Agrin induces phosphorylation of the nicotinic acetylcholine receptor

IgG1-3 MuSK Antibodies Inhibit AChR Cluster Formation, Restored by SHP2 Inhibitor, Despite Normal MuSK, DOK7, or AChR Subunit Phosphorylation

BACKGROUND AND OBJECTIVES

Up to 50% of patients with myasthenia gravis (MG) without acetylcholine receptor antibodies (AChR-Abs) have antibodies to muscle-specific kinase (MuSK). Most MuSK antibodies (MuSK-Abs) are IgG4 and inhibit agrin-induced MuSK phosphorylation, leading to impaired clustering of AChRs at the developing or mature neuromuscular junction. However, IgG1-3 MuSK-Abs also exist in MuSK-MG patients, and their potential mechanisms have not been explored fully.

METHODS

C2C12 myotubes were exposed to MuSK-MG plasma IgG1-3 or IgG4, with or without purified agrin. MuSK, Downstream of Kinase 7 (DOK7), and βAChR were immunoprecipitated and their phosphorylation levels identified by immunoblotting. Agrin and agrin-independent AChR clusters were measured by immunofluorescence and AChR numbers by binding of 125I-α-bungarotoxin. Transcriptomic analysis was performed on treated myotubes.

RESULTS

IgG1-3 MuSK-Abs impaired AChR clustering without inhibiting agrin-induced MuSK phosphorylation. Moreover, the well-established pathway initiated by MuSK through DOK7, resulting in βAChR phosphorylation, was not impaired by MuSK-IgG1-3 and was agrin-independent. Nevertheless, the AChR clusters did not form, and both the number of AChR microclusters that precede full cluster formation and the myotube surface AChRs were reduced. Transcriptomic analysis did not throw light on the pathways involved. However, the SHP2 inhibitor, NSC-87877, increased the number of microclusters and led to fully formed AChR clusters.

DISCUSSION

MuSK-IgG1-3 is pathogenic but seems to act through a noncanonical pathway. Further studies should throw light on the mechanisms involved at the neuromuscular junction.

Agrin expression is correlated with tumor development and poor prognosis in cholangiocarcinoma

Objective

This study examined the role of agrin in the development of cholangiocarcinoma (CCA).

Methods

Western blotting was performed to detect the expression of target genes. The correlation between agrin expression and prognosis was analyzed using the Kaplan–Meier method. Proliferation, migration, invasion, and tumorigenesis were examined in CCA cells and tissues using the Cell Counting Kit-8 assay, cell cycle analysis, transwell migration assay, and nude mouse tumorigenicity assay in vivo, respectively.

Results

Agrin expression was significantly upregulated in CCA tissues compared with that in adjacent non-tumor tissues, and agrin expression was correlated with poorer tumor characteristics such as portal vein tumor thrombus, intrahepatic metastasis, and worse survival. Forced agrin expression in CCA cells apparently promoted proliferation, colony formation, migration, invasion, and cell cycle progression, but agrin depletion had the opposite effects. Furthermore, agrin-depleted CCA cells developed fewer and smaller tumors than control cells in vivo. Mechanistic analyses indicated that agrin activated the Hippo signaling pathway and induced the translocation of YAP to the nucleus.

Conclusions

Agrin promoted CCA progression by activating the Hippo signaling pathway, suggesting its promise as a target for CCA therapy.

Alternative Splicing and the Intracellular Domain Mediate TM-agrin's Ability to Differentially Regulate the Density of Excitatory and Inhibitory Synapse-like Specializations in Developing CNS Neurons

Agrin is a multi-domain protein best known for its essential function during formation of the neuromuscular junction. Alternative mRNA splicing at sites named y and z in the C-terminal part of agrin regulates its interaction with a receptor complex consisting of the agrin-binding low-density lipoprotein receptor-related protein 4 (Lrp4) and the muscle-specific kinase (MuSK). Isoforms with inserts at both splice sites bind to Lrp4, activate MuSK and are synaptogenic at the neuromuscular junction. Agrin is also expressed as a transmembrane protein in the central nervous system (CNS) but its function during interneuronal synapse formation is unclear. Recently we demonstrated that transfection of a full-length cDNA coding for transmembrane agrin (TM-agrin) in cultured embryonic cortical neurons induced an Lrp4-dependent but MuSK-independent increase in dendritic glutamatergic synapses and an Lrp4- and MuSK-independent reduction of inhibitory synapses. Here we show that presynaptic specializations were similarly affected by TM-agrin overexpression. In addition, we mapped the regions within TM-agrin responsible for TM-agrin's effects on dendritic aggregates of synapse-associated proteins. We show that the presence of a four amino acid insert at splice site y is essential for the increase in the density of puncta containing the postsynaptic density protein 95 kDa. This effect was independent of splice site z. The reduction of the gephyrin puncta density was independent of the entire extracellular part of TM-agrin but required a highly conserved serine residue in the intracellular domain of TM-agrin. These results provide further evidence for a function of TM-agrin during CNS synaptogenesis and demonstrate that different domains and alternative splicing of TM-agrin differentially affect excitatory and inhibitory synapse formation in cultured embryonic CNS neurons.

