Molecular Determinants of Altered Ca 2+ Handling in Human Chronic Atrial Fibrillation

Fine tuning contractility: atrial sarcomere function in health and disease

The molecular mechanisms of sarcomere proteins underlie the contractile function of the heart. Although our understanding of the sarcomere has grown tremendously, the focus has been on ventricular sarcomere isoforms due to the critical role of the ventricle in health and disease. However, atrial-specific or -enriched myofilament protein isoforms, as well as isoforms that become expressed in disease, provide insight into ways this complex molecular machine is fine-tuned. Here, we explore how atrial-enriched sarcomere protein composition modulates contractile function to fulfill the physiological requirements of atrial function. We review how atrial dysfunction negatively affects the ventricle and the many cardiovascular diseases that have atrial dysfunction as a comorbidity. We also cover the pathophysiology of mutations in atrial-enriched contractile proteins and how they can cause primary atrial myopathies. Finally, we explore what is known about contractile function in various forms of atrial fibrillation. The differences in atrial function in health and disease underscore the importance of better studying atrial contractility, especially as therapeutics currently in development to modulate cardiac contractility may have different effects on atrial sarcomere function.

Protein Kinase D Plays a Crucial Role in Maintaining Cardiac Homeostasis by Regulating Post-Translational Modifications of Myofilament Proteins

Protein kinase D (PKD) enzymes play important roles in regulating myocardial contraction, hypertrophy, and remodeling. One of the proteins phosphorylated by PKD is titin, which is involved in myofilament function. In this study, we aimed to investigate the role of PKD in cardiomyocyte function under conditions of oxidative stress. To do this, we used mice with a cardiomyocyte-specific knock-out of Prkd1, which encodes PKD1 (Prkd1loxP/loxP; αMHC-Cre; PKD1 cKO), as well as wild type littermate controls (Prkd1loxP/loxP; WT). We isolated permeabilized cardiomyocytes from PKD1 cKO mice and found that they exhibited increased passive stiffness (Fpassive), which was associated with increased oxidation of titin, but showed no change in titin ubiquitination. Additionally, the PKD1 cKO mice showed increased myofilament calcium (Ca2+) sensitivity (pCa50) and reduced maximum Ca2+-activated tension. These changes were accompanied by increased oxidation and reduced phosphorylation of the small myofilament protein cardiac myosin binding protein C (cMyBPC), as well as altered phosphorylation levels at different phosphosites in troponin I (TnI). The increased Fpassive and pCa50, and the reduced maximum Ca2+-activated tension were reversed when we treated the isolated permeabilized cardiomyocytes with reduced glutathione (GSH). This indicated that myofilament protein oxidation contributes to cardiomyocyte dysfunction. Furthermore, the PKD1 cKO mice exhibited increased oxidative stress and increased expression of pro-inflammatory markers interleukin (IL)-6, IL-18, and tumor necrosis factor alpha (TNF-α). Both oxidative stress and inflammation contributed to an increase in microtubule-associated protein 1 light chain 3 (LC3)-II levels and heat shock response by inhibiting the mammalian target of rapamycin (mTOR) in the PKD1 cKO mouse myocytes. These findings revealed a previously unknown role for PKD1 in regulating diastolic passive properties, myofilament Ca2+ sensitivity, and maximum Ca2+-activated tension under conditions of oxidative stress. Finally, we emphasized the importance of PKD1 in maintaining the balance of oxidative stress and inflammation in the context of autophagy, as well as cardiomyocyte function.

N-Terminal Fragment of Cardiac Myosin Binding Protein C Modulates Cooperative Mechanisms of Thin Filament Activation in Atria and Ventricles

Cardiac myosin binding protein C (cMyBP-C) is one of the essential control components of the myosin cross-bridge cycle. The C-terminal part of cMyBP-C is located on the surface of the thick filament, and its N-terminal part interacts with actin, myosin, and tropomyosin, affecting both kinetics of the ATP hydrolysis cycle and lifetime of the cross-bridge, as well as calcium regulation of the actin-myosin interaction, thereby modulating contractile function of myocardium. The role of cMyBP-C in atrial contraction has not been practically studied. We examined effect of the N-terminal C0-C1-m-C2 (C0-C2) fragment of cMyBP-C on actin-myosin interaction using ventricular and atrial myosin in an in vitro motility assay. The C0-C2 fragment of cMyBP-C significantly reduced the maximum sliding velocity of thin filaments on both myosin isoforms and increased the calcium sensitivity of the actin-myosin interaction. The C0-C2 fragment had different effects on the kinetics of ATP and ADP exchange, increasing the affinity of ventricular myosin for ADP and decreasing the affinity of atrial myosin. The effect of the C0-C2 fragment on the activation of the thin filament depended on the myosin isoforms. Atrial myosin activates the thin filament less than ventricular myosin, and the C0-C2 fragment makes these differences in the myosin isoforms more pronounced.

Proteolytic degradation of atrial sarcomere proteins underlies contractile defects in atrial fibrillation

Aims

Atrial fibrillation (AFib) is the most common cardiac rhythm disturbance. Treatment of AFib involves restoration of the atrial electrical rhythm. Following rhythm restoration, a period of depressed mechanical function known as atrial stunning occurs that involves decreased blood flow velocity and reduced atrial contractility. This suggests that defects in contractility occur in AFib and are revealed upon restoration of rhythm. The aim of this project is to define the contractile remodeling that occurs in AFib.

Methods and Results

To assess contractile function, we used a canine atrial tachypacing model of induced AFib. Mass spectrometry analysis showed dysregulation of contractile proteins in samples from AFib compared to sinus rhythm atria. Atrial cardiomyocytes showed reduced force of contraction in skinned single cardiomyocyte calcium-force studies. There were no significant differences in myosin heavy chain isoform expression. Resting tension is decreased in the AFib samples correlating with reduced full-length titin in the sarcomere. We measured degradation of other myofilament proteins including cMyBP-C, actinin, and cTnI, showing significant degradation in the AFib samples compared to sinus rhythm atria. Many of the protein degradation products appeared as discrete cleavage products that are generated by calpain proteolysis. We assessed calpain activity and found it to be significantly increased. Skinned cardiomyocytes from AFib atria showed decreased troponin I phosphorylation, consistent with the increased calcium sensitivity that was found within these cardiomyocytes.

Conclusions

With these results it can be concluded that AFib causes alterations in contraction that can be explained by both molecular changes occurring in myofilament proteins and overall myofilament protein degradation. These results provide an understanding of the contractile remodeling that occurs in AFib and provides insight into the molecular explanation for atrial stunning and the increased risk of atrial thrombus and stroke in AFib.

Integrative human atrial modelling unravels interactive protein kinase A and Ca2+/calmodulin-dependent protein kinase II signalling as key determinants of atrial arrhythmogenesis

AIMS

Atrial fibrillation (AF), the most prevalent clinical arrhythmia, is associated with atrial remodelling manifesting as acute and chronic alterations in expression, function, and regulation of atrial electrophysiological and Ca2+-handling processes. These AF-induced modifications crosstalk and propagate across spatial scales creating a complex pathophysiological network, which renders AF resistant to existing pharmacotherapies that predominantly target transmembrane ion channels. Developing innovative therapeutic strategies requires a systems approach to disentangle quantitatively the pro-arrhythmic contributions of individual AF-induced alterations.

METHODS AND RESULTS

Here, we built a novel computational framework for simulating electrophysiology and Ca2+-handling in human atrial cardiomyocytes and tissues, and their regulation by key upstream signalling pathways [i.e. protein kinase A (PKA), and Ca2+/calmodulin-dependent protein kinase II (CaMKII)] involved in AF-pathogenesis. Populations of atrial cardiomyocyte models were constructed to determine the influence of subcellular ionic processes, signalling components, and regulatory networks on atrial arrhythmogenesis. Our results reveal a novel synergistic crosstalk between PKA and CaMKII that promotes atrial cardiomyocyte electrical instability and arrhythmogenic triggered activity. Simulations of heterogeneous tissue demonstrate that this cellular triggered activity is further amplified by CaMKII- and PKA-dependent alterations of tissue properties, further exacerbating atrial arrhythmogenesis.

CONCLUSIONS

Our analysis reveals potential mechanisms by which the stress-associated adaptive changes turn into maladaptive pro-arrhythmic triggers at the cellular and tissue levels and identifies potential anti-AF targets. Collectively, our integrative approach is powerful and instrumental to assemble and reconcile existing knowledge into a systems network for identifying novel anti-AF targets and innovative approaches moving beyond the traditional ion channel-based strategy.

PPP1R12C Promotes Atrial Hypocontractility in Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF)-the most common sustained cardiac arrhythmia-increases thromboembolic stroke risk 5-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function remain unknown. We tested the hypothesis that increased expression of PPP1R12C (protein phosphatase 1 regulatory subunit 12C)-the PP1 (protein phosphatase 1) regulatory subunit targeting MLC2a (atrial myosin light chain 2)-causes hypophosphorylation of MLC2a and results in atrial hypocontractility.

METHODS

Right atrial appendage tissues were isolated from human patients with AF versus sinus rhythm controls. Western blots, coimmunoprecipitation, and phosphorylation studies were performed to examine how the PP1c (PP1 catalytic subunit)-PPP1R12C interaction causes MLC2a dephosphorylation. In vitro studies of pharmacological MRCK (myotonic dystrophy kinase-related Cdc42-binding kinase) inhibitor (BDP5290) in atrial HL-1 cells were performed to evaluate PP1 holoenzyme activity on MLC2a. Cardiac-specific lentiviral PPP1R12C overexpression was performed in mice to evaluate atrial remodeling with atrial cell shortening assays, echocardiography, and AF inducibility with electrophysiology studies.

