Overexpression of FK-506 binding protein 12.0 modulates excitation contraction coupling in adult rabbit ventricular cardiomyocytes

Cellular and extracellular proteomic profiling of paradoxical low-flow low-gradient aortic stenosis myocardium

Aims

Patients with severe aortic stenosis (AS), low transvalvular flow (LF) and low gradient (LG) with normal ejection fraction (EF)-are referred to as paradoxical LF-LG AS (PLF-LG). PLF-LG patients develop more advanced heart failure symptoms and have a worse prognosis than patients with normal EF and high-gradient AS (NEF-HG). Despite its clinical relevance, the mechanisms underlying PLF-LG are still poorly understood.

Methods

Left ventricular (LV) myocardial biopsies of PLF-LG (n = 5) and NEF-HG patients (n = 6), obtained during transcatheter aortic valve implantation, were analyzed by LC-MS/MS after sequential extraction of cellular and extracellular matrix (ECM) proteins using a three-step extraction method. Proteomic data are available via ProteomeXchange with identifier PXD055391.

Results

73 cellular proteins were differentially abundant between the 2 groups. Among these, a network of proteins related to muscle contraction and arrhythmogenic cardiomyopathy (e.g., cTnI, FKBP1A and CACNA2D1) was found in PLF-LG. Extracellularly, upregulated proteins in PLF-LG were related to ATP synthesis and oxidative phosphorylation (e.g., ATP5PF, COX5B and UQCRB). Interestingly, we observed a 1.3-fold increase in cyclophilin A (CyPA), proinflammatory cytokine, in the extracellular extracts of PLF-LG AS patients (p < 0.05). Consistently, immunohistochemical analysis confirmed its extracellular localization in PLF-LG AS LV sections along with an increase in its receptor, CD147, compared to the NEF-HG AS patients. Levels of core ECM proteins, namely collagens and proteoglycans, were comparable between groups.

Conclusion

Our study pinpointed novel candidates and processes with potential relevance in the pathophysiology of PLF-LG. The role of CyPA in particular warrants further investigation.

Arrhythmia-Associated Calmodulin E105A Mutation Alters the Binding Affinity of CaM to a Ryanodine Receptor 2 CaM-Binding Pocket

Calmodulin (CaM) is a small, multifunctional calcium (Ca2+)-binding sensor that binds and regulates the open probability of cardiac ryanodine receptor 2 (RyR2) at both low and high cytosolic Ca2+ concentrations. Recent isothermal titration calorimetry (ITC) studies of a number of peptides that correspond to different regions of human RyR2 showed that two regions of human RyR2 (3584-3602aa and 4255-4271aa) bind with high affinity to CaM, suggesting that these two regions might contribute to a putative RyR2 intra-subunit CaM-binding pocket. Moreover, a previously characterized de novo long QT syndrome (LQTS)-associated missense CaM mutation (E105A) which was identified in a 6-year-old boy, who experienced an aborted first episode of cardiac arrest revealed that this mutation dysregulates normal cardiac function in zebrafish by a complex mechanism that involves alterations in both CaM-Ca2+ and CaM-RyR2 interactions. Herein, to gain further insight into how the CaM E105A mutation leads to severe cardiac arrhythmia, we generated large quantities of recombinant CaMWT and CaME105A proteins. We then performed ITC experiments to investigate and compare the interactions of CaMWT and CaME105A mutant protein with two synthetic peptides that correspond to the two aforementioned human RyR2 regions, which we have proposed to contribute to the RyR2 CaM-binding pocket. Our data reveal that the E105A mutation has a significant negative effect on the interaction of CaM with both RyR2 regions in the presence and absence of Ca2+, highlighting the potential contribution of these two human RyR2 regions to an RyR2 CaM-binding pocket, which may be essential for physiological CaM/RyR2 association and thus channel regulation.