Matrix Topography Regulates Synaptic Transmission at the Neuromuscular Junction

Recreation of a muscle that can be controlled by the nervous system would provide a major breakthrough for treatments of injury and diseases. However, the underlying basis of how neuron-muscle interfaces are formed is still not understood sufficiently. Here, it is hypothesized that substrate topography regulates neural innervation and synaptic transmission by mediating the cross-talk between neurons and muscles. This hypothesis is examined by differentiating neural stem cells on the myotubes, formed on the substrate with controlled groove width. The substrate with the groove width of 1600 nm, a similar size to the myofibril diameter, serves to produce larger and aligned myotubes than the flat substrate. The myotubes formed on the grooved substrate display increases in the acetylcholine receptor expression. Reciprocally, motor neuron progenitor cells differentiated from neural stem cells innervate the larger and aligned myotubes more actively than randomly oriented myotubes. As a consequence, mature and aligned myotubes respond to glutamate (i.e., an excitatory neurotransmitter) and curare (i.e., a neuromuscular antagonist) more rapidly and homogeneously than randomly oriented myotubes. The results of this study will be broadly useful for improving the quality of engineered muscle used in a series of applications including drug screening, regeneration therapies, and biological machinery assembly.

Exploring Missense Mutations in Tyrosine Kinases Implicated with Neurodegeneration

Protein kinases are one of the largest families of evolutionarily related proteins and the third most common protein class of human genome. All the protein kinases share the same structural organization. They are made up of an extracellular domain, transmembrane domain and an intra cellular kinase domain. Missense mutations in these kinases have been studied extensively and correlated with various neurological disorders. Individual mutations in the kinase domain affect the functions of protein. The enhanced or reduced expression of protein leads to hyperactivation or inactivation of the signalling pathways, resulting in neurodegeneration. Here, we present extensive analyses of missense mutations in the tyrosine kinase focussing on the neurodegenerative diseases encompassing structure function relationship. This is envisaged to enhance our understanding about the neurodegeneration and possible therapeutic measures.

Evaluation of the Nicotinic Acetylcholine Receptor-Associated Proteome at Baseline and Following Nicotine Exposure in Human and Mouse Cortex

Nicotinic acetylcholine receptors (nAChRs) support the initiation and maintenance of smoking, but the long-term changes occurring in the protein complex as a result of smoking and the nicotine in tobacco are not known. Human studies and animal models have also demonstrated that increasing cholinergic tone increases behaviors related to depression, suggesting that the nAChR-associated proteome could be altered in individuals with mood disorders. We therefore immunopurified nAChRs and associated proteins for quantitative proteomic assessment of changes in protein–protein interactions of high-affinity nAChRs containing the β2 subunit (β2*-nAChRs) from either cortex of mice treated with saline or nicotine, or postmortem human temporal cortex tissue from tobacco-exposed and nonexposed individuals, with a further comparison of diagnosed mood disorder to control subjects. We observed significant effects of nicotine exposure on the β2*-nAChR-associated proteome in human and mouse cortex, particularly in the abundance of the nAChR subunits themselves, as well as putative interacting proteins that make up core components of neuronal excitability (Na/K ATPase subunits), presynaptic neurotransmitter release (syntaxins, SNAP25, synaptotagmin), and a member of a known nAChR protein chaperone family (14-3-3ζ). These findings identify candidate-signaling proteins that could mediate changes in cholinergic signaling via nicotine or tobacco use. Further analysis of identified proteins will determine whether these interactions are essential for primary function of nAChRs at presynaptic terminals. The identification of differences in the nAChR-associated proteome and downstream signaling in subjects with various mood disorders may also identify novel etiological mechanisms and reveal new treatment targets.

■ REVIEW : Postsynaptic Anchoring of Receptors: A Cellular Approach to Neuronal and Muscular Sensitivity

Significant progress has been made toward the elucidation of the molecular mechanisms underlying the biogenesis and stabilization of postsynaptic membrane specializations at the neuromuscular junction of vertebrate skeletal muscle. The emerging picture reveals a continuous molecular link from the extracellular matrix within the synaptic cleft via integral and peripheral membrane proteins to the subsarcolemmal cytoskeleton. The formation and maintenance of synaptic contacts between neurons in the CNS might follow similar architectural principles but involve different molecules. The biogenesis of glycinergic postsynaptic membrane specializations depends on the widely expressed peripheral membrane protein gephyrin, which anchors the neurotransmitter receptor to underlying cytoskeletal elements in a dynamic manner. This anchoring mechanism could also contribute to the plasticity of glycinergic synapses. Other types of neurotransmitter receptors, like GABAA- and glutamate receptors, may have evolved different molecular mechanisms to ensure their localization in postsynaptic membrane specializations. The Neuroscientist 2:100-108, 1996

Sodium nitrate decreases agrin-induced acetylcholine receptor clustering

BACKGROUND

Humans are exposed to nitrate predominantly through diet with peak plasma concentrations within an hour after ingestion, but additional exposure is obtained from the environment, and minimally through de novo synthesis. Higher nitrate consumption has been associated with methemoglobinemia, spontaneous abortions, atherosclerosis, myocardial ischemia, septic and distressed lung, inflammatory bowel disease, amyotrophic lateral sclerosis, and neural tube defects. However, skeletal muscle development has not been examined.

