KCNN4 links PIEZO-dependent mechanotransduction to NLRP3 inflammasome activation

Immune cells sense the microenvironment to fine-tune their inflammatory responses. Patients with cryopyrin-associated periodic syndrome (CAPS), caused by mutations in the NLRP3 gene, develop autoinflammation triggered by nonantigenic cues such as from the environment. However, the underlying mechanisms are poorly understood. Here, we uncover that KCNN4, a calcium-activated potassium channel, links PIEZO-mediated mechanotransduction to NLRP3 inflammasome activation. Yoda1, a PIEZO1 agonist, lowered the threshold for NLRP3 inflammasome activation. PIEZO-mediated sensing of stiffness and shear stress increased NLRP3-dependent inflammation. Myeloid-specific deletion of PIEZO1/2 protected mice from gouty arthritis. Mechanistically, activation of PIEZO1 triggers calcium influx, which activates KCNN4 to evoke potassium efflux and promotes NLRP3 inflammasome activation. Activation of PIEZO signaling was sufficient to activate the inflammasome in cells expressing CAPS-causing NLRP3 mutants via KCNN4. Last, pharmacological inhibition of KCNN4 alleviated autoinflammation in cells of patients with CAPS and in mice bearing a CAPS mutation. Thus, PIEZO-dependent mechanical inputs boost inflammation in NLRP3-dependent diseases, including CAPS.

Cooling-induced cutaneous vasodilatation is mediated by small-conductance, calcium-activated potassium channels in tail arteries from male mice

Cooling causes cutaneous dilatation to restrain cold-induced constriction and prevent tissue injury. Cooling increases communication through myoendothelial gap junctions (MEGJs), thereby increasing endothelium-derived hyperpolarization (EDH)-type dilatation. EDH is initiated by calcium-activated potassium channels (KCa ) activated by endothelial stimuli or muscle-derived mediators traversing MEGJs (myoendothelial feedback). The goal of this study was to determine the individual roles of KCa with small (SK3) and intermediate (IK1) conductance in cooling-induced dilatation. Vasomotor responses of mice isolated cutaneous tail arteries were analyzed by pressure myography at 37°C and 28°C. Cooling increased acetylcholine-induced EDH-type dilatation during inhibition of NO and prostacyclin production. IK1 inhibition did not affect dilatations to acetylcholine, whereas SK3 inhibition inhibited dilatation at both temperatures. Cooling uncovered myoendothelial feedback to inhibit constrictions in U46619. IK1 inhibition did not affect U46619 constrictions, whereas SK3 inhibition abolished the inhibitory effect of cooling without affecting U46619 constriction at 37°C. Immunoblots confirmed SK3 expression, which was localized (immunofluorescence) to holes in the internal elastic lamina consistent with myoendothelial projections. Immunoblots and Immunofluorescence did not detect IK1. Studies in non-cutaneous arteries have highlighted the predominant role of IK1 in EDH-type dilatation. Cutaneous arteries are distinctly reliant on SK3, which may enable EDH-type dilation to be amplified by cooling.

Potassium Channel-blockers as Therapeutic Agents to Interfere with Bone Resorption of Periodontal Disease

Inflammatory lesions of periodontal disease contain all the cellular components, including abundant activated/memory T- and B-cells, necessary to control immunological interactive networks and to accelerate bone resorption by RANKL-dependent and -independent mechanisms. Blockade of RANKL function has been shown to ameliorate periodontal bone resorption and other osteopenic disorders without affecting inflammation. Development of therapies aimed at decreasing the expression of RANKL and pro-inflammatory cytokines by T-cells constitutes a promising strategy to ameliorate not only bone resorption, but also inflammation. Several reports have demonstrated that the potassium channels Kv1.3 and IKCa1, through the use of selective blockers, play important roles in T-cell-mediated events, including T-cell proliferation and the production of pro-inflammatory cytokines. More recently, a potassium channel-blocker for Kv1.3 has been shown to down-regulate bone resorption by decreasing the ratio of RANKL-to-OPG expression by memory-activated T-cells. In this article, we first summarize the mechanisms by which chronically activated/memory T-cells, in concert with B-cells and macrophages, trigger inflammatory bone resorption. Then, we describe the main structural and functional characteristics of potassium channels Kv1.3 and IKCa1 in some of the cells implicated in periodontal disease progression. Finally, this review elucidates some recent advances in the use of potassium channel-blockers of Kv1.3 and IKCa1 to ameliorate the clinical signs or side-effects of several immunological disorders and to decrease inflammatory bone resorption in periodontal disease. ABBREVIATIONS: AICD, activation-induced cell death; APC, antigen-presenting cells; B(K), large conductance; CRAC, calcium release-activated calcium channels; DC, dendritic cell; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IFN-γ, interferon-γ; IP3, inositol (1,4,5)-triphosphate; (K)ir, inward rectifier; JNK, c-Jun N-terminal kinase; I(K), intermediate conductance; LPS, lipopolysaccharide; L, ligand; MCSF, macrophage colony-stimulating factor; MHC, major histocompatibility complex; NFAT, nuclear factor of activated T-cells; RANK, receptor activator of nuclear factor-κB; TCM, central memory T-cells; TEM, effector memory T-cells; TNF, tumor necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand; OPG, osteoprotegerin; Omp29, 29-kDa outer membrane protein; PKC, protein kinase C; PLC, phospholipase C; RT-PCR, reverse-transcriptase polymerase chain-reaction; S(K), small conductance; TCR, T-cell receptor; and (K)v, voltage-gated.

IKs blockade reduces dispersion of repolarization in heart failure

BACKGROUND

The slow and rapid (I(Kr)) components of I(K) are major determinants of ventricular repolarization. Unlike I(Kr), which is homogeneously expressed across the transmural wall, I(Ks) expression is reduced in midmyocardial cells and presumably contributes significantly to transmural dispersion of repolarization. Increased dispersion of repolarization during pharmacologic blockade of I(Kr) is proarrhythmic, primarily due to relatively selective prolongation of midmyocardial cell action potential duration (APD). The mechanisms underlying proarrhythmia in heart disease associated with impaired repolarization, such as heart failure, are unknown. We hypothesize that, in contrast to I(Kr) blockade, I(Ks) blockade will have little effect on midmyocardial cells and hence decrease dispersion of repolarization in heart failure.

OBJECTIVES

The purpose of this study was to determine the effect of blockade of the slow component of the delayed rectifier current (I(Ks)) on arrhythmogenic dispersion of repolarization and proarrhythmia in heart failure.

METHODS

Optical action potentials were simultaneously recorded from 256 sites spanning the transmural wall of the arterially perfused canine wedge preparation. Hearts from dogs with heart failure induced by rapid pacing (n = 6) were compared with normals (n = 6).

RESULTS

Baseline dispersion of repolarization, as measured from the range of transmural APD during stimulation at a cycle length of 2,000 ms, was significantly higher in heart failure (75 +/- 24 ms) compared with controls (39 +/- 21 ms, P < .04). I(Ks) blockade with 30 microM chromanol decreased dispersion of repolarization by 40% (P < .02) in heart failure, reducing it to values found in normals. Decreased dispersion of repolarization was due to a larger, relatively selective, drug-induced APD prolongation of epicardial (23%) compared with midmyocardial cells (9%, P < .02). VT could not be induced in failing hearts under conditions of I(Ks) blockade, and no proarrhythmia was observed.

CONCLUSION

I(Ks) blockade significantly reduced heart failure-induced dispersion of repolarization to values seen in nonfailing hearts. By prolonging repolarization without increasing dispersion of repolarization, I(Ks) blockade may have antiarrhythmic effects without creating proarrhythmia.

Two distinct pathways account for EDHF-dependent dilatation in the gracilis artery of dyslipidaemic hApoB+/+ mice

1 A universal endothelium-derived hyperpolarising factor (EDHF--non-NO/non-PGI(2)) has not been identified. EDHF, however, is essential for the physiological control of resistance artery tone. The impact of dyslipidaemia (DL), a risk factor for cardiovascular diseases, on the nature and the efficacy of EDHF has not been evaluated yet. 2 Pressurised (80 mmHg) gracilis arterial segments isolated from mice expressing the human apoB-100 and C57Bl/6 wild-type (WT) mice were used. EDHF-dependent dilatations to acetylcholine (ACh) were measured in the presence of L-NNA (100 microM, NOS inhibitor) and indomethacin (10 microM, COX inhibitor). 3 Maximal EDHF-induced dilatations were increased in DL when compared to WT (95+/-2 versus 86+/-4% in WT; P<0.05). Combination of apamin and charybdotoxin strongly reduced (P<0.05) ACh-induced dilatation in WT (22+/-4%) and DL (25+/-5%). 4 Combined addition of barium (Ba(2+)) and ouabain abolished EDHF-induced dilatations in WT arteries (13+/-3%; P<0.05). In vessels isolated from DL mice, however, only the addition of 14,15-EEZE (a 14,15-EET antagonist) to Ba(2+) and ouabain prevented EDHF-induced dilatations (5+/-3% compared to 54+/-11% in the presence of combined Ba(2+) and ouabain; P<0.05). 5 Our data suggest that EDHF-mediated dilatation depends on the opening of endothelial SK(Ca) and IK(Ca) channels. This is associated with the opening of K(ir) channels and activation of the Na(+)/K(+)-ATPase pump on smooth muscle cells leading to dilatation. In arteries from DL mice, a cytochrome P450 metabolite likely to be 14,15-EET equally contributes to the dilatory action of ACh. The early increased efficacy of EDHF in arteries isolated from DL mice may originate from the duplication of the EDHF pathways.

