Expression of Kir4.1 and Kir5.1 inwardly rectifying potassium channels in oligodendrocytes, the myelinating cells of the CNS

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

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

Ion Channels and Ionotropic Receptors in Astrocytes: Physiological Functions and Alterations in Alzheimer's Disease and Glioblastoma

The human brain is composed of nearly one hundred billion neurons and an equal number of glial cells, including macroglia, i.e., astrocytes and oligodendrocytes, and microglia, the resident immune cells of the brain. In the last few decades, compelling evidence has revealed that glial cells are far more active and complex than previously thought. In particular, astrocytes, the most abundant glial cell population, not only take part in brain development, metabolism, and defense against pathogens and insults, but they also affect sensory, motor, and cognitive functions by constantly modulating synaptic activity. Not surprisingly, astrocytes are actively involved in neurodegenerative diseases (NDs) and other neurological disorders like brain tumors, in which they rapidly become reactive and mediate neuroinflammation. Reactive astrocytes acquire or lose specific functions that differently modulate disease progression and symptoms, including cognitive impairments. Astrocytes express several types of ion channels, including K+, Na+, and Ca2+ channels, transient receptor potential channels (TRP), aquaporins, mechanoreceptors, and anion channels, whose properties and functions are only partially understood, particularly in small processes that contact synapses. In addition, astrocytes express ionotropic receptors for several neurotransmitters. Here, we provide an extensive and up-to-date review of the roles of ion channels and ionotropic receptors in astrocyte physiology and pathology. As examples of two different brain pathologies, we focus on Alzheimer's disease (AD), one of the most diffuse neurodegenerative disorders, and glioblastoma (GBM), the most common brain tumor. Understanding how ion channels and ionotropic receptors in astrocytes participate in NDs and tumors is necessary for developing new therapeutic tools for these increasingly common neurological conditions.

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

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

Diverse functions of the inward-rectifying potassium channel Kir5.1 and its relationship with human diseases

The inward-rectifying potassium channel subunit Kir5.1, encoded by Kcnj16, can form functional heteromeric channels (Kir4.1/5.1 and Kir4.2/5.1) with Kir4.1 (encoded by Kcnj10) or Kir4.2 (encoded by Kcnj15). It is expressed in the kidneys, pancreas, thyroid, brain, and other organs. Although Kir5.1 cannot form functional homomeric channels in most cases, an increasing number of studies in recent years have found that the functions of this subunit should not be underestimated. Kir5.1 can confer intracellular pH sensitivity to Kir4.1/5.1 channels, which can act as extracellular potassium sensors in the renal distal convoluted tubule segment. This segment plays an important role in maintaining potassium and acid-base balances. This review summarizes the various pathophysiological processes involved in Kir5.1 and the expression changes of Kir5.1 as a differentially expressed gene in various cancers, as well as describing several other disease phenotypes caused by Kir5.1 dysfunction.

Gabapentin-Friend or foe?

BACKGROUND

Gabapentin is a recommended first-line agent for treating neuropathic pain; however, its efficacy rate is reportedly low, and the risk of adverse events is high. A plausible explanation for this lies with its wide range of actions, the entirety of which have yet to be fully elucidated.

METHODS

A review of the literature was conducted on gabapentin's known and proposed analgesic mechanisms of action, as well as potentially opposing or detrimental actions.

RESULTS

Gabapentin's classical analgesic mechanisms involve direct attenuation of excitatory neurotransmission in the spinal cord via inhibition of neuronal ion channels, while indirect mechanisms include descending inhibition and block of injury-evoked synaptogenesis. Glial effects have also been reported; however, whether they are neuroprotective or detrimental is unknown. Furthermore, data from animal models do not reflect clinical outcomes.

CONCLUSIONS

Gabapentin's clinical use should be reconsidered according to the net effects of its numerous assumed actions, including the tripartite synapse and oligodendrocyte effects. Whether it is doing more harm than good, especially in the scenarios of incomplete or loss of response, warrants consideration when prescribing gabapentin.

Destabilizing COXIV in Müller Glia Increases Retinal Glycolysis and Alters Scotopic Electroretinogram

Müller glia (MG), the principal glial cell of the retina, have a metabolism that defies categorization into glycolytic versus oxidative. We showed that MG mount a strong hypoxia response to ocular hypertension, raising the question of their relative reliance on mitochondria for function. To explore the role of oxidative phosphorylation (OXPHOS) in MG energy production in vivo, we generated and characterized adult mice in which MG have impaired cytochrome c oxidase (COXIV) activity through knockout of the COXIV constituent COX10. Histochemistry and protein analysis showed that COXIV protein levels were significantly lower in knockout mouse retina compared to control. Loss of COXIV activity in MG did not induce structural abnormalities, though oxidative stress was increased. Electroretinography assessment showed that knocking out COX10 significantly impaired scotopic a- and b-wave responses. Inhibiting mitochondrial respiration in MG also altered the retinal glycolytic profile. However, blocking OXPHOS in MG did not significantly exacerbate retinal ganglion cell (RGC) loss or photopic negative response after ocular hypertension (OHT). These results suggest that MG were able to compensate for reduced COXIV stability by maintaining fundamental processes, but changes in retinal physiology and metabolism-associated proteins indicate subtle changes in MG function.

