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Respiratory Physiology & Neurobiology Nov 2021The pathogenesis of obstructive sleep apnea (OSA) remains controversial. The role of anatomic stenosis is indisputable, and neural regulation of the upper airway remains...
The pathogenesis of obstructive sleep apnea (OSA) remains controversial. The role of anatomic stenosis is indisputable, and neural regulation of the upper airway remains to be elucidated. The upper airway maintains patency through the upper airway reflex. Lesions in any link of the reflex can increase the collapsibility of the upper airway. In this study, we investigated sensorimotor nerve lesions and their possible relationship with OSA. Tissue samples were obtained from the pharyngopalatine arch in 47 patients with OSA and 45 control participants to examine changes in the expression levels of myelin basic protein (MBP) and agrin through immunohistochemistry and western blotting. Downregulation of MBP in the mucosa reflects myelinated degeneration of mucosal sensory nerve axons, whereas upregulation of agrin in the neuromuscular junction reflects synaptic regeneration following denervation. The two neural factors correlate significantly with polysomnographic parameters, such as the apnea hypopnea index and lowest oxygen saturation. Our findings suggest that sensorimotor nerve damage in the upper airway of patients with OSA may be associated closely with the mechanism of OSA.
Topics: Adult; Agrin; Axons; Female; Humans; Male; Middle Aged; Myelin Basic Protein; Neuromuscular Junction; Patient Acuity; Polysomnography; Respiratory Mucosa; Sensory Receptor Cells; Sleep Apnea, Obstructive; Young Adult
PubMed: 34146730
DOI: 10.1016/j.resp.2021.103720 -
Molecular Neurobiology Dec 2022Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its...
Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.
Topics: Agrin; Muscle Fibers, Skeletal; Neurons; Heparin; Synapses
PubMed: 36197591
DOI: 10.1007/s12035-022-03056-2 -
Scientific Reports Sep 2015MuSK antibody-positive myasthenia gravis (MuSK-MG) accounts for 5 to 15% of autoimmune MG. MuSK and LRP4 are coreceptors for agrin in the signaling pathway that causes...
MuSK antibody-positive myasthenia gravis (MuSK-MG) accounts for 5 to 15% of autoimmune MG. MuSK and LRP4 are coreceptors for agrin in the signaling pathway that causes clustering of acetylcholine receptor (AChR). MuSK also anchors the acetylcholinesterase (AChE)/collagen Q (ColQ) complex to the synaptic basal lamina. We previously reported that anti-MuSK antibodies (MuSK-IgG) block binding of ColQ to MuSK and cause partial endplate AChE deficiency in mice. We here analyzed the physiological significance of binding of ColQ to MuSK and block of this binding by MuSK-IgG. In vitro plate-binding assay showed that MuSK-IgG blocked MuSK-LRP4 interaction in the presence of agrin. Passive transfer of MuSK-IgG to Colq-knockout mice attenuated AChR clustering, indicating that lack of ColQ is not the key event causing defective clustering of AChR in MuSK-MG. In three MuSK-MG patients, the MuSK antibodies recognized the first and fourth immunoglobulin-like domains (Ig1 and Ig4) of MuSK. In two other MuSK-MG patients, they recognized only the Ig4 domain. LRP4 and ColQ also bound to the Ig1 and Ig4 domains of MuSK. Unexpectedly, the AChE/ColQ complex blocked MuSK-LRP4 interaction and suppressed agrin/LRP4/MuSK signaling. Quantitative analysis showed that MuSK-IgG suppressed agrin/LRP4/MuSK signaling to a greater extent than ColQ.
Topics: Acetylcholinesterase; Agrin; Animals; Autoantibodies; Cell Line; Collagen; Epitopes; Gene Expression Regulation; Humans; Immunization, Passive; Immunoglobulin G; LDL-Receptor Related Proteins; Mice; Mice, Knockout; Models, Biological; Muscle Proteins; Neuromuscular Junction; Protein Binding; Protein Interaction Domains and Motifs; Receptor Protein-Tyrosine Kinases; Receptors, Cholinergic; Receptors, Nicotinic; Signal Transduction
PubMed: 26355076
DOI: 10.1038/srep13928 -
Development (Cambridge, England) Apr 2010Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and... (Review)
Review
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.
