PHOSPHOLIPASE D3 CONTRIBUTES TO ALZHEIMER’S DISEASE RISK VIA DISRUPTION OF Aβ CLEARANCE THROUGH THE LYSOSOME

Abstract

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β (Aβ) in the brain. We recently identified coding variants in the phospholipase D3 (PLD3) gene that double the risk for late onset AD. While the normal function of PLD3 is poorly understood, PLD3 is highly expressed in neurons and in brain regions most susceptible to amyloid pathology. We examined the impact of PLD3 risk variants on PLD3 and Aβ metabolism using CRISPR/Cas9 in induced pluripotent stem cells (iPSC). We then modeled the PLD3 expression patterns observed in AD brains in immortalized cell and AD mouse models. Lysosomal function was assessed in human brain tissue. PLD3 A442A disrupts a splicing enhancer binding site and reduces PLD3 splicing in human brains. Differentiation of PLD3 A442A and isogenic control iPSCs into cortical neurons produced cells that were morphologically similar. At the molecular level, PLD3 A442A neurons displayed a similar defect in PLD3 splicing as was observed in human brains and a significant increase in Aβ42/Aβ40 compared with isogenic control lines. Thus, PLD3 A442A is sufficient to alter PLD3 splicing and Aβ metabolism. PLD3 expression was significantly lower in AD brains compared with controls, and PLD3 expression was highly correlated with expression of lysosomal genes. Thus, we sought to determine whether PLD3 contributes to Aβ accumulation in AD via disrupted Aβ metabolism. We found that overexpression of PLD3 in immortalized cells decreased Aβ levels while shRNA silencing of PLD3 increased Aβ levels. In an AD mouse model, overexpression of PLD3 in hippocampal neurons produced decreased interstitial fluid (ISF) Aβ levels and accelerated Aβ turnover. Conversely, PLD3 silencing increased ISF Aβ and reduced Aβ turnover. Thus, reduced turnover of ISF Aβ may lead to Aβ accumulation. To begin to determine whether PLD3 influences Aβ turnover via the lysosome, we isolated lysosomal fractions from human AD and control brains. PLD3 was enriched in lysosomal subfractions and PLD3 distribution in these subfractions was altered in AD. Furthermore, PLD3 stability in the lysosomal fractions was disrupted in AD brains. These findings demonstrate that PLD3 may contribute to AD risk via altered Aβ clearance through the lysosome.