Peter Boknik

Molecular Determinants of Altered Ca2+ Handling in Human Chronic Atrial Fibrillation

Background— Abnormal Ca2+ handling may contribute to impaired atrial contractility and arrhythmogenesis in human chronic atrial fibrillation (cAF). Here, we assessed the phosphorylation levels of key proteins involved in altered Ca2+ handling and contractility in cAF patients. Methods and Results— Total and phosphorylation levels of Ca2+-handling and myofilament proteins were analyzed by Western blotting in right atrial appendages of 49 patients in sinus rhythm and 52 cAF patients. We found a higher total activity of type 1 (PP1) and type 2A phosphatases in cAF, which was associated with inhomogeneous changes of protein phosphorylation in the cellular compartments, ie, lower protein kinase A (PKA) phosphorylation of myosin binding protein-C (Ser-282 site) at the thick myofilaments but preserved PKA phosphorylation of troponin I at the thin myofilaments and enhanced PKA (Ser-16 site) and Ca2+-calmodulin protein kinase (Thr-17 site) phosphorylation of phospholamban. PP1 activity at sarcoplasmic reticulum is controlled by inhibitor-1 (I-1), which blocks PP1 in its PKA-phosphorylated form only. In cAF, the ratio of Thr-35–phosphorylated to total I-1 was 10-fold higher, which suggests that the enhanced phosphorylation of phospholamban may result from a stronger PP1 inhibition by PKA-hyperphosphorylated (activated) I-1. Conclusions— Altered Ca2+ handling in cAF is associated with impaired phosphorylation of myosin binding protein-C, which may contribute to the contractile dysfunction after cardioversion. The hyperphosphorylation of phospholamban probably results from enhanced inhibition of sarcoplasmic PP1 by hyperphosphorylated I-1 and may reinforce the leakiness of ryanodine channels in cAF. Restoration of sarcoplasmic reticulum–associated PP1 function may represent a new therapeutic option for treatment of atrial fibrillation.

Calcium-Handling Abnormalities Underlying Atrial Arrhythmogenesis and Contractile Dysfunction in Dogs With Congestive Heart Failure

Background—Congestive heart failure (CHF) is a common cause of atrial fibrillation. Focal sources of unknown mechanism have been described in CHF-related atrial fibrillation. The authors hypothesized that abnormal calcium (Ca2+) handling contributes to the CHF-related atrial arrhythmogenic substrate. Methods and Results—CHF was induced in dogs by ventricular tachypacing (240 bpm ×2 weeks). Cellular Ca2+-handling properties and expression/phosphorylation status of key Ca2+ handling and myofilament proteins were assessed in control and CHF atria. CHF decreased cell shortening but increased left atrial diastolic intracellular Ca2+ concentration ([Ca2+]i), [Ca2+]i transient amplitude, and sarcoplasmic reticulum (SR) Ca2+ load (caffeine-induced [Ca2+]i release). SR Ca2+ overload was associated with spontaneous Ca2+ transient events and triggered ectopic activity, which was suppressed by the inhibition of SR Ca2+ release (ryanodine) or Na+/Ca2+ exchange. Mechanisms underlying abnormal SR Ca2+ handling were then studied. CHF increased atrial action potential duration and action potential voltage clamp showed that CHF-like action potentials enhance Ca2+i loading. CHF increased calmodulin-dependent protein kinase II phosphorylation of phospholamban by 120%, potentially enhancing SR Ca2+ uptake by reducing phospholamban inhibition of SR Ca2+ ATPase, but it did not affect phosphorylation of SR Ca2+-release channels (RyR2). Total RyR2 and calsequestrin (main SR Ca2+-binding protein) expression were significantly reduced, by 65% and 15%, potentially contributing to SR dysfunction. CHF decreased expression of total and protein kinase A–phosphorylated myosin-binding protein C (a key contractile filament regulator) by 27% and 74%, potentially accounting for decreased contractility despite increased Ca2+ transients. Complex phosphorylation changes were explained by enhanced calmodulin-dependent protein kinase II&dgr; expression and function and type-1 protein-phosphatase activity but downregulated regulatory protein kinase A subunits. Conclusions—CHF causes profound changes in Ca2+-handling and -regulatory proteins that produce atrial fibrillation–promoting atrial cardiomyocyte Ca2+-handling abnormalities, arrhythmogenic triggered activity, and contractile dysfunction.

