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

Ginsenoside Rb1 ameliorates heart failure through DUSP-1-TMBIM-6-mediated mitochondrial quality control and gut flora interactions

BACKGROUND

There is currently no specific therapeutic drug available for heart failure in clinical practice. Numerous studies have validated the efficacy of Ginsenoside Rb1, an active component found in various herbal remedies used for heart failure treatment, in effectively ameliorating myocardial ischemia. However, the precise mechanism of action and molecular targets of Ginsenoside Rb1 remain unclear.

PURPOSE

This study aims to explore the molecular mechanisms through which Ginsenoside Rb1 synergistically modulates the gut flora and mitochondrial quality control network in heart failure by targeting the DUSP-1-TMBIM-6-VDAC1 axis.

STUDY DESIGN

This study utilized DUSP-1/VDAC1 knockout (DUSP-1-/-/VDAC1-/-) and DUSP-1/VDAC1 transgenic (DUSP-1+/+/VDAC1+/+) mouse models of heart failure, established through Transverse Aortic Constriction (TAC) surgery and genetic modification techniques. The mice were subsequently subjected to treatment with Ginsenoside Rb1.

METHODS

A series of follow-up multi-omics analyses were conducted, including assessments of intestinal flora, gene transcription sequencing, single-cell databases, and molecular biology assays of primary cardiomyocytes, to investigate the mechanism of action of Ginsenoside Rb1.

RESULTS

Ginsenoside Rb1 was found to have multiple regulatory mechanisms on mitochondria. Notably, DUSP-1 was discovered to be a crucial molecular target of Ginsenoside Rb1, controlling both intestinal flora and mitochondrial function. The regulatory effects of DUSP-1 on inflammation and mitochondrial quality control were mediated by changes in TMBIM-6 and VDAC1. Furthermore, NLRP3-mediated inflammatory responses were found to interact with mitochondrial quality control, exacerbating myocardial injury under stress conditions. Ginsenoside Rb1 modulated the DUSP-1-TMBIM-6-VDAC1 axis, inhibited the release of pro-inflammatory factors, altered the structural composition of the gut flora, and protected impaired heart function. These effects indirectly influenced the crosstalk between inflammation, mitochondria, and gut flora.

CONCLUSION

The DUSP-1-TMBIM-6-VDAC1 axis, an upstream pathway regulated by Ginsenoside Rb1, is a profound mechanism through which Ginsenoside Rb1 improves cardiac function in heart failure by modulating inflammation, mitochondria, and gut flora.

Early heart development: examining the dynamics of function-form emergence

During early embryonic development, the heart undergoes a remarkable and complex transformation, acquiring its iconic four-chamber structure whilst concomitantly contracting to maintain its essential function. The emergence of cardiac form and function involves intricate interplays between molecular, cellular, and biomechanical events, unfolding with precision in both space and time. The dynamic morphological remodelling of the developing heart renders it particularly vulnerable to congenital defects, with heart malformations being the most common type of congenital birth defect (∼35% of all congenital birth defects). This mini-review aims to give an overview of the morphogenetic processes which govern early heart formation as well as the dynamics and mechanisms of early cardiac function. Moreover, we aim to highlight some of the interplay between these two processes and discuss how recent findings and emerging techniques/models offer promising avenues for future exploration. In summary, the developing heart is an exciting model to gain fundamental insight into the dynamic relationship between form and function, which will augment our understanding of cardiac congenital defects and provide a blueprint for potential therapeutic strategies to treat disease.

Myofilament-based physiological regulatory compensation preserves diastolic function in failing hearts with severe Ca2+ handling deficits

