Myocyte hypertrophy in neonatal rat heart cultures and its regulation by serum and by catecholamines

Circadian control of histone turnover during cardiac development and growth

During postnatal cardiac hypertrophy, cardiomyocytes undergo mitotic exit, relying on DNA replication-independent mechanisms of histone turnover to maintain chromatin organization and gene transcription. In other tissues, circadian oscillations in nucleosome occupancy influence clock-controlled gene expression, suggesting a role for the circadian clock in temporal control of histone turnover and coordinated cardiomyocyte gene expression. We sought to elucidate roles for the master circadian transcription factor, Bmal1, in histone turnover, chromatin organization, and myocyte-specific gene expression and cell growth in the neonatal period. Bmal1 knockdown in neonatal rat ventricular myocytes decreased myocyte size, total cellular protein synthesis, and transcription of the fetal hypertrophic gene Nppb after treatment with serum or the α-adrenergic agonist phenylephrine. Depletion of Bmal1 decreased the expression of clock-controlled genes Per2 and Tcap, as well as Sik1, a Bmal1 target upregulated in adult versus embryonic hearts. Bmal1 knockdown impaired Per2 and Sik1 promoter accessibility as measured by micrococcal nuclease-quantitative PCR and impaired histone turnover as measured by metabolic labeling of acid-soluble chromatin fractions. Sik1 knockdown in turn decreased myocyte size, while simultaneously inhibiting natriuretic peptide B transcription and activating Per2 transcription. Linking these changes to chromatin remodeling, depletion of the replication-independent histone variant H3.3a inhibited myocyte hypertrophy and prevented phenylephrine-induced changes in clock-controlled gene transcription. Bmal1 is required for neonatal myocyte growth, replication-independent histone turnover, and chromatin organization at the Sik1 promoter. Sik1 represents a novel clock-controlled gene that coordinates myocyte growth with hypertrophic and clock-controlled gene transcription. Replication-independent histone turnover is required for transcriptional remodeling of clock-controlled genes in cardiac myocytes in response to growth stimuli.

Circadian Control of Histone Turnover During Cardiac Development and Growth

Rationale: During postnatal cardiac hypertrophy, cardiomyocytes undergo mitotic exit, relying on DNA replication-independent mechanisms of histone turnover to maintain chromatin organization and gene transcription. In other tissues, circadian oscillations in nucleosome occupancy influence clock-controlled gene expression, suggesting an unrecognized role for the circadian clock in temporal control of histone turnover and coordinate cardiomyocyte gene expression. Objective: To elucidate roles for the master circadian transcription factor, Bmal1, in histone turnover, chromatin organization, and myocyte-specific gene expression and cell growth in the neonatal period. Methods and Results: Bmal1 knockdown in neonatal rat ventricular myocytes (NRVM) decreased myocyte size, total cellular protein, and transcription of the fetal hypertrophic gene Nppb following treatment with increasing serum concentrations or the α-adrenergic agonist phenylephrine (PE). Bmal1 knockdown decreased expression of clock-controlled genes Per2 and Tcap, and salt-inducible kinase 1 (Sik1) which was identified via gene ontology analysis of Bmal1 targets upregulated in adult versus embryonic hearts. Epigenomic analyses revealed co-localized chromatin accessibility and Bmal1 localization in the Sik1 promoter. Bmal1 knockdown impaired Per2 and Sik1 promoter accessibility as measured by MNase-qPCR and impaired histone turnover indicated by metabolic labeling of acid-soluble chromatin fractions and immunoblots of total and chromatin-associated core histones. Sik1 knockdown basally increased myocyte size, while simultaneously impairing and driving Nppb and Per2 transcription, respectively. Conclusions: Bmal1 is required for neonatal myocyte growth, replication-independent histone turnover, and chromatin organization at the Sik1 promoter. Sik1 represents a novel clock-controlled gene that coordinates myocyte growth with hypertrophic and clock-controlled gene transcription.

Targeting transcription in heart failure via CDK7/12/13 inhibition

Heart failure with reduced ejection fraction (HFrEF) is associated with high mortality, highlighting an urgent need for new therapeutic strategies. As stress-activated cardiac signaling cascades converge on the nucleus to drive maladaptive gene programs, interdicting pathological transcription is a conceptually attractive approach for HFrEF therapy. Here, we demonstrate that CDK7/12/13 are critical regulators of transcription activation in the heart that can be pharmacologically inhibited to improve HFrEF. CDK7/12/13 inhibition using the first-in-class inhibitor THZ1 or RNAi blocks stress-induced transcription and pathologic hypertrophy in cultured rodent cardiomyocytes. THZ1 potently attenuates adverse cardiac remodeling and HFrEF pathogenesis in mice and blocks cardinal features of disease in human iPSC-derived cardiomyocytes. THZ1 suppresses Pol II enrichment at stress-transactivated cardiac genes and inhibits a specific pathologic gene program in the failing mouse heart. These data identify CDK7/12/13 as druggable regulators of cardiac gene transactivation during disease-related stress, suggesting that HFrEF features a critical dependency on transcription that can be therapeutically exploited.

Limonin stabilises SIRT6 by activating USP10 in cardiac hypertrophy

BACKGROUND AND PURPOSE

Limonin, a natural tetracyclic triterpenoid extract, exerts extensive pharmacological effects; however, its role in cardiac hypertrophy remains to be elucidated. We investigated the beneficial effects of limonin on cardiac hypertrophy and explored the potential mechanisms.

EXPERIMENTAL APPROACH

C57/BL6 male mice were subjected to aortic banding (AB) surgery and neonatal rat cardiac myocytes (NRCMs) were stimulated with phenylephrine (PE) to evaluate the effects of limonin on cardiac hypertrophy.

KEY RESULTS

Limonin markedly improved the cardiac function and heart weight in AB operation mice. In addition, limonin-treated mice and NRCMs produced fewer cardiac hypertrophy markers than those treated with the vehicle in hypertrophic groups. Sustained AB- or PE-stimulation impaired cardiac sirtuin 6 (SIRT6) protein levels, which were partially rescued by limonin and subsequently enhanced the activity of PPARα, and Sirt6 siRNA inhibited the anti-hypertrophic effects of limonin in vitro. Interestingly, limonin did not influence Sirt6 mRNA levels, but controlled its ubiquitin levels. Thus, the protein biosynthesis inhibitor, cycloheximide (CHX), and proteasome inhibitor, MG-132, were used to determine SIRT6 protein expression levels. Under PE stimulation, limonin increased SIRT6 protein levels in the presence of CHX, but it didn't influence SIRT6 expression in the presence of MG-132, suggesting that limonin promotes SIRT6 abundance by inhibiting its ubiquitination degradation. Furthermore, limonin inhibited the degradation of SIRT6 by activating ubiquitin-specific peptidase (Cuspidi et al.)-10, while USP10 siRNA abrogated the beneficial effects of limonin.

CONCLUSION AND IMPLICATIONS

Limonin mediates the ubiquitination and degradation of SIRT6 by activating USP10, providing an attractive therapeutic target for cardiac hypertrophy.

