Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Role of Fyn and the interleukin-6-STAT-3-autophagy axis in sarcopenia

Sarcopenia is the progressive loss of muscle mass wherein Fyn regulates STAT3 to decrease autophagy. To elucidate the role of inflammation in Fyn-STAT3-dependent autophagy and sarcopenia, here we aimed to investigate the underlying mechanisms using two mouse models of primary and secondary sarcopenia: (1) tail suspension and (2) sciatic denervation. In wild-type mice, the expression of Fyn and IL-6 increased significantly. The expression and phosphorylation levels of STAT3 were also significantly augmented, while autophagic activity was abolished. To investigate Fyn-dependency, we used tail suspension with Fyn-null mice. In tail-suspended wild-type mice, IL-6 expression was increased; however, it was abolished in Fyn-null mice, which maintained autophagy and the expression and ablation of STAT3 phosphorylation. In conclusion, Fyn was found to be associated with the IL-6-STAT3-autophagy axis in sarcopenia. This finding permits a better understanding of sarcopenia-associated metabolic diseases and the possible development of therapeutic interventions.

Distinct roles of the dystrophin-glycoprotein complex: α-dystrobrevin and α-syntrophin in the maintenance of the postsynaptic apparatus of the neuromuscular synapse

α-syntrophin (α-syn) and α-dystrobrevin (α-dbn), two components of the dystrophin-glycoprotein complex, are essential for the maturation and maintenance of the neuromuscular junction (NMJ) and mice deficient in either α-syn or α-dbn exhibit similar synaptic defects. However, the functional link between these two proteins and whether they exert distinct or redundant functions in the postsynaptic organization of the NMJ remain largely unknown. We generated and analyzed the synaptic phenotype of double heterozygote (α-dbn+/-, α-syn+/-), and double homozygote knockout (α-dbn-/-; α-syn-/-) mice and examined the ability of individual molecules to restore their defects in the synaptic phenotype. We showed that in double heterozygote mice, NMJs have normal synaptic phenotypes and no signs of muscular dystrophy. However, in double knockout mice (α-dbn-/-; α-syn-/-), the synaptic phenotype (the density, the turnover and the distribution of AChRs within synaptic branches) is more severely impaired than in single α-dbn-/- or α-syn-/- mutants. Furthermore, double mutant and single α-dbn-/- mutant mice showed more severe exercise-induced fatigue and more significant reductions in grip strength than single α-syn-/- mutant and wild-type. Finally, we showed that the overexpression of the transgene α-syn-GFP in muscles of double mutant restores primarily the abnormal extensions of membrane containing AChRs that extend beyond synaptic gutters and lack synaptic folds, whereas the overexpression of α-dbn essentially restores the abnormal dispersion of patchy AChR aggregates in the crests of synaptic folds. Altogether, these data suggest that α-syn and α-dbn act in parallel pathways and exert distinct functions on the postsynaptic structural organization of NMJs.

Mechanism of disease and therapeutic rescue of Dok7 congenital myasthenia

Congenital myasthenia (CM) is a devastating neuromuscular disease, and mutations in DOK7, an adaptor protein that is crucial for forming and maintaining neuromuscular synapses, are a major cause of CM^1,2. The most common disease-causing mutation (DOK7^{1124_1127 dup}) truncates DOK7 and leads to the loss of two tyrosine residues that are phosphorylated and recruit CRK proteins, which are important for anchoring acetylcholine receptors at synapses. Here we describe a mouse model of this common form of CM (Dok7^CM mice) and a mouse with point mutations in the two tyrosine residues (Dok7^2YF). We show that Dok7^CM mice had severe deficits in neuromuscular synapse formation that caused neonatal lethality. Unexpectedly, these deficits were due to a severe deficiency in phosphorylation and activation of muscle-specific kinase (MUSK) rather than a deficiency in DOK7 tyrosine phosphorylation. We developed agonist antibodies against MUSK and show that these antibodies restored neuromuscular synapse formation and prevented neonatal lethality and late-onset disease in Dok7^CM mice. These findings identify an unexpected cause for disease and a potential therapy for both DOK7 CM and other forms of CM caused by mutations in AGRIN, LRP4 or MUSK, and illustrate the potential of targeted therapy to rescue congenital lethality.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

Roles of tyrosine kinases and phosphatases in the formation and dispersal of acetylcholine receptor clusters

The formation of acetylcholine receptor (AChR) clusters at the postsynaptic muscle membrane in response to motor innervation is a key event in the development of the neuromuscular junction. The synaptic AChR clustering process is initiated by motor axon-released agrin, which activates a tyrosine kinase-based signaling pathway to cause AChR aggregation. In cultured muscle cells, AChR clustering is elicited by diverse nonneural signals, and this process is also mediated by tyrosine kinases. Conversely, the formation of new AChR clusters induced by innervation or nonneural stimuli is unfailingly associated with the dispersal of pre-existing AChR clusters, and this process is mediated by tyrosine phosphatases. In this review, we address how local kinase activation leads to global phosphatase action in muscle. More specifically, we discuss the roles of Src kinase and the SH2 domain-containing tyrosine phosphatase Shp-2 in establishing a regenerative mechanism to propagate the AChR cluster dispersing signal extrasynaptically and in defining the boundary of cluster formation subsynaptically.

Muscle-Specific Kinase Myasthenia Gravis

Thirty to fifty percent of patients with acetylcholine receptor (AChR) antibody (Ab)-negative myasthenia gravis (MG) have Abs to muscle specific kinase (MuSK) and are referred to as having MuSK-MG. MuSK is a 100 kD single-pass post-synaptic transmembrane receptor tyrosine kinase crucial to the development and maintenance of the neuromuscular junction. The Abs in MuSK-MG are predominantly of the IgG4 immunoglobulin subclass. MuSK-MG differs from AChR-MG, in exhibiting more focal muscle involvement, including neck, shoulder, facial and bulbar-innervated muscles, as well as wasting of the involved muscles. MuSK-MG is highly associated with the HLA DR14-DQ5 haplotype and occurs predominantly in females with onset in the fourth decade of life. Some of the standard treatments of AChR-MG have been found to have limited effectiveness in MuSK-MG, including thymectomy and cholinesterase inhibitors. Therefore, current treatment involves immunosuppression, primarily by corticosteroids. In addition, patients respond especially well to B cell depletion agents, e.g., rituximab, with long-term remissions. Future treatments will likely derive from the ongoing analysis of the pathogenic mechanisms underlying this disease, including histologic and physiologic studies of the neuromuscular junction in patients as well as information derived from the development and study of animal models of the disease.

