Discovery of circadian harmonics
We profiled global gene expression over two full days using Affymetrix microarrays. We identified rhythmic transcripts in the mouse liver and pituitary, as well as fibroblasts (NIH3T3) and osteosarcoma cells (U2OS). These data have been made freely available on CircaDB as a resource to the field. To our surprise, we found several hundred genes cycling with period lengths much shorter than 24 hours (Fig. 1). These ultradian rhythms had period lengths of ~8 and ~12 hours -- i.e., the second and third harmonics of 24 hour oscillations.
Subsequently, we have shown that these rhythms are found in tissues throughout the body. Moreover, they are found in fruit flies as well, suggesting that circadian harmonics are a common feature of animal transcriptional rhythms. At a mechanistic level, 12 hour rhythms require both a central and peripheral circadian oscillator, indicating that these rhythms are ultimately downstream of the conventional circadian clock. Typically, they are involved in cellular responses to stress, suggesting that ultradian transcriptional rhythms respond to twice daily stresses.
Department of Biology
University of Missouri, St. Louis