Spatiotemporal Control of CNS Myelination by Oligodendrocyte Programmed Cell Death through the TFEB-PUMA Axis

Galectin-3 alleviates demyelination by modulating microglial anti-inflammatory polarization through PPARγ-CD36 axis

Demyelination is characterized by disruption of myelin sheath and disorders in myelin formation. Currently, there are no effective therapeutic treatments available. Microglia, especially anti-inflammatory phenotype microglia are critical for remyelination. Galectin-3 (Gal-3), which is known to modulate microglia activation, is correlated with myelination. In this study, we aimed to elucidate the roles of Gal-3 during myelin formation and explore the efficiency and mechanism of rGal-3 administration in remyelination. We enrolled Gal-3 knockout (Lgals3 KO) mice and demonstrated Lgals3 KO causes demyelination during spontaneous myelinogenesis. We performed a cuprizone (CPZ) intoxication model and found Lgals3 KO aggravates demyelinated lesions and favors microglial pro-inflammatory phenotype polarization. Recombinant Gal-3 (rGal-3) administration alleviates CPZ intoxication and drives microglial towards anti-inflammatory phenotype. Additionally, RNA sequencing results reveal the correlation between Gal-3 and the PPARγ-CD36 axis. Thus, we performed SSO and GW9662 administration to inhibit the activation of the PPARγ-CD36 axis and found that rGal-3 administration modulates microglial phenotype polarization by regulating the PPARγ-CD36 axis. Together, our findings highlight the importance of Gal-3 in myelination and provide insights into rGal-3 administration for modulating microglial anti-inflammatory phenotype polarization through the PPARγ-CD36 axis.

BRG1 programs PRC2-complex repression and controls oligodendrocyte differentiation and remyelination

Chromatin-remodeling protein BRG1/SMARCA4 is pivotal for establishing oligodendrocyte (OL) lineage identity. However, its functions for oligodendrocyte-precursor cell (OPC) differentiation within the postnatal brain and during remyelination remain elusive. Here, we demonstrate that Brg1 loss profoundly impairs OPC differentiation in the brain with a comparatively lesser effect in the spinal cord. Moreover, BRG1 is critical for OPC remyelination after injury. Integrative transcriptomic/genomic profiling reveals that BRG1 exhibits a dual role by promoting OPC differentiation networks while repressing OL-inhibitory cues and proneuronal programs. Furthermore, we find that BRG1 interacts with EED/PRC2 polycomb-repressive-complexes to enhance H3K27me3-mediated repression at gene loci associated with OL-differentiation inhibition and neurogenesis. Notably, BRG1 depletion decreases H3K27me3 deposition, leading to the upregulation of BMP/WNT signaling and proneurogenic genes, which suppresses OL programs. Thus, our findings reveal a hitherto unexplored spatiotemporal-specific role of BRG1 for OPC differentiation in the developing CNS and underscore a new insight into BRG1/PRC2-mediated epigenetic regulation that promotes and safeguards OL lineage commitment and differentiation.

Extracellular vesicles produced by HIV-1 Nef-expressing cells induce myelin impairment and oligodendrocyte damage in the mouse central nervous system

HIV-associated neurocognitive disorders (HAND) are a spectrum of cognitive impairments that continue to affect approximately half of all HIV-positive individuals despite effective viral suppression through antiretroviral therapy (ART). White matter pathologies have persisted in the ART era, and the degree of white matter damage correlates with the degree of neurocognitive impairment in patients with HAND. The HIV protein Nef has been implicated in HAND pathogenesis, but its effect on white matter damage has not been well characterized. Here, utilizing in vivo, ex vivo, and in vitro methods, we demonstrate that Nef-containing extracellular vesicles (Nef EVs) disrupt myelin sheaths and inflict damage upon oligodendrocytes within the murine central nervous system. Intracranial injection of Nef EVs leads to reduced myelin basic protein (MBP) staining and a decreased number of CC1 + oligodendrocytes in the corpus callosum. Moreover, cerebellar slice cultures treated with Nef EVs exhibit diminished MBP expression and increased presence of unmyelinated axons. Primary mixed brain cultures and enriched oligodendrocyte precursor cell cultures exposed to Nef EVs display a decreased number of O4 + cells, indicative of oligodendrocyte impairment. These findings underscore the potential contribution of Nef EV-mediated damage to oligodendrocytes and myelin maintenance in the pathogenesis of HAND.

Small-molecule-induced epigenetic rejuvenation promotes SREBP condensation and overcomes barriers to CNS myelin regeneration

Remyelination failure in diseases like multiple sclerosis (MS) was thought to involve suppressed maturation of oligodendrocyte precursors; however, oligodendrocytes are present in MS lesions yet lack myelin production. We found that oligodendrocytes in the lesions are epigenetically silenced. Developing a transgenic reporter labeling differentiated oligodendrocytes for phenotypic screening, we identified a small-molecule epigenetic-silencing-inhibitor (ESI1) that enhances myelin production and ensheathment. ESI1 promotes remyelination in animal models of demyelination and enables de novo myelinogenesis on regenerated CNS axons. ESI1 treatment lengthened myelin sheaths in human iPSC-derived organoids and augmented (re)myelination in aged mice while reversing age-related cognitive decline. Multi-omics revealed that ESI1 induces an active chromatin landscape that activates myelinogenic pathways and reprograms metabolism. Notably, ESI1 triggered nuclear condensate formation of master lipid-metabolic regulators SREBP1/2, concentrating transcriptional co-activators to drive lipid/cholesterol biosynthesis. Our study highlights the potential of targeting epigenetic silencing to enable CNS myelin regeneration in demyelinating diseases and aging.

Oligodendrocyte Maturation Alters the Cell Death Mechanisms That Cause Demyelination

Myelinating oligodendrocytes die in human disease and early in aging. Despite this, the mechanisms that underly oligodendrocyte death are not resolved and it is also not clear whether these mechanisms change as oligodendrocyte lineage cells are undergoing differentiation and maturation. Here, we used a combination of intravital imaging, single-cell ablation, and cuprizone-mediated demyelination, in both female and male mice, to discover that oligodendrocyte maturation dictates the dynamics and mechanisms of cell death. After single-cell phototoxic damage, oligodendrocyte precursor cells underwent programmed cell death within hours, differentiating oligodendrocytes died over several days, while mature oligodendrocytes took weeks to die. Importantly cells at each maturation stage all eventually died but did so with drastically different temporal dynamics and morphological features. Consistent with this, cuprizone treatment initiated a caspase-3-dependent form of rapid cell death in differentiating oligodendrocytes, while mature oligodendrocytes never activated this executioner caspase. Instead, mature oligodendrocytes exhibited delayed cell death which was marked by DNA damage and disruption in poly-ADP-ribose subcellular localization. Thus, oligodendrocyte maturation plays a key role in determining the mechanism of death a cell undergoes in response to the same insult. This means that oligodendrocyte maturation is important to consider when designing strategies for preventing cell death and preserving myelin while also enhancing the survival of new oligodendrocytes in demyelinating conditions.

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.

