Blocking cardiac growth in hypertrophic cardiomyopathy induces cardiac dysfunction and decreased survival only in males

Cyclin D2 is a critical mediator of exercise-induced cardiac hypertrophy

A number of signaling pathways underlying pathological cardiac hypertrophy have been identified. However, few studies have probed the functional significance of these signaling pathways in the context of exercise or physiological pathways. Exercise studies were performed on females from six different genetic mouse models that have been shown to exhibit alterations in pathological cardiac adaptation and hypertrophy. These include mice expressing constitutively active glycogen synthase kinase-3β (GSK-3βS9A), an inhibitor of CaMK II (AC3-I), both GSK-3βS9A and AC3-I (GSK-3βS9A/AC3-I), constitutively active Akt (myrAkt), mice deficient in MAPK/ERK kinase kinase-1 (MEKK1-/-), and mice deficient in cyclin D2 (cyclin D2-/-). Voluntary wheel running performance was similar to NTG littermates for five of the mouse lines. Exercise induced significant cardiac growth in all mouse models except the cyclin D2-/- mice. Cardiac function was not impacted in the cyclin D2-/- mice and studies using a phospho-antibody array identified six proteins with increased phosphorylation (greater than 150%) and nine proteins with decreased phosphorylation (greater than 33% decrease) in the hearts of exercised cyclin D2-/- mice compared to exercised NTG littermate controls. Our results demonstrate that unlike the other hypertrophic signaling molecules tested here, cyclin D2 is an important regulator of both pathologic and physiological hypertrophy. Impact statement This research is relevant as the hypertrophic signaling pathways tested here have only been characterized for their role in pathological hypertrophy, and not in the context of exercise or physiological hypertrophy. By using the same transgenic mouse lines utilized in previous studies, our findings provide a novel and important understanding for the role of these signaling pathways in physiological hypertrophy. We found that alterations in the signaling pathways tested here had no impact on exercise performance. Exercise induced cardiac growth in all of the transgenic mice except for the mice deficient in cyclin D2. In the cyclin D2 null mice, cardiac function was not impacted even though the hypertrophic response was blunted and a number of signaling pathways are differentially regulated by exercise. These data provide the field with an understanding that cyclin D2 is a key mediator of physiological hypertrophy.

Computational Approach to Measuring Myocyte Disarray in Animal Models of Heart Disease

In cardiovascular disease research, studies often include measuring cardiac function and performing histological examination of heart tissue. After measuring contractility, hearts from animals such as mice and rats are often frozen or fixed, sliced, and stained to quantify the morphology of various structures such as extracellular matrix proteins, cell nuclei, and F-actin. Traditional scoring methods have largely consisted of assessing sections of images for the presence or absence of myocyte disarray. These approaches require unbiased manual assessment, which can require extra personnel, and are not scalable to the quantity of data that can be generated by modern automated experimental techniques. Here, we describe an automated image analysis approach for unbiased numerical measurement of myocyte disarray. We provide step-by-step instructions for image preparation as well as a basic Matlab script for measurements. © 2017 by John Wiley & Sons, Inc.

The Importance of Biological Sex and Estrogen in Rodent Models of Cardiovascular Health and Disease

Nearly one-third of deaths in the United States are caused by cardiovascular disease (CVD) each year. In the past, CVD was thought to mainly affect men, leading to the exclusion of women and female animals from clinical studies and preclinical research. In light of sexual dimorphisms in CVD, a need exists to examine baseline cardiac differences in humans and the animals used to model CVD. In humans, sex differences are apparent at every level of cardiovascular physiology from action potential duration and mitochondrial energetics to cardiac myocyte and whole-heart contractile function. Biological sex is an important modifier of the development of CVD with younger women generally being protected, but this cardioprotection is lost later in life, suggesting a role for estrogen. Although endogenous estrogen is most likely a mediator of the observed functional differences in both health and disease, the signaling mechanisms involved are complex and are not yet fully understood. To investigate how sex modulates CVD development, animal models are essential tools and should be useful in the development of therapeutics. This review will focus on describing the cardiovascular sexual dimorphisms that exist both physiologically and in common animal models of CVD.

