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Date of Award

Spring 2011

Author's School

College of Arts & Sciences

Author's Department/Program

Biology

Abstract

Alzheimer’s disease (AD) is a neurodegenerative disease that impairs cognition and causes behavioral changes. It is the most common cause of dementia with increasing frequency after the age of 65 and with a prevalence as high as 50% in people over the age of 85. The main pathological hallmarks of the AD brain are the presence of extracellular amyloid plaques and intracellular neurofibrillary tangles. It is believed that accumulation of amyloid β (Aβ) peptide leads to a cascade of neurotoxic events, resulting in Aβ deposition in the brain, formation of neurofibrillary tangles, synaptic and neuronal dysfunction, inflammation, and eventually neuronal cell death. How Aβ begins to accumulate in the brain remains unknown, but it is thought that amyloid plaque formation arises from an imbalance between Aβ production and Aβ clearance. Apolipoprotein E (apoE), which interacts with Aβ and alters its aggregation propensity and clearance, is recognized as an important genetic risk factor for Alzheimer’s disease. Recent studies in the Holtzman laboratory demonstrated that increased levels of the low-density lipoprotein receptor (LDLR) in the brain results in a marked decrease in apoE levels and also a significant reduction in Aβ aggregation and deposition. Therefore, modulation of LDLR levels, either genetically or pharmacologically, is a potential strategy for reducing amyloid plaque levels in the brain. Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that binds to and regulates LDLR levels in peripheral tissues by inducing its degradation. In this study, we considered the possibility that a decrease in PCSK9 levels or activity in the brain may lead to higher LDLR levels, and consequently, a reduction in apoE levels and Aβ plaque formation. To test this hypothesis, we assessed the levels of apoE and Aβ, as well as plaque load in APP/PS1/PCSK9+/+ and APP/PS1/PCSK9-/- mice that express high amounts of the human Aβ peptide. We measured Aβ and ApoE levels in the brain of seven-month-old mice by ELISA. Our results showed that the APP/PS1/PCSK9-/- mice had significantly lower levels of apoE in the plasma compared to the APP/PS1/PCSK9+/+ mice. APP/PS1/PCSK9-/- mice also displayed a trend toward having lower levels of Aβ in the cortex; however, the difference was not significant. We quantified plaque load by immunohistochemistry using an anti-Aβ antibody. We also performed staining with X-34, a molecule that specifically binds to fibrillar amyloid-β-containing plaques. Collectively, the Aβ and X-34 staining results showed that levels of Aβ and amyloid plaque deposition in the brain were significantly reduced in APP/PS1/PCSK9-/- mice compared to APP/PS1/PCSK9+/+ mice. Reduction in apoE levels in the plasma of APP/PS1/PCSK9-/- mice supports our hypothesis that PCSK9 affects apoE levels. However, we did not see a difference in apoE levels in the brain of PCSK9-/- mice compared to PCSK9+/+, suggesting that the mechanism by which PCSK9 regulates Aβ levels in the brain may be different than we initially proposed. Decreased Aβ levels and plaque load in the brains of APP/PS1/PCSK9-/- mice are consistent with our view that PCSK9 plays a key role in regulating amyloid plaque formation. Although the precise mechanism by which this occurs remains to be elucidated, these positive results offer exciting research directions for developing new treatments for AD.

Language

English (en)

Advisor/Committee Chair

Jacob Basak