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Madhavan R, Peng HB. Molecular regulation of postsynaptic differentiation at the neuromuscular junction. IUBMB Life 2005; 57:719-30. [PMID: 16511964 DOI: 10.1080/15216540500338739] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.
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Affiliation(s)
- Raghavan Madhavan
- Department of Biology, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
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52
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Levedakou EN, Chen XJ, Soliven B, Popko B. Disruption of the mouse Large gene in the enr and myd mutants results in nerve, muscle, and neuromuscular junction defects. Mol Cell Neurosci 2005; 28:757-69. [PMID: 15797722 DOI: 10.1016/j.mcn.2004.12.007] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2004] [Revised: 12/17/2004] [Accepted: 12/18/2004] [Indexed: 10/25/2022] Open
Abstract
The autosomal recessive neuromuscular disorder associated with the enervated (enr) mouse transgene insertion manifests impaired peripheral nerve regeneration due to defects in Schwann cells and resembles the myodystrophy (Large(myd)) phenotype. Here we show that the enr transgene has integrated into Chr 8 approximately 160 kb downstream from the 3' end of the Large gene disrupting its expression as confirmed by the lack of genetic complementation between Large(myd) and enr mice, the very low Large mRNA levels in enr tissues and hypoglycosylation of alpha-dystroglycan, a known substrate of LARGE. Mutant nerve conduction and grip strength were impaired whereas sodium channel clustering at the nodes of Ranvier was unaffected. Interestingly, the mutant neuromuscular junctions displayed abnormal acetylcholine receptor clustering with reduced immunostaining for beta-dystroglycan, laminin, agrin, MuSK, and to a lesser extent acetylcholinesterase and rapsyn. These data implicate LARGE in nerve, muscle, and neuromuscular junction function.
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Affiliation(s)
- Eleni N Levedakou
- Jack Miller Center for Peripheral Neuropathy, Department of Neurology, MC 2030, The University of Chicago, 5841 South Maryland Avenue, Chicago, IL 60637, USA
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53
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Abstract
PURPOSE OF REVIEW Congenital myasthenic syndromes are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. In this article, a strategy that leads to the diagnosis of congenital myasthenic syndromes is presented, and recent advances in the clinical, genetic and molecular aspects of congenital myasthenic syndrome are outlined. RECENT FINDINGS Besides the identification of new mutations in genes already known to be implicated in congenital myasthenic syndromes (genes for the acetylcholine receptor subunits and the collagen tail of acetylcholinesterase), mutations in other genes have more recently been discovered and characterized (genes for choline acetyltransferase, rapsyn, and the muscle sodium channel SCN4A). Fluoxetine has recently been proposed as an alternative treatment for 'slow channel' congenital myasthenic syndrome. SUMMARY The characterization of congenital myasthenic syndromes comprises two complementary steps: establishing the diagnosis and identifying the pathophysiological type of congenital myasthenic syndrome. Characterization of the type of congenital myasthenic syndrome has allowed it to be classified as caused by presynaptic, synaptic and postsynaptic defects. A clinically and muscle histopathologically oriented genetic study has identified several genes in which mutations cause the disease. Despite comprehensive characterization, the phenotypic expression of one given gene involved is variable, and the aetiology of many congenital myasthenic syndromes remains to be discovered.
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Affiliation(s)
- Daniel Hantaï
- Inserm U582 and Unité Clinique de Pathologie Neuromusculaire, Institut de Myologie, Hôpital de la Salpêtrière, Paris, France.
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54
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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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55
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Nazarian J, Bouri K, Hoffman EP. Intracellular expression profiling by laser capture microdissection: three novel components of the neuromuscular junction. Physiol Genomics 2004; 21:70-80. [PMID: 15623565 DOI: 10.1152/physiolgenomics.00227.2004] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The neuromuscular junction (NMJ) is a regionally specialized area of myofibers defined, in part, by specific gene expression from underlying myonuclei. We sought to obtain a more complete picture of the mRNA transcripts and proteins playing a role in NMJ formation and maintenance using laser capture microdissection (LCM) and to define expression profiles of the nuclear domain at the NMJ. NMJs (800) were isolated from normal mouse tibialis anterior muscle by LCM, with an equal amount of adjacent non-NMJ regions isolated. Many known components of the NMJ were found significantly differentially expressed. Three differentially expressed potential novel components of the NMJ were chosen for further study, and each was validated by immunostaining with and without blocking peptides (3/3), quantitative RT-PCR (3/3), and in situ hybridization (1/3). The three genes validated were dual-specificity phosphatase-6 (DUSP6), ribosomal receptor-binding protein-1 (RRBP1), and vacuolar protein sorting-26 (VPS26). Query of each of these novel components in a 27-time point in vivo muscle regeneration series showed expression commensurate with previously known NMJ markers (nestin, alpha-ACh receptor). Understanding and discovering elements responsible for the integrity and function of NMJs is relevant to understanding neuromuscular diseases such as spinal muscular atrophy. Our LCM-based mRNA expression profiling provided us with new means of identification of specific genes potentially responsible for NMJ stability and function and new candidates for involvement in disease pathogenesis.
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Affiliation(s)
- Javad Nazarian
- The Institute for Biomedical Sciences, George Washington University, Washington, District of Columbia, USA
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56
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Gervásio OL, Phillips WD. Increased ratio of rapsyn to ACh receptor stabilizes postsynaptic receptors at the mouse neuromuscular synapse. J Physiol 2004; 562:673-85. [PMID: 15550459 PMCID: PMC1665540 DOI: 10.1113/jphysiol.2004.077685] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
The metabolic turnover of nicotinic ACh receptors (AChR) at the neuromuscular synapse is regulated over a tenfold range by innervation status, muscle electrical activity and neural agrin, but the downstream effector of such changes has not been defined. The AChR-associated protein rapsyn is essential for forming AChR clusters during development. Here, rapsyn was tagged with enhanced green fluorescent protein (EGFP) to begin to probe its influence at the adult synapse. In C2 myotubes, rapsyn-EGFP participated with AChR in agrin-induced AChR cluster formation. When electroporated into the tibialis anterior muscle of young adult mice, rapsyn-EGFP accumulated in discrete subcellular structures, many of which colocalized with Golgi markers, consistent with the idea that rapsyn assembles with AChR in the exocytic pathway. Rapsyn-EGFP also targeted directly to the postsynaptic membrane where it occupied previously vacant rapsyn binding sites, thereby increasing the rapsyn to AChR ratio. At endplates displaying rapsyn-EGFP, the metabolic turnover of AChR (labelled with rhodamine-alpha-bungarotoxin) was slowed. Thus, the metabolic half-life of receptors at the synapse may be modulated by local changes in the subsynaptic ratio of rapsyn to AChR.
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Affiliation(s)
- Othon L Gervásio
- Department of Physiology (F13), Institute for Biomedical Research, The University of Sydney, NSW 2006 Australia
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57
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Cartaud A, Strochlic L, Guerra M, Blanchard B, Lambergeon M, Krejci E, Cartaud J, Legay C. MuSK is required for anchoring acetylcholinesterase at the neuromuscular junction. ACTA ACUST UNITED AC 2004; 165:505-15. [PMID: 15159418 PMCID: PMC2172359 DOI: 10.1083/jcb.200307164] [Citation(s) in RCA: 136] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
At the neuromuscular junction, acetylcholinesterase (AChE) is mainly present as asymmetric forms in which tetramers of catalytic subunits are associated to a specific collagen, collagen Q (ColQ). The accumulation of the enzyme in the synaptic basal lamina strictly relies on ColQ. This has been shown to be mediated by interaction between ColQ and perlecan, which itself binds dystroglycan. Here, using transfected mutants of ColQ in a ColQ-deficient muscle cell line or COS-7 cells, we report that ColQ clusterizes through a more complex mechanism. This process requires two heparin-binding sites contained in the collagen domain as well as the COOH terminus of ColQ. Cross-linking and immunoprecipitation experiments in Torpedo postsynaptic membranes together with transfection experiments with muscle-specific kinase (MuSK) constructs in MuSK-deficient myotubes or COS-7 cells provide the first evidence that ColQ binds MuSK. Together, our data suggest that a ternary complex containing ColQ, perlecan, and MuSK is required for AChE clustering and support the notion that MuSK dictates AChE synaptic localization at the neuromuscular junction.
