Mechanistic role of forces in the regulation of morphogenetic movements during gastrulation

The coordinated cellular movements are crucial for the assembly and morphogenesis of tissue during embryonic development. These movements are governed by chemical and mechanical inputs from the surrounding microenvironment. Mechanical forces are generated and sensed at cell-cell and cell-extracellular matrix (ECM) junctions. Cytoskeletal linkages at these junctions are essential for transduction of mechanical inputs. These mechanical cues are essential for guiding directed migration during morphogenesis. In this dissertation, I have investigated the mechanism by which cells balance forces between cadherin-based cell-cell adhesions and integrin-based focal adhesions (FAs) during collective migration in Xenopus embryos.
Xenopus mesendoderm cells migrate collectively along a fibronectin substrate at gastrulation, but how the adhesive and mechanical forces required for these movements are generated and transmitted is unclear. I demonstrated that mesendoderm cells organize into leader and follower cells. The traction forces are limited primarily to leading edge cells in mesendoderm explants, and that these forces are balanced by intercellular stresses in follower rows. This is further reflected in the morphology of these cells, with broad lamellipodial protrusions, mature focal adhesions and a gradient of activated Rac1 evident at the leading edge, while small protrusions, rapid turnover of immature focal adhesions and lack of a Rac1 activity gradient characterize cells in following rows. I established that keratin intermediate filaments (IFs) are necessary for the maintenance of the organization of mesendoderm tissue and loss of keratin IFs results in high traction stresses in follower row cells, misdirected protrusions and the formation of actin stress fibers anchored in streak-like focal adhesions. I propose that maintenance of mechanical integrity in the mesendoderm by keratin intermediate filaments is required to balance forces within the tissue to regulate collective cell movements. I also show that cadherin-associated protein, plakoglobin (PG; also known as gamma-catenin) is also essential to proper mesendoderm migration. PG is required for regulating the dynamics of FAs, the organization of actin and keratin IF network during mesendoderm migration. I propose that PG acts by maintaining forces balance between cadherin and integrin adhesions required for polarization of mesendoderm cells during migration.
The data presented in this study focuses on understanding the role of forces in the regulation of morphogenetic processes. I have identified a unique role for keratin IFs and PG in maintaining crosstalk between cadherin and integrin adhesions. Taken together these findings indicate that achieving the balance of forces between cellular junctions is integral to tissue morphogenesis.