Pathomechanisms in heart failure: the contractile connection

Spontaneous myogenic fasciculation associated with the lengthening of cardiac muscle in response to static preloading

Force enhancement is one kind of myogenic spontaneous fasciculation in lengthening preload striated muscles. In cardiac muscle, the role of this biomechanical event is not well established. The physiological passive property is an essential part for maintaining normal diastole in the heart. In excessive preload heart, force enhancement relative erratic passive properties may cause muscle decompensating, implicate in the development of diastolic dysfunction. In this study, the force enhancement occurrence in mouse cardiac papillary muscle was evaluated by a microstepping stretch method. The intracellular Ca²⁺ redistribution during occurrence of force enhancement was monitored in real-time by a Flou-3 (2 mM) indicator. The force enhancement amplitude, the enhancement of the prolongation time, and the tension-time integral were analyzed by myography. The results indicated that the force enhancement occurred immediately after active stretching and was rapidly enhanced during sustained static stretch. The presence of the force and the increase in the amplitude synchronized with the acquisition and immediate transfer of Ca²⁺ to adjacent fibres. In highly preloaded fibres, the enhancement exceeded the maximum passive tension (from 4.49 ± 0.43 N/mm² to 6.20 ± 0.51 N/mm²). The occurrence of force enhancement were unstable in each static stretch. The increased enhancement amplitude combined with the reduced prolongation time to induce a reduction in the tension-time integral. We concluded that intracellular Ca²⁺-synchronized force enhancement is one kind of interruption event in excessive preload cardiac muscle. During the cardiac muscle in its passive relaxation period, the occurrence of this interruption affected the rhythmic stability of the cardiac relaxation cycle.

Therapeutic Stalemate in Heart Failure With Preserved Ejection Fraction

The findings of randomized trials of neurohormonal modulation have been neutral in heart failure with preserved ejection fraction and consistently positive in heart failure with reduced ejection. Left ventricular remodeling promotes the development and progression of heart failure with preserved and reduced ejection fraction. However, different stimuli mediate left ventricular remodeling that is commonly concentric in heart failure with preserved ejection fraction and eccentric in heart failure with reduced ejection. The stimuli that promote concentric left ventricular remodeling may account for the neutral findings of neuhormonal modulation in heart failure with preserved ejection fraction. Low‐grade systemic inflammation‐induced microvascular endothelial dysfunction is currently the leading hypothesis behind the development and progression of heart failure with preserved ejection fraction. The hypothesis provided the rationale for several randomized controlled trials that have led to neutral findings. The trials and their limitations are reviewed.

Quantifying drug-induced structural toxicity in hepatocytes and cardiomyocytes derived from hiPSCs using a deep learning method

Cardiac and hepatic toxicity result from induced disruption of the functioning of cardiomyocytes and hepatocytes, respectively, which is tightly related to the organization of their subcellular structures. Cellular structure can be analyzed from microscopy imaging data. However, subtle or complex structural changes that are not easily perceived may be missed by conventional image-analysis techniques. Here we report the evaluation of PhenoTox, an image-based deep-learning method of quantifying drug-induced structural changes using human hepatocytes and cardiomyocytes derived from human induced pluripotent stem cells. We assessed the ability of the deep learning method to detect variations in the organization of cellular structures from images of fixed or live cells. We also evaluated the power and sensitivity of the method for detecting toxic effects of drugs by conducting a set of experiments using known toxicants and other methods of screening for cytotoxic effects. Moreover, we used PhenoTox to characterize the effects of tamoxifen and doxorubicin-which cause liver toxicity-on hepatocytes. PhenoTox revealed differences related to loss of cytochrome P450 3A4 activity, for which it showed greater sensitivity than a caspase 3/7 assay. Finally, PhenoTox detected structural toxicity in cardiomyocytes, which was correlated with contractility defects induced by doxorubicin, erlotinib, and sorafenib. Taken together, the results demonstrated that PhenoTox can capture the subtle morphological changes that are early signs of toxicity in both hepatocytes and cardiomyocytes.

Pathophysiology of Hypertensive Heart Disease: Beyond Left Ventricular Hypertrophy

PURPOSE OF REVIEW

Given that the life expectancy and the burden of hypertension are projected to increase over the next decade, hypertensive heart disease (HHD) may be expected to play an even more central role in the pathophysiology of cardiovascular disease (CVD). A broader understanding of the features and underlying mechanisms that constitute HHD therefore is of paramount importance.

