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.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K_2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K_2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K⁺ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K_2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

Rare diseases caused by abnormal calcium sensing and signalling

The calcium-sensing receptor (CaSR) provides the major mechanism for the detection of extracellular calcium concentration in several cell types, via the induction of G-protein-coupled signalling. Accordingly, CaSR plays a pivotal role in calcium homeostasis, and the CaSR gene defects are related to diseases characterized by serum calcium level changes. Activating mutations of the CaSR gene cause enhanced sensitivity to extracellular calcium concentration resulting in autosomal dominant hypocalcemia or Bartter-syndrome type V. Inactivating CaSR gene mutations lead to resistance to extracellular calcium. In these cases, familial hypocalciuric hypercalcaemia (FHH1) or neonatal severe hyperparathyroidism (NSHPT) can develop. FHH2 and FHH3 are associated with mutations of genes of partner proteins of calcium signal transduction. The common polymorphisms of the CaSR gene have been reported not to affect the calcium homeostasis itself; however, they may be associated with the increased risk of malignancies.

TRESK background potassium channel modifies the TRPV1-mediated nociceptor excitability in sensory neurons

BACKGROUND

TWIK-related spinal cord potassium channel (TRESK) background potassium channels have a key role in controlling resting membrane potential and excitability of sensory neurons. A frameshift mutation leading to complete loss of TRESK function has been identified in members of a family suffering from migraine with aura. In the present study, we examined the role of TRESK channels on nociceptor function in mice.

METHODS

Calcium imaging was used to investigate the role of TRESK channels in the modulation of the response evoked by transient receptor potential vanilloid 1 (TRPV1) receptor stimulation in dorsal root ganglion neurons. Release of calcitonin gene-related peptide from trigeminal afferents and changes in meningeal blood flow were also measured. Experiments were performed on wild-type and TRESK knockout animals.

RESULTS

Inhibition of TRESK increased the TRPV1-mediated calcium signal in dorsal root ganglion neurons and potentiated capsaicin-induced increases in calcitonin gene-related peptide release and meningeal blood flow. Activation of TRESK decreased the capsaicin sensitivity of sensory neurons, leading to an attenuation of capsaicin-induced increase in meningeal blood flow. In TRESK knockout animals, TRPV1-mediated nociceptive reactions were unaffected by pretreatment with TRESK modulators.

CONCLUSIONS

Pharmacological manipulation of TRESK channels influences the TRPV1-mediated functions of nociceptors. Altered TRESK function might contribute to trigeminal nociceptor sensitization in migraine patients.

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.

Two P domain potassium channels (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

The 4TM family of K channels mediate many of the background potassium currents observed in native cells. They are open across the physiological voltage-range and are regulated by a wide array of neurotransmitters and biochemical mediators. The pore-forming α-subunit contains two pore loop (P) domains and two subunits assemble to form one ion conduction pathway lined by four P domains. It is important to note that single channels do not have two pores but that each subunit has two P domains in its primary sequence; hence the name two P domain, or K2P channels (and not two-pore channels). Some of the K2P subunits can form heterodimers across subfamilies (e.g. K2P3.1 with K2P9.1). The nomenclature of 4TM K channels in the literature is still a mixture of IUPHAR and common names. The suggested division into subfamilies, described in the More detailed introduction, is based on similarities in both structural and functional properties within subfamilies and this explains the "common abbreviation" nomenclature in the tables below.

TRESK background potassium channel is not gated at the helix bundle crossing near the cytoplasmic end of the pore

Two-pore domain K⁺ channels (K_2P) are responsible for background K⁺ currents and regulate the resting membrane potential and cellular excitability. Their activity is controlled by a large variety of physicochemical factors and intracellular signaling pathways. The majority of these effects converge on the intracellular C-terminus of the channels, resulting in the modification of the gating at the selectivity filter. Another gating mechanism, the activation gate at the helix bundle crossing is also well documented in other K⁺ channel families, however, it remains uncertain whether this type of gating is functional in K_2P channels. The regulation of TWIK-related spinal cord K⁺ channel (TRESK) is different from the other K_2P channels. Regulatory factors acting via the C-terminus are not known, instead channel activity is modified by the phosphorylation/dephosphorylation of the unusually long intracellular loop between the 2^nd and 3^rd transmembrane segments. These unique structural elements of the regulation lead us to examine channel gating at the bundle crossing region. Ba²⁺ was applied to the intracellular side of excised membrane patches and the characteristics of the channel block were determined. We compared the kinetics of the development of Ba²⁺ block when the channels were phosphorylated (inhibited) or dephosphorylated (activated) and also in different mutants mimicking the two functional states. Neither the phosphorylation/dephosphorylation nor the point mutations influenced the development of Ba²⁺ block, suggesting that the conformational changes of the bundle crossing region do not contribute to the phosphorylation-dependent gating of TRESK.

