Voltage-gated K+ channels play a role in cAMP-stimulated neuritogenesis in mouse neuroblastoma N2A cells

M084 causes cell cycle arrest and inhibits voltage-gated Na+ and K+ channels in neuronal N2A cells

The effects of M084, an inhibitor of transient receptor potential (TRP) channels TRPC4 and TRPC5, on cell proliferation, voltage-gated K+ (Kv) channels and voltage-gated Na+ (Nav) channels were investigated in mouse neuronal N2A cells. Cell proliferation was measured by MTT assay and trypan blue exclusion test. Mitochondrial membrane potential was measured using JC-1 as a fluorescent probe. Cell cycle and ion channel activities were studied, respectively, using flow cytometry and voltage-clamp method. M084 (10-100 μM) concentration-dependently suppressed cell proliferation; M084 at 100 μM also arrested cell cycle at the G1 phase, and caused a decrease in mitochondrial membrane potential. The anti-proliferative effect of M084 was not mitigated by dorsomorphine (AMPK inhibitor), sodium salicylate (NF-κB inhibitor) and SP600125 (JNK inhibitor), but was alleviated by SB203580 (p38 inhibitor). M084 (3-100 μM) suppressed Nav and Kv currents in a concentration-dependent fashion with IC50 values of 9.1 and 29.2 μM, respectively. M084 (30 μM) caused left-shifts in inactivation curves of both Nav and Kv currents, and diminished peak amplitude of current injection-triggered membrane potential overshoot. In conclusion, M084 suppressed neuronal cell growth and inhibited their Nav and Kv channels.

Tannic Acid as an Ion Channel Modulator: An Understanding of Its Pharmacological Spectrum

Tannic acid (TA) is a polyphenol present in many plant foods and beverages such as green tea and wines. As a food additive, it has been recognized by Food and Drug Administration as generally safe. As a candidate drug, its pharmacological effects cover a wide spectrum, ranging from antibacterial, anticancer, cardioprotection to neuroprotection. TA has been shown to modulate a number of ion channels such as Ca2+-activated Cl- channels (CaCC), voltage-gated K+ (Kv) channels and transient receptor potential (TRP) channels, producing effects such as analgesia, antihypertensive effects and reduction of airway hypersensitivity. In this review we focus on how ion channel modulation by TA may account for the pharmacological effects of TA in various cells and organ systems. Further emphasis should be paid to factors, such as dosage and routes of administration, before the pharmacological actions of TA could be translated into therapeutic applications.

Tannic Acid Modulates Voltage-gated K + Channels to Promote Neuritogenesis in Neuronal N2A Cells

ABSTRACT

In a previous report, we showed that voltage-gated K + (Kv) Kv1 and Kv2 channels are involved in cAMP-induced neuritogenesis of mouse neuronal N2A cells. In this report, we examined the effects of tannic acid (TA) on Kv channels and neuritogenesis in N2A cells. TA (15 μM) mildly enhanced Kv currents at -30 to -20 mV but strongly inhibited Kv currents at higher voltages, causing a preferential activation of currents at low voltages. When enhancement and suppression of Kv currents (at -20 and +70 mV, respectively) by different concentrations of TA were analyzed, TA at 4 μM produced strong enhancement at -20 mV with relatively mild suppression at + 70 mV. TA (4 μM) also promoted neuritogenesis; such promotion was suppressed by a Kv channel blocker tetraethylammonium ion, or a combination of hongotoxin-1 (blocker of Kv1.1), UK 78282 (blocker of Kv1.4) and guangxitoxin 1E (blocker of Kv2.1). Our results demonstrate, for the first time, TA at low concentrations could modulate Kv channels and thereby promote neuritogenesis.

Decoding TGR5: A comprehensive review of its impact on cerebral diseases

Currently, unraveling the enigmatic realm of drug targets for cerebral disorders poses a formidable challenge. Takeda G protein-coupled receptor 5 (TGR5), also known as G protein-coupled bile acid receptor 1, is a specific bile acid receptor. Widely distributed across various tissues, TGR5 orchestrates a myriad of biological functions encompassing inflammation, energy metabolism, fatty acid metabolism, immune responses, cellular proliferation, apoptosis, and beyond. Alongside its well-documented implications in liver diseases, obesity, type 2 diabetes, tumors, and cardiovascular diseases, a growing body of evidence accentuates the pivotal role of TGR5 in cerebral diseases. Thus, this comprehensive review aimed to scrutinize the current insights into the pathological mechanisms involving TGR5 in cerebral diseases, while contemplating its potential as a promising therapeutic target for cerebral diseases.

Inhibition of Kv2.1 potassium channels by the antidepressant drug sertraline

Sertraline is a commonly used antidepressant of the selective serotonin reuptake inhibitors (SSRIs) class. In this study, we have used the patch-clamp technique to assess the effects of sertraline on Kv2.1 channels heterologously expressed in HEK-293 cells and on the voltage-gated potassium currents (IKv) of Neuro 2a cells, which are predominantly mediated by Kv2.1 channels. Our results reveal that sertraline inhibits Kv2.1 channels in a concentration-dependent manner. The sertraline-induced inhibition was not voltage-dependent and did not require the channels to be open. The kinetics of activation and deactivation were accelerated and decelerated, respectively, by sertraline. Moreover, the inhibition by this drug was use-dependent. Notably, sertraline significantly modified the inactivation mechanism of Kv2.1 channels; the steady-state inactivation was shifted to hyperpolarized potentials, the closed-state inactivation was enhanced and accelerated, and the recovery from inactivation was slowed, suggesting that this is the main mechanism by which sertraline inhibits Kv2.1 channels. Overall, this study provides novel insights into the pharmacological actions of sertraline on Kv2.1 channels, shedding light on the intricate interaction between SSRIs and ion channel function.

Automated Patch Clamp for the Detection of Tetrodotoxin in Pufferfish Samples

Tetrodotoxin (TTX) is a marine toxin responsible for many intoxications around the world. Its presence in some pufferfish species and, as recently reported, in shellfish, poses a serious health concern. Although TTX is not routinely monitored, there is a need for fast, sensitive, reliable, and simple methods for its detection and quantification. In this work, we describe the use of an automated patch clamp (APC) system with Neuro-2a cells for the determination of TTX contents in pufferfish samples. The cells showed an IC50 of 6.4 nM for TTX and were not affected by the presence of muscle, skin, liver, and gonad tissues of a Sphoeroides pachygaster specimen (TTX-free) when analysed at 10 mg/mL. The LOD achieved with this technique was 0.05 mg TTX equiv./kg, which is far below the Japanese regulatory limit of 2 mg TTX equiv./kg. The APC system was applied to the analysis of extracts of a Lagocephalus sceleratus specimen, showing TTX contents that followed the trend of gonads > liver > skin > muscle. The APC system, providing an in vitro toxicological approach, offers the advantages of being sensitive, rapid, and reliable for the detection of TTX-like compounds in seafood.