Vezatin is required for the maturation of the neuromuscular synapse

Key genes, such as Agrin, Lrp4, and MuSK, are required for the initial formation, subsequent maturation, and long-term stabilization of mammalian neuromuscular synapses. Additional molecules are thought to function selectively during the evolution and stabilization of these synapses, but these molecular players are largely unknown. Here, we used mass spectrometry to identify vezatin, a two-pass transmembrane protein, as an acetylcholine receptor (AChR)–associated protein, and we provide evidence that vezatin binds directly to AChRs. We show that vezatin is dispensable for the formation of synapses but plays a later role in the emergence of a topologically complex and branched shape of the synapse, as well as the stabilization of AChRs. In addition, neuromuscular synapses in vezatin mutant mice display premature signs of deterioration, normally found only during aging. Thus, vezatin has a selective role in the structural elaboration and postnatal maturation of murine neuromuscular synapses.

MACF1 links Rapsyn to microtubule- and actin-binding proteins to maintain neuromuscular synapses

Oury et al. show that the scaffolding protein MACF1 links Rapsyn, which binds acetylcholine receptors, to the microtubule- and actin-network at neuromuscular synapses. MACF1 thereby plays a role in synaptic maturation in mice, and mutations of MACF1 are associated with congenital myasthenia in humans.

Complex mechanisms are required to form neuromuscular synapses, direct their subsequent maturation, and maintain the synapse throughout life. Transcriptional and post-translational pathways play important roles in synaptic differentiation and direct the accumulation of the neurotransmitter receptors, acetylcholine receptors (AChRs), to the postsynaptic membrane, ensuring for reliable synaptic transmission. Rapsyn, an intracellular peripheral membrane protein that binds AChRs, is essential for synaptic differentiation, but how Rapsyn acts is poorly understood. We screened for proteins that coisolate with AChRs in a Rapsyn-dependent manner and show that microtubule actin cross linking factor 1 (MACF1), a scaffolding protein with binding sites for microtubules (MT) and actin, is concentrated at neuromuscular synapses, where it binds Rapsyn and serves as a synaptic organizer for MT-associated proteins, EB1 and MAP1b, and the actin-associated protein, Vinculin. MACF1 plays an important role in maintaining synaptic differentiation and efficient synaptic transmission in mice, and variants in MACF1 are associated with congenital myasthenia in humans.

Beta-2 Adrenergic Receptor Agonists Enhance AChR Clustering in C2C12 Myotubes: Implications for Therapy of Myasthenic Disorders

Background:

Congenital myasthenic syndromes (CMS) are a group of inherited neuromuscular transmission disorders causing fatiguable muscle weakness. ADRB2 agonists have been observed to provide therapeutic benefit where destabilisation of NMJ structures is part of the underlying pathology, such as in DOK7, COLQ and MuSK CMS as well as in slow channel syndrome. However, very little is known about the molecular mechanisms underlying the effects of ADRB2 agonists in CMS.

Objective:

In vitro investigation into whether an ADRB2 agonist affects the AChR clustering pathway and has the potential to increase the number and stability of AChR clusters.

Methods:

Cultured C2C12 mouse myotubes overexpressing the common DOK7 frameshift mutation c.1124_1127dupTGCC were incubated with salbutamol sulphate and the effect on AChR cluster numbers were investigated. Moreover, agrin-induced AChR clusters in C2C12 WT cells were left to disperse after agrin-wash-off, and the effects of incubation with salbutamol sulphate on AChR cluster numbers were explored.

Results:

Salbutamol sulphate induced a significant increase in the number of AChR clusters formed on C2C12 cells overexpressing c.1124_1127dupTGCC. Furthermore, significantly more clusters remained in C2C12 WT myotubes incubated with salbutamol sulphate following agrin wash-off.

Conclusions:

The results suggest that ADRB2 agonists directly affect proteins located at the neuromuscular junction and exert a stabilising effect on AChR clusters.

Fundamental Molecules and Mechanisms for Forming and Maintaining Neuromuscular Synapses

The neuromuscular synapse is a relatively large synapse with hundreds of active zones in presynaptic motor nerve terminals and more than ten million acetylcholine receptors (AChRs) in the postsynaptic membrane. The enrichment of proteins in presynaptic and postsynaptic membranes ensures a rapid, robust, and reliable synaptic transmission. Over fifty years ago, classic studies of the neuromuscular synapse led to a comprehensive understanding of how a synapse looks and works, but these landmark studies did not reveal the molecular mechanisms responsible for building and maintaining a synapse. During the past two-dozen years, the critical molecular players, responsible for assembling the specialized postsynaptic membrane and regulating nerve terminal differentiation, have begun to be identified and their mechanism of action better understood. Here, we describe and discuss five of these key molecular players, paying heed to their discovery as well as describing their currently understood mechanisms of action. In addition, we discuss the important gaps that remain to better understand how these proteins act to control synaptic differentiation and maintenance.

Agrin-LRP4-MuSK signaling as a therapeutic target for myasthenia gravis and other neuromuscular disorders

INTRODUCTION

Signal transduction at the neuromuscular junction (NMJ) is compromised in a diverse array of diseases including myasthenia gravis, Lambert-Eaton myasthenic syndrome, Isaacs' syndrome, congenital myasthenic syndromes, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. Except for sarcopenia, all are orphan diseases. In addition, the NMJ signal transduction is impaired by tetanus, botulinum, curare, α-bungarotoxin, conotoxins, organophosphate, sarin, VX, and soman to name a few. Areas covered: This review covers the agrin-LRP4-MuSK signaling pathway, which drives clustering of acetylcholine receptors (AChRs) and ensures efficient signal transduction at the NMJ. We also address diseases caused by autoantibodies against the NMJ molecules and by germline mutations in genes encoding the NMJ molecules. Expert opinion: Representative small compounds to treat the defective NMJ signal transduction are cholinesterase inhibitors, which exert their effects by increasing the amount of acetylcholine at the synaptic space. Another possible therapeutic strategy to enhance the NMJ signal transduction is to increase the number of AChRs, but no currently available drug has this functionality.

Diverging roles for Lrp4 and Wnt signaling in neuromuscular synapse development during evolution

In this study, Remédio et al. use mice and zebrafish to show that muscle prepatterning in mammals and zebrafish is established by different mechanisms. Their findings demonstrate that Agrin/Lrp4/MuSK signaling plays an essential role in neuromuscular synapse formation in both fish and mammals, whereas Wnt signaling is dispensable.

