Development of the neuromuscular junction, the highly specialized and precisely organized region that enables rapid and reliable stimulation of a muscle by a motor neuron, depends on complex interactions between nerve and muscle. Signaling from the neurally derived proteoglycan agrin to the postsynaptic receptor tyrosine kinase MuSK (muscle-specific kinase) is critical to aggregation of postsynaptic acetylcholine receptors (AChRs); however, the pathway linking agrin to MuSK has been unclear. Noting that mice with mutations in the gene that encodes low-density lipoprotein receptor (LDLR)-related protein 4 (Lrp4) have neuromuscular defects like those in mice with mutations in the gene encoding MuSK, Kim et al. investigated the role of Lrp4 in signaling from agrin to MuSK. Agrin failed to stimulate MuSK tyrosine phosphorylation in a muscle cell line derived from lrp4mitt mutant mice; agrin-dependent MuSK phosphorylation and AChR clustering were restored by retroviral infection with Lrp4. LDLR, which is in the same family as Lrp4, did not confer responsiveness to agrin, whereas a chimera containing the Lrp4 extracellular and transmembrane domains and the LDLR cytoplasmic domain did. BaF3 cells (a nonadherent pro-B cell line) expressing Lrp4 adhered to coverslips to which a 50-kD fragment of the neural splice form of agrin had been absorbed, and fluorescently labeled agrin bound to BaF3 cells that expressed Lrp4. Calcium enhanced agrin binding to Lrp4-expressing Baf3 cells, as did coexpression of MuSK or addition of the soluble MuSK extracellular domain. Lrp4-expressing cells formed aggregates with each other and with MuSK-expressing cells, and coimmunoprecipitation analysis indicated that MuSK and Lrp4 formed a complex when coexpressed. Coexpression with Lrp4 increased basal phosphorylation of MuSK in BaF3 cells and enabled agrin-dependent MuSK phosphorylation. The authors thus conclude that Lrp4 acts as an agrin receptor that associates with MuSK and enables its agrin-dependent activation.
N. Kim, A. L. Stiegler, T. O. Cameron, P. T. Hallock, A. M. Gomez, J. H. Huang, S. R. Hubbard, M. L. Dustin, S. J. Burden, Lrp4 is a receptor for agrin and forms a complex with MuSK. Cell 135, 334-342 (2008). [PubMed]