The Nodes of Ranvier: Molecular Assembly and Maintenance

Myelin basic protein mRNA levels affect myelin sheath dimensions, architecture, plasticity, and density of resident glial cells

Myelin Basic Protein (MBP) is essential for both elaboration and maintenance of CNS myelin, and its reduced accumulation results in hypomyelination. How different Mbp mRNA levels affect myelin dimensions across the lifespan and how resident glial cells may respond to such changes are unknown. Here, to investigate these questions, we used enhancer-edited mouse lines that accumulate Mbp mRNA levels ranging from 8% to 160% of wild type. In young mice, reduced Mbp mRNA levels resulted in corresponding decreases in Mbp protein accumulation and myelin sheath thickness, confirming the previously demonstrated rate-limiting role of Mbp transcription in the control of initial myelin synthesis. However, despite maintaining lower line specific Mbp mRNA levels into old age, both MBP protein levels and myelin thickness improved or fully normalized at rates defined by the relative Mbp mRNA level. Sheath length, in contrast, was affected only when mRNA levels were very low, demonstrating that sheath thickness and length are not equally coupled to Mbp mRNA level. Striking abnormalities in sheath structure also emerged with reduced mRNA levels. Unexpectedly, an increase in the density of all glial cell types arose in response to reduced Mbp mRNA levels. This investigation extends understanding of the role MBP plays in myelin sheath elaboration, architecture, and plasticity across the mouse lifespan and illuminates a novel axis of glial cell crosstalk.

Astrocytes at the intersection of ageing, obesity, and neurodegeneration

Once considered passive cells of the central nervous system (CNS), glia are now known to actively maintain the CNS parenchyma; in recent years, the evidence for glial functions in CNS physiology and pathophysiology has only grown. Astrocytes, a heterogeneous group of glial cells, play key roles in regulating the metabolic and inflammatory landscape of the CNS and have emerged as potential therapeutic targets for a variety of disorders. This review will outline astrocyte functions in the CNS in healthy ageing, obesity, and neurodegeneration, with a focus on the inflammatory responses and mitochondrial function, and will address therapeutic outlooks.

Development of myelination and axon diameter for fast and precise action potential conductance

Axons of globular bushy cells in the cochlear nucleus convey hyper-accurate signals to the superior olivary complex, the initial site of binaural processing via comparably thick axons and the calyx of the Held synapse. Bushy cell fibers involved in hyper-accurate binaural processing of low-frequency sounds are known to have an unusual internode length-to-axon caliber ratio (L/d) correlating with higher conduction velocity and superior temporal precision of action potentials. How the L/d-ratio develops and what determines this unusual myelination pattern is unclear. Here we describe a gradual developmental transition from very simple to complex, mature nodes of Ranvier on globular bushy cell axons during a 2-week period starting at postnatal day P6/7. The molecular composition of nodes matured successively along the axons from somata to synaptic terminals with morphologically and molecularly mature nodes appearing almost exclusively after hearing onset. Internodal distances are initially coherent with the canonical L/d-ratio of ~100. Several days after hearing onset, however, an over-proportional increase in axon caliber occurs in cells signaling low-frequency sounds which alters their L/d ratio to ~60. Hence, oligodendrocytes initially myelinating axons according to their transient axon caliber but a subsequent differential axon thickening after hearing onset results in the unusual myelination pattern.

The bone transcription factor Osterix controls extracellular matrix- and node of Ranvier-related gene expression in oligodendrocytes

Oligodendrocytes are the primary producers of many extracellular matrix (ECM)-related proteins found in the CNS. Therefore, oligodendrocytes play a critical role in the determination of brain stiffness, node of Ranvier formation, perinodal ECM deposition, and perineuronal net formation, all of which depend on the ECM. Nevertheless, the transcription factors that control ECM-related gene expression in oligodendrocytes remain unknown. Here, we found that the transcription factor Osterix (also known as Sp7) binds in proximity to genes important for CNS ECM and node of Ranvier formation and mediates their expression. Oligodendrocyte-specific ablation of Sp7 changes ECM composition and brain stiffness and results in aberrant node of Ranvier formation. Sp7 is known to control osteoblast maturation and bone formation. Our comparative analyses suggest that Sp7 plays a conserved biological role in oligodendrocytes and in bone-forming cells, where it mediates brain and bone tissue stiffness by controlling expression of ECM components.

Motor Activity-Induced White Matter Repair in White Matter Stroke

Subcortical white matter stroke (WMS) is a progressive disorder which is demarcated by the formation of small ischemic lesions along white matter tracts in the CNS. As lesions accumulate, patients begin to experience severe motor and cognitive decline. Despite its high rate of incidence in the human population, our understanding of the cause and outcome of WMS is extremely limited. As such, viable therapies for WMS remain to be seen. This study characterizes myelin recovery following stroke and motor learning-based rehabilitation in a mouse model of subcortical WMS. Following WMS, a transient increase in differentiating oligodendrocytes occurs within the peri-infarct in young male adult mice, which is completely abolished in male aged mice. Compound action potential recording demonstrates a decrease in conduction velocity of myelinated axons at the peri-infarct. Animals were then tested on one of three distinct motor learning-based rehabilitation strategies (skilled reach, restricted access to a complex running wheel, and unrestricted access to a complex running wheel) for their capacity to induce repair. These studies determined that unrestricted access to a complex running wheel alone increases the density of differentiating oligodendrocytes in infarcted white matter in young adult male mice, which is abolished in aged male mice. Unrestricted access to a complex running wheel was also able to enhance conduction velocity of myelinated axons at the peri-infarct to a speed comparable to naive controls suggesting functional recovery. However, there was no evidence of motor rehabilitation-induced remyelination or myelin protection.SIGNIFICANCE STATEMENT White matter stroke is a common disease with no medical therapy. A form of motor rehabilitation improves some aspects of white matter repair and recovery.

Reduced length of nodes of Ranvier and altered proteoglycan immunoreactivity in prefrontal white matter in major depressive disorder and chronically stressed rats

Major depressive disorder (MDD) and chronic unpredictable stress (CUS) in animals feature comparable cellular and molecular disturbances that involve neurons and glial cells in gray and white matter (WM) in prefrontal brain areas. These same areas demonstrate disturbed connectivity with other brain regions in MDD and stress-related disorders. Functional connectivity ultimately depends on signal propagation along WM myelinated axons, and thus on the integrity of nodes of Ranvier (NRs) and their environment. Various glia-derived proteoglycans interact with NR axonal proteins to sustain NR function. It is unclear whether NR length and the content of associated proteoglycans is altered in prefrontal cortex (PFC) WM of human subjects with MDD and in experimentally stressed animals. The length of WM NRs in histological sections from the PFC of 10 controls and 10 MDD subjects, and from the PFC of control and CUS rats was measured. In addition, in WM of the same brain region, five proteoglycans, tenascin-R and NR protein neurofascin were immunostained or their levels measured with western blots. Analysis of covariance and t-tests were used for group comparisons. There was dramatic reduction of NR length in PFC WM in both MDD and CUS rats. Proteoglycan BRAL1 immunostaining was reduced at NRs and in overall WM of MDD subjects, as was versican in overall WM. Phosphacan immunostaining and levels were increased in both in MDD and CUS. Neurofascin immunostaining at NRs and in overall WM was significantly increased in MDD. Reduced length of NRs and increased phosphacan and neurocan in MDD and stressed animals suggest that morphological and proteoglycan changes at NRs in depression may be related to stress exposure and contribute to connectivity alterations. However, differences between MDD and CUS for some NR related markers may point to other mechanisms affecting the structure and function of NRs in MDD.

