Johns Hopkins researchers have discovered that nitric oxide, a chemical messenger involved in bodily functions from erection to nerves' communication, also shuts down a protein involved in Parkinson's disease. The finding finally provides a biochemical link between Parkinson's disease (PD) that occurs in families and the vast majority of cases which occur randomly in the population, as well as giving researchers a brand new target for developing treatments to slow or stop the disease.
The protein in question is parkin, and earlier research had shown that mutations that cripple it occur in about a third of patients with familial PD, but rarely show up in the much more common sporadic cases of the disorder. In the absence of these mutations, however, scientists weren't sure how, or even whether, malfunction of parkin was involved in the disease.
In the April 23 issue of Science, the Hopkins team reports that nitric oxide (NO) attaches to parkin and reduces its normal ability to mark proteins — including itself — for destruction. However, in animal models of PD, there's so much NO on parkin that the protein doesn't work at all. Moreover, NO modification of parkin was two to three times higher in brain tissue from patients with PD than in those without the disease, the researchers report.
"In every tissue sample from patients, the level of NO on parkin was higher than the very highest level measured in brain tissue from people without the disease," says Ted Dawson, M.D., Ph.D., professor of neurology and neuroscience and co-director of the Program for Neural Regeneration and Repair in Hopkins' Institute for Cell Engineering. "This tells us that very effective NO scavengers, ones that cross the blood brain barrier and enter neurons, could be potential drugs to treat Parkinson's disease." While one doesn't yet exist, such a scavenger should mop up extra NO in the brain, he says, preventing it from blocking parkin's activity.
Other ways of reducing NO, such as preventing its production in cells, are less likely to work well because the molecule is so important to humans' normal function, from sending and receiving signals in the brain to relaxing and contracting blood vessels in order to control blood pressure. The researchers point out that, based on their work, NO modification of parkin is a normal process somehow gone awry in Parkinson's disease.
In normal cells and normal mice, postdoctoral fellow Kenny Chung found that NO is attached to parkin and regulates its activity. But in a mouse model of PD and in patients with PD or a similar condition called diffuse Lewy body disease, NO modification was so high parkin couldn't do its job at all. "We looked at NO modification of parkin step-by-step from the most basic level of biology — an in vitro protein system — all the way to patient tissue," says Chung, who is working on identifying the tools cells use to add NO to parkin.
Parkinson's disease is characterized by gradual loss of brain cells that make a chemical called dopamine. There is no cure, although treatments are available that help slow its progression. Genetic mutations in parkin that cause the protein to fail are thought to contribute to the disease in two ways. First, parkin's normal targets — including itself — are not marked for destruction as they should be. Second, any abnormal proteins can't be marked for destruction, either. Both failures likely contribute to protein build-up and the formation of protein gobs — so-called Lewy bodies — in cells that die in PD. By shutting down parkin proteins that are otherwise normal, excessive NO modification of parkin could contribute to PD in the same ways, the researchers say.
The research was funded by the United States Public Health Service, the Edward D. and Anna Mitchell Family Foundation, and the Mary Lou McIlhaney Scholar Award. Authors on the paper are Chung, Dawson, Bobby Thomas, Xiaojie Li, Olga Pletnikova, Juan Troncoso, Laura Marsh and Valina Dawson, all of Johns Hopkins. On the Web: http://www.sciencemag.org