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J. Biol. Chem., Vol. 283, Issue 46, 31673-31678, November 14, 2008

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CLOCK Is Required for Maintaining the Circadian Rhythms of Opsin mRNA Expression in Photoreceptor Cells^*

Ping Li, Shyam S. Chaurasia, Yan Gao, Aprell L. Carr, P. Michael Iuvone, and Lei Li^¶1

From the Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, the Department of Pharmacology, Emory University, Atlanta, Georgia 30322, and the ^¶Department of Physiology, Nankai University School of Medicine, China, Tianjin 300071

In zebrafish, the expression of long-wavelength cone (LC) opsin mRNA fluctuated rhythmically between the day and night. In a 24-h period, expression was high in the afternoon and low in the early morning. This pattern of fluctuation persisted in zebrafish that were kept in constant darkness, suggesting an involvement of circadian clocks. Functional expression of Clock, a circadian clock gene that contributes to the central circadian pacemaker, was found to play an important role in maintaining the circadian rhythms of LC opsin mRNA expression. In zebrafish embryos, in which the translation of Clock was inhibited by anti-Clock morpholinos, the circadian rhythms of LC opsin mRNA expression diminished. CLOCK may regulate the circadian rhythms of LC opsin mRNA expression via cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. In control retinas, the concentration of cAMP was high in the early morning and low in the remainder of the day and night. Inhibition of Clock translation abolished the fluctuation in the concentration of cAMP, thereby diminishing the circadian rhythms of opsin mRNA expression. Transient increase of cAMP concentrations in the early morning (i.e. by treating the embryos with 8-bromo-cAMP) restored the circadian rhythms of LC opsin mRNA expression in morpholino-treated embryos. Together, the data suggest that Clock plays important roles in regulating the circadian rhythms in photoreceptor cells.

Received for publication, May 20, 2008 , and in revised form, August 4, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grants R01 EY013147 (to L. L.), EY004864 and EY 014764 (to P. M. I.), and Core Grant for Vision Research P30 EY03630. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556. Tel.: 574-631-7488; Fax: 574-631-7413; E-mail: Li.78{at}nd.edu.

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