Heterogeneous expression of voltage-gated potassium channels of the shaker family (Kv1) in oligodendrocyte progenitors

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.

Intellectual Disability and Potassium Channelopathies: A Systematic Review

Intellectual disability (ID) manifests prior to adulthood as severe limitations to intellectual function and adaptive behavior. The role of potassium channelopathies in ID is poorly understood. Therefore, we aimed to evaluate the relationship between ID and potassium channelopathies. We hypothesized that potassium channelopathies are strongly associated with ID initiation, and that both gain- and loss-of-function mutations lead to ID. This systematic review explores the burden of potassium channelopathies, possible mechanisms, advancements using animal models, therapies, and existing gaps. The literature search encompassed both PubMed and Embase up to October 2019. A total of 75 articles describing 338 cases were included in this review. Nineteen channelopathies were identified, affecting the following genes: KCNMA1, KCNN3, KCNT1, KCNT2, KCNJ10, KCNJ6, KCNJ11, KCNA2, KCNA4, KCND3, KCNH1, KCNQ2, KCNAB1, KCNQ3, KCNQ5, KCNC1, KCNB1, KCNC3, and KCTD3. Twelve of these genes presented both gain- and loss-of-function properties, three displayed gain-of-function only, three exhibited loss-of-function only, and one had unknown function. How gain- and loss-of-function mutations can both lead to ID remains largely unknown. We identified only a few animal studies that focused on the mechanisms of ID in relation to potassium channelopathies and some of the few available therapeutic options (channel openers or blockers) appear to offer limited efficacy. In conclusion, potassium channelopathies contribute to the initiation of ID in several instances and this review provides a comprehensive overview of which molecular players are involved in some of the most prominent disease phenotypes.

Human immunodeficiency virus protein Tat induces oligodendrocyte injury by enhancing outward K+ current conducted by KV1.3

Brain white matter damage is frequently detected in patients infected with human immunodeficiency virus type 1 (HIV-1). White matter is composed of neuronal axons sheathed by oligodendrocytes (Ols), the myelin-forming cells in central nervous system. Ols are susceptible to HIV-1 viral trans-activator of transcription (Tat) and injury of Ols results in myelin sheath damage. It has been demonstrated that activation of voltage-gated K⁺ (K_V) channels induces cell apoptosis and Ols predominantly express K⁺ channel K_V1.3. It is our hypothesis that Tat injures Ols via activation of K_V1.3. To test this hypothesis, we studied the involvement of K_V1.3 in Tat-induced Ol/myelin injury both in vitro and ex vivo. Application of Tat to primary rat Ol cultures enhanced whole-cell K_V1.3 current recorded under voltage clamp configuration and confirmed by specific K_V1.3 antagonists Margatoxin (MgTx) and 5-(4-phenoxybutoxy) psoralen (PAP). The Tat enhancement of K_V1.3 current was associated with Tat-induced Ol apoptosis, which was blocked by MgTx and PAP or by siRNA knockdown of K_V1.3 gene. The Tat-induced Ol injury was validated in cultured rat brain slices, particularly in corpus callosum and striatum, that incubation of the slices with Tat resulted in myelin damage and reduction of myelin basic protein which were also blocked by aforementioned K_V1.3 antagonists. Further studies revealed that Tat interacts with K_V1.3 as determined by protein pull-down of recombinant GST-Tat with K_V1.3 expressed in rat brains and HEK293 cells. Such protein-protein interaction may alter channel protein phosphorylation, resultant channel activity and consequent Ol/myelin injury. Taken together, these results demonstrate an involvement of K_V1.3 in Tat- induced Ol/myelin injury, a potential mechanism for the pathogenesis of HIV-1-associated white matter damage.

Complexes of Peptide Blockers with Kv1.6 Pore Domain: Molecular Modeling and Studies with KcsA-Kv1.6 Channel

Potassium voltage-gated Kv1.6 channel, which is distributed primarily in neurons of central and peripheral nervous systems, is of significant physiological importance. To date, several high-affinity Kv1.6-channel blockers are known, but the lack of selective ones among them hampers the studies of tissue localization and functioning of Kv1.6 channels. Here we present an approach to advanced understanding of interactions of peptide toxin blockers with a Kv1.6 pore. It combines molecular modeling studies and an application of a new bioengineering system based on a KcsA-Kv1.6 hybrid channel for the quantitative fluorescent analysis of blocker-channel interactions. Using this system we demonstrate that peptide toxins agitoxin 2, kaliotoxin1 and OSK1 have similar high affinity to the extracellular vestibule of the K⁺-conducting pore of Kv1.6, hetlaxin is a low-affinity ligand, whereas margatoxin and scyllatoxin do not bind to Kv1.6 pore. Binding of toxins to Kv1.6 pore has considerable inverse dependence on the ionic strength. Model structures of KcsA-Kv1.6 and Kv1.6 complexes with agitoxin 2, kaliotoxin 1 and OSK1 were obtained using homology modeling and molecular dynamics simulation. Interaction interfaces, which are formed by 15-19 toxin residues and 10 channel residues, are described and compared. Specific sites of Kv1.6 pore recognition are identified for targeting of peptide blockers. Analysis of interactions between agitoxin 2 derivatives with point mutations (S7K, S11G, L19S, R31G) and KcsA-Kv1.6 confirms reliability of the calculated complex structure.

