Co-regulation of circadian clock genes and microRNAs in bone metabolism

Mammalian bone is constantly metabolized from the embryonic stage,and the maintenance of bone health depends on the dynamic balance between bone resorption and bone formation,mediated by osteoclasts and osteoblasts.It is widely recognized that circadian clock genes can regulate bone metabolism.In recent years,the regulation of bone metabolism by non-coding RNAs has become a hotspot of research.MicroRNAs can participate in bone catabolism and anabolism by targeting key factors related to bone metabolism,including circadian clock genes.However,research in this field has been conducted only in recent years and the mechanisms involved are not yet well established.Recent studies have focused on how to target circadian clock genes to treat some diseases,such as autoimmune diseases,but few have focused on the co-regulation of circadian clock genes and microRNAs in bone metabolic diseases.Therefore,in this paper we review the progress of research on the co-regulation of bone metabolism by circadian clock genes and microRNAs,aiming to provide new ideas for the prevention and treatment of bone metabolic diseases such as osteoporosis.

Role of circadian gene Clock during differentiation of mouse pluripotent stem cells

Biological rhythms controlled by the circadian clock are absent in embryonic stem cells （ESCs）. However, they start to develop during the differentiation of pluripotent ESCs to downstream cells. Conversely, biological rhythms in adult somatic cells disappear when they are reprogrammed into induced pluripotent stem cells （iPSCs）. These studies indicated that the development of biological rhythms in ESCs might be closely associated with the maintenance and differentiation of ESCs. The core circadian gene Clock is essential for regulation of biological rhythms. Its role in the development of biological rhythms of ESCs is totally unknown. Here, we used CRISPR/CAS9-mediated genetic editing techniques, to completely knock out the Clock expression in mouse ESCs. By AP, teratoma formation, quantitative real-time PCR and Immunofluorescent staining, we did not find any dif- ference between Clock knockout mESCs and wild type mESCs in morphology and pluripotent capability under the pluripotent state. In brief, these data indicated Clock did not influence the maintaining of pluripotent state. However, they exhibited decreased proliferation and increased apoptosis. Furthermore, the biological rhythms failed to develop in Clock knockout mESCs after spontaneous differentiation, which indicated that there was no compensational factor in most peripheral tissues as described in mice models before （DeBruyne et ah, 2007b）. After spontaneous differentiation, loss of CLOCK protein due to Clock gene silencing induced spontaneous differentiation of mESCs, indicating an exit from the pluripotent state, or its differentiating ability. Our findings indicate that the core circadian gene Clock may be essential during normal mESCs differentiation by regulating mESCs proliferation, apoptosis and activity.