RESULTS

In human patients with AF, PPP1R12C expression was increased 2-fold versus sinus rhythm controls (P=2.0×10-2; n=12 and 12 in each group) with >40% reduction in MLC2a phosphorylation (P=1.4×10-6; n=12 and 12 in each group). PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF (P=2.9×10-2 and 6.7×10-3, respectively; n=8 and 8 in each group). In vitro studies utilizing drug BDP5290, which inhibits T560-PPP1R12C phosphorylation, demonstrated increased PPP1R12C binding with both PP1c and MLC2a and dephosphorylation of MLC2a. Mice treated with lentiviral PPP1R12C vector demonstrated a 150% increase in left atrial size versus controls (P=5.0×10-6; n=12, 8, and 12), with reduced atrial strain and atrial ejection fraction. Pacing-induced AF in mice treated with lentiviral PPP1R12C vector was significantly higher than in controls (P=1.8×10-2 and 4.1×10-2, respectively; n=6, 6, and 5).

CONCLUSIONS

Patients with AF exhibit increased levels of PPP1R12C protein compared with controls. PPP1R12C overexpression in mice increases PP1c targeting to MLC2a and causes MLC2a dephosphorylation, which reduces atrial contractility and increases AF inducibility. These findings suggest that PP1 regulation of sarcomere function at MLC2a is a key determinant of atrial contractility in AF.

NADPH Oxidases and Oxidative Stress in the Pathogenesis of Atrial Fibrillation

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and its prevalence increases with age. The irregular and rapid contraction of the atria can lead to ineffective blood pumping, local blood stasis, blood clots, ischemic stroke, and heart failure. NADPH oxidases (NOX) and mitochondria are the main sources of reactive oxygen species in the heart, and dysregulated activation of NOX and mitochondrial dysfunction are associated with AF pathogenesis. NOX- and mitochondria-derived oxidative stress contribute to the onset of paroxysmal AF by inducing electrophysiological changes in atrial myocytes and structural remodeling in the atria. Because high atrial activity causes cardiac myocytes to expend extremely high energy to maintain excitation-contraction coupling during persistent AF, mitochondria, the primary energy source, undergo metabolic stress, affecting their morphology, Ca2+ handling, and ATP generation. In this review, we discuss the role of oxidative stress in activating AF-triggered activities, regulating intracellular Ca2+ handling, and functional and anatomical reentry mechanisms, all of which are associated with AF initiation, perpetuation, and progression. Changes in the extracellular matrix, inflammation, ion channel expression and function, myofibril structure, and mitochondrial function occur during the early transitional stages of AF, opening a window of opportunity to target NOX and mitochondria-derived oxidative stress using isoform-specific NOX inhibitors and mitochondrial ROS scavengers, as well as drugs that improve mitochondrial dynamics and metabolism to treat persistent AF and its transition to permanent AF.

Mechanisms of spontaneous Ca2+ release-mediated arrhythmia in a novel 3D human atrial myocyte model: II. Ca2+ -handling protein variation

Disruption of the transverse-axial tubule system (TATS) in diseases such as heart failure and atrial fibrillation occurs in combination with changes in the expression and distribution of key Ca2+ -handling proteins. Together this ultrastructural and ionic remodelling is associated with aberrant Ca2+ cycling and electrophysiological instabilities that underlie arrhythmic activity. However, due to the concurrent changes in TATs and Ca2+ -handling protein expression and localization that occur in disease it is difficult to distinguish their individual contributions to the arrhythmogenic state. To investigate this, we applied our novel 3D human atrial myocyte model with spatially detailed Ca2+ diffusion and TATS to investigate the isolated and interactive effects of changes in expression and localization of key Ca2+ -handling proteins and variable TATS density on Ca2+ -handling abnormality driven membrane instabilities. We show that modulating the expression and distribution of the sodium-calcium exchanger, ryanodine receptors and the sarcoplasmic reticulum (SR) Ca2+ buffer calsequestrin have varying pro- and anti-arrhythmic effects depending on the balance of opposing influences on SR Ca2+ leak-load and Ca2+ -voltage relationships. Interestingly, the impact of protein remodelling on Ca2+ -driven proarrhythmic behaviour varied dramatically depending on TATS density, with intermediately tubulated cells being more severely affected compared to detubulated and densely tubulated myocytes. This work provides novel mechanistic insight into the distinct and interactive consequences of TATS and Ca2+ -handling protein remodelling that underlies dysfunctional Ca2+ cycling and electrophysiological instability in disease. KEY POINTS: In our companion paper we developed a 3D human atrial myocyte model, coupling electrophysiology and Ca2+ handling with subcellular spatial details governed by the transverse-axial tubule system (TATS). Here we utilize this model to mechanistically examine the impact of TATS loss and changes in the expression and distribution of key Ca2+ -handling proteins known to be remodelled in disease on Ca2+ homeostasis and electrophysiological stability. We demonstrate that varying the expression and localization of these proteins has variable pro- and anti-arrhythmic effects with outcomes displaying dependence on TATS density. Whereas detubulated myocytes typically appear unaffected and densely tubulated cells seem protected, the arrhythmogenic effects of Ca2+ handling protein remodelling are profound in intermediately tubulated cells. Our work shows the interaction between TATS and Ca2+ -handling protein remodelling that underlies the Ca2+ -driven proarrhythmic behaviour observed in atrial fibrillation and may help to predict the effects of antiarrhythmic strategies at varying stages of ultrastructural remodelling.

Downregulation of FKBP5 Promotes Atrial Arrhythmogenesis

BACKGROUND

Atrial fibrillation (AF), the most common arrhythmia, is associated with the downregulation of FKBP5 (encoding FKBP5 [FK506 binding protein 5]). However, the function of FKBP5 in the heart remains unknown. Here, we elucidate the consequences of cardiomyocyte-restricted loss of FKBP5 on cardiac function and AF development and study the underlying mechanisms.

METHODS

Right atrial samples from patients with AF were used to assess the protein levels of FKBP5. A cardiomyocyte-specific FKBP5 knockdown mouse model was established by crossbreeding Fkbp5flox/flox mice with Myh6MerCreMer/+ mice. Cardiac function and AF inducibility were assessed by echocardiography and programmed intracardiac stimulation. Histology, optical mapping, cellular electrophysiology, and biochemistry were employed to elucidate the proarrhythmic mechanisms due to loss of cardiomyocyte FKBP5.

RESULTS

FKBP5 protein levels were lower in the atrial lysates of patients with paroxysmal AF or long-lasting persistent (chronic) AF. Cardiomyocyte-specific knockdown mice exhibited increased AF inducibility and duration compared with control mice. Enhanced AF susceptibility in cardiomyocyte-specific knockdown mice was associated with the development of action potential alternans and spontaneous Ca2+ waves, and increased protein levels and activity of the NCX1 (Na+/Ca2+-exchanger 1), mimicking the cellular phenotype of chronic AF patients. FKBP5-deficiency enhanced transcription of Slc8a1 (encoding NCX1) via transcription factor hypoxia-inducible factor 1α. In vitro studies revealed that FKBP5 negatively modulated the protein levels of hypoxia-inducible factor 1α by competitively interacting with heat-shock protein 90. Injections of the heat-shock protein 90 inhibitor 17-AAG normalized protein levels of hypoxia-inducible factor 1α and NCX1 and reduced AF susceptibility in cardiomyocyte-specific knockdown mice. Furthermore, the atrial cardiomyocyte-selective knockdown of FKBP5 was sufficient to enhance AF arrhythmogenesis.

CONCLUSIONS

This is the first study to demonstrate a role for the FKBP5-deficiency in atrial arrhythmogenesis and to establish FKBP5 as a negative regulator of hypoxia-inducible factor 1α in cardiomyocytes. Our results identify a potential molecular mechanism for the proarrhythmic NCX1 upregulation in chronic AF patients.

Pitx2c deficiency confers cellular electrophysiological hallmarks of atrial fibrillation to isolated atrial myocytes

AIMS

Atrial fibrillation (AF) has been associated with altered expression of the transcription factor Pitx2c and a high incidence of calcium release-induced afterdepolarizations. However, the relationship between Pitx2c expression and defective calcium homeostasis remains unclear and we here aimed to determine how Pitx2c expression affects calcium release from the sarcoplasmic reticulum (SR) and its impact on electrical activity in isolated atrial myocytes.

METHODS

To address this issue, we applied confocal calcium imaging and patch-clamp techniques to atrial myocytes isolated from a mouse model with conditional atrial-specific deletion of Pitx2c.

RESULTS

Our findings demonstrate that heterozygous deletion of Pitx2c doubles the calcium spark frequency, increases the frequency of sparks/site 1.5-fold, the calcium spark decay constant from 36 to 42 ms and the wave frequency from none to 3.2 min^-1. Additionally, the cell capacitance increased by 30% and both the SR calcium load and the transient inward current (I_TI) frequency were doubled. Furthermore, the fraction of cells with spontaneous action potentials increased from none to 44%. These effects of Pitx2c deficiency were comparable in right and left atrial myocytes, and homozygous deletion of Pitx2c did not induce any further effects on sparks, SR calcium load, I_TI frequency or spontaneous action potentials.

CONCLUSION

Our findings demonstrate that heterozygous Pitx2c deletion induces defects in calcium homeostasis and electrical activity that mimic derangements observed in right atrial myocytes from patients with AF and suggest that Pitx2c deficiency confers cellular electrophysiological hallmarks of AF to isolated atrial myocytes.

