Activation of sodium-dependent glutamate transporters regulates the morphological aspects of oligodendrocyte maturation via signaling through calcium/calmodulin-dependent kinase IIβ's actin-binding/-stabilizing domain

Identification of a Subpopulation of Astrocyte Progenitor Cells in the Neonatal Subventricular Zone: Evidence that Migration is Regulated by Glutamate Signaling

In mice engineered to express enhanced green fluorescent protein (eGFP) under the control of the entire glutamate transporter 1 (GLT1) gene, eGFP is found in all 'adult' cortical astrocytes. However, when 8.3 kilobases of the human GLT1/EAAT2 promoter is used to control expression of tdTomato (tdT), tdT is only found in a subpopulation of these eGFP-expressing astrocytes. The eGFP mice have been used to define mechanisms of transcriptional regulation using astrocytes cultured from cortex of 1-3 day old mice. Using the same cultures, we were never able to induce tdT+ expression. We hypothesized that these cells might not have migrated into the cortex by this age. In this study, we characterized the ontogeny of tdT+ cells, performed single-cell RNA sequencing (scRNA-seq), and tracked their migration in organotypic slice cultures. At postnatal day (PND) 1, tdT+ cells were observed in the subventricular zone and striatum but not in the cortex, and they did not express eGFP. At PND7, tdT+ cells begin to appear in the cortex with their numbers increasing with age. At PND1, scRNA-seq demonstrates that the tdT+ cells are molecularly heterogeneous, with a subpopulation expressing astrocytic markers, subsequently validated with immunofluorescence. In organotypic slices, tdT+ cells migrate into the cortex, and after 7 days they express GLT1, NF1A, and eGFP. An ionotropic glutamate receptor (iGluR) antagonist reduced by 50% the distance tdT+ cells migrate from the subventricular zone into the cortex. The pan-glutamate transport inhibitor, TFB-TBOA, increased, by sixfold, the number of tdT+ cells in the cortex. In conclusion, although tdT is expressed by non-glial cells at PND1, it is also expressed by glial progenitors that migrate into the cortex postnatally. Using this fluorescent labeling, we provide novel evidence that glutamate signaling contributes to the control of glial precursor migration.

Understanding the pathogenetic mechanisms underlying altered neuronal function associated with CAMK2B mutations

'Dominant mutations in CAMK2B, encoding a subunit of the calcium/calmodulin-dependent protein kinase II (CAMK2), a serine/threonine kinase playing a key role in synaptic plasticity, learning and memory, underlie a recently characterized neurodevelopmental disorder (MRD54) characterized by delayed psychomotor development, mild to severe intellectual disability, hypotonia, and behavioral abnormalities. Targeted therapies to treat MRD54 are currently unavailable. In this review, we revise current knowledge on the molecular and cellular mechanisms underlying the altered neuronal function associated with defective CAMKIIβ function. We also summarize the identified genotype-phenotype correlations and discuss the disease models that have been generated to profile the altered neuronal phenotype and understand the pathophysiology of this disease.

Lysophosphatidic acid signaling via LPA6 : A negative modulator of developmental oligodendrocyte maturation

The developmental process of central nervous system (CNS) myelin sheath formation is characterized by well-coordinated cellular activities ultimately ensuring rapid and synchronized neural communication. During this process, myelinating CNS cells, namely oligodendrocytes (OLGs), undergo distinct steps of differentiation, whereby the progression of earlier maturation stages of OLGs represents a critical step toward the timely establishment of myelinated axonal circuits. Given the complexity of functional integration, it is not surprising that OLG maturation is controlled by a yet fully to be defined set of both negative and positive modulators. In this context, we provide here first evidence for a role of lysophosphatidic acid (LPA) signaling via the G protein-coupled receptor LPA6 as a negative modulatory regulator of myelination-associated gene expression in OLGs. More specifically, the cell surface accessibility of LPA6 was found to be restricted to the earlier maturation stages of differentiating OLGs, and OLG maturation was found to occur precociously in Lpar6 knockout mice. To further substantiate these findings, a novel small molecule ligand with selectivity for preferentially LPA6 and LPA6 agonist characteristics was functionally characterized in vitro in primary cultures of rat OLGs and in vivo in the developing zebrafish. Utilizing this approach, a negative modulatory role of LPA6 signaling in OLG maturation could be corroborated. During development, such a functional role of LPA6 signaling likely serves to ensure timely coordination of circuit formation and myelination. Under pathological conditions as seen in the major human demyelinating disease multiple sclerosis (MS), however, persistent LPA6 expression and signaling in OLGs can be seen as an inhibitor of myelin repair. Thus, it is of interest that LPA6 protein levels appear elevated in MS brain samples, thereby suggesting that LPA6 signaling may represent a potential new druggable pathway suitable to promote myelin repair in MS.

