Neuronal activity regulates remyelination via glutamate signalling to oligodendrocyte progenitors

Shared patterns of glial transcriptional dysregulation link Huntington's disease and schizophrenia

Huntington's disease and juvenile-onset schizophrenia have long been regarded as distinct disorders. However, both manifest cell-intrinsic abnormalities in glial differentiation, with resultant astrocytic dysfunction and hypomyelination. To assess whether a common mechanism might underlie the similar glial pathology of these otherwise disparate conditions, we used comparative correlation network approaches to analyse RNA-sequencing data from human glial progenitor cells (hGPCs) produced from disease-derived pluripotent stem cells. We identified gene sets preserved between Huntington's disease and schizophrenia hGPCs yet distinct from normal controls that included 174 highly connected genes in the shared disease-associated network, focusing on genes involved in synaptic signalling. These synaptic genes were largely suppressed in both schizophrenia and Huntington's disease hGPCs, and gene regulatory network analysis identified a core set of upstream regulators of this network, of which OLIG2 and TCF7L2 were prominent. Among their downstream targets, ADGRL3, a modulator of glutamatergic synapses, was notably suppressed in both schizophrenia and Huntington's disease hGPCs. Chromatin immunoprecipitation sequencing confirmed that OLIG2 and TCF7L2 each bound to the regulatory region of ADGRL3, whose expression was then rescued by lentiviral overexpression of these transcription factors. These data suggest that the disease-associated suppression of OLIG2 and TCF7L2-dependent transcription of glutamate signalling regulators may impair glial receptivity to neuronal glutamate. The consequent loss of activity-dependent mobilization of hGPCs may yield deficient oligodendrocyte production, and hence the hypomyelination noted in these disorders, as well as the disrupted astrocytic differentiation and attendant synaptic dysfunction associated with each. Together, these data highlight the importance of convergent glial molecular pathology in both the pathogenesis and phenotypic similarities of two otherwise unrelated disorders, Huntington's disease and schizophrenia.

A preclinical mice model of multiple sclerosis based on the toxin-induced double-site demyelination of callosal and cerebellar fibers

BACKGROUND

Multiple sclerosis (MS) is an irreversible progressive CNS pathology characterized by the loss of myelin (i.e. demyelination). The lack of myelin is followed by a progressive neurodegeneration triggering symptoms as diverse as fatigue, motor, locomotor and sensory impairments and/or bladder, cardiac and respiratory dysfunction. Even though there are more than fourteen approved treatments for reducing MS progression, there are still no cure for the disease. Thus, MS research is a very active field and therefore we count with different experimental animal models for studying mechanisms of demyelination and myelin repair, however, we still lack a preclinical MS model assembling demyelination mechanisms with relevant clinical-like signs.

RESULTS

Here, by inducing the simultaneous demyelination of both callosal and cerebellar white matter fibers by the double-site injection of lysolecithin (LPC), we were able to reproduce CNS demyelination, astrocyte recruitment and increases levels of proinflammatory cytokines levels along with motor, locomotor and urinary impairment, as well as cardiac and respiratory dysfunction, in the same animal model. Single site LPC-injections either in corpus callosum or cerebellum only, fails in to reproduce such a complete range of MS-like signs.

CONCLUSION

We here report that the double-site LPC injections treatment evoke a complex MS-like mice model. We hope that this experimental approach will help to deepen our knowledge about the mechanisms of demyelinated diseases such as MS.

Dysregulated Cholinergic Signaling Inhibits Oligodendrocyte Maturation Following Demyelination

Dysregulation of oligodendrocyte progenitor cell (OPC) recruitment and oligodendrocyte differentiation contribute to failure of remyelination in human demyelinating diseases such as multiple sclerosis (MS). Deletion of muscarinic receptor enhances OPC differentiation and remyelination. However, the role of ligand-dependent signaling versus constitutive receptor activation is unknown. We hypothesized that dysregulated acetylcholine (ACh) release upon demyelination contributes to ligand-mediated activation hindering myelin repair. Following chronic cuprizone (CPZ)-induced demyelination (male and female mice), we observed a 2.5-fold increase in ACh concentration. This increase in ACh concentration could be attributed to increased ACh synthesis or decreased acetylcholinesterase-/butyrylcholinesterase (BChE)-mediated degradation. Using choline acetyltransferase (ChAT) reporter mice, we identified increased ChAT-GFP expression following both lysolecithin and CPZ demyelination. ChAT-GFP expression was upregulated in a subset of injured and uninjured axons following intraspinal lysolecithin-induced demyelination. In CPZ-demyelinated corpus callosum, ChAT-GFP was observed in Gfap+ astrocytes and axons indicating the potential for neuronal and astrocytic ACh release. BChE expression was significantly decreased in the corpus callosum following CPZ demyelination. This decrease was due to the loss of myelinating oligodendrocytes which were the primary source of BChE. To determine the role of ligand-mediated muscarinic signaling following lysolecithin injection, we administered neostigmine, a cholinesterase inhibitor, to artificially raise ACh. We identified a dose-dependent decrease in mature oligodendrocyte density with no effect on OPC recruitment. Together, these results support a functional role of ligand-mediated activation of muscarinic receptors following demyelination and suggest that dysregulation of ACh homeostasis directly contributes to failure of remyelination in MS.

Vagus nerve stimulation enhances remyelination and decreases innate neuroinflammation in lysolecithin-induced demyelination

BACKGROUND

Current treatments for Multiple Sclerosis (MS) poorly address chronic innate neuroinflammation nor do they offer effective remyelination. The vagus nerve has a strong regulatory role in inflammation and Vagus Nerve Stimulation (VNS) has potential to affect both neuroinflammation and remyelination in MS.

OBJECTIVE

This study investigated the effects of VNS on demyelination and innate neuroinflammation in a validated MS rodent model.

METHODS

Lysolecithin (LPC) was injected in the corpus callosum (CC) of 46 Lewis rats, inducing a demyelinated lesion. 33/46 rats received continuously-cycled VNS (cVNS) or one-minute per day VNS (1minVNS) or sham VNS from 2 days before LPC-injection until perfusion at 3 days post-injection (dpi) (corresponding with a demyelinated lesion with peak inflammation). 13/46 rats received cVNS or sham from 2 days before LPC-injection until perfusion at 11 dpi (corresponding with a partial remyelinated lesion). Immunohistochemistry and proteomics analyses were performed to investigate the extend of demyelination and inflammation.

RESULTS

Immunohistochemistry showed that cVNS significantly reduced microglial and astrocytic activation in the lesion and lesion border, and significantly reduced the Olig2+ cell count at 3 dpi. Furthermore, cVNS significantly improved remyelination with 57.4 % versus sham at 11 dpi. Proteomic gene set enrichment analyses showed increased activation of (glutamatergic) synapse pathways in cVNS versus sham, most pronounced at 3 dpi.

CONCLUSION

cVNS improved remyelination of an LPC-induced lesion. Possible mechanisms might include modulation of microglia and astrocyte activity, increased (glutamatergic) synapses and enhanced oligodendrocyte clearance after initial injury.

β-carbolines that enhance GABAA receptor response expressed in oligodendrocytes promote remyelination in an in vivo rat model of focal demyelination

Demyelination is typically followed by a remyelination process through mature oligodendrocytes (OLs) differentiated from precursor cells (OPCs) recruited into the lesioned areas, however, this event usually results in uncompleted myelination. Potentiation of the remyelination process is an important target for designing effective therapeutic strategies against white matter loss. Here, it was evaluated the remyelinating effect of different β-carbolines that present differential allosteric modulation on the GABAA receptor expressed in OLs. For this, we used a focalized demyelination model in the inferior cerebellar peduncle (i.c.p.) of rats (DRICP model), in which, demyelination by ethidium bromide (0.05%) stereotaxic injection was confirmed histologically by staining with Black-Gold II (BGII) and toluidine blue. In addition, a longitudinal analysis with diffusion-weighted magnetic resonance imaging (dMRI) was made by computing fractional anisotropy (FA), apparent diffusion coefficient (ADC) and diffusivity parameters to infer i.c.p. microstructural changes. First, dMRI analysis revealed FA decreases together with ADC and radial diffusivity (RD) increases after demyelination, which correlates with histological BGII observations. Then, we evaluated the effect produced by three allosteric GABAA receptor modulators, the N-butyl-β-carboline-3-carboxylate (β-CCB), ethyl 9H-pyrido [3,4-b]indole-3-carboxylate (β-CCE), and 4-ethyl-6,7-dimethoxy-9H-pyrido [3,4-b]indole-3-carboxylic acid methyl ester (DMCM). The results indicated that daily systemic β-CCB (1 mg/Kg) or β-CCE (1 mg/Kg) administration for 2 weeks, but not DMCM (0.35 mg/Kg), in lesioned animals increased FA and decreased ADC or RD, suggesting myelination improvement. This was supported by BGII staining analysis that showed a recovery of myelin content. Also, it was quantified by immunohistochemistry both NG2+ and CC1+ cellular population in the different experimental sceneries. Data indicated that either β-CCB or β-CCE, but not DMCM, produced an increase in the population of CC1+ cells in the lesioned area. Finally, it was also calculated the g-ratio of myelinated axons and observed a similar value in those lesioned animals treated with β-CCB or β-CCE compared to controls. Thus, using the DRICP model, it was observed that either β-CCB or β-CCE, positive modulators of the GABAA receptor in OLs, had a potent promyelinating effect.

