Oligodendrocyte precursor cells are accurate sensors of local K+ in mature gray matter

Postnatal Development of Synaptic Plasticity at Hippocampal CA1 Synapses: Correlation of Learning Performance with Pathway-Specific Plasticity

To determine the critical timing for learning and the associated synaptic plasticity, we analyzed developmental changes in learning together with training-induced plasticity. Rats were subjected to an inhibitory avoidance (IA) task prior to weaning. While IA training did not alter latency at postnatal day (PN) 16, there was a significant increase in latency from PN 17, indicating a critical day for IA learning between PN 16 and 17. One hour after training, acute hippocampal slices were prepared for whole-cell patch clamp analysis following the retrieval test. In the presence of tetrodotoxin (0.5 µM), miniature excitatory postsynaptic currents (mEPSCs) and inhibitory postsynaptic currents (mIPSCs) were sequentially recorded from the same CA1 neuron. Although no changes in the amplitude of mEPSCs or mIPSCs were observed at PN 16 and 21, significant increases in both excitatory and inhibitory currents were observed at PN 23, suggesting a specific critical day for training-induced plasticity between PN 21 and 23. Training also increased the diversity of postsynaptic currents at PN 23 but not at PN 16 and 21, demonstrating a critical day for training-induced increase in the information entropy of CA1 neurons. Finally, we analyzed the plasticity at entorhinal cortex layer III (ECIII)-CA1 or CA3-CA1 synapses for each individual rat. At either ECIII-CA1 or CA3-CA1 synapses, a significant correlation between mean α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartic acid (AMPA/NMDA) ratio and learning outcomes emerged at PN 23 at both synapses, demonstrating a critical timing for the direct link between AMPA receptor-mediated synaptic plasticity and learning efficacy. Here, we identified multiple critical periods with respect to training-induced synaptic plasticity and delineated developmental trajectories of learning mechanisms at hippocampal CA1 synapses.

Dmd mdx mice have defective oligodendrogenesis, delayed myelin compaction and persistent hypomyelination

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene, resulting in the loss of dystrophin, a large cytosolic protein that links the cytoskeleton to extracellular matrix receptors in skeletal muscle. Aside from progressive muscle damage, many patients with DMD also have neurological deficits of unknown etiology. To investigate potential mechanisms for DMD neurological deficits, we assessed postnatal oligodendrogenesis and myelination in the Dmdmdx mouse model. In the ventricular-subventricular zone (V-SVZ) stem cell niche, we found that oligodendrocyte progenitor cell (OPC) production was deficient, with reduced OPC densities and proliferation, despite a normal stem cell niche organization. In the Dmdmdx corpus callosum, a large white matter tract adjacent to the V-SVZ, we also observed reduced OPC proliferation and fewer oligodendrocytes. Transmission electron microscopy further revealed significantly thinner myelin, an increased number of abnormal myelin structures and delayed myelin compaction, with hypomyelination persisting into adulthood. Our findings reveal alterations in oligodendrocyte development and myelination that support the hypothesis that changes in diffusion tensor imaging seen in patients with DMD reflect developmental changes in myelin architecture.

Norepinephrine modulates calcium dynamics in cortical oligodendrocyte precursor cells promoting proliferation during arousal in mice

Oligodendrocytes, the myelinating cells of the central nervous system (CNS), are generated from oligodendrocyte precursor cells (OPCs) that express neurotransmitter receptors. However, the mechanisms that affect OPC activity in vivo and the physiological roles of neurotransmitter signaling in OPCs are unclear. In this study, we generated a transgenic mouse line that expresses membrane-anchored GCaMP6s in OPCs and used longitudinal two-photon microscopy to monitor OPC calcium (Ca2+) dynamics in the cerebral cortex. OPCs exhibit focal and transient Ca2+ increases within their processes that are enhanced during locomotion-induced increases in arousal. The Ca2+ transients occur independently of excitatory neuron activity, rapidly decline when OPCs differentiate and are inhibited by anesthesia, sedative agents or noradrenergic receptor antagonists. Conditional knockout of α1A adrenergic receptors in OPCs suppresses spontaneous and locomotion-induced Ca2+ increases and reduces OPC proliferation. Our results demonstrate that OPCs are directly modulated by norepinephrine in vivo to enhance Ca2+ dynamics and promote population homeostasis.

Oligodendrocyte precursor cells: the multitaskers in the brain

In the central nervous system, oligodendrocyte precursor cells (OPCs) are recognized as the progenitors responsible for the generation of oligodendrocytes, which play a critical role in myelination. Extensive research has shed light on the mechanisms underlying OPC proliferation and differentiation into mature myelin-forming oligodendrocytes. However, recent advances in the field have revealed that OPCs have multiple functions beyond their role as progenitors, exerting control over neural circuits and brain function through distinct pathways. This review aims to provide a comprehensive understanding of OPCs by first introducing their well-established features. Subsequently, we delve into the emerging roles of OPCs in modulating brain function in both healthy and diseased states. Unraveling the cellular and molecular mechanisms by which OPCs influence brain function holds great promise for identifying novel therapeutic targets for central nervous system diseases.

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

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

Inward rectifying Kir4.1 channels regulate oligodendrocyte precursor cell differentiation and CNS myelination in vivo

The functions of K_ir4.1 in oligodendrocyte development have been in controversial. We recently reported that inhibiting K_ir4.1 impeded oligodendrocyte precursor cell (OPC) differentiation and oligodendrocyte (OL) maturation, due to K_ir4.1 altering intracellular pH of OPCs through Na⁺/H⁺ exchangers. However, our conclusion was limited by in vitro observation, thereby it becomes necessary to seek in vivo evidence to determine the roles of K_ir4.1 on OPC development and CNS myelination. Here, we used Olig1-Cre to knockout K_ir4.1 in OPCs from the early developmental stage. We found that the cell-specific deletion of K_ir4.1 significantly impeded OPC differentiation and reduced the number of mature OLs in the cerebral cortex and the corpus callosum. Hence, our in vivo evidence supports that K_ir4.1 can regulate OPC differentiation and is essential to CNS myelination.

Kir4.1 is specifically expressed and active in non-myelinating Schwann cells

Non-myelinating Schwann cells (NMSC) play important roles in peripheral nervous system formation and function. However, the molecular identity of these cells remains poorly defined. We provide evidence that Kir4.1, an inward-rectifying K+ channel encoded by the KCNJ10 gene, is specifically expressed and active in NMSC. Immunostaining revealed that Kir4.1 is present in terminal/perisynaptic SCs (TPSC), synaptic glia at neuromuscular junctions (NMJ), but not in myelinating SCs (MSC) of adult mice. To further examine the expression pattern of Kir4.1, we generated BAC transgenic Kir4.1-CreERT2 mice and crossed them to the tdTomato reporter line. Activation of CreERT2 with tamoxifen after the completion of myelination onset led to robust expression of tdTomato in NMSC, including Remak Schwann cells (RSC) along peripheral nerves and TPSC, but not in MSC. In contrast, activating CreERT2 before and during the onset of myelination led to tdTomato expression in NMSC and MSC. These observations suggest that immature SC express Kir4.1, and its expression is then downregulated selectively in myelin-forming SC. In support, we found that while activating CreERT2 induces tdTomato expression in immature SC, it fails to induce tdTomato in MSC associated with sensory axons in culture. NMSC derived from neonatal sciatic nerve were shown to express Kir4.1 and exhibit barium-sensitive inwardly rectifying macroscopic K+ currents. Thus, this study identified Kir4.1 as a potential modulator of immature SC and NMSC function. Additionally, it established a novel transgenic mouse line to introduce or delete genes in NMSC.

