miR-182/183-Rasa1 axis induced macrophage polarization and redox regulation promotes repair after ischemic cardiac injury

Few therapies have produced significant improvement in cardiac structure and function after ischemic cardiac injury (ICI). Our possible explanation is activation of local inflammatory responses negatively impact the cardiac repair process following ischemic injury. Factors that can alter immune response, including significantly altered cytokine levels in plasma and polarization of macrophages and T cells towards a pro-reparative phenotype in the myocardium post-MI is a valid strategy for reducing infarct size and damage after myocardial injury. Our previous studies showed that cortical bone stem cells (CBSCs) possess reparative effects after ICI. In our current study, we have identified that the beneficial effects of CBSCs appear to be mediated by miRNA in their extracellular vesicles (CBSC-EV). Our studies showed that CBSC-EV treated animals demonstrated reduced scar size, attenuated structural remodeling, and improved cardiac function versus saline treated animals. These effects were linked to the alteration of immune response, with significantly altered cytokine levels in plasma, and polarization of macrophages and T cells towards a pro-reparative phenotype in the myocardium post-MI. Our detailed in vitro studies demonstrated that CBSC-EV are enriched in miR-182/183 that mediates the pro-reparative polarization and metabolic reprogramming in macrophages, including enhanced OXPHOS rate and reduced ROS, via Ras p21 protein activator 1 (RASA1) axis under Lipopolysaccharides (LPS) stimulation. In summary, CBSC-EV deliver unique molecular cargoes, such as enriched miR-182/183, that modulate the immune response after ICI by regulating macrophage polarization and metabolic reprogramming to enhance repair.

Effects of maternal hypothyroidism on postnatal cardiomyocyte proliferation and cardiac disease responses of the progeny

Maternal hypothyroidism (MH) could adversely affect the cardiac disease responses of the progeny. This study tested the hypothesis that MH reduces early postnatal cardiomyocyte (CM) proliferation so that the adult heart of MH progeny has a smaller number of larger cardiac myocytes, which imparts adverse cardiac disease responses following injury. Thyroidectomy (TX) was used to establish MH. The progeny from mice that underwent sham or TX surgery were termed Ctrl (control) or MH (maternal hypothyroidism) progeny, respectively. MH progeny had similar heart weight (HW) to body weight (BW) ratios and larger CM size consistent with fewer CMs at postnatal day 60 (P60) compared with Ctrl (control) progeny. MH progeny had lower numbers of EdU+, Ki67+, and phosphorylated histone H3 (PH3)+ CMs, which suggests they had a decreased CM proliferation in the postnatal timeframe. RNA-seq data showed that genes related to DNA replication were downregulated in P5 MH hearts, including bone morphogenetic protein 10 (Bmp10). Both in vivo and in vitro studies showed Bmp10 treatment increased CM proliferation. After transverse aortic constriction (TAC), the MH progeny had more severe cardiac pathological remodeling compared with the Ctrl progeny. Thyroid hormone (T4) treatment for MH mothers preserved their progeny's postnatal CM proliferation capacity and prevented excessive pathological remodeling after TAC. Our results suggest that CM proliferation during early postnatal development was significantly reduced in MH progeny, resulting in fewer CMs with hypertrophy in adulthood. These changes were associated with more severe cardiac disease responses after pressure overload.NEW & NOTEWORTHY Our study shows that compared with Ctrl (control) progeny, the adult progeny of mothers who have MH (MH progeny) had fewer CMs. This reduction of CM numbers was associated with decreased postnatal CM proliferation. Gene expression studies showed a reduced expression of Bmp10 in MH progeny. Bmp10 has been linked to myocyte proliferation. In vivo and in vitro studies showed that Bmp10 treatment of MH progeny and their myocytes could increase CM proliferation. Differences in CM number and size in adult hearts of MH progeny were linked to more severe cardiac structural and functional remodeling after pressure overload. T4 (synthetic thyroxine) treatment of MH mothers during their pregnancy, prevented the reduction in CM number in their progeny and the adverse response to disease stress.

Combining three independent pathological stressors induces a heart failure with preserved ejection fraction phenotype

Heart failure (HF) with preserved ejection fraction (HFpEF) is defined as HF with an ejection fraction (EF) ≥ 50% and elevated cardiac diastolic filling pressures. The underlying causes of HFpEF are multifactorial and not well-defined. A transgenic mouse with low levels of cardiomyocyte (CM)-specific inducible Cavβ2a expression (β2a-Tg mice) showed increased cytosolic CM Ca2+, and modest levels of CM hypertrophy, and fibrosis. This study aimed to determine if β2a-Tg mice develop an HFpEF phenotype when challenged with two additional stressors, high-fat diet (HFD) and Nω-nitro-l-arginine methyl ester (l-NAME, LN). Four-month-old wild-type (WT) and β2a-Tg mice were given either normal chow (WT-N, β2a-N) or HFD and/or l-NAME (WT-HFD, WT-LN, WT-HFD-LN, β2a-HFD, β2a-LN, and β2a-HFD-LN). Some animals were treated with the histone deacetylase (HDAC) (hypertrophy regulators) inhibitor suberoylanilide hydroxamic acid (SAHA) (β2a-HFD-LN-SAHA). Echocardiography was performed monthly. After 4 mo of treatment, terminal studies were performed including invasive hemodynamics and organs weight measurements. Cardiac tissue was collected. Four months of HFD plus l-NAME treatment did not induce a profound HFpEF phenotype in FVB WT mice. β2a-HFD-LN (3-Hit) mice developed features of HFpEF, including increased atrial natriuretic peptide (ANP) levels, preserved EF, diastolic dysfunction, robust CM hypertrophy, increased M2-macrophage population, and myocardial fibrosis. SAHA reduced the HFpEF phenotype in the 3-Hit mouse model, by attenuating these effects. The 3-Hit mouse model induced a reliable HFpEF phenotype with CM hypertrophy, cardiac fibrosis, and increased M2-macrophage population. This model could be used for identifying and preclinical testing of novel therapeutic strategies.NEW & NOTEWORTHY Our study shows that three independent pathological stressors (increased Ca2+ influx, high-fat diet, and l-NAME) together produce a profound HFpEF phenotype. The primary mechanisms include HDAC-dependent-CM hypertrophy, necrosis, increased M2-macrophage population, fibroblast activation, and myocardial fibrosis. A role for HDAC activation in the HFpEF phenotype was shown in studies with SAHA treatment, which prevented the severe HFpEF phenotype. This "3-Hit" mouse model could be helpful in identifying novel therapeutic strategies to treat HFpEF.

Sex-specific responses to slow progressive pressure overload in a large animal model of HFpEF

Approximately 50% of all heart failure (HF) diagnoses can be classified as HF with preserved ejection fraction (HFpEF). HFpEF is more prevalent in females compared with males, but the underlying mechanisms are unknown. We previously showed that pressure overload (PO) in male felines induces a cardiopulmonary phenotype with essential features of human HFpEF. The goal of this study was to determine if slow progressive PO induces distinct cardiopulmonary phenotypes in females and males in the absence of other pathological stressors. Female and male felines underwent aortic constriction (banding) or sham surgery after baseline echocardiography, pulmonary function testing, and blood sampling. These assessments were repeated at 2 and 4 mo postsurgery to document the effects of slow progressive pressure overload. At 4 mo, invasive hemodynamic studies were also performed. Left ventricle (LV) tissue was collected for histology, myofibril mechanics, extracellular matrix (ECM) mass spectrometry, and single-nucleus RNA sequencing (snRNAseq). The induced pressure overload (PO) was not different between sexes. PO also induced comparable changes in LV wall thickness and myocyte cross-sectional area in both sexes. Both sexes had preserved ejection fraction, but males had a slightly more robust phenotype in hemodynamic and pulmonary parameters. There was no difference in LV fibrosis and ECM composition between banded male and female animals. LV snRNAseq revealed changes in gene programs of individual cell types unique to males and females after PO. Based on these results, both sexes develop cardiopulmonary dysfunction but the phenotype is somewhat less advanced in females.NEW & NOTEWORTHY We performed a comprehensive assessment to evaluate the effects of slow progressive pressure overload on cardiopulmonary function in a large animal model of heart failure with preserved ejection fraction (HFpEF) in males and females. Functional and structural assessments were performed at the organ, tissue, cellular, protein, and transcriptional levels. This is the first study to compare snRNAseq and ECM mass spectrometry of HFpEF myocardium from males and females. The results broaden our understanding of the pathophysiological response of both sexes to pressure overload. Both sexes developed a robust cardiopulmonary phenotype, but the phenotype was equal or a bit less robust in females.

Cortical bone stem cells modify cardiac inflammation after myocardial infarction by inducing a novel macrophage phenotype

Acute damage to the heart, as in the case of myocardial infarction (MI), triggers a robust inflammatory response to the sterile injury that is part of a complex and highly organized wound-healing process. Cortical bone stem cell (CBSC) therapy after MI has been shown to reduce adverse structural and functional remodeling of the heart after MI in both mouse and swine models. The basis for these CBSC treatment effects on wound healing are unknown. The present experiments show that CBSCs secrete paracrine factors known to have immunomodulatory properties, most notably macrophage colony-stimulating factor (M-CSF) and transforming growth factor-β, but not IL-4. CBSC therapy increased the number of galectin-3+ macrophages, CD4+ T cells, and fibroblasts in the heart while decreasing apoptosis in an in vivo swine model of MI. Macrophages treated with CBSC medium in vitro polarized to a proreparative phenotype are characterized by increased CD206 expression, increased efferocytic ability, increased IL-10, TGF-β, and IL-1RA secretion, and increased mitochondrial respiration. Next generation sequencing revealed a transcriptome significantly different from M2a or M2c macrophage phenotypes. Paracrine factors from CBSC-treated macrophages increased proliferation, decreased α-smooth muscle actin expression, and decreased contraction by fibroblasts in vitro. These data support the idea that CBSCs are modulating the immune response to MI to favor cardiac repair through a unique macrophage polarization that ultimately reduces cell death and alters fibroblast populations that may result in smaller scar size and preserved cardiac geometry and function.NEW & NOTEWORTHY Cortical bone stem cell (CBSC) therapy after myocardial infarction alters the inflammatory response to cardiac injury. We found that cortical bone stem cell therapy induces a unique macrophage phenotype in vitro and can modulate macrophage/fibroblast cross talk.

Molecular Signature of HFpEF: Systems Biology in a Cardiac-Centric Large Animal Model

In this study the authors used systems biology to define progressive changes in metabolism and transcription in a large animal model of heart failure with preserved ejection fraction (HFpEF). Transcriptomic analysis of cardiac tissue, 1-month post-banding, revealed loss of electron transport chain components, and this was supported by changes in metabolism and mitochondrial function, altogether signifying alterations in oxidative metabolism. Established HFpEF, 4 months post-banding, resulted in changes in intermediary metabolism with normalized mitochondrial function. Mitochondrial dysfunction and energetic deficiencies were noted in skeletal muscle at early and late phases of disease, suggesting cardiac-derived signaling contributes to peripheral tissue maladaptation in HFpEF. Collectively, these results provide insights into the cellular biology underlying HFpEF progression.

