Overexpression of beta2-adrenergic receptors cAMP-dependent protein kinase phosphorylates and modulates slow delayed rectifier potassium channels expressed in murine heart: evidence for receptor/channel co-localization

Characterization and functional role of rapid- and slow-activating delayed rectifier K+ currents in atrioventricular node cells of guinea pigs

The atrioventricular (AV) node is the only conduction pathway where electrical impulse can pass from atria to ventricles and exhibits spontaneous automaticity. This study examined the function of the rapid- and slow-activating delayed rectifier K⁺ currents (I_Kr and I_Ks) in the regulation of AV node automaticity. Isolated AV node cells from guinea pigs were current- and voltage-clamped to record the action potentials and the I_Kr and I_Ks current. The expression of I_Kr or I_Ks was confirmed in the AV node cells by immunocytochemistry, and the positive signals of both channels were localized mainly on the cell membrane. The basal spontaneous automaticity was equally reduced by E4031 and HMR-1556, selective blockers of I_Kr and I_Ks, respectively. The nonselective β-adrenoceptor agonist isoproterenol markedly increased the firing rate of action potentials. In the presence of isoproterenol, the firing rate of action potentials was more effectively reduced by the I_Ks inhibitor HMR-1556 than by the I_Kr inhibitor E4031. Both E4031 and HMR-1556 prolonged the action potential duration and depolarized the maximum diastolic potential under basal and β-adrenoceptor-stimulated conditions. I_Kr was not significantly influenced by β-adrenoceptor stimulation, but I_Ks was concentration-dependently enhanced by isoproterenol (EC₅₀: 15 nM), with a significant negative voltage shift in the channel activation. These findings suggest that both the I_Kr and I_Ks channels might exert similar effects on regulating the repolarization process of AV node action potentials under basal conditions; however, when the β-adrenoceptor is activated, I_Ks modulation may become more important.

Insulin acts as an atypical KCNQ1/KCNE1-current activator and reverses long QT in insulin-resistant aged rats by accelerating the ventricular action potential repolarization through affecting the β3 -adrenergic receptor signaling pathway

Insufficient-heart function is associated with myocardial insulin resistance in the elderly, particularly associated with long-QT, in a dependency on dysfunctional KCNQ1/KCNE1-channels. So, we aimed to examine the contribution of alterations in KCNQ1/KCNE1-current (I_Ks ) to the aging-related remodeling of the heart as well as the role of insulin treatment on I_Ks in the aged rats. Prolonged late-phase action potential (AP) repolarization of ventricular cardiomyocytes from insulin-resistant 24-month-old rats was significantly reversed by in vitro treatment of insulin or PKG inhibitor (in vivo, as well) via recovery in depressed I_Ks . Although the protein level of either KCNQ1 or KCNE1 in cardiomyocytes was not affected with aging, PKG level was significantly increased in those cells. The inhibited I_Ks in β₃ -ARs-stimulated cells could be reversed with a PKG inhibitor, indicating the correlation between PKG-activation and β₃ -ARs activation. Furthermore, in vivo treatment of aged rats, characterized by β₃ -ARs activation, with either insulin or a PKG inhibitor for 2 weeks provided significant recoveries in I_Ks , prolonged late phases of APs, prolonged QT-intervals, and low heart rates without no effect on insulin resistance. In vivo insulin treatment provided also significant recovery in increased PKG and decreased PIP2 level, without the insulin effect on the KCNQ1 level in β₃ -ARs overexpressed cells. The inhibition of I_Ks in aged-rat cardiomyocytes seems to be associated with activated β₃ -ARs dependent remodeling in the interaction between KCNQ1 and KCNE1. Significant recoveries in ventricular-repolarization of insulin-treated aged cardiomyocytes via recovery in I_Ks strongly emphasize two important issues: (1) I_Ks can be a novel target in aging-associated remodeling in the heart and insulin may be a cardioprotective agent in the maintenance of normal heart function during the aging process. (2) This study is one of the first to demonstrate insulin's benefits on long-QT in insulin-resistant aged rats by accelerating the ventricular AP repolarization through reversing the depressed I_Ks via affecting the β₃ -ARs signaling pathway and particularly affecting activated PKG.

Selective activation of adrenoceptors potentiates IKs current in pulmonary vein cardiomyocytes through the protein kinase A and C signaling pathways

Delayed rectifier K⁺ current (I_Ks) is a key contributor to repolarization of action potentials. This study investigated the mechanisms underlying the adrenoceptor-induced potentiation of I_Ks in pulmonary vein cardiomyocytes (PVC). PVC were isolated from guinea pig pulmonary vein. The action potentials and I_Ks current were recorded using perforated and conventional whole-cell patch-clamp techniques. The expression of I_Ks was examined using immunocytochemistry and Western blotting. KCNQ1, a I_Ks pore-forming protein was detected as a signal band approximately 100 kDa in size, and its immunofluorescence signal was found to be mainly localized on the cell membrane. The I_Ks current in PVC was markedly enhanced by both β₁- and β₂-adrenoceptor stimulation with a negative voltage shift in the current activation, although the potentiation was more effectively induced by β₂-adrenoceptor stimulation than β₁-adrenoceptor stimulation. Both β-adrenoceptor-mediated increases in I_Ks were attenuated by treatment with the adenylyl cyclase (AC) inhibitor or protein kinase A (PKA) inhibitor. Furthermore, the I_Ks current was increased by α₁-adrenoceptor agonist but attenuated by the protein kinase C (PKC) inhibitor. PVC exhibited action potentials in normal Tyrode solution which was slightly reduced by HMR-1556 a selective I_Ks blocker. However, HMR-1556 markedly reduced the β-adrenoceptor-potentiated firing rate. The stimulatory effects of β- and α₁-adrenoceptor on I_Ks in PVC are mediated via the PKA and PKC signal pathways. HMR-1556 effectively reduced the firing rate under β-adrenoceptor activation, suggesting that the functional role of I_Ks might increase during sympathetic excitation under in vivo conditions.