Topics: Agrin; Animals; Collagen; Fibroblast Growth Factors; Glial Cell Line-Derived Neurotrophic Factor; Laminin; Motor Neurons; Muscle Fibers, Skeletal; Neuroglia; Neuromuscular Junction; Receptor Protein-Tyrosine Kinases; Receptors, Cholinergic; Receptors, LDL; Signal Transduction; Synapses; Synaptic Transmission; Wnt Proteins
PubMed: 20215342
DOI: 10.1242/dev.038711 -
Biochimica Et Biophysica Acta Oct 2013MuSK (muscle-specific kinase) is a receptor tyrosine kinase that plays a central signaling role in the formation of neuromuscular junctions (NMJs). MuSK is activated in... (Review)
Review
MuSK (muscle-specific kinase) is a receptor tyrosine kinase that plays a central signaling role in the formation of neuromuscular junctions (NMJs). MuSK is activated in a complex spatio-temporal manner to cluster acetylcholine receptors on the postsynaptic (muscle) side of the synapse and to induce differentiation of the nerve terminal on the presynaptic side. The ligand for MuSK is LRP4 (low-density lipoprotein receptor-related protein-4), a transmembrane protein in muscle, whose binding affinity for MuSK is potentiated by agrin, a neuronally derived heparan-sulfate proteoglycan. In addition, Dok7, a cytoplasmic adaptor protein, is also required for MuSK activation in vivo. This review focuses on the physical interplay between these proteins and MuSK for activation and downstream signaling, which culminates in NMJ formation. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.
Topics: Agrin; Animals; Gene Expression Regulation; Humans; LDL-Receptor Related Proteins; Models, Molecular; Muscle Proteins; Muscle, Skeletal; Nerve Endings; Neuromuscular Junction; Receptor Protein-Tyrosine Kinases; Receptors, Cholinergic; Synapses; Synaptic Transmission
PubMed: 23467009
DOI: 10.1016/j.bbapap.2013.02.034 -
Laboratory Investigation; a Journal of... Nov 2006Agrin is a multifunctional heparan sulfate proteoglycan originally discovered in the neuromuscular junctions and later observed in numerous other localizations. The...
Agrin is a multifunctional heparan sulfate proteoglycan originally discovered in the neuromuscular junctions and later observed in numerous other localizations. The presence of agrin in the liver, either healthy or diseased, has formerly not been reported. We detected agrin in minor amounts in the basement membranes of blood vessels and bile ducts in the healthy liver. The proliferation of bile ductules and the formation of new septal blood vessels in liver cirrhosis, as well as neoangiogenesis in the hepatocellular carcinoma (HCC) result in a dramatic increase in the quantity of agrin. Vascular and peribiliary basement membranes were strongly immunopositive for agrin in 29/29 human liver specimens with cirrhosis and HCC. However, sinusoidal walls of regenerative nodules in the cirrhotic liver consistently remained negative. Given the selectivity of agrin for tumor microvessels, agrin immunohistochemistry may prove helpful in recognizing malignant transformation in cirrhotic livers. Similar immunohistochemical observations were made on the liver of rats exposed to a combined cirrhosis/HCC induction treatment. In both human and rats, agrin probably originates from activated myofibroblasts, vascular smooth muscle cells and biliary epithelial cells. Increased agrin expression in human specimens, in the liver of 4/4 treated rats, as well as in isolated rat liver mesenchymal cells was verified by quantitative RT-PCR. Considering that agrin binds various growth factors, and it directly interacts with cell membrane receptors such as alphav-integrins, we hypothesize a stimulatory role for agrin in neoangiogenic processes such as tumor vascularization, and a supportive role in bile ductule proliferation.
Topics: Agrin; Animals; Basement Membrane; Bile Ducts, Intrahepatic; Carcinoma, Hepatocellular; Cell Proliferation; Disease Models, Animal; Fibroblasts; Fluorescent Antibody Technique, Indirect; Gene Expression Regulation, Neoplastic; Humans; Kupffer Cells; Liver; Liver Cirrhosis; Liver Neoplasms; Microcirculation; Muscle Cells; Neovascularization, Pathologic; RNA, Messenger; Rats; Tumor Cells, Cultured
PubMed: 16983329
DOI: 10.1038/labinvest.3700475 -
Nature Reviews. Neuroscience Feb 2003The formation of mature synaptic connections involves the targeted transport and aggregation of synaptic vesicles, the gathering of presynaptic release sites and the... (Review)
Review
The formation of mature synaptic connections involves the targeted transport and aggregation of synaptic vesicles, the gathering of presynaptic release sites and the clustering of postsynaptic neurotransmitter receptors and ion channels. Positional cues are required to orient the cytoskeleton in the direction of neuronal outgrowth, and also to direct the juxtaposition of synaptic protein complexes at the pre- and postsynaptic membranes. Both anterograde and retrograde factors are thought to contribute positional information during synaptic differentiation, and recent studies in vertebrates and invertebrates have begun to uncover a new role in this process for proteins that are essential for pattern formation in the early embryo.
Topics: Agrin; Animals; Axons; Body Patterning; Cell Differentiation; Drosophila; Mice; Nervous System; Neuromuscular Junction; Proto-Oncogene Proteins; Signal Transduction; Synapses; Transforming Growth Factor beta; Wnt Proteins; Xenopus; Zebrafish Proteins
PubMed: 12563282
DOI: 10.1038/nrn1036 -
Proceedings of the National Academy of... Feb 1999The precise orchestration of synaptic differentiation is critical for efficient information exchange in the nervous system. The nerve-muscle synapse forms in response to...
