Life and death of cardiac stem cells: a paradigm shift in cardiac biology

Concise review: stem cells, myocardial regeneration, and methodological artifacts

This review discusses the current controversy about the role that endogenous and exogenous progenitor cells have in cardiac homeostasis and myocardial regeneration following injury. Although great enthusiasm was created by the possibility of reconstituting the damaged heart, the opponents of this new concept of cardiac biology have interpreted most of the findings supporting this possibility as the product of technical artifacts. This article challenges this established, static view of cardiac growth and favors the notion that the mammalian heart has the inherent ability to replace its cardiomyocytes through the activation of a pool of resident primitive cells or the administration of hematopoietic stem cells.

Myocyte enlargement, differentiation, and proliferation kinetics in the fetal sheep heart

The generation of new myocytes is an essential process of in utero heart growth. Most, or all, cardiac myocytes lose their capacity for proliferation during the perinatal period through the process of terminal differentiation. An increasing number of studies focus on how experimental interventions affect cardiac myocyte growth in the fetal sheep. Nevertheless, fundamental questions about normal growth of the fetal heart remain unanswered. In this study, we determined that during the last third of gestation the hearts of fetal sheep grew primarily by four processes. 1) Myocyte proliferation contributed substantially to daily cardiac mass gain, and the number of cardiac myocytes continued to increase to term. 2) The (hitherto unrecognized) contribution to cardiac growth by the increase in myocyte size associated with the transition from mononucleation to binucleation (terminal differentiation) became considerable from approximately 115 days of gestational age (dGA) until term (145dGA). Because binucleation became the more frequent outcome of myocyte cell cycle activity after approximately 115dGA, the number of binucleated myocytes increased at the expense of the number of mononucleated myocytes. Both the interval between nuclear divisions and the duration of cell cycle activity in myocytes decreased substantially during this same period. Finally, cardiac growth was in part due to enlargement of 3) mononucleated and 4) binucleated myocytes, which grew in cross-sectional diameter but not length during the last third of gestation. These data on normal cardiac growth may enable a more detailed understanding of the consequences of experimental and pathological interventions in prenatal life.

The telomere-telomerase axis and the heart

The preservation of myocyte number and cardiac mass throughout life is dependent on the balance between cell death and cell division. Rapidly emerging evidence indicates that new myocytes can be formed through the activation and differentiation of resident cardiac progenitor cells. The critical issue is the identification of mechanisms that define the aging of cardiac progenitor cells and, ultimately, their inability to replace dying myocytes. The most reliable marker of cellular senescence is the modification of the telomere-telomerase axis, together with the expression of the cell cycle inhibitors p16INK4a and p53. Cellular senescence is characterized by biochemical events that occur within the cell. In this regard, one of the most relevant processes is represented by repeated oxidative stress that may evolve into the activation of the cell death program or result in the development of a senescent phenotype. Thus, the modulation of telomerase activity and the control of telomeric length, together with the attenuation of the formation of reactive oxygen species, may represent important therapeutic tools in regenerative medicine and in prevention of aging and diabetic cardiomyopathies.

Sequential growth of fetal sheep cardiac myocytes in response to simultaneous arterial and venous hypertension

