Small and large animal models in cardiac contraction research: advantages and disadvantages

In Vitro Methods to Model Cardiac Mechanobiology in Health and Disease

In vitro cardiac modeling has taken great strides in the past decade. While most cell and engineered tissue models have focused on cell and tissue contractile function as readouts, mechanobiological cues from the cell environment that affect this function, such as matrix stiffness or organization, are less well explored. In this study, we review two-dimensional (2D) and three-dimensional (3D) models of cardiac function that allow for systematic manipulation or precise control of mechanobiological cues under simulated (patho)physiological conditions while acquiring multiple readouts of cell and tissue function. We summarize the cell types used in these models and highlight the importance of linking 2D and 3D models to address the multiscale organization and mechanical behavior. Finally, we provide directions on how to advance in vitro modeling for cardiac mechanobiology using next generation hydrogels that mimic mechanical and structural environmental features at different length scales and diseased cell types, along with the development of new tissue fabrication and readout techniques. Impact statement Understanding the impact of mechanobiology in cardiac (patho)physiology is essential for developing effective tissue regeneration and drug discovery strategies and requires detailed cause-effect studies. The development of three-dimensional in vitro models allows for such studies with high experimental control, while integrating knowledge from complementary cell culture models and in vivo studies for this purpose. Complemented by the use of human-induced pluripotent stem cells, with or without predisposed genetic diseases, these in vitro models will offer promising outlooks to delineate the impact of mechanobiological cues on human cardiac (patho)physiology in a dish.

Human induced pluripotent stem cell-based platform for modeling cardiac ischemia

Ischemic heart disease is a major cause of death worldwide, and the only available therapy to salvage the tissue is reperfusion, which can initially cause further damage. Many therapeutics that have been promising in animal models have failed in human trials. Thus, functional human based cardiac ischemia models are required. In this study, a human induced pluripotent stem cell derived-cardiomyocyte (hiPSC-CM)-based platform for modeling ischemia-reperfusion was developed utilizing a system enabling precise control over oxygen concentration and real-time monitoring of the oxygen dynamics as well as iPS-CM functionality. In addition, morphology and expression of hypoxia-related genes and proteins were evaluated as hiPSC-CM response to 8 or 24 h hypoxia and 24 h reoxygenation. During hypoxia, initial decrease in hiPSC-CM beating frequency was observed, after which the CMs adapted to the conditions and the beating frequency gradually increased already before reoxygenation. During reoxygenation, the beating frequency typically first surpassed the baseline before settling down to the values close the baseline. Furthermore, slowing on the field potential propagation throughout the hiPSC-CM sheet as well as increase in depolarization time and decrease in overall field potential duration were observed during hypoxia. These changes were reversed during reoxygenation. Disorganization of sarcomere structures was observed after hypoxia and reoxygenation, supported by decrease in the expression of sarcomeric proteins. Furthermore, increase in the expression of gene encoding glucose transporter 1 was observed. These findings indicate, that despite their immature phenotype, hiPSC-CMs can be utilized in modeling ischemia-reperfusion injury.

MicroRNA-21 regulates right ventricular remodeling secondary to pulmonary arterial pressure overload

Right ventricular (RV) function is a critical determinant of survival in patients with pulmonary arterial hypertension (PAH). While miR-21 is known to associate with vascular remodeling in small animal models of PAH, its role in RV remodeling in large animal models has not been characterized. Herein, we investigated the role of miR-21 in RV dysfunction using a sheep model of PAH secondary to pulmonary arterial constriction (PAC). RV structural and functional remodeling were examined using ultrasound imaging. Our results showed that post PAC, RV strain significantly decreased at the basal region compared with t the control. Moreover, such dysfunction was accompanied by increases in miR-21 levels. To determine the role of miR-21 in RV remodeling secondary to PAC, we investigated the molecular alteration secondary to phenylephrine induced hypertrophy and miR21 overexpression in vitro using neonatal rat ventricular myocytes (NRVMs). We found that overexpression of miR-21 in the setting of hypertrophic stimulation augmented only the expression of proteins critical for mitosis but not cytokinesis. Strikingly, this molecular alteration was associated with an eccentric cellular hypertrophic phenotype similar to what we observed in vivo PAC animal model in sheep. Importantly, this hypertrophic change was diminished upon suppressing miR-21 in NRVMs. Collectively, our in vitro and in vivo data demonstrate that miR-21 is a critical contributor in the development of RV dysfunction and could represent a novel therapeutic target for PAH associated RV dysfunction.

Generation of Nonmosaic, Two-Pore Channel 2 Biallelic Knockout Pigs in One Generation by CRISPR-Cas9 Microinjection Before Oocyte Insemination

Studies of knockout (KO) mice with defects in the endolysosomal two-pore channels (TPCs) have shown TPCs to be involved in pathophysiological processes, including heart and muscle function, metabolism, immunity, cancer, and viral infection. With the objective of studying TPC2's pathophysiological roles for the first time in a large, more humanlike animal model, TPC2 KO pigs were produced using CRISPR-Cas9. A major problem using CRISPR-Cas9 to edit embryos is mosaicism; thus, we studied for the first time the effect of microinjection timing on mosaicism. Mosaicism was greatly reduced when in vitro produced embryos were microinjected before insemination, and surgical embryo transfer (ET) was performed using such embryos. All TPC2 KO fetuses and piglets born following ET (i.e., F0 generation) were nonmosaic biallelic KOs. The generation of nonmosaic animals greatly facilitates germ line transmission of the mutation, thereby aiding the rapid and efficient generation of KO animal lines for medical research and agriculture.

Mitochondrial Ca2+, redox environment and ROS emission in heart failure: Two sides of the same coin?

Heart failure (HF) is a progressive, debilitating condition characterized, in part, by altered ionic equilibria, increased ROS production and impaired cellular energy metabolism, contributing to variable profiles of systolic and diastolic dysfunction with significant functional limitations and risk of premature death. We summarize current knowledge concerning changes of intracellular Na+ and Ca2+ control mechanisms during the disease progression and their consequences on mitochondrial Ca2+ homeostasis and the shift in redox balance. Absent existing biological data, our computational modeling studies advance a new 'in silico' analysis to reconcile existing opposing views, based on different experimental HF models, regarding variations in mitochondrial Ca2+ concentration that participate in triggering and perpetuating oxidative stress in the failing heart and their impact on cardiac energetics. In agreement with our hypothesis and the literature, model simulations demonstrate the possibility that the heart's redox status together with cytoplasmic Na+ concentrations act as regulators of mitochondrial Ca2+ levels in HF and of the bioenergetics response that will ultimately drive ATP supply and oxidative stress. The resulting model predictions propose future directions to study the evolution of HF as well as other types of heart disease, and to develop novel testable mechanistic hypotheses that may lead to improved therapeutics.

Clinical Considerations for a Family with Dilated Cardiomyopathy, Sudden Cardiac Death, and a Novel TTN Frameshift Mutation

Dilated cardiomyopathy (DCM) is the leading indication for heart transplantation. TTN gene truncating mutations account for about 25% of familial DCM cases and for 18% of sporadic DCM cases. The clinical relevance of specific variants in TTN has been difficult to determine because of the sheer size of the protein for which TTN encodes, as well as existing extensive genetic variation. Clinicians should communicate novel clinically-relevant variants and genotype–phenotype associations, so that animal studies evaluating the molecular mechanisms are always conducted with a focus on clinical significance. In the present study, we report for the first time the novel truncating heterozygous variant NM_001256850.1:c.72777_72783del (p.Phe24259Leufs*51) in the TTN gene and its association with DCM in a family with sudden death. This variant occurs in the A-band region of the sarcomere, in a known mutational hotspot of the gene. Truncating titin variants that occur in this region are the most common cause of DCM and have been rarely reported in asymptomatic individuals, differently from other pathogenic TTN gene variants. Further studies are warranted to better understand this particular clinically-relevant variant.

