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Fyn Signaling in Ischemia-Reperfusion Injury: Potential and Therapeutic Implications. Mediators Inflamm 2022; 2022:9112127. [PMID: 36157893 PMCID: PMC9499810 DOI: 10.1155/2022/9112127] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2022] [Revised: 08/20/2022] [Accepted: 08/27/2022] [Indexed: 11/18/2022] Open
Abstract
Ischemic stroke caused by arterial occlusion is the most common type of stroke and is one of the leading causes of disability and death, with the incidence increasing each year. Fyn is a nonreceptor tyrosine kinase belonging to the Src family of kinases (SFKs), which is related to many normal and pathological processes of the nervous system, including neurodevelopment and disease progression. In recent years, more and more evidence suggests that Fyn may be closely related to cerebral ischemia-reperfusion, including energy metabolism disorders, excitatory neurotoxicity, intracellular calcium homeostasis, free radical production, and the activation of apoptotic genes. This paper reviews the role of Fyn in the pathological process of cerebral ischemia-reperfusion, including neuroexcitotoxicity and neuroinflammation, to explore how Fyn affects specific signal cascades and leads to cerebral ischemia-reperfusion injury. In addition, Fyn also promotes the production of superoxide and endogenous NO, so as to quickly react to produce peroxynitrite, which may also mediate cerebral ischemia-reperfusion injury, which is discussed in this paper. Finally, we revealed the treatment methods related to Fyn inhibitors and discussed its potential as a clinical treatment for ischemic stroke.
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Paz ML, Barrantes FJ. Cholesterol in myasthenia gravis. Arch Biochem Biophys 2021; 701:108788. [PMID: 33548213 DOI: 10.1016/j.abb.2021.108788] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2020] [Revised: 01/13/2021] [Accepted: 01/26/2021] [Indexed: 01/03/2023]
Abstract
The cholinergic neuromuscular junction is the paradigm peripheral synapse between a motor neuron nerve ending and a skeletal muscle fiber. In vertebrates, acetylcholine is released from the presynaptic site and binds to the nicotinic acetylcholine receptor at the postsynaptic membrane. A variety of pathologies among which myasthenia gravis stands out can impact on this rapid and efficient signaling mechanism, including autoimmune diseases affecting the nicotinic receptor or other synaptic proteins. Cholesterol is an essential component of biomembranes and is particularly rich at the postsynaptic membrane, where it interacts with and modulates many properties of the nicotinic receptor. The profound changes inflicted by myasthenia gravis on the postsynaptic membrane necessarily involve cholesterol. This review analyzes some aspects of myasthenia gravis pathophysiology and associated postsynaptic membrane dysfunction, including dysregulation of cholesterol metabolism in the myocyte brought about by antibody-receptor interactions. In addition, given the extensive therapeutic use of statins as the typical cholesterol-lowering drugs, we discuss their effects on skeletal muscle and the possible implications for MG patients under chronic treatment with this type of compound.
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Affiliation(s)
- Mariela L Paz
- Universidad de Buenos Aires, Facultad de Farmacia y Bioquímica, Departamento de Microbiología, Inmunología, Biotecnología y Genética, Cátedra de Inmunología, Buenos Aires, Argentina; CONICET, Universidad de Buenos Aires, Instituto de Estudios de la Inmunidad Humoral "Prof. Dr. Ricardo A. Margni" (IDEHU), Buenos Aires, Argentina
| | - Francisco J Barrantes
- Laboratory of Molecular Neurobiology, Biomedical Research Institute (BIOMED), UCA, CONICET, Av. Alicia Moreau de Justo 1600, C1107AFF, Buenos Aires, Argentina.
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Noël G, Tham DKL, Guadagno E, MacVicar B, Moukhles H. The Laminin-Induced Phosphorylation of PKCδ Regulates AQP4 Distribution and Water Permeability in Rat Astrocytes. Cell Mol Neurobiol 2020; 41:1743-1757. [PMID: 32851539 DOI: 10.1007/s10571-020-00944-w] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2020] [Accepted: 08/14/2020] [Indexed: 11/29/2022]
Abstract
In astrocytes, the water-permeable channel aquaporin-4 (AQP4) is concentrated at the endfeet that abut the blood vessels of the brain. The asymmetric distribution of this channel is dependent on the function of dystroglycan (DG), a co-expressed laminin receptor, and its associated protein complex. We have demonstrated that the addition of laminin to astrocytes in culture causes the clustering of AQP4, DG, and lipid rafts. The last, in particular, have been associated with the initiation of cell signaling. As laminin binding to DG in muscle cells can induce the tyrosine phosphorylation of syntrophin and laminin requires tyrosine kinases for acetylcholine receptor clustering in myotubes, we asked if signal transduction might also be involved in AQP4 clustering in astrocytes. We analyzed the timecourse of AQP4, DG, and monosialotetrahexosylganglioside (GM1) clustering in primary cultures of rat astrocytes following the addition of laminin, and determined that the clustering of DG precedes that of AQP4 and GM1. We also showed that laminin induces the formation of phosphotyrosine-rich clusters and that the tyrosine kinase inhibitor, genistein, disrupts the laminin-induced clustering of both β-DG and AQP4. Using the Kinexus antibody microarray chip, we then identified protein-serine kinase C delta (PKCδ) as one of the main proteins exhibiting high levels of tyrosine phosphorylation upon laminin treatment. Selective inhibitors of PKC and siRNA against PKCδ disrupted β-DG and AQP4 clustering, and also caused water transport to increase in astrocytes treated with laminin. Our results demonstrate that the effects of laminin on AQP4 localization and function are relayed, at least in part, through PKC signaling.
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Affiliation(s)
- Geoffroy Noël
- Department of Cellular and Physiological Sciences, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver, V6T 1Z3, Canada
| | - Daniel Kai Long Tham
- Department of Cellular and Physiological Sciences, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver, V6T 1Z3, Canada
| | - Eric Guadagno
- Department of Cellular and Physiological Sciences, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver, V6T 1Z3, Canada
| | - Brian MacVicar
- Department of Psychiatry, Djavad Mowafaghian Centre for Brain Health, University of British Columbia, 2215 Wesbrook Mall, Vancouver, V6T 1Z3, Canada
| | - Hakima Moukhles
- Department of Cellular and Physiological Sciences, Life Sciences Institute, University of British Columbia, 2350 Health Sciences Mall, Vancouver, V6T 1Z3, Canada.
