RESEARCHERS REVERSE DOPAMINERGIC NEURON LOSS IN PARKINSON MODELS  

                       [ORIGINAL ARTICLE]    Samson, Kurt

ARTICLE IN BRIEF

[check mark] By introducing a gene that manufactures an excess of a special cellular transfer protein, investigators countered alpha-synuclein's toxic gridlock and restored normal traffic to dopamine-producing neurons, according to a new study.

Working with rudimentary animal models of Parkinson disease (PD), a multicenter team of researchers found a way to protect and rescue neurons from degeneration and death. They did so by genetically bolstering proteins involved in a key transport mechanism between cells that fails due to the rogue activity of another protein's genetic mutation.

If replicated in human studies, the research could lead to new therapeutic approaches to PD, according to authors of the study published in the July 21 Science (2006;313: 24–328).

The researchers discovered that a genetic defect in PD causes a surplus of the protein alpha-synuclein to accumulate in nerve cells, blocking the internal traffic of proteins necessary for normal cellular activity in the substantia nigra. By introducing a gene that manufactures an excess of a special cellular transfer protein, they countered alpha-synuclein's toxic gridlock and restored normal traffic to dopamine-producing neurons.

“For the first time we were able to repair dopaminergic neuron loss, the specific cells that are damaged in Parkinson disease,” said study co-author Aaron Gitler, PhD, a Post-doctoral Fellow at the Whitehead Institute for Biomedical Research at the Massachusetts Institute of Technology in Cambridge, MA.

Researchers have long associated the presence of Lewy bodies with the loss of dopamine-producing neurons in the substantia nigra. Exactly how Lewy bodies contributed to the pathogenesis of the disease has been a puzzle, however.

LEWY BODIES: TOXIC OR PROTECTIVE?

“It's controversial right now whether Lewy bodies are toxic or protective. For example, forming clumps of toxic proteins might be a good defense mechanism to sequester a toxic protein away from the rest of the cell,” Dr. Gitler told Neurology          Today in a telephone interview. But, he added that mutations in the gene that encodes for the alpha-synuclein protein have been identified in families with early-onset PD, and some of these may increase the propensity to aggregate.

“Alpha-synuclein is the most abundant protein in Lewy bodies, but no one knew its function,” noted Dr. Gitler, who works in the laboratory of lead author Susan L. Lindquist, PhD, a Whitehead and Howard Hughes Medical Institute researcher and Professor of Biology at MIT.

“Our approach was simple. If all cells have to deal with aggregating proteins, the mechanisms employed to deal with them are likely highly conserved, and using simple model systems may allow us to uncover novel mechanisms to circumvent the toxic effects of these accumulations,” he explained.

THE YEAST FACTORY

Three years ago, the team first engineered yeast cells that manufactured human alpha-synuclein. They discovered that cells that made too much of the protein quickly withered and died. Excess alpha-synuclein, they observed, interfered with the shuttling of packets of proteins between the endoplasmic reticulum (ER), which produces proteins in cells, and the Golgi apparatus, which sorts, modifies, and directs them as needed.

They reasoned that if alpha-synuclein mutations impede this shuttling activity, genetically reinforcing ER-Golgi transport proteins might counter this interference.

The investigators identified 34 yeast strains that overexpressed Ypt1p, a protein involved in ER-Golgi trafficking. By genetically increasing expression of Ypt1p in yeast models of the disease, the team found they could suppress alpha-synuclein's interference in cellular transport and prevent cell degradation and death. They then discovered that the same process could be used in fruit flies, C. elegans, and in rat brain cells.

“We tried this a number of different ways, from creating transgenic animals that naturally overexpressed this protein, to injecting a copy of the gene for this transport protein into the neurons through a gene-therapy technique. In all cases the results were the same: cell death ceased, and the neurons were restored to normal health,” Dr. Gitler explained.

The researchers next genetically enhanced the activity of Ypt1ps mammalian counterpart, called Rab1, and found boosting Rab1 expression likewise countered alpha-synuclein toxicity in cultured rat neurons. Toxicity was determined by measuring the number of dopamine-producing neurons and by observing the physical status of cells.

The researchers are continuing to work their way up the evolutionary ladder, exploring whether other transport genes identified in the yeast factory can provide similar neuroprotective effects in Parkinson models.

THERAPEUTIC POSSIBILITIES

Instead of trying to protect dopamine-producing neurons themselves, current treatments for Parkinson disease try to restore dopamine levels in the brain or to treat symptoms of the disease. While the team's findings may offer new therapeutic possibilities, Dr. Gitler cautioned that the research is far from being applicable to PD in humans.

“I need to stress that we are in the real early days of this research. It's intriguing, but there are a lot of details that still need to be worked out.”

While the technique significantly suppressed toxicity in almost all cases, none saw complete suppression, he pointed out. “This confirms our yeast studies showing that other pathways are [also] affected by alpha-synuclein accumulation,” Dr. Gitler said. “In humans there are probably other pathways involved, given how many genes we found that modified alpha-synuclein toxicity, and also other proteins.”

The team has also screened some 150,000 chemical compounds and identified several that appear to reverse alpha-synuclein toxicity, findings the researchers plan to publish soon, Dr. Gitler told Neurology Today.

EXPERTS COMMENT

If the research is corroborated in further studies, the researchers may have identified the earliest point at which alpha-synuclein begins to affect the dopaminergic process on neurons, Ted Dawson, MD, PhD, told Neurology Today in a telephone interview. If so, it would make an ideal target for new therapeutic approaches, he said. Dr. Dawson is the Leonard and Madlyn Abramson Professor of Neurodegenerative Diseases at the Johns Hopkins University School of Medicine's Institute for Cell Engineering, in Baltimore, MD.

“This is a really interesting study,” Dr. Dawson continued. “Although there are always problems applying findings in worms and fruit flies to higher species, there's a distinct possibility that the findings can be replicated in living animals and humans. The fact that rat neurons in culture responded is encouraging. As things go, it appears very promising.”

If genetic misfolding causes alpha-synuclein's action on the ER-Golgi transport mechanism, it suggests a potential target for gene therapy. However, if chemical compounds can be used to counter the defect, their therapeutic potential would be even greater, Dr. Dawson said.

He noted that other research is also yielding important insights into alpha-synuclein activity in the dopamine transport process, as well as other avenues that might one day be used to prevent or arrest dopaminergic neuron loss.

“We shouldn't discount all the other research looking for ways to prevent alpha-synuclein, but this is the first paper up the pike in that direction. The primary importance is that they may have identified the earliest defect,” he said.

“I find it most compelling that if [the team] is correct in yeast and these other models, and if the ER-Golgi is truly the first step in the process, then we should focus on that for therapeutic intervention. But the question remains whether or not there are other pathways or anything further upstream.”