Year 3

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 final 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 in our current and future studies.