Features, Fates, and Functions of Oligodendrocyte Precursor Cells

Oligodendrocyte precursor cells (OPCs) are a central nervous system resident population of glia with a distinct molecular identity and an ever-increasing list of functions. OPCs generate oligodendrocytes throughout development and across the life span in most regions of the brain and spinal cord. This process involves a complex coordination of molecular checkpoints and biophysical cues from the environment that initiate the differentiation and integration of new oligodendrocytes that synthesize myelin sheaths on axons. Outside of their progenitor role, OPCs have been proposed to play other functions including the modulation of axonal and synaptic development and the participation in bidirectional signaling with neurons and other glia. Here, we review OPC identity and known functions and discuss recent findings implying other roles for these glial cells in brain physiology and pathology.

Remyelination in the Central Nervous System

The inability of the mammalian central nervous system (CNS) to undergo spontaneous regeneration has long been regarded as a central tenet of neurobiology. However, while this is largely true of the neuronal elements of the adult mammalian CNS, save for discrete populations of granule neurons, the same is not true of its glial elements. In particular, the loss of oligodendrocytes, which results in demyelination, triggers a spontaneous and often highly efficient regenerative response, remyelination, in which new oligodendrocytes are generated and myelin sheaths are restored to denuded axons. Yet remyelination in humans is not without limitation, and a variety of demyelinating conditions are associated with sustained and disabling myelin loss. In this work, we will (1) review the biology of remyelination, including the cells and signals involved; (2) describe when remyelination occurs and when and why it fails, including the consequences of its failure; and (3) discuss approaches for therapeutically enhancing remyelination in demyelinating diseases of both children and adults, both by stimulating endogenous oligodendrocyte progenitor cells and by transplanting these cells into demyelinated brain.

ANKS1B encoded AIDA-1 regulates social behaviors by controlling oligodendrocyte function

Heterozygous deletions in the ANKS1B gene cause ANKS1B neurodevelopmental syndrome (ANDS), a rare genetic disease characterized by autism spectrum disorder (ASD), attention deficit/hyperactivity disorder, and speech and motor deficits. The ANKS1B gene encodes for AIDA-1, a protein that is enriched at neuronal synapses and regulates synaptic plasticity. Here we report an unexpected role for oligodendroglial deficits in ANDS pathophysiology. We show that Anks1b-deficient mouse models display deficits in oligodendrocyte maturation, myelination, and Rac1 function, and recapitulate white matter abnormalities observed in ANDS patients. Selective loss of Anks1b from the oligodendrocyte lineage, but not from neuronal populations, leads to deficits in social preference and sensory reactivity previously observed in a brain-wide Anks1b haploinsufficiency model. Furthermore, we find that clemastine, an antihistamine shown to increase oligodendrocyte precursor cell maturation and central nervous system myelination, rescues deficits in social preference in 7-month-old Anks1b-deficient mice. Our work shows that deficits in social behaviors present in ANDS may originate from abnormal Rac1 activity within oligodendrocytes.

Oligodendrocyte progenitor cells in Alzheimer's disease: from physiology to pathology

Oligodendrocyte progenitor cells (OPCs) play pivotal roles in myelin formation and phagocytosis, communicating with neighboring cells and contributing to the integrity of the blood-brain barrier (BBB). However, under the pathological circumstances of Alzheimer's disease (AD), the brain's microenvironment undergoes detrimental changes that significantly impact OPCs and their functions. Starting with OPC functions, we delve into the transformation of OPCs to myelin-producing oligodendrocytes, the intricate signaling interactions with other cells in the central nervous system (CNS), and the fascinating process of phagocytosis, which influences the function of OPCs and affects CNS homeostasis. Moreover, we discuss the essential role of OPCs in BBB formation and highlight the critical contribution of OPCs in forming CNS-protective barriers. In the context of AD, the deterioration of the local microenvironment in the brain is discussed, mainly focusing on neuroinflammation, oxidative stress, and the accumulation of toxic proteins. The detrimental changes disturb the delicate balance in the brain, impacting the regenerative capacity of OPCs and compromising myelin integrity. Under pathological conditions, OPCs experience significant alterations in migration and proliferation, leading to impaired differentiation and a reduced ability to produce mature oligodendrocytes. Moreover, myelin degeneration and formation become increasingly active in AD, contributing to progressive neurodegeneration. Finally, we summarize the current therapeutic approaches targeting OPCs in AD. Strategies to revitalize OPC senescence, modulate signaling pathways to enhance OPC differentiation, and explore other potential therapeutic avenues are promising in alleviating the impact of AD on OPCs and CNS function. In conclusion, this review highlights the indispensable role of OPCs in CNS function and their involvement in the pathogenesis of AD. The intricate interplay between OPCs and the AD brain microenvironment underscores the complexity of neurodegenerative diseases. Insights from studying OPCs under pathological conditions provide a foundation for innovative therapeutic strategies targeting OPCs and fostering neurodegeneration. Future research will advance our understanding and management of neurodegenerative diseases, ultimately offering hope for effective treatments and improved quality of life for those affected by AD and related disorders.

Systemic and intrinsic functions of ATRX in glial cell fate and CNS myelination in male mice

Myelin, an extension of the oligodendrocyte plasma membrane, wraps around axons to facilitate nerve conduction. Myelination is compromised in ATR-X intellectual disability syndrome patients, but the causes are unknown. We show that loss of ATRX leads to myelination deficits in male mice that are partially rectified upon systemic thyroxine administration. Targeted ATRX inactivation in either neurons or oligodendrocyte progenitor cells (OPCs) reveals OPC-intrinsic effects on myelination. OPCs lacking ATRX fail to differentiate along the oligodendrocyte lineage and acquire a more plastic state that favors astrocytic differentiation in vitro and in vivo. ATRX chromatin occupancy in OPCs greatly overlaps with that of the chromatin remodelers CHD7 and CHD8 as well as H3K27Ac, a mark of active enhancers. Overall, our data indicate that ATRX regulates the onset of myelination systemically via thyroxine, and by promoting OPC differentiation and suppressing astrogliogenesis. These functions of ATRX identified in mice could explain white matter pathogenesis observed in ATR-X syndrome patients.

Activity-dependent oligodendrocyte calcium dynamics and their changes in Alzheimer's disease

Oligodendrocytes (OCs) form myelin around axons, which is dependent on neuronal activity. This activity-dependent myelination plays a crucial role in training and learning. Previous studies have suggested that neuronal activity regulates proliferation and differentiation of oligodendrocyte precursor cells (OPCs) and myelination. In addition, deficient activity-dependent myelination results in impaired motor learning. However, the functional response of OC responsible for neuronal activity and their pathological changes is not fully elucidated. In this research, we aimed to understand the activity-dependent OC responses and their different properties by observing OCs using in vivo two-photon microscopy. We clarified that the Ca2+ activity in OCs is neuronal activity dependent and differentially regulated by neurotransmitters such as glutamate or adenosine triphosphate (ATP). Furthermore, in 5-month-old mice models of Alzheimer's disease, a period before the appearance of behavioral abnormalities, the elevated Ca2+ responses in OCs are ATP dependent, suggesting that OCs receive ATP from damaged tissue. We anticipate that our research will help in determining the correct therapeutic strategy for neurodegenerative diseases beyond the synapse.