Dysfunction of NG2 glial cells affects neuronal plasticity and behavior

NG2 glia represents a distinct type of macroglial cells in the CNS and is unique among glia because they receive synaptic input from neurons. They are abundantly present in white and gray matter. While the majority of white matter NG2 glia differentiates into oligodendrocytes, the physiological impact of gray matter NG2 glia and their synaptic input are still ill defined. Here, we asked whether dysfunctional NG2 glia affect neuronal signaling and behavior. We generated mice with inducible deletion of the K⁺ channel Kir4.1 in NG2 glia and performed comparative electrophysiological, immunohistochemical, molecular and behavioral analyses. Kir4.1 was deleted at postnatal day 23-26 (recombination efficiency about 75%) and mice were investigated 3-8 weeks later. Notably, these mice with dysfunctional NG2 glia demonstrated improved spatial memory as revealed by testing new object location recognition while working and social memory remained unaffected. Focussing on the hippocampus, we found that loss of Kir4.1 potentiated synaptic depolarizations of NG2 glia and stimulated the expression of myelin basic protein while proliferation and differentiation of hippocampal NG2 glia remained largely unaffected. Mice with targeted deletion of the K⁺ channel in NG2 glia showed impaired long-term potentiation at CA3-CA1 synapses, which could be fully rescued by extracellular application of a TrkB receptor agonist. Our data demonstrate that proper NG2 glia function is important for normal brain function and behavior.

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).

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

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

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.

EAST/SeSAME Syndrome and Beyond: The Spectrum of Kir4.1- and Kir5.1-Associated Channelopathies

In 2009, two groups independently linked human mutations in the inwardly rectifying K+ channel Kir4.1 (gene name KCNJ10) to a syndrome affecting the central nervous system (CNS), hearing, and renal tubular salt reabsorption. The autosomal recessive syndrome has been named EAST (epilepsy, ataxia, sensorineural deafness, and renal tubulopathy) or SeSAME syndrome (seizures, sensorineural deafness, ataxia, intellectual disability, and electrolyte imbalance), accordingly. Renal dysfunction in EAST/SeSAME patients results in loss of Na+, K+, and Mg2+ with urine, activation of the renin-angiotensin-aldosterone system, and hypokalemic metabolic alkalosis. Kir4.1 is highly expressed in affected organs: the CNS, inner ear, and kidney. In the kidney, it mostly forms heteromeric channels with Kir5.1 (KCNJ16). Biallelic loss-of-function mutations of Kir5.1 can also have disease significance, but the clinical symptoms differ substantially from those of EAST/SeSAME syndrome: although sensorineural hearing loss and hypokalemia are replicated, there is no alkalosis, but rather acidosis of variable severity; in contrast to EAST/SeSAME syndrome, the CNS is unaffected. This review provides a framework for understanding some of these differences and will guide the reader through the growing literature on Kir4.1 and Kir5.1, discussing the complex disease mechanisms and the variable expression of disease symptoms from a molecular and systems physiology perspective. Knowledge of the pathophysiology of these diseases and their multifaceted clinical spectrum is an important prerequisite for making the correct diagnosis and forms the basis for personalized therapies.

Cell of all trades: oligodendrocyte precursor cells in synaptic, vascular, and immune function

Oligodendrocyte precursor cells (OPCs) are not merely a transitory progenitor cell type, but rather a distinct and heterogeneous population of glia with various functions in the developing and adult central nervous system. In this review, we discuss the fate and function of OPCs in the brain beyond their contribution to myelination. OPCs are electrically sensitive, form synapses with neurons, support blood-brain barrier integrity, and mediate neuroinflammation. We explore how sex and age may influence OPC activity, and we review how OPC dysfunction may play a primary role in numerous neurological and neuropsychiatric diseases. Finally, we highlight areas of future research.

Microglia-neuron interaction at nodes of Ranvier depends on neuronal activity through potassium release and contributes to remyelination

Microglia, the resident immune cells of the central nervous system, are key players in healthy brain homeostasis and plasticity. In neurological diseases, such as Multiple Sclerosis, activated microglia either promote tissue damage or favor neuroprotection and myelin regeneration. The mechanisms for microglia-neuron communication remain largely unkown. Here, we identify nodes of Ranvier as a direct site of interaction between microglia and axons, in both mouse and human tissues. Using dynamic imaging, we highlight the preferential interaction of microglial processes with nodes of Ranvier along myelinated fibers. We show that microglia-node interaction is modulated by neuronal activity and associated potassium release, with THIK-1 ensuring their microglial read-out. Altered axonal K⁺ flux following demyelination impairs the switch towards a pro-regenerative microglia phenotype and decreases remyelination rate. Taken together, these findings identify the node of Ranvier as a major site for microglia-neuron interaction, that may participate in microglia-neuron communication mediating pro-remyelinating effect of microglia after myelin injury.

Microglia are important for brain homeostasis and plasticity. The mechanisms underlying microglia-neuron interactions are still unclear. Here, the authors show that microglia preferentially interact with the nodes of Ranvier along axons. This interaction is modulated by neuronal activity and contributes to remyelination in mice.

Ion Channels as New Attractive Targets to Improve Re-Myelination Processes in the Brain

Multiple sclerosis (MS) is the most demyelinating disease of the central nervous system (CNS) characterized by neuroinflammation. Oligodendrocyte progenitor cells (OPCs) are cycling cells in the developing and adult CNS that, under demyelinating conditions, migrate to the site of lesions and differentiate into mature oligodendrocytes to remyelinate damaged axons. However, this process fails during disease chronicization due to impaired OPC differentiation. Moreover, OPCs are crucial players in neuro-glial communication as they receive synaptic inputs from neurons and express ion channels and neurotransmitter/neuromodulator receptors that control their maturation. Ion channels are recognized as attractive therapeutic targets, and indeed ligand-gated and voltage-gated channels can both be found among the top five pharmaceutical target groups of FDA-approved agents. Their modulation ameliorates some of the symptoms of MS and improves the outcome of related animal models. However, the exact mechanism of action of ion-channel targeting compounds is often still unclear due to the wide expression of these channels on neurons, glia, and infiltrating immune cells. The present review summarizes recent findings in the field to get further insights into physio-pathophysiological processes and possible therapeutic mechanisms of drug actions.

Combining whole-cell patch clamp and dye loading in acute brain slices with bulk RNA sequencing in embryonic to aged mice

Single-cell electrophysiological recordings combined with dye loading and immunohistochemistry provide unparalleled single-cell resolution of cell physiology, morphology, location, and protein expression. When correlated with bulk RNA sequencing, these data can define cell identity and function. Here, we describe a protocol to prepare acute brain slices from embryonic and postnatal mice for whole-cell patch clamp, dye loading and post-hoc immunohistochemistry, and cell isolation for bulk RNA sequencing. While we focus on oligodendrocyte precursor cells, this protocol is applicable to other brain cells. For complete details on the use and execution of this protocol, please refer to Spitzer et al. (2019).