RECENT FINDINGS

HHD is a condition that arises as a result of elevated blood pressure and constitutes a key underlying mechanism for cardiovascular morbidity and mortality. Historically, studies investigating HHD have primarily focused on left ventricular (LV) hypertrophy (LVH), but it is increasingly apparent that HHD encompasses a range of target-organ damage beyond LVH, including other cardiovascular structural and functional adaptations that may occur separately or concomitantly. HHD is characterized by micro- and macroscopic myocardial alterations, structural phenotypic adaptations, and functional changes that include cardiac fibrosis, and the remodeling of the atria and ventricles and the arterial system. In this review, we summarize the structural and functional alterations in the cardiac and vascular system that constitute HHD and underscore their underlying pathophysiology.

Rapamycin persistently improves cardiac function in aged, male and female mice, even following cessation of treatment

Even in healthy aging, cardiac morbidity and mortality increase with age in both mice and humans. These effects include a decline in diastolic function, left ventricular hypertrophy, metabolic substrate shifts, and alterations in the cardiac proteome. Previous work from our laboratory indicated that short-term (10-week) treatment with rapamycin, an mTORC1 inhibitor, improved measures of these age-related changes. In this report, we demonstrate that the rapamycin-dependent improvement of diastolic function is highly persistent, while decreases in both cardiac hypertrophy and passive stiffness are substantially persistent 8 weeks after cessation of an 8-week treatment of rapamycin in both male and female 22- to 24-month-old C57BL/6NIA mice. The proteomic and metabolomic abundance changes that occur after 8 weeks of rapamycin treatment have varying persistence after 8 further weeks without the drug. However, rapamycin did lead to a persistent increase in abundance of electron transport chain (ETC) complex components, most of which belonged to Complex I. Although ETC protein abundance and Complex I activity were each differentially affected in males and females, the ratio of Complex I activity to Complex I protein abundance was equally and persistently reduced after rapamycin treatment in both sexes. Thus, rapamycin treatment in the aged mice persistently improved diastolic function and myocardial stiffness, persistently altered the cardiac proteome in the absence of persistent metabolic changes, and led to persistent alterations in mitochondrial respiratory chain activity. These observations suggest that an optimal translational regimen for rapamycin therapy that promotes enhancement of healthspan may involve intermittent short-term treatments.

Engineered Cardiac Tissues Generated in the Biowire™ II: A Platform for Human-Based Drug Discovery

Recent advances in techniques to differentiate human induced pluripotent stem cells (hiPSCs) hold the promise of an unlimited supply of human derived cardiac cells from both healthy and disease populations. That promise has been tempered by the observation that hiPSC-derived cardiomyocytes (hiPSC-CMs) typically retain a fetal-like phenotype, raising concern about the translatability of the in vitro data obtained to drug safety, discovery and development studies. The Biowire™ II platform was used to generate 3D engineered cardiac tissues (ECTs) from hiPSC-CMs and cardiac fibroblasts. Long term electrical stimulation was employed to obtain ECTs that possess a phenotype like that of adult human myocardium including a lack of spontaneous beating, the presence of a positive force-frequency response from 1-4Hz and prominent post-rest potentiation. Pharmacology studies were performed in the ECTs to confirm the presence and functionality of pathways that modulate cardiac contractility in humans. Canonical responses were observed for compounds that act via the β-adrenergic/cAMP-mediated pathway, e.g. isoproterenol and milrinone; the L-type calcium channel, e.g. FPL64176 and nifedipine; and indirectly effect intracellular Ca2+ concentrations, e.g. digoxin. Expected positive inotropic responses were observed for compounds that modulate proteins of the cardiac sarcomere, e.g. omecamtiv mecarbil and levosimendan. ECTs generated in the BiowireTM II platform display adult-like properties and have canonical responses to cardiotherapeutic and cardiotoxic agents that affect contractility in humans via a variety of mechanisms. These data demonstrate that this human-based model can be used to assess the effects of novel compounds on contractility early in the drug discovery and development process.

Biomechanical Characterization of Human Pluripotent Stem Cell-Derived Cardiomyocytes by Use of Atomic Force Microscopy

Atomic force microscopy (AFM) is not only a high-resolution imaging technique but also a sensitive tool able to study biomechanical properties of bio-samples (biomolecules, cells) in native conditions-i.e., in buffered solutions (culturing media) and stable temperature (mostly 37 °C). Micromechanical transducers (cantilevers) are often used to map surface stiffness distribution, adhesion forces, and viscoelastic parameters of living cells; however, they can also be used to monitor time course of cardiomyocytes contraction dynamics (e.g. beating rate, relaxation time), together with other biomechanical properties. Here we describe the construction of an AFM-based biosensor setup designed to study the biomechanical properties of cardiomyocyte clusters, through the use of standard uncoated silicon nitride cantilevers. Force-time curves (mechanocardiograms, MCG) are recorded continuously in real time and in the presence of cardiomyocyte-contraction affecting drugs (e.g., isoproterenol, metoprolol) in the medium, under physiological conditions. The average value of contraction force and the beat rate, as basic biomechanical parameters, represent pharmacological indicators of different phenotype features. Robustness, low computational requirements, and optimal spatial sensitivity (detection limit 200 pN, respectively 20 nm displacement) are the main advantages of the presented method.