Electrical, Electromagnetic, Ultrasound Wave Therapies, and Electronic Implants for Neuronal Rejuvenation, Neuroprotection, Axonal Regeneration, and IOP Reduction

The peripheral nervous system (PNS) of mammals and nervous systems of lower organisms possess significant regenerative potential. In contrast, although neural plasticity can provide some compensation, the central nervous system (CNS) neurons and nerves of adult mammals generally fail to regenerate after an injury or damage. However, use of diverse electrical, electromagnetic and sonographic energy waves are illuminating novel ways to stimulate neuronal differentiation, proliferation, neurite growth, and axonal elongation/regeneration leading to various levels of functional recovery in animals and humans afflicted with disorders of the CNS, PNS, retina, and optic nerve. Tools such as acupuncture, electroacupuncture, electroshock therapy, electrical stimulation, transcranial magnetic stimulation, red light therapy, and low-intensity pulsed ultrasound therapy are demonstrating efficacy in treating many different maladies. These include wound healing, partial recovery from motor dysfunctions, recovery from ischemic/reperfusion insults and CNS and ocular remyelination, retinal ganglion cell (RGC) rejuvenation, and RGC axonal regeneration. Neural rejuvenation and axonal growth/regeneration processes involve activation or intensifying of the intrinsic bioelectric waves (action potentials) that exist in every neuronal circuit of the body. In addition, reparative factors released at the nerve terminals and via neuronal dendrites (transmitter substances), extracellular vesicles containing microRNAs and neurotrophins, and intercellular communication occurring via nanotubes aid in reestablishing lost or damaged connections between the traumatized tissues and the PNS and CNS. Many other beneficial effects of the aforementioned treatment paradigms are mediated via gene expression alterations such as downregulation of inflammatory and death-signal genes and upregulation of neuroprotective and cytoprotective genes. These varied techniques and technologies will be described and discussed covering cell-based and animal model-based studies. Data from clinical applications and linkage to human ocular diseases will also be discussed where relevant translational research has been reported.

Chronic demyelination and myelin repair after spinal cord injury in mice: A potential link for glutamatergic axon activity

Our prior work examining endogenous repair after spinal cord injury (SCI) in mice revealed that large numbers of new oligodendrocytes (OLs) are generated in the injured spinal cord, with peak oligodendrogenesis between 4 and 7 weeks post-injury (wpi). We also detected new myelin formation over 2 months post-injury (mpi). Our current work significantly extends these results, including quantification of new myelin through 6 mpi and concomitant examination of indices of demyelination. We also examined electrophysiological changes during peak oligogenesis and a potential mechanism driving OL progenitor cell (OPC) contact with axons. Results reveal peak in remyelination occurs during the 3rd mpi, and that myelin generation continues for at least 6 mpi. Further, motor evoked potentials significantly increased during peak remyelination, suggesting enhanced axon potential conduction. Interestingly, two indices of demyelination, nodal protein spreading and Nav1.2 upregulation, were also present chronically after SCI. Nav1.2 was expressed through 10 wpi and nodal protein disorganization was detectable throughout 6 mpi suggesting chronic demyelination, which was confirmed with EM. Thus, demyelination may continue chronically, which could trigger the long-term remyelination response. To examine a potential mechanism that may initiate post-injury myelination, we show that OPC processes contact glutamatergic axons in the injured spinal cord in an activity-dependent manner. Notably, these OPC/axon contacts were increased 2-fold when axons were activated chemogenetically, revealing a potential therapeutic target to enhance post-SCI myelin repair. Collectively, results show the surprisingly dynamic nature of the injured spinal cord over time and that the tissue may be amenable to treatments targeting chronic demyelination.

Neuronal activity and remyelination: new insights into the molecular mechanisms and therapeutic advancements

This article reviews the role of neuronal activity in myelin regeneration and the related neural signaling pathways. The article points out that neuronal activity can stimulate the formation and regeneration of myelin, significantly improve its conduction speed and neural signal processing ability, maintain axonal integrity, and support axonal nutrition. However, myelin damage is common in various clinical diseases such as multiple sclerosis, stroke, dementia, and schizophrenia. Although myelin regeneration exists in these diseases, it is often incomplete and cannot promote functional recovery. Therefore, seeking other ways to improve myelin regeneration in clinical trials in recent years is of great significance. Research has shown that controlling neuronal excitability may become a new intervention method for the clinical treatment of demyelinating diseases. The article discusses the latest research progress of neuronal activity on myelin regeneration, including direct or indirect stimulation methods, and the related neural signaling pathways, including glutamatergic, GABAergic, cholinergic, histaminergic, purinergic and voltage-gated ion channel signaling pathways, revealing that seeking treatment strategies to promote myelin regeneration through precise regulation of neuronal activity has broad prospects.

Disruption of neuronal RHEB signaling impairs oligodendrocyte differentiation and myelination through mTORC1-DLK1 axis

How neuronal signaling affects brain myelination remains poorly understood. We show dysregulated neuronal RHEB-mTORC1-DLK1 axis impairs brain myelination. Neuronal Rheb cKO impairs oligodendrocyte differentiation/myelination, with activated neuronal expression of the imprinted gene Dlk1. Neuronal Dlk1 cKO ameliorates myelination deficit in neuronal Rheb cKO mice, indicating that activated neuronal Dlk1 expression contributes to impaired myelination caused by Rheb cKO. The effect of Rheb cKO on Dlk1 expression is mediated by mTORC1; neuronal mTor cKO and Raptor cKO and pharmacological inhibition of mTORC1 recapitulate elevated neuronal Dlk1 expression. We demonstrate that both a secreted form of DLK1 and a membrane-bound DLK1 inhibit the differentiation of cultured oligodendrocyte precursor cells into oligodendrocytes expressing myelin proteins. Finally, neuronal expression of Dlk1 in transgenic mice reduces the formation of mature oligodendrocytes and myelination. This study identifies Dlk1 as an inhibitor of oligodendrocyte myelination and a mechanism linking altered neuronal signaling with oligodendrocyte dysfunction.

Neuroprotection in an Experimental Model of Multiple Sclerosis via Opening of Big Conductance, Calcium-Activated Potassium Channels

Big conductance calcium-activated (BK) channel openers can inhibit pathologically driven neural hyperactivity to control symptoms via hyperpolarizing signals to limit neural excitability. We hypothesized that BK channel openers would be neuroprotective during neuroinflammatory, autoimmune disease. The neurodegenerative disease was induced in a mouse experimental autoimmune encephalomyelitis model with translational value to detect neuroprotection in multiple sclerosis. Following the treatment with the BK channel openers, BMS-204253 and VSN16R, neuroprotection was assessed using subjective and objective clinical outcomes and by quantitating spinal nerve content. Treatment with BMS-204253 and VSN16R did not inhibit the development of relapsing autoimmunity, consistent with minimal channel expression via immune cells, nor did it change leukocyte levels in rodents or humans. However, it inhibited the accumulation of nerve loss and disability as a consequence of autoimmunity. Therefore, in addition to symptom control, BK channel openers have the potential to save nerves from excitotoxic damage and could be useful as either stand-alone neuroprotective agents or as add-ons to current disease-modifying treatments that block relapsing MS but do not have any direct neuroprotective activity.

Toward reframing brain-social dynamics: current assumptions and future challenges

Evolutionary analyses suggest that the human social brain and sociality appeared together. The two fundamental tools that accelerated the concurrent emergence of the social brain and sociality include learning and plasticity. The prevailing core idea is that the primate brain and the cortex in particular became reorganised over the course of evolution to facilitate dynamic adaptation to ongoing changes in physical and social environments. Encouraged by computational or survival demands or even by instinctual drives for living in social groups, the brain eventually learned how to learn from social experience via its massive plastic capacity. A fundamental framework for modeling these orchestrated dynamic responses is that social plasticity relies upon neuroplasticity. In the present article, we first provide a glimpse into the concepts of plasticity, experience, with emphasis on social experience. We then acknowledge and integrate the current theoretical concepts to highlight five key intertwined assumptions within social neuroscience that underlie empirical approaches for explaining the brain-social dynamics. We suggest that this epistemological view provides key insights into the ontology of current conceptual frameworks driving future research to successfully deal with new challenges and possible caveats in favour of the formulation of novel assumptions. In the light of contemporary societal challenges, such as global pandemics, natural disasters, violent conflict, and other human tragedies, discovering the mechanisms of social brain plasticity will provide new approaches to support adaptive brain plasticity and social resilience.

