Central Nervous System Remyelination Research Articles

Deciphering the role of sphingosine 1-phosphate in central nervous system myelination and repair.

Sphingosine 1-phosphate (S1P) is a bioactive lipid of the sphingolipid family and plays a pivotal role in the mammalian nervous system. Indeed, S1P is a therapeutic target for treating demyelinating diseases such as multiple sclerosis. Being part of an interconnected sphingolipid metabolic network, the amount of S1P available for signalling is equilibrated between its synthetic (sphingosine kinases 1 and 2) and degradative (sphingosine 1-phosphate lyase) enzymes. Once produced, S1P exerts its biological roles via signalling to a family of five G protein-coupled S1P receptors 1-5 (S1PR1-5). Despite significant progress, the precise roles that S1P metabolism and downstream signalling play in regulating myelin formation and repair remain largely opaque and somewhat controversial. Genetic or pharmacological studies adopting various model systems identify that stimulating S1P-S1PR signalling protects myelin-forming oligodendrocytes after central nervous system (CNS) injury and attenuates demyelination invivo. However, evidence to support its role in remyelination of the mammalian CNS is limited, although blocking S1P synthesis sheds light on the role of endogenous S1P in promoting CNS remyelination. This review focuses on summarising the current understanding of S1P in CNS myelin formation and repair, discussing the complexity of S1P-S1PR interaction and the underlying mechanism by which S1P biosynthesis and signalling regulates oligodendrocyte myelination in the healthy and injured mammalian CNS, raising new questions for future investigation.

Zinc-Finger Protein ZFP488 Regulates the Timing of Oligodendrocyte Myelination and Remyelination.

Oligodendrocyte myelination and remyelination after injury are intricately regulated by various intrinsic and extrinsic factors, including transcriptional regulators. Among these, the zinc-finger protein ZFP488 is an oligodendrocyte-enriched transcriptional regulator that promotes oligodendrocyte differentiation in the developing neural tube and in oligodendroglial cell lines. However, the specific in vivo genetic requirements for ZFP488 during oligodendrocyte development and remyelination have not been defined. To address this gap, we generated a lineage-traceable ZFP488 knock-out mouse line, wherein an H2b-GFP reporter replaces the ZFP488-coding region. Using these mice of either sex, we examined the dynamics of ZFP488 expression from the endogenous promoter in the developing central nervous system (CNS). We observed a unique expression pattern in the oligodendrocyte lineage, with ZFP488 expression particularly enriched in differentiated oligodendrocytes. ZFP488 loss resulted in delayed myelination in the developing CNS and impaired remyelination after demyelinating injury in the brain. Integrated transcriptomic and genomic profiling further revealed that ZFP488 loss decreased the expression of myelination-associated genes but not oligodendrocyte progenitor-associated genes, suggesting that ZFP488 serves as a positive regulator of myelination by regulating maturation programs. Thus, our genetic loss-of-function study revealed that ZFP488 regulates a stage-dependent differentiation program that controls the timing of CNS myelination and remyelination.

Innate immune training restores pro-reparative myeloid functions to promote remyelination in the aged central nervous system

The reduced ability of the central nervous system to regenerate with increasing age limits functional recovery following demyelinating injury. Previous work has shown that myelin debris can overwhelm the metabolic capacity of microglia, thereby impeding tissue regeneration in aging, but the underlying mechanisms are unknown. In a model of demyelination, we found that a substantial number of genes that were not effectively activated in aged myeloid cells displayed epigenetic modifications associated with restricted chromatin accessibility. Ablation of two class I histone deacetylases in microglia was sufficient to restore the capacity of aged mice to remyelinate lesioned tissue. We used Bacillus Calmette-Guerin (BCG), a live-attenuated vaccine, to train the innate immune system and detected epigenetic reprogramming of brain-resident myeloid cells and functional restoration of myelin debris clearance and lesion recovery. Our results provide insight into aging-associated decline in myeloid function and how this decay can be prevented by innate immune reprogramming.

