1004 – INSIGHT ON NOTCH MECHANISMS TO ORCHESTRATE THE DEVELOPMENT OF HEMATOPOIETIC STEM CELLS IN THE EMBRYO

Abstract

A conserved Notch function is crucial for the specification and generation of Hematopoietic Stem Cells (HSC) across evolutionary distinct organisms. By using genetically modified organisms, most of the elements that are functionally relevant have been identified mainly in mouse and zebrafish. However, due to the disruptive nature of these studies, it is still uncertain the physiological sequence of events that lead to and result from this specific Notch function. This is especially important in terms of reproducing this activity in vitro. The Notch system provides cells with a binary decision mechanism that takes place among neighboring cells. In vertebrate systems, several Notch ligands and receptors co-exist in the same cell and cells compete for activating the Notch receptor that will condition the fate of that cell. In the AGM, HSCs develop from hemogenic endothelial cells that reside in the ventral side of the dorsal aorta. Cells in the developing aortic endothelium co-express different types of Notch ligands and receptors. Several groups including ours have contributed to define the elements involved in this signal, but still several gaps prevent to have a complete understanding. In addition to the Notch1 receptor, Jagged1 and hes1, we have recently shown how Dll4 has also an important structural function in the hematopoietic cluster composition of the aorta. We provided data on how blocking Dll4 with a specific antibody impinges on the number of cells recruited into the cluster and the HSC activity. Thanks to current reagents and methodologies, we now have taken a step backwards to analyze the dynamics of Notch ligands and receptors at the protein and RNA level by single cell index sorting. We are characterizing the different hemogenic and hematopoietic clusters by the expression and activity of the Notch pathway. Latest results will be presented in the meeting.