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Xing G, Xiong WC, Mei L. Rapsyn as a signaling and scaffolding molecule in neuromuscular junction formation and maintenance. Neurosci Lett 2020; 731:135013. [DOI: 10.1016/j.neulet.2020.135013] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2020] [Accepted: 04/23/2020] [Indexed: 12/20/2022]
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2
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Rahman AA, Robinson AM, Brookes SJH, Eri R, Nurgali K. Rectal prolapse in Winnie mice with spontaneous chronic colitis: changes in intrinsic and extrinsic innervation of the rectum. Cell Tissue Res 2016; 366:285-299. [DOI: 10.1007/s00441-016-2465-z] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2016] [Accepted: 06/29/2016] [Indexed: 12/19/2022]
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3
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Wang J, Fu XQ, Lei WL, Wang T, Sheng AL, Luo ZG. Nuclear factor kappaB controls acetylcholine receptor clustering at the neuromuscular junction. J Neurosci 2010; 30:11104-13. [PMID: 20720118 PMCID: PMC6633475 DOI: 10.1523/jneurosci.2118-10.2010] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2010] [Revised: 06/29/2010] [Accepted: 07/02/2010] [Indexed: 11/21/2022] Open
Abstract
At the vertebrate neuromuscular junction (NMJ), acetylcholine receptor (AChR) clustering is stimulated by motor neuron-derived glycoprotein Agrin and requires a number of intracellular signal or structural proteins, including AChR-associated scaffold protein Rapsyn. Here, we report a role of nuclear factor kappaB (NF-kappaB), a well known transcription factor involved in a variety of immune responses, in regulating AChR clustering at the NMJ. We found that downregulating the expression of RelA/p65 subunit of NF-kappaB or inhibiting NF-kappaB activity by overexpression of mutated form of IkappaB (inhibitor kappaB), which is resistant to proteolytic degradation and thus constitutively keeps NF-kappaB inactive in the cytoplasma, impeded the formation of AChR clusters in cultured C2C12 muscle cells stimulated by Agrin. In contrast, overexpression of RelA/p65 promoted AChR clustering. Furthermore, we investigated the mechanism by which NF-kappaB regulates AChR clustering. Interestingly, we found that downregulating the expression of RelA/p65 caused a marked reduction in the protein and mRNA level of Rapsyn and upregulation of RelA/p65 enhanced Rapsyn promoter activity. Mutation of NF-kappaB binding site on Rapsyn promoter prevented responsiveness to RelA/p65 regulation. Moreover, forced expression of Rapsyn in RelA/p65 downregulated muscle cells partially rescued AChR clusters, suggesting that NF-kappaB regulates AChR clustering, at least partially through the transcriptional regulation of Rapsyn. In line with this notion, genetic ablation of RelA/p65 selectively in the skeletal muscle caused a reduction of AChR density at the NMJ and a decrease in the level of Rapsyn. Thus, NF-kappaB signaling controls AChR clustering through transcriptional regulation of synaptic protein Rapsyn.
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Affiliation(s)
- Jia Wang
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
| | - Xiu-Qing Fu
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
| | - Wen-Liang Lei
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
| | - Tong Wang
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
| | - Ai-Li Sheng
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
| | - Zhen-Ge Luo
- Institute of Neuroscience and State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
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4
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Wang J, Ruan NJ, Qian L, Lei WL, Chen F, Luo ZG. Wnt/beta-catenin signaling suppresses Rapsyn expression and inhibits acetylcholine receptor clustering at the neuromuscular junction. J Biol Chem 2008; 283:21668-75. [PMID: 18541538 DOI: 10.1074/jbc.m709939200] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
The dynamic interaction between positive and negative signals is necessary for remodeling of postsynaptic structures at the neuromuscular junction. Here we report that Wnt3a negatively regulates acetylcholine receptor (AChR) clustering by repressing the expression of Rapsyn, an AChR-associated protein essential for AChR clustering. In cultured myotubes, treatment with Wnt3a or overexpression of beta-catenin, the condition mimicking the activation of the Wnt canonical pathway, inhibited Agrin-induced formation of AChR clusters. Moreover, Wnt3a treatment promoted dispersion of AChR clusters, and this effect was prevented by DKK1, an antagonist of the Wnt canonical pathway. Next, we investigated possible mechanisms underlying Wnt3a regulation of AChR clustering in cultured muscle cells. Interestingly, we found that Wnt3a treatment caused a decrease in the protein level of Rapsyn. In addition, Rapsyn promoter activity in cultured muscle cells was inhibited by the treatment with Wnt3a or beta-catenin overexpression. Forced expression of Rapsyn driven by a promoter that is not responsive to Wnt3a prevented the dispersing effect of Wnt3a on AChR clusters, suggesting that Wnt3a indeed acts to disperse AChR clusters by down-regulating the expression of Rapsyn. The role of Wnt/beta-catenin signaling in dispersing AChR clusters was also investigated in vivo by electroporation of Wnt3a or beta-catenin into mouse limb muscles, where ectopic Wnt3a or beta-catenin caused disassembly of postsynaptic apparatus. Together, these results suggest that Wnt/beta-catenin signaling plays a negative role for postsynaptic differentiation at the neuromuscular junction, probably by regulating the expression of synaptic proteins, such as Rapsyn.
