A comparative freeze-fracture study of plasma membrane of dystrophic skeletal muscles in dy/dy mice with merosin (laminin 2) deficiency and mdx mice with dystrophin deficiency.

The intramembranous particle (IMP), orthogonal array (OA) and orthogonal array subunit particle (OASP) densities of skeletal muscle plasma membranes of merosin deficient dy/dy mice and their control mice at 7, 14 and 28 days after birth were analysed by freeze-fracture electron microscopy. Similar studies were performed on dystrophin-deficient mdx mice with mild muscle weakness at 28 days after birth for the comparison with those of dy/dy mice with severe muscle weakness at the same age. In the pre-clinical stage of dy/dy mice at 14 days after birth, the membranes showed a significantly decreased density of OAs (P<0.01 by Wilcoxon rank-sum test) as compared with control mice, while those in the clinical stage of dy/dy mice at 28 days after birth showed normal IMP density but a marked depletion of OA density (P<0.01). Moreover, at 28 days after birth, the reduction of OAs in the plasma membranes of dy/dy mice was more marked than that of mdx mice (P<0.05 by Wilcoxon rank-sum test). These results provided us with the information that the OA density was affected more severely with merosin deficiency than with dystrophin deficiency, and again supported our previously proposed concept that the clinical severity in muscular dystrophies correlated with the OA density.