Expression of K2P channels in sensory and motor neurons of the autonomic nervous system

Mechanical activation of vagal afferents involves opposing cation and TREK1 currents and NO regulation

Vagal afferents convey signals of mechanical stimulation in the gut to the brain, which is essential for the regulation of food intake. However, ion channels sensing mechanical stimuli are not fully understood. This study aimed to examine the ionic currents activated by mechanical stimulation and a possible neuro-modulatory role of nitric oxide on vagal afferents. Nodose neuronal currents and potentials, and intestinal afferent firing by mechanical stimulation were measured by whole-cell patch clamp, and in vitro afferent recording, respectively. Osmotically activated cation and two-pore domain K+ currents were identified in nodose neurons. The membrane potential displayed a biphasic change under hypotonic stimulation. Cation channel-mediated depolarization was followed by a hyperpolarization mediated by K+ channels. The latter was inhibited by l-methionine (TREK1 channel inhibitor) and l-NNA (nitric oxide synthase inhibitor). Correspondingly, mechanical stimulation activated opposing cation and TREK1 currents. NOS inhibition decreased TREK1 currents and potentiated jejunal afferent nerve firing induced by mechanical stimuli. This study suggested a novel activation mechanism of ion channels underlying adaptation under mechanical distension in vagal afferent neurons. The guts' ability to perceive mechanical stimuli is vital in determining how it responds to food intake. The mechanosensation through ion channels could initiate and control gut function.

Antiarrhythmic calcium channel blocker verapamil inhibits trek currents in sympathetic neurons

Background and Purpose: Verapamil, a drug widely used in certain cardiac pathologies, exert its therapeutic effect mainly through the blockade of cardiac L-type calcium channels. However, we also know that both voltage-dependent and certain potassium channels are blocked by verapamil. Because sympathetic neurons of the superior cervical ganglion (SCG) are known to express a good variety of potassium currents, and to finely tune cardiac activity, we speculated that the effect of verapamil on these SCG potassium channels could explain part of the therapeutic action of this drug. To address this question, we decided to study, the effects of verapamil on three different potassium currents observed in SCG neurons: delayed rectifier, A-type and TREK (a subfamily of K2P channels) currents. We also investigated the effect of verapamil on the electrical behavior of sympathetic SCG neurons.

Experimental Approach: We employed the Patch-Clamp technique to mouse SCG neurons in culture.

Key Results: We found that verapamil depolarizes of the resting membrane potential of SCG neurons. Moreover, we demonstrated that this drug also inhibits A-type potassium currents. Finally, and most importantly, we revealed that the current driven through TREK channels is also inhibited in the presence of verapamil.

Conclusion and Implications: We have shown that verapamil causes a clear alteration of excitability in sympathetic nerve cells. This fact undoubtedly leads to an alteration of the sympathetic-parasympathetic balance which may affect cardiac function. Therefore, we propose that these possible peripheral alterations in the autonomic system should be taken into consideration in the prescription of this drug.

Are TREK Channels Temperature Sensors?

Internal human body normal temperature fluctuates between 36.5 and 37.5°C and it is generally measured in the oral cavity. Interestingly, most electrophysiological studies on the functioning of ion channels and their role in neuronal behavior are carried out at room temperature, which usually oscillates between 22 and 24°C, even when thermosensitive channels are studied. We very often forget that if the core of the body reached that temperature, the probability of death from cardiorespiratory arrest would be extremely high. Does this mean that we are studying ion channels in dying neurons? Thousands of electrophysiological experiments carried out at these low temperatures suggest that most neurons tolerate this aggression quite well, at least for the duration of the experiments. This also seems to happen with ion channels, although studies at different temperatures indicate large changes in both, neuron and channel behavior. It is known that many chemical, physical and therefore physiological processes, depend to a great extent on body temperature. Temperature clearly affects the kinetics of numerous events such as chemical reactions or conformational changes in proteins but, what if these proteins constitute ion channels and these channels are specifically designed to detect changes in temperature? In this review, we discuss the importance of the potassium channels of the TREK subfamily, belonging to the recently discovered family of two-pore domain channels, in the transduction of thermal sensitivity in different cell types.

