Circadian Rhythms in Metabolic Organs and the Microbiota During Acute Fasting in Mice

The circadian clock regulates metabolism in anticipation of regular changes in the environment. It is found throughout the body, including in key metabolic organs such as the liver, adipose tissues, and intestine, where the timing of the clock is set largely by nutrient signaling. However, the circadian clocks of these tissues during the fasted state have not been completely characterized. Moreover, the sufficiency of a functioning host clock to produce diurnal rhythms in the composition of the microbiome in fasted animals has not been explored. To this end, mice were fasted 24 h prior to collection of key metabolic tissues and fecal samples for the analysis of circadian clock gene expression and microbiome composition. Rhythm characteristics were determined using CircaCompare software. We identify tissue-specific changes to circadian clock rhythms upon fasting, particularly in the brown adipose tissue, and for the first time demonstrate the rhythmicity of the microbiome in fasted animals.