Hematopoietic Stem Cell Niche Interactions In The Embryo Can Modulate The Size Of The Stem Cell Pool Into Adulthood

Hematopoietic stem cells (HSC) reside in the bone marrow niche and sustain the production of blood throughout life. The entire pool of these rare and important cells is generated during a brief window of embryonic development. HSC are produced by the hemogenic endothelium of the dorsal aorta, migrate to and expand in the fetal liver, and then migrate again to seed the bone marrow. The zebrafish is a highly conserved and well-established model for HSC development. Similar to mammals, HSC emerge from the dorsal aorta, but then colonize a vascular plexus in the tail of the embryo—the caudal hematopoietic tissue (CHT). It is difficult to directly observe the interactions between an endogenous HSC and its niche, so we have developed the CHT as a model for HSC-niche interactions. To track HSC in vivo we have generated a transgenic reporter using the previously described mouse Runx1 +23 kb intronic enhancer. The purity of the stem cell pool marked by this reporter was determined. Using adult-to-adult limiting dilution transplantation with as few as one Runx1+23 positive cell, we have estimated the HSC purity to be approximately 1/35 (without immune matching), or similar to Kit+Sca1+Lin- (KSL) in mouse. This is the most pure stem cell population defined in the zebrafish system. Using embryo-to-embryo transplantation, a technique that is unique to zebrafish, we sorted Runx1+23 positive cells from one group of embryos and transplanted them to another by injection directly into circulation. Embryos are then grown to adulthood and marrow is tested for long-term engraftment between 3 and 5 months. This transplantation technique precedes formation of the thymus, thereby removing any chance of immune rejection. Highly stringent dilution of HSC in our embryo-to-embryo transplants has estimated a stem cell purity of one in two cells. Next, we applied our highly specific reporter to visualize HSC migration to the CHT niche. After arrival of the HSC, we have described 5 distinct steps during colonization: 1) adherence; 2) extravasation; 3) abluminal migration; 4) endothelial niche formation (“cuddling”); and 5) cell fate decisions. Live imaging analysis of HSC together with endothelial and stromal transgenic reporters has allowed us to quantify the relationship between different cell types within the CHT. For example, we observe preferential localization of HSC in close proximity to cxcl12a positive stromal cells. Lastly, we have sought to identify the molecular mechanisms involved in interactions between HSC and their niche. A chemical genetic screen identified the natural product lycorine as a small molecule that increases hematopoiesis in the CHT and promotes HSC-endothelial cell interactions. Combined chemical treatment and live imaging revealed that lycorine significantly increased the residence time of HSC in the niche. To test if treatment during the window of CHT colonization (2-3 days post fertilization) had long-term effects on HSC and the stem cell pool, the compound was washed off at 3 days and the Runx1+23 positive population was quantified by FACS. At 7 days post fertilization, after colonization of the marrow, there was a sustained and significant increase in Runx1+23 positive HSC. Strikingly, after 3 months, when treated embryos were raised to adulthood, we discovered that the increased HSC-endothelial cell interactions we observed in the CHT niche had in fact had an impact on the number of HSC in the adult. Our studies establish that the Runx1+23 transgenic is a highly specific reporter of HSC both in the embryo and adult, and that we can use this reporter for in vivo observation of an endogenous HSC niche. Furthermore, we show that the size of the adult stem cell pool can be altered by a transient signal during development.