Heterogeneous Distribution of Isoactins in Cultured Vascular Smooth Muscle Cells Does Not Reflect Segregation of Contractile and Cytoskeletal Domains

Abstract

We have previously demonstrated that α-smooth muscle (α-SM) actin is predominantly distributed in the central region and β-non-muscle (β-NM) actin in the periphery of cultured rabbit aortic smooth muscle cells (SMCs). To determine whether this reflects a special form of segregation of contractile and cytoskeletal components in SMCs, this study systematically investigated the distribution relationship of structural proteins using high-resolution confocal laser scanning fluorescent microscopy. Not only isoactins but also smooth muscle myosin heavy chain, α-actinin, vinculin, and vimentin were heterogeneously distributed in the cultured SMCs. The predominant distribution of β-NM actin in the cell periphery was associated with densely distributed vinculin plaques and disrupted or striated myosin and α-actinin aggregates, which may reflect a process of stress fiber assembly during cell spreading and focal adhesion formation. The high-level labeling of α-SM actin in the central portion of stress fibers was related to continuous myosin and punctate α-actinin distribution, which may represent the maturation of the fibrillar structures. The findings also suggest that the stress fibers, in which actin and myosin filaments organize into sarcomere-like units with α-actinin-rich dense bodies analogous to Z-lines, are the contractile structures of cultured SMCs that link to the network of vimentin-containing intermediate filaments through the dense bodies and dense plaques.