Alternative Methods of Activation of the Stem Cell Niche Reveal a Spatial Localization of Primitive Cells in the Bone Marrow

Abstract 2396

Osteoblasts are key constituents of the murine hematopoietic stem cell (HSC) endosteal niche, evidenced by the fact that increasing their activity leads to an increase in the number of HSCs. However, recent studies have also suggested a role of the bone resorbing osteoclast in the HSC niche. In these studies we wished to examine whether activation of the osteoblast or the osteoclast had differing effects on the primitive hematopoietic cell populations. To achieve this we pharmacologically activated osteoclasts and osteoblasts using receptor activator of nuclear factor-κB ligand (RANK-L) and parathyroid hormone (PTH), respectively. Our in vitro results demonstrated that both PTH and RANK-L treatment of bone marrow stromal cell populations activated osteoblasts and osteoclasts. Yet, RANK-L treatment, which is able to expand the osteoclastic population to a much higher degree, demonstrated enhanced support of primitive hematopoietic cells in cobblestone area forming cell assays. We next examined the effects of in vivo treatment with RANK-L and PTH by performing histological analyses to study the effect of these treatments on bone composition, and examining the effects on different HSC sub-populations. These data suggested that the effects of our treatment regimens were different according to the region of the bone, with RANK-L treatment having no effect or leading to a reduction in trabecular bone in the metaphysis, but an increase in bone remodeling activity at the endosteal surface of the cortical bone in the diaphysis, while PTH treatment increases bone formation and remodeling activity both in the diaphysis and the trabecular bone of the metaphysis. Correlating with this was our observations that LT-HSC frequency was increased following RANK-L and PTH treatment, yet an increased ST-HSC frequency was only observed following PTH treatment. Direct examination of HSCs in specific regions of the bone indicated that RANK-L treatment preferentially increased the LT-HSCs in the diaphysis region of the bone marrow, while PTH treatment specifically increased the ST-HSC population in the metaphysis. These data not only reveal a supportive role for osteoclasts in the HSC niche suggesting that osteoclast interaction with osteoblasts is an essential component for maintaining HSC population in the bone marrow niche, they also suggest a possible structural organization for localization of LT-HSC vs. ST-HSCs in the long bone with LT-HSCs being more localized in the endosteal surface of the diaphysis area of the long bones as oppose to the ST-HSCs having a preference for the metaphysis of these bones.