Smooth muscle actin isoforms: A tug of war between contraction and compliance

In higher vertebrates, smooth muscle (SM) contains two tissue-specific actin isoforms: alpha-SMA and gamma-SMA, which predominate in vascular and visceral SM, respectively. Whether alpha-SMA has been extensively studied and recognized for its contractile activity in SM and SM-like cells such as myofibroblasts, myoepithelial and myoid cells, the distribution and role of gamma-SMA remained largely unknown. We developed a new specific monoclonal antibody against gamma-SMA and confirmed that gamma-SMA predominates in the visceral system and is minor in the vascular system, although more expressed in highly compliant veins than in stiff arteries. Contrary to alpha-SMA, gamma-SMA is absent from myofibroblasts in vitro, and in fibrotic diseases in vivo. We raised the hypothesis that, whereas alpha-SMA is responsible for the "contractile" activity, gamma-SMA would be involved in the "compliance" of SM and SM-like cells. Several models support this hypothesis, namely veins vs. arteries and the physiological modifications occurring in the uterus and mammary glands during pregnancy and lactation. Our results suggest that, in addition to enteric smooth muscles, gamma-SMA is expressed in all the tissues submitted to an important dilation including veins, gravid uterus, and lactating mammary glands. The hypothesis of two complementary mechanical roles for the two SMA isoforms is sustained by their different intracellular distributions and by functional assays. (c) 2013 Elsevier GmbH. All rights reserved.