Kir5.1 channels: potential role in epilepsy and seizure disorders

Inwardly rectifying potassium (Kir) channels are broadly expressed in many mammalian organ systems, where they contribute to critical physiological functions. However, the importance and function of the Kir5.1 channel (encoded by the KCNJ16 gene) have not been fully recognized. This review focuses on the recent advances in understanding the expression patterns and functional roles of Kir5.1 channels in fundamental physiological systems vital to potassium homeostasis and neurological disorders. Recent studies have described the role of Kir5.1-forming Kir channels in mouse and rat lines with mutations in the Kcnj16 gene. The animal research reveals distinct renal and neurological phenotypes, including pH and electrolyte imbalances, blunted ventilatory responses to hypercapnia/hypoxia, and seizure disorders. Furthermore, it was confirmed that these phenotypes are reminiscent of those in patient cohorts in which mutations in the KCNJ16 gene have also been identified, further suggesting a critical role for Kir5.1 channels in homeostatic/neural systems health and disease. Future studies that focus on the many functional roles of these channels, expanded genetic screening in human patients, and the development of selective small-molecule inhibitors for Kir5.1 channels, will continue to increase our understanding of this unique Kir channel family member.

Inward rectifier potassium (Kir) channels in the retina: living our vision

Channel proteins are vital for conducting ions throughout the body and are especially relevant to retina physiology. Inward rectifier potassium (Kir) channels are a class of K+ channels responsible for maintaining membrane potential and extracellular K+ concentrations. Studies of the KCNJ gene (that encodes Kir protein) expression identified the presence of all of the subclasses (Kir 1-7) of Kir channels in the retina or retinal-pigmented epithelium (RPE). However, functional studies have established the involvement of the Kir4.1 homotetramer and Kir4.1/5.1 heterotetramer in Müller glial cells, Kir2.1 in bipolar cells, and Kir7.1 in the RPE cell physiology. Here, we propose the potential roles of Kir channels in the retina based on the physiological contributions to the brain, pancreatic, and cardiac tissue functions. There are several open questions regarding the expressed KCNJ genes in the retina and RPE. For example, why does not the Kir channel subtype gene expression correspond with protein expression? Catching up with multiomics or functional "omics" approaches might shed light on posttranscriptional changes that might influence Kir subunit mRNA translation within the retina that guides our vision.

Astrocyte-neuron crosstalk through Hedgehog signaling mediates cortical synapse development

Neuron-glia interactions play a critical role in the regulation of synapse formation and circuit assembly. Here we demonstrate that canonical Sonic hedgehog (Shh) pathway signaling in cortical astrocytes acts to coordinate layer-specific synaptic connectivity. We show that the Shh receptor Ptch1 is expressed by cortical astrocytes during development and that Shh signaling is necessary and sufficient to promote the expression of genes involved in regulating synaptic development and layer-enriched astrocyte molecular identity. Loss of Shh in layer V neurons reduces astrocyte complexity and coverage by astrocytic processes in tripartite synapses; conversely, cell-autonomous activation of Shh signaling in astrocytes promotes cortical excitatory synapse formation. Furthermore, Shh-dependent genes Lrig1 and Sparc distinctively contribute to astrocyte morphology and synapse formation. Together, these results suggest that Shh secreted from deep-layer cortical neurons acts to specialize the molecular and functional features of astrocytes during development to shape circuit assembly and function.

Xie et al. show that Sonic hedgehog expressed by cortical layer V neurons signals to Ptch1-expressing astrocytes where it is necessary to promote the expression of layer-enriched astrocyte gene networks that control synapse formation and astrocyte morphological complexity.

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

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

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Envisioning the role of inwardly rectifying potassium (Kir) channel in epilepsy

Epilepsy is a devastating neurological disorder characterized by recurrent seizures attributed to the disruption of the dynamic excitatory and inhibitory balance in the brain. Epilepsy has emerged as a global health concern affecting about 70 million people worldwide. Despite recent advances in pre-clinical and clinical research, its etiopathogenesis remains obscure, and there are still no treatment strategies modifying disease progression. Although the precise molecular mechanisms underlying epileptogenesis have not been clarified yet, the role of ion channels as regulators of cellular excitability has increasingly gained attention. In this regard, emerging evidence highlights the potential implication of inwardly rectifying potassium (Kir) channels in epileptogenesis. Kir channels consist of seven different subfamilies (Kir1-Kir7), and they are highly expressed in both neuronal and glial cells in the central nervous system. These channels control the cell volume and excitability. In this review, we discuss preclinical and clinical evidence on the role of the several subfamilies of Kir channels in epileptogenesis, aiming to shed more light on the pathogenesis of this disorder and pave the way for future novel therapeutic approaches.

Dysfunction of oligodendrocyte inwardly rectifying potassium channel in a rat model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by the death of motor neurons in the spinal cord and the brain. Although this disease is characterized by motoneuron degeneration, non-neuronal cells such as oligodendrocytes play an important role in the disease onset and progression. The aim of our study was to examine functional properties of oligodendrocytes in the SOD1^G93A rat model of ALS with a particular focus on the inwardly rectifying potassium channel Kir4.1 that is abundantly expressed in these glial cells and plays a role in the regulation of extracellular K⁺ . First, we demonstrate that the expression of Kir4.1 is diminished in the spinal cord oligodendrocytes of the SOD1^G93A rat. Moreover, our data show an elevated number of dysmorphic oligodendrocytes in the ALS spinal cord that is indicative of a degenerative phenotype. In order to assess physiological properties of oligodendrocytes, we prepared cell cultures from the rat spinal cord. Oligodendrocytes isolated from the SOD1^G93A spinal cord display similar ramification of the processes as the control but express a lower level of Kir4.1. We further demonstrate an impairment of oligodendrocyte functional properties in ALS. Remarkably, whole-cell patch-clamp recordings revealed compromised membrane biophysical properties and diminished inward currents in the SOD1^G93A oligodendrocytes. In addition, the Ba²⁺ -sensitive Kir currents were decreased in ALS oligodendrocytes. Altogether, our findings provide the evidence of impaired Kir4.1 expression and function in oligodendrocytes of the SOD1^G93A spinal cord, suggesting oligodendrocyte Kir4.1 channel as a potential contributor to the ALS pathophysiology.

