Vassilios J. Bezzerides

Mechanism of Persistent Protein Kinase D1 Translocation and Activation

The specificity of many signal transduction pathways relies on the spatiotemporal features of each signaling step. G protein-coupled receptor-mediated activation of protein kinases leads to diverse cellular effects. Upon receptor activation, PKD1 and several C-type protein kinases (PKCs), translocate to the plasma membrane and become catalytically active. Here we show that, unlike PKCs, PKD1 remains active at the membrane for hours. The two DAG binding C1 domains of PKD1 have distinct functional roles in targeting and maintaining PKD1 at the plasma membrane. C1A achieves fast, maximal, and reversible translocation, while C1B translocates partially, but persistently, to the plasma membrane. The persistent localization requires the C1B domain of PKD1, which binds Galphaq. We incorporate the kinetics of PKD1 translocation into a three-state model that suggests how PKD1 binding to DAG and Galphaq uniquely encodes frequency-dependent PKD1 signaling.

Phenotypic screen quantifying differential regulation of cardiac myocyte hypertrophy identifies CITED4 regulation of myocyte elongation

Cardiac hypertrophy is controlled by a highly connected signaling network with many effectors of cardiac myocyte size. Quantification of the contribution of individual pathways to specific changes in shape and transcript abundance is needed to better understand hypertrophy signaling and to improve heart failure therapies. We stimulated cardiac myocytes with 15 hypertrophic agonists and quantitatively characterized differential regulation of 5 shape features using high-throughput microscopy and transcript levels of 12 genes using qPCR. Transcripts measured were associated with phenotypes including fibrosis, cell death, contractility, proliferation, angiogenesis, inflammation, and the fetal cardiac gene program. While hypertrophy pathways are highly connected, the agonist screen revealed distinct hypertrophy phenotypic signatures for the 15 receptor agonists. We then used k-means clustering of inputs and outputs to identify a network map linking input modules to output modules. Five modules were identified within inputs and outputs with many maladaptive outputs grouping together in one module: Bax, C/EBPβ, Serca2a, TNFα, and CTGF. Subsequently, we identified mechanisms underlying two correlations revealed in the agonist screen: correlation between regulators of fibrosis and cell death signaling (CTGF and Bax mRNA) caused by AngII; and myocyte proliferation (CITED4 mRNA) and elongation caused by Nrg1. Follow-up experiments revealed positive regulation of Bax mRNA level by CTGF and an incoherent feedforward loop linking Nrg1, CITED4 and elongation. With this agonist screen, we identified the most influential inputs in the cardiac hypertrophy signaling network for a variety of features related to pathological and protective hypertrophy signaling and shared regulation among cardiac myocyte phenotypes.

Saying Yes to Exercise and NO to Cardiac Injury

Clinical studies demonstrate benefits of exercise in both preventing cardiovascular disease in the general population and mitigating existing disease in cardiovascular patients.1,–,3 Systemic effects of exercise on skeletal muscle and peripheral vessels as well as metabolism and insulin sensitivity undoubtedly contribute to these benefits. However, growing evidence from animal studies suggests that exercise also modulates intrinsic cardiac signaling mechanisms that contribute to its benefits. These benefits are probably best documented in models of ischemic injury but also appear to extend to heart failure in at least some experimental4 and clinical2,3 settings. The effects of physical conditioning on ischemic injury can occur in as little as 3 to 5 days in experimental animal models.5 Interestingly the protective effects of exercise are not limited to the immediate postexercise time period as in ischemic preconditioning but can persist for days afterward.6 In the current issue of Circulation Research , Calvert et al7 investigated the role of nitric oxide (NO) metabolites and β3-adrenergic receptor (β3-AR) signaling in this context through elegant studies in wild-type and genetically modified mouse models. Their work provides new insights into the mechanisms underlying exercise-induced cardioprotection and underscores the potential therapeutic relevance of these pathways. Exercise increases expression of a variety of potentially protective proteins. These include heat-shock proteins (HSPs), both HSP27 and HSP90, as well as stress-related HSP72.8,9 Whereas overexpression of these proteins confers protection of mitochondria from myocardial ischemia-reperfusion injury (MI/R), the cardioprotective effects of exercise can also occur without significant elevations in HSPs.10 Other proposed cardioprotective proteins include upregulation of both mitochrondial and sarcolemmal ATP-sensitive potassium channels, catalase, superoxide dismutase and ER stress proteins.11 Exercise also induces activation of PI3-Kinase/Akt signaling, which is required for physiological …

Modeling Inherited Arrhythmia Disorders Using Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Inherited arrhythmia disorders (IADs) are a group of potentially lethal diseases that remain diagnostic and management challenges. Although the genetic basis for many of these disorders is well known, the pathogenicity of individual mutations and the resulting clinical outcomes are difficult to predict. Treatment options remain imperfect, and optimizing therapy for individual patients can be difficult. Recent advances in the derivation of induced pluripotent stem cells (iPSCs) from patients and creation of genetically engineered human models using CRISPR/Cas9 has the potential to dramatically advance translational arrhythmia research. In this review, we discuss the current state of modeling IADs using human iPSC-derived cardiomyocytes. We also discuss current limitations and areas for further study.

