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The GATOR complex regulates an essential response to meiotic double-stranded breaks in Drosophila

  1. Youheng Wei
  2. Lucia Bettedi
  3. Chun-Yuan Ting
  4. Kuikwon Kim
  5. Yingbiao Zhang
  6. Jiadong Cai
  7. Mary A Lilly  Is a corresponding author
  1. National Institute of Child Health and Human Development, National Institutes of Health, United States
  2. Yangzhou University, China
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Cite this article as: eLife 2019;8:e42149 doi: 10.7554/eLife.42149

Abstract

The TORC1 regulator GATOR1/SEACIT controls meiotic entry and early meiotic events in yeast. However, how metabolic pathways influence meiotic progression in metazoans remains poorly understood. Here we examine the role of the TORC1 regulators GATOR1 and GATOR2 in the response to meiotic double-stranded breaks (DSB) during Drosophila oogenesis. We find that in mutants of the GATOR2 component mio, meiotic DSBs trigger the constitutive downregulation of TORC1 activity and a permanent arrest in oocyte growth. Conversely, in GATOR1 mutants, high TORC1 activity results in the delayed repair of meiotic DSBs and the hyperactivation of p53. Unexpectedly, we found that GATOR1 inhibits retrotransposon expression in the presence of meiotic DSBs in a pathway that functions in parallel to p53. Thus, our studies have revealed a link between oocyte metabolism, the repair of meiotic DSBs and retrotransposon expression.

https://doi.org/10.7554/eLife.42149.001

Introduction

We are interested in understanding how metabolism impacts meiotic progression during oogenesis. Target of Rapamycin Complex 1 (TORC1) is a multi-protein complex that functions as a master regulator of metabolism (Loewith and Hall, 2011; Laplante and Sabatini, 2012a; Jewell and Guan, 2013). In the presence of adequate nutrients and positive upstream growth signals, TORC1, which contains the serine/threonine kinase Target of Rapamycin, becomes active and functions to stimulate growth and inhibit catabolic metabolism through the phosphorylation of down-stream effector proteins. The Seh1 Associated Complex Inhibits TORC1 (SEACIT), originally identified in yeast, inhibits TORC1 activity in response to amino acid limitation (Neklesa and Davis, 2009; Dokudovskaya et al., 2011; Wu and Tu, 2011; Bar-Peled et al., 2013; Panchaud et al., 2013). SEACIT, known as the GAP Activity Towards Rags complex 1 (GATOR1) in metazoans, is comprised of three highly conserved proteins Npr2/Nprl2, Npr3/Nprl3 and Iml1/Depdc5 (Bar-Peled et al., 2013; Panchaud et al., 2013). In Drosophila and mammals, depleting any of the three GATOR1 components results in increased TORC1 activity and growth, as well as a reduced response to amino acid starvation (Kowalczyk et al., 2012; Bar-Peled et al., 2013; Wei et al., 2014; Dutchak et al., 2015; Cai et al., 2016; Marsan et al., 2016). Thus, the role of the SEACIT/GATOR1 complex in the regulation of TORC1 activity is highly conserved in eukaryotes.

The multi-protein GATOR2 complex, known as Seh1 Associated Complex Activates TORC1 (SEACAT) in yeast, inhibits the activity of GATOR1 and thus functions to activate TORC1 (Bar-Peled et al., 2013; Wei et al., 2014) (Figure 1A). In metazoans, the GATOR2 complex functions in multiple amino acid sensing pathways (Bar-Peled et al., 2013; Panchaud et al., 2013; Chantranupong et al., 2014; Parmigiani et al., 2014; Kim et al., 2015; Cai et al., 2016). In tissue culture cells, depleting GATOR2 components results in the constitutive activation of GATOR1 and the permanent downregulation of TORC1 activity (Bar-Peled et al., 2013; Wei and Lilly, 2014). However, genetic studies of the role of individual GATOR2 components in Drosophila, indicate that the requirement for the GATOR2 complex is more nuanced when examined in the context of a multicellular animal (Iida and Lilly, 2004; Wei et al., 2016). For example, mutations in the GATOR2 component mio, result in a block to oocyte growth and differentiation, due to the constitutive downregulation of TORC1 activity in the female germline (Iida and Lilly, 2004; Wei et al., 2016). However, mio is not required to maintain TORC1 activity in most somatic tissues of Drosophila (Wei et al., 2016). Why there is a tissue specific requirement for mio in the female germline of Drosophila is currently unknown.

Figure 1 with 4 supplements see all
Mio prevents the constitutive downregulation of TORC1 activity in response to meiotic DSBs.

(A) The GATOR2 complex opposes the activity of the TORC1 inhibitor GATOR1. (B) Representative ovaries from wild type (WT), mio2, double-mutant mio2, mei-W681 and mei-W681 females. Scale bar, 1000 μm. (C) Western blot of p-S6K and total-S6K levels of whole ovaries prepared from WT, mio2 and mio2, mei-w681 and mei-W681 mutant females. (D) Quantification of p-S6K levels relative to total S6K. Unpaired student T-test was used to calculate the statistical significance. Error bars represent the standard deviation (SD) for three independent experiments. *p<0.05.

https://doi.org/10.7554/eLife.42149.002

In single celled eukaryotes, nutrient limitation often facilitates meiotic entry (van Werven and Amon, 2011). In the yeast Saccharomyces cerevisiae, the down-regulation of TORC1 by SEACIT/GATOR1 in response to amino acid stress promotes both meiotic entry and early meiotic progression (Deutschbauer et al., 2002; Jordan et al., 2007; Neklesa and Davis, 2009; Spielewoy et al., 2010). Surprisingly, as is observed in yeast, during Drosophila oogenesis the GATOR1 complex promotes meiotic entry (Wei et al., 2014). These data raise the intriguing possibility that in Drosophila the GATOR1 complex and low TORC1 activity may be critical to the regulation of additional events of the early meiotic cycle.

Here we report that the GATOR complex is critical to the response to meiotic DSB during Drosophila oogenesis. We find that restraining TORC1 activity via a pathway that involves both GATOR1 and the Tuberous sclerosis complex (TSC) promotes the timely repair of meiotic DSBs and prevents the hyperactivation of p53 in the female germline. Notably, the delayed repair of meiotic DSBs in GATOR1 mutants is due, at least in part, to the hyperactivation of the TORC1 target S6K. Conversely, our data indicate that the GATOR2 component Mio opposes the activity of GATOR1 in the female germline, thus preventing the constitutive downregulation of TORC1 activity and allowing for the growth and development of the oocyte in later stages of oogenesis. Thus, we have identified a regulatory loop required to modulate TORC1 activity in response to meiotic DSBs during Drosophila oogenesis. Finally, during the course of our studies, we observed that the GATOR1 complex prevents the derepression of retrotransposon expression in the presence of meiotic DSBs.

Results

Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs

The GATOR2 complex inhibits the TORC1 inhibitor GATOR1 (Figure 1A). Ovaries from mutants of the GATOR2 component mio, have reduced TORC1 activity and are severely growth restricted (Figure 1B–D) (Iida and Lilly, 2004; Wei et al., 2014). In our previous studies, we demonstrated that the mio ovarian phenotypes result from the constitutive downregulation of TORC1 activity via a GATOR1 dependent pathway (Wei et al., 2014). Thus, removing GATOR1 activity in the mio mutant background, as is observed in mio, nprl3 double mutants, results in increased TORC1 activity and rescues the mio ovarian phenotypes (Wei et al., 2014).

Surprisingly, mio mutants are also suppressed by blocking the formation of meiotic DSBs, with approximately 70% of mio ovaries achieving wild-type levels of growth when double mutant for genes required to generate meiotic DSBs (Figure 1B) (Iida and Lilly, 2004). One model to explain this observation is that meiotic DSBs promote the downregulation of TORC1 activity in the early meiotic cycle and that mio is required to oppose or attenuate this response. To test this idea, we examined if blocking the formation of meiotic DSBs in the mio mutant background resulted in increased TORC1 activity. Towards this end, we compared the phosphorylation status of S6 kinase, a downstream TORC1 target, in ovaries from mio2 single mutant versus mio, mei-W68 double mutant ovaries (Figure 1C,D). For these experiments, we used null alleles of both mio (mio2) and mei-W68 (mei-W681) (McKim and Hayashi-Hagihara, 1998; Iida and Lilly, 2004). mei-W68 (SPO11 homolog) is a highly-conserved enzyme required for the generation of meiotic DSBs (McKim and Hayashi-Hagihara, 1998; Sekelsky et al., 1999; Liu et al., 2002). We found that relative to ovaries from mio single mutants, mio, mei-W68 double mutants have increased levels of TORC1 activity as measured by the phosphorylation of S6K (Figure 1C,D). Notably, mio2, mei-W681 mutants have TORC1 activity levels similar to that observed in mei-W681 single mutants (Figure 1C,D). Why ovaries from mei-W681 single mutants have decreased levels of TORC1 activity relative to wild type ovaries is unclear. In addition to increasing TORC1 activity, blocking the formation of meiotic DSBs partially rescues mio mutant fertility, with mio2, mei-W681 double mutants laying approximately ten times more eggs than mio2 single mutants (Figure 1—figure supplement 1). Finally, mutants in the spnA homolog Rad51, which fail to repair meiotic DSBs, also have decreased TORC1 activity relative to wild-type ovaries (Figure 1—figure supplement 2). From these data, we conclude that the constitutive downregulation of TORC1 activity in mio mutants is potentiated, at least in part, by the presence of meiotic DSBs.

To refine when meiotic DSBs impact TORC1 activity during meiosis, we stained ovaries with antibodies against the phosphorylated form of the TORC1 target 4E-BP (Teleman et al., 2005). TORC1-mediated phosphorylation of 4E-BP, known as Thor in Drosophila, initiates cap-dependent translation by eIF4E (Gingras et al., 1998) (Teleman et al., 2005). In Drosophila, oogenesis begins in region 1 of the germarium when a germline stem cell divides to produce a cystoblast that undergoes four mitotic divisions, with incomplete cytokinesis, to produce a 16 cell interconnected germline cyst (Figure 2A) (de Cuevas et al., 1997). In late region 2a of the germarium, Spo11/Mei-W68 generates the meiotic DSBs that initiate meiotic recombination (McKim and Hayashi-Hagihara, 1998; Hunter, 2015). As meiosis proceeds, meiotic DSBs are repaired such that by late region 2b, only a small fraction of oocytes retain unrepaired DSBs (Mehrotra and McKim, 2006; Narbonne-Reveau and Lilly, 2009). We found that the levels of p4E-BP are low in the vast-majority of ovarian cysts undergoing mitotic divisions in region 1 (Figure 2A’,B). However, a small number (0.5 per germarium, n = 57) of region one ovarian cysts have dramatically higher levels of p4E-BP staining (Figure 2A”, arrowhead). These observations are consistent with the previously reported oscillation of TORC1 activity during the mitotic cell cycle in larval imaginal discs (Kim et al., 2017; Romero-Pozuelo et al., 2017). As ovarian cysts enter meiosis I in early region 2a of the germarium, the levels of p4E-BP are low (Figure 2A’,B). As ovarian cysts are encapsulated by follicle cells in germarial region 2b, the levels of p4E-BP begin to rise and remain above those observed in region one and early region 2a (Figure 2B, arrow). In contrast to what is observed in wild type, in mio mutant ovarian cysts, p4E-BP levels remain low in germline cells in region 2b and beyond (Figure 2C, arrow). Consistent with the western blot analysis, mio, mei-w68 double mutant ovarian cysts have an approximately three-fold increase in p4E-BP staining in region 2b of the germarium relative to mio mutants (Figure 2C,E and H) Notably, the increase in TORC1 activity in the mio, mei-w68 double mutants is restricted to the germline, consistent with blocking meiotic DSBs having cell autonomous effects on TORC1 activity in the germline. Additionally, consistent with our western blot analysis in Figure 1—figure supplement 2, ovarian cysts from spnA/Rad51 mutants, which do not repair meiotic DSBs, have reduced levels of p4E-BP staining (Figure 2F). Taken together these data strongly suggest that mio is required to oppose the downregulation of TORC1 activity triggered by the presence of meiotic DSBs.

Suppressing the production of meiotic DSBs increases p4E-BP staining in the female germline of mio mutants.

(A) Schematic representation of the Drosophila germarium. (A’A’) pseudo-color representation of p-4E-BP staining, arrowhead denotes a region one ovarian cyst with high p-4E-BP levels. Ovaries from (B) wild type (C) mio2, (D) mei-W681, (E) mio2; mei-W681, (F) spnA1, (G) thor2 females stained for C(3)G (cyan) to mark the synaptonemal complex, p-4E-BP (white), Vasa (magenta) to highlight the germline cytoplasm and DNA (Blue). (B) In wild-type ovarian cysts, p-4E-BP staining begins to increase in region 2b (arrow). (C) In mio mutant ovarian cysts, p-4E-BP levels remain low in region 2b and region 3. (D,E) mio, mei-W68 double mutants have p-4E-BP expression levels similar to mei-W68 single mutants. (F) spnA mutants, which fail to repair meiotic DNA breaks, have low levels of p-4E-BP staining (G) thor2/4E-BP null mutants serve as a negative control. (H) p-4E-BP intensity measurement of region 2b data (B)-(G). Scale bar: 7 μm. Unpaired T-student test was used to calculate statistical significance. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001.

https://doi.org/10.7554/eLife.42149.007

To examine if meiotic DSBs activate the downregulation of TORC1 activity via GATOR1, or activate a parallel TORC1 inhibitory pathway, we performed epistasis analysis with a null mutant of the GATOR1 component nprl3 (nprl31) and a mutant of mei-P22, a gene required for the generation of meiotic DSBs (McKim et al., 1998; Liu et al., 2002; Iida and Lilly, 2004). The mei-P22P22 mutation, which rescues the mio ovarian growth deficit, blocks the formation of meiotic DSBs resulting in meiotic exchange rates of zero or near zero (McKim et al., 1998; Liu et al., 2002; Iida and Lilly, 2004). We found that mei-P22P22 single mutants had pS6K levels that were not significantly different than ovaries from wild-type females when measured by western blot (Figure 1—figure supplement 3). This result is not surprising when one considers the anatomy of the Drosophila ovary. In wild-type ovaries, meiotic DSBs are present in only a small number of 16 cell cysts in the germarium. Moreover, meiotic DSBs are repaired prior to the rapid growth of the egg chamber. Thus, in wild type females, ovarian cysts that contain meiotic DSBs represent an exceedingly small percentage of the tissue in the ovary. Therefore, it is unlikely that increasing TORC1 activity in only a small number of germarial ovarian cysts would result in an increase in TORC1 activity in the ovary that could be observed by western blot. In contrast, note that ovaries from spnA mutants, which retain DSBs throughout oogenesis, have low TORC1 activity relative to wild type ovaries (Figure 1—figure supplement 2). Consistent with mei-P22 mutant ovaries not having increased TORC1 activity, we found that mei-P22, nprl3 double-mutant ovaries do not have pS6K levels above those of nprl3 single mutants. Thus, using epistasis analysis we were unable to definitely determine if meiotic DSBS trigger the downregulation of TORC1 activity via the GATOR1/TSC pathway.

