Basic Science and Pathogenesis.

Background: Alzheimer's disease (AD) is the memory-related neurodegenerative disorder, contributing to 70% of the cases globally. Synaptic dysfunction is a well-known early event that causes progressive cognitive decline in AD. The latest AD therapeutics on the forefront only offer a moderate symptomatic relief with significant off-target effects. Therefore, understanding the mechanism for AD pathogenesis and developing novel therapeutic targets are urgently needed. Our lab has recently reported an anomalistic increase in phospholipase D isoform 1 (PLD1), that breakdown phospholipids in AD postmortem brain samples, compared to control subjects. Moreover, the effect of elevated PLD1 driven by amyloid-β and tau deposits has been well-established in wild type and in 6-month-old 3xTg-AD model mice. In the present study, we assess the novel role of PLD1 in modulating cellular mechanisms involved in synaptic dysfunction in AD.

Method: Here, we studied the spatial and temporal expression of PLD1 in 3xTg-AD model mice, treated with a small molecule PLD1 inhibitor (VU0155069), in an age-dependent manner. Furthermore, the brain-region specific mechanisms of PLD1 were evaluated by utilizing adeno-associated viral 2 (AAV2) vectors via intracerebroventricular route in 18- and 24-month-old wild-type and 3xTg-AD model mice. Following VU0155069/AAV2 administration, the mice cohorts were subjected to behavioral studies specific to learning and memory, such as Y-maze, novel object recognition (NOR), and elevated plus maze. Synaptic dysfunctions were studied using high frequency stimulation long-term potentiation (HFS-LTP), by conventional electrophysiology and multi-electrode array (MEA). Finally, the synaptic strength in frozen synaptosomal (P2) fractions was determined by previously standardized novel in vitro assay called the Fluorescence-Assisted Single Synaptosome-Long Term Potentiation (FASS-LTP). Morphological changes in the synapse were assessed using ImageJ and IMARIS following Golgi-Cox staining, a gold standard for measuring dendritic spine integrity. To deduce the underlying mechanisms, we used brain spheroids developed from human tissue to test the effects of PLD1 inhibition.

Result: In WT aged mice we noted differential effects of PLD1 over expression and attenuation. Additionally, we corroborate our results with diseased aging seen in 3xTg-AD using pharmacological and molecular approaches with AAV2 vectors.

Conclusion: Our research provides a novel insight into how PLD1 contributes to progressive functional deficits associated with synaptic dysfunction by impinging on critical cellular signaling events compromised in early and late stages of Alzheimer's disease.