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Hunter DD, Manglapus MK, Bachay G, Claudepierre T, Dolan MW, Gesuelli KA, Brunken WJ. CNS synapses are stabilized trans-synaptically by laminins and laminin-interacting proteins. J Comp Neurol 2017; 527:67-86. [PMID: 29023785 DOI: 10.1002/cne.24338] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2017] [Revised: 09/21/2017] [Accepted: 09/29/2017] [Indexed: 01/05/2023]
Abstract
The retina expresses several laminins in the outer plexiform layer (OPL), where they may provide an extracellular scaffold for synapse stabilization. Mice with a targeted deletion of the laminin β2 gene (Lamb2) exhibit retinal disruptions: photoreceptor synapses in the OPL are disorganized and the retinal physiological response is attenuated. We hypothesize that laminins are required for proper trans-synaptic alignment. To test this, we compared the distribution, expression, association and modification of several pre- and post-synaptic elements in wild-type and Lamb2-null retinae. A potential laminin receptor, integrin α3, is at the presynaptic side of the wild-type OPL. Another potential laminin receptor, dystroglycan, is at the post-synaptic side of the wild-type OPL. Integrin α3 and dystroglycan can be co-immunoprecipitated with the laminin β2 chain, demonstrating that they may bind laminins. In the absence of the laminin β2 chain, the expression of many pre-synaptic components (bassoon, kinesin, among others) is relatively undisturbed although their spatial organization and anchoring to the membrane is disrupted. In contrast, in the Lamb2-null, β-dystroglycan (β-DG) expression is altered, co-localization of β-DG with dystrophin and the glutamate receptor mGluR6 is disrupted, and the post-synaptic bipolar cell components mGluR6 and GPR179 become dissociated, suggesting that laminins mediate scaffolding of post-synaptic components. In addition, although pikachurin remains associated with β-DG, pikachurin is no longer closely associated with mGluR6 or α-DG in the Lamb2-null. These data suggest that laminins act as links among pre- and post-synaptic laminin receptors and α-DG and pikachurin in the synaptic space to maintain proper trans-synaptic alignment.
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Affiliation(s)
- Dale D Hunter
- Department of Anatomy and Cellular Biology, Tufts University and Tufts Center for Vision Research, Boston, Massachusetts.,Department of Ophthalmology and the SUNY Eye Institute, Upstate Medical University, Syracuse, New York
| | - Mary K Manglapus
- Department of Anatomy and Cellular Biology, Tufts University and Tufts Center for Vision Research, Boston, Massachusetts
| | - Galina Bachay
- Department of Ophthalmology and the SUNY Eye Institute, Upstate Medical University, Syracuse, New York
| | - Thomas Claudepierre
- Department of Anatomy and Cellular Biology, Tufts University and Tufts Center for Vision Research, Boston, Massachusetts
| | - Michael W Dolan
- Department of Ophthalmology and the SUNY Eye Institute, Upstate Medical University, Syracuse, New York
| | - Kelly-Ann Gesuelli
- Department of Ophthalmology and the SUNY Eye Institute, Upstate Medical University, Syracuse, New York
| | - William J Brunken
- Department of Anatomy and Cellular Biology, Tufts University and Tufts Center for Vision Research, Boston, Massachusetts.,Department of Ophthalmology and the SUNY Eye Institute, Upstate Medical University, Syracuse, New York
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Vezina-Audette R, Tremblay M, Carbonetto S. Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture. Matrix Biol 2016; 57-58:106-123. [PMID: 27964993 DOI: 10.1016/j.matbio.2016.11.006] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2016] [Accepted: 11/16/2016] [Indexed: 12/11/2022]
Abstract
Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.
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Affiliation(s)
- Raphael Vezina-Audette
- Centre for Research in Neuroscience, and Dept. of Neurology, McGill University Health Centre, 1650, Cedar Ave., Montreal, Quebec, H3G 1A4, Canada
| | - Mathieu Tremblay
- Centre for Research in Neuroscience, and Dept. of Neurology, McGill University Health Centre, 1650, Cedar Ave., Montreal, Quebec, H3G 1A4, Canada
| | - Salvatore Carbonetto
- Centre for Research in Neuroscience, and Dept. of Neurology, McGill University Health Centre, 1650, Cedar Ave., Montreal, Quebec, H3G 1A4, Canada.
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Schulte C, Ripamonti M, Maffioli E, Cappelluti MA, Nonnis S, Puricelli L, Lamanna J, Piazzoni C, Podestà A, Lenardi C, Tedeschi G, Malgaroli A, Milani P. Scale Invariant Disordered Nanotopography Promotes Hippocampal Neuron Development and Maturation with Involvement of Mechanotransductive Pathways. Front Cell Neurosci 2016; 10:267. [PMID: 27917111 PMCID: PMC5114288 DOI: 10.3389/fncel.2016.00267] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2016] [Accepted: 11/01/2016] [Indexed: 11/18/2022] Open
Abstract
The identification of biomaterials which promote neuronal maturation up to the generation of integrated neural circuits is fundamental for modern neuroscience. The development of neural circuits arises from complex maturative processes regulated by poorly understood signaling events, often guided by the extracellular matrix (ECM). Here we report that nanostructured zirconia surfaces, produced by supersonic cluster beam deposition of zirconia nanoparticles and characterized by ECM-like nanotopographical features, can direct the maturation of neural networks. Hippocampal neurons cultured on such cluster-assembled surfaces displayed enhanced differentiation paralleled by functional changes. The latter was demonstrated by single-cell electrophysiology showing earlier action potential generation and increased spontaneous postsynaptic currents compared to the neurons grown on the featureless unnaturally flat standard control surfaces. Label-free shotgun proteomics broadly confirmed the functional changes and suggests furthermore a vast impact of the neuron/nanotopography interaction on mechanotransductive machinery components, known to control physiological in vivo ECM-regulated axon guidance and synaptic plasticity. Our results indicate a potential of cluster-assembled zirconia nanotopography exploitable for the creation of efficient neural tissue interfaces and cell culture devices promoting neurogenic events, but also for unveiling mechanotransductive aspects of neuronal development and maturation.
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Affiliation(s)
- Carsten Schulte
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di MilanoMilan, Italy; Fondazione FilareteMilan, Italy
| | - Maddalena Ripamonti
- Neurobiology of Learning Unit, Division of Neuroscience, Scientific Institute San Raffaele, Università Vita-Salute San Raffaele Milan, Italy
| | - Elisa Maffioli
- Fondazione FilareteMilan, Italy; Dipartimento di Medicina Veterinaria, Università degli Studi di MilanoMilan, Italy
| | - Martino A Cappelluti
- Fondazione FilareteMilan, Italy; SEMM - European School of Molecular MedicineMilan, Italy
| | - Simona Nonnis
- Dipartimento di Medicina Veterinaria, Università degli Studi di Milano Milan, Italy
| | - Luca Puricelli
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di Milano Milan, Italy
| | - Jacopo Lamanna
- Neurobiology of Learning Unit, Division of Neuroscience, Scientific Institute San Raffaele, Università Vita-Salute San Raffaele Milan, Italy
| | - Claudio Piazzoni
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di Milano Milan, Italy
| | - Alessandro Podestà
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di Milano Milan, Italy
| | - Cristina Lenardi
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di Milano Milan, Italy
| | - Gabriella Tedeschi
- Fondazione FilareteMilan, Italy; Dipartimento di Medicina Veterinaria, Università degli Studi di MilanoMilan, Italy
| | - Antonio Malgaroli
- Neurobiology of Learning Unit, Division of Neuroscience, Scientific Institute San Raffaele, Università Vita-Salute San Raffaele Milan, Italy
| | - Paolo Milani
- Dipartimento di Fisica, Centro Interdisciplinare Materiali e Interfacce Nanostrutturate, Università degli Studi di Milano Milan, Italy
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Mate SE, Brown KJ, Hoffman EP. Integrated genomics and proteomics of the Torpedo californica electric organ: concordance with the mammalian neuromuscular junction. Skelet Muscle 2011; 1:20. [PMID: 21798097 PMCID: PMC3156643 DOI: 10.1186/2044-5040-1-20] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2010] [Accepted: 05/04/2011] [Indexed: 11/25/2022] Open
Abstract
Background During development, the branchial mesoderm of Torpedo californica transdifferentiates into an electric organ capable of generating high voltage discharges to stun fish. The organ contains a high density of cholinergic synapses and has served as a biochemical model for the membrane specialization of myofibers, the neuromuscular junction (NMJ). We studied the genome and proteome of the electric organ to gain insight into its composition, to determine if there is concordance with skeletal muscle and the NMJ, and to identify novel synaptic proteins. Results Of 435 proteins identified, 300 mapped to Torpedo cDNA sequences with ≥2 peptides. We identified 14 uncharacterized proteins in the electric organ that are known to play a role in acetylcholine receptor clustering or signal transduction. In addition, two human open reading frames, C1orf123 and C6orf130, showed high sequence similarity to electric organ proteins. Our profile lists several proteins that are highly expressed in skeletal muscle or are muscle specific. Synaptic proteins such as acetylcholinesterase, acetylcholine receptor subunits, and rapsyn were present in the electric organ proteome but absent in the skeletal muscle proteome. Conclusions Our integrated genomic and proteomic analysis supports research describing a muscle-like profile of the organ. We show that it is a repository of NMJ proteins but we present limitations on its use as a comprehensive model of the NMJ. Finally, we identified several proteins that may become candidates for signaling proteins not previously characterized as components of the NMJ.
