LARGE glycans on dystroglycan function as a tunable matrix scaffold to prevent dystrophy

The dense glycan coat that surrounds every cell is essential for cellular development and physiological function(1), and it is becoming appreciated that its composition is highly dynamic. Post-translational addition of the polysaccharide repeating unit [-3-xylose-alpha 1,3-glucuronic acid-beta 1-](n) by like-acetylglucosaminyltransferase (LARGE) is required for the glycoprotein dystroglycan to function as a receptor for proteins in the extracellular matrix(2,3). Reductions in the amount of [-3-xylose-alpha 1,3-glucuronic acid-beta 1-](n) (hereafter referred to as LARGE-glycan) on dystroglycan result in heterogeneous forms of muscular dystrophy(4). However, neither patient nor mouse studies has revealed a clear correlation between glycosylation status and phenotype(5,6). This disparity can be attributed to our lack of knowledge of the cellular function of the LARGE-glycan repeat. Here we show that coordinated upregulation of Large and dystroglycan in differentiating mouse muscle facilitates rapid extension of LARGE-glycan repeat chains. Using synthesized LARGE-glycan repeats we show a direct correlation between LARGE-glycan extension and its binding capacity for extracellular matrix ligands. Blocking Large upregulation during muscle regeneration results in the synthesis of dystroglycan with minimal LARGE-glycan repeats in association with a less compact basement membrane, immature neuromuscular junctions and dysfunctional muscle predisposed to dystrophy. This was consistent with the finding that patients with increased clinical severity of disease have fewer LARGE-glycan repeats. Our results reveal that the LARGE-glycan of dystroglycan serves as a tunable extracellular matrix protein scaffold, the extension of which is required for normal skeletal muscle function.