Donald M Bers

The heart pumps blood against the mechanical afterload from arterial resistance, and increased afterload may alter cardiac electrophysiology and contribute to life-threatening arrhythmias. However, the cellular and molecular mechanisms underlying mechanoelectric coupling in cardiomyocytes remain unclear. We developed an innovative patch-clamp-in-gel technology to embed cardiomyocytes in a three-dimensional (3D) viscoelastic hydrogel that imposes an afterload during regular myocyte contraction. Here, we investigated how afterload affects action potentials, ionic currents, intracellular Ca2+ transients, and cell contraction of adult rabbit ventricular cardiomyocytes...

Aim : Reperfusion after myocardial ischemia causes cellular injury, in part due to changes in mitochondrial Ca2+ handling, oxidative stress, and myocyte energetics. We have previously shown that the 18-kDa translocator protein of the outer mitochondrial membrane (TSPO) can modulate Ca2+ handling. Here, we aim to evaluate the role of the TSPO in ischemia/reperfusion (I/R) injury. Methods : Rabbit ventricular myocytes underwent simulated acute ischemia (20 min) and reperfusion (at 15 min, 1 h, and 3 h) in the absence and presence of 50 μM PK11195, a TSPO inhibitor...

Sarcoplasmic reticulum (SR) Ca2+ cycling is tightly regulated by ryanodine receptor (RyR) Ca2+ release and sarco-endoplasmic reticulum Ca2+ -ATPase (SERCA) Ca2+ uptake during each excitation-contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca2+ alternans in the intact rabbit heart, which contributes to arrhythmogenic action potential duration (APD) alternans. Recent studies have also implicated impaired SERCA function, a key feature of heart failure, in cardiac alternans and arrhythmias...

KEY POINTS: Increasing sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump activity enhances sarcoplasmic reticulum calcium (Ca) load, which increases both ryanodine receptor opening and driving force of Ca release flux. Both of these effects promote Ca spark formation and wave propagation. However, increasing SERCA activity also accelerates local cytosolic Ca decay as the wave front travels to the next cluster, which limits wave propagation. As a result, increasing SERCA pump activity has a biphasic effect on the propensity of arrhythmogenic Ca waves, but a monotonic effect to increase Ca spark frequency and amplitude...

Rationale: Diabetic hyperglycemia is associated with cardiac dysfunction and increased arrhythmia risk, and calcium/calmodulin-dependent protein kinase II (CaMKII) function has been implicated. CaMKII activity is promoted by both oxidation and O linked β-N-acetylglucosamine (O GlcNAc) of known CaMKII sites. Objective: To investigate which post-translational modifications occur in human diabetic hearts and how they alter electrophysiological and Ca2+ handling properties in hyperglycemia. Methods and Results: We assessed echocardiography, electrophysiology, Ca2+ -handling, and protein expression in site-specific CaMKII mutant mice (O GlcNAc-resistant S280A and oxidation-resistant MM281/2VV knock-ins, and global and cardiac-specific knockouts), in myocytes subjected to acute hyperglycemia and angiotensin II (Ang-II) and mice after streptozotocin injections (to induce diabetes)...

No abstract text is available yet for this article.

Rationale: Increased myocardial activity of Ca/calmodulin-dependent kinase II (CaMKII) leads to heart failure (HF) and arrhythmias. In Drosophila neurons, interaction of CaMKII with Ca/CaM-dependent serine protein kinase (CASK) has been shown to inhibit CaMKII activity, but the consequences of this regulation for HF and ventricular arrhythmias are unknown. Objective: We hypothesize that CASK associates with CaMKII in human and mouse hearts thereby limiting CaMKII activity, and that altering CASK expression in mice changes CaMKII activity accordingly, with functional consequences for contractile function and arrhythmias...

Nuclear histone deacetylase 4 (HDAC4) represses MEF2-mediated transcription, implicated in the development of heart failure. CaMKII-dependent phosphorylation drives nucleus-to-cytoplasm HDAC4 shuttling, but protein kinase A (PKA) is also linked to HDAC4 translocation. However, the interplay of CaMKII and PKA in regulating adult cardiomyocyte HDAC4 translocation is unclear. Here we sought to determine the interplay of PKA- and CaMKII-dependent HDAC4 phosphorylation and translocation in adult mouse, rabbit and human ventricular myocytes...

AIM: Diabetic hyperglycemia is associated with increased arrhythmia risk. We aimed to investigate whether hyperglycemia alone can be accountable for arrhythmias or whether it requires the presence of additional pathological factors. METHODS AND RESULTS: Action potentials (APs) and arrhythmogenic spontaneous diastolic activities were measured in isolated murine ventricular, rabbit atrial and ventricular myocytes acutely exposed to high glucose. Acute hyperglycemia increased the short-term variability (STV) of action potential duration (APD), enhanced delayed afterdepolarizations and the inducibility of APD alternans during tachypacing in both murine and rabbit atrial and ventricular myocytes...

No abstract text is available yet for this article.

