TREK-1 Channel Activity Research Articles

Prolonged exposure to lactate causes TREK1 channel clustering in rat hippocampal astrocytes

Increased concentrations of lactate (15–30 mM) are associated with and found to be neuroprotective in various brain pathophysiology. In our earlier studies we showed that high levels of lactate can increase TREK1 channel activity and expression within 1 h. TREK1 channels are two pore domain leak potassium ion channels that are upregulated during cerebral ischemia, epilepsy and other brain pathologies. They play a prominent neuroprotective role against excitotoxicity. Although it has been previously shown that chronic application of lactate (6 h) causes increased gene transcription and protein expression, we observe clustering of TREK1 channels that is dependent on time of exposure (3–6 h) and concentration of lactate (15–30 mM). Using immunofluorescence techniques and image analysis, we show that the clustering of TREK1 channels is dependent on the actin cytoskeletal network of the astrocytes. Clustering of TREK1 channels can augment astrocytic functions during pathophysiological conditions and have significant implications in lactate mediated neuroprotection.

Activation of the TREK-1 Potassium Channel Improved Cognitive Deficits in a Mouse Model of Alzheimer's Disease by Modulating Glutamate Metabolism.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by cognitive dysfunction. The glutamate (Glu) metabolic pathway may be a major contributor to the memory dysfunction associated with AD. The TWIK-related potassium channel-1 (TREK-1) protects against brain ischemia, but any specific role for the channel in AD remains unknown. In this study, we used SAMP8 mice as an AD model and age-matched SAMR1 mice as controls. We explored the trends of changes in TREK-1 channel activity and the levels of AD-related molecules in the brains of SAMP8 mice. We found that the expression level of TREK-1 increased before 3months of age and then began to decline. The levels of Tau and Glu increased with age whereas the acetylcholine level decreased with age. α-Linolenic acid (ALA), an activator of the TREK-1 channel, significantly increased the TREK-1 level, and improved the learning and memory deficits of SAMP8 mice aged 6months. The mechanism in play may involve the Glu metabolic pathway. After activation of the TREK-1 channel, damaged neurons and astrocytes were rescued, the levels of Glu and the N-methyl-D-aspartate receptor were downregulated, and the level of glutamate transporter-1 was upregulated. These findings suggest that TREK-1 plays a crucial role in the pathological progression of AD; activation of the TREK-1 channel improved cognitive deficits in SAMP8 mice via a mechanism that involved Glu metabolism. The TREK-1 potassium channel may thus be a valuable therapeutic target in AD patients.

Transition between conformational states of the TREK-1 K2P channel promoted by interaction with PIP2

Members of the TREK family of two-pore domain potassium channels are highly sensitive to regulation by membrane lipids, including phosphatidylinositol-4,5-bisphosphate (PIP2). Previous studies have demonstrated that PIP2 increases TREK-1 channel activity; however, the mechanistic understanding of the conformational transitions induced by PIP2 remain unclear. Here, we used coarse-grained molecular dynamics and atomistic molecular dynamics simulations to model the PIP2-binding site on both the up and down state conformations of TREK-1. We also calculated the free energy of PIP2 binding relative to other anionic phospholipids in both conformational states using potential of mean force and free-energy-perturbation calculations. Our results identify state-dependent binding of PIP2 to sites involving the proximal C-terminus, and we show that PIP2 promotes a conformational transition from a down state toward an intermediate that resembles the up state. These results are consistent with functional data for PIP2 regulation, and together provide evidence for a structural mechanism of TREK-1 channel activation by phosphoinositides.

Mechanistic Insights into Volatile Anesthetic Modulation of K2P Channels

K2P potassium channels are known to be modulated by volatile anesthetic (VA) drugs and play important roles in clinically relevant effects that accompany general anesthesia. By utilizing a photoaffinity analog of the VA isoflurane, we have identified a VA binding site in the TREK1 K2P channel. The functional importance of the identified site was validated by mutagenesis and biochemical modification. Molecular dynamic simulations of TREK1 in the presence of VA found multiple neighboring residues on TREK1 TM2, TM3 and TM4 that contribute to anesthetic binding. Residues within the identified VA binding region play roles in the mechanisms by which heat, mechanical stretch, and other pharmacological modulators alter TREK1 channel activity and overlap with positions found to modulate TASK K2P channel VA sensitivity. Our findings define molecular contacts that mediate VA binding to TREK1 channels and suggest a mechanistic basis to explain how K2P channels are modulated by VAs.

