The TREK two P domain K+ channels

Bioluminescence Imaging of Potassium Ion Using a Sensory Luciferin and an Engineered Luciferase

Bioluminescent indicators are power tools for studying dynamic biological processes. In this study, we present the generation of novel bioluminescent indicators by modifying the luciferin molecule with an analyte-binding moiety. Specifically, we have successfully developed the first bioluminescent indicator for potassium ions (K+), which are critical electrolytes in biological systems. Our approach involved the design and synthesis of a K+-binding luciferin named potassiorin. Additionally, we engineered a luciferase enzyme called BRIPO (bioluminescent red indicator for potassium) to work synergistically with potassiorin, resulting in optimized K+-dependent bioluminescence responses. Through extensive validation in cell lines, primary neurons, and live mice, we demonstrated the efficacy of this new tool for detecting K+. Our research demonstrates an innovative concept of incorporating sensory moieties into luciferins to modulate luciferase activity. This approach has great potential for developing a wide range of bioluminescent indicators, advancing bioluminescence imaging (BLI), and enabling the study of various analytes in biological systems.

The functions of mechanosensitive ion channels in tooth and bone tissues

Tooth and bone are independent tissues with a close relationship. Both are composed of a highly calcified outer structure and soft inner tissue, and both are constantly under mechanical stress. In particular, the alveolar bone and tooth constitute an occlusion system and suffer from masticatory and occlusal force. Thus, mechanotransduction is a key process in many developmental, physiological and pathological processes in tooth and bone. Mechanosensitive ion channels such as Piezo1 and Piezo2 are important participants in mechanotransduction, but their functions in tooth and bone are poorly understood. This review summarizes our current understanding of mechanosensitive ion channels and their roles in tooth and bone tissues. Research in these areas may shed new light on the regulation of tooth and bone tissues and potential treatments for diseases affecting these tissues.

Trek2a regulates gnrh3 expression under control of melatonin receptor Mt1 and α2-adrenoceptor

Gonadotrophin-releasing hormone (GnRH) expression is associated with the two-pore domain potassium ion (K⁺) channel-related K⁺ (TREK) channel trek2a expression and melatonin levels. We aimed to investigate correlation of trek2a expression with gnrh3 expression, and regulatory mechanisms of trek2a expression by the melatonin receptor Mt1 and α₂-adrenoceptor which are regulated by melatonin. trek2a specific siRNA, Mt1 antagonist luzindole and α₂-adrenoceptor antagonist prazosin were administered into the adult zebrafish brain and gene expressions were examined by real-time PCR. trek2a specific siRNA administration significantly reduced expression levels of trek2a, gnrh3 and mt1. Luzindole administration suppressed trek2a and gnrh3 expressions. Prazosin administration reduced trek2a and gnrh3 expressions. It is suggested that Trek2a regulates gnrh3 expression under the control of Mt1 and α₂-adrenoceptor.

Enhanced Expression of TREK-1 Is Related with Chronic Constriction Injury of Neuropathic Pain Mouse Model in Dorsal Root Ganglion

Neuropathic pain is a complex state showing increased pain response with dysfunctional inhibitory neurotransmission. The TREK family, one of the two pore domain K⁺ (K2P) channel subgroups were focused among various mechanisms of neuropathic pain. These channels influence neuronal excitability and are thought to be related in mechano/thermosensation. However, only a little is known about the expression and role of TREK-1 and TREK-2, in neuropathic pain. It is performed to know whether TREK-1 and/ or 2 are positively related in dorsal root ganglion (DRG) of a mouse neuropathic pain model, the chronic constriction injury (CCI) model. Following this purpose, Reverse Transcription Polymerase Chain Reaction (RT-PCR) and western blot analyses were performed using mouse DRG of CCI model and compared to the sham surgery group. Immunofluorescence staining of isolectin- B4 (IB4) and TREK were performed. Electrophysiological recordings of single channel currents were analyzed to obtain the information about the channel. Interactions with known TREK activators were tested to confirm the expression. While both TREK-1 and TREK-2 mRNA were significantly overexpressed in DRG of CCI mice, only TREK-1 showed significant increase (~9 fold) in western blot analysis. The TREK-1-like channel recorded in DRG neurons of the CCI mouse showed similar current-voltage relationship and conductance to TREK-1. It was easily activated by low pH solution (pH 6.3), negative pressure, and riluzole. Immunofluorescence images showed the expression of TREK-1 was stronger compared to TREK-2 on IB4 positive neurons. These results suggest that modulation of the TREK-1 channel may have beneficial analgesic effects in neuropathic pain patients.