Holdemanella biformis improves glucose tolerance and regulates GLP-1 signaling in obese mice

Impaired glucose homeostasis in obesity is mitigated by enhancing the glucoregulatory actions of glucagon-like peptide 1 (GLP-1), and thus, strategies that improve GLP-1 sensitivity and secretion have therapeutic potential for the treatment of type 2 diabetes. This study shows that Holdemanella biformis, isolated from the feces of a metabolically healthy volunteer, ameliorates hyperglycemia, improves oral glucose tolerance and restores gluconeogenesis and insulin signaling in the liver of obese mice. These effects were associated with the ability of H. biformis to restore GLP-1 levels, enhancing GLP-1 neural signaling in the proximal and distal small intestine and GLP-1 sensitivity of vagal sensory neurons, and to modify the cecal abundance of unsaturated fatty acids and the bacterial species associated with metabolic health. Our findings overall suggest the potential use of H biformis in the management of type 2 diabetes in obesity to optimize the sensitivity and function of the GLP-1 system, through direct and indirect mechanisms.

Contribution of K2P Potassium Channels to Cardiac Physiology and Pathophysiology

Years before the first two-pore domain potassium channel (K2P) was cloned, certain ion channels had already been demonstrated to be present in the heart with characteristics and properties usually attributed to the TREK channels (a subfamily of K2P channels). K2P channels were later detected in cardiac tissue by RT-PCR, although the distribution of the different K2P subfamilies in the heart seems to depend on the species analyzed. In order to collect relevant information in this regard, we focus here on the TWIK, TASK and TREK cardiac channels, their putative roles in cardiac physiology and their implication in coronary pathologies. Most of the RNA expression data and electrophysiological recordings available to date support the presence of these different K2P subfamilies in distinct cardiac cells. Likewise, we show how these channels may be involved in certain pathologies, such as atrial fibrillation, long QT syndrome and Brugada syndrome.

KCNK18 Biallelic Variants Associated with Intellectual Disability and Neurodevelopmental Disorders Alter TRESK Channel Activity

The TWIK-related spinal cord potassium channel (TRESK) is encoded by KCNK18, and variants in this gene have previously been associated with susceptibility to familial migraine with aura (MIM #613656). A single amino acid substitution in the same protein, p.Trp101Arg, has also been associated with intellectual disability (ID), opening the possibility that variants in this gene might be involved in different disorders. Here, we report the identification of KCNK18 biallelic missense variants (p.Tyr163Asp and p.Ser252Leu) in a family characterized by three siblings affected by mild-to-moderate ID, autism spectrum disorder (ASD) and other neurodevelopment-related features. Functional characterization of the variants alone or in combination showed impaired channel activity. Interestingly, Ser252 is an important regulatory site of TRESK, suggesting that alteration of this residue could lead to additive downstream effects. The functional relevance of these mutations and the observed co-segregation in all the affected members of the family expand the clinical variability associated with altered TRESK function and provide further insight into the relationship between altered function of this ion channel and human disease.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Different sensitivity of action potential generation to the rate of depolarization in vagal afferent A-fiber versus C-fiber neurons

This study demonstrates that the action potential discharge in vagal afferent A-fiber neurons is about 20 times more sensitive to the rate of membrane depolarization compared to C-fiber neurons. The sensitivity of action potential generation to the depolarization rate in vagal sensory neurons is independent of the intensity of current stimuli but nearly abrogated by inhibiting the D-type potassium channel. These findings help better understand the mechanisms that control the activation of vagal afferent nerves.

Contribution of KCNQ and TREK Channels to the Resting Membrane Potential in Sympathetic Neurons at Physiological Temperature

The ionic mechanisms controlling the resting membrane potential (RMP) in superior cervical ganglion (SCG) neurons have been widely studied and the M-current (IM, KCNQ) is one of the key players. Recently, with the discovery of the presence of functional TREK-2 (TWIK-related K+ channel 2) channels in SCG neurons, another potential main contributor for setting the value of the resting membrane potential has appeared. In the present work, we quantified the contribution of TREK-2 channels to the resting membrane potential at physiological temperature and studied its role in excitability using patch-clamp techniques. In the process we have discovered that TREK-2 channels are sensitive to the classic M-current blockers linopirdine and XE991 (IC50 = 0.310 ± 0.06 µM and 0.044 ± 0.013 µM, respectively). An increase from room temperature (23 °C) to physiological temperature (37 °C) enhanced both IM and TREK-2 currents. Likewise, inhibition of IM by tetraethylammonium (TEA) and TREK-2 current by XE991 depolarized the RMP at room and physiological temperatures. Temperature rise also enhanced adaptation in SCG neurons which was reduced due to TREK-2 and IM inhibition by XE991 application. In summary, TREK-2 and M currents contribute to the resting membrane potential and excitability at room and physiological temperature in the primary culture of mouse SCG neurons.

