Vijay Renigunta

Interaction with 14-3-3 proteins promotes functional expression of the potassium channels TASK-1 and TASK-3.

The two‐pore‐domain potassium channels TASK‐1, TASK‐3 and TASK‐5 possess a conserved C‐terminal motif of five amino acids. Truncation of the C‐terminus of TASK‐1 strongly reduced the currents measured after heterologous expression in Xenopus oocytes or HEK293 cells and decreased surface membrane expression of GFP‐tagged channel proteins. Two‐hybrid analysis showed that the C‐terminal domain of TASK‐1, TASK‐3 and TASK‐5, but not TASK‐4, interacts with isoforms of the adapter protein 14‐3‐3. A pentapeptide motif at the extreme C‐terminus of TASK‐1, RRx(S/T)x, was found to be sufficient for weak but significant interaction with 14‐3‐3, whereas the last 40 amino acids of TASK‐1 were required for strong binding. Deletion of a single amino acid at the C‐terminal end of TASK‐1 or TASK‐3 abolished binding of 14‐3‐3 and strongly reduced the macroscopic currents observed in Xenopus oocytes. TASK‐1 mutants that failed to interact with 14‐3‐3 isoforms (V411*, S410A, S410D) also produced only very weak macroscopic currents. In contrast, the mutant TASK‐1 S409A, which interacts with 14‐3‐3‐like wild‐type channels, displayed normal macroscopic currents. Co‐injection of 14‐3‐3ζ cRNA increased TASK‐1 current in Xenopus oocytes by about 70 %. After co‐transfection in HEK293 cells, TASK‐1 and 14‐3‐3ζ (but not TASK‐1ΔC5 and 14‐3‐3ζ) could be co‐immunoprecipitated. Furthermore, TASK‐1 and 14‐3‐3 could be co‐immunoprecipitated in synaptic membrane extracts and postsynaptic density membranes. Our findings suggest that interaction of 14‐3‐3 with TASK‐1 or TASK‐3 may promote the trafficking of the channels to the surface membrane.

The Retention Factor p11 Confers an Endoplasmic Reticulum-Localization Signal to the Potassium Channel TASK-1

The interaction of the adaptor protein p11, also denoted S100A10, with the C‐terminus of the two‐pore‐domain K+ channel TASK‐1 was studied using yeast two‐hybrid analysis, glutathione S‐transferase pulldown, and co‐immunoprecipitation. We found that p11 interacts with a 40 amino‐acid region in the proximal C‐terminus of the channel. In heterologous expression systems, deletion of the p11‐interacting domain enhanced surface expression of TASK‐1. Attachment of the p11‐interacting domain to the cytosolic tail of the reporter protein CD8 caused retention/retrieval of the construct in the endoplasmic reticulum (ER). Attachment of the last 36 amino acids of p11 to CD8 also caused ER localization, which was abolished by removal or mutation of a putative retention motif (H/K)xKxxx, at the C‐terminal end of p11. Imaging of EGFP‐tagged TASK‐1 channels in COS cells suggested that wild‐type TASK‐1 was largely retained in the ER. Knockdown of p11 with siRNA enhanced trafficking of TASK‐1 to the surface membrane. Our results suggest that binding of p11 to TASK‐1 retards the surface expression of the channel, most likely by virtue of a di‐lysine retention signal at the C‐terminus of p11. Thus, the cytosolic protein p11 may represent a ‘retention factor’ that causes localization of the channel to the ER.

Intracellular traffic of the K+ channels TASK-1 and TASK-3: role of N- and C-terminal sorting signals and interaction with 14-3-3 proteins.

The two‐pore‐domain potassium channels TASK‐1 (KCNK3) and TASK‐3 (KCNK9) modulate the electrical activity of neurons and many other cell types. We expressed TASK‐1, TASK‐3 and related reporter constructs in Xenopus oocytes, mammalian cell lines and various yeast strains to study the mechanisms controlling their transport to the surface membrane and the role of 14‐3‐3 proteins. We measured potassium currents with the voltage‐clamp technique and fused N‐ and C‐terminal fragments of the channels to various reporter proteins to study changes in subcellular localisation and surface expression. Mutational analysis showed that binding of 14‐3‐3 proteins to the extreme C‐terminus of TASK‐1 and TASK‐3 masks a tri‐basic motif, KRR, which differs in several important aspects from canonical arginine‐based (RxR) or lysine‐based (KKxx) retention signals. Pulldown experiments with GST fusion proteins showed that the KRR motif in the C‐terminus of TASK‐3 channels was able to bind to COPI coatomer. Disabling the binding of 14‐3‐3, which exposes the KRR motif, caused localisation of the GFP‐tagged channel protein mainly to the Golgi complex. TASK‐1 and TASK‐3 also possess a di‐basic N‐terminal retention signal, KR, whose function was found to be independent of the binding of 14‐3‐3. Suppression of channel surface expression with dominant‐negative channel mutants revealed that interaction with 14‐3‐3 has no significant effect on the dimeric assembly of the channels. Our results give a comprehensive description of the mechanisms by which 14‐3‐3 proteins, together with N‐ and C‐terminal sorting signals, control the intracellular traffic of TASK‐1 and TASK‐3.