Schwann Cell Remyelination in the Multiple Sclerosis Central Nervous System

Multiple sclerosis (MS) is a central nervous system (CNS) demyelinating disease. Failure to remyelinate successfully is common in MS lesions, often with consequent neuronal/axonal damage. CNS myelin is normally produced by oligodendroglial cells. Remyelination by Schwann cells (SchC) has been reported in spinal cord demyelination, in which SchCs are in close proximity to CNS myelin. We identified an MS cerebral lesion that was remyelinated by SchCs. This prompted us to query the extent of SchC remyelination in the brain and spinal cords of additional autopsied MS specimens. CNS tissues were obtained from the autopsies of 14 MS cases. Remyelinated lesions were identified by Luxol fast blue-periodic-acid Schiff and solochrome cyanine staining. Deparaffinized sections containing remyelinated lesions were stained with anti-glial fibrillary acid protein to identify reactive astrocytes. Glycoprotein P zero (P0) is a protein exclusive to peripheral but not CNS myelin. Areas of SchC remyelination were identified by staining with anti-P0. Myelinated regions in the index case cerebral lesion were confirmed to be of SchC origin using anti-P0 staining. Subsequently, 64 MS lesions from 14 autopsied MS cases were examined, and 23 lesions in 6 cases showed remyelination by SchCs. Lesions from the cerebrum, brainstem, and spinal cord were examined in each case. When present, SchC remyelination was most commonly located adjacent to the venules and associated with a lower surrounding density of glial fibrillary acid protein+ reactive astrocytes than areas of only oligodendroglial cell remyelination. The difference was significant only for spinal cord and brainstem lesions but not for lesions located in the brain. In conclusion, we demonstrated SchC remyelination in the cerebrum, brainstem, and spinal cord of 6 autopsied MS cases. To our knowledge, this is the first report of supratentorial SchC remyelination in MS.

Emerging concepts in oligodendrocyte and myelin formation, inputs from the zebrafish model

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS), which are derived from OL precursor cells. Myelin insulates axons allowing the saltatory conduction of action potentials and also provides trophic and metabolic supports to axons. Interestingly, oligodendroglial cells have the capacity to sense neuronal activity, which regulates myelin sheath formation via the vesicular release of neurotransmitters. Neuronal activity-dependent regulation of myelination is mediated by specialized interaction between axons and oligodendroglia, involving both synaptic and extra-synaptic modes of communications. The zebrafish has provided key advantages for the study of the myelination process in the CNS. External development and transparent larval stages of this vertebrate specie combined with the existence of several transgenic reporter lines provided key advances in oligodendroglial cell biology, axo-glial interactions and CNS myelination. In this publication, we reviewed and discussed the most recent knowledge on OL development and myelin formation, with a focus on mechanisms regulating these fundamental biological processes in the zebrafish. Especially, we highlighted the critical function of axons and oligodendroglia modes of communications and calcium signaling in myelin sheath formation and growth. Finally, we reviewed the relevance of these knowledge's in demyelinating diseases and drug discovery of pharmacological compounds favoring myelin regeneration.

Neuronal activity and NIBS in developmental myelination and remyelination - current state of knowledge

Oligodendrocytes are responsible for myelinating central nervous system (CNS) axons. and rapid electrical transmission through saltatory conduction of action potentials. Myelination and myelin repair rely partially on oligodendrogenesis, which comprises. oligodendrocyte precursor cell (OPC) migration, maturation, and differentiation into. oligodendrocytes (OL). In multiple sclerosis (MS), demyelination occurs due to an. inflammatory cascade with auto-reactive T-cells. When oligodendrogenesis fails, remyelination becomes aberrant and conduction impairments are no longer restored. Although current disease modifying therapies have achieved results in modulating the. faulty immune response, disease progression continues because of chronic. inflammation, neurodegeneration, and failure of remyelination. Therapies have been. tried to promote remyelination. Modulation of neuronal activity seems to be a very. promising strategy in preclinical studies. Additionally, studies in people with MS. (pwMS) have shown symptom improvement following non-invasive brain stimulation. (NIBS) techniques. The aforementioned mechanisms are yet unknown and probably. involve both the activation of neurons and glial cells. Noting neuronal activity. contributes to myelin plasticity and that NIBS modulates neuronal activity; we argue. that NIBS is a promising research horizon for demyelinating diseases. We review the. hypothesized pathways through which NIBS may affect both neuronal activity in the. CNS and how the resulting activity can affect oligodendrogenesis and myelination.

Alterations in oligodendrocyte transcriptional networks reveal region-specific vulnerabilities to neurological disease

Neurological disease is characterized the by dysfunction of specific neuroanatomical regions. To determine whether region-specific vulnerabilities have a transcriptional basis at cell-type-specific resolution, we analyzed gene expression in mouse oligodendrocytes across various brain regions. Oligodendrocyte transcriptomes cluster in an anatomical arrangement along the rostrocaudal axis. Moreover, regional oligodendrocyte populations preferentially regulate genes implicated in diseases that target their region of origin. Systems-level analyses identify five region-specific co-expression networks representing distinct molecular pathways in oligodendrocytes. The cortical network exhibits alterations in mouse models of intellectual disability and epilepsy, the cerebellar network in ataxia, and the spinal network in multiple sclerosis. Bioinformatic analyses reveal potential molecular regulators of these networks, which were confirmed to modulate network expression in vitro in human oligodendroglioma cells, including reversal of the disease-associated transcriptional effects of a pathogenic Spinocerebellar ataxia type 1 allele. These findings identify targetable region-specific vulnerabilities to neurological disease mediated by oligodendrocytes.

Cancer neuroscience: State of the field, emerging directions

The nervous system governs both ontogeny and oncology. Regulating organogenesis during development, maintaining homeostasis, and promoting plasticity throughout life, the nervous system plays parallel roles in the regulation of cancers. Foundational discoveries have elucidated direct paracrine and electrochemical communication between neurons and cancer cells, as well as indirect interactions through neural effects on the immune system and stromal cells in the tumor microenvironment in a wide range of malignancies. Nervous system-cancer interactions can regulate oncogenesis, growth, invasion and metastatic spread, treatment resistance, stimulation of tumor-promoting inflammation, and impairment of anti-cancer immunity. Progress in cancer neuroscience may create an important new pillar of cancer therapy.

TREM2 mediates physical exercise-promoted neural functional recovery in rats with ischemic stroke via microglia-promoted white matter repair

BACKGROUND

The repair of white matter injury is of significant importance for functional recovery after ischemic stroke, and the up-regulation of triggering receptors expressed on myeloid cells 2 (TREM2) after ischemic stroke is neuroprotective and implicated in remyelination. However, the lack of effective therapies calls for the need to investigate the regenerative process of remyelination and the role of rehabilitation therapy. This study sought to investigate whether and how moderate physical exercise (PE) promotes oligodendrogenesis and remyelination in rats with transient middle cerebral artery occlusion (tMCAO).

METHODS

Male Sprague-Dawley rats (weighing 250-280 g) were subjected to tMCAO. AAV-shRNA was injected into the lateral ventricle to silence the Trem2 gene before the operation. The rats in the physical exercise group started electric running cage training at 48 h after the operation. The Morris water maze and novel object recognition test were used to evaluate cognitive function. Luxol fast blue staining, diffusion tensor imaging, and electron microscopy were used to observe myelin injury and repair. Immunofluorescence staining was applied to observe the proliferation and differentiation of oligodendrocyte precursor cells (OPCs). Expression of key molecules were detected using immunofluorescence staining, quantitative real-time polymerase chain reaction, Western blotting, and Enzyme-linked immunosorbent assay, respectively.

RESULTS

PE exerted neuroprotective efects by modulating microglial state, promoting remyelination and recovery of neurological function of rats over 35 d after stroke, while silencing Trem2 expression in rats suppressed the aforementioned effects promoted by PE. In addition, by leveraging the activin-A neutralizing antibody, we found a direct beneficial effect of PE on microglia-derived activin-A and its subsequent role on oligodendrocyte differentiation and remyelination mediated by the activin-A/Acvr axis.

CONCLUSIONS

The present study reveals a novel regenerative role of PE in white matter injury after stroke, which is mediated by upregulation of TREM2 and microglia-derived factor for oligodendrocytes regeneration. PE is an effective therapeutic approach for improving white matter integrity and alleviating neurological function deficits after ischemic stroke.

