The functions of the GATA factor <italic>pannier</italic> and its partner <italic>u-shaped</italic> in specifying the <italic>Drosophila</italic> heart.

Abstract

In <italic>Drosophila</italic>, heart progenitors arise from bilaterally symmetric positions in the dorsal mesoderm which migrate toward the dorsal midline and fuse to form a linear heart. The initial specification of the cardiac precursors requires the inductive signaling of Dpp and Wingless, which emanate from the dorsal ectoderm. The transfer of this signaling information from the ectoderm to the underlying mesoderm induces cardiac-specific differentiation in the presence of Tinman, a mesoderm-specific homeobox transcription factor. In this thesis, I present evidence that the GATA transcription factor, Pannier, and its binding partner U-shaped, cooperate in the process of heart development. <italic> pannier</italic> provides an essential function in the mesoderm for the initiation of cardiac-specific <italic>tinman</italic> expression and for the specification of the heart primordium. <italic>u-shaped</italic> also promotes heart development, but unlike <italic>pannier</italic>, only by maintaining <italic>tinman</italic> expression in the cardiogenic region. By contrast, pan-mesodermal overexpression of <italic>pannier</italic> ectopically expands <italic>tinman</italic> expression, whereas overexpression of <italic>u-shaped</italic> inhibits cardiogenesis, suggesting that <italic>u-shaped</italic> plays a dual role in heart development. Manipulation of <italic>pannier</italic> activity in either the ectoderm or the mesoderm affects cardiac specification, indicating that its function is required in both germlayers. I find that ectodermal <italic>pannier</italic> function is required for the maintenance of both Dpp and Wingless signaling activities. I also find that <italic>pannier</italic> and <italic>tinman</italic> together can significantly induce cardiac specification, however this synergistic induction can only be observed in the mesoderm. Synergistic induction by <italic> pannier</italic> and <italic>tinman</italic> is also abolished with co-expression of <italic>u-shaped</italic>, which further supports its dual role in cardiogenesis. <italic> pannier, tinman</italic> and <italic>u-shaped</italic> have been shown to interact genetically during <italic>Drosophila</italic> heart formation, however it is not known if this involves physical interactions. I utilized a bimolecular fluorescence complementation technique in order to visualize protein-protein interactions <italic>in vivo</italic> to address whether protein interactions are indeed required during cardiogenesis. After visualizing protein interactions in a cell culture system, I generated transgenic fly lines, which could be expressed in specific patterns throughout the embryo. The utilization of this complementation assay could help elucidate the precise regulatory networks involved in making a <italic>Drosophila</italic> heart.