With great power comes great responsibility: using mouse genetics to study cardiac hypertrophy and failure

The Microenvironment of the Pathogenesis of Cardiac Hypertrophy

Pathological cardiac hypertrophy is a key risk factor for the development of heart failure and predisposes individuals to cardiac arrhythmia and sudden death. While physiological cardiac hypertrophy is adaptive, hypertrophy resulting from conditions comprising hypertension, aortic stenosis, or genetic mutations, such as hypertrophic cardiomyopathy, is maladaptive. Here, we highlight the essential role and reciprocal interactions involving both cardiomyocytes and non-myocardial cells in response to pathological conditions. Prolonged cardiovascular stress causes cardiomyocytes and non-myocardial cells to enter an activated state releasing numerous pro-hypertrophic, pro-fibrotic, and pro-inflammatory mediators such as vasoactive hormones, growth factors, and cytokines, i.e., commencing signaling events that collectively cause cardiac hypertrophy. Fibrotic remodeling is mediated by cardiac fibroblasts as the central players, but also endothelial cells and resident and infiltrating immune cells enhance these processes. Many of these hypertrophic mediators are now being integrated into computational models that provide system-level insights and will help to translate our knowledge into new pharmacological targets. This perspective article summarizes the last decades' advances in cardiac hypertrophy research and discusses the herein-involved complex myocardial microenvironment and signaling components.

The dynamic interplay between cardiac mitochondrial health and myocardial structural remodeling in metabolic heart disease, aging, and heart failure

This review provides a holistic perspective on the bi-directional relationship between cardiac mitochondrial dysfunction and myocardial structural remodeling in the context of metabolic heart disease, natural cardiac aging, and heart failure. First, a review of the physiologic and molecular drivers of cardiac mitochondrial dysfunction across a range of increasingly prevalent conditions such as metabolic syndrome and cardiac aging is presented, followed by a general review of the mechanisms of mitochondrial quality control (QC) in the heart. Several important mechanisms by which cardiac mitochondrial dysfunction triggers or contributes to structural remodeling of the heart are discussed: accumulated metabolic byproducts, oxidative damage, impaired mitochondrial QC, and mitochondrial-mediated cell death identified as substantial mechanistic contributors to cardiac structural remodeling such as hypertrophy and myocardial fibrosis. Subsequently, the less studied but nevertheless important reverse relationship is explored: the mechanisms by which cardiac structural remodeling feeds back to further alter mitochondrial bioenergetic function. We then provide a condensed pathogenesis of several increasingly important clinical conditions in which these relationships are central: diabetic cardiomyopathy, age-associated declines in cardiac function, and the progression to heart failure, with or without preserved ejection fraction. Finally, we identify promising therapeutic opportunities targeting mitochondrial function in these conditions.

Ablation of cardiomyocyte-derived BDNF during development causes myocardial degeneration and heart failure in the adult mouse heart

Objective

Brain-derived neurotrophic factor (BDNF) and its receptor TrkB-T1 were recently found to be expressed in cardiomyocytes. However, the functional role of cardiomyocyte-derived BDNF in heart pathophysiology is not yet fully known. Recent studies revealed that BDNF-TrkB pathway plays a critical role to maintain integrity of cardiac structure and function, cardiac pathology and regeneration of myocardial infarction (MI). Therefore, the BDNF-TrkB pathway may be a novel target for myocardial pathophysiology in the adult heart.

Approach and results

In the present study, we established a cardiomyocyte-derived BDNF conditional knockout mouse in which BDNF expression in developing cardiomyocytes is ablated under the control of the Myosin heavy chain 6 (MYH6) promoter. The results of the present study show that ablation of cardiomyocyte-derived BDNF during development does not impair survival, growth or reproduction; however, in the young adult heart, it causes cardiomyocyte death, degeneration of the myocardium, cardiomyocyte hypertrophy, left atrial appendage thrombosis, decreased cardiac function, increased cardiac inflammation and ROS activity, and metabolic disorders, leading to heart failure (HF) in the adult heart and eventually resulting in a decrease in the one-year survival rate. In addition, ablation of cardiomyocyte-derived BDNF during the developmental stage leads to exacerbation of cardiac dysfunction and poor regeneration after MI in adult hearts.

Conclusion

Cardiomyocyte-derived BDNF is irreplaceable for maintaining the integrity of cardiac structure and function in the adult heart and regeneration after MI. Therefore, the BDNF-TrkB pathway will be a novel target for myocardial pathophysiology in the adult heart.

Piezo2 is not an indispensable mechanosensor in murine cardiomyocytes

A short-term increase in ventricular filling leads to an immediate (Frank-Starling mechanism) and a slower (Anrep effect) rise in cardiac contractility, while long-term increased cardiac load (e.g., in arterial hypertension) decreases contractility. Whether these answers to mechanical tension are mediated by specific sensors in cardiomyocytes remains elusive. In this study, the piezo2 protein was evaluated as a potential mechanosensor. Piezo2 was found to be upregulated in various rat and mouse cardiac tissues upon mechanical or pharmacological stress. To investigate its function, C57BL/6J mice with homozygous cardiomyocyte-specific piezo2 knockout [Piezo2-KO] were created. To this end, α-MHC-Cre mice were crossed with homozygous "floxed" piezo2 mice. α-MHC-Cre mice crossed with wildtype mice served as controls [WT-Cre⁺]. In cardiomyocytes of Piezo2-KO mice, piezo2 mRNA was reduced by > 90% and piezo2 protein was not detectable. Piezo2-KO mice displayed no morphological abnormalities or altered cardiac function under nonstressed conditions. In a subsequent step, hearts of Piezo2-KO or WT-Cre⁺-mice were stressed by either three weeks of increased afterload (angiotensin II, 2.5 mg/kg/day) or one week of hypercontractility (isoprenaline, 30 mg/kg/day). As expected, angiotensin II treatment in WT-Cre⁺-mice resulted in higher heart and lung weight (per body weight, + 38%, + 42%), lower ejection fraction and cardiac output (- 30%, - 39%) and higher left ventricular anterior and posterior wall thickness (+ 34%, + 37%), while isoprenaline led to higher heart weight (per body weight, + 25%) and higher heart rate and cardiac output (+ 24%, + 54%). The Piezo2-KO mice reacted similarly with the exception that the angiotensin II-induced increases in wall thickness were blunted and the isoprenaline-induced increase in cardiac output was slightly less pronounced. As cardiac function was neither severely affected under basal nor under stressed conditions in Piezo2-KO mice, we conclude that piezo2 is not an indispensable mechanosensor in cardiomyocytes.

Generation and characterization of a Myh6-driven Cre knockin mouse line

Gene deletion by the Cre-Loxp system has facilitated functional studies of many critical genes in mice, offering important insights and allowing deeper understanding on the mechanisms underlying organ development and diseases, such as heart development and diseases. In this study, we generated a Myh6-Cre knockin mouse model by inserting the IRES-Cre-wpre-polyA cassette between the translational stop codon and the 3' untranslated region of the endogenous Myh6 gene. By crossing knockin mice with the Rosa26 reporter lines, we found that Myh6-Cre targeted cardiomyocytes at the embryonic and postnatal stages. In addition, we were able to inactivate the desmosome gene Desmoplakin (Dsp) by breeding Myh6-Cre mice with a conditional Dsp^flox knockout mouse line, which resulted in embryonic lethality during the mid-term pregnancy. These results suggest that the new Myh6-Cre mouse line can serve as a robust tool to dissect the distinct roles of genes involved in heart development and function.