Tamm-Horsfall Glycoprotein Interacts with Renal Outer Medullary Potassium Channel ROMK2 and Regulates Its Function

Tamm-Horsfall glycoprotein (THGP) or Uromodulin is a membrane protein exclusively expressed along the thick ascending limb (TAL) and early distal convoluted tubule (DCT) of the nephron. Mutations in the THGP encoding gene result in Familial Juvenile Hyperuricemic Nephropathy (FJHN), Medullary Cystic Kidney Disease type 2 (MCKD-2), and Glomerulocystic Kidney Disease (GCKD). The physicochemical and biological properties of THGP have been studied extensively, but its physiological function in the TAL remains obscure. We performed yeast two-hybrid screening employing a human kidney cDNA library and identified THGP as a potential interaction partner of the renal outer medullary potassium channel (ROMK2), a key player in the process of salt reabsorption along the TAL. Functional analysis by electrophysiological techniques in Xenopus oocytes showed a strong increase in ROMK current amplitudes when co-expressed with THGP. The effect of THGP was specific for ROMK2 and did not influence current amplitudes upon co-expression with Kir2.x, inward rectifier potassium channels related to ROMK. Single channel conductance and open probability of ROMK2 were not altered by co-expression of THGP, which instead increased surface expression of ROMK2 as determined by patch clamp analysis and luminometric surface quantification, respectively. Despite preserved interaction with ROMK2, disease-causing THGP mutants failed to increase its current amplitude and surface expression. THGP−/− mice exhibited increased ROMK accumulation in intracellular vesicular compartments when compared with WT animals. Therefore, THGP modulation of ROMK function confers a new role of THGP on renal ion transport and may contribute to salt wasting observed in FJHN/MCKD-2/GCKD patients.

A Specific Two-pore Domain Potassium Channel Blocker Defines the Structure of the TASK-1 Open Pore

Two-pore domain potassium (K2P) channels play a key role in setting the membrane potential of excitable cells. Despite their role as putative targets for drugs and general anesthetics, little is known about the structure and the drug binding site of K2P channels. We describe A1899 as a potent and highly selective blocker of the K2P channel TASK-1. As A1899 acts as an open-channel blocker and binds to residues forming the wall of the central cavity, the drug was used to further our understanding of the channel pore. Using alanine mutagenesis screens, we have identified residues in both pore loops, the M2 and M4 segments, and the halothane response element to form the drug binding site of TASK-1. Our experimental data were used to validate a K2P open-pore homology model of TASK-1, providing structural insights for future rational design of drugs targeting K2P channels.

A semisynthetic fusicoccane stabilizes a protein-protein interaction and enhances the expression of K+ channels at the cell surface

Small-molecule stabilization of protein-protein interactions is an emerging field in chemical biology. We show how fusicoccanes, originally identified as fungal toxins acting on plants, promote the interaction of 14-3-3 proteins with the human potassium channel TASK-3 and present a semisynthetic fusicoccane derivative (FC-THF) that targets the 14-3-3 recognition motif (mode 3) in TASK-3. In the presence of FC-THF, the binding of 14-3-3 proteins to TASK-3 was increased 19-fold and protein crystallography provided the atomic details of the effects of FC-THF on this interaction. We also tested the functional effects of FC-THF on TASK channels heterologously expressed in Xenopus oocytes. Incubation with 10 μM FC-THF was found to promote the transport of TASK channels to the cell membrane, leading to a significantly higher density of channels at the surface membrane and increased potassium current.

Knock-out mice reveal the contributions of P2Y and P2X receptors to nucleotide-induced Ca2+ signaling in macrophages

Immune cell function is modulated by changes in extracellular nucleotide levels. Here we used reverse transcription-PCR analyses, single cell Ca2+ imaging, and knock-out mice to define the receptors mediating nucleotide-induced Ca2+ signaling in resident peritoneal macrophages. In Ca2+-free buffer, the potent (K0.5 <1 μm) stimulatory effect of UTP (or ATP) on endoplasmic reticulum (ER) Ca2+ release was abolished in cells isolated from P2Y2/P2Y4 double knock-out mice. Moreover, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{P}2\mathrm{Y}_{4}^{0{/}-}\) \end{document}, but not \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{P}2\mathrm{Y}_{2}^{-{/}-}\) \end{document}, macrophages responded to UTP. In \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{P}2\mathrm{Y}_{2}^{-{/}-}\) \end{document} macrophages, we could elicit Ca2+ responses to “pure” P2X receptor activation by applying ATP in buffer containing Ca2+. Purified UDP and ADP were ineffective agonists, although modest UDP-induced Ca2+ responses could be elicited in macrophages after “activation” with lipopolysaccharide and interferon-γ. Notably, in Ca2+-free buffer, UTP-induced Ca2+ transients decayed within 1 min, and there was no response to repeated agonist challenge. Measurements of ER [Ca2+] with mag-fluo-4 showed that ER Ca2+ stores were depleted under these conditions. When extracellular Ca2+ was available, ER Ca2+ stores refilled, but Ca2+ increased to only ∼40% of the initial value upon repeated UTP challenge. This apparent receptor desensitization persisted in GRK2+/- and GRK6-/- macrophages and after inhibition of candidate kinases protein kinase C and calmodulin-dependent kinase II. Initial challenge with UTP also reduced Ca2+ mobilization by complement component C5a (and vice versa). In conclusion, homologous receptor desensitization is not the major mechanism that rapidly dampens Ca2+ signaling mediated by P2Y2, the sole Gq-coupled receptor for UTP or ATP in macrophages. UDP responsiveness (P2Y6 receptor expression) increases following macrophage activation.