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Affiliation(s)
- Suzanne E Mate
- Department of Biochemistry and Molecular Genetics, IBS, George Washington University, Washington DC, USA
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Morini R, Becchetti A. Integrin receptors and ligand-gated channels. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2010; 674:95-105. [PMID: 20549943 DOI: 10.1007/978-1-4419-6066-5_9] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Plastic expression of different integrin subunits controls the different stages of neural development, whereas in the adult integrins regulate synaptic stability. Evidence of integrin-channel crosstalk exists for ionotropic glutamate receptors. As is often the case in other tissues, integrin engagement regulates channel activity through complex signaling pathways that often include tyrosine phosphorylation cascades. The specific pathways recruited by integrin activation depend on cerebral region and cell type. In turn, ion channels control integrin expression onto the plasma membrane and their ligand binding affinity. The most extensive studies concern the hippocampus and suggest implications for neuronal circuit plasticity. The physiological relevance of these findings depends on whether adhesion molecules, aside from determining tissue stability, contribute to synaptogenesis and the responsiveness of mature synapses, thus contributing to long-term circuit consolidation. Little evidence is available for other ligand-gated channels, with the exception of nicotinic receptors. These exert a variety of functions in neurons and non neural tissue, both in development and in the adult, by regulating cell cycle, synaptogenesis and synaptic circuit refinement. Detailed studies in epidermal keratinocytes have shed some light on the possible mechanisms through which ACh can regulate cell motility, which may be of general relevance for morphogenetic processes. As to the control of mature synapses, most results concern the integrinic control of nicotinic receptors in the neuromuscular junction. Following this lead, a few studies have addressed similar topics in adult cerebral synapses. However, pursuing and interpreting these results in the brain is especially difficult because of the complexity of the nicotinic roles and the widespread contribution of nonsynaptic, paracrine transmission. From a pathological point of view, considering the well-known contribution of both integrins and ligand-gated channels to synaptogenesis and neural regeneration, the above studies point to interesting implications for epileptogenesis.
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Affiliation(s)
- Raffaella Morini
- Department of Biotechnology and Biosciences, University of Milano-Bicocca, Piazza della Scienza 2, 20126 Milano, Italy
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Noël G, Tham DKL, Moukhles H. Interdependence of laminin-mediated clustering of lipid rafts and the dystrophin complex in astrocytes. J Biol Chem 2009; 284:19694-704. [PMID: 19451651 DOI: 10.1074/jbc.m109.010090] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Astrocyte endfeet surrounding blood vessels are active domains involved in water and potassium ion transport crucial to the maintenance of water and potassium ion homeostasis in brain. A growing body of evidence points to a role for dystroglycan and its interaction with perivascular laminin in the targeting of the dystrophin complex and the water-permeable channel, aquaporin 4 (AQP4), at astrocyte endfeet. However, the mechanisms underlying such compartmentalization remain poorly understood. In the present study we found that AQP4 resided in Triton X-100-insoluble fraction, whereas dystroglycan was recovered in the soluble fraction in astrocytes. Cholesterol depletion resulted in the translocation of a pool of AQP4 to the soluble fraction indicating that its distribution is indeed associated with cholesterol-rich membrane domains. Upon laminin treatment AQP4 and the dystrophin complex, including dystroglycan, reorganized into laminin-associated clusters enriched for the lipid raft markers GM1 and flotillin-1 but not caveolin-1. Reduced diffusion rates of GM1 in the laminin-induced clusters were indicative of the reorganization of raft components in these domains. In addition, both cholesterol depletion and dystroglycan silencing reduced the number and area of laminin-induced clusters of GM1, AQP4, and dystroglycan. These findings demonstrate the interdependence between laminin binding to dystroglycan and GM1-containing lipid raft reorganization and provide novel insight into the dystrophin complex regulation of AQP4 polarization in astrocytes.
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Affiliation(s)
- Geoffroy Noël
- Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver V6T 1Z3, Canada
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Rushton E, Rohrbough J, Broadie K. Presynaptic secretion of mind-the-gap organizes the synaptic extracellular matrix-integrin interface and postsynaptic environments. Dev Dyn 2009; 238:554-71. [PMID: 19235718 PMCID: PMC2677818 DOI: 10.1002/dvdy.21864] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
Mind-the-Gap (MTG) is required during synaptogenesis of the Drosophila glutamatergic neuromuscular junction (NMJ) to organize the postsynaptic domain. Here, we generate MTG::GFP transgenic animals to demonstrate MTG is synaptically targeted, secreted, and localized to punctate domains in the synaptic extracellular matrix (ECM). Drosophila NMJs form specialized ECM carbohydrate domains, with carbohydrate moieties and integrin ECM receptors occupying overlapping territories. Presynaptically secreted MTG recruits and reorganizes secreted carbohydrates, and acts to recruit synaptic integrins and ECM glycans. Transgenic MTG::GFP expression rescues hatching, movement, and synaptogenic defects in embryonic-lethal mtg null mutants. Targeted neuronal MTG expression rescues mutant synaptogenesis defects, and increases rescue of adult viability, supporting an essential neuronal function. These results indicate that presynaptically secreted MTG regulates the ECM-integrin interface, and drives an inductive mechanism for the functional differentiation of the postsynaptic domain of glutamatergic synapses. We suggest that MTG pioneers a novel protein family involved in ECM-dependent synaptic differentiation.