The Pitfalls of in vivo Cardiac Physiology in Genetically Modified Mice - Lessons Learnt the Hard Way in the Creatine Kinase System

In order to fully understand gene function, at some point, it is necessary to study the effects in an intact organism. The creation of the first knockout mouse in the late 1980's gave rise to a revolution in the field of integrative physiology that continues to this day. There are many complex choices when selecting a strategy for genetic modification, some of which will be touched on in this review, but the principal focus is to highlight the potential problems and pitfalls arising from the interpretation of in vivo cardiac phenotypes. As an exemplar, we will scrutinize the field of cardiac energetics and the attempts to understand the role of the creatine kinase (CK) energy buffering and transport system in the intact organism. This story highlights the confounding effects of genetic background, sex, and age, as well as the difficulties in interpreting knockout models in light of promiscuous proteins and metabolic redundancy. It will consider the dose-dependent effects and unintended consequences of transgene overexpression, and the need for experimental rigour in the context of in vivo phenotyping techniques. It is intended that this review will not only bring clarity to the field of cardiac energetics, but also aid the non-expert in evaluating and critically assessing data arising from in vivo genetic modification.

Loss of Endogenously Cycling Adult Cardiomyocytes Worsens Myocardial Function

RATIONALE

Endogenously cycling adult cardiomyocytes increase after myocardial infarction (MI) but remain scarce and are generally thought not to contribute to myocardial function. However, this broadly held assumption has not been tested, mainly because of the lack of transgenic reporters that restrict Cre expression to adult cardiomyocytes that reenter the cell cycle.

OBJECTIVE

We created and validated a new transgenic mouse, αMHC (alpha myosin heavy chain)-MerDreMer-Ki67p-RoxedCre (denoted αDKRC [cardiomyocyte-specific αMHC-MerDreMer-Ki67p-RoxedCre]) that restricts Cre expression to cycling adult cardiomyocytes and uniquely integrates spatial and temporal adult cardiomyocyte cycling events based on the DNA specificities of orthologous Dre and Cre recombinases. We then created αDKRC::DTA mice that expressed an inducible diphtheria toxin in adult cycling cardiomyocytes and examined the effects of ablating these endogenously cycling cardiomyocytes on myocardial function after ischemic-reperfusion (I/R) MI.

METHODS AND RESULTS

A tandem αDKRC transgene was designed, validated in cultured cells, and used to make transgenic mice. The αDKRC transgene integrated between MYH6 and MYH7 and did not disrupt expression of the surrounding genes. Compared with controls, αDKRC::RLTG (Rox-Lox-tdTomato-eGFP) mice treated with Tamoxifen expressed tdTomato+ in cardiomyocytes with rare Bromodeoxyuridine+, eGFP+ cardiomyocytes, consistent with reentry of the cell cycle. We then pretreated αDKRC::RLTG mice with Tamoxifen to activate the reporter before sham or reperfusion (I/R) MI surgeries. Compared with Sham surgery, the I/R MI group had increased single and paired eGFP+ (enhanced green fluorescent protein)+ cardiomyocytes predominantly in the border zones (5.8±0.5 versus 3.3±0.3 cardiomyocytes per 10-micron section, N=8-9 mice per group, n=16-24 sections per mouse), indicative of cycled cardiomyocytes. The single to paired eGFP+ cardiomyocyte ratio was ≈9 to 1 (5.2±0.4 single versus 0.6±0.2 paired cardiomyocytes) in the I/R MI group after MI, suggesting that cycling cardiomyocytes were more likely to undergo polyploidy than replication. The ablation of endogenously cycling adult cardiomyocytes in αDKRC::DTA (diphtheria) mice caused progressive worsening left ventricular chamber size and function after I/R MI, compared with controls.

CONCLUSIONS

Although scarce, endogenously cycling adult cardiomyocytes contribute to myocardial function after injury, suggesting that these cells may be physiologically relevant.

Role of extracellular signal-regulated kinase 1/2 signaling underlying cardiac hypertrophy

Cardiac hypertrophy is the result of increased myocardial cell size responding to an increased workload and developmental signals. These extrinsic and intrinsic stimuli as key drivers of cardiac hypertrophy have spurred efforts to target their associated signaling pathways. The extracellular signal-regulated kinases 1/2 (ERK1/2), as an essential member of mitogen-activated protein kinases (MAPKs), has been widely recognized for promoting cardiac growth. Several modified transgenic mouse models have been generated through either affecting the upstream kinase to change ERK1/2 activity, manipulating the direct role of ERK1/2 in the heart, or targeting phosphatases or MAPK scaffold proteins to alter total ERK1/2 activity in response to an increased workload. Using these models, both regulation of the upstream events and modulation of each isoform and indirect effector could provide important insights into how ERK1/2 modulates cardiomyocyte biology. Furthermore, a plethora of compounds, inhibitors, and regulators have emerged in consideration of ERK, or its MAPK kinases, are possible therapeutic targets against cardiac hypertrophic diseases. Herein, is a review of the available evidence regarding the exact role of ERK1/2 in regulating cardiac hypertrophy and a discussion of pharmacological strategy for treatment of cardiac hypertrophy.

A simple adeno-associated virus-based approach for the generation of cardiac genetic models in rats

Background: Heart failure is a major health problem and progress in this field relies on better understanding of the mechanisms and development of novel therapeutics using animal models. The rat may be preferable to the mouse as a cardiovascular disease model due to its closer physiology to humans and due to its large size that facilitates surgical and monitoring procedures. However, unlike the mouse, genetic manipulation of the rat genome is challenging. Methods: Here we developed a simple, refined, and robust cardiac-specific rat transgenic model based on an adeno-associated virus (AAV) 9 containing a cardiac troponin T promoter. This model uses a single intraperitoneal injection of AAV and does not require special expertise or equipment. Results: We characterize the AAV dose required to achieve a high cardiac specific level of expression of a transgene in the rat heart using a single intraperitoneal injection to neonates. We show that at this AAV dose GFP expression does not result in hypertrophy, a change in cardiac function or other evidence for toxicity. Conclusions: The model shown here allows easy and fast transgenic based disease modeling of cardiovascular disease in the rat heart, and can also potentially be expanded to deliver Cas9 and gRNAs or to deliver small hairpin (sh)RNAs to also achieve gene knockouts and knockdown in the rat heart.

Integrated network pharmacology and molecular docking approaches to reveal the synergistic mechanism of multiple components in Venenum Bufonis for ameliorating heart failure

Venenum Bufonis (VB), also called Chan Su in China, has been extensively used as a traditional Chinese medicine (TCM) for treating heart failure (HF) since ancient time. However, the active components and the potential anti-HF mechanism of VB remain unclear. In the current study, the major absorbed components and metabolites of VB after oral administration in rats were first collected from literatures. A total of 17 prototypes and 25 metabolites were gathered. Next, a feasible network-based pharmacological approach was developed and employed to explore the therapeutic mechanism of VB on HF based on the collected constituents. In total, 158 main targets were screened out and considered as effective players in ameliorating HF. Then, the VB components-main HF putative targets-main pathways network was established, clarifying the underlying biological process of VB on HF. More importantly, the main hubs were found to be highly enriched in adrenergic signalling in cardio-myocytes. After verified by molecular docking studies, four key targets (ATP1A1, GNAS, MAPK1 and PRKCA) and three potential active leading compounds (bufotalin, cinobufaginol and 19-oxo-bufalin) were identified, which may play critical roles in cardiac muscle contraction. This study demonstrated that the integrated strategy based on network pharmacology and molecular docking was helpful to uncover the synergistic mechanism of multiple constituents in TCM.

Long Noncoding RNA Ahit Protects Against Cardiac Hypertrophy Through SUZ12 (Suppressor of Zeste 12 Protein Homolog)-Mediated Downregulation of MEF2A (Myocyte Enhancer Factor 2A)

BACKGROUND

Long noncoding RNA (lncRNA) can regulate various physiological and pathological processes through multiple molecular mechanisms in cis and in trans. However, the role of lncRNAs in cardiac hypertrophy is yet to be fully elucidated.

