The oligodendrocyte precursor cell in health and disease

NEP1-40 alleviates behavioral phenotypes and promote oligodendrocyte progenitor cell differentiation in the hippocampus of cuprizone-induced demyelination mouse model

BACKGROUND

Studies have demonstrated that the failure of oligodendrocyte precursor cells (OPCs) differentiation as a major cause of remyelination failure in demyelinating disease. The reasons for this failure are not completely understood. We hypothesized that the present of myelin debris in CNS play an important role in poor OPCs differentiation in the mouse model of demyelinating disease.

METHODS

Mice were fed by the food mixed with normal or 0.2 % cuprizone (CPZ) for 6 weeks. Then the learning and memory impairment were tested by Morris water maze test. The spontaneous alternation behavior and depression-like symptoms were assessed by tail suspension test and open filed test. The number of OPCs and oligodendrocytes were counted by immunofluorescence. After exposed to CPZ for 6 weeks, the mice were then receiving stereotactic injection of NEP1-40 into the CA3 of hippocampus. The behavioral, learning and memory changes were assessed by tail suspension test and open field test. The differentiation of OPCs were detected by immunofluorescence and western blot.

RESULTS

The mice in CPZ group are more likely to show signs of depression and they showed impairment of long-term learning and memory function. The differentiation of OPCs were impaired in CPZ group. We found that mice treated with NEP1-40 showed less depression-like symptom in TST and higher locomotor activity in the OFT than the mice treated with PBS.

CONCLUSIONS

Our study suggest that NEP1-40 can promote OPC differentiation and survival. Further study should focus on the effect of NEP1-40 on the differentiation and survival of OPCs in vitro.

Diffuse Axonal Injury in the Rat Brain: Axonal Injury and Oligodendrocyte Activity Following Rotational Injury

Traumatic brain injury (TBI) commonly results in primary diffuse axonal injury (DAI) and associated secondary injuries that evolve through a cascade of pathological mechanisms. We aim at assessing how myelin and oligodendrocytes react to head angular-acceleration-induced TBI in a previously described model. This model induces axonal injuries visible by amyloid precursor protein (APP) expression, predominantly in the corpus callosum and its borders. Brain tissue from a total of 27 adult rats was collected at 24 h, 72 h and 7 d post-injury. Coronal sections were prepared for immunohistochemistry and RNAscope® to investigate DAI and myelin changes (APP, MBP, Rip), oligodendrocyte lineage cell loss (Olig2), oligodendrocyte progenitor cells (OPCs) (NG2, PDGFRa) and neuronal stress (HSP70, ATF3). Oligodendrocytes and OPCs numbers (expressed as percentage of positive cells out of total number of cells) were measured in areas with high APP expression. Results showed non-statistically significant trends with a decrease in oligodendrocyte lineage cells and an increase in OPCs. Levels of myelination were mostly unaltered, although Rip expression differed significantly between sham and injured animals in the frontal brain. Neuronal stress markers were induced at the dorsal cortex and habenular nuclei. We conclude that rotational injury induces DAI and neuronal stress in specific areas. We noticed indications of oligodendrocyte death and regeneration without statistically significant changes at the timepoints measured, despite indications of axonal injuries and neuronal stress. This might suggest that oligodendrocytes are robust enough to withstand this kind of trauma, knowledge important for the understanding of thresholds for cell injury and post-traumatic recovery potential.

Adenosine A2B receptors inhibit K+ currents and cell differentiation in cultured oligodendrocyte precursor cells and modulate sphingosine-1-phosphate signaling pathway

Oligodendrocytes are the only myelinating cells in the brain and differentiate from their progenitors (OPCs) throughout adult life. However, this process fails in demyelinating pathologies. Adenosine is emerging as an important player in OPC differentiation and we recently demonstrated that adenosine A_2A receptors inhibit cell maturation by reducing voltage-dependent K⁺ currents. No data are available to date about the A_2B receptor (A_2BR) subtype. The bioactive lipid mediator sphingosine-1-phosphate (S1P) and its receptors (S1P_1-5) are also crucial modulators of OPC development. An interaction between this pathway and the A_2BR is reported in peripheral cells. We studied the role of A_2BRs in modulating K⁺ currents and cell differentiation in OPC cultures and we investigated a possible interplay with S1P signaling. Our data indicate that the A_2BR agonist BAY60-6583 and its new analogue P453 inhibit K⁺ currents in cultured OPC and the effect was prevented by the A_2BR antagonist MRS1706, by K⁺ channel blockers and was differently modulated by the S1P analogue FTY720-P. An acute (10 min) exposure of OPCs to BAY60-6583 also increased the phosphorylated form of sphingosine kinase 1 (SphK1). A chronic (7 days) treatment with the same agonist decreased OPC differentiation whereas SphK1/2 inhibition exerted the opposite effect. Furthermore, A_2BR was overexpressed during OPC differentiation, an effect prevented by the pan SphK1/2 inhibitor VPC69047. Finally, A_2BR silenced cells showed increased cell maturation, decreased SphK1 expression and enhanced S1P lyase levels. We conclude that A_2BRs inhibit K⁺ currents and cell differentiation and positively modulate S1P synthesis in cultured OPCs.

Pericyte-Mediated Tissue Repair through PDGFRβ Promotes Peri-Infarct Astrogliosis, Oligodendrogenesis, and Functional Recovery after Acute Ischemic Stroke

Post-stroke functional recovery can occur spontaneously during the subacute phase; however, how post-stroke fibrotic repair affects functional recovery is highly debated. Platelet-derived growth factor receptor β (PDGFRβ)-expressing pericytes are responsible for post-stroke fibrotic repair within infarct areas; therefore, we examined peri-infarct neural reorganization and functional recovery after permanent middle cerebral artery occlusion (pMCAO) using pericyte-deficient Pdgfrb^+/- mice. Time-dependent reduction of infarct area sizes, i.e., repair, was significantly impaired in Pdgfrb^+/- mice with recovery of cerebral blood flow (CBF) in ischemic areas attenuated by defective leptomeningeal arteriogenesis and intrainfarct angiogenesis. Peri-infarct astrogliosis, accompanied by increased STAT3 phosphorylation, was attenuated in Pdgfrb^+/- mice. Pericyte-conditioned medium (PCM), particularly when treated with platelet-derived growth factor subunit B (PDGFB) homodimer (PDGF-BB; PCM/PDGF-BB), activated STAT3 and enhanced the proliferation and activity of cultured astrocytes. Although peri-infarct proliferation of oligodendrocyte (OL) precursor cells (OPCs) was induced promptly after pMCAO regardless of intrainfarct repair, OPC differentiation and remyelination were significantly attenuated in Pdgfrb^+/- mice. Consistently, astrocyte-CM (ACM) promoted OPC differentiation and myelination, which were enhanced remarkably by adding PCM/PDGF-BB to the medium. Post-stroke functional recovery correlated well with the extent and process of intrainfarct repair and peri-infarct oligodendrogenesis. Overall, pericyte-mediated intrainfarct fibrotic repair through PDGFRβ may promote functional recovery through enhancement of peri-infarct oligodendrogenesis as well as astrogliosis after acute ischemic stroke.

