201
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Wnt4 participates in the formation of vertebrate neuromuscular junction. PLoS One 2012; 7:e29976. [PMID: 22253844 PMCID: PMC3257248 DOI: 10.1371/journal.pone.0029976] [Citation(s) in RCA: 70] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2011] [Accepted: 12/07/2011] [Indexed: 01/27/2023] Open
Abstract
Neuromuscular junction (NMJ) formation requires the highly coordinated communication of several reciprocal signaling processes between motoneurons and their muscle targets. Identification of the early, spatially restricted cues in target recognition at the NMJ is still poorly documented, especially in mammals. Wnt signaling is one of the key pathways regulating synaptic connectivity. Here, we report that Wnt4 contributes to the formation of vertebrate NMJ in vivo. Results from a microarray screen and quantitative RT-PCR demonstrate that Wnt4 expression is regulated during muscle cell differentiation in vitro and muscle development in vivo, being highly expressed when the first synaptic contacts are formed and subsequently downregulated. Analysis of the mouse Wnt4−/− NMJ phenotype reveals profound innervation defects including motor axons overgrowing and bypassing AChR aggregates with 30% of AChR clusters being unapposed by nerve terminals. In addition, loss of Wnt4 function results in a 35% decrease of the number of prepatterned AChR clusters while Wnt4 overexpression in cultured myotubes increases the number of AChR clusters demonstrating that Wnt4 directly affects postsynaptic differentiation. In contrast, muscle structure and the localization of several synaptic proteins including acetylcholinesterase, MuSK and rapsyn are not perturbed in the Wnt4 mutant. Finally, we identify MuSK as a Wnt4 receptor. Wnt4 not only interacts with MuSK ectodomain but also mediates MuSK activation. Taken together our data reveal a new role for Wnt4 in mammalian NMJ formation that could be mediated by MuSK, a key receptor in synaptogenesis.
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202
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Henríquez JP, Salinas PC. Dual roles for Wnt signalling during the formation of the vertebrate neuromuscular junction. Acta Physiol (Oxf) 2012; 204:128-36. [PMID: 21554559 DOI: 10.1111/j.1748-1716.2011.02295.x] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Wnt proteins play prominent roles in different aspects of neuronal development culminating with the formation of complex neuronal circuits. Here, we discuss new studies addressing the function of Wnt signalling at the peripheral neuromuscular junction (NMJ). In both, invertebrate and vertebrate organisms, Wnt signalling promotes and also inhibits the assembly of the neuromuscular synapse. Here, we focus our attention on recent studies at the vertebrate NMJ that demonstrate that some Wnt proteins collaborate with the Agrin-MuSK signalling to induce post-synaptic differentiation. In contrast, Wnts that activate the Wnt/β-catenin signalling inhibit post-synaptic differentiation. The dual function of different Wnts might finely modulate the proper apposition of the pre- and post-synaptic terminals during NMJ formation and growth.
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Affiliation(s)
- J P Henríquez
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Chile.
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203
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Richman DP, Nishi K, Morell SW, Chang JM, Ferns MJ, Wollmann RL, Maselli RA, Schnier J, Agius MA. Acute severe animal model of anti-muscle-specific kinase myasthenia: combined postsynaptic and presynaptic changes. ACTA ACUST UNITED AC 2011; 69:453-60. [PMID: 22158720 DOI: 10.1001/archneurol.2011.2200] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022]
Abstract
OBJECTIVES To determine the pathogenesis of anti-muscle-specific kinase (MuSK) myasthenia, a newly described severe form of myasthenia gravis associated with MuSK antibodies characterized by focal muscle weakness and wasting and absence of acetylcholine receptor antibodies, and to determine whether antibodies to MuSK, a crucial protein in the formation of the neuromuscular junction (NMJ) during development, can induce disease in the mature NMJ. Design, Setting, and PARTICIPANTS Lewis rats were immunized with a single injection of a newly discovered splicing variant of MuSK, MuSK 60, which has been demonstrated to be expressed primarily in the mature NMJ. Animals were assessed clinically, serologically, and by repetitive stimulation of the median nerve. Muscle tissue was examined immunohistochemically and by electron microscopy. RESULTS Animals immunized with 100 μg of MuSK 60 developed severe progressive weakness starting at day 16, with 100% mortality by day 27. The weakness was associated with high MuSK antibody titers, weight loss, axial muscle wasting, and decrementing compound muscle action potentials. Light and electron microscopy demonstrated fragmented NMJs with varying degrees of postsynaptic muscle end plate destruction along with abnormal nerve terminals, lack of registration between end plates and nerve terminals, local axon sprouting, and extrajunctional dispersion of cholinesterase activity. CONCLUSIONS These findings support the role of MuSK antibodies in the human disease, demonstrate the role of MuSK not only in the development of the NMJ but also in the maintenance of the mature synapse, and demonstrate involvement of this disease in both presynaptic and postsynaptic components of the NMJ.
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Affiliation(s)
- David P Richman
- Department of Neurology, University of California-Davis, 1515 Newton Ct., Davis, CA 95616, USA.
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204
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Nishimune H. Molecular mechanism of active zone organization at vertebrate neuromuscular junctions. Mol Neurobiol 2011; 45:1-16. [PMID: 22135013 DOI: 10.1007/s12035-011-8216-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2011] [Accepted: 10/24/2011] [Indexed: 02/08/2023]
Abstract
Organization of presynaptic active zones is essential for development, plasticity, and pathology of the nervous system. Recent studies indicate a trans-synaptic molecular mechanism that organizes the active zones by connecting the pre- and the postsynaptic specialization. The presynaptic component of this trans-synaptic mechanism is comprised of cytosolic active zone proteins bound to the cytosolic domains of voltage-dependent calcium channels (P/Q-, N-, and L-type) on the presynaptic membrane. The postsynaptic component of this mechanism is the synapse organizer (laminin β2) that is expressed by the postsynaptic cell and accumulates specifically on top of the postsynaptic specialization. The pre- and the postsynaptic components interact directly between the extracellular domains of calcium channels and laminin β2 to anchor the presynaptic protein complex in front of the postsynaptic specialization. Hence, the presynaptic calcium channel functions as a scaffolding protein for active zone organization and as an ion-conducting channel for synaptic transmission. In contrast to the requirement of calcium influx for synaptic transmission, the formation of the active zone does not require the calcium influx through the calcium channels. Importantly, the active zones of adult synapses are not stable structures and require maintenance for their integrity. Furthermore, aging or diseases of the central and peripheral nervous system impair the active zones. This review will focus on the molecular mechanisms that organize the presynaptic active zones and summarize recent findings at the neuromuscular junctions and other synapses.
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Affiliation(s)
- Hiroshi Nishimune
- Department of Anatomy and Cell Biology, and Kansas Intellectual and Developmental Disabilities Research Center, University of Kansas Medical School, 3901 Rainbow Blvd., MS 3051, HLSIC Rm. 2073, Kansas City, KS 66160, USA.
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205
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Update on Wnt signaling in bone cell biology and bone disease. Gene 2011; 492:1-18. [PMID: 22079544 DOI: 10.1016/j.gene.2011.10.044] [Citation(s) in RCA: 288] [Impact Index Per Article: 22.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2011] [Revised: 10/13/2011] [Accepted: 10/20/2011] [Indexed: 12/17/2022]
Abstract
For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.
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206
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Niziolek PJ, Farmer T, Cui Y, Turner CH, Warman ML, Robling AG. High-bone-mass-producing mutations in the Wnt signaling pathway result in distinct skeletal phenotypes. Bone 2011; 49:1010-9. [PMID: 21855668 PMCID: PMC3412139 DOI: 10.1016/j.bone.2011.07.034] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/13/2011] [Revised: 07/18/2011] [Accepted: 07/21/2011] [Indexed: 10/17/2022]
Abstract
Mutations among genes that participate in the canonical Wnt signaling pathway can lead to drastically different skeletal phenotypes, ranging from severe osteoporosis to severe osteosclerosis. Many high-bone-mass (HBM) causing mutations that occur in the LRP5 gene appear to impart the HBM phenotype, in part, by increasing resistance to soluble Wnt signaling inhibitors, including sclerostin. Sost loss-of-function mutant mice (Sost knock-out) and Lrp5 gain-of-function mutant mice (Lrp5 HBM knock-in) have high bone mass. These mutants potentially would be predicted to be phenocopies of one another, because in both cases, the sclerostin-Lrp5 interaction is disrupted. We measured bone mass, size, geometry, architecture, and strength in bones from three different genetic mouse models (Sost knock-out, Lrp5 A214V knock-in, and Lrp5 G171V knock-in) of HBM. We found that all three mouse lines had significantly elevated bone mass in the appendicular skeleton and in the cranium. Sost mutants and Lrp5 A214V mutants were statistically indistinguishable from one another in most endpoints, whereas both were largely different from the Lrp5 G171V mutants. Lrp5 G171V mutants preferentially added bone endocortically, whereas Lrp5 A214V and Sost mutants preferentially added bone periosteally. Cranial thickness and cranial nerve openings were similarly altered in all three HBM models. We also assessed serum serotonin levels as a possible mechanism accounting for the observed changes in bone mass, but no differences in serum serotonin were found in any of the three HBM mouse lines. The skeletal dissimilarities of the Lrp5 G171V mutant to the other mutants suggest that other, non-sclerostin-associated mechanisms might account for the changes in bone mass resulting from this mutation.
