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

Spatiotemporal calcium dynamics orchestrate oligodendrocyte development and myelination

Oligodendrocyte lineage cells (OLCs), comprising oligodendrocyte precursor cells (OPCs) and oligodendrocytes, are pivotal in sculpting central nervous system (CNS) architecture and function. OPCs mature into oligodendrocytes, which enwrap axons with myelin sheaths that are critical for enhancing neural transmission. Notably, OLCs actively respond to neuronal activity, modulating neural circuit functions. Understanding neuron-OLC interactions is key to unraveling how OLCs contribute to CNS health and pathology. This review highlights insights from zebrafish and mouse models, revealing how synaptic and extrasynaptic pathways converge to shape spatiotemporal calcium (Ca2+) dynamics within OLCs. We explore how Ca2+ signal integration across spatial and temporal scales acts as a master regulator of OLC fate determination and myelin plasticity.

Oligodendrocyte precursor cells establish regulatory contacts with the soma of active neurons

Oligodendrocyte precursor cells (OPCs) can generate myelinating oligodendrocytes life-long, a process that is dependent on neuronal activity. However, it is possible that OPCs have additional functions, influencing neuronal functions directly. We have used a mouse genetic model of juvenile seizures and chemical induction of neuronal activity to examine the morphological and molecular changes in OPCs around activated principal neurons. We found an increase in process extension of OPCs specifically toward the soma of activated neurons. Moreover, we found that the close proximity of OPC processes around neurons expressing the immediate early gene c-Fos decreased the calcium transients in these neurons, indicating a regulative function of OPCs. Analyses of transcriptome and chromatin accessibility revealed significant changes in genes involved in transforming growth factor beta (TGFβ) signaling. Extracellular matrix genes, particularly those encoding type VI collagen, an established binding partner for the OPC surface protein NG2, was increased around active neurons. Our findings indicate that OPCs are an integral part of the neural network and may help to decrease the activity of neurons that have previously been over-excited, in order to protect these neurons.

Physiology of neuroglia of the central nervous system

Neuroglia of the central nervous system (CNS) are a diverse and highly heterogeneous population of cells of ectodermal, neuroepithelial origin (macroglia, that includes astroglia and oligodendroglia) and mesodermal, myeloid origin (microglia). Neuroglia are primary homeostatic cells of the CNS, responsible for the support, defense, and protection of the nervous tissue. The extended class of astroglia (which includes numerous parenchymal astrocytes, such as protoplasmic, fibrous, velate, marginal, etc., radial astrocytes such as Bergmann glia, Muller glia, etc., and ependymoglia lining the walls of brain ventricles and central canal of the spinal cord) is primarily responsible for overall homeostasis of the nervous tissue. Astroglial cells control homeostasis of ions, neurotransmitters, hormones, metabolites, and are responsible for neuroprotection and defense of the CNS. Oligodendroglia provide for myelination of axons, hence supporting and sustaining CNS connectome. Microglia are tissue macrophages adapted to the CNS environment which contribute to the host of physiologic functions including regulation of synaptic connectivity through synaptic pruning, regulation of neurogenesis, and even modifying neuronal excitability. Neuroglial cells express numerous receptors, transporters, and channels that allow neuroglia to perceive and follow neuronal activity. Activation of these receptors triggers intracellular ionic signals that govern various homeostatic cascades underlying glial supportive and defensive capabilities. Ionic signaling therefore represents the substrate of glial excitability.

Genetic Downregulation of GABAB Receptors from Oligodendrocyte Precursor Cells Protects Against Demyelination in the Mouse Spinal Cord

GABAergic signaling and GABAB receptors play crucial roles in regulating the physiology of oligodendrocyte-lineage cells, including their proliferation, differentiation, and myelination. Therefore, they are promising targets for studying how spinal oligodendrocyte precursor cells (OPCs) respond to injuries and neurodegenerative diseases like multiple sclerosis. Taking advantage of the temporally controlled and cell-specific genetic downregulation of GABAB receptors from OPCs, our investigation addresses their specific influence on OPC behavior in the gray and white matter of the mouse spinal cord. Our results show that, while GABAB receptors do not significantly alter spinal cord myelination under physiological conditions, they distinctly regulate the OPC differentiation and Ca2+ signaling. In addition, we investigate the impact of OPC-GABAB receptors in two models of toxic demyelination, namely, the cuprizone and the lysolecithin models. The genetic downregulation of OPC-GABAB receptors protects against demyelination and oligodendrocyte loss. Additionally, we observe the enhanced resilience to cuprizone-induced pathological alterations in OPC Ca2+ signaling. Our results provide valuable insights into the potential therapeutic implications of manipulating GABAB receptors in spinal cord OPCs and deepen our understanding of the interplay between GABAergic signaling and spinal cord OPCs, providing a basis for future research.