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

CNS Schwann cells display oligodendrocyte precursor-like potassium channel activation and antigenic expression in vitro

Central nervous system (CNS) injury triggers production of myelinating Schwann cells from endogenous oligodendrocyte precursors (OLPs). These CNS Schwann cells may be attractive candidates for novel therapeutic strategies aiming to promote endogenous CNS repair. However, CNS Schwann cells have been so far mainly characterized in situ regarding morphology and marker expression, and it has remained enigmatic whether they display functional properties distinct from peripheral nervous system (PNS) Schwann cells. Potassium channels (K+) have been implicated in progenitor and glial cell proliferation after injury and may, therefore, represent a suitable pharmacological target. In the present study, we focused on the function and expression of voltage-gated K+ channels Kv(1-12) and accessory β-subunits in purified adult canine CNS and PNS Schwann cell cultures using electrophysiology and microarray analysis and characterized their antigenic phenotype. We show here that K+ channels differed significantly in both cell types. While CNS Schwann cells displayed prominent K D-mediated K+ currents, PNS Schwann cells elicited K(D-) and K(A-type) K+ currents. Inhibition of K+ currents by TEA and Ba2+ was more effective in CNS Schwann cells. These functional differences were not paralleled by differential mRNA expression of Kv(1-12) and accessory β-subunits. However, O4/A2B5 and GFAP expressions were significantly higher and lower, respectively, in CNS than in PNS Schwann cells. Taken together, this is the first evidence that CNS Schwann cells display specific properties not shared by their peripheral counterpart. Both Kv currents and increased O4/A2B5 expression were reminiscent of OLPs suggesting that CNS Schwann cells retain OLP features during maturation.

Human glioma-initiating cells show a distinct immature phenotype resembling but not identical to NG2 glia

Glioma-initiating cells (GICs) represent a potential important therapeutic target because they are likely to account for the frequent recurrence of malignant gliomas; however, their identity remains unsolved. Here, we characterized the cellular lineage fingerprint of GICs through a combination of electrophysiology, lineage marker expression, and differentiation assays of 5 human patient-derived primary GIC lines. Most GICs coexpressed nestin, NG2 proteoglycan, platelet-derived growth factor receptor-α, and glial fibrillary acidic protein. Glioma-initiating cells could be partially differentiated into astrocytic but not oligodendroglial or neural lineages. We also demonstrate that GICs have a characteristic electrophysiologic profile distinct from that of well-characterized tumor bulk cells. Together, our results suggest that GICs represent a unique type of cells reminiscent of an immature phenotype that closely resembles but is not identical to NG2 glia with respect to marker expression and functional membrane properties.

Charybdotoxin and margatoxin acting on the human voltage-gated potassium channel hKv1.3 and its H399N mutant: an experimental and computational comparison

The effect of the pore-blocking peptides charybdotoxin and margatoxin, both scorpion toxins, on currents through human voltage-gated hK(v)1.3 wild-type and hK(v)1.3_H399N mutant potassium channels was characterized by the whole-cell patch clamp technique. In the mutant channels, both toxins hardly blocked current through the channels, although they did prevent C-type inactivation by slowing down the current decay during depolarization. Molecular dynamics simulations suggested that the fast current decay in the mutant channel was a consequence of amino acid reorientations behind the selectivity filter and indicated that the rigidity-flexibility in that region played a key role in its interactions with scorpion toxins. A channel with a slightly more flexible selectivity filter region exhibits distinct interactions with scorpion toxins. Our studies suggest that the toxin-channel interactions might partially restore rigidity in the selectivity filter and thereby prevent the structural rearrangements associated with C-type inactivation.

