Integrin Trafficking to Focal Adhesions

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ABSTRACT The ability of cells to assemble, adhere to and dynamically sense the extracellular matrix (ECM) is essential for multicellular life. Integrins, a family of heterodimeric αβ transmembrane adhesion receptors, bind specific ECM ligands via their ectodomains and permit bidirectional communication vital for cell adhesion, migration, differentiation, and survival. Proper integrin function is paramount for tissue morphogenesis, and is perturbed in cancer, skin disorders, musculoskeletal, cardiovascular and inflammatory diseases. A major site of integrin- matrix engagement is in dynamic micron-sized signaling platforms, called focal adhesions (FA). Integrins can laterally exchange into and out of FA, but also traffic to and from the cell surface and this trafficking influences cell migration, invasion and cancer metastasis. However, despite its importance, understanding at the cellular and mechanistic level of precisely where and how integrins undergo exocytic and endocytic traffic has been challenging due to difficulties directly visualizing integrin exo-endocytosis in live cells. In response to this challenge we recently generated `ecto-tagged' integrins containing the pH-sensitive fluorophore pHluorin or a chemical-genetic Halo-tag inserted into an extracellular loop. These ecto-tagged integrins provided the first direct views of integrin exocytosis and revealed that, contrary to initial expectations, integrin exocytosis occurs at a subset of FA. Drawing on our expertise in integrins (Calderwood) and live-cell imaging of membrane trafficking (Toomre), this multi-investigator R01 proposal seeks to test major new hypotheses arising from these results. In Aim 1 we test the hypothesis that integrins are selectively delivered to growing FA in a subunit-specific, ECM-regulated process. In Aim 2 we test the hypotheses that integrin endocytosis occurs at a distinct set of FA that are turning over and that endocytosed integrins are recycled to growing FA. Furthermore, we probe molecular mechanisms involved in recycling to FA. Finally, in Aim 3 we test the hypothesis that integrin-dependent fibronectin (FN) exocytosis also occurs at growing FA, while FN endocytosis occurs at FA that are turning over. Our experimental approaches combine novel ecto-tagged integrins and matrix constructs with new cleavable Halo dyes and pH-switching to follow exo-endocytosis in live cells so as to test new hypothesis about where integrin is delivered and the underlying mechanisms.