​​​​INVESTIGATING EARLY AΒ-DRIVEN SYNAPTIC DYSFUNCTIONS IN ALZHEIMER’S DISEASE: MORPHOLOGICAL AND FUNCTIONAL ALTERATIONS OF DENDRITIC SPINES OCCUR SIMULTANEOUSLY IN VITRO

One of the earliest pathological signs in Alzheimer’s disease (AD) is the loss of excitatory synapses. Paradoxically, early AD stages are also characterized by neuronal hyperexcitability. Although both events are described in human AD brains and animal models of AD, underlying mechanisms remain unclear. Dendritic spines are the post-synaptic sites of excitatory input, they are highly dynamic and as such, early morphological alterations may coincide with changes in their activity. In our cellular model, murine neurons expressing the human APP Swedish mutation shows reduced spine density. However, whether these initial morphological alterations are accompanied by functional impairments of individual spines is not known. Therefore, we aimed to characterize the functional properties of dendritic spines in APP Swedish-expressing neurons. To this end, we measured their spontaneous and evoked calcic oscillations using Xph15-Gcamp6f-GFP plasmid construct, expressing a post-synaptic density-bound calcium sensor. Primary cortical cultures from CD-1 mice embryos were co-transfected at DIV11-13 with hAPPswedish-t2a-LifeActin-mCherry. Imaging was performed by confocal microscopy. At 48H post-transfection, dendritic spine loss in APP Swedish neurons occurred simultaneously with a ~35% increase in their spontaneous calcium transient frequency compared to control. This phenomenon was dependent on the amyloidogenic processing of APP, as prevented by 24H treatment with β-secretase inhibitor IV (at 1µM). These results suggest an Aβ-dependent synaptotoxicity in early AD and show that synaptic loss coincides with hyperexcitability of the remaining spines. The consequences of these functional/morphological alterations and their temporal relationship are yet to be determined.