Pharmacological activation of PIEZO1 in human red blood cells prevents Plasmodium falciparum invasion

An inherited gain-of-function variant (E756del) in the mechanosensitive cationic channel PIEZO1 was shown to confer a significant protection against severe malaria. Here, we demonstrate in vitro that human red blood cell (RBC) infection by Plasmodium falciparum is prevented by the pharmacological activation of PIEZO1. Yoda1 causes an increase in intracellular calcium associated with rapid echinocytosis that inhibits RBC invasion, without affecting parasite intraerythrocytic growth, division or egress. Notably, Yoda1 treatment significantly decreases merozoite attachment and subsequent RBC deformation. Intracellular Na⁺/K⁺ imbalance is unrelated to the mechanism of protection, although delayed RBC dehydration observed in the standard parasite culture medium RPMI/albumax further enhances the resistance to malaria conferred by Yoda1. The chemically unrelated Jedi2 PIEZO1 activator similarly causes echinocytosis and RBC dehydration associated with resistance to malaria invasion. Spiky outward membrane projections are anticipated to reduce the effective surface area required for both merozoite attachment and internalization upon pharmacological activation of PIEZO1. Globally, our findings indicate that the loss of the typical biconcave discoid shape of RBCs, together with an altered optimal surface to volume ratio, induced by PIEZO1 pharmacological activation prevent efficient P. falciparum invasion.

A machine-learning algorithm integrating baseline serum proteomic signatures predicts exercise responsiveness in overweight males with prediabetes

The molecular transducers conferring the benefits of chronic exercise in diabetes prevention remain to be comprehensively investigated. Herein, serum proteomic profiling of 688 inflammatory and metabolic biomarkers in 36 medication-naive overweight and obese men with prediabetes reveals hundreds of exercise-responsive proteins modulated by 12-week high-intensity interval exercise training, including regulators of metabolism, cardiovascular system, inflammation, and apoptosis. Strong associations are found between proteins involved in gastro-intestinal mucosal immunity and metabolic outcomes. Exercise-induced changes in trefoil factor 2 (TFF2) are associated with changes in insulin resistance and fasting insulin, whereas baseline levels of the pancreatic secretory granule membrane major glycoprotein GP2 are related to changes in fasting glucose and glucose tolerance. A hybrid set of 23 proteins including TFF2 are differentially altered in exercise responders and non-responders. Furthermore, a machine-learning algorithm integrating baseline proteomic signatures accurately predicts individualized metabolic responsiveness to exercise training.

Adipose-targeted triiodothyronine therapy counteracts obesity-related metabolic complications and atherosclerosis with negligible side effects

Thyroid hormone (TH) is a thermogenic activator with anti-obesity potential. However, systemic TH administration has no obvious clinical benefits on weight reduction. Herein we selectively delivered triiodothyronine (T3) to adipose tissues by encapsulating T3 in liposomes modified with an adipose homing peptide (PLT3). Systemic T3 administration failed to promote thermogenesis in brown and white adipose tissues (WAT) due to a feedback suppression of sympathetic innervation. PLT3 therapy effectively obviated this feedback suppression on adrenergic inputs, and potently induced browning and thermogenesis of WAT, leading to alleviation of obesity, glucose intolerance, insulin resistance, and fatty liver in obese mice. Furthermore, PLT3 was much more effective than systemic T3 therapy in reducing hypercholesterolemia and atherosclerosis in apoE-deficient mice. These findings uncover WAT as a viable target mediating the therapeutic benefits of TH and provide a safe and efficient therapeutic strategy for obesity and its complications by delivering TH to adipose tissue.

Piezo1 and Piezo2 foster mechanical gating of K2P channels

Mechanoelectrical transduction is mediated by the opening of different types of force-sensitive ion channels, including Piezo1/2 and the TREK/TRAAK K2P channels. Piezo1 curves the membrane locally into an inverted dome that reversibly flattens in response to force application. Moreover, Piezo1 forms numerous preferential interactions with various membrane lipids, including cholesterol. Whether this structural architecture influences the functionality of neighboring membrane proteins is unknown. Here, we show that Piezo1/2 increase TREK/TRAAK current amplitude, slow down activation/deactivation, and remove inactivation upon mechanical stimulation. These findings are consistent with a mechanism whereby Piezo1/2 cause a local depletion of membrane cholesterol associated with a prestress of TREK/TRAAK channels. This regulation occurs in mouse fibroblasts between endogenous Piezo1 and TREK-1/2, both channel types acting in concert to delay wound healing. In conclusion, we demonstrate a community effect between different structural and functional classes of mechanosensitive ion channels.

