LEXINGTON, KY (August 3, 1998) -- A team of University of Kentucky Chandler Medical Center researchers has recently discovered that Par-4, a protein, may lead to nerve cell death in Alzheimer's disease and related brain disorders. Announced in the August issue of Nature Medicine, the research results show an increased level of the protein -- Par-4 -- in nerve cells in the brains of Alzheimer's disease patients, suggesting the protein may be a contributing factor to the disease.
Blocking Par-4 might be useful as a future treatment for the disease, said Mark Mattson, Ph.D., UK professor of anatomy and neurobiology. "The study's findings identify a new molecular target upon which to aim the emerging arsenal of weapons in the battle against Alzheimer's disease," Mattson said.
Par-4 may play a critical role in nerve cell death in Alzheimer's disease and related brain disorders. Cell death is a fundamental problem in age-related neurodegenerative disorders -- in Alzheimer's disease, nerve cells critical for learning and memory die.
Previous studies have shown that nerve cells in the brains of Alzheimer's patients die by apoptosis, a term used to describe programmed cell death. During the process of apoptosis, a cell shrinks and its DNA (string-like molecules that make up genes) becomes broken into small pieces. Each gene in DNA carries the information required to produce one specific protein. When the gene is expressed, or "switched on," the protein is made. During apoptosis, genes are activated that encode "killer proteins," and these proteins signal cells to begin dying.
The identity of the "killer proteins" and their place in the cascade of events leading to cell death has been a mystery to scientists for decades. The current research may fill one more piece in the puzzle.
Vivek Rangnekar, Ph.D., associate professor of microbiology and immunology, UK College of Medicine, discovered the gene that codes for Par-4 in 1993. He demonstrated several years ago that Par-4 was conspicuously present in prostate cancer cells undergoing apoptosis as a result of androgen(testosterone) removal. Testosterone removal is a common treatment for prostate cancer that can reduce the formation of tumors of the prostate gland in humans.
Rangnekar said that Mattson became aware of his cancer research and suggested that Par-4 might play a role in the same molecular processes that occur in neurodegenerative disorders.
To investigate the protein's effects, Mattson and Qing Guo, Ph.D., genetically engineered nerve cell cultures that blocked the ability of the Par-4 protein to function. The cells became remarkably resistant to death.
"This suggests the necessity of the Par-4 gene in the molecular cascade of events that signals the beginning of cell death," Mattson said. "Our cell culture data demonstrate that Par-4 is expressed during the early stages of the cell death process."
Factors such as environmental stimulants and genetic predisposition might trigger the cell death chain of events. Once set into motion, other genes and proteins help trigger the "switches" to signal cell death. Mattson and Rangnekar continued to pursue other factors that might interact with Par-4.
In earlier studies, Mattson's team gathered evidence that a protein called amyloid-beta protein, which abnormally accumulates in the brains of Alzheimer's patients by forming structures called plaques, can cause apoptosis in cultured rat and human neurons. Amyloid-beta protein increases free radical levels in cells and disrupts the ability of nerve cells to maintain proper calcium levels -- both are contributors to cell demise.
Cell culture experiments in the present study demonstrated that Par-4 made nerve cells more vulnerable to being killed by amyloid-beta protein, and that levels of free radicals and calcium were increased within the cells. When Par-4 was blocked, the neurons were resistant to being killed by amyloid-beta protein and showed lower levels of free radicals.
Other survival-promoting proteins in brain cells, called nerve growth factors, previously were shown to suppress free radical accumulation. They also helped stabilize calcium levels.
Nerve cells were deprived of these growth factors in the present study. Levels of Par-4 rapidly increased, the cells underwent apoptosis, then died. When Par-4 was blocked and the cells were deprived of growth factors, the cells lived, as was the case with the amyloid-beta protein.
Taking their case one step further, researchers then introduced a defective human presenilin-1 gene into cell cultures. People who inherit this defective gene develop Alzheimer's disease at a very early age. Presenilin-1 mutations appear to make nerve cells vulnerable to age-related changes in the brain, such as increased levels of amyloid-beta protein, oxidative stress (a state of damage caused by reactive, oxygen-containing chemicals that can affect a single molecule or an entire organism) and reduced energy availability.
The cell cultures that had the presenilin-1 mutations exhibited increased Par-4 levels following exposure to amyloid-beta protein. More important, however, the team found that blocking Par-4 expression counteracted the adverse effects of the presenilin-1 mutations.
"These findings suggest that Par-4 plays an important role in both the rare early-onset inherited forms of Alzheimer's disease and in the more common sporadic late-onset forms of the disease," Mattson said.
Future investigations may demonstrate the role of Par-4 in other disorders, including epilepsy, amyotrophic lateral sclerosis, Parkinson's disease and stroke. Researchers are also introducing the human Par-4 gene into fertilized mouse eggs. They hope to produce mice with propensities toward cancer and neurodegenerative disorders. These mouse models would provide a valuable tool for eventual drug development and continued research on the effects of Par-4 on cell death.
Other University of Kentucky Chandler Medical Center researchers involved in the study were: Weiming Fu; Jun Xie, M.D.; Hong Luo; Stephen F. Sells; and Vimala Bondada. The study was funded by National Institutes of Health grants.
Journal
Nature Medicine