Voltage-independent Cation Channels Research Articles

Cyclic nucleotide-regulated channels (CNG) in GtoPdb v.2023.1

Cyclic nucleotide-gated (CNG) channels are responsible for signalling in the primary sensory cells of the vertebrate visual and olfactory systems. CNG channels are voltage-independent cation channels formed as tetramers. Each subunit has 6TM, with the pore-forming domain between TM5 and TM6. CNG channels were first found in rod photoreceptors [83, 120], where light signals through rhodopsin and transducin to stimulate phosphodiesterase and reduce intracellular cyclic GMP level. This results in a closure of CNG channels and a reduced ‘dark current’. Similar channels were found in the cilia of olfactory neurons [181] and the pineal gland [71]. The cyclic nucleotides bind to a domain in the C terminus of the subunit protein: other channels directly binding cyclic nucleotides include hyperolarisation-activated, cyclic nucleotide-gated channels (HCN), ether-a-go-go and certain plant potassium channels.The HCN channels are cation channels that are activated by hyperpolarisation at voltages negative to ~-50 mV. The cyclic nucleotides cyclic AMP and cyclic GMP directly bind to the cyclic nucleotide-binding domain of HCN channels and shift their activation curves to more positive voltages, thereby enhancing channel activity. HCN channels underlie pacemaker currents found in many excitable cells including cardiac cells and neurons [65, 192]. In native cells, these currents have a variety of names, such as Ih, Iq and If. The four known HCN channels have six transmembrane domains and form tetramers. It is believed that the channels can form heteromers with each other, as has been shown for HCN1 and HCN4 [2]. High resolution structural studies of CNG and HCN channels has provided insight into the the gating processes of these channels [139, 146, 140]. A standardised nomenclature for CNG and HCN channels has been proposed by the NC-IUPHAR Subcommittee on voltage-gated ion channels [108].

Cyclic nucleotide-regulated channels (CNG) in GtoPdb v.2021.3

Cyclic nucleotide-gated (CNG) channels are responsible for signalling in the primary sensory cells of the vertebrate visual and olfactory systems. CNG channels are voltage-independent cation channels formed as tetramers. Each subunit has 6TM, with the pore-forming domain between TM5 and TM6. CNG channels were first found in rod photoreceptors [83, 120], where light signals through rhodopsin and transducin to stimulate phosphodiesterase and reduce intracellular cyclic GMP level. This results in a closure of CNG channels and a reduced ‘dark current’. Similar channels were found in the cilia of olfactory neurons [181] and the pineal gland [71]. The cyclic nucleotides bind to a domain in the C terminus of the subunit protein: other channels directly binding cyclic nucleotides include hyperolarisation-activated, cyclic nucleotide-gated channels (HCN), ether-a-go-go and certain plant potassium channels.The HCN channels are cation channels that are activated by hyperpolarisation at voltages negative to ~-50 mV. The cyclic nucleotides cyclic AMP and cyclic GMP directly bind to the cyclic nucleotide-binding domain of HCN channels and shift their activation curves to more positive voltages, thereby enhancing channel activity. HCN channels underlie pacemaker currents found in many excitable cells including cardiac cells and neurons [64, 192]. In native cells, these currents have a variety of names, such as Ih, Iq and If. The four known HCN channels have six transmembrane domains and form tetramers. It is believed that the channels can form heteromers with each other, as has been shown for HCN1 and HCN4 [2]. High resolution structural studies of CNG and HCN channels has provided insight into the the gating processes of these channels [139, 146, 140]. A standardised nomenclature for CNG and HCN channels has been proposed by the NC-IUPHAR Subcommittee on voltage-gated ion channels [108].

