PGE2 induced self-renewal in hematopoietic stem cells

Hematopoietic stem cell transplantation (HSCT) is the most prevalent and successful stem cell based therapy with 50 000 recipients annually worldwide most of whom are suffering from blood related malignancies. However, this treatment option is frequently limited by the lack of an appropriate donor because of graft versus host disease (GVHD). Human umbilical cord blood (hCB) can be used as an alternative, less immunogenic, source of hematopoietic stem cells but the relatively few stem cells in hCB pose a significant drawback, lengthening the reconstitution rate and risk for infections especially in adult patients. Leonard Zons lab has previously identified Prostaglandin E2 (PGE2) in a small molecule zebrafish in vivo screen as a potent enhancer of HSC formation (North et al. 2007). Subsequent studies showed that a short treatment of whole bone marrow or hCB with PGE2 ex vivo can enhance HSC engraftment during transplantation in zebrafish and mice. Recently a Phase I clinical trial was successfully completed in which dmPGE2 (a more stable form of PGE2) treated hCB significantly shortened repopulation rate and thereby lowered the risk for infections in patients (Cutler et al. 2013). The mechanism however of how PGE2 can both upregulate HSC formation during zebrafish development and enhance HSC engraftment in a transplantation setting is not known. In the zebrafish embryo PGE2 is known to increase HSC numbers but the exact developmental process affected remains to be determined. In preliminary studies the Zon lab has determined that dmPGE2 treatment functionally converts short-term (ST)-HSCs into long term (LT)-HSCs with enhanced self-renewal potential. The long lasting effects of this short (2 hour) treatment with dmPGE2 suggest epigenetic modification of target cell populations. I hypothesize that PGE2 is reprogramming progenitor cells into long term self-renewing HSCs during development in zebrafish and upon HSCT. I plan to test two predictions made from this hypothesis with the following series of experiments. Aim 1 is to test the prediction that in the zebrafish embryo PGE2 treatment increases HSC specification of progenitor cells (the hemogenic endothelium). I plan to define the effect of PGE2 on HSC clonal diversity using the brainbow system in zebrafish. Aim 2 is to test the prediction that PGE2 induces epigenetic changes which enhance self-renewal and long term engraftment in adult progenitor cells. I will determine PGE2 induced epigenetic changes of the self-renewal potential using ChIPSeq and DNA methylation studies in human CD34+ cells. Aim 3 is to establish the identity of PGE2s induced self-renewal network from aim 2 by Gain-of-Function (GOF)/Loss-of-Function (LOF) studies of key epigenetic factors via transplantation assays. Understanding the self-renewal gene network downstream of PGE2 will not only give fundamental insights into mechanisms controlling self-renewal in long term repopulating HSCs but may also have therapeutic relevance.

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