A Matter of State: Diversity in Oligodendrocyte Lineage Cells

Oligodendrocyte precursor cells (OPCs) give rise to oligodendrocytes which myelinate axons in the central nervous system. Although classically thought to be a homogeneous population, OPCs are reported to have different developmental origins and display regional and temporal diversity in their transcriptome, response to growth factors, and physiological properties. Similarly, evidence is accumulating that myelinating oligodendrocytes display transcriptional heterogeneity. Analyzing this reported heterogeneity suggests that OPCs, and perhaps also myelinating oligodendrocytes, may exist in different functional cell states. Here, we review the evidence indicating that OPCs and oligodendrocytes are diverse, and we discuss the implications of functional OPC states for myelination in the adult brain and for myelin repair.

A hypothesis for the role of axon demyelination in seizure generation

Loss of myelin and altered oligodendrocyte distribution in the cerebral cortex are commonly observed both in postsurgical tissue derived from different focal epilepsies (such as focal cortical dysplasias and tuberous sclerosis) and in animal models of focal epilepsy. Moreover, seizures are a frequent symptom in demyelinating diseases, such as multiple sclerosis, and in animal models of demyelination and oligodendrocyte dysfunction. Finally, the excessive activity reported in demyelinated axons may promote hyperexcitability. We hypothesize that the extracellular potassium rise generated during epileptiform activity may be amplified by the presence of axons without appropriate myelin coating and by alterations in oligodendrocyte function. This process could facilitate the triggering of recurrent spontaneous seizures in areas of altered myelination and could result in further demyelination, thus promoting epileptogenesis.

Apamin administration impact on miR-219 and miR-155-3p expression in cuprizone induced multiple sclerosis model

Multiple sclerosis (MS) is a chronic debilitating disease that attacks the central nervous system. This study aims to investigate miR-219 and miR-155-3p expression levels involved in the myelination process following the administration of apamin peptide in the model of multiple sclerosis disease. Forty-four 8 week C57BL/6 male mice (22 ± 5 g) randomly divided into six groups. Apamin (100 µg/kg/BW) was administered intraperitoneally as a co-treatment during phase I (demyelination) or post-treatment phase II (remyelination) twice a week in cuprizone induced MS model. At the end of study myelin content and microRNA expression levels were measured with LFB staining and quantitative Real-Time PCR method, respectively. It was observed that the intended microRNAs were dysregulated during the different phases of disease induction. After 6 weeks of cuprizone exposure, miR-219 downregulated in phase I in comparison with the negative control. On the other hand, the apamin co-treatment significantly inhibit the miR-155-3p upregulation during the phase I as compared with the cuprizone group (p < 0.0001). Apamin has more impact on the miR155-3p reduction in phase I than miR-219 elevation in phase II. It could be considered as a therapeutic option for decreasing plaque formation during the exacerbation phase of the MS disease. Apamin has more impact on the miR155-3p reduction in phase I than miR-219 elevation in phase II. It could be considered as a therapeutic option for decreasing plaque formation during the exacerbation phase of the MS disease.

Myelination of parvalbumin interneurons shapes the function of cortical sensory inhibitory circuits

Myelination of projection neurons by oligodendrocytes is key to optimize action potential conduction over long distances. However, a large fraction of myelin enwraps the axons of parvalbumin-positive fast-spiking interneurons (FSI), exclusively involved in local cortical circuits. Whether FSI myelination contributes to the fine‐tuning of intracortical networks is unknown. Here we demonstrate that FSI myelination is required for the establishment and maintenance of the powerful FSI-mediated feedforward inhibition of cortical sensory circuits. The disruption of GABAergic synaptic signaling of oligodendrocyte precursor cells prior to myelination onset resulted in severe FSI myelination defects characterized by longer internodes and nodes, aberrant myelination of branch points and proximal axon malformation. Consequently, high-frequency FSI discharges as well as FSI-dependent postsynaptic latencies and strengths of excitatory neurons were reduced. These dysfunctions generated a strong excitation-inhibition imbalance that correlated with whisker-dependent texture discrimination impairments. FSI myelination is therefore critical for the function of mature cortical inhibitory circuits.

Myelination optimizes conduction speed of excitatory neurons. However, whether myelination of interneurons (INs) refines cortical networks is unclear. Here, the authors show that INs myelination shapes feedforward inhibition of mouse cortical sensory circuits and impacts whisker-mediated behaviour.

Dysregulated Glial Differentiation in Schizophrenia May Be Relieved by Suppression of SMAD4- and REST-Dependent Signaling

Astrocytic differentiation is developmentally impaired in patients with childhood-onset schizophrenia (SCZ). To determine why, we used genetic gain- and loss-of-function studies to establish the contributions of differentially expressed transcriptional regulators to the defective differentiation of glial progenitor cells (GPCs) produced from SCZ patient-derived induced pluripotent cells (iPSCs). Negative regulators of the bone morphogenetic protein (BMP) pathway were upregulated in SCZ GPCs, including BAMBI, FST, and GREM1, whose overexpression retained SCZ GPCs at the progenitor stage. SMAD4 knockdown (KD) suppressed the production of these BMP inhibitors by SCZ GPCs and rescued normal astrocytic differentiation. In addition, the BMP-regulated transcriptional repressor REST was upregulated in SCZ GPCs, and its KD similarly restored normal glial differentiation. REST KD also rescued potassium-transport-associated gene expression and K⁺ uptake, which were otherwise deficient in SCZ glia. These data suggest that the glial differentiation defect in childhood-onset SCZ, and its attendant disruption in K⁺ homeostasis, may be rescued by targeting BMP/SMAD4- and REST-dependent transcription.

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.

The voltage-gated calcium channel CaV1.2 promotes adult oligodendrocyte progenitor cell survival in the mouse corpus callosum but not motor cortex

Throughout life, oligodendrocyte progenitor cells (OPCs) proliferate and differentiate into myelinating oligodendrocytes. OPCs express cell surface receptors and channels that allow them to detect and respond to neuronal activity, including voltage‐gated calcium channel (VGCC)s. The major L‐type VGCC expressed by developmental OPCs, CaV1.2, regulates their differentiation. However, it is unclear whether CaV1.2 similarly influences OPC behavior in the healthy adult central nervous system (CNS). To examine the role of CaV1.2 in adulthood, we conditionally deleted this channel from OPCs by administering tamoxifen to P60 Cacna1c^fl/fl (control) and Pdgfrα‐CreER:: Cacna1c^fl/fl (CaV1.2‐deleted) mice. Whole cell patch clamp analysis revealed that CaV1.2 deletion reduced L‐type voltage‐gated calcium entry into adult OPCs by ~60%, confirming that it remains the major L‐type VGCC expressed by OPCs in adulthood. The conditional deletion of CaV1.2 from adult OPCs significantly increased their proliferation but did not affect the number of new oligodendrocytes produced or influence the length or number of internodes they elaborated. Unexpectedly, CaV1.2 deletion resulted in the dramatic loss of OPCs from the corpus callosum, such that 7 days after tamoxifen administration CaV1.2‐deleted mice had an OPC density ~42% that of control mice. OPC density recovered within 2 weeks of CaV1.2 deletion, as the lost OPCs were replaced by surviving CaV1.2‐deleted OPCs. As OPC density was not affected in the motor cortex or spinal cord, we conclude that calcium entry through CaV1.2 is a critical survival signal for a subpopulation of callosal OPCs but not for all OPCs in the mature CNS.