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Affiliation(s)
- Paul J. Niziolek
- Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, USA
| | - Takeisha Farmer
- Department of Anatomy & Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA
| | - Yajun Cui
- Department of Genetics, Case Western Reserve University School of Medicine, Cleveland, OH, USA
| | - Charles H. Turner
- Department of Biomedical Engineering, Indiana University–Purdue University at Indianapolis, Indianapolis, IN, USA
| | - Matthew L. Warman
- Department of Orthopaedic Surgery, Children’s Hospital, Boston, MA, USA
- Howard Hughes Medical Institute, Department of Genetics, Harvard Medical School, Boston, MA, USA
| | - Alexander G. Robling
- Department of Anatomy & Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA
- Department of Biomedical Engineering, Indiana University–Purdue University at Indianapolis, Indianapolis, IN, USA
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207
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Gomez AM, Burden SJ. The extracellular region of Lrp4 is sufficient to mediate neuromuscular synapse formation. Dev Dyn 2011; 240:2626-33. [PMID: 22038977 DOI: 10.1002/dvdy.22772] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 10/05/2011] [Indexed: 11/09/2022] Open
Abstract
Neuromuscular synapse formation requires an exchange of signals between motor neurons and muscle. Agrin, supplied by motor neurons, binds to Lrp4 in muscle, stimulating phosphorylation of MuSK and recruitment of a signaling complex essential for synapse-specific transcription and anchoring of key proteins in the postsynaptic membrane. Lrp4, like the LDLR and other Lrp-family members, contains an intracellular region with motifs that can regulate receptor trafficking, as well as assembly of an intracellular signaling complex. Here, we show that the intracellular region of Lrp4 is dispensable for Agrin to stimulate MuSK phosphorylation and clustering of acetylcholine receptors in cultured myotubes. Moreover, muscle-selective expression of a Lrp4-CD4 chimera, composed of the extracellular and transmembrane regions of Lrp4 and the intracellular region of CD4, rescues neuromuscular synapse formation and the neonatal lethality of lrp4 mutant mice, demonstrating that Lrp4, lacking the Lrp4 intracellular region, is sufficient for presynaptic and postsynaptic differentiation.
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Affiliation(s)
- Andrea M Gomez
- Molecular Neurobiology Program, Helen L. and Martin S. Kimmel Center for Biology and Medicine at the Skirball Institute of Biomolecular Medicine, NYU Medical School, 540 First Avenue, New York, NY 10016, USA
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208
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Zhang W, Coldefy AS, Hubbard SR, Burden SJ. Agrin binds to the N-terminal region of Lrp4 protein and stimulates association between Lrp4 and the first immunoglobulin-like domain in muscle-specific kinase (MuSK). J Biol Chem 2011; 286:40624-30. [PMID: 21969364 DOI: 10.1074/jbc.m111.279307] [Citation(s) in RCA: 113] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Neuromuscular synapse formation depends upon coordinated interactions between motor neurons and muscle fibers, leading to the formation of a highly specialized postsynaptic membrane and a highly differentiated nerve terminal. Synapse formation begins as motor axons approach muscles that are prepatterned in the prospective synaptic region in a manner that depends upon Lrp4, a member of the LDL receptor family, and muscle-specific kinase (MuSK), a receptor tyrosine kinase. Motor axons supply Agrin, which binds Lrp4 and stimulates further MuSK phosphorylation, stabilizing nascent synapses. How Agrin binds Lrp4 and stimulates MuSK kinase activity is poorly understood. Here, we demonstrate that Agrin binds to the N-terminal region of Lrp4, including a subset of the LDLa repeats and the first of four β-propeller domains, which promotes association between Lrp4 and MuSK and stimulates MuSK kinase activity. In addition, we show that Agrin stimulates the formation of a functional complex between Lrp4 and MuSK on the surface of myotubes in the absence of the transmembrane and intracellular domains of Lrp4. Further, we demonstrate that the first Ig-like domain in MuSK, which shares homology with the NGF-binding region in Tropomyosin Receptor Kinase (TrKA), is required for MuSK to bind Lrp4. These findings suggest that Lrp4 is a cis-acting ligand for MuSK, whereas Agrin functions as an allosteric and paracrine regulator to promote association between Lrp4 and MuSK.
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Affiliation(s)
- Wei Zhang
- Molecular Neurobiology Program, Helen L. and Martin S. Kimmel Center for Biology and Medicine at the Skirball Institute of Biomolecular Medicine, New York University Medical School, New York, New York 10016, USA
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209
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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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210
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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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211
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Rimer M. Emerging roles for MAP kinases in agrin signaling. Commun Integr Biol 2011; 4:143-6. [PMID: 21655426 DOI: 10.4161/cib.4.2.14357] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2010] [Accepted: 12/02/2010] [Indexed: 11/19/2022] Open
Abstract
Information between neurons and the target cells they innervate passes through sites of functional contact called synapses. How synapses form and are altered by sensory or cognitive experience is central to understand nervous system function. Studies of synapse formation and plasticity have concentrated on a few "model" synapses. The vertebrate neuromuscular junction (NMJ), the synapse between a motoneuron in the spinal cord and a skeletal muscle fiber, is one such model synapse. The extracellular matrix proteoglycan agrin plays an essential organizing role at the NMJ. Agrin is also present at some synapses in the brain and in other organs in the periphery, but its function outside the NMJ is unclear. The core signaling pathway for agrin at the NMJ, which is still incompletely defined, includes molecules specifically involved in this cascade and molecules used in other signaling pathways in many cells. Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved components of intracellular signaling modules that control a myriad of cellular processes. This article reviews emerging evidence that suggests that MAPKs are involved in agrin signaling at the NMJ and in the putative functions of agrin in the formation of a subset of synapses in the brain.
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Affiliation(s)
- Mendell Rimer
- Department of Neuroscience & Experimental Therapeutics; College of Medicine; Texas A&M Health Science Center; College Station, TX USA
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212
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Higuchi O, Hamuro J, Motomura M, Yamanashi Y. Autoantibodies to low-density lipoprotein receptor-related protein 4 in myasthenia gravis. Ann Neurol 2011; 69:418-22. [PMID: 21387385 DOI: 10.1002/ana.22312] [Citation(s) in RCA: 315] [Impact Index Per Article: 24.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction, where acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low-density lipoprotein (LDL) receptor-related protein 4 (Lrp4) are essential. About 80% and 0% to 10% of patients with generalized MG have autoantibodies to AChR and MuSK, respectively, but pathogenic factors are elusive in others. Here we show that a proportion of AChR antibody-negative patients have autoantibodies to Lrp4. These antibodies inhibit binding of Lrp4 to its ligand and predominantly belong to the immunoglobulin G1 (IgG1) subclass, a complement activator. These findings together indicate the involvement of Lrp4 antibodies in the pathogenesis of AChR antibody-negative MG.
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Affiliation(s)
- Osamu Higuchi
- Division of Genetics, Department of Cancer Biology, the Institute of Medical Science, the University of Tokyo, Japan
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213
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Leupin O, Piters E, Halleux C, Hu S, Kramer I, Morvan F, Bouwmeester T, Schirle M, Bueno-Lozano M, Fuentes FJR, Itin PH, Boudin E, de Freitas F, Jennes K, Brannetti B, Charara N, Ebersbach H, Geisse S, Lu CX, Bauer A, Van Hul W, Kneissel M. Bone overgrowth-associated mutations in the LRP4 gene impair sclerostin facilitator function. J Biol Chem 2011; 286:19489-500. [PMID: 21471202 DOI: 10.1074/jbc.m110.190330] [Citation(s) in RCA: 209] [Impact Index Per Article: 16.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Humans lacking sclerostin display progressive bone overgrowth due to increased bone formation. Although it is well established that sclerostin is an osteocyte-secreted bone formation inhibitor, the underlying molecular mechanisms are not fully elucidated. We identified in tandem affinity purification proteomics screens LRP4 (low density lipoprotein-related protein 4) as a sclerostin interaction partner. Biochemical assays with recombinant proteins confirmed that sclerostin LRP4 interaction is direct. Interestingly, in vitro overexpression and RNAi-mediated knockdown experiments revealed that LRP4 specifically facilitates the previously described inhibitory action of sclerostin on Wnt1/β-catenin signaling. We found the extracellular β-propeller structured domain of LRP4 to be required for this sclerostin facilitator activity. Immunohistochemistry demonstrated that LRP4 protein is present in human and rodent osteoblasts and osteocytes, both presumed target cells of sclerostin action. Silencing of LRP4 by lentivirus-mediated shRNA delivery blocked sclerostin inhibitory action on in vitro bone mineralization. Notably, we identified two mutations in LRP4 (R1170W and W1186S) in patients suffering from bone overgrowth. We found that these mutations impair LRP4 interaction with sclerostin and its concomitant sclerostin facilitator effect. Together these data indicate that the interaction of sclerostin with LRP4 is required to mediate the inhibitory function of sclerostin on bone formation, thus identifying a novel role for LRP4 in bone.
