Regulation of smooth muscle actin expression and contraction in adult human mesenchymal stem cells.

Prior studies have demonstrated the expression of a contractile actin isoform, alpha-smooth muscle actin, in bone marrow stromal cells. One objective of the current study was to correlate contractility with alpha-smooth muscle actin expression in human bone marrow stroma-derived mesenchymal stem cells. A second objective was to determine the effects of transforming growth factor-beta1, platelet derived growth factor-BB, and a microfilament-modifying agent on alpha-smooth muscle actin expression and alpha-smooth muscle actin-enabled contraction. Adult human bone marrow stromal cells were passaged in monolayer and their inducibility to chondrocytic, osteoblastic, and adipogenic phenotypes was demonstrated. Western blot analysis was employed along with densitometry to quantify the alpha-smooth muscle actin content of the cells and their contractility was evaluated by their contraction of a type I collagen-glycosaminoglycan sponge-like matrix into which they were seeded. Transforming growth factor-beta1 (1 ng/ml) significantly increased and platelet-derived growth factor-BB (10 ng/ml) decreased alpha-smooth muscle actin expression and the contractility of the cells. Cytochalasin D also blocked cell contraction. There was a notably high correlation of cell-mediated contraction normalized to the DNA content of the matrices with alpha-smooth muscle actin content of the cells by linear regression analysis (R(2) = 0.88). These findings lay the groundwork for considering the role of alpha-smooth muscle actin-enabled contraction in mesenchymal stem cells and in their connective tissue cell progeny.