Characterization of membrane protein complex associated with dystrophin

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In the present study, we have investigated the biological functions of the dystroglycan complex in the nervous system. As results, we show that the nervous system alpha-dystroglycan has a molecular mass of 120 kDa instead of 15kDa of that of muscle. The nervous system alpha-dystroglycan binds laminin with high affinity and this binding is dependent on calcium and is inhibited by heparin and NaCl. The nervous system alpha-dystroglycan is presumed to be associated with Dp116 and utrophin as anchoring proteins, but not the sarcoglycan complex. We also show that 120 kDa alpha-dystroglycan is a Schwann cell receptor of laminin-2, the endoneurial isoform of laminin comprised of the alpha2, beta1 and gamma1 chains, in the peripheral nervous system. Schwann cellalpha-dystroglycan is also a receptor of agrin, an acetylcholine receptor-aggregating molecule having partial homology to laminin alpha chains in the C-terminus. Immunochemical analysis demonstrates that the peripheral nerve isoform of agrin is a 400 kDa component of the endoneurial basal lamina and is co-localized with alpha-dystroglycan surrounding the outermost layr of myelin sheath of peripheral nerve fibers. Blot overlay analysis demonstrates that both endogenous peripheral nerve agrin and laminin-2 bind to Schwann cell alpha-dystroglycan. Recombinant C-terminal fragment of the peripheral nerve isoform of agrin also binds to Schwann cell alpha-dystroglycan, confirming that the binding site for Schwann cell alpha-dystroglycan resides in the C-terminus of agrinmolecule. Furthermore, the binding of recombinant agrin C-terminal fragment to Schwann cell alpha-dystroglycan competes with that of laminin-2. All together, these results indicate that alpha-dystroglycan is a dual receptor for agrin and laminin-2 in the Schwann cell membrane. Currently, we are investigating the biological functions of the dystroglycan complex in the central nervous system.