Interaction of the Joining Region in Junctophilin-2 With the L-Type Ca2+ Channel Is Pivotal for Cardiac Dyad Assembly and Intracellular Ca2+ Dynamics

RATIONALE

Ca2+-induced Ca2+ release (CICR) in normal hearts requires close approximation of L-type calcium channels (LTCCs) within the transverse tubules (T-tubules) and RyR (ryanodine receptors) within the junctional sarcoplasmic reticulum. CICR is disrupted in cardiac hypertrophy and heart failure, which is associated with loss of T-tubules and disruption of cardiac dyads. In these conditions, LTCCs are redistributed from the T-tubules to disrupt CICR. The molecular mechanism responsible for LTCCs recruitment to and from the T-tubules is not well known. JPH (junctophilin) 2 enables close association between T-tubules and the junctional sarcoplasmic reticulum to ensure efficient CICR. JPH2 has a so-called joining region that is located near domains that interact with T-tubular plasma membrane, where LTCCs are housed. The idea that this joining region directly interacts with LTCCs and contributes to LTCC recruitment to T-tubules is unknown.

OBJECTIVE

To determine if the joining region in JPH2 recruits LTCCs to T-tubules through direct molecular interaction in cardiomyocytes to enable efficient CICR.

METHODS AND RESULTS

Modified abundance of JPH2 and redistribution of LTCC were studied in left ventricular hypertrophy in vivo and in cultured adult feline and rat ventricular myocytes. Protein-protein interaction studies showed that the joining region in JPH2 interacts with LTCC-α1C subunit and causes LTCCs distribution to the dyads, where they colocalize with RyRs. A JPH2 with induced mutations in the joining region (mutPG1JPH2) caused T-tubule remodeling and dyad loss, showing that an interaction between LTCC and JPH2 is crucial for T-tubule stabilization. mutPG1JPH2 caused asynchronous Ca2+-release with impaired excitation-contraction coupling after β-adrenergic stimulation. The disturbed Ca2+ regulation in mutPG1JPH2 overexpressing myocytes caused calcium/calmodulin-dependent kinase II activation and altered myocyte bioenergetics.

CONCLUSIONS

The interaction between LTCC and the joining region in JPH2 facilitates dyad assembly and maintains normal CICR in cardiomyocytes.

Abstract 342: Cortical Bone Stem Cell Therapy Alters Macrophage Phenotype and Reduces Cardiac Cell Death After Myocardial Infarction

Acute injuries to the heart, like myocardial infarction (MI), contribute to the development and pathology of heart failure (HF). Reperfusion of the ischemic heart greatly increases survival but results in reperfusion injury that can account for up to 50% of the final infarct size. The inflammatory response to MI-induced myocardial injury is thought to be responsible for the propagation of reperfusion injury into the infarct border zone, expanding myocardial damage. We have previously shown in a swine model of MI that intramyocardial injections of cortical bone-derived stem cells (CBSCs) into the infarct border zone has no acute cardioprotective effect but reduces scar size by half and prevents the decline of ejection fraction and LV dilation 3 months after MI. Our new preliminary data show that CBSCs have potent immunoregulatory capabilities. Therefore, we hypothesize that CBSC treatment has an effect on the immune response to MI that improves the wound healing response to myocardial injury and mitigates LV remodeling and infarct size 3 months later. To test this hypothesis, we characterized the effects of CBSC paracrine factors on macrophages in vitro and found that CBSC-treated macrophages express higher levels of CD206, produce more IL-1RA and IL-10, and phagocytose apoptotic myocytes more efficiently. In addition, macrophages were increased in CBSC-treated swine hearts 7 days after MI compared to controls with a corresponding increase in IL-1RA and TIMP-2. Apoptosis was decreased overall and in macrophages specifically in CBSC-treated animals. From these data we conclude CBSCs may exert an acute pro-reparative effect on the immune response after MI, reducing reperfusion injury and adverse remodeling resulting in improved functional outcomes at later time points.

HDAC inhibition improves cardiopulmonary function in a feline model of diastolic dysfunction

Heart failure with preserved ejection fraction (HFpEF) is a major health problem without effective therapies. This study assessed the effects of histone deacetylase (HDAC) inhibition on cardiopulmonary structure, function, and metabolism in a large mammalian model of pressure overload recapitulating features of diastolic dysfunction common to human HFpEF. Male domestic short-hair felines (n = 31, aged 2 months) underwent a sham procedure (n = 10) or loose aortic banding (n = 21), resulting in slow-progressive pressure overload. Two months after banding, animals were treated daily with suberoylanilide hydroxamic acid (b + SAHA, 10 mg/kg, n = 8), a Food and Drug Administration-approved pan-HDAC inhibitor, or vehicle (b + veh, n = 8) for 2 months. Echocardiography at 4 months after banding revealed that b + SAHA animals had significantly reduced left ventricular hypertrophy (LVH) (P < 0.0001) and left atrium size (P < 0.0001) versus b + veh animals. Left ventricular (LV) end-diastolic pressure and mean pulmonary arterial pressure were significantly reduced in b + SAHA (P < 0.01) versus b + veh. SAHA increased myofibril relaxation ex vivo, which correlated with in vivo improvements of LV relaxation. Furthermore, SAHA treatment preserved lung structure, compliance, blood oxygenation, and reduced perivascular fluid cuffs around extra-alveolar vessels, suggesting attenuated alveolar capillary stress failure. Acetylation proteomics revealed that SAHA altered lysine acetylation of mitochondrial metabolic enzymes. These results suggest that acetylation defects in hypertrophic stress can be reversed by HDAC inhibitors, with implications for improving cardiac structure and function in patients.

Cortical bone-derived stem cell therapy reduces apoptosis after myocardial infarction

Ischemic heart diseases such as myocardial infarction (MI) are the largest contributors to cardiovascular disease worldwide. The resulting cardiac cell death impairs function of the heart and can lead to heart failure and death. Reperfusion of the ischemic tissue is necessary but causes damage to the surrounding tissue by reperfusion injury. Cortical bone stem cells (CBSCs) have been shown to increase pump function and decrease scar size in a large animal swine model of MI. To investigate the potential mechanism for these changes, we hypothesized that CBSCs were altering cardiac cell death after reperfusion. To test this, we performed TUNEL staining for apoptosis and antibody-based immunohistochemistry on tissue from Göttingen miniswine that underwent 90 min of lateral anterior descending coronary artery ischemia followed by 3 or 7 days of reperfusion to assess changes in cardiomyocyte and noncardiomyocyte cell death. Our findings indicate that although myocyte apoptosis is present 3 days after ischemia and is lower in CBSC-treated animals, myocyte apoptosis accounts for <2% of all apoptosis in the reperfused heart. In addition, nonmyocyte apoptosis trends toward decreased in CBSC-treated hearts, and although CBSCs increase macrophage and T-cell populations in the infarct region, the occurrence of apoptosis in CD45⁺ cells in the myocardium is not different between groups. From these data, we conclude that CBSCs may be influencing cardiomyocyte and noncardiomyocyte cell death and immune cell recruitment dynamics in the heart after MI, and these changes may account for some of the beneficial effects conferred by CBSC treatment.NEW & NOTEWORTHY The following research explores aspects of cell death and inflammation that have not been previously studied in a large animal model. In addition, apoptosis and cell death have not been studied in the context of cell therapy and myocardial infarction. In this article, we describe interactions between cell therapy and inflammation and the potential implications for cardiac wound healing.

Abstract 826: Histone Deacetylase Inhibition Improves Heart Failure With Preserved Ejection Fraction Cardiopulmonary Phenotype and Induces Metabolomic Switch

Rationale: Approximately 50% of heart failure patients are diagnosed with Heart Failure with preserved Ejection Fraction (HFpEF), but there are currently no effective treatments.

Hypothesis: Treatment of a feline HFpEF animal model with a pan HDAC inhibitor, SAHA, will improve the cardiopulmonary phenotype and mediate changes in the metabolome and skeletal muscle composition.

Methods and results: Male domestic short hair cats (n=21, age 2mo) underwent either a sham procedure (n=5) or aortic constriction (n=16) using a customized pre-shaped band causing slow progressive pressure overload during maturation. At 2-months post-banding, banded animals received either daily treatment with 10mg/kg SAHA (b+SAHA) (n=8) or vehicle (b+veh) (n=8) for 2 months. At 4 months post-banding, b+ SAHA animals had significantly reduced LV wall thickness, LA size (LA/Ao), and improved LA function (LA EF) vs. b+ veh animals (fig). Invasive hemodynamics and lung mechanics were performed after 2 months of treatment. Banded animals had significantly increased filling pressures (LVEDP), increased pulmonary arterial pressures (mPAP), decreased lung compliance and arterial oxygenation (A-aDO 2 ). SAHA significantly reduced LVEDP, mPAP, and A-aDO 2 and increased lung compliance (fig). b+SAHA animals had an increase in the percentage of type 1 muscle fibers (increased oxidative capacity) compared to type 2 (fig). Blood-based metabolomics revealed SAHA-induced a metabolic shift towards increased lipolysis and mitochondrial oxidation.

Conclusion: Treatment with SAHA improved cardiopulmonary structure and function in banded animals and caused changes in the metabolome and skeletal muscle.

Caveolae-localized L-type Ca2+ channels do not contribute to function or hypertrophic signalling in the mouse heart

Aims

L-type Ca2+ channels (LTCCs) in adult cardiomyocytes are localized to t-tubules where they initiate excitation-contraction coupling. Our recent work has shown that a subpopulation of LTCCs found at the surface sarcolemma in caveolae of adult feline cardiomyocytes can also generate a Ca2+ microdomain that activates nuclear factor of activated T-cells signaling and cardiac hypertrophy, although the relevance of this paradigm to hypertrophy regulation in vivo has not been examined.

Methods and results

Here we generated heart-specific transgenic mice with a putative caveolae-targeted LTCC activator protein that was ineffective in initiating or enhancing cardiac hypertrophy in vivo. We also generated transgenic mice with cardiac-specific overexpression of a putative caveolae-targeted inhibitor of LTCCs, and while this protein inhibited caveolae-localized LTCCs without effects on global Ca2+ handling, it similarly had no effect on cardiac hypertrophy in vivo. Cardiac hypertrophy was elicited by pressure overload for 2 or 12 weeks or with neurohumoral agonist infusion. Caveolae-specific LTCC activator or inhibitor transgenic mice showed no greater change in nuclear factor of activated T-cells activity after 2 weeks of pressure overload stimulation compared with control mice.

