Mar. 30, 1999 St. Louis, March 30, 1999 -- Scientists have uncovered evidence about the workings of one of the gateways into the brain. Their findings suggest approaches to control the gateway with drugs, which could have implications for treating AIDS, depression, cancer and other diseases that affect the brain. Doctors have struggled to get many drugs across a main gateway to the brain known as the blood-brain barrier. But now, researchers have determined that a guardian protein called p-glycoprotein at this barrier collaborates with a similar protein to limit traffic through a second barrier to the brain. This structure, which lines cavities deep within the brain, is known as the choroid plexus.
"The choroid plexus may be a major surface of exchange for drugs traveling from the blood into the cerebrospinal fluid through this back-door barrier," says David Piwnica-Worms, M.D., Ph.D., professor of radiology and associate professor of molecular biology & pharmacology at Washington University School of Medicine in St. Louis. He is principal investigator of the study published in the March 30 issue of Proceedings of the National Academy of Sciences. Vallabhaneni V. Rao, Ph.D., a former instructor of radiology, was lead author of the study, which also involved collaborators at Yale University School of Medicine. Piwnica-Worms hopes to learn how to alter the properties of proteins that keep this back door shut. "It might be possible to enhance delivery of many drugs by blocking the transporter proteins at the choroid plexus in a selective and careful way," he says. These efforts might allow entry into the brain of the protease inhibitors that kill cells infected with HIV, for example. Some doctors think the brain serves as a safe haven for the virus, thwarting efforts to eradicate it.
The choroid plexus drew Piwnica-Worms' interest after images originally designed to highlight cancerous cells also revealed the tissue to be a potential location for the p-glycoprotein (Pgp), a known transporter of many molecules across cell surfaces. He was studying Pgp as a marker for cells that are less amenable to chemotherapy treatment. The transporter acts on certain cancer cells as the molecular equivalent of a bouncer at a bar, spitting chemotherapeutic drugs out of the cells before they can cause the intended damage.
A radioactive imaging compound called technetium-99m-SESTAMIBI can reveal whether cancer cells lack Pgp and therefore are more likely to respond to chemotherapy. Cancer cells without the bouncer on their surface will take the compound inside, where its decay produces photons of gamma rays detectable on images produced by a special scanner. Unexpectedly, Piwnica-Worms found that patients injected with the technetium compound sequester it within the choroid plexus, whose main function is to selectively pull substances out of nearby blood vessels to produce the cerebrospinal fluid that bathes nerve cells.
Piwnica-Worms knew that Pgp at the blood-brain barrier would keep the imaging agent out of the brain. So he determined whether Pgp also was the protein that prevented the agent from crossing this second gateway. "The fact that you see the compound in the choroid plexus -- but not in the cerebrospinal fluid -- told us there must be a barrier there holding the compound back from reaching the brain," he says.
He looked at the epithelial cell layer thought to serve as this barrier in the choroid plexus, which also is called the blood-cerebrospinal barrier, to determine whether Pgp or a related transporter was there. Both Pgp and the multidrug-resistance associated protein (MRP) are known to transport the technetium compound across cell membranes, so either could have prevented movement into the cerebrospinal fluid.
Using antibody imaging techniques, Piwnica-Worms determined that Pgp was present on the epithelial cells. However, the transporter sat on the surface that faced the cerebrospinal fluid. In contrast, MRP was on the cell surface closest to the bloodstream, where it appeared poised to pump compounds away from the brain, as Pgp does at the blood-brain barrier. Experiments using cells grown in special test tubes and selective inhibitors of the two transporters verified that Pgp and MRP could play the roles suggested by their positions on the epithelial cells. In addition, Pgp thwarted the ability of a common cancer drug called Taxol to enter the epithelial cells from the surface that normally faces the cerebrospinal fluid.
These findings suggest that doctors who want to treat brain ailments need to overcome both Pgp and MRP to succeed with drug treatments. Yet pharmaceutical companies have primarily tested drugs that block Pgp function as treatments for cancers in the brain and elsewhere. "We don't know what might happen if you inhibit both of these," Piwnica-Worms says.
The transporters' roles at the blood-cerebrospinal fluid barrier have yet to be characterized. Piwnica-Worms suggests that Pgp likely plays a different role at this gateway, perhaps as a sodium transporter or as a way to regulate cholesterol and related compounds inside the central nervous system. He will use animal models lacking one or both transporters to clarify their functions in normal and disease states.
Support for this research was provided by grants from the U.S. Department of Energy and the National Institutes of Health.
Rao, V.V., Dahlheimer, J.L., Bardgett, M.E., Snyder, A.Z., Finch, R.A., Sartorelli, A.C., Piwnica-Worms, D. Choroid plexus epithelial expression of MDR1 P-glycoprotein and multidrug resistance-associated protein contribute to the blood-cerebrospinal-fluid drug-permeability barrier. Proceedings of the National Academy of Sciences. 96 (7): 3900-3905, March 30, 1999.
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