Temporal coordination of diverse events is a hallmark of organismal development. This is illustrated by the juvenile-to-adult (J/A) transition of animals, in mammals also known as puberty. J/A transition involves coordinated morphological changes of sexual organs as well as various other tissues and organs, including skin (Lee and Houk, 2006). The molecular mechanisms that control the onset of J/A transition in humans are poorly understood, but have been well studied in the nematode Caenorhabditis elegans (Faunes and Larraín, 2016). In C. elegans, J/A transition is controlled by a cascade of regulators termed the heterochronic pathway, which coordinates somatic cell fate programs (Ambros and Horvitz, 1984). Orthologues of heterochronic genes have also been implicated in timing the onset of puberty in mammals including humans (Abreu et al., 2013; Corre et al., 2016; Ong et al., 2009; Perry et al., 2009; Sulem et al., 2009; Zhu et al., 2010) (reviewed in Faunes and Larraín, 2016; Moss and Romer-Seibert, 2014), indicating an evolutionary conservation of the molecular principles of temporal coordination of J/A transition events.

The C. elegans J/A transition has been particularly well studied in the epidermis, where it encompasses four events related to cell fates and molting (Ambros, 1989), illustrated in their order of occurrence in Figure 1A. First, skin progenitor cells called seam cells, which undergo asymmetric (self-renewal) divisions during larval stages, cease to do so in adult animals. The last seam cell division takes place during the transition from the third (L3) to the last (L4) larval stage. Second, during the mid-L4 stage, seam cells fuse into a syncytium, a state considered terminally differentiated (Sulston and Horvitz, 1977). Third, animals generate an adult cuticle, characterized by the presence of adult-specific collagens (Cox and Hirsh, 1985) and a microscopically visible structure known as adult alae (Singh and Sulston, 1978). Fourth, following shedding of the L4 cuticle, animals stop molting, the process of cuticle synthesis and shedding that happens at the end of each larval stage.

Figure 1

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Figure 1—source data 1

Quantification of unfused seam cell junctions, raw data related to Figure 1B.

https://cdn.elifesciences.org/articles/53387/elife-53387-fig1-data1-v1.xlsx

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Genetic screens have identified precocious and retarded mutations (Ambros and Horvitz, 1984), which cause animals to exhibit somatically adult features before reaching sexual maturity or retain juvenile somatic features after reaching sexual maturity, respectively. Thus, the identified factors, called heterechronic genes, regulate the initiation of J/A transition events. Among these genes, lin-29, encoding a transcription factor of the EGR/Krüppel family, is considered the downstream-most gene of the heterochronic pathway (Rougvie and Moss, 2013). Indeed, LIN-29 accumulates immediately prior to transition to adulthood during the last (L4) larval stage (Bettinger et al., 1996).

Current models of the heterochronic pathway depict a simple linear chain of events during the last larval stages that leads to upregulation of LIN-29 (Faunes and Larraín, 2016; Moss and Romer-Seibert, 2014; Rougvie and Moss, 2013). The miRNA let-7 accumulates during the L3 stage to inhibit synthesis of the RNA binding protein LIN-41 (Ding and Großhans, 2009; Reinhart et al., 2000). Since LIN-41 translationally represses lin-29 (Aeschimann et al., 2017), its decreased levels during L4 allow for accumulation of LIN-29. As all four epidermal J/A events require LIN-29 (Ambros and Horvitz, 1984; Bettinger et al., 1997), this pathway architecture can ensure their coordinated execution.

However, multiple lines of evidence challenge this simple linear model. First, LIN-29 occurs in two protein isoforms, LIN-29a and LIN-29b (Rougvie and Ambros, 1995), and LIN-41 appears to silence only lin-29a but not lin-29b (Aeschimann et al., 2017). Second, lin-41(0) mutant precocious phenotypes, unlike the retarded lin-29(0) mutant phenotypes, are only partially penetrant (Slack et al., 2000), indicating additional control of LIN-29 beyond repression by LIN-41. Third, let-7 mutations do not recapitulate all phenotypes of lin-29(0) (Ambros, 1989), as let-7 appears dispensable for proper timing of seam cell fusion (Hunter et al., 2013).