Transcriptomic, proteomic, and metabolomic analyses identify candidate pathways linking maternal cadmium exposure to altered neurodevelopment and behavior

Cadmium (Cd) is a ubiquitous toxic heavy metal of major public concern. Despite inefficient placental transfer, maternal Cd exposure impairs fetal growth and development. Increasing evidence from animal models and humans suggests maternal Cd exposure negatively impacts neurodevelopment; however, the underlying molecular mechanisms are unclear. To address this, we utilized multiple -omics approaches in a mouse model of maternal Cd exposure to identify pathways altered in the developing brain. Offspring maternally exposed to Cd presented with enlarged brains proportional to body weights at birth and altered behavior at adulthood. RNA-seq in newborn brains identified exposure-associated increases in Hox gene and myelin marker expression and suggested perturbed retinoic acid (RA) signaling. Proteomic analysis showed altered levels of proteins involved in cellular energy pathways, hypoxic response, and RA signaling. Consistent with transcriptomic and proteomic analyses, we identified increased levels of retinoids in maternally-exposed newborn brains. Metabolomic analyses identified metabolites with significantly altered abundance, supportive of changes to cellular energy pathways and hypoxia. Finally, maternal Cd exposure reduced mitochondrial DNA levels in newborn brains. The identification of multiple pathways perturbed in the developing brain provides a basis for future studies determining the mechanistic links between maternal Cd exposure and altered neurodevelopment and behavior.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Ion Channels and Electrophysiological Properties of Astrocytes: Implications for Emergent Stimulation Technologies

Astrocytes comprise a heterogeneous cell population characterized by distinct morphologies, protein expression and function. Unlike neurons, astrocytes do not generate action potentials, however, they are electrically dynamic cells with extensive electrophysiological heterogeneity and diversity. Astrocytes are hyperpolarized cells with low membrane resistance. They are heavily involved in the modulation of K⁺ and express an array of different voltage-dependent and voltage-independent channels to help with this ion regulation. In addition to these K⁺ channels, astrocytes also express several different types of Na⁺ channels; intracellular Na⁺ signaling in astrocytes has been linked to some of their functional properties. The physiological hallmark of astrocytes is their extensive intracellular Ca²⁺ signaling cascades, which vary at the regional, subregional, and cellular levels. In this review article, we highlight the physiological properties of astrocytes and the implications for their function and influence of network and synaptic activity. Furthermore, we discuss the implications of these differences in the context of optogenetic and DREADD experiments and consider whether these tools represent physiologically relevant techniques for the interrogation of astrocyte function.

Ethanol inhibition of lateral orbitofrontal cortex neuron excitability is mediated via dopamine D1/D5 receptor-induced release of astrocytic glycine

Recent findings from this laboratory demonstrate that ethanol reduces the intrinsic excitability of orbitofrontal cortex (OFC) neurons via activation of strychnine-sensitive glycine receptors. Although the mechanism linking ethanol to the release of glycine is currently unknown, astrocytes are a source of neurotransmitters including glycine and activation of dopamine D1-like receptors has been reported to enhance extracellular levels of glycine via a functional reversal of the astrocytic glycine transporter GlyT1. We recently reported that like ethanol, dopamine or a D1/D5 receptor agonist increases a tonic current in lateral OFC (lOFC) neurons. Therefore, in this study, we used whole-cell patch-clamp electrophysiology to examine whether ethanol inhibition of OFC spiking involves the release of glycine from astrocytes and whether this release is dopamine receptor dependent. Ethanol, applied acutely, decreased spiking of lOFC neurons and this effect was blocked by antagonists of GlyT1, the norepinephrine transporter or D1-like but not D2-like receptors. Ethanol enhanced the tonic current of OFC neurons and occluded the effect of dopamine suggesting that ethanol and dopamine may share a common pathway. Altering astrocyte function by suppressing intracellular astrocytic calcium signaling or blocking the astrocyte-specific Kir4.1 potassium channels reduced but did not completely abolish ethanol inhibition of OFC neuron firing. However, when both astrocytic calcium signaling and Kir4.1 channels were inhibited, ethanol had no effect on firing. Ethanol inhibition was also prevented by inhibitors of phospholipase C and conventional isoforms of protein kinase C (cPKC) previously shown to block D1R-induced GlyT1 reversal and PKC inhibition of Kir4.1 channels. Finally, the membrane potential of OFC astrocytes was depolarized by bath application of a Kir4.1 blocker, a D1 agonist or ethanol and ethanol effect was blocked by a D1 antagonist. Together, these findings suggest that acute ethanol inhibits OFC neuron excitability via a D1 receptor-mediated dysregulation of astrocytic glycine transport.

