Tie21 bone marrow endothelial cells regulate hematopoietic stem cell regeneration following radiation injury

Abstract

Hematopoietic stem cells (HSCs) reside in proximity to bone marrow endothelial cells (BM ECs) and maintenance of the HSC pool is dependent upon EC-mediated c-kit signaling. Here, we used genetic models to determine whether radioprotection of BM ECs could facilitate hematopoietic regeneration following radiation-induced myelosuppression. We developed mice bearing deletion of the proapoptotic proteins, BAK and BAX, in Tie21 ECs and HSCs (Tie2Bak/BaxFl/2 mice) and compared their hematopoietic recovery following total body irradiation (TBI) with mice which retained Bax in Tie21 cells. Mice bearing deletion of Bak and Bax in Tie21 cells demonstrated protection of BM HSCs, preserved BM vasculature, and 100% survival following lethal dose TBI. In contrast, mice that retained Bax expression in Tie21 cells demonstrated depletion of BM HSCs, disrupted BM vasculature, and 10% survival post-TBI. In a complementary study, VEcadherinBak/BaxFl/2 mice, which lack Bak and Bax in VEcadherin1 ECs, also demonstrated increased recovery of BM stem/progenitor cells following TBI compared to mice which retained Bax in VEcadherin1 ECs. Importantly, chimeric mice that lacked Bak and Bax in HSCs but retained Bak and Bax in BM ECs displayed significantly decreased HSC content and survival following TBI compared to mice lacking Bak and Bax in both HSCs and BM ECs. These data suggest that the hematopoietic response to ionizing radiation is dependent upon HSCautonomous responses but is regulated by BM EC-mediated mechanisms. Therefore, BM ECs may be therapeutically targeted as a means to augment hematopoietic reconstitution following myelosuppression. © 2012 AlphaMed Press.