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Affiliation(s)
- Annie Cartaud
- Biologie Cellulaire des Membranes, Institut Jacques Monod, UMR 7592 Centre National de la Recherche Scientifique (CNRS), Universités Paris 6 and Paris 7, 75251 Paris, Cedex 05, France
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58
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Bozzi M, Di Stasio E, Cicero DO, Giardina B, Paci M, Brancaccio A. The effect of an ionic detergent on the natively unfolded beta-dystroglycan ectodomain and on its interaction with alpha-dystroglycan. Protein Sci 2004; 13:2437-45. [PMID: 15295116 PMCID: PMC2280000 DOI: 10.1110/ps.04762504] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Dystroglycan (DG) is an adhesion complex, expressed in a wide variety of tissues, formed by an extracellular and a transmembrane subunit, alpha-DG and beta-DG, respectively, interacting noncovalently. Recently, we have shown that the recombinant ectodomain of beta-DG, beta-DG(654-750), behaves as a natively unfolded protein, as it is able to bind the C-terminal domain of alpha-DG, while not displaying a defined structural organization. We monitored the effect of a commonly used denaturing agent, the anionic detergent sodium dodecylsulphate (SDS), on beta-DG(654-750) using a number of biophysical techniques. Very low concentrations of SDS (< or =2 mM) affect both tryptophan fluorescence and circular dichroism of beta-DG, and significantly perturb the interaction with the alpha-DG subunit as shown by solid-phase binding assays and fluorescence titrations in solution. This result confirms, as recently proposed for natively unfolded proteins, that beta-DG(654-750) exists in a native state, which is crucial to fulfill its biological function. Two-dimensional NMR analysis shows that SDS does not induce any evident conformational rearrangement within the ectodomain of beta-DG. Its first 70 amino acids, which show a lower degree of mobility, interact with the detergent, but this does not change the amount of secondary structure, whereas the highly flexible and mobile C-terminal region of beta-DG(654-750) remains largely unaffected, even at a very high SDS concentration (up to 50 mM). Our data indicate that SDS can be used as a useful tool for investigating natively unfolded proteins, and confirm that the beta-DG ectodomain is an interesting model system.
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Affiliation(s)
- Manuela Bozzi
- Consiglio Nazionale delle Richerche (CNR), Istituto di Chimica del Riconoscimento Molecolare, c/o Istituto di Biochimica e Biochimica Clinica, Università Cattolica del Sacro Cuore, 00168 Rome, Italy
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59
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Andreux F, Hantaï D, Eymard B. [Congenital myasthenic syndromes: phenotypic expression and pathophysiological characterisation]. Rev Neurol (Paris) 2004; 160:163-76. [PMID: 15034473 DOI: 10.1016/s0035-3787(04)70887-5] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
Congenital Myasthenic Syndromes (CMS) are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. The twenty five past Years saw major advances in identifying different types of CMS due to abnormal presynaptic, synaptic, and postsynaptic proteins. CMS diagnosis requires two steps: 1) positive diagnosis supported by myasthenic signs beginning in neonatal period, efficacy of anticholinesterase medications, positive family history, negative tests for anti-acetylcholine receptor (AChR) antibodies, electromyographic studies (decremental response at low frequency, repetitive CMAP after one single stimulation); 2) pathophysiological characterisation of CMS implying specific studies: light and electron microscopic analysis of endplate (EP) morphology, estimation of the number of AChR per EP, acetylcholinesterase (AChE) expression, molecular genetic analysis. Most CMS are postsynaptic due to mutations in the AChR subunits genes that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh resulting respectively in Slow Channel Syndrome (characterized by a autosomal dominant transmission, repetitive CMAP, refractoriness to anticholinesterase medication) and fast channel, recessively transmitted. AChR deficiency without kinetic abnormalities is caused by recessive mutations in AChR genes (mostly epsilon subunit) or by primary rapsyn deficiency, a post synaptic protein involved in AChR concentration. Recently, mutations in SCN4A sodium channel have been reported in one patient. AChE deficiency is identified on the following data: recessive transmission, presence of repetitive CMAP, refractoriness to cholinesterase inhibitors, slow pupillary response to light and absent expression of the enzyme at EP. This synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) that anchors the catalytic subunits in the synaptic basal lamina. The most frequent presynaptic CMS is caused by mutations of choline acetyltransferase. Several CMS are still not characterized. Many EP molecules are potential etiological candidates. In these unidentified cases, other methods of investigations are required: linkage analysis, when sufficient number of informative relatives are available, microelectrophysiological studies performed in intercostal or anconeus muscles. Prognosis of CMS, depending on severity and evolution of symptoms, is difficult to assess, and it cannot not be simply derived from mutation identification. Most patients respond favourably to anticholinesterase medications or to 3,4 DAP which is effective not only in presynaptic but also in postsynaptic CMS. Specific therapies for slow channel CMS are quinidine and fluoxetine that normalize the prolonged opening episodes. Clinical benefits derived from the full characterisation of each case include genetic counselling and specific therapy.
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Affiliation(s)
- F Andreux
- INSERM 582 et Institut de Myologie, Hôpital de la Pitié-Salpêtrière
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60
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Banwell BL, Ohno K, Sieb JP, Engel AG. Novel truncating RAPSN mutations causing congenital myasthenic syndrome responsive to 3,4-diaminopyridine. Neuromuscul Disord 2004; 14:202-7. [PMID: 15036330 DOI: 10.1016/j.nmd.2003.11.004] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2003] [Revised: 10/30/2003] [Accepted: 11/03/2003] [Indexed: 11/28/2022]
Abstract
Rapsyn is essential for clustering the acetylcholine receptor at the postsynaptic membrane of the neuromuscular junction. Direct sequencing of RAPSN in two children with congenital myasthenic syndromes with no mutation in any of the AChR subunits identified two heterozygous recessive mutations in each: a previously characterized N88K mutation in both, and a second frameshifting mutation in Patient (Pt) 1 and a nonsense mutation in Pt 2. An intercostal muscle biopsy in Pt 1 revealed decreased AChRs per endplate and decreased amplitude of the miniature endplate potential, predicted consequences of rapsyn deficiency. Clinically, both children manifested with hypomotility in utero, fatigable ocular and limb weakness since birth, decreased strength during viral illness, decremental response on electromyography, and absence of AChR antibodies. Pt 1, however, had a more severe clinical course with recurrent episodes of respiratory failure, contractures, and craniofacial malformations. In both patients, treatment with pyridostigmine was of some benefit, but the addition of 3,4-diaminopyridine led to significant clinical improvement. Thus, rapsyn deficiency predicting similar consequences at the cellular level can result in phenotypes with marked differences in severity of symptoms, risk of respiratory failure, and presence of contractures and craniofacial malformations.
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Affiliation(s)
- Brenda L Banwell
- Department of Pediatrics (Neurology), The Hospital for Sick Children, University of Toronto, Canada
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61
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Eymard B, Ioos C, Barois A, Estournet B, Mayer M, Fournier E, Yasaki E, Prioleau C, Bauché S, Gaudon K, Leroy JP, Koenig J, Richard P, Hantaï D. Syndromes myasthéniques congénitaux dus à des mutations du gène de la rapsyne. Rev Neurol (Paris) 2004; 160:S78-84. [PMID: 15269664 DOI: 10.1016/s0035-3787(04)71009-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
Congenital myasthenic syndromes (CMS) are genetic diseases characterized by dysfunctional neuromuscular transmission and usually start during the neonatal period. Most are due to postsynaptic abnormalities, specifically to mutations in the acetylcholine receptor (AChR) genes. In 2002, the group of A Engel reported the first cases of CMS with mutations in the gene coding rapsyn, a postsynaptic molecule which stabilizes AChR aggregates at the neuromuscular junction. Since this first publication, more than 30 other cases, including six in France, have been reported. Study of these published cases allows us to distinguish three classes of phenotypes: 1) severe neonatal cases; 2) more benign cases, starting during infancy; 3) cases with facial malformations, involving Jewish patients originating from the Near-East. Comparison of the observations of other groups with our own has led us to the following conclusions: the N88K mutation is frequent (homozygous in 50% of cases); besides the N88K mutation, the second mutation varies considerably; heterozygous allelic cases (N88K + another mutation) are severe; there is probably a founder effect in the European population. There is phenotypic variability in the homozygous N88K cases, with benign cases and severe cases of early expression. A Engel and colleagues report that the seven cases of benign CMS with facial malformation, previously described in the Jewish population of Iraq and Iran, were caused by mutation in the promoter region of the rapsyn gene.
