CASQ2 (cardiac calsequestrin) is commonly believed to serve as the SR (sarcoplasmic reticulum) luminal Ca2+ sensor. Ablation of CASQ2 promotes SCWs (spontaneous Ca2+ waves) and CPVT (catecholaminergic polymorphic ventricular tachycardia) upon stress but not at rest. How SCWs and CPVT are triggered by stress in the absence of the CASQ2-based luminal Ca2+ sensor is an important unresolved question. In the present study, we assessed the role of the newly identified RyR2 (ryanodine receptor 2)-resident luminal Ca2+ sensor in determining SCW propensity, CPVT susceptibility and cardiac hypertrophy in Casq2-KO (knockout) mice. We crossbred Casq2-KO mice with RyR2 mutant (E4872Q+/−) mice, which lack RyR2-resident SR luminal Ca2+ sensing, to generate animals with both deficiencies. Casq2+/− and Casq2−/− mice showed stress-induced VTs (ventricular tachyarrhythmias), whereas Casq2+/−/E4872Q+/− and Casq2−/−/E4872Q+/− mice displayed little or no stress-induced VTs. Confocal Ca2+ imaging revealed that Casq2−/− hearts frequently exhibited SCWs after extracellular Ca2+ elevation or adrenergic stimulation, whereas Casq2−/−/E4872Q+/− hearts had few or no SCWs under the same conditions. Cardiac hypertrophy developed and CPVT susceptibility increased with age in Casq2−/− mice, but not in Casq2−/−/E4872Q+/− mice. However, the amplitudes and dynamics of voltage-induced Ca2+ transients in Casq2−/− and Casq2−/−/E4872Q+/− hearts were not significantly different. Our results indicate that SCWs, CPVT and hypertrophy in Casq2-null cardiac muscle are governed by the RyR2-resident luminal Ca2+ sensor. This implies that defects in CASQ2-based lumi-nal Ca2+ sensing can be overridden by the RyR2-resident luminal Ca2+ sensor. This makes this RyR2-resident sensor a promising molecular target for the treatment of Ca2+-mediated arrhythmias.

INTRODUCTION

In cardiac muscle cells, membrane depolarization induces the release of Ca2+ from the SR (sarcoplasmic reticulum) via the CICR (Ca2+-induced Ca2+ release) mechanism. In this process, a small Ca2+ influx through voltage-dependent L-type Ca2+ channels activates the cardiac SR Ca2+ release channel/RyR2 (ryanodine receptor 2). The resulting large SR Ca2+ release then drives muscle contraction [13]. However, SR Ca2+ release can also occur spontaneously under conditions of SR Ca2+ overload in the absence of membrane depolarization [49]. This SR Ca2+-overload-triggered spontaneous Ca2+ release may evoke propagating Ca2+ waves that can result in DADs (delayed afterdepolarizations) and trigger arrhythmias [8,1012]. These SCW (spontaneous Ca2+ wave)-triggered DADs are a major cause of VT (ventricular tachyarrhythmias) in heart failure [1214]. Such DADs are also thought to cause CPVT (catecholaminergic polymorphic ventricular tachycardia), which is linked to mutations in RyR2 and CASQ2 (cardiac calsequestrin) [15]. Indeed, CPVT-linked RyR2 or CASQ2 mutations enhance the propensity for SCWs [1623].

It is well established that when SR Ca2+ content reaches a critical level, RyR2 channels begin to open [2427]. How the elevating SR Ca2+ load opens the RyR2 channel and triggers SCWs is unclear. The RyR2 channel is thought to be regulated by an intra-SR Ca2+-sensing mechanism(s) that promotes RyR2 opening as SR Ca2+ content rises [2832]. The molecular nature of the intra-SR Ca2+ sensor is debated. It is commonly thought that CASQ2, a low-affinity high-capacity SR luminal Ca2+-binding protein, serves as the SR luminal Ca2+ sensor [29,33]. At low SR Ca2+ levels, CASQ2 associates with the triadin–junctin–RyR2 complex and inhibits RyR2 activity. At high SR Ca2+ loads, CASQ2 dissociates from the complex, relieving the inhibition [29,34]. Interestingly, cardiomyocytes isolated from Casq2-KO (knockout) mice still respond to SR Ca2+ overload and display overload-induced SCWs [35]. These findings indicate that there are other non-CASQ2-based SR luminal Ca2+-sensing mechanisms operating in cardiac cells. Furthermore, single purified native and recombinant CASQ2-free RyR2s in bilayers are still modulated by luminal Ca2+ [3640]. These observations indicate that the RyR2 channel is also controlled by a non-CASQ2-based, perhaps a RyR2-resident, luminal Ca2+ sensor.

We have recently identified a RyR2-resident luminal Ca2+ sensor on the channel protein itself [41]. We found that mutating Glu4872 located in the RyR2's helix bundle crossing completely abolishes luminal, but not cytosolic, Ca2+ activation of RyR2. This residue is part of a network of salt bridges involved in both luminal Ca2+ sensing and channel gating. Cardiomyocytes in heterozygous knockin mouse hearts expressing the RyR2-E4872Q mutant were resistant to store-overload-induced SCWs. Furthermore, the E4872Q heterozygous mutant mice are completely protected against stress-induced VTs [41]. Thus this newly discovered RyR2-resident luminal Ca2+ sensor is a key determinant of store-overload-induced SCWs and stress-induced VTs. The relationship between this RyR2-resident sensor and CASQ2-based sensing is entirely unexplored.