OLIG2 over-expression impairs proliferation of human Down syndrome neural progenitors

Mental retardation and early Alzheimer's disease (AD) have generally been attributed to progressive neuronal loss in the developing and mature Down syndrome (DS) brain. However, reduced neuronal production during development could also contribute to the smaller brain size and simplified gyral patterning seen in this disorder. Here, we show impairments in proliferation within the ventricular zone (VZ) of early DS fetal cortex and in cultured early passage DS human neural progenitors (HNPs). We find that the reduced proliferative rates correspond temporally with increased expression of the chromosome 21 (HSA21) associated, oligodendrocyte transcription factor OLIG2 at 14-18 weeks gestational age (GA) (period of neurogenesis). Moreover, the DS HNPs adopt more oligodendrocyte-specific features including increased oligodendrocyte marker expression, as well as a reduction in KCNA3 potassium channel expression and function. We further show that OLIG2 inhibition or over-expression regulates potassium channel expression levels and that activation or inhibition of these channels influences the rate of progenitor proliferation. Finally, neural progenitors from Olig2 over-expressing transgenic mice exhibit these same impairments in proliferation and potassium channel expression. These findings suggest that OLIG2 over-expression inhibits neural progenitor proliferation through changes in potassium channel activity, thereby contributing to the reduced neuronal numbers and brain size in DS.

Dalfampridine in multiple sclerosis: from symptomatic treatment to immunomodulation

Multiple sclerosis (MS) is a neurodegenerative disease that is deemed to affect more than 2.1 million people worldwide, and for which there is no cure. Early symptoms of MS are believed to result from axonal demyelination leading to slowing or blockade of impulse conduction. The blockade of K+ channels has been proven to improve conduction deficiencies secondary to demyelination in patients with MS. Dalfampridine is a K+ channel blocker that was recently approved by FDA for the symptomatic treatment of ambulation hardship in MS. Understanding the mechanisms by which Dalfampridine exerts its therapeutic effects is a complex issue as it blocks a wide variety of K+ channels that are distributed across multiple cell types in the nervous system but also in the immune system, and because of their molecular identities remaining unknown. This review describes Dalfampridine potential roles at the cellular and molecular levels in MS pathogenesis.

Potassium channel expression in adult murine neural progenitor cells

Neural progenitor cells (NPCs) are a source of new neurons and glia in the adult brain. Most NPCs reside in the forebrain subventricular zone (SVZ) and in the subgranular zone of the dentate gyrus, where they contribute to plasticity in the adult brain. To use their potential for repair, it is essential to identify the molecules that regulate their growth, migration and differentiation. Potassium (K+) channels are promising molecule candidates for NPC regulation as they are important components of signal transduction and their diversity is ideal to cover the complex functions required for cell proliferation and differentiation. There is increasing evidence that K+ channels influence cell growth and neurogenesis, however, very little is known regarding K+ channel distribution in NPCs. We therefore explored the expression of a variety of voltage-gated (Kv), inwardly rectifying (Kir) and two-pore (K2P) K+ channels in the SVZ of adult mice and in neurosphere cultures of NPCs during growth and differentiation. Immunocytochemical analysis revealed a differential expression pattern of K+ channels in nestin+ SVZ precursor cells, early SVZ doublecortin+ neurons and (sub)ependymal cells. These findings were confirmed in neurosphere cultures at the protein and mRNA levels. The expression of some K+ channel proteins, such as Kir4.1, Kir6.1, TREK1 or TASK1, suggests a role of K+ channels in the complex regulation of NPC proliferation, maturation and differentiation.

Editing of neurotransmitter receptor and ion channel RNAs in the nervous system

The central dogma of molecular biology defines the major route for the transfer of genetic information from genomic DNA to messenger RNA to three-dimensional proteins that affect structure and function. Like alternative splicing, the post-transcriptional conversion of adenosine to inosine (A-to-I) by RNA editing can dramatically expand the diversity of the transcriptome to generate multiple, functionally distinct protein isoforms from a single genomic locus. While RNA editing has been identified in virtually all tissues, such post-transcriptional modifications have been best characterized in RNAs encoding both ligand- and voltage-gated ion channels and neurotransmitter receptors. These RNA processing events have been shown to play an important role in the function of the encoded protein products and, in several cases, have been shown to be critical for the normal development and function of the nervous system.

Characterization of voltage-gated potassium channels in human neural progenitor cells

Background

Voltage-gated potassium (K_v) channels are among the earliest ion channels to appear during brain development, suggesting a functional requirement for progenitor cell proliferation and/or differentiation. We tested this hypothesis, using human neural progenitor cells (hNPCs) as a model system.

Methodology/Principal Findings

In proliferating hNPCs a broad spectrum of K_v channel subtypes was identified using quantitative real-time PCR with a predominant expression of the A-type channel K_v4.2. In whole-cell patch-clamp recordings K_v currents were separated into a large transient component characteristic for fast-inactivating A-type potassium channels (I_A) and a small, sustained component produced by delayed-rectifying channels (I_K). During differentiation the expression of I_A as well as A-type channel transcripts dramatically decreased, while I_K producing delayed-rectifiers were upregulated. Both K_v currents were differentially inhibited by selective neurotoxins like phrixotoxin-1 and α-dendrotoxin as well as by antagonists like 4-aminopyridine, ammoniumchloride, tetraethylammonium chloride and quinidine. In viability and proliferation assays chronic inhibition of the A-type currents severely disturbed the cell cycle and precluded proper hNPC proliferation, while the blockade of delayed-rectifiers by α-dendrotoxin increased proliferation.

