Isoform-Specific Effects of Apolipoprotein E on Markers of Inflammation and Toxicity in Brain Glia and Neuronal Cells In Vitro.

Jaclyn Iannucci,Abhik Sen,Paula Grammas

doi:10.3390/cimb43010018

Abstract

Mutations to the cholesterol transport protein apolipoprotein E (ApoE) have been identified as a major risk factor for the development of sporadic or late-onset Alzheimer’s disease (AD), with the e4 allele representing an increased risk and the rare e2 allele having a reduced risk compared to the primary e3 form. The reasons behind the change in risk are not entirely understood, though ApoE4 has been connected to inflammation and toxicity in both the brain and the periphery. The goal of this study was to better understand how the ApoE isoforms (ApoE2/3/4) confer differential AD-related risk by assessing cell-specific ApoE-related neuroinflammatory and neurotoxic effects. We compared the effects of ApoE isoforms in vitro on human astrocytes, a human immortalized microglia cell line (HMC3), and the human neuroblastoma cell line SH-SY5Y. Cells were treated for 24 h with or without recombinant ApoE2, ApoE3, or ApoE4 (20 nM) and inflammation and toxicity markers assessed. Our results indicated the expression of inflammatory cytokines IL-1β, TNFα, and IL-6 in human astrocytes was increased in response to all ApoE isoforms, with ApoE4 evoking the highest level of cytokine expression. In response to ApoE2 or ApoE3, microglial cells showed reduced levels of microglial activation markers TREM2 and Clec7a, while ApoE4 induced increased levels of both markers. ApoE2 promoted neuron survival through increased BDNF release from astrocytes. In addition, ApoE2 promoted, while ApoE4 reduced, neuronal viability. Overall, these results suggest that ApoE4 acts on cells in the brain to promote inflammation and neuronal injury and that the deleterious effects of ApoE4 on these cells may, in part, contribute to its role as a risk factor for AD.

Highlights

The most significant known genetic risk factor for sporadic or late-onset Alzheimer’s disease (AD) is an allele variant of apolipoprotein E (ApoE), a protein involved in lipid metabolism and cholesterol homeostasis [1]
We examined the effect of ApoE isoforms on the production of both the pro-inflammatory cytokine tumor necrosis factor-α (TNFα) as well as on IL-6, a complex cytokine with both pro- and anti3.2
Treatment with ApoE4 increases TNFα level (ApoE2 vs. ApoE4 p = 0.0915; the brain, we investigated the connection between ApoE isoform and brain derived neurotrophic factor (BDNF) expression levApoE3 vs. ApoE4 p = 0.1911), while exposure to ApoE2 appeared to decrease TNFα exels

Summary

Introduction

The most significant known genetic risk factor for sporadic or late-onset Alzheimer’s disease (AD) is an allele variant of apolipoprotein E (ApoE), a protein involved in lipid metabolism and cholesterol homeostasis [1]. The ApoE gene has three polymorphic alleles that occur in humans, e2, e3, and e4. These alleles occur at different frequencies, with e3 occurring most frequently at 65–70%, followed by e4 and e2 [1]. Population studies have shown that the e4 allele is associated with a significantly elevated likelihood for developing Alzheimer’s, while the e2 variant seems to have protective effects compared to e3 [1,2]. The relationship between ApoE alleles and AD has been well-established, the mechanisms behind these risk differences have not been well characterized

Objectives

Methods

Findings

Discussion

Conclusion