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Rab5 and Rab11 maintain hematopoietic homeostasis by restricting multiple signaling pathways in Drosophila

  1. Shichao Yu
  2. Fangzhou Luo
  3. Li Hua Jin  Is a corresponding author
  1. Department of Genetics, College of Life Sciences, Northeast Forestry University, China
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Cite this article as: eLife 2021;10:e60870 doi: 10.7554/eLife.60870

Abstract

The hematopoietic system of Drosophila is a powerful genetic model for studying hematopoiesis, and vesicle trafficking is important for signal transduction during various developmental processes; however, its interaction with hematopoiesis is currently largely unknown. In this article, we selected three endosome markers, Rab5, Rab7, and Rab11, that play a key role in membrane trafficking and determined whether they participate in hematopoiesis. Inhibiting Rab5 or Rab11 in hemocytes or the cortical zone (CZ) significantly induced cell overproliferation and lamellocyte formation in circulating hemocytes and lymph glands and disrupted blood cell progenitor maintenance. Lamellocyte formation involves the JNK, Toll, and Ras/EGFR signaling pathways. Notably, lamellocyte formation was also associated with JNK-dependent autophagy. In conclusion, we identified Rab5 and Rab11 as novel regulators of hematopoiesis, and our results advance the understanding of the mechanisms underlying the maintenance of hematopoietic homeostasis as well as the pathology of blood disorders such as leukemia.

Introduction

Drosophila is a powerful genetic model for studying hematopoiesis due to the conservation between its hematopoietic system and that of mammals, including conserved regulatory factors and signaling pathways (Yu et al., 2018a; Banerjee et al., 2019). By utilizing this model, we can also improve the understanding of the molecular mechanisms underlying some blood system diseases, such as leukemia. While two main waves of hematopoiesis occur in Drosophila during the entire life cycle, the existence of hematopoiesis in the adult stage is controversial (Ghosh et al., 2015; Sanchez Bosch et al., 2019). The first wave occurs in the embryonic head mesoderm, where two types of hemocytes, plasmatocytes and crystal cells, are derived (Holz et al., 2003). Plasmatocytes are macrophage-like cells that can kill invading pathogens by phagocytosis (Tepass et al., 1994), while crystal cells play an important role in wound healing via the melanization response (Lanot et al., 2001).

The lymph gland, consisting of a pair of anterior lobes and a series of posterior lobes, is the site of the second phase of hematopoiesis that occurs during the larval stage (Jung et al., 2005). During metamorphosis, the lymph gland dissociates and releases hemocytes into the circulating hemolymph (Grigorian et al., 2011). Three distinct zones are identified within the anterior lobe: the medullary zone (MZ), where prohemocytes (blood cell progenitors) reside; a cortical zone (CZ) consisting of mature hemocytes, including plasmatocytes and crystal cells; and a posterior signaling center (PSC), which controls lymph gland homeostasis under both normal conditions and immune challenge (Jung et al., 2005; Yu et al., 2018a). The balance between the maintenance and differentiation of the MZ is a complex biological process involving a series of internal and external regulators and signaling pathways, such as Wg, Janus kinase (JAK)/STAT, insulin, and ROS (Krzemień et al., 2007; Sinenko et al., 2009; Owusu-Ansah and Banerjee, 2009; Benmimoun et al., 2012). Upon wasp infestation, the lymph gland can produce another type of hemocyte, the lamellocyte, which is much larger than other hemocyte types and rare in healthy larvae (Lanot et al., 2001). Lamellocytes function mainly to encapsulate foreign objects that are too large to be phagocytosed by plasmatocytes (Rizki and Rizki, 1992). Previous studies have shown that the JAK/STAT, JNK, Toll, Notch, and ecdysone pathways contribute to lamellocyte formation (Sorrentino et al., 2002; Zettervall et al., 2004; Small et al., 2014); however, the mechanism by which lamellocyte fate is specified is incompletely understood.

Rab family proteins, members of the larger family of Ras-like GTPases, play key roles in regulating vesicle trafficking and are evolutionarily conserved in many organisms (Zhang et al., 2007). As small GTPases, Rab proteins cycle between GTP-bound and GDP-bound forms (Molendijk et al., 2004; Pfeffer and Aivazian, 2004). To date, 31 Rab proteins have been identified and shown to be critical in multiple biological processes (Zhang et al., 2007). Among these proteins, Rab5 (an early endosome marker), Rab7 (a late endosome marker), and Rab11 (a recycling endosome marker) are members of the ‘core Rabs’ family due to their crucial roles in vesicle transport and multiple developmental processes (Dunst et al., 2015). For instance, Rab7 participates in wing disc dorsal/ventral pattern formation (Wilkin et al., 2008), whereas Rab5 and Rab11 play key roles in cellularization and dorsal closure (Pelissier et al., 2003; Sasikumar and Roy, 2009; Mateus et al., 2011). However, only a few reports describe the functions of these Rabs in Drosophila hematopoiesis and indicate that the hemocyte-specific depletion of Rab5 and Rab11 leads to an increase in the circulating hemocyte count (Jean et al., 2012; Del Signore et al., 2017). Furthermore, a study in mammals showed that vacuolar protein sorting protein 33b (VPS33B) mediates hematopoiesis by regulating exosomal autocrine signaling in humans and interacts with RAB11A (Gu et al., 2016). No additional direct evidence has been found to indicate the relationships between the three core Rabs and hematopoiesis.

In this study, we downregulated Rab5, Rab7 and Rab11 individually in the hematopoietic system and found that inhibiting Rab5 or Rab11 but not Rab7 strongly induced cell overproliferation in the Drosophila blood system. In addition, inhibiting Rab5 or Rab11 in hemocytes and the CZ resulted in the loss of MZ quiescence and aberrant lamellocyte differentiation in lymph glands and circulating hemocytes due to activation of the JNK, Ras/EGFR, and Toll signaling pathways. Moreover, we found that during lamellocyte formation, JNK and Ras acted coordinately and that Toll acted downstream of JNK in this process. Finally, we showed that JNK-induced lamellocyte production upon disruption of Rab5/Rab11 GTPase activity was autophagy-dependent. These data advance our understanding of the relationship between vesicle transport and hematopoiesis as well as the mechanism underlying lamellocyte differentiation. Furthermore, our results may provide a fundamental basis for studying vesicle transport in the pathology of leukemia.

Results

Inhibiting Rab5 or Rab11 promotes cell proliferation in the blood system

To test whether Rab5, Rab7 or Rab11 functions in Drosophila hematopoiesis, we first inhibited the GTPase activity of these three Rabs in the blood system by crossing flies expressing the differentiated hemocyte-specific driver Hml>UAS-GFP with UAS-Rab5DN, UAS-Rab7DN, and UAS-Rab11DN flies and quantified the circulating hemocyte count. We observed a significant increase in hemocytes in Hml>UAS-GFP>UAS-Rab5DN and Hml>UAS-GFP>UAS-Rab11DN larvae compared with those in the corresponding controls; however, the hemocyte count in Hml>UAS-GFP>UAS-Rab7DN larvae did not obviously change (Figure 1A). Next, we confirmed these results by using three kinds of GTPase RNAi flies (Figure 1A). These results were consistent with those in previous reports (Jean et al., 2012; Del Signore et al., 2017), suggesting that Rab5 and Rab11 help control the circulating hemocyte count.

Figure 1 with 2 supplements see all
Inhibiting Rab5 or Rab11 promoted cell proliferation in circulating hemocytes and lymph glands.

(A) The number of circulating hemocytes increased upon the loss of Rab5 or Rab11 but not Rab7 in hemocytes. (B–P) Immunostaining for PH3 was performed in lymph glands from third instar larvae. The number of PH3-positive cells increased after the inactivation of Rab5 or Rab11 but not Rab7. The PH3-positive cell count per unit area was calculated as the PH3-positive cell count on a single anterior lobe divided by the area of the single anterior lobe; the quantification is shown in (B). Scale bar: 50 μm. ns, not significant; ***p<0.001 (one-way ANOVA).

Given that Rab5 and Rab11 can affect the numbers of circulating hemocytes, we sought to determine whether they control homeostasis of the lymph gland or circulating hemocytes. First, we dissected lymph glands and stained them with an anti-PH3 antibody to examine mitotic activity. Significantly more PH3-positive cells were observed in Hml>UAS-GFP>UAS-Rab5DN and Hml>UAS-GFP>UAS-Rab11DN lymph glands than in control (Hml>UAS-GFP>WT and UAS-RabDN X WT) lymph glands (Figure 1B,K,O); moreover, the anterior lobes were larger in Hml>UAS-GFP>UAS-Rab5/11DN lymph glands than in those of the control (Figure 1—figure supplement 1A–F,P). Consistent with the observation that Rab7 did not control the hemocyte count, the PH3-positive cell count was comparable between Hml>UAS-GFP>UAS-Rab7DN flies and the corresponding controls (Figure 1B,C,F,M). We also used RNAi lines to confirm these results (Figure 1L,N,P). In addition, the number of PH3-positive cells among Hml>UAS-GFP>UAS-Rab5/11DN circulating hemocytes increased during the second instar stage but not the third instar stage (Figure 1—figure supplement 1G–L,Q). Given that a decreased sessile hemocyte count is associated with an increased circulating hemocyte count (Zettervall et al., 2004), we then assessed the sessile hemocytes from Hml>UAS-Rab5/11DN larvae; however, the sessile hemocyte pattern was unchanged (Figure 1—figure supplement 1M–O), suggesting that the increased circulating hemocyte count upon inhibition of Rab5/Rab11 resulted from the high proliferation of hemocytes. The above results indicated that both Rab5 and Rab11 maintain hematopoietic homeostasis in Drosophila. Therefore, we next focused on the function of Rab5 and Rab11 in hematopoiesis.

Rab5 and Rab11 are expressed in the Drosophila hematopoietic system

To examine the expression pattern of Rab5 and Rab11 in the hematopoietic system, we stained lymph glands from Hml>UAS-GFP (a CZ marker) larvae with anti-Rab5 and anti-Rab11 antibodies. Both Rab5 and Rab11 were expressed widely in both GFP-positive and GFP-negative areas, indicating that they were localized in both the CZ and MZ (Figure 1—figure supplement 2A–B’’). By using col>UAS-GFP (a PSC marker), we observed higher levels of these two proteins in PSC cells (Figure 1—figure supplement 2C–D’’). Moreover, the Rab5 and Rab11 proteins were localized in the cytoplasms of circulating hemocytes, while this expression pattern was not observed in Hml>UAS-GFP>UAS-Rab5 RNAi or Hml>UAS-GFP>UAS-Rab11 RNAi hemocytes (Figure 1—figure supplement 2E–H’). These data suggested that Rab5 and Rab11 are widely distributed throughout the Drosophila hematopoietic system.

