Allosteric coupling between proximal C-terminus and selectivity filter is facilitated by the movement of transmembrane segment 4 in TREK-2 channel

Atomistic mechanism of coupling between cytosolic sensor domain and selectivity filter in TREK K2P channels

The two-pore domain potassium (K2P) channels TREK-1 and TREK-2 link neuronal excitability to a variety of stimuli including mechanical force, lipids, temperature and phosphorylation. This regulation involves the C-terminus as a polymodal stimulus sensor and the selectivity filter (SF) as channel gate. Using crystallographic up- and down-state structures of TREK-2 as a template for full atomistic molecular dynamics (MD) simulations, we reveal that the SF in down-state undergoes inactivation via conformational changes, while the up-state structure maintains a stable and conductive SF. This suggests an atomistic mechanism for the low channel activity previously assigned to the down state, but not evident from the crystal structure. Furthermore, experimentally by using (de-)phosphorylation mimics and chemically attaching lipid tethers to the proximal C-terminus (pCt), we confirm the hypothesis that moving the pCt towards the membrane induces the up-state. Based on MD simulations, we propose two gating pathways by which movement of the pCt controls the stability (i.e., conductivity) of the filter gate. Together, these findings provide atomistic insights into the SF gating mechanism and the physiological regulation of TREK channels by phosphorylation.

Membrane phospholipids control gating of the mechanosensitive potassium leak channel TREK1

Tandem pore domain (K2P) potassium channels modulate resting membrane potentials and shape cellular excitability. For the mechanosensitive subfamily of K2Ps, the composition of phospholipids within the bilayer strongly influences channel activity. To examine the molecular details of K2P lipid modulation, we solved cryo-EM structures of the TREK1 K2P channel bound to either the anionic lipid phosphatidic acid (PA) or the zwitterionic lipid phosphatidylethanolamine (PE). At the extracellular face of TREK1, a PA lipid inserts its hydrocarbon tail into a pocket behind the selectivity filter, causing a structural rearrangement that recapitulates mutations and pharmacology known to activate TREK1. At the cytoplasmic face, PA and PE lipids compete to modulate the conformation of the TREK1 TM4 gating helix. Our findings demonstrate two distinct pathways by which anionic lipids enhance TREK1 activity and provide a framework for a model that integrates lipid gating with the effects of other mechanosensitive K2P modulators.

The Polysite Pharmacology of TREK K2P Channels

K_2P (KCNK) potassium channels form "background" or "leak" currents that have critical roles in cell excitability control in the brain, cardiovascular system, and somatosensory neurons. Similar to many ion channel families, studies of K_2Ps have been limited by poor pharmacology. Of six K_2P subfamilies, the thermo- and mechanosensitive TREK subfamily comprising K_2P2.1 (TREK-1), K_2P4.1 (TRAAK), and K_2P10.1 (TREK-2) are the first to have structures determined for each subfamily member. These structural studies have revealed key architectural features that underlie K_2P function and have uncovered sites residing at every level of the channel structure with respect to the membrane where small molecules or lipids can control channel function. This polysite pharmacology within a relatively small (~70 kDa) ion channel comprises four structurally defined modulator binding sites that occur above (Keystone inhibitor site), at the level of (K_2P modulator pocket), and below (Fenestration and Modulatory lipid sites) the C-type selectivity filter gate that is at the heart of K_2P function. Uncovering this rich structural landscape provides the framework for understanding and developing subtype-selective modulators to probe K_2P function that may provide leads for drugs for anesthesia, pain, arrhythmia, ischemia, and migraine.

Convergence of Multiple Stimuli to a Single Gate in TREK1 and TRAAK Potassium Channels

Inhibitory potassium channels of the TREK1/TRAAK family are integrators of multiple stimuli, including temperature, membrane stretch, polyunsaturated fatty acids and pH. How these signals affect the gating of these channels is the subject of intense research. We have previously identified a cytoplasmic domain, pCt, which plays a major role in controlling channel activity. Here, we use pharmacology to show that the effects of pCt, arachidonic acid, and extracellular pH converge to the same gate within the channel. Using a state-dependent inhibitor, fluoxetine, as well as natural and synthetic openers, we provide further evidence that the “up” and “down” conformations identified by crystallography do not correspond to open and closed states of these channels.