Glutamatergic systems in neuropathic pain and emerging non-opioid therapies

Neuropathic pain, a disease of the somatosensory nervous system, afflicts many individuals and adequate management with current pharmacotherapies remains elusive. The glutamatergic system of neurons, receptors and transporters are intimately involved in pain but, to date, there have been few drugs developed that therapeutically modulate this system. Glutamate transporters, or excitatory amino acid transporters (EAATs), remove excess glutamate around pain transmitting neurons to decrease nociception suggesting that the modulation of glutamate transporters may represent a novel approach to the treatment of pain. This review highlights and summarizes (1) the physiology of the glutamatergic system in neuropathic pain, (2) the preclinical evidence for dysregulation of glutamate transport in animal pain models, and (3) emerging novel therapies that modulate glutamate transporters. Successful drug discovery requires continuous focus on basic and translational methods to fully elucidate the etiologies of this disease to enable the development of targeted therapies. Increasing the efficacy of astrocytic EAATs may serve as a new way to successfully treat those suffering from this devastating disease.

EAAT1-dependent slc1a3 Transcriptional Control depends on the Substrate Translocation Process

Glutamate, the major excitatory neurotransmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent transporters expressed in neurons and glial cells. The bulk of glutamate uptake activity occurs in glial cells through the sodium-dependent glutamate/aspartate transporter (EAAT1/GLAST) and glutamate transporter 1 (EAAT2/GLT-1). EAAT1/GLAST is the predominant transporter within the cerebellum. It is highly enriched in Bergmann glial cells that span the cerebellar cortex and wrap the most abundant glutamatergic synapses in the central nervous system, the synapse formed by the parallel fibers and the Purkinje cells. In the past years, it has become evident that Bergmann glial cells are involved in glutamatergic transmission. Glutamate transporters are tightly regulated due to their essential role in tripartite synapses. Glutamate regulates EAAT1/GLAST function and gene expression in a receptor-dependent and receptor-independent manner. Through the use of the non-metabolizable EAAT1/GLAST ligand, D-Aspartate, and the well-established chick cerebellar Bergmann glia primary culture, in this contribution, we demonstrate that EAAT1/GLAST down-regulates its expression and function at the transcriptional level through the activation of a signaling pathway that includes the phosphatidyl inositol 3 kinase (PI3K), the Ca2+/diacylglycerol dependent protein kinase PKC and the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB). These results favor the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

Summary statement

EAAT1/GLAST down-regulates its expression and function at the transcriptional level by activating a signaling pathway that includes PI3K, PKC and NF-κB, favoring the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

Deletion of the Sodium-Dependent Glutamate Transporter GLT-1 in Maturing Oligodendrocytes Attenuates Myelination of Callosal Axons During a Postnatal Phase of Central Nervous System Development

The sodium-dependent glutamate transporter GLT-1 (EAAT2, SLC1A2) has been well-described as an important regulator of extracellular glutamate homeostasis in the central nervous system (CNS), a function that is performed mainly through its presence on astrocytes. There is, however, increasing evidence for the expression of GLT-1 in CNS cells other than astrocytes and in functional roles that are mediated by mechanisms downstream of glutamate uptake. In this context, GLT-1 expression has been reported for both neurons and oligodendrocytes (OLGs), and neuronal presynaptic presence of GLT-1 has been implicated in the regulation of glutamate uptake, gene expression, and mitochondrial function. Much less is currently known about the functional roles of GLT-1 expressed by OLGs. The data presented here provide first evidence that GLT-1 expressed by maturing OLGs contributes to the modulation of developmental myelination in the CNS. More specifically, using inducible and conditional knockout mice in which GLT-1 was deleted in maturing OLGs during a peak period of myelination (between 2 and 4 weeks of age) revealed hypomyelinated characteristics in the corpus callosum of preferentially male mice. These characteristics included reduced percentages of smaller diameter myelinated axons and reduced myelin thickness. Interestingly, this myelination phenotype was not found to be associated with major changes in myelin gene expression. Taken together, the data presented here demonstrate that GLT-1 expressed by maturing OLGs is involved in the modulation of the morphological aspects associated with CNS myelination in at least the corpus callosum and during a developmental window that appears of particular vulnerability in males compared to females.