Spatiotemporal Profile and Morphological Changes of NG2 Glia in the CA1 Region of the Rat Hippocampus after Transient Forebrain Ischemia

Neuron-glial antigen-2 (NG2) glia undergo proliferation and morphological changes following brain insults. Here, we show that NG2 glia is activated in a characteristic time- and layer-specific manner in the ischemia-vulnerable CA1 region of the rat hippocampus. Resting NG2 glia of the pyramidal cell layer (somatic region) shared morphological features with those of the neighboring dendritic stratum radiatum. During the postischemic period, reactive NG2 glia of the pyramidal cell layer exhibited shortened, scarcely branched processes, while those of the stratum radiatum had multiple branching processes with their arborization being almost indiscernible 7~14 days after reperfusion. Immunoelectron microscopy demonstrated that NG2 immunoreactivity was specifically associated with the plasma membrane and the adjacent extracellular matrix of NG2 glia in the stratum radiatum at 14 days. NG2 glia also exhibited differences in their numbers and proliferation profiles in the two examined hippocampal strata after ischemia. In addition, induced NG2 expression in activated microglia/macrophages exhibited a characteristic strata-dependent pattern in the ischemic CA1 hippocampus. NG2 induction was prominent in macrophage-like phenotypes which were predominantly localized in the pyramidal cell layer, compared with activated stellate microglial cells in the stratum radiatum. Thus, our data demonstrate that activation of NG2 glia and the induction of NG2 expression in activated microglia/macrophages occur in a distinct time- and layer-specific manner in the ischemic CA1 hippocampus. These characteristic profiles of reactive NG2 glia could be secondary to the degeneration processes occurring in the cell bodies or dendritic domains of hippocampal CA1 pyramidal neurons after ischemic insults.

Insights into the pathogenesis of multiple system atrophy: focus on glial cytoplasmic inclusions

Multiple system atrophy (MSA) is a debilitating and fatal neurodegenerative disorder. The disease severity warrants urgent development of disease-modifying therapy, but the disease pathogenesis is still enigmatic. Neurodegeneration in MSA brains is preceded by the emergence of glial cytoplasmic inclusions (GCIs), which are insoluble α-synuclein accumulations within oligodendrocytes (OLGs). Thus, preventive strategies against GCI formation may suppress disease progression. However, although numerous studies have tried to elucidate the molecular pathogenesis of GCI formation, difficulty remains in understanding the pathological interaction between the two pivotal aspects of GCIs; α-synuclein and OLGs. The difficulty originates from several enigmas: 1) what triggers the initial generation and possible propagation of pathogenic α-synuclein species? 2) what contributes to OLG-specific accumulation of α-synuclein, which is abundantly expressed in neurons but not in OLGs? and 3) how are OLGs and other glial cells affected and contribute to neurodegeneration? The primary pathogenesis of GCIs may involve myelin dysfunction and dyshomeostasis of the oligodendroglial cellular environment such as autophagy and iron metabolism. We have previously reported that oligodendrocyte precursor cells are more prone to develop intracellular inclusions in the presence of extracellular fibrillary α-synuclein. This finding implies a possibility that the propagation of GCI pathology in MSA brains is mediated through the internalization of pathological α-synuclein into oligodendrocyte precursor cells. In this review, in order to discuss the pathogenesis of GCIs, we will focus on the composition of neuronal and oligodendroglial inclusions in synucleinopathies. Furthermore, we will introduce some hypotheses on how α-synuclein pathology spreads among OLGs in MSA brains, in the light of our data from the experiments with primary oligodendrocyte lineage cell culture. While various reports have focused on the mysterious source of α-synuclein in GCIs, insights into the mechanism which regulates the uptake of pathological α-synuclein into oligodendroglial cells may yield the development of the disease-modifying therapy for MSA. The interaction between glial cells and α-synuclein is also highlighted with previous studies of post-mortem human brains, cultured cells, and animal models, which provide comprehensive insight into GCIs and the MSA pathomechanisms.

Oligodendrocyte precursor cells transplantation protects blood-brain barrier in a mouse model of brain ischemia via Wnt/β-catenin signaling

Blood–brain barrier damage is a critical pathological feature of ischemic stroke. Oligodendrocyte precursor cells are involved in maintaining blood–brain barrier integrity during the development. However, whether oligodendrocyte precursor cell could sustain blood–brain barrier permeability during ischemic brain injury is unknown. Here, we investigate whether oligodendrocyte precursor cell transplantation protects blood–brain barrier integrity and promotes ischemic stroke recovery. Adult male ICR mice (n = 68) underwent 90 min transient middle cerebral artery occlusion. After ischemic assault, these mice received stereotactic injection of oligodendrocyte precursor cells (6 × 10⁵). Oligodendrocyte precursor cells transplantation alleviated edema and infarct volume, and promoted neurological recovery after ischemic stroke. Oligodendrocyte precursor cells reduced blood–brain barrier leakage via increasing claudin-5, occludin and β-catenin expression. Administration of β-catenin inhibitor blocked the beneficial effects of oligodendrocyte precursor cells. Wnt7a protein treatment increased β-catenin and claudin-5 expression in endothelial cells after oxygen–glucose deprivation, which was similar to the results of the conditioned medium treatment of oligodendrocyte precursor cells on endothelial cells. We demonstrated that oligodendrocyte precursor cells transplantation protected blood–brain barrier in the acute phase of ischemic stroke via activating Wnt/β-catenin pathway. Our results indicated that oligodendrocyte precursor cells transplantation was a novel approach to the ischemic stroke therapy.

Paired Related Homeobox Protein 1 Regulates Quiescence in Human Oligodendrocyte Progenitors

Human oligodendrocyte progenitor cells (hOPCs) persist into adulthood as an abundant precursor population capable of division and differentiation. The transcriptional mechanisms that regulate hOPC homeostasis remain poorly defined. Herein, we identify paired related homeobox protein 1 (PRRX1) in primary PDGFαR⁺ hOPCs. We show that enforced PRRX1 expression results in reversible G_1/0 arrest. While both PRRX1 splice variants reduce hOPC proliferation, only PRRX1a abrogates migration. hOPC engraftment into hypomyelinated shiverer/rag2 mouse brain is severely impaired by PRRX1a, characterized by reduced cell proliferation and migration. PRRX1 induces a gene expression signature characteristic of stem cell quiescence. Both IFN-γ and BMP signaling upregulate PRRX1 and induce quiescence. PRRX1 knockdown modulates IFN-γ-induced quiescence. In mouse brain, PRRX1 mRNA was detected in non-dividing OPCs and is upregulated in OPCs following demyelination. Together, these data identify PRRX1 as a regulator of quiescence in hOPCs and as a potential regulator of pathological quiescence.