We were interested in defining the upstream pathway that connects meiotic DSBs to the TORC1 regulatory machinery. Ataxia telangiectasia (ATM), known as telomere fusion (tefu) in Drosophila, regulates both the generation and the repair of meiotic DSBs (Joyce et al., 2011). atm/tefu mutants produce supernumerary meiotic DSBs. Therefore, we examined a downstream target and effector of atm/tefu, chk2, which has not been implicated in the generation of meiotic DSBs. chk2, known as loki in Drosophila, is a critical component of the DNA damage response pathway and has multiple targets involved in DNA repair, cell cycle progression and apoptosis (Zannini et al., 2014). We determined that removing chk2/loki activity in the mio mutant background, by generating double-mutants of loki and mio null alleles, partially rescued the mio mutant phenotype (Figure 1—figure supplement 4). Specifically, we found that mio2, loki6 double mutants are approximately twice the size of mio2 single mutants. In contrast, we previously demonstrated that the downregulation of TORC1 activity observed in the mio mutant is not triggered by the upstream activity of Ataxia Telangiectasia–Related (ATR), known as mei-41 in Drosophila (Iida and Lilly, 2004; Joyce et al., 2011). These data suggest that DSBs communicate to the TORC1 machinery at least in part through the checkpoint protein Chk2/Loki.

GATOR1 promotes the repair of meiotic DSB

In previous work we found that the GATOR1 complex downregulates TORC1 activity to facilitate meiotic entry in Drosophila ovarian cysts (Deutschbauer et al., 2002; Jordan et al., 2007; Neklesa and Davis, 2009; Spielewoy et al., 2010; Wei et al., 2014). However, the delay in meiotic entry observed in Drosophila GATOR1 mutants is not fully penetrant and therefore unlikely to be the sole cause of the infertility observed in GATOR1 mutant females (Figure 3—figure supplement 1A,B) (Wei et al., 2014). Considering our findings that meiotic DSBs serve to promote and/or reinforce low TORC1 activity after the mitotic/meiotic switch, we hypothesized that as is observed in yeast, the downregulation of TORC1 activity may be critical to the regulation of additional early meiotic events, including the repair of meiotic DSBs in Drosophila.

To test this hypothesis, we examined the behavior of meiotic DSBs in null alleles of nprl2, nprl3 and iml1 (Cai et al., 2016; Wei et al., 2016). During Drosophila oogenesis, the kinetics of DSB formation and repair can be followed using an antibody against the phosphorylated form of His2Av known as γ-His2Av (Madigan et al., 2002; Mehrotra and McKim, 2006). Drosophila ovarian cysts generate meiotic DSBs after the initiation of synaptonemal complex (SC) formation in region 2a of the germarium (Carpenter, 1975; Jang et al., 2003; Mehrotra and McKim, 2006). γ-H2Av nuclear foci are first observed in the two pro-oocytes, which are in early pachytene (Jang et al., 2003; Mehrotra and McKim, 2006). A small number of DSBs are also observed in the pro-nurse cells (Mehrotra and McKim, 2006; Narbonne-Reveau and Lilly, 2009). As meiosis proceeds and the DSBs are repaired, γ-H2Av-positive foci decrease in number and mostly disappear by late region 2b (Jang et al., 2003; Mehrotra and McKim, 2006). γ-H2Av signals are rarely detected in region three oocytes (Figure 3A, arrow). Analysis of mutants that fail to repair DSBs, and thus capture the total number of meiotic breaks, indicate that wild-type oocytes generate approximately 20–25 Spo11/Mei-W68 dependent breaks during oogenesis (Mehrotra and McKim, 2006; Joyce et al., 2011).

Figure 3 with 1 supplement see all
GATOR1 influences the steady state number and persistence of DSBs in early oocytes.

Ovaries from (A) wild type, (B) nprl21, (C) nprl31/Df, (D) iml1/Df, (E) nanos-GAL4; UAS-Nprl3; nprl31/Df, (F) mei-P22P22, nprl31, and (G) spnA1/spnA093 females were stained for C(3)G (green, A’–G’) and γ-H2Av (red, A–G). C(3)G marks the synaptonemal complex (SC) and is used to mark oocytes and follow meiotic progression. γ-H2Av marks DSBs. Scale bars, 10 μm. In wild type oocytes, meiotic DSBs are induced in region 2a and repaired by region 3 (arrow). In GATOR1 mutants, DSBs persist in region three oocytes. In nanos-GAL4; UAS-Nprl3; nprl31/Df oocytes, DSBs are repaired by region 3. mei-P22P22, nprl31 mutants have no DSBs. (H) Ovaries from wild type, nprl21, nprl31/Df, iml1/Df, spn-A1/spn-A093A, nprl21; spnA1/spnA093,mei-P22p22,nprl31 and nanos-GAL4; UAS-Nprl3; nprl31/Df flies were stained for C(3)G (green) and γ-H2Av (red). Representative immunofluorescent microphotographs of the γ-H2Av foci in region 2a oocyte are shown. (I) Percentage of region three oocytes with γ-H2Av foci. (J and K) Quantification of γ-H2Av foci in region 2a oocytes. Unpaired T-student test was used to calculate the statistical significance. Error bars represent SD from at least three independent experiments.**p<0.01, ****p<0.0001, ns: no significance. Note that the three GATOR1 mutants, nprl31, nprl21 and iml11 are null alleles (Cai et al., 2016; Wei et al., 2016).

https://doi.org/10.7554/eLife.42149.008

To determine if GATOR1 regulates the behavior of meiotic DSBs in Drosophila, we compared the pattern of γ-H2Av foci in wild-type versus GATOR1 mutant ovaries using antibodies against γ-H2Av and the SC component C(3)G, to highlight DSBs and oocytes respectively (Iida and Lilly, 2004; Mehrotra and McKim, 2006). We determined that while the majority of wild-type oocytes had repaired all of their DSBs and thus had no γ-H2Av foci by region 3 of the germarium, in GATOR1 mutants between 50–80% of region three oocytes are γ-H2Av positive (Figure 3A–E,H, arrow) (Mehrotra and McKim, 2006). Moreover, GATOR1 mutant oocytes had a significant increase in the steady state number of γ-H2Av foci per oocyte nucleus in region 2a of the germarium relative to wild-type oocytes (Figure 3J). From these data, we conclude that the GATOR1 complex influences the behavior of meiotic DSB during early oogenesis.

Next, we examined if the altered γ-H2Av pattern observed in GATOR1 mutants was dependent on the meiotic DSB machinery. Alternatively, the extra DSB may be induced during the premeiotic S phase, as is observed in mutants of the CycE/Cdk2 inhibitor dacapo (Hong et al., 2003). To address this question, we analyzed nprl3, mei-P22 double-mutants. As discussed above, mei-P22 is required for the formation of meiotic DSBs in Drosophila (Liu et al., 2002). We determined that double-mutant nprl3, mei-P22 oocytes had no γ-H2Av foci (Figure 3F,I and J). These data indicate that the increase in the steady state number of γ-H2Av foci, as well as the retention of these foci into region 3 of the germarium, are dependent on the meiotic DSB machinery.

During the development of wild-type Drosophila oocytes, the generation and repair of meiotic DSBs is asynchronous. Thus, the number of γ-H2Av foci observed in an oocyte at any single time point, is less than the total number of DSBs generated during the lifetime of the oocyte.

(Mehrotra and McKim, 2006; Joyce et al., 2011). We noticed that the number of γ-H2Av foci observed in GATOR1 mutant oocytes is never more than the 20–25 foci observed in mutants in the DSB repair pathway (Figure 3J) (Mehrotra and McKim, 2006). This observation suggested that the increase in the steady state number of DSBs observed in GATOR1 mutants may result from the delayed repair of meiotic DSBs, rather than the production of supernumerary Mei-W68/Spo11 induced DSBs. To test this hypothesis, we generated nprl2, spnA double mutants. Importantly, spnA mutants, including the spnA1/spnA093 transheterozygotes used here, fail to repair meiotic DSBs (Figure 3G) (Staeva-Vieira et al., 2003; Joyce and McKim, 2011). If nprl2 mutants make extra meiotic breaks, then the number of foci in the nprl2, spnA double mutants should be higher than either single mutant. However, we found that the nprl2, spnA double-mutants contained approximately the same number of γ-H2Av foci as spnA single mutants (Figure 3K). Thus, mutations in nprl2 do not result in the production of extra meiotic breaks. Taken together, our data strongly suggest that the GATOR1 complex influences the repair, rather than the production, of meiotic DSBs.

In Drosophila, the failure to repair meiotic DSBs activates an ATR-dependent checkpoint that disrupts dorsal ventral (DV) patterning in the egg (Ghabrial et al., 1998; Ghabrial and Schüpbach, 1999). We find that approximately 98% of eggs from nprl31 mothers exhibit no DV patterning defects (n = 308). Moreover, the approximately 2% of eggs from nprl3 mothers that have a possible DV patterning defect, are smaller and have a collapsed egg shell. Thus, the DV patterning defects observed in eggs from nprl3 mutant mothers may reflect a general problem in egg chamber growth and development. In contrast, 77% of eggs from mutant spnA1 mothers exhibit DV patterning defects (n = 271) (Staeva-Vieira et al., 2003). Why we do not observe DV patterning defects in GATOR1 mutants is unclear but may indicate that high TORC1 activity delays but does not block all aspects of meiotic DNA repair. Alternatively, high TORC1 activity may override the translational repression of the patterning gene gurken that drives the pattern defects observed in DNA repair mutants such as spnA (Abdu et al., 2002).

Co-depleting S6K rescues the increase in the steady state number of meiotic DSBs in iml1 germline depletions

TORC1 stimulates protein synthesis and cell growth through the phosphorylation of downstream effector proteins that promote anabolism and inhibit catabolism (Hay and Sonenberg, 2004; Wullschleger et al., 2006). We wanted to identify the pathways downstream of TORC1 that impact the repair of meiotic DSBs (Wei et al., 2016). Towards this end we examined the role of three well known downstream targets of TORC1 phosphorylation, S6K, Atg1 and 4E-BP in the regulation of meiotic DSBs (Laplante and Sabatini, 2012b). We determined that the depletion of iml1 by RNAi, recapitulates the iml1 mutant phenotype resulting in an increase in the steady state number of meiotic DSBs in the germarium (Figure 4). TORC1 phosphorylation activates S6K, a serine–threonine kinase that promotes translation and growth (Tavares et al., 2015). In mammals, S6K links growth control to the DNA damage response (Xie et al., 2018). To determine if the hyperactivation of S6K contributes to the defects in the repair of meiotic DSBs observed in GATOR1 mutants, we reduced the dose of the S6K gene by half in the iml1 germline depletions. Notably, reducing the dose of S6K lowered the steady state number of meiotic DSBs in iml1 RNAi depletions to levels observed in controls (Figure 4A,B). These data indicate that the delay in the repair of meiotic DSBs observed in GATOR1 mutant ovaries is at least in part the result of the hyperactivation of S6K.

Reducing the dose of S6K rescues the meiotic DSB phenotype in iml1 knockdowns.

(A) γ-H2Av foci in region 2a oocytes in the indicated genotypes. (B) Quantification of γ-H2Av foci in region 2a oocytes in the indicated genotypes. Note that meiotic DSBs were increased in nanos-GAL4; iml1RNAi females. Moreover, removing a single copy of S6K in the nanos-GAL4; iml1RNAi background reduced the steady state number of meiotic DSBs in nanos-GAL4; iml1RNAi; S6Kl-1/+ females. Unpaired T-student test was used to calculate statistical significance. *p<0.05, ****p<0.0001, n.s.: no significance. (B).

https://doi.org/10.7554/eLife.42149.010

High TORC1 inhibits the activation of autophagy via the inhibitory phosphorylation of Atg1 and Atg13 (Galluzzi et al., 2014). In our previous work, we determined that GATOR1 mutants fail to undergo autophagy in response to starvation due to inappropriately high TORC1 activity (Wei et al., 2016). Recent evidence suggests that blocking autophagy inhibits DSB repair through homologous recombination (Hewitt and Korolchuk, 2017). Therefore, to examine if a block to autophagy is responsible for the delay in the repair of meiotic DSBs we generated germline clones of a null allele of atg1, which is required for the activation of autophagy in Drosophila (Scott et al., 2004). We found that late region 2a oocytes in atg1Δ3D germline clones had approximately the same number of γ-H2Av foci as similarly staged wild-type oocytes (Figure 4B). From these data, we conclude that the increased number of meiotic DSBs observed in GATOR1 mutants is unlikely to be the result of a block to autophagy. Similarly, we found that null mutants of the translational inhibitor 4E-BP/thor, which is inhibited by TORC1 activity do not have an increased steady state number of meiotic DSBs. These data argue that the increase in the steady state number of meiotic DSBs in GATOR1 mutants is not the result of increased translation due to a block to 4E-BP translational inhibition.

