Molecular clock and bone metabolism : stimulation of osteoblast and chondrocyte differentiation by novel clock genes

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Previous studies showed that in cartilage, DNA synthesis, aggrecan synthesis and type II collagen synthesis have circadian rhythms. However, genes that show circadian expression in cartilage have not been identified.To investigate molecular mechanism underlying circadian regulation of gene expression in cartilage, we analyzed expression profiles genome-widely, using Affimetrix DNA microarrays. We identified 270 circadian genes in the cartilage. Most of these genes differed from those of circadian genes in other tissues, suggesting that cartilage has an unique circadian system. The mRNA levels of aggrecom, type II collagen, proline hydroxylase, chondroitin sulfotransferase as well as DEC2 showed circadian variation in cartilage in vivo. The circadian rhythms of gene expression should be important for skeletal growth.