The absence of CASQ2 has little effect in unstressed cardiomyocytes, but enhances the propensity for store-overload-induced SCWs and CPVT when cells are stressed [35]. An interesting and important question is how does elevated SR luminal Ca2+ promote the observed SCWs and CPVT in Casq2-KO mice when CASQ2-based luminal Ca2+ sensing is absent. In the light of our recent findings [41], we hypothesize that the RyR2-resident luminal Ca2+ sensor controls the occurrence of SCWs and stress-induced CPVT in Casq2-KO mice. To test this hypothesis, we generated mice lacking both CASQ2-based and RyR2-resident Ca2+ sensing (Casq2−/−/RyR2-E4872Q+/−) by cross-breeding the Casq2-KO mice with the RyR2-E4872Q+/− mutant mice. We found that the RyR2-E4872Q mutation markedly suppressed SR Ca2+-overload-induced SCWs, stress-triggered VTs and cardiac hypertrophy in Casq2-KO mice. Thus defects in CASQ2-based luminal Ca2+ sensing can be overridden by manipulating the function of the RyR2-resident luminal Ca2+ sensor.

MATERIALS AND METHODS

Animal studies

All animal studies were approved by the Institutional Animal Care and Use Committees at Rush University Medical Center and the University of Calgary, and were performed in accordance with NIH (National Institutes of Health) guidelines.

Generation of the Casq2-KO, RyR2-E4872Q mouse model (Casq2−/−/RyR2-E4872Q+/−)

RyR2-E4872Q+/− mice [41] were cross-bred with the Casq2-KO mice [35] to obtain the Casq2+/−/RyR2-E4872Q+/− mice and Casq2+/− littermates. Casq2−/−/RyR2-E4872Q+/− mice were then generated by breeding between the Casq2+/−/RyR2-E4872Q+/− mice. The Casq2-KO mice were a gift from Dr Bjorn Knollmann's laboratory at the Vanderbilt University School of Medicine, Division of Clinical Pharmacology, Nashville, TN, U.S.A.

ECG (electrocardiogram) recording and induction of VTs in anaesthetized mice

WT (wild-type), Casq2+/−, Casq2+/−/RyR2-E4872Q+/−, Casq2−/− and Casq2−/−/RyR2-E4872Q+/− mice were assessed for their susceptibility to drug-induced arrhythmias using ECG recording. Briefly, mice were lightly anaesthetized with isoflurane vapour (0.5–1%) and 95% O2. Anaesthetized mice were placed on a heating pad (27°C), and needle electrodes (BIOPAC Systems) were inserted subcutaneously into the right-upper limb and left-lower abdomen. The animals’ ECGs were continuously monitored. Once the heart rate stabilized, a 5–10 min baseline ECG recording was made in each animal. VTs were induced by an intraperitoneal injection of a mixture of adrenaline (also known as epinephrine) (1.6 mg/kg of body mass) and caffeine (120 mg/kg of body mass) (referred to as epi/caff in the present paper). For assessing the age-dependent susceptibility to CPVT, VTs were induced by injection (intraperitoneal) of adrenaline (1.6 mg/kg of body mass) alone. The ECGs were continuously recorded for 30 min immediately after injection [42]. To determine the duration (in seconds) of VT, the 30 min overall recording period was subdivided into ten consecutive 3 min recording periods (i.e. 0–3, 3–6, 6–9 … 21–24, 24–27 and 27–30 min). The VT duration (as percentages) within each 3 min period (as well as over the entire 30 min period) was then determined by calculating the percentage of time in VT (in seconds). VT was defined as three or more consecutive ectopic beats.

Post-mortem whole heart hypertrophy measurements

The HW/BW (heart weight/body weight) ratios of the Casq2−/− and Casq2−/−/RyR2-E4872Q+/− mice at 4–6 months of age were determined. Animals were killed by cervical dislocation after measuring the body mass. The hearts were rapidly excised and washed thoroughly in PBS [137 mM NaCl, 8 mM Na2HPO4, 1.5 mM KH2PO4 and 2.7 mM KCl (pH 7.4)] solution and then dried using absorbant paper. Connective tissues and fat surrounding the hearts were removed and the heart weight was then measured.

Confocal Ca2+ imaging of intact hearts in situ

Excised hearts were loaded with Rhod-2 AM (rhodamine 2 acetoxymethyl ester) (3–4 μM; ATT Bioquest) in KH (Krebs–Henseleit) solution (120 mM NaCl, 24 mM NaHCO3, 11.1 mM glucose, 5.4 mM KCl, 2.0 mM CaCl2, 1 mM MgCl2, 0.42 mM NaH2PO4, 10 mM taurine and 5 mM creatine, oxygenated with 95% O2 and 5% CO2) at room temperature for 40 min via a retrograde Langendorff perfusion system [43]. After Rhod-2 AM loading, hearts were transferred to another Langendorff apparatus (37°C) attached to a confocal microscope system. In situ confocal line-scan imaging of Ca2+ signals arising from epicardial myocytes was performed on hearts beating at sinus rhythm. To minimize motion artefacts during Ca2+ imaging, blebbistatin (3 μM; Sigma) with BDM (2,3-butanedione-monoxime, 10 mM; Sigma) or blebbistatin (5–10 μM) without BDM were added to the perfusion solution [44,45]. Line-scan images were acquired at a rate of 1.93 ms per line. In some experiments, hearts were perfused with KH solutions containing progressively higher Ca2+ levels (3, 4, 5, 6 and 7 mM) to induce SCWs. In other experiments, hearts perfused with the KH solution were fast paced (6 Hz) with 100 nM isoprenaline (also known as isoproterenol) to mimic physiological stress conditions and induce SCWs. The amplitude, time to peak, T50 (time to 50% decay) of Ca2+ transients while pacing at 6 Hz were assessed.