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Affiliation(s)
- Olivier Leupin
- Musculoskeletal Disease Area, Novartis Institutes for Biomedical Research, Novartis Pharma AG, Basel, Switzerland.
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214
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Woller B, Luiskandl S, Popovic M, Prieler BEM, Ikonge G, Mutzl M, Rehmann H, Herbst R. Rin-like, a novel regulator of endocytosis, acts as guanine nucleotide exchange factor for Rab5a and Rab22. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2011; 1813:1198-210. [PMID: 21419809 PMCID: PMC3096779 DOI: 10.1016/j.bbamcr.2011.03.005] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/05/2010] [Revised: 03/08/2011] [Accepted: 03/10/2011] [Indexed: 11/23/2022]
Abstract
RIN proteins serve as guanine nucleotide exchange factors for Rab5a. They are characterized by the presence of a RIN homology domain and a C-terminal Vps9 domain. Currently three family members have been described and analyzed. Here we report the identification of a novel RIN family member, Rin-like (Rinl), that represents a new interaction partner of the receptor tyrosine kinase MuSK, which is an essential key regulator of neuromuscular synapse development. Rinl is localized to neuromuscular synapses but shows the highest expression in thymus and spleen. Rinl preferentially binds to nucleotide-free Rab5a and catalyzes the exchange of GDP for GTP. Moreover, Rinl also binds GDP-bound Rab22 and increases the GDP/GTP exchange implicating Rinl in endocytotic processes regulated by Rab5a and Rab22. Interestingly, Rinl shows a higher catalytic rate for Rab22 compared to Rab5a. Rinl is closely associated with the cytoskeleton and thus contributes to the spatial control of Rab5a and Rab22 signaling at actin-positive compartments. Most importantly, overexpression of Rinl affects fluid-phase as well as EGFR endocytosis.
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Affiliation(s)
- Barbara Woller
- Center for Brain Research, Medical University of Vienna, Spitalgasse 4, A-1090 Vienna, Austria
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215
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Ohazama A, Porntaveetus T, Ota MS, Herz J, Sharpe PT. Lrp4: A novel modulator of extracellular signaling in craniofacial organogenesis. Am J Med Genet A 2011; 152A:2974-83. [PMID: 21108386 DOI: 10.1002/ajmg.a.33372] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The low-density lipoprotein (LDL) receptor family is a large evolutionarily conserved group of transmembrane proteins. It has been shown that LDL receptor family members can also function as direct signal transducers or modulators for a broad range of cellular signaling pathways. We have identified a novel mode of signaling pathway integration/coordination that occurs outside cells during development that involves an LDL receptor family member. Physical interaction between an extracellular protein (Wise) that binds BMP ligands and an Lrp receptor (Lrp4) that modulates Wnt signaling, acts to link these two pathways. Mutations in either Wise or Lrp4 in mice produce multiple, but identical abnormalities in tooth development that are linked to alterations in BMP and Wnt signaling. Teeth, in common with many other organs, develop by a series of epithelial-mesenchymal interactions, orchestrated by multiple cell signaling pathways. In tooth development, Lrp4 is expressed exclusively in epithelial cells and Wise mainly in mesenchymal cells. Our hypothesis, based on the mutant phenotypes, cell signaling activity changes and biochemical interactions between Wise and Lrp4 proteins, is that Wise and Lrp4 together act as an extracellular mechanism of coordinating BMP and Wnt signaling activities in epithelial-mesenchymal cell communication during development.
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Affiliation(s)
- Atsushi Ohazama
- Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London Bridge, London, UK
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216
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Hallock PT, Xu CF, Park TJ, Neubert TA, Curran T, Burden SJ. Dok-7 regulates neuromuscular synapse formation by recruiting Crk and Crk-L. Genes Dev 2011; 24:2451-61. [PMID: 21041412 DOI: 10.1101/gad.1977710] [Citation(s) in RCA: 85] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Agrin, released by motor neurons, promotes neuromuscular synapse formation by stimulating MuSK, a receptor tyrosine kinase expressed in skeletal muscle. Phosphorylated MuSK recruits docking protein-7 (Dok-7), an adaptor protein that is expressed selectively in muscle. In the absence of Dok-7, neuromuscular synapses fail to form, and mutations that impair Dok-7 are a major cause of congenital myasthenia in humans. How Dok-7 stimulates synaptic differentiation is poorly understood. Once recruited to MuSK, Dok-7 directly stimulates MuSK kinase activity. This unusual activity of an adapter protein is mediated by the N-terminal region of Dok-7, whereas most mutations that cause congenital myasthenia truncate the C-terminal domain. Here, we demonstrate that Dok-7 also functions downstream from MuSK, and we identify the proteins that are recruited to the C-terminal domain of Dok-7. We show that Agrin stimulates phosphorylation of two tyrosine residues in the C-terminal domain of Dok-7, which leads to recruitment of two adapter proteins: Crk and Crk-L. Furthermore, we show that selective inactivation of Crk and Crk-L in skeletal muscle leads to severe defects in neuromuscular synapses in vivo, revealing a critical role for Crk and Crk-L downstream from Dok-7 in presynaptic and postsynaptic differentiation.
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Affiliation(s)
- Peter T Hallock
- Molecular Neurobiology Program, Skirball Institute of Biomolecular Medicine, New York University Medical School, New York, New York 10016, USA
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217
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Dieckmann M, Dietrich MF, Herz J. Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family. Biol Chem 2011; 391:1341-63. [PMID: 20868222 DOI: 10.1515/bc.2010.129] [Citation(s) in RCA: 93] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.
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Affiliation(s)
- Marco Dieckmann
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9046, USA
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218
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Abstract
Synapse formation is a critical step in the assembly of neuronal circuits. Both secreted and membrane-associated proteins contribute to the assembly and maturation of synapses. In addition, neuronal activity regulates the formation of neuronal circuits through the stimulation of growth factor secretion and the localization of receptors such as NMDA and AMPA receptors (NMDAR and AMPAR, respectively). Little is known, however, about the role of activity in the localization and function of receptors for synaptogenic molecules. Wnts are secreted proteins that play a role in synapse formation by regulating pre- and postsynaptic assembly at central and peripheral synapses. Wnts can signal through different receptors including Frizzleds (Fzs), the LRP5/6 coreceptors, Ror and Ryk. Fz receptors have been shown to mediate Wnt function during synapse formation. At the cell surface, Fz receptors are located at synaptic and extrasynaptic sites. Importantly, synaptic localization of Fzs is regulated by neuronal activity in a Wnt-dependent manner. In this review, we discuss the function of Wnt-Fz signaling in the assembly of central and peripheral synapses and the evidence supporting a role for Wnt ligands and their Fz receptors in activity-mediated synapse formation.
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Affiliation(s)
- Macarena Sahores
- Department of Cell and Developmental Biology, University College London, United Kingdom
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219
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Gomez AM, Van Den Broeck J, Vrolix K, Janssen SP, Lemmens MAM, Van Der Esch E, Duimel H, Frederik P, Molenaar PC, Martínez-Martínez P, De Baets MH, Losen M. Antibody effector mechanisms in myasthenia gravis-pathogenesis at the neuromuscular junction. Autoimmunity 2010; 43:353-70. [PMID: 20380584 DOI: 10.3109/08916930903555943] [Citation(s) in RCA: 80] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies that are either directed to the muscle nicotinic acetylcholine receptor (AChR) or to the muscle-specific tyrosine kinase (MuSK). These autoantibodies define two distinct subforms of the disease-AChR-MG and MuSK-MG. Both AChR and MuSK are expressed on the postsynaptic membrane of the neuromuscular junction (NMJ), which is a highly specialized region of the muscle dedicated to receive and process signals from the motor nerve. Autoantibody binding to proteins of the postsynaptic membrane leads to impaired neuromuscular transmission and muscle weakness. Pro-inflammatory antibodies of the human IgG1 and IgG3 subclass modulate the AChR, cause complement activation, and attract lymphocytes; together acting to decrease levels of the AChR and AChR-associated proteins and to reduce postsynaptic folding. In patients with anti-MuSK antibodies, there is no evidence of loss of junctional folds and no apparent loss of AChR density. Anti-MuSK antibodies are predominantly of the IgG4 isotype, which functionally differs from other IgG subclasses in its anti-inflammatory activity. Moreover, IgG4 undergoes a posttranslational modification termed Fab arm exchange that prevents cross-linking of antigens. These findings suggest that MuSK-MG may be different in etiological and pathological mechanisms from AChR-MG. The effector functions of IgG subclasses on synapse structure and function are discussed in this review.