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Affiliation(s)
- Jia Wang
- Institute of Neuroscience, State Key Laboratory of Neuroscience, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yue Yang Road, Shanghai, China
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Lee Y, Rudell J, Yechikhov S, Taylor R, Swope S, Ferns M. Rapsyn carboxyl terminal domains mediate muscle specific kinase-induced phosphorylation of the muscle acetylcholine receptor. Neuroscience 2008; 153:997-1007. [PMID: 18436384 DOI: 10.1016/j.neuroscience.2008.03.009] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2007] [Revised: 03/03/2008] [Accepted: 03/06/2008] [Indexed: 11/17/2022]
Abstract
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.
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Affiliation(s)
- Y Lee
- Departments of Anesthesiology and Physiology and Membrane Biology, One Shields Avenue, University of California-Davis, Davis, CA 95616, USA
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6
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Chen F, Qian L, Yang ZH, Huang Y, Ngo ST, Ruan NJ, Wang J, Schneider C, Noakes PG, Ding YQ, Mei L, Luo ZG. Rapsyn interaction with calpain stabilizes AChR clusters at the neuromuscular junction. Neuron 2007; 55:247-60. [PMID: 17640526 DOI: 10.1016/j.neuron.2007.06.031] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2006] [Revised: 03/27/2007] [Accepted: 06/25/2007] [Indexed: 12/28/2022]
Abstract
Agrin induces, whereas acetylcholine (ACh) disperses, ACh receptor (AChR) clusters during neuromuscular synaptogenesis. Such counteractive interaction leads to eventual dispersal of nonsynaptic AChR-rich sites and formation of receptor clusters at the postjunctional membrane. However, the underlying mechanisms are not well understood. Here we show that calpain, a calcium-dependent protease, is activated by the cholinergic stimulation and is required for induced dispersion of AChR clusters. Interestingly, the AChR-associated protein rapsyn interacted with calpain in an agrin-dependent manner, and this interaction inhibited the protease activity of calpain. Disrupting the endogenous rapsyn/calpain interaction enhanced CCh-induced dispersion of AChR clusters. Moreover, the loss of AChR clusters in agrin mutant mice was partially rescued by the inhibition of calpain via overexpressing calpastatin, an endogenous calpain inhibitor, or injecting calpeptin, a cell-permeable calpain inhibitor. These results demonstrate that calpain participates in ACh-induced dispersion of AChR clusters, and rapsyn stabilizes AChR clusters by suppressing calpain activity.
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Affiliation(s)
- Fei Chen
- Institute of Neuroscience and Key Laboratory of Neurobiology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China
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Cho CH, Song W, Leitzell K, Teo E, Meleth AD, Quick MW, Lester RAJ. Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation. J Neurosci 2005; 25:3712-23. [PMID: 15814802 PMCID: PMC6725387 DOI: 10.1523/jneurosci.5389-03.2005] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2003] [Revised: 03/02/2005] [Accepted: 03/02/2005] [Indexed: 11/21/2022] Open
Abstract
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.
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Affiliation(s)
- Chang-Hoon Cho
- Department of Neurobiology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0021, USA
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Inoue K, Ueno S, Yamada J, Fukuda A. Characterization of newly cloned variant of rat glycine receptor α1 subunit. Biochem Biophys Res Commun 2005; 327:300-5. [PMID: 15629462 DOI: 10.1016/j.bbrc.2004.12.010] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2004] [Indexed: 11/25/2022]
Abstract
Responses to glycine, a major inhibitory neurotransmitter within the nervous system, are mediated by glycine receptors (GlyRs). Here, we report the cloning and analysis of a novel splicing variant of the GlyRalpha1 subunit. This variant, named GlyRalpha1del, has a truncated cytoplasmic region between transmembrane domains (TM)3 and TM4, and compared to other variants, the truncation is contributed by a different acceptor site in exon 9. We transfected GlyRalpha1 or GlyRalpha1del into HEK293 cells, and then examined the glycine-activated currents using a whole-cell patch-clamp recording technique. Maximal currents and current-voltage relationships showed no clear difference between GlyRalpha1del and GlyRalpha1. Moreover, dose-response curves indicated that the EC50 values for glycine differed significantly between the two GlyRalpha1 derivatives, although their Hill coefficients were similar. When present with other isoforms, GlyRalpha1del might alter the response to glycine or to other agonists, as this variant expands the potential heterogeneity among glycine receptors.
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Affiliation(s)
- Koichi Inoue
- Department of Physiology, Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan.