Motor axons approach muscles that are prepatterned in the prospective synaptic region. In mice, prepatterning of acetylcholine receptors requires Lrp4, a LDLR family member, and MuSK, a receptor tyrosine kinase. Lrp4 can bind and stimulate MuSK, strongly suggesting that association between Lrp4 and MuSK, independent of additional ligands, initiates prepatterning in mice. In zebrafish, Wnts, which bind the Frizzled (Fz)-like domain in MuSK, are required for prepatterning, suggesting that Wnts may contribute to prepatterning and neuromuscular development in mammals. We show that prepatterning in mice requires Lrp4 but not the MuSK Fz-like domain. In contrast, prepatterning in zebrafish requires the MuSK Fz-like domain but not Lrp4. Despite these differences, neuromuscular synapse formation in zebrafish and mice share similar mechanisms, requiring Lrp4, MuSK, and neuronal Agrin but not the MuSK Fz-like domain or Wnt production from muscle. Our findings demonstrate that evolutionary divergent mechanisms establish muscle prepatterning in zebrafish and mice.

Α-Dystrobrevin-1 recruits Grb2 and α-catulin to organize neurotransmitter receptors at the neuromuscular junction

Neuromuscular junctions (NMJs), the synapses made by motor neurons on muscle fibers, form during embryonic development but undergo substantial remodeling postnatally. Several lines of evidence suggest that α-dystrobrevin, a component of the dystrophin-associated glycoprotein complex (DGC), is a crucial regulator of the remodeling process and that tyrosine phosphorylation of one isoform, α-dystrobrevin-1, is required for its function at synapses. We identified a functionally important phosphorylation site on α-dystrobrevin-1, generated phosphorylation-specific antibodies to it and used them to demonstrate dramatic increases in phosphorylation during the remodeling period, as well as in nerve-dependent regulation in adults. We then identified proteins that bind to this site in a phosphorylation-dependent manner and others that bind to α-dystrobrevin-1 in a phosphorylation-independent manner. They include multiple members of the DGC, as well as α-catulin, liprin-α1, Usp9x, PI3K, Arhgef5 and Grb2. Finally, we show that two interactors, α-catulin (phosphorylation independent) and Grb2 (phosphorylation dependent) are localized to NMJs in vivo, and that they are required for proper organization of neurotransmitter receptors on myotubes.

Forced expression of muscle specific kinase slows postsynaptic acetylcholine receptor loss in a mouse model of MuSK myasthenia gravis

We investigated the influence of postsynaptic tyrosine kinase signaling in a mouse model of muscle‐specific kinase (MuSK) myasthenia gravis (MG). Mice administered repeated daily injections of IgG from MuSK MG patients developed impaired neuromuscular transmission due to progressive loss of acetylcholine receptor (AChR) from the postsynaptic membrane of the neuromuscular junction. In this model, anti‐MuSK‐positive IgG caused a reduction in motor endplate immunolabeling for phosphorylated Src‐Y418 and AChR β‐subunit‐Y390 before any detectable loss of MuSK or AChR from the endplate. Adeno‐associated viral vector (rAAV) encoding MuSK fused to enhanced green fluorescent protein (MuSK‐EGFP) was injected into the tibialis anterior muscle to increase MuSK synthesis. When mice were subsequently challenged with 11 daily injections of IgG from MuSK MG patients, endplates expressing MuSK‐EGFP retained more MuSK and AChR than endplates of contralateral muscles administered empty vector. Recordings of compound muscle action potentials from myasthenic mice revealed less impairment of neuromuscular transmission in muscles that had been injected with rAAV‐MuSK‐EGFP than contralateral muscles (empty rAAV controls). In contrast to the effects of MuSK‐EGFP, forced expression of rapsyn‐EGFP provided no such protection to endplate AChR when mice were subsequently challenged with MuSK MG IgG. In summary, the immediate in vivo effect of MuSK autoantibodies was to suppress MuSK‐dependent tyrosine phosphorylation of proteins in the postsynaptic membrane, while increased MuSK synthesis protected endplates against AChR loss. These results support the hypothesis that reduced MuSK kinase signaling initiates the progressive disassembly of the postsynaptic membrane scaffold in this mouse model of MuSK MG.

Mechanisms controlling neuromuscular junction stability

The neuromuscular junction (NMJ) is the synaptic connection between motor neurons and muscle fibers. It is involved in crucial processes such as body movements and breathing. Its proper development requires the guidance of motor axons toward their specific targets, the development of multi-innervated myofibers, and a selective synapse stabilization. It first consists of the removal of excessive motor axons on myofibers, going from multi-innervation to a single innervation of each myofiber. Whereas guidance cues of motor axons toward their specific muscular targets are well characterized, only few molecular and cellular cues have been reported as clues for selecting and stabilizing specific neuromuscular junctions. We will first provide a brief summary on NMJ development. We will then review molecular cues that are involved in NMJ stabilization, in both pre- and post-synaptic compartments, considering motor neurons and Schwann cells on the one hand, and muscle on the other hand. We will provide links with pathologies and highlight advances that can be brought both by basic research on NMJ development and clinical data resulting from the analyses of neurodegeneration of synaptic connections to obtain a better understanding of this process. The goal of this review is to highlight the findings toward understanding the roles of poly- or single-innervations and the underlying mechanisms of NMJ stabilization.

Role of extracellular matrix proteins and their receptors in the development of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ) remains the best-studied model for understanding the mechanisms involved in synaptogenesis, due to its relatively large size, its simplicity of patterning, and its unparalleled experimental accessibility. During neuromuscular development, each skeletal myofiber secretes and deposits around its extracellular surface an assemblage of extracellular matrix (ECM) proteins that ultimately form a basal lamina. This is also the case at the NMJ, where the motor nerve contributes additional factors. Before most of the current molecular components were known, it was clear that the synaptic ECM of adult skeletal muscles was unique in composition and contained factors sufficient to induce the differentiation of both pre- and postsynaptic membranes. Biochemical, genetic, and microscopy studies have confirmed that agrin, laminin (221, 421, and 521), collagen IV (α3-α6), collagen XIII, perlecan, and the ColQ-bound form of acetylcholinesterase are all synaptic ECM proteins with important roles in neuromuscular development. The roles of their many potential receptors and/or binding proteins have been more difficult to assess at the genetic level due to the complexity of membrane interactions with these large proteins, but roles for MuSK-LRP4 in agrin signaling and for integrins, dystroglycan, and voltage-gated calcium channels in laminin-dependent phenotypes have been identified. Synaptic ECM proteins and their receptors are involved in almost all aspects of synaptic development, including synaptic initiation, topography, ultrastructure, maturation, stability, and transmission.

