Voltage-gated Na+ channels in glia: properties and possible functions

Modulating voltage-gated sodium channels to enhance differentiation and sensitize glioblastoma cells to chemotherapy

BACKGROUND

Glioblastoma (GBM) stands as the most prevalent and aggressive form of adult gliomas. Despite the implementation of intensive therapeutic approaches involving surgery, radiation, and chemotherapy, Glioblastoma Stem Cells contribute to tumor recurrence and poor prognosis. The induction of Glioblastoma Stem Cells differentiation by manipulating the transcriptional machinery has emerged as a promising strategy for GBM treatment. Here, we explored an innovative approach by investigating the role of the depolarized resting membrane potential (RMP) observed in patient-derived GBM sphereforming cell (GSCs), which allows them to maintain a stemness profile when they reside in the G0 phase of the cell cycle.

METHODS

We conducted molecular biology and electrophysiological experiments, both in vitro and in vivo, to examine the functional expression of the voltage-gated sodium channel (Nav) in GSCs, particularly focusing on its cell cycle-dependent functional expression. Nav activity was pharmacologically manipulated, and its effects on GSCs behavior were assessed by live imaging cell cycle analysis, self-renewal assays, and chemosensitivity assays. Mechanistic insights into the role of Nav in regulating GBM stemness were investigated through pathway analysis in vitro and through tumor proliferation assay in vivo.

RESULTS

We demonstrated that Nav is functionally expressed by GSCs mainly during the G0 phase of the cell cycle, suggesting its pivotal role in modulating the RMP. The pharmacological blockade of Nav made GBM cells more susceptible to temozolomide (TMZ), a standard drug for this type of tumor, by inducing cell cycle re-entry from G0 phase to G1/S transition. Additionally, inhibition of Nav substantially influenced the self-renewal and multipotency features of GSCs, concomitantly enhancing their degree of differentiation. Finally, our data suggested that Nav positively regulates GBM stemness by depolarizing the RMP and suppressing the ERK signaling pathway. Of note, in vivo proliferation assessment confirmed the increased susceptibility to TMZ following pharmacological blockade of Nav.

CONCLUSIONS

This insight positions Nav as a promising prognostic biomarker and therapeutic target for GBM patients, particularly in conjunction with temozolomide treatment.

Characterization of hyperpolarization-activated cyclic nucleotide-gated channels in oligodendrocytes

Mature oligodendrocytes (OLG) are the myelin-forming cells of the central nervous system. Recent work has shown a dynamic role for these cells in the plasticity of neural circuits, leading to a renewed interest in voltage-sensitive currents in OLG. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and their respective current (Ih) were recently identified in mature OLG and shown to play a role in regulating myelin length. Here we provide a biochemical and electrophysiological characterization of HCN channels in cells of the oligodendrocyte lineage. We observed that mice with a nonsense mutation in the Hcn2 gene (Hcn2ap/ap) have less white matter than their wild type counterparts with fewer OLG and fewer oligodendrocyte progenitor cells (OPCs). Hcn2ap/ap mice have severe motor impairments, although these deficits were not observed in mice with HCN2 conditionally eliminated only in oligodendrocytes (Cnpcre/+; Hcn2F/F). However, Cnpcre/+; Hcn2F/F mice develop motor impairments more rapidly in response to experimental autoimmune encephalomyelitis (EAE). We conclude that HCN2 channels in OLG may play a role in regulating metabolism.

Nav1.7 as a chondrocyte regulator and therapeutic target for osteoarthritis

Osteoarthritis (OA) is the most common joint disease. Currently there are no effective methods that simultaneously prevent joint degeneration and reduce pain1. Although limited evidence suggests the existence of voltage-gated sodium channels (VGSCs) in chondrocytes2, their expression and function in chondrocytes and in OA remain essentially unknown. Here we identify Nav1.7 as an OA-associated VGSC and demonstrate that human OA chondrocytes express functional Nav1.7 channels, with a density of 0.1 to 0.15 channels per µm2 and 350 to 525 channels per cell. Serial genetic ablation of Nav1.7 in multiple mouse models demonstrates that Nav1.7 expressed in dorsal root ganglia neurons is involved in pain, whereas Nav1.7 in chondrocytes regulates OA progression. Pharmacological blockade of Nav1.7 with selective or clinically used pan-Nav channel blockers significantly ameliorates the progression of structural joint damage, and reduces OA pain behaviour. Mechanistically, Nav1.7 blockers regulate intracellular Ca2+ signalling and the chondrocyte secretome, which in turn affects chondrocyte biology and OA progression. Identification of Nav1.7 as a novel chondrocyte-expressed, OA-associated channel uncovers a dual target for the development of disease-modifying and non-opioid pain relief treatment for OA.

Characterization in Potent Modulation on Voltage-Gated Na+ Current Exerted by Deltamethrin, a Pyrethroid Insecticide

Deltamethrin (DLT) is a type-II pyrethroid ester insecticide used in agricultural and domestic applications as well as in public health. However, transmembrane ionic channels perturbed by this compound remain largely unclear, although the agent is thought to alter the gating characteristics of voltage-gated Na⁺ (Na_V) channel current. In this study, we reappraised whether and how it and other related compounds can make any further modifications on voltage-gated Na⁺ current (I_Na) in pituitary tumor (GH₃) cells. Cell exposure to DLT produced a differential and dose-dependent stimulation of peak (transient, I_Na(T)) or sustained (late, I_Na(L)) I_Na; consequently, the EC₅₀ value required for DLT-stimulated I_Na(T) or I_Na(L) was determined to be 11.2 or 2.5 μM, respectively. However, neither the fast nor slow component in the inactivation time constant of I_Na(T) activated by short depolarizing pulse was changed with the DLT presence; conversely, tefluthrin (Tef), a type-I pyrethroid insecticide, can accentuate I_Na with a slowing in inactivation time course of the current. The I_Na(L) augmented by DLT was attenuated by further application of either dapagliflozin (Dapa) or amiloride, but not by chlorotoxin. During pulse train (PT) stimulation, with the Tef or DLT presence, the cumulative inhibition of I_Na(T) became slowed; moreover, following PT stimuli, a large tail current with a slowly recovering process was observed. Alternatively, during rapid depolarizing pulse, the amplitude of I_Na(L) and tail I_Na (I_Na(Tail)) for each depolarizing pulse became progressively increased by adding DLT, not by Tef. The recovery time constant following PT stimulation with continued presence of Tef or DLT was shortened by further addition of Dapa. The voltage-dependent hysteresis (Hys_(V)) of persistent I_Na was differentially augmented by Tef or DLT. Taken together, the magnitude, gating, frequency dependence, as well as Hys_(V) behavior of I_Na exerted by the presence of DLT or Tef might exert a synergistic impact on varying functional activities of excitable cells in culture or in vivo.