In silico screening of GMQ-like compounds reveals guanabenz and sephin1 as new allosteric modulators of acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) are voltage-independent cation channels that detect decreases in extracellular pH. Dysregulation of ASICs underpins a number of pathologies. Of particular interest is ASIC3, which is recognised as a key sensor of acid-induced pain and is important in the establishment of pain arising from inflammatory conditions, such as rheumatoid arthritis. Thus, the identification of new ASIC3 modulators and the mechanistic understanding of how these compounds modulate ASIC3 could be important for the development of new strategies to counteract the detrimental effects of dysregulated ASIC3 activity in inflammation. Here, we report the identification of novel ASIC3 modulators based on the ASIC3 agonist, 2-guanidine-4-methylquinazoline (GMQ). Through a GMQ-guided in silico screening of Food and Drug administration (FDA)-approved drugs, 5 compounds were selected and tested for their modulation of rat ASIC3 (rASIC3) using whole-cell patch-clamp electrophysiology. Of the chosen drugs, guanabenz (GBZ), an α2-adrenoceptor agonist, produced similar effects to GMQ on rASIC3, activating the channel at physiological pH (pH 7.4) and potentiating its response to mild acidic (pH 7) stimuli. Sephin1, a GBZ derivative that lacks α2-adrenoceptor activity, has been proposed to act as a selective inhibitor of a regulatory subunit of the stress-induced protein phosphatase 1 (PPP1R15A) with promising therapeutic potential for the treatment of multiple sclerosis. However, we found that like GBZ, sephin1 activates rASIC3 at pH 7.4 and potentiates its response to acidic stimulation (pH 7), i.e. sephin1 is a novel modulator of rASIC3. Furthermore, docking experiments showed that, like GMQ, GBZ and sephin1 likely interact with the nonproton ligand sensor domain of rASIC3. Overall, these data demonstrate the utility of computational analysis for identifying novel ASIC3 modulators, which can be validated with electrophysiological analysis and may lead to the development of better compounds for targeting ASIC3 in the treatment of inflammatory conditions.

Cyclic nucleotide-regulated channels (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

Cyclic nucleotide-gated (CNG) channels are responsible for signalling in the primary sensory cells of the vertebrate visual and olfactory systems.CNG channels are voltage-independent cation channels formed as tetramers. Each subunit has 6TM, with the pore-forming domain between TM5 and TM6. CNG channels were first found in rod photoreceptors [69, 98], where light signals through rhodopsin and transducin to stimulate phosphodiesterase and reduce intracellular cyclic GMP level. This results in a closure of CNG channels and a reduced ‘dark current’. Similar channels were found in the cilia of olfactory neurons [153] and the pineal gland [60]. The cyclic nucleotides bind to a domain in the C terminus of the subunit protein: other channels directly binding cyclic nucleotides include HCN, eag and certain plant potassium channels.Hyperpolarisation-activated, cyclic nucleotide-gated (HCN)The hyperpolarisation-activated, cyclic nucleotide-gated (HCN) channels are cation channels that are activated by hyperpolarisation at voltages negative to ~-50 mV. The cyclic nucleotides cyclic AMP and cyclic GMP directly activate the channels and shift the activation curves of HCN channels to more positive voltages, thereby enhancing channel activity. HCN channels underlie pacemaker currents found in many excitable cells including cardiac cells and neurons [56, 164]. In native cells, these currents have a variety of names, such as Ih, Iq and If. The four known HCN channels have six transmembrane domains and form tetramers. It is believed that the channels can form heteromers with each other, as has been shown for HCN1 and HCN4 [2]. High resolution structural studies of CNG and HCN channels has provided insight into the the gating processes of these channels [117, 121]. A standardised nomenclature for CNG and HCN channels has been proposed by the NC-IUPHAR subcommittee on voltage-gated ion channels [88].

Acid-sensing cation channels

In both the CNS and peripheral nervous system (PNS), energy metabolism, neuronal activity, hypoxia, hypercapnia, and inflammation elicit pH fluctuations that may affect neuronal excitability and function. The acid-sensing ion channels (ASICs) are voltage-independent cation channels that are activated by acidic pH and are expressed in neurons throughout the body. ASICs belong to the degenerin/epithelial sodium (Na+) channel (DEG/ENaC) superfamily, which includes several amiloride-sensitive cation channels. ASICs are primarily permeable to Na+ and elicit cell depolarization, leading to secondary intracellular accumulation of calcium (Ca2+). ASICs in peripheral sensory neurons participate in nociception, mechanosensation, and taste transduction. In the CNS, ASICs are involved in synaptic plasticity, learning, fear conditioning, and maintenance of retinal integrity. Studies using ASIC channel blockers or transgenic mice indicate that ASICs may be involved in several neurologic disorders, including pain, cerebral ischemia, seizures, and neurodegeneration. ASICs may also have a role in anxiety and depression. Therefore, ASICs may represent novel targets for potential treatment of these disorders. The structure, function, modulation, and clinical implications of ASICs have been the subjects of several reviews.1–10

