Astrocytes regulate myelin clearance through recruitment of microglia during cuprizone-induced demyelination

Impaired Remyelination in a Mouse Model of Huntington Disease

White matter (WM) abnormalities are a well-established feature of Huntington disease (HD), although their nature is not fully understood. Here, we asked whether remyelination as a measure of WM plasticity is impaired in a model of HD. Using the cuprizone assay, we examined demyelination and remyelination responses in YAC128 HD mice. Treatment with 0.2% cuprizone (CPZ) for 6 weeks resulted in significant reduction in mature (GSTπ-positive) oligodendrocyte counts and FluoroMyelin staining in the corpus callosum, leading to similar demyelination states in YAC128 and wild-type (WT) mice. Six weeks following cessation of CPZ, we observed robust remyelination in WT mice as indicated by an increase in mature oligodendrocyte counts and FluoroMyelin staining. In contrast, YAC128 mice exhibited an impaired remyelination response. The increase in mature oligodendrocyte counts in YAC128 HD mice following CPZ cessation was lower than that of WT. Furthermore, there was no increase in FluoroMyelin staining compared to the demyelinated state in YAC128 mice. We confirmed these findings using electron microscopy where the CPZ-induced reduction in myelinated axons was reversed following CPZ cessation in WT but not YAC128 mice. Our findings demonstrate that remyelination is impaired in YAC128 mice and suggest that WM plasticity may be compromised in HD.

Astrocyte ablation induced by La-aminoadipate (L-AAA) potentiates remyelination in a cuprizone demyelinating mouse model

Chronic demyelination in the central nervous system (CNS) is accompanied by an increase in the number of reactive astrocytes and astrogliosis. There are controversial issues regarding astrocytes and their roles in demyelinating diseases in particular for multiple sclerosis (MS). We aimed to evaluate possible roles for pharmacologic astrocyte ablation strategy using La-aminoadipate (L-AAA) on remyelination in a cuprizone model of demyelination. Male C57BL/6 mice were fed with 0.2% cuprizone for 12 weeks followed by 2-week administration of L-AAA through a cannula inserted 1 mm above the corpus callosum. Rotarod test showed a significant decrease in the range of motor coordination deficits after ablation of astrocytes in mice receiving cuprizone. Results of Luxol fast blue (LFB) and transmission electron microscopy (TEM) for evaluation of myelin content within the corpus callosum revealed a noticeable rise in the percentage of myelinated areas and in the number of myelinated fibers after L-AAA administration in the animals. Astrocyte ablation reduced protein expressions for GFAP (an astrocyte marker) and Iba-1 (a microglial marker), but increased expression of Olig2 (an oligodendrocyte marker) assessed by immunofluorescence. Finally, expression of genes related to recruitment of microglia (astrocyte chemokines CXCL10 and CXCL12) and suppression of oligodendrocyte progenitor cell (OPC) differentiation (astrocyte peptides ET-1 and EDNRB) showed a considerable decrease after administration of L-AAA (for all p < 0.05). These results are indicative of improved remyelination after ablation of astrocytes possibly through hampering microgliosis and astrogliosis and a further rise in the number of matured Olig2+ cells.

Osmotic Demyelination: From an Oligodendrocyte to an Astrocyte Perspective

Osmotic demyelination syndrome (ODS) is a disorder of the central myelin that is often associated with a precipitous rise of serum sodium. Remarkably, while the myelin and oligodendrocytes of specific brain areas degenerate during the disease, neighboring neurons and axons appear unspoiled, and neuroinflammation appears only once demyelination is well established. In addition to blood‒brain barrier breakdown and microglia activation, astrocyte death is among one of the earliest events during ODS pathology. This review will focus on various aspects of biochemical, molecular and cellular aspects of oligodendrocyte and astrocyte changes in ODS-susceptible brain regions, with an emphasis on the crosstalk between those two glial cells. Emerging evidence pointing to the initiating role of astrocytes in region-specific degeneration are discussed.

Polarized microglia do not influence oligodendrocyte lineage cells via astrocytes

Microglia can adopt different activation patterns, ranging from a pro-inflammatory M1- to an anti-inflammatory M2-like phenotype in which they play crucial roles in various neuroinflammatory diseases. M2-like microglia are described to drive remyelination, whereas detrimental effects have been attributed to M1-like microglia. How polarized microglia might act on oligodendrocyte lineage cells indirectly by influencing astrocytes has not been studied in detail. In this study, conditioned media from polarized murine microglia were used to treat astrocytes and astrocytic gene expression was analyzed by microarray for genes known to influence oligodendrocyte lineage cells. Supernatants of astrocytes previously stimulated with soluble effectors from polarized microglia were used to investigate effects on oligodendrocyte precursor cells (OPC). Growth factors known to induce OPC proliferation, differentiation, and survival were upregulated in astrocytes treated with supernatants from M1-like microglia while M0- and M2-like microglia only had negligible effects on the expression of these factors in astrocytes. Despite the upregulation of these factors in M1 stimulated astrocytes there were no significant effects on OPC in vitro. All astrocyte supernatants induced proliferation of A2B5⁺ OPC and inhibited differentiation of OPC into mature oligodendrocytes. A trend toward enhanced migration of OPC was induced by M1 stimulated astrocytes. Our data suggest that M1-like microglia may potentially influence OPC and remyelination indirectly via astrocytes by inducing the expression of respective growth factors, however, this has no significant effect in addition to the already strong effects of unstimulated astrocytes on OPC. Nevertheless, the observed effect may be of relevance in other pathophysiological scenarios.

Changes in neurosteroidogenesis during demyelination and remyelination in cuprizone-treated mice

Changes of neurosteroids may be involved in the pathophysiology of multiple sclerosis (MS). The present study investigated whether changes of neurosteroidogenesis also occurred in the grey and white matter regions of the brain in mice subjected to cuprizone-induced demyelination. Accordingly, we compared the expression of neurosteroidogenic proteins, including steroidogenic acute regulatory protein (StAR), voltage-dependent anion channel (VDAC) and 18 kDa translocator protein (TSPO), as well as neurosteroidogenic enzymes, including the side chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase/isomerase and 5α-reductase (5α-R), during the demyelination and remyelination periods. Using immunohistochemistry and a quantitative polymerase chain reaction, we demonstrated a decreased expression of StAR, P450scc and 5α-R with respect to an increase astrocytic and microglial reaction and elevated levels of tumor necrosis factor (TNF)α during the cuprizone demyelination period in the hippocampus, cortex and corpus callosum. These parameters, as well as the glial reaction, were normalised after 2 weeks of spontaneous remyelination in regions containing grey matter. Conversely, persistent elevated levels of TNFα and low levels of StAR and P450scc were observed during remyelination in corpus callosum white matter. We conclude that neurosteroidogenesis/myelination status and glial reactivity are inversely related in the hippocampus and neocortex. Establishing a cause and effect relationship for the measured variables remains a future challenge for understanding the pathophysiology of MS.