Application of Carbon-13 Isotopomer Analysis to Assess Perinatal Myocardial Glucose Metabolism in Sheep

Ovine models of pregnancy have been used extensively to study maternal-fetal interactions and have provided considerable insight into nutrient transfer to the fetus. Ovine models have also been utilized to study congenital heart diseases. In this work, we demonstrate a comprehensive assessment of heart function and metabolism using a perinatal model of heart function with the addition of a [U-13C]glucose as tracer to study central energy metabolism. Using nuclear magnetic resonance spectroscopy, and metabolic modelling, we estimate myocardial citric acid cycle turnover (normalized for oxygen consumption), substrate selection, and anaplerotic fluxes. This methodology can be applied to studying acute and chronic effects of hormonal signaling in future studies.

A simple adeno-associated virus-based approach for the generation of cardiac genetic models in rats

Background: Heart failure is a major health problem and progress in this field relies on better understanding of the mechanisms and development of novel therapeutics using animal models. The rat may be preferable to the mouse as a cardiovascular disease model due to its closer physiology to humans and due to its large size that facilitates surgical and monitoring procedures. However, unlike the mouse, genetic manipulation of the rat genome is challenging. Methods: Here we developed a simple, refined, and robust cardiac-specific rat transgenic model based on an adeno-associated virus (AAV) 9 containing a cardiac troponin T promoter. This model uses a single intraperitoneal injection of AAV and does not require special expertise or equipment. Results: We characterize the AAV dose required to achieve a high cardiac specific level of expression of a transgene in the rat heart using a single intraperitoneal injection to neonates. We show that at this AAV dose GFP expression does not result in hypertrophy, a change in cardiac function or other evidence for toxicity. Conclusions: The model shown here allows easy and fast transgenic based disease modeling of cardiovascular disease in the rat heart, and can also potentially be expanded to deliver Cas9 and gRNAs or to deliver small hairpin (sh)RNAs to also achieve gene knockouts and knockdown in the rat heart.

Advances in Multidimensional Cardiac Biosensing Technologies: From Electrophysiology to Mechanical Motion and Contractile Force

Cardiovascular disease is currently a leading killer to human, while drug-induced cardiotoxicity remains the main cause of the withdrawal and attrition of drugs. Taking clinical correlation and throughput into account, cardiomyocyte is perfect as in vitro cardiac model for heart disease modeling, drug discovery, and cardiotoxicity assessment by accurately measuring the physiological multiparameters of cardiomyocytes. Remarkably, cardiomyocytes present both electrophysiological and biomechanical characteristics due to the unique excitation-contraction coupling, which plays a significant role in studying the cardiomyocytes. This review mainly focuses on the recent advances of biosensing technologies for the 2D and 3D cardiac models with three special properties: electrophysiology, mechanical motion, and contractile force. These high-performance multidimensional cardiac models are popular and effective to rebuild and mimic the heart in vitro. To help understand the high-quality and accurate physiologies, related detection techniques are highly demanded, from microtechnology to nanotechnology, from extracellular to intracellular recording, from multiple cells to single cell, and from planar to 3D models. Furthermore, the characteristics, advantages, limitations, and applications of these cardiac biosensing technologies, as well as the future development prospects should contribute to the systematization and expansion of knowledge.

Fast-relaxing cardiomyocytes exert a dominant role in the relaxation behavior of heterogeneous myocardium

Substantial variation in relaxation rate exists among cardiomyocytes within small volumes of myocardium; however, it is unknown how this variability affects the overall relaxation mechanics of heart muscle. In this study, we sought to modulate levels of cellular heterogeneity in a computational model, then validate those predictions using an engineered heart tissue platform. We formulated an in silico tissue model composed of half-sarcomeres with varied relaxation rates, incorporating single-cell cardiomyocyte experimental data. These model tissues randomly sampled relaxation parameters from two offset distributions of fast- and slow-relaxing populations of half-sarcomeres. Isometric muscle twitch simulations predicted a complex relationship between relaxation time and the proportion of fast-versus slow-relaxing cells in heterogeneous tissues. Specifically, a 50/50 mixture of fast and slow cells did not lead to relaxation time that was the mean of the relaxation times associated with the two pure cases. Rather, the mean relaxation time was achieved at a ratio of 70:30 slow:fast relaxing cells, suggesting a disproportionate impact of fast-relaxing cells on overall tissue relaxation. To examine whether this behavior persists in vitro, we constructed engineered heart tissues from two lines of fast- and slow-relaxing human iPSC-derived cardiomyocytes. Cell tracking via fluorescent nanocrystals confirmed the presence of both cell populations in the 50/50 mixed tissues at the time of mechanical characterization. Isometric muscle twitch relaxation times of these mixed-population engineered heart tissues showed agreement with the predictions from the model, namely that the measured relaxation rate of 50/50 mixed tissues more closely resembled that of tissues made with 100% fast-relaxing cells. Our observations suggest that cardiomyocyte diversity can play an important role in determining tissue-level relaxation.

Cardiac Optical Mapping in Situ in Swine Models: A View of the Current Situation

Optical mapping is recognized as a promising tool for the registration of electrical activity in the heart. Most cardiac optical mapping experiments are performed in ex vivo isolated heart models. However, the electrophysiological properties of the heart are highly influenced by the autonomic nervous system as well as humoral regulation; therefore, in vivo investigations of heart activity in large animals are definitely preferred. Furthermore, such investigations can be considered the last step before clinical application. Recently, two comprehensive studies have examined optical mapping approaches for pig hearts in situ (in vivo), likely advancing the methodological capacity to perform complex electrophysiological investigations of the heart. Both studies had the same aim, i.e., to develop high-spatiotemporal-resolution optical mapping suitable for registration of electrical activity of pig heart in situ, but the methods chosen were different. In this brief review, we analyse and compare the results of recent studies and discuss their translational potential for in situ cardiac optical mapping applications in large animals. We focus on the modes of blood circulation that are employed, the use of different voltage-sensitive dyes and their loading procedures, and ways of eliminating contraction artefacts. Finally, we evaluate the possible scenarios for optical mapping (OM) application in large animals in situ and infer which scenario is optimal.

ECG Restitution Analysis and Machine Learning to Detect Paroxysmal Atrial Fibrillation: Insight from the Equine Athlete as a Model for Human Athletes

Atrial fibrillation is the most frequent arrhythmia in both equine and human athletes. Currently, this condition is diagnosed via electrocardiogram (ECG) monitoring which lacks sensitivity in about half of cases when it presents in paroxysmal form. We investigated whether the arrhythmogenic substrate present between the episodes of paroxysmal atrial fibrillation (PAF) can be detected using restitution analysis of normal sinus-rhythm ECGs. In this work, ECG recordings were obtained during routine clinical work from control and horses with PAF. The extracted QT, TQ, and RR intervals were used for ECG restitution analysis. The restitution data were trained and tested using k-nearest neighbor (k-NN) algorithm with various values of neighbors k to derive a discrimination tool. A combination of QT, RR, and TQ intervals was used to analyze the relationship between these intervals and their effects on PAF. A simple majority vote on individual record (one beat) classifications was used to determine the final classification. The k-NN classifiers using two-interval measures were able to predict the diagnosis of PAF with area under the receiving operating characteristic curve close to 0.8 (RR, TQ with k ≥ 9) and 0.9 (RR, QT with k ≥ 21 or TQ, QT with k ≥ 25). By simultaneously using all three intervals for each beat and a majority vote, mean area under the curves of 0.9 were obtained for all tested k-values (3-41). We concluded that 3D ECG restitution analysis can potentially be used as a metric of an automated method for screening of PAF.

Cardiac sub-volume targeting demonstrates regional radiosensitivity in the mouse heart

BACKGROUND AND PURPOSE

Radiation-induced cardiac toxicity (RICT) remains one of the most critical dose limiting constraints in radiotherapy. Recent studies have shown higher doses to the base of the heart are associated with worse overall survival in lung cancer patients receiving radiotherapy. This work aimed to investigate the impact of sub-volume heart irradiation in a mouse model using small animal image-guided radiotherapy.