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Affiliation(s)
- Alejandro M Gomez
- Neuroimmunology Group, Department of Neuroscience, School of Mental Health and Neuroscience, Maastricht University, Maastricht, The Netherlands
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220
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Bolliger MF, Zurlinden A, Lüscher D, Bütikofer L, Shakhova O, Francolini M, Kozlov SV, Cinelli P, Stephan A, Kistler AD, Rülicke T, Pelczar P, Ledermann B, Fumagalli G, Gloor SM, Kunz B, Sonderegger P. Specific proteolytic cleavage of agrin regulates maturation of the neuromuscular junction. J Cell Sci 2010; 123:3944-55. [DOI: 10.1242/jcs.072090] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
During the initial stage of neuromuscular junction (NMJ) formation, nerve-derived agrin cooperates with muscle-autonomous mechanisms in the organization and stabilization of a plaque-like postsynaptic specialization at the site of nerve–muscle contact. Subsequent NMJ maturation to the characteristic pretzel-like appearance requires extensive structural reorganization. We found that the progress of plaque-to-pretzel maturation is regulated by agrin. Excessive cleavage of agrin via transgenic overexpression of an agrin-cleaving protease, neurotrypsin, in motoneurons resulted in excessive reorganizational activity of the NMJs, leading to rapid dispersal of the synaptic specialization. By contrast, expression of cleavage-resistant agrin in motoneurons slowed down NMJ remodeling and delayed NMJ maturation. Neurotrypsin, which is the sole agrin-cleaving protease in the CNS, was excluded as the physiological agrin-cleaving protease at the NMJ, because NMJ maturation was normal in neurotrypsin-deficient mice. Together, our analyses characterize agrin cleavage at its proteolytic α- and β-sites by an as-yet-unspecified protease as a regulatory access for relieving the agrin-dependent constraint on endplate reorganization during NMJ maturation.
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Affiliation(s)
- Marc F. Bolliger
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Andreas Zurlinden
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
- Neurotune AG, 8952 Schlieren, Switzerland
| | - Daniel Lüscher
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Lukas Bütikofer
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Olga Shakhova
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Maura Francolini
- Department of Medical Pharmacology, University of Milan, 20129 Milan, Italy
| | - Serguei V. Kozlov
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Paolo Cinelli
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Alexander Stephan
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Andreas D. Kistler
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Thomas Rülicke
- Institute of Laboratory Animal Science, University of Zurich, 8091 Zurich, Switzerland
| | - Pawel Pelczar
- Institute of Laboratory Animal Science, University of Zurich, 8091 Zurich, Switzerland
| | - Birgit Ledermann
- Institute of Laboratory Animal Science, University of Zurich, 8091 Zurich, Switzerland
| | - Guido Fumagalli
- Department of Medicine and Public Health, University of Verona, 37134 Verona, Italy
| | - Sergio M. Gloor
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Beat Kunz
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Peter Sonderegger
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
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221
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Feeding conditions control the expression of genes involved in sterol metabolism in peripheral blood mononuclear cells of normoweight and diet-induced (cafeteria) obese rats. J Nutr Biochem 2010; 21:1127-33. [DOI: 10.1016/j.jnutbio.2009.10.001] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2009] [Revised: 09/24/2009] [Accepted: 10/01/2009] [Indexed: 11/22/2022]
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222
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Ahn Y, Sanderson BW, Klein OD, Krumlauf R. Inhibition of Wnt signaling by Wise (Sostdc1) and negative feedback from Shh controls tooth number and patterning. Development 2010; 137:3221-31. [PMID: 20724449 DOI: 10.1242/dev.054668] [Citation(s) in RCA: 161] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Mice carrying mutations in Wise (Sostdc1) display defects in many aspects of tooth development, including tooth number, size and cusp pattern. To understand the basis of these defects, we have investigated the pathways modulated by Wise in tooth development. We present evidence that, in tooth development, Wise suppresses survival of the diastema or incisor vestigial buds by serving as an inhibitor of Lrp5- and Lrp6-dependent Wnt signaling. Reducing the dosage of the Wnt co-receptor genes Lrp5 and Lrp6 rescues the Wise-null tooth phenotypes. Inactivation of Wise leads to elevated Wnt signaling and, as a consequence, vestigial tooth buds in the normally toothless diastema region display increased proliferation and continuous development to form supernumerary teeth. Conversely, gain-of-function studies show that ectopic Wise reduces Wnt signaling and tooth number. Our analyses demonstrate that the Fgf and Shh pathways are major downstream targets of Wise-regulated Wnt signaling. Furthermore, our experiments revealed that Shh acts as a negative-feedback regulator of Wnt signaling and thus determines the fate of the vestigial buds and later tooth patterning. These data provide insight into the mechanisms that control Wnt signaling in tooth development and into how crosstalk among signaling pathways controls tooth number and morphogenesis.
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Affiliation(s)
- Youngwook Ahn
- Stowers Institute for Medical Research, Kansas City, MO 64110, USA
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223
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Rimer M. Modulation of agrin-induced acetylcholine receptor clustering by extracellular signal-regulated kinases 1 and 2 in cultured myotubes. J Biol Chem 2010; 285:32370-7. [PMID: 20696763 DOI: 10.1074/jbc.m110.144774] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Agrin released by motoneurons induces and/or maintains acetylcholine receptor (AChR) clustering and other aspects of postsynaptic differentiation at the vertebrate neuromuscular junction. Agrin acts by binding and activating a receptor complex containing LDL receptor protein 4 (Lrp4) and muscle-specific kinase (MuSK). Two critical downstream components of this signaling cascade, Dox-7 and rapsyn, have been identified. However, additional intracellular essential elements remain unknown. Prior observations by others and us suggested antagonistic interactions between agrin and neuregulin-1 (Nrg-1) signaling in cultured myotubes and developing muscle fibers in vivo. A hallmark of Nrg-1 signaling in skeletal muscle cells is the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). ERK1/2 are also activated in most cells by phorbol 12-myristate 13-acetate, a classical inhibitor of agrin-induced AChR clustering in myotubes. Here, it was investigated whether agrin activates ERK1/2 directly and whether such activation modulates agrin-induced AChR clustering. Agrin induced a rapid but transient activation of ERK1/2 in myotubes that was Lrp4/MuSK-dependent. However, blocking this ERK1/2 activation did not prevent but potentiated AChR clustering induced by agrin. ERK1/2 activation was dispensable for Nrg-1-mediated inhibition of the AChR clustering activity of agrin, but was indispensable for such activity by phorbol 12-myristate 13-acetate. Together, these results suggest agrin-induced activation of ERK1/2 is a negative modulator of agrin signaling in skeletal muscle cells.
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Affiliation(s)
- Mendell Rimer
- Department of Neuroscience and Experimental Therapeutics, College of Medicine, Texas A&M Health Science Center, College Station, Texas 77843, USA.
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224
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Mason JJ, Williams BO. SOST and DKK: Antagonists of LRP Family Signaling as Targets for Treating Bone Disease. J Osteoporos 2010; 2010:460120. [PMID: 20948575 PMCID: PMC2951123 DOI: 10.4061/2010/460120] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/03/2010] [Revised: 04/27/2010] [Accepted: 05/26/2010] [Indexed: 11/22/2022] Open
Abstract
The study of rare human genetic disorders has often led to some of the most significant advances in biomedical research. One such example was the body of work that resulted in the identification of the Low Density Lipoprotein-Related Protein (LRP5) as a key regulator of bone mass. Point mutations were identified that encoded forms of LRP5 associated with very high bone mass (HBM). HBM patients live to a normal age and do not appear to have increased susceptibility to carcinogenesis or other disease. Thus, devising methods to mimic the molecular consequences of this mutation to treat bone diseases associated with low bone mass is a promising avenue to pursue. Two groups of agents related to putative LRP5/6 functions are under development. One group, the focus of this paper, is based on antagonizing the functions of putative inhibitors of Wnt signaling, Dickkopf-1 (DKK1), and Sclerostin (SOST). Another group of reagents under development is based on the observation that LRP5 may function to control bone mass by regulating the secretion of serotonin from the enterrochromaffin cells of the duodenum.
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Affiliation(s)
- James J. Mason
- Center for Skeletal Disease Research, Van Andel Research Institute, 333 Bostwick NE, Grand Rapids, MI 49503, USA,Laboratory of Orthopaedic Cell and Tissue Mechanics, Van Andel Research Institute, 333 Bostwick NE, Grand Rapids, MI 49503, USA
| | - Bart O. Williams
- Center for Skeletal Disease Research, Van Andel Research Institute, 333 Bostwick NE, Grand Rapids, MI 49503, USA,Laboratory of Cell Signaling and Carcinogenesis, Van Andel Research Institute, 333 Bostwick NE, Grand Rapids, MI 49503, USA,*Bart O. Williams:
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225
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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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226
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Karner CM, Dietrich MF, Johnson EB, Kappesser N, Tennert C, Percin F, Wollnik B, Carroll TJ, Herz J. Lrp4 regulates initiation of ureteric budding and is crucial for kidney formation--a mouse model for Cenani-Lenz syndrome. PLoS One 2010; 5:e10418. [PMID: 20454682 PMCID: PMC2861670 DOI: 10.1371/journal.pone.0010418] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2010] [Accepted: 04/09/2010] [Indexed: 12/21/2022] Open
Abstract
Background Development of the kidney is initiated when the ureteric bud (UB) branches from the Wolffian duct and invades the overlying metanephric mesenchyme (MM) triggering the mesenchymal/epithelial interactions that are the basis of organ formation. Multiple signaling pathways must be integrated to ensure proper timing and location of the ureteric bud formation. Methods and Principal Findings We have used gene targeting to create an Lrp4 null mouse line. The mutation results in early embryonic lethality with a subpenetrant phenotype of kidney agenesis. Ureteric budding is delayed with a failure to stimulate the metanephric mesenchyme in a timely manner, resulting in failure of cellular differentiation and resulting absence of kidney formation in the mouse as well as comparable malformations in humans with Cenani-Lenz syndrome. Conclusion Lrp4 is a multi-functional receptor implicated in the regulation of several molecular pathways, including Wnt and Bmp signaling. Lrp4−/− mice show a delay in ureteric bud formation that results in unilateral or bilateral kidney agenesis. These data indicate that Lrp4 is a critical regulator of UB branching and lack of Lrp4 results in congenital kidney malformations in humans and mice.
