THE RELATIONSHIP BETWEEN PERLECAN AND DYSTROGLYCAN AND ITS IMPLICATION IN THE FORMATION OF THE NEUROMUSCULAR-JUNCTION

Perlecan is a major heparan-sulfate proteoglycan (HSPG) within the bas ement membrane surrounding skeletal muscle fibers. The C-terminus of i ts core protein contains three globular domain modules which are also found in laminin and agrin, two proteins that bind to dystroglycan (DG , cranin) on the muscle surface with these modules. In this study, we examined whether perlecan can also bind to DG and is involved in signa ling the formation of the neuromuscular junction (NMJ). By labeling cu ltured muscle cells with a polyclonal anti-perlecan antibody, this pro tein is found both within the extracellular matrix in a fibrillar netw ork and at the cell surface in a punctate pattern. In Xenopus muscle c ells, the cell-surface perlecan is precisely colocalized with DG. Both perlecan and DG are clustered at ACh receptor clusters induced by spi nal neurons or by beads coated with HB-GAM, a heparin-binding growth f actor. Blot overlay assays have shown that perlecan binds alpha-DG in a calcium and heparin-sensitive manner. Furthermore, perlecan is prese nt in muscle lysate immunoprecipitated with an anti-DG antibody. Immun olabeling also showed colocalization between HB-GAM and perlecan and b etween HB-GAM and DG. These data suggest that perlecan is anchored to muscle surface via DG-dystrophin complex. Since DG is also a site of a grin binding, the neural agrin secreted by motoneurons during NMJ form ation may compete with the pre-existing perlecan for cell surface bind ing. This competition may result in the presentation of perlecan-bound growth factors such as HB-GAM to effect synaptic induction.