miRNA suppression of a Notch repressor directs non-neuronal fate in Drosophila mechanosensory organs

Although there is abundant evidence that individual microRNA (miRNA) loci repress large cohorts of targets, large-scale knockout studies suggest that most miRNAs are phenotypically dispensable. Here, we identify a rare case of developmental cell specification that is highly dependent on miRNA control of an individual target. We observe that binary cell fate choice in the peripheral sensory organ lineage is controlled by the non-neuronally expressed cluster, with a majority of notum sensory organs exhibiting transformation of sheath cells into ectopic neurons. The defect phenocopies Notch loss of function during the sheath-neuron cell fate decision, suggesting the miRNAs facilitate Notch signaling. Consistent with this, knockouts are strongly enhanced by heterozygosity, and activated nuclear Notch is impaired in the miRNA mutant. Although Hairless (H) is the canonical nuclear Notch pathway inhibitor, and heterozygotes exhibit bristle cell fate phenotypes reflecting gain-of-Notch signaling, does not rescue mutants. Instead, we identify Insensible (Insb), another neural nuclear Notch pathway inhibitor, as a critical direct miR-279/996 target. Insb is posttranscriptionally restricted to neurons by these miRNAs, and its heterozygosity strongly suppresses ectopic peripheral nervous system neurons in mutants. Thus, proper assembly of multicellular mechanosensory organs requires a double-negative circuit involving miRNA-mediated suppression of a Notch repressor to assign non-neuronal cell fate.