Mechanistic insights into volatile anesthetic modulation of K2P channels.

K2P potassium channels are known to be modulated by volatile anesthetic (VA) drugs and play important roles in clinically relevant effects that accompany general anesthesia. Here, we utilize a photoaffinity analog of the VA isoflurane to identify a VA-binding site in the TREK1 K2P channel. The functional importance of the identified site was validated by mutagenesis and biochemical modification. Molecular dynamics simulations of TREK1 in the presence of VA found multiple neighboring residues on TREK1 TM2, TM3, and TM4 that contribute to anesthetic binding. The identified VA-binding region contains residues that play roles in the mechanisms by which heat, mechanical stretch, and pharmacological modulators alter TREK1 channel activity and overlaps with positions found to modulate TASK K2P channel VA sensitivity. Our findings define molecular contacts that mediate VA binding to TREK1 channels and suggest a mechanistic basis to explain how K2P channels are modulated by VAs.

PKC- and PKA-dependent phosphorylation modulates TREK-1 function in naïve and neuropathic rats.

PKC and PKA phosphorylation inhibit TREK-1 channels downstream of Gs protein-coupled receptor activation in vitro. However, the role of phosphorylation of TREK-1 in neuropathic pain is unknown. The purpose of this study was to investigate whether altered TREK-1 channel function by PKA and PKC modulators contributes to antiallodynia in neuropathic rats. Furthermore, we investigated if the in vitro described sites for PKC and PKA phosphorylation (S300 and S333, respectively) participate in the modulation of TREK-1 in naïve and neuropathic rats. L5/L6 spinal nerve ligation (SNL) induced tactile allodynia. Intrathecal injection of BL-1249 (TREK-1 activator) reversed nerve injury-induced tactile allodynia, whereas spadin (TREK-1 blocker) produced tactile allodynia in naïve rats and reversed the antiallodynic effect induced by BL-1249 in neuropathic rats. Intrathecal administration of rottlerin or Rp-cAMPs (PKC and PKA inhibitors, respectively) enhanced the antiallodynia observed with BL-1249 in neuropathic rats. In contrast, pretreatment with PdBu or forskolin (PKC and PKA activators, respectively) reduced the BL-1249-induced antiallodynia. Intrathecal injection of two high-activity TREK-1 recombinant channels, using a in vivo transfection method with lipofectamine, with mutations at PKC/PKA phosphosites (S300A and S333A) reversed tactile allodynia in neuropathic rats, with no effect in naïve rats. In contrast, transfection of two low-activity TREK-1 recombinant channels with phosphomimetic mutations at those sites (S300D and S333D) produced tactile allodynia in naïve rats and interfered with antiallodynic effects of rottlerin/BL-1249 or Rp-cAMPs/BL-1249. Data suggest that TREK-1 channel activity can be dynamically tuned in vivo by PKC/PKA to provoke allodynia and modulate its antiallodynic role in neuropathic pain.

Spadin Selectively Antagonizes Arachidonic Acid Activation of TREK-1 Channels

TREK-1 channel activity is a critical regulator of neuronal, cardiac, and smooth muscle physiology and pathology. The antidepressant peptide, spadin, has been proposed to be a TREK-1-specific blocker. Here we sought to examine the mechanism of action underlying spadin inhibition of TREK-1 channels. Heterologous expression in Xenopus laevis oocytes and electrophysiological analysis using two-electrode voltage clamp in standard bath solutions was used to characterize the pharmacological profile of wild-type and mutant murine TREK-1 and TREK-2 channels using previously established human K2P activators; arachidonic acid (AA), cis-4,7,10,13,16,19-docosahexaenoic acid (DHA), BL-1249, and cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC) and inhibitors; spadin and barium (Ba2+). Mouse TREK-1 and TREK-2 channel currents were both significantly increased by AA, BL-1249, and CDC, similar to their human homologs. Under basal conditions, both TREK-1 and TREK-2 currents were insensitive to application of spadin, but could be blocked by Ba2+. Spadin did not significantly inhibit either TREK-1 or TREK-2 currents either chemically activated by AA, BL-1249, or CDC, or structurally activated via a gating mutation. However, pre-exposure to spadin significantly perturbed the subsequent activation of TREK-1 currents by AA, but not TREK-2. Furthermore, spadin was unable to prevent activation of TREK-1 by BL-1249, CDC, or the related bioactive lipid, DHA. Spadin specifically antagonizes the activation of TREK-1 channels by AA, likely via an allosteric mechanism. Lack of intrinsic activity may explain the absence of clinical side effects during antidepressant therapy.