The effects of electrical stimulation on glial cell behaviour

Neural interface devices interact with the central nervous system (CNS) to substitute for some sort of functional deficit and improve quality of life for persons with disabilities. Design of safe, biocompatible neural interface devices is a fast-emerging field of neuroscience research. Development of invasive implant materials designed to directly interface with brain or spinal cord tissue has focussed on mitigation of glial scar reactivity toward the implant itself, but little exists in the literature that directly documents the effects of electrical stimulation on glial cells. In this review, a survey of studies documenting such effects has been compiled and categorized based on the various types of stimulation paradigms used and their observed effects on glia. A hybrid neuroscience cell biology-engineering perspective is offered to highlight considerations that must be made in both disciplines in the development of a safe implant. To advance knowledge on how electrical stimulation affects glia, we also suggest experiments elucidating electrochemical reactions that may occur as a result of electrical stimulation and how such reactions may affect glia. Designing a biocompatible stimulation paradigm should be a forefront consideration in the development of a device with improved safety and longevity.

Nav1.8 in keratinocytes contributes to ROS-mediated inflammation in inflammatory skin diseases

Reactive oxygen species (ROS)-activated proinflammatory signals in keratinocytes play a crucial role in the immunoregulation of inflammatory skin diseases, including rosacea and psoriasis. Nav1.8 is a voltage-gated sodium ion channel, and its abnormal expression in the epidermal layer contributes to pain hypersensitivity in the skin. However, whether and how epidermal Nav1.8 is involved in skin immunoregulation remains unclear. This study was performed to identify the therapeutic role of Nav1.8 in inflammatory skin disorders. We found that Nav1.8 expression was significantly upregulated in the epidermis of rosacea and psoriasis skin lesions. Nav1.8 knockdown ameliorated skin inflammation in LL37-and imiquimod-induced inflammation mouse models. Transcriptome sequencing results indicated that Nav1.8 regulated the expression of pro-inflammatory mediators (IL1β and IL6) in keratinocytes, thereby contributing to immune infiltration in inflammatory skin disorders. In vitro, tumor necrosis factor alpha (TNFα), a cytokine that drives the development of various inflammatory skin disorders, increased Nav1.8 expression in keratinocytes. Knockdown of Nav1.8 eliminated excess ROS production, thereby attenuating the TNFα-induced production of inflammatory mediators; however, a Nav1.8 blocker did not have the same effect. Mechanistically, Nav1.8 reduced superoxide dismutase 2 (SOD2) activity by directly binding to SOD2 to prevent its deacetylation and mitochondrial localization, subsequently inducing ROS accumulation. Collectively, our study describes a central role for Nav1.8 in regulating pro-inflammatory responses in the skin and indicates a novel therapeutic strategy for rosacea and psoriasis.

Neuronal activity drives pathway-specific depolarization of peripheral astrocyte processes

Astrocytes are glial cells that interact with neuronal synapses via their distal processes, where they remove glutamate and potassium (K⁺) from the extracellular space following neuronal activity. Astrocyte clearance of both glutamate and K⁺ is voltage dependent, but astrocyte membrane potential (V_m) is thought to be largely invariant. As a result, these voltage dependencies have not been considered relevant to astrocyte function. Using genetically encoded voltage indicators to enable the measurement of V_m at peripheral astrocyte processes (PAPs) in mice, we report large, rapid, focal and pathway-specific depolarizations in PAPs during neuronal activity. These activity-dependent astrocyte depolarizations are driven by action potential-mediated presynaptic K⁺ efflux and electrogenic glutamate transporters. We find that PAP depolarization inhibits astrocyte glutamate clearance during neuronal activity, enhancing neuronal activation by glutamate. This represents a novel class of subcellular astrocyte membrane dynamics and a new form of astrocyte-neuron interaction.

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

Slow recovery from the inactivation of voltage-gated sodium channel Nav1.3 in mouse taste receptor cells

Action potentials play an important role in neurotransmitter release in response to taste. Here, I have investigated voltage-gated Na⁺ channels, a primary component of action potentials, in respective cell types of mouse fungiform taste bud cells (TBCs) with in situ whole-cell clamping and single-cell RT-PCR techniques. The cell types of TBCs electrophysiologically examined were determined immunohistochemically using the type III inositol 1,4,5-triphoshate receptor as a type II cell marker and synaptosomal-associated protein 25 as a type III cell marker. I show that type II cells, type III cells, and TBCs not immunoreactive to these markers (likely type I cells) generate voltage-gated Na⁺ currents. The recovery following inactivation of these currents was well fitted with double exponential curves. The time constants in type III cells (~20 ms and ~ 1 s) were significantly slower than respective time constants in other cell types. RT-PCR analysis indicated the expression of Nav1.3, Nav1.5, Nav1.6, and β1 subunit mRNAs in TBCs. Pharmacological inhibition and single-cell RT-PCR studies demonstrated that type II and type III cells principally express tetrodotoxin (TTX)-sensitive Nav1.3 channels and that ~ 30% of type I cells express TTX-resistant Nav1.5 channels. The auxiliary β1 subunit that modulates gating kinetics was rarely detected in TBCs. As the β1 subunit co-expressed with an α subunit is known to accelerate the recovery from inactivation, it is likely that voltage-gated Na⁺ channels in TBCs may function without β subunits. Slow recovery from inactivation, especially in type III cells, may limit high-frequency firing in response to taste substances.

The Novel Direct Modulatory Effects of Perampanel, an Antagonist of AMPA Receptors, on Voltage-Gated Sodium and M-type Potassium Currents

Perampanel (PER) is a selective blocker of AMPA receptors showing efficacy in treating various epileptic disorders including brain tumor-related epilepsy and also potential in treating motor neuron disease. However, besides its inhibition of AMPA-induced currents, whether PER has any other direct ionic effects in different types of neurons remains largely unknown. We investigated the effects of PER and related compounds on ionic currents in different types of cells, including hippocampal mHippoE-14 neurons, motor neuron-like NSC-34 cells and U87 glioma cells. We found that PER differentially and effectively suppressed the amplitude of voltage-gated Na⁺ currents (I_Na) in mHippoE-14 cells. The IC₅₀ values required to inhibit peak and late I_Na were 4.12 and 0.78 μM, respectively. PER attenuated tefluthrin-induced increases in both amplitude and deactivating time constant of I_Na. Importantly, PER also inhibited the amplitude of M-type K⁺ currents (I_K(M)) with an IC₅₀ value of 0.92 μM. The suppression of I_K(M) was attenuated by the addition of flupirtine or ZnCl₂ but not by L-quisqualic acid or sorafenib. Meanwhile, in cell-attached configuration, PER (3 μM) decreased the activity of M-type K⁺ channels with no change in single-channel conductance but shifting the activation curve along the voltage axis in a rightward direction. Supportively, PER suppressed I_K(M) in NSC-34 cells and I_Na in U87 glioma cells. The inhibitory effects of PER on both I_Na and I_K(M), independent of its antagonistic effect on AMPA receptors, may be responsible for its wide-spectrum of effects observed in neurological clinical practice.

