Exploring the role of kit ligand at the onset of hematopoiesis

Abstract

Hematopoietic stem cells (HSCs) are generated during mammalian development from hemogenic endothelium in the major arteries through a process called endothelial-to-hematopoietic transition. The extrinsic and intrinsic factors that orchestrate this process are poorly understood. Kit ligand (KitL; also known as Stem Cell Factor/SCF) is a cytokine that plays a pivotal role in the adult bone marrow HSC niche, and it is thought to act on several cell types during embryogenesis. It is known that the absence of KitL causes a significant reduction in the HSC pool and leads to death in utero with severe anaemia. However, it is currently unclear how and when the hematopoietic defect originates in embryos lacking KitL, and what is the precise role of this cytokine in the first steps of hematopoiesis. Using a novel KitL transgenic reporter line, we show that KitL expressing cells are found in the microenvironment of all hematopoietic sites of the E8.5-E10.5 mouse embryo. We set up to investigate in detail the hematopoietic phenotype in Steel mutants at early stages. We show for the first time that Sl/Sl embryos display smaller and less proliferative hematopoietic clusters in the dorsal aorta and in the yolk sac. Already at E10.5, homozygous Steel mutants show a reduction in Pre-HSC numbers, fetal liver cellularity and yolk sac erythropoiesis. At E11.5 these mutants display decreased progenitor numbers in all hematopoietic sites. These data suggest that, contrary to earlier reports, Kit signalling plays a role in early hematopoiesis. Moreover, our data identify a previously unrecognized role for KitL in the maturation and/or expansion of hematopoietic progenitors belonging to the second yolk sac wave, that could contribute to the anaemia observed later in development. Our ongoing studies aim at elucidating the mechanism by which KitL exerts its role in early hematopoiesis, and identifying other candidate cytokines with possible synergistic effects. Hematopoietic stem cells (HSCs) are generated during mammalian development from hemogenic endothelium in the major arteries through a process called endothelial-to-hematopoietic transition. The extrinsic and intrinsic factors that orchestrate this process are poorly understood. Kit ligand (KitL; also known as Stem Cell Factor/SCF) is a cytokine that plays a pivotal role in the adult bone marrow HSC niche, and it is thought to act on several cell types during embryogenesis. It is known that the absence of KitL causes a significant reduction in the HSC pool and leads to death in utero with severe anaemia. However, it is currently unclear how and when the hematopoietic defect originates in embryos lacking KitL, and what is the precise role of this cytokine in the first steps of hematopoiesis. Using a novel KitL transgenic reporter line, we show that KitL expressing cells are found in the microenvironment of all hematopoietic sites of the E8.5-E10.5 mouse embryo. We set up to investigate in detail the hematopoietic phenotype in Steel mutants at early stages. We show for the first time that Sl/Sl embryos display smaller and less proliferative hematopoietic clusters in the dorsal aorta and in the yolk sac. Already at E10.5, homozygous Steel mutants show a reduction in Pre-HSC numbers, fetal liver cellularity and yolk sac erythropoiesis. At E11.5 these mutants display decreased progenitor numbers in all hematopoietic sites. These data suggest that, contrary to earlier reports, Kit signalling plays a role in early hematopoiesis. Moreover, our data identify a previously unrecognized role for KitL in the maturation and/or expansion of hematopoietic progenitors belonging to the second yolk sac wave, that could contribute to the anaemia observed later in development. Our ongoing studies aim at elucidating the mechanism by which KitL exerts its role in early hematopoiesis, and identifying other candidate cytokines with possible synergistic effects.