Inhibition of fibroblast activation protein partially restores the homeostatic phenotype of mechanoactivated synoviocytes.

OBJECTIVE

Synovial fibrosis, characterized by mechanoactivation of fibroblast-like synoviocytes (FLS), is thought to be a driver of osteoarthritis (OA). Fibroblast activation protein (FAP) is expressed by activated FLS and contributes to OA progression, but the mechanisms by which it does so are undefined. The objective of this study was to evaluate FAP expression in OA across species, assess FAP regulation in FLS, and determine how FAP inhibition alters the phenotype of mechanoactivated FLS.

METHODS

Synovium from human patients (n = 9) and animal models of OA (murine n = 5; porcine n = 8) was evaluated for FAP expression via immunofluorescence. An in vitro model of FLS mechanoactivation was established by plating FLS on polyacrylamide gels or glass +/-TGFβ (n = 5/group). Activated FLS were treated with a FAP inhibitor (FAPi) and phenotype was evaluated via immunofluorescence, RT-qPCR, collagen gel contraction, and FUNCAT (n = 5-7/group).

RESULTS

FAP was increased in mouse (MD: 10.3, 95% CI: [3.0, 17.6]), pig (8.7 [4.6, 12.8]), and human (24.5 [11.8, 37.3]) OA synovium and correlated with histological markers of synovial pathology. In vitro culture on stiff substrates activated FLS, and treatment with TGFβ induced FAP expression (1.8 [0.3, 3.4]). FAP inhibition partially rescued the morphological, transcriptional, and functional profile of activated FLS (e.g., reduced stress fiber formation (-0.2 [-0.35, -0.05]), αSMA expression (-2.9 [-4.9, -0.84], and contractility (-0.16 [-0.33, -0.0]).

CONCLUSIONS

The upregulation of FAP in OA synovium is conserved across species. FAP is a TGFβ-responsive driver of FLS mechanoactivation, and its inhibition partially restores the homeostatic function of FLS. Targeting FAP may represent a novel strategy to improve synovial health and mitigate joint degeneration in OA.