Carbon monoxide metabolism is essential for circadian transcription and dynamics

Circadian clocks are cell-autonomous oscillators regulating daily rhythms in a wide range of physiological, metabolic and behavioral processes. Conversely, metabolic signals such as redox state, NAD+/NADH and AMP/ADP ratios or heme feed back to and modulate circadian mechanisms to optimize energy utilization across the 24-hour cycle. We show that the signaling molecule carbon monoxide (CO) generated by rhythmic heme degradation is required for normal circadian rhythms as well as circadian metabolic outputs. CO suppresses circadian transcription by attenuating CLOCK/BMAL1 binding to target promoters. Pharmacological inhibition or genetic depletion of CO-producing heme oxygenases abrogate normal daily cycles in mammalian cells and Drosophila. In mouse hepatocytes, suppression of CO production leads to a global upregulation of CLOCK/BMAL1-dependent circadian gene expression and thereby misregulation of many metabolic processes including gluconeogenesis. Overall, 8 samples of HO1 knock-out mice and their wildtype littermates (BALB/c), untreated or treated with HO2-shRNA, were analyzed. Two replicates were used for each knock-out/knock-down group and for the normal controls.