Axonal energy metabolism, and the effects in aging and neurodegenerative diseases

Human studies consistently identify bioenergetic maladaptations in brains upon aging and neurodegenerative disorders of aging (NDAs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and Amyotrophic lateral sclerosis. Glucose is the major brain fuel and glucose hypometabolism has been observed in brain regions vulnerable to aging and NDAs. Many neurodegenerative susceptible regions are in the topological central hub of the brain connectome, linked by densely interconnected long-range axons. Axons, key components of the connectome, have high metabolic needs to support neurotransmission and other essential activities. Long-range axons are particularly vulnerable to injury, neurotoxin exposure, protein stress, lysosomal dysfunction, etc. Axonopathy is often an early sign of neurodegeneration. Recent studies ascribe axonal maintenance failures to local bioenergetic dysregulation. With this review, we aim to stimulate research in exploring metabolically oriented neuroprotection strategies to enhance or normalize bioenergetics in NDA models. Here we start by summarizing evidence from human patients and animal models to reveal the correlation between glucose hypometabolism and connectomic disintegration upon aging/NDAs. To encourage mechanistic investigations on how axonal bioenergetic dysregulation occurs during aging/NDAs, we first review the current literature on axonal bioenergetics in distinct axonal subdomains: axon initial segments, myelinated axonal segments, and axonal arbors harboring pre-synaptic boutons. In each subdomain, we focus on the organization, activity-dependent regulation of the bioenergetic system, and external glial support. Second, we review the mechanisms regulating axonal nicotinamide adenine dinucleotide (NAD+) homeostasis, an essential molecule for energy metabolism processes, including NAD+ biosynthetic, recycling, and consuming pathways. Third, we highlight the innate metabolic vulnerability of the brain connectome and discuss its perturbation during aging and NDAs. As axonal bioenergetic deficits are developing into NDAs, especially in asymptomatic phase, they are likely exaggerated further by impaired NAD+ homeostasis, the high energetic cost of neural network hyperactivity, and glial pathology. Future research in interrogating the causal relationship between metabolic vulnerability, axonopathy, amyloid/tau pathology, and cognitive decline will provide fundamental knowledge for developing therapeutic interventions.

LGI3/2-ADAM23 interactions cluster Kv1 channels in myelinated axons to regulate refractory period

Along myelinated axons, Shaker-type potassium channels (Kv1) accumulate at high density in the juxtaparanodal region, directly adjacent to the paranodal axon-glia junctions that flank the nodes of Ranvier. However, the mechanisms that control the clustering of Kv1 channels, as well as their function at this site, are still poorly understood. Here we demonstrate that axonal ADAM23 is essential for both the accumulation and stability of juxtaparanodal Kv1 complexes. The function of ADAM23 is critically dependent on its interaction with its extracellular ligands LGI2 and LGI3. Furthermore, we demonstrate that juxtaparanodal Kv1 complexes affect the refractory period, thus enabling high-frequency burst firing of action potentials. Our findings not only reveal a previously unknown molecular pathway that regulates Kv1 channel clustering, but they also demonstrate that the juxtaparanodal Kv1 channels that are concealed below the myelin sheath, play a significant role in modifying axonal physiology.

Higher throughput workflow with sensitive, reliable and automatic quantification of myelination in vitro suitable for drug screening

Multiple sclerosis (MS) is the most common demyelinating autoimmune disease of the central nervous system (CNS). Immune-mediated myelin and axonal damage that is accompanied by chronic axonal loss causing destruction of the myelin sheaths are hallmarks of MS. While great strides have been made in understanding the molecular underpinnings of re-/myelination, currently no remyelination therapy is available for MS. As myelination is a complex process that is not fully understood, we sought to develop a systematic, reliable, automated and quantitative higher throughput screening method. We aimed to quantitate myelin sheaths in vitro with high sensitivity at the single cell level suitable for testing small compound libraries. To this end, we miniaturised in vitro retinal ganglion cell-oligodendrocyte precursor cell (RGC-OPC) co-cultures into a multi-well plate format. This allowed us to maintain the reciprocal interaction of live axons and oligodendrocytes (OLs) to ensure compact myelin formation. To quantify our co-cultures, we developed a novel computer vision algorithm to precisely measure myelination. We demonstrated efficacy of our system with known pro-differentiating compounds BQ3020 and XAV939 which exhibited robust, efficient, and dose dependent effects on myelination. Through this combination of experimental and technical advances, we have developed a method allowing systematic and reliable testing of remyelinating compound efficacy.

Astrocytes as Context for the Involvement of Myelin and Nodes of Ranvier in the Pathophysiology of Depression and Stress-Related Disorders

Astrocytes, despite some shared features as glial cells supporting neuronal function in gray and white matter, participate and adapt their morphology and neurochemistry in a plethora of distinct regulatory tasks in specific neural environments. In the white matter, a large proportion of the processes branching from the astrocytes' cell bodies establish contacts with oligodendrocytes and the myelin they form, while the tips of many astrocyte branches closely associate with nodes of Ranvier. Stability of myelin has been shown to greatly depend on astrocyte-to-oligodendrocyte communication, while the integrity of action potentials that regenerate at nodes of Ranvier has been shown to depend on extracellular matrix components heavily contributed by astrocytes. Several lines of evidence are starting to show that in human subjects with affective disorders and in animal models of chronic stress there are significant changes in myelin components, white matter astrocytes and nodes of Ranvier that have direct relevance to connectivity alterations in those disorders. Some of these changes involve the expression of connexins supporting astrocyte-to-oligodendrocyte gap junctions, extracellular matrix components produced by astrocytes around nodes of Ranvier, specific types of astrocyte glutamate transporters, and neurotrophic factors secreted by astrocytes that are involved in the development and plasticity of myelin. Future studies should further examine the mechanisms responsible for those changes in white matter astrocytes, their putative contribution to pathological connectivity in affective disorders, and the possibility of leveraging that knowledge to design new therapies for psychiatric disorders.

The role of gangliosides in the organisation of the node of Ranvier examined in glycosyltransferase transgenic mice

Gangliosides are a family of sialic acid containing glycosphingolipids highly enriched in plasma membranes of the vertebrate nervous system. They are functionally diverse in modulating nervous system integrity, notably at the node of Ranvier, and also act as receptors for many ligands including toxins and autoantibodies. They are synthesised in a stepwise manner by groups of glycosyl- and sialyltransferases in a developmentally and tissue regulated manner. In this review, we summarise and discuss data derived from transgenic mice with different transferase deficiencies that have been used to determine the role of glycolipids in the organisation of the node of Ranvier. Understanding their role at this specialised functional site is crucial to determining differential pathophysiology following directed genetic or autoimmune injury to peripheral nerve nodal or paranodal domains, and revealing the downstream consequences of axo-glial disruption.