Cell reprogramming for oligodendrocytes: A review of protocols and their applications to disease modeling and cell-based remyelination therapies

Oligodendrocytes are a type of glial cells that produce a lipid-rich membrane called myelin. Myelin assembles into a sheath and lines neuronal axons in the brain and spinal cord to insulate them. This not only increases the speed and efficiency of nerve signal transduction but also protects the axons from damage and degradation, which could trigger neuronal cell death. Demyelination, which is caused by a loss of myelin and oligodendrocytes, is a prominent feature of many neurological conditions, including Multiple sclerosis (MS), spinal cord injuries (SCI), and leukodystrophies. Demyelination is followed by a time of remyelination mediated by the recruitment of endogenous oligodendrocyte precursor cells, their migration to the injury site, and differentiation into myelin-producing oligodendrocytes. Unfortunately, endogenous remyelination is not sufficient to overcome demyelination, which explains why there are to date no regenerative-based treatments for MS, SCI, or leukodystrophies. To better understand the role of oligodendrocytes and develop cell-based remyelination therapies, human oligodendrocytes have been derived from somatic cells using cell reprogramming. This review will detail the different cell reprogramming methods that have been developed to generate human oligodendrocytes and their applications to disease modeling and cell-based remyelination therapies. Recent developments in the field have seen the derivation of brain organoids from pluripotent stem cells, and protocols have been devised to incorporate oligodendrocytes within the organoids, which will also be reviewed.

Women in the field of multiple sclerosis: How they contributed to paradigm shifts

History is full of women who made enormous contributions to science. While there is little to no imbalance at the early career stage, a decreasing proportion of women is found as seniority increases. In the multiple sclerosis (MS) field, 44% of first authors and only 35% of senior authors were female. So, in this review, we highlight ground-breaking research done by women in the field of MS, focusing mostly on their work as principal investigators. MS is an autoimmune disorder of the central nervous system (CNS), with evident paradigm shifts in the understating of its pathophysiology. It is known that the immune system becomes overactivated and attacks myelin sheath surrounding axons. The resulting demyelination disrupts the communication signals to and from the CNS, which causes unpredictable symptoms, depending on the neurons that are affected. Classically, MS was reported to cause mostly physical and motor disabilities. However, it is now recognized that cognitive impairment affects more than 50% of the MS patients. Another shifting paradigm was the involvement of gray matter in MS pathology, formerly considered to be a white matter disease. Additionally, the identification of different T cell immune subsets and the mechanisms underlying the involvement of B cells and peripheral macrophages provided a better understanding of the immunopathophysiological processes present in MS. Relevantly, the gut-brain axis, recognized as a bi-directional communication system between the CNS and the gut, was found to be crucial in MS. Indeed, gut microbiota influences not only different susceptibilities to MS pathology, but it can also be modulated in order to positively act in MS course. Also, after the identification of the first microRNA in 1993, the role of microRNAs has been investigated in MS, either as potential biomarkers or therapeutic agents. Finally, concerning MS therapeutical approaches, remyelination-based studies have arisen on the spotlight aiming to repair myelin loss/neuronal connectivity. Altogether, here we emphasize the new insights of remarkable women that have voiced the impact of cognitive impairment, white and gray matter pathology, immune response, and that of the CNS-peripheral interplay on MS diagnosis, progression, and/or therapy efficacy, leading to huge breakthroughs in the MS field.

A subset of OPCs do not express Olig2 during development which can be increased in the adult by brain injuries and complex motor learning

Oligodendrocyte precursor cells (OPCs) are uniformly distributed in the mammalian brain; however, their function is rather heterogeneous in respect to their origin, location, receptor/channel expression and age. The basic helix-loop-helix transcription factor Olig2 is expressed in all OPCs as a pivotal determinant of their differentiation. Here, we identified a subset (2%-26%) of OPCs lacking Olig2 in various brain regions including cortex, corpus callosum, CA1 and dentate gyrus. These Olig2 negative (Olig2^neg ) OPCs were enriched in the juvenile brain and decreased subsequently with age, being rarely detectable in the adult brain. However, the loss of this population was not due to apoptosis or microglia-dependent phagocytosis. Unlike Olig2^pos OPCs, these subset cells were rarely labeled for the mitotic marker Ki67. And, accordingly, BrdU was incorporated only by a three-day long-term labeling but not by a 2-hour short pulse, suggesting these cells do not proliferate any more but were derived from proliferating OPCs. The Olig2^neg OPCs exhibited a less complex morphology than Olig2^pos ones. Olig2^neg OPCs preferentially remain in a precursor stage rather than differentiating into highly branched oligodendrocytes. Changing the adjacent brain environment, for example, by acute injuries or by complex motor learning tasks, stimulated the transition of Olig2^pos OPCs to Olig2^neg cells in the adult. Taken together, our results demonstrate that OPCs transiently suppress Olig2 upon changes of the brain activity.

Enhanced glaucomatous damage accompanied by glial response in a new multifactorial mouse model

Introduction

Glaucoma is a complex, multifactorial neurodegenerative disease, which can lead to blindness if left untreated. It seems that, among others, immune processes, elevated intraocular pressure (IOP), or a combination of these factors are responsible for glaucomatous damage. Here, we combined two glaucoma models to examine if a combination of risk factors (IOP and immune response) results in a more severe damage of retinal ganglion cells (RGCs) and the optic nerves as well as an additional glia activation.

Methods

Six-week-old wildtype (WT+ONA) and βB1-Connective Tissue Growth Factor (CTGF) mice (CTGF+ONA) were immunized with 1 mg ONA (optic nerve antigen). A WT and a CTGF control group (CTGF) received sodium chloride instead. IOP was measured before and every two weeks after immunization. After six weeks, electroretinogram (ERG) measurements were performed. Then, retinae and optic nerves were processed for (immuno-) histology. Further, mRNA levels of corresponding genes in optic nerve and retina were analyzed via RT-qPCR.

Results

Six weeks after immunization, the IOP in CTGF and CTGF+ONA mice was increased. The optic nerve of CTGF+ONA animals displayed the most severe cell inflammation, demyelination, and macroglia activation. Fewer numbers of oligodendrocytes were only observed in WT+ONA optic nerves, while more apoptotic cells triggered by the extrinsic pathway could be revealed in all three glaucoma groups. The number of microglia/macrophages was not altered within the optic nerves of all groups. The loss of neuronal cells, especially RGCs was most pronounced in CTGF+ONA retinae in the central part and this was accompanied by an enhanced activation of microglia/macrophages. Also, Müller cell activation could be noted in CTGF and CTGF+ONA retinae.

Discussion

In this new model, an additive degeneration could be noted in optic nerves as well as in the number of RGCs. These results suggest a potential additive role of high IOP and immune factors in glaucoma development, which will aid for understanding this multifactorial disease more precisely in the future.

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.

Enhancing axonal myelination in seniors: A review exploring the potential impact cannabis has on myelination in the aged brain

Consumption of cannabis is on the rise as public opinion trends toward acceptance and its consequent legalization. Specifically, the senior population is one of the demographics increasing their use of cannabis the fastest, but research aimed at understanding cannabis' impact on the aged brain is still scarce. Aging is characterized by many brain changes that slowly alter cognitive ability. One process that is greatly impacted during aging is axonal myelination. The slow degradation and loss of myelin (i.e., demyelination) in the brain with age has been shown to associate with cognitive decline and, furthermore, is a common characteristic of numerous neurological diseases experienced in aging. It is currently not known what causes this age-dependent degradation, but it is likely due to numerous confounding factors (i.e., heightened inflammation, reduced blood flow, cellular senescence) that impact the many cells responsible for maintaining overall homeostasis and myelin integrity. Importantly, animal studies using non-human primates and rodents have also revealed demyelination with age, providing a reliable model for researchers to try and understand the cellular mechanisms at play. In rodents, cannabis was recently shown to modulate the myelination process. Furthermore, studies looking at the direct modulatory impact cannabis has on microglia, astrocytes and oligodendrocyte lineage cells hint at potential mechanisms to prevent some of the more damaging activities performed by these cells that contribute to demyelination in aging. However, research focusing on how cannabis impacts myelination in the aged brain is lacking. Therefore, this review will explore the evidence thus far accumulated to show how cannabis impacts myelination and will extrapolate what this knowledge may mean for the aged brain.

