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Herbst R, Huijbers MG, Oury J, Burden SJ. Building, Breaking, and Repairing Neuromuscular Synapses. Cold Spring Harb Perspect Biol 2024; 16:a041490. [PMID: 38697654 PMCID: PMC11065174 DOI: 10.1101/cshperspect.a041490] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2024]
Abstract
A coordinated and complex interplay of signals between motor neurons, skeletal muscle cells, and Schwann cells controls the formation and maintenance of neuromuscular synapses. Deficits in the signaling pathway for building synapses, caused by mutations in critical genes or autoantibodies against key proteins, are responsible for several neuromuscular diseases, which cause muscle weakness and fatigue. Here, we describe the role that four key genes, Agrin, Lrp4, MuSK, and Dok7, play in this signaling pathway, how an understanding of their mechanisms of action has led to an understanding of several neuromuscular diseases, and how this knowledge has contributed to emerging therapies for treating neuromuscular diseases.
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Affiliation(s)
- Ruth Herbst
- Center for Pathophysiology, Infectiology and Immunology, Medical University of Vienna, 1090 Vienna, Austria
| | - Maartje G Huijbers
- Department of Human Genetics, Leiden University Medical Centre LUMC, 2300 RC Leiden, the Netherlands
- Department of Neurology, Leiden University Medical Centre LUMC, 2333 ZA Leiden, the Netherlands
| | - Julien Oury
- Helen L. and Martin S. Kimmel Center for Biology and Medicine at the Skirball Institute of Biomolecular Medicine, NYU School of Medicine, New York, New York 10016, USA
| | - Steven J Burden
- Neurology Department, Massachusetts General Hospital, Charlestown, Massachusetts 02129, USA
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Kamiya K, Tachiki T, Sato Y, Kouda K, Kajita E, Tamaki J, Kagamimori S, Iki M. Association between the 110-kDa C-terminal agrin fragment and skeletal muscle decline among community-dwelling older women. J Cachexia Sarcopenia Muscle 2023; 14:2253-2263. [PMID: 37562951 PMCID: PMC10570065 DOI: 10.1002/jcsm.13309] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/08/2022] [Revised: 06/14/2023] [Accepted: 07/11/2023] [Indexed: 08/12/2023] Open
Abstract
BACKGROUND C-terminal agrin fragment (CAF) is a biomarker for neuromuscular junction degradation. This study aimed to investigate whether 110-kDa CAF (CAF110) was associated with the presence and incidence of low muscle mass and strength. METHODS This cross-sectional retrospective cohort study comprised women aged ≥65 years. We measured muscle mass using a dual-energy X-ray absorptiometry scanner, hand-grip strength, and blood sampling between 2011 and 2012. A follow-up study with the same measurements was conducted between 2015 and 2017. Low muscle mass and strength were defined as an appendicular skeletal muscle mass index <5.4 kg/m2 and hand-grip strength <18 kg, respectively. The CAF110 level was measured using enzyme-linked immunosorbent assay kits. RESULTS In total, 515 women (74.3 ± 6.3 years) were included in this cross-sectional analysis. Of these, 101 (19.6%) and 128 (24.9%) women presented with low muscle mass and strength, respectively. For low muscle mass, the odds ratios (ORs) of the middle and highest CAF110 tertile groups, compared with the lowest group, were 1.93 (95% confidence interval: 1.09-3.43; P = 0.024) and 2.15 (1.22-3.80; P = 0.008), respectively. After adjusting for age, the ORs remained significant: 1.98 (1.11-3.52; P = 0.020) and 2.27 (1.28-4.03; P = 0.005), respectively. Low muscle strength ORs of all the CAF110 tertile groups were not significant. In the longitudinal analysis, 292 and 289 women were assessed for incidents of low muscle mass and strength, respectively. Of those, 34 (11.6%) and 20 (6.9%) women exhibited low muscle mass and strength, respectively. For incident low muscle mass, the crude OR of the CAF110 ≥ the median value group was marginally higher than that of the CAF110 < median value group (median [interquartile range]: 1.98 [0.94-4.17] (P = 0.072). After adjusting for age and baseline muscle mass, the OR was 2.22 [0.97-5.06] (P = 0.058). All low muscle strength ORs of the median categories of CAF110 were not significant. CONCLUSIONS CAF110 was not associated with low muscle strength. However, CAF110 may be a potential marker for the incidence of low muscle mass.
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Affiliation(s)
- Kuniyasu Kamiya
- Department of Hygiene and Public Health, Faculty of MedicineOsaka Medical and Pharmaceutical UniversityTakatsukiJapan
| | | | - Yuho Sato
- Department of Human LifeJin‐ai UniversityEchizenJapan
| | - Katsuyasu Kouda
- Department of Hygiene and Public HealthKansai Medical UniversityHirakataJapan
| | - Etsuko Kajita
- Faculty of NursingChukyo Gakuin UniversityMizunamiJapan
| | - Junko Tamaki
- Department of Hygiene and Public Health, Faculty of MedicineOsaka Medical and Pharmaceutical UniversityTakatsukiJapan
| | | | - Masayuki Iki
- Department of Public HealthKindai University Faculty of MedicineOsaka‐SayamaJapan
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Bushra S, Lin YN, Joudaki A, Ito M, Ohkawara B, Ohno K, Masuda A. Neural Isoforms of Agrin Are Generated by Reduced PTBP1-RNA Interaction Network Spanning the Neuron-Specific Splicing Regions in AGRN. Int J Mol Sci 2023; 24:ijms24087420. [PMID: 37108583 PMCID: PMC10139058 DOI: 10.3390/ijms24087420] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2023] [Revised: 04/12/2023] [Accepted: 04/14/2023] [Indexed: 04/29/2023] Open
Abstract
Agrin is a heparan sulfate proteoglycan essential for the clustering of acetylcholine receptors at the neuromuscular junction. Neuron-specific isoforms of agrin are generated by alternative inclusion of three exons, called Y, Z8, and Z11 exons, although their processing mechanisms remain elusive. We found, by inspection of splicing cis-elements into the human AGRN gene, that binding sites for polypyrimidine tract binding protein 1 (PTBP1) were extensively enriched around Y and Z exons. PTBP1-silencing enhanced the coordinated inclusion of Y and Z exons in human SH-SY5Y neuronal cells, even though three constitutive exons are flanked by these alternative exons. Deletion analysis using minigenes identified five PTBP1-binding sites with remarkable splicing repression activities around Y and Z exons. Furthermore, artificial tethering experiments indicated that binding of a single PTBP1 molecule to any of these sites represses nearby Y or Z exons as well as the other distal exons. The RRM4 domain of PTBP1, which is required for looping out a target RNA segment, was likely to play a crucial role in the repression. Neuronal differentiation downregulates PTBP1 expression and promotes the coordinated inclusion of Y and Z exons. We propose that the reduction in the PTPB1-RNA network spanning these alternative exons is essential for the generation of the neuron-specific agrin isoforms.
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Affiliation(s)
- Samira Bushra
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Ying-Ni Lin
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Atefeh Joudaki
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Mikako Ito
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Bisei Ohkawara
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Kinji Ohno
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
| | - Akio Masuda
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Aichi, Japan
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Nikolaou N, Gordon PM, Hamid F, Taylor R, Lloyd-Jones J, Makeyev EV, Houart C. Cytoplasmic pool of U1 spliceosome protein SNRNP70 shapes the axonal transcriptome and regulates motor connectivity. Curr Biol 2022; 32:5099-5115.e8. [PMID: 36384140 DOI: 10.1016/j.cub.2022.10.048] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2021] [Revised: 09/09/2022] [Accepted: 10/21/2022] [Indexed: 11/17/2022]
Abstract
Regulation of pre-mRNA splicing and polyadenylation plays a profound role in neurons by diversifying the proteome and modulating gene expression in response to physiological cues. Although most of the pre-mRNA processing is thought to occur in the nucleus, numerous splicing regulators are also found in neurites. Here, we show that U1-70K/SNRNP70, a component of the major spliceosome, localizes in RNA-associated granules in zebrafish axons. We identify the extra-nuclear SNRNP70 as an important regulator of motor axonal growth, nerve-dependent acetylcholine receptor (AChR) clustering, and neuromuscular synaptogenesis. This cytoplasmic pool has a protective role for a limited number of transcripts regulating their abundance and trafficking inside axons. Moreover, non-nuclear SNRNP70 regulates splice variants of transcripts such as agrin, thereby controlling synapse formation. Our results point to an unexpected, yet essential, function of non-nuclear SNRNP70 in axonal development, indicating a role of spliceosome proteins in cytoplasmic RNA metabolism during neuronal connectivity.
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Affiliation(s)
- Nikolas Nikolaou
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK; Department of Life Sciences, University of Bath, Bath BA2 7AY, UK.
| | - Patricia M Gordon
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK
| | - Fursham Hamid
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK
| | - Richard Taylor
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK
| | | | - Eugene V Makeyev
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK
| | - Corinne Houart
- Centre for Developmental Neurobiology MRC CNDD, IoPPN, Guy's Campus, King's College London, London SE1 1UL, UK.
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Deconstruction of Neurotrypsin Reveals a Multi-factorially Regulated Activity Affecting Myotube Formation and Neuronal Excitability. Mol Neurobiol 2022; 59:7466-7485. [PMID: 36197591 PMCID: PMC9616769 DOI: 10.1007/s12035-022-03056-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2022] [Accepted: 09/21/2022] [Indexed: 11/16/2022]
Abstract
Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.
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Dobrowolny G, Barbiera A, Sica G, Scicchitano BM. Age-Related Alterations at Neuromuscular Junction: Role of Oxidative Stress and Epigenetic Modifications. Cells 2021; 10:1307. [PMID: 34074012 PMCID: PMC8225025 DOI: 10.3390/cells10061307] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Revised: 05/19/2021] [Accepted: 05/22/2021] [Indexed: 12/11/2022] Open
Abstract
With advancing aging, a decline in physical abilities occurs, leading to reduced mobility and loss of independence. Although many factors contribute to the physio-pathological effects of aging, an important event seems to be related to the compromised integrity of the neuromuscular system, which connects the brain and skeletal muscles via motoneurons and the neuromuscular junctions (NMJs). NMJs undergo severe functional, morphological, and molecular alterations during aging and ultimately degenerate. The effect of this decline is an inexorable decrease in skeletal muscle mass and strength, a condition generally known as sarcopenia. Moreover, several studies have highlighted how the age-related alteration of reactive oxygen species (ROS) homeostasis can contribute to changes in the neuromuscular junction morphology and stability, leading to the reduction in fiber number and innervation. Increasing evidence supports the involvement of epigenetic modifications in age-dependent alterations of the NMJ. In particular, DNA methylation, histone modifications, and miRNA-dependent gene expression represent the major epigenetic mechanisms that play a crucial role in NMJ remodeling. It is established that environmental and lifestyle factors, such as physical exercise and nutrition that are susceptible to change during aging, can modulate epigenetic phenomena and attenuate the age-related NMJs changes. This review aims to highlight the recent epigenetic findings related to the NMJ dysregulation during aging and the role of physical activity and nutrition as possible interventions to attenuate or delay the age-related decline in the neuromuscular system.
