Jakub Sroubek

Programmed ventricular stimulation for risk stratification in the Brugada syndrome: A pooled analysis

Background— The role of programmed ventricular stimulation in identifying patients with Brugada syndrome at the highest risk for sudden death is uncertain. Methods and Results— We performed a systematic review and pooled analysis of prospective, observational studies of patients with Brugada syndrome without a history of sudden cardiac arrest who underwent programmed ventricular stimulation. We estimated incidence rates and relative hazards of cardiac arrest or implantable cardioverter-defibrillator shock. We analyzed individual-level data from 8 studies comprising 1312 patients who experienced 65 cardiac events (median follow-up, 38.3 months). A total of 527 patients were induced into arrhythmias with up to triple extrastimuli. Induction was associated with cardiac events during follow-up (hazard ratio, 2.66; 95% confidence interval [CI], 1.44–4.92, P<0.001), with the greatest risk observed among those induced with single or double extrastimuli. Annual event rates varied substantially by syncope history, presence of spontaneous type 1 ECG pattern, and arrhythmia induction. The lowest risk occurred in individuals without syncope and with drug-induced type 1 patterns (0.23%, 95% CI, 0.05–0.68 for no induced arrhythmia with up to double extrastimuli; 0.45%, 95% CI, 0.01–2.49 for induced arrhythmia), and the highest risk occurred in individuals with syncope and spontaneous type 1 patterns (2.55%, 95% CI, 1.58–3.89 for no induced arrhythmia; 5.60%, 95% CI, 2.98–9.58 for induced arrhythmia). Conclusions— In patients with Brugada syndrome, arrhythmias induced with programmed ventricular stimulation are associated with future ventricular arrhythmia risk. Induction with fewer extrastimuli is associated with higher risk. However, clinical risk factors are important determinants of arrhythmia risk, and lack of induction does not necessarily portend low ventricular arrhythmia risk, particularly in patients with high-risk clinical features.

PKA phosphorylation of HERG protein regulates the rate of channel synthesis

Acute changes in cAMP and protein kinase A (PKA) signaling can regulate ion channel protein activities such as gating. Effects on channels due to chronic PKA signaling, as in stress or disease states, are less understood. We examined the effects of prolonged PKA activity on the human ether-a-go-go-related gene (HERG) K(+) channel in stably transfected human embryonic kidney (HEK)293 cells. Sustained elevation of cAMP by either chlorophenylthiol (CPT)-cAMP or forskolin increased the HERG channel protein abundance two- to fourfold within 24 h, with measurable difference as early as 4 h. The cAMP-induced augmentation was not due to changes in transcription and was specific for HERG compared with other cardiac K(+) channels (Kv1.4, Kv1.5, Kir2.1, and KvLQT1). PKA activity was necessary for the effect on HERG protein and did not involve other cAMP signaling pathways. Direct PKA phosphorylation of the HERG protein was responsible for the cAMP-induced augmentation. Enhanced abundance of HERG protein was detected in endoplasmic reticulum-enriched, Golgi, and plasma membrane without significant changes in trafficking rates or patterns. An increase in the K(+) current density carried by the HERG channel was also observed, but with a delay, suggesting that traffic to the surface is rate-limiting traffic. Acceleration of the HERG protein synthesis rate was the primary factor in the cAMP/PKA effect with lesser effects on protein stability. These results provide evidence for a novel mechanism whereby phosphorylation of a nascent protein dictates its rate of synthesis, resetting its steady-state abundance.

Protein Kinase A Activity at the Endoplasmic Reticulum Surface Is Responsible for Augmentation of Human ether-a-go-go-related Gene Product (HERG)

Human ether-a-go-go-related gene product (HERG) is a cardiac potassium channel commonly implicated in the pathogenesis of the long QT syndrome, type 2 (LQT2). LQT2 mutations typically have incomplete penetrance and affect individuals at various stages of their lives; this may mirror variations in intracellular signaling and HERG regulation. Previous work showed that sustained protein kinase A (PKA) activity augments HERG protein abundance by a mechanism that includes enhanced protein translation. To investigate the subcellular site of this regulation, we generated site-specific probes to the cytoplasmic surface of the endoplasmic reticulum (ER), the presumed locale of channel synthesis. Real-time FRET-based indicators demonstrated both cAMP and PKA activity at the ER. A PKA inhibitor targeted to the ER surface (termed p4PKIg) completely abolished PKA-mediated augmentation of HERG in HEK293 cells as well as rat neonatal cardiomyocytes. Immunofluorescence co-localization, targeted FRET-based PKA biosensors, phospho-specific antibodies, and in vivo phosphorylation experiments confirmed that p4PKIg is preferentially active at the ER surface rather than the plasma membrane. Rerouting this inhibitor to the outer mitochondrial membrane diminishes its ability to block cAMP-dependent HERG induction. Our results support a model where PKA-dependent regulation of HERG synthesis occurs at the ER surface. Furthermore, reagents generated for this study provide novel experimental tools to probe compartmentalized cAMP/PKA signaling within cells.

