25 Jun 2006 (Medscape) - Howard Hughes Medical Institute researchers
have pinpointed defects in a critical cellular pathway that can lead
to the death of dopamine-producing nerve cells and ultimately
Parkinson's symptoms. Their studies have also identified a new way
to rescue dying neurons in several animal models of Parkinson's
disease.
According to the researchers, the findings offer a promising
opportunity for developing new drugs to treat the underlying causes
of Parkinson's disease and related neurodegenerative disorders.

The research team, which included Howard Hughes Medical Institute
investigators Susan L. Lindquist and Nancy M. Bonini, published
their findings in June 2006, in Science Express, which provides
electronic publication of selected Science papers in advance of
print. Lindquist is at the Whitehead Institute for Biomedical
Research and Bonini is at the University of Pennsylvania. Antony
Cooper of the University of Missouri-Kansas City and Aaron Gitler,
who is in Lindquist's laboratory, were co-lead authors on the paper.
Other co-authors were from Purdue University, the University of
Alabama, Medical College of Georgia and New York University.

The researchers' began their experiments seeking to clarify the role
of the protein alpha-synuclein in Parkinson's disease. It had long
been known that abnormalities in alpha-synuclein could cause a
lethal buildup of the protein in neurons. Researchers also knew that
accumulation of alpha-synuclein caused neurodegeneration in animal
models of Parkinson's disease, but little was known about alpha-
synuclein's normal cellular function or how it contributed to
disease. One major problem facing researchers, Lindquist said, was
that alpha-synuclein accumulation causes a range of abnormalities,
and it was not possible to sort out which were causes and which were
effects in Parkinson's disease pathology.

However, Lindquist's team developed a technique to switch on
overproduction of alpha-synuclein in yeast, so they could determine
which abnormalities arose earliest in the pathological process.
Those experiments by Cooper revealed that an important early defect
affected the machinery that transports proteins between two major
cellular organelles -- the endoplasmic reticulum (ER) and the Golgi
apparatus. The endoplasmic reticulum is the site of protein
production, and the Golgi apparatus is the cell's "post office,"
which modifies, sorts and adds the molecular addresses that
designate the specific destinations in the cell where proteins are
needed.

Lindquist and her colleagues had conducted a genetic screen in yeast
to discover genes whose activity affected the toxicity of alpha-
synuclein. That study showed that genes enhancing ER-to-Golgi
trafficking prevented alpha-synuclein toxicity. In particular, they
found that one protein, called Ypt1p, which is involved in
regulating trafficking could also be switched on to suppress alpha-
synuclein toxicity in yeast cells.

"Our findings indicated that this ER-to-Golgi trafficking pathway is
intimately coupled to the pathology, although in humans there are
likely others involved as well, given how many genes we found that
modified alpha-synuclein toxicity," said Lindquist. "But these
findings were so persuasive that we decided we needed to test
whether enhancing Ypt1p activity would suppress alpha-synuclein
toxicity in animal models of the disease. Fortunately, we had an
excellent team of collaborators with expertise in these models, who
could conduct these studies."

The researchers next studied whether enhancing activity of the
mammalian Ytp1p counterpart, called Rab1, suppressed alpha-synuclein
toxicity in the fruitfly Drosophila, the roundworm C. elegans and in
cultures of rat neurons. Bonini and her colleagues tested the effect
in fruitflies; co-author Guy Caldwell and his colleagues at the
University of Alabama performed the tests in roundworms; and co-
author Jean-Christophe Roche and his colleagues at Purdue performed
the tests in rat neurons. Caldwell is also coordinator of HHMI's
Undergraduate Research Intern Program at the University of Alabama.

"They all came back with the same answer," said Lindquist. "All saw
significant suppression of toxicity; although none saw complete
suppression, which confirms our yeast studies showing that other
pathways are affected by alpha-synuclein accumulation. However,
importantly, the results of our genetic screen have given us a way
to ask important questions about these other aspects of alpha-
synuclein toxicity," she said.

Lindquist also said the findings give important clues to why
dopamine-producing neurons in the brain are the most vulnerable
neurons to toxic alpha-synuclein accumulation. The death of such
neurons reduces brain dopamine levels, causing the tremors and other
symptoms of Parkinson's disease. Dopamine is one of many types of
neurotransmitter -- chemical signals that one neuron launches at its
neighbor to trigger a nerve impulse.

"Of all the neurotransmitters, dopamine has a higher potential for
being toxic," she said. "Its toxicity is normally prevented in the
neuron by sequestration within vesicles for transport from the ER.
But a defect in ER trafficking caused by alpha-synuclein
accumulation could cause the toxic buildup of dopamine to occur in
these neurons."

Lindquist and her colleagues believe their findings will guide the
search for new drugs that suppress alpha-synuclein toxicity by
enhancing the machinery of ER-to-Golgi transport. Thus, she said,
they have already conducted a screen of 150,000 compounds for those
with therapeutic potential.

"We have found compounds that reverse alpha-synuclein toxicity, and
we plan to publish those results soon," she said. "These findings
are exciting because they tell us we have a platform for discovering
new therapeutic strategies and for speeding the process of
discovering treatments for these disorders."

Current treatments for Parkinson's disease do not aim at protecting
the dopamine-producing neurons themselves. Rather, the treatments
seek to restore dopamine levels in the brain or to treat symptoms of
the disease.

Lindquist cautioned that the findings "have not by any means proven
that this mechanism of pathology or the compounds that affect it are
relevant to humans. However, given the fact that we've found the
same results in yeast, flies, worms and rat neurons, I would be very
surprised if we didn't find that they were relevant in humans," she
said.

Bonini added that the research team's findings illustrate the power
of animal models in revealing insight into Parkinson's
disease. "These results highlight the value and importance of very
simple model organisms in studying these disorders," she said. "For
example, yeast is only a single cell, not even a neuron, and yet it
can reveal proteins that modify the toxicity of alpha-synuclein. And
in flies, it is possible to study the effects of these proteins on
alpha-synuclein toxicity in dopaminergic neurons. Clearly, these
kinds of basic research collaborations, in which you can progress up
the evolutionary tree using multiple model organisms, will open the
door to new therapeutic opportunities for Parkinson's," said Bonini.