Ca(2+) signaling evoked by activation of Na(+) channels and Na(+)/Ca(2+) exchangers is required for GABA-induced NG2 cell migration

A New Acquaintance of Oligodendrocyte Precursor Cells in the Central Nervous System

Oligodendrocyte precursor cells (OPCs) are a heterogeneous multipotent population in the central nervous system (CNS) that appear during embryogenesis and persist as resident cells in the adult brain parenchyma. OPCs could generate oligodendrocytes to participate in myelination. Recent advances have renewed our knowledge of OPC biology by discovering novel markers of oligodendroglial cells, the myelin-independent roles of OPCs, and the regulatory mechanism of OPC development. In this review, we will explore the updated knowledge on OPC identity, their multifaceted roles in the CNS in health and diseases, as well as the regulatory mechanisms that are involved in their developmental stages, which hopefully would contribute to a further understanding of OPCs and attract attention in the field of OPC biology.

Voltage-Gated Ion Channels Are Transcriptional Targets of Sox10 during Oligodendrocyte Development

The transcription factor Sox10 is an important determinant of oligodendroglial identity and influences oligodendroglial development and characteristics at various stages. Starting from RNA-seq data, we here show that the expression of several voltage-gated ion channels with known expression and important function in oligodendroglial cells depends upon Sox10. These include the Nav1.1, Cav2.2, Kv1.1, and Kir4.1 channels. For each of the four encoding genes, we found at least one regulatory region that is activated by Sox10 in vitro and at the same time bound by Sox10 in vivo. Cell-specific deletion of Sox10 in oligodendroglial cells furthermore led to a strong downregulation of all four ion channels in a mouse model and thus in vivo. Our study provides a clear functional link between voltage-gated ion channels and the transcriptional regulatory network in oligodendroglial cells. Furthermore, our study argues that Sox10 exerts at least some of its functions in oligodendrocyte progenitor cells, in myelinating oligodendrocytes, or throughout lineage development via these ion channels. By doing so, we present one way in which oligodendroglial development and properties can be linked to neuronal activity to ensure crosstalk between cell types during the development and function of the central nervous system.

N-butyl-β-carboline-3-carboxylate (β-CCB) systemic administration promotes remyelination in the cuprizone demyelinating model in mice

Demyelination is generated in several nervous system illnesses. Developing strategies for effective clinical treatments requires the discovery of promyelinating drugs. Increased GABAergic signaling through γ-aminobutyric acid type A receptor (GABAAR) activation in oligodendrocytes has been proposed as a promyelinating condition. GABAAR expressed in oligodendroglia is strongly potentiated by n-butyl-β-carboline-3-carboxylate (β-CCB) compared to that in neurons. Here, mice were subjected to 0.3% cuprizone (CPZ) added in the food to induce central nervous system demyelination, a well-known model for multiple sclerosis. Then β-CCB (1 mg/Kg) was systemically administered to analyze the remyelination status in white and gray matter areas. Myelin content was evaluated using Black-Gold II (BGII) staining, immunofluorescence (IF), and magnetic resonance imaging (MRI). Evidence indicates that β-CCB treatment of CPZ-demyelinated animals promoted remyelination in several white matter structures, such as the fimbria, corpus callosum, internal capsule, and cerebellar peduncles. Moreover, using IF, it was observed that CPZ intake induced an increase in NG2+ and a decrease in CC1+ cell populations, alterations that were importantly retrieved by β-CCB treatment. Thus, the promyelinating character of β-CCB was confirmed in a generalized demyelination model, strengthening the idea that it has clinical potential as a therapeutic drug.

The Emerging Role of Microglial Hv1 as a Target for Immunomodulation in Myelin Repair

In the central nervous system (CNS), the myelin sheath ensures efficient interconnection between neurons and contributes to the regulation of the proper function of neuronal networks. The maintenance of myelin and the well-organized subtle process of myelin plasticity requires cooperation among myelin-forming cells, glial cells, and neural networks. The process of cooperation is fragile, and the balance is highly susceptible to disruption by microenvironment influences. Reactive microglia play a critical and complicated role in the demyelination and remyelination process. Recent studies have shown that the voltage-gated proton channel Hv1 is selectively expressed in microglia in CNS, which regulates intracellular pH and is involved in the production of reactive oxygen species, underlying multifaceted roles in maintaining microglia function. This paper begins by examining the molecular mechanisms of demyelination and emphasizes the crucial role of the microenvironment in demyelination. It focuses specifically on the role of Hv1 in myelin repair and its therapeutic potential in CNS demyelinating 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.

Pathological potential of oligodendrocyte precursor cells: terra incognita

Adult oligodendrocyte precursor cells (aOPCs), transformed from fetal OPCs, are idiosyncratic neuroglia of the central nervous system (CNS) that are distinct in many ways from other glial cells. OPCs have been classically studied in the context of their remyelinating capacity. Recent studies, however, revealed that aOPCs not only contribute to post-lesional remyelination but also play diverse crucial roles in multiple neurological diseases. In this review we briefly present the physiology of aOPCs and summarize current knowledge of the beneficial and detrimental roles of aOPCs in different CNS diseases. We discuss unique features of aOPC death, reactivity, and changes during senescence, as well as aOPC interactions with other glial cells and pathological remodeling during disease. Finally, we outline future perspectives for the study of aOPCs in brain pathologies which may instigate the development of aOPC-targeting therapeutic strategies.