Rebound effects of NCX3 pharmacological inhibition: A novel strategy to accelerate myelin formation in oligodendrocytes

The Na+/Ca2+ exchanger NCX3 is an important regulator of sodium and calcium homeostasis in oligodendrocyte lineage. To date, no information is available on the effects resulting from prolonged exposure to NCX3 blockers and subsequent drug washout in oligodendroglia. Here, we investigated, by means of biochemical, morphological and functional analyses, the pharmacological effects of the NCX3 inhibitor, the 5-amino-N-butyl-2-(4-ethoxyphenoxy)-benzamide hydrochloride (BED), on NCXs expression and activity, as well as intracellular [Na+]i and [Ca2+]i levels, during treatment and following drug washout both in human MO3.13 oligodendrocytes and rat primary oligodendrocyte precursor cells (OPCs). BED exposure antagonized NCX activity, induced OPCs proliferation and [Na+]i accumulation. By contrast, 2 days of BED washout after 4 days of treatment significantly upregulated low molecular weight NCX3 proteins, reversed NCX activity, and increased intracellular [Ca2+]i. This BED-free effect was accompanied by an upregulation of NCX3 expression in oligodendrocyte processes and accelerated expression of myelin markers in rat primary oligodendrocytes. Collectively, our findings show that the pharmacological inhibition of the NCX3 exchanger with BED blocker maybe followed by a rebound increase in NCX3 expression and reversal activity that accelerate myelin sheet formation in oligodendrocytes. In addition, they indicate that a particular attention should be paid to the use of NCX inhibitors for possible rebound effects, and suggest that further studies will be necessary to investigate whether selective pharmacological modulation of NCX3 exchanger may be exploited to benefit demyelination and remyelination in demyelinating diseases.

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

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

Myelination of regenerating optic nerve axons occurs in conjunction with an increase of the oligodendrocyte precursor cell population in the adult mice

Recently, regeneration of CNS tracts has been partially accomplished by strategies of intrinsic neuronal growth stimulation. However, restoration of function is dependent on proper myelination of regenerating axons. Previous work from our group (Goulart et al., 2018) has shown an increase in oligodendrocyte staining in the regenerating optic nerve, 2 weeks after crush, in animals that were submitted to conditional deletion of pten gene in retinal ganglion cells and intravitreal injection of zymosan + cAMP. Thus, in the present study we aimed to investigate the maturation of the oligodendroglial lineage and myelination during the regeneration of the optic nerve under the same conditions of our previous work. We showed that the combined treatment promoted an increase of myelinated fibers within the optic nerve, 12 weeks after lesion, as well as an increase in Sox10 positive cells. Early-OPCs, positive to A2B5, were also increased at 12 weeks, whereas O4 positive, late-OPCs, were increased from 2 until 12 weeks after crush. At 12 weeks after crush, the optic nerve of Regenerating group presented more CC1 positive oligodendrocytes and increased MRF positive myelinating oligodendrocytes, culminating in CTB traced regenerating axons superimposed to MBP staining, suggestive of myelination. Thus, our work showed that conditional deletion of pten gene in retinal ganglion cells and intravitreal inflammatory stimuli + cAMP stimulate full maturation of the olidodendroglial lineage, from OPC proliferation and differentiation to myelination of regenerating CNS axons.

The roles of neuron-NG2 glia synapses in promoting oligodendrocyte development and remyelination

NG2 immunopositive progenitor cells, also simply termed as NG2 glia and thought mainly to be oligodendrocyte precursor cells (OPCs), form synaptic connections with neurons in gray and white matters of brain. One of the most classical features of oligodendrocyte lineage cells is myelination, which will favor neuronal signaling transmission. Thus, is there a causal link between the specific synapses of neuron-NG2 glia and myelination? Building on this, here, we will discuss several relevant issues. First, in order to understand the synapses, it is necessary to integrate the definite inputs onto NG2 glia. We show that the synaptic activities and myelination are not synchronized, so the synapses are more likely to regulate early development of NG2 glia and prepare for myelination. Furthermore, several studies have suggested that the synapses also play a role in recovery of pathological conditions, such as multiple sclerosis (MS). Therefore, elucidating the activities of neuron-NG2 glia synapses will be beneficial for both physiological and pathological conditions. Graphical abstract The existence of neuron-NG2 glia synapses reveals that the neuronal activities projecting to NG2 glia is an elaborate regulation, and the signaling from neurons to NG2 glia is frequent in early stage. The neuron-NG2 glia synapses indirectly provide a basic condition to support myelination by extrasynaptic communication. The neuron-NG2 glia synapses also promote remyelination, and it occurs similar to physiological conditions.

