Mechanisms of Action of the KCa2-Negative Modulator AP30663, a Novel Compound in Development for Treatment of Atrial Fibrillation in Man.

AIMS: Small conductance Ca2+-activated K+ channels (SK channels, KCa2) are a new target for treatment of atrial fibrillation (AF). AP30663 is a small molecule inhibitor of KCa2 channels that is currently in clinical development for treatment of AF. The aim of this study is to present the electrophysiological profile and mechanism of action of AP30663 and its efficacy in prolonging atrial refractoriness in rodents, and by bioinformatic analysis investigate if genetic variants in KCNN2 or KCNN3 influence the expression level of these in human heart tissue. METHODS AND RESULTS: Whole-cell and inside-out patch-clamp recordings of heterologously expressed KCa2 channels revealed that AP30663 inhibits KCa2 channels with minor effects on other relevant cardiac ion channels. AP30663 modulates the KCa2.3 channel by right-shifting the Ca2+-activation curve. In isolated guinea pig hearts AP30663 significantly prolonged the atrial effective refractory period (AERP) with minor effects on the QT-interval corrected for heart rate. Similarly, in anaesthetized rats 5 and 10 mg/kg of AP30663 changed the AERP to 130.7±5.4% and 189.9±18.6 of baseline values. The expression quantitative trait loci analyses revealed that the genome wide association studies for AF SNP rs13376333 in KCNN3 is associated with increased mRNA expression of KCNN3 in human atrial appendage tissue. CONCLUSIONS: AP30663 is a novel negative allosteric modulator of KCa2 channels that concentration-dependently prolonged rodent atrial refractoriness with minor effects on the QT-interval. Moreover, AF associated SNPs in KCNN3 influence KCNN3 mRNA expression in human atrial tissue. These properties support continued development of AP30663 for treatment of AF in man.

The KCa2 Channel Inhibitor AP30663 Selectively Increases Atrial Refractoriness, Converts Vernakalant-Resistant Atrial Fibrillation and Prevents Its Reinduction in Conscious Pigs.

AIMS: To describe the effects of the KCa2 channel inhibitor AP30663 in pigs regarding tolerability, cardiac electrophysiology, pharmacokinetics, atrial functional selectivity, effectiveness in cardioversion of tachy-pacing induced vernakalant-resistant atrial fibrillation (AF), and prevention of reinduction of AF. METHODS AND RESULTS: Six healthy pigs with implanted pacemakers and equipped with a Holter monitor were used to compare the effects of increasing doses (0, 5, 10, 15, 20, and 25 mg/kg) of AP30663 on the right atrial effective refractory period (AERP) and on various ECG parameters, including the QT interval. Ten pigs with implanted neurostimulators were long-term atrially tachypaced (A-TP) until sustained vernakalant-resistant AF was present. 20 mg/kg AP30663 was tested to discover if it could successfully convert vernakalant-resistant AF to sinus rhythm (SR) and protect against reinduction of AF. Seven anesthetized pigs were used for pharmacokinetic experiments. Two pigs received an infusion of 20 mg/kg AP30663 over 60 min while five pigs received 5 mg/kg AP30663 over 30 min. Blood samples were collected before, during, and after infusion on AP30663. AP30663 was well-tolerated and prominently increased the AERP in pigs with little effect on ventricular repolarization. Furthermore, it converted A-TP induced AF that had become unresponsive to vernakalant, and it prevented reinduction of AF in pigs. Both a >30 ms increase of the AERP and conversion of AF occurred in different pigs at a free plasma concentration level of around 1.0-1.4 µM of AP30663, which was achieved at a dose level of 5 mg/kg. CONCLUSION: AP30663 has shown properties in animals that would be of clinical interest in man.

A new negative allosteric modulator, AP14145, for the study of small conductance calcium-activated potassium (KCa 2) channels.