Conclusions/Significance

These findings suggest that A-type potassium currents are essential for proper proliferation of immature multipotent hNPCs.

Expression of Kv1.2 in microglia and its putative roles in modulating production of proinflammatory cytokines and reactive oxygen species

Microglial cells are endowed with different potassium ion channels but their expression and specific functions have remained to be fully clarified. This study has shown Kv1.2 expression in the amoeboid microglia in the rat brain between 1 (P1) and 10 (P10) days of age. Kv1.2 expression was localized in the ramified microglia at P14 and was hardly detected at P21. In postnatal rats exposed to hypoxia, Kv1.2 immunoreactivity in microglia was markedly enhanced. Quantitative RT-PCR analysis confirmed Kv1.2 mRNA expression in microglial cells in vitro. It was further shown that Kv1.2 and protein expression coupled with that of interleukin 1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) was significantly increased when the cells were subjected to hypoxia. The same increase was observed in cells exposed to adenosine 5'-triphosphate (ATP) and lipopolysaccharide (LPS). Concomitantly, the intracellular potassium concentration decreased significantly. Blockade of Kv1.2 channel with rTityustoxin-Kalpha (TsTx) resulted in partial recovery of intracellular potassium concentration accompanied by a reduced expression of IL-1beta and TNF-alpha mRNA and protein expression and intracellular reactive oxygen species (ROS) production. We conclude that Kv1.2 in microglia modulates IL-1beta and TNF-alpha expression and ROS production probably by regulating the intracellular potassium concentration.

Re-expression of a developmentally restricted potassium channel in autoimmune demyelination: Kv1.4 is implicated in oligodendroglial proliferation

Mechanisms of lesion repair in multiple sclerosis are incompletely understood. To some degree, remyelination can occur, associated with an increase of proliferating oligodendroglial cells. Recently, the expression of potassium channels has been implicated in the control of oligodendrocyte precursor cell proliferation in vitro. We investigated the expression of Kv1.4 potassium channels in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, a model of multiple sclerosis. Confocal microscopy revealed expression of Kv1.4 in AN2-positive oligodendrocyte precursor cells and premyelinating oligodendrocytes in vitro but neither in mature oligodendrocytes nor in the spinal cords of healthy adult mice. After induction of myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, Kv1.4 immunoreactivity was detected in or around lesions already during disease onset with a peak early and a subsequent decrease in the late phase of the disease. Kv1.4 expression was confined to 2',3'-cyclic nucleotide 3'-phosphodiesterase-positive oligodendroglial cells, which were actively proliferating and ensheathed naked axons. After a demyelinating episode, the number of Kv1.4 and 2',3'-cyclic nucleotide 3'-phosphodiesterase double-positive cells was greatly reduced in ciliary neurotrophic factor knockout mice, a model with impaired lesion repair. In summary, the re-expression of an oligodendroglial potassium channel may have a functional implication on oligodendroglial cell cycle progression, thus influencing tissue repair in experimental autoimmune encephalomyelitis and multiple sclerosis.

Selective blockage of Kv1.3 and Kv3.1 channels increases neural progenitor cell proliferation

The modulation of cell proliferation in neural progenitor cells (NPCs) is believed to play a role in neuronal regeneration. Recent studies showed that K(+) channel activity influenced cell proliferation. Therefore, we examined NPCs for K(+) channels and tested whether NPC self renewing can be modulated by synthetic K(+) channel modulators. The whole-cell K(+) current was partly K(+) dependent and showed a cumulative inactivating component. Two tetra-ethyl-ammonium ion (TEA)-sensitive K(+) currents with different voltage dependencies ( = 65 microm, E(50) = -0.3 +/- 1.3 mV and = 8 mm, E(50) = -15.2 +/- 2.8 mV) and an almost TEA-insensitive current were identified. Kaliotoxin blocked approximately 50% of the entire K(+) currents (IC(50) = 0.25 nm). These properties resembled functional characteristics of K(v)1.4, K(v)1.3 and K(v)3.1 channels. Transcripts for these channels, as well as proteins for K(v)1.3 and K(v)3.1, were identified. Immunocytochemical staining revealed K(v)1.3 and K(v)3.1 K(+) channel expression in almost all NPCs. The blockage of K(v)3.1 by low concentrations of TEA, as well as the blockage of K(v)1.3 by Psora-4, increased NPC proliferation. These findings underline the regulatory role of K(+) channels on the cell cycle and imply that K(+) channel modulators might be used therapeutically to activate endogenous NPCs.

