The authors “give mechanistic insight for how time-of-day difference in blood-brain barrier permeability comes to be,” says Robert Dallmann, a circadian biologist and pharmacologist at University of Warwick in the United Kingdom who did not participate in the study. “They even show that time of treatment really makes a difference.”
Researchers and clinicians have found that the timing of medical treatments—including vaccines and chemotherapy for brain cancer—can influence their efficacy, but it is still unclear how the circadian clock exerts these effects.
To investigate the permeability of the BBB, a team led by Amita Sehgal of the University of Pennsylvania injected fruit flies with fluorescent dye at four-hour timepoints throughout the day. They found that more of the dye made it into the insects’ brains in the early evening than at other times (flies are most active during the day). When the researchers injected flies lacking the period gene, which encodes an essential element of the circadian clock, they observed no difference in the levels of dye in the flies’ brains over the course of the day, suggesting that permeability depends on the molecular clock.
In flies, the BBB is composed of two layers of glial cells: subperineural glia and perineural glia. Sehgal and colleagues observed that the increase in permeability at night was due to an overall decrease in active efflux—the movement of foreign substances out of the subperineural glia via transporters. They also found that the early evening decrease in efflux coincides with an increase in gap junctions connecting subperineural glia with perineural glia. Magnesium ions, which regulate efflux transporter function, diffuse from the subperineural glia into the perineural glia through these gap junctions, resulting in less transporter activity and a higher retention of drugs on the brain side of the barrier at night.
We may want to start paying more and more attention to chronobiology and drug administration for tough diseases. A drug that’s not very effective at 9 AM may be a blockbuster at 9 PM.—William Banks,
University of Washington
“There’s gap junctional communication between two cell types that accounts for the rhythmicity in one cell conferring rhythms on another cell,” says Sehgal.
The researchers fed the anti-seizure drug to a strain of flies that can be mechanically stimulated to have seizures. Flies that were medicated in the early evening showed faster seizure recovery times than flies that got the drug in the afternoon. In epileptic flies without a molecular clock, the drug worked just as fast regardless of the time of day it was received.
The magnesium-mediated mechanism is novel, “but it is not clear that it applies in the vertebrate context,” writes Roland Bainton, who studies the BBB at the University of California, San Francisco, and did not participate in the study, in an email to The Scientist. He cautions that the feeding assays used to deliver the anti-seizure drug to the fruit flies are “notoriously variable,” and adds that “the anatomic characteristics of insect and vertebrate BBB structures are quite distinct.”
Sehgal’s group is currently exploring whether the mechanism occurs in mammals. “We’ve done work in a human [cell] culture model, and find that the same is true there,” she says, “and we’re now working on the mouse blood-brain barrier . . . to identify the transporters that are rhythmic there.” She adds that understanding which transporters are oscillating could have implications for finding the optimal time to give patients drugs that are targeted to the central nervous system.
“Our invertebrate and vertebrate models are probably telling us a lot about what’s going on in human brains and human disease,” says William Banks, who studies the BBB at the University of Washington and was not involved in the work. “We may want to start paying more and more attention to chronobiology and drug administration for tough diseases. A drug that’s not very effective at 9 AM may be a blockbuster at 9 PM.”
“The clocks of people, unlike the clocks of flies, can be very different,” says Dallmann. “If you want to treat the people according to the clock, you probably have to treat them according to their clock, which is a further complication.”
See “Time, Flies”
S.L. Zhang et al., “A circadian clock in the blood-brain barrier regulates xenobiotic efflux,” Cell, doi:10.1016/j.cell.2018.02.017, 2018.