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Affiliation(s)
- Gabriella Dobrowolny
- Department of Anatomy, Histology, Forensic Medicine and Orthopaedics (DAHFMO)-Unit of Histology and Medical Embryology, Sapienza University of Rome, Laboratory Affiliated to Istituto Pasteur Italia-Fondazione Cenci Bolognetti, 00161 Rome, Italy;
| | - Alessandra Barbiera
- Department of Life Sciences and Public Health, Histology and Embryology Unit, Fondazione Policlinico Universitario A. Gemelli IRCCS, 00168 Rome, Italy; (A.B.); (G.S.)
| | - Gigliola Sica
- Department of Life Sciences and Public Health, Histology and Embryology Unit, Fondazione Policlinico Universitario A. Gemelli IRCCS, 00168 Rome, Italy; (A.B.); (G.S.)
| | - Bianca Maria Scicchitano
- Department of Life Sciences and Public Health, Histology and Embryology Unit, Fondazione Policlinico Universitario A. Gemelli IRCCS, 00168 Rome, Italy; (A.B.); (G.S.)
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Fuertes-Alvarez S, Izeta A. Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction. Aging Dis 2021; 12:494-514. [PMID: 33815879 PMCID: PMC7990373 DOI: 10.14336/ad.2020.0708] [Citation(s) in RCA: 22] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2020] [Accepted: 07/08/2020] [Indexed: 12/11/2022] Open
Abstract
Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.
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Affiliation(s)
- Sandra Fuertes-Alvarez
- 1Biodonostia, Tissue Engineering Group, Paseo Dr. Begiristain, s/n, San Sebastian 20014, Spain
| | - Ander Izeta
- 1Biodonostia, Tissue Engineering Group, Paseo Dr. Begiristain, s/n, San Sebastian 20014, Spain.,2Tecnun-University of Navarra, School of Engineering, Department of Biomedical Engineering and Science, Paseo Mikeletegi, 48, San Sebastian 20009, Spain
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Zhao Y, Peng HB. Roles of tyrosine kinases and phosphatases in the formation and dispersal of acetylcholine receptor clusters. Neurosci Lett 2020; 733:135054. [PMID: 32428606 DOI: 10.1016/j.neulet.2020.135054] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2020] [Revised: 05/09/2020] [Accepted: 05/12/2020] [Indexed: 10/24/2022]
Abstract
The formation of acetylcholine receptor (AChR) clusters at the postsynaptic muscle membrane in response to motor innervation is a key event in the development of the neuromuscular junction. The synaptic AChR clustering process is initiated by motor axon-released agrin, which activates a tyrosine kinase-based signaling pathway to cause AChR aggregation. In cultured muscle cells, AChR clustering is elicited by diverse nonneural signals, and this process is also mediated by tyrosine kinases. Conversely, the formation of new AChR clusters induced by innervation or nonneural stimuli is unfailingly associated with the dispersal of pre-existing AChR clusters, and this process is mediated by tyrosine phosphatases. In this review, we address how local kinase activation leads to global phosphatase action in muscle. More specifically, we discuss the roles of Src kinase and the SH2 domain-containing tyrosine phosphatase Shp-2 in establishing a regenerative mechanism to propagate the AChR cluster dispersing signal extrasynaptically and in defining the boundary of cluster formation subsynaptically.
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Affiliation(s)
- Yang Zhao
- Division of Life Science, the Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Special Administrative Region.
| | - H Benjamin Peng
- Division of Life Science, the Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Special Administrative Region; College of Life Science, National Tsing Hua University, Hsinchu, Taiwan, ROC.
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Oentaryo MJ, Tse ACK, Lee CW. Neuronal MT1-MMP mediates ECM clearance and Lrp4 cleavage for agrin deposition and signaling in presynaptic development. J Cell Sci 2020; 133:jcs246710. [PMID: 32591486 DOI: 10.1242/jcs.246710] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2020] [Accepted: 06/16/2020] [Indexed: 08/31/2023] Open
Abstract
Agrin is a crucial factor that induces postsynaptic differentiation at neuromuscular junctions (NMJs), but how secreted agrin is locally deposited in the context of extracellular matrix (ECM) environment and its function in presynaptic differentiation remain largely unclear. Here, we report that the proteolytic activity of neuronal membrane-type 1 matrix metalloproteinase (MT1-MMP; also known as MMP14) facilitates agrin deposition and signaling during presynaptic development at NMJs. Firstly, agrin deposition along axons exhibits a time-dependent increase in cultured neurons that requires MMP-mediated focal ECM degradation. Next, local agrin stimulation induces the clustering of mitochondria and synaptic vesicles, two well-known presynaptic markers, and regulates vesicular trafficking and surface insertion of MT1-MMP. MMP inhibitor or MT1-MMP knockdown suppresses agrin-induced presynaptic differentiation, which can be rescued by treatment with the ectodomain of low-density lipoprotein receptor-related protein 4 (Lrp4). Finally, neuronal MT1-MMP knockdown inhibits agrin deposition and nerve-induced acetylcholine receptor clustering in nerve-muscle co-cultures and affects synaptic structures at Xenopus NMJs in vivo Collectively, our results demonstrate a previously unappreciated role of agrin, as well as dual functions of neuronal MT1-MMP proteolytic activity in orchestrating agrin deposition and signaling, in presynaptic development.
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Affiliation(s)
- Marilyn Janice Oentaryo
- School of Biomedical Sciences, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong
| | - Anna Chung-Kwan Tse
- School of Biomedical Sciences, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong
| | - Chi Wai Lee
- School of Biomedical Sciences, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong
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Takamori M. Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability. Front Mol Neurosci 2020; 13:86. [PMID: 32547365 PMCID: PMC7272578 DOI: 10.3389/fnmol.2020.00086] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2020] [Accepted: 04/28/2020] [Indexed: 12/18/2022] Open
Abstract
Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca2+ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.
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Ohkawara B, Shen X, Selcen D, Nazim M, Bril V, Tarnopolsky MA, Brady L, Fukami S, Amato AA, Yis U, Ohno K, Engel AG. Congenital myasthenic syndrome-associated agrin variants affect clustering of acetylcholine receptors in a domain-specific manner. JCI Insight 2020; 5:132023. [PMID: 32271162 DOI: 10.1172/jci.insight.132023] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2019] [Accepted: 02/28/2020] [Indexed: 12/17/2022] Open
Abstract
Congenital myasthenic syndromes (CMS) are caused by mutations in molecules expressed at the neuromuscular junction. We report clinical, structural, ultrastructural, and electrophysiologic features of 4 CMS patients with 6 heteroallelic variants in AGRN, encoding agrin. One was a 7.9-kb deletion involving the N-terminal laminin-binding domain. Another, c.4744G>A - at the last nucleotide of exon 26 - caused skipping of exon 26. Four missense mutations (p.S1180L, p.R1509W, p.G1675S, and p.Y1877D) expressed in conditioned media decreased AChR clusters in C2C12 myotubes. The agrin-enhanced phosphorylation of MuSK was markedly attenuated by p.Y1877D in the LG3 domain and moderately attenuated by p.R1509W in the LG1 domain but not by the other 2 mutations. The p.S1180L mutation in the SEA domain facilitated degradation of secreted agrin. The p.G1675S mutation in the LG2 domain attenuated anchoring of agrin to the sarcolemma by compromising its binding to heparin. Anchoring of agrin with p.R1509W in the LG1 domain was similarly attenuated. Mutations of agrin affect AChR clustering by enhancing agrin degradation or by suppressing MuSK phosphorylation and/or by compromising anchoring of agrin to the sarcolemma of the neuromuscular junction.
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Affiliation(s)
- Bisei Ohkawara
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - XinMing Shen
- Department of Neurology, Mayo Clinic, Rochester, Minnesota, USA
| | - Duygu Selcen
- Department of Neurology, Mayo Clinic, Rochester, Minnesota, USA
| | - Mohammad Nazim
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Vera Bril
- Vera Bril, Department of Neurology, University of Toronto, Toronto, Canada
| | - Mark A Tarnopolsky
- Department of Pediatrics, McMaster University Medical Center, Hamilton, Ontario, Canada
| | - Lauren Brady
- Department of Pediatrics, McMaster University Medical Center, Hamilton, Ontario, Canada
| | - Sae Fukami
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Anthony A Amato
- Department of Neurology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA
| | - Uluc Yis
- Division of Child Neurology, Department of Pediatrics, School of Medicine, Dokuz Eylul University, Izmir, Turkey
| | - Kinji Ohno
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Andrew G Engel
- Department of Neurology, Mayo Clinic, Rochester, Minnesota, USA
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Guarino SR, Canciani A, Forneris F. Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers. Front Mol Biosci 2020; 6:156. [PMID: 31998752 PMCID: PMC6966886 DOI: 10.3389/fmolb.2019.00156] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2019] [Accepted: 12/13/2019] [Indexed: 12/20/2022] Open
Abstract
Synapse formation is a very elaborate process dependent upon accurate coordination of pre and post-synaptic specialization, requiring multiple steps and a variety of receptors and signaling molecules. Due to its relative structural simplicity and the ease in manipulation and observation, the neuromuscular synapse or neuromuscular junction (NMJ)-the connection between motor neurons and skeletal muscle-represents the archetype junction system for studying synapse formation and conservation. This junction is essential for survival, as it controls our ability to move and breath. NMJ formation requires coordinated interactions between motor neurons and muscle fibers, which ultimately result in the formation of a highly specialized post-synaptic architecture and a highly differentiated nerve terminal. Furthermore, to ensure a fast and reliable synaptic transmission following neurotransmitter release, ligand-gated channels (acetylcholine receptors, AChRs) are clustered on the post-synaptic muscle cell at high concentrations in sites opposite the presynaptic active zone, supporting a direct role for nerves in the organization of the post-synaptic membrane architecture. This organized clustering process, essential for NMJ formation and for life, relies on key signaling molecules and receptors and is regulated by soluble extracellular molecules localized within the synaptic cleft. Notably, several mutations as well as auto-antibodies against components of these signaling complexes have been related to neuromuscular disorders. The recent years have witnessed strong progress in the understanding of molecular identities, architectures, and functions of NMJ macromolecules. Among these, prominent roles have been proposed for neural variants of the proteoglycan agrin, its receptor at NMJs composed of the lipoprotein receptor-related protein 4 (LRP4) and the muscle-specific kinase (MuSK), as well as the regulatory soluble synapse-specific protease Neurotrypsin. In this review we summarize the current state of the art regarding molecular structures and (agrin-dependent) canonical, as well as (agrin-independent) non-canonical, MuSK signaling mechanisms that underscore the formation of neuromuscular junctions, with the aim of providing a broad perspective to further stimulate molecular, cellular and tissue biology investigations on this fundamental intercellular contact.
