3063 – APELIN RECEPTOR EXPRESSING CELLS ARE PART OF THE IAHC MICROENVIRONMENT AND ACCELERATE HEMATOPOIETIC DIFFERENTIATION

• Published in: Experimental hematology 2024-08, Vol.137, p.104385, Article 104385
• Main Authors: Dzierzak, Elaine Anne, Popravko, Anna, Vink, Chris, Schumacher, Linus, Forrester, Lesley, Mackintosh, Lorna
• Format: Article
• Language: English
• ISSN: 0301-472X
• DOI: 10.1016/j.exphem.2024.104385

Apelin receptor (AplnR/AR/APJ/AGTRLl1) is a class A, or rhodopsin-like, G-protein coupled receptor (GPCR) and is known to be highly expressed on embryonic aorta (AGM) CD31+CD45- cells. Transcriptomic signatures on highly enriched IAHC cells revealed a cluster of AplnR expressing CD31+ckit+ cells that are Runx1 negative/low expressing. Here, we isolate these AplnR expressing intra-aortic hematopoietic cluster cells (IAHC; CD31+cKit+) and characterize the phenotype and function so as to position them in relation to the heterogeneous hematopoietic populations co-existing within IAHCs. We show that AplnR is expressed during hematopoietic differentiation of mESC and that CD31+cKit+AplnR+ cells have no hematopoietic function. Although at odds with a previous report (Jackson et al.) that functional hematopoietic cells can be enriched by selection of AplnR+ cells, this can be explained by the fact that different subpopulations of hematopoietic cells and mesodermal cells express AplnR. We found that IAHCs consist of AplnR+ and AplnR- cells and that these cells are distributed through all major sites of embryonic hematopoiesis. AplnR expression can be gained and/or lost on CD31+cKit+ cells in culture, but this may not be the case in vivo. When sorted AplnR IAHC were tested for in vivo reconstitution of adult recipients, they were found to be negative for HSC activity. Although bearing a pre-HSC phenotype, they are not functional pre-HSCs. However, when tested in a co-culture system, we found that they accelerate the differentiation of AplnR negative pre-HSCs, strongly suggesting that in the embryonic aorta, AplnR+ IAHCs function as part of the hematopoietic microenvironment. Funding: Wellcome Trust Tissue Repair PhD training fellowship, Bloodwise/BCUK Grant 18010, ERC Advanced Grant 341096.