How to Use the Cuprizone Model to Study De- and Remyelination

Multiple sclerosis (MS) is an autoimmune and inflammatory disorder affecting the central nervous system whose cause is still largely unknown. Oligodendrocyte degeneration results in demyelination of axons, which can eventually be repaired by a mechanism called remyelination. Prevention of demyelination and the pharmacological support of remyelination are two promising strategies to ameliorate disease progression in MS patients. The cuprizone model is commonly employed to investigate oligodendrocyte degeneration mechanisms or to explore remyelination pathways. During the last decades, several different protocols have been applied, and all have their pros and cons. This article intends to offer guidance for conducting pre-clinical trials using the cuprizone model in mice, focusing on discovering new treatment approaches to prevent oligodendrocyte degeneration or enhance remyelination.

Transcription Factor EB: A Promising Therapeutic Target for Ischemic Stroke

Transcription factor EB (TFEB) is an important endogenous defensive protein that responds to ischemic stimuli. Acute ischemic stroke is a growing concern due to its high morbidity and mortality. Most survivors suffer from disabilities such as numbness or weakness in an arm or leg, facial droop, difficulty speaking or understanding speech, confusion, impaired balance or coordination, or loss of vision. Although TFEB plays a neuroprotective role, its potential effect on ischemic stroke remains unclear. This article describes the basic structure, regulation of transcriptional activity, and biological roles of TFEB relevant to ischemic stroke. Additionally, we explore the effects of TFEB on the various pathological processes underlying ischemic stroke and current therapeutic approaches. The information compiled here may inform clinical and basic studies on TFEB, which may be an effective therapeutic drug target for ischemic stroke.

White matter injury across neurodegenerative disease

Oligodendrocytes (OLs), the myelin-generating cells of the central nervous system (CNS), are active players in shaping neuronal circuitry and function. It has become increasingly apparent that injury to cells within the OL lineage plays a central role in neurodegeneration. In this review, we focus primarily on three degenerative disorders in which white matter loss is well documented: Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). We discuss clinical data implicating white matter injury as a key feature of these disorders, as well as shared and divergent phenotypes between them. We examine the cellular and molecular mechanisms underlying the alterations to OLs, including chronic neuroinflammation, aggregation of proteins, lipid dysregulation, and organellar stress. Last, we highlight prospects for therapeutic intervention targeting the OL lineage to restore function.

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.

Mitochondrial network reorganization and transient expansion during oligodendrocyte generation

Oligodendrocyte precursor cells (OPCs) give rise to myelinating oligodendrocytes of the central nervous system. This process persists throughout life and is essential for recovery from neurodegeneration. To better understand the cellular checkpoints that occur during oligodendrogenesis, we determined the mitochondrial distribution and morphometrics across the oligodendrocyte lineage in mouse and human cerebral cortex. During oligodendrocyte generation, mitochondrial content expanded concurrently with a change in subcellular partitioning towards the distal processes. These changes were followed by an abrupt loss of mitochondria in the oligodendrocyte processes and myelin, coinciding with sheath compaction. This reorganization and extensive expansion and depletion took 3 days. Oligodendrocyte mitochondria were stationary over days while OPC mitochondrial motility was modulated by animal arousal state within minutes. Aged OPCs also displayed decreased mitochondrial size, content, and motility. Thus, mitochondrial dynamics are linked to oligodendrocyte generation, dynamically modified by their local microenvironment, and altered in the aging brain.

Mitf is a Schwann cell sensor of axonal integrity that drives nerve repair

Schwann cells respond to acute axon damage by transiently transdifferentiating into specialized repair cells that restore sensorimotor function. However, the molecular systems controlling repair cell formation and function are not well defined, and consequently, it is unclear whether this form of cellular plasticity has a role in peripheral neuropathies. Here, we identify Mitf as a transcriptional sensor of axon damage under the control of Nrg-ErbB-PI3K-PI5K-mTorc2 signaling. Mitf regulates a core transcriptional program for generating functional repair Schwann cells following injury and during peripheral neuropathies caused by CMT4J and CMT4D. In the absence of Mitf, core genes for epithelial-to-mesenchymal transition, metabolism, and dedifferentiation are misexpressed, and nerve repair is disrupted. Our findings demonstrate that Schwann cells monitor axonal health using a phosphoinositide signaling system that controls Mitf nuclear localization, which is critical for activating cellular plasticity and counteracting neural disease.

The committed oligodendrocyte precursor cell, a newly-defined intermediate progenitor cell type in oligodendroglial lineage

In the central nervous system, oligodendrocytes (OLs) produce myelin sheaths that provide trophic support to neuronal axons and increase the propagation speed of action potential. OLs are constantly generated from OL precursor cells (OPCs) throughout life span. The production of myelinating OLs consists of three canonical stages: OPCs, newly-formed OLs (NFOs), and mature myelinating OLs. Recently, single-cell RNA transcriptomic analyses identified a new population of oligodendroglial cells, namely differentiation committed OPCs (COPs). COPs represent a critical intermediate population between OPCs and NFOs, as revealed by specific expression of G-protein coupled receptor 17 (GPR17). The dysregulation of COPs leads to the remyelination failure in demyelinating diseases and impairs the replacement of lost myelin sheaths due to aging. Hence, understanding the development of COPs and their underlying regulatory network will be helpful in establishing new strategies for promoting myelin repair in demyelinating diseases. This review summarizes the current knowledge on the development and functions of COPs under both physiological and pathological conditions. Overall, COPs function as "checkpoints" to prevent inappropriate precocious OL differentiation and myelination through expressing distinct regulatory factors. Deepening our understanding of COPs may not only advance our knowledge of how OL lineage progresses during development, but also open the door to new treatments for demyelinating diseases.

Cleavage of VAMP2/3 Affects Oligodendrocyte Lineage Development in the Developing Mouse Spinal Cord

In the developing and adult CNS, new oligodendrocytes (OLs) are generated from a population of cells known as oligodendrocyte precursor cells (OPCs). As they begin to differentiate, OPCs undergo a series of highly regulated changes to morphology, gene expression, and membrane organization. This stage represents a critical bottleneck in oligodendrogliogenesis, and the regulatory program that guides it is still not fully understood. Here, we show that in vivo toxin-mediated cleavage of the vesicle associated SNARE proteins VAMP2/3 in the OL lineage of both male and female mice impairs the ability of early OLs to mature into functional, myelinating OLs. In the developing mouse spinal cord, many VAMP2/3-cleaved OLs appeared to stall in the premyelinating, early OL stage, resulting in an overall loss of both myelin density and OL number. The Src kinase Fyn, a key regulator of oligodendrogliogenesis and myelination, is highly expressed among premyelinating OLs, but its expression decreases as OLs mature. We found that OLs with cleaved VAMP2/3 in the spinal cord white matter showed significantly higher expression of Fyn compared with neighboring control cells, potentially because of an extended premyelinating stage. Overall, our results show that functional VAMP2/3 in OL lineage cells is essential for proper myelin formation and plays a major role in controlling the maturation and terminal differentiation of premyelinating OLs.SIGNIFICANCE STATEMENT The production of mature oligodendrocytes (OLs) is essential for CNS myelination during development, myelin remodeling in adulthood, and remyelination following injury or in demyelinating disease. Before myelin sheath formation, newly formed OLs undergo a series of highly regulated changes during a stage of their development known as the premyelinating, or early OL stage. This stage acts as a critical checkpoint in OL development, and much is still unknown about the dynamic regulatory processes involved. In this study, we show that VAMP2/3, SNARE proteins involved in vesicular trafficking and secretion play an essential role in regulating premyelinating OL development and are required for healthy myelination in the developing mouse spinal cord.

