Myelinating satellite oligodendrocytes are integrated in a glial syncytium constraining neuronal high-frequency activity

Heterogeneity of mature oligodendrocytes in the central nervous system

Mature oligodendrocytes form myelin sheaths that are crucial for the insulation of axons and efficient signal transmission in the central nervous system. Recent evidence has challenged the classical view of the functionally static mature oligodendrocyte and revealed a gamut of dynamic functions such as the ability to modulate neuronal circuitry and provide metabolic support to axons. Despite the recognition of potential heterogeneity in mature oligodendrocyte function, a comprehensive summary of mature oligodendrocyte diversity is lacking. We delve into early 20 th -century studies by Robertson and Río-Hortega that laid the foundation for the modern identification of regional and morphological heterogeneity in mature oligodendrocytes. Indeed, recent morphologic and functional studies call into question the long-assumed homogeneity of mature oligodendrocyte function through the identification of distinct subtypes with varying myelination preferences. Furthermore, modern molecular investigations, employing techniques such as single cell/nucleus RNA sequencing, consistently unveil at least six mature oligodendrocyte subpopulations in the human central nervous system that are highly transcriptomically diverse and vary with central nervous system region. Age and disease related mature oligodendrocyte variation denotes the impact of pathological conditions such as multiple sclerosis, Alzheimer's disease, and psychiatric disorders. Nevertheless, caution is warranted when subclassifying mature oligodendrocytes because of the simplification needed to make conclusions about cell identity from temporally confined investigations. Future studies leveraging advanced techniques like spatial transcriptomics and single-cell proteomics promise a more nuanced understanding of mature oligodendrocyte heterogeneity. Such research avenues that precisely evaluate mature oligodendrocyte heterogeneity with care to understand the mitigating influence of species, sex, central nervous system region, age, and disease, hold promise for the development of therapeutic interventions targeting varied central nervous system pathology.

Nf1 mutation disrupts activity-dependent oligodendroglial plasticity and motor learning in mice

Neurogenetic disorders, such as neurofibromatosis type 1 (NF1), can cause cognitive and motor impairments, traditionally attributed to intrinsic neuronal defects such as disruption of synaptic function. Activity-regulated oligodendroglial plasticity also contributes to cognitive and motor functions by tuning neural circuit dynamics. However, the relevance of oligodendroglial plasticity to neurological dysfunction in NF1 is unclear. Here we explore the contribution of oligodendrocyte progenitor cells (OPCs) to pathological features of the NF1 syndrome in mice. Both male and female littermates (4-24 weeks of age) were used equally in this study. We demonstrate that mice with global or OPC-specific Nf1 heterozygosity exhibit defects in activity-dependent oligodendrogenesis and harbor focal OPC hyperdensities with disrupted homeostatic OPC territorial boundaries. These OPC hyperdensities develop in a cell-intrinsic Nf1 mutation-specific manner due to differential PI3K/AKT activation. OPC-specific Nf1 loss impairs oligodendroglial differentiation and abrogates the normal oligodendroglial response to neuronal activity, leading to impaired motor learning performance. Collectively, these findings show that Nf1 mutation delays oligodendroglial development and disrupts activity-dependent OPC function essential for normal motor learning in mice.

Deficit of satellite oligodendrocytes of neurons in the rostral part of the head of the caudate nucleus in schizophrenia

Increasing evidence implicates compromised myelin integrity and oligodendrocyte abnormalities in the dysfunction of neuronal networks in schizophrenia. We previously reported a deficiency of myelinating oligodendrocytes (OL), oligodendrocyte progenitors (OP) and satellite oligodendrocytes of neurons (Sat-OL) in the prefrontal cortex and the inferior parietal cortex - cortical hubs of the frontoparietal cognitive network and default mode network (DMN) altered in schizophrenia. Deficiency of OL and OP was also detected in the head of the caudate nucleus (HCN), which accumulates cortical projections from the associative cortex and is the central node of these networks. However, the number of Sat-Ol per neuron in schizophrenia has not been studied in the HCN. In the current study we estimated the number of Sat-Ol per neuron in the rostral part of the HCN in schizophrenia (n = 18) compared to healthy controls (n = 18) in the same section collection that was previously used to study the number Ol and OP. We found a significant decrease of the number of Sat-Ol per neuron (- 50%, p < 0.001) in schizophrenia as compared to normal controls. Considering that the rostral part of the HCN is an individual network-specific projection zone of the DMN, the deficit of Sat-Ol found in schizophrenia may be related to the dysfunctional DMN-HCN connections, which has been repeatedly described in schizophrenia. The dramatic decrease of the number of Sat-Ol per neuron may be partially related to a pronounced excess of dopamine concentration in the rostral part of the HCN in schizophrenia.

Long-term in vivo three-photon imaging reveals region-specific differences in healthy and regenerative oligodendrogenesis

The generation of new myelin-forming oligodendrocytes in the adult central nervous system is critical for cognitive function and regeneration following injury. Oligodendrogenesis varies between gray and white matter regions, suggesting that local cues drive regional differences in myelination and the capacity for regeneration. However, the layer- and region-specific regulation of oligodendrocyte populations is unclear due to the inability to monitor deep brain structures in vivo. Here we harnessed the superior imaging depth of three-photon microscopy to permit long-term, longitudinal in vivo three-photon imaging of the entire cortical column and subcortical white matter in adult mice. We find that cortical oligodendrocyte populations expand at a higher rate in the adult brain than those of the white matter. Following demyelination, oligodendrocyte replacement is enhanced in the white matter, while the deep cortical layers show deficits in regenerative oligodendrogenesis and the restoration of transcriptional heterogeneity. Together, our findings demonstrate that regional microenvironments regulate oligodendrocyte population dynamics and heterogeneity in the healthy and diseased brain.

Oligodendrocyte-axon metabolic coupling is mediated by extracellular K+ and maintains axonal health

The integrity of myelinated axons relies on homeostatic support from oligodendrocytes (OLs). To determine how OLs detect axonal spiking and how rapid axon-OL metabolic coupling is regulated in the white matter, we studied activity-dependent calcium (Ca2+) and metabolite fluxes in the mouse optic nerve. We show that fast axonal spiking triggers Ca2+ signaling and glycolysis in OLs. OLs detect axonal activity through increases in extracellular potassium (K+) concentrations and activation of Kir4.1 channels, thereby regulating metabolite supply to axons. Both pharmacological inhibition and OL-specific inactivation of Kir4.1 reduce the activity-induced axonal lactate surge. Mice lacking oligodendroglial Kir4.1 exhibit lower resting lactate levels and altered glucose metabolism in axons. These early deficits in axonal energy metabolism are associated with late-onset axonopathy. Our findings reveal that OLs detect fast axonal spiking through K+ signaling, making acute metabolic coupling possible and adjusting the axon-OL metabolic unit to promote axonal health.