Donepezil, a drug for Alzheimer's disease, promotes oligodendrocyte generation and remyelination

Myelin sheaths play important roles in neuronal functions. In the central nervous system (CNS), the myelin is formed by oligodendrocytes (OLs), which are differentiated from oligodendrocyte precursor cells (OPCs). In CNS demyelinating disorders such as multiple sclerosis (MS), the myelin sheaths are damaged and the remyelination process is hindered. Small molecule drugs that promote OPC to OL differentiation and remyelination may provide a new way to treat these demyelinating diseases. Here we report that donepezil, an acetylcholinesterase inhibitor (AChEI) developed for the treatment of Alzheimer's disease (AD), significantly promotes OPC to OL differentiation. Interestingly, other AChEIs, including huperzine A, rivastigmine, and tacrine, have no such effect, indicating that donepezil's effect in promoting OPC differentiation is not dependent on the inhibition of AChE. Donepezil also facilitates the formation of myelin sheaths in OPC-DRG neuron co-culture. More interestingly, donepezil also promotes the repair of the myelin sheaths in vivo and provides significant therapeutic effect in a cuprizone-mediated demyelination animal model. Donepezil is a drug that has been used to treat AD safely for many years; our findings suggest that it might be repurposed to treat CNS demyelinating diseases such as MS by promoting OPC to OL differentiation and remyelination.

Gintonin Enhances Proliferation, Late Stage Differentiation, and Cell Survival From Endoplasmic Reticulum Stress of Oligodendrocyte Lineage Cells

Although evidence on myelin diseases is steadily accumulating, effective preventive or therapeutic strategies against them have not been established so far. Ginseng is well known for its beneficial effects on health and diseases; however, detailed studies on ginseng’s effects on myelin-producing oligodendrocytes have not been performed yet. In this study, we investigated the function of gintonin—an active component of ginseng—on the proliferation, differentiation, and survival of oligodendrocyte lineage cells. We performed real-time percutaneous coronary intervention, Western blot, and immunocytochemistry on primary oligodendrocyte precursor cell cultures and in vitro myelinating co-cultures. Our results show that gintonin stimulates oligodendrocyte precursor cell proliferation. Gintonin’s effect was inhibited by Ki16425, an antagonist of lysophosphatidic acid 1/3 receptors. Interestingly, with regard to cell differentiation, gintonin facilitated late differentiation of oligodendrocyte development, but not early differentiation. Moreover, it showed protective effects on oligodendrocyte lineage cells against endoplasmic reticulum stress-induced cell death, potentially by modulating unfolded protein responses. Our results suggest that gintonin is a potential therapeutic candidate in the treatment of myelin diseases.

Scaffold-mediated sequential drug/gene delivery to promote nerve regeneration and remyelination following traumatic nerve injuries

Neural tissue regeneration following traumatic injuries is often subpar. As a result, the field of neural tissue engineering has evolved to find therapeutic interventions and has seen promising outcomes. However, robust nerve and myelin regeneration remain elusive. One possible reason may be the fact that tissue regeneration often follows a complex sequence of events in a temporally-controlled manner. Although several other fields of tissue engineering have begun to recognise the importance of delivering two or more biomolecules sequentially for more complete tissue regeneration, such serial delivery of biomolecules in neural tissue engineering remains limited. This review aims to highlight the need for sequential delivery to enhance nerve regeneration and remyelination after traumatic injuries in the central nervous system, using spinal cord injuries as an example. In addition, possible methods to attain temporally-controlled drug/gene delivery are also discussed for effective neural tissue regeneration.

Convergence of human cellular models and genetics to study neural stem cell signaling to enhance central nervous system regeneration and repair

Human central nervous system (CNS) regeneration is considered the holy grail of neuroscience research, and is one of the most pressing and difficult questions in biology and science. Despite more than 20 years of work in the field of neural stem cells (NSCs), the area remains in its infancy as our understanding of the fundamental mechanisms that can be leveraged to improve CNS regeneration in neurological diseases is still growing. Here, we focus on the recent lessons from lower organism CNS regeneration genetics and how such findings are starting to illuminate our understanding of NSC signaling pathways in humans. These findings will allow us to improve upon our knowledge of endogenous NSC function, the utility of exogenous NSCs, and the limitations of NSCs as therapeutic vehicles for providing relief from devastating human neurological diseases. We also discuss the limitations of activating NSC signaling for CNS repair in humans, especially the potential for tumor formation. Finally, we will review the recent advances in new culture techniques, including patient-derived cells and cerebral organoids to model the genetic regulation of signaling pathways controlling the function of NSCs during injury and disease states.

Agenesis and Hypomyelination of Corpus Callosum in Mice Lacking Nsun5, an RNA Methyltransferase

Williams-Beuren syndrome (WBS) is caused by microdeletions of 28 genes and is characterized by cognitive disorder and hypotrophic corpus callosum (CC). Nsun5 gene, which encodes cytosine-5 RNA methyltransferase, is located in the deletion loci of WBS. We have reported that single-gene knockout of Nsun5 (Nsun5-KO) in mice impairs spatial cognition. Herein, we report that postnatal day (PND) 60 Nsun5-KO mice showed the volumetric reduction of CC with a decline in the number of myelinated axons and loose myelin sheath. Nsun5 was highly expressed in callosal oligodendrocyte precursor cells (OPCs) and oligodendrocytes (OLs) from PND7 to PND28. The numbers of OPCs and OLs in CC of PND7-28 Nsun5-KO mice were significantly reduced compared to wild-type littermates. Immunohistochemistry and Western blot analyses of myelin basic protein (MBP) showed the hypomyelination in the CC of PND28 Nsun5-KO mice. The Nsun5 deletion suppressed the proliferation of OPCs but did not affect transition of radial glial cells into OPCs or cell cycle exit of OPCs. The protein levels, rather than transcriptional levels, of CDK1, CDK2 and Cdc42 in the CC of PND7 and PND14 Nsun5-KO mice were reduced. These findings point to the involvement of Nsun5 deletion in agenesis of CC observed in WBS.

Revealing Spatial and Temporal Patterns of Cell Death, Glial Proliferation, and Blood-Brain Barrier Dysfunction Around Implanted Intracortical Neural Interfaces

Improving the long-term performance of neural electrode interfaces requires overcoming severe biological reactions such as neuronal cell death, glial cell activation, and vascular damage in the presence of implanted intracortical devices. Past studies traditionally observe neurons, microglia, astrocytes, and blood-brain barrier (BBB) disruption around inserted microelectrode arrays. However, analysis of these factors alone yields poor correlation between tissue inflammation and device performance. Additionally, these studies often overlook significant biological responses that can occur during acute implantation injury. The current study employs additional histological markers that provide novel information about neglected tissue components-oligodendrocytes and their myelin structures, oligodendrocyte precursor cells, and BBB -associated pericytes-during the foreign body response to inserted devices at 1, 3, 7, and 28 days post-insertion. Our results reveal unique temporal and spatial patterns of neuronal and oligodendrocyte cell loss, axonal and myelin reorganization, glial cell reactivity, and pericyte deficiency both acutely and chronically around implanted devices. Furthermore, probing for immunohistochemical markers that highlight mechanisms of cell death or patterns of proliferation and differentiation have provided new insight into inflammatory tissue dynamics around implanted intracortical electrode arrays.