RNA editing modulates the binding of drugs and highly unsaturated fatty acids to the open pore of Kv potassium channels

The time course of inactivation of voltage‐activated potassium (Kv) channels is an important determinant of the firing rate of neurons. In many Kv channels highly unsaturated lipids as arachidonic acid, docosahexaenoic acid and anandamide can induce fast inactivation. We found that these lipids interact with hydrophobic residues lining the inner cavity of the pore. We analysed the effects of these lipids on Kv1.1 current kinetics and their competition with intracellular tetraethylammonium and Kvβ subunits. Our data suggest that inactivation most likely represents occlusion of the permeation pathway, similar to drugs that produce ‘open‐channel block’. Open‐channel block by drugs and lipids was strongly reduced in Kv1.1 channels whose amino acid sequence was altered by RNA editing in the pore cavity, and in Kv1.x heteromeric channels containing edited Kv1.1 subunits. We show that differential editing of Kv1.1 channels in different regions of the brain can profoundly alter the pharmacology of Kv1.x channels. Our findings provide a mechanistic understanding of lipid‐induced inactivation and establish RNA editing as a mechanism to induce drug and lipid resistance in Kv channels.

Structural determinants of Kvβ1.3‐induced channel inactivation: a hairpin modulated by PIP2

Inactivation of voltage‐gated Kv1 channels can be altered by Kvβ subunits, which block the ion‐conducting pore to induce a rapid (‘N‐type’) inactivation. Here, we investigate the mechanisms and structural basis of Kvβ1.3 interaction with the pore domain of Kv1.5 channels. Inactivation induced by Kvβ1.3 was antagonized by intracellular PIP2. Mutations of R5 or T6 in Kvβ1.3 enhanced Kv1.5 inactivation and markedly reduced the effects of PIP2. R5C or T6C Kvβ1.3 also exhibited diminished binding of PIP2 compared with wild‐type channels in an in vitro lipid‐binding assay. Further, scanning mutagenesis of the N terminus of Kvβ1.3 revealed that mutations of L2 and A3 eliminated N‐type inactivation. Double‐mutant cycle analysis indicates that R5 interacts with A501 and T480 of Kv1.5, residues located deep within the pore of the channel. These interactions indicate that Kvβ1.3, in contrast to Kvβ1.1, assumes a hairpin structure to inactivate Kv1 channels. Taken together, our findings indicate that inactivation of Kv1.5 is mediated by an equilibrium binding of the N terminus of Kvβ1.3 between phosphoinositides (PIPs) and the inner pore region of the channel.

The inhibition of the potassium channel TASK-1 in rat cardiac muscle by endothelin-1 is mediated by phospholipase C

AIMS The two-pore-domain potassium channel TASK-1 is robustly inhibited by the activation of receptors coupled to the Gα(q) subgroup of G-proteins, but the signal transduction pathway is still unclear. We have studied the mechanisms by which endothelin receptors inhibit the current carried by TASK-1 channels (I(TASK)) in cardiomyocytes. METHODS AND RESULTS Patch-clamp measurements were carried out in isolated rat cardiomyocytes. I(TASK) was identified by extracellular acidification to pH 6.0 and by the application of the TASK-1 blockers A293 and A1899. Endothelin-1 completely inhibited I(TASK) with an EC(50) of <10 nM; this effect was mainly mediated by endothelin-A receptors. Application of 20 nM endothelin-1 caused a significant increase in action potential duration under control conditions; this was significantly reduced after pre-incubation of the cardiomyocytes with 200 nM A1899. The inhibition of I(TASK) by endothelin-1 was not affected by inhibitors of protein kinase C or rho kinase, but was strongly reduced by U73122, an inhibitor of phospholipase C (PLC). The ability of endothelin-1 to activate PLC-mediated signalling pathways was examined in mammalian cells transfected with TASK-1 and the endothelin-A receptor using patch-clamp measurements and total internal reflection microscopy. U73122 prevented the inhibition of I(TASK) by endothelin-1 and blocked PLC-mediated signalling, as verified with a fluorescent probe for phosphatidylinositol-(4,5)-bisphosphate hydrolysis. CONCLUSION Our results show that I(TASK) in rat cardiomyocytes is controlled by endothelin-1 and suggest that the inhibition of TASK-1 via endothelin receptors is mediated by the activation of PLC. The prolongation of the action potential observed with 20 nM endothelin-1 was mainly due to the inhibition of I(TASK).