Mitochondrial Cardiomyopathy Caused by Elevated Reactive Oxygen Species and Impaired Cardiomyocyte Proliferation

Rationale: Although mitochondrial diseases often cause abnormal myocardial development, the mechanisms by which mitochondria influence heart growth and function are poorly understood. Objective: To investigate these disease mechanisms, we studied a genetic model of mitochondrial dysfunction caused by inactivation of Tfam (transcription factor A, mitochondrial), a nuclear-encoded gene that is essential for mitochondrial gene transcription and mitochondrial DNA replication. Methods and Results: Tfam inactivation by Nkx2.5Cre caused mitochondrial dysfunction and embryonic lethal myocardial hypoplasia. Tfam inactivation was accompanied by elevated production of reactive oxygen species (ROS) and reduced cardiomyocyte proliferation. Mosaic embryonic Tfam inactivation confirmed that the block to cardiomyocyte proliferation was cell autonomous. Transcriptional profiling by RNA-seq demonstrated the activation of the DNA damage pathway. Pharmacological inhibition of ROS or the DNA damage response pathway restored cardiomyocyte proliferation in cultured fetal cardiomyocytes. Neonatal Tfam inactivation by AAV9-cTnT-Cre caused progressive, lethal dilated cardiomyopathy. Remarkably, postnatal Tfam inactivation and disruption of mitochondrial function did not impair cardiomyocyte maturation. Rather, it elevated ROS production, activated the DNA damage response pathway, and decreased cardiomyocyte proliferation. We identified a transient window during the first postnatal week when inhibition of ROS or the DNA damage response pathway ameliorated the detrimental effect of Tfam inactivation. Conclusions: Mitochondrial dysfunction caused by Tfam inactivation induced ROS production, activated the DNA damage response, and caused cardiomyocyte cell cycle arrest, ultimately resulting in lethal cardiomyopathy. Normal mitochondrial function was not required for cardiomyocyte maturation. Pharmacological inhibition of ROS or DNA damage response pathways is a potential strategy to prevent cardiac dysfunction caused by some forms of mitochondrial dysfunction.

Inhibition of serum and glucocorticoid regulated kinase-1 as novel therapy for cardiac arrhythmia disorders

Alterations in sodium flux (INa) play an important role in the pathogenesis of cardiac arrhythmias and may also contribute to the development of cardiomyopathies. We have recently demonstrated a critical role for the regulation of the voltage-gated sodium channel NaV1.5 in the heart by the serum and glucocorticoid regulated kinase-1 (SGK1). Activation of SGK1 in the heart causes a marked increase in both the peak and late sodium currents leading to prolongation of the action potential duration and an increased propensity to arrhythmia. Here we show that SGK1 directly regulates NaV1.5 channel function, and genetic inhibition of SGK1 in a zebrafish model of inherited long QT syndrome rescues the long QT phenotype. Using computer-aided drug discovery coupled with in vitro kinase assays, we identified a novel class of SGK1 inhibitors. Our lead SGK1 inhibitor (5377051) selectively inhibits SGK1 in cultured cardiomyocytes, and inhibits phosphorylation of an SGK1-specific target as well as proliferation in the prostate cancer cell line, LNCaP. Finally, 5377051 can reverse SGK1’s effects on NaV1.5 and shorten the action potential duration in induced pluripotent stem cell (iPSC)-derived cardiomyocytes from a patient with a gain-of-function mutation in Nav 1.5 (Long QT3 syndrome). Our data suggests that SGK1 inhibitors warrant further investigation in the treatment of cardiac arrhythmias.

Channelopathy as a SUDEP Biomarker in Dravet Syndrome Patient-Derived Cardiac Myocytes

Summary Dravet syndrome (DS) is a severe developmental and epileptic encephalopathy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Most DS patients carry de novo variants in SCN1A, resulting in Nav1.1 haploinsufficiency. Because SCN1A is expressed in heart and in brain, we proposed that cardiac arrhythmia contributes to SUDEP in DS. We generated DS patient and control induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). We observed increased sodium current (INa) and spontaneous contraction rates in DS patient iPSC-CMs versus controls. For the subject with the largest increase in INa, cardiac abnormalities were revealed upon clinical evaluation. Generation of a CRISPR gene-edited heterozygous SCN1A deletion in control iPSCs increased INa density in iPSC-CMs similar to that seen in patient cells. Thus, the high risk of SUDEP in DS may result from a predisposition to cardiac arrhythmias in addition to seizures, reflecting expression of SCN1A in heart and brain.

