Induction of acetylcholine receptor gene expression by ARIA requires activation of mitogen-activated protein kinase

Identification of genetic risk loci and prioritization of genes and pathways for myasthenia gravis: a genome-wide association study

Myasthenia gravis is a chronic autoimmune disease characterized by autoantibody-mediated interference of signal transmission across the neuromuscular junction. We performed a genome-wide association study (GWAS) involving 1,873 patients diagnosed with acetylcholine receptor antibody-positive myasthenia gravis and 36,370 healthy individuals to identify disease-associated genetic risk loci. Replication of the discovered loci was attempted in an independent cohort from the UK Biobank. We also performed a transcriptome-wide association study (TWAS) using expression data from skeletal muscle, whole blood, and tibial nerve to test the effects of disease-associated polymorphisms on gene expression. We discovered two signals in the genes encoding acetylcholine receptor subunits that are the most common antigenic target of the autoantibodies: a GWAS signal within the cholinergic receptor nicotinic alpha 1 subunit (CHRNA1) gene and a TWAS association with the cholinergic receptor nicotinic beta 1 subunit (CHRNB1) gene in normal skeletal muscle. Two other loci were discovered on 10p14 and 11q21, and the previous association signals at PTPN22, HLA-DQA1/HLA-B, and TNFRSF11A were confirmed. Subgroup analyses demonstrate that early- and late-onset cases have different genetic risk factors. Genetic correlation analysis confirmed a genetic link between myasthenia gravis and other autoimmune diseases, such as hypothyroidism, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes. Finally, we applied Priority Index analysis to identify potentially druggable genes/proteins and pathways. This study provides insight into the genetic architecture underlying myasthenia gravis and demonstrates that genetic factors within the loci encoding acetylcholine receptor subunits contribute to its pathogenesis.

Regulation of Gene expression at the neuromuscular Junction

Gene expression in skeletal muscle is profoundly changed upon innervation. 50 years of research on the neuromuscular system have greatly increased our understanding of the mechanisms underlying these changes. By controlling the expression and the activity of key transcription factors, nerve-evoked electrical activity in the muscle fiber positively and negatively regulates the expression of hundreds of genes. Innervation also compartmentalizes gene expression into synaptic and extra-synaptic regions of muscle fibers. In addition, electrically-evoked, release of several factors (e.g. Agrin, Neuregulin, Wnt ligands) induce the clustering of synaptic proteins and of a few muscle nuclei. The sub-synaptic nuclei acquire a particular chromatin organization and develop a specific gene expression program dedicated to building and maintaining a functional neuromuscular synapse. Deciphering synapse-specific, transcriptional regulation started with the identification of the N-box, a six base pair element present in the promoters of the acetylcholine δ and ε subunits. Most genes with synapse-specific expression turned out to contain at least one N-box in their promoters. The N-box is a response element for the synaptic signals Agrin and Neuregulins as well as a binding site for transcription factors of the Ets family. The Ets transcription factors GABP and Erm are implicated in the activation of post-synaptic genes via the N-box. In muscle fibers, Erm expression is restricted to the NMJ whereas GABP is expressed in all muscle nuclei but phosphorylated and activated by the JNK and ERK signaling pathways in response to Agrin and Neuregulins. Post-synaptic gene expression also correlates with chromatin modifications at the genomic level as evidenced by the strong enrichment of decondensed chromatin and acetylated histones in sub-synaptic nuclei. Here we discuss these transcriptional pathways for synaptic specialization at NMJs.

Extracellular signal-regulated kinases 1 and 2 regulate neuromuscular junction and myofiber phenotypes in mammalian skeletal muscle

The neuromuscular junction is the synapse between a motor neuron of the spinal cord and a skeletal muscle fiber in the periphery. Reciprocal interactions between these excitable cells, and between them and others cell types present within the muscle tissue, shape the development, homeostasis and plasticity of skeletal muscle. An important aim in the field is to understand the molecular mechanisms underlying these cellular interactions, which include identifying the nature of the signals and receptors involved but also of the downstream intracellular signaling cascades elicited by them. This review focuses on work that shows that skeletal muscle fiber-derived extracellular signal-regulated kinases 1 and 2 (ERK1/2), ubiquitous and prototypical intracellular mitogen-activated protein kinases, have modulatory roles in the maintenance of the neuromuscular synapse and in the acquisition and preservation of fiber type identity in skeletal muscle.

Neuregulin-1/ErbB network: An emerging modulator of nervous system injury and repair

Neuregulin-1 (Nrg-1) is a member of the Neuregulin family of growth factors with essential roles in the developing and adult nervous system. Six different types of Nrg-1 (Nrg-1 type I-VI) and over 30 isoforms have been discovered; however, their specific roles are not fully determined. Nrg-1 signals through a complex network of protein-tyrosine kinase receptors, ErbB2, ErbB3, ErbB4 and multiple intracellular pathways. Genetic and pharmacological studies of Nrg-1 and ErbB receptors have identified a critical role for Nrg-1/ErbB network in neurodevelopment including neuronal migration, neural differentiation, myelination as well as formation of synapses and neuromuscular junctions. Nrg-1 signaling is best known for its characterized role in development and repair of the peripheral nervous system (PNS) due to its essential role in Schwann cell development, survival and myelination. However, our knowledge of the impact of Nrg-1/ErbB on the central nervous system (CNS) has emerged in recent years. Ongoing efforts have uncovered a multi-faceted role for Nrg-1 in regulating CNS injury and repair processes. In this review, we provide a timely overview of the most recent updates on Nrg-1 signaling and its role in nervous system injury and diseases. We will specifically highlight the emerging role of Nrg-1 in modulating the glial and immune responses and its capacity to foster neuroprotection and remyelination in CNS injury. Nrg-1/ErbB network is a key regulatory pathway in the developing nervous system; therefore, unraveling its role in neuropathology and repair can aid in development of new therapeutic approaches for nervous system injuries and associated disorders.

