Multiple functions for the catenin family member plakoglobin in cadherin-dependent adhesion, fibronectin matrix assembly and Xenopus gastrulation movements

Shaping an embryo requires tissue-scale cell rearrangements known as morphogenetic events. These force-dependent processes require cells to adhere to their neighbors, through cadherin-catenin complexes, and to their extracellular matrix substrates, through integrin-based focal contacts. Integrin receptors are not only important for attachment to the extracellular matrix, but also for its fibrillar assembly. Fibrillogenesis requires actomyosin contractility, regulated in part by cadherin-catenin complexes. One such catenin, plakoglobin, mediates the attachment of actin stress fibers to cadherin cytoplasmic tails through its interactions with actin-binding proteins. In Xenopus gastrulae, plakoglobin has been identified as an essential member in the force-induced collective migration of the mesendoderm tissue. In the current study, we have further characterized the role of plakoglobin in two additional morphogenetic processes, epiboly and convergent extension. Plakoglobin-deficient tadpoles are 40% shorter and gastrulae contain notochords that are 60% wider than stage-matched controls, indicating convergent extension defects. The radially intercalating ectoderm of morphant animal caps is nearly twice as thick as controls. Furthermore, morphant embryos exhibit a failure to assemble a fibronectin matrix at the notochord-somite-boundary or along the blastocoel roof. The loss of the fibronectin matrix, while not due to changes in overall patterning, is a result of a failure to assemble the soluble dimers into long fibrils. The force of attachment to a cadherin or fibronectin substrate is reduced in plakoglobin morphants, indicating defects in adhesion to both cadherin and fibronectin. These data suggest that plakoglobin regulates morphogenesis and fibronectin assembly through cell-cell and cell-matrix adhesion.