Polycystins and partners: proposed role in mechanosensitivity

Mutations of the two polycystins, PC1 and PC2, lead to polycystic kidney disease. Polycystins are able to form complexes with numerous families of proteins that have been suggested to participate in mechanical sensing. The proposed role of polycystins and their partners in the kidney primary cilium is to sense urine flow. A role for polycystins in mechanosensing has also been shown in other cell types such as vascular smooth muscle cells and cardiac myocytes. At the plasma membrane, polycystins interact with diverse ion channels of the TRP family and with stretch-activated channels (Piezos, TREKs). The actin cytoskeleton and its interacting proteins, such as filamin A, have been shown to be essential for these interactions. Numerous proteins involved in cell-cell and cell-extracellular matrix junctions interact with PC1 and/or PC2. These multimeric protein complexes are important for cell structure integrity, the transmission of force, as well as for mechanosensing and mechanotransduction. A group of polycystin partners are also involved in subcellular trafficking mechanisms. Finally, PC1 and especially PC2 interact with elements of the endoplasmic reticulum and are essential components of calcium homeostasis. In conclusion, we propose that both PC1 and PC2 act as conductors to tune the overall cellular mechanosensitivity.

Changes in sensory activity of ocular surface sensory nerves during allergic keratoconjunctivitis

Peripheral neural mechanisms underlying the sensations of irritation, discomfort, and itch accompanying the eye allergic response have not been hitherto analyzed. We explored this question recording the changes in the electrical activity of corneoconjunctival sensory nerve fibers of the guinea pig after an ocular allergic challenge. Sensitization was produced by i.p. ovalbumin followed by repeated application in the eye of 10% ovalbumin on days 14 to 18. Blinking and tearing rate were measured. Spontaneous and stimulus-evoked (mechanical, thermal, chemical) impulse activity was recorded from mechanonociceptor, polymodal nociceptor and cold corneoscleral sensory afferent fibers. After a single (day 14) or repeated daily exposures to the allergen during the following 3 to 4days, tearing and blinking rate increased significantly. Also, sensitization was observed in mechanonociceptors (transient reduction of mechanical threshold only on day 14) and in polymodal nociceptors (sustained enhancement of the impulse response to acidic stimulation). In contrast, cold thermoreceptors showed a significant decrease in basal ongoing activity and in the response to cooling. Treatment with the TRPV1 and TRPA1 blockers capsazepine and HC-030031 reversed the augmented blinking. Only capsazepine attenuated tearing rate increase and sensitization of the polymodal nociceptors response to CO2. Capsazepine also prevented the decrease in cold thermoreceptor activity caused by the allergic challenge. We conclude that changes in nerve impulse activity accompanying the ocular allergic response, primarily mediated by activation of nociceptor's TRPV1 and to a lesser degree by activation of TRPA1 channels, explain the eye discomfort sensations accompanying allergic episodes.

