Diurnal Changes in Protein Expression at the Blood-Brain Barrier in Mice

Circadian rhythms influence the transport function of the blood-brain barrier (BBB) and peripheral organs. However, the influence of circadian rhythms on protein expression in the BBB remains to be completely elucidated. Therefore, we aimed to investigate diurnal changes in protein expression in the mouse BBB using quantitative proteomics. Quantitative proteomics showed that the expression of 67, 10, and 20 proteins in the isolated mouse brain capillary fraction changed significantly at zeitgeber time (ZT) 6, 12, and 18, respectively, compared to ZT0. Among them, the levels of 44 proteins were significantly increased at ZT6 and then returned to the same level as ZT0 at ZT12 and ZT18. Gene ontology analysis indicated that the proteins significantly increased at ZT6 were majorly related to translation. The brain capillary endothelial cell-selective proteins sepiapterin reductase and vascular endothelial growth factor receptor 2 showed diurnal variation. In contrast, the expression of ABC transporters, SLC transporters, and receptors associated with receptor-mediated transcytosis, and tight junction proteins did not change within a day. The present findings demonstrated that protein expression related to transport function and physical barrier at the BBB was maintained throughout the day, although the proteins involved in some biological processes exhibited diurnal variation at the BBB.