Hormonal Signaling Actions on Kv7.1 (KCNQ1) Channels

Kv7 channels (Kv7.1-7.5) are voltage-gated K⁺ channels that can be modulated by five β-subunits (KCNE1-5). Kv7.1-KCNE1 channels produce the slow-delayed rectifying K⁺ current, I_Ks, which is important during the repolarization phase of the cardiac action potential. Kv7.2-7.5 are predominantly neuronally expressed and constitute the muscarinic M-current and control the resting membrane potential in neurons. Kv7.1 produces drastically different currents as a result of modulation by KCNE subunits. This flexibility allows the Kv7.1 channel to have many roles depending on location and assembly partners. The pharmacological sensitivity of Kv7.1 channels differs from that of Kv7.2-7.5 and is largely dependent upon the number of β-subunits present in the channel complex. As a result, the development of pharmaceuticals targeting Kv7.1 is problematic. This review discusses the roles and the mechanisms by which different signaling pathways affect Kv7.1 and KCNE channels and could potentially provide different ways of targeting the channel.

Targeting the β-adrenergic receptor in the clinical management of congenital long QT syndrome

The long QT syndrome (LQTS) is largely treated pharmacologically with β-blockers, despite the role of sympathetic activity in LQTS being poorly understood. Using the trigger-substrate model of cardiac arrhythmias in this review, we amalgamate current experimental and clinical data from both animal and human studies to explain the mechanism of adrenergic stimulation and blockade on LQT arrhythmic risk and hence assess the efficacy of β-adrenoceptor blockade in the management of LQTS. In LQTS1 and LQTS2, sympathetic stimulation increases arrhythmic risk by enhancing early afterdepolarizations and transmural dispersion of repolarization. β-Blockers successfully reduce cardiac events by reducing these triggers and substrates; however, these effects are less marked in LQTS2 compared with LQTS1. In LQTS3, clinical and experimental investigations of the effects of sympathetic stimulation and β-blocker use have produced contradictory findings, resulting in significant clinical uncertainty. We offer explanations for these contradicting results relating to study sample size, the dose of the β-blocker administered associated with its off-target Na⁺ channel effects, as well as the type of β-blocker used. We conclude that the antiarrhythmic efficacy of β-blockers is a genotype-specific phenomenon, and hence the use of β-blockers in clinical practice should be genotype dependent.

Gating and Regulation of KCNQ1 and KCNQ1 + KCNE1 Channel Complexes

The IKs channel complex is formed by the co-assembly of Kv7.1 (KCNQ1), a voltage-gated potassium channel, with its β-subunit, KCNE1 and the association of numerous accessory regulatory molecules such as PIP2, calmodulin, and yotiao. As a result, the IKs potassium current shows kinetic and regulatory flexibility, which not only allows IKs to fulfill physiological roles as disparate as cardiac repolarization and the maintenance of endolymph K⁺ homeostasis, but also to cause significant disease when it malfunctions. Here, we review new areas of understanding in the assembly, kinetics of activation and inactivation, voltage-sensor pore coupling, unitary events and regulation of this important ion channel complex, all of which have been given further impetus by the recent solution of cryo-EM structural representations of KCNQ1 alone and KCNQ1+KCNE3. Recently, the stoichiometric ratio of KCNE1 to KCNQ1 subunits has been confirmed to be variable up to a ratio of 4:4, rather than fixed at 2:4, and we will review the results and new methodologies that support this conclusion. Significant advances have been made in understanding differences between KCNQ1 and IKs gating using voltage clamp fluorimetry and mutational analysis to illuminate voltage sensor activation and inactivation, and the relationship between voltage sensor translation and pore domain opening. We now understand that the KCNQ1 pore can open with different permeabilities and conductance when the voltage sensor is in partially or fully activated positions, and the ability to make robust single channel recordings from IKs channels has also revealed the complicated pore subconductance architecture during these opening steps, during inactivation, and regulation by 1−4 associated KCNE1 subunits. Experiments placing mutations into individual voltage sensors to drastically change voltage dependence or prevent their movement altogether have demonstrated that the activation of KCNQ1 alone and IKs can best be explained using allosteric models of channel gating. Finally, we discuss how the intrinsic gating properties of KCNQ1 and IKs are highly modulated through the impact of intracellular signaling molecules and co-factors such as PIP2, protein kinase A, calmodulin and ATP, all of which modulate IKs current kinetics and contribute to diverse IKs channel complex function.

Nanometric targeting of type 9 adenylyl cyclase in heart

Adenylyl cyclases (ACs) convert ATP into the classical second messenger cyclic adenosine monophosphate (cAMP). Cardiac ACs, specifically AC5, AC6, and AC9, regulate cAMP signaling controlling functional outcomes such as heart rate, contractility and relaxation, gene regulation, stress responses, and glucose and lipid metabolism. With so many distinct functional outcomes for a single second messenger, the cell creates local domains of cAMP signaling to correctly relay signals. Targeting of ACs to A-kinase anchoring proteins (AKAPs) not only localizes ACs, but also places them within signaling nanodomains, where cAMP levels and effects can be highly regulated. Here we will discuss the recent work on the structure, regulation and physiological functions of AC9 in the heart, where it accounts for <3% of total AC activity. Despite the small contribution of AC9 to total cardiac cAMP production, AC9 binds and regulates local PKA phosphorylation of Yotiao-IKs and Hsp20, demonstrating a role for nanometric targeting of AC9.