GATOR1 mutants hyperactivate p53 in response to meiotic DSBs p53, a transcription factor that mediates a conserved response to genotoxic stress, regulates early meiotic events in multiple organisms (Stürzbecher et al., 1996; Lee et al., 1997; Linke et al., 2003; Mateo et al., 2016). During Drosophila oogenesis, the generation of meiotic DSBs results in the brief activation of p53 and the expression of downstream targets (Figure 5A) (Lu et al., 2010). To determine if GATOR1 mutants experience increased genotoxic stress during oogenesis, we used a reporter construct to assay p53 activity. The p53-GFPnls reporter construct contains the Green Fluorescent Protein (GFP) under the control of an enhancer from the p53 transcriptional target reaper (Lu et al., 2010). In wild-type ovaries, a faint signal from the p53-GFPnls reporter is first observed in region 2a of the germarium, concurrent with the generation of meiotic DSBs (Figure 5A, arrow) (Lu et al., 2010). As ovarian cysts continue to develop, the p53-GFPnls signal rapidly dissipates as meiotic DSBs are repaired (Figure 5A, arrowhead) (Lu et al., 2010). By region 3 of the germarium less than 5% of p53-GFPnls ovarian cysts contain detectable levels of GFP (Figure 5A). In contrast, the germaria of all three GATOR1 mutants exhibited a dramatic increase in both the strength and the duration of p53-GFPnls expression in the germarium, with strong GFP signal observed in nearly 80% of region three ovarian cysts (Figure 5A–D,F,G, arrowheads). Homozygous germline clones of the iml11 and nprl31 null alleles, hyperactivate p53 confirming that GATOR1 activity is required cell autonomously in the female germline (Figure 5—figure supplement 1).

Figure 5 with 1 supplement see all
GATOR1 prevents p53 hyperactivation in Drosophila early ovarian cysts Ovaries from (A) p53R-GFP, (B) nprl21; p53R-GFP, (C) p53R-GFP;nprl31/Df, (D) p53R-GFP; iml1/Df and (E) p53R-GFP; mei-P22p22, nprl31 were stained for GFP (green) and 1B1 (red).

Germarial regions are defined by 1B1 staining. In wild-type ovaries the p53-GFP reporter is briefly activated in region 2 (indicated by arrow). Note the low level of GFP staining. In contrast, in GATOR1 mutants, p53R-GFP is robustly activated with strong GFP signal often persisting into germarial region three and beyond. Additionally, in GATOR1 mutant germaria, p53R-GFP is frequently activated in germline stem cell (GSC) and daughter cystoblasts (CB). In mei-P22p22, nprl31 double mutant germaria, the hyperactivation of p53R-GFP is rescued in region 2a ovarian cysts. However, p53-GFP activation in GSC and CB is retained in the double mutants (asterisk) indicating that in these cells the activation of p53 is not contingent on the presence of meiotic DSBs. Scale bars, 10 μm. (F) Percentage of germaria with sustained p53R-GFP signal in region 3. (G) Percentage of germaria with high p53R-GFP signal in region 2. (H) Percentage of germaria with p53R-GFP expression in GSC and CB. Unpaired student T-test was used to calculate the statistical significance Error bars represent SD from at least three independent experiments. **p<0.01, ****p<0.0001.

https://doi.org/10.7554/eLife.42149.011

We predicted that the persistent hyperactivation of p53 in the female germ line of GATOR1 mutants is due to the delayed repair of meiotic DSBs. To test this model, we examined p53 activation in the mei-P22, nprl3 double mutant. Strikingly, inhibiting meiotic DSBs strongly suppressed the expression of p53-GFPnls in the nprl3 mutant background (Figure 5E). From these observations, we infer that in GATOR1 mutant ovaries, the hyperactivation of p53 is downstream of Spo11/Mei-W68-induced DBS. Moreover, we conclude that the GATOR1 complex is required to oppose genotoxic stress triggered by meiotic DSBs and/or other downstream events of meiotic recombination.

Recent evidence indicates that p53 is activated in germline stem cells of Drosophila after exposure to cellular stresses, including deregulated growth and ionizing radiation (Wylie et al., 2016; Ma et al., 2016). The GATOR1 complex inhibits TORC1 activity and is required to restrain growth in Drosophila (Wei et al., 2014; Cai et al., 2016). As was reported with other mutants that deregulate growth (Wylie et al., 2016), we found that the p53-GFPnls reporter construct is robustly activated in the germline stem cells and their near descendants in GATOR1 mutant females (Figure 5B–D,H). Consistent with the restriction of Mei-W68/Spo11 activity to meiotic cysts, the mei-P22, nprl3 double mutants retain p53-GFPnls expression in stem cells even though the meiotic activation of the p53- GFPnls reporter is lost in regions 2a and 2b of the germarium in the mei-P22, nprl3 double mutant (Figure 5E–G). Thus, an independent cellular stress, likely related to deregulated metabolism or growth, activates p53 in the germline stem cells and cystoblasts of GATOR1 mutant females.

Tsc1 germline depletions phenocopy GATOR1 mutants

The most parsimonious interpretation of our data is that in GATOR1 mutant ovaries, high TORC1 activity opposes the timely repair of meiotic DSBs and increases genotoxic stress. To test this model, we depleted the Tuberous sclerosis complex (TSC) component Tsc1 from the female germline. TSC is a potent inhibitor of TORC1 that directly inhibits the small GTPase Rheb, a critical activator of TORC1 (Inoki et al., 2003; Zhang et al., 2003). We determined that depleting Tsc1 in the female germline using RNAi, resulted in the robust expression of p53-GFPnls during the early meiotic cycle (Figure 6A–D). Moreover, as was observed in GATOR1 mutants, Tsc1RNAi oocytes had an increase in the steady state number of γ-H2Av foci as well as an increase in the percentage of oocytes that retained γ-H2Av positive into region 3 of the germarium (Figure 6E,F). Taken together our data support the model that the tight control of TORC1 activity by both GATOR1 and TSC is essential to the proper regulation of meiotic DSBs during Drosophila oogenesis.

The TORC1 inhibitor TSC1 promotes genomic stability in early oocytes.

Ovaries from (A) p53R-GFP; MTD >mCherry RNAi and (B) p53R-GFP; MTD >Tsc1 RNAi flies were stained for GFP (green) and 1B1 (red). In the mCherry RNAi (control) ovaries the p53-GFP expression is very low. In contrast, the Tsc1 RNAi ovaries have sustained GFP signal that persists into germarial region 3. Scale bars, 10 μm. (C) Percentage of germaria with strong p53R-GFP signal in region 2. (D) Percentage of germaria with sustained p53R-GFP signal in region 3. The γ-H2Av foci were determined in MTD >mCherry RNAi and MTD >Tsc1 RNAi ovaries. (E) Percentage of region three oocytes with γ-H2Av foci. (F) Quantification of γ-H2Av foci per oocyte in region 2a. Unpaired student T-test was used to calculate the statistical significance Error bars represent SD from at least three independent experiments. *p<0.05, ****p<0.0001.

https://doi.org/10.7554/eLife.42149.013

nprl3 mutant follicle cells are sensitive to genotoxic stress

In humans, mutations in components of TSC sensitize cells to multiple forms of genotoxic stress (Paterson et al., 1982; Deschavanne and Fertil, 1996; Lee et al., 2007; Pai et al., 2016). Thus, we predicted that the GATOR1 mutants, which have a two to three-fold increase in TORC1 activity, might have a globally diminished ability to respond to DNA damage. To test this model, we treated nprl3 mutant larvae with the mutagen Methyl Methane Sulfonate (MMS) and compared the percentage of mutant animals that survived to adulthood relative to sibling heterozygous controls. MMS generates an array of DNA lesions including DSBs (Magaña-Schwencke et al., 1982). Notably, we found that nprl3 mutant larvae were sensitive to DNA damage, with nprl3/Df transheterozygotes exhibiting a greater than 10-fold decrease in survival rates when exposed to 0.08% MMS (Table 1). These data support the idea that the GATOR1 complex plays a critical role in the response to genotoxic stress in both germline and somatic tissues.

Table 1
nprl2 and nprl3 larvae are sensitive to the mutagen Methyl Methane Sulfonate.
https://doi.org/10.7554/eLife.42149.014
MMS (0%)MMS (0.04%)MMS (0.08%)
Genotype% Obs. (# of Obs. / # of Total)% of expected progeny% Obs. (# of Obs. / # of Total)% of expected progeny% Obs. (# of Obs. / # of Total)% of expected progeny
nprl21/nprl2127.6 (172/624)55.2% (27.6/50)17.6 (99/561)35.2% (17.6/50)13.7 (42/307)27.4% (13.7/50)
nprl31/Df27.1 (822/3038) 81.4% (27.1/33.3)8 (198/2472)24% (8.0/33.3)2.4 (46/1925)7.2% (2.4/33.3)
  1. Eclosion after exposure to the mutagen Methyl Methane Sulfonate. Third instar larvae derived from heterozygous parents were treated with the indicated concentration of MMS and the surviving adult progeny were scored. % Obs represents the total percentage of adults of the indicated mutant genotype divided by the total number of adults scored. % of expected progeny represents the percentage of mutant adults observed (% Obs) divided by the expected percentage of mutant adults based on the parental cross. For nprl2, which is on the X chromosome, the expected percentage of mutant progeny was 50%. For nprl3, which is on the 3rd chromosome, the expected percentage of mutant progeny was 33.3%. The observation that nprl2 and nprl3 have a lower percent survival than would be predicted at 0% MMS reflects the fact that these mutants are partially lethal in the absence of mutagen.

    Obs.=Observed, # of Obs = Number of mutant adults scored, # of Total = Total number of adults scored.

Next, we wanted to determine if, as we observed in the female germline, mutations in GATOR1 components result in a delay in the repair of DSBs in somatic tissues. We exposed Drosophila females to 10 Gray (Gy) of γ-rays and then followed the dynamics of γ-H2Av staining in the somatically derived mitotically dividing follicle cells of the ovary. During Drosophila oogenesis, the somatic follicle cells divide mitotically until stage 6 of oogenesis at which point, they enter the endocycle. Prior to stage 6 of oogenesis wild-type and nprl3/Df follicle cells have very low levels of γ-H2Av staining (Figure 7A,B) (Hong et al., 2007). One hour after exposure to 10Gy of γ-H2Av both wild-type and nprl3/Df mutants have a dramatic increase in follicle cells with γ-H2Av foci (Figure 7A’,B’). However, 6 hr after irradiation while wild-type somatic cells have decreased numbers of γ-H2Av positive follicle cells due to rapid DNA repair, nprl3/Df females retain elevated numbers of follicle cells with γ-H2Av foci. (Figure 7A–C). Thus, as is observed in the female germline, in nprl3 mutants the somatic cells of the ovary exhibit a delay in the repair of DSBs.

Nprl3 promotes DNA repair in somatically derived follicle cells.

Egg chambers from fed (A) WT and (B) nprl31/Df females exposed to 10 Gy γ-irradiation. Ovaries were dissected at 0 hr (no irradiation) (A, B), 1 hr (A’, B’) and 6 hr post-irradiation (A’’, B’’) and stained with antibodies against γ-H2Av (dsDNA breaks, Red), C(3)G (cyan) and the DNA dye DAPI (blue). (C) Quantification of γ-H2Av positive follicle cells from the indicated time points and genotypes. Note that an increased percentage of nprl3/Df follicle cells contain γ-H2Av foci 6 hr post irradiation relative to controls. Scale bar: 7 μm. Arrows denote γ-H2Av positive follicle cells.

https://doi.org/10.7554/eLife.42149.015

GATOR1 inhibits retrotransposon expression in Drosophila

Genotoxic stress, resulting from DNA damage, has been implicated in transposon activation in multiple organisms (Bradshaw and McEntee, 1989; Walbot, 1992; Hagan et al., 2003; Beauregard et al., 2008). These results are consistent with the model that genotoxic stress promotes TE activation (McClintock, 1984; Harris et al., 2009; Wylie et al., 2016). Therefore, we wanted to determine if retrotransposons expression is derepressed in the ovaries of GATOR1 mutants which exhibit several phenotypes consistent with increased genotoxic stress. Towards this end, we used qRT-PCR to compare expression levels for multiple retrotransposons in wild type versus nprl2 and nprl3 mutant ovaries. We found that nprl2 and nprl3 mutant ovaries have increased expression of multiple retrotransposons including TAHRE, Het-A, Indefix and Gypsy (Figure 8A). In contrast to nprl2 and nprl3 mutants, Tsc1RNAi germline resulted in little or no increase in retrotransposon expression (Figure 8—figure supplement 1). From these results, we conclude that the GATOR1 components nprl2 and nprl3 oppose retrotransposon expression in the female germline of Drosophila.

Figure 8 with 1 supplement see all
GATOR1 opposes retrotransposon expression in parallel to p53.

