Impaired oligodendrogenesis in the white matter of aged mice following diffuse traumatic brain injury

Senescence is a negative prognostic factor for outcome and recovery following traumatic brain injury (TBI). TBI-induced white matter injury may be partially due to oligodendrocyte demise. We hypothesized that the regenerative capacity of oligodendrocyte precursor cells (OPCs) declines with age. To test this hypothesis, the regenerative capability of OPCs in young [(10 weeks ±2 (SD)] and aged [(62 weeks ±10 (SD)] mice was studied in mice subjected to central fluid percussion injury (cFPI), a TBI model causing widespread white matter injury. Proliferating OPCs were assessed by immunohistochemistry for the proliferating cell nuclear antigen (PCNA) marker and labeled by 5-ethynyl-2'-deoxyuridine (EdU) administered daily through intraperitoneal injections (50 mg/kg) from day 2 to day 6 after cFPI. Proliferating OPCs were quantified in the corpus callosum and external capsule on day 2 and 7 post-injury (dpi). The number of PCNA/Olig2-positive and EdU/Olig2-positive cells were increased at 2dpi (p < .01) and 7dpi (p < .01), respectively, in young mice subjected to cFPI, changes not observed in aged mice. Proliferating Olig2+/Nestin+ cells were less common (p < .05) in the white matter of brain-injured aged mice, without difference in proliferating Olig2+/PDGFRα+ cells, indicating a diminished proliferation of progenitors with different spatial origin. Following TBI, co-staining for EdU/CC1/Olig2 revealed a reduced number of newly generated mature oligodendrocytes in the white matter of aged mice when compared to the young, brain-injured mice (p < .05). We observed an age-related decline of oligodendrogenesis following experimental TBI that may contribute to the worse outcome of elderly patients following TBI.

A retroviral link to vertebrate myelination through retrotransposon-RNA-mediated control of myelin gene expression

Myelin, the insulating sheath that surrounds neuronal axons, is produced by oligodendrocytes in the central nervous system (CNS). This evolutionary innovation, which first appears in jawed vertebrates, enabled rapid transmission of nerve impulses, more complex brains, and greater morphological diversity. Here, we report that RNA-level expression of RNLTR12-int, a retrotransposon of retroviral origin, is essential for myelination. We show that RNLTR12-int-encoded RNA binds to the transcription factor SOX10 to regulate transcription of myelin basic protein (Mbp, the major constituent of myelin) in rodents. RNLTR12-int-like sequences (which we name RetroMyelin) are found in all jawed vertebrates, and we further demonstrate their function in regulating myelination in two different vertebrate classes (zebrafish and frogs). Our study therefore suggests that retroviral endogenization played a prominent role in the emergence of vertebrate myelin.

Compartmentalized ciliation changes of oligodendrocytes in aged mouse optic nerve

Primary cilia are microtubule-based sensory organelles that project from the apical surface of most mammalian cells, including oligodendrocytes, which are myelinating cells of the central nervous system (CNS) that support critical axonal function. Dysfunction of CNS glia is associated with aging-related white matter diseases and neurodegeneration, and ciliopathies are known to affect CNS white matter. To investigate age-related changes in ciliary profile, we examined ciliary length and frequency in the retinogeniculate pathway, a white matter tract commonly affected by diseases of aging but in which expression of cilia has not been characterized. We found expression of Arl13b, a marker of primary cilia, in a small group of Olig2-positive oligodendrocytes in the optic nerve, optic chiasm, and optic tract in young and aged C57BL/6 wild-type mice. While the ciliary length and ciliated oligodendrocyte cells were constant in young mice in the retinogeniculate pathway, there was a significant increase in ciliary length in the anterior optic nerve as compared to the aged animals. Morphometric analysis confirmed a specific increase in the ciliation rate of CC1+ /Olig2+ oligodendrocytes in aged mice compared with young mice. Thus, the prevalence of primary cilia in oligodendrocytes in the visual pathway and the age-related changes in ciliation suggest that they may play important roles in white matter and age-associated optic neuropathies.

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.

The Roles of Caloric Restriction Mimetics in Central Nervous System Demyelination and Remyelination

The dysfunction of myelinating glial cells, the oligodendrocytes, within the central nervous system (CNS) can result in the disruption of myelin, the lipid-rich multi-layered membrane structure that surrounds most vertebrate axons. This leads to axonal degeneration and motor/cognitive impairments. In response to demyelination in the CNS, the formation of new myelin sheaths occurs through the homeostatic process of remyelination, facilitated by the differentiation of newly formed oligodendrocytes. Apart from oligodendrocytes, the two other main glial cell types of the CNS, microglia and astrocytes, play a pivotal role in remyelination. Following a demyelination insult, microglia can phagocytose myelin debris, thus permitting remyelination, while the developing neuroinflammation in the demyelinated region triggers the activation of astrocytes. Modulating the profile of glial cells can enhance the likelihood of successful remyelination. In this context, recent studies have implicated autophagy as a pivotal pathway in glial cells, playing a significant role in both their maturation and the maintenance of myelin. In this Review, we examine the role of substances capable of modulating the autophagic machinery within the myelinating glial cells of the CNS. Such substances, called caloric restriction mimetics, have been shown to decelerate the aging process by mitigating age-related ailments, with their mechanisms of action intricately linked to the induction of autophagic processes.

Oligodendrocyte pathology in Huntington's disease: from mechanisms to therapeutics

Oligodendrocytes (OLGs), highly specialized glial cells that wrap axons with myelin sheaths, are critical for brain development and function. There is new recognition of the role of OLGs in the pathogenesis of neurodegenerative diseases (NDDs), including Huntington's disease (HD), a prototypic NDD caused by a polyglutamine tract expansion in huntingtin (HTT), which results in gain- and loss-of-function effects. Clinically, HD is characterized by a constellation of motor, cognitive, and psychiatric disturbances. White matter (WM) structures, representing myelin-rich regions of the brain, are profoundly affected in HD, and recent findings reveal oligodendroglia dysfunction as an early pathological event. Here, we focus on mechanisms that underlie oligodendroglial deficits and dysmyelination in the progression of the disease, highlighting the pathogenic contributions of mutant HTT (mHTT). We also discuss potential therapeutic implications involving these molecular pathways.

Oligodendrocyte precursor cells stop sensory axons regenerating into the spinal cord

Primary somatosensory axons stop regenerating as they re-enter the spinal cord, resulting in incurable sensory loss. What arrests them has remained unclear. We previously showed that axons stop by forming synaptic contacts with unknown non-neuronal cells. Here, we identified these cells in adult mice as oligodendrocyte precursor cells (OPCs). We also found that only a few axons stop regenerating by forming dystrophic endings, exclusively at the CNS:peripheral nervous system (PNS) borderline where OPCs are absent. Most axons stop in contact with a dense network of OPC processes. Live imaging, immuno-electron microscopy (immuno-EM), and OPC-dorsal root ganglia (DRG) co-culture additionally suggest that axons are rapidly immobilized by forming synapses with OPCs. Genetic OPC ablation enables many axons to continue regenerating deep into the spinal cord. We propose that sensory axons stop regenerating by encountering OPCs that induce presynaptic differentiation. Our findings identify OPCs as a major regenerative barrier that prevents intraspinal restoration of sensory circuits following spinal root injury.

Oligodendrocyte Progenitor Cell Transplantation Ameliorates Preterm Infant Cerebral White Matter Injury in Rats Model

Background

Cerebral white matter injury (WMI) is the most common brain injury in preterm infants, leading to motor and developmental deficits often accompanied by cognitive impairment. However, there is no effective treatment. One promising approach for treating preterm WMI is cell replacement therapy, in which lost cells can be replaced by exogenous oligodendrocyte progenitor cells (OPCs).