Shortened Spadin Analogs Display Better TREK-1 Inhibition, In Vivo Stability and Antidepressant Activity.

Depression is a devastating mental disorder that affects 20% of the population worldwide. Despite their proven efficacy, antidepressants present a delayed onset of action and serious adverse effects. Seven years ago, we described spadin (PE 12-28) as a promising endogenous peptide with antidepressant activity. Spadin specifically blocks the TREK-1 channel. Previously, we showed in vivo that, spadin activity disappeared beyond 7 h after administration. In order to improve in vivo spadin stability and bioavailability, we screened spadin analogs and derivatives. From the study of spadin blood degradation products, we designed a 7 amino-acid peptide, PE 22-28. In vitro studies on hTREK-1/HEK cells by using patch-clamp technique, showed that PE 22-28 displayed a better specificity and affinity for TREK-1 channel compared to spadin, IC50 of 0.12 nM vs. 40–60 nM for spadin. In the same conditions, we also pointed out that different modifications of its N or C-terminal ends maintained or abolished TREK-1 channel activity without affecting PE 22-28 affinity. In vivo, the antidepressant properties of PE 22-28 and its derivatives were demonstrated in behavioral models of depression, such as the forced swimming test. Mice treated with spadin-analogs showed a significant reduction of the immobility time. Moreover, in the novelty suppressed feeding test after a 4-day sub-chronic treatment PE 22-28 reduced significantly the latency to eat the food pellet. PE 22-28 and its analogs were able to induce neurogenesis after only a 4-day treatment with a prominent effect of the G/A-PE 22-28. On mouse cortical neurons, PE 22-28 and its derivatives enhanced synaptogenesis measured by the increase of PSD-95 expression level. Finally, the action duration of PE 22-28 and its analogs was largely improved in comparison with that of spadin, up to 23 h instead of 7 h. Taken together, our results demonstrated that PE 22-28 and its derivatives represent other promising molecules that could be an alternative to spadin in the treatment of depression.

The Mini-Spadin, an efficient alternate to Spadin in the depression treatment

ObjectivesWe previously discovered spadin as a new antidepressant drug concept. Spadin exerts its antidepressant actions on the TREK-1 potassium channel, a new antidepressant (AD) target. We have shown that spadin acts more rapidly in comparison to other ADs. We have pointed out that spadin induced neurogenesis after only 4-day treatments. We have demonstrated that spadin did not display side effects at the cardiac level and on TREK-1 controlled functions such as stroke, epilepsy or pain.ObjectivesWith the final goal to make spadin a drug for human clinic, our objective was to find analogs of spadin demonstrating a better affinity or a better in vivo stability or both.MethodsSeveral analogs of spadin were synthesized. Their ability to block the TREK-1 channel activity were first tested by electrophysiology on HEK293 cells stably transfected with TREK-1 channels. AD effects were measured by using the forced swim test and the novelty suppressed feeding test. Neurogenesis was investigated by measuring the expression level of the synaptic protein PSD-95 in in vitro cultured neurons.ResultsOur data allow us to identify a shortened spadin, called mini-spadin, that displayed the same AD properties as spadin and a 400 fold increase in the TREK-1 affinity. Mini-spadin increased the synaptogenesis marker PSD95 levels after only 24 hours of treatment, suggesting that like spadin, mini-spadin was able to induce neurogenesis and synaptogenesis.ConclusionsEven if further experiments are required, the mini-spadin appears to be more efficient than spadin offering a very promising alternate to spadin as human drug.Disclosure of interestThe authors have not supplied their declaration of competing interest.

Ischaemic concentrations of lactate increase TREK1 channel activity by interacting with a single histidine residue in the carboxy terminal domain.

