Reciprocity between phase shifts and amplitude changes in the mammalian circadian clock

doi:10.1073/pnas.0708877104

Reciprocity between phase shifts and amplitude changes in the mammalian circadian clock

*Regulatory Biology Laboratory and
^‖Laboratory of Genetics, The Salk Institute for Biological Studies, La Jolla, CA 92037;
^†FLI-Leibniz Institute for Age Research, Beutenbergstrasse 11, D-07745 Jena, Germany;
^§Departments of Psychiatry and Cell and Developmental Biology, University of California at San Diego, La Jolla, CA 92093; and
^¶Veterans Affairs San Diego Healthcare System, San Diego, CA 92161

Edited by Joseph S. Takahashi, Northwestern University, Evanston, IL, and approved October 23, 2007 (received for review September 19, 2007)

Abstract

Circadian rhythms help organisms adapt to predictable daily changes in their environment. Light resets the phase of the underlying oscillator to maintain the organism in sync with its surroundings. Light also affects the amplitude of overt rhythms. At a critical phase during the night, when phase shifts are maximal, light can reduce rhythm amplitude to nearly zero, whereas in the subjective day, when phase shifts are minimal, it can boost amplitude substantially. To explore the cellular basis for this reciprocal relationship between phase shift and amplitude change, we generated a photoentrainable, cell-based system in mammalian fibroblasts that shares several key features of suprachiasmatic nucleus light entrainment. Upon light stimulation, these cells exhibit calcium/cyclic AMP responsive element-binding (CREB) protein phosphorylation, leading to temporally gated acute induction of the Per2 gene, followed by phase-dependent changes in phase and/or amplitude of the PER2 circadian rhythm. At phases near the PER2 peak, photic stimulation causes little phase shift but enhanced rhythm amplitude. At phases near the PER2 nadir, on the other hand, the same stimuli cause large phase shifts but dampen rhythm amplitude. Real-time monitoring of PER2 oscillations in single cells reveals that changes in both synchrony and amplitude of individual oscillators underlie these phenomena.

Footnotes

**To whom correspondence should be addressed. E-mail: satchin{at}salk.edu
Author contributions: S.R.P. and N.T. contributed equally to this work; I.M.V. and S.P. designed research; S.R.P., N.T., D.K.W., and L.D.H. performed research; S.R.P., N.T., D.K.W., L.D.H., and S.P. analyzed data; and L.D.H. and S.P. wrote the paper.
↵ ^‡Present address: Faculty of Medicine, Kyorin University, 6-20-2 Shinkawa, Mitaka-shi, Tokyo 181-8611, Japan.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0708877104/DC1.