Hematopoietic Stem Cell Engraftment in Bone Marrow Is Dependent upon G_sα.

During development, hematopoietic stem cells (HSCs) translocate from the fetal liver to the bone marrow, which remains the site of hematopoiesis throughout adulthood. In the bone marrow the HSCs are located at the endosteal surface, where the osteoblasts are a key component of the stem cell niche. The exogenous signals that specifically direct HSCs to the bone marrow have been thought to include stimulation of the chemokine receptor CXCR4 by its cognate ligand stromal derived factor-1α (SDF-1α or CXCL12). However, experiments in which CXCR4^−/− fetal liver hematopoietic cells were transplanted into wild-type hosts demonstrated efficient engraftment of the HSCs in the bone marrow. In addition, treatment of HSCs with inhibitors of Gα_i-coupled signaling, which blocks transmigration towards SDF-1αin vitro, does not affect bone marrow homing and engraftment in vivo. Therefore, we examined whether G_sα-coupled mechanisms play a key role in the engraftment of the HSCs in the bone marrow environment. Utilizing an inducible-conditional knockout of G_sα, we found that deletion of the gene in hematopoietic bone marrow cells did not affect their ability to perform in the in vitro primitive CFU-C or LTC-IC assay systems. However, G_sα^−/− cells were unable to establish effective hematopoiesis in the bone marrow microenvironment in vivo in a competitive repopulation assay (41.1% contribution from wild-type cells versus 1.4% from knockout cells). These effects were not due to an inability of the cells to function in the bone marrow in vivo as deletion of G_sα following establishment of hematopoiesis had no effects on the HSCs. Examining the ability of the HSCs to home to the bone marrow, though, demonstrated that deletion of G_sα resulted in a marked impairment of the ability of the stem cells to localize to the marrow space (approximately 9-fold reduction in the level of primitive cell homing). Furthermore, treatment of BM MNCs with an activator of G_sα augmented the cells homing and thus engraftment potential. These studies demonstrate that G_sα is critical to the localization of HSCs to the bone marrow. Which receptors utilize this pathway in this context remains unknown. However, G_sα represents a previously unrecognized signaling pathway for homing and engraftment of HSCs to bone marrow. Pharmacologic activation of G_sα in HSC ex vivo prior to transplantation offers a potential method for enhancing stem cell engraftment efficiency.