Xerocytosis is caused by mutations that alter the kinetics of the mechanosensitive channel PIEZO1

Familial xerocytosis (HX) in humans is an autosomal disease that causes dehydration of red blood cells resulting in hemolytic anemia which has been traced to two individual mutations in the mechanosensitive ion channel, PIEZO1. Each mutation alters channel kinetics in ways that can explain the clinical presentation. Both mutations slowed inactivation and introduced a pronounced latency for activation. A conservative substitution of lysine for arginine (R2456K) eliminated inactivation and also slowed deactivation, indicating that this mutant's loss of charge is not responsible for HX. Fitting the current vs. pressure data to Boltzmann distributions showed that the half-activation pressure, P1/2, for M2225R was similar to that of WT, whereas mutations at position 2456 were left shifted. The absolute stress sensitivity was calibrated by cotransfection and comparison with MscL, a well-characterized mechanosensitive channel from bacteria that is driven by bilayer tension. The slope sensitivity of WT and mutant human PIEZO1 (hPIEZO1) was similar to that of MscL implying that the in-plane area increased markedly, by ∼6-20 nm(2) during opening. In addition to the behavior of individual channels, groups of hPIEZO1 channels could undergo simultaneous changes in kinetics including a loss of inactivation and a long (∼200 ms), silent latency for activation. These observations suggest that hPIEZO1 exists in spatial domains whose global properties can modify channel gating. The mutations that create HX affect cation fluxes in two ways: slow inactivation increases the cation flux, and the latency decreases it. These data provide a direct link between pathology and mechanosensitive channel dysfunction in nonsensory cells.

The mechanosensitive ion channel Piezo1 is inhibited by the peptide GsMTx4

Cells can respond to mechanical stress by gating mechanosensitive ion channels (MSCs). The cloning of Piezo1, a eukaryotic cation selective MSC, defines a new system for studying mechanical transduction at the cellular level. Because Piezo1 has electrophysiological properties similar to those of endogenous cationic MSCs that are selectively inhibited by the peptide GsMTx4, we tested whether the peptide targets Piezo1 activity. Extracellular GsMTx4 at micromolar concentrations reversibly inhibited ∼80% of the mechanically induced current of outside-out patches from transfected HEK293 cells. The inhibition was voltage insensitive, and as seen with endogenous MSCs, the mirror image d enantiomer inhibited like the l. The rate constants for binding and unbinding based on Piezo1 current kinetics provided association and dissociation rates of 7.0 × 10(5) M(-1) s(-1) and 0.11 s(-1), respectively, and a K(D) of ∼155 nM, similar to values previously reported for endogenous MSCs. Consistent with predicted gating modifier behavior, GsMTx4 produced an ∼30 mmHg rightward shift in the pressure-gating curve and was active on closed channels. In contrast, streptomycin, a nonspecific inhibitor of cationic MSCs, showed the use-dependent inhibition characteristic of open channel block. The peptide did not block currents of the mechanical channel TREK-1 on outside-out patches. Whole-cell Piezo1 currents were also reversibly inhibited by GsMTx4, and although the off rate was nearly identical to that of outside-out patches, differences were observed for the on rate. The ability of GsMTx4 to target the mechanosensitivity of Piezo1 supports the use of this channel in high-throughput screens for pharmacological agents and diagnostic assays.

TREK-1 channels do not mediate nitrergic neurotransmission in circular smooth muscle from the lower oesophageal sphincter

BACKGROUND AND PURPOSE

The ionic mechanisms underlying nitrergic inhibitory junction potentials (IJPs) in gut smooth muscle remain a matter of debate. Recently, it has been reported that opening of TWIK-related K(+) channel 1 (TREK-1) K(+) channels contributes to the nitrergic IJP in colonic smooth muscle. We investigated the effects of TREK-1 channel blockers on nitrergic neurotransmission in mouse and opossum lower oesophageal sphincter (LOS) circular smooth muscle (CSM).

EXPERIMENTAL APPROACH

The effects of TREK-1 channel blockers were characterized pharmacologically in murine and opossum gut smooth muscle using conventional intracellular and tension recordings.