TASK-1 (KCNK3) channels in the lung: from cell biology to clinical implications

TWIK-related acid-sensitive potassium channel 1 (TASK-1 encoded by KCNK3) belongs to the family of two-pore domain potassium channels. This gene subfamily is constitutively active at physiological resting membrane potentials in excitable cells, including smooth muscle cells, and has been particularly linked to the human pulmonary circulation. TASK-1 channels are sensitive to a wide array of physiological and pharmacological mediators that affect their activity such as unsaturated fatty acids, extracellular pH, hypoxia, anaesthetics and intracellular signalling pathways. Recent studies show that modulation of TASK-1 channels, either directly or indirectly by targeting their regulatory mechanisms, has the potential to control pulmonary arterial tone in humans. Furthermore, mutations in KCNK3 have been identified as a rare cause of both familial and idiopathic pulmonary arterial hypertension. This review summarises our current state of knowledge of the functional role of TASK-1 channels in the pulmonary circulation in health and disease, with special emphasis on current advancements in the field.

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.

Formation of Functional Heterodimers by TREK-1 and TREK-2 Two-pore Domain Potassium Channel Subunits

Two-pore domain (K2P) potassium channels are the major molecular correlates of the background (leak) K(+) current in a wide variety of cell types. They generally play a key role in setting the resting membrane potential and regulate the response of excitable cells to various stimuli. K2P channels usually function as homodimers, and only a few examples of heteromerization have been previously reported. Expression of the TREK (TWIK-related K(+) channel) subfamily members of K2P channels often overlaps in neurons and in other excitable cells. Here, we demonstrate that heterologous coexpression of TREK-1 and TREK-2 subunits results in the formation of functional heterodimers. Taking advantage of a tandem construct (in which the two different subunits were linked together to enforce heterodimerization), we characterized the biophysical and pharmacological properties of the TREK-1/TREK-2 current. The heteromer was inhibited by extracellular acidification and by spadin similarly to TREK-1, and its ruthenium red sensitivity was intermediate between TREK-1 and TREK-2 homodimers. The heterodimer has also been distinguished from the homodimers by its unique single channel conductance. Assembly of the two different subunits was confirmed by coimmunoprecipitation of epitope-tagged TREK-1 and TREK-2 subunits, coexpressed in Xenopus oocytes. Formation of TREK-1/TREK-2 channels was also demonstrated in native dorsal root ganglion neurons indicating that heterodimerization may provide greater diversity of leak K(+) conductances also in native tissues.

Differential sensitivity of TREK-1, TREK-2 and TRAAK background potassium channels to the polycationic dye ruthenium red

BACKGROUND AND PURPOSE

Pharmacological separation of the background potassium currents of closely related K2P channels is a challenging problem. We previously demonstrated that ruthenium red (RR) inhibits TASK-3 (K2 P 9.1), but not TASK-1 (K2 P 3.1) channels. RR has been extensively used to distinguish between TASK currents in native cells. In the present study, we systematically investigate the RR sensitivity of a more comprehensive set of K2 P channels.

EXPERIMENTAL APPROACH

K(+) 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

RR differentiates between two closely related members of the TREK subfamily. TREK-2 (K2 P 10.1) proved to be highly sensitive to RR (IC50 = 0.2 μM), whereas TREK-1 (K2 P 2.1) was not affected by the compound. We identified aspartate 135 (D135) as the target of the inhibitor in mouse TREK-2c. D135 lines the wall of the extracellular ion pathway (EIP), a tunnel structure through the extracellular cap characteristic for K2 P channels. TREK-1 contains isoleucine in the corresponding position. The mutation of this isoleucine (I110D) rendered TREK-1 sensitive to RR. The third member of the TREK subfamily, TRAAK (K2 P 4.1) was more potently inhibited by ruthenium violet, a contaminant in some RR preparations, than by RR. DRG neurons predominantly express TREK-2 and RR-resistant TREK-1 and TRESK (K2 P 18.1) background K(+) channels. We detected the RR-sensitive leak K(+) current component in DRG neurons.

