Chronic Multiple Sclerosis Lesions Research Articles

Susceptibility-sensitive MRI of multiple sclerosis lesions and the impact of normal-appearing white matter changes.

Susceptibility-sensitive magnetic resonance imaging (MRI) has gained importance in multiple sclerosis (MS) research because of its versatility, high resolution and excellent sensitivity to changes in tissue structure and composition. In particular, mapping of the resonance frequency of the MR signal and quantitative susceptibility mapping (QSM) have been explored for the description of MS lesions. Many current studies utilizing these techniques attribute increases in the MR frequency or QSM to elevated tissue iron content, in addition to myelin loss. However, this common interpretation is inconsistent with recent histopathological studies. Here, we investigate the nature of MR frequency shifts related to MS lesions by comparing post-mortem MRI data with histology, and contrast them with numerical simulations of the MR signal. We demonstrate that iron accumulation is not the driving source of the MR frequency or QSM image contrast in our sample; rather, most chronic MS lesions are characterized by advanced loss of both myelin and iron. Moreover, our results suggest that the appearance of MS lesions on MR frequency maps and QSM depends on changes in the non-lesional white matter surrounding the lesions. Understanding and accounting for these changes is essential for the quantitative interpretation of MR frequency or QSM data in white matter.

Staining of HLA-DR, Iba1 and CD68 in human microglia reveals partially overlapping expression depending on cellular morphology and pathology

HLA-DR, Iba1 and CD68 are widely used microglia markers in human tissue. However, due to differences in gene regulation, they may identify different activation stages of microglia. Here, we directly compared the expression of HLA-DR, Iba1 and CD68 in microglia with different phenotypes, ranging from ramified to amoeboid, to foamy phagocytizing macrophages, in adjacent sections immunocytochemically double stained for two of the markers. Material was used from patients diagnosed with multiple sclerosis (MS) and Alzheimer's disease (AD) patients and control subjects because together they contain all the microglia activation stages in an acute and a chronic inflammatory setting. We found a similar, yet not identical, overall expression pattern. All three markers were expressed by ramified/amoeboid microglia around chronic active MS lesions, but overlap between HLA-DR and Iba1 was limited. Foamy macrophages in the demyelinating rims of active MS lesions of MS expressed more HLA-DR and CD68 than Iba1. All markers were expressed by small microglia accumulations (nodules) in MS NAWM. Dense core AD plaques in the hippocampus were mostly associated with microglia expressing HLA-DR. Diffuse AD plaques were not specifically associated with microglia at all. These results indicate that microglia markers have different potential for neuropathological analysis, with HLA-DR and CD68 reflecting immune activation and response to tissue damage, and Iba1 providing a marker more suited for structural studies in the absence of pathology.

Pathogenic implications of distinct patterns of iron and zinc in chronic MS lesions

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS) in which oligodendrocytes, the CNS cells that stain most robustly for iron and myelin are the targets of injury. Metals are essential for normal CNS functioning, and metal imbalances have been linked to demyelination and neurodegeneration. Using a multidisciplinary approach involving synchrotron techniques, iron histochemistry and immunohistochemistry, we compared the distribution and quantification of iron and zinc in MS lesions to the surrounding normal appearing and periplaque white matter, and assessed the involvement of these metals in MS lesion pathogenesis. We found that the distribution of iron and zinc is heterogeneous in MS plaques, and with few remarkable exceptions they do not accumulate in chronic MS lesions. We show that brain iron tends to decrease with increasing age and disease duration of MS patients; reactive astrocytes organized in large astrogliotic areas in a subset of smoldering and inactive plaques accumulate iron and safely store it in ferritin; a subset of smoldering lesions do not contain a rim of iron-loaded macrophages/microglia; and the iron content of shadow plaques varies with the stage of remyelination. Zinc in MS lesions was generally decreased, paralleling myelin loss. Iron accumulates concentrically in a subset of chronic inactive lesions suggesting that not all iron rims around MS lesions equate with smoldering plaques. Upon degeneration of iron-loaded microglia/macrophages, astrocytes may form an additional protective barrier that may prevent iron-induced oxidative damage.