(A) Quantitative RT-PCR analysis of expression levels for retrotransposons in wild type, nprl21, nprl31/Df, nanos-GAL4; UAS-Nprl3; nprl31/Df, spnA1/spnA093 and spnA1/spnA1 ovaries. (B) Quantitative RT-PCR analysis of expression levels for transposons in wild type, nprl31 and mei-P22P22, nprl31 ovaries. (C) Quantitative RT-PCR analysis of expression levels for the transposons in wild type, p535A-1-4, nprl31/Df and nprl31/Df, p535A-1-4 ovaries. Rp49 is used for normalization. Fold expression levels are relative to wild type. Error bars represent SD of three independent experiments.

https://doi.org/10.7554/eLife.42149.016

An additional possible connection between retrotransposon expression and the delayed repair of meiotic DSBs is suggested by our analysis of the DNA repair protein spnA/Rad51. As discussed above, spnA is a homolog of the DNA repair protein Rad51 which is conserved from yeast to humans (Staeva-Vieira et al., 2003). Rad51 catalyzes strand exchange between homologous DNA molecules and thus facilitates homologous recombination. In Drosophila, spnA/Rad51 mutant females fail to repair meiotic DSBs (Staeva-Vieira et al., 2003). Intriguingly, we found that spnA/Rad51 mutant ovaries had increased levels of retrotransposon expression (Figure 8A). Recently it has been reported that depletions of Rad51 result in the activation of Long interspersed repeat element 1 (LINE1) retrotransposons in HeLa cells (Liu et al., 2018). Thus, our data are consistent with the model that the increased levels of retrotransposon expression observed in GATOR1 mutant ovaries may be due, at least in part, to the delay in the repair of meiotic DSBs.

In nprl3 mutant ovaries, the activation of p53 and the increase in γ-H2Av foci is dependent on the production of meiotic DSBs (Figures 3 and 4). To determine if the expression of retrotransposons in nprl3 mutant ovaries also requires the meiotic DSB machinery, we examined nprl3, mei-P22 double mutants. Using qRT-PCR we observed that retrotransposon expression was largely, but not completely, suppressed in nprl3, mei-P22 double-mutant ovaries. Thus, meiotic DSBs trigger the expression of retrotransposons during Drosophila oogenesis in the nprl3 mutant background (Figure 8B). However, it is important to note that these data suggest that the GATOR1 complex may also impact retrotransposon expression independent of meiotic DSBs as indicated by the relatively modest rescue of TAHRE over-expression observed in nprl3, mei-P22 double mutants (Figure 8B).

Finally, we wanted to determine if the GATOR1 complex inhibits the activation of retrotransposons through the p53 pathway. To answer this question, we performed epistasis analysis by generating double mutants that were homozygous for null alleles of both p53 and nprl3. Strikingly, the p53, nprl3 double mutant ovaries showed a dramatic increase in retrotransposon expression relative to either single mutant (Figure 8C). Thus, the p53 and nprl3 phenotypes are additive with respect to the inhibition of retrotransposon expression. These data strongly suggest that GATOR1 and p53 act through independent pathways to inhibit retrotransposon activation in the female germline during meiosis (Figure 8C).

Discussion

Recent evidence implicates metabolic pathways as important regulators of meiotic progression and gametogenesis (LaFever et al., 2010; Ferguson et al., 2012; Chi et al., 2016; Sieber et al., 2016; Guo et al., 2018). Here we define a role for the GATOR complex, a conserved regulator of TORC1 activity, in the regulation of two events that impact germline genome stability: the response to meiotic DSBs and the inhibition of retrotransposon expression.

The GATOR complex and the response to meiotic DSBs

We have previously shown that in Drosophila, mutations in the GATOR2 component mio, result in the constitutive activation of the GATOR1 pathway in the female germline but not in somatic tissues (Iida and Lilly, 2004; Wei et al., 2014). Here we demonstrate that the tissue specific requirement for mio during oogenesis is due, at least in part, to the generation of meiotic DBSs during oogenesis. In Drosophila, only the female germline undergoes meiotic recombination and thus experiences the genotoxic stress associated with developmentally programmed DSBs (Hughes et al., 2018). We show that in mio mutants, blocking the formation of meiotic DSBs prevents the constitutive downregulation of TORC1 activity thus allowing for the growth and development of the oocyte. These data are consistent with the model that meiotic DSBs trigger the activation of a TORC1 inhibitory pathway that must be opposed and/or attenuated by the GATOR2 component Mio (Figure 9).

A working model for the role of the GATOR complex in the response to meiotic DSBs.

(A) After ovarian cysts enter meiosis, meiotic DSBs function to activate and/or maintain a GATOR1/TSC dependent pathway to ensure low TORC1 activity in early prophase of meiosis I. Low TORC1 activity promotes the timely repair of meiotic DSBs. Currently, whether meiotic DSBs directly activate the GATOR1/TSC pathway or an alternative pathway that works in concert with, or in parallel to, GATOR1/TSC is not known. (B) Subsequently, the GATOR2 component Mio is required to attenuate the activity of the GATOR1/TSC pathway, thus allowing for increased TORC1 activity and the growth and development of the oocyte in later stages of oogenesis.

https://doi.org/10.7554/eLife.42149.018

While there are several possible models that might explain our data, we believe the most parsimonious explanation for our results is that the TORC1 inhibitory pathway activated by meiotic DSBs, involves both GATOR1 and TSC (Figure 9). This model is consistent with the ability of both GATOR1 and TSC depletions to rescue the mio mutant phenotype (Wei et al., 2014). Additionally, recent reports indicate that GATOR1 and TSC act in a common pathway to downregulate TORC1 activity in response to multiple upstream inhibitory inputs (Demetriades et al., 2014; Menon et al., 2014; Demetriades et al., 2016). Previously, we determined that in Drosophila, amino acid starvation induces a dramatic GATOR1/TSC dependent decrease in TORC1 activity in somatic tissues, that far exceeds any reduction in TORC1 activity observed in GATOR2 null mutants (Wei et al., 2014; Cai et al., 2016). This observation strongly suggests that, in addition to the removal of the GATOR2 inhibition of GATOR1, there is an activation step that is required to fully potentiate the GATOR1/TSC pathway.

Thus, based on our data we propose the following model (Figure 9). Meiotic DSBs activate, or are required to maintain, a GATOR1/TSC dependent pathway that downregulates TORC1 activity in the female germline (Figure 9A). The GATOR2 component Mio is required to oppose or turnoff this pathway to prevent the constitutive downregulation of TORC1 activity in later stages of oogenesis. While we believe our data support the role of the GATOR1/TSC pathway, we concede that an alternative regulator of TORC1 activity may also be critical to the downregulation of TORC1 activity in response to meiotic DSBs.

GATOR1 and TSC promote the repair of meiotic DSBs

Hyperactivation of TORC1 has been linked to defects in the DNA damage response in single celled and multicellular organisms (Begley et al., 2004; Feng et al., 2007; Klermund et al., 2014; Pai et al., 2016; Ma et al., 2018; Xie et al., 2018). The observation that meiotic DSBs likely promote the GATOR1 dependent downregulation of TORC1 activity during Drosophila oogenesis, suggested that limiting TORC1 activity may be important to the regulation of meiotic DSB repair. In our previous work, we found that GATOR1 mutant ovaries had TORC1 activity levels approximately three times higher than those observed in wild-type ovaries (Wei et al., 2014; Cai et al., 2016). Here we demonstrate that GATOR1 mutant ovaries exhibit multiple phenotypes consistent with the misregulation of meiotic DSB repair including, an increase in the steady state number of Mei-W68/Spo-11 induced DSBs, the retention of meiotic DSBs into later stages of oogenesis and the hyperactivation of p53. (Cai et al., 2016; Wei et al., 2016). Importantly, RNAi depletions of Tsc1 partially phenocopied the GATOR1 ovarian defects. Thus, the misregulation of meiotic DSBs observed in GATOR1 mutant oocytes are due to high TORC1 activity and not to a TORC1 independent function of the GATOR1 complex.

Epistasis analysis between the GATOR1 component nprl3 and the Rad51 homolog spnA, strongly suggest that GATOR1 impacts the repair, rather than the generation, of meiotic DSBs. We determined that double mutants of nprl2 and the Rad51 homolog spnA, which is required for the repair of meiotic DSBs, have approximately the same number of DSBs as spnA single mutants. These data are consistent with GATOR1 and spnA influencing the common process of DNA repair and are inconsistent with GATOR1 mutants producing supernumerary breaks.

Our observations on the role of the GATOR1 complex during Drosophila oogenesis are particularly intriguing in light of similar meiotic defects observed in a npr3 mutants in Saccharomyces cerevisiae (Jordan et al., 2007). In the sporulation proficient strain SK1, npr3 mutant cells enter meiosis and express the transcription factor and master regulator of gametogenesis IME1 with wild-type kinetics (Jordan et al., 2007). Subsequently, npr3 mutants exhibit a mild delay in the generation of meiotic DNA breaks, but a substantial delay in the repair of meiotic DSBs (Jordan et al., 2007). Thus, yeast and Drosophila SEACIT/GATOR1 mutants share a common meiotic phenotype, the delayed repair of meiotic DSBs. These results raise the intriguing possibility that low TORC1 activity may be a common feature of the early meiotic cycle in many organisms.

Notably, our data indicate that the delay in the repair of meiotic DSBs in GATOR1 mutants is due to the hyperactivation of the TORC1 downstream target S6K. S6K is a critical downstream effector of TORC1 that impacts multiple essential cellular processes including, but not limited to cell growth, energy balance and aging (Magnuson et al., 2012). Intriguingly, in mammals, S6K has been implicated in the regulation of the DNA damage response with hyperactivation of the TORC1-S6K pathway resulting in the accumulation of unrepaired DSBs and genome instability (Lai et al., 2010; Xie et al., 2018). Thus, similar to what is reported in mammals, our data are consistent with the model that the hyperactivity of the TORC1/S6K axis delays the repair of DSBs in Drosophila.

Finally, we determined that GATOR1 mutants have a diminished response to DSBs outside the female germline in somatic tissues of Drosophila. Similar to what is observed in TSC mutant cells in humans that have increased levels of TORC1 activity, we find that GATOR1 mutant embryos have a reduced ability to survive low levels of γ-irradiation (Paterson et al., 1982; Deschavanne and Fertil, 1996; Lee et al., 2007; Pai et al., 2016). Moreover, in the somatic follicle cells of the ovary we observed a delay in the repair of DSBs after adult females are exposed to low levels of γ-irradiation. Thus, in Drosophila inappropriately high TORC1 activity delays the repair of DSBs in both the germline and somatic tissues.

GATOR1 opposes retrotransposon expression

The initiation of homologous recombination through the programmed generation of DNA double-stranded breaks (DSBs) is a universal feature of meiosis (McKim and Hayashi-Hagihara, 1998; Gray and Cohen, 2016). DSBs represent a dangerous form of DNA damage that can result in dramatic and permanent changes to the germline genome (Alexander et al., 2010). To minimize this destructive potential, the generation and repair of meiotic DSBs is tightly controlled in space and time (Longhese et al., 2009). The activation of transposable elements represents an additional threat to genome integrity in germ line cells (Crichton et al., 2014; Toth et al., 2016). Genotoxic stress, resulting from DNA damage, has been implicated in the deregulation of transposons in multiple organisms (Bradshaw and McEntee, 1989; Walbot, 1992; Hagan et al., 2003; Beauregard et al., 2008). Thus, germ line cells may be at an increased risk for transposon derepression due to the genotoxic stress associated with meiotic recombination. Consistent with this hypothesis, germ line cells have evolved extensive surveillance systems to detect and silence transposons beyond the pathways present in most somatic tissues (Khurana and Theurkauf, 2010; Ku and Lin, 2014; Toth et al., 2016).

Previous studies have shown that DNA damage promotes the deregulation of retrotransposon in multiple organisms, including Drosophila (McClintock, 1984; Bradshaw and McEntee, 1989; Walbot, 1992; Hagan et al., 2003; Beauregard et al., 2008; Molla-Herman et al., 2015; Wylie et al., 2016). In line with these studies, we find that in GATOR1 mutants, the DSBs that initiate meiotic recombination trigger the deregulation of retrotransposon expression. Similarly, p53 mutant females derepress retrotransposon expression during oogenesis, but as observed in GATOR1 mutants, primarily in the presence of meiotic DSBs (Wylie et al., 2016). Double mutants of nprl3, p53 exhibit a dramatic increase in retrotransposon expression relative to either p53 or nprl3 single mutants, implying that p53 and GATOR1 act through independent pathways to repress retrotransposon expression in the female germline. One possibility is that both GATOR1 and p53 independently impact genome stability. Thus, disabling both pathways may have an additive effect on both genome stability and retrotransposon expression. Consistent with the hypothesis that genome instability drives retrotransposon expression, we find that mutants in spnA/Rad51, which fail to repair meiotic DSBs, also exhibit increased transcription of multiple retrotransposons. Intriguingly, the SpnA homolog Rad51, as well as other genes required for DNA repair, was recently identified in a high throughput screen for genes that suppress (Long Interspersed Element-1) LINE1 expression in mammalian tissue culture cells (Liu et al., 2018).

However, our data also suggest that the GATOR1 complex may influence retrotransposon expression independent of the regulation of TORC1 activity. While both GATOR1 and TSC are required for the efficient repair of meiotic DSBs, in contrast to GATOR1 mutant ovaries, we observed little to no increase in retrotransposon expression in the Tsc1 depleted ovaries. We believe reflects the incomplete depletion of Tsc1 by RNAi resulting in a reduced retention of meiotic DSBs relative to GATOR1 mutants (Figure 6). However, a second possibility is that the GATOR1 complex inhibits retrotransposon expression independent of TORC1 inhibition. As is observed with spnA the depletion of GATOR1 components, but not TSC components result in the activation of LINE1 expression in HeLa cells (Liu et al., 2018). Taken together, these data hint that the GATOR1 complex may impact retrotransposon expression in the germline via two independent pathways: First by promoting the repair of meiotic DSBs through the downregulation of TORC1 activity and second via a pathway that functions independent of TORC1 inhibition.

