By KRISTA CONGER
Mutant mice whose brains gradually become peppered with small holes resembling those found in prion disease lack a protein involved in disposing of cellular trash, say medical center researchers.
The finding may shed light on how diseases such as Creutzfeldt-Jakob, bovine spongiform encephalopathy (also known as mad cow disease), and scrapie wreak havoc in humans and other mammals, and lends further support to the growing notion that glitches in protein turnover may be the unifying element in many neurodegenerative disorders.
Although the scientists caution that the mutant mice don’t accumulate the misfolded protein, or prion, associated with the infectious forms of spongiform encephalopathy, the rodents’ brains are dead-ringers for the brains of people and cows who have died from the disease. They speculate that the mutation in a ubiquitin ligase — a protein that flags other proteins for destruction by the cell’s recycling machinery — may represent a downstream step in the cascade of events that leaves the brain looking somewhat like a kitchen sponge.
"No one really understands how or why spongy degeneration develops," said Gregory Barsh, MD, PhD, professor of pediatrics and of genetics, in whose laboratory the research was conducted. "Now we have a molecular handle with which to study it." The research is published in the Jan. 31 issue of Science.
Ubiquitin ligases are just a few members of a complex team of proteins that make up the ubiquitin pathway. Together they identify and physically tag abnormal, misfolded or simply worn-out proteins for dismantling in the cell’s recycling center. Until recently they were about as glamorous as garbage collectors.
The study of neurodegenerative diseases such as Parkinson’s, Huntington’s and Lou Gehrig’s upped the ubiquitin ante, however, with the discovery that patients’ brain cells share a common trait: large clumps of proteins seemingly begging in vain for destruction.
Prion diseases such as Creutzfeldt-Jakob, scrapie (a fatal disease affecting the central nervous system of sheep and goats), or bovine spongiform encephalopathy are also associated with accumulation of a specific protein. However, a direct role for the ubiquitin pathway has been identified only in an early onset form of Parkinson’s disease — until now.
"This is the first convincing evidence that links spongiform disease to a ubiquitin-dependent protein turnover defect," said Lin He, the graduate student who performed the work together with Teresa Gunn, a former postdoctoral fellow in the Barsh lab. "We’re really excited about it."
He and Gunn studied a mutation, mahoganoid, which changes the hair color of laboratory mice from brown to black. Previous research by He, Gunn and Barsh had shown that mice with a mutation in another coat color gene, Attractin, developed spongiform degeneration and body tremor. Because mahoganoid mutants share their darkened coat color with Attractin mutants, He and Gunn wondered if they would also develop holes in their brains. They did.
"In many ways, mahoganoid or Attractin mutant animals develop a prion-like disease without prions," said Barsh.
When He and Gunn cloned the gene responsible for the mayhem, they found that the mutants were unable to express a ubiquitin ligase they named Mahogunin. Mahogunin was also identified late last year by scientists at Columbia University School of Medicine. Further experiments by the Stanford researchers suggested that the Attractin gene either activates or permits the activity of Mahogunin in the pathway of destruction, and confirmed that Mahogunin can function as a ubiquitin ligase in vitro.
He, Gunn and Barsh speculate that the absence of Mahogunin leads to the buildup of a protein or proteins in the neurons, which causes damage and eventual neuronal death. The finding confirms many researchers’ long-standing suspicions that the removal of unwanted proteins is vital to the health of neurons in the brain.
"The theme has been there, but now here’s a striking example of how protein stability is critical in this disease as well," said Peter Jackson, PhD, Stanford associate professor of pathology who, together with a graduate student in his lab, Adam Eldridge, collaborated on the work. Now He and her colleagues face their next research hurdle in their attempt to connect the dots leading to neuronal degeneration and death: identifying the natural target for Mahogunin.
Additional collaborators include Stanley J. Watson Jr., MD, PhD, professor of psychiatry, and Xinyun Lu, PhD, research investigator, both from the University of Michigan School of Medicine; and Aaron Jolly, a student at Cornell University.
Experts assess future effects of age-related disease (3/21/01)
Stanford Report, February 5, 2003