Enhanced Ca2+-Dependent SK-Channel Gating and Membrane Trafficking in Human Atrial Fibrillation

BACKGROUND

Small-conductance Ca2+-activated K+ (SK)-channel inhibitors have antiarrhythmic effects in animal models of atrial fibrillation (AF), presenting a potential novel antiarrhythmic option. However, the regulation of SK-channels in human atrial cardiomyocytes and its modification in patients with AF are poorly understood and were the object of this study.

METHODS

Apamin-sensitive SK-channel current (ISK) and action potentials were recorded in human right-atrial cardiomyocytes from sinus rhythm control (Ctl) patients or patients with (long-standing persistent) chronic AF (cAF).

RESULTS

ISK was significantly higher, and apamin caused larger action potential prolongation in cAF- versus Ctl-cardiomyocytes. Sensitivity analyses in an in silico human atrial cardiomyocyte model identified IK1 and ISK as major regulators of repolarization. Increased ISK in cAF was not associated with increases in mRNA/protein levels of SK-channel subunits in either right- or left-atrial tissue homogenates or right-atrial cardiomyocytes, but the abundance of SK2 at the sarcolemma was larger in cAF versus Ctl in both tissue-slices and cardiomyocytes. Latrunculin-A and primaquine (anterograde and retrograde protein-trafficking inhibitors) eliminated the differences in SK2 membrane levels and ISK between Ctl- and cAF-cardiomyocytes. In addition, the phosphatase-inhibitor okadaic acid reduced ISK amplitude and abolished the difference between Ctl- and cAF-cardiomyocytes, indicating that reduced calmodulin-Thr80 phosphorylation due to increased protein phosphatase-2A levels in the SK-channel complex likely contribute to the greater ISK in cAF-cardiomyocytes. Finally, rapid electrical activation (5 Hz, 10 minutes) of Ctl-cardiomyocytes promoted SK2 membrane-localization, increased ISK and reduced action potential duration, effects greatly attenuated by apamin. Latrunculin-A or primaquine prevented the 5-Hz-induced ISK-upregulation.

CONCLUSIONS

ISK is upregulated in patients with cAF due to enhanced channel function, mediated by phosphatase-2A-dependent calmodulin-Thr80 dephosphorylation and tachycardia-dependent enhanced trafficking and targeting of SK-channel subunits to the sarcolemma. The observed AF-associated increases in ISK, which promote reentry-stabilizing action potential duration shortening, suggest an important role for SK-channels in AF auto-promotion and provide a rationale for pursuing the antiarrhythmic effects of SK-channel inhibition in humans.

Myosin Light Chain Dephosphorylation by PPP1R12C Promotes Atrial Hypocontractility in Atrial Fibrillation

Background

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, increases thromboembolic stroke risk five-fold. Although atrial hypocontractility contributes to stroke risk in AF, the molecular mechanisms reducing myofilament contractile function remain unknown. We tested the hypothesis that increased expression of PPP1R12C, the PP1 regulatory subunit targeting atrial myosin light chain 2 (MLC2a), causes hypophosphorylation of MLC2a and results in atrial hypocontractility.

Methods

Right atrial appendage tissues were isolated from human AF patients versus sinus rhythm (SR) controls. Western blots, co-immunoprecipitation, and phosphorylation studies were performed to examine how the PP1c-PPP1R12C interaction causes MLC2a de-phosphorylation. In vitro studies of pharmacologic MRCK inhibitor (BDP5290) in atrial HL-1 cells were performed to evaluate PP1 holoenzyme activity on MLC2a. Cardiac-specific lentiviral PPP1R12C overexpression was performed in mice to evaluate atrial remodeling with atrial cell shortening assays, echocardiography, and AF inducibility with EP studies.

Results

In human patients with AF, PPP1R12C expression was increased two-fold versus SR controls ( P =2.0×10 -2 , n=12,12 in each group) with > 40% reduction in MLC2a phosphorylation ( P =1.4×10 -6 , n=12,12 in each group). PPP1R12C-PP1c binding and PPP1R12C-MLC2a binding were significantly increased in AF ( P =2.9×10 -2 and 6.7×10 -3 respectively, n=8,8 in each group). In vitro studies utilizing drug BDP5290, which inhibits T560-PPP1R12C phosphorylation, demonstrated increased PPP1R12C binding with both PP1c and MLC2a, and dephosphorylation of MLC2a. Lenti-12C mice demonstrated a 150% increase in LA size versus controls ( P =5.0×10 -6 , n=12,8,12), with reduced atrial strain and atrial ejection fraction. Pacing-induced AF in Lenti-12C mice was significantly higher than controls ( P =1.8×10 -2 and 4.1×10 -2 respectively, n= 6,6,5).

Conclusions

AF patients exhibit increased levels of PPP1R12C protein compared to controls. PPP1R12C overexpression in mice increases PP1c targeting to MLC2a and causes MLC2a dephosphorylation, which reduces atrial contractility and increases AF inducibility. These findings suggest that PP1 regulation of sarcomere function at MLC2a is a key determinant of atrial contractility in AF.

Emerging role of heat shock proteins in cardiovascular diseases

Heat Shock Proteins (HSPs) are evolutionarily conserved proteins from prokaryotes to eukaryotes. They are ubiquitous proteins involved in key physiological and cellular pathways (viz. inflammation, immunity and apoptosis). Indeed, the survivability of the cells under various stressful conditions depends on appropriate levels of HSP expression. There is a growing line of evidence for the role of HSPs in regulating cardiovascular diseases (CVDs) (viz. hypertension, atherosclerosis, atrial fibrillation, cardiomyopathy and heart failure). Furthermore, studies indicate that a higher concentration of circulatory HSP antibodies correlate to CVDs; some are even potential markers for CVDs. The multifaceted roles of HSPs in regulating cellular signaling necessitate unraveling their links to pathophysiology of CVDs. This review aims to consolidate our understanding of transcriptional (via multiple transcription factors including HSF-1, NF-κB, CREB and STAT3) and post-transcriptional (via microRNAs including miR-1, miR-21 and miR-24) regulation of HSPs. The cytoprotective nature of HSPs catapults them to the limelight as modulators of cell survival. Yet another attractive prospect is the development of new therapeutic strategies against cardiovascular diseases (from hypertension to heart failure) by targeting the regulation of HSPs. Moreover, this review provides insights into how genetic variation of HSPs can contribute to the manifestation of CVDs. It would also offer a bird's eye view of the evolving role of different HSPs in the modulation and manifestation of cardiovascular disease.

Mechanistic and therapeutic role of Drp1 in the pathogenesis of stroke

Stroke had emerged as one of the leading causes of death and long-term disability across the globe. Emerging evidence suggests a significant increase in the incidence of stroke with age, which is further expected to increase dramatically owing to an ever-expanding elderly population. The current situation imposes a significant burden on the healthcare system and requires a deeper understanding of the underlying mechanisms and development of novel interventions. It is well established that mitochondrial dysfunction plays a pivotal role in the onset of stroke. Dynamin-related protein 1 (Drp1), is a key regulator of mitochondria fission, and plays a crucial role during the pathogenesis of stroke. Drp1 protein levels significantly increase after stroke potentially in a p38 mitogen-activated protein kinases (MAPK) dependent manner. Protein phosphatase 2A (PP2A) facilitate mitochondrial fission and cell death by dephosphorylating the mitochondrial fission enzyme Drp1 at the inhibitory phosphorylation site serine 637. Outer mitochondrial membrane A-Kinase Anchoring Proteins 1 (AKAP 1) and protein kinase A complex (PKA) complex inhibits Drp1-dependent mitochondrial fission by phosphorylating serine 637. Drp1 activation promotes the release of cytochrome C from mitochondria and therefore leads to apoptosis. In addition, Drp1 activation inhibits mitochondrial glutathione dependent free radical scavenging, which further enhances the ROS level and exacerbate mitochondrial dysfunction. Drp1 translocate p53 to mitochondrial membrane and leads to mitochondria-related necrosis. The current review article discusses the possible mechanistic pathways by which Drp1 can influence the pathogenesis of stroke. Besides, it will describe various inhibitors for Drp1 and their potential role as therapeutics for stroke in the future.

CaMKII as a Therapeutic Target in Cardiovascular Disease

CaMKII (the multifunctional Ca2+ and calmodulin-dependent protein kinase II) is a highly validated signal for promoting a variety of common diseases, particularly in the cardiovascular system. Despite substantial amounts of convincing preclinical data, CaMKII inhibitors have yet to emerge in clinical practice. Therapeutic inhibition is challenged by the diversity of CaMKII isoforms and splice variants and by physiological CaMKII activity that contributes to learning and memory. Thus, uncoupling the harmful and beneficial aspects of CaMKII will be paramount to developing effective therapies. In the last decade, several targeting strategies have emerged, including small molecules, peptides, and nucleotides, which hold promise in discriminating pathological from physiological CaMKII activity. Here we review the cellular and molecular biology of CaMKII, discuss its role in physiological and pathological signaling, and consider new findings and approaches for developing CaMKII therapeutics.

Electrical Remodelling in Cardiac Disease

The human heart responds to various diseases with structural, mechanical, and electrical remodelling processes [...].

