Characterizing functional stem cell-cardiomyocyte interactions

Cardiac Tissue Engineering: Inclusion of Non-cardiomyocytes for Enhanced Features

Engineered cardiac tissues (ECTs) are 3D physiological models of the heart that are created and studied for their potential role in developing therapies of cardiovascular diseases and testing cardio toxicity of drugs. Recreating the microenvironment of the native myocardium in vitro mainly involves the use of cardiomyocytes. However, ECTs with only cardiomyocytes (CM-only) often perform poorly and are less similar to the native myocardium compared to ECTs constructed from co-culture of cardiomyocytes and nonmyocytes. One important goal of co-culture tissues is to mimic the native heart's cellular composition, which can result in better tissue function and maturity. In this review, we investigate the role of nonmyocytes in ECTs and discuss the mechanisms behind the contributions of nonmyocytes in enhancement of ECT features.

Formation of an electrical coupling between differentiating cardiomyocytes

Human induced pluripotent stem cell–derived cardiomyocytes (hiPSC-CMs) serve as an indispensable platform for the study of human cardiovascular disease is human induced pluripotent stem cell–derived cardiomyocytes (hiPSC-CMs). While the possibility of reproducing rare pathologies, patient-specific selection of drugs, and other issues concerning single cardiomyocytes have been well studied, little attention has been paid to the properties of the whole syncytium of CMs, in which both the functionality of individual cells and the distribution of electrophysiological connections between them are essential. The aim of this work is to directly study the ability of hiPSC-CMs to form a functional syncytium that can stably conduct an excitation wave. For that purpose, syncytium forming hiPSC-CMs were harvested and seeded (transferred) on a new substrate on different days of differentiation. The excitation conduction in a sample was characterized by the stability of the wavefront using optical mapping data. We found that the cells transferred before the 20th day of differentiation were able to organize a functional syncytium capable of further development and stable excitation conduction at high stimulation frequencies, while the cells transferred after 20 days did not form a homogeneous syncytium, and multiple instabilities of the propagating wavefront were observed with the possibility of reentry formation.

Overcoming the Roadblocks to Cardiac Cell Therapy Using Tissue Engineering

Transplantations of various stem cells or their progeny have repeatedly improved cardiac performance in animal models of myocardial injury; however, the benefits observed in clinical trials have been generally less consistent. Some of the recognized challenges are poor engraftment of implanted cells and, in the case of human cardiomyocytes, functional immaturity and lack of electrical integration, leading to limited contribution to the heart's contractile activity and increased arrhythmogenic risks. Advances in tissue and genetic engineering techniques are expected to improve the survival and integration of transplanted cells, and to support structural, functional, and bioenergetic recovery of the recipient hearts. Specifically, application of a prefabricated cardiac tissue patch to prevent dilation and to improve pumping efficiency of the infarcted heart offers a promising strategy for making stem cell therapy a clinical reality.

Machine learning plus optical flow: a simple and sensitive method to detect cardioactive drugs

Current preclinical screening methods do not adequately detect cardiotoxicity. Using human induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs), more physiologically relevant preclinical or patient-specific screening to detect potential cardiotoxic effects of drug candidates may be possible. However, one of the persistent challenges for developing a high-throughput drug screening platform using iPS-CMs is the need to develop a simple and reliable method to measure key electrophysiological and contractile parameters. To address this need, we have developed a platform that combines machine learning paired with brightfield optical flow as a simple and robust tool that can automate the detection of cardiomyocyte drug effects. Using three cardioactive drugs of different mechanisms, including those with primarily electrophysiological effects, we demonstrate the general applicability of this screening method to detect subtle changes in cardiomyocyte contraction. Requiring only brightfield images of cardiomyocyte contractions, we detect changes in cardiomyocyte contraction comparable to – and even superior to – fluorescence readouts. This automated method serves as a widely applicable screening tool to characterize the effects of drugs on cardiomyocyte function.