Statistical analysis

All values shown are means±S.E.M. unless indicated otherwise. To test for differences between groups, we used Student's t test (two-tailed). Statistical analyses were carried out using SPSS version 15.0. P<0.05 was considered to be statistically significant.

RESULTS

The RyR2-E4872Q mutation suppresses CPVT in Casq2-KO mice

We have recently reported that the RyR2-E4872Q mutation eliminates CPVT in the RyR2-R4496C CPVT mouse model [41]. In the present study, we tested whether the RyR2-E4872Q mutation also suppresses CPVT in the Casq2-KO CPVT mouse model. RyR2-E4872Q+/− mice were cross-bred with Casq2-KO mice (Casq2−/−) to generate double mutant mice heterozygous for both genetic attributes (i.e. Casq2+/−/RyR2-E4872Q+/−). These mice were then crossed to produce Casq2−/−/RyR2-E4872Q+/− mice entirely lacking CASQ2. We then tested CPVT susceptibility of (i) WT, (ii) Casq2+/−, (iii) Casq2−/−, (iv) Casq2+/−/RyR2-E4872Q+/− and (v) Casq2−/−/RyR2-E4872Q+/− mice. CPVT susceptibility was determined from ECG measurements before and after injection (intraperitoneal) of either a mixture of epi/caff or adrenaline alone.

Figure 1 shows that epi/caff injection induced transient VTs in the Casq2+/− mice, which peaked at ~6 min and disappeared after ~20 min (Figures 1A and 1C). The same epi/caff injection also induced transient VTs in WT mice (Supplementary Figure S1 at http://www.biochemj.org/bj/461/bj4610099add.htm). However, The epi/caff injection induced little or no VTs in Casq2+/−/RyR2-E4872Q+/− mice (Figure 1B). Figure 1(C) shows post-injection VT duration (time in VT as a percentage) summary results collected during ten consecutive 3 min time segments. Figure 1(D) compares average VT duration over the entire 30 min period. VT duration (over 30 min) in the Casq2+/−/RyR2-E4872Q+/− mice (0.8±0.6%) was greatly reduced compared with that in Casq2+/− mice (32.8±4.8%) (P<0.001). WT mice showed 10.9±6.7% VT duration (over 30 min), which is significantly lower than that in Casq2+/− mice (P<0.05).

The RyR2-E4872Q mutation suppresses CPVT in heterozygous Casq2+/− mice

Figure 1
The RyR2-E4872Q mutation suppresses CPVT in heterozygous Casq2+/− mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of Casq2+/− (A) and Casq2+/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of epi/caff. The average data show the VT duration (as a percentage) during either 3 min (C) or 30 min (D) periods of ECG recording. ECG was recorded before and immediately after the injection of the triggering agent(s). A 30 min ECG recording starting immediately after the injection of the triggering agent(s) was used to determine the duration of VT. To determine the duration (in seconds) of VT, the 30 min overall recording period was subdivided into ten consecutive 3 min recording periods (i.e. 0–3, 3–6, 6–9 … 21–24, 24–27 and 27–30 min). The VT duration (as a percentage) within each 3 min period (as well as over the entire 30 min period) was then determined by calculating the percentage of time in VT (in seconds). VT was defined as three or more consecutive ectopic beats. (C) Summary results of VT duration (%) determined over each consecutive 3 min period. (D) VT duration (%) determined over the entire 30 min period of ECG recording. The boxes indicate regions that are time expanded below. Data shown are means±S.E.M. for WT (n=6), Casq2+/− (n=7), and Casq2+/−/E4872Q+/− (n=9) mice (*P<0.05, compared with Casq2+/−; **P< 0.001, compared with Casq2+/−).

Figure 1
The RyR2-E4872Q mutation suppresses CPVT in heterozygous Casq2+/− mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of Casq2+/− (A) and Casq2+/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of epi/caff. The average data show the VT duration (as a percentage) during either 3 min (C) or 30 min (D) periods of ECG recording. ECG was recorded before and immediately after the injection of the triggering agent(s). A 30 min ECG recording starting immediately after the injection of the triggering agent(s) was used to determine the duration of VT. To determine the duration (in seconds) of VT, the 30 min overall recording period was subdivided into ten consecutive 3 min recording periods (i.e. 0–3, 3–6, 6–9 … 21–24, 24–27 and 27–30 min). The VT duration (as a percentage) within each 3 min period (as well as over the entire 30 min period) was then determined by calculating the percentage of time in VT (in seconds). VT was defined as three or more consecutive ectopic beats. (C) Summary results of VT duration (%) determined over each consecutive 3 min period. (D) VT duration (%) determined over the entire 30 min period of ECG recording. The boxes indicate regions that are time expanded below. Data shown are means±S.E.M. for WT (n=6), Casq2+/− (n=7), and Casq2+/−/E4872Q+/− (n=9) mice (*P<0.05, compared with Casq2+/−; **P< 0.001, compared with Casq2+/−).