Brain region dependent molecular signatures and myelin repair following chronic demyelination

Multiple sclerosis (MS) is the most prevalent demyelinating disease of the central nervous system, characterized by myelin destruction, axonal degeneration, and progressive loss of neurological functions. Remyelination is considered an axonal protection strategy and may enable functional recovery, but the mechanisms of myelin repair, especially after chronic demyelination, remain poorly understood. Here, we used the cuprizone demyelination mouse model to investigate spatiotemporal characteristics of acute and chronic de- and remyelination and motor functional recovery following chronic demyelination. Extensive remyelination occurred after both the acute and chronic insults, but with less robust glial responses and slower myelin recovery in the chronic phase. Axonal damage was found at the ultrastructural level in the chronically demyelinated corpus callosum and in remyelinated axons in the somatosensory cortex. Unexpectedly, we observed the development of functional motor deficits after chronic remyelination. RNA sequencing of isolated brain regions revealed significantly altered transcripts across the corpus callosum, cortex and hippocampus. Pathway analysis identified selective upregulation of extracellular matrix/collagen pathways and synaptic signaling in the chronically de/remyelinating white matter. Our study demonstrates regional differences of intrinsic reparative mechanisms after a chronic demyelinating insult and suggests a potential link between long-term motor function alterations and continued axonal damage during chronic remyelination. Moreover, the transcriptome dataset of three brain regions and over an extended de/remyelination period provides a valuable platform for a better understanding of the mechanisms of myelin repair as well as the identification of potential targets for effective remyelination and neuroprotection for progressive MS.

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 GABA_B receptors (GABA_B Rs) both in vitro and in vivo, and that GABA_B R-mediated signaling enhances OPC differentiation and myelin protein expression in vitro. Our goal here was to evaluate the pro-remyelinating potential of GABA_B 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.

Restoration of spinal cord injury: From endogenous repairing process to cellular therapy

Spinal cord injury (SCI) disrupts neurological pathways and impacts sensory, motor, and autonomic nerve function. There is no effective treatment for SCI currently. Numerous endogenous cells, including astrocytes, macrophages/microglia, and oligodendrocyte, are involved in the histological healing process following SCI. By interfering with cells during the SCI repair process, some advancements in the therapy of SCI have been realized. Nevertheless, the endogenous cell types engaged in SCI repair and the current difficulties these cells confront in the therapy of SCI are poorly defined, and the mechanisms underlying them are little understood. In order to better understand SCI and create new therapeutic strategies and enhance the clinical translation of SCI repair, we have comprehensively listed the endogenous cells involved in SCI repair and summarized the six most common mechanisms involved in SCI repair, including limiting the inflammatory response, protecting the spared spinal cord, enhancing myelination, facilitating neovascularization, producing neurotrophic factors, and differentiating into neural/colloidal cell lines.

Adaptive and maladaptive myelination in health and disease

Within the past decade, multiple lines of evidence have converged to identify a critical role for activity-regulated myelination in tuning the function of neural networks. In this Review, we provide an overview of accumulating evidence that activity-regulated myelination is required for brain adaptation and learning across multiple domains. We then discuss dysregulation of activity-dependent myelination in the context of neurological disease, a novel frontier with the potential to uncover new mechanisms of disease pathogenesis and to develop new therapeutic strategies. Alterations in myelination and neural network function can result from deficient myelin plasticity that impairs neurological function or from maladaptive myelination, in which intact activity-dependent myelination contributes to the disease process by promoting pathological patterns of neuronal activity. These emerging mechanisms suggest new avenues for therapeutic intervention that could more fully address the complex interactions between neurons and oligodendroglia.

CNS remyelination and inflammation: From basic mechanisms to therapeutic opportunities

Remyelination, the myelin regenerative response that follows demyelination, restores saltatory conduction and function and sustains axon health. Its declining efficiency with disease progression in the chronic autoimmune disease multiple sclerosis (MS) contributes to the currently untreatable progressive phase of the disease. Although some of the bona fide myelin regenerative medicine clinical trials have succeeded in demonstrating proof-of-principle, none of these compounds have yet proceeded toward approval. There therefore remains a need to increase our understanding of the fundamental biology of remyelination so that existing targets can be refined and new ones discovered. Here, we review the role of inflammation, in particular innate immunity, in remyelination, describing its many and complex facets and discussing how our evolving understanding can be harnessed to translational goals.

Impact of amino acids on microglial activation and CNS remyelination

Amino acids and their derivatives function as building blocks as well as signaling molecules to modulate various cellular processes in living organisms. In mice, amino acids accumulate in demyelinated lesions and return to basal levels during remyelination. Studies have found that amino acids and their metabolites modulate immune activity in the central nervous system (CNS) and influence oligodendrocyte differentiation and remyelination efficiency. In this review, we discuss current studies on amino acid metabolism in the context of CNS remyelination. By understanding the mechanisms of amino acid signaling and metabolism in demyelinated lesions, we may deepen our understanding of compartmentalized CNS inflammation in demyelinating disease like multiple sclerosis (MS) and provide evidence to develop novel pharmacological therapies targeting amino acid metabolism to prevent disease worsening.

Prolonged, Low-Level Exposure to the Marine Toxin, Domoic Acid, and Measures of Neurotoxicity in Nonhuman Primates

BACKGROUND

The excitotoxic molecule, domoic acid (DA), is a marine algal toxin known to induce overt hippocampal neurotoxicity. Recent experimental and epidemiological studies suggest adverse neurological effects at exposure levels near the current regulatory limit (20 ppm, ∼0.075-0.1mg/kg). At these levels, cognitive effects occur in the absence of acute symptoms or evidence of neuronal death.

OBJECTIVES

This study aimed to identify adverse effects on the nervous system from prolonged, dietary DA exposure in adult, female Macaca fascicularis monkeys.

METHODS

Monkeys were orally exposed to 0, 0.075, and 0.15mg/kg per day for an average of 14 months. Clinical blood counts, chemistry, and cytokine levels were analyzed in the blood. In-life magnetic resonance (MR) imaging assessed volumetric and tractography differences in and between the hippocampus and thalamus. Histology of neurons and glia in the fornix, fimbria, internal capsule, thalamus, and hippocampus was evaluated. Hippocampal RNA sequencing was used to identify differentially expressed genes. Enrichment of gene networks for neuronal health, excitotoxicity, inflammation/glia, and myelin were assessed with Gene Set Enrichment Analysis.

RESULTS

Clinical blood counts, chemistry, and cytokine levels were not altered with DA exposure in nonhuman primates. Transcriptome analysis of the hippocampus yielded 748 differentially expressed genes (fold change≥1.5; p≤0.05), reflecting differences in a broad molecular profile of intermediate early genes (e.g., FOS, EGR) and genes related to myelin networks in DA animals. Between exposed and control animals, MR imaging showed comparable connectivity of the hippocampus and thalamus and histology showed no evidence of hypomyelination. Histological examination of the thalamus showed a larger microglia soma size and an extension of cell processes, but suggestions of a GFAP+astrocyte response showed no indication of astrocyte hypertrophy.

DISCUSSION

In the absence of overt hippocampal excitotoxicity, chronic exposure of Macaca fascicularis monkeys to environmentally relevant levels of DA suggested a subtle shift in the molecular profile of the hippocampus and the microglia phenotype in the thalamus that was possibly reflective of an adaptive response due to prolonged DA exposure. https://doi.org/10.1289/EHP10923.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

Role of Ketogenic Diets in Multiple Sclerosis and Related Animal Models: An Updated Review

Prescribing a ketogenic diet (KD) is a century-old dietary intervention mainly used in the context of intractable epilepsy. The classic KD and its variants regained popularity in recent decades, and they are considered potentially beneficial in a variety of neurological conditions other than epilepsy. Many patients with multiple sclerosis (MS) have attempted diet modification for better control of their disease, although evidence thus far remains insufficient to recommend a specific diet for these patients. The results of 3 pilot clinical trials of KD therapy for MS, as well as several related studies, have been reported in recent years. The preliminary findings suggest that KD is safe, feasible, and potentially neuroprotective and disease-modifying for patients with MS. Research on corresponding rodent models has also lent support to the efficacy of KD in the prevention and treatment of experimental autoimmune encephalomyelitis and toxin-induced inflammatory demyelinating conditions in the brain. Furthermore, the animal studies have yielded mechanistic insights into the molecular mechanisms of KD action in relevant situations, paving the way for precision nutrition. Herein we review and synthesize recent advances and also identify unresolved issues, such as the roles of adipokines and gut microbiota, in this field. Hopefully this panoramic view of current understanding can inform future research directions and clinical practice with regard to KD in MS and related conditions.