Low-level laser therapy modulates demyelination in mice

There are no effective therapies for remyelination. Low-level laser therapy (LLLT) has been found advantageous in neurogenesis promotion, cell death prevention, and modulation of inflammation in central and peripheral nervous system models. The purpose of this study was to analyse LLLT effects on cuprizone-induced demyelination. Mice were randomly distributed into three groups: Control Laser (CTL), Cuprizone (CPZ), and Cuprizone Laser (CPZL). Mice from CPZ and CPZL groups were exposed to a 0.2% cuprizone oral diet for four complete weeks. Six sessions of transcranial laser irradiation were applied on three consecutive days, during the third and fourth weeks, with parameters of 36 J/cm², 50 mW, 0.028 cm² spot area, continuous wave, 1 J, 20 s, 1.78 W/cm² in a single point equidistant between the eyes and ears of CTL and CPZL mice. Motor coordination was assessed by the rotarod test. Twenty-four hours after the last laser session, all animals were euthanized, and brains were extracted. Serum was obtained for lactate dehydrogenase toxicity testing. Histomorphological analyses consisted of Luxol Fast Blue staining and immunohistochemistry. The results showed that laser-treated animals presented motor performance improvement, attenuation of demyelination, increased number of oligodendrocyte precursor cells, modulated microglial and astrocytes activation, and a milder toxicity by cuprizone. Although further studies are required, it is suggested that LLLT represents a feasible therapy for demyelinating diseases.

IFN-γ Producing Th1 Cells Induce Different Transcriptional Profiles in Microglia and Astrocytes

Autoreactive T cells that infiltrate into the central nervous system (CNS) are believed to have a significant role in mediating the pathology of neuroinflammatory diseases like multiple sclerosis. Their interaction with microglia and astrocytes in the CNS is crucial for the regulation of neuroinflammatory processes. Our previous work demonstrated that effectors secreted by Th1 and Th17 cells have different capacities to influence the phenotype and function of glial cells. We have shown that Th1-derived effectors altered the phenotype and function of both microglia and astrocytes whereas Th17-derived effectors induced direct effects only on astrocytes but not on microglia. Here we investigated if effector molecules associated with IFN-γ producing Th1 cells induced different gene expression profiles in microglia and astrocytes. We performed a microarray analysis of RNA isolated from microglia and astrocytes treated with medium and Th-derived culture supernatants and compared the gene expression data. By using the criteria of 2-fold change and a false discovery rate of 0.01 (corrected p < 0.01), we demonstrated that a total of 2,106 and 1,594 genes were differentially regulated in microglia and astrocytes, respectively, in response to Th1-derived factors. We observed that Th1-derived effectors induce distinct transcriptional changes in microglia and astrocytes in addition to commonly regulated transcripts. These distinct transcriptional changes regulate peculiar physiological functions, and this knowledge can help to better understand T cell mediated neuropathologies.

Microglia-Astrocyte Crosstalk: An Intimate Molecular Conversation

Microglia-astrocyte crosstalk has recently been at the forefront of glial research. Emerging evidence illustrates that microglia- and astrocyte-derived signals are the functional determinants for the fates of astrocytes and microglia, respectively. By releasing diverse signaling molecules, both microglia and astrocytes establish autocrine feedback and their bidirectional conversation for a tight reciprocal modulation during central nervous system (CNS) insult or injury. Microglia, the constant sensors of changes in the CNS microenvironment and restorers of tissue homeostasis, not only serve as the primary immune cells of the CNS but also regulate the innate immune functions of astrocytes. Similarly, microglia determine the functions of reactive astrocytes, ranging from neuroprotective to neurotoxic. Conversely, astrocytes through their secreted molecules regulate microglial phenotypes and functions ranging from motility to phagocytosis. Altogether, the microglia-astrocyte crosstalk is fundamental to neuronal functions and dysfunctions. This review discusses the current understanding of the intimate molecular conversation between microglia and astrocytes and outlines its potential implications in CNS health and disease.

Neuroprotective effects of G-CSF administration in microglia-mediated reactive T cell activation in vitro

G-CSF is a growth factor that has known neuroprotective effects in a variety of experimental brain injury models. As both antigen-presenting microglia and reactive T cells are key components in the development and progression of EAE, the aim of this study is to investigate the neuroprotective effects of recombinant human G-CSF, as administered in microglia-mediated reactive T cell assay in vitro. Our results indicate that G-CSF treatment has no apparent effect for the resting un-activated microglia. G-CSF pre-protection of microglia increased protective cytokine IL-4 production and effectively inhibited the productions of NO and other inflammatory mediators (IFN-γ, TNF-α, IL-1β, IL-17, and chemokine MCP-1) after LPS stimulation. G-CSF suppressed the proliferative response of microglia-mediated MOG_35-55 reactive T cells. G-CSF-microglia-T cells increased IL-4 and IL-10 secretions and decreased IFN-γ, TNF-α, and IL-17 productions. G-CSF significantly elevated CD4⁺CD25⁺ regulatory T cell subset in microglia-mediated reactive T cells. Moreover, G-CSF inhibited MHC-II expression of microglia after LPS activation or in the interactions of microglia and reactive T cells. G-CSF administration induced the apoptosis and enhanced the G0/G1 to S phase transition and elevated the gene expression of apoptosis markers in microglia-mediated reactive T cells after stimulated by specific antigen MOG_35-55. These findings reveal that G-CSF administration potently neuroprotects the central nervous system (CNS) from immune-mediated damage in microglia-mediated reactive T cell activation. Apoptosis of reactive T cells in CNS is important in attenuating the development of autoimmune CNS diseases. G-CSF administration has neuroprotective effects in CNS and the potential to be a therapeutic agent in multiple sclerosis.

