Targeting of Calcineurin to an NFAT-like Docking Site Is Required for the Calcium-dependent Activation of the Background K+ Channel, TRESK

Critical contribution of the intracellular C-terminal region to TRESK channel activity is revealed by the epithelial Na+ current ratio (ENaR) method

TRESK (K_2P18.1) possesses unique structural proportions within the K_2P background potassium channel family. The previously described TRESK regulatory mechanisms are based on the long intracellular loop between the second and third transmembrane segments (TMS). However, the functional significance of the exceptionally short intracellular C-terminal region (iCtr) following the fourth TMS has not yet been examined. In the present study, we investigated TRESK constructs modified at the iCtr by two-electrode voltage clamp and the newly developed epithelial sodium current ratio (ENaR) method in Xenopus oocytes. The ENaR method allowed the evaluation of channel activity by exclusively using electrophysiology, and provided data that are otherwise not readily available under whole-cell conditions. TRESK homodimer was connected with two ENaC (epithelial Na⁺ channel) heterotrimers and the Na⁺ current was measured as an internal reference, proportional to the number of channels in the plasma membrane. Modifications of TRESK iCtr resulted in diverse functional effects, indicating a complex contribution of this region to K⁺ channel activity. Mutations of positive residues in proximal iCtr locked TRESK in a low activity, calcineurin-insensitive state, although this phosphatase binds to distant motifs in the loop region. Accordingly, mutations in proximal iCtr may prevent the transmission of modulation to the gating machinery. Replacing distal iCtr with a sequence designed to interact with the inner surface of the plasma membrane increased the activity of the channel to unprecedented levels, as indicated by ENaR and single channel measurements. In conclusion, the distal iCtr is a major positive determinant of TRESK function.

Multi-level integrative analysis of the roles of lncRNAs and differential mRNAs in the progression of chronic pancreatitis to pancreatic ductal adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is one of the most malignant tumors and approximately 5% of patients with chronic pancreatitis (CP) inevitably develop PDAC. This study aims explore the key gene regulation involved in the progression of CP to PDAC, with a particular emphasis on the function of lncRNAs.

RESULTS

A total of 103 pancreatic tissue samples collected from 11 to 92 patients with CP and PDAC, respectively, were included in this study. After normalizing and logarithmically converting the original data, differentially expressed lncRNAs (DElncRNAs) and mRNAs (DEGs) in each dataset were selected. To determine the main functional pathways of differential mRNAs, we further annotated DEGs using gene ontology (GO) and analyzed the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment. In addition, the interaction between lncRNA-miRNA-mRNA was clarified and the protein-protein interaction (PPI) network was constructed to screen for key modules and determine hub genes. Finally, quantitative real-time polymerase chain reaction (qPCR) was used to detect the changes in non-coding RNAs and key mRNAs in the pancreatic tissues of patients with CP and PDAC. In this study, 230 lncRNAs and 17,668 mRNAs were included. There were nine upregulated lncRNAs and 188 downregulated lncRNAs. Furthermore, 2334 upregulated differential mRNAs and 10,341 downregulated differential mRNAs were included in the enrichment analysis. From the KEGG enrichment analysis, cytokine-cytokine receptor interaction, calcium signaling pathway, cAMP signaling pathway, and nicotine addiction exhibited significant differences. Additionally, a total of 52 lncRNAs, 104 miRNAs, and 312 mRNAs were included in the construction of a potential lncRNA-miRNA-mRNA regulatory network. PPI network was established and two of the five central DEGs were created in this module, suggesting that lysophosphatidic acid receptor 1 (LPAR1) and regulator of calcineurin 2 (RCAN2) may play significant roles in the progression from CP to PDAC. Finally, the PCR results suggested that LINC01547/hsa-miR-4694-3p/LPAR1 and LINC00482/hsa-miR-6756-3p/RCAN2 play important roles in the carcinogenesis process of CP.

CONCLUSION

Two signaling axes critical in the progression of CP to PDAC were screened out. Our findings will be useful for novel insights into the molecular mechanism and potential diagnostic or therapeutic biomarkers for CP and PDAC.

Advances in Current Drugs and Formulations for the Management of Atopic Dermatitis

Atopic dermatitis (AD) is a chronic, relapsing inflammatory skin disease with a complex pathophysiology. Treatment of AD remains challenging owing to the presence of a wide spectrum of clinical phenotypes and limited response to existing therapies. However, recent genetic, immunological, and pathophysiological insights into the disease mechanism resulted in the invention of novel therapeutic drug candidates. This review provides a comprehensive overview of current therapies and assesses various novel drug delivery strategies currently under clinical investigation. Further, this review majorly emphasizes on various topical treatments including emollient therapies, barrier repair agents, topical corticosteroids (TCS), phosphodiesterase 4 (PDE4) inhibitors, calcineurin inhibitors, and Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway inhibitors. It also discusses biological and systemic therapies, upcoming treatments based on ongoing clinical trials. Additionally, this review scrutinized the use of pharmaceutical inactive ingredients in the approved topical dosage forms for AD treatment.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

Calcineurin Regulates Synaptic Plasticity and Nociceptive Transmission at the Spinal Cord Level

Calcineurin, the predominant Ca2+/calmodulin-dependent serine/threonine protein phosphatase (also known as protein phosphatase 2B), is highly expressed in immune T cells and the nervous system, including the dorsal root ganglion and spinal cord. It controls synaptic transmission and plasticity by maintaining the appropriate phosphorylation status of many ion channels present at presynaptic and postsynaptic sites. As such, normal calcineurin activity in neurons and synapses is mainly involved in negative feedback regulation in response to increased neuronal activity and intracellular Ca2+ levels. Calcineurin inhibitors (e.g., cyclosporine and tacrolimus) are widely used as immunosuppressants in tissue and organ transplantation recipients and for treating autoimmune diseases but can cause severe pain in some patients. Furthermore, diminished calcineurin activity at the spinal cord level may play a major role in the transition from acute to chronic neuropathic pain after nerve injury. Restoring calcineurin activity at the spinal cord level produces long-lasting pain relief in animal models of neuropathic pain. In this article, we provide an overview of recent studies on the critical roles of calcineurin in regulating glutamate NMDA and AMPA receptors, voltage-gated Ca2+ channels, potassium channels, and transient receptor potential channels expressed in the spinal dorsal horn and primary sensory neurons.

K2P18.1 translates T cell receptor signals into thymic regulatory T cell development

It remains largely unclear how thymocytes translate relative differences in T cell receptor (TCR) signal strength into distinct developmental programs that drive the cell fate decisions towards conventional (Tconv) or regulatory T cells (Treg). Following TCR activation, intracellular calcium (Ca²⁺) is the most important second messenger, for which the potassium channel K_2P18.1 is a relevant regulator. Here, we identify K_2P18.1 as a central translator of the TCR signal into the thymus-derived Treg (tTreg) selection process. TCR signal was coupled to NF-κB-mediated K_2P18.1 upregulation in tTreg progenitors. K_2P18.1 provided the driving force for sustained Ca²⁺ influx that facilitated NF-κB- and NFAT-dependent expression of FoxP3, the master transcription factor for Treg development and function. Loss of K_2P18.1 ion-current function induced a mild lymphoproliferative phenotype in mice, with reduced Treg numbers that led to aggravated experimental autoimmune encephalomyelitis, while a gain-of-function mutation in K_2P18.1 resulted in increased Treg numbers in mice. Our findings in human thymus, recent thymic emigrants and multiple sclerosis patients with a dominant-negative missense K_2P18.1 variant that is associated with poor clinical outcomes indicate that K_2P18.1 also plays a role in human Treg development. Pharmacological modulation of K_2P18.1 specifically modulated Treg numbers in vitro and in vivo. Finally, we identified nitroxoline as a K_2P18.1 activator that led to rapid and reversible Treg increase in patients with urinary tract infections. Conclusively, our findings reveal how K_2P18.1 translates TCR signals into thymic T cell fate decisions and Treg development, and provide a basis for the therapeutic utilization of Treg in several human disorders.