MATERIALS AND METHODS

C57BL/6 mice were irradiated with a single fraction of 16 Gy to the base, middle or apex of the heart using a small animal radiotherapy research platform. Cone beam CT and echocardiography were performed at baseline and at 10 week intervals until 50 weeks post-treatment. Structural and functional parameters were correlated with mean heart dose (MHD) and volume of heart receiving 5 Gy (V5).

RESULTS

All irradiated mice showed a time dependent increase in left ventricle wall thickness in diastole of ~0.2 mm detected at 10 weeks post-treatment, with the most significant and persistent changes occurring in the heart base-irradiated animals. Similarly, statistically different functional effects (p < 0.01) were observed in base-irradiated animals which showed the most significant decreases compared to controls. The observed functional changes did not correlate with MHD and V5 (R2 < 0.1), indicating that whole heart dosimetry parameters do not predict physiological changes resulting from cardiac sub-volume irradiation.

CONCLUSIONS

This is the first report demonstrating the structural and functional consequences of sub-volume targeting in the mouse heart and reverse translates clinical observations indicating the heart base as a critical radiosensitive region.

Multisite phosphorylation of the cardiac ryanodine receptor: a random or coordinated event?

Many proteins are phosphorylated at more than one phosphorylation site to achieve precise tuning of protein function and/or integrate a multitude of signals into the activity of one protein. Increasing the number of phosphorylation sites significantly broadens the complexity of molecular mechanisms involved in processing multiple phosphorylation sites by one or more distinct kinases. The cardiac ryanodine receptor (RYR2) is a well-established multiple phospho-target of kinases activated in response to β-adrenergic stimulation because this Ca²⁺ channel is a critical component of Ca²⁺ handling machinery which is responsible for β-adrenergic enhancement of cardiac contractility. Our review presents a selective overview of the extensive, often conflicting, literature which focuses on identifying reliable lines of evidence to establish if multiple RYR2 phosphorylation is achieved randomly or in a specific sequence, and whether phosphorylation at individual sites is functionally specific and additive or similar and can therefore be substituted.

Effects of fibrillin mutations on the behavior of heart muscle cells in Marfan syndrome

Marfan syndrome (MFS) is a systemic disorder of connective tissue caused by pathogenic variants in the fibrillin-1 (FBN1) gene. Myocardial dysfunction has been demonstrated in MFS patients and mouse models, but little is known about the intrinsic effect on the cardiomyocytes (CMs). In this study, both induced pluripotent stem cells derived from a MFS-patient and the line with the corrected FBN1 mutation were differentiated to CMs. Several functional analyses are performed on this model to study MFS related cardiomyopathy. Atomic force microscopy revealed that MFS CMs are stiffer compared to corrected CMs. The contraction amplitude of MFS CMs is decreased compared to corrected CMs. Under normal culture conditions, MFS CMs show a lower beat-to-beat variability compared to corrected CMs using multi electrode array. Isoproterenol-induced stress or cyclic strain demonstrates lack of support from the matrix in MFS CMs. This study reports the first cardiac cell culture model for MFS, revealing abnormalities in the behavior of MFS CMs that are related to matrix defects. Based on these results, we postulate that impaired support from the extracellular environment plays a key role in the improper functioning of CMs in MFS.

Effects of omecamtiv mecarbil on calcium-transients and contractility in a translational canine myocyte model

Omecamtiv mecarbil (OM) is a selective cardiac myosin activator (myotrope), currently in Phase 3 clinical investigation as a novel treatment for heart failure with reduced ejection fraction. OM increases cardiac contractility by enhancing interaction between myosin and actin in a calcium-independent fashion. This study aims to characterize the mechanism of action by evaluating its simultaneous effect on myocyte contractility and calcium-transients (CTs) in healthy canine ventricular myocytes. Left ventricular myocytes were isolated from canines and loaded with Fura-2 AM. With an IonOptix system, contractility parameters including amplitude and duration of sarcomere shortening, contraction and relaxation velocity, and resting sarcomere length were measured. CT parameters including amplitude at systole and diastole, velocity at systole and diastole, and duration at 50% from peak were simultaneously measured. OM was tested at 0.03, 0.1, 0.3, 1, and 3 µmol\L concentrations to simulate therapeutic human plasma exposure levels. OM and isoproterenol (ISO) demonstrated differential effects on CTs and myocyte contractility. OM increased contractility mainly by prolonging duration of contraction while ISO increased contractility mainly by augmenting the amplitude of contraction. ISO increased the amplitude and velocity of CT, shortened duration of CT concurrent with increasing myocyte contraction, while OM did not change the amplitude, velocity, and duration of CT up to 1 µmol\L. Decreases in relaxation velocity and increases in duration were present only at 3 µmol\L. In this translational myocyte model study, therapeutically relevant concentrations of OM increased contractility but did not alter intracellular CTs, a mechanism of action distinct from traditional calcitropes.

Role of the Carotid Body in an Ovine Model of Renovascular Hypertension

The carotid body is implicated as an important mediator and potential treatment target for hypertension. The mechanisms driving increased carotid body tonicity in hypertension are incompletely understood. Using a large preclinical animal model, which is crucial for translation, we hypothesized that carotid sinus nerve denervation would chronically decrease blood pressure in a renovascular ovine model of hypertension in which hypertonicity of the carotid body is associated with reduced common carotid artery blood flow. Adult ewes underwent either unilateral renal artery clipping or sham surgery. Two weeks later, flow probes were placed around the contralateral renal and common carotid arteries. Hypertension was accompanied by a significant reduction in common carotid blood flow but no change in renal blood flow. Carotid sinus nerve denervation significantly reduced blood pressure compared with sham. In both hypertensive and normotensive animals, carotid body stimulation using potassium cyanide caused dose-dependent increases in mean arterial pressure and common carotid conductance but a reduction in renal vascular conductance. These responses were not different between the animal groups. Taken together, our findings indicate that (1) the carotid body is activated in renovascular hypertension, and this is associated with reduced blood flow (decreased vascular conductance) in the common carotid artery and (2) the carotid body can differentially regulate blood flow to the common carotid and renal arteries. We suggest that in the ovine renovascular model, carotid body hypertonicity may be a product of reduced common carotid artery blood flow and plays an amplifying role with the kidney in the development of hypertension.

Discovery and characterization of ORM-11372, a novel inhibitor of the sodium-calcium exchanger with positive inotropic activity

Background and Purpose

The lack of selective sodium–calcium exchanger (NCX) inhibitors has hampered the exploration of physiological and pathophysiological roles of cardiac NCX 1.1. We aimed to discover more potent and selective drug like NCX 1.1 inhibitor.

Experimental Approach

A flavan series‐based pharmacophore model was constructed. Virtual screening helped us identify a novel scaffold for NCX inhibition. A distinctively different NCX 1.1 inhibitor, ORM‐11372, was discovered after lead optimization. Its potency against human and rat NCX 1.1 and selectivity against other ion channels was assessed. The cardiovascular effects of ORM‐11372 were studied in normal and infarcted rats and rabbits. Human cardiac safety was studied ex vivo using human ventricular trabeculae.

Key Results

ORM‐11372 inhibited human NCX 1.1 reverse and forward currents; IC₅₀ values were 5 and 6 nM respectively. ORM‐11372 inhibited human cardiac sodium 1.5 (I_Na) and hERG K_V11.1 currents (I_hERG) in a concentration‐dependent manner; IC₅₀ values were 23.2 and 10.0 μM. ORM‐11372 caused no changes in action potential duration; short‐term variability and triangulation were observed for concentrations of up to 10 μM. ORM‐11372 induced positive inotropic effects of 18 ± 6% and 35 ± 8% in anaesthetized rats with myocardial infarctions and in healthy rabbits respectively; no other haemodynamic effects were observed, except improved relaxation at the lowest dose.

