In C. elegans larvae, the execution of stage-specific developmental events is controlled by heterochronic genes, which include those encoding a set of transcription factors and the microRNAs that regulate the timing of their expression. Under adverse environmental conditions, developing larvae enter a stress-resistant, quiescent stage called ‘dauer’. Dauer larvae are characterized by the arrest of all progenitor cell lineages at a stage equivalent to the end of the second larval stage (L2). If dauer larvae encounter conditions favorable for resumption of reproductive growth, they recover and complete development normally, indicating that post-dauer larvae possess mechanisms to accommodate an indefinite period of interrupted development. For cells to progress to L3 cell fate, the transcription factor Hunchback-like-1 (HBL-1) must be downregulated. Here, we describe a quiescence-induced shift in the repertoire of microRNAs that regulate HBL-1. During continuous development, HBL-1 downregulation (and consequent cell fate progression) relies chiefly on three let-7 family microRNAs, whereas after quiescence, HBL-1 is downregulated primarily by the lin-4 microRNA in combination with an altered set of let-7 family microRNAs. We propose that this shift in microRNA regulation of HBL-1 expression involves an enhancement of the activity of lin-4 and let-7 microRNAs by miRISC modulatory proteins, including NHL-2 and LIN-46. These results illustrate how the employment of alternative genetic regulatory pathways can provide for the robust progression of progenitor cell fates in the face of temporary developmental quiescence.
During animal development, key developmental events, such as cell division and differentiation, occur in precisely timed sequences. The precision of developmental timing is ensured by networks of regulatory genes, such as the heterochronic genes of C. elegans that control stage-specific cell fate decisions in the worm (reviewed by Moss, 2007; Resnick et al., 2010; Rougvie, 2001). A potential challenge to the regulation of developmental timing is the interruption of development by cellular quiescence, a reversible non-proliferating state. In mammals, quiescence is an important feature of many adult stem cells. Despite lengthy quiescent periods, stem cells possess the capacity to maintain their tissue specificity and multipotency. Defects in this process can lead to a failure to maintain tissue homeostasis, and are thought to be an important aspect of the aging process (Sharpless and DePinho, 2007).
In C. elegans, a form of developmental and cellular quiescence occurs during the dauer larva stage, an optional developmentally arrested form that is adopted in response to unfavorable environmental cues. These cues are transduced through a complex signaling network, including TGFβ, insulin and nuclear hormone receptor pathways (Fielenbach and Antebi, 2008). Some key regulators of dauer quiescence, including the DAF-16/FoxO transcription factor, and the DAF-18/PTEN phosphatase, are also key regulators of quiescence in mammalian adult stem cells. Furthermore, the p21CIP/p27KIP cell-cycle inhibitors are required to maintain cell cycle arrest in both quiescent mammalian adult stem cells and quiescent progenitor cells in C. elegans dauer larvae (Hong et al., 1998; Tothova and Gilliland, 2007). Dauer quiescence occurs specifically after the second larval molt, and all cells exit the cell cycle for the duration of the quiescence. The length of dauer quiescence is variable, from hours to potentially months – longer than the entire lifespan of animals that developed continuously (Fielenbach and Antebi, 2008). If favorable environmental conditions are encountered, larvae recover from dauer quiescence and resume development. Remarkably, the pattern and sequence of cell divisions in post-dauer larvae are identical to those of continuously developing larvae (Fig. 1) (Braendle and Félix, 2008; Euling and Ambros, 1996; Liu and Ambros, 1991). Thus, progenitor cells in C. elegans larvae possess a capacity to maintain their precise state of cell fate specification during a lengthy developmental sequence.
A clue to the mechanism underlying the ability of C. elegans to accommodate dauer quiescence comes from the study of heterochronic genes, which have been studied primarily for their roles in the stem-cell-like ‘seam cell’ lineage of the hypodermis (reviewed by Moss, 2007; Resnick et al., 2010; Rougvie, 2001). At each larval stage, seam cells express a particular stage-specific cell fate, defined by a specific pattern and sequence of cell divisions (Fig. 1A). At adulthood, seam cells exit the cell cycle, differentiate, and secrete an adult-specific cuticular structure called ‘adult alae’. Furthermore, the seam cells and other hypodermal cells express an adult-specific collagen encoded by col-19 (Ambros and Horvitz, 1984; Liu et al., 1995). Heterochronic genes regulate stage-specific seam cell fates and, accordingly, mutations in these genes can result in either ‘precocious’ development, wherein events of a particular larval stage are skipped and later events occur precociously, or ‘retarded’ development, wherein events of a particular larval stage are reiterated in subsequent larval stages (Ambros and Horvitz, 1984).