Methods used to achieve different levels of the neuronal differentiation process in SH-SY5Y and Neuro2a cell lines: An integrative review

To study the process of neuronal differentiation, the human neuroblastoma (SH-SY5Y) and the murine neuroblastoma (Neuro2a) cell lines have proven to be effective models. For this approach, different protocols involving known neurotrophic factors and other molecules, such as retinoic acid (RA), have been assessed to better understand the neuronal differentiation process. Thus, the goal of this manuscript was to provide a brief overview of recent studies that have used protocols to promote neurodifferentiation in SH-SY5Y and Neuro2a cell lines and used acquired morphology and neuronal markers to validate whether differentiation was effective. The published results supply some guidance regarding the relationship between RA and neurotrophins for SH-SY5Y, as well a serum concentrations for both cell lines. Furthermore, they demonstrate the potential application of Neuro2a, which is critical for future research on neuronal differentiation.

Analysis of High-Risk Neuroblastoma Transcriptome Reveals Gene Co-Expression Signatures and Functional Features

Neuroblastoma represents a neoplastic expansion of neural crest cells in the developing sympathetic nervous system and is childhood's most common extracranial solid tumor. The heterogeneity of gene expression in different types of cancer is well-documented, and genetic features of neuroblastoma have been described by classification, development stage, malignancy, and progression of tumors. Here, we aim to analyze RNA sequencing datasets, publicly available in the GDC data portal, of neuroblastoma tumor samples from various patients and compare them with normal adrenal gland tissue from the GTEx data portal to elucidate the gene expression profile and regulation networks they share. Our results from the differential expression, weighted correlation network, and functional enrichment analyses that we performed with the count data from neuroblastoma and standard normal gland samples indicate that the analysis of transcriptome data from 58 patients diagnosed with high-risk neuroblastoma shares the expression pattern of 104 genes. More importantly, our analyses identify the co-expression relationship and the role of these genes in multiple biological processes and signaling pathways strongly associated with this disease phenotype. Our approach proposes a group of genes and their biological functions to be further investigated as essential molecules and possible therapeutic targets of neuroblastoma regardless of the etiology of individual tumors.

Afatinib triggers a Ni2+ -resistant Ca2+ influx pathway in A549 non-small cell lung cancer cells

Afatinib is used to treat non-small cell lung cancer cells (NSCLC), and its mechanism involves irreversible inhibition of epidermal growth factor receptor (EGFR) tyrosine kinase. In this study, we examined if afatinib had cytotoxic action against NSCLC other than inhibition of tyrosine kinase. Afatinib (1-30 μM) caused apoptotic death in A549 NSCLC in a concentration-dependent manner. Afatinib triggered Ca²⁺ influx without causing Ca²⁺ release, and the Ca²⁺ influx was unaffected by sodium orthovanadate (SOV, an inhibitor of tyrosine phosphatase), suggesting that afatinib-triggered Ca²⁺ response was unrelated to its inhibition of tyrosine kinase. Addition of afatinib also promoted Mn²⁺ influx. Ca²⁺ influx triggered by afatinib was resistant to SKF96365 and ruthenium red (two general blockers of TRP channels) and, unexpectedly, Ni²⁺ (a non-specific Ca²⁺ channel blocker). Afatinib caused an increase in mitochondrial Ca²⁺ level, an initial mitochondrial hyperpolarization (4 h) and followed by mitochondrial potential collapse (24-48 h). Afatinib-induced cell death was slightly but significantly alleviated in low extracellular Ca²⁺ condition or under pharmacological block of mitochondrial permeability transition pore (MPTP) opening by cyclosporin A. Therefore, in addition to tyrosine kinase inhibition as a major anti-cancer mechanism of afatinib, stimulation of an atypical Ca²⁺ influx pathway, mitochondrial Ca²⁺ overload, and potential collapse in part contribute to afatinib-induced cell death.

Integrin-KCNB1 potassium channel complexes regulate neocortical neuronal development and are implicated in epilepsy

Potassium (K⁺) channels are robustly expressed during prenatal brain development, including in progenitor cells and migrating neurons, but their function is poorly understood. Here, we investigate the role of voltage-gated K⁺ channel KCNB1 (Kv2.1) in neocortical development. Neuronal migration of glutamatergic neurons was impaired in the neocortices of KCNB1 null mice. Migratory defects persisted into the adult brains, along with disrupted morphology and synaptic connectivity. Mice developed seizure phenotype, anxiety, and compulsive behavior. To determine whether defective KCNB1 can give rise to developmental channelopathy, we constructed Knock In (KI) mice, harboring the gene variant Kcnb1^R312H (R312H mice) found in children with developmental and epileptic encephalopathies (DEEs). The R312H mice exhibited a similar phenotype to the null mice. Wild type (WT) and R312H KCNB1 channels made complexes with integrins α5β5 (Integrin_K⁺ channel_Complexes, IKCs), whose biochemical signaling was impaired in R312H brains. Treatment with Angiotensin II in vitro, an agonist of Focal Adhesion kinase, a key component of IKC signaling machinery, corrected the neuronal abnormalities. Thus, a genetic mutation in a K⁺ channel induces severe neuromorphological abnormalities through non-conducting mechanisms, that can be rescued by pharmacological intervention. This underscores a previously unknown role of IKCs as key players in neuronal development, and implicate developmental channelopathies in the etiology of DEEs.

Combining mKate2-Kv1.3 Channel and Atto488-Hongotoxin for the Studies of Peptide Pore Blockers on Living Eukaryotic Cells

The voltage-gated potassium Kv1.3 channel is an essential component of vital cellular processes which is also involved in the pathogenesis of some autoimmune, neuroinflammatory and oncological diseases. Pore blockers of the Kv1.3 channel are considered as potential drugs and are used to study Kv1 channels' structure and functions. Screening and study of the blockers require the assessment of their ability to bind the channel. Expanding the variety of methods used for this, we report on the development of the fluorescent competitive binding assay for measuring affinities of pore blockers to Kv1.3 at the membrane of mammalian cells. The assay constituents are hongotoxin 1 conjugated with Atto488, fluorescent mKate2-tagged Kv1.3 channel, which was designed to improve membrane expression of the channel in mammalian cells, confocal microscopy, and a special protocol of image processing. The assay is implemented in the "mix and measure", format and allows the screening of Kv1.3 blockers, such as peptide toxins, that bind to the extracellular vestibule of the K⁺-conducting pore, and analyzing their affinity.

Sensitivity of Ca2+-sensing receptor-transient receptor potential-mediated Ca2+ influx to extracellular acidity in bEND.3 endothelial cells

Ca²⁺-sensing receptors (CaSRs) are G protein-coupled receptors activated by elevated concentrations of extracellular Ca²⁺. In our previous works, we showed protein and functional expression of CaSR in mouse cerebral endothelial cell (EC) (bEND.3); the CaSR response (high Ca²⁺-elicited cytosolic [Ca²⁺] elevation) was unaffected by suppression of phospholipase C but in part involved Ca²⁺ influx through transient receptor potential V1 (TRPV1) channels. In this work, we investigated if extracellular acidity affected CaSR-mediated Ca²⁺ influx triggered by high (3 mM) Ca²⁺ (CaSR agonist), 3 mM spermine (CaSR agonist), and 10 mM cinacalcet (positive allosteric modulator of CaSR). Extracellular acidosis (pH 6.8 and pH 6.0) strongly suppressed cytosolic [Ca²⁺] elevation triggered by high Ca²⁺, spermine, and cinacalcet; acidosis also inhibited Mn²⁺ influx stimulated by high Ca²⁺ and cinacalcet. Purinoceptor-triggered Ca²⁺ response, however, was not suppressed by acidosis. Extracellular acidity also did not affect membrane potential, suggesting suppressed CaSR-mediated Ca²⁺ influx in acidity did not result from the reduced electrical driving force for Ca²⁺. Our results suggest Ca²⁺ influx through a putative CaSR-TRP complex in bEND.3 EC was sensitive to extracellular pH.