CONCLUSIONS AND IMPLICATIONS

We propose that RR may be useful for distinguishing TREK-2 (K2P 10.1) from TREK-1 (K2P 2.1) and other RR-resistant K2 P channels in native cells.

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.

Significance of biochemical markers in the diagnosis of neuroendocrine tumours and in the follow-up of patients

Circulating markers of neuroendocrine tumours are useful tools in the diagnosis of these tumours. Laboratory tests for general biomarkers have acceptable sensitivity for the recognition of neuroendocrine tumours as these biologically active proteins are typically synthesized by all types of neuroendocrine cells. Measurement of chromogranin A is widely used not only in the diagnosis of neuroendocrine tumours but it may predict the prognosis of the diseases and the effect of the antitumor therapy. It is also a useful tool for the detection of residual tumours. Neurendocrine tumours represent a heterogeneous group of tumours with the ability to secrete several hormones and, therefore, measurement of these hormones can also serve as neuroendocrine cell type-specific markers in routine clinical practice. In this review the authors summarize the significance of tumour markers in the diagnosis of neuroendocrine tumours as well as in the management and follow-up of patients with this disease. Orv. Hetil., 2014, 155(45), 1775–1782.

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.

[Effects and significance of estradiol in men]

The most important estrogen is estradiol in both men and women. In men elevated estradiol levels and associated metabolic disorders have been implicated in the development of common diseases including cardiovascular disorders, insulin resistance and type 2 diabetes mellitus, as increased estradiol associated with decreased testosterone levels increases the risk of these diseases. In this review the authors summarize the causes and consequences of androgen deficiency and estradiol excess, and they review recent studies on potential therapeutic strategies to correct increased estradiol levels in men.

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.

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.

TRESK background K(+) channel is inhibited by PAR-1/MARK microtubule affinity-regulating kinases in Xenopus oocytes

TRESK (TWIK-related spinal cord K⁺ channel, KCNK18) is a major background K⁺ channel of sensory neurons. Dominant-negative mutation of TRESK is linked to familial migraine. This important two-pore domain K⁺ channel is uniquely activated by calcineurin. The calcium/calmodulin-dependent protein phosphatase directly binds to the channel and activates TRESK current several-fold in Xenopus oocytes and HEK293 cells. We have recently shown that the kinase, which is responsible for the basal inhibition of the K⁺ current, is sensitive to the adaptor protein 14-3-3. Therefore we have examined the effect of the 14-3-3-inhibited PAR-1/MARK, microtubule-associated-protein/microtubule affinity-regulating kinase on TRESK in the Xenopus oocyte expression system. MARK1, MARK2 and MARK3 accelerated the return of TRESK current to the resting state after the calcium-dependent activation. Several other serine-threonine kinase types, generally involved in the modulation of other ion channels, failed to influence TRESK current recovery. MARK2 phosphorylated the primary determinant of regulation, the cluster of three adjacent serine residues (S274, 276 and 279) in the intracellular loop of mouse TRESK. In contrast, serine 264, the 14-3-3-binding site of TRESK, was not phosphorylated by the kinase. Thus MARK2 selectively inhibits TRESK activity via the S274/276/279 cluster, but does not affect the direct recruitment of 14-3-3 to the channel. TRESK is the first example of an ion channel phosphorylated by the dynamically membrane-localized MARK kinases, also known as general determinants of cellular polarity. These results raise the possibility that microtubule dynamics is coupled to the regulation of excitability in the neurons, which express TRESK background potassium channel.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

TRESK background K(+) channel is inhibited by phosphorylation via two distinct pathways

