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Nature Jul 2017The adult mammalian heart is non-regenerative owing to the post-mitotic nature of cardiomyocytes. The neonatal mouse heart can regenerate, but only during the first week...
The adult mammalian heart is non-regenerative owing to the post-mitotic nature of cardiomyocytes. The neonatal mouse heart can regenerate, but only during the first week of life. Here we show that changes in the composition of the extracellular matrix during this week can affect cardiomyocyte growth and differentiation in mice. We identify agrin, a component of neonatal extracellular matrix, as required for the full regenerative capacity of neonatal mouse hearts. In vitro, recombinant agrin promotes the division of cardiomyocytes that are derived from mouse and human induced pluripotent stem cells through a mechanism that involves the disassembly of the dystrophin-glycoprotein complex, and Yap- and ERK-mediated signalling. In vivo, a single administration of agrin promotes cardiac regeneration in adult mice after myocardial infarction, although the degree of cardiomyocyte proliferation observed in this model suggests that there are additional therapeutic mechanisms. Together, our results uncover a new inducer of mammalian heart regeneration and highlight fundamental roles of the extracellular matrix in cardiac repair.
Topics: Adaptor Proteins, Signal Transducing; Agrin; Animals; Animals, Newborn; Cell Cycle Proteins; Cell Proliferation; Dystroglycans; Extracellular Matrix Proteins; Female; Heart; Mice; Myocardial Infarction; Myocardium; Myocytes, Cardiac; Phosphoproteins; Regeneration; YAP-Signaling Proteins
PubMed: 28581497
DOI: 10.1038/nature22978 -
Cells Nov 2019The vertebrate skeletal neuromuscular junction (NMJ) has long served as a model system for studying synapse structure, function, and development. Over the last several... (Review)
Review
The vertebrate skeletal neuromuscular junction (NMJ) has long served as a model system for studying synapse structure, function, and development. Over the last several decades, a neuron-specific isoform of agrin, a heparan sulfate proteoglycan, has been identified as playing a central role in synapse formation at all vertebrate skeletal neuromuscular synapses. While agrin was initially postulated to be the inductive molecule that initiates synaptogenesis, this model has been modified in response to work showing that postsynaptic differentiation can develop in the absence of innervation, and that synapses can form in transgenic mice in which the agrin gene is ablated. In place of a unitary mechanism for neuromuscular synapse formation, studies in both mice and zebrafish have led to the proposal that two mechanisms mediate synaptogenesis, with some synapses being induced by nerve contact while others involve the incorporation of prepatterned postsynaptic structures. Moreover, the current model also proposes that agrin can serve two functions, to induce synaptogenesis and to stabilize new synapses, once these are formed. This review examines the evidence for these propositions, and concludes that it remains possible that a single molecular mechanism mediates synaptogenesis at all NMJs, and that agrin acts as a stabilizer, while its role as inducer is open to question. Moreover, if agrin does not act to initiate synaptogenesis, it follows that as yet uncharacterized molecular interactions are required to play this essential inductive role. Several alternatives to agrin for this function are suggested, including focal pericellular proteolysis and integrin signaling, but all require experimental validation.
Topics: Agrin; Animals; Humans; Models, Animal; Neurogenesis; Neuromuscular Junction; Neurons; Synapses
PubMed: 31744142
DOI: 10.3390/cells8111448 -
Cell Reports Sep 2022The neuromuscular junction (NMJ) is an essential synapse whose loss is a key hallmark of the neurodegenerative disease spinal muscular atrophy (SMA). Here, we show that...
The neuromuscular junction (NMJ) is an essential synapse whose loss is a key hallmark of the neurodegenerative disease spinal muscular atrophy (SMA). Here, we show that activity of the SMA-determining SMN protein in the assembly of U7 small nuclear ribonucleoprotein (snRNP)-which functions in the 3'-end processing of replication-dependent histone mRNAs-is required for NMJ integrity. Co-expression of U7-specific Lsm10 and Lsm11 proteins selectively enhances U7 snRNP assembly, corrects histone mRNA processing defects, and rescues key structural and functional abnormalities of neuromuscular pathology in SMA mice-including NMJ denervation, decreased synaptic transmission, and skeletal muscle atrophy. Furthermore, U7 snRNP dysfunction drives selective loss of the synaptic organizing protein Agrin at NMJs innervating vulnerable muscles of SMA mice. These findings reveal a direct contribution of U7 snRNP dysfunction to neuromuscular pathology in SMA and suggest a role for histone gene regulation in maintaining functional synaptic connections between motor neurons and muscles.