KEY RESULTS

In LOS, L-methionine depolarized the resting membrane potential (RMP) but did not inhibit the nitrergic IJP. Cumulative application of theophylline hyperpolarized the RMP and inhibited the nitrergic IJP concentration dependently. The induced membrane hyperpolarization was prevented by pre-application of caffeine, but not by 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one. 8-Br-cAMP significantly hyperpolarized membrane potential and increased the amplitude of the nitrergic IJP. In opossum LOS muscle strips, L-methionine increased resting tone but had no effect on nerve-mediated LOS relaxation. On the other hand, theophylline markedly inhibited tone. In CSM from mouse proximal colon, L-methionine caused modest inhibition of nitrergic IJPs.

CONCLUSIONS AND IMPLICATIONS

TREK-1 channels were not involved in the nitrergic IJP in LOS CSM. Not only does L-methionine have no effect on the nitrergic IJP or LOS relaxation, but the effect of theophylline appears to be due to interruption of Ca(2+)-releasing pathways (i.e. caffeine-like effect) rather than via blockade of TREK-1 channels.

Lysophospholipid receptors in cell signaling

There is increasing evidence that different phospholipids are involved in regulation of various cell processes and cell-cell interactions. Lysophospholipids (lysophosphatidic acid, lysophosphatidylcholine) and a number of lysosphingolipids play particular roles in these regulations. Their effects are mediated by specific G-protein-coupled receptors. G-Protein coupled signal transduction to the cell nucleus involving a chain of intracellular protein kinases induces the main effects in cells--growth, proliferation, survival, or apoptosis. This review summarizes recent data on various groups of lysophospholipid receptors and their cell signal transduction pathways.

Evidence for TREK-like tandem-pore domain channels in intrapulmonary chemoreceptor chemotransduction

Intrapulmonary chemoreceptors (IPC) are carbon dioxide sensing neurons that innervate the lungs of birds, control breathing pattern, and are inhibited by halothane and intracellular acidosis. TASK and TREK are subfamilies of tandem-pore domain potassium leak channels, important in setting resting membrane potential, that are affected by volatile anesthetics and acidosis. We hypothesized that such channels might underlie signal transduction in IPC. We treated mallard ducks with four volatile anesthetics in increasing concentrations to test their effects on IPC discharge through single cell, extracellular recording from vagal fibers. Isoflurane inhalation attenuated IPC discharge only at 8.25% inspired (alpha=0.05). Halothane attenuated IPC discharge significantly (alpha=0.05) at all treatment levels. Chloroform at 3.8%, 5.6%, and 8.25% significantly attenuated IPC discharge (alpha=0.05). Ether at 1.9%, 2.9%, and 3.8% significantly attenuated IPC discharge (alpha=0.05), abolishing IPC discharge at 3.8% inspired. The pharmacological signature of IPC discharge attenuation suggests that IPC express tandem-pore domain leak channels similar to TREK channels, which are inhibited by intracellular acidosis.

Stretch-activated channels: a mini-review. Are stretch-activated channels an ocular barometer?

All cells are subject to physical forces by virtue of their position in a dynamically changing environment. This review outlines the various putative 'mechanosensors', or sensors of pressure cells possess, and discusses in particular the role stretch-activated membrane channels play in pressure recognition and transduction. The widespread occurrence of these channels is discussed and these 'mechanosensors' are related to pressure-related diseases, in particular, glaucoma.