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Affiliation(s)
| | | | - Kendal Broadie
- Department of Biological Sciences, Vanderbilt Brain Institute, Vanderbilt Kennedy Center for Research on Human Development, Vanderbilt University, Nashville, Tennessee
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Nishimune H, Valdez G, Jarad G, Moulson CL, Müller U, Miner JH, Sanes JR. Laminins promote postsynaptic maturation by an autocrine mechanism at the neuromuscular junction. ACTA ACUST UNITED AC 2008; 182:1201-15. [PMID: 18794334 PMCID: PMC2542479 DOI: 10.1083/jcb.200805095] [Citation(s) in RCA: 105] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
A prominent feature of synaptic maturation at the neuromuscular junction (NMJ) is the topological transformation of the acetylcholine receptor (AChR)-rich postsynaptic membrane from an ovoid plaque into a complex array of branches. We show here that laminins play an autocrine role in promoting this transformation. Laminins containing the alpha4, alpha5, and beta2 subunits are synthesized by muscle fibers and concentrated in the small portion of the basal lamina that passes through the synaptic cleft at the NMJ. Topological maturation of AChR clusters was delayed in targeted mutant mice lacking laminin alpha5 and arrested in mutants lacking both alpha4 and alpha5. Analysis of chimeric laminins in vivo and of mutant myotubes cultured aneurally demonstrated that the laminins act directly on muscle cells to promote postsynaptic maturation. Immunohistochemical studies in vivo and in vitro along with analysis of targeted mutants provide evidence that laminin-dependent aggregation of dystroglycan in the postsynaptic membrane is a key step in synaptic maturation. Another synaptically concentrated laminin receptor, Bcam, is dispensable. Together with previous studies implicating laminins as organizers of presynaptic differentiation, these results show that laminins coordinate post- with presynaptic maturation.
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Affiliation(s)
- Hiroshi Nishimune
- Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
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12
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Shigemoto K, Kubo S, Jie C, Hato N, Abe Y, Ueda N, Kobayashi N, Kameda K, Mominoki K, Miyazawa A, Ishigami A, Matsuda S, Maruyama N. Myasthenia Gravis Experimentally Induced with Muscle-specific Kinase. Ann N Y Acad Sci 2008; 1132:93-8. [DOI: 10.1196/annals.1405.002] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
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Tyrosine phosphatases such as SHP-2 act in a balance with Src-family kinases in stabilization of postsynaptic clusters of acetylcholine receptors. BMC Neurosci 2007; 8:46. [PMID: 17605785 PMCID: PMC1924855 DOI: 10.1186/1471-2202-8-46] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2007] [Accepted: 07/02/2007] [Indexed: 01/31/2023] Open
Abstract
BACKGROUND Development of neural networks requires that synapses are formed, eliminated and stabilized. At the neuromuscular junction (NMJ), agrin/MuSK signaling, by triggering downstream pathways, causes clustering and phosphorylation of postsynaptic acetylcholine receptors (AChRs). Postnatally, AChR aggregates are stabilized by molecular pathways that are poorly characterized. Gain or loss of function of Src-family kinases (SFKs) disassembles AChR clusters at adult NMJs in vivo, whereas AChR aggregates disperse rapidly upon withdrawal of agrin from cultured src-/-;fyn-/- myotubes. This suggests that a balance between protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) such as those of the Src-family may be essential in stabilizing clusters of AChRs. RESULTS We have analyzed the role of PTPs in maintenance of AChR aggregates, by adding and then withdrawing agrin from cultured myotubes in the presence of PTP or PTK inhibitors and quantitating remaining AChR clusters. In wild-type myotubes, blocking PTPs with pervanadate caused enhanced disassembly of AChR clusters after agrin withdrawal. When added at the time of agrin withdrawal, SFK inhibitors destabilized AChR aggregates but concomitant addition of pervanadate rescued cluster stability. Likewise in src-/-;fyn-/- myotubes, in which agrin-induced AChR clusters form normally but rapidly disintegrate after agrin withdrawal, pervanadate addition stabilized AChR clusters. The PTP SHP-2, known to be enriched at the NMJ, associated and colocalized with MuSK, and agrin increased this interaction. Specific SHP-2 knockdown by RNA interference reduced the stability of AChR clusters in wild-type myotubes. Similarly, knockdown of SHP-2 in adult mouse soleus muscle by electroporation of RNA interference constructs caused disassembly of pretzel-shaped AChR-rich areas in vivo. Finally, we found that src-/-;fyn-/- myotubes contained elevated levels of SHP-2 protein. CONCLUSION Our data are the first to show that the fine balance between PTPs and SFKs is a key aspect in stabilization of postsynaptic AChR clusters. One phosphatase that acts in this equilibrium is SHP-2. Thus, PTPs such as SHP-2 stabilize AChR clusters under normal circumstances, but when these PTPs are not balanced by SFKs, they render clusters unstable.
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Weston CA, Teressa G, Weeks BS, Prives J. Agrin and laminin induce acetylcholine receptor clustering by convergent, Rho GTPase-dependent signaling pathways. J Cell Sci 2007; 120:868-75. [PMID: 17298982 DOI: 10.1242/jcs.03367] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
During neuromuscular junction formation, extracellular matrix-mediated signals cause muscle surface acetylcholine receptors (AChRs) to aggregate at synaptic sites. Two extracellular matrix proteins, agrin and laminin, have each been shown to initiate signaling pathways that culminate in AChR clustering in cultured muscle cells. Here we present evidence that laminin-induced AChR clustering is mediated by the activation of the Rho GTPases Cdc42, Rac and Rho. Clustering in response to laminin is blocked by the dominant negative mutants Cdc42N17, RacN17 and RhoN19, as well as by the Rho inhibitor C3 transferase. Moreover, laminin-induced AChR clustering is impaired by the Rho kinase inhibitor Y-27632. Agrin-induced AChR clustering has previously been shown to require activation of Cdc42, Rac and Rho. Therefore, although agrin and laminin use distinct transmembrane receptors to initiate AChR clustering, their signaling pathways converge at the level of Rho GTPase activation.
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Affiliation(s)
- Christi A Weston
- Department of Pharmacological Sciences, Stony Brook University, Stony Brook, NY 11794, USA
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15
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Bruneau E, Akaaboune M. The dynamics of the rapsyn scaffolding protein at individual acetylcholine receptor clusters. J Biol Chem 2007; 282:9932-9940. [PMID: 17283077 DOI: 10.1074/jbc.m608714200] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Rapsyn, a cytoplasmic receptor-associated protein, is required for the clustering of acetylcholine receptors (AChRs). Although AChR dynamics have been extensively studied, little is known about the dynamics of rapsyn. Here, we used a rapsyn-green fluorescent protein (GFP) fusion protein and quantitative fluorescent imaging to study the dynamics of rapsyn in transfected C2C12 myotubes. First, we found that rapsyn-GFP expression at clusters did not alter AChR aggregation, function, or turnover. Quantification of rapsyn immunofluorescence indicated that the expression of rapsyn-GFP proteins at clusters does not increase the overall rapsyn density compared with untransfected myotube clusters. Using time lapse imaging and fluorescence recovery after photobleaching, we demonstrated that the recovery of rapsyn-GFP fluorescence at clusters was very fast, with a halftime of about approximately 1.5 h (approximately 3 times faster than AChRs). Inhibition of protein kinase C significantly altered receptor insertion, but it had no effect on rapsyn insertion. When cells were treated with the broad spectrum kinase inhibitor staurosporine, receptor insertion was decreased even further. However, inhibition of protein kinase A had no effect on insertion of either rapsyn or receptors. Finally, when cells were treated with neural agrin, rapsyn and AChRs were both directed away from preexisting clusters and accumulated together in new small clusters. These results demonstrate the remarkable dynamism of rapsyn, which may underlie the stability and maintenance of the postsynaptic scaffold and suggest that the insertion of different postsynaptic proteins may be operating independently.
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Affiliation(s)
- Emile Bruneau
- Department of Molecular, Cellular and Developmental Biology and Program in Neuroscience, University of Michigan, Ann Arbor, Michigan 48109
| | - Mohammed Akaaboune
- Department of Molecular, Cellular and Developmental Biology and Program in Neuroscience, University of Michigan, Ann Arbor, Michigan 48109.