Rab5 and Rab11 in the CZ affect MZ maintenance and lamellocyte differentiation

Given that Rab5 and Rab11 affected the proliferation of lymph gland cells, we then used Hml>UAS-GFP to evaluate the changes in the CZ upon the downregulation of Rab5 or Rab11 GTPase activity. By determining the percentage of the GFP-positive area, we found that the CZ was larger in the Rab5DN and Rab11DN groups than in the control group (Figure 2A–D). This result was confirmed by using antibodies against the mature plasmatocyte marker P1, and expansion of P1-positive cells was observed in Hml>UAS-Rab5/11DN lymph glands, including in the anterior and posterior lobes (Figure 2E–G’). In addition, the MZ areas were decreased in lymph glands, as detected by domeMESO-lacZ (an MZ marker) (Figure 2H–K), indicating that Rab5 and Rab11 played a role in progenitor cell maintenance in a non-cell-autonomous manner. These results were confirmed by using UAS-RNAi flies (Figure 2—figure supplement 1A–G). Based on the current model, the non-cell-autonomous regulation of MZ maintenance is mediated by Pvr/STAT/Adgf-A signaling (Mondal et al., 2011). To examine whether the decreased MZ area was regulated by this signaling pathway, we overexpressed UAS-STAT92E and UAS-Adgf-A in Hml>UAS-Rab5/11DN larvae and stained the lymph glands with anti-P1 antibodies. However, the increased P1-positive area was not suppressed in Hml>UAS-Rab5/11DN>UAS-STAT92E or Hml>UAS-Rab5/11DN>UAS-Adgf-A lymph glands (Figure 2—figure supplement 2A–H), suggesting that the non-cell-autonomous regulation of MZ maintenance from the CZ in our study was independent of Pvr/STAT/Adgf-A signaling; the mechanism underlying this process warrants further investigation.

Figure 2 with 6 supplements see all
Inactivation of Rab5 or Rab11 promoted differentiation in circulating hemocytes and lymph glands.

(A–D) The percentage of the GFP-positive area in anterior lobes was increased in Hml>UAS-GFP>UAS-Rab5DN and Hml>UAS-GFP>UAS-Rab11DN lymph glands; the quantification is shown in (D). (E–G’) Immunostaining for the plasmatocyte marker P1 showed that the P1-positive area was increased upon the inactivation of Rab5 or Rab11 in both the anterior (E–G) and posterior lobes (E’–G’). (H–K) Analysis using the medullary zone (MZ) marker domeMESO-lacZ showed that the MZ area was decreased in Hml>UAS-Rab5DN and Hml>UAS-Rab11DN lymph glands. The proportion of the MZ area in the anterior lobe is shown in (K). (L–Y) Immunostaining for the lamellocyte marker L1 showed that the lamellocyte count was increased in lymph glands (L–N) and circulating hemocytes (O–Q) when Rab5 or Rab11 GTPase activity was disrupted. Aberrant lamellocyte differentiation was more severe in lymph glands (S–U) and circulating hemocytes (V–X) after the simultaneous disruption of Rab5 and Rab11. The lamellocyte frequency among total circulating hemocytes is shown in (R) and (Y). Scale bar: 50 μm. ***p<0.001 (one-way ANOVA).

Next, we examined two additional types of hemocytes, crystal cells, and lamellocytes, with anti-ProPO1 and anti-L1 antibodies, respectively. The numbers of crystal cells were comparable among the groups (Figure 2—figure supplement 3A–C,K); however, many lamellocytes were observed in Hml>UAS-GFP>UAS-Rab5/11DN lymph glands, although lamellocytes were rare in healthy larvae (Figure 2L–N). This phenotype was confirmed with UAS-RNAi flies (Figure 2—figure supplement 3D–F). Accordingly, the aberrant lamellocyte differentiation was rescued after overexpression of UAS-Rab5WT or UAS-Rab11CA (Figure 2—figure supplement 3G–J). Additionally, many lamellocytes were detected in the circulating hemolymph from Hml>UAS-GFP>UAS-Rab5/11DN larvae (Figure 2O–R), and some lamellocytes were GFP-positive (arrows in Figure 2P,Q), indicating that they were in a transition state between plasmatocytes and lamellocytes. Moreover, we used three additional hemocyte-specific Gal4 drivers, pxn-Gal4, He-Gal4, and srp-Gal4, to confirm lamellocyte formation. Consistent with our previous result, lamellocytes emerged in lymph glands and circulating hemocytes upon the disruption of Rab5 or Rab11 GTPase activity (Figure 2—figure supplement 4A–P). Because the Hml>UAS-Rab5DN and Hml>UAS-Rab11DN phenotypes were similar, we sought to understand the relationship between Rab5 and Rab11 in the induction of lamellocyte formation. Therefore, we simultaneously inactivated Rab5 and Rab11 in the CZ and observed more lamellocytes in the lymph glands and circulating hemocytes, as well as larger posterior lobes (Figure 2U), in these groups than in the Rab5DN and Rab11DN groups (Figure 2S–Y). In addition, constitutively active Rab11 suppressed the lamellocyte formation in Hml>UAS-Rab5DN lymph glands (Figure 2—figure supplement 5A,B). These results indicated that although Rab5 and Rab11 induce lamellocyte formation in an independent manner, enhanced recycling endosome activity can help alleviate defects in early endosomes during this process. To determine whether the decreased MZ area was due to an altered PSC, we analyzed PSC cells with anti-Antp antibodies (Figure 2—figure supplement 6A–C). However, we did not find differences in the distribution or count of PSC cells after inactivation of Rab5 and Rab11 in the CZ relative to those in the controls (Figure 2—figure supplement 6Y), suggesting that the roles of these two GTPases in regulating MZ cell quiescence are PSC-independent.

Next, we used the MZ- and PSC-specific Gal4 drivers dome-Gal4 UAS-GFP and Antp-Gal4, respectively, to inactivate Rab5 and Rab11 and stained lymph glands with antibodies against P1, L1, and Antp. However, the CZ area and the numbers of lamellocytes and PSC cells in the lymph gland were unchanged relative to those in the controls (Figure 2—figure supplement 6D–U,Z,AA). Moreover, the number of GFP-positive cells did not change when another PSC-specific Gal4 driver, col-Gal4 UAS-GFP, was used (Figure 2—figure supplement 6V–X,BB), indicating that Rab5 and Rab11 do not play a role in regulating lymph gland hematopoiesis in the MZ or PSC. Therefore, we focused our further studies on Rab5/Rab11 function in the CZ of the lymph gland. The above results suggested that Rab5 and Rab11 in the CZ not only affect cell proliferation but also play a role in mediating the balance between MZ maintenance and differentiation.

Inhibiting Rab5/Rab11 can induce lamellocytes by activating the JNK pathway

Next, we sought to determine the mechanism underlying lamellocyte induction after Rab5 or Rab11 inactivation. JAK/STAT signaling is a crucial regulator of lamellocyte formation, and overexpression of Drosophila JAK in the CZ induces a large number of lamellocytes in lymph glands (Terriente-Félix et al., 2017). We knocked down JAK (known as hop in Drosophila) using UAS-hop RNAi to determine whether the lamellocyte count was reduced in Hml>UAS-GFP>UAS-Rab5/11 RNAi lymph glands. However, lamellocytes were not repressed by the knockdown of hop (Figure 3—figure supplement 1A–F), indicating that JAK signaling is not required for lamellocyte differentiation upon inhibition of Rab5 or Rab11 activity.

We then focused on the JNK pathway, which contributes to specifying lamellocyte fate (Zettervall et al., 2004; Tokusumi et al., 2009). puckered (puc) is a target gene of JNK signaling in Drosophila, and we found that the puc-lacZ signal was significantly increased in Hml>UAS-GFP>UAS-Rab5/11 RNAi lymph glands (Figure 3A–C). We also stained lymph glands with an anti-p-JNK antibody and found that the positive signal was significantly increased in the Hml>UAS-GFP>UAS-Rab5/11DN groups compared with the control group (Figure 3D–F). Because the JNK signaling pathway is often associated with apoptosis (Li et al., 2019), apoptosis was assessed by transferase dUTP nick end labeling (TUNEL) assays. We did not observe a large number of apoptotic cells in Hml>UAS-GFP>UAS-Rab5/11DN lymph glands (Figure 3G–I). In addition, aberrant lamellocyte differentiation was not rescued after overexpression of the antiapoptotic protein p35 (Figure 3—figure supplement 1G–J). To further analyze the interaction between Rab5/Rab11 and the JNK signaling pathway, the Hml>UAS-GFP>UAS-Rab5 RNAi and Hml>UAS-GFP>UAS-Rab11 RNAi lines were crossed with a series of JNK-related transgenic flies, including UAS-puc (the negative regulator of the JNK pathway), UAS-bsk (JNK) RNAi, UAS-hep (JNKK) RNAi, UAS-TAK1 (JNKKK) RNAi, and UAS-TRAF1 (an upstream regulator of TAK1) RNAi flies, and the lamellocyte frequency was analyzed (Figure 3J–BB). The lamellocyte frequency was significantly decreased in both UAS-puc and UAS-bsk RNAi lymph glands (Figure 3P–R,V–X, BB). In addition, UAS-hep RNAi alleviated the aberrant lamellocyte differentiation induced by the knockdown of Rab5 but not Rab11 (Figure 3S,Y), whereas UAS-TAK1 RNAi and UAS-dTRAF1 RNAi failed to reduce the lamellocyte count (Figure 3T–U,Z–BB). We also confirmed this result in circulating hemocytes, showing that aberrant lamellocyte differentiation was rescued in Hml>UAS-GFP>UAS-Rab5/11 RNAi>UAS-bsk RNAi hemocytes (Figure 3—figure supplement 2A–E).

Figure 3 with 2 supplements see all
JNK signaling was activated upon Rab5 or Rab11 inactivation in the lymph gland.

(A–I) The JNK pathway activity was elevated in lymph glands upon Rab5 or Rab11 inactivation, as elucidated by the monitoring of JNK signaling with puc-lacZ (A–C) and anti-p-JNK antibodies (D–F). Apoptotic cells in Hml>UAS-GFP>UAS-Rab5DN and Hml>UAS-GFP>UAS-Rab11DN lymph glands were detected by TUNEL assays (G–I). (J–AA) Immunostaining for L1 (red) showed that aberrant lamellocyte differentiation was rescued in Hml>UAS-GFP>UAS-Rab5 RNAi>UAS-puc (Q), Hml>UAS-GFP>UAS-Rab11 RNAi>UAS-puc (W), Hml>UAS-GFP>UAS-Rab5 RNAi>UAS-bsk RNAi (R), Hml>UAS-GFP>UAS-Rab11 RNAi>UAS-bsk RNAi (X), and Hml>UAS-GFP>UAS-Rab5 RNAi>UAS-hep RNAi (S) lymph glands. The quantifications for (J–AA) are shown in BB. Scale bar: 50 μm. ns, not significant; ***p<0.001 (one-way ANOVA).

