Role of Sfrp4, a Wnt Antagonist, in Bone Repair and Regeneration

Abstract

Sfrp4 (Secreted Frizzled Receptor Protein 4) serves as a decoy receptor for Wnts and differently from sclerostin (Sost) that blocks canonical (c)Wnt signaling, it suppresses cWnt and non-cWnt cascades. Loss of function mutations in Sfrp4 cause Pyle disease, a skeletal disease-causing cortical thinning and fractures. Using Sfrp4-/- mice, we demonstrated cortical thinning is by decreased periosteal bone formation and increased endosteal remodeling through non-cWnt/Ror2/Jnk cascade activation. Given that periosteum contains stem cells which forms new bone to injury, we investigated role of activation of cWnt and non-cWnt signaling (Sfrp4-/- mice) in bone regeneration. We created calvarial critical defects (which don’t heal spontaneously) or subcritical defects (which heal spontaneously). We used Sost-/- mice for cWnt signaling activation. We confirmed cWnt activation in Sost-/- and Sfrp4-/- calvariae, and non-cWnt/Jnk activation only in Sfrp4-/- calvariae. MicroCT analyses indicate while cWnt activation (Sost deletion) favors bone regeneration (BV/TV(%)) within initial critical defect (p.0001) 6-wk after surgery, Sfrp4 deletion did not. In subcritical defects, cWnt activation (Sost-/- mice) led to accelerated bone regeneration (p.01), while Sfrp4-/- mice did not. Sfrp4 deletion leads to significant decrease in percentage and function of Cathepsin K (Ctsk+) labelled calvarial periosteal stem cells (PSCs). We investigated effect of Sfrp4 deletion in response to injury by using CtskCre;mTmG;wt and CtskCre;mTmG;Sfrp4-/- mice. Preliminary confocal analyses indicate Sfrp4 deletion impairs response of Ctsk+ PSCs. We explored whether activation of Ror2 cascade in Sfrp4 null mice might improve bone formation in subcritical defects. We generated mice lacking Ror2 receptor in Ctsk+ cells in Sfrp4 null background. Preliminary findings suggest Ror2 signaling does not impact response to injury found in Sfrp4-/- mice. Our findings demonstrate Wnt signaling fine-tuning is critical for bone responses and activation of non-cWnt signaling in Sfrp4-/- mice might be responsible for improper function of stem cells within sutures and periosteum.