Studying the Role of Mechanical Contraction in Cardiac Muscle Development Using Genetically Engineered Non-Contractile Human Stem Cell-Derived Cardiomyocytes

Abstract

Mechanical contraction is an inherent function of cardiomyocytes but much of the effect of contraction on multiple cellular functions and therapeutic potential remains unknown. Due to the complete lethality of mechanical quiescence in cardiomyocytes, many researchers have used antagonists to mimic and study the effect of mechanical inactivity in vitro. However, this poses a great barrier as the antagonists are toxic and transient and cannot be used for long-term studies. With the emergence of human induced pluripotent stem cells (hiPSCs), we are now able to circumvent this shortcoming by creating in vitro models to induce stable and long-term mechanical quiescence in human cardiomyocytes. The following dissertation reports on the effects of mechanical contraction in human cardiomyocytes on regenerative stem cell therapeutics after myocardial infarction (MI), proliferation, and early development cardiac biology. To evaluate if mechanical contraction from the cardiac grafts after MI contribute to the overall functional improvement, we transplanted non-contractile and contractile hiPSC-cardiomyocytes (hiPSC-CMs) into the infarcted rat hearts. At 3 months post transplantation, non-contractile cardiomyocytes were equipotent with contractile cardiomyocytes in preventing the decline of systolic function after MI. These results suggest that force production by cardiac grafts is not necessary to prevent decline in cardiac function post MI in rodents. However, during this study, we observed significantly larger graft sizes from non-contractile cardiomyocytes compared to contractile cardiomyocytes. To understand this difference in graft sizes, we investigated the effect of mechanical contraction on cardiomyocytes proliferation by studying the relationship between cyclin B1 and p53. We found that mechanical contraction, one of the most demanding metabolic activities, increases p53 activity that degrades cyclin B1 due to increased oxidative stress from contracting sarcomeres. This results in decreased cardiomyocyte proliferation that inhibits cardiac regeneration upon injury. Additionally, we further investigated if cardiomyocytes can form sarcomeres, the fundamental contractile units, in the absence of contraction to understand if mechanical activity is needed for structural development. By investigating the effect of contraction in areas ranging from therapeutic mechanism to developmental biology, these findings provide new understanding of how contraction affects cellular mechanisms and development.