UCSF

The influence of the biophysical environment in known models of cell patterning is an area of growing interest to determine how to properly control cell fate and recapitulate tissue organization. This dissertation uses BMP4-induced human embryonic stem cells (hESCs) on gels of defined soft stiffness as a model of gastrulation-stage embryos. Additionally, to probe how mechanical cues affect the regulation of cell fate, traction force microscopy is leveraged to measure stress distributions in tissues. The findings here described indicate that areas of higher tension correlate with early mesoderm differentiation, emphasizing a critical role for tissue architecture in early development. The elevated tension promotes the release of beta-catenin from adhesion junctions and further Wnt signaling through a force-dependent conformational exposure of the tyrosine 654 (Y654) of cadherin-bound beta-catenin. By using a dephosphomimetic mutant for beta-catenin (Y654F), expression of the early mesodermal marker Brachyury (T) is severely affected, indicating a mechanism whereby high cell-cell tension initiates and spatially restrict a release of beta-catenin from junctions to promote mesoderm specification. Furthermore, in order to understand how the mesodermal T signal becomes spatially restricted, this dissertation explored an underappreciated aspect of developmental systems, programmed cell death (PCD). The findings indicate that not only apoptosis may play a mechanical role in equilibrating tissue-level forces, but also that apoptotic cell recognition may play an inhibitory role in the expression of the mesoderm marker T and regulate the boundary of its signal through the phagocytic receptor MERTK. These results provide a framework to understand how mechanical cues and PCD can work in tandem to regulate the self-organization of tissues in developmental processes, such as gastrulation.