Severe dysfunction in cardiac muscle intracellular Ca2+ handling is a common pathway underlying heart failure. Here we used an inducible genetic model of severe Ca2+ cycling dysfunction by the targeted temporal gene ablation of the cardiac Ca2+ ATPase, SERCA2, in otherwise normal adult mice. In this model, in vivo heart performance was minimally affected initially, even though Serca2a protein was markedly reduced. The mechanism underlying the sustained in vivo heart performance in the weeks prior to complete heart pump failure and death is not clear and is important to understand. Studies were primarily focused on understanding how in vivo diastolic function could be relatively normal under conditions of marked Serca2a deficiency. Interestingly, data show increased cardiac troponin I (cTnI) serine 23/24 phosphorylation content in hearts upon Serca2a ablation in vivo. We report that hearts isolated from the Serca2-deficient mice retained near normal heart pump functional responses to β-adrenergic stimulation. Unexpectedly, using genetic complementation models, in concert with inducible Serca2 ablation, data show that Serca2a-deficient hearts that also lacked the central β-adrenergic signaling-dependent Serca2a negative regulator, phospholamban (PLN), had severe diastolic dysfunction that could still be corrected by β-adrenergic stimulation. Notably, integrating a serines 23/24-to-alanine PKA-refractory sarcomere incorporated cTnI molecular switch complex in mice deficient in Serca2 showed blunting of β-adrenergic stimulation-mediated enhanced diastolic heart performance. Taken together, these data provide evidence of a compensatory regulatory role of the myofilaments as a critical physiological bridging mechanism to aid in preserving heart diastolic performance in failing hearts with severe Ca2+ handling deficits.

SENP2-mediated SERCA2a deSUMOylation increases calcium overload in cardiomyocytes to aggravate myocardial ischemia/reperfusion injury

BACKGROUND

Sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) is a key protein that maintains myocardial Ca 2+ homeostasis. The present study aimed to investigate the mechanism underlying the SERCA2a-SUMOylation (small ubiquitin-like modifier) process after ischemia/reperfusion injury (I/RI) in vitro and in vivo .

METHODS

Calcium transient and systolic/diastolic function of cardiomyocytes isolated from Serca2a knockout (KO) and wild-type mice with I/RI were compared. SUMO-relevant protein expression and localization were detected by quantitative real-time PCR (RT-qPCR), Western blotting, and immunofluorescence in vitro and in vivo . Serca2a-SUMOylation, infarct size, and cardiac function of Senp1 or Senp2 overexpressed/suppressed adenovirus infected cardiomyocytes, were detected by immunoprecipitation, triphenyltetrazolium chloride (TTC)-Evans blue staining, and echocardiography respectively.

RESULTS

The results showed that the changes of Fura-2 fluorescence intensity and contraction amplitude of cardiomyocytes decreased in the I/RI groups and were further reduced in the Serca2a KO + I/RI groups. Senp1 and Senp2 messenger ribose nucleic acid (mRNA) and protein expression levels in vivo and in cardiomyocytes were highest at 6 h and declined at 12 h after I/RI. However, the highest levels in HL-1 cells were recorded at 12 h. Senp2 expression increased in the cytoplasm, unlike that of Senp1. Inhibition of Senp2 protein reversed the I/RI-induced Serca2a-SUMOylation decline, reduced the infarction area, and improved cardiac function, while inhibition of Senp1 protein could not restore the above indicators.

CONCLUSION

I/RI activated Senp1 and Senp2 protein expression, which promoted Serca2a-deSUMOylation, while inhibition of Senp2 expression reversed Serca2a-SUMOylation and improved cardiac function.

SGLT2 inhibitor dapagliflozin reduces endothelial dysfunction and microvascular damage during cardiac ischemia/reperfusion injury through normalizing the XO-SERCA2-CaMKII-coffilin pathways