Methods

This study developed a method to differentiate human neural stem cells (hNSCs) into human OPCs (hOPCs). The preterm WMI animal model was established in rats on postnatal day 3, and OLIG2+/NG2+/PDGFRα+/O4+ hOPCs were enriched and transplanted into the corpus callosum on postnatal day 10. Then, histological analysis and electron microscopy were used to detect lesion structure; behavioral assays were performed to detect cognitive function.

Results

Transplanted hOPCs survived and migrated throughout the major white matter tracts. Morphological differentiation of transplanted hOPCs was observed. Histological analysis revealed structural repair of lesioned areas. Re-myelination of the axons in the corpus callosum was confirmed by electron microscopy. The Morris water maze test revealed cognitive function recovery.

Conclusion

Our study showed that exogenous hOPCs could differentiate into CC1+ OLS in the brain of WMI rats, improving their cognitive functions.

Astrocytic response mediated by the CLU risk allele inhibits OPC proliferation and myelination in a human iPSC model

The C allele of rs11136000 variant in the clusterin (CLU) gene represents the third strongest known genetic risk factor for late-onset Alzheimer's disease. However, whether this single-nucleotide polymorphism (SNP) is functional and what the underlying mechanisms are remain unclear. In this study, the CLU rs11136000 SNP is identified as a functional variant by a small-scale CRISPR-Cas9 screen. Astrocytes derived from isogenic induced pluripotent stem cells (iPSCs) carrying the "C" or "T" allele of the CLU rs11136000 SNP exhibit different CLU expression levels. TAR DNA-binding protein-43 (TDP-43) preferentially binds to the "C" allele to promote CLU expression and exacerbate inflammation. The interferon response and CXCL10 expression are elevated in cytokine-treated C/C astrocytes, leading to inhibition of oligodendrocyte progenitor cell (OPC) proliferation and myelination. Accordingly, elevated CLU and CXCL10 but reduced myelin basic protein (MBP) expression are detected in human brains of C/C carriers. Our study uncovers a mechanism underlying reduced white matter integrity observed in the CLU rs11136000 risk "C" allele carriers.

Modeling epigenetic lesions that cause gliomas

Epigenetic lesions that disrupt regulatory elements represent potential cancer drivers. However, we lack experimental models for validating their tumorigenic impact. Here, we model aberrations arising in isocitrate dehydrogenase-mutant gliomas, which exhibit DNA hypermethylation. We focus on a CTCF insulator near the PDGFRA oncogene that is recurrently disrupted by methylation in these tumors. We demonstrate that disruption of the syntenic insulator in mouse oligodendrocyte progenitor cells (OPCs) allows an OPC-specific enhancer to contact and induce Pdgfra, thereby increasing proliferation. We show that a second lesion, methylation-dependent silencing of the Cdkn2a tumor suppressor, cooperates with insulator loss in OPCs. Coordinate inactivation of the Pdgfra insulator and Cdkn2a drives gliomagenesis in vivo. Despite locus synteny, the insulator is CpG-rich only in humans, a feature that may confer human glioma risk but complicates mouse modeling. Our study demonstrates the capacity of recurrent epigenetic lesions to drive OPC proliferation in vitro and gliomagenesis in vivo.

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

Purification and characterization of human neural stem and progenitor cells

The human brain undergoes rapid development at mid-gestation from a pool of neural stem and progenitor cells (NSPCs) that give rise to the neurons, oligodendrocytes, and astrocytes of the mature brain. Functional study of these cell types has been hampered by a lack of precise purification methods. We describe a method for prospectively isolating ten distinct NSPC types from the developing human brain using cell-surface markers. CD24-THY1-/lo cells were enriched for radial glia, which robustly engrafted and differentiated into all three neural lineages in the mouse brain. THY1hi cells marked unipotent oligodendrocyte precursors committed to an oligodendroglial fate, and CD24+THY1-/lo cells marked committed excitatory and inhibitory neuronal lineages. Notably, we identify and functionally characterize a transcriptomically distinct THY1hiEGFRhiPDGFRA- bipotent glial progenitor cell (GPC), which is lineage-restricted to astrocytes and oligodendrocytes, but not to neurons. Our study provides a framework for the functional study of distinct cell types in human neurodevelopment.

High-efficiency pharmacogenetic ablation of oligodendrocyte progenitor cells in the adult mouse CNS

Approaches to investigate adult oligodendrocyte progenitor cells (OPCs) by targeted cell ablation in the rodent CNS have limitations in the extent and duration of OPC depletion. We have developed a pharmacogenetic approach for conditional OPC ablation, eliminating >98% of OPCs throughout the brain. By combining recombinase-based transgenic and viral strategies for targeting OPCs and ventricular-subventricular zone (V-SVZ)-derived neural precursor cells (NPCs), we found that new PDGFRA-expressing cells born in the V-SVZ repopulated the OPC-deficient brain starting 12 days after OPC ablation. Our data reveal that OPC depletion induces V-SVZ-derived NPCs to generate vast numbers of PDGFRA+NG2+ cells with the capacity to proliferate and migrate extensively throughout the dorsal anterior forebrain. Further application of this approach to ablate OPCs will advance knowledge of the function of both OPCs and oligodendrogenic NPCs in health and disease.

Remyelination in neuromyelitis optica spectrum disorder is promoted by edaravone through mTORC1 signaling activation

Neuromyelitis optica spectrum disorder (NMOSD) is a severe inflammatory autoimmune disease of the central nervous system that is manifested as secondary myelin loss. Oligodendrocyte progenitor cells (OPCs) are the principal source of myelinating oligodendrocytes (OLs) and are abundant in demyelinated regions of NMOSD patients, thus possibly representing a cellular target for pharmacological intervention. To explore the therapeutic compounds that enhance myelination due to endogenous OPCs, we screened the candidate drugs in mouse neural progenitor cell (NPC)-derived OPCs. We identified drug edaravone, which is approved by the Food and Drug Administration (FDA), as a promoter of OPC differentiation into mature OLs. Edaravone enhanced remyelination in organotypic slice cultures and in mice, even when edaravone was administered following NMO-IgG-induced demyelination, and ameliorated motor impairment in a systemic mouse model of NMOSD. The results of mechanistic studies in NMO-IgG-treated mice and the biopsy samples of the brain tissues of NMOSD patients indicated that the mTORC1 signaling pathway was significantly inhibited, and edaravone promoted OPC maturation and remyelination by activating mTORC1 signaling. Furthermore, pharmacological activation of mTORC1 signaling significantly enhanced myelin regeneration in NMOSD. Thus, edaravone is a potential therapeutic agent that promotes lesion repair in NMOSD patients by enhancing OPC maturation.

TCF7l2, a nuclear marker that labels premyelinating oligodendrocytes and promotes oligodendroglial lineage progression

Clinical and basic neuroscience research is greatly benefited from the identification and characterization of lineage specific and developmental stage-specific markers. In the glial research community, histological markers that specifically label newly differentiated premyelinating oligodendrocytes are still scarce. Premyelinating oligodendrocyte markers, especially those of nuclear localization, enable researchers to easily quantify the rate of oligodendrocyte generation regardless of developmental ages. We propose that the transcription factor 7-like 2 (TCF7l2, mouse gene symbol Tcf7l2) is a useful nuclear marker that specifically labels newly generated premyelinating oligodendrocytes and promotes oligodendroglial lineage progression. Here, we highlight the controversial research history of TCF7l2 expression and function in oligodendroglial field and discuss previous experimental data justifying TCF7l2 as a specific nuclear marker for premyelinating oligodendrocytes during developmental myelination and remyelination. We conclude that TCF7l2 can be used alone or combined with pan-oligodendroglial lineage markers to identify newly differentiated or newly regenerated oligodendrocytes and quantify the rate of oligodendrocyte generation.