Figure 2 shows analogous experiments on homozygous Casq2-KO mice. Sample ECG recordings from Casq2−/− (Figure 2A) and Casq2−/−/RyR2-E4872Q+/− (Figure 2B) mice before and after epi/caff injection are shown. Casq2−/− mice exhibit a more severe phenotype compared with Casq2+/− mice with long periods of VTs lasting over 30 min (Figure 2C, and Supplementary Figure S2 at http://www.biochemj.org/bj/461/bj4610099add.htm). Casq2−/−/RyR2-E4872Q+/− mice displayed short periods of VTs that subsided after a few minutes (Figure 2C and Supplementary Figure S2). Figure 2(D) compares the average VT duration over the entire 30 min post-injection recording period in Casq2−/− and Casq2−/−/RyR2-E4872Q+/− mice. As was the case for Casq2+/− mice (Figure 1), the RyR2-E4872Q mutation significantly decreased VT duration in Casq2−/− mice (83.2±8.6% to 6.6±4.2%; P<0.001). Thus the RyR2-E4872Q mutation protects against stress-induced VTs in both heterozygous and homozygous Casq2-KO mice. These observations, together with our recent findings [41], suggest that the RyR2-resident luminal Ca2+ sensor governs CPVT with or without CASQ2 present.

The RyR2-E4872Q mutation protects against CPVT in homozygous Casq2−/− mice

Figure 2
The RyR2-E4872Q mutation protects against CPVT in homozygous Casq2−/− mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of Casq2−/− (A) and Casq2−/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of epi/caff. VT occurred intermittently and percentage of time in VT (VT duration) was measured in sequential 3 min periods post-injection (C). Average time in VT (VT duration) was measured (post-injection) over the entire 30 min recording period (D). Data shown are means±S.E.M. for eight mice for each group (**P<0.001, when compared with Casq2−/−).

Figure 2
The RyR2-E4872Q mutation protects against CPVT in homozygous Casq2−/− mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of Casq2−/− (A) and Casq2−/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of epi/caff. VT occurred intermittently and percentage of time in VT (VT duration) was measured in sequential 3 min periods post-injection (C). Average time in VT (VT duration) was measured (post-injection) over the entire 30 min recording period (D). Data shown are means±S.E.M. for eight mice for each group (**P<0.001, when compared with Casq2−/−).

Age-dependent CPVT in Casq2−/− mice is eliminated by the RyR2-E4872Q mutation

To determine whether there is an age-dependent progression of CPVT susceptibility in Casq2-KO mice, we performed stress tests in young and old Casq2−/− mice (2–3 months and 5–6 months respectively). Figures 3(A) and 3(B) shows that injection of adrenaline (1.6 mg/kg) alone (i.e. no caffeine) triggered sustained VTs in old Casq2−/− mice, but not when the old mice had the RyR2-E4872Q mutation. Injection of adrenaline alone did not trigger VTs in WT mice (n=6) (results not shown). Unlike in old Casq2−/− mice, VTs in young Casq2−/− mice subsided approximately 20 min after the adrenaline injection (Figure 3C, and Supplementary Figures S3 and S4 at http://www.biochemj.org/bj/461/bj4610099add.htm). Figure 3(D) compares the average VT duration over the entire 30 min post-injection recording period in the young and old Casq2−/− mice (21.6±5.9% and 97.1±0.9% respectively). The old Casq2−/− mice were significantly (P<0.001) more susceptible to VT than young Casq2−/− mice. When the RyR2-E4872Q mutation was present, there was no post-injection VT in Casq2−/− mice at either age (Figures 3C and 3D). Thus the RyR2-E4872Q mutation also limits age-dependent CPVT progression in Casq2-KO mice. Since the RyR2-E4872Q mutation protects against VTs induced by adrenaline alone (Figure 3) or by epi/caff (Figures 1 and 2) in Casq2-KO mice, the protective effect of the RyR2-E4872Q mutation is not caffeine-dependent.

Age-dependent susceptibility to CPVT in Casq2−/−, but not in Casq2−/−/E4872Q+/−, mice

Figure 3
Age-dependent susceptibility to CPVT in Casq2−/−, but not in Casq2−/−/E4872Q+/−, mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of 5–6-month-old Casq2−/− mice (A) and Casq2−/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of adrenaline (1.6 mg/kg of body mass) alone (no caffeine). VT duration (%) in 2–3-month-old and 5–6-month-old Casq2−/− and Casq2−/−/E4872Q+/− mice was measured within each of ten consecutive 3 min periods (C), as well as over the entire 30 min recording period (D). Note that adrenaline alone triggered long-lasting VTs in 5–6-month-old Casq2−/− mice, but only short and transient VTs in 2–3-month-old Casq2−/− mice. There were no VTs detected in Casq2−/−/E4872Q+/− mice aged 2–3 months or 5–6 months. Data shown are means±S.E.M. for six to nine mice for each group. **P<0.001, 2–3 months against 5–6 months Casq2−/− mice or Casq2−/−/E4872Q+/− against Casq2−/− mice within the same age group.

Figure 3
Age-dependent susceptibility to CPVT in Casq2−/−, but not in Casq2−/−/E4872Q+/−, mice

Representative ECG recordings (15 s trace at the top and 2 s trace at the bottom) of 5–6-month-old Casq2−/− mice (A) and Casq2−/−/E4872Q+/− (B) mice before (top panels) and 3–6 min after (bottom panels) an intraperitoneal injection of adrenaline (1.6 mg/kg of body mass) alone (no caffeine). VT duration (%) in 2–3-month-old and 5–6-month-old Casq2−/− and Casq2−/−/E4872Q+/− mice was measured within each of ten consecutive 3 min periods (C), as well as over the entire 30 min recording period (D). Note that adrenaline alone triggered long-lasting VTs in 5–6-month-old Casq2−/− mice, but only short and transient VTs in 2–3-month-old Casq2−/− mice. There were no VTs detected in Casq2−/−/E4872Q+/− mice aged 2–3 months or 5–6 months. Data shown are means±S.E.M. for six to nine mice for each group. **P<0.001, 2–3 months against 5–6 months Casq2−/− mice or Casq2−/−/E4872Q+/− against Casq2−/− mice within the same age group.