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Affiliation(s)
| | | | - Federico Forneris
- The Armenise-Harvard Laboratory of Structural Biology, Department Biology and Biotechnology, University of Pavia, Pavia, Italy
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13
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Handara G, Kröger S. Alternative Splicing and the Intracellular Domain Mediate TM-agrin's Ability to Differentially Regulate the Density of Excitatory and Inhibitory Synapse-like Specializations in Developing CNS Neurons. Neuroscience 2019; 419:60-71. [PMID: 31672640 DOI: 10.1016/j.neuroscience.2019.09.011] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2019] [Revised: 09/09/2019] [Accepted: 09/10/2019] [Indexed: 01/26/2023]
Abstract
Agrin is a multi-domain protein best known for its essential function during formation of the neuromuscular junction. Alternative mRNA splicing at sites named y and z in the C-terminal part of agrin regulates its interaction with a receptor complex consisting of the agrin-binding low-density lipoprotein receptor-related protein 4 (Lrp4) and the muscle-specific kinase (MuSK). Isoforms with inserts at both splice sites bind to Lrp4, activate MuSK and are synaptogenic at the neuromuscular junction. Agrin is also expressed as a transmembrane protein in the central nervous system (CNS) but its function during interneuronal synapse formation is unclear. Recently we demonstrated that transfection of a full-length cDNA coding for transmembrane agrin (TM-agrin) in cultured embryonic cortical neurons induced an Lrp4-dependent but MuSK-independent increase in dendritic glutamatergic synapses and an Lrp4- and MuSK-independent reduction of inhibitory synapses. Here we show that presynaptic specializations were similarly affected by TM-agrin overexpression. In addition, we mapped the regions within TM-agrin responsible for TM-agrin's effects on dendritic aggregates of synapse-associated proteins. We show that the presence of a four amino acid insert at splice site y is essential for the increase in the density of puncta containing the postsynaptic density protein 95 kDa. This effect was independent of splice site z. The reduction of the gephyrin puncta density was independent of the entire extracellular part of TM-agrin but required a highly conserved serine residue in the intracellular domain of TM-agrin. These results provide further evidence for a function of TM-agrin during CNS synaptogenesis and demonstrate that different domains and alternative splicing of TM-agrin differentially affect excitatory and inhibitory synapse formation in cultured embryonic CNS neurons.
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Affiliation(s)
- Gerry Handara
- Department of Physiological Genomics, Biomedical Center, Ludwig-Maximilians-University, Großhaderner Str. 9, D-82152 Planegg-Martinsried, Germany; Institute for Stem Cell Research, German Research Center for Environmental Health, Helmholtz Centre Munich, Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany
| | - Stephan Kröger
- Department of Physiological Genomics, Biomedical Center, Ludwig-Maximilians-University, Großhaderner Str. 9, D-82152 Planegg-Martinsried, Germany.
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14
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Eiber N, Rehman M, Kravic B, Rudolf R, Sandri M, Hashemolhosseini S. Loss of Protein Kinase Csnk2b/CK2β at Neuromuscular Junctions Affects Morphology and Dynamics of Aggregated Nicotinic Acetylcholine Receptors, Neuromuscular Transmission, and Synaptic Gene Expression. Cells 2019; 8:cells8080940. [PMID: 31434353 PMCID: PMC6721821 DOI: 10.3390/cells8080940] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2019] [Revised: 07/21/2019] [Accepted: 08/12/2019] [Indexed: 12/19/2022] Open
Abstract
The protein kinase Csnk2/CK2 is important for cell proliferation, differentiation, and survival. Previously, we showed that CK2 binds distinctive proteins at neuromuscular junctions (NMJs) of mice and phosphorylates some of them. CK2 likely stabilizes clustered nicotinic acetylcholine receptors (AChRs). In the absence of the β-subunit of CK2 in skeletal muscle fibers, mice develop an age-dependent decrease of grip strength accompanied by NMJ fragmentation and impairments of neuromuscular transmission. However, the precise role of CK2β regarding the clustering of AChRs and downstream signaling at NMJs is unknown. Here, we compared conditional CK2β-deficient mice with controls and found in the mutants (1) a lower decrement of endplate potentials after repetitive stimulation and decrements of nerve-evoked compound muscle action potentials decayed more rapidly after synaptic transmission was partially blocked, (2) that their muscle weakness was partially rescued by administration of an acetylcholine esterase inhibitor, (3) fragmented NMJs and impaired AChR clustering was detected in muscles and cultured muscle cells, (4) enlarged myonuclei, (5) impaired synaptic gene expression, and (6) a high turnover rate of their AChR clusters in vivo. Altogether, our data demonstrate a role for CK2 at the NMJ by maintaining a high density of AChRs and ensuring physiological synaptic gene expression.
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Affiliation(s)
- Nane Eiber
- Institute of Biochemistry, Medical Faculty, Friedrich-Alexander-University of Erlangen-Nürnberg, |91054 Erlangen, Germany
| | - Michael Rehman
- Institute of Biochemistry, Medical Faculty, Friedrich-Alexander-University of Erlangen-Nürnberg, |91054 Erlangen, Germany
- Weill Cornell Medical College, Department of Medicine, New York, NY 10065, USA
| | - Bojana Kravic
- Institute of Biochemistry, Medical Faculty, Friedrich-Alexander-University of Erlangen-Nürnberg, |91054 Erlangen, Germany
- Faculty of Biology, University of Duisburg-Essen, 45141 Essen, Germany
| | - Rüdiger Rudolf
- Institute of molecular- and cellular biology, University of Applied Sciences Mannheim, |68163 Mannheim, Germany
| | - Marco Sandri
- Department of Biomedical Science, University of Padova, 35122 Padova, Italy
| | - Said Hashemolhosseini
- Institute of Biochemistry, Medical Faculty, Friedrich-Alexander-University of Erlangen-Nürnberg, |91054 Erlangen, Germany.
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15
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Rudell JB, Maselli RA, Yarov-Yarovoy V, Ferns MJ. Pathogenic effects of agrin V1727F mutation are isoform specific and decrease its expression and affinity for HSPGs and LRP4. Hum Mol Genet 2019; 28:2648-2658. [PMID: 30994901 PMCID: PMC6687949 DOI: 10.1093/hmg/ddz081] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2019] [Revised: 03/27/2019] [Accepted: 04/15/2019] [Indexed: 12/17/2022] Open
Abstract
Agrin is a large extracellular matrix protein whose isoforms differ in their tissue distribution and function. Motoneuron-derived y+z+ agrin regulates the formation of the neuromuscular junction (NMJ), while y-z- agrin is widely expressed and has diverse functions. Previously we identified a missense mutation (V1727F) in the second laminin globular (LG2) domain of agrin that causes severe congenital myasthenic syndrome. Here, we define pathogenic effects of the agrin V1727F mutation that account for the profound dysfunction of the NMJ. First, by expressing agrin variants in heterologous cells, we show that the V1727F mutation reduces the secretion of y+z+ agrin compared to wild type, whereas it has no effect on the secretion of y-z- agrin. Second, we find that the V1727F mutation significantly impairs binding of y+z+ agrin to both heparin and the low-density lipoprotein receptor-related protein 4 (LRP4) coreceptor. Third, molecular modeling of the LG2 domain suggests that the V1727F mutation primarily disrupts the y splice insert, and consistent with this we find that it partially occludes the contribution of the y splice insert to agrin binding to heparin and LRP4. Together, these findings identify several pathogenic effects of the V1727F mutation that reduce its expression and ability to bind heparan sulfate proteoglycan and LRP4 coreceptors involved in the muscle-specific kinase signaling pathway. These defects primarily impair the function of neural y+z+ agrin and combine to cause a severe CMS phenotype, whereas y-z- agrin function in other tissues appears preserved.
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Affiliation(s)
- John B Rudell
- Department of Physiology and Membrane Biology, University of California Davis, Davis, CA, USA
| | - Ricardo A Maselli
- Department of Neurology, University of California Davis, Davis, CA, USA
| | - Vladimir Yarov-Yarovoy
- Department of Physiology and Membrane Biology, University of California Davis, Davis, CA, USA
| | - Michael J Ferns
- Department of Physiology and Membrane Biology, University of California Davis, Davis, CA, USA
- Department of Anesthesiology and Pain Medicine, University of California Davis, Davis, CA, USA
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16
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Huang ML, Tota EM, Lucas TM, Godula K. Influencing Early Stages of Neuromuscular Junction Formation through Glycocalyx Engineering. ACS Chem Neurosci 2018; 9:3086-3093. [PMID: 30095249 PMCID: PMC6395550 DOI: 10.1021/acschemneuro.8b00295] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Achieving molecular control over the formation of synaptic contacts in the nervous system can provide important insights into their regulation and can offer means for creating well-defined in vitro systems to evaluate modes of therapeutic intervention. Agrin-induced clustering of acetylcholine receptors (AChRs) at postsynaptic sites is a hallmark of the formation of the neuromuscular junction, a synapse between motoneurons and muscle cells. In addition to the cognate agrin receptor LRP4 (low-density lipoprotein receptor related protein-4), muscle cell heparan sulfate (HS) glycosaminoglycans (GAGs) have also been proposed to contribute to AChR clustering by acting as agrin co-receptors. Here, we provide direct evidence for the role of HS GAGs in agrin recruitment to the surface of myotubes, as well as their functional contributions toward AChR clustering. We also demonstrate that engineering of the myotube glycocalyx using synthetic HS GAG polymers can replace native HS structures to gain control over agrin-mediated AChR clustering.