Topics: Agrin; Animals; Histones; Mice; Muscular Atrophy, Spinal; Neurodegenerative Diseases; Neuromuscular Junction; RNA, Messenger; Ribonucleoprotein, U7 Small Nuclear
PubMed: 36130491
DOI: 10.1016/j.celrep.2022.111393 -
World Journal of Gastroenterology Jan 2016Proteoglycans are a group of molecules that contain at least one glycosaminoglycan chain, such as a heparan, dermatan, chondroitin, or keratan sulfate, covalently... (Review)
Review
Proteoglycans are a group of molecules that contain at least one glycosaminoglycan chain, such as a heparan, dermatan, chondroitin, or keratan sulfate, covalently attached to the protein core. These molecules are categorized based on their structure, localization, and function, and can be found in the extracellular matrix, on the cell surface, and in the cytoplasm. Cell-surface heparan sulfate proteoglycans, such as syndecans, are the primary type present in healthy liver tissue. However, deterioration of the liver results in overproduction of other proteoglycan types. The purpose of this article is to provide a current summary of the most relevant data implicating proteoglycans in the development and progression of human and experimental liver cancer. A review of our work and other studies in the literature indicate that deterioration of liver function is accompanied by an increase in the amount of chondroitin sulfate proteoglycans. The alteration of proteoglycan composition interferes with the physiologic function of the liver on several levels. This article details and discusses the roles of syndecan-1, glypicans, agrin, perlecan, collagen XVIII/endostatin, endocan, serglycin, decorin, biglycan, asporin, fibromodulin, lumican, and versican in liver function. Specifically, glypicans, agrin, and versican play significant roles in the development of liver cancer. Conversely, the presence of decorin could potentially provide protective effects.
Topics: Agrin; Animals; Carcinoma, Hepatocellular; Glycosaminoglycans; Glypicans; Heparan Sulfate Proteoglycans; Humans; Liver Neoplasms; Liver Neoplasms, Experimental; Proteoglycans; Syndecan-1; Versicans
PubMed: 26755884
DOI: 10.3748/wjg.v22.i1.379 -
Neuroscience Letters Jan 2020The receptor tyrosine kinase MuSK (muscle-specific kinase) is the key signaling molecule during the formation of a mature and functional neuromuscular junction (NMJ).... (Review)
Review
The receptor tyrosine kinase MuSK (muscle-specific kinase) is the key signaling molecule during the formation of a mature and functional neuromuscular junction (NMJ). Signal transduction events downstream of MuSK activation induce both pre- and postsynaptic differentiation, which, most prominently, includes the clustering of acetylcholine receptors (AChRs) at synaptic sites. MuSK activation requires a complex interplay between its co-receptor Lrp4 (low-density lipoprotein receptor-related protein-4), the motor neuron-derived heparan-sulfate proteoglycan Agrin and the intracellular adaptor protein Dok-7. A tight regulation of MuSK kinase activity is crucial for proper NMJ development. Defects in MuSK signaling are the cause of muscle weakness as reported in congenital myasthenic syndromes and myasthenia gravis. This review focuses on recent structure-based analyses of MuSK, Agrin, Lrp4 and Dok-7 interactions and their function during MuSK activation. Conclusions about the regulation of the MuSK kinase that were derived from molecular structures will be highlighted. In addition, the role of MuSK during development and disease will be discussed.
Topics: Agrin; Animals; Humans; LDL-Receptor Related Proteins; Muscle Proteins; Neuromuscular Junction; Neuromuscular Junction Diseases; Receptor Protein-Tyrosine Kinases; Receptors, Cholinergic
PubMed: 31811897
DOI: 10.1016/j.neulet.2019.134676 -
Nature Jul 2021Congenital myasthenia (CM) is a devastating neuromuscular disease, and mutations in DOK7, an adaptor protein that is crucial for forming and maintaining neuromuscular...