Adenosine A2B receptors differently modulate oligodendrogliogenesis and myelination depending on their cellular localization

Differentiation of oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes (OLs) is a key event for axonal myelination in the brain; this process fails during demyelinating pathologies. Adenosine is emerging as an important player in oligodendrogliogenesis, by activating its metabotropic receptors (A1R, A2AR, A2BR, and A3R). We previously demonstrated that the Gs-coupled A2BR reduced differentiation of primary OPC cultures by inhibiting delayed rectifier (IK) as well as transient (IA) outward K+ currents. To deepen the unclear role of this receptor subtype in neuron-OL interplay and in myelination process, we tested the effects of different A2BR ligands in a dorsal root ganglion neuron (DRGN)/OPC cocultures, a corroborated in vitro myelination assay. The A2BR agonist, BAY60-6583, significantly reduced myelin basic protein levels but simultaneously increased myelination index in DRGN/OPC cocultures analyzed by confocal microscopy. The last effect was prevented by the selective A2BR antagonists, PSB-603 and MRS1706. To clarify this unexpected data, we wondered whether A2BRs could play a functional role on DRGNs. We first demonstrated, by immunocytochemistry, that primary DRGN monoculture expressed A2BRs. Their selective activation by BAY60-6583 enhanced DRGN excitability, as demonstrated by increased action potential firing, decreased rheobase and depolarized resting membrane potential and were prevented by PSB-603. Throughout this A2BR-dependent enhancement of neuronal activity, DRGNs could release factors to facilitate myelination processes. Finally, silencing A2BR in DRGNs alone prevents the increased myelination induced by BAY60-6583 in cocultures. In conclusion, our data suggest a different role of A2BR during oligodendrogliogenesis and myelination, depending on their activation on neurons or oligodendroglial cells.

Membrane properties and coupling of macroglia in the optic nerve

We established a longitudinal acute slice preparation of transgenic mouse optic nerve to characterize membrane properties and coupling of glial cells by patch-clamp and dye-filling, complemented by immunohistochemistry. Unlike in cortex or hippocampus, the majority of EGFP + cells in optic nerve of the hGFAP-EGFP transgenic mouse, a tool to identify astrocytes, were characterized by time and voltage dependent K+-currents including A-type K+-currents, properties previously described for NG2 glia. Indeed, the majority of transgene expressing cells in optic nerve were immunopositive for NG2 proteoglycan, whereas only a minority show GFAP immunoreactivity. Similar physiological properties were seen in YFP + cells from NG2-YFP transgenic mice, indicating that in optic nerve the transgene of hGFAP-EGFP animals is expressed by NG2 glia instead of astrocytes. Using Cx43kiECFP transgenic mice as another astrocyte-indicator revealed that astrocytes had passive membrane currents. Dye-filling showed that hGFAP-EGFP+ cells in optic nerve were coupled to none or few neighboring cells while hGFAP-EGFP+ cells in the cortex form large networks. Similarly, dye-filling of NG2-YFP+ and Cx43-CFP+ cells in optic nerve revealed small networks. Our work shows that identification of astrocytes in optic nerve requires distinct approaches, that the cells express membrane current patterns distinct from cortex and that they form small networks.

Synaptic input and Ca2+ activity in zebrafish oligodendrocyte precursor cells contribute to myelin sheath formation

In the nervous system, only one type of neuron-glial synapse is known to exist: that between neurons and oligodendrocyte precursor cells (OPCs), yet their composition, assembly, downstream signaling and in vivo functions remain largely unclear. Here, we address these questions using in vivo microscopy in zebrafish spinal cord and identify postsynaptic molecules PSD-95 and gephyrin in OPCs. The puncta containing these molecules in OPCs increase during early development and decrease upon OPC differentiation. These puncta are highly dynamic and frequently assemble at 'hotspots'. Gephyrin hotspots and synapse-associated Ca2+ activity in OPCs predict where a subset of myelin sheaths forms in differentiated oligodendrocytes. Further analyses reveal that spontaneous synaptic release is integral to OPC Ca2+ activity, while evoked synaptic release contributes only in early development. Finally, disruption of the synaptic genes dlg4a/dlg4b, gphnb and nlgn3b impairs OPC differentiation and myelination. Together, we propose that neuron-OPC synapses are dynamically assembled and can predetermine myelination patterns through Ca2+ signaling.

