Involvement of Kv1.1 and Nav1.5 in proliferation of gastric epithelial cells

Roles of Microglial Ion Channel in Neurodegenerative Diseases

As the average age and life expectancy increases, the incidence of both acute and chronic central nervous system (CNS) pathologies will increase. Understanding mechanisms underlying neuroinflammation as the common feature of any neurodegenerative pathology, we can exploit the pharmacology of cell specific ion channels to improve the outcome of many CNS diseases. As the main cellular player of neuroinflammation, microglia play a central role in this process. Although microglia are considered non-excitable cells, they express a variety of ion channels under both physiological and pathological conditions that seem to be involved in a plethora of cellular processes. Here, we discuss the impact of modulating microglia voltage-gated, potential transient receptor, chloride and proton channels on microglial proliferation, migration, and phagocytosis in neurodegenerative diseases.

In Vivo Evidence for Voltage-Gated Sodium Channel Expression in Carcinomas and Potentiation of Metastasis

A wide body of evidence suggests that voltage-gated sodium channels (VGSCs) are expressed de novo in several human carcinomas where channel activity promotes a variety of cellular behaviours integral to the metastatic cascade. These include directional motility (including galvanotaxis), pH balance, extracellular proteolysis, and invasion. Contrary to the substantial in vitro data, however, evidence for VGSC involvement in the cancer process in vivo is limited. Here, we critically assess, for the first time, the available in vivo evidence, hierarchically from mRNA level to emerging clinical aspects, including protein-level studies, electrolyte content, animal tests, and clinical imaging. The evidence strongly suggests that different VGSC subtypes (mainly Nav1.5 and Nav1.7) are expressed de novo in human carcinoma tissues and generally parallel the situation in vitro. Consistent with this, tissue electrolyte (sodium) levels, quantified by clinical imaging, are significantly higher in cancer vs. matched non-cancer tissues. These are early events in the acquisition of metastatic potential by the cancer cells. Taken together, the multi-faceted evidence suggests that the VGSC expression has clinical (diagnostic and therapeutic) potential as a prognostic marker, as well as an anti-metastatic target. The distinct advantages offered by the VGSC include especially (1) its embryonic nature, demonstrated most clearly for the predominant neonatal Nav1.5 expression in breast and colon cancer, and (2) the specifically druggable persistent current that VGSCs develop under hypoxic conditions, as in growing tumours, which promotes invasiveness and metastasis.

Microglia in Alzheimer's Disease: A Role for Ion Channels

Alzheimer's disease is the most common form of dementia, it is estimated to affect over 40 million people worldwide. Classically, the disease has been characterized by the neuropathological hallmarks of aggregated extracellular amyloid-β and intracellular paired helical filaments of hyperphosphorylated tau. A wealth of evidence indicates a pivotal role for the innate immune system, such as microglia, and inflammation in the pathology of Alzheimer's disease. The over production and aggregation of Alzheimer's associated proteins results in chronic inflammation and disrupts microglial clearance of these depositions. Despite being non-excitable, microglia express a diverse array of ion channels which shape their physiological functions. In support of this, there is a growing body of evidence pointing to the involvement of microglial ion channels contributing to neurodegenerative diseases such as Alzheimer's disease. In this review, we discuss the evidence for an array of microglia ion channels and their importance in modulating microglial homeostasis and how this process could be disrupted in Alzheimer's disease. One promising avenue for assessing the role that microglia play in the initiation and progression of Alzheimer's disease is through using induced pluripotent stem cell derived microglia. Here, we examine what is already understood in terms of the molecular underpinnings of inflammation in Alzheimer's disease, and the utility that inducible pluripotent stem cell derived microglia may have to advance this knowledge. We outline the variability that occurs between the use of animal and human models with regards to the importance of microglial ion channels in generating a relevant functional model of brain inflammation. Overcoming these hurdles will be pivotal in order to develop new drug targets and progress our understanding of the pathological mechanisms involved in Alzheimer's disease.