METHODS

C2C12 skeletal muscle cell cultures were maintained, myoblasts were fused into myotubes, and then cultures were exposed to motor neuron derived agrin to enhance acetylcholine receptor (AChR) clustering. Untreated cultures were compared with cultures exposed to sodium nitrate at concentrations ranging from 10 ng/mL-100 μg/mL.

RESULTS

The results reported here demonstrate that 1 μg/mL sodium nitrate was sufficient to decrease the frequency of agrin-induced AChR clustering without affecting myotube formation. In addition, concentrations of sodium nitrate of 1 μg/mL or 100 μg/mL decreased gene expression of the myogenic transcription factor myogenin and AChR in correlation with the agrin-induced AChR clustering data.

CONCLUSIONS

These results reveal that sodium nitrate decreases the frequency of agrin-induced AChR clustering by a mechanism that includes myogenin and AChR gene expression. As a consequence sodium nitrate may pose a risk for skeletal muscle development and subsequent neuromuscular synapse formation in humans.

Regulation of muscle acetylcholine receptor turnover by β subunit tyrosine phosphorylation

At the neuromuscular junction (NMJ), the postsynaptic localization of muscle acetylcholine receptor (AChR) is regulated by neural signals and occurs via several processes including metabolic stabilization of the receptor. However, the molecular mechanisms that influence receptor stability remain poorly defined. Here, we show that neural agrin and the tyrosine phosphatase inhibitor, pervanadate slow the degradation of surface receptor in cultured muscle cells. Their action is mediated by tyrosine phosphorylation of the AChR β subunit, as agrin and pervandate had no effect on receptor half-life in AChR-β(3F/3F) muscle cells, which have targeted mutations of the β subunit cytoplasmic tyrosines. Moreover, in wild type AChR-β(3Y) muscle cells, we found a linear relationship between average receptor half-life and the percentage of AChR with phosphorylated β subunit, with half-lives of 12.7 and 23 h for nonphosphorylated and phosphorylated receptor, respectively. Surprisingly, pervanadate increased receptor half-life in AChR-β(3Y) myotubes in the absence of clustering, and agrin failed to increase receptor half-life in AChR-β(3F/3F) myotubes even in the presence of clustering. The metabolic stabilization of the AChR was mediated specifically by phosphorylation of βY390 as mutation of this residue abolished β subunit phosphorylation but did not affect δ subunit phosphorylation. Receptor stabilization also led to higher receptor levels, as agrin increased surface AChR by 30% in AChR-β(3Y) but not AChR-β(3F/3F) myotubes. Together, these findings identify an unexpected role for agrin-induced phosphorylation of β(Y390) in downregulating AChR turnover. This likely stabilizes AChR at developing synapses, and contributes to the extended half-life of AChR at adult NMJs.

Synaptic neurotransmitter-gated receptors

Since the discovery of the major excitatory and inhibitory neurotransmitters and their receptors in the brain, many have deliberated over their likely structures and how these may relate to function. This was initially satisfied by the determination of the first amino acid sequences of the Cys-loop receptors that recognized acetylcholine, serotonin, GABA, and glycine, followed later by similar determinations for the glutamate receptors, comprising non-NMDA and NMDA subtypes. The last decade has seen a rapid advance resulting in the first structures of Cys-loop receptors, related bacterial and molluscan homologs, and glutamate receptors, determined down to atomic resolution. This now provides a basis for determining not just the complete structures of these important receptor classes, but also for understanding how various domains and residues interact during agonist binding, receptor activation, and channel opening, including allosteric modulation. This article reviews our current understanding of these mechanisms for the Cys-loop and glutamate receptor families.

Developmental and stress-induced remodeling of cell–cell communication in the adrenal medullary tissue

The adrenal medullary tissue contributes to maintain body homeostasis in reaction to stressful environmental changes via the release of catecholamines into the blood circulation in response to splanchnic nerve activation. Accordingly, chromaffin cell stimulus-secretion coupling undergoes temporally restricted periods of anatomo- functional remodeling in response to prevailing hormonal requirements of the organism. The postnatal development of the adrenal medulla and response to stress are remarkable physiological situations in which the stimulus- secretion coupling is critically affected. Catecholamine secretion from rat chromaffin cells is under a dual control involving an incoming initial command arising from the sympathetic nervous system that releases acetylcholine at the splanchnic nerve terminal-chromaffin cell synapses and a local gap junction-mediated intercellular communication. Interestingly, these two communication pathways are functionally interconnected within the gland and exhibit coordinated plasticity mechanisms. This article reviews the physiological and molecular evidence that the adrenal medullary tissue displays anatomical and functional adaptative remodeling of cell–cell communications upon physiological (postnatal development) and/or physiopathological (stress) situations associated with specific needs in circulating catecholamine levels.