Enhanced NCLX-dependent mitochondrial Ca2+ efflux attenuates pathological remodeling in heart failure

Mitochondrial calcium (mCa2+) uptake couples changes in cardiomyocyte energetic demand to mitochondrial ATP production. However, excessive mCa2+ uptake triggers permeability transition and necrosis. Despite these established roles during acute stress, the involvement of mCa2+ signaling in cardiac adaptations to chronic stress remains poorly defined. Changes in NCLX expression are reported in heart failure (HF) patients and models of cardiac hypertrophy. Therefore, we hypothesized that altered mCa2+ homeostasis contributes to the hypertrophic remodeling of the myocardium that occurs upon a sustained increase in cardiac workload. The impact of mCa2+ flux on cardiac function and remodeling was examined by subjecting mice with cardiomyocyte-specific overexpression (OE) of the mitochondrial Na+/Ca2+ exchanger (NCLX), the primary mediator of mCa2+ efflux, to several well-established models of hypertrophic and non-ischemic HF. Cardiomyocyte NCLX-OE preserved contractile function, prevented hypertrophy and fibrosis, and attenuated maladaptive gene programs in mice subjected to chronic pressure overload. Hypertrophy was attenuated in NCLX-OE mice, prior to any decline in cardiac contractility. NCLX-OE similarly attenuated deleterious cardiac remodeling in mice subjected to chronic neurohormonal stimulation. However, cardiomyocyte NCLX-OE unexpectedly reduced overall survival in mice subjected to severe neurohormonal stress with angiotensin II + phenylephrine. Adenoviral NCLX expression limited mCa2+ accumulation, oxidative metabolism, and de novo protein synthesis during hypertrophic stimulation of cardiomyocytes in vitro. Our findings provide genetic evidence for the contribution of mCa2+ to early pathological remodeling in non-ischemic heart disease, but also highlight a deleterious consequence of increasing mCa2+ efflux when the heart is subjected to extreme, sustained neurohormonal stress.

High-Throughput Screening Platform in Postnatal Heart Cells and Chemical Probe Toolbox to Assess Cardiomyocyte Proliferation

Restoring lost heart muscle is an attractive goal for cardiovascular regenerative medicine. One appealing strategy is the therapeutic stimulation of cardiomyocyte proliferation, which inter alia remains challenging due to available assay technologies capturing the complex biology. Here, a high-throughput-formatted phenotypic assay platform was established using rodent whole heart-derived cells to preserve the cellular environment of cardiomyocytes. Several readouts allowed the quantification of cycling cardiomyocytes, including a transgenic H2B-mCherry system for unequivocal, automated detection of cardiomyocyte nuclei. A chemical genetics approach revealed pronounced species differences and furnished pan-kinase inhibitors 5 and 36 as potent and robust inducers of endoreplication and acytokinetic mitosis. Combined profiling of the commonly used p38 MAPK inhibitors SB203580 (1), SB239063 (2) and a novel set of skepinone-L (6) derivatives pointed to off-target effects beyond p38 that might be critical for effective cardiomyocyte cytokinesis. Kinome-focused screening eventually furnished TG003 (38) as a novel candidate for stimulating cardiomyocyte proliferation.

The Antiarrhythmic Activity of Novel Pyrrolidin-2-one Derivative S-75 in Adrenaline-Induced Arrhythmia

Arrhythmia is a quivering or irregular heartbeat that can often lead to blood clots, stroke, heart failure, and other heart-related complications. The limited efficacy and safety of antiarrhythmic drugs require the design of new compounds. Previous research indicated that pyrrolidin-2-one derivatives possess an affinity for α₁-adrenergic receptors. The blockade of α₁-adrenoceptor may play a role in restoring normal sinus rhythm; therefore, we aimed to verify the antiarrhythmic activity of novel pyrrolidin-2-one derivative S-75. In this study, we assessed the influence on sodium, calcium, potassium channels, and β₁-adrenergic receptors to investigate the mechanism of action of S-75. Lack of affinity for β₁-adrenoceptors and weak effects on ion channels decreased the role of these adrenoceptors and channels in the pharmacological activity of S-75. Next, we evaluated the influence of S-75 on normal ECG in rats and isolated rat hearts, and the tested derivative did not prolong the QT_c interval, which may confirm the lack of the proarrhythmic potential. We tested antiarrhythmic activity in adrenaline-, aconitine- and calcium chloride-induced arrhythmia models in rats. The studied compound showed prophylactic antiarrhythmic activity in the adrenaline-induced arrhythmia, but no significant activity in the model of aconitine- or calcium chloride-induced arrhythmia. In addition, S-75 was not active in the model of post-reperfusion arrhythmias of the isolated rat hearts. Conversely, the compound showed therapeutic antiarrhythmic properties in adrenaline-induced arrhythmia, reducing post-arrhythmogen heart rhythm disorders, and decreasing animal mortality. Thus, we suggest that the blockade of α₁-adrenoceptor might be beneficial in restoring normal heart rhythm in adrenaline-induced arrhythmia.

C1q/TNF-related protein 9 decreases cardiomyocyte hypoxia/reoxygenation-induced inflammation by inhibiting the TLR4/MyD88/NF-κB signaling pathway

C1q/TNF-related protein 9 (CTRP9) acts as an adipokine and has been reported to exert numerous biological functions, such as anti-inflammatory and anti-oxidative stress effects, in ischemic heart disease. In the present study, the role of CTRP9 in neonatal rat cardiomyocytes (NRCMs) following hypoxia/reoxygenation (H/R) and the underlying mechanism was investigated. Adenoviral vectors containing CTRP9 or green fluorescent protein were transfected into NRCMs. A H/R model was constructed 2 days after transfection by 2 h incubation under hypoxia followed by 4 h of reoxygenation. Lactate dehydrogenase (LDH), creatine kinase (CK) and CK-myocardial band (CK-MB) levels were detected by a biochemical analyzer using biochemical kits. In addition, cell viability was detected using trypan blue staining to determine the extent of cell injury. Inflammatory cytokines TNF-α, IL-6 and IL-10 were measured by ELISA. Western blotting and reverse transcription-quantitative PCR were used to evaluate the expression levels of CTRP9, toll-like receptor 4 (TLR4), myeloid differentiation primary response (MyD88) and NF-κB. The DNA binding activity of NF-κB was also detected using an electrophoretic mobility shift assay. The results indicated that transfection with adenoviral vectors containing CTRP9 could markedly enhance CTRP9 expression. CTRP9 overexpression increased cell viability and decreased the release of LDH, CK and CK-MB. In addition, CTRP9 overexpression reduced TNF-α and IL-6 levels whilst increasing IL-10 levels, but decreased the expression of TLR4, MyD88 and NF-κB. Furthermore, the DNA binding activity of NF-κB under H/R was also decreased by CTRP9 overexpression. In conclusion, the results of the present study suggested that CTRP9 could protect cardiomyocytes from H/R injury, which was at least partially due to the inhibition of the TLR4/MyD88/NF-κB signaling pathway to reduce the release of inflammatory cytokines.

A peptide of the N terminus of GRK5 attenuates pressure-overload hypertrophy and heart failure

Aberrant changes in gene expression underlie the pathogenesis and progression of pressure-overload heart failure, leading to maladaptive cardiac hypertrophy, ventricular remodeling, and contractile dysfunction. Signaling through the G protein G_q triggers maladaptation and heart failure, in part through the activation of G protein-coupled receptor kinase 5 (GRK5). Hypertrophic stimuli induce the accumulation of GRK5 in the nuclei of cardiomyocytes, where it regulates pathological gene expression through multiple transcription factors including NFAT. The nuclear targeting of GRK5 is mediated by an amino-terminal (NT) domain that binds to calmodulin (CaM). Here, we sought to prevent GRK5-mediated pathology in pressure-overload maladaptation and heart failure by expressing in cardiomyocytes a peptide encoding the GRK5 NT (GRK5nt) that encompasses the CaM binding domain. In cultured cardiomyocytes, GRK5nt expression abrogated G_q-coupled receptor-mediated hypertrophy, including attenuation of pathological gene expression and the transcriptional activity of NFAT and NF-κB. We confirmed that GRK5nt bound to and blocked Ca²⁺-CaM from associating with endogenous GRK5, thereby preventing GRK5 nuclear accumulation after pressure overload. We generated mice that expressed GRKnt in a cardiac-specific fashion (TgGRK5nt mice), which exhibited reduced cardiac hypertrophy, ventricular dysfunction, pulmonary congestion, and cardiac fibrosis after chronic transverse aortic constriction. Together, our data support a role for GRK5nt as an inhibitor of pathological GRK5 signaling that prevents heart failure.

