The Chemical Connection

Designs on Aging-Ready

By Carrie Arnold

In the summer of 1982, a handful of patients appeared in emergency rooms across California. One day, they were completely healthy; the next, they were unable to walk or talk. Many had tremors. The symptoms were classic Parkinson’s disease, but the young age of the patients and the abrupt onset of problems said otherwise. A scientific investigation revealed that these individuals had injected themselves with a new type of synthetic heroin that was contaminated with a chemical called MPTP.

The mystery caught the attention of a young Timothy Greenamyre, then a medical student at the University of Michigan, who followed the saga as it unfolded. MPTP, the researchers concluded, selectively and irreversibly damaged the dopamine-making neurons of the substantia nigra. To Greenamyre, this demonstrated that an environmental chemical could cause Parkinson’s disease. He began to wonder whether there might be others.

There were, including rotenone, a pesticide considered safe to humans as late as the early 2000s. Greenamyre would later achieve broad recognition in the Parkinson’s field for creating a rat model of Parkinson’s using rotenone, which has helped scientists uncover novel risks for the disease. More recently, Greenamyre’s work has opened the way to potential new therapeutics.

“If we could slow the progression of disease and expand the therapeutic window in which treatments work, it would be a huge advance,” says D. James Surmeier, a neuroscientist at Northwestern University.

When Greenamyre was in medical school, papers interrogating the MPTP problem began appearing. Greenamyre followed as molecular studies showed that MPTP killed cells by inhibiting a large complex of proteins called Complex 1. Without Complex 1, mitochondria couldn’t produce energy, and the affected cells died. Postmortem studies of people with Parkinson’s revealed their Complex 1 enzymes were also malfunctioning.

After graduating from Michigan and completing his residency there, Greenamyre accepted a faculty job at the University of Rochester and later moved to Emory University. Then, as now, his focus was on how the brain directed the body’s movements, and how that could go wrong. If MPTP could cause Parkinson’s symptoms in humans and animals by gumming up Complex 1, Greenamyre reasoned, maybe other Complex 1 inhibitors could, too. He selected rotenone, a pesticide used by home gardeners and wildlife biologists. Greenamyre infused rats daily with a rotenone solution and waited. It didn’t take long. After a week, the rats began showing the specific jerky movements that were Parkinson’s hallmarks. When Greenamyre peered at brain tissue under the microscope, he saw a dramatic loss of dopamine brain cells in the substantia nigra. In surviving brain cells, he found clumps of a protein known as α-synuclein that is characteristic of Parkinson’s. While still at Emory, Greenamyre published his findings in a 2000 paper in Nature Neuroscience.

“What got people’s interest—and ours, too—was the fact that for the first time in an animal model, we saw an accumulation and aggregation of α-synuclein,” Greenamyre says. “We had mitochondrial damage linked to pesticide exposure, linked with symptoms, linked with α-synuclein. It was the trifecta of Parkinson’s disease.”

For Parkinson’s scientists, the results were groundbreaking. “It may be the most important discovery he’s made. This work raised people’s awareness about the importance of environmental agents in promoting Parkinson’s. Things that we think are safe may not be safe,” says Surmeier.

Importantly for Parkinson’s scientists, the rotenone model captured the full molecular and cellular breadth of the disease. The work also bolstered support for two emerging hypotheses about Parkinson’s: that the disease was intimately connected to dysfunction in the mitochondria, and that pesticide exposure increased Parkinson’s risk.

In 2004, not long after publishing his groundbreaking rotenone paper, Greenamyre decamped from Atlanta to head up the Pittsburgh Institute for Neurodegenerative Diseases (PIND). For him, it was love at first sight. At many other universities, labs were siloed, each closed off in its own room with scientists toiling away behind closed doors. At Pitt, however, Greenamyre walked into an open floor plan designed to encourage creativity, where researchers from different labs mingled at benches throughout the space.

“One lab begins where another one has ended. There’s no walls, there’s no doors. It knocks down the inertia that stops you from going and knocking on somebody’s door to ask if they could help you with a problem or why their Western blots look so much better than yours,” says Greenamyre, who is now the Love Family Professor of Neurology.

Pitt neurologist Edward Burton, UPMC Professor of Movement Disorders, was drawn to Greenamyre’s kindness and understated enthusiasm. “Tim has been an outstanding mentor, scientific collaborator and colleague,” says Burton, professor of neurology and of microbiology and molecular genetics. “He is full of great ideas.”

His rotenone model let Greenamyre dissect the earliest molecular events on the long path to Parkinson’s and determine how different genetic variants interacted with the effects of exposures to pesticides.

“The billion-dollar question in Parkinson’s disease is, What is the initial thing that happens? What happens first? If we knew what happened, we could target Parkinson’s at the earliest stages,” says Sarah Berman, professor of neurology and of clinical and translational science at Pitt School of Medicine.

Painstaking studies over more than a decade in Greenamyre’s lab and others have fueled a far more complete understanding of Parkinson’s. His new work—revealing how oxidative stress builds up and damages neurons—is providing a career bookend that is perhaps as important as his early rotenone studies.

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