26 research outputs found

    Syndecan-3 and syndecan-4 are enriched in Schwann cell perinodal processes

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    BACKGROUND: Nodes of Ranvier correspond to specialized axonal domains where voltage-gated sodium channels are highly concentrated. In the peripheral nervous system, they are covered by Schwann cells microvilli, where three homologous cytoskeletal-associated proteins, ezrin, radixin and moesin (ERM proteins) have been found, to be enriched. These glial processes are thought to play a crucial role in organizing axonal nodal domains during development. However, little is known about the molecules present in Schwann cell processes that could mediate axoglial interactions. The aim of this study is to identify by immunocytochemistry transmembrane proteins enriched in Schwann cells processes that could interact, directly or indirectly, with axonal proteins. RESULTS: We show that syndecan-3 (S3) and syndecan-4 (S4), two proteoglycans expressed in Schwann cells, are enriched in perinodal processes in rat sciatic nerves. S3 labeling was localized in close vicinity of sodium channels as early as post-natal day 2, and highly concentrated at nodes of Ranvier in the adult. S4 immunoreactivity accumulated at nodes later, and was also prominent in internodal regions of myelinated fibers. Both S3 and S4 were co-localized with ezrin in perinodal processes. CONCLUSIONS: Our data identify S3 and S4 as transmembrane proteins specifically enriched in Schwann cell perinodal processes, and suggest that S3 may be involved in early axoglial interactions during development

    Selective Axonal Expression of the Kv1 Channel Complex in Pre-myelinated GABAergic Hippocampal Neurons.

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    In myelinated fibers, the voltage-gated sodium channels Nav1 are concentrated at the nodal gap to ensure the saltatory propagation of action potentials. The voltage-gated potassium channels Kv1 are segregated at the juxtaparanodes under the compact myelin sheath and may stabilize axonal conduction. It has been recently reported that hippocampal GABAergic neurons display high density of Nav1 channels remarkably in clusters along the axon before myelination (Freeman et al., 2015). In inhibitory neurons, the Nav1 channels are trapped by the ankyrinG scaffold at the axon initial segment (AIS) as observed in pyramidal and granule neurons, but are also forming "pre-nodes," which may accelerate conduction velocity in pre-myelinated axons. However, the distribution of the Kv1 channels along the pre-myelinated inhibitory axons is still unknown. In the present study, we show that two subtypes of hippocampal GABAergic neurons, namely the somatostatin and parvalbumin positive cells, display a selective high expression of Kv1 channels at the AIS and all along the unmyelinated axons. These inhibitory axons are also highly enriched in molecules belonging to the juxtaparanodal Kv1 complex, including the cell adhesion molecules (CAMs) TAG-1, Caspr2, and ADAM22 and the scaffolding protein 4.1B. Here, taking advantage of hippocampal cultures from 4.1B and TAG-1 knock-out mice, we observed that 4.1B is required for the proper positioning of Caspr2 and TAG-1 along the distal axon, and that TAG-1 deficiency induces alterations in the axonal distribution of Caspr2. However, the axonal expression of Kv1 channels and clustering of ankyrinG were not modified. In conclusion, this study allowed the analysis of the hierarchy between channels, CAMs and scaffolding proteins for their expression along hippocampal inhibitory axons before myelination. The early steps of channel compartmentalization preceding myelination may be crucial for stabilizing nerve impulses switching from a continuous to saltatory conduction during network development

    Association of TAG-1 with Caspr2 is essential for the molecular organization of juxtaparanodal regions of myelinated fibers

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    Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier. Here, we show the role in this process of TAG-1, a glycosyl-phosphatidyl-inositol–anchored cell adhesion molecule. In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems. In contrast, the localization of protein 4.1B, an axoplasmic partner of Caspr2, was only moderately altered. TAG-1, which is expressed in both neurons and glia, was able to associate in cis with Caspr2 and in trans with itself. Thus, a tripartite intercellular protein complex, comprised of these two proteins, appears critical for axo–glial contacts at juxtaparanodes. This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo–glial interactions