Neural stem cells and oligodendrocyte progenitor cells compete for remyelination in the corpus callosum

A major therapeutic goal in demyelinating diseases, such as Multiple Sclerosis, is to improve remyelination, thereby restoring effective axon conduction and preventing neurodegeneration. In the adult central nervous system (CNS), parenchymal oligodendrocyte progenitor cells (pOPCs) and, to a lesser extent, pre-existing oligodendrocytes (OLs) and oligodendrocytes generated from neural stem cells (NSCs) in the sub-ventricular zone (SVZ) are capable of forming new myelin sheaths. Due to their self-renewal capabilities and the ability of their progeny to migrate widely within the CNS, NSCs represent an additional source of remyelinating cells that may be targeted to supplement repair by pOPCs. However, in demyelinating disorders and disease models, the NSC contribution to myelin repair is modest and most evident in regions close to the SVZ. We hypothesized that NSC-derived cells may compete with OPCs to remyelinate the same axons, with pOPCs serving as the primary remyelinating cells due to their widespread distribution within the adult CNS, thereby limiting the contribution of NSC-progeny. Here, we have used a dual reporter, genetic fate mapping strategy, to characterize the contribution of pOPCs and NSC-derived OLs to remyelination after cuprizone-induced demyelination. We confirmed that, while pOPCs are the main remyelinating cells in the corpus callosum, NSC-derived cells are also activated and recruited to demyelinating lesions. Blocking pOPC differentiation genetically, resulted in a significant increase in the recruitment NSC-derived cells into the demyelinated corpus callosum and their differentiation into OLs. These results strongly suggest that pOPCs and NSC-progeny compete to repair white matter lesions. They underscore the potential significance of targeting NSCs to improve repair when the contribution of pOPCs is insufficient to affect full remyelination.

Activin A rescues preterm brain injury through a novel Noggin/BMP4/Id2 signaling pathway

Activin A (Act A) has been reported to promote oligodendrocyte progenitor cell (OPC) differentiation in vitro and improve neurological outcomes in adult mice. However, the roles and mechanisms of action of Act A in preterm brain injury are unknown. In the present study, P5 rats were subjected to hypoxia‑ischemia to establish a neonatal white matter injury (WMI) model and Act A was injected via the lateral ventricle. Pathological characteristics, OPC differentiation, myelination, and neurological performance were analyzed. Further, the involvement of the Noggin/BMP4/Id2 signaling pathway in the roles of Act A in WMI was explored. Act A attenuated pathological damage, promoted OPC differentiation, enhanced myelin sheath and myelinated axon formation, and improved neurological performance of WMI rats. Moreover, Act A enhanced noggin expression, which, in turn, inhibited the expression of bone morphogenetic protein 4 (BMP4) and inhibitor of DNA binding 2 (Id2). Furthermore, upregulation of Id2 completely abolished the rescue effects of Act A in WMI rats. In conclusion, the present findings suggested that Act A rescues preterm brain injury via targeting a novel Noggin/BMP4/Id2 signaling pathway.

Expression of myelin transcription factor 1 and lamin B receptor mediate neural progenitor fate transition in the zebrafish spinal cord pMN domain

The pMN domain is a restricted domain in the ventral spinal cord, defined by the expression of the olig2 gene. Though it is known that the pMN progenitor cells can sequentially generate motor neurons and oligodendrocytes, the lineages of these progenitors are controversial and how their progeny are generated is not well understood. Using single-cell RNA sequencing, here, we identified a previously unknown heterogeneity among pMN progenitors with distinct fates and molecular signatures in zebrafish. Notably, we characterized two distinct motor neuron lineages using bioinformatic analysis. We then went on to investigate specific molecular programs that regulate neural progenitor fate transition. We validated experimentally that expression of the transcription factor myt1 (myelin transcription factor 1) and inner nuclear membrane integral proteins lbr (lamin B receptor) were critical for the development of motor neurons and neural progenitor maintenance, respectively. We anticipate that the transcriptome features and molecular programs identified in zebrafish pMN progenitors will not only provide an in-depth understanding of previous findings regarding the lineage analysis of oligodendrocyte progenitor cells and motor neurons but will also help in further understanding of the molecular programming involved in neural progenitor fate transition.

A novel mouse model of diffuse midline glioma initiated in neonatal oligodendrocyte progenitor cells highlights cell-of-origin dependent effects of H3K27M

Diffuse midline glioma (DMG) is a type of lethal brain tumor that develops mainly in children. The majority of DMG harbor the K27M mutation in histone H3. Oligodendrocyte progenitor cells (OPCs) in the brainstem are candidate cells-of-origin for DMG, yet there is no genetically engineered mouse model of DMG initiated in OPCs. Here, we used the RCAS/Tv-a avian retroviral system to generate DMG in Olig2-expressing progenitors and Nestin-expressing progenitors in the neonatal mouse brainstem. PDGF-A or PDGF-B overexpression, along with p53 deletion, resulted in gliomas in both models. Exogenous overexpression of H3.3K27M had a significant effect on tumor latency and tumor cell proliferation when compared with H3.3WT in Nestin+ cells but not in Olig2+ cells. Further, the fraction of H3.3K27M-positive cells was significantly lower in DMGs initiated in Olig2+ cells relative to Nestin+ cells, both in PDGF-A and PDGF-B-driven models, suggesting that the requirement for H3.3K27M is reduced when tumorigenesis is initiated in Olig2+ cells. RNA-sequencing analysis revealed that the differentially expressed genes in H3.3K27M tumors were non-overlapping between Olig2;PDGF-B, Olig2;PDGF-A, and Nestin;PDGF-A models. GSEA analysis of PDGFA tumors confirmed that the transcriptomal effects of H3.3K27M are cell-of-origin dependent with H3.3K27M promoting epithelial-to-mesenchymal transition (EMT) and angiogenesis when Olig2 marks the cell-of-origin and inhibiting EMT and angiogenesis when Nestin marks the cell-of-origin. We did observe some overlap with H3.3K27M promoting negative enrichment of TNFA_Signaling_Via_NFKB in both models. Our study suggests that the tumorigenic effects of H3.3K27M are cell-of-origin dependent, with H3.3K27M being more oncogenic in Nestin+ cells than Olig2+ cells.

Genetically modified macrophages accelerate myelin repair

Preventing neurodegeneration‐associated disability progression in patients with multiple sclerosis (MS) remains an unmet therapeutic need. As remyelination prevents axonal degeneration, promoting this process in patients might enhance neuroprotection. In demyelinating mouse lesions, local overexpression of semaphorin 3F (Sema3F), an oligodendrocyte progenitor cell (OPC) attractant, increases remyelination. However, molecular targeting to MS lesions is a challenge. A clinically relevant paradigm for delivering Sema3F to demyelinating lesions could be to use blood‐derived macrophages as vehicles. Thus, we chose transplantation of genetically modified hematopoietic stem cells (HSCs) as means of obtaining chimeric mice with circulating Sema3F‐overexpressing monocytes. We demonstrated that Sema3F‐transduced HSCs stimulate OPC migration in a neuropilin 2 (Nrp2, Sema3F receptor)‐dependent fashion, which was conserved in middle‐aged OPCs. While demyelinating lesions induced in mice with Sema3F‐expressing blood cells showed no changes in inflammation and OPC survival, OPC recruitment was enhanced which accelerated the onset of remyelination. Our results provide a proof of concept that blood cells, particularly monocytes/macrophages, can be used to deliver pro‐remyelinating agents “at the right time and place,” suggesting novel means for remyelination‐promoting strategies in MS.