Concussion leads to widespread axonal sodium channel loss and disruption of the node of Ranvier

Despite being a major health concern, little is known about the pathophysiological changes that underly concussion. Nonetheless, emerging evidence suggests that selective damage to white matter axons, or diffuse axonal injury (DAI), disrupts brain network connectivity and function. While voltage-gated sodium channels (NaChs) and their anchoring proteins at the nodes of Ranvier (NOR) on axons are key elements of the brain's network signaling machinery, changes in their integrity have not been studied in context with DAI. Here, we utilized a clinically relevant swine model of concussion that induces evolving axonal pathology, demonstrated by accumulation of amyloid precursor protein (APP) across the white matter. Over a two-week follow-up post-concussion with this model, we found widespread loss of NaCh isoform 1.6 (Nav1.6), progressive increases in NOR length, the appearance of void and heminodes and loss of βIV-spectrin, ankyrin G, and neurofascin 186 or their collective diffusion into the paranode. Notably, these changes were in close proximity, yet distinct from APP-immunoreactive swollen axonal profiles, potentially representing a unique, newfound phenotype of axonal pathology in DAI. Since concussion in humans is non-fatal, the clinical relevance of these findings was determined through examination of post-mortem brain tissue from humans with higher levels of acute traumatic brain injury. Here, a similar loss of Nav1.6 and changes in NOR structures in brain white matter were observed as found in the swine model of concussion. Collectively, this widespread and progressive disruption of NaChs and NOR appears to be a form of sodium channelopathy, which may represent an important substrate underlying brain network dysfunction after concussion.

Ganglioside lipidomics of CNS myelination using direct infusion shotgun mass spectrometry

Gangliosides are present and concentrated in axons and implicated in axon-myelin interactions, but how ganglioside composition changes during myelin formation is not known. Here, we present a direct infusion (shotgun) lipidomics method to analyze gangliosides in small amounts of tissue reproducibly and with high sensitivity. We resolve the mouse ganglioside lipidome during development and adulthood and determine the ganglioside content of mice lacking the St3gal5 and B4galnt1 genes that synthesize most ganglioside species. Our results reveal substantial changes in the ganglioside lipidome during the formation of myelinated nerve fibers. In sum, we provide insights into the CNS ganglioside lipidome with a quantitative and sensitive mass spectrometry method. Since this method is compatible with global lipidomic profiling, it will provide insights into ganglioside function in physiology and pathology.

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A sensitive direct infusion mass spectrometry method for ganglioside lipidomics

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Quantification of gangliosides in CNS myelin development

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Generation of myelin in the absence of gangliosides

CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

Case report: Immunoadsorption therapy for anti-caspr1 antibody-associated nodopathy

Background and objectives

Several autoantibodies against proteins located at the node of Ranvier has been identified in patients with chronic inflammatory demyelinating polyneuropathy (CIDP) in the last few years. Then a new concept, autoimmune nodo-paranodopathies was proposed. Cases of Caspr1 autoantibodies are the most rare. Here we describe an anti-Caspr1 nodopathy patient, summarized his clinical, physiological and pathological features.

Case presentation

We present the case of a 56-year-old male patient with proprioceptive loss, ataxia, coarse tremor and distal limb weakness without any painess and cranial involvement. Electrophysiological studies showed prolonged distal motor latencies, conduction slowing and reduced amplitude distal compound muscle action potential (CMAP) amplitude. Antibodies against the nodes of Ranvier in serum samples revealed a positive finding for the anti-Caspr1 antibody (1:10).Myelinated fiber loss could be revealed in nerve biopsy. Longitudinal ultrathin sections of the nodal region was discovered in electron microscope, the paranodal/nodal architecture was destructed. It was lack of transverse bands and enlargement of the space between the axon and the paranodal loops was seen. The patient improved obviously after three times immunoadsorption(IA) therapy.

Conclusion

Anti-Caspr1 nodopathy patient may present atypical symptoms without any neuropathic pain and cranial palsy. The destruction of paranodal/nodal architecture could be observed in nerve biopsy, which may be caused by the lost of axoglial complex formed by NF155, CNTN1 and Caspr1. Antibodies detection is important for the diagnosis, while IA therapy could be regarded as an option for the patients allergic to rituximab (RTX).

Morphology changes in the cochlea of impulse noise-induced hidden hearing loss

BACKGROUND

This study was focused on impulse noise induces hidden hearing loss.

OBJECTIVES

This study was designed to determine the morphology changes of noise-induced hidden hearing loss (NIHHL).

METHOD

Fifteen guinea pigs were divided into three groups: noise-induced hidden hearing loss (NIHHL) group, noise-induced hearing loss (NIHL) group, and normal control group. For the NIHHL group, guinea pigs were exposed to 15 times of impulse noise with peak intensity of 163 dB SPL at one time. For the NIHL group, animals were exposed to two rounds of 100 times impulse noise, and the time interval is 24 h. Auditory brain response (ABR) was tested before, immediately, 24 h, one week, and one month after noise exposure to evaluate cochlear physiology changes. One month after noise exposure, all guinea pigs in three groups were sacrificed, and basement membranes were carefully dissected immediately after ABR tests. The cochlea samples were observed by transmission electron microscopy (TEM) to find out the morphology changes.

RESULT

The ABR results showed that 15 times of impulse noise exposure could cause NIHHL in guinea pigs and 200 times could cause completely hearing loss. Impulse noise exposure could cause a dramatic increase of mitochondria in the inner hair cell. The structures of ribbon synapse and heminode were also obviously impaired compared to the normal group. The nerve fiber and myelin sheath remained intact after impulse noise exposure.

CONCLUSION

This research revealed that impulse noise could cause hidden hearing loss, and the changes in inner hair cells, ribbon synapse, and heminode all played a vital role in the pathogenesis of hidden hearing loss.

Sphingolipid Players in Multiple Sclerosis: Their Influence on the Initiation and Course of the Disease

Sphingolipids (SLs) play a significant role in the nervous system, as major components of the myelin sheath, contributors to lipid raft formation that organize intracellular processes, as well as active mediators of transport, signaling and the survival of neurons and glial cells. Alterations in SL metabolism and content are observed in the course of central nervous system diseases, including multiple sclerosis (MS). In this review, we summarize the current evidence from studies on SLs (particularly gangliosides), which may shed new light upon processes underlying the MS background. The relevant aspects of these studies include alterations of the SL profile in MS, the role of antibodies against SLs and complexes of SL-ligand-invariant NKT cells in the autoimmune response as the core pathomechanism in MS. The contribution of lipid-raft-associated SLs and SL-laden extracellular vesicles to the disease etiology is also discussed. These findings may have diagnostic implications, with SLs and anti-SL antibodies as potential markers of MS activity and progression. Intriguing prospects of novel therapeutic options in MS are associated with SL potential for myelin repair and neuroprotective effects, which have not been yet addressed by the available treatment strategies. Overall, all these concepts are promising and encourage the further development of SL-based studies in the field of MS.

Diabetes Mellitus Is a Possible Risk Factor for Nodo-paranodopathy With Antiparanodal Autoantibodies

Background and Objectives

Nodo-paranodopathies are peripheral neuropathies with dysfunction of the node of Ranvier. Affected patients who are seropositive for antibodies against adhesion molecules like contactin-1 and neurofascin show distinct clinical features and a disruption of the paranodal complex. An axoglial dysjunction is also a characteristic finding of diabetic neuropathy. Here, we aim to investigate a possible association of antibody-mediated nodo-paranodopathy and diabetes mellitus (DM).

