Molecular organization of the extraocular muscle neuromuscular junction: Partial conservation of and divergence from the skeletal muscle prototype

PURPOSE. The phenotypically novel extraocular muscles (EOMs) exhibit fundamental differences in innervation and neuromuscular junction (NMJ) morphology from other skeletal muscles. In the current study, the morphology and molecular organization of NMJs of EOM singly innervated (SIF) and multiply innervated (MIF) fiber types were evaluated and the distribution of molecules involved in formation and maintenance of NMJs were specifically characterized. METHODS. Adult mouse EOM NMJ organization was examined by immunofluorescence and confocal microscopy. Differential cellular localization of components of two established synaptic signaling pathways, (1) neuregulin and erbB receptors 2, 3, and 4 and (2) agrin, MuSK, and rapsyn and select NMJ-associated structural proteins were studied for EOM SIF and MIF populations. Endplate topography and structure were also studied, using both confocal microscopy and transmission electron microscopy, with NMJ morphologic organization correlated with specific EOM fiber types. RESULTS. Confocal fluorescence microscopy demonstrated that, for NMJs of both EOM SIFs and MIFs, components of neuregulin and agrin pathways and the major components of the junctional dystrophin-glycoprotein complex (DGC) colocalized with acetylcholine receptor (AChR) aggregates. However, EOM exhibited novel fiber-type-specific extrasynaptic localization of two key DGC signaling-related molecules: a-dystrobrevin I (global MIFs) and syntrophin beta1 (global MIFs and orbital MIFs and SIFs). CONCLUSIONS. The data establish that the molecular organization of EOM SIF and MIF NMJs includes the same signaling and structural molecules previously characterized for other skeletal muscles. By contrast, divergence in-other aspects of the synaptic and nonsynaptic sarcolemmal organization of EOM fiber types may underlie the unique responses of these muscles in a variety of neuromuscular disorders.