Stem cell derived oligodendrocytes to study myelin diseases

Oligodendroglial pathology is central to de- and dysmyelinating, but also contributes to neurodegenerative and psychiatric diseases as well as brain injury. The understanding of oligodendroglial biology in health and disease has been significantly increased during recent years by experimental in vitro and in vivo preclinical studies as well as histological analyses of human tissue samples. However, for many of these diseases the underlying pathology is still not fully understood and treatment options are frequently lacking. This is at least partly caused by the limited access to human oligodendrocytes from patients to perform functional studies and drug screens. The induced pluripotent stem cell technology (iPSC) represents a possibility to circumvent this obstacle and paves new ways to study human disease and to develop new treatment options for so far incurable central nervous system (CNS) diseases. In this review, we summarize the differences between human and rodent oligodendrocytes, provide an overview of the different techniques to generate oligodendrocytes from human progenitor or stem cells and describe the results from studies using iPSC derived oligodendroglial lineage cells. Furthermore, we discuss future perspectives and challenges of the iPSC technology with respect to disease modeling, drug screen, and cell transplantation approaches.

Sodium channels enable fast electrical signaling and regulate phagocytosis in the retinal pigment epithelium

BACKGROUND

Voltage-gated sodium (Nav) channels have traditionally been considered a trademark of excitable cells. However, recent studies have shown the presence of Nav channels in several non-excitable cells, such as astrocytes and macrophages, demonstrating that the roles of these channels are more diverse than was previously thought. Despite the earlier discoveries, the presence of Nav channel-mediated currents in the cells of retinal pigment epithelium (RPE) has been dismissed as a cell culture artifact. We challenge this notion by investigating the presence and possible role of Nav channels in RPE both ex vivo and in vitro.

RESULTS

Our work demonstrates that several subtypes of Nav channels are found in human embryonic stem cell (hESC)-derived and mouse RPE, most prominently subtypes Nav1.4, Nav1.6, and Nav1.8. Whole cell patch clamp recordings from the hESC-derived RPE monolayers showed that the current was inhibited by TTX and QX-314 and was sensitive to the selective blockers of the main Nav subtypes. Importantly, we show that the Nav channels are involved in photoreceptor outer segment phagocytosis since blocking their activity significantly reduces the efficiency of particle internalization. Consistent with this role, our electron microscopy results and immunocytochemical analysis show that Nav1.4 and Nav1.8 accumulate on phagosomes and that pharmacological inhibition of Nav channels as well as silencing the expression of Nav1.4 with shRNA impairs the phagocytosis process.

CONCLUSIONS

Taken together, our study shows that Nav channels are present in RPE, giving this tissue the capacity of fast electrical signaling. The channels are critical for the physiology of RPE with an important role in photoreceptor outer segment phagocytosis.

Role of astroglia in diet-induced central neuroplasticity

Obesity, characterized by increased adiposity that develops when energy intake outweighs expenditure, is rapidly becoming a serious health crisis that affects millions of people worldwide and is associated with severe comorbid disorders including hypertension, cardiovascular disease, and type II diabetes. Obesity is also associated with the dysregulation of central neurocircuits involved in the control of autonomic, metabolic, and cognitive functions. Systemic inflammation associated with diet-induced obesity (DIO) has been proposed to be responsible for the development of these comorbidities as well as the dysregulation of central neurocircuits. A growing body of evidence suggests, however, that exposure to a high-fat diet (HFD) may cause neuroinflammation and astroglial activation even before systemic inflammation develops, which may be sufficient to cause dysregulation of central neurocircuits involved in energy homeostasis before the development of obesity. The purpose of this review is to summarize the current literature exploring astroglial-dependent modulation of central circuits following exposure to HFD and DIO, including not only dysregulation of neurocircuits involved in energy homeostasis and feeding behavior, but also the dysregulation of learning, memory, mood, and reward pathways.

Nav1.5 in astrocytes plays a sex-specific role in clinical outcomes in a mouse model of multiple sclerosis

Astrogliosis is a hallmark of neuroinflammatory disorders such as multiple sclerosis (MS). A detailed understanding of the underlying molecular mechanisms governing astrogliosis might facilitate the development of therapeutic targets. We investigated whether Nav1.5 expression in astrocytes plays a role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), a murine model of MS. We created a conditional knockout of Nav1.5 in astrocytes and determined whether this affects the clinical course of EAE, focal macrophage and T cell infiltration, and diffuse activation of astrocytes. We show that deletion of Nav1.5 from astrocytes leads to significantly worsened clinical outcomes in EAE, with increased inflammatory infiltrate in both early and late stages of disease, unexpectedly, in a sex-specific manner. Removal of Nav1.5 in astrocytes leads to increased inflammation in female mice with EAE, including increased astroglial response and infiltration of T cells and phagocytic monocytes. These cellular changes are consistent with more severe EAE clinical scores. Additionally, we found evidence suggesting possible dysregulation of the immune response-particularly with regard to infiltrating macrophages and activated microglia-in female Nav1.5 KO mice compared with WT littermate controls. Together, our results show that deletion of Nav1.5 from astrocytes leads to significantly worsened clinical outcomes in EAE, with increased inflammatory infiltrate in both early and late stages of disease, in a sex-specific manner.

Down-regulation of Inwardly Rectifying K+ Currents in Astrocytes Derived from Patients with Monge's Disease

Chronic mountain sickness (CMS) or Monge's disease is a disease in highlanders. These patients have a variety of neurologic symptoms such as migraine, mental fatigue, confusion, dizziness, loss of appetite, memory loss and neuronal degeneration. The cellular and molecular mechanisms underlying CMS neuropathology is not understood. In the previous study, we demonstrated that neurons derived from CMS patients' fibroblasts have a decreased expression and altered gating properties of voltage-gated sodium channel. In this study, we further characterize the electrophysiological properties of iPSC-derived astrocytes from CMS patients. We found that the current densities of the inwardly rectifying potassium (Kir) channels in CMS astrocytes (-5.7 ± 2.2 pA/pF at -140 mV) were significantly decreased as compared to non-CMS (-28.4 ± 3.4 pA/pF at -140 mV) and sea level subjects (-28.3 ± 5.3 pA/pF at -140 mV). We further demonstrated that the reduced Kir current densities in CMS astrocytes were caused by their decreased protein expression of Kir4.1 and Kir2.3 channels, while single channel properties (i.e., P_o, conductance) of Kir channel in CMS astrocytes were not altered. In addition, we found no significant differences of outward potassium currents between CMS and non-CMS astrocytes. As compared to non-CMS and sea level subjects, the K⁺ uptake ability in CMS astrocytes was significantly decreased. Taken together, our results suggest that down-regulation of Kir channels and the resulting decreased K⁺ uptake ability in astrocytes could be one of the major molecular mechanisms underlying the neurologic manifestations in CMS patients.