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Affiliation(s)
| | - Ember M. Tota
- Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
| | - Taryn M. Lucas
- Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
| | - Kamil Godula
- Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
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17
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Reinhard JR, Lin S, McKee KK, Meinen S, Crosson SC, Sury M, Hobbs S, Maier G, Yurchenco PD, Rüegg MA. Linker proteins restore basement membrane and correct LAMA2-related muscular dystrophy in mice. Sci Transl Med 2018; 9:9/396/eaal4649. [PMID: 28659438 DOI: 10.1126/scitranslmed.aal4649] [Citation(s) in RCA: 49] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2016] [Revised: 03/03/2017] [Accepted: 05/04/2017] [Indexed: 12/12/2022]
Abstract
LAMA2-related muscular dystrophy (LAMA2 MD or MDC1A) is the most frequent form of early-onset, fatal congenital muscular dystrophies. It is caused by mutations in LAMA2, the gene encoding laminin-α2, the long arm of the heterotrimeric (α2, β1, and γ1) basement membrane protein laminin-211 (Lm-211). We establish that despite compensatory expression of laminin-α4, giving rise to Lm-411 (α4, β1, and γ1), muscle basement membrane is labile in LAMA2 MD biopsies. Consistent with this deficit, recombinant Lm-411 polymerized and bound to cultured myotubes only weakly. Polymerization and cell binding of Lm-411 were enhanced by addition of two specifically designed linker proteins. One, called αLNNd, consists of the N-terminal part of laminin-α1 and the laminin-binding site of nidogen-1. The second, called mini-agrin (mag), contains binding sites for laminins and α-dystroglycan. Transgenic expression of mag and αLNNd in a mouse model for LAMA2 MD fully restored basement membrane stability, recovered muscle force and size, increased overall body weight, and extended life span more than five times to a maximum survival beyond 2 years. These findings provide a mechanistic understanding of LAMA2 MD and establish a strong basis for a potential treatment.
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Affiliation(s)
| | - Shuo Lin
- Biozentrum, University of Basel, 4056 Basel, Switzerland
| | - Karen K McKee
- Robert Wood Johnson Medical School, Rutgers University, Piscataway, NJ 08854, USA
| | - Sarina Meinen
- Biozentrum, University of Basel, 4056 Basel, Switzerland
| | - Stephanie C Crosson
- Robert Wood Johnson Medical School, Rutgers University, Piscataway, NJ 08854, USA
| | - Maurizio Sury
- Biozentrum, University of Basel, 4056 Basel, Switzerland
| | - Samantha Hobbs
- Robert Wood Johnson Medical School, Rutgers University, Piscataway, NJ 08854, USA
| | | | - Peter D Yurchenco
- Robert Wood Johnson Medical School, Rutgers University, Piscataway, NJ 08854, USA
| | - Markus A Rüegg
- Biozentrum, University of Basel, 4056 Basel, Switzerland.
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18
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Xi J, Yan C, Liu WW, Qiao K, Lin J, Tian X, Wu H, Lu J, Wong LJ, Beeson D, Zhao C. Novel SEA and LG2 Agrin mutations causing congenital Myasthenic syndrome. Orphanet J Rare Dis 2017; 12:182. [PMID: 29258548 PMCID: PMC5735900 DOI: 10.1186/s13023-017-0732-z] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2017] [Accepted: 12/04/2017] [Indexed: 11/17/2022] Open
Abstract
Background Congenital myasthenic syndrome caused by mutations in AGRN, a gene encoding a protein with a crucial function at the neuromuscular junction, is a rare disorder. There are few studies in this area. We here present two cases with novel mutations of AGRN of which we further investigated possible pathogenesis. Results Patient 1 had general limb weakness with fluctuation and deterioration in the afternoon and in hot weather. Patient 2 had early-onset weakness of lower extremities with suspected fluctuation in the early stages, which then progressed to the upper limbs. Both distal and proximal muscles were involved. Repetitive stimulation on EMG in both patients showed decrement in proximal and distal limbs. Patient 2 showed a marked response to salbutamol while Patient 1 did not. By targeted exome sequencing, two novel homozygous missense variants, p.L1176P and p.R1698C, in the SEA and LG2 domain of agrin were identified respectively. Further functional analysis revealed instability of the protein and impaired clustering of the acetylcholine receptor (AChR) by both mutations. Conclusions The mutations identified in AGRN in our study may cause congenital myasthenic syndrome by damaging protein stability and interfering with AChR clustering. These results broaden the understandings on the phenotype, genotype and pathogenesis of this rare disorder. Electronic supplementary material The online version of this article (10.1186/s13023-017-0732-z) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Jianying Xi
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China
| | - Chong Yan
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China
| | - Wei-Wei Liu
- Neurosciences Group, Nuffield Department of Clinical Neurosciences, Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, OX3 9DS, UK
| | - Kai Qiao
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China
| | - Jie Lin
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China
| | - Xia Tian
- Baylor Genetics Laboratories, Houston, Texas, USA
| | - Hui Wu
- Department of Neurology, Jing'an District Center Hospital of Shanghai, Shanghai, China
| | - Jiahong Lu
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China
| | - Lee-Jun Wong
- Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, USA
| | - David Beeson
- Neurosciences Group, Nuffield Department of Clinical Neurosciences, Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, OX3 9DS, UK.
| | - Chongbo Zhao
- Department of Neurology, Huashan Hospital, Fudan University, Shanghai, China.,Department of Neurology, Jing'an District Center Hospital of Shanghai, Shanghai, China
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19
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Kim JK, Caine C, Awano T, Herbst R, Monani UR. Motor neuronal repletion of the NMJ organizer, Agrin, modulates the severity of the spinal muscular atrophy disease phenotype in model mice. Hum Mol Genet 2017; 26:2377-2385. [PMID: 28379354 DOI: 10.1093/hmg/ddx124] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2016] [Accepted: 03/23/2017] [Indexed: 11/14/2022] Open
Abstract
Spinal muscular atrophy (SMA) is a common and often fatal neuromuscular disorder caused by low levels of the Survival Motor Neuron (SMN) protein. Amongst the earliest detectable consequences of SMN deficiency are profound defects of the neuromuscular junctions (NMJs). In model mice these synapses appear disorganized, fail to mature and are characterized by poorly arborized nerve terminals. Given one role of the SMN protein in orchestrating the assembly of spliceosomal snRNP particles and subsequently regulating the alternative splicing of pre-mRNAs, a plausible link between SMN function and the distal neuromuscular SMA phenotype is an incorrectly spliced transcript or transcripts involved in establishing or maintaining NMJ structure. In this study, we explore the effects of one such transcript-Z+Agrin-known to be a critical organizer of the NMJ. We confirm that low SMN protein reduces motor neuronal levels of Z+Agrin. Repletion of this isoform of Agrin in the motor neurons of SMA model mice increases muscle fiber size, enhances the post-synaptic NMJ area, reduces the abnormal accumulation of intermediate filaments in nerve terminals of the neuromuscular synapse and improves the innervation of muscles. While these effects are independent of changes in SMN levels or increases in motor neuron numbers they nevertheless have a significant effect on the overall disease phenotype, enhancing mean survival in severely affected SMA model mice by ∼40%. We conclude that Agrin is an important target of the SMN protein and that mitigating NMJ defects may be one strategy in treating human spinal muscular atrophy.
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Affiliation(s)
- Jeong-Ki Kim
- Department of Pathology and Cell Biology.,Center for Motor Neuron Biology and Disease, Columbia University Medical Center, New York, NY 10032, USA
| | - Charlotte Caine
- Department of Pathology and Cell Biology.,Center for Motor Neuron Biology and Disease, Columbia University Medical Center, New York, NY 10032, USA
| | - Tomoyuki Awano
- Department of Pathology and Cell Biology.,Center for Motor Neuron Biology and Disease, Columbia University Medical Center, New York, NY 10032, USA
| | - Ruth Herbst
- Center for Brain Research.,Institute of Immunology, Medical University of Vienna, 1090 Vienna
| | - Umrao R Monani
- Department of Pathology and Cell Biology.,Center for Motor Neuron Biology and Disease, Columbia University Medical Center, New York, NY 10032, USA.,Department of Neurology, Columbia University Medical Center, New York, NY 10032, USA
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20
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Mis K, Grubic Z, Lorenzon P, Sciancalepore M, Mars T, Pirkmajer S. In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle. Molecules 2017; 22:molecules22091418. [PMID: 28846617 PMCID: PMC6151842 DOI: 10.3390/molecules22091418] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2017] [Revised: 08/18/2017] [Accepted: 08/23/2017] [Indexed: 12/19/2022] Open
Abstract
Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.
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Affiliation(s)
- Katarina Mis
- Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia.
| | - Zoran Grubic
- Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia.
| | - Paola Lorenzon
- Department of Life Sciences, University of Trieste, via A. Fleming 22, I-34127 Trieste, Italy.
| | - Marina Sciancalepore
- Department of Life Sciences, University of Trieste, via A. Fleming 22, I-34127 Trieste, Italy.
| | - Tomaz Mars
- Department of Life Sciences, University of Trieste, via A. Fleming 22, I-34127 Trieste, Italy.
| | - Sergej Pirkmajer
- Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia.
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21
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Takamori M. Synaptic Homeostasis and Its Immunological Disturbance in Neuromuscular Junction Disorders. Int J Mol Sci 2017; 18:ijms18040896. [PMID: 28441759 PMCID: PMC5412475 DOI: 10.3390/ijms18040896] [Citation(s) in RCA: 13] [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] [Received: 02/21/2017] [Revised: 04/04/2017] [Accepted: 04/19/2017] [Indexed: 12/14/2022] Open
Abstract
In the neuromuscular junction, postsynaptic nicotinic acetylcholine receptor (nAChR) clustering, trans-synaptic communication and synaptic stabilization are modulated by the molecular mechanisms underlying synaptic plasticity. The synaptic functions are based presynaptically on the active zone architecture, synaptic vesicle proteins, Ca2+ channels and synaptic vesicle recycling. Postsynaptically, they are based on rapsyn-anchored nAChR clusters, localized sensitivity to ACh, and synaptic stabilization via linkage to the extracellular matrix so as to be precisely opposed to the nerve terminal. Focusing on neural agrin, Wnts, muscle-specific tyrosine kinase (a mediator of agrin and Wnts signalings and regulator of trans-synaptic communication), low-density lipoprotein receptor-related protein 4 (the receptor of agrin and Wnts and participant in retrograde signaling), laminin-network (including muscle-derived agrin), extracellular matrix proteins (participating in the synaptic stabilization) and presynaptic receptors (including muscarinic and adenosine receptors), we review the functional structures of the synapse by making reference to immunological pathogenecities in postsynaptic disease, myasthenia gravis. The synapse-related proteins including cortactin, coronin-6, caveolin-3, doublecortin, R-spondin 2, amyloid precursor family proteins, glia cell-derived neurotrophic factor and neurexins are also discussed in terms of their possible contribution to efficient synaptic transmission at the neuromuscular junction.
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Affiliation(s)
- Masaharu Takamori
- Neurological Center, Kanazawa-Nishi Hospital, Kanazawa, Ishikawa 920-0025, Japan.