Messenger RNA expression and immunological quantification of phospholamban and SR-Ca2+-ATPase in failing and nonfailing human hearts

OBJECTIVES Human heart failure is associated with prolonged relaxation and prolonged Ca2+ transients which indicates an impaired function of the sarcoplasmic reticulum (SR) and may be detrimental for cardiac function. Controversy exists whether the altered SR function is accompanied by changes in the expression of phospholamban (PLB) and cardiac SR-Ca(2+)-ATPase (SERCA2) on mRNA and/or protein levels. METHODS We determined mRNA and/or protein levels for PLB and SERCA2 in the same left ventricular tissue of patients undergoing heart transplantation due to idiopathic dilated cardiomyopathy (IDC) or ischemic cardiomyopathy (ICM) in comparison to left ventricular tissue from nonfailing human hearts (NF). Total protein extracts were prepared and subjected to SDS gel electrophoresis. PLB and SERCA2 were identified with specific antibodies. Total RNA was isolated and hybridized with 32P-labeled cDNAs for human PLB and rat SERCA2. RESULTS Hybridization revealed the three expected mRNAs with the PLB probe (3.3 kb, 1.9 kb and 0.6 kb) and a single band with the SERCA2 probe (4.5 kb). Determination of respective proteins by immunoblotting revealed unchanged protein levels for PLB and SERCA2, whereas the mRNA levels for PLB and SERCA2 were reduced by about 30% and 50%, respectively. CONCLUSIONS These data show the level of SERCA2 and PLB protein and mRNA in the same hearts. The reduced mRNA level of SERCA2 and PLB is in accordance with previous data. However, the unchanged protein levels may indicate that the diminished RNA expression is not accompanied by a corresponding decrease for these proteins in human heart failure. These data also show that the altered SR function in human heart failure cannot be explained by altered protein levels of PLB and SERCA2. Furthermore, it is suggested that extrapolations from cardiac mRNA levels to protein expression may be misleading.

Evidence for protein phosphatase inhibitor-1 playing an amplifier role in β-adrenergic signaling in cardiac myocytes

The protein phosphatase inhibitor‐1 (PPI‐1) inhibits phosphatase type‐1 (PP1) only when phosphorylated by protein kinase A and could play a pivotal role in the phosphorylation/dephosphorylation balance. Rat cardiac PPI‐1 was cloned by reverse transcriptase‐polymerase chain reaction, expressed in Eschericia coli, evaluated in phosphatase assays, and used to generate an antiserum. An adenovirus was constructed encoding PPI‐1 and green fluorescent protein (GFP) under separate cytomegalovirus promotors (AdPPI‐1/GFP). A GFP‐only virus (AdGFP) served as control. Engineered heart tissue (EHT) from neonatal rat cardiomyocytes and adult rat cardiac myocytes (ARCMs) were used as model systems. PPI‐1 expression was determined in human ventricular samples by Northern blots. Compared with AdGFP, AdPPI‐1/GFP‐infected neonatal rat cardiomyocytes displayed a 73% reduction in PP1 activity. EHTs infected with AdPPI‐1/GFP exhibited a fivefold increase in isoprenaline sensitivity. AdPPI‐1/GFP‐infected ARCMs displayed enhanced cell shortening as well as enhanced phospholamban phosphorylation when stimulated with 1 nM isoprenaline. PPI‐1 mRNA levels were reduced by 57±12% in failing hearts with dilated and ischemic cardiomyopathy (n=8 each) compared with nonfailing hearts (n=8). In summary, increased PPI‐1 expression enhances myocyte sensitivity to isoprenaline, indicating that PPI‐1 acts as an amplifier in β‐adrenergic signaling. Decreased PPI‐1 in failing human hearts could participate in desensitization of the cAMP pathway.