Radiation-induced neuropathological changes in the oligodendrocyte lineage with relevant clinical manifestations and therapeutic strategies

PURPOSE

With technological advancements in radiation therapy for tumors of the central nervous system (CNS), high doses of ionizing radiation can be delivered to the tumors with improved accuracy. Despite the reduction of ionizing radiation-induced toxicity to surrounding tissues of the CNS, a wide array of side effects still occurs, particularly late-delayed changes. These alterations, such as white matter damages and neurocognitive impairments, are often debilitative and untreatable, significantly affecting the quality of life of these patients, especially children. Oligodendrocytes, a major class of glial cells, have been identified to be one of the targets of radiation toxicity and are recognized be involved in late-delayed radiation-induced neuropathological changes. These cells are responsible for forming the myelin sheaths that surround and insulate axons within the CNS. Here, the effects of ionizing radiation on the oligodendrocyte lineage as well as the common clinical manifestations resulting from radiation-induced damage to oligodendrocytes will be discussed. Potential prophylactic and therapeutic strategies against radiation-induced oligodendrocyte damage will also be considered.

CONCLUSION

Oligodendrocytes and oligodendrocyte progenitor cells (OPCs) are radiosensitive cells of the CNS. Here, general responses of these cells to radiation exposure have been outlined. However, several findings have not been consistent across various studies. For instance, cognitive decline in irradiated animals was observed to be accompanied by obvious demyelination or white matter changes in several studies but not in others. Hence, further studies have to be conducted to elucidate the level of contribution of the oligodendrocyte lineage to the development of late-delayed effects of radiation exposure, as well as to classify the dose and brain region-specific responses of the oligodendrocyte lineage to radiation. Several potential therapeutic approaches against late-delayed changes have been discussed, such as the transplantation of OPCs into irradiated regions and implementation of exercise. Many of these approaches show promising results. Further elucidation of the mechanisms involved in radiation-induced death of oligodendrocytes and OPCs would certainly aid in the development of novel protective and therapeutic strategies against the late-delayed effects of radiation.

Isolation of neural stem and oligodendrocyte progenitor cells from the brain of live rats

Postnatal brain neural stem and progenitor cells (NSPCs) cluster in anatomically inaccessible stem cell niches, such as the subependymal zone (SEZ). Here, we describe a method for the isolation of NSPCs from live animals, which we term “milking.” The intracerebroventricular injection of a release cocktail, containing neuraminidase, integrin-β1-blocking antibody, and fibroblast growth factor 2, induces the controlled flow of NSPCs in the cerebrospinal fluid, where they are collected via liquid biopsies. Isolated cells retain key in vivo self-renewal properties and their cell-type profile reflects the cell composition of their source area, while the function of the niche is sustained even 8 months post-milking. By changing the target area more caudally, we also isolate oligodendrocyte progenitor cells (OPCs) from the corpus callosum. This novel approach for sampling NSPCs and OPCs paves the way for performing longitudinal studies in experimental animals, for more in vivo relevant cell culture assays, and for future clinical neuro-regenerative applications.

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Isolation of brain neural stem and oligodendrocyte progenitor cells from live rats

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Cells are induced to flow from their niche into the cerebrospinal fluid

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Neurogenesis persists despite long-term ependymal damage/loss

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Collected cells retain the properties of endogenous neural stem/progenitor cells

Met is required for oligodendrocyte progenitor cell migration in Danio rerio

During vertebrate central nervous system development, most oligodendrocyte progenitor cells (OPCs) are specified in the ventral spinal cord and must migrate throughout the neural tube until they become evenly distributed, occupying non-overlapping domains. While this process of developmental OPC migration is well characterized, the nature of the molecular mediators that govern it remain largely unknown. Here, using zebrafish as a model, we demonstrate that Met signaling is required for initial developmental migration of OPCs, and, using cell-specific knock-down of Met signaling, show that Met acts cell-autonomously in OPCs. Taken together, these findings demonstrate in vivo, the role of Met signaling in OPC migration and provide new insight into how OPC migration is regulated during development.

Chronic Exposure to Alcohol Inhibits New Myelin Generation in Adult Mouse Brain

Chronic alcohol consumption causes cognitive impairments accompanying with white matter atrophy. Recent evidence has shown that myelin dynamics remain active and are important for brain functions in adulthood. For example, new myelin generation is required for learning and memory functions. However, it remains undetermined whether alcohol exposure can alter myelin dynamics in adulthood. In this study, we examine the effect of chronic alcohol exposure on myelin dynamics by using genetic approaches to label newly generated myelin (NG2-CreERt; mT/mG). Our results indicated that alcohol exposure (either 5% or 10% in drinking water) for 3 weeks remarkably reduced mGFP + /NG2- new myelin and mGFP + /CC1 + new oligodendrocytes in the prefrontal cortex and corpus callosum of 6-month-old NG2-CreERt; mT/mG mice as compared to controls without changing the mGFP + /NG2 + oligodendrocyte precursor cells (OPCs) density, suggesting that alcohol exposure may inhibit oligodendrocyte differentiation. In support with these findings, the alcohol exposure did not significantly alter apoptotic cell number or overall MBP expression in the brains. Further, the alcohol exposure decreased the histone deacetylase1 (HDAC1) expression in mGFP + /NG2 + OPCs, implying epigenetic mechanisms were involved in the arrested OPC differentiation. Together, our results indicate that chronic exposure to alcohol can inhibit myelinogenesis in the adult mouse brain and that may contribute to alcohol-related cognitive impairments.

Oligodendrogenesis increases in hippocampal grey and white matter prior to locomotor or memory impairment in an adult mouse model of tauopathy

Myelin and axon losses are associated with cognitive decline in healthy ageing but are worse in people diagnosed with tauopathy. To determine whether tauopathy is also associated with enhanced myelin plasticity, we evaluated the behaviour of OPCs in mice that expressed a human pathological variant of microtubule-associated protein tau (MAPTP301S ). By 6 months of age (P180), MAPTP301S mice overexpressed hyperphosphorylated tau and had developed reactive gliosis in the hippocampus but had not developed overt locomotor or memory impairment. By performing cre-lox lineage tracing of adult OPCs, we determined that the number of newborn oligodendrocytes added to the hippocampus, entorhinal cortex and fimbria was equivalent in control and MAPTP301S mice prior to P150. However, between P150 and P180, significantly more new oligodendrocytes were added to these regions in the MAPTP301S mouse brain. This large increase in new oligodendrocyte number was not the result of increased OPC proliferation, nor did it alter oligodendrocyte density in the hippocampus, entorhinal cortex or fimbria, which was equivalent in P180 wild-type and MAPTP301S mice. Furthermore, the proportion of hippocampal and fimbria axons with myelin was unaffected by tauopathy. However, the proportion of myelinated axons that were ensheathed by immature myelin internodes was significantly increased in the hippocampus and fimbria of P180 MAPTP301S mice, when compared with their wild-type littermates. These data suggest that MAPTP301S transgenic mice experience significant oligodendrocyte turnover, with newborn oligodendrocytes compensating for myelin loss early in the development of tauopathy.

Safety of direct injection of oligodendrocyte progenitor cells into the spinal cord of uninjured Göttingen minipigs

OBJECTIVE

This study was conducted as a final proof-of-safety direct injection of oligodendrocyte progenitor cells into the uninjured spinal cord prior to translation to the human clinical trials.

METHODS

In this study, 107 oligodendrocyte progenitor cells (LCTOPC1, also known as AST-OPC1 and GRNOPC1) in 50-μL suspension were injected directly into the uninjured spinal cords of 8 immunosuppressed Göttingen minipigs using a specially designed stereotactic delivery device. Four additional Göttingen minipigs were given Hanks' Balanced Salt Solution and acted as the control group.