Background: Given the importance of microvascular injury in infarct formation and expansion, development of therapeutic strategies for microvascular protection against myocardial ischemia/reperfusion injury (IRI) is of great interest. Here, we explored the molecular mechanisms underlying the protective effects of the SGLT2 inhibitor dapagliflozin (DAPA) against cardiac microvascular dysfunction mediated by IRI. Methods: DAPA effects were evaluated both in vivo, in mice subjected to IRI, and in vitro, in human coronary artery endothelial cells (HCAECs) exposed to hypoxia/reoxygenation (H/R). DAPA pretreatment attenuated luminal stenosis, endothelial swelling, and inflammation in cardiac microvessels of IRI-treated mice. Results: In H/R-challenged HCAECs, DAPA treatment improved endothelial barrier function, endothelial nitric oxide synthase (eNOS) activity, and angiogenic capacity, and inhibited H/R-induced apoptosis by preventing cofilin-dependent F-actin depolymerization and cytoskeletal degradation. Inhibition of H/R-induced xanthine oxidase (XO) activation and upregulation, sarco(endo)plasmic reticulum calcium-ATPase 2 (SERCA2) oxidation and inactivation, and cytoplasmic calcium overload was further observed in DAPA-treated HCAECs. DAPA also suppressed calcium/Calmodulin (CaM)-dependent kinase II (CaMKII) activation and cofilin phosphorylation, and preserved cytoskeleton integrity and endothelial cell viability following H/R. Importantly, the beneficial effects of DAPA on cardiac microvascular integrity and endothelial cell survival were largely prevented in IRI-treated SERCA2-knockout mice. Conclusions: These results indicate that DAPA effectively reduces cardiac microvascular damage and endothelial dysfunction during IRI through inhibition of the XO-SERCA2-CaMKII-cofilin pathway.

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Brain-specific heterozygous loss-of-function of ATP2A2, endoplasmic reticulum Ca2+ pump responsible for Darier's disease, causes behavioral abnormalities and a hyper-dopaminergic state

A report of a family of Darier's disease with mood disorders drew attention when the causative gene was identified as ATP2A2 (or SERCA2), which encodes a Ca2+ pump on the endoplasmic reticulum (ER) membrane and is important for intracellular Ca2+ signaling. Recently, it was found that loss-of-function mutations of ATP2A2 confer a risk of neuropsychiatric disorders including depression, bipolar disorder and schizophrenia. In addition, a genome-wide association study found an association between ATP2A2 and schizophrenia. However, the mechanism of how ATP2A2 contributes to vulnerability to these mental disorders is unknown. Here, we analyzed Atp2a2 heterozygous brain-specific conditional knockout (hetero cKO) mice. The ER membranes prepared from the hetero cKO mouse brain showed decreased Ca2+ uptake activity. In Atp2a2 heterozygous neurons, decays of cytosolic Ca2+ level were slower than control neurons after depolarization. The hetero cKO mice showed altered behavioral responses to novel environments and impairments in fear memory, suggestive of enhanced dopamine signaling. In vivo dialysis demonstrated that extracellular dopamine levels in the NAc were indeed higher in the hetero cKO mice. These results altogether indicate that the haploinsufficiency of Atp2a2 in the brain causes prolonged cytosolic Ca2+ transients, which possibly results in enhanced dopamine signaling, a common feature of mood disorders and schizophrenia. These findings elucidate how ATP2A2 mutations causing a dermatological disease may exert their pleiotropic effects on the brain and confer a risk for mental disorders.

The First Heartbeat-Origin of Cardiac Contractile Activity

The amniote embryonic heart starts as a crescent of mesoderm that transitions through a midline linear heart tube in the course of developing into the four chambered heart. It is unusual in having to contract rhythmically while still undergoing extensive morphogenetic remodeling. Advances in imaging have allowed us to determine when during development this contractile activity starts. In the mouse, focal regions of contractions can be detected as early as the cardiac crescent stage. Calcium transients, required to trigger contraction, can be detected even earlier, prior to contraction. In this review, we outline what is currently known about how this early contractile function is initiated and the impact early contractile function has on cardiac development.

Sarcoplasmic reticulum Ca2+ -induced Ca2+ release regulates class IIa HDAC localization in mouse embryonic cardiomyocytes