Ginsenoside Rg1 Prevents PTSD-Like Behaviors in Mice Through Promoting Synaptic Proteins, Reducing Kir4.1 and TNF-α in the Hippocampus

Ginsenoside Rg1 is efficient to prevent or treat mental disorders. However, the mechanisms underlying the effects of ginsenoside Rg1 on post-traumatic stress disorder (PTSD) are still not known. In this study, single-prolonged stress (SPS) regime, as well as injection of lipopolysaccharide (LPS), was used to produce PTSD-like behaviors in C57 mice, and the effects of ginsenoside Rg1 (10, 20, 40 mg/kg/d, ip, for 14 days) on PTSD-like behaviors were evaluated. Our results showed that ginsenoside Rg1 promoted fear extinction and prevented depression-like behaviors in both LPS and SPS models. Importantly, ginsenoside Rg1 alleviated LPS- or SPS-stimulated expression of pro-inflammatory cytokines (IL-1β and TNF-α), activation of astrocytes and microglia, and reduction of hippocampal synaptic proteins (PSD95, Arc, and GluA1). Ginsenoside Rg1 also reduced the increase of hippocampal Kir4.1 and GluN2A induced by PTSD regime. Importantly, reducing hippocampal astroglial Kir4.1 expression promoted fear extinction and improved depression-like behaviors in LPS-treated mice. Additionally, intracerebroventricular injection of TNF-α caused an impairment of fear extinction and promoted Kir4.1 expression in the hippocampus. Together, our study reveals novel protective effects of ginsenoside Rg1 against PTSD-like behaviors in mice, likely via promoting synaptic proteins, reducing Kir4.1 and TNF-α in the hippocampus.

Divergent membrane properties of mouse cochlear glial cells around hearing onset

Spiral ganglion neurons (SGNs) are the primary afferent neurons of the auditory system, and together with their attendant glia, form the auditory nerve. Within the cochlea, satellite glial cells (SGCs) encapsulate the cell body of SGNs, whereas Schwann cells (SCs) wrap their peripherally- and centrally-directed neurites. Despite their likely importance in auditory nerve function and homeostasis, the physiological properties of auditory glial cells have evaded description. Here, we characterized the voltage-activated membrane currents of glial cells from the mouse cochlea. We identified a prominent weak inwardly rectifying current in SGCs within cochlear slice preparations (postnatal day P5-P6), which was also present in presumptive SGCs within dissociated cultures prepared from the cochleae of hearing mice (P14-P15). Pharmacological block by Ba²⁺ and desipramine suggested that channels belonging to the Kir4 family mediated the weak inwardly rectifying current, and post hoc immunofluorescence implicated the involvement of Kir4.1 subunits. Additional electrophysiological profiles were identified for glial cells within dissociated cultures, suggesting that glial subtypes may have specific membrane properties to support distinct physiological roles. Immunofluorescence using fixed cochlear sections revealed that although Kir4.1 is restricted to SGCs after the onset of hearing, these channels are more widely distributed within the glial population earlier in postnatal development (i.e., within both SGCs and SCs). The decrease in Kir4.1 immunofluorescence during SC maturation was coincident with a reduction of Sox2 expression and advancing neurite myelination. The data suggest a diversification of glial properties occurs in preparation for sound-driven activity in the auditory nerve.

Ion Channels as Therapeutic Targets in High Grade Gliomas

Simple Summary

Glioblastoma multiforme is an aggressive grade IV lethal brain tumour with a median survival of 14 months. Despite surgery to remove the tumour, and subsequent concurrent chemotherapy and radiotherapy, there is little in terms of effective treatment options. Because of this, exploring new treatment avenues is vital. Brain tumours are intrinsically electrically active; expressing unique patterns of ion channels, and this is a characteristic we can exploit. Ion channels are specialised proteins in the cell’s membrane that allow for the passage of positive and negatively charged ions in and out of the cell, controlling membrane potential. Membrane potential is a crucial biophysical signal in normal and cancerous cells. Research has identified that specific classes of ion channels not only move the cell through its cell cycle, thus encouraging growth and proliferation, but may also be essential in the development of brain tumours. Inhibition of sodium, potassium, calcium, and chloride channels has been shown to reduce the capacity of glioblastoma cells to grow and invade. Therefore, we propose that targeting ion channels and repurposing commercially available ion channel inhibitors may hold the key to new therapeutic avenues in high grade gliomas.

Abstract

Glioblastoma multiforme (GBM) is a lethal brain cancer with an average survival of 14–15 months even with exhaustive treatment. High grade gliomas (HGG) represent the leading cause of CNS cancer-related death in children and adults due to the aggressive nature of the tumour and limited treatment options. The scarcity of treatment available for GBM has opened the field to new modalities such as electrotherapy. Previous studies have identified the clinical benefit of electrotherapy in combination with chemotherapeutics, however the mechanistic action is unclear. Increasing evidence indicates that not only are ion channels key in regulating electrical signaling and membrane potential of excitable cells, they perform a crucial role in the development and neoplastic progression of brain tumours. Unlike other tissue types, neural tissue is intrinsically electrically active and reliant on ion channels and their function. Ion channels are essential in cell cycle control, invasion and migration of cancer cells and therefore present as valuable therapeutic targets. This review aims to discuss the role that ion channels hold in gliomagenesis and whether we can target and exploit these channels to provide new therapeutic targets and whether ion channels hold the mechanistic key to the newfound success of electrotherapies.