Precision medicine for heart failure based on molecular mechanisms: The 2019 ISHR Research Achievement Award Lecture

Heart failure is a leading cause of death, and the number of patients with heart failure continues to increase worldwide. To realize precision medicine for heart failure, its underlying molecular mechanisms must be elucidated. In this review summarizing the "The Research Achievement Award Lecture" of the 2019 XXIII ISHR World Congress held in Beijing, China, we would like to introduce our approaches for investigating the molecular mechanisms of cardiac hypertrophy, development, and failure, as well as discuss future perspectives.

Alterations of Ca2+ signaling and Ca2+ release sites in cultured ventricular myocytes with intact internal Ca2+ storage

Although cultured adult cardiac myocytes in combination with cell-level genetic modifications have been adopted for the study of protein function, the cellular alterations caused by the culture conditions themselves need to be clarified before we can interpret the effects of genetically altered proteins. We systematically compared the cellular morphology, global Ca²⁺ signaling, elementary Ca²⁺ release (sparks), and arrangement of ryanodine receptor (RyR) clusters in short-term (2 days)-cultured adult rat ventricular myocytes with those of freshly isolated myocytes. The transverse (t)-tubules were remarkably decreased (to ∼25%) by culture, and whole-cell capacitance was reduced by ∼35%. The magnitude of depolarization-induced Ca²⁺ transients decreased to ∼50%, and Ca²⁺ transient decay was slowed by culture. The culture did not affect sarcoplasmic reticulum (SR) Ca²⁺ loading. Therefore, fractional Ca²⁺ release was attenuated by culture. In the cultured cells, the L-type Ca²⁺ current (I_Ca) was smaller (∼50% of controls) and its inactivation was slower. In cultured myocytes, there were significantly fewer (∼50% of control) Ca²⁺ sparks, the local Ca²⁺ releases through RyR clusters, compared with in freshly isolated cells. Amplitude and kinetics (duration and time-to-peak) of individual sparks were similar, but they showed greater width in cultured cells. Immunolocalization analysis revealed that the cross-striation of RyRs distribution became weaker and less organized, and that the density of RyR clusters decreased in cultured myocytes. Our data suggest that the loss of t-tubules and generation of compromised Ca²⁺ transients and I_Ca in short-term adult ventricular cell culture are independent of SR Ca²⁺ loading status. In addition, the deteriorated arrangement of the RyR-clusters and their decreased density after short-term culture may be partly responsible for fewer Ca²⁺ sparks and a decrease in global Ca²⁺ release.

Salt-inducible kinase 1 maintains HDAC7 stability to promote pathologic cardiac remodeling

Salt-inducible kinases (SIKs) are key regulators of cellular metabolism and growth, but their role in cardiomyocyte plasticity and heart failure pathogenesis remains unknown. Here, we showed that loss of SIK1 kinase activity protected against adverse cardiac remodeling and heart failure pathogenesis in rodent models and cardiomyocytes derived from human induced pluripotent stem cells. We found that SIK1 phosphorylated and stabilized histone deacetylase 7 (HDAC7) protein during cardiac stress, an event that is required for pathologic cardiomyocyte remodeling. Gain- and loss-of-function studies of HDAC7 in cultured cardiomyocytes implicated HDAC7 as a prohypertrophic signaling effector that can induce c-Myc expression, indicating a functional departure from the canonical MEF2 corepressor function of class IIa HDACs. Taken together, our findings reveal what we believe to be a previously unrecognized role for a SIK1/HDAC7 axis in regulating cardiac stress responses and implicate this pathway as a potential target in human heart failure.

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

Heart failure in single right ventricle congenital heart disease: physiological and molecular considerations

Because of remarkable surgical and medical advances over the past several decades, there are growing numbers of infants and children living with single ventricle congenital heart disease (SV), where there is only one functional cardiac pumping chamber. Nevertheless, cardiac dysfunction (and ultimately heart failure) is a common complication in the SV population, and pharmacological heart failure therapies have largely been ineffective in mitigating the need for heart transplantation. Given that there are several inherent risk factors for ventricular dysfunction in the setting of SV in addition to probable differences in molecular adaptations to heart failure between children and adults, it is perhaps not surprising that extrapolated adult heart failure medications have had limited benefit in children with SV heart failure. Further investigations into the molecular mechanisms involved in pediatric SV heart failure may assist with risk stratification as well as development of targeted, efficacious therapies specific to this patient population. In this review, we present a brief overview of SV anatomy and physiology, with a focus on patients with a single morphological right ventricle requiring staged surgical palliation. Additionally, we discuss outcomes in the current era, risk factors associated with the progression to heart failure, present state of knowledge regarding molecular alterations in end-stage SV heart failure, and current therapeutic interventions. Potential avenues for improving SV outcomes, including identification of biomarkers of heart failure progression, implications of personalized medicine and stem cell-derived therapies, and applications of novel models of SV disease, are proposed as future directions.

Progranulin deficiency leads to enhanced age-related cardiac hypertrophy through complement C1q-induced β-catenin activation

AIMS

Age-related cardiac hypertrophy and subsequent heart failure are predicted to become increasingly serious problems in aging populations. Progranulin (PGRN) deficiency is known to be associated with accelerated aging in the brain. We aimed to evaluate the effects of PGRN deficiency on cardiac aging, including left ventricular hypertrophy.

METHODS AND RESULTS

Echocardiography was performed on wild-type (WT) and PGRN-knockout (KO) mice every 3 months from 3 to 18 months of age. Compared to that of WT mice, PGRN KO mice exhibited age-dependent cardiac hypertrophy and cardiac dysfunction at 18 months. Morphological analyses showed that the heart weight to tibia length ratio and cross-sectional area of cardiomyocytes at 18 months were significantly increased in PGRN KO mice relative to those in WT mice. Furthermore, accumulation of lipofuscin and increases in senescence markers were observed in the hearts of PGRN KO mice, suggesting that PGRN deficiency led to enhanced aging of the heart. Enhanced complement C1q (C1q) and activated β-catenin protein expression levels were also observed in the hearts of aged PGRN KO mice. Treatment of PGRN-deficient cardiomyocytes with C1q caused β-catenin activation and cardiac hypertrophy. Blocking C1q-induced β-catenin activation in PGRN-depleted cardiomyocytes attenuated hypertrophic changes. Finally, we showed that C1 inhibitor treatment reduced cardiac hypertrophy and dysfunction in old KO mice, possibly by reducing β-catenin activation. These results suggest that C1q is a crucial regulator of cardiac hypertrophy induced by PGRN ablation.

CONCLUSION

The present study demonstrates that PGRN deficiency enhances age-related cardiac hypertrophy via C1q-induced β-catenin activation. PGRN is a potential therapeutic target to prevent cardiac hypertrophy and dysfunction.

The N6-Methyladenosine mRNA Methylase METTL3 Controls Cardiac Homeostasis and Hypertrophy

BACKGROUND

N⁶-Methyladenosine (m6A) methylation is the most prevalent internal posttranscriptional modification on mammalian mRNA. The role of m6A mRNA methylation in the heart is not known.

METHODS

To determine the role of m6A methylation in the heart, we isolated primary cardiomyocytes and performed m6A immunoprecipitation followed by RNA sequencing. We then generated genetic tools to modulate m6A levels in cardiomyocytes by manipulating the levels of the m6A RNA methylase methyltransferase-like 3 (METTL3) both in culture and in vivo. We generated cardiac-restricted gain- and loss-of-function mouse models to allow assessment of the METTL3-m6A pathway in cardiac homeostasis and function.