The physiological metabolite, lactate and the two-pore domain leak potassium channel, TREK1 are known neuroprotectants against cerebral ischaemia. However, it is not known whether lactate interacts with TREK1 channel to provide neuroprotection. In this study we show that lactate increases TREK1 channel activity and hyperpolarizes CA1 stratum radiatum astrocytes in hippocampal slices. Lactate increases open probability and decreases longer close time of the human (h)TREK1 channel in a concentration dependent manner. Lactate interacts with histidine 328 (H328) in the carboxy terminal domain of hTREK1 channel to decrease its dwell time in the longer closed state. This interaction was dependent on the charge on H328. Lactate-insensitive mutant H328A hTREK1 showed pH sensitivity similar to wild-type hTREK1, indicating that the effect of lactate on hTREK1 is independent of pH change. A rise in lactate concentration and the leak potassium channel TREK1 have been independently associated with cerebral ischaemia. Recent literature suggests lactate to be neuroprotective and TREK1 knockout mice show an increased sensitivity to brain and spinal cord ischaemia; however, the connecting link between the two is missing. Therefore we hypothesized that lactate might interact with TREK1 channels. In the present study, we show that lactate at ischaemic concentrations (15-30 mm) at pH 7.4 increases TREK1 current in CA1 stratum radiatum astrocytes and causes membrane hyperpolarization. We confirm the intracellular action of lactate on TREK1 in hippocampal slices using monocarboxylate transporter blockers and at single channel level in cell-free inside-out membrane patches. The intracellular effect of lactate on TREK1 is specific since other monocarboxylates such as pyruvate and acetate at pH 7.4 failed to increase TREK1 current. Deletion and point mutation experiments suggest that lactate decreases the longer close dwell time incrementally with increase in lactate concentration by interacting with the histidine residue at position 328 (H328) in the carboxy terminal domain of the TREK1 channel. The interaction of lactate with H328 is dependent on the charge on the histidine residue since isosteric mutation of H328 to glutamine did not show an increase in TREK1 channel activity with lactate. This is the first demonstration of a direct effect of lactate on ion channel activity. The action of lactate on the TREK1 channel signifies a separate neuroprotective mechanism in ischaemia since it was found to be independent of the effect of acidic pH on channel activity.

TREK‐1 K+ Channels in the Cardiovascular System: Their Significance and Potential as a Therapeutic Target

Potassium (K(+) ) channels are important in cardiovascular disease both as drug targets and as a cause of underlying pathology. Voltage-dependent K(+) (K(V) ) channels are inhibited by the class III antiarrhythmic agents. Certain vasodilators work by opening K(+) channels in vascular smooth muscle cells (VSMCs), and K(+) channel activation may also be a route to improving endothelial function. The two-pore domain K(+) (K(2P) ) channels form a group of 15 known channels with an expanding list of functions in the cardiovascular system. One of these K(2P) channels, TREK-1, is the focus of this review. TREK-1 channel activity is tightly regulated by intracellular and extracellular pH, membrane stretch, polyunsaturated fatty acids (PUFAs), temperature, and receptor-coupled second messenger systems. TREK-1 channels are also activated by volatile anesthetics and some neuroprotectant agents, and they are inhibited by selective serotonin reuptake inhibitors (SSRIs) as well as amide local anesthetics. Some of the clinical cardiovascular effects and side effects of these drugs may be through their actions on TREK-1 channels. It has recently been suggested that TREK-1 channels have a role in mechano-electrical coupling in the heart. They also seem important in the vascular responses to PUFAs, and this may underlie some of the beneficial cardiovascular effects of the essential dietary fatty acids. Development of selective TREK-1 openers and inhibitors may provide promising routes for intervention in cardiovascular diseases.

Mood Stabilizers Activate TREK-1, but not TREK-2

Earlier studies have reported that channel activity of TREK-1 and TREK-2, members of two-pore domain K+ (K2P) channel family, are inhibited by antidepressants and antipsychotics such as fluoxetine, norfluoxetine, paroxetine, and chlorpromazine. This study was carried out to investigate the effect of mood stabilizers which are commonly used for treatment of bipolar disorder on TREK-1 and TREK-2 channels. In HEK-293A cells transfected with human TREK-1 or TREK-2, LiCl, gabapentin, valproic acid, and carbamazepin increased TREK-1 currents by 43±14%, 25±11%, 28±12%, and 82±11%, respectively, whereas these mood stabilizers have no effect on TREK-2 channel activity. The application of lamotrigine, a mood stabilizer inclined toward depression, failed to change TREK-1 channel activity. These results suggest that TREK-1 could be therapeutic potential target for treatment of bipolar disorder as well as depression.

