Distribution of α-Dystroglycan during Embryonic Nerve-Muscle Synaptogenesis

The distribution of α-dystroglycan (αDG) relative to acetylcholine receptors (AChRs) and neural agrin was examined by immunofluorescent staining with mAb IIH6 in cultures of nerve and muscle cells derived from Xenopus embryos. In Western blots probed with mAb IIH6, αDG was evident in membrane extracts of Xenopus muscle but not brain. αDG immunofluorescence was present at virtually all synaptic clusters of AChRs and neural agrin. Even microclusters of AChRs and agrin at synapses no older than 1-2 h (the earliest examined) had αDG associated with them. αDG was also colocalized at the submicrometer level with AChRs at nonsynaptic clusters that have little or no agrin. The number of large (>4 μm) nonsynaptic clusters of αDG, like the number of large nonsynaptic clusters of AChRs, was much lower on innervated than on noninnervated cells. When mAb IIH6 was included in the culture medium, the large nonsynaptic clusters appeared fragmented and less compact, but the accumulation of agrin and AChRs along nerve-muscle contacts was not prevented. It is concluded that during nerve-muscle synaptogenesis, αDG undergoes the same nerve-induced changes in distribution as AChRs. We propose a diffusion trap model in which the αDG-transmembrane complex participates in the anchoring and recruitment of AChRs and αDG during the formation of synaptic as well as nonsynaptic AChR clusters.