METHODS

A mouse lncRNA microarray was used to identify differentially expressed lncRNAs in the mouse hearts following transverse aortic constriction-induced pressure overload comparing to the sham-operated samples. The direct impact of one lncRNA, Ahit, on cardiomyocyte hypertrophy was characterized in neonatal rat cardiomyocytes in response to phenylephrine by targeted knockdown and overexpression. The in vivo function of Ahit was analyzed in mouse hearts by using cardiac-specific adeno-associated virus, serotype 9-short hairpin RNA to knockdown Ahit in combination with transverse aortic constriction. Using catRAPID program, an interaction between Ahit and SUZ12 (suppressor of zeste 12 protein homolog) was predicted and validated by RNA immunoprecipitation and immunoblotting following RNA pull-down. Chromatin immunoprecipitation was performed to determine SUZ12 or H3K27me3 occupancy on the MEF2A (myocyte enhancer factor 2A) promoter. Finally, the expression of human Ahit (leukemia-associated noncoding IGF1R activator RNA 1 [LUNAR1]) in the serum samples from patients of hypertrophic cardiomyopathy was tested by quantitative real-time polymerase chain reaction.

RESULTS

A previously unannotated lncRNA, antihypertrophic interrelated transcript (Ahit), was identified to be upregulated in the mouse hearts after transverse aortic constriction. Inhibition of Ahit induced cardiac hypertrophy, both in vitro and in vivo, associated with increased expression of MEF2A, a critical transcriptional factor involved in cardiac hypertrophy. In contrast, overexpression of Ahit significantly attenuated stress-induced cardiac hypertrophy in vitro. Furthermore, Ahit was significantly upregulated in serum samples of patients diagnosed with hypertensive heart disease versus nonhypertrophic hearts (1.46±0.17 fold, P=0.0325). Mechanistically, Ahit directly bound and recruited SUZ12, a core PRC2 (polycomb repressive complex 2) protein, to the promoter of MEF2A, triggering its trimethylation on H3 lysine 27 (H3K27me3) residues and mediating the downregulation of MEF2A, thereby preventing cardiac hypertrophy.

CONCLUSIONS

Ahit is a lncRNA with a significant role in cardiac hypertrophy regulation through epigenomic modulation. Ahit is a potential therapeutic target of cardiac hypertrophy.

Dystrophin and calcium current are decreased in cardiomyocytes expressing Cre enzyme driven by αMHC but not TNT promoter

The Cre/lox system is a potent technology to control gene expression in mouse tissues. However, cardiac-specific Cre recombinase expression alone can lead to cardiac alterations when no loxP sites are present, which is not well understood. Many loxP-like sites have been identified in the mouse genome that might be Cre sensitive. One of them is located in the Dmd gene encoding dystrophin, a protein important for the function and stabilization of voltage-gated calcium (Ca_v1.2) and sodium (Na_v1.5) channels, respectively. Here, we investigate whether Cre affects dystrophin expression and function in hearts without loxP sites in the genome. In mice expressing Cre under the alpha-myosin heavy chain (MHC-Cre) or Troponin T (TNT-Cre) promoter, we investigated dystrophin expression, Na_v1.5 expression, and Ca_v1.2 function. Compared to age-matched MHC-Cre^- mice, dystrophin protein level was significantly decreased in hearts from MHC-Cre⁺ mice of more than 12-weeks-old. Quantitative RT-PCR revealed decreased mRNA levels of Dmd gene. Unexpectedly, calcium current (I_CaL), but not Na_v1.5 protein expression was altered in those mice. Surprisingly, in hearts from 12-week-old and older TNT-Cre⁺ mice, neither I_CaL nor dystrophin and Na_v1.5 protein content were altered compared to TNT-Cre^-. Cre recombinase unpredictably affects cardiac phenotype, and Cre-expressing mouse models should be carefully investigated before experimental use.

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

Ablation of cardiac TIGAR preserves myocardial energetics and cardiac function in the pressure overload heart failure model

Despite the advances in medical therapy, the morbidity and mortality of heart failure (HF) remain unacceptably high. HF results from reduced metabolism–contraction coupling efficiency, so the modulation of cardiac metabolism may be an effective strategy for therapeutic interventions. Tumor suppressor p53 (TP53) and its downstream target TP53-induced glycolysis and apoptosis regulator (TIGAR) are known to modulate cardiac metabolism and cell fate. To investigate TIGAR’s function in HF, we compared myocardial, metabolic, and functional outcomes between TIGAR knockout (TIGAR−/−) mice and wild-type (TIGAR+/+) mice subjected to chronic thoracic transverse aortic constriction (TAC), a pressure-overload HF model. In wild-type mice hearts, p53 and TIGAR increased markedly during HF development. Eight weeks after TAC surgery, the left ventricular (LV) dysfunction, fibrosis, oxidative damage, and myocyte apoptosis were significantly advanced in wild-type than in TIGAR−/− mouse heart. Further, myocardial high-energy phosphates in wild-type hearts were significantly decreased compared with those of TIGAR−/− mouse heart. Glucose oxidation and glycolysis rates were also reduced in isolated perfused wild-type hearts following TAC than those in TIGAR−/− hearts, which suggest that the upregulation of TIGAR in HF causes impaired myocardial energetics and function. The effects of TIGAR knockout on LV function were also replicated in tamoxifen (TAM)-inducible cardiac-specific TIGAR knockout mice ( TIGARflox/flox/Tg(Myh6-cre/Esr1) mice). The ablation of TIGAR during pressure-overload HF preserves myocardial function and energetics. Thus, cardiac TIGAR-targeted therapy to increase glucose metabolism will be a novel strategy for HF.

NEW & NOTEWORTHY The present study is the first to demonstrate that TP53-induced glycolysis and apoptosis regulator (TIGAR) is upregulated in the myocardium during experimental heart failure (HF) in mice and that TIGAR knockout can preserve the heart function and myocardial energetics during HF. Reducing TIGAR to enhance myocardial glycolytic energy production is a promising therapeutic strategy for HF.

Cardiomyocyte glucocorticoid and mineralocorticoid receptors directly and antagonistically regulate heart disease in mice

Stress is increasingly associated with heart dysfunction and is linked to higher mortality rates in patients with cardiometabolic disease. Glucocorticoids are primary stress hormones that regulate homeostasis through two nuclear receptors, the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), both of which are present in cardiomyocytes. To examine the specific and coordinated roles that these receptors play in mediating the direct effects of stress on the heart, we generated mice with cardiomyocyte-specific deletion of GR (cardioGRKO), MR (cardioMRKO), or both GR and MR (cardioGRMRdKO). The cardioGRKO mice spontaneously developed cardiac hypertrophy and left ventricular systolic dysfunction and died prematurely from heart failure. In contrast, the cardioMRKO mice exhibited normal heart morphology and function. Despite the presence of myocardial stress, the cardioGRMRdKO mice were resistant to the cardiac remodeling, left ventricular dysfunction, and early death observed in the cardioGRKO mice. Gene expression analysis revealed the loss of gene changes associated with impaired Ca²⁺ handling, increased oxidative stress, and enhanced cell death and the presence of gene changes that limited the hypertrophic response and promoted cardiomyocyte survival in the double knockout hearts. Reexpression of MR in cardioGRMRdKO hearts reversed many of the cardioprotective gene changes and resulted in cardiac failure. These findings reveal a critical role for balanced cardiomyocyte GR and MR stress signaling in cardiovascular health. Therapies that shift stress signaling in the heart to favor more GR and less MR activity may provide an improved approach for treating heart disease.

Genetic Basis for Congenital Heart Disease: Revisited: A Scientific Statement From the American Heart Association

This review provides an updated summary of the state of our knowledge of the genetic contributions to the pathogenesis of congenital heart disease. Since 2007, when the initial American Heart Association scientific statement on the genetic basis of congenital heart disease was published, new genomic techniques have become widely available that have dramatically changed our understanding of the causes of congenital heart disease and, clinically, have allowed more accurate definition of the pathogeneses of congenital heart disease in patients of all ages and even prenatally. Information is presented on new molecular testing techniques and their application to congenital heart disease, both isolated and associated with other congenital anomalies or syndromes. Recent advances in the understanding of copy number variants, syndromes, RASopathies, and heterotaxy/ciliopathies are provided. Insights into new research with congenital heart disease models, including genetically manipulated animals such as mice, chicks, and zebrafish, as well as human induced pluripotent stem cell-based approaches are provided to allow an understanding of how future research breakthroughs for congenital heart disease are likely to happen. It is anticipated that this review will provide a large range of health care-related personnel, including pediatric cardiologists, pediatricians, adult cardiologists, thoracic surgeons, obstetricians, geneticists, genetic counselors, and other related clinicians, timely information on the genetic aspects of congenital heart disease. The objective is to provide a comprehensive basis for interdisciplinary care for those with congenital heart disease.

