Genetic control of cardiac morphogenesis and function in <italic>Drosophila melanogaster</italic>.

Abstract

The <italic>Drosophila</italic> heart is a simple and well-defined organ, which consists of a limited number of cells. In <italic>Drosophila</italic> embryos, the heart progenitors arise from bilateral positions in the dorsal mesoderm through signals from the ectoderm. Following specification, the myocardial cells undergo a typical mesenchyme-epithelium transition and become aligned in two rows of epithelial cells that migrate dorsally to establish the heart lumen at the dorsal midline. Although the genetic requirements for the initial formation of the cardiac mesoderm and cardiac cell specification have been studied in some detail, little is known about the morphogenetic and functional processes by which the cardiac cells assemble into a linear heart tube and eventually pump the hemolymph properly. In this thesis, I exploit the advanced system in genetics of <italic>Drosophila</italic> to investigate the molecular-cellular mechanisms underlying these late events during heart development and maturation. I first investigated the role of the Tbx20 homologs, <italic>neuromancer 1 </italic> and <italic>2</italic> (<italic>nmr1&2</italic>), which are prominently expressed in the forming heart, in <italic>Drosophila</italic> cardiac specification and morphogenesis. I generated <italic>nmr1</italic> deletion mutants by P-element excision and combined them with <italic>nmr2 </italic>-RNAi transgenes as a way of eliminating both Tbx20 gene functions in the mesoderm. These <italic>nmr</italic> double mutants exhibit cardiac specification defects that likely reflect multiple requirements during heart development. In moderate <italic>nmr</italic> mutants a normal number of myocardial cells is generated but they misalign at the dorsal midline during heart tube assembly and exhibit severe polarity defects. Interestingly, I found similar alignment defects in <italic>slit</italic> as well as in <italic>robo</italic> (its receptor) mutants. Therefore, in the following chapter, I determined the requirement of <italic>slit/robo</italic> in cardiac morphogenesis and uncovered Slit and Robo proteins in conjunction with transcription factor Neuromancer and cell polarity molecules contribute significantly to <italic> Drosophila</italic> heart morphogenesis by guiding heart cell alignment and/or inhibiting cell mixing between the bilateral compartments of heart cell progenitors, and by maintaining proper polarity of the myocardial epithelium. Lastly, I explored novel roles of early transcription factors, <italic>tinman</italic> (<italic>tin</italic>), <italic>neuromancer</italic> and <italic> pannier</italic> (<italic>pnr</italic>), in adult cardiac function and physiology. Based on the genetic interaction between <italic>tin</italic> and <italic> nmr</italic>, I conducted a screen for mutations that alter the cardiac stress response of flies heterozygous for these cardiac transcription factor genes. The new factors I identified in this screen, i.e. small GTPase cdc42, may not only uncover new single gene candidates for human heart disease, but also uncover genetic traits that alter the susceptibility to the expression of the disease.