Candidate CSPG4 mutations and induced pluripotent stem cell modeling implicate oligodendrocyte progenitor cell dysfunction in familial schizophrenia

Schizophrenia is highly heritable, yet its underlying pathophysiology remains largely unknown. Among the most well-replicated findings in neurobiological studies of schizophrenia are deficits in myelination and white matter integrity; however, direct etiological genetic and cellular evidence has thus far been lacking. Here, we implement a family-based approach for genetic discovery in schizophrenia combined with functional analysis using induced pluripotent stem cells (iPSCs). We observed familial segregation of two rare missense mutations in Chondroitin Sulfate Proteoglycan 4 (CSPG4) (c.391G > A [p.A131T], MAF 7.79 × 10-5 and c.2702T > G [p.V901G], MAF 2.51 × 10-3). The CSPG4A131T mutation was absent from the Swedish Schizophrenia Exome Sequencing Study (2536 cases, 2543 controls), while the CSPG4V901G mutation was nominally enriched in cases (11 cases vs. 3 controls, P = 0.026, OR 3.77, 95% CI 1.05-13.52). CSPG4/NG2 is a hallmark protein of oligodendrocyte progenitor cells (OPCs). iPSC-derived OPCs from CSPG4A131T mutation carriers exhibited abnormal post-translational processing (P = 0.029), subcellular localization of mutant NG2 (P = 0.007), as well as aberrant cellular morphology (P = 3.0 × 10-8), viability (P = 8.9 × 10-7), and myelination potential (P = 0.038). Moreover, transfection of healthy non-carrier sibling OPCs confirmed a pathogenic effect on cell survival of both the CSPG4A131T (P = 0.006) and CSPG4V901G (P = 3.4 × 10-4) mutations. Finally, in vivo diffusion tensor imaging of CSPG4A131T mutation carriers demonstrated a reduction of brain white matter integrity compared to unaffected sibling and matched general population controls (P = 2.2 × 10-5). Together, our findings provide a convergence of genetic and functional evidence to implicate OPC dysfunction as a candidate pathophysiological mechanism of familial schizophrenia.

Electrophysiological Properties of Adult Zebrafish Oligodendrocyte Progenitor Cells

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

Physiology of Oligodendroglia

Oligodendrocytes are the myelinating cells of the CNS, producing the insulating myelin sheath that facilitates rapid electrical conduction of axonal action potentials. Oligodendrocytes arise from oligodendrocyte progenitor cells (OPCs) under the control of multiple factors, including neurotransmitters and other neuron-derived factors. A significant population of OPCs persists in the adult CNS, where they are often referred to as NG2-glia, because they are identified by their expression of the NG2 chondroitin sulphate proteoglycan (CSPG4). In the adult brain, the primary function of NG2-glia is the life-long generation of oligodendrocytes to replace myelin lost through natural 'wear and tear' and pathology, as well as to provide new oligodendrocytes to myelinate new connections formed in response to new life experiences. NG2-glia contact synapses and respond to neurotransmitters and potassium released during neuronal transmission; to this end, NG2-glia (OPCs) express multiple neurotransmitter receptors and ion channels, with prominent roles being identified for glutamatergic signalling and potassium channels in oligodendrocyte differentiation. Myelinating oligodendrocytes also express a wide range of neurotransmitter receptors and ion channels, together with transporters and gap junctions; together, these have critical functions in cellular ion and water homeostasis, as well as metabolism, which is essential for maintaining myelin and axon integrity. An overriding theme is that oligodendrocyte function and myelination is not only essential for rapid axonal conduction, but is essential for learning and the long-term integrity of axons and neurones. Hence, myelination underpins cognitive function and the massive computing power of the human brain and myelin loss has devastating effects on CNS function. This chapter focuses on normal oligodendrocyte physiology.

Two-Step Generation of Oligodendrocyte Progenitor Cells From Mouse Fibroblasts for Spinal Cord Injury

Oligodendrocyte progenitor cells (OPCs) are attracting attention as the ideal cell therapy for spinal cord injury (SCI). Recently, advanced reprogramming and differentiation techniques have made it possible to generate therapeutic cells for treating SCI. In the present study, we used directly-induced neural stem cells (DNSCs) from fibroblasts to establish OPCs (DN-OPCs) capable of proliferation and confirmed their OPC-specific characteristics. Also, we evaluated the effect of transplanted DN-OPCs on SCI in rats. The DN-OPCs exhibited an OPC-specific phenotype and electrophysiological function and could be differentiated into oligodendrocytes. In the SCI model, transplanted DN-OPCs improved behavior recovery, and showed engraftment into the host spinal cord with expression of myelin basic protein. These results suggest that DN-OPCs could be a new source of potentially useful cells for treating SCI.

NG2 Glia: Novel Roles beyond Re-/Myelination

Neuron-glia antigen 2-expressing glial cells (NG2 glia) serve as oligodendrocyte progenitors during development and adulthood. However, recent studies have shown that these cells represent not only a transitional stage along the oligodendroglial lineage, but also constitute a specific cell type endowed with typical properties and functions. Namely, NG2 glia (or subsets of NG2 glia) establish physical and functional interactions with neurons and other central nervous system (CNS) cell types, that allow them to constantly monitor the surrounding neuropil. In addition to operating as sensors, NG2 glia have features that are expected for active modulators of neuronal activity, including the expression and release of a battery of neuromodulatory and neuroprotective factors. Consistently, cell ablation strategies targeting NG2 glia demonstrate that, beyond their role in myelination, these cells contribute to CNS homeostasis and development. In this review, we summarize and discuss the advancements achieved over recent years toward the understanding of such functions, and propose novel approaches for further investigations aimed at elucidating the multifaceted roles of NG2 glia.

Kir4.1 channels in NG2-glia play a role in development, potassium signaling, and ischemia-related myelin loss

The contribution of the inwardly rectifying K⁺ channel subtype Kir4.1 has been focused mainly on astrocytes, where they play important roles in the maintenance of resting membrane potential, extracellular K⁺ uptake, and facilitation of glutamate uptake in the central nervous system. Here, we report the role of Kir4.1 channels in NG2-glia during brain development, potassium signaling, and in an ischemic stroke disease model. Kir4.1 channels are widely expressed in NG2-glia during brain development. In the adult mouse hippocampus, Kir4.1 channels in NG2-glia constitute more than 80% of K⁺ channels inward currents. This large portion of Kir4.1 channel currents exhibits a deficit in NG2-glia as an initial response in a transient ischemic mouse model. Further evidence indicates that Kir4.1 deficits in NG2-glia potentially cause axonal myelin loss in ischemia through the association with oligodendrocyte-specific protein (OSP/Claudin-11), which unravels a potential therapeutic target in the treatment of ischemic stroke.