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22
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Dietary supplementation with bovine-derived milk fat globule membrane lipids promotes neuromuscular development in growing rats. Nutr Metab (Lond) 2017; 14:9. [PMID: 28127382 PMCID: PMC5259894 DOI: 10.1186/s12986-017-0161-y] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2016] [Accepted: 01/15/2017] [Indexed: 12/11/2022] Open
Abstract
Background The milk fat globule membrane (MFGM) is primarily composed of polar phospho- and sphingolipids, which have established biological effects on neuroplasticity. The present study aimed to investigate the effect of dietary MFGM supplementation on the neuromuscular system during post-natal development. Methods Growing rats received dietary supplementation with bovine-derived MFGM mixtures consisting of complex milk lipids (CML), beta serum concentrate (BSC) or a complex milk lipid concentrate (CMLc) (which lacks MFGM proteins) from post-natal day 10 to day 70. Results Supplementation with MFGM mixtures enriched in polar lipids (BSC and CMLc, but not CML) increased the plasma phosphatidylcholine (PC) concentration, with no effect on plasma phosphatidylinositol (PI), phosphatidylethanolamine (PE), phosphatidylserine (PS) or sphingomyelin (SM). In contrast, muscle PC was reduced in rats receiving supplementation with both BSC and CMLc, whereas muscle PI, PE, PS and SM remained unchanged. Rats receiving BSC and CMLc (but not CML) displayed a slow-to-fast muscle fibre type profile shift (MyHCI → MyHCIIa) that was associated with elevated expression of genes involved in myogenic differentiation (myogenic regulatory factors) and relatively fast fibre type specialisation (Myh2 and Nfatc4). Expression of neuromuscular development genes, including nerve cell markers, components of the synaptogenic agrin–LRP4 pathway and acetylcholine receptor subunits, was also increased in muscle of rats supplemented with BSC and CMLc (but not CML). Conclusions These findings demonstrate that dietary supplementation with bovine-derived MFGM mixtures enriched in polar lipids can promote neuromuscular development during post-natal growth in rats, leading to shifts in adult muscle phenotype. Electronic supplementary material The online version of this article (doi:10.1186/s12986-017-0161-y) contains supplementary material, which is available to authorized users.
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23
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Abstract
Alternative precursor-mRNA splicing is a key mechanism for regulating gene expression in mammals and is controlled by specialized RNA-binding proteins. The misregulation of splicing is implicated in multiple neurological disorders. We describe recent mouse genetic studies of alternative splicing that reveal its critical role in both neuronal development and the function of mature neurons. We discuss the challenges in understanding the extensive genetic programmes controlled by proteins that regulate splicing, both during development and in the adult brain.
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Affiliation(s)
- Celine K Vuong
- Molecular Biology Interdepartmental Graduate Program, University of California at Los Angeles, Los Angeles, California 90095, USA
| | - Douglas L Black
- Department of Microbiology, Immunology, and Molecular Genetics, University of California at Los Angeles, Los Angeles, California 90095, USA
| | - Sika Zheng
- Division of Biomedical Sciences, University of California at Riverside, Riverside, California 92521, USA
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24
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Kirsch J, Kröger S. ■ REVIEW : Postsynaptic Anchoring of Receptors: A Cellular Approach to Neuronal and Muscular Sensitivity. Neuroscientist 2016. [DOI: 10.1177/107385849600200211] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Significant progress has been made toward the elucidation of the molecular mechanisms underlying the biogenesis and stabilization of postsynaptic membrane specializations at the neuromuscular junction of vertebrate skeletal muscle. The emerging picture reveals a continuous molecular link from the extracellular matrix within the synaptic cleft via integral and peripheral membrane proteins to the subsarcolemmal cytoskeleton. The formation and maintenance of synaptic contacts between neurons in the CNS might follow similar architectural principles but involve different molecules. The biogenesis of glycinergic postsynaptic membrane specializations depends on the widely expressed peripheral membrane protein gephyrin, which anchors the neurotransmitter receptor to underlying cytoskeletal elements in a dynamic manner. This anchoring mechanism could also contribute to the plasticity of glycinergic synapses. Other types of neurotransmitter receptors, like GABAA- and glutamate receptors, may have evolved different molecular mechanisms to ensure their localization in postsynaptic membrane specializations. The Neuroscientist 2:100-108, 1996
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Affiliation(s)
- Joachim Kirsch
- Department of Morphology Johann Wolfgang Goethe-University Frankfurt, Federal Republic of Germany, Department of Neurochemistry
| | - Stephan Kröger
- Department of Neuroanatomy Max-Planck-Institute for Brain Research Frankfurt, Germany
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25
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Shen C, Xiong WC, Mei L. LRP4 in neuromuscular junction and bone development and diseases. Bone 2015; 80:101-108. [PMID: 26071838 DOI: 10.1016/j.bone.2015.05.012] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/28/2014] [Revised: 05/04/2015] [Accepted: 05/08/2015] [Indexed: 02/08/2023]
Abstract
Low-density lipoprotein receptor-related protein 4 (LRP4) is a member of the low-density lipoprotein receptor (LDLR) family. Recent studies have revealed multiple functions and complex signaling mechanisms of LRP4 in different organs and tissues. LPR4 mutation or malfunction has been implicated in neurological disorders including congenital myasthenic syndrome, myasthenia gravis, and diseases of bone or kidney. This article is part of a Special Issue entitled "Muscle Bone Interactions".
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Affiliation(s)
- Chengyong Shen
- Department of Neuroscience and Regenerative Medicine, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA; Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA
| | - Wen-Cheng Xiong
- Department of Neuroscience and Regenerative Medicine, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA; Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA
| | - Lin Mei
- Department of Neuroscience and Regenerative Medicine, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA; Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, GA 30912, USA.
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26
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Eldridge S, Nalesso G, Ismail H, Vicente-Greco K, Kabouridis P, Ramachandran M, Niemeier A, Herz J, Pitzalis C, Perretti M, Dell'Accio F. Agrin mediates chondrocyte homeostasis and requires both LRP4 and α-dystroglycan to enhance cartilage formation in vitro and in vivo. Ann Rheum Dis 2015; 75:1228-35. [PMID: 26290588 PMCID: PMC4760904 DOI: 10.1136/annrheumdis-2015-207316] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2015] [Accepted: 07/14/2015] [Indexed: 12/31/2022]
Abstract
Objectives Osteoarthritis (OA) is a leading cause of disability for which there is no cure. The identification of molecules supporting cartilage homeostasis and regeneration is therefore a major pursuit in musculoskeletal medicine. Agrin is a heparan sulfate proteoglycan which, through binding to low-density lipoprotein receptor-related protein 4 (LRP4), is required for neuromuscular synapse formation. In other tissues, it connects the cytoskeleton to the basement membrane through binding to α-dystroglycan. Prompted by an unexpected expression pattern, we investigated the role and receptor usage of agrin in cartilage. Methods Agrin expression pattern was investigated in human osteoarthritic cartilage and following destabilisation of the medial meniscus in mice. Extracellular matrix (ECM) formation and chondrocyte differentiation was studied in gain and loss of function experiments in vitro in three-dimensional cultures and gain of function in vivo, using an ectopic cartilage formation assay in nude mice. Receptor usage was investigated by disrupting LRP4 and α-dystroglycan by siRNA and blocking antibodies respectively. Results Agrin was detected in normal cartilage but was progressively lost in OA. In vitro, agrin knockdown resulted in reduced glycosaminoglycan content, downregulation of the cartilage transcription factor SOX9 and other cartilage-specific ECM molecules. Conversely, exogenous agrin supported cartilage differentiation in vitro and ectopic cartilage formation in vivo. In the context of cartilage differentiation, agrin used an unusual receptor repertoire requiring both LRP4 and α-dystroglycan. Conclusions We have discovered that agrin strongly promotes chondrocyte differentiation and cartilage formation in vivo. Our results identify agrin as a novel potent anabolic growth factor with strong therapeutic potential in cartilage regeneration.
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Affiliation(s)
- Suzanne Eldridge
- Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK
| | - Giovanna Nalesso
- Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK
| | - Habib Ismail
- Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK
| | | | - Panos Kabouridis
- The Francis Crick Institute (Mill Hill Laboratory), The Ridgeway, London, UK
| | - Manoj Ramachandran
- Department of Trauma and Orthopaedic Surgery, Royal London Hospital, London, UK
| | - Andreas Niemeier
- Department of Orthopaedics and IBMII: Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany
| | - Joachim Herz
- Department of Molecular Genetics, Center for Translational Neurodegeneration Research, University of Texas Southwestern Medical Center, Dallas, Texas, USA
| | - Costantino Pitzalis
- Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK
| | - Mauro Perretti
- Centre for Biochemical Pharmacology, William Harvey Research Institute, London, UK
| | - Francesco Dell'Accio
- Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK
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Tintignac LA, Brenner HR, Rüegg MA. Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting. Physiol Rev 2015; 95:809-52. [DOI: 10.1152/physrev.00033.2014] [Citation(s) in RCA: 224] [Impact Index Per Article: 24.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.
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Affiliation(s)
- Lionel A. Tintignac
- Biozentrum, University of Basel, Basel, Switzerland; Department of Biomedicine, University of Basel, Basel, Switzerland; and INRA, UMR866 Dynamique Musculaire et Métabolisme, Montpellier, France
| | - Hans-Rudolf Brenner
- Biozentrum, University of Basel, Basel, Switzerland; Department of Biomedicine, University of Basel, Basel, Switzerland; and INRA, UMR866 Dynamique Musculaire et Métabolisme, Montpellier, France
| | - Markus A. Rüegg
- Biozentrum, University of Basel, Basel, Switzerland; Department of Biomedicine, University of Basel, Basel, Switzerland; and INRA, UMR866 Dynamique Musculaire et Métabolisme, Montpellier, France
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28
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Abstract
The neuromuscular junction (NMJ) is a synapse between motor neurons and skeletal muscle fibers, and is critical for control of muscle contraction. Its formation requires neuronal agrin that acts by binding to LRP4 to stimulate MuSK. Mutations have been identified in agrin, MuSK, and LRP4 in patients with congenital myasthenic syndrome, and patients with myasthenia gravis develop antibodies against agrin, LRP4, and MuSK. However, it remains unclear whether the agrin signaling pathway is critical for NMJ maintenance because null mutation of any of the three genes is perinatal lethal. In this study, we generated imKO mice, a mutant strain whose LRP4 gene can be deleted in muscles by doxycycline (Dox) treatment. Ablation of the LRP4 gene in adult muscle enabled studies of its role in NMJ maintenance. We demonstrate that Dox treatment of P30 mice reduced muscle strength and compound muscle action potentials. AChR clusters became fragmented with diminished junctional folds and synaptic vesicles. The amplitude and frequency of miniature endplate potentials were reduced, indicating impaired neuromuscular transmission and providing cellular mechanisms of adult LRP4 deficiency. We showed that LRP4 ablation led to the loss of synaptic agrin and the 90 kDa fragments, which occurred ahead of other prejunctional and postjunctional components, suggesting that LRP4 may regulate the stability of synaptic agrin. These observations demonstrate that LRP4 is essential for maintaining the structural and functional integrity of the NMJ and that loss of muscle LRP4 in adulthood alone is sufficient to cause myasthenic symptoms.