REVIEW ■ : Molecular Basis of Congenital Myasthenic Syndromes: Mutations in the Acetylcholine Receptor

The congenital myasthenic syndromes include end-plate (EP) acetylcholinesterase deficiency, presynaptic abnormalities affecting the evoked release or size of transmitter quanta, and acetylcholine (ACh) receptor (AChR) channelopathies stemming from a kinetic abnormality and/or deficiency of AChR. A kinetic abnor mality predicts, and AChR deficiency may predict, one or more mutations in an AChR subunit gene. These clues have led to the identification of 53 mutations in different subunits of AChR in 55 kinships of the congenital myasthenic syndromes. The mutations either increase or decrease the response to ACh, produce AChR deficiency, or both. In the slow-channel syndromes, prolonged opening episodes of AChR cause cationic overloading of the EP and an EP myopathy; the mutations occur in different subunits and different domains of the subunits and have dominant positive effects. The M1 and M2 mutations slow channel closure, increase apparent affinity for ACh, and variably enhance desensitization, and the extracellular αG153S enhances affinity for ACh, promoting reopening of the diliganded receptor. In the low-affinity fast-channel syndrome, εP121L reduces affinity for ACh and reopening of the diliganded receptor, resulting in a de creased response to ACh and shorter burst durations. Severe EP AChR deficiency results from heterozy gous or homozygous mutations that terminate translation prematurely; these are concentrated in the ε subunit, probably because substitution of the fetal γ for the adult ε subunit can rescue the phenotype from fatal null mutations in ε. Variable AChR deficiency and variable functional abnormalities stem from hetero allelic nonsense and missense mutations in AChR subunit genes. NEUROSCIENTIST 4:185-194, 1998

Neuregulin in cardiovascular development and disease

Studies in genetically modified mice have demonstrated that neuregulin-1 (NRG-1), along with the erythroblastic leukemia viral oncogene homolog (ErbB) 2, 3, and 4 receptor tyrosine kinases, is necessary for multiple aspects of cardiovascular development. These observations stimulated in vitro and in vivo animal studies, implicating NRG-1/ErbB signaling in the regulation of cardiac cell biology throughout life. Cardiovascular effects of ErbB2-targeted cancer therapies provide evidence in humans that ErbB signaling plays a role in the maintenance of cardiac function. These and other studies suggest a conceptual model in which a key function of NRG-1/ErbB signaling is to mediate adaptations of the heart to physiological and pathological stimuli through activation of intracellular kinase cascades that regulate tissue plasticity. Recent work implicates NRG-1/ErbB signaling in the regulation of multiple aspects of cardiovascular biology, including angiogenesis, blood pressure, and skeletal muscle responses to exercise. The therapeutic potential of recombinant NRG-1 as a potential treatment for heart failure has been demonstrated in animal models and is now being explored in clinical studies. NRG-1 is found in human serum and plasma, and it correlates with some clinical parameters, suggesting that it may have value as an indicator of prognosis. In this review, we bring together this growing literature on NRG-1 and its significance in cardiovascular development and disease.

Neuregulin 1 up-regulates the expression of nicotinic acetylcholine receptors through the ErbB2/ErbB3-PI3K-MAPK signaling cascade in adult autonomic ganglion neurons

We investigated effects of Neuregulin 1 (NRG1) on the expression of nicotinic acetylcholine receptor (nAChR) in major pelvic ganglion (MPG) from adult rat. MPG neurons were found to express transcripts for type I and III NRG1s as well as α and β-type epidermal growth factor (EGF)-like domains. Of the four ErbB receptor isoforms, ErbB1, ErbB2, and ErbB3 were expressed in MPG neurons. Treating MPG with NRG1β significantly increased the transcript and protein level of the nAChR α3 and β4 subunits. Consistent with these molecular data, nicotinic currents (I(ACh) ) were significantly up-regulated in NRG1β-treated sympathetic and parasympathetic MPG neurons. In contrast, the type III NRG1 and the α form of the NRG1 failed to alter the I(ACh) . Inhibition of the ErbB2 tyrosine kinase completely abolished the effects of NRG1β on the I(ACh) . Stimulation of the ErbB receptors by NRG1β activated the phosphatidylinositol-3-kinase (PI3K) and mitogen-activated protein kinase (MAPK). Immunoblot analysis revealed that PI3K-mediated activation of Akt preceded Erk1/2 activation in NRG1β-treated MPG neurons. Furthermore, specific PI3K inhibitors abrogated the phosphorylation of Erk1/2, while inhibition of MEK did not prevent the phosphorylation of Akt. Taken together, these findings suggest that NRG1 up-regulates nAChR expression via the ErbB2/ErbB3-PI3K-MAPK signaling cascade and may be involved in maintaining the ACh-mediated synaptic transmission in adult autonomic ganglia.