Regulation of IKs Potassium Current by Isoproterenol in Adult Cardiomyocytes Requires Type 9 Adenylyl Cyclase

The subunits KCNQ1 and KCNE1 generate the slowly activating, delayed rectifier potassium current, I_Ks, that responds to sympathetic stimulation and is critical for human cardiac repolarization. The A-kinase anchoring protein Yotiao facilitates macromolecular complex formation between I_Ks and protein kinase A (PKA) to regulate phosphorylation of KCNQ1 and I_Ks currents following beta-adrenergic stimulation. We have previously shown that adenylyl cyclase Type 9 (AC9) is associated with a KCNQ1-Yotiao-PKA complex and facilitates isoproterenol-stimulated phosphorylation of KCNQ1 in an immortalized cell line. However, requirement for AC9 in sympathetic control of I_Ks in the heart was unknown. Using a transgenic mouse strain expressing the KCNQ1-KCNE1 subunits of I_Ks, we show that AC9 is the only adenylyl cyclase (AC) isoform associated with the KCNQ1-KCNE1-Yotiao complex in the heart. Deletion of AC9 resulted in the loss of isoproterenol-stimulated KCNQ1 phosphorylation in vivo, even though AC9 represents less than 3% of total cardiac AC activity. Importantly, a significant reduction of isoproterenol-stimulated I_Ks currents was also observed in adult cardiomyocytes from I_Ks-expressing AC9KO mice. AC9 and Yotiao co-localize with N-cadherin, a marker of intercalated disks and cell–cell junctions, in neonatal and adult cardiomyocytes, respectively. In conclusion, AC9 is necessary for sympathetic regulation of PKA phosphorylation of KCNQ1 in vivo and for functional regulation of I_Ks in adult cardiomyocytes.

Single channel kinetic analysis of the cAMP effect on IKs mutants, S209F and S27D/S92D

The I_Ks current is important in the heart’s response to sympathetic stimulation. β-adrenergic stimulation increases the amount of I_Ks and creates a repolarization reserve that shortens the cardiac action potential duration. We have recently shown that 8-CPT-cAMP, a membrane-permeable cAMP analog, changes the channel kinetics and causes it to open more quickly and more often, as well as to higher subconductance levels, which produces an increase in the I_Ks current. The mechanism proposed to underlie these kinetic changes is increased activation of the voltage sensors. The present study extends our previous work and shows detailed subconductance analysis of the effects of 8-CPT-cAMP on an enhanced gating mutant (S209F) and on a double pseudo-phosphorylated I_Ks channel (S27D/S92D). 8-CPT-cAMP still produced kinetic changes in S209F + KCNE1, further enhancing gating, while S27D/S92D + KCNE1 showed no significant response to 8-CPT-cAMP, suggesting that these last two mutations fully recapitulate the effect of channel phosphorylation by cAMP.

The IKs Channel Response to cAMP Is Modulated by the KCNE1:KCNQ1 Stoichiometry

The delayed potassium rectifier current, I_Ks, is assembled from tetramers of KCNQ1 and varying numbers of KCNE1 accessory subunits in addition to calmodulin. This channel complex is important in the response of the cardiac action potential to sympathetic stimulation, during which I_Ks is enhanced. This is likely due to channels opening more quickly, more often, and to greater sublevel amplitudes during adrenergic stimulation. KCNQ1 alone is unresponsive to cyclic adenosine monophosphate (cAMP), and thus KCNE1 is required for a functional effect of protein kinase A phosphorylation. Here, we investigate the effect that KCNE1 has on the response to 8-4-chlorophenylthio (CPT)-cAMP, a membrane-permeable cAMP analog, by varying the number of KCNE1 subunits present using fusion constructs of I_Ks with either one (EQQQQ) or two (EQQ) KCNE1 subunits in the channel complex with KCNQ1. These experiments use both whole-cell and single-channel recording techniques. EQQ (2:4, E1:Q1) shows a significant shift in V_1/2 of activation from 10.4 mV ± 2.2 in control to -2.7 mV ± 1.2 (p-value: 0.0024). EQQQQ (1:4, E1:Q1) shows a smaller change in response to 8-CPT-cAMP, 6.3 mV ± 2.3 to -3.2 mV ± 3.0 (p-value: 0.0435). As the number of KCNE1 subunits is reduced, the shift in the V_1/2 of activation becomes smaller. At the single-channel level, a similar graded change in subconductance occupancy and channel activity is seen in response to 8-CPT-cAMP: the less E1, the smaller the response. However, both constructs show a significant reduction of a similar magnitude in the first latency to opening (EQQ control: 0.90 s ± 0.07 to 0.71 s ± 0.06, p-value: 0.0032 and EQQQQ control: 0.94 s ± 0.09 to 0.56 s ± 0.07, p-value < 0.0001). This suggests that there are both E1-dependent and E1-independent effects of 8-CPT-cAMP on the channel.

cAMP-dependent regulation of IKs single-channel kinetics

The delayed potassium rectifier current, I_Ks , is composed of KCNQ1 and KCNE1 subunits and plays an important role in cardiac action potential repolarization. During β-adrenergic stimulation, 3'-5'-cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) phosphorylates KCNQ1, producing an increase in I_Ks current and a shortening of the action potential. Here, using cell-attached macropatches and single-channel recordings, we investigate the microscopic mechanisms underlying the cAMP-dependent increase in I_Ks current. A membrane-permeable cAMP analog, 8-(4-chlorophenylthio)-cAMP (8-CPT-cAMP), causes a marked leftward shift of the conductance-voltage relation in macropatches, with or without an increase in current size. Single channels exhibit fewer silent sweeps, reduced first latency to opening (control, 1.61 ± 0.13 s; cAMP, 1.06 ± 0.11 s), and increased higher-subconductance-level occupancy in the presence of cAMP. The E160R/R237E and S209F KCNQ1 mutants, which show fixed and enhanced voltage sensor activation, respectively, largely abolish the effect of cAMP. The phosphomimetic KCNQ1 mutations, S27D and S27D/S92D, are much less and not at all responsive, respectively, to the effects of PKA phosphorylation (first latency of S27D + KCNE1 channels: control, 1.81 ± 0.1 s; 8-CPT-cAMP, 1.44 ± 0.1 s, P < 0.05; latency of S27D/S92D + KCNE1: control, 1.62 ± 0.1 s; cAMP, 1.43 ± 0.1 s, nonsignificant). Using total internal reflection fluorescence microscopy, we find no overall increase in surface expression of the channel during exposure to 8-CPT-cAMP. Our data suggest that the cAMP-dependent increase in I_Ks current is caused by an increase in the likelihood of channel opening, combined with faster openings and greater occupancy of higher subconductance levels, and is mediated by enhanced voltage sensor activation.

