| A key event in the development of the vertebrate neuromuscular junction (NMJ) is the postsynaptic accumulation of acetylcholine receptors (AChRs) on muscle membrane in response to motor innervation. This process by which AChRs are clustered at the NMJ can be satisfactorily explained by the "diffusion-mediated trapping" model, which depicts the immobilization and aggregation of AChRs undergoing Brownian diffusion on the membrane by a subsynaptic cytoskeletal scaffold generated by nerve-derived signals. Interestingly, in cultured muscle cells, direct current (DC)-electric field stimulation can also induce AChR clustering. With electric field stimulation, the AChRs aggregate along the cathode-facing edge of cells, which has been explained by various models including diffusion-trapping and electromigration. Here, we critically examined the clustering of AChRs in response to DC electric field through single molecule tracking using quantum dots (QDs). Analyses of AChR trajectories and the calculation of their Hurst exponent and diffusion coefficients of mobile AChRs have shown that DC electric field-induced AChR clustering conforms to the diffusion-trap model.;These results led us to investigate the molecular mechanism for electric field-stimulated AChR trapping. Since AChR clustering at the NMJ requires the activation of agrin/MuSK (muscle-specific kinase) signaling, we examined whether electric field stimulation was also mediated through the same signaling pathway. Supporting this premise, our studies showed that MuSK activation, accompanied by src tyrosine kinase signaling and F-actin assembly, was involved in DC electric field induced AChR clustering. Rapsyn, a protein associated with AChR's cytoplasmic domain, also played an important role in this AChR clustering process.;As another manifestation of AChR surface dynamics, their crosslinking by autoantibodies in myasthenia gravis patients causes their internalization. This process can be mimicked by QD-induced AChR crosslinking and subsequent endocytosis in cultured Xenopus muscle cells. This allowed the tracking of the entire AChR internalization process. We found that this endocytic process involved clathrin-mediated signaling pathway that is influenced by synaptogenic signaling. Stimuli that promote postsynaptic development (AChR clustering) down-regulate crosslinking-induced AChR internalization. Taken together, my study suggests that the dynamics of surface AChRs can be controlled by physical stimuli that have impact on intracellular signaling pathways to effect the formation, maintenance and dispersal of the postsynaptic apparatus. |
