Published online before print April 13, 2009, doi: 10.1161/CIRCULATIONAHA.108.805804
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(Circulation. 2009;119:2179-2187.)
© 2009 American Heart Association, Inc.
Molecular Cardiology |
Mineralocorticoid Modulation of Cardiac Ryanodine Receptor Activity Is Associated With Downregulation of FK506-Binding Proteins
From INSERM, U637 (A.M.G., A.R., L.P., E.P., R.P., S.R., J.-P.B.), Université Montpellier, France; INSERM, U772 (Y.S.-M., C.L., F.J.), Collège de France, Paris, France; Wales Heart Research Institute (S.Z., F.A.L.), Cardiff University School of Medicine, Cardiff, United Kingdom; Department of Physiology (X.Z., H.H.V.), University of Wisconsin Medical School, Madison, Wis; and Département de l’Information Médicale (R.S., M.-C.P.), Hôpital Lapeyronie, CHU Montpellier, France.
Correspondence to Jean-Pierre Benitah, Laboratoire de physiopathologie cardiovasculaire, INSERM U637, CHU Arnaud de Villeneuve, 34295 Montpellier, France. E-mail jean-pierre.benitah{at}inserm.fr
Received July 16, 2007; accepted March 3, 2009.
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Abstract |
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Background— The mineralocorticoid pathway is involved in cardiac arrhythmias associated with heart failure through mechanisms that are incompletely understood. Defective regulation of the cardiac ryanodine receptor (RyR) is an important cause of the initiation of arrhythmias. Here, we examined whether the aldosterone pathway might modulate RyR function.
Methods and Results— Using the whole-cell patch clamp method, we observed an increase in the occurrence of delayed afterdepolarizations during action potential recordings in isolated adult rat ventricular myocytes exposed for 48 hours to aldosterone 100 nmol/L, in freshly isolated myocytes from transgenic mice with human mineralocorticoid receptor expression in the heart, and in wild-type littermates treated with aldosterone. Sarcoplasmic reticulum Ca2+ load and RyR expression were not altered; however, RyR activity, visualized in situ by confocal microscopy, was increased in all cells, as evidenced by an increased occurrence and redistribution to long-lasting and broader populations of spontaneous Ca2+ sparks. These changes were associated with downregulation of FK506-binding proteins (FKBP12 and 12.6), regulatory proteins of the RyR macromolecular complex.
Conclusions— We suggest that in addition to modulation of Ca2+ influx, overstimulation of the cardiac mineralocorticoid pathway in the heart might be a major upstream factor for aberrant Ca2+ release during diastole, which contributes to cardiac arrhythmia in heart failure.
Key Words: aldosterone • calcium • ryanodine receptor • hormones • ion channels • arrhythmia
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Introduction |
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During the past decade, research has focused on the actions of aldosterone in target organs beyond the kidney, expanding the role of the aldosterone pathway in cardiovascular pathogenesis.1 Indeed, mineralocorticoid receptors (MRs), which mainly underlie aldosterone action, have been detected in a range of nonrenal tissues, including the brain, blood vessels, and heart,2 which suggests a broader pattern of biological activity for aldosterone than previously anticipated. The pivotal role of aldosterone, causing sodium retention with expansion of the extracellular volume that results in deterioration of hemodynamic responsiveness and a fall in cardiac output, has long been recognized in heart failure (HF).3,4 In addition, accumulating experimental and clinical evidence suggests that aldosterone has direct adverse cardiac effects independent of its effects on blood pressure, especially an increased risk of arrhythmic death.3,4 Interestingly, major clinical trials involving MR antagonists have shown significant benefits on risk of cardiovascular events, in particular sudden death in HF.5,6 A pertinent question, therefore, is how activation of cardiac MR participates in life-threatening arrhythmias.
Clinical Perspective p 2187
Most fatal arrhythmias in experimental HF are initiated by nonreentrant mechanisms that arise from abnormal ventricular automaticity or triggered activity.7,8 The latter consists of either early afterdepolarizations (EADs) that occur in the plateau phase of the action potential (AP) or delayed afterdepolarizations (DADs) that occur at repolarized membrane potentials.9 EADs typically occur in the setting of prolonged repolarization due to alterations in ionic currents and to reactivation of the Ca2+ current (ICa). DADs are caused by membrane depolarization initiated by spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR). We have accumulated evidence, both ex vivo and in vivo, that modulation of Ca2+ influx is a central factor in the cardiac action of the aldosterone pathway10–14 and might be involved in EAD-related fatal ventricular tachyarrhythmia.14 Moreover, enhanced diastolic leak of Ca2+ via RyRs generates DADs and is a fundamental mechanism underlying several genetic or acquired arrhythmias.15,16 Therefore, we tested whether activation of the aldosterone pathway modulates diastolic RyR activity in heart.