Here, we address these discrepancies by studying the regulation of the two LIN-29 isoforms and their functions in J/A transition. LIN-29a and LIN-29b share most of their protein sequence with the exception of 142 amino acids at the N-terminus that are unique to LIN-29a (Figure 2A). Previous studies have suggested that these isoforms function redundantly: they share a common co-factor, MAB-10 (Harris and Horvitz, 2011), they have similar expression patterns, and they are interchangeable in complementation analysis (Bettinger et al., 1996; Bettinger et al., 1997). However, employing isoform-specific mutations and endogenous protein tagging, we show here that the lin-29a and lin-29b isoforms differ in function and expression patterns. The most striking functional difference occurs in seam cell fusion, which relies only on LIN-29b, but not on LIN-29a or MAB-10. Moreover, whereas lin-29a is regulated by LIN-41, the lin-29b isoform is regulated by the transcription factor HBL-1, which results in a distinct spatiotemporal expression of the two isoforms. The expression patterns alone can explain the unique phenotypic consequences of the isoform-specific mutations, whereas the sequence difference in the N-terminal portion of the two isoforms does not contribute to functional differences. Coordination of the activities of LIN-29a and LIN-29b is achieved through LIN-28, an RNA-binding protein that regulates both HBL-1 and let-7–LIN-41. Taken together, our findings help to reframe the regulatory logic that enables the heterochronic pathway to coordinate different events into an overall larval-to-adult transition program.

Figure 2

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In the four decades since the discovery of the first heterochronic genes (Ambros and Horvitz, 1984; Chalfie et al., 1981), accumulating genetic and molecular interaction data have facilitated the formulation of ever more refined heterochronic pathway models. A unifying concept of current heterochronic pathway models is that C. elegans J/A transition is triggered through a linear chain of events: let-7 silences LIN-41, and thereby relieves lin-29 from repression by LIN-41 (Ambros, 2011; Faunes and Larraín, 2016; Rougvie and Moss, 2013). The findings that we have presented here reveal that this concept requires revision. We demonstrate that LIN-29 occurs in two functionally distinct isoforms (Figure 11A) and that the functional separation does not depend on differences in protein sequence. Instead, it results from distinct temporal and, possibly, but not directly investigated by us, spatial, expression patterns (Figure 11B). Unique expression patterns of the isoforms are the result of isoform-specific regulation: whereas lin-29a is under direct translation control by LIN-41 (Aeschimann et al., 2017), lin-29b is repressed, directly or indirectly, by the transcription factor HBL-1 (Figure 11C). Hence, J/A transition relies on two separate arms of the heterochronic pathway rather than a linear chain of events.

Figure 11

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The RNA-binding protein LIN-28 regulates both lin-29 isoforms and thus coordinates their activities. Indeed, we propose that lin-28 functions as the downstream-most heterochronic gene before branching, consistent with its dual functions of post-transcriptional activation of hbl-1 expression (Vadla et al., 2012), and inhibition of let-7 biogenesis (Lehrbach et al., 2009; Van Wynsberghe et al., 2011), which shields lin-41 from repression by let-7. At the same time, we note that the spatial expression patterns of lin-29 isoforms upon LIN-28 depletion are predicted by the results of HBL-1 depletion but not entirely by those of LIN-41 depletion. Specifically, whereas upon lin-41 RNAi, LIN-29a accumulation in hyp7 precedes that in the seam, the converse is true in lin-28 mutant animals. While the different kinetics and extents of LIN-41 depletion resulting from the two distinct manipulations may account for some of these differences, the results also suggest that additional, as yet unidentified, regulatory mechanisms exist on which LIN-28 may impact. Indeed, tissue specificity of heterochronic regulation may deserve further attention. In particular, distinct mechanisms appear to repress hbl-1 in hyp7 versus seam cells: Whereas in hyp7, repression relies on post-transcriptional silencing of the hbl-1 mRNA by the let-7 sister miRNAs (miR-48, miR-84 and miR-241) (Abbott et al., 2005), in seam cells, LIN-46 sequesters HBL-1 in the cytoplasm after LIN-46 has itself been released from posttranscriptional repression by LIN-28 (Ilbay and Ambros, 2019; Ilbay et al., 2019).

Our findings and the revised heterochronic pathway model that we propose (Figure 11C) finally provide a mechanistic explanation for the previously reported redundancies between lin-41 and hbl-1 (Abrahante et al., 2003). However, they appear contradicted by other claims and findings from the published literature. First, the functional redundancy of LIN-29 isoforms inferred previously (Bettinger et al., 1997) disagrees with our finding of LIN-29 isoform specialization. Second, lin-41 was described to affect seam cell fusion in both loss-of-function (Großhans et al., 2005) and over-expression experiments (Slack et al., 2000). This contrasts with a previous report that the lin-41 regulator let-7 was dispensable for seam cell fusion (Hunter et al., 2013) and, more explicitly, our conclusion that seam cell fusion relies on HBL-1–LIN-29b but not on let-7–LIN-41–LIN-29a. Finally, the new model, just like previous ones, fails to accommodate the apparent dual and antagonistic functions of HBL-1, namely suppression of adult cell fates in larvae, and suppression of larval cell fates in adults (Lin et al., 2003).

