Stem Cell Therapies for Cardiovascular Disease

Types of Stem Cells and Mechanisms of Benefit

Stem cells can be classified according to their level of potency. A totipotent stem cell, such as a fertilized zygote, can lead to an entire organism. A pluripotent cell, such as an embryonic stem (ES) cell, leads to cells from all three germ layers but is unable to generate an organism. Multipotent stem cells, such as mesenchymal stem cells (MSCs), can lead to different types of cells from the same cell germ layer, such as adipocytes, bone, or cartilage cells. Skeletal muscle myoblasts, endothelial progenitor cells, and bone marrow mononuclear cells (BN-MNCs) are other examples of multipotent stem cells that have been studied in cardiac regeneration.

The signaling pathways that drive stem cells into cardiomyocyte fate are areas of intense research; better understanding of these pathways can be used as valuable therapeutic tools in enhancing cardiac regeneration. Differentiation is the process by which stem cells can become cardiomyocytes, whereas transdifferentiation is a process in which a somatic cell adopts alternative cell fates (e.g., a fibroblast adopting an endothelial cell fate). Another process by which stem cells can alter cardiac function is fusion. When stem cells fuse with somatic cells (e.g., cardiomyocytes), the resulting cells have characteristics of both cell types; however, the extent to which any clinical benefit can be achieved from this process is currently unclear. Another mechanism by which stem cells can enhance tissue regeneration is through a paracrine mechanism. After injection, stem cells are believed to release cytokines and/or growth factors that have physiological effects on other cells in the injured environment and affect repair. For instance, injection of MSCs into the injured heart affects the balance of Wnt signaling in the injured environment to modulate angiogenesis and fibrosis. Identification of key stem cell secreted molecules mediating pro-reparative effects on the injured heart may lead to development of new therapeutic strategies in which injection of key stem cell secreted molecules, rather than stem cells, may be sufficient to augment cardiac healing and obviate issues such as immune response, dose titration, and availability.

ES cells have the ability to give rise to any cell type of the organism, and under the appropriate conditions, can differentiate into cardiomyocytes. They originate from the inner cell mass of the blastocyst during development. Studies have shown that injection of ES cells leads to successful engraftment of them into the surrounding cardiac tissue, making this approach appealing. However, large-scale generation of ES cell–derived cardiomyocytes currently remains unrealistic, because this is a field still in its infancy and filled with several ethical and political challenges. Differentiation of bone marrow stem cells into functioning cardiomyocytes has been more challenging.

Bone marrow contains different types of progenitor stem cells, among which BM-MNCs and MSCs have been extensively studied. Injection of BM-MNCs into the diseased cardiac muscle leads to improvement of cardiac function. Although initial studies suggested that BM-MNCs transdifferentiated into myocytes, later studies suggested an indirect effect likely related to the paracrine effects of cells. A paracrine effect contributing to salutary effects on cardiac repair was confirmed for MSC injection after cardiac injury. In vitro studies suggested that these cells could also differentiate into beating cardiomyocytes, and these findings generated a lot of excitement that led to several preclinical studies. These studies suggested that injection of MSCs into the injured myocardium resulted in improved cardiac function despite the low number of mesenchymal-derived cardiomyocyte cells, which suggested a multifactorial effect similar to BM-MNCs.

The limited supply of ES cells and associated social challenges have led to other pathways to develop pluripotent stem cells. A type of cell that has attracted a lot of interest recently is the resident cardiac progenitor cell. The existence of these cells and their ability to differentiate into cardiomyocytes, as well as endothelial and smooth muscle cells, has shaken the long-standing belief that the heart is a fixed organ unable of regeneration. Several challenges remain to fully derive the potential benefits of these cells. First, different cell markers have been used to characterize these cells, and it is presently unclear if there are any biological differences between cells with different cell markers. Second, the number of these cells is small, and their role in normal cardiac function is not clear. However, injection of these cells into an infarcted heart leads to improvement of cardiac function. What makes these cells particularly attractive is their ability to differentiate into other cell types (e.g., endothelial and smooth muscle cells) because the regenerating cardiomyocytes will need new blood vessels and supporting cells to properly function. Also, use of these cells appears to avoid some of the ethical challenges that can arise with the ES cells. Their small number and the technical difficulties associated with successfully multiplying them have led to the development of other types and techniques, among which induced pluripotent stem cells merit special mention. These are ES cells derived from skin fibroblasts through genetic manipulation (through the overexpression of certain transcription factors). What makes this approach unique and revolutionary is the ability of skin fibroblasts to be reprogrammed into induced pluripotent stem cells that could be injected into an injured heart. Although this field and its technology are in their infancy, it holds great promise for future cardiovascular regeneration therapy.

Another type of stem cell that deserves special mention is the endothelial progenitor cell, especially for vascular regeneration. Injury of the vascular endothelium triggers a cascade of events that aims to reconstitute the endothelium via the proliferation of the remaining endothelial cells and differentiation of the endothelial progenitor cells. It is based on these premises that administration of endothelial progenitor cells might indirectly improve cardiac function by enhancing vasculature repair and angiogenesis.

Cardiospheres have also been tested in heart regeneration after myocardial infarction. Cardiospheres are three-dimensional multicellular structures from cardiac explant cultures in nonadhesive surfaces. It is currently believed that injected cardiospheres modulate scar formation via paracrine effects or secreted exosomes containing active molecules such as microRNA. The phase I clinical trial testing cardiospheres in humans did not show any left ventricular ejection fraction improvement; however, this trial did show a reduction of scar mass at 6 months and an increase in viable myocardium.

During this first wave of excitement, skeletal myoblasts have also been studied, with promising results in early tests, but with no significant benefit in clinical trials; therefore the use of these cells in cardiovascular regeneration is uncertain ( Table 6.1 ).