Multiple modalities converge on a common gate to control K2P channel function

Feeling the hidden mechanical forces in lipid bilayer is an original sense

Life's origin entails enclosing a compartment to hoard material, energy, and information. The envelope necessarily comprises amphipaths, such as prebiotic fatty acids, to partition the two aqueous domains. The self-assembled lipid bilayer comes with a set of properties including its strong anisotropic internal forces that are chemically or physically malleable. Added bilayer stretch can alter force vectors on embedded proteins to effect conformational change. The force-from-lipid principle was demonstrated 25 y ago when stretches opened purified Escherichia coli MscL channels reconstituted into artificial bilayers. This reductionistic exercise has rigorously been recapitulated recently with two vertebrate mechanosensitive K(+) channels (TREK1 and TRAAK). Membrane stretches have also been known to activate various voltage-, ligand-, or Ca(2+)-gated channels. Careful analyses showed that Kv, the canonical voltage-gated channel, is in fact exquisitely sensitive even to very small tension. In an unexpected context, the canonical transient-receptor-potential channels in the Drosophila eye, long presumed to open by ligand binding, is apparently opened by membrane force due to PIP2 hydrolysis-induced changes in bilayer strain. Being the intimate medium, lipids govern membrane proteins by physics as well as chemistry. This principle should not be a surprise because it parallels water's paramount role in the structure and function of soluble proteins. Today, overt or covert mechanical forces govern cell biological processes and produce sensations. At the genesis, a bilayer's response to osmotic force is likely among the first senses to deal with the capricious primordial sea.

Influence of the N terminus on the biophysical properties and pharmacology of TREK1 potassium channels

TWIK-related K(+) 1 (TREK1) potassium channels are members of the two-pore domain potassium channel family and contribute to background potassium conductances in many cell types, where their activity can be regulated by a variety of physiologic and pharmacologic mediators. Fenamates such as FFA (flufenamic acid; 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid), MFA [mefenamic acid; 2-(2,3-dimethylphenyl)aminobenzoic acid], NFA [niflumic acid; 2-{[3-(trifluoromethyl)phenyl]amino}nicotinic acid], and diclofenac [2-(2-(2,6-dichlorophenylamino)phenyl)acetic acid] and the related experimental drug BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine] enhance the activity of TREK1 currents, and we show that BL-1249 is the most potent of these compounds. Alternative translation initiation produces a shorter, N terminus truncated form of TREK1 with a much reduced open probability and a proposed increased permeability to sodium compared with the longer form. We show that both forms of TREK1 can be activated by fenamates and that a number of mutations that affect TREK1 channel gating occlude the action of fenamates but only in the longer form of TREK1. Furthermore, fenamates produce a marked enhancement of current through the shorter, truncated form of TREK1 and reveal a K(+)-selective channel, like the long form. These results provide insight into the mechanism of TREK1 channel activation by fenamates, and, given the role of TREK1 channels in pain, they suggest a novel analgesic mechanism for these compounds.

Recovery of current through mutated TASK3 potassium channels underlying Birk Barel syndrome

TASK3 (TWIK-related acid-sensitive K(+) channel 3) potassium channels are members of the two-pore-domain potassium channel family. They are responsible for background leak potassium currents found in many cell types. TASK3 channels are genetically imprinted, and a mutation in TASK3 (G236R) is responsible for Birk Barel mental retardation dysmorphism syndrome, a maternally transmitted developmental disorder. This syndrome may arise from a neuronal migration defect during development caused by dysfunctional TASK3 channels. Through the use of whole-cell electrophysiologic recordings, we have found that, although G236R mutated TASK3 channels give rise to a functional current, this current is significantly smaller in an outward direction when compared with wild-type (WT) TASK3 channels. In contrast to WT TASK3 channels, the current is inwardly rectifying. Furthermore, the current through mutated channels is differentially sensitive to a number of regulators, such as extracellular acidification, extracellular zinc, and activation of Gαq-coupled muscarinic (M3) receptors, compared with WT TASK3 channels. The reduced outward current through mutated TASK3_G236R channels can be overcome, at least in part, by both a gain-of-function additional mutation of TASK3 channels (A237T) or by application of the nonsteroidal anti-inflammatory drug flufenamic acid (FFA; 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid). FFA produces a significantly greater enhancement of current through mutated channels than through WT TASK3 channels. We propose that pharmacologic enhancement of mutated TASK3 channel current during development may, therefore, provide a potentially useful therapeutic strategy in the treatment of Birk Barel syndrome.

