Molecular Analysis of a Congenital Myasthenic Syndrome Due to a Pathogenic Variant Affecting the C-Terminus of ColQ.

Congenital Myasthenic Syndromes (CMSs) are rare inherited diseases of the neuromuscular junction characterized by muscle weakness. CMSs with acetylcholinesterase deficiency are due to pathogenic variants in COLQ, a collagen that anchors the enzyme at the synapse. The two COLQ N-terminal domains have been characterized as being biochemical and functional.

The collagen ColQ binds to LRP4 and regulates the activation of the Muscle-Specific Kinase-LRP4 receptor complex by agrin at the neuromuscular junction.

Collagen Q (ColQ) is a nonfibrillar collagen that plays a crucial role at the vertebrate neuromuscular junction (NMJ) by anchoring acetylcholinesterase to the synapse. ColQ also functions in signaling, as it regulates acetylcholine receptor clustering and synaptic gene expression, in a manner dependent on muscle-specific kinase (MuSK), a key protein in NMJ formation and maintenance. MuSK forms a complex with low-density lipoprotein receptor-related protein 4 (LRP4), its coreceptor for the proteoglycan agrin at the NMJ.

AChR β-Subunit mRNAs Are Stabilized by HuR in a Mouse Model of Congenital Myasthenic Syndrome With Acetylcholinesterase Deficiency.

Collagen Q (COLQ) is a specific collagen that anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction. So far, no mutation has been identified in the human gene but over 50 different mutations in the gene are causative for a congenital myasthenic syndrome (CMS) with AChE deficiency. Mice deficient for COLQ mimic most of the functional deficit observed in CMS patients.

Collagens at the vertebrate neuromuscular junction, from structure to pathologies.

The extracellular matrix at the neuromuscular junction is built upon components secreted by the motoneuron, the muscle cell and terminal Schwann cells, the cells constituting this specific synapse. This compartment contains glycoproteins, proteoglycans and collagens that form a dense and specialized layer, the synaptic basal lamina. A number of these molecules are known to play a crucial role in anterograde and retrograde signalings that are active in neuromuscular junction formation, maintenance and function.

Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency.

The collagen ColQ anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction (NMJ). It also binds MuSK and perlecan/dystroglycan, 2 signaling platforms of the postsynaptic domain. Mutations in ColQ cause a congenital myasthenic syndrome (CMS) with AChE deficiency.

MuSK frizzled-like domain is critical for mammalian neuromuscular junction formation and maintenance.

The muscle-specific kinase MuSK is one of the key molecules orchestrating neuromuscular junction (NMJ) formation. MuSK interacts with the Wnt morphogens, through its Frizzled-like domain (cysteine-rich domain [CRD]). Dysfunction of MuSK CRD in patients has been recently associated with the onset of myasthenia, common neuromuscular disorders mainly characterized by fatigable muscle weakness.

Developmental consequences of the ColQ/MuSK interactions.

CollagenQ (ColQ) is a specific collagen that anchors acetylcholinesterase (AChE) in the synaptic basal lamina of the neuromuscular junction (NMJ). Over 30 mutations in the COLQ gene have been identified that are responsible for a congenital myasthenic syndrome with AChE deficiency, highlighting the importance of this collagen in the physiology of the NMJ. The anchoring of AChE at the synapse requires the interaction of ColQ with MuSK (Muscle-Specific Kinase), a tyrosine kinase expressed on the muscle membrane that is necessary for the formation and the maintenance of the NMJ.

Existence of tenascin-C isoforms in rat that contain the alternatively spliced AD1 domain are developmentally regulated during hippocampal development.

Tenascin-C (TN-C) is a multimodular glycoprotein of the extracellular matrix which is important for the development of the nervous system and has a range of different functions which are mediated by the different protein domains present. TN-C contains eight constitutive fibronectin type III (FNIII) domains and a region of alternatively spliced FNIII domains. In the mouse and chick, six of these domains have been described and characterized, whereas in human there are nine of them.

Analysis of combinatorial variability reveals selective accumulation of the fibronectin type III domains B and D of tenascin-C in injured brain.