MLP and CARP are linked to chronic PKCα signalling in dilated cardiomyopathy

MLP (muscle LIM protein)-deficient mice count among the first mouse models for dilated cardiomyopathy (DCM), yet the exact role of MLP in cardiac signalling processes is still enigmatic. Elevated PKCα signalling activity is known to be an important contributor to heart failure. Here we show that MLP directly inhibits the activity of PKCα. In end-stage DCM, PKCα is concentrated at the intercalated disc of cardiomyocytes, where it is sequestered by the adaptor protein CARP in a multiprotein complex together with PLCβ1. In mice deficient for both MLP and CARP the chronic PKCα signalling chain at the intercalated disc is broken and they remain healthy. Our results suggest that the main role of MLP in heart lies in the direct inhibition of PKCα and that chronic uninhibited PKCα activity at the intercalated disc in the absence of functional MLP leads to heart failure.

Altered function of the muscle LIM protein (MLP) causes dilated cardiomyopathy in mice and humans. Lange et al. explain the molecular role of MLP in the heart by showing that it affects the signalling complex at the intercalated discs of failing hearts that consists of PKCα, PLCβ1 and CARP by inhibiting PKCα auto-phosphorylation and function.

Arrhythmogenic cardiomyopathy in a patient with a rare loss-of-function KCNQ1 mutation

BACKGROUND

Ventricular tachycardia (VT) is a common manifestation of advanced cardiomyopathies. In a subset of patients with dilated cardiomyopathy, VT is the initial and the cardinal manifestation of the disease. The molecular genetic basis of this subset of dilated cardiomyopathy is largely unknown.

METHODS AND RESULTS

We identified 10 patients with dilated cardiomyopathy who presented with VT and sequenced 14 common causal genes for cardiomyopathies and arrhythmias. Functional studies included cellular patch clamp, confocal microscopy, and immunoblotting. We identified nonsynonymous variants in 4 patients, including a rare missense p.R397Q mutation in the KCNQ1 gene in a 60-year-old man who presented with incessant VT and had mild cardiac dysfunction. The p.R397Q mutation was absent in an ethnically matched control group, affected a conserved amino acid, and was predicted by multiple algorithms to be pathogenic. Co-expression of the mutant KCNQ1 with its partner unit KCNE1 was associated with reduced tail current density of slowly activating delayed rectifier K(+) current (IKs). The mutation reduced membrane localization of the protein.

CONCLUSIONS

Dilated cardiomyopathy with an initial presentation of VT may be a forme fruste of arrhythmogenic cardiomyopathy caused by mutations in genes encoding the ion channels. The findings implicate KCNQ1 as a possible causal gene for arrhythmogenic cardiomyopathy.

β2 adrenergic receptor activation governs cardiac repolarization and arrhythmogenesis in a guinea pig model of heart failure

β2-AR activation increases the risk of sudden cardiac death (SCD) in heart failure (HF) patients. Non-selective β-AR blockers have greater benefits on survival than selective β1-AR blockers in chronic HF patients, indicating that β2-AR activation contributes to SCD in HF. This study investigated the role of β2-AR activation on repolarization and ventricular arrhythmia (VA) in the experimental HF model. The guinea pig HF was induced by descending aortic banding. The effective refractoriness period (ERP), corrected QT (QTc) and the incidence of VA were examined using Langendorff and programmed electrical stimulation. Ikr and APD were recorded by the whole cell patch clamp. Selective β2-AR agonist salbutamol significantly increased the incidence of VA, prolonged QTc and shortened ERP. These effects could be prevented by the selective β2-AR antagonist, ICI118551. Salbutamol prolonged APD90 and reduced Ikr in guinea pig HF myocytes. The antagonists of cAMP (Rp-cAMP) and PKA (KT5720) attenuated Ikr inhibition and APD prolongation induced by salbutamol. However, the antagonists of Gi protein (PTX) and PDE III (amrinone) showed opposite effects. This study indicates that β2-AR activation increases the incidence of VA in the experimental HF model via activation of Gs/cAMP/PKA and/or inhibition of Gi/PDE pathways.

Long-QT mutation p.K557E-Kv7.1: dominant-negative suppression of IKs, but preserved cAMP-dependent up-regulation

AIMS

Mutations in KCNQ1, encoding for Kv7.1, the α-subunit of the IKs channel, cause long-QT syndrome type 1, potentially predisposing patients to ventricular tachyarrhythmias and sudden cardiac death, in particular, during elevated sympathetic tone. Here, we aim at characterizing the p.Lys557Glu (K557E) Kv7.1 mutation, identified in a Dutch kindred, at baseline and during (mimicked) increased adrenergic tone.

METHODS AND RESULTS

K557E carriers had moderate QTc prolongation that augmented significantly during exercise. IKs characteristics were determined after co-expressing Kv7.1-wild-type (WT) and/or K557E with minK and Yotiao in Chinese hamster ovary cells. K557E caused IKs loss of function with slowing of the activation kinetics, acceleration of deactivation kinetics, and a rightward shift of voltage-dependent activation. Together, these contributed to a dominant-negative reduction in IKs density. Confocal microscopy and western blot indicated that trafficking of K557E channels was not impaired. Stimulation of WT IKs by 3'-5'-cyclic adenosine monophosphate (cAMP) generated strong current up-regulation that was preserved for K557E in both hetero- and homozygosis. Accumulation of IKs at fast rates occurred both in WT and in K557E, but was blunted in the latter. In a computational model, K557E showed a loss of action potential shortening during β-adrenergic stimulation, in accordance with the lack of QT shortening during exercise in patients.