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ), a peripheral synapse formed between motoneuron and skeletal muscle, is characterized by a protracted postnatal period of maturation and life-long maintenance. In neuromuscular disorders such as congenital myasthenic syndromes (CMSs), disruptions of NMJ maturation and/or maintenance are frequently observed. In particular, defective neuromuscular transmission associated with structural and molecular abnormalities at the pre- and postsynaptic membranes, as well as at the synaptic cleft, has been reported in these patients. Here, we review recent advances in the understanding of molecular and cellular events that mediate NMJ maturation and maintenance. The underlying regulatory mechanisms, including key molecular regulators at the presynaptic nerve terminal, synaptic cleft, and postsynaptic muscle membrane, are discussed.

Neuregulin-1 potentiates agrin-induced acetylcholine receptor clustering through muscle-specific kinase phosphorylation

At neuromuscular synapses, neural agrin (n-agrin) stabilizes embryonic postsynaptic acetylcholine receptor (AChR) clusters by signalling through the muscle-specific kinase (MuSK) complex. Live imaging of cultured myotubes showed that the formation and disassembly of primitive AChR clusters is a dynamic and reversible process favoured by n-agrin, and possibly other synaptic signals. Neuregulin-1 is a growth factor that can act through muscle ErbB receptor kinases to enhance synaptic gene transcription. Recent studies suggest that neuregulin-1-ErbB signalling can modulate n-agrin-induced AChR clustering independently of its effects on transcription. Here we report that neuregulin-1 increased the size of developing AChR clusters when injected into muscles of embryonic mice. We investigated this phenomenon using cultured myotubes, and found that in the ongoing presence of n-agrin, neuregulin-1 potentiates AChR clustering by increasing the tyrosine phosphorylation of MuSK. This potentiation could be blocked by inhibiting Shp2, a postsynaptic tyrosine phosphatase known to modulate the activity of MuSK. Our results provide new evidence that neuregulin-1 modulates the signaling activity of MuSK and hence might function as a second-order regulator of postsynaptic AChR clustering at the neuromuscular synapse. Thus two classic synaptic signalling systems (neuregulin-1 and n-agrin) converge upon MuSK to regulate postsynaptic differentiation.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

Tyrosine phosphorylation of tau regulates its interactions with Fyn SH2 domains, but not SH3 domains, altering the cellular localization of tau

Recent reports have demonstrated that interactions between the microtubule-associated protein tau and the nonreceptor tyrosine kinase Fyn play a critical role in mediating synaptic toxicity and neuronal loss in response to β-amyloid (Aβ) in models of Alzheimer's disease. Disruption of interactions between Fyn and tau may thus have the potential to protect neurons from Aβ-induced neurotoxicity. Here, we investigated tau and Fyn interactions and the potential implications for positioning of these proteins in membrane microdomains. Tau is known to bind to Fyn via its Src-homology (SH)3 domain, an association regulated by phosphorylation of PXXP motifs in tau. Here, we show that Pro216 within the PXXP(213-216) motif in tau plays an important role in mediating the interaction of tau with Fyn-SH3. We also show that tau interacts with the SH2 domain of Fyn, and that this association, unlike that of Fyn-SH3, is influenced by Fyn-mediated tyrosine phosphorylation of tau. In particular, phosphorylation of tau at Tyr18, a reported target of Fyn, is important for mediating Fyn-SH2-tau interactions. Finally, we show that tyrosine phosphorylation influences the localization of tau to detergent-resistant membrane microdomains in primary cortical neurons, and that this trafficking is Fyn-dependent. These findings may have implications for the development of novel therapeutic strategies aimed at disrupting the tau/Fyn-mediated synaptic dysfunction that occurs in response to elevated Aβ levels in neurodegenerative disease.

To build a synapse: signaling pathways in neuromuscular junction assembly

Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.

Neural cell adhesion molecule is required for stability of reinnervated neuromuscular junctions

Studies examining the etiology of motoneuron diseases usually focus on motoneuron death as the defining pathophysiology of the disease. However, impaired neuromuscular transmission and synapse withdrawal often precede cell death, raising the possibility that abnormalities in synaptic function contribute to disease onset. Although little is known about the mechanisms maintaining the synaptic integrity of neuromuscular junctions (NMJs), Drosophila studies suggest that Fasciclin II plays an important role. Inspired by these studies we used a reinnervation model of synaptogenesis to analyze neuromuscular function in mice lacking neural cell adhesion molecule (NCAM), the Fasciclin II vertebrate homolog. Our results showed that the recovery of contractile force was the same in wild-type and NCAM-/- mice at 1 month after nerve injury, indicating that endplates were appropriately reformed. This normality was only transient because the contractile force and myofiber number decreased at 3 months after injury in NCAM-/- mice. Both declined further 3 months later. Myofibers degenerated, not because motoneurons died but because synapses were withdrawn. Although neurotransmission was initially normal at reinnervated NCAM-/- NMJs, it was significantly compromised 3 months later. Interestingly, the selective ablation of NCAM from motoneurons, or muscle fibers, did not mimic the deficits observed in reinnervated NCAM-/- mice. Taken together, these results indicate that NCAM is required to maintain normal synaptic function at reinnervated NMJs, although its loss pre-synaptically or post-synaptically is not sufficient to induce synaptic destabilization. Consideration is given to the role of NCAM in terminal Schwann cells for maintaining synaptic integrity and how NCAM dysfunction may contribute to motoneuron disorders.

The function of cortactin in the clustering of acetylcholine receptors at the vertebrate neuromuscular junction

Background

Postsynaptic enrichment of acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction (NMJ) depends on the activation of the muscle receptor tyrosine MuSK by neural agrin. Agrin-stimulation of MuSK is known to initiate an intracellular signaling cascade that leads to the clustering of AChRs in an actin polymerization-dependent manner, but the molecular steps which link MuSK activation to AChR aggregation remain incompletely defined.

Methodology/Principal Findings

In this study we used biochemical, cell biological and molecular assays to investigate a possible role in AChR clustering of cortactin, a protein which is a tyrosine kinase substrate and a regulator of F-actin assembly and which has also been previously localized at AChR clustering sites. We report that cortactin was co-enriched at AChR clusters in situ with its target the Arp2/3 complex, which is a key stimulator of actin polymerization in cells. Cortactin was further preferentially tyrosine phosphorylated at AChR clustering sites and treatment of myotubes with agrin significantly enhanced the tyrosine phosphorylation of cortactin. Importantly, forced expression in myotubes of a tyrosine phosphorylation-defective cortactin mutant (but not wild-type cortactin) suppressed agrin-dependent AChR clustering, as did the reduction of endogenous cortactin levels using RNA interference, and introduction of the mutant cortactin into muscle cells potently inhibited synaptic AChR aggregation in response to innervation.

