Aging Reduces ATP-Binding Cassette Transporter Expression in Brain Microvessels of Mice

Background: ATP-binding cassette (ABC) transporters are expressed in the vascular walls of brain capillaries and remove toxic chemicals from the brain. The expression of ABC transporters in peripheral organs is transcriptionally regulated by clock genes and exhibits 24 h periodic fluctuations. In addition, clock gene outputs diminish with aging. In this study, we evaluated whether the expression of ABC transporters in the blood-brain barrier (BBB) of young mice had a 24 h cycle, and whether the expression of ABC transporters in the BBB decreased with age. Methods: Brain microvascular (BMV) fractions from the cerebral cortex of male C57BL/6J mice were prepared using dextran. BMV fractions from young mice (12 weeks old) were prepared every four hours to evaluate 24 h rhythmicity. BMV fractions from both young and aged mice (85 weeks old) were prepared when protein expression peaked (Zeitgeber Time 5). Protein and mRNA expression of ABC transporters in BMV fractions were measured. Results: In young mice, protein expression of P-glycoprotein, breast cancer resistance protein, and multidrug resistance protein 4 showed time-dependent variations with a peak in the light phase (Zeitgeber Time 5); mRNA expression showed no time-dependent variation. The protein expression of these transporters was lower in the BBB of aged mice than in that of young mice, although mRNA expression did not differ between young and aged mice. Conclusions: ABC transporter protein expression levels in BMV endothelial cells decreased with aging; however, mRNA levels did not change, which suggests changes in protein expression did not result from diminished clock gene output. Further studies are needed to elucidate the mechanisms by which ABC transporter expression in the BBB decreases with aging.