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29
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Erasso D, Tender G, Levitt RC, Cui JG. Agrin requires specific proteins to selectively activate γ-aminobutyric acid neurons for pain suppression. Exp Neurol 2014; 261:646-53. [DOI: 10.1016/j.expneurol.2014.08.014] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2014] [Revised: 08/11/2014] [Accepted: 08/14/2014] [Indexed: 01/23/2023]
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30
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Darabid H, Perez-Gonzalez AP, Robitaille R. Neuromuscular synaptogenesis: coordinating partners with multiple functions. Nat Rev Neurosci 2014; 15:630-1. [DOI: 10.1038/nrn3821] [Citation(s) in RCA: 111] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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31
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Perlecan antagonizes collagen IV and ADAMTS9/GON-1 in restricting the growth of presynaptic boutons. J Neurosci 2014; 34:10311-24. [PMID: 25080592 DOI: 10.1523/jneurosci.5128-13.2014] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
In the mature nervous system, a significant fraction of synapses are structurally stable over a long time scale. However, the mechanisms that restrict synaptic growth within a confined region are poorly understood. Here, we identified that in the C. elegans neuromuscular junction, collagens Type IV and XVIII, and the secreted metalloprotease ADAMTS/GON-1 are critical for growth restriction of presynaptic boutons. Without these components, ectopic boutons progressively invade into the nonsynaptic region. Perlecan/UNC-52 promotes the growth of ectopic boutons and functions antagonistically to collagen Type IV and GON-1 but not to collagen XVIII. The growth constraint of presynaptic boutons correlates with the integrity of the extracellular matrix basal lamina or basement membrane (BM), which surrounds chemical synapses. Fragmented BM appears in the region where ectopic boutons emerge. Further removal of UNC-52 improves the BM integrity and the tight association between BM and presynaptic boutons. Together, our results unravel the complex role of the BM in restricting the growth of presynaptic boutons and reveal the antagonistic function of perlecan on Type IV collagen and ADAMTS protein.
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32
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Steiner E, Enzmann GU, Lyck R, Lin S, Rüegg MA, Kröger S, Engelhardt B. The heparan sulfate proteoglycan agrin contributes to barrier properties of mouse brain endothelial cells by stabilizing adherens junctions. Cell Tissue Res 2014; 358:465-79. [PMID: 25107608 PMCID: PMC4210653 DOI: 10.1007/s00441-014-1969-7] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2014] [Accepted: 07/17/2014] [Indexed: 01/13/2023]
Abstract
Barrier characteristics of brain endothelial cells forming the blood–brain barrier (BBB) are tightly regulated by cellular and acellular components of the neurovascular unit. During embryogenesis, the accumulation of the heparan sulfate proteoglycan agrin in the basement membranes ensheathing brain vessels correlates with BBB maturation. In contrast, loss of agrin deposition in the vasculature of brain tumors is accompanied by the loss of endothelial junctional proteins. We therefore wondered whether agrin had a direct effect on the barrier characteristics of brain endothelial cells. Agrin increased junctional localization of vascular endothelial (VE)-cadherin, β-catenin, and zonula occludens-1 (ZO-1) but not of claudin-5 and occludin in the brain endothelioma cell line bEnd5 without affecting the expression levels of these proteins. This was accompanied by an agrin-induced reduction of the paracellular permeability of bEnd5 monolayers. In vivo, the lack of agrin also led to reduced junctional localization of VE-cadherin in brain microvascular endothelial cells. Taken together, our data support the notion that agrin contributes to barrier characteristics of brain endothelium by stabilizing the adherens junction proteins VE-cadherin and β-catenin and the junctional protein ZO-1 to brain endothelial junctions.
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Affiliation(s)
- Esther Steiner
- Theodor Kocher Institute, University of Bern, Freiestrasse 1, 3012, Bern, Switzerland
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33
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Zhang B, Shen C, Bealmear B, Ragheb S, Xiong WC, Lewis RA, Lisak RP, Mei L. Autoantibodies to agrin in myasthenia gravis patients. PLoS One 2014; 9:e91816. [PMID: 24632822 PMCID: PMC3954737 DOI: 10.1371/journal.pone.0091816] [Citation(s) in RCA: 96] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2014] [Accepted: 02/02/2014] [Indexed: 11/19/2022] Open
Abstract
To determine if patients with myasthenia gravis (MG) have antibodies to agrin, a proteoglycan released by motor neurons and is critical for neuromuscular junction (NMJ) formation, we collected serum samples from 93 patients with MG with known status of antibodies to acetylcholine receptor (AChR), muscle specific kinase (MuSK) and lipoprotein-related 4 (LRP4) and samples from control subjects (healthy individuals and individuals with other diseases). Sera were assayed for antibodies to agrin. We found antibodies to agrin in 7 serum samples of MG patients. None of the 25 healthy controls and none of the 55 control neurological patients had agrin antibodies. Two of the four triple negative MG patients (i.e., no detectable AChR, MuSK or LRP4 antibodies, AChR-/MuSK-/LRP4-) had antibodies against agrin. In addition, agrin antibodies were detected in 5 out of 83 AChR+/MuSK-/LRP4- patients but were not found in the 6 patients with MuSK antibodies (AChR-/MuSK+/LRP4-). Sera from MG patients with agrin antibodies were able to recognize recombinant agrin in conditioned media and in transfected HEK293 cells. These sera also inhibited the agrin-induced MuSK phosphorylation and AChR clustering in muscle cells. Together, these observations indicate that agrin is another autoantigen in patients with MG and agrin autoantibodies may be pathogenic through inhibition of agrin/LRP4/MuSK signaling at the NMJ.
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Affiliation(s)
- Bin Zhang
- Department of Neuroscience and Regenerative Medicine and Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, Georgia, United States of America
- Department of Physiology, Basic Medical School, Tongji Medical College, Huazhong University of Science & Technology, Wuhan, Hubei Province, P. R. China
| | - Chengyong Shen
- Department of Neuroscience and Regenerative Medicine and Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, Georgia, United States of America
- Charlie Norwood VA Medical Center, Augusta, Georgia, United States of America
| | - Beverly Bealmear
- Department of Neurology, School of Medicine, Wayne State University, Detroit, Michigan, United States of America
| | - Samia Ragheb
- Department of Neurology, School of Medicine, Wayne State University, Detroit, Michigan, United States of America
- Department of Biomedical Sciences, Oakland University William Beaumont School of Medicine, Rochester, Michigan, United States of America
| | - Wen-Cheng Xiong
- Department of Neuroscience and Regenerative Medicine and Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, Georgia, United States of America
- Charlie Norwood VA Medical Center, Augusta, Georgia, United States of America
| | - Richard A. Lewis
- Department of Neurology, School of Medicine, Wayne State University, Detroit, Michigan, United States of America
- Department of Neurology, Cedars-Sinai Medical Center, West Hollywood, California, United States of America
| | - Robert P. Lisak
- Department of Neurology, School of Medicine, Wayne State University, Detroit, Michigan, United States of America
- Department of Immunology and Microbiology, School of Medicine, Wayne State University, Detroit, Michigan, United States of America
| | - Lin Mei
- Department of Neuroscience and Regenerative Medicine and Department of Neurology, Medical College of Georgia, Georgia Regents University, Augusta, Georgia, United States of America
- Charlie Norwood VA Medical Center, Augusta, Georgia, United States of America
- * E-mail:
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Hettwer S, Lin S, Kucsera S, Haubitz M, Oliveri F, Fariello RG, Ruegg MA, Vrijbloed JW. Injection of a soluble fragment of neural agrin (NT-1654) considerably improves the muscle pathology caused by the disassembly of the neuromuscular junction. PLoS One 2014; 9:e88739. [PMID: 24520420 PMCID: PMC3919806 DOI: 10.1371/journal.pone.0088739] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2013] [Accepted: 01/10/2014] [Indexed: 01/13/2023] Open
Abstract
Treatment of neuromuscular diseases is still an unsolved problem. Evidence over the last years strongly indicates the involvement of malformation and dysfunction of neuromuscular junctions in the development of such medical conditions. Stabilization of NMJs thus seems to be a promising approach to attenuate the disease progression of muscle wasting diseases. An important pathway for the formation and maintenance of NMJs is the agrin/Lrp4/MuSK pathway. Here we demonstrate that the agrin biologic NT-1654 is capable of activating the agrin/Lrp4/MuSK system in vivo, leading to an almost full reversal of the sarcopenia-like phenotype in neurotrypsin-overexpressing (SARCO) mice. We also show that injection of NT-1654 accelerates muscle re-innervation after nerve crush. This report demonstrates that a systemically administered agrin fragment has the potential to counteract the symptoms of neuromuscular disorders.
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Affiliation(s)
| | - Shuo Lin
- Biozentrum, University of Basel, Basel, Switzerland
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35
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Abstract
Muscle-specific kinase (MuSK) is essential for each step in neuromuscular synapse formation. Before innervation, MuSK initiates postsynaptic differentiation, priming the muscle for synapse formation. Approaching motor axons recognize the primed, or prepatterned, region of muscle, causing motor axons to stop growing and differentiate into specialized nerve terminals. MuSK controls presynaptic differentiation by causing the clustering of Lrp4, which functions as a direct retrograde signal for presynaptic differentiation. Developing synapses are stabilized by neuronal Agrin, which is released by motor nerve terminals and binds to Lrp4, a member of the low-density lipoprotein receptor family, stimulating further association between Lrp4 and MuSK and increasing MuSK kinase activity. In addition, MuSK phosphorylation is stimulated by an inside-out ligand, docking protein-7 (Dok-7), which is recruited to tyrosine-phosphorylated MuSK and increases MuSK kinase activity. Mutations in MuSK and in genes that function in the MuSK signaling pathway, including Dok-7, cause congenital myasthenia, and autoantibodies to MuSK, Lrp4, and acetylcholine receptors are responsible for myasthenia gravis.
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36
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Role of skeletal muscle proteoglycans during myogenesis. Matrix Biol 2013; 32:289-97. [PMID: 23583522 DOI: 10.1016/j.matbio.2013.03.007] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2013] [Revised: 03/30/2013] [Accepted: 03/30/2013] [Indexed: 02/06/2023]
Abstract
Skeletal muscle formation during development and the adult mammal consists of a highly organised and regulated the sequence of cellular processes intending to form or repair muscle tissue. This sequence includes, cell proliferation, migration, and differentiation. Proteoglycans (PGs), macromolecules formed by a core protein and glycosaminoglycan chains (GAGs) present a great diversity of functions explained by their capacity to interact with different ligands and receptors forming part of their signalling complex and/or protecting them from proteolytic cleavage. Particularly attractive is the function of the different types of PGs present at the neuromuscular junction (NMJ). This review is focussed on the advances reached to understand the role of PGs during myogenesis and skeletal muscular dystrophies.