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

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

White matter disturbances in phenylketonuria: Possible underlying mechanisms

White matter pathologies, as well as intellectual disability, microcephaly, and other central nervous system injuries, are clinical traits commonly ascribed to classic phenylketonuria (PKU). PKU is an inherited metabolic disease elicited by the deficiency of phenylalanine hydroxylase. Accumulation of l-phenylalanine (Phe) and its metabolites is found in tissues and body fluids in phenylketonuric patients. In order to mitigate the clinical findings, rigorous dietary Phe restriction constitutes the core of therapeutic management in PKU. Myelination is the process whereby the oligodendrocytes wrap myelin sheaths around the axons, supporting the conduction of action potentials. White matter injuries are implicated in the brain damage related to PKU, especially in untreated or poorly treated patients. The present review summarizes evidence toward putative mechanisms driving the white matter pathology in PKU patients.

Glial cells in schizophrenia: a unified hypothesis

The cellular neurobiology of schizophrenia remains poorly understood. We discuss neuroimaging studies, pathological findings, and experimental work supporting the idea that glial cells might contribute to the development of schizophrenia. Experimental studies suggest that abnormalities in the differentiation competence of glial progenitor cells lead to failure in the morphological and functional maturation of oligodendrocytes and astrocytes. We propose that immune activation of microglial cells during development, superimposed upon genetic risk factors, could contribute to defective differentiation competence of glial progenitor cells. The resulting hypomyelination and disrupted white matter integrity might contribute to transmission desynchronisation and dysconnectivity, whereas the failure of astrocytic differentiation results in abnormal glial coverage and support of synapses. The delayed and deficient maturation of astrocytes might, in parallel, lead to disruption of glutamatergic, potassium, and neuromodulatory homoeostasis, resulting in dysregulated synaptic transmission. By highlighting a role for glial cells in schizophrenia, these studies potentially point to new mechanisms for disease modification.

A critical role for the inward rectifying potassium channel Kir7.1 in oligodendrocytes of the mouse optic nerve

Inward rectifying potassium channels (Kir) are a large family of ion channels that play key roles in ion homeostasis in oligodendrocytes, the myelinating cells of the central nervous system (CNS). Prominent expression of Kir4.1 has been indicated in oligodendrocytes, but the extent of expression of other Kir subtypes is unclear. Here, we used qRT-PCR to determine expression of Kir channel transcripts in the mouse optic nerve, a white matter tract comprising myelinated axons and the glia that support them. A novel finding was the high relative expression of Kir7.1, comparable to that of Kir4.1, the main glial Kir channel. Significantly, Kir7.1 immunofluorescence labelling in optic nerve sections and in isolated cells was localised to oligodendrocyte somata. Kir7.1 are known as a K⁺ transporting channels and, using patch clamp electrophysiology and the Kir7.1 blocker VU590, we demonstrated Kir7.1 channels carry a significant proportion of the whole cell potassium conductance in oligodendrocytes isolated from mouse optic nerves. Notably, oligodendrocytes are highly susceptible to ischemia/hypoxia and this is due at least in part to disruption of ion homeostasis. A key finding of this study is that blockade of Kir7.1 with VU590 compromised oligodendrocyte cell integrity and compounds oligodendroglial loss in ischemia/hypoxia in the oxygen-glucose deprivation (OGD) model in isolated intact optic nerves. These data reveal Kir7.1 channels are molecularly and functionally expressed in oligodendrocytes and play an important role in determining oligodendrocyte survival and myelin integrity.

Epsilon Toxin from Clostridium perfringens Causes Inhibition of Potassium inward Rectifier (Kir) Channels in Oligodendrocytes

Epsilon toxin (ETX), produced by Clostridium perfringens types B and D, causes serious neurological disorders in animals. ETX can bind to the white matter of the brain and the oligodendrocytes, which are the cells forming the myelin sheath around neuron axons in the white matter of the central nervous system. After binding to oligodendrocytes, ETX causes demyelination in rat cerebellar slices. We further investigated the effects of ETX on cerebellar oligodendrocytes and found that ETX induced small transmembrane depolarization (by ~ +6.4 mV) in rat oligodendrocytes primary cultures. This was due to partial inhibition of the transmembrane inward rectifier potassium current (Kir). Of the two distinct types of Kir channel conductances (~25 pS and ~8.5 pS) recorded in rat oligodendrocytes, we found that ETX inhibited the large-conductance one. This inhibition did not require direct binding of ETX to a Kir channel. Most likely, the binding of ETX to its membrane receptor activates intracellular pathways that block the large conductance Kir channel activity in oligodendrocyte. Altogether, these findings and previous observations pinpoint oligodendrocytes as a major target for ETX. This supports the proposal that ETX might be a cause for Multiple Sclerosis, a disease characterized by myelin damage.