RESULTS

Cell survival and no evidence of histological damage, abnormal inflammation, microbiological or immunological abnormalities, tumor formation, or unexpected morbidity or mortality were demonstrated.

CONCLUSIONS

These data strongly support the safety of intraparenchymal injection of LCTOPC1 into the spinal cord using a model anatomically similar to that of the human spinal cord. Furthermore, this research provides guidance for future clinical interventions, including mechanisms for precise positioning and anticipated volumes of biological payloads that can be safely delivered directly into uninjured portions of the spinal cord.

Donor cell memory confers a metastable state of directly converted cells

Generation of induced oligodendrocyte progenitor cells (iOPCs) from somatic fibroblasts is a strategy for cell-based therapy of myelin diseases. However, iOPC generation is inefficient, and the resulting iOPCs exhibit limited expansion and differentiation competence. Here we overcome these limitations by transducing an optimized transcription factor combination into a permissive donor phenotype, the pericyte. Pericyte-derived iOPCs (PC-iOPCs) are stably expandable and functionally myelinogenic with high differentiation competence. Unexpectedly, however, we found that PC-iOPCs are metastable so that they can produce myelination-competent oligodendrocytes or revert to their original identity in a context-dependent fashion. Phenotypic reversion of PC-iOPCs is tightly linked to memory of their original transcriptome and epigenome. Phenotypic reversion can be disconnected from this donor cell memory effect, and in vivo myelination can eventually be achieved by transplantation of O4⁺ pre-oligodendrocytes. Our data show that donor cell source and memory can contribute to the fate and stability of directly converted cells.

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.

In Vivo Expression of Reprogramming Factor OCT4 Ameliorates Myelination Deficits and Induces Striatal Neuroprotection in Huntington's Disease

White matter atrophy has been shown to precede the massive loss of striatal GABAergic neurons in Huntington’s disease (HD). This study investigated the effects of in vivo expression of reprogramming factor octamer-binding transcription factor 4 (OCT4) on neural stem cell (NSC) niche activation in the subventricular zone (SVZ) and induction of cell fate specific to the microenvironment of HD. R6/2 mice randomly received adeno-associated virus 9 (AAV9)-OCT4, AAV9-Null, or phosphate-buffered saline into both lateral ventricles at 4 weeks of age. The AAV9-OCT4 group displayed significantly improved behavioral performance compared to the control groups. Following AAV9-OCT4 treatment, the number of newly generated NSCs and oligodendrocyte progenitor cells (OPCs) significantly increased in the SVZ, and the expression of OPC-related genes and glial cell-derived neurotrophic factor (GDNF) significantly increased. Further, amelioration of myelination deficits in the corpus callosum was observed through electron microscopy and magnetic resonance imaging, and striatal DARPP32⁺ GABAergic neurons significantly increased in the AAV9-OCT4 group. These results suggest that in situ expression of the reprogramming factor OCT4 in the SVZ induces OPC proliferation, thereby attenuating myelination deficits. Particularly, GDNF released by OPCs seems to induce striatal neuroprotection in HD, which explains the behavioral improvement in R6/2 mice overexpressing OCT4.

Heterogeneity of glial progenitor cells during the neurogenesis-to-gliogenesis switch in the developing human cerebral cortex

The heterogeneity and molecular characteristics of progenitor cells, especially glial progenitors, in the developing human cerebral cortex remain elusive. Here, we find that EGFR expression begins to sharply increase after gestational week (GW) 20, which corresponds to the beginning stages of human gliogenesis. In addition, EGFR⁺ cells are mainly distributed in the germinal zone and frequently colocalize with the stemness marker SOX2 during this period. Then, by performing single-cell RNA sequencing on these EGFR⁺ cells, we successfully enriched and characterized various glial- and neuronal-lineage progenitor cells and validated their phenotypes in fixed slices. Notably, we identified two subgroups with molecular characteristics similar to those of astrocytes, and the immunostaining results show that these cells are mainly distributed in the outer subventricular zone and might originate from the outer radial glial cells. In short, the EGFR-sorting strategy and molecular signatures in the diverse lineages provide insights into human glial development.

Non-canonical Targets of HIF1a Impair Oligodendrocyte Progenitor Cell Function

Mammalian cells respond to insufficient oxygen through transcriptional regulators called hypoxia-inducible factors (HIFs). Although transiently protective, prolonged HIF activity drives distinct pathological responses in different tissues. Using a model of chronic HIF1a accumulation in pluripotent-stem-cell-derived oligodendrocyte progenitors (OPCs), we demonstrate that HIF1a activates non-canonical targets to impair generation of oligodendrocytes from OPCs. HIF1a activated a unique set of genes in OPCs through interaction with the OPC-specific transcription factor OLIG2. Non-canonical targets, including Ascl2 and Dlx3, were sufficient to block differentiation through suppression of the oligodendrocyte regulator Sox10. Chemical screening revealed that inhibition of MEK/ERK signaling overcame the HIF1a-mediated block in oligodendrocyte generation by restoring Sox10 expression without affecting canonical HIF1a activity. MEK/ERK inhibition also drove oligodendrocyte formation in hypoxic regions of human oligocortical spheroids. This work defines mechanisms by which HIF1a impairs oligodendrocyte formation and establishes that cell-type-specific HIF1a targets perturb cell function in response to low oxygen.

Schwann cell remyelination of the central nervous system: why does it happen and what are the benefits?

Myelin sheaths, by supporting axonal integrity and allowing rapid saltatory impulse conduction, are of fundamental importance for neuronal function. In response to demyelinating injuries in the central nervous system (CNS), oligodendrocyte progenitor cells (OPCs) migrate to the lesion area, proliferate and differentiate into new oligodendrocytes that make new myelin sheaths. This process is termed remyelination. Under specific conditions, demyelinated axons in the CNS can also be remyelinated by Schwann cells (SCs), the myelinating cell of the peripheral nervous system. OPCs can be a major source of these CNS-resident SCs—a surprising finding given the distinct embryonic origins, and physiological compartmentalization of the peripheral and central nervous system. Although the mechanisms and cues governing OPC-to-SC differentiation remain largely undiscovered, it might nevertheless be an attractive target for promoting endogenous remyelination. This article will (i) review current knowledge on the origins of SCs in the CNS, with a particular focus on OPC to SC differentiation, (ii) discuss the necessary criteria for SC myelination in the CNS and (iii) highlight the potential of using SCs for myelin regeneration in the CNS.

Select neurotrophins promote oligodendrocyte progenitor cell process outgrowth in the presence of chondroitin sulfate proteoglycans

Axonal damage and the subsequent interruption of intact neuronal pathways in the spinal cord are largely responsible for the loss of motor function after injury. Further exacerbating this loss is the demyelination of neighboring uninjured axons. The post-injury environment is hostile to repair, with inflammation, a high expression of chondroitin sulfate proteoglycans (CSPGs) around the glial scar, and myelin breakdown. Numerous studies have demonstrated that treatment with the enzyme chondroitinase ABC (cABC) creates a permissive environment around a spinal lesion that permits axonal regeneration. Neurotrophic factors like brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophic factor-3 (NT-3), and ciliary neurotrophic factor (CNTF) have been used to promote neuronal survival and stimulate axonal growth. CSPGs expressed near a lesion also inhibit migration and differentiation of endogenous oligodendrocyte progenitor cells (OPCs) in the spinal cord, and cABC treatment can neutralize this inhibition. This study examined the neurotrophins commonly used to stimulate axonal regeneration after injury and their potential effects on OPCs cultured in the presence of CSPGs. The results reveal differential effects on OPCs, with BDNF and GDNF promoting process outgrowth and NT-3 stimulating differentiation of OPCs, while CNTF appears to have no observable effect. This finding suggests that certain neurotrophic agents commonly utilized to stimulate axonal regeneration after a spinal injury may also have a beneficial effect on the endogenous oligodendroglial cells as well.

