EURON and THEME joint PhD meeting

94
EURON and THEME joint meeting 2011
γ-secretase dependent phagocytosis of Amyloid-beta
(Aβ) in microglial cells
Sonia Tosheva, Jochen Walter
Department of Neurology, University of Bonn.
Introduction
Alzheimer’s disease (AD) is the most common form of dementia characterized
by key features that include neurofibrillary tangels formation and depositions
of fibrillar amyloid β-peptides (Aβ) which form senile plaques in the brain. Aβ
is produced after amyloid precursor protein proteolytic cleavage via β and
γ-secretase pathways. γ-secretase cleaves many type I proteins thereby affecting
the protein stability and trafficking, cognitive functions, intracellular signaling
and calcium homeostasis. This enzyme is a complex of 4 individual proteins but
the catalytic role is due to Presenilins (PS). And missense mutations in the PS
genes are a major cause of early onset of familial AD (FAD).
To address the critical role of γ-secretase in amyloid plaque clearance, we focused
our work on studying the involvement of microglial cells in this process. The
microglial phagocytosis of Aβ can be triggered via several receptors like the Low
density lipoprotein receptor related protein1 (LRP1). LRP1 is a type I membrane
protein and a γ-secretase substrate. It interacts with APP and facilitates its trafficking
and processing. LRP1 uses Aβ as a ligand and controls its uptake and transport to
the lysosomes. There Aβ is degraded by proteolitic enzymes mainly Cathepsin D.
Methods
We chose different approaches to study the γ-secretase functions in microglial
cell lines- BV2 and ES derived microglia (EsDM). Genetic mutations of the
Presenilin (PS) gene and pharmacological component (DAPT) were used to inhibit
the γ-secretase activity.
Results
Our results showed that γ-secretase is involved in Aβ uptake and decrease in this
process was seen after γ-secretase inhibition. We reported that Aβ`s receptor
LRP1 is expressed in microglial cells where its processing and endocytosis is
γ-secretase dependent. Cells with inhibited γ-secretase activity demonstrated
impaired LRP1 endocytosis, and Aβ cellular uptake. This also correlated with a
decrease in degradation and changes in the lysosomal structure and localization.
Opposite effects were shown in microglia cells overexpressing PS1 where the rate
of Aβ uptake and its degradation was significantly high.