Conclusion and Implications

ORM‐11372, a unique, novel, and potent inhibitor of human and rat NCX 1.1, is a positive inotropic compound. NCX inhibition can induce clinically relevant improvements in left ventricular contractions without affecting relaxation, heart rate, or BP, without pro‐arrhythmic risk.

Echocardiographic reference intervals with allometric scaling of 823 clinically healthy rhesus macaques (Macaca mulatta)

BACKGROUND

Echocardiography is commonly used for assessing cardiac structure and function in various species including non-human primates. A few previous studies reported normal echocardiographic reference intervals of clinically healthy rhesus macaques under sedation. However, these studies were under-powered, and the techniques were not standardized. In addition, body weight, age, and sex matched reference intervals should be established as echocardiographic measurements are commonly influenced by these variables. The purpose of this study was to establish reference intervals for complete echocardiographic parameters based on a large cohort of clinically healthy rhesus macaques with wide ranges of weight and age distributions using allometric scaling.

RESULTS

A total of 823 rhesus macaques (ages 6 months to 31 years old; body weights 1.4 to 22.6 kg) were enrolled. Of these rhesus macaques, 421 were males and 402 were females. They were assessed with a complete echocardiographic examination including structural and functional evaluation under sedation with ketamine hydrochloride. The reference intervals of the key echocardiographic parameters were indexed to weight, age, and sex by calculating the coefficients of the allometric eq. Y = aM^b. On correlation matrix, body weight, age, sex, and heart rate were significantly correlated with various echocardiographic parameters and some of the parameters were strongly correlated with body weight and age. Multiple regression analysis revealed that heart rate and body weight statistically significantly predicted several echocardiographic parameters. Valve regurgitation including tricuspid, aortic, pulmonic, and mitral regurgitations without other cardiac structural and functional abnormalities are common in clinically healthy rhesus macaques under ketamine sedation.

CONCLUSIONS

In this study, the reference intervals of echocardiographic parameters were established by performing complete echocardiographic examinations on a large number of clinical healthy rhesus macaques. In addition, allometric scaling was performed based on their weight, and further indexed to age and sex. These allometrically scaled reference intervals can be used to accurately evaluate echocardiographic data in rhesus macaques and diagnose structural and functional evidence of cardiac disease.

Cardiac hypertrophy in a dish: a human stem cell based model

Cardiac hypertrophy is an important and independent risk factor for the development of heart failure. To better understand the mechanisms and regulatory pathways involved in cardiac hypertrophy, there is a need for improved in vitro models. In this study, we investigated how hypertrophic stimulation affected human induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMs). The cells were stimulated with endothelin-1 (ET-1) for 8, 24, 48, 72, or 96 h. Parameters including cell size, ANP-, proBNP-, and lactate concentration were analyzed. Moreover, transcriptional profiling using RNA-sequencing was performed to identify differentially expressed genes following ET-1 stimulation. The results show that the CMs increase in size by approximately 13% when exposed to ET-1 in parallel to increases in ANP and proBNP protein and mRNA levels. Furthermore, the lactate concentration in the media was increased indicating that the CMs consume more glucose, a hallmark of cardiac hypertrophy. Using RNA-seq, a hypertrophic gene expression pattern was also observed in the stimulated CMs. Taken together, these results show that hiPSC-derived CMs stimulated with ET-1 display a hypertrophic response. The results from this study also provide new molecular insights about the underlying mechanisms of cardiac hypertrophy and may help accelerate the development of new drugs against this condition.

Scalable Biomimetic Coaxial Aligned Nanofiber Cardiac Patch: A Potential Model for "Clinical Trials in a Dish"

Recent advances in cardiac tissue engineering have shown that human induced-pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) cultured in a three-dimensional (3D) micro-environment exhibit superior physiological characteristics compared with their two-dimensional (2D) counterparts. These 3D cultured hiPSC-CMs have been used for drug testing as well as cardiac repair applications. However, the fabrication of a cardiac scaffold with optimal biomechanical properties and high biocompatibility remains a challenge. In our study, we fabricated an aligned polycaprolactone (PCL)-Gelatin coaxial nanofiber patch using electrospinning. The structural, chemical, and mechanical properties of the patch were assessed by scanning electron microscopy (SEM), immunocytochemistry (ICC), Fourier-transform infrared spectroscopy (FTIR)-spectroscopy, and tensile testing. hiPSC-CMs were cultured on the aligned coaxial patch for 2 weeks and their viability [lactate dehydrogenase (LDH assay)], morphology (SEM, ICC), and functionality [calcium cycling, multielectrode array (MEA)] were assessed. Furthermore, particle image velocimetry (PIV) and MEA were used to evaluate the cardiotoxicity and physiological functionality of the cells in response to cardiac drugs. Nanofibers patches were comprised of highly aligned core-shell fibers with an average diameter of 578 ± 184 nm. Acellular coaxial patches were significantly stiffer than gelatin alone with an ultimate tensile strength of 0.780 ± 0.098 MPa, but exhibited gelatin-like biocompatibility. Furthermore, hiPSC-CMs cultured on the surface of these aligned coaxial patches (3D cultures) were elongated and rod-shaped with well-organized sarcomeres, as observed by the expression of cardiac troponin-T and α-sarcomeric actinin. Additionally, hiPSC-CMs cultured on these coaxial patches formed a functional syncytium evidenced by the expression of connexin-43 (Cx-43) and synchronous calcium transients. Moreover, MEA analysis showed that the hiPSC-CMs cultured on aligned patches showed an improved response to cardiac drugs like Isoproterenol (ISO), Verapamil (VER), and E4031, compared to the corresponding 2D cultures. Overall, our results demonstrated that an aligned, coaxial 3D cardiac patch can be used for culturing of hiPSC-CMs. These biomimetic cardiac patches could further be used as a potential 3D in vitro model for "clinical trials in a dish" and for in vivo cardiac repair applications for treating myocardial infarction.

Long-Chain Acylcarnitines and Cardiac Excitation-Contraction Coupling: Links to Arrhythmias

A growing number of metabolomic studies have associated high circulating levels of the amphiphilic fatty acid metabolites, long-chain acylcarnitines (LCACs), with cardiovascular disease (CVD) risk. These studies show that plasma LCAC levels can be correlated with the stage and severity of CVD and with indices of cardiac hypertrophy and ventricular function. Complementing these recent clinical associations is an extensive body of basic research that stems mostly from the twentieth century. These works, performed in cardiomyocyte and multicellular preparations from animal and cell models, highlight stereotypical derangements in cardiac electrophysiology induced by exogenous LCAC treatment that promote arrhythmic muscle behavior. In many cases, this is coupled with acute inotropic modulation; however, whether LCACs increase or decrease contractility is inconclusive. Linked to the electromechanical alterations induced by LCAC exposure is an array of effects on cardiac excitation-contraction coupling mechanisms that overload the cardiomyocyte cytosol with Na⁺ and Ca²⁺ ions. The aim of this review is to revisit this age-old literature and collate it with recent findings to provide a pathophysiological context for the growing body of metabolomic association studies that link circulating LCACs with CVD.

CRISPR-Cas systems: Overview, innovations and applications in human disease research and gene therapy

Genome editing is the modification of genomic DNA at a specific target site in a wide variety of cell types and organisms, including insertion, deletion and replacement of DNA, resulting in inactivation of target genes, acquisition of novel genetic traits and correction of pathogenic gene mutations. Due to the advantages of simple design, low cost, high efficiency, good repeatability and short-cycle, CRISPR-Cas systems have become the most widely used genome editing technology in molecular biology laboratories all around the world. In this review, an overview of the CRISPR-Cas systems will be introduced, including the innovations, the applications in human disease research and gene therapy, as well as the challenges and opportunities that will be faced in the practical application of CRISPR-Cas systems.