Wnt Signaling in Cardiac Disease

Wnt signaling encompasses multiple and complex signaling cascades and is involved in many developmental processes such as tissue patterning, cell fate specification, and control of cell division. Consequently, accurate regulation of signaling activities is essential for proper embryonic development. Wnt signaling is mostly silent in the healthy adult organs but a reactivation of Wnt signaling is generally observed under pathological conditions. This has generated increasing interest in this pathway from a therapeutic point of view. In this review article, the involvement of Wnt signaling in cardiovascular development will be outlined, followed by its implication in myocardial infarct healing, cardiac hypertrophy, heart failure, arrhythmias, and atherosclerosis. The initial experiments not always offer consensus on the effects of activation or inactivation of the pathway, which may be attributed to (i) the type of cardiac disease, (ii) timing of the intervention, and (iii) type of cells that are targeted. Therefore, more research is needed to determine the exact implication of Wnt signaling in the conditions mentioned above to exploit it as a powerful therapeutic target.

Role of biological sex in normal cardiac function and in its disease outcome - a review

Biological sex plays an important role in normal cardiac physiology as well as in the heart's response to cardiac disease. Women generally have better cardiac function and survival than do men in the face of cardiac disease; however, this is progressively lost when comparing postmenopausal women with age matched men. Animal model of cardiac disease mirror what is seen in humans. Sex hormones contribute significantly to sex based difference in cardiac functioning and in its disease outcome. Estrogen is considered to be cardioprotective, whereas testosterone is detrimental to heart function.

Role of the Wnt-Frizzled system in cardiac pathophysiology: a rapidly developing, poorly understood area with enormous potential

Abstract  The Wnt-Frizzled (Fzd) G-protein-coupled receptor system, involving 19 distinct Wnt ligands and 10 Fzd receptors, plays key roles in the development and functioning of many organ systems. There is increasing evidence that Wnt-Fzd signalling is important in regulating cardiac function. Wnt-Fzd signalling primarily involves a canonical pathway, with dishevelled-1-dependent nuclear translocation of β-catenin that derepresses Wnt-sensitive gene transcription, but can also include non-canonical pathways via phospholipase-C/Ca(2+) mobilization and dishevelled-protein activation of small GTPases. Wnt-Fzd effects vary with specific ligand/receptor interactions and associated downstream pathways. This paper reviews the biochemistry and physiology of the Wnt-Fzd complex, and presents current knowledge of Wnt signalling in cardiac remodelling processes such as hypertrophy and fibrosis, as well as disease states such as myocardial infarction (MI), heart failure and arrhythmias. Wnt signalling is activated during hypertrophy; inhibiting Wnt signalling by activating glycogen synthase kinase attenuates the hypertrophic response. Wnt signalling has complex and time-dependent actions post-MI, so that either beneficial or harmful effects might result from Wnt-directed interventions. Stem cell biology, a promising area for therapeutic intervention, is highly regulated by Wnt signalling. The Wnt system regulates fibroblast function, and is prominently altered in arrhythmogenic ventricular cardiomyopathy, a familial disease involving excess deposition of fibroadipose tissue. Wnt signalling controls connexin43 expression, thereby contributing to the regulation of cardiac electrical stability and arrhythmia generation. Although much has been learned about Wnt-Fzd signalling in hypertrophy and infarction, its role is poorly understood for a broad range of other heart disorders. Much more needs to be learned for its contributions to be fully appreciated, and to permit more effective exploitation of its enormous potential in therapeutic development.

Remodeling the cardiac transcriptional landscape with diet

The perception that soy food products and dietary supplements will have beneficial effects on cardiovascular health has led to a massive consumer market. However, we have previously noted that diet profoundly affects disease progression in a genetic model of hypertrophic cardiomyopathy (HCM). In this model, a soy-based diet negatively impacts cardiac function in male mice. Given the frequent connection between functional changes and transcriptional changes, we investigated the effect of diet (soy- vs. milk-based) on cardiac gene expression and how it is affected by the additional factors of sex and disease. We found that gene expression in the heart is altered more by diet than by sex or an inherited disease. We also found that the healthy male heart may be sensitized to dietary perturbations of gene expression in that it displays a gene expression profile more similar to diseased male and female hearts than to healthy female hearts. These observations may in part account for documented divergence in HCM phenotypes between males and females and between diets.

Selective activation of PI3Kalpha/Akt/GSK-3beta signalling and cardiac compensatory hypertrophy during recovery from heart failure

AIMS

Activation of phosphoinositide-3 kinase (PI3K) is essential for cell growth, relating to adaptive and maladaptive cardiac hypertrophy. This longitudinal canine study was designed to investigate the role of PI3Kalpha and PI3Kgamma in cardiac remodelling during congestive heart failure (CHF) and cardiac recovery (CR).

