Nell-1 regulates fibrocartilage stem cells and alleviates temporomandibular joint osteoarthritis progression by inhibiting hedgehog pathway

BACKGROUND: Nel-like molecule-1 (Nell-1) could prevent the damage of cartilage in osteoarthritis (OA), including temporomandibular joint osteoarthritis (TMJOA), though, the definite mechanism of the process is unclear. Fibrocartilage stem cells (FCSCs) were discovered in the temporomandibular joint (TMJ), and were associated with the TMJ health and disease. However, little information is available on the effect of Nell-1 on FCSCs fate in TMJOA progression. METHODS: In this study, TMJOA was induced in rabbits through collagenase injection. 8 weeks after injection, the left and right joints of the rabbits were injected with AAV9-CMV-Nell-1 and AAV9 vector virus supernatant respectively. FCSCs were isolated, cultured, and transfected with recombinant AdNell-1. Histology staining, immunofluorescence staining, micro-CT, qRT-PCR and western blot were applied to detect the condition of FCSCs and condyles in vitro and in vivo. RESULTS: In the collagenase injection group, we observed histological changes consistent with OA, meanwhile, the expression of Nell-1 was decreased, but Indian hedgehog (Ihh) and Sonic hedgehog (Shh) were significantly elevated in FCSCs and condylar cartilage. Further in vitro and in vivo analyses revealed that Nell-1 markedly reduced Ihh and Shh. In addition, Nell-1 induced the chondrogenic differentiation of FCSCs by stimulating the expression of Collagen II, Aggrecan and Sox-9. CONCLUSION: Nell-1 leads to improved chondrogenic capacity of FCSCs in TMJOA and demonstrated an inhibitory effect on Hh signal transduction while concurrently facilitating the process of cartilage remodeling. Collectively, Nell-1 may be a promising drug candidate for the treatment of TMJOA.