Role of lipid rafts in agrin-elicited acetylcholine receptor clustering

Emerging concepts of membrane organization point to the compartmentalization of the plasma membrane into distinct lipid microdomains. This lateral segregation within cellular membranes is based on cholesterol-sphingolipid-enriched microdomains or lipid rafts which can move laterally and assemble into large-scale domains to create plasma membrane specialized cellular structures at specific cell locations. Such domains are likely involved in the genesis of the postsynaptic specialization at the neuromuscular junction, which requires the accumulation of acetylcholine receptors (AChRs), through activation of the muscle specific kinase MuSK by the neurotropic factor agrin and the reorganization of the actin cytoskeleton. We used C2C12 myotubes as a model system to investigate whether agrin-elicited AChR clustering correlated with lipid rafts. In a previous study, using two-photon Laurdan confocal imaging, we showed that agrin-induced AChR clusters corresponded to condensed membrane domains: the biophysical hallmark of lipid rafts [F. Stetzkowski-Marden, K. Gaus, M. Recouvreur, A. Cartaud, J. Cartaud, Agrin elicits membrane condensation at sites of acetylcholine receptor clusters in C2C12 myotubes, J. Lipid Res. 47 (2006) 2121-2133]. We further demonstrated that formation and stability of AChR clusters depend on cholesterol. We also reported that three different extraction procedures (Triton X-100, pH 11 or isotonic Ca++, Mg++ buffer) generated detergent resistant membranes (DRMs) with similar cholesterol/GM1 ganglioside content, which are enriched in several signalling postsynaptic components, notably AChR, the agrin receptor MuSK, rapsyn and syntrophin. Upon agrin engagement, actin and actin-nucleation factors such as Arp2/3 and N-WASP were transiently recovered within raft fractions suggesting that the activation by agrin can trigger actin polymerization. Taken together, the present data suggest that AChR clustering at the neuromuscular junction relies upon a mechanism of raft coalescence driven by agrin-elicited actin polymerization.