Methods

We retrospectively analyzed clinical data of 227 patients with chronic inflammatory demyelinating polyradiculoneuropathy and Guillain-Barré syndrome from multiple centers in Germany who had undergone diagnostic testing for antiparanodal antibodies targeting neurofascin-155, pan-neurofascin, contactin-1–associated protein 1, and contactin-1. To study possible direct pathogenic effects of antiparanodal antibodies, we performed immunofluorescence binding assays on human pancreatic tissue sections.

Results

The frequency of DM was 33.3% in seropositive patients and thus higher compared with seronegative patients (14.1%, OR = 3.04, 95% CI = 1.31–6.80). The relative risk of DM in seropositive patients was 3.4-fold higher compared with the general German population. Seropositive patients with DM most frequently harbored anti–contactin-1 antibodies and had higher antibody titers than seropositive patients without DM. The diagnosis of DM preceded the onset of neuropathy in seropositive patients. No immunoreactivity of antiparanodal antibodies against pancreatic tissue was detected.

Discussion

We report an association of nodo-paranodopathy and DM. Our results suggest that DM may be a potential risk factor for predisposing to developing nodo-paranodopathy and argue against DM being induced by the autoantibodies. Our findings set the basis for further research investigating underlying immunopathogenetic connections.

Complement and Coagulation System Crosstalk in Synaptic and Neural Conduction in the Central and Peripheral Nervous Systems

Complement and coagulation are both key systems that defend the body from harm. They share multiple features and are similarly activated. They each play individual roles in the systemic circulation in physiology and pathophysiology, with significant crosstalk between them. Components from both systems are mapped to important structures in the central nervous system (CNS) and peripheral nervous system (PNS). Complement and coagulation participate in critical functions in neuronal development and synaptic plasticity. During pathophysiological states, complement and coagulation factors are upregulated and can modulate synaptic transmission and neuronal conduction. This review summarizes the current evidence regarding the roles of the complement system and the coagulation cascade in the CNS and PNS. Possible crosstalk between the two systems regarding neuroinflammatory-related effects on synaptic transmission and neuronal conduction is explored. Novel treatment based on the modulation of crosstalk between complement and coagulation may perhaps help to alleviate neuroinflammatory effects in diseased states of the CNS and PNS.

Impact of Neurofascin on Chronic Inflammatory Demyelinating Polyneuropathy via Changing the Node of Ranvier Function: A Review

The effective conduction of action potential in the peripheral nervous system depends on the structural and functional integrity of the node of Ranvier and paranode. Neurofascin (NF) plays an important role in the conduction of action potential in a saltatory manner. Two subtypes of NF, NF186, and NF155, are involved in the structure of the node of Ranvier. In patients with chronic inflammatory demyelinating polyneuropathy (CIDP), anti-NF antibodies are produced when immunomodulatory dysfunction occurs, which interferes with the conduction of action potential and is considered the main pathogenic factor of CIDP. In this study, we describe the assembling mechanism and anatomical structure of the node of Ranvier and the necessary cell adhesion molecules for its physiological function. The main points of this study are that we summarized the recent studies on the role of anti-NF antibodies in the changes in the node of Ranvier function and its impact on clinical manifestations and analyzed the possible mechanisms underlying the pathogenesis of CIDP.

Neurokinins and their receptors in the rat trigeminal system: Differential localization and release with implications for migraine pain

Substance P (SP) and calcitonin gene-related peptide (CGRP) have both been considered potential drug candidates in migraine therapy. In recent years, CGRP receptor inhibition has been established as an effective treatment, in particular as a prophylactic for chronic migraine. Curiously, inhibition of neurokinin receptor 1 (NK1R) failed to alleviate acute migraine attacks in clinical trials, and the neurokinins were consequently abandoned as potential antimigraine candidates. The reason behind this has remained enigmatic.

Utilizing immunohistochemistry and semi-quantitative cell counts the expression of neurokinins and their associated receptors was examined in the rat trigeminal ganglion.

Immunohistochemistry results revealed SP co-localization in CGRP positive neurons and C-fibres, where it mainly concentrated at boutons. Neurokinin A (NKA) was observed in a population of C-fibres and small neurons where it could co-localize with SP. In contrast, neurokinin B (NKB) did not co-localize with SP and was observed in large/medium sized neurons and Aδ-fibres. All neurokinin receptors (NK1-3R) were found to be expressed in a majority of trigeminal ganglion neurons and A-fibres.

The functional release of SP and CGRP in the trigeminovascular system was stimulated with either 60 mM K+ or 100 nM capsaicin and measured with an enzyme-linked immunosorbent assay (ELISA). ELISA results established that SP can be released locally from trigeminovascular system. The released SP was comparatively minor compared to the CGRP release from stimulated dura mater, trigeminal ganglion neurons and fibres. We hypothesize that SP and CGRP signalling pathways may work in tandem to exacerbate painful stimuli in the TGV system.

Distinct roles for the Charcot-Marie-tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings. However, adult Mtmr5-/- mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Consistently, Mtmr5 levels were highest during axon radial sorting and fell sharply after postnatal day seven. Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.

Osteocalcin attenuates oligodendrocyte differentiation and myelination via GPR37 signaling in the mouse brain

The bone-derived hormone osteocalcin (OCN) is crucial for brain development and neural cognitive functions, yet the exact roles of OCN in central nervous system (CNS) remain elusive. Here, we find that genetic deletion of OCN facilitates oligodendrocyte (OL) differentiation and hypermyelination in the CNS. Although dispensable for the proliferation of oligodendrocyte precursor cells (OPCs), OCN is critical for the myelination of OLs, which affects myelin production and remyelination after demyelinating injury. Genome-wide RNA sequencing analyses reveal that OCN regulates a number of G protein–coupled receptors and myelination-associated transcription factors, of which Myrf might be a key downstream effector in OLs. GPR37 is identified as a previously unknown receptor for OCN, thus regulating OL differentiation and CNS myelination. Overall, these findings suggest that OCN orchestrates the transition between OPCs and myelinating OLs via GPR37 signaling, and hence, the OCN/GPR37 pathway regulates myelin homeostasis in the CNS.