While the fetal heart grows by myocyte enlargement and proliferation, myocytes lose their capacity for proliferation in the perinatal period after terminal differentiation. The relationship between myocyte enlargement, proliferation, and terminal differentiation has not been studied under conditions of combined arterial and venous hypertension, as occurs in some clinical conditions. We hypothesize that fetal arterial and venous hypertension initially leads to cardiomyocyte proliferation, followed by myocyte enlargement. Two groups of fetal sheep received intravascular plasma infusions for 4 or 8 days (from 130 days gestation) to increase vascular pressures. Fetal hearts were arrested in diastole and dissociated. Myocyte size, terminal differentiation (%binucleation), and cell cycle activity (Ki-67[+] cells as a % of mononucleated myocytes) were measured. We found that chronic plasma infusion greatly increased venous and arterial pressures. Heart (but not body) weights were approximately 30% greater in hypertensive fetuses than controls. The incidence of cell cycle activity doubled in hypertensive fetuses compared with controls. After 4 days of hypertension, myocytes were (approximately 11%) longer, but only after 8 days were they wider (approximately 12%). After 8 days, %binucleation was approximately 50% greater in hypertensive fetuses. We observed two phases of cardiomyocyte growth and maturation in response to fetal arterial and venous hypertension. In the early phase, the incidence of cell cycle activity increased and myocytes elongated. In the later phase, the incidence of cell cycle activity remained elevated, %binucleation increased, and cross sections were greater. This study highlights unique fetal adaptations of the myocardium and the importance of experimental duration when interpreting fetal cardiac growth data.

Molecular and cellular basis of congenital heart disease

The cellular and molecular basis of congenital heart disease (CHD) is an evolving area of rapid discovery. This article introduced the basic mechanisms underlying cardiac development and CHD in order to permit a clear understanding of current diagnostics and therapeutics and their future development. It is clear that although significant advances have been made in understanding mechanisms controlling heart formation, the direct causes of CHD remain poorly defined. Future studies tha delineate the complexity of these mechanisms are required to provide a comprehensive understanding of the etiologies of CHD. Such understanding will lead to the development of novel approaches to prevention and therapy.

From cardiac repair to cardiac regeneration – ready to translate?

Cardiovascular disease is a major public health challenge in the western world. Mortality of acute events has improved, but more patients develop HF--a condition affecting up to 22 million people worldwide. Cell transplantation is the first therapy to attempt replacement of lost cardiomyocytes and vasculature to restore lost contractile function. Since the first reported functional repair after injection of autologous skeletal myoblasts into the injured heart in 1998, a variety of cell types have been proposed for transplantation in different stages of cardiovascular disease. Fifteen years of preclinical research and the rapid move into clinical studies have left us with promising results and a better understanding of cells as a potential clinical tool. Cell-based cardiac repair has been the first step, but cardiac regeneration remains the more ambitious goal. Promising new cell types and the rapidly evolving concept of adult stem and progenitor cell fate may enable us to move towards regenerating viable and functional myocardium. Meeting a multidisciplinary consensus will be required to translate these findings into safe and applicable clinical tools.

Diabetes promotes cardiac stem cell aging and heart failure, which are prevented by deletion of the p66shc gene

Diabetes leads to a decompensated myopathy, but the etiology of the cardiac disease is poorly understood. Oxidative stress is enhanced with diabetes and oxygen toxicity may alter cardiac progenitor cell (CPC) function resulting in defects in CPC growth and myocyte formation, which may favor premature myocardial aging and heart failure. We report that in a model of insulin-dependent diabetes mellitus, the generation of reactive oxygen species (ROS) leads to telomeric shortening, expression of the senescent associated proteins p53 and p16INK4a, and apoptosis of CPCs, impairing the growth reserve of the heart. However, ablation of the p66shc gene prevents these negative adaptations of the CPC compartment, interfering with the acquisition of the heart senescent phenotype and the development of heart failure with diabetes. ROS elicit 3 cellular reactions: low levels activate cell growth, intermediate quantities trigger cell apoptosis, and high amounts initiate cell necrosis. CPC replication predominates in diabetic p66shc-/-, whereas CPC apoptosis and myocyte apoptosis and necrosis prevail in diabetic wild type. Expansion of CPCs and developing myocytes preserves cardiac function in diabetic p66shc-/-, suggesting that intact CPCs can effectively counteract the impact of uncontrolled diabetes on the heart. The recognition that p66shc conditions the destiny of CPCs raises the possibility that diabetic cardiomyopathy is a stem cell disease in which abnormalities in CPCs define the life and death of the heart. Together, these data point to a genetic link between diabetes and ROS, on the one hand, and CPC survival and growth, on the other.