Expression of collagen VI α5 and α6 chains in human muscle and in Duchenne muscular dystrophy-related muscle fibrosis

Collagen VI is a major extracellular matrix (ECM) protein with a critical role in maintaining skeletal muscle functional integrity. Mutations in COL6A1, COL6A2 and COL6A3 genes cause Ullrich Congenital Muscular Dystrophy (UCMD), Bethlem Myopathy, and Myosclerosis. Moreover, Col6a1(-/-) mice and collagen VI deficient zebrafish display a myopathic phenotype. Recently, two additional collagen VI chains were identified in humans, the alpha 5 and alpha 6 chains, however their distribution patterns and functions in human skeletal muscle have not been thoroughly investigated yet. By means of immunofluorescence analysis, the alpha 6 chain was detected in the endomysium and perimysium, while the alpha 5 chain labeling was restricted to the myotendinous junctions. In normal muscle cultures, the alpha 6 chain was present in traces in the ECM, while the alpha 5 chain was not detected. In the absence of ascorbic acid, the alpha 6 chain was mainly accumulated into the cytoplasm of a sub-set of desmin negative cells, likely of interstitial origin, which can be considered myofibroblasts as they expressed alpha-smooth muscle actin. TGF-beta 1 treatment, a pro-fibrotic factor which induces trans-differentiation of fibroblasts into myofibroblasts, increased the alpha 6 chain deposition in the extracellular matrix after addition of ascorbic acid. In order to define the involvement of the alpha 6 chain in muscle fibrosis we studied biopsies of patients affected by Duchenne Muscular Dystrophy (DMD). We found that the alpha 6 chain was dramatically up-regulated in fibrotic areas where, in contrast, the alpha 5 chain was undetectable. Our results show a restricted and differential distribution of the novel alpha 6 and alpha 5 chains in skeletal muscle when compared to the widely distributed, homologous alpha 3 chain, suggesting that these new chains may play specific roles in specialized ECM structures. While the alpha 5 chain may have a specialized function in tissue areas subjected to tensile stress, the alpha 6 chain appears implicated in ECM remodeling during muscle fibrosis. (c) 2012 Elsevier B.V. All rights reserved.