Regenerating CNS myelin - from mechanisms to experimental medicines

TLR4 Associated Signaling Disrupters as a New Means to Overcome HERV-W Envelope-Mediated Myelination Deficits

Myelin repair in the adult central nervous system (CNS) is driven by successful differentiation of resident oligodendroglial precursor cells (OPCs) and thus constitutes a neurodegenerative process capable to compensate for functional deficits upon loss of oligodendrocytes and myelin sheaths as it is observed in multiple sclerosis (MS). The human endogenous retrovirus type W (HERV-W) represents an MS-specific pathogenic entity, and its envelope (ENV) protein was previously identified as a negative regulator of OPC maturation—hence, it is of relevance in the context of diminished myelin repair. We here focused on the activity of the ENV protein and investigated how it can be neutralized for improved remyelination. ENV-mediated activation of toll like receptor 4 (TLR4) increases inducible nitric oxide synthase (iNOS) expression, prompts nitrosative stress, and results in myelin-associated deficits, such as decreased levels of oligodendroglial maturation marker expression and morphological alterations. The intervention of TLR4 surface expression represents a potential means to rescue such ENV-dependent deficits. To this end, the rescue capacity of specific substances, either modulating V-ATPase activity or myeloid differentiation 2 (MD2)-mediated TLR4 glycosylation status, such as compound 20 (C20), L48H437, or folimycin, was analyzed, as these processes were demonstrated to be relevant for TLR4 surface expression. We found that pharmacological treatment can rescue the maturation arrest of oligodendroglial cells and their myelination capacity and can prevent iNOS induction in the presence of the ENV protein. In addition, downregulation of TLR4 surface expression was observed. Furthermore, mitochondrial integrity crucial for oligodendroglial cell differentiation was affected in the presence of ENV and ameliorated upon pharmacological treatment. Our study, therefore, provides novel insights into possible means to overcome myelination deficits associated with HERV-W ENV-mediated myelin deficits.

The Endocannabinoid System in Glial Cells and Their Profitable Interactions to Treat Epilepsy: Evidence from Animal Models

Epilepsy is one of the most common neurological conditions. Yearly, five million people are diagnosed with epileptic-related disorders. The neuroprotective and therapeutic effect of (endo)cannabinoid compounds has been extensively investigated in several models of epilepsy. Therefore, the study of specific cell-type-dependent mechanisms underlying cannabinoid effects is crucial to understanding epileptic disorders. It is estimated that about 100 billion neurons and a roughly equal number of glial cells co-exist in the human brain. The glial population is in charge of neuronal viability, and therefore, their participation in brain pathophysiology is crucial. Furthermore, glial malfunctioning occurs in a wide range of neurological disorders. However, little is known about the impact of the endocannabinoid system (ECS) regulation over glial cells, even less in pathological conditions such as epilepsy. In this review, we aim to compile the existing knowledge on the role of the ECS in different cell types, with a particular emphasis on glial cells and their impact on epilepsy. Thus, we propose that glial cells could be a novel target for cannabinoid agents for treating the etiology of epilepsy and managing seizure-like disorders.

Neural Cell Membrane-Coated Nanoparticles for Targeted and Enhanced Uptake by Central Nervous System Cells

Targeted drug delivery to specific neural cells within the central nervous system (CNS) plays important roles in treating neurological disorders, such as neurodegenerative (e.g., targeting neurons) and demyelinating diseases [e.g., targeting oligodendrocytes (OLs)]. However, the presence of many other cell types within the CNS, such as microglial and astrocytes, may lead to nonspecific uptake and subsequent side effects. As such, exploring an effective and targeted drug delivery system is of great necessity. Synthetic micro-/nanoparticles that have been coated with biologically derived cellular membranes have emerged as a new class of drug delivery vehicles. However, the use of neural cell-derived membrane coatings remains unexplored. Here, we utilized this technique and demonstrated the efficacy of targeted delivery by using four types of cell membranes that were derived from the CNS, namely, microglial, astrocytes, oligodendrocyte progenitor cells (OPCs), and cortical neurons. A successful cell membrane coating over poly(ε-caprolactone) nanoparticles (NPs) was confirmed using dynamic light scattering, zeta potential measurements, and transmission electron microscopy. Subsequently, an extensive screening of these cell membrane-coated NPs was carried out on various CNS cells. Results suggested that microglial and OLs were the most sensitive cell types toward cell membrane-coated NPs. Specifically, cell membrane-coated NPs significantly enhanced the uptake efficiency of OLs (p < 0.001). Additionally, a temporal uptake study indicated that the OLs took up microglial membrane-coated NPs (DPP-PCL-M Mem) most efficiently. Besides that, coating the NPs with four types of the CNS cell membrane did not result in obvious specific uptake in microglial but reduced the activation of microglial, especially for DPP-PCL-M Mem (p < 0.01). Taken together, DPP-PCL-M Mem were uptaken most efficiently in OLs and did not induce significant microglial activation and may be most suitable for CNS drug delivery applications.

The role of neural stem cells in regulating glial scar formation and repair

Glial scars are a common pathological occurrence in a variety of central nervous system (CNS) diseases and injuries. They are caused after severe damage and consist of reactive glia that form a barrier around the damaged tissue that leads to a non-permissive microenvironment which prevents proper endogenous regeneration. While there are a number of therapies that are able to address some components of disease, there are none that provide regenerative properties. Within the past decade, neural stem cells (NSCs) have been heavily studied due to their potent anti-inflammatory and reparative capabilities in disease and injury. Exogenously applied NSCs have been found to aid in glial scar healing by reducing inflammation and providing cell replacement. However, endogenous NSCs have also been found to contribute to the reactive environment by different means. Further understanding how NSCs can be leveraged to aid in the resolution of the glial scar is imperative in the use of these cells as regenerative therapies. To do so, humanised 3D model systems have been developed to study the development and maintenance of the glial scar. Herein, we explore the current work on endogenous and exogenous NSCs in the glial scar as well as the novel 3D stem cell–based technologies being used to model this pathology in a dish.

Remyelination: what are the prospects for regenerative therapies in multiple sclerosis?

Multiple sclerosis (MS) involves the immune system attacking the myelin sheaths surrounding axons and is a major cause of disability in working-age adults. Various approved therapies now provide reasonably good control over MS neuroinflammation, but none have a pronounced impact on the neurodegeneration associated with the disease. One prominent approach to fulfilling the unmet need for neuroprotective therapies, is the search for agents that promote 'remyelination', namely the generation of new oligodendrocytes that can form replacement myelin sheaths around denuded axons. In this article, I discuss some emerging targets for remyelinating therapies, mainly being pursued by recently formed small companies translating academic findings.

Myelin: A gatekeeper of activity-dependent circuit plasticity?

[Figure: see text].

Small molecule screening as an approach to encounter inefficient myelin repair

While current multiple sclerosis therapies are focused on immunomodulation, thereby slowing down disease progression, scientific interest has nowadays been shifted toward regenerative therapies aiming at reversing already existing deficits. The application of chemical compounds was proven to be valuable for the understanding of oligodendrogenesis and for exposing mechanisms that can boost remyelination. However, sufficient myelin repair has not been achieved yet, thus underscoring the need for more studies toward this unmet clinical goal. In this regard, many research groups have significantly contributed to the field via developing compound screening approaches or using single substances. We, here, present an overview of recent studies addressing the identification of myelin repair drugs and provide insights into technical aspects and identified substances.