Potassium Channelopathies and Gastrointestinal Ulceration

Potassium channels and transporters maintain potassium homeostasis and play significant roles in several different biological actions via potassium ion regulation. In previous decades, the key revelations that potassium channels and transporters are involved in the production of gastric acid and the regulation of secretion in the stomach have been recognized. Drugs used to treat peptic ulceration are often potassium transporter inhibitors. It has also been reported that potassium channels are involved in ulcerative colitis. Direct toxicity to the intestines from nonsteroidal anti-inflammatory drugs has been associated with altered potassium channel activities. Several reports have indicated that the long-term use of the antianginal drug Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, increases the chances of ulceration and perforation from the oral to anal regions throughout the gastrointestinal (GI) tract. Several of these drug features provide further insights into the role of potassium channels in the occurrence of ulceration in the GI tract. The purpose of this review is to investigate whether potassium channelopathies are involved in the mechanisms responsible for ulceration that occurs throughout the GI tract.

Sodium channels in astroglia and microglia

Voltage-gated sodium channels are required for electrogenesis in excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons, cardiac, and skeletal muscle cells. Cells that have not traditionally been considered to be excitable (nonexcitable cells), including glial cells, also express sodium channels in physiological conditions as well as in pathological conditions. These channels contribute to multiple functional roles that are seemingly unrelated to the generation of action potentials. Here, we discuss the dynamics of sodium channel expression in astrocytes and microglia, and review evidence for noncanonical roles in effector functions of these cells including phagocytosis, migration, proliferation, ionic homeostasis, and secretion of chemokines/cytokines. We also examine possible mechanisms by which sodium channels contribute to the activity of glial cells, with an eye toward therapeutic implications for central nervous system disease. GLIA 2016;64:1628-1645.

The Tau-Induced Reduction of mRNA Levels of Kv Channels in Human Neuroblastoma SK-N-SH Cells

Previous findings indicated that microtubule-binding protein tau and voltage-gated K(+) (Kv) channels exhibit a regulatory role in cell proliferation. However, the possible interaction of tau with Kv channels remained obscure. In this report, transfection of tau plasmids into human neuroblastoma SK-N-SH cells caused a significant reduction in the messenger RNA (mRNA) levels of several Kv channels, including Kv2.1, Kv3.1, Kv5.1, Kv9.2, and KCNH4. Correspondingly, the Kv currents recorded using patch-clamp techniques were substantially declined in the tau-transfected SK-N-SH cells. Moreover, tau induction and treatment with the Kv channel blocker TEA (tetraethylammonium) were able to improve proliferation rates of SK-N-SH cells by 43.1 and 66.2%, respectively. These data suggested that the tau-mediated alteration of Kv channels could be involved in its action on neural proliferation.

Effects of BKCa and Kir2.1 Channels on Cell Cycling Progression and Migration in Human Cardiac c-kit+ Progenitor Cells

Our previous study demonstrated that a large-conductance Ca2+-activated K+ current (BKCa), a voltage-gated TTX-sensitive sodium current (INa.TTX), and an inward rectifier K+ current (IKir) were heterogeneously present in most of human cardiac c-kit+ progenitor cells. The present study was designed to investigate the effects of these ion channels on cell cycling progression and migration of human cardiac c-kit+ progenitor cells with approaches of cell proliferation and mobility assays, siRNA, RT-PCR, Western blots, flow cytometry analysis, etc. It was found that inhibition of BKCa with paxilline, but not INa.TTX with tetrodotoxin, decreased both cell proliferation and migration. Inhibition of IKir with Ba2+ had no effect on cell proliferation, while enhanced cell mobility. Silencing KCa.1.1 reduced cell proliferation by accumulating the cells at G0/G1 phase and decreased cell mobility. Interestingly, silencing Kir2.1 increased the cell migration without affecting cell cycling progression. These results demonstrate the novel information that blockade or silence of BKCa channels, but not INa.TTX channels, decreases cell cycling progression and mobility, whereas inhibition of Kir2.1 channels increases cell mobility without affecting cell cycling progression in human cardiac c-kit+ progenitor cells.

Voltage-gated sodium channels and cancer: is excitability their primary role?