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9
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Strochlic L, Cartaud A, Cartaud J. The synaptic muscle-specific kinase (MuSK) complex: New partners, new functions. Bioessays 2005; 27:1129-35. [PMID: 16237673 DOI: 10.1002/bies.20305] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The muscle-specific kinase MuSK is part of an agrin receptor complex that stimulates tyrosine phosphorylation and drives clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). MuSK also regulates synaptic gene transcription in subsynaptic nuclei. Over the past few years, decisive progress has been made in the identification of MuSK effectors, helping to understand its function in the formation of the NMJ. Similarly to AChR, MuSK and several of its partners are the target of mutations responsible for diseases of the NMJ, such as congenital myasthenic syndromes. This minireview will focus on the multiple MuSK effectors so far identified that place MuSK at the center of a multifunctional signaling complex involved in the organization of the NMJ and associated disorders.
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Affiliation(s)
- Laure Strochlic
- Biologie Cellulaire des Membranes, Institut Jacques Monod, Paris, France
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Keramidas A, Moorhouse AJ, Schofield PR, Barry PH. Ligand-gated ion channels: mechanisms underlying ion selectivity. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2004; 86:161-204. [PMID: 15288758 DOI: 10.1016/j.pbiomolbio.2003.09.002] [Citation(s) in RCA: 162] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Anion/cation selectivity is a critical property of ion channels and underpins their physiological function. Recently, there have been numerous mutagenesis studies, which have mapped sites within the ion channel-forming segments of ligand-gated ion channels that are determinants of the ion selectivity. Site-directed mutations to specific amino acids within or flanking the M2 transmembrane segments of the anion-selective glycine, GABA(A) and GABA(C) receptors and the cation-selective nicotinic acetylcholine and serotonin (type 3) receptors have revealed discrete, equivalent regions within the ion channel that form the principal selectivity filter, leading to plausible molecular mechanisms and mathematical models to describe how ions preferentially permeate these channels. In particular, the dominant factor determining anion/cation selectivity seems to be the sign and exposure of charged amino acids lining the selectivity filter region of the open channel. In addition, the minimum pore diameter, which can be influenced by the presence of a local proline residue, also makes a contribution to such ion selectivity in LGICs with smaller diameters increasing anion/cation selectivity and larger ones decreasing it.
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Affiliation(s)
- Angelo Keramidas
- Department of Physiology and Pharmacology, School of Medical Sciences, The University of New South Wales, UNSW Sydney, NSW 2052, Australia
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11
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Borges LS, Lee Y, Ferns M. Dual role for calcium in agrin signaling and acetylcholine receptor clustering. JOURNAL OF NEUROBIOLOGY 2002; 50:69-79. [PMID: 11748634 DOI: 10.1002/neu.10020] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
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.
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Affiliation(s)
- Lucia S Borges
- Department of Neurology and Neurosurgery, McGill University, and Centre for Research in Neuroscience, Montreal General Hospital Research Institute, 1650 Cedar Ave, Montreal, Quebec, H3G 1A4, Canada
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12
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Leite JF, Cascio M. Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology. Mol Cell Neurosci 2001; 17:777-92. [PMID: 11358478 DOI: 10.1006/mcne.2001.0984] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.
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Affiliation(s)
- J F Leite
- Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA
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Src-class kinases act within the agrin/MuSK pathway to regulate acetylcholine receptor phosphorylation, cytoskeletal anchoring, and clustering. J Neurosci 2001. [PMID: 11356869 DOI: 10.1523/jneurosci.21-11-03806.2001] [Citation(s) in RCA: 83] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Synaptogenesis at the neuromuscular junction requires agrin-induced stable localization of acetylcholine receptors (AChRs) at the endplate. The effects of agrin are transduced by the muscle-specific receptor tyrosine kinase (MuSK). This study provides evidence that Src-class protein tyrosine kinases mediate the effects of agrin-activated MuSK to regulate clustering and anchoring of AChRs in skeletal muscle. MuSK was complexed with both Src and Fyn in the C2 mouse muscle cell line. These associations were enhanced by agrin and by increasing protein tyrosine phosphorylation with pervanadate. Coupling between MuSK and the Src-class kinases in vivo appeared to be caused by a phosphotyrosine-SH2 domain interaction because binding of MuSK to the SH2 domains of Fyn and Src in vitro was specific, enhanced by phosphorylation, and dependent on MuSK autophosphorylation. In addition, Src and Fyn phosphorylated MuSK. AChR phosphorylation, stimulated by agrin or pervanadate, was inhibited by blocking Src-class kinases with PP1. Furthermore, agrin-induced clustering and cytoskeletal anchoring of AChRs was dependent on Src-family kinases. These data support the conclusion that Fyn and Src act downstream of MuSK to regulate the stable localization of AChRs at the neuromuscular endplate during agrin-induced synaptogenesis.