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Methods |
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All experiments were performed according to European Union Council Directives (86/609/EEC) for the care of laboratory animals. A detailed Methods section can be found in the online-only Data Supplement.
Cell Isolation and Incubation
Isolated ventricular myocytes from adult male Wistar rats (250 to 350 g; Elevage Janvier, Le Genest Saint Isle, France) were incubated for 48 hours, with or without D-aldosterone 100 nmol/L (Sigma, Saint Quentin Fallaviere, France). In some experiments, RU28318 10 µmol/L was added to aldosterone.10,11
Human MR Transgenic Mice and In Vivo Aldosterone Exposure
Cardiac-specific expression of human MR (hMR) was obtained by crossing a tetO-hMR mouse strain with an 
-MHC-tTA transactivator mouse strain.14 Littermate wild-type, gender-matched mice (WT) were used as controls. In some of these, after pentobarbital sodium anesthesia (30 mg/kg) was administered, osmotic minipumps (model 2 ML4, Alza Corp, Mountain View, Calif) were implanted subcutaneously for constant delivery of D-aldosterone 50 µg/d (dissolved in 0.9% saline) over 3 weeks. Chronic aldosterone infusion significantly increased the plasma aldosterone concentration above control levels (from 278±77 [n=7] to 1374±150 [n=7] pg/mL, P<0.05). Mice were devoid of cardiac hypertrophy either on the whole-organ level (heart weight–to–body weight ratio 5.9±0.4 [n=15], 5.9±0.3 [n=11], and 5.3±0.4 mg/g [n=6] in WT, aldosterone-treated, and hMR mice, respectively; P>0.05) or at the cellular level (membrane capacitance 201.8±6.1 [n=30], 199.6±12.7 [n=21], and 199.0±11.0 pF [n=26] for isolated ventricular myocytes from WT, aldosterone-treated, and hMR mice, respectively; P>0.05).
AP Recording
The whole-cell patch-clamp method was used in the current-clamp configuration to record APs with solutions and protocol as described previously.11
Spontaneous Local SR Ca2+ Release: Ca2+ Sparks
Fluo-3AM–loaded cells were imaged in Tyrode solution (in mmol/L: NaCl 130, NaH2PO4 0.4, NaHCO3 5.8, CaCl2 1, MgCl2 0.5, KCl 5.4, glucose 22, HEPES 25, insulin 0.01, pH 7.4) with a confocal microscope in line-scan mode.11
Cell Lysate and SR-Enriched Membrane Fraction (SR Fraction)
Cell lysates were prepared with homogenization buffer (in mmol/L: sucrose 300, NaF 20, HEPES 20, aprotinin 5.2 10–4, benzamidine 0.5, leupeptin, 0.012, PMSF 0.1, pH 7.2) with a Potter-Elvehjem homogenizer and spun at 2000g for 10 minutes. SR fractions were isolated by ultracentrifugation at 40 000g for 30 minutes at 4°C.17 Protein concentration by the Bradford method and binding of [3H]ryanodine to SR fractions were assessed.17
Single-Channel Recordings
Analysis of RyR single-channel activity was done by fusing SR vesicles into planar lipid bilayers with Cs+ as a charge carrier.17 Only bilayers that contained a single channel were used. Channel activity was always tested for sensitivity to EGTA, which was added in the cis chamber at the end of each experiment.
Reverse-Transcription Polymerase Chain Reaction Analysis
Total RNA was extracted with Trizol (Invitrogen, Cergy Pontoise, France). First-strand cDNA was synthesized after DNaseI treatment (DNAfree, Ambion, Courtaboeuf, France) with 1 µg of total RNA, random hexamers (Amersham, Biosciences Europe GMBH, Orsay, France), and Superscript II reverse transcriptase (Invitrogen). Transcript levels were analyzed in triplicate by real-time polymerase chain reaction with an iCycler iQ apparatus (Bio-Rad, Marnes la Coquette, France) with a qPCR Core Kit for SYBR Green I (Eurogentec, Brussels, Belgium) that contained 500 nmol/L of specific primers (Data Supplement, Table I) and 3 µL of diluted template cDNA. Relative expression of FK506-binding proteins (FKBPs) and RyR was normalized by the geometric average of relative quantities for reference genes. Serial dilutions of pooled cDNA were used in each experiment to assess the efficiency of the polymerase chain reaction.
