Spatial distribution and molecular dynamics of dystrophin glycoprotein components at the neuromuscular junction in vivo

A bimolecular fluorescence complementation (BiFC) approach was used to study the molecular interactions between different components of the postsynaptic protein complex at the neuromuscular junction of living mice. We show that rapsyn forms complex with both alpha-dystrobrevin and alpha-syntrophin at the crests of junctional folds. The linkage of rapsyn to alpha-syntrophin and/or alpha-dystrobrevin is mediated by utrophin, a protein localized at acetylcholine receptor (AChR)-rich domains. In mice deficient in alpha-syntrophin, in which utrophin is no longer present at the synapse, rapsyn interaction with alpha-dystrobrevin was completely abolished. This interaction was completely restored when either utrophin or alpha-syntrophin was introduced into muscles deficient in alpha-syntrophin. However, in neuromuscular junctions deficient in alpha-dystrobrevin, in which utrophin is retained, complex formation between rapsyn and alpha-syntrophin was unaffected. Using fluorescence recovery after photobleaching, we found that alpha-syntrophin turnover is 5-7 times faster than that of AChRs, and loss of alpha-dystrobrevin has no effect on rapsyn and alpha-syntrophin half-life, whereas the half-life of AChR was significantly altered. Altogether, these results provide new insights into the spatial distribution of dystrophin glycoprotein components and their dynamics in living mice.