The heterochronic gene network can be thought of as a molecular timer that regulates stage-specific cell fate progression from the first larval stage (L1), through subsequent larval stages (L2-L4) to the adult (reviewed by Rougvie, 2001). During continuous development through the four larval stages, transcription factors that specify early cell fates are progressively downregulated by microRNAs (miRNAs), levels of which rise over time. In the first larval stage, the LIN-14 transcription factor is required for L1 cell fate. Expression of the lin-4 miRNA begins soon after the onset of larval development, and lin-4 levels rise during the L1 stage. When lin-4 levels are high enough, lin-4 downregulates lin-14, a direct lin-4 target, thereby permitting progression to L2 fate (Fig. 1B) (reviewed by Rougvie, 2001). In a similar manner, the Hunchback-like-1 (HBL-1) transcription factor is expressed in the first part of the L2 stage when it promotes L2 fates and prevents precocious expression of L3 fates. Levels of three redundant members of the let-7 family rise during the L2 stage and downregulate hbl-1, permitting progression to L3 cell fates (Fig. 1B) (Abbott et al., 2005). By contrast, the expression of post-dauer L3 cell fates should not rely on the same timer, because development is arrested throughout dauer quiescence, independent of the amount of time that elapses. In support of the expectation that post-dauer timing mechanisms are distinct from the timer that operates during continuous development, certain heterochronic genes that are normally required for stage-specific cell fate specification have been found to be dispensable in situations in which progenitor cells undergo a period of dauer quiescence (Abrahante et al., 2003; Liu and Ambros, 1991; Morita and Han, 2006). These observations indicate that alternative genetic mechanisms regulate stage-specific cell fate progression during dauer-interrupted development compared with continuous development (Liu and Ambros, 1991). However, little is known about the nature of the post-dauer developmental timing mechanisms, or how timing mechanisms shift from the continuous development mode to the post-dauer mode.
In this work, we probe the mechanisms that regulate developmental timing after dauer quiescence, focusing on the role of the three let-7 family miRNAs that downregulate hbl-1 during continuous development. We find that dauer quiescence shifts the requirement for progression to L3 cell fate from resting primarily on the let-7 family miRNAs to resting on the parallel action of two miRNA families: the lin-4 family and the let-7 family. We propose that there is a potentiation of the activity of both lin-4 and let-7 family miRNAs after dauer quiescence. This potentiation involves nhl-2, which encodes a miRISC modulator that enhances let-7 family miRNA activity, and lin-46, which encodes a putative scaffolding protein that, in this context, seems to enhance lin-4 miRNA activity.
MATERIALS AND METHODS
Nematode growth conditions
C. elegans Bristol strain was grown on standard nematode growth medium with Escherichia coli strain OP50 as a food source. Animals were maintained at 20°C, and all experiments were carried out at 20°C except where indicated. The complete genotype of all strains is listed in supplementary material Table S1. The genotype of all newly derived strains was confirmed by PCR genotyping.
lin-4 overexpression transgenes
Transgenic lines arEx1311 and arEx1313 were created by injecting N2 hermaphrodites with 1 ng/μl p915 [hsp16-2::lin-4], 1 ng/μl p716 [myo-3::mCherry] and 50 ng/μl N2 genomic DNA digested with PvuII. p915 was constructed using by PCR amplifying the lin-4 precursor from N2 genomic DNA, and cloning it into pPD49.78. The lin-4 precursor sequence used was the same sequence previously shown to rescue lin-4 phenotypes in vulval development (Li and Greenwald, 2010). Once lines were established by mCherry expression, the arrays were crossed into maIs105[col-19::GFP], nhl-2(ok818); maIs105, or lin-46(ma164) maIs105 backgrounds. To carry out mis-expression experiments, ten gravid adults of these strains were permitted to lay eggs on 60-mm nematode growth media (NGM) plates seeded with OP50 for two hours. The embryos developed ∼15 hours at 20°C and then were subject to heat-shock at 33°C for one hour. After heat-shock, the plates were returned to 20°C where development continued until larvae were scored during the L4 stage, ∼2 days after heat-shock. mCherry expression was used to determine whether the larvae carried the array and thus mis-expressed lin-4 upon heat-shock.
Characterization of developmental phenotypes
Retarded phenotypes were determined in young adult hermaphrodites that had either grown continuously or had interrupted their development with dauer quiescence. Young adult animals were scored for adult alae formation and/or col-19::GFP expression using a Zeiss Axio Imager D1 with an AxioCam MRm camera, and an X-Cite 120Q light source (EXFO Photonic Solutions). Continuously developing animals were grown with plenty of food and sparse population density. Populations were synchronized by allowing mothers to lay eggs for several hours, or by dissecting embryos out of gravid mothers for strains in which all hermaphrodites are egg-laying defective. Post-dauer populations were synchronized by selecting dauer larvae from a starved and crowded plate using 1% SDS, then adding the larvae to NGM plates seeded with OP50 to stimulate recovery. For certain strains, some animals with otherwise dauer-like morphology nevertheless displayed sensitivity to SDS treatment, perhaps owing to incomplete expression of the dauer differentiation programs (Liu and Ambros, 1989). In these cases, we collected dauer larvae manually based on their characteristic morphology rather than relying on SDS selection. hbl-1::GFP[ctIs39, ctIs37] or LIN-28::GFP[maIs108] expression in post-dauer-L3 larvae was ascertained by picking dauer larvae (identified by morphology) from crowded and starved plates and allowing them to recover for ∼14 hours at 20°C. At this point, the morphology of the gonad and vulval precursors resembled that of continuously developing L3 staged larvae. We used the presence of dauer alae on the cuticle to verify that these larvae had previously been in dauer quiescence.
