Cardiac recovery by stem and progenitor cells

Abstract

The adult human heart has a minimal ability to regenerate myocardium. Therefore, loss of viable cardiomyocytes in cardiac disease, such as myocardial infarction (MI), may lead to heart failure. After evaluating the regenerative potential of several stem cell sources of cardiac myocytes and vascular cells (chapter 2), we created a mouse model of MI and cell transplantation which allows long term phenotypical analysis of engrafted human stem and progenitor cells, and used magnetic resonance imaging (MRI) to monitor cardiac function (chapter 3). We thus performed the first long term study of human embryonic stem cell-derived cardiomyocytes (hESC-CM) in uninjured and infarcted mouse hearts, and found that hESC-CM survive, integrate and mature in the host myocardium for at least 12 weeks. HESC-CM transplantation improved heart function post-MI at 4 weeks compared to differentiated hESCs devoid of cardiomyocytes, but this was not sustained at 12 weeks (chapter 4). Comparing the effects of injection of hESC-CM at different dosages with hESC-non-CM derivatives, culture medium or no injection in the same experimental model, we demonstrate that both hESC-CM and hESC-non-CM provide long term functional improvement compared to vehicle- or no injection even though only cardiomyocytes formed persistent grafts. Importantly, increasing numbers of hESC-CM for transplantation resulted in no additional functional benefit. In addition, we confirmed evidence of a paracrine contribution of the transplanted hESC-CM by demonstrating increased vascularization in the infarcted heart associated specifically with transplantation of these cells (chapter 5). Because the grafted cells were usually surrounded by a fibrotic layer, we determined its composition with respect to extracellular matrix type and origin, and found a contribution of both host and donor cells. In addition, we characterized the integrin expression of hESC-CM in vitro and at various time points post-transplantation. Co-transplanted hESC-derived endothelial cells formed a capillary network through the fibrotic layer that communicated with the mouse vasculature, supporting extended graft survival and maturation over a 6 month period (chapter 6). We further specified the role and suitability of vasculogenic cells in cardiac regeneration using mice and human cells with a deficiency of endoglin, an accessory TGFbeta-receptor present on mononuclear cells (MNCs). Defects in mutant mice in vascularization and cardiac function post-MI could be rescued by intravenous injection of healthy donor MNCs, but not MNCs from patients with the gene mutation. This indicates that cardiovascular patients, who often have defects in their MNCs and endothelial progenitor cells, may benefit from transplantation of cells with high endoglin expression, and matched heterologous cells may be preferred (chapter 7). Finally, we show that human heart-derived cardiomyocyte progenitor cells (CMPCs) and CMPC-CM prevented cardiac dilatation and deterioration of cardiac function for at least 12 weeks after MI. In addition, CMPCs differentiated in vivo into cardiomyocytes, smooth muscle cells and endothelial cells (chapter 8). In conclusion, several cell types improve cardiac function but some problems need to be solved, and further investigation into the underlying mechanisms is required for optimization in the choice of cell number and type so that clinical strategies can be properly defined.