Norepinephrine regulates calcium signals and fate of oligodendrocyte precursor cells in the mouse cerebral cortex

Oligodendrocyte precursor cells (OPCs) generate oligodendrocytes, contributing to myelination and myelin repair. OPCs contact axons and respond to neuronal activity, but how the information relayed by the neuronal activity translates into OPC Ca2+ signals, which in turn influence their fate, remains unknown. We generated transgenic mice for concomitant monitoring of OPCs Ca2+ signals and cell fate using 2-photon microscopy in the somatosensory cortex of awake-behaving mice. Ca2+ signals in OPCs mainly occur within processes and confine to Ca2+ microdomains. A subpopulation of OPCs enhances Ca2+ transients while mice engaged in exploratory locomotion. We found that OPCs responsive to locomotion preferentially differentiate into oligodendrocytes, and locomotion-non-responsive OPCs divide. Norepinephrine mediates locomotion-evoked Ca2+ increases in OPCs by activating α1 adrenergic receptors, and chemogenetic activation of OPCs or noradrenergic neurons promotes OPC differentiation. Hence, we uncovered that for fate decisions OPCs integrate Ca2+ signals, and norepinephrine is a potent regulator of OPC fate.

Oligodendrocyte precursor cells stop sensory axons regenerating into the spinal cord

Primary somatosensory axons stop regenerating as they re-enter the spinal cord, resulting in incurable sensory loss. What arrests them has remained unclear. We previously showed that axons stop by forming synaptic contacts with unknown non-neuronal cells. Here, we identified these cells in adult mice as oligodendrocyte precursor cells (OPCs). We also found that only a few axons stop regenerating by forming dystrophic endings, exclusively at the CNS:peripheral nervous system (PNS) borderline where OPCs are absent. Most axons stop in contact with a dense network of OPC processes. Live imaging, immuno-electron microscopy (immuno-EM), and OPC-dorsal root ganglia (DRG) co-culture additionally suggest that axons are rapidly immobilized by forming synapses with OPCs. Genetic OPC ablation enables many axons to continue regenerating deep into the spinal cord. We propose that sensory axons stop regenerating by encountering OPCs that induce presynaptic differentiation. Our findings identify OPCs as a major regenerative barrier that prevents intraspinal restoration of sensory circuits following spinal root injury.

Chronic demyelination and myelin repair after spinal cord injury in mice: A potential link for glutamatergic axon activity

Our prior work examining endogenous repair after spinal cord injury (SCI) in mice revealed that large numbers of new oligodendrocytes (OLs) are generated in the injured spinal cord, with peak oligodendrogenesis between 4 and 7 weeks post-injury (wpi). We also detected new myelin formation over 2 months post-injury (mpi). Our current work significantly extends these results, including quantification of new myelin through 6 mpi and concomitant examination of indices of demyelination. We also examined electrophysiological changes during peak oligogenesis and a potential mechanism driving OL progenitor cell (OPC) contact with axons. Results reveal peak in remyelination occurs during the 3rd mpi, and that myelin generation continues for at least 6 mpi. Further, motor evoked potentials significantly increased during peak remyelination, suggesting enhanced axon potential conduction. Interestingly, two indices of demyelination, nodal protein spreading and Nav1.2 upregulation, were also present chronically after SCI. Nav1.2 was expressed through 10 wpi and nodal protein disorganization was detectable throughout 6 mpi suggesting chronic demyelination, which was confirmed with EM. Thus, demyelination may continue chronically, which could trigger the long-term remyelination response. To examine a potential mechanism that may initiate post-injury myelination, we show that OPC processes contact glutamatergic axons in the injured spinal cord in an activity-dependent manner. Notably, these OPC/axon contacts were increased 2-fold when axons were activated chemogenetically, revealing a potential therapeutic target to enhance post-SCI myelin repair. Collectively, results show the surprisingly dynamic nature of the injured spinal cord over time and that the tissue may be amenable to treatments targeting chronic demyelination.

Oligodendrocyte precursor cells: the multitaskers in the brain

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

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

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

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.