To further confirm the relationship between Rab5/Rab11 and Bsk in the induction of lamellocyte formation, hemocytes were simultaneously stained with anti-Hrs (the endosome marker) and anti-p-JNK antibodies. The accumulation of endosomes and elevated p-JNK levels were observed after disrupting Rab5/Rab11 GTPase activity (Figure 4A–C’’, N–O), and p-JNK and Hrs+ endosomes exhibited more colocalization than control endosomes (Figure 4A–C’’, P). Next, we utilized Rab5-Gal4 and Rab11-Gal4, which can be used to express genes under the control of the endogenous regulatory elements of Rab5 and Rab11 loci (Chan et al., 2011; Jin et al., 2012), to overexpress bsk and analyzed lamellocytes in the lymph glands. We observed a large number of lamellocytes in Rab5/Rab11>UAS-bsk lymph glands (Figure 4H–K), similar to the results observed when the JNK signaling pathway was activated in mature hemocytes using UAS-hepAct (Figure 4D,E). However, the lamellocyte count did not appreciably change upon overexpression of bsk in the CZ (Figure 4F), consistent with a previous report showing that overexpression of bsk in hemocytes promotes JNK phosphorylation but does not obviously affect hemocyte morphology, the sessile hemocyte population, or the number of circulating hemocytes (Williams, 2006). Furthermore, even after activation of bsk in the whole animal using a ubiquitous Gal4 driver, no lamellocytes were found in da>UAS-bsk lymph glands (Figure 4G). In addition, hemocytes from Rab5/Rab11>UAS-bsk, Hml>UAS-bsk, and Hml>UAS-hepAct larvae were stained with anti-p-JNK, anti-p-Jun (the phosphorylated form of the JNK pathway transcription factor, also known as Jra in Drosophila), and anti-matrix metalloproteinase 1 (Mmp1, a target gene in the JNK pathway that encodes a proteinase that cleaves proteins in the extracellular matrix) antibodies, respectively. Although the p-JNK levels were elevated in hemocytes from all experimental groups, the p-Jun and Mmp1 levels were increased in only Hml>UAS-hepAct and Rab5/11>UAS-bsk hemocytes; no positive signal was observed in Hml>UAS-bsk hemocytes (Figure 4—figure supplement 1A–U). Furthermore, we found more Hrs and p-JNK colocalization in Hml>UAS-hepAct hemocytes than in Hml>UAS-bsk hemocytes (Figure 4L–M’’, P). We then sought to determine why lamellocyte formation was observable in Rab5/11>UAS-bsk larvae but not in Hml>UAS-bsk or da>UAS-bsk larvae. First, according to the expression data from FlyBase, Rab5 and Rab11 are expressed at higher levels in tissues, including fat bodies, the gut and salivary glands, than the daughterless (da) gene; we confirmed this by crossing UAS-GFP with Rab5-Gal4, Rab11-Gal4, and da-Gal4 (data not shown). Therefore, we crossed Hml>UAS-bsk with ppl-Gal4 (fat body-specific driver), NP3084-gal4 (midgut-specific driver), and elav-Gal4 (nervous system-specific driver) and stained hemocytes with anti-L1 antibodies to assess systemic effects during the process. As expected, aberrant lamellocyte differentiation was observed in Hml>ppl>UAS-bsk and Hml>NP3084>UAS-bsk circulating hemocytes but not in Hml>elav>UAS-bsk circulating hemocytes (Figure 4—figure supplement 2A–D), suggesting that systemic effects existed in Rab5/Rab11>UAS-bsk larvae. Consistent with the above data, the p-Jun and Mmp1 levels were increased in Hml>UAS-Rab5/11DN hemocytes (Figure 5A–F). In addition, knocking down two transcription factors in the JNK pathway, Jra (Drosophila Jun) and Kay (Drosophila Fos), restored aberrant lamellocyte differentiation among Hml>UAS-Rab5/11DN hemocytes (Figure 5G–M). These results indicated that Rab5 and Rab11 restrict lamellocyte production by suppressing the JNK signaling pathway and act upstream of Bsk.

Figure 4 with 2 supplements see all
Inhibiting Rab5 or Rab11 induced high p-JNK levels in endosomes.

(A–C’’) An increased degree of Hrs (green) and p-JNK (red) colocalization was observed in Hml>UAS-Rab5DN and Hml>UAS-Rab11DN hemocytes. Merged images (Hrs+p-JNK+DAPI) are displayed in (A–C). (D–K) Massive lamellocyte formation was observed in lymph glands from Hml>UAS-hepAct (E), Rab5>UAS-bsk (I), and Rab11>UAS-bsk (K) larvae. (L–M’’) Hml>UAS-hepAct hemocytes exhibited more Hrs (green) and p-JNK (red) colocalization than Hml>UAS-bsk hemocytes. The Hrs and p-JNK levels are shown in (N) and (O), respectively. (P) The colocalization degree between Hrs and p-JNK is displayed as the Pearson correlation coefficient, which was analyzed with the Colocalization Finder plugin from ImageJ. Scale bars: 50 μm (lymph glands) and 10 μm (hemocytes). ns, not significant; ***p<0.001 (one-way ANOVA).

Inhibiting Rab5 or Rab11 in hemocytes increased the p-Jun and Mmp1 levels.

(A–F) Immunostaining of circulating hemocytes showed that the levels of p-Jun (A–C) and Mmp1 (D–F) were increased after the inactivation of Rab5 or Rab11. (G–M) The lamellocyte frequency, as analyzed by anti-L1 staining, was rescued in Hml>UAS-Rab5DN>UAS-jra/kay RNAi and Hml>UAS-Rab11DN>UAS-jra/kay RNAi circulating hemocytes. The percentage of L1-positive cells among total circulating hemocytes from (G–L) is shown in (M). Scale bar: 10 μm. ***p<0.001 (Student’s t-test).

Knockdown of Rab5/Rab11 can activate Ras/EGFR signaling to promote cell proliferation and coordinate with Bsk to regulate lamellocyte induction

Enhanced Ras/EGFR pathway activity can induce cell overproliferation (Zettervall et al., 2004). Given that downregulation of Rab5/Rab11 GTPase activity resulted in the high proliferation rate of lymph gland cells, we examined Ras/EGFR signaling with anti-p-Erk antibodies. Only a few positive cells were detected in control lymph glands; however, high p-Erk signaling was observed in Hml>UAS-Rab5/11DN lymph glands (Figure 6A–C). Consistent with this finding, the expanded anterior lobe areas were rescued when Ras85D was knocked down in Hml>UAS-Rab5/11DN larvae (Figure 6D–I, CC). In addition, the increase in circulating hemocytes was rescued (Figure 6DD). Several studies have indicated that Ras/EGFR signaling functions in the PSC and MZ to induce lamellocyte formation; however, overexpressing only Ras in hemocytes does not significantly affect lamellocytes (Zettervall et al., 2004; Dragojlovic-Munther and Martinez-Agosto, 2013; Louradour et al., 2017). Based on our previous data showing the lack of lamellocytes when bsk was overexpressed in the CZ, we sought to determine whether Bsk and Ras synergistically affect lamellocyte formation. Interestingly, the lamellocyte counts were increased in Hml>UAS-bsk>UAS-RasV12 lymph glands and circulating hemocytes, while fewer L1-positive cells were detected in those of Hml>UAS-bsk or Hml>UAS-RasV12 (Figure 6J–O, EE). Moreover, knockdown of Ras85D in Hml>UAS-Rab5/11DN larvae partially prevented the increase in the lamellocyte count in the lymph gland (Figure 6P–S) and in the circulating hemocytes (Figure 6—figure supplement 1A–E). We also observed higher p-JNK levels in endosomes in Hml>UAS-bsk>UAS-RasV12 hemocytes than in Hml>UAS-bsk or Hml>UAS-RasV12 hemocytes (Figure 6T–V’’, FF), and the p-Jun and Mmp1 levels were elevated in Hml>UAS-bsk>UAS-RasV12 hemocytes (Figure 6W–BB). These data suggested that Rab5/Rab11 suppresses Ras/EGFR signaling to restrict hemocyte proliferation and that Ras and Bsk coordinately promote high p-JNK levels in endosomes and eventually regulate lamellocyte induction.

Figure 6 with 1 supplement see all
Ras/EGFR signaling was enhanced upon Rab5 or Rab11 inactivation.

(A–C) Immunostaining of lymph glands showed high p-Erk (red) signals upon Rab5 or Rab11 inactivation. (D–I) Anterior lobe enlargement (visualized by DAPI staining) in Hml>UAS-Rab5/11DN lymph glands was rescued after knockdown of Ras85D. (CC) Quantification of the anterior lobe area from (D–I). (J–O) The lamellocyte count was increased in lymph glands (J–L) and circulating hemocytes (M–O) when bsk and Ras were simultaneously overexpressed in the cortical zone (CZ). (EE) The lamellocyte frequency in circulating hemocytes from (M–O). (P–S) The increased lamellocyte count in lymph glands after Rab5 or Rab11 inactivation was rescued by the knockdown of Ras levels using UAS-Ras85D RNAi. (T–BB) Colocalization between Hrs (green) and p-JNK (red) (T–V’’) and p-Jun and Mmp1 (W–BB) was increased in Hml>UAS-bsk>UAS-RasV12 hemocytes compared with control hemocytes. The colocalization degrees are shown in (FF). (DD) Quantification of the circulating hemocyte counts in third instar larvae showed that the increased hemocyte count in Hml>UAS-Rab5/11DN larvae was rescued by the downregulation of Ras85D. Scale bars: 50 μm (lymph glands) and 10 μm (hemocytes). ns, not significant; ***p<0.001 (one-way ANOVA).

Inhibiting Rab5/Rab11 activates the Toll pathway to induce lamellocyte formation

To assess whether the Toll pathway is involved in Rab5/Rab11-dependent lamellocyte induction, we first detected the two core components, Dif and Dorsal. We showed that Dif and Dorsal were activated in the nuclei of Hml>UAS-Rab5/11DN hemocytes, whereas nuclear localization of the two proteins was rare in the control (Figure 7A–C,E–G); this phenotype was also confirmed in Hml>UAS-Rab5/11 RNAi hemocytes (Figure 7—figure supplement 1A–F). In addition, Dif and Dorsal were also activated in Hml>UAS-hepAct hemocytes when JNK signaling was activated (Figure 7D,H); this result was consistent with a previous study showing that Toll acts downstream of the JNK pathway during wing development (Wu et al., 2015).