Decreased phosphorylation of δ and ε subunits of the acetylcholine receptor coincides with delayed postsynaptic maturation in PKC θ deficient mouse

Protein kinase C (PKC) activity is involved in the nicotinic acetylcholine receptor (nAChR) redistribution at the neuromuscular junction in vivo during postnatal maturation. Here we studied, in PKC theta (PKCtheta) deficient mice (KO), how the theta isoform of PKC is involved in the nAChR cluster maturation that is accompanied by the developmental activity-dependent neuromuscular synapse elimination process. We found that axonal elimination and dispersion of nAChR from the postsynaptic plaques and its redistribution to form the mature postsynaptic apparatus were delayed but not totally suppressed in PKCtheta deficient mice. Moreover, the delay in the maturation of the morphology of the nAChR clusters during the early postnatal synapse elimination period in the PKCtheta deficient mice coincides with a reduction in the PKCtheta-mediated phosphorylation on the delta subunit of the nAChR. In addition, we show evidence for PKCtheta regulation of PKA in normally phosphorylating the epsilon subunit of nAChR. We have also found that the theta isoform of PKC is located on the postsynaptic component of the neuromuscular junction but is also expressed by motoneurons in the spinal cord and in the motor nerve terminals. The results allow us to hypothesize that a spatially specific and opposing action of PKCtheta and PKA may result in activity-dependent alterations to synaptic connectivity at both the nerve inputs and the postsynaptic nAChR clusters.

Associated proteins: The universal toolbox controlling ligand gated ion channel function

Ligand gated ion channels are integral multimeric membrane proteins that can detect with high sensitivity the presence of a specific transmitter in the extracellular space and transduce this signal into an ion flux. While these receptors are widely expressed in the nervous system, their expression is not limited to neurons or their postsynaptic targets but extends to non-neuronal cells where they participate in many physiological responses. Cells have developed complex regulatory mechanisms allowing for the precise control and modulation of ligand gated ion channels. In this overview the roles of accessory subunits and associated proteins in these regulatory mechanisms are reviewed and their relevance illustrated by examples at different ligand gated ion channel types, with emphasis on nicotinic acetylcholine receptors. Dysfunction of ligand gated ion channels can result in neuromuscular, neurological or psychiatric disorders. A better understanding of the precise function of associated proteins and how they impact on ligand gated ion channels will provide new therapeutic opportunities for clinical intervention.

Ethanol decreases agrin-induced acetylcholine receptor clustering in C2C12 myotube culture

We investigated the effect of ethanol on skeletal muscle development using C2C12 cell culture. The ethanol concentrations of 10mM, 25mM, and 100mM, were tested because plasma samples of alcohol-dependent individuals fall within this range. We assessed two specific events in skeletal muscle development, the fusion of myoblasts to form myotubes and the acetylcholine receptor (AChR) clustering associated with neuromuscular synapse formation. We report that ethanol does not effect myotube formation or the viability of myoblasts or myotubes in C2C12 cell culture. However, ethanol does effect AChR clustering on C2C12 myotubes. As motor neurons approach skeletal muscle during development, agrin is released by motor neurons and induces AChR clustering on muscle fibers. In our experiments, agrin was applied to cell cultures during the period when myoblasts fuse to form myotubes. In cell cultures exposed to ethanol during myotube formation, agrin-induced AChR clustering was decreased compared to untreated cultures. In cell cultures exposed to ethanol during myoblast proliferation, with ethanol removed during myotube formation, agrin-induced AChR clustering was unaffected. We conclude that exposure to a physiologically relevant concentration of ethanol during the specific period of myotube formation decreases agrin-induced AChR clustering.

Na,K-ATPase activity regulates AMPA receptor turnover through proteasome-mediated proteolysis

Neuronal activity largely depends on two key components on the membrane: the Na,K-ATPase (NKA) that maintains the ion gradients and sets the foundation of excitability, and the ionotropic glutamatergic AMPA receptors (AMPARs) through which sodium influx forms the driving force for excitation. Because the frequent sodium transients from glutamate receptor activity need to be efficiently extruded, a functional coupling between NKA and AMPARs should be a necessary cellular device for synapse physiology. We show that NKA is enriched at synapses and associates with AMPARs. NKA dysfunction induces a rapid reduction in AMPAR cell-surface expression as well as total protein abundance, leading to a long-lasting depression in synaptic transmission. AMPAR proteolysis requires sodium influx, proteasomal activity and receptor internalization. These data elucidate a novel mechanism by which NKA regulates AMPAR turnover and thereby synaptic strength and brain function.

Epidemiological and immunological profile of muscle-specific kinase myasthenia gravis in Greece

OBJECTIVES

The purposes of this study were to determine the epidemiological characteristics of muscle-specific kinase-myasthenia gravis (MuSK-MG) in Greece and the IgG subclass of the anti-MuSK antibodies.

METHODS

This population-based study was performed on MuSK-MG patients in Greece between 1 January 1986 and 30 June 2006. Epidemiological and clinical data for 33 patients were collected. In addition, the distribution of anti-MuSK IgG autoantibody subclasses in the sera of 14 patients was determined by immunoprecipitation.

RESULTS

The average annual incidence was 0.32 patients/million population/year. On 1st July 2006, there were 33 prevalent cases, giving a point prevalence rate of 2.92/million (women 4.56 and men 1.25). In females, onset of MuSK-MG occurred after the age of 30, whilst, in males, the disease appears in any decade. The female:male incidence ratio was 3.33:1, whilst the prevalence ratio was 3.65:1. Most patients presented with involvement of the facial and bulbar muscles. Amongst about 800 MG patients seropositive for antibodies against either the AChR or MuSK, one patient was found to be seropositive for anti-MuSK antibodies and ambiguous for anti-acetylcholine receptor (anti-AChR) antibodies. The vast majority of anti-MuSK antibodies were IgG4, whilst total IgG4 levels in these patients were similar to those in two healthy controls.