Decoding DNA sequence-driven evolution of the human brain epigenome at cellular resolution

DNA-based evolutionary comparisons of regulatory genomic elements enable insight into functional changes, overcoming tissue inaccessibility. Here, we harnessed adult and fetal cortex single-cell ATAC-seq datasets to uncover DNA substitutions specific to the human and human-ancestral lineages within apes. We found that fetal microglia identity is evolutionarily divergent in all lineages, whereas other cell types are conserved. Using multiomic datasets, we further identified genes linked to multiple lineage-divergent gene regulatory elements and implicated biological pathways associated with these divergent features. We also uncovered patterns of transcription factor binding site evolution across lineages and identified expansion of bHLH-PAS factor targets in human-hominin lineages, and MEF2 factor targets in the ape lineage. Finally, conserved features were more enriched in brain disease variants, whereas there was no distinct enrichment on the human lineage compared to its ancestral lineages. Our study identifies major evolutionary patterns in the human brain epigenome at cellular resolution.

Autophagy collaborates with apoptosis pathways to control oligodendrocyte number

Oligodendrocytes are the sole myelin-producing cells in the central nervous system. Oligodendrocyte number is tightly controlled across diverse brain regions to match local axon type and number, yet the underlying mechanisms remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under physiological conditions, elicits premyelinating oligodendrocyte apoptosis during development. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy functions cell autonomously in the premyelinating oligodendrocyte to trigger cell apoptosis, and it genetically interacts with the TFEB pathway to limit oligodendrocyte number across diverse brain regions. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrogenesis.

TFEB is a central regulator of the aging process and age-related diseases

Old age is associated with a greater burden of disease, including neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, as well as other chronic diseases. Coincidentally, popular lifestyle interventions, such as caloric restriction, intermittent fasting, and regular exercise, in addition to pharmacological interventions intended to protect against age-related diseases, induce transcription factor EB (TFEB) and autophagy. In this review, we summarize emerging discoveries that point to TFEB activity affecting the hallmarks of aging, including inhibiting DNA damage and epigenetic modifications, inducing autophagy and cell clearance to promote proteostasis, regulating mitochondrial quality control, linking nutrient-sensing to energy metabolism, regulating pro- and anti-inflammatory pathways, inhibiting senescence and promoting cell regenerative capacity. Furthermore, the therapeutic impact of TFEB activation on normal aging and tissue-specific disease development is assessed in the contexts of neurodegeneration and neuroplasticity, stem cell differentiation, immune responses, muscle energy adaptation, adipose tissue browning, hepatic functions, bone remodeling, and cancer. Safe and effective strategies of activating TFEB hold promise as a therapeutic strategy for multiple age-associated diseases and for extending lifespan.

Transcriptomic atlas and interaction networks of brain cells in mouse CNS demyelination and remyelination

Demyelination is a hallmark of multiple sclerosis, leukoencephalopathies, cerebral vasculopathies, and several neurodegenerative diseases. The cuprizone mouse model is widely used to simulate demyelination and remyelination occurring in these diseases. Here, we present a high-resolution single-nucleus RNA sequencing (snRNA-seq) analysis of gene expression changes across all brain cells in this model. We define demyelination-associated oligodendrocytes (DOLs) and remyelination-associated MAFBhi microglia, as well as astrocytes and vascular cells with signatures of altered metabolism, oxidative stress, and interferon response. Furthermore, snRNA-seq provides insights into how brain cell types connect and interact, defining complex circuitries that impact demyelination and remyelination. As an explicative example, perturbation of microglia caused by TREM2 deficiency indirectly impairs the induction of DOLs. Altogether, this study provides a rich resource for future studies investigating mechanisms underlying demyelinating diseases.

Emerging mitochondrial-mediated mechanisms involved in oligodendrocyte development

Oligodendrocytes are the myelinating glia of the central nervous system and are generated after oligodendrocyte progenitor cells (OPCs) transition into pre-oligodendrocytes and then into myelinating oligodendrocytes. Myelin is essential for proper signal transmission within the nervous system and axonal metabolic support. Although the intrinsic and extrinsic factors that support the differentiation, survival, integration, and subsequent myelination of appropriate axons have been well investigated, little is known about how mitochondria-related pathways such as mitochondrial dynamics, bioenergetics, and apoptosis finely tune these developmental events. Previous findings suggest that changes to mitochondrial morphology act as an upstream regulatory mechanism of neural stem cell (NSC) fate decisions. Whether a similar mechanism is engaged during OPC differentiation has yet to be elucidated. Maintenance of mitochondrial dynamics is vital for regulating cellular bioenergetics, functional mitochondrial networks, and the ability of cells to distribute mitochondria to subcellular locations, such as the growing processes of oligodendrocytes. Myelination is an energy-consuming event, thus, understanding the interplay between mitochondrial dynamics, metabolism, and apoptosis will provide further insight into mechanisms that mediate oligodendrocyte development in healthy and disease states. Here we will provide a concise overview of oligodendrocyte development and discuss the potential contribution of mitochondrial mitochondrial-mediated mechanisms to oligodendrocyte bioenergetics and development.

Coordinated Regulation of Myelination by Growth Factor and Amino-acid Signaling Pathways

Myelin-forming oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) are essential for structural and functional homeostasis of nervous tissue. Albeit with certain similarities, the regulation of CNS and PNS myelination is executed differently. Recent advances highlight the coordinated regulation of oligodendrocyte myelination by amino-acid sensing and growth factor signaling pathways. In this review, we discuss novel insights into the understanding of differential regulation of oligodendrocyte and Schwann cell biology in CNS and PNS myelination, with particular focus on the roles of growth factor-stimulated RHEB-mTORC1 and GATOR2-mediated amino-acid sensing/signaling pathways. We also discuss recent progress on the metabolic regulation of oligodendrocytes and Schwann cells and the impact of their dysfunction on neuronal function and disease.

Oligodendrocyte-lineage cell exocytosis and L-type prostaglandin D synthase promote oligodendrocyte development and myelination

In the developing central nervous system, oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes, which form myelin around axons. Oligodendrocytes and myelin are essential for the function of the central nervous system, as evidenced by the severe neurological symptoms that arise in demyelinating diseases such as multiple sclerosis and leukodystrophy. Although many cell-intrinsic mechanisms that regulate oligodendrocyte development and myelination have been reported, it remains unclear whether interactions among oligodendrocyte-lineage cells (OPCs and oligodendrocytes) affect oligodendrocyte development and myelination. Here, we show that blocking vesicle-associated membrane protein (VAMP) 1/2/3-dependent exocytosis from oligodendrocyte-lineage cells impairs oligodendrocyte development, myelination, and motor behavior in mice. Adding oligodendrocyte-lineage cell-secreted molecules to secretion-deficient OPC cultures partially restores the morphological maturation of oligodendrocytes. Moreover, we identified L-type prostaglandin D synthase as an oligodendrocyte-lineage cell-secreted protein that promotes oligodendrocyte development and myelination in vivo. These findings reveal a novel autocrine/paracrine loop model for the regulation of oligodendrocyte and myelin development.