In embryonic cardiomyocytes, sarcoplasmic reticulum (SR)‐derived Ca²⁺ release is required to induce Ca²⁺ oscillations for contraction and to control cardiac development through Ca²⁺‐activated pathways. Here, our aim was to study how SR Ca²⁺ release regulates cytosolic and nuclear Ca²⁺ distribution and the subsequent effects on the Ca²⁺‐dependent localization of class IIa histone deacetylases (HDAC) and cardiac‐specific gene expression in embryonic cardiomyocytes. Confocal microscopy was used to study changes in Ca²⁺‐distribution and localization of immunolabeled HDAC4 and HDAC5 upon changes in SR Ca²⁺ release in mouse embryonic cardiomyocytes. Dynamics of translocation were also observed with a confocal microscope, using HDAC5‐green fluorescent protein transfected myocytes. Expression of class IIa HDACs in differentiating myocytes and changes in cardiac‐specific gene expression were studied using real‐time quantitative PCR. Inhibition of SR Ca²⁺ release caused a significant decrease in intranuclear Ca²⁺ concentration, a rapid nuclear import of HDAC5 and subnuclear redistribution of HDAC4. Endogenous localization of HDAC5 and HDAC4 was mostly cytosolic and at the nuclear periphery, respectively. Downregulated expression of cardiac‐specific genes was also observed upon SR Ca²⁺ release inhibition. Electrical stimulation of sarcolemmal Ca²⁺ influx was not sufficient to rescue either the HDAC localization or the gene expression changes. SR Ca²⁺ release controls subcellular Ca²⁺ distribution and regulates localization of HDAC4 and HDAC5 in embryonic cardiomyocytes. Changes in SR Ca²⁺ release also caused changes in expression of the developmental phase‐specific genes, which may be due to the changes in HDAC‐localization.

A role for calcium in resistin transcriptional activation in diabetic hearts

The adipokine resistin has been proposed to link obesity, insulin resistance and diabetes. We have previously reported that diabetic hearts express high levels of resistin while overexpression of resistin in adult rat hearts gives rise to a phenotype resembling diabetic cardiomyopathy. The transcriptional regulation of resistin in diabetic cardiac tissue is currently unknown. This study investigated the mechanism of resistin upregulation and the role of Serca2a in its transcriptional suppression. We demonstrate that restoration of Ca²⁺ homeostasis in diabetic hearts, through normalization of Serca2a function genetically and pharmacologically, suppressed resistin expression via inhibition of NFATc. H9c2 myocytes stimulated with high-glucose concentration or Ca²⁺ time-dependently increased NFATc and resistin expression while addition of the Ca²⁺ chelator BAPTA-AM attenuated this effect. NFATc expression was enhanced in hearts from ob/ob diabetic and from cardiac-specific Serca2a^−/− mice. Similarly, NFATc increased resistin expression in myocytes cultured in low glucose while the NFATc inhibitor VIVIT blocked glucose-induced resistin expression, suggesting that hyperglycemia/diabetes induces resistin expression possibly through NFATc activation. Interestingly, overexpression of Serca2a or VIVIT mitigated glucose-stimulated resistin and NFATc expression and enhanced AMPK activity, a downstream target of resistin signaling. NFATc direct activation of resistin was verified by resistin promoter luciferase activity and chromatin-immunoprecipitation analysis. Interestingly, activation of Serca2a by a novel agonist, CDN1163, mirrored the effects of AAV9-Serca2a gene transfer on resistin expression and its promoter activity and AMPK signaling in diabetic mice. These findings parse a role for Ca²⁺ in resistin transactivation and provide support that manipulation of Serca2a-NFATc-Resistin axis might be useful in hyper-resistinemic conditions.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2

Sarco(endo)plasmic reticulum Ca2+ -ATPase (SERCA)2 transports Ca2+ from the cytosol into the sarcoplasmic reticulum of cardiomyocytes and is essential for maintaining myocardial Ca2+ handling and thus the mechanical function of the heart. SERCA2 is a major ATP consumer in excitation-contraction coupling but is regarded to contribute to energetically efficient Ca2+ handling in the cardiomyocyte. Previous studies using cardiomyocyte-specific SERCA2 knockout (KO) mice have demonstrated that decreased SERCA2 activity reduces the Ca2+ transient amplitude and induces compensatory Ca2+ transport mechanisms that may lead to more inefficient Ca2+ transport. In this study, we examined the relationship between left ventricular (LV) function and myocardial O2 consumption (MVo2) in ex vivo hearts from SERCA2 KO mice to directly measure how SERCA2 elimination influences mechanical and energetic features of the heart. Ex vivo hearts from SERCA2 KO hearts developed mechanical dysfunction at 4 wk and demonstrated virtually no working capacity at 7 wk. In accordance with the reported reduction in Ca2+ transient amplitude in cardiomyocytes from SERCA2 KO mice, work-independent MVo2 was decreased due to a reduced energy cost of excitation-contraction coupling. As these hearts also showed a marked impairment in the efficiency of chemomechanical energy transduction (contractile efficiency, i.e, work-dependent MVo2), hearts from SERCA2 KO mice were found to be mechanically inefficient. This ex vivo evaluation of mechanical and energetic function in hearts from SERCA2 KO mice brings together findings from previous experimental and mathematical modeling-based studies and demonstrates that reduced SERCA2 activity not only leads to mechanical dysfunction but also to energetic dysfunction.

