Background Although many osteoarthritis (OA) patients show significant synovial involvement, consequences are largely unknown. We found highly induced expression of canonical Wnts 2b and 16 and WISP1, a downstream protein, in knee joints in two experimental murine OA models. Wnt signaling has been implicated in OA incidence and modulation of the β-catenin pathway leads to OA-like changes in cartilage. In addition, TGF-β signaling is critical in cartilage maintenance. TGF-β signals via both ALK5 and ALK1 and downstream via Smad 2/3 and Smad 1/5/8 respectively.
Objectives Investigate the potency of canonical Wnts, produced in the synovium, to induce OA pathology and whether canonical Wnts skew TGF-β signaling from the protective Smad 2/3 pathway towards the chondrocyte hypertrophy-inducing Smad 1/5/8 pathway.
Methods Pathway analysis of microarray data was done using DAVID software. Detection of Smad 2/3 and Smad 1/5/8 phosphorylation was done by Western blot analysis. In vivo overexpression of genes from the canonical Wnt signaling pathway was achieved by intra-articular injection of adenoviral vectors that specifically target synovial cells, due to their size. Joint pathology was assessed by histology. Gene expression was analyzed by qPCR.
Results Pathway analysis using DAVID showed that both the Wnt and TGF-β signaling pathway were enriched in the synovium of mice with collagenase-induced OA. To determine whether synovial overexpression of canonical Wnts leads to cartilage damage, we injected adenoviral vectors for Wnt8a and Wnt16 into murine knee joints. A significant induction of OA pathology was found at the medial margin of the medial tibial plateau 7 days after overexpression. The incidence was 92% for Wnt8a overexpression compared to 17% (both N=12) for the control virus and 80% for Wnt16 overexpression, but only 20% (both N=5) for the control virus. Because of their relatively small size, Wnts and WISP1 proteins can migrate into the cartilage and possibly alter chondrocyte phenotype. Synovial overexpression of Wnt8a and Wnt16 led to β-catenin accumulation in chondrocytes, a tell-tale sign of canonical Wnt signaling, indicating diffusion of Wnts to the cartilage. Moreover, in vitro overexpression of canonical Wnts and WISP1 in human chondrocytes led to a significant increase of collagen type I and a significant decrease in type II collagen expression, suggesting a loss of chondrocyte phenotype. Pre-incubation with Wnt3a or WISP1 alone or Wnt3a + WISP1 together resulted in decreased TGF-β-induced phosphorylation of Smad 2/3, whereas phosphorylation of Smad 1/5/8 was increased in vitro. This implies a shift towards dominant TGF-β signaling via the hypertrophy-inducing ALK1 pathway. Moreover, the expression of the anti-hypertrophic factor Sox9 was decreased after pre-incubation with Wnt3a and WISP1.
Conclusions Canonical Wnts produced in the synovium may play an important role in OA pathology by inducing β-catenin signaling in the cartilage followed by cartilage damage. Synovial overexpression of canonical Wnts, as found in experimental OA, may lead to chondrocyte phenotype changes, probably via modulation of the important TGF-β signaling pathway. This underlines that synovial Wnt/WISP1 expression may be a potential target for OA therapy.
Disclosure of Interest None Declared
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