However, closer examination reveals that the published data are fully compatible with the new model. First, the conclusion that lin-29b is functionally equivalent to lin-29a was based on complementation of lin-29(0) mutant heterochronic phenotypes by a lin-29b transgene. In these experiments, lin-29b was expressed from a multicopy array (Bettinger et al., 1997) and thus likely over-expressed. Hence, rather than contradicting our model, these data provide further support for our conclusion that it is lin-29 isoform expression, not protein sequence, that determines functional distinctions.

Second, although lin-29a is dispensable for seam cell fusion in a wild-type context, our experiments (Figure 8) clearly demonstrate that, if expressed sufficiently early, lin-29a can induce seam cell fusion. This may precisely be the scenario in LIN-41-depleted animals, where LIN-29a (and MAB-10) accumulate precociously (Figure 9A,B,D, Figure 9—figure supplement 1 and Aeschimann et al., 2017), thereby causing partial precocious seam cell fusion (Großhans et al., 2005).

Seam cell fusion failure in lin-41 overexpression animals was inferred from the presence of seam cell junctions at the L4/adult molt, rather than from direct observation of syncytium formation in mid-L4 (Slack et al., 2000). Therefore, the phenotype seems readily explained by re-appearance of junctions following a failure of syncytial nuclei to exit the division cycle. This scenario is supported by our finding that although seam cells fuse in a wild-type manner in both lin-41 overexpressing lin-41(∆LCS) animals (Figure 1B) and mab-10(0) lin-29a(∆) mutant animals (Figure 3A,C) in the L4 stage, mab-10(0) lin-29a(∆) mutant adults exhibit de novo generated seam cell junctions (Figure 3B).

Third, we propose that a similar scenario of intrasyncytial cell divisions following seam cell fusion also explains the perplexing observation that hbl-1 mutant adults exhibit seam cell junctions and a greater than wild-type number of seam cell nuclei (Lin et al., 2003), which, at the time, was interpreted as a retarded heterochronic phenotype. However, as we find that only LIN-29b accumulates precociously in HBL-1-depleted animals, we would expect them to undergo only a partial J/A transition at the L3 stage. Specifically, although seam cells fuse into a syncytium during L3, syncytial nuclei divide again in L4 and remain trapped in the seam (Abrahante et al., 2003). Hence, we propose that hbl-1 mutant animals exhibit a purely precocious phenotype and that extra nuclei and seam cell junctions observed in hbl-1 mutant adults are a consequence of an incomplete precocious J/A transition at the L3/L4 stage rather than reflection of a distinct, larval-fate suppressing function of HBL-1 in adults. Taken together, we find that the new model parsimoniously explains previously published and the present new data, and provides explanations for previously unaccounted phenotypes.

Ours and previous work (Abrahante et al., 2003; Lin et al., 2003) reveal a striking loss of coordination of J/A transition events in both HBL-1-depleted larvae and mab-10(0) lin-29a(∆) double mutant adults. Specifically, in both cases, seam cells differentiate, as evidenced by fusion and alae formation, but fail to arrest the cell division cycle. This reveals that in seam cells, cell cycle arrest is not necessary for differentiation; that is proliferation and differentiation are not simply antagonistic processes. This finding highlights a need for factors that coordinate the activities of the two arms of the heterochronic pathway, and thus overall J/A transition. We have identified LIN-28 as a relevant factor, but others may exist. Nonetheless, coordination of J/A transition events through upstream activities of the heterochronic pathway rather than the previously proposed ‘terminal’ let-7–LIN-41–LIN-29 axis would seem to entail a surprising lack of robustness, making the pathway vulnerable to perturbations that can uncouple individual J/A transition events. Hence, we will be curious to learn whether this architecture is owed to other, unknown constraints or evolutionary history, or whether it has a particular benefit for the animal. At any rate, we speculate that additional layers of regulation will facilitate coordinated execution of the J/A transition.

Accumulation of LIN-29 has long been considered a key event for triggering J/A transition. We support this key function of LIN-29 in J/A transition by showing that LIN-29 accumulation alone is sufficient to trigger seam cell fusion as early as the L1 stage. While opposite temporal transformation phenotypes for loss-of-function and gain-of-function alleles are considered a hallmark of core heterochronic genes (Moss and Romer-Seibert, 2014), rather surprisingly, this had not previously been shown for LIN-29, and it had indeed been argued that LIN-29 accumulation was insufficient to trigger precocious J/A transition (Slack et al., 2000). However, our results are further supported by recent molecular evidence showing that lin-29 is both necessary and sufficient for expression of adult collagens (Abete-Luzi and Eisenmann, 2018) and, in parallel to the present study, that precocious expression of lin-29 in late L2 suffices for seam cell fusion in L3 (Abete-Luzi et al., 2020).