Adipocyte Piezo1 mediates obesogenic adipogenesis through the FGF1/FGFR1 signaling pathway in mice

White adipose tissue (WAT) expansion in obesity occurs through enlargement of preexisting adipocytes (hypertrophy) and through formation of new adipocytes (adipogenesis). Adipogenesis results in WAT hyperplasia, smaller adipocytes and a metabolically more favourable form of obesity. How obesogenic WAT hyperplasia is induced remains, however, poorly understood. Here, we show that the mechanosensitive cationic channel Piezo1 mediates diet-induced adipogenesis. Mice lacking Piezo1 in mature adipocytes demonstrated defective differentiation of preadipocyte into mature adipocytes when fed a high fat diet (HFD) resulting in larger adipocytes, increased WAT inflammation and reduced insulin sensitivity. Opening of Piezo1 in mature adipocytes causes the release of the adipogenic fibroblast growth factor 1 (FGF1), which induces adipocyte precursor differentiation through activation of the FGF-receptor-1. These data identify a central feed-back mechanism by which mature adipocytes control adipogenesis during the development of obesity and suggest Piezo1-mediated adipocyte mechano-signalling as a mechanism to modulate obesity and its metabolic consequences.

Adipose tissue expansion occurs via enlargement of adipocytes as well as the generation of new fat cells, the latter being associated with more favorable metabolic outcomes. Here, the authors show that activation of adipocyte Piezo1 results in release of FGF1 and stimulates the differentiation of adipocyte precursor cells.

Piezo Ion Channels in Cardiovascular Mechanobiology

Mechanotransduction has a key role in vascular development, physiology, and disease states. Piezo1 is a mechanosensitive (MS) nonselective cationic channel that occurs in endothelial and vascular smooth muscle cells. It is activated by shear stress associated with increases in local blood flow, as well as by cell membrane stretch upon elevation of blood pressure. Here, we briefly review the pharmacological modulators of Piezo and discuss current understanding of the role of Piezo1 in vascular mechanobiology and associated clinical disorders, such as atherosclerosis and hypertension. Ultimately, we believe that this research will help identify novel therapeutic strategies for the treatment of vascular diseases.

Smooth muscle filamin A is a major determinant of conduit artery structure and function at the adult stage

Human mutations in the X-linked FLNA gene are associated with a remarkably diverse phenotype, including severe arterial morphological anomalies. However, the role for filamin A (FlnA) in vascular cells remains partially understood. We used a smooth muscle (sm)-specific conditional mouse model to delete FlnA at the adult stage, thus avoiding the developmental effects of the knock-out. Inactivation of smFlnA in adult mice significantly lowered blood pressure, together with a decrease in pulse pressure. However, both the aorta and carotid arteries showed a major outward hypertrophic remodeling, resistant to losartan, and normally occurring in hypertensive conditions. Notably, arterial compliance was significantly enhanced in the absence of smFlnA. Moreover, reactivity of thoracic aorta rings to a variety of vasoconstrictors was elevated, while basal contractility in response to KCl depolarization was reduced. Enhanced reactivity to the thromboxane A2 receptor agonist U46619 was fully reversed by the ROCK inhibitor Y27632. We discuss the possibility that a reduction in arterial stiffness upon smFlnA inactivation might cause a compensatory increase in conduit artery diameter for normalization of parietal tension, independently of the ROCK pathway. In conclusion, deletion of smFlnA in adult mice recapitulates the vascular phenotype of human bilateral periventricular nodular heterotopia, culminating in aortic dilatation.

Piezo1 in Smooth Muscle Cells Is Involved in Hypertension-Dependent Arterial Remodeling

The mechanically activated non-selective cation channel Piezo1 is a determinant of vascular architecture during early development. Piezo1-deficient embryos die at midgestation with disorganized blood vessels. However, the role of stretch-activated ion channels (SACs) in arterial smooth muscle cells in the adult remains unknown. Here, we show that Piezo1 is highly expressed in myocytes of small-diameter arteries and that smooth-muscle-specific Piezo1 deletion fully impairs SAC activity. While Piezo1 is dispensable for the arterial myogenic tone, it is involved in the structural remodeling of small arteries. Increased Piezo1 opening has a trophic effect on resistance arteries, influencing both diameter and wall thickness in hypertension. Piezo1 mediates a rise in cytosolic calcium and stimulates activity of transglutaminases, cross-linking enzymes required for the remodeling of small arteries. In conclusion, we have established the connection between an early mechanosensitive process, involving Piezo1 in smooth muscle cells, and a clinically relevant arterial remodeling.

