title: R-spondins are newly recognized players in osteoarthritis that regulate Wnt signaling in osteoblasts.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors: Unité de Recherche en Arthrose, Centre de Recherche du Centre Hospitalier de l'Université de Montréal, Hôpital Notre-Dame, Montréal, Québec, Canada.
authors: Élie
authors: É
authors: Abed
authors: Thomas F
authors: TF
authors: Chan
authors: Aline
authors: A
authors: Delalandre
authors: Johanne
authors: J
authors: Martel-Pelletier
authors: Jean-Pierre
authors: JP
authors: Pelletier
authors: Daniel
authors: D
authors: Lajeunesse
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002478:Cells, Cultured; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000378:metabolism* / Q000473:pathology; D010006:Osteoblasts / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D015398:Signal Transduction / Q000502:physiology*; D019699:Thrombospondins / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism / Q000494:pharmacology; D051153:Wnt Proteins / Q000378:metabolism*; D060449:Wnt Signaling Pathway / Q000502:physiology; D060508:Wnt3 Protein / Q000494:pharmacology; D051176:beta Catenin / Q000378:metabolism; D000098907:R-Spondins
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D019699:Thrombospondins; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; D060508:Wnt3 Protein; D051176:beta Catenin; C503243:RSPO1 protein, human; D000098907:R-Spondins; C557408:Rspo2 protein, human; C540523:Wnt11 protein, human