Glutamate Transporters: Expression and Function in Oligodendrocytes

Glutamate, the main excitatory neurotransmitter of the vertebrate central nervous system (CNS), is well known as a regulator of neuronal plasticity and neurodevelopment. Such glutamate function is thought to be mediated primarily by signaling through glutamate receptors. Thus, it requires a tight regulation of extracellular glutamate levels and a fine-tuned homeostasis that, when dysregulated, has been associated with a wide range of central pathologies including neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. In the mammalian CNS, extracellular glutamate levels are controlled by a family of sodium-dependent glutamate transporters belonging to the solute carrier family 1 (SLC1) that are also referred to as excitatory amino acid transporters (EAATs). The presumed main function of EAATs has been best described in the context of synaptic transmission where EAATs expressed by astrocytes and neurons effectively regulate extracellular glutamate levels so that synapses can function independently. There is, however, increasing evidence that EAATs are expressed by cells other than astrocytes and neurons, and that they exhibit functions beyond glutamate clearance. In this review, we will focus on the expression and functions of EAATs in the myelinating cells of the CNS, oligodendrocytes. More specifically, we will discuss potential roles of oligodendrocyte-expressed EAATs in contributing to extracellular glutamate homeostasis, and in regulating oligodendrocyte maturation and CNS myelination by exerting signaling functions that have traditionally been associated with glutamate receptors. In addition, we will provide some examples for how dysregulation of oligodendrocyte-expressed EAATs may be involved in the pathophysiology of neurologic diseases.

Glutamate Transporters and Mitochondria: Signaling, Co-compartmentalization, Functional Coupling, and Future Directions

In addition to being an amino acid that is incorporated into proteins, glutamate is the most abundant neurotransmitter in the mammalian CNS, the precursor for the inhibitory neurotransmitter γ-aminobutyric acid, and one metabolic step from the tricarboxylic acid cycle intermediate α-ketoglutarate. Extracellular glutamate is cleared by a family of Na⁺-dependent transporters. These transporters are variably expressed by all cell types in the nervous system, but the bulk of clearance is into astrocytes. GLT-1 and GLAST (also called EAAT2 and EAAT1) mediate this activity and are extremely abundant proteins with their expression enriched in fine astrocyte processes. In this review, we will focus on three topics related to these astrocytic glutamate transporters. First, these transporters co-transport three Na⁺ ions and a H⁺ with each molecule of glutamate and counter-transport one K⁺; they are also coupled to a Cl^- conductance. The movement of Na⁺ is sufficient to cause profound astrocytic depolarization, and the movement of H⁺ is linked to astrocytic acidification. In addition, the movement of Na⁺ can trigger the activation of Na⁺ co-transporters (e.g. Na⁺-Ca²⁺ exchangers). We will describe the ways in which these ionic movements have been linked as signals to brain function and/or metabolism. Second, these transporters co-compartmentalize with mitochondria, potentially providing a mechanism to supply glutamate to mitochondria as a source of fuel for the brain. We will provide an overview of the proteins involved, discuss the evidence that glutamate is oxidized, and then highlight some of the un-resolved issues related to glutamate oxidation. Finally, we will review evidence that ischemic insults (stroke or oxygen/glucose deprivation) cause changes in these astrocytic mitochondria and discuss the ways in which these changes have been linked to glutamate transport, glutamate transport-dependent signaling, and altered glutamate metabolism. We conclude with a broader summary of some of the unresolved issues.