Endothelin Receptor Signaling: New Insight Into Its Regulatory Mechanisms

The endothelin (ET) system consists of two G protein coupled–receptors (GPCRs), ET type A receptor (ETAR) and ET type B receptor (ETBR), and three endogenous ligands, ET-1, ET-2, and ET-3. Stimulation of ETRs with ET-1 induces an increase in intracellular Ca2+ concentration that is involved in a diverse array of physiological and pathophysiological processes, including vasoconstriction, and cell proliferation. Store-operated Ca2+ entry and receptor-operated Ca2+ entry triggered by activation of ETRs are regulated or modulated by endoplasmic reticulum Ca2+ sensor (stromal interaction molecule 1) and voltage-independent cation channels (transient receptor potential canonical channels and Orai1). The ET-1–induced Ca2+ mobilization results from activation of heterotrimeric G proteins by ETRs. In contrast, GPCR biology including modulation of receptor function and trafficking is regulated by a variety of GPCR interacting proteins (GIPs) that generally interact with the C-terminal domain of GPCRs. The ETR signaling is also regulated by GIPs such as Jun activation domain-binding protein 1. This review focuses on the regulatory mechanisms of the ETR signaling with special attention to the components involved in Ca2+ signaling and to GIPs in the signal transduction, modification, and degradation of ETRs.

Inhibitory Effect of Furosemide on Non-Selective Voltage-Independent Cation Channels in Human Erythrocytes

Background: Furosemide, a loop diuretic inhibiting the renal tubular Na⁺,K⁺,2Cl^- cotransporter, has been shown to decrease cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in platelets and erythrocytes. [Ca²⁺]_i in erythrocytes is a function of Ca²⁺ permeable cation channels. Activation of those channels e.g. by energy depletion or oxidative stress leads to increase of [Ca²⁺]_i, which in turn triggers eryptosis, a suicidal erythrocyte death characterized by cell membrane scrambling. The present study was performed to explore whether furosemide influences the cation channels and thus influences eryptosis. Methods: Cation channel activity was determined by whole-cell patch clamp, [Ca²⁺]_i utilizing Fluo3 fluorescence and annexin V binding to estimate cell membrane scrambling with phosphatidylserine exposure. Results: A 45 min exposure to furosemide (10 and 100 µM) slightly, but significantly decreased cation channel activity and [Ca²⁺]_i in human erythrocytes drawn from healthy individuals. ATP-depletion (> 3 hours, +37°C, 6 mM ionosine and 6 mM iodoacetic acid) enhanced the non-selective cation channel activity, increased [Ca²⁺]_i and triggered cell membrane scrambling, effects significantly blunted by furosemide (10 – 100 µM). Oxidative stress by exposure to tert-butylhydroperoxide (0.1 –1 mM) similarly enhanced the non-selective cation channels activity, increased [Ca²⁺]_i and triggered cell membrane scrambling, effects again significantly blunted by furosemide (10 – 100 µM). Conclusions: The present study shows for the first time that the loop diuretic furosemide applied at micromolar concentrations (10 – 100 µM) inhibits non-selective cation channel activity in and eryptosis of human erythrocytes.

Evaluation of potential synergistic action of a combined treatment with alpha-methyl-prednisolone and taurine on the mdx mouse model of Duchenne muscular dystrophy

Glucocorticoids are the sole drugs clinically used in Duchenne muscular dystrophy, in spite of the relevant side effects. Combination of glucocorticoids with synergistic drugs may be one strategy to lower doses and control side effects, meanwhile providing wider control of the complex pathology. This study is a preclinical evaluation of the effect of a combined treatment of α-methyl-prednisolone (PDN) with taurine, a safe aminoacid with positive effects on some pathology-related events. PDN (1 mg/kg/day i.p.) and taurine (1 g/kg/day orally) were administered either alone or in combination, for 4-8 weeks to male dystrophic mdx mice chronically exercised on a treadmill. Effects were assessed in vivo and ex vivo with a variety of methodological approaches. In vivo, each treatment significantly increased fore limb strength, a marked synergistic effect being observed with the combination PDN + taurine. Ex vivo, PDN + taurine completely restored the mechanical threshold, an electrophysiological index of calcium homeostasis, of extensor digitorum longus myofibres and the benefit was greater than for PDN alone. In parallel, the overactivity of voltage-independent cation channels in dystrophic myofibres was reduced. No effects were observed on plasma levels of creatine kinase, while lactate dehydrogenase was decreased by taurine and, to a minor extent, by PDN + taurine. A similar histology profile was observed in PDN and PDN + taurine-treated muscles. PDN + taurine significantly increased taurine level in fast-twitch muscle and brain, by high-pressure liquid chromatography analysis. The combination PDN + taurine has additive actions on in vivo and ex vivo functional end points, with less evident advantages on histopathology and biochemical markers of the disease.