Tenascin-C (Tnc) is a multimodular extracellular matrix glycoprotein that is markedly upregulated in CNS injuries where it is primarily secreted by reactive astrocytes. Different Tnc isoforms can be generated by the insertion of variable combinations of one to seven (in rats) alternatively spliced distinct fibronectin type III (FnIII) domains to the smallest variant. Each spliced FnIII repeat mediates specific actions on neurite outgrowth, neuron migration or adhesion.

Contributions of selective knockout studies to understanding cholinesterase disposition and function.

The complete knockout of the acetylcholinesterase gene (AChE) in the mouse yielded a surprising phenotype that could not have been predicted from deletion of the cholinesterase genes in Drosophila, that of a living, but functionally compromised animal. The phenotype of this animal showed a sufficient compromise in motor function that precluded precise characterization of central and peripheral nervous functional deficits. Since AChE in mammals is encoded by a single gene with alternative splicing, additional understanding of gene expression might be garnered from selected deletions of the alternatively spliced exons.

Targeting of acetylcholinesterase in neurons in vivo: a dual processing function for the proline-rich membrane anchor subunit and the attachment domain on the catalytic subunit.

Acetylcholinesterase (AChE) accumulates on axonal varicosities and is primarily found as tetramers associated with a proline-rich membrane anchor (PRiMA). PRiMA is a small transmembrane protein that efficiently transforms secreted AChE to an enzyme anchored on the outer cell surface. Surprisingly, in the striatum of the PRiMA knock-out mouse, despite a normal level of AChE mRNA, we find only 2-3% of wild type AChE activity, with the residual AChE localized in the endoplasmic reticulum, demonstrating that PRiMA in vivo is necessary for intracellular processing of AChE in neurons.

Identification of cis-acting elements involved in acetylcholinesterase RNA alternative splicing.

The 3' end of Acetylcholinesterase (AChE) pre-mRNA is processed by a complex mechanism of alternative splicing. Three different transcripts are generated and called R, H and T according respectively to the intron (intron 4') or exons (5 or 6) retained in the mature RNA. The relative expression of the specific transcripts depends on cell type, developmental stage or pathophysiological conditions.

Evidence for distinct leptomeningeal cell-dependent paracrine and EGF-linked autocrine regulatory pathways for suppression of fibrillar collagens in astrocytes.

A unique and unresolved property of the central nervous system is that its extracellular matrix lacks fibrillar elements. In the present report, we show that astrocytes secrete triple helices of fibrillar collagens type I, III and V in culture, while no astroglial collagen expression could be detected in vivo. We discovered two inhibitory mechanisms that could underlie this apparent discrepancy.

DSD-1-Proteoglycan/Phosphacan and receptor protein tyrosine phosphatase-beta isoforms during development and regeneration of neural tissues.

Interactions between neurons and glial cells play important roles in regulating key events of development and regeneration of the CNS. Thus, migrating neurons are partly guided by radial glia to their target, and glial scaffolds direct the growth and directional choice of advancing axons, e.g.

Cholinesterases and the resistance of the mouse diaphragm to the effect of tubocurarine.

Background: The diaphragm is resistant to competitive neuromuscular blocking agents. Because of the competitive mechanism of action of tubocurarine, the rate of hydrolysis of acetylcholine at the neuromuscular junction may modulate its neuromuscular blocking effect. The authors compared the neuromuscular blocking effect of tubocurarine on isolated diaphragm and extensor digitorum longus (EDL) muscles and quantified the acetylcholinesterase activity in hetero-oligomers.

The extracellular matrix glycoprotein Tenascin-C is expressed by oligodendrocyte precursor cells and required for the regulation of maturation rate, survival and responsiveness to platelet-derived growth factor.

Analysis of Tenascin-C (TN-C) knockout mice revealed novel roles for this extracellular matrix (ECM) protein in regulation of the developmental programme of oligodendrocyte precursor cells (OPCs), their maturation into myelinating oligodendrocytes and sensitivity to growth factors. A major component of the ECM of developing nervous tissue, TN-C was expressed in zones of proliferation, migration and morphogenesis. Examination of TN-C knockout mice showed roles for TN-C in control of OPC proliferation and migration towards zones of myelination [E.

Regulation of RPTPbeta/phosphacan expression and glycosaminoglycan epitopes in injured brain and cytokine-treated glia.

