Prolonged stretching of rat uteri causes hypoxia and inhibits contractility via potassium channel TREK1

In brief

During pregnancy, uterine kept quiescence along with uterine overdistention before labor. Prolonged stretching induced uterus myometrial hypoxia, increased TREK1 expression, and relaxed the myometrium, which may contribute to uterine quiescence and atony during pregnancy.

Abstract

The mechanisms underlying pre-labor uterine quiescence and uterine atony during overdistention are unclear. TREK1 (a two-pore domain potassium channel) and hypoxia-inducible factor-1α (HIF-1α) are activated by mechanical stretch, and their expression is upregulated by decreased uterine contractility. HIF-1α is a nuclear factor which regulates numerous target proteins, but whether it regulates TREK1 during the uterine stretch to cause uterine quiescence and/or atony is unclear. We investigated uterine contractility at different gestational stages in rats, as well as in non-pregnant uteri, which were induced by prolonged stretching and hypoxia. We also assessed the effects of incubating the uteri with or without echinomycin or l-methionine. Moreover, we analyzed HIF-1α and TREK1 expression levels in each group, as well as at various gestational stages of pregnant human uteri. We found that contractility was significantly decreased in pregnant uteri when compared with non-pregnant uteri, and this decrease was associated with increases in HIF-1α and TREK1 expression levels. HIF-1α and TREK1 expression levels in human uteri increased with the gestational length. Decreased uterine contractility and increased HIF-1α and TREK1 expression levels were also observed in non-pregnant rat uteri under 8 g of stretching tension or hypoxia. Inhibition of hypoxia with echinomycin restored normal uterine contractility, while HIF-1α and TREK1 protein expression remained reduced. TREK1 inhibition with l-methionine also restored uterine contractility under tension or hypoxia. In conclusion, we demonstrated that prolonged stretching induces myometrial hypoxia, increases TREK1 expression, and relaxes the myometrium, which may contribute to uterine quiescence and atony.

Bacterial lipopolysaccharide hyperpolarizes the membrane potential and is antagonized by the K2p channel blocker doxapram

Exposure of Drosophila skeletal muscle to bacterial lipopolysaccharides (LPS) rapidly and transiently hyperpolarizes membrane potential. However, the mechanism responsible for hyperpolarization remains unclear. The resting membrane potential of the cells is maintained through multiple mechanisms. This study investigated the possibility of LPS activating calcium-activated potassium channels (K_Ca) and/or K2p channels. 2-Aminoethyl diphenylborinate (2-APB), blocks uptake of Ca²⁺ into the endoplasmic reticulum (ER); thus, limiting release from ryanodine-sensitive internal stores to reduce the function of K_Ca channels. Exposure to 2-APB produces waves of hyperpolarization even during desensitization of the response to LPS and in the presence of doxapram. This finding in this study suggests that doxapram blocked the acid-sensitive K2p tandem-pore channel subtype known in mammals. Doxapram blocked LPS-induced hyperpolarization and depolarized the muscles as well as induced motor neurons to produce evoked excitatory junction potentials (EJPs). This was induced by depolarizing motor neurons, similar to the increase in extracellular K⁺ concentration. The hyperpolarizing effect of LPS was not blocked by decreased extracellular Ca²⁺or the presence of Cd²⁺. LPS appears to transiently activate doxapram sensitive K2p channels independently of K_Ca channels in hyperpolarizing the muscle. Septicemia induced by gram-negative bacteria results in an increase in inflammatory cytokines, primarily induced by bacterial LPS. Currently, blockers of LPS receptors in mammals are unknown; further research on doxapram and other K2p channels is warranted. (220 words).

Involvement of TREK-1 Channel in Cell Viability of H9c2 Rat Cardiomyoblasts Affected by Bupivacaine and Lipid Emulsion

Lipid emulsion (LE) therapy has been used to reduce overdose of bupivacaine (BPV)-induced cardiotoxicity. The TWIK-related potassium channel-1 (TREK-1) is inhibited by BPV and activated by polyunsaturated fatty acids, which are the main component in LE. These pharmacological properties inspired us to investigate whether the TREK-1 channel is associated with cell viability of H9c2 cardiomyoblasts affected by BPV and LE. Consistent with previous studies, BPV-induced cell death was reduced by LE treatment. The reduction in the TREK-1 expression level by BPV was alleviated by LE. The BPV cytotoxicity highly decreased in TREK-1 overexpressed cells but was the opposite in TREK-1 knocked-down cells. TREK-1 channel activators and inhibitors increased and decreased cell viability, respectively. BPV-induced depolarization of the plasma and mitochondrial membrane potential and increase in intracellular Ca²⁺ level were blocked by LE treatment. BPV-induced depolarization of membrane potential was reduced in TREK-1 overexpressed cells, indicating that TREK-1 channels mediate setting the resting membrane potentials as a background K⁺ channel in H9c2 cells. These results show that TREK-1 activity is involved in the BPV cytotoxicity and the antagonistic effect of LE in H9c2 cells and suggest that TREK-1 could be a target for action of BPV and LE.

[TWIK-related Acid Sensitive K + Channels in the Regulation of Respiration and Sleep]

TWIK-related acid-sensitive K ⁺ channel(TASK)is an important member of the two-pore-domain potassium channels family. It is widely expressed in the central nervous system and peripheral tissues and is extremely sensitive to hypoxia and pH changes in extracellular fluid. TASK participates in regulating the expression of respiratory center and the respiratory movement and also plays certain role in sleep regulation. This article reviews the recent advances in the roles of TASK in the regulation of respiration and sleep.

PD-L1 inhibits acute and chronic pain by suppressing nociceptive neuron activity via PD-1

Programmed cell death ligand-1 (PD-L1) is typically produced by cancer cells and suppresses immunity through the receptor PD-1 expressed on T cells. However, the role of PD-L1 and PD-1 in regulating pain and neuronal function is unclear. Here we report that both melanoma and normal neural tissues including dorsal root ganglion (DRG) produce PD-L1 that can potently inhibit acute and chronic pain. Intraplantar injection of PD-L1 evoked analgesia in naive mice via PD-1, whereas PD-L1 neutralization or PD-1 blockade induced mechanical allodynia. Mice lacking Pd1 (Pdcd1) exhibited thermal and mechanical hypersensitivity. PD-1 activation in DRG nociceptive neurons by PD-L1 induced phosphorylation of the tyrosine phosphatase SHP-1, inhibited sodium channels and caused hyperpolarization through activation of TREK2 K⁺ channels. PD-L1 also potently suppressed nociceptive neuron excitability in human DRGs. Notably, blocking PD-L1 or PD-1 elicited spontaneous pain and allodynia in melanoma-bearing mice. Our findings identify a previously unrecognized role of PD-L1 as an endogenous pain inhibitor and a neuromodulator.