IKCa-channel blockers. Part 2: discovery of cyclohexadienes

Novel cyclohexadienes have been identified as potent and specific IK(Ca)-channel blockers. In this communication we describe their synthesis as well as their chemical and biological properties. A selected derivative is being enriched in rat brain and reduces the infarct volume, intracranial pressure as well as the water content in a rat subdural hematoma model of traumatic brain injury after iv administration.

The phosphatidylinositol 3-phosphate phosphatase myotubularin- related protein 6 (MTMR6) is a negative regulator of the Ca2+-activated K+ channel KCa3.1

Myotubularins (MTMs) belong to a large subfamily of phosphatases that dephosphorylate the 3' position of phosphatidylinositol 3-phosphate [PI(3)P] and PI(3,5)P(2). MTM1 is mutated in X-linked myotubular myopathy, and MTMR2 and MTMR13 are mutated in Charcot-Marie-Tooth syndrome. However, little is known about the general mechanism(s) whereby MTMs are regulated or the specific biological processes regulated by the different MTMs. We identified a Ca(2+)-activated K channel, K(Ca)3.1 (also known as KCa4, IKCa1, hIK1, or SK4), that specifically interacts with the MTMR6 subfamily of MTMs via coiled coil (CC) domains on both proteins. Overexpression of MTMR6 inhibited K(Ca)3.1 channel activity, and this inhibition required MTMR6's CC and phosphatase domains. This inhibition is specific; MTM1, a closely related MTM, did not inhibit K(Ca)3.1. However, a chimeric MTM1 in which the MTM1 CC domain was swapped for the MTMR6 CC domain inhibited K(Ca)3.1, indicating that MTM CC domains are sufficient to confer target specificity. K(Ca)3.1 was also inhibited by the PI(3) kinase inhibitors LY294002 and wortmannin, and this inhibition was rescued by the addition of PI(3)P, but not other phosphoinositides, to the patch pipette solution. PI(3)P also rescued the inhibition of K(Ca)3.1 by MTMR6 overexpression. These data, when taken together, indicate that K(Ca)3.1 is regulated by PI(3)P and that MTMR6 inhibits K(Ca)3.1 by dephosphorylating the 3' position of PI(3)P, possibly leading to decreased PI(3)P in lipid microdomains adjacent to K(Ca)3.1. K(Ca)3.1 plays important roles in controlling proliferation by T cells, vascular smooth muscle cells, and some cancer cell lines. Thus, our findings not only provide unique insights into the regulation of K(Ca)3.1 channel activity but also raise the possibility that MTMs play important roles in the negative regulation of T cells and in conditions associated with pathological cell proliferation, such as cancer and atherosclerosis.

4-Chlorobenzo[F]isoquinoline (CBIQ), a novel activator of CFTR and ΔF508 CFTR

4-Chlorobenzo[F]isoquinoline (CBIQ) is a novel compound, here shown to activate both CFTR (cystic fibrosis transmembrane conductance regulator) Cl- ion channels and KCNN4, intermediate conductance, calcium-sensitive K+-channels, present in transporting epithelia by the use of heterologous expression systems. Earlier studies with other benzoquinolines, namely 7,8- and 5,6 benzoquinoline, showed they too could activate CFTR and KCNN4, but the evidence was only indirect. However this study also shows that CBIQ can also activate DeltaF508 CFTR, the most common mutant form of CFTR present in approximately 75% of patients with cystic fibrosis. This property is not shared with the other benzoquinolines. As activation of CFTR and KCNN4 work in unison to promote epithelial chloride secretion, CBIQ is a new chemical scaffold for developing agents that may be useful in cystic fibrosis.

The distribution of small and intermediate conductance calcium-activated potassium channels in the rat sensory nervous system

Small (SK) and intermediate (IK) conductance calcium-activated potassium channels are candidate ion channels for the regulation of excitability in nociceptive neurones. We have used unique peptide-directed antisera to describe the immunocytochemical distribution of the known isoforms of these ion channels in dorsal root ganglia (DRG) and spinal cord of the rat. These investigations sought to characterize further the phenotype and hence possible functions of nociceptive neurone subpopulations in the rat. In addition, using Western blotting, we sought to determine the level of protein expression of SK and IK channels in sensory nervous tissues following induction of inflammation (Freund's Complete Adjuvant (FCA) arthritis model) or nerve injury (chronic constriction injury model). We show that SK1, SK2, SK3 and IK1 are all expressed in DRG and spinal cord. Morphometric analysis revealed that SK1, SK2 and IK1 were preferentially localized to neurones having cell bodies <1000 microm2 (putative nociceptors) in DRG. Dual labeling immunocytochemistry showed that these ion channels co-localize with both CGRP and IB4, known markers of nociceptor sub-populations. SK2 was localized almost exclusively in the superficial laminae of the spinal cord dorsal horn, the region in which many sensory afferents terminate; the distribution of SK1 and IK1 was more widespread in spinal cord, although some preferential labeling within the dorsal horn was observed in the case of IK1. Here we show evidence for a distinctive pattern of expression for certain members of the calcium-activated potassium channel family in the rat DRG.

Blocking ion channel KCNN4 alleviates the symptoms of experimental autoimmune encephalomyelitis in mice

The KCNN4 potassium-ion channel has been reported to play an important role in regulating antigen-induced T cell effector functions in vitro. This study presents the first evidence that a selective KCNN4 blocker, TRAM-34, confers protection against experimental autoimmune encephalomyelitis (EAE) in the mouse model. Treatment with the KCNN4 blocker did not prevent infiltration of T cells in the spinal cord, but resulted in the reduction of both the protein and the message levels of TNF-alpha and IFN-gamma as well as the message levels of several other pro-inflammatory molecules in the spinal cord. Plasma concentrations of TRAM-34 within a 24-h period were between the in vitro IC(50) and IC(90) values for the KCNN4 channel. The effect of TRAM-34 was reversible, as indicated by the development of clinical EAE symptoms within 48 h after withdrawal of treatment. In summary, our data support the idea that KCNN4 channels play a critical role in the immune response during the development of MOG-induced EAE in C57BL/6 mice.

New insights on the voltage dependence of the KCa3.1 channel block by internal TBA

We present in this work a structural model of the open IKCa (K_Ca3.1) channel derived by homology modeling from the MthK channel structure, and used this model to compute the transmembrane potential profile along the channel pore. This analysis showed that the selectivity filter and the region extending from the channel inner cavity to the internal medium should respectively account for 81% and 16% of the transmembrane potential difference. We found however that the voltage dependence of the IKCa block by the quaternary ammonium ion TBA applied internally is compatible with an apparent electrical distance δ of 0.49 ± 0.02 (n = 6) for negative potentials. To reconcile this observation with the electrostatic potential profile predicted for the channel pore, we modeled the IKCa block by TBA assuming that the voltage dependence of the block is governed by both the difference in potential between the channel cavity and the internal medium, and the potential profile along the selectivity filter region through an effect on the filter ion occupancy states. The resulting model predicts that δ should be voltage dependent, being larger at negative than positive potentials. The model also indicates that raising the internal K⁺ concentration should decrease the value of δ measured at negative potentials independently of the external K⁺ concentration, whereas raising the external K⁺ concentration should minimally affect δ for concentrations >50 mM. All these predictions are born out by our current experimental results. Finally, we found that the substitutions V275C and V275A increased the voltage sensitivity of the TBA block, suggesting that TBA could move further into the pore, thus leading to stronger interactions between TBA and the ions in the selectivity filter. Globally, these results support a model whereby the voltage dependence of the TBA block in IKCa is mainly governed by the voltage dependence of the ion occupancy states of the selectivity filter.