Integrating multi-scale data to create a virtual physiological mouse heart

While the virtual physiological human (VPH) project has made great advances in human modelling, many of the tools and insights developed as part of this initiative are also applicable for facilitating mechanistic understanding of the physiology of a range of other species. This process, in turn, has the potential to provide human relevant insights via a different scientific path. Specifically, the increasing use of mice in experimental research, not yet fully complemented by a similar increase in computational modelling, is currently missing an important opportunity for using and interpreting this growing body of experimental data to improve our understanding of cardiac function. This overview describes our work to address this issue by creating a virtual physiological mouse model of the heart. We describe the similarities between human- and mouse-focused modelling, including the reuse of VPH tools, and the development of methods for investigating parameter sensitivity that are applicable across species. We show how previous results using this approach have already provided important biological insights, and how these can also be used to advance VPH heart models. Finally, we show an example application of this approach to test competing multi-scale hypotheses by investigating variations in length-dependent properties of cardiac muscle.

Prominent heart organ-level performance deficits in a genetic model of targeted severe and progressive SERCA2 deficiency

The cardiac SERCA2 Ca²⁺ pump is critical for maintaining normal Ca²⁺ handling in the heart. Reduced SERCA2a content and blunted Ca²⁺ reuptake are frequently observed in failing hearts and evidence implicates poor cardiac Ca²⁺ handling in the progression of heart failure. To gain insight into mechanism we investigated a novel genetic mouse model of inducible severe and progressive SERCA2 deficiency (inducible Serca2 knockout, SERCA2 KO). These mice eventually die from overt heart failure 7-10 weeks after knockout but as yet there have been no reports on intrinsic mechanical performance at the isolated whole heart organ level. Thus we studied whole-organ ex vivo function of hearts isolated from SERCA2 KO mice at one and four weeks post-knockout in adult animals. We found that isolated KO heart function was only modestly impaired one week post-knockout, when SERCA2a protein was 32% of normal. At four weeks post-knockout, function was severely impaired with near non-detectable levels of SERCA2. During perfusion with 10 mM caffeine, LV developed pressures were similar between 4-week KO and control hearts, and end-diastolic pressures were lower in KO. When hearts were subjected to ischemia-reperfusion injury, recovery was not different between control and KO hearts at either one or four weeks post-knockout. Our findings indicate that ex vivo function of isolated SERCA2 KO hearts is severely impaired long before symptoms appear in vivo, suggesting that physiologically relevant heart function in vivo can be sustained for weeks in the absence of robust SR Ca²⁺ flux.

Synchrony of cardiomyocyte Ca(2+) release is controlled by T-tubule organization, SR Ca(2+) content, and ryanodine receptor Ca(2+) sensitivity

Recent work has demonstrated that cardiomyocyte Ca(2+)release is desynchronized in several pathological conditions. Loss of Ca(2+) release synchrony has been attributed to t-tubule disruption, but it is unknown if other factors also contribute. We investigated this issue in normal and failing myocytes by integrating experimental data with a mathematical model describing spatiotemporal dynamics of Ca(2+) in the cytosol and sarcoplasmic reticulum (SR). Heart failure development in postinfarction mice was associated with progressive t-tubule disorganization, as quantified by fast-Fourier transforms. Data from fast-Fourier transforms were then incorporated in the model as a dyadic organization index, reflecting the proportion of ryanodine receptors located in dyads. With decreasing dyadic-organization index, the model predicted greater dyssynchrony of Ca(2+) release, which exceeded that observed in experimental line-scan images. Model and experiment were reconciled by reducing the threshold for Ca(2+) release in the model, suggesting that increased RyR sensitivity partially offsets the desynchronizing effects of t-tubule disruption in heart failure. Reducing the magnitude of SR Ca(2+) content and release, whether experimentally by thapsigargin treatment, or in the model, desynchronized the Ca(2+) transient. However, in cardiomyocytes isolated from SERCA2 knockout mice, RyR sensitization offset such effects. A similar interplay between RyR sensitivity and SR content was observed during treatment of myocytes with low-dose caffeine. Initial synchronization of Ca(2+) release during caffeine was reversed as SR content declined due to enhanced RyR leak. Thus, synchrony of cardiomyocyte Ca(2+) release is not only determined by t-tubule organization but also by the interplay between RyR sensitivity and SR Ca(2+) content.