Ageing impairs the regenerative capacity of regulatory T cells in mouse central nervous system remyelination

Myelin regeneration (remyelination) is essential to prevent neurodegeneration in demyelinating diseases such as Multiple Sclerosis, however, its efficiency declines with age. Regulatory T cells (Treg) recently emerged as critical players in tissue regeneration, including remyelination. However, the effect of ageing on Treg-mediated regenerative processes is poorly understood. Here, we show that expansion of aged Treg does not rescue age-associated remyelination impairment due to an intrinsically diminished capacity of aged Treg to promote oligodendrocyte differentiation and myelination in male and female mice. This decline in regenerative Treg functions can be rescued by a young environment. We identified Melanoma Cell Adhesion Molecule 1 (MCAM1) and Integrin alpha 2 (ITGA2) as candidates of Treg-mediated oligodendrocyte differentiation that decrease with age. Our findings demonstrate that ageing limits the neuroregenerative capacity of Treg, likely limiting their remyelinating therapeutic potential in aged patients, and describe two mechanisms implicated in Treg-driven remyelination that may be targetable to overcome this limitation.

Exosomal miR-23a-3p derived from human umbilical cord mesenchymal stem cells promotes remyelination in central nervous system demyelinating diseases by targeting Tbr1/Wnt pathway

Oligodendrocyte precursor cells are present in the adult central nervous system, and their impaired ability to differentiate into myelinating oligodendrocytes can lead to demyelination in patients with multiple sclerosis, accompanied by neurological deficits and cognitive impairment. Exosomes, small vesicles released by cells, are known to facilitate intercellular communication by carrying bioactive molecules. In this study, we utilized exosomes derived from human umbilical cord mesenchymal stem cells (HUMSCs-Exos). We performed sequencing and bioinformatics analysis of exosome-treated cells to demonstrate that HUMSCs-Exos can stimulate myelin gene expression in oigodendrocyte precursor cells. Functional investigations revealed that HUMSCs-Exos activate the Pi3k/Akt pathway and regulate the Tbr1/Wnt signaling molecules through the transfer of miR-23a-3p, promoting oligodendrocytes differentiation and enhancing the expression of myelin-related proteins. In an experimental autoimmune encephalomyelitis model, treatment with HUMSCs-Exos significantly improved neurological function and facilitated remyelination. This study provides cellular and molecular insights into the use of cell-free exosome therapy for central nervous system demyelination associated with multiple sclerosis, demonstrating its great potential for treating demyelinating and neurodegenerative diseases.

Human iPSC-derived endothelial cells promote CNS remyelination via BDNF and mTORC1 pathway.

Damage of myelin is a component of many diseases in the central nervous system (CNS). The activation and maturation of the quiescent oligodendrocyte progenitor cells (OPCs) are the crucial cellular processes for CNS remyelination, which is influenced by neuroinflammation in the lesion microenvironment. Endothelial cells derived from human induced pluripotent stem cells (hiPSC-ECs) have shown promise in restoring function in various preclinical animal models. Here we ask whether and whether transplantation of hiPSC-ECs could benefit remyelination in a mouse model of CNS demyelination. Our results show that in vitro, hiPSC-ECs increase OPC proliferation, migration and differentiation via secreted soluble factors including brain-derived neurotrophic factor (BDNF). hiPSC-ECs also promote the survival of oligodendrocyte lineage cells in vitro and in vivo. Transplantation of hiPSC-ECs into a toxin-induced demyelination lesion in mouse corpus callosum (CC) leads to increased density of oligodendrocyte lineage cells and level of myelin in demyelinated area, correlated with a decreased neuroinflammation and an increased proportion of pro-regenerative M2 phenotype in microglia/macrophages. The hiPSC-EC-exposed oligodendrocyte lineage cells showed significant increase in the level of phosphorylated S6 ribosomal protein (pS6) both in vitro and in vivo, indicating an involvement of mTORC1 pathway. These results suggest that hiPSC-ECs may benefit myelin protection and regeneration which providing a potential source of cell therapy for a wide range of diseases and injuries associated with myelin damage.

