Mechanosensitive Adhesive Networks Guide Collective Migration of Xenopus Mesendoderm

Developing embryos undergo dramatic cell and tissue rearrangements that are required for morphogenesis. These movements have long been thought to be governed by chemical signals encoded by genetic information. However, it has recently become clear that developing tissues also generate forces that are transmitted throughout the tissue, sensed by cells, and transduced into chemical guidance cues. Physical forces may thus act analogously to morphogens, working in a “concentration-dependent” manner and in conjunction with chemical signals to establish the shape and pattern of an embryo. In this dissertation, I explore the mechanisms by which cells sense, integrate, and respond to forces on cadherin- and integrin-based adhesion complexes, and the manner in which the resultant signals guide collective cell migration.
During gastrulation of the Xenopus embryo, traction forces are generated by mesendoderm cells as they migrate collectively across an extracellular matrix (ECM) composed of fibronectin. Tissue cohesion is maintained by cadherin-based cell-cell adhesions that resist and balance the traction forces generated by migration. I establish here that anisotropic tension on cadherin adhesions leads to the assembly of a mechanosensitive cadherin adhesion complex containing the intermediate filament protein keratin and the catenin-family protein plakoglobin. The spatial organization of keratin and plakoglobin is dependent on tension at cell-cell adhesions and adhesion to fibronectin. Furthermore, both keratin and plakoglobin are required for force-induced polarization of protrusions in mesendoderm cells. I demonstrate that the integrin associated protein focal adhesion kinase (FAK) is required for multiple morphogenetic events occurring in the early development of the Xenopus embryo. FAK is required for protrusive polarity of migrating mesendoderm cells, organization of keratin filaments and association between plakoglobin and cadherins. I propose that FAK acts by modulating the dynamic balance of traction forces and cell cohesion necessary for polarity within the collectively migrating tissue.
Although mechanical stimuli have emerged as key regulators of development and of pathologic events such as the metastatic invasion of tumor cells, the molecular mechanisms by which forces are sensed are not well understood. I describe here a mass spectrometry based screen designed to discover proteins involved in mechanotransduction by identifying proteins that undergo a conformational change in response to force on cell-cell or cell-ECM adhesions. Initial findings in this screen include proteins involved in the citric acid cycle and glycolysis, suggesting that tension on adhesion complexes may modulate energy production in the cell. These studies now set the stage for continued investigation into the components and mechanisms of adhesion-dependent mechanotransduction. Together the data presented here indicate that regulation of cell polarity in mesendoderm by anisotropic tension generated via the balance of tractive and cohesive forces represents an emergent property of this collectively migrating tissue.