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Affiliation(s)
- B Eymard
- INSERM U582 et Institut de Myologie, Hôpital de la Salpêtrière, Paris.
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62
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Bozzi M, Bianchi M, Sciandra F, Paci M, Giardina B, Brancaccio A, Cicero DO. Structural characterization by NMR of the natively unfolded extracellular domain of beta-dystroglycan: toward the identification of the binding epitope for alpha-dystroglycan. Biochemistry 2004; 42:13717-24. [PMID: 14622018 DOI: 10.1021/bi034867w] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Dystroglycan (DG) is an adhesion molecule playing a crucial role for tissue stability during both early embriogenesis and adulthood and is composed by two tightly interacting subunits: alpha-DG, membrane-associated and highly glycosylated, and the transmembrane beta-DG. Recently, by solid-phase binding assays and NMR experiments, we have shown that the C-terminal domain of alpha-DG interacts with a recombinant extracellular fragment of beta-DG (positions 654-750) independently from glycosylation and that the linear binding epitope is located between residues 550 and 565 of alpha-DG. In order to elucidate which moieties of beta-DG are specifically involved in the complex with alpha-DG, the ectodomain has been recombinantly expressed and purified in a labeled ((13)C,(15)N) form and studied by multidimensional NMR. Although it represents a natively unfolded protein domain, we obtained an almost complete backbone assignment. Chemical shift index, (1)H-(15)N heteronuclear single-quantum coherence and nuclear Overhauser effect (HSQC-NOESY) spectra and (3)J(HN,H)(alpha) coupling constant values confirm that this protein is highly disordered, but (1)H-(15)N steady-state NOE experiments indicate that the protein presents two regions of different mobility. The first one, between residues 659 and 722, is characterized by a limited degree of mobility, whereas the C-terminal portion, containing about 30 amino acids, is highly flexible. The binding of beta-DG(654-750) to the C-terminal region of the alpha subunit, alpha-DG(485-620), has been investigated, showing that the region of beta-DG(654-750) between residues 691 and 719 is involved in the interaction.
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Affiliation(s)
- Manuela Bozzi
- Istituto di Chimica del Riconoscimento Molecolare c/o Istituto di Biochimica e Biochimica Clinica, Consiglio Nazionale della Ricerche, Università Cattolica del Sacro Cuore, Largo Francesco Vito 1, 00168 Rome, Italy
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63
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Sotgia F, Bonuccelli G, Bedford M, Brancaccio A, Mayer U, Wilson MT, Campos-Gonzalez R, Brooks JW, Sudol M, Lisanti MP. Localization of phospho-beta-dystroglycan (pY892) to an intracellular vesicular compartment in cultured cells and skeletal muscle fibers in vivo. Biochemistry 2003; 42:7110-23. [PMID: 12795607 DOI: 10.1021/bi0271289] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
beta-Dystroglycan is a ubiquitously expressed integral membrane protein that undergoes tyrosine phosphorylation in an adhesion-dependent manner. Tyrosine 892 is now thought to be the principal site for recognition by the c-Src tyrosine kinase; however, little is known about the regulation of this phosphorylation event in vivo. Here, we generated a novel monoclonal antibody probe that recognizes only tyrosine 892 phosphorylated beta-dystroglycan (pY892). We show that upon tyrosine phosphorylation, beta-dystroglycan undergoes a profound change in its sub-cellular localization (e.g., from the plasma membrane to an internal membrane compartment). One possibility is that the net negative charge at position 892 causes the redistribution of beta-dystroglycan to this intracellular vesicular location. In support of this notion, mutation of tyrosine 892 to glutamate (Y892E) is sufficient to drive this intracellular localization, while other point mutants (Y892F and Y892A) remain at the plasma membrane. Interestingly, our colocalization studies with endosomal markers (EEA1, transferrin, and transferrin receptor) suggest that these phospho-beta-dystroglycan containing internal vesicles represent a subset of recycling endosomes. At the level of these internal vesicular structures, we find that tyrosine phosphorylated beta-dystroglycan is colocalized with c-Src. In addition, we demonstrate that known ligands for alpha-dystroglycan, namely, agrin and laminin, are able to induce the tyrosine phosphorylation of beta-dystroglycan. Finally, we show that tyrosine phosphorylated beta-dystroglycan is also detectable in skeletal muscle tissue lysates and is localized to an internal vesicular membrane compartment in skeletal muscle fibers in vivo. The generation of a phospho-specific beta-dystroglycan (pY892) mAb probe provides a new powerful tool for dissecting the role of dystroglycan phosphorylation in normal cellular functioning and in the pathogenesis of muscular dystrophies.
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Affiliation(s)
- Federica Sotgia
- Department of Molecular Pharmacology and The Albert Einstein Cancer Center, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, USA
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64
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Jayasinha V, Nguyen HH, Xia B, Kammesheidt A, Hoyte K, Martin PT. Inhibition of dystroglycan cleavage causes muscular dystrophy in transgenic mice. Neuromuscul Disord 2003; 13:365-75. [PMID: 12798792 DOI: 10.1016/s0960-8966(03)00040-3] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Dystroglycan (DG) is an essential component of the dystrophin-glycoprotein complex, a molecular scaffold that links the extracellular matrix to the actin cytoskeleton. Dystroglycan protein is post-translationally cleaved into alpha dystroglycan, a highly glycosylated peripheral membrane protein, and beta dystroglycan, a transmembrane protein. Despite clear evidence of the importance of dystroglycan and its associated proteins in muscular dystrophy, the purpose of dystroglycan proteolysis is unclear. By introducing a point mutation at the normal site of proteolysis (serine 654 to alanine, DGS654A), we have created a dystroglycan protein that is severely inhibited in its cleavage. Transgenic expression of DGS654A in mouse skeletal muscles inhibited the expression of endogenously cleaved dystroglycan, while overexpression of wild type dystroglycan by similar amounts did not. DGS654A animals had increased serum creatine kinase activity and most muscles had increased numbers of central nuclei. Overexpression of wild type dystroglycan, by contrast, caused no dystrophy by these measures. Dystrophy in DGS654A muscles correlated with reduced binding of antibodies that recognize glycosylated forms of alpha dystroglycan. Lastly, neuromuscular junctions in DGS654A muscles were aberrant in structure. These data show that aberrant processing of the dystroglycan polypeptide causes muscular dystrophy and suggest that dystroglycan processing is important for the proper glycosylation of alpha dystroglycan.
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Affiliation(s)
- Vianney Jayasinha
- Department of Neuroscience, Glycobiology Research and Training Center, School of Medicine, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0691, USA
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65
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Engel AG, Ohno K, Sine SM. Sleuthing molecular targets for neurological diseases at the neuromuscular junction. Nat Rev Neurosci 2003; 4:339-52. [PMID: 12728262 DOI: 10.1038/nrn1101] [Citation(s) in RCA: 132] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- Andrew G Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota 55905, USA.
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66
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Abstract
The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.
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Affiliation(s)
- Gabriela Bezakova
- Department of Pharmacology/Neurobiology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland
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67
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Huebsch KA, Maimone MM. Rapsyn-mediated clustering of acetylcholine receptor subunits requires the major cytoplasmic loop of the receptor subunits. JOURNAL OF NEUROBIOLOGY 2003; 54:486-501. [PMID: 12532399 DOI: 10.1002/neu.10177] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
During synaptogenesis at the neuromuscular junction, nicotinic acetylcholine receptors (AChRs) are organized into high-density postsynaptic clusters that are critical for efficient synaptic transmission. Rapsyn, an AChR associated cytoplasmic protein, is essential for the aggregation and immobilization of AChRs at the neuromuscular junction. Previous studies have shown that when expressed in nonmuscle cells, both assembled and unassembled AChR subunits are clustered by rapsyn, and the clustering of the alpha subunit is dependent on its major cytoplasmic loop. In the present study, we investigated the mechanism of rapsyn-induced clustering of the AChR beta, gamma, and delta subunits by testing mutant subunits for the ability to cocluster with rapsyn in transfected QT6 cells. For each subunit, deletion of the major cytoplasmic loop, between the third and fourth transmembrane domains, dramatically reduced coclustering with rapsyn. Furthermore, each major cytoplasmic loop was sufficient to mediate clustering of an unrelated transmembrane protein. The AChR subunit mutants lacking the major cytoplasmic loops could assemble into alphadelta dimers, but these were poorly clustered by rapsyn unless at least one mutant was replaced with its wild-type counterpart. These results demonstrate that the major cytoplasmic loop of each AChR subunit is both necessary and sufficient for mediating efficient clustering by rapsyn, and that only one such domain is required for rapsyn-mediated clustering of an assembly intermediate, the alphadelta dimer.