Engineered heart slices for electrophysiological and contractile studies

A major consideration in the design of engineered cardiac tissues for the faithful representation of physiological behavior is the recapitulation of the complex topography and biochemistry of native tissue. In this study we present engineered heart slices (EHS), which consist of neonatal rat ventricular cells (NRVCs) seeded onto thin slices of decellularized cardiac tissue that retain important aspects of native extracellular matrix (ECM). To form EHS, rat or pig ventricular tissue was sectioned into 300 μm-thick, 5 to 16 mm-diameter disks, which were subsequently decellularized using detergents, spread on coverslips, and seeded with NRVCs. The organized fiber structure of the ECM remained after decellularization and promoted cell elongation and alignment, resulting in an anisotropic, functional tissue that could be electrically paced. Contraction decreased at higher pacing rates, and optical mapping revealed electrical conduction that was anisotropic with a ratio of approximately 2.0, rate-dependent shortening of the action potential and slowing of conduction, and slowing of conduction by the sodium channel blocker lidocaine. Reentrant arrhythmias could also be pace-induced and terminated. EHS constitute an attractive in vitro cardiac tissue in which cardiac cells are cultured on thin slices of decellularized cardiac ECM that provide important biochemical, structural, and mechanical cues absent in traditional cell cultures.

An autologous platelet-rich plasma hydrogel compound restores left ventricular structure, function and ameliorates adverse remodeling in a minimally invasive large animal myocardial restoration model: a translational approach: Vu and Pal "Myocardial Repair: PRP, Hydrogel and Supplements"

AIMS

Cell-based myocardial restoration has not penetrated broad clinical practice yet due to poor cell retention and survival rates. In this study, we attempt a translational, large-scale restorative but minimally invasive approach in the pig, aiming at both structurally stabilizing the left ventricular (LV) wall and enhancing function following ischemic injury.

METHODS AND RESULTS

A myocardial infarction (MI) was created by permanent ligation of left circumflex coronary artery through a small lateral thoracotomy. Thirty-six Yorkshire pigs were randomized to receive transthoracic intramyocardial injection into both infarct and border zone areas with different compounds: 1) Hyaluronic acid-based hydrogel; 2) autologous platelet-rich plasma (PRP); 3) ascorbic acid-enriched hydrogel (50 mg/L), combined with IV ibuprofen (25 mg/kg) and allopurinol (25 mg/kg) (cocktail group); 4) PRP and cocktail (full-compound); or 5) saline (control). The latter two groups received daily oral ibuprofen (25 mg/kg) for 7 days and allopurinol (25 mg/kg) for 30 days, postoperatively. Hemodynamic and echocardiographic studies were carried out at baseline, immediately after infarction and at end-point. Eight weeks after MI, the full-compound group had better LV fractional area change, ejection fraction and smaller LV dimensions than the control group. Also, dp/dtmax was significantly higher in the full-compound group when the heart rate increased from 100 bpm to 160bpm in stress tests. Blood vessel density was higher in the full-compound group, compared to the other treatment groups.

CONCLUSIONS

A combination of PRP, anti-oxidant and anti-inflammatory factors with intramyocardial injection of hydrogel has the potential to structurally and functionally improve the injured heart muscle while attenuating adverse cardiac remodeling after acute myocardial infarction.

Stem cells can form gap junctions with cardiac myocytes and exert pro-arrhythmic effects

Stem cell therapy has been suggested to be a promising option for regeneration of injured myocardium, for example following a myocardial infarction. For clinical use cell-based therapies have to be safe and applicable and are aimed to renovate the architecture of the heart. Yet for functional and coordinated activity synchronized with the host myocardium stem cells have to be capable of forming electrical connections with resident cardiomyocytes. In this paper we discuss whether stem cells are capable of establishing functional electrotonic connections with cardiomyocytes and whether these may generate a risk for arrhythmias. Application of stem cells in the clinical setting with outcomes concerning arrhythmogenic safety and future perspectives will also briefly be touched upon.