Multiple Potential Molecular Contributors to Atrial Hypocontractility Caused by Atrial Tachycardia Remodeling in Dogs

Background—Atrial fibrillation impairs atrial contractility, inducing atrial stunning that promotes thromboembolic stroke. Action potential (AP)-prolonging drugs are reported to normalize atrial hypocontractility caused by atrial tachycardia remodeling (ATR). Here, we addressed the role of AP duration (APD) changes in ATR-induced hypocontractility. Methods and Results—ATR (7-day tachypacing) decreased APD (perforated patch recording) by ≈50%, atrial contractility (echocardiography, cardiomyocyte video edge detection), and [Ca2+]i transients. ATR AP waveforms suppressed [Ca2+]i transients and cell shortening of control cardiomyocytes; whereas control AP waveforms improved [Ca2+]i transients and cell shortening in ATR cells. However, ATR cardiomyocytes clamped with the same control AP waveform had ≈60% smaller [Ca2+]i transients and cell shortening than control cells. We therefore sought additional mechanisms of contractile impairment. Whole-cell voltage clamp revealed reduced ICaL; ICaL inhibition superimposed on ATR APs further suppressed [Ca2+]i transients in control cells. Confocal microscopy indicated ATR-impaired propagation of the Ca2+ release signal to the cell center in association with loss of t-tubular structures. Myofilament function studies in skinned permeabilized cardiomyocytes showed altered Ca2+ sensitivity and force redevelopment in ATR, possibly due to hypophosphorylation of myosin-binding protein C and myosin light-chain protein 2a (immunoblot). Hypophosphorylation was related to multiple phosphorylation system abnormalities where protein kinase A regulatory subunits were downregulated, whereas autophosphorylation and expression of Ca2+-calmodulin-dependent protein kinase II&dgr; and protein phosphatase 1 activity were enhanced. Recovery of [Ca2+]i transients and cell shortening occurred in parallel after ATR cessation. Conclusions—Shortening of APD contributes to hypocontractility induced by 1-week ATR but accounts for it only partially. Additional contractility-suppressing mechanisms include ICaL current reduction, impaired subcellular Ca2+ signal transmission, and altered myofilament function associated with abnormal myosin and myosin-associated protein phosphorylation. The complex mechanistic basis of the atrial hypocontractility associated with AF argues for upstream therapeutic targeting rather than interventions directed toward specific downstream pathophysiological derangements.

Distinct contractile and molecular differences between two goat models of atrial dysfunction: AV block-induced atrial dilatation and atrial fibrillation

Atrial dilatation is an independent risk factor for thromboembolism in patients with and without atrial fibrillation (AF). In many patients, atrial dilatation goes along with depressed contractile function of the dilated atria. While some mechanisms causing atrial contractile dysfunction in fibrillating atria have been addressed previously, the cellular and molecular mechanisms of atrial contractile remodeling in dilated atria are unknown. This study characterized in vivo atrial contractile function in a goat model of atrial dilatation and compared it to a goat model of AF. Differences in the underlying mechanisms were elucidated by studying contractile function, electrophysiology and sarcoplasmic reticulum (SR) Ca2+ load in atrial muscle bundles and by analyzing expression and phosphorylation levels of key Ca2+-handling proteins, myofilaments and the expression and activity of their upstream regulators. In 7 chronically instrumented, awake goats atrial contractile dysfunction was monitored during 3 weeks of progressive atrial dilatation after AV-node ablation (AV block goats (AVB)). In open chest experiments atrial work index (AWI) and refractoriness were measured (10 goats with AVB, 5 goats with ten days of AF induced by repetitive atrial burst pacing (AF), 10 controls). Isometric force of contraction (FC), transmembrane action potentials (APs) and rapid cooling contractures (RCC, a measure of SR Ca2+ load) were studied in right atrial muscle bundles. Total and phosphorylated Ca2+-handling and myofilament protein levels were quantified by Western blot. In AVB goats, atrial size increased by 18% (from 26.6+/-4.4 to 31.6+/-5.5 mm, n=7 p<0.01) while atrial fractional shortening (AFS) decreased (from 18.4+/-1.7 to 12.8+/-4.0% at 400 ms, n=7, p<0.01). In open chest experiments, AWI was reduced in AVB and in AF goats compared to controls (at 400 ms: 8.4+/-0.9, n=7, and 3.2+/-1.8, n=5, vs 18.9+/-5.3 mmxmmHg, n=7, respectively, p<0.05 vs control). FC of isolated right atrial muscle bundles was reduced in AVB (n=8) and in AF (n=5) goats compared to controls (n=9) (at 2 Hz: 2.3+/-0.5 and 0.7+/-0.2 vs 5.5+/-1.0 mN/mm2, respectively, p<0.05). APs were shorter in AF, but unchanged in AVB goats. RCCs were reduced in AVB and AF versus control (AVB, 3.4+/-0.5 and AF, 4.1+/-1.4 vs 12.2+/-3.2 mN/mm2, p<0.05). Protein levels of protein kinase A (PKA) phosphorylated phospholamban (PLB) were reduced in AVB (n=8) and AF (n=8) vs control (n=7) by 37.9+/-12.4% and 29.7+/-10.1%, respectively (p<0.01), whereas calmodulin-dependent protein kinase II (CaMKII) phosphorylated ryanodine channels (RyR2) were increased by 166+/-55% in AVB (n=8) and by 146+/-56% in AF (n=8) goats (p<0.01). PKA-phosphorylated myosin-binding protein-C and troponin-I were reduced exclusively in AVB goat atria (by 75+/-10% and 55+/-15%, respectively, n=8, p<0.05). Atrial dilatation developing during slow ventricular rhythm after complete AV block as well as AF-induced remodeling are associated with atrial contractile dysfunction. Both AVB and AF goat atria show decreased SR Ca2+ load, likely caused by PLB dephosphorylation and RYR2 hyperphosphorylation. While shorter APs further compromise contractility in AF goat atria, reduced myofilament phosphorylation may impair contractility in AVB goat atria. Thus, atrial hypocontractility appears to have distinct molecular contributors in different types of atrial remodeling.