Slowly eroding lesions in multiple sclerosis

Background: At autopsy, 20%–40% of chronic multiple sclerosis (MS) lesions are labeled “slowly expanding” and feature myelin phagocytosis at the lesion edge. As pathological lesion classification relies on a single, terminal time point, the rate of lesion expansion cannot be directly measured. Objective: To study long-term volume changes in individual MS lesions. Methods: Volumes of individual lesions on proton density magnetic resonance imaging (MRI) acquired between 1992 and 2015 were measured in 22 individuals (one lesion per person). After correction for acquisition protocol, a mixed model evaluated lesion volume changes. Results: The mean (standard deviation) lesion volume at baseline was 142 (82) mL, falling to 74 (51) mL after 16 (3) years. All lesions shrank over time. Change in lesion volume did not correlate with change in supratentorial brain volume (p = 0.33). In simulations, the results could be explained by a process of slow radial expansion superimposed on substantially more rapid resorption of damaged tissue. Conclusion: We noted sustained radiological contraction of MS lesions, a surprising result given that fresh myelin breakdown products within chronic active lesions are observed relatively frequently at autopsy. Therefore, the primary pathological process in chronic lesions, even those described as “slowly expanding,” is likely to be tissue loss.

Expression of IL-1β in rhesus EAE and MS lesions is mainly induced in the CNS itself.

BackgroundInterleukin (IL)-1β is a pro-inflammatory cytokine that plays a role in the pathogenesis of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), the animal model for MS. Yet, detailed studies on IL-1β expression in different stages of MS lesion development and a comparison of IL-1β expression in MS and EAE are lacking.MethodsHere, we performed an extensive characterization of IL-1β expression in brain tissue of MS patients, which included different MS lesion types, and in brain tissue of rhesus macaques with EAE.ResultsIn rhesus EAE brain tissue, we observed prominent IL-1β staining in MHC class II+ cells within perivascular infiltrates and at the edges of large demyelinating lesions. Surprisingly, staining was localized to resident microglia or differentiated macrophages rather than to infiltrating monocytes, suggesting that IL-1β expression is induced within the central nervous system (CNS). By contrast, IL-1β staining in MS brain tissue was much less pronounced. Staining was found in the parenchyma of active and chronic active MS lesions and in nodules of MHC class II+ microglia in otherwise normal appearing white matter. IL-1β expression was detected in a minority of the nodules only, which could not be distinguished by the expression of pro- and anti-inflammatory markers. These nodules were exclusively found in MS, and it remains to be determined whether IL-1β+ nodules are destined to progress into active lesions or whether they merely reflect a transient response to cellular stress.ConclusionsAlthough the exact localization and relative intensity of IL-1β expression in EAE and MS is different, the staining pattern in both neuroinflammatory disorders is most consistent with the idea that the expression of IL-1β during lesion development is induced in the tissue rather than in the periphery.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0605-8) contains supplementary material, which is available to authorized users.

The physiology of blood platelets and changes of their biological activities in multiple sclerosis.

Increasing evidence indicates that blood platelets contribute to diverse processes that extend beyond hemostasis. Many of the same mechanisms that play a role in hemostasis and thrombosis facilitate platelets the participation in other physiological and pathological processes, particularly in the inflammation, the immune response and central nervous system disorders. Platelets are involved in pathophysiology of central nervous system diseases, especially in the pathogenesis of multiple sclerosis, but their role appears to be neglected. Platelets contribute to the inflammation and cooperate with immune cells in inflammatory and immune responses. These blood cells were identified in inflamed spinal cord and in the brain in chronic active lesions of multiple sclerosis and in the related animal models referred as Experimental Autoimmune Encephalomyelitis. This review summarizes recent insights in the platelet activation accompanied by the exocytosis of bioactive compounds stored in granules, formation of platelet microparticles, expression of specific membrane receptors, synthesis of numerous biomediators, generation of free radicals, and introduces the mechanisms by which activated platelets may be involved in the pathophysiology of multiple sclerosis. Understanding the role of platelets in multiple sclerosis may be essential for improved therapies.

Small heat shock proteins are induced during multiple sclerosis lesion development in white but not grey matter.