Autophagy collaborates with apoptosis pathways to control myelination specificity and function

Oligodendrocytes are the sole myelin producing cells in the central nervous system. Oligodendrocyte numbers are tightly controlled across diverse brain regions to match local axon type and number, but the underlying mechanisms and functional significance remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under canonical settings, elicits premyelinating oligodendrocyte apoptosis during development and regulates critical aspects of nerve pulse propagation. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy acts in the TFEB-Bax/Bak pathway and elevates PUMA mRNA levels to trigger premyelinating oligodendrocyte apoptosis cell-autonomously. Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath numbers and fine-tune nerve pulse propagation. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrocyte number.

HIGHLIGHTS

Autophagy flux increases in the premyelinating and myelinating oligodendrocytesAutophagy promotes premyelinating oligodendrocyte (pre-OL) apoptosis to control myelination location and timing Autophagy acts in the TFEB-PUMA-Bax/Bak pathway and elevates PUMA mRNA levels to determine pre-OL fate Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath thickness and finetune nerve pulse propagation.

Neuronal ambient RNA contamination causes misinterpreted and masked cell types in brain single-nuclei datasets

Ambient RNA contamination in single-cell and single-nuclei RNA sequencing (snRNA-seq) is a significant problem, but its consequences are poorly understood. Here, we show that ambient RNAs in brain snRNA-seq datasets have a nuclear or non-nuclear origin with distinct gene set signatures. Both ambient RNA signatures are predominantly neuronal, and we find that some previously annotated neuronal cell types are distinguished by ambient RNA contamination. We detect pervasive neuronal ambient RNA contamination in all glial cell types unless glia and neurons are physically separated prior to sequencing. We demonstrate that this contamination can be removed in silico and show that previous single-nuclei RNA-seq-based annotations of immature oligodendrocytes are glial nuclei contaminated with ambient RNAs. After ambient RNA removal, we detect rare, committed oligodendrocyte progenitor cells not annotated in most previous adult human brain datasets. Together, these results provide an in-depth analysis of ambient RNA contamination in brain single-nuclei datasets.

Deciphering the Genetic Crosstalk between Microglia and Oligodendrocyte Precursor Cells during Demyelination and Remyelination Using Transcriptomic Data

Demyelinating disorders show impaired remyelination due to failure in the differentiation of oligodendrocyte progenitor cells (OPCs) into mature myelin-forming oligodendrocytes, a process driven by microglia-OPC crosstalk. Through conducting a transcriptomic analysis of microarray studies on the demyelination-remyelination cuprizone model and using human samples of multiple sclerosis (MS), we identified molecules involved in this crosstalk. Differentially expressed genes (DEGs) of specific regions/cell types were detected in GEO transcriptomic raw data after cuprizone treatment and in MS samples, followed by functional analysis with GO terms and WikiPathways. Additionally, microglia-OPC crosstalk between microglia ligands, OPC receptors and target genes was examined with the NicheNet model. We identified 108 and 166 DEGs in the demyelinated corpus callosum (CC) at 2 and 4 weeks of cuprizone treatment; 427 and 355 DEGs in the remyelinated (4 weeks of cuprizone treatment + 14 days of normal diet) compared to 2- and 4-week demyelinated CC; 252 DEGs in MS samples and 2730 and 12 DEGs in OPC and microglia of 4-week demyelinated CC. At this time point, we found 95 common DEGs in the CC and OPCs, and one common DEG in microglia and OPCs, mostly associated with myelin and lipid metabolism. Crosstalk analysis identified 47 microglia ligands, 43 OPC receptors and 115 OPC target genes, all differentially expressed in cuprizone-treated samples and associated with myelination. Our differential expression pipeline identified demyelination/remyelination transcriptomic biomarkers in studies using diverse platforms and cell types/tissues. Cellular crosstalk analysis yielded novel markers of microglia ligands, OPC receptors and target genes.

Spatially resolved gene regulatory and disease-related vulnerability map of the adult Macaque cortex

Single cell approaches have increased our knowledge about the cell type composition of the non-human primate (NHP), but a detailed characterization of area-specific regulatory features remains outstanding. We generated single-cell transcriptomic and chromatin accessibility (single-cell ATAC) data of 358,237 cells from prefrontal cortex (PFC), primary motor cortex (M1) and primary visual cortex (V1) of adult female cynomolgus monkey brain, and integrated this dataset with Stereo-seq (spatial enhanced resolution omics-sequencing) of the corresponding cortical areas to assign topographic information to molecular states. We identified area-specific chromatin accessible sites and their targeted genes, including the cell type-specific transcriptional regulatory network associated with excitatory neurons heterogeneity. We reveal calcium ion transport and axon guidance genes related to specialized functions of PFC and M1, identified the similarities and differences between adult macaque and human oligodendrocyte trajectories, and mapped the genetic variants and gene perturbations of human diseases to NHP cortical cells. This resource establishes a transcriptomic and chromatin accessibility combinatory regulatory landscape at a single-cell and spatially resolved resolution in NHP cortex.

The Alzheimer's Cell Atlas (TACA): A single-cell molecular map for translational therapeutics accelerator in Alzheimer's disease

Introduction

Recent advances in generating massive single-cell/nucleus transcriptomic data have shown great potential for facilitating the identification of cell type-specific Alzheimer's disease (AD) pathobiology and drug-target discovery for therapeutic development.

Methods

We developed The Alzheimer's Cell Atlas (TACA) by compiling an AD brain cell atlas consisting of over 1.1 million cells/nuclei across 26 data sets, covering major brain regions (hippocampus, cerebellum, prefrontal cortex, and so on) and cell types (astrocyte, microglia, neuron, oligodendrocytes, and so on). We conducted nearly 1400 differential expression comparisons to identify cell type-specific molecular alterations (e.g., case vs healthy control, sex-specific, apolipoprotein E (APOE) ε4/ε4, and TREM2 mutations). Each comparison was followed by protein-protein interaction module detection, functional enrichment analysis, and omics-informed target and drug (over 700,000 perturbation profiles) screening. Over 400 cell-cell interaction analyses using 6000 ligand-receptor interactions were conducted to identify the cell-cell communication networks in AD.

Results

All results are integrated into TACA (https://taca.lerner.ccf.org/), a new web portal with cell type-specific, abundant transcriptomic information, and 12 interactive visualization tools for AD.

Discussion

We envision that TACA will be a highly valuable resource for both basic and translational research in AD, as it provides abundant information for AD pathobiology and actionable systems biology tools for drug discovery.

