Constitutively active MuSK is clustered in the absence of agrin and induces ectopic postsynaptic-like membranes in skeletal muscle fibers.

In skeletal muscle fibers, neural agrin can direct the accumulation of acetylcholine receptors (AChR) and transcription of AChR subunit genes from the subsynaptic nuclei. Although the receptor tyrosine kinase MuSK is required for AChR clustering, it is less clear whether MuSK regulates gene transcription. To elucidate the role of MuSK in these processes, we constructed a constitutively active MuSK receptor, MuSKneuTMuSK, taking advantage of the spontaneous homodimerization of the transmembrane domain of neuT, an oncogenic variant of the neu/erbB2 receptor. In the extrasynaptic region of innervated muscle fibers, MuSKneuTMuSK formed highly concentrated aggregates that colocalized with AChR clusters. Associated with MuSK-induced AChR clusters was a normal complement of synaptic proteins. Moreover, transcription of the AChR-epsilon subunit gene was increased, albeit via an indirect mechanism by MuSK-induced aggregation of erbB receptors and neuregulin. Although neural agrin was not required, the activity of MuSKneuTMuSK was nevertheless potentiated by ectopic expression of a muscle agrin isoform inactive in AChR clustering. To define the role of the kinase domain in the formation of a postsynaptic-like membrane, a second fusion receptor, neuneuTMuSK, which included the MuSK kinase but not the MuSK extracellular domain, was expressed. Significantly, neuneuTMuSK induced AChR clusters that colocalized with aggregates of endogenous MuSK. Taken together, it was concluded that the MuSK kinase domain is sufficient to initiate the recruitment of additional MuSK receptors, which then develop into highly concentrated aggregates by means of a positive feedback loop to induce a postsynaptic membrane in the absence of neural agrin.