Title

Internalization of beta amyloid peptide by human mononuclear phagocytes: Its role in neurotoxicity

Date of Completion

January 2001

Keywords

Biology, Neuroscience|Health Sciences, Pharmacology

Degree

Ph.D.

Abstract

Alzheimer's disease (AD) is a progressive dementia ultimately leading to death. Characteristic of neuropathological findings of the disease at autopsy is the extracellular accretion of beta-amyloid (Aβ) protein into senile plaques (SPs). Reports of immune activated microglia, members of the mononuclear phagocyte system (mø) in intimate contact with these lesions have led to the theory that AD has a chronic inflammatory component. Furthermore, reports of the expression of inflammatory mediators in AD brain have implicated these brain mø as mediators of neural cell death in AD. These findings strongly suggest that mø-mediated toxicity in AD may result from mø interaction with Aβ. As phagocytic stimuli can activate mø to a cytopathic state, a key issue in AD etiopathogenesis is the relationship between phagocytosis of Aβ by mø, and mø-mediated neurotoxicity. The objective of this investigation was to examine the hypothesis that the phagocytosis of Aβ by mø is a prerequisite for neurotoxicity. While phagocytosis of Aβ by mø has been studied by a variety of methods, none have unequivocally established whether Aβ is internalized, or is just partially surrounded by mø processes. Additionally, the link between phagocytosis of Aβ by mø, and mø-mediated neurotoxicity remains undemonstrated. A novel approach employing confocal microscopy and three-dimensional image reconstruction was used to confirm that mø completely internalize Aβ. Results demonstrate that mø unequivocally phagoeytose Aβ, exerting a graded neurotoxic response that appears to be related to their extent of Aβ phagocytosis. Decreasing Aβ aggregate size resulted in increased aggregate internalization by mø, and Aβ present in high-speed supernatant fractions were potently neurotoxic, and taken up more avidly by mø than unfractionated Aβ. Both phagocytosis of Aβ and mø-mediated neurotoxicity were found to be partially dependent on mø generated reactive oxygen species (ROS). Collectively, these findings reveal that Aβ phagocytosis is associated with increased mø-mediated neurotoxicity, and highlights the prospect that internalization of Aβ by mø in the brain might contribute to the neuroinflarrimation observed in AD. ^