Behavioural alterations and morphological changes are attenuated by the lack of TRPA1 receptors in the cuprizone-induced demyelination model in mice

We have recently reported that the Transient Receptor Potential Ankyrin 1 (TRPA1) receptor deficiency significantly attenuated cuprizone-induced demyelination by reducing the apoptosis of mature oligodendrocytes. The aim of the present study was to gather additional data on the role of TRPA1 by investigating the time course of behavioural alterations and morphological changes in cuprizone-treated TRPA1 receptor gene-deficient mice. Demyelination was induced by feeding male wild-type (WT) and TRPA1 gene-deleted (TRPA1 KO) mice with 0.2% cuprizone for 6 weeks. Behavioural tests were performed once per week to follow cuprizone-induced functional changes. Mechanonociceptive thresholds were investigated by a dynamic plantar aesthesiometer and von Frey filaments. Motor performance was assessed by accelerating RotaRod and horizontal grid tests. For the study of spontaneous activity, the open field test was used. The time course of corpus callosum demyelination was also followed weekly by magnetic resonance imaging (MRI). Histological analysis of myelin loss was performed with Luxol Fast Blue (LFB) staining at week 3 and electron microscopy (EM) at week 6. Astrocyte and microglia accumulation at week 3 was assessed by immunohistochemistry (IHC). Cuprizone treatment induced no changes in mechanonociception or motor performance. In the open arena, cuprizone-treated mice spent more time with locomotion, their mean velocity was significantly higher and the distance they travelled was longer than untreated mice. No statistical difference was detected between WT and TRPA1 KO mice in these parameters. On the other hand, significantly increased rearing behaviour was induced in WT mice compared to TRPA1 KO animals. Morphological changes detected with MRI, LFB, IHC and EM analysis revealed reduced damage of the myelin and attenuated accumulation of astrocytes and microglia in cuprizone-treated TRPA1 KO animals, at each examined time point. Our recent data further suggest that inhibition of TRPA1 receptors could be a promising therapeutic approach to limit central nervous system damage in demyelinating diseases.

Experimental Demyelination and Axonal Loss Are Reduced in MicroRNA-146a Deficient Mice

Background

The cuprizone (CPZ) model of multiple sclerosis (MS) was used to identify microRNAs (miRNAs) related to in vivo de- and remyelination. We further investigated the role of miR-146a in miR-146a-deficient (KO) mice: this miRNA is differentially expressed in MS lesions and promotes differentiation of oligodendrocyte precursor cells (OPCs) during remyelination, but its role has not been examined during demyelination.

Methods

MicroRNAs were examined by Agilent Mouse miRNA Microarray in the corpus callosum during CPZ-induced demyelination and remyelination. Demyelination, axonal loss, changes in number of oligodendrocytes, OPCs, and macrophages/microglia was compared by histology/immunohistochemistry between KO and WT mice. Differential expression of target genes and proteins of miR-146a was analyzed in the transcriptome (4 × 44K Agilent Whole Mouse Genome Microarray) and proteome (liquid chromatography tandem mass spectrometry) of CPZ-induced de- and remyelination in WT mice. Levels of proinflammatory molecules in the corpus callosum were compared in WT versus KO mice by Meso Scale Discovery multiplex protein analysis.

Results

miR-146a was increasingly upregulated during CPZ-induced de- and remyelination. The absence of miR-146a in KO mice protected against demyelination, axonal loss, body weight loss, and atrophy of thymus and spleen. The number of CNP⁺ oligodendrocytes was increased during demyelination in the miR-146a KO mice, while there was a trend of increased number of NG2⁺ OPCs in the WT mice. miR-146a target genes, SNAP25 and SMAD4, were downregulated in the proteome of demyelinating corpus callosum in WT mice. Higher levels of SNAP25 were measured by ELISA in the corpus callosum of miR-146a KO mice, but there was no difference between KO and WT mice during demyelination. Multiplex protein analysis of the corpus callosum lysate revealed upregulated TNF-RI, TNF-RII, and CCL2 in the WT mice in contrast to KO mice. The number of Mac3⁺ and Iba1⁺ macrophages/microglia was reduced in the demyelinating corpus callosum of the KO mice.

Conclusion

During demyelination, absence of miR-146a reduced inflammatory responses, demyelination, axonal loss, the number of infiltrating macrophages, and increased the number of myelinating oligodendrocytes. The number of OPCs was slightly higher in the WT mice during remyelination, indicating a complex role of miR-146a during in vivo de- and remyelination.

Changes in the axo-glial junctions of the optic nerves of cuprizone-treated mice

Demyelination induced by cuprizone in mice has served a useful model system for the study of demyelinating diseases, such as multiple sclerosis. Severity of demyelination by cuprizone, however, varies across different regions of the central nervous system; the corpus callosum is sensitive, while the optic nerves are resistant. Here, we investigated the effects of cuprizone on optic nerves, focusing on the axo-glial junctions. Immunostaining for sodium channels, contactin-associated protein, neurofascins, and potassium channels revealed that there were no massive changes in the density and morphology of the axo-glial junctions in cuprizone-treated optic nerves. However, when we counted the number of incomplete junctional complexes, we observed increased numbers of isolated paranodes. These isolated paranodes were immunopositive for both axonal and glial membrane proteins, indicating that they were the contact sites between axons and glia. These were not associated with sodium channels or potassium channels, suggesting the absence of physiological functions. When teased axons from cuprizone-treated optic nerves were immunostained, the isolated paranodes were found at the internode region of the myelin. From these observations, we conclude that cuprizone induces new contacts between axons and myelins at the internode region.

The Role of Astrocytes in Multiple Sclerosis

The role traditionally assigned to astrocytes in the pathogenesis of multiple sclerosis (MS) lesions has been the formation of the glial scar once inflammation has subsided. Astrocytes are now recognized to be early and highly active players during lesion formation and key for providing peripheral immune cells access to the central nervous system. Here, we review the role of astrocytes in the formation and evolution of MS lesions, including the recently described functional polarization of astrocytes, discuss prototypical pathways for astrocyte activation, and summarize mechanisms by which MS treatments affect astrocyte function.

Exploring the genetics and non-cell autonomous mechanisms underlying ALS/FTLD

Although amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, was first described in 1874, a flurry of genetic discoveries in the last 10 years has markedly increased our understanding of this disease. These findings have not only enhanced our knowledge of mechanisms leading to ALS, but also have revealed that ALS shares many genetic causes with another neurodegenerative disease, frontotemporal lobar dementia (FTLD). In this review, we survey how recent genetic studies have bridged our mechanistic understanding of these two related diseases and how the genetics behind ALS and FTLD point to complex disorders, implicating non-neuronal cell types in disease pathophysiology. The involvement of non-neuronal cell types is consistent with a non-cell autonomous component in these diseases. This is further supported by studies that identified a critical role of immune-associated genes within ALS/FTLD and other neurodegenerative disorders. The molecular functions of these genes support an emerging concept that various non-autonomous functions are involved in neurodegeneration. Further insights into such a mechanism(s) will ultimately lead to a better understanding of potential routes of therapeutic intervention. Facts ALS and FTLD are severe neurodegenerative disorders on the same disease spectrum. Multiple cellular processes including dysregulation of RNA homeostasis, imbalance of proteostasis, contribute to ALS/FTLD pathogenesis. Aberrant function in non-neuronal cell types, including microglia, contributes to ALS/FTLD. Strong neuroimmune and neuroinflammatory components are associated with ALS/FTLD patients. Open Questions Why can patients with similar mutations have different disease manifestations, i.e., why do C9ORF72 mutations lead to motor neuron loss in some patients while others exhibit loss of neurons in the frontotemporal lobe? Do ALS causal mutations result in microglial dysfunction and contribute to ALS/FTLD pathology? How do microglia normally act to mitigate neurodegeneration in ALS/FTLD? To what extent do cellular signaling pathways mediate non-cell autonomous communications between distinct central nervous system (CNS) cell types during disease? Is it possible to therapeutically target specific cell types in the CNS?

