Saltatory Conduction Research Articles

Myelination and Node of Ranvier Formation in a Human Motoneuron-Schwann Cell Serum-Free Coculture.

Myelination and node of Ranvier formation play an important role in the rapid conduction of nerve impulses, referred to as saltatory conduction, along axons in the peripheral nervous system. We report a human-human myelination model using human primary Schwann cells (SCs) and human-induced pluripotent stem-cell-derived motoneurons utilizing a serum-free medium supplemented with ascorbate to induce myelination, where 41.6% of SCs expressed the master transcription factor for myelination, early growth response protein 2. After 30 days in coculture, myelin segments were visualized using immunocytochemistry for myelin basic protein surrounding neurofilament-stained motor neuron axons, which was confirmed via 3D confocal Raman microscopy, a viable alternative for transmission electron microscopy analysis. The myelination efficiency was 65%, and clusters of voltage-gated sodium channels and the paranodal protein contactin-associated protein 1 indicated node of Ranvier formation. This model has applications to study remyelination and demyelinating diseases, including Charcot-Marie Tooth disorder, Guillian-Barre syndrome, and anti-myelin-associated glycoprotein peripheral neuropathy.

Nodes of Ranvier during development and repair in the CNS.

Saltatory conduction of action potentials along myelinated axons depends on the nodes of Ranvier - small unmyelinated axonal domains where voltage-gated sodium channels are concentrated. Our knowledge of the complex molecular composition of these axonal domains continues to accumulate, although the mechanisms of nodal assembly, which have been elucidated in the PNS, remain only partially understood in the CNS. Besides the key role of the nodes in accelerating conduction, nodal variations are thought to allow the fine tuning of axonal conduction speed to meet information processing needs. In addition, through their multiple glial contacts, nodes seem to be important for neuron-glia interactions. As we highlight in this Review, the disorganization of axonal domains has been implicated in the pathophysiology of various neurological diseases. In multiple sclerosis, for example, nodal and perinodal disruption following demyelination, with subsequent changes in ion channel distribution, leads to altered axonal conduction and integrity. The nodal clusters regenerate concurrently with but also prior to remyelination, allowing the restoration of axonal conduction. In this article, we review current knowledge of the organization and function of nodes of Ranvier in the CNS. We go on to discuss dynamic changes in the nodes during demyelination and remyelination, highlighting the impact of these changes on neuronal physiology in health and disease as well as the associated therapeutic implications.

Bioinspired nervous signal transmission system based on two-dimensional laminar nanofluidics: From electronics to ionics

Mammalian nervous systems, as natural ionic circuitries, stand out in environmental perception and sophisticated information transmission, relying on protein ionic channels and additional necessary structures. Prosperously emerged ionic regulated biomimetic nanochannels exhibit great potentialities in various application scenarios, especially signal transduction. Most reported direct current systems possess deficiencies in informational density and variability, which are superiorities of alternating current (AC) systems and necessities in bioinspired nervous signal transmission. Here, inspired by myelinated saltatory conduction, alternating electrostatic potential controlled nanofluidics are constructed with a noncontact application pattern and MXene nanosheets. Under time-variant external stimuli, ions confined in the interlaminar space obtain the capability of carriers for the AC ionic circuit. The transmitted information is accessible from typical sine to a frequency-modulated binary signal. This work demonstrates the potentiality of the bioinspired nervous signal transmission between electronics and ionic nanofluidics, which might push one step forward to the avenue of AC ionics.

White matter alterations in Williams syndrome related to behavioral and motor impairments.

Myelin is the electrical insulator surrounding the neuronal axon that makes up the white matter (WM) of the brain. It helps increase axonal conduction velocity (CV) by inducing saltatory conduction. Damage to the myelin sheath and WM is associated with many neurological and psychiatric disorders. Decreasing myelin deficits, and thus improving axonal conduction, has the potential to serve as a therapeutic mechanism for reducing the severity of some of these disorders. Myelin deficits have been previously linked to abnormalities in social behavior, suggesting an interplay between brain connectivity and sociability. This review focuses on Williams syndrome (WS), a genetic disorder characterized by neurocognitive characteristics and motor abnormalities, mainly known for its hypersociability characteristic. We discuss fundamental aspects of WM in WS and how its alterations can affect motor abilities and social behavior. Overall, findings regarding changes in myelin genes and alterations in WM structure in WS suggest new targets for drug therapy aimed at improving conduction properties and altering brain-activity synchronization in this disorder.

