Insensible is a novel nuclear inhibitor of Notch activity in Drosophila

A nonneural miRNA cluster mediates hearing via repression of two neural targets

We show here that mir-279/996 are absolutely essential for development and function of Johnston's organ (JO), the primary proprioceptive and auditory organ in Drosophila Their deletion results in highly aberrant cell fate determination, including loss of scolopale cells and ectopic neurons, and mutants are electrophysiologically deaf. In vivo activity sensors and mosaic analyses indicate that these seed-related miRNAs function autonomously to suppress neural fate in nonneuronal cells. Finally, genetic interactions pinpoint two neural targets (elav and insensible) that underlie miRNA mutant JO phenotypes. This work uncovers how critical post-transcriptional regulation of specific miRNA targets governs cell specification and function of the auditory system.

Multilayered gene control drives timely exit from the stem cell state in uncommitted progenitors during Drosophila asymmetric neural stem cell division

Self-renewal genes maintain stem cells in an undifferentiated state by preventing the commitment to differentiate. Robust inactivation of self-renewal gene activity following asymmetric stem cell division allows uncommitted stem cell progeny to exit from an undifferentiated state and initiate the commitment to differentiate. Nonetheless, how self-renewal gene activity at mRNA and protein levels becomes synchronously terminated in uncommitted stem cell progeny is unclear. We demonstrate that a multilayered gene regulation system terminates self-renewal gene activity at all levels in uncommitted stem cell progeny in the fly neural stem cell lineage. We found that the RNA-binding protein Brain tumor (Brat) targets the transcripts of a self-renewal gene, deadpan (dpn), for decay by recruiting the deadenylation machinery to the 3' untranslated region (UTR). Furthermore, we identified a nuclear protein, Insensible, that complements Cullin-mediated proteolysis to robustly inactivate Dpn activity by limiting the level of active Dpn through protein sequestration. The synergy between post-transcriptional and transcriptional control of self-renewal genes drives timely exit from the stem cell state in uncommitted progenitors. Our proposed multilayered gene regulation system could be broadly applicable to the control of exit from stemness in all stem cell lineages.

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

Large-scale knockout studies suggest that most miRNAs are phenotypically dispensable. However, Kavaler et al. show here that developmental specification toward a non-neuronal fate in the Drosophila melanogaster peripheral sensory organ lineage depends critically on mir-279/996 repression of the Notch repressor Insensible.

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 Drosophila melanogaster peripheral sensory organ lineage is controlled by the non-neuronally expressed mir-279/996 cluster, with a majority of notum sensory organs exhibiting transformation of sheath cells into ectopic neurons. The mir-279/996 defect phenocopies Notch loss of function during the sheath–neuron cell fate decision, suggesting the miRNAs facilitate Notch signaling. Consistent with this, mir-279/996 knockouts are strongly enhanced by Notch heterozygosity, and activated nuclear Notch is impaired in the miRNA mutant. Although Hairless (H) is the canonical nuclear Notch pathway inhibitor, and H heterozygotes exhibit bristle cell fate phenotypes reflecting gain-of-Notch signaling, H/+ does not rescue mir-279/996 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 mir-279/996 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.

The CSL proteins, versatile transcription factors and context dependent corepressors of the notch signaling pathway

The Notch signaling pathway is a reiteratively used cell to cell communication pathway that triggers pleiotropic effects. The correct regulation of the pathway permits the efficient regulation of genes involved in cell fate decision throughout development. This activity relies notably on the CSL proteins, (an acronym for CBF-1/RBPJ-κ in Homo sapiens/Mus musculus respectively, Suppressor of Hairless in Drosophila melanogaster, Lag-1 in Caenorhabditis elegans) which is the unique transcription factor and DNA binding protein involved in this pathway. The CSL proteins have the capacity to recruit activation or repression complexes according to the cellular context. The aim of this review is to describe the different co-repressor proteins that interact directly with CSL proteins to form repression complexes thereby regulating the Notch signaling pathway in animal cells to give insights into the paralogous evolution of these co-repressors in higher eumetazoans and their subsequent effects at developmental processes.