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel, KCNK18) is directly regulated by the calcium/calmodulin-dependent phosphatase calcineurin and 14-3-3 adaptor proteins. The calcium signal robustly activates the channel via calcineurin, whereas the anchoring of 14-3-3 interferes with the return of the current to the resting state after the activation in Xenopus oocytes. In the present study, we report that the phosphorylation of TRESK at two distinct regulatory regions, the 14-3-3 binding site (Ser-264) and the cluster of three adjacent serine residues (Ser-274, Ser-276, and Ser-279), are responsible for channel inhibition. The phosphorylation of Ser-264 by protein kinase A accelerated the return of the current of S276E mutant TRESK to the resting state after the calcineurin-dependent activation. In the presence of 14-3-3, the basal current of the S276E mutant was reduced, and its calcineurin-dependent activation was augmented, suggesting that the direct binding of the adaptor protein to TRESK contributed to the basal inhibition of the channel under resting conditions. Unexpectedly, we found that 14-3-3 impeded the recovery of the current of S264E mutant TRESK to the resting state after the calcineurin-dependent activation, despite of the mutated 14-3-3 binding site. This suggests that 14-3-3 inhibited the kinase phosphorylating the regulatory cluster of Ser-274, Ser-276, and Ser-279, independently of the direct interaction between TRESK and 14-3-3. In conclusion, two distinct inhibitory kinase pathways converge on TRESK, and their effect on the calcineurin-dependent regulation is differentially modulated by the functional availability of 14-3-3.

Phosphorylation-dependent binding of 14-3-3 proteins controls TRESK regulation

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel) is reversibly activated by the calcium/calmodulin-dependent protein phosphatase, calcineurin. In the present study, we report that 14-3-3 proteins directly bind to the intracellular loop of TRESK and control the kinetics of the calcium-dependent regulation of the channel. Coexpression of 14-3-3eta with TRESK blocked, whereas the coexpression of a dominant negative form of 14-3-3eta accelerated the return of the K(+) current to the resting state after the activation mediated by calcineurin in Xenopus oocytes. The direct action of 14-3-3 was spatially restricted to TRESK, since 14-3-3eta was also effective, when it was tethered to the channel by a flexible polyglutamine-containing chain. The effect of both the coexpressed and chained 14-3-3 was alleviated by the microinjection of Ser(P)-Raf259 phosphopeptide that competes with TRESK for binding to 14-3-3. The gamma and eta isoforms of 14-3-3 controlled TRESK regulation, whereas the beta, zeta, epsilon, sigma, and tau isoforms failed to influence the mechanism significantly. Phosphorylation of serine 264 in mouse TRESK was required for the binding of 14-3-3eta. Because 14-3-3 proteins are ubiquitous, they are expected to control the duration of calcineurin-mediated TRESK activation in all the cell types that express the channel, depending on the phosphorylation state of serine 264. This kind of direct control of channel regulation by 14-3-3 is unique within the two-pore domain K(+) channel family.

Neonatal severe hyperparathyroidism associated with a novel de novo heterozygous R551K inactivating mutation and a heterozygous A986S polymorphism of the calcium-sensing receptor gene

INTRODUCTION

Neonatal severe hyperparathyroidism (NSHPT) is induced by inactivating mutations of human calcium-sensing receptor (CaSR). Only three heterozygous de novo inactivating mutations of CaSR causing NSHPT have been described. We report the case of a now 11-year-old boy with NSHPT and we characterize a novel inactivating mutation along with the results of some functional analyses.

PATIENT AND METHODS

As a neonate the patient presented the clinical syndrome of NSHPT. At 6 years of age persisting hypercalcaemia without clinical symptoms was documented, and the patient remained completely symptom free without parathyroid surgery until his present age of 11 years. The entire coding region of the CaSR gene of the patient and his family members was sequenced. Functional investigation was performed in HEK-293 cells, transiently transfected with wild type and mutant CaSR plasmid constructs.

RESULTS

Sequence analysis revealed a novel de novo heterozygous mutation at codon 551 (AGG-->AAG), predicting a change of arginine to lysine (R551K) and a known heterozygous polymorphism (A986S) on the same allele, which was inherited from the father. We demonstrated that the novel R551K mutation significantly reduced the calcium sensitivity of CaSR (EC50: from 3.38 +/- 0.62-6.10 +/- 0.83 mmol/l), which was not alleviated by the simultaneous presence of A986S polymorphism.

CONCLUSIONS

We present the fourth NSHPT case induced by a novel de novo heterozygous inactivating mutation (R551K) of the CaSR gene. The disease gradually reverted to a symptomless, benign condition resembling familial hypocalciuric hypercalcaemia without any surgical intervention.

