Increased osteoblast Gα₁₁ level compromises bone healing quality by suppressing high-density bone formation.

Abstract

Gα_q/11 proteins that couple G protein-coupled receptors to stimulate phospholipase C play important roles in the skeletal system as previously demonstrated by bone abnormalities when activating mutations or when overexpression of these G proteins occur in mice or humans. Here we investigated the effect of increased Gα₁₁ in osteoblastic cells on bone fracture repair in transgenic (G₁₁-Tg) mice in comparison to wild type (WT). Following stabilized tibial osteotomies in male mice, fracture healing was examined weekly over 4 weeks by micro-CT, histomorphometry, and gene expression analysis and bone biomechanics after 4 weeks. Histomorphometry showed diminished cartilage in G₁₁-Tg mice at peak soft callus formation. Histology showed diminished osteoblasts on the healing bone of G₁₁-Tg mice throughout the 4 weeks ending with lower bone volume and bone mineral content as seen by micro-CT. Consistent with the lower amounts of cartilage and bone, mRNAs encoding chondrocyte proteins, Sox 9, Col 2a1 and Col 10a1 were significantly lower in G₁₁-Tg compared to WT at week 1. From 2 to 4 weeks Runx2, the master regulator of osteoblasts, and osteocalcin, a mature osteoblast marker, were significantly lower in G₁₁-Tg than in WT. Osteoclasts were not significantly different between G₁₁-Tg and WT. After 4 weeks, torsion testing showed significantly lower yield torque and torsional stiffness in G₁₁-Tg compared to WT. Together our results show that increased Gα₁₁ inhibits endochondral bone development following fracture by suppressing both chondrocyte and osteoblast formation resulting in mechanically weaker bone.