Heart-directed Expression of a Human Cardiac Isoform of cAMP-Response Element Modulator in Transgenic Mice

The transcriptional activation mediated by cAMP-response element (CRE) and transcription factors of the CRE-binding protein (CREB)/CRE modulator (CREM) family represents an important mechanism of cAMP-dependent gene regulation possibly implicated in detrimental effects of chronic β-adrenergic stimulation in end-stage heart failure. We studied the cardiac role of CREM in transgenic mice with heart-directed expression of CREM-IbΔC-X, a human cardiac CREM isoform. Transgenic mice displayed atrial enlargement with atrial and ventricular hypertrophy, developed atrial fibrillation, and died prematurely. In vivo hemodynamic assessment revealed increased contractility of transgenic left ventricles probably due to a selective up-regulation of SERCA2, the cardiac Ca2+-ATPase of the sarcoplasmic reticulum. In transgenic ventricles, reduced phosphorylation of phospholamban and of the CREB was associated with increased activity of serine-threonine protein phosphatase 1. The density of β1-adrenoreceptor was increased, and messenger RNAs encoding transcription factor dHAND and small G-protein RhoB were decreased in transgenic hearts as compared with wild-type controls. Our results indicate that heart-directed expression of CREM-IbΔC-X leads to complex cardiac alterations, suggesting CREM as a central regulator of cardiac morphology, function, and gene expression.

Early impairment of calcium handling and altered expression of junctin in hearts of mice overexpressing the β1-adrenergic receptor

Chronic stimulation of cardiac β1‐adrenergic receptors contributes to disease progression and mortality in patients and animal models of heart failure. To search for the mechanism of adrenergic impairment of cardiac function in vivo, we studied transgenic mice with cardiac‐specific overexpression of β1‐adrenergic receptors. Transgenic mice with cardiac overexpression of β1‐adrenergic receptors showed progressive left ventricular fibrosis starting at 4 months of age. Left ventricular catheterization revealed a modest enhancement of contractility and relaxation at 2 months of age, followed by progressive dysfunction in both parameters and ultimately cardiac failure. When the effects of endogenous catecholamines were blocked by the β‐receptor antagonist propranolol, maximal rate of contractility (dp/dtmax) and maximal rate of relaxation (dp/dtmin) were significantly blunted in 2‐month‐old β1‐receptor transgenic mice. Isolated cardiomyocytes from these animals displayed markedly altered calcium transients with significant prolongation of the intracellular calcium transient compared with nontransgenic littermates. We determined the expression of sarcoplasmic reticulum proteins involved in calcium handling by RNase protection assay and by immunoblotting. Although the expression of calsequestrin, triadin, and phospholamban was not altered, we observed a progressive decrease in junctin abundance in β1‐receptor transgenic mice (P < 0.001 TG vs. WT). Altered expression of the sarcoplasmic reticulum protein junctin may be involved in the pathogenesis of cardiac failure after chronic stimulation of β1‐adrenergic receptors.