To obtain continuously developing L3 staged larvae, embryos isolated by hypochlorite treatment were placed on NGM plates seeded with OP50 and incubated at 20°C until larvae reached the L3 stage, as judged by the extent of gonad development. Post-dauer L3 staged larvae were isolated from crowded and starved plates incubated at 20°C by SDS-selection. SDS-resistant dauer larvae were recovered on fresh NGM plates seeded with OP50 and incubated at 20°C for ∼14 hours. RNA from these larvae was extracted using the Trizol reagent (Invitrogen). Levels of 107 microRNAs were quantified using miR-Taqman-based real-time PCR (Applied Biosystems), and data were analyzed as described (Karp et al., 2011). Three biological replicates were assayed for each strain.
lin-4 can substitute for let-7 family miRNAs after dauer quiescence
Three let-7 family miRNAs (miR-48, miR-84, miR-241) are functionally redundant and downregulate hbl-1 during the L2 stage in order to allow progression to L3 cell fates (Abbott et al., 2005). Mutant larvae that lack all of these miRNAs inappropriately express L2 cell fates during the L3 stage, thereby delaying subsequent larval cell fates. This ultimately causes a failure to produce adult-specific structures when animals reach reproductive maturity, a classic ‘retarded’ phenotype (Abbott et al., 2005; Ambros and Horvitz, 1984). Specifically, the retarded phenotype of the triple mutant mir-48(0) mir-241(0); mir-84(0) [hereafter ‘mir-48-241-84(0)’] includes (1) a reiteration of the L2-specific proliferative division of the seam cells during the L3 stage, (2) a failure to produce adult alae at the L4 molt, and (3) a failure of young adults to express the adult-specific collagen col-19 (Fig. 2) (Abbott et al., 2005; Liu et al., 1995). By contrast, L3 and subsequent cell fates are executed normally in mir-48-241-84(0) mutants that interrupt their development with dauer quiescence (Fig. 2). Therefore, although these let-7 family miRNAs are required for normal progression to L3 cell fate during continuous development, they are not crucial for development following a period of developmental and cellular quiescence.
The regulation of developmental timing following a variable and potentially lengthy quiescent period implies the need to utilize regulatory mechanisms that do not rely on either chronological time or continuous progression through development. Indeed, many heterochronic genes that are required during continuous development are dispensable after dauer quiescence (Abrahante et al., 2003; Liu and Ambros, 1991; Morita and Han, 2006). The question therefore arises: What developmental timing mechanism might operate in post-dauer development, such that the let-7 family miRNAs are not required? A simple hypothesis is that another miRNA might substitute for mir-48-241-84 after dauer quiescence. One candidate is lin-4, as the hbl-1 3′UTR contains at least one site complementary to the lin-4 miRNA. Furthermore, previous work has demonstrated that certain mutant strains carrying a lin-4(0) mutation can display a post-dauer retarded phenotype, albeit less severe than during continuous development (Liu and Ambros, 1991). Therefore, lin-4 might participate in a developmental timing mechanism that operates after dauer quiescence. To test this hypothesis, we constructed a strain that lacks lin-4, as well as mir-48-241-84. This strain contains another mutation, lin-14(n179), which reduces the activity of a lin-4 target that would otherwise block dauer formation (Liu and Ambros, 1991; Liu and Ambros, 1989). Note that the lin-14(n179) allele is included in all lin-4(0) strains and their controls discussed below, although for simplicity it is not always mentioned.
We find that lin-4(0); mir-48-241-84(0) displays a completely penetrant retarded phenotype during both continuous and dauer-interrupted development (Fig. 3); in particular, these animals exhibit a reiteration of L2 cell fates during post-dauer L3 and post-dauer L4 stages (supplementary material Fig. S1). The observation that post-dauer lin-4(0); mir-48-241-84(0) animals display a retarded phenotype whereas post-dauer mir-48-241-84(0) animals do not (Figs 2, 3) suggests that lin-4 is sufficient to promote L3 cell fate during dauer-interrupted development. As mir-48-241-84(0) display a retarded phenotype during continuous development even when lin-4 is present (Fig. 2), lin-4 is not sufficient to promote L3 cell fate during continuous development. Thus, dauer quiescence produces a shift in which miRNAs are important for L3 cell fate. In addition, post-dauer lin-4(0); mir-48-241-84(0) animals are more retarded than post-dauer lin-4(0) animals (Fig. 3), indicating that mir-48-241-84 function together with lin-4 to control post-dauer stage-specific cell fate progression.
