Mouse large can modify complex N- and mucin O-glycans on α-dystroglycan to induce laminin binding

The human LARGE gene encodes a protein with two putative glycosyltransferase domains and is required for the generation of functional α-dystroglycan (α-DG). Monoclonal antibodies IIH6 and VIA4-1 recognize the functional glycan epitopes of α-DG that are necessary for binding to laminin and other ligands. Overexpression of full-length mouse Large generated functionally glycosylated α-DG in Pro^-5 Chinese hamster ovary (CHO) cells, and the amount was increased by co-expression of protein:O-mannosyl N-acetylglucosaminyltransferase 1. However, functional α-DG represented only a small fraction of the α-DG synthesized by CHO cells or expressed from an α-DG construct. To identify features of the glycan epitopes induced by Large, the production of functionally glycosylated α-DG was investigated in several CHO glycosylation mutants. Mutants with defective transfer of sialic acid (Lec2), galactose (Lec8), or fucose (Lec13) to glycoconjugates, and the Lec15 mutant that cannot synthesize O-mannose glycans, all produced functionally glycosylated α-DG upon overexpression of Large. Laminin binding and the α-DG glycan epitopes were enhanced in Lec2 and Lec8 cells. In Lec15 cells, functional α-DG was increased by co-expression of core 2 N-acetylglucosaminyltransferase 1 with Large. Treatment with N-glycanase markedly reduced functionally glycosylated α-DG in Lec2 and Lec8 cells. The combined data provide evidence that Large does not transfer to Gal, Fuc, or sialic acid on α-DG nor induce the transfer of these sugars to α-DG. In addition, the data suggest that human LARGE may restore functional α-DG to muscle cells from patients with defective synthesis of O-mannose glycans via the modification of N-glycans and/or mucin O-glycans on α-DG.