Temperature sensitivity of two-pore (K2P) potassium channels

At normal body temperature, the two-pore potassium channels TREK-1 (K2P2.1/KCNK2), TREK-2 (K2P10.1/KCNK10), and TRAAK (K2P4.1/KCNK2) regulate cellular excitability by providing voltage-independent leak of potassium. Heat dramatically potentiates K2P channel activity and further affects excitation. This review focuses on the current understanding of the physiological role of heat-activated K2P current, and discusses the molecular mechanism of temperature gating in TREK-1, TREK-2, and TRAAK.

Mechano-gated ion channels in sensory systems

Living organisms sense their physical environment through cellular mechanotransduction, which converts mechanical forces into electrical and biochemical signals. In turn, signal transduction serves a wide variety of functions, from basic cellular processes as diverse as proliferation, differentiation, migration, and apoptosis up to some of the most sophisticated senses, including touch and hearing. Accordingly, defects in mechanosensing potentially lead to diverse diseases and disorders such as hearing loss, cardiomyopathies, muscular dystrophies, chronic pain, and cancer. Here, we review the status of mechanically activated ion channel discovery and discuss current challenges to define their properties and physiological functions.

Enhancement of TWIK-related acid-sensitive potassium channel 3 (TASK3) two-pore domain potassium channel activity by tumor necrosis factor α

TASK3 two-pore domain potassium (K2P) channels are responsible for native leak K channels in many cell types which regulate cell resting membrane potential and excitability. In addition, TASK3 channels contribute to the regulation of cellular potassium homeostasis. Because TASK3 channels are important for cell viability, having putative roles in both neuronal apoptosis and oncogenesis, we sought to determine their behavior under inflammatory conditions by investigating the effect of TNFα on TASK3 channel current. TASK3 channels were expressed in tsA-201 cells, and the current through them was measured using whole cell voltage clamp recordings. We show that THP-1 human myeloid leukemia monocytes, co-cultured with hTASK3-transfected tsA-201 cells, can be activated by the specific Toll-like receptor 7/8 activator, R848, to release TNFα that subsequently enhances hTASK3 current. Both hTASK3 and mTASK3 channel activity is increased by incubation with recombinant TNFα (10 ng/ml for 2-15 h), but other K2P channels (hTASK1, hTASK2, hTREK1, and hTRESK) are unaffected. This enhancement by TNFα is not due to alterations in levels of channel expression at the membrane but rather to an alteration in channel gating. The enhancement by TNFα can be blocked by extracellular acidification but persists for mutated TASK3 (H98A) channels that are no longer acid-sensitive even in an acidic extracellular environment. TNFα action on TASK3 channels is mediated through the intracellular C terminus of the channel. Furthermore, it occurs through the ASK1 pathway and is JNK- and p38-dependent. In combination, TNFα activation and TASK3 channel activity can promote cellular apoptosis.

A high-throughput functional screen identifies small molecule regulators of temperature- and mechano-sensitive K2P channels

K_2P (KCNK) potassium channels generate “leak” potassium currents that strongly influence cellular excitability and contribute to pain, somatosensation, anesthesia, and mood. Despite their physiological importance, K_2Ps lack specific pharmacology. Addressing this issue has been complicated by the challenges that the leak nature of K_2P currents poses for electrophysiology-based high-throughput screening strategies. Here, we present a yeast-based high-throughput screening assay that avoids this problem. Using a simple growth-based functional readout, we screened a library of 106,281 small molecules and identified two new inhibitors and three new activators of the mammalian K_2P channel K_2P2.1 (KCNK2, TREK-1). By combining biophysical, structure–activity, and mechanistic analysis, we developed a dihydroacridine analogue, ML67-33, that acts as a low micromolar, selective activator of temperature- and mechano-sensitive K_2P channels. Biophysical studies show that ML67-33 reversibly increases channel currents by activating the extracellular selectivity filter-based C-type gate that forms the core gating apparatus on which a variety of diverse modulatory inputs converge. The new K_2P modulators presented here, together with the yeast-based assay, should enable both mechanistic and physiological studies of K_2P activity and facilitate the discovery and development of other K_2P small molecule modulators.