Conclusion

Our results indicate that LTCCs in the caveolae microdomain do not affect cardiac function and are not necessary for the regulation of hypertrophic signaling in the adult mouse heart.

A Feline HFpEF Model with Pulmonary Hypertension and Compromised Pulmonary Function

Heart Failure with preserved Ejection Fraction (HFpEF) represents a major public health problem. The causative mechanisms are multifactorial and there are no effective treatments for HFpEF, partially attributable to the lack of well-established HFpEF animal models. We established a feline HFpEF model induced by slow-progressive pressure overload. Male domestic short hair cats (n = 20), underwent either sham procedures (n = 8) or aortic constriction (n = 12) with a customized pre-shaped band. Pulmonary function, gas exchange, and invasive hemodynamics were measured at 4-months post-banding. In banded cats, echocardiography at 4-months revealed concentric left ventricular (LV) hypertrophy, left atrial (LA) enlargement and dysfunction, and LV diastolic dysfunction with preserved systolic function, which subsequently led to elevated LV end-diastolic pressures and pulmonary hypertension. Furthermore, LV diastolic dysfunction was associated with increased LV fibrosis, cardiomyocyte hypertrophy, elevated NT-proBNP plasma levels, fluid and protein loss in pulmonary interstitium, impaired lung expansion, and alveolar-capillary membrane thickening. We report for the first time in HFpEF perivascular fluid cuff formation around extra-alveolar vessels with decreased respiratory compliance. Ultimately, these cardiopulmonary abnormalities resulted in impaired oxygenation. Our findings support the idea that this model can be used for testing novel therapeutic strategies to treat the ever growing HFpEF population.

Cortical Bone Stem Cell Therapy Preserves Cardiac Structure and Function After Myocardial Infarction

RATIONALE

Cortical bone stem cells (CBSCs) have been shown to reduce ventricular remodeling and improve cardiac function in a murine myocardial infarction (MI) model. These effects were superior to other stem cell types that have been used in recent early-stage clinical trials. However, CBSC efficacy has not been tested in a preclinical large animal model using approaches that could be applied to patients.

OBJECTIVE

To determine whether post-MI transendocardial injection of allogeneic CBSCs reduces pathological structural and functional remodeling and prevents the development of heart failure in a swine MI model.

METHODS AND RESULTS

Female Göttingen swine underwent left anterior descending coronary artery occlusion, followed by reperfusion (ischemia-reperfusion MI). Animals received, in a randomized, blinded manner, 1:1 ratio, CBSCs (n=9; 2×10⁷ cells total) or placebo (vehicle; n=9) through NOGA-guided transendocardial injections. 5-ethynyl-2'deoxyuridine (EdU)-a thymidine analog-containing minipumps were inserted at the time of MI induction. At 72 hours (n=8), initial injury and cell retention were assessed. At 3 months post-MI, cardiac structure and function were evaluated by serial echocardiography and terminal invasive hemodynamics. CBSCs were present in the MI border zone and proliferating at 72 hours post-MI but had no effect on initial cardiac injury or structure. At 3 months, CBSC-treated hearts had significantly reduced scar size, smaller myocytes, and increased myocyte nuclear density. Noninvasive echocardiographic measurements showed that left ventricular volumes and ejection fraction were significantly more preserved in CBSC-treated hearts, and invasive hemodynamic measurements documented improved cardiac structure and functional reserve. The number of EdU⁺ cardiac myocytes was increased in CBSC- versus vehicle- treated animals.

CONCLUSIONS

CBSC administration into the MI border zone reduces pathological cardiac structural and functional remodeling and improves left ventricular functional reserve. These effects reduce those processes that can lead to heart failure with reduced ejection fraction.

Remodeling of repolarization and arrhythmia susceptibility in a myosin-binding protein C knockout mouse model

Hypertrophic cardiomyopathy (HCM) is one of the most common genetic cardiac diseases and among the leading causes of sudden cardiac death (SCD) in the young. The cellular mechanisms leading to SCD in HCM are not well known. Prolongation of the action potential (AP) duration (APD) is a common feature predisposing hypertrophied hearts to SCD. Previous studies have explored the roles of inward Na⁺ and Ca²⁺ in the development of HCM, but the role of repolarizing K⁺ currents has not been defined. The objective of this study was to characterize the arrhythmogenic phenotype and cellular electrophysiological properties of mice with HCM, induced by myosin-binding protein C (MyBPC) knockout (KO), and to test the hypothesis that remodeling of repolarizing K⁺ currents causes APD prolongation in MyBPC KO myocytes. We demonstrated that MyBPC KO mice developed severe hypertrophy and cardiac dysfunction compared with wild-type (WT) control mice. Telemetric electrocardiographic recordings of awake mice revealed prolongation of the corrected QT interval in the KO compared with WT control mice, with overt ventricular arrhythmias. Whole cell current- and voltage-clamp experiments comparing KO with WT mice demonstrated ventricular myocyte hypertrophy, AP prolongation, and decreased repolarizing K⁺ currents. Quantitative RT-PCR analysis revealed decreased mRNA levels of several key K⁺ channel subunits. In conclusion, decrease in repolarizing K⁺ currents in MyBPC KO ventricular myocytes contributes to AP and corrected QT interval prolongation and could account for the arrhythmia susceptibility.NEW & NOTEWORTHY Ventricular myocytes isolated from the myosin-binding protein C knockout hypertrophic cardiomyopathy mouse model demonstrate decreased repolarizing K⁺ currents and action potential and QT interval prolongation, linking cellular repolarization abnormalities with arrhythmia susceptibility and the risk for sudden cardiac death in hypertrophic cardiomyopathy.

Role of STIM1 (Stromal Interaction Molecule 1) in Hypertrophy-Related Contractile Dysfunction

RATIONALE

Pathological increases in cardiac afterload result in myocyte hypertrophy with changes in myocyte electrical and mechanical phenotype. Remodeling of contractile and signaling Ca²⁺ occurs in pathological hypertrophy and is central to myocyte remodeling. STIM1 (stromal interaction molecule 1) regulates Ca²⁺ signaling in many cell types by sensing low endoplasmic reticular Ca²⁺ levels and then coupling to plasma membrane Orai channels to induce a Ca²⁺ influx pathway. Previous reports suggest that STIM1 may play a role in cardiac hypertrophy, but its role in electrical and mechanical phenotypic alterations is not well understood.

OBJECTIVE

To define the contributions of STIM1-mediated Ca²⁺ influx on electrical and mechanical properties of normal and diseased myocytes, and to determine whether Orai channels are obligatory partners for STIM1 in these processes using a clinically relevant large animal model of hypertrophy.

METHODS AND RESULTS

Cardiac hypertrophy was induced by slow progressive pressure overload in adult cats. Hypertrophied myocytes had increased STIM1 expression and activity, which correlated with altered Ca²⁺-handling and action potential (AP) prolongation. Exposure of hypertrophied myocytes to the Orai channel blocker BTP2 caused a reduction of AP duration and reduced diastolic Ca²⁺ spark rate. BTP2 had no effect on normal myocytes. Forced expression of STIM1 in cultured adult feline ventricular myocytes increased diastolic spark rate and prolonged AP duration. STIM1 expression produced an increase in the amount of Ca²⁺ stored within the sarcoplasmic reticulum and activated Ca²⁺/calmodulin-dependent protein kinase II. STIM1 expression also increased spark rates and induced spontaneous APs. STIM1 effects were eliminated by either BTP2 or by coexpression of a dominant negative Orai construct.

CONCLUSIONS

STIM1 can associate with Orai in cardiac myocytes to produce a Ca²⁺ influx pathway that can prolong the AP duration and load the sarcoplasmic reticulum and likely contributes to the altered electromechanical properties of the hypertrophied heart.

Acute Catecholamine Exposure Causes Reversible Myocyte Injury Without Cardiac Regeneration

RATIONALE

Catecholamines increase cardiac contractility, but exposure to high concentrations or prolonged exposures can cause cardiac injury. A recent study demonstrated that a single subcutaneous injection of isoproterenol (ISO; 200 mg/kg) in mice causes acute myocyte death (8%-10%) with complete cardiac repair within a month. Cardiac regeneration was via endogenous cKit(+) cardiac stem cell-mediated new myocyte formation.

OBJECTIVE

Our goal was to validate this simple injury/regeneration system and use it to study the biology of newly forming adult cardiac myocytes.

METHODS AND RESULTS

C57BL/6 mice (n=173) were treated with single injections of vehicle, 200 or 300 mg/kg ISO, or 2 daily doses of 200 mg/kg ISO for 6 days. Echocardiography revealed transiently increased systolic function and unaltered diastolic function 1 day after single ISO injection. Single ISO injections also caused membrane injury in ≈10% of myocytes, but few of these myocytes appeared to be necrotic. Circulating troponin I levels after ISO were elevated, further documenting myocyte damage. However, myocyte apoptosis was not increased after ISO injury. Heart weight to body weight ratio and fibrosis were also not altered 28 days after ISO injection. Single- or multiple-dose ISO injury was not associated with an increase in the percentage of 5-ethynyl-2'-deoxyuridine-labeled myocytes. Furthermore, ISO injections did not increase new myocytes in cKit(+/Cre)×R-GFP transgenic mice.

CONCLUSIONS

A single dose of ISO causes injury in ≈10% of the cardiomyocytes. However, most of these myocytes seem to recover and do not elicit cKit(+) cardiac stem cell-derived myocyte regeneration.

Abstract 84: Remodeling of Repolarization and Arrhythmia Susceptibility in a Myosin Binding Protein C Knockout Mouse Model

Background: Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiac disease (1:500 in the general population) and amongst the leading causes of sudden cardiac death (SCD) in the young. There is lack of an in-depth understanding of cellular mechanisms leading to SCD in HCM. Prior studies that explored the cellular electrophysiological alterations in HCM, focused on the role of calcium and sodium currents in the development of cardiac hypertrophy and arrhythmia. Repolarizing potassium currents in HCM have not been investigated in detail. Better definition of the cellular substrates for arrhythmias could improve risk stratification and lead to potential novel therapeutic targets for prevention of SCD.

Objective: To characterize the arrhythmogenic phenotype, cellular electrophysiological properties and repolarizing potassium currents in Myosin binding Protein C (MyBPC) knockout (KO) mice.

Methods and results: MyBPC KO mice demonstrate overt phenotype with severe hypertrophy and cardiac dysfunction compared to wild type (WT) control mice, as evidenced by echocardiography and heart weight/body weight analysis. Telemetric electrocardiographic recordings reveal prolongation of the QTc interval in the KO compared to WT control mice (53.2 ± 7.4 vs 30.5 ± 5 msec, n=4 KO, 4 WT, p< 0.05) with frequent arrhythmias including preexcitation, ventricular and supraventricular tachycardia. Whole cell current and voltage clamp studies, comparing KO to WT mice, demonstrate myocyte hypertrophy (469 ± 44 vs 234 ± 15 pF, n=18 KO,15 WT, p<0.01), action potential prolongation (Action potential duration (APD)50 of 30.4± vs 17.7 msec, APD90 of 125.7± vs 74.7 msec, n=11 KO, 12 WT, p <0.05) and decreased repolarizing potassium currents (n=15 KO, 14 WT): I k Peak (-120 to 60 mV) decrease of 33-39%, I k 1 (-120 to -60 mV) decrease of 16-30%, I ks (20 to 60 mV) decrease 21-23% and I to (0 to 60mV) decrease of 46-61%.