Palmitoylation targets the calcineurin phosphatase to the phosphatidylinositol 4-kinase complex at the plasma membrane

Calcineurin, the conserved protein phosphatase and target of immunosuppressants, is a critical mediator of Ca2+ signaling. Here, to discover calcineurin-regulated processes we examined an understudied isoform, CNAβ1. We show that unlike canonical cytosolic calcineurin, CNAβ1 localizes to the plasma membrane and Golgi due to palmitoylation of its divergent C-terminal tail, which is reversed by the ABHD17A depalmitoylase. Palmitoylation targets CNAβ1 to a distinct set of membrane-associated interactors including the phosphatidylinositol 4-kinase (PI4KA) complex containing EFR3B, PI4KA, TTC7B and FAM126A. Hydrogen-deuterium exchange reveals multiple calcineurin-PI4KA complex contacts, including a calcineurin-binding peptide motif in the disordered tail of FAM126A, which we establish as a calcineurin substrate. Calcineurin inhibitors decrease PI4P production during Gq-coupled GPCR signaling, suggesting that calcineurin dephosphorylates and promotes PI4KA complex activity. In sum, this work discovers a calcineurin-regulated signaling pathway which highlights the PI4KA complex as a regulatory target and reveals that dynamic palmitoylation confers unique localization, substrate specificity and regulation to CNAβ1.

NUAK2 and RCan2 participate in the p53 mutant pro-tumorigenic network

Most inactivating mutations in TP53 gene generates neomorphic forms of p53 proteins that experimental evidence and clinical observations suggest to exert gain-of-function effects. While massive effort has been deployed in the dissection of wild type p53 transcriptional programme, p53 mutant pro-tumorigenic gene network is still largely elusive. To help dissecting the molecular basis of p53 mutant GOF, we performed an analysis of a fully annotated genomic and transcriptomic human pancreatic adenocarcinoma to select candidate players of p53 mutant network on the basis their differential expression between p53 mutant and p53 wild-type cohorts and their prognostic value. We identified NUAK2 and RCan2 whose p53 mutant GOF-dependent regulation was further validated in pancreatic cancer cellular model. Our data demonstrated that p53R270H can physically bind RCan2 gene locus in regulatory regions corresponding to the chromatin permissive areas where known binding partners of p53 mutant, such as p63 and Srebp, bind. Overall, starting from clinically relevant data and progressing into experimental validation, our work suggests NUAK2 and RCan2 as novel candidate players of the p53 mutant pro-tumorigenic network whose prognostic and therapeutic interest might attract future studies.

TRESK channel contributes to depolarization-induced shunting inhibition and modulates epileptic seizures

Glutamatergic and GABAergic synaptic transmission controls excitation and inhibition of postsynaptic neurons, whereas activity of ion channels modulates neuronal intrinsic excitability. However, it is unclear how excessive neuronal excitation affects intrinsic inhibition to regain homeostatic stability under physiological or pathophysiological conditions. Here, we report that a seizure-like sustained depolarization can induce short-term inhibition of hippocampal CA3 neurons via a mechanism of membrane shunting. This depolarization-induced shunting inhibition (DShI) mediates a non-synaptic, but neuronal intrinsic, short-term plasticity that is able to suppress action potential generation and postsynaptic responses by activated ionotropic receptors. We demonstrate that the TRESK channel significantly contributes to DShI. Disruption of DShI by genetic knockout of TRESK exacerbates the sensitivity and severity of epileptic seizures of mice, whereas overexpression of TRESK attenuates seizures. In summary, these results uncover a type of homeostatic intrinsic plasticity and its underlying mechanism. TRESK might represent a therapeutic target for antiepileptic drugs.

KCNK18 Biallelic Variants Associated with Intellectual Disability and Neurodevelopmental Disorders Alter TRESK Channel Activity

The TWIK-related spinal cord potassium channel (TRESK) is encoded by KCNK18, and variants in this gene have previously been associated with susceptibility to familial migraine with aura (MIM #613656). A single amino acid substitution in the same protein, p.Trp101Arg, has also been associated with intellectual disability (ID), opening the possibility that variants in this gene might be involved in different disorders. Here, we report the identification of KCNK18 biallelic missense variants (p.Tyr163Asp and p.Ser252Leu) in a family characterized by three siblings affected by mild-to-moderate ID, autism spectrum disorder (ASD) and other neurodevelopment-related features. Functional characterization of the variants alone or in combination showed impaired channel activity. Interestingly, Ser252 is an important regulatory site of TRESK, suggesting that alteration of this residue could lead to additive downstream effects. The functional relevance of these mutations and the observed co-segregation in all the affected members of the family expand the clinical variability associated with altered TRESK function and provide further insight into the relationship between altered function of this ion channel and human disease.

The Background K+ Channel TRESK in Sensory Physiology and Pain

TRESK belongs to the K_2P family of potassium channels, also known as background or leak potassium channels due to their biophysical properties and their role regulating membrane potential of cells. Several studies to date have highlighted the role of TRESK in regulating the excitability of specific subtypes of sensory neurons. These findings suggest TRESK could be involved in pain sensitivity. Here, we review the different evidence available that involves the channel in pain and sensory perception, from studies knocking out the channel or overexpressing it to identified mutations that link the channel to migraine pain. In addition, the therapeutic possibilities are discussed, as targeting the channel seems an interesting therapeutic approach to reduce nociceptor activation and to decrease pain.

TRESK and TREK-2 two-pore-domain potassium channel subunits form functional heterodimers in primary somatosensory neurons

Two-pore-domain potassium channels (K_2P) are the major determinants of the background potassium conductance. They play a crucial role in setting the resting membrane potential and regulating cellular excitability. These channels form homodimers; however, a few examples of heterodimerization have also been reported. The K_2P channel subunits TRESK and TREK-2 provide the predominant background potassium current in the primary sensory neurons of the dorsal root and trigeminal ganglia. A recent study has shown that a TRESK mutation causes migraine because it leads to the formation of a dominant negative truncated TRESK fragment. Surprisingly, this fragment can also interact with TREK-2. In this study, we determined the biophysical and pharmacological properties of the TRESK/TREK-2 heterodimer using a covalently linked TRESK/TREK-2 construct to ensure the assembly of the different subunits. The tandem channel has an intermediate single-channel conductance compared with the TRESK and TREK-2 homodimers. Similar conductance values were recorded when TRESK and TREK-2 were coexpressed, demonstrating that the two subunits can spontaneously form functional heterodimers. The TRESK component confers calcineurin-dependent regulation to the heterodimer and gives rise to a pharmacological profile similar to the TRESK homodimer, whereas the presence of the TREK-2 subunit renders the channel sensitive to the selective TREK-2 activator T2A3. In trigeminal primary sensory neurons, we detected single-channel activity with biophysical and pharmacological properties similar to the TRESK/TREK-2 tandem, indicating that WT TRESK and TREK-2 subunits coassemble to form functional heterodimeric channels also in native cells.

