MuSK regulates neuromuscular junction Nav1.4 localization and excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly, the function of the NMJ is to transduce nerve action potentials into muscle fiber action potentials (MFAPs). Efficient neuromuscular transmission requires both cholinergic signaling, responsible for generation of endplate potentials (EPPs), and excitation, the amplification of the EPP by postsynaptic voltage-gated sodium channels (Nav1.4) to generate the MFAP. In contrast to the cholinergic component, the signaling pathways that organize Nav1.4 and mediate muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as the main organizer of acetylcholine receptors AChRs), also binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here, using mice lacking the MuSK Ig3 domain ('ΔIg3-MuSK'), we probed the role of this domain at the NMJ. NMJs formed in ΔIg3-MuSK animals with pre- and post- synaptic specializations aligned at all ages examined. However, the ΔIg3-MuSK postsynaptic apparatus was fragmented from the first weeks of life. Synaptic electrophysiology showed that spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable at WT and ΔIg3-MuSK NMJs. However, single fiber electromyography revealed that nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal as evidenced by jitter and blocking. Further, nerve-evoked compound muscle action potentials and muscle force production were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK NMJs, but not at the sarcolemma broadly, indicating that the observed excitability defects result from impaired synaptic localization of this ion channel. We propose that MuSK plays distinct, domain-specific roles at the NMJ: the Ig1 domain mediates agrin-LRP4 mediated AChR localization, while the Ig3 domain maintains postsynaptic Nav1.4 density, conferring the muscle excitability required to amplify cholinergic signals and trigger action potentials.Significance Statement The neuromuscular junction (NMJ) is required for nerve-evoked muscle contraction and movement, and its function is compromised during aging and disease. Though the mechanisms underlying neurotransmitter release and cholinergic response at this synapse have been studied extensively, the machinery necessary for nerve-evoked muscle excitation are incompletely characterized. We show that the Ig3 domain of MuSK (muscle-specific kinase) regulates NMJ structure and the localization of voltage-gated sodium channels necessary for nerve-evoked muscle fiber excitation and force production. This function of MuSK is structurally and mechanistically distinct from its role in organizing cholinergic machinery. The Ig3 domain of MuSK thus emerges as a target for selectively modulating excitability, which is defective in conditions such as congenital myasthenic syndromes and age-related muscle weakness.