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. 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.
It is also worth mentioning that multiple components of the InsR pathway, including age-1, pdk-1, akt-2, and daf-16, are predicted to be targets of the let-7 family miRNAs. 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. 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. 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). (A) Differential interference contrast (DIC) images showing L4 worms recovered from 4-d–starved L1 worms.
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- We further examined the functional relationship between miR-71 and DAF-16, a FOXO transcription factor acting critically and negatively downstream of AGE-1/PI3K in the InsR pathway.
- (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.
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- This result suggests that the high expression of miR-71 during L1 diapause is induced or maintained by other signaling pathways.
This result suggests that the high expression of miR-71 during L1 diapause is induced or maintained by other signaling pathways. We asked whether the expression of miR-71 was regulated by DAF-16, which is required during L1 diapause for long-term survival (2). It is possible that other miRNAs, including those in the let-7 family, control developmental timing in other tissues during the recovery phase after L1 starvation. We speculate that the expression of heterochronic genes controlling the L2/L3 programs, including that of hbl-1 and lin-42, are increased during L1 diapause to arrest the developmental progression, and miR-71 is probably required to suppress these “excess” signals during the recovery phase (Fig. S5). 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. 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).
- (E) Fluorescence and DIC images showing that the unc-31 3′UTR reporter was repressed in mir-71(+)worms (2/2 transgenic lines) but not in mir-71(lf) worms (4/4 transgenic lines).
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- The eggs were transferred to plates seeded with HB101 and bleached again 3 d later.
- Components of the InsR pathway, including age-1, have recently been predicted to be targets of miR-71 in its role in aging (14).
- It is also worth mentioning that multiple components of the InsR pathway, including age-1, pdk-1, akt-2, and daf-16, are predicted to be targets of the let-7 family miRNAs.
- We found that the 3′UTRs of several genes of the InsR pathway, including unc-31, age-1, pdk-1, akt-2, and sgk-1, contain predicted miR-71 targeting sites (as predicted by TargetScan and mirWIP).
- 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.