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Affiliation(s)
- Courtney M. Karner
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Martin F. Dietrich
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Eric B. Johnson
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Natalie Kappesser
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Christian Tennert
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Ferda Percin
- Department of Medical Genetics, Faculty of Medicine, Gazi University, Ankara, Turkey
| | - Bernd Wollnik
- Center for Molecular Medicine Cologne (CMMC) and Institute of Human Genetics, University of Cologne, Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD), Cologne, Germany
| | - Thomas J. Carroll
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Joachim Herz
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
- * E-mail:
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227
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Wu H, Xiong WC, Mei L. To build a synapse: signaling pathways in neuromuscular junction assembly. Development 2010; 137:1017-33. [PMID: 20215342 DOI: 10.1242/dev.038711] [Citation(s) in RCA: 379] [Impact Index Per Article: 27.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.
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Affiliation(s)
- Haitao Wu
- Program of Developmental Neurobiology, Institute of Molecular Medicine and Genetics, Department of Neurology, Medical College of Georgia, Augusta, GA 30912, USA
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228
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Ectodomains of the LDL receptor-related proteins LRP1b and LRP4 have anchorage independent functions in vivo. PLoS One 2010; 5:e9960. [PMID: 20383322 PMCID: PMC2850915 DOI: 10.1371/journal.pone.0009960] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2009] [Accepted: 03/02/2010] [Indexed: 01/20/2023] Open
Abstract
Background The low-density lipoprotein (LDL) receptor gene family is a highly conserved group of membrane receptors with diverse functions in developmental processes, lipoprotein trafficking, and cell signaling. The low-density lipoprotein (LDL) receptor-related protein 1b (LRP1B) was reported to be deleted in several types of human malignancies, including non-small cell lung cancer. Our group has previously reported that a distal extracellular truncation of murine Lrp1b that is predicted to secrete the entire intact extracellular domain (ECD) is fully viable with no apparent phenotype. Methods and Principal Findings Here, we have used a gene targeting approach to create two mouse lines carrying internally rearranged exons of Lrp1b that are predicted to truncate the protein closer to the N-terminus and to prevent normal trafficking through the secretary pathway. Both mutations result in early embryonic lethality, but, as expected from the restricted expression pattern of LRP1b in vivo, loss of Lrp1b does not cause cellular lethality as homozygous Lrp1b-deficient blastocysts can be propagated normally in culture. This is similar to findings for another LDL receptor family member, Lrp4. We provide in vitro evidence that Lrp4 undergoes regulated intramembraneous processing through metalloproteases and γ-secretase cleavage. We further demonstrate negative regulation of the Wnt signaling pathway by the soluble extracellular domain. Conclusions and Significance Our results underline a crucial role for Lrp1b in development. The expression in mice of truncated alleles of Lrp1b and Lrp4 with deletions of the transmembrane and intracellular domains leads to release of the extracellular domain into the extracellular space, which is sufficient to confer viability. In contrast, null mutations are embryonically (Lrp1b) or perinatally (Lrp4) lethal. These findings suggest that the extracellular domains of both proteins may function as a scavenger for signaling ligands or signal modulators in the extracellular space, thereby preserving signaling thresholds that are critical for embryonic development, as well as for the clear, but poorly understood role of LRP1b in cancer.
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229
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Jing L, Gordon LR, Shtibin E, Granato M. Temporal and spatial requirements of unplugged/MuSK function during zebrafish neuromuscular development. PLoS One 2010; 5:e8843. [PMID: 20107509 PMCID: PMC2809748 DOI: 10.1371/journal.pone.0008843] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2009] [Accepted: 12/15/2009] [Indexed: 01/22/2023] Open
Abstract
One of the earliest events in neuromuscular junction (NMJ) development is the accumulation of acetylcholine receptor (AChR) at the center of muscle cells. The unplugged/MuSK (muscle specific tyrosine kinase) gene is essential to initiate AChR clustering but also to restrict approaching growth cones to the muscle center, thereby coordinating pre- and postsynaptic development. To determine how unplugged/MuSK signaling coordinates these two processes, we examined the temporal and spatial requirements of unplugged/MuSK in zebrafish embryos using heat-shock inducible transgenes. Here, we show that despite its expression in muscle cells from the time they differentiate, unplugged/MuSK activity is first required just prior to the appearance of AChR clusters to simultaneously induce AChR accumulation and to guide motor axons. Furthermore, we demonstrate that ectopic expression of unplugged/MuSK throughout the muscle membrane results in wildtype-like AChR prepattern and neuromuscular synapses in the central region of muscle cells. We propose that AChR prepatterning and axonal guidance are spatio-temporally coordinated through common unplugged/MuSK signals, and that additional factor(s) restrict unplugged/MuSK signaling to a central muscle zone critical for establishing mid-muscle synaptogenesis.
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Affiliation(s)
- Lili Jing
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
- Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Laura R. Gordon
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Elena Shtibin
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
| | - Michael Granato
- Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America
- * E-mail:
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230
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Abstract
Neuromuscular diseases can affect the survival of peripheral neurons, their axons extending to peripheral targets, their synaptic connections onto those targets, or the targets themselves. Examples include motor neuron diseases such as amyotrophic lateral sclerosis, peripheral neuropathies, such as Charcot-Marie-Tooth diseases, myasthenias, and muscular dystrophies. Characterizing these phenotypes in mouse models requires an integrated approach, examining both the nerve and the muscle histologically, anatomically, and functionally by electrophysiology. Defects observed at these levels can be related back to onset, severity, and progression, as assessed by "quality-of-life measures" including tests of gross motor performance such as gait or grip strength. This chapter describes methods for assessing neuromuscular disease models in mice, and how interpretation of these tests can be complicated by the inter-relatedness of the phenotypes.
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231
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Madhavan R, Gong ZL, Ma JJ, Chan AWS, Peng HB. The function of cortactin in the clustering of acetylcholine receptors at the vertebrate neuromuscular junction. PLoS One 2009; 4:e8478. [PMID: 20041195 PMCID: PMC2793544 DOI: 10.1371/journal.pone.0008478] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2009] [Accepted: 12/03/2009] [Indexed: 11/18/2022] Open
Abstract
Background Postsynaptic enrichment of acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction (NMJ) depends on the activation of the muscle receptor tyrosine MuSK by neural agrin. Agrin-stimulation of MuSK is known to initiate an intracellular signaling cascade that leads to the clustering of AChRs in an actin polymerization-dependent manner, but the molecular steps which link MuSK activation to AChR aggregation remain incompletely defined. Methodology/Principal Findings In this study we used biochemical, cell biological and molecular assays to investigate a possible role in AChR clustering of cortactin, a protein which is a tyrosine kinase substrate and a regulator of F-actin assembly and which has also been previously localized at AChR clustering sites. We report that cortactin was co-enriched at AChR clusters in situ with its target the Arp2/3 complex, which is a key stimulator of actin polymerization in cells. Cortactin was further preferentially tyrosine phosphorylated at AChR clustering sites and treatment of myotubes with agrin significantly enhanced the tyrosine phosphorylation of cortactin. Importantly, forced expression in myotubes of a tyrosine phosphorylation-defective cortactin mutant (but not wild-type cortactin) suppressed agrin-dependent AChR clustering, as did the reduction of endogenous cortactin levels using RNA interference, and introduction of the mutant cortactin into muscle cells potently inhibited synaptic AChR aggregation in response to innervation. Conclusion Our results suggest a novel function of phosphorylation-dependent cortactin signaling downstream from agrin/MuSK in facilitating AChR clustering at the developing NMJ.