Mechanisms of oligodendrocyte progenitor developmental migration

Oligodendrocytes, the myelinating cells of the central nervous system (CNS), develop from oligodendrocyte progenitor cells (OPCs) that must first migrate extensively throughout the developing brain and spinal cord. Specified at particular times from discrete regions in the developing CNS, OPCs are one of the most migratory of cell types and disperse rapidly. A variety of factors act on OPCs to trigger intracellular changes that regulate their migration. We will discuss factors that act as long-range guidance cues, those that act to regulate cellular motility, and those that are critical in determining the final positioning of OPCs. In addition, recent evidence has identified the vasculature as the physical substrate used by OPCs for their migration. Several new findings relating to this oligodendroglial-vascular signaling axis reveal new insight on the relationship between OPCs and blood vessels in the developing and adult brain.

Monocytes in central nervous system remyelination

Remyelination failure with aging and progression of neurodegenerative disorders contributes to axonal dysfunction, highlighting the importance of understanding the mechanisms underpinning this process to develop regenerative therapies. Central nervous system (CNS) macrophages, encompassing both resident microglia and blood monocyte-derived cells, play a crucial role in driving successful remyelination. Although there has been a focus on the critical roles of microglia in remyelination, the specific contribution of monocyte-derived macrophages is still not fully understood. Until recently, the lack of tools enabling distinction between CNS macrophage populations has hindered our understanding of monocyte influence on remyelination. Recent advances have allowed for identification and characterization of monocyte populations in health, aging and in neurodegenerative conditions like multiple sclerosis, indicating heterogeneity of monocyte subsets impacted by both intrinsic and extrinsic factors. Here, we discuss the new tools enabling distinction between macrophage populations and advancements in understanding the importance of monocytes in remyelination, and reflect on the potential for therapeutic targeting of monocytes to promote remyelination.

Transcriptomic analysis of loss of Gli1 in neural stem cells responding to demyelination in the mouse brain

In the adult mammalian brain, Gli1 expressing neural stem cells reside in the subventricular zone and their progeny are recruited to sites of demyelination in the white matter where they generate new oligodendrocytes, the myelin forming cells. Remarkably, genetic loss or pharmacologic inhibition of Gli1 enhances the efficacy of remyelination by these neural stem cells. To understand the molecular mechanisms involved, we performed a transcriptomic analysis of this Gli1-pool of neural stem cells. We compared murine NSCs with either intact or deficient Gli1 expression from adult mice on a control diet or on a cuprizone diet which induces widespread demyelination. These data will be a valuable resource for identifying therapeutic targets for enhancing remyelination in demyelinating diseases like multiple sclerosis.

Proteomic and lipidomic profiling of demyelinating lesions identifies fatty acids as modulators in lesion recovery

After demyelinating injury of the central nervous system, resolution of the mounting acute inflammation is crucial for the initiation of a regenerative response. Here, we aim to identify fatty acids and lipid mediators that govern the balance of inflammatory reactions within demyelinating lesions. Using lipidomics, we identify bioactive lipids in the resolution phase of inflammation with markedly elevated levels of n-3 polyunsaturated fatty acids. Using fat-1 transgenic mice, which convert n-6 fatty acids to n-3 fatty acids, we find that reduction of the n-6/n-3 ratio decreases the phagocytic infiltrate. In addition, we observe accelerated decline of microglia/macrophages and enhanced generation of oligodendrocytes in aged mice when n-3 fatty acids are shuttled to the brain. Thus, n-3 fatty acids enhance lesion recovery and may, therefore, provide the basis for pro-regenerative medicines of demyelinating diseases in the central nervous system.

MedXercise: a promising strategy to promote remyelination

Multiple sclerosis is an inflammatory and demyelinating disease of the central nervous system. While remyelination facilitates functional recovery in animal models, it is limited in people with multiple sclerosis. Thus, multiple strategies have been put forth to promote remyelination, including exercise and medication. Exercise promotes the release of growth factors and induces protein-level changes, while remyelinating medications act through a variety of mechanisms to promote oligodendrocyte maturation within the lesion. In animal models, the combination of medication and exercise (Medication + eXercise = MedXercise) has an additive effect on remyelination and other pathological features of multiple sclerosis. In this review, we highlight the existing literature on the effects of exercise and medication on remyelination both independently and in combination.

Osteocalcin attenuates oligodendrocyte differentiation and myelination via GPR37 signaling in the mouse brain

The bone-derived hormone osteocalcin (OCN) is crucial for brain development and neural cognitive functions, yet the exact roles of OCN in central nervous system (CNS) remain elusive. Here, we find that genetic deletion of OCN facilitates oligodendrocyte (OL) differentiation and hypermyelination in the CNS. Although dispensable for the proliferation of oligodendrocyte precursor cells (OPCs), OCN is critical for the myelination of OLs, which affects myelin production and remyelination after demyelinating injury. Genome-wide RNA sequencing analyses reveal that OCN regulates a number of G protein–coupled receptors and myelination-associated transcription factors, of which Myrf might be a key downstream effector in OLs. GPR37 is identified as a previously unknown receptor for OCN, thus regulating OL differentiation and CNS myelination. Overall, these findings suggest that OCN orchestrates the transition between OPCs and myelinating OLs via GPR37 signaling, and hence, the OCN/GPR37 pathway regulates myelin homeostasis in the CNS.

Rnf43 is "lord of the ring" finger proteins in remyelination

Oligodendrocyte precursor cell differentiation into myelinating oligodendrocytes is critical for remyelination in the central nervous system after injury. In this issue of Neuron, Niu et al. (2021) detail a novel role for ring finger protein Rnf43, which is expressed in response to injury and is essential to promote remyelination in vivo.

Shenzhiling oral liquid protects the myelin sheath against Alzheimer's disease through the PI3K/Akt-mTOR pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Shenzhiling oral liquid (SZL), a traditional Chinese medicine (TCM) compound, is firstly approved by the Chinese Food and Drug Administration (CFDA) for the treatment of mild to moderate Alzheimer's disease (AD). SZL is composed of ten Chinese herbs, and the precise therapy mechanism of its action to AD is far from fully understood.

AIM OF THE STUDY

The purpose of this study was to observe whether SZL is an effective therapy for amyloid-beta (Aβ)-induced myelin sheath and oligodendrocytes impairments. Notably, the primary aim was to elucidate whether and through what underlying mechanism SZL protects the myelin sheath through the PI3K/Akt-mTOR signaling pathway in Aβ₄₂-induced OLN-93 oligodendrocytes in vitro.

MATERIALS AND METHODS

APP/PS1 mice were treated with SZL or donepezil continuously for three months, and Aβ₄₂-induced oligodendrocyte OLN-93 cells mimicking AD pathogenesis of myelin sheath impairments were incubated with SZL-containing serum or with donepezil. LC-MS/MS was used to analysis the active components of SZL and SZL-containing serum. The Y maze test was administered after 3 months of treatment, and the hippocampal tissues of the APP/PS1 mice were then harvested for observation of myelin sheath and oligodendrocyte morphology. Cell viability and toxicity were assessed using CCK-8 and lactate dehydrogenase (LDH) release assays, and flow cytometry was used to measure cell apoptosis. The expression of the myelin proteins MBP, PLP, and MAG and that of Aβ₄₂ and Aβ₄₀ in the hippocampi of APP/PS1 mice were examined after SZL treatment. Simultaneously, the expression of p-PI3K, PI3K, p-Akt, Akt, p-mTOR, and mTOR were also examined. The expression of proteins, including CNPase, Olig2, NKX2.2, MBP, PLP, MAG, MOG, p-PI3K, PI3K, p-Akt, Akt, p-mTOR, and mTOR, was determined by immunofluorescence and Western blot, and the corresponding gene expression was evaluated by qPCR in Aβ₄₂-induced OLN-93 oligodendrocytes.