Potassium currents in primary cultured astrocytes from the rat corpus callosum

The corpus callosum (CC) is the main white matter tract in the brain and is involved in interhemispheric communication. Using the whole-cell voltage-clamp technique, a study was made of K(+)-currents in primary cultured astrocytes from the CC of newborn rats. These cells were positive to glial fibrillary acidic protein after culturing in Dulbecco's Modified Eagle Medium (> 95% of cells) or in serum-free neurobasal medium with G5 supplement (> 99% of cells). Astrocytes cultured in either medium displayed similar voltage-activated ion currents. In 81% of astrocytes, the current had a transient component and a sustained component, which were blocked by 4-aminopyridine and tetraethylammonium, respectively; and both had a reversal potential of -66 mV, indicating that they were carried by K(+) ions. Based on the Ba(2+)-sensitivity and activation kinetics of the K(+)-current, two groups of astrocytes were discerned. One group (55% of cells) displayed a strong Ba(2+) blockade of the K(+)-current whose activation kinetics, time course of decay, and the current-voltage relationship were modified by Ba(2+). This current was greatly blocked (52%) by Ba(2+) in a voltage-dependent way. Another group (45% of cells) presented weak Ba(2+)-blockade, which was only blocked 24% by Ba(2+). The activation kinetics and time course of decay of this current component were unaffected by Ba(2+). These results may help to understand better the roles of voltage-activated K(+)-currents in astrocytes from the rat CC in particular and glial cells in general.

K+ channel KV3.1 associates with OSP/claudin-11 and regulates oligodendrocyte development

K(+) channels are differentially expressed throughout oligodendrocyte (Olg) development. K(V)1 family voltage-sensitive K(+) channels have been implicated in proliferation and migration of Olg progenitor cell (OPC) stage, and inward rectifier K+ channels (K(IR))4.1 are required for OPC differentiation to myelin-forming Olg. In this report we have identified a Shaw family K(+) channel, K(V)3.1, that is involved in proliferation and migration of OPC and axon myelination. Application of anti-K(V)3.1 antibody or knockout of Kv3.1 gene decreased the sustained K(+) current component of OPC by 50% and 75%, respectively. In functional assays block of K(V)3.1-specific currents or knockout of Kv3.1 gene inhibited proliferation and migration of OPC. Adult Kv3.1 gene-knockout mice had decreased diameter of axons and decreased thickness of myelin in optic nerves compared with age-matched wild-type littermates. Additionally, K(V)3.1 was identified as an associated protein of Olg-specific protein (OSP)/claudin-11 via yeast two-hybrid analysis, which was confirmed by coimmunoprecipitation and coimmunohistochemistry. In summary, the K(V)3.1 K(+) current accounts for a significant component of the total K(+) current in cells of the Olg lineage and, in association with OSP/claudin-11, plays a significant role in OPC proliferation and migration and myelination of axons.

Downregulation of platelet-derived growth factor-alpha receptor-mediated tyrosine kinase activity as a cellular mechanism for K+-channel regulation during oligodendrocyte development in situ

Oligodendrocyte maturation has been defined based on expression of developmentally regulated antigens. However, transitions at early stages of the lineage have not been functionally characterized fully in situ. Combining 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-promoter driven enhanced green fluorescent protein expression and whole-cell capacitance measurements permitted a reliable distinction between subcortical white matter NG2+ oligodendrocyte progenitors (OPs) and O4+ preoligodendrocytes (pre-OLs) in situ. We focused on K+ channels because their expression has been associated previously with the proliferation and differentiation potential of OPs. Using whole-cell patch clamp, we observed a downregulation of the delayed outward-rectifying current (IKDR) between the NG2+ and O4+ stages but no significant changes in transient K+-channel current (IKA) amplitude. Tyrosine kinase inhibition in NG2+ cells reduced IKDR amplitude with no effect on IKA, which mimicked the endogenous changes observed between OPs and pre-OLs. Tyrosine kinase inhibition also reduced the proliferative capacity of NG2+ OPs in slice cultures. Conversely, acute platelet-derived growth factor receptor-alpha (PDGFR-alpha) activation caused an increase of IKDR in NG2+ but not in O4+ cells. Consistent with this finding, PDGFR-alpha immunoreactivity was confined to NG2+ cells with undetectable levels in O4+ cells, suggesting that PDGFR-alpha signaling is absent in pre-OLs in situ. Importantly, the PDGF-induced increase of IKDR in NG2+ cells was prevented by tyrosine kinase inhibition. Together, these data indicate that PDGFR-alpha and tyrosine kinase activity act via a common pathway that influences functional expression of K+ channels and proliferative capacity of OPs in situ.