Congenital myasthenia (CM) is a devastating neuromuscular disease, and mutations in DOK7, an adaptor protein that is crucial for forming and maintaining neuromuscular synapses, are a major cause of CM. The most common disease-causing mutation (DOK7) truncates DOK7 and leads to the loss of two tyrosine residues that are phosphorylated and recruit CRK proteins, which are important for anchoring acetylcholine receptors at synapses. Here we describe a mouse model of this common form of CM (Dok7 mice) and a mouse with point mutations in the two tyrosine residues (Dok7). We show that Dok7 mice had severe deficits in neuromuscular synapse formation that caused neonatal lethality. Unexpectedly, these deficits were due to a severe deficiency in phosphorylation and activation of muscle-specific kinase (MUSK) rather than a deficiency in DOK7 tyrosine phosphorylation. We developed agonist antibodies against MUSK and show that these antibodies restored neuromuscular synapse formation and prevented neonatal lethality and late-onset disease in Dok7 mice. These findings identify an unexpected cause for disease and a potential therapy for both DOK7 CM and other forms of CM caused by mutations in AGRIN, LRP4 or MUSK, and illustrate the potential of targeted therapy to rescue congenital lethality.
Topics: Aging; Agrin; Animals; Animals, Newborn; Antibodies; Disease Models, Animal; Female; LDL-Receptor Related Proteins; Male; Mice; Molecular Targeted Therapy; Muscle Fibers, Skeletal; Muscle Proteins; Mutation; Myasthenic Syndromes, Congenital; Phosphorylation; Phosphotyrosine; Proto-Oncogene Proteins c-crk; Receptor Protein-Tyrosine Kinases; Recurrence; Synapses
PubMed: 34163073
DOI: 10.1038/s41586-021-03672-3 -
Proceedings of the National Academy of... Jun 2023MuSK is a receptor tyrosine kinase (RTK) that plays essential roles in the formation and maintenance of the neuromuscular junction. Distinct from most members of RTK...
MuSK is a receptor tyrosine kinase (RTK) that plays essential roles in the formation and maintenance of the neuromuscular junction. Distinct from most members of RTK family, MuSK activation requires not only its cognate ligand agrin but also its coreceptors LRP4. However, how agrin and LRP4 coactivate MuSK remains unclear. Here, we report the cryo-EM structure of the extracellular ternary complex of agrin/LRP4/MuSK in a stoichiometry of 1:1:1. This structure reveals that arc-shaped LRP4 simultaneously recruits both agrin and MuSK to its central cavity, thereby promoting a direct interaction between agrin and MuSK. Our cryo-EM analyses therefore uncover the assembly mechanism of agrin/LRP4/MuSK signaling complex and reveal how MuSK receptor is activated by concurrent binding of agrin and LRP4.
Topics: Receptors, Cholinergic; Agrin; LDL-Receptor Related Proteins; Signal Transduction; Neuromuscular Junction; Receptor Protein-Tyrosine Kinases
PubMed: 37252960
DOI: 10.1073/pnas.2300453120 -
Current Opinion in Clinical Nutrition... May 2016Denervation is a hallmark of age-related and other types of muscle wasting. This review focuses on recent insights and current viewpoints regarding the mechanisms and... (Review)
Review
PURPOSE OF REVIEW
Denervation is a hallmark of age-related and other types of muscle wasting. This review focuses on recent insights and current viewpoints regarding the mechanisms and clinical relevance of maintaining the neuromuscular junction to counteract muscle wasting resulting from aging or neural disease/damage.
RECENT FINDINGS
Activity-dependent regulation of autophagy, the agrin-muscle specific kinase-Lrp4 signaling axis, and sympathetic modulation are principal mechanisms involved in stabilizing the neuromuscular junction. These findings are derived from several animal models and were largely confirmed by human gene expression analysis as well as insights from rare neuromuscular diseases such as amyotrophic lateral sclerosis and congenital myasthenic syndromes. Based on these insights, agrin-derived fragments are currently being evaluated as biomarkers for age-related muscle wasting. Tuning of autophagy, of the agrin pathway, and of sympathetic input are being studied as clinical treatment of muscle wasting disorders.