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Affiliation(s)
- Raghavan Madhavan
- Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
| | - Zhuolin L. Gong
- Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
| | - Jin Jin Ma
- Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
| | - Ariel W. S. Chan
- Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
| | - H. Benjamin Peng
- Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
- * E-mail:
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232
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Choi HY, Dieckmann M, Herz J, Niemeier A. Lrp4, a novel receptor for Dickkopf 1 and sclerostin, is expressed by osteoblasts and regulates bone growth and turnover in vivo. PLoS One 2009; 4:e7930. [PMID: 19936252 PMCID: PMC2775917 DOI: 10.1371/journal.pone.0007930] [Citation(s) in RCA: 166] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2009] [Accepted: 10/27/2009] [Indexed: 12/14/2022] Open
Abstract
Lrp4 is a multifunctional member of the low density lipoprotein-receptor gene family and a modulator of extracellular cell signaling pathways in development. For example, Lrp4 binds Wise, a secreted Wnt modulator and BMP antagonist. Lrp4 shares structural elements within the extracellular ligand binding domain with Lrp5 and Lrp6, two established Wnt co-receptors with important roles in osteogenesis. Sclerostin is a potent osteocyte secreted inhibitor of bone formation that directly binds Lrp5 and Lrp6 and modulates both BMP and Wnt signaling. The anti-osteogenic effect of sclerostin is thought to be mediated mainly by inhibition of Wnt signaling through Lrp5/6 within osteoblasts. Dickkopf1 (Dkk1) is another potent soluble Wnt inhibitor that binds to Lrp5 and Lrp6, can displace Lrp5-bound sclerostin and is itself regulated by BMPs. In a recent genome-wide association study of bone mineral density a significant modifier locus was detected near the SOST gene at 17q21, which encodes sclerostin. In addition, nonsynonymous SNPs in the LRP4 gene were suggestively associated with bone mineral density. Here we show that Lrp4 is expressed in bone and cultured osteoblasts and binds Dkk1 and sclerostin in vitro. MicroCT analysis of Lrp4 deficient mutant mice revealed shortened total femur length, reduced cortical femoral perimeter, and reduced total femur bone mineral content (BMC) and bone mineral density (BMD). Lumbar spine trabecular bone volume per total volume (BV/TV) was significantly reduced in the mutants and the serum and urinary bone turnover markers alkaline phosphatase, osteocalcin and desoxypyridinoline were increased. We conclude that Lrp4 is a novel osteoblast expressed Dkk1 and sclerostin receptor with a physiological role in the regulation of bone growth and turnover, which is likely mediated through its function as an integrator of Wnt and BMP signaling pathways.
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Affiliation(s)
- Hong Y. Choi
- Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
| | - Marco Dieckmann
- Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
- Institut für Physiologische Chemie und Pathobiochemie, Johannes Gutenberg-Universität Mainz, Mainz, Germany
| | - Joachim Herz
- Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
- * E-mail:
| | - Andreas Niemeier
- Department of Orthopaedics and IBMII: Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany
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233
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Lee Y, Rudell J, Ferns M. Rapsyn interacts with the muscle acetylcholine receptor via alpha-helical domains in the alpha, beta, and epsilon subunit intracellular loops. Neuroscience 2009; 163:222-32. [PMID: 19482062 PMCID: PMC2728176 DOI: 10.1016/j.neuroscience.2009.05.057] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2009] [Revised: 05/01/2009] [Accepted: 05/25/2009] [Indexed: 12/12/2022]
Abstract
At the developing vertebrate neuromuscular junction, the acetylcholine receptor becomes aggregated at high density in the postsynaptic muscle membrane. Receptor localization is regulated by the motoneuron-derived factor, agrin, and requires an intracellular, scaffolding protein called rapsyn. However, it remains unclear where rapsyn binds on the acetylcholine receptor and how their interaction is regulated. In this study, we identified rapsyn's binding site on the acetylcholine receptor using chimeric constructs where the intracellular domain of CD4 was substituted for the major intracellular loop of each mouse acetylcholine receptor subunit. When expressed in heterologous cells, we found that rapsyn clustered and cytoskeletally anchored CD4-alpha, beta and epsilon subunit loops but not CD4-delta loop. Rapsyn-mediated clustering and anchoring was highest for beta loop, followed by epsilon and alpha, suggesting that rapsyn interacts with the loops with different affinities. Moreover, by making deletions within the beta subunit intracellular loop, we show that rapsyn interacts with the alpha-helical region, a secondary structural motif present in the carboxyl terminal portion of the subunit loops. When expressed in muscle cells, rapsyn co-immunoprecipitated together with a CD4-alpha helical region chimera, independent of agrin signaling. Together, these findings demonstrate that rapsyn interacts with the acetylcholine receptor via an alpha-helical structural motif conserved between the alpha, beta and epsilon subunits. Binding at this site likely mediates the critical rapsyn interaction involved in localizing the acetylcholine receptor at the neuromuscular junction.
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Affiliation(s)
- Y Lee
- Department of Anesthesiology and Physiology and Membrane Biology, University of California Davis, One Shields Avenue, Davis, CA 95616, USA
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234
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Weatherbee SD, Niswander LA, Anderson KV. A mouse model for Meckel syndrome reveals Mks1 is required for ciliogenesis and Hedgehog signaling. Hum Mol Genet 2009; 18:4565-75. [PMID: 19776033 DOI: 10.1093/hmg/ddp422] [Citation(s) in RCA: 110] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
Meckel syndrome (MKS) is a rare autosomal recessive disease causing perinatal lethality associated with a complex syndrome that includes occipital meningoencephalocele, hepatic biliary ductal plate malformation, postaxial polydactyly and polycystic kidneys. The gene mutated in type 1 MKS encodes a protein associated with the base of the cilium in vertebrates and nematodes. However, shRNA knockdown studies in cell culture have reported conflicting results on the role of Mks1 in ciliogenesis. Here we show that loss of function of mouse Mks1 results in an accurate model of human MKS, with structural abnormalities in the neural tube, biliary duct, limb patterning, bone development and the kidney that mirror the human syndrome. In contrast to cell culture studies, loss of Mks1 in vivo does not interfere with apical localization of epithelial basal bodies but rather leads to defective cilia formation in most, but not all, tissues. Analysis of patterning in the neural tube and the limb demonstrates altered Hedgehog (Hh) pathway signaling underlies some MKS defects, although both tissues show an expansion of the domain of response to Shh signaling, unlike the phenotypes seen in other mutants with cilia loss. Other defects in the skull, lung, rib cage and long bones are likely to be the result of the disruption of Hh signaling, and the basis of defects in the liver and kidney require further analysis. Thus the disruption of Hh signaling can explain many, but not all, of the defects caused by loss of Mks1.
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Affiliation(s)
- Scott D Weatherbee
- Developmental Biology Program, Sloan-Kettering Institute, New York, NY 10065, USA.
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235
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Abstract
Although WNTs have been long thought of as regulators of cell fate, recent studies highlight their involvement in crucial aspects of synaptic development in the nervous system. Particularly compelling are recent studies of the neuromuscular junction in nematodes, insects, fish and mammals. These studies place WNTs as major determinants of synapse differentiation and neurotransmitter receptor clustering.
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236
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ter Beek WP, Martínez-Martínez P, Losen M, de Baets MH, Wintzen AR, Verschuuren JJGM, Niks EH, van Duinen SG, Vincent A, Molenaar PC. The effect of plasma from muscle-specific tyrosine kinase myasthenia patients on regenerating endplates. THE AMERICAN JOURNAL OF PATHOLOGY 2009; 175:1536-44. [PMID: 19745065 DOI: 10.2353/ajpath.2009.090040] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Muscle-specific tyrosine kinase (MuSK) is essential for clustering of acetylcholine receptors (AChRs) at embryogenesis and likely also important for maintaining synaptic structure in adult muscle. In 5 to 7% of myasthenia gravis (MG) cases, the patients' blood contains antibodies to MuSK. To investigate the effect of MuSK-MG antibody on synapse regeneration, notexin was used to induce damage to the flexor digitorum brevis muscle. We administered aliquots of MuSK-MG patients' plasma to the flexor digitorum brevis twice daily for a period up to 21 days, and muscles were investigated ex vivo in contraction experiments. AChR levels were measured with (125)I-alpha-bungarotoxin, and endplates were studied with quantitative immunohistochemistry. In normal muscles and in 14-day regenerated muscles, MuSK plasma caused impairment of nerve stimulus-induced contraction in the presence of 0.35 and 0.5 mmol/L Ca(2+) with or without 100 to 400 nmol/L tubocurarine. Endplate size was decreased in regenerated muscles relative to controls; however, we did not observe such differences in muscle not treated with notexin. MuSK plasma had no effect on the amount and turnover rate of AChRs. Our results suggest that anti-MuSK antibodies influence the activity of MuSK molecules without reducing their number, thereby diminishing the size of the endplate and affecting the functioning of AChRs.
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Affiliation(s)
- W Pascale ter Beek
- Neurophysiology Section, Department of Neurology, Leiden University Medical Centre, Leiden, The Netherlands
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237
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Abstract
Clustering of acetylcholine receptors (AChR) in muscle fibers prior to innervation by motor neurons is thought to be involved in neuromuscular junction formation. Jing et al. now report in Neuron that this prepatterning of AChRs, via a novel MuSK-dependent Wnt pathway, may guide motor axons to the central region of muscle fibers for synapse formation in zebrafish.