Norfluoxetine inhibits TREK-2 K2P channels by multiple mechanisms including state-independent effects on the selectivity filter gate

The TREK subfamily of two-pore domain K⁺ (K2P) channels are inhibited by fluoxetine and its metabolite, norfluoxetine (NFx). Although not the principal targets of this antidepressant, TREK channel inhibition by NFx has provided important insights into the conformational changes associated with channel gating and highlighted the role of the selectivity filter in this process. However, despite the availability of TREK-2 crystal structures with NFx bound, the precise mechanisms underlying NFx inhibition remain elusive. NFx has previously been proposed to be a state-dependent inhibitor, but its binding site suggests many possible ways in which this positively charged drug might inhibit channel activity. Here we show that NFx exerts multiple effects on single-channel behavior that influence both the open and closed states of the channel and that the channel can become highly activated by 2-APB while remaining in the down conformation. We also show that the inhibitory effects of NFx are unrelated to its positive charge but can be influenced by agonists which alter filter stability, such as ML335, as well as by an intrinsic voltage-dependent gating process within the filter. NFx therefore not only inhibits channel activity by altering the equilibrium between up and down conformations but also can directly influence filter gating. These results provide further insight into the complex allosteric mechanisms that modulate filter gating in TREK K2P channels and highlight the different ways in which filter gating can be regulated to permit polymodal regulation.

Structural Insights into the Mechanisms and Pharmacology of K2P Potassium Channels

Leak currents, defined as voltage and time independent flows of ions across cell membranes, are central to cellular electrical excitability control. The K_2P (KCNK) potassium channel class comprises an ion channel family that produces potassium leak currents that oppose excitation and stabilize the resting membrane potential in cells in the brain, cardiovascular system, immune system, and sensory organs. Due to their widespread tissue distribution, K_2Ps contribute to many physiological and pathophysiological processes including anesthesia, pain, arrythmias, ischemia, hypertension, migraine, intraocular pressure regulation, and lung injury responses. Structural studies of six homomeric K_2Ps have established the basic architecture of this channel family, revealed key moving parts involved in K_2P function, uncovered the importance of asymmetric pinching and dilation motions in the K_2P selectivity filter (SF) C-type gate, and defined two K_2P structural classes based on the absence or presence of an intracellular gate. Further, a series of structures characterizing K_2P:modulator interactions have revealed a striking polysite pharmacology housed within a relatively modestly sized (~70 kDa) channel. Binding sites for small molecules or lipids that control channel function are found at every layer of the channel structure, starting from its extracellular side through the portion that interacts with the membrane bilayer inner leaflet. This framework provides the basis for understanding how gating cues sensed by different channel parts control function and how small molecules and lipids modulate K_2P activity. Such knowledge should catalyze development of new K_2P modulators to probe function and treat a wide range of disorders.

Development of Non-opioid Analgesics Targeting Two-pore Domain Potassium Channels

Two-pore domain potassium (K2P) channels are a diverse family of potassium channels. K2P channels generate background leak potassium currents to regulate cellular excitability and are thereby involved in a wide range of neurological disorders. K2P channels are modulated by a variety of physicochemical factors such as mechanical stretch, temperature, and pH. In the the peripheral nervous system (PNS), K2P channels are widely expressed in nociceptive neurons and play a critical roles in pain perception. In this review, we summarize the recent advances in the pharmacological properties of K2P channels, with a focus on the exogenous small-molecule activators targeting K2P channels. We emphasize the subtype-selectivity, cellular and in vivo pharmacological properties of all the reported small-molecule activators. The key underlying analgesic mechanisms mediated by K2P are also summarized based on the data in the literature from studies using small-molecule activators and genetic knock-out animals. We discuss advantages and limitations of the translational perspectives of K2P in pain medicine and provide outstanding questions for future studies in the end.