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16
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Saint-Amant L, Sprague SM, Hirata H, Li Q, Cui WW, Zhou W, Poudou O, Hume RI, Kuwada JY. The zebrafishennui behavioral mutation disrupts acetylcholine receptor localization and motor axon stability. Dev Neurobiol 2007; 68:45-61. [DOI: 10.1002/dneu.20569] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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17
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Milholland RBR, Dulla C, Gordon H. L-type calcium channels mediate acetylcholine receptor aggregation on cultured muscle. Dev Neurobiol 2007; 67:987-98. [PMID: 17565707 DOI: 10.1002/dneu.20397] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Agrin activation of muscle specific kinase (MuSK) initiates postsynaptic development on skeletal muscle that includes the aggregation of acetylcholine receptors (AChRs; Glass et al. [1996]: Cell 85: 513-523; Gautam et al. [1996]: Cell 85: 525-535). Although the agrin/MuSK signaling pathway remains largely unknown, changes in intracellular calcium levels are required for agrin-induced AChR aggregation (Megeath and Fallon [1998]: J Neurosci 18: 672-678). Here, we show that L-type calcium channels (L-CaChs) are required for full agrin-induced aggregation of AChRs and sufficient to induce agrin-independent AChR aggregation. Blockade of L-CaChs in muscle cultures inhibited agrin-induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Activation of L-CaChs in the absence of agrin induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Agrin responsiveness was significantly reduced in primary muscle cultures from the muscular dysgenesis mouse, a natural mutant, which does not express the L-CaCh. Our results establish a novel role for L-CaChs as important sources of the intracellular calcium necessary for the aggregation of AChRs.
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Affiliation(s)
- Rebecca B R Milholland
- Department of Cell Biology and Anatomy, University of Arizona Health Sciences Center, Tucson, Arizona 85724-5044, USA
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18
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Willmann R, Pun S, Stallmach L, Sadasivam G, Santos AF, Caroni P, Fuhrer C. Cholesterol and lipid microdomains stabilize the postsynapse at the neuromuscular junction. EMBO J 2006; 25:4050-60. [PMID: 16932745 PMCID: PMC1560359 DOI: 10.1038/sj.emboj.7601288] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2006] [Accepted: 07/25/2006] [Indexed: 11/08/2022] Open
Abstract
Stabilization and maturation of synapses are important for development and function of the nervous system. Previous studies have implicated cholesterol-rich lipid microdomains in synapse stabilization, but the underlying mechanisms remain unclear. We found that cholesterol stabilizes clusters of synaptic acetylcholine receptors (AChRs) in denervated muscle in vivo and in nerve-muscle explants. In paralyzed muscles, cholesterol triggered maturation of nerve sprout-induced AChR clusters into pretzel shape. Cholesterol treatment also rescued a specific defect in AChR cluster stability in cultured src(-/-);fyn(-/-) myotubes. Postsynaptic proteins including AChRs, rapsyn, MuSK and Src-family kinases were strongly enriched in lipid microdomains prepared from wild-type myotubes. Microdomain disruption by cholesterol-sequestering methyl-beta-cyclodextrin disassembled AChR clusters and decreased AChR-rapsyn interaction and AChR phosphorylation. Amounts of microdomains and enrichment of postsynaptic proteins into microdomains were decreased in src(-/-);fyn(-/-) myotubes but rescued by cholesterol treatment. These data provide evidence that cholesterol-rich lipid microdomains and SFKs act in a dual mechanism in stabilizing the postsynapse: SFKs enhance microdomain-association of postsynaptic components, whereas microdomains provide the environment for SFKs to maintain interactions and phosphorylation of these components.
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Affiliation(s)
- Raffaella Willmann
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Zürich, Switzerland
| | - San Pun
- Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - Lena Stallmach
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Zürich, Switzerland
| | - Gayathri Sadasivam
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Zürich, Switzerland
| | | | - Pico Caroni
- Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - Christian Fuhrer
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Zürich, Switzerland
- Department of Neurochemistry, Brain Research Institute, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland. Tel.: +41 44 635 33 10; Fax: +41 44 635 33 03; E-mail:
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19
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Tremblay MR, Carbonetto S. An Extracellular Pathway for Dystroglycan Function in Acetylcholine Receptor Aggregation and Laminin Deposition in Skeletal Myotubes. J Biol Chem 2006; 281:13365-13373. [PMID: 16531403 DOI: 10.1074/jbc.m600912200] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The dystroglycan (DG) complex is involved in agrin-induced acetylcholine receptor clustering downstream of muscle-specific kinase where it regulates the stability of acetylcholine receptor aggregates as well as assembly of the synaptic basement membrane. We have previously proposed that this entails coordinate extracellular and intracellular interactions of its two subunits, alpha- and beta-DG. To assess the contribution of the extracellular and intracellular portions of DG, we have used adenoviruses to express full-length and deletion mutants of beta-DG in myotubes derived from wild-type embryonic stem cells or from cells null for DG. We show that alpha-DG is properly glycosylated and targeted to the myotube surface in the absence of beta-DG. Extracellular interactions of DG modulate the size and the microcluster density of agrin-induced acetylcholine receptor aggregates and are responsible for targeting laminin to these clusters. Thus, the association of alpha- and beta-DG in skeletal muscle may coordinate independent roles in signaling. We discuss how DG may regulate synapses through extracellular signaling functions of its alpha subunit.
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Affiliation(s)
- Mathieu R Tremblay
- Department of Biology, McGill University, Montréal General Hospital Research Institute, Montréal, Québec H3G 1A4, Canada
| | - Salvatore Carbonetto
- Department of Biology, McGill University, Montréal General Hospital Research Institute, Montréal, Québec H3G 1A4, Canada; Center for Research in Neuroscience, McGill University, Montréal General Hospital Research Institute, Montréal, Québec H3G 1A4, Canada.
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20
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Morita A, Yamashita N, Sasaki Y, Uchida Y, Nakajima O, Nakamura F, Yagi T, Taniguchi M, Usui H, Katoh-Semba R, Takei K, Goshima Y. Regulation of dendritic branching and spine maturation by semaphorin3A-Fyn signaling. J Neurosci 2006; 26:2971-80. [PMID: 16540575 PMCID: PMC6673984 DOI: 10.1523/jneurosci.5453-05.2006] [Citation(s) in RCA: 132] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
A member of semaphorin family, semaphorin3A (Sema3A), acts as a chemorepellent or chemoattractant on a wide variety of axons and dendrites in the development of the nervous systems. We here show that Sema3A induces clustering of both postsynaptic density-95 (PSD-95) and presynaptic synapsin I in cultured cortical neurons without changing the density of spines or filopodia. Neuropilin-1 (NRP-1), a receptor for Sema3A, is present on both axons and dendrites. When the cultured neurons are exposed to Sema3A, the cluster size of PSD-95 is markedly enhanced, and an extensive colocalization of PSD-95 and NRP-1 or actin-rich protrusion is seen. The effects of Sema3A on spine morphology are blocked by PP2, an Src type tyrosine kinase inhibitor, but not by the PP3, the inactive-related compound. In the cultured cortical neurons from fyn(-/-) mice, dendrites bear few spines, and Sema3A does not induce PSD-95 cluster formation on the dendrites. Sema3A and its receptor genes are highly expressed during the synaptogenic period of postnatal days 10 and 15. The cortical neurons in layer V, but not layer III, show a lowered density of synaptic bouton-like structure on dendrites in sema3A- and fyn-deficient mice. The neurons of the double-heterozygous mice show the lowered spine density, whereas those of single heterozygous mice show similar levels of the spine density as the wild type. These findings suggest that the Sema3A signaling pathway plays an important role in the regulation of dendritic spine maturation in the cerebral cortex neurons.