Ion channels in T cells: from molecular pharmacology to therapy

Ion channels of a variety of cell types, such as cardiac and smooth muscle cells and neurons, serve as targets for many drugs used in therapy. T cells also express an assortment of ion channels that are in the focus of intensive research, as they may provide efficient ways to specifically manipulate T cell function and, consequently, immune responses. T cell activation relies on the operation of voltage-gated and Ca2+-activated potassium channels and Ca2+ release-activated Ca2+ channels. Many peptide toxin and small molecule blockers of these channels are known, but inhibitors of even higher affinity and selectivity would be needed for safe and effective clinical use. The recent discovery that the expression pattern of potassium channels in T cells is subset specific emphasizes the potential that these proteins have in immunomodulation. Compounds that could suppress T cells involved in autoimmunity without affecting T cells in normal immune responses would be of enormous value. In this paper the basic properties of these channels and compounds known to influence their operation are reviewed.

Selective blockade of the intermediate-conductance Ca2+-activated K+ channel suppresses proliferation of microvascular and macrovascular endothelial cells and angiogenesis in vivo

OBJECTIVE

Ca2+-activated K+ (K(Ca)) channels have been proposed to promote mitogenesis in several cell types. Here, we tested whether the intermediate-conductance K(Ca) channel (IKCa1) and the large-conductance K(Ca) channel (BK(Ca)) contribute to endothelial cell (EC) proliferation and angiogenesis.

MATERIAL AND RESULTS

Function and expression of IKCa1 and BK(Ca)/Slo were investigated by patch-clamp analysis and real-time RT-PCR in human umbilical vein ECs (HUVECs) and in dermal human microvascular ECs 1 (HMEC-1). HMEC-1 expressed IKCa1 and BK(Ca)/Slo, whereas HUVECs expressed IKCa1. A 48-hour exposure to basic fibroblast growth factor (bFGF) augmented IKCa1 current amplitudes and induced a 3-fold increase in IKCa1 mRNA expression in HUVECs and HMEC-1. Vascular endothelial growth factor (VEGF) was also effective in upregulating IKCa1. BK(Ca)/Slo expression and current amplitudes in HMEC-1 were not altered by bFGF. bFGF- and VEGF-induced EC proliferation was suppressed by charybdotoxin, clotrimazole, or the selective IKCa1 blocker 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), whereas inhibition of BK(Ca)/Slo by iberiotoxin was ineffective. In the Matrigel plug assay in mice, administration of TRAM-34 for 2 weeks significantly suppressed angiogenesis by approximately 85%.

CONCLUSIONS

bFGF and VEGF upregulate expression of IKCa1 in human ECs. This upregulation of IKCa1 seems to be required for mitogen-induced EC proliferation and angiogenesis in vivo. Selective IKCa1 blocker might be of therapeutic value to prevent tumor angiogenesis.

pH dependence of extracellular calcium sensing receptor activity determined by a novel technique

BACKGROUND

Increasing evidence points to the role of the extracellular Calcium Sensing Receptor (CaSR) as a multimodal receptor responding to diverse physiologic stimuli, such as extracellular divalent and polyvalent cations, amino acids, and ionic strength. Within the kidney, these stimuli converge on the CaSR to coordinate systemic calcium and water homeostasis. In this process, the impact of urinary pH changes on the activity of the CaSR has not yet been defined. We therefore performed the present study to analyze the pH sensitivity of the CaSR.

METHODS

To assess the activation state of the CaSR, we developed a new method based on the functional coupling between CaSR activity and gating of calcium sensitive potassium currents mediated by SK4 potassium channels. Two-electrode voltage clamping was used to determine whole cell currents in Xenopus oocytes heterologously expressing rat CaSR and rat SK4 potassium channels.

RESULTS

Coexpression of CaSR and SK4 gave rise to potassium currents that were dependent on CaSR-mediated intracellular calcium release, and thereby corresponded to the activation state of the CaSR. In presence of extracellular calcium, ambient alkalinization above pH 7.5 increased CaSR activity. Evaluation of the CaSR calcium sensitivity at various ambient proton concentrations revealed that this effect was due to a sensitization of the CaSR towards extracellular calcium.

CONCLUSION

Coexpression with SK4 potassium channels provides a fast and sensitive approach to evaluate CaSR activity in Xenopus oocytes. As disclosed by this novel technique, CaSR activity is regulated by extracellular pH.

Anuroctoxin, a New Scorpion Toxin of the α-KTx 6 Subfamily, Is Highly Selective for Kv1.3 over IKCa1 Ion Channels of Human T Lymphocytes

The physiological function of T lymphocytes can be modulated selectively by peptide toxins acting on Kv1.3 K(+) channels. Because Kv1.3-specific peptide toxins are considered to have a significant therapeutic potential in the treatment of autoimmune diseases, the discovery of new toxins is highly motivated. Through chromatographic procedures and electrophysiological assays, using patch-clamp methodology, the isolation of a novel peptide named anuroctoxin was accomplished using the venom of the Mexican scorpion Anuroctonus phaiodactylus. It has 35 amino acid residues with a molecular weight of 4082.8, tightly bound by four disulfide bridges whose complete covalent structure was determined. It has a pyroglutamic acid at the N-terminal region and an amidated C-terminal residue. Sequence comparison and phylogenetic clustering analysis classifies anuroctoxin into subfamily 6 of the alpha-KTx scorpion toxins (systematic name, alpha-KTx 6.12). Patch-clamp experiments show that anuroctoxin is a high-affinity blocker of Kv1.3 channels of human T lymphocytes with a K(d) of 0.73 nM, and it does not block the Ca(2+)-activated IKCa1 K(+) channels. These two channels play different but important roles in T-lymphocyte activation. Furthermore, the toxin practically does not inhibit Shaker IR, mKv1.1, and rKv2.1 channels, whereas the affinity of anuroctoxin for hKv1.2 is almost an order of magnitude smaller than for Kv1.3. The pharmacological profile and the selectivity of this new toxin for Kv1.3 over IKCa1 may provide an important tool for the modulation of the immune system, especially in cases in which selective inhibition of Kv1.3 is required.

Selective intermediate-/small-conductance calcium-activated potassium channel (KCNN4) blockers are potent and effective therapeutics in experimental brain oedema and traumatic brain injury caused by acute subdural haematoma

Early deterioration and death after brain injury is often the result of oedema in the injured and peri-lesional tissue. So far, no pharmacotherapy is available that exhibits significant brain oedema-reducing efficacy in patients. We selected two low molecular weight compounds from different chemical classes, a triazole (1-[(2-chlorophenyl)diphenylmethyl]-1,2,3-triazole) and a cyclohexadiene (methyl 4-[4-chloro-3-(trifluoromethyl)phenyl]-6-methyl-3-oxo-1,4,7-tetrahydroisobenzofuran-5-carboxylate) to characterize their pharmacological properties on KCNN4 channels (intermediate/small conductance calcium-activated potassium channel, subfamily N, member 4) in vitro as well as in vivo. In vitro we replaced potassium by rubidium (Rb+) and determined Rb+ fluxes evoked by 10 micro m of the calcium ionophore A23187 on C6BU1 rat glioma cells. Compared with known KCNN4 blockers, such as clotrimazole (IC50=360 +/- 12 nm) and charybdotoxin (IC50=3.3 +/- 1.9 nm), the triazole and cyclohexadiene were considerably more potent than clotrimazole and displayed similar potencies (IC50=12.1 +/- 8.8 and 13.3 +/- 4.7 nm, respectively). In the rat acute subdural haematoma model, both the triazole and cyclohexadiene displayed reduction of brain water content (-26% at 0.3 mg/kg and -24% at 0.01 mg/kg) and reduction of the intracranial pressure (-46% at 0.1 mg/kg and -60% at 0.003 mg/kg) after 24 h when administered as a 4-h infusion immediately after brain injury. When infarct volumes were determined after 7 days, the triazole as well as the cyclohexadiene displayed strong neuroprotective efficacy (-52% infarct volume reduction at 1.2 mg/kg and -43% at 0.04 mg/kg, respectively). It is concluded that blockade of KCNN4 channels is a new pharmacological approach to attenuate acute brain damage caused by traumatic brain injury.