Ets-1 regulates radial glia formation during vertebrate embryogenesis

Radial glia cells are the first distinguishable glial population derived from neural epithelial cells and serve as guides for migrating neurons and as neural progenitor cells in the developing brain. Despite their functional importance during neural development, the determination and differentiation of these cells remains poorly understood at the molecular level. Ets-1 and Ets-2, Ets (E26 transformation-specific) transcription factors, are vertebrate homologues of Drosophila pointed, which is expressed in a subset of glia cells and promotes different aspects of Drosophila glia cell differentiation. However, it remains unsolved that the function of Ets genes is conserved in vertebrate glia development. Here we report that Ets-1 but not Ets-2 is necessary for Xenopus radial glia formation and the activity of Ets-1 is sufficient for radial glia formation prior to neural tube closure. Furthermore, we show that Ras-MAPK (mitogen activated protein kinase) signaling, which acts as an upstream activator of Ets-1 in other biological processes, also regulates radial glia formation. A mutant form of Ets-1, which is not responsive to Ras-MAPK signaling, inhibits radial glia formation promoted by Ras-MAPK signaling. Together, our results show that Ets-1 activated by Ras-MAPK signaling promotes radial glia formation during Xenopus embryogenesis.

Molecular control of neuromuscular junction development

Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.

Modulation of agrin-induced acetylcholine receptor clustering by extracellular signal-regulated kinases 1 and 2 in cultured myotubes

Agrin released by motoneurons induces and/or maintains acetylcholine receptor (AChR) clustering and other aspects of postsynaptic differentiation at the vertebrate neuromuscular junction. Agrin acts by binding and activating a receptor complex containing LDL receptor protein 4 (Lrp4) and muscle-specific kinase (MuSK). Two critical downstream components of this signaling cascade, Dox-7 and rapsyn, have been identified. However, additional intracellular essential elements remain unknown. Prior observations by others and us suggested antagonistic interactions between agrin and neuregulin-1 (Nrg-1) signaling in cultured myotubes and developing muscle fibers in vivo. A hallmark of Nrg-1 signaling in skeletal muscle cells is the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). ERK1/2 are also activated in most cells by phorbol 12-myristate 13-acetate, a classical inhibitor of agrin-induced AChR clustering in myotubes. Here, it was investigated whether agrin activates ERK1/2 directly and whether such activation modulates agrin-induced AChR clustering. Agrin induced a rapid but transient activation of ERK1/2 in myotubes that was Lrp4/MuSK-dependent. However, blocking this ERK1/2 activation did not prevent but potentiated AChR clustering induced by agrin. ERK1/2 activation was dispensable for Nrg-1-mediated inhibition of the AChR clustering activity of agrin, but was indispensable for such activity by phorbol 12-myristate 13-acetate. Together, these results suggest agrin-induced activation of ERK1/2 is a negative modulator of agrin signaling in skeletal muscle cells.

LRP4 serves as a coreceptor of agrin

Neuromuscular junction (NMJ) formation requires agrin, a factor released from motoneurons, and MuSK, a transmembrane tyrosine kinase that is activated by agrin. However, how signal is transduced from agrin to MuSK remains unclear. We report that LRP4, a low-density lipoprotein receptor (LDLR)-related protein, is expressed specifically in myotubes and binds to neuronal agrin. Its expression enables agrin binding and MuSK signaling in cells that otherwise do not respond to agrin. Suppression of LRP4 expression in muscle cells attenuates agrin binding, agrin-induced MuSK tyrosine phosphorylation, and AChR clustering. LRP4 also forms a complex with MuSK in a manner that is stimulated by agrin. Finally, we showed that LRP4 becomes tyrosine-phosphorylated in agrin-stimulated muscle cells. These observations indicate that LRP4 is a coreceptor of agrin that is necessary for MuSK signaling and AChR clustering and identify a potential target protein whose mutation and/or autoimmunization may cause muscular dystrophies.

Neuregulin 1 in neural development, synaptic plasticity and schizophrenia

Schizophrenia is a highly debilitating mental disorder that affects approximately 1% of the general population, yet it continues to be poorly understood. Recent studies have identified variations in several genes that are associated with this disorder in diverse populations, including those that encode neuregulin 1 (NRG1) and its receptor ErbB4. The past few years have witnessed exciting progress in our knowledge of NRG1 and ErbB4 functions and the biological basis of the increased risk for schizophrenia that is potentially conferred by polymorphisms in the two genes. An improved understanding of the mechanisms by which altered function of NRG1 and ErbB4 contributes to schizophrenia might eventually lead to the development of more effective therapeutics.

Muscle-specific receptor tyrosine kinase endocytosis in acetylcholine receptor clustering in response to agrin

Agrin, a factor used by motoneurons to direct acetylcholine receptor (AChR) clustering at the neuromuscular junction, initiates signal transduction by activating the muscle-specific receptor tyrosine kinase (MuSK). However, the underlying mechanisms remain poorly defined. Here, we demonstrated that MuSK became rapidly internalized in response to agrin, which appeared to be required for induced AChR clustering. Moreover, we provided evidence for a role of N-ethylmaleimide sensitive factor (NSF) in regulating MuSK endocytosis and subsequent signaling in response to agrin stimulation. NSF interacts directly with MuSK with nanomolar affinity, and treatment of muscle cells with the NSF inhibitor N-ethylmaleimide, mutation of NSF, or suppression of NSF expression all inhibited agrin-induced AChR clustering. Furthermore, suppression of NSF expression and NSF mutation attenuate MuSK downstream signaling. Our study reveals a potentially novel mechanism that regulates agrin/MuSK signaling cascade.

NRG induces membrane targeting of Galphaz in muscle: implication in myogenesis

The neuregulins (NRGs) constitute a family of trophic factors that are known to play critical roles during neural development. We recently reported that Gbeta subunit regulates NRG-mediated signaling and gene transcription in cultured C2C12 myotubes. In this study, we demonstrated that NRG treatment of C2C12 myotubes stimulates a rapid translocation of Galphaz protein to the plasma membranes. In addition, Galphaz protein is localized to the postsynaptic regions at adult neuromuscular junctions and is prominently expressed in rat skeletal muscle during early postnatal stages. Interestingly, we found that expression of the constitutively activated Galphaz in C2C12 myoblasts attenuates myogenic differentiation. Taken together, our observations reveal an unanticipated role of Galphaz in mediating the actions of NRG during neural development.