    Protein 4.1B Contributes to the Organization of Peripheral Myelinated Axons

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    Neurons are characterized by extremely long axons. This exceptional cell shape is likely to depend on multiple factors including interactions between the cytoskeleton and membrane proteins. In many cell types, members of the protein 4.1 family play an important role in tethering the cortical actin-spectrin cytoskeleton to the plasma membrane. Protein 4.1B is localized in myelinated axons, enriched in paranodal and juxtaparanodal regions, and also all along the internodes, but not at nodes of Ranvier where are localized the voltage-dependent sodium channels responsible for action potential propagation. To shed light on the role of protein 4.1B in the general organization of myelinated peripheral axons, we studied 4.1B knockout mice. These mice displayed a mildly impaired gait and motility. Whereas nodes were unaffected, the distribution of Caspr/paranodin, which anchors 4.1B to the membrane, was disorganized in paranodal regions and its levels were decreased. In juxtaparanodes, the enrichment of Caspr2, which also interacts with 4.1B, and of the associated TAG-1 and Kv1.1, was absent in mutant mice, whereas their levels were unaltered. Ultrastructural abnormalities were observed both at paranodes and juxtaparanodes. Axon calibers were slightly diminished in phrenic nerves and preterminal motor axons were dysmorphic in skeletal muscle. βII spectrin enrichment was decreased along the axolemma. Electrophysiological recordings at 3 post-natal weeks showed the occurrence of spontaneous and evoked repetitive activity indicating neuronal hyperexcitability, without change in conduction velocity. Thus, our results show that in myelinated axons 4.1B contributes to the stabilization of membrane proteins at paranodes, to the clustering of juxtaparanodal proteins, and to the regulation of the internodal axon caliber

    Rôle de SCHIP-1 dans l'organisation et la fonction des fibres myélinisées

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    Les noeuds de Ranvier (NR) et les segments initiaux (SI) sont responsables de la génération et de la propagation rapide des potentiels d action le long de la fibre myélinisée, respectivement. Des canaux Na+ dépendants du voltage sont concentrés dans ces deux régions où ils font partie de complexes multimoléculaires incluant notamment les protéines du cytosquelette ankyrine G et spectrine bIV. Nos travaux décrivent l identification d un nouveau composant enrichi à la membrane axonale de ces deux régions : SCHIP-1. L ankyrine G et spectrine bIV sont toutes deux indispensables à la localisation de SCHIP-1 aux SI et/ou aux NR et la région C-terminale de SCHIP-1 est capable d interagir avec l ankyrine G. Nous avons produit des souris mutantes exprimant une forme tronquée de SCHIP-1, délétée de sa partie C-terminale. L étude de ces souris montre que la délétion de SCHIP-1 perturbe l organisation de la fibre myélinisée comme l indiquent notamment les anomalies de la morphologie et du nombre des NR et la diminution du nombre de fibres dans les nerfs périphériques. Les retentissements de la délétion de SCHIP-1 sur le phénotype des souris sont multiples. Les souris 10/ 10 accusent un léger retard de croissance et souffrent d une mauvaise coordination des mouvements. D autre part les souris SCHIP-1 10/ 10 ont des déficits importants d interactions sociales et montrent des comportements stéréotypés, deux des trois symptômes majeurs de l autisme, une observation à mettre en rapport avec le rôle possible de SCHIP-1 dans les troubles de type autisme chez l'humain.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

    Contacts cellulaires des fibres myélinisées du système nerveux périphérique

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    La myélinisation permet la propagation rapide des potentiels d'action. Elle fournit à l'axone une gaine de myéline isolante régulièrement interrompue au niveau des noeuds de Ranvier où sont concentrés les canaux Na+ dépendants du voltage. Dans le système nerveux périphérique, l'architecture et la fonction des fibres myélinisées sont dépendantes de l'établissement de contacts cellulaires finement régulés, entre les membranes d'une même cellule gliale myélinisante, entre l'axone et les cellules gliales, et entre les cellules gliales et la matrice extracellulaire. Des composants protéiques majeurs de l'ensemble de ces contacts ont été identifiés ces dernières années. Cette revue résume les connaissances actuelles dans ce domaine

