gms | German Medical Science

Objectives: Besides considerable improvement in fracture care, still 5-10% of all fractures heal poorly. Therefore, there is the urgent need to find new target molecules to improve fracture healing. The Wnt-signaling pathway and its molecules are promising candidates, since many studies have shown its strong osteoanabolic effect. One activator of the Wnt-signaling cascade, Wnt1, has recently gained attention for its extremely strong positive effect on the intact skeleton. However, the role of Wnt1 during fracture healing is unclear. Therefore, the aim of the present study was to investigate if Wnt1 might be a promising target to accelerate fracture healing, both in skeletally healthy and osteoporotic mice, which display a diminished healing capacity.

Methods: Transgenic animals for a temporary induction of Wnt1 specifically in osteoblasts by removal of Doxycyclin from the mouse diet were used (Col1a-rtTA+/tg and ptet-Wnt1+/tg). 12 weeks old female mice were anaesthetized and sham-operated or ovariectomized (OVX) to induce postmenopausal osteoporosis. 8 weeks later, the right femur was stabilized by an external fixator and osteotomized. Two days prior to the surgery, the mouse diet was changed to induce Wnt1-overexpression. After 14 and 21 days, mice were euthanized. Fractured and intact femurs were harvested and tested for their bending stiffness. Afterwards, bones were fixed in 4% PFA for µCT and histological analysis. Another subset of animals was euthanized on day 14 and the fracture callus was harvested for RNASeq analysis. Statistical analysis: One-way-ANOVA, Tukey's post-hoc test. p<0.05. n=5-8.

Results and conclusion: On day 14 after fracture, sham-OVX and OVX Wnt1-tg animals displayed an increased bone formation in the fracture callus and increased numbers and surface of osteoblasts compared to control mice. On day 21 after fracture, sham-OVX and OVX Wnt1-tg animals displayed significantly larger fracture calli with a 3-fold increased bone content. Absolute and relative bending stiffness of the fractured femurs were significantly increased. Histological analysis revealed that numbers and surface of osteoblasts were significantly increased, while number and surface of osteoclasts were significantly decreased. Furthermore, cartilage area was significantly higher in Wnt1-tg mice, indicating a delayed cartilage remodeling. RNASeq analysis of fracture callus tissue revealed that Hippo/YAP-signaling and BMP-signaling pathways were highly enriched in Wnt1-tg animals.

We found that inducible osteoblast-specific overexpression of Wnt1 during fracture healing very strongly increased osteoblast activity and bone formation in the fracture callus. These data indicates that Wnt1 is indeed a very promising candidate to accelerate fracture healing both under healthy and osteoporotic conditions. Molecular analyses revealed that activation of Hippo/YAP signaling and subsequent BMP overexpression might be the reason for the increased osteoblast activity and bone formation in the fracture callus.