Phospholipase D3 contributes to Alzheimer’s disease risk via disruption of Aβ clearance and microglia response to amyloid plaques

Abstract

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. AD is also the result of complex genetic architecture that can be leveraged to understand pathways central to disease processes. We have previously identified coding variants in the phospholipase D3 (PLD3) gene that double the late-onset AD risk. However, the mechanism by which PLD3 impacts AD risk is unknown. One AD risk variant, PLD3 p.A442A, disrupts a splicing enhancer-binding site and reduces PLD3 splicing in human brains. Using differentiated induced pluripotent stem cells from a PLD3 p.A442A carrier and CRISPR-reverted, isogenic control, we show that PLD3 p.A442A cortical neurons exhibit a PLD3 splicing defect and a significant increase in Aβ42 and Aβ40, both of which are corrected upon reversion of the risk allele in isogenic control neurons. Thus, PLD3 p.A442A is sufficient to alter PLD3 splicing and Aβ metabolism. While the normal function of PLD3 is poorly understood, PLD3 is highly expressed in neurons and brain regions most susceptible to amyloid pathology. PLD3 expression is significantly lower in AD brains than controls, suggesting that PLD3 may play a role in sporadic AD. Thus, we sought to determine whether PLD3 contributes to Aβ accumulation in AD. In a mouse model of amyloid accumulation, loss of Pld3 increases interstitial fluid (ISF) Aβ and reduces Aβ turnover. AAV-mediated overexpression of PLD3 in the hippocampus decreased ISF Aβ levels and accelerated Aβ turnover. To determine whether PLD3-mediated reduction of ISF Aβ impacts amyloid accumulation, we measured amyloid plaque abundance and size after significant Aβ deposition. We found that in the absence of Pld3, amyloid plaques were less compact and more diffuse. Additionally, we observed reduced recruitment of microglia to amyloid plaques in the absence of Pld3. PLD3 may impact amyloid accumulation and AD risk through disrupted microglia function as PLD3 is enriched in disease associated microglia in human brains. Together, our findings demonstrate that PLD3 regulates Aβ clearance through cell-autonomous and non-cell-autonomous pathways in a manner that likely contributes to AD risk.