Presentation and integration of multiple signals that modulate oligodendrocyte lineage progression and myelination

Myelination is critical for fast saltatory conduction of action potentials. Recent studies have revealed that myelin is not a static structure as previously considered but continues to be made and remodeled throughout adulthood in tune with the network requirement. Synthesis of new myelin requires turning on the switch in oligodendrocytes (OL) to initiate the myelination program that includes synthesis and transport of macromolecules needed for myelin production as well as the metabolic and other cellular functions needed to support this process. A significant amount of information is available regarding the individual intrinsic and extrinsic signals that promote OL commitment, expansion, terminal differentiation, and myelination. However, it is less clear how these signals are made available to OL lineage cells when needed, and how multiple signals are integrated to generate the correct amount of myelin that is needed in a given neural network state. Here we review the pleiotropic effects of some of the extracellular signals that affect myelination and discuss the cellular processes used by the source cells that contribute to the variation in the temporal and spatial availability of the signals, and how the recipient OL lineage cells might integrate the multiple signals presented to them in a manner dialed to the strength of the input.

The role of depolarizing activation of Na+-Ca2+ exchanger by oligodendrocyte progenitor cells in the effect of sevoflurane on myelination

AIMS

The aim of this study was to investigate the role of depolarizing activation of Na⁺-Ca²⁺ exchanger (NCX) by oligodendrocyte progenitor cells (OPC) in the effect of sevoflurane on myelination.

MAIN METHODS

On postnatal days 7, 8, and 9, mice were exposed to 3 % sevoflurane for 2 h per day. The proliferation, differentiation, and myelin sheath of OPC were observed with immunofluorescence, quantitative real-time polymerase chain reaction (QRT-PCR), and transmission electron microscopy (TEM) at various time points. The open field, Y maze, and new object recognition tests were used to measure spatial learning and memory. siRNA was used for the knockdown NCX1 in human OPC (HOPC) before sevoflurane exposure; the Transwell migration assay was used to measure cell migration ability and Fluo 4-AM was used to measure intracellular Ca2+ concentration.

KEY FINDINGS

Pretreatment with an NCX inhibitor attenuated the proliferation and differentiation of OPC induced by sevoflurane and induced a remarkable increase in platelet-derived growth factor receptor-alpha (PDGFRα), 2, 3-cyclic nucleotide 3-phosphodiesterase (CNPase), oligodendrocyte transcription factor 2 (Olig2), and homeodomain protein NK2 homeobox 2 (NKX2.2) levels. Pretreatment with an NCX inhibitor alleviated the sevoflurane-induced myelination disorder and cognitive impairment. The decreased cell migration and increased intracellular Ca²⁺ concentration observed in the siRNA-negative control group was reversed in the sevoflurane plus siRNA-NCX1 group.

SIGNIFICANCE

This study suggests that repeated sevoflurane exposure in newborn mice leads to depolarization of OPC, which leads to Ca²⁺ influx through NCX and affects OPC proliferation, migration, differentiation, and myelination, ultimately leading to cognitive impairment.

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

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

Novel Toolboxes for the Investigation of Activity-Dependent Myelination in the Central Nervous System

Myelination is essential for signal processing within neural networks. Emerging data suggest that neuronal activity positively instructs myelin development and myelin adaptation during adulthood. However, the underlying mechanisms controlling activity-dependent myelination have not been fully elucidated. Myelination is a multi-step process that involves the proliferation and differentiation of oligodendrocyte precursor cells followed by the initial contact and ensheathment of axons by mature oligodendrocytes. Conventional end-point studies rarely capture the dynamic interaction between neurons and oligodendrocyte lineage cells spanning such a long temporal window. Given that such interactions and downstream signaling cascades are likely to occur within fine cellular processes of oligodendrocytes and their precursor cells, overcoming spatial resolution limitations represents another technical hurdle in the field. In this mini-review, we discuss how advanced genetic, cutting-edge imaging, and electrophysiological approaches enable us to investigate neuron-oligodendrocyte lineage cell interaction and myelination with both temporal and spatial precision.

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

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

NG2 glia-derived GABA release tunes inhibitory synapses and contributes to stress-induced anxiety

NG2 glia, also known as oligodendrocyte precursor cells (OPCs), play an important role in proliferation and give rise to myelinating oligodendrocytes during early brain development. In contrast to other glial cell types, the most intriguing aspect of NG2 glia is their ability to directly sense synaptic inputs from neurons. However, whether this synaptic interaction is bidirectional or unidirectional, or its physiological relevance has not yet been clarified. Here, we report that NG2 glia form synaptic complexes with hippocampal interneurons and that selective photostimulation of NG2 glia (expressing channelrhodopsin-2) functionally drives GABA release and enhances inhibitory synaptic transmission onto proximal interneurons in a microcircuit. The mechanism involves GAD67 biosynthesis and VAMP-2 containing vesicular exocytosis. Further, behavioral assays demonstrate that NG2 glia photoactivation triggers anxiety-like behavior in vivo and contributes to chronic social defeat stress.

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.

Connecting Neuroinflammation and Neurodegeneration in Multiple Sclerosis: Are Oligodendrocyte Precursor Cells a Nexus of Disease?