Figure 7 with 1 supplement see all
The Toll pathway was activated upon Rab5 or Rab11 inactivation.

(A–H) Immunostaining of circulating hemocytes showed that Dif and Dorsal were activated in Hml>UAS-Rab5/11DN (B–C, F–G) and Hml>UAS-hepAct (D, H) hemocytes. (I–X) Detection of lamellocyte formation in hemocytes and lymph glands using anti-L1 antibodies (red) showed that UAS-Dif RNAi but not UAS-dorsal RNAi plays a significant role in restricting aberrant lamellocyte differentiation. The percentage of lamellocytes among circulating hemocytes is shown in (Y). Scale bars: 10 μm (hemocytes) and 50 μm (lymph glands). ns, not significant; ***p<0.001 (one-way ANONA).

Furthermore, we used UAS-dorsal RNAi and UAS-Dif RNAi to repress the Toll pathway and examined whether aberrant lamellocytes were rescued in lymph glands and hemocytes. Compared with controls, lamellocytes were decreased dramatically by knockdown of Dif but not dorsal in Hml>UAS-Rab5/11DN larvae (Figure 7I–Y). Based on our previous study showing that activation of Dif in lymph glands caused lamellocyte differentiation (Hao and Jin, 2017), we reasoned that the disruption of Rab5/Rab11 GTPase activities induces Toll signaling pathway activation, and Dif may play a more significant role in this process.

Autophagy participates in lamellocyte formation upon Rab5 or Rab11 inactivation

The JNK signaling pathway can trigger the induction of autophagy, and this process is mediated by dFOXO, the transcription factor in the JNK pathway (Chang and Neufeld, 2010). Overexpression of dFOXO in hemocytes causes lamellocyte formation in the absence of wasp parasitization (Tokusumi et al., 2017); we confirmed this result by using hemocytes from pxn>UAS-GFP>UAS-dFOXO larvae (Figure 8—figure supplement 1A,B,S). We also showed that dFOXO was activated in Hml>UAS-Rab5/11DN larvae and that the lamellocyte count was rescued by combined UAS-dFOXO RNAi (Figure 8—figure supplement 1C–J,T); we speculated that autophagy may play a role in this process. To further analyze the relationship between hematopoiesis and autophagy, we first detected the formation of autophagosomes in the Cg;Atg8a-mCherry line; generally, the presence of cytoplasmic Atg8 puncta indicates the occurrence of autophagy in a cell (Mauvezin et al., 2014). Under fed conditions, autophagosomes were scarce in the control group; however, large numbers of autophagosomes (visualized as red puncta in the cytoplasm) were observed in hemocytes of Cg;Atg8a-mCherry>UAS-Rab5/11DN larvae (Figure 8A–C). These results were confirmed with the UAS-Rab5/11 RNAi lines (Figure 8—figure supplement 1K–M) and suggested that the loss of Rab5 or Rab11 in hemocytes can induce autophagy under fed conditions. In addition, we found that autophagosome formation tended to occur in larger hemocytes that looked like lamellocytes. To investigate this observation, we labeled autophagosomes with an anti-Atg8 antibody and labeled lamellocytes with an anti-L1 antibody. Interestingly, Atg8 puncta were observed in Hml>UAS-Rab5/11DN (Figure 8D–F) and Hml>UAS-hepAct hemocytes (Figure 8—figure supplement 1N,O), and Atg8-positive autophagosomes were especially prominent in lamellocytes. Ref(2)P is the fly homolog of p62 and accumulates in cells when autophagy is restricted (Pankiv et al., 2007; Mauvezin et al., 2014). Thus, we used Cg;p62-HA to assess autophagy upon downregulation of Rab5 or Rab11. By measuring the fluorescence intensity of the HA tag, we showed that the p62 levels were decreased in Cg;p62-HA>UAS-Rab5/11DN hemocytes (Figure 8G–I,R); the same phenotype was observed in Cg;p62-HA>UAS-Rab5/11 RNAi larvae (Figure 8—figure supplement 1P–R,U). In addition, the p62 signals were even weaker in L1-positive cells (Figure 8H,I). Thus, we reasoned that autophagy may participate in lamellocyte formation.

Figure 8 with 2 supplements see all
Loss of Rab5 or Rab11 activated autophagy in hemocytes.

(A–C) Many autophagosomes were observed in Cg;Atg8a-mCherry>UAS-Rab5DN and Cg;Atg8a-mCherry>UAS-Rab11DN hemocytes. (D–I) Circulating hemocytes of Hml-Gal4, Hml>UAS-Rab5DN, and Hml>UAS-Rab11DN larvae were labeled with anti-Atg8 (red) and anti-L1 (green) antibodies, whereas those of Cg;p62-HA>WT, Cg;p62-HA>UAS-Rab5DN, and Cg;p62-HA>UAS-Rab11DN were labeled with anti-HA (green) and anti-L1 (red) antibodies (G–I). The arrows in (E and F) indicate lamellocytes with large numbers of autophagosomes. The average p62 levels per hemocyte as measured by evaluating HA fluorescence are shown in (R). (J–Q) The LysoTracker intensity (red) and lamellocyte differentiation (green) were examined in circulating hemocytes from Hml>Gal80ts>UAS-Rab5DN and Hml>Gal80ts>UAS-Rab11DN larvae after being shifted to 29°C for 0 hr, 12 hr, 24 hr, and 48 hr. In (M) and (Q), the asterisks indicate ‘intermediates’, and the arrows indicate ‘mature lamellocytes’. Scale bar: 10 μm. ***p<0.001 (one-way ANOVA).

Next, we analyzed lamellocyte induction and autophagy intensity after inactivation of Rab5 or Rab11 at different time points (0 hr, 12 hr, 24 hr, and 48 hr) with anti-L1 antibodies and LysoTracker, respectively (Figure 8J–Q). Under fed conditions, lysosomes are poorly stained by LysoTracker in the fat bodies from third instar larvae, whereas induction of autophagy leads to strong punctate staining with LysoTracker (Scott et al., 2004). Initially, lamellocytes were not found, and the LysoTracker signals were low (Figure 8J,N). After 12 hr, the fluorescence intensity of LysoTracker was increased, but lamellocytes were still rare among circulating hemocytes (Figure 8K,O). At 24 hr, lamellocytes began to emerge, accompanied by strong LysoTracker signals in hemocytes (Figure 8L,P). After 48 hr, two types of lamellocytes were observed among circulating hemocytes: (1) cells with punctate L1 signals in the cytoplasm but smaller than typical lamellocytes (we defined this cell type as ‘intermediates’ and indicated them with asterisks in the figure) and (2) large cells with strong L1 signals (we defined this cell type as ‘mature lamellocytes’ and indicated them with arrows in the figure) (Figure 8M,Q; Stofanko et al., 2010). Interestingly, the LysoTracker intensity was relatively high in ‘intermediates’ but significantly lower in mature lamellocytes, suggesting that autophagy may promote lamellocyte formation.

To test this hypothesis, we overexpressed Atg1 in hemocytes and analyzed lamellocyte production since enhanced Atg1 levels can directly induce autophagy and Atg1 is possibly regulated by FOXO (Scott et al., 2007; Chang and Neufeld, 2010). We used two sources of UAS-Atg1 (#2 and #3) and found large numbers of lamellocytes in both the Hml>UAS-Atg1 #2 and Hml>UAS-Atg1 #3 groups (Figure 9A–C,Y), suggesting that enhanced autophagy levels trigger the induction of lamellocyte formation. Next, we knocked down Atg1 and Atg8 separately in Hml>Rab5/11DN larvae and evaluated the lamellocyte count. We observed fewer lamellocytes among Hml>UAS-Rab5/11DN>UAS-Atg1 RNAi and Hml>UAS-Rab5/11DN>UAS-Atg8 RNAi hemocytes than among control hemocytes (Figure 9D–I,Z–AA). To further validate that JNK is required for autophagy activation in Hml>UAS-Rab5/11DN hemocytes, we repressed JNK signaling by using UAS-bsk RNAi or UAS-bskDN flies. The increased LysoTracker signals in Hml>UAS-Rab5/11DN>WT hemocytes were suppressed in Hml>UAS-Rab5/11DN>UAS-bsk RNAi and Hml>UAS-Rab5/11DN>UAS-bskDN hemocytes (Figure 9J–P), suggesting that this process is JNK-dependent. Accordingly, aberrant lamellocyte differentiation was rescued in Hml>UAS-Rab5/11DN>UAS-bsk RNAi and Hml>UAS-Rab5/11DN>UAS-bskDN circulating hemocytes (Figure 8—figure supplement 2A–G). Next, we investigated whether lysosomal function was crucial for autophagy activation and lamellocyte formation upon the inhibition of Rab5/Rab11. We then knocked down Syntaxin 17 (Syx17), a SNARE that is required for the fusion of autophagosomes and lysosomes (Takáts et al., 2013), in Hml>UAS-Rab5/11DN larvae and stained hemocytes with LysoTracker and anti-L1 antibodies. The LysoTracker intensity was decreased to a greater extent in Hml>UAS-Rab5/11DN>UAS-Syx17 RNAi hemocytes (Figure 9Q–T), indicating that the lysosomal activity/autophagy was suppressed. In addition, the increased lamellocyte count was suppressed when Syx17 was knocked down (Figure 9U–X, BB). This result indicated that autophagosome-lysosome fusion is critical for inducing autophagy and lamellocyte formation upon the loss of Rab5/Rab11. In conclusion, these data indicated that autophagy plays a role in lamellocyte formation upon the loss of Rab5 or Rab11.

The lamellocyte formation upon the loss of Rab5 or Rab11 was autophagy-dependent.

(A–I, U–X) The lamellocyte count was determined in circulating hemocytes by anti-L1 immunostaining. Overexpression of Atg1 in two sources of UAS-Atg1 flies resulted in massive lamellocyte production (A–C). In addition, knocking down Atg1 (E, H), Atg8 (F, I), or Syx17 (V, X) in Hml>UAS-Rab5/11DN larvae suppressed the increase in the lamellocyte count. (Y–BB) The lamellocyte numbers were quantified. (J–T) LysoTracker staining in circulating hemocytes showed that the increased LysoTracker intensity was suppressed in Hml>UAS-Rab5/Rab11DN>UAS-bsk RNAi, Hml>UAS-Rab5/Rab11DN>UAS-bskDN, and Hml>UAS-Rab5/Rab11DN>UAS-Syx17 RNAi hemocytes. Scale bar: 10 μm. ***p<0.001 (one-way ANOVA).

