Proliferation of synovial Mesenchymal Stromal/Stem Cells (MSCs) leads to synovial hyperplasia (SH) following Joint Surface Injury (JSI). Uncontrolled Yap activity causes tissue overgrowth due to modulation of MSC proliferation. We hypothesised that YAP plays a role in SH following JSI. A spatiotemporal analysis of Yap expression was performed using the JSI model in C57Bl/6 mice. Synovial samples from patients were similarly analysed. Gdf5-Cre;Yap1fl/fl;Tom mice were created to determine the effect YAP1 knockout in Gdf5 lineage cells on SH after JSI. In patients, Yap expression was upregulated in activated synovium, including a subset of CD55 positive fibroblast-like synoviocytes in the synovial lining (SL). Cells staining positive for the proliferation marker Ki67 expressed active YAP. In mice, Yap was highly expressed in injured knee joint synovium compared to controls. Yap mRNA levels at 2 (p<0.05) and 8 days (p<0.001) after injury were increased. Conditional Yap1 knockout in Gdf5 progeny cells prevented hyperplasia of synovial lining (SL) after JSI. Cellularity was significantly decreased in the SL but not in the sub-lining of injured Yap1 knockout- compared to control mice. The percentage of cells in synovium that were Tom+ increased in response to JSI in control and haplo-insufficient but not in YAP1 knockout mice (p<0.05). Modulation of YAP and proliferation of MSCs in the synovium after JSI provides a system to study the role of SH after trauma in re-establishing joint homeostasis and is a potential novel therapeutic target for the treatment of post traumatic OA

Tranexamic acid (TXA) is an anti-fibrinolytic medication commonly used to reduce peri-operative bleeding. Increasingly, topical administration as an intra-articular injection or peri-operative wash is being administered at concentrations between 10–100mg/ml. This study investigated effects of TXA on human periarticular tissues and primary cell cultures using clinically relevant concentrations. Tendon, synovium and cartilage obtained from routine orthopaedic surgeries were used ex vivo or cultured for in vitro studies using various concentrations of TXA. They were stained with 5-chloromethylfluorescein diacetate and propidium iodide and imaged using confocal microscopy to identify the proportion of live and dead cells. The in vitro effect of TXA on primary cultured tenocytes, synovial like fibroblast (FLS) cells and chondrocytes was investigated using cell viability assays (MTT), fluorescent microscopy and multi-protein apoptotic arrays for cell death. There was significant (p<0.01) increase in cell death in all tissue treated with 100mg/ml TXA, ex vivo. MTT assays revealed significant (p<0.05) decrease in cell viability following treatment with 50 or 100mg/ml of TXA within 4 hours of all cell types cultured in vitro. Additionally, there was significant (p<0.05) increase in cell apoptosis detected by fluorescent microscopy within 1 hour of exposure to TXA. Furthermore, multi-protein apoptotic arrays detected increased apoptotic proteins within 1 hour of TXA treatment in tenocytes and FLS cells. Our study provides evidence of TXA cytotoxicity to human peri-articular tissues ex vivo and in vitro at concentrations and durations of treatment routinely used in clinical environments. Clinicians should therefore show caution when considering use of topical TXA administration