Characterization of the Heteromeric Potassium Channel Formed by Kv2.1 and the Retinal Subunit Kv8.2 in Xenopus Oocytes

Kv8.2 (KCNV2) subunits do not form homotetrameric potassium channels, although they coassemble with Kv2.1 to constitute functional heteromers. High expression of Kv8.2 was reported in the human retina and its mutations were linked to the visual disorder “cone dystrophy with supernormal rod electroretinogram.” We detected abundant Kv8.2 expression in the photoreceptor layer of mouse retina, where Kv2.1 is also known to be present. When the two subunits were coexpressed in Xenopus oocytes in equal amounts, Kv8.2 abolished the current of Kv2.1. If the proportion of Kv8.2 was reduced then the current of heteromeric channels emerged. Kv8.2 shifted the steady-state activation of Kv2.1 to more negative potentials, without affecting the voltage dependence of inactivation. This gave rise to a window current within the −40 to −10 mV membrane potential range. Ba2+ inhibited the heteromeric channel and shifted its activation to more positive potentials. These electrophysiological and pharmacological properties resemble those of the voltage-gated K+ current (named IKx) described in amphibian retinal rods. Furthermore, oocytes expressing Kv2.1/Kv8.2 developed transient hyperpolarizing overshoots in current-clamp experiments, whereas those expressing only Kv2.1 failed to do so. Similar overshoots are characteristic responses of photoreceptors to light flashes. We demonstrated that Kv8.2 G476D, analogous to a disease-causing human mutation, eliminated Kv2.1 current, if the subunits were coexpressed equally. However, Kv8.2 G476D did not form functional heteromers under any conditions. Therefore we suggest that the custom-tailored current of Kv2.1/Kv8.2 functionally contributes to photoreception, and this is the reason that mutations of Kv8.2 lead to a genetic visual disorder.

Porcine circovirus type 2 and associated diseases in Romania--short communication

Porcine circovirus type 2 (PCV2) has been demonstrated to be the causal agent for postweaning multisystemic wasting syndrome (PMWS) and porcine dermatitis and nephropathy syndrome (PDNS). This report describes the first detection of PCV2 and associated diseases in a Romanian swine herd located in Transylvania. The clinical signs, pathological and histopathological changes observed in affected pigs were similar to those previously described for PDNS and PMWS. Polymerase chain reaction and in situ hybridisation were used for the detection of PCV2 nucleic acids from tissues and serum samples. Complete PCV2 genomes of both PMWS and PDNS cases were sequenced and analysed, and by comparing them with each other no genomic differences could be detected. The sequence analysis showed that the Romanian PCV2 was closely related to PCV2 identified in France and in Hungary.

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

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel) is activated in response to the calcium signal by the calcium/calmodulin-dependent protein phosphatase, calcineurin. In the present study we report that calcineurin also interacts with TRESK via an NFAT-like docking site, in addition to its enzymatic action. In its intracellular loop, mouse TRESK possesses the amino acid sequence, PQIVID, which is similar to the calcineurin binding consensus motif, PXIXIT (where X denotes any amino acids), necessary for NFAT (nuclear factor of activated T cells) activation and nuclear translocation. Mutations of the PQIVID sequence of TRESK to PQIVIA, PQIVAD, or PQAVAD increasingly deteriorated the calcium-dependent activation in the listed order and correspondingly reduced the benzocaine sensitivity (a property discriminating activated channels from resting ones), when it was measured after the calcium signal in Xenopus oocytes. Microinjection of VIVIT peptide, designed to inhibit the NFAT-calcineurin interaction specifically, also eliminated TRESK activation. The intracellular loop of TRESK, expressed as a GST fusion protein, bound constitutively active calcineurin in vitro. PQAVAD mutation as well as addition of VIVIT peptide to the reaction abrogated this calcineurin binding. Wild type calcineurin was recruited to GST-TRESK-loop in the presence of calcium and calmodulin. These results indicate that the PQIVID sequence is a docking site for calcineurin, and its occupancy is required for the calcium-dependent regulation of TRESK. Immunosuppressive compounds, developed to target the NFAT binding site of calcineurin, are also expected to interfere with TRESK regulation, in addition to their desired effect on NFAT.

