Induction of an osteoarthritis-like phenotype and degradation of phosphorylated Smad3 by Smurf2 in transgenic mice.
OBJECTIVE
To determine whether Smurf2, an E3 ubiquitin ligase known to inhibit transforming growth factor beta (TGFbeta) signaling, is expressed in human osteoarthritic (OA) cartilage and can initiate OA in mice.


METHODS
Human OA cartilage was obtained from patients undergoing knee arthroplasty. Samples were graded histologically using the Mankin scale and were examined immunohistochemically for Smurf2 expression. A transgene driven by the collagen 2alpha1 promoter was used to overexpress Smurf2 in mice. Smurf2 overexpression in mouse sternal chondrocytes was confirmed by reverse transcription-polymerase chain reaction and Western blotting. Changes in articular cartilage area, chondrocyte number, and chondrocyte diameter were assessed histomorphometrically using OsteoMeasure software. Alterations in type X collagen and matrix metalloproteinase 13 (MMP-13) in articular chondrocytes were examined by in situ hybridization and immunohistochemistry, respectively. Joint bone phenotypes were evaluated by microfocal computed tomography. The effects of Smurf2 overexpression on TGFbeta signaling were examined using a luciferase-based reporter and immunoprecipitation/Western blotting.


RESULTS
Human OA cartilage strongly expressed Smurf2 as compared with nonarthritic human cartilage. By 8 months of age, Smurf2-transgenic mice exhibited decreased articular cartilage area, fibrillation, clefting, eburnation, subchondral sclerosis, and osteophytes. Increased expression of type X collagen and MMP-13 were also detected in articular cartilage from transgenic mice. Transgenic sternal chondrocytes showed reduced TGFbeta signaling as well as decreased expression and increased ubiquitination of pSmad3.


CONCLUSIONS
Smurf2 is up-regulated during OA in humans, and Smurf2-transgenic mice spontaneously develop an OA-like phenotype that correlates with decreased TGFbeta signaling and increased pSmad3 degradation. Overall, these results suggest a role of Smurf2 in the pathogenesis of OA.
Wu
Qiuqian
Q
Department of Orthopaedics, University of Rochester Medical Center, Rochester, New York 14642, USA.
Kim
Kyung-Ok
KO
Sampson
Erik R
ER
Chen
Di
D
Awad
Hani
H
O'Brien
Todd
T
Puzas
J Edward
JE
Drissi
Hicham
H
Schwarz
Edward M
EM
O'Keefe
Regis J
RJ
Zuscik
Michael J
MJ
Rosier
Randy N
RN
D000818:Animals; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism; D004195:Disease Models, Animal; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D020370:Osteoarthritis, Knee / Q000378:metabolism* / Q000473:pathology*; D051900:Smad3 Protein / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism* / Q000502:physiology; D015854:Up-Regulation
D051900:Smad3 Protein; C497791:Smad3 protein, mouse; C471641:SMURF2 protein, human; C533325:Smurf2 protein, mouse; D044767:Ubiquitin-Protein Ligases