BACKGROUND AND PURPOSE: Small conductance calcium-activated potassium (KCa 2) channels represent a promising atrial-selective target for treatment of atrial fibrillation. Here, we establish the mechanism of KCa 2 channel inhibition by the new compound AP14145. EXPERIMENTAL APPROACH: Using site-directed mutagenesis, binding determinants for AP14145 inhibition were explored. AP14145 selectivity and mechanism of action were investigated by patch-clamp recordings of heterologously expressed KCa 2 channels. The biological efficacy of AP14145 was assessed by measuring atrial effective refractory period (AERP) prolongation in anaesthetized rats, and a beam walk test was performed in mice to determine acute CNS-related effects of the drug. KEY RESULTS: AP14145 was found to be an equipotent negative allosteric modulator of KCa 2.2 and KCa 2.3 channels (IC50  = 1.1 ± 0.3 µM). The presence of AP14145 (10 µM) increased the EC50 of Ca2+ on KCa 2.3 channels from 0.36 ± 0.02 to 1.2 ± 0.1 µM. The inhibitory effect strongly depended on two amino acids, S508 and A533 in the channel. AP14145 concentration-dependently prolonged AERP in rats. Moreover, AP14145 (10 mg·kg-1 ) did not trigger any apparent CNS effects in mice. CONCLUSIONS AND IMPLICATIONS: AP14145 is a negative allosteric modulator of KCa 2.2 and KCa 2.3 channels that shifted the calcium dependence of channel activation, an effect strongly dependent on two identified amino acids. AP14145 prolonged AERP in rats and did not trigger any acute CNS effects in mice. The understanding of how KCa 2 channels are inhibited, at the molecular level, will help further development of drugs targeting KCa 2 channels.

Preservation of cardiac function by prolonged action potentials in mice deficient of KChIP2.

Inherited ion channelopathies and electrical remodeling in heart disease alter the cardiac action potential with important consequences for excitation-contraction coupling. Potassium channel-interacting protein 2 (KChIP2) is reduced in heart failure and interacts under physiological conditions with both Kv4 to conduct the fast-recovering transient outward K(+) current (Ito,f) and with CaV1.2 to mediate the inward L-type Ca(2+) current (ICa,L). Anesthetized KChIP2(-/-) mice have normal cardiac contraction despite the lower ICa,L, and we hypothesized that the delayed repolarization could contribute to the preservation of contractile function. Detailed analysis of current kinetics shows that only ICa,L density is reduced, and immunoblots demonstrate unaltered CaV1.2 and CaVß2 protein levels. Computer modeling suggests that delayed repolarization would prolong the period of Ca(2+) entry into the cell, thereby augmenting Ca(2+)-induced Ca(2+) release. Ca(2+) transients in disaggregated KChIP2(-/-) cardiomyocytes are indeed comparable to wild-type transients, corroborating the preserved contractile function and suggesting that the compensatory mechanism lies in the Ca(2+)-induced Ca(2+) release event. We next functionally probed dyad structure, ryanodine receptor Ca(2+) sensitivity, and sarcoplasmic reticulum Ca(2+) load and found that increased temporal synchronicity of the Ca(2+) release in KChIP2(-/-) cardiomyocytes may reflect improved dyad structure aiding the compensatory mechanisms in preserving cardiac contractile force. Thus the bimodal effect of KChIP2 on Ito,f and ICa,L constitutes an important regulatory effect of KChIP2 on cardiac contractility, and we conclude that delayed repolarization and improved dyad structure function together to preserve cardiac contraction in KChIP2(-/-) mice.

Sinoatrial node dysfunction induces cardiac arrhythmias in diabetic mice.

BACKGROUND: The aim of this study was to probe cardiac complications, including heart-rate control, in a mouse model of type-2 diabetes. Heart-rate development in diabetic patients is not straight forward: In general, patients with diabetes have faster heart rates compared to non-diabetic individuals, yet diabetic patients are frequently found among patients treated for slow heart rates. Hence, we hypothesized that sinoatrial node (SAN) dysfunction could contribute to our understanding of the mechanism behind this conundrum and the consequences thereof. METHODS: Cardiac hemodynamic and electrophysiological characteristics were investigated in diabetic db/db and control db/+ mice. RESULTS: We found improved contractile function and impaired filling dynamics of the heart in db/db mice, relative to db/+ controls. Electrophysiologically, we observed comparable heart rates in the two mouse groups, but SAN recovery time was prolonged in diabetic mice. Adrenoreceptor stimulation increased heart rate in all mice and elicited cardiac arrhythmias in db/db mice only. The arrhythmias emanated from the SAN and were characterized by large RR fluctuations. Moreover, nerve density was reduced in the SAN region. CONCLUSIONS: Enhanced systolic function and reduced diastolic function indicates early ventricular remodeling in obese and diabetic mice. They have SAN dysfunction, and adrenoreceptor stimulation triggers cardiac arrhythmia originating in the SAN. Thus, dysfunction of the intrinsic cardiac pacemaker and remodeling of the autonomic nervous system may conspire to increase cardiac mortality in diabetic patients.