Potassium channels, memory T cells, and multiple sclerosis

Multiple sclerosis is a chronic inflammatory autoimmune disease of the central nervous system characterized by demyelination and axonal damage that result in disabling neurological deficits. Here the authors explain the rationale for the use of inhibitors of the Kv1.3 K+ channel in immune cells as a therapy for multiple sclerosis and other autoimmune disorders.

Cardiac performance in the zebrafish breakdance mutant

In the Tubingen screen a breakdance mutant of zebrafish (bre) was described as an arrhythmia, in which the ventricle beats only with every second atrial contraction (2:1 rhythm). Surprisingly, a careful analysis of the effect of the breakdance mutation on cardiac performance of the zebrafish during development between 3 d.p.f. and 14 d.p.f revealed that homozygous bre mutants did not always show the 2:1 rhythm. Cardiac activity was continuously recorded for a period of 20 min in each larva, and during this period we observed that heart rate randomly switched between the 2:1 rhythm and a 1:1 rhythm. Furthermore, at 28 degrees C and at 31 degrees C the expression of the 2:1 rhythm decreased with development. At 31 degrees C this was in part due to a significantly reduced survival rate of mutants beyond 4 d.p.f. Besides development, temperature had a marked effect on the expression of the 2:1 rhythm, and during the first days of development the expression of the 2:1 rhythm was significantly higher at elevated incubation temperatures. By contrast, in the 2:1 beating heart ventricular contraction rate was about 80 beats min(-1) throughout development irrespective of the temperature, and even in the 1:1 rhythm mutants showed a significant bradycardia at all three temperatures (25 degrees C, 28 degrees C or 31 degrees C). Compared to wild-type animals, cardiac output was significantly lower in bre mutants. Pressure traces recorded in the ventricle of mutants revealed a prolonged relaxation phase, indicating that the second pacemaker current could not be conveyed to the ventricle (AV-block). This phenotype is comparable to the human Long QT Syndrome, an arrhythmia caused by a modification of an ion channel involved in cardiac repolarization. The bradycardia and the modified temperature sensitivity of heart rate suggested that the activity of the pacemaker cells was also affected by this mutation.

K+ channel blockade impairs remyelination in the cuprizone model

The adult CNS has the capacity to remyelinate following metabolic, toxic and autoimmune demyelinating insults. In cuprizone-induced demyelination, spontaneous remyelination occurs after the cessation of cuprizone diet. We used the cuprizone model to investigate the role of glial K(+) channels in oligodendroglial (OLG) regeneration and remyelination in vivo. We found that treatment with 4-aminopyridine (4-AP), a broad-spectrum K(+) channel antagonist, results in: (1) decreased number of oligodendroglial progenitors (OP) and OLGs; (2) diminished astrogliosis; and (3) decreased remyelination in the corpus callosum based on the immunoreactivity to myelin basic protein (MBP), Rip monoclonal antibody, and by electron microscopy. Our findings support the concept that glial K(+) channels play an important role during OLG regeneration and remyelination, a crucial factor to be considered during the development of therapeutic strategies to facilitate recovery in demyelinating diseases and spinal cord injury.

Temporal and spatial patterns of Kv1.1 and Kv1.2 protein and gene expression in spinal cord white matter after acute and chronic spinal cord injury in rats: implications for axonal pathophysiology after neurotrauma

After spinal cord injury (SCI), surviving white matter axons display axonal dysfunction associated with demyelination and altered K+ channel activity. To clarify the molecular basis of posttraumatic axonal pathophysiology after SCI, we investigated the changes in expression and distribution of the axonal K+ channel subunits Kv1.1 and Kv1.2 in spinal cord white matter after in vivo SCI in the rat. Using Western blot analysis, we found an increased expression of Kv1.1 and Kv1.2 at 2 and 6 weeks after SCI. By real-time PCR we observed an increase in Kv1.1 and Kv1.2 mRNA levels 1 day after SCI, which persisted until 6 weeks. Confocal immunohistochemistry showed a markedly dispersed labelling of Kv1.1 and Kv1.2 along the injured axons, in contrast to the tight localization of these channels to the juxtaparanodes of noninjured axons. This redistribution of Kv1.1 and Kv1.2 occurred as early as 1 h postinjury along some injured axons, and persisted at 6 weeks postinjury. In parallel with the redistribution of Kv1.1 and 1.2, contactin-associated protein (Caspr), which is normally confined to a paranodal location, also displayed a more diffuse distribution along the injured spinal cord axons. Our results suggest that the increased expression of Kv1.1 and Kv1.2 proteins is transcriptionally regulated. In contrast, the redistribution of the axonal K+ channel subunits occurs very early postinjury and probably reflects a disruption of the juxtaparanodal axonal region due to physical trauma, as shown by altered localization of Caspr.