Conclusion

Our results suggest a novel function of phosphorylation-dependent cortactin signaling downstream from agrin/MuSK in facilitating AChR clustering at the developing NMJ.

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters

Background

A crucial event in the development of the vertebrate neuromuscular junction (NMJ) is the postsynaptic enrichment of muscle acetylcholine (ACh) receptors (AChRs). This process involves two distinct steps: the local clustering of AChRs at synapses, which depends on the activation of the muscle-specific receptor tyrosine kinase MuSK by neural agrin, and the global dispersal of aneural or "pre-patterned" AChR aggregates, which is triggered by ACh or by synaptogenic stimuli. We and others have previously shown that tyrosine phosphatases, such as the SH2 domain-containing phosphatase Shp2, regulate AChR cluster formation in muscle cells, and that tyrosine phosphatases also mediate the dispersal of pre-patterned AChR clusters by synaptogenic stimuli, although the specific phosphatases involved in this latter step remain unknown.

Results

Using an assay system that allows AChR cluster assembly and disassembly to be studied separately and quantitatively, we describe a previously unrecognized role of the tyrosine phosphatase Shp2 in AChR cluster disassembly. Shp2 was robustly expressed in embryonic Xenopus muscle in vivo and in cultured myotomal muscle cells, and treatment of the muscle cultures with an inhibitor of Shp2 (NSC-87877) blocked the dispersal of pre-patterned AChR clusters by synaptogenic stimuli. In contrast, over-expression in muscle cells of either wild-type or constitutively active Shp2 accelerated cluster dispersal. Significantly, forced expression in muscle of the Shp2-activator SIRPα1 (signal regulatory protein α1) also enhanced the disassembly of AChR clusters, whereas the expression of a truncated SIRPα1 mutant that suppresses Shp2 signaling inhibited cluster disassembly.

Conclusion

Our results suggest that Shp2 activation by synaptogenic stimuli, through signaling intermediates such as SIRPα1, promotes the dispersal of pre-patterned AChR clusters to facilitate the selective accumulation of AChRs at developing NMJs.

Rapsyn carboxyl terminal domains mediate muscle specific kinase-induced phosphorylation of the muscle acetylcholine receptor

At the developing vertebrate neuromuscular junction, postsynaptic localization of the acetylcholine receptor (AChR) is regulated by agrin signaling via the muscle specific kinase (MuSK) and requires an intracellular scaffolding protein called rapsyn. In addition to its structural role, rapsyn is also necessary for agrin-induced tyrosine phosphorylation of the AChR, which regulates some aspects of receptor localization. Here, we have investigated the molecular mechanism by which rapsyn mediates AChR phosphorylation at the rodent neuromuscular junction. In a heterologous COS cell system, we show that MuSK and rapsyn induced phosphorylation of beta subunit tyrosine 390 (Y390) and delta subunit Y393, as in muscle cells. Mutation of beta Y390 or delta Y393 did not inhibit MuSK/rapsyn-induced phosphorylation of the other subunit in COS cells, and mutation of beta Y390 did not inhibit agrin-induced phosphorylation of the delta subunit in Sol8 muscle cells; thus, their phosphorylation occurs independently, downstream of MuSK activation. In COS cells, we further show that MuSK-induced phosphorylation of the beta subunit was mediated by rapsyn, as MuSK plus rapsyn increased beta Y390 phosphorylation more than rapsyn alone and MuSK alone had no effect. Intriguingly, MuSK also induced tyrosine phosphorylation of rapsyn itself. We then used deletion mutants to map the rapsyn domains responsible for activation of cytoplasmic tyrosine kinases that phosphorylate the AChR subunits. We found that rapsyn C-terminal domains (amino acids 212-412) are both necessary and sufficient for activation of tyrosine kinases and induction of cellular tyrosine phosphorylation. Moreover, deletion of the rapsyn RING domain (365-412) abolished MuSK-induced tyrosine phosphorylation of the AChR beta subunit. Together, these findings suggest that rapsyn facilitates AChR phosphorylation by activating or localizing tyrosine kinases via its C-terminal domains.

Regulation of ACh receptor clustering by the tyrosine phosphatase Shp2

At the vertebrate neuromuscular junction (NMJ), postsynaptic aggregation of muscle acetylcholine receptors (AChRs) depends on the activation of MuSK, a muscle-specific tyrosine kinase that is stimulated by neural agrin and regulated by muscle-intrinsic tyrosine kinases and phosphatases. We recently reported that Shp2, a tyrosine phosphatase containing src homology two domains, suppressed MuSK-dependent AChR clustering in cultured myotubes, but how this effect of Shp2 is controlled has remained unclear. In this study, biochemical assays showed that agrin-treatment of C2 mouse myotubes enhanced the tyrosine phosphorylation of signal regulatory protein alpha1 (SIRPalpha1), a known activator of Shp2, and promoted SIRPalpha1's interaction with Shp2. Moreover, in situ experiments revealed that treatment of myotubes with the Shp2-selective inhibitor NSC-87877 increased spontaneous and agrin-induced AChR clustering, and that AChR clustering was also enhanced in myotubes ectopically expressing inactive (dominant-negative) Shp2; in contrast, AChR clustering was reduced in myotubes expressing constitutively active Shp2. Significantly, expression of truncated (nonShp2-binding) and full-length (Shp2-binding) forms of SIRPalpha1 in myotubes also increased and decreased AChR clustering, respectively, and coexpression of truncated SIRPalpha1 with active Shp2 and full-length SIRPalpha1 with inactive Shp2 reversed the actions of the exogenous Shp2 proteins on AChR clustering. These results suggest that SIRPalpha1 is a novel downstream target of MuSK that activates Shp2, which, in turn, suppresses AChR clustering. We propose that an inhibitory loop involving both tyrosine kinases and phosphatases sets the level of agrin/MuSK signaling and constrains it spatially to help generate high-density AChR clusters selectively at NMJs.

