Concurrent Oral 11 - Osteoarthritis [OP73-OP78]: OP73. Mechanical Load Drives Inflammatory Gene Expression and Disease in Murine OA

Abstract

Background: The role of mechanical factors in OA pathogenesis is undisputed, but how these factors drive the processes that lead to joint disease is unknown. What is clear is that OA is not simply a disease of cartilage attrition as a result of abnormal or repetitive wearing of the surfaces, but that it requires activation of pathways that lead to expression of degradative enzymes, which break down the matrix. We have established a murine surgical model of OA, that causes destabilization of the medial meniscus (DMM) and which produces robust cartilage degeneration within 8–12 weeks of surgery. The aim of this study was to examine the early expression of inflammatory genes in the joints of OA mice and to assess the influence of joint loading on gene expression and the development of OA. Methods: DMM surgery was performed on 10 week old male C57Bl6 mice by cutting the right menisco-tibial ligament. Some mice underwent sham surgery where the capsule of the joint was opened but the menicso-tibial ligament was left intact. The joints of these mice were either left for up to 12 weeks and sectioned for histological scoring or had RNA extracted at early time points post surgery. Microarray analysis was performed and regulated genes were selected and validated quantitatively by RT-PCR using Taqman high density microfluidic cards. A subtotal reduction in weight bearing through the ipsilateral hind limb was induced by cutting the sciatic nerve at the time of DMM surgery. Results: Compared with sham operated mice, DMM surgery strongly induced a number of genes within 6 h of surgery. Of these, the chemokine CCL2, TNF-stimulated gene 6 (TSG-6), IL-6, serum amyloid A (SAA) and arginase were the most highly regulated. Some metalloproteinases such as ADAMTS4, ADAMTS5 and MMP3 were regulated from 3 days, albeit less strongly. Mice that had undergone sciatic neurectomy exhibited abnormal gait; some weight was born through the limb, but the leg was maintained in full extension and walking was achieved by flexion at the hip. When DMM surgery was performed at the same time as sciatic neurectomy, the joints showed no evidence of OA even 12 post surgery. Analysis of early gene expression in these animals revealed a striking abrogation of greater than 80% of the inflammatory genes studied. Conclusions: These data show that there is an early inflammatory response in the joint in response to knee destabilization injury/surgery and importantly, indicate that inflammatory gene expression is dependent upon mechanical loading through the joint. We hypothesize that upon joint destabilization the perceived mechanical loads in the articular cartilage reach a threshold at which inflammatory genes are expressed. An intact meniscus (which helps dissipate load) or partial immobilization can reduce the perceived loads to below this threshold thus protecting the joint from degradation. Disclosure statement: All authors have declared no conflicts of interest.