Emerging roles for two-pore-domain potassium channels and their potential therapeutic impact

Abstract

A distinct gene family of widely distributed and well-modulated two-pore-domain background potassium (K(2P)) channels establish resting membrane potential and cell excitability. By using new mouse models in which K(2P)-channel genes are deleted, the contributions of these channels to important physiological functions are now being revealed. Here, we highlight results of recent studies using mice deleted for K(2P)-channel subunits that uncover physiological functions of these channels, mostly those of the TASK and TREK subgroup. Consistent with activation of these K(2P) channels by volatile anesthetics, TASK-1, TASK-3 and TREK-1 contribute to anesthetic-induced hypnosis and immobilization. The acid-sensitive TASK channels are not required for brainstem control of breathing by CO(2) or pH, despite widespread expression in respiratory-related neurons. TASK channels are necessary, however, for homeostatic regulation of adrenal aldosterone secretion. The heat-, stretch- and lipid-activated TREK-1 channels contribute to temperature and mechanical pain sensation, neuroprotection by polyunsaturated fatty acids and, unexpectedly, mood regulation. The alkaline-activated TASK-2 channel is necessary for HCO(3)(-) reabsorption and osmotic volume regulation in kidney proximal tubule cells. Development of compounds that selectively modulate K(2P) channels is crucial for verifying these results and assessing the efficacy of therapies targeting these interesting channels.

Figure 1

Properties and organization of the K_2P channel family. (a) The presumed topology for a subunit of the two-pore-domain family of background K⁺ channels; each subunit has two pore-forming (P) loops and four transmembrane (TM) domains. The functional channel forms as a dimer. (b) Whole-cell currents obtained from heterologous expression of TASK-1, a K_2P channel sensitive to extracellular pH. As illustrated in the inset, acidifying the bath pH (from 7.3 to 5.9) inhibited TASK-1 current, whereas alkalizing the bath (from 5.9 to 8.4) increased current. The I-V curves for TASK-1 obtained at different bath pH reverse near the K⁺ equilibrium potential (E_K) and are well-fitted by the GHK current equation, as expected for a potassium ‘leak’ current. (c) The 15 known human two-pore-domain K⁺-channel genes are presented in a phylogenetic tree and placed into six subgroups as described here; genes that have not produced functional channels are shown in gray. The more distant relationships between the subgroups have been omitted. Multiple naming schemes for these channels are in current use. The Human Genome Organization (http://www.hugo-international.org) uses KCNK preceding a number that reflects the order of discovery of each of the genes; the International Union of Basic and Clinical Pharmacology (www.iuphar.org) adopted a very similar scheme for naming the cognate channels, replacing the KCNK with a K_2P prefix. A third naming scheme that is in popular use employs a set of acronyms based on salient physiological or pharmacological properties (provided here). The acronyms are as follows: TWIK, tandem of P domains in a weak inwardly rectifying K⁺ channel; THIK, tandem-pore-domain halothane-inhibited K⁺ channel; TREK, TWIK-related K⁺-channel gene; TRAAK, TWIK-related arachidonic-acid-stimulated K⁺ channel; TASK, TWIK-related acid-sensitive K⁺ channel; TALK, TWIK-related alkaline-pH-activated K⁺ channel; and TRESK, TWIK-related spinal-cord K⁺ channel. Here, we focus on the TASK- and TREK-channel groups.

Figure 2

TASK channels regulate aldosterone production. Schematic illustrating role of TASK channels in aldosterone production from adrenal ZG cells, with proposed mechanisms for channel modulation and effects of channel deletion. Main panel: In ZG cells, TASK channels provide a background K⁺ current that maintains a negative membrane potential (Ψ), limiting the Ca²⁺ entry via T-type Ca²⁺ channels that drives aldosterone production and secretion. AngII-mediated inhibition of TASK channels by a Gα_q-linked angiotensin AT₁ receptor causes membrane depolarization, which, along with an independent calmodulin-dependent-protein-kinase-II-mediated activation of Ca²⁺ channels, promotes increased Ca²⁺ entry and aldosterone release [13]. Low Na⁺ diet: (a) in a wild-type mouse on a low-salt diet, elevated levels of AngII enhance ZG cell-membrane depolarization and Ca²⁺-channel activation, increasing aldosterone production. (b) In TASK^-/- mice, the absence of the background K⁺ conductance leads to a persistent strong depolarization of ZG cells; in this context, AngII-mediated Ca²⁺-channel activation greatly increases aldosterone production [14]. High Na⁺ diet: (c) on a high-salt diet, circulating AngII levels are low and constitutive TASK activity is high, driving the ZG cell-membrane potential to hyperpolarized levels that minimize Ca²⁺-channel activity and aldosterone production. (d) In TASK^-/- mice, the persistent membrane depolarization maintains Ca²⁺-channel activity and aldosterone production even in the absence of AngII [14].