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Affiliation(s)
- Kimberly A Huebsch
- Department of Cell and Developmental Biology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, New York 13210, USA
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68
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A role for the juxtamembrane domain of beta-dystroglycan in agrin-induced acetylcholine receptor clustering. J Neurosci 2003. [PMID: 12533599 DOI: 10.1523/jneurosci.23-02-00392.2003] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Synaptic differentiation results from an exchange of informational molecules between synaptic partners during development. At the vertebrate neuromuscular junction, agrin is one molecule presented by the presynaptic motor neuron that plays an instructive role in postsynaptic differentiation of the muscle cell, most notably in aggregation of acetylcholine receptors (AChRs). Although agrin is the best-characterized synaptogenic molecule, its mechanism of action remains uncertain, but clearly, it requires the receptor tyrosine kinase MuSK (muscle-specific kinase), the intracellular protein rapsyn, an Src-like kinase, and cytoskeletal components. In addition, the transmembrane protein dystroglycan interacts with the cytoskeleton and is implicated in agrin responsiveness. This alpha-beta heterodimer can bind agrin via its extracellular alpha subunit and associates with the membrane cytoskeleton via its beta subunit. In this study, we demonstrate that overexpression of the beta subunit of dystroglycan in cultured muscle cells inhibits agrin-induced AChR clustering. Deletion analysis and point mutagenesis demonstrate that the inhibition is mediated by an intracellular, juxtamembrane region composed of basic amino acids. Finally, the inhibition mediated by beta-dystroglycan extends to the minimal agrin fragment required for AChR clustering, suggesting that dystroglycan plays an important role in postsynaptic differentiation in response to agrin.
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69
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Abstract
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.
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Affiliation(s)
- Andrew G Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota 55905, USA.
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70
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Abstract
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.
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Affiliation(s)
- Andrew G Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA.
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71
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Akaaboune M, Grady RM, Turney S, Sanes JR, Lichtman JW. Neurotransmitter receptor dynamics studied in vivo by reversible photo-unbinding of fluorescent ligands. Neuron 2002; 34:865-76. [PMID: 12086635 DOI: 10.1016/s0896-6273(02)00739-0] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We show that fluorescently tagged ligands with high affinity for their targets can be reversibly unbound by focused laser excitation. By sequential unbinding and relabeling with different colors of alpha-bungarotoxin, we selectively labeled adjacent pools of acetylcholine receptors (AChRs) at neuromuscular junctions of adult mice. Timelapse imaging in vivo revealed that synaptic AChRs completely intermingle over approximately 4 days and many extrasynaptic AChRs are incorporated into the synapse each day. In mice that lacked alpha-dystrobrevin, a component of the dystrophin-glycoprotein complex, rates of AChR turnover, and intermingling were increased approximately 4- to 5-fold. These results demonstrate remarkable molecular dynamism underlying macroscopic stability of the postsynaptic membrane, and establish alpha-dystrobrevin as a key control point for regulation of mobility and turnover.
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Affiliation(s)
- Mohammed Akaaboune
- Department of Anatomy and Neurobiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, USA
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72
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Kunz S, Borrow P, Oldstone MBA. Receptor structure, binding, and cell entry of arenaviruses. Curr Top Microbiol Immunol 2002; 262:111-37. [PMID: 11987803 DOI: 10.1007/978-3-642-56029-3_5] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Affiliation(s)
- S Kunz
- Department of Neuropharmacology, Division of Virology, Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
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73
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Ohno K, Engel AG, Shen XM, Selcen D, Brengman J, Harper CM, Tsujino A, Milone M. Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome. Am J Hum Genet 2002; 70:875-85. [PMID: 11791205 PMCID: PMC379116 DOI: 10.1086/339465] [Citation(s) in RCA: 171] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2001] [Accepted: 01/04/2002] [Indexed: 01/22/2023] Open
Abstract
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.
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Affiliation(s)
- Kinji Ohno
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
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74
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Blake DJ, Weir A, Newey SE, Davies KE. Function and genetics of dystrophin and dystrophin-related proteins in muscle. Physiol Rev 2002; 82:291-329. [PMID: 11917091 DOI: 10.1152/physrev.00028.2001] [Citation(s) in RCA: 813] [Impact Index Per Article: 37.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.
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Affiliation(s)
- Derek J Blake
- Medical Research Council, Functional Genetics Unit, Department of Human Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
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75
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Huh KH, Fuhrer C. Clustering of nicotinic acetylcholine receptors: from the neuromuscular junction to interneuronal synapses. Mol Neurobiol 2002; 25:79-112. [PMID: 11890459 DOI: 10.1385/mn:25:1:079] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
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.
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Affiliation(s)
- Kyung-Hye Huh
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Switzerland
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76
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Liyanage Y, Hoch W, Beeson D, Vincent A. The agrin/muscle-specific kinase pathway: new targets for autoimmune and genetic disorders at the neuromuscular junction. Muscle Nerve 2002; 25:4-16. [PMID: 11754179 DOI: 10.1002/mus.1218] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The increasing understanding of the structural complexity of the neuromuscular junction (NMJ), and the processes that are important in its development, suggests many possible new disease targets. Here, we summarize briefly the genetic and autoimmune disorders that affect neuromuscular transmission, and the identified targets, including new evidence that antibodies to muscle-specific receptor tyrosine kinase (MuSK) are involved in the pathogenesis of acetylcholine receptor (AChR) antibody-negative myasthenia gravis. We then review the development of the NMJ, focusing on the important roles of nerve-derived agrin and MuSK in clustering of AChRs and other essential components of the NMJ.
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Affiliation(s)
- Yohan Liyanage
- Neurosciences Group, Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford OX3 9DS, UK
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77
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Rando TA. The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies. Muscle Nerve 2001; 24:1575-94. [PMID: 11745966 DOI: 10.1002/mus.1192] [Citation(s) in RCA: 281] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.
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Affiliation(s)
- T A Rando
- Department of Neurology and Neurological Sciences, Stanford University Medical Center, Room A-343, Stanford, California 94305-5235, USA.
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78
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Zaccaria ML, Di Tommaso F, Brancaccio A, Paggi P, Petrucci TC. Dystroglycan distribution in adult mouse brain: a light and electron microscopy study. Neuroscience 2001; 104:311-24. [PMID: 11377836 DOI: 10.1016/s0306-4522(01)00092-6] [Citation(s) in RCA: 121] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Dystroglycan, originally identified in muscle as a component of the dystrophin-associated glycoprotein complex, is a ubiquitously expressed cell-surface receptor that forms a transmembrane link between the extracellular matrix and the cytoskeleton. It contains two subunits, alpha and beta, formed by proteolytic cleavage of a common precursor. In the brain, different neuronal subtypes and glial cells may express dystroglycan in complex with distinct cytoplasmic proteins such as dystrophin, utrophin and their truncated forms. To examine the distribution of dystroglycan in adult mouse brain, we raised antibodies against the recombinant amino- and carboxyl-terminal domains of alpha-dystroglycan. On western blot, the antibodies recognized specifically alpha-dystroglycan in cerebellar extracts. Using light microscopy, alpha-dystroglycan was found in neurons of the cerebral cortex, hippocampus, olfactory bulb, basal ganglia, thalamus, hypothalamus, brainstem and cerebellum, where dystrophin and its truncated isoforms are also known to be present. Electron microscopy revealed that alpha-dystroglycan immunoreactivity was preferentially associated with the postsynaptic specializations. Dystroglycan immunostaining was also detected in perivascular astrocytes and in those facing the pia mater, where utrophin and dystrophin truncated isoforms are present. The cell body and endfeet of astrocytes around blood vessels and the endothelial cells at the blood-brain barrier also expressed dystroglycan. From these data, we suggest that dystroglycan, by bridging the extracellular matrix and the cytoskeleton, may play an important functional role at specialized intercellular contacts, synapses and the blood-brain barrier, whose structural and functional organization strictly depend on the integrity of the extracellular matrix-cytoskeleton linkage.