Voltage-gated sodium channels (Na_V) are molecular characteristics of excitable cells. Their activation, triggered by membrane depolarization, generates transient sodium currents that initiate action potentials in neurons and muscle cells. Sodium currents were discovered by Hodgkin and Huxley using the voltage clamp technique and reported in their landmark series of papers in 1952. It was only in the 1980's that sodium channel proteins from excitable membranes were molecularly characterized by Catterall and his collaborators. Non-excitable cells can also express Na_V channels in physiological conditions as well as in pathological conditions. These Na_V channels can sustain biological roles that are not related to the generation of action potentials. Interestingly, it is likely that the abnormal expression of Na_V in pathological tissues can reflect the re-expression of a fetal phenotype. This is especially true in epithelial cancer cells for which these channels have been identified and sodium currents recorded, while it was not the case for cells from the cognate normal tissues. In cancers, the functional activity of Na_V appeared to be involved in regulating the proliferative, migrative, and invasive properties of cells. This review is aimed at addressing the non-excitable roles of Na_V channels with a specific emphasis in the regulation of cancer cell biology.

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 .

The life and death of breast cancer cells: proposing a role for the effects of phytoestrogens on potassium channels

Changes in the regulation of potassium channels are increasingly implicated in the altered activity of breast cancer cells. Increased or reduced expression of a number of K(+) channels have been identified in numerous breast cancer cell lines and cancerous tissue biopsy samples, compared to normal tissue, and are associated with tumor formation and spread, enhanced levels of proliferation, and resistance to apoptotic stimuli. Through knockout or silencing of K(+) channel genes, and use of specific or more broad pharmacologic K(+) channel blockers, the growth of numerous cell lines, including breast cancer cells, has been modified. In this manner it has been proposed that in MCF7 breast cancer cells proliferation appears to be regulated by the activity of a number of K(+) channels, including the Ca(2+) activated K(+) channels, and the voltage-gated K(+) channels hEAG and K(v)1.1. The effect of phytoestrogens on K(+) channels has not been extensively studied but yields some interesting results. In a number of cell lines the phytoestrogen genistein inhibits K(+) current through several channels including K(v)1.3 and hERG. Where it has been used, structurally similar daidzein has little or no effect on K(+) channel activity. Since many K(+) channels have roles in proliferation and apoptosis in breast cancer cells, the impact of K(+) channel regulation by phytoestrogens is of potentially great relevance.

Dendrotoxin-κ suppresses tumor growth induced by human lung adenocarcinoma A549 cells in nude mice

Voltage-gated K(+) (Kv) channels have been considered to be a regulator of membrane potential and neuronal excitability. Recently, accumulated evidence has indicated that several Kv channel subtypes contribute to the control of cell proliferation in various types of cells and are worth noting as potential emerging molecular targets of cancer therapy. In the present study, we investigated the effects of the Kv1.1-specific blocker, dendrotoxin-κ (DTX-κ, on tumor formation induced by the human lung adenocarcinoma cell line A549 in a xenograft model. Kv1.1 mRNA and protein was expressed in A549 cells and the blockade of Kv1.1 by DTX-κ, reduced tumor formation in nude mice. Furthermore, treatment with DTX-κ significantly increased protein expression of p21(Waf1/Cip1), p27(Kip1), and p15(INK4B) and significantly decreased protein expression of cyclin D3 in tumor tissues compared to the control. These results suggest that DTX-κ has anti-tumor effects in A549 cells through the pathway governing G1-S transition.

Functional ion channels in stem cells

Bioelectrical signals generated by ion channels play crucial roles in excitation genesis and impulse conduction in excitable cells as well as in cell proliferation, migration and apoptosis in proliferative cells. Recent studies have demonstrated that multiple ion channels are heterogeneously present in different stem cells; however, patterns and phenotypes of ion channels are species- and/or origin-dependent. This editorial review focuses on the recent findings related to the expression of functional ion channels and the roles of these channels in regulation of cell proliferation in stem cells. Additional effort is required in the future to clarify the ion channel expression in different types of stem cells; special attention should be paid to the relationship between ion channels and stem cell proliferation, migration and differentiation.

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.

Characterization of multiple ion channels in cultured human cardiac fibroblasts

Background

Although fibroblast-to-myocyte electrical coupling is experimentally suggested, electrophysiology of cardiac fibroblasts is not as well established as contractile cardiac myocytes. The present study was therefore designed to characterize ion channels in cultured human cardiac fibroblasts.

