Individual Neuronal Subtypes Exhibit Diversity in CNS Myelination Mediated by Synaptic Vesicle Release

SummaryRegulation of myelination by oligodendrocytes in the CNS has important consequences for higher-order nervous system function (e.g., [1–4]), and there is growing consensus that neuronal activity regulates CNS myelination (e.g., [5–9]) through local axon-oligodendrocyte synaptic-vesicle-release-mediated signaling [10–12]. Recent analyses have indicated that myelination along axons of distinct neuronal subtypes can differ [13, 14], but it is not known whether regulation of myelination by activity is common to all neuronal subtypes or only some. This limits insight into how specific neurons regulate their own conduction. Here, we use a novel fluorescent fusion protein reporter to study myelination along the axons of distinct neuronal subtypes over time in zebrafish. We find that the axons of reticulospinal and commissural primary ascending (CoPA) neurons are among the first myelinated in the zebrafish CNS. To investigate how activity regulates myelination by different neuronal subtypes, we express tetanus toxin (TeNT) in individual reticulospinal or CoPA neurons to prevent synaptic vesicle release. We find that the axons of individual tetanus toxin expressing reticulospinal neurons have fewer myelin sheaths than controls and that their myelin sheaths are 50% shorter than controls. In stark contrast, myelination along tetanus-toxin-expressing CoPA neuron axons is entirely normal. These results indicate that while some neuronal subtypes modulate myelination by synaptic vesicle release to a striking degree in vivo, others do not. These data have implications for our understanding of how different neurons regulate myelination and thus their own function within specific neuronal circuits

Neuronal activity disrupts myelinated axon integrity in the absence of NKCC1b

Through a genetic screen in zebrafish, we identified a mutant with disruption to myelin in both the CNS and PNS caused by a mutation in a previously uncharacterized gene, slc12a2b, predicted to encode a Na+, K+, and Cl− (NKCC) cotransporter, NKCC1b. slc12a2b/NKCC1b mutants exhibited a severe and progressive pathology in the PNS, characterized by dysmyelination and swelling of the periaxonal space at the axon–myelin interface. Cell-type–specific loss of slc12a2b/NKCC1b in either neurons or myelinating Schwann cells recapitulated these pathologies. Given that NKCC1 is critical for ion homeostasis, we asked whether the disruption to myelinated axons in slc12a2b/NKCC1b mutants is affected by neuronal activity. Strikingly, we found that blocking neuronal activity completely prevented and could even rescue the pathology in slc12a2b/NKCC1b mutants. Together, our data indicate that NKCC1b is required to maintain neuronal activity–related solute homeostasis at the axon–myelin interface, and the integrity of myelinated axons

Adaptive myelination from fish to man

AbstractMyelinated axons with nodes of Ranvier are an evolutionary elaboration common to essentially all jawed vertebrates. Myelin made by Schwann cells in our peripheral nervous system and oligodendrocytes in our central nervous system has been long known to facilitate rapid energy efficient nerve impulse propagation. However, it is now also clear, particularly in the central nervous system, that myelin is not a simple static insulator but that it is dynamically regulated throughout development and life. New myelin sheaths can be made by newly differentiating oligodendrocytes, and mature myelin sheaths can be stimulated to grow again in the adult. Furthermore, numerous studies in models from fish to man indicate that neuronal activity can affect distinct stages of oligodendrocyte development and the process of myelination itself. This begs questions as to how these effects of activity are mediated at a cellular and molecular level and whether activity-driven adaptive myelination is a feature common to all myelinated axons, or indeed all oligodendrocytes, or is specific to cells or circuits with particular functions. Here we review the recent literature on this topic, elaborate on the key outstanding questions in the field, and look forward to future studies that incorporate investigations in systems from fish to man that will provide further insight into this fundamental aspect of nervous system plasticity.This article is part of a Special Issue entitled SI: Myelin Evolution

Transport d'ARN messagers et traduction locale dans les axones au cours du développement du système nerveux du poisson zèbre