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238
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Wnt signals organize synaptic prepattern and axon guidance through the zebrafish unplugged/MuSK receptor. Neuron 2009; 61:721-33. [PMID: 19285469 DOI: 10.1016/j.neuron.2008.12.025] [Citation(s) in RCA: 141] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2008] [Revised: 11/13/2008] [Accepted: 12/24/2008] [Indexed: 11/24/2022]
Abstract
Early during neuromuscular development, acetylcholine receptors (AChRs) accumulate at the center of muscle fibers, precisely where motor growth cones navigate and synapses eventually form. Here, we show that Wnt11r binds to the zebrafish unplugged/MuSK ectodomain to organize this central muscle zone. In the absence of such a zone, prepatterned AChRs fail to aggregate and, as visualized by live-cell imaging, growth cones stray from their central path. Using inducible unplugged/MuSK transgenes, we show that organization of the central muscle zone is dispensable for the formation of neural synapses, but essential for AChR prepattern and motor growth cone guidance. Finally, we show that blocking noncanonical dishevelled signaling in muscle fibers disrupts AChR prepatterning and growth cone guidance. We propose that Wnt ligands activate unplugged/MuSK signaling in muscle fibers to restrict growth cone guidance and AChR prepatterns to the muscle center through a mechanism reminiscent of the planar cell polarity pathway.
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239
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Cartaud J, Cartaud A. LRP4 : le co-récepteur de l’agrine enfin identifié ? Med Sci (Paris) 2009; 25:215-6. [DOI: 10.1051/medsci/2009253215] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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240
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Inoue A, Setoguchi K, Matsubara Y, Okada K, Sato N, Iwakura Y, Higuchi O, Yamanashi Y. Dok-7 activates the muscle receptor kinase MuSK and shapes synapse formation. Sci Signal 2009; 2:ra7. [PMID: 19244212 DOI: 10.1126/scisignal.2000113] [Citation(s) in RCA: 78] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The formation of the neuromuscular junction (NMJ) is orchestrated by the muscle-specific receptor tyrosine kinase MuSK and by neural agrin, an extracellular activator of MuSK. We previously showed that the MuSK-interacting protein Dok-7 is essential for neuromuscular synaptogenesis, although the mechanisms by which Dok-7 regulates MuSK activity and promotes synapse formation have been unclear. Here, we show that Dok-7 directly interacts with the cytoplasmic portion of MuSK and activates the receptor tyrosine kinase, and that neural agrin requires Dok-7 to activate MuSK. In vivo overexpression of Dok-7 increased MuSK activation and promoted NMJ formation. Furthermore, Dok-7 was required for the localization of MuSK in the central region of muscle, which is essential for the correct formation of NMJs in this region. These observations indicate that Dok-7 positively regulates neuromuscular synaptogenesis by controlling MuSK activity, its distribution, and its responsiveness to neural agrin.
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Affiliation(s)
- Akane Inoue
- Division of Genetics and Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan
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241
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Abstract
Two recent papers reported identification of a long-sought agrin coreceptor, Lrp4 (Kim et al. in Cell and Zhang et al. in Neuron). In this issue of Neuron, Linnoila et al. report the identification of a new player in the agrin-MuSK pathway, Tid1, which directly interacts with MuSK and is responsible for transducing signals from MuSK activation to AChR clustering, culminating in cross-linking to the subsynaptic cytoskeleton. These papers substantially reshape the agrin-MuSK-ACh hypothesis of neuromuscular synaptogenesis.
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242
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Abstract
The heparan sulfate proteoglycan agrin is best known for its essential role during formation, maintenance and regeneration of the neuromuscular junction. Mutations in agrin-interacting proteins are the genetic basis for a number of neuromuscular disorders. However, agrin is widely expressed in many tissues including neurons and glial cells of the brain, where its precise function is much less understood. Fewer synapses develop in brains that lack agrin, consistent with a function of agrin during CNS synaptogenesis. Recently, a specific transmembrane form of agrin (TM-agrin) was identified that is concentrated at that interneuronal synapses in the brain. Clustering or overexpression of TM-agrin leads to the formation of filopodia-like processes, which might be precursors for CNS synapses. Agrin is subject to defined and activity-dependent proteolytic cleavage by neurotrypsin at synapses and dysregulation of agrin processing might contribute to the development of mental retardation. This review summarizes what is known about the role of agrin during synapse formation at the neuromuscular junction and in the developing CNS and will discuss additional functions of agrin in the adult CNS, in particular during BBB formation, during recovery after traumatic brain injury and in the etiology of diseases, including Alzheimer’s disease and mental retardation.
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Affiliation(s)
- Stephan Kröger
- Department of Physiological Genomics, Ludwig-Maximilians University, Schillerstrasse 46, D-80336 Munich, Germany
| | - Heike Pfister
- Department of Physiological Genomics, Ludwig-Maximilians University, Schillerstrasse 46, D-80336 Munich, Germany
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243
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Ohazama A, Johnson EB, Ota MS, Choi HJ, Porntaveetus T, Oommen S, Itoh N, Eto K, Gritli-Linde A, Herz J, Sharpe PT. Lrp4 modulates extracellular integration of cell signaling pathways in development. PLoS One 2008; 3:e4092. [PMID: 19116665 PMCID: PMC2605561 DOI: 10.1371/journal.pone.0004092] [Citation(s) in RCA: 157] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2008] [Accepted: 11/21/2008] [Indexed: 11/18/2022] Open
Abstract
The extent to which cell signaling is integrated outside the cell is not currently appreciated. We show that a member of the low-density receptor-related protein family, Lrp4 modulates and integrates Bmp and canonical Wnt signalling during tooth morphogenesis by binding the secreted Bmp antagonist protein Wise. Mouse mutants of Lrp4 and Wise exhibit identical tooth phenotypes that include supernumerary incisors and molars, and fused molars. We propose that the Lrp4/Wise interaction acts as an extracellular integrator of epithelial-mesenchymal cell signaling. Wise, secreted from mesenchyme cells binds to BMP's and also to Lrp4 that is expressed on epithelial cells. This binding then results in the modulation of Wnt activity in the epithelial cells. Thus in this context Wise acts as an extracellular signaling molecule linking two signaling pathways. We further show that a downstream mediator of this integration is the Shh signaling pathway.
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Affiliation(s)
- Atsushi Ohazama
- Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London, United Kingdom
| | - Eric B. Johnson
- Department of Molecular Genetics, UT Southwestern Medical Center, Dallas, Texas, United States of America
| | - Masato S. Ota
- Section of Molecular Craniofacial Embryology, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
| | - Hong J. Choi
- Department of Molecular Genetics, UT Southwestern Medical Center, Dallas, Texas, United States of America
| | - Thantrira Porntaveetus
- Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London, United Kingdom
| | - Shelly Oommen
- Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London, United Kingdom
| | - Nobuyuki Itoh
- Department of Genetic Biochemistry, Kyoto University Graduate School of Pharmaceutical Sciences, Kyoto, Japan
| | - Kazuhiro Eto
- Section of Molecular Craniofacial Embryology, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
| | - Amel Gritli-Linde
- Department of Oral Biochemistry, Sahlgrenska Academy at Goteborg University, Goteborg, Sweden
| | - Joachim Herz
- Department of Molecular Genetics, UT Southwestern Medical Center, Dallas, Texas, United States of America
| | - Paul T. Sharpe
- Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London, United Kingdom
- * E-mail:
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244
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Dobbins GC, Luo S, Yang Z, Xiong WC, Mei L. alpha-Actinin interacts with rapsyn in agrin-stimulated AChR clustering. Mol Brain 2008; 1:18. [PMID: 19055765 PMCID: PMC2621155 DOI: 10.1186/1756-6606-1-18] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2008] [Accepted: 12/03/2008] [Indexed: 11/10/2022] Open
Abstract
AChR is concentrated at the postjunctional membrane at the neuromuscular junction. However, the underlying mechanism is unclear. We show that α-actinin, a protein known to cross-link F-actin, interacts with rapsyn, a scaffold protein essential for neuromuscular junction formation. α-Actinin, rapsyn, and surface AChR form a ternary complex. Moreover, the rapsyn-α-actinin interaction is increased by agrin, a factor known to stimulate AChR clustering. Downregulation of α-actinin expression inhibits agrin-mediated AChR clustering. Furthermore, the rapsyn-α-actinin interaction can be disrupted by inhibiting Abl and by cholinergic stimulation. Together these results indicate a role for α-actinin in AChR clustering.
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Affiliation(s)
- G Clement Dobbins
- Institute of Molecular Medicine and Genetics, Department of Neurobiology, Medical College of Georgia, Augusta, Georgia 30912, USA.