The pathology in neurodegenerative diseases is often accompanied by inflammation. It is well-known that many cells within the central nervous system (CNS) also contribute to ongoing neuroinflammation, which can promote neurodegeneration. Multiple sclerosis (MS) is both an inflammatory and neurodegenerative disease in which there is a complex interplay between resident CNS cells to mediate myelin and axonal damage, and this communication network can vary depending on the subtype and chronicity of disease. Oligodendrocytes, the myelinating cell of the CNS, and their precursors, oligodendrocyte precursor cells (OPCs), are often thought of as the targets of autoimmune pathology during MS and in several animal models of MS; however, there is emerging evidence that OPCs actively contribute to inflammation that directly and indirectly contributes to neurodegeneration. Here we discuss several contributors to MS disease progression starting with lesion pathology and murine models amenable to studying particular aspects of disease. We then review how OPCs themselves can play an active role in promoting neuroinflammation and neurodegeneration, and how other resident CNS cells including microglia, astrocytes, and neurons can impact OPC function. Further, we outline the very complex and pleiotropic role(s) of several inflammatory cytokines and other secreted factors classically described as solely deleterious during MS and its animal models, but in fact, have many neuroprotective functions and promote a return to homeostasis, in part via modulation of OPC function. Finally, since MS affects patients from the onset of disease throughout their lifespan, we discuss the impact of aging on OPC function and CNS recovery. It is becoming clear that OPCs are not simply a bystander during MS progression and uncovering the active roles they play during different stages of disease will help uncover potential new avenues for therapeutic intervention.

Oligodendroglial GABAergic Signaling: More Than Inhibition!

GABA is the main inhibitory neurotransmitter in the CNS acting at two distinct types of receptor: ligand-gated ionotropic GABA_A receptors and G protein-coupled metabotropic GABA_B receptors, thus mediating fast and slow inhibition of excitability at central synapses. GABAergic signal transmission has been intensively studied in neurons in contrast to oligodendrocytes and their precursors (OPCs), although the latter express both types of GABA receptor. Recent studies focusing on interneuron myelination and interneuron-OPC synapses have shed light on the importance of GABA signaling in the oligodendrocyte lineage. In this review, we start with a short summary on GABA itself and neuronal GABAergic signaling. Then, we elaborate on the physiological role of GABA receptors within the oligodendrocyte lineage and conclude with a description of these receptors as putative targets in treatments of CNS diseases.

Thermosensitive TRPV4 channels mediate temperature-dependent microglia movement

Movement is a key feature of the surveillance and protective roles of microglia. This dynamic process is highly modulated by the surrounding environment. We discovered that microglia movement is temperature dependent in vitro and in vivo. Our investigation of thermosensitive TRP channel involvement in this phenomenon revealed several candidates including TRPM2, TRPM4, and TRPV4 channels. Using pharmacological tools and transgenic mice, we showed that the temperature dependency of microglia movement mainly relies on TRPV4 channel activity. Understanding the mechanisms by which temperature modulates microglia movement will improve our comprehension of pathological processes and allow the identification of new leads for the treatment of brain pathologies.

Microglia maintain central nervous system homeostasis by monitoring changes in their environment (resting state) and by taking protective actions to equilibrate such changes (activated state). These surveillance and protective roles both require constant movement of microglia. Interestingly, induced hypothermia can reduce microglia migration caused by ischemia, suggesting that microglia movement can be modulated by temperature. Although several ion channels and transporters are known to support microglia movement, the precise molecular mechanism that regulates temperature-dependent movement of microglia remains unclear. Some members of the transient receptor potential (TRP) channel superfamily exhibit thermosensitivity and thus are strong candidates for mediation of this phenomenon. Here, we demonstrate that mouse microglia exhibit temperature-dependent movement in vitro and in vivo that is mediated by TRPV4 channels within the physiological range of body temperature. Our findings may provide a basis for future research into the potential clinical application of temperature regulation to preserve cell function via manipulation of ion channel activity.

Therapeutic Potential of GABAergic Signaling in Myelin Plasticity and Repair

Oligodendrocytes (OLs) produce myelin to insulate axons. This accelerates action potential propagation, allowing nerve impulse information to synchronize within complex neuronal ensembles and promoting brain connectivity. Brain plasticity includes myelination, a process that starts early after birth and continues throughout life. Myelin repair, followed by injury or disease, requires new OLs differentiated from a population derived from oligodendrocyte precursor cells (OPCs) that continue to proliferate, migrate and differentiate to preserve and remodel myelin in the adult central nervous system. OPCs represent the largest proliferative neural cell population outside the adult neurogenic niches in the brain. OPCs receive synaptic inputs from glutamatergic and GABAergic neurons throughout neurodevelopment, a unique feature among glial cells. Neuron-glia communication through GABA signaling in OPCs has been shown to play a role in myelin plasticity and repair. In this review we will focus on the molecular and functional properties of GABA_A receptors (GABA_ARs) expressed by OPCs and their potential role in remyelination.

Roles of Microglial Ion Channel in Neurodegenerative Diseases

As the average age and life expectancy increases, the incidence of both acute and chronic central nervous system (CNS) pathologies will increase. Understanding mechanisms underlying neuroinflammation as the common feature of any neurodegenerative pathology, we can exploit the pharmacology of cell specific ion channels to improve the outcome of many CNS diseases. As the main cellular player of neuroinflammation, microglia play a central role in this process. Although microglia are considered non-excitable cells, they express a variety of ion channels under both physiological and pathological conditions that seem to be involved in a plethora of cellular processes. Here, we discuss the impact of modulating microglia voltage-gated, potential transient receptor, chloride and proton channels on microglial proliferation, migration, and phagocytosis in neurodegenerative diseases.