Astrocytic TRPV4 Channels and Their Role in Brain Ischemia

Transient receptor potential cation channels subfamily V member 4 (TRPV4) are non-selective cation channels expressed in different cell types of the central nervous system. These channels can be activated by diverse physical and chemical stimuli, including heat and mechanical stress. In astrocytes, they are involved in the modulation of neuronal excitability, control of blood flow, and brain edema formation. All these processes are significantly impaired in cerebral ischemia due to insufficient blood supply to the tissue, resulting in energy depletion, ionic disbalance, and excitotoxicity. The polymodal cation channel TRPV4, which mediates Ca²⁺ influx into the cell because of activation by various stimuli, is one of the potential therapeutic targets in the treatment of cerebral ischemia. However, its expression and function vary significantly between brain cell types, and therefore, the effect of its modulation in healthy tissue and pathology needs to be carefully studied and evaluated. In this review, we provide a summary of available information on TRPV4 channels and their expression in healthy and injured neural cells, with a particular focus on their role in ischemic brain injury.

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

Recent Insights into the Functional Role of AMPA Receptors in the Oligodendrocyte Lineage Cells In Vivo

This review discusses the experimental findings of several recent studies which investigated the functional role of AMPA receptors (AMPARs) in oligodendrocyte lineage cells in vivo, in mice and in zebrafish. These studies provided valuable information showing that oligodendroglial AMPARs may be involved in the modulation of proliferation, differentiation, and migration of oligodendroglial progenitors, as well as survival of myelinating oligodendrocytes during physiological conditions in vivo. They also suggested that targeting the subunit composition of AMPARs may be an important strategy for treating diseases. However, at the same time, the experimental findings taken together still do not provide a clear picture on the topic. Hence, new ideas and new experimental designs are required for understanding the functional role of AMPARs in the oligodendrocyte lineage cells in vivo. It is also necessary to consider more closely the temporal and spatial aspects of AMPAR-mediated signalling in the oligodendrocyte lineage cells. These two important aspects are routinely discussed by neuronal physiologists studying glutamatergic synaptic transmission, but are rarely debated and thought about by researchers studying glial cells.

Separate optogenetic manipulation of Nerve/glial antigen 2 (NG2) glia and mural cells using the NG2 promoter

Nerve/glial antigen 2 (NG2) is a protein marker of NG2 glia and mural cells, and NG2 promoter activity is utilized to target these cells. However, the NG2 promoter cannot target NG2 glia and mural cells separately. This has been an obstacle for NG2 glia-specific manipulation. Here, we developed transgenic mice in which either cell type can be targeted using the NG2 promoter. We selected a tetracycline-controllable gene induction system for cell type-specific transgene expression, and generated NG2-tetracycline transactivator (tTA) transgenic lines. We crossed tTA lines with the tetO-ChR2 (channelrhodopsin-2)-EYFP line to characterize tTA-dependent transgene induction. We isolated two unique NG2-tTA mouse lines: one that induced ChR2-EYFP only in mural cells, likely due to the chromosomal position effect of NG2-tTA insertion, and the other that induced it in both cell types. We then applied a Cre-mediated set-subtraction strategy to the latter case and eliminated ChR2-EYFP from mural cells, resulting in NG2 glia-specific transgene induction. We further demonstrated that tTA-dependent ChR2 expression could manipulate cell function. Optogenetic mural cell activation decreased cerebral blood flow, as previously reported, indicating that tTA-mediated ChR2 expression was sufficient to impact cellular function. ChR2-mediated depolarization was observed in NG2 glia in acute hippocampal slices. In addition, ChR2-mediated depolarization of NG2 glia inhibited their proliferation but promoted their differentiation in juvenile mice. Since the tTA-tetO combination is expandable, the mural cell-specific NG2-tTA line and the NG2 glia-specific NG2-tTA line will permit us to conduct observational and manipulation studies to examine in vivo function of these cells separately.

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.

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.

L-Type Ca2+ Channels of NG2 Glia Determine Proliferation and NMDA Receptor-Dependent Plasticity