Discussion

Proper regulation of the hematopoietic system is crucial to the survival of both invertebrates and vertebrates. In humans, impaired hematopoietic homeostasis can cause several blood disorders, including leukemia. Multiple intrinsic and extrinsic signals contribute to the tight regulation of the blood system. In the last decade, Drosophila hematopoiesis has been extensively studied because it shares many conserved regulatory factors and signaling pathways with mammalian hematopoiesis (Yu et al., 2018a). However, the molecular mechanisms of hematopoiesis remain largely unknown. Vesicle trafficking is a critical component of signal transduction in multiple developmental processes, and aberrant membrane transport significantly alters the signal output. Several reports have described the interaction between vesicle transport and hematopoiesis and shown that the Ras GTPase family member ARF1 modulates Drosophila hemocyte homeostasis via multiple signaling pathways (Khadilkar et al., 2014; Khadilkar et al., 2017; Sinha et al., 2013), whereas Graf regulates plasmatocyte proliferation through the GPI-enriched endocytic compartment (GEEC) endocytosis of EGFR (Kim et al., 2017). In addition, loss of Vps35, a regulator of endocytosis, leads to increased hemocyte counts and aberrant lamellocyte differentiation, which is associated with Toll and EGFR activation (Korolchuk et al., 2007).

In this article, we focused on three Rab proteins, Rab5, Rab7, and Rab11, that play key roles in vesicle trafficking and determined whether they function in mediating hematopoietic homeostasis. Knockdown or inactivation of Rab5 or Rab11 but not Rab7 significantly induced cell overproliferation in both circulating hemocytes and lymph glands, consistent with previous reports revealing that Rab5 and Rab11 affect cell proliferation in other tissues (Takino et al., 2014; Nie et al., 2019). However, the increased lymph gland size and hemocyte count were rescued by inhibiting the Ras/EGFR pathway. We also showed that inactivation of Rab5 or Rab11 resulted in the loss of blood cell progenitor quiescence and aberrant differentiation of plasmatocytes and lamellocytes, which was dependent on non-cell-autonomous regulation from the CZ and independent of the PSC (Figure 10A). However, the crystal cell count in lymph glands was not significantly altered. Lamellocytes, a large and disc-shaped type of hemocyte, are rarely seen in healthy larvae; however, to resist wasp egg infestation, Drosophila will produce numerous lamellocytes that can encapsulate invading eggs, followed by the formation of a multilayer structure to kill them (Rizki and Rizki, 1992). PSC-derived ROS are important during this process. First, wasp parasitism elevates the ROS levels in the PSC, which can trigger the secretion of Spitz (an EGFR pathway ligand), subsequently inducing the conversion of circulating hemocytes into lamellocytes (Sinenko et al., 2011). In addition, a recent study showed that high ROS levels in PSC cells can also activate the Toll/NFkB pathway and eventually promote the differentiation of lamellocytes (Louradour et al., 2017). Moreover, starvation or a high-sugar diet results in lamellocyte differentiation in both lymph glands and circulating hemocytes (Shim et al., 2012; Yu et al., 2018b). Thus, the appearance of lamellocytes can be an indicator of a hazardous environment; however, the mechanism underlying the differentiation of lamellocytes is poorly understood.

Schematic diagram of lymph gland morphology and the regulatory network of signaling pathways upon Rab5/Rab11 inactivation.

(A) Schematic diagram of lymph glands from control and Rab5/Rab11-deficient third instar larvae. When Rab5 or Rab11 was downregulated in the cortical zone (CZ), the formation of many premature hemocytes and lamellocytes was induced in the anterior and posterior lobes, whereas the medullary zone (MZ) area was decreased. Different cell types are represented by different colors and shapes. (B) Inhibiting Rab5 or Rab11 led to high p-JNK levels in endosomes. Next, activation of multiple signaling pathways, including JNK, Ras/EGFR, and Toll, ultimately contributed to lamellocyte differentiation and cell overproliferation.

In this article, we sought to clarify the regulatory network involved in the induction of lamellocyte formation. In Hml>UAS-GFP>UAS-Rab5/11DN circulating hemocytes, a subset of L1+ cells was colabeled with GFP, further confirming the plasticity of plasmatocyte conversion into lamellocytes (Stofanko et al., 2010); however, the cause of this conversion is not fully understood. A previous directed screen for genes involved in Drosophila hemocyte activation suggested that the JAK/STAT, JNK, and Toll signaling pathways participate in lamellocyte formation (Zettervall et al., 2004). Via rescue assays using various UAS-RNAi flies, we showed that both Rab5 and Rab11 restrict lamellocyte induction by repressing JNK and Toll activation rather than JAK/STAT activation. Inactivation of Rab5 or Rab11 significantly promoted the phosphorylation of JNK in lymph glands and hemocytes; we also observed high p-JNK levels in Hrs-positive endosomes in circulating hemocytes.

Ras/EGFR is considered a key pathway in the regulation of cell proliferation and growth; however, the role of Ras/EGFR in the CZ is unclear. We showed an increase in p-Erk activity when either Rab5 or Rab11 was inactivated in the CZ, and deletion of Ras85D in the CZ rescued aberrant lamellocyte differentiation; however, overexpression of Ras failed to induce massive lamellocyte differentiation. Interestingly, when bsk and Ras were overexpressed simultaneously in the blood system, many lamellocytes were produced in circulating hemocytes and lymph glands. In addition, we detected increased p-JNK levels in endosomes and increased p-Jun and Mmp1 levels in Hml>UAS-bsk>UAS-RasV12 hemocytes, consistent with a previous article describing the role of Fos in regulating gut cell proliferation via the integration of EGFR and JNK signaling (Biteau and Jasper, 2011); however, the interaction between these two pathways warrants further study.

Toll signaling, a traditional innate immune signaling pathway, plays an important role in killing invading pathogens by triggering the production of multiple antimicrobial peptides (AMPs) (Lemaitre and Hoffmann, 2007). In addition, the Toll pathway is involved in lamellocyte formation. Our previous study reported that the loss of jumu, a member of the forkhead transcription factor family, induces lamellocyte differentiation via the nuclear translocation of Dif throughout the entire lymph gland (Hao and Jin, 2017). In this paper, downregulation of Dif rescued aberrant lamellocyte differentiation in Hml>UAS-Rab5/11DN lymph glands, further confirming the crucial role of Toll signaling molecules, especially Dif, in lymph gland hematopoiesis. Based on a study on Drosophila wings showing the interaction between JNK and Toll (Wu et al., 2015), our findings show that Dif acts as a downstream regulator of the JNK pathway in hemocyte activation. In summary, disruption of vesicle trafficking by inactivation of Rab5 or Rab11 in the blood system can activate multiple signaling pathways and consequently disrupt immune cell quiescence (Figure 10B). Endosomal recycling is crucial for receptor/ligand control. A previous study indicated that the loss of Rab5 leads to the stimulation of Eiger (the ligand of JNK pathway)/EGFR at the membrane of mutant cells, and Rab5 is important for eye disc development (Takino et al., 2014). In addition, another study showed that the activation of Rab11 GTPase in orcl mutants induces the stimulation of Spz, the Toll pathway ligand, and ORCL, which is required for maintaining hemocyte quiescence (Del Signore et al., 2017). Based on these studies, we reasoned that the phenotype observed in our study may also be due to the misregulation of various receptors, which warrants further investigation.

Autophagy is a conserved process in which cytoplasmic proteins or organelles are degraded in lysosomes. In Drosophila, only one report to date has revealed the function of Atg genes in hematopoiesis; this report showed that Atg6 is required for not only multiple vesicle trafficking pathways but also hematopoietic homeostasis (Shravage et al., 2013). Here, we showed that dFOXO expression and autophagy were induced when Rab5 or Rab11 was downregulated under fed conditions. In addition, we observed increased autophagy activity in lamellocytes and found that knockdown of dFOXO, Atg1, Atg8, and Syx17 restricted lamellocyte formation upon the loss of Rab5 or Rab11. Interestingly, we also observed very low lysosomal activity in fully mature lamellocytes and high lysosomal activity in intermediates. We reasoned that the formation of lamellocytes requires high autophagy levels and that after lamellocyte maturation, autophagy is halted and maintained at basal levels. Moreover, we are the first to prove that overexpression of Atg1 results in the appearance of a large number of lamellocytes. Previous studies have reported the role of Rab5 and Rab11 in autophagy in fat bodies and have shown that Rab5 promotes the degradation of autophagic cargo through lysosomal function (Hegedűs et al., 2016), whereas Rab11 facilitates the fusion of endosomes and autophagosomes (Szatmári et al., 2014). In this study, we demonstrated that deletion or inactivation of Rab5 or Rab11 induced autophagy under normal conditions in hemocytes and affected hematopoiesis; the underlying mechanism by which autophagy regulates hematopoiesis needs to be further clarified. In summary, we showed that maintaining a normal level of vesicle trafficking as well as autophagy in the blood system are important for hematopoietic homeostasis. We have identified Rab5 and Rab11 as novel regulators of hematopoiesis.

Materials and methods

Drosophila strains and culture conditions

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UAS-Rab5DN, UAS-Rab7DN, UAS-Rab11DN, UAS-Rab5WT, UAS-Rab11CA, puc-lacZ, UAS-hepAct, UAS-RasV12, UAS-bsk, UAS-Atg1 #2, UAS-p35, Rab5-Gal4, Rab11-Gal4, UAS-GFP, ppl-Gal4, and da-Gal4 were obtained from the Bloomington Drosophila Stock Center (BDSC). UAS-Rab5 RNAi, UAS-Rab7 RNAi, UAS-Rab11 RNAi, UAS-bsk RNAi, UAS-hep RNAi, UAS-dorsal RNAi, UAS-Dif RNAi, UAS-Atg1 RNAi, UAS-Atg8 RNAi, and UAS-hop RNAi were obtained from the Vienna Drosophila Resource Center. Hml-Gal4, UAS-jra RNAi, UAS-kay RNAi, UAS-dFOXO RNAi, UAS-Ras85d RNAi, UAS-Syx17 RNAi, elav-Gal4, and Tub-Gal4; Tub-Gal80ts were obtained from the Tsinghua Fly Center. Hml-Gal4;UAS-2xEGFP, UAS-STAT92E, UAS-Adgf-A, and Antp-Gal4 were gifts from Utpal Banerjee (Mandal et al., 2007); col-Gal4 UAS-mCD8GFP was a gift from Lucas Waltzer (Benmimoun et al., 2015); the srp-Gal4, domeMESO-lacZ, and dome-Gal4;UAS-2xEGFP lines were gifts from Jiwon Shim; and pxn-Gal4;UAS-GFP and He-Gal4;UAS-GFP were gifts from Norbert Perrimon. UAS-puc, UAS-bskDN, UAS-TRAF1 RNAi, and UAS-TAK1 RNAi were gifts from José Carlos Pastor-Pareja (Willsey et al., 2016). Cg;Atg8a-mCherry and Cg;p62-HA were gifts from Chao Tong. The UAS-Atg1 #3 line was gifted by Guangchao Chen (Chen et al., 2008). The UAS-dFOXO line was a gift from Pierre Léopold. The NP3084-Gal4 was a gift from Bing Zhou (Xiao et al., 2019). The w1118line (from BDSC) was used as the wild-type (WT) line in this article. For crosses performed with the UAS/Gal4 system, flies were allowed to lay eggs at 25°C; after the eggs hatched into first instar larvae, the larvae were transferred to 29°C. The other strains were reared at 25°C. All strains and crosses were cultured on standard cornmeal-yeast medium.

