Sandra G. Velleman1 and Macdonald Wick
The Ohio State University Department of Animal Sciences
Abstract
Skeletal muscle development is, in part, regulated by myoblast-extracellular matrix interactions mediated by the transmembrane integrin family of heterodimeric receptors. The avian genetic muscle weakness, Low Score Normal (LSN), exhibits modified myotube and sarcomere structure which may be associated with altered integrin expression. Protein expression of the β1 integrin subunit was measured during myogenic satellite cell proliferation and differentiation. In normal and LSN satellite cell cultures, β1 integrin expression was low during proliferation. In early differentiation, â1 integrin expression increased and was then downregulated. The LSN β1 integrin expression was significantly lower during differentiation. These results suggest that the decreased expression of β1 integrin during LSN myogenesis may be associated with modifications in muscle formation.
Introduction
Skeletal muscle differentiation is a complex process involving cell proliferation, differentiation, migration, cell-cell signaling, fusion of myoblasts, and subsequent development of myofibers. As muscle proceeds through this stage-specific development, the myogenic cells interact with the extracellular matrix environment. The extracellular matrix communicates information to the cell through cellular receptors, called integrins, which influence cell migration, adhesion, proliferation, and differentiation (Blaschuk and Holland, 1994; Boettiger et al., 1995; Chon et al., 1998).
The integrins are a widely expressed family of heterodimeric cell surface receptors containing β- and β-subunits linking the extracellular matrix to the cellular cytoskeleton. Integrins transduce biochemical signals from the extracellular matrix across the plasma membrane to modulate cellular function (Guan and Chen, 1996). The β1 family of integrin receptors is specific for several extracellular matrix molecules: e.g., Types I, II, and IV collagen, laminin, and fibronectin (Horwitz et al., 1985; Akiyama et al., 1989; Hall et al., 1990; Syfrig et al., 1991; Enomoto et al., 1993). Through the development of the β1 integrin cell substrate attachment (CSAT) monoclonal antibody, it was demonstrated that β1 containing integrins are important in the adhesion of muscle cells (Neff et al.,1982; Decker et al.,1984). Furthermore, muscle β1 integrins are also important in myoblast fusion (Rosen et al., 1992), and are located at sites of adhesion and migration (Jaffredo et al., 1988; Lakonishok et al., 1992), myotendon junctions (Bozyczko et al., 1989; Syfrig et al., 1991); and neuromuscular junctions (Bozyczko et al., 1989). Therefore, the precise developmental expression and localization is critical to normal muscle growth and development.
Transforming growth factor-beta (TGF-β) has been shown to be a regulator of β1 integrin subunit expression and may modulate the amount and distribution of αβ1 integrins (Heino et al., 1989). Modification of integrin subunit levels would likely affect the degree of cell adhesiveness to the extracellular matrix which would alter cell migration and other properties critical to muscle assembly. The chicken genetic muscle weakness, LSN, exhibits elevated RNA steady state levels of pectoral muscle TGF-β-1 and TGF-β-2 late in embryonic development (Velleman and Coy, 1998), and altered myotube morphology (Velleman and McFarland, 1999). These alterations in LSN TGF-β expression and myotube morphology are consistent with modified skeletal muscle assembly mediated by an integrin receptor pathway. Therefore, LSN may be a valuable model in furthering the understanding of how skeletal muscle differentiation affects integrin expression.