Robin: Hematopoiesis and stem cells during embryonic development

The Robin group studies the cellular and molecular mechanisms that lead to the production and expansion of hematopoietic stem cells during embryonic development.

Figure 1: Intra-aortic hematopoietic clusters attached to the endothelial layer of the mouse embryonic aorta (Orange: CD31)

Hematopoietic stem cells (HSCs) are self-renewing multipotent cells that produce all blood cell types during the entire life of an individual. Hence, they are the only cell type that can be used to replenish the bone marrow in patients with blood-related disorders. One major challenge in the field of stem cell research is to generate large quantities of these very rare cells in vitro for research and clinical use. This is extremely difficult at present because all steps leading to HSC generation in vivo, and the precise role of the supportive surrounding microenvironment, are far from been elucidated yet. The overall research goal of the lab is to elucidate the cellular and molecular events leading to HSC production and expansion during embryonic development. We notably focus on understanding the origin of HSCs during embryonic development, the mechanisms implicated in HSC generation and expansion, the composition and function of the different HSC niches during ontogeny, and the cellular and molecular signature of HSCs and precursors.

Figure 2: Scanning electron microscopy picture of an intra-aortic hematopoietic cluster (collaboration with Dr. M. Mommaas and J. Onderwater, LUMC)

Cellular and molecular characterization of hematopoietic stem cells (HSCs) and their precursors

All HSCs are generated during embryonic development from specialized endothelial cells endowed with a hemogenic potential. We and others have shown that hemogenic endothelial cells are present in the aorta where they generate numerous Intra-Aortic Hematopoietic Clusters (IAHCs) that contain very few HSCs. Therefore, the identity of most IAHC cells is unknown. We are interested in identifying all cell types present in IAHCs, and elucidating the successive steps leading to IAHC and HSC formation, at the cellular and molecular levels.

The movie shows the dynamic transition of a hemogenic endothelial cell into a hematopoietic stem/progenitor cell in the mouse embryonic aorta (Green: Ly6A, red: CD31)

Biomechanics of HSC formation

Our research aims to uncover the essential factors that regulate embryonic HSC production. At mouse embryonic day (E)10.5, HSCs are part of intra-aortic hematopoietic clusters (IAHCs) that are tightly attached to the aortic endothelium. We recently established a novel experimental imaging set-up to show the dynamic formation of HSCs directly in the aorta of the mouse embryo. During the endothelial into hematopoietic transition (EHT), we observed intense cellular movements and surface membrane modifications, highlighting important roles for cell adhesion molecules and microtubule (MT) cytoskeleton rearrangement. We aim to understand the basic biomechanics of embryonic HSC formation with a focus on MT regulatory proteins. These proteins are especially involved in cell polarity regulation, (asymmetric) cell division, adhesion, and migration. New transgenic mouse lines will also be generated to study IAHC formation in the mouse embryo.

Figure 3: Confocal microscopy picture of a chicken embryo (Red: MEP21, blue: Runx1, green: CD45)

Role and characterization of the hematopoietic stem cell niches during embryonic development

HSC emergence mainly occurs in the ventral aspect of the embryo aorta (zebrafish, chicken, mouse and human) and is tightly controlled in time and space by regulatory signals emitted by the surrounding microenvironment. The precise, specific and well-defined localization of HSC emergence in the aorta of the aforementioned species make all these models powerful tools to trigger the molecular signals coming from the surrounding tissues and restricting hematopoietic activity to specific areas. Using state-of-the-art technology, we aim to decipher genes that are (differentially) expressed in specific regions of the aorta during HSC generation. The comparison of all species will also to unravel conserved molecular pathways involved in the regulation of the first HSC generated during embryonic development.

Origin of hematopoietic stem cells (HSCs)

The anatomical site of mammalian HSC origin remains controversial in mammals because the blood is already circulating at the time of HSC detection (leading to the possibility of cell exchange between tissues). To solve this fundamental question, we are currently developing novel and challenging in vivo embryo rescue assay.