Immunostaining

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For lymph gland staining, lymph glands were dissected in ice-cold PBS. After fixation for 30 min in 4% paraformaldehyde, dissected tissues were incubated in blocking solution (PBS containing 0.1% Tween-20% and 5% goat serum) for 30 min and then incubated with primary antibodies diluted in blocking solution at 4°C overnight. After several rinses in PBST, lymph glands were sequentially incubated with secondary antibodies for 2 hr and 4'6-diamidino-2-phenylindole (DAPI) for 10 min before being mounted with Slowfade mounting reagent (Thermo Fisher). For the immunostaining of hemocytes, 10 larvae were bled in 10 μl of PBS and then transferred to an adhesive glass slide. After incubation for 30 min in a humidified chamber, hemocytes were fixed using 4% paraformaldehyde for 10 min, blocked in blocking buffer, and sequentially incubated with primary antibodies, secondary antibodies, and DAPI before being mounted. All samples were observed under a Zeiss Axioskop 2 Plus microscope or a Zeiss LSM510 confocal microscope. The following primary antibodies were used in the study: rabbit anti-PH3 (Millipore, RRID:AB_1977177), mouse anti-P1 and mouse anti-L1 (gifts from Istvan Andó); rabbit anti-ProPO1 (a gift from Erjun Ling); rabbit anti-Dif (a gift from Dominique Ferrandon); mouse anti-Antp (RRID:AB_528082), mouse anti-Dorsal (RRID:AB_528204), mouse anti-Hrs (RRID:AB_2618261) and mouse anti-Mmp1 (RRID:AB_579782) (Developmental Studies Hybridoma Bank); rabbit anti-Rab5 (RRID:AB_882240) and rabbit anti-GABARAP (anti-Atg8) (Abcam, RRID:AB_10861928); mouse anti-Rab11 (BD Biosciences, RRID:AB_397983); rabbit anti-HA (Sigma, RRID:AB_260070); rabbit anti-dFOXO (a gift from Pierre Léopold); rabbit anti-p-Erk (Cell Signaling Technology, RRID:AB_2315112); mouse anti-p-Jun (Santa Cruz, RRID:AB_629275), mouse anti-β-Gal (RRID:AB_430877), and mouse anti-p-JNK (RRID:AB_430864) (Promega). All Alexa Fluor 488- and Alexa Fluor 568-conjugated secondary antibodies were purchased from Thermo Fisher.

LysoTracker staining

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First, 5–10 larvae were bled in 10 μl of PBS, and circulating hemocytes were then incubated with 1 μM LysoTracker Red DND-99 (Thermo Fisher) for 30 min and with 4% paraformaldehyde for 10 min. Finally, the hemocytes were mounted with Slowfade mounting reagent and analyzed using a Zeiss Axioskop 2 Plus microscope or a Zeiss LSM 510 Meta confocal microscope. Flies carrying the Gal80ts element were first reared at 18°C and then shifted to 29°C for 0 hr, 12 hr, 24 hr, and 48 hr to activate the Gal4 driver.

Quantification of circulating hemocytes

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Groups of five wandering third instar larvae (at least 10 groups per treatment) were bled in 20 μl of PBS and transferred to a Neubauer Improved hemocytometer (Marienfeld) to quantify the number of circulating hemocytes per larva.

TUNEL assays

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TUNEL assays (Roche) were employed according to the manufacturer's instructions to detect apoptosis.

Statistical analysis

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The PH3+, ProPO+, L1+, or Antp+ cell counts were quantified with ImageJ. For the quantification of the area of GFP+ or domeMESO+ cells, the images were converted to eight bits and adjusted to obtain an identical threshold by using ImageJ. Then, the area with the identical threshold was measured as the fluorescence+ area. The fluorescence intensity of p62-HA was also analyzed with ImageJ. The p-values for all experiments were calculated with a two-tailed, unpaired Student’s t-test or one-way ANOVA using GraphPad Prism 6.0 software. The thresholds for statistical significance were established as *p<0.05, **p<0.01, and ***p<0.001. The error bars in all column diagrams indicate the means ± SDs. All staining experiments were performed at least three times independently. For each genotype in each independent experiment, at least 10 lymph glands or at least 10 images of hemocytes were analyzed. The detailed sample sizes are shown either directly on the bar or by dot plots.

References

    1. Tepass U
    2. Fessler LI
    3. Aziz A
    4. Hartenstein V
    (1994)
    Embryonic origin of hemocytes and their relationship to cell death in Drosophila
    Development 120:1829–1837.

Decision letter

  1. Jiwon Shim
    Reviewing Editor; Hanyang University, Republic of Korea
  2. Utpal Banerjee
    Senior Editor; University of California, Los Angeles, United States
  3. Lucas Waltzer
    Reviewer; University of Clermont Auvergne, France

In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses.

Acceptance summary:

This study dissects the role of Rab proteins in Drosophila hematopoietic homeostasis and shows that Rab5 and Rab11 are implicated in the hemocyte proliferation and differentiation through several genetic pathways, including the JNK, Toll, and Ras/EGFR. Moreover, inhibition of Rab/Rab11 alters autophagy in hemocytes, together activating differentiation of lamellocytes. Genetic interactions indicated in this study will promote further investigations of intricate balances in the endocytic machinery from the perspective of immunity and hematopoiesis.

Decision letter after peer review:

Thank you for submitting your article "Rab5 and Rab11 maintain hematopoietic homeostasis by restricting multiple signaling pathways in Drosophila" for consideration by eLife. Your article has been reviewed by three peer reviewers, one of whom is a member of our Board of Reviewing Editors, and the evaluation has been overseen by Utpal Banerjee as the Senior Editor. The following individual involved in review of your submission has agreed to reveal their identity: Lucas Waltzer (Reviewer #2).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

As the editors have judged that your manuscript is of interest, but as described below that additional experiments are required before it is published, we would like to draw your attention to changes in our revision policy that we have made in response to COVID-19 (https://elifesciences.org/articles/57162). First, because many researchers have temporarily lost access to the labs, we will give authors as much time as they need to submit revised manuscripts. We are also offering, if you choose, to post the manuscript to bioRxiv (if it is not already there) along with this decision letter and a formal designation that the manuscript is "in revision at eLife". Please let us know if you would like to pursue this option. (If your work is more suitable for medRxiv, you will need to post the preprint yourself, as the mechanisms for us to do so are still in development.)

Summary:

In this manuscript, Shichao and Jin investigated the hematopoietic phenotype driven by the loss of endosomal regulators, Rab5 and Rab11. Inhibition of Rab5 or Rab11 induces hyperproliferation of the lymph gland or circulating hemocytes and activates the lamellocyte differentiation. Multiple signaling pathways including the JNK, Ras/EGFR, Toll, and autophagy are altered by the loss of endocytic control, highlighting the importance of vesicle trafficking in the Drosophila hematopoiesis.

All the reviewers find that the study is well-executed, and they were quite enthusiastic about the manuscript. But, they also find that there are several conclusions to be significantly strengthened to clearly support the authors' claim. In addition to 5 essential revisions, it is recommended that the authors look for bringing new insights into the Rab5/11 mechanism.

Essential revisions:

1) Rab5/Rab11 and autophagy

A general concern is at the end of the study where autophagy is interrogated. Unlike the previous sections of the manuscript, this section appears more preliminary. A knowledge gap remains: what is the functional role of autophagy in hematopoiesis? Does the autophagy induction from Rab5/11 perturbation require functional lysosomes? If lysosome function is perturbed, does this suppress the increased autophagy phenotype? An alternative model posits that autophagy merely sequesters signaling molecules away from recycling endosome-cell surface cycling. If so, fusion of autophagsomes with lysosomes may not be necessary for the role of autophagy in lamellocyte formation. Can autophagosomal SNARES be knocked down or perturbed to see the impact on formation? Evidence is presented that Rab5/11 knockdown increases autophagy, and this is correlated with lamellocyte formation/JNK activation. This conclusion could be tuned-down or directly tested (using for instance Hep or Bsk RNAi).

2) An increase in the number of circulating hemocytes

The origin of the increased number of circulating hemocytes should be clarified. The authors do not observe an increase in proliferation in circulating hemocytes at the L3 stage (in contrast with the situation in the lymph gland). Increased proliferation may have occurred earlier. Alternatively, it is possible that the increase is merely due to a disruption of the sessile hemocyte compartment. These 2 possibilities could be easily assessed.

3) Rab5/Rab11-gal4 expression

Different phenotypes driven by Rab5/Rab11-gal4 require additional experiments to address why it leads to the discrepancy when compare to hemocyte-specific drivers. This could simply be due to different levels of overexpression (and a threshold effect to observe lamellocyte induction in response to JNK activation). Or it is possible that Rab5/11-gal4 is expressed in other tissues causing systemic effects. This needs to be assessed either by looking directly at Bsk accumulation or with a UAS-destablizedGFP. At present, the conclusion that bsk needs to be expressed "under endosomal control" to induce lamellocyte seems weak and the concept still ill-defined.

4) Mixture of lymph gland- and circulating hemocyte phenotypes

Lymph gland- and circulating hemocytes are intermingled, and some of the pathways, including the Toll pathway and autophagy, are validated only by circulating hemocytes. The mixture of two cell types throughout the manuscript makes it hard to follow the authors' rational. Moreover, hemocyte differentiation/proliferation mechanisms in two tissues may differ. The authors need to better align the two populations with more caution and confirm all the pathways in both cell types.

5) The non-cell-autonomous phenotype in the lymph gland

In Figure 2H-K, the authors showed that the expression of RNAi against Rab5 or Rab11 in the CZ decreases the MZ in a non-cell-autonomous manner. How do the authors explain this phenotype by the current model? This should be shown with additional experiments.

