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24 February 2009 - At the neuromuscular junction (NMJ), the interface between muscle and nerve, clustering of acetylcholine receptors (AChRs) at the postsynaptic membrane ensures efficient synaptic transmission. NMJ formation and AChR clustering require the muscle receptor tyrosine kinase MuSK and its extracellular activator, agrin. However, MuSK can cause NMJ formation in the absence of agrin, leading Inoue et al. to investigate the mechanisms by which MuSK is activated independently of agrin. They now show that the cytoplasmic protein Dok-7 can directly activate MuSK in vitro and in vivo and, when overexpressed in skeletal muscle, leads to the formation of NMJs with wider endplates and bearing more AChR clusters. Strikingly, agrin did not induce AChR clustering in myotubes from Dok-7–deficient mice unless Dok-7 was exogenously expressed, indicating that Dok-7 primes MuSK for subsequent further activation by agrin. Sci. Signal. 2, ra7 (2009).
Scientific abstract written by the authors of the work.
24 February 2009 - Dok-7 Activates the Muscle Receptor Kinase MuSK and Shapes Synapse Formation. The formation of the neuromuscular junction (NMJ) is orchestrated by the muscle-specific receptor tyrosine kinase MuSK and by neural agrin, an extracellular activator of MuSK. We previously showed that the MuSK-interacting protein Dok-7 is essential for neuromuscular synaptogenesis, although the mechanisms by which Dok-7 regulates MuSK activity and promotes synapse formation have been unclear. Here, we show that Dok-7 directly interacts with the cytoplasmic portion of MuSK and activates the receptor tyrosine kinase, and that neural agrin requires Dok-7 to activate MuSK. In vivo overexpression of Dok-7 increased MuSK activation and promoted NMJ formation. Furthermore, Dok-7 was required for the localization of MuSK in the central region of muscle, which is essential for the correct formation of NMJs in this region. These observations indicate that Dok-7 positively regulates neuromuscular synaptogenesis by controlling MuSK activity, its distribution, and its responsiveness to neural agrin. Sci. Signal. 2, ra7 (2009).
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