Human Kv1.6 current displays a C-type-like inactivation when re-expressed in cos-7 cells

The human Kv1.6K(+) channel was functionally re-expressed in COS-7 cells at different levels. Voltage-activated K(+) currents are recorded upon cell membrane depolarization independently of the level of Kv1.6 expression. The current acquires a fast inactivation when Kv1.6 expression is increased. Inactivation was not affected by divalent cations or by extracellular tetraethylammonium. We have characterized the inactivation properties in biophysical terms. The fraction of inactivated current and the kinetics of inactivation are increased as the cell becomes more depolarized. Inactivated current can be reactivated according to a bi-exponential function of time. Additional experiments indicate that Kv1.6 inactivation properties are close to those of a conventional C-type inactivation. This work suggests that the concentration of Kv1.6 channel in the cell membrane strongly modulates the kinetic properties of Kv1.6-induced K(+) current. The physiological implications of these modifications are discussed.

4-aminopyridine decreases progesterone production by porcine granulosa cells

BACKGROUND

Ion channels occur as large families of related genes with cell-specific expression patterns. Granulosa cells have been shown to express voltage-gated potassium channels from more than one family. The purpose of this study was to determine the effects of 4-aminopyridine (4-AP), an antagonist of KCNA but not KCNQ channels.

METHODS

Granulosa cells were isolated from pig follicles and cultured with 4-AP, alone or in combination with FSH, 8-CPT-cAMP, estradiol 17beta, and DIDS. Complimentary experiments determined the effects of 4-AP on the spontaneously established pig granulosa cell line PGC-2. Granulosa cell or PGC-2 function was assessed by radio-immunoassay of media progesterone accumulation. Cell viability was assessed by trypan blue exclusion. Drug-induced changes in cell membrane potential and intracellular potassium concentration were documented by spectrophotometric determination of DiBAC4(3) and PBFI fluorescence, respectively. Expression of proliferating cell nuclear antigen (PCNA) and steroidogenic acute regulatory protein (StAR) was assessed by immunoblotting. Flow cytometry was also used to examine granulosa cell viability and size.

RESULTS

4-AP (2 mM) decreased progesterone accumulation in the media of serum-supplemented and serum-free granulosa cultures, but inhibited cell proliferation only under serum-free conditions. 4-AP decreased the expression of StAR, the production of cAMP and the synthesis of estradiol by PGC-2. Addition of either 8-CPT-cAMP or estradiol 17beta to serum-supplemented primary cultures reduced the inhibitory effects of 4-AP. 4-AP treatment was also associated with increased cell size, increased intracellular potassium concentration, and hyperpolarization of resting membrane potential. The drug-induced hyperpolarization of resting membrane potential was prevented either by decreasing extracellular chloride or by adding DIDS to the media. DIDS also prevented 4-AP inhibition of progesterone production.

CONCLUSION

4-AP inhibits basal and FSH-stimulated progesterone production by pig granulosa cells via drug action at multiple interacting steps in the steroidogenic pathway. These inhibitory effects of 4-AP on steroidogenesis may reflect drug-induced changes in intracellular concentrations of K+and Cl- as well as granulosa cell resting membrane potential.

Regulation of Kv1 subunit expression in oligodendrocyte progenitor cells and their role in G1/S phase progression of the cell cycle

Proliferative oligodendrocyte progenitor cells (OPs) express large, delayed outward-rectifying K(+) currents (I(K)), whereas nondividing immature and mature oligodendrocytes display much smaller I(K). Here, we show that up-regulation of I(K) occurs in G(1) phase of the cell cycle in purified cultured OPs and is the result of an RNA synthesis-dependent, selective increase of the K(+) channel subunit proteins Kv1.3 and Kv1.5. In oligodendrocyte cells acutely isolated from developing rat brain, a decrease of cyclin D expression is observed as these cells mature along their lineage. This is accompanied by a decrease in Kv1.3 and Kv1.5 subunit expression, suggesting a role for these subunits in the proliferative potential of OPs in situ. I(K) expressed in OPs in subventricular zone and developing white matter in acutely isolated slice preparations were selectively blocked by antagonists of Kv1.3, illustrating the functional presence of this subunit in situ. Interestingly, Kv1.3 block inhibited S-phase entry of both purified OPs in culture and in tissue slice cultures. Thus, we employ both in vitro and in situ experimental approaches to show that (i) RNA-dependent synthesis of Kv1.3 and Kv1.5 subunit proteins occurs in G(1) phase of the OP cell cycle and is responsible for the observed increase in I(K), and (ii) currents through Kv1.3-containing channels play a crucial role in G(1)/S transition of proliferating OPs.