Conclusion: MyBPC KO mice demonstrate remodeling of repolarizing potassium currents, action potential and QTc intervals prolongation that could account for the arrhythmia susceptibility.

Abstract 53: Characterization of a Feline HFpEF Model Induced by Slow Progressive Pressure Overload

Rationale: The prevalence of Heart Failure with preserved Ejection Fraction (HFpEF) is almost equal to that of Heart Failure with reduced Ejection Fraction (HFrEF) and represents a major public health problem. There are no proven effective treatments for HFpEF, partially attributable to the lack of well-established animal models for HFpEF.

Hypothesis: Cats develop a HFpEF phenotype similar to patients after induction of slow progressive pressure overload.

Methods and Results: Female and male short hair cats, aged 2 months, underwent either aortic constriction, with a customized pre-shaped band, or sham procedure. In contrast to immediate pressure overload, this approach results in a slow progressive pressure overload during growth. Transthoracic echocardiography was performed at baseline and 1, 2, 3, and 4 months after banding. Concentric LV hypertrophy was observed and heart weight to body weight ratio significantly increased in banded male and female cats compared to sham (male: 8.5±1.6 vs. 4.5±0.4; female: 7.4±0.2 vs. 4.9±0.8; p<0.05). Wet/dry lung weight ratio, indicating pulmonary congestion, did not differ between groups. End-diastolic wall thickness was significantly increased in banded cats at 2, 3 and 4 months, while the end-diastolic diameter and systolic function did not differ between banded and sham cats at any time point. A significant left atrial (LA) enlargement was observed in banded cats at 3 and 4 months, as measured by an increased LA/aorta ratio and increased LA area. LA ejection fraction was significantly decreased after aortic constriction (male sham vs. banded: 75.4±0.2 vs. 47.3±0.2; p<0.01; female sham vs. band: 75.4±2.1 vs. 53.9±4.4; p<0.05). Diastolic function, assessed by tissue Doppler imaging, was impaired in banded cats.

Summary and Conclusion: Slow progressive pressure overload in cats induces severe LV concentric hypertrophy, LA enlargement and dysfunction without causing LV dilation or systolic dysfunction, mimicking many clinical phenotypes in HFpEF patients. Further characterization will include incorporating invasive measurements, exercise capacity and serum biomarkers and testing of novel therapies.

Abstract 2: Cortical Bone Stem Cells Derived Exosomes as Potent Modulator of Cardiac Immune Response and Repair After Injury

Rationale: Cortical bone derived stem cells (CBSCs) are known to have improved growth kinetics and myocardial repair properties that are superior to other known stem cell types used. Salutary effects of CBSCs in large are mediated by paracrine secretion. Since exosomes represent an active component of released factors we tested if CBSC derived exosomes (CBSCs-Exo) can recapitulate the beneficial reparative effects of CBSCs.

Objective: Determine CBSCs derived exosomes and their contents for myocardial repair.

Methods and Results: Exosomes were isolated from murine CBSCs by ultracentrifugation and had typical size (30-100nm), as validated by electron microscopy and dynamic light scattering. To determine cardiac therapeutic value, CBSCs- Exo (60μg) were injected into the border zone of the mouse heart after myocardial infarction (MI). Animals injected with CBSCs-Exo had reduced infarct size and increased myocyte survival after MI injury. Interestingly, serum levels of pro-inflammatory cytokines were significantly reduced along with decreased expression of CD68+ cells in animals receiving CBSCs-Exo versus control animals. Long term analysis of CBSC-Exo animals showed improved cardiac function and contractility compared to saline treated animals concurrent with enhanced angiogenesis 6 weeks after MI. Salutary effects of CBSC-Exo were confirmed in vitro. CBSCs-Exo increased cardiac protection in NRVMs after hypoxic challenge and enhanced tube formation in HUVECs. Simultaneously, treatment of bone marrow-derived macrophages stimulated with lipopolysaccharide (LPS) and treated with CBSCs-Exo showed increased polarization towards the M2 phenotype, demonstrating an immunomodulatory capacity of CBSCs-Exo. The underlying mechanism for beneficial effects was linked to increased packaging of cardioprotective miRs including miR125, miR20 and miR18a in CBSCs-Exo confirmed by MiRNA array analysis.

Conclusion: Exosomes derived from CBSCs provide a cell free system that retains the reparative power of CBSC. CBSCs-Exo augment cardiac function after myocardial injury recapitulating earlier findings with CBSCs. The packaging of cardioprotective and immune-modulatory miRs in CBSCs-Exo appears to enhance their reparative effects after MI.

Abstract 364: Cortical Bone Stem Cells Derived Exosomes as Potent Modulator of Cardiac Immune Response and Repair After Injury

Rationale: Cortical bone derived stem cells (CBSCs) are known to have improved growth kinetics and myocardial repair properties that are superior to other known stem cell types used. Salutary effects of CBSCs in large are mediated by paracrine secretion. Since exosomes represent an active component of released factors we tested if CBSC derived exosomes (CBSCs-Exo) can recapitulate the beneficial reparative effects of CBSCs.

Objective: Determine CBSCs derived exosomes and their contents for myocardial repair.

Methods and Results: Exosomes were isolated from murine CBSCs by ultracentrifugation and had typical size (30-100nm), as validated by electron microscopy and dynamic light scattering. To determine cardiac therapeutic value, CBSCs- Exo (60μg) were injected into the border zone of the mouse heart after myocardial infarction (MI). Animals injected with CBSCs-Exo had reduced infarct size and increased myocyte survival after MI injury. Interestingly, serum levels of pro-inflammatory cytokines were significantly reduced along with decreased expression of CD68+ cells in animals receiving CBSCs-Exo versus control animals. Long term analysis of CBSC-Exo animals showed improved cardiac function and contractility compared to saline treated animals concurrent with enhanced angiogenesis 6 weeks after MI. Salutary effects of CBSC-Exo were confirmed in vitro. CBSCs-Exo increased cardiac protection in NRVMs after hypoxic challenge and enhanced tube formation in HUVECs. Simultaneously, treatment of bone marrow-derived macrophages stimulated with lipopolysaccharide (LPS) and treated with CBSCs-Exo showed increased polarization towards the M2 phenotype, demonstrating an immunomodulatory capacity of CBSCs-Exo. The underlying mechanism for beneficial effects was linked to increased packaging of cardioprotective miRs including miR125, miR20 and miR18a in CBSCs-Exo confirmed by MiRNA array analysis.

Conclusion: Exosomes derived from CBSCs provide a cell free system that retains the reparative power of CBSC. CBSCs-Exo augment cardiac function after myocardial injury recapitulating earlier findings with CBSCs. The packaging of cardioprotective and immune-modulatory miRs in CBSCs-Exo appears to enhance their reparative effects after MI.

Unique Features of Cortical Bone Stem Cells Associated With Repair of the Injured Heart

RATIONALE

Adoptive transfer of multiple stem cell types has only had modest effects on the structure and function of failing human hearts. Despite increasing the use of stem cell therapies, consensus on the optimal stem cell type is not adequately defined. The modest cardiac repair and functional improvement in patients with cardiac disease warrants identification of a novel stem cell population that possesses properties that induce a more substantial improvement in patients with heart failure.

OBJECTIVE

To characterize and compare surface marker expression, proliferation, survival, migration, and differentiation capacity of cortical bone stem cells (CBSCs) relative to mesenchymal stem cells (MSCs) and cardiac-derived stem cells (CDCs), which have already been tested in early stage clinical trials.

METHODS AND RESULTS

CBSCs, MSCs, and CDCs were isolated from Gottingen miniswine or transgenic C57/BL6 mice expressing enhanced green fluorescent protein and were expanded in vitro. CBSCs possess a unique surface marker profile, including high expression of CD61 and integrin β4 versus CDCs and MSCs. In addition, CBSCs were morphologically distinct and showed enhanced proliferation capacity versus CDCs and MSCs. CBSCs had significantly better survival after exposure to an apoptotic stimuli when compared with MSCs. ATP and histamine induced a transient increase of intracellular Ca(2+) concentration in CBSCs versus CDCs and MSCs, which either respond to ATP or histamine only further documenting the differences between the 3 cell types.

CONCLUSIONS

CBSCs are unique from CDCs and MSCs and possess enhanced proliferative, survival, and lineage commitment capacity that could account for the enhanced protective effects after cardiac injury.

Abstract 157: Unique Features of Cortical Bone Stem Cells Associated With Enhanced Cardiac Repair

Rationale: Adoptive transfer of bone marrow and cardiac derived stem cells (CDCs) into failing human hearts has been shown to be safe, yet these cells have only induced modest improvements after myocardial infarction (MI). Recently we have shown in a mouse model that cortical bone derived stem cells (CBSCs) induced a greater enhancement of cardiac function after MI through enhanced paracrine signaling and transdifferentiation of CBSCs into new cardiac tissue. However, the reparative potential of CBSCs relative to other stem cell types including bone marrow derived mesenchymal stem cells (MSCs) and CDCs is not known.

Objective: To characterize surface marker expression, proliferation, survival, migration and differentiation capacity of swine CBSCs relative to MSCs and CDCs.

Methods and Results: CBSCs, MSCs and CDCs were isolated from Gottingen miniswine. CBSCs were morphologically distinct from MSCs and CDCs, with differences in length to width ratio and overall cell surface area. Cell surface marker profiling using RT-PCR analysis revealed that CBSCs express some of the classical MSC markers such as CD106, CD271, CD105, CD90 and CD29 and are negative for CD45 and CD11-b. CBSCs had an enhanced proliferation capacity versus CDCs and MSCs, measured by CyQuant assay. Concurrently CBSCs had significantly decreased population-doubling time (3.57 and 1.26 fold decrease) as compared to MSC and CDCs. CBSCs exhibit enhanced survival after exposure to apoptotic stimuli as compared to MSCs measured by Annexin-V staining. A significantly greater % of CBSCs expressed markers of cardiac lineage commitment when exposed to dexamethasone than did CDCs or MSCs. Markers of cardiac lineages including GATA-4, α SMA, Troponin T, sm22 were measured with RT-PCR and immunocytochemistry.

Conclusion: CBSCs have enhanced proliferative, survival capacity and cardiac lineage commitment versus CDCs and MSCs that could account for their enhanced effects on cardiac regeneration after myocardial infarction.