Mechanistic insights into volatile anesthetic modulation of K2P channels

K2P potassium channels are known to be modulated by volatile anesthetic (VA) drugs and play important roles in clinically relevant effects that accompany general anesthesia. Here, we utilize a photoaffinity analog of the VA isoflurane to identify a VA-binding site in the TREK1 K2P channel. The functional importance of the identified site was validated by mutagenesis and biochemical modification. Molecular dynamics simulations of TREK1 in the presence of VA found multiple neighboring residues on TREK1 TM2, TM3, and TM4 that contribute to anesthetic binding. The identified VA-binding region contains residues that play roles in the mechanisms by which heat, mechanical stretch, and pharmacological modulators alter TREK1 channel activity and overlaps with positions found to modulate TASK K2P channel VA sensitivity. Our findings define molecular contacts that mediate VA binding to TREK1 channels and suggest a mechanistic basis to explain how K2P channels are modulated by VAs.

Dual regulatory effects of PI(4,5)P2 on TREK-2 K+ channel through antagonizing interaction between the alkaline residues (K330 and R355-357) in the cytosolic C-terminal helix

TWIK-related two-pore domain K⁺ channel-2 (TREK-2) has voltageindependent activity and shows additional activation by acidic intracellular pH (pH_i) via neutralizing the E³³² in the cytoplasmic C terminal (Ct). We reported opposite regulations of TREK-2 by phosphatidylinositol 4,5-bisphosphate (PIP₂) via the alkaline K³³⁰ and triple Arg residues (R^355-357); inhibition and activation, respectively. The G³³⁴ between them appeared critical because its mutation (G³³⁴A) endowed hTREK-2 with tonic activity, similar to the mutation of the inhibitory K³³⁰ (K³³⁰A). To further elucidate the role of putative bent conformation at G³³⁴, we compared the dual mutation forms, K³³⁰A/G³³⁴A and G³³⁴A/R^355-7A, showing higher and lower basal activity, respectively. The results suggested that the tonic activity of G³³⁴A owes to a dominant influence from R^355-7. Since there are additional triple Arg residues (R^377-9) distal to R^355-7, we also examined the triple mutant (G³³⁴A/R^355-7A/R^377-9A) that showed tonic inhibition same with G³³⁴A/R^355-7A. Despite the state of tonic inhibition, the activation by acidic pH_i was preserved in both G³³⁴A/R^355-7A and G³³⁴A/R^355-7A/R^377-9A, similar to the R^355-7A. Also, the inhibitory effect of ATP could be commonly demonstrated under the activation by acidic pH_i in R^355-7A, G³³⁴A/R^355-7A, and G³³⁴A/R^355-7A/R^377-9A. These results suggest that the putative bent conformation at G³³⁴ is important to set the tug-of-war between K³³⁰ and R^355-7 in the PIP₂-dependent regulation of TREK-2.

Two-Pore Domain Potassium Channels as Drug Targets: Anesthesia and Beyond

Two-pore domain potassium (K2P) channels stabilize the resting membrane potential of both excitable and nonexcitable cells and, as such, are important regulators of cell activity. There are many conditions where pharmacological regulation of K2P channel activity would be of therapeutic benefit, including, but not limited to, atrial fibrillation, respiratory depression, pulmonary hypertension, neuropathic pain, migraine, depression, and some forms of cancer. Up until now, few if any selective pharmacological regulators of K2P channels have been available. However, recent publications of solved structures with small-molecule activators and inhibitors bound to TREK-1, TREK-2, and TASK-1 K2P channels have given insight into the pharmacophore requirements for compound binding to these sites. Together with the increasing availability of a number of novel, active, small-molecule compounds from K2P channel screening programs, these advances have opened up the possibility of rational activator and inhibitor design to selectively target K2P channels.

Pharmacologically reversible, loss of function mutations in the TM2 and TM4 inner pore helices of TREK-1 K2P channels

A better understanding of the gating of TREK two pore domain potassium (K2P) channels and their activation by compounds such as the negatively charged activator, flufenamic acid (FFA) is critical in the search for more potent and selective activators of these channels. Currents through wild-type and mutated human K2P channels expressed in tsA201 cells were measured using whole-cell patch-clamp recordings in the presence and absence of FFA. Mutation of the TM2.6 residue of TREK-1 to a phenylalanine (G171F) and a similar mutation of TM4.6 (A286F) substantially reduced current through TREK-1 channels. In complementary experiments, replacing the natural F residues at the equivalent position in TRESK channels, significantly enhanced current. Known, gain of function mutations of TREK-1 (G137I, Y284A) recovered current through these mutated channels. This reduction in current could be also be reversed pharmacologically, by FFA. However, an appropriate length MTS (MethaneThioSulfonate) cross-linking reagent (MTS14) restricted the activation of TREK-1_A286C channels by repeated application of FFA. This suggests that the cross-linker stabilises the channel in a conformation which blunts FFA activation. Pharmacologically reversible mutations of TREK channels will help to clarify the importance of these channels in pathophysiological conditions such as pain and depression.

