Moderate heart dysfunction in mice with inducible cardiomyocyte-specific excision of the Serca2 gene

Transcriptional mechanisms regulating Ca(2+) homeostasis

Ca(2+) is a dynamic cellular secondary messenger which mediates a vast array of cellular responses. Control over these processes is achieved via an extensive combination of pumps and channels which regulate the concentration of Ca(2+) within not only the cytosol but also all intracellular compartments. Precisely how these pumps and channels are regulated is only partially understood, however, recent investigations have identified members of the Early Growth Response (EGR) family of zinc finger transcription factors as critical players in this process. The roles of several other transcription factors in control of Ca(2+) homeostasis have also been demonstrated, including Wilms Tumor Suppressor 1 (WT1), Nuclear Factor of Activated T cells (NFAT) and c-myc. In this review, we will discuss not only how these transcription factors regulate the expression of the major proteins involved in control of Ca(2+) homeostasis, but also how this transcriptional remodeling of Ca(2+) homeostasis affects Ca(2+) dynamics and cellular responses.

Impact of long-term caloric restriction on cardiac senescence: caloric restriction ameliorates cardiac diastolic dysfunction associated with aging

Approximately half of older patients with congestive heart failure have normal left ventricular (LV) systolic but abnormal LV diastolic function. In mammalian hearts, aging is associated with LV diastolic dysfunction. Caloric restriction (CR) is expected to retard cellular senescence and to attenuate the physiological decline in organ function. Therefore, the aim of the present study was to investigate the impact of long-term CR on cardiac senescence, in particular the effect of CR on LV diastolic dysfunction associated with aging. Male 8-month-old Fischer344 rats were divided into ad libitum fed and CR (40% energy reduction) groups. LV function was evaluated by echocardiography and cardiac senescence was compared between the two groups at the age of 30-month-old. (1) Echocardiography showed similar LV systolic function, but better LV diastolic function in the CR group. (2) Histological analysis revealed that CR attenuated the accumulation of senescence-associated β-galactosidase and lipofuscin and reduced myocyte apoptosis. (3) In measurements of [Ca(2+)](i) transients, the time to 50% relaxation was significantly smaller in the CR group, whereas F/F(0) was similar. (4) CR attenuated the decrease in sarcoplasmic reticulum calcium ATPase 2 protein with aging. (5) CR suppressed the mammalian target of rapamycin (mTOR) pathway and increased the ratio of conjugated to cytosolic light chain 3, suggesting that autophagy is enhanced in the CR hearts. In conclusion, CR improves diastolic function in the senescent myocardium by amelioration of the age-associated deterioration in intracellular Ca(2+) handling. Enhanced autophagy via the suppression of mTOR during CR may retard cardiac senescence.

Cardiomyocyte-specific disruption of Serca2 in adult mice causes sarco(endo)plasmic reticulum stress and apoptosis

Reduced sarco(endo)plasmic reticulum (SR) Ca(2+) ATPase (SERCA2) contributes to the impaired cardiomyocyte Ca(2+) homeostasis observed in heart failure. We hypothesized that a reduction in SERCA2 also elicits myocardial ER/SR stress responses, including unfolded protein responses (UPR) and cardiomyocyte apoptosis, which may additionally contribute to the pathophysiology of this condition. Left ventricular myocardium from mice with cardiomyocyte-specific tamoxifen-inducible disruption of Serca2 (SERCA2 KO) was compared with aged-matched controls. In SERCA2 KO hearts, SERCA2 protein levels were markedly reduced to 2% of control values at 7 weeks following tamoxifen treatment. Serca2 disruption caused increased abundance of the ER stress-associated proteins CRT, GRP78, PERK, and eIF2α and increased phosphorylation of PERK and eIF2α, indicating UPR induction. Pro-apoptotic signaling was also activated in SERCA2 KO, as the abundance of CHOP, caspase 12, and Bax was increased. Indeed, TUNEL staining revealed an increased fraction of cardiomyocytes undergoing apoptosis in SERCA2 KO. ER-Tracker staining additionally revealed altered ER structure. These findings indicate that reduction in SERCA2 protein abundance is associated with marked ER/SR stress in cardiomyocytes, which induces UPR, apoptosis, and ER/SR structural alterations. This suggests that reduced SERCA2 abundance or function may contribute to the phenotype of heart failure also through induction of ER/SR stress responses.

