Year 2

The molecular characterization of hematopoietic stem cell (HSC) microenvironments (also termed niches) is a fundamental goal in the field of stem cell biology and regenerative medicine. Using an ensemble of systems biology approaches, we have established shared molecular commonalities in HSC niches from distinct temporal and spatial locations, both in the adult vertebrate animal and in the developing embryo. Our studies have identified several genes that are predicted to be important niche factors. Experimentally, we have validated the importance of many of these factors, and are now working to better understand their functions. To investigate if the molecular pathways identified in the mouse model were conserved in other vertebrate species, we validated the importance of several predicted key regulators by functional studies in the zebrafish embryo. In addition to extracting the molecular core dedicated to HSC support, we also identified specific gene signatures active in the embryonic sites of HSC emergence. More recently, we have extended these approaches to a comparison of gene signatures in the murine AGM region, the birthplace of mammalian HSCs. Using sensitive laser microdissection techniques, coupled with microarray analyses and our proven bioinformatics approaches, we now have assembled new data sets for functional testing. As described in our progress report, we have discovered a variety of factors specifically expressed by the ventral mesenchyme underlying the hemogenic endothelium of the dorsal aorta. We hope that these will include novel mediators of the endothelial to hematopoietic transition (EHT), which we will translate to our human pluripotent cell approaches over the next year.