The role of talin, a master regulator of integrin-dependent morphogenesis, in the Drosophila embryo.

The morphogenesis and maintenance of three-dimensional tissue architecture requires intricate, coordinated regulation of cell shape, position, and proliferation. Cell adhesion molecules, which mediate the attachment of cells both to one another (Cell-Cell adhesion) and to their surrounding extra-cellular matrix (Cell-ECM adhesion), are central regulators of morphogenesis during development and homeostasis. Integrins are the major family of cell- ECM adhesion receptors in metazoans and connect between ECM ligands and the actin cytoskeleton via an intracellular integrin adhesion complex (IAC). Integrins contribute to an impressive range of cellular processes and tissue behaviours, but the molecular mechanisms underlying diverse integrin function are not well described. In this thesis, I aim to shed light on this matter via a structure/function-based analysis of the core IAC component, Talin, in the context of Drosophila melanogaster embryogenesis. Talin forms a direct structural link between integrins and the actin cytoskeleton and has been implicated as a regulator of integrin affinity for ECM ligands in the context of single cells in culture. My analysis reveals that in the context of a developing, intact organism, the principal functions of talin are to control IAC assembly, dynamics, and linkage to the cytoskeleton. Using a unique combination of forward and reverse genetics, live imaging, and immunofluorescence, I have uncovered novel insights into the role of talin as a regulator of integrin-mediated adhesion in vivo. I show that different domains of talin contribute to fine- tuning of integrin function in a tissue- and process-specific manner. The specific findings of this work are as follows: (1) two structurally distinct interactions between integrin and talin confer a developmentally regulated switch between modes of transient and stable adhesion (2) autoinhibitory control of talin provides a way to downregulate adhesion during morphogenesis ii and (3) the talin FERM domain primarily confers its effect on integrins through promoting receptor clustering and adhesion scaffolding rather than acting to increase integrin ligand affinity. In general, my work demonstrates how the interactions that talin makes within the integrin adhesion complex has major consequences for integrin function and thus for developmental control of tissue formation and maintenance.Medicine, Faculty ofGraduat