A new study indicates that mutant Cu/Zn superoxide dismutase (SOD1) enzymes that are associated with an inherited form of Lou Gehrig's disease cause the protein to become sticky in tissues. Partial unfolding of the mutant protein can expose hydrophobic residues that may promote abnormal interactions with other proteins or membranes in the cell.
The research appears as the "Paper of the Week" in the August 19 issue of the Journal of Biological Chemistry, an American Society for Biochemistry and Molecular Biology journal.
Over 5,600 people in the U.S. are diagnosed with amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease each year. About 30,000 Americans have the disease at any given time, and 10% of cases are inherited.
"Amyotrophic lateral sclerosis is a neurodegenerative disorder in which neurons of the motor pathways in the brain and spinal cord die," explains Dr. Lawrence J. Hayward of the University of Massachusetts Medical School. "It typically strikes during middle age, and although it may start with only mild weakness, the symptoms can spread insidiously over months to impair mobility, speech and swallowing, and ultimately the muscles required for respiration."
Despite the prevalence of ALS, the biological mechanisms that kill the motor neurons in most patients are incompletely understood. However, for a fraction of inherited ALS patients, mutations in the gene for SOD1 cause the disease by creating a toxic enzyme. Evidence suggests that misfolding or partial unfolding of mutant SOD1 proteins in these patients might be key to the toxicity.
Hoping to learn more about how SOD1 contributes to ALS, Dr. Hayward began to study the properties of several ALS-causing SOD1 mutants in research sponsored by the National Institutes of Health and the ALS Association.
"Our efforts have focused upon trying to explain how over 100 different mutant forms of SOD1 cause inherited ALS," says Dr. Hayward. "The initial results were puzzling because some mutations had dramatic effects on copper and zinc binding, enzymatic activity, and stability of the protein, but many other mutations seemed to cause only subtle changes in these properties in vitro. Yet all of the mutants were known to be toxic in patients."
As a result of several additional experiments done in his lab and by other groups, Dr. Hayward suspected that the mutant proteins might be more vulnerable than the normal enzyme to specific stresses in tissues. In their Journal of Biological Chemistry paper, Dr. Hayward and his colleagues at the University of Massachusetts Medical School show that when the mutant SOD1 enzymes are exposed to reagents that can disrupt some of the protein's bonds or remove its metal ions, they become much stickier than the normal protein.
"The mutants, but not the normal SOD1, adhere to a hydrophobic or 'greasy' surface, and this property could promote abnormal interactions with other proteins or membranes in the cell," explains Dr. Hayward. "How well different tissues can handle this burden of sticky protein, especially during aging, may be one factor that determines which cell types are most vulnerable in the disease. It was interesting to us that the adherent forms were not restricted to the nervous system in the mouse models but were also seen in other tissues such as heart and skeletal muscle. It is possible that this property could contribute to abnormalities in muscle, while other tissues such as kidney do not accumulate hydrophobic SOD1 despite a high expression level of the mutants."
These results may lead to new treatments for some forms of ALS. For example, if researchers can minimize the hydrophobic exposure or can understand how certain tissues prevent build-up of the sticky forms of SOD1, they might be able to boost defenses in tissues known to be susceptible to mutant SOD1 accumulation.