RNA transcripts from WT or Bmal1-deficient blood-brain barrier throughout a 24 hr period

The blood-brain barrier (BBB) is critical for neural function as it restricts effects of peripheral molecules on the brain. We report here circadian regulation of the BBB in mammals. Efflux of xenobiotics by the BBB oscillates in mice, with highest levels during the active phase and lowest during the resting phase. This oscillation is abrogated in circadian clock mutants. To elucidate the mechanisms of circadian regulation, we profiled the transcriptome of brain endothelial cells; interestingly, we detected limited circadian regulation of transcription, with no evident oscillations in efflux transporters. We recapitulated the cycling of xenobiotic efflux using a human microvascular endothelial cell line to find that the molecular clock drives the cycling of intracellular magnesium through transcriptional regulation of TRPM7, thereby accounting for the rhythm in efflux. As many drugs are substrates of efflux transporters, these findings highlight the importance of considering circadian regulation when targeting therapeutics to the CNS. Overall design: 2 genotypes (control endothelium and Bmal-deficient endothelium) were assessed for RNA transcript levels over 6 time points across a circadian day with 2 replicates per time point