Long-term levosimendan treatment improves systolic function and myocardial relaxation in mice with cardiomyocyte-specific disruption of the Serca2 gene

In human heart failure (HF), reduced cardiac function has, at least partly, been ascribed to altered calcium homeostasis in cardiomyocytes. The effects of the calcium sensitizer levosimendan on diastolic dysfunction caused by reduced removal of calcium from cytosol in early diastole are not well known. In this study, we investigated the effect of long-term levosimendan treatment in a murine model of HF where the sarco(endo)plasmatic reticulum ATPase (Serca) gene is specifically disrupted in the cardiomyocytes, leading to reduced removal of cytosolic calcium. After induction of Serca2 gene disruption, these mice develop marked diastolic dysfunction as well as impaired contractility. SERCA2 knockout (SERCA2KO) mice were treated with levosimendan or vehicle from the time of KO induction. At the 7-wk end point, cardiac function was assessed by echocardiography and pressure measurements. Vehicle-treated SERCA2KO mice showed significantly diminished left-ventricular (LV) contractility, as shown by decreased ejection fraction, stroke volume, and cardiac output. LV pressure measurements revealed a marked increase in the time constant (τ) of isovolumetric pressure decay, showing impaired relaxation. Levosimendan treatment significantly improved all three systolic parameters. Moreover, a significant reduction in τ toward normalization indicated improved relaxation. Gene-expression analysis, however, revealed an increase in genes related to production of the ECM in animals treated with levosimendan. In conclusion, long-term levosimendan treatment improves both contractility and relaxation in a heart-failure model with marked diastolic dysfunction due to reduced calcium transients. However, altered gene expression related to fibrosis was observed.

Beta-adrenergic stimulation maintains cardiac function in Serca2 knockout mice

Previous studies on Serca2 knockout (KO) mice showed that cardiac function is sustained in vivo for several weeks after knockout, whereas SERCA protein levels decrease and calcium dynamics are significantly impaired. In this study, we reconcile observed cellular and organ level contractile function using a cardiac multiscale model. We identified and quantified the changes in cellular function that are both consistent with observations and able to compensate for the decrease in SERCA. Calcium transients were used as input for multiscale computational simulations to predict whole-organ response. Although this response matched experimental pressure-volume (PV) measurements in healthy mice, the reduced magnitude calcium transients observed in KO cells were insufficient to trigger ventricular ejection. To replicate the effects of elevated catecholamine levels observed in vivo, cells were treated with isoproterenol. Incorporation of the resulting measured β-adrenergically stimulated calcium transients into the model resulted in a close match with experimental PV loops. Changes in myofilament properties, when considered in isolation, were not able to increase tension development to levels consistent with measurements, further confirming the necessity of a high β-adrenergic state. Modeling additionally indicated that increased venous return observed in the KO mice helps maintain a high ejection fraction via the Frank-Starling effect. Our study shows that increased β-adrenergic stimulation is a potentially highly significant compensatory mechanism by which cardiac function is maintained in Serca2 KO mice, producing the increases in both systolic and diastolic calcium, consistent with the observed contractile function observed in experimental PV measurements.