RESULTS

We measured the level of m6A methylation on cardiomyocyte mRNA, and found a significant increase in response to hypertrophic stimulation, suggesting a potential role for m6A methylation in the development of cardiomyocyte hypertrophy. Analysis of m6A methylation showed significant enrichment in genes that regulate kinases and intracellular signaling pathways. Inhibition of METTL3 completely abrogated the ability of cardiomyocytes to undergo hypertrophy when stimulated to grow, whereas increased expression of the m6A RNA methylase METTL3 was sufficient to promote cardiomyocyte hypertrophy both in vitro and in vivo. Finally, cardiac-specific METTL3 knockout mice exhibit morphological and functional signs of heart failure with aging and stress, showing the necessity of RNA methylation for the maintenance of cardiac homeostasis.

CONCLUSIONS

Our study identified METTL3-mediated methylation of mRNA on N⁶-adenosines as a dynamic modification that is enhanced in response to hypertrophic stimuli and is necessary for a normal hypertrophic response in cardiomyocytes. Enhanced m6A RNA methylation results in compensated cardiac hypertrophy, whereas diminished m6A drives eccentric cardiomyocyte remodeling and dysfunction, highlighting the critical importance of this novel stress-response mechanism in the heart for maintaining normal cardiac function.

A high-throughput ratiometric method for imaging hypertrophic growth in cultured primary cardiac myocytes

Maladaptive hypertrophy of cardiac myocytes increases the risk of heart failure. The underlying signaling can be triggered and interrogated in cultured neonatal ventricular myocytes (NRVMs) using sophisticated pharmacological and genetic techniques. However, the methods for quantifying cell growth are, by comparison, inadequate. The lack of quantitative, calibratable and computationally-inexpensive high-throughput technology has limited the scope for using cultured myocytes in large-scale analyses. We present a ratiometric method for quantifying the hypertrophic growth of cultured myocytes, compatible with high-throughput imaging platforms. Protein biomass was assayed from sulforhodamine B (SRB) fluorescence, and image analysis calculated the quotient of signal from extra-nuclear and nuclear regions. The former readout relates to hypertrophic growth, whereas the latter is a reference for correcting protein-independent (e.g. equipment-related) variables. This ratiometric measure, when normalized to the number of cells, provides a robust quantification of cellular hypertrophy. The method was tested by comparing the efficacy of various chemical agonists to evoke hypertrophy, and verified using independent assays (myocyte area, transcripts of markers). The method's high resolving power and wide dynamic range were confirmed by the ability to generate concentration-response curves, track the time-course of hypertrophic responses with fine temporal resolution, describe drug/agonist interactions, and screen for novel anti-hypertrophic agents. The method can be implemented as an end-point in protocols investigating hypertrophy, and is compatible with automated plate-reader platforms for generating high-throughput data, thereby reducing investigator-bias. Finally, the computationally-minimal workflow required for obtaining measurements makes the method simple to implement in most laboratories.

•

Maladaptive hypertrophy of myocytes can lead to heart failure.

•

Common methods for tracking growth in cultured myocytes are inadequate.

•

We design and test a method for tracking myocyte hypertrophy in vitro.

•

The method provides a ratiometric index of growth for high throughput analyses.

•

Using the method, we characterize further details of (anti)hypertrophic responses.

FBXL10 regulates cardiac dysfunction in diabetic cardiomyopathy via the PKC β2 pathway

Diabetic cardiomyopathy (DCM) is a condition associated with significant structural changes including cardiac tissue necrosis, localized fibrosis, and cardiomyocyte hypertrophy. This study sought to assess whether and how FBXL10 can attenuate DCM using a rat streptozotocin (STZ)‐induced DCM model system. In the current study, we found that FBXL10 expression was significantly decreased in diabetic rat hearts. FBXL10 protected cells from high glucose (HG)‐induced inflammation, oxidative stress, and apoptosis in vitro. In addition, FBXL10 significantly activated PKC β2 signaling pathway in H9c2 cells and rat model. The cardiomyocyte‐specific overexpression of FBXL10 at 12 weeks after the initial STZ administration attenuated oxidative stress and inflammation, thereby reducing cardiomyocyte death and preserving cardiac function in these animals. Moreover, FBXL10 protected against DCM via activation of the PKC β2 pathway. In conclusion, FBXL has the therapeutic potential for the treatment of DCM.

Luteolin suppresses lipopolysaccharide‑induced cardiomyocyte hypertrophy and autophagy in vitro

Luteolin (LTL) serves essential roles in a wide variety of biological processes. Lipopolysaccharide (LPS) can lead to myocardial hypertrophy and autophagy. However, the roles of LTL on LPS-induced cardiomyocyte hypertrophy and autophagy in rat cardiomyocytes have not yet been fully elucidated. In the present study, the morphology of cultured rat cardiomyocytes was observed under an inverted microscope. Cell viability was detected by MTT assay. α-Actinin and microtubule-associated protein 1 light chain 3 (LC3) expression levels were measured by immunofluorescence assay. In addition, the expression levels of atrial natriuretic peptide/brain natriuretic peptide (ANP/BNP), LC3, and autophagy- and Wnt signaling pathway-associated genes were analyzed by reverse transcription-quantitative polymerase chain reaction or western blot assays. The results indicated that LTL increased the cell viability of cardiomyocytes treated with LPS. LTL decreased the expression of cardiac hypertrophy associated markers (ANP and BNP). LTL decreased α-actinin and LC3 expression levels in LPS-treated cardiomyocytes. It was also demonstrated that LTL suppressed the mRNA and protein expression levels of LPS-mediated autophagy and Wnt signaling pathway-associated genes. In addition, it was demonstrated that silencing of β-catenin inhibited LPS-induced cardiomyocyte hypertrophy and the formation of autophagosomes. Thus, the present study suggested that LTL protected against LPS-induced cardiomyocyte hypertrophy and autophagy in rat cardiomyocytes.

ADAP1 limits neonatal cardiomyocyte hypertrophy by reducing integrin cell surface expression

The ArfGAP with dual PH domains 1 (ADAP1) regulates the activation of the hypertrophic mitogen-activated protein kinase ERK1/2 pathway in non-cardiomyocytes. However, its role in cardiomyocytes is unknown. Our aim was to characterize the role of ADAP1 in the hypertrophic process of cardiomyocytes. We assessed the expression of ADAP1 in the hearts of adult and neonatal rats by RT-qPCR and Western blotting and showed that it is preferentially expressed in cardiomyocytes. Adenoviral-mediated ADAP1 overexpression in cultured rat neonatal ventricular cardiomyocytes limited their serum-induced hypertrophic response as measured by immunofluorescence microscopy. Furthermore, ADAP1 overexpression completely blocked phenylephrine- and Mek1 constitutively active (Mek1ca) mutant-induced hypertrophy in these cells. The anti-hypertrophic effect of ADAP1 was not caused by a reduction in protein synthesis, interference with the Erk1/2 pathway, or disruption of the fetal gene program activation, as assessed by nascent protein labeling, Western blotting, and RT-qPCR, respectively. An analysis of cultured cardiomyocytes by confocal microscopy revealed that ADAP1 partially re-organizes α-actinin into dense puncta, a phenomenon that is synergized by Mek1ca overexpression. Biotin labeling of cell surface proteins from cardiomyocytes overexpressing ADAP1 revealed that it reduces the surface expression of β1-integrin, an effect that is strongly potentiated by Mek1ca overexpression. Our findings provide insights into the anti-hypertrophic function of ADAP1 in cardiomyocytes.