TASK-1 potassium channel is not critically involved in mediating hypoxic pulmonary vasoconstriction of murine intra-pulmonary arteries

The two-pore domain potassium channel KCNK3 (TASK-1) is expressed in rat and human pulmonary artery smooth muscle cells. There, it is associated with hypoxia-induced signalling, and its dysfunction is linked to pathogenesis of human pulmonary hypertension. We here aimed to determine its role in hypoxic pulmonary vasoconstriction (HPV) in the mouse, and hence the suitability of this model for further mechanistic investigations, using appropriate inhibitors and TASK-1 knockout (KO) mice. RT-PCR revealed expression of TASK-1 mRNA in murine lungs and pre-acinar pulmonary arteries. Protein localization by immunohistochemistry and western blot was unreliable since all antibodies produced labelling also in TASK-1 KO organs/tissues. HPV was investigated by videomorphometric analysis of intra- (inner diameter: 25–40 μm) and pre-acinar pulmonary arteries (inner diameter: 41–60 μm). HPV persisted in TASK-1 KO intra-acinar arteries. Pre-acinar arteries developed initial HPV, but the response faded earlier (after 30 min) in KO vessels. This HPV pattern was grossly mimicked by the TASK-1 inhibitor anandamide in wild-type vessels. Hypoxia-provoked rise in pulmonary arterial pressure (PAP) in isolated ventilated lungs was affected neither by TASK-1 gene deficiency nor by the TASK-1 inhibitor A293. TASK-1 is dispensable for initiating HPV of murine intra-pulmonary arteries, but participates in sustained HPV specifically in pre-acinar arteries. This does not translate into abnormal rise in PAP. While there is compelling evidence that TASK-1 is involved in the pathogenesis of pulmonary arterial hypertension in humans, the mouse does not appear to serve as a suitable model to study the underlying molecular mechanisms.

Arginine Vasopressin Potentiates the Stimulatory Action of CRH on Pituitary Corticotropes via a Protein Kinase C-Dependent Reduction of the Background TREK-1 Current

The hypothalamic hormone arginine vasopressin (AVP) potentiates the stimulatory action of CRH on ACTH secretion from pituitary corticotropes, but the underlying mechanism is elusive. Using the perforated patch-clamp technique to monitor membrane potentials in mouse corticotropes, we found that AVP triggered a transient hyperpolarization that was followed by a sustained depolarization. The hyperpolarization was caused by intracellular Ca(2+) release that in turn activated the small conductance Ca(2+)-activated K(+) (SK) channels. The depolarization was due to the suppression of background TWIK-related K(+) (TREK)-1 channels. Direct activation of protein kinase C (PKC) reduced the TREK-1 current, whereas PKC inhibition attenuated the AVP-mediated reduction of the TREK-1 current, implicating the involvement of PKC. The addition of CRH (which stimulates the protein kinase A pathway) in the presence of AVP, or vice versa, resulted in further suppression of the TREK-1 current. In corticotropes with buffered cytosolic Ca(2+) concentration ([Ca(2+)]i), AVP evoked a sustained depolarization, and the coapplication of AVP and CRH caused a larger depolarization than that evoked by AVP or CRH alone. In cells with minimal perturbation of [Ca(2+)]i and background TREK-1 channels, CRH evoked a sustained depolarization that was superimposed with action potentials, and the subsequent coapplication of AVP and CRH triggered a transient hyperpolarization that was followed by a larger depolarization. In summary, AVP and CRH have additive effects on the suppression of the TREK-1 current, resulting in a more robust depolarization in corticotropes. We suggest that this mechanism contributes to the potentiating action of AVP on CRH-evoked ACTH secretion.

The molecular basis of primary aldosteronism: from chimeric gene to channelopathy

Primary aldosteronism (PA) is the most common endocrine cause of high blood pressure. Only a minority of the PA cases are familial and due to known (CYP11B2/CYP11B1 chimeric gene or mutations in the KCNJ5 gene) or unknown causes. In the most common sporadic cases the mechanisms by which the excess aldosterone production persists in spite of high blood pressure, sodium retention, suppression of the renin angiotensin system and low potassium levels, all factors that by themselves would be expected to shut off aldosterone production, were a puzzle for decades. Only recently the discovery of functional mutations and down-regulation of potassium channels provided some explanations. We herein reviewed these recent findings and their mechanistic implications. We also propose a clinical molecular classification of familial hyperaldosteronism, which can be important from the practical standpoint as it considers besides the molecular features also the responsiveness to treatment and the imaging features.

Electrophysiology and pharmacology of tandem domain potassium channel TREK-1 related BDNF synthesis in rat astrocytes

In the present study, the functional properties and pharmacology of two-pore domain potassium channel (K2P) TREK-1 in primary cultured rat brain astrocytes were investigated. Western blot, patch clamping techniques, and ELISA were used to detect the distribution and function of TREK-1 as well as the expression of brain-derived neurotrophic factor (BDNF) on the primary cultured astrocytes. It was shown that TREK-1 protein expressed in astrocytes was 2.4-fold higher than it was expressed in microglia. Single channel recording via patch clamping showed that the TREK-1 outward currents in astrocytes could be activated by arachidonic acid (AA) or chloroform with the conductance of 113 ± 14 and 120 ± 13 pS, respectively. The current was also sensitive to mechanical stretch and intracellular acidification. Negative pressure (-30 cm H2O) and acidification of intracellular solution (pH 6.8 or 6.3) both enhanced TREK-1 channel open probability significantly. Further pharmacological studies showed that TREK-1 antagonist penfluridol inhibited AA-induced currents, and both penfluridol and methionine (TREK-1 blockers) significantly increased BDNF level in astrocytes by 50 %. These results indicated that TREK-1 channel current was a major component of K2P currents in astrocytes. TREK-1 channels might play important roles in regulating the function of astrocytes and might be used as a drug target for neuroprotection.

A novel channelopathy in pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension is a devastating disease with high mortality. Familial cases of pulmonary arterial hypertension are usually characterized by autosomal dominant transmission with reduced penetrance, and some familial cases have unknown genetic causes.

METHODS

We studied a family in which multiple members had pulmonary arterial hypertension without identifiable mutations in any of the genes known to be associated with the disease, including BMPR2, ALK1, ENG, SMAD9, and CAV1. Three family members were studied with whole-exome sequencing. Additional patients with familial or idiopathic pulmonary arterial hypertension were screened for the mutations in the gene that was identified on whole-exome sequencing. All variants were expressed in COS-7 cells, and channel function was studied by means of patch-clamp analysis.

RESULTS

We identified a novel heterozygous missense variant c.608 G→A (G203D) in KCNK3 (the gene encoding potassium channel subfamily K, member 3) as a disease-causing candidate gene in the family. Five additional heterozygous missense variants in KCNK3 were independently identified in 92 unrelated patients with familial pulmonary arterial hypertension and 230 patients with idiopathic pulmonary arterial hypertension. We used in silico bioinformatic tools to predict that all six novel variants would be damaging. Electrophysiological studies of the channel indicated that all these missense mutations resulted in loss of function, and the reduction in the potassium-channel current was remedied by the application of the phospholipase inhibitor ONO-RS-082.

CONCLUSIONS

Our study identified the association of a novel gene, KCNK3, with familial and idiopathic pulmonary arterial hypertension. Mutations in this gene produced reduced potassium-channel current, which was successfully remedied by pharmacologic manipulation. (Funded by the National Institutes of Health.)

[TREK1 potassium channels and depression]

Major depression disorder is an increasing heavy burden in modem society, but its pathological mechanism is still vague. Recent evidence indicated that two pore potassium channel, TREK1, is one of the important drug targets of antidepressants. The structural and functional research progress of TREK1 potassium channel were reviewed with an emphasis on its roles in anti-depression, neuronal protection, and neuronal plasticity. The complicated interactions between TREK1 potassium channel and monoamine transmitters-receptors were also reviewed and future directions to explore the underline mechanism were also discussed.