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 oligodendrocyte growth cone and its actin cytoskeleton: A fundamental element for progenitor cell migration and CNS myelination

Cells of the oligodendrocyte (OLG) lineage engage in highly motile behaviors that are crucial for effective central nervous system (CNS) myelination. These behaviors include the guided migration of OLG progenitor cells (OPCs), the surveying of local environments by cellular processes extending from differentiating and pre-myelinating OLGs, and during the process of active myelin wrapping, the forward movement of the leading edge of the myelin sheath's inner tongue along the axon. Almost all of these motile behaviors are driven by actin cytoskeletal dynamics initiated within a lamellipodial structure that is located at the tip of cellular OLG/OPC processes and is structurally as well as functionally similar to the neuronal growth cone. Accordingly, coordinated stoichiometries of actin filament (F-actin) assembly and disassembly at these OLG/OPC growth cones have been implicated in directing process outgrowth and guidance, and the initiation of myelination. Nonetheless, the functional importance of the OLG/OPC growth cone still remains to be fully understood, and, as a unique aspect of actin cytoskeletal dynamics, F-actin depolymerization and disassembly start to predominate at the transition from myelination initiation to myelin wrapping. This review provides an overview of the current knowledge about OLG/OPC growth cones, and it proposes a model in which actin cytoskeletal dynamics in OLG/OPC growth cones are a main driver for morphological transformations and motile behaviors. Remarkably, these activities, at least at the later stages of OLG maturation, may be regulated independently from the transcriptional gene expression changes typically associated with CNS myelination.

D-Aspartate treatment attenuates myelin damage and stimulates myelin repair

Glutamate signaling may orchestrate oligodendrocyte precursor cell (OPC) development and myelin regeneration through the activation of glutamate receptors at OPC‐neuron synapses. D‐Aspartate is a D‐amino acid exerting modulatory actions at glutamatergic synapses. Chronic administration of D‐Aspartate has been proposed as therapeutic treatment in diseases related to myelin dysfunction and NMDA receptors hypofunction, including schizophrenia and cognitive deficits. Here, we show, by using an in vivo remyelination model, that administration of D‐Aspartate during remyelination improved motor coordination, accelerated myelin recovery, and significantly increased the number of small‐diameter myelinated axons. Chronically administered during demyelination, D‐Aspartate also attenuated myelin loss and inflammation. Interestingly, D‐Aspartate exposure stimulated OPC maturation and accelerated developmental myelination in organotypic cerebellar slices. D‐Aspartate promoting effects on OPC maturation involved the activation of glutamate transporters, AMPA and NMDA receptors, and the Na⁺/Ca²⁺ exchanger NCX3. While blocking NMDA or NCX3 significantly prevented D‐Aspartate‐induced [Ca²⁺]_i oscillations, blocking AMPA and glutamate transporters prevented both the initial and oscillatory [Ca²⁺]_i response as well as D‐Aspartate‐induced inward currents in OPC. Our findings reveal that D‐Aspartate treatment may represent a novel strategy for promoting myelin recovery.

Neuronal Nitric Oxide Synthase in Cultured Cerebellar Bergmann Glia: Glutamate-Dependent Regulation

Glutamate exerts its actions through the activation of membrane receptors expressed in neurons and glia cells. The signaling properties of glutamate transporters have been characterized recently, suggesting a complex array of signaling transactions triggered by presynaptic released glutamate. In the cerebellar molecular layer, glutamatergic synapses are surrounded by Bergmann glia cells, compulsory participants of glutamate turnover and supply to neurons. Since a glutamate-dependent increase in cGMP levels has been described in these cells and the nitric oxide-cGMP signaling cascade increases their glutamate uptake activity, we describe here the Bergmann glia expression of neuronal nitric oxide synthetase. An augmentation of neuronal nitric oxide synthase was found upon glutamate exposure. This effect is mediated by glutamate transporters and is related to an increase in the stability of the enzyme. These results strengthen the notion of a complex regulation of glial glutamate uptake that supports neuronal glutamate signaling.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as 'glutamosome'. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and β-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Development of an Embryonic Zebrafish Oligodendrocyte-Neuron Mixed Coculture System

During vertebrate neural development, oligodendrocytes insulate nerve axons with myelin sheaths. Zebrafish (Danio rerio) has emerged as a useful model organism for studying oligodendrocyte development. However, the absence of an in vitro culture system necessitates in vivo manipulations and analyses, which, in some instances, limits the questions that can be addressed. To fill this gap we developed a mixed coculture system for embryonic zebrafish neurons and oligodendrocyte-lineage cells. Cultures harvested from embryos ≥30 hours postfertilization (hpf) yielded oligodendrocyte progenitor cells (OPCs) positive for olig2 and sox10 transgenic reporters. Cultured OPCs exhibited dynamic, exploratory membrane processes, and cell morphologies resembled those established in vivo. Cells harvested from advanced stage embryos possessed more arborized processes than those from early stage embryos. Advanced stage (>60 hpf) embryo culture produced differentiated, mbp⁺ oligodendrocytes. Genetically tractable neuron subtypes extended neurites when harvested from embryos ≥19 hpf. Coculture produced juxtaposed oligodendrocytes and neurons, demonstrating the practical usefulness of this technique for future studies examining axon-oligodendrocyte interactions under defined conditions. We expect that zebrafish oligodendrocyte culture will complement existing in vivo strengths and may facilitate future studies elucidating the mechanisms of oligodendrocyte specification, proliferation, differentiation, motility, and axon-oligodendrocyte interactions that shape adult myelination patterns.

