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

Abstract

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 α5 and α6 chains, however their distribution patterns and functions in human skeletal muscle have not been thoroughly investigated yet. By means of immunofluorescence analysis, the α6 chain was detected in the endomysium and perimysium, while the α5 chain labeling was restricted to the myotendinous junctions. In normal muscle cultures, the α6 chain was present in traces in the ECM, while the α5 chain was not detected. In the absence of ascorbic acid, the α6 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 α-smooth muscle actin. TGF-β1 treatment, a pro-fibrotic factor which induces trans-differentiation of fibroblasts into myofibroblasts, increased the α6 chain deposition in the extracellular matrix after addition of ascorbic acid. In order to define the involvement of the α6 chain in muscle fibrosis we studied biopsies of patients affected by Duchenne Muscular Dystrophy (DMD). We found that the α6 chain was dramatically up-regulated in fibrotic areas where, in contrast, the α5 chain was undetectable. Our results show a restricted and differential distribution of the novel α6 and α5 chains in skeletal muscle when compared to the widely distributed, homologous α3 chain, suggesting that these new chains may play specific roles in specialized ECM structures. While the α5 chain may have a specialized function in tissue areas subjected to tensile stress, the α6 chain appears implicated in ECM remodeling during muscle fibrosis.

Col6a1 Null Mice as a Model to Study Skin Phenotypes in Patients with Collagen VI Related Myopathies: Expression of Classical and Novel Collagen VI Variants during Wound Healing

“…Indeed, using the bleomycin model of skin fibrosis, we also found up-regulation of the α3 chain in the dermis and of the α5 and α6 chains in blood vessels, indicating that collagen VI is generally up-regulated in fibrotic tissue. Similarly, the α6 chain is expressed at higher levels in fibrotic muscle of Duchenne muscular dystrophy patients [28]. Interestingly, it was recently shown that a proteolytic fragment of collagen VI α1 chain is significantly elevated in the serum of patients with chronic obstructive pulmonary disease or idiopathic pulmonary fibrosis [29] and in a rat model of liver fibrosis [30].…”

The expanded collagen VI family: new chains and new questions

“…COL6A6 mRNA is present in a wide range of fetal and adult tissues in mouse and human (7,8). Several recent papers have described the expression of the three new chains in mouse (22), and human skin and muscle (29,32). In skeletal muscle, a restricted and differential distribution of the α6(VI) chain was found, which is presumably connected with specific functions of the new chain in specialized ECM structures.…”

“…In human skeletal muscle the staining pattern is similar. Like the α3 chain, α6 is expressed in both the perimysium and endomysium with a discontinuous pattern around muscle fibers (32). While α3 is clearly part of the basement membrane, α6 appears to be a component of the interstitial tissue between adjacent fibers and absent from the basement membrane though there are some areas of overlapping expression.…”

“…While α3 is clearly part of the basement membrane, α6 appears to be a component of the interstitial tissue between adjacent fibers and absent from the basement membrane though there are some areas of overlapping expression. Since α1, α2 and α3 chains were found in basement membrane structures in mouse skeletal muscle by electron microscopy, the authors speculate that α1α2α3 heterotrimers represent the basement membrane form of collagen VI and that this form of collagen VI plays a critical role in basement membrane integrity (32). The α5(VI) chain in contrast, is present at the myotendinous junction and co-localizes with laminin 211, a basement membrane marker.…”

“…The α5(VI) chain in contrast, is present at the myotendinous junction and co-localizes with laminin 211, a basement membrane marker. α6(VI) was upregulated in skeletal muscle of patients with Duchenne muscular dystrophy ( DMD ) gene mutations and associated with increased fibrosis (32). …”