C-type natriuretic peptide induces inotropic and lusitropic effects in human 3D-engineered cardiac tissue: Implications for the regulation of cardiac function in humans.

The role of C-type natriuretic peptide (CNP) in the regulation of cardiac function in humans remains to be established as previous investigations have been confined to animal model systems. Here, we used well-characterized engineered cardiac tissues (ECTs) generated from human stem cell-derived cardiomyocytes and fibroblasts to study the acute effects of CNP on contractility. Application of CNP elicited a positive inotropic response as evidenced by increases in maximum twitch amplitude, maximum contraction slope and maximum calcium amplitude. This inotropic response was accompanied by a positive lusitropic response as demonstrated by reductions in time from peak contraction to 90% of relaxation and time from peak calcium transient to 90% of decay that paralleled increases in maximum contraction decay slope and maximum calcium decay slope. To establish translatability, CNP-induced changes in contractility were also assessed in rat ex vivo (isolated heart) and in vivo models. Here, the effects on force kinetics observed in ECTs mirrored those observed in both the ex vivo and in vivo model systems, whereas the increase in maximal force generation with CNP application was only detected in ECTs. In conclusion, CNP induces a positive inotropic and lusitropic response in ECTs, thus supporting an important role for CNP in the regulation of human cardiac function. The high degree of translatability between ECTs, ex vivo and in vivo models further supports a regulatory role for CNP and expands the current understanding of the translational value of human ECTs. NEW FINDINGS: What is the central question of this study? What are the acute responses to C-type natriuretic peptide (CNP) in human-engineered cardiac tissues (ECTs) on cardiac function and how well do they translate to matched concentrations in animal ex vivo and in vivo models? What is the main finding and its importance? Acute stimulation of ECTs with CNP induced positive lusitropic and inotropic effects on cardiac contractility, which closely reflected the changes observed in rat ex vivo and in vivo cardiac models. These findings support an important role for CNP in the regulation of human cardiac function and highlight the translational value of ECTs.

Fibrotic Signaling in Cardiac Fibroblasts and Vascular Smooth Muscle Cells: The Dual Roles of Fibrosis in HFpEF and CAD.

Patients with heart failure with preserved ejection fraction (HFpEF) and atherosclerosis-driven coronary artery disease (CAD) will have ongoing fibrotic remodeling both in the myocardium and in atherosclerotic plaques. However, the functional consequences of fibrosis differ for each location. Thus, cardiac fibrosis leads to myocardial stiffening, thereby compromising cardiac function, while fibrotic remodeling stabilizes the atherosclerotic plaque, thereby reducing the risk of plaque rupture. Although there are currently no drugs targeting cardiac fibrosis, it is a field under intense investigation, and future drugs must take these considerations into account. To explore similarities and differences of fibrotic remodeling at these two locations of the heart, we review the signaling pathways that are activated in the main extracellular matrix (ECM)-producing cells, namely human cardiac fibroblasts (CFs) and vascular smooth muscle cells (VSMCs). Although these signaling pathways are highly overlapping and context-dependent, effects on ECM remodeling mainly act through two core signaling cascades: TGF-ß and Angiotensin II. We complete this by summarizing the knowledge gained from clinical trials targeting these two central fibrotic pathways.

G protein-coupled receptor kinase 5 (GRK5) contributes to impaired cardiac function and immune cell recruitment in post-ischemic heart failure.

