Abstract
Multicellular organisms respond to changing environment which is primarily driven by light from the sun. Essential cyclical processes such as digestion, sleep, migration and breeding are controlled by set of genes know as circadian genes. The core circadian genes comprise of CLOCK, BMAL-1, PERIOD and CYRPTOCHROME that are expressed cyclically and they regulate expression of several genes downstream. The expression of circadian genes has been well studied in multicellular animals; however, it has been shown that stem cells also possess active circadian cycle genes. The circadian cycle genes have been studied in mouse embryonic stem cells and in adult human stem cells. However, there are only few reports of circadian cycle genes in human pluripotent stem cells. We used human embryonic stem cells to investigate the expression of CLOCK, BMAL-1, PERIOD and CYRPTOCHORME genes by RT-PCR at 6, 18 and 22 hours in undifferentiated and differentiated cells. We differentiated human embryonic stem cells spontaneously by adding 10% fetal bovine serum (FBS), and the cells primarily differentiated into ectoderm and mesoderm. We report that CLOCK and BMAL-1 are differentially expressed while PERIOD and CRYPTOCHROME show cyclicity in differentiated and undifferentiated cells. Our results show circadian genes are active in human embryonic stem cells and this needs to be further investigated as human pluripotent stem cells have potential to be used for cell therapy, where they need to synchronize with the body's circadian cycle.