Actomyosin Contractility is a Potent Suppressor of Mesoderm Induction by Human Pluripotent Stem Cells

The activation of WNT signaling in human pluripotent stem cells drives efficient conversion to lateral mesoderm, which can be further differentiated into cardiomyocytes. Stabilization of the WNT effector β-catenin promotes expression of mesoderm-specifying genes such as Brachyury (encoded by TBXT ) and Eomesodermin and drives an epithelial-mesenchymal transition (EMT) that occurs at 49-52 hrs post WNT activation. Interestingly, WNT activation also triggers an early contraction of stem cell colonies, with increased myosin light chain (MLC) phosphorylation, resulting in cellular morphological changes. Small molecule inhibitors and genetic approaches were applied to determine if contractility is related to mesoderm differentiation. Contrary to expectation, suppressing contractility accelerated differentiation; Brachyury induction was enhanced, and EMT initiated as early as 24 hrs post WNT activation. Conversely, expression of constitutively active Rho kinase or myosin light chain kinase, to increase contractility, efficiently blocked any induction of mesoderm, and cells fail to undergo EMT. Mechanistically, reduced contractility promoted active β-catenin levels in the cytoplasm and nucleus. We propose that the early contraction of human pluripotent stem cell colonies in response to WNT activation initially delays mesoderm induction by restraining accumulation of active β-catenin, suggesting a role for contractility as a temporal gate.