Abstract 258: Hypoxic and Normoxic Niches Regulate Cardiac Stem Cell Function

Abstract

In bone marrow niches, hypoxia favors stem cell quiescence and normoxia is required for stem cell proliferation and commitment. The objectives of this work were to determine whether a) differences in O 2 content regulate the function of cardiac stem cells (CSCs); and b) defects in tissue oxygenation with aging affects niche homeostasis. At 3 and 30 months of age, hypoxic and normoxic CSCs were found in the myocardium, but the senescent mouse heart was characterized by a 1.5-fold increase in hypoxic CSCs. At both ages, hypoxic CSCs were quiescent and lineage negative while cycling and early committed cells were restricted to the normoxic pool. Importantly, telomeres were longer in hypoxic than normoxic CSCs and this difference was more apparent in old mice. Telomere length did not change with age in hypoxic CSCs but decreased 40% in normoxic CSCs. As a consequence, at 3 and 30 months, the fraction of normoxic CSCs expressing the senescence-associated marker p16 INK4a was, respectively, 6-fold and 2.7-fold higher than that of hypoxic CSCs. Collectively, these findings indicate that the old heart contains a larger fraction of CSCs, which are forced in a quiescent state and do not participate in myocyte turnover. To define whether hypoxic CSCs can be activated, senescent mice were kept in an atmosphere of 70% O 2 to increase O 2 content in the hypoxic niches. Hyperoxia decreased significantly the fraction of hypoxic CSCs at 1 day and this response persisted at one week, possibly favoring myocyte formation. These in vivo results suggest that O 2 levels in CSCs may be regulated by the capillary network and the degree of tissue oxygenation. The distance between CSCs and the closest capillary was significantly higher for hypoxic than for normoxic CSCs. The numerical density of capillaries per mm 2 of myocardium computed within a radius of 30 μm from hypoxic CSCs decreased markedly with age. Thus, diffusion distance for O 2 to CSCs appears to constitute a major determinant in the maintenance of the hypoxic state of CSCs. Collectively, our data demonstrate that a balance between hypoxic and normoxic niches is present in the young heart but is disrupted later in life when capillary rarefaction expands the pool of hypoxic quiescent CSCs, which are no longer involved in myocyte replacement.