Changes at the nodal and perinodal axonal domains: a basis for multiple sclerosis pathology?

That's a Wrap! Molecular Drivers Governing Neuronal Nogo Receptor-Dependent Myelin Plasticity and Integrity

Myelin is a dynamic membrane that is important for coordinating the fast propagation of action potentials along small or large caliber axons (0.1-10 μm) some of which extend the entire length of the spinal cord. Due to the heterogeneity of electrical and energy demands of the variable neuronal populations, the axo-myelinic and axo-glial interactions that regulate the biophysical properties of myelinated axons also vary in terms of molecular interactions at the membrane interfaces. An important topic of debate in neuroscience is how myelin is maintained and modified under neuronal control and how disruption of this control (due to disease or injury) can initiate and/or propagate neurodegeneration. One of the key molecular signaling cascades that have been investigated in the context of neural injury over the past two decades involves the myelin-associated inhibitory factors (MAIFs) that interact with Nogo receptor 1 (NgR1). Chief among the MAIF superfamily of molecules is a reticulon family protein, Nogo-A, that is established as a potent inhibitor of neurite sprouting and axon regeneration. However, an understated role for NgR1 is its ability to control axo-myelin interactions and Nogo-A specific ligand binding. These interactions may occur at axo-dendritic and axo-glial synapses regulating their functional and dynamic membrane domains. The current review provides a comprehensive analysis of how neuronal NgR1 can regulate myelin thickness and plasticity under normal and disease conditions. Specifically, we discuss how NgR1 plays an important role in regulating paranodal and juxtaparanodal domains through specific signal transduction cascades that are important for microdomain molecular architecture and action potential propagation. Potential therapeutics designed to target NgR1-dependent signaling during disease are being developed in animal models since interference with the involvement of the receptor may facilitate neurological recovery. Hence, the regulatory role played by NgR1 in the axo-myelinic interface is an important research field of clinical significance that requires comprehensive investigation.

Brain atrophy in multiple sclerosis: mechanisms, clinical relevance and treatment options

Multiple sclerosis (MS) is an immune-mediated disease of the central nervous system characterized by focal or diffuse inflammation, demyelination, axonal loss and neurodegeneration. Brain atrophy can be seen in the earliest stages of MS, progresses faster compared to healthy adults, and is a reliable predictor of future physical and cognitive disability. In addition, it is widely accepted to be a valid, sensitive and reproducible measure of neurodegeneration in MS. Reducing the rate of brain atrophy has only recently been incorporated as a critical endpoint into the clinical trials of new or emerging disease modifying drugs (DMDs) in MS. With the advent of easily accessible neuroimaging softwares along with the accumulating evidence, clinicians may be able to use brain atrophy measures in their everyday clinical practice to monitor disease course and response to DMDs. In this review, we will describe the different mechanisms contributing to brain atrophy, their clinical relevance on disease presentation and course and the effect of current or emergent DMDs on brain atrophy and neuroprotection.

Simultaneous Ablation of Neuronal Neurofascin and Ankyrin G in Young and Adult Mice Reveals Age-Dependent Increase in Nodal Stability in Myelinated Axons and Differential Effects on the Lifespan

Nodes of Ranvier are unique regions where voltage-gated sodium channels are highly enriched to drive saltatory conduction. Genetic ablations in adult mice with loss of specific nodal proteins causes slow but progressive nodal deterioration associated with decreased nerve conduction and axonopathy. What has remained unaddressed is whether loss of nodal proteins at different time points in postnatal life follows similar timelines of nodal disorganization. Here we utilized simultaneous ablation of Neurofascin (NF186) and Ankyrin G (AnkG) in mice of both sexes at three specific time points. We report that concurrent ablation of these core nodal components at postnatal day 13 (P13) leads to accelerated nodal destabilization in comparison with P23, and this disorganization is even slower when ablated at P93. Ablation of NF186 with AnkG at P13 reduced the half-life of NF186 to 15 days compared to 1 month at P23, which increased to 2 months at P93, indicating increasing nodal stability. The half-life of AnkG at the nodes also increased with age but showed enhanced disappearance from the node in the absence of NF186, with a half-life of 3 days at P13 ablation. The nodal disorganization occurred in a sequential manner, with AnkG disappearing first from the nodal areas irrespective of the timing of ablation, and led to decreased nerve conduction and affected axonal health. Together, our studies reveal that nodes of Ranvier in myelinated axons continue to become more stable with age and suggest that nodal disorganization in adult human demyelinating disorders occurs slowly until neurological symptoms become evident.