Protein expression profiles in murine ventricles modeling catecholaminergic polymorphic ventricular tachycardia: effects of genotype and sex

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is associated with mutations in the cardiac ryanodine receptor (RyR2). These result in stress-induced ventricular arrhythmic episodes, with clinical symptoms and prognosis reported more severe in male than female patients. Murine homozygotic RyR2-P2328S (RyR2^S/S ) hearts replicate the proarrhythmic CPVT phenotype of abnormal sarcoplasmic reticular Ca²⁺ leak and disrupted Ca²⁺ homeostasis. In addition, RyR2^S/S hearts show decreased myocardial action potential conduction velocities (CV), all features implicated in arrhythmic trigger and substrate. The present studies explored for independent and interacting effects of RyR2^S/S genotype and sex on expression levels of molecular determinants of Ca²⁺ homeostasis (CASQ2, FKBP12, SERCA2a, NCX1, and Ca_V 1.2) and CV (Na_V 1.5, Connexin (Cx)-43, phosphorylated-Cx43, and TGF-β1) in mice. Expression levels of Ca²⁺ homeostasis proteins were not altered, hence implicating abnormal RyR2 function alone in disrupted cytosolic Ca²⁺ homeostasis. Furthermore, altered Na_V 1.5, phosphorylated Cx43, and TGF-β1 expression were not implicated in the development of slowed CV. By contrast, decreased Cx43 expression correlated with slowed CV, in female, but not male, RyR2^S/S mice. The CV changes may reflect acute actions of the increased cytosolic Ca²⁺ on Na_V 1.5 and Cx43 function.

Unbalance Between Sarcoplasmic Reticulum Ca2 + Uptake and Release: A First Step Toward Ca2 + Triggered Arrhythmias and Cardiac Damage

The present review focusses on the regulation and interplay of cardiac SR Ca²⁺ handling proteins involved in SR Ca²⁺ uptake and release, i.e., SERCa2/PLN and RyR2. Both RyR2 and SERCA2a/PLN are highly regulated by post-translational modifications and/or different partners' proteins. These control mechanisms guarantee a precise equilibrium between SR Ca²⁺ reuptake and release. The review then discusses how disruption of this balance alters SR Ca²⁺ handling and may constitute a first step toward cardiac damage and malignant arrhythmias. In the last part of the review, this concept is exemplified in different cardiac diseases, like prediabetic and diabetic cardiomyopathy, digitalis intoxication and ischemia-reperfusion injury.

Molecular basis for allosteric regulation of the type 2 ryanodine receptor channel gating by key modulators

As a switch for the release of Ca²⁺ from the sarco(endo)plasmic reticulum of cardiomyocytes, the type 2 ryanodine receptor (RyR2) is subject to sophisticated regulation by a broad spectrum of modulators. Dysregulation of RyR2-mediated Ca²⁺ release is linked to life-threatening cardiac arrhythmias. The regulatory mechanism of RyR2 by key modulators, such as Ca²⁺, FKBP12.6, ATP, and caffeine, remains unclear. This study provides important insights into the long-range allosteric regulation of RyR2 channel gating by these modulators and serves as an important framework for mechanistic understanding of the regulation of this key player in the excitation–contraction coupling of cardiac muscles.

The type 2 ryanodine receptor (RyR2) is responsible for releasing Ca²⁺ from the sarcoplasmic reticulum of cardiomyocytes, subsequently leading to muscle contraction. Here, we report 4 cryo-electron microscopy (cryo-EM) structures of porcine RyR2 bound to distinct modulators that, together with our published structures, provide mechanistic insight into RyR2 regulation. Ca²⁺ alone induces a contraction of the central domain that facilitates the dilation of the S6 bundle but is insufficient to open the pore. The small-molecule agonist PCB95 helps Ca²⁺ to overcome the barrier for opening. FKBP12.6 induces a relaxation of the central domain that decouples it from the S6 bundle, stabilizing RyR2 in a closed state even in the presence of Ca²⁺ and PCB95. Although the channel is open when PCB95 is replaced by caffeine and adenosine 5′-triphosphate (ATP), neither of the modulators alone can sufficiently counter the antagonistic effect to open the channel. Our study marks an important step toward mechanistic understanding of the sophisticated regulation of this key channel whose aberrant activity engenders life-threatening cardiac disorders.

