NERVE GROWTH FACTOR IS SUFFICIENT TO CAUSE MULTIPLE OSTEOARTHRITIS-RELEVANT PATHOLOGICAL FEATURES IN NAÏVE MURINE KNEE JOINTS.

BACKGROUND: Nerve growth factor (NGF), a key mediator of pain and inflammation, is increased in joints with osteoarthritis (OA). Neutralizing NGF with monoclonal antibodies has shown analgesic effects in painful knee OA, but clinical development was stopped due to side effects in the joints. Knowledge about the biological effects of long-term exposure of joint tissues to NGF is limited. Therefore, we aimed to explore the effects of repeated intra-articular (IA) injections of NGF into the knee joints of healthy mice on pain and sensitization, as well as joint innervation and structure. METHODS: We conducted five experiments in male C57BL/6 mice. In Experiment 1, NGF (50ng or 500ng) or vehicle was injected IA into the knee of naive wildtype (WT) mice, twice a week for 4 weeks. We assessed knee swelling, knee hyperalgesia and histopathology. In Experiment 2, mice were injected with 500ng NGF or vehicle, twice a week for 4 weeks and microCT of the knee was performed. In Experiment 3, Na(V)1.8-tdTomato reporter mice were injected with 500ng NGF or vehicle, twice a week for 4 weeks, and joint innervation was assessed. In Experiment 4, WT mice received 500ng NGF or vehicle twice a week for 4 weeks and were used for single cell RNA sequencing (scRNAseq) of the synovium. In Experiment 5, L3-L5 DRGs of mice that received 3 IA injections of 500ng NGF or vehicle twice a week were used for bulk RNA sequencing. RESULTS: Repeated bi-weekly IA injections of NGF caused knee hyperalgesia in naïve mice. NGF caused dose-dependent knee swelling, synovial pathology, increased bone mineral density and trabecular bone thickness in the medial subchondral bone, growth of pre-osteophytes in the medial compartment, but no cartilage degeneration. NGF injection caused sprouting of Na(V)1.8+ neurons in the medial but not the lateral synovium. ScRNAseq of the synovium revealed upregulated genes related to neuronal sprouting, synovial fibrosis and ossification, confirming histopathological findings. Bulk RNA seq of DRG showed upregulated pathways related to axonal growth. CONCLUSIONS: In healthy mouse knees, NGF induced mechanical sensitization, synovitis, neoinnervation in the medial synovium, subchondral bone changes and pre-osteophyte growth in the medial compartment, thus capturing many pathological changes observed in OA, except cartilage damage.