Physiological- or cell biological relevance of Rab5/11-mediated function

What is the physiological or cell biological relevance of the Rab5/Rab11 regulation shown in this study? It will be important to show whether the endocytic control indeed is critical for the immune response/hemocyte development to manage all the associated signals.

[Editors' note: further revisions were suggested prior to acceptance, as described below.]

Thank you for resubmitting your work entitled "Rab5 and Rab11 maintain hematopoietic homeostasis by restricting multiple signaling pathways in Drosophila" for further consideration by eLife. Your revised article has been evaluated by Utpal Banerjee (Senior Editor) and a Reviewing Editor.

The manuscript has been improved but there are some remaining issues that need to be addressed before acceptance, as outlined below:

Reviewer #1:

Yu and colleagues supplemented a large number of data and addressed most of the concerns raised by the reviewers. New data clearly resolved concerns raised and explained how circulating hemocytes are increased upon loss of Rab5/11 and why Rab5/11-gal4 differs from Hml-gal4. Despite the clarification, the biological relevance of Rab5/Rab11 signaling in hematopoiesis is still unclear. It is well-known that Rab5/Rab11 endosomal pathway controls multiple proteins involved in cellular signalings, including EGFR/Ras, Eiger/JNK, and Toll, and thus, in my opinion, it will be important to understanding why Rab5/Rab11-mediated control is significant in the blood.

1) Biological relevance

Although the authors extensively investigated the functions of Rab5/Rab11 in the blood cell signaling, it is still ambiguous how the endosomal control is maintained during development or immunity and why it is significant in hematopoiesis. For example, does Rab5/Rab11 control the proliferation of hemocytes in a specific developmental stage by repressing or activating Ras/Erk? Is Rab5/Rab11 inhibited by immune challenges to directly stimulate the lamellocyte differentiation? In which context Rab5/Rab11 gets modified and manipulates target signals similar to phenotypes derived by Rab5DN/Rab11DN or Rab5/Rab11 overexpression?

2) Schematic diagram

A schematic diagram shown in Figure 10B does not fully represent the genetic pathways identified in the results. For example, there is no evidence showing a direct activation of pERK by Rab5/Rab11. And even though Rasv12 alone could not activate the lamellocyte differentiation, active Ras and Bsk together can induce the lamellocyte. The authors introduced dFOXO as a target of JNK and showed that loss of dFOXO in Hml>Rab5/11DN rescues the lamellocyte phenotype. These points are missing. Or the diagram could be simplified to help understand the major points.

Reviewer #2:

The authors demonstrate that decreasing the activity of Rab5 or Rab11 in differentiated hemocytes leads to an increase in blood cell number and to the spurious differentiation of lamellocytes. They show that the JNK, Toll and ras/EGFR pathways are implicated in lamellocyte induction downstream of Rab5/11 and further investigate JNK-induced autophagy in this process. These results bring important new insights into the fine tuning of blood cell homeostasis by the endocytic machinery.

In my opinion, the revisions performed by the authors clearly address the major points that were raised during the review process. The new experiments that have been added bring convincing results, even when they are "negative" (as for the non-autonomous phenotype in the lymph gland). Notably, they highlight the functional importance of autophagy/lysosomes and clarify the origin of the increase in circulating blood cells. The authors have also appropriately taken into considerations most other comments. In sum, the revised version of the manuscript is clearly improved.

Reviewer #3 :

This study dissects the role of Rab proteins Rab5 and 11 on Drosophila hematopoietic homeostasis. Using RNAi and dominant negative tissue-specific genetic manipulation, they demonstrate that Rab5/11 influence Drosophila hemocyte proliferaction in a JNK-dependent, autophagy dependent manner. Strengths include thorough genetic dissection of several signaling pathways in both the hemocytes and associated lymph tissues.

Comments:

1) This study primarily dissects the roles of Rab5 and Rab11 proteins in hemocyte homeostasis. After the initial review, a major concern was operationally dissecting the functional role for autophagy in hematopoiesis. The new Syx17 RNAi data demonstrates a requirement for functional autophagosome-lysosome fusion for the elevated hemocyte levels.

2) For the increased hemocyte concerns, the anti-PH3 staining indicates increases mitotic proliferation, and appears conclusive.

3) The driver differences data indicate variability in expression between the Rab5/Rab11 and da drivers. The text has been amended in accordance with this new data.

4) Regarding the differences in analysis between lymph and circulating hemocytes: lymph glands do not appear to manifest strong autophagy after starvation. These tissue distinctions appear to now be more discussed in the revision.

5) non-cell-autonomous phenotype in lymph gland: the PVR/STAT92E/adenosine signaling pathway is now tested, and found to be independent of it.

Overall conclusion: in general the revision has addressed the majority of points. The new experiments that have been added, particularly the Syx17 work and PH3 staining, are conclusive if somewhat minimalistic. The text is also adjusted to tone-down or re-interpret sections that were issues before. Point 5 remains unaddressed, as the lymph gland non-cell-autonomous phenotype was independent of PVR/STAT93E/adenosine pathway. However, I feel fully addressing this in full is outside the scope of this revision.

https://doi.org/10.7554/eLife.60870.sa1

Author response

Essential revisions:

1) Rab5/Rab11 and autophagy

A general concern is at the end of the study where autophagy is interrogated. Unlike the previous sections of the manuscript, this section appears more preliminary. A knowledge gap remains: what is the functional role of autophagy in hematopoiesis? Does the autophagy induction from Rab5/11 perturbation require functional lysosomes? If lysosome function is perturbed, does this suppress the increased autophagy phenotype? An alternative model posits that autophagy merely sequesters signaling molecules away from recycling endosome-cell surface cycling. If so, fusion of autophagsomes with lysosomes may not be necessary for the role of autophagy in lamellocyte formation. Can autophagosomal SNARES be knocked down or perturbed to see the impact on formation? Evidence is presented that Rab5/11 knockdown increases autophagy, and this is correlated with lamellocyte formation/JNK activation. This conclusion could be tuned-down or directly tested (using for instance Hep or Bsk RNAi).

We thank the reviewers for the suggestions to further clarify the role of autophagy in lamellocyte formation. Syntaxin 17 (Syx17) is an autophagosomal SNARE that is important for the fusion of lysosomes and autophagosomes (Takáts et al., 2013). Inhibiting Syx17 disrupts the fusion of lysosomes and autophagosomes and subsequently blocks autolysosomal degradation. To examine whether autophagy induction from Rab5/Rab11 perturbation requires functional lysosomes, we knocked down Syx17 in Hml>UAS-Rab5/11DN larvae. We found that increased LysoTracker signals were decreased in Hml>UAS-Rab5/11DN>Syx 17 RNAi circulating hemocytes (Figure 9Q-T), showing that the increased lysosomal activity was blocked. In addition, the lamellocyte count was decreased in Hml>UAS-Rab5/11DN>Syx 17 RNAi circulating hemocytes (Figure 9U-X, BB). The above data indicated that lysosomal activity is critical to the role of autophagy in lamellocyte formation upon the inhibition of Rab5/Rab11.

To further validate that JNK is required for autophagy activation in Hml>UAS-Rab5/11DN hemocytes, we repressed JNK signaling by using UAS-bsk RNAi or UAS-bskDN flies. Accordingly, the increased LysoTracker signals were restored in Hml>UAS-Rab5/11DN>UAS-bsk RNAi and Hml>UAS-Rab5/11DN>UAS-bskDN hemocytes (Figure 9J-P), suggesting that this process is JNK-dependent.

Regarding the functional role of autophagy in hematopoiesis, 20 autophagy-related genes have been identified to date (https://flybase.org/reports/FBgg0000076), but only Atg6 has been shown to affect hematopoiesis (Shravage et al., 2013). Autophagy is associated with many biological processes, including the immune response, aging and neurodegeneration. We are also interested in the detailed role of autophagy in hematopoiesis and will further focus on the mechanisms underlying how autophagy regulates hematopoiesis.

2) An increase in the number of circulating hemocytes

The origin of the increased number of circulating hemocytes should be clarified. The authors do not observe an increase in proliferation in circulating hemocytes at the L3 stage (in contrast with the situation in the lymph gland). Increased proliferation may have occurred earlier. Alternatively, it is possible that the increase is merely due to a disruption of the sessile hemocyte compartment. These 2 possibilities could be easily assessed.

To examine whether increased proliferation occurred at an earlier stage, we stained hemocytes from Hml>UAS-GFP>UAS-Rab5/11 RNAi 2nd and 3rd instar larvae with anti-PH3 antibodies. When compared with the Hml>UAS-GFP>WT hemocytes, the PH3-positive hemocyte count was increased in Hml>UAS-GFP>UAS-Rab5/11 RNAi hemocytes at the L2 stage (Figure 1—figure supplement 1G-I, Q), whereas we did not observe significantly increased proliferation at the L3 stage (Figure 1—figure supplement 1J-L, Q). In addition, we also examined sessile hemocytes from Hml>GFP>UAS-Rab5/11DN larvae, but the sessile hemocyte count was not decreased compared with that in Hml>UAS-GFP>WT larvae (Figure 1—figure supplement 1M-O). These data indicated that increased circulating hemocytes upon the inhibition of Rab5/Rab11 resulted from the high proliferation of hemocytes and not from the decreased sessile hemocyte count.

3) Rab5/Rab11-gal4 expression

Different phenotypes driven by Rab5/Rab11-gal4 require additional experiments to address why it leads to the discrepancy when compare to hemocyte-specific drivers. This could simply be due to different levels of overexpression (and a threshold effect to observe lamellocyte induction in response to JNK activation). Or it is possible that Rab5/11-gal4 is expressed in other tissues causing systemic effects. This needs to be assessed either by looking directly at Bsk accumulation or with a UAS-destablizedGFP. At present, the conclusion that bsk needs to be expressed "under endosomal control" to induce lamellocyte seems weak and the concept still ill-defined.

Given that we observed lamellocytes in Rab5/Rab11>UAS-bsk lymph glands but not in da>UAS-bsk lymph glands, we crossed da-Gal4 and Rab5/Rab11-Gal4 with UAS-GFP to determine where there was a difference in expression in the whole larvae. As shown in the figure, the overall fluorescence intensity in Rab5/Rab11-Gal4 was comparable with that in da>UAS-GFP, however; the fluorescence intensity was stronger in salivary glands from Rab5/Rab11-Gal4 larvae (Author response image 1A-C). In addition, based on gene expression data from FlyBase, Rab5 or Rab11 is highly expressed in the gut, fat bodies and salivary glands (Rab5: https://flybase.org/reports/FBgn0014010; Rab11: https://flybase.org/reports/FBgn0015790), while the daughterless gene has a mild expression pattern in these tissues (da: https://flybase.org/reports/FBgn0267821). These data suggest that Rab5-Gal4 and Rab11-Gal4 are stronger drivers in some tissues.

