Mechanistic Target of Rapamycin Regulates the Oligodendrocyte Cytoskeleton during Myelination

Molecular Motors in Myelination and Their Misregulation in Disease

Molecular motors are cellular components involved in the intracellular transport of organelles and materials to ensure cell homeostasis. This is particularly relevant in neurons, where the synaptic components synthesized in the soma need to travel over long distances to their destination. They can walk on microtubules (kinesins and dyneins) or actin filaments (myosins), the major components of cell cytoskeleton. While kinesins mostly perform the anterograde transport of intracellular components toward the plus ends of microtubules located distally in cell processes, cytoplasmic dyneins allow the retrograde flux of intracellular cargo toward the minus ends of microtubules located at the cell soma. Axon myelination represents a major aspect of neuronal maturation and is essential for neuronal function, as it speeds up the transmission of electrical signals. Increasing evidence supports a role for molecular motors in the homeostatic control of myelination. This role includes the trafficking of myelin components along the processes of myelinating cells and local regulation of pathways that ensure axon wrapping. Dysfunctional control of the intracellular transport machinery has therefore been linked to several brain pathologies, including demyelinating diseases. These disorders include a broad spectrum of conditions characterized by pathological demyelination of axons within the nervous system, ultimately leading to axonal degeneration and neuronal death, with multiple sclerosis representing the most prevalent and studied condition. This review highlights the involvement of molecular motors in the homeostatic control of myelination. It also discusses studies that have yielded insights into the dysfunctional activity of molecular motors in the pathophysiology of multiple sclerosis.

Bioinformatics analysis reveals key mechanisms of oligodendrocytes and oligodendrocyte precursor cells regulation in spinal cord Injury

Despite extensive research, spinal cord injuries (SCI), which could cause severe sensory, motor and autonomic dysfunction, remain largely incurable. Oligodendrocytes and oligodendrocyte precursor cells (ODC/OPC) play a crucial role in neural morphological repair and functional recovery following SCI. We performed single-cell sequencing (scRNA-seq) on 59,558 cells from 39 mouse samples, combined with microarray data from 164 SCI samples and 3 uninjured samples. We further validated our findings using a large clinical cohort consisting of 38 SCI patients, 10 healthy controls, and 10 trauma controls, assessed with the American Spinal Cord Injury Association (ASIA) scale. We proposed a novel SCI classification model based on the expression of prognostic differentially expressed ODC/OPC differentiation-related genes (PDEODGs). This model includes three types: Low ODC/OPC Score Classification (LOSC), Median ODC/OPC Score Classification (MOSC), and High ODC/OPC Score Classification (HOSC). Considering the relationship between these subtypes and prognosis, we speculated that enhancing ODC/OPC differentiation and inhibiting inflammatory infiltration may improve outcomes. Additionally, we identified potential treatments for SCI that target key genes within these subtypes, offering promising implications for therapy.

Thioredoxin-1 protein interactions in neuronal survival and neurodegeneration

Neuronal cell death remains the principal pathophysiologic hallmark of neurodegenerative diseases and the main challenge for treatment strategies. Thioredoxin1 (Trx1) is a major cytoplasmic thiol oxidoreductase protein involved in redox signaling, hence a crucial player in maintaining neuronal health. Trx1 levels are notably reduced in neurodegenerative diseases including Alzheimer's and Parkinson's diseases, however, the impact of this decrease on neuronal physiology remains largely unexplored. This is mainly due to the nature of Trx1 redox regulatory role which is afforded by a rapid electron transfer to its oxidized protein substrates. During this reaction, Trx1 forms a transient bond with the oxidized disulfide bond in the substrate. This is a highly fast reaction which makes the identification of Trx1 substrates a technically challenging task. In this project, we utilized a transgenic mouse model expressing a Flag-tagged mutant form of Trx1 that can form stable disulfide bonds with its substrates, hence allowing identification of the Trx1 target proteins. Autophagy is a vital housekeeping process in neurons that is critical for degradation of damaged proteins under oxidative stress conditions and is interrupted in neurodegenerative diseases. Given Trx1's suggested involvement in autophagy, we aimed to identify potential Trx1 substrates following pharmacologic induction of autophagy in primary cortical neurons. Treatment with rapamycin, an autophagy inducer, significantly reduced neurite outgrowth and caused cytoskeletal alterations. Using immunoprecipitation and mass spectrometry, we have identified 77 Trx1 target proteins associated with a wide range of cellular functions including cytoskeletal organization and neurodegenerative diseases. Focusing on neuronal cytoskeleton organization, we identified a novel interaction between Trx1 and RhoB which was confirmed in genetic models of Trx1 downregulation in primary neuronal cultures and HT22 mouse immortalized hippocampal neurons. The applicability of these findings was also tested against the publicly available proteomic data from Alzheimer's patients. Our study uncovers a novel role for Trx1 in regulating neuronal cytoskeleton organization and provides a mechanistic explanation for its multifaceted role in the physiology and pathology of the nervous system, offering new insights into the molecular mechanisms underlying neurodegeneration.