AIMS: Myocardial infarction (MI) is the most common cause of heart failure (HF) worldwide. G protein-coupled receptor kinase 5 (GRK5) is upregulated in failing human myocardium and promotes maladaptive cardiac hypertrophy in animal models. However, the role of GRK5 in ischemic heart disease is still unknown. In this study, we evaluated whether myocardial GRK5 plays a critical role post-MI in mice and included the examination of specific cardiac immune and inflammatory responses. METHODS AND RESULTS: Cardiomyocyte-specific GRK5 overexpressing transgenic mice (TgGRK5) and non-transgenic littermate control (NLC) mice as well as cardiomyocyte-specific GRK5 knockout mice (GRK5cKO) and wild type (WT) were subjected to MI and, functional as well as structural changes together with outcomes were studied. TgGRK5 post-MI mice showed decreased cardiac function, augmented left ventricular dimension and decreased survival rate compared to NLC post-MI mice. Cardiac hypertrophy and fibrosis as well as fetal gene expression were increased post-MI in TgGRK5 compared to NLC mice. In TgGRK5 mice, GRK5 elevation produced immuno-regulators that contributed to the elevated and long-lasting leukocyte recruitment into the injured heart and ultimately to chronic cardiac inflammation. We found an increased presence of pro-inflammatory neutrophils and macrophages as well as neutrophils, macrophages and T-lymphocytes at 4-days and 8-weeks respectively post-MI in TgGRK5 hearts. Conversely, GRK5cKO mice were protected from ischemic injury and showed reduced early immune cell recruitment (predominantly monocytes) to the heart, improved contractility and reduced mortality compared to WT post-MI mice. Interestingly, cardiomyocyte-specific GRK2 transgenic mice did not share the same phenotype of TgGRK5 mice and did not have increased cardiac leukocyte migration and cytokine or chemokine production post-MI. CONCLUSIONS: Our study shows that myocyte GRK5 has a crucial and GRK-selective role on the regulation of leucocyte infiltration into the heart, cardiac function and survival in a murine model of post-ischemic HF, supporting GRK5 inhibition as a therapeutic target for HF.

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.

GDF11 Decreases Pressure Overload-Induced Hypertrophy, but Can Cause Severe Cachexia and Premature Death.

RATIONALE: Possible beneficial effects of GDF11 (growth differentiation factor 11) on the normal, diseased, and aging heart have been reported, including reversing aging-induced hypertrophy. These effects have not been well validated. High levels of GDF11 have also been shown to cause cardiac and skeletal muscle wasting. These controversies could be resolved if dose-dependent effects of GDF11 were defined in normal and aged animals as well as in pressure overload-induced pathological hypertrophy. OBJECTIVE: To determine dose-dependent effects of GDF11 on normal hearts and those with pressure overload-induced cardiac hypertrophy. METHODS AND RESULTS: Twelve- to 13-week-old C57BL/6 mice underwent transverse aortic constriction (TAC) surgery. One-week post-TAC, these mice received rGDF11 (recombinant GDF11) at 1 of 3 doses: 0.5, 1.0, or 5.0 mg/kg for up to 14 days. Treatment with GDF11 increased plasma concentrations of GDF11 and p-SMAD2 in the heart. There were no significant differences in the peak pressure gradients across the aortic constriction between treatment groups at 1 week post-TAC. Two weeks of GDF11 treatment caused dose-dependent decreases in cardiac hypertrophy as measured by heart weight/tibia length ratio, myocyte cross-sectional area, and left ventricular mass. GDF11 improved cardiac pump function while preventing TAC-induced ventricular dilation and caused a dose-dependent decrease in interstitial fibrosis (in vivo), despite increasing markers of fibroblast activation and myofibroblast transdifferentiation (in vitro). Treatment with the highest dose (5.0 mg/kg) of GDF11 caused severe body weight loss, with significant decreases in both muscle and organ weights and death in both sham and TAC mice. CONCLUSIONS: Although GDF11 treatment can reduce pathological cardiac hypertrophy and associated fibrosis while improving cardiac pump function in pressure overload, high doses of GDF11 cause severe cachexia and death. Use of GDF11 as a therapy could have potentially devastating actions on the heart and other tissues.

Diabetic Cardiomyopathy: Current and Future Therapies. Beyond Glycemic Control.

