Influence of Remodeled ECM and Co-culture with iPSC-Derived Cardiac Fibroblasts on the Mechanical Function of Micropatterned iPSC-Derived Cardiomyocytes

INTRODUCTION

In native heart tissue, functions of cardiac fibroblasts (CFs) include synthesis, remodeling, and degradation of the extracellular matrix (ECM) as well as secreting factors that regulate cardiomyocyte (CM) function. The influence of direct co-culture and CF-derived ECM on CM mechanical function are not fully understood.

METHODS

Here we use an engineered culture platform that provides control over ECM geometry and substrate stiffness to evaluate the influence of iPSC-CFs, and the ECM they produce, on the mechanical function of iPSC-CMs. Mechanical analysis was performed using digital image correlation to quantify maximum contractile strain, spontaneous contraction rate, and full-field organization of the contractions.

RESULTS

When cultured alone, iPSC-CFs produce and remodel the ECM into fibers following the underlying 15° chevron patterned ECM. The substrates were decellularized and confirmed to have highly aligned fibers that covered a large fraction of the pattern area before reseeding with iPSC-CMs, alone or in co-culture with iPSC-CFs. When seeded on decellularized ECM, larger maximum contractile strains were observed in the co-culture condition compared to the CM Only condition. No significant difference was found in contractile strain between the Matrigel and decellularized ECM conditions; however, the spontaneous contraction rate was lower in the decellularized ECM condition. A methodology for quantifying alignment of cell contraction across the entire field of view was developed based on trajectories approximating the cell displacements during contraction. Trajectory alignment was unaltered by changes in culture or ECM conditions.

CONCLUSIONS

These combined observations highlight the important role CFs play in vivo and the need for models that enable a quantitative approach to examine interactions between the CFs and CMs, as well as the interactions of these cells with the ECM.

Identifying Features of Cardiac Disease Phenotypes Based on Mechanical Function in a Catecholaminergic Polymorphic Ventricular Tachycardia Model

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by an arrhythmogenic mechanism involving disruption of calcium handling. This genetic disease can lead to sudden death in children and young adults during physical or emotional stress. Prior CPVT studies have focused on calcium handling, but mechanical functionality has rarely been investigated in vitro. In this research we combine stem cell-derived cardiomyocytes from a CPVT patient (RyR2-H2464D mutation) and a healthy familial control with an engineered culture platform to evaluate mechanical function of cardiomyocytes. Substrates with Young's modulus ranging from 10 to 50 kPa were used in conjunction with microcontact printing of ECM proteins into defined patterns for subsequent attachment. Digital Image Correlation (DIC) was used to evaluate collections of contracting cells. The amplitude of contractile strain was utilized as a quantitative indicator of functionality and disease severity. We found statistically significant differences: the maximum contractile strain was consistently higher in patient samples compared to control samples on all substrate stiffnesses. Additionally, the patient cell line had a statistically significantly slower intrinsic contraction rate than the control, which agrees with prior literature. Differences in mechanical strain have not been previously reported, and hypercontractility is not a known characteristic of CPVT. However, functional changes can occur as the disease progresses, thus this observation may not represent behavior observed in adolescent and adult patients. These results add to the limited studies of mechanical function of CPVT CMs reported in literature and identify functional differences that should be further explored.

Micropattern platform promotes extracellular matrix remodeling by human PSC-derived cardiac fibroblasts and enhances contractility of co-cultured cardiomyocytes

In native heart tissue, cardiac fibroblasts provide the structural framework of extracellular matrix (ECM) while also influencing the electrical and mechanical properties of cardiomyocytes. Recent advances in the field of stem cell differentiation have led to the availability of human pluripotent stem cell-derived cardiac fibroblasts (iPSC-CFs) in addition to cardiomyocytes (iPSC-CMs). Here we use a novel 2D in vitro micropatterned platform that provides control over ECM geometry and substrate stiffness. When cultured alone on soft micropatterned substrates, iPSC-CFs are confined to the micropatterned features and remodel the ECM into anisotropic fibers. Similar remodeling and ECM production occurs when cultured with iPSC-CMs in a co-culture model. In addition to modifications in the ECM, our results show that iPSC-CFs influence iPSC-CM function with accelerated Ca2+ transient rise-up time and greater contractile strains in the co-culture conditions compared to when iPSC-CMs are cultured alone. These combined observations highlight the important role cardiac fibroblasts play in vivo and the need for co-culture models like the one presented here to provide more representative in vitro cardiac constructs.

Aligned human cardiac syncytium for in vitro analysis of electrical, structural, and mechanical readouts

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have emerged as an exciting new tool for cardiac research and can serve as a preclinical platform for drug development and disease modeling studies. However, these aspirations are limited by current culture methods in which hPSC-CMs resemble fetal human cardiomyocytes in terms of structure and function. Herein we provide a novel in vitro platform that includes patterned extracellular matrix with physiological substrate stiffness and is amenable to both mechanical and electrical analysis. Micropatterned lanes promote the cellular and myofibril alignment of hPSC-CMs while the addition of micropatterned bridges enable formation of a functional cardiac syncytium that beats synchronously over a large two-dimensional area. We investigated the electrophysiological properties of the patterned cardiac constructs and showed they have anisotropic electrical impulse propagation, as occurs in the native myocardium, with speeds 2x faster in the primary direction of the pattern as compared to the transverse direction. Lastly, we interrogated the mechanical function of the pattern constructs and demonstrated the utility of this platform in recording the strength of cardiomyocyte contractions. This biomimetic platform with electrical and mechanical readout capabilities will enable the study of cardiac disease and the influence of pharmaceuticals and toxins on cardiomyocyte function. The platform also holds potential for high throughput evaluation of drug safety and efficacy, thus furthering our understanding of cardiovascular disease and increasing the translational use of hPSC-CMs.

Two-Dimensional Culture Systems to Enable Mechanics-Based Assays for Stem Cell-Derived Cardiomyocytes

Well-controlled 2D cell culture systems advance basic investigations in cell biology and provide innovative platforms for drug development, toxicity testing, and diagnostic assays. These cell culture systems have become more advanced in order to provide and to quantify the appropriate biomechanical and biochemical cues that mimic the milieu of conditions present in vivo. Here we present an innovative 2D cell culture system to investigate human stem cell-derived cardiomyocytes, the muscle cells of the heart responsible for pumping blood throughout the body. We designed our 2D cell culture platform to control intracellular features to produce adult-like cardiomyocyte organization with connectivity and anisotropic conduction comparable to the native heart, and combined it with optical microscopy to quantify cell-cell and cell-substrate mechanical interactions. We show the measurement of forces and displacements that occur within individual cells, between neighboring cells, and between cells and their surrounding matrix. This system has broad potential to expand our understanding of tissue physiology, with particular advantages for the study of the mechanically active heart. Furthermore, this technique should prove valuable in screening potential drugs for efficacy and testing for toxicity.

Calcium and cardiac arrest

Calcium has a central role in regulating both cardiac automaticity and myocardial contractility. The ability of calcium to increase tension in normal myocardium is well known. These properties of calcium have led to its use in the setting of cardiopulmonary resuscitation, especially in the presence of electromechanical dissociation or asystole, but evidence of benefit from this therapy is lacking. During cardiac arrest, disturbances in the control of calcium movement in myocardium would likely result in elevations in cytosolic calcium and the disturbances in myocardial function that occur with calcium overload. Administration of calcium and the subsequent elevation in serum calcium concentrations under these conditions may have further detrimental effects on the heart and vascular smooth muscle. The routine use of calcium in cardiac arrest is not recommended.