A Phase 1b, Open-Label Study to Evaluate the Safety and Tolerability of the Putative Remyelinating Agent, Liothyronine, in Individuals with MS

Thyroid hormones are essential during developmental myelination and may play a direct role in remyelination and repair in the adult central nervous system by promoting the differentiation of oligodendrocyte precursor cells into mature oligodendrocytes. Since tri-iodothyronine (T3) is believed to mediate the majority of important thyroid hormone actions, liothyronine (synthetic T3) has the potential to induce reparative mechanisms and limit neurodegeneration in multiple sclerosis (MS). We completed a phase 1b clinical trial to determine the safety and tolerability of ascending doses of liothyronine in individuals with relapsing and progressive MS. A total of 20 people with MS were enrolled in this single-center trial of oral liothyronine. Eighteen participants completed the 24-week study. Our study cohort included mostly women (11/20), majority relapsing MS (12/20), mean age of 46, and baseline median EDSS of 3.5. Liothyronine was tolerated well without treatment-related severe/serious adverse events or evidence of disease activation/clinical deterioration. The most common adverse events included gastrointestinal distress and abnormal thyroid function tests. No clinical thyrotoxicosis occurred. Importantly, we did not observe a negative impact on secondary clinical outcome measures. The CSF proteomic changes suggest a biological effect of T3 treatment within the CNS. We noted changes primarily in proteins associated with immune cell function and angiogenesis. Liothyronine appeared safe and was well tolerated in people with MS. A larger clinical trial will help assess whether liothyronine can promote oligodendrogenesis and enhance remyelination in vivo, limit axonal degeneration, or improve function.

Glial aging and its impact on central nervous system myelin regeneration.

Aging is a major risk factor for several neurodegenerative diseases and is associated with cognitive decline. In addition to affecting neuronal function, the aging process significantly affects the functional phenotype of the glial cell compartment, comprising oligodendrocyte lineage cells, astrocytes, and microglia. These changes result in a more inflammatory microenvironment, resulting in a condition that is favorable for neuron and synapse loss. In addition to facilitating neurodegeneration, the aging glial cell population has negative implications for central nervous system remyelination, a regenerative process that is of particular importance to the chronic demyelinating disease multiple sclerosis. This review will discuss the changes that occur with aging in the three main glial populations and provide an overview of the studies documenting the impact these changes have on remyelination.

Animal models to investigate the effects of inflammation on remyelination in multiple sclerosis.

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating, and neurodegenerative disease of the central nervous system (CNS). In people with MS, impaired remyelination and axonal loss lead to debilitating long-term neurologic deficits. Current MS disease-modifying drugs mainly target peripheral immune cells and have demonstrated little efficacy for neuroprotection or promoting repair. To elucidate the pathological mechanisms and test therapeutic interventions, multiple animal models have been developed to recapitulate specific aspects of MS pathology, particularly the acute inflammatory stage. However, there are few animal models that facilitate the study of remyelination in the presence of inflammation, and none fully replicate the biology of chronic demyelination in MS. In this review, we describe the animal models that have provided insight into the mechanisms underlying demyelination, myelin repair, and potential therapeutic targets for remyelination. We highlight the limitations of studying remyelination in toxin-based demyelination models and discuss the combinatorial models that recapitulate the inflammatory microenvironment, which is now recognized to be a major inhibitor of remyelination mechanisms. These models may be useful in identifying novel therapeutics that promote CNS remyelination in inflammatory diseases such as MS.

The ApoA-I mimetic peptide 5A enhances remyelination by promoting clearance and degradation of myelin debris.

The progressive nature of demyelinating diseases lies in the inability of the central nervous system (CNS) to induce proper remyelination. Recently, we and others demonstrated that a dysregulated innate immune response partially underlies failure of CNS remyelination. Extensive accumulation of myelin-derived lipids and an inability to process these lipids was found to induce a disease-promoting phagocyte phenotype. Hence, restoring the ability of these phagocytes to metabolize and efflux myelin-derived lipids represents a promising strategy to promote remyelination. Here, we show that ApoA-I mimetic peptide 5A, a molecule well known to promote activity of the lipid efflux transporter ABCA1, markedly enhances remyelination. Mechanistically, we find that the repair-inducing properties of 5A are attributable to increased clearance and metabolism of remyelination-inhibiting myelin debris via the fatty acid translocase protein CD36, which is transcriptionally controlled by the ABCA1-JAK2-STAT3 signaling pathway. Altogether, our findings indicate that 5A promotes remyelination by stimulating clearance and degradation of myelin debris.