The Background K+ Channel TRESK in Sensory Physiology and Pain

TRESK belongs to the K_2P family of potassium channels, also known as background or leak potassium channels due to their biophysical properties and their role regulating membrane potential of cells. Several studies to date have highlighted the role of TRESK in regulating the excitability of specific subtypes of sensory neurons. These findings suggest TRESK could be involved in pain sensitivity. Here, we review the different evidence available that involves the channel in pain and sensory perception, from studies knocking out the channel or overexpressing it to identified mutations that link the channel to migraine pain. In addition, the therapeutic possibilities are discussed, as targeting the channel seems an interesting therapeutic approach to reduce nociceptor activation and to decrease pain.

PIP2 Mediated Inhibition of TREK Potassium Currents by Bradykinin in Mouse Sympathetic Neurons

Bradykinin (BK), a hormone inducing pain and inflammation, is known to inhibit potassium M-currents (I_M) and to increase the excitability of the superior cervical ganglion (SCG) neurons by activating the Ca²⁺-calmodulin pathway. M-current is also reduced by muscarinic agonists through the depletion of membrane phosphatidylinositol 4,5-biphosphate (PIP₂). Similarly, the activation of muscarinic receptors inhibits the current through two-pore domain potassium channels (K2P) of the “Tandem of pore-domains in a Weakly Inward rectifying K⁺ channel (TWIK)-related channels” (TREK) subfamily by reducing PIP₂ in mouse SCG neurons (mSCG). The aim of this work was to test and characterize the modulation of TREK channels by bradykinin. We used the perforated-patch technique to investigate riluzole (RIL) activated currents in voltage- and current-clamp experiments. RIL is a drug used in the palliative treatment of amyotrophic lateral sclerosis and, in addition to blocking voltage-dependent sodium channels, it also selectively activates the K2P channels of the TREK subfamily. A cell-attached patch-clamp was also used to investigate TREK-2 single channel currents. We report here that BK reduces spike frequency adaptation (SFA), inhibits the riluzole-activated current (I_RIL), which flows mainly through TREK-2 channels, by about 45%, and reduces the open probability of identified single TREK-2 channels in cultured mSCG cells. The effect of BK on I_RIL was precluded by the bradykinin receptor (B₂R) antagonist HOE-140 (d-Arg-[Hyp³, Thi⁵, d-Tic⁷, Oic⁸]BK) but also by diC₈PIP₂ which prevents PIP₂ depletion when phospholipase C (PLC) is activated. On the contrary, antagonizing inositol triphosphate receptors (IP₃R) using 2-aminoethoxydiphenylborane (2-APB) or inhibiting protein kinase C (PKC) with bisindolylmaleimide did not affect the inhibition of I_RIL by BK. In conclusion, bradykinin inhibits TREK-2 channels through the activation of B₂Rs resulting in PIP₂ depletion, much like we have demonstrated for muscarinic agonists. This mechanism implies that TREK channels must be relevant for the capture of information about pain and visceral inflammation.

High-fat diet-induced vagal afferent dysfunction via upregulation of 2-pore domain potassium TRESK channel

Research shows that rats and humans on a high-fat diet (HFD) are less sensitive to satiety signals known to act via vagal afferent pathways. We hypothesize that HFD causes an upregulation of 2-pore domain potassium channels, resulting in hyperpolarization of nodose ganglia (NG) and decreased vagal response to satiety signals, which contribute to hyperphagia. We show that a 2-week HFD caused an upregulation of 2-pore domain TWIK-related spinal cord K+ (TRESK) and TWIK-related acid-sensitive K+ 1 (TASK1) channels by 330% ± 50% and 60% ± 20%, respectively, in NG. Patch-clamp studies of isolated NG neurons demonstrated a decrease in excitability. In vivo single-unit NG recordings showed that a 2-week HFD led to a 55% reduction in firing frequency in response to CCK-8 or leptin stimulation. NG electroporation with TRESK siRNA restored NG responsiveness to CCK-8 and leptin. Rats fed a 2-week HFD consumed ~40% more calories compared with controls. Silencing NG TRESK but not TASK1 channel expression in HFD-fed rats restored normal calorie consumption. In conclusion, HFD caused upregulation of TRESK channels, resulting in NG hyperpolarization and decreased vagal responsiveness to satiety signals. This finding provides a pharmacological target to prevent or treat HFD-induced hyperphagia.

