MuSK Regulates Neuromuscular Junction Nav1.4 Localization and Excitability

Area of Science:

• Neuroscience
• Muscle Physiology
• Molecular Biology

Background:

• The neuromuscular junction (NMJ) is vital for muscle contraction, relying on cholinergic signaling and postsynaptic excitation.
• While acetylcholine receptor organization by MuSK is known, pathways regulating Nav1.4 channels at the NMJ are unclear.
• MuSK's Ig3 domain binds BMPs and influences signaling, suggesting a role beyond acetylcholine receptor clustering.

Purpose of the Study:

• To investigate the specific role of the MuSK Ig3 domain in NMJ function and muscle excitability.
• To determine if the Ig3 domain is essential for postsynaptic Nav1.4 channel localization and function.

Main Methods:

• Utilized knockout mice lacking the MuSK Ig3 domain (ΔIg3-MuSK).
• Performed electrophysiological recordings (single fiber EMG, compound muscle action potentials) and measured muscle force.
• Assessed NMJ structure, acetylcholine receptor density, and Nav1.4 channel localization.

Main Results:

• ΔIg3-MuSK NMJs showed fragmented postsynaptic apparatus but normal acetylcholine receptor clustering.
• Muscle fibers from ΔIg3-MuSK mice exhibited abnormal action potentials (jitter, blocking) and reduced force.
• Nav1.4 channel levels were significantly reduced at synaptic sites in ΔIg3-MuSK NMJs.

Conclusions:

• MuSK's Ig1 domain mediates acetylcholine receptor localization, while the Ig3 domain is critical for postsynaptic Nav1.4 channel clustering.
• Impaired Nav1.4 localization due to Ig3 domain absence disrupts muscle excitability and NMJ transmission.
• These findings reveal distinct, domain-specific functions of MuSK in NMJ organization and muscle function.