An alternative to force

Researchers have discovered a synthetic small molecule that activates a mechanosensitive ion channel involved in a blood disorder.

Piezo1-dependent stretch-activated channels are inhibited by Polycystin-2 in renal tubular epithelial cells

Mechanical forces associated with fluid flow and/or circumferential stretch are sensed by renal epithelial cells and contribute to both adaptive or disease states. Non-selective stretch-activated ion channels (SACs), characterized by a lack of inactivation and a remarkably slow deactivation, are active at the basolateral side of renal proximal convoluted tubules. Knockdown of Piezo1 strongly reduces SAC activity in proximal convoluted tubule epithelial cells. Similarly, overexpression of Polycystin-2 (PC2) or, to a greater extent its pathogenic mutant PC2-740X, impairs native SACs. Moreover, PC2 inhibits exogenous Piezo1 SAC activity. PC2 coimmunoprecipitates with Piezo1 and deletion of its N-terminal domain prevents both this interaction and inhibition of SAC activity. These findings indicate that renal SACs depend on Piezo1, but are critically conditioned by PC2.

Sensing pressure with ion channels

Opening of stretch-activated ion channels (SACs) is the earliest event occurring in mechanosensory transduction. The molecular identity of mammalian SACs has long remained a mystery. Only very recently, Piezo1 and Piezo2 have been shown to be essential components of distinct SACs and moreover, purified Piezo1 forms cationic channels when reconstituted into artificial bilayers. In line with these findings, dPiezo was demonstrated to act in the Drosophila mechanical nociception pathway. Finally, the 3D structure of the two-pore domain potassium channel (K(2P)), TRAAK [weakly inward rectifying K⁺ channel (TWIK)-related arachidonic acid stimulated K⁺ channel], has recently been solved, providing valuable information about pharmacology, selectivity and gating mechanisms of stretch-activated K⁺ channels (SAKs). These recent findings allow a better understanding of the molecular basis of molecular and cellular mechanotransduction.

Mechanoprotection by polycystins against apoptosis is mediated through the opening of stretch-activated K(2P) channels

How renal epithelial cells respond to increased pressure and the link with kidney disease states remain poorly understood. Pkd1 knockout or expression of a PC2 pathogenic mutant, mimicking the autosomal dominant polycystic kidney disease, dramatically enhances mechanical stress-induced tubular apoptotic cell death. We show the presence of a stretch-activated K(+) channel dependent on the TREK-2 K(2P) subunit in proximal convoluted tubule epithelial cells. Our findings further demonstrate that polycystins protect renal epithelial cells against apoptosis in response to mechanical stress, and this function is mediated through the opening of stretch-activated K(2P) channels. Thus, to our knowledge, we establish for the first time, both in vitro and in vivo, a functional relationship between mechanotransduction and mechanoprotection. We propose that this mechanism is at play in other important pathologies associated with apoptosis and in which pressure or flow stimulation is altered, including heart failure or atherosclerosis.

Polycystins and renovascular mechanosensory transduction

Autosomal dominant polycystic kidney disease is a common disorder, affecting approximately one in 1,000 individuals. This disease is characterized by the presence of renal and extrarenal cysts, as well as by cardiovascular abnormalities, including hypertension and intracranial aneurysms. Mutations in the PKD1 gene account for 85% of cases, whereas mutations in PKD2 account for the remaining 15% of cases. Findings from the past 10 years indicate that polycystins, the products of the PKD genes, have a key role in renal and vascular mechanosensory transduction. In the primary cilium of renal, nodal, and endothelial cells, polycystins are proposed to act as flow sensors. In addition, the ratio of polycystin-1 to polycystin-2 regulates pressure sensing in arterial myocytes. In this Review, we summarize the data indicating that polycystins are key molecules in mechanotransduction. Moreover, we discuss the role of nucleotide release and autocrine and/or paracrine purinergic signaling in both fluid flow and pressure responses. Finally, we discuss the possible role of altered mechanosensory transduction in the etiology of polycystic kidney disease.