Membrane localisation diversity of TPK channels and their physiological role

Potassium (K) is one of the major nutrients that is essential for plant growth and development. The majority of cellular K+ resides in the vacuole and tonoplast K+ channels of the TPK (Two Pore K) family are main players in cellular K+ homeostasis. All TPK channels were previously reported to be expressed in the tonoplast of the large central lytic vacuole (LV) except for one isoform in Arabidopsis that resides in the plasma membrane. However, plant cells often contain more than one type of vacuole that coexist in the same cell. We recently showed that two TPK isoforms (OsTPKa and OsTPKb) from Oryza sativa localise to different vacuoles with OsTPKa predominantly found in the LV tonoplast and OsTPKb primarily in smaller compartments that resemble small vacuoles (SVs). Our study further revealed that it is the C-terminal domain that determines differential targeting of OsTPKa and OsTPKb. Three C-terminal amino acids were particularly relevant for targeting TPKs to their respective endomembranes. In this addendum we further evaluate how the different localisation of TPKa and TPKb impact on their physiological role and how TPKs provide a potential tool to study the physiology of different types of vacuole.

Reciprocal regulation of TREK-1 channels by arachidonic acid and CRH in mouse corticotropes

Arachidonic acid (AA) is generated in the anterior pituitary gland upon stimulation by the ACTH secretagogue, CRH. Using the patch clamp technique, we examined the action of AA on the excitability of single pituitary corticotropes obtained from a transgenic mouse strain that expresses the enhanced green fluorescent protein driven by the proopiomelanocortin promoter. CRH evoked depolarization, but AA caused hyperpolarization. Under voltage clamp condition, AA caused a rapid inhibition of the delayed rectifier K(+) current and then increased a background K(+) current. Inhibition of AA metabolism did not prevent the activation of the K(+) current by AA, suggesting a direct action of AA. The sensitivity of the AA-activated K(+) current to fluoxetine, chlorpromazine, extracellular acidification, diphenylbutylpiperidine antipsychotics, and the membrane permeable cAMP analog [8-(4-chlorophenylthio)-cAMP] suggest that the current is mediated via TWIK-related K(+) channel (TREK)-1 channels. Activation of the CRH receptors that are coupled to the adenylate cyclase pathway suppressed the activation of TREK-1 current by AA and reversed the AA-mediated hyperpolarization. Intracellular acidification (pH 7.0) increased the basal amplitude of TREK-1 current and resulted in hyperpolarizaton. CRH suppressed the basal TREK-1 current in cells with intracellular acidification and caused depolarization. Our finding indicates that TREK-1 channels are important in setting the resting potential in corticotropes. The opposing actions of CRH and AA on the excitability of corticotropes raise the possibility that AA may act as a negative feedback regulator to reduce the stimulatory action of CRH and thus prevent excessive ACTH release during chronic stress.

Roles of tandem-pore K+ channels in plants - a puzzle still to be solved

The group of voltage-independent K(+) channels in Arabidopsis thaliana consists of six members, five tandem-pore channels (TPK1-TPK5) and a single K(ir)-like channel (KCO3). All TPK/KCO channels are located at the vacuolar membrane except for TPK4, which was shown to be a plasma membrane channel in pollen. The vacuolar channels interact with 14-3-3 proteins (also called General Regulating Factors, GRFs), indicating regulation at the level of protein-protein interactions. Here we review current knowledge about these ion channels and their genes, and highlight open questions that need to be urgently addressed in future studies to fully appreciate the physiological functions of these ion channels.

The role of TASK1 in aldosterone production and its expression in normal adrenal and aldosterone-producing adenomas

OBJECTIVES

Aldosterone production in the adrenal glomerulosa is mainly regulated by angiotensin II and K+. Adrenal glomerulosa cells are uniquely sensitive to extracellular K+. Genetic deletion of subunits of K+-selective leak-channels (KCNK), TASK1 and/or TASK3, in mice generates animals with hyperaldosteronism and histological changes in the adrenal cortex. Herein, we studied the expression of TASK1 in human adrenocortical cells, as well as its role in aldosterone production in H295R cells.

DESIGN

TASK1 expression was investigated by comparative microarray analysis of aldosterone-producing adenomas (APA) and normal adrenals (NAs). The effects of TASK1 knockdown by siRNA transfection were investigated in H295R cells. Fluo-4 fluorescent measurements of intracellular Ca2+ and pharmacological inhibition of Ca2+ -dependent calmodulin kinases (CaMK) were performed to better define the effects of TASK1 on Ca2+ signalling pathways.

RESULTS

Microarray analysis of APA and NA showed similar expression of TASK1 between these two groups. However, in APA, NA and H295R cells the expression of TASK1 was predominant when compared with other KCNK family members. Knockdown of TASK1 (with siRNA) induced the expression of steroidogenic acute regulatory (StAR) protein and aldosterone synthase (CYP11B2), and also stimulated pregnenolone and aldosterone production. Cells transfected with siTASK1 had increased intracellular Ca2+, leading to activation of CaMK and increased expression of CYP11B2.

CONCLUSIONS

Our study reveals the predominant expression of TASK1 over other KCNK family genes in the human adrenal cortex. Herein, we also described the role of TASK1 in the regulation of human aldosterone production through regulation of intracellular Ca2+ and CaMK signalling pathways.

TASK1 and TASK3 potassium channels: determinants of aldosterone secretion and adrenocortical zonation

Potassium channels control the membrane voltage of aldosterone-producing zona glomerulosa cells. They are responsible for the unique K(+) sensitivity of these cells and are important molecular targets of angiotensin II signaling. Among the 78 pore-forming K(+) channels in human genome only a few are found in adrenal glands. The 2-P-domain K(+) channels TASK1 and TASK3 are strongly expressed in the adrenal cortex and produce a background K(+) conductance, which is pivotal for the regulation of the aldosterone secretion in zona glomerulosa cells. Disruption of the TASK1 gene in mice resulted in an autonomous aldosterone production and caused a remarkable aberrant expression of aldosterone synthase in zona fasciculata cells that normally produce glucocorticoids. After puberty, only in male mice aldosterone production was switched off in the zona fasciculata and regular zonation of aldosterone synthase occurred. In double mutant TASK1(-/-)/TASK3(-/-) mice, also adult male mice displayed primary hyperaldosteronism. Therefore, these knockout mice are interesting models to study mechanisms of autonomous aldosterone production and adrenocortical zonation. These data suggest that modifications of the adrenocortical K(+) conductances could also contribute to autonomic aldosterone production and primary hyperaldosteronism in humans.

Role of TREK-1 potassium channel in bladder overactivity after partial bladder outlet obstruction in mouse

PURPOSE

Mouse models of partial bladder outlet obstruction cause bladder hypertrophy. Expression of a number of ion channels is altered in hypertrophic detrusor muscle, resulting in bladder dysfunction. We determined whether mechanosensitive TREK-1 channels are present in the murine bladder and whether their expression is altered in partial bladder outlet obstruction, resulting in abnormal filling responses.

MATERIALS AND METHODS

Partial bladder outlet obstruction was surgically induced in CD-1 mice and the mice recovered for 14 days. Cystometry was done to evaluate bladder pressure responses during filling at 25 microl per minute in partial bladder outlet obstruction mice and sham operated controls. TREK-1 channel expression was determined at the mRNA and protein levels by quantitative reverse transcriptase-polymerase chain reaction and Western blotting, respectively, and localized in the bladder wall using immunohistochemistry.

