Phase-response curve of cellular oscillators in peripheral tissues: analysis by real-time monitoring of <I>Bmal1</I> expression by bioluminescence reporter.

In mammals, the circadian pacemaker in the suprachiasmatic nucleus (SCN) is entrained by photic time cues. Explants of peripheral organs also exhibit circadian rhythms in clock gene expression, indicating the presence of peripheral clocks. However, the mechanisms are not known how the peripheral clocks entrain to the circadian pacemaker in the SCN. Cultured fibroblasts treated by high concentration of serum or dexamethasone (Dex) exhibit circadian rhythms in clock gene expression, which is considered as a model system of the peripheral clock. In the present experiment, we constructed a bioluminescent reporter gene Bmal1-LUC which expresses firefly luciferase under the control of clock gene Bmal1, and continuously monitored the luciferase activity from the stable Rat-1 fibroblast cell line expressing Bmal1-LUC. Rat-1 fibroblasts showed a type 0 phase-response curve to Dex pulses. In addition, a similar type 0 phase-response curve was obtained by vehicle pulses. These results suggest that the cellular oscillators in the rat fibroblasts are desynchronized without stimulation and become arrhythmic as a whole system, which can, however, be reset and synchronized to each other by a single stimulus. [Jpn J Physiol 55 Suppl:S200 (2005)]