Per2 plays a major role in the control of liver glycogen metabolism and fasting glycaemia

and aims: Recent evidence suggests that obesity-induced derangements of the expression of molecular components of the circadian clock may be implicated in the development of glucose intolerance. The aim of this study was to investigate the role of the clock protein period 2 (Per2), a key component of the circadian clock whose expression is deranged during obesity, in glucose homeostasis in lean and obese mice. Materials and methods: To investigate the role of Per2 in glucose metabolism in-vivo we used mice bearing a targeted gene mutation in the Per2 gene (Per2brdm) and thus unable to express a functional Per2 protein. Mice were hosted in our standard mouse facility in a 12-hour light and 12-hour dark cycle. Wt and Per2brdm mice were fed with either standard chow diet or high-fat diet for 24 weeks and analyzed for glucose homeostasis. Glucose tolerance test (GTT) and insulin tolerance test (ITT) were performed on mice food deprived for 7 hours. For pyruvate tolerance test (PTT) mice were fasted for 14 hours. Glucose levels were measured over 12 hours starvation timecourses during the light and the dark phase. Fed and fasting blood glucose and hepatic glycogen content were measured over different circadian timepoints. To investigate the role of Per2 in the control of liver gene expression we performed DNA-microarray analysis of RNA preparations from liver of Per2brdm and WT mice. Results were validated by QPCR. Results: Our results suggest that Per2 loss of function is not a basic requirement for the development of obesity-induced insulin resistance. Per2brdm mice show similar glucose tolerance compared to WT mice when placed on chow diet, and Per2 loss of function does not predispose to high-fat dietinduced insulin resistance. However, we have identified an important role for Per2 in the control of fasting glycemia and hepatic glycogen metabolism. Per2brdm mice show a decreased fasting glycemia compared to WT controls during the light phase. This difference is observed after 4 hours of food deprivation, is maximal at 7-8 hours of starvation, and disappears after about 12 hours of starvation. Gene expression-signature analysis from the microarray data indicates decreased expression of genes involved in gluconeogenesis and glycogen metabolism in livers from Per2brdm mice compared to livers from WT mice. PTT was performed to evaluate whole body gluconeogenesis. The results show that WT and Per2brdm mice display similar gluconeogenic potential in a PTT test. To evaluate the role of Per2 in glycogen metabolism we measured hepatic glycogen content in fed or 8 hours fasted WT and Per2brdm mice. The results show that fed Per2brdm mice display lower liver glycogen content during the light phase compared to WT controls. This difference is more pronounced in mice that were starved for 8 hours with Per2brdm mice displaying less than a third of the hepatic glycogen content compared to control WT mice (p-value < 0.005). Conclusion: Our data suggest that Per2 loss of function is not a major cause or a predisposition factor to impaired glucose tolerance. Nonetheless our results suggest that Per2 plays an important role in the control of fasting glycemia and in hepatic glycogen metabolism. Supported by: Swiss National Science Foundation