Wnt3a reestablishes osteogenic capacity to bone grafts from aged animals.

Background: Age-relatedfattydegenerationofthebonemarrowcontributestodelayedfracture-healingandosteoporosisrelatedfracturesintheelderly.Themechanismsunderlyingthisfattychangeareunknown,buttheymayrelatetothelevelof Wnt signaling within the aged marrow cavity. Methods: Transgenic mice were used inconjunctionwitha syngeneic bone-graftmodel tofollowthefatesof cellsinvolvedin the engraftment. Immunohistochemistry along with quantitative assays were used to evaluate Wnt signaling and adipogenic and osteogenicgeneexpressioninbonegraftsfromyoungandagedmice.LiposomalWnt3aprotein(L-Wnt3a)wastestedforitsability to restore osteogenic potential to aged bone grafts in critical-size defect models created in mice and in rabbits. Radiography, microquantitative computed tomography (micro-CT) reconstruction, histology, and histomorphometric measurements were used to quantify bone-healing resulting from L-Wnt3a or a control substance (liposomal phosphate-buffered saline solution [L-PBS]). Results: Expression profiling of cells in a bone graft demonstrated a shift away from an osteogenic gene profile and toward an adipogenic one with age. This age-related adipogenic shift was accompanied by a significant reduction (p < 0.05) in Wnt signaling and a loss in osteogenic potential. In both large and small animal models, osteogenic competence was restored to aged bone grafts by a brief incubation with the stem-cell factor Wnt3a. In addition, liposomal Wnt3a significantly reduced cell death in the bone graft, resulting in significantly more osseous regenerate in comparison with controls. Conclusions: Liposomal Wnt3a enhances cell survival and reestablishes the osteogenic capacity of bone grafts from aged animals. Clinical Relevance: We developed an effective, clinically applicable, regenerative medicine-based strategy for revitalizing bone grafts from aged patients.