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.
Furthermore, the observed derepression of individual genes by mir-71(lf) seemed too weak to account for the phenotype, consistent with the idea that a prominent phenotype of an miRNA mutation is caused by the collective effect of changing expression in many genes, an important property of miRNA-mediated gene regulation. (F) Fluorescence and DIC images showing that an hbl-1 3′UTR reporter was repressed in mir-71(+) worms and slightly derepressed in mir-71(lf) mutants. (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.
(D) Fractions of worms that carry 3′UTR reporter transgene and show no GFP expression GFP(−), weak GFP expression GFP(+/−), and comparable GFP expression to mCherry GFP(+). We found that the mRNA level of UNC-31 was up-regulated by about 20% in mir-71(lf) (Fig. 3A). These results suggest that a significant portion of the miR-71 activities in L1 diapause survival may be devoted to regulating the activities of UNC-31–mediated InsR/PI3K signaling and that the rest of miR-71 activity may regulate UNC-31–independent pathways.