Canonical TRP channels and mechanotransduction: from physiology to disease states

Mechano-gated ion channels play a key physiological role in cardiac, arterial, and skeletal myocytes. For instance, opening of the non-selective stretch-activated cation channels in smooth muscle cells is involved in the pressure-dependent myogenic constriction of resistance arteries. These channels are also implicated in major pathologies, including cardiac hypertrophy or Duchenne muscular dystrophy. Seminal work in prokaryotes and invertebrates highlighted the role of transient receptor potential (TRP) channels in mechanosensory transduction. In mammals, recent findings have shown that the canonical TRPC1 and TRPC6 channels are key players in muscle mechanotransduction. In the present review, we will focus on the functional properties of TRPC1 and TRPC6 channels, on their mechano-gating, regulation by interacting cytoskeletal and scaffolding proteins, physiological role and implication in associated diseases.

A polycystin-2 (TRPP2) dimerization domain essential for the function of heteromeric polycystin complexes

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in two genes, PKD1 and PKD2, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Earlier work has shown that PC1 and PC2 assemble into a polycystin complex implicated in kidney morphogenesis. PC2 also assembles into homomers of uncertain functional significance. However, little is known about the molecular mechanisms that direct polycystin complex assembly and specify its functions. We have identified a coiled coil in the C-terminus of PC2 that functions as a homodimerization domain essential for PC1 binding but not for its self-oligomerization. Dimerization-defective PC2 mutants were unable to reconstitute PC1/PC2 complexes either at the plasma membrane (PM) or at PM-endoplasmic reticulum (ER) junctions but could still function as ER Ca(2+)-release channels. Expression of dimerization-defective PC2 mutants in zebrafish resulted in a cystic phenotype but had lesser effects on organ laterality. We conclude that C-terminal dimerization of PC2 specifies the formation of polycystin complexes but not formation of ER-localized PC2 channels. Mutations that affect PC2 C-terminal homo- and heteromerization are the likely molecular basis of cyst formation in ADPKD.

Polycystin-1 and -2 dosage regulates pressure sensing

Autosomal-dominant polycystic kidney disease, the most frequent monogenic cause of kidney failure, is induced by mutations in the PKD1 or PKD2 genes, encoding polycystins TRPP1 and TRPP2, respectively. Polycystins are proposed to form a flow-sensitive ion channel complex in the primary cilium of both epithelial and endothelial cells. However, how polycystins contribute to cellular mechanosensitivity remains obscure. Here, we show that TRPP2 inhibits stretch-activated ion channels (SACs). This specific effect is reversed by coexpression with TRPP1, indicating that the TRPP1/TRPP2 ratio regulates pressure sensing. Moreover, deletion of TRPP1 in smooth muscle cells reduces SAC activity and the arterial myogenic tone. Inversely, depletion of TRPP2 in TRPP1-deficient arteries rescues both SAC opening and the myogenic response. Finally, we show that TRPP2 interacts with filamin A and demonstrate that this actin crosslinking protein is critical for SAC regulation. This work uncovers a role for polycystins in regulating pressure sensing.

Molecular basis of the mammalian pressure-sensitive ion channels: focus on vascular mechanotransduction

Mechano-gated ion channels are implicated in a variety of neurosensory functions ranging from touch sensitivity to hearing. In the heart, rhythm disturbance subsequent to mechanical effects is also associated with the activation of stretch-sensitive ion channels. Arterial autoregulation in response to hemodynamic stimuli, a vital process required for protection against hypertension-induced injury, is similarly dependent on the activity of force-sensitive ion channels. Seminal work in prokaryotes and invertebrates, including the nematode Caenorhabditis elegans and the fruit fly drosophila, greatly helped to identify the molecular basis of volume regulation, hearing and touch sensitivity. In mammals, more recent findings have indicated that members of several structural family of ion channels, namely the transient receptor potential (TRP) channels, the amiloride-sensitive ENaC/ASIC channels and the potassium channels K2P and Kir are involved in cellular mechanotransduction. In the present review, we will focus on the molecular and functional properties of these channel subunits and will emphasize on their role in the pressure-dependent arterial myogenic constriction and the flow-mediated vasodilation.

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.

Cardiovascular polycystins: insights from autosomal dominant polycystic kidney disease and transgenic animal models

Mutations in the PKD1 and PKD2 polycystin genes are responsible for autosomal dominant polycystic kidney disease (ADPKD), one of the most prevalent genetic kidney disorders. ADPKD is a multisystem disease characterized by the formation of numerous fluid-filled cysts in the kidneys, the pancreas, and the liver. Moreover, major cardiovascular manifestations are common complications in ADPKD. Intracranial aneurysms and arterial hypertension are among the leading causes of mortality in this disease. In the present review, we summarize our current understanding of the role of polycystins in the development, maintenance, and function of the cardiovascular system.