Altered functional properties of a missense variant in the TRESK K+ channel (KCNK18) associated with migraine and intellectual disability

Mutations in the KCNK18 gene that encodes the TRESK K2P potassium channel have previously been linked with typical familial migraine with aura. Recently, an atypical clinical case has been reported in which a male individual carrying the p.Trp101Arg (W101R) missense mutation in the KCNK18 gene was diagnosed with intellectual disability and migraine with brainstem aura. Here we report the functional characterization of this new missense variant. This mutation is located in a highly conserved residue close to the selectivity filter, and our results show although these mutant channels retain their K⁺ selectivity and calcineurin-dependent regulation, the variant causes an overall dramatic loss of TRESK channel function as well as an initial dominant-negative effect when co-expressed with wild-type channels in Xenopus laevis oocytes. The dramatic functional consequences of this mutation thereby support a potentially pathogenic role for this variant and provide further insight into the relationship between the structure and function of this ion channel.

Calcineurin Inhibition Causes α2δ-1-Mediated Tonic Activation of Synaptic NMDA Receptors and Pain Hypersensitivity

Calcineurin inhibitors, such as tacrolimus (FK506) and cyclosporine, are widely used as standard immunosuppressants in organ transplantation recipients. However, these drugs can cause severe pain in patients, commonly referred to as calcineurin inhibitor-induced pain syndrome (CIPS). Although calcineurin inhibition increases NMDAR activity in the spinal cord, the underlying mechanism remains enigmatic. Using an animal model of CIPS, we found that systemic administration of FK506 in male and female mice significantly increased the amount of α2δ-1-GluN1 complexes in the spinal cord and the level of α2δ-1-bound GluN1 proteins in spinal synaptosomes. Treatment with FK506 significantly increased the frequency of mEPSCs and the amplitudes of monosynaptic EPSCs evoked from the dorsal root and puff NMDAR currents in spinal dorsal horn neurons. Inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-NMDAR interaction with α2δ-1Tat peptide completely reversed the effects of FK506. In α2δ-1 gene KO mice, treatment with FK506 failed to increase the frequency of NMDAR-mediated mEPSCs and the amplitudes of evoked EPSCs and puff NMDAR currents in spinal dorsal horn neurons. Furthermore, systemic administration of gabapentin or intrathecal injection of α2δ-1Tat peptide reversed thermal and mechanical hypersensitivity in FK506-treated mice. In addition, genetically deleting GluN1 in dorsal root ganglion neurons or α2δ-1 genetic KO similarly attenuated FK506-induced thermal and mechanical hypersensitivity. Together, our findings indicate that α2δ-1-bound NMDARs mediate calcineurin inhibitor-induced tonic activation of presynaptic and postsynaptic NMDARs at the spinal cord level and that presynaptic NMDARs play a prominent role in the development of CIPS.SIGNIFICANCE STATEMENT Calcineurin inhibitors are immunosuppressants used to prevent rejection of transplanted organs and tissues. However, these drugs can cause severe, unexplained pain. We showed that calcineurin inhibition enhances physical interaction between α2δ-1 and NMDARs and their synaptic trafficking in the spinal cord. α2δ-1 is essential for calcineurin inhibitor-induced aberrant activation of presynaptic and postsynaptic NMDARs in the spinal cord. Furthermore, inhibiting α2δ-1 or disrupting α2δ-1-NMDAR interaction reduces calcineurin inhibitor-induced pain hypersensitivity. Eliminating NMDARs in primary sensory neurons or α2δ-1 KO also attenuates calcineurin inhibitor-induced pain hypersensitivity. This new information extends our mechanistic understanding of the role of endogenous calcineurin in regulating synaptic plasticity and nociceptive transmission and suggests new strategies for treating this painful condition.

Identifying New Substrates and Functions for an Old Enzyme: Calcineurin

Biological processes are dynamically regulated by signaling networks composed of protein kinases and phosphatases. Calcineurin, or PP3, is a conserved phosphoserine/phosphothreonine-specific protein phosphatase and member of the PPP family of phosphatases. Calcineurin is unique, however, in its activation by Ca2+ and calmodulin. This ubiquitously expressed phosphatase controls Ca2+-dependent processes in all human tissues, but is best known for driving the adaptive immune response by dephosphorylating the nuclear factor of the activated T-cells (NFAT) family of transcription factors. Therefore, calcineurin inhibitors, FK506 (tacrolimus), and cyclosporin A serve as immunosuppressants. We describe some of the adverse effects associated with calcineurin inhibitors that result from inhibition of calcineurin in nonimmune tissues, illustrating the many functions of this enzyme that have yet to be elucidated. In fact, calcineurin has essential roles beyond the immune system, from yeast to humans, but since its discovery more than 30 years ago, only a small number of direct calcineurin substrates have been shown (∼75 proteins). This is because of limitations in current methods for identification of phosphatase substrates. Here we discuss recent insights into mechanisms of calcineurin activation and substrate recognition that have been critical in the development of novel approaches for identifying its targets systematically. Rather than comprehensively reviewing known functions of calcineurin, we highlight new approaches to substrate identification for this critical regulator that may reveal molecular mechanisms underlying toxicities caused by calcineurin inhibitor-based immunosuppression.

Synaptic vesicle generation from activity-dependent bulk endosomes requires a dephosphorylation-dependent dynamin-syndapin interaction

Activity‐dependent bulk endocytosis generates synaptic vesicles (SVs) during intense neuronal activity via a two‐step process. First, bulk endosomes are formed direct from the plasma membrane from which SVs are then generated. SV generation from bulk endosomes requires the efflux of previously accumulated calcium and activation of the protein phosphatase calcineurin. However, it is still unknown how calcineurin mediates SV generation. We addressed this question using a series of acute interventions that decoupled the generation of SVs from bulk endosomes in rat primary neuronal culture. This was achieved by either disruption of protein–protein interactions via delivery of competitive peptides, or inhibition of enzyme activity by known inhibitors. SV generation was monitored using either a morphological horseradish peroxidase assay or an optical assay that monitors the replenishment of the reserve SV pool. We found that SV generation was inhibited by, (i) peptides that disrupt calcineurin interactions, (ii) an inhibitor of dynamin I GTPase activity and (iii) peptides that disrupt the phosphorylation‐dependent dynamin I–syndapin I interaction. Peptides that disrupted syndapin I interactions with eps15 homology domain‐containing proteins had no effect. This revealed that (i) calcineurin must be localized at bulk endosomes to mediate its effect, (ii) dynamin I GTPase activity is essential for SV fission and (iii) the calcineurin‐dependent interaction between dynamin I and syndapin I is essential for SV generation. We therefore propose that a calcineurin‐dependent dephosphorylation cascade that requires both dynamin I GTPase and syndapin I lipid‐deforming activity is essential for SV generation from bulk endosomes.

Pyrethroids inhibit K2P channels and activate sensory neurons: basis of insecticide-induced paraesthesias

Supplemental Digital Content is Available in the Text.

Inhibition of K_2P potassium channels by pyrethroid insecticides contribute to activate primary sensory neurons to cause paraesthesias and painful sensations.

Pyrethroid insecticides are widely used for pest control in agriculture or in human public health commonly as a topical treatment for scabies and head lice. Exposure to pyrethroids such as permethrin or tetramethrin (TM) causes sensory alterations such as transient pain, burning, stinging sensations, and paraesthesias. Despite the well-known effects of pyrethroids on sodium channels, actions on other channels that control sensory neuron excitability are less studied. Given the role of 2-pore domain potassium (K_2P) channels in modulating sensory neuron excitability and firing, both in physiological and pathological conditions, we examined the effect of pyrethroids on K_2P channels mainly expressed in sensory neurons. Through electrophysiological and calcium imaging experiments, we show that a high percentage of TM-responding neurons were nociceptors, which were also activated by TRPA1 and/or TRPV1 agonists. This pyrethroid also activated and enhanced the excitability of peripheral saphenous nerve fibers. Pyrethroids produced a significant inhibition of native TRESK, TRAAK, TREK-1, and TREK-2 currents. Similar effects were found in transfected HEK293 cells. At the behavioral level, intradermal TM injection in the mouse paw produced nocifensive responses and caused mechanical allodynia, demonstrating that the effects seen on nociceptors in culture lead to pain-associated behaviors in vivo. In TRESK knockout mice, pain-associated behaviors elicited by TM were enhanced, providing further evidence for a role of this channel in preventing excessive neuronal activation. Our results indicate that inhibition of K_2P channels facilitates sensory neuron activation and increases their excitability. These effects contribute to the generation of paraesthesias and pain after pyrethroid exposure.

