Genetic control of cardiac cell type specification in <italic>Drosophila</italic>.

Abstract

How cell type diversity of an organ is generated is a central question in organogenesis. During <italic>Drosophila</italic> heart formation, cardiac progenitors are specified in precise mesodermal positions, giving rise to multiple cell types in a highly ordered arrangement. Here, I studied the mechanisms by which positional information conveyed by signaling pathways and transcription factors work together to confine the expression of the homeobox gene <italic> even-skipped</italic> (<italic>eve</italic>) to a small region of the cardiac mesoderm. By manipulating both expression patterns and binding sites for transcription factors, I found that a complex combination of regulatory activities converge on a single enhancer of <italic>eve</italic> to generate precisely targeted gene expression. Secreted signals from the ectoderm, such as <italic>wg</italic> and <italic>dpp</italic>, and intrinsic factors in the mesoderm, such as Tinman, directly activate <italic>eve</italic> through this enhancer, whereas <italic> ladybird early</italic> (<italic>lbe</italic>) directly represses <italic> eve</italic>. Furthermore, I show that <italic>eve</italic> and <italic>lbe </italic> mutually repress each other to maintain their cell type specifications. Sequence comparison of the eve mesodermal enhancer in five <italic>Drosophila </italic> species revealed that functional binding sites are highly conserved over 60 million years of evolution. I have unveiled additional unknown conserved regions and tested their effects on enhancer activity. Two of them mediate strong repression whereas a third is required for early activation. We are using these regions as baits in Yeast one-hybrid screens with embryonic cDNA libraries to uncover novel regulators of cell specification in the cardiac mesoderm. Asymmetric cell division is involved in generating the cardiac cell type diversity. By examining the pattern of cardiac cell fates in a series of cell cycle modifications, I show that cardiac progenitors can still differentiate when they are unable to divide. Interestingly, the undivided progenitors of asymmetric cell divisions adopt a myogenic cell fate. In contrast, when <italic> Notch</italic> is ectopically activated or <italic>numb</italic> is mutated, these undivided progenitors adopt a non-myogenic, pericardial cell fate, indicating that the <italic>Notch</italic> pathway functions in parallel to cell division genes to generate the diversity of cardiac cell types.