Microglia depletion diminishes key elements of the leukotriene pathway in the brain of Alzheimer\u2019s Disease mice

J Michael,J Attems,L Aigner,H Mrowetz,P Schernthaner,R Poupardin,M S Unger

doi:10.1186/s40478-020-00989-4

Abstract

Leukotrienes (LTs) contribute to the neuropathology of chronic neurodegenerative disorders including Alzheimer’s Disease (AD), where they mediate neuroinflammation and neuronal cell-death. In consequence, blocking the action of Leukotrienes (LTs) ameliorates pathologies and improves cognitive function in animal models of neurodegeneration. Surprisingly, the source of Leukotrienes (LTs) in the brain is largely unknown. Here, we identified the Leukotriene (LT) synthesis rate-limiting enzyme 5-Lipoxygenase (5-Lox) primarily in neurons and to a lesser extent in a subpopulation of microglia in human Alzheimer´s Disease (AD) hippocampus brain sections and in brains of APP Swedish PS1 dE9 (APP-PS1) mice, a transgenic model for Alzheimer´s Disease (AD) pathology. The 5-Lipoxygenase (5-Lox) activating protein (FLAP), which anchors 5-Lipoxygenase (5-Lox) to the membrane and mediates the contact to the substrate arachidonic acid, was confined exclusively to microglia with the entire microglia population expressing 5-Lipoxygenase activating protein (FLAP). To define the contribution of microglia in the Leukotriene (LT) biosynthesis pathway, we ablated microglia using the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 in wildtype (WT) and APP-PS1 mice. Microglia ablation not only diminished the expression of FLAP and of the Leukotriene (LT) receptor Cysteinylleukotriene receptor 1 (CysLTR1), as expected based on their microglia cell type-specific expression, but also drastically reduced 5-Lipoxygenase (5-Lox) mRNA expression in the brain and its protein expression in neurons, in particular in wildtype (WT) mice. In conclusion i) microglia are key in Leukotriene (LT) biosynthesis, and ii) they regulate neuronal 5-Lipoxygenase (5-Lox) expression implying a yet unknown signaling mechanism between neurons and microglia.

Highlights

Leukotrienes (LTs) are elevated in the brain in aging, after injury, and in neurodegenerative diseases such as Alzheimer’s Disease (AD) [1–4]
Cell-type specific 5-Lox and FLAP expression in Alzheimer’s disease (AD) and AD transgenic mouse brains LTs play a pivotal role in AD pathology, yet it is unclear which cell types are involved in LT biosynthesis in the brain
In the present study we show that FLAP, a key activator molecule of the LT biosynthesis pathway, is expressed in all microglia and not in any other cell type of the brain. 5-Lox is present in a microglia subpopulation, in particular in the context of AD pathology

Summary

Introduction

Leukotrienes (LTs) are elevated in the brain in aging, after injury, and in neurodegenerative diseases such as Alzheimer’s Disease (AD) [1–4]. Increased levels of LTs contribute to age- and disease-related brain pathologies such as i) neuroinflammation and microglia / astroglia activation [5– 7], ii) neuronal damage [8, 9], and iii) blood-brain-barrier (BBB) permeability [10–12]. LT signaling has been recognized as therapeutic target in acute and chronic neurodegenerative diseases (for review see [13, 14]). In the context of AD, various in vivo studies demonstrated that genetic and pharmacological interventions reducing LT production or LT signaling ameliorate pathological burdens such as amyloid and tau load and improve cognitive function [2, 15–18]. Human genetics data illustrated that a single nucleotide polymorphism in the gene for 5-lox activating protein (FLAP), a key protein in the activation of LT synthesis, correlates with an increased risk for AD suggesting that LTs might contribute to AD development [19].

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