Amanda Patel

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.

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.

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.

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.

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.

Revisiting TRPC1 and TRPC6 mechanosensitivity

This article addresses whether TRPC1 or TRPC6 is an essential component of a mammalian stretch-activated mechano-sensitive Ca2+ permeable cation channel (MscCa). We have transiently expressed TRPC1 and TRPC6 in African green monkey kidney (COS) or Chinese hamster ovary (CHO) cells and monitored the activity of the stretch-activated channels using a fast pressure clamp system. Although both TRPC1 and TRPC6 are highly expressed at the protein level, the amplitude of the mechano-sensitive current is not significantly altered by overexpression of these subunits. In conclusion, although several TRPC channel members, including TRPC1 and TRPC6, have been recently proposed to form MscCa in vertebrate cells, the functional expression of these TRPC subunits in heterologous systems remains problematic.

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

TREK‐1 (KCNK2 or K2P2.1) is a mechanosensitive K2P channel that is opened by membrane stretch as well as cell swelling. Here, we demonstrate that membrane phospholipids, including PIP2, 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‐, pHi‐ and voltage‐dependent gating.