Decreased abundance of TRESK two-pore domain potassium channels in sensory neurons underlies the pain associated with bone metastasis

Bone metastasis–associated VEGF suppresses neuronal K + channels and increases pain in rats.

Anionic Phospholipids Bind to and Modulate the Activity of Human TRESK Background K+ Channel

The background K⁺ channel TRESK regulates sensory neuron excitability, and changes in its function/expression contribute to neuronal hyperexcitability after injury/inflammation, making it an attractive therapeutic target for pain-related disorders. Factors that change lipid bilayer composition/properties (including volatile anesthetics, chloroform, chlorpromazine, shear stress, and cell swelling/shrinkage) modify TRESK current, but despite the importance of anionic phospholipids (e.g., PIP₂) in the regulation of many ion channels, it remains unknown if membrane lipids affect TRESK function. We describe that both human and rat TRESK contain potential anionic phospholipid binding sites (apbs) in the large cytoplasmic loop, but only the human channel is able to bind to multilamellar vesicles (MLVs), enriched with anionic phospholipids, suggesting an electrostatically mediated interaction. We mapped the apbs to a short stretch of 14 amino acids in the loop, located at the membrane-cytosol interface. Disruption of electrostatic lipid-TRESK interactions inhibited hTRESK currents, while subsequent application of Folch Fraction MLVs or a PIP₂ analog activated hTRESK, an effect that was absent in the rat ortholog. Strikingly, channel activation by anionic phospholipids was conferred to rTRESK by replacing the equivalent rat sequence with the human apbs. Finally, in the presence of a calcineurin inhibitor, stimulation of a G_q/11-linked GPCR reduced hTRESK current, revealing a likely inhibitory effect of membrane lipid hydrolysis on hTRESK activity. This novel regulation of hTRESK by anionic phospholipids is a characteristic of the human channel that is not present in rodent orthologs. This must be considered when extrapolating results from animal models and may open the door to the development of novel channel modulators as analgesics.

Bioelectric-calcineurin signaling module regulates allometric growth and size of the zebrafish fin

The establishment of relative size of organs and structures is paramount for attaining final form and function of an organism. Importantly, variation in the proportions of structures frequently underlies adaptive change in morphology in evolution and maybe a common mechanism underlying selection. However, the mechanism by which growth is integrated within tissues during development to achieve proper proportionality is poorly understood. We have shown that signaling by potassium channels mediates coordinated size regulation in zebrafish fins. Recently, calcineurin inhibitors were shown to elicit changes in zebrafish fin allometry as well. Here, we identify the potassium channel kcnk5b as a key player in integrating calcineurin’s growth effects, in part through regulation of the cytoplasmic C-terminus of the channel. We propose that the interaction between Kcnk5b and calcineurin acts as a signaling node to regulate allometric growth. Importantly, we find that this regulation is epistatic to inherent mechanisms instructing overall size as inhibition of calcineurin is able to bypass genetic instruction of size as seen in sof and wild-type fins, however, it is not sufficient to re-specify positional memory of size of the fin. These findings integrate classic signaling mediators such as calcineurin with ion channel function in the regulation of size and proportion during growth.

Verapamil Inhibits TRESK (K2P18.1) Current in Trigeminal Ganglion Neurons Independently of the Blockade of Ca2+ Influx

Tandem pore domain weak inward rectifier potassium channel (TWIK)-related spinal cord K⁺ (TRESK; K_2P18.1) channel is the only member of the two-pore domain K⁺ (K_2P) channel family that is activated by an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and linked to migraines. This study was performed to identify the effect of verapamil, which is an L-type Ca²⁺ channel blocker and a prophylaxis for migraines, on the TRESK channel in trigeminal ganglion (TG) neurons, as well as in a heterologous system. Single-channel and whole-cell currents were recorded in TG neurons and HEK-293 cells transfected with mTRESK using patch-clamping techniques. In TG neurons, changes in [Ca²⁺]_i were measured using the fluo-3-AM Ca²⁺ indicator. Verapamil, nifedipine, and NiCl₂ inhibited the whole-cell currents in HEK-293 cells overexpressing mTRESK with IC₅₀ values of 5.2, 54.3, and >100 μM, respectively. The inhibitory effect of verapamil on TRESK channel was also observed in excised patches. In TG neurons, verapamil (10 μM) inhibited TRESK channel activity by approximately 76%. The TRESK channel activity was not dependent on the presence of extracellular Ca²⁺. In addition, the inhibitory effect of verapamil on the TRESK channel remained despite the absence of extracellular Ca²⁺. These findings show that verapamil inhibits the TRESK current independently of the blockade of Ca²⁺ influx in TG neurons. Verapamil will be able to exert its pharmacological effects by modulating TRESK, as well as Ca²⁺ influx, in TG neurons in vitro. We suggest that verapamil could be used as an inhibitor for identifying TRESK channel in TG neurons.

Role of lysophosphatidic acid in ion channel function and disease

Lysophosphatidic acid (LPA) is a bioactive phospholipid that exhibits a wide array of functions that include regulation of protein synthesis and adequate development of organisms. LPA is present in the membranes of cells and in the serum of several mammals and has also been shown to participate importantly in pathophysiological conditions. For several decades it was known that LPA produces some of its effects in cells through its interaction with specific G protein-coupled receptors, which in turn are responsible for signaling pathways that regulate cellular function. Among the target proteins for LPA receptors are ion channels that modulate diverse aspects of the physiology of cells and organs where they are expressed. However, recent studies have begun to unveil direct effects of LPA on ion channels, highlighting this phospholipid as a direct agonist and adding to the knowledge of the field of lipid-protein interactions. Moreover, the roles of LPA in pathophysiological conditions associated with the function of some ion channels have also begun to be clarified, and molecular mechanisms have been identified. This review focuses on the effects of LPA on ion channel function under normal and pathological conditions and highlights our present knowledge of the mechanisms by which it regulates the function and expression of N- and T-type Ca⁺⁺ channels; M-type K⁺ channel and inward rectifier K⁺ channel subunit 2.1; transient receptor potential (TRP) melastatin 2, TRP vanilloid 1, and TRP ankyrin 1 channels; and TWIK-related K⁺ channel 1 (TREK-1), TREK-2, TWIK-related spinal cord K⁺ channel (TRESK), and TWIK-related arachidonic acid-stimulated K⁺ channel (TRAAK).

Regulation of the phosphatase PP2B by protein-protein interactions

Protein dephosphorylation is important for regulating cellular signaling in a variety of contexts. Protein phosphatase-2B (PP2B), or calcineurin, is a widely expressed serine/threonine phosphatase that acts on a large cross section of potential protein substrates when activated by increased levels of intracellular calcium in concert with calmodulin. PxIxIT and LxVP targeting motifs are important for maintaining specificity in response to elevated calcium. In the present study, we describe the mechanism of PP2B activation, discuss its targeting by conserved binding motifs and review recent advances in the understanding of an A-kinase anchoring protein 79/PP2B/protein kinase A complex's role in synaptic long-term depression. Finally, we discuss potential for targeting PP2B anchoring motifs for therapeutic benefit.

