Environmental Factor, February 2011, National Institute of Environmental Health Sciences
Miller explores the longest signaling pathway in endothelial cells
By Melissa Kerr
In his 2010 review, Miller pointed to the environmental and therapeutic implications of his research. "Understanding of transporter function and regulation at the human BBB is critical before we can determine to what extent signaling can be manipulated to improve drug delivery to the CNS and to enhance neuroprotection," he wrote. (Photo courtesy of Steve McCaw)
The brain's most effective defense against neurotoxicants is also the greatest hindrance in the delivery of potentially life-saving medicines to the central nervous system (CNS). In his Jan. 4 talk on the intricacies of the blood-brain barrier (BBB) and its seemingly paradoxical role in human health, NIEHS Senior Investigator and Acting Scientific Director David Miller, Ph.D., made a presentation about his research into "what we think is the longest signaling pathway that has been worked out in endothelial cells" and its role in BBB regulation.
Attendees at the monthly meeting of the NIEHS Receptor Mechanism Discussion Group filled the Institute's Executive Conference Room, as Miller and colleagues discussed the results of his research, the roadblocks, and the potential if researchers can discover how to successfully manipulate the BBB (see text box).
The brain protects itself
Miller described the CNS as the "final frontier" in pharmaceutical therapy because receptors along the BBB cannot distinguish between neurotoxicants and therapeutic drugs, responding to both as potentially harmful xenobiotics. The luminal plasma membrane of the brain capillary endothelium acts as a filter to regulate the exchange of material between the blood and the brain and tightens in response to expression and activity of what are known as ATP-binding cassette (ABC) transporters, especially P-glycoprotein (see related story(http://www.niehs.nih.gov/news/newsletter/2010/june/science-p-glycoprotein.cfm)).
Endothelial cells lining the capillaries within the brain and the transporters they express govern uptake, distribution, and excretion through the tight barriers, regulating the movement of ions, water, nutrients, and waste across the BBB. Miller and his team have been targeting how to manipulate receptors in these multiple signaling pathways to advance medicinal therapy of CNS-related diseases, better understand the mode of action of environmental neurotoxicants, and discover how harmful proteins that build up in the brain return to the blood stream for elimination.
"P-glycoprotein is the major obstacle to getting therapeutic drugs into the brain," said Miller, "because it's at the right place to stop things from getting across the endothelium." P-glycoprotein (Pgp) is a transporter found in high concentrations within the brain capillary endothelium.
As Miller explained, "Basically, what we're looking for is a narrow window of time to knock down p-glycoprotein activity. Then you come in with whatever drug you're trying to get into the brain, and later p-glycoprotein activity rebounds and restores protection."
Untying a knot within the metabolic pathway
In their early experiments, Miller's group studied mice genetically engineered to knock out Pgp expression. When these mice were treated with the opiate methadone, the researchers found seven times higher levels in the brain than in wild-type mice, confirming the importance of Pgp in BBB regulation. These results led Miller and his team to look for a way to suppress the activity of Pgp directly, but they found that drugs that specifically inhibit the transporter itself, such as PSC833, work but are moderately toxic, essentially eliminating them as viable candidates for drug development.
As an alternate plan, Miller and his group are targeting p-glycoprotein signals that maintain basal transporter activity. One signaling pathway the team found is turned on by a protein called tumor necrosis factor-alpha that reacts with receptor TNF-R1 and signals through multiple steps to cause a loss of Pgp activity. A second strategy involves manipulating expression of vascular endothelial growth factor, which also signals loss of Pgp activity
Although the team's results have helped to elucidate extensive signaling pathways in animal models, it is not yet clear how their results will extrapolate to humans. And Miller readily concedes that there are many more twists and turns in the world's longest signaling pathway to explore in his group's quest to unravel the mysteries of the BBB.
(Melissa Kerr studies chemistry at North Carolina Central University. She is currently an intern in the NIEHS Office of Communications and Public Liaison.)
More findings from the Miller group's work on the BBB
Working from memory and without the help of a computer sidelined by technical problems, Miller developed his topic, "Receptor driven: outside in, out, in, signaling at the blood brain barrier," during a talk co-hosted by NIEHS Principal Investigators John Cidlowski, Ph.D. and Ken Korach, Ph.D. In the course of his presentation, Miller referred to several papers from his group, including the following:
- Miller DS.(http://www.ncbi.nlm.nih.gov/pubmed/20417575) 2010. Regulation of P-glycoprotein and other ABC drug transporters at the blood-brain barrier. Trends Pharmacol Sci 31(6):246-254.
- Wang X, Hawkins BT, Miller DS.(http://www.ncbi.nlm.nih.gov/pubmed/21048045) 2010. Aryl hydrocarbon receptor-mediated up-regulation of ATP-driven xenobiotic efflux transporters at the blood-brain barrier. FASEB J; doi: 10.1096/fj.10-169227 [Online 3 November 2010]. Story(http://www.niehs.nih.gov/news/newsletter/2011/january/science-study.cfm)
- Hartz AM, Miller DS, Bauer B.(http://www.ncbi.nlm.nih.gov/pubmed/20101004?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=1) 2010. Restoring blood-brain barrier P-glycoprotein reduces brain amyloid-beta in a mouse model of Alzheimer's disease. Mol Pharmacol 77(5):715-723. Story(http://www.niehs.nih.gov/news/newsletter/2010/april/science-are.cfm)
- Hawkins BT, Sykes DB, Miller DS.(http://www.ncbi.nlm.nih.gov/pubmed/20107068) 2010. Rapid, reversible modulation of blood-brain barrier P-glycoprotein transport activity by vascular endothelial growth factor. J Neurosci 30(4):1417-1425. Summary(http://www.niehs.nih.gov/news/newsletter/2010/april/science-intramural.cfm)
- Hartz AM, Madole EK, Miller DS, Bauer B.(http://www.ncbi.nlm.nih.gov/pubmed/20460386) 2010. Estrogen receptor beta signaling through phosphatase and tensin homolog/phosphoinositide 3-kinase/Akt/glycogen synthase kinase 3 down-regulates blood-brain barrier breast cancer resistance protein. J Pharmacol Exp Ther 334(2):467-476. Summary(http://www.niehs.nih.gov/news/newsletter/2010/october/science-intramural.cfm)
- Bauer B, Hartz AM, Pekcec A, Toellner K, Miller DS, Potschka H.(http://www.ncbi.nlm.nih.gov/pubmed/18094072?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum) 2008. Seizure-induced up-regulation of P-glycoprotein at the blood-brain barrier through glutamate and cyclooxygenase-2 signaling. Mol Pharmacol73(5):1444-1453. Story(http://www.niehs.nih.gov/news/newsletter/2008/may/epilepsy.cfm)