Alzheimer's Disease Research
Malcolm Leissring, Ph.D.
Mayo Clinic Jacksonville
Title: Catabolism Of Amyloid Beta And Tau By Cathepsin D In Vivo
Non-Technical Title: Can Alzheimer Disease Be Treated With The Enzyme Cathepsin D?
Duration: April 1, 2009 - March 31, 2012
Award Type: Standard
Award Amount: $400,000
We have discovered that deletion of an enzyme known as cathepsin D (CatD) in mice results in very large and very selective increases in the two toxic protein species that are most closely linked to two characteristics of Alzheimer's disease (AD): (1) the amyloid beta-protein (specifically the 42-amino acid form, amyloid beta42), which deposits in the extracellular "plaques" that characterize AD, and (2) the microtubule-associated protein, tau, which makes up the neurofibrillary tangles that form inside neurons in AD and other diseases. We hypothesize that CatD plays a protective role in preventing AD by (directly or indirectly) breaking down amyloid beta42 and tau. We will test this hypothesis by increasing or decreasing CatD levels in mice that accumulate either amyloid beta 42 or tau.
The brains of patients with Alzheimer's disease (AD) are characterized by abnormal accumulations of "plaques," which are extracellular deposits composed primarily of a protein fragment known as amyloid beta 42 and "tangles," which are fibrils that form inside neurons that are composed principally of a different protein known as tau. We have discovered that both of these proteins are destroyed by a specific enzyme known as cathepsin D (CatD), a finding that suggests it would play a protective role in preventing AD. Surprisingly, however, many years of research on the role of CatD in AD suggest that it might instead contribute to the disease. To discriminate between these two possibilities, we will increase or decrease CatD levels in mice that accumulate either amyloid beta42 or tau, then determine what the results are. If CatD is protective, as we hypothesize, then it might be possible to treat AD by increasing CatD levels with novel drugs, or by introducing the enzyme into patients directly.
This award supported a study investigating the role of an enzyme known as Cathepsin D (CatD) in Alzheimer's disease (AD). Dr. Leissring’s team found that CatD potently destroys the two major, toxic proteins that accumulate excessively in AD: the amyloid ß-protein (Aß), the major constituent of "senile plaques" that litter the brains of AD patients, and the tau protein, which makes up "tangles" that kill off brain cells in AD and other disorders. Deleting the gene for CatD in a mouse model of AD resulted in the largest increase in Aß ever recorded, the formation of exceptionally large plaques at very early ages, and also led to changes to the tau protein. The team’s findings are all consistent with a protective role for CatD in the disease process. Collectively, the team in this study identified a major new player in the complex process that leads to Alzheimer's disease. The research completed by Dr. Leissring’s team will help to assist in the diagnosis, detection, prevention and, hopefully, development of successful treatments for this devastating brain disorder.
During the first reporting period of this award, which covers ~10 months of activity, we accomplished four major goals. First, we delivered a gene-changing treatment that increased the activity of CatD in non-AD mice and quantified the degree of that increase in terms of overall levels and activity of CatD, and whether this results in premature death. Second, we attempted the same genetic increase in CatD expression/activity in our first mouse model of AD (the results of which will be available within a few months). Third, we created a new genetic-changing treatment that is intended to reduce activity of CatD and characterized its effects in cultured cells. Fourth, we prepared and bred a second mouse model of AD mice that will be required to complete this project. Finally—in very significant advances that are exceptionally important to this project, but not explicitly part of the specific aims—we made significant progress characterizing mice that lack CatD (a third model), determining the underlying mechanism that affects beta-amyloid. We also analyzed mutations in human cathepsin D. Data from both of these experiments in the fourth goal suggest that CatD is significantly associated with a risk for late-onset AD.