Fibroblast growth factor 7 promotes synovial fibrosis in osteoarthritis by activating the FGFR1 signaling pathway

Synovial fibrosis is an important pathological change in osteoarthritis (OA) and is closely associated with disease progression. During the progression of synovial fibrosis in OA, fibroblasts differentiate into myofibroblasts, leading to abnormal deposition of extracellular matrix (ECM) and resulting in clinical symptoms such as joint pain and limited mobility. This study aims to investigate the role and cellular and molecular mechanisms of the fibroblast growth factor 7 (FGF7)-fibroblast growth factor receptor 1 (FGFR1) signaling axis in OA synovial fibrosis.

① By mining publicly available synovial single-cell sequencing data, combined with differential analysis, enrichment analysis, fibrosis scoring, correlation analysis, and virtual knockout techniques, the relationship between FGF7/FGFR1 and OA synovial fibrosis was explored. ② An OA mouse model was established via destabilization of the medial meniscus (DMM), and the mice receiving sham surgery served as the control group. Recombinant FGF7 protein (2 μg/10 μL) or PBS was injected into the joint cavity once weekly for 4 weeks. Functional assessments were performed at 4 (n=6) and 8 weeks (n=5) after modeling via behavioral testing, including gait analysis, hindlimb balance test, and knee joint range-of-motion evaluation. Histopathological analyses, including HE, Masson's trichrome, and Sirius Red staining, as well as immunostaining for collagen type Ⅰ alpha 1 chain (COL1A1) and alpha-smooth muscle actin (α-SMA), were conducted to evaluate synovial fibrosis and inflammatory changes. ③ EdU, CCK-8 assay, wound-healing assay, qPCR, and Western blotting were applied to investigate the role of FGF7 in fibroblast-to-myofibroblast differentiation.

① FGF7 expression was positively correlated with synovial fibrosis scores. ② Exogenous FGF7 markedly aggravated OA-related joint dysfunction in DMM mice. At 4 weeks post-DMM, compared with the PBS group, the FGF7 group showed decreased distribution of right hindlimb ground contact time (P<0.01), right hindlimb weight-bearing (P<0.01), and knee joint range-of-motion (P<0.001), with a similar trend observed at 8 weeks post-DMM. ③ FGF7 promoted ECM deposition and upregulated fibrosis markers in the synovial tissues. Histological analysis revealed that FGF7 treatment significantly aggravated synovial inflammation at 4 weeks (P<0.0001) and 8 weeks (P<0.01), and promoted ECM deposition compared with the PBS group. COL1A1 immunohistochemical staining indicated increased type Ⅰ collagen deposition in the FGF7 group at 4 (P<0.05) and 8 weeks (P<0.0001), and immunofluorescence assay showed increased α-SMA expression in the FGF7 group at 4 (P<0.001) and 8 weeks (P<0.01). ④ In in vitro study, EdU and CCK-8 assays indicated that FGF7 promoted fibroblast proliferation (P<0.05); the scratch assay showed that FGF7 enhanced fibroblast migration (P<0.01); FGF7 upregulated α-SMA (P<0.001) and COL1A1 (P<0.001) expression in fibroblasts. ⑤Mechanistically, FGF7 exerted pro-fibrotic effects through the FGFR1 pathway. FGF7 activated p-FGFR1 (P<0.05) and p-ERK (P<0.01) expression in fibroblasts; BGJ398 inhibited the FGF7-induced upregulation of p-FGFR1 (P<0.05), p-ERK (P<0.05), COL1A1 (P<0.05), and α-SMA (P<0.01) in fibroblasts.

Exogenous FGF7 exacerbates synovial fibrosis in OA mice. Our findings revealed that FGF7 promotes fibroblast proliferation, migration, and differentiation into myofibroblasts via activation of the FGFR1 pathway, thereby providing a potential novel target for the treatment of OA synovial fibrosis.