IntroductionThe important protective role of small heat-shock proteins (HSPs) in regulating cellular survival and migration, counteracting protein aggregation, preventing apoptosis, and regulating inflammation in the central nervous system is now well-recognized. Yet, their role in the neuroinflammatory disorder multiple sclerosis (MS) is largely undocumented. With the exception of alpha B-crystallin (HSPB5), little is known about the roles of small HSPs in disease. ResultsHere, we examined the expression of four small HSPs during lesion development in MS, focussing on their cellular distribution, and regional differences between white matter (WM) and grey matter (GM). It is well known that MS lesions in these areas differ markedly in their pathology, with substantially more intense blood-brain barrier damage, leukocyte infiltration and microglial activation typifying WM but not GM lesions.We analysed transcript levels and protein distribution profiles for HSPB1, HSPB6, HSPB8 and HSPB11 in MS lesions at different stages, comparing them with normal-appearing brain tissue from MS patients and non-neurological controls. During active stages of demyelination in WM, and especially the centre of chronic active MS lesions, we found significantly increased expression of HSPB1, HSPB6 and HSPB8, but not HSPB11. When induced, small HSPs were exclusively found in astrocytes but not in oligodendrocytes, microglia or neurons. Surprisingly, while the numbers of astrocytes displaying high expression of small HSPs were markedly increased in actively demyelinating lesions in WM, no such induction was observed in GM lesions. This difference was particularly obvious in leukocortical lesions covering both WM and GM areas.ConclusionsSince induction of small HSPs in astrocytes is apparently a secondary response to damage, their differential expression between WM and GM likely reflects differences in mediators that accompany demyelination in either WM or GM during MS. Our findings also suggest that during MS, cortical structures fail to benefit from the protective actions of small HSPs.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-015-0267-2) contains supplementary material, which is available to authorized users.

Heterogeneity of acute multiple sclerosis lesions on sodium (23Na) MRI

Background: Advanced magnetic resonance imaging (MRI) techniques provide a window into pathological processes in multiple sclerosis (MS). Nevertheless, to date only few studies have performed sodium MRI in MS. Objectives: We analysed total sodium concentration (TSC) in hyperacute, acute and chronic lesions in MS with 23Na MRI. Methods: 23Na MRI and 1H MRI were performed in 65 MS patients and 10 healthy controls (HC). Mean TSC was quantified in all MS lesions with a diameter of >5 mm and in the normal appearing white and grey matter (NAWM, NAGM). Results: TSC in the NAWM and the NAGM of MS patients was significantly higher compared to HC (WM: 37.51 ± 2.65 mM versus 35.17 ± 3.40 mM; GM: 43.64 ± 2.75 mM versus 40.09 ± 4.64 mM). Acute and chronic MS lesions showed elevated TSC levels of different extent (contrast-enhancing lesions (49.07 ± 6.99 mM), T1 hypointense lesions (45.06 ± 6.26 mM) and remaining T1 isointense lesions (39.88 ± 5.54 mM)). However, non-enhancing hyperacute lesions with a reduced apparent diffusion coefficient showed a TSC comparable to the NAWM (37.22 ± 4.62 mM). Conclusions: TSC is not only a sensitive marker of the severity of chronic tissue abnormalities in MS but is also highly sensitive to opening of the blood–brain barrier and vasogenic tissue oedema in contrast-enhancing lesions.

Chronic T2 Lesions in Multiple Sclerosis are Heterogeneous Regarding Phase MR Imaging.

Phase imaging provides additional information on multiple sclerosis (MS) lesions and may in combination with mean diffusivity (MD) and magnetization transfer ratio (MTR) help differentiating heterogeneity of MS lesion pathology. Magnetic resonance imaging (MRI) was performed in 23 MS patients including diffusion tensor imaging (DTI), magnetization transfer imaging (MTI), and SWI. Mean values (MTR, MD, and homodyne filtered phase) from 138 chronic MS lesions and normal appearing white matter (NAWM) were obtained and correlations examined. For explorative analysis, a divisive hierarchical clustering algorithm was applied. Phase characteristics were an independent characteristic of chronic T2 lesions, as MTR and MD were not correlated with phase values (R = - 0.23, R = - 0.18). Dependent on MTR, MD, and phase, cluster analysis led to five lesion groups. Of the two groups with phase values close to NAWM, one presented with highest MD and most severe MTR decrease (p = 0.01), the other with slight MD increase and MTR decrease. Two lesion groups with highest phase values (p = 0.01) displayed slightly increased MD and moderate decrease in MTR. Clinical data including EDSS, disease duration, and age did not differ significantly between groups. Increased phase is predominantly detectable in lesions with clear MTR decrease but only moderate MD increase. Phase images seem to represent an independent parameter for MS lesion characterization and may provide additional information on MS lesion heterogeneity.