3,5-T2 and 3,3',5-T3 Regulate Cerebellar Thyroid Hormone Signalling and Myelin Molecular Dynamics in Tilapia

In contrast to mammalian adults, myelination in teleosts occurs throughout their lifespan and most of the progenitor cells are originated in the cerebellum. To understand the role that thyroid hormones (THs) play in juvenile cerebellar myelination in teleosts, we identified and localised the expression of genes involved in TH signalling (mct8, oatp1c1, dio2, dio3, thraa and l-thrb1) and analysed the effects of the two bioactive THs, T2 and T3, upon their regulation, as well as upon some structural components of the myelination process. Ex vivo approaches using organotypic cerebellar cultures followed by FISH and qPCR showed gene-specific localisation and regulation of TH signalling genes in the cerebellar nuclei. In vivo approaches using methimazole (MMI)-treated juvenile tilapias replaced with low doses of T3 and T2 showed by immunofluorescence that myelin fibres in the cerebellum are more abundant in the granular layer and that their visible size is reduced after MMI treatment but partially restored with TH replacement, suggesting that low doses of TH promote the re-myelination process in an altered condition. Together, our data support the idea that T2 and T3 promote myelination via different pathways and prompt T2 as a target for further analysis as a promising therapy for hypomyelination.

Uncovering the biology of myelin with optical imaging of the live brain

Myelin has traditionally been considered a static structure that is produced and assembled during early developmental stages. While this characterization is accurate in some contexts, recent studies have revealed that oligodendrocyte generation and patterns of myelination are dynamic and potentially modifiable throughout life. Unique structural and biochemical properties of the myelin sheath provide opportunities for the development and implementation of multimodal label-free and fluorescence optical imaging approaches. When combined with genetically encoded fluorescent tags targeted to distinct cells and subcellular structures, these techniques offer a powerful methodological toolbox for uncovering mechanisms of myelin generation and plasticity in the live brain. Here, we discuss recent advances in these approaches that have allowed the discovery of several forms of myelin plasticity in developing and adult nervous systems. Using these techniques, long-standing questions related to myelin generation, remodeling, and degeneration can now be addressed.

Intrinsic mutant HTT-mediated defects in oligodendroglia cause myelination deficits and behavioral abnormalities in Huntington disease

White matter abnormalities are a nearly universal pathological feature of neurodegenerative disorders including Huntington disease (HD). A long-held assumption is that this white matter pathology is simply a secondary outcome of the progressive neuronal loss that manifests with advancing disease. Using a mouse model of HD, here we show that white matter and myelination abnormalities are an early disease feature appearing before the manifestation of any behavioral abnormalities or neuronal loss. We further show that selective inactivation of mutant huntingtin (mHTT) in the NG2+ oligodendrocyte progenitor cell population prevented myelin abnormalities and certain behavioral deficits in HD mice. Strikingly, the improvements in behavioral outcomes were seen despite the continued expression of mHTT in nonoligodendroglial cells including neurons, astrocytes, and microglia. Using RNA-seq and ChIP-seq analyses, we implicate a pathogenic mechanism that involves enhancement of polycomb repressive complex 2 (PRC2) activity by mHTT in the intrinsic oligodendroglial dysfunction and myelination deficits observed in HD. Our findings challenge the long-held dogma regarding the etiology of white matter pathology in HD and highlight the contribution of epigenetic mechanisms to the observed intrinsic oligodendroglial dysfunction. Our results further suggest that ameliorating white matter pathology and oligodendroglial dysfunction may be beneficial for HD.

Oligodendrocyte precursor cells as a therapeutic target for demyelinating diseases

The mechanisms regulating differentiation of multipotent oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes (OLs) are critical to our understanding of myelination and remyelination. Following acute demyelination in the central nervous system, adult OPCs migrate to the injury site, differentiate into OLs and generate new myelin sheaths. A common feature of regenerative processes is the fact that remyelination efficiency declines with aging and, accounts for the observation that chronic demyelinating diseases like multiple sclerosis (MS) are characterized by an ineffective remyelination. Without doubt, impairment of OPC differentiation is an essential determinant of the aging effects in remyelination. However, spontaneous remyelination is limited in demyelinating diseases such as MS, owing in part to the failure of adult OPCs to differentiate into myelinating OLs. The inability to restore myelin after injury compromises axon integrity and renders them vulnerable to degeneration. Although the genes that regulate the proliferation and differentiation of OPCs during development have been intensively studied, relatively little is known about the molecular signals that regulate the function of adult OPCs after demyelination. Elucidating the mechanisms regulating OPC differentiation are key to identifying pharmacological targets for remyelination-enhancing therapy. This review will discuss OPC biology, myelination, and possible pharmacological targets for promoting the differentiation of OPCs as a strategy to enhance remyelination, including the potential for nanoscale delivery.

Differential roles of epigenetic regulators in the survival and differentiation of oligodendrocyte precursor cells

During development or after brain injury, oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes to supplement the number of oligodendrocytes. Although mechanisms of OPC differentiation have been extensively examined, the role of epigenetic regulators, such as histone deacetylases (HDACs) and DNA methyltransferase enzymes (DNMTs), in this process is still mostly unknown. Here, we report the differential roles of epigenetic regulators in OPC differentiation. We prepared primary OPC cultures from neonatal rat cortex. Our cultured OPCs expressed substantial amounts of mRNA for HDAC1, HDAC2, DNMT1, and DNMT3a. mRNA levels of HDAC1 and HDAC2 were both decreased by the time OPCs differentiated into myelin-basic-protein expressing oligodendrocytes. However, DNMT1 or DNMT3a mRNA level gradually decreased or increased during the differentiation step, respectively. We then knocked down those regulators in cultured OPCs with siRNA technique before starting OPC differentiation. While HDAC1 knockdown suppressed OPC differentiation, HDAC2 knockdown promoted OPC differentiation. DNMT1 knockdown also suppressed OPC differentiation, but unlike HDAC1/2, DNMT1-deficient cells showed cell damage during the later phase of OPC differentiation. On the other hand, when OPCs were transfected with siRNA for DNMT3a, the number of OPCs was decreased, indicating that DNMT3a may participate in OPC survival/proliferation. Taken together, these data demonstrate that each epigenetic regulator has different phase-specific roles in OPC survival and differentiation.

Studying Human Neurological Disorders Using Induced Pluripotent Stem Cells: From 2D Monolayer to 3D Organoid and Blood Brain Barrier Models

Neurological disorders have emerged as a predominant healthcare concern in recent years due to their severe consequences on quality of life and prevalence throughout the world. Understanding the underlying mechanisms of these diseases and the interactions between different brain cell types is essential for the development of new therapeutics. Induced pluripotent stem cells (iPSCs) are invaluable tools for neurological disease modeling, as they have unlimited self-renewal and differentiation capacity. Mounting evidence shows: (i) various brain cells can be generated from iPSCs in two-dimensional (2D) monolayer cultures; and (ii) further advances in 3D culture systems have led to the differentiation of iPSCs into organoids with multiple brain cell types and specific brain regions. These 3D organoids have gained widespread attention as in vitro tools to recapitulate complex features of the brain, and (iii) complex interactions between iPSC-derived brain cell types can recapitulate physiological and pathological conditions of blood-brain barrier (BBB). As iPSCs can be generated from diverse patient populations, researchers have effectively applied 2D, 3D, and BBB models to recapitulate genetically complex neurological disorders and reveal novel insights into molecular and genetic mechanisms of neurological disorders. In this review, we describe recent progress in the generation of 2D, 3D, and BBB models from iPSCs and further discuss their limitations, advantages, and future ventures. This review also covers the current status of applications of 2D, 3D, and BBB models in drug screening, precision medicine, and modeling a wide range of neurological diseases (e.g., neurodegenerative diseases, neurodevelopmental disorders, brain injury, and neuropsychiatric disorders). © 2019 American Physiological Society. Compr Physiol 9:565-611, 2019.

