Primitive adult hematopoietic stem cells can function as osteoblast precursors

Abstract

Osteoblasts are continually recruited from stem cell pools to maintain bone. Although their immediate precursor is a plasticadherent mesenchymal stem cell able to generate tissues other than bone, increasing evidence suggests the existence of a more primitive cell that can differentiate to both hematopoietic and mesenchymal cells. We show here that the ''side population'' (SP) of marrow stem cells, defined by their ability to rapidly expel a DNA-binding dye and to regenerate the hematopoietic compartment, can differentiate to osteoblasts through a mesenchymal intermediate. When transplanted into lethally irradiated mice, single gene-marked murine SP cells reconstituted depleted osteoprogenitor pools, such that a large proportion of the osteogenic cells in the epiphysis of long bone carried the donor SP cell marker. These findings suggest that the developmental capacity of SP cells is not restricted to the hematopoietic lineages but extends to osteogenic differentiation. This property not only elucidates a previously unrecognized step in osteoblast development, but also has intriguing implications for the use of SP cells in clinical orthopedics and stem cell-based disorders of bone. Skeletal bone is unique among human tissues. It is continuously remodeled throughout life in a process that requires the recruitment and proliferation of stem cells with the capacity to differentiate to functional osteoblasts, which then deposit and mineralize extracellular bone matrix (1, 2). The identification of an osteogenic stem cell with competency for both self-renewal and robust differentiation to bone-forming osteoblasts has been elusive. Several studies have documented the ability of cells in whole bone marrow to form osteoblasts in vitro (3) and in vivo (4). Pittenger et al. (3) isolated mesenchymal stem cells (MSCs) that were able to differentiate to chondrocytes, adipocytes, and osteoblasts in culture, whereas the results of serial transplantation of single bone marrow-derived stem cells can be interpreted to suggest the existence of rare long-term repopulating cells that can regenerate not only the entire hematopoietic system, but also several different mesenchymal lineages (5, 6).Our efforts to identify an osteogenic stem cell have focused on a side population (SP) of bone marrow cells that display strong hematopoietic reconstituting activity, as measured by competitive repopulation assays (7,8). These so-called SP cells, which can be identified by their unique capacity to efflux fluorescent DNA-binding dye (7,8), also have a limited capacity to differentiate in vivo to skeletal myocytes (9) as well as vascular endothelial cells (10), suggesting multilineage potential. To test the candidacy of these adult stem cells as progenitors of the osteoblast lineage, we tracked the fate of gene-marked SP both in vitro and in vivo. Donor-derived mesenchymal progenitors differentiated to osteoblasts in clonogenic medium, and immunostaining of long-bone sections after transplantation of SP cells into lethally irradiated mice demonst...

Characterization of circulating osteoblast lineage cells in humans

“…These findings thus lent considerable credence to the previous work of Long et al [3,4] and established that, at least in the experimental paradigm used by Dominici and colleagues [9], non-adherent bone marrow cells have a >10-fold more robust bone-repopulating activity than do adherent bone marrow stromal cells. Moreover, the findings were also consistent with previous work by Olmsted-Davis et al [10] suggesting the presence of a unique progenitor cell with both hematopoietic and osteoblastic differentiation potential in the non-adherent subset of bone marrow cells.…”

Hematopoietic cells and osteoblasts are derived from a common marrow progenitor after bone marrow transplantation

“…Thus, a reasonable hypothesis is that the hematopoietic marrow harbors a stem cell with osteogenic potential. Consistent with this idea is the observation, reported over a decade ago, that nonadherent CD34 bone marrow cells can differentiate to osteoblasts (12) and the quite recent report that murine bone marrow side population (SP) cells can engraft in bone after transplantation (13). Moreover, in our human cell therapy trials, donor osteoblast engraftment was demonstrated after transplantation of unmanipulated bone marrow (14), but the percentage of such engraftment could not be improved by transplanting as many as 5 ϫ 10 6 isolated plastic-adherent marrow stromal cells per kg of body weight, a cell number that greatly exceeds the marrow stromal cell content of unmanipulated marrow (15).…”

A small interfering RNA targeting Lnk accelerates bone fracture healing with early neovascularization

“…It has been reported that BM side population cells, which contain hematopoietic repopulating cells, can also engraft in the bone after transplantation 39 and that the non-adherent population of BM cells, including KSL cells, contains primitive cells able to generate both hematopoietic and osteocytic lineage cells. 40 Kwon et al 11 reported that Lnkdeficient EPCs (KSLs) are more potent in BM-EPC kinetics, including the ability of cell growth, endothelial commitment, mobilization and recruitment for vascular regeneration. Calvi et al 41 also found a parallel expansion of HSCs when the number of OBs was increased by parathyroid hormone infusion.…”