Spontaneous Facet Joint Osteoarthritis in NFAT1-Mutant Mice

ObjectivesAlthough rodent models of traumatically or chemically induced intervertebral facet joint osteoarthritis (FJOA) were previously described, the characteristics of spontaneous FJOA animal models have not been documented. This study aimed to identify the characteristics of a murine model of spontaneous FJOA and its underlying mechanisms. MethodsThe lumbar facet joints of mutant mice carrying a disrupted NFAT1 (nuclear factor of activated T cells 1) allele and of wild-type control mice were examined by histochemistry, quantitative gene expression analysis, immunohistochemistry, and histomorphometry using a novel FJOA scoring system at 2, 6, 12, and 18 months of age. The reproducibility of the FJOA scoring system was analyzed by inter-observer and intra-observer variability tests. Tissue-specific histomorphometric and gene expression changes were statistically analyzed. ResultsNFAT1-mutant facet joints displayed dysfunction of articular chondrocytes and synovial cells with aberrant gene and protein expression in cartilage and synovium as early as 2 months, followed by osteoarthritic structural changes such as articular surface fissuring and chondro-osteophyte formation at 6 months. Deeper cartilage lesions, synovitis, separation of cartilage from thickened subchondral bone, and tissue-specific molecular and cellular alterations in NFAT1-mutant facet joints became evident at 12 and 18 months. Osteoarthritic structural changes were not detected in wild-type facet joints at any ages, though age-related cartilage degeneration was observed at 18 months. ConclusionsUsing NFAT1-mutant mice, this study has identified for the first time an animal model of spontaneous FJOA with age-dependent osteoarthritic characteristics, developed the first FJOA scoring system, and elucidated the molecular mechanisms of NFAT1 mutation-mediated FJOA.