Differential modulation of the juxtaparanodal complex in Multiple Sclerosis

Pathological ultrastructural alterations of myelinated axons in normal appearing white matter in progressive multiple sclerosis

Multiple sclerosis (MS) pathophysiology includes inflammation, demyelination and neurodegeneration, but the exact mechanisms of disease initiation and progression are unknown. A major feature of lesions is lack of myelin, which increases axonal energy demand and requires adaptation in number and size of mitochondria. Outside lesions, subtle and diffuse alterations are observed in normal appearing white matter (NAWM) and normal appearing grey matter (NAGM), including increased oxidative stress, reduced axon density and changes in myelin composition and morphology. On an ultrastructural level, only limited data is available on alterations in myelinated axons. We generated large scale 2D scanning transmission electron microscopy images ('nanotomy') of non-demyelinated brain tissue of control and progressive MS donors, accessible via an open-access online repository. We observed a reduced density of myelinated axons in NAWM, without a decrease in cross-sectional axon area. Small myelinated axons were less frequently and large myelinated axons were more frequently present in NAWM, while the g-ratio was similar. The correlation between axonal mitochondrial radius and g-ratio was lost in NAWM, but not in NAGM. Myelinated axons in control GM and NAGM had a similar g-ratio and radius distribution. We hypothesize that axonal loss in NAWM is likely compensated by swelling of the remaining myelinated axons and subsequent adjustment of myelin thickness to maintain their g-ratio. Failure of axonal mitochondria to adjust their size and fine-tuning of myelin thickness may render NAWM axons and their myelin more susceptible to injury.

The Node of Ranvier as an Interface for Axo-Glial Interactions: Perturbation of Axo-Glial Interactions in Various Neurological Disorders

The action potential conduction along the axon is highly dependent on the healthy interactions between the axon and myelin-producing glial cells. Myelin, which facilitates action potential, is the protective insulation around the axon formed by Schwann cells and oligodendrocytes in the peripheral (PNS) and central nervous system (CNS), respectively. Myelin is a continuous structure with intermittent gaps called nodes of Ranvier, which are the sites enriched with ion channels, transmembrane, scaffolding, and cytoskeletal proteins. Decades-long extensive research has identified a comprehensive proteome with strictly regularized localization at the node of Ranvier. Concurrently, axon-glia interactions at the node of Ranvier have gathered significant attention as the pathophysiological targets for various neurodegenerative disorders. Numerous studies have shown the alterations in the axon-glia interactions culminating in neurological diseases. In this review, we have provided an update on the molecular composition of the node of Ranvier. Further, we have discussed in detail the consequences of disruption of axon-glia interactions during the pathogenesis of various CNS and PNS disorders.

Pathology of Initial Axon Segments in Chronic Inflammatory Demyelinating Polyradiculoneuropathy and Related Disorders

The diagnosis of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is based on a combination of clinical, electrodiagnostic and laboratory features. The different entities of the disease include chronic immune sensory polyradiculopathy (CISP) and autoimmune nodopathies. It is debatable whether CIDP occurring in the course of other conditions, i.e., monoclonal IgG or IgA gammopathy, should be treated as a separate disease entity from idiopathic CIDP. This study aims to evaluate the molecular differences of the nodes of Ranvier and the initial axon segment (AIS) and juxtaparanode region (JXP) as the potential cause of phenotypic variation of CIDP while also seeking new pathomechanisms since JXP is sequestered behind the paranode and autoantibodies may not access the site easily. The authors initially present the structure of the different parts of the neuron and its functional significance, then discuss the problem of whether damage to the juxtaparanodal region, Schwann cells and axons could cause CIDP or if these damages should be separated as separate disease entities. In particular, AIS's importance for modulating neural excitability and carrying out transport along the axon is highlighted. The disclosure of specific pathomechanisms, including novel target antigens, in the heterogeneous CIDP syndrome is important for diagnosing and treating these patients.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

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.

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.

Neuroinflammation in the normal-appearing white matter (NAWM) of the multiple sclerosis brain causes abnormalities at the nodes of Ranvier

Changes to the structure of nodes of Ranvier in the normal-appearing white matter (NAWM) of multiple sclerosis (MS) brains are associated with chronic inflammation. We show that the paranodal domains in MS NAWM are longer on average than control, with Kv1.2 channels dislocated into the paranode. These pathological features are reproduced in a model of chronic meningeal inflammation generated by the injection of lentiviral vectors for the lymphotoxin-α (LTα) and interferon-γ (IFNγ) genes. We show that tumour necrosis factor (TNF), IFNγ, and glutamate can provoke paranodal elongation in cerebellar slice cultures, which could be reversed by an N-methyl-D-aspartate (NMDA) receptor blocker. When these changes were inserted into a computational model to simulate axonal conduction, a rapid decrease in velocity was observed, reaching conduction failure in small diameter axons. We suggest that glial cells activated by pro-inflammatory cytokines can produce high levels of glutamate, which triggers paranodal pathology, contributing to axonal damage and conduction deficits.

