Phospholipase D2 ablation ameliorates Alzheimer-linked synaptic dysfunction and cognitive deficits: Mechanistic insights from lipidomic analysis

Growing evidence implicates aberrant lipid signaling in AD. While phospholipases A2 and C have been recently shown to mediate key actions of amyloid β-peptide (Aβ), the role of phospholipase D (PLD), which produces phosphatidic acid (PA), has so far remained elusive. Aβ oligomers were prepared using synthetic Aβ1-42. PC12 cells were transfected with a GFP-PLD2 construct and localization of GFP-PLD2 was analyzed by confocal microscopy after treatment with Aβ oligomers. PLD activity after Aβ oligomer treatment was measured using transphosphatidylation reaction catalyzed by PLD in the presence of 1-butanol. LTP was evaluated in the CA1 region of hippocampal slices from Pld2+/+ and Pld2-/- mice after treatment with Aβ oligomers. Pld2 mutant mice were crossed with the Tg2576 (Swedish APP, SwAPP) line to generate transgenic mice lacking PLD2. Contextual fear conditioning and radial-arm-water-maze (RAWM) behavior was tested at 5-6 and 11-12 months of age, respectively. After the RAWM, forebrains were processed for biochemical analysis. ELISA was performed to measure the levels of soluble and insoluble Aβ40/42. Lipidomics analysis of forebrain extracts was performed using high performance liquid chromatography coupled to electrospray ionization mass spectrometry. We show that oligomeric Aβ induces the internalization of GFP-PLD2 in PC12 cells. Acute treatments with oligomers enhance PLD activity in WT cultured neurons, but not in Pld2 mutant neurons. Oligomeric Aβ fails to reduce LTP in PLD2-deficient hippocampi. Furthermore, Pld2 ablation rescues memory deficits in a transgenic model of AD despite a significant Aβ load, suggesting PLD2 is necessary for the synaptotoxic action of Aβ. A lipidomics analysis of Pld2 mutant brains in the presence or absence of the SwAPP transgene unmasks striking crosstalks between PA and other lipids, which have been previously implicated in AD pathogenesis. Finally, this lipid analysis shows an exquisite acyl chain specificity and plasticity in the perturbation of PA metabolism, with the notable elevation in SwAPP brains of a PLD product previously linked to degeneration. Collectively, our results point to specific molecular species of PA and interconnected lipid metabolic pathways as key mediators of AD pathogenesis and identify PLD2 as a novel potential target for therapeutics.