Immunoblots
Immunoblots were prepared from cell lysates with anti-FKBP12/12.6 (1:1000, Santa-Cruz Biotechnology Inc, Santa Cruz, Calif), anti-RyR (1:3000, Affinity BioReagents, Golden, Colo), and anti-actin (1:20 000, Sigma) antibodies. FKBP12/12.6-RyR interaction was assessed on SR fractions with anti-FKBP12/12.6 (1:200) and anti-RyR (1:1000) antibodies from Affinity BioReagents. Immunoblots of RyR phosphorylation were performed with anti–RyR-PS2809 (1:5000) antibody from Badrilla (Leeds, West Yorkshire, United Kingdom) and anti–RyR-PS2815 (1:5000) antibody generously provided by Dr A.R. Marks (Columbia University, New York, NY).
Statistical Analysis
Preliminary descriptive analyses include frequencies for categorical variables and mean±SD for continuous variables. A conditional hierarchical linear model was used (SAS/UNIX statistical software, SAS Institute, Cary, NC; proc mixed) to compare continuous variables between groups to take into account the multiple observations per animal. The group was a fixed effect, and animals were a random effect nested in the group; in the case of repeated measurements on cells, we add a random effect for cell in animal.
Data are presented as mean±SEM and were compared with Student’s t test (for 2 groups) or ANOVA (for >2 groups, followed by a post hoc pairwise Tukey’s honestly significant difference test). Significance was defined at P<0.05.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
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Results |
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Aldosterone Pathway Increases Occurrence of DADs
During AP recordings, at 0.1-Hz cycle length in ventricular myocytes isolated from transgenic mice expressing hMR in the heart (hMR mice; Figure 1A), we observed oscillations in membrane potential after completion of the driven AP (Figure 1A, middle). Eventually, the DAD was large enough to reach the threshold to trigger spontaneous AP (Figure 1A, right). The occurrence of DADs was greatly enhanced by activation of the aldosterone pathway (Figure 1B). Ex vivo, after 48 hours’ exposure of isolated rat ventricular myocytes to aldosterone 100 nmol/L, the occurrence of DADs increased compared with control myocytes kept 48 hours in the absence of aldosterone. Likewise, in vivo, increases of DAD occurrence appeared after 3-week minipump infusion of aldosterone in WT mice or in hMR mice compared with untreated WT littermates.
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Modulation of Ca2+ Sparks by Aldosterone Pathway
DADs are commonly initiated by nonelectrically driven, spontaneous Ca2+ release from the SR via the RyRs.8 We thus examined the properties of RyRs in situ by visualizing spontaneous Ca2+ sparks, which reflect brief and local Ca2+ release events that occur when RyRs open.18,19 Figure 2A shows line-scan images of cells isolated from WT, aldosterone-treated, and hMR mice. Either elevated circulating aldosterone or cardiac hMR expression resulted in an 
1.6-fold increase of Ca2+ spark frequency (supplemental Table II; Figure 2B). No differences in mean values for Ca2+ spark amplitude (Figure 2C) or rise time (Figure 2D) were seen among the 3 groups. As seen in Figure 2A, in addition to classic Ca2+ sparks characterized by a brief, localized increase in the fluorescence signal (a half-time of decay of 30 ms and a diameter of 2 µm),18 we also observed the appearance of widened (>4 µm) and long-lasting Ca2+ release events (>80 ms). A mixed-effects model revealed increases of both averaged Ca2+ spark full widths and full durations at half-maximal amplitude in aldosterone-treated and hMR mice (supplemental Table II), which might reflect differences in population proportions. The distributions of full width and full duration at half maximum were empirically fitted to bimodal gaussian functions (Figure 2E and 2F). The distributions of full width at half maximum showed a prominent mode near 2.8 µm and a second minor mode near 4.3 µm, regardless of the conditions; however, the proportion of events that showed a larger width was greater in aldosterone-treated and hMR mice than in WT mice (Table). Moreover, events that lasted >50 ms were more frequent in aldosterone-treated and hMR mice than in WT mice. Distributions of log(full duration at half maximum) were fitted by 2 gaussian distributions with 2 distinct peaks at 32 and 87 ms, but the frequencies of long-lasting signals were greater in aldosterone-treated and hMR mice than in WT mice (Table). No correlation between Ca2+ spark spatiotemporal characteristics was observed. This is consistent with the occurrence of 3 different populations of Ca2+ sparks, the redistribution of which to long-lasting and more widespread populations was increased by activation of the cardiac aldosterone pathway.