Transient upregulation of Nav1.6 expression in the genu of corpus callosum following middle cerebral artery occlusion in the rats

Focal ischemic stroke can lead to brain damage and cause human disability and death. Increased excitatory transmission and reduced neuronal inhibition are important pathological alterations in the cerebral ischemia, which can induce abnormal brain excitability. Nav1.6 is a key determinant of neuronal excitability in the nervous system. Here we investigate the expression of Nav1.6 at protein and mRNA levels in the rats subjected to middle cerebral artery occlusion (MCAO). Nav1.6 expression at mRNA levels in the ischemic and contralateral hemispheres of MCAO rats were persistently decreased at 6h, 12h and 24h after reperfusion compared to the sham-operated rats. However, a prominent, dynamic increase of Nav1.6 immunoreactivity in reactive astrocytes was observed in the genu of corpus callosum (GCC) of MCAO rats in the acute phase, reaching the peak at 6h after reperfusion, rapidly dropping at 12h and 24h after reperfusion. Furthermore, the upregulation of Nav1.6 expression was strongly correlated with the severity of reactive astrogliosis. Collectively, these findings suggest that this upregulated astrocytic sodium channel expression in the GCC of MCAO rats may contribute to the functional roles of reactive astrocytes in response to brain ischemia.

Remarkable alterations of Nav1.6 in reactive astrogliosis during epileptogenesis

Voltage-gated sodium channels (VGSCs) play a vital role in controlling neuronal excitability. Nav1.6 is the most abundantly expressed VGSCs subtype in the adult central nervous system and has been found to contribute to facilitate the hyperexcitability of neurons after electrical induction of status epilepticus (SE). To clarify the exact expression patterns of Nav1.6 during epileptogenesis, we examined the expression of Nav1.6 at protein and mRNA levels in two distinct animal models of temporal lobe epilepsy (TLE) including a post-SE model induced by kainic acid (KA) intrahippocampal injection and a kindling model evoked by pentylenetetrazole (PTZ). A prominent, seizure intensity-dependent increase of Nav1.6 expression in reactive astrocytes was observed in ipsilateral hippocampus of post-SE rats, reaching the peak at 21 days after SE, a time point during the latent stage of epileptogenesis. However, Nav1.6 with low expression level was selectively expressed in the hippocampal neurons rather than astrocytes in PTZ-kindled animals. This seizure-related increase of a VGSCs subtype in reactive astrocytes after SE may represent a new mechanism for signal communication between neuron and glia in the course of epileptogenesis, facilitating the neuronal hyperexcitability.

Lamotrigine Reduces Inflammatory Response and Ameliorates Executive Function Deterioration in an Alzheimer's-Like Mouse Model

Alzheimer's disease (AD) has been described in the literature, to be associated with impairment of executive function which develops early in the course of disease, and an effective treatment for this clinical feature remains elusive. Preclinical studies have implied that lamotrigine, an antiepileptic agent, could be a potential treatment for executive dysfunction in AD patients. Although there have been promising results in previous studies with lamotrigine, executive function has never been measured using animal models. The aim of the present study was to evaluate the effects of lamotrigine on executive function and determine whether lamotrigine can attenuate inflammatory response in an AD mouse model. Nontransgenic and transgenic mice were treated with lamotrigine (0 or 30 mg/kg/day) in a standard laboratory chow diet starting at 3 months of age. After 6 months of continuous lamotrigine administration, there was a marked improvement in executive function and a significant attenuation in the expression of proinflammatory cytokines. These results suggest that lamotrigine could ameliorate executive dysfunction and brain inflammatory response in the mouse model of AD and early lamotrigine intervention may be a promising therapeutic strategy for AD.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Do Action Potentials Regulate Myelination?

A variety of anatomical features suggest that functional activity in the nervous system can influence the process of myelination, yet direct evidence of this is lacking. Research by Zalc and colleagues shows that myelination of optic nerve is inhibited by a neurotoxin that blocks action potential activity and is stimulated by a toxin that increases impulse activity, suggesting that impulse activity is necessary for initiating myelination during development of the optic nerve. Research by Fields and colleagues, using electrical stimulation of axons, shows that low frequency impulse activity inhibits myelination of dorsal root ganglion neurons, but high frequency impulse activity has no effect. This results from reduced expression of a cell adhesion molecule on the stimulated axons that is critical for inducing myelination. Together these studies support the conclusion that impulse activity can influence the process of myelination, probably through more than one molecular mechanism operating during discrete steps in the myelination process.

Dynamics of sodium channel Nav1.5 expression in astrocytes in mouse models of multiple sclerosis

Astrocytes actively participate in the response of the central nervous system to injury, including in multiple sclerosis. Astrocytes can play both beneficial and detrimental roles in response to neuroinflammation; however, in extreme cases, astrogliosis can result in the formation of a glial scar, which can impede the regeneration of injured neurons. Although astrocytes do not express the voltage-gated sodium channel Nav1.5 in the nonpathological human brain, they exhibit robust upregulation of Nav1.5 within acute and chronic multiple sclerosis lesions. Recent work has indicated that Nav1.5 contributes to the pathways that regulate glial scar formation in vitro through modulation of intracellular Ca levels. However, the temporal dynamics of astrocytic Nav1.5 channel expression in response to neuroinflammatory pathologies has not been investigated. We examined astrocytes from mice with monophasic and chronic-relapsing (CR) experimental autoimmune encephalomyelitis (EAE) by immunohistochemical analysis to determine whether Nav1.5 is expressed in these cells, and whether the expression correlates with the severity of disease and/or phases of relapse and remission. Our results demonstrate that Nav1.5 is upregulated in astrocytes in situ in a temporal manner that correlates with disease severity in both monophasic and CR EAE. Further, in CR EAE, Nav1.5 expression is upregulated during relapses and subsequently attenuated during periods of remission. These observations are consistent with the suggestion that Nav1.5 can play a role in the response of astrocytes to inflammatory pathologies in the central nervous system and suggest Nav1.5 may be a potential therapeutic target to modulate reactive astrogliosis in vivo.