Feier Song and colleagues have examined Kir4.1 channels in the mouse brain, and found global expression of functional channels during development. They also show that depletion of Kir4.1 channels impacts demyelination in ischemic stroke

Changes in white matter in mice resulting from low-frequency brain stimulation

Recent reports have begun to elucidate mechanisms by which learning and experience produce white matter changes in the brain. We previously reported changes in white matter surrounding the anterior cingulate cortex in humans after 2-4 weeks of meditation training. We further found that low-frequency optogenetic stimulation of the anterior cingulate in mice increased time spent in the light in a light/dark box paradigm, suggesting decreased anxiety similar to what is observed following meditation training. Here, we investigated the impact of this stimulation at the cellular level. We found that laser stimulation in the range of 1-8 Hz results in changes to subcortical white matter projection fibers in the corpus callosum. Specifically, stimulation resulted in increased oligodendrocyte proliferation, accompanied by a decrease in the g-ratio within the corpus callosum underlying the anterior cingulate cortex. These results suggest that low-frequency stimulation can result in activity-dependent remodeling of myelin, giving rise to enhanced connectivity and altered behavior.

Oligodendrocytes control potassium accumulation in white matter and seizure susceptibility

The inwardly rectifying K⁺ channel K_ir4.1 is broadly expressed by CNS glia and deficits in K_ir4.1 lead to seizures and myelin vacuolization. However, the role of oligodendrocyte K_ir4.1 channels in controlling myelination and K⁺ clearance in white matter has not been defined. Here, we show that selective deletion of K_ir4.1 from oligodendrocyte progenitors (OPCs) or mature oligodendrocytes did not impair their development or disrupt the structure of myelin. However, mice lacking oligodendrocyte K_ir4.1 channels exhibited profound functional impairments, including slower clearance of extracellular K⁺ and delayed recovery of axons from repetitive stimulation in white matter, as well as spontaneous seizures, a lower seizure threshold, and activity-dependent motor deficits. These results indicate that K_ir4.1 channels in oligodendrocytes play an important role in extracellular K⁺ homeostasis in white matter, and that selective loss of this channel from oligodendrocytes is sufficient to impair K⁺ clearance and promote seizures.

Dissipation of transmembrane potassium gradient is the main cause of cerebral ischemia-induced depolarization in astrocytes and neurons

Membrane potential (V_M) depolarization occurs immediately following cerebral ischemia and is devastating for the astrocyte homeostasis and neuronal signaling. Previously, an excessive release of extracellular K⁺ and glutamate has been shown to underlie an ischemia-induced V_M depolarization. Ischemic insults should impair membrane ion channels and disrupt the physiological ion gradients. However, their respective contribution to ischemia-induced neuronal and glial depolarization and loss of neuronal excitability are unanswered questions. A short-term oxygen-glucose deprivation (OGD) was used for the purpose of examining the acute effect of ischemic conditions on ion channel activity and physiological K⁺ gradient in neurons and glial cells. We show that a 30 min OGD treatment exerted no measurable damage to the function of membrane ion channels in neurons, astrocytes, and NG2 glia. As a result of the resilience of membrane ion channels, neuronal spikes last twice as long as our previously reported 15 min time window. In the electrophysiological analysis, a 30 min OGD-induced dissipation of transmembrane K⁺ gradient contributed differently in brain cell depolarization: severe in astrocytes and neurons, and undetectable in NG2 glia. The discrete cellular responses to OGD corresponded to a total loss of 69% of the intracellular K⁺ contents in hippocampal slices as measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). A major brain cell depolarization mechanism identified here is important for our understanding of cerebral ischemia pathology. Additionally, further understanding of the resilient response of NG2 glia to ischemia-induced intracellular K⁺ loss and depolarization should facilitate the development of future stroke therapy.

Heterogeneity and function of hippocampal macroglia

The contribution of glial cells to normal and impaired hippocampal function is increasingly being recognized, although important questions as to the mechanisms that these cells use for their crosstalk with neurons and capillaries are still unanswered or lead to controversy. Astrocytes in the hippocampus are morphologically variable and a single cell contacts with its processes more than 100,000 synapses. They predominantly express inward rectifier K⁺ channels and transporters serving homeostatic function but may also release gliotransmitters to modify neuronal signaling and brain circulation. Intracellular Ca²⁺ transients are key events in the interaction of astrocytes with neurons and the vasculature. Hippocampal NG2 glia represent a population of cells with proliferative capacity throughout adulthood. Intriguingly, they receive direct synaptic input from pyramidal neurons and interneurons and express a multitude of ion channels and receptors. Despite in-depth knowledge about the features of these transmembrane proteins, the physiological impact of NG2 glial cells and their synaptic input remain nebulous. Because of the low abundance of oligodendrocytes in the hippocampus, limited information is available about their specific properties. Given the multitude of signaling molecules expressed by the various types of hippocampal glial cells (and because of space constraints), we focus, in this review, on those properties that are considered key for the interaction of the respective cell type with its neighborhood.

On Myelinated Axon Plasticity and Neuronal Circuit Formation and Function

Studies of activity-driven nervous system plasticity have primarily focused on the gray matter. However, MRI-based imaging studies have shown that white matter, primarily composed of myelinated axons, can also be dynamically regulated by activity of the healthy brain. Myelination in the CNS is an ongoing process that starts around birth and continues throughout life. Myelin in the CNS is generated by oligodendrocytes and recent evidence has shown that many aspects of oligodendrocyte development and myelination can be modulated by extrinsic signals including neuronal activity. Because modulation of myelin can, in turn, affect several aspects of conduction, the concept has emerged that activity-regulated myelination represents an important form of nervous system plasticity. Here we review our increasing understanding of how neuronal activity regulates oligodendrocytes and myelinated axons in vivo, with a focus on the timing of relevant processes. We highlight the observations that neuronal activity can rapidly tune axonal diameter, promote re-entry of oligodendrocyte progenitor cells into the cell cycle, or drive their direct differentiation into oligodendrocytes. We suggest that activity-regulated myelin formation and remodeling that significantly change axonal conduction properties are most likely to occur over timescales of days to weeks. Finally, we propose that precise fine-tuning of conduction along already-myelinated axons may also be mediated by alterations to the axon itself. We conclude that future studies need to analyze activity-driven adaptations to both axons and their myelin sheaths to fully understand how myelinated axon plasticity contributes to neuronal circuit formation and function.

Different patterns of neuronal activity trigger distinct responses of oligodendrocyte precursor cells in the corpus callosum

In the developing and adult brain, oligodendrocyte precursor cells (OPCs) are influenced by neuronal activity: they are involved in synaptic signaling with neurons, and their proliferation and differentiation into myelinating glia can be altered by transient changes in neuronal firing. An important question that has been unanswered is whether OPCs can discriminate different patterns of neuronal activity and respond to them in a distinct way. Here, we demonstrate in brain slices that the pattern of neuronal activity determines the functional changes triggered at synapses between axons and OPCs. Furthermore, we show that stimulation of the corpus callosum at different frequencies in vivo affects proliferation and differentiation of OPCs in a dissimilar way. Our findings suggest that neurons do not influence OPCs in “all-or-none” fashion but use their firing pattern to tune the response and behavior of these nonneuronal cells.