RSK4 and PAK5 Are Novel Candidate Genes in Diabetic Rat Kidney and Brain

The orphan hepatic nuclear factor (HNF) HNF4alpha is of pivotal importance for liver development and hepatocellular differentiation and plays an essential role in a regulatory circuitry to control a wide range of metabolic processes. It also targets genes in other organs, including pancreas, kidney, intestine, and colon; promotes expression of an epithelial phenotype; triggers de novo formation of functional tight junctions; and contributes to epithelial cell polarity. In particular, HNF4alpha dysfunction leads to metabolic disorders, including diabetes. We used the chromatin immunoprecipitation (ChIP) cloning procedure and a bioinformatic approach to search for candidate genes associated with impaired liver, pancreas, and kidney function. We identified two novel targets regulated by HNF4alpha, which participate in the control, at least in part, in cell-cycle regulation and are members of the mitogen-activated kinase pathway. In multiple ChIP assays, ribosomal S6 kinase 4 (RSK4) and p21-activated kinase 5 (PAK5) were confirmed, and in vitro binding of HNF4alpha was evidenced by electrophoretic mobility shift assays (EMSA) using oligonucleotides, which harbor novel binding sites. We also used EMSA to probe for binding sites in promoters of HNF1alpha, apolipoprotein B, alpha1-antitrypsin, and angiotensinogen. We further studied RSK4 and PAK5 kinase expression in streptozotocin-induced diabetic rat kidney and brain and observed significant repression of HNF4alpha, RSK4, and PAK5 as determined by quantitative real-time reverse transcriptase-polymerase chain reaction. RSK4 and PAK5 may provide a molecular rationale for late-stage complications in disease, and further studies are warranted to explore these targets for the treatment of diabetic nephro- and neuropathy, frequently seen in patients with HNF4alpha dysfunction.

Hypotonic stress activates an intermediate conductance K+ channel in human colonic crypt cells

AIM

To investigate the effect of hypotonic stress on human colonic crypts cells in terms of ion channel activity and intracellular Ca2+ concentration.

METHODS

Single crypts were isolated from biopsies taken during colonoscopy. The patch clamp technique was used (in the cell-attached mode) to observe the activity of ion channels during hypotonic stress. Calcium measurements were made using the fluophores Fluo 3 or 4.

RESULTS

The intermediate conductance (29 pS), Ca2+ -sensitive, K+ channel (also known as KCNN4) previously described (Sandle et al. 1994) was seen in 54 of 149 patches (36%) when the crypts were bathed in normal extracellular solution (290 mOsm kg(-1)). Forty-one patches could be used for further analysis. Activation of one or several 29 pS channels was seen in 15 of 41 patches (39%) after 30 s to 4 min of exposure to hypotonic solution (160 mOsm kg(-1)). The open probability increased from 0.0043 in control solution to 0.44 at 5 min of hypotonic stress. When the crypts were exposed to hypotonic solution, an increase in intracellular Ca2+ could be seen. The increase in intracellular Ca2+ emanates mainly from intracellular stores.

CONCLUSION

The 29 pS K+ channel takes part in volume regulation in human colonic crypt cells. The activation of this channel is mediated through an increase in intracellular Ca2+.

Activation of human IK and SK Ca2+ -activated K+ channels by NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime)

We have identified and characterized the compound NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime) as a potent activator of human Ca2+ -activated K+ channels of SK and IK types, whereas it is devoid of effect on BK type channels. IK- and SK-channels have previously been reported to be activated by the benzimidazolinone, 1-EBIO and more potently by its dichloronated-analogue, DC-EBIO. NS309 is at least 1000 times more potent than 1-EBIO and at least 30 times more potent than DC-EBIO when the compounds are compared on the same cell.

Physiological Roles of the Intermediate Conductance, Ca2+-activated Potassium Channel Kcnn4

Three broad classes of Ca(2+)-activated potassium channels are defined by their respective single channel conductances, i.e. the small, intermediate, and large conductance channels, often termed the SK, IK, and BK channels, respectively. SK channels are likely encoded by three genes, Kcnn1-3, whereas IK and most BK channels are most likely products of the Kcnn4 and Slo (Kcnma1) genes, respectively. IK channels are prominently expressed in cells of the hematopoietic system and in organs involved in salt and fluid transport, including the colon, lung, and salivary glands. IK channels likely underlie the K(+) permeability in red blood cells that is associated with water loss, which is a contributing factor in the pathophysiology of sickle cell disease. IK channels are also involved in the activation of T lymphocytes. The fluid-secreting acinar cells of the parotid gland express both IK and BK channels, raising questions about their particular respective roles. To test the physiological roles of channels encoded by the Kcnn4 gene, we constructed a mouse deficient in its expression. Kcnn4 null mice were of normal appearance and fertility, their parotid acinar cells expressed no IK channels, and their red blood cells lost K(+) permeability. The volume regulation of T lymphocytes and erythrocytes was severely impaired in Kcnn4 null mice but was normal in parotid acinar cells. Despite the loss of IK channels, activated fluid secretion from parotid glands was normal. These results confirm that IK channels in red blood cells, T lymphocytes, and parotid acinar cells are indeed encoded by the Kcnn4 gene. The role of these channels in water movement and the subsequent volume changes in red blood cells and T lymphocytes is also confirmed. Surprisingly, Kcnn4 channels appear to play no required role in fluid secretion and regulatory volume decrease in the parotid gland.

Intermediate-conductance calcium-activated potassium channels in enteric neurones of the mouse: pharmacological, molecular and immunochemical evidence for their role in mediating the slow afterhyperpolarization

Calcium-activated potassium channels are critically important in modulating neuronal cell excitability. One member of the family, the intermediate-conductance potassium (IK) channel, is not thought to play a role in neurones because of its predominant expression in non-excitable cells such as erythrocytes and lymphocytes, in smooth muscle tissues, and its lack of apparent expression in brain. In the present study, we demonstrate that IK channels are localized on specific neurones in the mouse enteric nervous system where they mediate the slow afterhyperpolarization following an action potential. IK channels were localized by immunohistochemistry on intrinsic primary afferent neurones, identified by their characteristic Dogiel type II morphology. The slow afterhyperpolarization recorded from these cells was abolished by the IK channel blocker clotrimazole. RT-PCR and western analysis of extracts from the colon revealed an IK channel transcript and protein identical to the IK channel expressed in other cell types. These results indicate that IK channels are expressed in neurones where they play an important role in modulating firing properties.

Voltage dependence of ATP-dependent K+ current in rat cardiac myocytes is affected by IK1 and IK(ACh)

In this study we have investigated the voltage dependence of ATP-dependent K+ current (I(K(ATP))) in atrial and ventricular myocytes from hearts of adult rats and in CHO cells expressing Kir6.2 and SUR2A. The current-voltage relation of 2,4-dinitrophenole (DNP) -induced I(K(ATP)) in atrial myocytes and expressed current in CHO cells was linear in a voltage range between 0 and -100 mV. In ventricular myocytes, the background current-voltage relation of which is dominated by a large constitutive inward rectifier (I(K1)), the slope conductance of I(K(ATP)) was reduced at membrane potentials negative to E(K) (around -50 mV), resulting in an outwardly rectifying I-V relation. Overexpression of Kir2.1 by adenoviral gene transfer, a subunit contributing to I(K1) channels, in atrial myocytes resulted in a large I(K1)-like background current. The I-V relation of I(K(ATP)) in these cells showed a reduced slope conductance negative to E(K) similar to ventricular myocytes. In atrial myocytes with an increased background inward-rectifier current through Kir3.1/Kir3.4 channels (I(K(ACh))), irreversibly activated by intracellular loading with GTP-gamma-S, the I-V relation of I(K(ATP)) showed a reduced slope negative to E(K), as in ventricular myocytes and atrial myocytes overexpressing Kir2.1. It is concluded that the voltage dependencies of membrane currents are not only dependent on the molecular composition of the charge-carrying channel complexes but can be affected by the activity of other ion channel species. We suggest that the interference between inward I(K(ATP)) and other inward rectifier currents in cardiac myocytes reflects steady-state changes in K+ driving force due to inward K+ current.

The K+ channel iKCA1 potentiates Ca2+ influx and degranulation in human lung mast cells

BACKGROUND

Human lung and blood-derived mast cells express a Ca2+-activated K+ channel (KCA) that has electrophysiological properties resembling the intermediate conductance KCA (iKCA1). This channel is predicted to enhance IgE-dependent mast cell responses.

OBJECTIVE

To confirm the identity of this channel as iKCA1 in human lung mast cells and to examine the effect of an iKCA1 opener, 1-ethyl-2-benzimidazolinone (1-EBIO), on Ca2+ influx and degranulation after IgE-dependent activation.

METHODS

iKCA1 expression was examined by using RT-PCR. Ion currents were measured by using the patch clamp technique in human peripheral blood-derived mast cells, freshly isolated human lung mast cells (HLMCs), and long-term cultured HLMCs (LTHLMCs). Currents were manipulated with the specific iKCA1 opener 1-EBIO and the iKCA1 blockers clotrimazole and TRAM-34. Ratiometric Ca2+ imaging was performed on single fura-2-loaded cells, and histamine release was measured by radioenzymatic assay.