T-bet deficiency attenuates cardiac remodelling in rats

Previous studies have suggested the involvement of CD4 + T lymphocytes in cardiac remodelling. T-bet can direct Th1 lineage commitment. This study aimed to investigate the functional significance of T-bet in cardiac remodelling induced by pressure overload using T-bet global knockout rats. Increased T-bet levels were observed in rodent and human hypertrophied hearts. T-bet deficiency resulted in a less severe hypertrophic phenotype in rats. CD4 + T-lymphocyte reconstitution in T-bet-/- rats resulted in aggravated cardiac remodelling. T-cell homing molecule expression and cytokine secretion were altered in T-bet-deficient rat hearts. Administration of exogenous interferon-γ (IFN-γ) offset T-bet deficiency-mediated cardioprotection. Cardiomyocytes cultured in T-bet-/- CD4 + T-cell-conditioned media showed a reduced hypertrophic response after hypertrophic stimuli, which was abolished by an IFN-γ-neutralizing antibody. Taken together, our findings show that T-bet deficiency attenuates pressure overload-induced cardiac remodelling in rats. Specifically, targeting T-bet in T cells may be of great importance for the treatment of pathological cardiac remodelling and heart failure.

Experimental Models of Cardiovascular Diseases: Overview

Cardiovascular disease is one of the most common causes of deaths in clinics. Experimental models of cardiovascular diseases are essential to understand disease mechanism, to provide accurate diagnoses, and to develop new therapies. Large numbers of experimental models have been proposed and replicated by many laboratories in the past. Models with significant advantages are chosen and became more popular. Particularly, feasibility, reproducibility, and human disease resemblance are the common key factors for frequently used cardiovascular disease models. In this chapter, we provide a brief overview of these experimental models used for in vitro, in vivo, and in silico studies of cardiovascular diseases.

Rosiglitazone promotes cardiac hypertrophy and alters chromatin remodeling in isolated cardiomyocytes

Rosiglitazone is an anti-diabetic agent that raised a major controversy over its cardiovascular adverse effects. There is in vivo evidence that Rosiglitazone promotes cardiac hypertrophy by PPAR-γ-independent mechanisms. However, whether Rosiglitazone directly alters hypertrophic growth in cardiac cells is unknown. Chromatin remodeling by histone post-translational modifications has emerged as critical for many cardiomyopathies. Based on these observations, this study was initiated to investigate the cardiac hypertrophic effect of Rosiglitazone in a cellular model of primary neonatal rat cardiomyocytes (NRCM). We assessed whether the drug alters cardiac hypertrophy and its relationship with histone H3 phosphorylation. Our study showed that Rosiglitazone is a mild pro-hypertrophic agent. Rosiglitazone caused a significant increase in the release of brain natriuretic peptide (BNP) into the cell media and also increased cardiomyocytes surface area and atrial natriuretic peptide (ANP) protein expression significantly. These changes correlated with increased cardiac phosphorylation of p38 MAPK and enhanced phosphorylation of H3 at serine 10 globally and at one cardiac hypertrophic gene locus. These results demonstrate that Rosiglitazone causes direct cardiac hypertrophy in NRCM and alters H3 phosphorylation status. They suggest a new mechanism of Rosiglitazone cardiotoxicity implicating chromatin remodeling secondary to H3 phosphorylation, which activate the fetal cardiac gene program.

Biology of the cardiac myocyte in heart disease

Cardiac hypertrophy is a major risk factor for heart failure, and it has been shown that this increase in size occurs at the level of the cardiac myocyte. Cardiac myocyte model systems have been developed to study this process. Here we focus on cell culture tools, including primary cells, immortalized cell lines, human stem cells, and their morphological and molecular responses to pathological stimuli. For each cell type, we discuss commonly used methods for inducing hypertrophy, markers of pathological hypertrophy, advantages for each model, and disadvantages to using a particular cell type over other in vitro model systems. Where applicable, we discuss how each system is used to model human disease and how these models may be applicable to current drug therapeutic strategies. Finally, we discuss the increasing use of biomaterials to mimic healthy and diseased hearts and how these matrices can contribute to in vitro model systems of cardiac cell biology.

CTRP3 attenuates cardiac dysfunction, inflammation, oxidative stress and cell death in diabetic cardiomyopathy in rats

AIMS/HYPOTHESIS

Oxidative stress, inflammation and cell death are closely involved in the development of diabetic cardiomyopathy (DCM). C1q/tumour necrosis factor-related protein-3 (CTRP3) has anti-inflammatory properties but its role in DCM remains largely unknown. The aims of this study were to determine whether CTRP3 could attenuate DCM and to clarify the underlying mechanisms.

METHODS

Streptozotocin (STZ) was injected intraperitoneally to induce diabetes in Sprague-Dawley rats. Cardiomyocyte-specific CTRP3 overexpression was achieved using an adeno-associated virus system 12 weeks after STZ injection.

RESULTS

CTRP3 expression was significantly decreased in diabetic rat hearts. Knockdown of CTRP3 in cardiomyocytes at baseline resulted in increased oxidative injury, inflammation and apoptosis in vitro. Cardiomyocyte-specific overexpression of CTRP3 decreased oxidative stress and inflammation, attenuated myocyte death and improved cardiac function in rats treated with STZ. CTRP3 significantly activated AMP-activated protein kinase α (AMPKα) and Akt (protein kinase B) in H9c2 cells. CTRP3 protected against high-glucose-induced oxidative stress, inflammation and apoptosis in vitro. AMPKα deficiency abolished the protective effects of CTRP3 in vitro and in vivo. Furthermore, we found that CTRP3 activated AMPKα via the cAMP-exchange protein directly activated by cAMP (EPAC)-mitogen-activated protein kinase kinase (MEK) pathway.

CONCLUSIONS/INTERPRETATION

CTRP3 protected against DCM via activation of the AMPKα pathway. CTRP3 has therapeutic potential for the treatment of DCM.

Transcription Factor 7-like 2 Mediates Canonical Wnt/β-Catenin Signaling and c-Myc Upregulation in Heart Failure

BACKGROUND

How canonical Wnt/β-catenin signals in adult hearts, especially in different diseased states, remains unclear. The proto-oncogene, c-Myc, is a Wnt target and an early response gene during cardiac stress. It is not clear whether c-Myc is activated or how it is regulated during heart failure.

METHODS AND RESULTS

We investigated canonical Wnt/β-catenin signaling and how it regulated c-Myc expression in failing hearts of human ischemic heart disease, idiopathic dilated cardiomyopathy, and murine desmin-related cardiomyopathy. Our data demonstrated that canonical Wnt/β-catenin signaling was activated through nuclear accumulation of β-catenin in idiopathic dilated cardiomyopathy, ischemic heart disease, and murine desmin-related cardiomyopathy when compared with nonfailing controls and transcription factor 7-like 2 (TCF7L2) was the main β-catenin partner of the T-cell factor (TCF) family in adult hearts. We further revealed that c-Myc mRNA and protein levels were significantly elevated in failing hearts by real-time reverse transcription polymerase chain reaction, Western blotting, and immunohistochemical staining. Immunoprecipitation and confocal microscopy further showed that β-catenin interacted and colocalized with TCF7L2. More importantly, chromatin immunoprecipitation confirmed that β-catenin and TCF7L2 were recruited to the regulatory elements of c-Myc. This recruitment was associated with increased histone H3 acetylation and transcriptional upregulation of c-Myc. With lentiviral infection, TCF7L2 overexpression increased c-Myc expression and cardiomyocyte size, whereas shRNA-mediated knockdown of TCF7L2 suppressed c-Myc expression and cardiomyocyte growth in cultured neonatal rat cardiomyocytes.

CONCLUSIONS

This study indicates that TCF7L2 mediates canonic Wnt/β-catenin signaling and c-Myc upregulation during abnormal cardiac remodeling in heart failure and suppression of Wnt/β-catenin to c-Myc axis can be explored for preventing and treating heart failure.

Asiatic Acid Protects against Cardiac Hypertrophy through Activating AMPKα Signalling Pathway

Background: AMPactivated protein kinase α (AMPKα) is closely involved in the process of cardiac hypertrophy. Asiatic acid (AA), a pentacyclic triterpene, was found to activate AMPKα in our preliminary experiment. However, its effects on the development of cardiac hypertrophy remain unclear. The present study was to determine whether AA could protect against cardiac hypertrophy.