Theoretical and simulation studies on voltage-gated sodium channels

Voltage-gated sodium (Na_v) channels are indispensable membrane elements for the generation and propagation of electric signals in excitable cells. The successes in the crystallographic studies on prokaryotic Na_v channels in recent years greatly promote the mechanistic investigation of these proteins and their eukaryotic counterparts. In this paper, we mainly review the progress in computational studies, especially the simulation studies, on these proteins in the past years.

Voltage-gated sodium channel Nav 1.5 contributes to astrogliosis in an in vitro model of glial injury via reverse Na+ /Ca2+ exchange

Astrogliosis is a prominent feature of many, if not all, pathologies of the brain and spinal cord, yet a detailed understanding of the underlying molecular pathways involved in the transformation from quiescent to reactive astrocyte remains elusive. We investigated the contribution of voltage-gated sodium channels to astrogliosis in an in vitro model of mechanical injury to astrocytes. Previous studies have shown that a scratch injury to astrocytes invokes dual mechanisms of migration and proliferation in these cells. Our results demonstrate that wound closure after mechanical injury, involving both migration and proliferation, is attenuated by pharmacological treatment with tetrodotoxin (TTX) and KB-R7943, at a dose that blocks reverse mode of the Na(+) /Ca(2+) exchanger (NCX), and by knockdown of Nav 1.5 mRNA. We also show that astrocytes display a robust [Ca(2+) ]i transient after mechanical injury and demonstrate that this [Ca(2+) ]i response is also attenuated by TTX, KB-R7943, and Nav 1.5 mRNA knockdown. Our results suggest that Nav 1.5 and NCX are potential targets for modulation of astrogliosis after injury via their effect on [Ca(2+) ]i .

How do astrocytes shape synaptic transmission? Insights from electrophysiology

A major breakthrough in neuroscience has been the realization in the last decades that the dogmatic view of astroglial cells as being merely fostering and buffering elements of the nervous system is simplistic. A wealth of investigations now shows that astrocytes actually participate in the control of synaptic transmission in an active manner. This was first hinted by the intimate contacts glial processes make with neurons, particularly at the synaptic level, and evidenced using electrophysiological and calcium imaging techniques. Calcium imaging has provided critical evidence demonstrating that astrocytic regulation of synaptic efficacy is not a passive phenomenon. However, given that cellular activation is not only represented by calcium signaling, it is also crucial to assess concomitant mechanisms. We and others have used electrophysiological techniques to simultaneously record neuronal and astrocytic activity, thus enabling the study of multiple ionic currents and in depth investigation of neuro-glial dialogues. In the current review, we focus on the input such approach has provided in the understanding of astrocyte-neuron interactions underlying control of synaptic efficacy.

Expression of sodium channel α subunits 1.1, 1.2 and 1.6 in rat hippocampus after kainic acid-induced epilepsy

Voltage-gated Na(+) channels control neuronal excitability and are the primary target for the majority of anti-epileptic drugs. This study investigates the (sub)cellular expression patterns of three important brain-associated Na(+) channel α subunits: NaV1.1, NaV1.2 and NaV1.6 during epileptogenesis (induced by kainic acid) using time points that cover the period from induction to the chronic phase of epilepsy. NaV1.1 immunoreactivity was persistently reduced at 1 day, 3 weeks and 2 months after SE in CA1 and CA3. About 50% of the NaV1.1-positive interneurons was lost at one day after SE in all regions investigated. In the hilus a similar reduction in NeuN-positive neurons was found, while in the CA1 and CA3 region the loss in NeuN-positive neurons only reached 15% in the chronic phase of epilepsy. This implies a stronger shift in the balance between excitation and inhibition toward excitation in the CA1 and CA3 region than in the hilus. NaV1.2 immunoreactivity in the inner molecular layer of the dentate gyrus was lower than control at 1 day after SE. It increased at 3 weeks and 2 months after SE in the inner molecular layer and overlapped with sprouted mossy fibers. NaV1.6 immunoreactivity in the dendritic region of CA1 and CA3 was persistently reduced at all time-points during epileptogenesis. Some astrocytes expressed NaV1.1 and NaV1.6 at 3 weeks after SE. Expression data alone are not sufficient to explain changes in network stability, or infer causality in epileptogenesis. These results demonstrate that hippocampal sub-regional expression of NaV1.1, NaV1.2 and NaV1.6 Na(+) channel α subunits is altered during epileptogenesis in a time and location specific way. This implies that understanding epileptogenesis has to take into account several distinct and type-specific changes in sodium channel expression.

Targeted delivery of antibody-based therapeutic and imaging agents to CNS tumors: crossing the blood-brain barrier divide

INTRODUCTION

Brain tumors are inherently difficult to treat in large part due to the cellular blood-brain barriers (BBBs) that limit the delivery of therapeutics to the tumor tissue from the systemic circulation. Virtually no large molecules, including antibody-based proteins, can penetrate the BBB. With antibodies fast becoming attractive ligands for highly specific molecular targeting to tumor antigens, a variety of methods are being investigated to enhance the access of these agents to intracranial tumors for imaging or therapeutic applications.

AREAS COVERED

This review describes the characteristics of the BBB and the vasculature in brain tumors, described as the blood-brain tumor barrier (BBTB). Antibodies targeted to molecular markers of central nervous system (CNS) tumors will be highlighted, and current strategies for enhancing the delivery of antibodies across these cellular barriers into the brain parenchyma to the tumor will be discussed. Noninvasive imaging approaches to assess BBB/BBTB permeability and/or antibody targeting will be presented as a means of guiding the optimal delivery of targeted agents to brain tumors.

EXPERT OPINION

Preclinical and clinical studies highlight the potential of several approaches in increasing brain tumor delivery across the BBB divide. However, each carries its own risks and challenges. There is tremendous potential in using neuroimaging strategies to assist in understanding and defining the challenges to translating and optimizing molecularly targeted antibody delivery to CNS tumors to improve clinical outcomes.

Neuronal activity promotes myelination via a cAMP pathway

Neuronal activity promotes myelination in vivo and in vitro. However, the molecular events that mediate activity-dependent myelination are not completely understood. Seven, daily 1 h sessions of patterned electrical stimulation (ESTIM) promoted myelin segment formation in mixed cultures of dorsal root ganglion (DRG) neurons and oligodendrocytes (OLs); the increase in myelination was frequency-dependent. Myelin segment formation was also enhanced following exposure of DRGs to ESTIM prior to OL addition, suggesting that ESTIM promotes myelination in a manner involving neuron-specific signaling. Cyclic adenosine monophosphate (cAMP) levels in DRGs were increased three-fold following ESTIM, and artificially increasing cAMP mimicked the ability of ESTIM to promote myelination. Alternatively, inhibiting the cAMP pathway suppressed ESTIM-induced myelination. We used compartmentalized, microfluidic platforms to isolate DRG soma from OLs and assessed cell-type specific effects of ESTIM on myelination. A selective increase or decrease in DRG cAMP levels resulted in enhanced or suppressed myelination, respectively. This work describes a novel role for the cAMP pathway in neurons that results in enhanced myelination.

