Transforming growth factor b (TGF-b) is well known for its important function in hematopoietic stem cell (HSC) quiescence. However, the molecular mechanism underlining this function remains obscure. Endoglin (Eng), a type III receptor for the TGF-b superfamily, has been shown to selectively mark long-term HSCs; however, its necessity in adult HSCs is unknown due to embryonic lethality. Using conditional deletion of Eng combined with serial transplantation, we show that this TGF-b receptor is critical to maintain the HSC pool. Transplantation of Eng-deleted whole bone marrow or purified HSCs into lethally irradiated mice results in a profound engraftment defect in tertiary and quaternary recipients. Cell cycle analysis of primary grafts revealed decreased frequency of HSCs in G 0 , suggesting that lack of Eng impairs reentry of HSCs to quiescence. Using cytometry by time of flight (CyTOF) to evaluate the activity of signaling pathways in individual HSCs, we find that Eng is required within the Lin2Sca 1 Kit 1 –CD482 CD150 1 fraction for canonical and noncanonical TGF-b signaling, as indicated by decreased phosphorylation of SMAD2/3 and the p38 MAPK-activated protein kinase 2, respectively. These findings support an essential role for Eng in positively modulating TGF-b signaling to ensure maintenance of HSC quiescence.
Bibliographical noteFunding Information:
This work was supported by National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (NHLBI) grants R01-HL085840 and U01-HL100407, and NIH, National Institute of Arthritis and Musculoskeletal and Skin Diseases grants R01-AR055299 and R01-AR071439 (R.C.R.P.), and NIH, NHLBI grant T32HL007062 (J.B.). J.B. is the recipient of a Hartwell Foundation Fellowship.
© 2019 by The American Society of Hematology.