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37
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Cossins J, Belaya K, Zoltowska K, Koneczny I, Maxwell S, Jacobson L, Leite MI, Waters P, Vincent A, Beeson D. The search for new antigenic targets in myasthenia gravis. Ann N Y Acad Sci 2013; 1275:123-8. [PMID: 23278587 DOI: 10.1111/j.1749-6632.2012.06833.x] [Citation(s) in RCA: 58] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Around 80% of myasthenia gravis patients have antibodies against the acetylcholine receptor, and 0-60% of the remaining patients have antibodies against the muscle-specific tyrosine kinase, MuSK. Another recently identified antigen is low-density lipoprotein receptor-related protein 4 (Lrp4). To improve the existing assays and widen the search for new antigenic targets, we have employed cell-based assays in which candidate target proteins are expressed on the cell surface of transfected cells and probed with patient sera. These assays, combined with use of myotube cultures to explore the effects of the antibodies, enable us to begin to identify new antigenic targets and test antibody pathogenicity in vitro.
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Affiliation(s)
- Judith Cossins
- Nuffield Department of Clinical Neurosciences, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom.
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38
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Zong Y, Jin R. Structural mechanisms of the agrin-LRP4-MuSK signaling pathway in neuromuscular junction differentiation. Cell Mol Life Sci 2012. [PMID: 23178848 DOI: 10.1007/s00018-012-1209-9] [Citation(s) in RCA: 54] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin-LRP4-MuSK signaling pathway.
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Affiliation(s)
- Yinong Zong
- Center for Neuroscience, Aging, and Stem Cell Research, Sanford-Burnham Medical Research Institute, 10901 North Torrey Pines Road, La Jolla, CA, 92037, USA
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39
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Singhal N, Martin PT. Role of extracellular matrix proteins and their receptors in the development of the vertebrate neuromuscular junction. Dev Neurobiol 2012; 71:982-1005. [PMID: 21766463 DOI: 10.1002/dneu.20953] [Citation(s) in RCA: 101] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The vertebrate neuromuscular junction (NMJ) remains the best-studied model for understanding the mechanisms involved in synaptogenesis, due to its relatively large size, its simplicity of patterning, and its unparalleled experimental accessibility. During neuromuscular development, each skeletal myofiber secretes and deposits around its extracellular surface an assemblage of extracellular matrix (ECM) proteins that ultimately form a basal lamina. This is also the case at the NMJ, where the motor nerve contributes additional factors. Before most of the current molecular components were known, it was clear that the synaptic ECM of adult skeletal muscles was unique in composition and contained factors sufficient to induce the differentiation of both pre- and postsynaptic membranes. Biochemical, genetic, and microscopy studies have confirmed that agrin, laminin (221, 421, and 521), collagen IV (α3-α6), collagen XIII, perlecan, and the ColQ-bound form of acetylcholinesterase are all synaptic ECM proteins with important roles in neuromuscular development. The roles of their many potential receptors and/or binding proteins have been more difficult to assess at the genetic level due to the complexity of membrane interactions with these large proteins, but roles for MuSK-LRP4 in agrin signaling and for integrins, dystroglycan, and voltage-gated calcium channels in laminin-dependent phenotypes have been identified. Synaptic ECM proteins and their receptors are involved in almost all aspects of synaptic development, including synaptic initiation, topography, ultrastructure, maturation, stability, and transmission.
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Affiliation(s)
- Neha Singhal
- Center for Gene Therapy, The Research Institute at Nationwide Children's Hospital, Department of Pediatrics, Ohio State University College of Medicine, Columbus, Ohio 43205, USA
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40
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Maselli RA, Fernandez JM, Arredondo J, Navarro C, Ngo M, Beeson D, Cagney Ó, Williams DC, Wollmann RL, Yarov-Yarovoy V, Ferns MJ. LG2 agrin mutation causing severe congenital myasthenic syndrome mimics functional characteristics of non-neural (z-) agrin. Hum Genet 2012; 131:1123-35. [PMID: 22205389 PMCID: PMC4795461 DOI: 10.1007/s00439-011-1132-4] [Citation(s) in RCA: 61] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2011] [Accepted: 12/11/2011] [Indexed: 10/14/2022]
Abstract
We describe a severe form of congenital myasthenic syndrome (CMS) caused by two heteroallelic mutations: a nonsense and a missense mutation in the gene encoding agrin (AGRN). The identified mutations, Q353X and V1727F, are located at the N-terminal and at the second laminin G-like (LG2) domain of agrin, respectively. A motor-point muscle biopsy demonstrated severe disruption of the architecture of the neuromuscular junction (NMJ), including: dispersion and fragmentation of endplate areas with normal expression of acetylcholinesterase; simplification of postsynaptic membranes; pronounced reduction of the axon terminal size; widening of the primary synaptic cleft; and, collection of membranous debris material in the primary synaptic cleft and in the subsynaptic cytoplasm. Expression studies in heterologous cells revealed that the Q353X mutation abolished expression of full-length agrin. Moreover, the V1727F mutation decreased agrin-induced clustering of the acetylcholine receptor (AChR) in cultured C2 muscle cells by >100-fold, and phosphorylation of the MuSK receptor and AChR beta subunit by ~tenfold. Surprisingly, the V1727F mutant also displayed increased binding to α-dystroglycan but decreased binding to a neural (z+) agrin-specific antibody. Our findings demonstrate that agrin mutations can associate with a severe form of CMS and cause profound distortion of the architecture and function of the NMJ. The impaired ability of V1727F agrin to activate MuSK and cluster AChRs, together with its increased affinity to α-dystroglycan, mimics non-neural (z-) agrin and are important determinants of the pathogenesis of the disease.
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MESH Headings
- Acetylcholinesterase/metabolism
- Adult
- Agrin/chemistry
- Agrin/genetics
- Agrin/metabolism
- Base Sequence
- Cell Line
- Codon, Nonsense
- Dystroglycans/metabolism
- Female
- HEK293 Cells
- Humans
- Male
- Models, Molecular
- Muscle Fibers, Skeletal/metabolism
- Muscle Fibers, Skeletal/pathology
- Muscle, Skeletal/metabolism
- Muscle, Skeletal/pathology
- Muscle, Skeletal/physiopathology
- Mutation, Missense
- Myasthenic Syndromes, Congenital/genetics
- Myasthenic Syndromes, Congenital/metabolism
- Neuromuscular Junction/metabolism
- Neuromuscular Junction/pathology
- Pedigree
- Receptors, Cholinergic/genetics
- Receptors, Cholinergic/metabolism
- Sequence Analysis, DNA
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Affiliation(s)
- Ricardo A. Maselli
- Department of Neurology, University of California Davis, Davis, California, 95616 USA
| | - Jose M. Fernandez
- Service of Clinical Neurophysiology. University Hospital of Vigo, Vigo (Pontevedra), Spain
| | - Juan Arredondo
- Department of Neurology, University of California Davis, Davis, California, 95616 USA
| | - Carmen Navarro
- Department of Pathology, University Hospital of Vigo and CIBERER (Centro de Investigacion Biomedica en Red en Enfermedades Raras), Vigo (Pontevedra), Spain
| | - Maian Ngo
- Department of Neurology, University of California Davis, Davis, California, 95616 USA
| | - David Beeson
- Neurosciences Group, Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DS, UK
| | - Órla Cagney
- Department of Neurology, University of California Davis, Davis, California, 95616 USA
| | - D. Colette Williams
- Veterinary Medical Teaching Hospital, University of California Davis, Davis, California, 95616, USA
| | - Robert L. Wollmann
- Department of Pathology, University of Chicago, Chicago, Illinois, 60637, USA
| | - Vladimir Yarov-Yarovoy
- Department of Physiology and Membrane Biology, University of California Davis, Davis, California 95616, USA
| | - Michael J Ferns
- Department of Physiology and Membrane Biology, University of California Davis, Davis, California 95616, USA
- Department of Anesthesiology, University of California Davis, Davis, California 95616, USA
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41
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Abstract
Wnt proteins are best known for their profound roles in cell patterning, because they are required for the embryonic development of all animal species studied to date. Besides regulating cell fate, Wnt proteins are gaining increasing recognition for their roles in nervous system development and function. New studies indicate that multiple positive and negative Wnt signaling pathways take place simultaneously during the formation of vertebrate and invertebrate neuromuscular junctions. Although some Wnts are essential for the formation of NMJs, others appear to play a more modulatory role as part of multiple signaling pathways. Here we review the most recent findings regarding the function of Wnts at the NMJ from both vertebrate and invertebrate model systems.
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Affiliation(s)
- Kate Koles
- Department of Neurobiology, University of Massachusetts Medical School, Worcester, 01605, USA
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Kabouridis PS, Pimentel TA, Brancaleone V, D'Acquisto F, Oliani SM, Perretti M, Jury EC. Distinct localization of T cell Agrin during antigen presentation--evidence for the expression of Agrin receptor(s) in antigen-presenting cells. FEBS J 2012; 279:2368-80. [PMID: 22530934 DOI: 10.1111/j.1742-4658.2012.08615.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Agrin is over-expressed by activated and autoimmune T cells, and synergizes with the T cell receptor (TCR) to augment cell activation. In the present study, we show that Agrin accumulates to distinct areas of the plasma membrane and that cell activation causes its redistribution. During antigen presentation, Agrin primarily accumulates to the periphery of the mature immunological synapse, mostly in lamellipodia-like protrusions that wrap around the antigen-presenting cell and, conversely, anti-Agrin sera induced a significant redistribution of TCR at the plasma membrane. We also provide evidence for the expression of Agrin receptors in peripheral blood monocytes, dendritic cells and a fraction of B cells. Interestingly, interferon-α treatment, which induces the expression of Agrin in T cells, also augmented Agrin binding to monocytes. Stimulation of monocytes with recombinant Agrin induced the clustering of surface receptors, including major histocompatibility complex class II, activation of intracellular signalling cascades, as well as enhanced dsRNA-induced expression of pro-inflammatory cytokines interleukin-6 and tumour necrosis factor-α. Collectively, these results confirm the location of Agrin at the immunological synapse between T cells and antigen-presenting cells and justify further characterization of its receptors in the immune system.
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Affiliation(s)
- Panagiotis S Kabouridis
- William Harvey Research Institute, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, UK.