Long-term in vivo three-photon imaging reveals region-specific differences in healthy and regenerative oligodendrogenesis

The generation of new myelin-forming oligodendrocytes in the adult CNS is critical for cognitive function and regeneration following injury. Oligodendrogenesis varies between gray and white matter regions suggesting that local cues drive regional differences in myelination and the capacity for regeneration. Yet, the determination of regional variability in oligodendrocyte cell behavior is limited by the inability to monitor the dynamics of oligodendrocytes and their transcriptional subpopulations in white matter of the living brain. Here, we harnessed the superior imaging depth of three-photon microscopy to permit long-term, longitudinal in vivo three-photon imaging of an entire cortical column and underlying subcortical white matter without cellular damage or reactivity. Using this approach, we found that the white matter generated substantially more new oligodendrocytes per volume compared to the gray matter, yet the rate of population growth was proportionally higher in the gray matter. Following demyelination, the white matter had an enhanced population growth that resulted in higher oligodendrocyte replacement compared to the gray matter. Finally, deep cortical layers had pronounced deficits in regenerative oligodendrogenesis and restoration of the MOL5/6-positive oligodendrocyte subpopulation following demyelinating injury. Together, our findings demonstrate that regional microenvironments regulate oligodendrocyte population dynamics and heterogeneity in the healthy and diseased brain.

Glial Populations in the Human Brain Following Ischemic Injury

There is a growing interest in glial cells in the central nervous system due to their important role in maintaining brain homeostasis under physiological conditions and after injury. A significant amount of evidence has been accumulated regarding their capacity to exert either pro-inflammatory or anti-inflammatory effects under different pathological conditions. In combination with their proliferative potential, they contribute not only to the limitation of brain damage and tissue remodeling but also to neuronal repair and synaptic recovery. Moreover, reactive glial cells can modulate the processes of neurogenesis, neuronal differentiation, and migration of neurons in the existing neural circuits in the adult brain. By discovering precise signals within specific niches, the regulation of sequential processes in adult neurogenesis holds the potential to unlock strategies that can stimulate the generation of functional neurons, whether in response to injury or as a means of addressing degenerative neurological conditions. Cerebral ischemic stroke, a condition falling within the realm of acute vascular disorders affecting the circulation in the brain, stands as a prominent global cause of disability and mortality. Extensive investigations into glial plasticity and their intricate interactions with other cells in the central nervous system have predominantly relied on studies conducted on experimental animals, including rodents and primates. However, valuable insights have also been gleaned from in vivo studies involving poststroke patients, utilizing highly specialized imaging techniques. Following the attempts to map brain cells, the role of various transcription factors in modulating gene expression in response to cerebral ischemia is gaining increasing popularity. Although the results obtained thus far remain incomplete and occasionally ambiguous, they serve as a solid foundation for the development of strategies aimed at influencing the recovery process after ischemic brain injury.

Transfer of nuclear and ribosomal material from Sox10-lineage cells to neurons in the mouse brain

Material transfer is an essential form of intercellular communication to exchange information and resources between cells. Material transfer between neurons and from glia to neurons has been demonstrated to support neuronal survival and activity. Understanding the extent of material transfer in the healthy nervous system is limited. Here we report that in the mouse central nervous system (CNS), neurons receive nuclear and ribosomal material of Sox10-lineage cell (SOL) origin. We show that transfer of SOL-derived material to neurons is region dependent, establishes during postnatal brain maturation, and dynamically responds to LPS-induced neuroinflammation in the adult mouse brain. We identified satellite oligodendrocyte-neuron pairs with loss of plasma membrane integrity between nuclei, suggesting direct material transfer. Together, our findings provide evidence of regionally coordinated transfer of SOL-derived nuclear and ribosomal material to neurons in the mouse CNS, with potential implications for the understanding and modulation of neuronal function and treatment of neurological disorders.

Direct association with the vascular basement membrane is a frequent feature of myelinating oligodendrocytes in the neocortex

BACKGROUND

Oligodendrocyte lineage cells interact with the vasculature in the gray matter. Physical and functional interactions between blood vessels and oligodendrocyte precursor cells play an essential role in both the developing and adult brain. Oligodendrocyte precursor cells have been shown to migrate along the vasculature and subsequently detach from it during their differentiation to oligodendrocytes. However, the association of mature oligodendrocytes with blood vessels has been noted since the discovery of this glial cell type almost a century ago, but this interaction remains poorly explored.

RESULTS

Here, we systematically investigated the extent of mature oligodendrocyte interaction with the vasculature in mouse brain. We found that ~ 17% of oligodendrocytes were in contact with blood vessels in the neocortex, the hippocampal CA1 region and the cerebellar cortex. Contacts were made mainly with capillaries and sparsely with larger arterioles or venules. By combining light and serial electron microscopy, we demonstrated that oligodendrocytes are in direct contact with the vascular basement membrane, raising the possibility of direct signaling pathways and metabolite exchange with endothelial cells. During experimental remyelination in the adult, oligodendrocytes were regenerated and associated with blood vessels in the same proportion compared to control cortex, suggesting a homeostatic regulation of the vasculature-associated oligodendrocyte population.

CONCLUSIONS

Based on their frequent and close association with blood vessels, we propose that vasculature-associated oligodendrocytes should be considered as an integral part of the brain vasculature microenvironment. This particular location could underlie specific functions of vasculature-associated oligodendrocytes, while contributing to the vulnerability of mature oligodendrocytes in neurological diseases.

Glia Connect Inflammation and Neurodegeneration in Multiple Sclerosis

Multiple sclerosis (MS) is regarded as a chronic inflammatory disease that leads to demyelination and eventually to neurodegeneration. Activation of innate immune cells and other inflammatory cells in the brain and spinal cord of people with MS has been well described. However, with the innovation of technology in glial cell research, we have a deep understanding of the mechanisms of glial cells connecting inflammation and neurodegeneration in MS. In this review, we focus on the role of glial cells, including microglia, astrocytes, and oligodendrocytes, in the pathogenesis of MS. We mainly focus on the connection between glial cells and immune cells in the process of axonal damage and demyelinating neuron loss.

Glia Connect Inflammation and Neurodegeneration in Multiple Sclerosis

Multiple sclerosis (MS) is regarded as a chronic inflammatory disease that leads to demyelination and eventually to neurodegeneration. Activation of innate immune cells and other inflammatory cells in the brain and spinal cord of people with MS has been well described. However, with the innovation of technology in glial cell research, we have a deep understanding of the mechanisms of glial cells connecting inflammation and neurodegeneration in MS. In this review, we focus on the role of glial cells, including microglia, astrocytes, and oligodendrocytes, in the pathogenesis of MS. We mainly focus on the connection between glial cells and immune cells in the process of axonal damage and demyelinating neuron loss.

Kir4.1 channel activation in NG2 glia contributes to remyelination in ischemic stroke

BACKGROUND

Stroke is one of the most common neurological diseases in the world and is clinically manifested by transient or permanent brain dysfunction. It has a high mortality and disability rate, which severely affects people's health and diminishes the quality of life. However, there is no efficient treatment that can be considered curative and there are other less well-known theories of pathogenesis. Therefore, it is imperative to gain a full understanding of the pathophysiology of ischemia and to seek new therapeutic strategies.

