Hormonal activation of let-7-C microRNAs via EcR is required for adult Drosophila melanogaster morphology and function

Abstract

Steroid hormones and their nuclear receptors drive developmental transitions in diverse organisms, including mammals. In this study, we show that the Drosophila steroid hormone 20-hydroxyecdysone (20E) and its nuclear receptor directly activate transcription of the evolutionarily conserved let-7-complex (let-7-C) locus, which encodes the co-transcribed microRNAs miR-100, let-7 and miR-125. These small RNAs post-transcriptionally regulate the expression of target genes, and are required for the remodeling of the Drosophila neuromusculature during the larval-to-adult transition. Deletion of three 20E responsive elements located in the let-7-C locus results in reduced levels of let-7-C microRNAs, leading to neuromuscular and behavioral defects in adults. Given the evolutionary conservation of let-7-C microRNA sequences and temporal expression profiles, these findings indicate that steroid hormone-coupled control of let-7-C microRNAs is part of an ancestral pathway controlling the transition from larval-to-reproductive animal forms.

Regulation of Drosophila circadian rhythms by miRNA let-7 is mediated by a regulatory cycle

“…Previous studies showed that miR-100 expression was delayed and its expression level was reduced during development compared to let-7 and miR-125 (ref. 37). In this study, we found that the expression of pri-let-7-C did not exhibit cycling.…”

“…The presence of a GFP signal in clock neurons in let-7-C-GAL4:UAS-EGFP flies further supports the action of let-7 in the central pacemaker. To further investigate the expression of let-7 and its target Cwo in the brain, we also analysed two transcriptional reporters that contain almost the entire first intron of let-7-C (let-7-Cp12.5kb::lacZ) or just the 20-OH ecdysteroid (20E)-responsive 3.3 kb fragment (let-7-Cp3.3kb::lacZ) with cwo-Gal4 driving GFP 37 . The Cwo reporter was prominently expressed in the LNvs, the PI neurons and the SG (Fig.…”

“…Moreover, a transgenic line in which each of the ecdysone response elements (EcREs) sequences in the let-7-C locus were deleted 37 was used to confirm the necessity of EcREs for circadian let-7-C expression. In contrast to the wild-type lacZ reporter, expression of the mutated let-7-Cp12.5kbnEcRE1-3::lacZ reporter was strongly blocked in the pacemaker neurons (Fig.…”

“…EcR-mediated signalling in adult flies has an important role in the formation of long-term memory) and sleep regulation 44,45 , and defective long-term memory and sleep in EcR mutants is thought to be caused by insufficient EcR-mediated signalling. Previous studies showed that deletion of EcREs in the pri-let-7-C locus caused a reduction and delay of let-7-C miRNA expression in adult flies and is associated with a let-7-C mutant phenotype 37 . EcREs in intron 1 of the let-7-C locus play an important role in onset and maintenance of let-7-C expression in a tissue-specific manner 37 .…”

“…Previous studies showed that deletion of EcREs in the pri-let-7-C locus caused a reduction and delay of let-7-C miRNA expression in adult flies and is associated with a let-7-C mutant phenotype 37 . EcREs in intron 1 of the let-7-C locus play an important role in onset and maintenance of let-7-C expression in a tissue-specific manner 37 . In addition, the let-7-C locus was found to be a direct transcriptional target of 20E because pri-let-7-C RNA could be detected in KC167 cells 30 min after exposure to 20E 37 .…”