Senescent Fibroblasts Drive FAP/OLN Imbalance Through mTOR Signaling to Exacerbate Inflammation and Bone Resorption in Periodontitis

Fibroblast activation protein (FAP), predominantly expressed in activated fibroblasts, plays a key role in inflammatory bone diseases, but its role in periodontitis remains unclear. Accordingly, this study identified a positive association between FAP levels and periodontitis susceptibility using Mendelian randomization analysis. Human and mouse periodontitis tissues show elevated FAP and reduced osteolectin (OLN), an endogenous FAP inhibitor, indicating a FAP/OLN imbalance. Single-cell RNA sequencing revealed gingival fibroblasts (GFs) as the primary FAP and OLN source, with periodontitis-associated GFs showing increased reactive oxygen species, cellular senescence, and mTOR pathway activation. Rapamycin treatment restored the FAP/OLN balance in GFs. Recombinant FAP increased pro-inflammatory cytokine secretion and osteoclast differentiation in macrophages, exacerbating periodontal damage, whereas FAP inhibition reduced macrophage inflammation, collagen degradation, and bone resorption in experimental periodontitis. Therefore, senescent fibroblasts drive the FAP/OLN imbalance through mTOR activation, contributing to periodontitis progression. Consequently, targeting FAP may offer a promising therapeutic strategy for periodontitis.