Disease Associated Mutations in KIR Proteins Linked to Aberrant Inward Rectifier Channel Trafficking

The ubiquitously expressed family of inward rectifier potassium (K_IR) channels, encoded by KCNJ genes, is primarily involved in cell excitability and potassium homeostasis. Channel mutations associate with a variety of severe human diseases and syndromes, affecting many organ systems including the central and peripheral neural system, heart, kidney, pancreas, and skeletal muscle. A number of mutations associate with altered ion channel expression at the plasma membrane, which might result from defective channel trafficking. Trafficking involves cellular processes that transport ion channels to and from their place of function. By alignment of all K_IR channels, and depicting the trafficking associated mutations, three mutational hotspots were identified. One localized in the transmembrane-domain 1 and immediately adjacent sequences, one was found in the G-loop and Golgi-export domain, and the third one was detected at the immunoglobulin-like domain. Surprisingly, only few mutations were observed in experimentally determined Endoplasmic Reticulum (ER)exit-, export-, or ER-retention motifs. Structural mapping of the trafficking defect causing mutations provided a 3D framework, which indicates that trafficking deficient mutations form clusters. These “mutation clusters” affect trafficking by different mechanisms, including protein stability.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Oligodendrocytes express synaptic proteins that modulate myelin sheath formation

Vesicular release from neurons promotes myelin sheath growth on axons. Oligodendrocytes express proteins that allow dendrites to respond to vesicular release at synapses, suggesting that axon-myelin contacts use similar communication mechanisms as synapses to form myelin sheaths. To test this, we used fusion proteins to track synaptic vesicle localization and membrane fusion in zebrafish during developmental myelination and investigated expression and localization of PSD95, a dendritic post-synaptic protein, within oligodendrocytes. Synaptic vesicles accumulate and exocytose at ensheathment sites with variable patterning and most sheaths localize PSD95 with patterning similar to exocytosis site location. Disruption of candidate PDZ-binding transsynaptic adhesion proteins in oligodendrocytes cause variable effects on sheath length and number. One candidate, Cadm1b, localizes to myelin sheaths where both PDZ binding and extracellular adhesion to axons mediate sheath growth. Our work raises the possibility that axon-glial communication contributes to myelin plasticity, providing new targets for mechanistic unraveling of developmental myelination.

Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis

Acute and chronic homeostatic pH regulation is critical for the maintenance of optimal cellular function. Renal mechanisms dominate global pH regulation over longer time frames, and rapid adjustments in ventilation compensate for acute pH and CO2 changes. Ventilatory CO2 and pH chemoreflexes are primarily determined by brain chemoreceptors with intrinsic pH sensitivity likely driven by K+ channels. Here, we studied acute and chronic pH regulation in Kcnj16 mutant Dahl salt-sensitive (SS Kcnj16-/-) rats; Kcnj16 encodes the pH-sensitive inwardly rectifying K+ 5.1 (Kir5.1) channel. SS Kcnj16-/- rats hyperventilated at rest, likely compensating for a chronic metabolic acidosis. Despite their resting hyperventilation, SS Kcnj16-/- rats showed up to 45% reduction in the ventilatory response to graded hypercapnic acidosis vs. controls. SS Kcnj16-/- rats chronically treated with bicarbonate or the carbonic anhydrase inhibitor hydrochlorothiazide had partial restoration of arterial pH, but there was a further reduction in the ventilatory response to hypercapnic acidosis. SS Kcnj16-/- rats also had a nearly absent hypoxic ventilatory response, suggesting major contributions of Kir5.1 to O2- and CO2-dependent chemoreflexes. Although previous studies demonstrated beneficial effects of a high-K+ diet (HKD) on cardiorenal phenotypes in SS Kcnj16-/- rats, HKD failed to restore the observed ventilatory phenotypes. We conclude that Kir5.1 is a key regulator of renal H+ handling and essential for acute and chronic regulation of arterial pH as determinants of the ventilatory CO2 chemoreflex.-Puissant, M. M., Muere, C., Levchenko, V., Manis, A. D., Martino, P., Forster, H. V., Palygin, O., Staruschenko, A., Hodges, M. R. Genetic mutation of Kcnj16 identifies Kir5.1-containing channels as key regulators of acute and chronic pH homeostasis.

Ventral midbrain astrocytes display unique physiological features and sensitivity to dopamine D2 receptor signaling

Astrocytes are ubiquitous CNS cells that support tissue homeostasis through ion buffering, neurotransmitter recycling, and regulation of CNS vasculature. Yet, despite the essential functional roles they fill, very little is known about the physiology of astrocytes in the ventral midbrain, a region that houses dopamine-releasing neurons and is critical for reward learning and motivated behaviors. Here, using a combination of whole-transcriptome sequencing, histology, slice electrophysiology, and calcium imaging, we performed the first functional and molecular profiling of ventral midbrain astrocytes and observed numerous differences between these cells and their telencephalic counterparts, both in their gene expression profile and in their physiological properties. Ventral midbrain astrocytes have very low membrane resistance and inward-rectifying potassium channel-mediated current, and are extensively coupled to surrounding oligodendrocytes through gap junctions. They exhibit calcium responses to glutamate but are relatively insensitive to norepinephrine. In addition, their calcium activity can be dynamically modulated by dopamine D2 receptor signaling. Taken together, these data indicate that ventral midbrain astrocytes are physiologically distinct from astrocytes in cortex and hippocampus. This work provides new insights into the extent of functional astrocyte heterogeneity within the adult brain and establishes the foundation for examining the impact of regional astrocyte differences on dopamine neuron function and susceptibility to degeneration.

Physiology of Oligodendroglia

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

Distal tubule basolateral potassium channels: cellular and molecular mechanisms of regulation

PURPOSE OF REVIEW

Multiple clinical and translational evidence support benefits of high potassium diet; however, there many uncertainties underlying the molecular and cellular mechanisms determining effects of dietary potassium. Kir4.1 and Kir5.1 proteins form a functional heteromer (Kir4.1/Kir5.1), which is the primary inwardly rectifying potassium channel on the basolateral membrane of both distal convoluted tubule (DCT) and the collecting duct principal cells. The purpose of this mini-review is to summarize latest advances in our understanding of the evolution, physiological relevance and mechanisms controlling these channels.