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.

Novel role of the nociceptin system as a regulator of glutamate transporter expression in developing astrocytes

Our previous results showed that oligodendrocyte development is regulated by both nociceptin and its G-protein coupled receptor, the nociceptin/orphanin FQ receptor (NOR). The present in vitro and in vivo findings show that nociceptin plays a crucial conserved role regulating the levels of the glutamate/aspartate transporter GLAST/EAAT1 in both human and rodent brain astrocytes. This nociceptin-mediated response takes place during a critical developmental window that coincides with the early stages of astrocyte maturation. GLAST/EAAT1 upregulation by nociceptin is mediated by NOR and the downstream participation of a complex signaling cascade that involves the interaction of several kinase systems, including PI-3K/AKT, mTOR, and JAK. Because GLAST is the main glutamate transporter during brain maturation, these novel findings suggest that nociceptin plays a crucial role in regulating the function of early astrocytes and their capacity to support glutamate homeostasis in the developing brain.

Expression Profiling of the MAP Kinase Phosphatase Family Reveals a Role for DUSP1 in the Glioblastoma Stem Cell Niche

The dual specificity phosphatases (DUSPs) constitute a family of stress-induced enzymes that provide feedback inhibition on mitogen-activated protein kinases (MAPKs) critical in key aspects of oncogenic signaling. While described in other tumor types, the landscape of DUSP mRNA expression in glioblastoma (GB) remains largely unexplored. Interrogation of the REpository for Molecular BRAin Neoplasia DaTa (REMBRANDT) revealed induction (DUSP4, DUSP6), repression (DUSP2, DUSP7-9), or mixed (DUSP1, DUSP5, DUSP10, DUSP15) DUSP transcription of select DUSPs in bulk tumor specimens. To resolve features specific to the tumor microenvironment, we searched the Ivy Glioblastoma Atlas Project (Ivy GAP) repository, which highlight DUSP1, DUSP5, and DUSP6 as the predominant family members induced within pseudopalisading and perinecrotic regions. The inducibility of DUSP1 in response to hypoxia, dexamethasone, or the chemotherapeutic agent camptothecin was confirmed in GB cell lines and tumor-derived stem cells (TSCs). Moreover, we show that loss of DUSP1 expression is a characteristic of TSCs and correlates with expression of tumor stem cell markers in situ (ABCG2, PROM1, L1CAM, NANOG, SOX2). This work reveals a dynamic pattern of DUSP expression within the tumor microenvironment that reflects the cumulative effects of factors including regional ischemia, chemotherapeutic exposure among others. Moreover, our observation regarding DUSP1 dysregulation within the stem cell niche argue for its importance in the survival and proliferation of this therapeutically resistant population.

Induced Neural Activity Promotes an Oligodendroglia Regenerative Response in the Injured Spinal Cord and Improves Motor Function after Spinal Cord Injury

Myelination in the central nervous system (CNS) is a dynamic process that includes birth of oligodendrocyte progenitor cells (OPCs), their differentiation into oligodendrocytes, and ensheathment of axons. Regulation of myelination by neuronal activity has emerged as a new mechanism of CNS plasticity. Activity-dependent myelination has been shown to regulate sensory, motor, and cognitive functions. In this work, we aimed to employ this mechanism of CNS plasticity by utilizing induced neuronal activity to promote remyelination and functional recovery in a subchronic model of spinal cord injury (SCI). We used a mild contusive SCI at T10, which demyelinates surviving axons of the dorsal corticospinal tract (dCST), to investigate the effects of induced neuronal activity on oligodendrogenesis, remyelination, and motor function after SCI. Neuronal activity was induced through epidural electrodes that were implanted over the primary motor (M1) cortex. Induced neuronal activity increased the number of proliferating OPCs. Additionally, induced neuronal activity in the subchronic stages of SCI increased the number of oligodendrocytes, and enhanced myelin basic protein (MBP) expression and myelin sheath formation in dCST. The oligodendroglia regenerative response could have been mediated by axon-OPC synapses, the number of which increased after induced neuronal activity. Further, M1-induced neuronal activation promoted recovery of hindlimb motor function after SCI. Our work is a proof of principle demonstration that epidural electrical stimulation as a mode of inducing neuronal activity throughout white matter tracts of the CNS could be used to promote remyelination and functional recovery after CNS injuries and demyelination disorders.