Genetic Knockout of TRPM2 Increases Neuronal Excitability of Hippocampal Neurons by Inhibiting Kv7 Channel in Epilepsy

Epilepsy is a chronic brain disease that makes serious cognitive and motor retardation. Ion channels affect the occurrence of epilepsy in various ways, but the mechanisms have not yet been fully elucidated. Transient receptor potential melastain2 (TRPM2) ion channel is a non-selective cationic channel that can permeate Ca²⁺ and critical for epilepsy. Here, TRPM2 gene knockout mice were used to generate a chronic kindling epilepsy model by PTZ administration in mice. We found that TRPM2 knockout mice were more susceptible to epilepsy than WT mice. Furthermore, the neuronal excitability in the hippocampal CA1 region of TRPM2 knockout mice was significantly increased. Compared with WT group, there were no significant differences in the input resistance and after hyperpolarization of CA1 neurons in TRPM2 knockout mice. Firing adaptation rate of hippocampal CA1 pyramidal neurons of TRPM2 knockout mice was lower than that of WT mice. We also found that activation of Kv7 channel by retigabine reduced the firing frequency of action potential in the hippocampal pyramidal neurons of TRPM2 knockout mice. However, inhibiting Kv7 channel increased the firing frequency of action potential in hippocampal pyramidal neurons of WT mice. The data suggest that activation of Kv7 channel can effectively reduce epileptic seizures in TRPM2 knockout mice. We conclude that genetic knockout of TRPM2 in hippocampal CA1 pyramidal neurons may increase neuronal excitability by inhibiting Kv7 channel, affecting the susceptibility to epilepsy. These findings may provide a potential therapeutic target for epilepsy.

Suppression of Ca2+ oscillations by SERCA inhibition in human alveolar type 2 A549 cells: rescue by ochratoxin A but not CDN1163

AIMS

Lung type 2 alveolar cells, by secreting surfactant to lower surface tension, contribute to enhance lung compliance. Stretching, as a result of lung expansion, triggers type 1 alveolar cell to release ATP, which in turn stimulates Ca²⁺-dependent surfactant secretion by neighboring type 2 cells. In this report, we studied ATP-triggered Ca²⁺ signaling in human alveolar type 2 A549 cells.

MAIN METHODS

Ca²⁺ signaling was examined using microfluorimetric measurement with fura-2 as fluorescent dye.

KEY FINDINGS

Ca²⁺ oscillations triggered by ATP relied on inositol 1,4,5-trisphosphate-induced Ca²⁺ release and store-operated Ca²⁺ entry. Pathological conditions such as influenza virus infection and diabetes reportedly inhibit sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA). We found that a very mild inhibition of SERCA by cyclopiazonic acid (CPA) sufficed to decrease Ca²⁺ oscillation frequency and the percentage of cells exhibiting Ca²⁺ oscillations. Ochratoxin A (OTA), an activator of SERCA, could prevent the suppressive effects by CPA. Inhibition of SERCA by hydrogen peroxide also suppressed Ca²⁺ oscillations. Interestingly, hydrogen peroxide-induced inhibition was prevented by OTA but aggravated by CDN1163, an allosteric activator of SERCA. CDN1163 also had an untoward effect of releasing intracellular Ca²⁺.

SIGNIFICANCE

Different modes of activation of SERCA may determine the outcome of rescue of Ca²⁺ oscillations in case of SERCA inhibition in alveolar type 2 cells.

Propofol Causes Sustained Ca2+ Elevation in Endothelial Cells by Stimulating Ryanodine Receptor and Suppressing Plasmalemmal Ca2+ Pump

ABSTRACT

Propofol, a general anesthetic administered intravenously, may cause pain at the injection site. The pain is in part due to irritation of vascular endothelial cells. We here investigated the effects of propofol on Ca2+ transport and pain mediator release in human umbilical vein endothelial cells (EA.hy926). Propofol mobilized Ca2+ from cyclopiazonic acid (CPA)-dischargeable pool but did not cause Ca2+ release from the lysosomal Ca2+ stores. Propofol-elicited Ca2+ release was suppressed by 100 μM ryanodine, suggesting the participation of ryanodine receptor channels. Propofol did not affect ATP-triggered Ca2+ release but abolished the Ca2+ influx triggered by ATP; in addition, propofol also suppressed store-operated Ca2+ entry elicited by CPA. Ca2+ clearance during CPA-induced Ca2+ discharge was unaffected by a low Na+ (50 mM) extracellular solution, but strongly suppressed by 5 mM La3+ (an inhibitor of plasmalemmal Ca2+ pump), suggesting Ca2+ extrusion was predominantly through the plasmalemmal Ca2+ pump. Propofol mimicked the effect of La3+ in suppressing Ca2+ clearance. Propofol also stimulated release of pain mediators, namely, reactive oxygen species and bradykinin. Our data suggest propofol elicited Ca2+ release and repressed Ca2+ clearance, causing a sustained cytosolic [Ca2+]i elevation. The latter may cause reactive oxygen species and bradykinin release, resulting in pain.

Lipotoxicity in human lung alveolar type 2 A549 cells: Mechanisms and protection by tannic acid

Palmitic acid (PA) is a saturated free fatty acid which, when being excessive, accounts for lipotoxicity. Using human lung A549 cells as a model for lung alveolar type 2 epithelial cells, we found that challenge of A549 cells with PA resulted in apoptotic cell death, as reflected by positive annexin V and PI staining, and also appearance of cleaved caspase-3. PA treatment also caused depletion of intracellular Ca²⁺ store, endoplasmic reticulum (ER) stress, and oxidative stress. Tannic acid (TA), a polyphenol present in wines and many beverages, alleviated PA-induced ER stress, oxidative stress and apoptotic death. Thus, our results suggest PA lipotoxicity in lung alveolar type 2 epithelial cells could be protected by TA.

A basal level of γ-linolenic acid depletes Ca2+ stores and induces endoplasmic reticulum and oxidative stresses to cause death of breast cancer BT-474 cells

Gamma-linolenic acid (GLA), a natural fatty acid obtained from oils of various vegetables and seeds, has been demonstrated as an anticancer agent. In this work, we investigated the anticancer effects of GLA on breast cancer BT-474 cells. GLA at 30 μM, a concentration reportedly within the range of circulating concentrations in clinical studies, caused apoptotic cell death. GLA caused an elevation in mitochondrial Ca²⁺ level and a decrease in mitochondrial membrane potential. GLA treatment depleted cyclopiazonic acid (CPA)-sensitive Ca²⁺ store and triggered substantial Ca²⁺ influx. Intracellular Ca²⁺ release triggered by GLA was suppressed by 3 μM xestospongin C (XeC, IP₃ receptor-channel blocker) and 100 μM ryanodine (ryanodine receptor-channel blocker), suggesting that the Ca²⁺ release was via IP₃ receptor-channel and ryanodine receptor-channel. Increased expressions of p-eIF2α and CHOP were observed in GLA-treated cells, suggesting GLA-treated cells had increased expressions of p-eIF2α and CHOP, which suggest endoplasmic reticulum (ER) stress. In addition, GLA elicited increased production of reactive oxygen species. Taken together, our results suggest a basal level of GLA induced apoptotic cell death by causing Ca²⁺ overload, mitochondrial dysfunction, Ca²⁺ store depletion, ER stress, and oxidative stress. This is the first report to show that GLA caused Ca²⁺ store depletion and ER stress. GLA-induced Ca²⁺ store depletion resulted from opening of IP₃ receptor-channel and ryanodine receptor-channel.