Sodium accumulation in SERCA knockout-induced heart failure

In cardiomyocytes, a major decrease in the level of sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) can severely impair systolic and diastolic functions. In mice with cardiomyocyte-specific conditional excision of the Serca2 gene (SERCA2 KO), end-stage heart failure developed between four and seven weeks after gene deletion combined with [Na(+)](i) elevation and intracellular acidosis. In this study, to investigate the underpinning changes in Ca(2+) dynamics and metabolic homeostasis, we developed data-driven mathematical models of Ca(2+) dynamics in the ventricular myocytes of the control, four-week, and seven-week SERCA2 knockout (KO) mice. The seven-week KO model showed that elevated [Na(+)](i) was due to increased Na(+) influxes through the Na(+)/Ca(2+) exchanger (NCX) and the Na(+)/H(+) exchanger, with the latter exacerbated by intracellular acidosis. Furthermore, NCX upregulation in the seven-week KO model resulted in increased ATP consumption for ion transport. Na(+) accumulation in the SERCA KO due to NCX upregulation and intracellular acidosis potentially play a role in the development of heart failure, by initiating a reinforcing cycle involving: a mismatch between ATP demand and supply; an increasingly compromised metabolism; a decreased pH(i); and, finally, an even greater [Na(+)](i) elevation.

Slowed relaxation and preserved maximal force in soleus muscles of mice with targeted disruption of the Serca2 gene in skeletal muscle

Sarcoplasmic reticulum Ca(2+) ATPases (SERCAs) play a major role in muscle contractility by pumping Ca(2+) from the cytosol into the sarcoplasmic reticulum (SR) Ca(2+) store, allowing muscle relaxation and refilling of the SR with releasable Ca(2+). Decreased SERCA function has been shown to result in impaired muscle function and disease in human and animal models. In this study, we present a new mouse model with targeted disruption of the Serca2 gene in skeletal muscle (skKO) to investigate the functional consequences of reduced SERCA2 expression in skeletal muscle. SkKO mice were viable and basic muscle structure was intact. SERCA2 abundance was reduced in multiple muscles, and by as much as 95% in soleus muscle, having the highest content of slow-twitch fibres (40%). The Ca(2+) uptake rate was significantly reduced in SR vesicles in total homogenates. We did not find any compensatory increase in SERCA1 or SERCA3 abundance, or altered expression of several other Ca(2+)-handling proteins. Ultrastructural analysis revealed generally well-preserved muscle morphology, but a reduced volume of the longitudinal SR. In contracting soleus muscle in vitro preparations, skKO muscles were able to fully relax, but with a significantly slowed relaxation time compared to controls. Surprisingly, the maximal force and contraction rate were preserved, suggesting that skKO slow-twitch fibres may be able to contribute to the total muscle force despite loss of SERCA2 protein. Thus it is possible that SERCA-independent mechanisms can contribute to muscle contractile function.

Exercise training before cardiac-specific Serca2 disruption attenuates the decline in cardiac function in mice

In the heart, function of the sarco(endo)plasmic Ca2+-ATPase (SERCA2) is closely linked to contractility, cardiac function, and aerobic fitness. SERCA2 function can be increased by high-intensity interval training, whereas reduced SERCA2 abundance is associated with impaired cardiac function. The working hypothesis was, therefore, that exercise training before cardiomyocyte-specific disruption of the Serca2 gene would delay the onset of cardiac dysfunction in mice. Before Serca2 gene disruption by tamoxifen, untreated SERCA2 knockout mice ( Serca2flox/flox Tg-αMHC-MerCreMer; S2KO), and SERCA2 FF control mice ( Serca2flox/flox, S2FF) were exercise trained by high-intensity interval treadmill running for 6 wk. Both genotypes responded to training, with comparable increases in maximal oxygen uptake (V̇o2max; 17%), left ventricle weight (15%), and maximal running speed (40%). After exercise training, cardiac-specific Serca2 gene disruption was induced in both exercise trained and sedentary S2KO mice. In trained S2KO, cardiac function decreased less rapidly than in sedentary S2KO. V̇o2max remained higher in trained S2KO the first 15 days after gene disruption. Six weeks after Serca2 disruption, cardiac output was higher in trained compared with sedentary S2KO mice. An exercise-training program attenuates the decline in cardiac performance induced by acute cardiac Serca2 gene disruption, indicating that mechanisms other than SERCA2 contribute to the favorable effect of exercise training.

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.

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.

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.