These new findings, shown in an animal study, provide further evidence that Pin1 (prolyl isomerase) is essential to protect individuals from age-related neurodegeneration and for the first time establish a direct link between amyloid plaques and tau tangles, the two abnormal structures that are considered the pathological hallmarks of this devastating disease. Led by researchers at Beth Israel Deaconess Medical Center (BIDMC) and Harvard Medical School, the study appears in the March 23, 2006 issue of the journal Nature.
"A century ago, in 1906, the German doctor Alois Alzheimer first observed an abundance of these plaques and tangles in the brains of Alzheimer's patients," explains the study's senior author, Kun Ping Lu, MD, PhD, an investigator in the Division of Cancer Cell Biology at BIDMC and Associate Professor of Medicine at Harvard Medical School.
"Throughout the years, intensive studies have been done to find out the causes of these two major lesions, but the exact relationship between the two has remained controversial and elusive," he adds. "Coupled with recent independent studies showing that genetic changes in the human Pin1 gene are associated with reduced Pin1 protein levels as well as an increased risk of Alzheimer's disease, these new results suggest that lack of sufficient Pin1 enzyme may be a key culprit in the onset of Alzheimer's disease."
Lu, together with Tony Hunter from the Salk Institute, first identified the Pin1 enzyme in 1995. Eight years later, in 2003, Lu and his colleagues demonstrated that Pin1 promoted dephosphorylation of tau, thereby "detangling" the protein which had become knotted and overburdened with excess phosphate molecules. They also confirmed that when Pin1 was missing, neurons in the regions of the brain responsible for memory would collapse under the weight of the tau protein tangles, ultimately leading to age-dependent neurodegeneration.
In this new study, Lu and his coauthors hypothesized that Pin1 might be acting in a similar fashion to regulate APP (amyloid precursor protein) cleavage and amyloid beta production, thereby preventing the formation of plaques.
Amyloid precursor protein is cleaved or processed in two different pathways in the brain, explains Lu. In the normal pathway, it divides into small segments that are important for neurons' growth and survival. However, in the pathological pathway, APP is broken into fragments of two types of amyloid beta peptides - Abeta40 and Abeta42, with Abeta42 being extremely toxic. Unlike the segments in the normal pathway, the amyloid peptide fragments are prone to clump together, leading to the creation of the troublesome plaques.
Lu and his colleagues began by examining the relationship between APP and Pin1 using a powerful microscopic technique known as nuclear magnetic resonance (NMR) spectroscopy.
The researchers, in collaboration with Linda Nicholson at Cornell University, observed through NMR that when APP was phosphorylated (a process in which an extra phosphate is acquired) it became misshapen and could not be easily restored to its original condition. However, once Pin1 was introduced, APP returned to its normal shape - with dramatic speed.
"We had previously proposed that Pin1 was regulating protein function by greatly accelerating structural changes, but this activity had never actually been visualized," explains Lu. "Now, for the first time, we were able to actually see this process as it occurred."
Using cell models the authors next went on to examine the effects of Pin1 on Abeta production, says Lu explaining, "We were able to show that while upregulation of Pin1 reduced Abeta generation, removing the Pin1 gene dramatically increased Abeta production by shifting APP processing away from the normal pathway into the pathological pathway."
In order to understand the relative importance of Pin1 on the production of different amyloid peptides in the brain, the scientists then examined changes in APP processing and production of different Abeta peptides in two separate mouse models: Pin1 knockout mice and mice in which Pin1 had been knocked out and mutant APP (which causes the early onset of Alzheimer's disease) simultaneously overexpressed.
"Because the effects of Pin1 knockout are age-dependent, we compared Abeta levels in the brains of the mice at different ages," explains Lu. And indeed their results showed that Pin1 knockout again switched APP processing away from the normal pathway into the pathological pathway in the mouse brains. Moreover, he adds, while Pin1 knockout did not significantly change the levels of Abeta40, it did selectively increase levels of insoluble Abeta42 by a significant 30 to 50 percent in two separate mouse models, to an extent that is similar to what would be found in the brains of Alzheimer's patients and in mouse models of the disease.
The most common cause of dementia among the elderly, Alzheimer's disease affects an estimated 4 million individuals in the United States, a number that is expected to increase significantly in the coming years with the aging of the baby boomer generation.
"As was earlier shown with tau proteins, it appears that Pin1 acts to restore misshapen amyloid precursor proteins to their original healthy shape, possibly preventing the onset of neurodegeneration and development of dementia," says D. Stephen Snyder, PhD, of the Etiology of Alzheimer's Disease program at the National Institute on Aging, which supported this study. "This finding offers important new insights into the molecular events that lead to Alzheimer's as we work to develop therapies for the treatment of this widespread disease."
Study coauthors include BIDMC investigators Lucia Pastorino, PhD, Anyang Sun, PhD, Xiao Zhen Zhou, MD, Martin Alastik, PhD, Greg Finn, PhD, Gerburg Wulf, MD, PhD, and Jormay Lim, PhD; Pei-Jung Lu, PhD, of Kaohsiung Veterans General Hospital in Taiwan, China; Shi-Hua Li, PhD, and Xiaojiang Li, PhD, of Emory University in Atlanta, Georgia; Weiming Xia, PhD, of Brigham and Women's Hospital, Boston; and Linda Nicholson, PhD, of Cornell University, Ithaca, New York.
This study was funded in part by grants from the National Institutes of Health, the National Science Foundation and the Taiwan National Science Council.
Beth Israel Deaconess Medical Center is a patient care, teaching and research affiliate of Harvard Medical School, and ranks fourth in National Institutes of Health funding among independent hospitals nationwide. BIDMC is clinically affiliated with the Joslin Diabetes Center and is a research partner of Dana-Farber/Harvard Cancer Care Center. BIDMC is the official hospital of the Boston Red Sox. For more information, visit www.bidmc.harvard.edu.