Synaptic differentiation is defective in mice lacking acetylcholine receptor beta-subunit tyrosine phosphorylation

Agrin activates MuSK, a receptor tyrosine kinase expressed in skeletal muscle, leading to tyrosine phosphorylation of the acetylcholine receptor (AChR) beta-subunit and clustering of AChRs. The importance of AChR beta-subunit tyrosine phosphorylation in clustering AChRs and regulating synaptic differentiation is poorly understood. We generated mice with targeted mutations in the three intracellular tyrosines of the AChR beta-subunit (AChR-beta(3F/3F)). Mice lacking AChR beta-subunit tyrosine phosphorylation thrive postnatally and have no overt behavioral defects, indicating that AChR beta-subunit tyrosine phosphorylation is not essential for the formation of neuromuscular synapses. Nonetheless, the size of synapses and the density of synaptic AChRs are reduced in AChR- beta(3F/3F) mutant mice. Moreover, synapses are structurally simplified and the organization of postjunctional folds is aberrant in mice lacking tyrosine phosphorylation of the AChR beta-subunit. Furthermore, mutant AChRs cluster poorly in response to agrin and are readily extracted from the cell surface of cultured myotubes by non-ionic detergent. These data indicate that tyrosine phosphorylation of the AChR beta-subunit has an important role in organizing AChRs and regulating synaptic differentiation.

Phosphoinositide 3-kinase acts through RAC and Cdc42 during agrin-induced acetylcholine receptor clustering

The formation of the neuromuscular junction (NMJ) is regulated by the nerve-derived heparan sulfate proteoglycan agrin and the muscle-specific kinase MuSK. Agrin induces a signal transduction pathway via MuSK, which promotes the reorganization of the postsynaptic muscle membrane. Activation of MuSK leads to the phosphorylation and redistribution of acetylcholine receptors (AChRs) and other postsynaptic proteins to synaptic sites. The accumulation of high densities of AChRs at postsynaptic regions represents a hallmark of NMJ formation and is required for proper NMJ function. Here we show that phosphoinositide 3-kinase (PI3-K) represents a component of the agrin/MuSK signaling pathway. Muscle cells treated with specific PI3-K inhibitors are unable to form full-size AChR clusters in response to agrin and AChR phosphorylation is reduced. Moreover, agrin-induced activation of Rac and Cdc42 is impaired in the presence of PI3-K inhibitors. PI3-K is localized to the postsynaptic muscle membrane consistent with a role during agrin/MuSK signaling. These results put PI3-K downstream of MuSK as regulator of AChR phosphorylation and clustering. Its role during agrin-stimulated Rac and Cdc42 activation suggests a critical function during cytoskeletal reorganizations, which lead to the redistribution of actin-anchored AChRs.

Tyrosine phosphatases such as SHP-2 act in a balance with Src-family kinases in stabilization of postsynaptic clusters of acetylcholine receptors

BACKGROUND

Development of neural networks requires that synapses are formed, eliminated and stabilized. At the neuromuscular junction (NMJ), agrin/MuSK signaling, by triggering downstream pathways, causes clustering and phosphorylation of postsynaptic acetylcholine receptors (AChRs). Postnatally, AChR aggregates are stabilized by molecular pathways that are poorly characterized. Gain or loss of function of Src-family kinases (SFKs) disassembles AChR clusters at adult NMJs in vivo, whereas AChR aggregates disperse rapidly upon withdrawal of agrin from cultured src-/-;fyn-/- myotubes. This suggests that a balance between protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) such as those of the Src-family may be essential in stabilizing clusters of AChRs.

RESULTS

We have analyzed the role of PTPs in maintenance of AChR aggregates, by adding and then withdrawing agrin from cultured myotubes in the presence of PTP or PTK inhibitors and quantitating remaining AChR clusters. In wild-type myotubes, blocking PTPs with pervanadate caused enhanced disassembly of AChR clusters after agrin withdrawal. When added at the time of agrin withdrawal, SFK inhibitors destabilized AChR aggregates but concomitant addition of pervanadate rescued cluster stability. Likewise in src-/-;fyn-/- myotubes, in which agrin-induced AChR clusters form normally but rapidly disintegrate after agrin withdrawal, pervanadate addition stabilized AChR clusters. The PTP SHP-2, known to be enriched at the NMJ, associated and colocalized with MuSK, and agrin increased this interaction. Specific SHP-2 knockdown by RNA interference reduced the stability of AChR clusters in wild-type myotubes. Similarly, knockdown of SHP-2 in adult mouse soleus muscle by electroporation of RNA interference constructs caused disassembly of pretzel-shaped AChR-rich areas in vivo. Finally, we found that src-/-;fyn-/- myotubes contained elevated levels of SHP-2 protein.

CONCLUSION

Our data are the first to show that the fine balance between PTPs and SFKs is a key aspect in stabilization of postsynaptic AChR clusters. One phosphatase that acts in this equilibrium is SHP-2. Thus, PTPs such as SHP-2 stabilize AChR clusters under normal circumstances, but when these PTPs are not balanced by SFKs, they render clusters unstable.

Analysis of a Shc family adaptor protein, ShcD/Shc4, that associates with muscle-specific kinase

Shc family proteins serve as phosphotyrosine adaptor molecules in various receptor-mediated signaling pathways. In mammals, three distinct Shc genes have been described that encode proteins characterized by two phosphotyrosine-interaction modules, an amino-terminal phosphotyrosine binding (PTB) domain and a carboxy-terminal Src homology 2 domain. Here, we report the analysis of an uncharacterized fourth Shc family protein, ShcD/Shc4, that is expressed in adult brain and skeletal muscle. Consistent with this expression pattern, we find that ShcD can associate via its PTB domain with the phosphorylated muscle-specific kinase (MuSK) receptor tyrosine kinase and undergo tyrosine phosphorylation downstream of activated MuSK. Interestingly, additional sites of tyrosine phosphorylation, including a novel Grb2 binding site, are present on ShcD that are not found in other Shc family proteins. Activation of MuSK upon agrin binding at the neuromuscular junction (NMJ) induces clustering and tyrosine phosphorylation of acetylcholine receptors (AChRs) required for synaptic transmission. ShcD is coexpressed with MuSK in the postsynaptic region of the NMJ, and in cultured myotubes stimulated with agrin, expression of ShcD appears to be important for early tyrosine phosphorylation of the AChR. Thus, we have characterized a new member of the Shc family of docking proteins, which may mediate a specific aspect of signaling downstream of the MuSK receptor.

Agrin and laminin induce acetylcholine receptor clustering by convergent, Rho GTPase-dependent signaling pathways

During neuromuscular junction formation, extracellular matrix-mediated signals cause muscle surface acetylcholine receptors (AChRs) to aggregate at synaptic sites. Two extracellular matrix proteins, agrin and laminin, have each been shown to initiate signaling pathways that culminate in AChR clustering in cultured muscle cells. Here we present evidence that laminin-induced AChR clustering is mediated by the activation of the Rho GTPases Cdc42, Rac and Rho. Clustering in response to laminin is blocked by the dominant negative mutants Cdc42N17, RacN17 and RhoN19, as well as by the Rho inhibitor C3 transferase. Moreover, laminin-induced AChR clustering is impaired by the Rho kinase inhibitor Y-27632. Agrin-induced AChR clustering has previously been shown to require activation of Cdc42, Rac and Rho. Therefore, although agrin and laminin use distinct transmembrane receptors to initiate AChR clustering, their signaling pathways converge at the level of Rho GTPase activation.