RESULTS

Obstructed bladders showed about a 2-fold increase in weight vs sham operated bladders. TREK-1 channel protein expression on Western blots from bladder smooth muscle strip homogenates was significantly decreased in obstructed mice. Immunohistochemistry revealed a significant decrease in TREK-1 channel immunoreactivity in detrusor smooth muscle in obstructed mice. On cystometry the TREK-1 channel blocker L-methioninol induced a significant increase in premature contractions during filling in sham operated mice. L-methioninol had no significant effect in obstructed mice, which showed an overactive detrusor phenotype.

CONCLUSIONS

TREK-1 channel down-regulation in detrusor myocytes is associated with bladder overactivity in a murine model of partial bladder outlet obstruction.

Task2 potassium channels set central respiratory CO2 and O2 sensitivity

Task2 K(+) channel expression in the central nervous system is surprisingly restricted to a few brainstem nuclei, including the retrotrapezoid (RTN) region. All Task2-positive RTN neurons were lost in mice bearing a Phox2b mutation that causes the human congenital central hypoventilation syndrome. In plethysmography, Task2(-/-) mice showed disturbed chemosensory function with hypersensitivity to low CO(2) concentrations, leading to hyperventilation. Task2 probably is needed to stabilize the membrane potential of chemoreceptive cells. In addition, Task2(-/-) mice lost the long-term hypoxia-induced respiratory decrease whereas the acute carotid-body-mediated increase was maintained. The lack of anoxia-induced respiratory depression in the isolated brainstem-spinal cord preparation suggested a central origin of the phenotype. Task2 activation by reactive oxygen species generated during hypoxia could silence RTN neurons, thus contributing to respiratory depression. These data identify Task2 as a determinant of central O(2) chemoreception and demonstrate that this phenomenon is due to the activity of a small number of neurons located at the ventral medullary surface.

Topical review. Dental pain and odontoblasts: facts and hypotheses

Dental pain arises from exposed dentin following bacterial, chemical, or mechanical erosion of enamel and/or recession of gingiva. Thus, dentin tissue and more specifically patent dentinal tubules represent the first structure involved in dentin sensitivity. Interestingly, the architecture of dentin could allow for the transfer of information to the underlying dental pulp via odontoblasts (dentin-forming cells), via their apical extension bathed in the dentinal fluid running in the tubules, or via a dense network of trigeminal sensory axons intimately related to odontoblasts. Therefore, external stimuli causing dentinal fluid movements and odontoblasts and/or nerve complex responses may represent a unique mechanosensory system bringing a new role for odontoblasts as sensor cells. How cells sense signals and how the latter are transmitted to axons represent the main questions to be resolved. However, several lines of evidence have demonstrated that odontoblasts express mechano- and/or thermosensitive transient receptor potential ion channels (TRPV1, TRPV2, TRPV3, TRPV4, TRPM3, KCa, TREK-1) that are likely to sense heat and/or cold or movements of dentinal fluid within tubules. Added to this, voltage-gated sodium channels confer excitable properties of odontoblasts in vitro in response to injection of depolarizing currents. In vivo, sodium channels co-localize with nerve terminals at the apical pole of odontoblasts and correlate with the spatial distribution of stretch-activated KCa channels. This highlights the terminal web as the pivotal zone of the pulp/dentin complex for sensing external stimuli. Crosstalk between odontoblasts and axons may take place by the release of mediators in the gap space between odontoblasts and axons in view of evidence for nociception-transducing receptors on trigeminal afferent fibers and expression of putative effectors by odontoblasts. Finally, how axons are guided to the target cells and which kind of signaling molecules are involved is extensively discussed in this review.

The response of the tandem pore potassium channel TASK-3 (K(2P)9.1) to voltage: gating at the cytoplasmic mouth

Although the tandem pore potassium channel TASK-3 is thought to open and shut at its selectivity filter in response to changes of extracellular pH, it is currently unknown whether the channel also shows gating at its inner, cytoplasmic mouth through movements of membrane helices M2 and M4. We used two electrode voltage clamp and single channel recording to show that TASK-3 responds to voltage in a way that reveals such gating. In wild-type channels, P(open) was very low at negative voltages, but increased with depolarisation. The effect of voltage was relatively weak and the gating charge small, 0.17. Mutants A237T (in M4) and N133A (in M2) increased P(open) at a given voltage, increasing mean open time and the number of openings per burst. In addition, the relationship between P(open) and voltage was shifted to less positive voltages. Mutation of putative hinge glycines (G117A, G231A), residues that are conserved throughout the tandem pore channel family, reduced P(open) at a given voltage, shifting the relationship with voltage to a more positive potential range. None of these mutants substantially affected the response of the channel to extracellular acidification. We have used the results from single channel recording to develop a simple kinetic model to show how gating occurs through two classes of conformation change, with two routes out of the open state, as expected if gating occurs both at the selectivity filter and at its cytoplasmic mouth.

Long-chain polyunsaturated fatty acids as anti-HIV supplementation during breastfeeding

OBJECTIVE

Breastfeeding by human immunodeficiency virus (HIV)-positive mothers is an unavoidable practice in some very poor countries. It has been suggested that long-chain polyunsaturated fatty acids (LC-PUFAs) in breast milk, such as arachidonic acid, act as natural, protective ingredients against HIV transmission. The objective of this study was to identify the protective mechanism of LC-PUFAs in cells susceptible to HIV infection (e.g. human CD4+ T cells, HeLa cells).

RESULTS

LC-PUFAs are bioactive molecules capable of activating the cellular protective machinery via modulation of endogenous background K+ or KCNK channels. KCNK channel expression contributes significantly to the stability of the cell membrane potential. During HIV-1 infection, degradation of the KCNK channel is accelerated, and the cell membrane potential becomes pathologically depolarized. From studying functionally distinct KCNK mutants, we found that the degree of membrane potential depolarization was directly proportional to the release efficiency of HIV-1 virions. On the other hand, supplementation of KCNK channel modulators such as arachidonic acid (AA) and docosahexaenoic acid (DHA) at micromolar doses could restore hyperpolarization and stability of the cell membrane potential when endogenous KCNK channels are partially knocked down (mimicking the depolarized state of an HIV-1-infected cell).

CONCLUSION

The protective mechanism of LC-PUFAs against HIV spread involves stimulation of the endogenous KCNK channels. Our work suggests that supplementation with AA and DHA may be beneficial in reducing the risk of HIV-1 transmission, particularly during the period of breastfeeding.