Diabetes mellitus and the associated complications represent a global burden on human health and economics. Cardiovascular diseases are the leading cause of death in diabetic patients, who have a 2-5 times higher risk of developing heart failure than age-matched non-diabetic patients, independent of other comorbidities. Diabetic cardiomyopathy is defined as the presence of abnormal cardiac structure and performance in the absence of other cardiac risk factors, such coronary artery disease, hypertension, and significant valvular disease. Hyperglycemia, hyperinsulinemia, and insulin resistance mediate the pathological remodeling of the heart, characterized by left ventricle concentric hypertrophy and perivascular and interstitial fibrosis leading to diastolic dysfunction. A change in the metabolic status, impaired calcium homeostasis and energy production, increased inflammation and oxidative stress, as well as an accumulation of advanced glycation end products are among the mechanisms implicated in the pathogenesis of diabetic cardiomyopathy. Despite a growing interest in the pathophysiology of diabetic cardiomyopathy, there are no specific guidelines for diagnosing patients or structuring a treatment strategy in clinical practice. Anti-hyperglycemic drugs are crucial in the management of diabetes by effectively reducing microvascular complications, preventing renal failure, retinopathy, and nerve damage. Interestingly, several drugs currently in use can improve cardiac health beyond their ability to control glycemia. GLP-1 receptor agonists and sodium-glucose co-transporter 2 inhibitors have been shown to have a beneficial effect on the cardiovascular system through a direct effect on myocardium, beyond their ability to lower blood glucose levels. In recent years, great improvements have been made toward the possibility of modulating the expression of specific cardiac genes or non-coding RNAs in vivo for therapeutic purpose, opening up the possibility to regulate the expression of key players in the development/progression of diabetic cardiomyopathy. This review summarizes the pathogenesis of diabetic cardiomyopathy, with particular focus on structural and molecular abnormalities occurring during its progression, as well as both current and potential future therapies.

Long-Term Caloric Restriction Improves Cardiac Function, Remodeling, Adrenergic Responsiveness, and Sympathetic Innervation in a Model of Postischemic Heart Failure.

BACKGROUND: Caloric restriction (CR) has been described to have cardioprotective effects and improve functional outcomes in animal models and humans. Chronic ischemic heart failure (HF) is associated with reduced cardiac sympathetic innervation, dysfunctional ß-adrenergic receptor signaling, and decreased cardiac inotropic reserve. We tested the effects of a long-term CR diet, started late after myocardial infarction on cardiac function, sympathetic innervation, and ß-adrenergic receptor responsiveness in a rat model of postischemic HF. METHODS AND RESULTS: Adult male rats were randomly assigned to myocardial infarction or sham operation and 4 weeks later were further randomized to a 1-year CR or normal diet. One year of CR resulted in a significant reduction in body weight, heart weight, and heart weight/tibia length ratio when compared with normal diet in HF groups. At the end of the study period, echocardiography and histology revealed that HF animals under the CR diet had ameliorated left ventricular remodeling compared with HF rats fed with normal diet. Invasive hemodynamic showed a significant improvement of cardiac inotropic reserve in CR HF rats compared with HF-normal diet animals. Importantly, CR dietary regimen was associated with a significant increase of cardiac sympathetic innervation and with normalized cardiac ß-adrenergic receptor levels in HF rats when compared with HF rats on the standard diet. CONCLUSIONS: We demonstrate, for the first time, that chronic CR, when started after HF established, can ameliorate cardiac dysfunction and improve inotropic reserve. At the molecular level, we find that chronic CR diet significantly improves sympathetic cardiac innervation and ß-adrenergic receptor levels in failing myocardium.

Notch signaling modulates the electrical behavior of cardiomyocytes.

Notch receptor signaling is active during cardiac development and silenced in myocytes after birth. Conversely, outward K+ Kv currents progressively appear in postnatal myocytes leading to shortening of the action potential (AP) and acquisition of the mature electrical phenotype. In the present study, we tested the possibility that Notch signaling modulates the electrical behavior of cardiomyocytes by interfering with Kv currents. For this purpose, the effects of Notch receptor activity on electrophysiological properties of myocytes were evaluated using transgenic mice with inducible expression of the Notch1 intracellular domain (NICD), the functional fragment of the activated Notch receptor, and in neonatal myocytes after inhibition of the Notch transduction pathway. By patch clamp, NICD-overexpressing cells presented prolonged AP duration and reduced upstroke amplitude, properties that were coupled with reduced rapidly activating Kv and fast Na+ currents, compared with cells obtained from wild-type mice. In cultured neonatal myocytes, inhibition of the proteolitic release of NICD with a γ-secretase antagonist increased transcript levels of the Kv channel-interacting proteins 2 (KChIP2) and enhanced the density of Kv currents. Collectively, these results indicate that Notch signaling represents an important regulator of the electrophysiological behavior of developing and adult myocytes by repressing, at least in part, repolarizing Kv currents. NEW & NOTEWORTHY We investigated the effects of Notch receptor signaling on the electrical properties of cardiomyocytes. Our results indicate that the Notch transduction pathway interferes with outward K+ Kv currents, critical determinants of the electrical repolarization of myocytes.

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.