Neural agrin: a synaptic stabiliser

Neural agrin is a heparan sulphate proteoglycan first defined by its ability to induce the clustering of acetylcholine receptors (AChRs) on cultured muscle cells. Neural agrin activates the transmembrane Muscle Specific Kinase (MuSK) on the postsynaptic muscle cell to stabilise the developing neuromuscular synapse. Three biological mechanisms for agrin/MuSK signalling are briefly discussed: selective transcription of synaptic genes such as MuSK itself, to reinforce developing postsynaptic clusters of AChRs; initiation of second messenger signalling pathways that can induce the formation of AChR clusters and retrograde signalling downstream of agrin/MuSK that may transform the growth cone of the motor axon into a stable differentiated nerve terminal, specialised for regulated exocytosis of neurotransmitter. Here we briefly review some key mechanisms through which neural agrin acts to foster the formation of mature neuromuscular synapses.

Cholesterol and lipid microdomains stabilize the postsynapse at the neuromuscular junction

Stabilization and maturation of synapses are important for development and function of the nervous system. Previous studies have implicated cholesterol-rich lipid microdomains in synapse stabilization, but the underlying mechanisms remain unclear. We found that cholesterol stabilizes clusters of synaptic acetylcholine receptors (AChRs) in denervated muscle in vivo and in nerve-muscle explants. In paralyzed muscles, cholesterol triggered maturation of nerve sprout-induced AChR clusters into pretzel shape. Cholesterol treatment also rescued a specific defect in AChR cluster stability in cultured src(-/-);fyn(-/-) myotubes. Postsynaptic proteins including AChRs, rapsyn, MuSK and Src-family kinases were strongly enriched in lipid microdomains prepared from wild-type myotubes. Microdomain disruption by cholesterol-sequestering methyl-beta-cyclodextrin disassembled AChR clusters and decreased AChR-rapsyn interaction and AChR phosphorylation. Amounts of microdomains and enrichment of postsynaptic proteins into microdomains were decreased in src(-/-);fyn(-/-) myotubes but rescued by cholesterol treatment. These data provide evidence that cholesterol-rich lipid microdomains and SFKs act in a dual mechanism in stabilizing the postsynapse: SFKs enhance microdomain-association of postsynaptic components, whereas microdomains provide the environment for SFKs to maintain interactions and phosphorylation of these components.

Casein kinase 2-dependent serine phosphorylation of MuSK regulates acetylcholine receptor aggregation at the neuromuscular junction

The release of Agrin by motoneurons activates the muscle-specific receptor tyrosine kinase (MuSK) as the main organizer of subsynaptic specializations at the neuromuscular junction. MuSK downstream signaling is largely undefined. Here we show that protein kinase CK2 interacts and colocalizes with MuSK at post-synaptic specializations. We observed CK2-mediated phosphorylation of serine residues within the kinase insert (KI) of MuSK. Inhibition or knockdown of CK2, or exchange of phosphorylatable serines by alanines within the KI of MuSK, impaired acetylcholine receptor (AChR) clustering, whereas their substitution by residues that imitate constitutive phosphorylation led to aggregation of AChRs even in the presence of CK2 inhibitors. Impairment of AChR cluster formation after replacement of MuSK KI with KIs of other receptor tyrosine kinases correlates with potential CK2-dependent serine phosphorylation within KIs. MuSK activity was unchanged but AChR stability decreased in the presence of CK2 inhibitors. Muscle-specific CK2beta knockout mice develop a myasthenic phenotype due to impaired muscle endplate structure and function. This is the first description of a regulatory cross-talk between MuSK and CK2 and of a role for the KI of the receptor tyrosine kinase MuSK for the development of subsynaptic specializations.

Shp2 Is Dispensable in the Formation and Maintenance of the Neuromuscular Junction

SHP2, a protein tyrosine phosphatase with two SH2 domains, has been implicated in regulating acetylcholine receptor (AChR) gene expression and cluster formation in cultured muscle cells. To understand the role of SHP2 in neuromuscular junction (NMJ) formation in vivo, we generated mus cle-specific deficient mice by using a loxP/Cre strategy since Shp2 null mutation causes embryonic lethality. Shp2(floxed/floxed) mice were crossed with mice expressing the Cre gene under the control of the human skeletal alpha-actin (HSA) promoter. Expression of SHP2 was reduced or diminished specifically in skeletal muscles of the conditional knockout (CKO) mice. The mutant mice were viable and fertile, without apparent muscle defects. The mRNA of the AChR alpha subunit and AChR clusters in CKO mice were localized in a narrow central region surrounding the phrenic nerve primary branches, without apparent change in intensity. AChR clusters colocalized with markers of synaptic vesicles and Schwann cells, suggesting proper differentiation of presynaptic terminals and Schwann cells. In comparison with age-matched littermates, no apparent difference was observed in the size and length of AChR clusters in CKO mice. Both the frequency and amplitude of mEPPs in CKO mice were similar to those in controls, suggesting normal neurotransmission when SHP2 was deficient. These results suggest that Shp2 is not required for NMJ formation and/or maintenance.

Defective neuromuscular synaptogenesis in mice expressing constitutively active ErbB2 in skeletal muscle fibers

We overexpressed a constitutively active form of the neuregulin receptor ErbB2 (CAErbB2) in skeletal muscle fibers in vivo and in vitro by tetracycline-inducible expression. Surprisingly, CAErbB2 expression during embryonic development was lethal and impaired synaptogenesis yielding a phenotype with loss of synaptic contacts, extensive axonal sprouting, and diffuse distribution of acetylcholine receptor (AChR) transcripts, reminiscent of agrin-deficient mice. CAErbB2 expression in cultured myotubes inhibited the formation and maintenance of agrin-induced AChR clusters, suggesting a muscle- and not a nerve-origin for the defect in CAErbB2-expressing mice. Levels of tyrosine phosphorylated MuSK, the signaling component of the agrin receptor, were similar, while tyrosine phosphorylation of AChRbeta subunits was dramatically reduced in CAErbB2-expressing embryos relative to controls. Thus, a gain-of-function manipulation of ErbB2 signaling pathways renders an agrin-deficient-like phenotype that uncouples MuSK and AChR tyrosine phosphorylation.

Molecular regulation of postsynaptic differentiation at the neuromuscular junction

The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.

