Control of cell polarity and migration by non-centrosomal microtubules

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Project Summary/Abstract The coordinated and regulated remodeling of the actin and microtubule (MT) cytoskeleton is required for cell migration for developmental processes and homeostatic maintenance, as well as during the body’s response to external insults and disease states including heart disease and tumor metastasis. During cell migration, actin filaments assemble and become linked to focal adhesion (FA) complexes, while MTs undergo dynamic instability that is locally controlled by MT- associated proteins (MAPs). These two processes enable cells to establish a leading-edge and a trailing-edge, and to migrate with directional persistence. Despite many advances in our understanding of the functional implications of MAPs on MTs and MT-FA interactions, it remains unknown how exactly MT organization is spatially and temporally coordinated with FAs to promote directional cell movement. A recent discovery showing that non-centrosomal MTs are both sufficient and required to drive polarized cell migration has established a paradigm shift, suggesting that non- centrosomal MTs are primed to function in a way that is distinct from MTs nucleated by the centrosome. The finding underscores the need to determine how cytoskeletal proteins identify and regulate non-centrosomal versus centrosomal MT dynamics and effects on polarity and migration. This gap in knowledge impacts our understanding of fundamental processes, including how signaling molecules simultaneously regulate families of proteins to achieve complex tasks, such as guiding persistent cell migration. The small GTPase, Rac1, is a key signaling protein that is spatially controlled to promote FA formation, MT growth, and actin filament assembly, resulting in leading edge advance. Rac1 signaling is complemented by the molecular motor protein, myosin-II, which organizes actin stress fibers, promotes FA maturation, and generates forces that pull the trailing-edge of the cell forward. Thus, Rac1 and myosin-II are spatially and temporally controlled to drive directional cell movement. One targeted MT effector protein, MCAK, is locally inhibited by Rac1 to promote leading-edge MT growth and cell polarity, and MCAKs effects on MT dynamics are sensitive to myosin-II contractility. Despite this knowledge, how Rac1 and myosin-II contribute to the organization of FAs, MTs, and actin is not well understood. Preliminary evidence demonstrates that FA-associated MTs are predominantly of non-centrosomal origin and that Rac1 activity enhances the association of two different families of MAPs, CAMSAPs and septins, which increase non-centrosomal MT growth into FAs. Here, we will test the hypothesis that Rac1 and myosin-II promote the association of CAMSAP and septins with non-centrosomal MTs, which inhibits MCAK-mediated MT disassembly and drives MT-FA interactions. Our approach will incorporate a team of undergraduate researchers using fluorescence microscopy of live endothelial cells to determine: (1) how Rac1 and myosin-II regulate CAMSAP and septin interactions with non-centrosomal MTs, (2) how MCAK disassembly of non-centrosomal MTs controls MT dynamics and FA size, and (3) how septins promote MT growth into FAs. These investigations will provide critical advances to the field of cell migration by functionally linking Rac1 and myosin-II with cytoskeletal effector proteins that control non-centrosomal MT growth into FAs and the regulation of cell migration in health and disease. ! !