Brain region-specific enhancement of remyelination and prevention of demyelination by the CSF1R kinase inhibitor BLZ945

Multiple sclerosis (MS) is a chronic inflammatory disease affecting the central nervous system (CNS). While multiple effective immunomodulatory therapies for MS exist today, they lack the scope of promoting CNS repair, in particular remyelination. Microglia play a pivotal role in regulating myelination processes, and the colony-stimulating factor 1 (CSF-1) pathway is a key regulator for microglia differentiation and survival. Here, we investigated the effects of the CSF-1 receptor kinase inhibitor, BLZ945, on central myelination processes in the 5-week murine cuprizone model by non-invasive and longitudinal magnetic resonance imaging (MRI) and histology. Therapeutic 2-week BLZ945 treatment caused a brain region-specific enhancement of remyelination in the striatum/cortex, which was absent in the corpus callosum/external capsule. This beneficial effect correlated positively with microglia reduction, increased oligodendrocytes and astrogliosis. Prophylactic BLZ945 treatment prevented excessive demyelination in the corpus callosum by reducing microglia and increasing oligondendrocytes. In the external capsule oligodendrocytes were depleted but not microglia and a buildup of myelin debris and axonal damage was observed. A similar microglial dysfunction in the external capsule with an increase of myelin debris was obvious in triggering receptor expressed on myeloid cells 2 (TREM2) knock-out mice treated with cuprizone. Finally, therapeutic BLZ945 treatment did not change the disease course in experimental autoimmune encephalomyelitis mice, a peripherally driven neuroinflammation model. Taken together, our data suggest that a short-term therapeutic inhibition of the CSF-1 receptor pathway by BLZ945 in the murine cuprizone model enhances central remyelination by modulating neuroinflammation. Thus, microglia-modulating therapies could be considered clinically for promoting myelination in combination with standard-of-care treatments in MS patients.

Animal models of multiple sclerosis: Focus on experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic, progressive disorder of the central nervous system (CNS) that affects more than two million people worldwide. Several animal models resemble MS pathology; the most employed are experimental autoimmune encephalomyelitis (EAE) and toxin- and/or virus-induced demyelination. In this review we will summarize our knowledge on the utility of different animal models in MS research. Although animal models cannot replicate the complexity and heterogeneity of the MS pathology, they have proved to be useful for the development of several drugs approved for treatment of MS patients. This review focuses on EAE because it represents both clinical and pathological features of MS. During the past decades, EAE has been effective in illuminating various pathological processes that occur during MS, including inflammation, CNS penetration, demyelination, axonopathy, and neuron loss mediated by immune cells.

Quantitative analysis of lipid debris accumulation caused by cuprizone induced myelin degradation in different CNS areas

Degradation of myelin sheath is thought to be the cause of neurodegenerative diseases, such as multiple sclerosis (MS), but definitive agreement on the mechanism of how myelin is lost is currently lacking. Autoimmune initiation of MS has been recently questioned by proposing that the immune response is a consequence of oligodendrocyte degeneration. To study the process of myelin breakdown, we induced demyelination with cuprizone and applied coherent anti-Stokes Raman scattering (CARS) microscopy, a non-destructive label-free method to image lipid structures in living tissue. We confirmed earlier results showing a brain region dependent myelin destructive effect of cuprizone. In addition, high resolution in situ CARS imaging revealed myelin debris forming lipid droplets alongwith myelinated axon fibers. Quantification of lipid debris with custom-made software for segmentation and three dimensional reconstruction revealed brain region dependent accumulation of lipid drops inversely correlated with the thickness of myelin sheaths. Finally, we confirmed that in situ CARS imaging is applicable to living human brain tissue in brain slices derived from a patient. Thus, CARS microscopy is potent tool for quantitative monitoring of myelin degradation in unprecedented spatiotemporal resolution during oligodendrocyte damage. We think that the accumulation of lipid drops around degrading myelin might be instrumental in triggering subsequent inflammatory processes.

Toll-Like Receptor 2-Mediated Glial Cell Activation in a Mouse Model of Cuprizone-Induced Demyelination

Multiple sclerosis (MS) is a chronic degenerative disease of the central nervous system that is characterized by myelin abnormalities, oligodendrocyte pathology, and concomitant glia activation. The factors triggering gliosis and demyelination are currently not well characterized. New findings suggest an important role of the innate immune response in the initiation and progression of active demyelinating lesions. Especially during progressive disease, aberrant glia activation rather than the invasion of peripheral immune cells is accountable for progressive neuronal injury. The innate immune response can be induced by pathogen-associated or danger-associated molecular patterns, which are identified by pattern recognition receptors (PRRs), including the Toll-like receptors (TLRs). In this study, we used the cuprizone model in mice to investigate the expression of TLR2 during the course of cuprizone-induced demyelination. In addition, we used TLR2-deficient mice to analyze the functional role of TLR2 activation during cuprizone-induced demyelination and reactive gliosis. We show a significantly increased expression of TLR2 in the corpus callosum and hippocampus of cuprizone-intoxicated mice. The absence of receptor signaling in TLR2-deficient mice resulted in less severe reactive astrogliosis in the corpus callosum and cortex. In addition, microglia activation was ameliorated in the corpus callosum of TLR2-deficient mice, but augmented in the cortex compared to wild-type littermates. Extent of demyelination and loss of mature oligodendrocytes was comparable in both genotypes. These results suggest that the TLR2 orchestrates glia activation during gray and white matter demyelination in the presence of an intact blood-brain barrier. Future studies now have to address the underlying mechanisms of the region-specific TLR2-mediated glia activation.

Neuroprotective effects of ellagic acid on cuprizone-induced acute demyelination through limitation of microgliosis, adjustment of CXCL12/IL-17/IL-11 axis and restriction of mature oligodendrocytes apoptosis

CONTEXT

Ellagic acid (EA) is a natural phenol antioxidant with various therapeutic activities. However, the efficacy of EA has not been examined in neuropathologic conditions.

OBJECTIVE

In vivo neuroprotective effects of EA on cuprizone (cup)-induced demyelination were evaluated.