Thermosensitivity of the two-pore domain K+ channels TREK-2 and TRAAK

TREK-1, TREK-2 and TRAAK are members of the two-pore domain K+ (K2P) channel family and are activated by membrane stretch and free fatty acids. TREK-1 has been shown to be sensitive to temperature in expression systems. We studied the temperature-sensitivity of TREK-2 and TRAAK in COS-7 cells and in neuronal cells. In transfected COS-7 cells, TREK-2 and TRAAK whole-cell currents increased approximately 20-fold as the bath temperature was raised from 24 degrees C to 42 degrees C. Similarly, in cell-attached patches of COS-7 cells, channel activity was very low, but increased progressively as the bath temperature was raised from 24 degrees C to 42 degrees C. The thresholds for activation of TREK-2 and TRAAK were approximately 25 degrees C and approximately 31 degrees C, respectively. Other K2P channels such as TASK-3 and TRESK-2 were not significantly affected by an increase in temperature from 24 degrees C to 37 degrees C. When the C-terminus of TREK-2 was replaced with that of TASK-3, its sensitivity to free fatty acids and protons was abolished, but the mutant could still be activated by heat. At 37 degrees C, TREK-1, TREK-2 and TRAAK were sensitive to arachidonic acid, pH and membrane stretch in both cell-attached and inside-out patches. In cerebellar granule and dorsal root ganglion neurones, TREK-1, TREK-2 and TRAAK were generally inactive in the cell-attached state at 24 degrees C, but became very active at 37 degrees C. In cell-attached patches of ventricular myocytes, TREK-1 was also normally closed at 24 degrees C, but was active at 37 degrees C. These results show that TREK-2 and TRAAK are also temperature-sensitive channels, are active at physiological body temperature, and therefore would contribute to the background K+ conductance and regulate cell excitability in response to various physical and chemical stimuli.

Tandem-pore domain potassium channels are functionally expressed in retinal (Müller) glial cells

Tandem-pore domain (2P-domain) K+-channels regulate neuronal excitability, but their function in glia, particularly, in retinal glial cells, is unclear. We have previously demonstrated the immunocytochemical localization of the 2P-domain K+ channels TASK-1 and TASK-2 in retinal Müller glial cells of amphibians. The purpose of the present study was to determine whether these channels were functional, by employing whole-cell recording from frog and mammalian (guinea pig, rat and mouse) Müller cells and confocal microscopy to monitor swelling in rat Müller cells. TASK-like immunolabel was localized in these cells. The currents mediated by 2P-domain channels were studied in isolation after blocking Kir, K(A), K(D), and BK channels. The remaining cell conductance was mostly outward and was depressed by acid pH, bupivacaine, methanandamide, quinine, and clofilium, and activated by alkaline pH in a manner consistent with that described for TASK channels. Arachidonic acid (an activator of TREK channels) had no effect on this conductance. Blockade of the conductance with bupivacaine depolarized the Müller cell membrane potential by about 50%. In slices of the rat retina, adenosine inhibited osmotic glial cell swelling via activation of A1 receptors and subsequent opening of 2P-domain K+ channels. The swelling was strongly increased by clofilium and quinine (inhibitors of 2P-domain K+ channels). These data suggest that 2P-domain K+ channels are involved in homeostasis of glial cell volume, in activity-dependent spatial K+ buffering and may play a role in maintenance of a hyperpolarized membrane potential especially in conditions where Kir channels are blocked or downregulated.

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.

Single-channel properties and pH sensitivity of two-pore domain K+ channels of the TALK family

The two-pore K2P channel family comprises TASK, TREK, TWIK, TRESK, TALK, and THIK subfamilies, and TALK-1, TALK-2, and TASK-2 are functional members of the TALK subfamily. Here we report for the first time the single-channel properties of TALK-2 and its pHo sensitivity, and compare them to those of TALK-1 and TASK-2. In transfected COS-7 cells, the three TALK K2P channels could be identified easily by their differences in single-channel conductance and gating kinetics. The single-channel conductances of TALK-1, TALK-2, and TASK-2 in symmetrical 150 mM KCl were 21, 33, and 70 pS (-60 mV), respectively. TALK-2 was sensitive mainly to the alkaline range (pH 7-10), whereas TALK-1 and TASK-2 were sensitive to a wider pHo range (6-10). The effect of pH changes was mainly on the opening frequency. Thus, members of the TALK family expressed in native tissues may be identified based on their single-channel kinetics and pHo sensitivity.