The versatile nature of the calcium-permeable cation channel TRPP2

TRPP2 is a member of the transient receptor potential (TRP) superfamily of cation channels, which is mutated in autosomal dominant polycystic kidney disease (ADPKD). TRPP2 is thought to function with polycystin 1-a large integral protein-as part of a multiprotein complex involved in transducing Ca(2+)-dependent information. TRPP2 has been implicated in various biological functions including cell proliferation, sperm fertilization, mating behaviour, mechanosensation and asymmetric gene expression. Although its function as a Ca(2+)-permeable cation channel is well established, its precise role in the plasma membrane, the endoplasmic reticulum and the cilium is controversial. Recent studies suggest that TRPP2 function is highly dependent on the subcellular compartment of expression, and is regulated by many interactions with adaptor proteins. This review summarizes the most pertinent evidence about the properties of TRPP2 channels, focusing on the compartment-specific functions of mammalian TRPP2.

Desensitization of mechano-gated K2P channels

The neuronal mechano-gated K2P channels TREK-1 and TRAAK show pronounced desensitization within 100 ms of membrane stretch. Desensitization persists in the presence of cytoskeleton disrupting agents, upon patch excision, and when channels are expressed in membrane blebs. Mechanosensitive currents evoked with a variety of complex stimulus protocols were globally fit to a four-state cyclic kinetic model in detailed balance, without the need to introduce adaptation of the stimulus. However, we show that patch stress can be a complex function of time and stimulation history. The kinetic model couples desensitization to activation, so that gentle conditioning stimuli do not cause desensitization. Prestressing the channels with pressure, amphipaths, intracellular acidosis, or the E306A mutation reduces the peak-to-steady-state ratio by changing the preexponential terms of the rate constants, increasing the steady-state current amplitude. The mechanical responsivity can be accounted for by a change of in-plane area of approximately 2 nm2 between the closed and open conformations. Desensitization and its regulation by chemical messengers is predicted to condition the physiological role of K2P channels.

Cross-talk between the mechano-gated K2P channel TREK-1 and the actin cytoskeleton

TREK-1 (KCNK2) is a K(2P) channel that is highly expressed in fetal neurons. This K(+) channel is opened by a variety of stimuli, including membrane stretch and cellular lipids. Here, we show that the expression of TREK-1 markedly alters the cytoskeletal network and induces the formation of actin- and ezrin-rich membrane protrusions. The genetic inactivation of TREK-1 significantly alters the growth cone morphology of cultured embryonic striatal neurons. Cytoskeleton remodelling is crucially dependent on the protein kinase A phosphorylation site S333 and the interactive proton sensor E306, but is independent of channel permeation. Conversely, the actin cytoskeleton tonically represses TREK-1 mechano-sensitivity. Thus, the dialogue between TREK-1 and the actin cytoskeleton might influence both synaptogenesis and neuronal electrogenesis.

The TREK K2P channels and their role in general anaesthesia and neuroprotection

Two-pore-domain K+ (K(2P)) channels are a diverse and highly regulated superfamily of channels that are thought to provide baseline regulation of membrane excitability. Of these, the TREK channels are expressed highly in the human CNS, and can be activated by temperature, membrane stretch and internal acidosis. In addition, TREK channels are sensitively activated by certain polyunsaturated fatty acids that have been shown to have neuroprotective activity and by volatile and gaseous general anaesthetics. New data derived from studies of knockout animals suggest that TREK-1 might have an important role in the general anaesthetic properties of volatile agents, such as halothane, and provide an explanation for the neuroprotective properties of polyunsaturated fatty acids.

Lysophosphatidic Acid-operated K+ Channels

Lysophosphatidic acid (LPA) is an abundant cellular lipid with a myriad of biological effects. It plays an important role in both inter- and intracellular signaling. Activation of the LPA1-3 G-protein-coupled receptors explains many of the extracellular effects of LPA, including cell growth, differentiation, survival, and motility. However, LPA also acts intracellularly, activating the nuclear hormone receptor peroxisome proliferator-activated receptor-gamma that regulates gene transcription. This study shows that the novel subfamily of mechano-gated K2P channels comprising TREK-1, TREK-2, and TRAAK is strongly activated by intracellular LPA. The LPA-activated 2P domain K+ channels are intracellular ligand-gated K+ channels such as the Ca2+- or the ATP-sensitive K+ channels. LPA reversibly converts these mechano-gated, pH- and voltage-sensitive channels into leak conductances. Gating conversion of the 2P domain K+ channels by intracellular LPA represents a novel form of ion channel regulation. Thus, the TREK and TRAAK channels should be included in the LPA-associated physiological and disease states.