White matter and neurological disorders

The central nervous system is simply divided into two distinct anatomical regions based on the color of tissues, i.e. the gray and white matter. The gray matter is composed of neuronal cell bodies, glial cells, dendrites, immune cells, and the vascular system, while the white matter is composed of concentrated myelinated axonal fibers extending from neuronal soma and glial cells, such as oligodendrocyte precursor cells (OPCs), oligodendrocytes, astrocytes, and microglia. As neuronal cell bodies are located in the gray matter, great attention has been focused mainly on the gray matter regarding the understanding of the functions of the brain throughout the neurophysiological areas, leading to a scenario in which the function of the white matter is relatively underestimated or has not received much attention. However, increasing evidence shows that the white matter plays highly significant and pivotal functions in the brain based on the fact that its abnormalities are associated with numerous neurological diseases. In this review, we will broadly discuss the pathways and functions of myelination, which is one of the main processes that modulate the functions of the white matter, as well as the manner in which its abnormalities are related to neurological disorders.

Neuron-oligodendroglia interactions: Activity-dependent regulation of cellular signaling

Oligodendrocyte lineage cells (oligodendroglia) and neurons engage in bidirectional communication throughout life to support healthy brain function. Recent work shows that changes in neuronal activity can modulate proliferation, differentiation, and myelination to support the formation and function of neural circuits. While oligodendroglia express a diverse collection of receptors for growth factors, signaling molecules, neurotransmitters and neuromodulators, our knowledge of the intracellular signaling pathways that are regulated by neuronal activity remains largely incomplete. Many of the pathways that modulate oligodendroglia behavior are driven by changes in intracellular calcium signaling, which may differentially affect cytoskeletal dynamics, gene expression, maturation, integration, and axonal support. Additionally, activity-dependent neuron-oligodendroglia communication plays an integral role in the recovery from demyelinating injuries. In this review, we summarize the modalities of communication between neurons and oligodendroglia and explore possible roles of activity-dependent calcium signaling in mediating cellular behavior and myelination.

GABAA Receptors are Selectively Expressed in NG2 Glia of the Cerebellar White Matter

The cerebellum is involved in the coordination of movement. Its cellular composition is dominated by GABAergic neuronal types, and glial cells are known to express functional receptors. GABAergic signaling regulates cell proliferation, differentiation, and migration during neurodevelopment. However, little is known about the functional expression of GABA receptors in the cerebellar white matter (WM). Thus, the aim of this study was to test whether glial cells express functional GABA receptors during postnatal development (P7-P9) of cerebellar WM. Immunofluorescence showed that half of the astrocytes express GAD67, suggesting that glial cells synthesize GABA. Calcium imaging in cerebellar slices revealed that GABA and the GABA_A agonist muscimol evoked calcium transients in sulforhodamine B negative cells, whereas the GABA_B agonist baclofen failed to evoke responses in cerebellar WM. Whole-cell patch-clamp recordings of GFAP⁺ cells showed dye coupling and a passive current-voltage relation typical of astrocytes. Surprisingly, these cells did not respond to muscimol. Two additional populations were identified as GFAP^- cells. The first population showed dye coupling, slow decaying inward and outward currents with no voltage dependence, and did not respond to GABA_A agonists. The second population showed an outward-rectifying current-voltage relationship and responded to muscimol, but dye coupling was absent. These cells received synaptic input and were NG2⁺, but evoked calcium waves failed to modulate the frequency of spontaneous postsynaptic currents (sPSCs) or signaling into NG2 glia. We conclude that GABA_A receptor-mediated signaling is selective for NG2 glia in the WM of the cerebellum.

Calcium Signaling in the Oligodendrocyte Lineage: Regulators and Consequences

Cells of the oligodendrocyte lineage express a wide range of Ca²⁺ channels and receptors that regulate oligodendrocyte progenitor cell (OPC) and oligodendrocyte formation and function. Here we define those key channels and receptors that regulate Ca²⁺ signaling and OPC development and myelination. We then discuss how the regulation of intracellular Ca²⁺ in turn affects OPC and oligodendrocyte biology in the healthy nervous system and under pathological conditions. Activation of Ca²⁺ channels and receptors in OPCs and oligodendrocytes by neurotransmitters converges on regulating intracellular Ca²⁺, making Ca²⁺ signaling a central candidate mediator of activity-driven myelination. Indeed, recent evidence indicates that localized changes in Ca²⁺ in oligodendrocytes can regulate the formation and remodeling of myelin sheaths and perhaps additional functions of oligodendrocytes and OPCs. Thus, decoding how OPCs and myelinating oligodendrocytes integrate and process Ca²⁺ signals will be important to fully understand central nervous system formation, health, and function.

Sodium-Calcium Exchangers of the SLC8 Family in Oligodendrocytes: Functional Properties in Health and Disease

The solute carrier 8 (SLC8) family of sodium-calcium exchangers (NCXs) functions as an essential regulatory system that couples opposite fluxes of sodium and calcium ions across plasmalemmal membranes. NCXs, thereby, play key roles in maintaining an ion homeostasis that preserves cellular integrity. Hence, alterations in NCX expression and regulation have been found to lead to ionic imbalances that are often associated with intracellular calcium overload and cell death. On the other hand, intracellular calcium has been identified as a key driver for a multitude of downstream signaling events that are crucial for proper functioning of biological systems, thus highlighting the need for a tightly controlled balance. In the CNS, NCXs have been primarily characterized in the context of synaptic transmission and ischemic brain damage. However, a much broader picture is emerging. NCXs are expressed by virtually all cells of the CNS including oligodendrocytes (OLGs), the cells that generate the myelin sheath. With a growing appreciation of dynamic calcium signals in OLGs, NCXs are becoming increasingly recognized for their crucial roles in shaping OLG function under both physiological and pathophysiological conditions. In order to provide a current update, this review focuses on the importance of NCXs in cells of the OLG lineage. More specifically, it provides a brief introduction into plasmalemmal NCXs and their modes of activity, and it discusses the roles of OLG expressed NCXs in regulating CNS myelination and in contributing to CNS pathologies associated with detrimental effects on OLG lineage cells.