Low molecular weight heparin as an anticoagulation strategy for left-sided ablation procedures

OBJECTIVE This quality improvement study was implemented to demonstrate consistent and reliable post procedure anticoagulation for patients undergoing left-sided ablations. We evaluated the safety and efficacy of anticoagulation practice during a transition from anticoagulation with overnight infusion of unfractionated heparin to a single subcutaneous injection of low molecular weight heparin. METHODS Outcomes for patients who received unfractionated heparin from January 2014 to October 2014, were compared with outcomes of patients who received low molecular weight heparin from October 2014 to October 2015. Complications prepractice and postpractice change were documented and compared to establish confidence in the practice change and confirm the safety of the anticoagulation therapy management. RESULTS There were no differences in the type or frequency of complications/adverse events demonstrated between the patients who had received unfractionated heparin for anticoagulation prophylaxis and those who received low molecular weight heparin. No thromboembolic events were reported or documented with either anticoagulation strategy. After confidence in the safety and efficacy of the practice change was established, a decision was made to discharge patients home the same day as there procedure, effectively reducing inpatient bed days and overall costs. CONCLUSION Administration of low molecular weight heparin provides predictable anticoagulation and equally safe as unfractionated heparin when administered to patients post left-sided ablation. A secondary gain has been reduction of procedural costs by elimination of the previously required inpatient observation stay.

Cardiac Events During Competitive, Recreational, and Daily Activities in Children and Adolescents With Long QT Syndrome

Background The 2005 Bethesda Conference Guidelines advise patients with long QT syndrome against competitive sports. We assessed cardiac event rates during competitive and recreational sports, and daily activities among treated long QT syndrome patients. Methods and Results Long QT syndrome patients aged ≥4 years treated with anti‐adrenergic therapy were included. Demographics included mechanism of presentation, corrected QT interval pretreatment, symptom history, medication compliance, and administration of QT‐prolonging medications. Corrected QT interval ≥550 ms or prior cardiac arrest defined high risk. Sports were categorized by cardiovascular demand per the 2005 Bethesda Conference Guidelines. Each was classified as recreational or competitive. One hundred seventy‐two patients (90; 52% female) with median age 15.2 years (interquartile range 11.4, 19.4) were included. Evaluation was performed for family history (102; 59%), incidental finding (34; 20%), and symptoms (36; 21%). Median corrected QT interval was 474 ms (interquartile range 446, 496) and 14 patients (8%) were deemed high risk. Treatment included β‐blockers (171; 99%), implantable cardioverter‐defibrillator (27; 16%), left cardiac sympathetic denervation (7; 4%), and pacemaker (3; 2%). Sports participation was recreational (66; 38%) or competitive (106; 62%), with 92 (53%) exercising against the Bethesda Conference Guidelines. There were no cardiac events in competitive athletes and no deaths. There were 13 cardiac events in 9 previously symptomatic patients during either recreational exercise or activities of daily life. Conclusions In this cohort of appropriately managed children with long QT syndrome, cardiac event rates were low and occurred during recreational but not competitive activities. This study further supports the need for increased assessment of arrhythmia risk during exercise in this patient population.

Genotype-phenotype-guided medical and surgical intervention in long QT syndrome

Introduction Long QT syndrome (LQTS) is an inherited cardiac arrhythmia disorder characterized by QT prolongation and/or abnormal T-wave morphology on the electrocardiogram (ECG) and symptoms including syncope, cardiac arrest, and sudden cardiac death. Beta-blockers have long been accepted as the first line of treatment, and are highly effective, especially in patients with LQTS type 1. This type of LQTS is related to abnormalities in the KvLQT1, which controls the slow delayed rectifier potassium current of cardiac repolarization, where cardiac events are typically triggered by adrenergic stimulation. An alternative antiadrenergic treatment strategy is the use of left cardiac sympathetic denervation (LCSD), initially advocated as an effective treatment in addition to beta-blockade for those with recurrent symptoms and appropriate implantable cardioverter-defibrillator discharges, but more recently used in patients who are intolerant to beta-blockade. The effects of LCSD are to both interrupt the main source of myocardial norepinephrine, thereby limiting the catecholaminergic activation of dysfunctional KvLQT1 channels, and to increase myocardial refractoriness and fibrillatory threshold. This report details patient-specific treatment strategies used in a child with LQTS type 1 who had complications on beta-blocker therapy. The strategies were based on genetic results and continual assessment of the phenotype.