CONCLUSION

K557E causes IKs loss of function with reduced fast rate-dependent current accumulation. cAMP-dependent stimulation of mutant IKs is preserved, but incapable of fully compensating for the baseline current reduction, explaining the long QT intervals at baseline and the abnormal QT accommodation during exercise in affected patients.

Phosphorylation of rat aquaporin-4 at Ser(111) is not required for channel gating

Aquaporin 4 (AQP4) is the predominant water channel in the mammalian brain and is mainly expressed in the perivascular glial endfeet at the brain-blood interface. AQP4 has been described as an important entry and exit site for water during formation of brain edema and regulation of AQP4 is therefore of therapeutic interest. Phosphorylation of some aquaporins has been proposed to regulate their water permeability via gating of the channel itself. Protein kinase (PK)-dependent phosphorylation of Ser(111) has been reported to increase the water permeability of AQP4 expressed in an astrocytic cell line. This possibility was, however, questioned based on the crystal structure of the human AQP4. Our study aimed to resolve if Ser(111) was indeed a site involved in phosphorylation-mediated gating of AQP4. The water permeability of AQP4-expressing Xenopus oocytes was not altered by a range of activators and inhibitors of PKG and PKA. Mutation of Ser(111) to alanine or aspartate (to prevent or mimic phosphorylation) did not change the water permeability of AQP4. PKG activation had no effect on the water permeability of AQP4 in primary cultures of rat astrocytes. Molecular dynamics simulations of a phosphorylation of AQP4.Ser(111) recorded no phosphorylation-induced change in water permeability. A phospho-specific antibody, exclusively recognizing AQP4 when phosphorylated on Ser(111) , failed to detect phosphorylation in cell lysate of rat brain stimulated by conditions proposed to induce phosphorylation of this residue. Thus, our data indicate a lack of phosphorylation of Ser(111) and of phosphorylation-dependent gating of AQP4.

Sex differences in repolarization and slow delayed rectifier potassium current and their regulation by sympathetic stimulation in rabbits

Slow delayed rectifier potassium current (IKs) is important in action potential (AP) repolarization and repolarization reserve. We tested the hypothesis that there are sex-specific differences in IKs, AP, and their regulation by β-adrenergic receptors (β-AR's) using whole-cell patch-clamp. AP duration (APD90) was significantly longer in control female (F) than in control male (M) myocytes. Isoproterenol (ISO, 500 nM) shortened APD90 comparably in M and F, and was largely reversed by β1-AR blocker CGP 20712A (CGP, 300 nM). Inhibition of IKs with chromanol 293B (10 μM) resulted in less APD prolongation in F at baseline (3.0 vs 8.9 %, p < 0.05 vs M) and even in the presence of ISO (5.4 vs 20.9 %, p < 0.05). This suggests that much of the ISO-induced APD abbreviation in F is independent of IKs. In F, baseline IKs was 42 % less and was more weakly activated by ISO (19 vs 68 % in M, p < 0.01). ISO enhancement of IKs was comparably attenuated by CGP in M and F. After ovariectomy, IKs in F had greater enhancement by ISO (72 %), now comparable to control M. After orchiectomy, IKs in M was only slightly enhanced by ISO (23 %), comparable to control F. Pretreatment with thapsigargin (to block SR Ca release) had bigger impact on ISO-induced APD shortening in F than that in M (p < 0.01). In conclusion, we found that there are sex differences in IKs, AP, and their regulation by β-AR's that are modulated by sex hormones, suggesting the potential for sex-specific antiarrhythmic therapy.

Sexual dimorphism and oestrogen regulation of KCNE3 expression modulates the functional properties of KCNQ1 K⁺ channels

The KCNQ1 potassium channel associates with various KCNE ancillary subunits that drastically affect channel gating and pharmacology. Co-assembly with KCNE3 produces a current with nearly instantaneous activation, some time-dependent activation at very positive potentials, a linear current-voltage relationship and a 10-fold higher sensitivity to chromanol 293B. KCNQ1:KCNE3 channels are expressed in colonic crypts and mediate basolateral K(+) recycling required for Cl(-) secretion. We have previously reported the female-specific anti-secretory effects of oestrogen via KCNQ1:KCNE3 channel inhibition in colonic crypts. This study was designed to determine whether sex and oestrogen regulate the expression and function of KCNQ1 and KCNE3 in rat distal colon. Colonic crypts were isolated from Sprague-Dawley rats and used for whole-cell patch-clamp and to extract total RNA and protein. Sheets of epithelium were used for short-circuit current recordings. KCNE1 and KCNE3 mRNA and protein abundance were significantly higher in male than female crypts. No expression of KCNE2 was found and no difference was observed in KCNQ1 expression between male and female (at oestrus) colonic crypts. Male crypts showed a 2.2-fold higher level of association of KCNQ1 and KCNE3 compared to female cells. In female colonic crypts, KCNQ1 and KCNE3 protein expression fluctuated throughout the oestrous cycle and 17β-oestradiol (E2 10 nM) produced a rapid (<15 min) dissociation of KCNQ1 and KCNE3 in female crypts only. Whole-cell K(+) currents showed a linear current-voltage relationship in male crypts, while K(+) currents in colonic crypts isolated from females displayed voltage-dependent outward rectification. Currents in isolated male crypts and epithelial sheets were 10-fold more sensitive to specific KCNQ1 inhibitors, such as chromanol 293B and HMR-1556, than in female. The effect of E2 on K(+) currents mediated by KCNQ1 with or without different β-subunits was assayed from current-voltage relations elicited in CHO cells transfected with KCNQ1 and KCNE3 or KCNE1 cDNA. E2 (100 nM) reduced the currents mediated by the KCNQ1:KCNE3 potassium channel and had no effect on currents via KCNQ1:KCNE1 or KCNQ1 alone. Currents mediated by the complex formed by KCNQ1 and the mutant KCNE3-S82A β-subunit (mutation of the site for PKCδ-promoted phosphorylation and modulation of the activity of KCNE3) showed rapid run-down and insensitivity to E2. Together, these data suggest that oestrogen regulates the expression of the KCNE1 and KCNE3 and with it the gating and pharmacological properties of the K(+) conductance required for Cl(-) secretion. The decreased association of the KCNQ1:KCNE3 channel complex promoted by oestrogen exposure underlies the molecular mechanism for the sexual dimorphism and oestrous cycle dependence of the anti-secretory actions of oestrogen in the intestine.