The Effects of Astrocyte and Oligodendrocyte Lineage Cell Interaction on White Matter Injury under Chronic Cerebral Hypoperfusion

Oligodendrocytes (OLGs) differentiate from oligodendrocyte-precursor-cells (OPCs) for myelination in white matter. This differentiation is maintained by cell-cell interactions through trophic factors such as brain-derived-neurotrophic-factor (BDNF). However, differentiation is impaired when white matter injury occurs in a chronic cerebral hypoperfusion model. Thus, we examined the effects of the interaction between astrocyte and oligodendrocyte lineage cells on myelination regarding the mechanism of impairment. A microcoil was applied to the bilateral common carotid arteries in male C57BL/6 mice as an in vivo cerebral chronic hypoperfusion model (BCAS model). A nonlethal concentration of CoCl2 was added to the primary cell culture from the postnatal rat cortex and incubated in vitro. White matter injury progressed in the BCAS model as myelin decreased. The numbers of OPCs and astrocytes increased after the operation, whereas that of OLGs decreased at day 28. BDNF continuously decreased until day 28. Differentiation was disrupted under the stressed conditions in the cell culture, but improved after administration of astrocyte-conditioned medium containing BDNF. Astrocytes with BDNF underwent differentiation, but differentiation was impaired under the stressed conditions due to the reduction of BDNF. We examined S100B regarding the mechanism of impairment. S100B is mainly expressed by mature astrocytes, and has neuroprotective and neurotoxic effects inside and outside of cells. GFAP-positive astrocytes increased in the corpus callosum in the BCAS model, whereas the number of mature astrocytes continued to decrease, resulting in reduced BDNF. The reduction in mature astrocytes due to the discharge of S100B in ischemic conditions caused the reduction in BDNF.

Lesion Dynamics Under Varying Paracrine PDGF Signaling in Brain Tissue

Paracrine PDGF signaling is involved in many processes in the body, both normal and pathological, including embryonic development, angiogenesis, and wound healing as well as liver fibrosis, atherosclerosis, and cancers. We explored this seemingly dual (normal and pathological) role of PDGF mathematically by modeling the release of PDGF in brain tissue and then varying the dynamics of this release. Resulting simulations show that by varying the dynamics of a PDGF source, our model predicts three possible outcomes for PDGF-driven cellular recruitment and lesion growth: (1) localized, short duration of growth, (2) localized, chronic growth, and (3) widespread chronic growth. Further, our model predicts that the type of response is much more sensitive to the duration of PDGF exposure than the maximum level of that exposure. This suggests that extended duration of paracrine PDGF signal during otherwise normal processes could potentially lead to lesions having a phenotype consistent with pathologic conditions.

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.

Isolation and Culture of Oligodendrocytes

Primary cultures of brain-derived rodent cells are widely used to study molecular and cellular mechanisms in neurobiology. In this chapter, we describe methods of purifying and culturing oligodendroglial cells from mouse perinatal brains. In addition, we describe methods of coculturing the purified oligodendrocytes with neurons. When prepared and cultured according to these protocols, many essential aspects of the biology of oligodendrocytes, such as their proliferation, differentiation, and myelination, can be studied in culture.

Pericytes in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory demyelinating disease that affects the central nervous system (CNS), particularly, in young adults. Current MS treatments aim to reduce demyelination; however, these have limited efficacy, display side effects and lack of regenerative activities. Oligodendrocyte progenitor cells (OPCs) represents the major source for new myelin. Upon demyelination, OPCs get activated, proliferate, migrate towards the lesion, and differentiate into remyelinating oligodendrocytes. Although myelin repair (remyelination) represents a robust response to myelin damage, during MS, this regenerative phenomenon decays in efficiency or even fails. CNS-resident pericytes (CNS-PCs) are essential for vascular homeostasis regulating blood-brain barrier (BBB) permeability and stability as well as endothelial cells (ECs) function during angiogenesis and neovascularization. Recent studies indicate that CNS-PCs also play a crucial role regulating OPC function during remyelination, and very importantly, these cells are substantially affected in MS. This chapter summarizes important aspects of MS and CNS remyelination as well as it provides new insights supporting the contribution of CNS-PCs to myelin regeneration and to MS pathology. Currently, there is evidence arguing in favor of CNS-PCs as novel therapeutic targets for the development of future treatments for MS.

Progesterone effects on oligodendrocyte differentiation in injured spinal cord

Spinal cord lesions result in chronic demyelination as a consequence of secondary injury. Although oligodendrocyte precursor cells proliferate the differentiation program fails. Successful differentiation implies progressive decrease of transcriptional inhibitors followed by upregulation of activators. Progesterone emerges as an anti-inflammatory and pro-myelinating agent which improves locomotor outcome after spinal cord injury. In this study, we have demonstrated that spinal cord injury enhanced oligodendrocyte precursor cell number and decreased mRNA expression of transcriptional inhibitors (Id2, Id4, hes5). However, mRNA expression of transcriptional activators (Olig2, Nkx2.2, Sox10 and Mash1) was down-regulated 3 days post injury. Interestingly, a differentiation factor such as progesterone increased transcriptional activator mRNA levels and the density of Olig2- expressing oligodendrocyte precursor cells. The differentiation program is regulated by extracellular signals which modify transcriptional factors and epigenetic players. As TGFβ1 is a known oligodendrocyte differentiation factor which is regulated by progesterone in reproductive tissues, we assessed whether TGFβ1 could mediate progesterone remyelinating actions after the lesion. Notwithstanding that astrocyte, oligodendrocyte precursor and microglial cell density increased after spinal cord injury, the number of these cells which expressed TGFβ1 remained unchanged regarding sham operated rats. However, progesterone treatment increased TGFβ1 mRNA expression and the number of astrocytes and microglial TGFβ1 expressing cells which would indirectly enhance oligodendrocyte differentiation. Therefore, TGFβ1 arises as a potential mediator of progesterone differentiating effects on oligodendrocyte linage.