Molecular determinants of chemical modulation of two-pore domain potassium channels

The K⁺ ion channels comprising the two-pore domain (K2P) family have specific biophysical roles in generating the critical regulatory K⁺ current. Ion flow through K2P channels and, implicitly, channel regulation is mediated by diverse metabolic and physical inputs such as mechanical stimulation, interaction with lipids or endogenous regulators, intra- or extracellular pH, and phosphorylation, while their function can be finely tuned by chemical compounds. In the latter category, some drug-channel interactions can lead to side effects or have clinical action, while identifying novel chemical modulators of K2Ps is an area of intense research. Due to their cellular and therapeutic importance, much attention was turned to these channels in recent years and several experimental approaches have pinpointed the molecular determinants of K2P chemical modulation. Given their unique structural features and properties, chemical modulators act on K2P channels in multiple and diverse ways. In this review, the particularities of K2P modulation by chemical compounds, such as binding modality, affinity, or position, are identified, synthesized, and linked to structural and functional properties in order to refer to how activators and blockers modify channel function and vice versa, focusing on specificity related to protein structure (and its modification) and cross-linking information among different subfamilies.

Mechanism of Mechanosensitive Gating of the TREK-2 Potassium Channel

The mechanism of mechanosensitive gating of ion channels underlies many physiological processes, including the sensations of touch, hearing, and pain perception. TREK-2 is the best-studied mechanosensitive member of the two-pore domain potassium channel family. Apart from pressure sensing, it responds to a diverse range of stimuli. Two states, termed "up" and "down," are known from x-ray structural crystallographic studies and have been suggested to differ in conductance. However, the structural details of the gating behavior are largely unknown. In this work, we used molecular dynamics simulations to study the conductance of the states as well as the effect of mechanical membrane stretch on the channel. We find that the down state is less conductive than the up state. The introduction of membrane stretch in the simulations shifts the state of the channel toward an up configuration, independent of the starting configuration, and also increases its conductance. The correlation of the selectivity filter state and the conductance supports a model in which the selectivity filter gates by a carbonyl flip. This gate is stabilized by the pore helices. We suggest a modulation of these helices by an interface to the transmembrane helices. Membrane pressure changes the conformation of the transmembrane helices directly and consequently also influences the channel conductance.

Triple arginine residues in the proximal C-terminus of TREK K+ channels are critical for biphasic regulation by phosphatidylinositol 4,5-bisphosphate

TWIK-related two-pore domain K⁺ channels (TREKs) are activated by acidic intracellular pH (pH_i), membrane stretch, temperature, and arachidonic acid (AA). Phosphatidylinositol 4,5-bisphosphate (PIP₂) exerts concentration-dependent biphasic regulations, which have been observed: inhibition by high PIP₂, activation by partial decrease of PIP₂, and inhibition by depletion of PIP₂. Consistently, the stimulation of voltage-sensitive PIP₂ phosphatase (Dr-VSP) induces initial activation and subsequent inhibition of TREKs. Lys in the proximal C-terminus (pCt) is responsible for the inhibition by high PIP₂, which is generated by phosphatidylinositol kinases with ATP; its neutralizing mutation [K³³⁰A of human TREK-2 (hTREK-2)] induces tonic high activity, irrespective of ATP. Here we focus on triple successive Arg in pCt (R3-pCt) as a candidate region for the stimulatory regulation by lower PIP₂. Their neutralized mutant (R3A-pCt; RRR^340-2A and RRR^355-7A in hTREK-1 and -2, respectively) showed negligible basal current and was not affected by ATP removal or by Dr-VSP activation. Phosphatidic acid, a phospholipid agonist of TREKs, did not activate R3A-pCt. In contrast, acidic pH_i, AA, and high temperature activated R3A-pCt normally, whereas activation by membrane stretch was attenuated. In hTREK-2, combined neutralizations of the inhibitory K³³⁰ and R3-pCt (K³³⁰A/RRR^355-7A) did not recover the suppressed current. In contrast, combined neutralization of pH_i-sensing Glu (E³³²A/R^355-7A) induced tonic high current and no further activation by pH_i. Interestingly, when the Gly between K³³⁰/E³³² and R3-pCt was mutated (G³³⁴A), hTREK-2 was tonic activated with reversed responses to ATP and acidic pH_i. Therefore, we propose that the PIP₂-dependent converse regulation of TREKs by Lys and R3-pCt with Gly implies structural flexibility.