Selective and state-dependent activation of TRESK (K2P 18.1) background potassium channel by cloxyquin

BACKGROUND AND PURPOSE

Cloxyquin (5-cloroquinolin-8-ol) has been described as an activator of TRESK (K_2P 18.1, TWIK-related spinal cord K⁺ channel) background potassium channel. We have examined the specificity of the drug by testing several K_2P channels. We have investigated the mechanism of cloxyquin-mediated TRESK activation, focusing on the differences between the physiologically relevant regulatory states of the channel.

EXPERIMENTAL APPROACH

Potassium currents were measured by two-electrode voltage clamp in Xenopus oocytes and by whole-cell patch clamp in mouse dorsal root ganglion (DRG) neurons.

KEY RESULTS

Cloxyquin (100 µM) activated mouse and human TRESK 4.4 ± 0.3 (n = 28) and 3.9 ± 0.3-fold (n = 8), respectively. The drug selectively targeted TRESK in the K_2P channel family and exerted state-dependent effects. TRESK was potently activated by cloxyquin in the resting state. However, after robust activation of the current by the calcium signal, evoked by stimulation of Gq-coupled receptors, the compound did not influence mouse TRESK and only slightly affected the human channel. The constitutively active mutant channels, mimicking the dephosphorylated state (S276A) or containing altered channel pore (F156A and F364A), were not further stimulated by cloxyquin. In a subpopulation of isolated DRG neurons, cloxyquin substantially activated the background potassium current.

CONCLUSIONS AND IMPLICATIONS

Cloxyquin activates TRESK by a Ca²⁺ /calcineurin-independent mechanism. The drug is specific for TRESK within the K_2P channel family and useful for studying TRESK currents in native cells. The state-dependent pharmacological profile of this channel should be considered in the development of therapeutics for migraine and other nociceptive disorders.

Investigating the human Calcineurin Interaction Network using the πɸLxVP SLiM

Ser/thr phosphorylation is the primary reversible covalent modification of proteins in eukaryotes. As a consequence, it is the reciprocal actions of kinases and phosphatases that act as key molecular switches to fine tune cellular events. It has been well documented that ~400 human ser/thr kinases engage substrates via consensus phosphosite sequences. Strikingly, we know comparatively little about the mechanism by which ~40 human protein ser/thr phosphatases (PSPs) dephosphorylate ~15000 different substrates with high specificity. The identification of substrates of the essential PSP calcineurin (CN) has been exceptionally challenging and only a small fraction has been biochemically confirmed. It is now emerging that CN binds regulators and substrates via two short linear motifs (SLiMs), the well-studied PxIxIT SLiM and the LxVP SLiM, which remains controversial at the molecular level. Here we describe the crystal structure of CN in complex with its substrate NFATc1 and show that the LxVP SLiM is correctly defined as πɸLxVP. Bioinformatics studies using the πɸLxVP SLiM resulted in the identification of 567 potential CN substrates; a small subset was experimentally confirmed. This combined structural-bioinformatics approach provides a powerful method for dissecting the CN interaction network and for elucidating the role of CN in human health and disease.

Increased Activation of the TRESK K+ Mediates Vago-Vagal Reflex Malfunction in Diabetic Rats

BACKGROUND & AIMS

Patients with diabetes have defects in the vagal afferent pathway that result in abnormal gastrointestinal function. We investigated whether selective increased activation of the 2-pore domain potassium channel TRESK (2-pore-domain weak inward-rectifying potassium channel-related spinal cord potassium channel) contributes to nodose ganglia (NG) malfunction, disrupting gastrointestinal function in diabetic rats.

METHODS

We conducted whole-cell current-clamp and single-unit recordings in NG neurons from diabetes-prone BioBreeding/Worcester rats and streptozotocin-induced diabetic (STZ-D) rats and compared them with control rats. NG neurons in rats or cultured NG neurons were exposed to pharmacologic antagonists and/or transfected with short hairpin or small interfering RNAs that reduced expression of TRESK. We then made electrophysiologic recordings and studied gastrointestinal functions.

RESULTS

We observed reduced input resistance, hyperpolarized membrane potential, and increased current threshold to elicit action potentiation in NG neurons of STZ-D rats compared with controls. NG neuron excitability was similarly altered in diabetes-prone rats. In vivo single-unit NG neuronal discharges in response to 30 and 60 pmol cholecystokinin octapeptide were significantly lower in STZ-D rats compared with controls. Reducing expression of the TRESK K+ channel restored NG excitability in vitro and in vivo, as well as cholecystokinin 8-stimulated secretion of pancreatic enzymes and secretin-induced gastrointestinal motility, which are mediated by vago-vagal reflexes. These abnormalities resulted from increased intracellular Ca2+ in the NG, activating calcineurin, which, in turn, bound to an nuclear factor of activated T cell-like docking site on the TRESK protein, resulting in neuronal membrane hyperpolarization.

CONCLUSIONS

In 2 rate models of diabetes, we found that activation of the TRESK K+ channel reduced NG excitability and disrupted gastrointestinal functions.

Aristolochic acid, a plant extract used in the treatment of pain and linked to Balkan endemic nephropathy, is a regulator of K2P channels

Background and Purpose

Aristolochic acid (AristA) is found in plants used in traditional medicines to treat pain. We investigated the action of AristA on TREK and TRESK, potassium (K2P) channels, which are potential therapeutic targets in pain. Balkan endemic nephropathy (BEN) is a renal disease associated with AristA consumption. A mutation of TASK‐2 (K_2P5.1) channels (T108P) is seen in some patients susceptible to BEN, so we investigated how both this mutation and AristA affected TASK‐2 channels.

Experimental Approach

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 AristA.

Key Results

TREK‐1‐ and TREK‐2‐mediated currents were enhanced by AristA (100 μM), whereas TRESK was inhibited. Inhibition of TRESK did not depend on the phosphorylation of key intracellular serines but was completely blocked by mutation of bulky residues in the inner pore (F145A_F352A). The TASK‐2_T108P mutation markedly reduced both current density and ion selectivity. A related mutation (T108C) had similar but less marked effects. External alkalization and application of flufenamic acid enhanced TASK‐2 and TASK‐2_T108C current but did not affect TASK‐2_T108P current. AristA (300 μM) produced a modest enhancement of TASK‐2 current.

Conclusions and Implications

Enhancement of TREK‐1 and TREK‐2 and inhibition of TRESK by AristA may contribute to therapeutically useful effects of this compound in pain. Whilst AristA is unlikely to interact directly with TASK‐2 channels in BEN, loss of functional TASK‐2 channels may indirectly increase susceptibility to AristA toxicity.

Short linear motifs - ex nihilo evolution of protein regulation

Short sequence motifs are ubiquitous across the three major types of biomolecules: hundreds of classes and thousands of instances of DNA regulatory elements, RNA motifs and protein short linear motifs (SLiMs) have been characterised. The increase in complexity of transcriptional, post-transcriptional and post-translational regulation in higher Eukaryotes has coincided with a significant expansion of motif use. But how did the eukaryotic cell acquire such a vast repertoire of motifs? In this review, we curate the available literature on protein motif evolution and discuss the evidence that suggests SLiMs can be acquired by mutations, insertions and deletions in disordered regions. We propose a mechanism of ex nihilo SLiM evolution – the evolution of a novel SLiM from “nothing” – adding a functional module to a previously non-functional region of protein sequence. In our model, hundreds of motif-binding domains in higher eukaryotic proteins connect simple motif specificities with useful functions to create a large functional motif space. Accessible peptides that match the specificity of these motif-binding domains are continuously created and destroyed by mutations in rapidly evolving disordered regions, creating a dynamic supply of new interactions that may have advantageous phenotypic novelty. This provides a reservoir of diversity to modify existing interaction networks. Evolutionary pressures will act on these motifs to retain beneficial instances. However, most will be lost on an evolutionary timescale as negative selection and genetic drift act on deleterious and neutral motifs respectively. In light of the parallels between the presented model and the evolution of motifs in the regulatory segments of genes and (pre-)mRNAs, we suggest our understanding of regulatory networks would benefit from the creation of a shared model describing the evolution of transcriptional, post-transcriptional and post-translational regulation.

