Circadian and light-induced regulation of the expression of core clock proteins in common carp

The unique circadian clock system of fish consists of central and peripheral pacemakers that are directly regulated by light. The functioning of this clock machinery is based on cyclic changes in the expression of clock proteins that form transcriptional-translational loops. In this study, we performed an analysis of the circadian changes in Cry1, Cry2 and Clock protein levels in the nervous system, immune system and other peripheral organs of common carp. Protein-level analyses revealed that Cry1, Cry2 and Clock proteins show diurnal expression patterns in both central and peripheral tissues, with tissue-specific variations. We have shown that both Cry variants may play important roles in the clock mechanism, but their importance appears to be organ specific, with Cry2 and Cry1 likely playing important roles in central and peripheral oscillators, respectively. However, further genetic analysis will be required to clearly define the role of these proteins in the carp clock mechanism. We have found that the central oscillator function in carp is primarily performed by the retina, which is characterized by strong endogenous rhythmicity of activity. We have also observed that light availability clearly modulates the circadian clock mechanism in fish and probably indirectly other physiological functions by regulating the expression of key proteins that determine clock activity. Post-translational modifications of clock proteins, which are crucial for their functionality, affect their circadian rhythmicity.