The Na+/Ca2+ exchangers in demyelinating diseases

Intracellular [Na⁺]_i and [Ca²⁺]_i imbalance significantly contribute to neuro-axonal dysfunctions and maladaptive myelin repair or remyelination failure in chronic inflammatory demyelinating diseases such as multiple sclerosis. Progress in recent years has led to significant advances in understanding how [Ca²⁺]_i signaling network drive degeneration or remyelination of demyelinated axons. The Na⁺/Ca²⁺ exchangers (NCXs), a transmembrane protein family including three members encoded by ncx1, ncx2, and ncx3 genes, are emerging important regulators of [Na⁺]_i and [Ca²⁺]_i both in neurons and glial cells. Here we review recent advance highlighting the role of NCX exchangers in axons and myelin-forming cells, i.e. oligodendrocytes, which represent the major targets of the aberrant inflammatory attack in multiple sclerosis. The contribution of NCX subtypes to axonal pathology and myelin synthesis will be discussed. Although a definitive understanding of mechanisms regulating axonal pathology and remyelination failure in chronic demyelinating diseases is still lacking and requires further investigation, current knowledge suggest that NCX activity plays a crucial role in these processes. Defining the relative contributions of each NCX transporter in axon pathology and myelinating glia will constitute not only a major advance in understanding in detail the intricate mechanism of neurodegeneration and remyelination failure in demyelinating diseases but also will help to identify neuroprotective or remyelinating strategies targeting selective NCX exchangers as a means of treating MS.

The voltage-gated sodium channel Nav1.7 associated with endometrial cancer

Background: Endometrial cancer is the most common gynecologic malignancy in women in the developed countries. Despite recent progress in functional characterization of voltage-gated sodium channel (Na_v) in multiple cancers, very little was known about the expression of Na_v in human endometrial cancer. The present study sought to determine the role of Na_v and molecular nature of this channel in the endometrial cancer.

Methods: PCR approach was introduced to determine expression level of Na_v subunits in endometrial cancer specimens. Pharmacological agents were used to investigate Na_v function in endometrial cancer cells. Flow cytometry were used to test cancer apoptosis, and invasion assays were applied to test tumor metastasis.

Results: Transcriptional levels of the all Na_v α and β subunits were determined by real time-PCR in endometrial cancer with pair tissues of carcinoma and adjacent nonneoplastic tissue, Na_v1.7 was the most highly expressed Na_v subtype in endometrial cancer tissues. Na_v1.7 level was closely associated with tumor size, local lymph node metastasis, and 5-year and 10-year survival ratio. Inhibition of this channel by Na_v1.7 blocker PF-05089771, promoted cancer apoptosis and attenuated cancer cell invasion.

Conclusion: These results establish a relationship between voltage-gated sodium channel protein and endometrial cancer, and suggest that Na_v1.7 is a potential prognostic biomarker and could serve as a novel therapeutic target for endometrial cancer.

Sodium Channel Nav1.3 Is Expressed by Polymorphonuclear Neutrophils during Mouse Heart and Kidney Ischemia In Vivo and Regulates Adhesion, Transmigration, and Chemotaxis of Human and Mouse Neutrophils In Vitro

BACKGROUND

Voltage-gated sodium channels generate action potentials in excitable cells, but they have also been attributed noncanonical roles in nonexcitable cells. We hypothesize that voltage-gated sodium channels play a functional role during extravasation of neutrophils.

METHODS

Expression of voltage-gated sodium channels was analyzed by polymerase chain reaction. Distribution of Nav1.3 was determined by immunofluorescence and flow cytometry in mouse models of ischemic heart and kidney injury. Adhesion, transmigration, and chemotaxis of neutrophils to endothelial cells and collagen were investigated with voltage-gated sodium channel inhibitors and lidocaine in vitro. Sodium currents were examined with a whole cell patch clamp.

RESULTS

Mouse and human neutrophils express multiple voltage-gated sodium channels. Only Nav1.3 was detected in neutrophils recruited to ischemic mouse heart (25 ± 7%, n = 14) and kidney (19 ± 2%, n = 6) in vivo. Endothelial adhesion of mouse neutrophils was reduced by tetrodotoxin (56 ± 9%, unselective Nav-inhibitor), ICA121431 (53 ± 10%), and Pterinotoxin-2 (55 ± 9%; preferential inhibitors of Nav1.3, n = 10). Tetrodotoxin (56 ± 19%), ICA121431 (62 ± 22%), and Pterinotoxin-2 (59 ± 22%) reduced transmigration of human neutrophils through endothelial cells, and also prevented chemotactic migration (n = 60, 3 × 20 cells). Lidocaine reduced neutrophil adhesion to 60 ± 9% (n = 10) and transmigration to 54 ± 8% (n = 9). The effect of lidocaine was not increased by ICA121431 or Pterinotoxin-2.

CONCLUSIONS

Nav1.3 is expressed in neutrophils in vivo; regulates attachment, transmigration, and chemotaxis in vitro; and may serve as a relevant target for antiinflammatory effects of lidocaine.