Antagonistic Effect of a Cytoplasmic Domain on the Basal Activity of Polymodal Potassium Channels

TREK/TRAAK channels are polymodal K⁺ channels that convert very diverse stimuli, including bioactive lipids, mechanical stretch and temperature, into electrical signals. The nature of the structural changes that regulate their activity remains an open question. Here, we show that a cytoplasmic domain (the proximal C-ter domain, pCt) exerts antagonistic effects in TREK1 and TRAAK. In basal conditions, pCt favors activity in TREK1 whereas it impairs TRAAK activity. Using the conformation-dependent binding of fluoxetine, we show that TREK1 and TRAAK conformations at rest are different, and under the influence of pCt. Finally, we show that depleting PIP₂ in live cells has a more pronounced inhibitory effect on TREK1 than on TRAAK. This differential regulation of TREK1 and TRAAK is related to a previously unrecognized PIP₂-binding site (R329, R330, and R331) present within TREK1 pCt, but not in TRAAK pCt. Collectively, these new data point out pCt as a major regulatory domain of these channels and suggest that the binding of PIP₂ to the pCt of TREK1 results in the stabilization of the conductive conformation in basal conditions.

The gating cycle of a K+ channel at atomic resolution

C-type inactivation in potassium channels helps fine-tune long-term channel activity through conformational changes at the selectivity filter. Here, through the use of cross-linked constitutively open constructs, we determined the structures of KcsA’s mutants that stabilize the selectivity filter in its conductive (E71A, at 2.25 Å) and deep C-type inactivated (Y82A at 2.4 Å) conformations. These structural snapshots represent KcsA’s transient open-conductive (O/O) and the stable open deep C-type inactivated states (O/I), respectively. The present structures provide an unprecedented view of the selectivity filter backbone in its collapsed deep C-type inactivated conformation, highlighting the close interactions with structural waters and the local allosteric interactions that couple activation and inactivation gating. Together with the structures associated with the closed-inactivated state (C/I) and in the well-known closed conductive state (C/O), this work recapitulates, at atomic resolution, the key conformational changes of a potassium channel pore domain as it progresses along its gating cycle.

The effects of stretch activation on ionic selectivity of the TREK-2 K2P K+ channel

The TREK-2 (KCNK10) K2P potassium channel can be regulated by variety of polymodal stimuli including pressure. In a recent study, we demonstrated that this mechanosensitive K⁺ channel responds to changes in membrane tension by undergoing a major structural change from its ‘down’ state to the more expanded ‘up’ state conformation. These changes are mostly restricted to the lower part of the protein within the bilayer, but are allosterically coupled to the primary gating mechanism located within the selectivity filter. However, any such structural changes within the filter also have the potential to alter ionic selectivity and there are reports that some K2Ps, including TREK channels, exhibit a dynamic ionic selectivity. In this addendum to our previous study we have therefore examined whether the selectivity of TREK-2 is altered by stretch activation. Our results reveal that the filter remains stable and highly selective for K⁺ over Na⁺ during stretch activation, and that permeability to a range of other cations (Rb⁺, Cs⁺ and NH₄⁺) also does not change. The asymmetric structural changes that occur during stretch activation therefore allow the channel to respond to changes in membrane tension without a loss of K⁺ selectivity.

