Pro-osteogenic Effects of WNT in a Mouse Model of Bone Formation Around Femoral Implants

Wnt signaling maintains homeostasis in the bone marrow cavity: if Wnt signaling is inhibited then bone volume and density would decline. In this study, we identified a population of Wnt-responsive cells as osteoprogenitor in the intact trabecular bone region, which were responsible for bone development and turnover. If an implant was placed into the long bone, this Wnt-responsive population and their progeny contributed to osseointegration. We employed Axin2Cre^CreERT2/+;R26^mTmG/+ transgenic mouse strain in which Axin2-positive, Wnt-responsive cells, and their progeny are permanently labeled by GFP upon exposure to tamoxifen. Each mouse received femoral implants placed into a site prepared solely by drilling, and a single-dose liposomal WNT3A protein was used in the treatment group. A lineage tracing strategy design allowed us to identify cells actively expressing Axin2 in response to Wnt signaling pathway. These tools demonstrated that Wnt-responsive cells and their progeny comprise a quiescent population residing in the trabecular region. In response to an implant placed, this population becomes mitotically active: cells migrated into the peri-implant region, up-regulated the expression of osteogenic proteins. Ultimately, those cells gave rise to osteoblasts that produced significantly more new bone in the peri-implant region. Wnt-responsive cells directly contributed to implant osseointegration. Using a liposomal WNT3A protein therapeutic, we showed that a single application at the time of implant placed was sufficient to accelerate osseointegration. The Wnt-responsive cell population in trabecular bone, activated by injury, ultimately contributes to implant osseointegration. Liposomal WNT3A protein therapeutic accelerates implant osseointegration in the long bone.