Ion Channels and Thermosensitivity: TRP, TREK, or Both?

Controlling body temperature is a matter of life or death for most animals, and in mammals the complex thermoregulatory system is comprised of thermoreceptors, thermosensors, and effectors. The activity of thermoreceptors and thermoeffectors has been studied for many years, yet only recently have we begun to obtain a clear picture of the thermosensors and the molecular mechanisms involved in thermosensory reception. An important step in this direction was the discovery of the thermosensitive transient receptor potential (TRP) cationic channels, some of which are activated by increases in temperature and others by a drop in temperature, potentially converting the cells in which they are expressed into heat and cold receptors. More recently, the TWIK-related potassium (TREK) channels were seen to be strongly activated by increases in temperature. Hence, in this review we want to assess the hypothesis that both these groups of channels can collaborate, possibly along with other channels, to generate the wide range of thermal sensations that the nervous system is capable of handling.

Tandem pore TWIK-related potassium channels and neuroprotection

TWIK-related potassium channels (TREK) belong to a subfamily of the two-pore domain potassium channels family with three members, TREK1, TREK2 and TWIK-related arachidonic acid-activated potassium channels. The two-pore domain potassium channels is the last big family of channels being discovered, therefore it is not surprising that most of the information we know about TREK channels predominantly comes from the study of heterologously expressed channels. Notwithstanding, in this review we pay special attention to the limited amount of information available on native TREK-like channels and real neurons in relation to neuroprotection. Mainly we focus on the role of free fatty acids, lysophospholipids and other neuroprotective agents like riluzole in the modulation of TREK channels, emphasizing on how important this modulation may be for the development of new therapies against neuropathic pain, depression, schizophrenia, epilepsy, ischemia and cardiac complications.

TREK-1 channels regulate pressure sensitivity and calcium signaling in trabecular meshwork cells

The trabecular meshwork (TM) plays a fundamental role in intraocular pressure regulation, but its mechanotransduction pathway is poorly understood. Yarishkin et al. show that the mechanosensing channel TREK-1 regulates TM membrane potential, pressure sensitivity, calcium homeostasis, and impedance.

Mechanotransduction by the trabecular meshwork (TM) is an essential component of intraocular pressure regulation in the vertebrate eye. This process is compromised in glaucoma but is poorly understood. In this study, we identify transient receptor potential vanilloid isoform 4 (TRPV4) and TWIK-related potassium channel-1 (TREK-1) as key molecular determinants of TM membrane potential, pressure sensitivity, calcium homeostasis, and transcellular permeability. We show that resting membrane potential in human TM cells is unaffected by “classical” inhibitors of voltage-activated, calcium-activated, and inwardly rectifying potassium channels but is depolarized by blockers of tandem-pore K⁺ channels. Using gene profiling, we reveal the presence of TREK-1, TASK-1, TWIK-2, and THIK transcripts in TM cells. Pressure stimuli, arachidonic acid, and TREK-1 activators hyperpolarize these cells, effects that are antagonized by quinine, amlodipine, spadin, and short-hairpin RNA–mediated knockdown of TREK-1 but not TASK-1. Activation and inhibition of TREK-1 modulates [Ca²⁺]_TM and lowers the impedance of cell monolayers. Together, these results suggest that tensile homeostasis in the TM may be regulated by balanced, pressure-dependent activation of TRPV4 and TREK-1 mechanotransducers.

