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Hypoxia (20 min) and reoxygenation (30 min) were simulated on isolated rat cardiomyocytes to evaluate the cytoprotective effect of selective [[delta].sub.2]-opioid receptor agonist deltorphin II, opioid receptor antagonist naloxone methiodide, [mu]-opioid receptor antagonist CTAP, [kappa]-opioid receptor antagonist nor-binaltorphimine, [[epsilon].sub.1]-opioid receptor antagonist BNTX, and [[delta].sub.2]- opioid receptors naltriben. Deltorphin II was administered 5 min before reoxygenation, antagonists were administered 10 min before reoxygenation. The cytoprotective effect of deltorphin II was assessed by the number of cardiomyocytes survived after hypoxia/reoxygenation, as well as by the lactate dehydrogenase content in the incubation medium. It has been established that the cytoprotective effect of deltorphin II occurs at a concentration of 64 nmol/liter and is associated with activation of [[delta].sub.2]-opioid receptors. Key Words: cardiomyocytes; hypoxia/reoxygenation; opioid receptors

Cardiovascular disease is the leading cause of death worldwide. Advanced insights into disease mechanisms and therapeutic strategies require a deeper understanding of the molecular processes involved in the healthy heart. Knowledge of the full repertoire of cardiac cells and their gene expression profiles is a fundamental first step in this endeavour. Here, using state-of-the-art analyses of large-scale single-cell and single-nucleus transcriptomes, we characterize six anatomical adult heart regions. Our results highlight the cellular heterogeneity of cardiomyocytes, pericytes and fibroblasts, and reveal distinct atrial and ventricular subsets of cells with diverse developmental origins and specialized properties. We define the complexity of the cardiac vasculature and its changes along the arterio-venous axis. In the immune compartment, we identify cardiac-resident macrophages with inflammatory and protective transcriptional signatures. Furthermore, analyses of cell-to-cell interactions highlight different networks of macrophages, fibroblasts and cardiomyocytes between atria and ventricles that are distinct from those of skeletal muscle. Our human cardiac cell atlas improves our understanding of the human heart and provides a valuable reference for future studies. Single-cell and single-nucleus RNA sequencing are used to construct a cellular atlas of the human heart that will aid further research into cardiac physiology and disease.

Cardiomyocyte transplantation after heart attack improves contractile function in monkeys. Pluripotent stem cell–derived cardiomyocyte grafts can remuscularize substantial amounts of infarcted myocardium and beat in synchrony with the heart, but in some settings cause ventricular arrhythmias. It is unknown whether human cardiomyocytes can restore cardiac function in a physiologically relevant large animal model. Here we show that transplantation of ∼750 million cryopreserved human embryonic stem cell–derived cardiomyocytes (hESC-CMs) enhances cardiac function in macaque monkeys with large myocardial infarctions. One month after hESC-CM transplantation, global left ventricular ejection fraction improved 10.6 ± 0.9% vs. 2.5 ± 0.8% in controls, and by 3 months there was an additional 12.4% improvement in treated vs. a 3.5% decline in controls. Grafts averaged 11.6% of infarct size, formed electromechanical junctions with the host heart, and by 3 months contained ∼99% ventricular myocytes. A subset of animals experienced graft-associated ventricular arrhythmias, shown by electrical mapping to originate from a point-source acting as an ectopic pacemaker. Our data demonstrate that remuscularization of the infarcted macaque heart with human myocardium provides durable improvement in left ventricular function.

The relation between ischemia and heart failure is well demonstrated, and several studies suggested that realizing the physiological role of autophagy will be of great importance. Luteoloside (Lut) is one of the main components of Lonicera japonica flos and exhibits antioxidant, anti-inflammatory, and cardioprotective properties. To determine if Lut pretreatment enhanced autophagy by 14-3-3[eta] expression and the AMPK[alpha]-mTOR/ULK1 pathway and protected the neonatal rat cardiomyocytes (NRCMs) against anoxia damage, NRCMs were treated using 20 [mu]M Lut for 36 h, and the anoxia damage model was established using NRCMs. The indexes reflecting the condition of NRCMs, oxidative stress level, and mitochondrial function were evaluated. In addition, the expression and phosphorylation of 14-3-3[eta] and AMPK[alpha]/mTOR/ULK1, and autophagy markers (LC3II, P62) and the abundance of autophagy lysosomes were detected. Results revealed that Lut pretreatment alleviated anoxia- induced damage in NRCMs, that is, Lut pretreatment could increase cell viability, decrease LDH activity and apoptosis, suppressed ROS generation and oxidative stress, restored intracellular ATP levels, stabilized MMP levels, and inhibited mPTP opening. Furthermore, Lut pretreatment could enhance autophagy via upregulating 14-3-3[eta], LC3II expression and increasing p-AMPK[alpha]/AMPK[alpha] and p-ULK1/ULK1 level, whereas P62 expression and p-mTOR/mTOR level decreased; the fluorescence intensity of autolysosomes also increased. However, in the NRCMs treated with pAD/14-3-3[eta] RNAi or incubated with 3-MA (an autophagy inhibitor), the abovementioned effects of Lut pretreatment were reduced. Taken together, Lut pretreatment could enhance autophagy by upregulating 14-3-3[eta] expression to influence the AMPK[alpha]-mTOR/ ULK1 pathway against anoxia-induced damage in NRCMs.