Characterization of Ca2+-Sensing Receptor-Mediated Ca2+ Influx in Microvascular bEND.3 Endothelial Cells

Ca²⁺-sensing receptors (CaSR), activated by elevated concentrations of extracellular Ca²⁺, have been known to regulate functions of thyroid cells, neurons, and endothelial cells (EC). In this report, we studied CaSR-mediated Ca²⁺ influx in mouse cerebral microvascular EC (bEND.3 cells). Cytosolic free Ca²⁺ concentration and Mn²⁺ influx were measured by fura-2 microfluorometry. High (3 mM) Ca²⁺ (CaSR agonist), 3 mM spermine (CaSR agonist), and 10 μM cinacalcet (positive allosteric modulator of CaSR) all triggered Ca²⁺ influx; however, spermine, unlike high Ca²⁺ and cinacalcet, did not promote Mn²⁺ influx and its response was poorly sensitive to SKF 96365, a TRP channel blocker. Consistently, 2-aminoethoxydiphenyl borate and ruthenium red (two other general TRP channel blockers) suppressed Ca²⁺ influx triggered by cinacalcet and high Ca²⁺ but not by spermine. Ca²⁺ influx triggered by high Ca²⁺, spermine, and cinacalcet was similarly suppressed by A784168, a potent and selective TRPV1 antagonist. Our results suggest that CaSR activation triggered Ca²⁺ influx via TRPV1 channels; intriguingly, pharmacological, and permeability properties of such Ca²⁺ influx depended on the stimulating ligands.

Perturbation of Ca2+ stores and store-operated Ca2+ influx by lidocaine in neuronal N2A and NG108-15 cells

In this report we examined the effects of lidocaine on Ca²⁺ homeostasis of neuronal cells using microfluorimetric measurement of cytosolic Ca²⁺ with fura 2 as probe. In mouse neuroblastoma N2A cells, 10 mM lidocaine caused Ca²⁺ release from the cyclopiazonic acid (CPA)-dischargeable pool and abolished ATP-triggered Ca²⁺ release. Lidocaine-triggered Ca²⁺ release was not affected by xestospongin C (XeC), an inositol 1,4,5-trisphosphate receptor (IP3R) inhibitor. N2A cells did not have functional ryanodine receptors (RYR) (absence of caffeine response) and we used differentiated NG108-15 cells (presence of caffeine response) for further experiments. Caffeine-triggered Ca²⁺ release was unaffected by a brief lidocaine exposure, but was eliminated after a prolonged treatment of lidocaine, suggesting lidocaine abolished caffeine action possibly not by interfering caffeine binding but via Ca²⁺ store depletion. Lidocaine-elicited Ca²⁺ release was unaffected by XeC or a high concentration of ryanodine, suggesting Ca²⁺ release was not via IP3R or RYR. Lidocaine did not affect nigericin-dischargeable lysosomal Ca²⁺ stores. Lastly, we observed that lidocaine suppressed CPA-induced store-operated Ca²⁺ influx in both N2A cells and differentiated NG108-15 cells. Our results suggest two novel actions of lidocaine in neuronal cells, namely, depletion of Ca²⁺ store (via an IP3R- and RYR-independent manner) and suppression of store-operated Ca²⁺ influx.

The versatile Kv channels in the nervous system: actions beyond action potentials

Voltage-gated K+ (Kv) channel opening repolarizes excitable cells by allowing K+ efflux. Over the last two decades, multiple Kv functions in the nervous system have been found to be unrelated to or beyond the immediate control of excitability, such as shaping action potential contours or regulation of inter-spike frequency. These functions include neuronal exocytosis and neurite formation, neuronal cell death, regulation of astrocyte Ca2+, glial cell and glioma proliferation. Some of these functions have been shown to be independent of K+ conduction, that is, they suggest the non-canonical functions of Kv channels. In this review, we focus on neuronal or glial plasmalemmal Kv channel functions which are unrelated to shaping action potentials or immediate control of excitability. Similar functions in other cell types will be discussed to some extent in appropriate contexts.

Polyamine stimulation perturbs intracellular Ca2+ homeostasis and decreases viability of breast cancer BT474 cells

Intracellular polyamines such as spermine and spermidine are essential to cell growth in normal and especially in cancer cells. However, whether extracellular polyamines affect cancer cell survival is unknown. We therefore examined the actions of extracellular polyamines on breast cancer BT474 cells. Our data showed that spermine, spermidine, and putrescine decreased cell viability by apoptosis. These polyamines also elicited Ca2+ signals, but the latter were unlikely triggered via Ca2+-sensing receptor (CaSR) as BT474 cells have been demonstrated previously to lack CaSR expression. Spermine-elicited Ca2+ response composed of both Ca2+ release and Ca2+ influx. Spermine caused a complete discharge of the cyclopiazonic acid (CPA)-sensitive Ca2+ pool and, expectedly, endoplasmic reticulum (ER) stress. The Ca2+ influx pore opened by spermine was Mn2+-impermeable, distinct from the CPA-triggered store-operated Ca2+ channel, which was Mn2+-permeable. Spermine cytotoxic effects were not due to oxidative stress, as spermine did not trigger reactive oxygen species formation. Our results therefore suggest that spermine acted on a putative polyamine receptor in BT474 cells, causing cytotoxicity by Ca2+ overload, Ca2+ store depletion, and ER stress.

Quercetin depletes intracellular Ca2+ stores and blunts ATP-triggered Ca2+ signaling in bEnd.3 endothelial cells

Quercetin is a flavonol polyphenol widely found in many vegetables, grains, and fruits. Quercetin has been shown to inhibit proliferation and invasion of various glioma cells and is regarded as a potential anticancer agent against glioma. However, whether and how this drug could affect brain blood vessels and endothelial cells (EC) are less understood. Further, there is hitherto no report on how quercetin affects brain EC Ca²⁺ homeostasis. In this report, we investigated the effects of quercetin on Ca²⁺ homeostasis in mouse brain bEnd.3 EC. We demonstrated that quercetin raised cytosolic Ca²⁺ level in a concentration-dependent manner. Quercetin-triggered Ca²⁺ signal composed of both internal Ca²⁺ release and extracellular Ca²⁺ influx. Quercetin caused Ca²⁺ release from the endoplasmic reticulum, and consistently, inhibition of inositol 1,4,5-trisphosphate receptor (IP3R) by xestospongin C (XeC) suppressed quercetin-triggered Ca²⁺ release. Quercetin also caused Ca²⁺ release from lysosomes, an observation in concordance with the inhibition of quercetin-triggered Ca²⁺ release by trans-Ned-19, a blocker of two-pore channels. As quercetin depleted intracellular Ca²⁺ storage, it suppressed ATP-induced Ca²⁺ release and thereby blunted ATP-triggered Ca²⁺ signaling. In addition, quercetin co-treatment significantly suppressed ATP-stimulated nitric oxide release. Our work therefore showed, for the first time, quercetin perturbed intracellular Ca²⁺ stores and strongly suppressed ATP-triggered response in bEnd.3 cells.