Dynamics of sodium channel Nav1.5 expression in astrocytes in mouse models of multiple sclerosis.

Astrocytes actively participate in the response of the central nervous system to injury, including in multiple sclerosis. Astrocytes can play both beneficial and detrimental roles in response to neuroinflammation; however, in extreme cases, astrogliosis can result in the formation of a glial scar, which can impede the regeneration of injured neurons. Although astrocytes do not express the voltage-gated sodium channel Nav1.5 in the nonpathological human brain, they exhibit robust upregulation of Nav1.5 within acute and chronic multiple sclerosis lesions. Recent work has indicated that Nav1.5 contributes to the pathways that regulate glial scar formation in vitro through modulation of intracellular Ca levels. However, the temporal dynamics of astrocytic Nav1.5 channel expression in response to neuroinflammatory pathologies has not been investigated. We examined astrocytes from mice with monophasic and chronic-relapsing (CR) experimental autoimmune encephalomyelitis (EAE) by immunohistochemical analysis to determine whether Nav1.5 is expressed in these cells, and whether the expression correlates with the severity of disease and/or phases of relapse and remission. Our results demonstrate that Nav1.5 is upregulated in astrocytes in situ in a temporal manner that correlates with disease severity in both monophasic and CR EAE. Further, in CR EAE, Nav1.5 expression is upregulated during relapses and subsequently attenuated during periods of remission. These observations are consistent with the suggestion that Nav1.5 can play a role in the response of astrocytes to inflammatory pathologies in the central nervous system and suggest Nav1.5 may be a potential therapeutic target to modulate reactive astrogliosis in vivo.

PO-0846 Tumefactive Demyelinating Lesions In Juvenile-onset Multiple Sclerosis

Background The pathogenesis of large demyelinating lesions is still controversial. Atypical tumefactive demyelinating lesions (TDL) associated with acute inflammation, peri-lesional oedema and gadolinium ring enhancement are infrequently described in patients with juvenile-onset multiple sclerosis (MS). Objective To describe the clinical, imaging and micro-structural metrics of TDLs and chronic MS lesions in patients with juvenile-onset MS. Methods Ten patients diagnosed with MS were analysed for the presence of TDLs and chronic non enhancing MS lesions. The MS lesions were defined by a region of interest encircling the lesion centre on 2–3 consecutive slices. DTI images were acquired along 31 independent orientations using a single shot echo-planar imaging sequence. Results Four patients with 6 TDL, developed acute neurological symptomatology. The two girls presented with acute ataxia and aphasia, and the two boys with severe ataxia. Three patients progressed rapidly to develop seizures, became stuporotic and were admitted to the paediatric intensive care unit. Brain MRI demonstrated six TDLs. Analysis of the whole group (10 patients) disclosed 21 chronic non enhancing lesions. Assessment of DTI metrics of TDL as compared to chronic MS lesions disclosed significant differences. Conclusion TDL are a possible presentation of demyelinating disorders, posing a diagnostic and therapeutic dilemma towards neoplastic lesions. The micro-structural analysis of TDL suggests a severe tissue disruption probably due to the acute inflammatory process and oedema. Our analysis provides metrical tools that together with MR spectroscopy and perfusion may aid to identify accurately TDLs, potentially sparing young patients unnecessary and possibly debilitating brain biopsy.

The EIIIA domain from astrocyte-derived fibronectin mediates proliferation of oligodendrocyte progenitor cells following CNS demyelination.

