Alzheimer’s disease (AD) is characterized by the presence of aggregates of
amyloid beta (Aβ) in senile plaques and tau in neurofibrillary tangles, as well as marked
neuron and synapse loss. Of these pathological changes, synapse loss correlates most
strongly with cognitive decline. Understanding the contributions of different risk
factors, toxic proteins, and protein networks to synaptic dysfunction and loss is
essential to understanding and one day curing this disease.
Oligomeric species of both Aβ and tau are implicated in synapse, however the
interaction between them requires further exploration. The first aim of this thesis was
to investigate the interaction of Aβ and tau in a novel mouse model AD. In this model
APP/PS1 mice were crossed with mice expressing full length wild type human tau
(hTau). Expression of hTau in APP/PS1 mice increased plaque size by~50% and
increased plaque-associated dystrophic neurites. However, no increase in neurite
curvature, neuron loss, or synapse loss was observed in the hTau APP/PS1 animals
compared with APP/PS1 alone.
The underlying cause of most cases of AD is not known, however genetic risk
factors have been identified, the strongest of which is the APOE e4 allele. APOE e4 is
associated with increased risk of developing AD and increased rates of cognitive decline
compared to the more common APOE e3 allele. The second aim of this thesis was to
detect differences in the AD synaptic proteome compared with controls and to also
investigate the effect of an APOE e4 allele on those changes. Unbiased label free LC-MS/
MS based proteomics of synapses isolated from human AD and control post-mortem
brains of known APOE genotypes was used. Of the 1043 proteins detected in
20 synaptic preparations 17% (173) were found to differ significantly (p<0.05, fold
change >1.2) in AD compared with control. A significant sub-set of these proteins were
affected by APOE e4 allele genotype. One of these was Clusterin which was not only
increased in the AD synapse but further increased in cases with an APOE e4 allele.
Clusterin is closely related to ApoE has also been genetically linked to AD in genome-wide
association studies. Aim three was to further investigate the involvement of Clusterin at the
synapse and the interaction of ApoE with Clusterin using array tomography. Array
tomography confirmed an increase in Clusterin co-localization with presynapses and
postsynapses in AD cases compared with controls and found a further increase in cases
with an APOE e4 allele. Array tomography also found an increase in synapses which co-localized
with Clusterin and Aβ together in cases with an APOE e4 allele. This implies
that Clusterin is important in Aβ mediated synapse loss in AD.
To further investigate the role of synapse loss in AD aim 4 of this thesis was to
develop a novel human based model of Aβ mediated synapse loss. This model uses
cortical neurons derived from induced pluripotent stem cells from a control individual
that are challenged with Aβ extracted from brains from AD and control individuals. This
model shows a significant and concentration dependent reduction in the number of
synapses in response Aβ from AD brain but not to control brain extract or AD brain
extract immunodepleted of Aβ.
The work presented in this thesis has investigated two novel models of AD to
assess the effect of known toxic proteins in AD related synapse degeneration. This work
also shows that profound protein changes occur at the synapse in AD and that many of
these are affected by APOE genotype. Many of these changes potentially cause or
contribute to synaptic dysfunction in AD and therefore could be important for
therapeutic interventions