ScienceDaily (Apr. 30, 2009) — Scientists have identified a protein that 
appears not only to be central to the process that causes Parkinson's disease 
but could also play a role in muting the high from methamphetamine and other 
addictive drugs. 
The action of the protein, known as organic cation transporter 3 or oct3, 
fills a longstanding gap in scientists' understanding of the brain damage that 
causes symptoms like tremor, stiffness, slowness of movement and postural 
instability. While these are found mainly in patients with Parkinson's 
disease, there are more than three dozen other known causes of this array of 
symptoms, known as "parkinsonism."
In a paper published online this week in the Proceedings of the National 
Academy of Sciences, scientists at the University of Rochester Medical Center 
and Columbia University have shown that oct3, a protein that shepherds 
molecules into and out of cells, plays a critical role, bringing toxic 
chemicals to the doorstep of the brain cells that die in patients with 
Parkinson's disease. The team found that oct3 is involved in the brain's 
response to addictive drugs like methamphetamine as well.
Precisely what causes Parkinson's disease remains largely a mystery. Some 
cases have a known genetic basis, and most others are attributed to 
environmental causes or a combination of gene-environment interactions. 
Doctors know that symptoms of Parkinson's stem from the death of a very small, 
specialized group of brain cells known as dopamine neurons, which produce a 
chemical needed by another area of the brain to help us move freely. It's not 
until most of those brain cells have already died that patients begin to show 
symptoms.
For decades, scientists have been trying to understand why those cells die. 
The latest paper supports a role for astrocytes, a type of cell that is the 
most common in the brain but which has been often overlooked by scientists 
focused more on cells known as neurons that send electrical signals. 
Astrocytes' role in Parkinson's is no surprise to brain experts who have also 
identified them as a player in Alzheimer's disease, amyotrophic lateral 
sclerosis, epilepsy, and other diseases.
"Astrocytes are definitely much more than support cells in the brain," said 
Kim Tieu, Ph.D., a corresponding author of the paper and assistant professor 
in the Department of Environmental Medicine at the University of Rochester 
Medical Center. "Scientists are discovering their involvement in many 
diseases. The latest results point to their role in Parkinson's disease."
Tieu initiated the study while a post-doctoral research associate in the 
laboratory of Serge Przedborski, M.D., Ph.D., the Page and William Black 
Professor of Neurology at Columbia University and a corresponding author. They 
chose to study how the brain handles a chemical known as MPTP, which 
ultimately damages the exact same brain cells that are injured in patients 
with Parkinson's disease. While MPTP does not cause Parkinson's disease, 
scientists regularly use it as a model for the disease because it causes an 
identical type of brain damage.
In the brain, MPTP is converted primarily in astrocytes to a chemical called 
MPP+, which is deadly to dopamine neurons. More than 20 years ago, as a 
graduate student with Solomon Snyder, M.D., Jonathan Javitch, M.D., Ph.D., now 
professor of psychiatry and pharmacology at Columbia and an author on the 
current paper, concluded that MPP+ is released from astrocytes before it kills 
dopaminergic neurons. But exactly how MPP+ is freed from astrocytes was 
unknown.
In this week's PNAS paper, the scientists finger oct3 as the shepherd that 
escorts toxic MPP+ out of the astrocytes and into the space surrounding 
dopamine neurons. That's where another molecule known as the dopamine 
transporter picks it up and brings it into the neuron itself.
When the team blocked or genetically removed oct3 in mice, the dopamine 
neurons in the brains did not die despite the presence of MPTP in the brain. 
Without oct3, MPP+ remained sequestered inside astrocytes and did not affect 
the dopamine neurons. And when oct3 was present in the usual amounts, dopamine 
neurons died as expected.
"The neurons affected in Parkinson's disease don't live in isolation in the 
brain," said Przedborski. "You must understand the brain environment as a 
whole to understand disease. For many years, people had a neuron-centric view 
of neurodegenerative diseases. But more and more scientists are realizing that 
if you wish to understand the process of neurodegeneration, you must take into 
account the astrocytes, the microglia, as well as the neurons. Astrocytes 
maintain an intimate relationship with neurons, and to understand one, you 
have to understand the other."
The team also analyzed brain tissue from people who died of Parkinson's 
disease and found that oct3 is active in astrocytes in the brain region 
affected by Parkinson's disease. They found the same thing in mice, where the 
absence of oct3 correlated exactly to areas of the brain where neurons were 
not damaged.
The team also showed that oct3 plays a role in the brain's response to 
methamphetamine. Oct3 is critical for helping astrocytes soak up excess 
dopamine in the space around neurons. When dopamine isn't removed as quickly 
or thoroughly as usual, people can feel euphoric, but they can also experience 
brain damage. The finding that oct3 may play a role matches other scientists' 
observations that people in whom oct3 activity is reduced have a higher 
potential for addiction.
The molecule might also offer a new target for treating depression. Many anti-
depressants work by allowing the brain chemical serotonin to stay available in 
the brain longer than it otherwise would. Since one of oct3's functions is to 
remove serotonin from the brain, blocking it may offer a new avenue to treat 
depression.
The chemicals that the team used to block oct3 in mice would be toxic in 
people, and there is no drug available for people now that blocks or boosts 
oct3, Tieu and Przedborski said. But such a drug might be useful for 
Parkinson's, drug addiction, and depression.
"How you choose to manipulate the function of oct3 depends on the source of 
the toxic molecules," said Tieu, who is also a scientist in the University's 
Center for Neural Development and Disease. "You would try to lessen its 
effects in a condition where it makes a toxic molecule available to vulnerable 
cells, as illustrated in the current model of Parkinson's disease. But in the 
case of drug addiction, you might try to increase it, to lessen the impact of 
a drug like methamphetamine."
Other authors at the University of Rochester include post-doctoral research 
associates Mei Cui, Ph.D., Radha Aras, Ph.D., and Mamata Hatwar, Ph.D.; 
graduate student Whitney Christian; medical and graduate student Phillip 
Rappold; former undergraduate student Joseph Panza; and Ned Ballatori, Ph.D., 
professor of environmental medicine. At Columbia, Vernice Jackson-Lewis, 
Ph.D., associate research scientist, also contributed to the research. The 
work was funded by the National Institute of Environmental Health Sciences.

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