Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers

Abstract

Synapse formation is a very elaborate process dependent upon accurate coordination of pre and post-synaptic specialization, requiring multiple steps and a variety of receptors and signaling molecules. Due to its relative structural simplicity and the ease in manipulation and observation, the neuromuscular synapse or neuromuscular junction (NMJ)-the connection between motor neurons and skeletal muscle-represents the archetype junction system for studying synapse formation and conservation. This junction is essential for survival, as it controls our ability to move and breath. NMJ formation requires coordinated interactions between motor neurons and muscle fibers, which ultimately result in the formation of a highly specialized post-synaptic architecture and a highly differentiated nerve terminal. Furthermore, to ensure a fast and reliable synaptic transmission following neurotransmitter release, ligand-gated channels (acetylcholine receptors, AChRs) are clustered on the post-synaptic muscle cell at high concentrations in sites opposite the presynaptic active zone, supporting a direct role for nerves in the organization of the post-synaptic membrane architecture. This organized clustering process, essential for NMJ formation and for life, relies on key signaling molecules and receptors and is regulated by soluble extracellular molecules localized within the synaptic cleft. Notably, several mutations as well as auto-antibodies against components of these signaling complexes have been related to neuromuscular disorders. The recent years have witnessed strong progress in the understanding of molecular identities, architectures, and functions of NMJ macromolecules. Among these, prominent roles have been proposed for neural variants of the proteoglycan agrin, its receptor at NMJs composed of the lipoprotein receptor-related protein 4 (LRP4) and the muscle-specific kinase (MuSK), as well as the regulatory soluble synapse-specific protease Neurotrypsin. In this review we summarize the current state of the art regarding molecular structures and (agrin-dependent) canonical, as well as (agrin-independent) non-canonical, MuSK signaling mechanisms that underscore the formation of neuromuscular junctions, with the aim of providing a broad perspective to further stimulate molecular, cellular and tissue biology investigations on this fundamental intercellular contact.

Recent insights into neuromuscular junction biology in Duchenne muscular dystrophy: Impacts, challenges, and opportunities

“…While many additional signalling pathways have been identified that play a role in synapse specialization, including neuregulin/ErbB, Wnt/β-catenin, and Hippo/Yes-associated protein signalling cascades [40] , the remainder of this section will emphasize molecules that have been examined in the DMD context and how they develop, maintain, and remodel synaptic structure and function. Readers interested in much more comprehensive surveys of NMJ maturation are enthusiastically referred to these recent, expert reviews [ 20 , 22 , 23 , 37 , 40 , 41 ].…”

How to Build and to Protect the Neuromuscular Junction: The Role of the Glial Cell Line-Derived Neurotrophic Factor

“…It involves the neuron-secreted proteoglycan agrin with its receptor, the lipoprotein receptor-related protein 4 (LRP4), the muscle-specific kinase (MuSK) and the regulatory soluble synapse-specific protease Neurotrypsin. Since MuSK is not able to directly bind Agrin, LRP4 mediates the process by forming a stable receptor/co-receptor assembly (for a complete review see [ 1 ]). The complex interplay between motor axons, terminal Schwann cells and the muscle fibers of the peripheral nervous system (PNS), is reminiscent of the tripartite synapse in the central nervous system (CNS) where pre- and post-synaptic neurons and astrocytes interact ( Figure 1 ).…”

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

“…Lrp4 (Pink frame, Figure 1) which plays a pivotal role in mediating agrin (via green-limit, MuSK lg1/2, Figure 1)-and Wnt (via red-limit, MuSK CRD, Figure 1)-signalings for AChR clustering at the postsynaptic membrane and also serves the retrograde signaling from muscle to nerve (Barik et al, 2014a;Guarino et al, 2020). The molecular structure of Lrp4 consists of a large extracellular domain that contains eight low-density lipoprotein receptor domains class A (LDLa), EGF-like domains and four β-propeller domains; its N-terminal region (including the last few LDLa repeats and the first β-propeller domain) interacts with agrin (Zhang et al, 2011;Zong et al, 2012); in its third β-propeller domain, the edge part mediates the MuSK signaling and the central part mediates the Wnt signaling (Ohkawara et al, 2014).…”

“…Wnt 11 (Wu et al, 2010;Zhang et al, 2012a), Wnt 9a (Zhang et al, 2012a), and Wnt4 (Strochlic et al, 2012;Zhang et al, 2012a) are derived from muscles. The binding of the Ig4 domain (CRD) of MuSK (red-limit in pink MuSK, Figure 1) with Wnts contributes to AChR clustering through the non-canonical pathway mediated by the Dishevelled protein (Dvl; Pink frame, Figure 1), leading to the formation of AChR microclusters at the early non-innervated stage, termed as the AChR prepatterning (Kummer et al, 2006) which forms the clusters at the central part of the muscle and guides the incoming axon (Gray frame, Figure 1; Luo et al, 2002;Jing et al, 2009;Stiegler et al, 2009;Wu et al, 2010;Gordon et al, 2012;Koles and Budnik, 2012;Barik et al, 2014b;Burden et al, 2018;Guarino et al, 2020). The motoneuron-derived Wnt7a and Wnt7b serve nerve terminal development (Shen et al, 2018).…”