Because lin-4 can substitute more effectively for mir-48-241-84 during dauer-interrupted than continuous development (Figs 2, 3), we hypothesized that lin-4 in combination with the let-7 family miRNAs is more important after dauer quiescence than during continuous development. If this hypothesis is correct, the loss of this combination of miRNAs should produce a stronger phenotype after dauer quiescence than during continuous development. Because lin-4(0); mir-48-241-84(0) mutants already display a completely penetrant retarded phenotype during continuous development, we constructed a mutant strain lacking lin-4 that was also mutant for mir-84, one member of the let-7 family. These lin-4(0); mir-84(0) animals display a moderately retarded phenotype during continuous development, comparable to that of lin-4(0). By contrast, this same strain displays a dramatic retarded phenotype after dauer quiescence (Fig. 3). Thus, lin-4 and mir-84 are more crucial for the proper progression of cell fates during post-dauer development than they are for the same progression of cell fates during continuous development. Note that the severely retarded post-dauer phenotype of lin-4(0); mir-84(0) animals is the result of loss of both lin-4 and mir-84 because the control strains [lin-4(0); mir-84(+) and lin-4(+); mir-84(0)] are significantly less retarded than the lin-4(0); mir-84(0) double mutant after dauer quiescence (Fig. 3). Loss of mir-48 or mir-241 also enhances the retarded phenotype of lin-4(0) post-dauer animals, consistent with the functional redundancy of the three let-7 family miRNAs (Table 2, lines 9 vs 17, 10 vs 16, P≤0.015, Fisher’s Exact Test). By contrast, the lin-4 family member mir-237 does not appear to be important for post-dauer developmental timing (Table 2, lines 5, 6). We interpret the above findings to mean that lin-4 and let-7 family miRNAs act in parallel to ensure cell fate progression after quiescence.
Potentiation of let-7 family miRNA activity by NHL-2 after dauer quiescence
The observations that lin-4 can substitute for mir-48-241-84 during dauer-interrupted development, but not during continuous development, and that lin-4 and mir-84 are together more important for dauer-interrupted development than in continuous development suggests a shift in the programming of microRNA regulation of L2-to-L3 cell fate progression after dauer quiescence. This shift could involve changes in the levels of lin-4 and let-7 family miRNAs and/or changes in their activities. For example, if lin-4 were more abundant post-dauer than during continuous development, that could explain why lin-4 can substitute for mir-48-241-84 only after dauer quiescence. However, wild-type dauer and post-dauer larvae do not express higher levels of mature lin-4 or let-7 family microRNAs than do continuously developing larvae. On the contrary, certain let-7 family miRNAs, particularly miR-48 and miR-241, are reduced in level in dauer and post-dauer L3 stages (Table 1) (Bethke et al., 2009; Hammell et al., 2009a; Karp et al., 2011).
Because our results indicate that lin-4 and let-7 family miRNAs play crucial roles in the expression of stage-specific cell fates after dauer quiescence, yet the levels of these mature miRNAs are either unaffected or reduced after dauer quiescence, we reasoned that protein factors might enhance the activity of the miRNAs after dauer quiescence, perhaps by potentiating the activity of the miRNA-induced silencing complex (miRISC). The miRISC is the miRNA-protein complex that binds to target messenger RNAs and leads to downregulation of the expression of protein from the target mRNA (Krol et al., 2010). A candidate factor that might enhance post-dauer miRNA activity is nhl-2, which encodes a miRISC modulator. nhl-2 has been shown to potentiate the activity of the let-7 family miRNAs during continuous development, without affecting their levels (Hammell et al., 2009b). If nhl-2 activity also enhances let-7 family miRNA activity after dauer quiescence, then loss of nhl-2 should have a similar effect as loss of mir-48-241-84 in combination with lin-4(0). Indeed, lin-4(0); nhl-2(0) animals display a dramatic post-dauer retarded phenotype (Fig. 4A,B; supplementary material Fig. S2). Furthermore, the lin-4(0); nhl-2(0) retarded phenotype is stronger after dauer quiescence than it is in animals that developed continuously (Fig. 4A,B), similar to the lin-4(0); mir-84(0) mutant strain (Fig. 3). These results suggest that nhl-2 acts in parallel to lin-4 during post-dauer development, perhaps to potentiate the activity of certain let-7 family miRNAs.
We wondered next whether nhl-2 might also affect lin-4 activity. During continuous development, nhl-2 is known to affect activity of several miRNAs in addition to let-7 family members, but its effect on lin-4 is unknown (Hammell et al., 2009b). In order to address this question, we investigated whether nhl-2 was required for a lin-4 gain-of-function phenotype. The assay we used is similar to the mir-48 gain-of-function assay that was used previously to demonstrate that nhl-2 potentiates let-7 family miRNA activity (Hammell et al., 2009b). In brief, mis-expression of miR-48 causes precocious phenotypes, presumably owing to premature downregulation of hbl-1 (Abbott et al., 2005; Li et al., 2005). These precocious phenotypes are suppressed by loss of nhl-2, implying that nhl-2 is required for full miR-48 activity (Hammell et al., 2009b). To create an analogous experiment, we mis-expressed lin-4 using the heat-shock promoter. lin-4 mis-expression during continuous development caused precocious phenotypes, as expected if lin-4 targets such as lin-14 and lin-28 are downregulated prematurely (supplementary material Fig. S3). However, loss of nhl-2 has little to no effect on the lin-4 mis-expression phenotype, suggesting that lin-4 activity is not affected by nhl-2, at least during continuous development (supplementary material Fig. S3). Because both lin-14(0) and lin-28(0) precocious phenotypes are completely suppressed by post-dauer development (Liu and Ambros, 1991), we could only perform this experiment for continuously developing animals.
