Sub-physiological oxygen levels optimal for growth and survival of human atrial cardiac stem cells.

Cardiac stem cells reside in niches where the oxygen levels are close to 3%. For cytotherapy, cells are conventionally expanded in ambient oxygen (21% O2) which represents hyperoxia compared to the oxygen tension of niches. Cardiosphere-derived cells (CDCs) are then transplanted to host tissue with lower-O2 levels. The high-O2 gradient can reduce the efficacy of cultured cells. Based on the assumption that minimizing injury due to O2 gradients will enhance the yield of functionally efficient cells, CDCs were cultured in 3% O2 and compared with cells maintained in ambient O2. CDCs were isolated from human right atrial explants and expanded in parallel in 21 and 3% oxygen and compared with regard to survival, proliferation, and retention of stemness. Increased cell viability even in the tenth passage and enhanced cardiosphere formation was observed in cells expanded in 3% O2. The cell yield from seven passages was fourfold higher for cells cultured in 3% O2. Preservation of stemness in hypoxic environment was evident from the proportion of c-kit-positive cells and reduced myogenic differentiation. Hypoxia promoted angiogenesis and reduced the tendency to differentiate to noncardiac lineages (adipocytes and osteocytes). Mimicking the microenvironment at transplantation, when shifted to 5% O2, viability and proliferation rate were significantly higher for CDCs expanded in 3% O2. Expansion of CDCs, from atria in sub-physiological oxygen, helps in obtaining a higher yield of healthy cells with better preservation of stem cell characteristics. The cells so cultured are expected to improve engraftment and facilitate myocardial regeneration.

Differential response of human cardiac stem cells and bone marrow mesenchymal stem cells to hypoxia-reoxygenation injury.

Cardiosphere-derived cells (CDCs) and bone marrow mesenchymal stem cells (MSCs) are popularly used in stem cell therapy for myocardial regeneration. The cell type that survives and maintains stem cell characteristics in the adverse microenvironment following ischemia-reperfusion injury is presumed to be ideal for transplantation. The study was therefore aimed at identifying the cell type with relatively greater resistance to ischemia-reperfusion injury. CDCs were isolated from the right atrial appendage and MSCs from bone marrow of patients who underwent coronary artery bypass graft surgery. Ischemia-reperfusion injury was simulated in vitro by subjecting the cells to hypoxia (0.5% O2) followed by reintroduction of oxygen (HR injury). Greater resistance of CDCs to HR injury was apparent from the decreased expression of senescence markers and lower proportion of apoptotic cells (one-sixth of that in MSCs). HR injury retarded cell cycle progression in MSCs. Consequent to HR injury, cell migration and secretion of stromal-derived growth factor were stimulated, significantly in CDCs. The differentiation to myocyte lineage and angiogenesis assessed by tube formation ability was better for CDCs. Release of vascular endothelial growth factor was relatively more in CDCs and was further stimulated by HR injury. Differentiation to osteogenic and angiogenic lineage was stimulated by HR injury in MSCs. Compared to MSCs, CDCs appear to be the cell of choice for promoting myocardial regeneration by virtue of its survival capacity in the event of ischemic insult along with higher proliferation rate, migration efficiency, release of growth factors with paracrine effects and differentiation to cardiac lineage.