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Borges LS, Ferns M. Agrin-induced phosphorylation of the acetylcholine receptor regulates cytoskeletal anchoring and clustering. J Cell Biol 2001; 153:1-12. [PMID: 11285269 PMCID: PMC2185523 DOI: 10.1083/jcb.153.1.1] [Citation(s) in RCA: 89] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2000] [Accepted: 01/31/2001] [Indexed: 12/19/2022] Open
Abstract
At the developing neuromuscular junction, a motoneuron-derived factor called agrin signals through the muscle-specific kinase receptor to induce postsynaptic aggregation of the acetylcholine receptor (AChR). The agrin signaling pathway involves tyrosine phosphorylation of the AChR beta subunit, and we have tested its role in receptor localization by expressing tagged, tyrosine-minus forms of the beta subunit in mouse Sol8 myotubes. We find that agrin-induced phosphorylation of the beta subunit occurs only on cell surface AChR, and that AChR-containing tyrosine-minus beta subunit is targeted normally to the plasma membrane. Surface AChR that is tyrosine phosphorylated is less detergent extractable than nonphosphorylated AChR, indicating that it is preferentially linked to the cytoskeleton. Consistent with this, we find that agrin treatment reduces the detergent extractability of AChR that contains tagged wild-type beta subunit but not tyrosine-minus beta subunit. In addition, agrin-induced clustering of AChR containing tyrosine-minus beta subunit is reduced in comparison to wild-type receptor. Thus, we find that agrin-induced phosphorylation of AChR beta subunit regulates cytoskeletal anchoring and contributes to the clustering of the AChR, and this is likely to play an important role in the postsynaptic localization of the receptor at the developing synapse.
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Affiliation(s)
- L S Borges
- Department of Neurology & Neurosurgery, McGill University, Montreal, Quebec H3A 2T5, Canada
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Poea S, Guyon T, Bidault J, Bruand C, Mouly V, Berrih-Aknin S. Modulation of acetylcholine receptor expression in seronegative myasthenia gravis. Ann Neurol 2001. [DOI: 10.1002/1531-8249(200011)48:5<696::aid-ana2>3.0.co;2-s] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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16
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Dai Z, Luo X, Xie H, Peng HB. The actin-driven movement and formation of acetylcholine receptor clusters. J Cell Biol 2000; 150:1321-34. [PMID: 10995438 PMCID: PMC2150690 DOI: 10.1083/jcb.150.6.1321] [Citation(s) in RCA: 109] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2000] [Accepted: 07/18/2000] [Indexed: 11/22/2022] Open
Abstract
A new method was devised to visualize actin polymerization induced by postsynaptic differentiation signals in cultured muscle cells. This entails masking myofibrillar filamentous (F)-actin with jasplakinolide, a cell-permeant F-actin-binding toxin, before synaptogenic stimulation, and then probing new actin assembly with fluorescent phalloidin. With this procedure, actin polymerization associated with newly induced acetylcholine receptor (AChR) clustering by heparin-binding growth-associated molecule-coated beads and by agrin was observed. The beads induced local F-actin assembly that colocalized with AChR clusters at bead-muscle contacts, whereas both the actin cytoskeleton and AChR clusters induced by bath agrin application were diffuse. By expressing a green fluorescent protein-coupled version of cortactin, a protein that binds to active F-actin, the dynamic nature of the actin cytoskeleton associated with new AChR clusters was revealed. In fact, the motive force generated by actin polymerization propelled the entire bead-induced AChR cluster with its attached bead to move in the plane of the membrane. In addition, actin polymerization is also necessary for the formation of both bead and agrin-induced AChR clusters as well as phosphotyrosine accumulation, as shown by their blockage by latrunculin A, a toxin that sequesters globular (G)-actin and prevents F-actin assembly. These results show that actin polymerization induced by synaptogenic signals is necessary for the movement and formation of AChR clusters and implicate a role of F-actin as a postsynaptic scaffold for the assembly of structural and signaling molecules in neuromuscular junction formation.
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Affiliation(s)
- Z Dai
- Department of Cell Biology and Anatomy, University of North Carolina, Chapel Hill, North Carolina 27599, USA
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17
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Nghiêm HO, Bettendorff L, Changeux JP. Specific phosphorylation of Torpedo 43K rapsyn by endogenous kinase(s) with thiamine triphosphate as the phosphate donor. FASEB J 2000; 14:543-54. [PMID: 10698970 DOI: 10.1096/fasebj.14.3.543] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
43K rapsyn is a peripheral protein specifically associated with the nicotinic acetylcholine receptor (nAChR) present in the postsynaptic membrane of the neuromuscular junction and of the electrocyte, and is essential for its clustering. Here, we demonstrate a novel specific phosphorylation of 43K rapsyn by endogenous protein kinase(s) present in Torpedo electrocyte nAChR-rich membranes and identify thiamine triphosphate (TTP) as the phosphate donor. In the presence of Mg(2+) and [gamma-(32)P]-TTP, 43K rapsyn is specifically phosphorylated with a (32)P-half-maximal incorporation at approximately 5-25 microM TTP. The presence of TTP in the cytosol and of 43K rapsyn at the cytoplasmic face of the postsynaptic membrane, together with TTP-dependent phosphorylation of 43K rapsyn without added exokinases, suggests that TTP-dependent-43K-rapsyn phosphorylation may occur in vivo. In addition, phosphoamino acid and chemical stability analysis suggests that the residues phosphorylated are predominantly histidines. Inhibition of phosphorylation by Zn(2+) suggests a possible control of 43K rapsyn phosphorylation state by its zinc finger domain. Endogenous kinase(s) present in rodent brain membranes can also use [gamma-(32)P]-TTP as a phosphodonor. The use of a phosphodonor (TTP) belonging to the thiamine family but not to the classical (ATP, GTP) purine triphosphate family represents a novel phosphorylation pathway possibly important for synaptic proteins.
