The role of MuSK in synapse formation and neuromuscular disease

Abstract

Muscle-specific kinase (MuSK) is essential for each step in neuromuscular synapse formation. Before innervation, MuSK initiates postsynaptic differentiation, priming the muscle for synapse formation. Approaching motor axons recognize the primed, or prepatterned, region of muscle, causing motor axons to stop growing and differentiate into specialized nerve terminals. MuSK controls presynaptic differentiation by causing the clustering of Lrp4, which functions as a direct retrograde signal for presynaptic differentiation. Developing synapses are stabilized by neuronal Agrin, which is released by motor nerve terminals and binds to Lrp4, a member of the low-density lipoprotein receptor family, stimulating further association between Lrp4 and MuSK and increasing MuSK kinase activity. In addition, MuSK phosphorylation is stimulated by an inside-out ligand, docking protein-7 (Dok-7), which is recruited to tyrosine-phosphorylated MuSK and increases MuSK kinase activity. Mutations in MuSK and in genes that function in the MuSK signaling pathway, including Dok-7, cause congenital myasthenia, and autoantibodies to MuSK, Lrp4, and acetylcholine receptors are responsible for myasthenia gravis.

Figure 1.

During development, motor axons (green) approach muscle in which AChR gene expression (blue) and clustering of AChRs (red) is enhanced in the prospective synaptic region of muscle. Motor axons form synapses in the prepatterned region of muscle. Motor axons release Agrin, which binds Lrp4 and stimulates MuSK, enhancing postsynaptic differentiation; and ACh, which depolarizes muscle, suppressing AChR expression. The combination of the two antagonist signals sharpens the prepattern so that AChR expression is restricted to synaptic sites.

Figure 2.

In the absence of MuSK, AChRs (red) fail to cluster and motor axons (green) fail to stop and differentiate. Motor axons and nerve terminals were stained with antibodies to neurofilament (NF) and synaptophysin (Syn), respectively.

Figure 3.

Motor axons release Agrin and ACh. Agrin binds to Lrp4, which stimulates association between Lrp4 and MuSK and MuSK phosphorylation. Once phosphorylated in the juxtamembrane region, MuSK recruits Dok-7, which forms a dimer and stabilizes a dimer of MuSK. Tyrosine phosphorylation of Dok-7 stimulates recruitment of Crk/Crk-L, which activates a poorly understood signaling pathway that leads to the anchoring of Rapsyn and AChRs at sites where MuSK is activated.

Figure 4.

The domain organization of MuSK and Lrp4. (A) The extracellular region of MuSK contains three Ig-like domains and a Frizzled-like domain (Fz). The intracellular region of MuSK contains a juxtamembrane region (JM), a kinase domain (KD), and a short cytoplasmic tail. The crystal structures of the first two Ig-like domains (Stiegler et al. 2006), the Frizzled-like domain (Stiegler et al. 2009), and the intracellular region (Till et al. 2002) have been determined, but their arrangement in the context of the full kinase is not known and is depicted here only for illustrative purposes. (B) The extracellular region of Lrp4 contains eight LDLa repeats, two EGF-like domains, and four BP domains with embedded EGF-like domains. The crystal structure of the first BP domain in Lrp4 has been determined (Zong et al. 2012), whereas the structures of all other domains in Lrp4 have been inferred from the structures of these domains in other Lrp family members (Rudenko et al. 2002). The arrangement of these domains in the context of the full receptor is not known and is shown only for illustrative purposes.

Figure 5.

Neuromuscular diseases: autoimmune myasthenia gravis and congenital myasthenia.