NG2 (nerve/glial antigen 2) glia are uniformly distributed in the gray and white matter of the central nervous system (CNS). They are the major proliferating cells in the brain and can differentiate into oligodendrocytes. NG2 glia do not only receive synaptic input from excitatory and inhibitory neurons, but also secrete growth factors and cytokines, modulating CNS homeostasis. They express several receptors and ion channels that play a role in rapidly responding upon synaptic signals and generating fast feedback, potentially regulating their own properties. Ca²⁺ influx via voltage-gated Ca²⁺ channels (VGCCs) induces an intracellular Ca²⁺ rise initiating a series of cellular activities. We confirmed that NG2 glia express L-type VGCCs in the white and gray matter during CNS development, particularly in the early postnatal stage. However, the function of L-type VGCCs in NG2 glia remains elusive. Therefore, we deleted L-type VGCC subtypes Cav1.2 and Cav1.3 genes conditionally in NG2 glia by crossbreeding NG2-CreERT2 knock-in mice to floxed Cav1.2 and flexed Cav1.3 transgenic mice. Our results showed that ablation of Cav1.2 and Cav1.3 strongly inhibited the proliferation of cortical NG2 glia, while differentiation in white and gray matter was not affected. As a consequence, no difference on myelination could be detected in various brain regions. In addition, we observed morphological alterations of the nodes of Ranvier induced by VGCC-deficient NG2 glia, i.e., shortened paired paranodes in the corpus callosum. Furthermore, deletion of Cav1.2 and Cav1.3 largely eliminated N-methyl-D-aspartate (NMDA)-dependent long-term depression (LTD) and potentiation in the hippocampus while the synaptic input to NG2 glia from axons remained unaltered. We conclude that L-type VGCCs of NG2 glia are essential for cell proliferation and proper structural organization of nodes of Ranvier, but not for differentiation and myelin compaction. In addition, L-type VGCCs of NG2 glia contribute to the regulation of long-term neuronal plasticity.

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.

Early life adversity targets the transcriptional signature of hippocampal NG2+ glia and affects voltage gated sodium (Nav) channels properties

The precise mechanisms underlying the detrimental effects of early life adversity (ELA) on adult mental health remain still elusive. To date, most studies have exclusively targeted neuronal populations and not considered neuron-glia crosstalk as a crucially important element for the integrity of stress-related brain function. Here, we have investigated the impact of ELA, in the form of a limited bedding and nesting material (LBN) paradigm, on a glial subpopulation with unique properties in brain homeostasis, the NG2+ cells. First, we have established a link between maternal behavior, activation of the offspring's stress response and heterogeneity in the outcome to LBN manipulation. We further showed that LBN targets the hippocampal NG2+ transcriptome with glucocorticoids being an important mediator of the LBN-induced molecular changes. LBN altered the NG2+ transcriptome and these transcriptional effects were correlated with glucocorticoids levels. The functional relevance of one LBN-induced candidate gene, Scn7a, could be confirmed by an increase in the density of voltage-gated sodium (Na_v) channel activated currents in hippocampal NG2+ cells. Scn7a remained upregulated until adulthood in LBN animals, which displayed impaired cognitive performance. Considering that Na_v channels are important for NG2+ cell-to-neuron communication, our findings provide novel insights into the disruption of this process in LBN mice.

Local Efficacy of Glutamate Uptake Decreases with Synapse Size

Synaptically released glutamate is largely cleared by glutamate transporters localized on perisynaptic astrocyte processes. Therefore, the substantial variability of astrocyte coverage of individual hippocampal synapses implies that the efficacy of local glutamate uptake and thus the spatial fidelity of synaptic transmission is synapse dependent. By visualization of sub-diffraction-limit perisynaptic astrocytic processes and adjacent postsynaptic spines, we show that, relative to their size, small spines display a stronger coverage by astroglial transporters than bigger neighboring spines. Similarly, glutamate transients evoked by synaptic stimulation are more sensitive to pharmacological inhibition of glutamate uptake at smaller spines, whose high-affinity N-methyl-D-aspartate receptors (NMDARs) are better shielded from remotely released glutamate. At small spines, glutamate-induced and NMDAR-dependent Ca²⁺ entry is also more strongly increased by uptake inhibition. These findings indicate that spine size inversely correlates with the efficacy of local glutamate uptake and thereby likely determines the probability of synaptic crosstalk.

Regenerative neurogenic response from glia requires insulin-driven neuron-glia communication

Understanding how injury to the central nervous system induces de novo neurogenesis in animals would help promote regeneration in humans. Regenerative neurogenesis could originate from glia and glial neuron-glia antigen-2 (NG2) may sense injury-induced neuronal signals, but these are unknown. Here, we used Drosophila to search for genes functionally related to the NG2 homologue kon-tiki (kon), and identified Islet Antigen-2 (Ia-2), required in neurons for insulin secretion. Both loss and over-expression of ia-2 induced neural stem cell gene expression, injury increased ia-2 expression and induced ectopic neural stem cells. Using genetic analysis and lineage tracing, we demonstrate that Ia-2 and Kon regulate Drosophila insulin-like peptide 6 (Dilp-6) to induce glial proliferation and neural stem cells from glia. Ectopic neural stem cells can divide, and limited de novo neurogenesis could be traced back to glial cells. Altogether, Ia-2 and Dilp-6 drive a neuron-glia relay that restores glia and reprogrammes glia into neural stem cells for regeneration.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

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

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

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

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Myelin calcium transients are independent from neuronal activity

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Adaxonal and paranodal myelin contained mitochondria

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Calcium transients require opening of mitochondrial permeability transition pores

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.