RESULTS

LC-MS/MS detected a total of 126 active compounds in SZL-containing serum, including terpenoids, flavones, phenols, phenylpropanoids and phenolic acids. SZL treatment significantly improved memory and cognition in APP/PS1 mice and decreased the G-ratio of myelin sheath, alleviated myelin sheath and oligodendrocyte impairments by decreasing Aβ₄₂ and Aβ₄₀ accumulation and increasing the expression of myelin proteins MBP, PLP, MAG, and PI3K/Akt-mTOR signaling pathway associated protein in the hippocampi of APP/PS1 mice. SZL-containing serum also significantly reversed the OLN-93 cell injury induced by Aβ₄₂ by increasing cell viability and enhanced the expression of MBP, PLP, MAG, and MOG. Meanwhile, SZL-containing serum facilitated the maturation and differentiation of oligodendrocytes in Aβ₄₂-induced OLN-93 cells by heightening the expression of CNPase, Olig2 and NKX2.2. SZL-containing serum treatment also fostered the expression of p-PI3K, PI3K, p-Akt, Akt, p-mTOR, and mTOR, indicating an activating PI3K/Akt-mTOR signaling pathway in OLN-93 cells. Furthermore, the effects of SZL on myelin proteins, p-Akt, and p-mTOR were clearly inhibited by LY294002 and/or rapamycin, antagonists of PI3K and m-TOR, respectively.

CONCLUSIONS

Our findings indicate that SZL exhibits a neuroprotective effect on the myelin sheath by promoting the expression of myelin proteins during AD, and its mechanism of action is closely related to the activation of the PI3K/Akt-mTOR signaling pathway.

TREM2-dependent lipid droplet biogenesis in phagocytes is required for remyelination

Upon demyelinating injury, microglia orchestrate a regenerative response that promotes myelin repair, thereby restoring rapid signal propagation and protecting axons from further damage. Whereas the essential phagocytic function of microglia for remyelination is well known, the underlying metabolic pathways required for myelin debris clearance are poorly understood. Here, we show that cholesterol esterification in male mouse microglia/macrophages is a necessary adaptive response to myelin debris uptake and required for the generation of lipid droplets upon demyelinating injury. When lipid droplet biogenesis is defective, innate immune cells do not resolve, and the regenerative response fails. We found that triggering receptor expressed on myeloid cells 2 (TREM2)-deficient mice are unable to adapt to excess cholesterol exposure, form fewer lipid droplets, and build up endoplasmic reticulum (ER) stress. Alleviating ER stress in TREM2-deficient mice restores lipid droplet biogenesis and resolves the innate immune response. Thus, we conclude that TREM2-dependent formation of lipid droplets constitute a protective response required for remyelination to occur.

BMP receptor blockade overcomes extrinsic inhibition of remyelination and restores neurovascular homeostasis

Extrinsic inhibitors at sites of blood–brain barrier disruption and neurovascular damage contribute to remyelination failure in neurological diseases. However, therapies to overcome the extrinsic inhibition of remyelination are not widely available and the dynamics of glial progenitor niche remodelling at sites of neurovascular dysfunction are largely unknown. By integrating in vivo two-photon imaging co-registered with electron microscopy and transcriptomics in chronic neuroinflammatory lesions, we found that oligodendrocyte precursor cells clustered perivascularly at sites of limited remyelination with deposition of fibrinogen, a blood coagulation factor abundantly deposited in multiple sclerosis lesions. By developing a screen (OPC-X-screen) to identify compounds that promote remyelination in the presence of extrinsic inhibitors, we showed that known promyelinating drugs did not rescue the extrinsic inhibition of remyelination by fibrinogen. In contrast, bone morphogenetic protein type I receptor blockade rescued the inhibitory fibrinogen effects and restored a promyelinating progenitor niche by promoting myelinating oligodendrocytes, while suppressing astrocyte cell fate, with potent therapeutic effects in chronic models of multiple sclerosis. Thus, abortive oligodendrocyte precursor cell differentiation by fibrinogen is refractory to known promyelinating compounds, suggesting that blockade of the bone morphogenetic protein signalling pathway may enhance remyelinating efficacy by overcoming extrinsic inhibition in neuroinflammatory lesions with vascular damage.

Safety and efficacy of bexarotene in patients with relapsing-remitting multiple sclerosis (CCMR One): a randomised, double-blind, placebo-controlled, parallel-group, phase 2a study

BACKGROUND

Progressive disability in multiple sclerosis occurs because CNS axons degenerate as a late consequence of demyelination. In animals, retinoic acid receptor RXR-gamma agonists promote remyelination. We aimed to assess the safety and efficacy of a non-selective retinoid X receptor agonist in promoting remyelination in people with multiple sclerosis.

METHODS

This randomised, double-blind, placebo-controlled, parallel-group, phase 2a trial (CCMR One) recruited patients with relapsing-remitting multiple sclerosis from two centres in the UK. Eligible participants were aged 18-50 years and had been receiving dimethyl fumarate for at least 6 months. Via a web-based system run by an independent statistician, participants were randomly assigned (1:1), by probability-weighted minimisation using four binary factors, to receive 300 mg/m2 of body surface area per day of oral bexarotene or oral placebo for 6 months. Participants, investigators, and outcome assessors were masked to treatment allocation. MRI scans were done at baseline and at 6 months. The primary safety outcome was the number of adverse events and withdrawals attributable to bexarotene. The primary efficacy outcome was the patient-level change in mean lesional magnetisation transfer ratio between baseline and month 6 for lesions that had a baseline magnetisation transfer ratio less than the within-patient median. We analysed the primary safety outcome in the safety population, which comprised participants who received at least one dose of their allocated treatment. We analysed the primary efficacy outcome in the intention-to-treat population, which comprised all patients who completed the study. This study is registered in the ISRCTN Registry, 14265371, and has been completed.

FINDINGS

Between Jan 17, 2017, and May 17, 2019, 52 participants were randomly assigned to receive either bexarotene (n=26) or placebo (n=26). Participants who received bexarotene had a higher mean number of adverse events (6·12 [SD 3·09]; 159 events in total) than did participants who received placebo (1·63 [SD 1·50]; 39 events in total). All bexarotene-treated participants had at least one adverse event, which included central hypothyroidism (n=26 vs none on placebo), hypertriglyceridaemia (n=24 vs none on placebo), rash (n=13 vs one on placebo), and neutropenia (n=10 vs none on placebo). Five (19%) participants on bexarotene and two (8%) on placebo discontinued the study drug due to adverse events. One episode of cholecystitis in a placebo-treated participant was the only serious adverse event. The change in mean lesional magnetisation transfer ratio was not different between the bexarotene group (0·25 percentage units [pu; SD 0·98]) and the placebo group (0·09 pu [0·84]; adjusted bexarotene-placebo difference 0·16 pu, 95% CI -0·39 to 0·71; p=0·55).

INTERPRETATION

We do not recommend the use of bexarotene to treat patients with multiple sclerosis because of its poor tolerability and negative primary efficacy outcome. However, statistically significant effects were seen in some exploratory MRI and electrophysiological analyses, suggesting that other retinoid X receptor agonists might have small biological effects that could be investigated in further studies.

FUNDING

Multiple Sclerosis Society of the United Kingdom.

