ALDH1A3 Is the Key Isoform That Contributes to Aldehyde Dehydrogenase Activity and Affects in Vitro Proliferation in Cardiac Atrial Appendage Progenitor Cells

High aldehyde dehydrogenase (ALDH(hi)) activity has been reported in normal and cancer stem cells. We and others have shown previously that human ALDH(hi) cardiac atrial appendage cells are enriched with stem/progenitor cells. The role of ALDH in these cells is poorly understood but it may come down to the specific ALDH isoform(s) expressed. This study aimed to compare ALDH(hi) and ALDH(lo) atrial cells and to identify the isoform(s) that contribute to ALDH activity, and their functional role. Methods and Results: Cells were isolated from atrial appendage specimens from patients with ischemic and/or valvular heart disease undergoing heart surgery. ALDH(hi) activity assessed with the Aldefluor reagent coincided with primitive surface marker expression (CD34(+)). Depending on their ALDH activity, RT-PCR analysis of ALDH(hi) and ALDH(lo) cells demonstrated a differential pattern of pluripotency genes (Oct 4, Nanog) and genes for more established cardiac lineages (Nkx2.5, Tbx5, Mef2c, GATA4). ALDH(hi) cells, but not ALDH(lo) cells, formed clones and were culture-expanded. When cultured under cardiac differentiation conditions, ALDH(hi) cells gave rise to a higher number of cardiomyocytes compared with ALDH(lo) cells. Among 19 ALDH isoforms known in human, ALDH1A3 was most highly expressed in ALDH(hi) atrial cells. Knocking down ALDH1A3, but not ALDH1A1, ALDH1A2, ALDH2, ALDH4A1, or ALDH8A1 using siRNA decreased ALDH activity and cell proliferation in ALDH(hi) cells. Conversely, overexpressing ALDH1A3 with a retroviral vector increased proliferation in ALDH(lo) cells. Conclusions: ALDH1A3 is the key isoform responsible for ALDH activity in ALDH(hi) atrial appendage cells, which have a propensity to differentiate into cardiomyocytes. ALDH1A3 affects in vitro proliferation of these cells.