A Novel Extracellular Protease-Mediated Mechanism Regulating FGF Signaling During Branching Morphogenesis

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Biomedical Genetics, 2016.Branching morphogenesis is the developmental process that underlies the formation of branched organs such as the lung, vasculature, kidney, prostate, pancreas, mammary gland, and salivary gland. During the branching process, growth factor ligands of the receptor tyrosine kinase family act as inductive signals to form new branches and control tip cell selection, and a lateral inhibition mechanism is required to restrict the tip domain and regulate the number of branches that form. Fibroblast growth factor (FGF) signaling is important for many developmental processes and regulates many of the cellular events driving branching morphogenesis such as cell proliferation, migration, and epithelial rearrangements. Consequently, FGF signaling regulates many types of branching morphogenesis such as the formation of the mammalian lung, prostate, pancreas, and salivary gland. Using the structural and genetic accessibility of the Drosophila tracheal system, we have identified an extracellular protease-mediated mechanism regulating FGF signaling during development and branching morphogenesis. We have shown that the Drosophila matrix metalloprotease Mmp2 cleaves the cell surface bound heparan sulfate proteoglycan Dally, which releases an N-terminal protein fragment, N-Dally, that acts as an inhibitor of FGF signaling in the neighboring target cells. This proteolytic event converts Dally from a cell surface bound FGF co-receptor to an extracellular diffusible inhibitor of FGF signaling. N-Dally inhibits FGF signaling by binding to Dally in the neighboring target cells and promoting the internalization and endocytosis of the co-receptor Dally-Btl/FGFR complex. Consistent with other examples of lateral inhibition, the cells that secrete the inhibitor N-Dally remain resistant to their own inhibition and continue to activate FGF signaling. The C-terminal fragment of Dally, C-Dally, which remains on the cell surface after Mmp2 cleaves Dally, retains the ability to act as an FGF co-receptor but is resistant to N-Dally inhibition. The mechanism we have identified regulating FGF signaling in the Drosophila tracheal system may give insight into understanding how development and morphogenesis occur in the more complex organ systems of vertebrates. FGF signaling is often deregulated resulting in developmental defects and cancer. N-Dally, characterized here as extracellular inhibitor of FGF signaling, might offer strategies to control FGF signaling in a therapeutic setting