Identification of candidate receptors mediating the osteoanabolic influence of Wnt1

Although WNT signaling pathways are of central importance for osteoblast regulation, Wnt1 is the only known ligand for which an osteoanabolic function has been demonstrated based on human genetic evidence. The therapeutic potential of Wnt1 was further documented in a transgenic model allowing inducible Wnt1 expression in osteoblasts, which produced a marked increase in trabecular and cortical bone mass within three weeks. However, the molecular mechanisms and target cells of Wnt1 action remain unclear. In this study, we analyzed transgenic mice with Wnt1 induction for 3, 5, or 7 d and confirmed rapid osteoanabolic effects in the spine and tibia. We also observed strong recruitment of osteoblasts and progenitor cells to bone surfaces. Cell culture experiments revealed that Wnt1 primarily acts on mesenchymal progenitor cells, as only the mesenchymal cell line ST2, not MC3T3-E1 osteoblasts, responded to Wnt1. Comparing receptor expression between these lines identified several Fzd genes, including Fzd4, with higher expression in ST2 cells. Knockdown by siRNA confirmed that FZD4, together with LRP5 and LRP6, mediates Wnt1-responsiveness in ST2 cells. Although Fzd4 deletion did not impair the in vivo anabolic effects of Wnt1 overexpression, Fzd4-heterozygous mice displayed moderately reduced bone mass at 24 wk. These findings demonstrate that Wnt1 strongly influences mesenchymal osteoprogenitor cells and identify FZD4 as one candidate receptor mediating these effects.