Gastrodin promotes CNS myelination via a lncRNA Gm7237/miR-142a/MRF pathway

Treatment of central nervous system (CNS) demyelination is greatly hindered by lack of the knowledge regarding to underlying molecular mechanisms as well as therapeutic agents. Here, we report a novel small molecule agent, gastrodin (GAS), which can significantly promote CNS myelination in in vivo mice models. By using high-throughput sequencing analysis, we discover a key long non-coding RNA Gm7237 that can enhance CNS myelination and is up-regulated by GAS. Through using bioinformatic analysis and experimental validations, we further unravel that microRNA-142a (miR-142a) and its target myelin gene regulatory factor (MRF) is under the direct regulation by Gm7237. Finally, we demonstrate that Gm7237/miR-142a/MRF axis is the key pathway involved in CNS myelination mediated by GAS. Overall, our results provide not only a novel agent for therapeutic treatment of CNS demyelination but also a molecular basis responsible for GAS-promoted CNS myelination.

Oligodendroglial ring finger protein Rnf43 is an essential injury-specific regulator of oligodendrocyte maturation

Oligodendrocyte (OL) maturation arrest in human white matter injury contributes significantly to the failure of endogenous remyelination in multiple sclerosis (MS) and newborn brain injuries such as hypoxic ischemic encephalopathy (HIE) that cause cerebral palsy. In this study, we identify an oligodendroglial-intrinsic factor that controls OL maturation specifically in the setting of injury. We find a requirement for the ring finger protein Rnf43 not in normal development but in neonatal hypoxic injury and remyelination in the adult mammalian CNS. Rnf43, but not the related Znrf3, is potently activated by Wnt signaling in OL progenitor cells (OPCs) and marks activated OPCs in human MS and HIE. Rnf43 is required in an injury-specific context, and it promotes OPC differentiation through negative regulation of Wnt signal strength in OPCs at the level of Fzd1 receptor presentation on the cell surface. Inhibition of Fzd1 using UM206 promotes remyelination following ex vivo and in vivo demyelinating injury.

Oligodendrocyte progenitors as environmental biosensors

The past decade has seen an important revision of the traditional concept of the role and function of glial cells. From "passive support" for neurons, oligodendrocyte lineage cells are now recognized as metabolic exchangers with neurons, a cellular interface with blood vessels and responders to gut-derived metabolites or changes in the social environment. In the developing brain, the differentiation of neonatal oligodendrocyte progenitors (nOPCs) is required for normal brain function. In adulthood, the differentiation of adult OPCs (aOPCs) serves an important role in learning, behavioral adaptation and response to myelin injury. Here, we propose the concept of OPCs as environmental biosensors, which "sense" chemical and physical stimuli over time and adjust to the new challenges by modifying their epigenome and consequent transcriptome. Because epigenetics defines the ability of the cell to "adapt" gene expression to changes in the environment, we propose a model of OPC differentiation resulting from time-dependent changes of the epigenomic landscape in response to declining mitogens, raising hormone levels, neuronal activity, changes in space constraints or stiffness of the extracellular matrix. We propose that the intrinsically different functional properties of aOPCs compared to nOPCs result from the accrual of "epigenetic memories" of distinct events, which are "recorded" in the nuclei of OPCs as histone and DNA marks, defining a "unique epigenomic landscape" over time.

CHPG enhances BDNF and myelination in cuprizone-treated mice through astrocytic metabotropic glutamate receptor 5

It is well recognized that astrocytes can produce factors known to affect the myelination process. One such factor, brain-derived neurotrophic factor (BDNF), can enhance the differentiation of oligodendrocyte lineage cells following a demyelinating lesion. Our previous work indicated that enhancing astrocyte-derived BDNF via injection of a general agonist of Group I/II metabotropic glutamate receptors (mGluRs) into the lesion increased myelin proteins in the cuprizone model of demyelination after 4 hr. To determine if this observation has potential therapeutic significance, we now use a more specific mGluR agonist, 2-chloro-5-hydroxyphenylglycine (CHPG), which binds to mGluR5, to examine effects on myelination through the clinically relevant approach of a peripheral injection. In initial studies, intraperitoneal injection of CHPG resulted in an increase in myelin proteins within the lesioned corpus callosum. These effects were blocked when either BDNF or the CHPG receptor, mGluR5, was deleted from glial fibrillary acidic protein (GFAP)+ astrocytes or when the BDNF receptor, tropomyosin receptor kinase B (TrkB), was deleted from proteolipid protein (PLP)+ oligodendrocytes. Moreover, injection of CHPG over 2 weeks not only elevated BDNF and myelin proteins, but also enhanced myelination and reversed behavioral deficits. Interestingly, effects on myelin and myelin proteins were not seen in the control animals, indicating that a lesion is critical in eliciting effects. Taken together, the data suggest that the mGluR agonist CHPG may be a potential therapeutic strategy for treating demyelinating diseases and that it works by enhancing the release of BDNF from astrocytes.

Revisiting remyelination: Towards a consensus on the regeneration of CNS myelin

The biology of CNS remyelination has attracted considerable interest in recent years because of its translational potential to yield regenerative therapies for the treatment of chronic and progressive demyelinating diseases such as multiple sclerosis (MS). Critical to devising myelin regenerative therapies is a detailed understanding of how remyelination occurs. The accepted dogma, based on animal studies, has been that the myelin sheaths of remyelination are made by oligodendrocytes newly generated from adult oligodendrocyte progenitor cells in a classical regenerative process of progenitor migration, proliferation and differentiation. However, recent human and a growing number of animal studies have revealed a second mode of remyelination in which mature oligodendrocytes surviving within an area of demyelination are able to regenerate new myelin sheaths. This discovery, while opening up new opportunities for therapeutic remyelination, has also raised the question of whether there are fundamental differences in myelin regeneration between humans and some of the species in which experimental remyelination studies are conducted. Here we review how this second mode of remyelination can be integrated into a wider and revised framework for understanding remyelination in which apparent species differences can be reconciled but that also raises important questions for future research.

Id proteins: emerging roles in CNS disease and targets for modifying neural stemcell behavior

Neural stem/progenitor cells (NSPCs) are found in the adult brain and spinal cord, and endogenous or transplanted NSPCs contribute to repair processes and regulate immune responses in the CNS. However, the molecular mechanisms of NSPC survival and integration as well as their fate determination and functionality are still poorly understood. Inhibitor of DNA binding (Id) proteins are increasingly recognized as key determinants of NSPC fate specification. Id proteins act by antagonizing the DNA-binding activity of basic helix-loop-helix (bHLH) transcription factors, and the balance of Id and bHLH proteins determines cell fate decisions in numerous cell types and developmental stages. Id proteins are central in responses to environmental changes, as they occur in CNS injury and disease, and cellular responses in adult NSPCs implicate Id proteins as prime candidates for manipulating stemcell behavior. Here, we outline recent advances in understanding Id protein pleiotropic functions in CNS diseases and propose an integrated view of Id proteins and their promise as potential targets in modifying stemcell behavior to ameliorate CNS disease.

Oligodendrocytes and Microglia: Key Players in Myelin Development, Damage and Repair

Oligodendrocytes, the myelin-making cells of the CNS, regulate the complex process of myelination under physiological and pathological conditions, significantly aided by other glial cell types such as microglia, the brain-resident, macrophage-like innate immune cells. In this review, we summarize how oligodendrocytes orchestrate myelination, and especially myelin repair after damage, and present novel aspects of oligodendroglial functions. We emphasize the contribution of microglia in the generation and regeneration of myelin by discussing their beneficial and detrimental roles, especially in remyelination, underlining the cellular and molecular components involved. Finally, we present recent findings towards human stem cell-derived preclinical models for the study of microglia in human pathologies and on the role of microbiome on glial cell functions.