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245
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Zhang B, Luo S, Wang Q, Suzuki T, Xiong WC, Mei L. LRP4 serves as a coreceptor of agrin. Neuron 2008; 60:285-97. [PMID: 18957220 DOI: 10.1016/j.neuron.2008.10.006] [Citation(s) in RCA: 408] [Impact Index Per Article: 25.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2008] [Revised: 10/09/2008] [Accepted: 10/08/2008] [Indexed: 11/17/2022]
Abstract
Neuromuscular junction (NMJ) formation requires agrin, a factor released from motoneurons, and MuSK, a transmembrane tyrosine kinase that is activated by agrin. However, how signal is transduced from agrin to MuSK remains unclear. We report that LRP4, a low-density lipoprotein receptor (LDLR)-related protein, is expressed specifically in myotubes and binds to neuronal agrin. Its expression enables agrin binding and MuSK signaling in cells that otherwise do not respond to agrin. Suppression of LRP4 expression in muscle cells attenuates agrin binding, agrin-induced MuSK tyrosine phosphorylation, and AChR clustering. LRP4 also forms a complex with MuSK in a manner that is stimulated by agrin. Finally, we showed that LRP4 becomes tyrosine-phosphorylated in agrin-stimulated muscle cells. These observations indicate that LRP4 is a coreceptor of agrin that is necessary for MuSK signaling and AChR clustering and identify a potential target protein whose mutation and/or autoimmunization may cause muscular dystrophies.
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Affiliation(s)
- Bin Zhang
- Program of Developmental Neurobiology, Institute of Molecular Medicine and Genetics, Department of Neurology, Medical College of Georgia, Augusta, GA 30912, USA
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246
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Herz J, Chen Y, Masiulis I, Zhou L. Expanding functions of lipoprotein receptors. J Lipid Res 2008; 50 Suppl:S287-92. [PMID: 19017612 DOI: 10.1194/jlr.r800077-jlr200] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Lipoprotein receptors are evolutionarily ancient proteins that are expressed on the surface of many cell types. Beginning with the appearance of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors evolved. Originally, these cell surface proteins were thought to merely mediate the traffic of lipids and nutrients between cells and, in some cases, by functioning as scavenger receptors, remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular space and the cell surface. Over the last decade, this picture has fundamentally changed. We now appreciate that many of these receptors are not mere cargo transporters; they are deeply embedded in the machinery by which cells communicate with each other. By physically interacting and coevolving with fundamental signaling pathways, lipoprotein receptors have occupied essential and surprisingly diverse functions that are indispensable for integrating the complex web of cellular signal input during development and in differentiated tissues.
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Affiliation(s)
- Joachim Herz
- Department of Molecular Genetics, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9046, USA.
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247
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Kim N, Stiegler AL, Cameron TO, Hallock PT, Gomez AM, Huang JH, Hubbard SR, Dustin ML, Burden SJ. Lrp4 is a receptor for Agrin and forms a complex with MuSK. Cell 2008; 135:334-42. [PMID: 18848351 DOI: 10.1016/j.cell.2008.10.002] [Citation(s) in RCA: 497] [Impact Index Per Article: 31.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2008] [Revised: 09/12/2008] [Accepted: 10/01/2008] [Indexed: 12/14/2022]
Abstract
Neuromuscular synapse formation requires a complex exchange of signals between motor neurons and skeletal muscle fibers, leading to the accumulation of postsynaptic proteins, including acetylcholine receptors in the muscle membrane and specialized release sites, or active zones in the presynaptic nerve terminal. MuSK, a receptor tyrosine kinase that is expressed in skeletal muscle, and Agrin, a motor neuron-derived ligand that stimulates MuSK phosphorylation, play critical roles in synaptic differentiation, as synapses do not form in their absence, and mutations in MuSK or downstream effectors are a major cause of a group of neuromuscular disorders, termed congenital myasthenic syndromes (CMS). How Agrin activates MuSK and stimulates synaptic differentiation is not known and remains a fundamental gap in our understanding of signaling at neuromuscular synapses. Here, we report that Lrp4, a member of the LDLR family, is a receptor for Agrin, forms a complex with MuSK, and mediates MuSK activation by Agrin.
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Affiliation(s)
- Natalie Kim
- Molecular Neurobiology Program, Skirball Institute of Biomolecular Medicine, Helen and Martin Kimmel Center for Biology and Medicine, NYU Medical School, New York, NY 10016, USA
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248
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Meltrin beta/ADAM19 interacting with EphA4 in developing neural cells participates in formation of the neuromuscular junction. PLoS One 2008; 3:e3322. [PMID: 18830404 PMCID: PMC2552171 DOI: 10.1371/journal.pone.0003322] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2008] [Accepted: 09/11/2008] [Indexed: 12/16/2022] Open
Abstract
Background Development of the neuromuscular junction (NMJ) is initiated by the formation of postsynaptic specializations in the central zones of muscles, followed by the arrival of motor nerve terminals opposite the postsynaptic regions. The post- and presynaptic components are then stabilized and modified to form mature synapses. Roles of ADAM (A Disintegrin And Metalloprotease) family proteins in the formation of the NMJ have not been reported previously. Principal Findings We report here that Meltrin β, ADAM19, participates in the formation of the NMJ. The zone of acetylcholine receptor α mRNA distribution was broader and excess sprouting of motor nerve terminals was more prominent in meltrin β–deficient than in wild-type embryonic diaphragms. A microarray analysis revealed that the preferential distribution of ephrin-A5 mRNA in the synaptic region of muscles was aberrant in the meltrin β–deficient muscles. Excess sprouting of motor nerve terminals was also found in ephrin-A5 knockout mice, which lead us to investigate a possible link between Meltrin β and ephrin-A5-Eph signaling in the development of the NMJ. Meltrin β and EphA4 interacted with each other in developing motor neurons, and both of these proteins localized in the NMJ. Coexpression of Meltrin β and EphA4 strongly blocked vesicular internalization of ephrin-A5–EphA4 complexes without requiring the protease activity of Meltrin β, suggesting a regulatory role of Meltrin β in ephrin-A5-Eph signaling. Conclusion Meltrin β plays a regulatory role in formation of the NMJ. The endocytosis of ephrin-Eph complexes is required for efficient contact-dependent repulsion between ephrin and Eph. We propose that Meltrin β stabilizes the interaction between ephrin-A5 and EphA4 by regulating endocytosis of the ephrinA5-EphA complex negatively, which would contribute to the fine-tuning of the NMJ during development.
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Kim ML, Chandrasekharan K, Glass M, Shi S, Stahl MC, Kaspar B, Stanley P, Martin PT. O-fucosylation of muscle agrin determines its ability to cluster acetylcholine receptors. Mol Cell Neurosci 2008; 39:452-64. [PMID: 18775496 DOI: 10.1016/j.mcn.2008.07.026] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2008] [Revised: 07/29/2008] [Accepted: 07/30/2008] [Indexed: 10/21/2022] Open
Abstract
Protein O-fucosyltransferase 1 (Pofut1) transfers fucose to serine or threonine on proteins, including Notch receptors, that contain EGF repeats with a particular consensus sequence. Here we demonstrate that agrin is O-fucosylated in a Pofut1-dependent manner, and that this glycosylation can regulate agrin function. Fucosylation of recombinant C45 agrin, both active (neural, z8) and inactive (muscle, z0) splice forms, was eliminated when agrin was overexpressed in Pofut1-deficient cells or by mutation of a consensus site for Pofut1 fucosylation (serine 1726 in the EGF4 domain). Loss of O-fucosylation caused a gain of function for muscle agrin such that it stimulated AChR clustering and MuSK phosphorylation in cultured myotubes at levels normally only found with the neural splice form. Deletion of Pofut1 in cultured primary myotubes and in adult skeletal muscle increased AChR aggregation. In addition, Pofut1 gene and protein expression and Pofut1 activity of the EGF4 domain of agrin were modulated during neuromuscular development. These data are consistent with a role for Pofut1 in AChR aggregation during synaptogenesis via the regulation of the synaptogenic activity of muscle agrin.
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Affiliation(s)
- Mi-Lyang Kim
- Center for Gene Therapy, The Research Institute at Nationwide Children's Hospital, Columbus, OH 43205, USA
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Muscle-wide secretion of a miniaturized form of neural agrin rescues focal neuromuscular innervation in agrin mutant mice. Proc Natl Acad Sci U S A 2008; 105:11406-11. [PMID: 18685098 DOI: 10.1073/pnas.0801683105] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Agrin and its receptor MuSK are required for the formation of the postsynaptic apparatus at the neuromuscular junction (NMJ). In the current model the local deposition of agrin by the nerve and the resulting local activation of MuSK are responsible for creating and maintaining the postsynaptic apparatus including clusters of acetylcholine receptors (AChRs). Concomitantly, the release of acetylcholine (ACh) and the resulting depolarization disperses those postsynaptic structures that are not apposed by the nerve and thus not stabilized by agrin-MuSK signaling. Here we show that a miniaturized form of agrin, consisting of the laminin-binding and MuSK-activating domains, is sufficient to fully restore NMJs in agrin mutant mice when expressed by developing muscle. Although miniagrin is expressed uniformly throughout muscle fibers and induces ectopic AChR clusters, the size and the number of those AChR clusters contacted by the motor nerve increase during development. We provide experimental evidence that this is due to ACh, because the AChR agonist carbachol stabilizes AChR clusters in organotypic cultures of embryonic diaphragms. In summary, our results show that agrin function in NMJ development requires only two small domains, and that this function does not depend on the local deposition of agrin at synapses. Finally, they suggest a novel local function of ACh in stabilizing postsynaptic structures.
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