Cell-cell adhesions are fundamental to the integrity and multicellular character of the vertebrate organism. The cohesive interactions between cells are particularly important to developmental morphogenesis during vertebrate gastrulation, where dynamic gross tissue movements of large cellular populations occur. Movements of groups of cells, so-called collective cell migration, provide an essential mechanism for relocating whole tissues while maintaining tissue integrity. One such motile population in the gastrulating Xenopus frog embryo, the mesendoderm, migrates anteriorly to populate the presumptive head of the embryo. In mesendoderm, cell cohesion is provided by the classical cadherin protein C-cadherin. Through C-cadherin cohesions, these cells push and pull against one another as they migrate, creating dynamic physical cell-on-cell interactions. We have previously shown that applying mechanical tension specifically on C-cadherin-mediated cell-cell adhesions specifies directional protrusive behavior and migration of individual cells. This function of cadherins requires both their signaling and physical cohesive capabilities, and association with keratin intermediate filaments appears to be an important step in the mechanotransduction process. The functional mechanism of keratin in specifying migratory polarity induced by mechanical stimulation of C-cadherin remains unknown. In this project we will examine the overall hypothesis that mechanical stress on cadherin-mediated cell-cell adhesions alters keratin intermediate filament organization and consequential signaling through cytoskeletal regulatory proteins essential to cell polarity within migratory tissues. The first specific aim will establish regulation of Rac1, a central mediator of lamellipodial cell protrusions, as a mechanism for migratory polarity specification induced by mechanically stimulated C-cadherin adhesions. In specific aim 2 we will determine whether keratin intermediate filaments negatively regulate local cellular protrusive behavior through suppression of Rac1 activity. As part of this aim, we will develop novel molecular tools to selectively depolymerize intermediate filaments in a photoinducible, spatiotemporal-restricted manner. In specific aim 3 we will explore the role of cytoskeletal adaptor and crosslinking proteins, specifically plakophilin-3, in enabling force-induced C-cadherin-dependent keratin intermediate filament recruitment and signaling of migratory cell polarity. This R15 project grant will seek to involve students, both undergraduate and graduate, in a multidisciplinary research program that uniquely combines aspects of developmental biology, cell biology, and biomechanics. The use of Xenopus laevis, a classic developmental biology organism, will provide students with a readily accessible model that is easy to manipulate and yet a powerful system for answering questions of cell and developmental biology.