Genetic and Tissue Engineering Approaches to Modeling the Mechanics of Human Heart Failure for Drug Discovery

Heart failure is the leading cause of death in the western world and as such, there is a great need for new therapies. Heart failure has a variable presentation in patients and a complex etiology; however, it is fundamentally a condition that affects the mechanics of cardiac contraction, preventing the heart from generating sufficient cardiac output under normal operating pressures. One of the major issues hindering the development of new therapies has been difficulties in developing appropriate in vitro model systems of human heart failure that recapitulate the essential changes in cardiac mechanics seen in the disease. Recent advances in stem cell technologies, genetic engineering, and tissue engineering have the potential to revolutionize our ability to model and study heart failure in vitro. Here, we review how these technologies are being applied to develop personalized models of heart failure and discover novel therapeutics.

Network-based predictions of in vivo cardiac hypertrophy

Cardiac hypertrophy is a common response of cardiac myocytes to stress and a predictor of heart failure. While in vitro cell culture studies have identified numerous molecular mechanisms driving hypertrophy, it is unclear to what extent these mechanisms can be integrated into a consistent framework predictive of in vivo phenotypes. To address this question, we investigate the degree to which an in vitro-based, manually curated computational model of the hypertrophy signaling network is able to predict in vivo hypertrophy of 52 cardiac-specific transgenic mice. After minor revisions motivated by in vivo literature, the model concordantly predicts the qualitative responses of 78% of output species and 69% of signaling intermediates within the network model. Analysis of four double-transgenic mouse models reveals that the computational model robustly predicts hypertrophic responses in mice subjected to multiple, simultaneous perturbations. Thus the model provides a framework with which to mechanistically integrate data from multiple laboratories and experimental systems to predict molecular regulation of cardiac hypertrophy.

Insertional Mutagenesis Confounds the Mechanism of the Morbid Phenotype of a PLNR9C Transgenic Mouse Line

BACKGROUND

A mouse line with heterozygous transgenic expression of phospholamban carrying a substitution of cysteine for arginine 9 (TgPLN^R9C) under the control of α-myosin heavy chain (αMHC) promoter features dilated cardiomyopathy, heart failure, and premature death.

METHODS AND RESULTS

Determination of transgene chromosomal localization by conventional methods shows that in this line the transgenic array of 13 PLN^R9C expression cassettes, arranged in a head-to-tail tandem orientation, have integrated into the bidirectional promoter of the αMHC (Myh6) gene and the gene for the regulatory noncoding RNA Myheart (Mhrt), both of which are known to be involved in cardiac development and pathology. Expression of the noncoding RNA Mhrt in TgPLN^R9C mice exhibits profound deregulation, despite the presence of the second, intact allele.

CONCLUSIONS

The TgPLN^R9C mouse strain is, in the best case, a functionally ambiguous phenocopy of the human PLN^R9C heterozygote, because a similar constellation of genetically programmed events can not occur in a patient. Publications featuring "cardiac-specific overexpression" are focused on the phenotype and typically forgo the definition of the transgene integration site or transgene temporal expression profile, so caution should be exercised in attributing clinical relevance to pathologic phenomena observed in αMHC-driven transgenes.

Mdm2 regulates cardiac contractility by inhibiting GRK2-mediated desensitization of β-adrenergic receptor signaling

The oncoprotein Mdm2 is a RING domain-containing E3 ubiquitin ligase that ubiquitinates G protein-coupled receptor kinase 2 (GRK2) and β-arrestin2, thereby regulating β-adrenergic receptor (βAR) signaling and endocytosis. Previous studies showed that cardiac Mdm2 expression is critical for controlling p53-dependent apoptosis during early embryonic development, but the role of Mdm2 in the developed adult heart is unknown. We aimed to identify if Mdm2 affects βAR signaling and cardiac function in adult mice. Using Mdm2/p53-KO mice, which survive for 9-12 months, we identified a critical and potentially novel role for Mdm2 in the adult mouse heart through its regulation of cardiac β1AR signaling. While baseline cardiac function was mostly similar in both Mdm2/p53-KO and wild-type (WT) mice, isoproterenol-induced cardiac contractility in Mdm2/p53-KO was significantly blunted compared with WT mice. Isoproterenol increased cAMP in left ventricles of WT but not of Mdm2/p53-KO mice. Additionally, while basal and forskolin-induced calcium handling in isolated Mdm2/p53-KO and WT cardiomyocytes were equivalent, isoproterenol-induced calcium handling in Mdm2/p53-KO was impaired. Mdm2/p53-KO hearts expressed 2-fold more GRK2 than WT. GRK2 polyubiquitination via lysine-48 linkages was significantly reduced in Mdm2/p53-KO hearts. Tamoxifen-inducible cardiomyocyte-specific deletion of Mdm2 in adult mice also led to a significant increase in GRK2, and resulted in severely impaired cardiac function, high mortality, and no detectable βAR responsiveness. Gene delivery of either Mdm2 or GRK2-CT in vivo using adeno-associated virus 9 (AAV9) effectively rescued β1AR-induced cardiac contractility in Mdm2/p53-KO. These findings reveal a critical p53-independent physiological role of Mdm2 in adult hearts, namely, regulation of GRK2-mediated desensitization of βAR signaling.

Loss of TRADD attenuates pressure overload-induced cardiac hypertrophy through regulating TAK1/P38 MAPK signalling in mice

We investigated the role of tumour necrosis factor receptor (TNFR)-associated death domain (TRADD) on pressure overload-induced cardiac hypertrophy and the underlying molecular mechanisms by using a TRADD deficiency mice model. 6-8 weeks wild-type and TRADD knockout mice were performed to transverse aorta constriction (TAC) or sham operation (6-8 mice for each group). 14 days after TAC, cardiac function was measured by echocardiography, as well as by pathological and molecular analyses of heart samples. The expressions of cardiac hypertrophic and fibrotic markers were detected by qPCR. Phosphorylated and total TAK1, Akt, and p38 MAPK levels were examined by Western blotting. The ratios of lung or heart/body weight, wall thickness/chamber diameter of left ventricular and cross area of cardiomyocyte were significantly reduced in TRADD knockout (KO) mice than those of wild-type mice after TAC. Moreover, cardiac hypertrophic and fibrotic markers were downregulated in TRADD knockout mice than those of wild-type mice following TAC. Protein expression analysis showed phosphorylated TAK1, p38 MAPK and AKT were upregulated after TAC in both wild-type and TRADD KO mice, phosphorylation of TAK1 and p38 MAPK was reduced more remarkably after TRADD deficiency, while phosphorylated AKT expression was similar between TRADD KO and wild-type mice following TAC. Our data suggest that TRADD KO blunts pressure overload-induced cardiac hypertrophy through mediating TAK1/p38 MAPK but not AKT phosphorylation in mice.

Operationalizing Precision Cardiovascular Medicine: Three Innovations

For precision medicine to become a reality, we propose three changes. First, healthcare deliverables must be prioritized, enabling translation of knowledge to the clinic. Second, physicians and patients must be convinced to participate, requiring additional infrastructure in health systems. Third, discovery science must evolve to shift the preclinical landscape for innovation. We propose a change in the fundamental relationship between basic and clinical science: rather than two distinct entities between which concepts must be translated, we envision a natural hybrid of these approaches, wherein discovery science and clinical trials coincide in the same health systems and patient populations.