Atrial fibrillation and electrophysiology in transgenic mice with cardiac-restricted overexpression of FKBP12

Cardiomyocyte-restricted overexpression of FK506-binding protein 12 transgenic (αMyHC-FKBP12) mice develop spontaneous atrial fibrillation (AF). The aim of the present study is to explore the mechanisms underlying the occurrence of AF in αMyHC-FKBP12 mice. Spontaneous AF was documented by telemetry in vivo and Langendorff-perfused hearts of αMyHC-FKBP12 and littermate control mice in vitro. Atrial conduction velocity was evaluated by optical mapping. The patch-clamp technique was applied to determine the potentially altered electrophysiology in atrial myocytes. Channel protein expression levels were evaluated by Western blot analyses. Spontaneous AF was recorded in four of seven αMyHC-FKBP12 mice but in none of eight nontransgenic (NTG) controls. Atrial conduction velocity was significantly reduced in αMyHC-FKBP12 hearts compared with NTG hearts. Interestingly, the mean action potential duration at 50% but not 90% was significantly prolonged in αMyHC-FKBP12 atrial myocytes compared with their NTG counterparts. Consistent with decreased conduction velocity, average peak Na+ current ( INa) density was dramatically reduced and the INa inactivation curve was shifted by approximately +7 mV in αMyHC-FKBP12 atrial myocytes, whereas the activation and recovery curves were unaltered. The Nav1.5 expression level was significantly reduced in αMyHC-FKBP12 atria. Furthermore, we found increases in atrial Cav1.2 protein levels and peak L-type Ca2+ current density and increased levels of fibrosis in αMyHC-FKBP12 atria. In summary, cardiomyocyte-restricted overexpression of FKBP12 reduces the atrial Nav1.5 expression level and mean peak INa, which is associated with increased peak L-type Ca2+ current and interstitial fibrosis in atria. The combined electrophysiological and structural changes facilitated the development of local conduction block and altered action potential duration and spontaneous AF. NEW & NOTEWORTHY This study addresses a long-standing riddle regarding the role of FK506-binding protein 12 in cardiac physiology. The work provides further evidence that FK506-binding protein 12 is a critical component for regulating voltage-gated sodium current and in so doing has an important role in arrhythmogenic physiology, such as atrial fibrillation.

Sensitized signalling between L-type Ca2+ channels and ryanodine receptors in the absence or inhibition of FKBP12.6 in cardiomyocytes

Aims

The heart contraction is controlled by the Ca2+-induced Ca2+ release (CICR) between L-type Ca2+ channels and ryanodine receptors (RyRs). The FK506-binding protein FKBP12.6 binds to RyR subunits, but its role in stabilizing RyR function has been debated for long. Recent reports of high-resolution RyR structure show that the HD2 domain that binds to the SPRY2 domain of neighbouring subunit in FKBP-bound RyR1 is detached and invisible in FKBP-null RyR2. The present study was to test the consequence of FKBP12.6 absence on the in situ activation of RyR2.

Methods and results

Using whole-cell patch-clamp combined with confocal imaging, we applied a near threshold depolarization to activate a very small fraction of LCCs, which in turn activated RyR Ca2+ sparks stochastically. FKBP12.6-knockout and FK506/rapamycin treatments increased spark frequency and LCC-RyR coupling fidelity without altering LCC open probability. Neither FK506 nor rapamycin further altered LCC-RyR coupling fidelity in FKBP12.6-knockout cells. In loose-seal patch-clamp experiments, the LCC-RyR signalling kinetics, indexed by the delay for a LCC sparklet to trigger a RyR spark, was accelerated after FKBP12.6 knockout and FK506/rapamycin treatments. These results demonstrated that RyRs became more sensitive to Ca2+ triggers without FKBP12.6. Isoproterenol (1 μM) further accelerated the LCC-RyR signalling in FKBP12.6-knockout cells. The synergistic sensitization of RyRs by catecholaminergic signalling and FKBP12.6 dysfunction destabilized the CICR system, leading to chaotic Ca2+ waves and ventricular arrhythmias.

Conclusion

FKBP12.6 keeps the RyRs from over-sensitization, stabilizes the potentially regenerative CICR system, and thus may suppress the life-threatening arrhythmogenesis.

