Characterization of pathways interacting with TGF-β; signaling during dorsal closure in Drosophila

Dorsal closure (DC) is the sealing of a hole in the dorsal epidermis during Drosophila embryogenesis that involves the coordinated morphogenesis of two tissues, and is a popular model system for the study of wound healing and developmental epithelial fusions. Several signaling pathways play critical roles in the regulation of DC, with the majority of research on these pathways being focused on the dorsal most epidermal (DME) cells of the closing epidermis. Recent results indicate that the tissue occupying the dorsal hole, the amnioserosa, plays an active role in DC, but little is known about how signaling in this tissue regulates DC. In this thesis, I demonstrate a requirement for the Transforming Growth Factor-b (TGF-β) pathway in the amnioserosa during DC. Through the loss-of-function analysis of TGF-β pathway components I demonstrate that the TGF-β pathway is required for cell shape change in the amnioserosa, at least in part by regulating expression of non-muscle myosin II heavy chain from the zipper (zip) gene. The zip gene is essential for DC and its expression is upregulated by TGF-β signaling in both the DME cells and amnioserosa to supply myosin for morphogenesis. My analysis indicates that TGF-β signaling is necessary but not sufficient for zip expression, prompting a search for additional inputs. My data demonstrate that the ACK family tyrosine kinases, DACK and PR2, effectors for the small GTPase Cdc42, cooperate with TGF-βsignaling to drive zip expression during DC. Based on my results, I have developed a model for the coordinated morphogenesis of the epidermis and the amnioserosa in which ACK-dependent signaling from the amnioserosa, together with the TGF-β ligand Dpp secreted from the DME cells of the epidermis, ensure coordinated cell shape in these two tissues through regulation of zip. Furthermore, I have identified the hormone ecdysone and EGFR signaling as candidate participants in ACK-dependent signaling regulating zip expression. EGFR signaling additionally impacts on TGF-β signaling during DC by regulating expression of Dpp in the epidermis, but how this is achieved need to be further investigated.