Using blood vessel organoids to investigate acquisition of hemogenic potential during human development.

Hematopoietic stem cells (HSCs), responsible for generating all blood cells, are produced during embryonic development in the aorta of the aorta-gonad-mesonephros (AGM) region. This process involves specialized endothelial cells (ECs), called hemogenic endothelial cells (HECs), undergoing an endothelial-to-hematopoietic transition (EHT), which leads to the formation of HSC precursors (pre-HSCs) that mature into functional HSCs. However, the regulation of HEC specification and EHT remains poorly understood. The suportive microenvironment in the AGM includes various factors like growth factors, cell interactions, and biomechanical forces, such as shear stress induced by blood flow, all of which are crucial for HSC generation. In vitro production of HSCs from pluripotent stem cells has been challenging, posing a barrier to addressing the shortage of HSCs for clinical treatment of blood-related diseases. Blood vessel organoids (BVOs), derived from induced pluripotent stem cells (iPSCs), offer a promising model for studying HEC specification and EHT, as they consist of a network of arterial ECs. The first objective of this proposal is to develop a modified protocol for BVO culture to induce the formation of HECs, EHT, and the generation of (pre-)HSCs from human iPSCs. Immunostaining, flow cytometry, single-cell RNA-sequencing, and functional hematopoietic assays will be used to evaluate the potential of human (h)BVOs. Data on hBVOs will be compared with published data from human AGM populations. The second objective is to study the impact of shear stress on EHT by using a new RUNX1-GFP reporter iPSC line and time-lapse confocal microscopy. These hBVOs will be cultured in a microfluidic system to simulate pulsatile blood flow, mimicking in vivo conditions in the AGM. This approach will help develop animal-free, high-throughput models for studying and modulating endothelial and hematopoietic production in vitro, with potential applications for future clinical therapies.