Methods: Mice subjected to aortic banding were orally given AA (10 or 30mg/kg) for 7 weeks. In the inhibitory experiment, Compound C was intraperitoneally injected for 3 weeks after surgery.

Results: Our results showed that AA markedly inhibited hypertrophic responses induced by pressure overload or angiotensin II. AA also suppressed cardiac fibrosis in vivo and accumulation of collagen in vitro. The protective effects of AA were mediated by activation of AMPKα and inhibition of the mammalian target of rapamycin (mTOR) pathway and extracellular signal-regulated kinase (ERK) in vivo and in vitro. However, AA lost the protective effects after AMPKα inhibition or gene deficiency.

Conclusions: AA protects against cardiac hypertrophy by activating AMPKα, and has the potential to be used for the treatment of heart failure.

Pioglitazone Protected against Cardiac Hypertrophy via Inhibiting AKT/GSK3β and MAPK Signaling Pathways

Peroxisome proliferator activated receptor γ (PPARγ) has been closely involved in the process of cardiovascular diseases. This study was to investigate whether pioglitazone (PIO), a PPARγ agonist, could protect against pressure overload-induced cardiac hypertrophy. Mice were orally given PIO (2.5 mg/kg) from 1 week after aortic banding and continuing for 7 weeks. The morphological examination and biochemical analysis were used to evaluate the effects of PIO. Neonatal rat ventricular cardiomyocytes were also used to verify the protection of PIO against hypertrophy in vitro. The results in our study demonstrated that PIO remarkably inhibited hypertrophic response induced by aortic banding in vivo. Besides, PIO also suppressed cardiac fibrosis in vivo. PIO treatment also inhibited the activation of protein kinase B (AKT)/glycogen synthase kinase-3β (GSK3β) and mitogen-activated protein kinase (MAPK) in the heart. In addition, PIO alleviated angiotensin II-induced hypertrophic response in vitro. In conclusion, PIO could inhibit cardiac hypertrophy via attenuation of AKT/GSK3β and MAPK pathways.

Dynamic Myofibrillar Remodeling in Live Cardiomyocytes under Static Stretch

An increase in mechanical load in the heart causes cardiac hypertrophy, either physiologically (heart development, exercise and pregnancy) or pathologically (high blood pressure and heart-valve regurgitation). Understanding cardiac hypertrophy is critical to comprehending the mechanisms of heart development and treatment of heart disease. However, the major molecular event that occurs during physiological or pathological hypertrophy is the dynamic process of sarcomeric addition, and it has not been observed. In this study, a custom-built second harmonic generation (SHG) confocal microscope was used to study dynamic sarcomeric addition in single neonatal CMs in a 3D culture system under acute, uniaxial, static, sustained stretch. Here we report, for the first time, live-cell observations of various modes of dynamic sarcomeric addition (and how these real-time images compare to static images from hypertrophic hearts reported in the literature): 1) Insertion in the mid-region or addition at the end of a myofibril; 2) Sequential addition with an existing myofibril as a template; and 3) Longitudinal splitting of an existing myofibril. The 3D cell culture system developed on a deformable substrate affixed to a stretcher and the SHG live-cell imaging technique are unique tools for real-time analysis of cultured models of hypertrophy.

Simvastatin reverses cardiomyocyte hypertrophy via the upregulation of phosphatase and tensin homolog expression

The aim of the present study was to investigate the effects of simvastatin on the protein kinase B (PKB) signaling pathway and the expression of phosphatase and tensin homolog (PTEN). The effects of simvastatin were analyzed by administering the drug orally to male spontaneously hypertensive rats (SHRs) at a dose of 10 mg/kg/day, while the control animals received an equal volume of saline. The systolic pressure (mmHg) of the rat tail artery was measured prior to the initiation of the experiment, and once a week until the end of the experiment. At the end of the experiment, the animals were euthanized and the hearts were removed. The left ventricular and interventricular septum were weighed, after which the left ventricular mass/body mass ratio was calculated. In addition, cardiomyocytes isolated from Sprague Dawley rats were cultured with 15% fetal bovine serum to induce hypertrophy, following which the cells were treated with different doses of simvastatin. The in vitro effects were assessed by measuring the surface area of the cardiomyocytes, while the rate of protein synthesis was measured using a 3H-leucine incorporation assay and western blot analyses. Simvastatin was demonstrated to inhibit cardiomyocyte hypertrophy in the in vivo and in vitro experiments. Notably, simvastatin increased PTEN expression and inhibited PKB expression in the SHR model, as well as in the cardiomyocytes in culture. In addition, the use of PTEN antisense oligodeoxynucleotides was revealed to inhibit the effects of simvastatin on cardiomyocytes. Therefore, these results indicated that simvastatin was able to reverse cardiomyocyte hypertrophy in vivo and in vitro, possibly by increasing the expression of PTEN.

Exogenous Nkx2.5- or GATA-4-transfected rabbit bone marrow mesenchymal stem cells and myocardial cell co-culture on the treatment of myocardial infarction in rabbits

The present study aimed to investigate the effects of Nkx2.5 or GATA-4 transfection with myocardial extracellular environment co-culture on the transformation of bone marrow mesenchymal stem cells (BMSCs) into differentiated cardiomyocytes. Nkx2.5 or GATA-4 were transfected into myocardial extracellular environment co-cultured BMSCs, and then injected into the periphery of infarcted myocardium of a myocardial infarction rabbit model. The effects of these gene transfections and culture on the infarcted myocardium were observed and the results may provide an experimental basis for the efficient myocardial cell differentiation of BMSCs. The present study also suggested that these cells may provide a source and clinical basis for myocardial injury repair via stem cell transplantation. The present study examined whether Nkx2.5 or GATA-4 exogenous gene transfection with myocardial cell extracellular environment co-culture were able to induce the differentiation of BMSCs into cardiac cells. In addition, the effect of these transfected BMSCs on the repair of the myocardium following myocardial infarction was determined using New Zealand rabbit models. The results demonstrated that myocardial cell differentiation was significantly less effective following exogenous gene transfection of Nkx2.5 or GATA-4 alone compared with that of transfection in combination with extracellular environment co-culture. In addition, the results of the present study showed that exogenous gene transfection of Nkx2.5 or GATA-4 into myocardial cell extracellular environment co-cultured BMSCs was able to significantly enhance the ability to repair, mitigating the death of myocardial cells and activation of the myocardium in rabbits with myocardial infarction compared with those of the rabbits transplanted with untreated BMSCs. In conclusion, the exogenous Nkx2.5 and GATA-4 gene transfection into myocardial extracellular environment co-cultured BMSCs induced increased differentiation into myocardial cells compared with that of gene transfection alone. Furthermore, significantly enhanced reparative effects were observed in the myocardium of rabbits following treatment with Nkx2.5- or GATA-4-transfected myocardial cell extracellular environment co-cultured BMSCs compared with those treated with untreated BMSCs.

Isolation and cryopreservation of neonatal rat cardiomyocytes

Cell culture has become increasingly important in cardiac research, but due to the limited proliferation of cardiomyocytes, culturing cardiomyocytes is difficult and time consuming. The most commonly used cells are neonatal rat cardiomyocytes (NRCMs), which require isolation every time cells are needed. The birth of the rats can be unpredictable. Cryopreservation is proposed to allow for cells to be stored until needed, yet freezing/thawing methods for primary cardiomyocytes are challenging due to the sensitivity of the cells. Using the proper cryoprotectant, dimethyl sulfoxide (DMSO), cryopreservation was achieved. By slowly extracting the DMSO while thawing the cells, cultures were obtained with viable NRCMs. NRCM phenotype was verified using immunocytochemistry staining for α-sarcomeric actinin. In addition, cells also showed spontaneous contraction after several days in culture. Cell viability after thawing was acceptable at 40-60%. In spite of this, the methods outlined allow one to easily cryopreserve and thaw NRCMs. This gives researchers a greater amount of flexibility in planning experiments as well as reducing the use of animals.