Phosphatidylinositol (4,5)bisphosphate inhibits K+-efflux channel activity in NT1 tobacco cultured cells

In the animal world, the regulation of ion channels by phosphoinositides (PIs) has been investigated extensively, demonstrating a wide range of channels controlled by phosphatidylinositol (4,5)bisphosphate (PtdInsP2). To understand PI regulation of plant ion channels, we examined the in planta effect of PtdInsP2 on the K+-efflux channel of tobacco (Nicotiana tabacum), NtORK (outward-rectifying K channel). We applied a patch clamp in the whole-cell configuration (with fixed "cytosolic" Ca2+ concentration and pH) to protoplasts isolated from cultured tobacco cells with genetically manipulated plasma membrane levels of PtdInsP2 and cellular inositol (1,4,5)trisphosphate: "Low PIs" had depressed levels of these PIs, and "High PIs" had elevated levels relative to controls. In all of these cells, K channel activity, reflected in the net, steady-state outward K+ currents (IK), was inversely related to the plasma membrane PtdInsP2 level. Consistent with this, short-term manipulations decreasing PtdInsP2 levels in the High PIs, such as pretreatment with the phytohormone abscisic acid (25 microM) or neutralizing the bath solution from pH 5.6 to pH 7, increased IK (i.e. NtORK activity). Moreover, increasing PtdInsP2 levels in controls or in abscisic acid-treated high-PI cells, using the specific PI-phospholipase C inhibitor U73122 (2.5-4 microM), decreased NtORK activity. In all cases, IK decreases stemmed largely from decreased maximum attainable NtORK channel conductance and partly from shifted voltage dependence of channel gating to more positive potentials, making it more difficult to activate the channels. These results are consistent with NtORK inhibition by the negatively charged PtdInsP2 in the internal plasma membrane leaflet. Such effects are likely to underlie PI signaling in intact plant cells.

Both TASK-3 and TREK-1 two-pore loop K channels are expressed in H295R cells and modulate their membrane potential and aldosterone secretion

The rate of aldosterone synthesis by adrenal glomerulosa cells relies on the selective permeability of the glomerulosa cell to K(+) ions. In rodent and bovine adrenal glomerulosa cells, this background potassium current is provided by a two-pore loop potassium (K2P) channel: largely TASK-3 in the rat and TREK-1 in the cow. The nature of the K2P channel in the human adrenal cortex is not known, and we have addressed this issue here using the H295R human adrenal cell line. We show that these cells express mRNA and protein for both TASK-3 and TREK-1 K2P channels. Using a potentiometric dye (FMP), we also show that TASK-3 and TREK-1 channel modulators can affect the membrane potential of H295R cells. Transfecting H295R cells with TASK-3 or TREK-1 dominant-negative mutants (TASK-3 G95E or TREK-1 G144E) produced depolarization of H295R cells and altered K-stimulated aldosterone secretion. Finally, transfection of a constitutively active mutant of Galpha(q) into H295R cells (GTPase-deficient Galpha(q)-QL) depolarized them and increased basal aldosterone secretion. Taken together, our data support both TASK-3 and TREK-1 as being functionally operational in the H295R cell line. This suggests that human adrenal glomerulosa cells may utilize both of these K2P channels for their background potassium current.

Protective effects of TREK-1 against oxidative injury induced by SNP and H2O2

AIM

TREK-1 (TWIK-related K+ channel-1) is a 2-pore-domain K+ channel subtype. The present study investigated the role of TREK-1 in cell death induced by oxidative stress.

METHODS

The cell viability of wild-type Chinese hamster ovary (CHO) and TREK-1-transfected CHO cells (TREK-1/CHO cells) was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in the presence of sodium nitroprusside (SNP) or hydrogen peroxide (H2O2). Apoptosis of wild-type CHO and TREK-1/CHO cells was detected using Hoechst33342 staining.

RESULTS

Both SNP and H2O2 caused dose- and time-dependent growth inhibition of wild-type CHO and TREK-1/ CHO cells. Following a 12 h exposure to SNP, the 50% inhibition (IC(50)) values for wild-type CHO and TREK-1/CHO cells were calculated as 0.69 mmol/L and 1.14 mmol/L, respectively. The IC(50) values were 0.07 mmol/L and 0.09 mmol/L in H2O2-treated wild-type CHO and TREK-1/CHO cells, respectively, following 12 h exposure to H2O2. Moreover, SNP/H2O2 induced less apoptosis in TREK-1/ CHO cells than that in wild-type CHO cells (P<0.05).

CONCLUSION

The results demonstrated that TREK-1 played a protective role against oxidative injury.

Cutaneous neurovascular interaction involved in tactile sensation

The sense of touch is one of the most vital; still, it is incompletely understood. We review the afferent function that allows for the relay of sensory information from the periphery (the skin) to the central nervous system. Within this afferent function, we examine the different integrating levels including several candidates for cutaneous transducers, the conduction of the information via the afferent nervous fibres and the transmission of the sensory stimuli to higher brain structures, resulting in the perception of the different senses. We then examine the efferent system that stimulates the skin by secreting neurotransmitters. Finally, we discuss the tools available to study the cutaneous neurovascular interaction and conclude on a novel test that assesses this interaction triggered by the application of a local non noxious pressure (tactile stimulation).

Serotonin increases GABA release in rat entorhinal cortex by inhibiting interneuron TASK-3 K+ channels

Whereas the entorhinal cortex (EC) receives profuse serotonergic innervations from the raphe nuclei in the brain stem and is critically involved in the generation of temporal lobe epilepsy, the function of serotonin (5-hydroxytryptamine, 5-HT) in the EC and particularly its roles in temporal lobe epilepsy are still elusive. Here we explored the cellular and molecular mechanisms underlying 5-HT-mediated facilitation of GABAergic transmission and depression of epileptic activity in the superficial layers of the EC. Application of 5-HT increased sIPSC frequency and amplitude recorded from the principal neurons in the EC with no effects on mIPSCs recorded in the presence of TTX. However, 5-HT reduced the amplitude of IPSCs evoked by extracellular field stimulation and in synaptically connected interneuron and pyramidal neuron pairs. Application of 5-HT generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in presynaptic interneurons suggesting that 5-HT still increases GABA release whereas the depressant effects of 5-HT on evoked IPSCs could be explained by 5-HT-induced reduction in action potential amplitude. The depolarizing effect of 5-HT was mediated by inhibition of TASK-3 K(+) channels in interneurons and required the functions of 5-HT(2A) receptors and Galpha(q/11) but was independent of phospholipase C activity. Application of 5-HT inhibited low-Mg(2+)-induced seizure activity in slices via 5-HT(1A) and 5-HT(2A) receptors suggesting that 5-HT-mediated depression of neuronal excitability and increase in GABA release contribute to its anti-epileptic effects in the EC.

Sipatrigine could have therapeutic potential for major depression and bipolar depression through antagonism of the two-pore-domain K+ channel TREK-1

Major depressive disorder (MDD) is a chronic, recurring and potentially life-threatening mental illness. Current treatments are inadequate - many depression medications, although safe and effective, generally have a slow onset of clinical benefit and around half of the MDD patients do not show full remission with optimized treatment. Therefore, there is still a need for the development of faster-acting and more effective medication for MDD. Recent studies have demonstrated that the TREK-1 protein, one of the 17 members of the two-pore domain K+ (K2P) potassium channel family, is inhibited by the antidepressant fluoxetine. Deletion of TREK-1 in mice caused a substantially reduced elevation of corticosterone levels under stress, and produced behaviour similar to that of naive animals treated with fluoxetine in various behavioural tests. These findings suggested that the blocker of the TREK-1 channel might potentially be a new type of antidepressant. Sipatrigine (BW619C89), a neuroprotective agent, has been found to be a potent antagonist of TREK-1. Its related compound, lamotrigine, has been approved for the treatment of bipolar depression and is used to supplement antidepressant medication in patients with treatment-resistant depression. Furthermore, in addition to its antagonistic effect on TREK-1, sipatrigine is also a glutamate release inhibitor. Excessive glutamatergic neurotransmission is associated with depressive-like behaviours and inhibiting glutamate neurotransmission may be implicated in antidepressant therapeutic mechanisms. From the above findings of the effects of sipatrigine on TREK-1 and glutamate neurotransmission, it is hypothesised that sipatrigine could have potential therapeutic effects for MDD or bipolar depression. Further evaluation of its antidepressant therapeutic and toxic effects in animal models is needed before clinical application.

