Myofibrillar and cytoskeletal assembly in neonatal rat cardiac myocytes cultured on laminin and collagen

Neonatal rat cardiomyocytes were cultured on extracellular matrix components laminin and collagens I+III to examine effects of extracellular matrix on the assembly of cytoskeletal proteins during myofibrillogenesis. Myofibril assembly was visualized by immunofluorescence of marker proteins for myofibrils (f-actin for I bands and α-actinin for Z bands), focal adhesions (vinculin), and transmembrane extracellular matrix receptors (β₁ integrin) as cells spread for various times in culture. By 4 h in culture, f-actin appeared organized into nonstriated stress-fiber-like structures while α-actinin, vinculin and β₁ integrin were localized in small streaks and beads. Subsequently, striated patterns were observed sequentially in the intracellular cytoskeletal components α-actinin, vinculin, f-actin, and then in the transmembrane β₁ integrin receptor. These data support an earlier model for sarcomerogenesis in which stress-fiber-like structures serve as initial scaffolds upon which α-actinin and then vinculin-containing costameres are assembled. This sequential and temporal assembly was the same on both laminin and collagens I+III. A quantitative difference, however, was apparent on the 2 matrices. There was an increased appearance on collagens I+III of rosettes (also called podosomes or cortical actin-containing bodies in other cells) which consisted of an f-actin core surrounded by α-actinin, vinculin and β₁ integrin rims. The increased incidence of rosettes in neonatal myocytes on collagens I+III suggests that these cytoskeletal complexes are involved in recognition and interaction with extracellular matrix components.