The RyR2-E4872Q mutation prevents cardiac hypertrophy in Casq2−/− mice

Casq2−/− mice are known to develop modest cardiac hypertrophy that increases their HW/BW ratio by ~10% [35]. Why the Casq2-KO induces cardiac hypertrophy is unknown. Since the RyR2-E4872Q mutation protects against CPVT in Casq2−/− mice, it could also limit cardiac hypertrophy in Casq2−/− mice. To test this possibility, we measured the HW/BW ratio in WT, Casq2−/− and Casq2−/−/RyR2-E4872Q+/− mice. Consistent with a previous study [35], we found a slight, but significant, increase in the HW/BW ratio in Casq2−/− mice compared with their WT littermates (4.9±0.1 compared with 4.0±0.1; P<0.001) (Figure 4). Interestingly, Casq2−/−/RyR2-E4872Q+/− mice exhibited HW/BW ratios (4.1±0.1) similar to those observed with WT mice, but significantly different from those of the Casq2−/− mice (P<0.001) (Figure 4). Thus the RyR2-E4872Q mutation (i.e. limiting SCWs and CPVT) prevents the development of cardiac hypertrophy in Casq2-KO mice. These data indicate that the RyR2-resident luminal Ca2+ sensor is also an important determinant of Casq2-KO-induced cardiac hypertrophy.

The RyR2-E4872Q mutation prevents cardiac hypertrophy in Casq2−/− mice

Figure 4
The RyR2-E4872Q mutation prevents cardiac hypertrophy in Casq2−/− mice

HW/BW ratios of 4–6-month-old WT, Casq2−/− and Casq2−/−/E4872Q+/− mice. Note that the HW/BW ratio of Casq2−/− mice was increased compared with that of WT mice, whereas the HW/BW ratio of Casq2−/−/E4872Q+/− mice was similar to that of WT mice. Data shown are means±S.E.M. from WT (n=27), Casq2−/− (n=28) and Casq2−/−/E4872Q+/− (n=20) mice (*P<0.001 compared with WT or Casq2−/−/E4872Q+/−).

Figure 4
The RyR2-E4872Q mutation prevents cardiac hypertrophy in Casq2−/− mice

HW/BW ratios of 4–6-month-old WT, Casq2−/− and Casq2−/−/E4872Q+/− mice. Note that the HW/BW ratio of Casq2−/− mice was increased compared with that of WT mice, whereas the HW/BW ratio of Casq2−/−/E4872Q+/− mice was similar to that of WT mice. Data shown are means±S.E.M. from WT (n=27), Casq2−/− (n=28) and Casq2−/−/E4872Q+/− (n=20) mice (*P<0.001 compared with WT or Casq2−/−/E4872Q+/−).

The RyR2-E4872Q mutation does not alter the depolarization- induced Ca2+ transient

The amplitude and dynamics of Ca2+ release evoked by depolarization (6 Hz) in Casq2−/− and Casq2−/−/RyR2-E4872Q+/− hearts were measured using confocal line-scan Ca2+ imaging. Figure 5 shows that there are no significant differences in Ca2+ transient amplitude (ΔF/F0; 1.09±0.03 compared with 1.05±0.05), time-to-peak (30.6±0.23 ms compared with 30.1±0.25 ms) or T50 (52.3±0.36 ms compared with 53.1±0.28 ms) in Casq2−/− and Casq2−/−/RyR2-E4872Q+/− hearts respectively. Therefore the RyR2-E4872Q mutation does not significantly alter the amplitude and dynamics of depolarization-induced Ca2+ release in Casq2-KO hearts.

The RyR2-E4872Q mutation does not affect depolarization-induced Ca2+ transients in intact Casq2−/− hearts

Figure 5
The RyR2-E4872Q mutation does not affect depolarization-induced Ca2+ transients in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM in KH solution and Langendorff-perfused with blebbistatin (5 μM) to inhibit muscle contraction. The perfused heart was paced at 6 Hz, and Ca2+ transients were monitored by line-scan confocal Ca2+ imaging. Representative images/traces of Casq2−/− (A) and Casq2−/−/E4872Q+/− hearts (B) are shown. Short markers on the left indicate cell boundaries within the intact heart. Average data of the amplitude (C), time to peak (D) and T50 (E) of Ca2+ transients in Casq2−/− and Casq2−/−/E4872Q+/− hearts are shown. Data shown are means±S.E.M. for 75 Casq2−/− and 60 Casq2−/−/E4872Q+/−cardiomyocytes in nine hearts for each group. Note that there are no significant differences in amplitude, time to peak or T50 between Casq2−/− and Casq2−/−/E4872Q+/− hearts.

Figure 5
The RyR2-E4872Q mutation does not affect depolarization-induced Ca2+ transients in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM in KH solution and Langendorff-perfused with blebbistatin (5 μM) to inhibit muscle contraction. The perfused heart was paced at 6 Hz, and Ca2+ transients were monitored by line-scan confocal Ca2+ imaging. Representative images/traces of Casq2−/− (A) and Casq2−/−/E4872Q+/− hearts (B) are shown. Short markers on the left indicate cell boundaries within the intact heart. Average data of the amplitude (C), time to peak (D) and T50 (E) of Ca2+ transients in Casq2−/− and Casq2−/−/E4872Q+/− hearts are shown. Data shown are means±S.E.M. for 75 Casq2−/− and 60 Casq2−/−/E4872Q+/−cardiomyocytes in nine hearts for each group. Note that there are no significant differences in amplitude, time to peak or T50 between Casq2−/− and Casq2−/−/E4872Q+/− hearts.