Zinc and Mercuric Ions Distinguish TRESK from the Other Two-Pore-Domain K+Channels

TWIK-related spinal cord K+ channel (TRESK) is the most recently cloned two-pore-domain potassium (2PK+) channel, regulated by the calcium/calmodulin-dependent protein phosphatase calcineurin. Functional identification of endogenous TRESK and its distinction from the other 2PK+ channels, producing similar background K+ current, are impeded by the lack of specific inhibitors. Therefore, we searched for antagonists selective against TRESK among the mouse 2PK+ channels by screening more than 200 substances. Mibefradil, zinc, and mercuric ions inhibited TRESK expressed in Xenopus laevis oocytes with IC50 values lower than 10 microM. The specificity of the identified agents was determined by measuring their effects on mouse TALK-1, TASK-1, TASK-2, TASK-3, THIK-1, TRAAK, TREK-1, and TREK-2. Mibefradil failed to discriminate well among the functional 2PK+ channels; however, Zn2+ and Hg2+ exerted a significantly stronger inhibitory effect on TRESK than on the other channels. Sensitivity to zinc but insensitivity to ruthenium red were distinctive features of TRESK. Whereas both Zn2+ and Hg2+ were selective blockers of TRESK among the mouse 2PK+ channels, human TRESK was resistant to Zn2+; it was blocked only by Hg2+. His132 of mouse TRESK was partly responsible for this difference. Mouse TRESK expressed in COS-7 cells was also inhibited by Zn2+ and Hg2+, and TRESK single-channel current was diminished in outside-out patches, indicating that the action of the ions was membrane-delimited, most probably targeting the channel itself. Thus, both Zn2+ and Hg2+ are expected to inhibit endogenous TRESK in isolated mouse cells, and these ions can be applied to identify the calcineurin-activated 2PK+ channel in its natural environment.

PIP2 hydrolysis underlies agonist-induced inhibition and regulates voltage gating of two-pore domain K+ channels

Two-pore (2-P) domain potassium channels are implicated in the control of the resting membrane potential, hormonal secretion, and the amplitude, frequency and duration of the action potential. These channels are strongly regulated by hormones and neurotransmitters. Little is known, however, about the mechanism underlying their regulation. Here we show that phosphatidylinositol 4,5-bisphosphate (PIP2) gating underlies several aspects of 2-P channel regulation. Our results demonstrate that all four 2-P channels tested, TASK1, TASK3, TREK1 and TRAAK are activated by PIP2. We show that mechanical stimulation may promote PIP2 activation of TRAAK channels. For TREK1, TASK1 and TASK3 channels, PIP2 hydrolysis underlies inhibition by several agonists. The kinetics of inhibition by the PIP2 scavenger polylysine, and the inhibition by the phosphatidylinositol 4-kinase inhibitor wortmannin correlated with the level of agonist-induced inhibition. This finding suggests that the strength of channel PIP2 interactions determines the extent of PLC-induced inhibition. Finally, we show that PIP2 hydrolysis modulates voltage dependence of TREK1 channels and the unrelated voltage-dependent KCNQ1 channels. Our results suggest that PIP2 is a common gating molecule for K+ channel families despite their distinct structures and physiological properties.

The two-pore domain K+ channel, TRESK, is activated by the cytoplasmic calcium signal through calcineurin

Agonist-induced cytoplasmic calcium signals often have profound effects on the membrane potential during cellular activation. In the present study, we report that cytoplasmic calcium elevation can regulate the membrane potential by a novel mechanism. TRESK, a recently described member of the two-pore domain potassium (2PK(+)) channel family, was activated 5-15-fold after stimulation of various Ca(2+)-mobilizing receptors in Xenopus oocytes. Extracellular application of ionomycin, as well as the microinjection of inositol 1,4,5-trisphosphate or calcium, also evoked TRESK activation, whereas microinjection of EGTA or pretreatment of the oocytes with thapsigargin prevented the receptor-mediated effect. These data indicate that TRESK is activated by increased cytoplasmic calcium concentration. However, application of Ca(2+) to inside-out membrane patches failed to influence TRESK single channel activity, suggesting that cytoplasmic factors are also required for the regulation. Cyclosporin A and FK506, specific inhibitors of the calcium/calmodulin-dependent protein phosphatase (calcineurin), completely eliminated TRESK activation. Coexpression of a constitutively active form of calcineurin with TRESK increased the basal background K(+) current and attenuated the response of the channel to the calcium signal, indicating that TRESK was activated by the permanent calcineurin activity. Serine 276 was identified as the major functional target of calcineurin in TRESK by alanine-scanning mutagenesis. This is the first example of calcineurin being involved in the regulation of a two-pore domain K(+) channel, and thus, TRESK channels may regulate the excitability of neurons and other cell types in response to Ca(2+)-mobilizing hormones and neurotransmitters in a manner that is sensitive to immunosuppressive drugs.