New wave-type mechanism of saltatory conduction in myelinated axons and micro-saltatory conduction in C fibres

We present a new wave-type model of saltatory conduction in myelinated axons. Poor conductivity in the neuron cytosol limits electrical current signal velocity according to cable theory, to 1–3 m/s, whereas saltatory conduction occurs with a velocity of 100–300 m/s. We propose a wave-type mechanism for saltatory conduction in the form of the kinetics of an ionic plasmon-polariton being the hybrid of the electro-magnetic wave and of the synchronized ionic plasma oscillations in myelinated segments along an axon. The model agrees with observations and allows for description of the regulatory role of myelin. It explains also the mechanism of conduction deficiency in demyelination syndromes such as multiple sclerosis. The recently observed micro-saltatory conduction in ultrathin unmyelinated C fibers with periodic ion gate clusters is also explained.

Accumulation of Neurofascin at Nodes of Ranvier Is Regulated by a Paranodal Switch.

The paranodal junctions flank mature nodes of Ranvier and provide a barrier between ion channels at the nodes and juxtaparanodes. These junctions also promote node assembly and maintenance by mechanisms that are poorly understood. Here, we examine their role in the accumulation of NF186, a key adhesion molecule of PNS and CNS nodes. We previously showed that NF186 is initially targeted/accumulates via its ectodomain to forming PNS (hemi)nodes by diffusion trapping, whereas it is later targeted to mature nodes by a transport-dependent mechanism mediated by its cytoplasmic segment. To address the role of the paranodes in this switch, we compared accumulation of NF186 ectodomain and cytoplasmic domain constructs in WT versus paranode defective (i.e., Caspr-null) mice. Both pathways are affected in the paranodal mutants. In the PNS of Caspr-null mice, diffusion trapping mediated by the NF186 ectodomain aberrantly persists into adulthood, whereas the cytoplasmic domain/transport-dependent targeting is impaired. In contrast, accumulation of NF186 at CNS nodes does not undergo a switch; it is predominantly targeted to both forming and mature CNS nodes via its cytoplasmic domain and requires intact paranodes. Fluorescence recovery after photobleaching analysis indicates that the paranodes provide a membrane diffusion barrier that normally precludes diffusion of NF186 to nodes. Linkage of paranodal proteins to the underlying cytoskeleton likely contributes to this diffusion barrier based on 4.1B and βII spectrin expression in Caspr-null mice. Together, these results implicate the paranodes as membrane diffusion barriers that regulate targeting to nodes and highlight differences in the assembly of PNS and CNS nodes.SIGNIFICANCE STATEMENT Nodes of Ranvier are essential for effective saltatory conduction along myelinated axons. A major question is how the various axonal proteins that comprise the multimeric nodal complex accumulate at this site. Here we examine how targeting of NF186, a key nodal adhesion molecule, is regulated by the flanking paranodal junctions. We show that the transition from diffusion-trapping to transport-dependent accumulation of NF186 requires the paranodal junctions. We also demonstrate that these junctions are a barrier to diffusion of axonal proteins into the node and highlight differences in PNS and CNS node assembly. These results provide new insights into the mechanism of node assembly and the pathophysiology of neurologic disorders in which impaired paranodal function contributes to clinical disability.

Ermin is a p116RIP -interacting protein promoting oligodendroglial differentiation and myelin maintenance.

Myelin sheaths, which insulate the axons and ensure saltatory conduction of the nerve impulse, are generated and maintained via largely uncharacterized mechanisms. Ermin is an oligodendrocyte-specific protein associated with the cytoskeleton, but how it regulates cytoskeletal remodeling during oligodendrocyte differentiation and its role in myelin maintenance are not clear. To address this, we generated mice constitutively deficient for Ermn, the Ermin-coding gene. We found that aged Ermn-knockout mice exhibit an aberrant myelin architecture, with splitting of myelin layers, peeling of the myelin sheath from axons, and breakdown of myelinated fibers. As a result, these mice had remarkably impaired motor coordination. Ermn knockout also accelerated cuprizone-induced demyelination and exacerbated the associated movement disorders. Ermin was found to contribute to oligodendrocyte morphogenesis by associating with the myosin phosphatase Rho interacting protein (Mprip/p116RIP ) and inactivating RhoA, a GTPase that controls cytoskeletal rearrangement in differentiating cells. These findings provide novel insights into the mechanisms regulating oligodendroglial differentiation, the maintenance of the myelin sheaths, and remyelination.