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Affiliation(s)
- H O Nghiêm
- CNRS UA D-1284, Neurobiologie Moléculaire, Institut Pasteur, 75724 Paris Cedex, France.
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Abstract
At the neuromuscular junction, aggregates of acetylcholine receptors (AChRs) are anchored in the muscle membrane by association with rapsyn and other postsynaptic proteins. We have investigated the interactions between the AChR and these proteins in cultured C2 myotubes before and after treatment with agrin, a nerve-derived protein that induces AChRs to cluster. When AChRs were isolated from detergent extracts of untreated C2 myotubes, they were associated with rapsyn and, to a lesser degree, with utrophin, beta-dystroglycan, MuSK, and src-related kinases, but not with syntrophin. Treatment with agrin increased the association of AChRs with MuSK, a receptor tyrosine kinase that forms part of the agrin receptor complex, without affecting other interactions. Analysis of rapsyn-deficient myotubes, which do not form protein clusters in response to agrin, revealed that rapsyn is required for association of the AChR with utrophin and beta-dystroglycan, and for the agrin-induced increase in association with MuSK, but not for constitutive interactions with MuSK and src-related kinases. In rapsyn -/- myotubes, agrin caused normal tyrosine phosphorylation of AChR-associated and total MuSK, whereas phosphorylation of the AChR beta subunit, both constitutive and agrin-induced, was strongly reduced. These results show first that aneural myotubes contain preassembled AChR protein complexes that may function in the assembly of the postsynaptic apparatus, and second that rapsyn, in addition to its role in AChR phosphorylation, mediates selected protein interactions with the AChR and serves as a link between the AChR and the dystrophin/utrophin glycoprotein complex.
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Mohamed AS, Swope SL. Phosphorylation and cytoskeletal anchoring of the acetylcholine receptor by Src class protein-tyrosine kinases. Activation by rapsyn. J Biol Chem 1999; 274:20529-39. [PMID: 10400682 DOI: 10.1074/jbc.274.29.20529] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Src class protein-tyrosine kinases bind to and phosphorylate the nicotinic acetylcholine receptor of skeletal muscle. This study provided evidence for the functional importance of Src kinases in regulating the nicotinic acetylcholine receptor at the neuromuscular junction. Three Src class kinases, Fyn, Fyk, and Src, each formed a complex with the endplate-specific cytoskeletal protein rapsyn. In addition, cellular phosphorylation by each kinase was stimulated by rapsyn in heterologous transfected cells. Several lines of evidence supported rapsyn as a substrate for Src kinases. Most importantly, rapsyn regulation of Fyn, Fyk, and Src resulted in phosphorylation of the nicotinic acetylcholine receptor beta and delta subunits and anchoring of the receptor to the cytoskeleton. Both nicotinic acetylcholine receptor phosphorylation and cytoskeletal anchoring were blocked by the Src kinase-selective inhibitor herbimycin A. Rapsyn alone also induced a modest increase in nicotinic acetylcholine receptor phosphorylation and cytoskeletal translocation. However, inhibition by herbimycin A and a catalytically inactive dominant negative Src demonstrated that the effects of rapsyn were mediated by endogenous Src kinases. These data support the importance of Src class kinases for stabilization of the nicotinic acetylcholine receptor at the endplate during synaptic differentiation at the neuromuscular junction.
