Previous studies showed that the release of postdocking calcium-regulated dense-core vesicles, the insulin receptor (InsR) pathway, the AMPK pathway, and protein chaperones are required for the long-term survival of starved L1 worms (2–4). Unlike dauer diapause, L1 diapause is not accompanied by life cycle changes and has not been shown to require certain signaling pathways that control the formation of dauer diapause such as TGF-β signaling (daf-1, daf-7) and nuclear hormone receptor (daf-12) (2, 3). The coordinated entrance into developmental arrest, long-term survival, and the reinitiation of development upon food availability are important biological processes to investigate. Different organisms have developed versatile growth arrest strategies to overcome starvation-induced metabolic and developmental problems. The presented results indicate that interactions between multiple miRNAs and likely a large number of their mRNA targets in multiple pathways regulate the response to starvation-induced L1 diapause.
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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).
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.
S1A indicated a dominant role of intestinal miRNAs in regulating L1 starvation survival. We used a dual-color 3′UTR reporter system (18) to test the computational, prediction-based hypothesis that the 3′UTRs of age-1 and unc-31 are directly regulated by miR-71 (Fig. 3B and Materials and Methods). Among these potential miRNA targets, the predicted miR-71–targeting sites in the 3′UTRs of age-1 and unc-31 are conserved between C.
- The eggs were transferred to plates seeded with HB101 and bleached again 3 d later.
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- Elegans Genetic Center (reference 257) and an N2 strain from the laboratory stock, respectively.
- (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).
- 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).
- We identified 10 miRNA mutants that showed reduced survival rates with a stringent standard, as well as a few miRNA mutants with slightly increased survival rates (Table S1, Fig. 1D, and Fig. S1B).