Immunohistochemical Localization of GFAP and Glutamate Regulatory Proteins in Chick Retina and Their Levels of Expressions in Altered Photoperiods

Moderate to intense light is reported to damage the chick retina, which is cone dominated. Light damage alters neurotransmitter pools, such as those of glutamate. Glutamate level in the retina is regulated by glutamate-aspartate transporter (GLAST) and glutamine synthetase (GS). We examined immunolocalization patterns and the expression levels of both markers and of glial fibrillary acidic protein (GFAP, a marker of neuronal stress) in chick retina exposed to 2000 lux under 12-h light:12-h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod), and 24L:0D (constant light) at post-hatch day 30. Retinal damage (increased death of photoreceptors and inner retinal neurons and Müller cell hypertrophy) and GFAP expression in Müller cells were maximal in 24L:0D condition compared to that seen in 12L:12D and 18L:6D conditions. GS was present in Müller cells and GLAST expressed in Müller cell processes and photoreceptor inner segments. GLAST expression was decreased in 24L:0D condition, and the expression levels between 12L:12D and 18L:6D, though increased marginally, were statistically insignificant. Similar was the case with GS expression that significantly decreased in 24L:0D condition. Our previous study with chicks exposed to 2000 lux reported increased retinal glutamate level in 24L:0D condition. The present results indicate that constant light induces decreased expressions of GLAST and GS, a condition that might aggravate glutamate-mediated neurotoxicity and delay neuroprotection in a cone-dominated retina.

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.

Glutamine/Glutamate Transporters in Glial Cells: Much More Than Participants of a Metabolic Shuttle

Glial glutamine and glutamate transporters play an important role in glial/neuronal interactions. An excellent model to establish the role of these membrane proteins is the cerebellum. The most abundant glutamatergic synapse in the central nervous system is present in the molecular layer of the cerebellar cortex, and it is entirely wrapped by Bergmann glial cells. The recycling of glutamate involves glutamate and glutamine transporters enriched in these radial glial processes. The functional properties of amino acid glial transporters allow, in an activity-dependent manner, the conformation of protein complexes important for the adequate support of glutamatergic neurotransmission. A detailed description of the most important features of glial glutamate and glutamine transporters follows, and a working model of the molecular mechanisms by which these glutamate and glutamine binding proteins interact, and by these means might modulate cerebellar glutamatergic transactions, is presented.

Extracellular cues influencing oligodendrocyte differentiation and (re)myelination

There is an increasing number of neurologic disorders found to be associated with loss and/or dysfunction of the CNS myelin sheath, ranging from the classic demyelinating disease, multiple sclerosis, through CNS injury, to neuropsychiatric diseases. The disabling burden of these diseases has sparked a growing interest in gaining a better understanding of the molecular mechanisms regulating the differentiation of the myelinating cells of the CNS, oligodendrocytes (OLGs), and the process of (re)myelination. In this context, the importance of the extracellular milieu is becoming increasingly recognized. Under pathological conditions, changes in inhibitory as well as permissive/promotional cues are thought to lead to an overall extracellular environment that is obstructive for the regeneration of the myelin sheath. Given the general view that remyelination is, even though limited in human, a natural response to demyelination, targeting pathologically 'dysregulated' extracellular cues and their downstream pathways is regarded as a promising approach toward the enhancement of remyelination by endogenous (or if necessary transplanted) OLG progenitor cells. In this review, we will introduce the extracellular cues that have been implicated in the modulation of (re)myelination. These cues can be soluble, part of the extracellular matrix (ECM) or mediators of cell-cell interactions. Their inhibitory and permissive/promotional roles with regard to remyelination as well as their potential for therapeutic intervention will be discussed.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

Transcriptional Regulation of Glutamate Transporters: From Extracellular Signals to Transcription Factors