Increased TASK channel-mediated currents underlie high-fat diet induced vagal afferent dysfunction

We have previously demonstrated that satiety sensing vagal afferent neurons are less responsive to meal-related stimuli in obesity because of reduced electrical excitability. As leak K⁺ currents are key determinants of membrane excitability, we hypothesized that leak K⁺ currents are increased in vagal afferents during obesity. Diet-induced obesity was induced by feeding C57Bl/6J mice a high-fat diet (HFF) (60% energy from fat) for 8-10 wk. In vitro extracellular recordings were performed on jejunal afferent nerves. Whole cell patch-clamp recordings were performed on mouse nodose ganglion neurons. Leak K⁺ currents were isolated using ion substitution and pharmacological blockers. mRNA for TWIK-related acid-sensitive K⁺ (TASK) subunits was measured using quantitative real-time PCR. Intestinal afferent responses to nutrient (oleate) and non-nutrient (ATP) stimuli were significantly decreased in HFF mice. Voltage clamp experiments revealed the presence of a voltage-insensitive resting potassium conductance that was increased by external alkaline pH and halothane, known properties of TASK currents. In HFF neurons, leak K⁺ current was approximately doubled and was reduced by TASK1 and TASK3 inhibitors. The halothane sensitive current was similarly increased. Quantitative PCR revealed the presence of mRNA encoding TASK1 (KCNK3) and TASK3 (KCNK9) channels in nodose neurons. TASK3 transcript was significantly increased in HFF mice. The reduction in vagal afferent excitability in obesity is due in part to an increase of resting (leak) K⁺ conductance. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a therapeutic target for obesity treatment. NEW & NOTEWORTHY This study characterized the electrophysiological properties and gene expression of the TWIK-related acid-sensitive K⁺ (TASK) channel in vagal afferent neurons. TASK conductance was increased and contributed to decreased excitability in diet-induced obesity. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a promising therapeutic target.

Effect of serum electrolytes within normal ranges on QTc prolongation: a cross-sectional study in a Chinese rural general population

Background

Many previous clinical studies have reported that prolongation of the QT interval corrected for heart rate (QTc) is associated with an increased risk of sudden cardiac death and all-cause mortality. This study aimed to explore associations between serum electrolytes and QTc prolongation in the north-eastern Chinese rural general population.

Methods

We performed a cross-sectional study including 10,334 (4820 men and 5514 women) from the general population aged ≥35 years in the Liaoning Province from 2012 to 2013. Anthropometric measurements, laboratory examinations and self-reported lifestyle factor information, echocardiography and electrocardiogram were collected by trained personnel. The associations between serum electrolytes and QTc prolongation were tested using multiple linear regression and logistic regression analyses.

Results

The mean QTc interval were 415.6 ± 18.8 and 470.1 ± 23.1 ms in normal group and QTc prolongation group respectively. The prevalence of QTc prolongation increased significantly with a decrease in serum potassium and an increase in magnesium. Stepwise multiple linear regression showed that age, hypertension, waist circumference were prominently positive associated with QTc interval both in male and female population. But serum potassium was significantly inversely associated with QTc interval. Serum magnesium and calcium also showed a positive relationship with QTc interval. Furthermore, multiple logistic regression found that lower quartile of serum potassium had higher risk for QTc prolongation, especially in female population (Q2 vs. Q4: OR: 1.54, 95%CI: 1.01–2.35; Q1 vs. Q4: OR: 2.02, 95%CI: 1.36–3.01). In addition, the higher serum magnesium increased the risk of QTc prolongation, which was significantly only in male population.

Conclusions

In present Chinese rural general population, even with normal range, a decrease in serum potassium and an increase in serum magnesium are important risk factors for QTc prolongation.

Electronic supplementary material

The online version of this article (10.1186/s12872-018-0906-1) contains supplementary material, which is available to authorized users.

Verapamil Inhibits TRESK (K2P18.1) Current in Trigeminal Ganglion Neurons Independently of the Blockade of Ca2+ Influx