Cre-loxP DNA recombination is possible with only minimal unspecific transcriptional changes and without cardiomyopathy in Tg(alphaMHC-MerCreMer) mice

Cre-loxP technology for conditional gene inactivation is a powerful tool in cardiovascular research. Induction of gene inactivation can be carried out by per oral or intraperitoneal tamoxifen administration. Unintended transient cardiomyopathy following tamoxifen administration for gene inactivation has recently been reported. We aimed to develop a protocol for tamoxifen-induced gene inactivation with minimal effects on gene transcription and in vivo cardiac function, allowing studies of acute loss of the targeted gene. In mRNA microarrays, 35% of the 34,760 examined genes were significantly regulated in MCM(+/0) compared with wild type. In MCM(+/0), we found a correlation between tamoxifen dose and degree of gene regulation. Comparing one and four intraperitoneal injections of 40 mg·kg(-1)·day(-1) tamoxifen, regulated genes were reduced to 1/5 in the single injection group. Pronounced alteration in protein abundance and acute cardiomyopathy were observed after the four-injection protocols but not the one-injection protocol. For verification of gene inactivation following one injection of tamoxifen, this protocol was applied to MCM(+/0)/Serca2(fl/fl). Serca2 mRNA levels and protein abundance followed the same pattern of decline with one and four tamoxifen injections. The presence of the MCM transgene induced major alterations of gene expression while administration of tamoxifen induced additional but less gene regulation. Thus nonfloxed MCM(+/0) should be considered as controls for mice that carry both a floxed gene of interest and the MCM transgene. One single tamoxifen injection administered to MCM(+/0)/Serca2(fl/fl) was sufficient for target gene inactivation, without acute cardiomyopathy, allowing acute studies subsequent to gene inactivation.

Circulating cytokine levels in mice with heart failure are etiology dependent

Objectives: The aim of this study was to examine whether alterations in circulating cytokine levels are dependent on the etiology of myocardial hypertrophy and heart failure (HF). Background: Several heart diseases are associated with altered levels of circulating cytokines. Cytokines are regarded as possible therapeutic targets or biomarkers, but such approaches are currently not in clinical use. If alterations in circulating cytokines are etiology dependent, this should be taken into consideration when using cytokines as disease markers and therapeutic targets. Methods: The serum levels of 25 cytokines were quantified with Luminex and/or ELISA in four murine models of heart disease: banding of the ascending aorta (AB) or the pulmonary artery (PB), myocardial infarction (MI), and a cardiomyopathy model with inducible cardiomyocyte-specific knockout of the sarco(endo)plasmatic reticulum Ca2+-ATPase (SERCA2KO). Results: No increase in circulating cytokine levels were found in mice 1 wk after AB, although substantial myocardial hypertrophy was present. After 1 wk of MI, only interleukin (IL)-18 was increased. In the SERCA2KO mice with HF, circulating levels of IL-1α, IL-2, IL-3, IL-6, IL-9, IL-10, IL-12p40, eotaxin, granulocyte-colony stimulating factor (G-CSF), interferon-γ, monocyte chemoattractant protein-1, macrophage inflammatory protein-1β were increased, and in mice with PB, IL-1α, IL-6, G-CSF, and monokine induced by gamma-interferon showed elevated levels. Conclusions: Serum levels of cytokines in mice with HF vary depending on the etiology. Increased serum levels of several cytokines were found in models with increased right ventricular afterload, suggesting that the cytokine responses result primarily from systemic congestion.