CONCLUSIONS

The incidence and prevalence of MuSK-MG in Greece are amongst the highest reported previously for other countries. MuSK-MG in Greece affects both sexes, but mainly females. The main epidemiological indices were calculated. The vast majority of anti-MuSK antibodies were IgG4.

[Synapse formation and regeneration]

Synapse formation is probably the key process in neural development allowing signal transmission between nerve cells. As an interesting model of synapse maturation, we considered first the neuromuscular junction (NMJ), whose development is particularly dependent on intercellular interactions between the motor nerve and the skeletal muscle. Nerve and muscle have distinct roles in synaptic compartment differentiation. The initial steps of this differentiation and motor endplate formation require several postsynaptic molecular agents including agrin, the tyrosine kinase receptor MuSK and rapsyn. The agrin or motoneuron dependence of this process continues to be debated while the following steps of axonal growth and postsynaptic apparatus maintenance essentially depend on neuronal agrin and a neuron-specific signal dispersing ectopic AChR aggregate remainders, possibly mediated by acetylcholine itself. Neuregulin is essentially involved in Schwann's cell survival and guidance for axonal growth. In this paper, we will discuss the similarities between Central Nervous System (CNS) synaptic formation and Motor innervation. The limited ability of the CNS to create new synapses after nervous system injury will be then discussed with a final consideration of some new strategies elaborated to circumvent the limitations of lesion extension processes.

Identification of a motif in the acetylcholine receptor beta subunit whose phosphorylation regulates rapsyn association and postsynaptic receptor localization

At the neuromuscular junction, the acetylcholine receptor (AChR) is specifically clustered in the postsynaptic membrane via interactions with rapsyn and other scaffolding proteins. However, it remains unclear where these proteins bind on the AChR and how the interactions are regulated. Here, we define a phosphorylation-dependent binding site on the receptor that mediates agrin-induced clustering. Using chimeric proteins in which CD4 is fused to the large intracellular loop of each of the AChR subunits we found that agrin induced clustering of only chimeras containing the beta subunit loop. By making deletions in the beta loop we defined a 20 amino-acid sequence that is sufficient for clustering. The sequence contains a conserved tyrosine (Y390) whose phosphorylation is induced by agrin and whose mutation abolished clustering of beta loop chimeras and their ability to inhibit agrin-induced clustering of the endogenous AChR. Phosphorylation of the AChR beta subunit is correlated with increased rapsyn/AChR binding, suggesting that the effect of betaY390 phosphorylation on clustering is mediated by rapsyn. Indeed, we found that rapsyn associated with CD4-beta loop chimeras in a phosphorylation-dependent manner, and that agrin increased the ratio of rapsyn binding to wild type AChR but not to AChR-beta(3F/3F), which lacks beta loop tyrosine phosphorylation sites. Together, these findings suggest that agrin-induced phosphorylation of the beta subunit motif increases the stoichiometry of rapsyn binding to the AChR, thereby helping to stably cluster the receptor and anchor it at high density in the postsynaptic membrane.

Neural agrin increases postsynaptic ACh receptor packing by elevating rapsyn protein at the mouse neuromuscular synapse

Fluorescence resonance energy transfer (FRET) experiments at neuromuscular junctions in the mouse tibialis anterior muscle show that postsynaptic acetylcholine receptors (AChRs) become more tightly packed during the first month of postnatal development. Here, we report that the packing of AChRs into postsynaptic aggregates was reduced in 4-week postnatal mice that had reduced amounts of the AChR-associated protein, rapsyn, in the postsynaptic membrane (rapsyn(+/-) mice). We hypothesize that nerve-derived agrin increases postsynaptic expression and targeting of rapsyn, which then drives the developmental increase in AChR packing. Neural agrin treatment elevated the expression of rapsyn in C2 myotubes by a mechanism that involved slowing of rapsyn protein degradation. Similarly, exposure of synapses in postnatal muscle to exogenous agrin increased rapsyn protein levels and elevated the intensity of anti-rapsyn immunofluorescence, relative to AChR, in the postsynaptic membrane. This increase in the rapsyn-to-AChR immunofluorescence ratio was associated with tighter postsynaptic AChR packing and slowed AChR turnover. Acute blockade of synaptic AChRs with alpha-bungarotoxin lowered the rapsyn-to-AChR immunofluorescence ratio, suggesting that AChR signaling also helps regulate the assembly of extra rapsyn in the postsynaptic membrane. The results suggest that at the postnatal neuromuscular synapse agrin signaling elevates the expression and targeting of rapsyn to the postsynaptic membrane, thereby packing more AChRs into stable, functionally-important AChR aggregates.

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.

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.

