Background Previously, we found strong upregulation of various members of the Wnt signaling pathway in knee joints in two osteoarthritis (OA) models. Wnt signaling has been implicated in OA through activation of the β-catenin pathway. Modulation of this pathway leads to OA-like changes in cartilage. However, the role of the synovium in the induction of OA pathology under the influence of Wnt signaling is unclear.
Objectives To investigate the potential of Wnt signaling to increase the expression of cartilage-degrading enzymes in the synovium.
Methods Pathway analysis of microarray data from the synovium of a collagenase-induced OA mouse model was performed using DAVID software. In vivo synovial overexpression of genes was achieved by intra-articular injection of adenoviral vectors. Human OA synovial tissue was collected from joint replacement surgery and stimulated directly or after outgrowth of fibroblasts. Monocytes were isolated from buffy coats and stimulated directly or after differentiation into macrophages. Joint pathology was assessed by histology. Gene expression was analyzed by qPCR.
Results Pathway analysis showed that Wnt signaling was enriched in the synovium during experimental OA. To determine if Wnt signaling increased the expression of cartilage-degrading enzymes, we stimulated human OA synovium with Wnt3a or WISP1. We found increased expression of MMP3, MMP9 and MMP13, whereas expression of the MMP inhibitors TIMP1 and 3 was not altered. Next, we investigated which cell-type in the synovium might have caused the increased MMP expression. Whereas stimulation of human synovial OA fibroblasts with Wnt3a or WISP1 increased the expression of MMPs, stimulation of M1 and M2 macrophages did not alter MMP expression. The expression of TIMPs was not altered in both cell types. In contrast to mature macrophages, stimulation of monocytes, present in the early phases of our OA models, strongly increased the expression of MMP3, 9 and 13, whereas TIMP levels were not altered. Next, we hypothesized that increased Wnt signaling is present in OA synovium, which we investigated by blocking this pathway. Inhibition by FrzB and DKK-1, a specific inhibitor for canonical Wnt signaling, led to decreased MMP expression in human synovial specimen. To determine if synovial overexpression of Wnt proteins leads to cartilage damage in vivo, we injected adenoviral vectors for Wnt8a and Wnt16 into murine knee joints, which specifically target the synovium. Seven days after overexpression, we found a significant induction of OA pathology at the medial tibia, a preferential site for damage in experimental OA. Lesions were found in 92% (n=12) of the knee joints after Wnt8a overexpression compared to 17% (N=12) for its control virus and 80% (N=5) for Wnt16 overexpression, but only 20% (N=5) for its control virus. Furthermore, we found VDIPEN neo-epitopes, reflecting MMP activity, in the cartilage and especially around the erosive cartilage lesions.
Conclusions Synovial expression of members of the Wnt signaling pathway may play an important role in OA pathology by increasing the expression of MMPs in the synovium. In addition, synovium-specific overexpression of Wnt signaling members induces cartilage damage in vivo. This underlines synovial Wnt/WISP1 expression to be a potential target for OA therapy.
Disclosure of Interest None declared