Post-Transcriptional Control of Human Ether-a-go-go-Related Gene Potassium Channel Protein by α-Adrenergic Receptor Stimulation

Stimulation of α1-adrenoreceptors (α1-AR) acutely alters ion channel behavior via several signaling pathways [calcium and protein kinase C (PKC)]. Little is known about sustained α1-adrenergic/PKC signaling and channel regulation as may occur during cardiovascular disease states. Here we describe the effects of prolonged α1A-AR and PKC activity on human ether-a-go-go-related gene (HERG) K+ channels (Kv11.1) expressed in a heterologous expression system. Stimulation of α1A-AR with phenylephrine or direct activation of PKC with phorbol ester increased HERG channel protein abundance and K+ current density in a time- and dose-dependent manner. Channel augmentation reached a steady-state plateau within 24 h with a 2- to 6-fold induction. Phorbol ester and moderate α1A-AR stimulation enhanced HERG abundance in a PKC-dependent fashion but with stronger α1A-adrenergic stimulation; protein kinase A (PKA)-dependent activity also contributed. Comparable channel induction of other cardiac K+ channels was not seen in this system. Comparison of wild-type HERG and channels with either mutated PKC phosphorylation sites (HERGΔPKC) or mutated PKA phosphorylation sites (HERGΔPKA) suggested that the mechanisms of augmentation of HERG by the two kinases were partially overlapping. The PKC-dependent effect was largely due to enhanced synthetic rates. Stimulation of α1-AR in cultured rat neonatal cardiac myocytes also enhanced the abundance of ERG channels. These findings show that α1A-AR stimulation is capable of influencing the balance of HERG channel synthesis and degradation via multiple signaling pathways, a process that may have relevance in cardiac diseases and treatment.

A-Kinase Anchoring Protein Targeting of Protein Kinase A and Regulation of HERG Channels

Adrenergic stimulation of the heart initiates a signaling cascade in cardiac myocytes that increases the concentration of cAMP. Although cAMP elevation may occur over a large area of a target-organ cell, its effects are often more restricted due to local concentration of its main effector, protein kinase A (PKA), through A-kinase anchoring proteins (AKAPs). The HERG potassium channel, which produces the cardiac rapidly activating delayed rectifying K+ current (IKr), is a target for cAMP/PKA regulation. PKA regulation of the current may play a role in the pathogenesis of hereditary and acquired abnormalities of the channel leading to cardiac arrhythmia. We examined the possible role for AKAP-mediated regulation of HERG channels. Here, we report that the PKA-RII-specific AKAP inhibitory peptide AKAP-IS perturbs the distribution of PKA-RII and diminishes the PKA-dependent phosphorylation of HERG protein. The functional consequence of AKAP-IS is a reversal of cAMP-dependent regulation of HERG channel activity. In further support of AKAP-mediated targeting of kinase to HERG, PKA activity was coprecipitated from HERG expressed in HEK cells. Velocity gradient centrifugation of solubilized porcine cardiac membrane proteins showed that several PKA-RI and PKA-RII binding proteins cosediment with ERG channels. A physical association of HERG with several specific AKAPs with known cardiac expression, however, was not demonstrable in heterologous cotransfection studies. These results suggest that one or more AKAP(s) targets PKA to HERG channels and may contribute to the acute regulation of IKr by cAMP.

Post-transcriptional control of HERG potassium channel protein by α-adrenergic receptor stimulation.

Stimulation of α1-adrenoreceptors (α1-AR) acutely alters ion channel behavior via several signaling pathways [calcium and protein kinase C (PKC)]. Little is known about sustained α1-adrenergic/PKC signaling and channel regulation as may occur during cardiovascular disease states. Here we describe the effects of prolonged α1A-AR and PKC activity on human ether-a-go-go-related gene (HERG) K+ channels (Kv11.1) expressed in a heterologous expression system. Stimulation of α1A-AR with phenylephrine or direct activation of PKC with phorbol ester increased HERG channel protein abundance and K+ current density in a time- and dose-dependent manner. Channel augmentation reached a steady-state plateau within 24 h with a 2- to 6-fold induction. Phorbol ester and moderate α1A-AR stimulation enhanced HERG abundance in a PKC-dependent fashion but with stronger α1A-adrenergic stimulation; protein kinase A (PKA)-dependent activity also contributed. Comparable channel induction of other cardiac K+ channels was not seen in this system. Comparison of wild-type HERG and channels with either mutated PKC phosphorylation sites (HERGΔPKC) or mutated PKA phosphorylation sites (HERGΔPKA) suggested that the mechanisms of augmentation of HERG by the two kinases were partially overlapping. The PKC-dependent effect was largely due to enhanced synthetic rates. Stimulation of α1-AR in cultured rat neonatal cardiac myocytes also enhanced the abundance of ERG channels. These findings show that α1A-AR stimulation is capable of influencing the balance of HERG channel synthesis and degradation via multiple signaling pathways, a process that may have relevance in cardiac diseases and treatment.

