Defining the role of p75 neurotrophin receptor (p75NTR) in the development of Alzheimer's disease

Author: Khalil Saadipour

Saadipour, Khalil, 2014 Defining the role of p75 neurotrophin receptor (p75NTR) in the development of Alzheimer's disease, Flinders University, School of Medicine

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Abstract

The dysregulation of neurotrophins and their receptors plays a crucial role in the pathological process of sporadic Alzheimer's disease (AD). Here, we investigated the potential functions of p75NTR in the development of AD. We have found that p75NTR interacts with APP and Aβ, as a p75NTR ligand, promotes the interaction. To address the significance of this p75NTR/APP interaction in AD, we discovered that p75NTR transfection increased amyloidogenic processing of APP in CHOAPP695. Aβ enhances APP amyloidogenic processing in mouse cortical neurons of AD/p75+/+, but not in AD/p75-/- neurons via upregulation of APP and BACE1 expression. Aβ42 increases the internalization of APP and the internalization of BACE1 through p75NTR. In addition, Aβ and proNGF increased the APP/BACE1 interaction. The Aβ42/p75NTR association regulates the phosphorylation of APP-Thr668 and phosphorylation of Tau in mouse cortical neurons. It was shown that Sortilin interacts with BACE1, mediates retrograde trafficking of BACE1 and promotes Aβ generation. We have elucidated that BACE1, the rate-limiting enzyme processing APP, interacts with p75NTR, as a co-receptor for Sortilin, and regulates its proteolytic processing. Our results present that BACE1 interacts with p75ECD. Aβ and proNGF significantly enhanced the BACE1/p75NTR interaction. The ratio of p75ECD/p75FL in BACE+/+ mouse brain was significantly higher than in BACE-/- mouse brain. p75ECD is increased in cell lysates, but reduced in culture medium, of HEK-293T cells co-transfected with BACE1/p75NTR plasmids. To address the physiological function of p75ECD in AD, we found that p75ECD significantly rescued Aβ and proNTs-induced impairment of neurite outgrowth in cortical neurons. The neurotrophin receptor p75NTR mediates both neurotrophic and neurodegenerative signals and its ectodomain shedding from the cell surface are physiologically regulated. We have conducted an in vivo study to investigate the effects of p75ECD-Fc recombinant protein on cognitive function and neuropathology features of AD in an AD mouse model. Our data showed that i.p delivery of p75ECD-Fc was not effective on cognitive function in APPswe/PS1DE9 (AD) mouse. p75ECD-Fc improved the process of learning, but not memory impairment in tau pathology-related tyrosine phosphorylation (PR5) mouse model. p75ECD-Fc significantly decreased the size and number of Aβ plaques in AD mouse brain through inhibition of BACE1 expression. p75ECD-Fc significantly reduced GFAP levels in AD mouse. Moreover, p75ECD-Fc was not effective in restoring the level of synaptic proteins, including the vesicle-associated membrane protein (VAMP2) and synaptosomal-associated protein 25 (SNAP-25) in AD mouse brain. p75ECD-Fc did not change ChAT levels, but it significantly reduced Tau phosphorylation and inhibited BACE1 expression in PR5 mouse brain. We further investigated the expression and regulation of Sortilin, as a p75NTR co-receptor, in AD. Our data showed that Sortilin expression is significantly increased in human AD brains and in brains of 6-month old APPswe/PS1dE9 transgenic mice in comparison with relevant control groups. Aβ42 enhanced the protein and mRNA expression level of Sortilin in SH-SY5Y cells. In addition, proBDNF also significantly increased the mRNA and protein expression of Sortilin. We found the inhibition of p75NTR and ROCK, but not JNK, suppressed constitutive and Aβ42-induced expression of Sortilin. Taken together, the full length of p75NTR mediates APP processing and contributes to AD pathogenesis via Aβ-induced upregulation of BACE1, APP and Sortilin, whereas the p75ECD fragment is a novel neurotrophic molecule and protects the brain from toxicity induced by Aβ and proNTs.

Keywords: p75NTR,Sortilin,APP,Amyloid beta,Alzheimer's disease
Subject: Medicine thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of Medicine
Supervisor: Damien J, Keating