Undiscovered Physiology of Transcript and Protein Networks

The past two decades have witnessed a rapid evolution in our ability to measure RNA and protein from biological systems. As a result, new principles have arisen regarding how information is processed in cells, how decisions are made, and the role of networks in biology. This essay examines this technological evolution, reviewing (and critiquing) the conceptual framework that has emerged to explain how RNA and protein networks control cellular function. We identify how future investigations into transcriptomes, proteomes, and other cellular networks will enable development of more robust, quantitative models of cellular behavior whilst also providing new avenues to use knowledge of biological networks to improve human health. © 2016 American Physiological Society. Compr Physiol 6:1851-1872, 2016.

CaMKII-dependent phosphorylation regulates basal cardiac pacemaker function via modulation of local Ca2+ releases

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) due to gradual change of the membrane potential called diastolic depolarization (DD). Spontaneous, submembrane local Ca(2+) releases (LCR) from ryanodine receptors (RyR) occur during late DD and activate an inward Na(+)/Ca(2+)exchange current to boost the DD rate and fire an action potential (AP). Here we studied the extent of basal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation and the role of basal CaMKII-dependent protein phosphorylation in generation of LCRs and regulation of normal automaticity of intact rabbit SANC. The basal level of activated (autophosphorylated) CaMKII in rabbit SANC surpassed that in ventricular myocytes (VM) by approximately twofold, and this was accompanied by high basal level of protein phosphorylation. Specifically, phosphorylation of phospholamban (PLB) at the CaMKII-dependent Thr(17) site was approximately threefold greater in SANC compared with VM, and RyR phosphorylation at CaMKII-dependent Ser(2815) site was ∼10-fold greater in the SA node, compared with that in ventricle. CaMKII inhibition reduced phosphorylation of PLB and RyR, decreased LCR size, increased LCR periods (time from AP-induced Ca(2+) transient to subsequent LCR), and suppressed spontaneous SANC firing. Graded changes in CaMKII-dependent phosphorylation (indexed by PLB phosphorylation at the Thr(17)site) produced by CaMKII inhibition, β-AR stimulation or phosphodiesterase inhibition were highly correlated with changes in SR Ca(2+) replenishment times and LCR periods and concomitant changes in spontaneous SANC cycle lengths (R(2) = 0.96). Thus high basal CaMKII activation modifies the phosphorylation state of Ca(2+) cycling proteins PLB, RyR, L-type Ca(2+) channels (and likely others), adjusting LCR period and characteristics, and ultimately regulates both normal and reserve cardiac pacemaker function.

Chromosomal mapping of the αMHC-MerCreMer transgene in mice reveals a large genomic deletion

Transgenic mice expressing a tamoxifen-inducible Cre recombinase specifically in cardiomyocytes were generated in 2001 and are in widespread use, having been employed in >150 published studies. However, several groups recently have reported that tamoxifen administration to these mice can have off-target effects that include cardiac dysfunction, fibrosis, and death. For this reason, among others, we considered it important to better characterize the transgene (termed herein, CM-MCM) and its chromosomal location(s). Cytogenetic analysis positioned the CM-MCM transgene within the C band of chromosome 19, and more precise mapping, using genome walking and DNA sequencing, showed that transgene insertion is in the C1 region. Using the genome walking data, we have developed PCR assays that not only identify mice that carry the transgene, but also distinguish homozygous animals (CM-MCM(Tg/Tg)) from hemizygous (CM-MCM(Tg/0)), permitting the rapid assessment of transgene zygosity and, thereby, helping to minimize off-target tamoxifen-induced effects. Substantial rearrangement/duplication of transgene elements is present, and transgene integration was accompanied by the deletion of a 19,500 bp fragment of genomic DNA that contains the promoter, exon 1 and part of intron 1 of the APOBEC1 complementation factor (A1cf) gene, as well as several elements that are predicted to regulate chromosomal architecture. A1cf protein expression is ablated by the deletion and, therefore, homozygous mice are functionally A1cf knockout. The implications of this unexpected finding are discussed.

Cardiac concentric hypertrophy promoted by activated Met receptor is mitigated in vivo by inhibition of Erk1,2 signalling with Pimasertib

Cardiac hypertrophy is a major risk factor for heart failure. Hence, its attenuation represents an important clinical goal. Erk1,2 signalling is pivotal in the cardiac response to stress, suggesting that its inhibition may be a good strategy to revert heart hypertrophy. In this work, we unveiled the events associated with cardiac hypertrophy by means of a transgenic model expressing activated Met receptor. c-Met proto-oncogene encodes for the tyrosine kinase receptor of Hepatocyte growth factor and is a strong inducer of Ras-Raf-Mek-Erk1,2 pathway. We showed that three weeks after the induction of activated Met, the heart presents a remarkable concentric hypertrophy, with no signs of congestive failure and preserved contractility. Cardiac enlargement is accompanied by upregulation of growth-regulating transcription factors, natriuretic peptides, cytoskeletal proteins, and Extracellular Matrix remodelling factors (Timp1 and Pai1). At a later stage, cardiac hypertrophic remodelling results into heart failure with preserved systolic function. Prevention trial by suppressing activated Met showed that cardiac hypertrophy is reversible, and progression to heart failure is prevented. Notably, treatment with Pimasertib, Mek1 inhibitor, attenuates cardiac hypertrophy and remodelling. Our results suggest that modulation of Erk1.2 signalling may constitute a new therapeutic approach for treating cardiac hypertrophies.

Modeling GATAD1-Associated Dilated Cardiomyopathy in Adult Zebrafish

Animal models have played a critical role in validating human dilated cardiomyopathy (DCM) genes, particularly those that implicate novel mechanisms for heart failure. However, the disease phenotype may be delayed due to age-dependent penetrance. For this reason, we generated an adult zebrafish model, which is a simpler vertebrate model with higher throughput than rodents. Specifically, we studied the zebrafish homologue of GATAD1, a recently identified gene for adult-onset autosomal recessive DCM. We showed cardiac expression of gatad1 transcripts, by whole mount in situ hybridization in zebrafish embryos, and demonstrated nuclear and sarcomeric I-band subcellular localization of Gatad1 protein in cardiomyocytes, by injecting a Tol2 plasmid encoding fluorescently-tagged Gatad1. We next generated gatad1 knock-out fish lines by TALEN technology and a transgenic fish line that expresses the human DCM GATAD1-S102P mutation in cardiomyocytes. Under stress conditions, longitudinal studies uncovered heart failure (HF)-like phenotypes in stable KO mutants and a tendency toward HF phenotypes in transgenic lines. Based on these efforts of studying a gene-based inherited cardiomyopathy model, we discuss the strengths and bottlenecks of adult zebrafish as a new vertebrate model for assessing candidate cardiomyopathy genes.

mTOR, cardiomyocytes and inflammation in cardiac hypertrophy

Mammalian target of rapamycin (mTOR) is an evolutionary conserved kinase that senses the nutrient and energy status of cells, the availability of growth factors, stress stimuli and other cellular and environmental cues. It responds by regulating a range of cellular processes related to metabolism and growth in accordance with the available resources and intracellular needs. mTOR has distinct functions depending on its assembly in the structurally distinct multiprotein complexes mTORC1 or mTORC2. Active mTORC1 enhances processes including glycolysis, protein, lipid and nucleotide biosynthesis, and it inhibits autophagy. Reported functions for mTORC2 after growth factor stimulation are very diverse, are tissue and cell-type specific, and include insulin-stimulated glucose transport and enhanced glycogen synthesis. In accordance with its cellular functions, mTOR has been demonstrated to regulate cardiac growth in response to pressure overload and is also known to regulate cells of the immune system. The present manuscript presents recently obtained insights into mechanisms whereby mTOR may change anabolic, catabolic and stress response pathways in cardiomocytes and discusses how mTOR may affect inflammatory cells in the heart during hemodynamic stress. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

IRF8 suppresses pathological cardiac remodelling by inhibiting calcineurin signalling

Interferon regulatory factor 8 (IRF8) is known to affect the innate immune response, for example, by regulating the differentiation and function of immune cells. However, whether IRF8 can influence cardiac hypertrophy is unknown. Here we show that IRF8 levels are decreased in human dilated/hypertrophic cardiomyopathic hearts and in murine hypertrophic hearts. Mice overexpressing Irf8 specifically in the heart are resistant to aortic banding (AB)-induced cardiac hypertrophy, whereas mice lacking IRF8 either globally or specifically in cardiomyocytes develop an aggravated phenotype induced by pressure overload. Mechanistically, we show that IRF8 directly interacts with NFATc1 to prevent NFATc1 translocation and thus inhibits the hypertrophic response. Inhibition of NFATc1 ameliorates the cardiac abnormalities in IRF8(-/-) mice after AB. In contrast, constitutive activation of NFATc1 nullifies the protective effects of IRF8 on cardiac hypertrophy in IRF8-overexpressing mice. Our results indicate that IRF8 is a potential therapeutic target in pathological cardiac hypertrophy.

