Abstract
About eight million people living in the United States have had one myocardial infarction. Myocardial infarctions result in the formation of scar tissue in the heart, which is incapable of pumping like healthy heart muscle. Introducing healthy heart muscle to the site of injury is a potential therapy for myocardial infarction, which can be achieved by generating functional cardiomyocytes from stem cells. Directed differentiation from human induced pluripotent stem cells (hiPSCs) into cardiomyocytes ( CMs) provides a virtually unlimited source of CMs for potential autologous cell transplantation therapy. However, hiPSC-CMs exhibit immature characteristics, which include small cell size, immature sarcomere structure and immature Ca2+ handling properties, similar to neonatal CMs. In vivo transplantations studies with these immature cardiomyocytes have demonstrated arrhythmias due to the mismatch of conduction velocities and location of the gap junctions, which is due to the small cell size of the transplanted cells compared to the host cells. CMs that have exited the cell cycle after birth can take 10-20 years to increase their size to that of mature CMs via a process known as physiological hypertrophy. This phenomenon has also been observed in athletes where physiological hypertrophy can be induced in days to months as a response to athletic training. Physiological hypertrophy is induced when growth factors bind to their receptors on the sarcolemma and activate the Akt pathway. Therefore, in this study we demonstrate that, using growth factors that have shown higher expression in athletes, physiological hypertrophy can be facilitated in hiPSC-CMs. Using flow cytometry and gene expression analysis, in this study we demonstrated that an increase in cell size can be achieved when approximately 50-60 day old hiPSC-CMs were treated with IGF-1, FGF-1 and FGF-2 as compared to untreated control. The increased cell size was coupled with uncompromised contractile function demonstrated by the increased expression of contractile proteins MLC2a and MLC2v and improved Ca2+ handling properties of the cells and increase activation of Akt pathway in treated cells. Fetal cardiac genes ANF and MHCa did not increase in expression significantly indicating that the hypertrophy induced was physiological and not pathalogical. Our results indicate that biochemical induction of physiological hypertrophy is possible and can result in the maturation of hiPSC-CMs.