METHODS

We first examined Kir4.1 channel and myelin based protein (MBP) expression in brain tissues from acute ischemic patients by Western blotting. We then established a transient ischemic mouse model (tMCAO) to conduct molecular, cell biological, transmission electron microscopy and pharmacokinetic studies, as well as in Kir4.1 cKO mice. Finally, neuroimaging and behavioral analyses were used to examine whether activation of Kir4.1 channel by luteolin could contribute to neuronal functional recovery in ischemic stroke.

FINDINGS

In acute ischemic stroke patients, we first demonstrated that Kir4.1 ion channels were greatly impaired and a severe demyelination of axons occurred in ischemic infarction area of cerebral cortex in these patients. Further evidence showed that the deficits of Kir4.1 channels in NG2 glia led to the myelin loss of axons in a transient ischemic mouse model (tMCAO). Treating ischemic mice with a natural botanical extract, luteolin augmented Kir4.1 channel currents in NG2 glia and consequently promoted remyelination of axons, alleviated the infarction area and ultimately improved motor function in a series of behavioral tests.

INTERPRETATION

Targeting Kir4.1 ion channels expressed in NG2 glial cells by luteolin treatment highlights an effective therapeutic strategy for a prompt brain functional recovery in ischemic stroke.

FUNDING

This work was supported by grants from the Ministry of Science and Technology China Brain Initiative (2022ZD0204702, to X.T.), the National Natural Science Foundation of China (82271466, 82171279, 31970904 and 31571063), the Program for Professor of Special Appointment (Eastern Scholar for Dr. X.T.) at Shanghai Institutions for Higher Learning (1510000084), Shanghai Pujiang Talent Award (15PJ1404600), Shanghai Municipal Science and Technology Major Project (2018SHZDZX05) and Shanghai Science and Technology Project (17411954000).

Oligodendrocyte precursor cells ingest axons in the mouse neocortex

Neurons in the developing brain undergo extensive structural refinement as nascent circuits adopt their mature form. This physical transformation of neurons is facilitated by the engulfment and degradation of axonal branches and synapses by surrounding glial cells, including microglia and astrocytes. However, the small size of phagocytic organelles and the complex, highly ramified morphology of glia have made it difficult to define the contribution of these and other glial cell types to this crucial process. Here, we used large-scale, serial section transmission electron microscopy (TEM) with computational volume segmentation to reconstruct the complete 3D morphologies of distinct glial types in the mouse visual cortex, providing unprecedented resolution of their morphology and composition. Unexpectedly, we discovered that the fine processes of oligodendrocyte precursor cells (OPCs), a population of abundant, highly dynamic glial progenitors, frequently surrounded small branches of axons. Numerous phagosomes and phagolysosomes (PLs) containing fragments of axons and vesicular structures were present inside their processes, suggesting that OPCs engage in axon pruning. Single-nucleus RNA sequencing from the developing mouse cortex revealed that OPCs express key phagocytic genes at this stage, as well as neuronal transcripts, consistent with active axon engulfment. Although microglia are thought to be responsible for the majority of synaptic pruning and structural refinement, PLs were ten times more abundant in OPCs than in microglia at this stage, and these structures were markedly less abundant in newly generated oligodendrocytes, suggesting that OPCs contribute substantially to the refinement of neuronal circuits during cortical development.

Spread of pathological human Tau from neurons to oligodendrocytes and loss of high-firing pyramidal neurons in aging mice

Intracellular aggregation of hyperphosphorylated Tau (pTau) in the brain is associated with cognitive and motor impairments, and ultimately neurodegeneration. We investigate how human pTau affects cells and network activity in the hippocampal formation of the THY-Tau22 tauopathy model mice in vivo. We find that pTau preferentially accumulates in deep-layer pyramidal neurons, leading to neurodegeneration, and we establish that pTau spreads to oligodendrocytes. During goal-directed virtual navigation in aged transgenic mice, we detect fewer high-firing prosubicular pyramidal cells, but the firing population retains its coupling to theta oscillations. Analysis of network oscillations and firing patterns of pyramidal and GABAergic neurons recorded in head-fixed and freely moving mice suggests preserved neuronal coordination. In spatial memory tests, transgenic mice have reduced short-term familiarity, but spatial working and reference memory are surprisingly normal. We hypothesize that unimpaired subcortical network mechanisms maintain cortical neuronal coordination, counteracting the widespread pTau aggregation, loss of high-firing cells, and neurodegeneration.

Adaptive and maladaptive myelination in health and disease

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

Heterogeneity and regulation of oligodendrocyte morphology

Oligodendrocytes form multiple myelin sheaths in the central nervous system (CNS), which increase nerve conduction velocity and are necessary for basic and higher brain functions such as sensory function, motor control, and learning. Structures of the myelin sheath such as myelin internodal length and myelin thickness regulate nerve conduction. Various parts of the central nervous system exhibit different myelin structures and oligodendrocyte morphologies. Recent studies supported that oligodendrocytes are a heterogenous population of cells and myelin sheaths formed by some oligodendrocytes can be biased to particular groups of axons, and myelin structures are dynamically modulated in certain classes of neurons by specific experiences. Structures of oligodendrocyte/myelin are also affected in pathological conditions such as demyelinating and neuropsychiatric disorders. This review summarizes our understanding of heterogeneity and regulation of oligodendrocyte morphology concerning central nervous system regions, neuronal classes, experiences, diseases, and how oligodendrocytes are optimized to execute central nervous system functions.

Reduced number of satellite oligodendrocytes of pyramidal neurons in layer 5 of the prefrontal cortex in schizophrenia

Neuroimaging, genetic and molecular biological studies have shown impaired intra-cortical myelination in patients with schizophrenia, particularly in the prefrontal cortex. Previously we reported a significant deficit of oligodendrocytes and oligodendrocyte clusters in layers 3 and 5 of the prefrontal cortex, Brodmann area 10 (BA10) in schizophrenia. In this current study, we investigate the number of oligodendrocyte satellites (Sat-Ol) per pyramidal neuron in layer 5 of BA10 in schizophrenia (n = 17) as compared to healthy controls (n = 20) in the same section collection as previously used to study the numerical density (Nv) of oligodendrocytes and oligodendrocyte clusters. We find a significant reduction (- 39%, p < 0.001) in the number of Sat-Ol per neuron in schizophrenia as compared to the control group. The number of Sat-Ol per neuron did not correlate with the Nv of oligodendrocytes or with the Nv of oligodendrocyte clusters. Our previous studies of the inferior parietal lobule (BA39 and BA40), demonstrated significant decrease of the number of Sat-Ol only in patient subgroups with poor and fair insight. Additionally, correlation pattern between number of Sat-Ol and Nv of oligodendrocytes and oligodendrocyte clusters was similar between the two functionally interconnected cortical areas, BA10 and BA40, whereas in BA39, strong significant correlations were revealed between the number of Sat-Ol and Nv of oligodendrocyte clusters (0.9 ≤ R ≥ 0.66; p < 0.001). These data suggest that that specific features of Sat-Ol alterations patterns may be associated with specific activity-driven plasticity of corresponding networks in the brain of people with schizophrenia.