The RyR2-E4872Q mutation diminishes SCWs in intact Casq2−/− hearts

Enhanced propensity for SCWs is believed to cause CPVT in Casq2−/− mice. Since RyR2-E4872Q mutation suppresses SR Ca2+-overload-induced SCWs in WT hearts [41], it is possible that the mutation also suppresses SCWs (and consequently CPVT) in Casq2−/− mice. To test this possibility, SCWs in intact Casq2−/− hearts were induced by increasing the extracellular Ca2+ concentration (2 to 3, 4, 5, 6 or 7 mM). Intracellular Ca2+ dynamics were measured using confocal line-scan Ca2+ imaging of intact hearts. Figure 6(A) shows sample line-scan images from Casq2−/− (top panel) and Casq2−/−/RyR2-E4872Q+/− (bottom panel) hearts at 4 mM extracellular Ca2+. Periodic Ca2+ transients occurred at the sinus rhythm, which varied from heart to heart. SCWs were evident (see arrowheads) in the Casq2−/− heart, but not in the Casq2−/−/RyR2-E4872Q+/− heart. Figure 6(B) shows an analogous experiment with 7 mM extracellular Ca2+ present. In the 7 mM case, SCWs are observed in both model hearts, but are less frequent with the RyR2-E4872Q mutation present (9.1±2.0 compared with 2.8±0.8 Hz/100 μm; P<0.01). Figure 6(C) shows SCW frequency summary results. SCW frequency in Casq2-KO hearts increased with extracellular Ca2+, but at all extracellular Ca2+ levels tested, the RyR2-E4872Q mutation significantly decreased SCW frequency. To avoid motion artefacts, muscle contraction was inhibited by blebbistatin (3 μM) plus BDM (10 mM). Since BDM has been reported to be an RyR2 agonist, control studies were carried out without BDM present (Supplementary Figure S5 at http://www.biochemj.org/bj/461/bj4610099add.htm). The RyR2-E4872Q mutation suppressed SCWs in Casq2−/− hearts in the presence or absence of BDM.

The RyR2-E4872Q mutation suppresses SCWs induced by elevating extracellular Ca2+ in intact Casq2−/− hearts

Figure 6
The RyR2-E4872Q mutation suppresses SCWs induced by elevating extracellular Ca2+ in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM and Langendorff-perfused with KH solution containing extracellular Ca2+ ([Ca2+]o) from 2 to 7mM to induce SR Ca2+ overload, and blebbistatin (3 μM) plus BDM (10 mM) to inhibit muscle contraction. Intracellular Ca2+ dynamics in epicardial ventricular myocytes in intact hearts under sinus rhythm was monitored by using line-scan confocal Ca2+ imaging. Representative line-scan Ca2+ images of Casq2−/− and Casq2−/−/E4872Q+/− hearts at 4 mM (A) and 7 mM (B) [Ca2+]o are shown. Arrowheads show the occurrence of SCWs. Short markers on the left indicate cell boundaries within the intact heart. (C) Frequency of SCWs at different [Ca2+]o in Casq2−/− and Casq2−/−/E4872Q+/− mouse hearts. Data shown are means±S.E.M. for four to five mouse hearts for each group (*P<0.05 and **P<0.01, compared with Casq2−/−).

Figure 6
The RyR2-E4872Q mutation suppresses SCWs induced by elevating extracellular Ca2+ in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM and Langendorff-perfused with KH solution containing extracellular Ca2+ ([Ca2+]o) from 2 to 7mM to induce SR Ca2+ overload, and blebbistatin (3 μM) plus BDM (10 mM) to inhibit muscle contraction. Intracellular Ca2+ dynamics in epicardial ventricular myocytes in intact hearts under sinus rhythm was monitored by using line-scan confocal Ca2+ imaging. Representative line-scan Ca2+ images of Casq2−/− and Casq2−/−/E4872Q+/− hearts at 4 mM (A) and 7 mM (B) [Ca2+]o are shown. Arrowheads show the occurrence of SCWs. Short markers on the left indicate cell boundaries within the intact heart. (C) Frequency of SCWs at different [Ca2+]o in Casq2−/− and Casq2−/−/E4872Q+/− mouse hearts. Data shown are means±S.E.M. for four to five mouse hearts for each group (*P<0.05 and **P<0.01, compared with Casq2−/−).

To eliminate the influence of the intrinsic sinus rhythm on SCW occurrence, as well as to promote SR Ca2+ overload, we ablated the AV node by electro-cautery and then perfused hearts with 100 nM isoprenaline. AV node-ablated Casq2−/− or Casq2−/−/RyR2-E4872Q+/− hearts were paced at 6 Hz to standardize the SR Ca2+ load and then the pace was reduced to 0.6 Hz. Figures 7(A) and 7(B) shows sample confocal line-scan images during the 0.6 Hz pacing period with SCW marked (see arrows). Figure 7(C) compares average SCW frequency in Casq2−/− (2.0±0.3 Hz/100 μm) and Casq2−/−/RyR2-E4872Q+/− (0.1±0.0 Hz/100 μm) hearts. The presence of the RyR2-E4872Q mutation significantly (P<0.001) reduced the occurrence of SCWs in Casq2-KO hearts. Taken together, these results show that the RyR2-resident luminal Ca2+ sensor is a key determinant of the occurrence of SCWs in Casq2-KO hearts.