To test whether nhl-2 can affect the activity of miRNAs in addition to miR-48-241-84 (such as lin-4) after dauer quiescence, we observed the post-dauer phenotype of nhl-2(0); mir-48-241-84(0) mutant animals, in comparison to the post-dauer phenotype of mir-48-241-84(0). If nhl-2 activity is important only to enhance miR-48-241-84 activity after dauer quiescence, then we would not expect to see any effect of removing nhl-2 when mir-48-241-84 are absent. However, we do find a moderate enhancement of the post-dauer phenotype of mir-48-241-84(0) when nhl-2 is removed (Table 2, lines 37, 38). This indicates that nhl-2 might indeed enhance the activity of miRNAs other than miR-48-241-84, such as one or more of the remaining let-7 family miRNAs (let-7, miR-793, miR-794, miR-795) (Ibáñez-Ventoso et al., 2008), and/or lin-4.
The C. elegans genome encodes four members of the TRIM-NHL family, NHL-1-3 and LIN-41, plus a related protein, NCL-1; (reviewed by Loedige and Filipowicz, 2009). Quadruple mutants ncl-1(0); nhl-1(0); nhl-2(0); nhl-3(0) do not display any post-dauer retarded phenotype, indicating that the TRIM-NHL proteins do not function redundantly for directing stage-specific cell fate during post-dauer development (Table 2, lines 21, 22). This is similar to the lack of redundancy observed in ncl-1(0); nhl-1(0); nhl-2(0); nhl-3(0) mutant animals that develop continuously (Hammell et al., 2009b; Hyenne et al., 2008).
lin-46 promotes post-dauer cell fate progression in parallel to let-7 family miRNAs
As nhl-2 potentiates the activity of let-7 family miRNAs, but does not appear to potentiate lin-4 activity, the question arises as to what factor might enhance lin-4 activity after dauer quiescence, such that lin-4 can substitute for mir-48-241-84 during post-dauer development. One candidate is lin-46, which promotes the proper timing of L2 and L3 cell fates during continuous development in parallel with nhl-2 and mir-48-241-84. LIN-46 shares sequence homology with the gephyrin scaffolding protein, but the molecular mechanism by which LIN-46 regulates developmental timing is unknown (Abbott et al., 2005; Hammell et al., 2009a; Pepper et al., 2004). We first tested whether lin-46 could also contribute to stage-specific cell fates after dauer quiescence. lin-46(0); mir-48-241-84(0) animals display a severely retarded post-dauer phenotype, indicating that lin-46 is required for the post-dauer suppression of mir-48-241-84(0) retarded phenotypes (Table 2, lines 7, 25). Furthermore, lin-46(0); mir-84(0) mutant animals display a retarded defect after dauer quiescence, but not during continuous development, indicating that these two genes (lin-46 and mir-84) are together more important for the regulation of stage-specific cell fates in post-dauer animals than they are in continuously developing animals (Fig. 4C). Finally, lin-46 also acts in parallel to nhl-2 after dauer quiescence, because nhl-2(0); lin-46(0) animals display a retarded post-dauer phenotype, whereas the single mutants do not (Table 2, lines 18, 21, 35). Therefore, similar to continuous development, lin-46 acts after dauer quiescence in parallel to mir-48-241-84 and nhl-2 to promote cell fate progression.
The genetic relationship between lin-46 and let-7 family miRNAs is similar to the relationship between lin-4 and let-7 family miRNAs. First, lin-46(0) and lin-4(0) both cause a similar enhancement of the retarded phenotype of mir-48-241-84(0) during continuous or post-dauer development (Fig. 3, Table 2) (Abbott et al., 2005). Second, lin-46(0) and lin-4(0) both cause a retarded post-dauer phenotype when combined with mir-84(0) (Fig. 3, Fig. 4C). These observations suggest that lin-46 might act in the same genetic pathway as lin-4. If this were the case, loss of lin-46 should not enhance the post-dauer retarded phenotypes caused by loss of lin-4. With respect to alae formation, this is indeed what we see (Table 2, lines 10, 31, and 33, 34; P=0.2 and P=0.1, respectively, Fisher’s Exact Test). However, using the same strains (as for lines 10 and 31 of Table 1) we find that lin-46(0) can moderately but statistically significantly (P=0.0007) enhance the post-dauer retarded col-19::GFP defects observed: 68% (n=34) of lin-4(0) young adults display defects in col-19::GFP expression in seam cells, compared with 100% (n=30) of lin-4(0); lin-46(0) young adults. This is consistent with additional roles for lin-46 with respect to the regulation of col-19 expression. We propose that lin-46 acts in the same genetic pathway as lin-4 to regulate the timing of adult alae formation, and also acts downstream of or in parallel with lin-4 to affect col-19 expression.