Potassium channel Kir 4.1 regulates oligodendrocyte differentiation via intracellular pH regulation

In humans, loss-of-function mutations of Kcnj10 in SeSAME/EAST syndrome, which encodes the inwardly rectifying K⁺ channel 4.1 (K_ir 4.1), causes progressive neurological decline. Despite its rich expression in oligodendrocyte (OL) lineage cells and an emerging link with demyelinating disease, the function of K_ir 4.1 in OLs is unclear. Here we show a novel role of K_ir 4.1 in OL development. K_ir 4.1 expression is markedly greater in OLs than in OL precursor cells (OPCs), and the down-regulation of K_ir 4.1 impairs OL maturation by affecting OPC differentiation. Interestingly, K_ir 4.1 regulates the intracellular pH of OPCs and OLs via the Na⁺ /H⁺ exchanger, which underlies impeded OPC differentiation by K_ir 4.1 inhibition. Furthermore, K_ir 4.1 regulates GSK3β and SOX10, two molecules critical to OPC development. Collectively, our work opens a new avenue to understanding the functions of K_ir 4.1 and intracellular pH in OLs.

Maladaptive myelination promotes generalized epilepsy progression

Activity-dependent myelination can fine-tune neural network dynamics. Conversely, aberrant neuronal activity, as occurs in disorders of recurrent seizures (epilepsy), could promote maladaptive myelination, contributing to pathogenesis. In this study, we tested the hypothesis that activity-dependent myelination resulting from absence seizures, which manifest as frequent behavioral arrests with generalized electroencephalography (EEG) spike-wave discharges, promote thalamocortical network hypersynchrony and contribute to epilepsy progression. We found increased oligodendrogenesis and myelination specifically within the seizure network in two models of generalized epilepsy with absence seizures (Wag/Rij rats and Scn8a^+/mut mice), evident only after epilepsy onset. Aberrant myelination was prevented by pharmacological seizure inhibition in Wag/Rij rats. Blocking activity-dependent myelination decreased seizure burden over time and reduced ictal synchrony as assessed by EEG coherence. These findings indicate that activity-dependent myelination driven by absence seizures contributes to epilepsy progression; maladaptive myelination may be pathogenic in some forms of epilepsy and other neurological diseases.

Microglia form satellites with different neuronal subtypes in the adult murine central nervous system

Microglia are the innate immune cells of the central nervous system (CNS). In the adult uncompromised CNS, they have a highly ramified morphology and continuously extend and retract their processes. A subpopulation of microglial cells forms close soma-to-soma contacts with neurons and have been termed satellite microglia, yet the role of such interaction is largely unknown. Here, we analyzed the distribution of satellite microglia in different areas of the CNS of adult male mice applying transgenic- and immunolabeling of neuronal subtypes and microglia followed by three-dimensional imaging analysis. We quantified satellite microglia associated with GABAergic and glutamatergic neurons in the somatosensory cortex, striatum, and thalamus; with dopaminergic and serotonergic neurons in the basal forebrain and raphe nucleus, respectively; and with cerebellar Purkinje cell neurons. Satellite microglia in the retina were assessed qualitatively. Microglia form satellites with all neuronal subtypes studied, whereas a preference for a specific neuron subtype was not found. The occurrence and frequency of satellite microglia is determined by the histo-architectural organization of the brain area and the densities of neuronal somata therein.

DNA methylation changes in glial cells of the normal-appearing white matter in Multiple Sclerosis patients

Multiple Sclerosis (MS), the leading cause of non-traumatic neurological disability in young adults, is a chronic inflammatory and neurodegenerative disease of the central nervous system (CNS). Due to the poor accessibility to the target organ, CNS-confined processes underpinning the later progressive form of MS remain elusive thereby limiting treatment options. We aimed to examine DNA methylation, a stable epigenetic mark of genome activity, in glial cells to capture relevant molecular changes underlying MS neuropathology. We profiled DNA methylation in nuclei of non-neuronal cells, isolated from 38 post-mortem normal-appearing white matter (NAWM) specimens of MS patients (n = 8) in comparison to white matter of control individuals (n = 14), using Infinium MethylationEPIC BeadChip. We identified 1,226 significant (genome-wide adjusted P-value < 0.05) differentially methylated positions (DMPs) between MS patients and controls. Functional annotation of the altered DMP-genes uncovered alterations of processes related to cellular motility, cytoskeleton dynamics, metabolic processes, synaptic support, neuroinflammation and signaling, such as Wnt and TGF-β pathways. A fraction of the affected genes displayed transcriptional differences in the brain of MS patients, as reported by publically available transcriptomic data. Cell type-restricted annotation of DMP-genes attributed alterations of cytoskeleton rearrangement and extracellular matrix remodelling to all glial cell types, while some processes, including ion transport, Wnt/TGF-β signaling and immune processes were more specifically linked to oligodendrocytes, astrocytes and microglial cells, respectively. Our findings strongly suggest that NAWM glial cells are highly altered, even in the absence of lesional insult, collectively exhibiting a multicellular reaction in response to diffuse inflammation.

Evidence for glutamine synthetase function in mouse spinal cord oligodendrocytes

Glutamine synthetase (GS) is a key enzyme that metabolizes glutamate into glutamine. While GS is highly enriched in astrocytes, expression in other glial lineages has been noted. Using a combination of reporter mice and cell type-specific markers, we show that GS is expressed in myelinating oligodendrocytes (OL) but not oligodendrocyte progenitor cells of the mouse and human ventral spinal cord. To investigate the role of GS in mature OL, we used a conditional knockout (cKO) approach to selectively delete GS-encoding gene (Glul) in OL, which caused a significant decrease in glutamine levels on mouse spinal cord extracts. GS cKO mice (CNP-cre+ :Glulfl/fl ) showed no differences in motor neuron numbers, size or axon density; OL differentiation and myelination in the ventral spinal cord was normal up to 6 months of age. Interestingly, GS cKO mice showed a transient and specific decrease in peak force while locomotion and motor coordination remained unaffected. Last, GS expression in OL was increased in chronic pathological conditions in both mouse and humans. We found a disease-stage dependent increase of OL expressing GS in the ventral spinal cord of SOD1(G93A) mouse model of amyotrophic lateral sclerosis. Moreover, we showed that GLUL transcripts levels were increased in OL in leukocortical tissue from multiple sclerosis but not control patients. These findings provide evidence towards OL-encoded GS function in spinal cord sensorimotor axis, which is dysregulated in chronic neurological diseases.

Delivery Platforms for CRISPR/Cas9 Genome Editing of Glial Cells in the Central Nervous System

Glial cells (astrocytes, oligodendrocytes, and microglia) are emerging as key players in several physiological and pathological processes of the central nervous system (CNS). Astrocytes and oligodendrocytes are not only supportive cells that release trophic factors or regulate energy metabolism, but they also actively modulate critical neuronal processes and functions in the tripartite synapse. Microglia are defined as CNS-resident cells that provide immune surveillance; however, they also actively contribute to shaping the neuronal microenvironment by scavenging cell debris or regulating synaptogenesis and pruning. Given the many interconnected processes coordinated by glial cells, it is not surprising that both acute and chronic CNS insults not only cause neuronal damage but also trigger complex multifaceted responses, including neuroinflammation, which can critically contribute to the disease progression and worsening of symptoms in several neurodegenerative diseases. Overall, this makes glial cells excellent candidates for targeted therapies to treat CNS disorders. In recent years, the application of gene editing technologies has redefined therapeutic strategies to treat genetic and age-related neurological diseases. In this review, we discuss the advantages and limitations of clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-based gene editing in the treatment of neurodegenerative disorders, focusing on the development of viral- and nanoparticle-based delivery methods for in vivo glial cell targeting.