Tandem pore domain weak inward rectifier potassium channel (TWIK)-related spinal cord K⁺ (TRESK; K_2P18.1) channel is the only member of the two-pore domain K⁺ (K_2P) channel family that is activated by an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and linked to migraines. This study was performed to identify the effect of verapamil, which is an L-type Ca²⁺ channel blocker and a prophylaxis for migraines, on the TRESK channel in trigeminal ganglion (TG) neurons, as well as in a heterologous system. Single-channel and whole-cell currents were recorded in TG neurons and HEK-293 cells transfected with mTRESK using patch-clamping techniques. In TG neurons, changes in [Ca²⁺]_i were measured using the fluo-3-AM Ca²⁺ indicator. Verapamil, nifedipine, and NiCl₂ inhibited the whole-cell currents in HEK-293 cells overexpressing mTRESK with IC₅₀ values of 5.2, 54.3, and >100 μM, respectively. The inhibitory effect of verapamil on TRESK channel was also observed in excised patches. In TG neurons, verapamil (10 μM) inhibited TRESK channel activity by approximately 76%. The TRESK channel activity was not dependent on the presence of extracellular Ca²⁺. In addition, the inhibitory effect of verapamil on the TRESK channel remained despite the absence of extracellular Ca²⁺. These findings show that verapamil inhibits the TRESK current independently of the blockade of Ca²⁺ influx in TG neurons. Verapamil will be able to exert its pharmacological effects by modulating TRESK, as well as Ca²⁺ influx, in TG neurons in vitro. We suggest that verapamil could be used as an inhibitor for identifying TRESK channel in TG neurons.

Activation of TREK currents by riluzole in three subgroups of cultured mouse nodose ganglion neurons

Two-pore domain potassium channels (K2P) constitute major candidates for the regulation of background potassium currents in mammalian cells. Channels of the TREK subfamily are also well positioned to play an important role in sensory transduction due to their sensitivity to a large number of physiological and physical stimuli (pH, mechanical, temperature). Following our previous report describing the molecular expression of different K2P channels in the vagal sensory system, here we confirm that TREK channels are functionally expressed in neurons from the mouse nodose ganglion (mNG). Neurons were subdivided into three groups (A, Ah and C) based on their response to tetrodotoxin and capsaicin. Application of the TREK subfamily activator riluzole to isolated mNG neurons evoked a concentration-dependent outward current in the majority of cells from all the three subtypes studied. Riluzole increased membrane conductance and hyperpolarized the membrane potential by approximately 10 mV when applied to resting neurons. The resting potential was similar in all three groups, but C cells were clearly less excitable and showed smaller hyperpolarization-activated currents at -100 mV and smaller sustained currents at -30 mV. Our results indicate that the TREK subfamily of K2P channels might play an important role in the maintenance of the resting membrane potential in sensory neurons of the autonomic nervous system, suggesting its participation in the modulation of vagal reflexes.

The antioxidant effects of riluzole on the APRE-19 celll model injury-induced by t-BHP

Background

Age-related macular degeneration (AMD) causes the dysfunction of the retinal pigment epithelial (RPE) cells. In this study, we examined the effects of riluzole, a sustained activator of the TRAAK potassium channel, on human RPE (ARPE-19) cells in an oxidant-induced cell-injury model and elucidate the mechanism of riluzole on RPE cell apoptosis.

Methods

The follow four groups of ARPE-19 cells were treated with riluzole and/or tert-butyl hydroperoxide (t-BHP) for 24.0 h: control, t-BHP, riluzole, and t-BHP + riluzole. Cell apoptosis was measured by flow cytometry, and Western blotting was performed to analyze the expression of the weakly inward rectifying potassium (TRAAK) channel. Finally, the mitochondrial membrane potential (Δψm) was detected by flow cytometry, and cytochrome C (Cyt-c) release was assessed by Western blotting.

Results

The viability of the cells in the cotreated group was significantly higher (85.6 ± 3.1%) than that in the t-BHP group (66.2 ± 2.5%). In addition, the cells in the cotreated group had a higher effect on increasing the expression of TRAAK than the t-BHP group. The results also showed that Cyt-c translocation significantly decreased and Δψm increased in the cotreated group.

Conclusions

These results demonstrate that riluzole protects RPE cells from apoptosis. The protection mechanism of riluzole could be from stabilizing mitochondrial Δψm and preventing the release of Cyt-c. Changes in TRAAK expression might also contribute to the protection of RPE cells.

