Eric Honoré

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.

An oxygen‐, acid‐ and anaesthetic‐sensitive TASK‐like background potassium channel in rat arterial chemoreceptor cells

1 The biophysical and pharmacological properties of an oxygen‐sensitive background K+ current in rat carotid body type‐I cells were investigated and compared with those of recently cloned two pore domain K+ channels. 2 Under symmetrical K+ conditions the oxygen‐sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. 3 Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. 4 Oxygen‐sensitive current was inhibited by 10 mM barium (57 % inhibition), 200 μM zinc (53 % inhibition), 200 μM bupivacaine (55 % inhibition) and 1 mM quinidine (105 % inhibition). 5 The general anaesthetic halothane (1.5 %) increased the oxygen‐sensitive K+ current (by 176 %). Halothane (3 mM) also stimulated single channel activity in inside‐out patches (by 240 %). Chloroform had no effect on background K+ channel activity. 6 Acidosis (pH 6.4) inhibited the oxygen‐sensitive background K+ current (by 56 %) and depolarised type‐I cells. 7 The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK‐1. Using in situ hybridisation TASK‐1 mRNA was found to be expressed in type‐I cells. We conclude that the oxygen‐ and acid‐sensitive background K+ channel of carotid body type‐I cells is likely to be an endogenous TASK‐1‐like channel.

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.

The neuronal background K2P channels: focus on TREK1

Two-pore-domain K+ (K2P) 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, K2P 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 K2P channel, has a key role in the cellular mechanisms of neuroprotection, anaesthesia, pain and depression.

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.

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.

TRAAK Is a Mammalian Neuronal Mechano-gated K+Channel

The novel structural class of mammalian channels with four transmembrane segments and two pore regions comprise background K+ channels (TWIK-1, TREK-1, TRAAK, TASK, and TASK-2) with unique physiological functions (1-6). Unlike its counterparts, TRAAK is only expressed in neuronal tissues, including brain, spinal cord, and retina (1). This report shows that TRAAK, which was known to be activated by arachidonic acid (3), is also opened by membrane stretch. Mechanical activation of TRAAK is induced by a convex curvature of the plasma membrane and can be mimicked by the amphipathic membrane crenator trinitrophenol. Cytoskeletal elements are negative tonic regulators of TRAAK. Membrane depolarization and membrane crenation synergize with stretch-induced channel opening. Finally, TRAAK is reversibly blocked by micromolar concentrations of gadolinium, a well known blocker of stretch-activated channels. Mechanical activation of TRAAK in the central nervous system may play an important role during growth cone motility and neurite elongation.

Kv2.1/Kv9.3, a novel ATP‐dependent delayed‐rectifier K+ channel in oxygen‐sensitive pulmonary artery myocytes

The molecular structure of oxygen‐sensitive delayed‐rectifier K+ channels which are involved in hypoxic pulmonary artery (PA) vasoconstriction has yet to be elucidated. To address this problem, we identified the Shab K+ channel Kv2.1 and a novel Shab‐like subunit Kv9.3, in rat PA myocytes. Kv9.3 encodes an electrically silent subunit which associates with Kv2.1 and modulates its biophysical properties. The Kv2.1/9.3 heteromultimer, unlike Kv2.1, opens in the voltage range of the resting membrane potential of PA myocytes. Moreover, we demonstrate that the activity of Kv2.1/Kv9.3 is tightly controlled by internal ATP and is reversibly inhibited by hypoxia. In conclusion, we propose that metabolic regulation of the Kv2.1/Kv9.3 heteromultimer may play an important role in hypoxic PA vasoconstriction and in the possible development of PA hypertension.

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.