Neurotransmitter acetylcholine negatively regulates neuromuscular synapse formation by a Cdk5-dependent mechanism

Synapse formation requires interactions between pre- and postsynaptic cells to establish the connection of a presynaptic nerve terminal with the neurotransmitter receptor-rich postsynaptic apparatus. At developing vertebrate neuromuscular junctions, acetylcholine receptor (AChR) clusters of nascent postsynaptic apparatus are not apposed by presynaptic nerve terminals. Two opposing activities subsequently promote the formation of synapses: positive signals stabilize the innervated AChR clusters, whereas negative signals disperse those that are not innervated. Although the nerve-derived protein agrin has been suggested to be a positive signal, the negative signals remain elusive. Here, we show that cyclin-dependent kinase 5 (Cdk5) is activated by ACh agonists and is required for the ACh agonist-induced dispersion of the AChR clusters that have not been stabilized by agrin. Genetic elimination of Cdk5 or blocking ACh production prevents the dispersion of AChR clusters in agrin mutants. Therefore, we propose that ACh negatively regulates neuromuscular synapse formation through a Cdk5-dependent mechanism.

The synaptic muscle-specific kinase (MuSK) complex: New partners, new functions

The muscle-specific kinase MuSK is part of an agrin receptor complex that stimulates tyrosine phosphorylation and drives clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). MuSK also regulates synaptic gene transcription in subsynaptic nuclei. Over the past few years, decisive progress has been made in the identification of MuSK effectors, helping to understand its function in the formation of the NMJ. Similarly to AChR, MuSK and several of its partners are the target of mutations responsible for diseases of the NMJ, such as congenital myasthenic syndromes. This minireview will focus on the multiple MuSK effectors so far identified that place MuSK at the center of a multifunctional signaling complex involved in the organization of the NMJ and associated disorders.

The interplay between Src family kinases and receptor tyrosine kinases

Src family tyrosine kinases (SFKs) are involved in a diverse array of physiological processes, as highlighted in this review. An overview of how SFKs interact with, and participate in signaling from, receptor tyrosine kinases (RTKs) is discussed. And also, how SFKs are activated by RTKs, and how SFKs, in turn, can activate RTKs, as well as how SFKs can promote signaling from growth factor receptors in a number of ways including participation in signaling pathways required for DNA synthesis, control of receptor turnover, actin cytoskeleton rearrangements and motility, and survival are discussed.

A single pulse of agrin triggers a pathway that acts to cluster acetylcholine receptors

Agrin triggers signaling mechanisms of high temporal and spatial specificity to achieve phosphorylation, clustering, and stabilization of postsynaptic acetylcholine receptors (AChRs). Agrin transiently activates the kinase MuSK; MuSK activation has largely vanished when AChR clusters appear. Thus, a tyrosine kinase cascade acts downstream from MuSK, as illustrated by the agrin-evoked long-lasting activation of Src family kinases (SFKs) and their requirement for AChR cluster stabilization. We have investigated this cascade and report that pharmacological inhibition of SFKs reduces early but not later agrin-induced phosphorylation of MuSK and AChRs, while inhibition of Abl kinases reduces late phosphorylation. Interestingly, SFK inhibition applied selectively during agrin-induced AChR cluster formation caused rapid cluster dispersal later upon agrin withdrawal. We also report that a single 5-min agrin pulse, followed by extensive washing, triggered long-lasting MuSK and AChR phosphorylation and efficient AChR clustering. Following the pulse, MuSK phosphorylation increased and, beyond a certain level, caused maximal clustering. These data reveal novel temporal aspects of tyrosine kinase action in agrin signaling. First, during AChR cluster formation, SFKs initiate early phosphorylation and an AChR stabilization program that acts much later. Second, a kinase mechanism rapidly activated by agrin acts thereafter autonomously in agrin's absence to further increase MuSK phosphorylation and cluster AChRs.

Agrin-induced AChR aggregate formation requires cGMP and aggregate maturation requires activation of cGMP-dependent protein kinase

Previously, it was demonstrated that agrin acting through the gaseous, signaling molecule, nitric oxide (NO), induces the formation of AChR aggregates on myotubes in culture. Soluble guanylyl cyclase (sGC), which is present at the neuromuscular junction, is a common target of NO. Therefore, we hypothesized that sGC and cGMP are involved in the agrin signaling cascade. Inhibition of sGC hindered AChR aggregation in both agrin- and NO donor-treated cultured myotubes; whereas, a cGMP analogue was able to induce the formation of AChR aggregates on naïve muscle cells. Due to the presence of cyclic GMP-dependent protein kinase (PKG) at the neuromuscular junction, we tested the ability of a PKG inhibitor to alter the agrin signaling cascade. PKG inhibition did not prevent nascent AChR aggregate formation; however, these aggregates were diffuse and composed of numerous microaggregates consistent with incomplete maturation. Thus, we conclude that cGMP is important for the initiation of AChR aggregation, while PKG is involved in the maturation of AChR aggregates.

ATP potentiates agrin-induced AChR aggregation in cultured myotubes: activation of RhoA in P2Y1 nucleotide receptor signaling at vertebrate neuromuscular junctions

At vertebrate neuromuscular junctions, ATP is known to stabilize acetylcholine in the synaptic vesicles and to be co-released with it. We have shown previously that a nucleotide receptor, P2Y(1) receptor, is localized at the nmjs, and we propose that this mediates a trophic role for synaptic ATP there. In cultured myotubes, the activation of P2Y(1) receptors modulated agrin-induced acetylcholine receptor (AChR) aggregation in a potentiation manner. This potentiation effect in agrin-induced AChR aggregation was reduced by antagonizing the P2Y(1) receptors. The guanosine triphosphatase RhoA was shown to be responsible for this P2Y(1)-potentiated effect. The localization of RhoA in rat and chicken skeletal muscles was restricted at the neuromuscular junctions. Application of P2Y(1) agonists in cultured myotubes induced RhoA activation, which showed an additive effect with agrin-induced RhoA activation. Over-expression of dominant-negative mutant of RhoA in cultured myotubes diminished the agrin-induced AChR aggregation, as well as the potentiation effect of P2Y(1)-specific agonist. Application of UTP in the cultures also triggered similar responses as did 2-methylthioadenosine 5'-diphosphate, suggesting the involvement of other subtypes of P2Y receptors. These results demonstrate that RhoA could serve as a downstream mediator of signaling mediated by P2Y(1) receptor and agrin, which therefore synergizes the effects of the two neuron-derived trophic factors in modulating the formation and/or maintenance of post-synaptic apparatus at the neuromuscular junctions.