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New Ways of “Seeing” the Mechanistic Heterogeneity of Multiple Sclerosis Plaque Pathogenesis

Meltzer, Ethan I. MD; Costello, Fiona E. MD; Frohman, Elliot M. MD, PhD, FAAN, FANA; Frohman, Teresa C. MSPS, PA-C, FANA

Journal of Neuro-Ophthalmology: March 2018 - Volume 38 - Issue 1 - p 91–100
doi: 10.1097/WNO.0000000000000633
Disease of the Year: Multiple Sclerosis
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Background: Over the past few decades, we have witnessed a transformation with respect to the principles and pathobiological underpinnings of multiple sclerosis (MS). From the traditional rubric of MS as an inflammatory and demyelinating disorder restricted to central nervous system (CNS) white matter, our contemporary view has evolved to encompass a broader understanding of the variable mechanisms that contribute to tissue injury, in a disorder now recognized to affect white and grey matter compartments.

Evidence Acquisition: A constellation of inflammation, ion channel derangements, bioenergetic supply: demand mismatches within the intra-axonal compartment, and alterations in the dynamics and oximetry of blood flow in CNS tissue compartments are observed in MS. These findings have raised questions regarding how histopathologic heterogeneity may influence the diverse clinical spectrum of MS; and, accordingly, how individual treatment needs vary from 1 patient to the next.

Results: We are now on new scaffolding in MS; one that promises to translate key clinical and laboratory observations to the application of emerging patient-centered therapies.

Conclusions: This review highlights our current knowledge of the underlying disease mechanisms in MS, explores the inflammatory and neurodegenerative consequences of tissue damage, and examines physiologic factors that contribute to bioenergetic homeostasis within the CNS of affected patients.

Department of Neurology (EM), Partner's Neurology Residency Training Program, Massachusetts General and Brigham and Women's Hospitals, Harvard Medical School, Massachusetts; The Department of Neurology (TCF, EMF), The Dell Medical School, The University of Texas, Austin, Texas; and Department of Neurology (FC), University of Calgary, Calgary, Canada.

Address correspondence to Teresa C. Frohman, MPAS, PA-C, FANA, Multiple Sclerosis & Neuroimmunology Center, The Dell Medical School, University of Texas, Austin, TX 78712; E-mail:teresa.frohman@austin.utexas.edu[LINESEPARATOR]

F. Costello has received consultancy fees from Clene and EMD Serono. The remaining authors report no conflicts of interest.

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OVERVIEW: CURRENT CHALLENGES IN MULTIPLE SCLEROSIS

The pathophysiology of multiple sclerosis (MS) has advanced greatly since being first described by Charcot in 1868. Yet, challenges remain with respect to predicting who might be affected, and how to reverse the course of this disease once it has become established. However, the past several decades have seen advances in our understanding of processes that influence the course of the disease both in the early, inflammatory phase; and, later, in the progressive stages of MS (1).

One of the most tantalizing prospects of the future of MS research involves early detection of presymptomatic individuals at high risk of developing the disease. Patients with radiographically isolated syndromes, who have incidental lesions detected on magnetic resonance imaging (MRI) that resemble MS plaques, have a 34% chance of attaining a clinical diagnosis of MS within 5 years (2). In addition, large epidemiologic studies have shown increased health care utilization in the years leading up the first identified clinical demyelinating event (3). The close temporal relationship between these 2 observations would suggest a causative association, and one that bears further investigation, particularly if the preceding changes in health and quality of life contribute to the acquisition of disease-related disability. Given that patients with MS develop disability not just with acute, symptomatic attacks, but also with smoldering subclinical disease, this is an area ripe for exploration. This is especially true, given the controversy surrounding MS therapies as being truly disease modifying, vs not, with respect to disease progression (4). Arguably, we can promote neuroprotective, preventative, performance enhancing, and even restorative properties of tissue preservation and reorganization by more effectively treating MS at its earliest stages. This iconoclastic approach challenges the longstanding belief that once the “ignition switch” has been activated in MS, the pathobiological sequence of events set into motion will become perpetual, characterized by an unrelenting series of pathogenically coupled processes that all terminate on tissue destruction.

Early localization of disease-specific targets may be key to ameliorating the disease process in MS, with focus on both clinical and subclinical components. In reality, a broad burden of tissue injury affecting noneloquent sites throughout the white and grey matter leads to brain atrophy, which over time eclipses the impact of lesions in highly eloquent regions of the central nervous system (CNS) (“The Elegance of Eloquence”) that are clinically apparent. Unfortunately, the diffuse tissue burden that is exacted on the MS patient's brain and spinal cord goes underappreciated for years, potentially because of histopathologic heterogeneity within lesions. Otherwise stated, subclinical MS lesions occur in regions of CNS pathway redundancy, and harbor varied proportions of demyelination, remyelination, and astrogliosis. Consequently, patients may temporarily compensate despite their increasing burden of disease, before reaching a point of precipitous neurological decline.

