Expression of α-Smooth Muscle Actin Determines the Fate of Mesenchymal Stromal Cells

Pro-fibrotic microenvironments of scars and tumors characterized by increased stiffness stimulate mesenchymal stromal cells (MSCs) to express alpha-smooth muscle actin (alpha-SMA). We investigated whether incorporation of a-SMA into contractile stress fibers regulates human MSC fate. Sorted alpha-SMA-positive MSCs exhibited high contractile activity, low clonogenicity, and differentiation potential limited to osteogenesis. Knockdown of alpha-SMA was sufficient to restore clonogenicity and adipogenesis in MSCs. Conversely, a-SMA overexpression induced YAP translocation to the nucleus and reduced the high clonogenicity and adipogenic potential of alpha-SMA-negative MSCs. Inhibition of YAP rescued the decreased adipogenic differentiation potential induced by alpha-SMA, establishing a mechanistic link between matrix stiffness, alpha-SMA, YAP, and MSC differentiation. Consistent with in vitro findings, nuclear localization of YAP was positively correlated in a-SMA expressing stromal cells of adiposarcoma and osteosarcoma. We propose that a-SMA mediated contraction plays a critical role in mechanically regulating MSC fate by controlling YAP/TAZ activation.