Complexes formed with integrin-α5 and KCNB1 potassium channel wild type or epilepsy-susceptibility variants modulate cellular plasticity via Ras and Akt signaling

Voltage-gated potassium (K⁺) channel subfamily B member 1 (KCNB1, Kv2.1) and integrin-α5 form macromolecular complexes-named integrin-α5-KCNB1 complexes (IKCs)-in the human brain, but their function was poorly understood. Here we report that membrane depolarization triggered IKC intracellular signals mediated by small GTPases of the Ras subfamily and protein kinase B (Akt) to advance the development of filopodia and lamellipodia in Chinese hamster ovary cells, stimulate their motility, and enhance neurite outgrowth in mouse neuroblastoma Neuro2a cells. Five KCNB1 mutants (L211P, R312H G379R, G381R, and F416L) linked to severe infancy or early-onset epileptic encephalopathy exhibited markedly defective conduction. However, although L211P, G379R, and G381R normally engaged Ras/Akt and stimulated cell migration, R312H and F416L failed to activate Ras/Akt signaling and did not enhance cell migration. Taken together, these data suggest that IKCs modulate cellular plasticity via Ras and Akt signaling. As such, defective IKCs may cause epilepsy through mechanisms other than dysregulated excitability such as, for example, abnormal neuronal development and resulting synaptic connectivity.-Yu, W., Shin, M. R., Sesti, F. Complexes formed with integrin-α5 and KCNB1 potassium channel wild type or epilepsy-susceptibility variants modulate cellular plasticity via Ras and Akt signaling.

Antagonism of Ca2+-sensing receptors by NPS 2143 is transiently masked by p38 activation in mouse brain bEND.3 endothelial cells

Ca²⁺-sensing receptors (CaSR) are G protein-coupled receptors which are activated by a rise in extracellular Ca²⁺. CaSR activation has been known to inhibit parathyroid hormone release and stimulate calcitonin release from parathyroid glands and thyroid parafollicular C cells, respectively. The roles of CaSR in other cell types including endothelial cells (EC) are much less understood. In this work, we demonstrated protein and functional expression of CaSR in mouse cerebral EC (bEND.3). Unexpectedly, CaSR response (high Ca²⁺-elicited cytosolic [Ca²⁺] elevation) was unaffected by edelfosine or U73122 but strongly suppressed by SK&F 96365, ruthenium red, and 2-aminoethoxydiphenyl borate (2-APB), suggesting involvement of TRPV and TRPC channels but not Gq-phospholipase C. Acute application of NPS2143, a negative allosteric modulator of CaSR, suppressed CaSR response. However, a 40-min NPS2143 pre-treatment surprisingly enhanced CaSR response. After 4-24 h of application, this enhancement faded away and suppression of CaSR response was observed again. Similar results were obtained when La³⁺ and Sr²⁺ were used as CaSR agonists. The transient NPS 2143 enhancement effect was abolished by SB203580, a p38 inhibitor. Consistently, NPS 2143 triggered a transient p38 activation. Taken together, results suggest that in bEND.3 cells, NPS 2143 caused acute suppression of CaSR response, but then elicited a transient enhancement of CaSR response in a p38-dependent manner. NPS 2143 effects on CaSR in bEND.3 cells therefore depended on drug exposure time. These findings warrant cautious use of this agent as a CaSR modulator and potential cardiovascular drug.

Valproic acid inhibits ATP-triggered Ca2+ release via a p38-dependent mechanism in bEND.3 endothelial cells

Valproic acid (VA) is currently used to treat epilepsy and bipolar disorder. It has also been demonstrated to promote neuroprotection and neurogenesis. Although beneficial actions of VA on brain blood vessels have also been demonstrated, the effects of VA on brain endothelial cell (EC) Ca²⁺ signaling are hitherto unreported. In this report, we examined the effects of VA on agonist-triggered Ca²⁺ signaling in mouse cortical bEND.3 EC. While VA (100 μm) did not cause an acute inhibition of ATP-triggered Ca²⁺ signaling, a 30-min VA treatment strongly suppressed ATP-triggered intracellular Ca²⁺ release; however, such treatment did not affect Ca²⁺ release triggered by cyclopiazonic acid, an inhibitor of SERCA Ca²⁺ pump, suggesting there was no reduction in Ca²⁺ store size. VA-activated p38 signaling, and VA-induced inhibition of ATP-triggered Ca²⁺ release was prevented by SB203580, a p38 inhibitor, suggesting VA caused the inhibition by activating p38. Remarkably, VA treatment did not affect acetylcholine-triggered Ca²⁺ release, suggesting VA may not inhibit inositol 1,4,5-trisphosphate-induced Ca²⁺ release per se, and may not act directly on Gq or phospholipase C. Taken together, our results suggest VA treatment, via a p38-dependent mechanism, led to an inhibition of purinergic receptor-effector coupling.

Voltage-gated K+ channels promote BT-474 breast cancer cell migration

Objective

A variety of ion channels have been implicated in breast cancer proliferation and metastasis. Voltage-gated K⁺ (Kv) channels not only cause repolarization in excitable cells, but are also involved in multiple cellular functions in non-excitable cells. In this study we investigated the role of Kv channels in migration of BT474 breast cancer cells.

Methods

Transwell technique was used to separate migratory cells from non-migratory ones and these two groups of cells were subject to electrophysiological examinations and microfluorimetric measurements for cytosolic Ca²⁺. Cell migration was examined in the absence or presence of Kv channel blockers.

Results

When compared with non-migratory cells, migratory cells had much higher Kv current densities, but rather unexpectedly, more depolarized membrane potential and reduced Ca²⁺ influx. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed the presence of Kv1.1, Kv1.3, Kv1.5, Kv2.1, Kv3.3, Kv3.4 and Kv4.3 channels. Cell migration was markedly inhibited by tetraethylammonium (TEA), a delayed rectifier Kv channel blocker, but not by 4-aminopyridine, an A-type Kv channel blocker.

Conclusions

Taken together, our results show that increased Kv channel expression played a role in BT474 cell migration, and Kv channels could be considered as biomarkers or potential therapeutic targets for breast cancer metastasis. The mechanism(s) by which Kv channels enhanced migration appeared unrelated to membrane hyperpolarization and Ca²⁺ influx.

Eicosapentaenoic acid triggers Ca2+ release and Ca2+ influx in mouse cerebral cortex endothelial bEND.3 cells

Eicosapentaenoic acid (EPA), an omega-3 fatty acid abundant in fish oil, protects endothelial cells (EC) from lipotoxicity and triggers EC NO release. The latter is related to an elevation of cytosolic Ca2+. Although EPA has been shown to cause human EC cytosolic Ca2+ elevation, the mechanism is unclear. Microfluorimetric imaging was used here to measure free cytosolic Ca2+ concentration. EPA was shown to cause intracellular Ca2+ release in mouse cerebral cortex endothelial bEND.3 cells; interestingly, the EPA-sensitive intracellular Ca2+ pool(s) appeared to encompass and was larger than the Ca2+ pool mobilized by sarcoplasmic-endoplasmic reticulum Ca2+-ATPase inhibition by cyclopiazonic acid. EPA also opened a Ca2+ influx pathway pharmacologically distinct from store-operated Ca2+ influx. Surprisingly, EPA-triggered Ca2+ influx was Ni2+-insensitive; and EPA did not trigger Mn2+ influx. Further, EPA-triggered Ca2+ influx did not involve Na+-Ca2+ exchangers. Thus, our results suggest EPA triggered unusual mechanisms of Ca2+ release and Ca2+ influx in EC.