RECENT FINDINGS

Kir4.1 and Kir5.1 channels play a critical role in determining electrolyte homeostasis in the kidney and blood pressure, respectively. It was reported that Kir4.1/Kir5.1 serves as potassium sensors in the distal nephron responding to variations in dietary intake and hormonal stimuli. Global and kidney specific knockouts of either channel resulted in hypokalemia and severe cardiorenal phenotypes. Furthermore, knock out of Kir5.1 in Dahl salt-sensitive rat background revealed the crucial role of the Kir4.1/Kir5.1 channel in salt-induced hypertension.

SUMMARY

Here, we focus on reviewing novel experimental evidence of the physiological function, expression and hormonal regulation of renal basolateral inwardly rectifying potassium channels. Further investigation of molecular and cellular mechanisms controlling Kir4.1 and Kir4.1/Kir5.1-mediating pathways and development of specific compounds targeting these channels function is essential for proper control of electrolyte homeostasis and blood pressure.

Expression of Kir2.1 Inward Rectifying Potassium Channels in Optic Nerve Glia: Evidence for Heteromeric Association with Kir4.1 and Kir5.1

Inward rectifying potassium (Kir) channels comprise a large family with diverse biophysical properties. A predominant feature of central nervous system (CNS) glia is their expression of Kir4.1, which as homomers are weakly rectifying channels, but form strongly rectifying channels as heteromers with Kir2.1. However, the extent of Kir2.1 expression and their association with Kir4.1 in glia throughout the CNS is unclear. We have examined this in astrocytes and oligodendrocytes of the mouse optic nerve, a typical CNS white matter tract. Western blot and immunocytochemistry demonstrates that optic nerve astrocytes and oligodendrocytes express Kir2.1 and that it co-localises with Kir4.1. Co-immunoprecipitation analysis provided further evidence that Kir2.1 associate with Kir4.1 and, moreover, Kir2.1 expression was significantly reduced in optic nerves and brains from Kir4.1 knock-out mice. In addition, optic nerve glia express Kir5.1, which may associate with Kir2.1 to form silent channels. Immunocytochemical and co-immunoprecipitation analyses indicate that Kir2.1 associate with Kir5.1 in optic nerve glia, but not in the brain. The results provide evidence that astrocytes and oligodendrocytes may express heteromeric Kir2.1/Kir4.1 and Kir2.1/Kir5.1 channels, together with homomeric Kir2.1 and Kir4.1 channels. In astrocytes, expression of multiple Kir channels is the biophysical substrate for the uptake and redistribution of K+ released during neuronal electrical activity known as ‘potassium spatial buffering’. Our findings suggest a similar potential role for the diverse Kir channels expressed by oligodendrocytes, which by way of their myelin sheaths are intimately associated with the sites of action potential propagation and axonal K+ release.

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

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

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

Oligodendrocytes control potassium accumulation in white matter and seizure susceptibility

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

Potassium channel dysfunction in neurons and astrocytes in Huntington's disease

Huntington's disease (HD) is a late-onset fatal neurodegenerative disease, characterized by progressive movement disorders, psychiatric symptoms, and cognitive impairment. The cytosine-adenine-guanine (CAG) triplet expansion encoding glutamine present in the protein huntingtin (Htt), produces widespread neuronal and glial pathology. Mutant huntingtin (mHtt) nuclear aggregates are the primary cause of cortical and striatal neuron degeneration, neuronal inflammation, apoptosis and eventual cell loss. The precise mechanisms underlying the pathogenesis of neurodegeneration in HD remain poorly understood and HD patients have no current cure. Potassium channels are widely expressed in most cell types. In neurons, they play a crucial role in setting the resting membrane potential, mediating the rapid repolarization phase of the action potential and controlling sub-threshold oscillations of membrane potentials. In glial cells, their major contributions are maintaining the resting membrane potential and buffering extracellular K⁺ . Thus, potassium channels have an essential function in both physiological and pathological brain conditions. This review summarizes recent progress on potassium channels involved in the pathology of HD by using different HD mouse models. Exploring the dysfunction of potassium channels in the brain illustrates new approaches for targeting this channel for the treatment of HD.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Myelin dynamics: protecting and shaping neuronal functions

Myelinating glial cells are well-known to insulate axons and to speed up action potential propagation. Through adjustments in the axonal coverage with myelin, myelin sheath thickness and possibly nodal/internode length oligodendrocytes are involved in fine-tuning the brain's computational power throughout life. Be it motor skill learning or social behaviors in higher vertebrates, proper myelination is critical in shaping brain functions. Neurons rely on their myelinating partners not only for setting conduction speed, but also for regulating the ionic environment and fueling their energy demands with metabolites. Also, long-term axonal integrity and neuronal survival are maintained by oligodendrocytes and loss of this well-coordinated axon-glial interplay contributes to neuropsychiatric diseases. Better insight into how myelination and oligodendrocyte functions are constantly fine-tuned in the adult CNS, which includes sensing of neuronal activity and adjusting glial metabolic support, will be critical for understanding higher brain functions and cognitive decline associated with myelin abnormalities in the aging brain.