Large Animal Models of Heart Failure: A Translational Bridge to Clinical Success

•

Preclinical large animal models play a critical and expanding role in translating basic science findings to the development and clinical approval of novel cardiovascular therapeutics.

•

This state-of-the-art review outlines existing methodologies and physiological phenotypes of several HF models developed in large animals. A comprehensive list of porcine, ovine, and canine models of disease are presented, and the translational importance of these studies to clinical success is highlighted through a brief overview of recent devices approved by the FDA alongside associated clinical trials and preclinical animal reports.

•

Increasing the use of large animal models of HF holds significant potential for identifying new mechanisms underlying this disease and providing valuable information regarding the safety and efficacy of new therapies, thus, improving physiological and economical translation of animal research to the successful treatment of human HF.

Preclinical large animal models of heart failure (HF) play a critical and expanding role in translating basic science findings to the development and clinical approval of novel therapeutics and devices. The complex combination of cardiovascular events and risk factors leading to HF has proved challenging for the development of new treatments for these patients. This state-of-the-art review presents historical and recent studies in porcine, ovine, and canine models of HF and outlines existing methodologies and physiological phenotypes. The translational importance of large animal studies to clinical success is also highlighted with an overview of recent devices approved by the Food and Drug Administration, together with preclinical HF animal studies used to aid both development and safety and/or efficacy testing. Increasing the use of large animal models of HF holds significant potential for identifying the novel mechanisms underlying the clinical condition and to improving physiological and economical translation of animal research to successfully treat human HF.

Development of a pro-arrhythmic ex vivo intact human and porcine model: cardiac electrophysiological changes associated with cellular uncoupling

We describe a human and large animal Langendorff experimental apparatus for live electrophysiological studies and measure the electrophysiological changes due to gap junction uncoupling in human and porcine hearts. The resultant ex vivo intact human and porcine model can bridge the translational gap between smaller simple laboratory models and clinical research. In particular, electrophysiological models would benefit from the greater myocardial mass of a large heart due to its effects on far-field signal, electrode contact issues and motion artefacts, consequently more closely mimicking the clinical setting. Porcine (n = 9) and human (n = 4) donor hearts were perfused on a custom-designed Langendorff apparatus. Epicardial electrograms were collected at 16 sites across the left atrium and left ventricle. A total of 1 mM of carbenoxolone was administered at 5 ml/min to induce cellular uncoupling, and then recordings were repeated at the same sites. Changes in electrogram characteristics were analysed. We demonstrate the viability of a controlled ex vivo model of intact porcine and human hearts for electrophysiology with pharmacological modulation. Carbenoxolone reduces cellular coupling and changes contact electrogram features. The time from stimulus artefact to (-dV/dt)_max increased between baseline and carbenoxolone (47.9 ± 4.1–67.2 ± 2.7 ms) indicating conduction slowing. The features with the largest percentage change between baseline and carbenoxolone were fractionation + 185.3%, endpoint amplitude − 106.9%, S-endpoint gradient + 54.9%, S point − 39.4%, RS ratio + 38.6% and (-dV/dt)_max − 20.9%. The physiological relevance of this methodological tool is that it provides a model to further investigate pharmacologically induced pro-arrhythmic substrates.

Catheter Ablation Lesion Visualization With Intracardiac Strain Imaging in Canines and Humans

Catheter ablation is a common treatment for arrhythmia, but can fail if lesion lines are noncontiguous. Identification of gaps and nontransmural lesions can reduce the likelihood of treatment failure and recurrent arrhythmia. Intracardiac myocardial elastography (IME) is a strain imaging technique that provides visualization of the lesion line. Estimation of lesion size and gap resolution were evaluated in an open-chest canine model ( n = 3 ), and clinical feasibility was investigated in patients undergoing ablation to treat typical cavotricuspid isthmus (CTI) atrial flutter ( n = 5 ). A lesion line consisting of three lesions and two gaps was generated on the canine left ventricle via epicardial ablation. One lesion was generated in one canine right ventricle. Average lesion and gap areas were measured with high agreement (33 ± 14 and 30 ± 15 mm², respectively) when compared against gross pathology (34 ± 19 and 26 ± 11 mm², respectively). Gaps as small as 11 mm² (3.6 mm on epicardial surface) were identifiable. Absolute error and relative error in estimated lesion area were 9.3 ± 8.4 mm² and 31% ± 34%; error in estimated gap area was 11 ± 9.0 mm² and 40% ± 29%. Flutter patients were imaged throughout the procedure. Strain was shown to be capable of differentiating between baseline and after ablation completion as confirmed by conduction block. In all patients, strain decreased in the CTI after ablation (mean paired difference of -17% ± 11%, ). IME could potentially become a useful ablation monitoring tool in health facilities.

From Human Pluripotent Stem Cells to 3D Cardiac Microtissues: Progress, Applications and Challenges

The knowledge acquired throughout the years concerning the in vivo regulation of cardiac development has promoted the establishment of directed differentiation protocols to obtain cardiomyocytes (CMs) and other cardiac cells from human pluripotent stem cells (hPSCs), which play a crucial role in the function and homeostasis of the heart. Among other developments in the field, the transition from homogeneous cultures of CMs to more complex multicellular cardiac microtissues (MTs) has increased the potential of these models for studying cardiac disorders in vitro and for clinically relevant applications such as drug screening and cardiotoxicity tests. This review addresses the state of the art of the generation of different cardiac cells from hPSCs and the impact of transitioning CM differentiation from 2D culture to a 3D environment. Additionally, current methods that may be employed to generate 3D cardiac MTs are reviewed and, finally, the adoption of these models for in vitro applications and their adaptation to medium- to high-throughput screening settings are also highlighted.

Studies to Elucidate the Mechanism of Cardio Protective and Hypotensive Activities of Anogeissus acuminata (Roxb. ex DC.) in Rodents

Anogeissus acuminata (Roxb. ex DC.) is a folkloric medicinal plant in Asia; including Pakistan; used as a traditional remedy for cardiovascular disorders. This study was planned to establish a pharmacological basis for the trivial uses of Anogeissus acuminata in certain medical conditions related to cardiovascular systems and to explore the underlying mechanisms. Mechanistic studies suggested that crude extract of Anogeissus acuminata (Aa.Cr) produced in vitro cardio-relaxant and vasorelaxant effects in isolated paired atria and aorta coupled with in vivo decrease in blood pressure by invasive method; using pressure and force transducers connected to Power Lab Data Acquisition System. Moreover; Aa.Cr showed positive effects in left ventricular hypertrophy in Sprague Dawley rats observed hemodynamically by a decrease in cardiac cell size and fibrosis; along with absence of inflammatory cells; coupled with reduced levels of angiotensin converting enzyme (ACE) and renin concentration along with increased concentrations of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP). In Acute Myocardial Infarction (AMI) model; creatine kinase (CK), creatine kinase-MB (CK-MB) and lactic acid dehydrogenase (LDH levels) were found to be decreased; along with decreased necrosis; edema and recruitment of inflammatory cells histologically. In vivo and ex vivo studies of Anogeissus acuminata provided evidence of vasorelaxant; hypotensive and cardioprotective properties facilitated through blockage of voltage-gated Ca⁺⁺ ion channel; validating its use in cardiovascular diseases.

Small animal models of heart failure

Heart disease is a major cause of death worldwide with increasing prevalence, which urges the development of new therapeutic strategies. Over the last few decades, numerous small animal models have been generated to mimic various pathomechanisms contributing to heart failure (HF). Despite some limitations, these animal models have greatly advanced our understanding of the pathogenesis of the different aetiologies of HF and paved the way to understanding the underlying mechanisms and development of successful treatments. These models utilize surgical techniques, genetic modifications, and pharmacological approaches. The present review discusses the strengths and limitations of commonly used small animal HF models, which continue to provide crucial insight and facilitate the development of new treatment strategies for patients with HF.