Prolonged Cre expression driven by the α-myosin heavy chain promoter can be cardiotoxic

Studying the importance of genetic factors in a desired cell type or tissue necessitates the use of precise genetic tools. With the introduction of bacteriophage Cre recombinase/loxP mediated DNA editing and promoter-specific Cre expression, it is feasible to generate conditional knockout mice in which particular genes are disrupted in a cell type-specific manner in vivo. In cardiac myocytes, this is often achieved through α-myosin heavy chain promoter (αMyHC)-driven Cre expression in conjunction with a loxP-site flanked gene of interest. Recent studies in other cell types demonstrate toxicity of Cre expression through induction of DNA damage. However, it is unclear to what extent the traditionally used αMyHC-Cre line [1] may exhibit cardiotoxicity. Further, the genotype of αMyHC-Cre(+/-) is not often included as a control group in cardiac myocyte-specific knockout studies. Here we present evidence that these αMyHC-Cre(+/-) mice show molecular signs of cardiac toxicity by 3months of age and exhibit decreased cardiac function by 6months of age compared to wild-type littermates. Hearts from αMyHC-Cre(+/-) mice also display evidence of fibrosis, inflammation, and DNA damage. Interestingly, some of the early functional changes observed in αMyHC-Cre(+/-) mice are sexually dimorphic. Given the high level of Cre recombinase expression resulting from expression from the αMyHC promoter, we asked if degenerate loxP-like sites naturally exist in the mouse genome and if so, whether they are affected by Cre in the absence of canonical loxP-sites. Using a novel bioinformatics search tool, we identified 619 loxP-like sites with 4 or less mismatches to the canonical loxP-site. 227 sites overlapped with annotated genes and 55 of these genes were expressed in cardiac muscle. Expression of ~26% of the 27 genes tested was disrupted in αMyHC-Cre(+/-) mice indicating potential targeting by Cre. Taken together, these results highlight both the importance of using αMyHC-Cre mice as controls in conditional knockout studies as well as the need for a less cardiotoxic Cre driver for the field.

A Murine Myh6MerCreMer Knock-In Allele Specifically Mediates Temporal Genetic Deletion in Cardiomyocytes after Tamoxifen Induction

A mouse model that mediates temporal, specific, and efficient myocardial deletion with Cre-LoxP technology will be a valuable tool to determine the function of genes during heart formation. Mhy6 encodes a cardiac muscle specific protein: alpha-myosin heavy chain. Here, we generated a new Myh6-MerCreMer (Myh6(MerCreMer/+)) inducible Cre knock-in mouse by inserting a MerCreMer cassette into the Myh6 start codon. By crossing knock-in mice with Rosa26 reporter lines, we found the Myh6(MerCreMer/+) mice mediate complete Cre-LoxP recombination in cardiomyocytes after tamoxifen induction. X-gal staining and immunohistochemistry analysis revealed that Myh6-driven Cre recombinase was specifically activated in cardiomyocytes at embryonic and adult stages. Furthermore, echocardiography showed that Myh6(MerCreMer/+) mice maintained normal cardiac structure and function before and after tamoxifen administration. These results suggest that the new Myh6(MerCreMer/+) mouse can serve as a robust tool to dissect the roles of genes in heart development and function. Additionally, myocardial progeny during heart development and after cardiac injury can be traced using this mouse line.

CaM Kinase II mediates maladaptive post-infarct remodeling and pro-inflammatory chemoattractant signaling but not acute myocardial ischemia/reperfusion injury

CaMKII was suggested to mediate ischemic myocardial injury and adverse cardiac remodeling. Here, we investigated the roles of different CaMKII isoforms and splice variants in ischemia/reperfusion (I/R) injury by the use of new genetic CaMKII mouse models. Although CaMKIIδC was upregulated 1 day after I/R injury, cardiac damage 1 day after I/R was neither affected in CaMKIIδ-deficient mice, CaMKIIδ-deficient mice in which the splice variants CaMKIIδB and C were re-expressed, nor in cardiomyocyte-specific CaMKIIδ/γ double knockout mice (DKO). In contrast, 5 weeks after I/R, DKO mice were protected against extensive scar formation and cardiac dysfunction, which was associated with reduced leukocyte infiltration and attenuated expression of members of the chemokine (C-C motif) ligand family, in particular CCL3 (macrophage inflammatory protein-1α, MIP-1α). Intriguingly, CaMKII was sufficient and required to induce CCL3 expression in isolated cardiomyocytes, indicating a cardiomyocyte autonomous effect. We propose that CaMKII-dependent chemoattractant signaling explains the effects on post-I/R remodeling. Taken together, we demonstrate that CaMKII is not critically involved in acute I/R-induced damage but in the process of post-infarct remodeling and inflammatory processes.

Generation of a tamoxifen inducible Tnnt2MerCreMer knock-in mouse model for cardiac studies

Tnnt2, encoding thin-filament sarcomeric protein cardiac troponin T, plays critical roles in heart development and function in mammals. To develop an inducible genetic deletion strategy in myocardial cells, we generated a new Tnnt2:MerCreMer (Tnnt2(MerCreMer/+)) knock-in mouse. Rosa26 reporter lines were used to examine the specificity and efficiency of the inducible Cre recombinase. We found that Cre was specifically and robustly expressed in the cardiomyocytes at embryonic and adult stages following tamoxifen induction. The knock-in allele on Tnnt2 locus does not impact cardiac function. These results suggest that this new Tnnt2(MerCreMer/+) mouse could be applied towards the temporal genetic deletion of genes of interests in cardiomyocytes with Cre-LoxP technology. The Tnnt2(MerCreMer/+) mouse model also provides a useful tool to trace myocardial lineage during development and repair after cardiac injury.

Novel Roles of GATA4/6 in the Postnatal Heart Identified through Temporally Controlled, Cardiomyocyte-Specific Gene Inactivation by Adeno-Associated Virus Delivery of Cre Recombinase

GATA4 and GATA6 are central cardiac transcriptional regulators. The postnatal, stage-specific function of the cardiac transcription factors GATA4 and GATA6 have not been evaluated. In part, this is because current Cre-loxP approaches to cardiac gene inactivation require time consuming and costly breeding of Cre-expressing and “floxed” mouse lines, often with limited control of the extent or timing of gene inactivation. We investigated the stage-specific functions of GATA4 and GATA6 in the postnatal heart by using adeno-associated virus serotype 9 to control the timing and extent of gene inactivation by Cre. Systemic delivery of recombinant, adeno-associated virus 9 (AAV9) expressing Cre from the cardiac specific Tnnt2 promoter was well tolerated and selectively and efficiently recombined floxed target genes in cardiomyocytes. AAV9:Tnnt2-Cre efficiently inactivated Gata4 and Gata6. Neonatal Gata4/6 inactivation caused severe, rapidly lethal systolic heart failure. In contrast, Gata4/6 inactivation in adult heart caused only mild systolic dysfunction but severe diastolic dysfunction. Reducing the dose of AAV9:Tnnt2-Cre generated mosaics in which scattered cardiomyocytes lacked Gata4/6. This mosaic knockout revealed that Gata4/6 are required cell autonomously for physiological cardiomyocyte growth. Our results define novel roles of GATA4 and GATA6 in the neonatal and adult heart. Furthermore, our data demonstrate that evaluation of gene function hinges on controlling the timing and extent of gene inactivation. AAV9:Tnnt2-Cre is a powerful tool for controlling these parameters.