Two-pore Domain Potassium Channels in Astrocytes

Two-pore domain potassium (K_2P) channels have a distinct structure and channel properties, and are involved in a background K⁺ current. The 15 members of the K_2P channels are identified and classified into six subfamilies on the basis of their sequence similarities. The activity of the channels is dynamically regulated by various physical, chemical, and biological effectors. The channels are expressed in a wide variety of tissues in mammals in an isoform specific manner, and play various roles in many physiological and pathophysiological conditions. To function as channels, the K_2P channels form dimers, and some isoforms form heterodimers that provide diversity in channel properties. In the brain, TWIK1, TREK1, TREK2, TRAAK, TASK1, and TASK3 are predominantly expressed in various regions, including the cerebral cortex, dentate gyrus, CA1-CA3, and granular layer of the cerebellum. TWIK1, TREK1, and TASK1 are highly expressed in astrocytes, where they play specific cellular roles. Astrocytes keep leak K⁺ conductance, called the passive conductance, which mainly involves TWIK1-TREK1 heterodimeric channel. TWIK1 and TREK1 also mediate glutamate release from astrocytes in an exocytosis-independent manner. The expression of TREK1 and TREK2 in astrocytes increases under ischemic conditions, that enhance neuroprotection from ischemia. Accumulated evidence has indicated that astrocytes, together with neurons, are involved in brain function, with the K_2P channels playing critical role in these astrocytes.

Formation of Functional Heterodimers by TREK-1 and TREK-2 Two-pore Domain Potassium Channel Subunits

Two-pore domain (K2P) potassium channels are the major molecular correlates of the background (leak) K(+) current in a wide variety of cell types. They generally play a key role in setting the resting membrane potential and regulate the response of excitable cells to various stimuli. K2P channels usually function as homodimers, and only a few examples of heteromerization have been previously reported. Expression of the TREK (TWIK-related K(+) channel) subfamily members of K2P channels often overlaps in neurons and in other excitable cells. Here, we demonstrate that heterologous coexpression of TREK-1 and TREK-2 subunits results in the formation of functional heterodimers. Taking advantage of a tandem construct (in which the two different subunits were linked together to enforce heterodimerization), we characterized the biophysical and pharmacological properties of the TREK-1/TREK-2 current. The heteromer was inhibited by extracellular acidification and by spadin similarly to TREK-1, and its ruthenium red sensitivity was intermediate between TREK-1 and TREK-2 homodimers. The heterodimer has also been distinguished from the homodimers by its unique single channel conductance. Assembly of the two different subunits was confirmed by coimmunoprecipitation of epitope-tagged TREK-1 and TREK-2 subunits, coexpressed in Xenopus oocytes. Formation of TREK-1/TREK-2 channels was also demonstrated in native dorsal root ganglion neurons indicating that heterodimerization may provide greater diversity of leak K(+) conductances also in native tissues.

KCNK18 (TRESK) genetic variants in Italian patients with migraine

OBJECTIVES

To evaluate the prevalence of KCNK18 gene mutations in a dataset of Italian migraineurs, with and without aura, and in healthy controls, and to investigate in silico the functional effects of the mutations.

BACKGROUND

A role for the KCNK18 gene encoding for TRESK, a member of the family of potassium channel, has been recently suggested in migraine with aura.

METHODS

We sequenced the KCNK18 gene in 425 migraineurs (255 with aura and 170 without aura) and 247 healthy controls.

RESULTS

Five genetic variants (R10G, C110R, Y163Y, S231P, and F372L) were found in 13 (5.1%) out of 255 migraine with aura patients, and 6 variants (R10G, D46D, C110R, Y163Y, S178T, and S231P) were identified in 12 (7.1%) out of 170 migraine without aura patients. In 2.8% of controls, the R10G and L20V substitutions were found. In silico analysis suggested that C110R, S178T, S231P, and F372L mutations may have potential damaging effect on channel function, whereas the remaining mutations may have low damaging effect.

CONCLUSIONS

Our study shows the presence of several KCNK18 gene mutations in both migraine with aura and migraine without aura. However, the precise role of this gene in migraine predisposition deserves further studies.