Figure 3

Neuroprotection mediated by TREK-1 channels. In cerebral ischemia models, TREK-1^-/- mice have reduced survival rates and are insensitive to the neuroprotective actions of PUFAs [36]. Two mechanisms conspire to account for this neuroprotection. (a) During ischemia in neurons, TREK-1 activation by intracellular acidification and by PUFAs leads to membrane hyperpolarization, decreasing activity and lowering metabolic demand. In TREK-1 knockouts, the neurons would be more depolarized and unresponsive to PUFAs. Blue indicates a more negative membrane potential and less activity; red indicates a depolarized state with more activity. (b) In cerebral blood vessels, TREK-1 is necessary for actions of endogenous vasodilators (e.g. acetylcholine) in endothelial cells (EC) and is activated directly by PUFAs in vascular smooth muscle cells (VSMCs), leading to vasodilation and enhanced collateral blood flow [45].

Figure 4

Depression-resistant phenotype in TREK-1 knockout mice. Serotonin-synthesizing neurons of the dorsal raphe nucleus have widespread ascending projections to terminal fields in the forebrain (red in brain schematic). In wild-type mice, firing activity in dorsal raphe neurons yields basal levels of serotonin in the raphe nucleus and in terminal fields (indicated by red dots). It is possible to evoke behavioral correlates of despair and helplessness in these wild-type animals that can be ameliorated by treatment with antidepressants (e.g. SSRIs) [43] by virtue of their ability to inhibit the 5-HT transporter and increase serotonin tone in terminal fields [61]. Note that chronic treatment with SSRIs leads to a functional desensitization of 5-HT_1A-receptor-dependent, Kir-channel-mediated autoinhibition of dorsal raphe neurons, enabling those cells to maintain normal discharge rates despite increased local serotonin levels [61]. In TREK-1^-/- mice, action-potential discharge rate is doubled in dorsal raphe neurons and this leads to increased serotonin levels in terminal fields that yield a depression-resistant phenotype and occlude further effects of SSRI treatment [43]. In these animals, chronically high levels of serotonin presumably desensitize 5-HT_1A-receptor-dependent autoinhibition of raphe neurons to permit high basal firing rates [61]. The increase in excitability of serotonin neurons in TREK-1^-/- mice and accompanying depression-resistant phenotype could reflect loss of the channel directly from dorsal raphe neurons [43]. Inhibition of TREK channels might, thus, provide an alternative approach to antidepressant therapy. Note that both TASK-1 and TASK-3 are also expressed in dorsal raphe neurons [25], indicating that inhibition of those channels might represent an additional therapeutic mechanism to enhance serotonin neuron activity.

Figure 5

Summary of physiological roles for TASK and TREK-1 channels. (a) TASK channels are inhibited by extracellular acidification (pHe), local anesthetics and Gα_q-linked G-protein-coupled receptors (GPCRs). They are activated by phospholipids and by volatile anesthetics. In knockout mice, a role for TASK channels has been demonstrated in aldosterone production from adrenal ZG cells [14] and in immobilizing actions of inhaled anesthetics [39,40]. A marked increase in nocturnal activity has also been found in TASK-3^-/- mice [39]. Other prominent ideas remain to be tested in these mice, including effects on glucosensing [28,51], immune-cell proliferation [55] and heart-rate regulation [52,53]. (b) TREK-1 channels are activated by PUFAs, phospholipids, stretch, volatile anesthetics, intracellular acidification (pHi) and raised temperature (T°C). They are inhibited by Gα_s- and Gα_q-linked GPCRs through protein kinase (PK)A and PKC, but activated by a PKG-mediated mechanism [5]. In knockout mice, a role for TREK-1 has been demonstrated in neuroprotection [36] and cerebral artery vasodilation [45], in hypnotic and immobilizing actions of inhaled anesthetics [36], in depression [43] and in pain sensation [42]. A role for TREK-1 in tumorigenesis has been suggested but not tested in knockout mice [57].