Atrial fibrillation: Epidemiology, pathophysiology, and clinical complications (literature review)

The last three decades have been characterized by an exponential increase in knowledge and advances in the clinical management of atrial fibrillation. The purpose of the study is to provide an overview of the pathogenesis of nonvalvular atrial fibrillation and a comprehensive investigation of the epidemiological data associated with various risk factors for atrial fibrillation. The leading research methods are analysis and synthesis, comparison, observation, induction and deduction, and grouping method. Research has shown that old age, male gender, and European descent are important risk factors for developing atrial fibrillation. Other modifiable risk factors include a sedentary lifestyle, smoking, obesity, diabetes mellitus, obstructive sleep apnea, and high blood pressure predisposing to atrial fibrillation, and each has been shown to induce structural and electrical atrial remodeling. Both heart failure and myocardial infarction increase the risk of developing atrial fibrillation and vice versa creating feedback that increases mortality. The review is a comprehensive study of the epidemiological data linking nonmodifiable and modifiable risk factors for atrial fibrillation, and the pathophysiological data supporting the relationship between each risk factor and the occurrence of atrial fibrillation. This may be necessary for the practice of the treatment of the cardiac system.

Pacemaker activity and ion channels in the sinoatrial node cells: MicroRNAs and arrhythmia

The primary pacemaking activity of the heart is determined by a spontaneous action potential (AP) within sinoatrial node (SAN) cells. This unique AP generation relies on two mechanisms: membrane clocks and calcium clocks. Nonhomologous arrhythmias are caused by several functional and structural changes in the myocardium. MicroRNAs (miRNAs) are essential regulators of gene expression in cardiomyocytes. These miRNAs play a vital role in regulating the stability of cardiac conduction and in the remodeling process that leads to arrhythmias. Although it remains unclear how miRNAs regulate the expression and function of ion channels in the heart, these regulatory mechanisms may support the development of emerging therapies. This study discusses the spread and generation of AP in the SAN as well as the regulation of miRNAs and individual ion channels. Arrhythmogenicity studies on ion channels will provide a research basis for miRNA modulation as a new therapeutic target.

Spatial Distribution of Calcium Sparks Determines Their Ability to Induce Afterdepolarizations in Human Atrial Myocytes

Analysis of the spatio-temporal distribution of calcium sparks showed a preferential increase in sparks near the sarcolemma in atrial myocytes from patients with atrial fibrillation (AF), linked to higher ryanodine receptor (RyR2) phosphorylation at s2808 and lower calsequestrin-2 levels. Mathematical modeling, incorporating modulation of RyR2 gating, showed that only the observed combinations of RyR2 phosphorylation and calsequestrin-2 levels can account for the spatio-temporal distribution of sparks in patients with and without AF. Furthermore, we demonstrate that preferential calcium release near the sarcolemma is key to a higher incidence and amplitude of afterdepolarizations in atrial myocytes from patients with AF.

Changes in cAMP signaling are associated with age-related downregulation of spontaneously beating atrial tissue energetic indices

The prevalence of atria-related diseases increases exponentially with age and is associated with ATP supply-to-demand imbalances. Because evidence suggests that cAMP regulates ATP supply-to-demand, we explored aged-associated alterations in atrial ATP supply-to-demand balance and its correlation with cAMP levels. Right atrial tissues driven by spontaneous sinoatrial node impulses were isolated from aged (22-26 months) and adult (3-4 months) C57/BL6 mice. ATP demand increased by addition of isoproterenol or 3-Isobutyl-1-methylxanthine (IBMX) and decreased by application of carbachol. Each drug was administrated at the dose that led to a maximal change in beating rate (X_max) and to 50% of that maximal change in adult tissue (X₅₀). cAMP, NADH, NAD + NADH, and ATP levels were measured in the same tissue. The tight correlation between cAMP levels and the beating rate (i.e., the ATP demand) demonstrated in adult atria was altered in aged atria. cAMP levels were lower in aged compared to adult atrial tissue exposed to X₅₀ of ISO or IBMX, but this difference narrowed at X_max. Neither ATP nor NADH levels correlated with ATP demand in either adult or aged atria. Baseline NADH levels were lower in aged as compared to adult atria, but were restored by drug perturbations that increased cAMP levels. Reduction in Ca²⁺-activated adenylyl cyclase-induced decreased cAMP and prolongation of the spontaneous beat interval of adult atrial tissue to their baseline levels in aged tissue, brought energetics indices to baseline levels in aged tissue. Thus, cAMP regulates right atrial ATP supply-to-demand matching and can restore age-associated ATP supply-to-demand imbalance.

Human cardiac myosin-binding protein C phosphorylation- and mutation-dependent structural dynamics monitored by time-resolved FRET

Cardiac myosin-binding protein C (cMyBP-C) is a thick filament-associated protein of the sarcomere and a potential therapeutic target for treating contractile dysfunction in heart failure. Mimicking the structural dynamics of phosphorylated cMyBP-C by small-molecule drug binding could lead to therapies that modulate cMyBP-C conformational states, and thereby function, to improve contractility. We have developed a human cMyBP-C biosensor capable of detecting intramolecular structural changes due to phosphorylation and mutation. Using site-directed mutagenesis and time-resolved fluorescence resonance energy transfer (TR-FRET), we substituted cysteines in cMyBP-C N-terminal domains C0 through C2 (C0-C2) for thiol-reactive fluorescent probe labeling to examine C0-C2 structure. We identified a cysteine pair that upon donor-acceptor labeling reports phosphorylation-sensitive structural changes between the C1 domain and the tri-helix bundle of the M-domain that links C1 to C2. Phosphorylation reduced FRET efficiency by ~18%, corresponding to a ~11% increase in the distance between probes and a ~30% increase in disorder between them. The magnitude and precision of phosphorylation-mediated TR-FRET changes, as quantified by the Z'-factor, demonstrate the assay's potential for structure-based high-throughput screening of compounds for cMyBP-C-targeted therapies to improve cardiac performance in heart failure. Additionally, by probing C1's spatial positioning relative to the tri-helix bundle, these findings provide new molecular insight into the structural dynamics of phosphoregulation as well as mutations in cMyBP-C. Biosensor sensitivity to disease-relevant mutations in C0-C2 was demonstrated by examination of the hypertrophic cardiomyopathy mutation R282W. The results presented here support a screening platform to identify small molecules that regulate N-terminal cMyBP-C conformational states.

Identification of Phosphorylation and Other Post-Translational Modifications in the Central C4C5 Domains of Murine Cardiac Myosin Binding Protein C

Cardiac myosin binding protein C (cMyBPC) is a critical multidomain protein that modulates myosin cross bridge behavior and cardiac contractility. cMyBPC is principally regulated by phosphorylation of the residues within the M-domain of its N-terminus. However, not much is known about the phosphorylation or other post-translational modification (PTM) landscape of the central C4C5 domains. In this study, the presence of phosphorylation outside the M-domain was confirmed in vivo using mouse models expressing cMyBPC with nonphosphorylatable serine (S) to alanine substitutions. Purified recombinant mouse C4C5 domain constructs were incubated with 13 different kinases, and samples from the 6 strongest kinases were chosen for mass spectrometry analysis. A total of 26 unique phosphorylated peptides were found, representing 13 different phosphorylation sites including 10 novel sites. Parallel reaction monitoring and subsequent mutagenesis experiments revealed that the S690 site (UniProtKB O70468) was the predominant target of PKA and PKG1. We also report 6 acetylation and 7 ubiquitination sites not previously described in the literature. These PTMs demonstrate the possibility of additional layers of regulation and potential importance of the central domains of cMyBPC in cardiac health and disease. Data are available via ProteomeXchange with identifier PXD031262.

Emerging Antiarrhythmic Drugs for Atrial Fibrillation

Atrial fibrillation (AF), the most common cardiac arrhythmia worldwide, is driven by complex mechanisms that differ between subgroups of patients. This complexity is apparent from the different forms in which AF presents itself (post-operative, paroxysmal and persistent), each with heterogeneous patterns and variable progression. Our current understanding of the mechanisms responsible for initiation, maintenance and progression of the different forms of AF has increased significantly in recent years. Nevertheless, antiarrhythmic drugs for the management of AF have not been developed based on the underlying arrhythmia mechanisms and none of the currently used drugs were specifically developed to target AF. With the increased knowledge on the mechanisms underlying different forms of AF, new opportunities for developing more effective and safer AF therapies are emerging. In this review, we provide an overview of potential novel antiarrhythmic approaches based on the underlying mechanisms of AF, focusing both on the development of novel antiarrhythmic agents and on the possibility of repurposing already marketed drugs. In addition, we discuss the opportunity of targeting some of the key players involved in the underlying AF mechanisms, such as ryanodine receptor type-2 (RyR2) channels and atrial-selective K⁺-currents (I_K2P and I_SK) for antiarrhythmic therapy. In addition, we highlight the opportunities for targeting components of inflammatory signaling (e.g., the NLRP3-inflammasome) and upstream mechanisms targeting fibroblast function to prevent structural remodeling and progression of AF. Finally, we critically appraise emerging antiarrhythmic drug principles and future directions for antiarrhythmic drug development, as well as their potential for improving AF management.

Protein kinases in cardiovascular diseases

ABSTRACT

Cardiovascular disease (CVD) remains the leading cause of death worldwide. Therefore, exploring the mechanism of CVDs and critical regulatory factors is of great significance for promoting heart repair, reversing cardiac remodeling, and reducing adverse cardiovascular events. Recently, significant progress has been made in understanding the function of protein kinases and their interactions with other regulatory proteins in myocardial biology. Protein kinases are positioned as critical regulators at the intersection of multiple signals and coordinate nearly every aspect of myocardial responses, regulating contractility, metabolism, transcription, and cellular death. Equally, reconstructing the disrupted protein kinases regulatory network will help reverse pathological progress and stimulate cardiac repair. This review summarizes recent researches concerning the function of protein kinases in CVDs, discusses their promising clinical applications, and explores potential targets for future treatments.

