Three distinct mechanisms, Notch instructive, permissive, and independent, regulate the expression of two different pericardial genes to specify cardiac cell subtypes in Drosophila melanogaster

Abstract

The development of a complex organ involves the specification and differentiation of diverse cell types. Two major cell types, contractile cardial cells (CCs) and nephrocytic pericardial cells (PCs), comprise the Drosophila heart, with binding sites for Suppressor of Hairless [Su(H)], an integral transcription factor in the Notch signaling pathway, enriched in the enhancers of genes that are specifically expressed in PCs. Here we show three distinct mechanisms regulating the expression of two PC-specific genes, Holes in muscle (Him), and Zn finger homeodomain 1 (zfh1). Him is regulated in a Notch-permissive manner: Su(H) forms a repressor complex with co-repressors that binds to the Him enhancer, repressing transcription in CCs; Notch signaling alleviates this repression in PCs to allow Him transcription. In contrast, zfh1 is transcribed by a Notch-instructive mechanism in most PCs: mere alleviation of repression by preventing the binding of the Su(H) repressor complex to the zfh1 enhancer is not sufficient to activate transcription; zfh1 transcription requires the presence of an activator complex formed by the binding of the Notch intracellular domain to Su(H). A third, Notch-independent pathway activates transcription from the same zfh1 enhancer in the remaining, even skipped-expressing, PCs. Our results illustrate how the same feature, enrichment of Su(H) binding sites in PC-specific gene enhancers, is utilized in two distinct ways to contribute to the same overall goal, the activation of the pericardial gene program, and present an example of a pleiotropic enhancer that is regulated by two independent mechanisms.