As lin-46 acts in parallel with mir-48-241-84 but might act in the same pathway as lin-4, one possibility is that lin-46 might affect lin-4 levels. Specifically, the post-dauer retarded phenotype observed in lin-46(0); mir-48-241-84(0) mutant animals could be due to reduced lin-4 expression. To test this hypothesis, levels of mature miRNAs were quantified in lin-46(0); mir-48-241-84(0) and mir-48-241-84(0) post-dauer L3 staged larvae using Taqman real-time qRT-PCR. No statistically significant difference in the levels of lin-4 or its homolog miR-237 were found between these two mutant strains (Fig. 4D).
We next examined whether lin-46 could potentiate lin-4 activity in a manner analogous to the potentiation of miR-48-241-84 activity by nhl-2 (though not necessarily via the same molecular mechanism). We addressed this question using the same lin-4(gf) assay that we used to assess whether nhl-2 could affect lin-4 activity. We found that lin-46 was required for the complete expression of lin-4(gf) phenotypes during continuous development (supplementary material Fig. S3), and that, in this context, lin-46 appears to potentiate lin-4 activity via the lin-14 3′UTR (supplementary material Fig. S4).
LIN-46 and NHL-2 potentiate downregulation of HBL-1 by lin-4 and let-7 family miRNA pathways after dauer quiescence
Downregulation of HBL-1 is required for progression to L3 cell fates during continuous development, and mir-48-241-84, lin-46 and nhl-2 all contribute to hbl-1 downregulation (Abbott et al., 2005; Hammell et al., 2009a). HBL-1 is also downregulated during dauer quiescence, via a mechanism involving the hbl-1 3′UTR (Karp and Ambros, 2011). To test whether mis-expression of HBL-1 causes the retarded postdauer phenotypes of lin-4(0); mir-48-241-84(0) and lin-4(0); mir-84(0) animals, lin-4(0); mir-48-241-84(0); hbl-1(lf) and lin-4(0); mir-84(0); hbl-1(lf) strains were constructed. hbl-1 null alleles are not available, and RNAi suggests that they would probably be embryonic lethal (Fay et al., 1999). In both cases, reduction of hbl-1 partially suppressed the retarded phenotypes of both post-dauer and continuously developing animals (Fig. 3). This suggests that the retarded phenotype of lin-4(0); mir-48-241-84(0) mutants results from mis-expression of HBL-1. The observed incomplete suppression by hbl-1(lf) could be due to mis-expression of additional targets, or to residual hbl-1 activity from the hbl-1(lf) allele. It is noteworthy that the same hbl-1(lf) allele very efficiently suppressed the lin-46(0); mir-48-241-84(0) post-dauer phenotypes (Table 2, lines 25, 26). This suggests a lesser degree of hbl-1 mis-expression during post-dauer development in lin-46(0); mir-48-241-84(0) animals compared with lin-4(0); mir-48-241-84(0) animals. Finally, the retarded post-dauer phenotypes of nhl-2(0); lin-46(0) are also suppressed by reduction of hbl-1 (Table 2, lines 35, 36). This supports the idea that lin-46 and nhl-2 might function in the same pathways as lin-4 and let-7 family miRNAs to regulate HBL-1 during post-dauer development.
Consistent with a direct role for lin-4 in downregulation of HBL-1, the hbl-1 3′UTR contains a putative lin-4 binding site. However, during continuous development, lin-4 has not been shown to contribute substantially to the downregulation of HBL-1 between the L2 and L3 stages, although lin-4 does affect hbl-1 expression at later stages (Abrahante et al., 2003; Lin et al., 2003). Our results indicate that for proper development after dauer quiescence, lin-4 must act together with let-7 family miRNAs to repress HBL-1 levels; animals multiply deficient for lin-4 and mir-48-241-84 exhibit persistent expression of the hbl-1 reporter during the post-dauer L3 stage (Fig. 5E,F). Loss of lin-4 alone can also cause persistent expression of the hbl-1 reporter (Fig. 5I,J), consistent with the retarded phenotypes observed in lin-4(0) animals after dauer quiescence (Fig. 3) (Liu and Ambros, 1991).
We also tested two known lin-4 targets, lin-14 and lin-28, for their involvement in post-dauer retarded phenotypes (Lee et al., 1993; Moss et al., 1997; Wightman et al., 1993). The lin-14 and lin-28 3′UTRs contain putative binding sites for let-7 family as well as lin-4 miRNAs. To test lin-14, we made use of a complex lin-14 allele (n355n679), which lacks most of the lin-14 3′UTR, rendering it insensitive to miRNAs. In particular, all predicted lin-4 and let-7 family binding sites are deleted (G. Hayes and G. Ruvkun, personal communication) (Reinhart and Ruvkun, 2001). In addition, this allele includes a mis-sense mutation that lowers lin-14 activity sufficiently to allow dauer formation to occur (Liu and Ambros, 1989; Reinhart and Ruvkun, 2001). If overexpression of lin-14 is the cause of the post-dauer retarded phenotypes observed when lin-4 and let-7 family miRNAs are deleted, then removing lin-4 or mir-84 should not affect post-dauer phenotypes in a lin-14(n355n679) background. Instead, loss of lin-4, mir-84 or both miRNAs enhanced the retarded phenotype of lin-14(n355n679) during both continuous and post-dauer development (Fig. 5K). Indeed, lin-4(0); lin-14(n355n679) mir-84(0) animals display a more retarded phenotype if their development is interrupted by dauer quiescence (Fig. 5K), similar to lin-4(0); lin-14(n179) mir-84(0) animals (Fig. 3). Therefore, these retarded phenotypes are caused by target genes other than lin-14, probably hbl-1. Additionally, lin-46(0) enhances the post-dauer retarded phenotype of lin-14(n355n679) (Table 2, lines 39, 40; P=0.0002, Fisher’s Exact Test). This is consistent with lin-46 promoting post-dauer cell fate progression through a mechanism that does not involve the lin-14 3′UTR, such as through lin-4 regulation of hbl-1.