FK506-binding proteins 12 and 12.6 (FKBPs) as regulators of cardiac Ryanodine Receptors: Insights from new functional and structural knowledge

Ryanodine Receptors (RyRs) are intracellular Ca²⁺ channels that mediate Ca²⁺ flux from the sarco(endo)plasmic reticulum in many cell types. The interaction of RyRs with FK506-binding proteins (FKBPs) has been proposed as an important regulatory mechanism, where the loss of this interaction leads to channel dysfunction. In the heart, phosphorylation of RyR has been suggested to disrupt the RyR-FKBP interaction promoting altered Ca²⁺ signaling, heart failure and arrhythmias. However, the functional result of FKBP interaction with RyR and how this interaction is regulated remains highly controversial. Recently, high resolution structures of RyR have provided novel aspects to the ongoing debate. This review will discuss the most recent functional data in light of these new structures.

Semi-automated program for analysis of local Ca2+ spark release with application for classification of heart cell type

Local Ca²⁺ spark releases are essential to the Ca²⁺ cycling process. Thus, they play an important role in ventricular and atrial cell contraction, as well as in sinoatrial cell automaticity. Characterizing their properties in healthy cells from different regions in the heart can reveal the basic biophysical differences among these regions. We designed a semi-automatic Matlab Graphical User Interface (called Sparkalyzer) to characterize parameters of Ca²⁺ spark release from any major cardiac tissue, as recorded in line-scan mode with a confocal laser-scanning microscope. We validated the algorithm on experimental images from rabbit sinoatrial, atrial, and ventricular cells loaded with Fluo-4 AM. The program characterizes general image parameters of Ca²⁺ transients and sparks: spark duration, which indicates for how long the spark provides Ca²⁺ to the closed intracellular mechanisms (typical value: 25±1, 23±1, 26±1ms for sinoatrial, atrial, and ventricular cells, respectively); spark amplitude, which indicates the amount of Ca²⁺ released by a single spark (1.6±0.1, 1.6±0.2, 1.4±0.1F/F₀ for sinoatrial, atrial, and ventricular cells, respectively); spark length, which is the length of the Ca²⁺ wavelets fired out of a row of ryanodine receptors (5±0.1, 5±0.2, 3.4±0.3μm for sinoatrial, atrial, or ventricular cells, respectively) and number of sparks (0.14±0.02, 0.025±0.01, 0.02±0.01 for 1μm in 1s for sinoatrial, atrial, and ventricular cells, respectively). This method is reliable for Ca²⁺ spark analysis of sinoatrial, atrial, or ventricular cells. Moreover, by examining the average value of Ca²⁺ spark characteristics and their scattering around the mean, atrial, ventricular and sinoatrial cells can be differentiated.

The role of FK506-binding proteins 12 and 12.6 in regulating cardiac function

Specifically, FK506-binding proteins 12 (FKBP12) and 12.6 (FKBP12.6) are cis-trans peptidyl prolyl isomerases that are expressed in the heart. Both FKBP12 and FKBP12.6 were previously known to interact with ryanodine receptors in striated muscles. Although FKBP12 is abundantly present in the heart, its function in the heart is largely uncertain. Recently, by generating FKBP12 transgenic overexpression and cardiac-restricted knockout mice, we showed that FKBP12 is critically important in regulating trans-sarcolemmal ionic currents, predominately the voltage-gated Na+ current, I(Na), but it appears to be less important for regulating cardiac ryanodine receptor function. Similar genetic approaches also confirm the role of FKBP12.6 in regulating cardiac ryanodine receptors. The current study demonstrated that FKBP12 and FKBP12.6 have very different physiologic functions in the heart.

The effect of K201 on isolated working rabbit heart mechanical function during pharmacologically induced Ca2+ overload

BACKGROUND AND PURPOSE

Reduced cardiac contractility has been associated with disrupted myocardial Ca(2+) signalling. The 1,4 benzothiazepine K201 (JTV-519) acts on several Ca(2+) handling proteins and improves cardiac contractility in vivo in a variety of animal models of myocardial dysfunction. However, it is unclear whether this improvement depends on the systemic effects of K201 or if K201 reverses the effects of Ca(2+) dysregulation, regardless of the cause.

EXPERIMENTAL APPROACH

The effect of K201 on cardiac mechanical function was assessed in isolated working hearts from adult rabbits, using a ventricular pressure-volume catheter. In separate experiments, the effect of K201 was investigated in hearts following pharmacologically induced Ca(2+) overload using elevated extracellular [Ca(2+) ] ([Ca(2+) ](o) ) and β-adrenoceptor stimulation.

