Silent but not dumb: how cellular trafficking and pore gating modulate expression of TWIK1 and THIK2

Gating mechanism of the two-pore-domain potassium channel THIK1

TWIK-related halothane-inhibited potassium channel (THIK1) maintains the resting membrane potential and regulates potassium efflux in microglia. It is a potential therapeutic target for neurodegenerative disorders, neuropathic pain and inflammation. However, the mechanism underlying its function remains unclear. Here we used cryo-electron microscopy to solve the structures of full-length human THIK1, revealing two inner gates and a C-type selectivity filter gate, distinct from other two-pore-domain potassium channels. One inner gate, formed by a short helix in the distal C terminus, introduces a unique gating mechanism involving the distal cytoplasmic domain. The other, beneath the selectivity filter, is constricted by Y273 in the M4 helix, dividing the cavity. In addition, the selectivity filter gate is modulated by polyunsaturated fatty acids. These structural insights into THIK1 gating, through the distal C-terminal helices, hydrophilic residues and selectivity filter, advance our understanding of THIK1's role in microglial homeostasis and neuropathologies.

X-ray irradiation reduces ATP-dependent activation of NLRP3 inflammasome by inhibiting TWIK2 activity in macrophages

BACKGROUND

The spleen, as the body's largest peripheral immune organ and a crucial source of circulating monocytes, plays a significant role in the acute inflammatory response of spleen-derived macrophages to diseases. Therefore, studying the impact and mechanism of X-ray irradiation on spleen-derived macrophages' inflammatory responses is of great importance.

METHOD

Extracted and identified mice splenic macrophages were divided into four groups: control group, LPS and ATP co-stimulated non-irradiated group, LPS and ATP co-stimulated group irradiated after 6 h, and LPS and ATP co-stimulated group irradiated after 12 h In the LPS and ATP co-stimulated groups, LPS (1μg/ml) and ATP (5mmol/L) were added to establish an inflammatory model in mice splenic macrophages. The irradiated groups were exposed to a medical linear accelerator (Elekta Synergy), while the non-irradiated groups were placed under the light source for the same duration without irradiation. Protein extraction was performed in each group at 6 h and 12 h post-treatment for subsequent analysis using Western blot, ELISA, RT-qPCR and other relevant methods.

RESULTS

(1) Compared with the non-irradiated group, the cell activity in the groups irradiated for 6 h and 12 h at 8 Gy showed a significant increase (P<0.01). (2) In the LPS and ATP co-stimulated groups irradiated after 6 h and 12 h, the expression of NLRP3 mRNA and protein, IL-18 and IL-1β showed a notable decrease compared to the LPS and ATP co-stimulated non-irradiated group (P<0.05). Additionally, caspase-1 expression of caspase-1 mRNA and protein in the 12 h post-irradiation group also decreased considerably when compared with the LPS and ATP co-stimulated non-irradiated group (P < 0.05). In the groups irradiated after 6 h and 12 h, (3) there was a remarkable decrease in the expression of TWIK mRNA and TWIK2, (4) as well as Gq mRNA and protein, when compared to the LPS and ATP co-stimulated non-irradiated group (P < 0.05). Particularly, the 12 h post-irradiation group exhibited a notable reduction in PKC expression (P < 0.05).

CONCLUSION

X-ray irradiation is capable of inhibiting the activation of ATP-dependent NLRP3 inflammasomes in splenic macrophages.

Cryo-EM structure of the human THIK-1 K2P K+ channel reveals a lower Y gate regulated by lipids and anesthetics

THIK-1 (KCNK13) is a halothane-inhibited and anionic-lipid-activated two-pore domain (K2P) K+ channel implicated in microglial activation and neuroinflammation, and a current target for the treatment of neurodegenerative disorders, for example Alzheimer's disease and amyothropic lateral sclerosis (ALS). However, compared to other K2P channels, little is known about the structural and functional properties of THIK-1. Here we present a 3.16-Å-resolution cryo-EM structure of human THIK-1 that reveals several distinct features, in particular, a tyrosine in M4 that contributes to a lower 'Y gate' that opens upon activation by physiologically relevant G-protein-coupled receptor and lipid signaling pathways. We demonstrate that linoleic acid bound within a modulatory pocket adjacent to the filter influences channel activity, and that halothane inhibition involves a binding site within the inner cavity, both resulting in conformational changes to the Y gate. Finally, the extracellular cap domain contains positively charged residues that line the ion exit pathway and contribute to the distinct biophysical properties of this channel. Overall, our results provide structural insights into THIK-1 function and identify distinct regulatory sites that expand its potential as a drug target for the modulation of microglial function.