High-intensity exercise training in mice with cardiomyocyte-specific disruption of Serca2

Several lines of evidence indicate that the sarco(endo)plasmic reticulum ATPase type 2 (SERCA2) is essential for maintaining myocardial calcium handling and cardiac pump function. Hence, a reduction in SERCA2 abundance is expected to reduce work performance and maximal oxygen uptake (V̇o2max) and to limit the response to exercise training. To test this hypothesis, we compared V̇o2max and exercise capacity in mice with cardiac disruption of Serca2 (SERCA2 KO) with control mice (SERCA2 FF). We also determined whether the effects on V̇o2max and exercise capacity could be modified by high-intensity aerobic exercise training. Treadmill running at 85–90% of V̇o2max started 2 wk after Serca2 gene disruption and continued for 4 wk. V̇o2max and maximal running speed were measured weekly in a metabolic chamber. Cardiac function was assessed by echocardiography during light anesthesia. In sedentary SERCA2 KO mice, the aerobic capacity was reduced by 50% and running speed by 28%, whereas trained SERCA2 KO mice were able to maintain maximal running speed despite a 36% decrease in V̇o2max. In SERCA2 FF mice, both V̇o2max and maximal running speed increased by training, while no changes occurred in the sedentary group. Left ventricle dimensions remained unchanged by training in both genotypes. In contrast, training induced right ventricle hypertrophy in SERCA2 KO mice. In conclusion, the SERCA2 protein is essential for sustaining cardiac pump function and exercise capacity. Nevertheless, SERCA2 KO mice were able to maintain maximal running speed in response to exercise training despite a large decrease in V̇o2max.

Genetic background affects function and intracellular calcium regulation of mouse hearts

AIMS

The genetic background is currently under close scrutiny when determining cardiovascular disease progression and response to therapy. However, this factor is rarely considered in physiological studies, where it could influence the normal behaviour and adaptive responses of the heart. We aim to test the hypothesis that genetic strain variability is associated with differences in excitation-contraction coupling mechanisms, in particular those involved in cytoplasmic Ca(2+) regulation, and that they are concomitant to differences in whole-heart function and cell morphology.

METHODS AND RESULTS

We studied 8- to 10-week-old male C57BL/6, BALB/C, FVB, and SV129 mice. Echocardiography and radiotelemetry were used to assess cardiac function in vivo. FVB mice had increased left ventricular ejection fraction and fractional shortening with significantly faster heart rate (HR) and lack of diurnal variation of HR. Confocal microscopy, sarcomere length tracking, and epifluorescence were used to investigate cell volume, t-tubule density, contractility, and Ca(2+) handling in isolated ventricular myocytes. Sarcomere relaxation and time-to-peak of the Ca(2+) transient were prolonged in BALB/C myocytes, with more frequent Ca(2+) sparks and significantly higher sarcoplasmic reticulum (SR) Ca(2+) leak. There were no strain differences in the contribution of different Ca(2+) extrusion mechanisms. SV129 had reduced SR Ca(2+) leak with elevated SR Ca(2+) content and smaller cell volume and t-tubule density compared with myocytes from other strains.

CONCLUSION

These results demonstrate that a different genetic background is associated with physiological differences in cardiac function in vivo and differences in morphology, contractility, and Ca(2+) handling at the cellular level.

There goes the neighborhood: pathological alterations in T-tubule morphology and consequences for cardiomyocyte Ca2+ handling

T-tubules are invaginations of the cardiomyocyte membrane into the cell interior which form a tortuous network. T-tubules provide proximity between the electrically excitable cell membrane and the sarcoplasmic reticulum, the main intracellular Ca²⁺ store. Tight coupling between the rapidly spreading action potential and Ca²⁺ release units in the SR membrane ensures synchronous Ca²⁺ release throughout the cardiomyocyte. This is a requirement for rapid and powerful contraction. In recent years, it has become clear that T-tubule structure and composition are altered in several pathological states which may importantly contribute to contractile defects in these conditions. In this review, we describe the “neighborhood” of proteins in the dyadic cleft which locally controls cardiomyocyte Ca²⁺ homeostasis and how alterations in T-tubule structure and composition may alter this neighborhood during heart failure, atrial fibrillation, and diabetic cardiomyopathy. Based on this evidence, we propose that T-tubules have the potential to serve as novel therapeutic targets.