Loss of K+ currents in heart failure is accentuated in KChIP2 deficient mice.

INTRODUCTION: KV 4 together with KV Channel-Interacting Protein 2 (KChIP2) mediate the fast recovering transient outward potassium current (I(to,f)) in the heart. KChIP2 is downregulated in human heart failure (HF), potentially underlying the loss of I(to,f). We investigated remodeling associated with HF hypothesizing that KChIP2 plays a central role in the modulation of outward K(+) currents in HF. METHODS AND RESULTS: HF was induced by aortic banding in wild-type (WT) and KChIP2 deficient (KChIP2(-/-)) mice, evaluated by echocardiography. Action potentials were measured by floating microelectrodes in intact hearts. Ventricular cardiomyocytes were isolated and whole-cell currents were recorded by patch clamp. Left ventricular action potentials in KChIP2(-/-) mice were prolonged in a rate dependent manner, consistent with patch-clamp data showing loss of a fast recovering outward K(+) current and upregulation of the slow recovering I(to,s) and I(Kur). HF decreased all outward K(+) currents in WT mice and did not change the relative contribution of I(to,f) in WT mice. Compared to WT HF, KChIP2(-/-) HF had a larger reduction of K(+) -current density. However, the relative APD prolongation caused by HF was shorter for KChIP2(-/-) compared with WT, and the APs of the 2 HF mouse types were indistinguishable. CONCLUSION: I(to,f) is just one of many K(+) currents being downregulated in murine HF. The downregulation of repolarizing currents in HF is accentuated in KChIP2(-/-) mice. However, the prolongation of APs associated with HF is less in KChIP2(-/-) compared to WT, suggesting other compensatory mechanism(s) in the KChIP2(-/-) mouse.

Development of heart failure is independent of K+ channel-interacting protein 2 expression.

Abnormal ventricular repolarization in ion channelopathies and heart disease is a major cause of ventricular arrhythmias and sudden cardiac death. K(+) channel-interacting protein 2 (KChIP2) expression is significantly reduced in human heart failure (HF), contributing to a loss of the transient outward K(+) current (Ito). We aim to investigate the possible significance of a changed KChIP2 expression on the development of HF and proarrhythmia. Transverse aortic constrictions (TAC) and sham operations were performed in wild-type (WT) and KChIP2(-/-) mice. Echocardiography was performed before and every 2 weeks after the operation. Ten weeks post-surgery, surface ECG was recorded and we paced the heart in vivo to induce arrhythmias. Afterwards, tissue from the left ventricle was used for immunoblotting. Time courses of HF development were comparable in TAC-operated WT and KChIP2(-/-) mice. Ventricular protein expression of KChIP2 was reduced by 70% after 10 weeks TAC in WT mice. The amplitudes of the J and T waves were enlarged in KChIP2(-/-) control mice. Ventricular effective refractory period, RR, QRS and QT intervals were longer in mice with HF compared to sham-operated mice of either genotype. Pacing-induced ventricular tachycardia (VT) was observed in 5/10 sham-operated WT mice compared with 2/10 HF WT mice with HF. Interestingly, and contrary to previously published data, sham-operated KChIP2(-/-) mice were resistant to pacing-induced VT resulting in only 1/10 inducible mice. KChIP2(-/-) with HF mice had similar low vulnerability to inducible VT (1/9). Our results suggest that although KChIP2 is downregulated in HF, it is not orchestrating the development of HF. Moreover, KChIP2 affects ventricular repolarization and lowers arrhythmia susceptibility. Hence, downregulation of KChIP2 expression in HF may be antiarrhythmic in mice via reduction of the fast transient outward K(+) current.