Neuregulin-1 Enhances Depolarization-Induced GABA Release

Neuregulin-1 (NRG1), a regulator of neural development, has been shown to regulate neurotransmission at excitatory synapses. Although ErbB4, a key NRG1 receptor, is expressed in glutamic acid decarboxylase (GAD)-positive neurons, little is known about its role in GABAergic transmission. We show that ErbB4 is localized at GABAergic terminals of the prefrontal cortex. Our data indicate a role of NRG1, both endogenous and exogenous, in regulation of GABAergic transmission. This effect was blocked by inhibition or mutation of ErbB4, suggesting the involvement of ErbB4. Together, these results indicate that NRG1 regulates GABAergic transmission via presynaptic ErbB4 receptors, identifying a novel function of NRG1. Because both NRG1 and ErbB4 have emerged as susceptibility genes of schizophrenia, these observations may suggest a mechanism for abnormal GABAergic neurotransmission in this disorder.

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.

Erbin inhibits RAF activation by disrupting the sur-8-Ras-Raf complex

Erbin is a member of the LAP (leucine-rich repeat (LRR) and PDZ domain) family. It inhibits Ras-mediated activation of ERK in response to growth factors. In this study, we investigated the mechanisms by which Erbin regulates the Ras-Raf-MEK pathway. The N-terminal LRR domain was necessary and sufficient to inhibit neuregulin-activated expression of epsilon416-Luc, a reporter of ERK activation. On the other hand, Erbin had no effect on Ras activation, but it attenuated neuregulin-induced Raf activation, suggesting that Erbin may regulate Raf activation by Ras. Via the LRR domain, Erbin interacts with Sur-8, a scaffold protein necessary for the Ras-Raf complex. Expression of Erbin attenuated the interaction of Sur-8 with active Ras and Raf. Moreover, Erbin-shRNA, which suppressed Erbin expression at mRNA and protein levels, increased the interaction of Sur-8 with Ras and Raf, ERK activation, and neuregulin-induced expression of endogenous acetylcholine receptor epsilon-subunit mRNA. These results demonstrate a regulatory role of Erbin in the Ras-Raf-MEK pathway, suggesting that Erbin may inhibit ERK activation by disrupting the Sur-8-Ras/Raf interaction.

Inhibition of MuSK expression by CREB interacting with a CRE-like element and MyoD

The type I receptor-like protein tyrosine kinase MuSK is essential for the neuromuscular junction formation. MuSK expression is tightly regulated during development, but the underlying mechanisms were unclear. Here we identified a novel mechanism by which MuSK expression may be regulated. A cyclic AMP response element (CRE)-like element in the 5'-flanking region of the MuSK gene binds to CREB1 (CRE-binding protein 1). Mutation of this element increases the MuSK promoter activity, suggesting a role for CREB1 in attenuation of MuSK expression. Interestingly, CREB mutants unable to bind to DNA also inhibit MuSK promoter activity, suggesting a CRE-independent inhibitory mechanism. In agreement, CREB1 could inhibit a mutant MuSK transgene reporter whose CRE site was mutated. We provide evidence that CREB interacts directly with MyoD, a myogenic factor essential for MuSK expression in muscle cells. Suppression of CREB expression by small interfering RNA increases MuSK promoter activity. These results demonstrate an important role for CREB1 in the regulation of MuSK expression.

Neuregulin-induced expression of the acetylcholine receptor requires endocytosis of ErbB receptors

Neuregulin-induced expression of the acetylcholine receptor (AChR) contributes to high concentration of the receptor at the neuromuscular junction (NMJ). Neuregulin-1 activates ErbB tyrosine kinases and subsequently intracellular kinases including Erk that is required for induced AChR expression. Recent studies demonstrate that ligand-induced internalization may regulate signaling of various receptor tyrosine kinases. However, the role of induced ErbB endocytosis in regulating AChR expression was unclear. Here we provide evidence that ErbB tyrosine kinases became rapidly internalized in response to neuregulin. The internalization required the kinase activity of ErbB proteins and involved a clathrin-dependent endocytic pathway. Moreover, neuregulin-induced Erk activation and AChR expression were attenuated when ErbB endocytosis was blocked. These results indicate that ErbB proteins undergo endocytosis in response to neuregulin, and this process is required for neuregulin signaling and induced AChR expression.

Regulation of NMDA receptors by neuregulin signaling in prefrontal cortex

Recent linkage studies have identified a significant association of the neuregulin gene with schizophrenia, but how neuregulin is involved in schizophrenia is primarily unknown. Aberrant NMDA receptor functions have been implicated in the pathophysiology of schizophrenia. Therefore, we hypothesize that neuregulin, which is present in glutamatergic synaptic vesicles, may affect NMDA receptor functions via actions on its ErbB receptors enriched in postsynaptic densities, hence participating in emotional regulation and cognitive processes that are impaired in schizophrenia. To test this, we examined the regulation of NMDA receptor currents by neuregulin signaling pathways in prefrontal cortex (PFC), a prominent area affected in schizophrenia. We found that bath perfusion of neuregulin significantly reduced whole-cell NMDA receptor currents in acutely isolated and cultured PFC pyramidal neurons and decreased NMDA receptor-mediated EPSCs in PFC slices. The effect of neuregulin was mainly blocked by application of the ErbB receptor tyrosine kinase inhibitor, phospholipase C (PLC) inhibitor, IP3 receptor (IP3R) antagonist, or Ca2+ chelators. The neuregulin regulation of NMDA receptor currents was also markedly attenuated in cultured neurons transfected with mutant forms of Ras or a dominant-negative form of MEK1 (mitogen-activated protein kinase kinase 1). Moreover, the neuregulin effect was prevented by agents that stabilize or disrupt actin polymerization but not by agents that interfere with microtubule assembly. Furthermore, neuregulin treatment increased the abundance of internalized NMDA receptors in cultured PFC neurons, which was also sensitive to agents affecting actin cytoskeleton. Together, our study suggests that both PLC/IP3R/Ca2+ and Ras/MEK/ERK (extracellular signal-regulated kinase) signaling pathways are involved in the neuregulin-induced reduction of NMDA receptor currents, which is likely through enhancing NR1 internalization via an actin-dependent mechanism.