Separate mechanisms cause anemia in ischemic vs. nonischemic murine heart failure

In ischemic congestive heart failure (CHF), anemia is associated with poor prognosis. Whether anemia develops in nonischemic CHF is uncertain. The hematopoietic inhibitors TNF-α and nitric oxide (NO) are activated in ischemic CHF. We examined whether mice with ischemic or nonischemic CHF develop anemia and whether TNF-α and NO are involved. We studied mice ( n = 7–9 per group) with CHF either due to myocardial infarction (MI) or to overexpression of the Ca2+-binding protein calsequestrin (CSQ) or to induced cardiac disruption of the sarcoplasmic reticulum Ca2+-ATPase 2 gene (SERCA2 KO). Hematopoiesis was analyzed by colony formation of CD34+bone marrow cells. Hemoglobin concentration was 14.0 ± 0.4 g/dl (mean ± SD) in controls, while it was decreased to 10.1 ± 0.4, 9.7 ± 0.4, and 9.6 ± 0.3 g/dl in MI, CSQ, and SERCA2 KO, respectively ( P < 0.05). Colony numbers per 100,000 CD34+cells in the three CHF groups were reduced to 33 ± 3 (MI), 34 ± 3 (CSQ), and 39 ± 3 (SERCA2 KO) compared with 68 ± 4 in controls ( P < 0.05). Plasma TNF-α nearly doubled in MI, and addition of anti-TNF-α antibody normalized colony formation. Inhibition of colony formation was completely abolished with blockade of endothelial NO synthase in CSQ and SERCA2 KO, but not in MI. In conclusion, the mechanism of anemia in CHF depends on the etiology of cardiac disease; whereas TNF-α impairs hematopoiesis in CHF following MI, NO inhibits blood cell formation in nonischemic murine CHF.

Reduced SERCA2 abundance decreases the propensity for Ca2+ wave development in ventricular myocytes

AIMS

To describe the overall role of reduced sarcoplasmic reticulum Ca(2+) ATPase (SERCA2) for Ca(2+) wave development.

METHODS AND RESULTS

SERCA2 knockout [Serca2(flox/flox) Tg(alphaMHC-MerCreMer); KO] mice allowing inducible cardiomyocyte-specific disruption of the Serca2 gene in adult mice were compared with Serca(flox/flox) (FF) control mice. Six days after Serca2 gene disruption, SERCA2 protein abundance was reduced by 53% in KO compared with FF, whereas SERCA2 activity in field-stimulated, Fluo-5F AM-loaded cells was reduced by 42%. Baseline Ca(2+) content of the sarcoplasmic reticulum (SR) and Ca(2+) transient amplitude and rate constant of decay measured in whole-cell voltage-clamped cells were decreased in KO to 75, 81, and 69% of FF values. Ca(2+) waves developed in only 31% of KO cardiomyocytes compared with 57% of FF when external Ca(2+) was raised (10 mM), although SR Ca(2+) content needed for waves to develop was 79% of FF values. In addition, waves propagated at a 15% lower velocity in KO cells. Ventricular extrasystoles (VES) occurred with lower frequency in SERCA2 KO mice (KO: 3 +/- 1 VES/h vs. FF: 8 +/- 1 VES/h) (P < 0.05 for all results).

CONCLUSION

Reduced SERCA2 abundance resulted in decreased amplitude and decay rate of Ca(2+) transients, reduced SR Ca(2+) content, and decreased propensity for Ca(2+) wave development.

Sodium accumulation promotes diastolic dysfunction in end-stage heart failure following Serca2 knockout

