Background: The extracellular matrix (ECM) is a dynamically remodelled tissue that influences cell behaviour, provides structural integrity to cells and tissues and regulates growth factor bioavailability. Extracellular proteases such as the matrix metalloproteinases (MMP) and the A Disintegrin-like and Metalloproteinase domain with Thrombospondin-1 repeats (ADAMTS) remodel the ECM, altering its composition to reflect changes in physiology and pathology. Versican (Vcan), an ECM proteoglycan, is the major component of the primitive “transitional matrix” which typically requires remodelling by ADAMTS proteoglycanases, as exemplified in several ADAMTS transgenic mouse models. Reduced processing of versican in ADAMTS-knockout mice is strongly associated with congenital defects such as cleft palate, myxomatous heart valves and interdigital webbing.
Muscular dystrophies present with inadequate muscle fibre regeneration and excessive fibrosis or an accumulation of ECM. Dystrophic patients feature an accumulation of versican in their skeletal muscle ECM, compared to healthy controls. The role of ADAMTS-mediated versicanase activity during skeletal muscle development and regeneration is poorly defined, however Adamts5, the strongest versicanase in vitro, is strikingly co-expressed with Vcan in developing mouse skeletal muscle. Therefore, we hypothesised that ADAMTS proteolytic activity directed towards versican could mediate skeletal muscle development and regeneration.
Results: Adamts proteoglycanases and Vcan mRNA were significantly up-regulated during in vitro myogenesis whilst ADAMTS5 and cleaved versican were co-localised to the pericellular matrix of fusing myoblasts. Adamts5 silencing using targeted siRNA resulted in decreased myoblast fusion, impairing mature muscle fibre formation. Further, the addition of versican had an additive effect on Adamts5 siRNA-mediated inhibition of myoblast fusion. Finally, a particle exclusion assay showed that silencing of Adamts5 and thus reduced versican processing, resulted in ECM accumulation providing a mechanism for the observed inhibition of myoblast fusion. Our data suggest the clearance of versican by ADAMTS proteoglycanases is necessary for myoblast fusion, and may have implications for therapeutic development to ameliorate skeletal muscle repair in muscular dystrophy patients.