(Re)building the nervous system: A review of neuron-glia interactions from development to disease

Neuron-glia interactions are fundamental to the development and function of the nervous system. During development, glia, including astrocytes, microglia, and oligodendrocytes, influence neuronal differentiation and migration, synapse formation and refinement, and myelination. In the mature brain, glia are crucial for maintaining neural homeostasis, modulating synaptic activity, and supporting metabolic functions. Neurons, inherently vulnerable to various stressors, rely on glia for protection and repair. However, glia, in their reactive state, can also promote neuronal damage, which contributes to neurodegenerative and neuropsychiatric diseases. Understanding the dual role of glia-as both protectors and potential aggressors-sheds light on their complex contributions to disease etiology and pathology. By appropriately modulating glial activity, it may be possible to mitigate neurodegeneration and restore neuronal function. In this review, which originated from the International Society for Neurochemistry (ISN) Advanced School in 2019 held in Montreal, Canada, we first describe the critical importance of glia in the development and maintenance of a healthy nervous system as well as their contributions to neuronal damage and neurological disorders. We then discuss potential strategies to modulate glial activity during disease to protect and promote a properly functioning nervous system. We propose that targeting glial cells presents a promising therapeutic avenue for rebuilding the nervous system.

Regulators of Oligodendrocyte Differentiation

Myelination has evolved as a mechanism to ensure fast and efficient propagation of nerve impulses along axons. Within the central nervous system (CNS), myelination is carried out by highly specialized glial cells, oligodendrocytes. The formation of myelin is a prolonged aspect of CNS development that occurs well into adulthood in humans, continuing throughout life in response to injury or as a component of neuroplasticity. The timing of myelination is tightly tied to the generation of oligodendrocytes through the differentiation of their committed progenitors, oligodendrocyte precursor cells (OPCs), which reside throughout the developing and adult CNS. In this article, we summarize our current understanding of some of the signals and pathways that regulate the differentiation of OPCs, and thus the myelination of CNS axons.

The role of TSC1 and TSC2 proteins in neuronal axons

Tuberous Sclerosis Complex 1 and 2 proteins, TSC1 and TSC2 respectively, participate in a multiprotein complex with a crucial role for the proper development and function of the nervous system. This complex primarily acts as an inhibitor of the mechanistic target of rapamycin (mTOR) kinase, and mutations in either TSC1 or TSC2 cause a neurodevelopmental disorder called Tuberous Sclerosis Complex (TSC). Neurological manifestations of TSC include brain lesions, epilepsy, autism, and intellectual disability. On the cellular level, the TSC/mTOR signaling axis regulates multiple anabolic and catabolic processes, but it is not clear how these processes contribute to specific neurologic phenotypes. Hence, several studies have aimed to elucidate the role of this signaling pathway in neurons. Of particular interest are axons, as axonal defects are associated with severe neurocognitive impairments. Here, we review findings regarding the role of the TSC1/2 protein complex in axons. Specifically, we will discuss how TSC1/2 canonical and non-canonical functions contribute to the formation and integrity of axonal structure and function.

EAAT3 impedes oligodendrocyte remyelination in chronic cerebral hypoperfusion-induced white matter injury

BACKGROUND

Chronic cerebral hypoperfusion-induced demyelination causes progressive white matter injury, although the pathogenic pathways are unknown.

METHODS

The Single Cell Portal and PanglaoDB databases were used to analyze single-cell RNA sequencing experiments to determine the pattern of EAAT3 expression in CNS cells. Immunofluorescence (IF) was used to detect EAAT3 expression in oligodendrocytes and oligodendrocyte progenitor cells (OPCs). EAAT3 levels in mouse brains were measured using a western blot at various phases of development, as well as in traumatic brain injury (TBI) and intracerebral hemorrhage (ICH) mouse models. The mouse bilateral carotid artery stenosis (BCAS) model was used to create white matter injury. IF, Luxol Fast Blue staining, and electron microscopy were used to investigate the effect of remyelination. 5-Ethynyl-2-Deoxy Uridine staining, transwell chamber assays, and IF were used to examine the effects of OPCs' proliferation, migration, and differentiation in vivo and in vitro. The novel object recognition test, the Y-maze test, the rotarod test, and the grid walking test were used to examine the impact of behavioral modifications.

RESULTS

A considerable amount of EAAT3 was expressed in OPCs and mature oligodendrocytes, according to single-cell RNA sequencing data. During multiple critical phases of mouse brain development, there were no substantial changes in EAAT3 levels in the hippocampus, cerebral cortex, or white matter. Furthermore, neither the TBI nor ICH models significantly affected the levels of EAAT3 in the aforementioned brain areas. The chronic white matter injury caused by BCAS, on the other hand, resulted in a strikingly high level of EAAT3 expression in the oligodendroglia and white matter. Correspondingly, blocking EAAT3 assisted in the recovery of cognitive and motor impairment as well as the restoration of cerebral blood flow following BCAS. Furthermore, EAAT3 suppression was connected to improved OPCs' survival and proliferation in vivo as well as faster OPCs' proliferation, migration, and differentiation in vitro. Furthermore, this study revealed that the mTOR pathway is implicated in EAAT3-mediated remyelination.

CONCLUSIONS

Our findings provide the first evidence that abnormally high levels of oligodendroglial EAAT3 in chronic cerebral hypoperfusion impair OPCs' pro-remyelination actions, hence impeding white matter repair and functional recovery. EAAT3 inhibitors could be useful in the treatment of ischemia demyelination.