Transient astrocyte-like NG2 glia subpopulation emerges solely following permanent brain ischemia

NG2 glia display wide proliferation and differentiation potential under physiological and pathological conditions. Here, we examined these two features following different types of brain disorders such as focal cerebral ischemia (FCI), cortical stab wound (SW), and demyelination (DEMY) in 3‐month‐old mice, in which NG2 glia are labeled by tdTomato under the Cspg4 promoter. To compare NG2 glia expression profiles following different CNS injuries, we employed single‐cell RT‐qPCR and self‐organizing Kohonen map analysis of tdTomato‐positive cells isolated from the uninjured cortex/corpus callosum and those after specific injury. Such approach enabled us to distinguish two main cell populations (NG2 glia, oligodendrocytes), each of them comprising four distinct subpopulations. The gene expression profiling revealed that a subpopulation of NG2 glia expressing GFAP, a marker of reactive astrocytes, is only present transiently after FCI. However, following less severe injuries, namely the SW and DEMY, subpopulations mirroring different stages of oligodendrocyte maturation markedly prevail. Such injury‐dependent incidence of distinct subpopulations was also confirmed by immunohistochemistry. To characterize this unique subpopulation of transient astrocyte‐like NG2 glia, we used single‐cell RNA‐sequencing analysis and to disclose their basic membrane properties, the patch‐clamp technique was employed. Overall, we have proved that astrocyte‐like NG2 glia are a specific subpopulation of NG2 glia emerging transiently only following FCI. These cells, located in the postischemic glial scar, are active in the cell cycle and display a current pattern similar to that identified in cortical astrocytes. Astrocyte‐like NG2 glia may represent important players in glial scar formation and repair processes, following ischemia.

Human Oligodendrocytes and Myelin In Vitro to Evaluate Developmental Neurotoxicity

Neurodevelopment is uniquely sensitive to toxic insults and there are concerns that environmental chemicals are contributing to widespread subclinical developmental neurotoxicity (DNT). Increased DNT evaluation is needed due to the lack of such information for most chemicals in common use, but in vivo studies recommended in regulatory guidelines are not practical for the large-scale screening of potential DNT chemicals. It is widely acknowledged that developmental neurotoxicity is a consequence of disruptions to basic processes in neurodevelopment and that testing strategies using human cell-based in vitro systems that mimic these processes could aid in prioritizing chemicals with DNT potential. Myelination is a fundamental process in neurodevelopment that should be included in a DNT testing strategy, but there are very few in vitro models of myelination. Thus, there is a need to establish an in vitro myelination assay for DNT. Here, we summarize the routes of myelin toxicity and the known models to study this particular endpoint.

Oligodendrocyte Secreted Factors Shape Hippocampal GABAergic Neuron Transcriptome and Physiology

Oligodendrocytes form myelin for central nervous system axons and release factors which signal to neurons during myelination. Here, we ask how oligodendroglial factors influence hippocampal GABAergic neuron physiology. In mixed hippocampal cultures, GABAergic neurons fired action potentials (APs) of short duration and received high frequencies of excitatory synaptic events. In purified neuronal cultures without glial cells, GABAergic neuron excitability increased and the frequency of synaptic events decreased. These effects were largely reversed by adding oligodendrocyte conditioned medium (OCM). We compared the transcriptomic signature with the electrophysiological phenotype of single neurons in these three culture conditions. Genes expressed by single pyramidal or GABAergic neurons largely conformed to expected cell-type specific patterns. Multiple genes of GABAergic neurons were significantly downregulated by the transition from mixed cultures containing glial cells to purified neuronal cultures. Levels of these genes were restored by the addition of OCM to purified cultures. Clustering genes with similar changes in expression between different culture conditions revealed processes affected by oligodendroglial factors. Enriched genes are linked to roles in synapse assembly, AP generation, and transmembrane ion transport, including of zinc. These results provide new insight into the molecular targets by which oligodendrocytes influence neuron excitability and synaptic function.

Glial Cells Promote Myelin Formation and Elimination

Building a functional nervous system requires the coordinated actions of many glial cells. In the vertebrate central nervous system (CNS), oligodendrocytes myelinate neuronal axons to increase conduction velocity and provide trophic support. Myelination can be modified by local signaling at the axon-myelin interface, potentially adapting sheaths to support the metabolic needs and physiology of individual neurons. However, neurons and oligodendrocytes are not wholly responsible for crafting the myelination patterns seen in vivo. Other cell types of the CNS, including microglia and astrocytes, modify myelination. In this review, I cover the contributions of non-neuronal, non-oligodendroglial cells to the formation, maintenance, and pruning of myelin sheaths. I address ways that these cell types interact with the oligodendrocyte lineage throughout development to modify myelination. Additionally, I discuss mechanisms by which these cells may indirectly tune myelination by regulating neuronal activity. Understanding how glial-glial interactions regulate myelination is essential for understanding how the brain functions as a whole and for developing strategies to repair myelin in disease.

Building a (w)rapport between neurons and oligodendroglia: Reciprocal interactions underlying adaptive myelination

Myelin, multilayered lipid-rich membrane extensions formed by oligodendrocytes around neuronal axons, is essential for fast and efficient action potential propagation in the central nervous system. Initially thought to be a static and immutable process, myelination is now appreciated to be a dynamic process capable of responding to and modulating neuronal function throughout life. While the importance of this type of plasticity, called adaptive myelination, is now well accepted, we are only beginning to understand the underlying cellular and molecular mechanisms by which neurons communicate experience-driven circuit activation to oligodendroglia and precisely how changes in oligodendrocytes and their myelin refine neuronal function. Here, we review recent findings addressing this reciprocal relationship in which neurons alter oligodendroglial form and oligodendrocytes conversely modulate neuronal function.

Neuron-Oligodendrocyte Communication in Myelination of Cortical GABAergic Cells

Axonal myelination by oligodendrocytes increases the speed and reliability of action potential propagation, and so plays a pivotal role in cortical information processing. The extent and profile of myelination vary between different cortical layers and groups of neurons. Two subtypes of cortical GABAergic neurons are myelinated: fast-spiking parvalbumin-expressing cells and somatostatin-containing cells. The expression of pre-nodes on the axon of these inhibitory cells before myelination illuminates communication between oligodendrocytes and neurons. We explore the consequences of myelination for action potential propagation, for patterns of neuronal connectivity and for the expression of behavioral plasticity.

Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons

The myelination of axons by oligodendrocytes is a highly complex cell-to-cell interaction. Oligodendrocytes and axons have a reciprocal signaling relationship in which oligodendrocytes receive cues from axons that direct their myelination, and oligodendrocytes subsequently shape axonal structure and conduction. Oligodendrocytes are necessary for the maturation of excitatory domains on the axon including nodes of Ranvier, help buffer potassium, and support neuronal energy metabolism. Disruption of the oligodendrocyte-axon unit in traumatic injuries, Alzheimer's disease and demyelinating diseases such as multiple sclerosis results in axonal dysfunction and can culminate in neurodegeneration. In this review, we discuss the mechanisms by which demyelination and loss of oligodendrocytes compromise axons. We highlight the intra-axonal cascades initiated by demyelination that can result in irreversible axonal damage. Both the restoration of oligodendrocyte myelination or neuroprotective therapies targeting these intra-axonal cascades are likely to have therapeutic potential in disorders in which oligodendrocyte support of axons is disrupted.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Decreasing brain iron in multiple sclerosis: The difference between concentration and content in iron MRI

Increased brain iron concentration is often reported concurrently with disease development in multiple sclerosis (MS) and other neurodegenerative diseases. However, it is unclear whether the higher iron concentration in patients stems from an influx of iron into the tissue or a relative reduction in tissue compartments without much iron. By taking into account structural volume, we investigated tissue iron content in the deep gray matter (DGM) over 2 years, and compared findings to previously reported changes in iron concentration. 120 MS patients and 40 age‐ and sex‐matched healthy controls were included. Clinical testing and MRI were performed both at baseline and after 2 years. Overall, iron content was calculated from structural MRI and quantitative susceptibility mapping in the thalamus, caudate, putamen, and globus pallidus. MS patients had significantly lower iron content than controls in the thalamus, with progressive MS patients demonstrating lower iron content than relapsing–remitting patients. Over 2 years, iron content decreased in the DGM of patients with MS, while it tended to increase or remain stable among controls. In the thalamus, decreasing iron content over 2 years was associated with disability progression. Our study showed that temporally increasing magnetic susceptibility in MS should not be considered as evidence for iron influx because it may be explained, at least partially, by disease‐related atrophy. Declining DGM iron content suggests that, contrary to the current understanding, iron is being removed from the DGM in patients with MS.

A role of oligodendrocytes in information processing

Myelinating oligodendrocytes enable fast propagation of action potentials along the ensheathed axons. In addition, oligodendrocytes play diverse non-canonical roles including axonal metabolic support and activity-dependent myelination. An open question remains whether myelination also contributes to information processing in addition to speeding up conduction velocity. Here, we analyze the role of myelin in auditory information processing using paradigms that are also good predictors of speech understanding in humans. We compare mice with different degrees of dysmyelination using acute multiunit recordings in the auditory cortex, in combination with behavioral readouts. We find complex alterations of neuronal responses that reflect fatigue and temporal acuity deficits. We observe partially discriminable but similar deficits in well myelinated mice in which glial cells cannot fully support axons metabolically. We suggest a model in which myelination contributes to sustained stimulus perception in temporally complex paradigms, with a role of metabolically active oligodendrocytes in cortical information processing.

Myelin plasticity: sculpting circuits in learning and memory

Throughout our lifespan, new sensory experiences and learning continually shape our neuronal circuits to form new memories. Plasticity at the level of synapses has been recognized and studied for decades, but recent work has revealed an additional form of plasticity - affecting oligodendrocytes and the myelin sheaths they produce - that plays a crucial role in learning and memory. In this Review, we summarize recent work characterizing plasticity in the oligodendrocyte lineage following sensory experience and learning, the physiological and behavioural consequences of manipulating that plasticity, and the evidence for oligodendrocyte and myelin dysfunction in neurodevelopmental disorders with cognitive symptoms. We also discuss the limitations of existing approaches and the conceptual and technical advances that are needed to move forward this rapidly developing field.

Remyelination in multiple sclerosis: from basic science to clinical translation

The treatment of multiple sclerosis has been transformed by the successful development of immunotherapies that efficiently reduce disease activity and related clinical relapses during the relapsing-remitting phase of the disease. However, the prevention of disability progression, which is due to axonal and neuronal damage and loss, has yet to be achieved and is therapeutically challenging, particularly during the progressive phase of the disease. One strategy to counteract neurodegeneration is to promote neuroprotection by enhancing myelin regeneration, hence restoring nerve conduction and metabolic support to the axon. Animal studies have provided targets for interventions to improve brain and spinal cord remyelination, paving the way for the translation of this research to humans. From these initial and promising forays, further problems have emerged, including questions on how best to design these clinical trials and appropriately measure the outcomes. Solving these problems will need additional work before efficacious pro-remyelination therapies will be ready for people with multiple sclerosis, but there is a real sense of hope that researchers are getting closer to a successful therapy.

Cellular Environment Remodels the Genomic Fabrics of Functional Pathways in Astrocytes

We profiled the transcriptomes of primary mouse cortical astrocytes cultured alone or co-cultured with immortalized precursor oligodendrocytes (Oli-neu cells). Filters between the cell types prevented formation of hetero-cellular gap junction channels but allowed for free exchange of the two culture media. We previously reported that major functional pathways in the Oli-neu cells are remodeled by the proximity of non-touching astrocytes and that astrocytes and oligodendrocytes form a panglial transcriptomic syncytium in the brain. Here, we present evidence that the astrocyte transcriptome likewise changes significantly in the proximity of non-touching Oli-neu cells. Our results indicate that the cellular environment strongly modulates the transcriptome of each cell type and that integration in a heterocellular tissue changes not only the expression profile but also the expression control and networking of the genes in each cell phenotype. The significant decrease of the overall transcription control suggests that in the co-culture astrocytes are closer to their normal conditions from the brain. The Oli-neu secretome regulates astrocyte genes known to modulate neuronal synaptic transmission and remodels calcium, chemokine, NOD-like receptor, PI3K-Akt, and thyroid hormone signaling, as well as actin-cytoskeleton, autophagy, cell cycle, and circadian rhythm pathways. Moreover, the co-culture significantly changes the gene hierarchy in the astrocytes.

Cuprizone-induced oligodendrocyte loss and demyelination impairs recording performance of chronically implanted neural interfaces

Biological inflammation induced during penetrating cortical injury can disrupt functional neuronal and glial activity within the cortex, resulting in potential recording failure of chronically implanted neural interfaces. Oligodendrocytes provide critical support for neuronal health and function through direct contact with neuronal soma and axons within the cortex. Given their fundamental role to regulate neuronal activity via myelin, coupled with their heightened vulnerability to metabolic brain injury due to high energetic demands, oligodendrocytes are hypothesized as a possible source of biological failure in declining recording performances of intracortical microelectrode devices. To determine the extent of their contribution to neuronal activity and function, a cuprizone-inducible model of oligodendrocyte depletion and demyelination in mice was performed prior to microelectrode implantation. At 5 weeks of cuprizone exposure, mice demonstrated significantly reduced cortical oligodendrocyte density and myelin expression. Mice were then implanted with functional recording microelectrodes in the visual cortex and neuronal activity was evaluated up to 7 weeks alongside continued cuprizone administration. Cuprizone-induced oligodendrocyte loss and demyelination was associated with significantly reduced recording performances at the onset of implantation, which remained relatively stable over time. In contast, recording performances for mice on a normal diet were intially elevated before decreasing over time to the recording level of tcuprizone-treated mice. Further electrophysiological analysis revealed deficits in multi-unit firing rates, frequency-dependent disruptions in neuronal oscillations, and altered laminar communication within the cortex of cuprizone-treated mice. Post-mortem immunohistochemistry revealed robust depletion of oligodendrocytes around implanted microelectrode arrays alongside comparable neuronal densities to control mice, suggesting that oligodendrocyte loss was a possible contributor to chronically impaired device performances. This study highlights potentially significant contributions from the oligodendrocyte lineage population concerning the biological integration and long-term functional performance of neural interfacing technology.