Methods and Findings

A whole-cell patch voltage clamp technique and RT-PCR were employed to determine ion channels expression and their molecular identities. We found that multiple ion channels were heterogeneously expressed in human cardiac fibroblasts. These include a big conductance Ca²⁺-activated K⁺ current (BK_Ca) in most (88%) human cardiac fibroblasts, a delayed rectifier K⁺ current (IK_DR) and a transient outward K⁺ current (I_to) in a small population (15 and 14%, respectively) of cells, an inwardly-rectifying K⁺ current (I_Kir) in 24% of cells, and a chloride current (I_Cl) in 7% of cells under isotonic conditions. In addition, two types of voltage-gated Na⁺ currents (I_Na) with distinct properties were present in most (61%) human cardiac fibroblasts. One was a slowly inactivated current with a persistent component, sensitive to tetrodotoxin (TTX) inhibition (I_Na.TTX, IC₅₀ = 7.8 nM), the other was a rapidly inactivated current, relatively resistant to TTX (I_Na.TTXR, IC₅₀ = 1.8 µM). RT-PCR revealed the molecular identities (mRNAs) of these ion channels in human cardiac fibroblasts, including KCa.1.1 (responsible for BK_Ca), Kv1.5, Kv1.6 (responsible for IK_DR), Kv4.2, Kv4.3 (responsible for I_to), Kir2.1, Kir2.3 (for I_Kir), Clnc3 (for I_Cl), Na_V1.2, Na_V1.3, Na_V1.6, Na_V1.7 (for I_Na.TTX), and Na_V1.5 (for I_Na.TTXR).

Conclusions

These results provide the first information that multiple ion channels are present in cultured human cardiac fibroblasts, and suggest the potential contribution of these ion channels to fibroblast-myocytes electrical coupling.

Characterization of ion channels in human preadipocytes

Ion channels participate in regulation of cell proliferation. However, though preadipocyte (the progenitor of fat cell) is a type of highly proliferating cells, ion channel expression and their role in proliferation is not understood in human preadipocytes. The present study was designed to characterize ion channels using whole-cell patch clamp technique, RT-PCR, and Western blotting. It was found that a 4-aminopyridine- (4-AP) sensitive transient outward K(+) current (I(to)) was present in a small population of (32.0%) cells, and an outward "noisy" big conductance Ca(2+)-activated K(+) current (I(KCa)) was present in most (92.7%) preadipocytes. The noisy current was inhibited by the big conductance I(KCa) channel blocker paxilline (1 microM), and enhanced by the Ca(2+) ionophore A23187 (5 microM) and the big conductance I(KCa) channel activator NS1619 (10 microM). RT-PCR and Western blot revealed the molecular identities (i.e., KCa1.1 and Kv4.2) of the functional ionic currents I(KCa) and I(to). Blockade of I(KCa) or I(to) with paxilline or 4-AP reduced preadipocyte proliferation, and similar results were obtained with specific siRNAs targeting to KCa1.1 and Kv4.2. Flow cytometric analysis showed ion channel blockade or knockdown of KCa1.1 or Kv4.2 with specific siRNA increased the cell number of G0/G1 phase. The present study demonstrates for the first time that two types of functional ion channel currents, I(to) and big conductance I(KCa), are present in human preadipocytes and that these two types of ion channels participate in regulating proliferation of human preadipocytes.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Involvement of voltage-gated K+ and Na+ channels in gastric epithelial cell migration