KEY RESULTS

K201 induced a concentration-dependent decline in systolic function (peak pressure, dP/dt(max) and preload recruitable stroke work), lusitropy (reduced dP/dt(min) and increased end diastolic pressure) and stroke volume, independent of decreased heart rate. In separate experiments, mechanical function in hearts exposed to 4.5 mmol·L(-1) [Ca(2+) ](o) and 150 nmol·L(-1) isoprenaline declined until cessation of aortic flow (in 6 out of 11 hearts). However, all hearts perfused with the addition of 1 µmol·L(-1) K201 maintained aortic flow and demonstrated significantly improved peak systolic pressures, dP/dt(max) and dP/dt(min) .

CONCLUSIONS AND IMPLICATIONS

K201 significantly improved mechanical function of the heart during Ca(2+) overload. This suggests that K201 can limit the detrimental effects of elevated intracellular Ca(2+) and exert beneficial effects on cardiac contractile function, independent of systemic effects previously observed in vivo.

Isolated rabbit working heart function during progressive inhibition of myocardial SERCA activity

RATIONALE

The extent to which sarcoplasmic reticulum Ca(2+)ATPase (SERCA) activity alone determines left ventricular (LV) pump function is unknown.

OBJECTIVE

To correlate SERCA activity with hemodynamic function of rabbit LV during thapsigargin perfusion.

METHODS AND RESULTS

Isolated rabbit hearts were perfused in working heart configuration, and LV pump function was assessed using a pressure-volume catheter. Rapid and complete (>95%) inhibition of SERCA was associated with a moderate decrease in cardiac function (to 70%-85% of control). Further decrease in cardiac function to 50%-75% of control occurred over the next ≈ 30 minutes despite no detectable further inhibition of SERCA activity. Analysis of the 20 seconds prior to pump failure revealed a rapid decrease in end diastolic volume. Intermediate levels of SERCA function (≈ 50% of control) had only minor hemodynamic effects. Parallel experiments in field-stimulated isolated ventricular cardiomyocytes monitored intracellular Ca(2+) and cell shortening. On perfusion with thapsigargin, Ca(2+) transient amplitude and cell shortening fell to ≈ 70% of control followed by increased diastolic Ca(2+) concentration and diastolic cell shortening to achieve a new steady state.

CONCLUSIONS

The relationship between SERCA activity and LV function in the rabbit is highly nonlinear. In the short term, only moderate effects on LV pump function were observed despite almost complete (>95%) reduction in SERCA activity. The terminal decline of function was associated with sudden sustained increase in diastolic tone comparable to the sustained contraction observed in isolated cardiomyocytes. Secondary increases of intracellular Ca(2+) and Na(+) following complete SERCA inhibition eventually limit contractile function and precipitate LV pump failure.

FKBP12 activates the cardiac ryanodine receptor Ca2+-release channel and is antagonised by FKBP12.6

Changes in FKBP12.6 binding to cardiac ryanodine receptors (RyR2) are implicated in mediating disturbances in Ca(2+)-homeostasis in heart failure but there is controversy over the functional effects of FKBP12.6 on RyR2 channel gating. We have therefore investigated the effects of FKBP12.6 and another structurally similar molecule, FKBP12, which is far more abundant in heart, on the gating of single sheep RyR2 channels incorporated into planar phospholipid bilayers and on spontaneous waves of Ca(2+)-induced Ca(2+)-release in rat isolated permeabilised cardiac cells. We demonstrate that FKBP12 is a high affinity activator of RyR2, sensitising the channel to cytosolic Ca(2+), whereas FKBP12.6 has very low efficacy, but can antagonise the effects of FKBP12. Mathematical modelling of the data shows the importance of the relative concentrations of FKBP12 and FKBP12.6 in determining RyR2 activity. Consistent with the single-channel results, physiological concentrations of FKBP12 (3 µM) increased Ca(2+)-wave frequency and decreased the SR Ca(2+)-content in cardiac cells. FKBP12.6, itself, had no effect on wave frequency but antagonised the effects of FKBP12.We provide a biophysical analysis of the mechanisms by which FK-binding proteins can regulate RyR2 single-channel gating. Our data indicate that FKBP12, in addition to FKBP12.6, may be important in regulating RyR2 function in the heart. In heart failure, it is possible that an alteration in the dual regulation of RyR2 by FKBP12 and FKBP12.6 may occur. This could contribute towards a higher RyR2 open probability, 'leaky' RyR2 channels and Ca(2+)-dependent arrhythmias.

