The agrin/MuSK signaling pathway is spatially segregated from the neuregulin/ErbB receptor signaling pathway at the neuromuscular junction

The neuregulin/erbB receptor and agrin/MuSK pathways are critical for commu nication between the nerve, muscle, and Schwann cell that establishes the p recise topological arrangement at the vertebrate neuromuscular junction (NM J). ErbB2, erbB3, and erbB4 as well as neuregulin, agrin, and MuSK are know n to be concentrated at the NMJ. Here we have examined NMJs from gastrocnem ius muscle of adult rat using immunofluorescence confocal microscopy to cha racterize in detail the distribution of these proteins relative to the dist ribution of acetylcholine receptors (AChRs). We have determined that erbB2 and erbB4 are enriched in the depths of the secondary junctional folds on t he postsynaptic muscle membrane. In contrast, erbB3 at the NMJ was concentr ated at presynaptic terminal Schwann cells. This distribution strongly argu es that erbB2/erbB4 heterodimers are the functional postsynaptic neuregulin receptors of the NMJ. Neuregulin was localized to the axon terminal, secon dary folds, and terminal Schwann cells, where it was in a position to signa l through erbB receptors. MuSK was concentrated in the postsynaptic primary gutter region where it was codistributed with AChRs. Agrin was present at the axon terminal and in the basal lamina associated with the primary gutte r region, but not in the secondary junctional folds. The differential distr ibutions of the neuregulin and agrin signaling pathways argue against neure gulin and erbB receptors being localized to the NMJ via direct interactions with either agrin or MuSK.