Drosophila melanogaster is a simple, tractable invertebrate model for heart development. The similarities of the Drosophila dorsal vessel to vertebrate cardiac formation provide opportunities to study complex processes, in a system whose embryonic development does not hinge on heart function. The dorsal vessel has an aorta and a chamber termed the heart. Crucial structures for the proper functioning of the dorsal vessel are the inflow tracts, or ostia, located in the heart chamber.
The question remains as to how formation of ostia differ from non-ostial cardiac cells. Initial heart specification requires the homeotic selector protein Abdominal A (Abd-A). Ostial cells specifically express the COUP-TFII homolog Seven-up (Svp), while neighboring non-ostial cardiac cells express Tinman (Tin)/Nkx2.5. We are addressing the contribution of signaling glycoprotein Wingless (Wg)/Wnt in formation of Drosophila ostia.
Our approach was to analyze the roles of three genes: svp, abd-A, and wg. Ostia do not form in svp mutants, while wg is partially required for ostia formation. Expressing ectopic Abd-A throughout the cardiac tube results in ostia formation in the aorta. By contrast, if Abd-A is expressed in Svp-expressing cells, ostia do not form. We hypothesize that ostia formation involves a combination of signals from the Wg pathway and cells expressing Tinman (Tin)/Nkx2.5. When Wg signaling is inhibited in the neighboring Tin cells, ostia do not form, suggesting that Wg signals paracrinely. Our findings support an additional role for Wg after initial heart cell specification and interactions between ostial and non-ostial cells are required for inflow tract formation.