Disparities in the association of the ryanodine receptor and the FK506-binding proteins in mammalian heart

The FK506-binding proteins (FKBP12 and FKBP12.6; also known as FKBP1A and FKBP1B, respectively) are accessory subunits of the ryanodine receptor (RyR) Ca(2+) release channel. Aberrant RyR2-FKBP12.6 interactions have been proposed to be the underlying cause of channel dysfunction in acquired and inherited cardiac disease. However, the stoichiometry of the RyR2 association with FKBP12 or FKBP12.6 in mammalian heart is currently unknown. Here, we describe detailed quantitative analysis of cardiac stoichiometry between RyR2 and FKBP12 or FKBP12.6 using immunoblotting and [(3)H]ryanodine-binding assays, revealing striking disparities between four mammalian species. In mouse and pig heart, RyR2 is found complexed with both FKBP12 and FKBP12.6, although the former is the most abundant isoform. In rat heart, RyR2 is predominantly associated with FKBP12.6, whereas in rabbit it is associated with FKBP12 only. Co-immunoprecipitation experiments demonstrate RyR2-specific interaction with both FKBP isoforms in native cardiac tissue. Assuming four FKBP-binding sites per RyR2 tetramer, only a small proportion of available sites are occupied by endogenous FKBP12.6. FKBP interactions with RyR2 are very strong and resistant to drug (FK506, rapamycin and cyclic ADPribose) and redox (H(2)O(2) and diamide) treatment. By contrast, the RyR1-FKBP12 association in skeletal muscle is readily disrupted under oxidative conditions. This is the first study to directly assess association of endogenous FKBP12 and FKBP12.6 with RyR2 in native cardiac tissue. Our results challenge the widespread perception that RyR2 associates exclusively with FKBP12.6 to near saturation, with important implications for the role of the FK506-binding proteins in RyR2 pathophysiology and cardiac disease.

A negative screen for mutations in calstabin 1 and 2 genes in patients with dilated cardiomyopathy

Background

Calstabins 1 and 2 bind to Ryanodine receptors regulating muscle excitation-contraction coupling. Mutations in Ryanodine receptors affecting their interaction with calstabins lead to different cardiac pathologies. Animal studies suggest the involvement of calstabins with dilated cardiomyopathy.

Results

We tested the hypothesis that calstabins mutations may cause dilated cardiomyopathy in humans screening 186 patients with idiopathic dilated cardiomyopathy for genetic alterations in calstabins 1 and 2 genes (FKBP12 and FKBP12.6). No missense variant was found. Five no-coding variations were found but not related to the disease.

Conclusions

These data corroborate other studies suggesting that mutations in FKBP12 and FKBP12.6 genes are not commonly related to cardiac diseases.

K201 (JTV-519) alters the spatiotemporal properties of diastolic Ca(2+) release and the associated diastolic contraction during β-adrenergic stimulation in rat ventricular cardiomyocytes

K201 has previously been shown to reduce diastolic contractions in vivo during β-adrenergic stimulation and elevated extracellular calcium concentration ([Ca²⁺]_o). The present study characterised the effect of K201 on electrically stimulated and spontaneous diastolic sarcoplasmic reticulum (SR)-mediated Ca²⁺ release and contractile events in isolated rat cardiomyocytes during β-adrenergic stimulation and elevated [Ca²⁺]_o. Parallel experiments using confocal microscopy examined spontaneous diastolic Ca²⁺ release events at an enhanced spatiotemporal resolution. 1.0 μmol/L K201 in the presence of 150 nmol/L isoproterenol (ISO) and 4.75 mmol/L [Ca²⁺]_o significantly decreased the amplitude of diastolic contractions to ~16% of control levels. The stimulated free Ca²⁺ transient amplitude was significantly reduced, but stimulated cell shortening was not significantly altered. When intracellular buffering was taken into account, K201 led to an increase in action potential-induced SR Ca²⁺ release. Myofilament sensitivity to Ca²⁺ was not changed by K201. Confocal microscopy revealed diastolic events composed of multiple Ca²⁺ waves (2–3) originating at various points along the cardiomyocyte length during each diastolic period. 1.0 μmol/L K201 significantly reduced the (a) frequency of diastolic events and (b) initiation points/diastolic interval in the remaining diastolic events to 61% and 71% of control levels respectively. 1.0 μmol/L K201 can reduce the probability of spontaneous diastolic Ca²⁺ release and their associated contractions which may limit the propensity for the contractile dysfunction observed in vivo.