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Affiliation(s)
- M L Zaccaria
- Dipartimento di Biologia Cellulare e dello Sviluppo, Università "La Sapienza", 00185, Rome, Italy
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79
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Donnadieu E, Revy P, Trautmann A. Imaging T-cell antigen recognition and comparing immunological and neuronal synapses. Immunology 2001; 103:417-25. [PMID: 11529931 PMCID: PMC1783261 DOI: 10.1046/j.1365-2567.2001.01268.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2001] [Accepted: 04/27/2001] [Indexed: 12/15/2022] Open
Affiliation(s)
- E Donnadieu
- Laboratoire d'Immuno-Pharmacologie, CNRS UPR 415, ICGM, 22 rue Méchain, 75014 Paris, France
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80
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Sciandra F, Schneider M, Giardina B, Baumgartner S, Petrucci TC, Brancaccio A. Identification of the beta-dystroglycan binding epitope within the C-terminal region of alpha-dystroglycan. EUROPEAN JOURNAL OF BIOCHEMISTRY 2001; 268:4590-7. [PMID: 11502221 DOI: 10.1046/j.1432-1327.2001.02386.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Dystroglycan is a receptor for extracellular matrix proteins that plays a crucial role during embryogenesis in addition to adult tissue stabilization. A precursor product of a single gene is post-translationally cleaved to form two different subunits, alpha and beta. The extracellular alpha-dystroglycan is a membrane-associated, highly glycosylated protein that binds to various extracellular matrix molecules, whereas the transmembrane beta-dystroglycan binds, via its cytosolic domain, to dystrophin and many other proteins. alpha- and beta-Dystroglycan interact tightly but noncovalently. We have previously shown that the N-terminal region of beta-dystroglycan, beta-DG(654-750), binds to the C-terminal region of murine alpha-dystroglycan independently from glycosylation. Preparing a series of deleted recombinant fragments and using solid-phase binding assays, the C-terminal sequence of alpha-dystroglycan containing the binding epitope for beta-dystroglycan has been defined more precisely. We found that a region of 36 amino acids, from position 550-585, is required for binding the extracellular region, amino acids 654-750 of beta-dystroglycan. Recently, a dystroglycan-like gene was identified in Drosophila that showed a moderate degree of conservation with vertebrate dystroglycan (31% identity, 48% similarity). Surprisingly, the Drosophila sequence contains a region showing a higher degree of identity and conservation (45% and 66%) that coincides with the 550-585 sequence of vertebrate alpha-dystroglycan. We have expressed this Drosophila dystroglycan fragment and measured its binding to the extracellular region of vertebrate (murine) beta-dystroglycan (Kd = 6 +/- 1 microM). These data confirm the proper identification of the beta-dystroglycan binding epitope and stress the importance of this region during evolution. This finding might help the rational design of dystroglycan-specific binding drugs, that could have important biomedical applications.
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Affiliation(s)
- F Sciandra
- Center for Receptor Chemistry (CNR) Institute of Chemistry and Clinical Chemistry, Catholic University of Rome, Italy
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81
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Bartoli M, Ramarao MK, Cohen JB. Interactions of the rapsyn RING-H2 domain with dystroglycan. J Biol Chem 2001; 276:24911-7. [PMID: 11342559 DOI: 10.1074/jbc.m103258200] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Rapsyn, a peripheral membrane protein of skeletal muscle, is necessary for the formation of the highly organized structure of the vertebrate neuromuscular junction. For mice lacking rapsyn, there is a failure of postsynaptic specialization characterized by an absence of nicotinic acetylcholine receptors (nAChRs) and other integral and peripheral membrane proteins such as beta-dystroglycan and utrophin. Dystroglycan is necessary for the formation of the mature neuromuscular junction and has been shown to interact directly with rapsyn. Previous studies with rapsyn fragments and mutants, expressed in 293T cells along with nAChRs, establish that the rapsyn tetratricopeptide repeat (TPR) domain is involved in self-association and its coiled-coil domain is necessary for nAChR clustering. The function of the rapsyn RING-H2 domain, which is not necessary for rapsyn self-association or nAChR clustering, is unknown. To further characterize these domains, we have used a yeast two-hybrid assay to test for interactions at the plasma membrane between rapsyn domains and a nAChR beta-subunit fragment, the beta-dystroglycan cytoplasmic domain, or rapsyn domains. The rapsyn coiled-coil domain interacts with the nAChR beta-subunit cytoplasmic domain, but not with itself, other rapsyn domains, or beta-dystroglycan. The RING-H2 domain interacts only with the beta-dystroglycan cytoplasmic domain. Furthermore, when expressed in 293T cells, a rapsyn construct containing as few as two TPRs and the RING-H2 domain self-associates and clusters dystroglycan, but not nAChRs. These results emphasize the modular character of the rapsyn structural domains.
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Affiliation(s)
- M Bartoli
- Department of Neurobiology, Harvard Medical School, 220 Longwood Ave., Boston, MA 02115, USA
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82
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Strochlic L, Cartaud A, Labas V, Hoch W, Rossier J, Cartaud J. MAGI-1c: a synaptic MAGUK interacting with muSK at the vertebrate neuromuscular junction. J Cell Biol 2001; 153:1127-32. [PMID: 11381096 PMCID: PMC2174332 DOI: 10.1083/jcb.153.5.1127] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The muscle-specific receptor tyrosine kinase (MuSK) forms part of a receptor complex, activated by nerve-derived agrin, that orchestrates the differentiation of the neuromuscular junction (NMJ). The molecular events linking MuSK activation with postsynaptic differentiation are not fully understood. In an attempt to identify partners and/or effectors of MuSK, cross-linking and immunopurification experiments were performed in purified postsynaptic membranes from the Torpedo electrocyte, a model system for the NMJ. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis was conducted on both cross-link products, and on the major peptide coimmunopurified with MuSK; this analysis identified a polypeptide corresponding to the COOH-terminal fragment of membrane-associated guanylate kinase (MAGUK) with inverted domain organization (MAGI)-1c. A bona fide MAGI-1c (150 kD) was detected by Western blotting in the postsynaptic membrane of Torpedo electrocytes, and in a high molecular mass cross-link product of MuSK. Immunofluorescence experiments showed that MAGI-1c is localized specifically at the adult rat NMJ, but is absent from agrin-induced acetylcholine receptor clusters in myotubes in vitro. In the central nervous system, MAGUKs play a primary role as scaffolding proteins that organize cytoskeletal signaling complexes at excitatory synapses. Our data suggest that a protein from the MAGUK family is involved in the MuSK signaling pathway at the vertebrate NMJ.
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Affiliation(s)
- Laure Strochlic
- Biologie Cellulaire des Membranes, Institut Jacques Monod, UMR 7592 CNRS, Universités Paris 6 et Paris 7, 75251 Paris, France
| | - Annie Cartaud
- Biologie Cellulaire des Membranes, Institut Jacques Monod, UMR 7592 CNRS, Universités Paris 6 et Paris 7, 75251 Paris, France
| | - Valérie Labas
- Neurobiologie et Diversité Cellulaire, UMR 7637 CNRS, Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris, 75005 Paris, France
| | - Werner Hoch
- Department of Biochemistry, Max Planck Institute for Developmental Biology, D 72076, Tübingen, Germany
| | - Jean Rossier
- Neurobiologie et Diversité Cellulaire, UMR 7637 CNRS, Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris, 75005 Paris, France
| | - Jean Cartaud
- Biologie Cellulaire des Membranes, Institut Jacques Monod, UMR 7592 CNRS, Universités Paris 6 et Paris 7, 75251 Paris, France
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83
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Dalloz C, Claudepierre T, Rodius F, Mornet D, Sahel J, Rendon A. Differential Distribution of the Members of the Dystrophin Glycoprotein Complex in Mouse Retina: Effect of the mdx3Cv Mutation. Mol Cell Neurosci 2001; 17:908-20. [PMID: 11358487 DOI: 10.1006/mcne.2001.0978] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Dystrophin glycoprotein complex (DGC) assembly and function require mediation by dystrophin in skeletal muscle. The existence of such complexes and the correlation with DMD phenotypes are not yet established in the central nervous system. Here we have studied the expression of DMD gene mRNAs and proteins in retina from C57BL/6 and mdx(3Cv) mouse strains. Then we have comparatively investigated the localization of dystrophin and dystrophin-associated proteins (DAPs) in both strains to analyze the repercussion of the mdx(3Cv) mutation on the retinal distributions of alpha/beta-dystroglycan, alpha1-syntrophin, alpha-dystrobrevin, and delta/gamma-sarcoglycan. Results showed that DMD gene product deficiency affects the expression of dystroglycan assembly exclusively at the outer plexiform layer without an apparent effect on the other DAPs. We conclude that the localization of members of the DGC could be independent of the presence of the DMD gene products and/or utrophin.