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.

microRNA in Cardiovascular Aging and Age-Related Cardiovascular Diseases.

Over the last decades, life expectancy has significantly increased although several chronic diseases persist in the population, with aging as the leading risk factor. Despite improvements in diagnosis and treatment, many elderlies suffer from cardiovascular problems that are much more frequent in an older, more fragile organism. In the long term, age-related cardiovascular diseases (CVDs) contribute to the decline of quality of life and ability to perform normal activities of daily living. microRNAs (miRNAs) are a class of small non-coding RNAs that regulate gene expression at the posttranscriptional level in both physiological and pathological conditions. In this review, we will focus on the role of miRNAs in aging and age-related CVDs as heart failure, hypertension, atherosclerosis, atrial fibrillation, and diabetes mellitus. miRNAs are key regulators of complex biological mechanisms, representing an exciting potential therapeutic target in CVDs. Moreover, one major challenge in geriatric medicine is to find reliable biomarkers for diagnosis, prognosis, and prediction of the response to specific drugs. miRNAs represent a very promising tool due to their stability in the circulation and unique signature in CVDs. However, further studies are needed to investigate their translational potential in the real clinical practice.

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.

Hyperglycemia induces defective Ca2+ homeostasis in cardiomyocytes.

Diabetes and other metabolic conditions characterized by elevated blood glucose constitute important risk factors for cardiovascular disease. Hyperglycemia targets myocardial cells rendering ineffective mechanical properties of the heart, but cellular alterations dictating the progressive deterioration of cardiac function with metabolic disorders remain to be clarified. In the current study, we examined the effects of hyperglycemia on cardiac function and myocyte physiology by employing mice with high blood glucose induced by administration of streptozotocin, a compound toxic to insulin-producing ß-cells. We found that hyperglycemia initially delayed the electrical recovery of the heart, whereas cardiac function became defective only after ~2 mo with this condition and gradually worsened with time. Prolonged hyperglycemia was associated with increased chamber dilation, thinning of the left ventricle (LV), and myocyte loss. Cardiomyocytes from hyperglycemic mice exhibited defective Ca2+ transients before the appearance of LV systolic defects. Alterations in Ca2+ transients involved enhanced spontaneous Ca2+ releases from the sarcoplasmic reticulum (SR), reduced cytoplasmic Ca2+ clearance, and declined SR Ca2+ load. These defects have important consequences on myocyte contraction, relaxation, and mechanisms of rate adaptation. Collectively, our data indicate that hyperglycemia alters intracellular Ca2+ homeostasis in cardiomyocytes, hindering contractile activity and contributing to the manifestation of the diabetic cardiomyopathy. NEW & NOTEWORTHY: We have investigated the effects of hyperglycemia on cardiomyocyte physiology and ventricular function. Our results indicate that defective Ca2+ handling is a critical component of the progressive deterioration of cardiac performance of the diabetic 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.

Myocyte repolarization modulates myocardial function in aging dogs.

Studies of myocardial aging are complex and the mechanisms involved in the deterioration of ventricular performance and decreased functional reserve of the old heart remain to be properly defined. We have studied a colony of beagle dogs from 3 to 14 yr of age kept under a highly regulated environment to define the effects of aging on the myocardium. Ventricular, myocardial, and myocyte function, together with anatomical and structural properties of the organ and cardiomyocytes, were evaluated. Ventricular hypertrophy was not observed with aging and the structural composition of the myocardium was modestly affected. Alterations in the myocyte compartment were identified in aged dogs, and these factors negatively interfere with the contractile reserve typical of the young heart. The duration of the action potential is prolonged in old cardiomyocytes contributing to the slower electrical recovery of the myocardium. Also, the remodeled repolarization of cardiomyocytes with aging provides inotropic support to the senescent muscle but compromises its contractile reserve, rendering the old heart ineffective under conditions of high hemodynamic demand. The defects in the electrical and mechanical properties of cardiomyocytes with aging suggest that this cell population is an important determinant of the cardiac senescent phenotype. Collectively, the delayed electrical repolarization of aging cardiomyocytes may be viewed as a critical variable of the aging myopathy and its propensity to evolve into ventricular decompensation under stressful conditions.

Late Na(+) current and protracted electrical recovery are critical determinants of the aging myopathy.