TWIK-Related Spinal Cord K⁺ Channel Expression Is Increased in the Spinal Dorsal Horn after Spinal Nerve Ligation

PURPOSE

The TWIK-related spinal cord K⁺ channel (TRESK) has recently been discovered and plays an important role in nociceptor excitability in the pain pathway. Because there have been no reports on the TRESK expression or its function in the dorsal horn of the spinal cord in neuropathic pain, we analyzed TRESK expression in the spinal dorsal horn in a spinal nerve ligation (SNL) model.

MATERIALS AND METHODS

We established a SNL mouse model by using the L5-6 spinal nerves ligation. We used real-time polymerase chain reaction and immunohistochemistry to investigate TRESK expression in the dorsal horn and L5 dorsal rot ganglion (DRG).

RESULTS

The SNL group showed significantly higher expression of TRESK in the ipsilateral dorsal horn under pain, but low expression in L5 DRG. Double immunofluorescence staining revealed that immunoreactivity of TRESK was mostly restricted in neuronal cells, and that synapse markers GAD67 and VGlut2 appeared to be associated with TRESK expression. We were unable to find a significant association between TRESK and calcineurin by double immunofluorescence.

CONCLUSION

TRESK in spinal cord neurons may contribute to the development of neuropathic pain following injury.

Role of leak potassium channels in pain signaling

Potassium (K(+)) channels are membrane proteins that allow rapid and selective flow of K(+) ions across the cell membrane, generating electrical signals in neurons. Thus, K(+) channels play a critical role in determining the neuronal excitability. Two-pore domain (K2P) "leak" K(+) channels give rise to leak K(+) currents that are responsible for the resting membrane potential and input resistance. The wide expression of leak K(+) channels in the central and peripheral nervous system suggests that these channels are critically involved in pain signaling and behavior. Indeed, it has become apparent in the past decade that the leak K(+) channels play essential roles in the development of pain. In this review, we describe evidence for the roles of TASK1, TASK3, TREK1, TREK2, TRAAK and TRESK channels in pain signaling and behavior. Furthermore, we describe the possible involvement of TASK2 and TWIK1 channels in pain.

Activation of TRESK channels by the inflammatory mediator lysophosphatidic acid balances nociceptive signalling

In dorsal root ganglia (DRG) neurons TRESK channels constitute a major current component of the standing outward current IK_SO. A prominent physiological role of TRESK has been attributed to pain sensation. During inflammation mediators of pain e.g. lysophosphatidic acid (LPA) are released and modulate nociception. We demonstrate co-expression of TRESK and LPA receptors in DRG neurons. Heterologous expression of TRESK and LPA receptors in Xenopus oocytes revealed augmentation of basal K⁺ currents upon LPA application. In DRG neurons nociception can result from TRPV1 activation by capsaicin or LPA. Upon co-expression in Xenopus oocytes LPA simultaneously increased both depolarising TRPV1 and hyperpolarising TRESK currents. Patch-clamp recordings in cultured DRG neurons from TRESK[wt] mice displayed increased IK_SO after application of LPA whereas under these conditions IK_SO in neurons from TRESK[ko] mice remained unaltered. Under current-clamp conditions LPA application differentially modulated excitability in these genotypes upon depolarising pulses. Spike frequency was attenuated in TRESK[wt] neurons and, in contrast, augmented in TRESK[ko] neurons. Accordingly, excitation of nociceptive neurons by LPA is balanced by co-activation of TRESK channels. Hence excitation of sensory neurons is strongly controlled by the activity of TRESK channels, which therefore are good candidates for the treatment of pain disorders.

Properties, regulation, pharmacology, and functions of the K₂p channel, TRESK

TWIK-related spinal cord K(+) channel (TRESK) is the gene product of KCNK18, the last discovered leak potassium K2P channel gene. Under resting conditions, TRESK is constitutively phosphorylated at two regulatory regions. Protein kinase A (PKA) and microtubule affinity-regulating (MARK) kinases can be applied in experiments to phosphorylate these sites of TRESK expressed in Xenopus oocytes, respectively. Upon generation of a calcium signal, TRESK is dephosphorylated and thereby activated by calcineurin. In this process, the binding of calcineurin to the channel by non-catalytic interacting sites is essential. The phosphorylation/dephosphorylation regulatory process is modified by 14-3-3 proteins. Human, but not murine TRESK is also activated by protein kinase C. TRESK is expressed most abundantly in sensory neurons of the dorsal root ganglia (DRG) and trigeminal ganglia, and the channel modifies certain forms of nociceptive afferentation. In a large pedigree, a dominant negative mutant TRESK allele was found to co-segregate perfectly with migraine phenotype. While this genetic defect may be responsible only for a very small fraction of migraine cases, specific TRESK activation is expected to exert beneficial effect in common forms of the disease.

The LQLP calcineurin docking site is a major determinant of the calcium-dependent activation of human TRESK background K+ channel

Calcium-dependent activation of human TRESK (TWIK-related spinal cord K(+) channel, K2P18.1) depends on direct targeting of calcineurin to the PQIIIS motif. In the present study we demonstrate that TRESK also contains another functionally relevant docking site for the phosphatase, the LQLP amino acid sequence. Combined mutations of the PQIIIS and LQLP motifs were required to eliminate the calcium-dependent regulation of the channel. In contrast to the alanine substitutions of PQIIIS, the mutation of LQLP to AQAP alone did not significantly change the amplitude of TRESK activation evoked by the substantial elevation of cytoplasmic calcium concentration. However, the AQAP mutation slowed down the response to high calcium. In addition, modest elevation of [Ca(2+)], which effectively regulated the wild type channel, failed to activate TRESK-AQAP. This indicates that the AQAP mutation diminished the sensitivity of TRESK to calcium. Even if PQIIIS was replaced by the PVIVIT sequence of high calcineurin binding affinity, the effect of the AQAP mutation was clearly detected in this TRESK-PVIVIT context. Substitution of the LQLP region with the corresponding fragment of NFAT transcription factor, perfectly matching the previously described LXVP calcineurin-binding consensus sequence, increased the calcium-sensitivity of TRESK-PVIVIT. Thus the enhancement of the affinity of TRESK for calcineurin by the incorporation of PVIVIT could not compensate for or prevent the effects of LQLP sequence modifications, suggesting that the two calcineurin-binding regions play distinct roles in the regulation. Our results indicate that the LQLP site is a fundamental determinant of the calcium-sensitivity of human TRESK.

