A Characteristic Expression Pattern of Core Circadian Genes in the Diurnal Tree Shrew

The day-active tree shrew may serve as an animal model of human-like diurnal rhythms. However, the molecular basis for circadian rhythms in this species has remained unclear. In the present study, we investigated the expression patterns of core circadian genes involved in transcriptional/translational feedback loops (TTFLs) in both central and peripheral tissues of the tree shrew. The expression of 12 core circadian genes exhibited similar rhythmic patterns in the olfactory bulb, prefrontal cortex, hippocampus, and cerebellum, while the hypothalamus exhibited the weakest oscillations. The rhythms in peripheral tissues, especially the liver, were much more robust than those in brain tissues. ARNTL and NPAS2 were weakly rhythmic in brain tissues but exhibited almost the strongest rhythmicity in peripheral tissues. CLOCK and CRY2 exhibited the weakest rhythms in both central and peripheral tissues, while NR1D1 and CIART exhibited robust rhythms in both tissues. Most of these circadian genes were highly expressed at light/dark transitions in both brain and peripheral tissues, such as ARNTL and NPAS2 peaking at dusk while PERs peaking at dawn. Additionally, the peripheral clock was phase-advanced relative to the brain clock, as there was a significant advance (2-4 h) for PER3, DBP, NR1D1 and NR1D2. Furthermore, these genes exhibited an anti-phasic relationship between the diurnal tree shrew and the nocturnal mouse (i.e., 12-h phasing differential). Collectively, our findings demonstrate a characteristic expression pattern of core circadian genes in the tree shrew, which may provide a means for elucidating molecular mechanisms of diurnal rhythms.