Agrin-induced acetylcholine receptor clustering is mediated by the small guanosine triphosphatases Rac and Cdc42

During neuromuscular junction formation, agrin secreted from motor neurons causes muscle cell surface acetylcholine receptors (AChRs) to cluster at sy naptic sites by mechanisms that are insufficiently understood. The Rho fami ly of small guanosine triphosphatases (GTPases), including Rac and Cdc42, c an mediate focal reorganization of the cell periphery in response to extrac ellular signals. Here, we investigated the role of Rac and Cdc42 in couplin g agrin signaling to AChR clustering. We found that agrin causes marked mus cle-specific activation of Rac and Cdc42 in differentiated myotubes, as det ected by biochemical measurements. Moreover, this activation is crucial for AChR clustering, since the expression of dominant interfering mutants of e ither Rac or Cdc42 in myotubes blocks agrin-induced AChR clustering. In con trast, constitutively active Rac and Cdc42 mutants cause AChR to aggregate in the absence of agrin. By indicating that agrin-dependent activation of R ac and Cdc42 constitutes a critical step in the signaling pathway leading t o AChR clustering, these findings suggest a novel role for these Rho-GTPase s: the coupling of neuronal signaling to a key step in neuromuscular synapt ogenesis.