Modulation of circadian rhythms in articular cartilage by heat pulses
ObjectivePrior studies have shown that disruption of the circadian clock leads to cartilage degeneration in mice while shift work is associated with higher risk of osteoarthritis (OA) in humans. In this study we investigated the potential of heat pulses to restore dampened circadian rhythms in articular cartilage. MethodsFemoral head cartilage explants and primary chondrocytes were isolated from PER2::LUC mice. Human femoral condyle cartilage was obtained from osteoarthritic patients undergoing total knee replacement. Tissues and cells were exposed to heat shock at various temperatures (37-43 {degrees}C) and incubation lengths. Bioluminescence from explants and cells was recorded in real-time. RNA sequencing and qPCR were used to assess gene expression changes in response to heat. ResultsWe established that a 60-min pulse at 43 {degrees}C was sufficient to restore dampened PER2::LUC rhythms in mouse cartilage explants or in primary chondrocytes. Transcriptome analysis in mouse articular cartilage showed an up-regulation of genes encoding heat shock proteins and collagens, and a transient down-regulation of Sox9, Runx2, Per1, Clock and Cry2. Heat induced the expression of circadian clock genes in human osteoarthritic knee cartilage. Mechanistically, inhibition of HSP90 activity or perturbation of F-actin polymerisation blocked the heat-induced resynchronisation of circadian rhythms. ConclusionTogether, these data have contributed to a greater understanding of the multifaceted nature of the connections between circadian timekeeping, heat stress responses and homeostasis in articular cartilage. These findings also suggest that time-prescribed temperature increases could be developed into a non-invasive intervention to slow down tissue ageing and restore homeostasis in osteoarthritic joints by improving circadian oscillations of cartilage rhythmic pathways.