The vinculin/sarcomeric-α-actinin/α-actin nexus in cultured cardiac myocytes

Abstract

Experiments are described supporting the proposition that the assembly of stress fibers in non-muscle cells and the assembly of myofibrils in cardiac cells share conserved mechanisms. Double staining with a battery of labeled antibodies against membrane-associated proteins, myofibrillar proteins, and stress fiber proteins reveals the following: (a) dissociated, cultured cardiac myocytes reconstitute intercalated discs consisting of adherens junctions (AJs) and desmosomes at sites of cell-cell contact and sub-sarcolemmal adhesion plaques (SAPs) at sites of cell-substrate contact; (b) each AJ or SAP associates proximally with a striated myofibril, and conversely every striated myofibril is capped at either end by an AJ or a SAP; (C) the invariant association between a given myofibril and its SAP is especially prominent at the earliest stages of myofibrillogenesis; nascent myofibrils are capped by oppositely oriented SAPs; (d) the insertion of nascent myofibrils into AJs or into SAPs invariably involves vinculin, α-actin, and sarcomeric α-actinin (s-α-actinin); (e) AJs are positive for A-CAM but negative for talin and integrin; SAPs lack A-CAM but are positive for talin and integrin; (f) in cardiac cells all α-actinin-containing structures invariably are positive for the sarcomeric isoform, α-actin and related sarcomeric proteins; they lack non-s - α-actinin, γ-actin, and caldesmon; (g) in fibroblasts all α-actinin-containing structures are positive for the non-sarcomeric isoform, γ-actin, and related non-sarcomeric proteins, including caldesmon; and (h) myocytes differ from all other types of adherent cultured cells in that they do not assemble authentic stress fibers; instead they assemble stress fiber-like structures of linearly aligned I-Z-I-like complexes consisting exclusively of sarcomeric proteins.