Genes encoding components of the GATOR1 complex are often deleted in cancers (Lerman and Minna, 2000; Ji et al., 2002; Ueda et al., 2006; Bar-Peled et al., 2013). As is observed in GATOR1 mutants, cancer cells frequently have increased TORC1 activity, increased genomic instability and increased retrotransposon expression. Thus, in the future it will be important to identify the molecular mechanism by which the GATOR1 complex influences both the response to genotoxic stress and the expression of retrotransposons under both normal and pathological conditions.

Materials and methods

Key resources table
Reagent type or
resource
DesignationSource or referenceIdentifiersAdditional
information
Gene (Drosophila melanogaster)Atg1FlyBaseFBgn0260945
Gene (Drosophila melanogaster)Iml1FlyBaseFBgn0035227
Gene (Drosophila melanogaster)LokiFlyBaseFBgn0019686
Gene (Drosophila melanogaster)MioFlyBaseFBgn0031399
Gene (Drosophila melanogaster)Mei-W68FlyBaseFBgn0002716
Gene (Drosophila melanogaster)Mei-P22FlyBaseFBgn0016036
Gene (Drosophila melanogaster)Nprl2FlyBaseFBgn0030800
Gene (Drosophila melanogaster)Nprl3FlyBaseFBgn0036397
Gene (Drosophila melanogaster)p53FlyBaseFBgn0039044
Gene (Drosophila melanogaster)Spn-AFlyBaseFBgn0003479
Gene (Drosophila melanogaster)S6KFlyBaseFBgn0283472
Gene (Drosophila melanogaster)ThorFlyBaseFBgn0261560
Gene (Drosophila melanogaster)Tsc1FlyBaseFBgn0026317
Genetic reagent (Drosophila melanogaster)Atg1Δ3D, FRT 2A/Tm3, sb1FlyBase, PMID: 24098761FBal0176392
Genetic reagent (Drosophila melanogaster)iml11FlyBase, PMID: 27672113FBal0325028FlyBase symbol: iml11
Genetic reagent (Drosophila melanogaster)lokiP6FlyBase, PMID: 14729967FBal0216721FlyBase symbol: lokp6
Genetic reagent (Drosophila melanogaster)mio2FlyBase, PMID: 14973288FBal0158954FlyBase symbol: mio2
Genetic reagent (Drosophila melanogaster)mei-P22P22Bloomington Drosophila Stock CenterBDSC:4931Genotype: y1w1/Dp(1;Y)y+; mei-P22P22; svspa-pol
Genetic reagent (Drosophila melanogaster)mei-W681Bloomington Drosophila Stock CenterBDSC:4962FlyBase symbol: mei-W861
Genetic reagent (Drosophila melanogaster)nprl21FlyBase, PMID: 27672113FBal0325026FlyBase symbol: nprl21
Genetic reagent (Drosophila melanogaster)nprl31FlyBase, PMID: 27166823FBal0319815FlyBase symbol: nprl31
Genetic reagent (Drosophila melanogaster)p535A-1-4Bloomington Drosophila Stock CenterBDSC:6815Genotype: y1 w1118; p535A-1-4
Genetic reagent (Drosophila melanogaster)S6KI-1Bloomington Drosophila Stock CenterBDSC:32552Genotype: y[1] w[*]; S6k[l-1]/TM6B, P{y[+t7.7] ry[+t7.2]=Car20y}TPN1, Tb[1]
Genetic reagent (Drosophila melanogaster)spnA1Bloomington Drosophila Stock CenterBDSC:3322Genotype: Dp(1;Y)BS; ru1 st1 e1 spn-A1 ca1/TM3, Sb1
Genetic reagent (Drosophila melanogaster)spnA093AFlyBase, PMID: 14592983FBal0151428FlyBase symbol: spn-A093A
Genetic reagent (Drosophila melanogaster)thor2Bloomington Drosophila Stock CenterBDSC:9559Genotype: y[1] w[*]; Thor[2]
Genetic reagent (Drosophila melanogaster)Df(3L)ED4238Bloomington Drosophila Stock CenterBDSC:8052Genotype: w1118; Df(3L)ED4238, P{3'.RS5+3.3'}ED4238/TM6C, cu1 Sb1
Genetic reagent (Drosophila melanogaster)Df(3L)ED4515Bloomington Drosophila Stock CenterBDSC:9071Genotype: w1118; Df(3L)ED4515, P{3'.RS5+3.3'}ED4515/TM6C, cu1 Sb1
Genetic reagent (Drosophila melanogaster)nanos-Gal4FlyBase, PMID: 9501989FBrf0100715FlyBase symbol: GAL4VP16.nos.UTR
Genetic reagent (Drosophila melanogaster)p53R-GFPFlyBase, PMID: 20522776FBrf0210965FlyBase symbol: GFPrpr.p53R.Tag:NLS(Unk)
Genetic reagent (Drosophila melanogaster)MTD-Gal4Bloomington Drosophila Stock CenterBDSC:31777Genotype: P{otu-GAL4::VP16.R}1, w*; P{GAL4-nos.NGT}40; P{GAL4::VP16-nos.UTR}CG6325MVD1
Genetic reagent (Drosophila melanogaster)Tsc1 RNAiBloomington Drosophila Stock CenterBDSC:35144Genotype: y1 sc* v1; P{TRiP.GL00012}attP2
Genetic reagent (Drosophila melanogaster)Iml1 RNAiBloomington Drosophila Stock CenterBDSC:57492Genotype: y[1] sc[*] v[1] sev[21]; P{y[+t7.7] v[+t1.8]=TRiP.HMC04806}attP40
Genetic reagent (Drosophila melanogaster)mCherry RNAiBloomington Drosophila Stock CenterBDSC:35787Genotype: y[1] sc[*] v[1] sev[21]; P{y[+t7.7] v[+t1.8]=UAS mCherry.VALIUM10}attP2
Genetic reagent (Drosophila melanogaster)UAS-Nprl3FlyBase, PMID: 27672113FBrf0234182FlyBase symbol: Nprl3UASp.Tag:FLAG,Tag:HA
Genetic reagent (Drosophila melanogaster)FRT80BBloomington Drosophila Stock CenterBDSC:1988Genotype: w[*]; P{ry[+t7.2]=neoFRT}80B ry[506]
Genetic reagent (Drosophila melanogaster)FRT2ABloomington Drosophila Stock CenterBDSC:1997Genotype: w[*]; P{w[+mW.hs]=FRT(w[hs])}2A
Genetic reagent (Drosophila melanogaster)FRT2A, ubi-GFPnlsBloomington Drosophila Stock CenterBDSC: 5825Genotype: w[1118]; P{w[+mC]=Ubi GFP.nls}3L1 P{Ubi-GFP.nls}3L2 P{w[+mW.hs]=FRT(w[hs])}2A
Genetic reagent (Drosophila melanogaster)hsFLP; FRT80B,lacZBloomington Drosophila Stock CenterBDSC:6341Genotype: P{hsFLP}22, y1 w*; P{arm-lacZ.V}70 C P{neoFRT}80B
Genetic reagent (Drosophila melanogaster)UAS-mCherry RNAiBloomington Drosophila Stock CenterBDSC:35785Genotype: y1 sc* v1; P{VALIUM20-mCherry}attP2
Genetic reagent (Drosophila melanogaster)nos-FlpThis paper.Lilly Lab.
AntibodyGoat anti-Rabbit, mouse Alexa 488–568- 594–647- secondariesThermo FisherImmunofluorescence (1:1000)
Antibodyanti-dS6K (Guinea pig polyclonal)PMID: 20444422Western Blot (1:5000)
Antibodyanti-phospho-Thr398-S6K (Rabbit polyclonal)Cell Signaling Technologies9209; RRID:AB_2269804Western Blot (1:1000)
Antibodyanti-GFP (Rabbit polyclonal)InvitrogenA11122; RRID:AB_221569Immunofluorescence (1:1000)
Antibodyanti-g-H2Av (Rabbit poly clonal)Active Motif39117; RRID:AB_2793161Immunofluorescence (1:1000)
Antibodyanti-C(3)G (clone 1A8 and 1G2) (Mouse monoclonal)PMID: 15767569Immunofluorescence (1:200)
Antibodyanti-C(3)G (Rabbit polyclonal)PMID: 12588841Immunofluorescence (1:3000)
Antibodyanti-1B1 (Mouse monoclonal)Developmental Studies Hybridoma Bank1B1; RRID:AB_528070Immunofluorescence (1:100)
Antibodyanti-g-H2Av (Mouse monoclonal)Developmental Studies Hybridoma BankUNC93-5.2.1; RRID:AB_2618077Immunofluorescence (1:5000)
Antibodyanti-Phospho-4E-BP1 (Thr37/46) (236B4) (Rabbit monoclonal)Cell Signaling Technologies2855;RRID:AB_560835Immunofluorescence (1:200)
Commercial assay or kitECLPerkinElmerNEL105001EA
Chemical compound, drugmethyl methanesulfonate (MMS)Sigma129925–5G
Commercial assay or kitRNAeasy KitQiagen74104
Commercial assay or kitcDNA Reverse Transcription KitThermo Fischer11752
Commercial assay or kitPower SYBR green mastermixThermo FischerA25742

Fly stocks

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All fly stocks were maintained at 25°C on standard media. The p53R-GFP transgenic line was a gift from John M. Abrams (Lu et al., 2010). The germline specific driver nanos-Gal4 was obtained from Ruth Lehmann (Van Doren et al., 1998). The spnA093A stock was a gift from Ruth Lehmann (Staeva-Vieira et al., 2003). The nprl21, nprl31, iml11, and UAS-Nprl3 were described previously (Cai et al., 2016; Wei et al., 2016). The stocks w1118; Df(3L)ED4515, P{3'.RS5+3.3'}ED4515/TM6C, cu1 Sb1 (BDSC#9071), w1118; Df(3L)ED4238, P{3'.RS5+3.3'}ED4238/TM6C, cu1 Sb1 (BDSC#8052), w1118; P{neoFRT}82B P{Ubi-mRFP.nls}3R (BDSC#30555), P{hsFLP}22, y1 w*; P{arm-lacZ.V}70 C P{neoFRT}80B (BDSC#6341), MTD-GAL4 (P{w[+mC]=otu-GAL4::VP16.R}1, w[*] P{w[+mC]=GAL4 nos.NGT}40; P{w[+mC]=GAL4::VP16 nos.UTR}CG6325[MVD1], BDSC#31777), UAS-Tsc1 RNAi (y1 sc* v1; P{TRiP.GL00012}attP2, BDSC#35144), UAS-mCherry RNAi (y1 sc* v1; P{VALIUM20-mCherry}attP2), y1 w1118; p535A-1-4 (BDSC#6815), y1 w1/Dp(1;Y)y+; mei-P22P22; svspa-pol (BDSC#4931), mei-W681 (BDSC#4932) and Dp(1;Y)BS; ru1 st1 e1 spn-A1 ca1/TM3, Sb1 (BDSC#3322) were obtained from Bloomington Stock Center.

Western blot analysis

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The protocol was adapted from Bjedov et al. (2010). Briefly 6 pairs of ovaries were freshly dissected in cell insect media and homogenized in 30 μl of 4x Laemmli loading sample buffer (Invitrogen, #NP0008) containing 10x sample reducing agent (Invitrogen, #NP009). Extracts were cleared by centrifugation and boiled for 10 min at 90°C. 10 μl of protein extract was loaded per lane on polyacrylamide gel (Invitrogen, #NP0335). Proteins were separated and transferred to nitrocellulose membrane. Primary antibodies used were as follows: guinea pig anti-dS6K (gift of Aurelius Teleman,1:5,000,) (Hahn et al., 2010) and rabbit anti-phospho-Thr398-S6K (Cell Signaling Technologies #9209, 1:1,000). HRP- conjugated secondary antibodies (Jackson Immunoresearch, AffiniPure anti-rabbit #111-005-144 and anti-guinea pig #106-005-003) were used. Blots were developed using the ECL detection system (PerkinElmer, #NEL105001EA). Western blots were analyzed using ImageJ program (US National Institutes of Health).

Immunofluorescence and microscopy

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Immunofluorescence was performed as previously described (Hong et al., 2003; Iida and Lilly, 2004). Primary antibodies used were as follows: rabbit anti-GFP (Invitrogen, 1:1000); rabbit anti-γ−H2Av (Active Motif, 1:500); rabbit anti-C(3)G 1:3000 (Hong et al., 2003); mouse anti-1B1 (Developmental Studies Hybridoma Bank, 1:100); mouse anti-γ-H2Av (Developmental Studies Hybridoma Bank, 1:5000); mouse anti-C(3)G (kindly provided by R. Scott Hawley, 1:200) (Page and Hawley, 2001). Alexa-488 and Alex-594 (Invitrogen, 1:1000) secondary antibodies were used for fluorescence. After staining, ovaries were mounted in prolong gold antifade reagent with DAPI (Life Technology). Images were acquired on either a Leica SP5 confocal microscope or Zeiss LSM 880 with Airyscan confocal microscope.

γ-H2Av foci quantification

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To score the number of γ-H2Av foci per oocyte, ovaries were stained with antibodies against C(3)G and γ-H2Av as well as the DNA dye DAPI. Pro-oocytes and oocytes were identified by the pattern of anti-C(3)G staining (Page and Hawley, 2001). Multiple Z-sections encompassing an entire region 2a pro-oocyte nucleus were acquired by Leica SP5 confocal microscope or Zeiss LSM 880 with Airyscan confocal microscopy. The obtained z-stacks of images were deconvolved to remove out-of-focus light and z-distortion with Huygens Professional software (Scientific Volume Imaging) and clearly defined γ-H2Av were counted manually in the ovarian cyst with the highest levels of γ-H2Av staining. Alternatively, 3D images were rendered by using Imaris software (Bitplane) and a graph workstation equipped with NVIDIA Quadro 3D vision system. Clearly defined γ-H2Av foci were visualized and counted by using Imaris spots module or ImageJ to define the total number of foci per nucleus.