METHODS AND RESULTS

All dogs were surgically instrumented. Congestive heart failure was induced by cardiac pacing for 3-4 weeks and CR was allowed by terminating pacing for 5-6 weeks after induction of HF. Control dogs had sham surgery, but did not undergo pacing. Left ventricular (LV) contractile function was depressed in CHF and restored to 80-90% of the normal level in CR, with a 25% increase in LV weight. The expression of PI3Kgamma was increased four-fold in CHF, but returned to control levels in CR. In contrast, the expression of PI3Kalpha in CHF was not different from that in controls, but increased three-fold in CR and was accompanied by increases in phosphorylation of Akt (five-fold), GSK-3beta (five-fold), beta-catenin (three-fold), mTOR (two-fold), and P70S6K (two-fold).

CONCLUSION

Our results indicate that PI3K isoforms are regulated differently during the course of CHF/CR and that the selective activation of PI3Kalpha, through Akt, GSK-3beta, and mTOR signalling pathways, may be involved in the development of cardiac compensatory hypertrophy and functional restoration.

Constitutive glycogen synthase kinase-3alpha/beta activity protects against chronic beta-adrenergic remodelling of the heart

Aims

Glycogen synthase kinase 3 (GSK-3) signalling is implicated in the growth of the heart during development and in response to stress. However, its precise role remains unclear. We set out to characterize developmental growth and response to chronic isoproterenol (ISO) stress in knockin (KI) mice lacking the critical N-terminal serines, 21 of GSK-3α and 9 of GSK-3β respectively, required for inactivation by upstream kinases.

Methods and results

Between 5 and 15 weeks, KI mice grew more rapidly, but normalized heart weight and contractile performance were similar to wild-type (WT) mice. Isolated hearts of both genotypes responded comparably to acute ISO infusion with increases in heart rate and contractility. In WT mice, chronic subcutaneous ISO infusion over 14 days resulted in cardiac hypertrophy, interstitial fibrosis, and impaired contractility, accompanied by foetal gene reactivation. These effects were all significantly attenuated in KI mice. Indeed, ISO-treated KI hearts demonstrated reversible physiological remodelling traits with increased stroke volume and a preserved contractile response to acute adrenergic stimulation. Furthermore, simultaneous pharmacological inhibition of GSK-3 in KI mice treated with chronic subcutaneous ISO recapitulated the adverse remodelling phenotype seen in WT hearts.

Conclusion

Expression of inactivation-resistant GSK-3α/β does not affect eutrophic myocardial growth but protects against pathological hypertrophy induced by chronic adrenergic stimulation, maintaining cardiac function and attenuating interstitial fibrosis. Accordingly, strategies to prevent phosphorylation of Ser-21/9, and consequent inactivation of GSK-3α/β, may enable a sustained cardiac response to chronic β-agonist stimulation while preventing pathological remodelling.

Sex-based cardiac physiology

Biological sex plays an important role in normal cardiac physiology as well as in the heart's response to cardiac disease. Women generally have better cardiac function and survival than do men in the face of cardiac disease; however, this sex difference is lost when comparing postmenopausal women with age-matched men. Animal models of cardiac disease mirror what is seen in humans. Sex steroid hormones contribute significantly to sex-based differences in cardiac disease outcomes. Estrogen is generally considered to be cardioprotective, whereas testosterone is thought to be detrimental to heart function. Environmental estrogen-like molecules, such as phytoestrogens, can also affect cardiac physiology in both a positive and a negative manner.

Role of animal models in HCM research

Hypertrophic cardiomyopathy (HCM) is a complex cardiovascular genetic disorder characterized by marked clinical and genetic heterogeneity. Major advances have been made in the clinical characterization of patients with HCM and in identifying causative gene mutations. However, many questions remain regarding the underlying disease mechanisms. Furthermore, in a disease where no pharmacological treatments currently exists which can either prevent or cause regression of disease, processes to identify novel therapies are the crucial next steps. Animal models of HCM have already proved to be universally useful in confirming gene causation and dissecting out key molecular pathways involved in the development of HCM and its sequelae, including heart failure and sudden death. These findings have led to studies in animal models investigating novel therapeutic approaches in HCM, specifically targeting the development and progression of cardiac hypertrophy, fibrosis, and heart failure. This review will provide a brief summary of some of the key animal models of HCM and how these models have been utilized to understand disease mechanisms and to investigate new potential therapies. Ongoing studies using animal models of HCM will lead to a greater understanding of disease pathogenesis and will facilitate the translation of these findings to improved clinical outcomes in HCM patients.