Upregulation of Kv 1.4 protein and gene expression after chronic spinal cord injury

After spinal cord injury (SCI), white matter tracts are characterized by demyelination and increased sensitivity to the K(+) channel blocker 4-aminopyridine (4-AP). These effects appear to contribute to neurological impairment after SCI, although the molecular changes in K(+) channel subunit expression remain poorly understood. We examined changes in gene expression of the 4-AP-sensitive voltage-gated K(+) channel Kv 1.4 after chronic SCI in the rat. Quantitative immunoblotting showed that Kv 1.4 protein was significantly increased at 6 weeks, but not at 1 week, after SCI in spinal cord white matter. Kv 1.4 was localized to astrocytes, oligodendrocytes, and oligodendrocyte progenitor cells but not to axons in both the normal and the injured spinal cord white matter. Because glial cells proliferate after SCI, we used immunogold electron microscopy to quantify Kv 1.4 protein in individual glial cells and found a sixfold increase of Kv 1.4 in cells of the oligodendrocyte lineage after chronic injury. Finally, quantitative in situ hybridization showed that Kv 1.4 mRNA was significantly upregulated in spinal cord white matter, but not gray matter, after SCI. In summary, we show that Kv 1.4 is expressed in glial cells and not in axons in the rat spinal cord white matter and that its expression is markedly increased in cells of the oligodendrocyte lineage after chronic SCI. Given that K(+) channels play a role in glial cell proliferation, cells exhibiting changes in Kv 1.4 expression may represent proliferating oligodendroglia in the chronically injured spinal cord.

Molecular and functional characterization of ERG, KCNQ, and KCNE subtypes in rat stomach smooth muscle

Contribution of K(+) channels derived from the expression of ERG, KCNQ, and KCNE subtypes, which are responsible for rapidly and slowly activating delayed rectifier K(+) currents (I(Kr) and I(Ks), respectively) in cardiac myocytes, to membrane currents was examined in stomach circular smooth muscle cells (SMCs). The region-qualified multicell RT-PCR showed that ERG1/KCNE2 transcripts were expressed in rat stomach fundus and antrum SMCs and that KCNQ1/KCNE1 transcripts were expressed in antrum but not fundus. Western blotting and immunocytochemical analyses indicate that ERG1 proteins were substantially expressed in both regions, whereas KCNE1 proteins were faintly expressed in antrum and not in fundus. Both I(Kr)- and I(Ks)-like currents susceptible to E-4031 and indapamide, respectively, were identified in circular SMCs of antrum but only I(Kr)-like current was identified in fundus. It is strongly suggested that I(Kr)- and I(Ks)-like currents functionally identified in rat stomach SMCs are attributable to the expression of ERG1/KCNE2 and KCNQ1/KCNE1, respectively. The membrane depolarization by 1 microM E-4031 indicates the contribution of K(+) channels encoded by ERG1/KCNE2 to the resting membrane potential in stomach SMCs.

Molecular basis of voltage-dependent potassium currents in porcine granulosa cells

The major objective of this study was to elucidate the molecular bases for K(+) current diversity in porcine granulosa cells (GC). Two delayed rectifier K(+) currents with distinct electrophysiological and pharmacological properties were recorded from porcine GC by using whole-cell patch clamp: 1) a slowly activating, noninactivating current (I(Ks)) antagonized by clofilium, 293B, L-735,821, and L-768,673; and 2) an ultrarapidly activating, slowly inactivating current (I(Kur)) antagonized completely by clofilium and 4-aminopyridine and partially by tetraethylammonium, charybdotoxin, dendrotoxin, and kaliotoxin. The molecular identity of the K(+) channel genes underlying I(Ks) and I(Kur) was examined using reverse transcription-polymerase chain reaction and immunoblotting to detect K(+) channel transcripts and proteins. We found that GC could express multiple voltage-dependent K(+) (Kv) channel subunits, including KCNQ1, KCNE1, Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, Kvbeta1.3, and Kvbeta2. Coimmunoprecipitation was used to establish the hetero-oligomeric nature of granulosa cell Kv channels. KCNE1 and KCNQ1 were coassociated in GC, and their expression coincided with the expression of I(Ks). Extensive coassociation of the various Kv alpha- and beta-subunits was also documented, suggesting that the diverse electrophysiological and pharmacological properties of I(Kur) currents may reflect variation in the composition and stoichiometry of the channel assemblies, as well as differences in post-translational modification of contributing Kv channel subunits. Our findings provide an essential background for experimental definition of granulosa K(+) channel function(s). It will be critical to define the functional roles of specific GC K(+) channels, because these proteins may represent either novel targets for assisted reproduction or potential sites of drug toxicity.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.