TASK-2: a K2P K(+) channel with complex regulation and diverse physiological functions

TASK-2 (K2P5.1) is a two-pore domain K(+) channel belonging to the TALK subgroup of the K2P family of proteins. TASK-2 has been shown to be activated by extra- and intracellular alkalinization. Extra- and intracellular pH-sensors reside at arginine 224 and lysine 245 and might affect separate selectivity filter and inner gates respectively. TASK-2 is modulated by changes in cell volume and a regulation by direct G-protein interaction has also been proposed. Activation by extracellular alkalinization has been associated with a role of TASK-2 in kidney proximal tubule bicarbonate reabsorption, whilst intracellular pH-sensitivity might be the mechanism for its participation in central chemosensitive neurons. In addition to these functions TASK-2 has been proposed to play a part in apoptotic volume decrease in kidney cells and in volume regulation of glial cells and T-lymphocytes. TASK-2 is present in chondrocytes of hyaline cartilage, where it is proposed to play a central role in stabilizing the membrane potential. Additional sites of expression are dorsal root ganglion neurons, endocrine and exocrine pancreas and intestinal smooth muscle cells. TASK-2 has been associated with the regulation of proliferation of breast cancer cells and could become target for breast cancer therapeutics. Further work in native tissues and cells together with genetic modification will no doubt reveal the details of TASK-2 functions that we are only starting to suspect.

Domain-swapped chain connectivity and gated membrane access in a Fab-mediated crystal of the human TRAAK K+ channel

TRAAK (TWIK-related arachidonic acid-stimulated K(+) channel, K2P4.1) K(+) ion channels are expressed predominantly in the nervous system to control cellular resting membrane potential and are regulated by mechanical and chemical properties of the lipid membrane. TRAAK channels are twofold symmetric, which precludes a direct extension of gating mechanisms that close canonical fourfold symmetric K(+) channels. We present the crystal structure of human TRAAK in complex with antibody antigen-binding fragments (Fabs) at 2.75-Å resolution. In contrast to a previous structure, this structure reveals a domain-swapped chain connectivity enabled by the helical cap that exchanges two opposing outer helices 180° around the channel. An unrelated conformational change of an inner helix seals a side opening to the membrane bilayer and is associated with structural changes around the K(+)-selectivity filter that may have implications for mechanosensitivity and gating of TRAAK channels.

Using yeast to study potassium channel function and interactions with small molecules

Analysis of ion channel mutants is a widely used approach for dissecting ion channel function and for characterizing the mechanisms of action of channel-directed modulators. Expression of functional potassium channels in potassium-uptake-deficient yeast together with genetic selection approaches offers an unbiased, high-throughput, activity-based readout that can rapidly identify large numbers of active ion channel mutants. Because of the assumption-free nature of the method, detailed biophysical analysis of the functional mutants from such selections can provide new and unexpected insights into both ion channel gating and ion channel modulator mechanisms. Here, we present detailed protocols for generation and identification of functional mutations in potassium channels using yeast selections in the potassium-uptake-deficient strain SGY1528. This approach is applicable for the analysis of structure-function relationships of potassium channels from a wide range of sources including viruses, bacteria, plants, and mammals and can be used as a facile way to probe the interactions between ion channels and small-molecule modulators.

The influence of cold temperature on cellular excitability of hippocampal networks

The hippocampus plays an important role in short term memory, learning and spatial navigation. A characteristic feature of the hippocampal region is its expression of different electrical population rhythms and activities during different brain states. Physiological fluctuations in brain temperature affect the activity patterns in hippocampus, but the underlying cellular mechanisms are poorly understood. In this work, we investigated the thermal modulation of hippocampal activity at the cellular network level. Primary cell cultures of mouse E17 hippocampus displayed robust network activation upon light cooling of the extracellular solution from baseline physiological temperatures. The activity generated was dependent on action potential firing and excitatory glutamatergic synaptic transmission. Involvement of thermosensitive channels from the transient receptor potential (TRP) family in network activation by temperature changes was ruled out, whereas pharmacological and immunochemical experiments strongly pointed towards the involvement of temperature-sensitive two-pore-domain potassium channels (K_2P), TREK/TRAAK family. In hippocampal slices we could show an increase in evoked and spontaneous synaptic activity produced by mild cooling in the physiological range that was prevented by chloroform, a K_2P channel opener. We propose that cold-induced closure of background TREK/TRAAK family channels increases the excitability of some hippocampal neurons, acting as a temperature-sensitive gate of network activation. Our findings in the hippocampus open the possibility that small temperature variations in the brain in vivo, associated with metabolism or blood flow oscillations, act as a switch mechanism of neuronal activity and determination of firing patterns through regulation of thermosensitive background potassium channel activity.