Functional enhancement of AT1R potency in the presence of the TPαR is revealed by a comprehensive 7TM receptor co-expression screen.

BACKGROUND: Functional cross-talk between seven transmembrane (7TM) receptors can dramatically alter their pharmacological properties, both in vitro and in vivo. This represents an opportunity for the development of novel therapeutics that potentially target more specific biological effects while causing fewer adverse events. Although several studies convincingly have established the existence of 7TM receptor cross-talk, little is known about the frequencey and biological significance of this phenomenon. METHODOLOGY/PRINCIPAL FINDINGS: To evaluate the extent of synergism in 7TM receptor signaling, we took a comprehensive approach and co-expressed 123 different 7TM receptors together with the angiotensin II type 1 receptor (AT1R) and analyzed how each receptor affected the angiotensin II (AngII) response. To monitor the effect we used integrative receptor activation/signaling assay called Receptor Selection and Amplification Technology (R-SAT). In this screen the thromboxane A2α receptor (TPαR) was the only receptor which significantly enhanced the AngII-mediated response. The TPαR-mediated enhancement of AngII signaling was significantly reduced when a signaling deficient receptor mutant (TPαR R130V) was co-expressed instead of the wild-type TPαR, and was completely blocked both by TPαR antagonists and COX inhibitors inhibiting formation of thromboxane A2 (TXA2). CONCLUSIONS/SIGNIFICANCE: We found a functional enhancement of AT1R only when co-expressed with TPαR, but not with 122 other 7TM receptors. In addition, the TPαR must be functionally active, indicating the AT1R enhancement is mediated by a paracrine mechanism. Since we only found one receptor enhancing AT1R potency, our results suggest that functional augmentation through 7TM receptor cross-talk is a rare event that may require specific conditions to occur.

5'-AMP Activated Protein Kinase is Involved in the Regulation of Myocardial ß-Oxidative Capacity in Mice.

5'-adenosine monophosphate-activated protein kinase (AMPK) is considered central in regulation of energy status and substrate utilization within cells. In heart failure the energetic state is compromised and substrate metabolism is altered. We hypothesized that this could be linked to changes in AMPK activity and we therefore investigated mitochondrial oxidative phosphorylation capacity from the oxidation of long- and medium-chain fatty acids (LCFA and MCFA) in cardiomyocytes from young and old mice expressing a dominant negative AMPKα2 (AMPKα2-KD) construct and their wildtype (WT) littermates. We found a 35-45% (P < 0.05) lower mitochondrial capacity for oxidizing MCFA in AMPKα2-KD of both age-groups, compared to WT. This coincided with marked decreases in protein expression (19/29%, P < 0.05) and activity (14/21%, P < 0.05) of 3-hydroxyacyl-CoA-dehydrogenase (HAD), in young and old AMPKα2-KD mice, respectively, compared to WT. Maximal LCFA oxidation capacity was similar in AMPKα2-KD and WT mice independently of age implying that LCFA-transport into the mitochondria was unaffected by loss of AMPK activity or progressing age. Expression of regulatory proteins of glycolysis and glycogen breakdown showed equivocal effects of age and genotype. These results illustrate that AMPK is necessary for normal mitochondrial function in the heart and that decreased AMPK activity may lead to an altered energetic state as a consequence of reduced capacity to oxidize MCFA. We did not identify any clear aging effects on mitochondrial function.

The angiotensin II type 1 receptor antagonist Losartan binds and activates bradykinin B2 receptor signaling.