Cryopreservation of neonatal cardiomyocytes

Cardiomyocytes are frequently used for in vitro models for cardiac research. The isolation of cells is time-consuming and, due to the cells limited proliferative abilities, must be performed frequently. To reduce the time requirements and the impact on research animals, we describe a method for cryopreserving neonatal rat cardiomyocytes (NRCMs), and subsequently thawing them for use in assays.

Stable, covalent attachment of laminin to microposts improves the contractility of mouse neonatal cardiomyocytes

The mechanical output of contracting cardiomyocytes, the muscle cells of the heart, relates to healthy and disease states of the heart. Culturing cardiomyocytes on arrays of elastomeric microposts can enable inexpensive and high-throughput studies of heart disease at the single-cell level. However, cardiomyocytes weakly adhere to these microposts, which limits the possibility of using biomechanical assays of single cardiomyocytes to study heart disease. We hypothesized that a stable covalent attachment of laminin to the surface of microposts improves cardiomyocyte contractility. We cultured cells on polydimethylsiloxane microposts with laminin covalently bonded with the organosilanes 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane with glutaraldehyde. We measured displacement of microposts induced by the contractility of mouse neonatal cardiomyocytes, which attach better than mature cardiomyocytes to substrates. We observed time-dependent changes in contractile parameters such as micropost deformation, contractility rates, contraction and relaxation speeds, and the times of contractions. These parameters were affected by the density of laminin on microposts and by the stability of laminin binding to micropost surfaces. Organosilane-mediated binding resulted in higher laminin surface density and laminin binding stability. 3-glycidoxypropyltrimethoxysilane provided the highest laminin density but did not provide stable protein binding with time. Higher surface protein binding stability and strength were observed with 3-aminopropyltriethoxysilane with glutaraldehyde. In cultured cardiomyocytes, contractility rate, contraction speeds, and contraction time increased with higher laminin stability. Given these variations in contractile function, we conclude that binding of laminin to microposts via 3-aminopropyltriethoxysilane with glutaraldehyde improves contractility observed by an increase in beating rate and contraction speed as it occurs during the postnatal maturation of cardiomyocytes. This approach is promising for future studies to mimic in vivo tissue environments.

The phosphatidylinositol(4,5)bisphosphate-binding sequence of transient receptor potential channel canonical 4α is critical for its contribution to cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy requires a source of Ca(2+) distinct from the Ca(2+) that regulates contraction. The canonical transient receptor potential channel (TrpC) family, a family of cation channels regulated by activation of phospholipase C (PLC), has been implicated in this response. Cardiomyocyte hypertrophy downstream of Gq-coupled receptors is mediated specifically by PLCβ1b that is scaffolded onto a SH3 and ankyrin repeat protein 3 (Shank3) complex at the sarcolemma. TrpC4 exists as two splice variants (TrpC4α and TrpC4β) that differ only in an 84-residue sequence that binds to phosphatidylinositol(4,5)bisphosphate (PIP2), the substrate of PLCβ1b. In neonatal rat cardiomyocytes, TrpC4α, but not TrpC4β, coimmunoprecipitated with both PLCβ1b and Shank3. Heightened PLCβ1b expression caused TrpC4α, but not TrpC4β, translocation to the sarcolemma, where it colocalized with PLCβ1b. When overexpressed in cardiomyocytes, TrpC4α, but not TrpC4β, increased cell area (893 ± 18 to 1497 ± 29 mm(2), P < 0.01) and marker gene expression (atrial natriuretic peptide increased by 409 ± 32%, and modulatory calcineurin inhibitory protein 1 by 315 ± 28%, P < 0.01). Dominant-negative TrpC4 reduced hypertrophy initiated by PLCβ1b, or PLCβ1b-coupled receptor activation, by 72 ± 8% and 39 ± 5 %, respectively. We conclude that TrpC4α is selectively involved in mechanisms downstream of PLCβ1b culminating in cardiomyocyte hypertrophy, and that the hypertrophic response is dependent on the TrpC4α splice variant-specific sequence that binds to PIP2.

Tom70 serves as a molecular switch to determine pathological cardiac hypertrophy

Pathological cardiac hypertrophy is an inevitable forerunner of heart failure. Regardless of the etiology of cardiac hypertrophy, cardiomyocyte mitochondrial alterations are always observed in this context. The translocases of mitochondrial outer membrane (Tom) complex governs the import of mitochondrial precursor proteins to maintain mitochondrial function under pathophysiological conditions; however, its role in the development of pathological cardiac hypertrophy remains unclear. Here, we showed that Tom70 was downregulated in pathological hypertrophic hearts from humans and experimental animals. The reduction in Tom70 expression produced distinct pathological cardiomyocyte hypertrophy both in vivo and in vitro. The defective mitochondrial import of Tom70-targeted optic atrophy-1 triggered intracellular oxidative stress, which led to a pathological cellular response. Importantly, increased Tom70 levels provided cardiomyocytes with full resistance to diverse pro-hypertrophic insults. Together, these results reveal that Tom70 acts as a molecular switch that orchestrates hypertrophic stresses and mitochondrial responses to determine pathological cardiac hypertrophy.

Serum supplemented culture medium masks hypertrophic phenotypes in human pluripotent stem cell derived cardiomyocytes

It has been known for over 20 years that foetal calf serum can induce hypertrophy in cultured cardiomyocytes but this is rarely considered when examining cardiomyocytes derived from pluripotent stem cells (PSC). Here, we determined how serum affected cardiomyocytes from human embryonic- (hESC) and induced pluripotent stem cells (hiPSC) and hiPSC from patients with hypertrophic cardiomyopathy linked to a mutation in the MYBPC3 gene. We first confirmed previously published hypertrophic effects of serum on cultured neonatal rat cardiomyocytes demonstrated as increased cell surface area and beating frequency. We then found that serum increased the cell surface area of hESC- and hiPSC-derived cardiomyocytes and their spontaneous contraction rate. Phenylephrine, which normally induces cardiac hypertrophy, had no additional effects under serum conditions. Likewise, hiPSC-derived cardiomyocytes from three MYBPC3 patients which had a greater surface area than controls in the absence of serum as predicted by their genotype, did not show this difference in the presence of serum. Serum can thus alter the phenotype of human PSC derived cardiomyocytes under otherwise defined conditions such that the effects of hypertrophic drugs and gene mutations are underestimated. It is therefore pertinent to examine cardiac phenotypes in culture media without or in low concentrations of serum.

Modulation of Fas-Fas Ligand Interaction Rehabilitates Hypoxia-Induced Apoptosis of Mesenchymal Stem Cells in Ischemic Myocardium Niche

Mesenchymal stem cells (MSCs) have the potential to repair and regenerate ischemic heart tissue; however, the poor viability of transplanted MSCs in the ischemic region is a major obstacle to their therapeutic use. This cell death is caused by Fas and Fas ligand (FasL) interactions under harsh conditions. To investigate improving the survival and therapeutic effects of MSCs, we focused our research on Fas-FasL-mediated cell death. In this study, we found that the poor viability of transplanted MSCs was caused by Fas-FasL interactions between host ischemic myocardial cells and implanted MSCs. In addition, we found that increased Fas expression and the corresponding decrease of cell survival were in close relation to hypoxic MSCs treated with FasL and H2O2. When MSCs were treated with a recombinant Fas/Fc chimera (Fas/Fc) inhibiting Fas-FasL interactions, the expressions of proapoptotic proteins including caspase-8, caspase-3, Bax, and cytochrome-c were attenuated, and the survival of MSCs was recovered. In ischemia-reperfusion injury models, the interaction between FasL in ischemic heart and Fas in implanted MSCs caused a loss of transplanted MSCs, whereas the inhibition of this interaction by Fas/Fc treatment improved cell survival and restored heart function. Thus, our study suggests that Fas-FasL interactions are responsible for activating cell death signaling in implanted stem cells and could be a potential target for improving therapeutic efficacy of stem cells in treating ischemic heart diseases.