Berberine Reduces cAMP-Induced Chloride Secretion in T84 Human Colonic Carcinoma Cells through Inhibition of Basolateral KCNQ1 Channels

Berberine is a plant alkaloid with multiple pharmacological actions, including antidiarrhoeal activity and has been shown to inhibit Cl(-) secretion in distal colon. The aims of this study were to determine the molecular signaling mechanisms of action of berberine on Cl(-) secretion and the ion transporter targets. Monolayers of T84 human colonic carcinoma cells grown in permeable supports were placed in Ussing chambers and short-circuit current measured in response to secretagogues and berberine. Whole-cell current recordings were performed in T84 cells using the patch-clamp technique. Berberine decreased forskolin-induced short-circuit current in a concentration-dependent manner (IC(50) 80 ± 8 μM). In apically permeabilized monolayers and whole-cell current recordings, berberine inhibited a cAMP-dependent and chromanol 293B-sensitive basolateral membrane K(+) current by 88%, suggesting inhibition of KCNQ1 K(+) channels. Berberine did not affect either apical Cl(-) conductance or basolateral Na(+)-K(+)-ATPase activity. Berberine stimulated p38 MAPK, PKCα and PKA, but had no effect on p42/p44 MAPK and PKCδ. However, berberine pre-treatment prevented stimulation of p42/p44 MAPK by epidermal growth factor. The inhibitory effect of berberine on Cl(-) secretion was partially blocked by HBDDE (∼65%), an inhibitor of PKCα and to a smaller extent by inhibition of p38 MAPK with SB202190 (∼15%). Berberine treatment induced an increase in association between PKCα and PKA with KCNQ1 and produced phosphorylation of the channel. We conclude that berberine exerts its inhibitory effect on colonic Cl(-) secretion through inhibition of basolateral KCNQ1 channels responsible for K(+) recycling via a PKCα-dependent pathway.

O-glycosylation of the cardiac I(Ks) complex

Post-translational modifications of the KCNQ1–KCNE1 (Kv7) K+ channel complex are vital for regulation of the cardiac IKs current and action potential duration. Here, we show the KCNE1 regulatory subunit is O-glycosylated with mucin-type glycans in vivo. As O-linked glycosylation sites are not recognizable by sequence gazing, we designed a novel set of glycosylation mutants and KCNE chimeras and analysed their glycan content using deglycosylation enzymes. Our results show that KCNE1 is exclusively O-glycosylated at Thr-7, which is also required for N-glycosylation at Asn-5. For wild type KCNE1, the overlapping N- and O-glycosylation sites are innocuous for subunit biogenesis; however, mutation of Thr-7 to a non-hydroxylated residue yielded mostly unglycosylated protein and a small fraction of mono-N-glycosylated protein. The compounded hypoglycosylation was equally deleterious for KCNQ1–KCNE1 cell surface expression, demonstrating that KCNE1 O-glycosylation is a post-translational modification that is integral for the proper biogenesis and anterograde trafficking of the cardiac IKs complex. The enzymatic assays and panel of glycosylation mutants used here will be valuable for identifying the different KCNE1 glycoforms in native cells and determining the roles N- and O-glycosylation play in KCNQ1–KCNE1 function and localization in cardiomyocytes,

Inhibition of the heterotetrameric K+ channel KCNQ1/KCNE1 by the AMP-activated protein kinase

The heterotetrameric K(+)-channel KCNQ1/KCNE1 is expressed in heart, skeletal muscle, liver and several epithelia including the renal proximal tubule. In the heart, it contributes to the repolarization of cardiomyocytes. The repolarization is impaired in ischemia. Ischemia stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase, sensing energy depletion and stimulating several cellular mechanisms to enhance energy production and to limit energy utilization. AMPK has previously been shown to downregulate the epithelial Na(+) channel ENaC, an effect mediated by the ubiquitin ligase Nedd4-2. The present study explored whether AMPK regulates KCNQ1/KCNE1. To this end, cRNA encoding KCNQ1/KCNE1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1 + AMPKβ1 + AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. KCNQ1/KCNE1 activity was determined in two electrode voltage clamp experiments. Moreover, KCNQ1 abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced KCNQ1/KCNE1-mediated currents and reduced KCNQ1 abundance in the cell membrane. Similarly, Nedd4-2 decreased KCNQ1/KCNE1-mediated currents and KCNQ1 protein abundance in the cell membrane. Activation of AMPK in isolated perfused proximal renal tubules by AICAR (10 mM) was followed by significant depolarization. In conclusion, AMPK is a potent regulator of KCNQ1/KCNE1.