Current thinking on the mechanisms by which multiple sclerosis gives rise to cumulative neurological disability revolves largely around focal lesions of inflammation and demyelination. However, some of the debilitating symptoms, such as severe fatigue, might be better explained by a more diffuse pathology. This study shows that paranodes in the white matter become abnormal as a result of neuroinflammation, which may be the result of the action of cytokines that cause glia to release glutamate.

The function of contactin-2/TAG-1 in oligodendrocytes in health and demyelinating pathology

The oligodendrocyte maturation process and the transition from the pre-myelinating to the myelinating state are extremely important during development and in pathology. In the present study, we have investigated the role of the cell adhesion molecule CNTN2/TAG-1 on oligodendrocyte proliferation, differentiation, myelination, and function during development and under pathological conditions. With the combination of in vivo, in vitro, ultrastructural, and electrophysiological methods, we have mapped the expression of CNTN2 protein in the oligodendrocyte lineage during the different stages of myelination and its involvement on oligodendrocyte maturation, branching, myelin-gene expression, myelination, and axonal function. The cuprizone model of central nervous system demyelination was further used to assess CNTN2 in pathology. During development, CNTN2 can transiently affect the expression levels of myelin and myelin-regulating genes, while its absence results in reduced oligodendrocyte branching, hypomyelination of fiber tracts and impaired axonal conduction. In pathology, CNTN2 absence does not affect the extent of de- and remyelination. However during remyelination, a novel, CNTN2-independent mechanism is revealed that is able to recluster voltage gated potassium channels (VGKCs) resulting in the improvement of fiber conduction.

Remyelinating Pharmacotherapies in Multiple Sclerosis

We have witnessed major successes in the development of effective immunomodulatory therapies capable of reducing adaptive immune-mediated myelin damage in MS over the last 30 years. However, until it is possible to prevent MS or initiate treatment before it has already caused lesions there is a need to repair myelin damage to prevent further axonal loss. The past decade has brought remarkable advances in our understanding of oligodendrocyte biology and the related search for remyelinating therapies in humans. In this review, we first outline the basic biology of central nervous system myelin and remyelination, including a discussion of the major identified pathways and targets that might help yield CNS remyelinating drugs. In conjunction, we provide an overview of techniques that have helped identify compounds capable of promoting oligodendrocyte precursor cell differentiation and myelination. This includes the methods for both initial in vitro screening and subsequent in vivo confirmation of the target. We then review methods proposed to quantify human remyelination in vivo, including visual evoked potentials and putative imaging modalities. As the remyelination era approaches, with the announcement of the first positive trial in remyelination, we are now tasked with answering new questions regarding patient-specific factors (e.g., age) that may influence the extent and optimal therapeutic window for remyelination.

Formation and disruption of functional domains in myelinated CNS axons

Communication in the central nervous system (CNS) occurs through initiation and propagation of action potentials at excitable domains along axons. Action potentials generated at the axon initial segment (AIS) are regenerated at nodes of Ranvier through the process of saltatory conduction. Proper formation and maintenance of the molecular structure at the AIS and nodes are required for sustaining conduction fidelity. In myelinated CNS axons, paranodal junctions between the axolemma and myelinating oligodendrocytes delineate nodes of Ranvier and regulate the distribution and localization of specialized functional elements, such as voltage-gated sodium channels and mitochondria. Disruption of excitable domains and altered distribution of functional elements in CNS axons is associated with demyelinating diseases such as multiple sclerosis, and is likely a mechanism common to other neurological disorders. This review will provide a brief overview of the molecular structure of the AIS and nodes of Ranvier, as well as the distribution of mitochondria in myelinated axons. In addition, this review highlights important structural and functional changes within myelinated CNS axons that are associated with neurological dysfunction.

Structural Characterization of the Extracellular Domain of CASPR2 and Insights into Its Association with the Novel Ligand Contactin1

Contactin-associated protein-like 2 (CNTNAP2) encodes for CASPR2, a multidomain single transmembrane protein belonging to the neurexin superfamily that has been implicated in a broad range of human phenotypes including autism and language impairment. Using a combination of biophysical techniques, including small angle x-ray scattering, single particle electron microscopy, analytical ultracentrifugation, and bio-layer interferometry, we present novel structural and functional data that relate the architecture of the extracellular domain of CASPR2 to a previously unknown ligand, Contactin1 (CNTN1). Structurally, CASPR2 is highly glycosylated and has an overall compact architecture. Functionally, we show that CASPR2 associates with micromolar affinity with CNTN1 but, under the same conditions, it does not interact with any of the other members of the contactin family. Moreover, by using dissociated hippocampal neurons we show that microbeads loaded with CASPR2, but not with a deletion mutant, co-localize with transfected CNTN1, suggesting that CNTN1 is an endogenous ligand for CASPR2. These data provide novel insights into the structure and function of CASPR2, suggesting a complex role of CASPR2 in the nervous system.