No transcriptional evidence for active Nav channels in two classes of cancer cell

Voltage-gated sodium channel (Na_v) expression in non-excitable cells has raised questions regarding their non-canonical roles. Interestingly, a growing body of evidence also points towards the prevalence of aberrant Na_v expression in malignant tumors, potentially opening a new therapeutic window. In this study, the transcriptional consequences of channel inhibition were investigated in non-small cell lung carcinoma H460 and neuroblastoma SH-SYSY cell lines, that both express Na_v1.7. Channel activity was blocked by the application of both selective, ProTx-II, and non-selective, tetrodotoxin, inhibitors. Global gene expression profiling did not point to any statistically significant inhibition-associated perturbation of the transcriptome. A small subset of genes that showed relatively consistent changes across multiple treatments were further assayed in the context of a multiplex bead expression array which failed to recapitulate the changes seen in the global array. We conclude that there is no robust transcriptional signature associated with the inhibition of two sodium channel expressing cancer cell lines and consequently sodium channel inhibition will not lend itself to therapeutic approaches such as transcription-based drug repurposing.

Ganaxolone enhances microglial clearance activity and promotes remyelination in focal demyelination in the corpus callosum of ovariectomized rats

Aim

Experimental studies have shown that the progesterone metabolite, allopregnanolone, is endowed with promyelinating effects. The mechanisms underlying these promyelinating effects are not well understood. Therefore, we explored the impact of allopregnanolone's synthetic analogue, ganaxolone, on remyelination and microglial activation following focal demyelination in the corpus callosum of ovariectomized rats.

Methods

Ovariectomized adult Sprague Dawley rats received a stereotaxic injection of 2 µL of 1% lysolecithin solution in the corpus callosum followed by daily injections of either ganaxolone (intraperitoneal injection [i.p.], 2.5 mg/kg) or vehicle. The demyelination lesion was assessed 3 and 7 days postdemyelination insult using Luxol fast blue staining and transmission electron microscopy. The expression levels of myelin proteins (MBP, MAG, MOG, CNPase) were explored using Western blot. The inflammatory response and clearance of damaged myelin were evaluated using immunofluorescent staining (Iba1, dMBP, GFAP) and multiplex enzyme‐linked immunosorbent assay (IL‐1β, TNF‐α, IL‐4, IL‐10, IL‐6).

Results

Systemic administration of ganaxolone promoted remyelination of lysolecithin‐induced demyelination, upregulated the expression of major myelin proteins, and enhanced microglial clearance of damaged myelin. Astrocytosis, as well as locally produced pro‐ and antiinflammatory cytokines, was not affected by ganaxolone treatment.

Conclusion

Ganaxolone promotes remyelination in response to focal demyelination of the corpus callosum of ovariectomized rats. This effect is, at least in part, mediated by enhancing microglial clearance of myelin debris, which creates a conducive environment for a successful remyelination process.

Prenatal Environment That Affects Neuronal Migration

Migration of neurons starts in the prenatal period and continues into infancy. This developmental process is crucial for forming a proper neuronal network, and the disturbance of this process results in dysfunction of the brain such as epilepsy. Prenatal exposure to environmental stress, including alcohol, drugs, and inflammation, disrupts neuronal migration and causes neuronal migration disorders (NMDs). In this review, we summarize recent findings on this topic and specifically focusing on two different modes of migration, radial, and tangential migration during cortical development. The shared mechanisms underlying the NMDs are discussed by comparing the molecular changes in impaired neuronal migration under exposure to different types of prenatal environmental stress.

Glutamate versus GABA in neuron-oligodendroglia communication

In the central nervous system (CNS), myelin sheaths around axons are formed by glial cells named oligodendrocytes (OLs). In turn, OLs are generated by oligodendrocyte precursor cells (OPCs) during postnatal development and in adults, according to a process that depends on the proliferation and differentiation of these progenitors. The maturation of OL lineage cells as well as myelination by OLs are complex and highly regulated processes in the CNS. OPCs and OLs express an array of receptors for neurotransmitters, in particular for the two main CNS neurotransmitters glutamate and GABA, and are therefore endowed with the capacity to respond to neuronal activity. Initial studies in cell cultures demonstrated that both glutamate and GABA signaling mechanisms play important roles in OL lineage cell development and function. However, much remains to be learned about the communication of glutamatergic and GABAergic neurons with oligodendroglia in vivo. This review focuses on recent major advances in our understanding of the neuron-oligodendroglia communication mediated by glutamate and GABA in the CNS, and highlights the present controversies in the field. We discuss the expression, activation modes and potential roles of synaptic and extrasynaptic receptors along OL lineage progression. We review the properties of OPC synaptic connectivity with presynaptic glutamatergic and GABAergic neurons in the brain and consider the implication of glutamate and GABA signaling in activity-driven adaptive myelination.

The Neuromodulator Adenosine Regulates Oligodendrocyte Migration at Motor Exit Point Transition Zones

During development, oligodendrocyte progenitor cells (OPCs) migrate extensively throughout the spinal cord. However, their migration is restricted at transition zones (TZs). At these specialized locations, unique glial cells in both zebrafish and mice play a role in preventing peripheral OPC migration, but the mechanisms of this regulation are not understood. To elucidate the mechanisms that mediate OPC segregation at motor exit point (MEP) TZs, we performed an unbiased small-molecule screen. Using chemical screening and in vivo imaging, we discovered that inhibition of A2a adenosine receptors (ARs) causes ectopic OPC migration out of the spinal cord. We provide in vivo evidence that neuromodulation, partially mediated by adenosine, influences OPC migration specifically at the MEP TZ. This work opens exciting possibilities for understanding how OPCs reach their final destinations during development and identifies mechanisms that could promote their migration in disease.