Novel role for cyclin-dependent kinase 2 in neuregulin-induced acetylcholine receptor epsilon subunit expression in differentiated myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

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.

Neuregulin-1 induces expression of Egr-1 and activates acetylcholine receptor transcription through an Egr-1-binding site

Localization of acetylcholine receptors (AChRs) to neuromuscular synapses is mediated, in part, through selective transcription of AChR genes in myofiber synaptic nuclei. Neuregulin-1 (NRG-1) and its receptors, ErbBs, are concentrated at synaptic sites, and NRG-1 activates AChR synthesis in cultured muscle cells, suggesting that NRG-1-ErbB signaling functions to activate synapse-specific transcription. Previous studies have demonstrated that NRG-1-induced transcription is conferred by cis-acting elements located within 100 bp of 5' flanking DNA from the AChR epsilon subunit gene, and that it requires a GABP binding site within this region. To determine whether additional regulatory elements have a role in NRG-1 responsiveness, we used transcriptional reporter assays in a muscle cell line, and we identified an element that is required for NRG-1-induced transcription (neuregulin response element, NRE). Proteins from myotube extracts bind the NRE and NRG-1 treatment of the cells stimulates this binding. The ability of NRG-1 to stimulate formation of a protein-DNA complex with the NRE requires induction of protein expression. The complex contains early growth response-1 (Egr-1), a member of the Egr family of transcription factors, because proteins in the complex bind specifically to an Egr consensus site, and formation of the complex is inhibited by antibodies to Egr-1. NRG-1 induces expression of Egr-1 in myotubes, which presumably is responsible for the ability of NRG-1 to stimulate protein binding to the NRE. These results suggest that NRG-1 signaling in myotubes involves induction of Egr-1 expression, which in turn serves to activate transcription of the AChR epsilon subunit gene.

Neuronal cyclin-dependent kinase 5: role in nervous system function and its specific inhibition by the Cdk5 inhibitory peptide

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is involved in the regulation of the cell cycle. As their name suggests, the Cdks require association with activator proteins called cyclins for their activity. Cdk5, however, is unique to this family of proline-directed serine/threonine kinases on two accounts. Firstly, Cdk5 has not been found to function in the cell cycle and, although expressed in a number of tissues, its activity is restricted to the nervous system. Secondly, unlike the other members of the Cdk family, Cdk5 is not activated by association with a cyclin, although it can bind them. Instead, Cdk5 is activated by the activator proteins p35 and p39 that are structurally distinct from cyclins and have, for the most part, a neuronal-specific expression pattern. In the past decade of research on Cdk5, it is now established that Cdk5 activity is critical for the proper formation and function of the brain. Moreover, its role as a central kinase, phosphorylating its substrates in its 'cross-talk' control of other kinase and signal transduction pathways, has also been determined. In addition to the normal physiological role of Cdk5, the kinase has been implicated in certain neurodegenerative disorders. For example, Cdk5 associates with the proteolytic, more active p25 fragment that is derived through the cleavage of p35. In turn, the p25/Cdk5 complex aberrantly phosphorylates its substrates tau and neurofilaments, which has been implicated in the pathogenesis of these disorders. Here, we attempt to review the past decade of research on Cdk5 from our laboratory and others, on the roles of Cdk5 in nervous system function. Additionally, our research has recently uncovered a possible therapeutic avenue of research, focusing on inhibition of aberrant Cdk5 hyperactivity which may well be used to treat the symptoms of a number of neurodegenerative diseases. The elucidation of a specific inhibitor of p25/Cdk5, termed CIP, also inhibits p25/Cdk5-mediated tau phosphorylation. This may well provide us with avenues of research focusing on the inhibition of pathologically damaging p25/Cdk5 species.

Neuregulin Signaling on Glucose Transport in Muscle Cells

Neuregulin-1, a growth factor that potentiates myogenesis induces glucose transport through translocation of glucose transporters, in an additive manner to insulin, in muscle cells. In this study, we examined the signaling pathway required for a recombinant active neuregulin-1 isoform (rhHeregulin-beta(1), 177-244, HRG) to stimulate glucose uptake in L6E9 myotubes. The stimulatory effect of HRG required binding to ErbB3 in L6E9 myotubes. PI3K activity is required for HRG action in both muscle cells and tissue. In L6E9 myotubes, HRG stimulated PKBalpha, PKBgamma, and PKCzeta activities. TPCK, an inhibitor of PDK1, abolished both HRG- and insulin-induced glucose transport. To assess whether PKB was necessary for the effects of HRG on glucose uptake, cells were infected with adenoviruses encoding dominant negative mutants of PKBalpha. Dominant negative PKB reduced PKB activity and insulin-stimulated glucose transport but not HRG-induced glucose transport. In contrast, transduction of L6E9 myotubes with adenoviruses encoding a dominant negative kinase-inactive PKCzeta abolished both HRG- and insulin-stimulated glucose uptake. In soleus muscle, HRG induced PKCzeta, but not PKB phosphorylation. HRG also stimulated the activity of p70S6K, p38MAPK, and p42/p44MAPK and inhibition of p42/p44MAPK partially repressed HRG action on glucose uptake. HRG did not affect AMPKalpha(1) or AMPKalpha(2) activities. In all, HRG stimulated glucose transport in muscle cells by activation of a pathway that requires PI3K, PDK1, and PKCzeta, but not PKB, and that shows cross-talk with the MAPK pathway. The PI3K, PDK1, and PKCzeta pathway can be considered as an alternative mechanism, independent of insulin, to induce glucose uptake.