RESULTS

Both fresh HLMCs and LTHLMCs expressed iKCA1 mRNA. The iKCA1 opener 1-EBIO induced iKCA1 currents in 89% of human peripheral blood-derived mast cells, 12% of fresh HLMCs, and 67% of LTHLMCs, which were blocked by the iKCA1 blockers clotrimazole and TRAM-34. After cell activation with a suboptimal concentration of anti-IgE, 1-EBIO enhanced the IgE-dependent rise in cytosolic-free Ca2+ and potentiated IgE-dependent histamine release.

CONCLUSION

Opening of iKCA1 enhances IgE-dependent Ca2+ influx and histamine release in HLMCs. Inhibition of iKCA1 may provide a novel approach to the treatment of mast cell-mediated disease.

Block of maurotoxin and charybdotoxin on human intermediate-conductance calcium-activated potassium channels (hIKCa1)

Using human intermediate-conductance calcium-activated potassium (hIKCa1) channels as a model we aimed to characterize structural differences between maurotoxin (MTX) and charybdotoxin (CTX) and to gain new insights into the molecular determinants that define the interaction of these pore-blocking peptides with hIKCa1 channel. We report here that the block of MTX, but not of CTX on current through hIKCa1 channels is pH0 dependent. The replacement of histidine 236 from hIKCa1 channel with a smaller amino acid, cystein, did not change MTX binding affinity, however, partially affected the pH0 dependency of its block at low pH0. In contrast, CTX binding affinity to the hIKCa1_H236C channel mutant was increased suggesting that His236 might play a role in the binding of CTX, but has only a weak influence in the binding of MTX to hIKCa1 channels.

Distribution of dehydration rates generated by maximal Gardos-channel activation in normal and sickle red blood cells

The Ca(2+)-activated K+ channels of human red blood cells (RBCs) (Gardos channels, hIK1, hSK4) can mediate rapid cell dehydration, of particular relevance to the pathophysiology of sickle cell disease. Previous investigations gave widely discrepant estimates of the number of Gardos channels per RBC, from as few as 1 to 3 to as many as 300, with large cell-to-cell differences, suggesting that RBCs could differ extensively in their susceptibility to dehydration by elevated Ca2+. Here we investigated the distribution of dehydration rates induced by maximal and uniform Ca2+ loads in normal (AA) and sickle (SS) RBCs by measuring the time-dependent changes in osmotic fragility and RBC volume distributions. We found a remarkable conservation of osmotic lysis and volume distribution profiles during Ca(2+)-induced dehydration, indicating overall uniformity of dehydration rates among AA and SS RBCs. In light of these results, alternative interpretations were suggested for the previously proposed low estimates and heterogeneity of channel numbers per cell. The results support the view that stochastic Ca2+ permeabilization rather than Gardos-channel variation is the main determinant selecting which SS cells dehydrate through Gardos channels in each sickling episode.

K+ channel expression during B cell differentiation: implications for immunomodulation and autoimmunity

Using whole-cell patch-clamp, fluorescence microscopy and flow cytometry, we demonstrate a switch in potassium channel expression during differentiation of human B cells from naive to memory cells. Naive and IgD(+)CD27(+) memory B cells express small numbers of the voltage-gated Kv1.3 and the Ca(2+)-activated intermediate-conductance IKCa1 channel when quiescent, and increase IKCa1 expression 45-fold upon activation with no change in Kv1.3 levels. In contrast, quiescent class-switched memory B cells express high levels of Kv1.3 ( approximately 2000 channels/cell) and maintain their Kv1.3(high) expression after activation. Consistent with their channel phenotypes, proliferation of naive and IgD(+)CD27(+) memory B cells is suppressed by the specific IKCa1 inhibitor TRAM-34 but not by the potent Kv1.3 blocker Stichodactyla helianthus toxin, whereas the proliferation of class-switched memory B cells is suppressed by Stichodactyla helianthus toxin but not TRAM-34. These changes parallel those reported for T cells. Therefore, specific Kv1.3 and IKCa1 inhibitors may have use in therapeutic manipulation of selective lymphocyte subsets in immunological disorders.

Mapping of Maurotoxin Binding Sites on hKv1.2, hKv1.3, and hIKCa1 Channels

Maurotoxin (MTX) is a potent blocker of human voltage-activated Kv1.2 and intermediate-conductance calcium-activated potassium channels, hIKCa1. Because its blocking affinity on both channels is similar, although the pore region of these channels show only few conserved amino acids, we aimed to characterize the binding sites of MTX in these channels. Investigating the pH(o) dependence of MTX block on current through hKv1.2 channels, we concluded that the block is less pH(o) - sensitive than for hIKCa1 channels. Using mutant cycle analysis and computer docking, we tried to identify the amino acids through which MTX binds to hKv1.2 and hIKCa1 channels. We report that MTX interacts with hKv1.2 mainly through six strong interactions. Lys(23) from MTX protrudes into the channel pore interacting with the GYGD motif, whereas Tyr(32) and Lys(7) interact with Val(381), Asp(363), and Glu(355), stabilizing the toxin onto the channel pore. Because only Val(381), Asp(363), and the GYGD motif are conserved in hIKCa1 channels, and the replacement of His(399) from hKv1.3 channels with a threonine makes this channel MTX-sensitive, we concluded that MTX binds to all three channels through the same amino acids. Glu(355), although important, is not essential in MTX recognition. A negatively charged amino acid in this position could better stabilize the toxin-channel interaction and could explain the pH(o) sensitivity of MTX block on current through hIKCa1 versus hKv1.2 channels.

Characterization of the outer pore region of the apamin-sensitive Ca2+-activated K+ channel rSK2

We have studied the interaction between the SK2 channel and different scorpion toxins in order to find similarity and differences to other K+ channels. Beside apamin, ScTX is a high affinity blocker of the SK2 channel, whereas CTX is unable to block current through SK2. In order to prove that the ScTX affinity can be explained by the character of the different residues in the outer pore of the SK channels we introduced point mutations that render SK2 K+ channel SK1 and SK3 like. Directed by the results of the toxin receptor on the ShakerK+ channel, we changed single amino acids of the SK2 K+ channel that should render it sensitive to other peptide toxins like CTX a blocker of the IK channel, or KTX a blocker of the voltage-dependent channel Kv1.1 and Kv1.3. Amino acids V342G, S344E, and G384D of SK2 were changed to amino acids known from ShakerK+ channel to improve Shaker K+ channel CTX sensitivity. Interestingly SK2 V342G became CTX sensitive with a Kd of 19 nM and was also KTX sensitive Kd=97 nM. SK2 S344E (KdCTX = 105 nM,KdKTX = 144 nM) and G348D (KdCTX = 31 nM,Kd KTX = 89 nM) became also CTX and KTX sensitive with a lower affinity. The mutant channels SK V342G, SK2 S344E and SK2 G348D showed reduced ScTX sensitivity (Kd = 6 nM,Kd = 48 nM, and Kd = 12 nM). Because the exchange of a single residue could create a new high affinity binding site for CTX and KTX we concluded that the outer vestibule around position V342, S344, and G348 of the SK2 K+ channel pore is very similar to those of voltage-gated K+ channels such as the Shaker K+ channel, Kv1.1 and Kv1.3 channels and also to the prokaryotic KcsA channel. From mutant cycle analysis of KTX position H34 and SK2 position V342G, S344E, and G348D we could deduce that KTX binds in a similar way to SK2 channel mutant pore than to the Kv1.1 pore. We have studied the interaction between the SK2 channel and different scorpion toxins in order to find similarity and differences to other K+ channels. Beside apamin, ScTX is a high affinity blocker of the SK2 channel, whereas CTX is unable to block current through SK2. In order to prove that the ScTX affinity can be explained by the character of the different residues in the outer pore of the SK channels we introduced point mutations that render SK2 K+ channel SK1 and SK3 like. Directed by the results of the toxin receptor on the ShakerK+ channel, we changed single amino acids of the SK2 K+ channel that should render it sensitive to other peptide toxins like CTX a blocker of the IK channel, or KTX a blocker of the voltage-dependent channel Kv1.1 and Kv1.3. Amino acids V342G, S344E, and G384D of SK2 were changed to amino acids known from ShakerK+ channel to improve Shaker K+ channel CTX sensitivity. Interestingly SK2 V342G became CTX sensitive with a Kd of 19 nM and was also KTX sensitive Kd = 97 nM. SK2 S344E (KdCTX = 105 nM,KdKTX = 144 nM) and G348D (KdCTX = 31 nM,Kd KTX = 89 nM) became also CTX and KTX sensitive with a lower affinity. The mutant channels SK V342G, SK2 S344E and SK2 G348D showed reduced ScTX sensitivity (Kd = 6 nM,Kd = 48 nM, and Kd = 12 nM). Because the exchange of a single residue could create a new high affinity binding site for CTX and KTX we concluded that the outer vestibule around position V342, S344, and G348 of the SK2 K+ channel pore is very similar to those of voltage-gated K+ channels such as the Shaker K+ channel, Kv1.1 and Kv1.3 channels and also to the prokaryotic KcsA channel. From mutant cycle analysis of KTX position H34 and SK2 position V342G, S344E, and G348D we could deduce that KTX binds in a similar way to SK2 channel mutant pore than to the Kv1.1 pore.

