Tyrosine-phosphorylated and nonphosphorylated isoforms of α-dystrobrevin : roles in skeletal muscle and its neuromuscular and myotendinous junctions

α-Dystrobrevin (DB), a cytoplasmic component of the dystrophin–glycoprotein complex, is found throughout the sarcolemma of muscle cells. Mice lacking αDB exhibit muscular dystrophy, defects in maturation of neuromuscular junctions (NMJs) and, as shown here, abnormal myotendinous junctions (MTJs). In normal muscle, alternative splicing produces two main αDB isoforms, αDB1 and αDB2, with common NH₂-terminal but distinct COOH-terminal domains. αDB1, whose COOH-terminal extension can be tyrosine phosphorylated, is concentrated at the NMJs and MTJs. αDB2, which is not tyrosine phosphorylated, is the predominant isoform in extrajunctional regions, and is also present at NMJs and MTJs. Transgenic expression of either isoform in αDB^−/− mice prevented muscle fiber degeneration; however, only αDB1 completely corrected defects at the NMJs (abnormal acetylcholine receptor patterning, rapid turnover, and low density) and MTJs (shortened junctional folds). Site-directed mutagenesis revealed that the effectiveness of αDB1 in stabilizing the NMJ depends in part on its ability to serve as a tyrosine kinase substrate. Thus, αDB1 phosphorylation may be a key regulatory point for synaptic remodeling. More generally, αDB may play multiple roles in muscle by means of differential distribution of isoforms with distinct signaling or structural properties.