The role of Akt/GSK-3beta signaling in familial hypertrophic cardiomyopathy

Mutations in cardiac troponin T (TnT) are a cause of familial hypertrophic cardiomyopathy (FHC). Transgenic mice expressing a missense mutation (R92Q) or a splice site donor mutation (Trunc) in the cardiac TnT gene have mutation-specific phenotypes but mice of both models have smaller hearts compared to wild type and exhibit hemodynamic dysfunction. Because growth-related signaling pathways in the hearts of mice expressing TnT mutations are not known, we evaluated the impact of increased Akt or glycogen synthase kinase-3beta (GSK-3beta) activity in both mutant TnT mice; molecules that increase heart size via physiologic pathways and block pathologic growth, respectively. Expression of activated Akt dramatically augments heart size in both R92Q and Trunc mice; however, this increase in heart size is not beneficial, since Akt also increases fibrosis in both TnT mutants and causes some pathologic gene expression shifts in the R92Q mice. Activated GSK-3beta results in further decreases in left ventricular size in both R92Q and Trunc hearts, but this decrease is associated with significant mutation-specific phenotypes. Among many pathologic consequences, activating GSK-3beta in R92Q hearts decreases phosphorylation of troponin I and results in early mortality. In contrast, increased GSK-3beta activity in Trunc hearts does not significantly impact cardiac phenotypes. These findings demonstrate that increased Akt and its downstream target, GSK-3beta can impact both cardiac size and phenotype in a mutation-specific manner. Moreover, increased activity of these molecules implicated in beneficial cardiac phenotypes exacerbates the progression of disease in the R92Q TnT mutant.

Other Transgenic Animal Models Used in Cardiovascular Studies

Previous chapters have described a large number of transgenic animal models used to study specific cardiovascular syndromes. This chapter will fill in some gaps. Many of these transgenic animals were developed to study normal and/or abnormal physiological responses in other organ systems, or to study basic biochemical and molecular reactions or pathways. These models were then discovered to also have effects on the cardiovascular system, some of them unanticipated.

A word of caution, particularly when highly inbred mouse strains are used to develop transgenic models - not all strains of a particular species are created equal. When cardiovascular parameters of age- and sex-matched A/J and C57BL/6J inbred mice were compared the C57BL/6J mice demonstrated eccentric physiologic ventricular hypertrophy, increased ventricular function, lower heart rates, and increased exercise endurance.¹

Are transgenic mice the 'alkahest' to understanding myocardial hypertrophy and failure?

Murine transgenesis using cardioselective promoters has become increasingly common in studies of cardiac hypertrophy and heart failure, with expression mediated by pronuclear microinjection being the commonest format. Without wishing to decry their usefulness, in our view, such studies are not necessarily as unambiguous as sometimes portrayed and clarity is not always their consequence. We describe broadly the types of approach undertaken in the heart and point out some of the drawbacks. We provide three arbitrarily-chosen examples where, in spite of a number of often-independent studies, no consensus has yet been achieved. These include glycogen synthase kinase 3, the extracellular signal-regulated kinase pathway and the ryanodine receptor 2. We believe that the transgenic approach should not be viewed in an empyreal light and, depending on the questions asked, we suggest that other experimental systems provide equal (or even more) valuable outcomes.

Deletion of GSK-3beta in mice leads to hypertrophic cardiomyopathy secondary to cardiomyoblast hyperproliferation

Based on extensive preclinical data, glycogen synthase kinase-3 (GSK-3) has been proposed to be a viable drug target for a wide variety of disease states, ranging from diabetes to bipolar disorder. Since these new drugs, which will be more powerful GSK-3 inhibitors than lithium, may potentially be given to women of childbearing potential, and since it has controversially been suggested that lithium therapy might be linked to congenital cardiac defects, we asked whether GSK-3 family members are required for normal heart development in mice. We report that terminal cardiomyocyte differentiation was substantially blunted in Gsk3b(-/-) embryoid bodies. While GSK-3alpha-deficient mice were born without a cardiac phenotype, no live-born Gsk3b(-/-) pups were recovered. The Gsk3b(-/-) embryos had a double outlet RV, ventricular septal defects, and hypertrophic myopathy, with near obliteration of the ventricular cavities. The hypertrophic myopathy was caused by cardiomyocyte hyperproliferation without hypertrophy and was associated with increased expression and nuclear localization of three regulators of proliferation - GATA4, cyclin D1, and c-Myc. These studies, which we believe are the first in mammals to examine the role of GSK-3alpha and GSK-3beta in the heart using loss-of-function approaches, implicate GSK-3beta as a central regulator of embryonic cardiomyocyte proliferation and differentiation, as well as of outflow tract development. Although controversy over the teratogenic effects of lithium remains, our studies suggest that caution should be exercised in the use of newer, more potent drugs targeting GSK-3 in women of childbearing age.