Identification of novel small molecule modulators of K2P18.1 two-pore potassium channel

Two-pore domain potassium (K2P) channels are responsible for background potassium (K+) current, which is crucial for the maintenance of resting membrane potential. K2P18.1, also called TWIK-related spinal cord K+ channel (TRESK) or KCNK18, is thought to be a major contributor to background K+ currents, particularly in sensory neurons where it is abundantly expressed. Despite its critical role and potential therapeutic implication, pharmacological tools for probing K2P18.1 activity remain unavailable. Here, we report a high-throughput screen against a collection of bioactive compounds that yielded 26 inhibitors and 8 activators of K2P18.1 channel activity with more than 10-fold selectivity over the homologous channel K2P9.1. Among these modulators, the antihistamine loratadine inhibited K2P18.1 activity with IC50 of 0.49±0.23 µM and is considerably more potent than existing K2P18.1 inhibitors. Importantly, the inhibition by loratadine remains equally efficacious upon potentiation of K2P18.1 by calcium signaling. Furthermore, the loratadine effect is dependent on transmembrane residues F145 and F352, providing orthogonal evidence that the inhibition is caused by a direct compound-channel interaction. This study reveals new pharmacological modulators of K2P18.1 activity useful in dissecting native K2P18.1 function.

Tubulin binds to the cytoplasmic loop of TRESK background K⁺ channel in vitro

The cytoplasmic loop between the second and third transmembrane segments is pivotal in the regulation of TRESK (TWIK-related spinal cord K+ channel, K2P18.1, KCNK18). Calcineurin binds to this region and activates the channel by dephosphorylation in response to the calcium signal. Phosphorylation-dependent anchorage of 14-3-3 adaptor protein also modulates TRESK at this location. In the present study, we identified molecular interacting partners of the intracellular loop. By an affinity chromatography approach using the cytoplasmic loop as bait, we have verified the specific association of calcineurin and 14-3-3 to the channel. In addition to these known interacting proteins, we observed substantial binding of tubulin to the intracellular loop. Successive truncation of the polypeptide and pull-down experiments from mouse brain cytosol narrowed down the region sufficient for the binding of tubulin to a 16 amino acid sequence: LVLGRLSYSIISNLDE. The first six residues of this sequence are similar to the previously reported tubulin-binding region of P2X2 purinergic receptor. The tubulin-binding site of TRESK is located close to the protein kinase A (PKA)-dependent 14-3-3-docking motif of the channel. We provide experimental evidence suggesting that 14-3-3 competes with tubulin for the binding to the cytoplasmic loop of TRESK. It is intriguing that the 16 amino acid tubulin-binding sequence includes the serines, which were previously shown to be phosphorylated by microtubule-affinity regulating kinases (MARK kinases) and contribute to channel inhibition. Although tubulin binds to TRESK in vitro, it remains to be established whether the two proteins also interact in the living cell.

Spatial and temporal analysis of NADPH oxidase-generated hydrogen peroxide signals by novel fluorescent reporter proteins

AIMS

Hydrogen peroxide (H2O2) is an emerging signaling molecule with diverse regulatory functions. Despite its significance, the spatial and temporal organization of H2O2 signals within cells is basically unknown. Our limited knowledge about H2O2 signals is largely due to the lack of appropriate techniques for measuring intracellular H2O2. The aim of the current study was to develop novel fluorescent reporter proteins for the measurement of intracellular H2O2.

RESULTS

We developed two novel, fluorescence resonance energy transfer-based redox probes that undergo opposite emission ratio changes upon exposure to H2O2. We have successfully used these sensors to measure H2O2 production by NADPH oxidases (Nox). Moreover, we targeted these probes to specific cellular compartments or incorporated them into oxidase complexes to detect H2O2 at different, well-defined loci.

INNOVATION

Studying Nox2- and dual oxidase 1 (Duox1)-expressing cells, we provide the first analysis of how NADPH-oxidase generated H2O2 signals radiate within and between cells.

CONCLUSION

Our results suggest that H2O2 produced by Noxs can induce redox changes in the intracellular milieu of Nox/Duox-expressing cells while simultaneously transmitting paracrine effects to neighboring cells.

TRESK potassium channel in human T lymphoblasts

TRESK (TWIK-related spinal cord K(+)) channel, encoded by KCNK18 gene, belongs to the double-pore domain K(+) channel family and in normal conditions is expressed predominantly in the central nervous system. In our previous patch-clamp study on Jurkat T lymphoblasts we have characterized highly selective K(+) channel with pharmacological profile identical to TRESK. In the present work, the presence of KCNK18 mRNA was confirmed in T lymphoblastic cell lines (Jurkat, JCaM, H9) but not in resting peripheral blood lymphocytes of healthy donors. Positive immunostaining for TRESK was demonstrated in lymphoblastic cell lines, in germinal centers of non-tumoral lymph nodes, and in clinical samples of T acute lymphoblastic leukemias/lymphomas. Besides detection in the plasma membrane, intracellular TRESK localization was also revealed. Possible involvement of TRESK channel in lymphocyte proliferation and tumorigenesis is discussed.

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.

Identification of blocker binding site in mouse TRESK by molecular modeling and mutational studies

TWIK (tandem-pore domain weak inward rectifying K(+))-related spinal cord K(+) channel, TRESK, a member of the tandem-pore domain K(+) channel family, is the most recently cloned K(2P) channel. TRESK is highly expressed in dorsal root ganglion neuron, a pain sensing neuron, which is a target for analgesics. In this study, a reliable 3D structure for transmembrane (TM) region of mouse TRESK (mTRESK) was constructed, and then the reasonable blocker binding mode of the protein was investigated. The 3D structure of the mTRESK built by homology modeling method was validated with recommend value of stereochemical quality. Based on the validated structure, K(+) channel blocker-bound conformation was obtained by molecular docking and 5ns MD simulation with DPPC lipid bilayer. Our docking study provides the plausible binding mode of known blockers with key interacting residues, especially, F156 and F364. Finally, these modeling results were verified by experimental study with mutation from phenylalanine to alanine (F156A, F364A and F156A/F364A) at the TM2 and TM4. This is the first modeling study for TRESK that can provide structural information of the protein including ligand binding information. These results can be useful in structure based drug design for finding new blockers of the TRESK as potential therapeutic target of pain treatment.

Calcineurin-inhibitor pain syndrome

BACKGROUND

There has been increased recognition of calcineurin, a phosphoprotein serine/threonine phosphatase enzyme, in the regulation of many physiologic systems. Calcineurin mediates activation of lymphocytes, which play a role in immune response. Widely distributed in the central nervous system, calcinuerin also plays an important role in sensory neural function, via its role in the regulation of newly discovered 2-pore potassium channels, which greatly influence neuronal resting membrane potentials. Calcinuerin inhibition is the mechanism of action of immunomodulatory drugs such as cyclosporine and tacrolimus, which are widely used in transplantation medicine to prevent rejection. While important for immunosuppression, the use of calcineurin inhibitors has been associated with the development of a new pain syndrome called the calcineurin pain syndrome, which appears to be an untoward complication of the interruption of the physiologic function of calcineurin.

METHODS

This is a narrative review focusing on the epidemiology, pathophysiology, characterization of a newly recognized pain syndrome associated with the use of calcineurin inhibitors.

RESULTS

The use of immunosuppressants however is associated with several well-known toxicities to which the calcineurin pain syndrome can be added. The development of this syndrome most likely involves altered nociceptive processing due to the effect of calcineurin inhibition on neuronal firing, as well as effects of calcineurin on vascular tone. The most striking aspect of the treatment of this syndrome is the response to calcium channel blockers, which suggest that the effects of calcineurin inhibition on vascular tone play an important role in the development of the calcineurin pain syndrome.

CONCLUSION

The calcineurin syndrome is a newly recognized complication associated with the use of calcineurin inhibitors. There is no standard therapy at this time but anecdotal reports suggest the effectiveness of calcium channel blockers.