Abstract 311: Single-Dose Isoproterenol does not Depress Cardiac Function in Mice

Rationale: Myocardial injury after repeated or continuous administration of isoproterenol (ISO) in preclinical models has been widely described in the literature by our lab and others. Recent controversial reports using a one-time dose of ISO, to mimic a Takotsubo-like cardiomyopathy, demonstrated pronounced and reversible depression of cardiac function at one-week post injection with widespread myocyte death followed by robust myocardial regeneration and recovery of cardiac function.

Hypothesis: Single-dose ISO does not produce depression of cardiac function

Methods and Results: C57Bl/6 mice were given a single subcutaneous injection of vehicle (saline) or 5, 200, or 300 mg/kg of ISO. Cardiac function was measured using transthoracic echocardiography with cardiac strain analysis at baseline prior to ISO injection and after 1, 7, 14, and 28 days post-injection. Animals were sacrificed after 1, 7, and 28 days post-injection for evaluation of gross heart weight (HW), HW/body weight (BW) and HW/tibia length (TL). Left ventricular (LV) functional measurements revealed no significant differences in global systolic function (ejection fraction and fractional shortening) between vehicle- or ISO-treated animals at any concentration. Additionally, no significant differences were detected between vehicle- or ISO-treated animals in any cardiac dimensions measured by echocardiography (LV cross-sectional area, internal diameter, end-diastolic or end-systolic volumes) or in gross HW, HW/BW or HW/TL. LV global cardiac strain was also not significantly different between vehicle and ISO-treated animals at any time point. When apical regions of the LV endocardium (the area most predominantly affected by Takotsubo cardiomyopathy) were specifically examined using strain analysis, no significant differences could be detected between vehicle and ISO-treated animals at any time point.

Conclusion: Single-dose ISO injury in a mouse model does not produce any depression of cardiac function at 1 week post injection.

Autologous c-Kit+ Mesenchymal Stem Cell Injections Provide Superior Therapeutic Benefit as Compared to c-Kit+ Cardiac-Derived Stem Cells in a Feline Model of Isoproterenol-Induced Cardiomyopathy

BACKGROUND

Cardiac- (CSC) and mesenchymal-derived (MSC) CD117+ isolated stem cells improve cardiac function after injury. However, no study has compared the therapeutic benefit of these cells when used autologously.

METHODS

MSCs and CSCs were isolated on day 0. Cardiomyopathy was induced (day 28) by infusion of L-isoproterenol (1,100 ug/kg/hour) from Alzet minipumps for 10 days. Bromodeoxyuridine (BrdU) was infused via minipumps (50 mg/mL) to identify proliferative cells during the injury phase. Following injury (day 38), autologous CSC (n = 7) and MSC (n = 4) were delivered by intracoronary injection. These animals were compared to those receiving sham injections by echocardiography, invasive hemodynamics, and immunohistochemistry.

RESULTS

Fractional shortening improved with CSC (26.9 ± 1.1% vs. 16.1 ± 0.2%, p = 0.01) and MSC (25.1 ± 0.2% vs. 12.1 ± 0.5%, p = 0.01) as compared to shams. MSC were superior to CSC in improving left ventricle end-diastolic (LVED) volume (37.7 ± 3.1% vs. 19.9 ± 9.4%, p = 0.03) and ejection fraction (27.7 ± 0.1% vs. 19.9 ± 0.4%, p = 0.02). LVED pressure was less in MSC (6.3 ± 1.3 mmHg) as compared to CSC (9.3 ± 0.7 mmHg) and sham (13.3 ± 0.7); p = 0.01. LV BrdU+ myocytes were higher in MSC (0.17 ± 0.03%) than CSC (0.09 ± 0.01%) and sham (0.06 ± 01%); p < 0.001.

CONCLUSIONS

Both CD117+ isolated CSC and MSC therapy improve cardiac function and attenuate pathological remodeling. However, MSC appear to confer additional benefit.

Transient receptor potential channels contribute to pathological structural and functional remodeling after myocardial infarction

RATIONALE

The cellular and molecular basis for post-myocardial infarction (MI) structural and functional remodeling is not well understood.

OBJECTIVE

Our aim was to determine if Ca2+ influx through transient receptor potential canonical (TRPC) channels contributes to post-MI structural and functional remodeling.

METHODS AND RESULTS

TRPC1/3/4/6 channel mRNA increased after MI in mice and was associated with TRPC-mediated Ca2+ entry. Cardiac myocyte-specific expression of a dominant-negative (loss-of-function) TRPC4 channel increased basal myocyte contractility and reduced hypertrophy and cardiac structural and functional remodeling after MI while increasing survival in mice. We used adenovirus-mediated expression of TRPC3/4/6 channels in cultured adult feline myocytes to define mechanistic aspects of these TRPC-related effects. TRPC3/4/6 overexpression in adult feline myocytes induced calcineurin (Cn)-nuclear factor of activated T-cells (NFAT)-mediated hypertrophic signaling, which was reliant on caveolae targeting of TRPCs. TRPC3/4/6 expression in adult feline myocytes increased rested state contractions and increased spontaneous sarcoplasmic reticulum Ca2+ sparks mediated by enhanced phosphorylation of the ryanodine receptor. TRPC3/4/6 expression was associated with reduced contractility and response to catecholamines during steady-state pacing, likely because of enhanced sarcoplasmic reticulum Ca2+ leak.

CONCLUSIONS

Ca2+ influx through TRPC channels expressed after MI activates pathological cardiac hypertrophy and reduces contractility reserve. Blocking post-MI TRPC activity improved post-MI cardiac structure and function.

Imatinib activates pathological hypertrophy by altering myocyte calcium regulation

BACKGROUND

Imatinib mesylate is a selective tyrosine-kinase inhibitor used in the treatment of multiple cancers, most notably chronic myelogenous leukemia. There is evidence that imatinib can induce cardiotoxicity in cancer patients. Our hypothesis is that imatinib alters calcium regulatory mechanisms and can contribute to development of pathological cardiac hypertrophy.

METHODS AND RESULTS

Neonatal rat ventricular myocytes (NRVMs) were treated with clinical doses (low: 2 μM; high: 5 μM) of imatinib and assessed for molecular changes. Imatinib increased peak systolic Ca(2+) and Ca(2+) transient decay rates and Western analysis revealed significant increases in phosphorylation of phospholamban (Thr-17) and the ryanodine receptor (Ser-2814), signifying activation of calcium/calmodulin-dependent kinase II (CaMKII). Imatinib significantly increased NRVM volume as assessed by Coulter counter, myocyte surface area, and atrial natriuretic peptide abundance seen by Western. Imatinib induced cell death, but did not activate the classical apoptotic program as assessed by caspase-3 cleavage, indicating a necrotic mechanism of death in myocytes. We expressed AdNFATc3-green fluorescent protein in NRVMs and showed imatinib treatment significantly increased nuclear factor of activated T cells translocation that was inhibited by the calcineurin inhibitor FK506 or CaMKII inhibitors.

CONCLUSION

These data show that imatinib can activate pathological hypertrophic signaling pathways by altering intracellular Ca(2+) dynamics. This is likely a contributing mechanism for the adverse cardiac effects of imatinib.

Sorafenib cardiotoxicity increases mortality after myocardial infarction

RATIONALE

Sorafenib is an effective treatment for renal cell carcinoma, but recent clinical reports have documented its cardiotoxicity through an unknown mechanism.

OBJECTIVE

Determining the mechanism of sorafenib-mediated cardiotoxicity.

METHODS AND RESULTS

Mice treated with sorafenib or vehicle for 3 weeks underwent induced myocardial infarction (MI) after 1 week of treatment. Sorafenib markedly decreased 2-week survival relative to vehicle-treated controls, but echocardiography at 1 and 2 weeks post MI detected no differences in cardiac function. Sorafenib-treated hearts had significantly smaller diastolic and systolic volumes and reduced heart weights. High doses of sorafenib induced necrotic death of isolated myocytes in vitro, but lower doses did not induce myocyte death or affect inotropy. Histological analysis documented increased myocyte cross-sectional area despite smaller heart sizes after sorafenib treatment, further suggesting myocyte loss. Sorafenib caused apoptotic cell death of cardiac- and bone-derived c-kit+ stem cells in vitro and decreased the number of BrdU+ (5-bromo-2'-deoxyuridine+) myocytes detected at the infarct border zone in fixed tissues. Sorafenib had no effect on infarct size, fibrosis, or post-MI neovascularization. When sorafenib-treated animals received metoprolol treatment post MI, the sorafenib-induced increase in post-MI mortality was eliminated, cardiac function was improved, and myocyte loss was ameliorated.

CONCLUSIONS

Sorafenib cardiotoxicity results from myocyte necrosis rather than from any direct effect on myocyte function. Surviving myocytes undergo pathological hypertrophy. Inhibition of c-kit+ stem cell proliferation by inducing apoptosis exacerbates damage by decreasing endogenous cardiac repair. In the setting of MI, which also causes large-scale cell loss, sorafenib cardiotoxicity dramatically increases mortality.

Abstract 242: Bone-derived Stem Cells Repair The Heart After Myocardial Infarction Through Transdifferentiation And Paracrine Signaling Mechanisms

Rationale: Autologous bone marrow- or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials but has only offered limited functional improvements. Finding the optimal stem cell type best suited for cardiac regeneration remains a key goal toward improving clinical outcomes.

Objective: To determine the mechanism by which novel bone-derived stem cells support the injured heart.

Methods and Results: Cortical bone stem cells (CBSCs) were isolated from EGFP+ transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction (MI) with injection of CBSCs (n=57) or saline (n=59). Cardiac function was monitored using echocardiography with strain analysis. EGFP+ stem cells in vivo were shown to express only 2/8 factors tested (basic fibroblast growth factor and vascular endothelial growth factor) and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, attenuated adverse remodeling, and decreased infarct size relative to saline-treated MI controls. By 6 weeks post-MI, EGFP+ cardiomyocytes, vascular smooth muscle cells and endothelial cells could be identified on histology. Isolated EGFP+ myocytes were smaller, more frequently mononucleated, and demonstrated fractional shortening and calcium currents indistinguishable from EGFP- myocytes from the same hearts.

Conclusions: CBSCs improve survival, cardiac function, and attenuate remodeling by 1) secreting the proangiogenic factors bFGF and VEGF (stimulating endogenous neovascularization), and 2) differentiating into functional adult myocytes and vascular cells.

Abstract 278: Exercise Training Ameliorates LV Dysfunction in Mice with a Calcium Influx Mediated Cardiomyopathy

Background— Ca 2+ influx through L-type Ca 2+ channels (LTCCs) induces Ca 2+ release from the SR to induce and regulate cardiac contraction. We have characterized a mouse model with cardiac-specific expression of the β2a subunit of the LTCC (β2a), leading to pathological hypertrophy due to excessive LTCC Ca 2+ entry. Chronic exercise causes physiological hypertrophy in normal hearts. We determined if swim training improved or further deranged the performance of β2a mice.