Research progress of two-pore potassium channel in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion injury (MIRI) is a secondary injury caused by restoring blood flow after acute myocardial infarction, which may lead to serious arrhythmia and heart damage. In recent years, the role of potassium channels in MIRI has attracted much attention, especially the members of the two-pore domain potassium (K2P) channel family. K2P channel has unique structure and function, and the formation of its heterodimer increases its functional diversity. This paper reviews the structural characteristics, types, expression and physiological functions of K2P channel in the heart. In particular, we pay attention to whether members of the subfamily such as TWIK, TREK, TASK, TALK, THIK and TRESK participate in MIRI and their related mechanisms. Future research will help to reveal the molecular mechanism of K2P channel in MIRI and provide new strategies for the treatment of cardiovascular diseases.

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

Glial-derived neurotrophic factor regulates the expression of TREK2 in rat primary sensory neurons leading to attenuation of axotomy-induced neuropathic pain

TREK2 is a member of the 2-pore domain family of K+ channels (K2P) preferentially expressed by unmyelinated, slow-conducting and non-peptidergic isolectin B4-binding (IB4+) primary sensory neurons of the dorsal root ganglia (DRG). IB4+ neurons depend on the glial-derived neurotrophic factor (GDNF) family of ligands (GFL's) to maintain their phenotype. In our previous work, we demonstrated that 7 days after spinal nerve axotomy (SNA) of the L5 DRG, TREK2 moves away from the cell membrane resulting in a more depolarised resting membrane potential (Em). Given that axotomy deprives DRG neurons from peripherally-derived GFL's, we hypothesized that they might control the expression of TREK2. Using a combination of immunohistochemistry, immunocytochemistry, western blotting, in vivo pharmacological manipulation and behavioral tests we examined the ability of the GFL's (GDNF, neurturin and artemin) and their selective receptors (GFRα1, GFRα2 and GFRα3) to regulate the expression and function of TREK2 in the DRG. We found that TREK2 correlated strongly with the three receptors normally and ipsilaterally for all GFR's after SNA. GDNF, but not NGF, neurturin or artemin up-regulated the expression of TREK2 in cultured DRG neurons. In vivo continuous, subcutaneous administration of GDNF restored the subcellular distribution of TREK2 ipsilaterally and reversed mechanical and cold allodynia 7 days after SNA. This is the first demonstration that GDNF controls the expression of a K2P channel in nociceptors. As TREK2 controls the Em of C-nociceptors affecting their excitability, our finding has therapeutic potential in the treatment of chronic pain.

Endolysosomal cation channels point the way towards precision medicine of cancer and infectious diseases

Infectious diseases and cancer are among the key medical challenges that humankind is facing today. A growing amount of evidence suggests that ion channels in the endolysosomal system play a crucial role in the pathology of both groups of diseases. The development of advanced patch-clamp technologies has allowed us to directly characterize ion fluxes through endolysosomal ion channels in their native environments. Endolysosomes are essential organelles for intracellular transport, digestion and metabolism, and maintenance of homeostasis. The endolysosomal ion channels regulate the function of the endolysosomal system through four basic mechanisms: calcium release, control of membrane potential, pH change, and osmolarity regulation. In this review, we put particular emphasis on the endolysosomal cation channels, including TPC2 and TRPML2, which are particularly important in monocyte function. We discuss existing endogenous and synthetic ligands of these channels and summarize current knowledge of their impact on channel activity and function in different cell types. Moreover, we summarize recent findings on the importance of TPC2 and TRPML2 channels as potential drug targets for the prevention and treatment of the emerging infectious diseases and cancer.