Dimethylarginine dimethylaminohydrolase 1 as a novel regulator of oligodendrocyte differentiation in the central nervous system remyelination

Remyelination is a regenerative process that restores the lost neurological function and partially depends on oligodendrocyte differentiation. Differentiation of oligodendrocytes spontaneously occurs after demyelination, depending on the cell intrinsic mechanisms. By combining a loss-of-function genomic screen with a web-resource-based candidate gene identification approach, we identified that dimethylarginine dimethylaminohydrolase 1 (DDAH1) is a novel regulator of oligodendrocyte differentiation. Silencing DDAH1 in oligodendrocytes prevented the expression of myelin basic protein in mouse oligodendrocyte culture with the change in expression of genes annotated with oligodendrocyte development. DDAH1 inhibition attenuated spontaneous remyelination in a cuprizone-induced demyelinated mouse model. Conversely, increased DDAH1 expression enhanced remyelination capacity in experimental autoimmune encephalomyelitis. These results provide a novel therapeutic option for demyelinating diseases by modulating DDAH1 activity.

Stem Cell Therapies for Progressive Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system characterized by demyelination and axonal degeneration. MS patients typically present with a relapsing-remitting (RR) disease course, manifesting as sporadic attacks of neurological symptoms including ataxia, fatigue, and sensory impairment. While there are several effective disease-modifying therapies able to address the inflammatory relapses associated with RRMS, most patients will inevitably advance to a progressive disease course marked by a gradual and irreversible accrual of disabilities. Therapeutic intervention in progressive MS (PMS) suffers from a lack of well-characterized biological targets and, hence, a dearth of successful drugs. The few medications approved for the treatment of PMS are typically limited in their efficacy to active forms of the disease, have little impact on slowing degeneration, and fail to promote repair. In looking to address these unmet needs, the multifactorial therapeutic benefits of stem cell therapies are particularly compelling. Ostensibly providing neurotrophic support, immunomodulation and cell replacement, stem cell transplantation holds substantial promise in combatting the complex pathology of chronic neuroinflammation. Herein, we explore the current state of preclinical and clinical evidence supporting the use of stem cells in treating PMS and we discuss prospective hurdles impeding their translation into revolutionary regenerative medicines.

Live imaging of remyelination in the adult mouse corpus callosum

Oligodendrocyte precursor cells (OPCs) retain the capacity to remyelinate axons upon demyelinating injury. However, mode of cell division and differentiation dynamics of individual OPCs in deep brain structures, such as the corpus callosum, remains unknown. Using in vivo two-photon imaging in a focal model of demyelination, we show that OPCs undergo several rounds of symmetric and asymmetric cell divisions before producing a subset of daughter cells that differentiates into myelinating oligodendrocytes. The data presented here characterize the behavior of OPC clones and delineate the cellular principles that lead to remyelination.

Oligodendrocyte precursor cells (OPCs) retain the capacity to remyelinate axons in the corpus callosum (CC) upon demyelination. However, the dynamics of OPC activation, mode of cell division, migration, and differentiation on a single-cell level remain poorly understood due to the lack of longitudinal observations of individual cells within the injured brain. After inducing focal demyelination with lysophosphatidylcholin in the CC of adult mice, we used two-photon microscopy to follow for up to 2 mo OPCs and their differentiating progeny, genetically labeled through conditional recombination driven by the regulatory elements of the gene Achaete-scute homolog 1. OPCs underwent several rounds of symmetric and asymmetric cell divisions, producing a subset of daughter cells that differentiates into myelinating oligodendrocytes. While OPCs continue to proliferate, differentiation into myelinating oligodendrocytes declines with time, and death of OPC-derived daughter cells increases. Thus, chronic in vivo imaging delineates the cellular principles leading to remyelination in the adult brain, providing a framework for the development of strategies to enhance endogenous brain repair in acute and chronic demyelinating disease.

Δ9 -Tetrahydrocannabinol promotes functional remyelination in the mouse brain

BACKGROUND AND PURPOSE

Research on demyelinating disorders aims to find novel molecules that are able to induce oligodendrocyte precursor cell differentiation to promote central nervous system remyelination and functional recovery. Δ⁹ -Tetrahydrocannabinol (THC), the most prominent active constituent of the hemp plant Cannabis sativa, confers neuroprotection in animal models of demyelination. However, the possible effect of THC on myelin repair has never been studied.

EXPERIMENTAL APPROACH

By using oligodendroglia-specific reporter mouse lines in combination with two models of toxin-induced demyelination, we analysed the effect of THC on the processes of oligodendrocyte regeneration and functional remyelination.

KEY RESULTS

We show that THC administration enhanced oligodendrocyte regeneration, white matter remyelination and motor function recovery. THC also promoted axonal remyelination in organotypic cerebellar cultures. THC remyelinating action relied on the induction of oligodendrocyte precursor differentiation upon cell cycle exit and via CB₁ cannabinoid receptor activation.

CONCLUSIONS AND IMPLICATIONS

Overall, our study identifies THC administration as a promising pharmacological strategy aimed to promote functional CNS remyelination in demyelinating disorders.

Co-Ultramicronized Palmitoylethanolamide/Luteolin-Induced Oligodendrocyte Precursor Cell Differentiation is Associated With Tyro3 Receptor Upregulation

Remyelination in patients with multiple sclerosis frequently fails, especially in the chronic phase of the disease promoting axonal and neuronal degeneration and progressive disease disability. Drug-based therapies able to promote endogenous remyelination capability of oligodendrocytes are thus emerging as primary approaches to multiple sclerosis. We have recently reported that the co-ultramicronized composite of palmitoylethanolamide and the flavonoid luteolin (PEALut) promotes oligodendrocyte precursor cell (OPC) maturation without affecting proliferation. Since TAM receptor signaling has been reported to be important modulator of oligodendrocyte survival, we here evaluated the eventual involvement of TAM receptors in PEALut-induced OPC maturation. The mRNAs related to TAM receptors -Tyro3, Axl, and Mertk- were all present at day 2 in vitro. However, while Tyro3 gene expression significantly increased upon cell differentiation, Axl and Mertk did not change during the first week in vitro. Tyro3 gene expression developmental pattern resembled that of MBP myelin protein. In OPCs treated with PEALut the developmental increase of Tyro3 mRNA was significantly higher as compared to vehicle while was reduced gene expression related to Axl and Mertk. Rapamycin, an inhibitor of mTOR, prevented oligodendrocyte growth differentiation and myelination. PEALut, administered to the cultures 30 min after rapamycin, prevented the alteration of mRNA basal expression of the TAM receptors as well as the expression of myelin proteins MBP and CNPase. Altogether, data obtained confirm that PEALut promotes oligodendrocyte differentiation as shown by the increase of MBP and CNPase and Tyro3 mRNAs as well as CNPase and Tyro3 immunostainings. The finding that these effects are reduced when OPCs are exposed to rapamycin suggests an involvement of mTOR signaling in PEALut effects.

