title: BAPX-1/NKX-3.2 acts as a chondrocyte hypertrophy molecular switch in osteoarthritis.
abstract: OBJECTIVE
  Osteoarthritis (OA) development involves a shift of the articular chondrocyte phenotype toward hypertrophic differentiation via still poorly characterized mechanisms. The purpose of this study was to test our hypothesis that the function of BAPX-1/NKX-3.2 is impaired in OA chondrocytes and leads directly to loss of hypertrophic protection of the articular chondrocyte, which is central in the changing chondrocyte phenotype that drives OA.
  
  
  METHODS
  Human articular chondrocytes (HACs; from healthy and OA donors) and SW-1353 chondrocytic cells were exposed to bone morphogenetic protein 7 (BMP-7), interleukin-1β (IL-1β), tumor necrosis factor, or OA synovial fluid (SF; 20% [volume/volume]). Loss-of-function and gain-of-function experiments for BAPX-1/NKX-3.2 were performed. Mouse experimental models of OA were used, and (immuno)histochemistry of tissue sections was performed. Gene and protein expression of BAPX-1/NKX-3.2 and chondrogenic, hypertrophic, and OA-related mediators were determined by real-time quantitative polymerase chain reaction analysis and immunoblotting. In addition, alkaline phosphatase (AP) activity and prostaglandin E2 levels were measured.
  
  
  RESULTS
  BAPX-1/NKX-3.2 expression correlated negatively with expression of chondrocyte hypertrophic markers (RUNX-2, COL10A1, AP), cartilage-degrading enzymes (matrix metalloproteinase 13, ADAMTS-5), and mediators of inflammation (cyclooxygenase 2, IL-6) in healthy and OA chondrocytes, as well as in OA induced chondrocytes. BAPX-1/NKX-3.2 positivity was diminished in articular chondrocytes in the knee joints of mice with experimental OA. Knockdown of BAPX-1/NKX-3.2 in HACs did not influence the expression of SOX9, COL2A1, or aggrecan, but led to an acute hypertrophic shift in the HAC phenotype. Overexpression of BAPX-1/NKX-3.2 decreased hypertrophic gene expression in HACs. Furthermore, the hypertrophic OA chondrocyte phenotype could be counteracted by overexpression of BAPX-1/NKX-3.2 and by BMP-7 in a BAPX-1/NKX-3.2 dependent manner.
  
  
  CONCLUSIONS
  Our findings indicate that BAPX-1/NKX-3.2 is a molecular switch that is involved in controlling the hypertrophic phenotype of the postdevelopmental (OA) articular chondrocyte.
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Marjolein M J
authors.initials: MM
authors.lastname: Caron
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Pieter J
authors.initials: PJ
authors.lastname: Emans
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Don A M
authors.initials: DA
authors.lastname: Surtel
authors.affiliation: Radboud University Medical Centre, Nijmegen, The Netherlands.
authors.forename: Peter M
authors.initials: PM
authors.lastname: van der Kraan
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Lodewijk W
authors.initials: LW
authors.lastname: van Rhijn
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Tim J M
authors.initials: TJ
authors.lastname: Welting
mesh_terms: D000818:Animals; D001169:Arthritis, Experimental / Q000378:metabolism* / Q000473:pathology; D055419:Bone Morphogenetic Protein 7 / Q000494:pharmacology; D002358:Cartilage, Articular / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D002454:Cell Differentiation; D049108:Cell Enlargement / Q000187:drug effects; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D018398:Homeodomain Proteins / Q000378:metabolism*; D006801:Humans; D053583:Interleukin-1beta / Q000494:pharmacology; D051379:Mice; D010003:Osteoarthritis / Q000378:metabolism* / Q000473:pathology; D014157:Transcription Factors / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
chemical_list: D055419:Bone Morphogenetic Protein 7; D018398:Homeodomain Proteins; D053583:Interleukin-1beta; C491536:NKX3-2 protein, human; D014157:Transcription Factors; D014409:Tumor Necrosis Factor-alpha