The ligand for osteoprotegerin has been identified, and it is a TNF-related cytokine that replaces the requirement for stromal cells, vitamin D3, and glucocorticoids in the coculture model of in vitro osteoclastogenesis. OPG ligand (OPGL) binds to a unique hematopoeitic progenitor cell that is committed to the osteoclast lineage and stimulates the rapid induction of genes that typify osteoclast development. OPGL directly activates isolated mature osteoclasts in vitro, and short-term administration into normal adult mice results in osteoclast activation associated with systemic hypercalcemia. These data suggest that OPGL is an osteoclast differentiation and activation factor. The effects of OPGL are blocked in vitro and in vivo by OPG, suggesting that OPGL and OPG are key extracellular regulators of osteoclast development.

Wingless (Wnt) is a potent morphogen demonstrated in multiple cell lineages to promote the expansion and maintenance of stem and progenitor cell populations. Wnt effects are highly context dependent, and varying effects of Wnt signaling on hematopoietic stem cells (HSCs) have been reported. We explored the impact of Wnt signaling in vivo, specifically in the context of the HSC niche by using an osteoblast-specific promoter driving expression of the paninhibitor of canonical Wnt signaling, Dickkopf1 (Dkk1). Here we report that Wnt signaling was markedly inhibited in HSCs and, unexpectedly given prior reports, reduction in HSC Wnt signaling resulted in reduced p21Cip1 expression, increased cell cycling, and a progressive decline in regenerative function after transplantation. This effect was microenvironment determined, but irreversible if the cells were transferred to a normal host. Wnt pathway activation in the niche is required to limit HSC proliferation and preserve the reconstituting function of endogenous hematopoietic stem cells.

To determine the role of PTHrP in fetal calcium metabolism, blood calcium was measured in mice homozygous (HOM) 45 Ca accumulation in HOM P THrP-ablated fetuses, but P THrP-(1-34), PTH-(1-84), and the diluent had no effect. Finally, similar studies were performed on fetal mice that lacked the PTH͞PTHrP receptor gene. Ionized calcium was significantly reduced in HOM PTH͞PTHrP receptor-ablated fetuses. However, 5 min after maternal injection of 45 Ca and 51 Cr, relative accumulation of 45 Ca was significantly increased in these fetuses. It was concluded that PTHrP is an important regulator of fetal blood calcium and placental calcium transport. In addition, the bioactivity of PTHrP for placental calcium transport is specified by a mid-molecular region that does not use the PTH͞PTHrP receptor.

Wnt signaling is essential for osteogenesis and also functions as an adipogenic switch, but it is not known if interrupting wnt signaling via knockout of β-catenin from osteoblasts would cause bone marrow adiposity. Here, we determined whether postnatal deletion of β-catenin in preosteoblasts, through conditional cre expression driven by the osterix promoter, causes bone marrow adiposity. Postnatal disruption of β-catenin in the preosteoblasts led to extensive bone marrow adiposity and low bone mass in adult mice. In cultured bone marrow-derived cells isolated from the knockout mice, adipogenic differentiation was dramatically increased, whereas osteogenic differentiation was significantly decreased. As myoblasts, in the absence of wnt/β-catenin signaling, can be reprogrammed into the adipocyte lineage, we sought to determine whether the increased adipogenesis we observed partly resulted from a cell-fate shift of preosteoblasts that had to express osterix, (lineage-committed early osteoblasts), from the osteoblastic to the adipocyte lineage. Using lineage tracing both in vivo and in vitro we demonstrated that the loss of β-catenin from preosteoblasts caused a cell-fate shift of these cells from osteoblasts to adipocytes, a shift that may at least partly contribute to the bone marrow adiposity and low bone mass in the knockout mice. These novel findings indicate that wnt/β-catenin signaling exerts control over the fate of lineage-committed early osteoblasts, with respect to their differentiation into osteoblastic vs. adipocytic populations in bone, and thus offers potential insight into the origin of bone marrow adiposity.