A2 B Adenosine Receptors and Sphingosine 1-Phosphate Signaling Cross-Talk in Oligodendrogliogenesis

Oligodendrocyte-formed myelin sheaths allow fast synaptic transmission in the brain. Impairments in the process of myelination, or demyelinating insults, might cause chronic diseases such as multiple sclerosis (MS). Under physiological conditions, remyelination is an ongoing process throughout adult life consisting in the differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes (OLs). During pathological events, this process fails due to unfavorable environment. Adenosine and sphingosine kinase/sphingosine 1-phosphate signaling axes (SphK/S1P) play important roles in remyelination processes. Remarkably, fingolimod (FTY720), a sphingosine analog recently approved for MS treatment, plays important roles in OPC maturation. We recently demonstrated that the selective stimulation of A_{2 B} adenosine receptors (A_{2 B} Rs) inhibit OPC differentiation in vitro and reduce voltage-dependent outward K⁺ currents (I _K ) necessary to OPC maturation, whereas specific SphK1 or SphK2 inhibition exerts the opposite effect. During OPC differentiation A_{2 B} R expression increases, this effect being prevented by SphK1/2 blockade. Furthermore, selective silencing of A_{2 B} R in OPC cultures prompts maturation and, intriguingly, enhances the expression of S1P lyase, the enzyme responsible for irreversible S1P catabolism. Finally, the existence of an interplay between SphK1/S1P pathway and A_{2 B} Rs in OPCs was confirmed since acute stimulation of A_{2 B} Rs activates SphK1 by increasing its phosphorylation. Here the role of A_{2 B} R and SphK/S1P signaling during oligodendrogenesis is reviewed in detail, with the purpose to shed new light on the interaction between A_{2 B} Rs and S1P signaling, as eventual innovative targets for the treatment of demyelinating disorders.

Demyelination and remyelination detected in an alternative cuprizone mouse model of multiple sclerosis with 7.0 T multiparameter magnetic resonance imaging

The aim of this study was to investigate the mechanisms underlying demyelination and remyelination with 7.0 T multiparameter magnetic resonance imaging (MRI) in an alternative cuprizone (CPZ) mouse model of multiple sclerosis (MS). Sixty mice were divided into six groups (n = 10, each), and these groups were imaged with 7.0 T multiparameter MRI and treated with an alternative CPZ administration schedule. T₂-weighted imaging (T₂WI), susceptibility-weighted imaging (SWI), and diffusion tensor imaging (DTI) were used to compare the splenium of the corpus callosum (sCC) among the groups. Prussian blue and Luxol fast blue staining were performed to assess pathology. The correlations of the mean grayscale value (mGSV) of the pathology results and the MRI metrics were analyzed to evaluate the multiparameter MRI results. One-way ANOVA and post hoc comparison showed that the normalized T₂WI (T₂-nor), fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) values were significantly different among the six groups, while the mean phase (Φ) value of SWI was not significantly different among the groups. Correlation analysis showed that the correlation between the T₂-nor and mGSV was higher than that among the other values. The correlations among the FA, RD, MD, and mGSV remained instructive. In conclusion, ultrahigh-field multiparameter MRI can reflect the pathological changes associated with and the underlying mechanisms of demyelination and remyelination in MS after the successful establishment of an acute CPZ-induced model.

Effects of FGFR Tyrosine Kinase Inhibition in OLN-93 Oligodendrocytes

Fibroblast growth factor (FGF) signaling is involved in the pathogenesis of multiple sclerosis (MS). Data from neuropathology studies suggest that FGF signaling contributes to the failure of remyelination in MS. In MOG_35–55-induced EAE, oligodendrocyte-specific deletion of FGFR1 and FGFR2 resulted in a less severe disease course, reduced inflammation, myelin and axon degeneration and changed FGF/FGFR and BDNF/TrkB signaling. Since signaling cascades in oligodendrocytes could not be investigated in the EAE studies, we here aimed to characterize FGFR-dependent oligodendrocyte-specific signaling in vitro. FGFR inhibition was achieved by application of the multi-kinase-inhibitor dovitinib and the FGFR1/2/3-inhibitor AZD4547. Both substances are potent inhibitors of FGF signaling; they are effective in experimental tumor models and patients with malignancies. Effects of FGFR inhibition in oligodendrocytes were studied by immunofluorescence microscopy, protein and gene analyses. Application of the tyrosine kinase inhibitors reduced FGFR1, phosphorylated ERK and Akt expression, and it enhanced BDNF and TrkB expression. Furthermore, the myelin proteins CNPase and PLP were upregulated by FGFR inhibition. In summary, inhibition of FGFR signaling in oligodendrocytes can be achieved by application of tyrosine kinase inhibitors. Decreased phosphorylation of ERK and Akt is associated with an upregulation of BDNF/TrkB signaling, which may be responsible for the increased production of myelin proteins. Furthermore, these data suggest that application of FGFR inhibitors may have the potential to promote remyelination in the CNS.

Human Endogenous Retrovirus as Therapeutic Targets in Neurologic Disease

Human endogenous retroviruses (HERVs) are ancient retroviral DNA sequences established into germline. They contain regulatory elements and encoded proteins few of which may provide benefits to hosts when co-opted as cellular genes. Their tight regulation is mainly achieved by epigenetic mechanisms, which can be altered by environmental factors, e.g., viral infections, leading to HERV activation. The aberrant expression of HERVs associates with neurological diseases, such as multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS), inflammatory processes and neurodegeneration. This review summarizes the recent advances on the epigenetic mechanisms controlling HERV expression and the pathogenic effects triggered by HERV de-repression. This article ends by describing new, promising therapies, targeting HERV elements, one of which, temelimab, has completed phase II trials with encouraging results in treating MS. The information gathered here may turn helpful in the design of new strategies to unveil epigenetic failures behind HERV-triggered diseases, opening new possibilities for druggable targets and/or for extending the use of temelimab to treat other associated diseases.

Human Glial Progenitor Cells Effectively Remyelinate the Demyelinated Adult Brain

Neonatally transplanted human glial progenitor cells (hGPCs) can myelinate the brains of myelin-deficient shiverer mice, rescuing their phenotype and survival. Yet, it has been unclear whether implanted hGPCs are similarly able to remyelinate the diffusely demyelinated adult CNS. We, therefore, ask if hGPCs could remyelinate both congenitally hypomyelinated adult shiverers and normal adult mice after cuprizone demyelination. In adult shiverers, hGPCs broadly disperse and differentiate as myelinating oligodendrocytes after subcortical injection, improving both host callosal conduction and ambulation. Implanted hGPCs similarly remyelinate denuded axons after cuprizone demyelination, whether delivered before or after demyelination. RNA sequencing (RNA-seq) of hGPCs back from cuprizone-demyelinated brains reveals their transcriptional activation of oligodendrocyte differentiation programs, while distinguishing them from hGPCs not previously exposed to demyelination. These data indicate the ability of transplanted hGPCs to disperse throughout the adult CNS, to broadly myelinate regions of dysmyelination, and also to be recruited as myelinogenic oligodendrocytes later in life, upon demyelination-associated demand.

Treg cell-derived osteopontin promotes microglia-mediated white matter repair after ischemic stroke

The precise mechanisms underlying the beneficial effects of regulatory T (Treg) cells on long-term tissue repair remain elusive. Here, using single-cell RNA sequencing and flow cytometry, we found that Treg cells infiltrated the brain 1 to 5 weeks after experimental stroke in mice. Selective depletion of Treg cells diminished oligodendrogenesis, white matter repair, and functional recovery after stroke. Transcriptomic analyses revealed potent immunomodulatory effects of brain-infiltrating Treg cells on other immune cells, including monocyte-lineage cells. Microglia depletion, but not T cell lymphopenia, mitigated the beneficial effects of transferred Treg cells on white matter regeneration. Mechanistically, Treg cell-derived osteopontin acted through integrin receptors on microglia to enhance microglial reparative activity, consequently promoting oligodendrogenesis and white matter repair. Increasing Treg cell numbers by delivering IL-2:IL-2 antibody complexes after stroke improved white matter integrity and rescued neurological functions over the long term. These findings reveal Treg cells as a neurorestorative target for stroke recovery.