The angiotensin II type 1 receptor (AT1R) blocker (ARB) Losartan has cardioprotective effects during ischemia-reperfusion injury and inhibits reperfusion arrhythmias -effects that go beyond the benefits of lowering blood pressure. The renin-angiotensin and kallikrein-kinin systems are intricately connected and some of the cardioprotective effects of Losartan are abolished by blocking the bradykinin B2 receptor (B2R) signaling. In this study, we investigated the ability of six clinically available ARBs to specifically bind and activate the B2R. First, we investigated their ability to activate phosphoinositide (PI) hydrolysis in COS-7 cells transiently expressing the B2R. We found that only Losartan activated the B2R, working as a partial agonist compared to the endogenous ligand bradykinin. This effect was blocked by the B2R antagonist HOE 140. A competitive binding analysis revealed that Losartan does not significantly compete with bradykinin and does not change the binding affinity of bradykinin on the B2R. Furthermore, Losartan but not Candesartan mimicked the ability of bradykinin to increase the recovery of contractile force after metabolic stress in rat atrial tissue strips. In conclusion, Losartan is a partial agonist of the B2R through direct binding and activation, suggesting that B2R agonism could partly explain the beneficial effects of Losartan.

Lack of evidence for AT1R/B2R heterodimerization in COS-7, HEK293, and NIH3T3 cells: how common is the AT1R/B2R heterodimer?

It has been suggested previously ( AbdAlla, S., Lother, H., and Quitterer, U. (2000) Nature 407, 94-98 ) that the angiotensin II type 1 receptor (AT1R) and the bradykinin B2 receptor (B2R) form constitutive heterodimers. Furthermore they demonstrate that AT1R signaling significantly increases in the presence of the B2R. These findings suggest that heterodimerization and potentiation of AT1R signaling is a universal phenomenon that occurs as a natural consequence of simultaneous expression of the two receptors. Hence this potential interaction is of great pharmacological and biological interest that adds an additional layer of complexity to the understanding of the cross-talk between the renin-angiotensin and kallikrein-kinin systems. Given the remarkable significance of this finding, scientists from four independent research groups have set out to reproduce and further examine the potential AT1R/B2R interaction. We have investigated functional potentiation by the B2R of AT1R signaling in three different cell lines using multiple assays including phosphoinositide hydrolysis, ERK activation, beta-arrestin recruitment, and receptor selection and amplification technology, and we have examined dimerization using bioluminescence resonance energy transfer and regulated secretion/aggregation technology. However, although both the AT1Rs and B2Rs were functional in our systems and the systems were fine tuned to detect small changes in receptor function, we failed to detect any functional modulation by or physical interaction between the two receptor proteins. In contrast to the previous observations, our data collectively suggest that AT1R/B2R heterodimerization does not occur as a natural consequence of their simultaneous expression in the same cell nor does the B2R influence the AT1R signaling.

Expression of interleukin-15 in human skeletal muscle effect of exercise and muscle fibre type composition.

The cytokine interleukin-15 (IL-15) has been demonstrated to have anabolic effects in cell culture systems. We tested the hypothesis that IL-15 is predominantly expressed by type 2 skeletal muscle fibres, and that resistance exercise regulates IL-15 expression in muscle. Triceps brachii, vastus lateralis quadriceps and soleus muscle biopsies were obtained from normally physically active, healthy, young male volunteers (n = 14), because these muscles are characterized by having different fibre-type compositions. In addition, healthy, normally physically active male subjects (n = 8) not involved in any kind of resistance exercise underwent a heavy resistance exercise protocol that stimulated the vastus lateralis muscle and biopsies were obtained from this muscle pre-exercise as well as 6, 24 and 48 h post-exercise. IL-15 mRNA levels were twofold higher in the triceps (type 2 fibre dominance) compared with the soleus muscle (type 1 fibre dominance), but Western blotting and immunohistochemistry revealed that muscle IL-15 protein content did not differ between triceps brachii, quadriceps and soleus muscles. Following resistance exercise, IL-15 mRNA levels were up-regulated twofold at 24 h of recovery without any changes in muscle IL-15 protein content or plasma IL-15 at any of the investigated time points. In conclusion, IL-15 mRNA level is enhanced in skeletal muscles dominated by type 2 fibres and resistance exercise induces increased muscular IL-15 mRNA levels. IL-15 mRNA levels in skeletal muscle were not paralleled by similar changes in muscular IL-15 protein expression suggesting that muscle IL-15 may exist in a translationally inactive pool.