Identification of a region in the TASK3 two pore domain potassium channel that is critical for its blockade by methanandamide

BACKGROUND AND PURPOSE

The TASK subfamily of two pore domain potassium channels (K2P) encodes for leak K currents, contributing to the resting membrane potential of many neurons and regulating their excitability. TASK1 and TASK3 channels are regulated by a number of pharmacological and physiological mediators including cannabinoids such as methanandamide. In this study, we investigate how methanandamide blocks these channels.

EXPERIMENTAL APPROACH

Currents through wild type and mutated TASK1 and TASK3 channels expressed in modified HEK-293 cells were measured using whole-cell electrophysiological recordings in the presence and absence of methanandamide.

KEY RESULTS

Methanandamide (3 microM) produced substantial block of hTASK1, hTASK3 and mTASK3 channels but was most potent at blocking hTASK3 channels. Block of these channels was irreversible unless cells were washed with buffer containing bovine serum albumin. Mutation of the distal six amino acids of TASK1 did not alter methanandamide inhibition, whilst C terminal truncation of TASK3 channels caused a small but significant reduction of inhibition. However, deletion of six amino acids (VLRFLT) at the interface between the final transmembrane domain and cytoplasmic C terminus of TASK3 channels gave functional currents that were no longer inhibited by methanandamide or by activation of GPCRs.

CONCLUSIONS AND IMPLICATIONS

Methanandamide potently blocked TASK3 and TASK1 channels and both methanandamide and G protein-mediated inhibition converged on the same intracellular gating pathway. Physiologically, methanandamide block of TASK1 and TASK3 channels may underpin a number of CNS effects of cannabinoids that are not mediated through activation of CB1 or CB2 receptors.

The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1: contributions of the pore domains

We have investigated the contribution to ionic selectivity of residues in the selectivity filter and pore helices of the P1 and P2 domains in the acid sensitive potassium channel TASK-1. We used site directed mutagenesis and electrophysiological studies, assisted by structural models built through computational methods. We have measured selectivity in channels expressed in Xenopus oocytes, using voltage clamp to measure shifts in reversal potential and current amplitudes when Rb+ or Na+ replaced extracellular K+. Both P1 and P2 contribute to selectivity, and most mutations, including mutation of residues in the triplets GYG and GFG in P1 and P2, made channels non-selective. We interpret the effects of these--and of other mutations--in terms of the way the pore is likely to be stabilised structurally. We show also that residues in the outer pore mouth contribute to selectivity in TASK-1. Mutations resulting in loss of selectivity (e.g. I94S, G95A) were associated with slowing of the response of channels to depolarisation. More important physiologically, pH sensitivity is also lost or altered by such mutations. Mutations that retained selectivity (e.g. I94L, I94V) also retained their response to acidification. It is likely that responses both to voltage and pH changes involve gating at the selectivity filter.

The presence of arachidonic acid-activated K+ channel, TREK-1, in human periodontal ligament fibroblasts

Human periodontal ligament (PDL) fibroblasts expressed following two-pore-domain K(+) channels, TWIK-2 > TREK-1 > TWIK-1 >> TASK-1 > TRAAK > TASK-2. TREK-2 message was not detectable. We found the presence of arachidonic acid-activated and mechanical stress-sensitive K(+) channel, TREK-1, in the PDL fibroblasts by patch-clamp technique. It was also found the significant increase of intracellular concentration of arachidonic acid upon the application of cyclic stretch. Therefore, we suppose that the mechanical stretch due to the mastication activates phospholipase A(2) to release arachidonic acid (AA) from membrane, then, the released AA activates TREK-1. Thus, TREK-1 K(+) channels may play a protective role to maintain the negative membrane potential of PDL fibroblasts against the environmental stimuli.

Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels

The mammalian family of two-pore domain K+ (K2P) channel proteins are encoded by 15 KCNK genes and subdivided into six subfamilies on the basis of sequence similarities: TWIK, TREK, TASK, TALK, THIK, and TRESK. K2P channels are expressed in cells throughout the body and have been implicated in diverse cellular functions including maintenance of the resting potential and regulation of excitability, sensory transduction, ion transport, and cell volume regulation, as well as metabolic regulation and apoptosis. In recent years K2P channel isoforms have been identified as important targets of several widely employed drugs, including: general anesthetics, local anesthetics, neuroprotectants, and anti-depressants. An important goal of future studies will be to identify the basis of drug actions and channel isoform selectivity. This goal will be facilitated by characterization of native K2P channel isoforms, their pharmacological properties and tissue-specific expression patterns. To this end the present review examines the biophysical, pharmacological, and functional characteristics of cloned mammalian K2P channels and compares this information with the limited data available for native K2P channels in order to determine criteria which may be useful in identifying ionic currents mediated by native channel isoforms and investigating their pharmacological and functional characteristics.

Reduced inhibition of cortical glutamate and GABA release by halothane in mice lacking the K+ channel, TREK-1

BACKGROUND AND PURPOSE

Deletion of TREK-1, a two-pore domain K(+) channel (K(2P)) activated by volatile anaesthetics, reduces volatile anaesthetic potency in mice, consistent with a role for TREK-1 as an anaesthetic target. We used TREK-1 knockout mice to examine the presynaptic function of TREK-1 in transmitter release and its role in the selective inhibition of glutamate vs GABA release by volatile anaesthetics.

EXPERIMENTAL APPROACH

The effects of halothane on 4-aminopyridine-evoked and basal [(3)H]glutamate and [(14)C]GABA release from cerebrocortical nerve terminals isolated from TREK-1 knockout (KO) and littermate wild-type (WT) mice were compared. TREK-1 was quantified by immunoblotting of nerve terminal preparations.

KEY RESULTS

Deletion of TREK-1 significantly reduced the potency of halothane inhibition of 4-aminopyridine-evoked release of both glutamate and GABA without affecting control evoked release or the selective inhibition of glutamate vs GABA release. TREK-1 deletion also reduced halothane inhibition of basal glutamate release, but did not affect basal GABA release.

CONCLUSIONS AND IMPLICATIONS

The reduced sensitivity of glutamate and GABA release to inhibition by halothane in TREK-1 KO nerve terminals correlates with the reduced anaesthetic potency of halothane in TREK-1 KO mice observed in vivo. A presynaptic role for TREK-1 was supported by the enrichment of TREK-1 in isolated nerve terminals determined by immunoblotting. This study represents the first evidence for a link between an anaesthetic-sensitive 2-pore domain K(+) channel and presynaptic function, and provides further support for presynaptic mechanisms in determining volatile anaesthetic action.