Inhibition of Cation Channels in Human Erythrocytes by Spermine

In erythrocytes, spermine concentration decreases gradually with age, which is paralleled by increases of cytosolic Ca²+ concentration, with subsequent cell shrinkage and cell membrane scrambling. Cytosolic Ca²+ was estimated from fluo-3 fluorescence, cell volume from forward scatter, cell membrane scrambling from annexin V binding and cation channel activity with whole-cell patch-clamp in human erythrocytes. Extracellular spermine exerted a dual effect on erythrocyte survival. At 200 μM spermine blunted the increase of intracellular Ca²+, cell shrinkage and annexin V binding following 48 h exposure of cells at +37 °C. In contrast, short exposure (10-30 min) of cells to 2 mM spermine was accompanied by increased cytosolic Ca²+ and annexin binding. Intracellular addition of spermine at subphysiological concentration (0.2 μM) significantly decreased the conductance of monovalent cations (Na+, K+, NMDG+) and of Ca²+. Moreover, spermine (0.2 μM) blunted the stimulation of voltage-independent cation channels by Cl⁻ removal. Spermine (0.2 and 200 μM) added to the extracellular bath solution similarly inhibited the cation conductance in Cl⁻-containing bath solution. The effect of 0.2 μM spermine, but not the effect of 200 μM, was rapidly reversible. Acute addition (250 μM) of a naphthyl acetyl derivative of spermine (200 μM) again significantly decreased basal cation conductance in NaCl bath solution and inhibited voltage-independent cation channels. Spermine is a powerful regulator of erythrocyte cation channel cytosolic Ca²+ activity and, thus, cell survival.

Current perspectives on acid-sensing ion channels: new advances and therapeutic implications

Acid-sensing ion channels (ASICs) form a family of voltage-independent cation channels that predominantly conduct Na+ ions, and were identified at the molecular level a little more than a decade ago. ASICs are activated by extracellular acidification within the physiological range, and they form effective proton sensors in both central and peripheral sensory neurons. A combination of genetic and pharmacologic approaches has revealed their implication in an increasing number of physiological and pathophysiological processes – most of them associated with extracellular pH fluctuations, ranging from synaptic plasticity, learning, memory, fear, depression, seizure termination and neuronal degeneration to nociception and mechanosensation. ASICs, therefore, emerge as new potential therapeutic targets in the management of psychiatric disorders, stroke, neurodegenerative diseases and pain.

Depolarization‐activated calcium channels shape the calcium signatures induced by low‐temperature stress

Depolarization‐activated calcium channels shape the calcium signatures induced by low‐temperature stress

Dopamine Neuron Responses Depend Exponentially on Pacemaker Interval

Midbrain dopamine neuron activity results from the integration of the responses to metabo- and ionotropic receptors with the postsynaptic excitability of these intrinsic pacemakers. Interestingly, intrinsic pacemaker rate varies greatly between individual dopamine neurons and is subject to short- and long-term regulation. Here responses of substantia nigra dopamine neurons to defined dynamic-clamp stimuli were measured to quantify the impact of cell-to-cell variation in intrinsic pacemaker rate. Then this approach was repeated in single dopamine neurons in which pacemaker rate was altered by activation of muscarinic receptors or current injection. These experiments revealed a dramatic exponential dependence on pacemaker interval for the responses to voltage-gated A-type K+ channels, voltage-independent cation channels and ionotropic synapses. Likewise, responses to native metabotropic (GABAb and mGluR1) inhibitory synapses depended steeply on pacemaker interval. These results show that observed variations in dopamine neuron pacemaker rate are functionally significant because they produce a >10-fold difference in responses to diverse stimuli. Both the magnitude and the mathematical form of the relationship between pacemaker interval and responses were not previously anticipated.

Heterogeneous distribution of TRPC proteins in the embryonic cortex

The present study was initiated to gain some information about the tissue distribution of transient receptor potential proteins of C-type (TRPC), a family of voltage-independent cation channels, at the beginning of neurogenesis in the telencephalon of embryonic mice. The mRNAs of all known TRPCs (TRPC1-TRPC7) could be found in the cortex at E13. TRPC1, TRPC3 and TRPC5 were the main isoforms, whereas the mRNAs for TRPC2, TRPC4, TRPC6 and TRPC7 were less abundant. The distribution throughout the cortical wall of TRPC1, TRPC3 and TRPC6 was studied by means of immuno-histochemistry. The data collected pointed to a heterogeneous expression of the channels. Three groups were identified. The first one comprises TRPC1, specifically found in the preplate but only in some post-mitotic neurons. It was mainly observed in a subset of cells distinct from the Cajal-Retzius cells. The second group is composed of TRPC3. It was found in non-neuronal cells and also in dividing (5-bromo-2'-deoxyuridine-positive) cells, indicating that TRPC3 is present in precursor cells. The third group contains TRPC6 detected in neuronal and in dividing non-neuronal cells. Double immunostaining experiments showed that TRPC3-positive cells also express TRPC6. Collectively, this report highlights a specific TRPC expression pattern in the immature cortical wall.

