Expression, function and regulation of the Him gene during Drosophila heart development

I have analysed the regulation and function of the Him gene to gain new insights into Drosophila heart development and its controlling factors. My results show that Him is important during the early specification of pericardial cells and cardioblasts. Loss of Him leads to a reduced number in both of these cell types by the end of embryogenesis. Over-expression of Him throughout the heart results in supernumerary pericardial cells. Him is expressed in all embryonic pericardial cells from embryonic stage 12 to approximately stage 15. I have identified an enhancer fragment that reproduces this expression pattern. Phylogenetic footprinting revealed three highly conserved regions within this sequence. I undertook an extensive mutational analysis of this enhancer to identify regulatory elements within it. I identified Tinman as a direct activator of Him expression. My data indicate that Him is activated in a widespread area of the dorsal mesoderm and the amnioserosa and is actively limited to the pericardial cells. A 5 bp mutation within the enhancer sequence allows for expression within the cardioblasts. Both heart cell types develop from the dorsal mesoderm and some share immediate progenitors. By stage 13, Him is pericardial cell specific and Mef2 is cardioblast specific. This is essential for normal heart development. If Him is not excluded from the cardioblasts, expression of the muscle-cell specific differentiation gene myosin is disrupted, similar to what has been described for Mef2 null mutants. If Mef2 is expressed in pericardial cells, the larval development of the pericardial cells is severely disturbed. A possible explanation for these data is that Him is part of a genetic program that prevents the premature differentiation of heart cells and its down-regulation permits the pericardial cells to undergo their correct development.