Developmental cues for the maturation of metabolic, electrophysiological and calcium handling properties of human pluripotent stem cell-derived cardiomyocytes

Human pluripotent stem cells (hPSCs), including embryonic and induced pluripotent stem cells, are abundant sources of cardiomyocytes (CMs) for cell replacement therapy and other applications such as disease modeling, drug discovery and cardiotoxicity screening. However, hPSC-derived CMs display immature structural, electrophysiological, calcium-handling and metabolic properties. Here, we review various biological as well as physical and topographical cues that are known to associate with the development of native CMs in vivo to gain insights into the development of strategies for facilitated maturation of hPSC-CMs.

Effects of daily hemodialysis on heart rate variability: results from the Frequent Hemodialysis Network (FHN) Daily Trial

BACKGROUND

End-stage renal disease is associated with reduced heart rate variability (HRV), components of which generally are associated with advanced age, diabetes mellitus and left ventricular hypertrophy. We hypothesized that daily in-center hemodialysis (HD) would increase HRV.

METHODS

The Frequent Hemodialysis Network (FHN) Daily Trial randomized 245 patients to receive 12 months of six versus three times per week in-center HD. Two hundred and seven patients had baseline Holter recordings. HRV measures were calculated from 24-h Holter electrocardiograms at both baseline and 12 months in 131 patients and included low-frequency power (LF, a measure of sympathetic modulation), high-frequency power (HF, a measure of parasympathetic modulation) and standard deviation (SD) of the R-R interval (SDNN, a measure of beat-to-beat variation).

RESULTS

Baseline to Month 12 change in LF was augmented by 50% [95% confidence interval (95% CI) 6.1-112%, P =0.022] and LF + HF was augmented by 40% (95% CI 3.3-88.4%, P = 0.03) in patients assigned to daily hemodialysis (DHD) compared with conventional HD. Changes in HF and SDNN were similar between the randomized groups. The effects of DHD on LF were attenuated by advanced age and diabetes mellitus (predefined subgroups). Changes in HF (r = -0.20, P = 0.02) and SDNN (r = -0.18, P = 0.04) were inversely associated with changes in left ventricular mass (LVM).

CONCLUSIONS

DHD increased the LF component of HRV. Reduction of LVM by DHD was associated with increased vagal modulation of heart rate (HF) and with increased beat-to-beat heart rate variation (SDNN), suggesting an important functional correlate to the structural effects of DHD on the heart in uremia.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

Soluble Concentrations of the Interleukin Receptor Family Member ST2 and β-Blocker Therapy in Chronic Heart Failure

Background—

Concentrations of soluble (s)ST2 predict prognosis in heart failure. We recently found changing doses of β-blocker (BB) may affect sST2 concentrations. It remains unclear whether sST2 concentrations identify benefit of BB therapy, however.

Methods and Results—

A total of 151 subjects with heart failure attributable to left ventricular systolic dysfunction were examined in this post hoc analysis; >96% were taking BB at enrollment. Medication regimen and sST2 values were obtained during 10 months. Cardiovascular events were examined as a function of baseline sST2 status (low ≤35 versus high >35 ng/mL) and final achieved BB dose (high ≥50 versus low <50 mg daily equivalent dose of metoprolol succinate). Patients with low sST2 titrated to high-dose BB had the lowest cardiovascular event rate at 0.53 events ( P =0.001), and lowest cumulative hazard ( P =0.003). Those with low sST2/low-dose BB, or high sST2/high-dose BB had intermediate outcomes (0.92 and 1.19 events). Patients with high sST2 treated with low-dose BB had the highest cardiovascular event rate (2.08 events) and the highest cumulative hazard. Compared with low sST2/high-dose BB, those with high sST2 treated with low-dose BB had an odds ratio of 6.77 ( P <0.001) for a cardiovascular event. Patients with low sST2/low-dose BB or high sST2/high-dose BB had intermediate odds ratios for cardiovascular events ( P =0.18 and 0.02). Similar results were found for heart failure hospitalization and cardiovascular death.

Conclusions—

Although BB therapy exerted dose-related benefits across all study participants, sST2 measurement identifies patients with chronic heart failure who may particularly benefit from higher BB doses.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov . Unique identifier: NCT00351390.

Identification and visualization of stimulus-specific transcriptional activity in cardiac hypertrophy in mice

Identification of specific signaling pathways for cardiac hypertrophy in living animals is challenging because no methods have been established to directly observe sequential molecular signaling events at the transcriptional level during pathogenesis. Here, our aim was to develop a useful method for monitoring the specific signaling pathways involved in the development of cardiac hypertrophy in vivo. Expression profiling of the left ventricle by microarray was performed in 2 different mouse models of cardiac hypertrophy: mechanical pressure overload by transverse aortic constriction (TAC) and neurohumoral activation by angiotensin II (Ang II) infusion. To annotate the information on transcription factor-binding sites, we collected promoter sequences and identified significantly frequent transcription factor-binding sites in the promoter regions of coregulated genes from both models (P < 0.05, binomial probability). Finally, we injected a firefly luciferase vector plasmid containing each transcription factor-binding site into the left ventricle in both models. In the TAC and Ang II models, we selected 379 and 12 upregulated genes, respectively. Twenty binding sites for transcription factors, including activator protein 4, were identified in the TAC model, and 4 sites for transcription factors, including ecotropic viral integration 1, were identified in the Ang II model. GATA-binding sites were noted in both models of cardiac hypertrophy. Using the firefly luciferase reporter, we demonstrated the enhancement of transcriptional activity during the progression of cardiac hypertrophy using in vivo imaging in live mice. These results suggested that our approach was useful for the identification of unique transcription factors that characterize different models of cardiac hypertrophy in vivo.

Roles for ATF6 and the sarco/endoplasmic reticulum protein quality control system in the heart

The hypertrophic growth of cardiac myocytes is a highly dynamic process that underlies physiological and pathological adaptation of the heart. Accordingly, a better understanding of the molecular underpinnings of cardiac myocyte hypertrophy is required in order to fully appreciate the causes and functional consequences of the changes in the size of the healthy and diseased heart. Hypertrophy is driven by increases in cardiac myocyte protein, which must be balanced by cellular ability to maintain protein quality in order to avoid maladaptive accumulation of toxic misfolded proteins. Recent studies have shown that the endoplasmic reticulum (ER), which, in cardiac myocytes, comprises the sarco/endoplasmic reticulum (SR/ER), is the site of most protein synthesis. Thus, the protein quality control machinery located at the SR/ER is likely to be an important determinant of whether the heart responds adaptively to hypertrophic growth stimuli. The SR/ER-transmembrane protein, ATF6, serves a critical protein quality control function as a first responder to the accumulation of potentially toxic, misfolded proteins. Misfolded proteins transform ATF6 into a transcription factor that regulates a gene program that is partly responsible for enhancing protein quality control. Two ATF6-inducible genes that have been studied in the heart and shown to be adaptive are RCAN1 and Derl3, which encode proteins that decrease protein-folding demand, and enhance degradation of misfolded proteins, respectively. Thus, the ATF6-regulated SR/ER protein quality control system is important for maintaining protein quality during growth, making ATF6, and other components of the system, potentially attractive targets for the therapeutic management pathological cardiac hypertrophy. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".