Ion Channels in Plants

Since the first recordings of single potassium channel activities in the plasma membrane of guard cells more than 25 years ago, patch-clamp studies discovered a variety of ion channels in all cell types and plant species under inspection. Their properties differed in a cell type- and cell membrane-dependent manner. Guard cells, for which the existence of plant potassium channels was initially documented, advanced to a versatile model system for studying plant ion channel structure, function, and physiology. Interestingly, one of the first identified potassium-channel genes encoding the Shaker-type channel KAT1 was shown to be highly expressed in guard cells. KAT1-type channels from Arabidopsis thaliana and its homologs from other species were found to encode the K+-selective inward rectifiers that had already been recorded in early patch-clamp studies with guard cells. Within the genome era, additional Arabidopsis Shaker-type channels appeared. All nine members of the Arabidopsis Shaker family are localized at the plasma membrane, where they either operate as inward rectifiers, outward rectifiers, weak voltage-dependent channels, or electrically silent, but modulatory subunits. The vacuole membrane, in contrast, harbors a set of two-pore K+ channels. Just very recently, two plant anion channel families of the SLAC/SLAH and ALMT/QUAC type were identified. SLAC1/SLAH3 and QUAC1 are expressed in guard cells and mediate Slow- and Rapid-type anion currents, respectively, that are involved in volume and turgor regulation. Anion channels in guard cells and other plant cells are key targets within often complex signaling networks. Here, the present knowledge is reviewed for the plant ion channel biology. Special emphasis is drawn to the molecular mechanisms of channel regulation, in the context of model systems and in the light of evolution.

State-independent intracellular access of quaternary ammonium blockers to the pore of TREK-1

We previously reported that TREK-1 gating by internal pH and pressure occurs close to or within the selectivity filter. These conclusions were based upon kinetic measurements of high-affinity block by quaternary ammonium (QA) ions that appeared to exhibit state-independent accessibility to their binding site within the pore. Intriguingly, recent crystal structures of two related K2P potassium channels were also both found to be open at the helix bundle crossing. However, this did not exclude the possibility of gating at the bundle crossing and it was suggested that side-fenestrations within these structures might allow state-independent access of QA ions to their binding site. In this addendum to our original study we demonstrate that even hydrophobic QA ions do not access the TREK-1 pore via these fenestrations. Furthermore, by using a chemically reactive QA ion immobilized within the pore via covalent cysteine modification we provide additional evidence that the QA binding site remains accessible to the cytoplasm in the closed state. These results support models of K2P channel gating which occur close to or within the selectivity filter and do not involve closure at the helix bundle crossing.

Optical control of endogenous proteins with a photoswitchable conditional subunit reveals a role for TREK1 in GABA(B) signaling

Selective ligands are lacking for many neuronal signaling proteins. Photoswitched tethered ligands (PTLs) have enabled fast and reversible control of specific proteins containing a PTL anchoring site and have been used to remote control overexpressed proteins. We report here a scheme for optical remote control of native proteins using a "photoswitchable conditional subunit" (PCS), which contains the PTL anchoring site as well as a mutation that prevents it from reaching the plasma membrane. In cells lacking native subunits for the protein, the PCS remains nonfunctional internally. However, in cells expressing native subunits, the native subunit and PCS coassemble, traffic to the plasma membrane, and place the native protein under optical control provided by the coassembled PCS. We apply this approach to the TREK1 potassium channel, which lacks selective, reversible blockers. We find that TREK1, typically considered to be a leak channel, contributes to the hippocampal GABA(B) response.

Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains

The two-pore domain potassium channel K_2P2.1 (TREK-1) responds to extracellular and intracellular stimuli, including pH and temperature. This study elucidates how the intracellular sensor element relays metabolic and thermal stimuli to the extracellular C-type gating element.

K_2P2.1 (TREK-1) is a polymodal two-pore domain leak potassium channel that responds to external pH, GPCR-mediated phosphorylation signals, and temperature through the action of distinct sensors within the channel. How the various intracellular and extracellular sensory elements control channel function remains unresolved. Here, we show that the K_2P2.1 (TREK-1) intracellular C-terminal tail (Ct), a major sensory element of the channel, perceives metabolic and thermal commands and relays them to the extracellular C-type gate through transmembrane helix M4 and pore helix 1. By decoupling Ct from the pore-forming core, we further demonstrate that Ct is the primary heat-sensing element of the channel, whereas, in contrast, the pore domain lacks robust temperature sensitivity. Together, our findings outline a mechanism for signal transduction within K_2P2.1 (TREK-1) in which there is a clear crosstalk between the C-type gate and intracellular Ct domain. In addition, our findings support the general notion of the existence of modular temperature-sensing domains in temperature-sensitive ion channels. This marked distinction between gating and sensory elements suggests a general design principle that may underlie the function of a variety of temperature-sensitive channels.