Exercise Training Protocols in Rabbits Applied in Cardiovascular Research

Simple Summary

Several animal models have been used to understand the physiological adaptations produced by exercise training in the healthy and diseased cardiovascular system. Among those, the protocols for acute and chronic exercise in rabbits present several advantages compared to other large animal models. In addition, the rabbit model has important physiological similarities with humans. On the other hand, the design of the training protocol is a key factor to induce the physiological adaptations. Here, we review the different training protocols used in rabbits and the different physiological adaptations produced in the cardiovascular system, in normal and pathological conditions.

Abstract

Rabbit exercise protocols allow for the evaluation of physiological and biomechanical changes and responses to episodes of acute or chronic exercise. The observed physiological changes are normal responses to stress, that is, adaptive responses to maintain or restore homeostasis after acute exercise. Indeed, the rabbit model is advantageous since (a) it has important physiological similarities in terms of the functioning of multiple organ systems, and can quickly induce alterations in pathophysiological conditions that resemble those of humans, and (b) it allows the implementation of a low-cost model in comparison with other large animals. When designing an exercise training protocol for rabbits, it is important to consider variables such as race, gender, age and, especially, training parameters such as volume, intensity, or rest, among others, to determine the outcome of the research. Therefore, the objective of this review is to identify and analyze exercise training protocols in rabbits in different experimental applications and the various physiological adaptations that are presented, with special focus in cardiovascular adaptations.

Micromechanobiology: Focusing on the Cardiac Cell-Substrate Interface

Engineered, in vitro cardiac cell and tissue systems provide test beds for the study of cardiac development, cellular disease processes, and drug responses in a dish. Much effort has focused on improving the structure and function of engineered cardiomyocytes and heart tissues. However, these parameters depend critically on signaling through the cellular microenvironment in terms of ligand composition, matrix stiffness, and substrate mechanical properties-that is, matrix micromechanobiology. To facilitate improvements to in vitro microenvironment design, we review how cardiomyocytes and their microenvironment change during development and disease in terms of integrin expression and extracellular matrix (ECM) composition. We also discuss strategies used to bind proteins to common mechanobiology platforms and describe important differences in binding strength to the substrate. Finally, we review example biomaterial approaches designed to support and probe cell-ECM interactions of cardiomyocytes in vitro, as well as open questions and challenges.

4D physiologically adaptable cardiac patch: A 4-month in vivo study for the treatment of myocardial infarction

There has been considerable progress in engineering cardiac scaffolds for the treatment of myocardial infarction (MI). However, it is still challenging to replicate the structural specificity and variability of cardiac tissues using traditional bioengineering approaches. In this study, a four-dimensional (4D) cardiac patch with physiological adaptability has been printed by beam-scanning stereolithography. By combining a unique 4D self-morphing capacity with expandable microstructure, the specific design has been shown to improve both the biomechanical properties of the patches themselves and the dynamic integration of the patch with the beating heart. Our results demonstrate improved vascularization and cardiomyocyte maturation in vitro under physiologically relevant mechanical stimulation, as well as increased cell engraftment and vascular supply in a murine chronic MI model. This work not only potentially provides an effective treatment method for MI but also contributes a cutting-edge methodology to enhance the structural design of complex tissues for organ regeneration.

Optical mapping of the pig heart in situ under artificial blood circulation

The emergence of optical imaging has revolutionized the investigation of cardiac electrical activity and associated disorders in various cardiac pathologies. The electrical signals of the heart and the propagation pathways are crucial for elucidating the mechanisms of various cardiac pathological conditions, including arrhythmia. The synthesis of near-infrared voltage-sensitive dyes and the voltage sensitivity of the FDA-approved dye Cardiogreen have increased the importance of optical mapping (OM) as a prospective tool in clinical practice. We aimed to develop a method for the high-spatiotemporal-resolution OM of the large animal hearts in situ using di-4-ANBDQBS and Cardiogreen under patho/physiological conditions. OM was adapted to monitor cardiac electrical behaviour in an open-chest pig heart model with physiological or artificial blood circulation. We detail the methods and display the OM data obtained using di-4-ANBDQBS and Cardiogreen. Activation time, action potential duration, repolarization time and conduction velocity maps were constructed. The technique was applied to track cardiac electrical activity during regional ischaemia and arrhythmia. Our study is the first to apply high-spatiotemporal-resolution OM in the pig heart in situ to record cardiac electrical activity qualitatively under artificial blood perfusion. The use of an FDA-approved voltage-sensitive dye and artificial blood perfusion in a swine model, which is generally accepted as a valuable pre-clinical model, demonstrates the promise of OM for clinical application.

Multiparametric Mechanistic Profiling of Inotropic Drugs in Adult Human Primary Cardiomyocytes

Effects of non-cardiac drugs on cardiac contractility can lead to serious adverse events. Furthermore, programs aimed at treating heart failure have had limited success and this therapeutic area remains a major unmet medical need. The challenges in assessing drug effect on cardiac contractility point to the fundamental translational value of the current preclinical models. Therefore, we sought to develop an adult human primary cardiomyocyte contractility model that has the potential to provide a predictive preclinical approach for simultaneously predicting drug-induced inotropic effect (sarcomere shortening) and generating multi-parameter data to profile different mechanisms of action based on cluster analysis of a set of 12 contractility parameters. We report that 17 positive and 9 negative inotropes covering diverse mechanisms of action exerted concentration-dependent increases and decreases in sarcomere shortening, respectively. Interestingly, the multiparametric readout allowed for the differentiation of inotropes operating via distinct mechanisms. Hierarchical clustering of contractility transient parameters, coupled with principal component analysis, enabled the classification of subsets of both positive as well as negative inotropes, in a mechanism-related mode. Thus, human cardiomyocyte contractility model could accurately facilitate informed mechanistic-based decision making, risk management and discovery of molecules with the most desirable pharmacological profile for the correction of heart failure.

A model of respiratory syncytial virus (RSV) infection of infants in newborn lambs

Many animal models have been established for respiratory syncytial virus (RSV) infection of infants with the purpose of studying the pathogenesis, immunological response, and pharmaceutical testing and the objective of finding novel therapies and preventive measures. This review centers on a neonatal lamb model of RSV infection that has similarities to RSV infection of infants. It includes a comprehensive description of anatomical and immunological similarities between ovine and human lungs along with comparison of pulmonary changes and immune responses with RSV infection. These features make the newborn lamb an effective model for investigating key aspects of RSV infection in infants. The importance of RSV lamb model application in preclinical therapeutic trials and current updates on new studies with the RSV-infected neonatal lamb are also highlighted.

Cholesterol induced autophagy via IRE1/JNK pathway promotes autophagic cell death in heart tissue

BACKGROUND

Cardiovascular diseases (CVDs), with highest mortality and morbidity rates, are the major cause of death in the world. Due to the limited information on heart tissue changes, mediated by hypercholesterolemia, we planned to investigate molecular mechanisms of endoplasmic reticulum (ER) stress and related cell death in high cholesterol fed rabbit model and possible beneficial effects of α-tocopherol.

METHODS

Molecular changes in rabbit heart tissue and cultured cardiomyocytes (H9c2 cells) were measured by western blotting, qRT-PCR, immunflouresence and flow cytometry experiments. Histological modifications were assessed by light and electron microscopes, while degradation of mitochondria was quantified through confocal microscope.

RESULTS

Feeding rabbits 2% cholesterol diet for 8 weeks and treatment of cultured cardiomyocytes with 10 μg/mL cholesterol for 3 h induced excessive autophagic activity via IRE1/JNK pathway. While no change in ER-associated degradation (ERAD) and apoptotic cell death were determined, electron and confocal microscopy analyses in cholesterol supplemented rabbits revealed significant parameters of autophagic cell death, including cytoplasmic autophagosomes, autolysosomes and organelle loss in juxtanuclear area as well as mitochondria engulfment by autophagosome. Either inhibition of ER stress or JNK in cultured cardiomyocytes or α-tocopherol supplementation in rabbits could counteract the effects of cholesterol.

