Formation of the Drosophila salivary gland

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The Drosophila salivary gland (SG) is an ideal model for revealing the molecular and cellular events underlying formation and physiological specialization of epithelial tubular organs, such as the lungs, kidneys, and secretory glands of humans. The SG is a simple tubular organ that forms using the same morphogenetic changes as more complicated organs of higher animals, including changes in cell shape, adhesion and movement. The SG is also the largest secretory organ in the embryo providing an ideal model for how cells achieve high-level secretory capacity and how changes in capacity are coordinated with the expression of secretory content. We have discovered four key transcription factors that play major roles in the morphogenesis and physiological specialization of the SG, and we have identified many/most of their transcriptional targets using genome-wide approaches. In this proposal, we explore how these proteins function both independently and as part of larger complexes to regulate distinct aspects of epithelial tube development. In Specific aim #1, we use genome-wide in vivo DNA binding assays to test our model that the levels of expression of SG specific gene products is mediated through the coordinate binding of three key transcription factors – Fkh, the Drosophila FoxA orthologue, Sage, a less highly conserved bHLH protein expressed in only the SG, and Sens, a zinc-finger transcription factor whose SG expression requires Fkh and Sage. We ask if CrebA, a bZip transcription factor that increases secretory capacity, also boosts levels of SG target gene expression directly or indirectly. We will identify binding sites for each protein, both in WT SGs and in SGs mutant for each other transcription factor. The biological relevance of specific cis acting sites will be validated in a representative subset of known target genes. These studies will reveal if we have identified the major factors controlling SG gene expression, and tests mechanistic models of enhancer organization and function in a system where the key major players and their downstream targets are known and can be manipulated. In Specific aims #2 and #3, we focus on the Sage, Sens and CrebA – independent functions of Fkh in controlling formation of epithelial tubes. We have identified Fkh target genes that when mutant disrupt early stages of tube morphogenesis. We ask how the products of these early Fkh target genes interface with membrane and cytoskeletal proteins to coordinate changes in cell shape and arrangement during tube internalization. We further use the Fkh binding data from aim #1 to identify additional key morphogenetic regulators.