Nav1.5 sodium channels in macrophages in multiple sclerosis lesions

BACKGROUND

Macrophages are dynamic participants in destruction of white matter in active multiple sclerosis (MS) plaques. Regulation of phagocytosis and myelin degradation along endosomal pathways in macrophages is highly-orchestrated and critically-dependent upon acidification of endosomal lumena. Evidence from in vitro studies with macrophages and THP-1 cells suggests that sodium channel Nav1.5 is present in the limiting membrane of maturing endosomes where it plays a prominent role in the accumulation of protons. However, a contribution of the Nav1.5 channel to macrophage-mediated events in vivo has not been demonstrated.

METHOD

We examined macrophages within active MS lesions by immunohistochemistry to determine whether Nav1.5 is expressed in these cells in situ and, if expressed, whether it is localized to specific compartments along the endocytic pathway.

RESULTS

Our results demonstrate that Nav1.5 is expressed within macrophages in active MS lesions, and that it is preferentially expressed in late endosomes and phagolysosomes (Rab7(+), LAMP-1(+)), and sparsely expressed in early (EEA-1(+)) endosomes. Triple-immunolabeling studies showed localization of Nav1.5 within Rab7(+) endosomes containing proteolipid protein, a myelin marker, in macrophages within active MS plaques.

CONCLUSIONS

These observations support the suggestion that Nav1.5 contributes to the phagocytic pathway of myelin degradation in macrophages in vivo within MS lesions.

Strategies for enhancing antibody delivery to the brain

Antibodies and antibody conjugates have emerged as important tools for cancer therapy. However, a major therapeutic challenge for the use of antibodies is their inability to cross the blood-brain barrier (BBB) to reach tumors localized in the central nervous system (CNS). Multiple methods have been developed to enhance antibody delivery to the CNS, including direct injection, mechanical or biochemical disruption of the BBB, conjugation to a 'molecular Trojan horse', cationization, encapsulation in nanoparticles and liposomes, and more recently, stem cell-mediated antibody delivery. In this review, we discuss each of these approaches, highlighting their successes and the obstacles that remain to be overcome.

Voltage-gated sodium channels as therapeutic targets in epilepsy and other neurological disorders

Voltage-gated sodium channels (VGSCs) are key mediators of intrinsic neuronal and muscle excitability. Abnormal VGSC activity is central to the pathophysiology of epileptic seizures, and many of the most widely used antiepileptic drugs, including phenytoin, carbamazepine, and lamotrigine, are inhibitors of VGSC function. These antiepileptic drugs might also be efficacious in the treatment of other nervous system disorders, such as migraine, multiple sclerosis, neurodegenerative diseases, and neuropathic pain. In this Review, we summarise the structure and function of VGSCs and their involvement in the pathophysiology of several neurological disorders. We also describe the biophysical and molecular bases for the mechanisms of action of antiepileptic VGSC blockers and discuss the efficacy of these drugs in the treatment of epileptic and non-epileptic disorders. Overall, clinical and experimental data indicate that these drugs are efficacious for a range of diseases, and that the development of drugs with enhanced selectivity for specific VGSC isoforms might be an effective and novel approach for the treatment of several neurological diseases.

Astrocytes within multiple sclerosis lesions upregulate sodium channel Nav1.5

Astrocytes are prominent participants in the response of the central nervous system to injury, including neuroinflammatory insults. Rodent astrocytes in vitro have been shown to express voltage-gated sodium channels in a dynamic manner, with a switch in expression of tetrodotoxin-sensitive to tetrodotoxin-resistant channels in reactive astrocytes. However, the expression of sodium channels in human astrocytes has not been studied, and it is not known whether there are changes in the expression of sodium channels in reactive astrocytes of the human central nervous system. Here, we demonstrate a focal and robust upregulation of sodium channel Nav1.5 in reactive astrocytes at the borders of, and within, active and chronic multiple sclerosis lesions. Nav1.5 was only detectable at very low levels in astrocytes within multiple sclerosis macroscopically normal-appearing white matter or in normal control brain. Nav1.1, Nav1.2, Nav1.3 and Nav1.6 showed little or no expression in astrocytes within normal control tissue and limited upregulation in active multiple sclerosis lesions. Nav1.5 was also expressed at high levels in astrocytes in tissue surrounding new and old cerebrovascular accidents and brain tumours. These results demonstrate the expression of Nav1.5 in human astrocytes and show that Nav1.5 expression is dynamic in these cells. Our observations suggest that the upregulated expression of Nav1.5 in astrocytes may provide a compensatory mechanism, which supports sodium/potassium pump-dependent ionic homoeostasis in areas of central nervous system injury.

Sodium channel activity modulates multiple functions in microglia

Microglia provide surveillance in the central nervous system and become activated following tissue insult. Detailed mechanisms by which microglia detect and respond to their environment are not fully understood, but it is known that microglia express a number of surface receptors and ion channels, including voltage-gated sodium channels, that participate in transduction of external stimuli to intra-cellular responses. To determine whether activated microglia are affected by the activity of sodium channels, we examined the expression of sodium channel isoforms in cultured microglia and the action of sodium channel blockade on multiple functions of activated microglia. Rat microglia in vitro express tetrodotoxin (TTX)-sensitive sodium channels Nav1.1 and Nav1.6 and the TTX-resistant channel Nav1.5, but not detectable levels of Nav1.2, Nav1.3, Nav1.7, Nav1.8, and Nav1.9. Sodium channel blockade with phenytoin (40 microM) and TTX (0.3 microM) significantly reduced by 50-60% the phagocytic activity of microglia activated with lipopolysaccharide (LPS); blockade with 10 microM TTX did not further reduce phagocytic activity. Phenytoin attenuated by approximately 50% the release of IL-1 alpha, IL-1 beta, and TNF-alpha from LPS-stimulated microglia, but had minimal effects on the release of IL-2, IL-4, IL-6, IL-10, MCP-1, and TGF-alpha. TTX (0.3 microM) reduced, but to a smaller extent, the release of IL-1 alpha, IL-1 beta, and TNF-alpha from activated microglia. Phenytoin and TTX also significantly decreased by approximately 50% adenosine triphosphate-induced migration by microglia; studies with microglia cultured from med mice (which lack Nav1.6) indicate that Nav1.6 plays a role in microglial migration. The results demonstrate that the activity of sodium channels contributes to effector roles of activated microglia.