CONCLUSION

Our findings underline the essential role of hypercholesterolemia in stimulating IRE1/JNK branch of ER stress response which then leads to autophagic cell death in heart tissue. Results also showed α-tocopherol as a promising regulator of autophagic cell death in cardiomyocytes.

Potential Therapeutic Value of Urotensin II Receptor Antagonist in Chronic Kidney Disease and Associated Comorbidities

Chronic kidney disease (CKD) remains a common disorder, leading to growing health and economic burden without curative treatment. In diabetic patients, CKD may result from a combination of metabolic and nonmetabolic-related factors, with mortality mainly driven by cardiovascular events. The marked overactivity of the urotensinergic system in diabetic patients implicates this vasoactive peptide as a possible contributor to the pathogenesis of renal as well as heart failure. Previous preclinical studies with urotensin II (UII) antagonists in chronic kidney disease were based on simple end points that did not reflect the complex etiology of the disease. Given this, our studies revisited the therapeutic value of UII antagonism in CKD and extensively characterized 1-({[6-{4-chloro-3-[3-(dimethylamino)propoxy]phenyl}-5-(2-methylphenyl)pyridin-2-yl]carbonyl}amino) cyclohexanecarboxylic acid hydrochloride (SAR101099), a potent, selective, and orally long-acting UII receptor competitive antagonist, inhibiting not only UII but also urotensin-related peptide activities. SR101099 treatment more than halved proteinurea and albumin/creatinine ratio in spontaneously hypertensive stroke-prone (SHR-SP) rats fed with salt/fat diet and Dahl-salt-sensitive rats, respectively, and it halved albuminuria in streptozotocin-induced diabetes rats. Importantly, these effects were accompanied by a decrease in mortality of 50% in SHR-SP and of 35% in the Dahl salt-sensitive rats. SAR101099 was also active on CKD-related cardiovascular pathologies and partly preserved contractile reserve in models of heart failure induced by myocardial infarction or ischemia/reperfusion in rats and pigs, respectively. SAR101099 exhibited a good safety/tolerability profile at all tested doses in clinical phase-I studies. Together, these data suggest that CKD patient selection considering comorbidities together with new stratification modalities should unveil the urotensin antagonists' therapeutic potential. SIGNIFICANCE STATEMENT: Chronic kidney disease (CKD) is a pathology with growing health and economic burden, without curative treatment. For years, the impact of urotensin II receptor (UT) antagonism to treat CKD may have been compromised by available tools or models to deeper characterize the urotensinergic system. New potent, selective, orally long-acting cross-species UT antagonist such as SAR101099 exerting reno- and cardioprotective effects could offer novel therapeutic opportunities. Its preclinical and clinical results suggest that UT antagonism remains an attractive target in CKD on top of current standard of care.

Conditional Up-Regulation of SERCA2a Exacerbates RyR2-Dependent Ventricular and Atrial Arrhythmias

Ryanodine receptor 2 (RyR2) and SERCA2a are two major players in myocyte calcium (Ca) cycling that are modulated physiologically, affected by disease and thus considered to be potential targets for cardiac disease therapy. However, how RyR2 and SERCA2a influence each others’ activities, as well as the primary and secondary consequences of their combined manipulations remain controversial. In this study, we examined the effect of acute upregulation of SERCA2a on arrhythmogenesis by conditionally overexpressing SERCA2a in a mouse model featuring hyperactive RyR2s due to ablation of calsequestrin 2 (CASQ2). CASQ2 knock-out (KO) mice were crossbred with doxycycline (DOX)-inducible SERCA2a transgenic mice to generate KO-TG mice. In-vivo ECG studies have shown that induction of SERCA2a (DOX+) overexpression markedly exacerbated both ventricular and atrial arrhythmias in vivo, compared with uninduced KO-TG mice (DOX-). Consistent with that, confocal microscopy in both atrial and ventricular myocytes demonstrated that conditional upregulation of SERCA2a enhanced the rate of occurrence of diastolic Ca release events. Additionally, deep RNA sequencing identified 17 downregulated genes and 5 upregulated genes in DOX+ mice, among which Ppp1r13l, Clcn1, and Agt have previously been linked to arrhythmias. Our results suggest that conditional upregulation of SERCA2a exacerbates hyperactive RyR2-mediated arrhythmias by further elevating diastolic Ca release.

Strategies for targeting the cardiac sarcomere: avenues for novel drug discovery

Introduction: Heart failure remains one of the largest clinical challenges in the United States. Researchers have continually searched for more effective heart failure treatments that target the cardiac sarcomere but have found few successes despite numerous expensive cardiovascular clinical trials. Among many reasons, the high failure rate of cardiovascular clinical trials may be partly due to incomplete characterization of a drug candidate's complex interaction with cardiac physiology.Areas covered: In this review, the authors address the issue of preclinical cardiovascular studies of sarcomere-targeting heart failure therapies. The authors consider inherent tradeoffs made between mechanistic transparency and physiological fidelity for several relevant preclinical techniques at the atomic, molecular, heart muscle fiber, whole heart, and whole-organism levels. Thus, the authors suggest a comprehensive, bottom-up approach to preclinical cardiovascular studies that fosters scientific rigor and hypothesis-driven drug discovery.Expert opinion: In the authors' opinion, the implementation of hypothesis-driven drug discovery practices, such as the bottom-up approach to preclinical cardiovascular studies, will be imperative for the successful development of novel heart failure treatments. However, additional changes to clinical definitions of heart failure and current drug discovery culture must accompany the bottom-up approach to maximize the effectiveness of hypothesis-driven drug discovery.

Myocardial Contractility: Historical and Contemporary Considerations

The term myocardial contractility is thought to have originated more than 125 years ago and has remained and enigma ever since. Although the term is frequently used in textbooks, editorials and contemporary manuscripts its definition remains illusive often being conflated with cardiac performance or inotropy. The absence of a universally accepted definition has led to confusion, disagreement and misconceptions among physiologists, cardiologists and safety pharmacologists regarding its definition particularly in light of new discoveries regarding the load dependent kinetics of cardiac contraction and their translation to cardiac force-velocity and ventricular pressure-volume measurements. Importantly, the Starling interpretation of force development is length-dependent while contractility is length independent. Most historical definitions employ an operational approach and define cardiac contractility in terms of the hearts mechanical properties independent of loading conditions. Literally defined the term contract infers that something has become smaller, shrunk or shortened. The addition of the suffix “ility” implies the quality of this process. The discovery and clinical investigation of small molecules that bind to sarcomeric proteins independently altering force or velocity requires that a modern definition of the term myocardial contractility be developed if the term is to persist. This review reconsiders the historical and contemporary interpretations of the terms cardiac performance and inotropy and recommends a modern definition of myocardial contractility as the preload, afterload and length-independent intrinsic kinetically controlled, chemo-mechanical processes responsible for the development of force and velocity.

The Degree of t-System Remodeling Predicts Negative Force-Frequency Relationship and Prolonged Relaxation Time in Failing Human Myocardium

The normally positive cardiac force-frequency relationship (FFR) becomes flat or negative in chronic heart failure (HF). Here we explored if remodeling of the cardiomyocyte transverse tubular system (t-system) is associated with alterations in FFR and contractile kinetics in failing human myocardium. Left-ventricular myocardial slices from 13 failing human hearts were mounted into a biomimetic culture setup. Maximum twitch force (F), 90% contraction duration (CD₉₀), time to peak force (TTP) and time to relaxation (TTR) were determined at 37°C and 0.2–2 Hz pacing frequency. F_1Hz/F₀._5Hz and F_2Hz/F₀._5Hz served as measures of FFR, intracellular cardiomyocyte t-tubule distance (ΔTT) as measure of t-system remodeling. Protein levels of SERCA2, NCX1, and PLB were quantified by immunoblotting. F_1Hz/F₀._5Hz (R² = 0.82) and F_2Hz/F₀._5Hz (R² = 0.5) correlated negatively with ΔTT, i.e., samples with severe t-system loss exhibited a negative FFR and reduced myocardial wall tension at high pacing rates. PLB levels also predicted F_1Hz/F₀._5Hz, but to a lesser degree (R² = 0.49), whereas NCX1 was not correlated (R² = 0.02). CD₉₀ correlated positively with ΔTT (R² = 0.39) and negatively with SERCA2/PLB (R² = 0.42), indicating that both the t-system and SERCA activity are important for contraction kinetics. Surprisingly, ΔTT was not associated with TTP (R² = 0) but rather with TTR (R² = 0.5). This became even more pronounced when interaction with NCX1 expression was added to the model (R² = 0.79), suggesting that t-system loss impairs myocardial relaxation especially when NCX1 expression is low. The degree of t-system remodeling predicts FFR inversion and contraction slowing in failing human myocardium. Moreover, together with NCX, the t-system may be important for myocardial relaxation.