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GENETIC AND ENVIRONMENTAL FACTORS

Although the landscape of genetic and environmental factors associated with a high predilection for the development of MS has been expanding, none have emerged that constitute a predominant and independent predictor for the future development of the disease. Likewise, a number of protective factors and influences have been demonstrated to exert counterbalancing effects on the risk of disease occurrence. Various genetic variations within the major histocompatibility complex (MHC), which code for immunoregulatory gene products (e.g., such as the HLA-DR2 1501B1 haplotype), confer an elevated risk of developing MS, but not to an extent that warrants routine population screening (5). Although there is an expanding complement of genetic components that augment risk predilection for MS, such factors are insufficient to fully explain disease causation. Even with the “double endowment” of the most strongly linked genetic haplotypes, epigenetic effects (e.g., posttranslational modifications), along with a diversity of categorical factors seem necessary to appreciate the pathobiological basis of susceptibility for developing MS. In particular, we know that environmental influences, nutritional factors, lifestyle choices (e.g., smoking), and infectious exposures, (Epstein Barr virus [EBV]) are capable of eliciting a set of tissue injury pathways in the context of compromised immune regulatory networks (6). Future clinical trials may focus on identifying at risk individuals for purposes of implementing pre-emptive treatment strategies capable of uncoupling mechanisms of causation. This approach would contrast with current practices, which are firmly telescoped on reducing and/or mitigating exacerbations, and decreasing MRI lesion burden, with hopes of slowing the progression of disease.

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PATHOPHYSIOLOGY

Inflammatory Demyelinating Lesions

The typical active MS plaque is believed to be located around a central vein (Fig. 1). The inflammatory response that ensues is first triggered by immune cells interacting with antigen-presenting cells in the perivascular spaces that surround these veins, with conventional histopathology showing varying stages of the inflammatory response. In contrast to later stages in MS dominated by progressive disease, early lesions in MS are characterized by breakdown of the blood-brain barrier (8). In an active lesion, there is an inflammatory infiltrate of macrophages and glial cells (although CD8+ T cells and B cells are also seen) with destruction of myelin. Axon loss is present, and there is reactive gliosis (9). Although the histopathological composition of the MS plaque was first reported on autopsy, modern MRI techniques have now become sufficiently sophisticated to identify the majority of lesions identified within postmortem tissue (10). Moreover, the so-called “normal appearing white matter” in the CNS can be demonstrated to harbor elements of tissue pathology visualized with nonconventional MRI techniques (e.g., diffusion tensor and magnetization transfer imaging), as well as with histopathological confirmation. Systematic biopsy studies have also revealed that histopathological subtypes seem to have discrete pathological features, which may impact tissue damage and treatment response (1).

FIG. 1

FIG. 1

Not all MS lesions are created equally. Some lesions show early remyelination and are called “shadow plaques.” These lesions are protective against further axon loss. Alternatively, some lesions become inactive and consist of a demyelinated core, whereas others show features of chronic, slowly expanding demyelination. The latter can be seen on 7T MRI as an enlarging rim of activated microglia/macrophages containing iron breakdown products (11). It remains unclear why some lesions convert to a more chronic inflammatory process, whereas other lesions are capable to exhibit some degree of histological (and, in turn clinical) recovery. Based on the histological and radiological role of demyelination as a factor contributing to MS disease activity, it comes as no surprise that research efforts have focused on remyelinating agents. Yet, studies have shown mixed results. In a randomized, controlled, double-blind cross-over trial looking at clemastine fumarate (an antihistamine), treated patients with a history of optic neuropathy demonstrated improved P100 latencies on visual evoked potential testing, presumably because of remyelination (12). Yet, an antibody to a leucine-rich repeat and immunoglobulin domain-containing neurite outgrowth inhibitor receptor-interacting protein-1 (LINGO-1), which acts to downregulate oligodendrocyte differentiation, has shown no definitive evidence of improved remyelination (13).

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Axonal Damage

After inflammatory demyelination, axons are at risk for early damage and transection (as many as 11,000 axons transected per mm3) (14–16). It is not known whether axon damage is the direct result of immune cell attack, or merely a byproduct of an inflammatory microenvironment (15). Eventually, axon loss continues in the chronic phase of the disease, possibly because of the loss of myelin and glial support (16). This is supported by evidence from mouse models that show axonal degeneration when myelin lacks 2 membrane proteolipids, although myelin sheath architecture is preserved. This observation would imply that axonal survival is dependent on glial support (17). In addition, the local microenvironment of inflammatory lesions contains many substances that can potentially damage axons, regardless of myelin health, including free radicals, cytokines and chemokines, oxidative products, and free enzymes (18) (Figs. 2 & 3). Proposed mechanisms of axonal damage include elevated levels of nitric oxide, CD8+ T-cell–mediated axonal transection, and glutamate-mediated excitotoxicity. The end result of axonal damage is the clinical hallmark of MS varied symptoms over time and space that are often fluctuating in nature, as it is amplified by Uhthoff's phenomenon (19).