Sequence-based bioinformatic prediction and QUASEP identify genomic imprinting of the KCNK9 potassium channel gene in mouse and human

Genomic imprinting is the epigenetic marking of gene subsets resulting in monoallelic or predominant expression of one of the two parental alleles according to their parental origin. We describe the systematic experimental verification of a prioritized 16 candidate imprinted gene set predicted by sequence-based bioinformatic analyses. We used Quantification of Allele-Specific Expression by Pyrosequencing (QUASEP) and discovered maternal-specific imprinted expression of the Kcnk9 gene as well as strain-dependent preferential expression of the Rarres1 gene in E11.5 (C57BL/6 x Cast/Ei)F1 and informative (C57BL/6 x Cast/Ei) x C57BL/6 backcross mouse embryos. For the remaining 14 candidate imprinted genes, we observed biallelic expression. In adult mouse tissues, we found that Kcnk9 expression was restricted to the brain and also was maternal-specific. QUASEP analysis of informative human fetal brain samples further demonstrated maternal-specific imprinted expression of the human KCNK9 orthologue. The CpG islands associated with the mouse and human Kcnk9/KCNK9 genes were not differentially methylated, but strongly hypomethylated. Thus, we speculate that mouse Kcnk9 imprinting may be regulated by the maternal germline differentially methylated region in Peg13, an imprinted non-coding RNA gene in close proximity to Kcnk9 on distal mouse chromosome 15. Our data have major implications for the proposed role of Kcnk9 in neurodevelopment, apoptosis and tumourigenesis, as well as for the efficiency of sequence-based bioinformatic predictions of novel imprinted genes.

Differential effects of volatile and intravenous anesthetics on the activity of human TASK-1

Volatile anesthetics have been shown to activate various two-pore (2P) domain K(+) (K(2P)) channels such as TASK-1 and TREK-1 (TWIK-related acid-sensitive K(+) channel), and mice deficient in these channels are resistant to halothane-induced anesthesia. Here, we investigated whether K(2P) channels were also potentially important targets of intravenous anesthetics. Whole cell patch-clamp techniques were used to determine the effects of the commonly used intravenous anesthetics etomidate and propofol on the acid-sensitive K(+) current in rat ventricular myocytes (which strongly express TASK-1) and selected human K(2P) channels expressed in Xenopus laevis oocytes. In myocytes, etomidate decreased both inward rectifier K(+) (K(ir)) current (I(K1)) and acid-sensitive outward K(+) current at positive potentials, suggesting that this drug may inhibit TASK channels. Indeed, in addition to inhibiting guinea pig Kir2.1 expressed in oocytes, etomidate inhibited human TASK-1 (and TASK-3) in a concentration-dependent fashion. Propofol had no effect on human TASK-1 (or TASK-3) expressed in oocytes. Moreover, we showed that, similar to the known effect of halothane, sevoflurane and the purified R-(-)- and S-(+)-enantiomers of isoflurane, without stereoselectivity, activated human TASK-1. We conclude that intravenous and volatile anesthetics have dissimilar effects on K(2P) channels. Human TASK-1 (and TASK-3) are insensitive to propofol but are inhibited by supraclinical concentrations of etomidate. In contrast, stimulatory effects of sevoflurane and enantiomeric isoflurane on human TASK-1 can be observed at clinically relevant concentrations.

Recent advance and possible future in TREK-2: a two-pore potassium channel may involved in the process of NPP, brain ischemia and memory impairment

TREK-2, a new member of the mechanosensitive tandem-pore K+ channel family, share 65% amino acid sequence identity and some similar basic electrophysiological and pharmacological properties with TREK-1. It also has some specific regulatory pathway and tissue distribution contrasted with TREK-1 and TRAAK. TREK-2 distributes extensively in CNS and periphery tissue. It can be regulated by G-protein-coupled receptor (GPCR) and may involve in several of physiological and pathophysiological conditions. The long-chain unsaturated free fatty acids such as arachidonic acid (AA), PHi, pressure and temperature can increase the activity of TREK-2. The purpose of this review is to present the recent study and possible importance of TREK-2 in neuropathic pain, thereby emphasizing TREK-2 as one of the important mechanisms underlying. This information should be very useful and prospective for effective chronic pain therapy and future analgesic drug development. This review also further predicts the role of TREK-2 in brain ischemia, memory and other tissue. The specific location and function of TREK-2 in these tissues need further study.

[TREK-1: a potential target for novel antidepressants]

The neurotransmitter serotonin (5-HT: 5-hydroxytryptamin) was suggested to be involved in the pathogenesis of depression as well as in the mechanisms of antidepressant treatments. However, the molecular mechanisms underlying the pathophysiology or treatment of depression are still poorly understood. A recent paper has shown that deletion of the two-pore domain potassium channel TREK-1 results in an antidepressant-like phenotype. TREK-1 -deficient mice behave as if they have been treated with an antidepressant drug, such as fluoxetine. Moreover, TREK-1-deficient mice showed a reduced elevation of corticosterone level under stress, an increased efficacy of 5-HT neurotransmission and an increased fluoxetine-induced neurogenesis in the hippocampus. Selective serotonin reuptake inhibitors (SSRIs) inhibited not only the 5-HT transporter but also the TREK-1 channel. In this article, we review the molecular and functional properties of the TREK-1 channel, which is a potential target for novel antidepressants.

Deafness associated changes in two-pore domain potassium channels in the rat inferior colliculus

Two-pore potassium channels can influence neuronal excitability by regulating background leakage of potassium ions and resting membrane potential. The present study used quantitative real time PCR and in situ hybridization to determine if the decreased activity from deafness would induce changes in two-pore potassium channel subunit expression in the rat inferior colliculus (IC). Ten subunits were assessed with quantitative real-time PCR at 3 days, 3 weeks and 3 months following bilateral cochlear ablation. TASK-1, TASK-5 and THIK-2 showed significant decreases in expression at all three times assessed. TASK-5, relatively specific to auditory neurons, had the greatest decrease. TWIK-1 was significantly decreased at 3 weeks and 3 months following deafness and TREK-2 was only significantly decreased at 3 days. TASK-3, TWIK-2, THIK-1, TRAAK and TREK-1 did not show any significant changes in gene expression. In situ hybridization was used to examine TASK-1, TASK-5, TWIK-1 and THIK-2 in the central nucleus, dorsal cortex and lateral (external) cortex of the IC in normal hearing animals and at 3 weeks following deafening. All four subunits showed expression in neurons throughout IC subdivisions in normal hearing rats, with TASK-5 having the greatest overall number of labeled neurons. There was no co-localization of subunit expression with glial fibrillary acidic protein immunostaining, indicating no expression in glia. Three weeks following deafening there was a significant decrease in the number of neurons expressing TASK-1 and THIK-2 in the IC, while TASK-5 had significant decreases in the central nucleus and dorsal cortex and TWIK-1 in the lateral and dorsal cortices.

Therapeutic potential of neuronal two-pore domain potassium-channel modulators

Two-pore domain potassium (K2P) channels are expressed in cells throughout the body and give rise to leak potassium currents which control the excitability of these cells. Although not inhibited by classical potassium channel-blocking drugs, such as tetraethylammonium and 4-aminopyridine, K2P channels are regulated by a diverse array of pharmacological mediators. There are six main families of K2P channels and among these certain members of the TREK family (ie, TREK-1 and TREK-2) are activated by general anesthetic agents such as halothane, xenon and nitrous oxide. In addition, all members of the TREK familyare activated by neuroprotective agents, such as riluzole, polyunsaturated fatty acids and lysophospholipids, suggesting that these channels play an important role in neuroprotection. TREK channels are also inhibited by chlorpromazine, local anesthetics and the antidepressant fluoxetine. Furthermore, all members of the TASK family are inhibited by cannabinoids and local anesthetics, and TASK-3 is selectively inhibited by ruthenium red. Thus, the diversity and physiological importance of K2P channels suggest that the development of selective compounds to target these proteins has therapeutic potential for CNS disorders such as stroke, depression and epilepsy.

