Casein Kinase 1δ Activity: A Key Element in the Zebrafish Circadian Timing System

Zebrafish have become a popular model for studies of the circadian timing mechanism. Taking advantage of its rapid development of a functional circadian clock and the availability of light-entrainable clock-containing cell lines, much knowledge has been gained about the circadian clock system in this species. However, the post-translational modifications of clock proteins, and in particular the phosphorylation of PER proteins by Casein kinase I delta and epsilon (CK1 delta and CK1 epsilon), have so far not been examined in the zebrafish. Using pharmacological inhibitors for CK1 delta and CK1 epsilon, a pan-CK1 delta/epsilon inhibitor PF-670462, and a CK1 epsilon -selective inhibitor PF-4800567, we show that CK1 delta activity is crucial for the functioning of the circadian timing mechanism of zebrafish, while CK1 epsilon plays a minor role. The CK1 delta/epsilon inhibitor disrupted circadian rhythms of promoter activity in the circadian clock-containing zebrafish cell line, PAC-2, while the CK1 epsilon inhibitor had no effect. Zebrafish larvae that were exposed to the CK1 delta/epsilon inhibitor showed no rhythms of locomotor activity while the CK1 epsilon inhibitor had only a minor effect on locomotor activity. Moreover, the addition of the CK1 delta/epsilon inhibitor disrupted rhythms of aanat2 mRNA expression in the pineal gland. The pineal gland is considered to act as a central clock organ in fish, delivering a rhythmic hormonal signal, melatonin, which is regulated by AANAT2 enzymatic activity. Therefore, CK1 delta plays a key role in the circadian timing system of the zebrafish. Furthermore, the effect of CK1 delta inhibition on rhythmic locomotor activity may reflect its effect on the function of the central clock in the pineal gland as well as its regulation of peripheral clocks.