Characterising the effects of genetic risk factors of Alzheimer’s disease on synaptic transmission: a functional and structural analysis

Pathological changes in neural network activity play a key role in Alzheimer’s Disease but the underlying processes remain unclear. Key genetic risk factors are correlated with the abnormal increase in production of the A?1-42 peptide and the expression of the E4 isoform of the Apolipoprotein E. We began by characterizing how elevated levels of A?1- 42 affect pre-synaptic activity. Employing a functional approach coupled with a strong EM-based ultrastructure readout in CA3-CA1 hippocampal synapses derived from the AD transgenic mouse model APPSwe/Ind, we show an enhancement in the recycling fraction when compared to WT terminals. Spatial analysis of the retrieved vesicles revealed a preferential localization of the functional pool in APPSwe/Ind synapses around the peri-active zone (AZ) area, suggesting an organizational correlate of functional retrieval deficits. Complementary experiments monitoring glutamate activity using the genetically encoded reporter iGluSnFr showed that A?1-42 treatment over the course of 24h was sufficient to bring about a deficit in neurotransmitter release and clearance; the reported impairments were exacerbated after long-term (96h-120h) incubation. By using pharmacological interventions aimed at dampening synaptic over- activity (Levetiracetam) and improving vesicle turn-over (Roscovitine) we were able to partially rescue the reported deficits in neurotransmitter activity. Using the sypHy reporter, 24h incubation with addition of oligomeric A?1-42 revealed deficits in vesicle turn-over in ApoE-transgenic mice-derived ApoE3 cultures, while ApoE4 neurons displayed no effect, while ApoE-transgenic mice reported a propensity of ApoE4 animals to maintain an elevated level of functional vesicles at a later age (8 months). Our findings suggest that synapses show hyperactive function in activity-evoked vesicle recruitment with A?1-42 treatment but that retrieval pathways become overwhelmed, significantly limiting ongoing signaling. Further investigation on the A?1-42 dependent spatial and functional deficits are of vital importance to further elucidate the role of A?1-42 in AD and intervene with therapeutic approaches.