A recent study showed that the expression of miR-71 was significantly increased relative to other miRNAs in starved L1 worms (15). However, miR-71 does not appear to regulate all postembryonic development during L1 diapause recovery. Unlike classical heterochronic miRNAs such as lin-4 and let-7, the role of miR-71 in vulval cell division is essential in animals recovering from starvation-induced L1 diapause, but not in animals hatched on plates with food. As pointed out above, multiple miRNAs in addition to miR-71 and the let-7 family miRNAs have roles in L1 diapause, and they may regulate the expression of many diverse targets that may include, but are not limited to, factors involved in UNC-31–InsR-signaling activities.
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Non-Unc stable transgenic lines were maintained, and the expression of GFP and mCherry were observed under a Zeiss Axiovision II microscope. Three days later, the number of worms that were L2 or older was recorded as number of survived worms (Ns), and the survival rate was calculated as Ns/Np, which is an estimation of survived worms in the whole population. MT12993 mir-71(n4115) worms were outcrossed with N2 for four generations before any test except the initial screen.
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- If the 3′UTR of age-1 or unc-31 is repressed by miR-71, the GFP expression will be repressed in tissues where miR-71 is expressed in wild-type worms, but derepressed in the same tissues of mir-71(lf) worms.
- This result suggests that the high expression of miR-71 during L1 diapause is induced or maintained by other signaling pathways.
- Components of the InsR pathway, including age-1, have recently been predicted to be targets of miR-71 in its role in aging (14).
- On one hand, we showed that deletions of a good number of miRNAs have varying impacts on the L1 diapause survival rate, although they may effect the rate through different mechanisms.
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- (B) Survival rate of single and double mutants to indicate the functional relationship between ain-1 and age-1.
- When late, first larval stage (L1) worms sense unfavorable conditions, they enter an alternative and long-lived larval stage called dauer larvae (or dauer diapause).
The primers that were used to amplify the 3′UTR of candidate genes are available upon request. 3′UTRs of genes of interest were cloned into the modified pPD129.57 vector as described previously (18). The data for 3′UTR expression and for VPC timing were analyzed using χ2 test.
(E) DIC images showing that hbl-1(RNAi) caused precocious VPC divisions in late L2/early L3 in both wild-type and mir-71(lf) worms recovered from 4 d of L1 starvation. Note that the daf-16(lf) worms recovering from 3 d of L1 starvation displayed a ∼12-h delay in overall development and that the mir-71(lf); daf-16(lf) double mutants displayed an ∼24-h delay. (C) Bar graph showing that the delayed VPC timing defects of mir-71(lf) worms was suppressed by an unc-31(lf) mutation and partially suppressed by an age-1(rf) mutation. In worms that recovered from 4 d of L1 starvation, we also found that a significant portion of the mir-71(lf) mutants displayed egg-laying defects and overproliferating or precociously reflexed gonads.
The two ain-1 loss-of-function alleles displayed significant reductions in L1 starvation survival rate. We further found that this survival rate reduction of ain-1 mutants was overcome by ectopic expression of the AIN-2 protein in the intestine but not in the muscle (Fig. 1A and Fig. S1A). We found that ain-1 but not ain-2 mutants displayed a significant reduction in L1 starvation survival rate compared with that of wild type (Fig. 1 A and D). Furthermore, a recent study suggests that the expression of certain miRNAs is differentially regulated by starvation-induced dauer diapause (15). Consistent with these ideas, several recent lines of evidence suggest that miRNA let-7 and the heterochronic genes lin-42 and hbl-1 are required to regulate the starvation-induced dauer diapause (10–12) and that a number of miRNAs including lin-4 and mir-71 are involved in regulating life span (13, 14).
