MicroRNAs Regulate Broad-Z3, Which Together with Chinmo Maintains Adult Lineage Neurons in an Immature State.

During metamorphosis, arrested immature neurons born during larval development differentiate into their functional adult form. This differentiation coincides with the downregulation of two zinc-finger transcription factors, Chronologically Inappropriate Morphogenesis (Chinmo) and the Z3 isoform of Broad (Br-Z3). Here, we show that is regulated by two microRNAs, and , that are activated at the larval-to-pupal transition and are known to also regulate The 3'UTR contains functional binding sites for both and that confers sensitivity to both of these microRNAs, as determined by deletion analysis in reporter assays. Forced expression of and miRNAs leads to early silencing of Br-Z3 and Chinmo and is associated with inappropriate neuronal sprouting and outgrowth. Similar phenotypes were observed by the combined but not separate depletion of and Because persistent Br-Z3 was not detected in mutants, this work suggests a model in which and activation at the onset of metamorphosis may act as a failsafe mechanism that ensures the coordinated silencing of both and needed for the timely outgrowth of neurons arrested during larval development. The and binding site sequences are conserved across species and possibly other insects as well, suggesting that this functional relationship is evolutionarily conserved.