Function and regulation of phosphatase 1 in healthy and diseased heart

Reversible phosphorylation of ion channels and calcium-handling proteins provides precise post-translational regulation of cardiac excitation and contractility. Serine/threonine phosphatases govern dephosphorylation of the majority of cardiac proteins. Accordingly, dysfunction of this regulation contributes to the development and progression of heart failure and atrial fibrillation. On the molecular level, these changes include alterations in the expression level and phosphorylation status of Ca²⁺ handling and excitation-contraction coupling proteins provoked by dysregulation of phosphatases. The serine/threonine protein phosphatase PP1 is one a major player in the regulation of cardiac excitation-contraction coupling. PP1 essentially impacts on cardiac physiology and pathophysiology via interactions with the cardiac ion channels Cav1.2, NKA, NCX and KCNQ1, sarcoplasmic reticulum-bound Ca²⁺ handling proteins such as RyR2, SERCA and PLB as well as the contractile proteins MLC2, TnI and MyBP-C. PP1 itself but also PP1-regulatory proteins like inhibitor-1, inhibitor-2 and heat-shock protein 20 are dysregulated in cardiac disease. Therefore, they represent interesting targets to gain more insights in heart pathophysiology and to identify new treatment strategies for patients with heart failure or atrial fibrillation. We describe the genetic and holoenzymatic structure of PP1 and review its role in the heart and cardiac disease. Finally, we highlight the importance of the PP1 regulatory proteins for disease manifestation, provide an overview of genetic models to study the role of PP1 for the development of heart failure and atrial fibrillation and discuss possibilities of pharmacological interventions.

Abnormal Calcium Handling in Atrial Fibrillation Is Linked to Changes in Cyclic AMP Dependent Signaling

Both, the decreased L-type Ca²⁺ current (I_Ca,L) density and increased spontaneous Ca²⁺ release from the sarcoplasmic reticulum (SR), have been associated with atrial fibrillation (AF). In this study, we tested the hypothesis that remodeling of 3′,5′-cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) signaling is linked to these compartment-specific changes (up- or down-regulation) in Ca²⁺-handling. Perforated patch-clamp experiments were performed in atrial myocytes from 53 patients with AF and 104 patients in sinus rhythm (Ctl). A significantly higher frequency of transient inward currents (I_TI) activated by spontaneous Ca²⁺ release was confirmed in myocytes from AF patients. Next, inhibition of PKA by H-89 promoted a stronger effect on the I_TI frequency in these myocytes compared to myocytes from Ctl patients (7.6-fold vs. 2.5-fold reduction), while the β-agonist isoproterenol (ISO) caused a greater increase in Ctl patients (5.5-fold vs. 2.1-fold). I_Ca,L density was larger in myocytes from Ctl patients at baseline (p < 0.05). However, the effect of ISO on I_Ca,L density was only slightly stronger in AF than in Ctl myocytes (3.6-fold vs. 2.7-fold). Interestingly, a significant reduction of I_Ca,L and Ca²⁺ sparks was observed upon Ca²⁺/Calmodulin-dependent protein kinase II inhibition by KN-93, but this inhibition had no effect on I_TI. Fluorescence resonance energy transfer (FRET) experiments showed that although AF promoted cytosolic desensitization to β-adrenergic stimulation, ISO increased cAMP to similar levels in both groups of patients in the L-type Ca²⁺ channel and ryanodine receptor compartments. Basal cAMP signaling also showed compartment-specific regulation by phosphodiesterases in atrial myocytes from 44 Ctl and 43 AF patients. Our results suggest that AF is associated with opposite changes in compartmentalized PKA/cAMP-dependent regulation of I_Ca,L (down-regulation) and I_TI (up-regulation).

Serine-threonine protein phosphatase regulation of Cx43 dephosphorylation in arrhythmogenic disorders

Regulation of cell-to-cell communication in the heart by the gap junction protein Connexin43 (Cx43) involves modulation of Cx43 phosphorylation state by protein kinases, and dephosphorylation by protein phosphatases. Dephosphorylation of Cx43 has been associated with impaired intercellular coupling and enhanced arrhythmogenesis in various pathologic states. While there has been extensive study of the protein kinases acting on Cx43, there has been limited studies of the protein phosphatases that may underlie Cx43 dephosphorylation. The focus of this review is to introduce serine-threonine protein phosphatase regulation of Cx43 phosphorylation state and cell-to-cell communication, and its impact on arrhythmogenesis in the setting of chronic heart failure and myocardial ischemia, as well as on atrial fibrillation. We also discuss the therapeutic potential of modulating protein phosphatases to treat arrhythmias in these clinical settings.

MicroRNAs and Calcium Signaling in Heart Disease

In hearts, calcium (Ca²⁺) signaling is a crucial regulatory mechanism of muscle contraction and electrical signals that determine heart rhythm and control cell growth. Ca²⁺ signals must be tightly controlled for a healthy heart, and the impairment of Ca²⁺ handling proteins is a key hallmark of heart disease. The discovery of microRNA (miRNAs) as a new class of gene regulators has greatly expanded our understanding of the controlling module of cardiac Ca²⁺ cycling. Furthermore, many studies have explored the involvement of miRNAs in heart diseases. In this review, we aim to summarize cardiac Ca²⁺ signaling and Ca²⁺-related miRNAs in pathological conditions, including cardiac hypertrophy, heart failure, myocardial infarction, and atrial fibrillation. We also discuss the therapeutic potential of Ca²⁺-related miRNAs as a new target for the treatment of heart diseases.

Atrial Fibrillation and Ischemic Stroke: A Clinical Review

Atrial fibrillation (AF) is an important risk factor for ischemic stroke resulting in a fivefold increased stroke risk and a twofold increased mortality. Our understanding of stroke mechanisms in AF has evolved since the concept of atrial cardiopathy was introduced as an underlying pathological change, with both AF and thromboembolism being common manifestations and outcomes. Despite the strong association with stroke, there is no evidence that screening for AF in asymptomatic patients improves clinical outcomes; however, there is strong evidence that patients with embolic stroke of undetermined source may require long-term monitoring to detect silent or paroxysmal AF. Stroke prevention in patients at risk, assessed by the CHA₂DS₂-VASc score, was traditionally achieved with warfarin; however, direct oral anticoagulants have solidified their role as safe and effective alternatives. Additionally, left atrial appendage exclusion has emerged as a viable option in patients intolerant of anticoagulation. When patients with AF have an acute stroke, the timing of initiation or resumption of anticoagulation for secondary stroke prevention has to be balanced against the risk of hemorrhagic conversion. Multiple randomized clinical trials are currently underway to determine the best timing for administration of anticoagulants following acute ischemic stroke.

Pleiotropic, non-cell death-associated effects of inhibitors of receptor-interacting protein kinase 1 in the heart

Inhibition of receptor-interacting protein kinase 1 (RIP1) has been recognized as a compelling tool for limiting necroptosis. Recent findings have indicated that RIP1 inhibitor, necrostatin-1 (Nec-1), is also able to modify heart function under non-cell death conditions. In this study, we investigated its underlying molecular mechanisms and compared with those of novel pharmacologically improved agents (Nec-1s and GSK'772) and its inactive analog (Nec-1i). Heart function was examined in Langendorff-perfused rat hearts. Certain proteins regulating myocardial contraction-relaxation cycle and oxidative stress (OS) were evaluated by immunoblotting and as the extent of lipid peroxidation, protein carbonylation and nitration, respectively. In spite of the increase of left ventricular developed pressure (LVDP) due to treatment by both Nec-1 and Nec-1i, only the former agent increased the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II delta (CaMKIIδ) at threonine 287 and cardiac myosin-binding protein-C (cMyBPc) at serine 282. In contrast, Nec-1s did not elicit such changes, while it also increased LVDP. GSK'772 activated CaMKIIδ-phospholamban (PLN) axis. Neither protein kinase A (PKA) nor its selected molecular targets, such as serine 16 phosphorylated PLN and sarco/endoplasmic reticulum Ca²⁺-ATPase 2a (SERCA2a), were affected by either RIP1 inhibitor. Nec-1, like other necrostatins (Nec-1i, Nec-1s), but not GSK'772, elevated protein tyrosine nitration without affecting other markers of OS. In conclusion, this study indicated for the first time that Nec-1 may affect basal heart function by the modulation of OS and activation of some proteins of contraction-relaxation cycle.

THE ROLE OF PHOSPHORYLATION IN ATRIAL FIBRILLATION: A FOCUS ON MASS SPECTROMETRY APPROACHES

Atrial fibrillation (AF) is the most common arrhythmia worldwide. It is associated with significant increases in morbidity in the form of stroke and heart failure, and a doubling in all-cause mortality. The pathophysiology of AF is incompletely understood, and this has contributed to a lack of effective treatments and disease-modifying therapies. An important cellular process that may explain how risk factors give rise to AF includes post-translational modification (PTM) of proteins. As the most commonly occurring PTM, protein phosphorylation is especially relevant. Although many methods exist for studying protein phosphorylation, a common and highly resolute technique is mass spectrometry (MS). This review will discuss recent evidence surrounding the role of protein phosphorylation in the pathogenesis of AF. MS-based technology to study phosphorylation and uses of MS in other areas of medicine such as oncology will also be presented. Based on these data, future goals and experiments will be outlined that utilize MS technology to better understand the role of phosphorylation in AF and elucidate its role in AF pathophysiology. This may ultimately allow for the development of more effective AF therapies.

Mapping genetic changes in the cAMP-signaling cascade in human atria

AIM

To obtain a quantitative expression profile of the main genes involved in the cAMP-signaling cascade in human control atria and in different cardiac pathologies.

