A Critical Role for BMP Signaling in Adult Hematopoietic Stem Cells

Abstract 1194

The Bone Morphogenetic Proteins (BMPs), which belong to the TGF-beta superfamily of ligands, figure prominently during development and are involved in a wide variety of biological processes throughout life. BMP ligands signal via Type I and Type II receptors, both of which are required at the cell surface for propagation of the signal intra-cellularly. Upon receptor activation, both the Smad1/5/8 pathway and the Tak1 MAPK circuitry can be activated, ultimately leading to transcriptional regulation of target genes (Blank et al., Development 2009). Although the BMP pathway plays a role during embryonic development of hematopoiesis, its role in adult hematopoiesis has remained elusive. Previous studies of the Smad1/5/8 pathway have indicated that this pathway is not involved in regulation of adult hematopoietic stem cells (HSCs) in vivo. However, previously published findings demonstrate that the BMP Type II receptor (BmprII) is highly expressed in HSCs, suggesting that BMPs may still play a role in adult HSC regulation via Smad-independent mechanisms. To fully elucidate the role of BMP signaling in hematopoietic cells, we utilized a conditional knockout mouse model targeted to the BmprII gene by Vav-Cre-mediated deletion. Steady state hematopoiesis was essentially normal in BmprII knockouts, but the more primitive LSK population in the bone marrow (BM) was significantly reduced in knockouts compared to littermate controls at 16 weeks of age (0.107% of BM vs. 0.133%, p≤0.05, n=8–10). This reduction in primitive cells translated functionally into a reduced colony forming capacity in vitro (86 colonies/90 000 cells plated vs. 112/90 000 cells plated for controls, p≤0.05, n=8–10). Additionally, when hematopoietic cells were challenged in vivo by transplanting 0.2×10e6 knockout or littermate control whole BM cells in a competitive fashion with 0×10e6 wild type whole BM cells into lethally irradiated recipient mice, the regenerative capacity of BmprII knockout cells was significantly reduced both short term in peripheral blood, at 4 weeks post transplantation (36.5% vs. 48.6% donor-derived cells, p≤0.05, n=7 donors per genotype), and long term in the BM at 16 weeks post transplantation (40.9% vs. 63.4% donor-derived cells, p≤0.05, n=7 donors per genotype). Furthermore, we found a reduction in the myeloid compartment in the BM of BmprII donor recipients at 16 weeks post transplantation (40.3% vs. 64.5% Gr1+/Mac1+ cells of the donor population, p≤0.05, n=7 donors per genotype) coupled with an increase in B-lymphoid cells (46.7% vs. 26.3% B220+ cells of the donor population, p≤0.05, n=7 donors per genotype). To quantify more primitive cells, LSK SLAM FACS analysis was performed, revealing a significant decrease in the numbers of LSK cells (3508 cells vs. 12022 cells per femur, p≤0.05, n=7 donors per genotype), as well as LSK SLAM cells (542 vs. 3023 cells per femur, p≤0.05) derived from BmprII donors. Our studies indicate that the BMP circuitry plays a critical role in HSC regulation and that inactivation of this pathway at the receptor level results in a reduced regenerative capacity in vivo.