R-spondins are newly recognized players in osteoarthritis that regulate Wnt signaling in osteoblasts

Clinical and in vitro studies suggest that altered osteogenesis or bone remodeling is involved in the progression and/or onset of osteoarthritis (OA). Wnt signaling plays a key role in osteogenesis via the canonical Wnt/β-catenin signaling pathway. Two of the R-spondins, Rspo-1 and Rspo-2, a family of 4 proteins unrelated to other Wnt ligands that act as Wnt agonists, are present in bone tissues. The purpose of this study was to investigate the potential role of Rspo-1 and Rspo-2 in OA osteoblasts.

Primary human normal and OA osteoblasts were prepared from tibial plateaus. The expression of Rspo-1 and Rspo-2 was evaluated by quantitative reverse transcription-polymerase chain reaction analysis. Western blot analysis was used to determine Rspo-2, β-catenin, and phospho-β-catenin levels. Wnt/β-catenin signaling was evaluated using the TOPflash T cell factor (TCF)/lymphoid enhancer factor (LEF) luciferase reporter assay. Mineralization was evaluated by alizarin red staining.

The expression of Rspo-1 was similar in normal and OA osteoblasts, whereas the expression and production of Rspo-2 were reduced in OA osteoblasts due to elevated levels of transforming growth factor β1 in these cells. The reduced Wnt-3a-dependent TOPflash TCF/LEF luciferase reporter activity in OA osteoblasts as compared to normal osteoblasts was corrected by the addition of recombinant human Rspo-2. Wnt-3a-dependent β-catenin levels were also corrected in OA osteoblasts by Rspo-2 addition. Wnt-3a alone increased the mineralization of OA osteoblasts, which was further increased by Rspo-2.

Reduced Rspo-2 levels in OA osteoblasts are responsible, at least in part, for their reduced Wnt/β-catenin signaling and abnormal mineralization. As Rspo-2 is a secreted soluble protein, this could lead to potential new avenues of treatment of OA.