SUMMARY
Basic research has revealed that maintenance of neuromuscular junctions and a few signaling pathways are important in the context of age-dependent and other forms of muscle wasting. These findings have recently started to enter clinical practice, but further research needs to substantiate and refine our knowledge.
Topics: Animals; Autophagy; Gene Expression Regulation; Humans; Models, Biological; Muscle Proteins; Muscular Atrophy; Nerve Degeneration; Nerve Tissue Proteins; Neuromuscular Junction; Neuromuscular Junction Diseases; Wasting Syndrome; Wnt Signaling Pathway
PubMed: 26870889
DOI: 10.1097/MCO.0000000000000267 -
Cell Adhesion & Migration 2013The heterotrimeric laminins are a defining component of all basement membranes and self-assemble into a cell-associated network. The three short arms of the cross-shaped... (Review)
Review
The heterotrimeric laminins are a defining component of all basement membranes and self-assemble into a cell-associated network. The three short arms of the cross-shaped laminin molecule form the network nodes, with a strict requirement for one α, one β and one γ arm. The globular domain at the end of the long arm binds to cellular receptors, including integrins, α-dystroglycan, heparan sulfates and sulfated glycolipids. Collateral anchorage of the laminin network is provided by the proteoglycans perlecan and agrin. A second network is then formed by type IV collagen, which interacts with the laminin network through the heparan sulfate chains of perlecan and agrin and additional linkage by nidogen. This maturation of basement membranes becomes essential at later stages of embryo development.
Topics: Agrin; Animals; Basement Membrane; Collagen Type IV; Cytoskeleton; Dystroglycans; Extracellular Matrix; Heparan Sulfate Proteoglycans; Humans; Integrin alpha Chains; Laminin; Polymerization; Protein Binding; Protein Interaction Mapping; Protein Structure, Tertiary
PubMed: 23076216
DOI: 10.4161/cam.21831 -
Severe congenital myasthenic syndromes caused by agrin mutations affecting secretion by motoneurons.Acta Neuropathologica Oct 2022Congenital myasthenic syndromes (CMS) are predominantly characterized by muscle weakness and fatigability and can be caused by a variety of mutations in genes required...
Congenital myasthenic syndromes (CMS) are predominantly characterized by muscle weakness and fatigability and can be caused by a variety of mutations in genes required for neuromuscular junction formation and maintenance. Among them, AGRN encodes agrin, an essential synaptic protein secreted by motoneurons. We have identified severe CMS patients with uncharacterized p.R1671Q, p.R1698P and p.L1664P mutations in the LG2 domain of agrin. Overexpression in primary motoneurons cultures in vitro and in chick spinal motoneurons in vivo revealed that the mutations modified agrin trafficking, leading to its accumulation in the soma and/or in the axon. Expression of mutant agrins in cultured cells demonstrated accumulation of agrin in the endoplasmic reticulum associated with induction of unfolded protein response (UPR) and impaired secretion in the culture medium. Interestingly, evaluation of the specific activity of individual agrins on AChR cluster formation indicated that when secreted, mutant agrins retained a normal capacity to trigger the formation of AChR clusters. To confirm agrin accumulation and secretion defect, iPS cells were derived from a patient and differentiated into motoneurons. Patient iPS-derived motoneurons accumulated mutant agrin in the soma and increased XBP1 mRNA splicing, suggesting UPR activation. Moreover, co-cultures of patient iPS-derived motoneurons with myotubes confirmed the deficit in agrin secretion and revealed a reduction in motoneuron survival. Altogether, we report the first mutations in AGRN gene that specifically affect agrin secretion by motoneurons. Interestingly, the three patients carrying these mutations were initially suspected of spinal muscular atrophy (SMA). Therefore, in the presence of patients with a clinical presentation of SMA but without mutation in the SMN1 gene, it can be worth to look for mutations in AGRN.
Topics: Agrin; Humans; Motor Neurons; Mutation; Myasthenic Syndromes, Congenital; Neuromuscular Junction
PubMed: 35948834
DOI: 10.1007/s00401-022-02475-8