MATERIAL AND METHODS

C57BL/6 J mice were fed with chow containing 0.2% cup for 4 weeks to induce oligodendrocytes (OLGs) depletion predominantly in the corpus callosum (CC). EA was administered at different doses (40 or 80 mg/kg body weight/day/i.p.) from the first day of cup diet. Oligodendrocytes apoptosis [TUNEL assay and myelin oligodendrocyte glycoprotein (MOG+)/caspase-3+ cells), gliosis (H&E staining, glial fibrillary acidic protein (GFAP+) and macrophage-3 (Mac-3+) cells) and inflammatory markers (interleukin 17 (IL-17), interleukin 11 (IL-11) and stromal cell-derived factor 1 α (SDF-1α) or CXCL12] during cup intoxication were examined.

RESULTS

High dose of EA (EA-80) increased mature oligodendrocytes population (MOG+ cells, p < 0.001), and decreased apoptosis (p < 0.05) compared with the cup mice. Treatment with both EA doses did not show any considerable effects on the expression of CXCL12, but significantly down-regulated the expression of IL-17 and up-regulated the expression of IL-11 in mRNA levels compared with the cup mice. Only treatment with EA-80 significantly decreased the population of active macrophage (MAC-3+ cells, p < 0.001) but not reactive astrocytes (GFAP+ cells) compared with the cup mice.

DISCUSSION AND CONCLUSION

In this model, EA-80 effectively reduces lesions via reduction of neuroinflammation and toxic effects of cup on mature OLGs. EA is a suitable therapeutic agent for moderate brain damage in neurodegenerative diseases such as multiple sclerosis.

Blood-brain barrier hyperpermeability precedes demyelination in the cuprizone model

In neuroinflammatory disorders such as multiple sclerosis, the physiological function of the blood-brain barrier (BBB) is perturbed, particularly in demyelinating lesions and supposedly secondary to acute demyelinating pathology. Using the toxic non-inflammatory cuprizone model of demyelination, we demonstrate, however, that the onset of persistent BBB impairment precedes demyelination. In addition to a direct effect of cuprizone on endothelial cells, a plethora of inflammatory mediators, which are mainly of astroglial origin during the initial disease phase, likely contribute to the destabilization of endothelial barrier function in vivo. Our study reveals that, at different time points of pathology and in different CNS regions, the level of gliosis correlates with the extent of BBB hyperpermeability and edema. Furthermore, in mutant mice with abolished type 3 CXC chemokine receptor (CXCR3) signaling, inflammatory responses are dampened and BBB dysfunction ameliorated. Together, these data have implications for understanding the role of BBB permeability in the pathogenesis of demyelinating disease.

Regenerating CNS myelin - from mechanisms to experimental medicines

Although the core concept of remyelination - based on the activation, migration, proliferation and differentiation of CNS progenitors - has not changed over the past 20 years, our understanding of the detailed mechanisms that underlie this process has developed considerably. We can now decorate the central events of remyelination with a host of pathways, molecules, mediators and cells, revealing a complex and precisely orchestrated process. These advances have led to recent drug-based and cell-based clinical trials for myelin diseases and have opened up hitherto unrecognized opportunities for drug-based approaches to therapeutically enhance remyelination.

Achievements and obstacles of remyelinating therapies in multiple sclerosis

Remyelination in the CNS is the natural process of damage repair in demyelinating diseases such as multiple sclerosis (MS). However, remyelination becomes inadequate in many people with MS, which results in axonal degeneration and clinical disability. Enhancement of remyelination is a logical therapeutic goal; nevertheless, all currently licensed therapies for MS are immunomodulatory and do not support remyelination directly. Several molecular pathways have been identified as potential therapeutic targets to induce remyelination, and some of these have now been assessed in proof-of-concept clinical trials. However, trial design faces several obstacles: optimal clinical or paraclinical outcome measures to assess remyelination remain ill-defined, and identification of the ideal timing of therapy is also a crucial issue. In addition, realistic expectations are needed concerning the probable benefits of such therapies. Nevertheless, approaches that enhance remyelination are likely to be protective for axons and so could prevent long-term neurodegeneration. Future MS treatment paradigms, therefore, are likely to comprise a combinatorial approach that involves both immunomodulatory and regenerative treatments.

Effectors of Th1 and Th17 cells act on astrocytes and augment their neuroinflammatory properties

Background

Autoreactive Th1 and Th17 cells are believed to mediate the pathology of multiple sclerosis in the central nervous system (CNS). Their interaction with microglia and astrocytes in the CNS is crucial for the regulation of the neuroinflammation. Previously, we have shown that only Th1 but not Th17 effectors activate microglia. However, it is not clear which cells are targets of Th17 effectors in the CNS.

Methods

To understand the effects driven by Th17 cells in the CNS, we induced experimental autoimmune encephalomyelitis in wild-type mice and CD4⁺ T cell-specific integrin α4-deficient mice where trafficking of Th1 cells into the CNS was affected. We compared microglial and astrocyte response in the brain and spinal cord of these mice. We further treated astrocytes with supernatants from highly pure Th1 and Th17 cultures and assessed the messenger RNA expression of neurotrophic factors, cytokines and chemokines, using real-time PCR. Data obtained was analyzed using the Kruskal-Wallis test.

Results

We observed in α4-deficient mice weak microglial activation but comparable astrogliosis to that of wild-type mice in the regions of the brain populated with Th17 infiltrates, suggesting that Th17 cells target astrocytes and not microglia. In vitro, in response to supernatants from Th1 and Th17 cultures, astrocytes showed altered expression of neurotrophic factors, pro-inflammatory cytokines and chemokines. Furthermore, increased expression of chemokines in Th1- and Th17-treated astrocytes enhanced recruitment of microglia and transendothelial migration of Th17 cells in vitro.

Conclusion

Our results demonstrate the delicate interaction between T cell subsets and glial cells and how they communicate to mediate their effects. Effectors of Th1 act on both microglia and astrocytes whereas Th17 effectors preferentially target astrocytes to promote neuroinflammation.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-0978-3) contains supplementary material, which is available to authorized users.

The Healthy and Diseased Microenvironments Regulate Oligodendrocyte Properties: Implications for Regenerative Medicine

White matter disorders are characterized by deficient myelin or myelin loss, lead to a range of neurologic dysfunctions, and can result in early death. Oligodendrocytes, which are responsible for white matter formation, are the first targets for treatment. However, many studies indicate that failure of white matter repair goes beyond the intrinsic incapacity of oligodendrocytes to (re)generate myelin and that failed interactions with neighboring cells or factors in the diseased microenvironment can underlie white matter defects. Moreover, most of the white matter disorders show specific white matter pathology caused by different disease mechanisms. Herein, we review the factors within the cellular and the extracellular microenvironment regulating oligodendrocyte properties and discuss stem cell tools to identify microenvironmental factors of importance to the development of improved regenerative medicine for patients with white matter disorders.