Control of Channel Clustering by Cleavage

Enrichment of sodium channels at nodes of Ranvier, a hallmark of myelinated axons, underlies effective saltatory conduction. In this issue of Neuron, Eshed-Eisenbach etal. (2020) demonstrate that proteolysis of gliomedin, which drives initial channel clustering, provides a novel mechanism to ensure fidelity of channel localization to nodes.

PP1C and PP2A are p70S6K Phosphatases Whose Inhibition Ameliorates HLD12-Associated Inhibition of Oligodendroglial Cell Morphological Differentiation.

Myelin sheaths created by oligodendroglial cells encase neuronal axons to achieve saltatory conduction and protect axons. Pelizaeus-Merzbacher disease (PMD) is a prototypic, hereditary demyelinating oligodendroglial disease of the central nervous system (CNS), and is currently known as hypomyelinating leukodystrophy 1 (HLD1). HLD12 is an autosomal recessive disorder responsible for the gene that encodes vacuolar protein sorting-associated protein 11 homolog (VPS11). VPS11 is a member of the molecular group controlling the early endosome antigen 1 (EEA1)- and Rab7-positive vesicle-mediated protein trafficking to the lysosomal compartments. Herein, we show that the HLD12-associated Cys846-to-Gly (C846G) mutation of VPS11 leads to its aggregate formation with downregulated signaling through 70 kDa S6 protein kinase (p70S6K) in the oligodendroglial cell line FBD-102b as the model. In contrast, wild-type proteins are localized in both EEA1- and Rab7-positive vesicles. Cells harboring the C846G mutant constructs decrease differentiated phenotypes with web-like structures following differentiation, whereas parental cells exhibit them suitably. It is of note that we identify PP1C and PP2A as the protein phosphatases for phosphorylated Thr-389 of p70S6K essential for kinase activation in cells. The respective knockdown experiments or inhibitor treatment stimulates phosphorylation of p70S6K and ameliorates the inhibition of morphological differentiation, as well as the formation of protein aggregates. These results indicate that inhibition of p70S6K phosphatases PP1C and PP2A improves the defective morphological differentiation associated with HLD12 mutation, thereby hinting at amelioration based on a possible molecular and cellular pathological mechanism underlying HLD12.

Saltatory Conduction: Jumping to New Conclusions.

Rapid and efficient saltatory action potential conduction depends on the myelin sheath and clustered Na+ channels at nodes of Ranvier. A new study convincingly shows that the periaxonal space is a necessary conductive component to accurately model myelinated axon physiology and saltatory conduction.

An alternative mechanism of early nodal clustering and myelination onset in GABAergic neurons of the central nervous system.

In vertebrates, fast saltatory conduction along myelinated axons relies on the node of Ranvier. How nodes assemble on CNS neurons is not yet fully understood. We previously described that node-like clusters can form prior to myelin deposition in hippocampal GABAergic neurons and are associated with increased conduction velocity. Here, we used a live imaging approach to characterize the intrinsic mechanisms underlying the assembly of these clusters prior to myelination. We first demonstrated that their components can partially preassemble prior to membrane targeting and determined the molecular motors involved in their trafficking. We then demonstrated the key role of the protein β2Nav for node-like clustering initiation. We further assessed the fate of these clusters when myelination proceeds. Our results shed light on the intrinsic mechanisms involved in node-like clustering prior to myelination and unravel a potential role of these clusters in node of Ranvier formation and in guiding myelination onset.

Generation of Oligodendrocytes and Oligodendrocyte-Conditioned Medium for Co-Culture Experiments.

In the central nervous system, oligodendrocytes are well-known for their role in axon myelination, that accelerates the propagation of action potentials through saltatory conduction. Moreover, an increasing number of reports suggest that oligodendrocytes interact with neurons beyond myelination, notably through the secretion of soluble factors. Here, we present a detailed protocol allowing purification of oligodendroglial lineage cells from glial cell cultures also containing astrocytes and microglial cells. The method relies on overnight shaking at 37 °C, which allows selective detachment of the overlying oligodendroglial cells and microglial cells, and the elimination of microglia by differential adhesion. We then describe the culture of oligodendrocytes and production of oligodendrocyte-conditioned medium (OCM). We also provide the kinetics of OCM treatment or oligodendrocytes addition to purified hippocampal neurons in co-culture experiments, studying oligodendrocyte-neuron interactions.