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21
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Shigemoto K, Kubo S, Maruyama N, Hato N, Yamada H, Jie C, Kobayashi N, Mominoki K, Abe Y, Ueda N, Matsuda S. Induction of myasthenia by immunization against muscle-specific kinase. J Clin Invest 2006; 116:1016-24. [PMID: 16557298 PMCID: PMC1409737 DOI: 10.1172/jci21545] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2004] [Accepted: 02/07/2006] [Indexed: 11/17/2022] Open
Abstract
Muscle-specific kinase (MuSK) is critical for the synaptic clustering of nicotinic acetylcholine receptors (AChRs) and plays multiple roles in the organization and maintenance of neuromuscular junctions (NMJs). MuSK is activated by agrin, which is released from motoneurons, and induces AChR clustering at the postsynaptic membrane. Although autoantibodies against the ectodomain of MuSK have been found in a proportion of patients with generalized myasthenia gravis (MG), it is unclear whether MuSK autoantibodies are the causative agent of generalized MG. In the present study, rabbits immunized with MuSK ectodomain protein manifested MG-like muscle weakness with a reduction of AChR clustering at the NMJs. The autoantibodies activated MuSK and blocked AChR clustering induced by agrin or by mediators that do not activate MuSK. Thus MuSK autoantibodies rigorously inhibit AChR clustering mediated by multiple pathways, an outcome that broadens our general comprehension of the pathogenesis of MG.
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Affiliation(s)
- Kazuhiro Shigemoto
- Department of Preventive Medicine, Ehime University School of Medicine, Ehime, Japan.
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22
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Sadasivam G, Willmann R, Lin S, Erb-Vögtli S, Kong XC, Rüegg MA, Fuhrer C. Src-family kinases stabilize the neuromuscular synapse in vivo via protein interactions, phosphorylation, and cytoskeletal linkage of acetylcholine receptors. J Neurosci 2006; 25:10479-93. [PMID: 16280586 PMCID: PMC6725837 DOI: 10.1523/jneurosci.2103-05.2005] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Postnatal stabilization and maturation of the postsynaptic membrane are important for development and function of the neuromuscular junction (NMJ), but the underlying mechanisms remain poorly characterized. We examined the role of Src-family kinases (SFKs) in vivo. Electroporation of kinase-inactive Src constructs into soleus muscles of adult mice caused NMJ disassembly: acetylcholine receptor (AChR)-rich areas became fragmented; the topology of nerve terminal, AChRs, and synaptic nuclei was disturbed; and occasionally nerves started to sprout. Electroporation of kinase-overactive Src produced similar but milder effects. We studied the mechanism of SFK action using cultured src(-/-);fyn(-/-) myotubes, focusing on clustering of postsynaptic proteins, their interaction with AChRs, and AChR phosphorylation. Rapsyn and the utrophin-glycoprotein complex were recruited normally into AChR-containing clusters by agrin in src(-/-);fyn(-/-) myotubes. But after agrin withdrawal, clusters of these proteins disappeared rapidly in parallel with AChRs, revealing that SFKs are of general importance in postsynaptic stability. At the same time, AChR interaction with rapsyn and dystrobrevin and AChR phosphorylation decreased after agrin withdrawal from mutant myotubes. Unexpectedly, levels of rapsyn protein were increased in src(-/-);fyn(-/-) myotubes, whereas rapsyn-cytoskeleton interactions were unaffected. The overall cytoskeletal link of AChRs was weak but still strengthened by agrin in mutant cells, consistent with the normal formation but decreased stability of AChR clusters. These data show that correctly balanced activity of SFKs is critical in maintaining adult NMJs in vivo. SFKs hold the postsynaptic apparatus together through stabilization of AChR-rapsyn interaction and AChR phosphorylation. In addition, SFKs control rapsyn levels and AChR-cytoskeletal linkage.
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Affiliation(s)
- Gayathri Sadasivam
- Department of Neurochemistry, Brain Research Institute, University of Zürich, CH-8057 Zürich, Switzerland
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23
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Campagna JA, Fallon J. Lipid rafts are involved in C95 (4,8) agrin fragment-induced acetylcholine receptor clustering. Neuroscience 2006; 138:123-32. [PMID: 16377091 DOI: 10.1016/j.neuroscience.2005.11.019] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2005] [Revised: 09/22/2005] [Accepted: 11/08/2005] [Indexed: 11/26/2022]
Abstract
During development of the neuromuscular junction, high densities of acetylcholine receptors accumulate beneath the overlying nerve terminal. A defining feature of mature synapses is the sharp demarcation of acetylcholine receptor density, which is approximately 1000-fold higher in the postsynaptic as compared with the contiguous extrasynaptic muscle membrane. These high densities of receptors accumulate by at least four mechanisms, re-distribution of existing surface receptors, local synthesis of new receptors, decreased turnover of synaptic receptors, and limitation of diffusion of sub-neural, aggregated receptors. The limitation of receptor diffusion within the membrane is likely in part due to the anchoring of acetylcholine receptor complexes to components of the cytoskeleton. Here we have tested the idea that lipid rafts--mobile, cholesterol enriched microdomains within the lipid bilayer--are another mechanism by which acetylcholine receptors are clustered in the postsynaptic apparatus. Using mouse C2C12 cells, a muscle cell line, we show that a carboxy terminal 95 amino acid fragment [C95 (4,8)] of the extracellular matrix molecule agrin that is essential for nerve-induced postsynaptic differentiation, promotes the redistribution of acetylcholine receptors into lipid rafts. Disruption of lipid rafts before agrin treatment largely inhibits de novo agrin-induced acetylcholine receptor clustering. Moreover, mature acetylcholine receptor clusters are destabilized if lipid rafts are disrupted. These results show that lipid rafts are important in both the initial clustering and later stabilization of agrin-induced acetylcholine receptor clusters and also suggest that lipid rafts may contribute to the postsynaptic localization of acetylcholine receptors in vivo.
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Affiliation(s)
- J A Campagna
- Department of Anesthesia, Longnecker Anesthesia Research Laboratory, University of Pennsylvania, Philadelphia, PA 19104, USA.
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24
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Smirnov SP, Barzaghi P, McKee KK, Ruegg MA, Yurchenco PD. Conjugation of LG Domains of Agrins and Perlecan to Polymerizing Laminin-2 Promotes Acetylcholine Receptor Clustering. J Biol Chem 2005; 280:41449-57. [PMID: 16219760 DOI: 10.1074/jbc.m508939200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Neuromuscular junction (NMJ) assembly is characterized by the clustering and neuronal alignment of acetylcholine receptors (AChRs). In this study we have addressed post-synaptic contributions to assembly that may arise from the NMJ basement membrane with cultured myotubes. We show that the cell surface-binding LG domains of non-neural (muscle) agrin and perlecan promote AChR clustering in the presence of laminin-2. This type of AChR clustering occurs with a several hour lag, requires muscle-specific kinase (MuSK), and is accompanied by tyrosine phosphorylation of MuSK and betaAChR. It also requires conjugation of the agrin or perlecan to laminin together with laminin polymerization. Furthermore, AChR clustering can be mimicked with antibody binding to non-neural agrin, supporting a mechanism of ligand aggregation. Neural agrin, in addition to its unique ability to cluster AChRs through its B/z sequence insert, also exhibits laminin-dependent AChR clustering, the latter enhancing and stabilizing its activity. Finally, we show that type IV collagen, which lacks clustering activity on its own, stabilizes laminin-dependent AChR clusters. These findings provide evidence for cooperative and partially redundant MuSK-dependent functions of basement membrane in AChR assembly that can enhance neural agrin activity yet operate in its absence. Such interactions may contribute to the assembly of aneural AChR clusters that precede neural agrin release as well as affect later NMJ development.