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

L-type calcium channels mediate acetylcholine receptor aggregation on cultured muscle

Agrin activation of muscle specific kinase (MuSK) initiates postsynaptic development on skeletal muscle that includes the aggregation of acetylcholine receptors (AChRs; Glass et al. [1996]: Cell 85: 513-523; Gautam et al. [1996]: Cell 85: 525-535). Although the agrin/MuSK signaling pathway remains largely unknown, changes in intracellular calcium levels are required for agrin-induced AChR aggregation (Megeath and Fallon [1998]: J Neurosci 18: 672-678). Here, we show that L-type calcium channels (L-CaChs) are required for full agrin-induced aggregation of AChRs and sufficient to induce agrin-independent AChR aggregation. Blockade of L-CaChs in muscle cultures inhibited agrin-induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Activation of L-CaChs in the absence of agrin induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Agrin responsiveness was significantly reduced in primary muscle cultures from the muscular dysgenesis mouse, a natural mutant, which does not express the L-CaCh. Our results establish a novel role for L-CaChs as important sources of the intracellular calcium necessary for the aggregation of AChRs.

Brain-derived neurotrophic factor rapidly increases NMDA receptor channel activity through Fyn-mediated phosphorylation

Brain-derived neurotrophic factor (BDNF) is a potent modulator of hippocampal synaptic plasticity. Previously, we found that one of the targets of BDNF modulation is NR2B-containing NMDA receptors. Furthermore, exposure to the trophin rapidly increases NMDA receptor activity and enhances tyrosine phosphorylation of NR2B in cortical and hippocampal postsynaptic densities (PSDs), potentially linking receptor phosphorylation to synaptic plasticity. To define the specific NR2B residue(s) regulated by BDNF, we focused on tyrosine 1472, phosphorylation of which increases after LTP. BDNF rapidly increased phosphorylation in cortical PSDs. The tyrosine kinase Fyn is critical since BDNF-dependent phosphorylation was abolished in Fyn knockout mice. Single-channel patch clamp recordings showed that Fyn is required for the increase in NMDA receptor activity elicited by BDNF. Collectively, our results suggest that BDNF enhances phosphorylation of NR2B tyrosine 1472 through activation of Fyn, leading to alteration of NMDA receptor activity and increased synaptic transmission.

The dynamics of recycled acetylcholine receptors at the neuromuscular junction in vivo

At the peripheral neuromuscular junction (NMJ), a significant number of nicotinic acetylcholine receptors (AChRs) recycle back into the postsynaptic membrane after internalization to intermingle with not-yet-internalized ;pre-existing' AChRs. However, the way in which these receptor pools are maintained and regulated at the NMJ in living animals remains unknown. Here, we demonstrate that recycled receptors in functional synapses are removed approximately four times faster than pre-existing receptors, and that most removed recycled receptors are replaced by new recycled ones. In denervated NMJs, the recycling of AChRs is significantly depressed and their removal rate increased, whereas direct muscle stimulation prevents their loss. Furthermore, we show that protein tyrosine phosphatase inhibitors cause the selective accumulation of recycled AChRs in the peri-synaptic membrane without affecting the pre-existing AChR pool. The inhibition of serine/threonine phosphatases, however, has no effect on AChR recycling. These data show that recycled receptors are remarkably dynamic, and suggest a potential role for tyrosine dephosphorylation in the insertion and maintenance of recycled AChRs at the postsynaptic membrane. These findings may provide insights into long-term recycling processes at less accessible synapses in the central nervous system in vivo.

An Extracellular Pathway for Dystroglycan Function in Acetylcholine Receptor Aggregation and Laminin Deposition in Skeletal Myotubes

The dystroglycan (DG) complex is involved in agrin-induced acetylcholine receptor clustering downstream of muscle-specific kinase where it regulates the stability of acetylcholine receptor aggregates as well as assembly of the synaptic basement membrane. We have previously proposed that this entails coordinate extracellular and intracellular interactions of its two subunits, alpha- and beta-DG. To assess the contribution of the extracellular and intracellular portions of DG, we have used adenoviruses to express full-length and deletion mutants of beta-DG in myotubes derived from wild-type embryonic stem cells or from cells null for DG. We show that alpha-DG is properly glycosylated and targeted to the myotube surface in the absence of beta-DG. Extracellular interactions of DG modulate the size and the microcluster density of agrin-induced acetylcholine receptor aggregates and are responsible for targeting laminin to these clusters. Thus, the association of alpha- and beta-DG in skeletal muscle may coordinate independent roles in signaling. We discuss how DG may regulate synapses through extracellular signaling functions of its alpha subunit.

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.

Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis

In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve–chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction–mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family–related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Reduced Glycosaminoglycan Sulfation Diminishes the Agrin Signal Transduction Pathway

Proteoglycans consist of a protein core complexed to glycosaminoglycan (GAG) side chains, are abundant in skeletal muscle cell membranes and basal lamina, and have important functions in neuromuscular synapse development. Treatment with chlorate results in the undersulfation of heparan sulfate and chondroitin sulfate GAGs in cell culture. In addition, chlorate treatment decreases the frequency of spontaneous acetylcholine receptor (AChR) clustering in skeletal muscle cell culture. AChRs and other molecules cluster to form the postsynaptic component of neuromuscular synapses. Chlorate treatment is shown here to decrease the frequency of agrin-induced AChR clustering and agrin-induced tyrosine phosphorylation of the AChR beta-subunit. These data suggest that reduced GAG chain sulfation decreases the frequency of AChR clustering by diminishing the agrin signal transduction pathway.