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), Proportion (p, in %), and Coefficient of Determination (r2) of Gaussian Fits to Ca2+ Spark Parameters
Although the effects of aldosterone in vivo are presumably due to its direct cardiomyocyte effect, aldosterone might cause release of second messengers from noncardiac cells that can affect Ca2+ sparks. To exclude this possibility, we examined the properties of spontaneous Ca2+ sparks of isolated rat ventricular myocytes kept 48 hours with or without aldosterone 100 nmol/L. Three different populations of Ca2+ sparks were observed (Figure 3): The normal Ca2+ sparks were narrow and brief (Figure 3A); the second population was substantially wider in space but only slightly longer in duration (Figure 3B); and the third population had the same width as the normal population but was much longer in duration (Figure 3C). Analysis of the distributions of full width at half maximum (Figure 3D) and full duration at half maximum (Figure 3E) showed that ex vivo aldosterone exposure increased the occurrence of both wider and longer populations (Table; supplemental Table II). In addition, we observed a 1.9-fold increase in Ca2+ spark frequency (supplemental Table II; Figure 3F), whereas mean Ca2+ spark amplitude (Figure 3G) and rise time (Figure 3H) were not affected. Coincubation with a specific MR antagonist, RU28318,11–13 prevented aldosterone-induced changes in Ca2+ spark properties (Figure 3).
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Aldosterone Effects on Ca2+ Sparks Might Arise From Modulation of RyR Intrinsic Properties
The open probability of RyRs is influenced by the amount of Ca2+ stored in the SR.20 A potential increase of SR Ca2+ load by aldosterone could therefore underlie the observed effects on Ca2+ sparks. SR Ca2+ load was estimated by rapid caffeine application (10 mmol/L) in the same intact cells used for Ca2+ sparks.11,21 No difference (P>0.05) was observed in hMR mice (peak ratio of fluorescence intensity [F/F0] 4.0±0.1, n=56) or aldosterone-treated mice (3.8±0.2, n=43) compared with WT mice (4.1±0.1, n=29). In addition, no difference in caffeine-evoked Ca2+ transients was noted after ex vivo aldosterone exposure (3.7±0.2 and 4.0±0.2 in control [n=13] and aldosterone-treated [n=13] myocytes, respectively; P>0.05). These results are consistent with the absence of alteration in Ca2+ spark amplitude and suggest that aldosterone does not modulate RyR activity (and hence, Ca2+ sparks) by altering SR content.
Ca2+ sparks result from the opening of clusters of RyRs; thus, increased Ca2+ spark frequency might result from increased RyR expression. Immunoblot analysis of cell lysates showed similar RyR amounts; after normalization against control, ratios of RyR to actin levels were 1±0.3 (n=8), 0.95±0.3 (n=4), and 0.82±0.3 (n=4) in WT, WT plus aldosterone, and hMR mice, respectively (P>0.05) and 1±0.2 (n=8) and 0.94±0.2 (n=4) in rat ventricular myocytes kept 48 hours without and with aldosterone, respectively (P>0.05). RyR expression of isolated rat cardiomyocytes kept 48 hours with or without aldosterone, examined by [3H]ryanodine binding to SR fractions, indicated unchanged high-affinity binding site for ryanodine (Kd values in nmol/L: 1.5±0.2 and 1.8±0.4 after 48 hours’ incubation with [n=6] and without aldosterone [n=6], respectively; P>0.05) and similar expression levels of RyR after aldosterone exposure (maximal receptor density 0.24±0.04 and 0.29±0.03 pmol/mg protein, respectively; P>0.05).
These results show that activation of the aldosterone pathway in cardiac myocytes altered Ca2+ spark properties without modifying SR Ca2+ content or RyR expression. Therefore, aldosterone modulation of Ca2+ sparks might arise from changes in the intrinsic activity of the RyR complex. As an index, we fused SR fractions from control and aldosterone-treated rat cardiomyocytes into planar lipid bilayers to record single-channel activity of RyR. In addition to classic bimodal gating,20 characterized by sequences of flickering openings with low (Figure 4A) and high (Figure 4B) open probability, clearly distinct, long, relatively stable openings with similar unitary current amplitude were observed (Figure 4C). Measurements from holding potentials from –35 to 35 mV showed that current-voltage relationships had similar conductances of 639, 632, and 640 pS (Figures 4A, 4B, and 4C, respectively). These results suggest that all recorded channel activity corresponded to RyRs; however, although the burst and long-lasting modes were observed in channels with and without aldosterone treatment, these modes occurred more frequently after aldosterone exposure (Figure 4D).