The versatile regulation of K2P channels by polyanionic lipids of the phosphoinositide and fatty acid metabolism

Work over the past three decades has greatly advanced our understanding of the regulation of K_ir K⁺ channels by polyanionic lipids of the phosphoinositide (e.g., PIP₂) and fatty acid metabolism (e.g., oleoyl-CoA). However, comparatively little is known regarding the regulation of the K_2P channel family by phosphoinositides and by long-chain fatty acid–CoA esters, such as oleoyl-CoA. We screened 12 mammalian K_2P channels and report effects of polyanionic lipids on all tested channels. We observed activation of members of the TREK, TALK, and THIK subfamilies, with the strongest activation by PIP₂ for TRAAK and the strongest activation by oleoyl-CoA for TALK-2. By contrast, we observed inhibition for members of the TASK and TRESK subfamilies. Our results reveal that TASK-2 channels have both activatory and inhibitory PIP₂ sites with different affinities. Finally, we provided evidence that PIP₂ inhibition of TASK-1 and TASK-3 channels is mediated by closure of the recently identified lower X-gate as critical mutations within the gate (i.e., L244A, R245A) prevent PIP₂-induced inhibition. Our findings establish that K⁺ channels of the K_2P family are highly sensitive to polyanionic lipids, extending our knowledge of the mechanisms of lipid regulation and implicating the metabolism of these lipids as possible effector pathways to regulate K_2P channel activity.

Transcriptomic profile of cationic channels in human pulmonary arterial hypertension

The dysregulation of K⁺ channels is a hallmark of pulmonary arterial hypertension (PAH). Herein, the channelome was analyzed in lungs of patients with PAH in a public transcriptomic database. Sixty six (46%) mRNA encoding cationic channels were dysregulated in PAH with most of them downregulated (83%). The principal component analysis indicated that dysregulated cationic channel expression is a signature of the disease. Changes were very similar in idiopathic, connective tissue disease and congenital heart disease associated PAH. This analysis 1) is in agreement with the widely recognized pathophysiological role of TASK1 and K_V1.5, 2) supports previous preliminary reports pointing to the dysregulation of several K⁺ channels including the downregulation of K_V1.1, K_V1.4, K_V1.6, K_V7.1, K_V7.4, K_V9.3 and TWIK2 and the upregulation of K_Ca1.1 and 3) points to other cationic channels dysregulated such as Kv7.3, TALK2, Ca_V1 and TRPV4 which might play a pathophysiological role in PAH. The significance of other changes found in Na⁺ and TRP channels remains to be investigated.

PACAP Induces Light Aversion in Mice by an Inheritable Mechanism Independent of CGRP

The neuropeptides CGRP (calcitonin gene-related peptide) and PACAP (pituitary adenylate cyclase-activating polypeptide) have emerged as mediators of migraine, yet the potential overlap of their mechanisms remains unknown. Infusion of PACAP, like CGRP, can cause migraine in people, and both peptides share similar vasodilatory and nociceptive functions. In this study, we have used light aversion in mice as a surrogate for migraine-like photophobia to compare CGRP and PACAP and ask whether CGRP or PACAP actions were dependent on each other. Similar to CGRP, PACAP induced light aversion in outbred CD-1 mice. The light aversion was accompanied by increased resting in the dark, but not anxiety in a light-independent open field assay. Unexpectedly, about one-third of the CD-1 mice did not respond to PACAP, which was not seen with CGRP. The responder and nonresponder phenotypes were stable, inheritable, and not sex linked, although there was a trend for greater responses among male mice. RNA-sequencing analysis of trigeminal ganglia yielded hierarchical clustering of responder and nonresponder mice and revealed a number of candidate genes, including greater expression of the Trpc5 and Kcnk12 ion channels and glycoprotein hormones and receptors in a subset of male responder mice. Importantly, an anti-PACAP monoclonal antibody could block PACAP-induced light aversion but not CGRP-induced light aversion. Conversely, an anti-CGRP antibody could not block PACAP-induced light aversion. Thus, we propose that CGRP and PACAP act by independent convergent pathways that cause a migraine-like symptom in mice.SIGNIFICANCE STATEMENT The relationship between the neuropeptides CGRP (calcitonin gene-related peptide) and PACAP (pituitary adenylate cyclase-activating polypeptide) in migraine is relevant given that both peptides can induce migraine in people, yet to date only drugs that target CGRP are available. Using an outbred strain of mice, we were able to show that most, but not all, mice respond to PACAP in a preclinical photophobia assay. Our finding that CGRP and PACAP monoclonal antibodies do not cross-inhibit the other peptide indicates that CGRP and PACAP actions are independent and suggests that PACAP-targeted drugs may be effective in patients who do not respond to CGRP-based therapeutics.

