Nicotinic Acetylcholine Receptor Stability at the NMJ Deficient in α-SyntrophinIn Vivo

Abstract

α-Syntrophin (α-syn), a scaffold protein, links signaling molecules to the dystrophin-glycoprotein complex. Absence of α-syn from the DGC is known to lead to structurally aberrant neuromuscular junctions (NMJs) with few acetylcholine receptors (AChRs) clustered at synaptic sites. Using α-syn knock-out mice, we show that during the first postnatal week, α-syn is not required for synapse formation. However, at postnatal day 6 (P6)-P7, the structural integrity of the postsynaptic apparatus is altered, the turnover rate of AChRs increases significantly, and the number/density of AChRs is impaired. At the adult α-syn(-/-) NMJ, the turnover rate of AChRs is ∼ 4 times faster than wild-type synapses, and most removed receptors are targeted to degradation as few AChRs recycled to synaptic sites. Biochemical analyses show that in muscle cells of adult knock-out α-syn mice, total AChRs and scaffold protein rapsyn are significantly reduced, the 89 kDa and 75 kDa isoforms of tyrosine phosphorylated α-dystrobrevin (α-dbn) 1 (which are required for the maintenance and stability of AChR in α-dbn(-/-) synapses) are barely detectable. Electroporation of GFP-α-dbn1 in α-syn(-/-) muscle cells partially restored receptor density, turnover rate, and the structural integrity of the postsynaptic apparatus, whereas expression of rapsyn-GFP failed to rescue the α-syn(-/-) synaptic phenotype. These results demonstrate that α-syn is required for the maturation and stability of the postsynaptic apparatus and suggest that α-syn may act via α-dbn1.