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FKBP12/12.6 Downregulation by Mineralocorticoid Pathway
Taken together, the present results indicate that aldosterone through MR increases the likelihood that RyRs will open abnormally during diastole, which might reflect changes in intrinsic properties of the RyR. Activity of RyRs is tightly regulated by several accessory proteins that form a macromolecular signaling complex with RyR.15,16 Among these ancillary proteins, FKBP12 and FKBP12.6 have been involved in the incidence of subsets of cardiac Ca2+ sparks with longer duration and wider spatial spread, along with increased frequency.21–23 Therefore, we assessed the expression of FKBP12/12.6 protein levels. Compared with the respective controls, immunoblot analysis of cell lysates showed decreases in the amount of FKBP12/12.6 relative to actin levels after in vivo or ex vivo aldosterone exposure and in hMR mice (Figure 5A). The RyR-FKBP12/12.6 association was assessed indirectly by measurement of the ratio of FKBP12/12.6 to RyR detected in the SR fractions.24–26 Immunoblot analysis indicated that the relative amount of FKBP12/12.6 to RyR was decreased in the hearts of hMR mice or WT mice treated with aldosterone compared with control WT (Figure 5B). Because RyR phosphorylation could influence the FKBP12/12.6-RyR association,15,16 we examined RyR phosphorylation status after activation of the mineralocorticoid pathway. RyR contains multiple phosphorylation sites, including RyR-Ser2815 (phosphorylated by CaMKII)27 and RyR-Ser2809 (phosphorylated by both CaMKII and PKA).28 Using 2 different phosphospecific antibodies (RyR-PS2815 and RyR-PS2809) on SR fractions, we observed no difference in the ratio of phosphorylated RyR to total RyR among groups (Figure 5C). To distinguish FKBP12 versus FKBP12.6 expression, real-time quantitative polymerase chain reaction was used to assess mRNA levels with specific primers. At the mRNA level, decreased expression of both FKBP12 and FKBP12.6 was observed in the 3 models studied compared with respective controls (Figure 5D), whereas mRNA levels of RyR were not altered (data not shown).
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Discussion |
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Long-term aldosterone exposure ex vivo or in vivo, or cardiac hMR expression in transgenic mice, increases the occurrence of DADs, in line with abnormal diastolic openings of RyR, which are associated with downregulation of FKBP12 and FKBP12.6. Although the link might be circumstantial, several lines of evidence suggest that overstimulation of the cardiac mineralocorticoid pathway may be a major upstream factor for aberrant Ca2+ release during diastole and may contribute to cardiac arrhythmia in HF.
Numerous experimental and clinical studies indicate that the aldosterone pathway participates in cardiac alterations associated with hypertension, HF, diabetes mellitus, and other pathologies.1,3–6,12,14 Notably, inappropriate activation of cardiac MR is a likely participant in the development of poor outcomes for patients with HF, especially those associated with cardiac arrhythmia.5,6 Interestingly, hMR mice showed high mortality and an increased occurrence of arrhythmia.14 One of the possible cellular mechanisms for those arrhythmias is triggered activity caused by EADs and DADs, both of which are commonly associated with intracellular Ca2+ mishandling. We previously reported that along with AP lengthening, EADs occur in hMR mice.14 The incidence of EADs was 1%, 11%, and 14% in ventricular myocytes isolated from WT, aldosterone-treated, and hMR littermate mice, respectively. In addition, we observed that 10% of rat ventricular myocytes incubated for 48 hours with aldosterone displayed EADs, whereas none were observed in control. Here, we found that the incidence of DAD was approximately 2-fold higher, thus constituting a substantial mechanism for aldosterone pathway–induced arrhythmias.
