Sfrp4 Deficiency Prevents Bone Regeneration of Calvarial Critical Defect

Objectives: Our studies have established that loss of function mutations in the Wnt inhibitor Secreted Frizzled Receptor Protein 4 (SFRP4) are the cause of Pyle’s disease, a rare skeletal disease characterized by limb deformity and fragility fractures and a variable degree of cranial abnormalities, mainly characterized by decreased calvarial thickness and increased porosity (OMIM-265900). Sfrp4 binds directly to Wnts, interfering with their ability to interact with their receptor complexes thus blocking both canonical and non-canonical Wnt cascades. Evidences have shown that stem cells in the calvarium suture and periosteum support bone repair in response to injury. Due to limitations, reconstruction of large craniofacial defects remains challenging. If a key role for canonical Wnt signaling in this process has been demonstrated, our understanding of the role of non-canonical Wnt signaling is limited. To examine whether Sfrp4 impacts cells involved in bone repair and regeneration in adult bones, we performed an injury repair model.
Methods: We created a 2.3mm diameter calvarial critical defect in the left parietal bone 2mm away from the sagittal suture. Micro-CT analysis 6 weeks post-surgery was performed to assess new bone formation within the defect (BV/TV%). After micro-CT analysis, skulls were decalcified and processed for paraffin embedding and sectioning and H & E staining performed. We used Sost^-/- mice in which critical-sized defects heal, as a control for our methods, and as a model for canonical Wnt signaling activation.
Results: Confirming previous findings, activation of canonical Wnt signaling (Sost deletion) allows for bone regeneration within the initial defect (Figure 1A). In contrast, deletion of Sfrp4 (thereby activation of both canonical and non-canonical Wnt signaling) does not in male and female mice (Figure 1B). These data suggest that non-canonical Wnt signaling might be responsible for improper function of stem cells within the sutures and/or progenitors in the periosteum and needs to be shut-down by Sfrp4 to ensure bone repair.
Conclusions: These data therefore suggest that non-canonical Wnt signaling might be responsible for improper function of stem cells within the sutures and/or progenitors in the periosteum and needs to be shut-down by Sfrp4 to ensure bone repair.