A phospholipid sensor controls mechanogating of the K+ channel TREK-1

TREK-1 (KCNK2 or K(2P)2.1) is a mechanosensitive K(2P) channel that is opened by membrane stretch as well as cell swelling. Here, we demonstrate that membrane phospholipids, including PIP(2), control channel gating and transform TREK-1 into a leak K(+) conductance. A carboxy-terminal positively charged cluster is the phospholipid-sensing domain that interacts with the plasma membrane. This region also encompasses the proton sensor E306 that is required for activation of TREK-1 by cytosolic acidosis. Protonation of E306 drastically tightens channel-phospholipid interaction and leads to TREK-1 opening at atmospheric pressure. The TREK-1-phospholipid interaction is critical for channel mechano-, pH(i)- and voltage-dependent gating.

The lipid-activated two-pore domain K+ channel TREK-1 is resistant to hypoxia: implication for ischaemic neuroprotection

TREK-1 is a member of the two-pore domain potassium (K(2P)) channel family that is mechano-, heat, pH, voltage and lipid sensitive. It is highly expressed in the central nervous system and probably encodes one of the previously described arachidonic acid-activated K(+) channels. Polyunsaturated fatty acids and lysophospholipids protect the brain against global ischaemia. Since both lipids are openers of TREK-1, it has been suggested that this K(2P) channel is directly involved in neuroprotection. Recently, however, this view has been challenged by a report claiming that TREK-1 and its activation by arachidonic acid is inhibited by hypoxia. In the present study, we demonstrate that the bubbling of saline with gases results in the loss of arachidonic acid from solution. Using experimental conditions which obviate this experimental artefact we demonstrate that TREK-1 is resistant to hypoxia and is strongly activated by arachidonic acid even at low P(O(2)) (< 4 Torr). Furthermore, hypoxia fails to affect basal as well as 2,4,6-trinitrophenol- and acid-stimulated TREK-1 currents. These data are supportive for a possible role of TREK-1 in ischaemic neuroprotection and in cell signalling via arachidonic acid.

K+-dependent cerebellar granule neuron apoptosis. Role of task leak K+ channels

Rat mature cerebellar granule, unlike hippocampal neurons, die by apoptosis when cultured in a medium containing a physiological concentration of K+ but survive under high external K+ concentrations. Cell death in physiological K+ parallels the developmental expression of the TASK-1 and TASK-3 subunits that encode the pH-sensitive standing outward K+ current IKso. Genetic transfer of the TASK subunits in hippocampal neurons, lacking IKso, induces cell death, while their genetic inactivation protects cerebellar granule neurons. Neuronal death of cultured rat granule neurons is also prevented by conditions that specifically reduce K+ efflux through the TASK-3 channels such as extracellular acidosis and ruthenium red. TASK leak K+ channels thus play an important role in K+-dependent apoptosis of cerebellar granule neurons in culture.

An intracellular proton sensor commands lipid- and mechano-gating of the K(+) channel TREK-1

The 2P domain K(+) channel TREK-1 is widely expres sed in the nervous system. It is opened by a variety of physical and chemical stimuli including membrane stretch, intracellular acidosis and polyunsaturated fatty acids. This activation can be reversed by PKA-mediated phosphorylation. The C-terminal domain of TREK-1 is critical for its polymodal function. We demonstrate that the conversion of a specific glutamate residue (E306) to an alanine in this region locks TREK-1 in the open configuration and abolishes the cAMP/PKA down-modulation. The E306A substitution mimics intracellular acidosis and rescues both lipid- and mechano-sensitivity of a loss-of-function truncated TREK-1 mutant. We conclude that protonation of E306 tunes the TREK-1 mechanical setpoint and thus sets lipid sensitivity.