S100A8/A9 induces microglia activation and promotes the apoptosis of oligodendrocyte precursor cells by activating the NF-κB signaling pathway

S100A8/A9, a heterodimer complex composed of calcium-binding proteins S100A8 and S100A9, is significantly increased in the serum of multiple sclerosis (MS) patients. Relevant reports have revealed that MS pathology is commonly associated with the activation of microglial cells and the damage of oligodendrocyte precursor cells (OPCs). Moreover, microglia activation following stimulation increases the expression of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), which further exacerbate the damage to OPCs. In this study, we were the first to confirm that S100A8/A9 treatment induced the activation, proliferation and migration of the murine microglia cell line BV-2; moreover, this treatment caused the cells to switch from an anti-inflammatory activated (M2) phenotype to a pro-inflammatory activated (M1) phenotype. Meanwhile, the level of the phosphorylated nuclear factor-κB (p-NF-κB) P65 protein was remarkably elevated, and the production of pro-inflammatory factors (IL-1β, TNF-α, MMP-9) and chemokines (CCL2, CCL3, CXCL10) was also increased in the S100A8/A9-treated BV-2 microglial cells. Inhibition of NF-κB P65 phosphorylation reversed the effects of S100A8/A9 on the production of pro-inflammatory factors and chemokines. We also explored the effects of S100A8/A9 and S100A8/A9-activated BV-2 microglial cells on the viability of OPCs. The results showed that both the S100A8/A9 complex and the conditioned medium (CM) of the S100A8/A9-activated BV-2 microglial cells resulted in OPC apoptosis, which was more pronounced in the case of the CM treatment. However, OPC apoptosis in the CM group was obviously decreased through the inhibition of NF-κB p65 phosphorylation. This study indicates that S100A8/A9 induces the activation of BV-2 microglial cells and promotes the production of pro-inflammatory factors by activating the NF-κB signaling pathway, which further exacerbates OPC damage.

Oligodendrocytes: Development, Physiology and Glucose Metabolism

The glutamate-glutamine cycle is an outstanding example of how essential neuronal-glial interactions are for brain function. For several decades, this and other metabolic cycles in the brain have only included neurons and astrocytes but not oligodendrocytes, the myelinating cells of the central nervous system (CNS). Recent data revealed that oligodendrocytes are highly metabolically active cells in the brain and, therefore, should not be ignored. Using ¹³C-labelled glucose in combination with nuclear magnetic resonance spectroscopy (MRS) and/or mass spectrometry (MS) it is possible to characterize metabolic functions in primary oligodendrocyte cultures. Mature rat oligodendrocytes avidly metabolize glucose in the cytosol and pyruvate derived from glucose in mitochondria. Moreover, they seem to have the ability of performing anaplerosis from pyruvate, which might enable them to synthesize metabolites de novo and transfer them to neighbouring cells. All these original findings highlight the importance of investigating oligodendrocyte metabolism separately from that of astrocytes and neurons to be able to discern the roles played by the individual partners. This is of particular importance in the white matter where the number of oligodendrocytes is considerable. The present book chapter provides some background on oligodendrocyte biology and physiology and summarizes the not very extensive information published on glucose metabolism in oligodendrocytes.

Kir4.1 channels in NG2-glia play a role in development, potassium signaling, and ischemia-related myelin loss

The contribution of the inwardly rectifying K⁺ channel subtype Kir4.1 has been focused mainly on astrocytes, where they play important roles in the maintenance of resting membrane potential, extracellular K⁺ uptake, and facilitation of glutamate uptake in the central nervous system. Here, we report the role of Kir4.1 channels in NG2-glia during brain development, potassium signaling, and in an ischemic stroke disease model. Kir4.1 channels are widely expressed in NG2-glia during brain development. In the adult mouse hippocampus, Kir4.1 channels in NG2-glia constitute more than 80% of K⁺ channels inward currents. This large portion of Kir4.1 channel currents exhibits a deficit in NG2-glia as an initial response in a transient ischemic mouse model. Further evidence indicates that Kir4.1 deficits in NG2-glia potentially cause axonal myelin loss in ischemia through the association with oligodendrocyte-specific protein (OSP/Claudin-11), which unravels a potential therapeutic target in the treatment of ischemic stroke.

Feier Song and colleagues have examined Kir4.1 channels in the mouse brain, and found global expression of functional channels during development. They also show that depletion of Kir4.1 channels impacts demyelination in ischemic stroke

Spinal Cord Stem Cells In Their Microenvironment: The Ependyma as a Stem Cell Niche

The ependyma of the spinal cord is currently proposed as a latent neural stem cell niche. This chapter discusses recent knowledge on the developmental origin and nature of the heterogeneous population of cells that compose this stem cell microenviroment, their diverse physiological properties and regulation. The chapter also reviews relevant data on the ependymal cells as a source of plasticity for spinal cord repair.

Therapeutic Strategies for Leukodystrophic Disorders Resulting from Perinatal Asphyxia: Focus on Myelinating Oligodendrocytes

Perinatal asphyxia results from the action of different risk factors like complications during pregnancy, preterm delivery, or long and difficult labor. Nowadays, it is still the leading cause of neonatal brain injury known as hypoxic-ischemic encephalopathy (HIE) and resulting neurological disorders. A temporal limitation of oxygen, glucose, and trophic factors supply results in alteration of neural cell differentiation and functioning and/or leads to their death. Among the affected cells are oligodendrocytes, responsible for myelinating the central nervous system (CNS) and formation of white matter. Therefore, one of the major consequences of the experienced HIE is leukodystrophic diseases resulting from oligodendrocyte deficiency or malfunctioning. The therapeutic strategies applied after perinatal asphyxia are aimed at reducing brain damage and promoting the endogenous neuroreparative mechanisms. In this review, we focus on the biology of oligodendrocytes and discuss present clinical treatments in the context of their efficiency in preserving white matter structure and preventing cognitive and behavioral deficits after perinatal asphyxia.

Morphological characterization of NG2 glia and their association with neuroglial cells in the 3-nitropropionic acid-lesioned striatum of rat

Our aim was to examine the spatiotemporal profiles and phenotypic characteristics of neuron-glia antigen 2 (NG2) glia and their associations with neuroglial cells in striatal lesions due to the mitochondrial toxin 3-nitropropionic acid (3-NP). In control striatum, weak NG2 immunoreactivity was restricted to resting NG2 glia with thin processes, but prominent NG2 expression was noted on activated microglia/macrophages, and reactive NG2 glia in the lesion core after 3-NP injection. Activation of NG2 glia, including enhanced proliferation and morphological changes, had a close spatiotemporal relationship with infiltration of activated microglia into the lesion core. Thick and highly branched processes of reactive NG2 glia formed a cellular network in the astrocyte-free lesion core and primarily surrounded developing cavities 2–4 weeks post-lesion. NG2 glia became associated with astrocytes in the lesion core and the border of cavities over the chronic interval of 4–8 weeks. Immunoelectron microscopy indicated that reactive NG2 glia had large euchromatic nuclei with prominent nucleoli and thick and branched processes that ramified distally. Thus, our data provide detailed information regarding the morphologies of NG2 glia in the lesion core, and support the link between transformation of NG2 glia to the reactive form and microglial activation/recruitment in response to brain insults.

Using biomaterials to promote pro-regenerative glial phenotypes after nervous system injuries

Trauma to either the central or peripheral nervous system (PNS) often leads to significant loss of function and disability in patients. This high rate of long-term disability is due to the overall limited regenerative potential of nervous tissue, even though the PNS has more regenerative potential than the central nervous system (CNS). The supporting glial cells in the periphery, Schwann cells, are part of the reason for the improved recovery observed in the PNS. In the CNS, the glial populations, astrocytes and oligodendrocytes (OLs), do not have as much potential to promote regeneration and are at times inhibitory to neuronal growth. In particular, the inhibitory roles astrocytes play following trauma has led to a historical focus on neurons and OLs instead of astrocytes. Recently, this focus has shifted as new, regenerative astrocyte phenotypes have been described. From these observations, glial cells clearly play critical roles in native recovery pathways in both the CNS and PNS. This makes the ability to manipulate both transplanted and native glial cell phenotypes a potentially successful strategy to improve nerve injury outcomes. This review focuses on factors that cause glial cells to adopt repair phenotypes and biomaterials that manipulate and/or harness these glial phenotypes.