Effect of glial cells on remyelination after spinal cord injury

Remyelination plays a key role in functional recovery of axons after spinal cord injury. Glial cells are the most abundant cells in the central nervous system. When spinal cord injury occurs, many glial cells at the lesion site are immediately activated, and different cells differentially affect inflammatory reactions after injury. In this review, we aim to discuss the core role of oligodendrocyte precursor cells and crosstalk with the rest of glia and their subcategories in the remyelination process. Activated astrocytes influence proliferation, differentiation, and maturation of oligodendrocyte precursor cells, while activated microglia alter remyelination by regulating the inflammatory reaction after spinal cord injury. Understanding the interaction between oligodendrocyte precursor cells and the rest of glia is necessary when designing a therapeutic plan of remyelination after spinal cord injury.

Lineage tracing reveals dynamic changes in oligodendrocyte precursor cells following cuprizone-induced demyelination

The regeneration of oligodendrocytes is a crucial step in recovery from demyelination, as surviving oligodendrocytes exhibit limited structural plasticity and rarely form additional myelin sheaths. New oligodendrocytes arise through the differentiation of platelet-derived growth factor receptor α (PDGFRα) expressing oligodendrocyte progenitor cells (OPCs) that are widely distributed throughout the CNS. Although there has been detailed investigation of the behavior of these progenitors in white matter, recent studies suggest that disease burden in multiple sclerosis (MS) is more strongly correlated with gray matter atrophy. The timing and efficiency of remyelination in gray matter is distinct from white matter, but the dynamics of OPCs that contribute to these differences have not been defined. Here, we used in vivo genetic fate tracing to determine the behavior of OPCs in gray and white matter regions in response to cuprizone-induced demyelination. Our studies indicate that the temporal dynamics of OPC differentiation varies significantly between white and gray matter. While OPCs rapidly repopulate the corpus callosum and mature into CC1 expressing mature oligodendrocytes, OPC differentiation in the cingulate cortex and hippocampus occurs much more slowly, resulting in a delay in remyelination relative to the corpus callosum. The protracted maturation of OPCs in gray matter may contribute to greater axonal pathology and disease burden in MS.

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies.

The novel synthetic microneurotrophin BNN27 protects mature oligodendrocytes against cuprizone-induced death, through the NGF receptor TrkA

BNN27, a member of a chemical library of C17-spiroepoxy derivatives of the neurosteroid DHEA, has been shown to regulate neuronal survival through its selective interaction with NGF receptors (TrkA and p75NTR ), but its role on glial populations has not been studied. Here, we present evidence that BNN27 provides trophic action (rescue from apoptosis), in a TrkA-dependent manner, to mature oligodendrocytes when they are challenged with the cuprizone toxin in culture. BNN27 treatment also increases oligodendrocyte maturation and diminishes microglia activation in vitro. The effect of BNN27 in the cuprizone mouse model of demyelination in vivo has also been investigated. In this model, that does not directly involve the adaptive immune system, BNN27 can protect from demyelination without affecting the remyelinating process. BNN27 preserves mature oligodendrocyte during demyelination, while reducing microgliosis and astrogliosis. Our findings suggest that BNN27 may serve as a lead molecule to develop neurotrophin-like blood-brain barrier (BBB)-permeable protective agents of oligodendrocyte populations and myelin, with potential applications in the treatment of demyelinating disorders.

Hyperpolarized 13C MR metabolic imaging can detect neuroinflammation in vivo in a multiple sclerosis murine model

Proinflammatory mononuclear phagocytes (MPs) play a crucial role in the progression of multiple sclerosis (MS) and other neurodegenerative diseases. Despite advances in neuroimaging, there are currently limited available methods enabling noninvasive detection of MPs in vivo. Interestingly, upon activation and subsequent differentiation toward a proinflammatory phenotype MPs undergo metabolic reprogramming that results in increased glycolysis and production of lactate. Hyperpolarized (HP) ¹³C magnetic resonance spectroscopic imaging (MRSI) is a clinically translatable imaging method that allows noninvasive monitoring of metabolic pathways in real time. This method has proven highly useful to monitor the Warburg effect in cancer, through MR detection of increased HP [1-¹³C]pyruvate-to-lactate conversion. However, to date, this method has never been applied to the study of neuroinflammation. Here, we questioned the potential of ¹³C MRSI of HP [1-¹³C]pyruvate to monitor the presence of neuroinflammatory lesions in vivo in the cuprizone mouse model of MS. First, we demonstrated that ¹³C MRSI could detect a significant increase in HP [1-¹³C]pyruvate-to-lactate conversion, which was associated with a high density of proinflammatory MPs. We further demonstrated that the increase in HP [1-¹³C]lactate was likely mediated by pyruvate dehydrogenase kinase 1 up-regulation in activated MPs, resulting in regional pyruvate dehydrogenase inhibition. Altogether, our results demonstrate a potential for ¹³C MRSI of HP [1-¹³C]pyruvate as a neuroimaging method for assessment of inflammatory lesions. This approach could prove useful not only in MS but also in other neurological diseases presenting inflammatory components.

An Efficient Platform for Astrocyte Differentiation from Human Induced Pluripotent Stem Cells

Growing evidence implicates the importance of glia, particularly astrocytes, in neurological and psychiatric diseases. Here, we describe a rapid and robust method for the differentiation of highly pure populations of replicative astrocytes from human induced pluripotent stem cells (hiPSCs), via a neural progenitor cell (NPC) intermediate. We evaluated this protocol across 42 NPC lines (derived from 30 individuals). Transcriptomic analysis demonstrated that hiPSC-astrocytes from four individuals are highly similar to primary human fetal astrocytes and characteristic of a non-reactive state. hiPSC-astrocytes respond to inflammatory stimulants, display phagocytic capacity, and enhance microglial phagocytosis. hiPSC-astrocytes also possess spontaneous calcium transient activity. Our protocol is a reproducible, straightforward (single medium), and rapid (<30 days) method to generate populations of hiPSC-astrocytes that can be used for neuron-astrocyte and microglia-astrocyte co-cultures for the study of neuropsychiatric disorders.

Neurotrophy and immunomodulation of induced neural stem cell grafts in a mouse model of closed head injury

Closed head injury (CHI) usually results in severe and permanent neurological impairments, which are caused by several intertwined phenomena, such as cerebral edema, blood-brain barrier (BBB) disruption, neuronal loss, astroglial scarring and inflammation. We previously reported that induced neural stem cells (iNSCs), similar to neural stem cells (NSCs), can accelerate neurological recovery in vivo and produce neurotrophic factors in vitro. However, the effects of iNSC neurotrophy following CHI were not determined. Moreover, whether iNSCs have immunomodulatory properties is unknown. Mouse models of CHI were established using a standardized weight-drop device and assessed by neurological severity score (NSS). Although these models fail to mimic the complete spectrum of human CHI, they reproduce impairment in neurological function observed in clinical patients. Syngeneic iNSCs or NSCs were separately transplanted into the brains of CHI mice at 12h after CHI. Neurological impairment post-CHI was evaluated by several tests. Animals were sacrificed for morphological and molecular biological analyses. We discovered that iNSC administration promoted neurological functional recovery in CHI mice and reduced cerebral edema, BBB disruption, cell death and astroglial scarring following trauma. Implanted iNSCs could up-regulate brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) levels to support the survival of existing neurons after CHI. In addition, engrafted iNSCs decreased immune cell recruitment and pro-inflammatory cytokine expression in the brain post-injury. Moreover, we found significant nuclear factor-kappaB (NF-κB) inhibition in the presence of iNSC grafts. In short, iNSCs exert neurotrophic and immunomodulatory effects that mitigate CHI-induced neurological impairment.