Glutamate delta-1 receptor regulates oligodendrocyte progenitor cell differentiation and myelination in normal and demyelinating conditions

In this study, we investigated the role of glutamate delta 1 receptor (GluD1) in oligodendrocyte progenitor cell (OPC)-mediated myelination during basal (development) and pathophysiological (cuprizone-induced demyelination) conditions. Initially, we sought to determine the expression pattern of GluD1 in OPCs and found a significant colocalization of GluD1 puncta with neuron-glial antigen 2 (NG2, OPC marker) in the motor cortex and dorsal striatum. Importantly, we found that the ablation of GluD1 led to an increase in the number of myelin-associated glycoprotein (MAG+) cells in the corpus callosum and motor cortex at P40 without affecting the number of NG2+ OPCs, suggesting that GluD1 loss selectively facilitates OPC differentiation rather than proliferation. Further, deletion of GluD1 enhanced myelination in the corpus callosum and motor cortex, as indicated by increased myelin basic protein (MBP) staining at P40, suggesting that GluD1 may play an essential role in the developmental regulation of myelination during the critical window period. In contrast, in cuprizone-induced demyelination, we observed reduced MBP staining in the corpus callosum of GluD1 KO mice. Furthermore, cuprizone-fed GluD1 KO mice showed more robust motor deficits. Collectively, our results demonstrate that GluD1 plays a critical role in OPC regulation and myelination in normal and demyelinating conditions.

Updated clinical recommendations for the management of tuberous sclerosis complex associated epilepsy

Children with tuberous sclerosis complex (TSC), may experience a variety of seizure types in the first year of life, most often focal seizure sand epileptic spasms. Drug resistance is seen early in many patients, and the management of TSC associated epilepsy remain a major challenge for clinicians. In 2018 clinical recommendations for the management of TSC associated epilepsy were published by a panel of European experts. In the last five years considerable progress has been made in understanding the neurobiology of epileptogenesis and three interventional randomized controlled trials have changed the therapeutic approach for the management of TSC associated epilepsy. Pre-symptomatic treatment with vigabatrin may delay seizure onset, may reduce seizure severity and reduce the risk of epileptic encephalopathy. The efficacy of mTOR inhibition with adjunctive everolimus was documented in patients with TSC associated refractory seizures and cannabidiol could be another therapeutic option. Epilepsy surgery has significantly improved seizure outcome in selected patients and should be considered early in all patients with drug resistant epilepsy. There is a need to identify patients who may have a higher risk of developing epilepsy and autism spectrum disorder (ASD). In the recent years significant progress has been made owing to the early identification of risk factors for the development of drug-resistant epilepsy. Better understanding of the mechanism underlying epileptogenesis may improve the management for TSC-related epilepsy. Developmental neurobiology and neuropathology give opportunities for the implementation of concepts related to clinical findings, and an early genetic diagnosis and use of EEG and MRI biomarkers may improve the development of pre-symptomatic and disease-modifying strategies.

The AMPK activator metformin improves recovery from demyelination by shifting oligodendrocyte bioenergetics and accelerating OPC differentiation

Multiple Sclerosis (MS) is a chronic disease characterized by immune-mediated destruction of myelinating oligodendroglia in the central nervous system. Loss of myelin leads to neurological dysfunction and, if myelin repair fails, neurodegeneration of the denuded axons. Virtually all treatments for MS act by suppressing immune function, but do not alter myelin repair outcomes or long-term disability. Excitingly, the diabetes drug metformin, a potent activator of the cellular "energy sensor" AMPK complex, has recently been reported to enhance recovery from demyelination. In aged mice, metformin can restore responsiveness of oligodendrocyte progenitor cells (OPCs) to pro-differentiation cues, enhancing their ability to differentiate and thus repair myelin. However, metformin's influence on young oligodendroglia remains poorly understood. Here we investigated metformin's effect on the temporal dynamics of differentiation and metabolism in young, healthy oligodendroglia and in oligodendroglia following myelin damage in young adult mice. Our findings reveal that metformin accelerates early stages of myelin repair following cuprizone-induced myelin damage. Metformin treatment of both isolated OPCs and oligodendrocytes altered cellular bioenergetics, but in distinct ways, suppressing oxidative phosphorylation and enhancing glycolysis in OPCs, but enhancing oxidative phosphorylation and glycolysis in both immature and mature oligodendrocytes. In addition, metformin accelerated the differentiation of OPCs to oligodendrocytes in an AMPK-dependent manner that was also dependent on metformin's ability to modulate cell metabolism. In summary, metformin dramatically alters metabolism and accelerates oligodendroglial differentiation both in health and following myelin damage. This finding broadens our knowledge of metformin's potential to promote myelin repair in MS and in other diseases with myelin loss or altered myelination dynamics.

Compound from Magnolia officinalis Ameliorates White Matter Injury by Promoting Oligodendrocyte Maturation in Chronic Cerebral Ischemia Models

Chronic cerebral hypoperfusion leads to white matter injury (WMI), which subsequently causes neurodegeneration and even cognitive impairment. However, due to the lack of treatment specifically for WMI, novel recognized and effective therapeutic strategies are urgently needed. In this study, we found that honokiol and magnolol, two compounds derived from Magnolia officinalis, significantly facilitated the differentiation of primary oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes, with a more prominent effect of the former compound. Moreover, our results demonstrated that honokiol treatment improved myelin injury, induced mature oligodendrocyte protein expression, attenuated cognitive decline, promoted oligodendrocyte regeneration, and inhibited astrocytic activation in the bilateral carotid artery stenosis model. Mechanistically, honokiol increased the phosphorylation of serine/threonine kinase (Akt) and mammalian target of rapamycin (mTOR) by activating cannabinoid receptor 1 during OPC differentiation. Collectively, our study indicates that honokiol might serve as a potential treatment for WMI in chronic cerebral ischemia.