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MESH Headings
- Animals
- Calcium-Binding Proteins
- Cytoskeletal Proteins/genetics
- Cytoskeletal Proteins/metabolism
- Dystroglycans
- Dystrophin/genetics
- Dystrophin/metabolism
- Dystrophin-Associated Proteins
- Gene Expression/physiology
- Membrane Glycoproteins/genetics
- Membrane Glycoproteins/metabolism
- Membrane Proteins/genetics
- Membrane Proteins/metabolism
- Mice
- Mice, Inbred C57BL/embryology
- Mice, Inbred C57BL/genetics
- Mice, Inbred C57BL/metabolism
- Mice, Inbred mdx/abnormalities
- Mice, Inbred mdx/genetics
- Mice, Inbred mdx/metabolism
- Muscle Proteins/genetics
- Muscle Proteins/metabolism
- Muscular Dystrophy, Duchenne/complications
- Muscular Dystrophy, Duchenne/genetics
- Muscular Dystrophy, Duchenne/metabolism
- Mutation/genetics
- RNA, Messenger/metabolism
- Retina/abnormalities
- Retina/metabolism
- Retina/physiopathology
- Retinal Diseases/genetics
- Retinal Diseases/metabolism
- Retinal Diseases/physiopathology
- Sarcoglycans
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Affiliation(s)
- C Dalloz
- Laboratoire de Physiopathologie Cellulaire et Moléculaire de la Rétine, Médicale A, INSERM EMI 99-18, CHRU, 1 Place de l'Hôpital, 67091 Strasbourg Cedex, France
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84
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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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85
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Gieseler K, Mariol MC, Bessou C, Migaud M, Franks CJ, Holden-Dye L, Ségalat L. Molecular, genetic and physiological characterisation of dystrobrevin-like (dyb-1) mutants of Caenorhabditis elegans. J Mol Biol 2001; 307:107-17. [PMID: 11243807 DOI: 10.1006/jmbi.2000.4480] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Dystrobrevins are protein components of the dystrophin complex, whose disruption leads to Duchenne muscular dystrophy and related diseases. The Caenorhabditis elegans dystrobrevin gene (dyb-1) encodes a protein 38 % identical with its mammalian counterparts. The C. elegans dystrobrevin is expressed in muscles and neurons. We characterised C. elegans dyb-1 mutants and showed that: (1) their behavioural phenotype resembles that of dystrophin (dys-1) mutants; (2) the phenotype of dyb-1 dys-1 double mutants is not different from the single ones; (3) dyb-1 mutants are more sensitive than wild-type animals to reductions of acetylcholinesterase levels and have an increased response to acetylcholine; (4) dyb-1 mutations alone do not lead to muscle degeneration, but synergistically produce a progressive myopathy when combined with a mild MyoD/hlh-1 mutation. All together, these findings further substantiate the role of dystrobrevins in cholinergic transmission and as functional partners of dystrophin.
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Affiliation(s)
- K Gieseler
- CGMC, CNRS-UMR 5534, Université Lyon1, 43 bld du 11 Novembre, 69622 Villeurbanne cedex, France
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86
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Ramarao MK, Bianchetta MJ, Lanken J, Cohen JB. Role of rapsyn tetratricopeptide repeat and coiled-coil domains in self-association and nicotinic acetylcholine receptor clustering. J Biol Chem 2001; 276:7475-83. [PMID: 11087759 DOI: 10.1074/jbc.m009888200] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Rapsyn, a 43-kDa peripheral membrane protein of skeletal muscle, is essential for clustering nicotinic acetylcholine receptors (nAChR) in the postsynaptic membrane. Previous studies with rapsyn NH(2)-terminal fragments fused to green fluorescent protein, expressed in 293T cells along with nAChRs, establish the following: Rapsyn-(1-90), containing the myristoylated amino terminus and two tetratricopeptide repeats (TPRs), was sufficient for self-association at the plasma membrane; rapsyn-(1-287), containing seven TPRs, did not cluster nAChRs; whereas rapsyn-(1-360)(,) containing a coiled-coil domain (rapsyn-(298-331)), clustered nAChRs. To further analyze the role of rapsyn structural domains in self-association and nAChR clustering, we have characterized the clustering properties of additional rapsyn mutants containing deletions and substitutions within the TPR and coiled-coil domains. A mutant lacking the coiled-coil domain alone (rapsyn-(black triangle288-348)), failed to cluster nAChRs. Within the coiled-coil domain neutralization of the charged side chains was tolerated, while alanine substitutions of large hydrophobic residues resulted in the loss of nAChR clustering. Rapsyn self-association requires at least two TPRs, as a single TPR (TPR1 or TPR2 alone) was not sufficient. While TPRs 1 and 2 are sufficient for self-association, they are not necessary, as TPRs 3-7 also formed clusters similar to wild-type rapsyn. Fragments containing TPRs co-localized with full-length rapsyn, while the expressed coiled-coil or RING-H2 domain did not. These results are discussed in terms of a homology model of rapsyn, based on the three-dimensional structure of the TPR domain of protein phosphatase 5.
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Affiliation(s)
- M K Ramarao
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
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87
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Jacobson C, Côté PD, Rossi SG, Rotundo RL, Carbonetto S. The dystroglycan complex is necessary for stabilization of acetylcholine receptor clusters at neuromuscular junctions and formation of the synaptic basement membrane. J Cell Biol 2001; 152:435-50. [PMID: 11157973 PMCID: PMC2195998 DOI: 10.1083/jcb.152.3.435] [Citation(s) in RCA: 158] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
The dystrophin-associated protein (DAP) complex spans the sarcolemmal membrane linking the cytoskeleton to the basement membrane surrounding each myofiber. Defects in the DAP complex have been linked previously to a variety of muscular dystrophies. Other evidence points to a role for the DAP complex in formation of nerve-muscle synapses. We show that myotubes differentiated from dystroglycan-/- embryonic stem cells are responsive to agrin, but produce acetylcholine receptor (AChR) clusters which are two to three times larger in area, about half as dense, and significantly less stable than those on dystroglycan+/+ myotubes. AChRs at neuromuscular junctions are similarly affected in dystroglycan-deficient chimeric mice and there is a coordinate increase in nerve terminal size at these junctions. In culture and in vivo the absence of dystroglycan disrupts the localization to AChR clusters of laminin, perlecan, and acetylcholinesterase (AChE), but not rapsyn or agrin. Treatment of myotubes in culture with laminin induces AChR clusters on dystroglycan+/+, but not -/- myotubes. These results suggest that dystroglycan is essential for the assembly of a synaptic basement membrane, most notably by localizing AChE through its binding to perlecan. In addition, they suggest that dystroglycan functions in the organization and stabilization of AChR clusters, which appear to be mediated through its binding of laminin.
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Affiliation(s)
- C Jacobson
- Department of Biology, McGill University/Center for Neuroscience Research, Montréal General Hospital Research Institute, Montréal, Québec H3G 1A4, Canada
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88
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Marchand S, Stetzkowski-Marden F, Cartaud J. Differential targeting of components of the dystrophin complex to the postsynaptic membrane. Eur J Neurosci 2001. [DOI: 10.1046/j.1460-9568.2001.01373.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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89
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Marchand S, Stetzkowski-Marden F, Cartaud J. Differential targeting of components of the dystrophin complex to the postsynaptic membrane. Eur J Neurosci 2001. [DOI: 10.1111/j.1460-9568.2001.01373.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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90
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Newey SE, Gramolini AO, Wu J, Holzfeind P, Jasmin BJ, Davies KE, Blake DJ. A novel mechanism for modulating synaptic gene expression: differential localization of alpha-dystrobrevin transcripts in skeletal muscle. Mol Cell Neurosci 2001; 17:127-40. [PMID: 11161474 DOI: 10.1006/mcne.2000.0918] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Alpha-dystrobrevin is a dystrophin-related and -associated protein that is involved in synapse maturation and is required for normal muscle function. There are three protein isoforms in skeletal muscle, alpha-dystrobrevin-1, -2, and -3 that are encoded by the single alpha-dystrobrevin gene. To understand the role of these proteins in muscle we have investigated the localisation and transcript distribution of the different alpha-dystrobrevin isoforms. Alpha-dystrobrevin-1 and -2 are concentrated at the neuromuscular junction and are both recruited into agrin-induced acetylcholine receptor clusters in cultured myotubes. We also demonstrate that all alpha-dystrobrevin mRNAs are transcribed from a single promoter in skeletal muscle. However, only transcripts encoding alpha-dystrobrevin-1 are preferentially accumulated at postsynaptic sites. These data suggest that the synaptic accumulation of alpha-dystrobrevin-1 mRNA occurs posttranscriptionally, identifying a novel mechanism for synaptic gene expression. Taken together, these results indicate that different isoforms possess distinct roles in synapse formation and possibly in the pathogenesis of muscular dystrophy.