Alterations in trans-sarcolemmal and sarcoplasmic reticulum (SR) Ca(2+) fluxes may contribute to impaired cardiomyocyte contraction and relaxation in heart failure. We investigated the mechanisms underlying heart failure progression in mice with conditional, cardiomyocyte-specific excision of the SR Ca(2+)-ATPase (SERCA) gene. At 4 weeks following gene deletion (4-week KO) cardiac function remained near normal values. However, end-stage heart failure developed by 7 weeks (7-week KO) as systolic and diastolic performance declined. Contractions in isolated myocytes were reduced between 4- and 7-week KO, and relaxation was slowed. Ca(2+) transients were similarly altered. Reduction in Ca(2+) transient magnitude resulted from complete loss of SR Ca(2+) release between 4- and 7-week KO, due to loss of a small remaining pool of SERCA2. Declining SR Ca(2+) release was partly offset by increased L-type Ca(2+) current, which was facilitated by AP prolongation in 7-week KO. Ca(2+) entry via reverse-mode Na(+)-Ca(2+) exchange (NCX) was also enhanced. Up-regulation of NCX and plasma membrane Ca(2+)-ATPase increased Ca(2+) extrusion rates in 4-week KO. Diastolic dysfunction in 7-week KO resulted from further SERCA2 loss, but also impaired NCX-mediated Ca(2+) extrusion following Na(+) accumulation. Reduced Na(+)-K(+)-ATPase activity contributed to the Na(+) gain. Normalizing [Na(+)] by dialysis increased the Ca(2+) decline rate in 7-week KO beyond 4-week values. Thus, while SERCA2 loss promotes both systolic and diastolic dysfunction, Na(+) accumulation additionally impairs relaxation in this model. Our observations indicate that if cytosolic Na(+) gain is prevented, up-regulated Ca(2+) extrusion mechanisms can maintain near-normal diastolic function in the absence of SERCA2.

Tamoxifen administration routes and dosage for inducible Cre-mediated gene disruption in mouse hearts

Tissue-specific and time-dependent control of in vivo gene disruption may be achieved using conditional knockout strategies in transgenic mice. Fusion of mutant estrogen receptor ligand-binding domains to Cre recombinase (Cre-ER(T), MerCreMer) combined with cardiac-directed gene expression has been used to generate several cardiac-specific tamoxifen-inducible Cre-expressing mouse lines. Such mice have successfully been used to generate Cre-loxP-mediated gene disruption in an inducible manner in the myocardium in vivo. However, information is sparse regarding the tamoxifen dosage, the time course of gene disruption and whether different administration routes differ in efficiency in obtaining gene disruption in the myocardium. We have evaluated these parameters in Serca2 ( flox/flox ) Tg(alphaMHC-MerCreMer) transgenic mice (SERCA2 KO). Serca2 mRNA transcript abundance was used as a sensitive indicator of Cre-loxP-dependent gene disruption in the myocardium. We found that 2 i.p. injections of tamoxifen in oil (1 mg/day, approximate total dose 80 mg/kg) was sufficient for efficient gene disruption with maximal reduction of Serca2 mRNA as early as 4 days after tamoxifen induction. Moreover, a simple protocol using tamoxifen-supplemented non-pelleted dry feed p.o. was comparable to i.p. injections in inducing gene disruption. These improvements may significantly improve animal welfare and reduce the workload in the production of cardiac conditional knockout mice.

Factors controlling the activity of the SERCA2a pump in the normal and failing heart

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

Mice carrying a conditional Serca2(flox) allele for the generation of Ca(2+) handling-deficient mouse models

Sarco(endo)plasmic reticulum calcium ATPases (SERCA) are cellular pumps that transport Ca(2+) into the sarcoplasmic reticulum (SR). Serca2 is the most widely expressed gene family member. The very early embryonic lethality of Serca2(null) mouse embryos has precluded further evaluation of loss of Serca2 function in the context of organ physiology. We have generated mice carrying a conditional Serca2(flox) allele which allows disruption of the Serca2 gene in an organ-specific and/or inducible manner. The model was tested by mating Serca2(flox) mice with MLC-2v(wt/Cre) mice and with alphaMHC-Cre transgenic mice. In heterozygous Serca2(wt/flox)MLC-2v(wt/Cre) mice, the expression of SERCA2a and SERCA2b proteins were reduced in the heart and slow skeletal muscle, in accordance with the expression pattern of the MLC-2v gene. In Serca2(flox/flox) Tg(alphaMHC-Cre) embryos with early homozygous cardiac Serca2 disruption, normal embryonic development and yolk sac circulation was maintained up to at least embryonic stage E10.5. The Serca2(flox) mouse is the first murine conditional gene disruption model for the SERCA family of Ca(2+) ATPases, and should be a powerful tool for investigating specific physiological roles of SERCA2 function in a range of tissues and organs in vivo both in adult and embryonic stages.