Androgens show sex-dependent differences in myelination in immune and non-immune murine models of CNS demyelination

Neuroprotective, anti-inflammatory, and remyelinating properties of androgens are well-characterized in demyelinated male mice and men suffering from multiple sclerosis. However, androgen effects mediated by the androgen receptor (AR), have been only poorly studied in females who make low androgen levels. Here, we show a predominant microglial AR expression in demyelinated lesions from female mice and women with multiple sclerosis, but virtually undetectable AR expression in lesions from male animals and men with multiple sclerosis. In female mice, androgens and estrogens act in a synergistic way while androgens drive microglia response towards regeneration. Transcriptomic comparisons of demyelinated mouse spinal cords indicate that, regardless of the sex, androgens up-regulate genes related to neuronal function integrity and myelin production. Depending on the sex, androgens down-regulate genes related to the immune system in females and lipid catabolism in males. Thus, androgens are required for proper myelin regeneration in females and therapeutic approaches of demyelinating diseases need to consider male-female differences.

mTORC2 Loss in Oligodendrocyte Progenitor Cells Results in Regional Hypomyelination in the Central Nervous System

In the CNS, oligodendrocyte progenitor cells (OPCs) differentiate into mature oligodendrocytes to generate myelin, an essential component for normal nervous system function. OPC differentiation is driven by signaling pathways, such as mTOR, which functions in two distinct complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), containing Raptor or Rictor, respectively. In the current studies, mTORC2 signaling was selectively deleted from OPCs in PDGFRα-Cre X Rictor^fl/fl mice. This study examined developmental myelination in male and female mice, comparing the impact of mTORC2 deletion in the corpus callosum and spinal cord. In both regions, Rictor loss in OPCs resulted in early reduction in myelin RNAs and proteins. However, these deficits rapidly recovered in spinal cord, where normal myelin was noted at P21 and P45. By contrast, the losses in corpus callosum resulted in severe hypomyelination and increased unmyelinated axons. The hypomyelination may result from decreased oligodendrocytes in the corpus callosum, which persisted in animals as old as postnatal day 350. The current studies focus on uniquely altered signaling pathways following mTORC2 loss in developing oligodendrocytes. A major mTORC2 substrate is phospho-Akt-S473, which was significantly reduced throughout development in both corpus callosum and spinal cord at all ages measured, yet this had little impact in spinal cord. Loss of mTORC2 signaling resulted in decreased expression of actin regulators, such as gelsolin in corpus callosum, but only minimal loss in spinal cord. The current study establishes a regionally specific role for mTORC2 signaling in OPCs, particularly in the corpus callosum.SIGNIFICANCE STATEMENT mTORC1 and mTORC2 signaling has differential impact on myelination in the CNS. Numerous studies identify a role for mTORC1, but deletion of Rictor (mTORC2 signaling) in late-stage oligodendrocytes had little impact on myelination in the CNS. However, the current studies establish that deletion of mTORC2 signaling from oligodendrocyte progenitor cells results in reduced myelination of brain axons. These studies also establish a regional impact of mTORC2, with little change in spinal cord in these conditional Rictor deletion mice. Importantly, in both brain and spinal cord, mTORC2 downstream signaling targets were impacted by Rictor deletion. Yet, these signaling changes had little impact on myelination in spinal cord, while they resulted in long-term alterations in myelination in brain.

Thyroid hormone-dependent oligodendroglial cell lineage genomic and non-genomic signaling through integrin receptors

Multiple sclerosis (MS) is a heterogeneous autoimmune disease whereby the pathological sequelae evolve from oligodendrocytes (OLs) within the central nervous system and are targeted by the immune system, which causes widespread white matter pathology and results in neuronal dysfunction and neurological impairment. The progression of this disease is facilitated by a failure in remyelination following chronic demyelination. One mediator of remyelination is thyroid hormone (TH), whose reliance on monocarboxylate transporter 8 (MCT8) was recently defined. MCT8 facilitates the entry of THs into oligodendrocyte progenitor cell (OPC) and pre-myelinating oligodendrocytes (pre-OLs). Patients with MS may exhibit downregulated MCT8 near inflammatory lesions, which emphasizes an inhibition of TH signaling and subsequent downstream targeted pathways such as phosphoinositide 3-kinase (PI3K)-Akt. However, the role of the closely related mammalian target of rapamycin (mTOR) in pre-OLs during neuroinflammation may also be central to the remyelination process and is governed by various growth promoting signals. Recent research indicates that this may be reliant on TH-dependent signaling through β1-integrins. This review identifies genomic and non-genomic signaling that is regulated through mTOR in TH-responsive pre-OLs and mature OLs in mouse models of MS. This review critiques data that implicates non-genomic Akt and mTOR signaling in response to TH-dependent integrin receptor activation in pre-OLs. We have also examined whether this can drive remyelination in the context of neuroinflammation and associated sequelae. Importantly, we outline how novel therapeutic small molecules are being designed to target integrin receptors on oligodendroglial lineage cells and whether these are viable therapeutic options for future use in clinical trials for MS.