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.

Reduced expression of nicotinic AChRs in myotubes from spinal muscular atrophy I patients

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by degeneration of motoneurons and skeletal muscle atrophy. In its most severe form, it leads to death before the age of 2 years. While primary degeneration of motor neurons is well established in this disease, and this results in neurogenic atrophy of skeletal muscle, we have previously reported evidence for a primary muscle defect. In this study, we used primary cultures of embryonic human skeletal muscle cells from patients with SMA and from controls to examine the effects of muscle fiber differentiation in the absence of a nerve component. Cultured SMA skeletal muscle cells are unable to fuse correctly to form multinuclear myotubes, the precursors of the myofibers. We also show that agrin-induced aggregates of nicotinic acetylcholine receptors, one of the earliest steps of neuromuscular junction formation, cannot be visualized by confocal microscopy on cells from SMA patients. In binding experiments, we demonstrate that this lack of clustering is due to defective expression of the nicotinic acetylcholine receptors in the myotubes of SMA patients whereas the affinity of alpha-bungarotoxin for its receptor remains unchanged regardless of muscle cell type (SMA or control). These observations suggest that muscle cells from SMA patients have intrinsic abnormalities that may affect proper formation of the neuromuscular junction.

Nicotine decreases agrin signaling and acetylcholine receptor clustering in C2C12 myotube culture

The clustering of acetylcholine receptors (AChRs) in skeletal muscle fibers is a critical event in neuromuscular synaptogenesis. AChRs in concert with other molecules form postsynaptic scaffolds in response to agrin released from motor neurons as motor neurons near skeletal muscle fibers in development. Agrin drives an intracellular signaling pathway that precedes AChR clustering and includes the tyrosine phosphorylation of AChRs. In C2C12 myotube culture, agrin application stimulates the agrin signaling pathway and AChR clustering. Previous studies have determined that the frequency of spontaneous AChR clustering is decreased and AChRs are partially inactivated when bound by the acetylcholine agonist nicotine. We hypothesized that nicotine interferes with AChR clustering and consequent postsynaptic scaffold formation. In the present study, C2C12 myoblasts were cultured with growth medium to stimulate proliferation and then differentiation medium to stimulate fusion into myotubes. They were bathed in a physiologically relevant concentration of nicotine and then subject to agrin treatment after myotube formation. Our results demonstrate that nicotine decreases agrin-induced tyrosine phosphorylation of AChRs and decreases the frequency of spontaneous as well as agrin-induced AChR clustering. We conclude that nicotine interferes with postsynaptic scaffold formation by preventing the tyrosine phosphorylation of AChRs, an agrin signaling event that precedes AChR clustering.

Neuregulin Inhibits Acetylcholine Receptor Aggregation in Myotubes

The high local concentration of acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction results from their aggregation by the agrin/MuSK signaling pathway and their synthetic up-regulation by the neuregulin/ErbB pathway. Here, we show a novel role for the neuregulin/ErbB pathway, the inhibition of AChR aggregation on the muscle surface. Treatment of C2C12 myotubes with the neuregulin epidermal growth factor domain decreased the number of both spontaneous and agrin-induced AChR clusters, in part by increasing the rate of cluster disassembly. Upon cluster disassembly, AChRs were internalized into caveolae (as identified by caveolin-3). Time-lapse microscopy revealed that individual AChR clusters fragmented into puncta, and application of neuregulin accelerated the rate at which AChR clusters decreased in area without affecting the density of AChRs remaining in individual clusters (as measured by the fluorescence intensity/unit area). We propose that this novel action of neuregulin regulates synaptic competition at the developing neuromuscular junction.

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.

Regulation of acetylcholine receptor clustering by the tumor suppressor APC

At the developing neuromuscular junction, motor neuron-derived agrin triggers the differentiation of postsynaptic membrane into a highly specialized structure, where the nicotinic acetylcholine receptors (AChRs) are aggregated into high-density clusters. Agrin acts by activating the muscle-specific kinase MuSK and inducing coaggregation of the 43-kDa protein rapsyn with AChRs on muscle cell membrane. The signaling mechanism downstream of MuSK is poorly defined. We report here that the mouse tumor suppressor protein adenomatous polyposis coli (APC) has a role in AChR clustering and that the Wnt/beta-catenin pathway may crosstalk with agrin signaling cascade during synapse formation.

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.

Lithium inhibits a late step in agrin-induced AChR aggregation

Agrin activates an intracellular signaling pathway to induce the formation of postsynaptic specializations on muscle fibers. In myotubes in culture, this pathway has been shown to include autophosphorylation of the muscle-specific kinase MuSK, activation of Src-family kinases, tyrosine phosphorylation of the acetylcholine receptor (AChR) beta subunit, a decrease in receptor detergent extractability, and the accumulation of AChRs into high-density aggregates. Here we report that treating chick myotubes with lithium prevented any detectable agrin-induced change in AChR distribution without affecting the number of AChRs or the agrin-induced change in AChR tyrosine phosphorylation and detergent extractability. Lithium treatment also increased the rate at which AChR aggregates disappeared when agrin was removed. The effects of lithium developed slowly over the course of approximately 12 h. Thus, sensitivity to lithium identifies a late step in the agrin signaling pathway, after agrin-induced MuSK and AChR phosphorylation, that is necessary for the recruitment of AChRs into visible aggregates.

