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These results suggest that miRNAs act in the intestine, and possibly in other tissues, to promote L1 starvation survival. MicroRNAs (miRNAs) are well known for their functions in controlling developmental timing in the nematode (5, 6). Upon entering L1 diapause, RNA polymerase II quickly accumulates and pauses at promoter regions, and this accumulation was speculated to stop transcription and facilitate the immediate reinitiation of gene expression when food becomes available (2).
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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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. (B) The severely reduced survival rate of the mir-71(lf) mutant was suppressed by a null allele of unc-31(e928). The effect observed in ain-1(lf) mutants is likely the consequence of the combined effects of attenuating functions of these individual miRNAs. Previous studies indicate that the InsR pathway plays a dominant role in regulating L1 starvation survival and that reducing the activity of the insulin receptor daf-2, the PI3Kinase age-1, or the upstream regulator unc-31 results in increased L1 starvation survival rate (2, 3).
(B) Survival rate of single and double mutants to indicate the functional relationship between ain-1 and age-1. 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).
Mutating miR-71 drastically reduces the survival rate of animals in L1 diapause, and the effect can be suppressed by mutations of insulin receptor pathway genes age-1 and unc-31. 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 revery play login to food starvation. Here we show that compromising overall microRNA (miRNA) functions or mutating certain individual miRNAs impairs the long-term survival of nematodes during starvation-induced L1 diapause. Third-party accounts will also be restored if third-party backup was enabled on the old device.
We next examined the relationship between miR-71 and UNC-31, which functions upstream of AGE-1 during L1 diapause by regulating calcium-regulated dense-core vesicle fusion and the release of an insulin-like ligand (3). 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). 1A because the ain-1 mutations reduce, but do not eliminate, miRISC functions. The overall effect of miRNAs on L1 starvation survival is expected to be significantly stronger than that reflected by the data in Fig.
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. We further examined worms recovering from 4 d of L1 starvation and found that around 90% of the mir-71(lf) mutants displayed retarded vulval precursor cell (VPC) division, compared with less than 5% in wild type (Fig. 4A). 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). (H and I) Fluorescence images (H) and statistical data (I) showing that the M cell diveded in fed animals but remained undivided in 4-, 7-, or 11-d–starved L1 wild-type and mir-71(lf) worms. (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). We found that the poor survival rate of daf-16(mu86)(lf) was further decreased by mir-71(lf) (Fig. 2C), consistent with the notion that a portion of miR-71 activities regulate genes that act in parallel to UNC-31–mediated InsR/PI3K signaling for long-term survival during L1 diapause.
(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. 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).