4-Chloro-benzo[F]isoquinoline (CBIQ) activates CFTR chloride channels and KCNN4 potassium channels in Calu-3 human airway epithelial cells

1 Calu-3 cells have been used to investigate the actions of 4-chloro-benzo[F]isoquinoline (CBIQ) on short-circuit current (SCC) in monolayers, whole-cell recording from single cells and by patch clamping. 2 CBIQ caused a sustained, reversible and repeatable increase in SCC in Calu-3 monolayers with an EC50 of 4.0 microm. Simultaneous measurements of SCC and isotopic fluxes of 36Cl- showed that CBIQ caused electrogenic chloride secretion. 3 Apical membrane permeabilisation to allow recording of basolateral membrane conductance in the presence of a K+ gradient suggested that CBIQ activated the intermediate-conductance calcium-sensitive K(+)-channel (KCNN4). Permeabilisation of the basolateral membranes of epithelial monolayers in the presence of a Cl- gradient suggested that CBIQ activated the Cl(-)-channel CFTR in the apical membrane. 4 Whole-cell recording in the absence of ATP/GTP of Calu-3 cells showed that CBIQ generated an inwardly rectifying current sensitive to clotrimazole. In the presence of the nucleotides, a more complex I/V relation was found that was partially sensitive to glibenclamide. The data are consistent with the presence of both KCNN4 and CFTR in Calu-3. 5 Isolated inside-out patches from Calu-3 cells revealed clotrimazole-sensitive channels with a conductance of 12 pS at positive potentials after activation with CBIQ and demonstrating inwardly rectifying properties, consistent with the known properties of KCNN4. Cell-attached patches showed single channel events with a conductance of 7 pS and a linear I/V relation that were further activated by CBIQ by an increase in open state probability, consistent with known properties of CFTR. It is concluded that CBIQ activates CFTR and KCNN4 ion channels in Calu-3 cells.

The human red cell voltage-regulated cation channel. The interplay with the chloride conductance, the Ca(2+)-activated K(+) channel and the Ca(2+) pump

The activation/deactivation kinetics of the human erythrocyte voltage-dependent cation channel was characterized at the single-channel level using inside-out patches. It was found that the time dependence for voltage activation after steps to positive membrane potentials was slow ( t(1/2) about 30 s), whereas the deactivation was fast ( t(1/2) about 15 ms). Both activation and deactivation of this channel were also demonstrated in intact red cells in suspension. At very positive membrane potentials generated by suspension in extracellular low Cl(-) concentrations, the cation conductance switched on with a time constant of about 2 min. Deactivation of the cation channel was clearly demonstrated during transient activation of the Gárdos channel elicited by Ca(2+) influx via the cation channel and ensuing efflux via the Ca(2+) pump. Thus, the voltage-dependent cation channel, the Gárdos channel and the Ca(2+) pump constitute a coupled feedback-regulated system that may become operative under physiological conditions.

Modulation of Gardos channel activity by oxidants and oxygen tension: effects of 1-chloro-2,4-dinitrobenzene and phenazine methosulphate

We compare the effects of 1-chloro-2,4-dinitrobenzene (CDNB) and phenazine methosulphate (PMS) on Gardos channel activity in normal human red cells. Both stimulate channel activity, both are dependent on the presence of extracellular Ca2+, and neither is affected by inhibitors of protein (de)phosphorylation. Of the two, PMS has a considerably greater effect. In addition, a major difference is that whilst CDNB has a greater stimulatory effect in oxygenated cells, by contrast, PMS is more effective in deoxygenated cells. These actions are correlated with ca. 30% inhibition of the plasma membrane Ca2+ pump (PMCA) and an increased sensitivity of the Gardos channel to Ca2+ (EC50 falling to about 150 nM). These findings are important in understanding how oxidants alter red cell cation permeability and may be relevant to the abnormal permeability phenotype shown by deoxygenated sickle cells.

Pharmacology of the human red cell voltage-dependent cation channel

The activation and pharmacological modulation of the nonselective voltage-dependent cation (NSVDC) channel from human erythrocytes were studied. Basic channel activation was achieved by suspending red cells in a low Cl(-) Ringer (2 mM), where a positive membrane potential (V(m) = E(Cl)) immediately developed. Voltage- and time-dependent activation of the NSVDC channel occurred, reaching a cation conductance (g+) of 1.5-2.0 microS cm(-2). In the presence of the classical Gárdos channel blocker clotrimazole (0-50 microM), activation occurred faster, and g+ saturated dose-dependently (EC50 = 14 microM) at a value of about 4 microS cm(-2). The clotrimazole analogues TRAM-34, econazole, and miconazole also stimulated the channel, whereas the chemically more distant Gárdos channel inhibitors nitrendipine and cetiedil had no effects. Although the potency for modulation of the NSVDC channel is much lower than the IC50 value for Gárdos channel inhibition, clotrimazole (and its analogues) constitutes the first chemical class of positive modulators of the NSVDC channel. This may be an important pharmacological "fingerprint" in the identification of the cloned equivalent of the erythrocyte channel.

Modulation of calcium-dependent chloride secretion by basolateral SK4-like channels in a human bronchial cell line

The human bronchial cell line16HBE14o- was used as a model of airway epithelial cells to study the Ca(2+)-dependent Cl(-) secretion and the identity of K(Ca) channels involved in the generation of a favorable driving force for Cl(-) exit. After ionomycin application, a calcium-activated short-circuit current ( I(sc)) developed, presenting a transient peak followed by a plateau phase. Both phases were inhibited to different degrees by NFA, glybenclamide and NPPB but DIDS was only effective on the peak phase. (86)Rb effluxes through both apical and basolateral membranes were stimulated by calcium, blocked by charybdotoxin, clotrimazole and TPA. 1-EBIO, a SK-channel opener, stimulated (86)Rb effluxes. Block of basolateral K(Ca) channels resulted in I(sc) inhibition but, while reduced, I(sc) was still observed if mucosal Cl(-) was lowered. Among SK family members, only SK4 and SK1 mRNAs were detected by RT-PCR. KCNQ1 mRNAs were also identified, but involvement of K(cAMP) channels in Cl(-) secretion was unlikely, since cAMP application had no effect on (86)Rb effluxes. Moreover, chromanol 293B or clofilium, specific inhibitors of KCNQ1 channels, had no effect on cAMP-dependent I(sc). In conclusion, two distinct components of Cl(-) secretion were identified by a pharmacological approach after a Cai2+ rise. K(Ca) channels presenting the pharmacology of SK4 channels are present on both apical and basolateral membranes, but it is the basolateral SK4-like channels that play a major role in calcium-dependent chloride secretion in 16HBE14o- cells.

Functional and molecular identification of intermediate-conductance Ca(2+)-activated K(+) channels in breast cancer cells: association with cell cycle progression

We have previously reported that the hEAG K(+) channels are responsible for the potential membrane hyperpolarization that induces human breast cancer cell progression into the G1 phase of the cell cycle. In the present study, we evaluate the role and functional expression of the intermediate-conductance Ca(2+)-activated K(+) channel, hIK1-like, in controlling cell cycle progression. Our results demonstrate that hIK1 current density increased in cells synchronized at the end of the G1 or S phase compared with those in the early G1 phase. This increased current density paralleled the enhancement in hIK1 mRNA levels and the highly negative membrane potential. Furthermore, in cells synchronized at the end of G1 or S phases, basal cytosolic Ca(2+) concentration ([Ca(2+)](i)) was also higher than in cells arrested in early G1. Blocking hIK1 channels with a specific blocker, clotrimazole, induced both membrane potential depolarization and a decrease in the [Ca(2+)](i) in cells arrested at the end of G1 and S phases but not in cells arrested early in the G1 phase. Blocking hIK1 with clotrimazole also induced cell proliferation inhibition but to a lesser degree than blocking hEAG with astemizole. The two drugs were essentially additive, inhibiting MCF-7 cell proliferation by 82% and arresting >90% of cells in the G1 phase. Thus, although the progression of MCF-7 cells through the early G1 phase is dependent on the activation of hEAG K(+) channels, when it comes to G1 and checkpoint G1/S transition, the membrane potential appears to be primarily dependent on the hIK1-activity level.