Our knowledge of pluripotent stem cell (PSC) biology has advanced to the point where we now can generate most cells of the human body in the laboratory. PSC-derived cardiomyocytes can be generated routinely with high yield and purity for disease research and drug development, and these cells are now gradually entering the clinical research phase for the testing of heart regeneration therapies. However, a major hurdle for their applications is the immature state of these cardiomyocytes. In this Review, we describe the structural and functional properties of cardiomyocytes and present the current approaches to mature PSC-derived cardiomyocytes. To date, the greatest success in maturation of PSC-derived cardiomyocytes has been with transplantation into the heart in animal models and the engineering of 3D heart tissues with electromechanical conditioning. In conventional 2D cell culture, biophysical stimuli such as mechanical loading, electrical stimulation and nanotopology cues all induce substantial maturation, particularly of the contractile cytoskeleton. Metabolism has emerged as a potent means to control maturation with unexpected effects on electrical and mechanical function. Different interventions induce distinct facets of maturation, suggesting that activating multiple signalling networks might lead to increased maturation. Despite considerable progress, we are still far from being able to generate PSC-derived cardiomyocytes with adult-like phenotypes in vitro. Future progress will come from identifying the developmental drivers of maturation and leveraging them to create more mature cardiomyocytes for research and regenerative medicine.In this Review, Murry and colleagues describe the hallmarks of cardiomyocyte maturation and the current approaches to mature stem cell-derived cardiomyocytes, highlighting challenges and future directions to generate cardiomyocytes with an optimal maturation state for use in research and regenerative medicine.

Cardiac tissues generated from human induced pluripotent stem cells (iPSCs) can serve as platforms for patient-specific studies of physiology and disease 1 – 6 . However, the predictive power of these models is presently limited by the immature state of the cells 1 , 2 , 5 , 6 . Here we show that this fundamental limitation can be overcome if cardiac tissues are formed from early-stage iPSC-derived cardiomyocytes soon after the initiation of spontaneous contractions and are subjected to physical conditioning with increasing intensity over time. After only four weeks of culture, for all iPSC lines studied, such tissues displayed adult-like gene expression profiles, remarkably organized ultrastructure, physiological sarcomere length (2.2 µm) and density of mitochondria (30%), the presence of transverse tubules, oxidative metabolism, a positive force–frequency relationship and functional calcium handling. Electromechanical properties developed more slowly and did not achieve the stage of maturity seen in adult human myocardium. Tissue maturity was necessary for achieving physiological responses to isoproterenol and recapitulating pathological hypertrophy, supporting the utility of this tissue model for studies of cardiac development and disease. A tissue culture system that provides an increasing intensity of electromechanical stimulation over time enables an in vitro model of cardiac tissue derived from human induced pluripotent stem cells to develop many of the characteristics of adult cardiac tissue.

Cardiomyocyte loss after injury results in adverse remodelling and fibrosis, inevitably leading to heart failure. The ERBB2–Neuregulin and Hippo–YAP signalling pathways are key mediators of heart regeneration, yet the crosstalk between them is unclear. We demonstrate that transient overexpression of activated ERBB2 in cardiomyocytes (OE CMs) promotes cardiac regeneration in a heart failure model. OE CMs present an epithelial–mesenchymal transition (EMT)-like regenerative response manifested by cytoskeletal remodelling, junction dissolution, migration and extracellular matrix turnover. We identified YAP as a critical mediator of ERBB2 signalling. In OE CMs, YAP interacts with nuclear-envelope and cytoskeletal components, reflecting an altered mechanical state elicited by ERBB2. We identified two YAP-activating phosphorylations on S352 and S274 in OE CMs, which peak during metaphase, that are ERK dependent and Hippo independent. Viral overexpression of YAP phospho-mutants dampened the proliferative competence of OE CMs. Together, we reveal a potent ERBB2-mediated YAP mechanotransduction signalling, involving EMT-like characteristics, resulting in robust heart regeneration.Aharonov et al. use in vivo genetic approaches to show that ErBB2-mediated YAP activation initiates epithelial–mesenchymal transition-like processes and dedifferentiation of cardiomyocytes to drive heart regeneration.

The past few decades have generated growing recognition that the immune system makes an important contribution to cardiac development, composition and function. Immune cells infiltrate the heart at gestation and remain in the myocardium, where they participate in essential housekeeping functions throughout life. After myocardial infarction or in response to infection, large numbers of immune cells are recruited to the heart to remove dying tissue, scavenge pathogens and promote healing. Under some circumstances, immune cells can cause irreversible damage, contributing to heart failure. This Review focuses on the role of the immune system in the heart under both homeostatic and perturbed conditions.

The epicardium and its derivatives provide trophic and structural support for the developing and adult heart. Here we tested the ability of human embryonic stem cell (hESC)-derived epicardium to augment the structure and function of engineered heart tissue in vitro and to improve efficacy of hESC-cardiomyocyte grafts in infarcted athymic rat hearts. Epicardial cells markedly enhanced the contractility, myofibril structure and calcium handling of human engineered heart tissues, while reducing passive stiffness compared with mesenchymal stromal cells. Transplanted epicardial cells formed persistent fibroblast grafts in infarcted hearts. Cotransplantation of hESC-derived epicardial cells and cardiomyocytes doubled graft cardiomyocyte proliferation rates in vivo, resulting in 2.6-fold greater cardiac graft size and simultaneously augmenting graft and host vascularization. Notably, cotransplantation improved systolic function compared with hearts receiving either cardiomyocytes alone, epicardial cells alone or vehicle. The ability of epicardial cells to enhance cardiac graft size and function makes them a promising adjuvant therapeutic for cardiac repair.