Catalpol Inhibits Ischemia-Induced Premyelinating Oligodendrocyte Damage through Regulation of Intercellular Calcium Homeostasis via Na⁺/Ca2+ Exchanger 3

The heightened vulnerability of premyelinating oligodendrocytes (PreOLs) in response to hypoxia⁻ischemia may contribute to perinatal white matter injury and subsequent neurobehavioral dysfunction. Intracellular Ca²⁺ overload is considered a crucial mechanism predisposing PreOLs to ischemic injury. We previously reported that catalpol, an iridoid glycoside extracted from Rehmannia root, inhibits intracellular Ca²⁺ overload of PreOLs in an in vitro ischemia model. However, the exact underlying mechanisms remain elusive. In the present study, we aimed to investigate the protective effects of catalpol on PreOLs and to explore the underlying mechanisms involved in the modulation of intracellular Ca²⁺ homeostasis. Postnatal day 2 (P2) Sprague-Dawley (SD) rats subjected to bilateral common carotid artery ligation followed by exposure to 8% oxygen for 10 min were used as a rat model of neonatal hypoxia⁻ischemia. We found that catalpol significantly improved behavioral functions and prevented PreOL loss and myelination deficit after hypoxia⁻ischemia. Our in vitro studies also confirmed the direct effects of catalpol on oxygen-glucose deprivation (OGD)-induced cell death and arrested maturation of PreOLs. Moreover, we demonstrated that catalpol significantly inhibited intracellular Ca²⁺ overload and promoted the expression of Na⁺/Ca²⁺ exchanger 3 (NCX3). Finally, we found that catalpol significantly reduced mitochondrial damage and subsequent extracellular signal-regulated kinase 1/2 (ERK1/2) and poly-ADP-ribose polymerase-1 (PARP-1) activation. Treatment with NCX3-preferring inhibitor 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943) significantly reversed the protective effects of catalpol on PreOLs under OGD. Overall, our data suggest that catalpol protects PreOLs from ischemic injury through regulation of intercellular Ca²⁺ homeostasis via upregulation of NCX3 activity.

α2 isoform of Na+,K+-ATPase via Na+,Ca2+ exchanger modulates myelin basic protein synthesis in oligodendrocyte lineage cells in vitro

Oligodendrocytes in the CNS myelinate neuronal axons, facilitating rapid propagation of action potentials. Myelin basic protein (MBP) is an essential component of myelin and its absence results in severe hypomyelination. In oligodendrocyte lineage cell (OLC) monocultures MBP synthesis starts at DIV4. Ouabain (10 nM), a Na⁺,K⁺-ATPase (NKA) blocker, stimulates MBP synthesis. As OLCs express the α2 isoform of NKA (α2-NKA) that has a high affinity for ouabain, we hypothesized that α2-NKA mediates this effect. Knockdown of α2-NKA with small interfering (si)RNA (α2-siRNA) significantly potentiated MBP synthesis at DIV4 and 5. This effect was completely blocked by KB-R7943 (1 μM), a Na⁺,Ca²⁺ exchanger (NCX) antagonist. α2-NKA ablation increased the frequency of NCX-mediated spontaneous Ca²⁺ transients ([Ca²⁺]t) at DIV4, whereas in control OLC cultures comparable frequency of [Ca²⁺]t was observed at DIV5. At DIV6 almost no [Ca²⁺]t were observed either in control or in α2-siRNA-treated cultures. Immunocytochemical analyses showed that α2-NKA co-localizes with MBP in proximal processes of immature OLCs but is only weakly present in MBP-enriched membrane sheets. Knockdown of α2-NKA in cortical slice cultures did not change MBP levels but reduced co-localization of neurofilament- and MBP-positive compartments. We conclude that α2-NKA activity in OLCs affects NCX-mediated [Ca²⁺]t and the onset of MBP synthesis. We suggest therefore that neuronal activity, presumably in form of local extracellular [K⁺] changes, might locally influence NCX-mediated [Ca²⁺]t in OLC processes thus triggering local MBP synthesis in the vicinity of an active axon.

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.

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.

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.

Roles of NG2-glia in ischemic stroke

Recent studies have shown that a widely distributed class of glial cells, termed NG2-glia, engages in rapid signaling with surrounding neurons through direct synaptic contacts in the developing and mature central nervous system (CNS). This unique glial cell group has a typical function of proliferating and differentiating into oligodendrocytes during early development of the brain, which is crucial to axon myelin formation. Therefore, NG2-glia are also called oligodendrocyte precursor cells (OPCs). In vitro and in vivo studies reveal that NG2-glia expressing receptors and ion channels demonstrate functional significance for rapid signaling with neuronal synapses and modulation of neuronal activities in both physiological and pathological conditions. Although it is well known that NG2-glia play an important role in demyelinating diseases such as multiple sclerosis, little is known about how NG2-glia or OPCs impact neurons and brain function following ischemic injury. This review summarizes recent progress on the roles of NG2-glia in ischemic stroke and illustrates new approaches for targeting NG2-glia in the brain to treat this disease.

Astrocyte Sodium Signalling and Panglial Spread of Sodium Signals in Brain White Matter

In brain grey matter, excitatory synaptic transmission activates glutamate uptake into astrocytes, inducing sodium signals which propagate into neighboring astrocytes through gap junctions. These sodium signals have been suggested to serve an important role in neuro-metabolic coupling. So far, it is unknown if astrocytes in white matter-that is in brain regions devoid of synapses-are also able to undergo such intra- and intercellular sodium signalling. In the present study, we have addressed this question by performing quantitative sodium imaging in acute tissue slices of mouse corpus callosum. Focal application of glutamate induced sodium transients in SR101-positive astrocytes. These were largely unaltered in the presence of ionotropic glutamate receptors blockers, but strongly dampened upon pharmacological inhibition of glutamate uptake. Sodium signals induced in individual astrocytes readily spread into neighboring SR101-positive cells with peak amplitudes decaying monoexponentially with distance from the stimulated cell. In addition, spread of sodium was largely unaltered during pharmacological inhibition of purinergic and glutamate receptors, indicating gap junction-mediated, passive diffusion of sodium between astrocytes. Using cell-type-specific, transgenic reporter mice, we found that sodium signals also propagated, albeit less effectively, from astrocytes to neighboring oligodendrocytes and NG2 cells. Again, panglial spread was unaltered with purinergic and glutamate receptors blocked. Taken together, our results demonstrate that activation of sodium-dependent glutamate transporters induces sodium signals in white matter astrocytes, which spread within the astrocyte syncytium. In addition, we found a panglial passage of sodium signals from astrocytes to NG2 cells and oligodendrocytes, indicating functional coupling between these macroglial cells in white matter.