Integrative genomics of microglia implicates DLG4 (PSD95) in the white matter development of preterm infants

Preterm birth places infants in an adverse environment that leads to abnormal brain development and cerebral injury through a poorly understood mechanism known to involve neuroinflammation. In this study, we integrate human and mouse molecular and neuroimaging data to investigate the role of microglia in preterm white matter damage. Using a mouse model where encephalopathy of prematurity is induced by systemic interleukin-1β administration, we undertake gene network analysis of the microglial transcriptomic response to injury, extend this by analysis of protein-protein interactions, transcription factors and human brain gene expression, and translate findings to living infants using imaging genomics. We show that DLG4 (PSD95) protein is synthesised by microglia in immature mouse and human, developmentally regulated, and modulated by inflammation; DLG4 is a hub protein in the microglial inflammatory response; and genetic variation in DLG4 is associated with structural differences in the preterm infant brain. DLG4 is thus apparently involved in brain development and impacts inter-individual susceptibility to injury after preterm birth.Inflammation mediated by microglia plays a key role in brain injury associated with preterm birth, but little is known about the microglial response in preterm infants. Here, the authors integrate molecular and imaging data from animal models and preterm infants, and find that microglial expression of DLG4 plays a role.

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Metabotropic Glutamate Receptors Protect Oligodendrocytes from Acute Ischemia in the Mouse Optic Nerve

Studies by Bruce Ransom and colleagues have made a major contribution to show that white matter is susceptible to ischemia/hypoxia. White matter contains axons and the glia that support them, notably myelinating oligodendrocytes, which are highly vulnerable to ischemic-hypoxic damage. Previous studies have shown that metabotropic GluRs (mGluRs) are cytoprotective for oligodendrocyte precursor cells and immature oligodendrocytes, but their potential role in adult white matter was unresolved. Here, we report that group 1 mGluR1/5 and group 2 mGluR3 subunits are expressed in optic nerves from mice aged postnatal day (P)8-12 and P30-35. We demonstrate that activation of group 1 mGluR protects oligodendrocytes against oxygen-glucose deprivation (OGD) in developing and young adult optic nerves. In contrast, group 2 mGluR are shown to be protective for oligodendrocytes against OGD in postnatal but not young adult optic nerves. The cytoprotective effect of group 1 mGluR requires activation of PKC, whilst group 2 mGluR are dependent on negatively regulating adenylyl cyclase and cAMP. Our results identify a role for mGluR in limiting injury of oligodendrocytes in developing and young adult white matter, which may be useful for protecting oligodendrocytes in neuropathologies involving excitoxicity and ischemia/hypoxia.

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

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

Potassium channel gene associations with joint processing speed and white matter impairments in schizophrenia

Patients with schizophrenia show decreased processing speed on neuropsychological testing and decreased white matter integrity as measured by diffusion tensor imaging, two traits shown to be both heritable and genetically associated indicating that there may be genes that influence both traits as well as schizophrenia disease risk. The potassium channel gene family is a reasonable candidate to harbor such a gene given the prominent role potassium channels play in the central nervous system in signal transduction, particularly in myelinated axons. We genotyped members of the large potassium channel gene family focusing on putatively functional single nucleotide polymorphisms (SNPs) in a population of 363 controls, 194 patients with schizophrenia spectrum disorder (SSD) and 28 patients with affective disorders with psychotic features who completed imaging and neuropsychological testing. We then performed three association analyses using three phenotypes - processing speed, whole-brain white matter fractional anisotropy (FA) and schizophrenia spectrum diagnosis. We extracted SNPs showing an association at a nominal P value of <0.05 with all three phenotypes in the expected direction: decreased processing speed, decreased FA and increased risk of SSD. A single SNP, rs8234, in the 3' untranslated region of voltage-gated potassium channel subfamily Q member 1 (KCNQ1) was identified. Rs8234 has been shown to affect KCNQ1 expression levels, and KCNQ1 levels have been shown to affect neuronal action potentials. This exploratory analysis provides preliminary data suggesting that KCNQ1 may contribute to the shared risk for diminished processing speed, diminished white mater integrity and increased risk of schizophrenia.

Store-operated calcium entry is essential for glial calcium signalling in CNS white matter

‘Calcium signalling’ is the ubiquitous response of glial cells to multiple extracellular stimuli. The primary mechanism of glial calcium signalling is by release of calcium from intracellular stores of the endoplasmic reticulum (ER). Replenishment of ER Ca²⁺ stores relies on store-operated calcium entry (SOCE). However, despite the importance of calcium signalling in glial cells, little is known about their mechanisms of SOCE. Here, we investigated SOCE in glia of the mouse optic nerve, a typical CNS white matter tract that comprises bundles of myelinated axons and the oligodendrocytes and astrocytes that support them. Using quantitative RT-PCR, we identified Orai1 channels, both Stim1 and Stim2, and the transient receptor potential M3 channel (TRPM3) as the primary channels for SOCE in the optic nerve, and their expression in both astrocytes and oligodendrocytes was demonstrated by immunolabelling of optic nerve sections and cultures. The functional importance of SOCE was demonstrated by fluo-4 calcium imaging on isolated intact optic nerves and optic nerve cultures. Removal of extracellular calcium ([Ca²⁺]_o) resulted in a marked depletion of glial cytosolic calcium ([Ca²⁺]_i), which recovered rapidly on restoration of [Ca²⁺]_o via SOCE. 2-aminoethoxydiphenylborane (2APB) significantly decreased SOCE and severely attenuated ATP-mediated calcium signalling. The results provide evidence that Orai/Stim and TRPM3 are important components of the ‘calcium toolkit’ that underpins SOCE and the sustainability of calcium signalling in white matter glia.