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.

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

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

Probing disrupted neurodevelopment in autism using human stem cell-derived neurons and organoids: An outlook into future diagnostics and drug development

Autism spectrum disorders (ASDs) represent a spectrum of neurodevelopmental disorders characterized by impaired social interaction, repetitive or restrictive behaviors, and problems with speech. According to a recent report by the Centers for Disease Control and Prevention, one in 68 children in the US is diagnosed with ASDs. Although ASD-related diagnostics and the knowledge of ASD-associated genetic abnormalities have improved in recent years, our understanding of the cellular and molecular pathways disrupted in ASD remains very limited. As a result, no specific therapies or medications are available for individuals with ASDs. In this review, we describe the neurodevelopmental processes that are likely affected in the brains of individuals with ASDs and discuss how patient-specific stem cell-derived neurons and organoids can be used for investigating these processes at the cellular and molecular levels. Finally, we propose a discovery pipeline to be used in the future for identifying the cellular and molecular deficits and developing novel personalized therapies for individuals with idiopathic ASDs.

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

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

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.

Missing in Action: Dysfunctional RNA Metabolism in Oligodendroglial Cells as a Contributor to Neurodegenerative Diseases?

The formation of myelin around axons by oligodendrocytes (OL) poses an enormous synthetic and energy challenge for the glial cell. Local translation of transcripts, including the mRNA for the essential myelin protein Myelin Basic Protein (MBP) at the site of myelin deposition has been recognised as an efficient mechanism to assure proper myelin sheath assembly. Oligodendroglial precursor cells (OPCs) form synapses with neurons and may localise many additional mRNAs in a similar fashion to synapses between neurons. In some diseases in which demyelination occurs, an abundance of OPCs is present but there is a failure to efficiently remyelinate and to synthesise MBP. This compromises axonal survival and function. OPCs are especially sensitive to cellular stress as occurring in neurodegenerative diseases, which can impinge on their ability to translate mRNAs into protein. Stress causes the build up of cytoplasmic stress granules (SG) in which many RNAs are sequestered and translationally stalled until the stress ceases. Chronic stress in particular could convert this initially protective reaction of the cell into damage, as persistence of SG may lead to pathological aggregate formation or long-term translation block of SG-associated RNAs. The recent recognition that many neurodegenerative diseases often exhibit an early white matter pathology with a proliferation of surviving OPCs, renders a study of the stress-associated processes in oligodendrocytes and OPCs especially relevant. Here, we discuss a potential dysfunction of RNA regulation in myelin diseases such as Multiple Sclerosis (MS) and Vanishing white matter disease (VWM) and potential contributions of OL dysfunction to neurodegenerative diseases such as Amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Fragile X syndrome (FXS).

NG2 Glia: Novel Roles beyond Re-/Myelination

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

Deficient neurotrophic factors of CSPG4-type neural cell exosomes in Alzheimer disease

Exosomes derived from chondroitin sulfate proteoglycan (CSPG) 4 type neural precursor cells (CSPG4Es) were purified from human plasma by sequential immunoabsorption with anti-CSPG4 and anti-platelet growth factor receptor α mAb to characterize the potential in vivo roles of CSPG4 cells in neuronal repair. Hepatocyte growth factor, fibroblast growth factors (FGFs)-2 and -13, and type 1 insulin-like growth factor (IGF-1), which enhance neuronal survival and functions, were quantified in CSPG4E extracts. For CSPG4Es of 24 healthy control subjects, mean levels of hepatocyte growth factor, FGF-13, and IGF-1, but not FGF-2, were significantly higher by up to 7-fold than in their neuronal-derived exosomes, and mean levels of all 4 growth factors were significantly higher by up to 8-fold than in their astrocyte-derived exosomes. Mean CSPG4E levels of all growth factors were significantly lower in patients with mild Alzheimer disease (AD) ( n = 24) than in age- and sex-matched cognitively normal control subjects ( n = 24). Mean CSPG4E levels of all growth factors were also significantly lower in 15 patients at the stage of moderate dementia from AD (AD₂) and at their preclinical stage 3 to 8 yr earlier (AD₁), with no differences between values at stages AD₁ and AD₂. Current findings suggest that CSPG4 cells export in exosomes higher levels of neurotrophic factors than neurons or astrocytes and that CSPG4E neurotrophic factors are diminished early in AD, with no significant progression of decreases later in the course.-Goetzl, E. J., Nogueras-Ortiz, C., Mustapic, M., Mullins, R. J., Abner, E. L., Schwartz, J. B., Kapogiannis, D. Deficient neurotrophic factors of CSPG4-type neural cell exosomes in Alzheimer disease.