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Affiliation(s)
- A S Mohamed
- Department of Neurology, Division of Neuroscience, Georgetown Institute for Cognitive and Computational Sciences, Georgetown University Medical Center, Washington, D.C. 20007-2197, USA
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Filippova N, Dudley R, Weiss DS. Evidence for phosphorylation-dependent internalization of recombinant human rho1 GABAC receptors. J Physiol 1999; 518 ( Pt 2):385-99. [PMID: 10381587 PMCID: PMC2269426 DOI: 10.1111/j.1469-7793.1999.0385p.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/1999] [Accepted: 04/13/1999] [Indexed: 11/26/2022] Open
Abstract
1. Recombinant wild-type or mutant human rho1 GABA receptors were expressed in human embryonic kidney (HEK) 293 or monkey COS-7 cells and studied using the patch clamp technique. 2. Standard whole-cell recordings with 4 mM Mg-ATP in the patch pipette induced a time-dependent decrease in the GABA-activated current (IGABA) amplitude that was not the result of a decrease in GABA sensitivity. In contrast, IGABA remained stable when recordings were obtained using the perforated patch configuration or with standard whole-cell recording and no Mg-ATP in the patch pipette. 3. The inhibitors of serine/threonine protein kinases KN-62 (20 microM) or staurosporine (20 nM) prevented the time-dependent decrease in the amplitude of IGABA seen in the presence of ATP. Alkaline phosphatase (220 U ml-1), when added to the patch pipette in the absence of ATP, induced a transient potentiation of IGABA. Although the protein kinase C (PKC) activator 4beta-phorbol 12-myristate, 13-acetate (PMA) did not reduce the amplitude of IGABA, inclusion of the catalytic domain of PKC in the recording pipette accelerated the time-dependent decrease in current amplitude. These data suggest that phosphorylation is involved in the regulation of the amplitude of IGABA. 4. Mutation of the three PKC consensus sequences of the rho1 receptor had no significant effect on the decline in IGABA, indicating that direct phosphorylation of these putative sites on the rho1 receptor does not underlie the time-dependent decrease in amplitude. 5. In COS-7 cells transfected with wild-type rho1 receptors, the amplitude of IGABA had completely recovered to the original value when the same cells were repatched after 30-40 min, indicating that the decline in IGABA was a reversible process. 6. The inhibitor of actin filament formation cytochalasin B, when added to the patch pipette in the absence of ATP, induced a time-dependent inactivation suggesting that the actin cytoskeleton may play a role in the regulation of the amplitude. 7. Coincident with the decrease in the amplitude of IGABA, the cell capacitance significantly decreased in the presence of ATP in the patch pipette. This decrease in capacitance was not observed in the absence of Mg-ATP. The decrease in the membrane surface area suggests that receptor internalization could be a potential mechanism for the observed inactivation. 8. At 32 C, compared with 22 C, the rate and magnitude of the decline was increased dramatically. In contrast, at 16 C, no significant change in IGABA was observed over the 20 min recording time. This marked temperature sensitivity is consistent with receptor internalization as a mechanism for the time-dependent decline in IGABA. 9. The specificity of the decrease in IGABA was assessed by coexpressing the voltage-dependent potassium channel Kv1.4 along with the rho1 receptor in HEK293 cells. The amplitude of the potassium current (IKv1.4) exhibited very little decrement in comparison to IGABA suggesting that the putative GABA receptor internalization was not the consequence of a non-specific membrane retrieval.
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Affiliation(s)
- N Filippova
- Department of Neurobiology, University of Alabama at Birmingham School of Medicine, 1719 Sixth Avenue South, CIRC 410, Birmingham, AL 35294-0021, USA
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21
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Swope SL, Moss SJ, Raymond LA, Huganir RL. Regulation of ligand-gated ion channels by protein phosphorylation. ADVANCES IN SECOND MESSENGER AND PHOSPHOPROTEIN RESEARCH 1999; 33:49-78. [PMID: 10218114 DOI: 10.1016/s1040-7952(99)80005-6] [Citation(s) in RCA: 254] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB
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Affiliation(s)
- S L Swope
- Department of Neurology, Georgetown Institute for Cognitive and Computational Neuroscience, Georgetown University Medical Center, Washington, DC 20007, USA
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Balasubramanian S, Huganir RL. Characterization of phosphotyrosine containing proteins at the cholinergic synapse. FEBS Lett 1999; 446:95-102. [PMID: 10100622 DOI: 10.1016/s0014-5793(99)00171-4] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Tyrosine phosphorylation has been associated with several aspects of the regulation of cholinergic synaptic function, including nicotinic acetylcholine receptor (AChR) desensitization as well as the synthesis and clustering of synaptic components. While some progress has been made in elucidating the molecular events initiating such signals, the downstream targets of these tyrosine kinase pathways have yet to be characterized. In this paper we have used molecular cloning techniques to identify proteins which are tyrosine phosphorylated at the cholinergic synapse. Phosphotyrosine containing proteins (PYCPs) were isolated from the electric organ of Torpedo californica by anti-phosphotyrosine immunoaffinity chromatography. Peptide sequencing and expression cloning then identified the isolated proteins. The proteins identified included heat shock protein 90, type III intermediate filament from Torpedo electric organ, alpha-fodrin, beta-tubulin, actin and rapsyn. These tyrosine phosphorylated proteins may play a role in the regulation of synaptic function by tyrosine kinases.