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43
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Abstract
Synapses are the fundamental units of neural circuits that enable complex behaviors. The neuromuscular junction (NMJ), a synapse formed between a motoneuron and a muscle fiber, has contributed greatly to understanding of the general principles of synaptogenesis as well as of neuromuscular disorders. NMJ formation requires neural agrin, a motoneuron-derived protein, which interacts with LRP4 (low-density lipoprotein receptor-related protein 4) to activate the receptor tyrosine kinase MuSK (muscle-specific kinase). However, little is known of how signals are transduced from agrin to MuSK. Here, we present the first crystal structure of an agrin-LRP4 complex, consisting of two agrin-LRP4 heterodimers. Formation of the initial binary complex requires the z8 loop that is specifically present in neuronal, but not muscle, agrin and that promotes the synergistic formation of the tetramer through two additional interfaces. We show that the tetrameric complex is essential for neuronal agrin-induced acetylcholine receptor (AChR) clustering. Collectively, these results provide new insight into the agrin-LRP4-MuSK signaling cascade and NMJ formation and represent a novel mechanism for activation of receptor tyrosine kinases.
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Bogdanik LP, Burgess RW. A valid mouse model of AGRIN-associated congenital myasthenic syndrome. Hum Mol Genet 2011; 20:4617-33. [PMID: 21890498 DOI: 10.1093/hmg/ddr396] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Congenital myasthenic syndromes (CMS) are inherited diseases affecting the neuromuscular junction (NMJ). Mutations in AGRIN (AGRN) and other genes in the AGRIN signaling pathway cause CMS, and gene targeting studies in mice confirm the importance of this pathway for NMJ formation. However, these mouse mutations are complete loss-of-function alleles that result in an embryonic failure of NMJ formation, and homozygous mice do not survive postpartum. Therefore, mouse models of AGRIN-related CMS that would allow preclinical testing or studies of postnatal disease progression are lacking. Using chemical mutagenesis in mice, we identified a point mutation in Agrn that results in a partial loss-of-function allele, creating a valid model of CMS. The mutation changes phenylalanine 1061 to serine in the SEA domain of AGRIN, a poorly characterized motif shared by other extracellular proteoglycans. NMJs in homozygous mice progressively degrade postnataly. Severity differs with genetic background, in different muscles, and in different regions within a muscle in a pattern matching mouse models of motor neuron disease. Mutant NMJs have decreased acetylcholine receptor density and an increased subsynaptic reticulum, evident by electron microscopy. Synapses eventually denervate and the muscles atrophy. Molecularly, several factors contribute to the partial loss of AGRIN's function. The mutant protein is found at NMJs, but is processed differently than wild-type, with decreased glycosylation, changes in sensitivity to the protease neurotrypsin and other proteolysis, and less efficient externalization and secretion. Therefore, the Agrn point mutation is a model for CMS caused by Agrn mutations and potentially other related neuromuscular diseases.
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45
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Pevzner A, Schoser B, Peters K, Cosma NC, Karakatsani A, Schalke B, Melms A, Kröger S. Anti-LRP4 autoantibodies in AChR- and MuSK-antibody-negative myasthenia gravis. J Neurol 2011; 259:427-35. [PMID: 21814823 DOI: 10.1007/s00415-011-6194-7] [Citation(s) in RCA: 203] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2011] [Revised: 06/27/2011] [Accepted: 07/14/2011] [Indexed: 11/25/2022]
Abstract
Myasthenia gravis (MG) is an autoimmune disorder characterized by a defect in synaptic transmission at the neuromuscular junction causing fluctuating muscle weakness with a decremental response to repetitive nerve stimulation or altered jitter in single-fiber electromyography (EMG). Approximately 80% of all myasthenia gravis patients have autoantibodies against the nicotinic acetylcholine receptor in their serum. Autoantibodies against the tyrosine kinase muscle-specific kinase (MuSK) are responsible for 5-10% of all myasthenia gravis cases. The autoimmune target in the remaining cases is unknown. Recently, low-density lipoprotein receptor-related protein (LRP4) has been identified as the agrin receptor. LRP4 interacts with agrin, and the binding of agrin activates MuSK, which leads to the formation of most if not all postsynaptic specializations, including aggregates containing acetylcholine receptors (AChRs) in the junctional plasma membrane. In the present study we tested if autoantibodies against LRP4 are detectable in patients with myasthenia gravis. To this end we analyzed 13 sera from patients with generalized myasthenia gravis but without antibodies against AChR or MuSK. The results showed that 12 out of 13 antisera from double-seronegative MG patients bound to proteins concentrated at the neuromuscular junction of adult mouse skeletal muscle and that approximately 50% of the tested sera specifically bound to HEK293 cells transfected with human LRP4. Moreover, 4 out of these 13 sera inhibited agrin-induced aggregation of AChRs in cultured myotubes by more than 50%, suggesting a pathogenic role regarding the dysfunction of the neuromuscular endplate. These results indicate that LRP4 is a novel target for autoantibodies and is a diagnostic marker in seronegative MG patients.
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Affiliation(s)
- Alexandra Pevzner
- Department of Physiological Genomics, Institute for Physiology, Ludwig-Maximilians University, Pettenkoferstrasse 12, 80336, Munich, Germany
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46
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Meinen S, Lin S, Thurnherr R, Erb M, Meier T, Rüegg MA. Apoptosis inhibitors and mini-agrin have additive benefits in congenital muscular dystrophy mice. EMBO Mol Med 2011; 3:465-79. [PMID: 21674808 PMCID: PMC3377088 DOI: 10.1002/emmm.201100151] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2010] [Revised: 04/29/2011] [Accepted: 05/17/2011] [Indexed: 11/18/2022] Open
Abstract
Mutations in LAMA2 cause a severe form of congenital muscular dystrophy, called MDC1A. Studies in mouse models have shown that transgenic expression of a designed, miniaturized form of the extracellular matrix molecule agrin (‘mini-agrin’) or apoptosis inhibition by either overexpression of Bcl2 or application of the pharmacological substance omigapil can ameliorate the disease. Here, we tested whether mini-agrin and anti-apoptotic agents act on different pathways and thus exert additive benefits in MDC1A mouse models. By combining mini-agrin with either transgenic Bcl2 expression or oral omigapil application, we show that the ameliorating effect of mini-agrin, which acts by restoring the mechanical stability of muscle fibres and, thereby, reduces muscle fibre breakdown and concomitant fibrosis, is complemented by apoptosis inhibitors, which prevent the loss of muscle fibres. Treatment of mice with both agents results in improved muscle regeneration and increased force. Our results show that the combination of mini-agrin and anti-apoptosis treatment has beneficial effects that are significantly bigger than the individual treatments and suggest that such a strategy might also be applicable to MDC1A patients.
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Patel TR, Meier M, Li J, Morris G, Rowe AJ, Stetefeld J. T-shaped arrangement of the recombinant agrin G3-IgG Fc protein. Protein Sci 2011; 20:931-40. [PMID: 21448912 PMCID: PMC3104224 DOI: 10.1002/pro.628] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2011] [Revised: 03/14/2011] [Accepted: 03/16/2011] [Indexed: 12/12/2022]
Abstract
Agrin is a large heparin sulphate proteoglycan with multiple domains, which is located in the extracellular matrix. The C-terminal G3 domain of agrin is functionally one of the most important domains. It harbors an α-dystroglycan binding site and carries out acetylcholine receptor clustering activities. In the present study, we have fused the G3 domain of agrin to an IgG Fc domain to produce a G3-Fc fusion protein that we intend to use as a tool to investigate new binding partners of agrin. As a first step of the study, we have characterized the recombinant fusion protein using a multidisciplinary approach using dynamic light scattering, analytical ultracentrifugation and small angle X-ray scattering (SAXS). Interestingly, our SAXS analysis using the high-resolution structures of G3 and Fc domain as models indicates that the G3-Fc protein forms a T-shaped molecule with the G3 domains extruding perpendicularly from the Fc scaffold. To validate our models, we have used the program HYDROPRO to calculate the hydrodynamic properties of the solution models. The calculated values are in excellent agreement with those determined experimentally.
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Affiliation(s)
- Trushar R Patel
- Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada
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48
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Barros CS, Franco SJ, Müller U. Extracellular matrix: functions in the nervous system. Cold Spring Harb Perspect Biol 2011; 3:a005108. [PMID: 21123393 DOI: 10.1101/cshperspect.a005108] [Citation(s) in RCA: 260] [Impact Index Per Article: 20.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.
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Affiliation(s)
- Claudia S Barros
- The Scripps Research Institute, Department of Cell Biology, Dorris Neuroscience Center, La Jolla, California 92037, USA
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49
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Tseng CN, Zhang L, Wu SL, Wang WF, Wang ZZ, Cascio M. Asparagine of z8 insert is critical for the affinity, conformation, and acetylcholine receptor-clustering activity of neural agrin. J Biol Chem 2010; 285:27641-51. [PMID: 20566625 DOI: 10.1074/jbc.m110.130625] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Agrin isoforms with different bioactivities are synthesized by the nerve and the muscle. Neural agrin containing an 8-amino acid insert (z8) introduced by alternative splicing is the active form that induces synaptic differentiation at the neuromuscular junction. In addition to alternative splicing, extracellular calcium is also required for the activity of neural agrin. To understand better how the activity of agrin is regulated by alternative splicing, we have applied alanine substitution mutagenesis to the z8 insert and the calcium binding site in the minimally functional AgG3z8 fragment. Single alanine substitutions in the 4th through the 7th amino acid of the z8 splice insert significantly reduced the function of agrin, in terms of acetylcholine receptor clustering activity and the affinity for binding to the muscle surface. Mutation of the asparagine at the 4th position drastically reduces bioactivity such that it is equivalent to that of muscle form AgG3z0. These reduced activity mutants also show reduced magnitudes of the calcium-induced CD spectrum change from that observed in AgG3z8 fragments, indicating that cross-talk between calcium and the z8 insert is critical for the normal activity of agrin. However, removal of Ca(2+) binding via mutation of both aspartic acids in the calcium binding site did not totally eliminate the activity of AgG3z8. These results suggest a model wherein the z8 insert is a Ca(2+)-responsive allosteric element that is essential in forming an active conformation in neuronal agrin.
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Affiliation(s)
- Chao-Neng Tseng
- Department of Biomedical Science and Environmental Biology, Kaohsiung Medical University, Kaohsiung City 807, Taiwan
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50
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Jury EC, Kabouridis PS. New role for Agrin in T cells and its potential importance in immune system regulation. Arthritis Res Ther 2010; 12:205. [PMID: 20398335 PMCID: PMC2888195 DOI: 10.1186/ar2957] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Agrin plays a crucial role in the maintenance of the neuromuscular junction. However, it is expressed in other tissues as well, including T lymphocytes, where cell activation induces its expression. Agrin from activated T cells has the capacity to induce aggregation of key receptors and to regulate signalling. Interestingly, T cells isolated from patients with systemic lupus erythematosus over-express Agrin and its co-stimulation with the T cell receptor enhances production of pathogenic cytokines. These early studies point to an important function for Agrin in T cell biology and make the case for a more thorough and systematic investigation into its role in the immune system.
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Affiliation(s)
- Elizabeth C Jury
- Centre for Rheumatology, Royal Free and University College Medical School, University College London, London W1P 4JF, UK.
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