Florian Lesage

TASK, a human background K+ channel to sense external pH variations near physiological pH.

TASK is a new member of the recently recognized TWIK K+ channel family. This 395 amino acid polypeptide has four transmembrane segments and two P domains. In adult human, TASK transcripts are found in pancreas

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.

A mammalian two pore domain mechano‐gated S‐like K+ channel

Aplysia S‐type K+ channels of sensory neurons play a dominant role in presynaptic facilitation and behavioural sensitization. They are closed by serotonin via cAMP‐dependent phosphorylation, whereas they are opened by arachidonic acid, volatile general anaesthetics and mechanical stimulation. We have identified a cloned mammalian two P domain K+ channel sharing the properties of the S channel. In addition, the recombinant channel is opened by lipid bilayer amphipathic crenators, while it is closed by cup‐formers. The cytoplasmic C‐terminus contains a charged region critical for chemical and mechanical activation, as well as a phosphorylation site required for cAMP inhibition.

TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure.

A new human weakly inward rectifying K+ channel, TWIK‐1, has been isolated. This channel is 336 amino acids long and has four transmembrane domains. Unlike other mammalian K+ channels, it contains two pore‐forming regions called P domains. Genes encoding structural homologues are present in the genome of Caenorhabditis elegans. TWIK‐1 currents expressed in Xenopus oocytes are time‐independent and present a nearly linear I‐V relationship that saturated for depolarizations positive to O mV in the presence of internal Mg2+. This inward rectification is abolished in the absence of internal Mg2+. TWIK‐1 has a unitary conductance of 34 pS and a kinetic behaviour that is dependent on the membrane potential. In the presence of internal Mg2+, the mean open times are 0.3 and 1.9 ms at −80 and +80 mV, respectively. The channel activity is up‐regulated by activation of protein kinase C and down‐regulated by internal acidification. Both types of regulation are indirect. TWIK‐1 channel activity is blocked by Ba2+(IC50=100 microM), quinine (IC50=50 microM) and quinidine (IC50=95 microM). This channel is of particular interest because its mRNA is widely distributed in human tissues, and is particularly abundant in brain and heart. TWIK‐1 channels are probably involved in the control of background K+ membrane conductances.

Cloning, functional expression and brain localization of a novel unconventional outward rectifier K+ channel.

Human TWIK‐1, which has been cloned recently, is a new structural type of weak inward rectifier K+ channel. Here we report the structural and functional properties of TREK‐1, a mammalian TWIK‐1‐related K+ channel. Despite a low amino acid identity between TWIK‐1 and TREK‐1 (approximately 28%), both channel proteins share the same overall structural arrangement consisting of two pore‐forming domains and four transmembrane segments (TMS). This structural similarity does not give rise to a functional analogy. K+ currents generated by TWIK‐1 are inwardly rectifying while K+ currents generated by TREK‐1 are outwardly rectifying. These channels have a conductance of 14 pS. TREK‐1 currents are insensitive to pharmacological agents that block TWIK‐1 activity such as quinine and quinidine. Extensive inhibitions of TREK‐1 activity are observed after activation of protein kinases A and C. TREK‐1 currents are sensitive to extracellular K+ and Na+. TREK‐1 mRNA is expressed in most tissues and is particularly abundant in the lung and in the brain. Its localization in this latter tissue has been studied by in situ hybridization. TREK‐1 expression is high in the olfactory bulb, hippocampus and cerebellum. These results provide the first evidence for the existence of a K+ channel family with four TMS and two pore domains in the nervous system of mammals. They also show that different members in this structural family can have totally different functional properties.

A neuronal two P domain K channel stimulated by arachidonic acid and polyunsaturated fatty acids

TWIK‐1, TREK‐1 and TASK K+ channels comprise a class of pore‐forming subunits with four membrane‐spanning segments and two P domains. Here we report the cloning of TRAAK, a 398 amino acid protein which is a new member of this mammalian class of K+ channels. Unlike TWIK‐1, TREK‐1 and TASK which are widely distributed in many different mouse tissues, TRAAK is present exclusively in brain, spinal cord and retina. Expression of TRAAK in Xenopus oocytes and COS cells induces instantaneous and non‐inactivating currents that are not gated by voltage. These currents are only partially inhibited by Ba2+ at high concentrations and are insensitive to the other classical K+ channel blockers tetraethylammonium, 4‐aminopyridine and Cs+. A particularly salient feature of TRAAK is that they can be stimulated by arachidonic acid (AA) and other unsaturated fatty acids but not by saturated fatty acids. These channels probably correspond to the functional class of fatty acid‐stimulated K+ currents that recently were identified in native neuronal cells but have not yet been cloned. These TRAAK channels might be essential in normal physiological processes in which AA is known to play an important role, such as synaptic transmission, and also in pathophysiological processes such as brain ischemia. TRAAK channels are stimulated by the neuroprotective drug riluzole.

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°C rise enhances TREK‐1 current amplitude by ∼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.

Inner Ear Defects Induced by Null Mutationof the isk Gene

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.

Cloning and Expression of a Novel pH-sensitive Two Pore Domain K+ Channel from Human Kidney

A complementary DNA encoding a novel K+ channel, called TASK-2, was isolated from human kidney and its gene was mapped to chromosome 6p21. TASK-2 has a low sequence similarity to other two pore domain K+ channels, such as TWIK-1, TREK-1, TASK-1, and TRAAK (18–22% of amino acid identity), but a similar topology consisting of four potential membrane-spanning domains. In transfected cells, TASK-2 produces noninactivating, outwardly rectifying K+ currents with activation potential thresholds that closely follow the K+equilibrium potential. As for the related TASK-1 and TRAAK channels, the outward rectification is lost at high external K+concentration. The conductance of TASK-2 was estimated to be 14.5 picosiemens in physiological conditions and 59.9 picosiemens in symmetrical conditions with 155 mm K+. TASK-2 currents are blocked by quinine (IC50 = 22 μm) and quinidine (65% of inhibition at 100 μm) but not by the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine, and Cs+. They are only slightly sensitive to Ba2+, with less than 17% of inhibition at 1 mm. As TASK-1, TASK-2 is highly sensitive to external pH in the physiological range. 10% of the maximum current was recorded at pH 6.5 and 90% at pH 8.8. Unlike all other cloned channels with two pore-forming domains, TASK-2 is essentially absent in the brain. In human and mouse, TASK-2 is mainly expressed in the kidney, where in situ hybridization shows that it is localized in cortical distal tubules and collecting ducts. This localization, as well as its functional properties, suggest that TASK-2 could play an important role in renal K+ transport.

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.