The RyR2-E4872Q mutation suppresses spontaneous Ca2+ waves induced by fast pacing and isoprenaline in intact Casq2−/− hearts

Figure 7
The RyR2-E4872Q mutation suppresses spontaneous Ca2+ waves induced by fast pacing and isoprenaline in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM and Langendorff-perfused with 2 mM extracellular Ca2+, 100 nM ISO (isoprenaline) and blebbistatin (10 μM) in KH solution. Representative Ca2+ images of Casq2−/− heart (A) and Casq2−/−/E4872Q+/− hearts (B) during 6 Hz pacing and after switching the pacing from 6 to 0.6 Hz in the presence of 100 nM ISO are shown. Arrowheads show the occurrence of SCWs. Short markers on the left indicate cell boundaries within the intact heart. (C) Frequency of SCWs after switching the pacing frequency from 6 to 0.6 Hz in the presence of 100 nM ISO. Data shown are means±S.E.M. for three to four mouse hearts for each group (**P<0.001, compared with Casq2−/−).

Figure 7
The RyR2-E4872Q mutation suppresses spontaneous Ca2+ waves induced by fast pacing and isoprenaline in intact Casq2−/− hearts

Intact hearts isolated from Casq2−/− and Casq2−/−/E4872Q+/− mice were loaded with Rhod-2 AM and Langendorff-perfused with 2 mM extracellular Ca2+, 100 nM ISO (isoprenaline) and blebbistatin (10 μM) in KH solution. Representative Ca2+ images of Casq2−/− heart (A) and Casq2−/−/E4872Q+/− hearts (B) during 6 Hz pacing and after switching the pacing from 6 to 0.6 Hz in the presence of 100 nM ISO are shown. Arrowheads show the occurrence of SCWs. Short markers on the left indicate cell boundaries within the intact heart. (C) Frequency of SCWs after switching the pacing frequency from 6 to 0.6 Hz in the presence of 100 nM ISO. Data shown are means±S.E.M. for three to four mouse hearts for each group (**P<0.001, compared with Casq2−/−).

DISCUSSION

A novel and important finding of the present study is that the RyR2-resident luminal Ca2+ sensor determines whether SCWs and Ca2+-triggered arrhythmias occur in the absence of the CASQ2-based luminal Ca2+ regulation. We have shown recently that this RyR2 luminal Ca2+ sensor located in the helix bundle crossing (the proposed gate) of the channel is responsible for luminal Ca2+ activation of the RyR2 channel and the occurrence of SCWs in the heart. Furthermore, we demonstrate that a RyR2 luminal Ca2+ sensing mutation, RyR2-E4872Q, completely abolishes stress-induced VTs in a mouse model harbouring a CPVT-linked RyR2 mutation R4496C [41]. In the present study, we show that the RyR2-E4872Q mutation diminishes SCWs and stress-induced VTs in Casq2-KO mice. Thus manipulating the RyR2-resident luminal Ca2+ sensor can protect against stress-induced VTs that arise from either a RyR2 mutation or CASQ2 deficiency. Thus the RyR2-resident luminal Ca2+ sensor represents a promising therapeutic target for suppressing Ca2+-triggered arrhythmias.

Another novel finding is that the RyR2-E4872Q mutation prevented cardiac hypertrophy in Casq2-KO mice. The reason that Casq2-KO mice develop cardiac hypertrophy is not entirely clear. It is possible that CASQ2 deficiency enhances diastolic spontaneous SR Ca2+ leak, which could drive Ca2+-dependent cardiac remodelling [46,47]. Thus suppressing the RyR2-resident luminal Ca2+ sensor may prevent the cardiac remodelling by limiting diastolic SR Ca2+ leak. Another interesting finding is that Casq2-KO mice become more susceptible to CPVT with age. Again, the mechanism for this age-dependent progression of CPVT in Casq2-KO mice is unknown. It is also unclear whether or not CPVT progresses with age in humans. Unlike mice that do not usually die from CPVT, humans succumb to CPVT at a young age, making it difficult to study the age-dependent progression of CPVT in humans. Interestingly, CPVT progression in mice occurs concurrently with the development of cardiac hypertrophy, suggesting that the two may be associated. In any event, manipulating the RyR2-resident luminal Ca2+ sensor limits both processes, and thus may generally be used to control Ca2+-mediated cardiomyopathies.

It has long been recognized that the Ca2+ level inside the SR critically regulates SR Ca2+ release [2427], but the molecular basis of that regulation is still debated. The RyR2 channel is likely to be modulated by SR Ca2+ load via some luminal Ca2+-sensing mechanisms [2832]. CASQ2 is commonly believed to act as the luminal Ca2+ sensor responsible for the regulation of Ca2+ release by SR luminal Ca2+ [29,33]. However, cardiac SR Ca2+ release in Casq2-KO mice is still governed by SR Ca2+ load [35]. This implies that other intra-SR Ca2+ control mechanisms exist. The recently identified RyR2-resident luminal Ca2+ sensor [41] probably explains why the absence of CASQ2 does not eliminate regulation of RyR2 by SR luminal Ca2+. Although ablating the CASQ2-based luminal Ca2+ sensor does not abolish luminal Ca2+ regulation of SR Ca2+ release, disabling the RyR2-resident luminal Ca2+ sensor eliminates SR luminal Ca2+ sensing with or without CASQ2 present. These observations indicate that even in the absence of the previously known luminal Ca2+ sensor CASQ2, the RyR2-resident luminal Ca2+ sensor still works as an effective regulator of RyR2 and its absence leads to suppression of SCWs and CPVT. These data suggest that regulation of RyR2 by luminal Ca2+ may involve a hierarchy of intra-SR luminal Ca2+ sensors. In this hierarchy, the RyR2-resident luminal Ca2+ sensor overrides the CASQ2-based luminal Ca2+ sensor. This hierarchy of sensors is consistent with our observation that the RyR2-resident luminal Ca2+ sensor mutation E4872Q, protects against both RyR2- and CASQ2-associated CPVT.