Early blockade of injured primary sensory afferents reduces glial cell activation in two rat neuropathic pain models

Satellite glial cells in the dorsal root ganglion (DRG), like the better-studied glia cells in the spinal cord, react to peripheral nerve injury or inflammation by activation, proliferation, and release of messengers that contribute importantly to pathological pain. It is not known how information about nerve injury or peripheral inflammation is conveyed to the satellite glial cells. Abnormal spontaneous activity of sensory neurons, observed in the very early phase of many pain models, is one plausible mechanism by which injured sensory neurons could activate neighboring satellite glial cells. We tested effects of locally inhibiting sensory neuron activity with sodium channel blockers on satellite glial cell activation in a rat spinal nerve ligation (SNL) model. SNL caused extensive satellite glial cell activation (as defined by glial fibrillary acidic protein [GFAP] immunoreactivity) which peaked on day 1 and was still observed on day 10. Perfusion of the axotomized DRG with the Na channel blocker tetrodotoxin (TTX) significantly reduced this activation at all time points. Similar findings were made with a more distal injury (spared nerve injury model), using a different sodium channel blocker (bupivacaine depot) at the injury site. Local DRG perfusion with TTX also reduced levels of nerve growth factor (NGF) in the SNL model on day 3 (when activated glia are an important source of NGF), without affecting the initial drop of NGF on day 1 (which has been attributed to loss of transport from target tissues). Local perfusion in the SNL model also significantly reduced microglia activation (OX-42 immunoreactivity) on day 3 and astrocyte activation (GFAP immunoreactivity) on day 10 in the corresponding dorsal spinal cord. The results indicate that early spontaneous activity in injured sensory neurons may play important roles in glia activation and pathological pain.

Characterization of acid-sensing ion channel expression in oligodendrocyte-lineage cells

Acid-sensing ion channels (ASICs) are widely expressed in neurons, where they serve in pain and mechanical sensation, and contribute to learning and memory. Six ASIC subunit proteins form homo- or heteromeric channel complexes with distinct physiological properties. Of such complexes, only monomeric ASIC1a channels are Ca2+ permeable. Prior pharmacologic and genetic studies have shown that ASIC1a channel inactivation markedly diminishes CNS susceptibility to ischemic damage. Here, we characterize ASIC expression in oligodendrocyte lineage cells (OLC) by molecular, electrophysiological, calcium imaging, and immunofluorescence techniques. ASIC1a, ASIC2a, and ASIC4 mRNAs were expressed in cultured rat OLC, with steady-state levels of each of these mRNAs several-fold higher in oligodendroglial progenitors than in mature oligodendroglia. ASIC transcripts were also detected in brain white matter, and ASIC1a protein expression was detected in white matter oligodendroglia. Inactivating, proton-gated, amiloride-sensitive OLC currents were detected by whole-cell voltage clamp. These currents showed profound tachyphylaxis with slow recovery, and were predominantly blocked by psalmotoxin, indicating that homomeric ASIC1a comprised a large fraction of functional ASIC in the cultured OLC. ASIC activation substantially depolarized OLC plasma membrane in current clamp studies, and elicited transient elevations in intracellular Ca2+ in imaging studies. Thus, OLC ASIC1a channels provide a means by which an acid shift in CNS extracellular pH, by diminishing plasma membrane potential and increasing Ca2+ permeability, can activate OLC signaling pathways, and may contribute to OLC vulnerability to CNS ischemia.

Specificity, affinity and efficacy of iota-conotoxin RXIA, an agonist of voltage-gated sodium channels Na(V)1.2, 1.6 and 1.7

The excitotoxic conopeptide iota-RXIA induces repetitive action potentials in frog motor axons and seizures upon intracranial injection into mice. We recently discovered that iota-RXIA shifts the voltage-dependence of activation of voltage-gated sodium channel Na(V)1.6 to a more hyperpolarized level. Here, we performed voltage-clamp experiments to examine its activity against rodent Na(V)1.1 through Na(V)1.7 co-expressed with the beta1 subunit in Xenopus oocytes and Na(V)1.8 in dissociated mouse DRG neurons. The order of sensitivity to iota-RXIA was Na(V)1.6 > 1.2 > 1.7, and the remaining subtypes were insensitive. The time course of iota-RXIA-activity on Na(V)1.6 during exposure to different peptide concentrations were well fit by single-exponential curves that provided k(obs). The plot of k(obs)versus [iota-RXIA] was linear, consistent with a bimolecular reaction with a K(d) of approximately 3 microM, close to the steady-state EC(50) of approximately 2 microM. iota-RXIA has an unusual residue, D-Phe, and the analog with an L-Phe instead, iota-RXIA[L-Phe44], had a two-fold lower affinity and two-fold faster off-rate than iota-RXIA on Na(V)1.6 and furthermore was inactive on Na(V)1.2. iota-RXIA induced repetitive action potentials in mouse sciatic nerve with conduction velocities of both A- and C-fibers, consistent with the presence of Na(V)1.6 at nodes of Ranvier as well as in unmyelinated axons. Sixteen peptides homologous to iota-RXIA have been identified from a single species of Conus, so these peptides represent a rich family of novel sodium channel-targeting ligands.

How do mutant Nav1.1 sodium channels cause epilepsy?

Voltage-gated sodium channels comprise pore-forming alpha subunits and auxiliary beta subunits. Nine different alpha subtypes, designated Nav1.1-Nav1.9 have been identified in excitable cells. Nav1.1, 1.2 and 1.6 are major subtypes in the adult mammalian brain. More than 200 mutations in the Nav1.1 alpha subtype have been linked to inherited epilepsy syndromes, ranging in severity from the comparatively mild disorder Generalized Epilepsy with Febrile Seizures Plus to the epileptic encephalopathy Severe Myoclonic Epilepsy of Infancy. Studies using heterologous expression and functional analysis of recombinant Nav1.1 channels suggest that epilepsy mutations in Nav1.1 may cause either gain-of-function or loss-of-function effects that are consistent with either increased or decreased neuronal excitability. How these diverse effects lead to epilepsy is poorly understood. This review summarizes the data on sodium channel mutations and epilepsy and builds a case for the hypothesis that most Nav1.1 mutations have their ultimate epileptogenic effects by reducing Nav1.1-mediated whole cell sodium currents in GABAergic neurons, resulting in widespread loss of brain inhibition, an ideal background for the genesis of epileptic seizures.