Effects of silver nanoparticles on the interactions of neuron- and glia-like cells: Toxicity, uptake mechanisms, and lysosomal tracking

Silver nanoparticles (AgNPs) are commonly used nanomaterials in consumer products. Previous studies focused on its effects on neurons; however, little is known about their effects and uptake mechanisms on glial cells under normal or activated states. Here, ALT astrocyte-like, BV-2 microglia and differentiated N2a neuroblastoma cells were directly or indirectly exposed to 10 nm AgNPs using mono- and co-culture system. A lipopolysaccharide (LPS) was pretreated to activate glial cells before AgNP treatment for mimicking NP exposure under brain inflammation. From mono-culture, ALT took up the most AgNPs and had the lowest cell viability within three cells. Moreover, AgNPs induced H₂ O₂ and NO from ALT/activated ALT and BV-2, respectively. However, AgNPs did not induce cytokines release (IL-6, TNF-α, MCP-1). LPS-activated BV-2 took up more AgNPs than normal BV-2, while the induction of ROS and cytokines from activated cells were diminished. Ca²⁺ -regulated clathrin- and caveolae-independent endocytosis and phagocytosis were involved in the AgNP uptake in ALT, which caused more rapid NP translocation to lysosome than in macropinocytosis and clathrin-dependent endocytosis-involved BV-2. AgNPs directly caused apoptosis and necrosis in N2a cells, while by indirect NP exposure to bottom chamber ALT or BV-2 in Transwell, more apoptotic upper chamber N2a cells were observed. Cell viability of BV-2 also decreased in an ALT-BV-2 co-culturing study. The damaged cells correlated to NP-mediated H₂ O₂ release from ALT or NO from BV-2, which indicates that toxic response of AgNPs to neurons is not direct, but indirectly arises from AgNP-induced soluble factors from other glial cells.

Perturbation of Akt Signaling, Mitochondrial Potential, and ADP/ATP Ratio in Acidosis-Challenged Rat Cortical Astrocytes

Cells switch to anaerobic glycolysis when there is a lack of oxygen during brain ischemia. Extracellular pH thus drops and such acidosis causes neuronal cell death. The fate of astrocytes, mechanical, and functional partners of neurons, in acidosis is less studied. In this report, we investigated the signaling in acidosis-challenged rat cortical astrocytes and whether these signals were related to mitochondrial dysfunction and cell death. Exposure to acidic pH (6.8, 6.0) caused Ca²⁺ release and influx, p38 MAPK activation, and Akt inhibition. Mitochondrial membrane potential was hyperpolarized after astrocytes were exposed to acidic pH as soon as 1 h and lasted for 24 h. Such mitochondrial hyperpolarization was prevented by SC79 (an Akt activator) but not by SB203580 (a p38 inhibitor) nor by cytosolic Ca²⁺ chelation by BAPTA, suggesting that only the perturbation in Akt signaling was causally related to mitochondrial hyperpolarization. SC79, SB203580, and BAPTA did not prevent acidic pH-induced cell death. Acidic pH suppressed ROS production, thus ruling out the role of ROS in cytotoxicity. Interestingly, pH 6.8 caused an increase in ADP/ATP ratio and apoptosis; pH 6.0 caused a further increase in ADP/ATP ratio and necrosis. Therefore, astrocyte cell death in acidosis did not result from mitochondrial potential collapse; in case of acidosis at pH 6.0, necrosis might partly result from mitochondrial hyperpolarization and subsequent suppressed ATP production. J. Cell. Biochem. 118: 1108-1117, 2017. © 2016 Wiley Periodicals, Inc.

Indirect effects of TiO2 nanoparticle on neuron-glial cell interactions

Although, titanium dioxide nanoparticles (TiO2NPs) are nanomaterials commonly used in consumer products, little is known about their hazardous effects, especially on central nervous systems. To examine this issue, ALT astrocyte-like, BV-2 microglia and differentiated N2a neuroblastoma cells were exposed to 6 nm of 100% anatase TiO2NPs. A lipopolysaccharide (LPS) was pre-treated to activate glial cells before NP treatment for mimicking NP exposure under brain injury. We found that ALT and BV-2 cells took up more NPs than N2a cells and caused lower cell viability. TiO2NPs induced IL-1β in the three cell lines and IL-6 in N2a. LPS-activated BV-2 took up more TiO2NPs than normal BV-2 and released more intra/extracellular reactive oxygen species (ROS), IL-1β, IL-6 and MCP-1 than did activated BV-2. Involvement of clathrin- and caveolae-dependent endocytosis in ALT and clathrin-dependent endocytosis and phagocytosis in BV-2 both had a slow NP translocation rate to lysosome, which may cause slow ROS production (after 24 h). Although TiO2NPs did not directly cause N2a viability loss, by indirect NP exposure to the bottom chamber of LPS-activated BV-2 in the Transwell system, they caused late apoptosis and loss of cell viability in the upper N2a chamber due to H2O2 and/or TNF-α release from BV-2. However, none of the adverse effects in N2a or BV-2 cells was observed when TiO2NPs were exposed to ALT-N2a or ALT-BV-2 co-culture. These results demonstrate that neuron damage can result from TiO2NP-mediated ROS and/or cytokines release from microglia, but not from astrocytes.

Parthenolide-Induced Cytotoxicity in H9c2 Cardiomyoblasts Involves Oxidative Stress

BACKGROUND

Cardiac cellular injury as a consequence of ischemia and reperfusion involves nuclear factor-κB (NF-κ B), amongst other factors, and NF-κ B inhibitors could substantially reduce myocardial infarct size. Parthenolide, a sesquiterpene lactone compound which could inhibit NF-κ B, has been shown to ameliorate myocardial reperfusion injury but may also produce toxic effects in cardiomyocytes at high concentrations. The aim of this study was to examine the cytotoxic effects of this drug on H9c2 cardiomyoblasts, which are precursor cells of cardiomyocytes.

METHODS

Cell viability and apoptosis were examined by MTT and TUNEL assay, respectively, and protein expression was analyzed by western blot. Reactive oxygen species (ROS) production was measured using DCFH-DA as dye. Cytosolic Ca(2+) concentration and mitochondrial membrane potential were measured microfluorimetrically using, respectively, fura 2 and rhodamine 123 as dyes.

RESULTS

Parthenolide caused apoptosis at 30 μ M, as judged by TUNEL assay and Bax and cytochrome c translocation. It also caused collapse of mitochondrial membrane potential and endoplasmic reticulum stress. Parthenolide triggered ROS formation, and vitamin C (antioxidant) partially alleviated parthenolide-induced cell death.

CONCLUSIONS

The results suggested that parthenolide at high concentrations caused cytotoxicity in cardiomyoblasts in part by inducing oxidative stress, and demonstrated the imperative for cautious and appropriate use of this agent in cardioprotection.

KEY WORDS

Cardiomyoblast; Endoplasmic reticulum stress; Oxidative stress; Parthenolide; Reperfusion injury.