Mechanisms of beta-adrenergic modulation of I(Ks) in the guinea-pig ventricle: insights from experimental and model-based analysis

Detailed understanding of I(Ks) gating complexity may provide clues regarding the mechanisms of repolarization instability and the resulting arrhythmias. We developed and tested a kinetic model to interpret physiologically relevant I(Ks) properties, including pause-dependence and modulation by beta-adrenergic receptors (beta-AR). I(Ks) gating was evaluated in guinea-pig ventricular myocytes at 36 degrees C in control and during beta-AR stimulation (0.1 micromol/L isoprenaline (ISO)). We tested voltage dependence of steady-state conductance (Gss), voltage dependence of activation and deactivation time constants (tau(act), tau(deact)), and pause-dependence of tau(act) during repetitive activations (tau(react)). The I(Ks) model was developed from the Silva and Rudy formulation. Parameters were optimized on control and ISO experimental data, respectively. ISO strongly increased Gss and its voltage dependence, changed the voltage dependence of tau(act) and tau(deact), and modified the pause-dependence of tau(react). A single set of model parameters reproduced all experimental data in control. Modification of only three transition rates led to a second set of parameters suitable to fit all ISO data. Channel unitary conductance and density were unchanged in the model, thus implying increased open probability as the mechanism of ISO-induced Gss enhancement. The new I(Ks) model was applied to analyze ISO effect on repolarization rate-dependence. I(Ks) kinetics and its beta-AR modulation were entirely reproduced by a single Markov chain of transitions (for each channel monomer). Model-based analysis suggests that complete opening of I(Ks) channels within a physiological range of potentials requires concomitant beta-AR stimulation. Transient redistribution of state occupancy, in addition to direct modulation of transition rates, may underlie beta-AR modulation of I(Ks) time dependence.

cAMP stimulates apical exocytosis of the renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the thick ascending limb: role of protein kinase A

The apical renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 mediates NaCl absorption by the thick ascending limb (TAL) of Henle's loop. cAMP stimulates NKCC2 by enhancing steady-state apical membrane levels of this protein; however, the trafficking and signaling mechanisms by which this occurs have not been studied. Here, we report that stimulation of endogenous cAMP levels with either forskolin/3-isobutyl-1-methylxanthine (IBMX) or the V2 receptor agonist [deamino-Cys(1),d-Arg(8)]vasopressin increases steady-state surface NKCC2 and that the protein kinase A (PKA) inhibitor H-89 blocks this effect. Confocal imaging of apical surface NKCC2 in isolated perfused TALs confirmed a stimulatory effect of cAMP on apical trafficking that was blocked by PKA inhibition. Selective stimulation of PKA with the agonist N(6)-benzoyl-cAMP (500 microm) stimulated steady-state surface NKCC2, whereas the Epac-selective agonist 8-p-chlorophenylthio-2'-O-methyl-cAMP (100 and 250 microm) had no effect. To explore the trafficking mechanism by which cAMP increases apical NKCC2, we measured cumulative apical membrane exocytosis and NKCC2 exocytic insertion in TALs. By monitoring apical FM1-43 fluorescence, we observed rapid stimulation of apical exocytosis (2 min) by forskolin/IBMX. We also found constitutive exocytic insertion of NKCC2 in TALs over time, which was increased by 3-fold in the presence of forskolin/IBMX. PKA inhibition blunted cAMP-stimulated exocytic insertion but did not affect the rate of constitutive exocytosis. We conclude that cAMP stimulates steady-state apical surface NKCC2 by stimulating exocytic insertion and that this process is highly dependent on PKA but not Epac.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

The cardiac IKs potassium channel macromolecular complex includes the phosphodiesterase PDE4D3

The cardiac I(Ks) potassium channel is a macromolecular complex consisting of alpha-(KCNQ1) and beta-subunits (KCNE1) and the A kinase-anchoring protein (AKAP) Yotiao (AKAP-9), which recruits protein kinase A) and protein phosphatase 1 to the channel. Here, we have tested the hypothesis that specific cAMP phosphodiesterase (PDE) isoforms of the PDE4D family that are expressed in the heart are also part of the I(Ks) signaling complex and contribute to its regulation by cAMP. PDE4D isoforms co-immunoprecipitated with I(Ks) channels in hearts of mice expressing the I(Ks) channel. In myocytes isolated from these mice, I(Ks) was increased by pharmacological PDE inhibition. PDE4D3, but not PDE4D5, co-immunoprecipitated with the I(Ks) channel only in Chinese hamster ovary cells co-expressing AKAP-9, and PDE4D3, but not PDE4D5, co-immunoprecipitated with AKAP-9. Functional experiments in Chinese hamster ovary cells expressing AKAP-9 and either PDE4D3 or PDE4D5 isoforms revealed modulation of the I(Ks) response to cAMP by PDE4D3 but not PDE4D5. We conclude that PDE4D3, like protein kinase A and protein phosphatase 1, is recruited to the I(Ks) channel via AKAP-9 and contributes to its critical regulation by cAMP.

Adrenergic regulation of a key cardiac potassium channel can contribute to atrial fibrillation: evidence from an I Ks transgenic mouse

Inherited gain-of-function mutations of genes coding for subunits of the heart slow potassium (I Ks) channel can cause familial atrial fibrillation (AF). Here we consider a potentially more prevalent mechanism and hypothesize that beta-adrenergic receptor (beta-AR)-mediated regulation of the I Ks channel, a natural gain-of-function pathway, can also lead to AF. Using a transgenic I Ks channel mouse model, we studied the role of the channel and its regulation by beta-AR stimulation on atrial arrhythmias. In vivo administration of isoprenaline (isoproterenol) predisposes I Ks channel transgenic mice but not wild-type (WT) littermates that lack I Ks to prolonged atrial arrhythmias. Patch-clamp analysis demonstrated expression and isoprenaline-mediated regulation of I Ks in atrial myocytes from transgenic but not WT littermates. Furthermore, computational modelling revealed that beta-AR stimulation-dependent accumulation of open I Ks channels accounts for the pro-arrhythmic substrate. Our results provide evidence that beta-AR-regulated I Ks channels can play a role in AF and imply that specific I Ks deregulation, perhaps through disruption of the I Ks macromolecular complex necessary for beta-AR-mediated I Ks channel regulation, may be a novel therapeutic strategy for treating this most common arrhythmia.

