High temperature requirement A1 serine protease (HtrA1) Losartan Osteoarthritis (OA) Temporomandibular joint (TMJ) Transforming growth factor-beta 1 (TGF-b1) s u m m a r y Objective: Transforming growth factor beta 1 (TGF-b1) is implicated in osteoarthritis (OA). The purpose of this study was to explore the ability of Losartan to inhibit the inflammatory signaling pathway of TGF-b1 observed during osteoarthritic progression in the temporomandibular joint (TMJ) and knee joint using a genetic mouse model. Methods: A murine OA model displaying the heterozygous chondrodysplasia gene (cho/þ), a col11a1 mutation, was used to test this hypothesis. Following a 7-month treatment period with Losartan, the synovial joints were analyzed for histopathological improvement comparing two experimental groups. Tissues were fixed in paraformaldehyde, processed to paraffin section, and stained with Safranin O and Fast Green to visualize proteoglycans and collagen proteins in cartilage. Using the Modified Mankin scoring system, the degree of staining and OA progression were evaluated. Results: Results show heterozygous animals receiving Losartan having diminished degeneration of TMJ condylar and knee joint articular cartilage. This was confirmed in the TMJ and knee by a statistically significant decrease in the Mankin histopathology score. Decreased expression of HtrA1, a key regulator to the TGF-b1 signaling pathway, was demonstrated in vitro as well as in vivo, via Losartan inhibition. Conclusion: Using a genetic mouse model of OA, this study demonstrated the utility of Losartan to improve treatment of human OA in the TMJ and knee joint through inhibition of the TGF-b1 signaling cascade. We further demonstrated inhibition of HtrA1, the lowering of Mankin scores to wild type control levels, and the limiting of OA progressive damage with treatment of Losartan.