Spatial profiles of electrical mismatch determine vulnerability to conduction failure across a host-donor cell interface

BACKGROUND

Electrophysiological mismatch between host cardiomyocytes and donor cells can directly affect the electrical safety of cardiac cell therapies; however, the ability to study host-donor interactions at the microscopic scale in situ is severely limited. We systematically explored how action potential (AP) differences between cardiomyocytes and other excitable cells modulate vulnerability to conduction failure in vitro.

METHODS AND RESULTS

AP propagation was optically mapped at 75 μm resolution in micropatterned strands (n=152) in which host neonatal rat ventricular myocytes (AP duration=153.2±2.3 ms, conduction velocity=22.3±0.3 cm/s) seamlessly interfaced with genetically engineered excitable donor cells expressing inward rectifier potassium (Kir2.1) and cardiac sodium (Na(v)1.5) channels with either weak (conduction velocity=3.1±0.1 cm/s) or strong (conduction velocity=22.1±0.4 cm/s) electrical coupling. Selective prolongation of engineered donor cell AP duration (31.9-139.1 ms) by low-dose BaCl2 generated a wide range of host-donor repolarization time (RT) profiles with maximum gradients (∇RT(max)) of 5.5 to 257 ms/mm. During programmed stimulation of donor cells, the vulnerable time window for conduction block across the host-donor interface most strongly correlated with ∇RT(max). Compared with well-coupled donor cells, the interface composed of poorly coupled cells significantly shortened the RT profile width by 19.7% and increased ∇RT(max) and vulnerable time window by 22.2% and 19%, respectively. Flattening the RT profile by perfusion of 50 μmol/L BaCl2 eliminated coupling-induced differences in vulnerability to block.

CONCLUSIONS

Our results quantify how the degree of electrical mismatch across a cardiomyocyte-donor cell interface affects vulnerability to conduction block, with important implications for the design of safe cardiac cell and gene therapies.

Electronic "expression" of the inward rectifier in cardiocytes derived from human-induced pluripotent stem cells

BACKGROUND

Human-induced pluripotent stem cell (h-iPSC)-derived cardiac myocytes are a unique model in which human myocyte function and dysfunction are studied, especially those from patients with genetic disorders. They are also considered a major advance for drug safety testing. However, these cells have considerable unexplored potential limitations when applied to quantitative action potential (AP) analysis. One major factor is spontaneous activity and resulting variability and potentially anomalous behavior of AP parameters.

OBJECTIVE

To demonstrate the effect of using an in silico interface on electronically expressed I(K1), a major component lacking in h-iPSC-derived cardiac myocytes.

METHODS

An in silico interface was developed to express synthetic I(K1) in cells under whole-cell voltage clamp.

RESULTS

Electronic I(K1) expression established a physiological resting potential, eliminated spontaneous activity, reduced spontaneous early and delayed afterdepolarizations, and decreased AP variability. The initiated APs had the classic rapid upstroke and spike and dome morphology consistent with data obtained with freshly isolated human myocytes as well as the readily recognizable repolarization attributes of ventricular and atrial cells. The application of 1 µM of BayK-8644 resulted in anomalous AP shortening in h-iPSC-derived cardiac myocytes. When I(K1) was electronically expressed, BayK-8644 prolonged the AP, which is consistent with the existing results on native cardiac myocytes.

CONCLUSIONS

The electronic expression of I(K1) is a simple and robust method to significantly improve the physiological behavior of the AP and electrical profile of h-iPSC-derived cardiac myocytes. Increased stability enables the use of this preparation for a controlled quantitative analysis of AP parameters, for example, drug responsiveness, genetic disorders, and dynamic behavior restitution profiles.