Dose-dependent Effects of Esmolol-epinephrine Combination Therapy in Myocardial Ischemia and Reperfusion Injury

BACKGROUND

Animal studies on cardiac arrest found that a combination of epinephrine with esmolol attenuates post-resuscitation myocardial dysfunction. Based on these findings, we hypothesized that esmololepinephrine combination therapy would be superior to a reported cardioprotective esmolol therapy alone in a mouse model of myocardial ischemia and reperfusion (IR) injury.

METHODS

C57BL/6J mice were subjected to 60 min of myocardial ischemia and 120 min of reperfusion. Mice received either saline, esmolol (0.4 mg/kg/h), epinephrine (0.05 mg/kg/h), or esmolol combined with epinephrine (esmolol: 0.4 mg/kg/h or 0.8 mg/kg/h and epinephrine: 0.05 mg/kg/h) during reperfusion. After reperfusion, infarct sizes in the area-at-risk and serum cardiac troponin-I levels were determined. Hemodynamic effects of drugs infused were determined by measurements of heart rate (HR) and mean arterial blood pressure (MAP) via a carotid artery catheter.

RESULTS

Esmolol during reperfusion resulted in robust cardioprotection (esmolol vs. saline: 24.3±8% vs. 40.6±3% infarct size), which was abolished by epinephrine co-administration (38.1±15% infarct size). Increasing the esmolol dose, however, was able to restore esmolol-cardioprotection in the epinephrine-esmolol (18.6±8% infarct size) co-treatment group with improved hemodynamics compared to the esmolol group (epinephrine-esmolol vs. esmolol: MAP 80 vs. 75 mmHg, HR 452 vs. 402 beats/min).

CONCLUSION

These results confirm earlier studies on esmolol-cardioprotection from myocardial IR-injury and demonstrate that a dose optimized epinephrine-esmolol co-treatment maintains esmolol-cardioprotection with improved hemodynamics compared to esmolol treatment alone. These findings might have implications for current clinical practice in hemodynamically unstable patients suffering from myocardial ischemia.

Human pluripotent stem cell-derived cardiomyocytes for studying energy metabolism

Cardiomyocyte energy metabolism is altered in heart failure, and primary defects of metabolic pathways can cause heart failure. Studying cardiac energetics in rodent models has principal shortcomings, raising the question to which extent human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) can provide an alternative. As metabolic maturation of CM occurs mostly after birth during developmental hypertrophy, the immaturity of hiPSC-CM is an important limitation. Here we shortly review the physiological drivers of metabolic maturation and concentrate on methods to mature hiPSC-CM with the goal to benchmark the metabolic state of hiPSC-CM against in vivo data and to see how far known abnormalities in inherited metabolic disorders can be modeled in hiPSC-CM. The current data indicate that hiPSC-CM, despite their immature, approximately mid-fetal state of energy metabolism, faithfully recapitulate some basic metabolic disease mechanisms. Efforts to improve their metabolic maturity are underway and shall improve the validity of this model.

Brugada Syndrome: Oligogenic or Mendelian Disease?

Brugada syndrome (BrS) is diagnosed by a coved-type ST-segment elevation in the right precordial leads on the electrocardiogram (ECG), and it is associated with an increased risk of sudden cardiac death (SCD) compared to the general population. Although BrS is considered a genetic disease, its molecular mechanism remains elusive in about 70-85% of clinically-confirmed cases. Variants occurring in at least 26 different genes have been previously considered causative, although the causative effect of all but the SCN5A gene has been recently challenged, due to the lack of systematic, evidence-based evaluations, such as a variant's frequency among the general population, family segregation analyses, and functional studies. Also, variants within a particular gene can be associated with an array of different phenotypes, even within the same family, preventing a clear genotype-phenotype correlation. Moreover, an emerging concept is that a single mutation may not be enough to cause the BrS phenotype, due to the increasing number of common variants now thought to be clinically relevant. Thus, not only the complete list of genes causative of the BrS phenotype remains to be determined, but also the interplay between rare and common multiple variants. This is particularly true for some common polymorphisms whose roles have been recently re-evaluated by outstanding works, including considering for the first time ever a polygenic risk score derived from the heterozygous state for both common and rare variants. The more common a certain variant is, the less impact this variant might have on heart function. We are aware that further studies are warranted to validate a polygenic risk score, because there is no mutated gene that connects all, or even a majority, of BrS cases. For the same reason, it is currently impossible to create animal and cell line genetic models that represent all BrS cases, which would enable the expansion of studies of this syndrome. Thus, the best model at this point is the human patient population. Further studies should first aim to uncover genetic variants within individuals, as well as to collect family segregation data to identify potential genetic causes of BrS.

Advances in Preclinical Research Models of Radiation-Induced Cardiac Toxicity

Radiation therapy (RT) is an important component of cancer therapy, with >50% of cancer patients receiving RT. As the number of cancer survivors increases, the short- and long-term side effects of cancer therapy are of growing concern. Side effects of RT for thoracic tumors, notably cardiac and pulmonary toxicities, can cause morbidity and mortality in long-term cancer survivors. An understanding of the biological pathways and mechanisms involved in normal tissue toxicity from RT will improve future cancer treatments by reducing the risk of long-term side effects. Many of these mechanistic studies are performed in animal models of radiation exposure. In this area of research, the use of small animal image-guided RT with treatment planning systems that allow more accurate dose determination has the potential to revolutionize knowledge of clinically relevant tumor and normal tissue radiobiology. However, there are still a number of challenges to overcome to optimize such radiation delivery, including dose verification and calibration, determination of doses received by adjacent normal tissues that can affect outcomes, and motion management and identifying variation in doses due to animal heterogeneity. In addition, recent studies have begun to determine how animal strain and sex affect normal tissue radiation injuries. This review article discusses the known and potential benefits and caveats of newer technologies and methods used for small animal radiation delivery, as well as how the choice of animal models, including variables such as species, strain, and age, can alter the severity of cardiac radiation toxicities and impact their clinical relevance.

Different degradation rates of nanofiber vascular grafts in small and large animal models

Nanofiber vascular grafts have been shown to create neovessels made of autologous tissue, by in vivo scaffold biodegradation over time. However, many studies on graft materials and biodegradation have been conducted in vitro or in small animal models, instead of large animal models, which demonstrate different degradation profiles. In this study, we compared the degradation profiles of nanofiber vascular grafts in a rat model and a sheep model, while controlling for the type of graft material, the duration of implantation, fabrication method, type of circulation (arterial/venous), and type of surgery (interposition graft). We found that there was significantly less remaining scaffold (i.e., faster degradation) in nanofiber vascular grafts implanted in the sheep model compared with the rat model, in both the arterial and the venous circulations, at 6 months postimplantation. In addition, there was more extracellular matrix deposition, more elastin formation, more mature collagen, and no calcification in the sheep model compared with the rat model. In conclusion, studies comparing degradation of vascular grafts in large and small animal models remain limited. For clinical translation of nanofiber vascular grafts, it is important to understand these differences.