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To test the hypothesis that these developmental timing genes mediate the regulatory role of miR-71 in larval development during recovery from starvation-induced L1 diapause, we examined whether knocking down HBL-1 function can suppress the retarded VPC timing defect of mir-71(lf). Reduction-of-function mutation (rf) in the age-1/PI3 kinase gene, age-1(hx546), made worms long-lived in the L1 starvation assay and was able to suppress the reduced L1 survival rate of mir-71(lf); the revery play login rate of the double mutants was comparable to that of wild type (Fig. 2A). Our genetic analysis indicated that for both L1 diapause survival and developmental recovery functions, miR-71 regulates expressions of genes in both the insulin receptor-dependent and -independent pathways.
(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. Note that the daf-16(lf) worms recovering from 3 d of L1 starvation displayed a ∼12-h delay in overall development and that the mir-71(lf); daf-16(lf) double mutants displayed an ∼24-h delay. (C) Bar graph showing that the delayed VPC timing defects of mir-71(lf) worms was suppressed by an unc-31(lf) mutation and partially suppressed by an age-1(rf) mutation. 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.
MiR-71 regulates vulval cell division during recovery of starved L1 worms. These results indicate that miR-71 is not essential for arresting seam cell or M-cell divisions during L1 diapause, suggesting that miR-71 function is distinct from DAF-16 function. DAF-16 (the FOXO homolog in C. elegans) has been shown to play an important role in cell cycle arrest and developmental progression partly by promoting cki-1 expression in some somatic cells during L1 arrest (2).
The primers that were used to amplify the 3′UTR of candidate genes are available upon request. 3′UTRs of genes of interest were cloned into the modified pPD129.57 vector as described previously (18). The data for 3′UTR expression and for VPC timing were analyzed using χ2 test.
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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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- The numbers on each image indicate how many worms of the examined ones displayed the indicated phenotype.
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- (Right panels) The gonad of the same animals in the Left panels to indicate the similar developmental stage.
- (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 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. 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.
- Non-Unc stable transgenic lines were maintained, and the expression of GFP and mCherry were observed under a Zeiss Axiovision II microscope.
- Worms strains were grown and maintained at 20 °C as described (29).
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- Consistent with these ideas, several recent lines of evidence suggest that miRNA let-7 and the heterochronic genes lin-42 and hbl-1 are required to regulate the starvation-induced dauer diapause (10–12) and that a number of miRNAs including lin-4 and mir-71 are involved in regulating life span (13, 14).
- 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.
- Images were pseudocolored in Photoshop CS3 (Adobe) and assembled in Illustrator CS3 (Adobe).
miR-71 Likely Directly Represses the Expression of age-1 and unc-31 by Acting on Their 3′ Untranslated Regions.
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. 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.
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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.
Fig. 2.
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Be sure to enable third-party account backup and restore if you use Duo Mobile to generate passcodes for logging into applications like Instagram, Facebook, Snapchat, or other web services. To compare the survival rates between strains, we simulated the survival rate of each genotype to 100 arbitrary “individual worms” and performed the log-rank test in Graphpad Prism 4. 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.
This will be followed by an ‘ex post evaluation’ in 2028, once the measures included in the recovery plans are fully implemented. The RRF Regulation requires that the Commission provides the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions with a mid-term evaluation on the implementation of the Recovery and Resilience Facility. Member States can also amend their plan if they can demonstrate that objective circumstances render the implementation of certain milestones and targets unfeasible. The RRF is also crucial for implementing the REPowerEU plan – the Commission’s response to the socio-economic hardships and global energy market disruption caused by Russia’s invasion of Ukraine.
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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). Furthermore, a recent study suggests that the expression of certain miRNAs is differentially regulated by starvation-induced dauer diapause (15). Consistent with these ideas, several recent lines of evidence suggest that miRNA let-7 and the heterochronic genes lin-42 and hbl-1 are required to regulate the starvation-induced dauer diapause (10–12) and that a number of miRNAs including lin-4 and mir-71 are involved in regulating life span (13, 14).
Consistent with the observation described above, the 4-d–starved mir-71(lf) mutants recovering on the RNAi control plates displayed the highly penetrant retarded defect in VPC division. If this were true, the starved mir-71(lf); daf-16(lf) double-mutant worms should show a slow growth phenotype similar to that of daf-16(lf) worms, but no specific VPC timing defect. (H) Fluorescence and DIC images showing that a lin-42 3′UTR reporter was repressed in mir-71(+) worms (2/2 transgenic lines) and prominently derepressed in mir-71(−) worms (2/2 transgenic lines). 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.
Elegans Genetic Center (reference 257) and an N2 strain from the laboratory stock, respectively. Wild-type strains A and B are an N2 strain recently obtained from the C. (A) Survival rate curves of wild-type and mutant strains, as indicated. This is consistent with the previous reports that AIN-1 and AIN-2 are functional homologs with overlapping biochemical roles (16, 17). The roles of InsRs have also been implicated in arresting the cell cycle in germ cells and a portion of somatic cells during L1 diapause (2, 4). Contributed new reagents/analytic tools; X.Z., R.Z., and M.H.
(D) A representative chart of the L1 starvation survival rates of different miRNA mutants. However, it remains unclear how, and to what extent, miRNAs coordinate animal survival and development in response to stresses. However, the mechanisms that coordinate the long-term survival, overall developmental arrest, and reinitiation remain to be investigated. However, when newly hatched L1 worms encounter an environment with no food, developmental programs arrest and the worm enters L1 diapause.
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.
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