Ruthenium red inhibits TASK-3 potassium channel by interconnecting glutamate 70 of the two subunits

TASK channels are highly pH-sensitive two-pore-domain background potassium channels expressed in the central nervous system and in some peripheral tissues. Their current can be regulated by receptor-mediated activation of phospholipase C and also by pharmacological means. We have reported previously that the cationic dye, ruthenium red (RR), inhibited homodimeric TASK-3 (kcnk9), whereas TASK-1 (kcnk3) homodimer and TASK-1/TASK-3 heterodimer were not affected by this compound. In the present study, we identify the molecular determinant of the RR-mediated TASK-3 inhibition. Mutation of the negatively charged Glu 70 of TASK-3 to Arg (E70R) or Cys (E70C) abolished the inhibitory action of RR. When two TASK-3 coding sequences were concatenated, and the entire homodimer was expressed as a single polypeptide chain, the resulting tandem channel was also sensitive to RR. Mutation of Glu 70 in either the first (E70R) or the second (E465R) linked subunit prevented the action of the inhibitor. Together with the Hill coefficient of 1.0 for TASK-3 inhibition, these data indicate that simultaneous binding of one polycationic RR molecule to Glu 70 of both subunits is required for the inhibitory action. The pivotal role of this residue in the inhibitory mechanism of RR is confirmed by the gained RR sensitivity of the mutant TASK-1 in which Lys 70 was changed to Glu. Our results indicate that RR inhibits TASK-3 by tethering its two subunits and identify amino acid 70 as a possible target for designing selective inhibitors against the different TASK channels.

TASK-3 dominates the background potassium conductance in rat adrenal glomerulosa cells

In a preceding study we showed that the highly negative resting membrane potential of rat adrenal glomerulosa cells is related to background potassium channel(s), which belong to the two-pore domain channel family. TWIK-related acid-sensitive K+ channel (TASK-1) expression was found in glomerulosa tissue, and the currents elicited by injection of glomerulosa mRNA (I(glom)) or TASK-1 cRNA (I(TASK-1)) showed remarkable similarity in Xenopus laevis oocytes. However, based on the different sensitivity of these currents to acidification, we concluded that TASK-1 may be responsible for a maximum of 25% of the weakly pH-dependent glomerulosa background K+ current. Here we demonstrate that TASK-3, a close relative of TASK-1, is expressed abundantly in glomerulosa cells. Northern blot detected TASK-3 message in adrenal glomerulosa, but not in other tissues. Quantitative RT-PCR experiments indicated even higher mRNA expression of TASK-3 than TASK-1 in glomerulosa tissue. Similarly to the glomerulosa background current, the current expressed by injection of TASK-3 cRNA (I(TASK-3)) was less acid-sensitive than I(TASK-1). Ruthenium red in the micromolar range inhibited I(glom) and I(TASK-3), but not I(TASK-1). Like I(TASK-1), I(TASK-3) was inhibited by stimulation of AT1a angiotensin II receptor coexpressed with the potassium channel. The high level of expression and its pharmacological properties suggest that TASK-3 dominates the resting potassium conductance of glomerulosa cells.

Formation of functional heterodimers between the TASK-1 and TASK-3 two-pore domain potassium channel subunits

The potassium channels in the two-pore domain family are widely expressed and regulate the excitability of neurons and other excitable cells. These channels have been shown to function as dimers, but heteromerization between the various channel subunits has not yet been reported. Here we demonstrate that two members of the TASK subfamily of potassium channels, TASK-1 and TASK-3, can form functional heterodimers when expressed in Xenopus laevis oocytes. To recognize the two TASK channel types, we took advantage of the higher sensitivity of TASK-1 over TASK-3 to physiological pH changes and the discriminating sensitivity of TASK-3 to the cationic dye ruthenium red. These features were clearly observed when the channels were expressed individually. However, when TASK-1 and TASK-3 were expressed together, the resulting current showed intermediate pH sensitivity and ruthenium red insensitivity (characteristic of TASK-1), indicating the formation of TASK-1/TASK-3 heterodimers. Expression of a tandem construct in which TASK-3 and TASK-1 were linked together yielded currents with features very similar to those observed when coexpressing the two channels. The tandem construct also responded to AT(1a) angiotensin II receptor stimulation with an inhibition that was weaker than the inhibition of homodimeric TASK-1 and greater than that shown by TASK-3. Expression of epitope-tagged channels in mammalian cells showed their primary presence in the plasma membrane consistent with their function in this location. Heteromerization of two-pore domain potassium channels may provide a greater functional diversity and additional means by which they can be regulated in their native tissues.