Genetic pathogenesis of the epileptogenic lesions in Tuberous Sclerosis Complex: Therapeutic targeting of the mTOR pathway

Tuberous sclerosis complex (TSC) is a genetic multisystem disease due to the mutation in one of the two genes TSC1 and TSC2, affecting several organs and systems and carrying a significant risk of early onset and refractory seizures. The pathogenesis of this complex disorder is now well known, with most of TSC-related manifestations being a consequence of the overactivation of the mammalian Target of Rapamycin (mTOR) complex. The discovery of this underlying mechanism paved the way for the use of a class of drugs called mTOR inhibitors including rapamycin and everolimus and specifically targeting this pathway. Rapamycin has been widely used in different animal models of TSC-related epilepsy and proved to be able not only to suppress seizures but also to prevent the development of epilepsy, thus demonstrating an antiepileptogenic potential. In some models, it also showed some benefit on neuropsychiatric manifestations associated with TSC. Everolimus has recently been approved by the US Food and Drug Administration and the European Medical Agency for the treatment of refractory seizures associated with TSC starting from the age of 2 years. It demonstrated a clear benefit when compared to placebo on reducing the frequency of different seizure types and exerting a higher effect in younger children. In conclusion, mTOR cascade can be a potentially major cause of TSC-associated epilepsy and neurodevelopmental disability, and additional research should investigate if early suppression of abnormal mTOR signal with mTOR inhibitors before seizure onset can be a more efficient approach and an effective antiepileptogenic and disease-modifying strategy in infants with TSC.

Protocadherin 15 suppresses oligodendrocyte progenitor cell proliferation and promotes motility through distinct signalling pathways

Oligodendrocyte progenitor cells (OPCs) express protocadherin 15 (Pcdh15), a member of the cadherin superfamily of transmembrane proteins. Little is known about the function of Pcdh15 in the central nervous system (CNS), however, Pcdh15 expression can predict glioma aggression and promote the separation of embryonic human OPCs immediately following a cell division. Herein, we show that Pcdh15 knockdown significantly increases extracellular signal-related kinase (ERK) phosphorylation and activation to enhance OPC proliferation in vitro. Furthermore, Pcdh15 knockdown elevates Cdc42-Arp2/3 signalling and impairs actin kinetics, reducing the frequency of lamellipodial extrusion and slowing filopodial withdrawal. Pcdh15 knockdown also reduces the number of processes supported by each OPC and new process generation. Our data indicate that Pcdh15 is a critical regulator of OPC proliferation and process motility, behaviours that characterise the function of these cells in the healthy CNS, and provide mechanistic insight into the role that Pcdh15 might play in glioma progression.

Protocadherin 15 promotes lamellipodial and filopodial dynamics in oligodendrocyte progenitor cells by regulating Cdc42-Arp2/3 activity, but also suppresses ERK1/2 phosphorylation to reduce proliferation.

Cholesterol biosynthesis defines oligodendrocyte precursor heterogeneity between brain and spinal cord

Brain and spinal cord oligodendroglia have distinct functional characteristics, and cell-autonomous loss of individual genes can result in different regional phenotypes. However, a molecular basis for these distinctions is unknown. Using single-cell analysis of oligodendroglia during developmental myelination, we demonstrate that brain and spinal cord precursors are transcriptionally distinct, defined predominantly by cholesterol biosynthesis. We further identify the mechanistic target of rapamycin (mTOR) as a major regulator promoting cholesterol biosynthesis in oligodendroglia. Oligodendroglia-specific loss of mTOR decreases cholesterol biosynthesis in both the brain and the spinal cord, but mTOR loss in spinal cord oligodendroglia has a greater impact on cholesterol biosynthesis, consistent with more pronounced deficits in developmental myelination. In the brain, mTOR loss results in a later adult myelin deficit, including oligodendrocyte death, spontaneous demyelination, and impaired axonal function, demonstrating that mTOR is required for myelin maintenance in the adult brain.

Using single-cell RNA sequencing, Khandker et al. reveal that oligodendroglia in the brain and spinal cord are distinct. These differences arise from mechanisms regulating cholesterol acquisition, necessary for maintenance of the lipid-rich myelin sheath, and involve mTOR in the regulation of cholesterol biosynthesis in oligodendroglia.

p70S6 kinase regulates oligodendrocyte differentiation and is active in remyelinating lesions

The p70 ribosomal S6 kinases (p70 ribosomal S6 kinase 1 and p70 ribosomal S6 kinase 2) are downstream targets of the mechanistic target of rapamycin signalling pathway. p70 ribosomal S6 kinase 1 specifically has demonstrated functions in regulating cell size in Drosophila and in insulin-sensitive cell populations in mammals. Prior studies demonstrated that the mechanistic target of the rapamycin pathway promotes oligodendrocyte differentiation and developmental myelination; however, how the immediate downstream targets of mechanistic target of rapamycin regulate these processes has not been elucidated. Here, we tested the hypothesis that p70 ribosomal S6 kinase 1 regulates oligodendrocyte differentiation during developmental myelination and remyelination processes in the CNS. We demonstrate that p70 ribosomal S6 kinase activity peaks in oligodendrocyte lineage cells at the time when they transition to myelinating oligodendrocytes during developmental myelination in the mouse spinal cord. We further show p70 ribosomal S6 kinase activity in differentiating oligodendrocytes in acute demyelinating lesions induced by lysophosphatidylcholine injection or by experimental autoimmune encephalomyelitis in mice. In demyelinated lesions, the expression of the p70 ribosomal S6 kinase target, phosphorylated S6 ribosomal protein, was transient and highest in maturing oligodendrocytes. Interestingly, we also identified p70 ribosomal S6 kinase activity in oligodendrocyte lineage cells in active multiple sclerosis lesions. Consistent with its predicted function in promoting oligodendrocyte differentiation, we demonstrate that specifically inhibiting p70 ribosomal S6 kinase 1 in cultured oligodendrocyte precursor cells significantly impairs cell lineage progression and expression of myelin basic protein. Finally, we used zebrafish to show in vivo that inhibiting p70 ribosomal S6 kinase 1 function in oligodendroglial cells reduces their differentiation and the number of myelin internodes produced. These data reveal an essential function of p70 ribosomal S6 kinase 1 in promoting oligodendrocyte differentiation during development and remyelination across multiple species.