Central nervous system remyelination by oligodendrocyte progenitor cells (OPCs) ultimately fails in the majority of multiple sclerosis (MS) lesions. Remyelination benefits from transient expression of factors that promote migration and proliferation of OPCs, which may include fibronectin (Fn). Fn is present in demyelinated lesions in two major forms; plasma Fn (pFn), deposited following blood‐brain barrier disruption, and cellular Fn, synthesized by resident glial cells and containing alternatively spliced domains EIIIA and EIIIB. Here, we investigated the distinctive roles that astrocyte‐derived Fn (aFn) and pFn play in remyelination. We used an inducible Cre‐lox recombination strategy to selectively remove pFn, aFn or both from mice, and examined the impact on remyelination of toxin‐induced demyelinated lesions of spinal cord white matter. This approach revealed that astrocytes are a major source of Fn in demyelinated lesions. Furthermore, following aFn conditional knockout, the number of OPCs recruited to the demyelinated lesion decreased significantly, whereas OPC numbers were unaltered following pFn conditional knockout. However, remyelination completed normally following conditional knockout of aFn and pFn. Both the EIIIA and EIIIB domains of aFn were expressed following demyelination, and in vitro assays demonstrated that the EIIIA domain of aFn mediates proliferation of OPCs, but not migration. Therefore, although the EIIIA domain from aFn mediates OPC proliferation, aFn is not essential for successful remyelination. Since previous findings indicated that astrocyte‐derived Fn aggregates in chronic MS lesions inhibit remyelination, aFn removal may benefit therapeutic strategies to promote remyelination in MS. GLIA 2015;63:242–256

Differential loss of KIR4.1 immunoreactivity in multiple sclerosis lesions.

Serum antibodies against the glial potassium channel KIR4.1 are found in a subpopulation of multiple sclerosis (MS) patients. Little is known about the expression of KIR4.1 in human normal brain tissue and in MS lesions. We analyzed the expression pattern of KIR4.1 in normal brain tissue and MS lesions of the subcortical white matter by immunohistochemistry. Markers of related glial proteins, myelin, and inflammatory cells were analyzed in parallel. KIR4.1 is expressed in oligodendrocytes and astrocytes in the adult human brain. In oligodendrocytes, KIR4.1 appears as a homotetramer channel, in astrocytes as homo- and heterotetramer channels together with KIR5.1. In acute MS lesions, KIR4.1 immunoreactivity (IR) was differentially lost on periplaque oligodendrocytes and perivascular astrocytes. In part of acute lesions, complement activation, apoptotic KIR4.1(+) glial cells, and phagocytes containing KIR4.1(+) fragments accompanied loss of glial KIR4.1 IR. Periplaque reactive astrocytes showed enhanced IR for both KIR4.1 and KIR5.1. In chronic active MS lesions, apart from a general loss of oligodendrocytes in the demyelinated area, we observed a decrease of astroglial KIR4.1 but not glial fibrillary acidic protein IR. In chronic inactive and remyelinating MS lesions, KIR4.1 IR was restored on astrocytes and found in a subset of presumably new myelinating oligodendrocytes. The expression profile of KIR4.1 in glial cells and stage-dependent alterations of KIR4.1 IR in MS lesions are compatible with an immune response against KIR4.1 at least in a subset of MS patients.

Oligodendroglia in cortical multiple sclerosis lesions decrease with disease progression, but regenerate after repeated experimental demyelination

Cerebral cortex shows a high endogenous propensity for remyelination. Yet, widespread subpial cortical demyelination (SCD) is a common feature in progressive multiple sclerosis (MS) and can already be found in early MS. In the present study, we compared oligodendroglial loss in SCD in early and chronic MS. Furthermore, we addressed in an experimental model whether repeated episodes of inflammatory SCD could alter oligodendroglial repopulation and subsequently lead to persistently demyelinated cortical lesions. NogoA+ mature oligodendrocytes and Olig2+ oligodendrocyte precursor cells were examined in SCD in patients with early and chronic MS, normal-appearing MS cortex, and control cortex as well as in the rat model of repeated targeted cortical experimental autoimmune encephalomyelitis (EAE). NogoA+ and Olig2+ cells were significantly reduced in SCD in patients with chronic, but not early MS. Repeated induction of SCD in rats resulted only in a transient loss of NogoA+, but not Olig2+ cells during the demyelination phase. This phase was followed by complete oligodendroglial repopulation and remyelination, even after four episodes of demyelination. Our data indicate efficient oligodendroglial repopulation in subpial cortical lesions in rats after repeated SCD that was similar to early, but not chronic MS cases. Accordingly, four cycles of experimental de- and remyelination were not sufficient to induce sustained remyelination failure as found in chronic cortical MS lesions. This suggests that alternative mechanisms contribute to oligodendrocyte depletion in chronic cortical demyelination in MS.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-014-1260-8) contains supplementary material, which is available to authorized users.