RyR2-linked CPVT in humans is inherited in an autosomal dominant fashion, whereas CASQ2-associated CPVT in humans is inherited in an autosomal recessive manner [15]. Consistent with human CPVT, stress-induced VTs can be readily observed in heterozygous CPVT-linked RyR2 mutant mice or in homozygous CPVT-linked Casq2-mutant or Casq2-KO mice. At variance with human CPVT, heterozygous Casq2-KO mice also displayed stress-induced VTs [48]. Consistent with this previous observation, we found that heterozygous Casq2-KO mice are also susceptible to CPVT under our triggering conditions (i.e. with epi/caff). CPVT sensitivity of heterozygous Casq2-KO mice has been explained by the modest reduction in CASQ2 protein expression [48]. It is thought that the reduced CASQ2 expression alters the CASQ2–RyR2 interaction and RyR2 function [48]. However, CASQ2 is also an intra-SR Ca2+ buffer and a reduction in CASQ2 expression could also alter intra-SR Ca2+ dynamics and, in turn, function of the RyR2-resident luminal Ca2+ sensor. CPVT-causing CASQ2 mutations typically result in a truncated CASQ2, reduced CASQ2 expression levels and/or altered CASQ2 Ca2+-binding capacity [15]. These diverse phenotypes may alter intra-SR Ca2+ buffering power and thus the SR Ca2+ dynamics that could influence the function of the RyR2-resident luminal Ca2+ sensor. Thus the existence of the RyR2-resident luminal Ca2+-sensing mechanism makes defining the relative contributions of CASQ2 buffering and the CASQ2–RyR2 interaction more important than ever.

In summary, RyR2 function is likely to be governed by the hierarchical operation of multiple intra-SR Ca2+-sensing mecha-nisms. Removal of RyR2-resident luminal Ca2+ sensing eliminates SCWs and CPVT with or without CASQ2 present. CASQ2 deficiency induces cardiac hypertrophy and increases CPVT susceptibility with age in mice only when RyR2-resident luminal Ca2+ sensing is present. This dominant action of the RyR2-resident luminal Ca2+ sensor makes it a promising molecular target for the treatment of Ca2+-triggered arrhythmias.

Abbreviations

     
  • BDM

    2,3-butanedione-monoxime

  •  
  • CASQ2

    cardiac calsequestrin

  •  
  • CPVT

    catecholaminergic polymorphic ventricular tachycardia

  •  
  • DAD

    delayed afterdepolarization

  •  
  • ECG

    electrocardiogram

  •  
  • epi/caff

    mixture of adrenaline (epinephrine) and caffeine

  •  
  • HW/BW

    heart weight/body weight

  •  
  • KH

    Krebs–Henseleit

  •  
  • KO

    knockout

  •  
  • Rhod-2 AM

    rhodamine 2 acetoxymethyl ester

  •  
  • RyR2

    ryanodine receptor 2

  •  
  • SCW

    spontaneous Ca2+ wave

  •  
  • SR

    sarcoplasmic reticulum

  •  
  • T50

    time to 50% decay

  •  
  • VT

    ventricular tachyarrhythmia

  •  
  • WT

    wild-type

AUTHOR CONTRIBUTION

Jingqun Zhang, Biyi Chen, Qiang Zhou, Long-Sheng Song and S.R. Wayne Chen designed the research; Jingqun Zhang, Biyi Chen, Xiaowei Zhong, Tao Mi, Ang Guo, Qiang Zhou, Zhen Tan and Guogen Wu performed the research; Jingqun Zhang, Biyi Chen, Xiaowei Zhong, Alexander W. Chen, Long-Sheng Song and S.R. Wayne Chen analysed the data; and Jingqun Zhang, Michael Fill, Long-Sheng Song and S.R. Wayne Chen wrote the paper.

We thank Dr Bjorn Knollmann for kindly providing the Casq2-KO mice.

FUNDING

This work was supported by the National Institutes of Health [grant number R01HL057832 (to M.F.)], [grant number R01HL090905 (to L.-S.S.)] and [grant number R01HL75210 (to S.R.W.C.)], the Heart and Stroke Foundation of Alberta, Northwest Territories and Nunavut, the Canadian Institutes of Health Research, the Canada Foundation for Innovation (CFI), and the Heart and Stroke Foundation/Libin Professorship in Cardiovascular Research (to S.R.W.C.). X.Z. is a recipient of the Alberta Innovates-Health Solutions (AIHS) Graduate Studentship Award.

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Author notes

1

These authors contributed equally to this work.

2

Present address: Department of Cardiology of Tongji Hospital, Tongji Medical School, Huazhong University of Science and Technology, Wuhan, China.

3

Present address: Department of Geriatrics of Tongji Hospital, Tongji Medical School, Huazhong University of Science and Technology, Wuhan, China.

4

S.R. Wayne Chen is an Alberta Innovates-Health Solutions (AIHS) scientist.