Differentiation Hotspots on a Cell Cycle Related Continuum.

Directed differentiation is defined as the ability to program a stem cell at the most primitive level while it still has its reproductive and full proliferative potential. This is in contrast to ex-vivo expansion where the stem cells are forced into specific lineage commitments, limiting the overall therapeutic utility. We have demonstrated differentiation “hotspots” on a cell cycle continuum (Exp Heme35:96, 2007). In this work we showed marked but reversible increases in differentiation potential to megakryocyte and granulocytes at different phases of a single cytokine induced cell cycle passage of highly purified quiescent murine lineagenegative rhodaminelowHoeschtlow (LRH) marrow stem cells. We have reproducibly induced directed stem cell differentiation by capitalizing on inherent changes in sensitivities to inductive cytokine signals in the context of cell cycle position. These cells, when exposed to thrombopoietin, FLT3-ligand and steel factor, synchronously pass through cell cycle. We have found that using a differentiation cytokine cocktail of G-CSF at 0.075ng/ml, GM-CSF at 0.0375ng/ml and steel factor at 50ng/ml, we were able to see enhanced megakaryopoiesis occurring 14-days after culture in those LRH stem cells that were in early to mid S-phase at time of inductive signaling. We have now shown that a megakaryocyte hotspot clusters around 32 hours; the G1/S interface, and that dramatic reversible changes in differentiation potential occur over one hour time intervals. We have confirmed this data by looking at LRH cells through cell cycle transit after initial cell division showing that a megakaryocyte hotspot occurs in two sequential cell cycles and still tied to S-phase at time of inductive signaling of the daughter cells. This hotspot has been demonstrated on a clonal basis, although the kinetics of the hotspot shifts when clonal as opposed to population studies are carried out. An important issue is whether in vitro cytokine exposure, separate from cell cycle status, determines the existence of the hotspot. To address this, we used Hoechst 33342 dye content to assist in separation of different cell cycle fractions (G0–1, early, mid and late components of S, G2/M) of lineage negative Sca-1+ stem cells, a cycling stem/progenitor cell population in which approximately 20% of the cells are in S-phase at isolation. These cells were only exposed to the differentiation cytokines and showed a megakaryocyte hotspot present in only early S-phase cells after 14-days of culture, showing that in vitro cell cycle phase determined the presence of the hotspot, separate from cytokine exposure. These data indicate that differentiation potential of marrow stem cells exists on a cell cycle related continuum and that this potential can be demonstrated on a single cell basis. This suggests a continuum model of stem cell regulation at the stem cell level as opposed to a pure hierarchical model.