The role of the molecular circadian clock in healthy and osteoarthritic chondrocytes

Background Osteoarthritis (OA) is a progressive, degenerative, and currently incurable joint disorder, and the molecular details of disease development and progression have not been fully identified. Recent research indicates that a dysregulated molecular clock in articular chondrocytes may contribute to OA pathogenesis. The circadian rhythm is not only determined by the central clock located in the hypothalamus but it is also adjusted by tissue- and cell- specific molecular clocks. The molecular basis of the circadian clock is the rhythmic expression and activity of conserved clock genes and transcription factors (Clock, Bmal1, Per2/3 and Cry1/2, cRev), which also regulate a wide range of downstream genes. It has recently been shown that the disruption of Bmal1, a key component of the molecular clock, induces an OA- like phenotype. However, the putative links between the molecular clock and the regulation of chondrogenesis have not been studied. Aims To assess the expression pattern of the circadian clock genes (Clock, Bmal1/2, Per2/3, Cry1/2, cRev) and key chondrogenic/osteogenic marker genes (Sox6, Sox9, Runx2) during cartilage formation and identify the time points for clockwork stimulation. Methods Chicken eggs were incubated for 4 days based on the Hamburger-Hamilton staging. Distal limb buds of the embryos were collected under the microscope. The high density micromass chicken cell cultures were cultured for 6-day-long and then serum shocked to synchronise the molecular clock. Samples were collected at various time points post- synchronisation and then stored at -80 degrees. RNA was the isolated and reverse transcribed to cDNA. Primers were designed and tested for specificity. Following serial dilutions, gene expressions were analysed using RT-qPCR (absolute quantification using SYBR Green method). Results There were statistically significant changes at various time-points of both the clock genes and key chondrogenic marker genes (One-way repeated measures ANOVA, N=3, *P < 0.05, **P < 0.01, ***P < 0.001). Conclusions The molecular clock is ticking during the early stages of in-vitro chondrogenesis; differentiating chondrocytes in chicken limb bud-derived micromass cultures contain self- sustained circadian clocks. Similarly, the molecular clockwork may have a profound influence on chondrogenesis (Sox6, Sox9, Runx2).egységes, osztatlanáltalános orvosango