“The MuSK-BMP pathway: a novel regulator of synaptic excitation and the myonuclear transcriptome”
Dr. Lauren A. Fish
The neuromuscular junction (NMJ) is the site of information exchange between the nervous system and muscle, and is compromised in aging and neurodegenerative diseases. Neuromuscular transmission requires both cholinergic signaling mediated by acetylcholine receptors (AChRs) and the generation of muscle action potentials by voltage-gated Nav1.4 sodium channels at the NMJ. We previously showed that muscle-specific kinase (MuSK), known for its Ig1 domain-mediated role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptomic responses. To assess the role of the MuSK-BMP pathway at the NMJ, we used mice lacking the BMP-binding MuSK Ig3 domain (“∆Ig3-MuSK”). Synapses form normally in ∆Ig3-MuSK muscle and denervation is not observed at any age examined. However, the ∆Ig3-MuSK postsynaptic apparatus becomes fragmented beginning shortly after birth and has fewer, shallower junctional folds. Ex-vivo electrophysiology showed that spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents are normal at ∆Ig3-MuSK NMJs. However, single fiber electromyography revealed increased jitter and blocking of nerve-evoked muscle action potentials. Nerve-evoked compound muscle action potentials and muscle torque force are also reduced in ∆Ig3-MuSK mice. Further, Nav1.4 levels are reduced at the ∆Ig3-MuSK NMJ, indicating that the excitability defects are due to impaired voltage-gated sodium channel localization. We therefore propose that MuSK has two distinct roles at the NMJ: an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and a BMP co-receptor required for maintaining proper Nav1.4 density and muscle excitability.
We also recently showed that MuSK-BMP signaling shapes transcriptional profiles of fast, and particularly, slow muscles. We next used single-nuclei RNA-sequencing to further characterize the role of the MuSK-BMP pathway in shaping the transcriptome of soleus (slow muscle) myonuclei. Differential expression analysis revealed coordinated changes in the transcriptomes of all ∆Ig3-MuSK myonuclei types except NMJ myonuclei, including downregulation of MuSK and several metabolic genes and upregulation of muscle-specific genes with known roles in myogenesis and muscle fiber differentiation. Gene set enrichment and gene ontology overrepresentation analyses confirmed that upregulated genes are involved in processes related to myogenesis and muscle hypertrophy, and downregulated genes are largely involved in fatty acid and lipid metabolism. These data suggest that the MuSK-BMP pathway regulates genes involved in key signaling pathways in muscle tissue relevant to sarcopenia, insulin resistance, and other metabolic disorders.