Several chondroitin sulfate proteoglycans (CSPGs) are upregulated after CNS injury and are thought to limit axonal regeneration in the adult mammalian CNS. Therefore, we examined the expression of the CSPG, receptor protein tyrosine phosphatase beta (RPTPbeta)/phosphacan, after a knife lesion to the cerebral cortex and after treatment of glial cultures with regulatory factors. The three splice variants of this CSPG gene, the secreted isoform, phosphacan, and the two transmembrane isoforms, the long and short RPTPbeta, were examined.

Activation of ionotropic glutamate receptors reduces the production of transforming growth factor-beta2 by developing neurons.

Neuronal cultures derived from developing rat cerebral cortex were used to investigate the influence of glutamate receptors on the neuronal production of transforming growth factor-beta2 (TGFbeta2), a multifunctional cytokine that modulates neuronal and glial growth. Long-term exposure (48 h) of cortical neurons to selective antagonists of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors markedly increased TGFbeta2 levels in the culture medium. Conversely, treatment with NMDA or kainate reduced TGFbeta2 to levels below those in untreated cultures.

Comparing astrocytic cell lines that are inhibitory or permissive for axon growth: the major axon-inhibitory proteoglycan is NG2.

Astrocytes, oligodendrocytes, and oligodendrocyte/type 2 astrocyte progenitors (O2A cells) can all produce molecules that inhibit axon regeneration. We have shown previously that inhibition of axon growth by astrocytes involves proteoglycans. To identify inhibitory mechanisms, we created astrocyte cell lines that are permissive or nonpermissive and showed that nonpermissive cells produce inhibitory chondroitin sulfate proteoglycans (CS-PGs).

Identification of CSF-1 as a brain macrophage migratory activity produced by astrocytes.

Intraparenchymal migration of macrophages occurs in the CNS during development or as a consequence of tissue injuries. In the present study, we have shown, by using an in vitro chemotaxis assay, that cultured rat astrocytes obtained from the developing cerebral cortex and striatum produce soluble factors, which attract purified brain macrophages. The effect of astrocyte-derived factors on macrophages was strongly reduced in the presence of antibodies neutralizing colony-stimulating factor 1 (CSF-1, also called M-CSF), and recombinant CSF-1 was found to act as a chemotactic agent on brain macrophages.

Neurons promote macrophage proliferation by producing transforming growth factor-beta2.

The infiltration of bone marrow-derived macrophages into the CNS contributes to growth and reactions of microglia during development or after brain injury. The proliferation of microglial cells is stimulated by colony-stimulating factor 1 (CSF-1), an astrocyte-produced growth factor that acts on mononuclear phagocytes. In the present study, we have shown, using an in vitro model system, that rodent neurons obtained from the developing cerebral cortex produce a soluble factor that strongly enhances the proliferation of macrophages cultured in the presence of CSF-1.

Recruitment of brain macrophages: roles of cytokines and extracellular matrix proteins produced by glial or neuronal cells.

Brain macrophages are a subpopulation of microglial cells which occur in the developing or in the injured CNS. These cells actively contribute to CNS tissue remodeling by acting on neuronal and macroglial lineages. Recruitment of brain macrophages is promoted by transformation of resting microglial phenotypes, infiltration of the tissue by exogenous macrophage precursors and local proliferation of phagocytes.

Immunohistochemical detection of thrombospondin in microglia in the developing rat brain.

The development of microglia involves the expression of a phenotype displaying phagocytic behaviour termed brain macrophage or amoeboid microglial cell. We have previously shown that rat brain macrophages purified in vitro secrete thrombospondin, an extracellular matrix protein, which acts on cultured neuronal cells by promoting neurite growth. In the present study, the expression of thrombospondin was investigated in tissue sections of the developing rat forebrain in relation to the distribution of microglia.

Cor triatriatum dexter in an adult diagnosed by transesophageal echocardiography: a case report.

Cor triatriatum dexter is a rare congenital heart malformation in which a persistent right sinus venosus valve divides the right atrium into two chambers. Before echocardiography, this anomaly has been rarely diagnosed before surgery or death. This is a case of cor triatriatum dexter in an adult with lifelong exertional cyanosis and dyspnea.