Highlights

We compiled an Alzheimer's disease (AD) brain cell atlas consisting of more than 1.1 million cells/nuclei transcriptomes from 26 data sets, covering major brain regions (cortex, hippocampus, cerebellum) and cell types (e.g., neuron, oligodendrocyte, astrocyte, and microglia).We conducted over 1400 differential expression (DE) comparisons to identify cell type-specific gene expression alterations. Major comparison types are (1) AD versus healthy control; (2) sex-specific DE, (3) genotype-driven DE (i.e., apolipoprotein E [APOE] ε4/ε4 vs APOE ε3/ε3; TREM2R47H vs common variants) analysis; and (4) others. Each comparison was further followed by (1) human protein-protein interactome network module analysis, (2) pathway enrichment analysis, and (3) gene-set enrichment analysis.For drug screening, we conducted gene set enrichment analysis for all the comparisons with over 700,000 drug perturbation profiles connecting more than 10,000 human genes and 13,000 drugs/compounds.A total of over 400 analyses of cell-cell interactions against 6000 experimentally validated ligand-receptor interactions were conducted to reveal the disease-relevant cell-cell communications in AD.

Oligodendroglial macroautophagy is essential for myelin sheath turnover to prevent neurodegeneration and death

Although macroautophagy deficits are implicated across adult-onset neurodegenerative diseases, we understand little about how the discrete, highly evolved cell types of the central nervous system use macroautophagy to maintain homeostasis. One such cell type is the oligodendrocyte, whose myelin sheaths are central for the reliable conduction of action potentials. Using an integrated approach of mouse genetics, live cell imaging, electron microscopy, and biochemistry, we show that mature oligodendrocytes require macroautophagy to degrade cell autonomously their myelin by consolidating cytosolic and transmembrane myelin proteins into an amphisome intermediate prior to degradation. We find that disruption of autophagic myelin turnover leads to changes in myelin sheath structure, ultimately impairing neural function and culminating in an adult-onset progressive motor decline, neurodegeneration, and death. Our model indicates that the continuous and cell-autonomous maintenance of the myelin sheath through macroautophagy is essential, shedding insight into how macroautophagy dysregulation might contribute to neurodegenerative disease pathophysiology.

Oligodendrocytes assemble myelin and support the axons they myelinate. Aber et al. report that oligodendrocytes coordinate autophagy and endocytosis to turn over myelin. The absence of oligodendroglial autophagy causes myelin abnormalities, behavioral dysfunction, glial and neurodegeneration, and death, demonstrating the importance of this process for a healthy CNS.

Ral GTPases are critical regulators of spinal cord myelination and homeostasis

Efficient myelination supports nerve conduction and axonal health throughout life. In the central nervous system, oligodendrocytes (OLs) carry out this demanding anabolic duty in part through biosynthetic pathways controlled by mTOR. We identify Ral GTPases as critical regulators of mouse spinal cord myelination and myelin maintenance. Ablation of Ral GTPases (RalA, RalB) in OL-lineage cells impairs timely onset and radial growth of developmental myelination, accompanied by increased endosomal/lysosomal abundance. Further examinations, including transcriptomic analyses of Ral-deficient OLs, were consistent with mTORC1-related deficits. However, deletion of the mTOR signaling-repressor Pten in Ral-deficient OL-lineage cells is unable to rescue mTORC1 activation or developmental myelination deficiencies. Induced deletion of Ral GTPases in OLs of adult mice results in late-onset myelination defects and tissue degeneration. Together, our data indicate critical roles for Ral GTPases to promote developmental spinal cord myelination, to ensure accurate mTORC1 signaling, and to protect the healthy state of myelin-axon units over time.

Galectin-3 administration drives remyelination after hypoxic-ischemic induced perinatal white matter injury

Hypoxic-ischemic (HI) induced perinatal white matter injury (PWMI) is a major cause of neurologic disabilities characterized by selective oligodendroglial death and myelin disruption. Galectin-3 (Gal-3) modulates postnatal subventricular zone gliogenesis and attenuates ischemic injury. However, the association between Gal-3 and myelin formation still remains unclear. In this study, we first perform Gal-3 knockdown (KD) to identify the importance of Gal-3 on myelin formation. Our results show impeded myelin formation, manifested by Olig2/CC1 (+) mature oligodendrocytes number, expression of oligodendroglial maturation-associated markers (MBP and CNPase), and myelin thickness and integrity. Then we perform recombinant Gal-3 (rGal-3) administration by intracerebroventricular injection. Notably, although rGal-3 administration shows no beneficial effect on oligodendrogenesis and myelin formation under normal condition, our results show that rGal-3 administration attenuates cognitive deficits and drives remyelination after PWMI, which are coupled to signs of enhanced myelin resiliency and cognition. Also, our results indicates that the significant increases in substrates for remyelination of rGal-3 administration are accompanied by enhanced Iba-1 (microglia marker)/ Mrc1 (M2 marker) (+) microglia and decreased Iba-1/ iNOS (M1 marker) (+) microglia. Altogether, our data in this research confirm the association between Gal-3 and myelin formation, underscore its position for the capacity for remyelination and restoration of function, and unveils the efficacy of rGal-3 administration with anti-inflammatory phenotype microglia (M2 microglia) activation. Thus, the findings suggest that Gal-3 plays a significant role in myelin formation and remyelination restoration.

Spatial and temporal heterogeneity in the lineage progression of fine oligodendrocyte subtypes

Background

Oligodendrocytes are glial cells that support and insulate axons in the central nervous system through the production of myelin. Oligodendrocytes arise throughout embryonic and early postnatal development from oligodendrocyte precursor cells (OPCs), and recent work demonstrated that they are a transcriptional heterogeneous cell population, but the regional and functional implications of this heterogeneity are less clear. Here, we apply in situ sequencing (ISS) to simultaneously probe the expression of 124 marker genes of distinct oligodendrocyte populations, providing comprehensive maps of the corpus callosum, cingulate, motor, and somatosensory cortex in the brain, as well as gray matter (GM) and white matter (WM) regions in the spinal cord, at postnatal (P10), juvenile (P20), and young adult (P60) stages. We systematically compare the abundances of these populations and investigate the neighboring preference of distinct oligodendrocyte populations.

Results

We observed that oligodendrocyte lineage progression is more advanced in the juvenile spinal cord compared to the brain, corroborating with previous studies. We found myelination still ongoing in the adult corpus callosum while it was more advanced in the cortex. Interestingly, we also observed a lateral-to-medial gradient of oligodendrocyte lineage progression in the juvenile cortex, which could be linked to arealization, as well as a deep-to-superficial gradient with mature oligodendrocytes preferentially accumulating in the deeper layers of the cortex. The ISS experiments also exposed differences in abundances and population dynamics over time between GM and WM regions in the brain and spinal cord, indicating regional differences within GM and WM, and we found that neighboring preferences of some oligodendroglia populations are altered from the juvenile to the adult CNS.

Conclusions

Overall, our ISS experiments reveal spatial heterogeneity of oligodendrocyte lineage progression in the brain and spinal cord and uncover differences in the timing of oligodendrocyte differentiation and myelination, which could be relevant to further investigate functional heterogeneity of oligodendroglia, especially in the context of injury or disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01325-z.