Glycogen synthase kinase 3 (GSK3) in the heart: a point of integration in hypertrophic signalling and a therapeutic target? A critical analysis

Glycogen synthase kinase 3 (GSK3, of which there are two isoforms, GSK3alpha and GSK3beta) was originally characterized in the context of regulation of glycogen metabolism, though it is now known to regulate many other cellular processes. Phosphorylation of GSK3alpha(Ser21) and GSK3beta(Ser9) inhibits their activity. In the heart, emphasis has been placed particularly on GSK3beta, rather than GSK3alpha. Importantly, catalytically-active GSK3 generally restrains gene expression and, in the heart, catalytically-active GSK3 has been implicated in anti-hypertrophic signalling. Inhibition of GSK3 results in changes in the activities of transcription and translation factors in the heart and promotes hypertrophic responses, and it is generally assumed that signal transduction from hypertrophic stimuli to GSK3 passes primarily through protein kinase B/Akt (PKB/Akt). However, recent data suggest that the situation is far more complex. We review evidence pertaining to the role of GSK3 in the myocardium and discuss effects of genetic manipulation of GSK3 activity in vivo. We also discuss the signalling pathways potentially regulating GSK3 activity and propose that, depending on the stimulus, phosphorylation of GSK3 is independent of PKB/Akt. Potential GSK3 substrates studied in relation to myocardial hypertrophy include nuclear factors of activated T cells, beta-catenin, GATA4, myocardin, CREB, and eukaryotic initiation factor 2Bvarepsilon. These and other transcription factor substrates putatively important in the heart are considered. We discuss whether cardiac pathologies could be treated by therapeutic intervention at the GSK3 level but conclude that any intervention would be premature without greater understanding of the precise role of GSK3 in cardiac processes.

Inhibition of glycogen synthase kinase 3beta during heart failure is protective

Glycogen synthase kinase (GSK)-3, a negative regulator of cardiac hypertrophy, is inactivated in failing hearts. To examine the histopathological and functional consequence of the persistent inhibition of GSK-3beta in the heart in vivo, we generated transgenic mice with cardiac-specific overexpression of dominant negative GSK-3beta (Tg-GSK-3beta-DN) and tetracycline-regulatable wild-type GSK-3beta. GSK-3beta-DN significantly reduced the kinase activity of endogenous GSK-3beta, inhibited phosphorylation of eukaryotic translation initiation factor 2B epsilon, and induced accumulation of beta-catenin and myeloid cell leukemia-1, confirming that GSK-3beta-DN acts as a dominant negative in vivo. Tg-GSK-3beta-DN exhibited concentric hypertrophy at baseline, accompanied by upregulation of the alpha-myosin heavy chain gene and increases in cardiac function, as evidenced by a significantly greater Emax after dobutamine infusion and percentage of contraction in isolated cardiac myocytes, indicating that inhibition of GSK-3beta induces well-compensated hypertrophy. Although transverse aortic constriction induced a similar increase in hypertrophy in both Tg-GSK-3beta-DN and nontransgenic mice, Tg-GSK-3beta-DN exhibited better left ventricular function and less fibrosis and apoptosis than nontransgenic mice. Induction of the GSK-3beta transgene in tetracycline-regulatable wild-type GSK-3beta mice induced left ventricular dysfunction and premature death, accompanied by increases in apoptosis and fibrosis. Overexpression of GSK-3beta-DN in cardiac myocytes inhibited tumor necrosis factor-alpha-induced apoptosis, and the antiapoptotic effect of GSK-3beta-DN was abrogated in the absence of myeloid cell leukemia-1. These results suggest that persistent inhibition of GSK-3beta induces compensatory hypertrophy, inhibits apoptosis and fibrosis, and increases cardiac contractility and that the antiapoptotic effect of GSK-3beta inhibition is mediated by myeloid cell leukemia-1. Thus, downregulation of GSK-3beta during heart failure could be compensatory.

Glycogen synthase kinase-3beta -- actively inhibiting hypertrophy

A number of signaling pathways are involved in the regulation of cardiac hypertrophy and remodeling. One that serves as an integrator of upstream inputs, and as a transducer to downstream effectors, is the protein kinase, glycogen synthase kinase-3beta. In this work we review the role of this putative "nodal point" in the response of the heart to growth stimuli, both physiologic and pathologic.