High-Frequency Microdomain Ca2+ Transients and Waves during Early Myelin Internode Remodeling

Ensheathment of axons by myelin is a highly complex and multi-cellular process. Cytosolic calcium (Ca²⁺) changes in the myelin sheath have been implicated in myelin synthesis, but the source of this Ca²⁺ and the role of neuronal activity is not well understood. Using one-photon Ca²⁺ imaging, we investigated myelin sheath formation in the mouse somatosensory cortex and found a high rate of spontaneous microdomain Ca²⁺ transients and large-amplitude Ca²⁺ waves propagating along the internode. The frequency of Ca²⁺ transients and waves rapidly declines with maturation and reactivates during remyelination. Unexpectedly, myelin microdomain Ca²⁺ transients occur independent of neuronal action potential generation or network activity but are nearly completely abolished when the mitochondrial permeability transition pores are blocked. These findings are supported by the discovery of mitochondria organelles in non-compacted myelin. Together, the results suggest that myelin microdomain Ca²⁺ signals are cell-autonomously driven by high activity of mitochondria during myelin remodeling.

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Developing myelin sheaths show high rates of calcium transients and calcium waves

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Myelin calcium transients are independent from neuronal activity

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Adaxonal and paranodal myelin contained mitochondria

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Calcium transients require opening of mitochondrial permeability transition pores

A roadmap for potassium buffering/dispersion via the glial network of the CNS

Glia use multiple mechanisms to mediate potassium fluxes that support neuronal function. In addition to changes in potassium levels within synapses, these ions are dynamically dispersed through the interstitial parenchyma, perivascular spaces, leptomeninges, cerebrospinal fluid, choroid plexus, blood, vitreous, and endolymph. Neural circuits drive diversity in the glia that buffer potassium and this is reciprocal. Glia mediate buffering of potassium locally at glial-neuronal interfaces and via widespread networked connections. Control of potassium levels in the central nervous system is mediated by mechanisms operating at various loci with complexity that is difficult to model. However, major components of networked glial buffering are known. The role that potassium buffering plays in homeostasis of the CNS underlies some pathologic phenomena. An overview of potassium fluxes in the CNS is relevant for understanding consequences of pathogenic sequence variants in genes that encode potassium buffering proteins. Potassium flows in the CNS are described as follows: K1, the coordinated potassium fluxes within the astrocytic cradle around the synapse; K2, temporary storage of potassium within astrocytic processes in proposed microdomains; K3, potassium fluxes between oligodendrocytes and astrocytes; K4, potassium fluxes between astrocytes; K5, astrocytic potassium flux mediation of neurovasular coupling; K6, CSF delivery of potassium to perivascular spaces with dispersion to interstitial fluid between astrocytic endfeet; K7, astrocytic delivery of potassium to CSF and K8, choroid plexus (modified glia) regulation of potassium at the blood-CSF barrier. Components, mainly potassium channels, transporters, connexins and modulators, and the pathogenic sequence variants of their genes with the associated diseases are described.

Downregulation of Astrocytic Kir4.1 Potassium Channels Is Associated with Hippocampal Neuronal Hyperexcitability in Type 2 Diabetic Mice

Epilepsy, characterized by recurrent seizures, affects 1% of the general population. Interestingly, 25% of diabetics develop seizures with a yet unknown mechanism. Hyperglycemia downregulates inwardly rectifying potassium channel 4.1 (Kir4.1) in cultured astrocytes. Therefore, the present study aims to determine if downregulation of functional astrocytic Kir4.1 channels occurs in brains of type 2 diabetic mice and could influence hippocampal neuronal hyperexcitability. Using whole-cell patch clamp recording in hippocampal brain slices from male mice, we determined the electrophysiological properties of stratum radiatum astrocytes and CA1 pyramidal neurons. In diabetic mice, astrocytic Kir4.1 channels were functionally downregulated as evidenced by multiple characteristics including depolarized membrane potential, reduced barium-sensitive Kir currents and impaired potassium uptake capabilities of hippocampal astrocytes. Furthermore, CA1 pyramidal neurons from diabetic mice displayed increased spontaneous activity: action potential frequency was ≈9 times higher in diabetic compared with non-diabetic mice and small EPSC event frequency was significantly higher in CA1 pyramidal cells of diabetics compared to non-diabetics. These differences were apparent in control conditions and largely pronounced in response to the pro-convulsant 4-aminopyridine. Our data suggest that astrocytic dysfunction due to downregulation of Kir4.1 channels may increase seizure susceptibility by impairing astrocytic ability to maintain proper extracellular homeostasis.

Mechanical regulation of oligodendrocyte biology

Oligodendrocytes (OL) are a subset of glial cells in the central nervous system (CNS) comprising the brain and spinal cord. The CNS environment is defined by complex biochemical and biophysical cues during development and response to injury or disease. In the last decade, significant progress has been made in understanding some of the key biophysical factors in the CNS that modulate OL biology, including their key role in myelination of neurons. Taken together, those studies offer translational implications for remyelination therapies, pharmacological research, identification of novel drug targets, and improvements in methods to generate human oligodendrocyte progenitor cells (OPCs) and OLs from donor stem cells in vitro. This review summarizes current knowledge of how various physical and mechanical cues affect OL biology and its implications for disease, therapeutic approaches, and generation of human OPCs and OLs.

Connexins-Based Hemichannels/Channels and Their Relationship with Inflammation, Seizures and Epilepsy

Connexins (Cxs) are a family of 21 protein isoforms, eleven of which are expressed in the central nervous system, and they are found in neurons and glia. Cxs form hemichannels (connexons) and channels (gap junctions/electric synapses) that permit functional and metabolic coupling between neurons and astrocytes. Altered Cx expression and function is involved in inflammation and neurological diseases. Cxs-based hemichannels and channels have a relevance to seizures and epilepsy in two ways: First, this pathological condition increases the opening probability of hemichannels in glial cells to enable gliotransmitter release, sustaining the inflammatory process and exacerbating seizure generation and epileptogenesis, and second, the opening of channels favors excitability and synchronization through coupled neurons. These biological events highlight the global pathological mechanism of epilepsy, and the therapeutic potential of Cxs-based hemichannels and channels. Therefore, this review describes the role of Cxs in neuroinflammation and epilepsy and examines how the blocking of channels and hemichannels may be therapeutic targets of anti-convulsive and anti-epileptic treatments.

