Secreted Signaling Molecules at the Neuromuscular Junction in Physiology and Pathology

Abstract

: Signal transduction at the neuromuscular junction (NMJ) is affected in many human diseases, including congenital myasthenic syndromes (CMS), myasthenia gravis, Lambert-Eaton myasthenic syndrome, Isaacs' syndrome, Schwartz-Jampel syndrome, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. The NMJ is a prototypic cholinergic synapse between the motor neuron and the skeletal muscle. Synaptogenesis of the NMJ has been extensively studied, which has also been extrapolated to further understand synapse formation in the central nervous system. Studies of genetically engineered mice have disclosed crucial roles of secreted molecules in the development and maintenance of the NMJ. In this review, we focus on the secreted signaling molecules which regulate the clustering of acetylcholine receptors (AChRs) at the NMJ. We first discuss the signaling pathway comprised of neural agrin and its receptors, low-density lipoprotein receptor-related protein 4 (Lrp4) and muscle-specific receptor tyrosine kinase (MuSK). This pathway drives the clustering of acetylcholine receptors (AChRs) to ensure efficient signal transduction at the NMJ. We also discuss three secreted molecules (Rspo2, Fgf18, and connective tissue growth factor (Ctgf)) that we recently identified in the Wnt/β-catenin and fibroblast growth factors (FGF) signaling pathways. The three secreted molecules facilitate the clustering of AChRs by enhancing the agrin-Lrp4-MuSK signaling pathway.

Keywords: FGF signaling; Wnt/β-catenin signaling; acetylcholine receptor; neuromuscular junction.

Figure 1

Schematic of the neuromuscular junction and representative signaling molecules involved in nicotinic acetylcholine receptor (AChR) clustering by muscle-specific receptor tyrosine kinase (MuSK) phosphorylation, and/or by gene expression. Agrin binds to lipoprotein receptor-related protein 4 (Lrp4) to phosphorylate MuSK (Section 3). Connective tissue growth factor (Ctgf) also binds to Lrp4 to enhance MuSK phosphorylation (Section 3). Wnt modulates MuSK phosphorylation by associating with either Frizzled (not shown) or Lrp4, or both (Section 5). Rspo2 binds to Lgr5 to enhance MuSK phosphorylation (Section 5). Fibroblast growth factor 18 (Fgf18) binds to Fgfrs (Section 6), and neuregulin-1 (Nrg1) binds to ErbBs (Section 7), to enhance MuSK phosphorylation and induce expression of neuromuscular junction (NMJ)-specific genes.

Figure 2

Structures and disease-causing missense mutations of agrin, Lrp4, and MuSK. The domains of agrin, Lrp4, and MuSK are indicated by squares and ovals, and are drawn to scale. Bars on the top indicate the positions where binding to a partner occurs. Mutations in congenital myasthenic syndrome (CMS) are indicated in red, and mutations in Cenani–Lenz syndactyly syndrome (CLSS), sclerosteosis type 2-like symptoms (SOST2), Richter syndrome (RS), and low-trauma fracture (LTF) are indicated in black. (A) Human agrin domains. SS, secretion signal peptide; NtA, N-terminal Agrin; FS, follistatin-like domain; LE, laminin EGF–like domain; S/T, serine/threonine-rich domain; SEA, a sperm protein, enterokinase, and agrin domain; EG, EGF-like domain; and LG1-3, laminin G-like domain 1–3. A/y and B/z are alternative exons included in the neuronal isoforms of agrin. (B) Human Lrp4 domains. LDLa, low-density lipoprotein receptor domain class A; EG, EGF-like domain; YWTD, β-propeller domain; TM, transmembrane domain. (C) Human MuSK domains. SS, secretion signal peptide; Ig1-3, immunoglobulin domain 1–3; Fz-CRD, Frizzled-like cysteine-rich domain; TM, transmembrane domain; and KD, kinase domain.