Methods and Results— β2a (n=12) and WT (n=20) mice were swim trained for 21 consecutive days. ECHO was performed at beginning and end of swim training. Ejection fraction (EF) and fractional shortening (FS) increased significantly with swim training in WT animals. (EF pre-swim: 57±2.02% vs. post-swim: 70±2.88%; FS pre-swim: 30±1.34% vs. post-swim: 39±2.42%). β2a mice were hypercontractile before swimming and had no significant change after training (EF pre-swim: 70±1.75% vs. post-swim: 74.06±2.72%; FS pre-training: 38±2.38% vs. post-training: 42±2.22%). Contractile performance was significantly greater in β2a mice versus WT before but not after training. There was a significant increase in HW/BW [mg/gm] ratio in trained WT (n=8) versus sedentary WT (n=8) (swim: 5.75±0.19 vs. sed: 4.63±0.2) but training did not cause additional hypertrophy in β2a (n=8) versus sedentary (n=7) (swim: 5.98±0.31 vs. sed: 5.96±0.25). Isolated WT and β2a hearts were placed on a Langendorff apparatus and a balloon was inserted into the LV to record isovolumic pressure. Each heart underwent 15 min of ischemia followed by 30 min of reperfusion. End diastolic pressure (EDP) increased in all hearts during ischemia. There were no protective effects of training on ischemia-induced increases in EDP in WT hearts. EDP increased more in sedentary β2a during ischemia but this effect was eliminated in trained β2a hearts. LV developed pressure (LVDP) fell with ischemia and partially recovered with reperfusion. LVDP recovery was significantly greater in trained versus sedentary WT hearts. Swim training also significantly improved LVDP recovery in β2a hearts.

Conclusions— Chronic exercise training reduces pathological hypertrophy and damaging effects of ischemia in a murine model of LTCC overexpression.

Bone-derived stem cells repair the heart after myocardial infarction through transdifferentiation and paracrine signaling mechanisms

RATIONALE

Autologous bone marrow-derived or cardiac-derived stem cell therapy for heart disease has demonstrated safety and efficacy in clinical trials, but functional improvements have been limited. Finding the optimal stem cell type best suited for cardiac regeneration is the key toward improving clinical outcomes.

OBJECTIVE

To determine the mechanism by which novel bone-derived stem cells support the injured heart.

METHODS AND RESULTS

Cortical bone-derived stem cells (CBSCs) and cardiac-derived stem cells were isolated from enhanced green fluorescent protein (EGFP+) transgenic mice and were shown to express c-kit and Sca-1 as well as 8 paracrine factors involved in cardioprotection, angiogenesis, and stem cell function. Wild-type C57BL/6 mice underwent sham operation (n=21) or myocardial infarction with injection of CBSCs (n=67), cardiac-derived stem cells (n=36), or saline (n=60). Cardiac function was monitored using echocardiography. Only 2/8 paracrine factors were detected in EGFP+ CBSCs in vivo (basic fibroblast growth factor and vascular endothelial growth factor), and this expression was associated with increased neovascularization of the infarct border zone. CBSC therapy improved survival, cardiac function, regional strain, attenuated remodeling, and decreased infarct size relative to cardiac-derived stem cells- or saline-treated myocardial infarction controls. By 6 weeks, EGFP+ cardiomyocytes, vascular smooth muscle, and endothelial cells could be identified in CBSC-treated, but not in cardiac-derived stem cells-treated, animals. EGFP+ CBSC-derived isolated myocytes were smaller and more frequently mononucleated, but were functionally indistinguishable from EGFP- myocytes.

CONCLUSIONS

CBSCs improve survival, cardiac function, and attenuate remodeling through the following 2 mechanisms: (1) secretion of proangiogenic factors that stimulate endogenous neovascularization, and (2) differentiation into functional adult myocytes and vascular cells.

Ca(2+) influx through L-type Ca(2+) channels and transient receptor potential channels activates pathological hypertrophy signaling

Common cardiovascular diseases such as hypertension and myocardial infarction require that myocytes develop greater than normal force to maintain cardiac pump function. This requires increases in [Ca(2+)]. These diseases induce cardiac hypertrophy and increases in [Ca(2+)] are known to be an essential proximal signal for activation of hypertrophic genes. However, the source of "hypertrophic" [Ca(2+)] is not known and is the topic of this study. The role of Ca(2+) influx through L-type Ca(2+) channels (LTCC), T-type Ca(2+) channels (TTCC) and transient receptor potential (TRP) channels on the activation of calcineurin (Cn)-nuclear factor of activated T cells (NFAT) signaling and myocyte hypertrophy was studied. Neonatal rat ventricular myocytes (NRVMs) and adult feline ventricular myocytes (AFVMs) were infected with an adenovirus containing NFAT-GFP, to determine factors that could induce NFAT nuclear translocation. Four millimolar Ca(2+) or pacing induced NFAT nuclear translocation. This effect was blocked by Cn inhibitors. In NRVMs Nifedipine (Nif, LTCC antagonist) blocked high Ca(2+)-induced NFAT nuclear translocation while SKF-96365 (TRP channel antagonist) and Nickel (Ni, TTCC antagonist) were less effective. The relative potency of these antagonists against Ca(2+) induced NFAT nuclear translocation (Nif>SKF-96365>Ni) was similar to their effects on Ca(2+) transients and the LTCC current. Infection of NRVM with viruses containing TRP channels also activated NFAT-GFP nuclear translocation and caused myocyte hypertrophy. TRP effects were reduced by SKF-96365, but were more effectively antagonized by Nif. These experiments suggest that Ca(2+) influx through LTCCs is the primary source of Ca(2+) to activate Cn-NFAT signaling in NRVMs and AFVMs. While TRP channels cause hypertrophy, they appear to do so through a mechanism involving Ca(2+) entry via LTCCs.

Abstract 194: Increasing Expression of Connexin43 in Bone Marrow Stem Cells Enhances Their Protective Effects on the Infarcted Heart

Introduction: Coupling of stem cells (SCs) injected into the infarcted heart to endogenous cells that survived injury may enhance SC survival and engraftment and improve their protective effects. Connexin43 (Cx43) gap junctions are some of the first proteins expressed by differentiating SCs transplanted into the heart after myocardial infarction (MI), suggesting that cell coupling is critical for SC survival and differentiation. Hypothesis: Overexpression of Cx43 in SCs will enhance their ability to engraft, differentiate, and improve cardiac function and survival after transplant into the infarcted heart.

Methods: Adult mouse SCs isolated from compact bone were sorted for c-kit surface marker using magnetic beads. SCs were transduced with PLL3.7 lentivirus encoding the Cx43 gene (GJA1) at MOI=300 to produce overexpression, which was confirmed by Western analysis on SC lysates. MI was induced in 12-week-old male C57BL/6 mice by ligating the left anterior descending coronary artery, and 40,000 SCs in 20 uL saline were administered immediately after MI via intramyocardial injection. A total of 69 animals were used: 18 received Cx43 SCs, 30 received non-transduced SCs (NTSC), 11 received only saline, and 10 underwent sham surgery. Echocardiography was performed 1 and 2 weeks post-MI.

Results: Animals receiving Cx43 SCs had improved survival (90.0%) versus NTSC (76.7%) and saline controls (70.0%). The Cx43 SC group displayed improved ejection fraction versus NTSC and saline controls at 1 week (48.7% v. 43.3%, p=0.04; 30.1%, p<0.0001) and 2 weeks post-MI (45.7% v. 40.4%, p=0.08; 26.8%, p=0.0004). Similar improvements in fractional shortening, stroke volume, and cardiac output were observed. Administration of Cx43 SC and NTSC attenuated remodeling with no difference between the two groups. Both decreased end-diastolic and systolic volumes, increased thickness of the infarct-related anterior wall, and decreased heart weight (HW), HW/body weight and HW/tibia length ratios at 1 and 2 weeks post-MI.

Conclusion: All SCs improved survival and cardiac function, and attenuated remodeling. Transplantation of Cx43 SCs improved survival and cardiac function even more than NTSCs, suggesting that Cx43 may play a crucial role in engraftment and differentiation.

Abstract 111: High Activity Gating of Caveolae-Targeted L-Type Ca 2+ Channels Can Initiate Pathological Hypertrophy

Introduction: The source of Ca 2+ that activates pathological cardiac hypertrophy is not clearly defined. We hypothesize that high activity gating of L-type Ca 2+ channels (LTCCs) localized in caveolae signaling microdomains stabilized by caveolin-3 (Cav-3) provides the Ca 2+ influx that locally activates calcineurin (Cn)-mediated nuclear factor of activated T-cells (NFAT) to induce hypertrophic gene expression. We generated novel reagents that specifically activate or inhibit the gating of LTCCs in caveolae for analysis of the hypertrophic program, as well as any potential effect on ICa and contraction.

Methods and Results: We targeted the known LTCC inhibitory protein Rem or an LTCC subunit (β2a) that promotes high activity gating specifically to caveolae by fusing them to a caveolin-binding domain peptide (termed Cav-Rem and Cav-β2a). We infected adult feline left ventricular myocytes (AFLVMs) with adenoviruses containing Cav-Rem or Cav-β2a for membrane localization and functional studies. NFAT nuclear translocation was determined by co-infecting AFLVMs with ad-NFAT-GFP and either ad-Cav-Rem or ad-Cav-β2a and pacing cells to induce Ca2+ influx mediated nuclear NFAT-GFP translocation. Membrane fractionation experiments showed that Cav-3 membrane domains contain 26.2 +/- 12.7% of membrane targeted LTCCs and blocking these with Cav-Rem eliminated a small fraction of the LTCC current (<15%) and almost all Ca2+ influx induced NFAT nuclear translocation (>90%), but did not reduce myocyte contractility. Conversely, selective enhancement of LTCC activity within caveolae with Cav-β2a caused a significant increase in NFAT nuclear translocation (>70%) but had no significant effect on contractility.

Conclusions: We provide proof of concept that specific Cav-targeted reagents can be used to enhance or inhibit LTCC activity within caveolae microdomains to amplify or block the hypertrophic response. Our results suggest that high activity gating of LTCCs, which is known to be present in the hypertrophied failing human heart, can activate signaling pathways linked to cardiac hypertrophy. Selectively inhibiting these caveolae localized LTCCs could be a novel mechanism to block pathological hypertrophy without reducing cardiac contractility.

A characterization and targeting of the infarct border zone in a swine model of myocardial infarction

INTRODUCTION

Novel therapies for myocardial infarction (MI) involving stem cells, gene therapy, biomaterials, or revascularization strategies have shown promise in animal studies and clinical trials, but results have been limited partially due to the injection of therapeutics into ischemic myocardium that cannot support their mechanism of action. Accurate targeting of therapeutics precisely to the infarct border zone (BZ) may be essential for effective repair of the ischemic heart.