METHODS AND RESULTS

Expression of 48 target genes playing a relevant role in the cAMP-signaling cascade was assessed by RT-qPCR. 113 samples were obtained from right atrial appendages (RAA) of patients in sinus rhythm (SR) with or without atrium dilation, paroxysmal atrial fibrillation (AF), persistent AF or heart failure (HF); and left atrial appendages (LAA) from patients in SR or with AF. Our results show that right and left atrial appendages in donor hearts or from SR patients have similar expression values except for AC7 and PDE2A. Despite the enormous chamber-dependent variability in the gene-expression changes between pathologies, several distinguishable patterns could be identified. PDE8A, PI3Kγ and EPAC2 were upregulated in AF. Different phosphodiesterase (PDE) families showed specific pathology-dependent changes.

CONCLUSION

By comparing mRNA-expression patterns of the cAMP-signaling cascade related genes in right and left atrial appendages of human hearts and across different pathologies, we show that 1) gene expression is not significantly affected by cardioplegic solution content, 2) it is appropriate to use SR atrial samples as controls, and 3) many genes in the cAMP-signaling cascade are affected in AF and HF but only few of them appear to be chamber (right or left) specific.

TOPIC

Genetic changes in human diseased atria.

TRANSLATIONAL PERSPECTIVE

The cyclic AMP signaling pathway is important for atrial function. However, expression patterns of the genes involved in the atria of healthy and diseased hearts are still unclear. We give here a general overview of how different pathologies affect the expression of key genes in the cAMP signaling pathway in human right and left atria appendages. Our study may help identifying new genes of interest as potential therapeutic targets or clinical biomarkers for these pathologies and could serve as a guide in future gene therapy studies.

Stress-driven cardiac calcium mishandling via a kinase-to-kinase crosstalk

Calcium homeostasis in the cardiomyocyte is critical to the regulation of normal cardiac function. Abnormal calcium dynamics such as altered uptake by the sarcoplasmic reticulum (SR) Ca²⁺-ATPase and increased diastolic SR calcium leak are involved in the development of maladaptive cardiac remodeling under pathological conditions. Ca²⁺/calmodulin-dependent protein kinase II-δ (CaMKIIδ) is a well-recognized key molecule in calcium dysregulation in cardiomyocytes. Elevated cellular stress is known as a common feature during pathological remodeling, and c-jun N-terminal kinase (JNK) is an important stress kinase that is activated in response to intrinsic and extrinsic stress stimuli. Our lab recently identified specific actions of JNK isoform 2 (JNK2) in CaMKIIδ expression, activation, and CaMKIIδ-dependent SR Ca²⁺ mishandling in the stressed heart. This review focuses on the current understanding of cardiac SR calcium handling under physiological and pathological conditions as well as the newly identified contribution of the stress kinase JNK2 in CaMKIIδ-dependent SR Ca²⁺ abnormal mishandling. The new findings identifying dual roles of JNK2 in CaMKIIδ expression and activation are also discussed in this review.

The crosstalk between cardiomyocyte calcium and inflammasome signaling pathways in atrial fibrillation

Atrial fibrillation (AF) is the most frequent arrhythmia in adults. The prevalence and incidence of AF is going to increase substantially over the next few decades. Because AF increases the risk of stroke, heart failure, dementia, and others, it severely impacts the quality of life, morbidity, and mortality. Although the pathogenesis of AF is multifaceted and complex, focal ectopic activity and reentry are considered as the fundamental proarrhythmic mechanisms underlying AF development. Over the past 2 decades, large amount of evidence points to the key role of intracellular Ca²⁺ dysregulation in both initiation and maintenance of AF. More recently, emerging evidence reveal that NLRP3 (NACHT, LRR, PYD domain-containing 3) inflammasome pathway contributes to the substrate of both triggered activity and reentry, ultimately promoting AF. In this article, we review the current state of knowledge on Ca²⁺ signaling and NLRP3 inflammasome activity in AF. We also discuss the potential crosstalk between these two quintessential contributors to AF promotion.

Mechanisms underlying pathological Ca2+ handling in diseases of the heart

Cardiomyocyte contraction relies on precisely regulated intracellular Ca²⁺ signaling through various Ca²⁺ channels and transporters. In this article, we will review the physiological regulation of Ca²⁺ handling and its role in maintaining normal cardiac rhythm and contractility. We discuss how inherited variants or acquired defects in Ca²⁺ channel subunits contribute to the development or progression of diseases of the heart. Moreover, we highlight recent insights into the role of protein phosphatase subunits and striated muscle preferentially expressed protein kinase (SPEG) in atrial fibrillation, heart failure, and cardiomyopathies. Finally, this review summarizes current drug therapies and new advances in genome editing as therapeutic strategies for the cardiac diseases caused by aberrant intracellular Ca²⁺ signaling.

Atrial Myocyte NLRP3/CaMKII Nexus Forms a Substrate for Postoperative Atrial Fibrillation

RATIONALE

Postoperative atrial fibrillation (POAF) is a common and troublesome complication of cardiac surgery. POAF is generally believed to occur when postoperative triggers act on a preexisting vulnerable substrate, but the underlying cellular and molecular mechanisms are largely unknown.

OBJECTIVE

To identify cellular POAF mechanisms in right atrial samples from patients without a history of atrial fibrillation undergoing open-heart surgery.

METHODS AND RESULTS

Multicellular action potentials, membrane ion-currents (perforated patch-clamp), or simultaneous membrane-current (ruptured patch-clamp) and [Ca²⁺]_i-recordings in atrial cardiomyocytes, along with protein-expression levels in tissue homogenates or cardiomyocytes, were assessed in 265 atrial samples from patients without or with POAF. No indices of electrical, profibrotic, or connexin remodeling were noted in POAF, but Ca²⁺-transient amplitude was smaller, although spontaneous sarcoplasmic reticulum (SR) Ca²⁺-release events and L-type Ca²⁺-current alternans occurred more frequently. CaMKII (Ca²⁺/calmodulin-dependent protein kinase-II) protein-expression, CaMKII-dependent phosphorylation of the cardiac RyR2 (ryanodine-receptor channel type-2), and RyR2 single-channel open-probability were significantly increased in POAF. SR Ca²⁺-content was unchanged in POAF despite greater SR Ca²⁺-leak, with a trend towards increased SR Ca²⁺-ATPase activity. Patients with POAF also showed stronger expression of activated components of the NLRP3 (NACHT, LRR, and PYD domains-containing protein-3)-inflammasome system in atrial whole-tissue homogenates and cardiomyocytes. Acute application of interleukin-1β caused NLRP3-signaling activation and CaMKII-dependent RyR2/phospholamban hyperphosphorylation in an immortalized mouse atrial cardiomyocyte cell-line (HL-1-cardiomyocytes) and enhanced spontaneous SR Ca²⁺-release events in both POAF cardiomyocytes and HL-1-cardiomyocytes. Computational modeling showed that RyR2 dysfunction and increased SR Ca²⁺-uptake are sufficient to reproduce the Ca²⁺-handling phenotype and indicated an increased risk of proarrhythmic delayed afterdepolarizations in POAF subjects in response to interleukin-1β.

CONCLUSIONS

Preexisting Ca²⁺-handling abnormalities and activation of NLRP3-inflammasome/CaMKII signaling are evident in atrial cardiomyocytes from patients who subsequently develop POAF. These molecular substrates sensitize cardiomyocytes to spontaneous Ca²⁺-releases and arrhythmogenic afterdepolarizations, particularly upon exposure to inflammatory mediators. Our data reveal a potential cellular and molecular substrate for this important clinical problem.

Molecular Basis of Atrial Fibrillation Pathophysiology and Therapy: A Translational Perspective

Atrial fibrillation (AF) is a highly prevalent arrhythmia, with substantial associated morbidity and mortality. There have been significant management advances over the past 2 decades, but the burden of the disease continues to increase and there is certainly plenty of room for improvement in treatment options. A potential key to therapeutic innovation is a better understanding of underlying fundamental mechanisms. This article reviews recent advances in understanding the molecular basis for AF, with a particular emphasis on relating these new insights to opportunities for clinical translation. We first review the evidence relating basic electrophysiological mechanisms to the characteristics of clinical AF. We then discuss the molecular control of factors leading to some of the principal determinants, including abnormalities in impulse conduction (such as tissue fibrosis and other extra-cardiomyocyte alterations, connexin dysregulation and Na+-channel dysfunction), electrical refractoriness, and impulse generation. We then consider the molecular drivers of AF progression, including a range of Ca2+-dependent intracellular processes, microRNA changes, and inflammatory signaling. The concept of key interactome-related nodal points is then evaluated, dealing with systems like those associated with CaMKII (Ca2+/calmodulin-dependent protein kinase-II), NLRP3 (NACHT, LRR, and PYD domains-containing protein-3), and transcription-factors like TBX5 and PitX2c. We conclude with a critical discussion of therapeutic implications, knowledge gaps and future directions, dealing with such aspects as drug repurposing, biologicals, multispecific drugs, the targeting of cardiomyocyte inflammatory signaling and potential considerations in intervening at the level of interactomes and gene-regulation. The area of molecular intervention for AF management presents exciting new opportunities, along with substantial challenges.