We propose that, in the unperturbed system, the temporal order of the individual epidermal J/A transition events is driven by differences in their LIN-29 dose sensitivity and the spatiotemporal differences in lin-29 isoform expression (Figure 11A and B). Specifically, we hypothesize that seam cell fusion rely on LIN-29 accumulation in only the seam, explaining its early occurrence and dependence on only LIN-29b, which accumulates early in this tissue. By contrast, accumulation of LIN-29b alone appears insufficient to drive cessation of seam cell division cycles. Although the last seam cell division normally happens at the L3-to-L4 molt, it is unclear when these cells exit the cell cycle (Figure 11A). In fact, our data suggest the possibility of a gradual effect where LIN-29a and LIN-29b can, individually, and more extensively jointly, delay division, that is slow down the cell cycle, but where terminating it requires full LIN-29 activity, and thus MAB-10. Consistent with this notion, exit from the mitotic cycle is the only epidermal J/A transition event for which we observed a defect in mab-10(0) single mutant animals, but the additional divisions occurred much later than in the other mutant combinations (data not shown). We note that others reported also extra molts in, mostly older and male, mab-10 mutant animals (Harris and Horvitz, 2011), which we did not investigate. An interesting possibility is that, akin to seam cell division, in the absence of MAB-10, LIN-29a and/or LIN-29b may suffice to delay the occurrence of the next molt rather than to prevent it entirely, and in all animals.

Finally, although both isoforms (but not MAB-10) are required for wild-type alae formation, their individual loss causes qualitatively different defects. We speculate that this may reflect an involvement of hyp7, in addition to seam cells, in alae formation, either directly or through an effect on seam cells that remains to be elucidated. The fact that MAB-10 function is not required for wild-type alae formation, but enhances precocious alae formation in lin-41 mutant animals (Harris and Horvitz, 2011), is then in agreement with the idea that it has a generic function in boosting LIN-29 activity, rather than a specific effect on a subset of LIN-29-regulated targets required for only certain processes. We propose that genome-wide identification of the targets of the lin-29 isoforms and mab-10 will allow further testing of this notion in the future. Tissue-specific identification of targets in particular would provide a more detailed understanding of when, where and how different J/A events are triggered. Already, the findings presented in this study have helped to revise our understanding of the fundamental regulatory architecture that temporally controls events occurring during the transition from a juvenile to an adult animal.

All data generated or analysed during this study are included in the manuscript and supporting files. Source data files have been provided for Figures 1, 3–6 and Figure 6—figure supplement 2.

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Author details

1. Chiara Azzi

   1. Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
   2. University of Basel, Basel, Switzerland

   Contribution

   Conceptualization, Formal analysis, Investigation, Writing - original draft, Writing - review and editing, designed, performed, and analyzed confocal microscopy, phenotypic analysis, luciferase assay, RT-qPCR and western blot experiments; created transgenic worm lines

   Contributed equally with

   Florian Aeschimann

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-2240-4618

2. Florian Aeschimann

   1. Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
   2. University of Basel, Basel, Switzerland

   Present address

   CSL Behring, Research, CSL Biologics Research Center, Bern, Switzerland

   Contribution

   Conceptualization, Formal analysis, Investigation, Writing - review and editing, designed, performed, and analyzed confocal microscopy, phenotypic analysis and western blot experiments; created transgenic worm lines

   Contributed equally with

   Chiara Azzi

   Competing interests

   The author is now affiliated with CSL Behring, Research, CSL Biologics Research Center, however all work was conducted when affiliated with Friedrich Miescher Institute for Biomedical Research and University of Basel.

   "This ORCID iD identifies the author of this article:" 0000-0001-5213-034X

3. Anca Neagu

   Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland

   Contribution

   Resources, Formal analysis, Investigation, performed and analyzed the precocious expression experiment and created transgenic worm lines

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-0643-5238

4. Helge Großhans

   1. Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
   2. University of Basel, Basel, Switzerland

   Contribution

   Conceptualization, Supervision, Funding acquisition, Investigation, Writing - original draft, Project administration, Writing - review and editing

   For correspondence

   helge.grosshans@fmi.ch

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-8169-6905

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