Quantitative T-wave alternans analysis for sudden cardiac death risk assessment and guiding therapy: answered and unanswered questions ☆,☆☆ For: Proceedings of ICE2015 Comandatuba, Brazil, Sudden Death Symposium

This review addresses current questions regarding use of T-wave alternans to stratify risk for sudden cardiac death. Both of the currently available commercial methodologies, namely, the frequency-domain spectral method and the time-domain modified moving average (MMA) method, are supported by guideline statements, cleared by the US FDA, and covered by the US Center for Medicare and Medicaid services. Similar numbers of patients have been enrolled in predictive studies; odds ratios generated by the two methods are similar including in a head-to-head study. However, in two prospective studies, prediction by TWA with the spectral method was negative, likely due to withdrawal of beta-blockade before the test with later resumption, while all studies with MMA have achieved prediction when the commercial software was used appropriately. Questions currently undergoing investigation include TWAs potential to guide ICD implantation, to track changes in risk during cardiac disease progression, and to evaluate the adequacy of medical therapy.

The use of Bcl-2 over-expression to stabilize hybridomas specific to the HERG potassium channel

We encountered a high degree of clonal hybridoma loss in the course of generating antibodies specific for the hERG potassium channel. A protein that is crucial for controlling heart rhythm, is abundant in parts of the brain and is abnormally expressed in some tumors. Intracellular domains of the protein were used for immunogens and generated adequate antibody responses in mice. Subsequent hybridomas created using Ag8 myeloma fusion partner yielded clones that secreted specific antibody but none could be successfully maintained in culture. A variety of mechanisms, including polyploidy inherent to hybridoma development or production of cytotoxic antibodies, may be responsible for eventual loss of cell viability by mechanisms that may include apoptosis. When spleen cells were fused to the NSO myeloma cell line that stably over-expresses the anti-apoptotic protein Bcl-2, hybridoma clones were generated that remained viable in culture with high level of hERG-specific antibody production. When the parental NSO cell line not over-expressing Bcl-2 was used, no stable hybridomas were produced. Antibodies secreted by NSO-Bcl-2 hybridomas were specific for hERG and performed well in immunoblot, immunoprecipitation and immunofluorescence assays. This work demonstrates a feasible option when faced with antigens that seem to be associated with clonal instability in the process of generating monoclonal antibodies.

Primary Prevention Implantable Cardiac Defibrillator Trials: What Have We Learned?

Patients with impaired left ventricular systolic function frequently die suddenly because of arrhythmic and nonarrhythmic causes. Nine trials have evaluated the utility of implantable cardioverter-defibrillator (ICDs) primary prevention of sudden cardiac death. Individuals with stable ischemic heart disease (no recent myocardial infarction), especially those with inducible arrhythmias, seem to derive the highest mortality benefit from prophylactic ICD use. The role of ICDs in other patient populations is much less clear and may even be harmful. The use of antiarrhythmic medications has not been shown to improve survival in any patient population at risk for sudden death.

Programmed Ventricular Stimulation for Risk Stratification in the Brugada SyndromeCLINICAL PERSPECTIVE: A Pooled Analysis

Background— The role of programmed ventricular stimulation in identifying patients with Brugada syndrome at the highest risk for sudden death is uncertain. Methods and Results— We performed a systematic review and pooled analysis of prospective, observational studies of patients with Brugada syndrome without a history of sudden cardiac arrest who underwent programmed ventricular stimulation. We estimated incidence rates and relative hazards of cardiac arrest or implantable cardioverter-defibrillator shock. We analyzed individual-level data from 8 studies comprising 1312 patients who experienced 65 cardiac events (median follow-up, 38.3 months). A total of 527 patients were induced into arrhythmias with up to triple extrastimuli. Induction was associated with cardiac events during follow-up (hazard ratio, 2.66; 95% confidence interval [CI], 1.44–4.92, P<0.001), with the greatest risk observed among those induced with single or double extrastimuli. Annual event rates varied substantially by syncope history, presence of spontaneous type 1 ECG pattern, and arrhythmia induction. The lowest risk occurred in individuals without syncope and with drug-induced type 1 patterns (0.23%, 95% CI, 0.05–0.68 for no induced arrhythmia with up to double extrastimuli; 0.45%, 95% CI, 0.01–2.49 for induced arrhythmia), and the highest risk occurred in individuals with syncope and spontaneous type 1 patterns (2.55%, 95% CI, 1.58–3.89 for no induced arrhythmia; 5.60%, 95% CI, 2.98–9.58 for induced arrhythmia). Conclusions— In patients with Brugada syndrome, arrhythmias induced with programmed ventricular stimulation are associated with future ventricular arrhythmia risk. Induction with fewer extrastimuli is associated with higher risk. However, clinical risk factors are important determinants of arrhythmia risk, and lack of induction does not necessarily portend low ventricular arrhythmia risk, particularly in patients with high-risk clinical features.