Molecular basis of the voltage-dependent gating of TREK-1, a mechano-sensitive K(+) channel

TREK-1 is a member of the mammalian two P domain K(+) channel family. Mouse TREK-1 activity, in transiently transfected COS cells, is reduced at negative resting membrane potentials by both an external Mg(2+) block and an intrinsic voltage-dependent gating mechanism leading to a strong outward rectification. Deletional and chimeric analysis demonstrates that the carboxy terminal domain of TREK-1, but not the PKA phosphorylation site S333, is responsible for voltage-dependent gating. Since the same region is also critically required for TREK-1 mechano-gating, both mechanisms might be functionally linked. Preferential opening of TREK-1 at depolarized potentials will greatly affect action potential duration, recovery from inactivation and neuronal repetitive firing activity.

Lipid and mechano-gated 2P domain K(+) channels

The two pore domain K(+) channels TREK and TRAAK are opened by membrane stretch. The activating mechanical force comes from the bilayer membrane and is independent of the cytoskeleton. Emerging work shows that mechano-gated TREK and TRAAK are opened by various lipids, including long chain polyunsaturated anionic fatty acids and neutral cone-shaped lysophospholipids. TREK-1 shares the properties of the Aplysia neuronal S channel, a presynaptic background K(+) channel involved in behavioral sensitization, a simple form of learning.

Molecular physiology of oxygen-sensitive potassium channels

Physiological adaptation to acute hypoxia involves oxygen-sensing by a variety of specialized cells including carotid body type I cells, pulmonary neuroepithelial body cells, pulmonary artery myocytes and foetal adrenomedullary chromaffin cells. Hypoxia induces depolarization by closing a specific set of potassium channels and triggers cellular responses. Molecular biology strategies have recently allowed the identification of the K+ channel subunits expressed in these specialized cells. Several voltage-gated K+ channel subunits comprising six transmembrane segments and a single pore domain (Kv1.2, Kv1.5, Kv2.1, Kv3.1, Kv3.3, Kv4.2 and Kv9.3) are reversibly blocked by hypoxia when expressed in heterologous expression systems. Additionally, the background K+ channel subunit TASK-1, which comprises four transmembrane segments and two pore domains, is also involved in both oxygen- and acid-sensing in peripheral chemoreceptors. Progress is currently being made to identify the oxygen sensors. Regulatory beta subunits may play an important role in the modulation of Kv channel subunits by oxygen.

Properties and modulation of mammalian 2P domain K+ channels

Mammalian 2P domain K(+) channels are responsible for background or 'leak' K(+) currents. These channels are regulated by various physical and chemical stimuli, including membrane stretch, temperature, acidosis, lipids and inhalational anaesthetics. Furthermore, channel activity is tightly controlled by membrane receptor stimulation and second messenger phosphorylation pathways. Several members of this novel family of K(+) channels are highly expressed in the central and peripheral nervous systems in which they are proposed to play an important physiological role. The pharmacological modulation of this novel class of ion channels could be of interest for both general anaesthesia and ischaemic neuroprotection.

The endocannabinoid anandamide is a direct and selective blocker of the background K(+) channel TASK-1

TASK-1 encodes an acid- and anaesthetic-sensitive background K(+) current, which sets the resting membrane potential of both cerebellar granule neurons and somatic motoneurons. We demonstrate that TASK-1, unlike the other two pore (2P) domain K(+) channels, is directly blocked by submicromolar concentrations of the endocannabinoid anandamide, independently of the CB1 and CB2 receptors. In cerebellar granule neurons, anandamide also blocks the TASK-1 standing-outward K(+) current, IKso, and induces depolarization. Anandamide-induced neurobehavioural effects are only partly reversed by antagonists of the cannabinoid receptors, suggesting the involvement of alternative pathways. TASK-1 constitutes a novel sensitive molecular target for this endocannabinoid.

TWIK-2, an inactivating 2P domain K+ channel

We cloned human and rat TWIK-2 and expressed this novel 2P domain K(+) channel in transiently transfected COS cells. TWIK-2 is highly expressed in the gastrointestinal tract, the vasculature, and the immune system. Rat TWIK-2 currents are about 15 times larger than human TWIK-2 currents, but both exhibit outward rectification in a physiological K(+) gradient and mild inward rectification in symmetrical K(+) conditions. TWIK-2 currents are inactivating at depolarized potentials, and the kinetic of inactivation is highly temperature-sensitive. TWIK-2 shows an extremely low conductance, which prevents the visualization of discrete single channel events. The inactivation and rectification are intrinsic properties of TWIK-2 channels. In a physiological K(+) gradient, TWIK-2 is half inhibited by 0.1 mm Ba(2+), quinine, and quinidine. Finally, cysteine 53 in the M1P1 external loop is required for functional expression of TWIK-2 but is not critical for subunit self-assembly. TWIK-2 is the first reported 2P domain K(+) channel that inactivates. The base-line, transient, and delayed activities of TWIK-2 suggest that this novel 2P domain K(+) channel may play an important functional role in cell electrogenesis.