Why monkeys do not get multiple sclerosis (spontaneously): An evolutionary approach

The goal of this review is to apply an evolutionary lens to understanding the origins of multiple sclerosis (MS), integrating three broad observations. First, only humans are known to develop MS spontaneously. Second, humans have evolved large brains, with characteristically large amounts of metabolically costly myelin. This myelin is generated over long periods of neurologic development—and peak MS onset coincides with the end of myelination. Third, over the past century there has been a disproportionate increase in the rate of MS in young women of childbearing age, paralleling increasing westernization and urbanization, indicating sexually specific susceptibility in response to changing exposures. From these three observations about MS, a life history approach leads us to hypothesize that MS arises in humans from disruption of the normal homeostatic mechanisms of myelin production and maintenance, during our uniquely long myelination period. This review will highlight under-explored areas of homeostasis in brain development, that are likely to shed new light on the origins of MS and to raise further questions about the interactions between our ancestral genes and modern environments.

Pathological Endogenous α-Synuclein Accumulation in Oligodendrocyte Precursor Cells Potentially Induces Inclusions in Multiple System Atrophy

Glial cytoplasmic inclusions (GCIs), commonly observed as α-synuclein (α-syn)-positive aggregates within oligodendrocytes, are the pathological hallmark of multiple system atrophy. The origin of α-syn in GCIs is uncertain; there is little evidence of endogenous α-syn expression in oligodendrocyte lineage cells, oligodendrocyte precursor cells (OPCs), and mature oligodendrocytes (OLGs). Here, based on in vitro analysis using primary rat cell cultures, we elucidated that preformed fibrils (PFFs) generated from recombinant human α-syn trigger multimerization and an upsurge of endogenous α-syn in OPCs, which is attributable to insufficient autophagic proteolysis. RNA-seq analysis of OPCs revealed that α-syn PFFs interfered with the expression of proteins associated with neuromodulation and myelination. Furthermore, we detected cytoplasmic α-syn inclusions in OLGs through differentiation of OPCs pre-incubated with PFFs. Overall, our findings suggest the possibility of endogenous α-syn accumulation in OPCs that contributes to GCI formation and perturbation of neuronal/glial support in multiple system atrophy brains.

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Endogenous α-syn in OPCs drastically increases via seeding from exogenous α-syn PFFs

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Exogenous α-syn PFFs do not induce SNCA mRNA overproduction but autophagic impairment

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Exogenous α-syn PFFs compromise gene regulation for neuronal/glial modulation in OPCs

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α-Syn PFF-treated OPCs differentiate into OLGs containing α-syn aggregates

Caffeoylquinic Acid Enhances Proliferation of Oligodendrocyte Precursor Cells

This report evaluates the protective effect of caffeoylquinic acid (CA) injury to oligodendrocyte precursor cells (OPCs) by promoting the formation of oligodendrocytes. Neonatal rat brain was used to isolate primary OPCs and non-lethal CoCl2 was used to induce hypoxic stress to inhibit the differentiation of OPCs. Differentiation of OPCs was estimated by survival assay and the expressions of myelin-basic-protein (MBP). Moreover, the effect of CA on the Akt signanling pathway was also estimated in the presence and absence of LY294002 (PI3K/Akt inhibitor) and adrenomedullin (AM) receptor antagonist (AM22-52) by using western blot assay. It was observed that CA enhances the differentiation OPCs in CoCl2 induced hypoxic stress condition. However treatment with CA in presence of LY294002 and AM22-52 was not able to enhance the differentiation of OPCs. Moreover treatment with CA significantly enhances the phosphorylation of Akt and presence of LY294002 and AM22-52 inhibits it. This report concludes that CA effectively attenuates the injury of white matter (OPCs) by enhancing the differentiation of OPCs. It enhances the formation of oligodendrocytes by activating AM receptor and thereby accelerates the regeneration of neuron in pathological condition.

Fabricating PLGA microparticles with high loads of the small molecule antioxidant N-acetylcysteine that rescue oligodendrocyte progenitor cells from oxidative stress

Reactive oxygen species (ROS), encompassing all oxygen radical or non-radical oxidizing agents, play key roles in disease progression. Controlled delivery of antioxidants is therapeutically relevant in such oxidant-stressed environments. Encapsulating small hydrophilic molecules into hydrophobic polymer microparticles via traditional emulsion methods has long been a challenge due to rapid mass transport of small molecules out of particle pores. We have developed a simple alteration to the existing water-in-oil-in-water (W/O/W) drug encapsulation method that dramatically improves loading efficiency: doping external water phases with drug to mitigate drug diffusion out of the particle during fabrication. PLGA microparticles with diameters ranging from 0.6 to 0.9 micrometers were fabricated, encapsulating high loads of 0.6-0.9 µm diameter PLGA microparticles were fabricated, encapsulating high loads of the antioxidant N-acetylcysteine (NAC), and released active, ROS-scavenging NAC for up to 5 weeks. Encapsulation efficiencies, normalized to the theoretical load of traditional encapsulation without doping, ranged from 96% to 400%, indicating that NAC-loaded external water phases not only prevented drug loss due to diffusion, but also doped the particles with additional drug. Antioxidant-doped particles positively affected the metabolism of oligodendrocyte progenitor cells (OPCs) under H₂ O₂ -mediated oxidative stress when administered both before (protection) or after (rescue) injury. Antioxidant doped particles improved outcomes of OPCs experiencing multiple doses of H₂ O₂ by increasing the intracellular glutathione content and preserving cellular viability relative to the injury control. Furthermore, antioxidant-doped particles preserve cell number, number of process extensions, cytoskeletal morphology, and nuclear size of H₂ O₂ -stressed OPCs relative to the injury control. These NAC-doped particles have the potential to provide temporally-controlled antioxidant therapy in neurodegenerative disorders such as multiple sclerosis (MS) that are characterized by continuous oxidative stress.

Myelin plasticity, neural activity, and traumatic neural injury

The possibility that adult organisms exhibit myelin plasticity has recently become a topic of great interest. Many researchers are exploring the role of myelin growth and adaptation in daily functions such as memory and motor learning. Here we consider evidence for three different potential categories of myelin plasticity: the myelination of previously bare axons, remodeling of existing sheaths, and the removal of a sheath with replacement by a new internode. We also review evidence that points to the importance of neural activity as a mechanism by which oligodendrocyte precursor cells (OPCs) are cued to differentiate into myelinating oligodendrocytes, which may potentially be an important component of myelin plasticity. Finally, we discuss demyelination in the context of traumatic neural injury and present an argument for altering neural activity as a potential therapeutic target for remyelination following injury. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 108-122, 2018.