Dynamic Calcium Release From Endoplasmic Reticulum Mediated by Ryanodine Receptor 3 Is Crucial for Oligodendroglial Differentiation

Increased intracellular Ca²⁺ in oligodendrocyte progenitor cells (OPCs) is important to initiate their differentiation, but the intracellular Ca²⁺ channel involved in this process remains unclear. As a Ca²⁺-induced Ca²⁺ release (CICR) channel that mediates endoplasmic reticulum (ER) Ca²⁺ release, the role of ryanodine receptors (RyRs) in oligodendroglial development is unexplored. In the present study, we observed that among the three mammalian isoforms, oligodendroglial lineage cells selectively expressed RyR3. Strong RyR3-positive signal was distributed all over the cytoplasm and processes in OPCs and/or immature OLs (imOLs), whereas it gradually decreased and was located mainly around the perinuclear region in mature oligodendrocytes (OLs). In addition, RyR3-mediated intracellular Ca²⁺ waves following caffeine stimulation were correlated with the expression pattern of RyR3, in which high flat Ca²⁺ fluctuations and oscillatory Ca²⁺ waves were more frequently recorded in OPCs and/or imOLs than in OLs. Through further functional exploration, we demonstrated that pretreatment with the RyR antagonist ryanodine could neutralize the increase in intracellular Ca²⁺ induced by OPC differentiation and reduce the number of mature OLs. Moreover, gene-level knockdown of RyR3 by lentivirus in OPCs resulted in inhibition of OPC differentiation. Taken together, our results provide new insight into the crucial role of RyR3-mediated ER Ca²⁺ release in the regulation of OPC differentiation and/or myelination.

Ca 2+ activity signatures of myelin sheath formation and growth in vivo

During myelination, individual oligodendrocytes initially over-produce short myelin sheaths, which are either retracted or stabilized. By live-imaging oligodendrocyte Ca2+ activity in vivo, we find that high-amplitude, long-duration Ca2+ transients in sheaths prefigure retractions, mediated by calpain. Following stabilization, myelin sheaths grow along axons, and we find that higher-frequency Ca2+ transient activity in sheaths precedes faster elongation. Our data implicate local Ca2+ signaling in regulating distinct stages of myelination.

Regulation of developing myelin sheath elongation by oligodendrocyte calcium transients in vivo

How action potentials regulate myelination by oligodendrocytes is uncertain. We show that neuronal activity raises [Ca2+]i in developing oligodendrocytes in vivo and that myelin sheath elongation is promoted by a high frequency of [Ca2+]i transients and prevented by [Ca2+]i buffering. Sheath elongation occurs ~1 h after [Ca2+]i elevation. Sheath shortening is associated with a low frequency of [Ca2+]i transients but with longer duration [Ca2+]i bursts. Thus, [Ca2+]i controls myelin sheath development.

Heterogeneity and function of hippocampal macroglia

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

On Myelinated Axon Plasticity and Neuronal Circuit Formation and Function

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

Oligodendroglia-lineage cells in brain plasticity, homeostasis and psychiatric disorders

Adult oligodendrocyte progenitor cells are uniformly distributed in both gray and white matter, displaying robust proliferative and migratory potential during health and disease. Recently, developments in new experimental approaches have brought about several novel insights about NG2-glia and myelinating oligodendrocytes, indicating a diverse toolkit of functions in experience-dependent myelination and homeostasis in the adult CNS. In this review, we summarize some of the topical studies that highlight newly emerging findings implicating oligodendroglia-lineage cells in brain plasticity, homeostasis and pathophysiology of neuropsychiatric disorders.

