The role of Wnt10b in post natal bone homeostasis and maintenance of mesenchymal progenitor cells

Mesenchymal Progenitor Cells (MPCs) contribute to the development and maintenance of bone, fat, muscle and cartilage in mammals. Several lines of evidence implicate canonical Wnt signaling in maintenance of these stem cell populations. Wnt10b is a canonical Wnt ligand expressed in developing bone, calvarial osteoblasts, and multipotential mesenchymal progenitors isolated from adult bone marrow. Here we demonstrate that Wnt10b null mice exhibit defects in both intramembranous and endochondral ossification that present as cranial suture agenesis at post natal day four and an age progressive loss of trabecular bone starting between one and two months of age. Maintenance of normal adult bone requires both copies of the Wnt10b gene as heterozygous animals express similar reductions in trabecular bone density in aged mice. No difference was observed in osteoclast number or activity as assessed by analysis of serum CTx levels indicating that the age progressive osteopenia in Wnt10b null mice is not due to increased bone resorption but rather is the result of decreased bone deposition. Using in vitro colony forming unit assays we show that the loss in trabecular bone and suture agenesis is associated with a reduction in the number of bone marrow derived mesenchymal progenitors and MPC lineage derived osteoblasts and adipoblasts and calvaria derived osteoprogenitors. Analysis of osteogenic gene expression in primary bone marrow stromal cells and calvarial osteoblasts demonstrates reductions in expression of several osteoblast differentiation markers but no change in Runx2, an osteoblast associated transcription factor expressed in multi-potent MPC. Additionally, Wnt10b null bone marrow derived progenitors and calvarial derived osteoblasts express reduced levels of bone morphogenic proteins (Bmp's) and Bmp response genes. Stimulation of the Wnt pathway in primary calvarial osteoblasts results in up regulation of Bmp4 and associated Bmp responsive genes showing that Wnt10b is sufficient to induce expression of Bmp4 and Bmp target genes in calvarial osteoblasts. Taken together, this work indicates that Wnt10b is a critical Wnt signaling ligand, required for mesenchymal progenitor activity in both calvarial morphogenesis and maintenance of adult bone and suggest a role for Wnt10b in maintenance of immature osteo and mesenchymal progenitors capable of depositing bone.