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miR-71 Is Not Required for Arresting Seam Cell or M-Cell Divisions During L1 Diapause.
Individual GFP reporter constructs for candidate genes (4 ng/μL) and the mCherry internal control plasmid (4 ng/μL) were mixed with unc-119 rescuing plasmid (20 ng/μL) and pBluescript KS+ (72 ng/μL) and coinjected into unc-119(ed3) and mir-71(n4115); unc-119(ed3) worms following standard protocols (32). Knocking down lit-1 by RNAi in mir-71(lf); lin-42(lf) double mutants caused no significant suppression of the VPC timing defects of mir-71(lf) worms. 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.
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Although the complete removal of miRNA functions causes embryonic lethality or infertility in worms, a partial disruption of overall miRNA functions by mutating either ain-1 or ain-2 provides an effective way to investigate miRNA functions (16, 17). However, we found that the reporter transgene with the lin-42 3′UTR was significantly repressed in wild-type worms, but derepressed in the mir-71(lf) worms (Fig. 4 H and I). This is consistent with hbl-1 being one of the downstream targets of miR-71, although this modest effect alone is not expected to account for the vulval developmental phenotype in mir-71 mutant. In starved L1 worms, we detected only a slight increase in the mRNA level of hbl-1 in mir-71 mutants compared with that in wild type (∼10%), which may not be biologically significant. In contrast, the mir-71(lf) mutant worms recovering on hbl-1(RNAi) displayed precocious VPC divisions similar to that seen in wild type (Fig. 4E).
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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.
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.}