Author response image 1

Moreover, to examine whether systemic effects exist, we crossed Hml>UAS-bsk with NP3084-Gal4 (midgut-specific driver), ppl-Gal4 (fat body-specific driver) and elav-Gal4 (nervous system-specific driver) and stained hemocytes with anti-L1. We observed lamellocyte formation in Hml>NP3084>UAS-bsk and Hml>ppl>UAS-bsk hemocytes (Figure 4—figure supplement 2A-D), indicating that the interaction between hemocytes and fat bodies or midguts may play a role in aberrant lamellocyte differentiation. The above results showed that the different phenotypes in Hml>UAS-bsk and Rab5/Rab11>UAS-bsk may result from systemic effects, so our conclusion that bsk expression under Rab5/Rab11 control is crucial for lamellocyte formation is not correct. We have deleted the previous conclusion “These data indicated that activation of JNK under endosomal control can activate downstream transcription factors in the JNK pathway, resulting in massive lamellocyte production.” from our paper.

4) Mixture of lymph gland- and circulating hemocyte phenotypes

Lymph gland- and circulating hemocytes are intermingled, and some of the pathways, including the Toll pathway and autophagy, are validated only by circulating hemocytes. The mixture of two cell types throughout the manuscript makes it hard to follow the authors' rational. Moreover, hemocyte differentiation/proliferation mechanisms in two tissues may differ. The authors need to better align the two populations with more caution and confirm all the pathways in both cell types.

We previously confirmed the Toll pathway in lamellocyte differentiation in lymph glands, showing that Dif played a more important role in Rab5/Rab11DN-induced lamellocyte formation (Figure 7I-P). To examine autophagy in lymph glands, we starved Hml>UAS-GFP larvae for 4 h, 6 h and 8 h and stained lymph glands with anti-Atg8. Compared with lymph glands from fed larvae, we could not see significant autophagosome formation after starvation (Author response image 2A-D). In contrast, after starvation for 4 h, 6 h or 8 h, we observed obvious autophagosome formation in hemocytes (Author response image 2E-H), showing that hemocytes are more sensitive to starvation and are probably a better model for studying the relationship between autophagy and hematopoiesis. In addition, we also confirmed the JNK and Ras/EGFR pathways in lamellocyte formation in circulating hemocytes (Figure 3—figure supplement 2; Figure 6—figure supplement 1).

Author response image 2

5) The non-cell-autonomous phenotype in the lymph gland

In Figure 2H-K, the authors showed that the expression of RNAi against Rab5 or Rab11 in the CZ decreases the MZ in a non-cell-autonomous manner. How do the authors explain this phenotype by the current model? This should be shown with additional experiments.

Based on the current model, CZ-to-MZ progenitor maintenance is mainly regulated by PVR/STAT92E/adenosine signaling (Mondal et al., 2011). To examine whether the non-cell-autonomous regulation from the CZ in our study is dependent on the PVR/STAT92E/adenosine signal, we crossed UAS-STAT92E and UAS-Adgf-A with Hml>UAS-Rab5/11DN and stained the lymph gland with anti-P1. However, the increased P1-positive area in Hml>UAS-Rab5/11DN lymph glands was not restored by UAS-STAT92E and UAS-Adgf-A, indicating that this non-cell-autonomous regulation from the CZ is independent of the PVR/STAT92E/adenosine signal (Figure 2—figure supplement 2). The underlying mechanism warrants future investigation.

Physiological- or cell biological relevance of Rab5/11-mediated function

What is the physiological or cell biological relevance of the Rab5/Rab11 regulation shown in this study? It will be important to show whether the endocytic control indeed is critical for the immune response/hemocyte development to manage all the associated signals.

Rab proteins are important for signal transduction. The inhibition of Rab5/Rab11 leads to aberrant signal transduction and eventually activates multiple innate immunity-associated signaling pathways. Activation of multiple pathways will result in abnormal hematopoiesis, including overproliferation and overdifferentiation of hemocytes. We believe that endocytic control is important for immune response/hemocyte development based on the following:

1) In addition to the UAS-Rab5 RNAi and UAS-Rab11 RNAi lines, we utilized the UAS-Rab5DN and UAS-Rab11DN lines, which are defective for GTPase activity (Zhang et al., 2007). In our study, when we overexpressed UAS-Rab5/Rab11DN in the blood system by using Hml-Gal4, we observed aberrant hematopoiesis, including increased hemocyte counts, an expanded CZ area and massive lamellocyte formation; we confirmed this phenotype using Rab RNAi lines. These results showed that not only the expression level of Rab5/Rab11 but also Rab5/Rab11 GTPase activity play crucial roles in maintaining hematopoietic homeostasis.

2) Many studies have shown that the inhibition of Rab5/Rab11, essential genes in endocytosis, can activate innate immunity-related signaling pathways, including the JNK, BMP, and Ras/EGFR pathways (Tang et al., 2017; Takino et al., 2014; Bhuin and Roy, 2010). A previous study showed that knocking down ARF1 or inhibiting ARF1 GTPase resulted in an increased Hrs-positive endosome count and the accumulation of Notch signaling in Hrs-positive endosomes, which eventually activated the Notch pathway (Khadilkar et al., 2014). Similarly, we also observed an increased Hrs-positive endosome count in Hml>UAS-Rab5/11DN, which means that defective Rab5/Rab11 GTPase led to abnormal endocytic activity. We also showed higher degrees of colocalization between Hrs and p-JNK and observed activation of p-JNK signaling. These data indicated that the endocytic control of Rab5/Rab11 is crucial for maintaining the homeostasis of signaling pathways.

Tang, Y., Geng, Q., Chen, D., Zhao, S., Liu, X., Wang, Z., 2017. Germline Proliferation Is Regulated by Somatic Endocytic Genes via JNK and BMP Signaling in Drosophila. Genetics 206(1), 189-197.

Bhuin, T., Roy, J.K., 2010. Rab11 regulates JNK and Raf/MAPK-ERK signalling pathways during Drosophila wing development. Cell Biol. Int. 34(11), 1113-1118.

Zhang, J., Schulze, K.L., Hiesinger, P.R., Suyama, K., Wang, S., Fish, M., Acar, M., Hoskins, R.A., Bellen, H.J., Scott, M.P., 2007. Thirty-One Flavors of Drosophila Rab Proteins. Genetics 176(2), 1307-1322.

[Editors' note: further revisions were suggested prior to acceptance, as described below.]

Reviewer #1:

Yu and colleagues supplemented a large number of data and addressed most of the concerns raised by the reviewers. New data clearly resolved concerns raised and explained how circulating hemocytes are increased upon loss of Rab5/11 and why Rab5/11-gal4 differs from Hml-gal4. Despite the clarification, the biological relevance of Rab5/Rab11 signaling in hematopoiesis is still unclear. It is well-known that Rab5/Rab11 endosomal pathway controls multiple proteins involved in cellular signalings, including EGFR/Ras, Eiger/JNK, and Toll, and thus, in my opinion, it will be important to understanding why Rab5/Rab11-mediated control is significant in the blood.

1) Biological relevance

Although the authors extensively investigated the functions of Rab5/Rab11 in the blood cell signaling, it is still ambiguous how the endosomal control is maintained during development or immunity and why it is significant in hematopoiesis. For example, does Rab5/Rab11 control the proliferation of hemocytes in a specific developmental stage by repressing or activating Ras/Erk? Is Rab5/Rab11 inhibited by immune challenges to directly stimulate the lamellocyte differentiation? In which context Rab5/Rab11 gets modified and manipulates target signals similar to phenotypes derived by Rab5DN/Rab11DN or Rab5/Rab11 overexpression?

We thank the reviewer for these points. We believe that there could be an interaction between Rab5/Rab11 and immunity (the lamellocyte formation upon wasp challenge). We will focus on these issues in our future study.

2) Schematic diagram

A schematic diagram shown in Figure 10B does not fully represent the genetic pathways identified in the results. For example, there is no evidence showing a direct activation of pERK by Rab5/Rab11. And even though Rasv12 alone could not activate the lamellocyte differentiation, active Ras and Bsk together can induce the lamellocyte. The authors introduced dFOXO as a target of JNK and showed that loss of dFOXO in Hml>Rab5/11DN rescues the lamellocyte phenotype. These points are missing. Or the diagram could be simplified to help understand the major points.

Given that there is no evidence showing loss of Rab5/Rab11 could directly activate p-Erk, we used the dotted line and one question mark in our revised version to display the relationship between Rab5/Rab11 and p-Erk (Figure 10B). In addition, we showed dFOXO as the target of the JNK pathway to facilitate lamellocyte formation (Figure 10B). We also simplified the schematic diagram to make it clearer.

https://doi.org/10.7554/eLife.60870.sa2

Article and author information

Author details

  1. Shichao Yu

    Department of Genetics, College of Life Sciences, Northeast Forestry University, Harbin, China
    Contribution
    Investigation, Visualization, Writing - original draft
    Competing interests
    No competing interests declared
  2. Fangzhou Luo

    Department of Genetics, College of Life Sciences, Northeast Forestry University, Harbin, China
    Contribution
    Investigation, Visualization
    Competing interests
    No competing interests declared
  3. Li Hua Jin

    Department of Genetics, College of Life Sciences, Northeast Forestry University, Harbin, China
    Contribution
    Supervision, Funding acquisition, Project administration, Writing - review and editing
    For correspondence
    lhjin2000@hotmail.com
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-5912-9800

Funding

National Natural Science Foundation of China (31772521)

  • Li Hua Jin

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

Acknowledgements

We thank Utpal Banerjee, Jiwon Shim, Lei Xue, Lucas Waltzer, Norbert Perrimon, José Carlos Pastor-Pareja, Chao Tong, Bing Zhou, and Guangchao Chen for gifting the fly strains and Istvan Andó, Dominique Ferrandon, Erjun Ling, Pierre Léopold, and the Developmental Studies Hybridoma Bank for providing the antibodies. We thank the Bloomington Drosophila Stock Center, Vienna Drosophila RNAi Center, and TsingHua Fly Center for providing the fly stocks. We also thank Neal Silverman for the valuable suggestions on this paper. This work was supported by the National Natural Science Foundation of China (31772521).

Senior Editor

  1. Utpal Banerjee, University of California, Los Angeles, United States

Reviewing Editor

  1. Jiwon Shim, Hanyang University, Republic of Korea

Reviewer

  1. Lucas Waltzer, University of Clermont Auvergne, France

Publication history

  1. Received: July 8, 2020
  2. Accepted: February 8, 2021
  3. Accepted Manuscript published: February 9, 2021 (version 1)
  4. Version of Record published: February 18, 2021 (version 2)

Copyright

© 2021, Yu et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

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