Glutamate is the predominant excitatory neurotransmitter in the mammalian CNS. It mediates essentially all rapid excitatory signaling. Dysfunction of glutamatergic signaling contributes to developmental, neurologic, and psychiatric diseases. Extracellular glutamate is cleared by a family of five Na(+)-dependent glutamate transporters. Two of these transporters (GLAST and GLT-1) are relatively selectively expressed in astrocytes. Other of these transporters (EAAC1) is expressed by neurons throughout the nervous system. Expression of the last two members of this family (EAAT4 and EAAT5) is almost exclusively restricted to specific populations of neurons in cerebellum and retina, respectively. In this review, we will discuss our current understanding of the mechanisms that control transcriptional regulation of the different members of this family. Over the last two decades, our understanding of the mechanisms that regulate expression of GLT-1 and GLAST has advanced considerably; several specific transcription factors, cis-elements, and epigenetic mechanisms have been identified. For the other members of the family, little or nothing is known about the mechanisms that control their transcription. It is assumed that by defining the mechanisms involved, we will advance our understanding of the events that result in cell-specific expression of these transporters and perhaps begin to define the mechanisms by which neurologic diseases are changing the biology of the cells that express these transporters. This approach might provide a pathway for developing new therapies for a wide range of essentially untreatable and devastating diseases that kill neurons by an excitotoxic mechanism.

The Autotaxin-Lysophosphatidic Acid Axis Modulates Histone Acetylation and Gene Expression during Oligodendrocyte Differentiation

During development, oligodendrocytes (OLGs), the myelinating cells of the CNS, undergo a stepwise progression during which OLG progenitors, specified from neural stem/progenitor cells, differentiate into fully mature myelinating OLGs. This progression along the OLG lineage is characterized by well synchronized changes in morphology and gene expression patterns. The latter have been found to be particularly critical during the early stages of the lineage, and they have been well described to be regulated by epigenetic mechanisms, especially by the activity of the histone deacetylases HDAC1 and HDAC2. The data presented here identify the extracellular factor autotaxin (ATX) as a novel upstream signal modulating HDAC1/2 activity and gene expression in cells of the OLG lineage. Using the zebrafish as an in vivo model system as well as rodent primary OLG cultures, this functional property of ATX was found to be mediated by its lysophospholipase D (lysoPLD) activity, which has been well characterized to generate the lipid signaling molecule lysophosphatidic acid (LPA). More specifically, the lysoPLD activity of ATX was found to modulate HDAC1/2 regulated gene expression during a time window coinciding with the transition from OLG progenitor to early differentiating OLG. In contrast, HDAC1/2 regulated gene expression during the transition from neural stem/progenitor to OLG progenitor appeared unaffected by ATX and its lysoPLD activity. Thus, together, our data suggest that an ATX-LPA-HDAC1/2 axis regulates OLG differentiation specifically during the transition from OLG progenitor to early differentiating OLG and via a molecular mechanism that is evolutionarily conserved from at least zebrafish to rodent.

SIGNIFICANCE STATEMENT

The formation of the axon insulating and supporting myelin sheath by differentiating oligodendrocytes (OLGs) in the CNS is considered an essential step during vertebrate development. In addition, loss and/or dysfunction of the myelin sheath has been associated with a variety of neurologic diseases in which repair is limited, despite the presence of progenitor cells with the potential to differentiate into myelinating OLGs. This study characterizes the autotaxin-lysophosphatidic acid signaling axis as a modulator of OLG differentiation in vivo in the developing zebrafish and in vitro in rodent OLGs in culture. These findings provide novel insight into the regulation of developmental myelination, and they are likely to lead to advancing studies related to the stimulation of myelin repair under pathologic conditions.

Glutamatergic Transmission: A Matter of Three

Glutamatergic transmission in the vertebrate brain requires the involvement of glia cells, in a continuous molecular dialogue. Glial glutamate receptors and transporters are key molecules that sense synaptic activity and by these means modify their physiology in the short and long term. Posttranslational modifications that regulate protein-protein interactions and modulate transmitter removal are triggered in glial cells by neuronal released glutamate. Moreover, glutamate signaling cascades in these cells are linked to transcriptional and translational control and are critically involved in the control of the so-called glutamate/glutamine shuttle and by these means in glutamatergic neurotransmission. In this contribution, we summarize our current understanding of the biochemical consequences of glutamate synaptic activity in their surrounding partners and dissect the molecular mechanisms that allow neurons to take control of glia physiology to ensure proper glutamate-mediated neuronal communication.