Molecular physiology of pH-sensitive background K(2P) channels

Background K(2P) channels are tightly regulated by different stimuli including variations of external and internal pH. pH sensitivity relies on proton-sensing residues that influence channel gating and activity. Gene inactivation in the mouse is a revealing implication of K(2P) channels in many physiological functions ranging from hormone secretion to central respiratory adaptation. Surprisingly, only a few phenotypic traits of these mice have yet been directly related to the pH sensitivity of K(2P) channels.

TWIK1, a unique background channel with variable ion selectivity

TWIK1 belongs to the family of background K(+) channels with two pore domains. In native and transfected cells, TWIK1 is detected mainly in recycling endosomes. In principal cells in the kidney, TWIK1 gene inactivation leads to the loss of a nonselective cationic conductance, an unexpected effect that was attributed to adaptive regulation of other channels. Here, we show that TWIK1 ion selectivity is modulated by extracellular pH. Although TWIK1 is K(+) selective at neutral pH, it becomes permeable to Na(+) at the acidic pH found in endosomes. Selectivity recovery is slow after restoration of a neutral pH. Such hysteresis makes plausible a role of TWIK1 as a background channel in which selectivity and resulting inhibitory or excitatory influences on cell excitability rely on its recycling rate between internal acidic stores and the plasma membrane. TWIK1(-/-) pancreatic β cells are more polarized than control cells, confirming a depolarizing role of TWIK1 in kidney and pancreatic cells.

Covalent modification of a volatile anesthetic regulatory site activates TASK-3 (KCNK9) tandem-pore potassium channels

TASK-3 (KCNK9) tandem-pore potassium channels provide a volatile anesthetic-activated and Gα(q) protein- and acidic pH-inhibited potassium conductance important in neuronal excitability. Met-159 of TASK-3 is essential for anesthetic activation and may contribute to the TASK-3 anesthetic binding site(s). We hypothesized that covalent occupancy of an anesthetic binding site would irreversibly activate TASK-3. We introduced a cysteine at residue 159 (M159C) and studied the rate and effect of Cys-159 modification by N-ethylmaleimide (NEM), a cysteine-selective alkylating agent. TASK-3 channels were transiently expressed in Fischer rat thyroid cells, and their function was studied in an Ussing chamber. NEM irreversibly activated M159C TASK-3, with minimal effects on wild-type TASK-3. NEM-modified M159C channels were resistant to inhibition by both acidic pH and active Gα(q) protein. M159C channels that were first inhibited by Gα(q) protein were more-slowly activated by NEM, which suggests protection of Cys-159, and similar results were observed with isoflurane activation of wild-type TASK-3. M159W and M159F TASK-3 mutants behaved like NEM-modified M159C channels, with increased basal currents and resistance to inhibition by active Gα(q) protein or acidic pH. TASK-3 wild-type/M159C dimers expressed as a single polypeptide demonstrated that modification of a single Cys-159 was sufficient for TASK-3 activation, and M159F/M159C and M159W/M159C dimers provided evidence for cross-talk between subunits. The data are consistent with residue 159 contributing to an anesthetic regulatory site or sites, and they suggest that volatile anesthetics, through perturbations at a single site, increase TASK-3 channel activity and disrupt its regulation by active Gα(q) protein, a determinant of central nervous system arousal and consciousness.

Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel

TRAAK channels, members of the two-pore domain K(+) (potassium ion) channel family K2P, are expressed almost exclusively in the nervous system and control the resting membrane potential. Their gating is sensitive to polyunsaturated fatty acids, mechanical deformation of the membrane, and temperature changes. Physiologically, these channels appear to control the noxious input threshold for temperature and pressure sensitivity in dorsal root ganglia neurons. We present the crystal structure of human TRAAK at a resolution of 3.8 angstroms. The channel comprises two protomers, each containing two distinct pore domains, which create a two-fold symmetric K(+) channel. The extracellular surface features a helical cap, 35 angstroms tall, that creates a bifurcated pore entryway and accounts for the insensitivity of two-pore domain K(+) channels to inhibitory toxins. Two diagonally opposed gate-forming inner helices form membrane-interacting structures that may underlie this channel's sensitivity to chemical and mechanical properties of the cell membrane.