Exosomes-Loaded Electroconductive Hydrogel Synergistically Promotes Tissue Repair after Spinal Cord Injury via Immunoregulation and Enhancement of Myelinated Axon Growth

Electroconductive hydrogels are very attractive candidates for accelerated spinal cord injury (SCI) repair because they match the electrical and mechanical properties of neural tissue. However, electroconductive hydrogel implantation can potentially aggravate inflammation, and hinder its repair efficacy. Bone marrow stem cell-derived exosomes (BMSC-exosomes) have shown immunomodulatory and tissue regeneration effects, therefore, neural tissue-like electroconductive hydrogels loaded with BMSC-exosomes are developed for the synergistic treatment of SCI. These exosomes-loaded electroconductive hydrogels modulate microglial M2 polarization via the NF-κB pathway, and synergistically enhance neuronal and oligodendrocyte differentiation of neural stem cells (NSCs) while inhibiting astrocyte differentiation, and also increase axon outgrowth via the PTEN/PI3K/AKT/mTOR pathway. Furthermore, exosomes combined electroconductive hydrogels significantly decrease the number of CD68-positive microglia, enhance local NSCs recruitment, and promote neuronal and axonal regeneration, resulting in significant functional recovery at the early stage in an SCI mouse model. Hence, the findings of this study demonstrate that the combination of electroconductive hydrogels and BMSC-exosomes is a promising therapeutic strategy for SCI repair.

Oscillatory calcium release and sustained store-operated oscillatory calcium signaling prevents differentiation of human oligodendrocyte progenitor cells

Endogenous remyelination in demyelinating diseases such as multiple sclerosis is contingent upon the successful differentiation of oligodendrocyte progenitor cells (OPCs). Signaling via the Gα_q-coupled muscarinic receptor (M_1/3R) inhibits human OPC differentiation and impairs endogenous remyelination in experimental models. We hypothesized that calcium release following Gα_q-coupled receptor (G_qR) activation directly regulates human OPC (hOPC) cell fate. In this study, we show that specific G_qR agonists activating muscarinic and metabotropic glutamate receptors induce characteristic oscillatory calcium release in hOPCs and that these agonists similarly block hOPC maturation in vitro. Both agonists induce calcium release from endoplasmic reticulum (ER) stores and store operated calcium entry (SOCE) likely via STIM/ORAI-based channels. siRNA mediated knockdown (KD) of obligate calcium sensors STIM1 and STIM2 decreased the magnitude of muscarinic agonist induced oscillatory calcium release and attenuated SOCE in hOPCs. In addition, STIM2 expression was necessary to maintain the frequency of calcium oscillations and STIM2 KD reduced spontaneous OPC differentiation. Furthermore, STIM2 siRNA prevented the effects of muscarinic agonist treatment on OPC differentiation suggesting that SOCE is necessary for the anti-differentiative action of muscarinic receptor-dependent signaling. Finally, using a gain-of-function approach with an optogenetic STIM lentivirus, we demonstrate that independent activation of SOCE was sufficient to significantly block hOPC differentiation and this occurred in a frequency dependent manner while increasing hOPC proliferation. These findings suggest that intracellular calcium oscillations directly regulate hOPC fate and that modulation of calcium oscillation frequency may overcome inhibitory Gα_q-coupled signaling that impairs myelin repair.

Newly Identified Deficiencies in the Multiple Sclerosis Central Nervous System and Their Impact on the Remyelination Failure

The pathogenesis of multiple sclerosis (MS) remains enigmatic and controversial. Myelin sheaths in the central nervous system (CNS) insulate axons and allow saltatory nerve conduction. MS brings about the destruction of myelin sheaths and the myelin-producing oligodendrocytes (ODCs). The conundrum of remyelination failure is, therefore, crucial in MS. In this review, the roles of epidermal growth factor (EGF), normal prions, and cobalamin in CNS myelinogenesis are briefly summarized. Thereafter, some findings of other authors and ourselves on MS and MS-like models are recapitulated, because they have shown that: (a) EGF is significantly decreased in the CNS of living or deceased MS patients; (b) its repeated administration to mice in various MS-models prevents demyelination and inflammatory reaction; (c) as was the case for EGF, normal prion levels are decreased in the MS CNS, with a strong correspondence between liquid and tissue levels; and (d) MS cobalamin levels are increased in the cerebrospinal fluid, but decreased in the spinal cord. In fact, no remyelination can occur in MS if these molecules (essential for any form of CNS myelination) are lacking. Lastly, other non-immunological MS abnormalities are reviewed. Together, these results have led to a critical reassessment of MS pathogenesis, partly because EGF has little or no role in immunology.

Neuron to Oligodendrocyte Precursor Cell Synapses: Protagonists in Oligodendrocyte Development and Myelination, and Targets for Therapeutics

The development of neuronal circuitry required for cognition, complex motor behaviors, and sensory integration requires myelination. The role of glial cells such as astrocytes and microglia in shaping synapses and circuits have been covered in other reviews in this journal and elsewhere. This review summarizes the role of another glial cell type, oligodendrocytes, in shaping synapse formation, neuronal circuit development, and myelination in both normal development and in demyelinating disease. Oligodendrocytes ensheath and insulate neuronal axons with myelin, and this facilitates fast conduction of electrical nerve impulses via saltatory conduction. Oligodendrocytes also proliferate during postnatal development, and defects in their maturation have been linked to abnormal myelination. Myelination also regulates the timing of activity in neural circuits and is important for maintaining the health of axons and providing nutritional support. Recent studies have shown that dysfunction in oligodendrocyte development and in myelination can contribute to defects in neuronal synapse formation and circuit development. We discuss glutamatergic and GABAergic receptors and voltage gated ion channel expression and function in oligodendrocyte development and myelination. We explain the role of excitatory and inhibitory neurotransmission on oligodendrocyte proliferation, migration, differentiation, and myelination. We then focus on how our understanding of the synaptic connectivity between neurons and OPCs can inform future therapeutics in demyelinating disease, and discuss gaps in the literature that would inform new therapies for remyelination.

Dysmyelination by Oligodendrocyte-Specific Ablation of Ninj2 Contributes to Depressive-Like Behaviors

Depression is a mental disorder affecting more than 300 million people in the world. Abnormalities in white matter are associated with the development of depression. Here, the authors show that mice with oligodendrocyte-specific deletion of Nerve injury-induced protein 2 （Ninj2) exhibit depressive-like behaviors. Loss of Ninj2 in oligodendrocytes inhibits oligodendrocyte development and myelination, and impairs neuronal structure and activities. Ninj2 competitively inhibits TNFα/TNFR1 signaling pathway by directly binding to TNFR1 in oligodendrocytes. Loss of Ninj2 activates TNFα-induced necroptosis, and increases C-C Motif Chemokine Ligand 2 (Ccl2) production, which might mediate the signal transduction from oligodendrocyte to neurons. Inhibition of necroptosis by Nec-1s administration synchronously restores oligodendrocyte development, improves neuronal excitability, and alleviates depressive-like behaviors. This study thus illustrates the role of Ninj2 in the development of depression and myelination, reveals the relationship between oligodendrocytes and neurons, and provides a potential therapeutic target for depression.

White matter abnormalities and multivariate pattern analysis in anti-NMDA receptor encephalitis

OBJECTIVE

This study aimed to investigate white matter (WM) microstructural alterations and their relationship correlation with disease severity in anti-NMDA receptor (NMDAR) encephalitis. Multivariate pattern analysis (MVPA) was applied to discriminate between patients and healthy controls and explore potential imaging biomarkers.

METHODS

Thirty-two patients with anti-NMDAR encephalitis and 26 matched healthy controls underwent diffusion tensor imaging. Tract-based spatial statistics and atlas-based analysis were used to determine WM microstructural alterations between the two groups. MVPA, based on a machine-learning algorithm, was applied to classify patients and healthy controls.

