The Broad Transcription Factor Links Hormonal Signaling, Gene Expression, and Cellular Morphogenesis Events During Drosophila Imaginal Disc Development.

Imaginal disc morphogenesis during metamorphosis in Drosophila melanogaster provides an excellent model to uncover molecular mechanisms by which hormonal signals effect physical changes during development. The broad (br) Z2 isoform encodes a transcription factor required for disc morphogenesis in response to 20-hydroxyecdysone, yet how it accomplishes this remains largely unknown. Here, we use functional studies of amorphic br 5 mutants and a transcriptional target approach to identify processes driven by br and its regulatory targets in leg imaginal discs. br 5 mutants fail to properly remodel their basal extracellular matrix (ECM) between 4 and 7 hr after puparium formation. Additionally, br 5 mutant discs do not undergo the cell shape changes necessary for leg elongation and fail to elongate normally when exposed to the protease trypsin. RNA-sequencing of wild-type and br 5 mutant leg discs identified 717 genes differentially regulated by br, including a large number of genes involved in glycolysis, and genes that encode proteins that interact with the ECM. RNA interference-based functional studies reveal that several of these genes are required for adult leg formation, particularly those involved in remodeling the ECM. Additionally, br Z2 expression is abruptly shut down at the onset of metamorphosis, and expressing it beyond this time results in failure of leg development during the late prepupal and pupal stages. Taken together, our results suggest that br Z2 is required to drive ECM remodeling, change cell shape, and maintain metabolic activity through the midprepupal stage, but must be switched off to allow expression of pupation genes.