IMP 2.0: a multi-species functional genomics portal for integration, visualization and prediction of protein functions and networks

IMP (Integrative Multi-species Prediction), originally released in 2012, is an interactive web server that enables molecular biologists to interpret experimental results and to generate hypotheses in the context of a large cross-organism compendium of functional predictions and networks. The system provides biologists with a framework to analyze their candidate gene sets in the context of functional networks, expanding or refining their sets using functional relationships predicted from integrated high-throughput data. IMP 2.0 integrates updated prior knowledge and data collections from the last three years in the seven supported organisms (Homo sapiens, Mus musculus, Rattus norvegicus, Drosophila melanogaster, Danio rerio, Caenorhabditis elegans, and Saccharomyces cerevisiae) and extends function prediction coverage to include human disease. IMP identifies homologs with conserved functional roles for disease knowledge transfer, allowing biologists to analyze disease contexts and predictions across all organisms. Additionally, IMP 2.0 implements a new flexible platform for experts to generate custom hypotheses about biological processes or diseases, making sophisticated data-driven methods easily accessible to researchers. IMP does not require any registration or installation and is freely available for use at http://imp.princeton.edu.

Role of cardiac- and myeloid-MyD88 signaling in endotoxin shock: a study with tissue-specific deletion models

BACKGROUND

Myeloid differentiation factor 88 (MyD88) is an adaptor molecule critical for host innate immunity. Studies have shown that signaling via MyD88 contributes to cytokine storm, cardiac dysfunction, and high mortality during endotoxin shock.However, the specific contribution of MyD88 signaling of immune and cardiac origins to endotoxin shock remains unknown.

METHODS

Tissue-specific MyD88 deletion models: Cre-recombinase transgenic mice with α-myosin heavy chain (α-MHC) or lysozyme M promoters were cross-bred with MyD88-loxP (MyD88fl/fl) mice, respectively, to generate cardiomyocyte- (α-MHCMyD88−/−) or myeloid-specific (Lyz-MyD88−/−) MyD88 deletion models and their respective MyD88fl/fl littermates. Endotoxin shock model: Mice were subjected to 15 mg/kg lipopolysaccharide (intraperitoneal injection). Cardiac function was measured by echocardiography and cytokines by multiplex assay and quantitative reverse transcription-polymerase chain reaction.

RESULTS

α-MHC-MyD88−/− mice had 61 and 87% reduction in MyD88 gene and protein expression in cardiomyocytes,respectively, whereas Lyz-MyD88−/− had 73 and 67% decrease, respectively, in macrophages (n=3 per group). After lipopolysaccharide treatment, the two groups of MyD88fl/fl littermates had 46% (n=10) and 60% (n=15) of mortality, respectively.Both α-MHC-MyD88−/− and Lyz-MyD88−/− mice had markedly improved survival. Compared with the MyD88fl/fl littermates, Lyz-MyD88−/− mice had warmer body temperature, attenuated systemic and cardiac inflammatory cytokine production,and significantly improved cardiac function, whereas α-MHC-MyD88−/− mice had decreased myocardial inducible nitricoxide synthase induction and modestly preserved cardiac function.

CONCLUSIONS

Both cardiomyocyte- and myeloid-MyD88 signaling play a role in cardiac dysfunction and mortality during endotoxin shock. Myeloid-MyD88 signaling plays a predominant role in systemic and cardiac inflammation after endotoxin challenge.

Non-genomic effects of spironolactone and eplerenone in cardiomyocytes of neonatal Wistar rats: do they evoke cardioprotective pathways?

Mineralocorticoid receptor (MR) antagonists of aldosterone (spironolactone and eplerenone) display beneficial effects in the treatment of cardiopathies; however, many of these responses are independent of this antagonism. The mechanisms of action of these drugs are not well known; few studies have comparatively evaluated whether eplerenone as well as spironolactone display cardioprotective effects independent of the blockade of aldosterone. To study these mechanisms, which lead to cardioprotective responses, and to evaluate comparatively their effects in vitro, we have evaluated the proliferative effect of spironolactone and eplerenone in primary culture of cardiomyocytes and fibroblasts of neonatal Wistar rats in the presence and absence of aldosterone. Spironolactone and eplerenone promoted proliferation of cardiomyocyte even in the absence of aldosterone, suggesting a signaling pathway independent of the antagonism over aldosterone. Spironolactone was able to reduce the proliferation of fibroblasts and to reverse the proliferation promoted by aldosterone, which was also displayed by eplerenone. To elucidate the biochemical pathways evoked by these drugs, we sought to analyze Ca(2+), cAMP, and cGMP, and the activity of PKC and ERK1/2. Spironolactone and eplerenone increased the levels of Ca(2+), cGMP and activity of ERK 1/2, and reversed the action of aldosterone on the activity of PKC and ERK1/2. Interestingly, only spironolactone increased the levels of cAMP. Our data support the fact that in addition to aldosterone, both spironolactone and eplerenone display rapid responses (non-genomic) such as an increase on cAMP, Ca(2+), and cGMP by spironolactone, and Ca(2+) and cGMP by eplerenone. We have observed a more consistent cardioprotection promoted by spironolactone; however, these effects have yet to be tested clinically. Therefore, our data show that these drugs do not only act as an antagonist of MR, but could lead to a new pharmacological classification of these drugs.

Functional implications of mitofusin 2-mediated mitochondrial-SR tethering

Cardiomyocyte mitochondria have an intimate physical and functional relationship with sarcoplasmic reticulum (SR). Under normal conditions mitochondrial ATP is essential to power SR calcium cycling that drives phasic contraction/relaxation, and changes in SR calcium release are sensed by mitochondria and used to modulate oxidative phosphorylation according to metabolic need. When perturbed, mitochondrial-SR calcium crosstalk can evoke programmed cell death. Physical proximity and functional interplay between mitochondria and SR are maintained in part through tethering of these two organelles by the membrane protein mitofusin 2 (Mfn2). Here we review and discuss findings from our two laboratories that derive from genetic manipulation of Mfn2 and closely related Mfn1 in mouse hearts and other experimental systems. By comparing the findings of our two independent research efforts we arrive at several conclusions that appear to be strongly supported, and describe a few areas of incomplete understanding that will require further study. In so doing we hope to clarify some misconceptions regarding the many varied roles of Mfn2 as both physical trans-organelle tether and mitochondrial fusion protein. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease."

Rapid and highly efficient inducible cardiac gene knockout in adult mice using AAV-mediated expression of Cre recombinase

AIMS

Inducible gene targeting in mice using the Cre/LoxP system has become a valuable tool to analyse the roles of specific genes in the adult heart. However, the commonly used Myh6-MerCreMer system requires time-consuming breeding schedules and is potentially associated with cardiac side effects, which may result in transient cardiac dysfunction. The aim of our study was to establish a rapid and simple system for cardiac gene inactivation in conditional knockout mice by gene transfer of a Cre recombinase gene using adeno-associated viral vectors of serotype 9 (AAV9).

METHODS AND RESULTS

AAV9 vectors expressing Cre under the control of a human cardiac troponin T promoter (AAV-TnT-Cre) enabled a highly efficient Cre/LoxP switching in cardiomyocytes 2 weeks after injection into 5- to 6-week-old ROSA26-LacZ reporter mice. Recombination efficiency was at least as high as observed with the Myh6-MerCreMer system. No adverse side effects were detected upon application of AAV-TnT-Cre. As proof of principle, we studied AAV-TnT-Cre in a conditional knockout model (Srf-flex1 mice) to deplete the myocardium of the transcription factor serum response factor (SRF). Four weeks after AAV-TnT-Cre injection, a strong decrease in the cardiac expression of SRF mRNA and protein was observed. Furthermore, mice developed a severe cardiac dysfunction with increased interstitial fibrosis in accordance with the central role of SRF for the expression of contractile and calcium trafficking proteins in the heart.