Phosphorylation of cardiac myosin-binding protein-C contributes to calcium homeostasis

Cardiac myosin-binding protein-C (cMyBP-C) is highly phosphorylated under basal conditions. However, its phosphorylation level is decreased in individuals with heart failure. The necessity of cMyBP-C phosphorylation for proper contractile function is well-established, but the physiological and pathological consequences of decreased cMyBP-C phosphorylation in the heart are not clear. Herein, using intact adult cardiomyocytes from mouse models expressing phospho-ablated (AAA) and phosphomimetic (DDD) cMyBP-C as well as controls, we found that cMyBP-C dephosphorylation is sufficient to reduce contractile parameters and calcium kinetics associated with prolonged decay time of the calcium transient and increased diastolic calcium levels. Isoproterenol stimulation reversed the depressive contractile and Ca²⁺-kinetic parameters. Moreover, caffeine-induced calcium release yielded no difference between AAA/DDD and controls in calcium content of the sarcoplasmic reticulum. On the other hand, sodium-calcium exchanger function and phosphorylation levels of calcium-handling proteins were significantly decreased in AAA hearts compared with controls. Stress conditions caused increases in both spontaneous aftercontractions in AAA cardiomyocytes and the incidence of arrhythmias in vivo compared with the controls. Treatment with omecamtiv mecarbil, a positive cardiac inotropic drug, rescued the contractile deficit in AAA cardiomyocytes, but not the calcium-handling abnormalities. These findings indicate a cascade effect whereby cMyBP-C dephosphorylation causes contractile defects, which then lead to calcium-cycling abnormalities, resulting in aftercontractions and increased incidence of cardiac arrhythmias under stress conditions. We conclude that improvement of contractile deficits alone without improving calcium handling may be insufficient for effective management of heart failure.

Atrial fibrillation risk loci interact to modulate Ca2+-dependent atrial rhythm homeostasis

Atrial fibrillation (AF), defined by disorganized atrial cardiac rhythm, is the most prevalent cardiac arrhythmia worldwide. Recent genetic studies have highlighted a major heritable component and identified numerous loci associated with AF risk, including the cardiogenic transcription factor genes TBX5, GATA4, and NKX2-5. We report that Tbx5 and Gata4 interact with opposite signs for atrial rhythm controls compared with cardiac development. Using mouse genetics, we found that AF pathophysiology caused by Tbx5 haploinsufficiency, including atrial arrhythmia susceptibility, prolonged action potential duration, and ectopic cardiomyocyte depolarizations, were all rescued by Gata4 haploinsufficiency. In contrast, Nkx2-5 haploinsufficiency showed no combinatorial effect. The molecular basis of the TBX5/GATA4 interaction included normalization of intra-cardiomyocyte calcium flux and expression of calcium channel genes Atp2a2 and Ryr2. Furthermore, GATA4 and TBX5 showed antagonistic interactions on an Ryr2 enhancer. Atrial rhythm instability caused by Tbx5 haploinsufficiency was rescued by a decreased dose of phospholamban, a sarco/endoplasmic reticulum Ca2+-ATPase inhibitor, consistent with a role for decreased sarcoplasmic reticulum calcium flux in Tbx5-dependent AF susceptibility. This work defines a link between Tbx5 dose, sarcoplasmic reticulum calcium flux, and AF propensity. The unexpected interactions between Tbx5 and Gata4 in atrial rhythm control suggest that evaluating specific interactions between genetic risk loci will be necessary for ascertaining personalized risk from genetic association data.

Metformin regulates adiponectin signalling in epicardial adipose tissue and reduces atrial fibrillation vulnerability

Epicardial adipose tissue (EAT) remodelling is closely related to the pathogenesis of atrial fibrillation (AF). We investigated whether metformin (MET) prevents AF‐dependent EAT remodelling and AF vulnerability in dogs. A canine AF model was developed by 6‐week rapid atrial pacing (RAP), and electrophysiological parameters were measured. Effective refractory periods (ERP) were decreased in the left and right atrial appendages as well as in the left atrium (LA) and right atrium (RA). MET attenuated the RAP‐induced increase in ERP dispersion, cumulative window of vulnerability, AF inducibility and AF duration. RAP increased reactive oxygen species (ROS) production and nuclear factor kappa‐B (NF‐κB) phosphorylation; up‐regulated interleukin‐6 (IL‐6), tumour necrosis factor‐α (TNF‐α) and transforming growth factor‐β1 (TGF‐β1) levels in LA and EAT; decreased peroxisome proliferator‐activated receptor gamma (PPARγ) and adiponectin (APN) expression in EAT and was accompanied by atrial fibrosis and adipose infiltration. MET reversed these alterations. In vitro, lipopolysaccharide (LPS) exposure increased IL‐6, TNF‐α and TGF‐β1 expression and decreased PPARγ/APN expression in 3T3‐L1 adipocytes, which were all reversed after MET administration. Indirect coculture of HL‐1 cells with LPS‐stimulated 3T3‐L1 conditioned medium (CM) significantly increased IL‐6, TNF‐α and TGF‐β1 expression and decreased SERCA2a and p‐PLN expression, while LPS + MET CM and APN treatment alleviated the inflammatory response and sarcoplasmic reticulum Ca²⁺ handling dysfunction. MET attenuated the RAP‐induced increase in AF vulnerability, remodelling of atria and EAT adipokines production profiles. APN may play a key role in the prevention of AF‐dependent EAT remodelling and AF vulnerability by MET.

Altered calcium handling produces reentry-promoting action potential alternans in atrial fibrillation-remodeled hearts

Atrial fibrillation (AF) alters atrial cardiomyocyte (ACM) Ca2+ handling, promoting ectopic beat formation. We examined the effects of AF-associated remodeling on Ca2+-related action potential dynamics and consequences for AF susceptibility. AF was maintained electrically in dogs by right atrial (RA) tachypacing. ACMs isolated from AF dogs showed increased Ca2+ release refractoriness, spontaneous Ca2+ spark frequency, and cycle length (CL) threshold for Ca2+ and action potential duration (APD) alternans versus controls. AF increased the in situ CL threshold for Ca2+/APD alternans and spatial dispersion in Ca2+ release recovery kinetics, leading to spatially discordant alternans associated with reentrant rotor formation and susceptibility to AF induction/maintenance. The clinically available agent dantrolene reduced Ca2+ leak and CL threshold for Ca2+/APD alternans in ACMs and AF dog right atrium, while suppressing AF susceptibility; caffeine increased Ca2+ leak and CL threshold for Ca2+/APD alternans in control dog ACMs and RA tissues. In vivo, the atrial repolarization alternans CL threshold was increased in AF versus control, as was AF vulnerability. Intravenous dantrolene restored repolarization alternans threshold and reduced AF vulnerability. Immunoblots showed reduced expression of total and phosphorylated ryanodine receptors and calsequestrin in AF and unchanged phospholamban/SERCA expression. Thus, along with promoting spontaneous ectopy, AF-induced Ca2+ handling abnormalities favor AF by enhancing vulnerability to repolarization alternans, promoting initiation and maintenance of reentrant activity; dantrolene provides a lead molecule to target this mechanism.

Linking cellular energy state to atrial fibrillation pathogenesis: Potential role of adenosine monophosphate-activated protein kinase

Adenosine monophosphate-activated protein kinase (AMPK) is the cellular stress-sensing molecule. Apart from maintaining cellular energy balance, AMPK controls expression and regulation of ion channels and ion transporters, including cytosolic Ca²⁺ handling proteins. Emerging evidence suggests that metabolic impairment plays a crucial role in the pathogenesis of atrial fibrillation. AMPK activation is thought to be protective by preventing metabolic stress, favorably modulating membrane electrophysiology including cytosolic Ca²⁺ dynamics; preventing cellular growth; and hypertrophic remodeling. This review considers current concepts and evidence from clinical and experimental studies regarding the role of AMPK in atrial fibrillation.

The 4q25 variant rs13143308T links risk of atrial fibrillation to defective calcium homoeostasis

AIMS

Single nucleotide polymorphisms on chromosome 4q25 have been associated with risk of atrial fibrillation (AF) but the exiguous knowledge of the mechanistic links between these risk variants and underlying electrophysiological alterations hampers their clinical utility. Here, we tested the hypothesis that 4q25 risk variants cause alterations in the intracellular calcium homoeostasis that predispose to spontaneous electrical activity.

METHODS AND RESULTS

Western blotting, confocal calcium imaging, and patch-clamp techniques were used to identify mechanisms linking the 4q25 risk variants rs2200733T and rs13143308T to defects in the calcium homoeostasis in human atrial myocytes. Our findings revealed that the rs13143308T variant was more frequent in patients with AF and that myocytes from carriers of this variant had a significantly higher density of calcium sparks (14.1 ± 4.5 vs. 3.1 ± 1.3 events/min, P = 0.02), frequency of transient inward currents (ITI) (1.33 ± 0.24 vs. 0.26 ± 0.09 events/min, P < 0.001) and incidence of spontaneous membrane depolarizations (1.22 ± 0.26 vs. 0.56 ± 0.17 events/min, P = 0.001) than myocytes from patients with the normal rs13143308G variant. These alterations were linked to higher sarcoplasmic reticulum calcium loading (10.2 ± 1.4 vs. 7.3 ± 0.5 amol/pF, P = 0.01), SERCA2 expression (1.37 ± 0.13 fold, P = 0.03), and RyR2 phosphorylation at ser2808 (0.67 ± 0.08 vs. 0.47 ± 0.03, P = 0.01) but not at ser2814 (0.28 ± 0.14 vs. 0.31 ± 0.14, P = 0.61) in patients carrying the rs13143308T risk variant. Furthermore, the presence of a risk variant or AF independently increased the ITI frequency and the increase in the ITI frequency observed in carriers of the risk variants was exacerbated in those with AF. By contrast, the presence of a risk variant did not affect the amplitude or properties of the L-type calcium current in patients with or without AF.