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Affiliation(s)
- Sergei P Smirnov
- Department of Pathology and Laboratory Medicine, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854, USA
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25
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Noël G, Belda M, Guadagno E, Micoud J, Klöcker N, Moukhles H. Dystroglycan and Kir4.1 coclustering in retinal Müller glia is regulated by laminin-1 and requires the PDZ-ligand domain of Kir4.1. J Neurochem 2005; 94:691-702. [PMID: 16033419 DOI: 10.1111/j.1471-4159.2005.03191.x] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Inwardly rectifying potassium (Kir) channels in Müller glia play a critical role in the spatial buffering of potassium ions that accumulate during retinal activity. To this end, Kir channels show a polarized subcellular distribution with the predominant channel subunit in Müller glia, Kir4.1, clustered in the endfeet of these cells at the inner limiting membrane. However, the molecular mechanisms underlying their distribution have yet to be identified. Here, we show that laminin, agrin and alpha-dystroglycan (DG) codistribute with Kir4.1 at the inner limiting membrane in the retina and that laminin-1 induces the clustering of alpha-DG, syntrophin and Kir4.1 in Müller cell cultures. In addition, we found that alpha-DG clusters were enriched for agrin and sought to investigate the role of agrin in their formation using recombinant C-agrins. Both C-agrin 4,8 and C-agrin 0,0 failed to induce alpha-DG clustering and neither of them potentiated the alpha-DG clustering induced by laminin-1. Finally, our data reveal that deletion of the PDZ-ligand domain of Kir4.1 prevents their laminin-induced clustering. These findings indicate that both laminin-1 and alpha-DG are involved in the distribution of Kir4.1 to specific Müller cell membrane domains and that this process occurs via a PDZ-domain-mediated interaction. Thus, in the basal lamina laminin is an essential regulator involved in clearing excess potassium released during neuronal activity, thereby contributing to the maintenance of normal synaptic transmission in the retina.
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Affiliation(s)
- Geoffrey Noël
- Département de Sciences Biologiques, Université de Montréal, Montréal, Québec, Canada
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26
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Schwander M, Shirasaki R, Pfaff SL, Müller U. Beta1 integrins in muscle, but not in motor neurons, are required for skeletal muscle innervation. J Neurosci 2005; 24:8181-91. [PMID: 15371519 PMCID: PMC6729792 DOI: 10.1523/jneurosci.1345-04.2004] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
In vitro studies have provided evidence that beta1 integrins in motor neurons promote neurite outgrowth, whereas beta1 integrins in myotubes regulate acetylcholine receptor (AChR) clustering. Surprisingly, using genetic studies in mice, we show here that motor axon outgrowth and neuromuscular junction (NMJ) formation in large part are unaffected when the integrin beta1 gene (Itgb1) is inactivated in motor neurons. In the absence of Itgb1 expression in skeletal muscle, interactions between motor neurons and muscle are defective, preventing normal presynaptic differentiation. Motor neurons fail to terminate their growth at the muscle midline, branch excessively, and develop abnormal nerve terminals. These defects resemble the phenotype of agrin-null mice, suggesting that signaling molecules such as agrin, which coordinate presynaptic and postsynaptic differentiation, are not presented properly to nerve terminals. We conclude that Itgb1 expression in muscle, but not in motor neurons, is critical for NMJ development.
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Affiliation(s)
- Martin Schwander
- Department of Cell Biology and Institute for Childhood and Neglected Disease, The Scripps Research Institute, La Jolla, California 92037, USA
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27
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McDonnell KMW, Grow WA. Reduced Glycosaminoglycan Sulfation Diminishes the Agrin Signal Transduction Pathway. Dev Neurosci 2004; 26:1-10. [PMID: 15509893 DOI: 10.1159/000080706] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2003] [Accepted: 11/04/2003] [Indexed: 12/13/2022] Open
Abstract
Proteoglycans consist of a protein core complexed to glycosaminoglycan (GAG) side chains, are abundant in skeletal muscle cell membranes and basal lamina, and have important functions in neuromuscular synapse development. Treatment with chlorate results in the undersulfation of heparan sulfate and chondroitin sulfate GAGs in cell culture. In addition, chlorate treatment decreases the frequency of spontaneous acetylcholine receptor (AChR) clustering in skeletal muscle cell culture. AChRs and other molecules cluster to form the postsynaptic component of neuromuscular synapses. Chlorate treatment is shown here to decrease the frequency of agrin-induced AChR clustering and agrin-induced tyrosine phosphorylation of the AChR beta-subunit. These data suggest that reduced GAG chain sulfation decreases the frequency of AChR clustering by diminishing the agrin signal transduction pathway.
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Affiliation(s)
- Kelly M W McDonnell
- Department of Anatomy, Arizona College of Osteopathic Medicine, Midwestern University, Glendale, Ariz. 85308, USA
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28
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Mittaud P, Camilleri AA, Willmann R, Erb-Vögtli S, Burden SJ, Fuhrer C. A single pulse of agrin triggers a pathway that acts to cluster acetylcholine receptors. Mol Cell Biol 2004; 24:7841-54. [PMID: 15340048 PMCID: PMC515067 DOI: 10.1128/mcb.24.18.7841-7854.2004] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Agrin triggers signaling mechanisms of high temporal and spatial specificity to achieve phosphorylation, clustering, and stabilization of postsynaptic acetylcholine receptors (AChRs). Agrin transiently activates the kinase MuSK; MuSK activation has largely vanished when AChR clusters appear. Thus, a tyrosine kinase cascade acts downstream from MuSK, as illustrated by the agrin-evoked long-lasting activation of Src family kinases (SFKs) and their requirement for AChR cluster stabilization. We have investigated this cascade and report that pharmacological inhibition of SFKs reduces early but not later agrin-induced phosphorylation of MuSK and AChRs, while inhibition of Abl kinases reduces late phosphorylation. Interestingly, SFK inhibition applied selectively during agrin-induced AChR cluster formation caused rapid cluster dispersal later upon agrin withdrawal. We also report that a single 5-min agrin pulse, followed by extensive washing, triggered long-lasting MuSK and AChR phosphorylation and efficient AChR clustering. Following the pulse, MuSK phosphorylation increased and, beyond a certain level, caused maximal clustering. These data reveal novel temporal aspects of tyrosine kinase action in agrin signaling. First, during AChR cluster formation, SFKs initiate early phosphorylation and an AChR stabilization program that acts much later. Second, a kinase mechanism rapidly activated by agrin acts thereafter autonomously in agrin's absence to further increase MuSK phosphorylation and cluster AChRs.