Reactive astrocytes and perivascular macrophages express NLRP3 inflammasome in active demyelinating lesions of multiple sclerosis and necrotic lesions of neuromyelitis optica and cerebral infarction

AbstractObjectiveInflammasome, activated by pathogen‐derived and host‐derived danger signals, constitutes a multimolecular signaling complex that serves as a platform for caspase‐1 (CASP1) activation and interleukin‐1β (IL‐1B) maturation. Mice deficient for NLRP3 inflammasome components are resistant to experimental autoimmune encephalomyelitis (EAE), suggesting a pro‐inflammatory role of NLRP3 inflammasome. However, at present, a pathological role of NLRP3 inflammasome in multiple sclerosis (MS) brains remains unknown.MethodsWe studied the expression of NLRP3 inflammasome components in active demyelinating lesions of MS by immunohistochemistry.ResultsReactive astrocytes and perivascular macrophages expressed all three components of NLRP3 inflammasome – NLRP3, ASC and CASP1 – along with IL‐1B in active demyelinating lesions of MS, active necrotic lesions of neuromyelitis optica (NMO) and acute necrotic lesions of cerebral infarction. In contrast, the levels of expression of NLRP3, ASC, CASP1, and IL‐1B were greatly reduced in chronic inactive lesions of MS and gliotic lesions of cerebral infarction. Furthermore, the great majority of ramified and amoeboid microglia did not express NLRP3 in active and inactive MS lesions.ConclusionsNLRP3 inflammasome could be activated chiefly in reactive astrocytes and infiltrating macrophages under the condition of active destruction of brain tissues that potentially provides a danger signal.

Natalizumab-Associated Progressive Multifocal Leukoencephalopathy in a Patient With Multiple Sclerosis: A Postmortem Study

Natalizumab, a monoclonal antibody directed against α4 integrins, has, to date, been associated with 399 cases of progressive multifocal leukoencephalopathy (PML) worldwide in patients receiving treatment for multiple sclerosis (MS). Because of the limited number of histologic studies, the possible interplay between MS and PML lesions has not been investigated. We report the clinical, radiologic, and histologic findings of an MS patient who developed PML after 32 months of natalizumab monotherapy. After withdrawal of natalizumab, she received plasma exchange, mefloquine, and mirtazapine but died soon thereafter. Postmortem examination was restricted to examination of the brain and spinal cord. Extensive PML lesions, characterized by the presence of JC virus DNA were found in the cerebral white matter and neocortex. Sharply demarcated areas of active PML lesions contained prominent inflammatory infiltrates composed of approximately equal numbers of CD4-positive and CD8-positive T cells, consistent with an immune reconstitution inflammatory syndrome. Conversely, all MS lesions identified were hypocellular, long-standing inactive plaques characterized by myelin loss, relative axonal preservation, and gliosis and, importantly, were devoid of JC virus DNA and active inflammation. Chronic inactive MS lesions were separate and distinct from nearby PML lesions. This case demonstrates the coexistence and apparent lack of interplay between chronic inactive MS and PML lesions, and that immune reconstitution inflammatory syndrome seems to affect the shape and appearance of PML but not MS lesions.

Connexin pathology in chronic multiple sclerosis and experimental autoimmune encephalomyelitis

AbstractCentral nervous system (CNS) glia are extensively coupled through gap junctions (GJ) formed by a highly cell‐specific set of connexins. Recently characterized inherited human disorders and respective animal models highlight the crucial role of glial GJ in CNS myelination and homeostasis. These emerging roles of connexins have generated interest into their possible involvement in common acquired demyelinating disorders, especially multiple sclerosis (MS). Analysis of post‐mortem brain samples from MS patients, including morphological, biochemical, and gene expression studies, showed significant alteration in glial connexins not only in lesions, but also extending into the normal appearing white matter (NAWM). Compared with non‐MS cases, the oligodendrocyte connexins, Cx32 and Cx47, were reduced in and around chronic MS lesions, whereas Cx43, the major astrocytic partner in oligodendrocyte–astrocyte (O/A) GJ, was increased, reflecting astrogliosis. Furthermore, in the NAWM, Cx32 GJ were significantly reduced along myelinated fibers, whereas Cx47 showed increased expression mainly in oligodendrocyte precursor cells (OPC). However, OPC showed only limited connectivity to astrocytes. These alterations were replicated in the experimental autoimmune encephalomyelitis (EAE) model, with persistent loss of Cx47 and Cx32 GJ even away from lesions. Additionally, Cx43 was severely reduced in acute EAE with disruption of O/A GJ, followed by increased Cx43 expression at chronic stages of astrogliosis, but without complete re‐establishment of GJ to oligodendrocytes. Thus, loss of oligodendrocyte GJ in myelinated fibers, and disrupted O/A GJ connectivity in MS lesions and in NAWM in the setting of persistent inflammation and astrogliosis emerge as important aspects of MS pathology, and might have implications for disease progression.