A simple technique for isolating healthy heart cells from mouse models

Single heart cells of mouse models provide powerful tools for heart research. However, their isolation is not easy, and it imposes a significant bottleneck on their use in cellular studies of the heart. Aiming to overcome this problem, this report introduces a novel technique that reproducibly isolates healthy heart cells from mouse models. Using simple devices that ensure easy handling and the rapid aortic cannulation of a small mouse heart, cell isolation was done under physiological conditions without using the "KB" medium or 2,3-butanedione monoxime (BDM). The isolated cells consistently had a healthy appearance and a high viability of 75 +/- 5% (mean +/- SD) in Tyrode solution containing 1.8 mM Ca2+. After 8 h of storage at 37 degrees C, they still had a viability of 45 +/- 12%. The cells showed normal contraction properties when field-stimulated, and they generated normal action potentials and membrane currents under the whole-cell clamp condition. The beta-adrenergic signal transduction of the cells was also normal when it was examined with the isoproterenol enhancement of the L-type Ca2+ current.

Interactions between GABA-B1 receptors and Kir 3 inwardly rectifying potassium channels

gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian brain. It acts via both ionotropic GABA-A and metabotropic GABA-B receptors. We evaluated the interaction of receptors with members of the inwardly rectifying potassium (Kir 3) channel family, which also play an important role in neuronal transmission and membrane excitability. These channels are functionally regulated by GABA-B receptors. Possible physical interactions between GABA-B receptor and Kir 3 channels expressed in HEK cells were evaluated using Bioluminescence Resonance Energy Transfer (BRET) experiments, co-immunoprecipitation and confocal microscopy. Our data indicate that Kir 3 channels and Gbetagamma subunits can interact with the GABA-B(1) subunits independently of the GABA-B(2) subunit or Kir 3.4 which are ultimately responsible for their targetting to the cell surface. Thus signalling complexes containing GABA-B receptors, G proteins and Kir channels are formed shortly after biosynthesis most likely in the endoplasmic reticulum.

Cellular distribution of the potassium channel KCNQ1 in normal mouse kidney

Mechanisms of K(+) secretion and absorption along the collecting duct are not understood fully. Because KCNQ1 participates in K(+) secretion within the inner ear and stomach, distribution of KCNQ1 in mouse kidney was studied using Northern and Western analyses, RT-PCR of isolated tubules, and immunohistochemistry. Northern blots demonstrated KCNQ1 transcripts in whole kidney. RT-PCR showed KCNQ1 mRNA in isolated distal convoluted tubule (DCT), connecting segment (CNT), collecting ducts (CD), and glomeruli. Immunoblots of kidney and stomach revealed a approximately 75-kDa protein, the expected mobility for KCNQ1. KCNQ1 was detected by immunohistochemistry throughout the distal nephron and CD. Thick ascending limbs exhibited weak basolateral immunolabel. In DCT and CNT cells, immunolabel was intense and basolateral, although KCNQ1 label was stronger in late than in early DCT. Initial collecting tubule and cortical CD KCNQ1 immunolabel was predominantly diffuse, but many cells exhibited discrete apical label. Double-labeling experiments demonstrated that principal cells, type B intercalated cells, and a few type A intercalated cells exhibited distinct apical KCNQ1 immunolabel. In inner medullary CD, principal cells exhibited distinct basolateral KCNQ1 immunolabel, whereas intercalated cells showed diffuse cytoplasmic staining. Thus KCNQ1 protein is widely distributed in mouse distal nephron and CD, with significant axial and cellular heterogeneity in location and intensity. These findings suggest that KCNQ1 has cell-specific roles in renal ion transport and may participate in K(+) secretion and/or absorption along the thick ascending limb, DCT, connecting tubule, and CD.

Heterotrimeric G proteins form stable complexes with adenylyl cyclase and Kir3.1 channels in living cells

Bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several G-protein subunits (Gαs, Gαi, Gβ1 and Gγ2) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically Gβ1-YFP1-158 and Gγ2-YFP159-238, which heterodimerize to produce fluorescent YFP-Gβ1γ2). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-Gγ2, GFP-Gβ1 or YFP-Gβ1γ2. Gα subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and Gβγ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.

Role of Nitric Oxide in Ca2+Sensitivity of the Slowly Activating Delayed Rectifier K+Current in Cardiac Myocytes

Sarcolemmal Ca2+ entry is a vital step for contraction of cardiomyocytes, but Ca2+ overload is harmful and may trigger arrhythmias and/or apoptosis. To maintain the amount of Ca2+ entry within an appropriate range, cardiomyocytes have feedback systems that tightly regulate ion channel activities in response to the changes in intracellular Ca2+ concentration ([Ca2+]i), thereby regulating Ca2+ entry. In guinea pig ventricular myocytes, Ca2+ ionophore, A23187, induced suppression of the L-type Ca2+ currents (I(Ca,L)) and enhancement of the slowly activating delayed rectifier K(+) currents (I(Ks)). At a low stimulation rate, I(Ca,L) suppression and I(Ks) enhancement contributed to the A23187-induced APD shortening with a similar magnitude, whereas at a high stimulation rate, I(Ks) enhancement dominantly contributed to APD shortening. I(Ks) enhancement induced by A23187 was attributable to actions of nitric oxide (NO), because they were inhibited by an inhibitor of NO synthase (NOS) and by a NO scavenger. A23187-induced alterations of APD and I(Ks) were strongly suppressed by a NOS3 inhibitor, but barely affected by a NOS1 inhibitor, suggesting that NOS3 was responsible for NO release in this phenomenon. Inhibition of calmodulin (CaM), but not Akt, blocked the enhancement of I(Ks) by A23187. Thus, CaM-dependent NOS3 activation confers the selective Ca2+-sensitivity on I(Ks). Ca2+-induced I(Ks) enhancement and resultant APD shortening potentially act as a physiological regulatory mechanism of Ca2+ recycling, because they were observed at a physiological range of [Ca2+]i in cardiac myocytes and are induced by physiologically relevant Ca2+ loading, such as digitalis application and rise in extracellular Ca2+ concentration.