Regional expression of phospholamban in the human heart.

BACKGROUND Several independent lines of evidence indicate that phospholamban (PLB) expression correlates positively with depression of force of contraction and duration of contraction in isolated cardiac preparations of several animal species. Here, we studied whether PLB levels correlate with attenuation of contractility and enhancement of contractile time parameters in different parts of the human heart. METHODS Force of contraction was measured in isolated electrically driven atrial and ventricular preparations from human hearts. Ca(2+)-uptake by human atrial and ventricular homogenates was assayed at different ionized Ca(2+)-concentrations. Protein expression of PLB and the sarcoplasmic Ca(2+)-ATPase (SERCA) was measured in homogenates by quantitative immunoblotting using specific antibodies. PLB mRNA expression was quantified in human cardiac preparations by Northern blot analysis. RESULTS The duration of contraction in isolated preparations of human right ventricle (RV) was double that found in right atrial preparations (RA) (620 +/- 25 ms versus 308 +/- 15 ms). In RA, PLB expression was reduced by 44% at the protein level and by 34% at the mRNA level compared to RV. In contrast, the SERCA protein content was increased by 104% in RA compared to RV. Ca(2+)-uptake at low ionized Ca(2+)-concentration, where the inhibiting effect of PLB is maximal, amounted to 1.39 +/- 0.28 nmol Ca2+/mg protein in RA and to 0.62 +/- 0.09 nmol Ca2+/mg protein in RV (n = 6 both). CONCLUSIONS It is suggested that duration of contraction is shorter in human atrium versus ventricle due to the combined effect of decreased PLB levels (which inhibits SERCA function) and increased SERCA levels. The lower relative ratio of PLB to SERCA leads to less inhibition of SERCA and increased Ca(2+)-uptake which enhances relaxation and contraction in human atrium.

Increased effects of C-type natriuretic peptide on cardiac ventricular contractility and relaxation in guanylyl cyclase A-deficient mice

OBJECTIVE The natriuretic peptides (NPs), atrial (ANP), B-type (BNP), and C-type (CNP) natriuretic peptides as well as their respective receptor-guanylyl cyclases (GC-A for ANP and BNP, and GC-B for CNP) are expressed in the heart. However, the local role of NPs in the regulation of cardiac contractility and the mutual interactions of NPs remain controversial. In the present study we evaluated the effects of ANP and CNP on cardiac function of wild-type (GC-A +/+) and GC-A-deficient (GC-A -/-) mice. METHODS The effects of NPs and their molecular mechanisms were assessed in the isolated perfused mouse working heart preparation. RESULTS In GC-A +/+ hearts, CNP exerted a biphasic action: an immediate increase in inotropy and lusitropy, followed by a slowly developing negative inotropic effect. These effects were mimicked by the cGMP-analogue, 8-pCPT-cGMP. In contrast, ANP did not affect cardiac function. In GC-A -/- hearts, the immediate contractile responses to CNP and 8-pCPT-cGMP were significantly enhanced. CNP increased cardiac cGMP levels and stimulated phospholamban (PLB) phosphorylation; the effect on PLB, but not cGMP, was enhanced in GC-A -/- hearts. In addition, cardiac expression of cGMP-dependent protein kinase (cGK I) was significantly increased in GC-A -/- mice. CONCLUSION CNP exerts a biphasic, initially positive inotropic and lusitropic, then negative inotropic effect in isolated working mouse hearts. A putative mechanism contributing to the immediate contractile responses is cGMP/cGK I-dependent phosphorylation of PLB and subsequent activation of the sarcoplasmic reticulum Ca(2+)-pump. ANP has no direct effects on cardiac contractility but chronic absence of its receptor, GC-A, results in increased responsiveness to CNP.