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We then compared the expression of a hbl-1 3′UTR reporter (18) in the mir-71(lf) mutants with that in wild type and found that the expression of this reporter was slightly derepressed at L3 in the mir-71 mutant (Fig. 4 F and G). (D) Bar graph showing that the delayed VPC timing defect of mir-71(lf) worms was enhanced by daf-16(lf) after 1 or 3 d of L1 starvation. (B) Bar graph showing the correlation between the severity of the retarded vulval precursor cell (VPC) timing defect of mir-71(lf) mutants and the duration of L1 starvation.
This result is consistent with the observation that miR-71 is specifically required for the starvation-induced stress response (Fig. S5). For example, we observed a robust retarded mutant phenotype in the vulval lineage but did not see obvious defects in seam cell differentiation or alae formation. It seems plausible that miRNAs that control developmental timing are also involved in regulating the metabolic rate through repressing the InsR pathway activity.
Consistent with the observation described above, the 4-d–starved mir-71(lf) mutants recovering on the RNAi control plates displayed the highly penetrant retarded defect in VPC division. If this were true, the starved mir-71(lf); daf-16(lf) double-mutant worms should show a slow growth phenotype similar to that of daf-16(lf) worms, but no specific VPC timing defect. (H) Fluorescence and DIC images showing that a lin-42 3′UTR reporter was repressed in mir-71(+) worms (2/2 transgenic lines) and prominently derepressed in mir-71(−) worms (2/2 transgenic lines). We thus asked whether miR-71 was required for the reinitiation of developmental programs during the recovery phase after L1 starvation. These results suggest that miR-71 regulates the expression of unc-31 and age-1 through their 3′UTRs. Note that there are extra GFP-positive cells (red arrows) in mir-71(lf) mutants.
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To investigate the roles of miRNAs in animal survival during starvation-induced L1 diapause, we impaired the overall miRISC function with loss-of-function (lf) mutants of ain-1 (ku322, ku425, and tm3681) and ain-2(tm2432) and examined their L1 starvation survival rate (Materials and Methods). The strong suppression of the mir-71(lf) defect by hbl-1(RNAi), and the relatively weak effect of miR-71 on hbl-1 expression, are consistent with the idea that miR-71 exerts its role by modulating activities of multiple genes related to hbl-1 function in developmental timing. In contrast, the nuclear-localized GFP expression under the control of the 3′UTR of age-1(Fig. 3 C and D) or unc-31 (Fig. 3 E and F) was strongly repressed in the control worms, but prominently derepressed in mir-71(lf) mutant worms. If the 3′UTR of age-1 or unc-31 is repressed by miR-71, the GFP expression will be repressed in tissues where miR-71 is expressed in wild-type worms, but derepressed in the same tissues of mir-71(lf) worms. (A) The mir-71(n4115, lf) mutant displayed severe reduction in L1 starvation survival rate, and the reduced survival rate of mir-71(lf) was suppressed by a reduction-of-function allele of age-1(hx546). (C) The reduced L1 starvation survival rate of ain-1(lf) mutants was significantly suppressed by a null allele of unc-31.
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These results compelled us to examine specific interactions between individual miRNAs and their targets to gain mechanistic insights. This result suggests that miR-71 likely functions upstream of, or in parallel to, HBL-1 in regulating VPC timing. Moreover, the expression of hbl-1 is repressed by let-7 family miRNAs at L3 during normal development, and the hyperactivity of hbl-1 caused by failure of miRNA regulation leads to retarded development (26).
- For example, we observed a robust retarded mutant phenotype in the vulval lineage but did not see obvious defects in seam cell differentiation or alae formation.
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- (C) The poor survival rate of daf-16(mu86, null) was enhanced by mir-71(lf).
- Unlike classical heterochronic miRNAs such as lin-4 and let-7, the role of miR-71 in vulval cell division is essential in animals recovering from starvation-induced L1 diapause, but not in animals hatched on plates with food.
- To determine the functional relationship of miR-71 with LIN-42 and LIT-1, mir-71(lf); lin-42(lf) L1 worms were starved for 4 d and recovered on lit-1(RNAi) plates.
- The reporter is strongly expressed in H and V cells in both wild-type and mir-71(lf) worms.
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