PKC-dependent activation of human K(2P) 18.1 K(+) channels

BACKGROUND AND PURPOSE

Two-pore-domain K(+) channels (K(2P) ) mediate K(+) background currents that modulate the membrane potential of excitable cells. K(2P) 18.1 (TWIK-related spinal cord K(+) channel) provides hyperpolarizing background currents in neurons. Recently, a dominant-negative loss-of-function mutation in K(2P) 18.1 has been implicated in migraine, and activation of K(2P) 18.1 channels was proposed as a therapeutic strategy. Here we elucidated the molecular mechanisms underlying PKC-dependent activation of K(2P) 18.1 currents.

EXPERIMENTAL APPROACH

Human K(2P) 18.1 channels were heterologously expressed in Xenopus laevis oocytes, and currents were recorded with the two-electrode voltage clamp technique.

KEY RESULTS

Stimulation of PKC using phorbol 12-myristate-13-acetate (PMA) activated the hK(2P) 18.1 current by 3.1-fold in a concentration-dependent fashion. The inactive analogue 4α-PMA had no effect on channel activity. The specific PKC inhibitors bisindolylmaleimide I, Ro-32-0432 and chelerythrine reduced PMA-induced channel activation indicating that PKC is involved in this effect of PMA. Selective activation of conventional PKC isoforms with thymeleatoxin (100 nM) did not reproduce K(2P) 18.1 channel activation. Current activation by PMA was not affected by pretreatment with CsA (calcineurin inhibitor) or KT 5720 (PKA inhibitor), ruling out a significant contribution of calcineurin or cross-talk with PKA to the PKC-dependent hK(2P) 18.1 activation. Finally, mutation of putative PKC phosphorylation sites did not prevent PMA-induced K(2P) 18.1 channel activation.

CONCLUSIONS AND IMPLICATIONS

We demonstrated that activation of hK(2P) 18.1 (TRESK) by PMA is mediated by PKC stimulation. Hence, PKC-mediated activation of K(2P) 18.1 background currents may serve as a novel molecular target for migraine treatment.

Calcium channel blockers act through nuclear factor Y to control transcription of key cardiac genes

First-generation calcium channel blockers such as verapamil are a widely used class of antihypertensive drugs that block L-type calcium channels. We recently discovered that they also reduce cardiac expression of proapoptotic thioredoxin-interacting protein (TXNIP), suggesting that they may have unappreciated transcriptional effects. By use of TXNIP promoter deletion and mutation studies, we found that a CCAAT element was mediating verapamil-induced transcriptional repression and identified nuclear factor Y (NFY) to be the responsible transcription factor as assessed by overexpression/knockdown and luciferase and chromatin immunoprecipitation assays in cardiomyocytes and in vivo in diabetic mice receiving oral verapamil. We further discovered that increased NFY-DNA binding was associated with histone H4 deacetylation and transcriptional repression and mediated by inhibition of calcineurin signaling. It is noteworthy that the transcriptional control conferred by this newly identified verapamil-calcineurin-NFY signaling cascade was not limited to TXNIP, suggesting that it may modulate the expression of other NFY targets. Thus, verapamil induces a calcineurin-NFY signaling pathway that controls cardiac gene transcription and apoptosis and thereby may affect cardiac biology in previously unrecognized ways.

Na(+)/H(+) exchanger 1 directly binds to calcineurin A and activates downstream NFAT signaling, leading to cardiomyocyte hypertrophy

The calcineurin A (CaNA) subunit was identified as a novel binding partner of plasma membrane Na(+)/H(+) exchanger 1 (NHE1). CaN is a Ca(2+)-dependent phosphatase involved in many cellular functions, including cardiac hypertrophy. Direct binding of CaN to the (715)PVITID(720) sequence of NHE1, which resembles the consensus CaN-binding motif (PXIXIT), was observed. Overexpression of NHE1 promoted serum-induced CaN/nuclear factor of activated T cells (NFAT) signaling in fibroblasts, as indicated by enhancement of NFAT promoter activity and nuclear translocation, which was attenuated by NHE1 inhibitor. In neonatal rat cardiomyocytes, NHE1 stimulated hypertrophic gene expression and the NFAT pathway, which were inhibited by a CaN inhibitor, FK506. Importantly, CaN activity was strongly enhanced with increasing pH, so NHE1 may promote CaN/NFAT signaling via increased intracellular pH. Indeed, Na(+)/H(+) exchange activity was required for NHE1-dependent NFAT signaling. Moreover, interaction of CaN with NHE1 and clustering of NHE1 to lipid rafts were also required for this response. Based on these results, we propose that NHE1 activity may generate a localized membrane microdomain with higher pH, thereby sensitizing CaN to activation and promoting NFAT signaling. In cardiomyocytes, such signaling can be a pathway of NHE1-dependent hypertrophy.

TRESK: the lone ranger of two-pore domain potassium channels

TRESK (TWIK-related spinal cord K(+) channel, KCNK18) belongs to the two-pore domain (K2P) background (leak) potassium channel family. Unlike other K2P channels, TRESK is activated by the calcium signal in heterologous expression systems. The activation is mediated by the calcium/calmodulin-dependent protein phosphatase, calcineurin. TRESK is abundantly expressed in dorsal root and trigeminal ganglia. The active ingredient of Sichuan pepper, sanshool, has been suggested to evoke tingling paresthesia by inhibiting the channel in a mechanoreceptor subpopulation of sensory neurons. Recently, dominant-negative mutation of human TRESK was found to be linked to migraine with aura in a large pedigree. It is hoped that future TRESK agonists may prevent or ameliorate the debilitating symptoms of migraine. It will be interesting to see whether the calcineurin-activated K(+) channel maintains normal excitability in the cerebral cortex thereby arresting cortical spreading depression (CSD), or prevents migraine attack only in the trigeminovascular (TGVS) system.

A Role for K2P Channels in the Operation of Somatosensory Nociceptors

The ability to sense mechanical, thermal, and chemical stimuli is critical to normal physiology and the perception of pain. Contact with noxious stimuli triggers a complex series of events that initiate innate protective mechanisms designed to minimize or avoid injury. Extreme temperatures, mechanical stress, and chemical irritants are detected by specific ion channels and receptors clustered on the terminals of nociceptive sensory nerve fibers and transduced into electrical information. Propagation of these signals, from distant sites in the body to the spinal cord and the higher processing centers of the brain, is also orchestrated by distinct groups of ion channels. Since their identification in 1995, evidence has emerged to support roles for K2P channels at each step along this pathway, as receptors for physiological and noxious stimuli, and as determinants of nociceptor excitability and conductivity. In addition, the many subtypes of K2P channels expressed in somatosensory neurons are also implicated in mediating the effects of volatile, general anesthetics on the central and peripheral nervous systems. Here, I offer a critical review of the existing data supporting these attributes of K2P channel function and discuss how diverse regulatory mechanisms that control the activity of K2P channels act to govern the operation of nociceptors.

TRESK gene recombinant adenovirus vector inhibits capsaicin-mediated substance P release from cultured rat dorsal root ganglion neurons

The present study was conducted to determine whether the activation of TRESK in the dorsal root ganglion (DRG) by the TRESK gene recombinant adenovirus vector inhibits the capsaicin-evoked substance P (SP) release using a radioimmunoassay. TRESK is an outwardly rectifying K⁺ current channel that contributes to the resting potential and is the most important background potassium channel in DRG. Previous studies have shown that neuropathic pain (NP) is closely related to the regulation of certain potassium channels in DRG neurons, while DRG-released SP is important in the peripheral mechanism of NP. In the present study, the TRESK gene adenovirus vector significantly enhanced the TRESK mRNA and protein of the cultured rat DRG neurons. Radioimmunoassay analysis revealed that the capsaicin-mediated SP release was significantly inhibited by the TRESK gene recombinant adenovirus vector in rat DRG neurons. These findings suggest that TRESK plays a role in adjusting the release of SP in DRG, which is related to NP.