Purpose: Single injurious mechanical impact to joints, as seen in sports injury, is known to be associated with a high risk of later development of osteoarthritis (OA). Irreversible chondrocyte death is initiated following impact and results in progressive breakdown of articular cartilage. Promotion of chondrocyte survival is therefore a necessary target in the prevention of development of further OA. Urocortin1 (Ucn) has recently been shown to have a pro-survival effect on chondrocytes in both the presence and absence of pro-apoptotic stimuli. The aim of this study was to investigate the role of Ucn in response to mechanical impact.
Methods: Full thickness porcine cartilage explants from lateral and medial femoral condyles were impacted with a 500g weight using a custom-made drop tower device. Impacted explants were treated with human Ucn (10-7M, 30 minutes prior to or following impact), non-selective cation channel blocker Gadolinium (500μM, 30 minutes following impact), or mechanosensitive ion channel blocker GsMTx4 (40μM, 2 hours prior to impact) and cultured for 72hrs. Cell viability was assessed with a CMFDA:PI live dead assay, using both macro confocal microscopy, as a means of avoiding bias, and high magnification confocal microscopy techniques. Lactate dehydrogenase (LDH) assays were conducted at 72hrs to provide a quantitative read out of cell death for each explant. Calcium influx was assessed 1hr post impact by Fluo-4 AM staining and visualised by multiphoton microscopy.
Results: Porcine explants were impacted from a range of heights (10-70mm) in order to establish the correct energy of impact for inducing cell death without excess damage and fracture of the cartilage explant. Cell death increased in a dose dependent manner with height. 50mm was chosen as optimal height for all further experiments. Pre-treatment of explants with Ucn had a significant increase on cell viability when compared to impacted alone, as seen by quantification of the CMFDA:PI ratio on macro confocal (p=0.0042) and by high magnification confocal microscopy (p=0.0015). This increase in cell viability was confirmed by LDH assay (p=0.0063). Post-impact treatment of the explants with Ucn also resulted in a significant increase in cell survival, seen in both CMFDA:PI ratio (p=0.0009) and LDH assay (p=0.0053). We have previously found in primary human chondrocytes that Ucn exerts its protective effect on chondrocytes by the inhibition of excessive calcium influx. Calcium influx was therefore visualised in impacted explants treated pre- and post-impact with Ucn. A significant decrease in the levels of calcium staining was seen in both of these conditions when compared to explants impacted in the absence of Ucn (p=0.0073, p=0.05). In vitro work also highlighted a role for the mechanosensitive ion channel Piezo1 in cell death associated with the absence of Ucn. Explants were therefore treated with both Gadolinium and GsMTx4 post-impact and cell viability assessed by multiphoton microscopy. An increase in cell viability was observed in both conditions, comparable to those seen with addition of Ucn.
Conclusions: A novel role for Ucn in the protection of articular chondrocytes maintained in cartilage explants has been shown and validated by two microscopy techniques and LDH assay. Ucn has also been shown to reduce calcium influx in impacted explants at 60 minutes post impact, and increased cell viability was observed by blocking mechanosensitive ion channels. These data suggest the importance of regulation of calcium influx in Ucn-mediated cell protection. The pro-survival effect of Ucn was seen both pre- and post-impact, highlighting the potential prophylactic and therapeutic use of Ucn for treating sudden injury to cartilage, and subsequent progression to post-traumatic OA.