Abstract 4 Umbilical Cord-Derived Mesenchymal Stromal Cells Suppress Neuroinflammation and Promote Remyelination in the Spinal Cord

IntroductionMesenchymal stromal cells (MSCs) are one of the most widely tested cell therapies due to their ability to suppress inflammation. Demyelinating diseases of the central nervous system (CNS) are commonly accompanied by neuroinflammation, including infiltrating proinflammatory leukocytes and activation of resident macrophages (microglia and border-associated macrophages in the meninges). The precise mechanism of how MSCs suppress neuroinflammation is unclear; however, we recently described a novel pathway where MSCs physically interact with macrophages to reprogram the immune response through the transfer of a cytoplasmic organelle called a processing body (p-body).ObjectiveWe set out to study how human umbilical cord tissue-derived MSCs interact with CNS microglia and border macrophages to protect in a mouse model of spinal cord demyelination.MethodsWe manufactured several MSC lines under GMP conditions and tested their ability to interact with CNS macrophages and promote remyelination in multiple preclinical assays. To determine if p-bodies are critical for these steps, we depleted p-bodies using CRISPR/Cas9 targeting to DDX6. To test for effects on acute demyelination, we injected lysophosphatidylcholine (LPC) into the spinal cord or added LPC to organotypic cerebellar slice cultures. To test direct effects on microglia activation, we treated primary microglia or a microglia cell line with lipopolysaccharide.ResultsWe determined that MSC promoted remyelination in the spinal cord and that this was dependent on p-bodies. MSC directly contacted CNS macrophages and inhibited their activation. Using primary microglia and a microglia cell line, we determined that MSCs suppress the release of proinflammation cytokines, IL-6, and TNF.DiscussionWe determined that MSCs suppress microglial cell activation and enhance remyelination of the spinal cord. Our study offers insight into a novel mechanism about how MSCs can alter the immune response via transferring p-bodies and enhance remyelination in CNS. Furthermore, our results may lead to a quick and simple potency assay to establish the efficacy of MSCs for suppressing neuroinflammation.

GABAB receptor agonist baclofen promotes central nervous system remyelination.

Promoting remyelination is considered as a potential neurorepair strategy to prevent/limit the development of permanent neurological disability in patients with multiple sclerosis (MS). To this end, a number of clinical trials are investigating the potential of existing drugs to enhance oligodendrocyte progenitor cell (OPC) differentiation, a process that fails in chronic MS lesions. We previously reported that oligodendroglia express GABAB receptors (GABAB Rs) both in vitro and in vivo, and that GABAB R-mediated signaling enhances OPC differentiation and myelin protein expression in vitro. Our goal here was to evaluate the pro-remyelinating potential of GABAB R agonist baclofen (Bac), a clinically approved drug to treat spasticity in patients with MS. We first demonstrated that Bac increases myelin protein production in lysolecithin (LPC)-treated cerebellar slices. Importantly, Bac administration to adult mice following induction of demyelination by LPC injection in the spinal cord resulted in enhanced OPC differentiation and remyelination. Thus, our results suggest that Bac repurposing should be considered as a potential therapeutic strategy to stimulate remyelination in patients with MS.

Hedgehog Signaling in CNS Remyelination.

Remyelination is a fundamental repair process in the central nervous system (CNS) that is triggered by demyelinating events. In demyelinating diseases, oligodendrocytes (OLs) are targeted, leading to myelin loss, axonal damage, and severe functional impairment. While spontaneous remyelination often fails in the progression of demyelinating diseases, increased understanding of the mechanisms and identification of targets that regulate myelin regeneration becomes crucial. To date, several signaling pathways have been implicated in the remyelination process, including the Hedgehog (Hh) signaling pathway. This review summarizes the current data concerning the complicated roles of the Hh signaling pathway in the context of remyelination. We will highlight the open issues that have to be clarified prior to bringing molecules targeting the Hh signaling to demyelinating therapy.

PRMT1 is required for the generation of MHC-associated microglia and remyelination in the central nervous system.

Remyelination failure in multiple sclerosis leads to progressive demyelination and inflammation, resulting in neurodegeneration and clinical decline. Microglia are innate immune cells that can acquire a regenerative phenotype to promote remyelination, yet little is known about the regulators controlling the regenerative microglia activation. Herein, using a cuprizone (CPZ)-diet induced de- and remyelination mice model, we identify PRMT1 as a driver for MHC-associated microglia population required for remyelination in the central nervous system. The loss of PRMT1, but not PRMT5, in microglia resulted in impairment of the remyelination with a reduction of oligoprogenitor cell number and prolonged microgliosis and astrogliosis. Using single-cell RNA sequencing, we found eight distinct microglial clusters during the CPZ diet, and PRMT1 depleted microglia hindered the formation of the MHC-associated cluster, expressing MHCII and CD11c. Mechanistically, PRMT1-KO microglia displayed reduced the H3K27ac peaks at the promoter regions of the MHC- and IFN-associated genes and further suppressed gene expression during CPZ diet. Overall, our findings demonstrate that PRMT1 is a critical regulator of the MHC- and IFN-associated microglia, necessary for central nervous system remyelination.