RESULTS

Patients exhibited significantly reduced fractional anisotropy in the corpus callosum, fornix, cingulum, anterior limb of the internal capsule, and corona radiata. Moreover, mean diffusivity was increased in the anterior corona radiata and body of the corpus callosum. On the other hand, radial diffusivity was increased in the anterior limb of the internal capsule, cingulum, corpus callosum, corona radiata, and fornix. WM changes in the cingulum, fornix, and retrolenticular part of the internal capsule were correlated with disease severity. The accuracy, sensitivity, and specificity of fractional anisotropy-based classification were each 78.33%, while they were 67.71, 65.83, and 70% for radial diffusivity.

CONCLUSION

Widespread WM lesions were detected in anti-NMDAR encephalitis. The correlation between WM abnormalities and disease severity suggests that these alterations may serve a key role in the pathophysiological mechanisms of anti-NMDAR encephalitis. The combination of tract-based spatial statistics and MVPA may provide more specific and complementary information at the group and individual levels.

Human iPSC-Derived Astrocytes: A Powerful Tool to Study Primary Astrocyte Dysfunction in the Pathogenesis of Rare Leukodystrophies

Astrocytes are very versatile cells, endowed with multitasking capacities to ensure brain homeostasis maintenance from brain development to adult life. It has become increasingly evident that astrocytes play a central role in many central nervous system pathologies, not only as regulators of defensive responses against brain insults but also as primary culprits of the disease onset and progression. This is particularly evident in some rare leukodystrophies (LDs) where white matter/myelin deterioration is due to primary astrocyte dysfunctions. Understanding the molecular defects causing these LDs may help clarify astrocyte contribution to myelin formation/maintenance and favor the identification of possible therapeutic targets for LDs and other CNS demyelinating diseases. To date, the pathogenic mechanisms of these LDs are poorly known due to the rarity of the pathological tissue and the failure of the animal models to fully recapitulate the human diseases. Thus, the development of human induced pluripotent stem cells (hiPSC) from patient fibroblasts and their differentiation into astrocytes is a promising approach to overcome these issues. In this review, we discuss the primary role of astrocytes in LD pathogenesis, the experimental models currently available and the advantages, future evolutions, perspectives, and limitations of hiPSC to study pathologies implying astrocyte dysfunctions.

New Epidermal-Growth-Factor-Related Insights Into the Pathogenesis of Multiple Sclerosis: Is It Also Epistemology?

Recent findings showing that epidermal growth factor (EGF) is significantly decreased in the cerebrospinal fluid (CSF) and spinal cord (SC) of living or deceased multiple sclerosis (MS) patients, and that its repeated administration to rodents with chemically- or virally-induced demyelination of the central nervous system (CNS) or experimental allergic encephalomyelitis (EAE) prevents demyelination and inflammatory reactions in the CNS, have led to a critical reassessment of the MS pathogenesis, partly because EGF is considered to have little or no role in immunology. EGF is the only myelinotrophic factor that has been tested in the CSF and spinal cord of MS patients, and it has been shown there is a good correspondence between liquid and tissue levels. This review: (a) briefly summarises the positive EGF effects on neural stem cells, oligodendrocyte cell lineage, and astrocytes in order to explain, at least in part, the biological basis of the myelin loss and remyelination failure in MS; and (b) after a short analysis of the evolution of the principle of cause-effect in the history of Western philosophy, highlights the lack of any experimental immune-, toxin-, or virus-mediated model that precisely reproduces the histopathological features and “clinical” symptoms of MS, thus underlining the inapplicability of Claude Bernard's crucial sequence of “observation, hypothesis, and hypothesis testing.” This is followed by a discussion of most of the putative non-immunologically-linked points of MS pathogenesis (abnormalities in myelinotrophic factor CSF levels, oligodendrocytes (ODCs), astrocytes, extracellular matrix, and epigenetics) on the basis of Popper's falsification principle, and the suggestion that autoimmunity and phologosis reactions (surely the most devasting consequences of the disease) are probably the last links in a chain of events that trigger the reactions. As it is likely that there is a lack of other myelinotrophic growth factors because myelinogenesis is controlled by various CNS and extra-CNS growth factors and other molecules within and outside ODCs, further studies are needed to investigate the role of non-immunological molecules at the time of the onset of the disease. In the words of Galilei, the human mind should be prepared to understand what nature has created.

Eccentric rehabilitation induces white matter plasticity and sensorimotor recovery in chronic spinal cord injury

Experience-dependent white matter plasticity offers new potential for rehabilitation-induced recovery after neurotrauma. This first-in-human translational experiment combined myelin water imaging in humans and genetic fate-mapping of oligodendrocyte lineage cells in mice to investigate whether downhill locomotor rehabilitation that emphasizes eccentric muscle actions promotes white matter plasticity and recovery in chronic, incomplete spinal cord injury (SCI). In humans, of 20 individuals with SCI that enrolled, four passed the imaging screen and had myelin water imaging before and after a 12-week (3 times/week) downhill locomotor treadmill training program (SCI + DH). One individual was excluded for imaging artifacts. Uninjured control participants (n = 7) had two myelin water imaging sessions within the same day. Changes in myelin water fraction (MWF), a histopathologically-validated myelin biomarker, were analyzed in a priori motor learning and non-motor learning brain regions and the cervical spinal cord using statistical approaches appropriate for small sample sizes. PDGFRα-CreER^T2:mT/mG mice, that express green fluorescent protein on oligodendrocyte precursor cells and subsequent newly-differentiated oligodendrocytes upon tamoxifen-induced recombination, were either naive (n = 6) or received a moderate (75 kilodyne), contusive SCI at T9 and were randomized to downhill training (n = 6) or unexercised groups (n = 6). We initiated recombination 29 days post-injury, seven days prior to downhill training. Mice underwent two weeks of daily downhill training on the same 10% decline grade used in humans. Between-group comparison of functional (motor and sensory) and histological (oligodendrogenesis, oligodendroglial/axon interaction, paranodal structure) outcomes occurred post-training. In humans with SCI, downhill training increased MWF in brain motor learning regions (postcentral, precuneus) and mixed motor and sensory tracts of the ventral cervical spinal cord compared to control participants (P < 0.05). In mice with thoracic SCI, downhill training induced oligodendrogenesis in cervical dorsal and lateral white matter, increased axon-oligodendroglial interactions, and normalized paranodal structure in dorsal column sensory tracts (P < 0.05). Downhill training improved sensorimotor recovery in mice by normalizing hip and knee motor control and reducing hyperalgesia, both of which were associated with new oligodendrocytes in the cervical dorsal columns (P < 0.05). Our findings indicate that eccentric-focused, downhill rehabilitation promotes white matter plasticity and improved function in chronic SCI, likely via oligodendrogenesis in nervous system regions activated by the training paradigm. Together, these data reveal an exciting role for eccentric training in white matter plasticity and sensorimotor recovery after SCI.

A Novel Vitronectin Peptide Facilitates Differentiation of Oligodendrocytes from Human Pluripotent Stem Cells (Synthetic ECM for Oligodendrocyte Differentiation)

Simple Summary

Oligodendrocyte (OD) is a cell type of great interest in the regenerative medicine for several neurological diseases. This study provides a new defined coating material for the differentiation of ODs from human pluripotent stem cells. A new peptide named VNP2, designed by in silico simulation, can be readily produced in a large amount and stably immobilized on the bottom of culture vessel. Upon using for differentiation of ODs, VNP2 promoted the differentiation efficiency more than the conventional coating materials did. Furthermore, transcriptomic analysis revealed molecular clues for the differentiation promoting activity of VNP2. Therefore, this peptide may be used as a favored coating material for the culture and differentiation of ODs.

Abstract

Differentiation of oligodendrocytes (ODs) presents a challenge in regenerative medicine due to their role in various neurological diseases associated with dysmyelination and demyelination. Here, we designed a peptide derived from vitronectin (VN) using in silico docking simulation and examined its use as a synthetic substrate to support the differentiation of ODs derived from human pluripotent stem cells. The designed peptide, named VNP2, promoted OD differentiation induced by the overexpression of SOX10 in OD precursor cells compared with Matrigel and full-length VN. ODs differentiated on VNP2 exhibited greater contact with axon-mimicking nanofibers than those differentiated on Matrigel. Transcriptomic analysis revealed that the genes associated with morphogenesis, cytoskeleton remodeling, and OD differentiation were upregulated in cells grown on VNP2 compared with cells grown on Matrigel. This new synthetic VN-derived peptide can be used to develop a culture environment for efficient OD differentiation.

Myelin: A gatekeeper of activity-dependent circuit plasticity?

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