Oligodendrocytes are glial cells of the central nervous system. One of their major tasks is to enwrap neuronal axons with myelin, providing electrical insulation of axons and a dramatic increase in the speed of nerve impulse propagation. Oligodendrocytes develop from oligodendrocyte precursor cells (OPCs). Self-renewal of OPCs, their differentiation into oligodendrocytes, and the process of myelin synthesis are influenced by neuronal activity. Furthermore, OPCs receive glutamatergic synaptic input from neurons. Neuronal activity in vivo is highly variable depending on the brain region, input stimulus, and/or behavioral task that an animal or human has to perform in everyday life. Therefore, it is important to understand whether different types of neuronal activity affect development and function of oligodendrocyte lineage cells in a distinct way. In this study, we demonstrate that the amount and the timing of glutamate release at synapses between neurons and OPCs, the properties of the subsequent ionic current through glutamate receptors in OPC membrane, as well as the extent of OPCs’ self-renewal and differentiation into oligodendrocytes differ depending on the frequency and duration of neuronal activity. Hence, the pattern of neuronal activity rather than just presence or absence of activity is an important parameter that determines development and function of oligodendroglial cells.

Oligodendrocyte progenitor cells are paired with GABA neurons in the mouse dorsal cortex: Unbiased stereological analysis

Oligodendrocyte progenitor cells (OPC) are glial cells that differentiate into myelinating oligodendrocytes during early stages of post-natal life. However, OPCs persist beyond developmental myelination and represent an important population of cycling cells in the gray and white matter of the adult brain. While adult OPCs form unique territories that are maintained through self-avoidance, some cortical OPCs appear to position their cell body very close to that of a neuron, forming what are known as OPC-neuron pairs. We used unbiased systematic stereological analysis of the NG2-CreER^TM:EYFP reporter mouse to determine that close to 170,000 OPC-neuron pairs can be found in the dorsal portion of the adult neocortex, with approximately 40% of OPCs and 4% of neurons in pairs. Through stereological analysis, we also determined that reference memory training does not change the prevalence of OPC-neuron pairs or the proportion of OPCs and neurons that form them. GABAergic agent administration did not affect the proportion of OPCs and neurons that can be found in pairs. However, the GABA_B-receptor agonist baclofen and the GABA_A receptor antagonist picrotoxin significantly increased the estimated number of pairs when compared to the control group and the GABA_B-receptor antagonist (i.e. saclofen) group. Density of OPC-neuron pairs was increased by the GABA_A receptor antagonist picrotoxin. Finally, histological analysis of OPC-neuron pairs suggested that in the dorsal portion of the cortex, GABAergic interneurons represent the most common neuronal component of the pairs, and that calbindin, calretinin and parvalbumin GABAergic interneurons found in the cortex take part in these pairs. Using previous estimates of the number of GABAergic neurons in the rodent cortex, we estimate that roughly one in four GABAergic neurons are paired with an OPC.

A specific GABAergic synapse onto oligodendrocyte precursors does not regulate cortical oligodendrogenesis

In the brain, neurons establish bona fide synapses onto oligodendrocyte precursor cells (OPCs), but the function of these neuron-glia synapses remains unresolved. A leading hypothesis suggests that these synapses regulate OPC proliferation and differentiation. However, a causal link between synaptic activity and OPC cellular dynamics is still missing. In the developing somatosensory cortex, OPCs receive a major type of synapse from GABAergic interneurons that is mediated by postsynaptic γ2-containing GABA_A receptors. Here we genetically silenced these receptors in OPCs during the critical period of cortical oligodendrogenesis. We found that the inactivation of γ2-mediated synapses does not impact OPC proliferation and differentiation or the propensity of OPCs to myelinate their presynaptic interneurons. However, this inactivation causes a progressive and specific depletion of the OPC pool that lacks γ2-mediated synaptic activity without affecting the oligodendrocyte production. Our results show that, during cortical development, the γ2-mediated interneuron-to-OPC synapses do not play a role in oligodendrogenesis and suggest that these synapses finely tune OPC self-maintenance capacity. They also open the interesting possibility that a particular synaptic signaling onto OPCs plays a specific role in OPC function according to the neurotransmitter released, the identity of presynaptic neurons or the postsynaptic receptors involved.

Conditional Deletion of the L-Type Calcium Channel Cav1.2 in Oligodendrocyte Progenitor Cells Affects Postnatal Myelination in Mice

To determine whether L-type voltage-operated Ca²⁺ channels (L-VOCCs) are required for oligodendrocyte progenitor cell (OPC) development, we generated an inducible conditional knock-out mouse in which the L-VOCC isoform Cav1.2 was postnatally deleted in NG2-positive OPCs. A significant hypomyelination was found in the brains of the Cav1.2 conditional knock-out (Cav1.2^KO) mice specifically when the Cav1.2 deletion was induced in OPCs during the first 2 postnatal weeks. A decrease in myelin proteins expression was visible in several brain structures, including the corpus callosum, cortex, and striatum, and the corpus callosum of Cav1.2^KO animals showed an important decrease in the percentage of myelinated axons and a substantial increase in the mean g-ratio of myelinated axons. The reduced myelination was accompanied by an important decline in the number of myelinating oligodendrocytes and in the rate of OPC proliferation. Furthermore, using a triple transgenic mouse in which all of the Cav1.2^KO OPCs were tracked by a Cre reporter, we found that Cav1.2^KO OPCs produce less mature oligodendrocytes than control cells. Finally, live-cell imaging in early postnatal brain slices revealed that the migration and proliferation of subventricular zone OPCs is decreased in the Cav1.2^KO mice. These results indicate that the L-VOCC isoform Cav1.2 modulates oligodendrocyte development and suggest that Ca²⁺ influx mediated by L-VOCCs in OPCs is necessary for normal myelination.

SIGNIFICANCE STATEMENT

Overall, it is clear that cells in the oligodendrocyte lineage exhibit remarkable plasticity with regard to the expression of Ca²⁺ channels and that perturbation of Ca²⁺ homeostasis likely plays an important role in the pathogenesis underlying demyelinating diseases. To determine whether voltage-gated Ca²⁺ entry is involved in oligodendrocyte maturation and myelination, we used a conditional knock-out mouse for voltage-operated Ca²⁺ channels in oligodendrocyte progenitor cells. Our results indicate that voltage-operated Ca²⁺ channels can modulate oligodendrocyte development in the postnatal brain and suggest that voltage-gated Ca²⁺ influx in oligodendroglial cells is critical for normal myelination. These findings could lead to novel approaches to intervene in neurodegenerative diseases in which myelin is lost or damaged.

Inwardly Rectifying K+ Currents in Cultured Oligodendrocytes from Rat Optic Nerve are Insensitive to pH

Inwardly rectifying K⁺ (Kir) channel expression signals at an advanced stage of maturation during oligodendroglial differentiation. Knocking down their expression halts the generation of myelin and produces severe abnormalities in the central nervous system. Kir4.1 is the main subunit involved in the tetrameric structure of Kir channels in glial cells; however, the precise composition of Kir channels expressed in oligodendrocytes (OLs) remains partially unknown, as participation of other subunits has been proposed. Kir channels are sensitive to H⁺; thus, intracellular acidification produces Kir current inhibition. Since Kir subunits have differential sensitivity to H⁺, we studied the effect of intracellular acidification on Kir currents expressed in cultured OLs derived from optic nerves of 12-day-old rats. Unexpectedly, Kir currents in OLs (2-4 DIV) did not change within the pH range of 8.0-5.0, as observed when using standard whole-cell voltage-clamp recording or when preserving cytoplasmic components with the perforated patch-clamp technique. In contrast, low pH inhibited astrocyte Kir currents, which was consistent with the involvement of the Kir4.1 subunit. The H⁺-insensitivity expressed in OL Kir channels was not intrinsic because Kir cloning showed no difference in the sequence reported for the Kir4.1, Kir2.1, or Kir5.1 subunits. Moreover, when Kir channels were heterologously expressed in Xenopus oocytes they behaved as expected in their general properties and sensitivity to H⁺. It is therefore concluded that Kir channel H⁺-sensitivity in OLs is modulated through an extrinsic mechanism, probably by association with a modulatory component or by posttranslational modifications.