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Affiliation(s)
- S E Newey
- Department of Human Anatomy and Genetics, MRC Functional Genetics Unit, University of Oxford, South Parks Road, Oxford, OX1 3QX, United Kingdom
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91
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Heathcote RD, Ekman JM, Campbell KP, Godfrey EW. Dystroglycan overexpression in vivo alters acetylcholine receptor aggregation at the neuromuscular junction. Dev Biol 2000; 227:595-605. [PMID: 11071777 DOI: 10.1006/dbio.2000.9906] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Dystroglycan is a member of the transmembrane dystrophin glycoprotein complex in muscle that binds to the synapse-organizing molecule agrin. Dystroglycan binding and AChR aggregation are mediated by two separate domains of agrin. To test whether dystroglycan plays a role in receptor aggregation at the neuromuscular junction, we overexpressed it by injecting rabbit dystroglycan RNA into one- or two-celled Xenopus embryos. We measured AChR aggregation in myotomes by labeling them with rhodamine-alpha-bungarotoxin followed by confocal microscopy and image analysis. Dystroglycan overexpression decreased AChR aggregation at the neuromuscular junction. This result is consistent with dystroglycan competition for agrin without signaling AChR aggregation. It also supports the hypothesis that dystroglycan is not the myotube-associated specificity component, (MASC) a putative coreceptor needed for agrin to activate muscle-specific kinase (MuSK) and signal AChR aggregation. Dystroglycan was distributed along the surface of muscle membranes, but was concentrated at the ends of myotomes, where AChRs normally aggregate at synapses. Overexpressed dystroglycan altered AChR aggregation in a rostral-caudal gradient, consistent with the sequential development of neuromuscular synapses along the embryo. Increasing concentrations of dystroglycan RNA did not further decrease AChR aggregation, but decreased embryo survival. Development often stopped during gastrulation, suggesting an essential, nonsynaptic role of dystroglycan during this early period of development.
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Affiliation(s)
- R D Heathcote
- Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, USA
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92
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Losasso C, Di Tommaso F, Sgambato A, Ardito R, Cittadini A, Giardina B, Petrucci TC, Brancaccio A. Anomalous dystroglycan in carcinoma cell lines. FEBS Lett 2000; 484:194-8. [PMID: 11078877 DOI: 10.1016/s0014-5793(00)02157-8] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
Dystroglycan is a receptor responsible for crucial interactions between extracellular matrix and cytoplasmic space. We provide the first evidence that dystroglycan is truncated. In HC11 normal murine and the 184B5 non-tumorigenic mammary human cell lines, the expected beta-dystroglycan 43 kDa band was found but human breast T47D, BT549, MCF7, colon HT29, HCT116, SW620, prostate DU145 and cervical HeLa cancer cells expressed an anomalous approximately 31 kDa beta-dystroglycan band. alpha-Dystroglycan was udetectable in most of the cell lines in which beta-dystroglycan was found as a approximately 31 kDa species. An anomalous approximately 31 kDa beta-dystroglycan band was also observed in N-methyl-N-nitrosurea-induced primary rat mammary tumours. Reverse transcriptase polymerase chain reaction experiments confirmed the absence of alternative splicing events and/or expression of eventual dystroglycan isoforms. Using protein extraction procedures at low- and high-ionic strength, we demonstrated that both the 43 kDa and approximately 31 kDa beta-dystroglycan bands harbour their transmembrane segment.
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Affiliation(s)
- C Losasso
- Centro Chimica dei Recettori (CNR), Istituto di Chimica e Chimica Clinica, Università Cattolica del Sacro Cuore, Rome, Italy
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93
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Claudepierre T, Dalloz C, Mornet D, Matsumura K, Sahel J, Rendon A. Characterization of the intermolecular associations of the dystrophin-associated glycoprotein complex in retinal Muller glial cells. J Cell Sci 2000; 113 Pt 19:3409-17. [PMID: 10984432 DOI: 10.1242/jcs.113.19.3409] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The abnormal retinal neurotransmission observed in Duchenne muscular dystrophy patients has been attributed to altered expression of C-terminal products of the dystrophin gene in this tissue. Muller glial cells from rat retina express dystrophin protein Dp71, utrophin and the members of the dystrophin-associated glycoprotein complex (DGC), namely beta-dystroglycan, delta- and gamma-sarcoglycans and alpha1-syntrophin. The DGC could function in muscle as a link between the cystoskeleton and the extracellular matrix, as well as a signaling complex. However, other than in muscle the composition and intermolecular associations among members of the DGC are still unknown. Here we demonstrate that Dp71 and/or utrophin from rat retinal Muller glial cells form a complex with beta-dystroglycan, delta-sarcoglycan and alpha1-syntrophin. We also show that beta-dystroglycan is associated with alpha-dystrobrevin-1 and PSD-93 and that anti-PSD antibodies coimmunoprecipitated alpha-syntrophin with PSD-93. By overlay experiments we also found that Dp71and/or utrophin and alpha-dystroglycan from Muller cells could bind to actin and laminin, respectively. These results indicate that the DGC could have both structural and signaling functions in retina. On the basis of our accumulated evidence, we propose a hypothetical model for the molecular organization of the dystrophin-associated glycoprotein complex in retinal Muller glial cells, which would be helpful for understanding its function in the central nervous system.
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Affiliation(s)
- T Claudepierre
- Inserm EMI 99-18, Laboratoire de Physiopathologie Moléculaire et Cellulaire de la Rétine, CHRU, France
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94
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Lück G, Hoch W, Hopf C, Blottner D. Nitric oxide synthase (NOS-1) coclustered with agrin-induced AChR-specializations on cultured skeletal myotubes. Mol Cell Neurosci 2000; 16:269-81. [PMID: 10995553 DOI: 10.1006/mcne.2000.0873] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Previously we reported that neuronal nitric oxide synthase type-1 (NOS-1) is expressed in skeletal myotubes in vitro. In the present paper we sought to determine whether agrin-induced membrane specializations known to include the nicotinic acetylcholine receptor (AChR) on cultured myotubes may also contain NOS-1 and related molecules. After treatment with various agrin constructs containing the full C-terminally AChR-clustering domain (fragments N2, N4), but not with fragment C2 (truncated), NOS-1 expressed in the cytosol of mouse C2C12 skeletal myotubes coclustered with AChR, 43K rapsyn, MuSK, and the dystrophin/utrophin glycoprotein-complex (DUGC). Agrin-induced specializations also included coaggregates of N-methyl-d-aspartic acid (NMDA)-receptor, alpha-sodium (NaCh), or Shaker-type K+ channel (KCh)/PSD-95 complexes, and NOS-1. We conclude that agrin is crucial for recruitment of preassembled multimolecular membrane clusters, including AChR, NMDAR, and ion channels linked to NOS-1. Coassembly of NOS-1 to postsynaptic molecules may reflect site-specific NO-signaling pathways in neuromuscular junction formation and functions.
