Multipotent stem and blast cells reversibly arrest cell division and differentiation in response to harsh environments to maintain homeostasis in animals. When C. elegans L1 larvae are hatched in the absence of food, both germline stem and somatic blast cells are prevented from initiating their postembryonic developmental events and sustained at developmentally quiescent state (L1 diapause). Nutrients reactivate these undifferentiated cells via the insulin/IGF pathway to release them from the developmental dormancy (1-3). Recent studies have begun to show that some of miRNAs are involved in regulation of stem cell fate. Thus, we speculated that there may be a microRNA that acts during L1 diapause to keep quiescence in stem and blast cells under the control of the insulin/IGF pathway, by antagonizing genes that promote progression of L1 development. In order to identify such a miRNA, we screened for miRNA mutants that inappropriately initiate L1 development, defects caused by constitutive activation of the insulin/IGF pathway (1,2,4). We found that, among mutants of 111 miRNAs, only animals deleting
mir-235 showed striking defects in maintaining L1 diapause.
mir-235 mutants initiate L1 developmental processes in somatic blast cells such as P and M cells, and eventually undergo L1 molt even under starvation conditions. These defects were significantly restored by introduction of the genomic fragment containing
mir-235. In contrast, restoration of the phenotypes was completely diminished when the
mir-235 genomic fragment was mutated in its seed sequence. These observations indicate that
mir-235 is required for maintaining L1 diapause. Northern blot analysis indicated that the amount of mature
mir-235 was upregulated during L1 diapause and downregulated in response to nutrients. Furthermore, this nutrient-dependent downregulation was abolished in
daf-2/insulin receptor mutants. Altogether, these findings indicate that downregulation of
mir-235 by nutrients via the insulin/IGF pathway is a critical step for exit from L1 diapause. The role of
mir-235 in arresting L1 developmental programs is reminiscent of that of CKI-1, a cyclin-dependent kinase inhibitor, in arresting cell cycle during L1 diapause (5). Thus, identification of
mir-235 targets may reveal the core machinery for L1 developmental progression. 1: Curr. Biol. 16, 773. 2: Curr. Biol. 16, 780. 3; Cell 128, 577. 4: IWM 2005 Fukuyama et al. 5: Development 125, 3585. Development 126, 4861.