METHODS

Ischemia-reperfusion MI was induced in Yorkshire swine by inflation of an angioplasty balloon in the left anterior descending coronary artery. Fluorescent microspheres were injected into the BZ under NOGA catheter guidance, and this location was identified grossly then examined by immunohistochemistry and Western analysis.

RESULTS

Analysis of the infarct zone two hours post-MI revealed a frankly necrotic region devoid of contractile proteins with marked activation of caspase-3. The NOGA-defined BZ closely approximates the grossly-defined BZ and contains intact myocytes and vasculature. Western analysis detected Akt expression and levels of Ca(2+) handling proteins equivalent to that of viable tissues.

CONCLUSIONS

Histological and Western analysis revealed that NOGA mapping precisely identifies grossly and molecularly defined infarct BZ at a location where there are still viable cells and vessels capable of supporting novel therapeutic strategies.

Severe hyperhomocysteinemia promotes bone marrow-derived and resident inflammatory monocyte differentiation and atherosclerosis in LDLr/CBS-deficient mice

RATIONALE

Hyperhomocysteinemia (HHcy) accelerates atherosclerosis and increases inflammatory monocytes (MC) in peripheral tissues. However, its causative role in atherosclerosis is not well established and its effect on vascular inflammation has not been studied. The underlying mechanism is unknown.

OBJECTIVE

This study examined the causative role of HHcy in atherogenesis and its effect on inflammatory MC differentiation.

METHODS AND RESULTS

We generated a novel HHcy and hyperlipidemia mouse model, in which cystathionine β-synthase (CBS) and low-density lipoprotein receptor (LDLr) genes were deficient (Ldlr(-/-) Cbs(-/+)). Severe HHcy (plasma homocysteine (Hcy)=275 μmol/L) was induced by a high methionine diet containing sufficient basal levels of B vitamins. Plasma Hcy levels were lowered to 46 μmol/L from 244 μmol/L by vitamin supplementation, which elevated plasma folate levels. Bone marrow (BM)-derived cells were traced by the transplantation of BM cells from enhanced green fluorescent protein (EGFP) transgenic mice after sublethal irradiation of the recipient. HHcy accelerated atherosclerosis and promoted Ly6C(high) inflammatory MC differentiation of both BM and tissue origins in the aortas and peripheral tissues. It also elevated plasma levels of TNF-α, IL-6, and MCP-1; increased vessel wall MC accumulation; and increased macrophage maturation. Hcy-lowering therapy reversed HHcy-induced lesion formation, plasma cytokine increase, and blood and vessel inflammatory MC (Ly6C(high+middle)) accumulation. Plasma Hcy levels were positively correlated with plasma levels of proinflammatory cytokines. In primary mouse splenocytes, L-Hcy promoted rIFNγ-induced inflammatory MC differentiation, as well as increased TNF-α, IL-6, and superoxide anion production in inflammatory MC subsets. Antioxidants and folic acid reversed L-Hcy-induced inflammatory MC differentiation and oxidative stress in inflammatory MC subsets.

CONCLUSIONS

HHcy causes vessel wall inflammatory MC differentiation and macrophage maturation of both BM and tissue origins, leading to atherosclerosis via an oxidative stress-related mechanism.

Validation of transcatheter left ventricular electromechanical mapping for assessment of cardiac function and targeted transendocardial injection in a porcine ischemia-reperfusion model

Ischemic heart disease, despite advances in treatment, remains the major cause of mortality worldwide. NOGA 3D left ventricular electromechanical mapping allows accurate determination of cardiac function and precise identification of sites of injury. In a porcine model of ischemia-reperfusion injury, we validate the use of the NOGA mapping system for assessment of cardiac function along with the Myostar injection catheter for directed delivery of therapeutics to localized target sites in the setting of acute myocardial injury.

Hyperphosphorylation of the cardiac ryanodine receptor at serine 2808 is not involved in cardiac dysfunction after myocardial infarction

RATIONALE

Abnormal behavior of the cardiac ryanodine receptor (RyR2) has been linked to cardiac arrhythmias and heart failure (HF) after myocardial infarction (MI). It has been proposed that protein kinase A (PKA) hyperphosphorylation of the RyR2 at a single residue, Ser-2808, is a critical mediator of RyR dysfunction, depressed cardiac performance, and HF after MI.

OBJECTIVE

We used a mouse model (RyRS2808A) in which PKA hyperphosphorylation of the RyR2 at Ser-2808 is prevented to determine whether loss of PKA phosphorylation at this site averts post MI cardiac pump dysfunction.

METHODS AND RESULTS

MI was induced in wild-type (WT) and S2808A mice. Myocyte and cardiac function were compared in WT and S2808A animals before and after MI. The effects of the PKA activator Isoproterenol (Iso) on L-type Ca(2+) current (I(CaL)), contractions, and [Ca(2+)](I) transients were also measured. Both WT and S2808A mice had depressed pump function after MI, and there were no differences between groups. MI size was also identical in both groups. L type Ca(2+) current, contractions, Ca(2+) transients, and SR Ca(2+) load were also not significantly different in WT versus S2808A myocytes either before or after MI. Iso effects on Ca(2+) current, contraction, Ca(2+) transients, and SR Ca(2+) load were identical in WT and S2808A myocytes before and after MI at both low and high concentrations.

CONCLUSIONS

These results strongly support the idea that PKA phosphorylation of RyR-S2808 is irrelevant to the development of cardiac dysfunction after MI, at least in the mice used in this study.

A caveolae-targeted L-type Ca²+ channel antagonist inhibits hypertrophic signaling without reducing cardiac contractility

RATIONALE

The source of Ca(2+) to activate pathological cardiac hypertrophy is not clearly defined. Ca(2+) influx through the L-type Ca(2+) channels (LTCCs) determines "contractile" Ca(2+), which is not thought to be the source of "hypertrophic" Ca(2+). However, some LTCCs are housed in caveolin-3 (Cav-3)-enriched signaling microdomains and are not directly involved in contraction. The function of these LTCCs is unknown.

OBJECTIVE

To test the idea that LTCCs in Cav-3-containing signaling domains are a source of Ca(2+) to activate the calcineurin-nuclear factor of activated T-cell signaling cascade that promotes pathological hypertrophy.

METHODS AND RESULTS

We developed reagents that targeted Ca(2+) channel-blocking Rem proteins to Cav-3-containing membranes, which house a small fraction of cardiac LTCCs. Blocking LTCCs within this Cav-3 membrane domain eliminated a small fraction of the LTCC current and almost all of the Ca(2+) influx-induced NFAT nuclear translocation, but it did not reduce myocyte contractility.

CONCLUSIONS

We provide proof of concept that Ca(2+) influx through LTCCs within caveolae signaling domains can activate "hypertrophic" signaling, and this Ca(2+) influx can be selectively blocked without reducing cardiac contractility.

Repair of the injured adult heart involves new myocytes potentially derived from resident cardiac stem cells

RATIONALE

The ability of the adult heart to generate new myocytes after injury is not established.

OBJECTIVE

Our purpose was to determine whether the adult heart has the capacity to generate new myocytes after injury, and to gain insight into their source.

METHODS AND RESULTS

Cardiac injury was induced in the adult feline heart by infusing isoproterenol (ISO) for 10 days via minipumps, and then animals were allowed to recover for 7 or 28 days. Cardiac function was measured with echocardiography, and proliferative cells were identified by nuclear incorporation of 5-bromodeoxyuridine (BrdU; 7-day minipump infusion). BrdU was infused for 7 days before euthanasia at days 10, 17, and 38 or during injury and animals euthanized at day 38. ISO caused reduction in cardiac function with evidence of myocyte loss from necrosis. During this injury phase there was a significant increase in the number of proliferative cells in the atria and ventricle, but there was no increase in BrdU+ myocytes. cKit+ cardiac progenitor cells were BrdU labeled during injury. During the first 7 days of recovery there was a significant reduction in cellular proliferation (BrdU incorporation) but a significant increase in BrdU+ myocytes. There was modest improvement in cardiac structure and function during recovery. At day 38, overall cell proliferation was not different than control, but increased numbers of BrdU+ myocytes were found when BrdU was infused during injury.

CONCLUSIONS

These studies suggest that ISO injury activates cardiac progenitor cells that can differentiate into new myocytes during cardiac repair.

Calcium influx through Cav1.2 is a proximal signal for pathological cardiomyocyte hypertrophy

Pathological cardiac hypertrophy (PCH) is associated with the development of arrhythmia and congestive heart failure. While calcium (Ca(2+)) is implicated in hypertrophic signaling pathways, the specific role of Ca(2+) influx through the L-type Ca(2+) channel (I(Ca-L)) has been controversial and is the topic of this study. To determine if and how sustained increases in I(Ca-L) induce PCH, transgenic mouse models with low (LE) and high (HE) expression levels of the β2a subunit of Ca(2+) channels (β2a) and in cultured adult feline (AF) and neonatal rat (NR) ventricular myocytes (VMs) infected with an adenovirus containing a β2a-GFP were used. In vivo, β2a LE and HE mice had increased heart weight to body weight ratio, posterior wall and interventricular septal thickness, tissue fibrosis, myocyte volume, and cross-sectional area and the expression of PCH markers in a time- and dose-dependent manner. PCH was associated with a hypercontractile phenotype including enhanced I(Ca-L), fractional shortening, peak Ca(2+) transient, at the myocyte level, greater ejection fraction, and fractional shortening at the organ level. In addition, LE mice had an exaggerated hypertrophic response to transverse aortic constriction. In vitro overexpression of β2a in cultured AFVMs increased I(Ca-L), cell volume, protein synthesis, NFAT, and HDAC translocations and in NRVMs increased surface area. These effects were abolished by the blockade of I(Ca-L), intracellular Ca(2+), calcineurin, CaMKII, and SERCA. In conclusion, increasing I(Ca-L) is sufficient to induce PCH through the calcineurin/NFAT and CaMKII/HDAC pathways. Both cytosolic and SR/ER-nuclear envelop Ca(2+) pools were shown to be involved.