Macroautophagy in CNS health and disease

Macroautophagy is an evolutionarily conserved process that delivers diverse cellular contents to lysosomes for degradation. As our understanding of this pathway grows, so does our appreciation for its importance in disorders of the CNS. Once implicated primarily in neurodegenerative events owing to acute injury and ageing, macroautophagy is now also linked to disorders of neurodevelopment, indicating that it is essential for both the formation and maintenance of a healthy CNS. In parallel to understanding the significance of macroautophagy across contexts, we have gained a greater mechanistic insight into its physiological regulation and the breadth of cargoes it can degrade. Macroautophagy is a broadly used homeostatic process, giving rise to questions surrounding how defects in this single pathway could cause diseases with distinct clinical and pathological signatures. To address this complexity, we herein review macroautophagy in the mammalian CNS by examining three key features of the process and its relationship to disease: how it functions at a basal level in the discrete cell types of the brain and spinal cord; which cargoes are being degraded in physiological and pathological settings; and how the different stages of the macroautophagy pathway intersect with diseases of neurodevelopment and adult-onset neurodegeneration.

Young CSF restores oligodendrogenesis and memory in aged mice via Fgf17

Recent understanding of how the systemic environment shapes the brain throughout life has led to numerous intervention strategies to slow brain ageing^1-3. Cerebrospinal fluid (CSF) makes up the immediate environment of brain cells, providing them with nourishing compounds^4,5. We discovered that infusing young CSF directly into aged brains improves memory function. Unbiased transcriptome analysis of the hippocampus identified oligodendrocytes to be most responsive to this rejuvenated CSF environment. We further showed that young CSF boosts oligodendrocyte progenitor cell (OPC) proliferation and differentiation in the aged hippocampus and in primary OPC cultures. Using SLAMseq to metabolically label nascent mRNA, we identified serum response factor (SRF), a transcription factor that drives actin cytoskeleton rearrangement, as a mediator of OPC proliferation following exposure to young CSF. With age, SRF expression decreases in hippocampal OPCs, and the pathway is induced by acute injection with young CSF. We screened for potential SRF activators in CSF and found that fibroblast growth factor 17 (Fgf17) infusion is sufficient to induce OPC proliferation and long-term memory consolidation in aged mice while Fgf17 blockade impairs cognition in young mice. These findings demonstrate the rejuvenating power of young CSF and identify Fgf17 as a key target to restore oligodendrocyte function in the ageing brain.

Abnormal oligodendrocyte function in schizophrenia explains the long latent interval in some patients

A puzzling feature of schizophrenia, is the long latency between the beginning of neuropathological changes and the clinical presentation that may be two decades later. Abnormalities in oligodendrocyte function may explain this latency, because mature oligodendrocytes produce myelination, and if myelination were abnormal from the outset, it would cause the synaptic dysfunction and abnormal neural tracts that are underpinning features of schizophrenia. The hypothesis is that latency is caused by events that occur in some patients as early as in-utero or infancy, because clones of abnormal, myelinating oligodendrocytes may arise at that time; their number doubles every ~2 years, so their geometric increase between birth and age twenty, when clinical presentation occurs, is about 1000-fold plus the effect of compounding. For those patients in particular, the long latency is because of a small but ongoing increase in volume of the resulting, abnormally myelinated neural tracts until, after a long latent interval, a critical mass is reached that allows the full clinical features of schizophrenia. During latency, there may be behavioral aberrancies because of abnormally myelinated neural tracts but they are insufficiently numerous for the clinical syndrome. The occurrence of behavioral symptoms during the long latent period, substantiates the hypothesis that abnormal oligodendrocytes explain the latency in some patients. Treatment with fingolimod or siponimod benefits both oligodendrocytes and neural tracts. Clinical trial would validate their potential benefit in appropriate patients with schizophrenia and, concurrently, would validate the hypothesis.

Oligodendrocytes and Myelin: Active players in Neurodegenerative brains?

Oligodendrocytes (OLs) are a major type of glial cells in the central nervous system that generate multiple myelin sheaths to wrap axons. Myelin ensures fast and efficient propagation of action potentials along axons and supports neurons with nourishment. The decay of OLs and myelin has been implicated in age-related neurodegenerative diseases and these changes are generally considered as an inevitable result of neuron loss and axon degeneration. Noticeably, OLs and myelin undergo dynamic changes in healthy adult brains, that is, newly formed OLs are continuously added throughout life from the differentiation of oligodendrocyte precursor cells (OPCs) and the pre-existing myelin sheaths may undergo degeneration or remodeling. Increasing evidence has shown that changes in OLs and myelin are present in the early stages of neurodegenerative diseases, and even prior to significant neuronal loss and functional deficits. More importantly, oligodendroglia-specific manipulation, by either deletion of the disease gene or enhancement of myelin renewal, can alleviate functional impairments in neurodegenerative animal models. These findings underscore the possibility that OLs and myelin are not passively but actively involved in neurodegenerative diseases and may play an important role in modulating neuronal function and survival. In this review, we summarize recent work characterizing OL and myelin changes in both healthy and neurodegenerative brains and discuss the potential of targeting oligodendroglial cells in treating neurodegenerative diseases. This article is protected by copyright. All rights reserved.

Insights Into Central Nervous System Glial Cell Formation and Function From Zebrafish

The term glia describes a heterogenous collection of distinct cell types that make up a large proportion of our nervous system. Although once considered the glue of the nervous system, the study of glial cells has evolved significantly in recent years, with a large body of literature now highlighting their complex and diverse roles in development and throughout life. This progress is due, in part, to advances in animal models in which the molecular and cellular mechanisms of glial cell development and function as well as neuron-glial cell interactions can be directly studied in vivo in real time, in intact neural circuits. In this review we highlight the instrumental role that zebrafish have played as a vertebrate model system for the study of glial cells, and discuss how the experimental advantages of the zebrafish lend themselves to investigate glial cell interactions and diversity. We focus in particular on recent studies that have provided insight into the formation and function of the major glial cell types in the central nervous system in zebrafish.

Replenishing the Aged Brains: Targeting Oligodendrocytes and Myelination?

Aging affects almost all the aspects of brain functions, but the mechanisms remain largely undefined. Increasing number of literatures have manifested the important role of glial cells in regulating the aging process. Oligodendroglial lineage cell is a major type of glia in central nervous system (CNS), composed of mature oligodendrocytes (OLs), and oligodendroglia precursor cells (OPCs). OLs produce myelin sheaths that insulate axons and provide metabolic support to meet the energy demand. OPCs maintain the population throughout lifetime with the abilities to proliferate and differentiate into OLs. Increasing evidence has shown that oligodendroglial cells display active dynamics in adult and aging CNS, which is extensively involved in age-related brain function decline in the elderly. In this review, we summarized present knowledge about dynamic changes of oligodendroglial lineage cells during normal aging and discussed their potential roles in age-related functional decline. Especially, focused on declined myelinogenesis during aging and underlying mechanisms. Clarifying those oligodendroglial changes and their effects on neurofunctional decline may provide new insights in understanding aging associated brain function declines.

