Neural regulation of alpha-dystroglycan biosynthesis and glycosylation in skeletal muscle

alpha-Dystroglycan (alpha-DG) is part of a complex of cell surface proteins linked to dystrophin or utrophin, which is distributed over the myofiber s urface and is concentrated at neuromuscular junctions, In laminin overlays of muscle extracts from developing chick hindlimb muscle, alpha-DG first ap pears at embryonic day (E) 10 with an apparent molecular mass of 120 kDa, B y E15 it is replaced by smaller (similar to 100 kDa) and larger (similar to 150 kDa) isoforms, The larger form increases in amount and in molecular ma ss (>200 kDa) as the muscle is innervated and the postsynaptic membrane dif ferentiates (E10-E20), and then decreases dramatically in amount after hatc hing. In myoblasts differentiating in culture the molecular mass of alpha-D G is not significantly increased by their replication, fusion, or different iation into myotubes. Monoclonal antibody IIH6, which recognizes a carbohyd rate epitope on alpha-DG, preferentially binds to the larger forms, suggest ing that the core protein is differentially glycosylated beginning at E16. Consistent with prior observations implicating the IIH6 epitope in laminin binding, the smaller forms of alpha-DG bind more weakly to laminin affinity columns than the larger ones. In blots of adult rat skeletal muscle probed with radiolabeled laminin or monoclonal antibody IIH6, alpha-DG appears as a >200-kDa band that decreases in molecular mass but increases in intensit y following denervation, Northern blots reveal a single mRNA transcript, in dicating that the reduction in molecular mass of alpha-DG after denervation is not obviously a result of alternative splicing but is likely due to pos ttranslational modification of newly synthesized molecules. The regulation of alpha-DG by the nerve and its increased affinity for laminin suggest tha t glycosylation of this protein may be important in myofiber-basement membr ane interactions during development and after denervation.