Implication of geranylgeranyltransferase I in synapse formation

Agrin activates the transmembrane tyrosine kinase MuSK to mediate acetylcholine receptor (AChR) clustering at the neuromuscular junction (NMJ). However, the intracellular signaling mechanism downstream of MuSK is poorly characterized. This study provides evidence that geranylgeranyltransferase I (GGT) is an important signaling component in the Agrin/MuSK pathway. Agrin causes a rapid increase in tyrosine phosphorylation of the alpha(G/F) subunit of GGT and in GGT activity. Inhibition of GGT activity or expression prevents muscle cells from forming AChR clusters in response to Agrin and attenuates the formation of neuromuscular synapses in spinal neuron-muscle cocultures. Importantly, transgenic mice expressing an alpha(G/F) mutant demonstrate NMJ defects with wider endplate bands and smaller AChR plaques. These results support the notion that prenylation is necessary for AChR clustering and the NMJ formation and/or maintenance, revealing an active role of GGT in Agrin/MuSK signaling.

Regulation of MuSK expression by a novel signaling pathway

MuSK is a receptor tyrosine kinase essential for neuromuscular junction formation. Expression of the MuSK gene is tightly regulated during development and at the neuromuscular junction. However, little is known about molecular mechanisms regulating its gene expression. Here we report a characterization of the promoter of the mouse MuSK gene. The transcription of MuSK starts at multiple sites with a major site 51 nt upstream of the translation start site. We have identified an E-box-like cis-element that is both required and sufficient for differentiation-dependent transcription. Interestingly, the promoter activity of the MuSK gene did not respond to neuregulin, a factor believed to mediate the synapse-specific transcription of acetylcholine receptor subunit genes. Rather, MuSK expression is increased in muscle cells stimulated with Wnt or at conditions when the Wnt signaling was activated. These results suggest a novel mechanism for the MuSK synapse-specific expression.

Congenital Myasthenic Syndromes: Multiple Molecular Targets at the Neuromuscular Junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

Downregulation and increased turnover of beta-amyloid precursor protein in skeletal muscle cultures by neuregulin-1

The beta-amyloid precursor protein (betaAPP) is found in skeletal muscle localized to the base of the postsynaptic folds of the neuromuscular junction; yet here, as well as in neurons, its function remains enigmatic. Here we report that the motor nerve-derived trophic factor neuregulin-1 (NRG1) regulates both steady-state betaAPP levels as well as the metabolism of the cell surface-associated protein in cultured muscle cells. These two effects occur over two discernible time scales. At short times (minutes to hours), NRG1 increases the rate of internalization and apparent degradation of cell surface betaAPP while reducing the release of soluble APP to the medium. At longer times (hours to days), NRG1 causes a decrease in mRNA for betaAPP with a concomitant reduction in steady-state protein levels. These are novel findings for this trophic factor originally identified as inducing the expression of nicotinic acetylcholine receptors and other important synaptic proteins in skeletal muscle. They suggest that betaAPP may play a receptor or signal transduction role at the neuromuscular junction since other receptor protein's actions are terminated in a similar fashion. The effects of NRG1 on betaAPP metabolism are overcome by inhibitors of both the phosphatidylinositol-3 (PI3) kinase and mitogen-activated protein (MAP) kinase pathways, yet are distinct from those activated during induction of nicotinic acetylcholine receptor biosynthesis. BetaAPP should be added to the list of specialized post-neuromuscular junction proteins that are regulated by cholinergic terminal-derived factors critical to synaptogenesis.

Ligand-dependent recruitment of the ErbB4 signaling complex into neuronal lipid rafts

Neuregulin (NRG) regulates synapse formation and synaptic plasticity, but little is known about the regulation of NRG signaling at synapses. Here we show that the NRG receptor ErbB4 was localized in anatomically defined postsynaptic densities in the brain. In cultured cortical neurons, ErbB4 was recruited to the neuronal lipid raft fraction after stimulation by NRG. Along with ErbB4, adaptor proteins Grb2 and Shc were translocated to lipid rafts by NRG stimulation. In transfected human embryonic kidney 293 cells, the partitioning of ErbB4 into a detergent-insoluble fraction that includes lipid rafts was increased by PSD-95 (postsynaptic density-95), through interaction of the ErbB4 C terminus with the PDZ [PSD-95/Discs large/zona occludens-1] domains of PSD-95. Disruption of lipid rafts inhibited NRG-induced activation of Erk and prevented NRG-induced blockade of induction of long-term potentiation at hippocampal CA1 synapses. Thus, our results indicate that NRG stimulation causes translocation of ErbB4 into lipid rafts and that lipid rafts are necessary for signaling by ErbB4.