Laminin regulates oligodendrocyte development and myelination

Oligodendrocytes are the cells that myelinate axons and provide trophic support to neurons in the CNS. Their dysfunction has been associated with a group of disorders known as demyelinating diseases, such as multiple sclerosis. Oligodendrocytes are derived from oligodendrocyte precursor cells, which differentiate into premyelinating oligodendrocytes and eventually mature oligodendrocytes. The development and function of oligodendrocytes are tightly regulated by a variety of molecules, including laminin, a major protein of the extracellular matrix. Accumulating evidence suggests that laminin actively regulates every aspect of oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination. How can laminin exert such diverse functions in oligodendrocytes? It is speculated that the distinct laminin isoforms, laminin receptors, and/or key signaling molecules expressed in oligodendrocytes at different developmental stages are the reasons. Understanding molecular targets and signaling pathways unique to each aspect of oligodendrocyte biology will enable more accurate manipulation of oligodendrocyte development and function, which may have implications in the therapies of demyelinating diseases. Here in this review, we first introduce oligodendrocyte biology, followed by the expression of laminin and laminin receptors in oligodendrocytes and other CNS cells. Next, the functions of laminin in oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination, are discussed in detail. Last, key questions and challenges in the field are discussed. By providing a comprehensive review on laminin's roles in OL lineage cells, we hope to stimulate novel hypotheses and encourage new research in the field.

Heterogeneity and Molecular Markers for CNS Glial Cells Revealed by Single-Cell Transcriptomics

Glial cells, including astrocytes, oligodendrocytes, and microglia, are the major components in the central nervous system (CNS). Studies have revealed the heterogeneity of each glial cell type and that they each may play distinct roles in physiological processes and/or neurological diseases. Single-cell sequencing (scRNA-seq) technology developed in recent years has extended our understanding of glial cell heterogeneity from the perspective of transcriptome profiling. This review summarizes the marker genes of major glial cells in the CNS and reveals their heterogeneity in different species, CNS regions, developmental stages, and pathological states (Alzheimer's disease and spinal cord injury), expanding our knowledge of glial cell heterogeneity on both molecular and functional levels.

Shaping of Regional Differences in Oligodendrocyte Dynamics by Regional Heterogeneity of the Pericellular Microenvironment

Oligodendrocyte precursor cells (OPCs) are glial cells that differentiate into mature oligodendrocytes (OLs) to generate new myelin sheaths. While OPCs are distributed uniformly throughout the gray and white matter in the developing and adult brain, those in white matter proliferate and differentiate into oligodendrocytes at a greater rate than those in gray matter. There is currently lack of evidence to suggest that OPCs comprise genetically and transcriptionally distinct subtypes. Rather, the emerging view is that they exist in different cell and functional states, depending on their location and age. Contrary to the normal brain, demyelinated lesions in the gray matter of multiple sclerosis brains contain more OPCs and OLs and are remyelinated more robustly than those in white matter. The differences in the dynamic behavior of OL lineage cells are likely to be influenced by their microenvironment. There are regional differences in astrocytes, microglia, the vasculature, and the composition of the extracellular matrix (ECM). We will discuss how the regional differences in these elements surrounding OPCs might shape their phenotypic variability in normal and demyelinated states.

A metabolome atlas of the aging mouse brain

The mammalian brain relies on neurochemistry to fulfill its functions. Yet, the complexity of the brain metabolome and its changes during diseases or aging remain poorly understood. Here, we generate a metabolome atlas of the aging wildtype mouse brain from 10 anatomical regions spanning from adolescence to old age. We combine data from three assays and structurally annotate 1,547 metabolites. Almost all metabolites significantly differ between brain regions or age groups, but not by sex. A shift in sphingolipid patterns during aging related to myelin remodeling is accompanied by large changes in other metabolic pathways. Functionally related brain regions (brain stem, cerebrum and cerebellum) are also metabolically similar. In cerebrum, metabolic correlations markedly weaken between adolescence and adulthood, whereas at old age, cross-region correlation patterns reflect decreased brain segregation. We show that metabolic changes can be mapped to existing gene and protein brain atlases. The brain metabolome atlas is publicly available ( https://mouse.atlas.metabolomics.us/ ) and serves as a foundation dataset for future metabolomic studies.

Defective myelination in an RNA polymerase III mutant leukodystrophic mouse

RNA polymerase (Pol) III synthesizes abundant short noncoding RNAs that have essential functions in protein synthesis, secretion, and other processes. Despite the ubiquitous functions of these RNAs, mutations in Pol III subunits cause Pol III-related leukodystrophy, an early-onset neurodegenerative disease. The basis of this neural sensitivity and the mechanisms of disease pathogenesis are unknown. Here we show that mice expressing pathogenic mutations in the largest Pol III subunit, Polr3a, specifically in Olig2-expressing cells, have impaired growth and developmental delay, deficits in cognitive, sensory, and fine sensorimotor function, and hypomyelination in multiple regions of the cerebrum and spinal cord. These phenotypes reflect a subset of clinical features seen in patients. In contrast, the gross motor defects and cerebellar hypomyelination that are common features of severely affected patients are absent in the mice, suggesting a relatively mild form of the disease in this conditional model. Our results show that disease pathogenesis in the mice involves defects that reduce both the number of mature myelinating oligodendrocytes and the ability of these cells to produce a myelin sheath of normal thickness. The findings suggest unique sensitivities of oligodendrogenesis and myelination to perturbations of Pol III transcription.

Postnatal myelination of the immature rat cingulum is regulated by GABAB receptor activity

Myelination of axons in the neonatal brain is a highly complex process primarily achieved by oligodendroglial cells (OLs). OLs express receptors for γ-aminobutyric acid (GABA) which is released from cortical interneurons on a basal level, while glial cells can be a source of GABA, too. We investigated GABA-induced oligodendroglial maturation, proliferation, apoptosis, and myelin production after pharmacological inhibition of GABA_A and GABA_B in the neonatal rat brain. Daily injections of the reverse GABA_A receptor agonist (DMCM) and the GABA_B receptor antagonist (CGP35348) were performed from postnatal day 6 (P6) to P11. MBP expression was examined by Western blots and immunohistochemistry. Furthermore, we determined the number of CC1⁺ OLIG2⁺ and CNP⁺ OLIG2⁺ cells to assess maturation, the number of PCNA⁺ OLIG2⁺ oligodendrocytes to assess proliferation, the number of oligodendrocyte precursor cells (PDGFRα⁺ OLIG2⁺ ), and apoptosis of OLs (CASP3A⁺ OLIG2⁺ ) as well as apoptotic cells in total (CASP3A⁺ DAPI⁺ ) at P11 and P15. In addition, we analyzed the expression Pdgfrα and CNP. MBP expression was significantly reduced after CGP treatment at P15. In the same animal group, CNP expression and CNP⁺ OLIG2⁺ cells decreased temporarily at P11. At P15, the proliferation of PCNA⁺ OLIG2⁺ cells and the number of PDGFRα⁺ OLIG2⁺ cells increased after GABA_B receptor antagonization whereas no significant differences were visible in the Pdgfrα gene expression. No changes in apoptotic cell death were observed. CGP treatment induced a transient maturational delay at P11 and deficits in myelin expression at P15 with increased oligodendroglial proliferation. Our in vivo study indicates GABA_B receptor activity as a potential modulator of oligodendroglial development.