Morphological interrelationship between astrocytes and nerve endings in the rat spinal trigeminal nucleus caudalis

It has been reported that the spinal trigeminal nucleus caudalis (Sp5C), which receives nociceptive information from the oro-facial regions, has four laminae. To clarify the role of glial cells in the transmission of the nociceptive information, the present study was conducted to examine the detailed distribution of astrocytes in each lamina and also to investigate a morphological interrelationship between the astrocytes and nerve endings in the rat Sp5C. After the preparation of the serial cryostat sections, immunohistochemistry for glial fibrillary acidic protein (GFAP) was employed to identify the astrocytes, and immunohistochemistry for substance P (SP), calcitonin gene-related peptide (CGRP), was used for the nerve endings. We also employed double-labeling immunofluorescence and electron microscopic immunohistochemistry for the GFAP/SP or GFAP/CGRP. GFAP-positive reactions were observed in all laminae of the Sp5C, and SP- or CGRP-positive nerve endings were observed in the lamina I and II. Additionally, we clarified the presence of GFAP/SP- or GFAP/CGRP-positive reactions by the double-labeling immunofluorescence and demonstrated the morphological interrelationship between the astrocytes and nerve endings by the double-labeling electron microscopic immunohistochemistry. These findings suggest that astrocytes might play some roles in the transmission of nociceptive information from the oro-facial region.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Expression of Nav1.6 sodium channels by Schwann cells at neuromuscular junctions: Role in the motor endplate disease phenotype

In addition to their role in action potential generation and fast synaptic transmission in neurons, voltage-dependent sodium channels can also be active in glia. Terminal Schwann cells (TSCs) wrap around the nerve terminal arborization at the neuromuscular junction, which they contribute to shape during development and in the postdenervation processes. Using fluorescent in situ hybridization (FISH), immunofluorescence, and confocal microscopy, we detected the neuronal Nav1.6 sodium channel transcripts and proteins in TSCs in normal adult rats and mice. Nav1.6 protein co-localized with the Schwann cell marker S-100 but was not detected in the SV2-positive nerve terminals. The med phenotype in mice is due to a mutation in the SCN8A gene resulting in loss of Nav1.6 expression. It leads to early onset in postnatal life of defects in neuromuscular transmission with minimal alteration of axonal conduction. Strikingly, in mutant mice, the nonmyelinated pre-terminal region of axons showed abundant sprouting at neuromuscular junctions, and most of the alpha-bungarotoxin-labeled endplates were devoid of S-100- or GFAP-positive TSCs. Using specific antibodies against the Nav1.2 and Nav1.6 sodium channels, ankyrin G and Caspr 1, and a pan sodium channel antibody, we found that a similar proportion of ankyrin G-positive nodes of Ranvier express sodium channels in mutant and wild-type animals and that nodal expression of Nav1.2 persists in med mice. Our data supports the hypothesis that the lack of expression of Nav1.6 in Schwann cells at neuromuscular junctions might play a role in the med phenotype.

Dynamics of forward and reverse transport by the glial glycine transporter, glyt1b

Glycine is a coagonist at the N-methyl-D-aspartate receptor. Changes in extracellular glycine concentration may modulate N-methyl-D-aspartate receptor function and excitatory synaptic transmission. The GLYT1 glycine transporter is present in glia surrounding excitatory synapses, and plays a key role in regulating extracellular glycine concentration. We investigated the kinetic and other biophysical properties of GLYT1b, stably expressed in CHO cells, using whole-cell patch-clamp techniques. Application of glycine produced an inward current, which decayed within a few seconds to a steady-state level. When glycine was removed, a transient outward current was observed, consistent with reverse transport of accumulated glycine. The outward current was enhanced by elevating intracellular or lowering extracellular [Na(+)], and was modulated by changes in extracellular [glycine] and time of glycine application. We developed a model of GLYT1b function, which accurately describes the time course of the transporter current under a range of experimental conditions. The model predicts that glial uptake of glycine will decay toward zero during a sustained period of elevated glycine concentration. This property of GLYT1b may permit spillover from glycinergic terminals to nearby excitatory terminals during a prolonged burst of inhibitory activity, and reverse transport may extend the period of elevated glycine concentration beyond the end of the inhibitory burst.

Potential effects of tetrodotoxin exposure to human glial cells postulated using microarray approach

Sodium channels play an important role in many neurological disorders and also in prostate cancer. Tetrodotoxin (TTX), a blocker of voltage-gated sodium channels has been chiefly used as a molecular probe for the study and characterization of these channels. The regulation of gene expression in response for the exposure of TTX to glial cells which are reported to be involved in neurodegenerative process is poorly understood. Therefore, the present study aims to develop a repository of genes and map it on a few pivotal neurodegenerative pathways to speculate the effect of TTX. Using Affymetrix GeneChip (HG-U133A), we have selected a subset of 692 differentially expressed genes, several of which are-cullin 4A (CUL4A), ubiquitin carrier protein (E2-EPF), proteasome (prosome, macropain) subunit, beta type, 8 (large multifunctional protease 7) (PSMB8), protein tyrosine phosphatase type IVA (PTP4A1), intercellular adhesion molecule 1 (ICAM1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caspase 1 (CASP1). These genes, which facilitate some of the neurodegenerative pathways, such as ubiquitin, proteasome, inflammation and kinases, were identified to be up- or down-regulated for the TTX treatment. Thus, the selected genes were further examined on ubiquitin-proteasome mediated inflammatory responses pathway as ample evidence for the role of glial cell-mediated inflammation in the neurodegenerative process are available. In summary, our result provides a basic understanding of the differentially expressed genes along with one of the possible pathway which may have been modulated by the exposure of TTX.

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

Molecular characterization of voltage-gated sodium channels in human gliomas

Voltage-sensitive sodium channels appear to be an electrophysiological hallmark of gliomas. However, the expression of channel subtypes is unclear in these tumors. In this study different gliomas were investigated for the expression of sodium channel subtypes Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.4, Na(v)1.6, and Na(x)(Na(v)2.1) using RT-PCR. At least one subtype of channels could be detected in each tumor. High-grade gliomas expressed fewer sodium channel subtypes and these at weaker levels than low-grade tumors. Expression of Na(v)1.6, the most abundant isoform in the CNS, was almost absent in the gliomas except the pilocytic variant. Our study gives clear evidence for a differential expression of sodium channel subtypes in gliomas and indicates a predominant expression of channels related to malignancy grades.

NaX sodium channel is expressed in non-myelinating Schwann cells and alveolar type II cells in mice

Na(x) is an extracellular sodium-level-sensitive sodium channel expressed in the circumventricular organs in the brain, essential loci for the sodium-level homeostasis in body fluids. Here, we examined the localization of Na(x) throughout the visceral organs at the cellular level. In visceral organs including lung, heart, intestine, bladder, kidney and tongue, a subset of Schwann cells within the peripheral nerve trunks were highly positive for Na(x). An electron microscopic study indicated that these Na(x)-positive cells were non-myelinating Schwann cells. In the lung, Na(x)-positive signals were also observed in the alveolar type II cells, which actively absorb sodium and water to aid gas exchange through the alveolar surface. It was thus suggested that the Na(x) sodium channel is involved in controlling the local extracellular sodium level through sodium absorption activity.