Physiological roles of heteromerization: focus on the two-pore domain potassium channels

Potassium channels form the largest family of ion channels with more than 80 members involved in cell excitability and signalling. Most of them exist as homomeric channels, whereas specific conditions are required to obtain heteromeric channels. It is well established that heteromerization of voltage-gated and inward rectifier potassium channels affects their function, increasing the diversity of the native potassium currents. For potassium channels with two pore domains (K_2P ), homomerization has long been considered the rule, their polymodal regulation by a wide diversity of physical and chemical stimuli being responsible for the adaptation of the leak potassium currents to cellular needs. This view has recently evolved with the accumulation of evidence of heteromerization between different K_2P subunits. Several functional intragroup and intergroup heteromers have recently been identified, which contribute to the functional heterogeneity of this family. K_2P heteromerization is involved in the modulation of channel expression and trafficking, promoting functional and signalling diversity. As illustrated in the Abstract Figure, heteromerization of TREK1 and TRAAK provides the cell with more possibilities of regulation. It is becoming increasingly evident that K_2P heteromers contribute to important physiological functions including neuronal and cardiac excitability. Since heteromerization also affects the pharmacology of K_2P channels, this understanding helps to establish K_2P heteromers as new therapeutic targets for physiopathological conditions.

The Phosphodiesterase Inhibitor IBMX Blocks the Potassium Channel THIK-1 from the Extracellular Side

The two-pore domain potassium channel (K2P-channel) THIK-1 has several predicted protein kinase A (PKA) phosphorylation sites. In trying to elucidate whether THIK-1 is regulated via PKA, we expressed THIK-1 channels in a mammalian cell line (CHO cells) and used the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) as a pharmacological tool to induce activation of PKA. Using the whole-cell patch-clamp recording, we found that THIK-1 currents were inhibited by application of IBMX with an IC50 of 120 µM. Surprisingly, intracellular application of IBMX or of the second messenger cAMP via the patch pipette had no effect on THIK-1 currents. In contrast, extracellular application of IBMX produced a rapid and reversible inhibition of THIK-1. In patch-clamp experiments with outside-out patches, THIK-1 currents were also inhibited by extracellular application of IBMX. Expression of THIK-1 channels in Xenopus oocytes was used to compare wild-type channels with mutated channels. Mutation of the putative PKA phosphorylation sites did not change the inhibitory effect of IBMX on THIK-1 currents. Mutational analysis of all residues of the (extracellular) helical cap of THIK-1 showed that mutation of the arginine residue at position 92, which is in the linker between cap helix 2 and pore helix 1, markedly reduced the inhibitory effect of IBMX. This flexible linker region, which is unique for each K2P-channel subtype, may be a possible target of channel-specific blockers. SIGNIFICANCE STATEMENT: The potassium channel THIK-1 is strongly expressed in the central nervous system. We studied the effect of 3-isobutyl-1-methyl-xanthine (IBMX) on THIK-1 currents. IBMX inhibits breakdown of cAMP and thus activates protein kinase A (PKA). Surprisingly, THIK-1 current was inhibited when IBMX was applied from the extracellular side of the membrane, but not from the intracellular side. Our results suggest that IBMX binds directly to the channel and that the inhibition of THIK-1 current was not related to activation of PKA.

