R-spondin-2 is a Wnt agonist that regulates osteoblast activity and bone mass

The R-spondin family of proteins are Wnt agonists, and the complete embryonic disruption of Rspo2 results in skeletal developmental defects that recapitulate the phenotype observed with Lrp5/6 deficiency. Previous work has shown that R-spondin-2 (Rspo2, RSPO2) is both highly expressed in Wnt-stimulated pre-osteoblasts and its overexpression induces osteoblast differentiation in the same cells, supporting its putative role as a positive autocrine regulator of osteoblastogenesis. However, the role of Rspo2 in regulating osteoblastogenesis and bone formation in postnatal bone has not been explored. Here we show that limb-bud progenitor cells from Rspo2 knockout mice undergo reduced mineralization during osteoblastogenesis in vitro and have a corresponding alteration in their osteogenic gene expression profile. We also generated the first Rspo2 conditional knockout (Rspo2(floxed)) mouse and disrupted Rspo2 expression in osteoblast-lineage cells by crossing to the Osteocalcin-Cre mouse line (Ocn-Cre + Rspo2(f/f)). Ocn-Cre + Rspo2(f/f) male and female mice at 1, 3, and 6 months were examined. Ocn-Cre + Rspo2(f/f) mice are decreased in overall body size compared to their control littermates and have decreased bone mass. Histomorphometric analysis of 1-month-old mice revealed a similar number of osteoblasts and mineralizing surface per bone surface with a simultaneous decrease in mineral apposition and bone formation rates. Consistent with this observation, serum osteocalcin in 3-month-old Ocn-Cre + Rspo2(f/f) was reduced, and bone marrow-mesenchymal stem cells from Ocn-Cre + Rspo2(f/f) mice undergo less mineralization in vitro. Finally, gene expression analysis and immunohistochemistry of mature bone shows reduced beta-catenin signaling in Ocn-Cre + Rspo2(f/f). Overall, RSPO2 reduces osteoblastogenesis and mineralization, leading to reduced bone mass.