Cyclic AMP-dependent protein kinase A and CREB are involved in neuregulin-induced synapse-specific expression of acetylcholine receptor gene

Neuregulin is reported to stimulate synapse-specific transcription of acetylcholine receptor (AChR) genes in the skeletal muscle fiber by multiple signaling pathways such as ERK, PI3K, and JNK. The co-localization of PKA mRNA with AChR and ErbBs, receptors for neuregulin, at the confined region of synapse implicates the putative role of PKA in neuregulin-induced AChR gene expression. In the present study, we found that mRNA and protein of a regulatory subunit of PKA (PKARIalpha) were concentrated at synaptic sites of the rat sternomastoid muscle fiber, while those of ERK and PI3K were uniformly distributed throughout the muscle fiber. Neuregulin (100 ng/ml) increased both PKA activity in the nucleus and AChRdelta subunit gene transcription in cultured Sol8 myotubes. These increases were significantly blocked by a specific PKA inhibitor H-89 (100 nM) and an adenylcyclase inhibitor SQ 22536 (200 microM) (72.5% and 60.1%, respectively). Furthermore, neuregulin phosphorylated CREB, a well-known down-stream transcription factor of PKA. While H-89 inhibited CREB phosphorylation, H-89 and PD098059 (50 microM), a specific MEK1/2 inhibitor, did not inhibit the phosphorylation of ERK and CREB, respectively, suggesting no cross-talk between PKA and ERK pathways. In conclusion, neuregulin increases AChRdelta subunit gene transcription, in part, by the activation of PKA/CREB, an alternative route to the previously reported ERK signaling pathway.

Casein kinase II phosphorylates translation initiation factor 5 (eIF5) in Saccharomyces cerevisiae

Eukaryotic translation initiation factor 5 (eIF5) interacts with the 40S initiation complex (40S-eIF3-mRNA-Met-tRNA(f)-eIF2-GTP) to promote the hydrolysis of ribosome-bound GTP. In Saccharomyces cerevisiae, eIF5 is encoded by a single-copy essential gene, TIF5, that is required for cell growth and viability. In this work, we show that eIF5 immunoprecipitated from cell-free extracts of (32)P-labelled yeast cells is phosphorylated on multiple serine residues. Phosphopeptide mapping reveals four major sites of phosphorylation that appear to be identical to recombinant yeast eIF5 sites phosphorylated in vitro by casein kinase II. Furthermore, analysis of eIF5 isolated from a yeast strain having a conditional mutant of casein kinase II indicates that phosphorylation of eIF5 is completely abolished at the non-permissive temperature. Additionally, haploid yeast strains were constructed to contain Ser-to-Ala mutations at the five casein kinase II consensus sequences in eIF5; in these cells, eIF5 phosphorylation was absent. Surprisingly, substitution of the TIF5 gene mutated at these sites for the wild-type gene had no obvious effect on cell growth under normal growth conditions. The implications of these results in eIF5 function are discussed.

Erbin suppresses the MAP kinase pathway

We present evidence here that Erbin is a negative regulator of the Ras-Raf-Erk signaling pathway. Expression of Erbin decreases transcription of the AChR epsilon-subunit gene, an event that is mediated by Erk activation. Although it interacts with the ErbB2 C terminus through the PDZ domain, Erbin has no effect on ErbB2 tyrosine phosphorylation or binding to the adaptor proteins Shc and Grb2. In contrast, expression of Erbin greatly impairs activation of Erk, but not Akt, by ligands that activate receptor tyrosine kinases. Moreover, Erbin inhibits the Erk activation by active Ras, while it fails to do so in the presence of active Raf-1. Erbin associates with active Ras, but not inactive Ras nor Raf. Consistently, Erbin interferes with the interaction between Ras and Raf both in vivo and in vitro. Finally, overexpression of Erbin leads to inhibition of NGF-induced neuronal differentiation of PC12 cells, whereas down-regulation of endogenous Erbin by specific siRNA exhibits an opposite effect. Collectively, our study has identified Erbin as a novel suppressor of the Ras signaling by disrupting the Ras-Raf interaction.

Congenital myasthenic syndromes: progress over the past decade

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic basal lamina, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A basal lamina CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

The spectrum of congenital myasthenic syndromes

The past decade saw remarkable advances in defining the molecular and genetic basis of the congenital myasthenic syndromes. These advances would not have been possible without antecedent clinical observations, electrophysiologic analysis, and careful morphologic studies that pointed to candidate genes or proteins. For example, a kinetic abnormality of the acetylcholine receptor (AChR) detected at the single channel level pointed to a kinetic mutation in an AChR subunit; endplate AChR deficiency suggested mutations residing in an AChR subunit or in rapsyn; absence of acetylcholinesterase (AChE) from the endplate predicted mutations in the catalytic or collagen-tailed subunit of this enzyme; and a history of abrupt episodes of apnea associated with a stimulation dependent decrease of endplate potentials and currents implicated proteins concerned with ACh resynthesis or vesicular filling. Discovery of mutations in endplate-specific proteins also prompted expression studies that afforded proof of pathogenicity, provided clues for rational therapy, lead to precise structure function correlations, and highlighted functionally significant residues or molecular domains that previous systematic mutagenesis studies had failed to detect. An overview of the spectrum of the congenital myasthenic syndromes suggests that most are caused by mutations in AChR subunits, and particularly in the epsilon subunit. Future studies will likely uncover new types of CMS that reside in molecules governing quantal release, organization of the synaptic basal lamina, and expression and aggregation of AChR on the postsynaptic junctional folds.

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.