Human oligodendrocyte myelination potential; relation to age and differentiation

OBJECTIVE

Myelin regeneration in the human central nervous system relies on progenitor cells within the tissue parenchyma, with possible contribution from previously myelinating oligodendrocytes. In multiple sclerosis, a demyelinating disorder, variables affecting remyelination efficiency include age, severity of initial injury, and progenitor cell properties. Our aim was to investigate the effects of age and differentiation on the myelination potential of human oligodendrocyte lineage cells.

METHODS

We derived viable primary oligodendrocyte lineage cells from surgical resections of pediatric and adult brain tissue. Ensheathment capacity using nanofiber assays and transcriptomic profiles from RNA sequencing were compared between A2B5+ antibody-selected progenitors and mature oligodendrocytes (non-selected cells).

RESULTS

We demonstrate that pediatric progenitor and mature cells ensheathed nanofibers more robustly than did adult progenitor and mature cells respectively. Within both age groups, the percentage of fibers ensheathed and ensheathment length per fiber were greater for A2B5+ progenitors. Gene expression of oligodendrocyte progenitor markers PDGFRA and PTPRZ1 were higher in A2B5+ vs A2B5- cells and in pediatric A2B5+ vs adult A2B5+ cells. p38 MAP kinases and actin cytoskeleton-associated pathways were upregulated in pediatric cells; both have been shown to regulate OL process outgrowth. Significant upregulation of "cell senescence" genes was detected in pediatric samples; this could reflect their role in development and the increased susceptibility of pediatric oligodendrocytes to activating cell death responses to stress.

INTERPRETATION

Our findings identify specific biological pathways relevant to myelination that are differentially enriched in human pediatric and adult oligodendrocyte lineage cells and suggest potential targets for remyelination enhancing therapies. This article is protected by copyright. All rights reserved.

The Akt-mTOR Pathway Drives Myelin Sheath Growth by Regulating Cap-Dependent Translation

In the vertebrate CNS, oligodendrocytes produce myelin, a specialized membrane, to insulate and support axons. Individual oligodendrocytes wrap multiple axons with myelin sheaths of variable lengths and thicknesses. Myelin grows at the distal ends of oligodendrocyte processes, and multiple lines of work have provided evidence that mRNAs and RNA binding proteins localize to myelin, together supporting a model where local translation controls myelin sheath growth. What signal transduction mechanisms could control this? One strong candidate is the Akt-mTOR pathway, a major cellular signaling hub that coordinates transcription, translation, metabolism, and cytoskeletal organization. Here, using zebrafish as a model system, we found that Akt-mTOR signaling promotes myelin sheath growth and stability during development. Through cell-specific manipulations to oligodendrocytes, we show that the Akt-mTOR pathway drives cap-dependent translation to promote myelination and that restoration of cap-dependent translation is sufficient to rescue myelin deficits in mTOR loss-of-function animals. Moreover, an mTOR-dependent translational regulator was phosphorylated and colocalized with mRNA encoding a canonically myelin-translated protein in vivo, and bioinformatic investigation revealed numerous putative translational targets in the myelin transcriptome. Together, these data raise the possibility that Akt-mTOR signaling in nascent myelin sheaths promotes sheath growth via translation of myelin-resident mRNAs during development.SIGNIFICANCE STATEMENT In the brain and spinal cord, oligodendrocytes extend processes that tightly wrap axons with myelin, a protein- and lipid-rich membrane that increases electrical impulses and provides trophic support. Myelin membrane grows dramatically following initial axon wrapping in a process that demands protein and lipid synthesis. How protein and lipid synthesis is coordinated with the need for myelin to be generated in certain locations remains unknown. Our study reveals that the Akt-mTOR signaling pathway promotes myelin sheath growth by regulating protein translation. Because we found translational regulators of the Akt-mTOR pathway in myelin, our data raise the possibility that Akt-mTOR activity regulates translation in myelin sheaths to deliver myelin on demand to the places it is needed.

mTOR Signaling Regulates Metabolic Function in Oligodendrocyte Precursor Cells and Promotes Efficient Brain Remyelination in the Cuprizone Model

In demyelinating diseases, such as multiple sclerosis, primary loss of myelin and subsequent neuronal degeneration throughout the CNS impair patient functionality. While the importance of mechanistic target of rapamycin (mTOR) signaling during developmental myelination is known, no studies have yet directly examined the function of mTOR signaling specifically in the oligodendrocyte (OL) lineage during remyelination. Here, we conditionally deleted Mtor from adult oligodendrocyte precursor cells (OPCs) using Ng2-CreERT in male adult mice to test its function in new OLs responsible for remyelination. During early remyelination after cuprizone-induced demyelination, mice lacking mTOR in adult OPCs had unchanged OL numbers but thinner myelin. Myelin thickness recovered by late-stage repair, suggesting a delay in myelin production when Mtor is deleted from adult OPCs. Surprisingly, loss of mTOR in OPCs had no effect on efficiency of remyelination after lysophosphatidylcholine lesions in either the spinal cord or corpus callosum, suggesting that mTOR signaling functions specifically in a pathway dysregulated by cuprizone to promote remyelination efficiency. We further determined that cuprizone and inhibition of mTOR cooperatively compromise metabolic function in primary rat OLs undergoing differentiation. Together, our results support the conclusion that mTOR signaling in OPCs is required to overcome the metabolic dysfunction in the cuprizone-demyelinated adult brain.SIGNIFICANCE STATEMENT Impaired remyelination by oligodendrocytes contributes to the progressive pathology in multiple sclerosis, so it is critical to identify mechanisms of improving remyelination. The goal of this study was to examine mechanistic target of rapamycin (mTOR) signaling in remyelination. Here, we provide evidence that mTOR signaling promotes efficient remyelination of the brain after cuprizone-mediated demyelination but has no effect on remyelination after lysophosphatidylcholine demyelination in the spinal cord or brain. We also present novel data revealing that mTOR inhibition and cuprizone treatment additively affect the metabolic profile of differentiating oligodendrocytes, supporting a mechanism for the observed remyelination delay. These data suggest that altered metabolic function may underlie failure of remyelination in multiple sclerosis lesions and that mTOR signaling may be of therapeutic potential for promoting remyelination.