Sex differences in mechanisms of cardiac excitation-contraction coupling in rat ventricular myocytes

Components of excitation-contraction (E-C) coupling were compared in ventricular myocytes isolated from 3-mo-old male and female rats. Ca2+ concentrations (fura-2) and cell shortening (edge detector) were measured simultaneously (37°C). Membrane potential and ionic currents were measured with microelectrodes. Action potentials were similar in male and female myocytes, but contractions were smaller and slower in females. In voltage-clamped cells, peak contractions were smaller in females than in males (5.1 ± 0.7% vs. 7.7 ± 0.8% diastolic length, P < 0.05). Similarly, Ca2+ transients were smaller in females than in males and the rate of rise of the Ca2+ transient was slower in females. Despite smaller contractions and Ca2+ transients in females, Ca2+ current density was similar in both groups. Sarcoplasmic reticulum Ca2+ content, assessed with caffeine, did not differ between the sexes. However, E-C coupling gain (rate of Ca2+ release/Ca2+ current) was smaller in females than in males (157.0 ± 15.6 vs. 338.4 ± 54.3 (nM/s)/(pA/pF), P < 0.05). To determine whether the reduced gain in female cells was due to changes in unitary Ca2+ release, spontaneous Ca2+ sparks were evaluated (fluo-4, 37°C). Spark frequencies and widths were similar in both groups, but spark amplitudes were smaller in females than in males (0.56 ± 0.01 vs. 0.64 ± 0.01 ΔF/F0, P < 0.05). Spark durations also were shorter in females than in males (full duration at half-maximum = 14.86 ± 0.17 vs. 16.25 ± 0.27 ms, P < 0.05). These observations suggest that decreases in the size and duration of Ca2+ sparks contributes to the decrease in E-C coupling gain in female myocytes. Thus, differences in cardiac contractile function arise, in part, from differences in unitary Ca2+ release between the sexes.

Mapping the ryanodine receptor FK506-binding protein subunit using fluorescence resonance energy transfer

The 12-kDa FK506-binding proteins (FKBP12 and FKBP12.6) are regulatory subunits of ryanodine receptor (RyR) Ca(2+) release channels. To investigate the structural basis of FKBP interactions with the RyR1 and RyR2 isoforms, we used site-directed fluorescent labeling of FKBP12.6, ligand binding measurements, and fluorescence resonance energy transfer (FRET). Single-cysteine substitutions were introduced at five positions distributed over the surface of FKBP12.6. Fluorescent labeling at position 14, 32, 49, or 85 did not affect high affinity binding to the RyR1. By comparison, fluorescent labeling at position 41 reduced the affinity of FKBP12.6 binding by 10-fold. Each of the five fluorescent FKBPs retained the ability to inhibit [(3)H]ryanodine binding to the RyR1, although the maximal extent of inhibition was reduced by half when the label was attached at position 32. The orientation of FKBP12.6 bound to the RyR1 and RyR2 was examined by measuring FRET from the different labeling positions on FKBP12.6 to an acceptor attached within the RyR calmodulin subunit. FRET was dependent on the position of fluorophore attachment on FKBP12.6; however, for any given position, the distance separating donors and acceptors bound to RyR1 versus RyR2 did not differ significantly. Our results show that FKBP12.6 binds to RyR1 and RyR2 in the same orientation and suggest new insights into the discrete structural domains responsible for channel binding and inhibition. FRET mapping of RyR-bound FKBP12.6 is consistent with the predictions of a previous cryoelectron microscopy study and strongly supports the proposed structural model.

