Protein Kinases in the Photic Signaling of the Mammalian Circadian Clock

Circadian clocks drive biological rhythms in physiology and behavior, providing a selective advantage by enabling organisms to synchronize to the 24 h environmental day. This process depends on light-dark transitions as the main signal that shifts the phase of the clock. In mammals, the light input reaches the master circadian clock in the hypothalamic suprachiasmatic nucleus through glutamatergic afferents from the retina, resulting in phase-shifts of the overt rhythms which depend on the time of the day at which light is applied, leading to changes in the activity of circadian core clock genes (i.e., Per1). This circadian gating of the synchronizing effect of light is dependent on the specific activation of signal transduction pathways involving several kinases acting on protein effectors. Protein phosphorylation is also an important regulatory mechanism essential for the generation and maintenance of circadian rhythms and plays a crucial role in the degradation and the appropriate turnover of PER proteins. In this work, we review the role of the main kinases implicated in the function of the master clock, with emphasis in those involved in circadian photic entrainment.