Conditional Deletion of the L-Type Calcium Channel Cav1.2 in NG2-Positive Cells Impairs Remyelination in Mice

Exploring the molecular mechanisms that drive the maturation of oligodendrocyte progenitor cells (OPCs) during the remyelination process is essential to developing new therapeutic tools to intervene in demyelinating diseases such as multiple sclerosis. To determine whether L-type voltage-gated calcium channels (L-VGCCs) are required for OPC development during remyelination, we generated an inducible conditional knock-out mouse in which the L-VGCC isoform Cav1.2 was deleted in NG2-positive OPCs (Cav1.2^KO). Using the cuprizone (CPZ) model of demyelination and mice of either sex, we establish that Cav1.2 deletion in OPCs leads to less efficient remyelination of the adult brain. Specifically, Cav1.2^KO OPCs mature slower and produce less myelin than control oligodendrocytes during the recovery period after CPZ intoxication. This reduced remyelination was accompanied by an important decline in the number of myelinating oligodendrocytes and in the rate of OPC proliferation. Furthermore, during the remyelination phase of the CPZ model, the corpus callosum of Cav1.2^KO animals presented a significant decrease in the percentage of myelinated axons and a substantial increase in the mean g-ratio of myelinated axons compared with controls. In addition, in a mouse line in which the Cav1.2^KO OPCs were identified by a Cre reporter, we establish that Cav1.2^KO OPCs display a reduced maturational rate through the entire remyelination process. These results suggest that Ca²⁺ influx mediated by L-VGCCs in oligodendroglial cells is necessary for normal remyelination and is an essential Ca²⁺ channel for OPC maturation during the remyelination of the adult brain.SIGNIFICANCE STATEMENT Ion channels implicated in oligodendrocyte differentiation and maturation may induce positive signals for myelin recovery. Voltage-gated Ca²⁺ channels (VGCCs) are important for normal myelination by acting at several critical steps during oligodendrocyte progenitor cell (OPC) development. To determine whether voltage Ca²⁺ entry is involved in oligodendrocyte differentiation and remyelination, we used a conditional knockout mouse for VGCCs in OPCs. Our results indicate that VGCCs can modulate oligodendrocyte maturation in the demyelinated brain and suggest that voltage-gated Ca²⁺ influx in OPCs is critical for remyelination. These findings could lead to novel approaches for obtaining a better understanding of the factors that control OPC maturation in order to stimulate this pool of progenitors to replace myelin in demyelinating diseases.

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.

Should We Stop Saying 'Glia' and 'Neuroinflammation'?

Central nervous system (CNS) therapeutics based on the theoretical framework of neuroinflammation have only barely succeeded. We argue that a problem may be the wrong use of the term 'neuroinflammation' as a distinct nosological entity when, based on recent evidence, it may not explain CNS disease pathology. Indeed, the terms 'neuroinflammation' and 'glia' could be obsolete. First, unbiased molecular profiling of CNS cell populations and individual cells reveals striking phenotypic heterogeneity in health and disease. Second, astrocytes, microglia, oligodendrocytes, and NG2 cells may contribute to higher-brain functions by performing actions beyond housekeeping. We propose that CNS diseases be viewed as failed circuits caused in part by disease-specific dysfunction of cells traditionally called 'glia', and hence, favor therapies promoting their functional recovery.

Rebound from Inhibition: Self-Correction against Neurodegeneration?

Neural networks play a critical role in establishing constraints on excitability in the central nervous system. Several recent studies have suggested that network dysfunction in the brain and spinal cord are compromised following insult by a neurodegenerative trigger and might precede eventual neuronal loss and neurological impairment. Early intervention of network excitability and plasticity might therefore be critical in resetting hyperexcitability and preventing later neuronal damage. Here, the behavior of neurons that generate burst firing upon recovery from inhibitory input or intrinsic membrane hyperpolarization (rebound neurons) is examined in the context of neural networks that underlie rhythmic activity observed in areas of the brain and spinal cord that are vulnerable to neurodegeneration. In a non-inflammatory rodent model of spongiform neurodegenerative disease triggered by retrovirus infection of glia, rebound neurons are particularly vulnerable to neurodegeneration, likely due to an inherently low calcium buffering capacity. The dysfunction of rebound neurons translates into a dysfunction of rhythmic neural circuits, compromising normal neurological function and leading to eventual morbidity. Understanding how virus infection of glia can mediate dysfunction of rebound neurons, induce hyperexcitability and loss of rhythmic function, pathologic features observed in neurodegenerative disorders ranging from epilepsy to motor neuron disease, might therefore suggest a common pathway for early therapeutic intervention.