That link is Protein Disulphide Isomerase (PDI), a chaperone protein that is necessary for proper protein folding in times of cellular stress. Published in today's issue of Nature, these findings revealed that in patients with Alzheimer's and Parkinson's Disease, overproduction of free radicals, specifically nitric oxide (NO), causes inhibition of PDI by a reaction called S-nitrosylation, thereby reducing PDI's neuroprotective benefits. This data provides the first molecular link between NO free radicals and protein misfolding, which is currently thought to be a common pathway in the pathogenesis of virtually all neurodegenerative conditions. Such conditions also include ALS (or Lou Gehrig's disease), Huntington's disease, and many others. Understanding the PDI pathway may lead to the development of new therapeutic approaches for these neurodegenerative diseases and other disorders associated with abnormal protein accumulations due to cellular stress.
"To our knowledge, this is the first published evidence of a link between protein misfolding due to enzymatic machinery malfunction found in a number of degenerative diseases and free radical stress in nerve cells," said Stuart A. Lipton, M.D., Ph.D., Professor and Director of the Del E. Webb Center for Neurosciences and Aging at the Burnham Institute and senior author of the study. Dr. Lipton is also a clinical neurologist in La Jolla. "Our data demonstrate a previously unrecognized relationship between NO and protein misfolding in degenerative disorders, showing that PDI can be a target of NO in cellular models of Parkinson's disease and human neurodegenerative disease."
A protein's structure determines its function. Genetic defects as well as exposure to free radicals or possibly other types of cellular stress can cause small structural defects that lead to protein misfolding. If the misfolded proteins cannot be refolded properly or degraded, they may build up in the cell to cause dysfunction. Defects in either the protein folding or degradation pathways can lead to accumulation of misfolded proteins. The accumulation of misfolded proteins is a common pathogenic mechanism in many diseases, including neurodegenerative disorders.
In normal circumstances, PDI levels increase in response to accumulation of misfolded proteins due to cellular stress. PDI acts as a chaperone for aggregated proteins, rearranging their chemical bonds and thus refolding the proteins to function normally. The new research by Dr. Lipton and his colleagues shows that molecules related to the free radical NO, which is present in elevated levels in neurodegenerative diseases, attacks PDI via a chemical S-nitroyslation reaction, altering PDI's structure and blocking its normal neuroprotective function, which ultimately leads to nerve cell injury and even death. These new results also show that this altered form of PDI is present in elevated amounts in patients with Alzheimer's and Parkinson's Disease, indicating that it is a potential marker for the disease as well as a potential therapeutic target.