Although many Ca2+-handling proteins are involved in DAD-related arrhythmias, abnormal opening of the RyR during diastole is an essential component.7,8 We show here that the aldosterone pathway increased the occurrence and changed the spatiotemporal properties of spontaneous Ca2+ sparks. Similar alterations were found either ex vivo by incubation of adult cardiac myocytes with aldosterone or in vivo in mouse hearts, either after chronic delivery of aldosterone or after hMR expression. These consistent findings suggest that direct activation of cardiac MR by aldosterone modifies the Ca2+ spark phenotype, independent of other compensatory changes in vivo. Indeed, the ex vivo effects of aldosterone are prevented by a specific MR antagonist. In addition, hMR mice have increased aldosterone receptor activity, presumably due to physiological aldosterone levels, as assessed by prevention of phenotype effects with MR antagonist.14 Moreover, MR antagonists also prevent other aldosterone-induced cardiac effects.10,11,13
Although we cannot exclude that the effects might be secondary to other modulations in myocyte Ca2+ handling, several lines of evidence suggest that activation of MR by aldosterone directly affects RyR activity. The increase in Ca2+ spark frequency is not due to an increase in SR Ca2+ load but rather might reflect a modulation of the intrinsic properties of the RyR complex. In addition to the increase in Ca2+ spark frequency, we observed a redistribution of Ca2+ sparks to long-lasting and broader populations. These "abnormal" Ca2+ sparks were also seen in control conditions but at a much lower frequency. Others have also found Ca2+ sparks that are longer or wider than the classic Ca2+ sparks in control conditions.18,21,29–31 The incidence of wide (>2 µm) and long (60 to 80 ms) Ca2+ sparks is increased in left ventricular hypertrophy in the dog without alteration in Ca2+ spark amplitude.32 Here, aldosterone exposure or cardiac hMR expression increased the proportions of long and wide Ca2+ sparks from a range of 4% to 6% to a range of 9% to 19% and from a range of 4% to 6% to a range of 16% to 30%, respectively, which constitutes a substantial effect. This indicates that aldosterone does not induce a new kind of Ca2+ spark but modifies the activity of functional release units.
Along with this alteration, we observed a significant downregulation of FKBP12 and FKBP12.6 expression without variation in the density of RyRs. Even if other alterations contributed to the observed modifications of Ca2+ spark characteristics, no other RyR-associated proteins have been shown to induce similar modulations to FKBP. FKBP12/12.6 binding to RyRs modulates the Ca2+-flux properties of the channel complex; in particular, it regulates RyR open probability and stability at rest20,33,34 (but see also Xiao et al35). FKBP12.6 removal by pharmacological approaches22,36,37 or in transgenic animal models,23 and conversely, adenoviral short-term FKBP12.6 overexpression,21,38 modulated Ca2+ spark frequency and the occurrence of longer and wider Ca2+ sparks in a manner similar to that observed here. We showed that activation of the cardiac aldosterone pathway produced a decreased expression of FKBPs. In addition, we assessed the RyR-FKBP12/12.6 association in hMR transgenic and WT mice treated with aldosterone. The present results suggest that in mouse hearts, activation of the aldosterone pathway results in substantially reduced RyR-FKBP12/12.6 interaction, which could be the cause of the observed RyR-mediated Ca2+ leak. These effects are not associated with alteration of RyR phosphorylation status but might reflect a genomic regulation of FKBP12/12.6 by the cardiac aldosterone pathway, as evidenced by the decrease in mRNA levels of FKBP12 and FKBP12.6. Partial loss of FKBP12.6 from RyR in HF has been shown to cause diastolic Ca2+ leak, which may result in a greater tendency for DAD to occur and consequent triggered arrhythmias.15,16,23,39–45 In addition, myocardial FKBP12.6 overexpression prevents triggered arrhythmias in normal hearts, probably by reducing diastolic SR Ca2+ leakage.46 Beyond the controversial hypothesis that excess RyR activity due to hyperphosphorylation reduces RyR affinity for FKBP12.6,47 most of the studies showed a reduction of FKBP protein level in HF.23,39–45 Thus, we suggest that activation of the aldosterone pathway might be an essential step in the cascade of molecular events leading to FKBP deficiency that causes RyR Ca2+ leakage and triggers malignant cardiac arrhythmias in HF. Taken together, the present findings may explain in part why the use of MR antagonists in addition to optimal medical therapy is associated with improved survival and fewer sudden cardiac deaths in patients.
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Acknowledgments |
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Sources of Funding
This work was supported by Agence Nationale de la Recherche (project No. A05071FS/APV05030FSA), Institut National de la Santé et de la Recherche Médicale (INSERM), the British Heart Foundation (PG/05/077), and the European Union (FPG, Life Science Genomics and Biotechnology for Health, #CT 2005 No. 018802, CONTICA).
Disclosures
None.
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The online-only Data Supplement is available with this article at http://circ.ahajournals.org/cgi/content/full/CIRCULATIONAHA.108.805804/DC1.
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