Markov modeling reveals novel intracellular modulation of the human TREK-2 selectivity filter

Two-pore domain potassium (K2P) channel ion conductance is regulated by diverse stimuli that directly or indirectly gate the channel selectivity filter (SF). Recent crystal structures for the TREK-2 member of the K2P family reveal distinct “up” and “down” states assumed during activation via mechanical stretch. We performed 195 μs of all-atom, unbiased molecular dynamics simulations of the TREK-2 channel to probe how membrane stretch regulates the SF gate. Markov modeling reveals a novel “pinched” SF configuration that stretch activation rapidly destabilizes. Free-energy barrier heights calculated for critical steps in the conduction pathway indicate that this pinched state impairs ion conduction. Our simulations predict that this low-conductance state is accessed exclusively in the compressed, “down” conformation in which the intracellular helix arrangement allosterically pinches the SF. By explicitly relating structure to function, we contribute a critical piece of understanding to the evolving K2P puzzle.

Selective Small Molecule Activators of TREK-2 Channels Stimulate Dorsal Root Ganglion c-Fiber Nociceptor Two-Pore-Domain Potassium Channel Currents and Limit Calcium Influx

The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential in dorsal root ganglia c-fiber nociceptors, which tunes electrical excitability and nociception. Thus, TREK-2 channels are considered a potential therapeutic target for treating pain; however, there are currently no selective pharmacological tools for TREK-2 channels. Here we report the identification of the first TREK-2 selective activators using a high-throughput fluorescence-based thallium (Tl+) flux screen (HTS). An initial pilot screen with a bioactive lipid library identified 11-deoxy prostaglandin F2α as a potent activator of TREK-2 channels (EC50 ≈ 0.294 μM), which was utilized to optimize the TREK-2 Tl+ flux assay (Z' = 0.752). A HTS was then performed with 76 575 structurally diverse small molecules. Many small molecules that selectively activate TREK-2 were discovered. As these molecules were able to activate single TREK-2 channels in excised membrane patches, they are likely direct TREK-2 activators. Furthermore, TREK-2 activators reduced primary dorsal root ganglion (DRG) c-fiber Ca2+ influx. Interestingly, some of the selective TREK-2 activators such as 11-deoxy prostaglandin F2α were found to inhibit the K2P channel TREK-1. Utilizing chimeric channels containing portions of TREK-1 and TREK-2, the region of the TREK channels that allows for either small molecule activation or inhibition was identified. This region lies within the second pore domain containing extracellular loop and is predicted to play an important role in modulating TREK channel activity. Moreover, the selective TREK-2 activators identified in this HTS provide important tools for assessing human TREK-2 channel function and investigating their therapeutic potential for treating chronic pain.

Intersubunit Concerted Cooperative and cis-Type Mechanisms Modulate Allosteric Gating in Two-Pore-Domain Potassium Channel TREK-2

In response to diverse stimuli, two-pore-domain potassium channel TREK-2 regulates cellular excitability, and hence plays a key role in mediating neuropathic pain, mood disorders and ischemia through. Although more and more input modalities are found to achieve their modulations via acting on the channel, the potential role of subunit interaction in these modulations remains to be explored. In the current study, the deletion (lack of proximal C-terminus, ΔpCt) or point mutation (G312A) was introduced into TREK-2 subunits to limit K⁺ conductance and used to report subunit stoichiometry. The constructs were then combined with wild type (WT) subunit to produce concatenated dimers with defined composition, and the gating kinetics of these channels to 2-Aminoethoxydiphenyl borate (2-APB) and extracellular pH (pH_o) were characterized. Our results show that combination of WT and ΔpCt/G312A subunits reserves similar gating properties to that of WT dimmers, suggesting that the WT subunit exerts dominant and positive effects on the mutated one, and thus the two subunits controls channel gating via a concerted cooperative manner. Further introduction of ΔpCt into the latter subunit of heterodimeric channel G312A-WT or G312A-G312A attenuated their sensitivity to 2-APB and pH_o alkalization, implicating that these signals were transduced by a cis-type mechanism. Together, our findings elucidate the mechanisms for how the two subunits control the pore gating of TREK-2, in which both intersubunit concerted cooperative and cis-type manners modulate the allosteric regulations induced by 2-APB and pH_o alkalization.