Neuron-glia cross talk revealed in reverberating networks by simultaneous extracellular recording of spikes and astrocytes' glutamate transporter and K+ currents

In neocortex networks, we simultaneously captured spikes and the slower astrocytes' K⁺and glutamate transporter (GluT) currents with the use of individual MEA electrodes. Inward and outward K⁺currents in different regions of the glial syncytium suggested that spatial buffering was operant. Moreover, in organotypic slices from ventral tegmental area and prefrontal cortex, the large GluT current amplitudes allowed to measure transporter currents with a single electrode. Our method allows direct study of the dynamic interplay of different cell types in excitable and nonexcitable tissue.

Astrocytes uptake synaptically released glutamate with electrogenic transporters (GluT) and buffer the spike-dependent extracellular K⁺ excess with background K⁺ channels. We studied neuronal spikes and the slower astrocytic signals on reverberating neocortical cultures and organotypic slices from mouse brains. Spike trains and glial responses were simultaneously captured from individual sites of multielectrode arrays (MEA) by splitting the recorded traces into appropriate filters and reconstructing the original signal by deconvolution. GluT currents were identified by using dl-threo-β-benzyloxyaspartate (TBOA). K⁺ currents were blocked by 30 μM Ba²⁺, suggesting a major contribution of inwardly rectifying K⁺ currents. Both types of current were tightly correlated with the spike rate, and their astrocytic origin was tested in primary cultures by blocking glial proliferation with cytosine β-d-arabinofuranoside (AraC). The spike-related, time-locked inward and outward K⁺ currents in different regions of the astrocyte syncytium were consistent with the assumptions of the spatial K⁺ buffering model. In organotypic slices from ventral tegmental area and prefrontal cortex, the GluT current amplitudes exceeded those observed in primary cultures by several orders of magnitude, which allowed to directly measure transporter currents with a single electrode. Simultaneously measuring cell signals displaying widely different amplitudes and kinetics will help clarify the neuron-glia interplay and make it possible to follow the cross talk between different cell types in excitable as well as nonexcitable tissue.

The role of glial-specific Kir4.1 in normal and pathological states of the CNS

Kir4.1 is an inwardly rectifying K(+) channel expressed exclusively in glial cells in the central nervous system. In glia, Kir4.1 is implicated in several functions including extracellular K(+) homeostasis, maintenance of astrocyte resting membrane potential, cell volume regulation, and facilitation of glutamate uptake. Knockout of Kir4.1 in rodent models leads to severe neurological deficits, including ataxia, seizures, sensorineural deafness, and early postnatal death. Accumulating evidence indicates that Kir4.1 plays an integral role in the central nervous system, prompting many laboratories to study the potential role that Kir4.1 plays in human disease. In this article, we review the growing evidence implicating Kir4.1 in a wide array of neurological disease. Recent literature suggests Kir4.1 dysfunction facilitates neuronal hyperexcitability and may contribute to epilepsy. Genetic screens demonstrate that mutations of KCNJ10, the gene encoding Kir4.1, causes SeSAME/EAST syndrome, which is characterized by early onset seizures, compromised verbal and motor skills, profound cognitive deficits, and salt-wasting. KCNJ10 has also been linked to developmental disorders including autism. Cerebral trauma, ischemia, and inflammation are all associated with decreased astrocytic Kir4.1 current amplitude and astrocytic dysfunction. Additionally, neurodegenerative diseases such as Alzheimer disease and amyotrophic lateral sclerosis demonstrate loss of Kir4.1. This is particularly exciting in the context of Huntington disease, another neurodegenerative disorder in which restoration of Kir4.1 ameliorated motor deficits, decreased medium spiny neuron hyperexcitability, and extended survival in mouse models. Understanding the expression and regulation of Kir4.1 will be critical in determining if this channel can be exploited for therapeutic benefit.

Intracellular ion signaling influences myelin basic protein synthesis in oligodendrocyte precursor cells

Myelination in the central nervous system depends on axon-oligodendrocyte precursor cell (OPC) interaction. We suggest that myelin synthesis may be influenced by [Na⁺]i and [Ca²⁺]i signaling in OPCs. Experiments were performed in mouse cultured OPCs at day in vitro (DIV) 2-6 or acute slices of the corpus callosum at postnatal days (P) 10-30. Synthesis of Myelin Basic Protein (MBP), an "executive molecule of myelin", was used as readout of myelination. Immunohistological data revealed that MBP synthesis in cultured OPCs starts around DIV4. Transient elevations of resting [Ca²⁺]i and [Na⁺]i levels were observed in the same temporal window (DIV4-5). At DIV4, but not at DIV2, both extracellular [K⁺] ([K⁺]e) elevation (+5mM) and partial Na⁺,K⁺-ATPase (NKA) inhibition elicited [Na⁺]i and [Ca²⁺]i transients. These responses were blocked with KB-R7943 (1μM), a blocker of Na⁺-Ca²⁺ exchanger (NCX), indicating an involvement of NCX which operates in reverse mode. Treatment of OPCs with culture medium containing elevated [K⁺] (+5mM, 24h) or ouabain (500nM, 24h) increased resting [Ca²⁺]i and facilitated MBP synthesis. Blockade of NCX with KB-R7943 (1μM, 12h) reduced resting [Ca²⁺]i and decreased MBP synthesis. Similar to the results obtained in OPC cultures, OPCs in acute callosal slices demonstrated an increase in resting [Ca²⁺]i and [Na⁺]i levels during development. NCX blockade induced [Ca²⁺]i and [Na⁺]i responses in OPCs at P20-30 but not at P10. We conclude that local [Na⁺]i and/or membrane potential changes can modulate Ca²⁺ influx through NCX and in turn MBP synthesis. Thus neuronal activity-induced changes in [K⁺]e may via NCX and NKA modulate myelination.

Adaptive myelination from fish to man

Myelinated axons with nodes of Ranvier are an evolutionary elaboration common to essentially all jawed vertebrates. Myelin made by Schwann cells in our peripheral nervous system and oligodendrocytes in our central nervous system has been long known to facilitate rapid energy efficient nerve impulse propagation. However, it is now also clear, particularly in the central nervous system, that myelin is not a simple static insulator but that it is dynamically regulated throughout development and life. New myelin sheaths can be made by newly differentiating oligodendrocytes, and mature myelin sheaths can be stimulated to grow again in the adult. Furthermore, numerous studies in models from fish to man indicate that neuronal activity can affect distinct stages of oligodendrocyte development and the process of myelination itself. This begs questions as to how these effects of activity are mediated at a cellular and molecular level and whether activity-driven adaptive myelination is a feature common to all myelinated axons, or indeed all oligodendrocytes, or is specific to cells or circuits with particular functions. Here we review the recent literature on this topic, elaborate on the key outstanding questions in the field, and look forward to future studies that incorporate investigations in systems from fish to man that will provide further insight into this fundamental aspect of nervous system plasticity. This article is part of a Special Issue entitled SI: Myelin Evolution.