The precise orchestration of synaptic differentiation is critical for efficient information exchange in the nervous system. The nerve-muscle synapse forms in response to agrin, which is secreted from the motor nerve terminal and induces the clustering of acetylcholine receptors (AChRs) and other elements of the postsynaptic apparatus on the subjacent muscle cell surface. In view of the highly restricted spatial localization and the plasticity of neuromuscular junctions, it seems likely that synapse formation and maintenance are regulated by additional, as-yet-unidentified factors. Here, we tested whether neurotrophins modulate the agrin-induced differentiation of postsynaptic specializations. We show that both brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) inhibit agrin-induced AChR clustering on cultured myotubes. Nerve growth factor and NT-3 are without effect. Muscle cells express full-length TrkB, the cognate receptor for BDNF and NT-4. Direct activation of this receptor by anti-TrkB antibodies mimicked the BDNF/NT-4 inhibition of agrin-induced AChR clustering. This BDNF/NT-4 inhibition is likely to be an intrinsic mechanism for regulating AChR clustering, because neutralization of endogenous TrkB ligands resulted in elevated levels of AChR clustering even in the absence of added agrin. Finally, high concentrations of agrin can occlude the BDNF/NT-4 inhibition of AChR clustering. These results indicate that an interplay between agrin and neurotrophins can regulate the formation of postsynaptic specializations. They also suggest a mechanism for the suppression of postsynaptic specializations at nonjunctional regions.
Topics: Agrin; Animals; Brain-Derived Neurotrophic Factor; Cell Differentiation; Cells, Cultured; Chick Embryo; Models, Neurological; Motor Neurons; Muscle Fibers, Skeletal; Nerve Growth Factors; Neuromuscular Junction; Peptide Fragments; RNA, Messenger; Rats; Receptor Protein-Tyrosine Kinases; Receptor, Ciliary Neurotrophic Factor; Receptors, Cholinergic; Receptors, Nerve Growth Factor; Recombinant Proteins; Synapses; Transcription, Genetic
PubMed: 9927702
DOI: 10.1073/pnas.96.3.1112 -
Primary structure and high expression of human agrin in basement membranes of adult lung and kidney.European Journal of Biochemistry May 1998Agrin is a heparan sulfate proteoglycan involved in the development of the neuromuscular junction during embryogenesis. In addition to this well-characterized function,...
Agrin is a heparan sulfate proteoglycan involved in the development of the neuromuscular junction during embryogenesis. In addition to this well-characterized function, agrin may have additional functions in other tissues and during other stages in development. In this study we present the cDNA sequence of human agrin, and demonstrate a high agrin content in adult basement membranes. The N-terminal domain of human agrin is highly similar to that of chick agrin, suggesting a similar function in laminin binding. The presence of three SGXG sequences supports serine-linked glycosylation of the core protein, two sites being particularly favorable for heparan sulfate attachment. Comparison of levels of agrin mRNA in fetal and adult human tissues showed a remarkable upregulation in adult kidney and lung. In both tissues truncated agrin transcripts were detected, lacking the region that encodes the laminin-binding domain. The high transcription levels in lung and kidney corresponded with the accumulation of agrin in the alveolar and glomerular basement membranes, suggesting a filtration-associated function. These data provide new directions for investigating the role of agrin in its different physiological environments, including the basement membranes of the neuromuscular junction, kidney and lung.
Topics: Age Factors; Agrin; Basement Membrane; Cloning, Molecular; Fluorescent Antibody Technique; Gene Expression Regulation; Heparan Sulfate Proteoglycans; Humans; Kidney; Lung; Molecular Sequence Data; RNA, Messenger; Sequence Analysis, DNA; Transcription, Genetic
PubMed: 9652404
DOI: 10.1046/j.1432-1327.1998.2540123.x -
Trends in Cancer Apr 2017Agrin is utilized by motor neurons to stimulate the LRP4-MuSK receptor in muscles for neuromuscular junction (NMJ) formation. Recent studies of cancer have identified...
Agrin is utilized by motor neurons to stimulate the LRP4-MuSK receptor in muscles for neuromuscular junction (NMJ) formation. Recent studies of cancer have identified novel functions of the low-density lipoprotein receptor-related protein 4-muscle-specific kinase (LRP4-MuSK) pathway. Agrin may act as a mechanotransduction signal in the extracellular matrix (ECM) to coordinate the cross-talk between the LRP4-MuSK pathway and integrin-focal adhesion pathway. Ensuing Yes-associated protein (YAP) activation promotes hepatocellular carcinoma (HCC). Here, we discuss the implications of the converged pathways in NMJ formation and liver cancer.
Topics: Adaptor Proteins, Signal Transducing; Agrin; Animals; Cell Cycle Proteins; Humans; Mice; Neuromuscular Junction; Phosphoproteins; Signal Transduction; YAP-Signaling Proteins
PubMed: 28718435
DOI: 10.1016/j.trecan.2017.03.005