C-Abl-induced Olig2 phosphorylation regulates the proliferation of oligodendrocyte precursor cells.

Oligodendrocytes (OLs), the myelinating cells in the central nervous system (CNS), are differentiated from OL progenitor cells (OPCs). The proliferation of existing OPCs is indispensable for myelination during CNS development and remyelination in response to demyelination stimulation. The transcription factor Olig2 is required for the specification of OLs and is expressed in the OL lineage. However, the post-translational modification of Olig2 in the proliferation of OPCs is poorly understood. Herein, we identified that c-Abl directly phosphorylates Olig2 mainly at the Tyr137 site, and that Olig2 phosphorylation is essential for OPC proliferation. The expression levels of c-Abl gradually decreased with brain development; moreover, c-Abl was highly expressed in OPCs. OL-specific c-Abl knockout at the developmental stage led to an insufficient proliferation of OPCs, a decreased expression of myelin-related genes, and myelination retardation. Accordingly, a c-Abl-specific kinase inhibitor suppressed OPC proliferation in vitro. Furthermore, we observed that OL-specific c-Abl knockout reduced OPC proliferation and remyelination in a cuprizone model of demyelination. In addition, we found that nilotinib, a clinically used c-Abl inhibitor, decreased the expression of myelin basic protein (Mbp) and motor coordination in mice, indicating a neurological side effect of a long-term administration of the c-Abl inhibitor. Thus, we identified the important role of c-Abl in OLs during developmental myelination and remyelination in a disease model.

Group I PAKs in myelin formation and repair of the central nervous system: what, when, and how.

p21-activated kinases (PAKs) are a family of cell division control protein 42/ras-related C3 botulinum toxin substrate 1 (Cdc42/Rac1)-activated serine/threonine kinases. Group I PAKs (PAK1-3) have distinct activation mechanisms from group II PAKs (PAK4-6) and are the focus of this review. In transformed cancer cells, PAKs regulate a variety of cellular processes and molecular pathways which are also important for myelin formation and repair in the central nervous system (CNS). De novo mutations in group I PAKs are frequently seen in children with neurodevelopmental defects and white matter anomalies. Group I PAKs regulate virtually every aspect of neuronal development and function. Yet their functions in CNS myelination and remyelination remain incompletely defined. Herein, we highlight the current understanding of PAKs in regulating cellular and molecular pathways and discuss the status of PAK-regulated pathways in oligodendrocyte development. We point out outstanding questions and future directions in the research field of group I PAKs and oligodendrocyte development.

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.

Δ9‐Tetrahydrocannabinol promotes functional remyelination in the mouse brain

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

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.

Myelin Repair: From Animal Models to Humans.

It is widely thought that brain repair does not occur, but myelin regeneration provides clear evidence to the contrary. Spontaneous remyelination may occur after injury or in multiple sclerosis (MS). However, the efficiency of remyelination varies considerably between MS patients and between the lesions of each patient. Myelin repair is essential for optimal functional recovery, so a profound understanding of the cells and mechanisms involved in this process is required for the development of new therapeutic strategies. In this review, we describe how animal models and modern cell tracing and imaging methods have helped to identify the cell types involved in myelin regeneration. In addition to the oligodendrocyte progenitor cells identified in the 1990s as the principal source of remyelinating cells in the central nervous system (CNS), other cell populations, including subventricular zone-derived neural progenitors, Schwann cells, and even spared mature oligodendrocytes, have more recently emerged as potential contributors to CNS remyelination. We will also highlight the conditions known to limit endogenous repair, such as aging, chronic inflammation, and the production of extracellular matrix proteins, and the role of astrocytes and microglia in these processes. Finally, we will present the discrepancies between observations in humans and in rodents, discussing the relationship of findings in experimental models to myelin repair in humans. These considerations are particularly important from a therapeutic standpoint.