TREK-1 channels regulate pressure sensitivity and calcium signaling in trabecular meshwork cells

The trabecular meshwork (TM) plays a fundamental role in intraocular pressure regulation, but its mechanotransduction pathway is poorly understood. Yarishkin et al. show that the mechanosensing channel TREK-1 regulates TM membrane potential, pressure sensitivity, calcium homeostasis, and impedance.

Mechanotransduction by the trabecular meshwork (TM) is an essential component of intraocular pressure regulation in the vertebrate eye. This process is compromised in glaucoma but is poorly understood. In this study, we identify transient receptor potential vanilloid isoform 4 (TRPV4) and TWIK-related potassium channel-1 (TREK-1) as key molecular determinants of TM membrane potential, pressure sensitivity, calcium homeostasis, and transcellular permeability. We show that resting membrane potential in human TM cells is unaffected by “classical” inhibitors of voltage-activated, calcium-activated, and inwardly rectifying potassium channels but is depolarized by blockers of tandem-pore K⁺ channels. Using gene profiling, we reveal the presence of TREK-1, TASK-1, TWIK-2, and THIK transcripts in TM cells. Pressure stimuli, arachidonic acid, and TREK-1 activators hyperpolarize these cells, effects that are antagonized by quinine, amlodipine, spadin, and short-hairpin RNA–mediated knockdown of TREK-1 but not TASK-1. Activation and inhibition of TREK-1 modulates [Ca²⁺]_TM and lowers the impedance of cell monolayers. Together, these results suggest that tensile homeostasis in the TM may be regulated by balanced, pressure-dependent activation of TRPV4 and TREK-1 mechanotransducers.

Recombinant tandem of pore-domains in a Weakly Inward rectifying K+ channel 2 (TWIK2) forms active lysosomal channels

Recombinant TWIK2 channels produce weak basal background K⁺ currents. Current amplitudes depend on the animal species the channels have been isolated from and on the heterologous system used for their re-expression. Here we show that this variability is due to a unique cellular trafficking. We identified three different sequence signals responsible for the preferential expression of TWIK2 in the Lamp1-positive lysosomal compartment. Sequential inactivation of tyrosine-based (Y₃₀₈ASIP) and di-leucine-like (E₂₆₆LILL and D₂₈₂EDDQVDIL) trafficking motifs progressively abolishes the targeting of TWIK2 to lysosomes, and promotes its functional relocation at the plasma membrane. In addition, TWIK2 contains two N-glycosylation sites (N₇₉AS and N₈₅AS) on its luminal side, and glycosylation is necessary for expression in lysosomes. As shown by electrophysiology and electron microscopy, TWIK2 produces functional background K⁺ currents in the endolysosomes, and its expression affects the number and mean size of the lysosomes. These results show that TWIK2 is expressed in lysosomes, further expanding the registry of ion channels expressed in these organelles.

mGluR3 Activation Recruits Cytoplasmic TWIK-1 Channels to Membrane that Enhances Ammonium Uptake in Hippocampal Astrocytes

TWIK-1 two-pore domain K⁺ channels are highly expressed in mature hippocampal astrocytes. While the TWIK-1 activity is readily detectable on astrocyte membrane, the majority of channels are retained in the intracellular compartments, which raises an intriguing question of whether the membrane TWIK-1 channels could be dynamically regulated for functions yet unknown. Here, the regulation of TWIK-1 membrane expression by G_i/G_o-coupled metabotropic glutamate receptor 3 (mGluR3) and its functional impact on ammonium uptake has been studied. Activation of mGluR3 induced a marked translocation of TWIK-1 channels from the cytoplasm to the membrane surface. Consistent with our early observation that membrane TWIK-1 behaves as nonselective monovalent cation channel, mGluR3-mediated TWIK-1 membrane expression was associated with a depolarizing membrane potential (V _M). As TWIK-1 exhibits a discernibly high permeability to ammonium (NH₄⁺), a critical substrate in glutamate-glutamine cycle for neurotransmitter replenishment, regulation of NH₄⁺ uptake capacity by TWIK-1 membrane expression was determined by response of astrocyte V _M to bath application of 5 mM NH₄Cl. Stimulation of mGluR3 potentiated NH₄⁺-induced V _M depolarization by ∼30 % in wild type, but not in TWIK-1 knockout astrocytes. Furthermore, activation of mGluR3 mediated a coordinated translocation of TWIK-1 channels with recycling endosomes toward astrocyte membrane and the mGluR3-mediated potentiation of NH₄⁺ uptake required a functional Rab-mediated trafficking pathway. Altogether, we demonstrate that the activation of mGluR3 up-regulates the membrane expression of TWIK-1 that in turn enhances NH₄⁺ uptake in astrocytes, a mechanism potentially important for functional coupling of astrocyte glutamate-glutamine cycle with the replenishment of neurotransmitters in neurons.

