Synaptic differentiation is defective in mice lacking acetylcholine receptor -subunit tyrosine phosphorylation

Agrin activates MuSK, a receptor tyrosine kinase expressed in skeletal muscle, leading to tyrosine phosphorylation of the acetylcholine receptor (AChR) -subunit and clustering of AChRs. The importance of AChR -subunit tyrosine phosphorylation in clustering AChRs and regulating synaptic differentiation is poorly understood. We generated mice with targeted mutations in the three intracellular tyrosines of the AChR -subunit (AChR-^3F/3F). Mice lacking AChR -subunit tyrosine phosphorylation thrive postnatally and have no overt behavioral defects, indicating that AChR -subunit tyrosine phosphorylation is not essential for the formation of neuromuscular synapses. Nonetheless, the size of synapses and the density of synaptic AChRs are reduced in AChR-^3F/3F mutant mice. Moreover, synapses are structurally simplified and the organization of postjunctional folds is aberrant in mice lacking tyrosine phosphorylation of the AChR -subunit. Furthermore, mutant AChRs cluster poorly in response to agrin and are readily extracted from the cell surface of cultured myotubes by non-ionic detergent. These data indicate that tyrosine phosphorylation of the AChR -subunit has an important role in organizing AChRs and regulating synaptic differentiation.