Impaired myelin production due to an intrinsic failure of oligodendrocytes in mTORpathies

AIMS

We aim to evaluate if the myelin pathology observed in epilepsy-associated focal cortical dysplasia type 2B (FCD2B) and-histologically indistinguishable-cortical tubers of tuberous sclerosis complex (TSC) is primarily related to the underlying malformation or constitutes a secondary phenomenon due to the toxic microenvironment created by epileptic seizures. We also aim to investigate the possible beneficial effect of the mTOR pathway regulator everolimus on white matter pathology.

METHODS

Primary mixed glial cell cultures derived from epilepsy surgery specimens of one TSC and seven FCD2B patients were grown on polycaprolactone fibre matrices and analysed using immunofluorescence and electron microscopy. Unaffected white matter from three age-matched epilepsy patients with mild malformations of cortical development (mMCD) and one with FCD3D served as controls. Additionally, TSC2 knock-out was performed using an oligodendroglial cell line. Myelination capacities of nanofibre grown cells in an inflammatory environment after mTOR-inhibitor treatment with everolimus were further investigated.

RESULTS

Reduced oligodendroglial turnover, directly related to a lower myelin content was found in the patients' primary cells. In our culture model of myelination dynamics, primary cells grown under 'inflammatory condition' showed decreased myelination, that was repaired by treatment with everolimus.

CONCLUSIONS

Results obtained in patient-derived primary oligodendroglial and TSC2 knock-out cells suggest that maturation of oligodendroglia and production of a proper myelin sheath seem to be impaired as a result of mTOR pathway disturbance. Hence, oligodendroglial pathology may reflect a more direct effect of the abnormal genetic programme rather than to be an inactive bystander of chronic epilepsy.

SKAP2 as a new regulator of oligodendroglial migration and myelin sheath formation

Oligodendroglial progenitor cells (OPCs) are highly proliferative and migratory cells, which differentiate into complex myelin forming and axon ensheathing mature oligodendrocytes during myelination. Recent studies indicate that the oligodendroglial cell population is heterogeneous on transcriptional and functional level depending on the location in the central nervous system. Here, we compared intrinsic properties of OPC from spinal cord and brain on functional and transcriptional level. Spinal cord OPC demonstrated increased migration as well as differentiation capacity. Moreover, transcriptome analysis revealed differential expression of several genes between both OPC populations. In spinal cord OPC, we confirmed upregulation of SKAP2, a cytoplasmatic adaptor protein known for its implication in cytoskeletal remodeling and migration in other cell types. Recent findings suggest that actin dynamics determine not only oligodendroglial migration, but also differentiation: Whereas actin polymerization is important for process extension, actin destabilization and depolymerization is required for myelin sheath formation. Downregulation or complete lack of SKAP2 in OPC resulted in reduced migration and impaired morphological maturation in oligodendrocytes. In contrast, overexpression of SKAP2 as well as constitutively active SKAP2 increased OPC migration suggesting that SKAP2 function is dependent on activation by phosphorylation. Furthermore, lack of SKAP2 enhanced the positive effect on OPC migration after integrin activation suggesting that SKAP2 acts as modulator of integrin dependent migration. In summary, we demonstrate the presence of intrinsic differences between spinal cord and brain OPC and identified SKAP2 as a new regulator of oligodendroglial migration and sheath formation.

Cofilin and Neurodegeneration: New Functions for an Old but Gold Protein

Cofilin is an actin-binding protein that plays a major role in the regulation of actin dynamics, an essential cellular process. This protein has emerged as a crucial molecule for functions of the nervous system including motility and guidance of the neuronal growth cone, dendritic spine organization, axonal branching, and synaptic signalling. Recently, other important functions in cell biology such as apoptosis or the control of mitochondrial function have been attributed to cofilin. Moreover, novel mechanisms of cofilin function regulation have also been described. The activity of cofilin is controlled by complex regulatory mechanisms, with phosphorylation being the most important, since the addition of a phosphate group to cofilin renders it inactive. Due to its participation in a wide variety of key processes in the cell, cofilin has been related to a great variety of pathologies, among which neurodegenerative diseases have attracted great interest. In this review, we summarized the functions of cofilin and its regulation, emphasizing how defects in these processes have been related to different neurodegenerative diseases.