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Affiliation(s)
- S Balasubramanian
- Department of Neuroscience, Howard Hughes Medical Institute, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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23
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Camus G, Ludosky MA, Bignami F, Marchand S, Cartaud J, Cartaud A. Developmental regulation of tyrosine phosphorylation of the nicotinic acetylcholine receptor in Torpedo electrocyte. Mol Cell Neurosci 1999; 13:69-78. [PMID: 10049532 DOI: 10.1006/mcne.1998.0728] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Tyrosine phosphorylation is thought to play a critical role in the clustering of acetylcholine receptors (AChR) at the developing neuromuscular junction. Yet, in vitro approaches have led to conflicting conclusions regarding the function of tyrosine phosphorylation of AChR beta subunit in AChR clustering. In this work, we followed in situ the time course of tyrosine phosphorylation of AChR in developing Torpedo electrocyte. We observed that tyrosine phosphorylation of the AChR beta and delta subunits occurs at a late stage of embryonic development after the accumulation of AChRs and rapsyn in the membrane and the onset of innervation. Interestingly, in the mature postsynaptic membrane, we observed two populations of AChR differing both in their phosphotyrosine content and distribution. Our data are consistent with the notion that tyrosine phosphorylation of the AChR is related to downstream events in the pathway regulating AChR accumulation rather than to initial clustering events.
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Affiliation(s)
- G Camus
- Département de Biologie Supramoléculaire et Cellulaire, Institut Jacques Monod, UMR 7592, CNRS et Universités Paris VI et Paris VII, 2, Place Jussieu, Paris Cédex 05, 75251, France
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24
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Bezakova G, Bloch RJ. The zinc finger domain of the 43-kDa receptor-associated protein, rapsyn: role in acetylcholine receptor clustering. Mol Cell Neurosci 1998; 11:274-88. [PMID: 9698394 DOI: 10.1006/mcne.1998.0688] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We injected rat myotubes with proteins and antibodies to assess the importance of the zinc finger (ZnF) domain of the 43-kDa receptor-associated protein, rapsyn, in clustering acetylcholine receptors (AChR). Injection of rat myotubes with a fusion protein containing the ZnF domain of rapsyn disrupted AChR clusters. Clusters were unaffected by a fusion protein containing a double mutant that does not bind zinc. Similar results were obtained with the purified wild type and mutant ZnF domains. The ZnF of HIV-1 nucleocapsid protein had no effect. AChR clusters were also disrupted in myotubes injected with antibodies to the ZnF domain, followed by injection of anti-antibodies. Injection of antibodies directed against a different rapsyn epitope or against the cytoplasmic domain of the AChR had no effect. In transfection experiments with HEK 293 cells, the ZnF domain failed to associate with membrane aggregates containing full-length rapsyn, AChR, or rapsyn and AChR together. We conclude that the ZnF domain of rapsyn provides a binding site essential for AChR clustering, but that this site is unlikely to be involved in high affinity binding of rapsyn to itself or to AChR.
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Affiliation(s)
- G Bezakova
- Department of Physiology, University of Maryland School of Medicine, 660 West Redwood Street, Baltimore, Maryland, 21201, USA
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25
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Meyer G, Wallace BG. Recruitment of a nicotinic acetylcholine receptor mutant lacking cytoplasmic tyrosine residues in its beta subunit into agrin-induced aggregates. Mol Cell Neurosci 1998; 11:324-33. [PMID: 9698397 DOI: 10.1006/mcne.1998.0689] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
During synaptogenesis at the vertebrate skeletal neuromuscular junction, acetylcholine receptors (AChRs) form high-density aggregates opposite the presynaptic terminal in response to nerve-derived agrin. Agrin has been shown to stimulate tyrosine phosphorylation of a muscle-specific receptor tyrosine kinase MuSK and of the AChR beta subunit, and tyrosine kinase inhibitors and a tyrosine kinase-deficient mutant of MuSK prevent AChR aggregation. To evaluate the role of tyrosine phosphorylation of the AChR beta subunit in receptor aggregation, we replaced all three putative cytoplasmic tyrosine residues of the AChR beta subunit with phenylalanine residues and expressed the mutant receptors in cultured myotubes. Upon agrin treatment, transfected myotubes formed AChR aggregates that contained receptors with mutant beta subunits. Thus, AChRs can be recruited into agrin-induced specializations by protein-protein interactions that do not depend on tyrosine phosphorylation of the AChR beta subunit.
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Affiliation(s)
- G Meyer
- Department of Physiology and Biophysics, C240, University of Colorado Health Sciences Center, 4200 East Ninth Avenue, Denver, Colorado, 80262, USA
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26
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Colledge M, Froehner SC. Signals mediating ion channel clustering at the neuromuscular junction. Curr Opin Neurobiol 1998; 8:357-63. [PMID: 9687350 DOI: 10.1016/s0959-4388(98)80061-5] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
High densities of acetylcholine receptors and sodium channels in the crests and troughs of the postsynaptic folds, respectively, ensure reliable neuromuscular signalling. Clustering of both ion channels is mediated by agrin. In the case of acetylcholine receptors, agrin activates the tyrosine kinase receptor muscle-specific kinase (MuSK), initiating a process requiring rapsyn and possibly also receptor phosphorylation. In many respects, the interactions between agrin and MuSK and their downstream effectors are atypical of conventional receptor tyrosine kinase signalling systems. A new understanding of the structural features of rapsyn involved in receptor clustering, as well as syntrophin's role in sodium channel targeting, has recently been revealed. Perhaps the most surprising result of the past year with regard to synaptogenesis is a negative one--mice lacking both dystrophin and utrophin have nearly normal neuromuscular junctions.