Kinetics of FKBP12.6 binding to ryanodine receptors in permeabilized cardiac myocytes and effects on Ca sparks

RATIONALE

FK506-binding proteins FKBP12.6 and FKBP12 are associated with cardiac ryanodine receptors (RyR2), and cAMP-dependent protein kinase A (PKA)-dependent phosphorylation of RyR2 was proposed to interrupt FKBP12.6-RyR2 association and activate RyR2. However, the function of FKBP12.6/12 and role of PKA phosphorylation in cardiac myocytes are controversial.

OBJECTIVE

To directly measure in situ binding of FKBP12.6/12 to RyR2 in ventricular myocytes, with simultaneous Ca sparks measurements as a RyR2 functional index.

METHODS AND RESULTS

We used permeabilized rat and mouse ventricular myocytes, and fluorescently-labeled FKBP12.6/12. Both FKBP12.6 and FKBP12 concentrate at Z-lines, consistent with RyR2 and Ca spark initiation sites. However, only FKBP12.6 inhibits resting RyR2 activity. Assessment of fluorescent FKBP binding in myocyte revealed a high FKBP12.6-RyR2 affinity (K(d)=0.7+/-0.1 nmol/L) and much lower FKBP12-RyR2 affinity (K(d)=206+/-70 nmol/L). Fluorescence recovery after photobleach confirmed this K(d) difference and showed that it is mediated by k(off). RyR2 phosphorylation by PKA did not alter binding kinetics or affinity of FKBP12.6/12 for RyR2. Using quantitative immunoblots, we determined endogenous [FKBP12] in intact myocytes is approximately 1 micromol/L (similar to [RyR]), whereas [FKBP12.6] is <or=150 nmol/L.

CONCLUSIONS

Only 10% to 20% of endogenous myocyte RyR2s have FKBP12.6 associated, but virtually all myocyte FKBP12.6 is RyR2-bound (because of very high affinity). FKBP12.6 but not FKBP12 inhibits basal RyR2 activity. PKA-dependent RyR2 phosphorylation has no significant effect on binding of either FKBP12 or 12.6 to RyR2 in myocytes.

Age-associated changes in excitation-contraction coupling are more prominent in ventricular myocytes from male rats than in myocytes from female rats

We evaluated effects of age on components of excitation-contraction (EC) coupling in ventricular myocytes from male and female rats to examine sex differences in mechanisms responsible for age-related contractile dysfunction. Myocytes were isolated from anesthetized young adult (approximately 3 mo) and aged (approximately 24 mo) Fischer 344 rats. Ca(2+) concentrations and contractions were measured simultaneously (37 degrees C, 2 Hz). Fractional shortening declined with age in males (6.7 +/- 0.6% to 2.4 +/- 0.4%; P < 0.05), as did peak Ca(2+) transients (47.7 +/- 4.6 to 28.1 +/- 2.1 nM; P < 0.05) and Ca(2+) current densities (-7.7 +/- 0.7 to -6.2 +/- 0.5 pA/pF; P < 0.05). Although sarcoplasmic reticulum (SR) Ca(2+) content was similar regardless of age in males, EC coupling gain declined significantly with age to 55.8 +/- 7.8% of values in younger males. In contrast with results in males, contraction and Ca(2+) transient amplitudes were unaffected by age in females. Ca(2+) current density declined with age in females (-7.5 +/- 0.5 to -5.1 +/- 0.7 pA/pF; P < 0.05), but SR Ca(2+) content actually increased dramatically (49.0 +/- 7.5 to 147.3 +/- 28.5 nM; P < 0.05). Even so, EC coupling gain was not affected by age in female myocytes. Age also promoted hypertrophy of male myocytes more than female myocytes. Age and sex differences in EC coupling were largely maintained when conditioning pulse frequency was increased to 4 Hz. Contractions, Ca(2+) transients, and EC coupling gain were also smaller in young females than in young males. Thus age-dependent changes are more prominent in myocytes from males than females. Increased SR Ca(2+) content may compensate for reduced Ca(2+) current to preserve contractile function in aged females, which may limit the detrimental effects of age on cardiac contractile function.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.