Abstract
Heart disease is a global health and economic burden. Specifically, atrial fibrillation, a common heart disease, affected 2.7 to 6.1 million people in the United States in 2010 and is expected to rise to upwards of 12.1 million in 2030. The sinoatrial node (SAN)–which is located in the right atrium and functions as the native pacemaker of the heart–can be compromised in patients that have atrial fibrillation and many types of rhythm disorders. SAN dysfunction is estimated to affect 403 to 666 per million people; of these, approximately 63 per million per year require the implantation of a pacemaker. Pacemaker implantation is a common method of treatment for rhythm disorders caused by SAN dysfunction, but has drawbacks such as high cost, device lifespan, and surgical risks. Therefore, a biopacemaker composed of human cardiomyocytes (CM) is a potential solution that could help patients have better outcomes and longer-term benefits.
Previous research has been aimed at successful differentiation of human induced pluripotent stem cells (hiPSCs) into CMs with future therapeutic efforts in mind. Unfortunately, when differentiating hiPSC into CMs on standard culture wares, current protocols achieve a heterogeneous population of cardiac cells, which can include endothelial cells, smooth muscle cells, and fibroblasts in addition to CMs. Moreover, the CM population is also heterogeneous in subtypes, in that we get cells of both contractile (atrial and ventricular) and pacemaking functional cell types—with pacemaking cells in the minority of this heterogeneous population. With these obstacles, further research was aimed at reproducing a larger and homogenous population of pacemaking CMs for treatment of SAN dysfunction.
Although many avenues of research look at the addition of various small molecules and their effect on hiPSC derived CM (hiPSC-CM) subtype differentiation, there is a growing interest in mimicking the extracellular matrix (ECM) as a way of insulating the pacemaking CMs from imposed cyclic mechanical strain by cardiac contractions. The heart has a natural strain of approximately 12% in the right atrium during right atrium systole. The SAN and left ventricle (LV) have been shown to have different ultrastructure and ECM protein composition. While the LV appears more mesh-like and flexible, the SAN ultrastructure appears rope-like and is stiffer, which may be protecting CMs and guiding them during development in a unique way that differs from the LV or other contractile regions of the heart.
The goal of this project was to test the protective effects of the porcine SAN ECM of the on implanted hiPSC-derived CMs in response to mechanical strain, with the contractile porcine LV ECM as a control. First, a decellularization protocol needed to be optimized for porcine tissue that would provide a suitable environment for recellularization. Following, we optimized the adherence and mechanical strain ramp-up to ensure cells and tissues remained attached to the stretch chambers during imposed mechanical strain. Finally, we were able to put this together to test the effects of mechanical strain on hiPSC-CM protein expression. Our final step was accomplished by recellularizing hiPSC-CM in both LV and SAN decellularized porcine ECMs and subjecting them to cyclic mechanical strain in vitro that mimics physiological strain conditions. The ability to promote and sustain the development of hiPSCs into CMs of the pacemaking subtype can lead efforts toward bioengineered pacemakers and cell therapies in the future.