Published in the September 25, 2003 issue of the journal Neuron, their studies in Drosophila, the fruit fly, showed that a protein called huntingtin is critical for normal neuronal transportation. When the protein is defective, however, it appears to physically blocks traffic in the narrow axons that are the long pipes of the nerve cells.
Although defective huntingtin genes have previously been linked to Huntington's disease, this is the first study to illustrate that the defective protein may cause neuronal damage by aggregating (sticking together) and blocking axonal traffic.
"These findings support our hypothesis that blockage of neuronal transportation is related to several neurodegenerative diseases," said the study's senior author, Lawrence S.B. Goldstein, Ph.D., UCSD professor of cellular and molecular medicine and a Howard Hughes Medical Institute investigator. "In a previous studies (Nature, Dec. 6, 2001 and Neuron, Nov. 8, 2001), we provided evidence that a protein linked to plaque accumulation in Alzheimer's disease is involved in brain cellular trafficking."
In the current study, the Goldstein team removed or reduced the normal huntingtin gene in fruit fly larva. As a result, they found that vesicle traffic up and down the axon was disrupted, indicating that huntingtin's normal function was related to the transport machinery.
Next, the researchers looked at the defective, or pathogenic version of the huntingtin protein, and of other proteins that cause polyglutamine diseases, which are neurodegenerative disorders similar to Huntington's disease. They found that the disease-causing versions of all these genes inhibited the transport machinery, while non-disease versions did not.
"There may be two things going on here," Goldstein said. "The aggregates encoded by the defective genes may be physically blocking traffic in these narrow pipes, like a plumbing problem. Or, the genes may be binding to components of the machinery, interfering with their normal function."
A devastating, degenerative brain disorder with no known treatment or cure, Huntington's disease causes brain deterioration, leading to an inability to walk, talk and reason. Huntington's disease, and the lesser known polyglutamine diseases, typically begin in adulthood and progress over 10 to 30 years.
The UCSD study was supported by a grant from the National Institutes of Health. The paper's first author was Shermali Guanwardena, Ph.D., UCSD Department of Cellular and Molecular Medicine. Additional authors were Lu-Shiun Her, Ph.D., Richard G. Brusch, B.A., Ingrid R. Niesman and Louis Sintasath, B.A., UCSD Department of Cellular and Molecular Medicine; Robert A. Laymon, M.S., UCSD Department of Cellular and Molecular Medicine and Howard Hughes Medical Institute; Beth Gordesky-Gold, Department of Biology, University of Pennsylvania; and Nancy M. Bonini, Ph.D., Howard Hughes Medical Institute and Department of Biology, University of Pennsylvania.