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Affiliation(s)
- G Lück
- Department of Anatomy 1, Neurobiology Unit, University Hospital Benjamin Franklin, Freie Universität Berlin, Königin-Luise-Strasse 15, Berlin, D-14195, Germany
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95
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Abstract
Rapsyn is a protein that interacts with the cytoplasmic face of the nicotinic acetylcholine receptors (AChR) to cluster them within postsynaptic membrane of muscle. Here we show that intracellular AChRs are also affected by rapsyn. When rapsyn was co-transfected with AChR into QT-6 fibroblasts, (125)I-alpha-bungarotoxin binding indicated a reduction in the fraction of AChRs expressed on the cell surface, compared to cells expressing AChRs alone. Double fluorescent labeling showed that intracellular AChRs accumulated in patches at the cell periphery, beneath rapsyn-associated cell surface AChR clusters. These patches were observed even when cells were grown in medium containing excess unlabelled alpha-bungarotoxin to mask internalized AChRs, suggesting that they arose from hindered trafficking of newly formed AChRs to the cell surface. Similarly, in the muscle cell line, C2, overexpression of rapsyn resulted in the co-localization of aggregates of intracellular alpha-bungarotoxin binding sites with rapsyn beneath cell surface AChR microaggregates. The results indicate that rapsyn can modify the trafficking of AChRs within the cell and suggest a role in selectively targeting newly synthesized intracellular AChRs to the postsynaptic membrane.
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Affiliation(s)
- H Han
- Institute for Biomedical Research, Department of Physiology, University of Sydney, Sydney, NSW Australia
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96
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Cartaud J, Cartaud A, Kordeli E, Ludosky MA, Marchand S, Stetzkowski-Marden F. The torpedo electrocyte: a model system to study membrane-cytoskeleton interactions at the postsynaptic membrane. Microsc Res Tech 2000; 49:73-83. [PMID: 10757880 DOI: 10.1002/(sici)1097-0029(20000401)49:1<73::aid-jemt8>3.0.co;2-l] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Many aspects of the organization of the electromotor synapse of electric fish resemble the nerve-muscle junction. In particular, the postsynaptic membrane in both systems share most of their proteins. As a remarquable source of cholinergic synapses, the Torpedo electrocyte model has served to identify the most important components involved in synaptic transmission such as the nicotinic acetylcholine receptor and the enzyme acetylcholinesterase, as well as proteins associated with the subsynaptic cytoskeleton and the extracellular matrix involved in the assembly of the postsynaptic membrane, namely the 43-kDa protein-rapsyn, the dystrophin/utrophin complex, agrin, and others. This review encompasses some representative experiments that helped to clarify essential aspects of the supramolecular organization and assembly of the postsynaptic apparatus of cholinergic synapses.
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Affiliation(s)
- J Cartaud
- Biologie Cellulaire des Membranes, Institut Jacques Monod, UMR 9275, CNRS, Universités Paris 6 et Paris7, 75251 Paris Cedex 05, France.
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97
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Mehler MF. Brain dystrophin, neurogenetics and mental retardation. BRAIN RESEARCH. BRAIN RESEARCH REVIEWS 2000; 32:277-307. [PMID: 10751678 DOI: 10.1016/s0165-0173(99)00090-9] [Citation(s) in RCA: 134] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.
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Affiliation(s)
- M F Mehler
- Departments of Neurology, Neuroscience and Psychiatry, the Einstein Comprehensive Cancer Center, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
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98
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Lebakken CS, Venzke DP, Hrstka RF, Consolino CM, Faulkner JA, Williamson RA, Campbell KP. Sarcospan-deficient mice maintain normal muscle function. Mol Cell Biol 2000; 20:1669-77. [PMID: 10669744 PMCID: PMC85350 DOI: 10.1128/mcb.20.5.1669-1677.2000] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Sarcospan is an integral membrane component of the dystrophin-glycoprotein complex (DGC) found at the sarcolemma of striated and smooth muscle. The DGC plays important roles in muscle function and viability as evidenced by defects in components of the DGC, which cause muscular dystrophy. Sarcospan is unique among the components of the complex in that it contains four transmembrane domains with intracellular N- and C-terminal domains and is a member of the tetraspan superfamily of proteins. Sarcospan is tightly linked to the sarcoglycans, and together these proteins form a subcomplex within the DGC. Stable expression of sarcospan at the sarcolemma is dependent upon expression of the sarcoglycans. Here we describe the generation and analysis of mice carrying a null mutation in the Sspn gene. Surprisingly, the Sspn-deficient muscle maintains expression of other components of the DGC at the sarcolemma, and no gross histological abnormalities of muscle from the mice are observed. The Sspn-deficient muscle maintains sarcolemmal integrity as determined by serum creatine kinase and Evans blue uptake assays, and the Sspn-deficient muscle maintains normal force and power generation capabilities. These data suggest either that sarcospan is not required for normal DGC function or that the Sspn-deficient muscle is compensating for the absence of sarcospan, perhaps by utilizing another protein to carry out its function.
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Affiliation(s)
- C S Lebakken
- Departments of Physiology and Biophysics and Neurology, Howard Hughes Medical Institute, Iowa City, Iowa 52242, USA
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99
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Bowe MA, Mendis DB, Fallon JR. The small leucine-rich repeat proteoglycan biglycan binds to alpha-dystroglycan and is upregulated in dystrophic muscle. J Cell Biol 2000; 148:801-10. [PMID: 10684260 PMCID: PMC2169361 DOI: 10.1083/jcb.148.4.801] [Citation(s) in RCA: 125] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The dystrophin-associated protein complex (DAPC) is necessary for maintaining the integrity of the muscle cell plasma membrane and may also play a role in coordinating signaling events at the cell surface. The alpha-/beta-dystroglycan subcomplex of the DAPC forms a critical link between the cytoskeleton and the extracellular matrix. A ligand blot overlay assay was used to search for novel dystroglycan binding partners in postsynaptic membranes from Torpedo electric organ. An approximately 125-kD dystroglycan-binding polypeptide was purified and shown by peptide microsequencing to be the Torpedo ortholog of the small leucine-rich repeat chondroitin sulfate proteoglycan biglycan. Biglycan binding to alpha-dystroglycan was confirmed by coimmunoprecipitation with both native and recombinant alpha-dystroglycan. The biglycan binding site was mapped to the COOH-terminal third of alpha-dystroglycan. Glycosylation of alpha-dystroglycan is not necessary for this interaction, but binding is dependent upon the chondroitin sulfate side chains of biglycan. In muscle, biglycan is detected at both synaptic and nonsynaptic regions. Finally, biglycan expression is elevated in muscle from the dystrophic mdx mouse. These findings reveal a novel binding partner for alpha-dystroglycan and demonstrate a novel avenue for interaction of the DAPC and the extracellular matrix. These results also raise the possibility of a role for biglycan in the pathogenesis, and perhaps the treatment, of muscular dystrophy.
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Affiliation(s)
- Mark A. Bowe
- Department of Neuroscience, Brown University, Providence, Rhode Island 02912
| | - Duane B. Mendis
- Department of Neuroscience, Brown University, Providence, Rhode Island 02912
| | - Justin R. Fallon
- Department of Neuroscience, Brown University, Providence, Rhode Island 02912
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Zaccaria ML, De Stefano ME, Gotti C, Petrucci TC, Paggi P. Selective reduction in the nicotinic acetylcholine receptor and dystroglycan at the postsynaptic apparatus of mdx mouse superior cervical ganglion. J Neuropathol Exp Neurol 2000; 59:103-12. [PMID: 10749099 DOI: 10.1093/jnen/59.2.103] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Our previous data suggested that in mouse sympathetic superior cervical ganglion (SCG) the dystrophin-dystroglycan complex may be involved in the stabilization of the nicotinic acetylcholine receptor (nAChR) clusters. Here we used SCG of dystrophic mdx mice, which express only the shorter isoforms of dystrophin (Dys), to investigate whether the lack of the full-length dystrophin (Dp427) could affect the localization of the dystroglycan and the alpha3 nAChR subunit (alpha3AChR) at the postsynaptic apparatus. We found a selective reduction in intraganglionic postsynaptic specializations immunopositive for alpha3AChR and for alpha- and beta-dystroglycan compared with the wild-type. Moreover, in mdx mice, unlike the wild-type, the disassembly of intraganglionic synapses induced by postganglionic nerve crush occurred at the slower rate and was not preceded by the loss of immunoreactivity for Dys isoforms, beta-dystroglycan, and alpha3AChR. These data indicate that the absence of Dp427 at the intraganglionic postsynaptic apparatus of mdx mouse SCG interferes with the presence of both dystroglycan and nAChR clusters at these sites and affects the rate of synapse disassembly induced by postganglionic nerve crush. Moreover, they suggest that the decrease in ganglionic nAChR may be one of the factors responsible for autonomic imbalance described in Duchenne muscular dystrophy patients.
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Affiliation(s)
- M L Zaccaria
- Dipartimento di Biologia Cellulare e dello Sviluppo, Università La Sapienza, Rome, Italy
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