Cell-Based Reparative Therapies for Multiple Sclerosis

The strong rationale for cell-based therapy in multiple sclerosis is based on the ability of stem and precursor cells of neural and mesenchymal origin to attenuate neuroinflammation, to facilitate endogenous repair processes, and to participate directly in remyelination, if directed towards a myelin-forming fate. However, there are still major gaps in knowledge regarding induction of repair in chronic multiple sclerosis lesions, and whether transplanted cells can overcome the multiple environmental inhibitory factors which underlie the failure of endogenous repair. Major challenges in clinical translation include the determination of the optimal cellular platform, the route of cell delivery, and candidate patients for treatment.

Myelin inhibits oligodendroglial maturation and regulates oligodendrocytic transcription factor expression

Myelin loss is a hallmark of multiple sclerosis (MS) and promoting central nervous system myelin repair has become a major therapeutic target. Despite the presence of oligodendrocytes precursors cells (OPCs) in chronic lesions of MS, remyelination often fails. The mechanism underlying this failure of remyelination remains unknown, but it is hypothesized that environmental cues act to inhibit the maturation/differentiation of oligodendroglia, preventing remyelination. The rate of CNS remyelination is correlated to the speed of phagocytosis of myelin debris, which is present following demyelination and trauma. Thus, myelin debris could inhibit CNS remyelination. Here, we demonstrate that OPCs cultured on myelin were robustly inhibited in their maturation, as characterized by the decreased expression of immature and mature oligodendrocytes markers, the impaired production of myelin gene products, as well as their stalled morphological complexity relative to OPCs cultured on a control substrate. OPCs in contact with myelin stopped proliferating and decreased the expression of OPC markers to a comparable degree as cells grown on a control substrate. The expression of two transcription factors known to prevent OPC differentiation and maturation were increased in cells that were in contact with myelin: inhibitor of differentiation family (ID) members 2 and 4. Overexpression of ID2 and ID4 in OPCs was previously reported to decrease the percentage of cells expressing mature oligodendrocyte markers. However, knockdown of ID2 and/or ID4 in OPCs did not increase oligodendroglial maturation on or off of myelin, suggesting that contact with myelin regulates additional regulatory elements.

Oligodendrocyte Progenitor Cell Susceptibility to Injury in Multiple Sclerosis

Remyelination in multiple sclerosis (MS) is often incomplete. In experimental models, oligodendrocyte progenitor cells (OPCs) rather than previously myelinating oligodendrocytes (OLs) are responsible for remyelination. This study compares the relative susceptibility of adult human OPCs and mature OLs to injury in actively demyelinating MS lesions and under invitro stress conditions. In all lesions (n = 20), the number of OLs (Olig2 weak/NogoA positive) was reduced compared to control white matter (mean 38± 4% of control value). In 11 cases, OPC numbers (Olig2 strong; NogoA negative) were also decreased; in eight of these, the reduction was greater for OPCs than for OLs. In the other nine samples, OPC numbers were greater than control white matter, indicating ongoing OPC migration and/or proliferation. Analysis of co-cultures with rat dorsal root ganglia neurons confirmed that OPCs were more capable of contacting and ensheathing axons than OLs. In isolated culture under stress conditions (withdrawal of serum/glucose and/or antioxidants), OPCs showed increased cell death and reduced process extension compared to OLs. Under all culture conditions, OPCs up-regulated expression of genes in the extrinsic proapoptotic pathway, and had increased susceptibility to tumor necrosis factor-induced cell death as compared to OLs. Our data suggest that susceptibility of OPCs to injury within the MS lesion environment contributes to the limited remyelination in MS.