Oligodendrocytes as A New Therapeutic Target in Schizophrenia: From Histopathological Findings to Neuron-Oligodendrocyte Interaction

Imaging and postmortem studies have revealed disturbed oligodendroglia-related processes in patients with schizophrenia and provided much evidence for disturbed myelination, irregular gene expression, and altered numbers of oligodendrocytes in the brains of schizophrenia patients. Oligodendrocyte deficits in schizophrenia might be a result of failed maturation and disturbed regeneration and may underlie the cognitive deficits of the disease, which are strongly associated with impaired long-term outcome. Cognition depends on the coordinated activity of neurons and interneurons and intact connectivity. Oligodendrocyte precursors form a synaptic network with parvalbuminergic interneurons, and disturbed crosstalk between these cells may be a cellular basis of pathology in schizophrenia. However, very little is known about the exact axon-glial cellular and molecular processes that may be disturbed in schizophrenia. Until now, investigations were restricted to peripheral tissues, such as blood, correlative imaging studies, genetics, and molecular and histological analyses of postmortem brain samples. The advent of human-induced pluripotent stem cells (hiPSCs) will enable functional analysis in patient-derived living cells and holds great potential for understanding the molecular mechanisms of disturbed oligodendroglial function in schizophrenia. Targeting such mechanisms may contribute to new treatment strategies for previously treatment-resistant cognitive symptoms.

Two adhesive systems cooperatively regulate axon ensheathment and myelin growth in the CNS

Central nervous system myelin is a multilayered membrane produced by oligodendrocytes to increase neural processing speed and efficiency, but the molecular mechanisms underlying axonal selection and myelin wrapping are unknown. Here, using combined morphological and molecular analyses in mice and zebrafish, we show that adhesion molecules of the paranodal and the internodal segment work synergistically using overlapping functions to regulate axonal interaction and myelin wrapping. In the absence of these adhesive systems, axonal recognition by myelin is impaired with myelin growing on top of previously myelinated fibers, around neuronal cell bodies and above nodes of Ranvier. In addition, myelin wrapping is disturbed with the leading edge moving away from the axon and in between previously formed layers. These data show how two adhesive systems function together to guide axonal ensheathment and myelin wrapping, and provide a mechanistic understanding of how the spatial organization of myelin is achieved.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Activity-dependent neuronal Klotho enhances astrocytic aerobic glycolysis

Mutations of the β-glucuronidase protein α-Klotho have been associated with premature aging, and altered cognitive function. Although highly expressed in specific areas of the brain, Klotho functions in the central nervous system remain unknown. Here, we show that cultured hippocampal neurons respond to insulin and glutamate stimulation by elevating Klotho protein levels. Conversely, AMPA and NMDA antagonism suppress neuronal Klotho expression. We also provide evidence that soluble Klotho enhances astrocytic aerobic glycolysis by hindering pyruvate metabolism through the mitochondria, and stimulating its processing by lactate dehydrogenase. Pharmacological inhibition of FGFR1, Erk phosphorylation, and monocarboxylic acid transporters prevents Klotho-induced lactate release from astrocytes. Taken together, these data suggest Klotho is a potential new player in the metabolic coupling between neurons and astrocytes. Neuronal glutamatergic activity and insulin modulation elicit Klotho release, which in turn stimulates astrocytic lactate formation and release. Lactate can then be used by neurons and other cells types as a metabolic substrate.

Perineuronal oligodendrocytes in health and disease: the journey so far

Perineuronal oligodendrocytes (pn-Ols) are located in the cerebral gray matter in close proximity to neuronal perikarya and less frequently near dendrites and neurites. Although their morphology is indistinguishable from that of other oligodendrocytes, it is not known if pn-Ols have a similar or different cell signature from that of typical myelinating oligodendroglial cells. In this review, we discussed the potential roles of these cells in myelination under normal and pathophysiologic conditions as functional and nutritional supporters of neurons, as restrainers of neuronal firing, and as possible players in glutamate-glutamine homeostasis. We also highlighted the occurrences in which perineuronal oligodendroglia are altered, such as in experimental demyelination, multiple sclerosis, cerebral ischemia, epilepsy, Alzheimer’s disease, schizophrenia, major depression, and bipolar disorder.

Neuronal input triggers Ca2+ influx through AMPA receptors and voltage-gated Ca2+ channels in oligodendrocytes

Communication between neurons and developing oligodendrocytes (OLs) leading to OL Ca²⁺ rise is critical for axon myelination and OL development. Here, we investigate signaling factors and sources of Ca²⁺ rise in OLs in the mouse brainstem. Glutamate puff or axon fiber stimulation induces a Ca²⁺ rise in pre‐myelinating OLs, which is primarily mediated by Ca²⁺‐permeable AMPA receptors. During glutamate application, inward currents via AMPA receptors and elevated extracellular K⁺ caused by increased neuronal activity collectively lead to OL depolarization, triggering Ca²⁺ influx via P/Q‐ and L‐type voltage‐gated Ca²⁺ (Ca_v) channels. Thus, glutamate is a key signaling factor in dynamic communication between neurons and OLs that triggers Ca²⁺ transients via AMPARs and Ca_v channels in developing OLs. The results provide a mechanism for OL Ca²⁺ dynamics in response to neuronal input, which has implications for OL development and myelination.

Loss of Shh signaling in the neocortex reveals heterogeneous cell recovery responses from distinct oligodendrocyte populations

The majority of oligodendrocytes in the neocortex originate from neural progenitors that reside in the dorsal forebrain. We recently showed that Sonic Hedgehog (Shh) signaling in these dorsal progenitors is required to produce normal numbers of neocortical oligodendrocytes during embryonic development. Conditional deletion of the Shh signaling effector, Smo, in dorsal progenitors caused a dramatic reduction in oligodendrocyte numbers in the embryonic neocortex. In the current study, we show that the depleted oligodendrocyte lineage in Smo conditional mutants is able to recover to control numbers over time. This eventual recovery is achieved in part by expansion of the ventrally-derived wild-type lineage that normally makes up a minority of the total oligodendrocyte population. However, we find that the remaining dorsally-derived mutant cells also increase in numbers over time to contribute equally to the recovery of the total population. Additionally, we found that the ways in which the dorsal and ventral sources cooperate to achieve recovery is different for distinct populations of oligodendrocyte-lineage cells. Oligodendrocyte precursor cells (OPCs) in the neocortical white matter recover completely by expansion of the remaining dorsally-derived Smo mutant cells. On the other hand, mature oligodendrocytes in the white and gray matter recover through an equal contribution from dorsal mutant and ventral wild-type lineages. Interestingly, the only population that did not make a full recovery was OPCs in the gray matter. We find that gray matter OPCs are less proliferative in Smo cKO mutants compared to controls, which may explain their inability to fully recover. Our data indicate that certain populations of the dorsal oligodendrocyte lineage are more affected by loss of Shh signaling than others. Furthermore, these studies shed new light on the complex relationship between dorsal and ventral sources of oligodendrocytes in the developing neocortex.