FIG. 2

FIG. 2

FIG. 3

FIG. 3

Arguably, this phenomenon first described by Uhthoff represents a modern paradigm to model the impaired integrity of axonal conduction after demyelinating injury as being affected by both abnormal voltage-gated sodium channels, as well as current leak from potassium efflux from uncovered potassium channels.

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Grey Matter Involvement

There are 4 types of cortical lesions seen in patients with MS including: leukocortical, intracortical, subpial, and cortex spanning types. Leukocortical lesions are at the grey matter and white matter junction. Intracortical lesions are characterized by perivascular demyelination expanding from a cortical vessel. Subpial lesions show demyelination along the outer surface of the gyri. Last, some lesions span the entirety of the cortex (20). In progressive MS, subpial lesions are the most common (8). All cortical lesions are associated more prominently with either primary or secondary progressive stages of MS as opposed to relapsing-remitting MS (21). Furthermore, several studies have shown that cortical demyelination and neurodegeneration are relatively specific to MS. The proposed mechanism to explain these changes relates to local inflammatory infiltrate and oxidative stress, and not simply to white matter damage with a dying back axonopathy culminating in grey matter degeneration (22).

Cortical demyelination is more pronounced in areas associated with meningeal inflammation within the sulci (23), suggesting that at least part of the pathophysiology is related to local flow of cerebrospinal fluid (CSF) and circulating factors in the subarachnoid space. However, it remains unknown whether or not cortical injury is being driven by a diffusing autoantibody or cytokine being produced in the CSF compartment and penetrating into the cortex; or, if cortical injury could be due to secondary tissue destruction from oxidative injury as a byproduct of overlying inflammation (23).

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B Cells

Patients with higher synthesis of CSF IgG oligoclonal bands at onset of disease have been shown to have worse disability and more severe grey matter pathology (24). In some selected populations, there is an increase in peripheral B-cell activity related to cytokine variance, in particular B-cell activating factor. This upregulation is possibly why therapies targeting B-cells are beneficial in MS (25). The benefits of plasma exchange in some patients with MS also underscore the prospects that soluble factor(s) (e.g., antibody, cytokine, chemokine, etc.) may figure prominently in both disease pathogenesis and course perpetuation (e.g., secondary progressive MS).

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Hypoxia and Mitochondrial Dysfunction

Demyelination of axons drastically increases their metabolic needs. For instance, Na+/K+ ATPase pump leak out into the ambient environment in the absence of myelin. Energy is needed to maintain homeostasis in this context (25). Recent studies have shown that demyelinated axons have a large increase in mitochondrial content, and remyelinated axons have a modest increase in mitochondrial content (26). These observations likely reflect compensatory mechanisms aimed to protect against the increased energy needs of the axon. However, mitochondrial content is susceptible to oxidative stress, and patients with progressive MS exhibit mitochondrial DNA deletions, which leads to respiratory-deficient neurons. The end effect is self-propagating injury: not only are axons destroyed in the acute demyelinating attack, but those that remain have a higher metabolic need, constituting a mismatch between supply and demand. Accordingly, therapies that target mitochondrial dysfunction are potential future avenues of treatment (27).

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B Cell Meningeal Follicles

Within the past decade, we have found evidence of a lymphatic system for the CNS, which may play a role in either initiating or perpetuating an inflammation (28). This observation has countered one of the tenets of a traditional dogma that has long held that the brain is immunologically privileged. Many systemic autoimmune diseases, such as Crohn disease, are characterized by ectopic lymphoid follicles. These are produced by the locally activated B-cells undergoing clonal expansion because of antigen presentation from dendritic cells as well as interactions with T-cells. Ectopic germinal follicles are also found in the meninges in MS, and may serve as a reservoir of cells that proliferate an inflammatory response (29). Evidence that these follicular structures are not just the byproduct of a larger inflammatory response is that subpial cortical lesions that are associated with B-cell follicle structures demonstrate a gradient of cell loss in the adjacent underyling cortex. There is greater cell loss in cortical layers I and II, with decreasing cell loss further from the cortical surface (30).

An emerging cardinal hallmark of MS is that of meningeal inflammation and its associated subpial demyelination. The extent of this meningeal inflammation corresponds to a severe MS clinical phenotype, including both primary and secondary progressive disease subtypes (20,31,32). Inflammatory meningeal infiltrates are detected by MRI in 33% of patients with progressive MS compared with historical rates seen on histopathology postmortem of 40% (20). The development of improved brain MRI techniques (high-resolution 3T postcontrast T2 FLAIR sequences with delayed image acquisition, and 7T sequences) now shows these structures in vivo (33). Continued improvement in MRI technology will allow for the detection of more subtle leptomeningeal enhancement on a longitudinal basis, and allow for observation of treatment response.