Astrocyte heterogeneity across the brain and spinal cord occurs developmentally, in adulthood and in response to demyelination

Astrocytes have emerged as essential regulators of function and response to injury in the brain and spinal cord, yet very little is known about regional differences that exist. Here we compare the expression of key astroglial markers (glial fibrillary acidic protein (GFAP) and Aldehyde Dehydrogenase-1 Family Member L1 (ALDH1L1)) across these disparate poles of the neuraxis, tracking their expression developmentally and in the context of demyelination. In addition, we document changes in the astrocyte regulatory cytokine interleukin 6 (IL-6), and its signaling partner signal transducer and activator of transcription 3 (STAT3), in vivo and in vitro. Results demonstrate that GFAP expression is higher in the developing and adult spinal cord relative to brain. Comparisons between GFAP and ALDH1L1 expression suggest elevations in spinal cord GFAP during the early postnatal period reflect an accelerated appearance of astrocytes, while elevations in adulthood reflect higher expression by individual astrocytes. Notably, increases in spinal cord compared to whole brain GFAP were paralleled by higher levels of IL-6 and STAT3. Equivalent elevations in GFAP, GFAP/ALDH1L1 ratios, and in IL-6, were observed in primary astrocyte cultures derived from spinal cord compared to cortex. Also, higher levels of GFAP were observed in the spinal cord compared to the brain after focal demyelinating injury. Altogether, these studies point to key differences in astrocyte abundance and the expression of GFAP and IL-6 across the brain and spinal cord that are positioned to influence regional specialization developmentally and responses occurring in the context of injury and disease.

Reduced cuprizone-induced cerebellar demyelination in mice with astrocyte-targeted production of IL-6 is associated with chronically activated, but less responsive microglia

BACKGROUND

Cerebellar pathology is a frequent feature of multiple sclerosis (MS), a demyelinating and neuroinflammatory disease of the central nervous system (CNS). Interleukin (IL)-6 is a multifunctional cytokine with a potential role in MS. Here we studied cuprizone-induced cerebellar pathology in transgenic mice with astrocyte-targeted production of IL-6 (GFAP-IL6), specifically focusing on demyelination, oligodendrocyte depletion and microglial cell response.

RESULTS

Over the course of cuprizone treatment, when compared with WT mice, GFAP-IL6Tg showed a reduced demyelination in the deep lateral cerebellar nuclei (LCN). The oligodendrocyte numbers in the LCN were comparable between WT and GFAP-IL6Tg mice after 4-6weeks of cuprizone treatment, however after the chronic cuprizone treatment (12weeks) we detected higher numbers of oligodendrocytes in GFAP-IL6Tg mice. Contrary to strong cuprizone-induced microglial activation in the LCN of WT mice, GFAP-IL6Tg mice had minimal cuprizone-induced microglial changes, despite an already existing reactive microgliosis in control GFAP-IL6Tg not present in control WT mice.

CONCLUSIONS

Our results show that chronic transgenic production of IL-6 reduced cuprizone-induced cerebellar demyelination and induced a specific activation state of the resident microglia population (Iba1⁺, CD11b⁺, MHCII⁺, CD68^-), likely rendering them less responsive to subsequent injury signals.

Modulating inflammation and neuroprotection in multiple sclerosis

Multiple sclerosis (MS) is a neurological disorder of the central nervous system with a presentation and disease course that is largely unpredictable. MS can cause loss of balance, impaired vision or speech, weakness and paralysis, fatigue, depression, and cognitive impairment. Immunomodulation is a major target given the appearance of focal demyelinating lesions in myelin-rich white matter, yet progression and an increasing appreciation for gray matter involvement, even during the earliest phases of the disease, highlights the need to afford neuroprotection and limit neurodegenerative processes that correlate with disability. This review summarizes key aspects of MS pathophysiology and histopathology with a focus on neuroimmune interactions in MS, which may facilitate neurodegeneration through both direct and indirect mechanisms. There is a focus on processes thought to influence disease progression and the role of oxidative stress and mitochondrial dysfunction in MS. The goals and efficacy of current disease-modifying therapies and those in the pipeline are discussed, highlighting recent advances in our understanding of pathways mediating disease progression to identify and translate both immunomodulatory and neuroprotective therapeutics from the bench to the clinic.

Acid sphingomyelinase deficiency enhances myelin repair after acute and chronic demyelination

The cuprizone animal model, also known as the toxic demyelination model, is a well-reproducible model of demyelination- and remyelination in mice, and has been useful in studying important aspect of human demyelinating diseases, including multiple sclerosis. In this study, we investigated the role of acid sphingomyelinase in demyelination and myelin repair by inducing acute and chronic demyelination with 5- or 12-week cuprizone treatment, followed by a 2-week cuprizone withdrawal phase to allow myelin repair. Sphingolipids, in particular ceramide and the enzyme acid sphingomyelinase, which generates ceramide from sphingomyelin, seem to be involved in astrocyte activation and neuronal damage in multiple sclerosis. We used immunohistochemistry to study glial reaction and oligodendrocyte distribution in acid sphingomyelinase deficient mice and wild-type C57BL/6J littermates at various time intervals after demyelination and remyelination. Axonal injury was quantified using amyloid precursor protein and synaptophysin, and gene expression and protein levels were measured using gene analysis and Western blotting, respectively. Our results show that mice lacking acid sphingomyelinase had a significant increase in myelin recovery and a significantly higher oligodendrocyte cell count after 2 weeks remyelination compared to wild-type littermates. Detrimental astroglial distribution was also significantly reduced in acid sphingomyelinase deficient animals. We obtained similar results in experiments using amitriptyline to inhibit acid sphingomyelinase. These findings suggest that acid sphingomyelinase plays a significant role in myelin repair, and its inhibition by amitriptyline may constitute a novel therapeutic approach for multiple sclerosis patients.