Syndromic Deafness Gene ATP6V1B2 Controls Degeneration of Spiral Ganglion Neurons Through Modulating Proton Flux

ATP6V1B2 encodes the V1B2 subunit in V-ATPase, a proton pump responsible for the acidification of lysosomes. Mutations in this gene cause DDOD syndrome, DOORS syndrome, and Zimmermann-Laband syndrome, which share overlapping feature of congenital sensorineural deafness, onychodystrophy, and different extents of intellectual disability without or with epilepsy. However, the underlying mechanisms remain unclear. To investigate the pathological role of mutant ATP6V1B2 in the auditory system, we evaluated auditory brainstem response, distortion product otoacoustic emissions, in a transgenic line of mice carrying c.1516 C > T (p.Arg506∗) in Atp6v1b2, Atp6v1b2 Arg506*/Arg506* . To explore the pathogenic mechanism of neurodegeneration in the auditory pathway, immunostaining, western blotting, and RNAscope analyses were performed in Atp6v1b2Arg506*/Arg506* mice. The Atp6v1b2Arg506*/Arg506* mice showed hidden hearing loss (HHL) at early stages and developed late-onset hearing loss. We observed increased transcription of Atp6v1b1 in hair cells of Atp6v1b2Arg506*/Arg506* mice and inferred that Atp6v1b1 compensated for the Atp6v1b2 dysfunction by increasing its own transcription level. Genetic compensation in hair cells explains the milder hearing impairment in Atp6v1b2Arg506*/Arg506* mice. Apoptosis activated by lysosomal dysfunction and the subsequent blockade of autophagic flux induced the degeneration of spiral ganglion neurons and further impaired the hearing. Intraperitoneal administration of the apoptosis inhibitor, BIP-V5, improved both phenotypical and pathological outcomes in two live mutant mice. Based on the pathogenesis underlying hearing loss in Atp6v1b2-related syndromes, systemic drug administration to inhibit apoptosis might be an option for restoring the function of spiral ganglion neurons and promoting hearing, which provides a direction for future treatment.

Peripheral Nerve Development and the Pathogenesis of Peripheral Neuropathy: the Sorting Point

Nerve development requires a coordinated sequence of events and steps to be accomplished for the generation of functional peripheral nerves to convey sensory and motor signals. Any abnormality during development may result in pathological structure and function of the nerve, which evolves in peripheral neuropathy. In this review, we will briefly describe different steps of nerve development while we will mostly focus on the molecular mechanisms involved in radial sorting of axons, one of these nerve developmental steps. We will summarize current knowledge of molecular pathways so far reported in radial sorting and their possible interactions. Finally, we will describe how disruption of these pathways may result in human neuropathies.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Analog Transmission of Action Potential Fine Structure in Spiral Ganglion Axons

Action potential waveforms generated at the axon initial segment (AIS) are specialized between and within neuronal classes. But is the fine structure of each electrical event retained when transmitted along myelinated axons or is it rapidly and uniformly transmitted to be modified again at the axon terminal? To address this issue action potential axonal transmission was evaluated in a class of primary sensory afferents that possess numerous types of voltage-gated ion channels underlying a complex repertoire of endogenous firing patterns. In addition to their signature intrinsic electrophysiological heterogeneity, spiral ganglion neurons are uniquely designed. The bipolar, myelinated somata of type I neurons are located within the conduction pathway, requiring that action potentials generated at the first heminode must be conducted through their electrically excitable membrane. We utilized this unusual axonal-like morphology to serve as a window into action potential transmission to compare locally-evoked action potential profiles to those generated peripherally at their glutamatergic synaptic connections with hair cell receptors. These comparisons showed that the distinctively-shaped somatic action potentials were highly correlated with the nodally-generated, invading ones for each neuron. This result indicates that the fine structure of the action potential waveform is maintained axonally, thus supporting the concept that analog signaling is incorporated into each digitally-transmitted action potential in the specialized primary auditory afferents.

An iPSC model of hereditary sensory neuropathy-1 reveals L-serine-responsive deficits in neuronal ganglioside composition and axoglial interactions

Hereditary sensory neuropathy type 1 (HSN1) is caused by mutations in the SPTLC1 or SPTLC2 sub-units of the enzyme serine palmitoyltransferase, resulting in the production of toxic 1-deoxysphingolipid bases (DSBs). We used induced pluripotent stem cells (iPSCs) from patients with HSN1 to determine whether endogenous DSBs are neurotoxic, patho-mechanisms of toxicity and response to therapy. HSN1 iPSC-derived sensory neurons (iPSCdSNs) endogenously produce neurotoxic DSBs. Complex gangliosides, which are essential for membrane micro-domains and signaling, are reduced, and neurotrophin signaling is impaired, resulting in reduced neurite outgrowth. In HSN1 myelinating cocultures, we find a major disruption of nodal complex proteins after 8 weeks, which leads to complete myelin breakdown after 6 months. HSN1 iPSC models have, therefore, revealed that SPTLC1 mutation alters lipid metabolism, impairs the formation of complex gangliosides, and reduces axon and myelin stability. Many of these changes are prevented by l-serine supplementation, supporting its use as a rational therapy.

Recent insights on astrocyte mechanisms in CNS homeostasis, pathology, and repair

Astrocytes play essential roles in development, homeostasis, injury, and repair of the central nervous system (CNS). Their development is tightly regulated by distinct spatial and temporal cues during embryogenesis and into adulthood throughout the CNS. Astrocytes have several important responsibilities such as regulating blood flow and permeability of the blood-CNS barrier, glucose metabolism and storage, synapse formation and function, and axon myelination. In CNS pathologies, astrocytes also play critical parts in both injury and repair mechanisms. Upon injury, they undergo a robust phenotypic shift known as "reactive astrogliosis," which results in both constructive and deleterious outcomes. Astrocyte activation and migration at the site of injury provides an early defense mechanism to minimize the extent of injury by enveloping the lesion area. However, astrogliosis also contributes to the inhibitory microenvironment of CNS injury and potentiate secondary injury mechanisms, such as inflammation, oxidative stress, and glutamate excitotoxicity, which facilitate neurodegeneration in CNS pathologies. Intriguingly, reactive astrocytes are increasingly a focus in current therapeutic strategies as their activation can be modulated toward a neuroprotective and reparative phenotype. This review will discuss recent advancements in knowledge regarding the development and role of astrocytes in the healthy and pathological CNS. We will also review how astrocytes have been genetically modified to optimize their reparative potential after injury, and how they may be transdifferentiated into neurons and oligodendrocytes to promote repair after CNS injury and neurodegeneration.

Dynamic early clusters of nodal proteins contribute to node of Ranvier assembly during myelination of peripheral neurons

Voltage-gated sodium channels cluster in macromolecular complexes at nodes of Ranvier to promote rapid nerve impulse conduction in vertebrate nerves. Node assembly in peripheral nerves is thought to be initiated at heminodes at the extremities of myelinating Schwann cells, and fusion of heminodes results in the establishment of nodes. Here we show that assembly of 'early clusters' of nodal proteins in the murine axonal membrane precedes heminode formation. The neurofascin (Nfasc) proteins are essential for node assembly, and the formation of early clusters also requires neuronal Nfasc. Early clusters are mobile and their proteins are dynamically recruited by lateral diffusion. They can undergo fusion not only with each other but also with heminodes, thus contributing to the development of nodes in peripheral axons. The formation of early clusters constitutes the earliest stage in peripheral node assembly and expands the repertoire of strategies that have evolved to establish these essential structures.

Animal Models of Multiple Sclerosis

Animal models with high translational validity are essential tools in understanding disease pathogenesis and in the development of therapeutic strategies. Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system characterized by progressive neurological deficits and socioeconomic burden. Experimental autoimmune encephalomyelitis (EAE) is the most extensively utilized animal model of MS, with well-characterized rodent and non-human primate variants. The EAE model is typically induced by either active immunization with myelin-derived proteins or peptides in adjuvant or by passive transfer of activated myelin-specific CD4⁺ T lymphocytes. To date, the EAE model has been an essential tool in the development of at least seven U.S. Food and Drug Administration (FDA)-approved immunomodulatory drugs for the treatment of MS, including glatiramer acetate, fingolimod, and natalizumab. However, the translational validity of the EAE model is frequently compromised due to poor study design, inconsistent clinical scoring endpoints, and inappropriate statistical calculations. No single animal model accurately reflects the complexity of human MS pathogenesis. Beyond EAE, multiple additional animal models are described, including Theiler's murine encephalomyelitis virus and cuprizone-induced demyelination, which facilitate the study of pathogen-induced CNS autoimmunity and remyelination, respectively. This overview summarizes several of the most frequently used animal models of MS and highlights key factors that significantly influence the experimental outcome and affect translational validity. © 2021 Wiley Periodicals LLC.