Role of Ectosomes in the Design of the Myelinated Axon: Structural Find

The evolution of vertebrates required an energy efficient mechanism of fast nerve conduction. Two events: formation of a myelin sheath ­– an insulator ­– and a segmented mode of covering the axon by myelin fulfilled this need. This arrangement, coordinated by oligodendrocytes and neurons, generates saltatory nerve conduction, but the mechanism is indeterminate. Disruption of this structure impairs nervous system function. We performed a developmental study of embryonic avian optic nerve, using transmission electron microscopy and 3D-electron tomography. We present structural evidence – in vivo – of three sequential events that set a platform for oligodendrocyte/neuron communication. First, they embrace, thereby positioning their plasma membranes face to face; second, they create a shared compartment for interchanging information; third, they release and exchange ectosomes to, we posit, transduce signals for structuring the myelinated axon. These data identity the earliest developments following oligodendrocyte/neuron encounter, set the stage to identifying their cross-talk and searching for the aetiology of nervous system diseases.

Neuroinflammation in Demyelinating Diseases: Oxidative Stress as a Modulator of Glial Cross-Talk.

Myelin is a specialized membrane allowing for saltatory conduction of action potentials in neurons, an essential process to achieve the normal communication across the nervous system. Accordingly, in diseases characterized by the loss of myelin and myelin forming cells -oligodendrocytes in the CNS-, patients show severe neurological disabilities. After a demyelinated insult, microglia, astrocytes and oligodendrocyte precursor cells invade the lesioned area initiating a spontaneous process of myelin repair (i.e. remyelination). A preserved hallmark of this neuroinflammatory scenario is a local increase of oxidative stress, where several cytokines and chemokines are released by glial and other cells. This generates an environment that determines cell interaction resulting in oligodendrocyte maturity and the ability to synthesize new myelin. Herein we review the main features of the regulatory aspect of these molecules based on recent findings and propose new putative signal molecules involved in the remyelination process, focused in the etiology of Multiple Sclerosis, one of the main demyelinating diseases causing disabilities in the population.

Cryo-ET Workflow for Understanding Ion Channels Localization on the Nodes of Ranvier

The localization of ion channels on myelinated axon is closely related with the saltatory conduction of action potential (AP). Abnormal changes in these channels contribute to multiple mental diseases. The development of cryo-Electron Tomography (cryo-ET) has provided a promising prospect for peering into ion channels in their native environment at high resolution. Previous achievements are reviewed here on cryo-ET. Accordingly, a cryo-ET workflow is designed for understanding ion channels localization in myelinated axon, especially nodes of Ranvier, which are significant for the saltatory conduction involved in the propagation of high-speed AP. The workflow is divided into six parts: the preparation of neural cultures with myelin, antibodies and immunofluorescence staining, frozen-hydrated sample preparation, cryo-ET imaging, cryo-correlative light and electron microscopy (cryo-CLEM) imaging, three-dimensional (3D) reconstruction and refinement. The purpose is to conceive a possible solution for the problems related to ion channel compounds including localization, conformation dynamics, accessory structures of ion channel and transient regulatory factors, and thus provide insights into treating neurological diseases caused by abnormal ion channels activity.

TBBPA targets converging key events of human oligodendrocyte development resulting in two novel AOPs

Myelinating oligodendrocytes (OLs) establish saltatory nerve conduction during white matter development. Thus, interference with oligodendrogenesis leads to an adverse outcome on brain performance in the child due to aberrant myelination. An intertwined network of hormonal, transcriptional and biosynthetic processes regulates OL development, thereby simultaneously creating various routes of interference for environmental toxicants. The flame retardant tetrabromobisphenol A (TBBPA) is debated as an endocrine disruptor, especially of the thyroid hormone (TH) system. We identified how TBBPA interferes with the establishment of a population of maturing OLs by two independent modes-of-action (MoA), dependent and independent of TH signaling. Combining the previously published oligodendrocyte maturation assay (NPC6) with large-scale transcriptomics, we describe TBBPA as a TH disruptor, impairing human OL maturation in vitro by dysregulation of oligodendrogenesis-associated genes (i.e., MBP, KLF9 and EGR1). Furthermore, TBBPA disrupts a gene expression network regulating cholesterol homeostasis, reducing OL numbers independently of TH signaling. These two MoA converge in a novel putative adverse outcome pathway (AOP) network on the key event (KE) hypomyelination. Comparative analyses of human and rat neural progenitor cells (NPCs) revealed that human oligodendrogenesis is more sensitive to endocrine disruption by TBBPA. Therefore, ethical, cost-efficient and species-overarching in vitro assays are needed for developmental neurotoxicity hazard assessment. By incorporation of large-scale transcriptomic analyses, we brought the NPC6 assay to a higher readiness level for future applications in a regulatory context. The combination of phenotypic and transcriptomic analyses helps to study MoA to eventually build AOPs for a better understanding of neurodevelopmental toxicity.

Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit

SummaryThe propagation of electrical impulses along axons is highly accelerated by the myelin sheath and produces saltating or “jumping” action potentials across internodes, from one node of Ranvier to the next. The underlying electrical circuit, as well as the existence and role of submyelin conduction in saltatory conduction remain, however, elusive. Here, we made patch-clamp and high-speed voltage-calibrated optical recordings of potentials across the nodal and internodal axolemma of myelinated neocortical pyramidal axons combined with electron microscopy and experimentally constrained cable modeling. Our results reveal a nanoscale yet conductive periaxonal space, incompletely sealed at the paranodes, which separates the potentials across the low-capacitance myelin sheath and internodal axolemma. The emerging double-cable model reproduces the recorded evolution of voltage waveforms across nodes and internodes, including rapid nodal potentials traveling in advance of attenuated waves in the internodal axolemma, revealing a mechanism for saltation across time and space.

The role of chromatin remodeling complexes in Schwann cell development.

Schwann cells develop from neural crest cells in an ordered series of events and give rise to myelinating and nonmyelinating subtypes. In their mature state, myelinating Schwann cells produce myelin sheaths that provide trophic support to axons and allow saltatory conduction in the vertebrate peripheral nervous system. Each step of Schwann cell development requires defined changes in chromatin structure that are catalyzed by chromatin remodeling complexes. Over the last years, all major types of chromatin remodeling complexes have been detected in Schwann cells and several have been functionally analyzed. SWI/SNF-type, CHD-type, and INO80/SWR-type chromatin remodelers in particular have been shown to interact with multiple cell-type specific transcription factors and histone modifiers and to be important regulators of Schwann cell development. As a result of different recruitment strategies, each chromatin remodeler targets defined genomic areas and impacts unique mechanisms at specific stages of Schwann cell development. Chromatin remodeling complexes undoubtedly constitute essential components of the Schwann cell regulatory network.

Use of Nerve Conduction Assessments to Evaluate Drug-Induced Peripheral Neuropathy in Nonclinical Species—A Brief Review

The peripheral nervous system (PNS) is subject to a wide range of structural and functional insults including direct damage to axons, loss of myelin, and progressive deficits in saltatory conduction. Drugs that damage the PNS often result in neuropathies that impact the structure and function of targeted nerves. In most cases, both sensory and motor neurons are affected with damage initially evident in the distal extremities. Drug-induced neuropathies are potentially reversible following cessation of treatment, but early stages of neuropathy can be subclinical and asymptomatic making diagnosis difficult. Nerve biopsy is highly validated and provides definitive evidence of nerve injury and corresponding severity; however, it is limited in some respects and electrophysiological measures can complement histopathological assessments and provide a functional measure of potential toxicity. In a drug development setting, nerve conduction assessments are valuable to monitor nerve function longitudinally if nerve damage is suspected or confirmed, and importantly, can be used to monitor progression and/or recovery of a drug-induced neuropathy. This review will summarize the methodology used in nerve conduction assessments as well as discuss data interpretation and considerations for use in nonclinical species. Finally, the use of nerve conduction assessments in nonclinical drug development is discussed.

The Molecular Basis for Remyelination Failure in Multiple Sclerosis

Myelin sheaths in the central nervous system (CNS) insulate axons and thereby allow saltatory nerve conduction, which is a prerequisite for complex brain function. Multiple sclerosis (MS), the most common inflammatory autoimmune disease of the CNS, leads to the destruction of myelin sheaths and the myelin-producing oligodendrocytes, thus leaving behind demyelinated axons prone to injury and degeneration. Clinically, this process manifests itself in significant neurological symptoms and disability. Resident oligodendroglial precursor cells (OPCs) and neural stem cells (NSCs) are present in the adult brain, and can differentiate into mature oligodendrocytes which then remyelinate the demyelinated axons. However, for multiple reasons, in MS the regenerative capacity of these cell populations diminishes significantly over time, ultimately leading to neurodegeneration, which currently remains untreatable. In addition, microglial cells, the resident innate immune cells of the CNS, can contribute further to inflammatory and degenerative axonal damage. Here, we review the molecular factors contributing to remyelination failure in MS by inhibiting OPC and NSC differentiation or modulating microglial behavior.