Expression profiles of cholesterol metabolism-related genes are altered during development of experimental autoimmune encephalomyelitis in the rat spinal cord

Increased evidence suggests that dysregulation of cholesterol metabolism may be a key event contributing to progression of multiple sclerosis (MS). Using an experimental autoimmune encephalomyelitis (EAE) model of MS we revealed specific changes in the mRNA and protein expression of key molecules involved in the maintaining of cholesterol homeostasis in the rat spinal cord: 3-hydroxy-3-methylglutaryl-coenzyme-A reductase (HMGCR), apolipoprotein E (ApoE) and cholesterol 24-hydroxylase (CYP46A1) during the course of disease. The presence of myelin lipid debris was seen only at the peak of EAE in demyelination loci being efficiently removed during the recovery period. Since CYP46A1 is responsible for removal of cholesterol excess, we performed a detailed profiling of CYP46A1 expression and revealed regional and temporal specificities in its distribution. Double immunofluorescence staining demonstrated CYP46A1 localization with neurons, infiltrated macrophages, microglia and astrocytes in the areas of demyelination, suggesting that these cells play a role in cholesterol turnover in EAE. We propose that alterations in the regulation of cholesterol metabolism at the onset and peak of EAE may add to the progression of disease, while during the recovery period may have beneficial effects contributing to the regeneration of myelin sheath and restoration of neuronal function.

Acutely damaged axons are remyelinated in multiple sclerosis and experimental models of demyelination

Remyelination is in the center of new therapies for the treatment of multiple sclerosis to resolve and improve disease symptoms and protect axons from further damage. Although remyelination is considered beneficial in the long term, it is not known, whether this is also the case early in lesion formation. Additionally, the precise timing of acute axonal damage and remyelination has not been assessed so far. To shed light onto the interrelation between axons and the myelin sheath during de- and remyelination, we employed cuprizone- and focal lysolecithin-induced demyelination and performed time course experiments assessing the evolution of early and late stage remyelination and axonal damage. We observed damaged axons with signs of remyelination after cuprizone diet cessation and lysolecithin injection. Similar observations were made in early multiple sclerosis lesions. To assess the correlation of remyelination and axonal damage in multiple sclerosis lesions, we took advantage of a cohort of patients with early and late stage remyelinated lesions and assessed the number of APP- and SMI32- positive damaged axons and the density of SMI31-positive and silver impregnated preserved axons. Early de- and remyelinating lesions did not differ with respect to axonal density and axonal damage, but we observed a lower axonal density in late stage demyelinated multiple sclerosis lesions than in remyelinated multiple sclerosis lesions. Our findings suggest that remyelination may not only be protective over a long period of time, but may play an important role in the immediate axonal recuperation after a demyelinating insult.

Formyl Peptide Receptor 1-Mediated Glial Cell Activation in a Mouse Model of Cuprizone-Induced Demyelination

Multiple sclerosis (MS) is a chronic degenerative disease of the central nervous system that is characterized by myelin abnormalities, oligodendrocyte pathology, and concomitant glia activation. Unclear are the factors triggering gliosis and demyelination. New findings suggest an important role of the innate immune response in the initiation and progression of active demyelinating lesions. The innate immune response is induced by pathogen-associated or danger-associated molecular patterns, which are identified by pattern recognition receptors (PRRs), including the G-protein coupled with formyl peptide receptors (FPRs). Glial cells, the immune cells of the central nervous system, also express the PRRs. In this study, we used the cuprizone mice model to investigate the expression of the FPR1 in the course of cuprizone-induced demyelination In addition, we used FPR1-deficient mice to analyze glial cell activation through immunohistochemistry and real-time RT-PCR in cuprizone model. Our results revealed a significantly increased expression of FPR1 in the cortex of cuprizone-treated mice. FPR1-deficient mice showed a slight but significant decrease of demyelination in the corpus callosum compared to the wild-type mice. Furthermore, FPR1 deficiency resulted in reduced glial cell activation and mRNA expression of microglia/macrophages markers, as well as pro- and anti-inflammatory cytokines in the cortex, compared to wild-type mice after cuprizone-induced demyelination. Combined together, these results suggest that the FPR1 is an important part of the innate immune response in the course of cuprizone-induced demyelination.

Phosphorylation of αB-crystallin supports reactive astrogliosis in demyelination

The small heat shock protein αB-crystallin (CRYAB) has been implicated in multiple sclerosis (MS) pathogenesis. Earlier studies have indicated that CRYAB inhibits inflammation and attenuates clinical disease when administered in the experimental autoimmune encephalomyelitis model of MS. In this study, we evaluated the role of CRYAB in primary demyelinating events. Using the cuprizone model of demyelination, a noninflammatory model that allows the analysis of glial responses in MS, we show that endogenous CRYAB expression is associated with increased severity of demyelination. Moreover, we demonstrate a strong correlation between the expression of CRYAB and the extent of reactive astrogliosis in demyelinating areas and in in vitro assays. In addition, we reveal that CRYAB is differentially phosphorylated in astrocytes in active demyelinating MS lesions, as well as in cuprizone-induced lesions, and that this phosphorylation is required for the reactive astrocyte response associated with demyelination. Furthermore, taking a proteomics approach to identify proteins that are bound by the phosphorylated forms of CRYAB in primary cultured astrocytes, we show that there is clear differential binding of protein targets due to the specific phosphorylation of CRYAB. Subsequent Ingenuity Pathway Analysis of these targets reveals implications for intracellular pathways and biological processes that could be affected by these modifications. Together, these findings demonstrate that astrocytes play a pivotal role in demyelination, making them a potential target for therapeutic intervention, and that phosphorylation of CRYAB is a key factor supporting the pathogenic response of astrocytes to oligodendrocyte injury.

Glial Cells and Their Function in the Adult Brain: A Journey through the History of Their Ablation

Glial cells, consisting of microglia, astrocytes, and oligodendrocyte lineage cells as their major components, constitute a large fraction of the mammalian brain. Originally considered as purely non-functional glue for neurons, decades of research have highlighted the importance as well as further functions of glial cells. Although many aspects of these cells are well characterized nowadays, the functions of the different glial populations in the brain under both physiological and pathological conditions remain, at least to a certain extent, unresolved. To tackle these important questions, a broad range of depletion approaches have been developed in which microglia, astrocytes, or oligodendrocyte lineage cells (i.e., NG2-glia and oligodendrocytes) are specifically ablated from the adult brain network with a subsequent analysis of the consequences. As the different glial populations are very heterogeneous, it is imperative to specifically ablate single cell populations instead of inducing cell death in all glial cells in general. Thanks to modern genetic manipulation methods, the approaches can now directly be targeted to the cell type of interest making the ablation more specific compared to general cell ablation approaches that have been used earlier on. In this review, we will give a detailed summary on different glial ablation studies, focusing on the adult mouse central nervous system and the functional readouts. We will also provide an outlook on how these approaches could be further exploited in the future.