MyelTracer: A Semi-Automated Software for Myelin g-Ratio Quantification

In the central and peripheral nervous systems, the myelin sheath promotes neuronal signal transduction. The thickness of the myelin sheath changes during development and in disease conditions like multiple sclerosis. Such changes are routinely detected using electron microscopy through g-ratio quantification. While g-ratio is one of the most critical measurements in myelin studies, a major drawback is that g-ratio quantification is extremely laborious and time-consuming. Here, we report the development and validation of MyelTracer, an installable, stand-alone software for semi-automated g-ratio quantification based on the Open Computer Vision Library (OpenCV). Compared with manual g-ratio quantification, using MyelTracer produces consistent results across multiple tissues and animal ages, as well as in remyelination after optic nerve crush, and reduces total quantification time by 40-60%. With g-ratio measurements via MyelTracer, a known hypomyelination phenotype can be detected in a Williams syndrome mouse model. MyelTracer is easy to use and freely available for Windows and Mac OS X (https://github.com/HarrisonAllen/MyelTracer).

Premyelinating Oligodendrocytes: Mechanisms Underlying Cell Survival and Integration

In the central nervous system, oligodendrocytes produce myelin sheaths that enwrap neuronal axons to provide trophic support and increase conduction velocity. New oligodendrocytes are produced throughout life through a process referred to as oligodendrogenesis. Oligodendrogenesis consists of three canonical stages: the oligodendrocyte precursor cell (OPC), the premyelinating oligodendrocyte (preOL), and the mature oligodendrocyte (OL). However, the generation of oligodendrocytes is inherently an inefficient process. Following precursor differentiation, a majority of premyelinating oligodendrocytes are lost, likely due to apoptosis. If premyelinating oligodendrocytes progress through this survival checkpoint, they generate new myelinating oligodendrocytes in a process we have termed integration. In this review, we will explore the intrinsic and extrinsic signaling pathways that influence preOL survival and integration by examining the intrinsic apoptotic pathways, metabolic demands, and the interactions between neurons, astrocytes, microglia, and premyelinating oligodendrocytes. Additionally, we will discuss similarities between the maturation of newly generated neurons and premyelinating oligodendrocytes. Finally, we will consider how increasing survival and integration of preOLs has the potential to increase remyelination in multiple sclerosis. Deepening our understanding of premyelinating oligodendrocyte biology may open the door for new treatments for demyelinating disease and will help paint a clearer picture of how new oligodendrocytes are produced throughout life to facilitate brain function.

Live imaging of remyelination in the adult mouse corpus callosum

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

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

Potential neuron-autonomous Purkinje cell degeneration by 2',3'-cyclic nucleotide 3'-phosphodiesterase promoter/Cre-mediated autophagy impairments

Studies of neuroglial interaction largely depend on cell-specific gene knockout (KO) experiments using Cre recombinase. However, genes known as glial-specific genes have recently been reported to be expressed in neuroglial stem cells, leading to the possibility that a glia-specific Cre driver results in unwanted gene deletion in neurons, which may affect sound interpretation. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) is generally considered to be an oligodendrocyte (OL) marker. Accordingly, Cnp promoter-controlled Cre recombinase has been used to create OL-specific gene targeting mice. However, in this study, using Rosa26-tdTomato-reporter/Cnp-Cre mice, we found that many forebrain neurons and cerebellar Purkinje neurons belong to the lineages of Cnp-expressing neuroglial stem cells. To answer whether gene targeting by Cnp-Cre can induce neuron-autonomous defects, we conditionally deleted an essential autophagy gene, Atg7, in Cnp-Cre mice. The Cnp-Cre-mediated Atg7 KO mice showed extensive p62 inclusion in neurons, including cerebellar Purkinje neurons with extensive neurodegeneration. Furthermore, neuronal areas showing p62 inclusion in Cnp-Cre-mediated Atg7 KO mice overlapped with the neuronal lineage of Cnp-expressing neuroglial stem cells. Moreover, Cnp-Cre-mediated Atg7-KO mice did not develop critical defects in myelination. Our results demonstrate that a large population of central neurons are derived from Cnp-expressing neuroglial stem cells; thus, conditional gene targeting using the Cnp promoter, which is known to be OL-specific, can induce neuron-autonomous phenotypes.

TFEB protein expression is reduced in aged brains and its overexpression mitigates senescence-associated biomarkers and memory deficits in mice

Identification of molecules and molecular pathways that can ameliorate aging-associated decline in cognitive function is crucial. Here we report that the protein levels of transcription factor EB (TFEB) were markedly reduced in both the cytosolic and nuclear fractions of the frontal cortex and hippocampus at 18-months of age relative to 6 months in the normal male wild-type mice. In the transgenic mice with ectopic expression of flag-TFEB in neurons, we observed that the levels of actin-normalized PGC1α and mtTFA were significantly increased in both the cortex and the hippocampus. Additionally, we confirmed increased mitochondria numbers in the flag-TFEB mice by transmission electron microscopy. Most importantly, TFEB expression in the 18-month-old transgenic mice mitigated markers of senescence including P16INK4a, γ-H2AX, and lamin B1, and improved memory skills implying that TFEB may exert an anti-aging effect by modulating neuronal senescence. Taken together these data strongly support that TFEB can be a useful therapeutic target for brain senescent cells to help overcome the age-related issues in cognition and possibly, achieve healthy aging.

Fractalkine-Dependent Microglial Pruning of Viable Oligodendrocyte Progenitor Cells Regulates Myelination

Oligodendrogenesis occurs during early postnatal development, coincident with neurogenesis and synaptogenesis, raising the possibility that microglia-dependent pruning mechanisms that modulate neurons regulate myelin sheath formation. Here we show a population of ameboid microglia migrating from the ventricular zone into the corpus callosum during early postnatal development, termed “the fountain of microglia,” phagocytosing viable oligodendrocyte progenitor cells (OPCs) before onset of myelination. Fractalkine receptor-deficient mice exhibit a reduction in microglial engulfment of viable OPCs, increased numbers of oligodendrocytes, and reduced myelin thickness but no change in axon number. These data provide evidence that microglia phagocytose OPCs as a homeostatic mechanism for proper myelination. A hallmark of hypomyelinating developmental disorders such as periventricular leukomalacia and of adult demyelinating diseases such as multiple sclerosis is increased numbers of oligodendrocytes but failure to myelinate, suggesting that microglial pruning of OPCs may be impaired in pathological states and hinder myelination.

Nemes-Baran et al. show that ameboid microglia engulf living oligodendrocyte progenitor cells (OPCs) during brain development. Fractalkine receptor-deficient microglia exhibit a reduction in engulfment of OPCs, resulting in a surplus of oligodendrocytes and impaired myelination. These data provide evidence that microglia phagocytose OPCs as a homeostatic mechanism required for normal myelination.

Oxidative stress and impaired oligodendrocyte precursor cell differentiation in neurological disorders

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

Senescent Microglia: The Key to the Ageing Brain?

Ageing represents the single biggest risk factor for development of neurodegenerative disease. Despite being such long-lived cells, microglia have been relatively understudied for their role in the ageing process. Reliably identifying aged microglia has proven challenging, not least due to the diversity of cell populations, and the limitations of available models, further complicated by differences between human and rodent cells. Consequently, the literature contains multiple descriptions and categorisations of microglia with neurotoxic phenotypes, including senescence, without any unifying markers. The role of microglia in brain homeostasis, particularly iron storage and metabolism, may provide a key to reliable identification.