Muscarinic modulation of TREK currents in mouse sympathetic superior cervical ganglion neurons

Muscarinic receptors play a key role in the control of neurotransmission in the autonomic ganglia, which has mainly been ascribed to the regulation of potassium M-currents and voltage-dependent calcium currents. Muscarinic agonists provoke depolarization of the membrane potential and a reduction in spike frequency adaptation in postganglionic neurons, effects that may be explained by M-current inhibition. Here, we report the presence of a riluzole-activated current (IRIL ) that flows through the TREK-2 channels, and that is also inhibited by muscarinic agonists in neurons of the mouse superior cervical ganglion (mSCG). The muscarinic agonist oxotremorine-M (Oxo-M) inhibited the IRIL by 50%, an effect that was abolished by pretreatment with atropine or pirenzepine, but was unaffected in the presence of himbacine. Moreover, these antagonists had similar effects on single-channel TREK-2 currents. IRIL inhibition was unaffected by pretreatment with pertussis toxin. The protein kinase C blocker bisindolylmaleimide did not have an effect, and neither did the inositol triphosphate antagonist 2-aminoethoxydiphenylborane. Nevertheless, the IRIL was markedly attenuated by the phospholipase C (PLC) inhibitor ET-18-OCH3. Finally, the phosphatidylinositol-3-kinase/phosphatidylinositol-4-kinase inhibitor wortmannin strongly attenuated the IRIL , whereas blocking phosphatidylinositol 4,5-bisphosphate (PIP2 ) depletion consistently prevented IRIL inhibition by Oxo-M. These results demonstrate that TREK-2 currents in mSCG neurons are inhibited by muscarinic agonists that activate M1 muscarinic receptors, reducing PIP2 levels via a PLC-dependent pathway. The similarities between the signaling pathways regulating the IRIL and the M-current in the same neurons reflect an important role of this new pathway in the control of autonomic ganglia excitability.

Properties, regulation, pharmacology, and functions of the K₂p channel, TRESK

TWIK-related spinal cord K(+) channel (TRESK) is the gene product of KCNK18, the last discovered leak potassium K2P channel gene. Under resting conditions, TRESK is constitutively phosphorylated at two regulatory regions. Protein kinase A (PKA) and microtubule affinity-regulating (MARK) kinases can be applied in experiments to phosphorylate these sites of TRESK expressed in Xenopus oocytes, respectively. Upon generation of a calcium signal, TRESK is dephosphorylated and thereby activated by calcineurin. In this process, the binding of calcineurin to the channel by non-catalytic interacting sites is essential. The phosphorylation/dephosphorylation regulatory process is modified by 14-3-3 proteins. Human, but not murine TRESK is also activated by protein kinase C. TRESK is expressed most abundantly in sensory neurons of the dorsal root ganglia (DRG) and trigeminal ganglia, and the channel modifies certain forms of nociceptive afferentation. In a large pedigree, a dominant negative mutant TRESK allele was found to co-segregate perfectly with migraine phenotype. While this genetic defect may be responsible only for a very small fraction of migraine cases, specific TRESK activation is expected to exert beneficial effect in common forms of the disease.

Tubulin binds to the cytoplasmic loop of TRESK background K⁺ channel in vitro

The cytoplasmic loop between the second and third transmembrane segments is pivotal in the regulation of TRESK (TWIK-related spinal cord K+ channel, K2P18.1, KCNK18). Calcineurin binds to this region and activates the channel by dephosphorylation in response to the calcium signal. Phosphorylation-dependent anchorage of 14-3-3 adaptor protein also modulates TRESK at this location. In the present study, we identified molecular interacting partners of the intracellular loop. By an affinity chromatography approach using the cytoplasmic loop as bait, we have verified the specific association of calcineurin and 14-3-3 to the channel. In addition to these known interacting proteins, we observed substantial binding of tubulin to the intracellular loop. Successive truncation of the polypeptide and pull-down experiments from mouse brain cytosol narrowed down the region sufficient for the binding of tubulin to a 16 amino acid sequence: LVLGRLSYSIISNLDE. The first six residues of this sequence are similar to the previously reported tubulin-binding region of P2X2 purinergic receptor. The tubulin-binding site of TRESK is located close to the protein kinase A (PKA)-dependent 14-3-3-docking motif of the channel. We provide experimental evidence suggesting that 14-3-3 competes with tubulin for the binding to the cytoplasmic loop of TRESK. It is intriguing that the 16 amino acid tubulin-binding sequence includes the serines, which were previously shown to be phosphorylated by microtubule-affinity regulating kinases (MARK kinases) and contribute to channel inhibition. Although tubulin binds to TRESK in vitro, it remains to be established whether the two proteins also interact in the living cell.