Engineered tissue geometry and Plakophilin-2 regulate electrophysiology of human iPSC-derived cardiomyocytes

Engineered heart tissues have been created to study cardiac biology and disease in a setting that more closely mimics in vivo heart muscle than 2D monolayer culture. Previously published studies suggest that geometrically anisotropic micro-environments are crucial for inducing "in vivo like" physiology from immature cardiomyocytes. We hypothesized that the degree of cardiomyocyte alignment and prestress within engineered tissues is regulated by tissue geometry and, subsequently, drives electrophysiological development. Thus, we studied the effects of tissue geometry on electrophysiology of micro-heart muscle arrays (μHM) engineered from human induced pluripotent stem cells (iPSCs). Elongated tissue geometries elicited cardiomyocyte shape and electrophysiology changes led to adaptations that yielded increased calcium intake during each contraction cycle. Strikingly, pharmacologic studies revealed that a threshold of prestress and/or cellular alignment is required for sodium channel function, whereas L-type calcium and rapidly rectifying potassium channels were largely insensitive to these changes. Concurrently, tissue elongation upregulated sodium channel (NaV1.5) and gap junction (Connexin 43, Cx43) protein expression. Based on these observations, we leveraged elongated μHM to study the impact of loss-of-function mutation in Plakophilin 2 (PKP2), a desmosome protein implicated in arrhythmogenic disease. Within μHM, PKP2 knockout cardiomyocytes had cellular morphology similar to what was observed in isogenic controls. However, PKP2-/- tissues exhibited lower conduction velocity and no functional sodium current. PKP2 knockout μHM exhibited geometrically linked upregulation of sodium channel but not Cx43, suggesting that post-translational mechanisms, including a lack of ion channel-gap junction communication, may underlie the lower conduction velocity observed in tissues harboring this genetic defect. Altogether, these observations demonstrate that simple, scalable micro-tissue systems can provide the physiologic stresses necessary to induce electrical remodeling of iPS-CM to enable studies on the electrophysiologic consequences of disease-associated genomic variants.

Robust, Automated Analysis of Electrophysiology in Induced Pluripotent Stem Cell-Derived Micro-Heart Muscle for Drug Toxicity

Drugs are often removed from clinical trials or market progression owing to their unforeseen effects on cardiac action potential and calcium handling. Induced pluripotent stem cell-derived cardiomyocytes and tissues fabricated from these cells are promising as screening tools for early identification of these potential cardiac liabilities. In this study, we describe an automated, open-source MATLAB-based analysis software for calculating cardiac action potentials and calcium transients from fluorescent reporters. We first identified the most robust manner in which to automatically identify the initiation point for action potentials and calcium transients in a user-independent manner, and used this approach to quantify the duration and morphology of these signals. We then demonstrate the software by assessing changes to action potentials and calcium transients in our micro-heart muscles after exposure to hydroxychloroquine, an antimalarial drug with known cardiac liability. Consistent with clinical observations, our system predicted mild action potential prolongation. However, we also observed marked calcium transient suppression, highlighting the advantage of testing multiple physiologic readouts in cardiomyocytes rather than relying on heterologous overexpression of single channels such as the human ether-a-go-go-related gene channel. This open-source software can serve as a useful, high-throughput tool for analyzing cardiomyocyte physiology from fluorescence imaging.

Interplay of Genotype and Substrate Stiffness in Driving the Hypertrophic Cardiomyopathy Phenotype in iPSC-Micro-Heart Muscle Arrays

INTRODUCTION

In clinical and animal studies, Hypertrophic Cardiomyopathy (HCM) shares many similarities with non-inherited cardiac hypertrophy induced by pressure overload (hypertension). This suggests a potential role for mechanical stress in priming tissues with mutation-induced changes in the sarcomere to develop phenotypes associated with HCM, including hypercontractility and aberrant calcium handling. Here, we tested the hypothesis that heterozygous loss of function of Myosin Binding Protein C (MYBCP3 +/- , mutations in which account for almost 50% of inherited HCM) combines with environmental stiffness to drive HCM phenotypes.

METHODS

We differentiated isogenic control (WTC) and MYBPC3 +/- iPSC into cardiomyocytes using small molecule manipulation of Wnt signaling, and then purified them using lactate media. The purified cardiomyocytes were seeded into "dog bone" shaped stencil molds to form micro-heart muscle arrays (μHM). To mimic changes in myocardial stiffness stemming from pressure overload, we varied the rigidity of the substrates μHM contract against. Stiffness levels ranged from those corresponding to fetal (5 kPa), healthy (15 kPa), pre-fibrotic (30 kPa) to fibrotic (65 kPa) myocardium. Substrates were embedded with a thin layer of fluorescent beads to track contractile force, and parent iPSC were engineered to express the genetic calcium indicator, GCaMP6f. High speed video microscopy and image analysis were used to quantify calcium handling and contractility of μHM.

RESULTS

Substrate rigidity triggered physiological adaptation for both genotypes. However, MYBPC3 +/- μHM showed a lower tolerance to substrate stiffness with the peak traction on 15 kPa, while WTC μHM had peak traction on 30 kPa. MYBPC3 +/- μHM exhibited hypercontractility, which was exaggerated by substrate rigidity. MYBPC3 +/- μHM hypercontractility was associated with longer rise times for calcium uptake and force development, along with higher overall Ca2+ intake.

CONCLUSION

We found MYBPC3 +/- mutations cause iPSC-μHM to exhibit hypercontractility, and also a lower tolerance for mechanical stiffness. Understanding how genetics work in combination with mechanical stiffness to trigger and/or exacerbate pathophysiology may lead to more effective therapies for HCM.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at (10.1007/s12195-021-00684-x).

Elastomer-Grafted iPSC-Derived Micro Heart Muscles to Investigate Effects of Mechanical Loading on Physiology

Mechanical loading plays a critical role in cardiac pathophysiology. Engineered heart tissues derived from human induced pluripotent stem cells (iPSCs) allow rigorous investigations of the molecular and pathophysiological consequences of mechanical cues. However, many engineered heart muscle models have complex fabrication processes and require large cell numbers, making it difficult to use them together with iPSC-derived cardiomyocytes to study the influence of mechanical loading on pharmacology and genotype-phenotype relationships. To address this challenge, simple and scalable iPSC-derived micro-heart-muscle arrays (μHM) have been developed. "Dog-bone-shaped" molds define the boundary conditions for tissue formation. Here, we extend the μHM model by forming these tissues on elastomeric substrates with stiffnesses spanning from 5 to 30 kPa. Tissue assembly was achieved by covalently grafting fibronectin to the substrate. Compared to μHM formed on plastic, elastomer-grafted μHM exhibited a similar gross morphology, sarcomere assembly, and tissue alignment. When these tissues were formed on substrates with different elasticity, we observed marked shifts in contractility. Increased contractility was correlated with increases in calcium flux and a slight increase in cell size. This afterload-enhanced μHM system enables mechanical control of μHM and real-time tissue traction force microscopy for cardiac physiology measurements, providing a dynamic tool for studying pathophysiology and pharmacology.