CONCLUSIONS

Here, we identify the 4q25 variant rs13143308T as a genetic risk marker for AF, specifically associated with excessive calcium release and spontaneous electrical activity linked to increased SERCA2 expression and RyR2 phosphorylation.

Cardiomyocyte calcium handling in health and disease: Insights from in vitro and in silico studies

Calcium (Ca²⁺) plays a central role in cardiomyocyte excitation-contraction coupling. To ensure an optimal electrical impulse propagation and cardiac contraction, Ca²⁺ levels are regulated by a variety of Ca²⁺-handling proteins. In turn, Ca²⁺ modulates numerous electrophysiological processes. Accordingly, Ca²⁺-handling abnormalities can promote cardiac arrhythmias via various mechanisms, including the promotion of afterdepolarizations, ion-channel modulation and structural remodeling. In the last 30 years, significant improvements have been made in the computational modeling of cardiomyocyte Ca²⁺ handling under physiological and pathological conditions. However, numerous questions involving the Ca²⁺-dependent regulation of different macromolecular complexes, cross-talk between Ca²⁺-dependent regulatory pathways operating over a wide range of time scales, and bidirectional interactions between electrophysiology and mechanics remain to be addressed by in vitro and in silico studies. A better understanding of disease-specific Ca²⁺-dependent proarrhythmic mechanisms may facilitate the development of improved therapeutic strategies. In this review, we describe the fundamental mechanisms of cardiomyocyte Ca²⁺ handling in health and disease, and provide an overview of currently available computational models for cardiomyocyte Ca²⁺ handling. Finally, we discuss important uncertainties and open questions about cardiomyocyte Ca²⁺ handling and highlight how synergy between in vitro and in silico studies may help to answer several of these issues.

Cardiac Myosin-Binding Protein C-From Bench to Improved Diagnosis of Acute Myocardial Infarction

Chest pain is responsible for 6-10% of all presentations to acute healthcare providers. Triage is inherently difficult and heavily reliant on the quantification of cardiac Troponin (cTn), as a minority of patients with an ultimate diagnosis of acute myocardial infarction (AMI) present with clear diagnostic features such as ST-elevation on the electrocardiogram. Owing to slow release and disappearance of cTn, many patients require repeat blood testing or present with stable but elevated concentrations of the best available biomarker and are thus caught at the interplay of sensitivity and specificity.We identified cardiac myosin-binding protein C (cMyC) in coronary venous effluent and developed a high-sensitivity assay by producing an array of monoclonal antibodies and choosing an ideal pair based on affinity and epitope maps. Compared to high-sensitivity cardiac Troponin (hs-cTn), we demonstrated that cMyC appears earlier and rises faster following myocardial necrosis. In this review, we discuss discovery and structure of cMyC, as well as the migration from a comparably insensitive to a high-sensitivity assay facilitating first clinical studies. This assay was subsequently used to describe relative abundance of the protein, compare sensitivity to two high-sensitivity cTn assays and test diagnostic performance in over 1900 patients presenting with chest pain and suspected AMI. A standout feature was cMyC's ability to more effectively triage patients. This distinction is likely related to the documented greater abundance and more rapid release profile, which could significantly improve the early triage of patients with suspected AMI.

Atrial Cardiomyopathy: An Unexplored Limb of Virchow's Triad for AF Stroke Prophylaxis

The most dreaded complication of atrial fibrillation is stroke, and 70–80% of patients with AF-related stroke die or become disabled. The mechanisms of thromboembolism in AF are multifactorial, with evidence demonstrating that all three criteria of Virchow's triad are satisfied in AF: abnormal stasis of blood, endothelial damage, and hypercoagulability. Mechanistic insights into the latter two limbs have resulted in effective stroke prophylactic therapies (left atrial appendage occlusion and oral anticoagulants); however, despite these advances, there remains an excess of stroke in the AF population that may be due, in part, to a lack of mechanistic understanding of atrial hypocontractility resulting in abnormal stasis of blood within the atrium. These observations support the emerging concept of atrial cardiomyopathy as a cause of stroke. In this Review, we evaluate molecular, translational, and clinical evidence for atrial cardiomyopathy as a cause for stroke from AF, and present a rationale for further investigation of this largely unaddressed limb of Virchow's triad in AF.

Activation of endogenous protein phosphatase 1 enhances the calcium sensitivity of the ryanodine receptor type 2 in murine ventricular cardiomyocytes

KEY POINTS

Increased protein phosphatase 1 (PP-1) activity has been found in end stage human heart failure. Although PP-1 has been extensively studied, a detailed understanding of its role in the excitation-contraction coupling mechanism, in normal and diseased hearts, remains elusive. The present study investigates the functional effect of the PP-1 activity on local Ca²⁺ release events in ventricular cardiomyocytes, by using an activating peptide (PDP3) for the stimulation of the endogenous PP-1 protein. We report that acute de-phosphorylation may increase the sensitivity of RyR2 channels to Ca²⁺ in situ, and that the RyR2-serine2808 phosphorylation site may mediate such a process. Our approach unmasks the functional importance of PP-1 in the regulation of RyR2 activity, suggesting a potential role in the generation of a pathophysiological sarcoplasmic reticulum Ca²⁺ leak in the diseased heart.

ABSTRACT

Changes in cardiac ryanodine receptor (RyR2) phosphorylation are considered to be important regulatory and disease related post-translational protein modifications. The extent of RyR2 phosphorylation is mainly determined by the balance of the activities of protein kinases and phosphatases, respectively. Increased protein phosphatase-1 (PP-1) activity has been observed in heart failure, although the regulatory role of this enzyme on intracellular Ca²⁺ handling remains poorly understood. To determine the physiological and pathophysiological significance of increased PP-1 activity, we investigated how the PP-1 catalytic subunit (PP-1c) alters Ca²⁺ sparks in permeabilized cardiomyocytes and we also applied a PP-1-disrupting peptide (PDP3) to specifically activate endogenous PP-1, including the one anchored on the RyR2 macromolecular complex. We compared wild-type and transgenic mice in which the usually highly phosphorylated site RyR2-S2808 has been ablated to investigate its involvement in RyR2 modulation (S2808A^+/+ ). In wild-type myocytes, PP-1 increased Ca²⁺ spark frequency by two-fold, followed by depletion of the sarcoplasmic reticulum Ca²⁺ store. Similarly, PDP3 transiently increased spark frequency and decreased sarcoplasmic reticulum Ca²⁺ load. RyR2 Ca²⁺ sensitivity, which was assessed by Ca²⁺ spark recovery analysis, was increased in the presence of PDP3 compared to a negative control peptide. S2808A^+/+ cardiomyocytes did not respond to both PP-1c and PDP3 treatment. Our results suggest an increased Ca²⁺ sensitivity of RyR2 upon de-phosphorylation by PP-1. Furthermore, we have confirmed the S2808 site as a target for PP-1 and as a potential link between RyR2s modulation and the cellular response.

Enhanced cardiomyocyte reactive oxygen species signaling promotes ibrutinib-induced atrial fibrillation

Atrial fibrillation (AF) occurs in up to 11% of cancer patients treated with ibrutinib. The pathophysiology of ibrutinib promoted AF is complicated, as there are multiple interactions involved; the detailed molecular mechanisms underlying this are still unclear. Here, we aimed to determine the electrophysiological and molecular mechanisms of burst-pacing-induced AF in ibrutinib-treated mice. The results indicated differentially expressed proteins in ibrutinib-treated mice, identified through proteomic analysis, were found to play a role in oxidative stress-related pathways. Finally, treatment with an inhibitor of NADPH oxidase (NOX) prevented and reversed AF development in ibrutinib-treated mice. It was showed that the related protein expression of reactive oxygen species (ROS) in the ibrutinib group was significantly increased, including NOX2, NOX4, p22-phox, XO and TGF-β protein expression. It was interesting that ibrutinib group also significantly increased the expression of ox-CaMKII, p-CaMKII (Thr-286) and p-RyR2 (Ser2814), causing enhanced abnormal sarcoplasmic reticulum (SR) Ca²⁺ release and mitochondrial structures, as well as atrial fibrosis and atrial hypertrophy in ibrutinib-treated mice, and apocynin reduced the expression of these proteins. Ibrutinib-treated mice were also more likely to develop AF, and AF occurred over longer periods. In conclusion, our study has established a pathophysiological role for ROS signaling in atrial cardiomyocytes, and it may be that ox-CaMKII and p-CaMKII (Thr-286) are activated by ROS to increase AF susceptibility following ibrutinib treatment. We have also identified the inhibition of NOX as a potential novel AF therapy approach.

Cardiac Myosin Binding Protein-C Phosphorylation Mitigates Age-Related Cardiac Dysfunction: Hope for Better Aging?

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation prevents aging-related cardiac dysfunction. We tested this hypothesis by aging genetic mouse models of hypophosphorylated cMyBP-C, wild-type equivalent, and phosphorylated-mimetic cMyBP-C for 18 to 20 months. Phosphorylated-mimetic cMyBP-C mice exhibited better survival, better preservation of systolic and diastolic functions, and unchanging wall thickness. Wild-type equivalent mice showed decreasing cMyBP-C phosphorylation along with worsening cardiac function and hypertrophy approaching those found in hypophosphorylated cMyBP-C mice. Intact papillary muscle experiments suggested that cMyBP-C phosphorylation increased cross-bridge detachment rates as the underlying mechanism. Thus, phosphorylating cMyBP-C is a novel mechanism with potential to treat aging-related cardiac dysfunction.