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Affiliation(s)
- Peggy Mittaud
- Brain Research Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
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29
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Jones MA, Werle MJ. Agrin-induced AChR aggregate formation requires cGMP and aggregate maturation requires activation of cGMP-dependent protein kinase. Mol Cell Neurosci 2004; 25:195-204. [PMID: 15019937 DOI: 10.1016/j.mcn.2003.10.013] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2002] [Revised: 09/24/2003] [Accepted: 10/21/2003] [Indexed: 10/26/2022] Open
Abstract
Previously, it was demonstrated that agrin acting through the gaseous, signaling molecule, nitric oxide (NO), induces the formation of AChR aggregates on myotubes in culture. Soluble guanylyl cyclase (sGC), which is present at the neuromuscular junction, is a common target of NO. Therefore, we hypothesized that sGC and cGMP are involved in the agrin signaling cascade. Inhibition of sGC hindered AChR aggregation in both agrin- and NO donor-treated cultured myotubes; whereas, a cGMP analogue was able to induce the formation of AChR aggregates on naïve muscle cells. Due to the presence of cyclic GMP-dependent protein kinase (PKG) at the neuromuscular junction, we tested the ability of a PKG inhibitor to alter the agrin signaling cascade. PKG inhibition did not prevent nascent AChR aggregate formation; however, these aggregates were diffuse and composed of numerous microaggregates consistent with incomplete maturation. Thus, we conclude that cGMP is important for the initiation of AChR aggregation, while PKG is involved in the maturation of AChR aggregates.
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Affiliation(s)
- Melissa A Jones
- Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, KS 66160, USA
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Kummer TT, Misgeld T, Lichtman JW, Sanes JR. Nerve-independent formation of a topologically complex postsynaptic apparatus. ACTA ACUST UNITED AC 2004; 164:1077-87. [PMID: 15037598 PMCID: PMC2172060 DOI: 10.1083/jcb.200401115] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
As the mammalian neuromuscular junction matures, its acetylcholine receptor (AChR)–rich postsynaptic apparatus is transformed from an oval plaque into a pretzel-shaped array of branches that precisely mirrors the branching pattern of the motor nerve terminal. Although the nerve has been believed to direct postsynaptic maturation, we report here that myotubes cultured aneurally on matrix-coated substrates form elaborately branched AChR-rich domains remarkably similar to those seen in vivo. These domains share several characteristics with the mature postsynaptic apparatus, including colocalization of multiple postsynaptic markers, clustering of subjacent myonuclei, and dependence on the muscle-specific kinase and rapsyn for their formation. Time-lapse imaging showed that branched structures arise from plaques by formation and fusion of AChR-poor perforations through a series of steps mirroring that seen in vivo. Multiple fluorophore imaging showed that growth occurs by circumferential, asymmetric addition of AChRs. Analysis in vivo revealed similar patterns of AChR addition during normal development. These results reveal the sequence of steps by which a topologically complex domain forms on a cell and suggest an unexpected nerve-independent role for the postsynaptic cell in generating this topological complexity.
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Affiliation(s)
- Terrance T Kummer
- Dept. of Anatomy and Neurobiology, Washington University School of Medicine, 660 South Euclid Ave., St. Louis, MO 63110, USA
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McDearmon EL, Combs AC, Ervasti JM. Core 1 glycans on alpha-dystroglycan mediate laminin-induced acetylcholine receptor clustering but not laminin binding. J Biol Chem 2003; 278:44868-73. [PMID: 12952987 DOI: 10.1074/jbc.m307026200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Although unique O-linked oligosaccharides on alpha-dystroglycan are important for binding to a variety of extracellular ligands, the function(s) of more generic carbohydrate structures on alpha-dystroglycan remain unclear. Recent studies suggest a role for glycoconjugates bearing the core 1 disaccharide Galbeta(1-3)GalNAc in acetylcholine receptor (AChR) clustering on the surface of muscle cells. Here, we report experiments demonstrating that the core 1-specific lectin jacalin almost completely abrogated laminin-induced AChR clustering in C2C12 myotubes and that alpha-dystroglycan was the predominant jacalin-binding protein detected in C2C12 myotube lysates. Although jacalin likely inhibited laminin-induced AChR clustering by directly binding to alpha-dystroglycan, jacalin had no effect on laminin binding to the myotube surface or to alpha-dystroglycan. Like jacalin, peanut agglutinin lectin also binds the core 1 disaccharide but not when it is terminally sialylated as expressed on alpha-dystroglycan. We show that C2C12 alpha-dystroglycan bound to peanut agglutinin only after digestion with neuraminidase. Simultaneous treatment of myotubes with neuraminidase and endo-O-glycosidase diminished alpha-dystroglycan binding to peanut agglutinin and inhibited neuraminidase-induced AChR clustering. We conclude that sialylated core 1 oligosaccharides of alpha-dystroglycan are important for laminin-induced AChR clustering and that their function in this process is distinct from the established role of alpha-dystroglycan oligosaccharides in laminin binding.
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Affiliation(s)
- Erin L McDearmon
- Graduate Program in Molecular and Cellular Pharmacology, University of Wisconsin-Madison Medical School, 53706, USA
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Affiliation(s)
- Alexander Dityatev
- Zentrum für Molekulare Neurobiologie, University of Hamburg, Martinistr. 52, 20246 Hamburg, Germany.
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Moransard M, Borges LS, Willmann R, Marangi PA, Brenner HR, Ferns MJ, Fuhrer C. Agrin regulates rapsyn interaction with surface acetylcholine receptors, and this underlies cytoskeletal anchoring and clustering. J Biol Chem 2003; 278:7350-9. [PMID: 12486121 DOI: 10.1074/jbc.m210865200] [Citation(s) in RCA: 80] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The acetylcholine receptor (AChR)-associated protein rapsyn is essential for neuromuscular synapse formation and clustering of AChRs, but its mode of action remains unclear. We have investigated whether agrin, a key nerve-derived synaptogenic factor, influences rapsyn-AChR interactions and how this affects clustering and cytoskeletal linkage of AChRs. By precipitating AChRs and probing for associated rapsyn, we found that in denervated diaphragm rapsyn associates with synaptic as well as with extrasynaptic AChRs showing that rapsyn interacts with unclustered AChRs in vivo. Interestingly, synaptic AChRs are associated with more rapsyn suggesting that clustering of AChRs may require increased interaction with rapsyn. In similar experiments in cultured myotubes, rapsyn interacted with intracellular AChRs and with unclustered AChRs at the cell surface, although surface interactions are much more prominent. Remarkably, agrin induces recruitment of additional rapsyn to surface AChRs and clustering of AChRs independently of the secretory pathway. This agrin-induced increase in rapsyn-AChR interaction strongly correlates with clustering, because staurosporine and herbimycin blocked both the increase and clustering. Conversely, laminin and calcium induced both increased rapsyn-AChR interaction and AChR clustering. Finally, time course experiments revealed that the agrin-induced increase occurs with AChRs that become cytoskeletally linked, and that this precedes receptor clustering. Thus, we propose that neural agrin controls postsynaptic aggregation of the AChR by enhancing rapsyn interaction with surface AChRs and inducing cytoskeletal anchoring and that this is an important precursor step for AChR clustering.
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Affiliation(s)
- Martijn Moransard
- Department of Neurochemistry, Brain Research Institute, University of Zürich, CH-8057 Zürich, Switzerland
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