Working with human neurons and fruit flies, researchers at Johns Hopkins have identified and then shut down a biological process that appears to trigger a particular form of Parkinson's disease present in a large number of patients. A report on the study, in the April 10 issue of the journal Cell, could lead to new treatments for this disorder.
"Drugs such as L-dopa can, for a time, manage symptoms of Parkinson's disease, but as the disease worsens, tremors give way to immobility and, in some cases, to dementia. Even with good treatment, the disease marches on," says Ted Dawson, M.D., Ph.D., professor of neurology and director of the Johns Hopkins Institute for Cell Engineering,
Dawson says the new research builds on a growing body of knowledge about the origins of Parkinson's disease, whose symptoms appear when dopamine-producing nerve cells in the brain degenerate. Further evidence for a role of genetics in Parkinson's disease appeared a decade ago when researchers identified key mutations in an enzyme known as leucine-rich repeat kinase 2, or LRRK2 — pronounced "lark2." When that enzyme was cloned, Dawson, together with his wife and longtime collaborator Valina Dawson, Ph.D., professor of neurology and member of the Institute for Cell Engineering, discovered that LRRK2 was a kinase, a type of enzyme that transfers phosphate groups to proteins and turns proteins on or off to change their activity.
Over the years, it was found that blocking kinase activity in mutated LRRK2 halted degeneration, while enhancing it made things worse. But nobody knew what proteins LRRK2 was acting on.
"For nearly a decade, scientists have been trying to figure out how mutations in LRRK2 cause Parkinson's disease," said Margaret Sutherland, Ph.D., a program director at the National Institute of Neurological Disorders and Stroke. "This study represents a clear link between LRRK2 and a pathogenic mechanism linked to Parkinson's disease."
Dawson went fishing for the right proteins using LRRK2 as bait. When his team began to identify those proteins, Dawson says they were surprised to discover that many were linked to the cellular machinery, like ribosomes, that make proteins. Nobody, says Dawson, suspected that LRRK2 might be involved at such a basic level as protein manufacture.
Unsure if they were right, the team then tested the proteins they identified to see which of them, if any, LRRK2 could add phosphate groups to. They came up with three ribosomal protein candidates — s11, s15 and s27. They then altered each ribosomal protein to see what would happen. It turned out that mutating s15 in a manner that blocked LRRK2 phosphorylation protected nerve cells taken from rats, humans and fruit flies from death. In other words, s15 appeared to be the much sought-after target of LRRK2, Dawson says.
"When you go fishing, you want to catch fish. We just happened to catch a big one," Dawson says.
With the protein now identified, Dawson's team is tackling further experiments to find out how excess protein production causes dopamine neurons to degenerate. And they want to see what happens when they block LRRK2 from phosphorylating the s15 protein in mice, to build on their findings from fruit flies and nerve cells grown in a dish.
"There's a big chasm between animal disease models and human treatments," says Ian Martin, Ph.D., a neuroscientist in Dawson's lab and the lead author on the paper. "But it's exciting. I think it definitely could turn into something real, hopefully in my lifetime."
Other authors on the paper include Jungwoo Wren Kim, Byoung Dae Lee, Hochul Kang, Jin-Chong Xu, Hao Jia, Jeannette Stankowski, Min-Sik Kim, Jun Zhong, Manoj Kumar, Shaida Andrabi, Yulan Xiong, Dennis Dickson and Akhilesh Pandey from the Johns Hopkins University School of Medicine and Zbigniew Wszolek of the Mayo Clinic.
The Johns Hopkins University is part of the Morris K. Udall Centers of Excellence for Parkinson's Disease. This work was supported by the National Institute of Neurological Disorders and Stroke (P50 NS038377 and P50 NS072187), the JPB Foundation, the Maryland Stem Cell Research Fund, the Adrienne Helis Malvin Medical Research Foundation and the Diana Helis Henry Medical Research Foundation.
Funding for a portion of the research described in this article was provided by Merck KGAA. Under a licensing agreement between Merck KGAA and The Johns Hopkins University, Dawson and the university shared fees received by the university on licensing of some of the reagents described in this article. Dawson was also a paid consultant to Merck KGAA. The terms of this arrangement are being managed by The Johns Hopkins University in accordance with its conflict of interest policies.