Effects of Ca 2+, K + and cGMP on Na + uptake in pepper plants

The effects of Ca 2+, K + and cGMP on Na + influx in pepper plants ( Capsicum annum L.) were studied. Calcium and K + inhibited Na + net uptake and had no effect on Na + efflux. The inhibition of Na + uptake by Ca 2+ showed a K i of 0.46 mM and micromolar concentrations of the membrane permeable cyclic nucleotide 8-bromo-cGMP inhibited Na + uptake. These results suggest that transporters with low discrimination between K + and Na +, sensitive to Ca 2+ and to cyclic nucleotides, may constitute important pathways for Na + influx. As suggested for other plant species, non-selective voltage-independent cation channels may mediate a large amount of Na + influx into pepper plants.

Crosstalk between voltage-independent Ca2+ channels and L-type Ca2+ channels in A7r5 vascular smooth muscle cells at elevated intracellular pH: evidence for functional coupling between L-type Ca2+ channels and a 2-APB-sensitive cation channel.

This study was designed to investigate the role of voltage-independent and voltage-dependent Ca2+ channels in the Ca2+ signaling associated with intracellular alkalinization in A7r5 vascular smooth muscle cells. Extracellular administration of ammonium chloride (20 mmol/L) resulted in elevation of intracellular pH and activation of a sustained Ca2+ entry that was inhibited by 2-amino-ethoxydiphenyl borate (2-APB, 200 micromol/L) but not by verapamil (10 micro;mol/L). Alkalosis-induced Ca2+ entry was mediated by a voltage-independent cation conductance that allowed permeation of Ca2+ (PCa/PNa approximately 6), and was associated with inhibition of L-type Ca2+ currents. Alkalosis-induced inhibition of L-type Ca2+ currents was dependent on the presence of extracellular Ca2+ and was prevented by expression of a dominant-negative mutant of calmodulin. In the absence of extracellular Ca2+, with Ba2+ or Na+ as charge carrier, intracellular alkalosis failed to inhibit but potentiated L-type Ca2+ channel currents. Inhibition of Ca2+ currents through voltage-independent cation channels by 2-APB prevented alkalosis-induced inhibition of L-type Ca2+ currents. Similarly, 2-APB prevented vasopressin-induced activation of nonselective cation channels and inhibition of L-type Ca2+ currents. We suggest the existence of a pH-controlled Ca2+ entry pathway that governs the activity of smooth muscle L-type Ca2+ channels due to control of Ca2+/calmodulin-dependent negative feedback regulation. This Ca2+ entry pathway exhibits striking similarity with the pathway activated by stimulation of phospholipase-C-coupled receptors, and may involve a similar type of cation channel. We demonstrate for the first time the tight functional coupling between these voltage-independent Ca2+ channels and classical voltage-gated L-type Ca2+ channels.

Maitotoxin induces insertion of different ion channels into the Xenopus oocyte plasma membrane via Ca(2+)-stimulated exocytosis.

The activation of cation channels in oocytes of Xenopus laevis by the marine poison maitotoxin (MTX) was monitored as membrane current (I(m)), conductance (Gm) and membrane surface area determined by continuous measurements of membrane capacitance (Cm). When MTX (25 pM) was added to the bathing solution there was an abrupt, large increase in inward membrane currents. Current/voltage relationships (I/V curves) were linear and suggested activation of voltage-independent non-selective cation channels (NSCC). MTX-induced Ca(2+)-sensitive currents were mainly carried by Na+ and were suppressed by low (0 mM) or high (40 mM) external Ca2+ concentrations and removal of Na+. Gadolinium (Gd3+, 10-500 microM) also had inhibitory effects, demonstrating the possible involvement of stretch-activated cation channels (SACC). In a high concentration (500 microM), amiloride substantially reduced the MTX-activated current while lower amiloride concentrations (50-100 microM) stimulated the current further. Continuous measurements of Cm revealed that MTX induced exocytotic delivery and functional insertion of new channel proteins into the plasma membrane, indicated by a Ca(2+)-dependent increase in membrane surface area by around 28%. From these data we conclude that MTX activates NSCC that require relatively high concentrations of amiloride to be blocked. Furthermore, MTX possibly stimulates activation of Gd(3+)- and Ca(2+)-sensitive mechanosensitive cation channels. Stimulation of these channels is achieved by exocytotic delivery and functional insertion of new channels into the plasma membrane in a pathway that depends on the presence of extracellular Ca2+.