Epithelial cell migration plays an important role in gastrointestinal mucosal repair. We previously reported that multiple functional ion channels, including a Ba(2+)-sensitive K(+) inward rectifier K(ir)1.2, 4-aminopyridine (4-AP)-sensitive voltage-gated K(+) channels K(v)1.1, K(v)1.6 and K(v)2.1, and a nifedipine-sensitive, tetrodotoxin (TTX)-insensitive voltage-gated Na(+) channel Na(v)1.5 were expressed in a non-transformed rat gastric epithelial cell line (RGM-1). In the present study, we further investigated whether these ion channels are involved in the modulation of gastric epithelial cell migration. Cell migration was determined by monolayer wound healing assay. Results showed that blockade of K(v) with 4-AP or Na(v)1.5 with nifedipine inhibited RGM-1 cell migration in the absence or presence of epidermal growth factor (EGF), which effectively stimulated RGM-1 cell migration. Moreover, high concentration of TTX mimicked the action of nifedipine, suggesting that the action of nifedipine was mediated through specific blockade of Na(v)1.5. In contrast, inhibition of K(ir)1.2 with Ba(2+), either in basal or EGF-stimulated condition, had no effect on RGM-1 cell migration. In conclusion, the present study demonstrates for the first time that voltage-gated K(+) and Na(+) channels are involved in the modulation of gastric epithelial cell migration.

Cell cycle-dependent expression of potassium channels and cell proliferation in rat mesenchymal stem cells from bone marrow

OBJECTIVE

Recently, our team has demonstrated that voltage-gated delayed rectifier K(+) current (IK(DR)) and Ca(2+)-activated K(+) current (I(KCa)) are present in rat bone marrow-derived mesenchymal stem cells; however, little is known of their physiological roles. The present study was designed to investigate whether functional expression of IK(DR) and I(KCa) would change with cell cycle progression, and whether they could regulate proliferation in undifferentiated rat mesenchymal stem cells (MSCs).

MATERIALS AND METHODS

Membrane potentials and ionic currents were recorded using whole-cell patch clamp technique, cell cycling was analysed by flow cytometry, cell proliferation was assayed with DNA incorporation method and the related genes were down-regulated by RNA interference (RNAi) and examined using RT-PCR.

RESULTS

It was found that membrane potential hyperpolarized, and cell size increased during the cell cycle. In addition, IK(DR) decreased, while I(KCa) increased during progress from G(1) to S phase. RT-PCR revealed that the mRNA levels of Kv1.2 and Kv2.1 (likely responsible for IK(DR)) reduced, whereas the mRNA level of KCa3.1 (responsible for intermediate-conductance I(KCa)) increased with the cell cycle progression. Down-regulation of Kv1.2, Kv2.1 or KCa3.1 with the specific RNAi, targeted to corresponding gene inhibited proliferation of rat MSCs.

CONCLUSION

These results demonstrate that membrane potential, IK(DR) and I(KCa) channels change with cell cycle progression and corresponding alteration of gene expression. IK(DR) and intermediate-conductance I(KCa) play an important role in maintaining membrane potential and they participate in modulation of proliferation in rat MSCs.

Sodium channel currents in rat hippocampal NG2 glia: characterization and contribution to resting membrane potential

We have recently reported that most of NG2 glycoprotein expressing glial cells, or NG2 glia, in rat hippocampus persistently express sodium channel currents (I(Na)) during development, but little is known about its function. We report here that hippocampal NG2 glia recorded in either acute slices or freshly isolated preparations from postnatal days (P) 7-21 rats express low density I(Na) (9.5-15.7 pA/pF) that is characterized by a fast activation and rapid inactivation kinetics with a tetrodotoxin (TTX) IC(50) value of 39.3 nM. The I(Na) expression correlated with a approximately 25 mV more depolarized resting membrane potential (RMP) as compared with non-I(Na)-expressing GLAST(+) astrocytes in situ at the same age. In the presence of the sodium channel blocker TTX (0.1 microM), these depolarized RMPs were negatively shifted by an average of 19 mV and 16 mV for I(Na)-expressing glia recordings from in situ and freshly isolated preparations, respectively. The I(Na) expressing glia actually showed a positive RMP (+12 mV) in the absence of potassium conductance that was inhibited to 0 mV by 0.1 microM TTX. Analysis of the I(Na) activation/inactivation curves yields an I(Na) "window current" at -40+/-20 mV, implying a persistent I(Na) component being active around the NG2 glia RMP of approximately -45 mV. According to the constant-field equation analysis, this active I(Na) component leads to a pNa/pK ratio of 0.14 at RMP which is approximately threefold higher than astrocytes (0.05). These results indicate that a TTX sensitive I(Na) component in NG2 glia contributes significantly to the depolarized NG2 glia RMP in the developing brain.