Changes in the proliferative capacity of NG2 cell subpopulations during postnatal development of the mouse hippocampus

Besides astrocytes and oligodendrocytes, NG2 proteoglycan-expressing cells (NG2 glia) represent a third subtype of macroglia in the brain. Originally described as oligodendrocyte precursor cells, they feature several characteristics not expected from mere progenitor cells, including synaptic connections with neurons. There is accumulating evidence that the properties of NG2 glia differ between different brain regions and developmental stages. To further analyze this proposed heterogeneity, we studied electrophysiological properties, transcript and protein expression, distribution and proliferative capacity of NG2 glia during postnatal development, focusing on the hippocampus and corpus callosum. All NG2 glia displayed a 'complex' current pattern consisting of voltage- and time-dependent in- and outward currents. In juvenile mice, Kir current densities and rectification index were highly variable and on average significantly lower than in adult animals. Single cell RT-PCR analyses of electrophysiologically characterized cells demonstrated that different glial genes were expressed at variable extent, independent of developmental stage and genetic background. In the hippocampus proper and the corpus callosum, the density of NG2 glia was highest at postnatal days (P)10-12, decreased by ~50 % at P25-35 and then remained stable in adults (P80-90). Interestingly, co-expression of NG2 and S100β, a marker for mature astrocytes, increased from 7 % at P10-12 to 27 % at P25-35 in the hippocampus proper, and then dropped again in the stratum radiatum at P80-90. In the dentate gyrus and corpus callosum, co-expression of NG2 and S100β was very low (3 %) and constant throughout development. Age-related differences were also observed with Ki-67, a proliferation marker. In NG2 glia of the CA1 region, its expression decreased from 16 % at P10-12 to 9 % (P25-35) and then 3 % (P80-90). Triple-stainings revealed that Ki-67 was also expressed in 2-3 % of NG2/S100β-positive cells in the juvenile and mature stratum radiatum, indicating that the latter, in contrast to S100β-positive astrocytes, still host proliferative potential. Taken together, we found significant differences in transcript and protein expression, electrophysiological properties and proliferative capacity of NG2 glia in the mouse forebrain, suggesting the co-existence of several subpopulations of NG2 glia. Our data thus support the idea of a substantial regional and developmental heterogeneity in this subtype of macroglia.

Activity-dependent calcium signalling in oligodendrocyte generation

Throughout postnatal life oligodendrocyte progenitor cells proliferate and differentiate into mature myelinating oligodendrocytes in the central nervous system. Neuronal activity is a major external signal controlling this process. Neurotransmitters, or other signalling molecules released in response to neuronal activity, evoke transient increases in intracellular calcium in oligodendrocyte progenitor cells. As calcium can mediate cellular processes, including the transcription of genes involved in oligodendrocyte progenitor cell division and maturation, a rise in intracellular calcium may be a key signal translating changes in neuronal activity into changes in oligodendrocyte progenitor cell behaviour. Here we review recent advances in our understanding of how neuronal activity can evoke calcium signalling in oligodendrocyte progenitor cells.

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

Satellite oligodendrocytes (s-OLs) are closely apposed to the soma of neocortical layer 5 pyramidal neurons but their properties and functional roles remain unresolved. Here we show that s-OLs form compact myelin and action potentials of the host neuron evoke precisely timed Ba²⁺-sensitive K⁺ inward rectifying (Kir) currents in the s-OL. Unexpectedly, the glial K⁺ inward current does not require oligodendrocytic Kir4.1. Action potential-evoked Kir currents are in part mediated by gap–junction coupling with neighbouring OLs and astrocytes that form a syncytium around the pyramidal cell body. Computational modelling predicts that glial Kir constrains the perisomatic [K⁺]_o increase most importantly during high-frequency action potentials. Consistent with these predictions neurons with s-OLs showed a reduced probability for action potential burst firing during [K⁺]_o elevations. These data suggest that s-OLs are integrated into a glial syncytium for the millisecond rapid K⁺ uptake limiting activity-dependent [K⁺]_o increase in the perisomatic neuron domain.

Satellite oligodendrocytes (s-OLs) are characterised by their close proximity to neocortical pyramidal cells. Here, the authors find that s-OLs myelinate axons and activity of host neurons evokes inward K⁺ currents in s-OLs which may work to modulate action potential burst firing by buffering extracellular K⁺ levels.

Maturation and electrophysiological properties of human pluripotent stem cell-derived oligodendrocytes

Rodent‐based studies have shown that the membrane properties of oligodendrocytes play prominent roles in their physiology and shift markedly during their maturation from the oligodendrocyte precursor cell (OPC) stage. However, the conservation of these properties and maturation processes in human oligodendrocytes remains unknown, despite their dysfunction being implicated in human neurodegenerative diseases such as multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). Here, we have defined the membrane properties of human oligodendrocytes derived from pluripotent stem cells as they mature from the OPC stage, and have identified strong conservation of maturation‐specific physiological characteristics reported in rodent systems. We find that as human oligodendrocytes develop and express maturation markers, they exhibit a progressive decrease in voltage‐gated sodium and potassium channels and a loss of tetrodotoxin‐sensitive spiking activity. Concomitant with this is an increase in inwardly rectifying potassium channel activity, as well as a characteristic switch in AMPA receptor composition. All these steps mirror the developmental trajectory observed in rodent systems. Oligodendrocytes derived from mutant C9ORF72‐carryng ALS patient induced pluripotent stem cells did not exhibit impairment to maturation and maintain viability with respect to control lines despite the presence of RNA foci, suggesting that maturation defects may not be a primary feature of this mutation. Thus, we have established that the development of human oligodendroglia membrane properties closely resemble those found in rodent cells and have generated a platform to enable the impact of human neurodegenerative disease‐causing mutations on oligodendrocyte maturation to be studied. Stem Cells2016;34:1040–1053

Aging affects new cell production in the adult hippocampus: A quantitative anatomic review

In the last century, cognitive impairment in elderly people was considered as the consequence of neuronal death. However, later analyses indicated that age-related reduction in neuron number was limited to specific regions of the central nervous system, and was irrelevant to brain dysfunction in both humans and non-human animals. Recent studies have indicated that progressive diminution of neural plasticity across an individual's life span may underlie age-related brain dysfunction. To date, various factors have been shown to contribute to neural plasticity. In particular, substantial data supports the importance of production of new cells in the adult brain: the rate of hippocampal neurogenesis wanes radically during aging; similarly, white matter homeostasis via oligodendrogenesis is also affected by aging. This review briefly summarizes quantitative studies on adult hippocampal neurogenesis and oligodendrogenesis. Although the hippocampus is traditionally recognized as the memory center of the brain, it has started to emerge as an integrator of cognition and emotion. One of the current research highlights is that diverse functions of the hippocampus are topographically embedded along its longitudinal and transverse axes. Here we discuss alterations in adult neurogenesis and oligodendrogenesis during aging from a topographic view point. The quantitative anatomic approach to age-related alterations in production of new cells in the hippocampus may give a novel insight into how brain functions suffer from aging.