Protein Kinase C Activation Drives a Differentiation Program in an Oligodendroglial Precursor Model through the Modulation of Specific Biological Networks

Protein kinase C (PKC) activation induces cellular reprogramming and differentiation in various cell models. Although many effectors of PKC physiological actions have been elucidated, the molecular mechanisms regulating oligodendrocyte differentiation after PKC activation are still unclear. Here, we applied a liquid chromatography–mass spectrometry (LC–MS/MS) approach to provide a comprehensive analysis of the proteome expression changes in the MO3.13 oligodendroglial cell line after PKC activation. Our findings suggest that multiple networks that communicate and coordinate with each other may finally determine the fate of MO3.13 cells, thus identifying a modular and functional biological structure. In this work, we provide a detailed description of these networks and their participating components and interactions. Such assembly allows perturbing each module, thus describing its physiological significance in the differentiation program. We applied this approach by targeting the Rho-associated protein kinase (ROCK) in PKC-activated cells. Overall, our findings provide a resource for elucidating the PKC-mediated network modules that contribute to a more robust knowledge of the molecular dynamics leading to this cell fate transition.

Reduction in pericyte coverage leads to blood-brain barrier dysfunction via endothelial transcytosis following chronic cerebral hypoperfusion

Background

Chronic cerebral hypoperfusion (CCH) is the leading cause of cerebral small vessel disease (CSVD). CCH is strongly associated with blood–brain barrier (BBB) dysfunction and white matter lesions (WMLs) in CSVD. However, the effects of CCH on BBB integrity and components and the cellular and molecular mechanisms underlying the effects of BBB dysfunction remain elusive. Whether maintaining BBB integrity can reverse CCH-induced brain damage has also not been explored.

Methods

In this study, we established a rat model of CSVD via permanent bilateral common carotid artery occlusion (2VO) to mimic the chronic hypoperfusive state of CSVD. The progression of BBB dysfunction and components of the BBB were assessed using immunostaining, Western blotting, transmission electron microscopy (TEM) and RNA sequencing. We also observed the protective role of imatinib, a tyrosine kinase inhibitor, on BBB integrity and neuroprotective function following CCH. The data were analyzed using one-way or two-way ANOVA.

Results

We noted transient yet severe breakdown of the BBB in the corpus callosum (CC) following CCH. The BBB was severely impaired as early as 1 day postoperation and most severely impaired 3 days postoperation. BBB breakdown preceded neuroinflammatory responses and the formation of WMLs. Moreover, pericyte loss was associated with BBB impairment, and the accumulation of serum protein was mediated by increased endothelial transcytosis in the CC. RNA sequencing also revealed increased transcytosis genes expression. BBB dysfunction led to brain damage through regulation of TGF-β/Smad2 signaling. Furthermore, imatinib treatment ameliorated serum protein leakage, oligodendrocyte progenitor cell (OPC) activation, endothelial transcytosis, microglial activation, and aberrant TGF-β/Smad2 signaling activation.

Conclusions

Our results indicate that reduced pericyte coverage leads to increased BBB permeability via endothelial transcytosis. Imatinib executes a protective role on the BBB integrity via inhibition of endothelial transcytosis. Maintenance of BBB integrity ameliorates brain damage through regulation of TGF-β/Smad2 signaling following CCH; therefore, reversal of BBB dysfunction may be a promising strategy for CSVD treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00255-2.

Identification of First-in-Class Inhibitors of Kallikrein-Related Peptidase 6 That Promote Oligodendrocyte Differentiation

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system (CNS) that causes severe motor, sensory, and cognitive impairments. Kallikrein-related peptidase (KLK)6 is the most abundant serine protease secreted in the CNS, mainly by oligodendrocytes, the myelin-producing cells of the CNS, and KLK6 is assumed to be a robust biomarker of MS, since it is highly increased in the cerebrospinal fluid (CSF) of MS patients. Here, we report the design and biological evaluation of KLK6's low-molecular-weight inhibitors, para-aminobenzyl derivatives. Interestingly, selected hit compounds were selective of the KLK6 proteolytic network encompassing KLK1 and plasmin that also participate in the development of MS physiopathology. Moreover, hits were found noncytotoxic on primary cultures of murine neurons and oligodendrocyte precursor cells (OPCs). Among them, two compounds (32 and 42) were shown to promote the differentiation of OPCs into mature oligodendrocytes in vitro constituting thus emerging leads for the development of regenerative therapies.

Oligodendrocytes are susceptible to Zika virus infection in a mouse model of perinatal exposure: Implications for CNS complications

Some children with proven intrauterine Zika virus (ZIKV) infection who were born asymptomatic subsequently manifested neurodevelopmental delays, pointing to impairment of development perinatally and postnatally. To model this, we infected postnatal day (P) 5-6 (equivalent to the perinatal period in humans) susceptible mice with a mammalian cell-propagated ZIKV clinical isolate from the Brazilian outbreak in 2015. All infected mice appeared normal up to 4 days post-intraperitoneal inoculation (dpi), but rapidly developed severe clinical signs at 5-6 dpi. All nervous tissue examined at 5/6 dpi appeared grossly normal. However, anti-ZIKV positive cells were observed in the optic nerve, brain, and spinal cord; predominantly in white matter. Co-labeling with cell type specific markers demonstrated oligodendrocytes and astrocytes support productive infection. Rarely, ZIKV positive neurons were observed. In spinal cord white matter, which we examined in detail, apoptotic cells were evident; the density of oligodendrocytes was significantly reduced; and there was localized microglial reactivity including expression of the NLRP3 inflammasome. Together, our observations demonstrate that a clinically relevant ZIKV isolate can directly impact oligodendrocytes. As primary oligodendrocyte cell death can lead later to secondary autoimmune demyelination, our observations may help explain neurodevelopmental delays in infants appearing asymptomatic at birth and commend lifetime surveillance.

Oxidative stress and impaired oligodendrocyte precursor cell differentiation in neurological disorders

Oligodendrocyte precursor cells (OPCs) account for 5% of the resident parenchymal central nervous system glial cells. OPCs are not only a back-up for the loss of oligodendrocytes that occurs due to brain injury or inflammation-induced demyelination (remyelination) but are also pivotal in plastic processes such as learning and memory (adaptive myelination). OPC differentiation into mature myelinating oligodendrocytes is controlled by a complex transcriptional network and depends on high metabolic and mitochondrial demand. Mounting evidence shows that OPC dysfunction, culminating in the lack of OPC differentiation, mediates the progression of neurodegenerative disorders such as multiple sclerosis, Alzheimer's disease and Parkinson's disease. Importantly, neurodegeneration is characterised by oxidative and carbonyl stress, which may primarily affect OPC plasticity due to the high metabolic demand and a limited antioxidant capacity associated with this cell type. The underlying mechanisms of how oxidative/carbonyl stress disrupt OPC differentiation remain enigmatic and a focus of current research efforts. This review proposes a role for oxidative/carbonyl stress in interfering with the transcriptional and metabolic changes required for OPC differentiation. In particular, oligodendrocyte (epi)genetics, cellular defence and repair responses, mitochondrial signalling and respiration, and lipid metabolism represent key mechanisms how oxidative/carbonyl stress may hamper OPC differentiation in neurodegenerative disorders. Understanding how oxidative/carbonyl stress impacts OPC function may pave the way for future OPC-targeted treatment strategies in neurodegenerative disorders.