Postnatal Loss of Neuronal and Glial Neurofascins Differentially Affects Node of Ranvier Maintenance and Myelinated Axon Function

Intricate molecular interactions between neurons and glial cells underlie the creation of unique domains that are essential for saltatory conduction of action potentials by myelinated axons. Previously, the cell surface adhesion molecule Neurofascin (Nfasc) has been shown to have a dual-role in the establishment of axonal domains from both the glial and neuronal interface. While the neuron-specific isoform of Neurofascin (NF186) is indispensable for clustering of voltage-gated sodium channels at nodes of Ranvier; the glial-specific isoform of Neurofascin (NF155) is required for myelinating glial cells to organize the paranodal domain. Although many studies have addressed the individual roles of NF155 and NF186 in assembling paranodes and nodes, respectively; critical questions about their roles in the maintenance and long-term health of the myelinated axons remain, which we aimed to address in these studies. Here using spatiotemporal ablation of Neurofascin in neurons alone or together with myelinating glia, we report that loss of NF186 individually from postnatal mice leads to progressive nodal destabilization and axonal degeneration. While individual ablation of paranodal NF155 does not disrupt nodes of Ranvier; loss of NF186 combined with NF155 causes more accelerated nodal destabilization than loss of NF186 alone, providing strong evidence regarding a supporting role for paranodes in nodal maintenance. In both cases of NF186 loss, myelinating axons show ultrastructural changes and degeneration. Our studies reveal that long-term maintenance of nodes and ultimately the health of axons is correlated with the stability of NF186 within the nodal complex and the presence of auxiliary paranodes.

Compromised axon initial segment integrity in EAE is preceded by microglial reactivity and contact

Axonal pathology is a key contributor to long-term disability in multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS), but the mechanisms that underlie axonal pathology in MS remain elusive. Evidence suggests that axonal pathology is a direct consequence of demyelination, as we and others have shown that the node of Ranvier disassembles following loss of myelin. In contrast to the node of Ranvier, we now show that the axon initial segment (AIS), the axonal domain responsible for action potential initiation, remains intact following cuprizone-induced cortical demyelination. Instead, we find that the AIS is disrupted in the neocortex of mice that develop experimental autoimmune encephalomyelitis (EAE) independent of local demyelination. EAE-induced mice demonstrate profound compromise of AIS integrity with a progressive disruption that corresponds to EAE clinical disease severity and duration, in addition to cortical microglial reactivity. Furthermore, treatment with the drug didox results in attenuation of AIS pathology concomitantly with microglial reversion to a less reactive state. Together, our findings suggest that inflammation, but not demyelination, disrupts AIS integrity and that therapeutic intervention may protect and reverse this pathology. GLIA 2016;64:1190-1209.

Mechanisms of sodium channel clustering and its influence on axonal impulse conduction

The efficient propagation of action potentials along nervous fibers is necessary for animals to interact with the environment with timeliness and precision. Myelination of axons is an essential step to ensure fast action potential propagation by saltatory conduction, a process that requires highly concentrated voltage-gated sodium channels at the nodes of Ranvier. Recent studies suggest that the clustering of sodium channels can influence axonal impulse conduction in both myelinated and unmyelinated fibers, which could have major implications in disease, particularly demyelinating pathology. This comprehensive review summarizes the mechanisms governing the clustering of sodium channels at the peripheral and central nervous system nodes and the specific roles of their clustering in influencing action potential conduction. We further highlight the classical biophysical parameters implicated in conduction timing, followed by a detailed discussion on how sodium channel clustering along unmyelinated axons can impact axonal impulse conduction in both physiological and pathological contexts.

Axonal degeneration in multiple sclerosis: defining therapeutic targets by identifying the causes of pathology

Current therapeutics in multiple sclerosis (MS) target the putative inflammation and immune attack on CNS myelin. Despite their effectiveness in blunting the relapse rate in MS patients, such therapeutics do not prevent MS disease progression. Importantly, specific clinical dilemma arises through inability to predict MS progression and thereby therapeutically target axonal injury during MS, limiting permanent disability. The current review identifies immune and neurobiological principles that govern the sequelae of axonal degeneration during MS disease progression. Defining the specific disease arbiters, inflammatory and autoimmune, oligodendrocyte dystrophy and degenerative myelin, we discuss a basis for a molecular mechanism in axons that may be targeted therapeutically, in spatial and temporal manner to limit axonal degeneration and thereby halt progression of MS.