Le neurone est une cellule extrêmement polarisée, dont les compartiments sont soit situés à grande distance du corps cellulaire (cône de croissance en développement), soit régulés de façon individuelle (épine dendritique mature). La localisation de transcrits dans ces compartiments, qui peuvent être traduits rapidement et localement en réponse à des stimuli externes localisés, permettrait une régulation spatiale et temporelle fine du protéome subcellulaire. Bien que de nombreuses études, menées sur des neurones en culture, aient montré que la traduction locale était nécessaire dans des fonctions comme le guidage axonal ou la régénération axonale, les évidences de l'existence d'un transport axonal d'ARNm dans le contexte d'un organisme entier restent très limitées. Au cours de mon travail de thèse, j'ai mis en évidence la présence d'ARN messagers dans les axones en développement du poisson zèbre. Cette localisation axonale est une propriété spécifique de certains ARN messagers puisque d'autres ARNm exprimés fortement dans les neurones sont restreints au corps cellulaire et exclus de l'axone. Mes résultats suggèrent qu'il s'agit d'un transport axonal dépendant des microtubules. Afin d'étudier les mécanismes de ce processus, nous avons mis au point un système rapporteur qui, lorsqu'il est exprimé dans des neurones isolés, permet d'évaluer le transport axonal d'ARNm. À l'aide de ce système rapporteur, nous avons montré que les mécanismes de transport des ARNm étaient conservés chez les vertébrés. Enfin, j'ai identifié un motif (zipcode) nécessaire et suffisant pour le transport axonal de l'ARN messager tubuline ß5.The neuron is an extremely polarised cell, whose compartments are located away from the cell body (the developing growth cone), or are regulated individually (the mature dendritic spines). The localisation of transcripts, which can be translated rapidly and locally in response to external localised cues, allow a fine spatial and temporal regulation of the subcellular proteome. Although many studies conducted on neuronal culture have shown that local translation is necessary for functions such as axon guidance or regeneration, evidence of mRNA axonal transport in the context of a whole developing organism remains very limited. During my PhD, I have shown the presence of messenger RNA in the developing axons of zebrafish embryo. This axonal localisation is a specific propriety of certain mRNA species, as others mRNA highly expressed in neurons are restricted to the cell body and excluded from the axons. My results suggest that this is a microtubule-dependent axonal transport. In order to investigate those processes, I set up a reporter system which, when expressed by isolated neurons, allow to evaluate the axonal transport of mRNA. Taking advantage of this reporter system, I demonstrated that the mechanisms of mRNA transport are conserved among vertebrate species. Finally, I identified a motif (zipcode) necessary and sufficient for axonal transport of tubulin ß5 mRNA.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Editorial: Physiology of Myelin Forming Cells, From Myelination to Neural Modulators

International audienc

Zebrafish Embryonic Neurons Transport Messenger RNA to Axons and Growth Cones In Vivo

International audienceAlthough mRNA was once thought to be excluded from the axonal compartment, the existence of protein synthesis in growing or regenerating axons in culture is now generally accepted. However, its extent and functional importance remain a subject of intense investigation. Furthermore, unambiguous evidence of mRNA axonal transport and local translation in vivo, in the context of a whole developing organism is still lacking. Here, we provide direct evidence of the presence of mRNAs of the tubb5, nefma, and stmnb2 genes in several types of axons in the developing zebrafish (Danio rerio) embryo, with frequent accumulation at the growth cone. We further show that axonal localization of mRNA is a specific property of a subset of genes, as mRNAs of the huc and neurod genes, abundantly expressed in neurons, were not found in axons. We set up a reporter system in which the 3' untranslated region (UTR) of candidate mRNA, fused to a fluorescent protein coding sequence, was expressed in isolated neurons of the zebrafish embryo. Using this reporter, we identified in the 3'UTR of tubb5 mRNA a motif necessary and sufficient for axonal localization. Our work thus establishes the zebrafish as a model system to study axonal transport in a whole developing vertebrate organism, provides an experimental frame to assay this transport in vivo and to study its mechanisms, and identifies a new zipcode involved in axonal mRNA localization