Many roles for oligodendrocyte precursor cells in physiology and pathology

Oligodendrocyte precursor cells (OPCs) are a fourth resident glial cell population in the mammalian central nervous system. They are evenly distributed throughout the gray and white matter and continue to proliferate and generate new oligodendrocytes (OLs) throughout life. They were understudied until a few decades ago when immunolabeling for NG2 and platelet-derived growth factor receptor alpha revealed cells that are distinct from mature OLs, astrocytes, neurons, and microglia. In this review, we provide a summary of the known properties of OPCs with some historical background, followed by highlights from recent studies that suggest new roles for OPCs in certain pathological conditions.

FGF/FGFR Pathways in Multiple Sclerosis and in Its Disease Models

Multiple sclerosis (MS) is a chronic inflammatory and neurodegenerative disease of the central nervous system (CNS) affecting more than two million people worldwide. In MS, oligodendrocytes and myelin sheaths are destroyed by autoimmune-mediated inflammation, while remyelination is impaired. Recent investigations of post-mortem tissue suggest that Fibroblast growth factor (FGF) signaling may regulate inflammation and myelination in MS. FGF2 expression seems to correlate positively with macrophages/microglia and negatively with myelination; FGF1 was suggested to promote remyelination. In myelin oligodendrocyte glycoprotein (MOG)35-55-induced experimental autoimmune encephalomyelitis (EAE), systemic deletion of FGF2 suggested that FGF2 may promote remyelination. Specific deletion of FGF receptors (FGFRs) in oligodendrocytes in this EAE model resulted in a decrease of lymphocyte and macrophage/microglia infiltration as well as myelin and axon degeneration. These effects were mediated by ERK/Akt phosphorylation, a brain-derived neurotrophic factor, and downregulation of inhibitors of remyelination. In the first part of this review, the most important pharmacotherapeutic principles for MS will be illustrated, and then we will review recent advances made on FGF signaling in MS. Thus, we will suggest application of FGFR inhibitors, which are currently used in Phase II and III cancer trials, as a therapeutic option to reduce inflammation and induce remyelination in EAE and eventually MS.

Overcoming the inhibitory microenvironment surrounding oligodendrocyte progenitor cells following experimental demyelination

Chronic demyelination in the human CNS is characterized by an inhibitory microenvironment that impairs recruitment and differentiation of oligodendrocyte progenitor cells (OPCs) leading to failed remyelination and axonal atrophy. By network-based transcriptomics, we identified sulfatase 2 (Sulf2) mRNA in activated human primary OPCs. Sulf2, an extracellular endosulfatase, modulates the signaling microenvironment by editing the pattern of sulfation on heparan sulfate proteoglycans. We found that Sulf2 was increased in demyelinating lesions in multiple sclerosis and was actively secreted by human OPCs. In experimental demyelination, elevated OPC Sulf1/2 expression directly impaired progenitor recruitment and subsequent generation of oligodendrocytes thereby limiting remyelination. Sulf1/2 potentiates the inhibitory microenvironment by promoting BMP and WNT signaling in OPCs. Importantly, pharmacological sulfatase inhibition using PI-88 accelerated oligodendrocyte recruitment and remyelination by blocking OPC-expressed sulfatases. Our findings define an important inhibitory role of Sulf1/2 and highlight the potential for modulation of the heparanome in the treatment of chronic demyelinating disease.

Demyelination results in impairments in oligodendrocyte progenitor cell recruitment. Here the authors identify sulfatase 1/2 as a potential modulator of myelination by modulating the microenvironment around oligodendrocyte progenitor cells.

One-step Reprogramming of Human Fibroblasts into Oligodendrocyte-like Cells by SOX10, OLIG2, and NKX6.2

Limited access to human oligodendrocytes impairs better understanding of oligodendrocyte pathology in myelin diseases. Here, we describe a method to robustly convert human fibroblasts directly into oligodendrocyte-like cells (dc-hiOLs), which allows evaluation of remyelination-promoting compounds and disease modeling. Ectopic expression of SOX10, OLIG2, and NKX6.2 in human fibroblasts results in rapid generation of O4⁺ cells, which further differentiate into MBP⁺ mature oligodendrocyte-like cells within 16 days. dc-hiOLs undergo chromatin remodeling to express oligodendrocyte markers, ensheath axons, and nanofibers in vitro, respond to promyelination compound treatment, and recapitulate in vitro oligodendroglial pathologies associated with Pelizaeus-Merzbacher leukodystrophy related to PLP1 mutations. Furthermore, DNA methylome analysis provides evidence that the CpG methylation pattern significantly differs between dc-hiOLs derived from fibroblasts of young and old donors, indicating the maintenance of the source cells’ “age.” In summary, dc-hiOLs represent a reproducible technology that could contribute to personalized medicine in the field of myelin diseases.

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SOX10, OLIG2, and NKX6.2 directly convert human fibroblasts into dc-hiOLs in 16 days

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dc-hiOLs express key oligodendrocyte markers

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dc-hiOLs preserve the epigenetic age of donor cells

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dc-hiOLs from PMD patients show maturation deficit and vulnerability to cell death

Age-dependent decline in remyelination capacity is mediated by apelin–APJ signaling

Age-related regeneration failure in the central nervous system can occur as a result of a decline in remyelination efficacy. The responsiveness of myelin-forming cells to signals for remyelination is affected by aging-related epigenetic modification; however, the molecular mechanism is not fully clarified. In the present study, we report that the apelin receptor (APJ) mediates remyelination efficiency with age. APJ expression in myelin-forming cells is correlated with age-associated changes in remyelination efficiency, and the activation of APJ promotes remyelination through the translocation of myelin regulatory factor. APJ signaling activation promoted remyelination in both aged mice with toxin-induced demyelination and mice with experimental autoimmune encephalomyelitis. In human cells, APJ activation enhanced the expression of remyelination markers. Impaired oligodendrocyte function in aged animals can be reversibly reactivated; thus, the results demonstrate that dysfunction of the apelin-APJ system mediates remyelination failure in aged animals, and that their myelinating function can be reactivated by APJ activation.

The Current Challenges for Drug Discovery in CNS Remyelination

The myelin sheath wraps around axons, allowing saltatory currents to be transmitted along neurons. Several genetic, viral, or environmental factors can damage the central nervous system (CNS) myelin sheath during life. Unless the myelin sheath is repaired, these insults will lead to neurodegeneration. Remyelination occurs spontaneously upon myelin injury in healthy individuals but can fail in several demyelination pathologies or as a consequence of aging. Thus, pharmacological intervention that promotes CNS remyelination could have a major impact on patient’s lives by delaying or even preventing neurodegeneration. Drugs promoting CNS remyelination in animal models have been identified recently, mostly as a result of repurposing phenotypical screening campaigns that used novel oligodendrocyte cellular models. Although none of these have as yet arrived in the clinic, promising candidates are on the way. Many questions remain. Among the most relevant is the question if there is a time window when remyelination drugs should be administrated and why adult remyelination fails in many neurodegenerative pathologies. Moreover, a significant challenge in the field is how to reconstitute the oligodendrocyte/axon interaction environment representative of healthy as well as disease microenvironments in drug screening campaigns, so that drugs can be screened in the most appropriate disease-relevant conditions. Here we will provide an overview of how the field of in vitro models developed over recent years and recent biological findings about how oligodendrocytes mature after reactivation of their staminal niche. These data have posed novel questions and opened new views about how the adult brain is repaired after myelin injury and we will discuss how these new findings might change future drug screening campaigns for CNS regenerative drugs.