Progressive disorganization of paranodal junctions and compact myelin due to loss of DCC expression by oligodendrocytes

Paranodal axoglial junctions are critical for maintaining the segregation of axonal domains along myelinated axons; however, the proteins required to organize and maintain this structure are not fully understood. Netrin-1 and its receptor Deleted in Colorectal Cancer (DCC) are proteins enriched at paranodes that are expressed by neurons and oligodendrocytes. To identify the specific function of DCC expressed by oligodendrocytes in vivo, we selectively eliminated DCC from mature myelinating oligodendrocytes using an inducible cre regulated by the proteolipid protein promoter. We demonstrate that DCC deletion results in progressive disruption of the organization of axonal domains, myelin ultrastructure, and myelin protein composition. Conditional DCC knock-out mice develop balance and coordination deficits and exhibit decreased conduction velocity. We conclude that DCC expression by oligodendrocytes is required for the maintenance and stability of myelin in vivo, which is essential for proper signal conduction in the CNS.

Differential roles of microglia and monocytes in the inflamed central nervous system

Phagocytic monocyte-derived macrophages associate with the nodes of Ranvier and initiate demyelination while microglia clear debris and display a suppressed metabolic gene signature in EAE.

In the human disorder multiple sclerosis (MS) and in the model experimental autoimmune encephalomyelitis (EAE), macrophages predominate in demyelinated areas and their numbers correlate to tissue damage. Macrophages may be derived from infiltrating monocytes or resident microglia, yet are indistinguishable by light microscopy and surface phenotype. It is axiomatic that T cell–mediated macrophage activation is critical for inflammatory demyelination in EAE, yet the precise details by which tissue injury takes place remain poorly understood. In the present study, we addressed the cellular basis of autoimmune demyelination by discriminating microglial versus monocyte origins of effector macrophages. Using serial block-face scanning electron microscopy (SBF-SEM), we show that monocyte-derived macrophages associate with nodes of Ranvier and initiate demyelination, whereas microglia appear to clear debris. Gene expression profiles confirm that monocyte-derived macrophages are highly phagocytic and inflammatory, whereas those arising from microglia demonstrate an unexpected signature of globally suppressed cellular metabolism at disease onset. Distinguishing tissue-resident macrophages from infiltrating monocytes will point toward new strategies to treat disease and promote repair in diverse inflammatory pathologies in varied organs.

Demyelination in multiple sclerosis

This review, focused on demyelination in multiple sclerosis, is divided in two parts. The first part addresses the many and not exclusive mechanisms leading to demyelination in the central nervous system. Although the hypothesis that a primary oligodendrocyte or myelin injury induces a secondary immune response in the central nervous system is still a matter of debate, most recent advances underline the influence of a primary immune response against myelin antigen(s), with a diversity of potential targets. Whereas multiple sclerosis was long considered as a T cell-mediated disease, the role of B lymphocytes is now increasingly recognized, and the influence of antibodies on tissue damage actively investigated. The second part of the review describes the axonal consequences of demyelination. Segmental demyelination results in conduction block or slowing of conduction through adaptative responses, notably related to modifications in the distribution of voltage gated sodium channels along the denuded axon. If demyelination persists, these changes, as well as the loss of trophic and metabolic support, will lead to irreversible axonal damage and loss. In this respect, favouring early myelin repair, during a window of time when axonal damage is still reversible, might pave the way for neuroprotection.

Node of Ranvier disruption as a cause of neurological diseases

Dysfunction and/or disruption of nodes of Ranvier are now recognized as key contributors to the pathophysiology of various neurological diseases. One reason is that the excitable nodal axolemma contains a high density of Nav (voltage-gated Na⁺ channels) that are required for the rapid and efficient saltatory conduction of action potentials. Nodal physiology is disturbed by altered function, localization, and expression of voltage-gated ion channels clustered at nodes and juxtaparanodes, and by disrupted axon–glial interactions at paranodes. This paper reviews recent discoveries in molecular/cellular neuroscience, genetics, immunology, and neurology that highlight the critical roles of nodes of Ranvier in health and disease.

Physiopathology of symptoms and signs in multiple sclerosis

The physiopathology of symptoms and signs in multiple sclerosis (MS) is a less divulged topic albeit its importance in the patients' management.

OBJECTIVE

It was to summarize the main biophysical and biochemical mechanisms which produce the clinical manifestations in MS.

RESULTS

The mechanisms underpinning neurological deficits are described in the relapsing and in the progressive phases, stressing inflammatory and neurodegenerative components, especially demyelination, axonal damage and conduction impairment. Transient worsening based in Uhthoff's phenomenon, mechanisms producing positive symptoms, as paraesthesias and Lhermitte sign due to axonal hyperexcitability and ephaptic interactions, and development of cortical symptoms will also be addressed. The variety of processes leading to neural repair and functional recovery in the remitting phase is focused, as remyelination and adaptive changes due to neural plasticity.

CONCLUSION

The awareness of mechanisms producing symptoms in MS emphasises the role of symptomatic and rehabilitation therapies in the improvement of patients' well-being.