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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On the other hand, the role of a particular miRNA (miR-71) is executed by repressing the expression of many genes in multiple pathways. 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. Instead, many specific physiological functions, such as the starvation-induced stress response, are regulated by a miRNA-target network, often involving multiple miRNAs and a large number of their targets. We found that the known developmental timing genes, hbl-1, lin-42, and lit-1, were at the top of the list (TargetScan). To understand how miR-71 affects VPC division, we searched its predicted targets for potential genes involved in regulating developmental timing. These results indicate that miR-71 plays a significant role in larval development of animals recovering from L1 diapause and likely does so by regulating the expression of components of the insulin receptor/DAF-16 pathway, as well as factors acting downstream, or in parallel to, DAF-16.
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Whereas the vulva of wild-type worms developed into the pyramidal stage (81 of 82 worms), the P6.p of mir-71(n4115, lf) mutant worms divided only once (83 of 89 worms). The computation-based prediction that age-1 and pdk-1 are potential targets of miR-71 was also reported in a recent study focusing on miRNA functions in aging where the mRNA level of pdk-1 was shown to be up-regulated in mir-71 worms (14). (C) Fluorescence and differential interference contrast (DIC) images showing that the age-1 3′UTR reporter was repressed in mir-71(+) worms (3/4 transgenic lines) but not in mir-71(lf) worms (4/4 transgenic lines). The transcript level of unc-31 was increased in mir-71(lf) worms, compared with that of wild-type controls that were normalized to the value of 1. MiR-71 represses the expression of age-1 and unc-31 through the actions on their 3′UTR, but miR-71 is not required for arresting M cell division during L1 diapause.
{Among short-lived miRNA mutants, a mir-71 deletion mutant, mir-71(n4115) (referred to as mir-71(lf) hereafter), displayed a severe reduction in L1 starvation survival rate (Table S1 and Fig. 2A). We found that the reduced survival rate of ain-1 was suppressed by either reduction of age-1 function or loss of unc-31 function (Fig. 1 B and C), suggesting that a significant portion of the overall miRNA functions in L1 diapause is upstream of, or in parallel to, the InsR pathway. In this study, we addressed the questions of whether and how miRNAs impact developmental arrest and the long-term survival of early L1 stage worms in response to food starvation.}
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