SK3-1C, a Dominant-negative Suppressor of SKCa and IKCa Channels

Small conductance Ca2+-activated K+ channels, products of the SK1-SK3 genes, regulate membrane excitability both within and outside the nervous system. We report the characterization of a SK3 variant (SK3-1C) that differs from SK3 by utilizing an alternative first exon (exon 1C) in place of exon 1A used by SK3, but is otherwise identical to SK3. Quantitative RT-PCR detected abundant expression of SK3-1C transcripts in human lymphoid tissues, skeletal muscle, trachea, and salivary gland but not the nervous system. SK3-1C did not produce functional channels when expressed alone in mammalian cells, but suppressed SK1, SK2, SK3, and IKCa1 channels, but not BKCa or KV channels. Confocal microscopy revealed that SK3-1C sequestered SK3 protein intracellularly. Dominant-inhibitory activity of SK3-1C was not due to a nonspecific calmodulin sponge effect since overexpression of calmodulin did not reverse SK3-1C-mediated intracellular trapping of SK3 protein, and calmodulin-Ca2+-dependent inactivation of CaV channels was not affected by SK3-1C overexpression. Deletion analysis identified a dominant-inhibitory segment in the SK3-1C C terminus that resembles tetramerization-coiled-coiled domains reported to enhance tetramer stability and selectivity of multimerization of many K+ channels. SK3-1C may therefore suppress calmodulin-gated SKCa/IKCa channels by trapping these channel proteins intracellularly via subunit interactions mediated by the dominant-inhibitory segment and thereby reduce functional channel expression on the cell surface. Such family-wide dominant-negative suppression by SK3-1C provides a powerful mechanism to titrate membrane excitability and is a useful approach to define the functional in vivo role of these channels in diverse tissues by their targeted silencing.

Role of the NH2 terminus in the assembly and trafficking of the intermediate conductance Ca2+-activated K+ channel hIK1

The role of the NH(2)-terminal leucine zipper and dileucine motifs of hIK1 in the assembly, trafficking, and function of the channel was investigated using cell surface immunoprecipitation, co-immunoprecipitation (Co-IP), immunoblot, and whole-cell patch clamp techniques. Mutation of the NH(2)-terminal leucine zipper at amino acid positions 18 and 25 (L18A/L25A) resulted in a complete loss of steady-state protein expression, cell surface expression, and whole-cell current density. Inhibition of proteasomal degradation with lactacystin restored L18A/L25A protein expression, although this channel was not expressed at the cell surface as assessed by cell surface immunoprecipitation and whole-cell patch clamp. In contrast, inhibitors of lysosomal degradation (leupeptin/pepstatin) and endocytosis (chloroquine) had little effect on L18A/L25A protein expression or localization. Further studies confirmed the rapid degradation of this channel, having a time constant of 19.0 +/- 1.3 min compared with 3.2 +/- 0.8 h for wild type hIK1. Co-expression studies demonstrated that the L18A/L25A channel associates with wild type channel, thereby attenuating its expression at the cell surface. Co-IP studies confirmed this association. However, L18A/L25A channels failed to form homotetrameric channels, as assessed by Co-IP, suggesting the NH(2) terminus plays a role in tetrameric channel assembly. As with the leucine zipper, mutation of the dileucine motif to alanines, L18A/L19A, resulted in a near complete loss in steady-state protein expression with the protein being similarly targeted to the proteasome for degradation. In contrast to our results on the leucine zipper, however, both chloroquine and growing the cells at the permissive temperature of 27 degrees C restored expression of L18A/L19A at the cell surface, suggesting that the defect in the channel trafficking is the result of a subtle folding error. In conclusion, we demonstrate that the NH(2) terminus of hIK1 contains overlapping leucine zipper and dileucine motifs essential for channel assembly and trafficking to the plasma membrane.

Role of Ca2+-activated K+ channels in human erythrocyte apoptosis

Exposure of erythrocytes to the Ca2+ ionophore ionomycin has recently been shown to induce cell shrinkage, cell membrane blebbing, and breakdown of phosphatidylserine asymmetry, all features typical of apoptosis of nucleated cells. Although breakdown of phosphatidylserine asymmetry is thought to result from activation of a Ca2+-sensitive scramblase, the mechanism and role of cell shrinkage have not been explored. The present study was performed to test whether ionomycin-induced activation of Ca2+-sensitive Gardos K+ channels and subsequent cell shrinkage participate in ionomycin-induced breakdown of phosphatidylserine asymmetry of human erythrocytes. According to on-cell patch-clamp experiments, ionomycin (1 microM) induces activation of inwardly rectifying K+-selective channels in the erythrocyte membrane. Fluorescence-activated cell sorter analysis reveals that ionomycin leads to a significant decrease of forward scatter, reflecting cell volume, an effect blunted by an increase of extracellular K+ concentration to 25 mM and exposure to the Gardos K+ channel blockers charybdotoxin (230 nM) and clotrimazole (5 microM). As reflected by annexin binding, breakdown of phosphatidylserine asymmetry is triggered by ionomycin, an effect again blunted, but not abolished, by an increase of extracellular K+ concentration and exposure to charybdotoxin (230 nM) and clotrimazole (5 microM). Similar to ionomycin, glucose depletion leads (within 55 h) to annexin binding of erythrocytes, an effect again partially reversed by an increase of extracellular K+ concentration and exposure to charybdotoxin. K-562 human erythroleukemia cells similarly respond to ionomycin with cell shrinkage and annexin binding, effects blunted by antisense, but not sense, oligonucleotides against the small-conductance Ca2+-activated K+ channel isoform hSK4 (KCNN4). The experiments disclose a novel functional role of Ca2+-sensitive K+ channels in erythrocytes, i.e., their participation in regulation of erythrocyte apoptosis.

Effects of fluoranthene, a polycyclic aromatic hydrocarbon, on cAMP-dependent anion secretion in human airway epithelia

The human respiratory tract is constantly exposed to polycyclic aromatic hydrocarbons (PAHs) through inhalation of atmospheric pollutants. We examined the effects of three PAHs (benzo[a]pyrene, anthracene, and fluoranthene) on the airway ion transport, which is essential for lung defense and normal airway function, using human airway epithelia (Calu-3). These three PAHs had no significant effect on the basal short-circuit current (I(sc)). However, fluoranthene (1-100 microM) applied in the apical compartment potentiated I(sc) in response to cAMP-related agents (isoproterenol, forskolin, and 8-bromo-cAMP). The effects of fluoranthene were unaffected by ellipticine, a PAH receptor antagonist. Estimation of the anionic composition of I(sc) revealed that isoproterenol increased both HCO(3)(-) and Cl(-) transport in the control, whereas it potentiated only Cl(-) transport in the presence of fluoranthene. The fluoranthene-induced modulations of these anion transporters were counteracted by charybdotoxin (ChTx, a hIK-1 channel blocker). Fluoranthene gradually augmented the ChTx-sensitive K(+) current (I(K)) across the basolateral membrane, accompanied by a sustained increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)). In the presence of fluoranthene, however, a much larger hIK-1-dependent I(K) was identified by the application of 8-bromo-cAMP without concomitant elevation of [Ca(2+)](i). These results suggest that fluoranthene switches from cAMP-dependent HCO(3)(-) secretion to Cl(-) secretion through the hIK-1 channel, whose sensitivity to protein kinase A may be up-regulated by the sustained [Ca(2+)](i) elevation produced by this chemical.