TREK-1 is a heat-activated background K(+) channel

Peripheral and central thermoreceptors are involved in sensing ambient and body temperature, respectively. Specialized cold and warm receptors are present in dorsal root ganglion sensory fibres as well as in the anterior/preoptic hypothalamus. The two-pore domain mechano-gated K(+) channel TREK-1 is highly expressed within these areas. Moreover, TREK-1 is opened gradually and reversibly by heat. A 10 degrees C rise enhances TREK-1 current amplitude by approximately 7-fold. Prostaglandin E2 and cAMP, which are strong sensitizers of peripheral and central thermoreceptors, reverse the thermal opening of TREK-1 via protein kinase A-mediated phosphorylation of Ser333. Expression of TREK-1 in peripheral sensory neurons as well as in central hypothalamic neurons makes this K(+) channel an ideal candidate as a physiological thermoreceptor.

Lysophospholipids open the two-pore domain mechano-gated K(+) channels TREK-1 and TRAAK

The two-pore (2P) domain K(+) channels TREK-1 and TRAAK are opened by membrane stretch as well as arachidonic acid (AA) (Patel, A. J., Honoré, E., Maingret, F., Lesage, F., Fink, M., Duprat, F., and Lazdunski, M. (1998) EMBO J. 17, 4283-4290; Maingret, F., Patel, A. J., Lesage, F., Lazdunski, M., and Honoré, E. (1999) J. Biol. Chem. 274, 26691-26696; Maingret, F., Fosset, M., Lesage, F., Lazdunski, M. , and Honoré, E. (1999) J. Biol. Chem. 274, 1381-1387. We demonstrate that lysophospholipids (LPs) and platelet-activating factor also produce large specific and reversible activations of TREK-1 and TRAAK. LPs activation is a function of the size of the polar head and length of the acyl chain but is independent of the charge of the molecule. Bath application of lysophosphatidylcholine (LPC) immediately opens TREK-1 and TRAAK in the cell-attached patch configuration. In excised patches, LPC activation is lost, whereas AA still produces maximal opening. The carboxyl-terminal region of TREK-1, but not the amino terminus and the extracellular loop M1P1, is critically required for LPC activation. LPC activation is indirect and may possibly involve a cytosolic factor, whereas AA directly interacts with either the channel proteins or the bilayer and mimics stretch. Opening of TREK-1 and TRAAK by fatty acids and LPs may be an important switch in the regulation of synaptic function and may also play a protective role during ischemia and inflammation.

Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel

TREK-1 is a member of the novel structural class of K(+) channels with four transmembrane segments and two pore domains in tandem (1,2). TREK-1 is opened by membrane stretch and arachidonic acid. It is also an important target for volatile anesthetics (2,3). Here we show that internal acidification opens TREK-1. Indeed, lowering pH(i) shifts the pressure-activation relationship toward positive values and leads to channel opening at atmospheric pressure. The pH(i)-sensitive region in the carboxyl terminus of TREK-1 is the same that is critically involved in mechano-gating as well as arachidonic acid activation. A convergence, which is dependent on the carboxyl terminus, occurs between mechanical, fatty acids and acidic stimuli. Intracellular acidosis, which occurs during brain and heart ischemia, will induce TREK-1 opening with subsequent K(+) efflux and hyperpolarization.

Inhalational anesthetics activate two-pore-domain background K+ channels

Volatile anesthetics produce safe, reversible unconsciousness, amnesia and analgesia via hyperpolarization of mammalian neurons. In molluscan pacemaker neurons, they activate an inhibitory synaptic K+ current (IKAn), proposed to be important in general anesthesia. Here we show that TASK and TREK-1, two recently cloned mammalian two-P-domain K+ channels similar to IKAn in biophysical properties, are activated by volatile general anesthetics. Chloroform, diethyl ether, halothane and isoflurane activated TREK-1, whereas only halothane and isoflurane activated TASK. Carboxy (C)-terminal regions were critical for anesthetic activation in both channels. Thus both TREK-1 and TASK are possibly important target sites for these agents.