MMS sensitivity assay

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The assay was performed as described in Ghabrial et al. (1998). Briefly, 10 males and 10 virgin females were mated in vials for 2 days at 25°C. Parents were transferred into German food (Genesee Scientific, Cat#66–115, Day 1) vials for 24 hr at 25°C and allowed to lay eggs. On Day two the parents were removed, and eggs were allowed to mature for 24 hr. Subsequently, the first and second instar larvae were treated with 250 μL of either 0.04% or 0.08% of the mutagen methyl methane sulfonate (MMS) (Sigma, Cat#129925–5G). Control larvae were treated with 250 μL water. After eclosion, the number of heterozygous and homozygous mutant flies were determined, and the percentage of each genotype was calculated.

Gamma irradiation assay

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Wild type and mutant flies were fed wet yeast for two days and vials containing flies were exposed to 10Gy γ-IR in a Mark-1 γ-irradiator (JL Shepherd and Associates, San Fernando, CA). After irradiation, flies were incubated at 25°C and ovaries were collected for immunostaining assays at indicated time points.

Retrotransposon expression analysis

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Total RNA was isolated from dissected ovaries using the RNeasy Kit (Qiagen) and treated with DNase. cDNA was generated using High-capacity cDNA Reverse Transcription Kit (Thermo Fisher). Real-time PCR was performed with Power SYBR green Mastermix (Thermo Fisher) using the following primers:

  • Rp49 Forward: CCGCTTCAAGGGACAGTATC;

  • Rp49 Reverse: GACAATCTCCTTGCGCTTCT;

  • TAHRE Forward: CTGTTGCACAAAGCCAAGAA;

  • TAHRE Reverse: GTTGGTAATGTTCGCGTCCT;

  • Het-A Forward: TCCAACTTTGTAACTCCCAGC;

  • Het-A Reverse: TTCTGGCTTTGGATTCCTCG;

  • Idefix Forward: TGAAGAAAAGAAGGGCGAGA;

  • Idefix Reverse: TTCTGCTGTTGATGCTTTGG;

  • Gypsy Forward: CCAGGTCGGGCTGTTATAGG;

  • Gypsy Reverse: GAACCGGTGTACTCAAGAGC.

The rp49 was used for normalization.

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Decision letter

  1. Bruce Edgar
    Reviewing Editor; University of Utah, United States
  2. Jessica K Tyler
    Senior Editor; Weill Cornell Medicine, United States

In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.

Thank you for submitting your article "The GATOR complex regulates an essential response to meiotic double-stranded breaks in Drosophila" for consideration by eLife. Your article has been reviewed by three peer reviewers, and the evaluation has been overseen by a Reviewing Editor and Jessica Tyler as the Senior Editor. The reviewers have opted to remain anonymous.

I have appended the reviews in their entirety below. As you can see, the reviews are reasonably well aligned, and the reviewers were favorable overall about your interesting study. However they have requested significant revisions that we hope can clarify some outstanding issues. These requests seem reasonable and I suggest you do your best to address those that are technically straightforward within a reasonable timeframe. While I agree with the reviewers that looking further into the GATOR dependency, determining whether DSBs in WT inhibit TOR, and how DSBs mechanistically affect TOR (is it via S6K?) are important directions, delineating a complete, precise mechanism is not necessarily essential for this paper to be published in eLife, so long as the conclusions are clear and warranted.

Reviewer #1:

In this manuscript, Wei and colleagues delineate a pathway whereby double-stranded breaks (DSBs) inhibit mTORC1 activity via the GATOR1 complex, and this in turn allows repair of the double-stranded breaks. Overall, this is an interesting finding. A couple of the causality links of this story, however, need to be further tested to make the story solid:

1) Figure 1C shows that inhibition of DSB formation leads to increased mTORC1 activity in a mutant condition where mTORC1 activity is aberrantly low due to loss-of-function of GATOR2 (mio2 mutation). However, this does not show that in the wildtype condition DSBs affect mTOR activity. If the model is correct, the DSBs that occur normally in the ovary should be inhibiting mTORC1. Removing these DSBs should alleviate this suppression. So does the mei-W681 mutation increase S6K phosphorylation in the ovary compared to wildtype animals?

2) The role of GATOR1 in mediating the effect of DSBs on mTORC1 needs to be tested more rigorously using loss-of-function epistasis. From the data in Figure 1C, the authors conclude DSBs affect mTORC1 via GATOR1. However, the data do not exclude the alternate possibility that DSBs and GATOR1 affect mTORC1 via parallel pathways. One way to test this is to assay S6K phosphorylation in ovaries of the following genotypes:

- WT

- mei-W681

- iml1- (GATOR1 loss-of-function)

- iml1-, mei-W691 double mutants

In addition, this manuscript is lacking molecular mechanisms. How does mTORC1 activity affect DSB repair? Is it through delaying cell cycle progression? Is it through affecting transcription or translation of proteins required for repair? Alternatively, the upstream mechanism how DSBs affect GATOR1 activity is also missing. This may be a significant amount of work. Hence, it is an editorial decision how much molecular mechanism is wished in the present manuscript.

Reviewer #2:

Wei and coworkers describe some interesting connections between TOR signaling, meiotic double strand breaks and genotoxic stress in the Drosophila ovary. This work builds on their previous characterization of the TORC1 regulators GATOR1 and GATOR2, the latter of which was shown to have germline-specific functions. Here they demonstrate that TORC1 activity is sensitive to DSB formation in the context of Mio/GATOR2 mutant ovaries, providing a possible explanation for these germline-specific effects. Under conditions of high TORC1 activity in GATOR1 mutants, resolution of meiotic DSBs is delayed, leading to increases in p53 activity, DNA damage sensitivity, and retrotransposon expression. Interestingly, some but not all of these effects are phenocopied by germline depletion of Tsc1, indicating some possible TORC1-independent functions of GATOR1.

This work is timely and well described, and adds to a growing literature examining the role of metabolism in gametogenesis. Overall the data are lovely and appear well controlled. Additional insight into the mechanisms behind these novel observations would strengthen the manuscript, and experiments that probe these issues in somatic cells would make it of more general interest to eLife readers.

1) The authors demonstrate that in mio mutants, disruption of DSB formation leads to increased TORC1 activity, and they repeatedly describe this result as a GATOR-dependent response. However, the observation that TORC1 activity responds to the presence or absence of DSBs in ovaries lacking mio demonstrates that GATOR2 is not required for this response (in contrast to the model depicted in Figure 6), and would suggest that it is likely independent of GATOR1 as well. The role of GATOR1 is not directly tested here, and DSBs could in principle affect TORC1 activity through any one of multiple positive or negative TORC1 regulators/components. Without evidence that GATOR1 is specifically/selectively involved in this signal, I don't see the justification for calling this a GATOR1-dependent effect in the Title, Abstract, etc.

2) The requirement for other well characterized TORC1 inputs (e.g., nutrient and insulin pathways) should be addressed. A more thorough probing of the mechanism by which DSBs affect TORC1 activity should be included here, and the effects of mei-P22 mutation on TORC1 activity should be examined in otherwise wildtype animals and other mutations/conditions that lead to low TORC1 signaling.

3) The proximal signals linking DSBs to TORC1 downregulation also should be addressed, by examining the genetic requirements for DSB checkpoint components in this response. Do exogenous, non-meiotic DSBs lead to similar effects, perhaps also in somatic cells, or does this require the context of a delayed synaptonemal complex?

4) Similarly, the manuscript elegantly shows that increased TORC1 activity results in delayed or defective repair of meiotic DSBs, but is lacking in mechanistic detail. Here, examining the localization and activity of repair pathway components could prove insightful. Again, whether these observations reflect a general effect of high TORC1 activity on DSB repair or is limited to germline-specific lesions should be tested, as previous studies have described that low TORC1 activity can impair DSB repair/DNA damage response.

Reviewer #3:

This paper analyzes the interesting intersection between the regulation of metabolism and meiotic progression. It is well known that entry of meiosis in budding yeast is regulated by nutrient availability. The PI has previously published that similar pathways, involving TORC1 and the GATOR1/2 complexes, regulate meiosis in Drosophila. This paper is interesting because it connects these regulators of metabolism to double strand break formation as well as p53 and stress responses. The data are convincing and support the conclusions. The most significant results are that DSBs trigger a stress response, which must be modulated by regulating the TORC1 pathway and preventing hyperactivation of p53. The data are interesting with significant impact in areas of regulating gametogenesis and meiotic recombination. There are several issues raised below that are mostly asking for more clarity in describing mutants and cytological results. A couple of experiments are suggested, which entail looking at a couple additional genotypes in the assays presented and thus should not be technically challenging unless there are viability issues. These are meant to generalize the results to the entire pathway presented in Figure 1 but are not essential for the main conclusions of the paper.

In several places are statements that TORC1 activity facilitates entry into meiosis (e.g. subsections “GATOR1 promotes the repair of meiotic DSB” and “GATOR2 opposes a GATOR1 dependent response to meiotic DSBs”). This seems to be mostly taken yeast results. In the absence of GATOR1 in flies, it might be predicted that oocytes either don't form oocytes or meiotic entry does not occur. In fact, Wei et al. reported a delay in meiotic entry in GATOR1 mutants. My question is whether the observed delay in meiotic entry may be more accurately described as a defect in progression. It is also critical to know if the mutants being analyzed in this paper are null alleles. Hypomorphs or RNAi could have leaky phenotypes when it comes to meiotic entry. In subsection “GATOR1 mutants hyperactivate p53 in response to meiotic DSBs”, a nprl3 is referred to as null, but this is in passing and this information should be much earlier. The authors should be careful to state whether the mutants are null and result in loss of GATOR1 or GATOR2 activity.

Paragraph three of subsection “GATOR1 promotes the repair of meiotic DSB”: Is the persistence of DSBs in region 3 oocytes associated with dorsal-ventral polarity defects? This is observed in DSB repair mutants like spnA.

Subsection “GATOR1 mutants hyperactivate p53 in response to meiotic DSBs”: The authors show that GATOR1 mutants hyperactivate p53. To show this is part of the TORC pathway, an experiment with p53 expression in a TORC mutant would be useful. Also, can the authors add p53 be in the model of Figure 6?

Subsection “GATOR1 inhibits retrotransposon expression inDrosophila”: The most confusing results concern the relationship between the GATOR1, p53 and TEs. The effect of these mutants on TE expression is DSB dependent. However, while nprl2/3 mutants have elevated TE expression, they also have elevated, rather than reduced, p53. One explanation is that the TORC pathway is downstream of p53. However, the phenotype of the p53 nprl3 double is additive with respect to TE expression. As mentioned above, the correct interpretation of these experiments depends on both mutants being null alleles. It also suggests the elevated p53 levels in the nprl mutants is not the cause of the increased TE expression. More comment on these discrepancies would be appreciated. It would also be helpful to see a GATOR1-2 (mio, nprl3) double mutant, or a mio single, to determine if TE regulation is dependent on the pathway.

Subsection “GATOR2 opposes a GATOR1 dependent response to meiotic DSBs”: this seems to be speculation (reasonable, but still a hypothesis) but stated more like fact – that the function of Mio is required to regulate the response to meiotic DSBs. If this were true, the mei-P22, mio double would be as fertile as mei-P22 single. Is that true? Can the authors comment on the small ovary phenotype? When does developmental arrest occur and is that related to some kind of checkpoint? Is it temporally separate from the induction and repair of DSBs?

Subsection “GATOR2 opposes a GATOR1 dependent response to meiotic DSBs”: "…regulation of meiotic DSB repair." and "…meiotic repair of DSBs…"

Subsection “GATOR1 opposes retrotransposon expression”: the implication is that TE expression is upregulated by genotoxic stress, correct? Is that known or a correlation with DSB formation.

Figure 2G gives the impression that many of the mutants have more DSBs in region 2A than wildtype. This could be due to asynchrony and variation within the different pro-oocytes of region 2A. Thus, there should be nprl mutant pro-oocytes that have the same number of foci as wild-type, the difference being that as the oocytes progress, the number of foci increases because the DSBs are not repaired. The authors should correct his by showing oocytes of equivalent stage (difficult) or showing all pro-oocytes from one germarium to show how the progression of foci number changes with time.

Figure 2H: Add spnA to the graph. There appear to be two differences between the spnA and the GATOR phenotypes. First, the γ-H2Av foci persist in the nurse cells as well as the oocyte in spnA mutants. Second, 100% of spnA oocytes have region 3 foci.

Figure 6 and legend: The figure lacks a panel A. It might be better to put the germarium schematic at the top.

https://doi.org/10.7554/eLife.42149.021

Author response

Reviewer #1:

In this manuscript, Wei and colleagues delineate a pathway whereby double-stranded breaks (DSBs) inhibit mTORC1 activity via the GATOR1 complex, and this in turn allows repair of the double-stranded breaks. Overall, this is an interesting finding. A couple of the causality links of this story, however, need to be further tested to make the story solid:

1) Figure 1C shows that inhibition of DSB formation leads to increased mTORC1 activity in a mutant condition where mTORC1 activity is aberrantly low due to loss-of-function of GATOR2 (mio2 mutation). However, this does not show that in the wildtype condition DSBs affect mTOR activity. If the model is correct, the DSBs that occur normally in the ovary should be inhibiting mTORC1. Removing these DSBs should alleviate this suppression. So does the mei-W681 mutation increase S6K phosphorylation in the ovary compared to wildtype animals?