ATP-dependent regulation of SK4/IK1-like currents in rat submandibular acinar cells: possible role of cAMP-dependent protein kinase

SK4/IK1 encodes an intermediate conductance, Ca2+ -activated K+ channel and fulfills a variety of physiological functions in excitable and nonexcitable cells. Although recent studies have provided evidence for the presence of SK4/IK1 channels in salivary acinar cells, the regulatory mechanisms and the physiological function of the channel remain unknown in these cells. Using molecular and electrophysiological techniques, we examined whether cytosolic ATP-dependent regulation of native SK4/IK1-like channel activity would involve endogenous cAMP-dependent protein kinase (PKA) in rat submandibular acinar (RSA) cells. Electrophysiological properties of tetraethylammonium (TEA) (10 mM)-insensitive, Ca2+ -dependent K+ currents in macropatches excised from RSA cells matched those of whole cell currents recorded from human embryonic kidney-293 cells heterologously expressing rat SK4/IK1 (rSK4/IK1) cloned from RSA cells. In outside-out macropatches, activity of native SK4/IK1-like channels, defined as a charybdotoxin (100 nM)-blockable current in the presence of TEA (10 mM) in the bathing solution, ran down unless both ATP and Mg2+ were present in the pipette solution. The nonhydrolyzable ATP analog AMP-PNP failed to support the channel activity as ATP did. The addition of Rp-cAMPS (10 microM), a PKA inhibitor, to the pipette solution containing ATP/Mg2+ induced a rundown of the Ca2+ -dependent K+ currents. Inclusion of cAMP (1 mM) into the pipette solution (1 microM free Ca2+) containing ATP/Mg2+ caused a gradual increase in the currents, the effect being pronounced for the currents induced by 0.1 microM free Ca2+. Forskolin (1 microM), an adenylyl cyclase activator, also increased the currents induced by 0.1 microM free Ca2+. In inside-out macropatches, cytosolic ATP/Mg2+ increased both the maximum current (proportional to the maximum channel activity) and Ca2+ sensitivity of current activation. Collectively, these results suggest that ATP-dependent regulation of native SK4/IK1-like channels, at least in part, is mediated by endogenous PKA in RSA cells.

Blockade of the intermediate-conductance calcium-activated potassium channel as a new therapeutic strategy for restenosis

BACKGROUND

Angioplasty stimulates proliferation and migration of vascular smooth muscle cells (VSMC), leading to neointimal thickening and vascular restenosis. In a rat model of balloon catheter injury (BCI), we investigated whether alterations in expression of Ca2+-activated K+ channels (KCa) contribute to intimal hyperplasia and vascular restenosis.

METHODS AND RESULTS

Function and expression of KCa in mature medial and neointimal VSMC were characterized in situ by combined single-cell RT-PCR and patch-clamp analysis. Mature medial VSMC exclusively expressed large-conductance KCa (BKCa) channels. Two weeks after BCI, expression of BKCa was significantly reduced in neointimal VSMC, whereas expression of intermediate-conductance KCa (IKCa1) channels was upregulated. In the aortic VSMC cell line, A7r5 epidermal growth factor (EGF) induced IKCa1 upregulation and EGF-stimulated proliferation was suppressed by the selective IKCa1 blocker TRAM-34. Daily in vivo administration of TRAM-34 to rats significantly reduced intimal hyperplasia by approximately 40% at 1, 2, and 6 weeks after BCI. Two weeks of treatment with the related compound clotrimazole was equally effective. Reduction of intimal hyperplasia was accompanied by decreased neointimal cell content, with no change in the rate of apoptosis or collagen content.

CONCLUSIONS

The switch toward IKCa1 expression may promote excessive neointimal VSMC proliferation. Blockade of IKCa1 could therefore represent a new therapeutic strategy to prevent restenosis after angioplasty.

Potassium channels as therapeutic targets for autoimmune disorders

In human T-lymphocytes, the voltage-gated Kv1.3 channel and the Ca(2+)-activated IKCa1 channel play an important role in Ca2+ signaling, activation, adhesion and migration and have, therefore, long been regarded as attractive targets for immunotherapy. Here, we review the pharmacology of the two channels, their expression pattern in naïve and memory T-cells and their functional roles during T-cell activation, and explain the rationale for the use of Kv1.3 blockers for the therapy of T-cell mediated autoimmune diseases.

4-Phenyl-4H-pyrans as IK(Ca) channel blockers

4-Phenyl-4H-pyrans have been identified as potent and specific IK(Ca) channel blockers. Their synthesis and structure-activity relationships are described. A selected derivative, rac-11, reduces the infarct volume in a rat subdural hematoma model of traumatic brain injury after iv administration.

K+ channels and T-cell synapses: the molecular background for efficient immunomodulation is shaping up

Diverse K(+) channels exhibit differential expression and function in human lymphocytes. Voltage-gated and Ca(2+)-activated K(+) channels have a significant impact on both T-cell activation and the overall immune response. A recent study on the plasma membrane organization of Kv1.3 K(+) channels in human T cells provides new insights into the immunoregulatory functions of K(+) channels, which might result in new and improved immune suppression protocols targeted to ion channels and/or T-cell signaling and accessory molecules.

Small-conductance calcium-activated K+ channels are expressed in pancreatic islets and regulate glucose responses

Glucose-stimulated insulin secretion is associated with transients of intracellular Ca(2+) concentration [Ca(2+)](i) in the pancreatic beta-cell. We identified the expression and function of specific small-conductance Ca(2+)-activated K(+) (SK) channel genes in insulin-secreting cells. The presence of mRNA for SK1, -2, -3, and -4 (intermediate-conductance Ca(2+)-activated K(+) 1 [IK1]) channels was demonstrated by RT-PCR in rodent islets and insulinoma cells. SK2 and -3 proteins in mouse islets were detected by immunoblot and immunocytochemistry. In the tTA-SK3 tet-off mouse, a normal amount of SK3 protein was present in islets, but it became undetectable after exposure to doxycycline (DOX), which inhibits the transcription of the tTA-SK3 gene. The SK/IK channel-blockers apamin, dequalinium, and charybdotoxin caused increases in average [Ca(2+)](i) levels and in frequency of [Ca(2+)](i) oscillations in wild-type mouse islets. In SK3-tTA tet-off mice, the addition of apamin with glucose and tetraethylammonium (TEA) caused a similar elevation in [Ca(2+)](i), which was greatly diminished after DOX suppression of SK3 expression. We conclude that SK1, -2, -3, and IK1 (SK4) are expressed in islet cells and insulin-secreting cells and are able to influence glucose-induced calcium responses, thereby regulating insulin secretion.

Identification of transcription coactivator OCA-B-dependent genes involved in antigen-dependent B cell differentiation by cDNA array analyses

The tissue-specific transcriptional coactivator OCA-B is required for antigen-dependent B cell differentiation events, including germinal center formation. However, the identity of OCA-B target genes involved in this process is unknown. This study has used large-scale cDNA arrays to monitor changes in gene expression patterns that accompany mature B cell differentiation. B cell receptor ligation alone induces many genes involved in B cell expansion, whereas B cell receptor and helper T cell costimulation induce genes associated with B cell effector function. OCA-B expression is induced by both B cell receptor ligation alone and helper T cell costimulation, suggesting that OCA-B is involved in B cell expansion as well as B cell function. Accordingly, several genes involved in cell proliferation and signaling, such as Lck, Kcnn4, Cdc37, cyclin D3, B4galt1, and Ms4a11, have been identified as OCA-B-dependent genes. Further studies on the roles played by these genes in B cells will contribute to an understanding of B cell differentiation.

Active K+ secretion through multiple KCa-type channels and regulation by IKCa channels in rat proximal colon

Colonic K+ secretion stimulated by cholinergic agents requires activation of muscarinic receptors and the release of intracellular Ca2+. However, the precise mechanisms by which this rise in Ca2+ leads to K+ efflux across the apical membrane are poorly understood. In the present study, Northern blot analysis of rat proximal colon revealed the presence of transcripts encoding rSK2 [small conductance (SK)], rSK4 [intermediate conductance (IK)], and rSlo [large conductance (BK)] Ca2+-activated K+ channels. In dietary K+-depleted animals, only rSK4 mRNA was reduced in the colon. On the basis of this observation, a cDNA encoding the K+ channel rSK4 was cloned from a rat colonic cDNA library. Transfection of this cDNA into Chinese hamster ovary (CHO) cells led to the expression of Ca2+-activated K+ channels that were blocked by the IK channel inhibitor clotrimazole (CLT). Confocal immunofluorescence confirmed the presence of IK channels in proximal colonic crypts, and Western blotting demonstrated that IK protein sorted to both the apical and basolateral surfaces of colonic epithelia. In addition, transcellular active K+ secretion was studied on epithelial strips of rat proximal colon using unidirectional 86Rb+ fluxes. The addition of thapsigargin or carbachol to the serosal surface enhanced net 86Rb+ secretion. The mucosal addition of CLT completely inhibited carbachol-induced net 86Rb+ secretion. In contrast, only partial inhibition was observed with the BK and SK channel inhibitors, iberiotoxin and apamin, respectively. Finally, in parallel with the reduction in SK4 message observed in animals deprived of dietary K+, carbachol-induced 86Rb+ secretion was abolished in dietary K+-depleted animals. These results suggest that the rSK4 channel mediates K+ secretion induced by muscarinic agonists in the rat proximal colon and that transcription of the rSK4 channel is downregulated to prevent K+ loss during dietary K+ depletion.