    Contacts cellulaires des fibres myélinisées du système nerveux périphérique

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    L’architecture et la fonction des fibres myélinisées sont dépendantes de l’établissement de contacts cellulaires finement réglés entre les membranes d’une même cellule gliale myélinisante, entre l’axone et les cellules gliales, et entre les cellules gliales et la matrice extracellulaire. Des composants protéiques majeurs de l’ensemble de ces contacts ont été identifiés ces dernières années. Des progrès importants ont notamment été faits dans l’identification de complexes moléculaires impliqués dans les contacts axo-gliaux au niveau des noeuds de Ranvier. Le rôle capital de certains composants des contacts dans le maintien de l’intégrité structurale et fonctionnelle des fibres a été démontré par la production de modèles murins. Dans certains cas, des mutations des gènes correspondants ont été identifiées chez des patients atteints de neuropathies périphériques telles que les maladies de Charcot-Marie-Tooth (CMT).Myelination allows the fast propagation of action potentials at a low energetic cost. It provides an insulating myelin sheath regularly interrupted at nodes of Ranvier where voltage-gated Na+ channels are concentrated. In the peripheral nervous system, the normal function of myelinated fibers requires the formation of highly differentiated and organized contacts between the myelinating Schwann cells, the axons and the extracellular matrix. Some of the major molecular complexes that underlie these contacts have been identified. Compact myelin which forms the bulk of the myelin sheath results from the fusion of the Schwann cell membranes through the proteins P0, PMP22 and MBP. The basal lamina of myelinating Schwann cells contains laminin-2 which associates with the glial complex dystroglycan/DPR2/L-periaxin. Non compact myelin, found in paranodal loops, periaxonal and abaxonal regions, and Schmidt-Lanterman incisures, presents reflexive adherens junctions, tight junctions and gap junctions, which contain cadherins, claudins and connexins, repectively. Axo-glial contacts determine the formation of distinct domains on the axon, the node, the paranode, and the juxtaparanode. At the paranodes, the glial membrane is tightly attached to the axolemma by septate-like junctions. Paranodal and juxtaparanodal axoglial complexes comprise an axonal transmembrane protein of the NCP family associated in cis and in trans with cell adhesion molecules of the immunoglobulin superfamily (IgSF-CAM). At nodes, axonal complexes are composed of Na+ channels and IgSF-CAMs. Schwann cell microvilli, which loosely cover the node, contain ERM proteins and the proteoglycans syndecan-3 and -4. The fundamental role of the cellular contacts in the normal function of myelinated fibers has been supported by rodent models and the detection of genetic alterations in patients with peripheral demyelinating neuropathies such as Charcot-Marie-Tooth diseases. Understanding more precisely their molecular basis now appears essential as a requisite step to further examine their involvement in the pathogenesis of peripheral neuropathies in general

    Axonal targeting of Caspr2 in hippocampal neurons via selective somatodendritic endocytosis.

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    International audienceContactin-associated protein 2 (Caspr2) is a neuronal membrane protein that is mutated in autism and related disorders. Although it is highly enriched at juxtaparanodes of Ranvier where it is essential for Shaker-type K(+) channel clustering, little is known about its function and regulation. In the present study, we examined the polarized expression of Caspr2 in hippocampal neurons using extracellular hemagglutinin (HA)-tagged Caspr2 constructs. We found that Caspr2 was targeted to the axonal surface, but colocalized with early endosomes in the somatodendritic compartment. The inhibition of endocytosis using a Dynamin-1 mutant or treatment with Dynasore prevented Caspr2 internalization from the dendrites and cell body. We identified a short sequence included into the 4.1B-binding domain that is required for the endocytosis of Caspr2. This sequence contains a protein kinase C (PKC) substrate motif on Thr1292, and point mutation of this residue or treatment with a PKC inhibitor prevented the somatodendritic internalization of Caspr2. Thus, the PKC-dependent trafficking of Caspr2 underlies its polarized expression in hippocampal neurons
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