We performed two sets of experiments to address the reviewer’s question. First, as suggested by reviewers #1 and #2, we examined TORC1 activity in mei-w681single mutants by western blot and determined that mei-w681 mutant ovaries do not have increased TORC1 activity relative to wild-type ovaries (Figure 1). A similar result was observed in mei-P22 mutants, which also fail to generate meiotic DSBs (Figure 1—figure supplement 3). We believe this observation is not surprising when one considers the anatomy of the Drosophila ovary. In wild-type ovaries, meiotic DSBs are present in only a small number of 16-cell cysts in the germarium and are repaired prior to the rapid growth of the egg chamber. Thus, ovarian cysts that contain meiotic DSBs represent an exceedingly small percentage of the tissue in the ovary. Thus, we reasoned it is unlikely that increasing TORC1 activity in only a small number of germarial ovarian cysts would result in an increase in TORC1 activity in the ovary that could be observed by western blot. However, please note that spnA/Rad51 mutants, which retain meiotic DSBs throughout oogenesis, have reduced TORC1 activity relative to wild-type ovaries as measured by p-S6K levels assayed by western blot. These experiments are described in the second paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs”, Figure 1 and Figure 1—figure supplement 2.

To obtain better resolution as to when and where meiotic DSBs impact TORC1 activity during oogenesis, we used an antibody against the phosphorylated form of the downstream TORC1 target 4E-BP. Briefly, we determined that “In contrast to what is observed in wild type, in mio mutant ovarian cysts, p4E-BP levels remain low in germline cells in region 2b and beyond (Figure 2C, arrow). Consistent with the western blot analysis, mio, mei-w68 double mutant ovarian cysts have an approximately three-fold increase in p4E-BP staining in region 2b of the germarium relative to mio mutants (Figure 2C, E and H) Notably, the increase in TORC1 activity in the mio, mei-w68 double mutants is restricted to the germline, consistent with blocking meiotic DSBs having cell autonomous effects on TORC1 activity in the germline. Additionally, consistent with our western blot analysis in Figure 1—figure supplement 2, ovarian cysts from spnA/Rad51 mutants, which do not repair meiotic DSBs, have reduced levels of p4E-BP staining (Figure 2F). Taken together these data strongly suggest that mio is required to oppose/attenuate the downregulation of TORC1 activity triggered by the presence of meiotic DSBs.” These new results are described in the third paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 2.

2) The role of GATOR1 in mediating the effect of DSBs on mTORC1 needs to be tested more rigorously using loss-of-function epistasis. From the data in Figure 1C, the authors conclude DSBs affect mTORC1 via GATOR1. However, the data do not exclude the alternate possibility that DSBs and GATOR1 affect mTORC1 via parallel pathways. One way to test this is to assay S6K phosphorylation in ovaries of the following genotypes:

- WT

- mei-W681

- iml1- (GATOR1 loss-of-function)

- iml1-, mei-W691 double mutants

As suggested by the reviewer we performed epistasis analysis using the GATOR1 component nprl3 and mei-P22 a gene required for the generation of meiotic DSBs. We found that p-S6K levels were not significantly different in wild-type and mei-p22 mutant ovaries. Moreover, mei-p22, nprl3 double mutants had lower levels of p-S6K than nprl3 single mutants. Thus, blocking the formation of meiotic DSBs does not in and of itself result in increased TORC1 activity. This experiment is described in the fourth paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and summarized in Figure 1—figure supplement 3.

In addition, this manuscript is lacking molecular mechanisms. How does mTORC1 activity affect DSB repair? Is it through delaying cell cycle progression? Is it through affecting transcription or translation of proteins required for repair?

In order to examine why high TORC1 activity delays the repair of meiotic DSBs we examined three downstream targets of TORC1, atg1, 4E-BP/thor and S6K. Atg1 and 4E-BP are inhibited by TORC1 activity while S6K is activated by TORC1. Our results suggest that the delay in the repair of meiotic DSBs observed in GATOR1 mutants is due, at least in part, to the increased activity of S6K. Our negative results were also informative. We determined that null mutants of atg1, which isrequired for autophagy, as well as null mutants of the translational inhibitor 4E-BP/thor, do not alter the kinetics of meiotic DSB repair during oogenesis. Thus, we concluded it is highly unlikely that high TORC1 activity delays the repair of meiotic DSBs through the constitutive inhibition of autophagy or the inhibition of the translational inhibitor 4E-BP. These experiments are detailed in the first paragraph of the subsection “Co-depleting S6K rescues the increase in the steady state number of meiotic DSBs in iml1 germline depletions” and Figure 4.

Alternatively, the upstream mechanism how DSBs affect GATOR1 activity is also missing. This may be a significant amount of work. Hence, it is an editorial decision how much molecular mechanism is wished in the present manuscript.

Although we believe that defining the upstream pathway connecting meiotic DSBs to the TORC1 regulatory machinery is beyond the scope of this manuscript, we did explore the role of the ATM downstream target and checkpoint protein, Chk2. Chk2 is known as Loki in Drosophila. Specifically, we determined that removing Chk2/Loki activity partially rescues the mio mutant phenotype. From this result we conclude that Chk2/Loki is partially responsible for the downregulation of TORC1 activity observed in mio mutants. These results are presented in the last paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 1—figure supplement 4.

Reviewer #2:

[…] This work is timely and well described, and adds to a growing literature examining the role of metabolism in gametogenesis. Overall the data are lovely and appear well controlled. Additional insight into the mechanisms behind these novel observations would strengthen the manuscript, and experiments that probe these issues in somatic cells would make it of more general interest to eLife readers.

1) The authors demonstrate that in mio mutants, disruption of DSB formation leads to increased TORC1 activity, and they repeatedly describe this result as a GATOR-dependent response. However, the observation that TORC1 activity responds to the presence or absence of DSBs in ovaries lacking mio demonstrates that GATOR2 is not required for this response (in contrast to the model depicted in Figure 6), and would suggest that it is likely independent of GATOR1 as well. The role of GATOR1 is not directly tested here, and DSBs could in principle affect TORC1 activity through any one of multiple positive or negative TORC1 regulators/components. Without evidence that GATOR1 is specifically/selectively involved in this signal, I don't see the justification for calling this a GATOR1-dependent effect in the Title, Abstract, etc.

In trying to present all theoretical possible explanations for our data, we presented an overly complicated model that reflected possible roles for different GATOR2 components (such as wdr24) that were not examined in the current manuscript. This has now been corrected. The current model shows that Mio is not involved in regulating the initial downregulation of TORC1 activity in response to meiotic DSBs but is required to attenuate or turn off this response. This model is consistent with all of our data. The model is presented in Figure 9 and discussed in the Discussion subsection “The GATOR complex and the response to meiotic DSBs”.

We agree with reviewer #2 that we have not formally proven that the GATOR1/TSC pathway is activated by the presence of meiotic DSBs and have now adjusted the text accordingly. Importantly, we have altered the Abstract and have corrected our language throughout the manuscript to reflect the speculative nature of the connection between meiotic DSBs and the direct activation of a GATOR1/TSC dependent pathway. However, we strongly disagree that the most logical explanation for our results is that the response to meiotic DSBs activates a pathway that is completely independent of the GATOR/TSC pathway. We believe a much simpler explanation is that the regulation of TORC1 activity in the female germline requires two steps: (1) the activation of a GATOR1/TSC dependent pathway by meiotic DSBs and (2) the failure of Mio to attenuate or turnoff this activated pathway. This would be consistent with our published epistasis analysis which demonstrated that double mutants of mio and GATOR1 or TSC components rescue the mio mutant phenotypes (Wei et al., 2014). It would also be consistent with the observation that GATOR1 is the only identified downstream target of the GATOR2 complex. These ideas are now presented in the Discussion as a model.

I don't see the justification for calling this a GATOR1-dependent effect in the Title.

Please note that we do not reference a GATOR1-dependent effect in the Title.

The title states:

“The GATOR complex regulates an essential response to meiotic double-stranded breaks in Drosophila

We believe the title accurately reflects our data in that (1) the GATOR1 complex impacts the repair of meiotic DSBs and (2) the GATOR2 component Mio is required to attenuate a TORC1 inhibitory pathway that is activated by meiotic DSBs.

2) The requirement for other well characterized TORC1 inputs (e.g., nutrient and insulin pathways) should be addressed.

Our apologies to the reviewer we are not sure what experiment is being suggested here. Please note that TSC, a well-known target of Akt1, is a component of the Insulin signaling pathway. However, we believe the examination of how meiotic DSBs impact insulin/Tor signaling beyond TSC and the GATOR complex are outside the scope of this manuscript.

A more thorough probing of the mechanism by which DSBs affect TORC1 activity should be included here.

Although we believe that defining the upstream pathway connecting meiotic DSBs to the TORC1 regulatory machinery is beyond the scope of this manuscript, we did explore the role of the ATM downstream target and checkpoint protein, Chk2. Chk2 is known as Loki in Drosophila. Specifically, we determined that removing Chk2/Loki activity partially rescues the mio mutant phenotype. From this result we conclude that Chk2/Loki is partially responsible for the downregulation of TORC1 activity observed in mio mutants. These results are presented in the last paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 1—figure supplement 4.

The effects of mei-P22 mutation on TORC1 activity should be examined in otherwise wildtype animals and other mutations/conditions that lead to low TORC1 signaling.

We performed two sets of experiments to address the reviewer’s question. First, as suggested by reviewers #1 and #2, we examined TORC1 activity in mei-w681single mutants by western blot and determined that, mei-w681 mutant ovaries do not have increased TORC1 activity relative to wild-type ovaries (Figure 1). A similar result was observed in mei-P22 mutants, which also fail to generate meiotic DSBs (Figure 1—figure supplement 3). We believe this observation is not surprising when one considers the anatomy of the Drosophila ovary. In wild-type ovaries, meiotic DSBs are present in only a small number of 16-cell cysts in the germarium and are repaired prior to the rapid growth of the egg chamber. Thus, ovarian cysts that contain meiotic DSBs represent an exceedingly small percentage of the tissue in the ovary. Thus, we reasoned it is unlikely that increasing TORC1 activity in only a small number of germarial ovarian cysts would result in an increase in TORC1 activity in the ovary that could be observed by western blot. However, please note that spnA/Rad51 mutants, which retain meiotic DSBs throughout oogenesis, have reduced TORC1 activity relative to wild-type ovaries as measured by p-S6K levels assayed by western blot. These experiments are described in the second paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 1 and Figure 1—figure supplement 2.

To obtain better resolution as to when and where meiotic DSBs impact TORC1 activity during oogenesis, we used an antibody against the phosphorylated form of the downstream TORC1 target 4E-BP. Briefly, we determined that “In contrast to what is observed in wild type, in mio mutant ovarian cysts, p4E-BP levels remain low in germline cells in region 2b and beyond (Figure 2C, arrow). Consistent with the western blot analysis, mio, mei-w68 double mutant ovarian cysts have an approximately three-fold increase in p4E-BP staining in region 2b of the germarium relative to mio mutants (Figure 2C, E and H) Notably, the increase in TORC1 activity in the mio, mei-w68 double mutants is restricted to the germline, consistent with blocking meiotic DSBs having cell autonomous effects on TORC1 activity in the germline. Additionally, consistent with our western blot analysis in Figure 1—figure supplement 2, ovarian cysts from spnA/Rad51 mutants, which do not repair meiotic DSBs, have reduced levels of p4E-BP staining (Figure 2F). Taken together these data strongly suggest that mio is required to oppose/attenuate the downregulation of TORC1 activity triggered by the presence of meiotic DSBs.” These new results are described in the third paragraph of the subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 2.

We believe that examining how mutations in additional components of the TORC1 machinery are impacted by blocking the formation of meiotic DSBs would be technically challenging, and beyond the scope of this manuscript.

3) The proximal signals linking DSBs to TORC1 downregulation also should be addressed, by examining the genetic requirements for DSB checkpoint components in this response. Do exogenous, non-meiotic DSBs lead to similar effects, perhaps also in somatic cells, or does this require the context of a delayed synaptonemal complex?

Our examination of the role of Chk2 is described above. Additionally, we have previously published that ATR does not link DSBs to TORC1 downregulation in mio mutants (Iida and Lilly, 2004).

Whether DSBs in GATOR2 mutants trigger the constitutive downregulation of TORC1 activity in somatic cells will be an interesting area for future analysis.

4) Similarly, the manuscript elegantly shows that increased TORC1 activity results in delayed or defective repair of meiotic DSBs, but is lacking in mechanistic detail.

In order to examine why high TORC1 activity delays the repair of meiotic DSBs we examined three downstream targets of TORC1, atg1, 4E-BP/thor and S6K. Atg1 and 4E-BP are inhibited by TORC1 activity while S6K is activated by TORC1. Our results suggest that the delay in the repair of meiotic DSBs observed in GATOR1 mutants is due, at least in part, to the increased activity of S6K. Our negative results were also informative. We determined that null mutants of atg1, which isrequired for autophagy, as well as null mutants of the translational inhibitor 4E-BP, do not alter the kinetics of meiotic DSB repair during oogenesis. Thus, we concluded it is highly unlikely that high TORC1 activity delays the repair of meiotic DSBs through the constitutive inhibition of autophagy or the inhibition of the translational inhibitor 4E-BP. These experiments are detailed in the first paragraph of the subsection “Co-depleting S6K rescues the increase in the steady state number of meiotic DSBs in iml1 germline depletions” and Figure 4.

Here, examining the localization and activity of repair pathway components could prove insightful.

We agree that the suggested experiments might provide important information for how TORC1 activity impacts DSB repair. However, because of the lack of available reagents these experiments would be extremely challenging and are beyond the scope of this manuscript.

Again, whether these observations reflect a general effect of high TORC1 activity on DSB repair or is limited to germline-specific lesions should be tested, as previous studies have described that low TORC1 activity can impair DSB repair/DNA damage response.

To assess the general role of the GATOR1 complex in the regulation of DSB repair we examined the repair of γ-irradiation induced DSBs in the somatically derived follicle cells of the ovary by monitoring γ-H2Av staining. From this experiment we determined that high TORC1 activity can impact the repair of DSBs in somatic cell populations. Thus, both inappropriately low and high TORC1 activity can impair DSB repair. These experiments are detailed in the subsection “nprl3 mutant follicle cells are sensitive to genotoxic stress” and Figure 7.