Properties of single two-pore domain TREK-2 channels expressed in mammalian cells

TREK-2 (K2P10.1), a member of the two-pore domain K+ (K2P) channel family, provides the background K+ conductance in many cell types, and is a target of neurotransmitters that act on receptors coupled to Gs and Gq. We report here that TREK-2 exhibits small (TREK-2S) and large (TREK-2L) conductance phenotypes when expressed in mammalian cell lines (COS-7, HEK293, HeLa) and in Xenopus oocytes. TREK-2S phenotype shows a noisy open state with a mean conductance of 54 pS (+40 mV). TREK-2L phenotype shows a full open state (202 pS) with several short-lived sub-conductance levels. Both phenotypes were strongly activated by arachidonic acid, membrane stretch (-40 mmHg) and intracellular acidification (pH 6.4). Phosphorylation of TREK-2 produced by treatment of cells with activators of protein kinases A and C, and okadaic acid (a serine/threonine phosphatase inhibitor) decreased the current contributed by TREK-2S and TREK-2L, and caused partial switching of conductance levels from those of TREK-2S and TREK-2L to more intermediate values. Under this condition, TREK-2 exhibited six conducting levels and one closed level. TREK-2 mutants in which putative protein kinases A and C phosphorylation sites were mutated to alanines (S326A, S359A, S326A/S359A) displayed mostly TREK-2S and TREK-2L phenotypes. However, S326D and S359D mutants (as well as the double mutants) that mimic the phosphorylated state showed all six conducting levels and low channel activity. The S326A and S359A mutants did not significantly affect the intrinsic voltage dependence of TREK-2 in Mg2+-free solution. Phenotypes resembling TREK-2S and TREK-2L were also observed in cerebellar granule neurons that express TREK-2 mRNA. These results show that TREK-2 exhibits two primary modes of gating that give rise to two channel phenotypes under dephosphorylated conditions, and that its phosphorylation shifts the gating mode to include intermediate conducting levels. This represents a novel mechanism by which receptor agonists modulate the function of a K+ channel to alter cell excitability.

The neuronal background K2P channels: focus on TREK1

Two-pore-domain K(+) (K(2P)) channel subunits are made up of four transmembrane segments and two pore-forming domains that are arranged in tandem and function as either homo- or heterodimeric channels. This structural motif is associated with unusual gating properties, including background channel activity and sensitivity to membrane stretch. Moreover, K(2P) channels are modulated by a variety of cellular lipids and pharmacological agents, including polyunsaturated fatty acids and volatile general anaesthetics. Recent in vivo studies have demonstrated that TREK1, the most thoroughly studied K(2P) channel, has a key role in the cellular mechanisms of neuroprotection, anaesthesia, pain and depression.

Serotonin Inhibits Neuronal Excitability by Activating Two-Pore Domain K+ Channels in the Entorhinal Cortex

The entorhinal cortex (EC) is regarded as the gateway to the hippocampus; the superficial layers (layers I-III) of the EC convey the cortical input projections to the hippocampus, whereas deep layers of the EC relay hippocampal output projections back to the superficial layers of the EC or to other cortical regions. The superficial layers of the EC receive strong serotonergic projections from the raphe nuclei. However, the function of serotonin in the EC is still elusive. In the present study, we examined the molecular and cellular mechanisms underlying serotonin-mediated inhibition of the neuronal excitability in the superficial layers (layers II and III) of the EC. Application of serotonin inhibited the excitability of stellate and pyramidal neurons in the superficial layers of the EC by activating the TWIK-1 type of the two-pore domain K(+) channels. The effects of 5-HT were mediated via 5-HT(1A) receptors and required the function of Galpha(i3) subunit and protein kinase A. Serotonin-mediated inhibition of EC activity resulted in an inhibition of hippocampal function. Our study provides a cellular mechanism that might at least partially explain the roles of serotonin in many physiological functions and neurological diseases.

Up- and down-regulation of the mechano-gated K(2P) channel TREK-1 by PIP (2) and other membrane phospholipids

TREK-1 is an unconventional K(+) channel that is activated by both physical and chemical stimuli. In this study, we show that the inner leaflet membrane phospholipids, including PIP(2), exert a mixed stimulatory and inhibitory effect on TREK-1. Intra-cellular phospholipids inhibit basal channel activity and activation by membrane stretch, intra-cellular acidosis and arachidonic acid. However, binding of endogenous negative inner leaflet phospholipids with poly-lysine reduces inhibition and reveals channel stimulation by exogenous intra-cellular phospholipids. A similar effect is observed with PI, PE, PS and PA, unlike DG, demonstrating that the phosphate at position 3 is required although the global charge of the molecule is not critical. Inhibition depends on the distal C-terminal domain that conditions channel mechano-sensitivity, but is independent of the positively charged PIP(2) stimulatory site in the proximal C-terminal domain. This is, to our knowledge, the first report of an ion channel dually regulated by membrane phospholipids.

[Molecular mechanism of ischemic brain injuries and perspectives of drug therapies for neuroprotection]

Ischemic brain injury is a critical condition in the management of patients during anesthesia and intensive care. It is not rare that pathological conditions such as cerebral ischemia, head trauma and low oxygen result in marked impairment of cerebral function, even if the patient's life is saved. We sometimes encounter sudden changes in a patient's condition not only during anesthesia, but also in intensive care unit with transient low-oxygen and ischemic conditions accompanying serious shock. We have been studying the mechanisms to counteract pathological conditions leading to neuronal cell death that have been exposed to such emergency conditions, and to discover therapeutic methods to minimize the brain damage after insult. With advances in the understanding of the mechanism of neuronal cell death, technology in intensive care for salvaging neuronal cell that are at the brink of death and for recovery of brain function has progressed. However, a breakthrough has not been achieved in the development of effective therapy. Protection of the brain from terminal impairment and preservation of function will be an important issue. To achieve this goal, it is critical to clarify the susceptible mechanisms causing ischemic brain damage. This report discusses the importance of the calcineurin/immunophilin signal transduction mechanism as a new mechanism that is involved in the induction of ischemic brain damage and refers the status-quo of cerebral protection by drug therapy.

Antipsychotics inhibit TREK but not TRAAK channels

Schizophrenia is a chronic mental illness affecting 0.4% of the population. Existing antipsychotic drugs are mainly used to treat positive symptoms such as hallucinations but have only poor effects on negative symptoms such as cognitive deficits or depression. TREK and TRAAK channels are two P domain background potassium channels activated by polyunsaturated fatty acids and mechanical stress. TREK but not TRAAK channels are regulated by Gs- and Gq-coupled pathways. The inactivation of the TREK-1 but not the TRAAK channel in mice results in a depression-resistant phenotype. In addition, it has been shown that antidepressants such as fluoxetine or paroxetine directly inhibit TREK channel activity. Here we show that different antipsychotic drugs directly inhibit TREK currents with IC(50) values of approximately 1 to approximately 20 microM. No effect is seen on TRAAK channel activity. We conclude that TREK channels might be involved in the therapeutic action of antipsychotics or in their secondary effects. Furthermore, TREK channels could play a role in the pathophysiology of psychiatric disorders such as depression and schizophrenia.