Glial Cells Promote Myelin Formation and Elimination

Building a functional nervous system requires the coordinated actions of many glial cells. In the vertebrate central nervous system (CNS), oligodendrocytes myelinate neuronal axons to increase conduction velocity and provide trophic support. Myelination can be modified by local signaling at the axon-myelin interface, potentially adapting sheaths to support the metabolic needs and physiology of individual neurons. However, neurons and oligodendrocytes are not wholly responsible for crafting the myelination patterns seen in vivo. Other cell types of the CNS, including microglia and astrocytes, modify myelination. In this review, I cover the contributions of non-neuronal, non-oligodendroglial cells to the formation, maintenance, and pruning of myelin sheaths. I address ways that these cell types interact with the oligodendrocyte lineage throughout development to modify myelination. Additionally, I discuss mechanisms by which these cells may indirectly tune myelination by regulating neuronal activity. Understanding how glial-glial interactions regulate myelination is essential for understanding how the brain functions as a whole and for developing strategies to repair myelin in disease.

iPSC-derived myelinoids to study myelin biology of humans

Myelination is essential for central nervous system (CNS) formation, health, and function. Emerging evidence of oligodendrocyte heterogeneity in health and disease and divergent CNS gene expression profiles between mice and humans supports the development of experimentally tractable human myelination systems. Here, we developed human iPSC-derived myelinating organoids ("myelinoids") and quantitative tools to study myelination from oligodendrogenesis through to compact myelin formation and myelinated axon organization. Using patient-derived cells, we modeled a monogenetic disease of myelinated axons (Nfasc155 deficiency), recapitulating impaired paranodal axo-glial junction formation. We also validated the use of myelinoids for pharmacological assessment of myelination-both at the level of individual oligodendrocytes and globally across whole myelinoids-and demonstrated reduced myelination in response to suppressed synaptic vesicle release. Our study provides a platform to investigate human myelin development, disease, and adaptive myelination.

Completion of neuronal remodeling prompts myelination along developing motor axon branches

Neuronal remodeling and myelination are two fundamental processes during neurodevelopment. How they influence each other remains largely unknown, even though their coordinated execution is critical for circuit function and often disrupted in neuropsychiatric disorders. It is unclear whether myelination stabilizes axon branches during remodeling or whether ongoing remodeling delays myelination. By modulating synaptic transmission, cytoskeletal dynamics, and axonal transport in mouse motor axons, we show that local axon remodeling delays myelination onset and node formation. Conversely, glial differentiation does not determine the outcome of axon remodeling. Delayed myelination is not due to a limited supply of structural components of the axon-glial unit but rather is triggered by increased transport of signaling factors that initiate myelination, such as neuregulin. Further, transport of promyelinating signals is regulated via local cytoskeletal maturation related to activity-dependent competition. Our study reveals an axon branch-specific fine-tuning mechanism that locally coordinates axon remodeling and myelination.

Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons

The myelination of axons by oligodendrocytes is a highly complex cell-to-cell interaction. Oligodendrocytes and axons have a reciprocal signaling relationship in which oligodendrocytes receive cues from axons that direct their myelination, and oligodendrocytes subsequently shape axonal structure and conduction. Oligodendrocytes are necessary for the maturation of excitatory domains on the axon including nodes of Ranvier, help buffer potassium, and support neuronal energy metabolism. Disruption of the oligodendrocyte-axon unit in traumatic injuries, Alzheimer's disease and demyelinating diseases such as multiple sclerosis results in axonal dysfunction and can culminate in neurodegeneration. In this review, we discuss the mechanisms by which demyelination and loss of oligodendrocytes compromise axons. We highlight the intra-axonal cascades initiated by demyelination that can result in irreversible axonal damage. Both the restoration of oligodendrocyte myelination or neuroprotective therapies targeting these intra-axonal cascades are likely to have therapeutic potential in disorders in which oligodendrocyte support of axons is disrupted.

Formation of the node of Ranvier by Schwann cells is under control of transcription factor Sox10

The transcription factor Sox10 is an essential regulator of genes that code for structural components of the myelin sheath and for lipid metabolic enzymes in both types of myelinating glia in the central and peripheral nervous systems. In an attempt to characterize additional Sox10 target genes in Schwann cells, we identified in this study a strong influence of Sox10 on the expression of genes associated with adhesion in the MSC80 Schwann cell line. These included the genes for Gliomedin, Neuronal cell adhesion molecule and Neurofascin that together constitute essential Schwann cell contributions to paranode and node of Ranvier. Using bioinformatics and molecular biology techniques we provide evidence that Sox10 directly activates these genes by binding to conserved regulatory regions. For activation, Sox10 cooperates with Krox20, a transcription factor previously identified as the central regulator of Schwann cell myelination. Both the activating function of Sox10 as well as its cooperation with Krox20 were confirmed in vivo. We conclude that the employment of Sox10 and Krox20 as regulators of structural myelin sheath components and genes associated with the node of Ranvier is one way of ensuring a biologically meaningful coordinated formation of both structures during peripheral myelination.

Neurofascin Is a Novel Component of Rod Photoreceptor Synapses in the Outer Retina

Neural circuit formation is an intricate and complex process where multiple neuron types must come together to form synaptic connections at a precise location and time. How this process is orchestrated during development remains poorly understood. Cell adhesion molecules are known to play a pivotal role in assembling neural circuits. They serve as recognition molecules between corresponding synaptic partners. In this study, we identified a new player in assembling neural circuits in the outer retina, the L1-family cell adhesion molecule Neurofascin (Nfasc). Our data reveals Nfasc is expressed in the synaptic layer where photoreceptors make synaptic connections to their respective partners. A closer examination of Nfasc expression shows high levels of expression in rod bipolars but not in cone bipolars. Disruption of Nfasc using a conditional knockout allele results in selective loss of pre- and post-synaptic proteins in the rod synaptic layer but not in the cone synaptic layer. Electron microscopic analysis confirms that indeed there are abnormal synaptic structures with less dendrites of rod bipolars innervating rod terminals in loss of Nfasc animals. Consistent with these findings, we also observe a decrease in rod-driven retinal responses with disruption of Nfasc function but not in cone-driven responses. Taken together, our data suggest a new role of Nfasc in rod synapses within the mouse outer retina.