Electrophysiological and contractile function of cardiomyocytes derived from human embryonic stem cells

Human embryonic stem cells have emerged as the prototypical source from which cardiomyocytes can be derived for use in drug discovery and cell therapy. However, such applications require that these cardiomyocytes (hESC-CMs) faithfully recapitulate the physiology of adult cells, especially in relation to their electrophysiological and contractile function. We review what is known about the electrophysiology of hESC-CMs in terms of beating rate, action potential characteristics, ionic currents, and cellular coupling as well as their contractility in terms of calcium cycling and contraction. We also discuss the heterogeneity in cellular phenotypes that arises from variability in cardiac differentiation, maturation, and culture conditions, and summarize present strategies that have been implemented to reduce this heterogeneity. Finally, we present original electrophysiological data from optical maps of hESC-CM clusters.

Size and ionic currents of unexcitable cells coupled to cardiomyocytes distinctly modulate cardiac action potential shape and pacemaking activity in micropatterned cell pairs

BACKGROUND

Cardiac cell therapies can yield electric coupling of unexcitable donor cells to host cardiomyocytes with functional consequences that remain unexplored.

METHODS AND RESULTS

We micropatterned cell pairs consisting of a neonatal rat ventricular myocyte (NRVM) coupled to an engineered human embryonic kidney 293 (HEK293) cell expressing either connexin-43 (Cx43 HEK) or inward rectifier potassium channel 2.1 (Kir2.1) and Cx43 (Kir2.1+Cx43 HEK). The NRVM-HEK contact length was fixed yielding a coupling strength of 68.9±9.7 nS, whereas HEK size was systematically varied. With increase in Cx43 HEK size, NRVM maximal diastolic potential was reduced from -71.7±0.6 mV in single NRVMs to -35.1±1.3 mV in pairs with an HEK:NRVM cell surface area ratio of 1.7±0.1, whereas the action potential upstroke ([dV(m)/dt](max)) and duration decreased to 1.6±0.7% and increased to 177±32% in single NRVM values, respectively (n=21 cell pairs). Pacemaking occurred in all NRVM-Cx43 HEK pairs with cell surface area ratios of 1.1 to 1.9. In contrast, NRVMs, coupled with Kir2.1+Cx43 HEKs of increasing size, had similar maximal diastolic potentials, exhibited no spontaneous activity, and showed a gradual decrease in action potential duration (n=23). Furthermore, coupling single NRVMs to a dynamic clamp model of HEK cell ionic current reproduced the cardiac maximal diastolic potentials and pacemaking rates recorded in cell pairs, whereas reproducing changes in (dV(m)/dt)(max) and action potential duration required coupling to an HEK model that also included cell membrane capacitance.

CONCLUSIONS

Size and ionic currents of unexcitable cells electrically coupled to cardiomyocytes distinctly affect cardiac action potential shape and initiation with important implications for the safety of cardiac cell and gene therapies.

Laser-patterned stem-cell bridges in a cardiac muscle model for on-chip electrical conductivity analyses

Following myocardial infarction there is an irreversible loss of cardiomyocytes that results in the alteration of electrical propagation in the heart. Restoration of functional electrical properties of the damaged heart muscle is essential to recover from the infarction. While there are a few reports that demonstrate that fibroblasts can form junctions that transmit electrical signals, a potential alternative using the injection of stem cells has emerged as a promising cellular therapy; however, stem-cell electrical conductivity within the cardiac muscle fiber is unknown. In this study, an in vitro cardiac muscle model was established on an MEA-based biochip with multiple cardiomyocytes that mimic cardiac tissue structure. Using a laser beam, stem cells were inserted adjacent to each muscle fiber (cell bridge model) and allowed to form cell-cell contact as determined by the formation of gap junctions. The electrical conductivity of stem cells was assessed and compared with the electrical conductivities of cardiomyocytes and fibroblasts. Results showed that stem cell-myocyte contacts exhibited higher and more stable conduction velocities than myocyte-fibroblast contacts, which indicated that stem cells have higher electrical compatibility with native cardiac muscle fibers than cardiac fibroblasts.