Unfortunately, we could not perform an equivalent experiment to test lin-28, because of the lack of availability of an miRNA-independent allele. LIN-28 expression was therefore examined in wild-type and mutant post-dauer animals. A rescuing LIN-28 reporter is mis-expressed in both lin-4(0) and lin-4(0); mir-48-241-84(0) animals during the post-dauer L3 stage (Fig. 5L). Therefore, mis-expression of LIN-28 could contribute to post-dauer retarded phenotypes in strains that lack lin-4. However, loss of lin-28 does not suppress the post-dauer retarded phenotypes of lin-46(0); mir-48-241-84(0) (Table 2, lines 25, 27), consistent with the idea that hbl-1 mis-expression is the major cause of the post-dauer retarded phenotypes in this strain.
Dauer quiescence affects the requirement for core miRISC components
We wondered whether dauer quiescence could affect the requirement for core components of the miRISC. Two core components of the miRISC are Argonaute proteins and GW182 proteins (reviewed by Krol et al., 2010). In C. elegans, each of these protein classes is represented by two partially redundant homologs: the Argonautes ALG-1 and ALG-2, and the GW182 proteins AIN-1 and AIN-2. Although simultaneous loss of both alg-1 and alg-2 or simultaneous loss of both ain-1 and ain-2 causes embryonic lethality, single mutants are viable (Ding et al., 2005; Grishok et al., 2001; Zhang et al., 2007). However, alg-1(0) and ain-1(0) single mutant animals display various pleiotropic defects, including notably retarded alae defects, consistent with a reduced ability of let-7 family miRNAs to downregulate hbl-1 in these backgrounds (Brenner et al., 2010; Ding et al., 2005; Grishok et al., 2001; Hammell et al., 2009b; Zhang et al., 2007). Indeed, an hbl-1 reporter is mis-expressed (albeit at low penetrance) in ain-1(0) mutant larvae (Hammell et al., 2009b).
By contrast, the alg-1(0) and ain-1(0) retarded phenotypes are not evident after dauer quiescence, suggesting that alg-2 and ain-2 substitute more effectively for their counterparts after dauer quiescence (Table 2, lines 41, 42, 44). However, alg-2(0) mutant animals do not display retarded post-dauer phenotypes (Table 2, line 43), indicating that the lack of alg-1(0) post-dauer phenotype cannot be explained by a specificity of alg-2 for post-dauer development. Interestingly, lin-46(0); alg-1(0) double mutants display a penetrant post-dauer retarded phenotype (Table 2, line 46). By contrast, nhl-2(0); alg-1(0) double mutants display only a mild post-dauer retarded phenotype, similar to alg-1(0) single mutants (Table 2, lines 41, 42, 45). Thus, like mir-48-241-84, alg-1 appears to act in the same pathway as nhl-2 but in parallel to lin-46. One possibility therefore is that alg-1(0) retarded phenotypes observed during continuous development could be due primarily to a reduction in the activity of the let-7 family miRNAs.
C. elegans larvae develop through one of two alternative life histories depending on environmental conditions: animals with ample resources develop continuously through the four larval stages, whereas in response to certain stresses the progression of developmental events is interrupted between the L2 and L3 stages by dauer larva quiescence, which is of indefinite length (Fielenbach and Antebi, 2008). Strikingly, precisely the same sequence of cell fate events occurs during the L3 and L4 stages of post-dauer development as during the L3 and L4 stages of continuous development. The interruption of development by dauer between the L2 and L3 would pose a challenge if developmental timing mechanisms controlling L2-to-L3 cell fate transitions were to rely on a strictly time-critical sequence of molecular events, as appears to occur during continuous development (Abbott et al., 2005; Rougvie, 2001). Previous findings had shown that, in fact, alternative mechanisms are employed during continuous vs post-dauer development to control the timing of L3 cell fates (Liu and Ambros, 1991). That previous study was not able to identify the molecular basis of these alternative developmental timing mechanisms. Here, we provide insight into the manner in which heterochronic gene activity is modulated in order to accommodate the interruption of development by dauer quiescence. We provide evidence that the post-dauer suppression of mir-48-241-84(0) retarded phenotypes results, at least in part, from a potentiation of lin-4 activity after dauer quiescence. Our data indicate that dauer quiescence causes a shift from reliance primarily on let-7 family miRNAs for the expression of L3 cell fates, to a reliance on parallel activity by both lin-4 and let-7 family miRNAs and, furthermore, that this shift requires the activity of NHL-2 and LIN-46. Indeed, disruption of both of these parallel pathways causes more severe phenotypes after dauer quiescence than in continuously developing animals. As described below, we propose that the quiescence-specific regulation of miRNA expression on the one hand, and activity on the other hand contributes to the robustness with which cell fate transitions occur despite the interruption of development by dauer quiescence.