The Role of KV7.3 in Regulating Osteoblast Maturation and Mineralization

KCNQ (KV7) channels are voltage-gated potassium (KV) channels, and the function of KV7 channels in muscles, neurons, and sensory cells is well established. We confirmed that overall blockade of KV channels with tetraethylammonium augmented the mineralization of bone-marrow-derived human mesenchymal stem cells during osteogenic differentiation, and we determined that KV7.3 was expressed in MG-63 and Saos-2 cells at the mRNA and protein levels. In addition, functional KV7 currents were detected in MG-63 cells. Inhibition of KV7.3 by linopirdine or XE991 increased the matrix mineralization during osteoblast differentiation. This was confirmed by alkaline phosphatase, osteocalcin, and osterix in MG-63 cells, whereas the expression of Runx2 showed no significant change. The extracellular glutamate secreted by osteoblasts was also measured to investigate its effect on MG-63 osteoblast differentiation. Blockade of KV7.3 promoted the release of glutamate via the phosphorylation of extracellular signal-regulated kinase 1/2-mediated upregulation of synapsin, and induced the deposition of type 1 collagen. However, activation of KV7.3 by flupirtine did not produce notable changes in matrix mineralization during osteoblast differentiation. These results suggest that KV7.3 could be a novel regulator in osteoblast differentiation.

Antigenic sites on the HN domain of botulinum neurotoxin A stimulate protective antibody responses against active toxin

Botulinum neurotoxins (BoNTs) are the most toxic substances known. BoNT intoxicates cells in a highly programmed fashion initiated by binding to the cell surface, internalization and enzymatic cleavage of substrate, thus, inhibiting synaptic exocytosis. Over the past two decades, immunological significance of BoNT/A C-terminal heavy chain (H_C) and light chain (L_C) domains were investigated extensively leading to important findings. In the current work, we explored the significance of BoNT/A heavy chain N-terminal (H_N) region as a vaccine candidate. Mice were immunized with recombinant H_N519–845 generating antibodies (Abs) that were found to be protective against lethal dose of BoNT/A. Immuno-dominant regions of H_N519–845 were identified and individually investigated for antibody response along with synthetic peptides within those regions, using in vivo protection assays against BoNT/A. Results were confirmed by patch-clamp analysis where anti-H_N antibodies were studied for the ability to block toxin-induced channel formation. This data strongly indicated that H_N519–593 is an important region in generating protective antibodies and should be valuable in a vaccine design. These results are the first to describe and dissect the protective activity of the BoNT/A H_N domain.

Microarray data analysis of neuroblastoma: Expression of SOX2 downregulates the expression of MYCN

The present study aimed to identify the genes directly or indirectly correlated with the amplification of MYCN in neuroblastoma (NB). Microarray data (GSE53371) were downloaded from Gene Expression Omnibus, and included 10 NB cell lines with MYCN amplification and 10 NB cell lines with normal MYCN copy numbers. Differentially expressed genes (DEGs) were identified using the Linear Models for Microarray Data package, and a false discovery rate of <0.05 and |log2FC (fold change)|>1 were selected as cut‑off criteria. Hierarchical clustering analysis and Gene Ontology analysis were respectively performed for the DEGs using the Pheatmap package in R language and The Database for Annotation, Visualization and Integrated Discovery. A protein‑protein interaction network (PPI) was constructed for the DEGs using the Search Tool for the Retrieval of Interacting Genes database. Pathway analysis was performed for the DEGs in the PPI network using the WEB‑based GEne SeT AnaLysis Toolkit. The correlation between MYCN and the key gene associated with MYCN was determined using Pearson's correlation coefficient. In total, 137 downregulated and 35 upregulated DEGs were identified. Functional enrichment analysis indicated that KCNMB4 was involved in the regulation of action potential in neuron term, and the FOS, GLI3 and GLI1 genes were involved in the extracellular matrix‑receptor interaction pathway. The PPI network and correlation analysis revealed that the expression of SOX2 was directly correlated with the expression of MYCN, and the correlation coefficient of SOX2 and MYCN was ‑0.83. Therefore, SOX2, KCNMB4, FOS, GLI3 and GLI1 may be involved in the pathogenesis of NB, with the expression of SOX2 downregulating the expression of MYCN.

Role of voltage-gated K(+) channels in regulating Ca(2+) entry in rat cortical astrocytes

Astrocytes have multiple functions such as provision of nourishment and mechanical support to the nervous system, helping to clear extracellular metabolites of neurons and modulating synaptic transmission by releasing gliotransmitters. In excitable cells, voltage-gated K(+) (Kv) channels serve to repolarize during action potentials. Astrocytes are considered non-excitable cells since they are not able to generate action potentials. There is an abundant expression of various Kv channels in astrocytes but the functions of these Kv channels remain unclear. We examined whether these astrocyte Kv channels regulate astrocyte "excitability" in the form of cytosolic Ca(2+) signaling. Electrophysiological examination revealed that neonatal rat cortical astrocytes possessed both delayed rectifier type and A-type Kv channels. Pharmacological blockade of both delayed rectifier Kv channels by TEA and A-type Kv channels by quinidine significantly suppressed store-operated Ca(2+) influx; however, TEA alone or quinidine alone did not suffice to cause such suppression. TEA and quinidine together dramatically enhanced current injection-triggered membrane potential overshoot (depolarization); either drug alone caused much smaller enhancements. Taken together, the results suggest both delayed rectifier and A-type Kv channels regulate astrocyte Ca(2+) signaling via controlling membrane potential.

Overexpression of tau downregulated the mRNA levels of Kv channels and improved proliferation in N2A cells

Microtubule binding protein tau has a crucial function in promoting the assembly and stabilization of microtubule. Besides tuning the action potentials, voltage-gated K+ channels (Kv) are important for cell proliferation and appear to play a role in the development of cancer. However, little is known about the possible interaction of tau with Kv channels in various tissues. In the present study, tau plasmids were transiently transfected into mouse neuroblastoma N2A cells to explore the possible linkages between tau and Kv channels. This treatment led to a downregulation of mRNA levels of several Kv channels, including Kv2.1, Kv3.1, Kv4.1, Kv9.2, and KCNH4, but no significant alteration was observed for Kv5.1 and KCNQ4. Furthermore, the macroscopic currents through Kv channels were reduced by 36.5% at +60 mV in tau-transfected N2A cells. The proliferation rates of N2A cells were also improved by the induction of tau expression and the incubation of TEA (tetraethylammonium) for 48 h by 120.9% and 149.3%, respectively. Following the cotransfection with tau in HEK293 cells, the mRNA levels and corresponding currents of Kv2.1 were significantly declined compared with single Kv2.1 transfection. Our data indicated that overexpression of tau declined the mRNA levels of Kv channels and related currents. The effects of tau overexpression on Kv channels provided an alternative explanation for low sensitivity to anti-cancer chemicals in some specific cancer tissues.