CONCLUSIONS

AAV9-mediated expression of Cre is a promising approach for rapid and efficient conditional cardiac gene knockout in adult mice.

In vivo proton T1 relaxation times of mouse myocardial metabolites at 9.4 T

PURPOSE

Proton magnetic resonance spectroscopy ((1) H-MRS) for quantitative in vivo assessment of mouse myocardial metabolism requires accurate acquisition timing to minimize motion artifacts and corrections for T1 -dependent partial saturation effects. In this study, mouse myocardial water and metabolite T1 relaxation time constants were quantified.

METHODS

Cardiac-triggered and respiratory-gated PRESS-localized (1) H-MRS was employed at 9.4 T to acquire signal from a 4-µL voxel in the septum of healthy mice (n = 10) while maintaining a steady state of magnetization using dummy scans during respiratory gates. Signal stability was assessed via standard deviations (SD) of zero-order phases and amplitudes of water spectra. Saturation-recovery experiments were performed to determine T1 values.

RESULTS

Phase SD did not vary for different repetition times (TR), and was 13.1° ± 4.5°. Maximal amplitude SD was 14.2% ± 5.1% at TR = 500 ms. Myocardial T1 values (mean ± SD) were quantified for water (1.71 ± 0.25 s), taurine (2.18 ± 0.62 s), trimethylamine from choline-containing compounds and carnitine (1.67 ± 0.25 s), creatine-methyl (1.34 ± 0.19 s), triglyceride-methylene (0.60 ± 0.15 s), and triglyceride-methyl (0.90 ± 0.17 s) protons.

CONCLUSION

This work provides in vivo quantifications of proton T1 values for mouse myocardial water and metabolites at 9.4 T.

Genomic DNA recombination with cell-penetrating peptide-tagged cre protein in mouse skeletal and cardiac muscle

The Cre-loxP recombination system has been used to promote DNA recombination both in vitro and in vivo. For in vivo delivery, Cre expression is commonly achieved through the use of tissue/cell type-specific promoters, viral infection, or drug inducible transcription and protein translocation to promote targeted DNA excision. The development of cell permeable (or penetrating) peptide tagged proteins has facilitated the delivery of Cre recombinase protein into cells in culture, organotypic slide culture, or in living animals. In this report, we generated bacterially expressed, his-tagged Cre protein with either a cardiac targeting peptide or an antennapedia peptide at the C-terminus and demonstrated efficient uptake and recombination in both cell culture and mice. To facilitate delivery to cardiac and skeletal muscle, we mixed proteins with pluronic F-127 hydrogel and delivered Cre protein into reporter Rosa26mTmG mouse skeletal muscle or Rosa26LacZ cardiac muscle via ultrasound guided injection. Activation of reporter gene expression indicated that these Cre proteins were enzymatically active. Recombination events were detected only in the vicinity of injection areas. In conclusion, we have developed a method to deliver enzymatically active Cre protein locally to skeletal muscle and cardiac muscle that may be adapted for use with other proteins.

Mouse models of heart failure: cell signaling and cell survival

Heart failure is one of the paramount global causes of morbidity and mortality. Despite this pandemic need, the available clinical counter-measures have not altered substantially in recent decades, most notably in the context of pharmacological interventions. Cell death plays a causal role in heart failure, and its inhibition poses a promising approach that has not been thoroughly explored. In previous approaches to target discovery, clinical failures have reflected a deficiency in mechanistic understanding, and in some instances, failure to systematically translate laboratory findings toward the clinic. Here, we review diverse mouse models of heart failure, with an emphasis on those that identify potential targets for pharmacological inhibition of cell death, and on how their translation into effective therapies might be improved in the future.

Essential role of stress hormone signaling in cardiomyocytes for the prevention of heart disease

Heart failure is a leading cause of death in humans, and stress is increasingly associated with adverse cardiac outcomes. Glucocorticoids are primary stress hormones, but their direct role in cardiovascular health and disease is poorly understood. To determine the in vivo function of glucocorticoid signaling in the heart, we generated mice with cardiomyocyte-specific deletion of the glucocorticoid receptor (GR). These mice are born at the expected Mendelian ratio, but die prematurely from spontaneous cardiovascular disease. By 3 mo of age, mice deficient in cardiomyocyte GR display a marked reduction in left ventricular systolic function, as evidenced by decreases in ejection fraction and fractional shortening. Heart weight and left ventricular mass are elevated, and histology revealed cardiac hypertrophy without fibrosis. Removal of endogenous glucocorticoids and mineralocorticoids neither augmented nor lessened the hypertrophic response. Global gene expression analysis of knockout hearts before pathology onset revealed aberrant regulation of a large cohort of genes associated with cardiovascular disease as well as unique disease genes associated with inflammatory processes. Genes important for maintaining cardiac contractility, repressing cardiac hypertrophy, promoting cardiomyocyte survival, and inhibiting inflammation had decreased expression in the GR-deficient hearts. These findings demonstrate that a deficiency in cardiomyocyte glucocorticoid signaling leads to spontaneous cardiac hypertrophy, heart failure, and death, revealing an obligate role for GR in maintaining normal cardiovascular function. Moreover, our findings suggest that selective activation of cardiomyocyte GR may represent an approach for the prevention of heart disease.

Rodent models of heart failure: an updated review

Heart failure (HF) is one of the major health and economic burdens worldwide, and its prevalence is continuously increasing. The study of HF requires reliable animal models to study the chronic changes and pharmacologic interventions in myocardial structure and function and to follow its progression toward HF. Indeed, during the past 40 years, basic and translational scientists have used small animal models to understand the pathophysiology of HF and find more efficient ways of preventing and managing patients suffering from congestive HF (CHF). Each species and each animal model has advantages and disadvantages, and the choice of one model over another should take them into account for a good experimental design. The aim of this review is to describe and highlight the advantages and drawbacks of some commonly used HF rodents models, including both non-genetically and genetically engineered models, with a specific subchapter concerning diastolic HF models.

Moderate and high amounts of tamoxifen in αMHC-MerCreMer mice induce a DNA damage response, leading to heart failure and death

Numerous mouse models have utilized Cre-loxP technology to modify gene expression. Adverse effects of Cre recombinase activity have been reported, including in the heart. However, the mechanisms associated with cardiac Cre toxicity are largely unknown. Here, we show that expression of Cre in cardiomyocytes induces a DNA damage response, resulting in cardiomyocyte apoptosis, cardiac fibrosis and cardiac dysfunction. In an effort to increase the recombination efficiency of a widely used tamoxifen-sensitive Cre transgene under control of the α-myosin-heavy-chain promoter (αMHC-MerCreMer), we observed myocardial dysfunction and decreased survival, which were dependent on the dose of tamoxifen injected. After excluding a Cre-independent contribution by tamoxifen, we found that Cre induced myocardial fibrosis, activation of pro-fibrotic genes and cardiomyocyte apoptosis. Examination of the molecular mechanisms showed activation of DNA damage response signaling and p53 stabilization in the absence of loxP sites, suggesting that Cre induced illegitimate DNA breaks. Cardiomyocyte apoptosis was also induced by expressing Cre using adenoviral transduction, indicating that the effect was not dependent on genomic integration of the transgene. Cre-mediated homologous recombination at loxP sites was dose-dependent and had a ceiling effect at ∼80% of cardiomyocytes showing recombination. By titrating the amount of tamoxifen to maximize recombination while minimizing animal lethality, we determined that 30 μg tamoxifen/g body weight/day injected on three consecutive days is the optimal condition for the αMHC-MerCreMer system to induce recombination in the Rosa26-lacZ strain. Our results further highlight the importance of experimental design, including the use of appropriate genetic controls for Cre expression.