Remyelinating Pharmacotherapies in Multiple Sclerosis

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

Mechanical plasticity during oligodendrocyte differentiation and myelination

In the central nervous system, oligodendrocyte precursor cells are exclusive in their potential to differentiate into myelinating oligodendrocytes. Oligodendrocyte precursor cells migrate within the parenchyma and extend cell membrane protrusions that ultimately evolve into myelinating sheaths able to wrap neuronal axons and significantly increase their electrical conductivity. The subcellular force generating mechanisms driving morphological and functional transformations during oligodendrocyte differentiation and myelination remain elusive. In this review, we highlight the mechanical processes governing oligodendrocyte plasticity in a dynamic interaction with the extracellular matrix.

The Role of the Oligodendrocyte Lineage in Acute Brain Trauma

An acute brain injury is commonly characterized by an extended cellular damage. The post-injury process of scar formation is largely determined by responses of various local glial cells and blood-derived immune cells. The role of astrocytes and microglia have been frequently reviewed in the traumatic sequelae. Here, we summarize the diverse contributions of oligodendrocytes (OLs) and their precursor cells (OPCs) in acute injuries. OLs at the lesion site are highly sensitive to a damaging insult, provoked by Ca²⁺ overload after hyperexcitation originating from increased levels of transmitters. At the lesion site, differentiating OPCs can replace injured oligodendrocytes to guarantee proper myelination that is instrumental for healthy brain function. In contrast to finally differentiated and non-dividing OLs, OPCs are the most proliferative cells of the brain and their proliferation rate even increases after injury. There exist even evidence that OPCs might also generate some type of astrocyte beside OLs. Thereby, OPCs can contribute to the generation and maintenance of the glial scar. In the future, detailed knowledge of the molecular cues that help to prevent injury-evoked glial cell death and that control differentiation and myelination of the oligodendroglial lineage will be pivotal in developing novel therapeutic approaches.

A Novel Three-Dimensional Culture System for Oligodendrocyte Precursor Cells

Oligodendrocytes are generated from oligodendrocyte precursor cells (OPCs). Mechanisms of OPC differentiation have been extensively examined with two-dimensional cell culture systems. However, these cellular events may be more accurately represented using a three-dimensional (3D) model. In this study, we report the development of a novel 3D OPC culture system using gels composed of a mixture of collagen and hyaluronan, wherein cultured rat primary OPCs can proliferate and differentiate into oligodendrocytes. Our data show that the gel concentration and cell-seeding density are critical factors for the numbers of OPCs and oligodendrocytes in our 3D culture system. In addition, Notch signaling, which supports cell-to-cell communication, may also be important for OPC function in our system because a Notch inhibitor DAPT suppressed OPC proliferation and differentiation. Taken together, cultured rat OPCs can grow in collagen-/hyaluronan-based gels, and our novel 3D OPC culture system may offer a useful platform for examining the mechanisms of OPC function in vitro.

Spinal cord injuries: how could cell therapy help?

INTRODUCTION

Spinal cord injury (SCI) is a devastating condition, where regenerative failure and cell loss lead to paralysis. The heterogeneous and time-sensitive pathophysiology has made it difficult to target tissue repair. Despite many medical advances, there are no effective regenerative therapies. As stem cells offer multi-targeted and environmentally responsive benefits, cell therapy is a promising treatment approach. Areas covered: This review highlights the cell therapies being investigated for SCI, including Schwann cells, olfactory ensheathing cells, mensenchymal stem/stromal cells, neural precursors, oligodendrocyte progenitors, embryonic stem cells, and induced pluripotent stem cells. Through mechanisms of cell replacement, scaffolding, trophic support and immune modulation, each approach targets unique features of SCI pathology. However, as the injury is multifaceted, it is increasingly recognized that a combinatorial approach will be necessary to treat SCI. Expert opinion: Most preclinical studies, and an increasing number of clinical trials, are finding that single cell therapies have only modest benefits after SCI. These considerations, alongside issues of therapy cost-effectiveness, need to be addressed at the bench. In addition to exploring combinatorial strategies, researchers should consider cell reproducibility and storage parameters when designing animal experiments. Equally important, clinical trials must follow strict regulatory guidelines that will enable transparency of results.

Myelin changes in Alexander disease

INTRODUCTION

Alexander disease (AxD) is a type of leukodystrophy. Its pathological basis, along with myelin loss, is the appearance of Rosenthal bodies, which are cytoplasmic inclusions in astrocytes. Mutations in the gene coding for GFAP have been identified as a genetic basis for AxD. However, the mechanism by which these variants produce the disease is not understood.

DEVELOPMENT

The most widespread hypothesis is that AxD develops when a gain of function mutation causes an increase in GFAP. However, this mechanism does not explain myelin loss, given that experimental models in which GFAP expression is normal or mutated do not exhibit myelin disorders. This review analyses other possibilities that may explain this alteration, such as epigenetic or inflammatory alterations, presence of NG2 (+) - GFAP (+) cells, or post-translational modifications in GFAP that are unrelated to increased expression.

CONCLUSIONS

The different hypotheses analysed here may explain the myelin alteration affecting these patients, and multiple mechanisms may coexist. These theories raise the possibility of designing therapies based on these mechanisms.

Roles of NG2-glia in ischemic stroke

Recent studies have shown that a widely distributed class of glial cells, termed NG2-glia, engages in rapid signaling with surrounding neurons through direct synaptic contacts in the developing and mature central nervous system (CNS). This unique glial cell group has a typical function of proliferating and differentiating into oligodendrocytes during early development of the brain, which is crucial to axon myelin formation. Therefore, NG2-glia are also called oligodendrocyte precursor cells (OPCs). In vitro and in vivo studies reveal that NG2-glia expressing receptors and ion channels demonstrate functional significance for rapid signaling with neuronal synapses and modulation of neuronal activities in both physiological and pathological conditions. Although it is well known that NG2-glia play an important role in demyelinating diseases such as multiple sclerosis, little is known about how NG2-glia or OPCs impact neurons and brain function following ischemic injury. This review summarizes recent progress on the roles of NG2-glia in ischemic stroke and illustrates new approaches for targeting NG2-glia in the brain to treat this disease.

Promoting in vivo remyelination with small molecules: a neuroreparative pharmacological treatment for Multiple Sclerosis

Multiple Sclerosis (MS) is a neurodegenerative disease where immune-driven demyelination occurs with inefficient remyelination, but therapies are limited, especially those to enhance repair. Here, we show that the dual phosphodiesterase (PDE)7- glycogen synthase kinase (GSK)3 inhibitor, VP3.15, a heterocyclic small molecule with good pharmacokinetic properties and safety profile, improves in vivo remyelination in mouse and increases both adult mouse and adult human oligodendrocyte progenitor cell (OPC) differentiation, in addition to its immune regulatory action. The dual inhibition is synergistic, as increasing intracellular levels of cAMP by cyclic nucleotide PDE inhibition both suppresses the immune response and increases remyelination, and in addition, inhibition of GSK3 limits experimental autoimmune encephalomyelitis in mice. This combination of an advantageous effect on the immune response and an enhancement of repair, plus demonstration of its activity on adult human OPCs, leads us to propose dual PDE7-GSK3 inhibition, and specifically VP3.15, as a neuroprotective and neuroreparative disease-modifying treatment for MS.