Inhibition of TASK-1 potassium channel by phospholipase C

The two-pore-domain K(+) channel, TASK-1, was recently shown to be a target of receptor-mediated regulation in neurons and in adrenal glomerulosa cells. Here, we demonstrate that TASK-1 expressed in Xenopus laevis oocytes is inhibited by different Ca(2+)-mobilizing agonists. Lysophosphatidic acid, via its endogenous receptor, and ANG II and carbachol, via their heterologously expressed ANG II type 1a and M(1) muscarinic receptors, respectively, inhibit TASK-1. This effect can be mimicked by guanosine 5'-O-(3-thiotriphosphate), indicating the involvement of GTP-binding protein(s). The phospholipase C inhibitor U-73122 reduced the receptor-mediated inhibition of TASK-1. Downstream signals of phospholipase C action (inositol 1,4,5-trisphosphate, cytoplasmic Ca(2+) concentration, and diacylglycerol) do not mediate the inhibition. Unlike the G(q)-coupled receptors, stimulation of the G(i)-activating M(2) muscarinic receptor coexpressed with TASK-1 results in an only minimal decrease of the TASK-1 current. However, additional coexpression of phospholipase C-beta(2) (which is responsive also to G(i) beta gamma-subunits) renders M(2) receptor activation effective. This indicates the significance of phospholipase C activity in the receptor-mediated inhibition of TASK-1.

TASK (TWIK-related acid-sensitive K+ channel) is expressed in glomerulosa cells of rat adrenal cortex and inhibited by angiotensin II

The present study was conducted to explore the possible contribution of a recently described leak K+ channel, TASK (TWIK-related acid-sensitive K+ channel), to the high resting K+ conductance of adrenal glomerulosa cells. Northern blot analysis showed the strongest TASK message in adrenal glomerulosa (capsular) tissue among the examined tissues including heart and brain. Single-cell PCR demonstrated TASK expression in glomerulosa cells. In patch-clamp experiments performed on isolated glomerulosa cells the inward current at -100 mV in 30 mM [K+] (reflecting mainly potassium conductance) was pH sensitive (17+/-2% reduction when the pH changed from 7.4 to 6.7). In Xenopus oocytes injected with mRNA prepared from adrenal glomerulosa tissue the expressed K+ current at -100 mV was virtually insensitive to tetraethylammonium (3 mM) and 4-aminopyridine (3 mM). Ba2+ (300 microM) and Cs+ (3 mM) induced voltage-dependent block. Lidocaine (1 mM) and extracellular acidification from pH 7.5 to 6.7 inhibited the current (by 28% and 16%, respectively). This inhibitory profile is similar (although it is not identical) to that of TASK expressed by injecting its cRNA. In oocytes injected with adrenal glomerulosa mRNA, TASK antisense oligonucleotide reduced significantly the expression of K+ current at -100 mV, while the sense oligonucleotide failed to have inhibitory effect. Application of angiotensin II (10 nM) both in isolated glomerulosa cells and in oocytes injected with adrenal glomerulosa mRNA inhibited the K+ current at -100 mV. Similarly, in oocytes coexpressing TASK and ATla angiotensin II receptor, angiotensin II inhibited the TASK current. These data together indicate that TASK contributes to the generation of high resting potassium permeability of glomerulosa cells, and this background K+ channel may be a target of hormonal regulation.

Cloning and functional expression of the cytoplasmic form of rat aminopeptidase P

A rat cytoplasmic aminopeptidase P was purified from liver cytosol with a procedure including an affinity elution step with 3 microM inositol 1,3,4-trisphosphate. Proteolytic fragments were generated, sequenced and the enzyme was cloned from a rat liver cDNA library. The structure shows high (87.8% and 95.5%, respectively) sequence identity at the nucleotide and amino acid levels with the previously described human putative cytoplasmic aminopeptidase P. The cloned rat enzyme was functionally expressed in Escherichia coli and also in COS-1 cells. Western blot analysis, using an antibody generated against the recombinant protein, and Northern blot hybridization showed ubiquitous expression of the protein in different tissues with the highest expression level in the testis.