{beta}1-Adrenergic receptor activation induces mouse cardiac myocyte death through both L-type calcium channel-dependent and -independent pathways

Cardiac diseases persistently increase the contractility demands of cardiac myocytes, which require activation of the sympathetic nervous system and subsequent increases in myocyte Ca(2+) transients. Persistent exposure to sympathetic and/or Ca(2+) stress is associated with myocyte death. This study examined the respective roles of persistent beta-adrenergic receptor (beta-AR) agonist exposure and high Ca(2+) concentration in myocyte death. Ventricular myocytes (VMs) were isolated from transgenic (TG) mice with cardiac-specific and inducible expression of the beta(2a)-subunit of the L-type Ca(2+) channel (LTCC). VMs were cultured, and the rate of myocyte death was measured in the presence of isoproterenol (ISO), other modulators of Ca(2+) handling and the beta-adrenergic system, and inhibitors of caspases and reactive oxygen species generation. The rate of myocyte death was greater in TG vs. wild-type myocytes and accelerated by ISO in both groups, although ISO did not increase LTCC current (I(Ca-L)) in TG-VMs. Nifedipine, an LTCC antagonist, only partially prevented myocyte death. These results suggest both LTCC-dependent and -independent mechanisms in ISO induced myocyte death. ISO increased the contractility of wild type and TG-VMs by enhancing sarcoplasmic reticulum function and inhibiting sarco(endo)plasmic reticulum Ca(2+)-ATPase, Na(+)/Ca(2+) exchanger, and CaMKII partially protected myocyte from death induced by both Ca(2+) and ISO. Caspase and reactive oxygen species inhibitors did not, but beta(2)-AR activation did, reduce myocyte death induced by enhanced I(Ca-L) and ISO stimulation. Our results suggest that catecholamines induce myocyte necrosis primarily through beta(1)-AR-mediated increases in I(Ca-L), but other mechanisms are also involved in rodents.

c-Kit+ bone marrow stem cells differentiate into functional cardiac myocytes

The utility of bone marrow cells (BMCs) to regenerate cardiac myocytes is controversial. The present study examined the capacity of different types of BMCs to generate functional cardiac myocytes. Isolated c-kit(+) BMCs (BMSCs), c-kit(+) and crude BMCs from the adult feline femur were membrane stained with PKH26 dye or infected with a control enhanced green fluorescence protein transcript (EGFP)-adenovirus prior to co-culture upon neonatal rat ventricular myocytes (NRVM). Co-cultured cells were immuno-stained for c-kit, alpha-tropomyosin, alpha-actinin, connexin 43 (Cx43) and Ki67 and analyzed with confocal microscopy. Electrophysiology of BMSC derived myocytes were compared to NRVMs within the same culture dish. Gap junction function was analyzed by fluorescence recovery after photo-bleaching (FRAP). BMCs proliferated and differentiated into cardiac myocytes during the first 48 hours of co-culturing. These newly formed cardiac myocytes were able to contract spontaneously or synchronously with neighboring NRVMs. The myogenic rate of c-kit(+) BMSCs was significantly greater than c-kit(+) and crude BMCs (41.2 +/- 2.1, 6.1 +/- 1.2, and 17.1 +/- 1.5%, respectively). The newly formed cardiac myocytes exhibited an immature electrophysiological phenotype until they became electrically coupled to NRVMs through functional gap junctions. BMSCs did not become functional myocytes in the absence of NRVMs. In conclusion, c-kit(+) BMSCs have the ability to transdifferentiate into functional cardiac myocytes.

Increased cardiac myocyte progenitors in failing human hearts

BACKGROUND

Increasing evidence, derived mainly from animal models, supports the existence of endogenous cardiac renewal and repair mechanisms in adult mammalian hearts that could contribute to normal homeostasis and the responses to pathological insults.

METHODS AND RESULTS

Translating these results, we isolated small c-kit+ cells from 36 of 37 human hearts using primary cell isolation techniques and magnetic cell sorting techniques. The abundance of these cardiac progenitor cells was increased nearly 4-fold in patients with heart failure requiring transplantation compared with nonfailing controls. Polychromatic flow cytometry of primary cell isolates (<30 microm) without antecedent c-kit enrichment confirmed the increased abundance of c-kit+ cells in failing hearts and demonstrated frequent coexpression of CD45 in these cells. Immunocytochemical characterization of freshly isolated, c-kit-enriched human cardiac progenitor cells confirmed frequent coexpression of c-kit and CD45. Primary cardiac progenitor cells formed new human cardiac myocytes at a relatively high frequency after coculture with neonatal rat ventricular myocytes. These contracting new cardiac myocytes exhibited an immature phenotype and frequent electric coupling with the rat myocytes that induced their myogenic differentiation.

CONCLUSIONS

Despite the increased abundance and cardiac myogenic capacity of cardiac progenitor cells in failing human hearts, the need to replace these organs via transplantation implies that adverse features of the local myocardial environment overwhelm endogenous cardiac repair capacity. Developing strategies to improve the success of endogenous cardiac regenerative processes may permit therapeutic myocardial repair without cell delivery per se.

Reduced effects of BAY K 8644 on L-type Ca2+ current in failing human cardiac myocytes are related to abnormal adrenergic regulation

Abnormal L-type Ca(2+) channel (LTCC, also named Cav1.2) density and regulation are important contributors to depressed contractility in failing hearts. The LTCC agonist BAY K 8644 (BAY K) has reduced inotropic effects on failing myocardium. We hypothesized that BAY K effects on the LTCC current (I(CaL)) in failing myocytes would be reduced because of increased basal activity. Since support of the failing heart with a left ventricular assist device (LVAD) improves contractility and adrenergic responses, we further hypothesized that BAY K effects on I(CaL) would be restored in LVAD-supported failing hearts. We tested our hypotheses in human ventricular myocytes (HVMs) isolated from nonfailing (NF), failing (F), and LVAD-supported failing hearts. We found that 1) BAY K had smaller effects on I(CaL) in F HVMs compared with NF HVMs; 2) BAY K had diminished effects on I(CaL) in NF HVM pretreated with isoproterenol (Iso) or dibutyryl cyclic AMP (DBcAMP); 3) BAY K effects on I(CaL) in F HVMs pretreated with acetylcholine (ACh) were normalized; 4) Iso had no effect on NF HVMs pretreated with BAY K; 5) BAY K effects on I(CaL) in LVAD HVMs were similar to those in NF HVMs; 6) BAY K effects were reduced in LVAD HVMs pretreated with Iso or DBcAMP; 7) Iso had no effect on I(CaL) in LVAD HVMs pretreated with BAY K. Collectively, these results suggest that the decreased BAY K effects on LTCC in F HVMs are caused by increased basal channel activity, which should contribute to abnormal contractility reserve.

Reduced sarcoplasmic reticulum Ca(2+) load mediates impaired contractile reserve in right ventricular pressure overload

Myocardial contractile reserve is significantly attenuated in patients with advanced heart failure. The aim of this study was to identify mechanisms of impaired contractile reserve in a large animal model that closely mimics human myocardial failure. Progressive right ventricular hypertrophy and failure were induced by banding the pulmonary artery in kittens. Isometric contractile force was measured in right ventricular trabeculae (n=115) from age-matched Control and Banded feline hearts. Rapid cooling contractures (RCC) were used to determine sarcoplasmic reticulum (SR) Ca(2+) load while assessing the ability of changes in rate, adrenergic stimulation and bath Ca(2+) to augment contractility. The positive force-frequency relationship and robust pre- and post-receptor adrenergic responses observed in Control trabeculae were closely paralleled by increases in RCC amplitude and the RCC2/RCC1 ratio. Conversely, the severely blunted force-frequency and adrenergic responses in Banded trabeculae were paralleled by an unchanged RCC amplitude and RCC2/RCC1 ratio. Likewise, supraphysiologic levels of bath Ca(2+) were associated with severely reduced contractility and RCC amplitude in Banded trabeculae compared to Controls. There were no differences in myofilament Ca(2+) sensitivity or length-dependent increases in contractility between Control and Banded trabeculae. There was a significant decrease in SR Ca(2+)-ATPase pump abundance and phosphorylation of phospholamban and ryanodine receptor in Banded trabeculae compared with Controls. A reduced ability to increase SR Ca(2+) load is the primary mechanism of reduced contractile reserve in failing feline myocardium. The similarity of impaired contractile reserve phenomenology in this feline model and transplanted hearts suggests mechanistic relevance to human myocardial failure.

Impaired contractile reserve in severe mitral valve regurgitation with a preserved ejection fraction

BACKGROUND

Impaired contractile reserve in chronic MR results from load-independent, myocyte contractile abnormalities.

AIMS

Investigate the mechanisms of contractile dysfunction in chronic mitral valve regurgitation (MR).

METHODS

Mild MR was produced in eight dogs followed by pacing induced left ventricular (LV) dilatation over eight months. In-vivo LV dP/dt was measured at several pacing rates. Contractile function was measured in isolated LV trabeculae and myocytes at several stimulation rates and during changes in extracellular [Ca2+]. Identical studies were performed with six control dogs.

RESULTS

Chronic MR resulted in a preserved ejection fraction with decreased dP/dt (p<0.01). LV trabeculae demonstrated significantly lower developed force and a negative force-frequency relation with chronic MR (p<0.05). Myocytes exhibited a negative shortening-frequency relationship in both groups with a greater decline with chronic MR (p<0.001) paralleled by decreases in peak [Ca2+](i) transients. Increases in extracellular [Ca2+] abrogated the defects in force generation in trabeculae from animals with chronic MR.

CONCLUSION

Even with a preserved EF, chronic severe MR results in a significant reduction in intrinsic contractile function and reserve. Functional impairment was load-independent reflecting a predominant defect in calcium cycling rather than impaired peak force generating capacity due to myofibrillar attenuation.

Adolescent feline heart contains a population of small, proliferative ventricular myocytes with immature physiological properties

Recent studies suggest that rather than being terminally differentiated, the adult heart is a self-renewing organ with the capacity to generate new myocytes from cardiac stem/progenitor cells (CS/PCs). This study examined the hypotheses that new myocytes are generated during adolescent growth, to increase myocyte number, and these newly formed myocytes are initially small, mononucleated, proliferation competent, and have immature properties. Ventricular myocytes (VMs) and cKit(+) (stem cell receptor) CS/PCs were isolated from 11- and 22-week feline hearts. Bromodeoxyuridine incorporation (in vivo) and p16(INK4a) immunostaining were measured to assess myocyte cell cycle activity and senescence, respectively. Telomerase activity, contractions, Ca(2+) transients, and electrophysiology were compared in small mononucleated (SMMs) and large binucleated (LBMs) myocytes. Heart mass increased by 101% during adolescent growth, but left ventricular myocyte volume only increased by 77%. Most VMs were binucleated (87% versus 12% mononucleated) and larger than mononucleated myocytes. A greater percentage of SMMs was bromodeoxyuridine positive (SMMs versus LBMs: 3.1% versus 0.8%; P<0.05), and p16(INK4a) negative and small myocytes had greater telomerase activity than large myocytes. Contractions and Ca(2+) transients were prolonged in SMMs versus LBMs and Ca(2+) release was disorganized in SMMs with reduced transient outward current and T-tubule density. The T-type Ca(2+) current, usually seen in fetal/neonatal VMs, was found exclusively in SMMs and in myocytes derived from CS/PC. Myocyte number increases during adolescent cardiac growth. These new myocytes are initially small and functionally immature, with patterns of ion channel expression normally found in the fetal/neonatal period.