Endogenous GABA controls oligodendrocyte lineage cell number, myelination, and CNS internode length

Adjusting the thickness and internodal length of the myelin sheath is a mechanism for tuning the conduction velocity of axons to match computational needs. Interactions between oligodendrocyte precursor cells (OPCs) and developing axons regulate the formation of myelin around axons. We now show, using organotypic cerebral cortex slices from mice expressing eGFP in Sox10‐positive oligodendrocytes, that endogenously released GABA, acting on GABA_A receptors, greatly reduces the number of oligodendrocyte lineage cells. The decrease in oligodendrocyte number correlates with a reduction in the amount of myelination but also an increase in internode length, a parameter previously thought to be set by the axon diameter or to be a property intrinsic to oligodendrocytes. Importantly, while TTX block of neuronal activity had no effect on oligodendrocyte lineage cell number when applied alone, it was able to completely abolish the effect of blocking GABA_A receptors, suggesting that control of myelination by endogenous GABA may require a permissive factor to be released from axons. In contrast, block of AMPA/KA receptors had no effect on oligodendrocyte lineage cell number or myelination. These results imply that, during development, GABA can act as a local environmental cue to control myelination and thus influence the conduction velocity of action potentials within the CNS. GLIA 2017;65:309–321

NG2 glial cells integrate synaptic input in global and dendritic calcium signals

Synaptic signaling to NG2-expressing oligodendrocyte precursor cells (NG2 cells) could be key to rendering myelination of axons dependent on neuronal activity, but it has remained unclear whether NG2 glial cells integrate and respond to synaptic input. Here we show that NG2 cells perform linear integration of glutamatergic synaptic inputs and respond with increasing dendritic calcium elevations. Synaptic activity induces rapid Ca²⁺ signals mediated by low-voltage activated Ca²⁺ channels under strict inhibitory control of voltage-gated A-type K⁺ channels. Ca²⁺ signals can be global and originate throughout the cell. However, voltage-gated channels are also found in thin dendrites which act as compartmentalized processing units and generate local calcium transients. Taken together, the activity-dependent control of Ca²⁺ signals by A-type channels and the global versus local signaling domains make intracellular Ca²⁺ in NG2 cells a prime signaling molecule to transform neurotransmitter release into activity-dependent myelination.

DOI:http://dx.doi.org/10.7554/eLife.16262.001

Glial Na(+) -dependent ion transporters in pathophysiological conditions

Sodium dynamics are essential for regulating functional processes in glial cells. Indeed, glial Na(+) signaling influences and regulates important glial activities, and plays a role in neuron-glia interaction under physiological conditions or in response to injury of the central nervous system (CNS). Emerging studies indicate that Na(+) pumps and Na(+) -dependent ion transporters in astrocytes, microglia, and oligodendrocytes regulate Na(+) homeostasis and play a fundamental role in modulating glial activities in neurological diseases. In this review, we first briefly introduced the emerging roles of each glial cell type in the pathophysiology of cerebral ischemia, Alzheimer's disease, epilepsy, Parkinson's disease, Amyotrophic Lateral Sclerosis, and myelin diseases. Then, we discussed the current knowledge on the main roles played by the different glial Na(+) -dependent ion transporters, including Na(+) /K(+) ATPase, Na(+) /Ca(2+) exchangers, Na(+) /H(+) exchangers, Na(+) -K(+) -Cl(-) cotransporters, and Na(+) - HCO3- cotransporter in the pathophysiology of the diverse CNS diseases. We highlighted their contributions in cell survival, synaptic pathology, gliotransmission, pH homeostasis, and their role in glial activation, migration, gliosis, inflammation, and tissue repair processes. Therefore, this review summarizes the foundation work for targeting Na(+) -dependent ion transporters in glia as a novel strategy to control important glial activities associated with Na(+) dynamics in different neurological disorders. GLIA 2016;64:1677-1697.

The Epithelial Sodium Channel and the Processes of Wound Healing

The epithelial sodium channel (ENaC) mediates passive sodium transport across the apical membranes of sodium absorbing epithelia, like the distal nephron, the intestine, and the lung airways. Additionally, the channel has been involved in the transduction of mechanical stimuli, such as hydrostatic pressure, membrane stretch, and shear stress from fluid flow. Thus, in vascular endothelium, it participates in the control of the vascular tone via its activity both as a sodium channel and as a shear stress transducer. Rather recently, ENaC has been shown to participate in the processes of wound healing, a role that may also involve its activities as sodium transporter and as mechanotransducer. Its presence as the sole channel mediating sodium transport in many tissues and the diversity of its functions probably underlie the complexity of its regulation. This brief review describes some aspects of ENaC regulation, comments on evidence about ENaC participation in wound healing, and suggests possible regulatory mechanisms involved in this participation.

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.

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.