Proof to date regarding the role of B-cell activity in MS, is the successful application of B-cell depleting therapy in patients with progressive disease (34). There is a germinal center-like reaction and clonal expansion by the intrathecal presence of somatic hypermutation of IgM, which can be found in the CSF. This supports the theory of antigen-driven affinity maturation within the protected compartment of the CNS (35). The production of oligoclonal IgM bands (both lipid-specific and to sperm-associated antigen 16, a protein expressed on astrocytes) is associated with higher disability (36). A self-sustaining inflammatory process contained behind the walls of the blood-brain barrier could also be the cause of the limited success for other systemic therapies in the treatment of progressive MS.

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Neurodegeneration

What tips the balance from the early relapsing-remitting course, to the unrelenting, and more treatment-recalcitrant progressive phase of the disease remains an enigmatic question in MS. Early in the course of MS, a peripheral immune response is likely focal; with chronicity an expanded repertoire of potential targets becomes established (i.e., epitope spreading), resulting in a much more formidable disease state to treat (37). Ongoing inflammatory mechanisms seem to be inextricably linked with evidence of demyelination, neurodegeneration, brain atrophy, and the corresponding set of clinical manifestations that are a derivative of these processes. In fact, patients without ongoing inflammation exhibit rates of neurodegeneration which match those of control subjects (38). These phases are not fully independent. For instance, patients with poor early relapse recovery exhibit a shorter latency for the onset of a predominantly progressive disease course. This observation implicates constitutive disease activity as the principal catalyst for the perpetuation of both the clinical and paraclinical facets of disease progression (39,40). Continuous atrophy, axonal loss, and ultimately neurodegeneration progress, even in the latest epochs of the disease course, corroborating findings from postmortem studies of patients with MS showing chronic plaques with on-going axonal injury (41).

The overarching pathophysiology of chronic axonal loss is based on a chronic energy imbalance between supply and demand. Demyelination causes a shift from saltatory, to membrane conduction of electrical propagation along “lengths” of segmental demyelination. This, in turn, leads to a redistribution of Na+ channels along the length of the axon, in contrast to the normal and strategically localized concentration of Na+ channels being distributed at the nodes of Ranvier and at the paranodal interface (42). In the face of chronic demyelination, there occurs a highly relevant diminution of intra-axonal adenosine triphosphate (ATP). This bioenergetic derangement impacts the necessary reversal of the Na+/Ca2+ channel needed to reestablish normal ionic concentrations, after the insertion of new sodium channels as an adaptation to restore axonal conduction. Although the process of recalibrating ionic concentrations is imperative, it is also very expensive in terms of energy utilization. Furthermore, the pathophysiologic consequences of this biochemical reorganization of the intra-axonal milieu, is also attended by excessive concentration of intra-axonal calcium. The escalation of intracellular levels of Ca2+ induces proteolytic enzymes and impairs mitochondrial function, in conjunction with the liberation of excitatory amino acids such as glutamate. This cascade of events accelerates tissue damage and disorganization, making repair and recovery increasingly more formidable in the CNS (18). In addition, the impaired mitochondria that remain are more susceptible to both ionic and hypoxic insults, creating a “currency” deficit, and an inability to maintain normal homeostasis. This eventually increases the vulnerability of demyelinated axons to undergo deconstruction (e.g., breakdown of intra-axonal microtubules and neurofilaments) and consequent degeneration (43).

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FUTURE DIRECTIONS

The Eye as a Window Into the Brain

The retina represents the front of the brain and can now be interrogated noninvasively with objective methods of assessment. A potential overview of how the mechanistic heterogeneity in MS plaque pathogenesis in the brain can be “seen through the eye” is outlined in Table 1 (44,45). With this approach, the eye represents a window through which we can characterize pathophysiologic signatures that correlate with changes in other discrete functional CNS systems, and with whole brain structure. Accordingly, the interrogation of structure–function relationships within the eye will provide deeper insights into mechanisms of CNS injury and repair, in MS and other degenerative disorders. In the field of MS, there is a great need for better biomarkers that are sensitive to subclinical manifestations of the disease, which ultimately govern disability. Interrogating the visual system with multiparametric modeling techniques that couple the most robust assessments of retinal architecture with physiological processing metrics will provide objective, valid, and reproducible means to elucidate pathophysiologic signatures of brain dysfunction in MS. Ultimately, exploring the link between eye and brain will enhance our understanding of subtle aspects of disease progression, and ultimately refine our ability to test the efficacy of new treatments in patients with MS.

TABLE 1

TABLE 1

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