ErbB2 is required for muscle spindle and myoblast cell survival

Signaling mediated by ErbB2 is thought to play a critical role in numerous developmental processes. However, due to the embryonic lethality associated with the germ line inactivation of erbB2, its role in adult tissues remains largely obscure. Given the expression of ErbB2 at the neuromuscular junction, we have created a muscle-specific knockout to assess its role there. This resulted in viable mice with a progressive defect in proprioception due to loss of muscle spindles. Interestingly, a partial reduction of ErbB2 levels also reduced the number of muscle spindles. Although histological analysis of the muscle revealed an otherwise normal architecture, induction of muscle injury revealed a defect in muscle regeneration. Consistent with these observations, primary myoblasts lacking ErbB2 exhibit extensive apoptosis upon differentiation into myofibers. Taken together, these results illustrate a dual role for ErbB2 in both muscle spindle maintenance and survival of myoblasts.

Compartmentalized NRG signaling and PDZ domain-containing proteins in synapse structure and function

The synapse-specific synthesis of the acetylcholine receptor (AChR) is mediated by multiple mechanisms including compartmentalized signaling induced by neuregulin (NRG). This paper presents evidence that NRG receptors--ErbB receptor tyrosine kinases interact with distinct PDZ domain-containing proteins that are localized at the neuromuscular junction (NMJ). ErbB4 associates with the PSD-95 (also known as SAP90)-family members including PSD-95, SAP97, and SAP102 whereas ErbB2 interacts with Erbin and PICK1. Although, ErbB kinases are concentrated at the NMJ, they are not colocalized with the AChR in cultured muscle cells even in the presence of agrin. Co-expression of PSD-95 causes ErbB4 to form clusters in COS cells. We propose that PDZ domain-containing proteins play a role in anchoring ErbB proteins at the neuromuscular junction, and/or mediating downstream signaling pathways. Such mechanisms could be important for the maintenance and function of the synapse.

Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.

Clustering of nicotinic acetylcholine receptors: from the neuromuscular junction to interneuronal synapses

Fast and accurate synaptic transmission requires high-density accumulation of neurotransmitter receptors in the postsynaptic membrane. During development of the neuromuscular junction, clustering of acetylcholine receptors (AChR) is one of the first signs of postsynaptic specialization and is induced by nerve-released agrin. Recent studies have revealed that different mechanisms regulate assembly vs stabilization of AChR clusters and of the postsynaptic apparatus. MuSK, a receptor tyrosine kinase and component of the agrin receptor, and rapsyn, an AChR-associated anchoring protein, play crucial roles in the postsynaptic assembly. Once formed, AChR clusters and the postsynaptic membrane are stabilized by components of the dystrophin/utrophin glycoprotein complex, some of which also direct aspects of synaptic maturation such as formation of postjunctional folds. Nicotinic receptors are also expressed across the peripheral and central nervous system (PNS/CNS). These receptors are localized not only at the pre- but also at the postsynaptic sites where they carry out major synaptic transmission. In neurons, they are found as clusters at synaptic or extrasynaptic sites, suggesting that different mechanisms might underlie this specific localization of nicotinic receptors. This review summarizes the current knowledge about formation and stabilization of the postsynaptic apparatus at the neuromuscular junction and extends this to explore the synaptic structures of interneuronal cholinergic synapses.

Congenital myasthenic syndromes: genetic defects of the neuromuscular junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or the resynthesis of ACh. Insufficient resynthesis of ACh is now known to be caused by mutations that reduce the expression, catalytic efficiency, or both of choline acetyltransferase. The synaptic CMS are caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent ColQ from associating with catalytic subunits or from insertion into the synaptic basal lamina. With one exception, postsynaptic CMS identified to date are associated with a kinetic abnormality or decreased expression of the acetylcholine receptor (AChR). Numerous mutations have now been identified in subunits of AChR that alter the kinetics or surface expression of the receptor. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most mutations that reduce surface expression of AChR reside in the receptor's epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. Null mutations in both alleles of other AChR subunits are likely lethal, owing to absence of a substituting subunit.

Neuregulin-heparan-sulfate proteoglycan interactions produce sustained erbB receptor activation required for the induction of acetylcholine receptors in muscle

Neuregulins bind to and activate members of the EGF receptor family of tyrosine kinases that initiate a signaling cascade that induces acetylcholine receptor synthesis in the postsynaptic membrane of neuromuscular synapses. In addition to an EGF-like domain, sufficient for receptor binding and tyrosine auto-phosphorylation, many spliced forms also have an IG-like domain that binds HSPGs and maintains a high concentration of neuregulin at synapses. Here, we show that the IG-like domain functions to keep the EGF-like domain at sufficiently high concentrations for a sufficiently long period of time necessary to induce acetylcholine receptor gene expression in primary chick myotubes. Using recombinant neuregulins with and without the IG-like domain, we found that IG-like domain binding to endogenous HSPGs produces a 4-fold increase in receptor phosphorylation. This enhancement of activity was blocked by soluble heparin or by pretreatment of muscle cells with heparitinase. We show that at least 12-24 h of neuregulin exposure was required to turn on substantial acetylcholine receptor gene expression and that the erbB receptors need to be kept phosphorylated during this time. The need for sustained erbB receptor activation may be the reason why neuregulins are so highly concentrated in the extracellular matrix of synapses.

Targeting transcription to the neuromuscular synapse

Concomitant with innervation, genes coding for components of the neuromuscular junction become exclusively expressed in subsynaptic nuclei. A six-base pair element, the N box, can confer synapse-specific transcription to the acetylcholine nicotinic receptor delta and epsilon subunit, utrophin, and acetylcholine esterase genes. N box-dependent synaptic expression is stimulated by the nerve-derived signal agrin and the trophic factor neuregulin, which triggers the MAPK and JNK signaling pathways, to ultimately allow activation by the N box binding Ets transcription factor GABP.