Regionally Specific Human Pre-Oligodendrocyte Progenitor Cells Produce Both Oligodendrocytes and Neurons after Transplantation in a Chronically Injured Spinal Cord Rat Model after Glial Scar Ablation

Chronic spinal cord injury (SCI) is a devastating medical condition. In the acute phase after injury, there is cell loss resulting in chronic axonal damage and loss of sensory and motor function including loss of oligodendrocytes that results in demyelination of axons and further dysfunction. In the chronic phase, the inhibitory environment within the lesion including the glial scar can arrest axonal growth and regeneration and can also potentially affect transplanted cells. We hypothesized that glial scar ablation (GSA) along with cell transplantation may be required as a combinatorial therapy to achieve functional recovery, and therefore we proposed to examine the survival and fate of human induced pluripotent stem cell (iPSC) derived pre-oligodendrocyte progenitor cells (pre-OPCs) transplanted in a model of chronic SCI, whether this was affected by GSA, and whether this combination of treatments would result in functional recovery. In this study, chronically injured athymic nude (ATN) rats were allocated to one of three treatment groups: GSA only, pre-OPCs only, or GSA+pre-OPCs. We found that human iPSC derived pre-OPCs were multi-potent and retained the ability to differentiate into mainly oligodendrocytes or neurons when transplanted into the chronically injured spinal cords of rats. Twelve weeks after cell transplantation, we observed that more of the transplanted cells differentiated into oligodendrocytes when the glial scar was ablated compared with no GSA. Further, we also observed that a higher percentage of transplanted cells differentiated into V2a interneurons and motor neurons in the pre-OPCs only group when compared with GSA+pre-OPCs. This suggests that the local environment created by ablation of the glial scar may have a significant effect on the fate of cells transplanted into the injury site.

Oligodendrocyte precursor cell maturation: role of adenosine receptors

Oligodendrocyte-formed myelin sheaths allow fast synaptic transmission in the brain and their degeneration leads to demyelinating diseases such as multiple sclerosis. Remyelination requires the differentiation of oligodendrocyte progenitor cells into mature oligodendrocytes but, in chronic neurodegenerative disorders, remyelination fails due to adverse environment. Therefore, a strategy to prompt oligodendrocyte progenitor cell differentiation towards myelinating oligodendrocytes is required. The neuromodulator adenosine, and its receptors (A₁, A_2A, A_2B and A₃ receptors: A₁R, A_2AR, A_2BR and A₃R), are crucial mediators in remyelination processes. It is known that A₁Rs facilitate oligodendrocyte progenitor cell maturation and migration whereas the A₃Rs initiates apoptosis in oligodendrocyte progenitor cells. Our group of research contributed to the field by demonstrating that A_2AR and A_2BR inhibit oligodendrocyte progenitor cell maturation by reducing voltage-dependent K⁺ currents necessary for cell differentiation. The present review summarizes the possible role of adenosine receptor ligands as potential therapeutic targets in demyelinating pathologies such as multiple sclerosis.

HIFα Regulates Developmental Myelination Independent of Autocrine Wnt Signaling

The developing CNS is exposed to physiological hypoxia, under which hypoxia-inducible factor α (HIFα) is stabilized and plays a crucial role in regulating neural development. The cellular and molecular mechanisms of HIFα in developmental myelination remain incompletely understood. A previous concept proposes that HIFα regulates CNS developmental myelination by activating the autocrine Wnt/β-catenin signaling in oligodendrocyte progenitor cells (OPCs). Here, by analyzing a battery of genetic mice of both sexes, we presented in vivo evidence supporting an alternative understanding of oligodendroglial HIFα-regulated developmental myelination. At the cellular level, we found that HIFα was required for developmental myelination by transiently controlling upstream OPC differentiation but not downstream oligodendrocyte maturation and that HIFα dysregulation in OPCs but not oligodendrocytes disturbed normal developmental myelination. We demonstrated that HIFα played a minor, if any, role in regulating canonical Wnt signaling in the oligodendroglial lineage or in the CNS. At the molecular level, blocking autocrine Wnt signaling did not affect HIFα-regulated OPC differentiation and myelination. We further identified HIFα-Sox9 regulatory axis as an underlying molecular mechanism in HIFα-regulated OPC differentiation. Our findings support a concept shift in our mechanistic understanding of HIFα-regulated CNS myelination from the previous Wnt-dependent view to a Wnt-independent one and unveil a previously unappreciated HIFα-Sox9 pathway in regulating OPC differentiation.SIGNIFICANCE STATEMENT Promoting disturbed developmental myelination is a promising option in treating diffuse white matter injury, previously called periventricular leukomalacia, a major form of brain injury affecting premature infants. In the developing CNS, hypoxia-inducible factor α (HIFα) is a key regulator that adapts neural cells to physiological and pathologic hypoxic cues. The role and mechanism of HIFα in oligodendroglial myelination, which is severely disturbed in preterm infants affected with diffuse white matter injury, is incompletely understood. Our findings presented here represent a concept shift in our mechanistic understanding of HIFα-regulated developmental myelination and suggest the potential of intervening with an oligodendroglial HIFα-mediated signaling pathway to mitigate disturbed myelination in premature white matter injury.

Oligodendrocyte Intrinsic miR-27a Controls Myelination and Remyelination

Remyelination requires the generation of new oligodendrocytes (OLs), which are derived from oligodendrocyte progenitor cells (OPCs). Maturation of OPCs into OLs is a multi-step process. Here, we describe a microRNA expressed by OLs, miR-27a, as a regulator of OL development and survival. Increased levels of miR-27a were found in OPCs associated with multiple sclerosis (MS) lesions and in animal models of demyelination. Increased levels of miR-27a led to inhibition of OPC proliferation by cell-cycle arrest, as well as impaired differentiation of human OPCs (hOPCs) and myelination by dysregulating the Wnt-β-catenin signaling pathway. In vivo administration of miR-27a led to suppression of myelinogenic signals, leading to loss of endogenous myelination and remyelination. Our findings provide evidence supporting a critical role for a steady-state level of OL-specific miR-27a in supporting multiple steps in the complex process of OPC maturation and remyelination.

Particles Containing Cells as a Strategy to Promote Remyelination in Patients With Multiple Sclerosis

The repair of demyelinated lesions is a key objective in multiple sclerosis research. Remyelination fundamentally depends on oligodendrocyte progenitor cells (OPC) reaching the lesion; this is influenced by numerous factors including age, disease progression time, inflammatory activity, and the pool of OPCs available, whether they be NG2 cells or cells derived from neural stem cells. Administering OPCs has been proposed as a potential cell therapy; however, these cells can only be administered directly. This article discusses the potential administration of OPCs encapsulated within hydrogel particles composed of biocompatible biomaterials, via the nose-to-brain pathway. We also discuss conditions for the indication of this therapy, and such related issues as the influence on endogenous remyelination, migration of OPCs to demyelinated areas, and the immune response, given the autoimmune nature of multiple sclerosis. Chitosan and derivatives constitute the most promising biomaterial for this purpose, although these issues must be addressed. In conclusion, this line of research may yield an alternative to the remyelinating drugs currently being studied.

