Cell Cycle Analysis and Synchronization of Pluripotent Hematopoietic Progenitor Stem Cells

Open Access
Publisher Website
Google Scholar
Abstract
Hematopoietic stem cells purified from mouse bone marrow are quiescent with less than 2% of Lin− Hoechstlow/Rhodaminelow (Lin− Holow/Rholow) and 10% to 15% of Lin−/Sca+ cells in S phase. These cells enter proliferative cycle and progress through G1 and into S phase in the presence of cytokines and 5% heat-inactivated fetal calf serum (HI-FCS). Cytokine-stimulated Lin− Holow/Rholow cells took 36 to 40 hours to complete first division and only 12 hours to complete each of 5 subsequent divisions. These cells require 16 to 18 hours to transit through G0 /G1 period and 28 to 30 hours to enter into mid-S phase during the first cycle. Up to 56% of Lin− Rholow/Holow cells are high-proliferative potential (7 factor-responsive) colony-forming cells (HPP-CFC). At isolation, HPP-CFC are quiescent, but after 28 to 30 hours of culture, greater than 60% are in S phase. Isoleucine-deprivation of Lin−Holow/Rholow cells in S phase of first cycle reversibly blocked them from entering into second cycle. After the release from isoleucine-block, these cells exhibited a G1 period of less than 2 hours and entered into mid-S phase by 12 hours. Thus, the duration of G1 phase of the cells in second cycle is 4 to 5 times shorter than that observed in their first cycle. Similar cell cycle kinetics are observed with Lin−/Sca+ population of bone marrow cells. Stem cell factor (SCF ) alone, in the presence of HI-FCS, is as effective as a cocktail of 2 to 7 cytokines in inducing quiescent Lin−/Sca+ cells to enter into proliferative cycle. Aphidicolin treatment reversibly blocked cytokine-stimulated Lin−/Sca+ cells at G1 /S boundary, allowing their tight synchrony as they progress through first S phase and enter into second G1 . For these cells also, SCF alone is sufficient for their progression through S phase. These studies indicate a very short G1 phase for stem cells induced to proliferate and offer experimental approaches to synchronize murine hematopoietic stem cells.