Insights Into the Role and Potential of Schwann Cells for Peripheral Nerve Repair From Studies of Development and Injury

Peripheral nerve injuries arising from trauma or disease can lead to sensory and motor deficits and neuropathic pain. Despite the purported ability of the peripheral nerve to self-repair, lifelong disability is common. New molecular and cellular insights have begun to reveal why the peripheral nerve has limited repair capacity. The peripheral nerve is primarily comprised of axons and Schwann cells, the supporting glial cells that produce myelin to facilitate the rapid conduction of electrical impulses. Schwann cells are required for successful nerve regeneration; they partially “de-differentiate” in response to injury, re-initiating the expression of developmental genes that support nerve repair. However, Schwann cell dysfunction, which occurs in chronic nerve injury, disease, and aging, limits their capacity to support endogenous repair, worsening patient outcomes. Cell replacement-based therapeutic approaches using exogenous Schwann cells could be curative, but not all Schwann cells have a “repair” phenotype, defined as the ability to promote axonal growth, maintain a proliferative phenotype, and remyelinate axons. Two cell replacement strategies are being championed for peripheral nerve repair: prospective isolation of “repair” Schwann cells for autologous cell transplants, which is hampered by supply challenges, and directed differentiation of pluripotent stem cells or lineage conversion of accessible somatic cells to induced Schwann cells, with the potential of “unlimited” supply. All approaches require a solid understanding of the molecular mechanisms guiding Schwann cell development and the repair phenotype, which we review herein. Together these studies provide essential context for current efforts to design glial cell-based therapies for peripheral nerve regeneration.

Implication of Contactins in Demyelinating Pathologies

Demyelinating pathologies comprise of a variety of conditions where either central or peripheral myelin is attacked, resulting in white matter lesions and neurodegeneration. Myelinated axons are organized into molecularly distinct domains, and this segregation is crucial for their proper function. These defined domains are differentially affected at the different stages of demyelination as well as at the lesion and perilesion sites. Among the main players in myelinated axon organization are proteins of the contactin (CNTN) group of the immunoglobulin superfamily (IgSF) of cell adhesion molecules, namely Contactin-1 and Contactin-2 (CNTN1, CNTN2). The two contactins perform their functions through intermolecular interactions, which are crucial for myelinated axon integrity and functionality. In this review, we focus on the implication of these two molecules as well as their interactors in demyelinating pathologies in humans. At first, we describe the organization and function of myelinated axons in the central (CNS) and the peripheral (PNS) nervous system, further analyzing the role of CNTN1 and CNTN2 as well as their interactors in myelination. In the last section, studies showing the correlation of the two contactins with demyelinating pathologies are reviewed, highlighting the importance of these recognition molecules in shaping the function of the nervous system in multiple ways.

Differential Contribution of Cadm1-Cadm3 Cell Adhesion Molecules to Peripheral Myelinated Axons

Cell adhesion proteins of the Cadm (SynCAM/Necl) family regulate myelination and the organization of myelinated axons. In the peripheral nervous system (PNS), intercellular contact between Schwann cells and their underlying axons is believed to be mediated by binding of glial Cadm4 to axonal Cadm3 or Cadm2. Nevertheless, given that distinct neurons express different combinations of the Cadm proteins, the identity of the functional axonal ligand for Cadm4 remains to be determined. Here, we took a genetic approach to compare the phenotype of Cadm4 null mice, which exhibit abnormal distribution of Caspr and Kv1 potassium channels, with mice lacking different combinations of Cadm1-Cadm3 genes. We show that in contrast to mice lacking the single Cadm1, Cadm2, or Cadm3 genes, genetic ablation of all three phenocopies the abnormalities detected in the absence of Cadm4. Similar defects were observed in double mutant mice lacking Cadm3 and Cadm2 (i.e., Cadm3 ^-/- /Cadm2 ^-/-) or Cadm3 and Cadm1 (i.e., Cadm3 ^-/- /Cadm1 ^-/-), but not in mice lacking Cadm1 and Cadm2 (i.e., Cadm1 ^-/- /Cadm2 ^-/-). Furthermore, axonal organization abnormalities were also detected in Cadm3 null mice that were heterozygous for the two other axonal Cadms. Our results identify Cadm3 as the main axonal ligand for glial Cadm4, and reveal that its absence could be compensated by the combined action of Cadm2 and Cadm1.SIGNIFICANCE STATEMENT Myelination by Schwann cells enables fast conduction of action potentials along motor and sensory axons. In these nerves, Schwann cell-axon contact is mediated by cell adhesion molecules of the Cadm family. Cadm4 in Schwann cells regulates axonal ensheathment and myelin wrapping, as well as the organization of the axonal membrane, but the identity of its axonal ligands is not clear. Here, we reveal that Cadm mediated axon-glia interactions depend on a hierarchical adhesion code that involves multiple family members. Our results provide important insights into the molecular mechanisms of axon-glia communication, and the function of Cadm proteins in PNS myelin.

Assembly and Function of the Juxtaparanodal Kv1 Complex in Health and Disease

The precise axonal distribution of specific potassium channels is known to secure the shape and frequency of action potentials in myelinated fibers. The low-threshold voltage-gated Kv1 channels located at the axon initial segment have a significant influence on spike initiation and waveform. Their role remains partially understood at the juxtaparanodes where they are trapped under the compact myelin bordering the nodes of Ranvier in physiological conditions. However, the exposure of Kv1 channels in de- or dys-myelinating neuropathy results in alteration of saltatory conduction. Moreover, cell adhesion molecules associated with the Kv1 complex, including Caspr2, Contactin2, and LGI1, are target antigens in autoimmune diseases associated with hyperexcitability such as encephalitis, neuromyotonia, or neuropathic pain. The clustering of Kv1.1/Kv1.2 channels at the axon initial segment and juxtaparanodes is based on interactions with cell adhesion molecules and cytoskeletal linkers. This review will focus on the trafficking and assembly of the axonal Kv1 complex in the peripheral and central nervous system (PNS and CNS), during development, and in health and disease.

Hypomyelinating leukodystrophies - unravelling myelin biology

Hypomyelinating leukodystrophies constitute a subset of genetic white matter disorders characterized by a primary lack of myelin deposition. Most patients with severe hypomyelination present in infancy or early childhood and develop severe neurological deficits, but the clinical presentation can also be mild with onset of symptoms in adolescence or adulthood. MRI can be used to visualize the process of myelination in detail, and MRI pattern recognition can provide a clinical diagnosis in many patients. Next-generation sequencing provides a definitive diagnosis in 80-90% of patients. Genes associated with hypomyelination include those that encode structural myelin proteins but also many that encode proteins involved in RNA translation and some lysosomal proteins. The precise pathomechanisms remain to be elucidated. Improved understanding of the process of myelination, the metabolic axonal support functions of myelin and the proposed contribution of myelin to CNS plasticity provide possible explanations as to why almost all patients with hypomyelination experience slow clinical decline after a long phase of stability. In this Review, we provide an overview of the hypomyelinating leukodystrophies, the advances in our understanding of myelin biology and of the genes involved in these disorders, and the insights these advances have provided into their clinical presentations and evolution.

More Than Mortar: Glia as Architects of Nervous System Development and Disease

Glial cells are an essential component of the nervous system of vertebrates and invertebrates. In the human brain, glia are as numerous as neurons, yet the importance of glia to nearly every aspect of nervous system development has only been expounded over the last several decades. Glia are now known to regulate neural specification, synaptogenesis, synapse function, and even broad circuit function. Given their ubiquity, it is not surprising that the contribution of glia to neuronal disease pathogenesis is a growing area of research. In this review, we will summarize the accumulated evidence of glial participation in several distinct phases of nervous system development and organization-neural specification, circuit wiring, and circuit function. Finally, we will highlight how these early developmental roles of glia contribute to nervous system dysfunction in neurodevelopmental and neurodegenerative disorders.