Previous work has shown that levels of certain mature let-7 family miRNAs are reduced before and during dauer quiescence, owing to the activity of the dauer-promoting form of the DAF-12 nuclear hormone receptor (Bethke et al., 2009; Hammell et al., 2009a; Karp et al., 2011). This reduction of let-7 family levels is important to allow the continued expression of HBL-1 throughout the extended pre-dauer L2d stage, which is >50% longer than the rapid L2 stage of continuous development (Fig. 6A,B) (Bethke et al., 2009; Golden and Riddle, 1984; Hammell et al., 2009a; Karp and Ambros, 2011). However, as larvae recover from dauer quiescence and prepare to express L3 cell fates, levels of miR-48 and miR-241 remain low relative to continuously developing L3-staged larvae (Table 1, Fig. 6B) (Karp et al., 2011). Despite these lower levels of some let-7 family miRNAs, hbl-1 is efficiently downregulated in post-dauer-L3 staged larvae, in a 3′UTR-dependent manner (Fig. 5). By contrast, lin-4 levels remain constant after the L1 stage, and are not affected by dauer quiescence (Fig. 6) (Karp et al., 2011). Perhaps this stable expression is the reason that reliance on lin-4 for progression to L3 cell fates increases after dauer quiescence. Finally, we propose that lin-46 and nhl-2 potentiate the activity of lin-4 and let-7 family miRNAs, respectively, to regulate the proper progression to L3 cell fates after dauer quiescence (Table 2, Fig. 4). As the expression of rescuing LIN-46 and NHL-2 reporters (Hammell et al., 2009b; Pepper et al., 2004) is not obviously increased during or after dauer quiescence (not shown), it will be interesting to discover at what level these factors are regulated during dauer-interrupted development.
In conclusion, we have identified a mechanism that enables cell fate progression to be robust to the interruption of development by a period of quiescence through alteration of let-7 family and lin-4 microRNA activities. Because lin-4 and let-7 miRNA families are evolutionarily conserved, similar relationships among these microRNAs could be operative in the context of developmental contingencies in other organisms (Ibáñez-Ventoso et al., 2008). let-7 and miR-125 (the lin-4 homolog in other species) family microRNAs are generally, though not exclusively, associated with promoting differentiation in a variety of cell types, analogous to their roles in cell fate progression in C. elegans (Mallanna and Rizzino, 2010; Nimmo and Slack, 2009). In flies, certain miRNAs have been shown to provide robustness to environmental stress, and the importance of miRNAs in the response to various types of stress has been proposed in mammals as well (Kosik, 2010; Leung and Sharp, 2010; Li et al., 2009). In general, such stress-buffering roles for microRNAs could reflect the sort of conditional shift in microRNA activity on an mRNA target like that described here. These findings also underscore the importance of protein co-factors that modulate miRNA activity. As potentiation of miRNA activity by the NHL-2 homolog TRIM32 has been described in mammals (Schwamborn et al., 2009), it will be interesting to explore whether TRIM32 and other miRNA co-factors are important for the robustness of cell fate specification in mammalian stem/progenitor cells to lengthy quiescent periods.
We thank Allison Abbott for initial observations concerning post-dauer suppression of let-7 family mutants; Ji Li for helpful discussions and reagents; Richie Ruiz and Cindy Zhou for technical assistance; and Iva Greenwald, Oliver Hobert and Ji Li for comments on the manuscript. We are very grateful to Iva Greenwald (HHMI/Columbia University Medical Center), in whose laboratory the final experiments were performed by X.K. as an HHMI Research Specialist. Some nematode strains used in this work were provided by the Caenorhabditis Genetics Center, which is funded by the NIH National Center for Research Resources (NCRR).
This work was supported by National Institutes of Health grants [R01GM30428, R01GM088365 to V.A.; and F32 GM73307 to X.K.]. At Columbia University, X.K. was supported by HHMI as a Research Specialist in the laboratory of Iva Greenwald. Core resources supported by the Diabetes Endocrinology Research Center grant (DK32520) were also used. V.A. is a member of the University of Massachusetts Medical School Diabetes and Endocrinology Research Center (UMass DERC) [DK32520]. Deposited in PMC for release after 12 months.
Competing interests statement
The authors declare no competing financial interests.
Supplementary material available online at http://dev.biologists.org/lookup/suppl/doi:10.1242/dev.075986/-/DC1
- Accepted April 13, 2012.
- © 2012.