Structure and activation of MuSK, a receptor tyrosine kinase central to neuromuscular junction formation

Abstract

MuSK (muscle-specific kinase) is a receptor tyrosine kinase that plays a central signaling role in the formation of neuromuscular junctions (NMJs). MuSK is activated in a complex spatio-temporal manner to cluster acetylcholine receptors on the postsynaptic (muscle) side of the synapse and to induce differentiation of the nerve terminal on the presynaptic side. The ligand for MuSK is LRP4 (low-density lipoprotein receptor-related protein-4), a transmembrane protein in muscle, whose binding affinity for MuSK is potentiated by agrin, a neuronally derived heparan-sulfate proteoglycan. In addition, Dok7, a cytoplasmic adaptor protein, is also required for MuSK activation in vivo. This review focuses on the physical interplay between these proteins and MuSK for activation and downstream signaling, which culminates in NMJ formation. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Keywords: Acetylcholine receptor; Agrin; LRP4; MuSK; Neuromuscular junction.

Figure 1. Schematic representation of the key components involved in the formation of the neuromuscular junction

Motor neurons secret agrin, a heparan-sulfate proteoglycan, into the synaptic basal lamina, which binds to LRP4 on the myotube surface and activates the receptor tyrosine kinase MuSK. MuSK activation is also driven by Dok7, a dimeric cytoplasmic adaptor protein. Activated MuSK phosphorylates downstream signaling components, which leads to rapsyn-dependent clustering of acetylcholine receptors (AChRs). The neurotransmitter acetylcholine, secreted by motor neurons, binds to the clustered AChRs and initiates muscle contraction.

Figure 2. MuSK architecture and structure

(A) Schematic diagram of MuSK. The extracellular region comprises three immunoglobulin (Ig)-like domains and a Frizzled-like cysteine-rich domain (Fz-CRD). The cytoplasmic domain contains the tyrosine kinase domain (TKD). Autophosphorylation (in trans) occurs on Tyr553 in the juxtamembrane region and Tyr750/754/755 in the activation loop of the kinase domain. (B) Crystal structure of Ig1-2 (PDB ID: 2IEP), highlighting in Ig1 a solvent-exposed disulfide bridge and adjacent Ile96. The second protomer in the crystallographic asymmetric unit is shown in light colors, and the domains are labeled with a prime (’). (C) Dimer of Ig1-2 mediated by Leu83 in Ig1. The view is approximately 90° from that in (B), from the top. Coloring and labeling as in (B).

Figure 3. LRP4 architecture and agrin interaction

(A) Schematic diagram of LRP4. The extracellular region consists of eight LDLa repeats, two EGF-like domains, and four β propeller-EGF units. The short cytoplasmic region is devoid of folded domains. (B) Crystal structure of agrin (LG3 domain) bound to the first β propeller-EGF unit of LRP4 (PDB ID: 3V64). Coloring of LRP4 as in (A). The z8 splicing insert of agrin is colored red. (C) 2:2 complex of agrin and LRP4 observed in the crystal structure. Same coloring as in (B). The domains in the second 1:1 agrin:LRP4 complex are labeled with a prime.