Abstract 126: Functional Properties of Cardiac Stem Cell Niches in the Young and Old Heart

Abstract

Cardiac stem cells (CSCs) are clustered in interstitial structures, in which cardiomyocytes and fibroblasts act as supporting cells. However, the effects of the niche microenvironment on the functional properties of CSCs are largely unknown and whether young and senescent myocytes affect differently CSC fate remains to be determined. To address this issue, cardiac niches were examined in young and old Fischer 344 rats. Clusters composed of two or more lineage negative CSCs were included in the analysis. Unexpectedly, from 4 to 28 months of age, the number of CSCs per niche increased nearly 2-fold. This expansion of the CSC pool took place despite the increase in the fraction of dying CSCs with age. Apoptosis of CSCs was restricted to cells expressing the senescence-associated protein p16 INK4a . However, the fraction of CSCs positive for p16 INK4a increased significantly, documenting that the process of clearance of senescent CSCs was inefficient in the old myocardium. The fraction of p16 INK4a -positive CSCs undergoing apoptosis decreased 3.5-fold with age, resulting in accumulation of senescent cells within the niche. These observations indicate that the pool of functionally competent CSCs able to contribute to myocyte turnover decreased with age. In an attempt to compensate for the higher myocyte loss occurring in aged hearts, p16 INK4a -negative CSCs proliferated 4-fold more in old than in young rats, experiencing progressive telomeric shortening. To define whether the fate of CSCs was dictated by the properties of the neighboring myocytes in the senescent heart, myocytes and CSCs were co-cultured for 3 days. With respect to CSCs plated alone, old myocytes enhanced 2.2-fold BrdU incorporation in co-cultured CSCs. This effect was less apparent with young myocytes. No significant differences in CSC apoptosis were seen. Comparable results were obtained when CSCs and myocytes were plated in Transwell systems. Importantly, old myocytes released more IGF-1 than young myocytes; blockade of IGF-1 receptor in CSCs attenuated significantly the positive impact of myocytes on CSC replication. Collectively, our findings indicate that the rate of accumulation of old CSCs is greater than the rate of their death leading to the formation of senescent niches and organ aging.