Increased laminin A expression in regenerating myofibers in neuromuscular disorders.

Laminin is a basement membrane (BM) glycoprotein composed of three of five subunits, the A, M, B1, B2, and the S chain. Four forms of laminin, A-B1-B2, A-S-B2, M-B1-B2, and M-S-B2, have been identified. Laminin is implicated in various biological processes such as cell adhesion and differentiation. We studied immunohistochemically the expression of the four laminin subunits A, M, B1, B2 as well as of neural cell adhesion molecule (N-CAM, CD56), a marker of regenerating myofibers, in various neuromuscular disorders. In normal muscle, the predominant subunits of myofiber laminin were M, B1, and B2. The A chain was only faintly expressed in myofiber BM. In inflammatory myopathies and dystrophinopathies myofiber laminin A expression was greatly increased. An average of 80% and 63% of laminin A-positive myofibers in inflammatory myopathies and dystrophinopathies, respectively, were additionally CD56 positive. Laminin A and CD56 expression in denervating diseases and mitochondrial myopathies were negligible. Expression of M, B1, and B2 subunits did not seem to be altered in the diseased conditions examined above. The data suggest that laminin A is upregulated in inflammatory myopathies and dystrophinopathies and, most markedly in regenerating myofibers.