Reviewer #3:

[…] There are several issues raised below that are mostly asking for more clarity in describing mutants and cytological results. A couple of experiments are suggested, which entail looking at a couple additional genotypes in the assays presented and thus should not be technically challenging unless there are viability issues. These are meant to generalize the results to the entire pathway presented in Figure 1 but are not essential for the main conclusions of the paper.

In several places are statements that TORC1 activity facilitates entry into meiosis (e.g. subsections “GATOR1 promotes the repair of meiotic DSB” and “GATOR2 opposes a GATOR1 dependent response to meiotic DSBs”). This seems to be mostly taken yeast results. In the absence of GATOR1 in flies, it might be predicted that oocytes either don't form oocytes or meiotic entry does not occur. In fact, Wei et al. reported a delay in meiotic entry in GATOR1 mutants. My question is whether the observed delay in meiotic entry may be more accurately described as a defect in progression.

This is an excellent question. However, at this point in time we do not have the reagents to distinguish between a delay and a defect in progression. I am not certain what assay we could use to distinguish between these two possibilities. Because GATOR1 mutants frequently undergo an extra mitotic division, we hypothesized that GATOR1 mutants delay meiotic entry. Yet, we can’t definitively rule out a defect in progression.

It is also critical to know if the mutants being analyzed in this paper are null alleles. Hypomorphs or RNAi could have leaky phenotypes when it comes to meiotic entry. In subsection “GATOR1 mutants hyperactivate p53 in response to meiotic DSBs”, a nprl3 is referred to as null, but this is in passing and this information should be much earlier. The authors should be careful to state whether the mutants are null and result in loss of GATOR1 or GATOR2 activity.

We thank the reviewer for pointing out this error. The GATOR1 (nprl2, nprl3 and iml1) and mio alleles used in this study are all null alleles. We have now made this clear the first time the alleles are discussed.

Paragraph three of subsection “GATOR1 promotes the repair of meiotic DSB”: Is the persistence of DSBs in region 3 oocytes associated with dorsal-ventral polarity defects? This is observed in DSB repair mutants like spnA.

As suggested by the reviewer, we determined that eggs laid by nprl3 null mutant females do not have DV patterning defects. This may reflect the fact that the repair of meiotic DSBs is delayed but not blocked in nprl3 mutants. Alternatively, high TORC1 activity may override the translational repression of Gurken that drives the pattern defects observed in DNA repair mutants such as spnA. These results are presented in subsection “GATOR1 promotes the repair of meiotic DSB”.

Subsection “GATOR1 mutants hyperactivate p53 in response to meiotic DSBs”: The authors show that GATOR1 mutants hyperactivate p53. To show this is part of the TORC pathway, an experiment with p53 expression in a TORC mutant would be useful.

As noted by reviewer #2, previous work has established that some TORC1 activity is required for DSB repair (Ma et al., 2018). Thus, we would predict that TORC1 mutants would activate p53 in the female germline. Additionally, we have previously demonstrated that TORC1 null mutants result in a very early arrest of ovarian cyst development, often prior to the final mitotic cyst division (Wei et al., 2014). Moreover, we have observed that a small fraction of TORC1 mutants cysts undergo apoptosis. Finally, in unpublished data we see a slight activation of p53 in some GATOR2 mutants, including mio, which may be the result of low TORC1 activity. For all of these reasons, we believe the result from the suggested experiment would be difficult to interpret.

Also, can the authors add p53 be in the model of Figure 6?

I think this would be difficult to do without causing confusion. We simplified our previous model at the request of the other reviewers. Currently, we believe that p53 is activated in GATOR1 mutants due to the delay in the repair of meiotic DSBs.

Subsection “GATOR1 inhibits retrotransposon expression in Drosophila”: The most confusing results concern the relationship between the GATOR1, p53 and TEs. The effect of these mutants on TE expression is DSB dependent. However, while nprl2/3 mutants have elevated TE expression, they also have elevated, rather than reduced, p53. One explanation is that the TORC pathway is downstream of p53. However, the phenotype of the p53 nprl3 double is additive with respect to TE expression. As mentioned above, the correct interpretation of these experiments depends on both mutants being null alleles. It also suggests the elevated p53 levels in the nprl mutants is not the cause of the increased TE expression. More comment on these discrepancies would be appreciated.

We appreciate the reviewer’s comments and have now clearly stated that the epistasis analysis between p53 and nprl3 was performed with null alleles. We realized that the introduction to these experiments was extremely confusing. We have now reorganized as well as rewritten components of this section of the manuscript.

Our data indicate that p53 is necessary but not sufficient for repression of retrotransposon expression in the female germline. This result is consistent with our epistasis analysis, which demonstrated that GATOR1 and p53 function to suppress retrotransposon expression via independent pathways (Figure 8). Because they are functioning in independent pathways, increasing p53 activity would not be predicted to rescue the GATOR1 phenotypes.

As currently outlined in the discussion subsection “GATOR1 opposes retrotransposon expression”, our data support the model that genotoxic stress, due to the delay in the repair of meiotic DSBs, is upstream of both increased p53 activity and derepressed retrotransposon expression.

It would also be helpful to see a GATOR1-2 (mio nprl) double mutant, or a mio single, to determine if TE regulation is dependent on the pathway.

It is unclear what one might predict for a mio single mutant, decreased retrotransposon expression perhaps? Because we have not yet examined how mio and other GATOR2 components impact genome stability, we believe these experiments are beyond the scope of the manuscript. As noted above, low TORC1 activity, disrupts the response to DSBs in mammalian cells. Consistent with these reports, in unpublished data we see a slight activation of p53 in some GATOR2 mutants, including mio. Thus, we believe it would be difficult to interpret the results from these experiments.

Subsection “GATOR2 opposes a GATOR1 dependent response to meiotic DSBs”: this seems to be speculation (reasonable, but still a hypothesis) but stated more like fact – that the function of Mio is required to regulate the response to meiotic DSBs. If this were true, the mei-P22 mio double would be as fertile as mei-P22 single. Is that true?

To address this question, we examined mio, mei-w68 double mutants. We determined that blocking the formation of meiotic DSBs partially rescues the fertility deficit in mio. We propose the partial nature of the rescue may reflect the requirement for mio at multiple times during oogenesis as well as the decreased fertility associated with blocking the formation of meiotic DSBs. Notably, the GATOR2 complex has been reported to play a role in spindle assembly and thus may impact the construction of the efficiency of the meiotic divisions (Platani et al., 2015). Please note we tried to be conservative in the interpretation of this result stating: “we demonstrate that the tissue specific requirement for Mio during oogenesis is due, at least in part, to the generation of meiotic DBSs during oogenesis.” These results are presented in subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 1—figure supplement 1.

Please note, in mio mutantsDSBs trigger a small egg chamber phenotype. These data indicate that mio is required to regulate a physiological response to meiotic DSBs. We do not claim that mio directlyregulates the repair of meiotic DSBs.

Can the authors comment on the small ovary phenotype. When does developmental arrest occur and is that related to some kind of checkpoint. Is it temporally separate from the induction and repair of DSBs.

We observe problems with oocyte specification in mio mutants beginning in late region 2a of the germarium (Iida and Lilly, 2004). However, while the oocyte fails to maintain the oocyte fate, other aspects of egg chamber development continue for some time including the endoreplication of nurse cell nuclei. Thus, it is difficult to establish exactly when mio egg chambers arrest their development. We have not been able to temporally separate the induction of meiotic DSBs and the failure to properly maintain oocyte fate in mio mutants. Both these events occur as early as region 2a.

We explored the role of the ATM downstream target and checkpoint protein, Chk2. Chk2 is known as Loki in Drosophila. Specifically, we determined that removing Chk2/Loki activity partially rescues the mio mutant phenotype. From this result we conclude that Chk2/Loki is partially responsible for the downregulation of TORC1 activity observed in mio mutants. These results are presented in subsection “Mio prevents the constitutive inhibition of TORC1 activity in response to meiotic DSBs” and Figure 1—figure supplement 4.

Additionally, we have previously published that ATR does not link DSBs to TORC1 downregulation in mio mutants (Iida and Lilly, 2004).

Subsection “GATOR1 opposes retrotransposon expression”: the implication is that TE expression is upregulated by genotoxic stress, correct? Is that known or a correlation with DSB formation.

It has been noted by Barbara McClintock and others, that genotoxic stress activates retrotransposons although the exact mechanism of activation remains elusive (Beauregard, Curcio, and Belfort, 2008; Bradshaw and McEntee, 1989; Hagan, Sheffield, and Rudin, 2003; Walbot, 1992). We proposed that “germ line cells may be at an increased risk for transposon derepression due to the genotoxic stress associated with meiotic recombination.” To test the model that unrepaired meiotic DSBs trigger retrotransposon expression we examined retrotransposon transcript levels in spnA/Rad51 mutant ovaries and determined that they were increased relative to wild type (Figure 8). These data are consistent with work from HeLa cells that LINE1 element activity is increased in Rad51 depleted cells (Liu et al., 2018). This work is presented in subsection “GATOR1 inhibits retrotransposon expression in Drosophila” and Figure 8.

Figure 2G gives the impression that many of the mutants have more DSBs in region 2A than wildtype. This could be due to asynchrony and variation within the different pro-oocytes of region 2A. Thus, there should be nprl mutant pro-oocytes that have the same number of foci as wild-type, the difference being that as the oocytes progress, the number of foci increases because the DSBs are not repaired. The authors should correct his by showing oocytes of equivalent stage (difficult) or showing all pro-oocytes from one germarium to show how the progression of foci number changes with time.

We found that GATOR1 mutants had an increase in the number of γ-H2Av foci relative to wild type. This is not just an impression. We used a modified method of counting γ-H2Av foci that was originally presented in Mehrotra and McKim (Mehrotra and McKim, 2006). For both mutant and wild type germaria, we reported the number of foci in the ovarian cyst that had the highest levels of γ-H2Av staining in the oocyte in region 2A of the germarium. As can be seen in Figure 3, this number can be the same as is observed in wild type germaria. However, on average, the number of foci in the GATOR1 mutants is higher. GATOR1 mutant germarium are somewhat misshapen, at least in part, because of the large number of 32 cell cysts. Thus, it would be difficult to establish equivalent stages in the germarium in mutant and wild type. Additionally, it is not clear how scoring all of the oocytes within a cyst, which would be a large amount of work, might alter our interpretation of the role of GATOR1 in the response to meiotic DSBs.

Figure 2H: Add spnA to the graph. There appear to be two differences between the spnA and the GATOR phenotypes. First, the γ-H2Av foci persist in the nurse cells as well as the oocyte in spnA mutants. Second, 100% of spnA oocytes have region 3 foci.

We did not quantify the retention of γ-H2av foci phenotype, γ-H2av foci also persist in the nurse cells in GATOR1 mutants although this is to a lesser degree then observed in spnA mutants (Figure 3). As suggested by the reviewer we added spnA mutants to the graph in Figure 3I-K.

Figure 6 and legend: The figure lacks a Panel A. It might be better to put the germarium schematic at the top.

We have altered and simplified the model as suggested by several reviewers (Figure 9).

https://doi.org/10.7554/eLife.42149.022

Article and author information

Author details

  1. Youheng Wei

    1. Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    2. College of Bioscience and Biotechnology, Yangzhou University, Yangzhou, China
    Contribution
    Conceptualization, Data curation, Formal analysis, Validation, Investigation, Methodology, Writing—review and editing
    Contributed equally with
    Lucia Bettedi
    Competing interests
    No competing interests declared
  2. Lucia Bettedi

    Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    Contribution
    Data curation, Formal analysis, Validation, Investigation, Writing—review and editing
    Contributed equally with
    Youheng Wei
    Competing interests
    No competing interests declared
  3. Chun-Yuan Ting

    Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    Contribution
    Data curation, Formal analysis, Validation, Investigation, Methodology
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-2302-4203
  4. Kuikwon Kim

    Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    Contribution
    Data curation, Formal analysis, Investigation, Methodology
    Competing interests
    No competing interests declared
  5. Yingbiao Zhang

    Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    Contribution
    Formal analysis, Validation, Investigation, Methodology
    Competing interests
    No competing interests declared
  6. Jiadong Cai

    College of Bioscience and Biotechnology, Yangzhou University, Yangzhou, China
    Contribution
    Investigation
    Competing interests
    No competing interests declared
  7. Mary A Lilly

    Cell Biology and Neurobiology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, United States
    Contribution
    Conceptualization, Resources, Formal analysis, Funding acquisition, Investigation, Writing—original draft, Project administration, Writing—review and editing
    For correspondence
    lillym@helix.nih.gov
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-1564-619X

Funding

Eunice Kennedy Shriver National Institute of Child Health and Human Development (Intramural Program HD00163 16)

  • Mary A Lilly

National Science Foundation of Jiangsu Province (BK20181456)

  • Youheng Wei

National Natural Science Foundation of China (31872287)

  • Youheng Wei

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Acknowledgements

Multiple stocks used in this study were obtained from the Bloomington Drosophila Stock Center (supported by NIH Grant P40OD018537). Research reported in this publication was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development Intramural Research program (to MAL, HD00163 16), the Natural Science Foundation of Jiangsu Province (to YW, BK20181456) and the National Natural Science Foundation of China (to YW, 31872287).

Senior Editor

  1. Jessica K Tyler, Weill Cornell Medicine, United States

Reviewing Editor

  1. Bruce Edgar, University of Utah, United States

Publication history

  1. Received: September 18, 2018
  2. Accepted: October 13, 2019
  3. Accepted Manuscript published: October 25, 2019 (version 1)
  4. Version of Record published: November 6, 2019 (version 2)

Copyright

This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

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