Chapter 7 - Regulation of the Mechano‐Gated K2P Channel TREK‐1 by Membrane Phospholipids

This chapter discusses the regulation of the mechano-gated K(2P) channel, TREK-1 by membrane phospholipids. TREK-1 (KCNK2 or K2P2.1) is a polymodal K(+) channel that is activated by membrane stretch, intracellular acidosis, heat, and cellular lipids, such as arachidonic acid (AA). Phospholipids, including PIP2, exert a dual dose-dependent effect on TREK-1. Low concentrations transform the mechanogated K(+) channel TREK-1 into a leak K(+) channel. The phospholipid-sensing domain is a positively charged cluster in the proximal C-terminal domain. This region also encompasses the proton sensor E306 that is required for the activation of TREK-1 by cytosolic acidosis. Protonation of E306 increases channel-phospholipid interaction leading to TREK-1 opening without direct-mechanical stimulation. At higher concentrations, intracellular phospholipids inhibit channel activation by stretch, intracellular acidosis, and AA. Binding endogenous negative inner leaflet phospholipids with polylysine reduces the inhibition and reveals channel stimulation by exogenous intracellular phospholipids. Both stimulatory and inhibitory effects are observed with phosphatidylinositol (PI), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidic acid (PA), but not diacylglycerol (DG), suggesting that the phosphate at position 3 is required, although the net charge is not critical. Membrane phospholipids, including PIP2, are major regulators of TREK-1 channel activity.

Expression of the mechanosensitive 2PK+ channel TREK‐1 in human osteoblasts

TREK-1 is a mechanosensitive member of the two-pore domain potassium channel family (2PK+) that is also sensitive to lipids, free fatty acids (including arachidonic acid), temperature, intracellular pH, and a range of clinically relevant compounds including volatile anaesthetics. TREK-1 is known to be expressed at high levels in excitable tissues, such as the nervous system, the heart and smooth muscle, where it is believed to play a prominent role in controlling resting cell membrane potential and electrical excitability. In this report, we use RT-PCR, Western blotting and immunohistochemistry to confirm that human derived osteoblasts and MG63 cells express TREK-1 mRNA and protein. In addition, we show gene expression of TREK2c and TRAAK channels. Furthermore, whole cell patch clamp electrophysiology demonstrates that these cells express a spontaneously active, outwardly rectifying potassium "background leak" current that shares many similarities to TREK-1. The outward current is largely insensitive to TEA and Ba2+, and is sensitive to application of lysophosphatidylcholine (LPC). In addition, blocking TREK-1 channel activity is shown to upregulate bone cell proliferation. It is concluded that human osteoblasts functionally express TREK-1 and that these channels contribute, at least in part, to the resting membrane potential of human osteoblast cells. We hypothesise a possible role for TREK-1 in mechanotransduction, leading to bone remodelling.

Sequential Phosphorylation Mediates Receptor- and Kinase-induced Inhibition of TREK-1 Background Potassium Channels

Background potassium channels determine membrane potential and input resistance and serve as prominent effectors for modulatory regulation of cellular excitability. TREK-1 is a two-pore domain background K+ channel (KCNK2, K2P2.1) that is sensitive to a variety of physicochemical and humoral factors. In this work, we used a recombinant expression system to show that activation of G alpha(q)-coupled receptors leads to inhibition of TREK-1 channels via protein kinase C (PKC), and we identified a critical phosphorylation site in a key regulatory domain that mediates inhibition of the channel. In HEK 293 cells co-expressing TREK-1 and either the thyrotropin-releasing hormone receptor (TRHR1) or the Orexin receptor (Orx1R), agonist stimulation induced robust channel inhibition that was suppressed by a bisindolylmaleimide PKC inhibitor but not by a protein kinase A blocker ((R(p))-cAMP-S). Channel inhibition by agonists or by direct activators of PKC (phorbol dibutyrate) and PKA (forskolin) was disrupted not only by alanine or aspartate mutations at an identified PKA site (Ser-333) in the C terminus, but also at a more proximal regulatory site in the cytoplasmic C terminus (Ser-300); S333A and S300A mutations enhanced basal TREK-1 current, whereas S333D and S300D substitutions mimicked phosphorylation and strongly diminished currents. When studied in combination, TREK-1 current density was enhanced in S300A/S333D but reduced in S300D/S333A mutant channels. Channel mutants were expressed and appropriately targeted to cell membranes. Together, these data support a sequential phosphorylation model in which receptor-induced kinase activation drives modification at Ser-333 that enables subsequent phosphorylation at Ser-300 to inhibit TREK-1 channel activity.