Too much protein may kill brain cells as Parkinson's progresses

Scientists may have discovered how the most common genetic cause of Parkinson's disease destroys brain cells and devastates many patients worldwide. The study was partially funded by the National Institutes of Health's National Institute of Neurological Disorders and Stroke (NINDS); the results may help scientists develop new therapies.

"This may be a major discovery for Parkinson's disease patients," said Ted Dawson, M.D., Ph.D., director of the Johns Hopkins University (JHU) Morris K. Udall Center of Excellence for Parkinson's Disease, Baltimore, MD. Dr. Dawson and his wife Valina Dawson, Ph.D., director of the JHU Stem Cell and Neurodegeneration Programs at the Institute for Cell Engineering, led the study published in Cell.

The investigators found that mutations in a gene called leucine-rich repeat kinase 2 (LRRK2; pronounced "lark two" or "lurk two") may increase the rate at which LRRK2 tags ribosomal proteins, which are key components of protein-making machinery inside cells. This could cause the machinery to manufacture too many proteins, leading to cell death.

"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 NINDS. "This study represents a clear link between LRRK2 and a pathogenic mechanism linked to Parkinson's disease."

Affecting more than half a million people in the United States, Parkinson's disease is a degenerative disorder that attacks nerve cells in many parts of the nervous system, most notably in a brain region called the substantia nigra, which releases dopamine, a chemical messenger important for movement. Initially, Parkinson's disease causes uncontrolled movements; including trembling of the hands, arms, or legs. As the disease gradually worsens, patients lose ability to walk, talk or complete simple tasks.

For the majority of cases of Parkinson's disease, a cause remains unknown. Mutations in the LRRK2 gene are a leading genetic cause. They have been implicated in as many as 10 percent of inherited forms of the disease and in about 4 percent of patients who have no family history. One study showed that the most common LRRK2 mutation, called G2019S, may be the cause of 30-40 percent of all Parkinson's cases in people of North African Arabic descent.

LRRK2 is a kinase enzyme, a type of protein found in cells that tags molecules with chemicals called phosphate groups. The process of phosphorylation helps regulate basic nerve cell function and health. Previous studies suggest that disease-causing mutations, like the G2019S mutation, increase the rate at which LRRK2 tags molecules. Identifying the molecules that LRRK2 tags provides clues as to how nerve cells may die during Parkinson's disease.

In this study, the researchers used LRRK2 as bait to fish out the proteins that it normally tags. Multiple experiments performed on human kidney cells suggested that LRRK2 tags ribosomal proteins. These proteins combine with other molecules, called ribonucleic acids, to form ribosomes, which are the cell's protein-making factories.