Progesterone through Progesterone Receptor B Isoform Promotes Rodent Embryonic Oligodendrogenesis

Oligodendrocytes are the myelinating cells of the central nervous system (CNS). These cells arise during the embryonic development by the specification of the neural stem cells to oligodendroglial progenitor cells (OPC); newly formed OPC proliferate, migrate, differentiate, and mature to myelinating oligodendrocytes in the perinatal period. It is known that progesterone promotes the proliferation and differentiation of OPC in early postnatal life through the activation of the intracellular progesterone receptor (PR). Progesterone supports nerve myelination after spinal cord injury in adults. However, the role of progesterone in embryonic OPC differentiation as well as the specific PR isoform involved in progesterone actions in these cells is unknown. By using primary cultures obtained from the embryonic mouse spinal cord, we showed that embryonic OPC expresses both PR-A and PR-B isoforms. We found that progesterone increases the proliferation, differentiation, and myelination potential of embryonic OPC through its PR by upregulating the expression of oligodendroglial genes such as neuron/glia antigen 2 (NG2), sex determining region Y-box9 (SOX9), myelin basic protein (MBP), 2′,3′-cyclic-nucleotide 3′-phosphodiesterase (CNP1), and NK6 homeobox 1 (NKX 6.1). These effects are likely mediated by PR-B, as they are blocked by the silencing of this isoform. The results suggest that progesterone contributes to the process of oligodendrogenesis during prenatal life through specific activation of PR-B.

Blocking the Thrombin Receptor Promotes Repair of Demyelinated Lesions in the Adult Brain

Myelin loss limits neurological recovery and myelin regeneration and is critical for restoration of function. We recently discovered that global knock-out of the thrombin receptor, also known as Protease Activated Receptor 1 (PAR1), accelerates myelin development. Here we demonstrate that knocking out PAR1 also promotes myelin regeneration. Outcomes in two unique models of myelin injury and repair, that is lysolecithin or cuprizone-mediated demyelination, showed that PAR1 knock-out in male mice improves replenishment of myelinating cells and remyelinated nerve fibers and slows early axon damage. Improvements in myelin regeneration in PAR1 knock-out mice occurred in tandem with a skewing of reactive astrocyte signatures toward a prorepair phenotype. In cell culture, the promyelinating effects of PAR1 loss of function are consistent with possible direct effects on the myelinating potential of oligodendrocyte progenitor cells (OPCs), in addition to OPC-indirect effects involving enhanced astrocyte expression of promyelinating factors, such as BDNF. These findings highlight previously unrecognized roles of PAR1 in myelin regeneration, including integrated actions across the oligodendrocyte and astroglial compartments that are at least partially mechanistically linked to the powerful BDNF-TrkB neurotrophic signaling system. Altogether, findings suggest PAR1 may be a therapeutically tractable target for demyelinating disorders of the CNS.SIGNIFICANCE STATEMENT Replacement of oligodendroglia and myelin regeneration holds tremendous potential to improve function across neurological conditions. Here we demonstrate Protease Activated Receptor 1 (PAR1) is an important regulator of the capacity for myelin regeneration across two experimental murine models of myelin injury. PAR1 is a G-protein-coupled receptor densely expressed in the CNS, however there is limited information regarding its physiological roles in health and disease. Using a combination of PAR1 knock-out mice, oligodendrocyte monocultures and oligodendrocyte-astrocyte cocultures, we demonstrate blocking PAR1 improves myelin production by a mechanism related to effects across glial compartments and linked in part to regulatory actions toward growth factors such as BDNF. These findings set the stage for development of new clinically relevant myelin regeneration strategies.

Oligodendrocytes in Development, Myelin Generation and Beyond

Oligodendrocytes are the myelinating cells of the central nervous system (CNS) that are generated from oligodendrocyte progenitor cells (OPC). OPC are distributed throughout the CNS and represent a pool of migratory and proliferative adult progenitor cells that can differentiate into oligodendrocytes. The central function of oligodendrocytes is to generate myelin, which is an extended membrane from the cell that wraps tightly around axons. Due to this energy consuming process and the associated high metabolic turnover oligodendrocytes are vulnerable to cytotoxic and excitotoxic factors. Oligodendrocyte pathology is therefore evident in a range of disorders including multiple sclerosis, schizophrenia and Alzheimer’s disease. Deceased oligodendrocytes can be replenished from the adult OPC pool and lost myelin can be regenerated during remyelination, which can prevent axonal degeneration and can restore function. Cell population studies have recently identified novel immunomodulatory functions of oligodendrocytes, the implications of which, e.g., for diseases with primary oligodendrocyte pathology, are not yet clear. Here, we review the journey of oligodendrocytes from the embryonic stage to their role in homeostasis and their fate in disease. We will also discuss the most common models used to study oligodendrocytes and describe newly discovered functions of oligodendrocytes.

GFAP Mutations in Astrocytes Impair Oligodendrocyte Progenitor Proliferation and Myelination in an hiPSC Model of Alexander Disease

Alexander disease (AxD) is a leukodystrophy that primarily affects astrocytes and is caused by mutations in the astrocytic filament gene GFAP. While astrocytes are thought to have important roles in controlling myelination, AxD animal models do not recapitulate critical myelination phenotypes and it is therefore not clear how AxD astrocytes contribute to leukodystrophy. Here, we show that AxD patient iPSC-derived astrocytes recapitulate key features of AxD pathology such as GFAP aggregation. Moreover, AxD astrocytes inhibit proliferation of human iPSC-derived oligodendrocyte progenitor cells (OPCs) in co-culture and reduce their myelination potential. CRISPR/Cas9-based correction of GFAP mutations reversed these phenotypes. Transcriptomic analyses of AxD astrocytes and postmortem brains identified CHI3L1 as a key mediator of AxD astrocyte-induced inhibition of OPC activity. Thus, this iPSC-based model of AxD not only recapitulates patient phenotypes not observed in animal models, but also reveals mechanisms underlying disease pathology and provides a platform for assessing therapeutic interventions.

Central Nervous System Remyelination: Roles of Glia and Innate Immune Cells

In diseases such as multiple sclerosis (MS), inflammation can injure the myelin sheath that surrounds axons, a process known as demyelination. The spontaneous regeneration of myelin, called remyelination, is associated with restoration of function and prevention of axonal degeneration. Boosting remyelination with therapeutic intervention is a promising new approach that is currently being tested in several clinical trials. The endogenous regulation of remyelination is highly dependent on the immune response. In this review article, we highlight the cell biology of remyelination and its regulation by innate immune cells. For the purpose of this review, we discuss the roles of microglia, and also astrocytes and oligodendrocyte progenitor cells (OPCs) as they are being increasingly recognized to have immune cell functions.

Brain White Matter: A Substrate for Resilience and a Substance for Subcortical Small Vessel Disease

Age-related brain white matter disease is a form of small vessel disease (SVD) that may be associated with lacunar and other small subcortical infarcts, cerebral microbleeds, and perivascular spaces. This common form of cerebrovascular disease may manifest clinically as cognitive impairment of varying degrees and difficulty with mobility. Whereas some persons show cognitive decline and mobility failure when there are brain white matter hyperintensities (WMH) and acute stroke, others recover, and not everyone with brain white matter disease is disabled. Thus, repair or compensation of brain white matter may be possible, and furthermore, certain vascular risks, such as raised blood pressure, are targets for prevention of white matter disease or are administered to reduce the burden of such disease. Vascular risk modification may be useful, but alone may not be sufficient to prevent white matter disease progression. In this chapter, we specifically focus on WMH of vascular origin and explore white matter development, plasticity, and enduring processes of myelination across the health span in the context of experimental and human data, and compare and contrast resilient brain white matter propensity to a diseased white matter state. We conclude with thoughts on novel ways one might study white matter resilience, and predict future healthy cognitive and functional outcomes.

