Karin van der Horn

Mutations in APC or beta-catenin inappropriately activate the transcription factor Tcf4, thereby transforming intestinal epithelial cells. Here it is shown that one of the target genes of Tcf4 in epithelial cells is Tcf1. The most abundant Tcf1 isoforms lack a beta-catenin interaction domain. Tcf1(-/-) mice develop adenomas in the gut and mammary glands. Introduction of a mutant APC allele into these mice substantially increases the number of these adenomas. Tcf1 may act as a feedback repressor of beta-catenin-Tcf4 target genes and thus may cooperate with APC to suppress malignant transformation of epithelial cells.

Apoptosis is an essential process in embryonic tissue remodeling and adult tissue homeostasis. Within the adult hematopoietic system, it allows for tight regulation of hematopoietic cell subsets. Previously, it was shown that B-cell leukemia 2 (Bcl-2) overexpression in the adult increases the viability and activity of hematopoietic cells under normal and/or stressful conditions. However, a role for apoptosis in the embryonic hematopoietic system has not yet been established. Since the first hematopoietic stem cells (HSCs) are generated within the aortagonad-mesonephros (AGM; an actively remodeling tissue) region beginning at embryonic day 10.5, we examined this tissue for expression of apoptosis-related genes and ongoing apoptosis. Here, we show expression of several proapoptotic and antiapoptotic genes in the AGM. We also generated transgenic mice overexpressing Bcl-2 under the control of the transcriptional regulatory elements of the HSC marker stem cell antigen-1 (Sca-1), to test for the role of cell survival in the regulation of AGM HSCs. We provide evidence for increased numbers and viability of Sca-1(+) cells in the AGM and subdissected midgestation aortas, the site where HSCs are localized. Most important, our in vivo transplantation data show that Bcl-2 overexpression increases AGM and fetal liver HSC activity, strongly suggesting that apoptosis plays a role in HSC development.

Hematopoietic stem cells (HSCs) are at the foundation of the hematopoietic hierarchy and give rise to all blood lineages in the adult organism. A thorough understanding of the molecular, cellular, and developmental biology of HSCs is of fundamental importance, but is also clinically relevant for the advancement of cell replacement therapies and transplantation protocols in blood-related genetic disease and leukemias. While the major anatomical sites of hematopoiesis change during ontogeny, it was long believed that the developmental origin of the adult mammalian hematopoietic system was the yolk sac. However, current studies have shown that the first adult-type HSCs are autonomously generated in the intrabody portion of the mouse embryo, the aorta-gonads-mesonephros (AGM) region, and sublocalize to the dorsal aorta. HSCs are also found in the other large embryonic vessels, the vitelline and umbilical arteries. The intraluminal hematopoietic clusters along these vessels, together with the role of the Runx1 transcription factor in cluster and HSC formation and the HSC/endothelial/mesenchymal Runxl expression pattern, strongly suggest a vascular endothelial/mesenchymal origin for the first HSCs. Moreover, a transgenic mouse line expressing the GFP marker under the control of the Sca-1 transcriptional regulatory elements (GFP expression marks all HSCs) shows a clear localization of GFP-expressing cells to the endothelial cell layer of the dorsal aorta. Thus, highly enriched GFP-positive AGM HSCs will serve as a basis for the future examination of the cellular and molecular factors involved in the induction and expansion of adult HSCs.

BACKGROUND: Hematopoietic progenitors are generated in the yolk sac and aorta-gonad-mesonephros region during early mouse development. At embryonic day 10.5 the first hematopoietic stem cells emerge in the aorta-gonad-mesonephros. Subsequently, hematopoietic stem cells and progenitors are found in the fetal liver. The fetal liver is a potent hematopoietic site, playing an important role in the expansion and differentiation of hematopoietic progenitors and hematopoietic stem cells. However, little is known concerning the regulation of fetal liver hematopoietic stem cells. In particular, the role of cytokines such as interleukin-1 in the regulation of hematopoietic stem cells in the embryo has been largely unexplored. Recently, we observed that the adult pro-inflammatory cytokine interleukin-1 is involved in regulating aorta-gonad-mesonephros hematopoietic progenitor and hematopoietic stem cell activity. Therefore, we set out to investigate whether interleukin-1 also plays a role in regulating fetal liver progenitor cells and hematopoietic stem cells. DESIGN AND METHODS: We examined the interleukin-1 ligand and receptor expression pattern in the fetal liver. The effects of interleukin-1 on hematopoietic progenitor cells and hematopoietic stem cells were studied by FACS and transplantation analyses of fetal liver explants, and in vivo effects on hematopoietic stem cell and progenitors were studied in Il1r1(-/-) embryos. RESULTS: We show that fetal liver hematopoietic progenitor cells express the IL-1RI and that interleukin-1 increases fetal liver hematopoiesis, progenitor cell activity and promotes hematopoietic cell survival. Moreover, we show that in Il1r1(-/-) embryos, hematopoietic stem cell activity is impaired and myeloid progenitor activity is increased. CONCLUSIONS: The IL-1 ligand and receptor are expressed in the midgestation liver and act in the physiological regulation of fetal liver hematopoietic progenitor cells and hematopoietic stem cells.