Overexpression of nPKC theta is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubes

Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin-induced nAChR clustering in C(2)C(12) cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two-fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC alpha, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC theta, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin-induced nAChR cluster formation we overexpressed an nPKC theta -green fluorescent protein (GFP) fusion protein in C(2)C(12) myotubes. The number of nAChR clusters was significantly reduced in nPKC theta-GFP compared to GFP overexpressing myotubes at less-than-maximal clustering concentrations of agrin. These data indicate that nPKC theta activity inhibits nAChR cluster formation. To examine whether nPKC theta activation by phorbol esters regulates agrin-induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC theta-GFP compared to GFP-overexpressing myotubes, suggesting that stimulating nPKC theta activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC theta activity inhibits agrin-induced nAChR cluster formation and disrupts preexisting agrin-induced nAChR clusters, suggest that nPKC theta activity is inhibitory for agrin function.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.

MuSK glycosylation restrains MuSK activation and acetylcholine receptor clustering

MuSK, a muscle-specific receptor tyrosine kinase that is activated by agrin, has a critical role in neuromuscular synapse formation. In cultured myotubes, agrin stimulates the rapid phosphorylation of MuSK, leading to MuSK activation and tyrosine phosphorylation and clustering of acetylcholine receptors. Agrin, however, fails to stimulate tyrosine phosphorylation of MuSK that is force-expressed in myoblasts and fibroblasts, indicating that myotubes contain an additional activity that is required for agrin to stimulate MuSK. Certain glycosyltransferases are expressed selectively at synaptic sites in skeletal muscle, raising the possibility that carbohydrate modifications of MuSK, catalyzed by glycosyltransferases expressed selectively in myotubes, may be essential for agrin to bind and activate MuSK. We identifed two N-linked glycosylation sites in MuSK, and we expressed MuSK mutants lacking one or both N-linked sites into MuSK mutant myotubes to determine whether N-linked carbohydrate modifications of MuSK have a role in MuSK activation. We found that N-linked glycosylation restrains ligand-independent tyrosine phosphorylation of MuSK and downstream signaling but is not necessary for agrin to stimulate MuSK.

The Zebrafish motility mutant twitch once reveals new roles for rapsyn in synaptic function

Upon touch, twitch once zebrafish respond with one or two swimming strokes instead of typical full-blown escapes. This use-dependent fatigue is shown to be a consequence of a mutation in the tetratricopeptide domain of muscle rapsyn, inhibiting formation of subsynaptic acetylcholine receptor clusters. Physiological analysis indicates that reduced synaptic strength, attributable to loss of receptors, is augmented by a potent postsynaptic depression not seen at normal neuromuscular junctions. The synergism between these two physiological processes is causal to the use-dependent muscle fatigue. These findings offer insights into the physiological basis of human myasthenic syndrome and reveal the first demonstration of a role for rapsyn in regulating synaptic function.

Regulation of AChR clustering by Dishevelled interacting with MuSK and PAK1

An important aspect of synapse development is the clustering of neurotransmitter receptors in the postsynaptic membrane. Although MuSK is required for acetylcholine receptor (AChR) clustering at the neuromuscular junction (NMJ), the underlying molecular mechanisms remain unclear. We report here that in muscle cells, MuSK interacts with Dishevelled (Dvl), a signaling molecule important for planar cell polarity. Disruption of the MuSK-Dvl interaction inhibits Agrin- and neuron-induced AChR clustering. Expression of dominant-negative Dvl1 in postsynaptic muscle cells reduces the amplitude of spontaneous synaptic currents at the NMJ. Moreover, Dvl1 interacts with downstream kinase PAK1. Agrin activates PAK, and this activation requires Dvl. Inhibition of PAK1 activity attenuates AChR clustering. These results demonstrate important roles of Dvl and PAK in Agrin/MuSK-induced AChR clustering and reveal a novel function of Dvl in synapse development.

Dual role for calcium in agrin signaling and acetylcholine receptor clustering

Agrin is a motoneuron-derived factor that initiates neuromuscular synapse formation; however, the signaling pathway underlying postsynaptic differentiation is not yet understood. We have investigated the role of calcium in agrin signaling through the MuSK receptor tyrosine kinase and in the intracellular signaling cascade that leads to AChR phosphorylation and clustering. We find that agrin- and neuramindase-induced MuSK activation in cultured myotubes is completely blocked by removal of extracellular calcium, but only slightly reduced by clamping of intracellular calcium transients with BAPTA. Following agrin's activation of MuSK, we find that the downstream tyrosine phosphorylation of the AChR beta-subunit was inhibited by BAPTA but not by a slower acting chelator, EGTA. Similarly, agrin-induced clustering of the AChR was blocked by BAPTA but not EGTA. These findings indicate that extracellular calcium is required for the formation of a MuSK signaling complex, and that intracellular calcium regulates phosphorylation and clustering of the AChR in the postsynaptic membrane.