Much more than a leak: structure and function of K₂p-channels

Over the last decade, we have seen an enormous increase in the number of experimental studies on two-pore-domain potassium channels (K2P-channels). The collection of reviews and original articles compiled for this special issue of Pflügers Archiv aims to give an up-to-date summary of what is known about the physiology and pathophysiology of K2P-channels. This introductory overview briefly describes the structure of K2P-channels and their function in different organs. Its main aim is to provide some background information for the 19 reviews and original articles of this special issue of Pflügers Archiv. It is not intended to be a comprehensive review; instead, this introductory overview focuses on some unresolved questions and controversial issues, such as: Do K2P-channels display voltage-dependent gating? Do K2P-channels contribute to the generation of action potentials? What is the functional role of alternative translation initiation? Do K2P-channels have one or two or more gates? We come to the conclusion that we are just beginning to understand the extremely complex regulation of these fascinating channels, which are often inadequately described as 'leak channels'.

The family of K2P channels: salient structural and functional properties

Potassium channels participate in many biological functions, from ion homeostasis to generation and modulation of the electrical membrane potential. They are involved in a large variety of diseases. In the human genome, 15 genes code for K(+) channels with two pore domains (K2P ). These channels form dimers of pore-forming subunits that produce background conductances finely regulated by a range of natural and chemical effectors, including signalling lipids, temperature, pressure, pH, antidepressants and volatile anaesthetics. Since the cloning of TWIK1, the prototypical member of this family, a lot of work has been carried out on their structure and biology. These studies are still in progress, but data gathered so far show that K2P channels are central players in many processes, including ion homeostasis, hormone secretion, cell development and excitability. A growing number of studies underline their implication in physiopathological mechanisms, such as vascular and pulmonary hypertension, cardiac arrhythmias, nociception, neuroprotection and depression. This review gives a synthetic view of the most noticeable features of these channels.

The role of protein-protein interactions in the intracellular traffic of the potassium channels TASK-1 and TASK-3

The intracellular transport of membrane proteins is controlled by trafficking signals: Short peptide motifs that mediate the contact with COPI, COPII or various clathrin-associated coat proteins. In addition, many membrane proteins interact with accessory proteins that are involved in the sorting of these proteins to different intracellular compartments. In the K2P channels, TASK-1 and TASK-3, the influence of protein-protein interactions on sorting decisions has been studied in some detail. Both TASK paralogues interact with the adaptor protein 14-3-3; TASK-1 interacts, in addition, with the adaptor protein p11 (S100A10) and the endosomal SNARE protein syntaxin-8. The role of these interacting proteins in controlling the intracellular traffic of the channels and the underlying molecular mechanisms are summarised in this review. In the case of 14-3-3, the interacting protein masks a retention signal in the C-terminus of the channel; in the case of p11, the interacting protein carries a retention signal that localises the channel to the endoplasmic reticulum; and in the case of syntaxin-8, the interacting protein carries an endocytosis signal that complements an endocytosis signal of the channel. These examples illustrate some of the mechanisms by which interacting proteins may determine the itinerary of a membrane protein within a cell and suggest that the intracellular traffic of membrane proteins may be adapted to the specific functions of that protein by multiple protein-protein interactions.