MuSK induces in vivo acetylcholine receptor clusters in a ligand-independent manner.
Muscle-specific receptor tyrosine kinase (MuSK) is required for the formation of the neuromuscular junction. Using direct gene transfer into single fibers, MuSK was expressed extrasynaptically in innervated rat muscle in vivo to identify its contribution to synapse formation. Spontaneous MuSK kinase activity leads, in the absence of its putative ligand neural agrin, to the appearance of epsilon-subunit-specific transcripts, the formation of acetylcholine receptor clusters, and acetylcholinesterase aggregates. Expression of kinase-inactive MuSK did not result in the formation of acetylcholine receptor (AChR) clusters, whereas a mutant MuSK lacking the ectodomain did induce AChR clusters. The contribution of endogenous MuSK was excluded by using genetically altered mice, where the kinase domain of the MuSK gene was flanked by loxP sequences and could be deleted upon expression of Cre recombinase. This allowed the conditional inactivation of endogenous MuSK in single muscle fibers and prevented the induction of ectopic AChR clusters. Thus, the kinase activity of MuSK initiates signals that are sufficient to induce the formation of AChR clusters. This process does not require additional determinants located in the ectodomain.
Sander
A
A
Abteilung Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, D-69120 Heidelberg, Germany.
Hesser
B A
BA
Witzemann
V
V
D000818:Animals; D003001:Cloning, Molecular; D005786:Gene Expression Regulation; D049452:Green Fluorescent Proteins; D008024:Ligands; D008164:Luminescent Proteins / Q000032:analysis / Q000235:genetics; D008954:Models, Biological; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000235:genetics*; D011993:Recombinant Fusion Proteins / Q000032:analysis; D011994:Recombinant Proteins / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013569:Synapses / Q000502:physiology
D008024:Ligands; D008164:Luminescent Proteins; D009124:Muscle Proteins; D011950:Receptors, Cholinergic; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; C051609:peripheral membrane protein 43K; D049452:Green Fluorescent Proteins; C095714:MUSK protein, human; D020794:Receptor Protein-Tyrosine Kinases