Rationale: The mechanisms underlying atrial fibrillation (AF), the most common clinical arrhythmia, are poorly understood. Nucleoplasmic Ca2+ regulates gene-expression, but the nature and significance of nuclear Ca2+ -changes in AF are largely unknown. Objective: To elucidate mechanisms by which AF alters atrial cardiomyocyte (CM) nuclear Ca2+ ([Ca2+ ]Nuc ) and Ca2+ /calmodulin-dependent protein kinase-II (CaMKII)-related signaling. Methods and Results: Atrial CMs were isolated from control and AF-dogs (kept in AF by atrial tachypacing [600 bpm x 1 week])...

RATIONALE: We recently discovered pivotal contributions of stress kinase JNK2 (c-Jun N-terminal kinase isoform 2) in increased risk of atrial fibrillation through enhanced diastolic sarcoplasmic reticulum (SR) calcium (Ca2+ ) leak via RyR2 (ryanodine receptor isoform 2). However, the role of JNK2 in the function of the SERCA2 (SR Ca2+ -ATPase), essential in maintaining SR Ca2+ content cycling during each heartbeat, is completely unknown. OBJECTIVE: To test the hypothesis that JNK2 increases SERCA2 activity SR Ca2+ content and exacerbates an arrhythmic SR Ca2+ content leak-load relationship...

Chronic hyperglycemia and diabetes lead to impaired cardiac repolarization, K+ channel remodeling and increased arrhythmia risk. However, the exact signaling mechanism by which diabetic hyperglycemia regulates cardiac K+ channels remains elusive. Here, we show that acute hyperglycemia increases inward rectifier K+ current (IK1 ), but reduces the amplitude and inactivation recovery time of the transient outward K+ current (Ito ) in mouse, rat, and rabbit myocytes. These changes were all critically dependent on intracellular O-GlcNAcylation...

RATIONALE: β1 ARs (β1 -adrenoceptors) exist at intracellular membranes and OCT3 (organic cation transporter 3) mediates norepinephrine entry into cardiomyocytes. However, the functional role of intracellular β1 AR in cardiac contractility remains to be elucidated. OBJECTIVE: Test localization and function of intracellular β1 AR on cardiac contractility. METHODS AND RESULTS: Membrane fractionation, super-resolution imaging, proximity ligation, coimmunoprecipitation, and single-molecule pull-down demonstrated a pool of β1 ARs in mouse hearts that were associated with sarco/endoplasmic reticulum Ca2+ -ATPase at the sarcoplasmic reticulum (SR)...

RATIONALE: CaMKII (Ca2+ -Calmodulin dependent protein kinase) δC activation is implicated in pathological progression of heart failure (HF) and CaMKIIδC transgenic mice rapidly develop HF and arrhythmias. However, little is known about early spatio-temporal Ca2+ handling and CaMKII activation in hypertrophy and HF. OBJECTIVE: To measure time- and location-dependent activation of CaMKIIδC signaling in adult ventricular cardiomyocytes, during transaortic constriction (TAC) and in CaMKIIδC transgenic mice...

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have enormous potential for the study of human cardiac disorders. However, their physiological immaturity severely limits their utility as a model system and their adoption for drug discovery. Here, we describe maturation media designed to provide oxidative substrates adapted to the metabolic needs of human iPSC (hiPSC)-CMs. Compared with conventionally cultured hiPSC-CMs, metabolically matured hiPSC-CMs contract with greater force and show an increased reliance on cardiac sodium (Na+ ) channels and sarcoplasmic reticulum calcium (Ca2+ ) cycling...

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death...

RATIONALE: Mitochondrial Ca2+ loading augments oxidative metabolism to match functional demands during times of increased work or injury. However, mitochondrial Ca2+ overload also directly causes mitochondrial rupture and cardiomyocyte death during ischemia-reperfusion injury by inducing mitochondrial permeability transition pore opening. The MCU (mitochondrial Ca2+ uniporter) mediates mitochondrial Ca2+ influx, and its activity is modulated by partner proteins in its molecular complex, including the MCUb subunit...

BACKGROUND: Rapid delayed rectifier K+ current (IKr ) and late Na+ current (INaL ) significantly shape the cardiac action potential (AP). Changes in their magnitudes can cause either long or short QT syndromes associated with malignant ventricular arrhythmias and sudden cardiac death. METHODS: Physiological self AP-clamp was used to measure INaL and IKr during the AP in rabbit and porcine ventricular cardiomyocytes to test our hypothesis that the balance between IKr and INaL affects repolarization stability in health and disease conditions...

Rationale: Diabetes mellitus (DM) is a complex, multisystem disease, affecting large populations worldwide. Chronic CaMKII activation may occur in DM and be arrhythmogenic. Diabetic hyperglycemia was shown to activate CaMKII by (1) O-linked attachment of N-acetylglucosamine (O-GlcNAc) at S280 leading to arrhythmia and (2) a reactive-oxygen species (ROS) mediated oxidation of CaMKII, that can increase post-infarction mortality. Objective: To test whether high extracellular [glucose] (Hi-Glu) promotes ventricular myocyte ROS generation and the role played by CaMKII...