Novel sGC Stimulators and sGC Activators for the Treatment of Heart Failure

The burden of heart failure (HF) increases worldwide with an aging population, and there is a high unmet medical need in both, heart failure with reduced ejection fraction (HFrEF) and with preserved ejection fraction (HFpEF). The nitric oxide (NO) pathway is a key regulator in the cardiovascular system and modulates vascular tone and myocardial performance. Disruption of the NO-cyclic guanosine monophosphate (cGMP) signaling axis and impaired cGMP formation by endothelial dysfunction could lead to vasotone dysregulation, vascular and ventricular stiffening, fibrosis, and hypertrophy resulting in a decline of heart as well as kidney function. Therefore, the NO-cGMP pathway is a treatment target in heart failure. Exogenous NO donors such as nitrates have long been used for treatment of cardiovascular diseases but turned out to be limited by increased oxidative stress and tolerance. More recently, novel classes of drugs were discovered which enhance cGMP production by targeting the NO receptor soluble guanylate cyclase (sGC). These compounds, the so-called sGC stimulators and sGC activators, are able to increase the enzymatic activity of sGC to generate cGMP independently of NO and have been developed to target this important signaling cascade in the cardiovascular system.This review will focus on the role of sGC in cardiovascular (CV) physiology and disease and the pharmacological potential of sGC stimulators and sGC activators therein. Preclinical data will be reviewed and summarized, and available clinical data with riociguat and vericiguat, novel direct sGC stimulators, will be presented. Vericiguat is currently being studied in a Phase III clinical program for the treatment of heart failure with reduced ejection fraction (HFrEF).

Phenotypic assays for analyses of pluripotent stem cell-derived cardiomyocytes

Stem cell-derived cardiomyocytes (CMs) hold great hopes for myocardium regeneration because of their ability to produce functional cardiac cells in large quantities. They also hold promise in dissecting the molecular principles involved in heart diseases and also in drug development, owing to their ability to model the diseases using patient-specific human pluripotent stem cell (hPSC)-derived CMs. The CM properties essential for the desired applications are frequently evaluated through morphologic and genotypic screenings. Even though these characterizations are necessary, they cannot in principle guarantee the CM functionality and their drug response. The CM functional characteristics can be quantified by phenotype assays, including electrophysiological, optical, and/or mechanical approaches implemented in the past decades, especially when used to investigate responses of the CMs to known stimuli (eg, adrenergic stimulation). Such methods can be used to indirectly determine the electrochemomechanics of the cardiac excitation-contraction coupling, which determines important functional properties of the hPSC-derived CMs, such as their differentiation efficacy, their maturation level, and their functionality. In this work, we aim to systematically review the techniques and methodologies implemented in the phenotype characterization of hPSC-derived CMs. Further, we introduce a novel approach combining atomic force microscopy, fluorescent microscopy, and external electrophysiology through microelectrode arrays. We demonstrate that this novel method can be used to gain unique information on the complex excitation-contraction coupling dynamics of the hPSC-derived CMs.

Physiologically inspired cardiac scaffolds for tailored in vivo function and heart regeneration

Tissue engineering is well suited for the treatment of cardiac disease due to the limited regenerative capacity of native cardiac tissue and the loss of function associated with endemic cardiac pathologies, such as myocardial infarction and congenital heart defects. However, the physiological complexity of the myocardium imposes extensive requirements on tissue therapies intended for these applications. In recent years, the field of cardiac tissue engineering has been characterized by great innovation and diversity in the fabrication of engineered tissue scaffolds for cardiac repair and regeneration to address these problems. From early approaches that attempted only to deliver cardiac cells in a hydrogel vessel, significant progress has been made in understanding the role of each major component of cardiac living tissue constructs (namely cells, scaffolds, and signaling mechanisms) as they relate to mechanical, biological, and electrical in vivo performance. This improved insight, accompanied by modern material science techniques, allows for the informed development of complex scaffold materials that are optimally designed for cardiac applications. This review provides a background on cardiac physiology as it relates to critical cardiac scaffold characteristics, the degree to which common cardiac scaffold materials fulfill these criteria, and finally an overview of recent in vivo studies that have employed this type of approach.