The Infantile Leukoencephalopathy-Associated Mutation of C11ORF73/HIKESHI Proteins Generates de novo Interactive Activity with Filamin A, Inhibiting Oligodendroglial Cell Morphological Differentiation

Genetic hypomyelinating diseases are a heterogeneous group of disorders involving the white matter. One infantile hypomyelinating leukoencephalopathy is associated with the homozygous variant (Cys4-to-Ser (C4S)) of the c11orf73 gene. Methods: We observed that in mouse oligodendroglial FBD-102b cells, the C4S mutant proteins but not the wild type ones of C11orf73 are microscopically localized in the lysosome. And, they downregulate lysosome-related signaling in an immunoblotting technique. Results: The C4S mutant proteins specifically interact with Filamin A, which is known to anchor transmembrane proteins to the actin cytoskeleton; the C4S mutant proteins and Filamin A are also observed in the lysosome fraction. While parental FBD-102b cells and cells harboring the wild type constructs exhibit morphological differentiation, cells harboring C4S mutant constructs do not. It may be that morphological differentiation is inhibited because expression of these C4S mutant proteins leads to defects in the actin cytoskeletal network involving Filamin A. Conclusions: The findings that leukoencephalopathy-associated C11ORF73 mutant proteins specifically interact with Filamin A, are localized in the lysosome, and inhibit morphological differentiation shed light on the molecular and cellular pathological mechanisms that underlie infantile hypomyelinating leukoencephalopathy.

The impact of trophic and immunomodulatory factors on oligodendrocyte maturation: Potential treatments for encephalopathy of prematurity

Encephalopathy of prematurity (EoP) is a major cause of morbidity in preterm neonates, causing neurodevelopmental adversities that can lead to lifelong impairments. Preterm birth-related insults, such as cerebral oxygen fluctuations and perinatal inflammation, are believed to negatively impact brain development, leading to a range of brain abnormalities. Diffuse white matter injury is a major hallmark of EoP and characterized by widespread hypomyelination, the result of disturbances in oligodendrocyte lineage development. At present, there are no treatment options available, despite the enormous burden of EoP on patients, their families, and society. Over the years, research in the field of neonatal brain injury and other white matter pathologies has led to the identification of several promising trophic factors and cytokines that contribute to the survival and maturation of oligodendrocytes, and/or dampening neuroinflammation. In this review, we discuss the current literature on selected factors and their therapeutic potential to combat EoP, covering a wide range of in vitro, preclinical and clinical studies. Furthermore, we offer a future perspective on the translatability of these factors into clinical practice.

Intrinsic and extrinsic regulators of oligodendrocyte progenitor proliferation and differentiation

Oligodendrocytes are highly specialized glial cells, responsible for producing myelin in the central nervous system (CNS). The multi-stage process of oligodendrocyte development is tightly regulated to ensure proper lineage progression of oligodendrocyte progenitor cells (OPCs) to mature myelin producing oligodendrocytes. This developmental process involves complex interactions between several intrinsic signaling pathways that are modulated by an array of extrinsic factors. Understanding these regulatory processes is of crucial importance, as it may help to identify specific molecular targets both to enhance plasticity in the normal CNS and to promote endogenous recovery following injury or disease. This review describes two major regulators that play important functional roles in distinct phases of oligodendrocyte development: OPC proliferation and differentiation. Specifically, we highlight the roles of the extracellular astrocyte/radial glia-derived protein Endothelin-1 in OPC proliferation and the intracellular Akt/mTOR pathway in OPC differentiation. Lastly, we reflect on how recent advances in neuroscience and scientific technology will enable greater understanding into how intrinsic and extrinsic regulators interact to generate oligodendrocyte diversity.

Tuberous Sclerosis Complex as Disease Model for Investigating mTOR-Related Gliopathy During Epileptogenesis

Tuberous sclerosis complex (TSC) represents the prototypic monogenic disorder of the mammalian target of rapamycin (mTOR) pathway dysregulation. It provides the rational mechanistic basis of a direct link between gene mutation and brain pathology (structural and functional abnormalities) associated with a complex clinical phenotype including epilepsy, autism, and intellectual disability. So far, research conducted in TSC has been largely neuron-oriented. However, the neuropathological hallmarks of TSC and other malformations of cortical development also include major morphological and functional changes in glial cells involving astrocytes, oligodendrocytes, NG2 glia, and microglia. These cells and their interglial crosstalk may offer new insights into the common neurobiological mechanisms underlying epilepsy and the complex cognitive and behavioral comorbidities that are characteristic of the spectrum of mTOR-associated neurodevelopmental disorders. This review will focus on the role of glial dysfunction, the interaction between glia related to mTOR hyperactivity, and its contribution to epileptogenesis in TSC. Moreover, we will discuss how understanding glial abnormalities in TSC might give valuable insight into the pathophysiological mechanisms that could help to develop novel therapeutic approaches for TSC or other pathologies characterized by glial dysfunction and acquired mTOR hyperactivation.

R-Ras GTPases Signaling Role in Myelin Neurodegenerative Diseases

Myelination is required for fast and efficient synaptic transmission in vertebrates. In the central nervous system, oligodendrocytes are responsible for creating myelin sheaths that isolate and protect axons, even throughout adulthood. However, when myelin is lost, the failure of remyelination mechanisms can cause neurodegenerative myelin-associated pathologies. From oligodendrocyte progenitor cells to mature myelinating oligodendrocytes, myelination is a highly complex process that involves many elements of cellular signaling, yet many of the mechanisms that coordinate it, remain unknown. In this review, we will focus on the three major pathways involved in myelination (PI3K/Akt/mTOR, ERK1/2-MAPK, and Wnt/β-catenin) and recent advances describing the crosstalk elements which help to regulate them. In addition, we will review the tight relation between Ras GTPases and myelination processes and discuss its potential as novel elements of crosstalk between the pathways. A better understanding of the crosstalk elements orchestrating myelination mechanisms is essential to identify new potential targets to mitigate neurodegeneration.