Abstract 1265: Cardiac Stem Cell Niches Control Cardiomyogenesis in the Adult Mouse Heart
Cardiac stem cells (CSCs) have been identified in the adult heart, but the microenvironment that protects the slow-cycling, undifferentiated, and self-renewing compartment of CSCs remains to be determined. The objective of this study was to document whether the adult myocardium possesses interstitial structures with the architectural organization of stem cell niches. Although a niche may include a single undifferentiated cell, we have directed our search to groups of cells that included both CSCs and early committed cells (LCCs). These nests were found to be present in the atria, base-mid-region and apex and consisted of lineage negative (Lin−) c-kitPOS, MDR1POS or Sca-1POS cells assembled with committed cells at various stages of differentiation. CSCs-LCCs were physically connected to myocytes and fibroblasts by adherens and gap junctions expressing N- and E-cadherin and connexin 43 and 45, respectively. The formation of functionally competent gap junctions between CSCs-LCCs and myocytes or fibroblasts was documented by two-photon microscopy in living cells. The green fluorescent dye calcein translocated from c-kitPOS-CSCs-LCCs to myocytes and fibroblasts but not to endothelial cells, providing evidence that myocytes and fibroblasts function as supporting cells in cardiac niches. Importantly, calcein transfer was inhibited by the gap junction blocker heptanol. The interstitium surrounding the cells within the niches consisted mainly of fibronectin and α2-laminin. These extracellular matrix proteins accumulated and surrounded undifferentiated CSCs that typically expressed the α4 integrin subunit on their plasma membrane. BrdU pulse-chase assay was employed to determine the growth kinetics of CSCs and the rate of myocyte turnover. As expected in an organ in steady state, asymmetric division of CSCs prevailed and replicating CSCs gave rise to one daughter CSC and one daughter committed cell. The half-life of myocytes averaged 150 days in the atria, 250 days at the base-mid-region and 100 days at the apex and it was inversely correlated with the level of hemodynamic stress. Thus, CSCs take long-term residence in the heart and cardiac niches provide the necessary, permissive milieu for survival, growth, self-renewal and differentiation of CSCs.