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Affiliation(s)
- M Colledge
- Department of Physiology, University of North Carolina at Chapel Hill 27599-7545, USA
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27
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Sanes JR, Apel ED, Gautam M, Glass D, Grady RM, Martin PT, Nichol MC, Yancopoulos GD. Agrin receptors at the skeletal neuromuscular junction. Ann N Y Acad Sci 1998; 841:1-13. [PMID: 9668217 DOI: 10.1111/j.1749-6632.1998.tb10905.x] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- J R Sanes
- Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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28
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Phillips WD, Vladeta D, Han H, Noakes PG. Rapsyn and agrin slow the metabolic degradation of the acetylcholine receptor. Mol Cell Neurosci 1997; 10:16-26. [PMID: 9361285 DOI: 10.1006/mcne.1997.0634] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Rapsyn is a 43-kDa cytoplasmic protein that clusters nicotinic acetylcholine receptors (AChR) in the postsynaptic membrane. Here we examine the effect of rapsynmediated AChR clustering on the metabolic stability of the AChR. When transfected into QT-6 fibroblasts, cell surface AChRs (alpha, beta, epsilon, and delta subunit combination) pulse labeled with 125I-alpha-bungarotoxin were degraded with a half-life of 16.4 +/- 1.1 h (mean +/- SEM). Cotransfection of rapsyn with AChR caused extensive AChR clustering and increased AChR half-life to 20.5 +/- 1.0 h. Anti-AChR antibodies such as mab 35 cause an increased AChR degradation often associated with myasthenia gravis: 80.8 +/- 2.5% of AChRs labeled at zero time were degraded over a 12-h period. Contransfection of rapsyn reduced this AChR loss to 66.4 +/- 3.8%. Rapsyn also reduced normal AChR degradation, from 53.2 +/- 2.1 to 44.2 +/- 2.2%. Muscle cell lines from wild-type myotubes displayed few AChR clusters, but treatment with neural agrin increased the number of AChR clusters 30-fold. Clustering was accompanied by reductions in AChR degradation (both in the presence and absence of mab 35) similar in magnitude to those produced by overexpression of rapsyn in QT-6 cells. In rapsyn-deficient myotubes, treatment with neural agrin neither caused AChR clustering nor reduced AChR degradation. Thus neural agrin may slow AChR degradation by inducing the rapsyn-dependent clustering of AChRs.
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Affiliation(s)
- W D Phillips
- Department of Physiology, University of Sydney, NSW, Australia
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Abstract
The motoneuron is the central neuron whose development is best understood. Recent research has provided much new information about the molecules involved in aspects of motoneuron development first outlined by classic embryology studies. Over the past year, progress has been particularly apparent in the following areas: motoneuron induction and control of motoneuron identity; factors that guide motor axon outgrowth; neurotrophic factors for motoneurons; and early steps in the formation of the neuromuscular junction.
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Affiliation(s)
- O deLapeyrière
- INSERM U. 382, Institut de Biologie du Développement de Marseille (CNRS-INSERM, Université de la Méditerranée), France.
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Apel ED, Glass DJ, Moscoso LM, Yancopoulos GD, Sanes JR. Rapsyn is required for MuSK signaling and recruits synaptic components to a MuSK-containing scaffold. Neuron 1997; 18:623-35. [PMID: 9136771 DOI: 10.1016/s0896-6273(00)80303-7] [Citation(s) in RCA: 197] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Agrin-induced clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane is a key step in synaptogenesis at the neuromuscular junction. The receptor tyrosine kinase MuSK is a component of the agrin receptor, while the cytoplasmic protein rapsyn is necessary for the clustering of AChRs and all other postsynaptic membrane components studied to date. We show here that MuSK remains concentrated at synaptic sites in rapsyn-deficient mutant mice, suggesting that MuSK forms a primary structural scaffold to which rapsyn attaches other synaptic components. Using nonmuscle cells, we show that rapsyn-MuSK interactions are mediated by the ectodomain of MuSK, suggesting the existence of a transmembrane intermediate. In addition to rapsyn's structural role, we demonstrate that it is required for an early step in MuSK signaling, AChR phosphorylation. This signaling requires the kinase domain of MuSK, but not its ectodomain. Thus, MuSK may interact with rapsyn in multiple ways to play both structural and signaling roles in agrin-induced differentiation.
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Affiliation(s)
- Elizabeth D Apel
- Department of Molecular Biology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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