The aging myopathy manifests itself with diastolic dysfunction and preserved ejection fraction. We raised the possibility that, in a mouse model of physiological aging, defects in electromechanical properties of cardiomyocytes are important determinants of the diastolic characteristics of the myocardium, independently from changes in structural composition of the muscle and collagen framework. Here we show that an increase in the late Na(+) current (INaL) in aging cardiomyocytes prolongs the action potential (AP) and influences temporal kinetics of Ca(2+) cycling and contractility. These alterations increase force development and passive tension. Inhibition of INaL shortens the AP and corrects dynamics of Ca(2+) transient, cell contraction and relaxation. Similarly, repolarization and diastolic tension of the senescent myocardium are partly restored. Thus, INaL offers inotropic support, but negatively interferes with cellular and ventricular compliance, providing a new perspective of the biology of myocardial aging and the aetiology of the defective cardiac performance in the elderly.

Hormone replacement therapy enhances IGF-1 signaling in skeletal muscle by diminishing miR-182 and miR-223 expressions: a study on postmenopausal monozygotic twin pairs.

MiRNAs are fine-tuning modifiers of skeletal muscle regulation, but knowledge of their hormonal control is lacking. We used a co-twin case-control study design, that is, monozygotic postmenopausal twin pairs discordant for estrogen-based hormone replacement therapy (HRT) to explore estrogen-dependent skeletal muscle regulation via miRNAs. MiRNA profiles were determined from vastus lateralis muscle of nine healthy 54-62-years-old monozygotic female twin pairs discordant for HRT (median 7 years). MCF-7 cells, human myoblast cultures and mouse muscle experiments were used to confirm estrogen's causal role on the expression of specific miRNAs, their target mRNAs and proteins and finally the activation of related signaling pathway. Of the 230 miRNAs expressed at detectable levels in muscle samples, qPCR confirmed significantly lower miR-182, miR-223 and miR-142-3p expressions in HRT using than in their nonusing co-twins. Insulin/IGF-1 signaling emerged one common pathway targeted by these miRNAs. IGF-1R and FOXO3A mRNA and protein were more abundantly expressed in muscle samples of HRT users than nonusers. In vitro assays confirmed effective targeting of miR-182 and miR-223 on IGF-1R and FOXO3A mRNA as well as a dose-dependent miR-182 and miR-223 down-regulations concomitantly with up-regulation of FOXO3A and IGF-1R expression. Novel finding is the postmenopausal HRT-reduced miRs-182, miR-223 and miR-142-3p expression in female skeletal muscle. The observed miRNA-mediated enhancement of the target genes' IGF-1R and FOXO3A expression as well as the activation of insulin/IGF-1 pathway signaling via phosphorylation of AKT and mTOR is an important mechanism for positive estrogen impact on skeletal muscle of postmenopausal women.

c-Kit-positive cardiac stem cells nested in hypoxic niches are activated by stem cell factor reversing the aging myopathy.

RATIONALE: Hypoxia favors stem cell quiescence, whereas normoxia is required for stem cell activation, but whether cardiac stem cell (CSC) function is regulated by the hypoxic/normoxic state of the cell is currently unknown. OBJECTIVE: A balance between hypoxic and normoxic CSCs may be present in the young heart, although this homeostatic control may be disrupted with aging. Defects in tissue oxygenation occur in the old myocardium, and this phenomenon may expand the pool of hypoxic CSCs, which are no longer involved in myocyte renewal. METHODS AND RESULTS: Here, we show that the senescent heart is characterized by an increased number of quiescent CSCs with intact telomeres that cannot re-enter the cell cycle and form a differentiated progeny. Conversely, myocyte replacement is controlled only by frequently dividing CSCs with shortened telomeres; these CSCs generate a myocyte population that is chronologically young but phenotypically old. Telomere dysfunction dictates their actual age and mechanical behavior. However, the residual subset of quiescent young CSCs can be stimulated in situ by stem cell factor reversing the aging myopathy. CONCLUSIONS: Our findings support the notion that strategies targeting CSC activation and growth interfere with the manifestations of myocardial aging in an animal model. Although caution has to be exercised in the translation of animal studies to human beings, our data strongly suggest that a pool of functionally competent CSCs persists in the senescent heart and that this stem cell compartment can promote myocyte regeneration effectively, partly correcting the aging myopathy.