Challenges in identifying the best source of stem cells for cardiac regeneration therapy

The overall clinical cardiac regeneration experience suggests that stem cell therapy can be safely performed, but it also underlines the need for reproducible results for their effective use in a real-world scenario. One of the significant challenges is the identification and selection of the best suited stem cell type for regeneration therapy. Bone marrow mononuclear cells, bone marrow-derived mesenchymal stem cells, resident or endogenous cardiac stem cells, endothelial progenitor cells and induced pluripotent stem cells are some of the stem cell types which have been extensively tested for their ability to regenerate the lost myocardium. While most of these cell types are being evaluated in clinical trials for their safety and efficacy, results show significant heterogeneity in terms of efficacy. The enthusiasm surrounding regenerative medicine in the heart has been dampened by the reports of poor survival, proliferation, engraftment, and differentiation of the transplanted cells. Therefore, the primary challenge is to create clearcut evidence on what actually drives the improvement of cardiac function after the administration of stem cells. In this review, we provide an overview of different types of stem cells currently being considered for cardiac regeneration and discuss why associated factors such as practicality and difficulty in cell collection should also be considered when selecting the stem cells for transplantation. Next, we discuss how the experimental variables (type of disease, marker-based selection and use of different isolation techniques) can influence the study outcome. Finally, we provide an outline of the molecular and genetic approaches to increase the functional ability of stem cells before and after transplantation.