17β-estradiol downregulated the expression of TASK-1 channels in mouse neuroblastoma N2A cells

TASK channels, an acid-sensitive subgroup of two pore domain K⁺ (K2P) channels family, were widely expressed in a variety of neural tissues, and exhibited potent functions such as the regulation of membrane potential. The steroid hormone estrogen was able to interact with K⁺ channels, including voltage-gated K⁺ (Kv) and large conductance Ca²⁺-activated (BK) K⁺ channels, in different types of cells like cardiac myocytes and neurons. However, it is unclear about the effects of estrogen on TASK channels. In the present study, the expressions of two members of acid-sensitive TASK channels, TASK-1 and TASK-2, were detected in mouse neuroblastoma N2A cells by RT-PCR. Extracellular acidification (pH 6.4) weakly but statistically significantly inhibited the outward background current by 22.9 % at a holding potential of 0 mV, which inactive voltage-gated K⁺ currents, suggesting that there existed the functional TASK channels in the membrane of N2A cells. Although these currents were not altered by the acute application of 100 nM 17β-estradiol, incubation with 10 nM 17β-estradiol for 48 h reduced the mRNA level of TASK-1 channels by 40.4 % without any effect on TASK-2 channels. The proliferation rates of N2A cells were also increased by treatment with 10 nM 17β-estradiol for 48 h. These data implied that N2A cells expressed functional TASK channels and chronic exposure to 17β-estradiol downregulated the expression of TASK-1 channels and improved cell proliferation. The effect of 17β-estradiol on TASK-1 channels might be an alternative mechanism for the neuroprotective action of 17β-estradiol.

Moderate superoxide production is an early promoter of mitochondrial biogenesis in differentiating N2a neuroblastoma cells

Reactive oxygen species (ROS) have been widely considered as harmful for cell development and as promoters of cell aging by increasing oxidative stress. However, ROS have an important role in cell signaling and they have been demonstrated to be beneficial by triggering hormetic signals, which could protect the organism from later insults. In the present study, N2a murine neuroblastoma cells were used as a paradigm of cell-specific (neural) differentiation partly mediated by ROS. Differentiation was triggered by the established treatments of serum starvation, forskolin or dibutyryl cyclic AMP. A marked differentiation, expressed as the development of neurites, was detected by fixation and staining with coomassie brilliant blue after 48 h treatment. This was accompanied by an increase in mitochondrial mass detected by mitotracker green staining, an increased expression of the peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1-alpha (PGC-1α) and succinate dehydrogenase activity as detected by MTT. In line with these results, an increase in free radicals, specifically superoxide anion, was detected in differentiating cells by flow cytometry. Superoxide scavenging by MnTBAP and MAPK inhibition by PD98059 partially reversed differentiation and mitochondrial biogenesis. In this way, we demonstrated that mitochondrial biogenesis and differentiation are mediated by superoxide and MAPK cues. Our data suggest that differentiation and mitochondrial biogenesis in N2a cells are part of a hormetic response which is triggered by a modest increase of superoxide anion concentration within the mitochondria.

Toxic role of K+ channel oxidation in mammalian brain

Potassium (K(+)) channels are essential to neuronal signaling and survival. Here we show that these proteins are targets of reactive oxygen species in mammalian brain and that their oxidation contributes to neuropathy. Thus, the KCNB1 (Kv2.1) channel, which is abundantly expressed in cortex and hippocampus, formed oligomers upon exposure to oxidizing agents. These oligomers were ∼10-fold more abundant in the brain of old than young mice. Oxidant-induced oligomerization of wild-type KCNB1 enhanced apoptosis in neuronal cells subject to oxidative insults. Consequently, a KCNB1 variant resistant to oxidation, obtained by mutating a conserved cysteine to alanine, (C73A), was neuroprotective. The fact that oxidation of KCNB1 is toxic, argues that this mechanism may contribute to neuropathy in conditions characterized by high levels of oxidative stress, such as Alzheimer's disease (AD). Accordingly, oxidation of KCNB1 channels was exacerbated in the brain of a triple transgenic mouse model of AD (3xTg-AD). The C73A variant protected neuronal cells from apoptosis induced by incubation with β-amyloid peptide (Aβ(1-42)). In an invertebrate model (Caenorhabditis elegans) that mimics aspects of AD, a C73A-KCNB1 homolog (C113S-KVS-1) protected specific neurons from apoptotic death induced by ectopic expression of human Aβ(1-42). Together, these data underscore a novel mechanism of toxicity in neurodegenerative disease.

Ca2+ store depletion and endoplasmic reticulum stress are involved in P2X7 receptor-mediated neurotoxicity in differentiated NG108-15 cells

P2X7 receptor (P2X7R) activation by extracellular ATP triggers influx of Na(+) and Ca(2+), cytosolic Ca(2+) overload and consequently cytotoxicity. Whether disturbances in endoplasmic reticulum (ER) Ca(2+) homeostasis and ER stress are involved in P2X7R-mediated cell death is unknown. In this study, a P2X7R agonist (BzATP) was used to activate P2X7R in differentiated NG108-15 neuronal cells. In a concentration-dependent manner, application of BzATP (10-100 µM) immediately raised cytosolic Ca(2+) concentration ([Ca(2+)]i) and caused cell death after a 24-h incubation. P2X7R activation for 2 h did not cause cell death but resulted in a sustained reduction in ER Ca2+ pool size, as evidenced by a diminished cyclopiazonic acid-induced Ca(2+) discharge (fura 2 assay) and a lower fluorescent signal in cells loaded with Mag-fura 2 (ER-specific Ca(2+)-fluorescent dye). Furthermore, P2X7R activation (2 h) led to the appearance of markers of ER stress [phosphorylated α subunit of eukaryotic initiation factor 2 (p-eIF2α) and C/EBP homologous protein (CHOP)] and apoptosis (cleaved caspase 3). Xestospongin C (XeC), an antagonist of inositol-1,4,5-trisphosphate (IP3) receptor (IP3R), strongly inhibited BzATP-triggered [Ca(2+)]i elevation, suggesting that the latter involved Ca(2+) release via IP3R. XeC pretreatment not only attenuated the reduction in Ca(2+) pool size in BzATP-treated cells, but also rescued cell death and prevented BzATP-induced appearance of ER stress and apoptotic markers. These novel observations suggest that P2X7R activation caused not only Ca(2+) overload, but also Ca(2+) release via IP3R, sustained Ca(2+) store depletion, ER stress and eventually apoptotic cell death.

Inhibitory effects of magnolol on voltage-gated Na+ and K+ channels of NG108-15 cells

Magnolol, a polyphenolic compound isolated from Houpu, a Chinese herb from the bark of Magnolia officinalis, has been reported to have in vitro and in vivo neuroprotective effects. In spite of these reported beneficial effects, studies on the direct impact of magnolol on neuronal ion channels have been scarce. Whether magnolol affects voltage-gated Na(+) channels (VGSC) and voltage-gated K(+) (Kv) channels is unknown. Using the whole-cell voltage-clamp method, we studied the effects of magnolol on voltage-gated ion channels in neuronal NG108-15 cells. Magnolol inhibited VGSC channels with mild state-dependence (IC(50) of 15 and 30 μM, at holding potentials of -70 and -100 mV, respectively). No frequency-dependence was observed in magnolol block. Magnolol caused a left-shift of 18 mV in the steady-state inactivation curve but did not affect the voltage-dependence of activation. Magnolol inhibited Kv channels with an IC(50) of 21 μM, and it caused a 20-mV left-shift in the steady-state inactivation curve without affecting the voltage-dependence of activation. In conclusion, magnolol is an inhibitor of both VGSC and Kv channels and these inhibitory effects may in part contribute to some of the reported neuroprotective effects of magnolol.