Tissue Transglutaminase Promotes Early Differentiation of Oligodendrocyte Progenitor Cells

Demyelinated lesions of the central nervous system are characteristic for multiple sclerosis (MS). Remyelination is not very effective, particular at later stages of the disease, which results in a chronic neurodegenerative character with worsening of symptoms. Previously, we have shown that the enzyme Tissue Transglutaminase (TG2) is downregulated upon differentiation of oligodendrocyte progenitor cells (OPCs) into myelin-forming oligodendrocytes and that TG2 knock-out mice lag behind in remyelination after cuprizone-induced demyelination. Here, we examined whether astrocytic or oligodendroglial TG2 affects OPCs in a cell-specific manner to modulate their differentiation, and therefore myelination. Our findings indicate that human TG2-expressing astrocytes did not modulate OPC differentiation and myelination. In contrast, persistent TG2 expression upon OPC maturation or exogenously added recombinant TG2 accelerated OPC differentiation and myelin membrane formation. Continuous exposure of recombinant TG2 to OPCs at different consecutive developmental stages, however, decreased OPC differentiation and myelin membrane formation, while it enhanced myelination in dorsal root ganglion neuron-OPC co-cultures. In MS lesions, TG2 is absent in OPCs, while human OPCs show TG2 immunoreactivity during brain development. Exposure to the MS-relevant pro-inflammatory cytokine IFN-γ increased TG2 expression in OPCs and prolonged expression of endogenous TG2 upon differentiation. However, despite the increased TG2 levels, OPC maturation was not accelerated, indicating that TG2-mediated OPC differentiation may be counteracted by other pathways. Together, our data show that TG2, either endogenously expressed, or exogenously supplied to OPCs, accelerates early OPC differentiation. A better understanding of the role of TG2 in the OPC differentiation process during MS is of therapeutic interest to overcome remyelination failure.

Generation of Oligodendrocyte Progenitor Cells From Mouse Bone Marrow Cells

Oligodendrocyte progenitor cells (OPCs) are a subtype of glial cells responsible for myelin regeneration. Oligodendrocytes (OLGs) originate from OPCs and are the myelinating cells in the central nervous system (CNS). OLGs play an important role in the context of lesions in which myelin loss occurs. Even though many protocols for isolating OPCs have been published, their cellular yield remains a limit for clinical application. The protocol proposed here is novel and has practical value; in fact, OPCs can be generated from a source of autologous cells without gene manipulation. Our method represents a rapid, and high-efficiency differentiation protocol for generating mouse OLGs from bone marrow-derived cells using growth-factor defined media. With this protocol, it is possible to obtain mature OLGs in 7–8 weeks. Within 2–3 weeks from bone marrow (BM) isolation, after neurospheres formed, the cells differentiate into Nestin⁺ Sox2⁺ neural stem cells (NSCs), around 30 days. OPCs specific markers start to be expressed around day 38, followed by RIP⁺O4⁺ around day 42. CNPase⁺ mature OLGs are finally obtained around 7–8 weeks. Further, bone marrow-derived OPCs exhibited therapeutic effect in shiverer (Shi) mice, promoting myelin regeneration and reducing the tremor. Here, we propose a method by which OLGs can be generated starting from BM cells and have similar abilities to subventricular zone (SVZ)-derived cells. This protocol significantly decreases the timing and costs of the OLGs differentiation within 2 months of culture.

Selective Neural Deletion of the Atg7 Gene Reduces Irradiation-Induced Cerebellar White Matter Injury in the Juvenile Mouse Brain by Ameliorating Oligodendrocyte Progenitor Cell Loss

Radiotherapy is an effective tool for treating brain tumors, but irradiation-induced toxicity to the normal brain tissue remains a major problem. Here, we investigated if selective neural autophagy related gene 7 (Atg7) deletion has a persistent effect on irradiation-induced juvenile mouse brain injury. Ten-day-old Atg7 knockout under a nestin promoter (KO) mice and wild-type (WT) littermates were subjected to a single dose of 6 Gy whole-brain irradiation. Cerebellar volume, cell proliferation, microglia activation, inflammation, and myelination were evaluated in the cerebellum at 5 days after irradiation. We found that neural Atg7 deficiency partially prevented myelin disruption compared to the WT mice after irradiation, as indicated by myelin basic protein staining. Irradiation induced oligodendrocyte progenitor cell (OPC) loss in the white matter of the cerebellum, and Atg7 deficiency partly prevented this. The mRNA expression of oligodendrocyte and myelination-related genes (Olig2, Cldn11, CNP, and MBP) was higher in the cerebellum in Atg7 KO mice compared with WT littermates. The total cerebellar volume was significantly reduced after irradiation in both Atg7 KO and WT mice. Atg7-deficient cerebellums were in a regenerative state before irradiation, as judged by the increased OPC-related and neurogenesis-related transcripts and the increased numbers of microglia; however, except for the OPC parameters these were the same in both genotypes after irradiation. Finally, there was no significant change in the number of astrocytes in the cerebellum after irradiation. These results suggest that selective neural Atg7 deficiency reduces irradiation-induced cerebellar white matter injury in the juvenile mouse brain, secondary to prevention of OPC loss.

Electrophysiological Properties of Adult Zebrafish Oligodendrocyte Progenitor Cells

Low remyelination efficiency after spinal cord injury (SCI) is a major restraint to successful axonal and functional regeneration in mammals. In contrast, adult zebrafish can: (i) regenerate oligodendrocytes and myelin sheaths within 2 weeks post lesion; (ii) re-grow axonal projections across the lesion site and (iii) recover locomotor function within 6 weeks after spinal cord transection. However, little is known about the intrinsic properties of oligodendrocyte progenitor cells (OPCs), the remyelinating cells of the central nervous system (CNS). Here, we demonstrate that purified OPCs from the adult zebrafish spinal cord are electrically active. They functionally express voltage-gated K⁺ and Na⁺ channels, glutamate receptors and exhibit depolarizing, tetrodotoxin (TTX)-sensitive spikes, as previously seen in rodent and human OPCs. Furthermore, we show that the percentage of zebrafish OPCs exhibiting depolarizing spikes and Na_v-mediated currents is lower as compared to rodent white matter OPCs, where these membrane characteristics have been shown to underlie OPC injury susceptibility. These findings imply that adult zebrafish OPCs resemble electrical properties found in mammals and represent a relevant cell type towards understanding the biology of the primary cells targeted in remyelination therapies for non-regenerative species. The in vitro platform introduced in this study could be used in the future to: (i) elucidate how membrane characteristics of zebrafish OPCs change upon injury and (ii) identify potential signaling components underlying OPC injury recognition.

Single-Cell Transcriptomics Uncovers Glial Progenitor Diversity and Cell Fate Determinants during Development and Gliomagenesis

The identity and degree of heterogeneity of glial progenitors and their contributions to brain tumor malignancy remain elusive. By applying lineage-targeted single-cell transcriptomics, we uncover an unanticipated diversity of glial progenitor pools with unique molecular identities in developing brain. Our analysis identifies distinct transitional intermediate states and their divergent developmental trajectories in astroglial and oligodendroglial lineages. Moreover, intersectional analysis uncovers analogous intermediate progenitors during brain tumorigenesis, wherein oligodendrocyte-progenitor intermediates are abundant, hyper-proliferative, and progressively reprogrammed toward a stem-like state susceptible to further malignant transformation. Similar actively cycling intermediate progenitors are prominent components in human gliomas with distinct driver mutations. We further unveil lineage-driving networks underlying glial fate specification and identify Zfp36l1 as necessary for oligodendrocyte-astrocyte lineage transition and glioma growth. Together, our results resolve the dynamic repertoire of common and divergent glial progenitors during development and tumorigenesis and highlight Zfp36l1 as a molecular nexus for balancing glial cell-fate decision and controlling gliomagenesis.