Is MS affecting the CNS only? Lessons from clinic to myelin pathophysiology

MS is regarded as a disease of the CNS where a combination of demyelination, inflammation, and axonal degeneration results in neurologic disability. However, various studies have also shown that the peripheral nervous system (PNS) can be involved in MS, expanding the consequences of this disorder outside the brain and spinal cord, and providing food for thought to the still unanswered questions about MS origin and treatment. Here, we review the emerging concept of PNS involvement in MS by looking at it from a clinical, molecular, and biochemical point of view. Clinical, pathologic, electrophysiologic, and imaging studies give evidence that the PNS is functionally affected during MS and suggest that the disease might be part of a spectrum of demyelinating disorders instead of being a distinct entity. At the molecular level, similarities between the anatomic structure of the myelin and its interaction with axons in CNS and PNS are evident. In addition, a number of biochemical alterations that affect the myelin during MS can be assumed to be shared between CNS and PNS. Involvement of the PNS as a relevant disease target in MS pathology may have consequences for reaching the diagnosis and for therapeutic approaches of patients with MS. Hence, future MS studies should pay attention to the involvement of the PNS, i.e., its myelin, in MS pathogenesis, which could advance MS research.

Drosophila Voltage-Gated Sodium Channels Are Only Expressed in Active Neurons and Are Localized to Distal Axonal Initial Segment-like Domains

In multipolar vertebrate neurons, action potentials (APs) initiate close to the soma, at the axonal initial segment. Invertebrate neurons are typically unipolar with dendrites integrating directly into the axon. Where APs are initiated in the axons of invertebrate neurons is unclear. Voltage-gated sodium (NaV) channels are a functional hallmark of the axonal initial segment in vertebrates. We used an intronic Minos-Mediated Integration Cassette to determine the endogenous gene expression and subcellular localization of the sole NaV channel in both male and female Drosophila, para Despite being the only NaV channel in the fly, we show that only 23 ± 1% of neurons in the embryonic and larval CNS express para, while in the adult CNS para is broadly expressed. We generated a single-cell transcriptomic atlas of the whole third instar larval brain to identify para expressing neurons and show that it positively correlates with markers of differentiated, actively firing neurons. Therefore, only 23 ± 1% of larval neurons may be capable of firing NaV-dependent APs. We then show that Para is enriched in an axonal segment, distal to the site of dendritic integration into the axon, which we named the distal axonal segment (DAS). The DAS is present in multiple neuron classes in both the third instar larval and adult CNS. Whole cell patch clamp electrophysiological recordings of adult CNS fly neurons are consistent with the interpretation that Nav-dependent APs originate in the DAS. Identification of the distal NaV localization in fly neurons will enable more accurate interpretation of electrophysiological recordings in invertebrates.SIGNIFICANCE STATEMENT The site of action potential (AP) initiation in invertebrates is unknown. We tagged the sole voltage-gated sodium (NaV) channel in the fly, para, and identified that Para is enriched at a distal axonal segment. The distal axonal segment is located distal to where dendrites impinge on axons and is the likely site of AP initiation. Understanding where APs are initiated improves our ability to model neuronal activity and our interpretation of electrophysiological data. Additionally, para is only expressed in 23 ± 1% of third instar larval neurons but is broadly expressed in adults. Single-cell RNA sequencing of the third instar larval brain shows that para expression correlates with the expression of active, differentiated neuronal markers. Therefore, only 23 ± 1% of third instar larval neurons may be able to actively fire NaV-dependent APs.

Schwann-cell-derived CMTM6 restricts radial axonal growth

Myelinating Schwann cells control the diameter of the axons they ensheath by an unknown mechanism. In a recent article in Nature Communications, Eichel and colleagues identify the tetraspan protein CMTM6 as a regulator of axonal caliber.

Mitochondrial dynamics and bioenergetics regulated by netrin-1 in oligodendrocytes

Mitochondria are dynamic organelles that produce energy and molecular precursors that are essential for myelin synthesis. Unlike in neurons, mitochondria in oligodendrocytes increase intracellular movement in response to glutamatergic activation and are more susceptible to oxidative stress than in astrocytes or microglia. The signaling pathways that regulate these cell type-specific mitochondrial responses in oligodendrocytes are not understood. Here, we visualized mitochondria migrating through thin cytoplasmic channels crossing myelin basic protein-positive compacted membranes and localized within paranodal loop cytoplasm. We hypothesized that local extracellular enrichment of netrin-1 might regulate the recruitment and function of paranodal proteins and organelles, including mitochondria. We identified rapid recruitment of mitochondria and paranodal proteins, including neurofascin 155 (NF155) and the netrin receptor deleted in colorectal carcinoma (DCC), to sites of contact between oligodendrocytes and netrin-1-coated microbeads in vitro. We provide evidence that Src-family kinase activation and Rho-associated protein kinase (ROCK) inhibition downstream of netrin-1 induces mitochondrial elongation, hyperpolarization of the mitochondrial inner membrane, and increases glycolysis. Our findings identify a signaling mechanism in oligodendrocytes that is sufficient to locally recruit paranodal proteins and regulate the subcellular localization, morphology, and function of mitochondria.

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 et al. (2020) demonstrate that proteolysis of gliomedin, which drives initial channel clustering, provides a novel mechanism to ensure fidelity of channel localization to nodes.

Delayed short-term tamoxifen treatment does not promote remyelination or neuron sparing after spinal cord injury

The tamoxifen-dependent Cre/lox system in transgenic mice has become an important research tool across all scientific disciplines for manipulating gene expression in specific cell types. In these mouse models, Cre-recombination is not induced until tamoxifen is administered, which allows researchers to have temporal control of genetic modifications. Interestingly, tamoxifen has been identified as a potential therapy for spinal cord injury (SCI) and traumatic brain injury patients due to its neuroprotective properties. It is also reparative in that it stimulates oligodendrocyte differentiation and remyelination after toxin-induced demyelination. However, it is unknown whether tamoxifen is neuroprotective and neuroreparative when administration is delayed after SCI. To properly interpret data from transgenic mice in which tamoxifen treatment is delayed after SCI, it is necessary to identify the effects of tamoxifen alone on anatomical and functional recovery. In this study, female and male mice received a moderate mid-thoracic spinal cord contusion. Mice were then gavaged with corn oil or a high dose of tamoxifen from 19-22 days post-injury, and sacrificed 42 days post-injury. All mice underwent behavioral testing for the duration of the study, which revealed that tamoxifen treatment did not impact hindlimb motor recovery. Similarly, histological analyses revealed that tamoxifen had no effect on white matter sparing, total axon number, axon sprouting, glial reactivity, cell proliferation, oligodendrocyte number, or myelination, but tamoxifen did decrease the number of neurons in the dorsal and ventral horn. Semi-thin sections confirmed that axon demyelination and remyelination were unaffected by tamoxifen. Sex-specific responses to tamoxifen were also assessed, and there were no significant differences between female and male mice. These data suggest that delayed tamoxifen administration after SCI does not change functional recovery or improve tissue sparing in female or male mice.