Background
Neuro-inflammation has been implicated in the progression of both acute and chronic neurological diseases. Resident cells of the central nervous system (CNS) detect soluble amyloid-β (Aβ) through the toll-like receptors (TLRs), involving Myd88 and interferon regulatory factor (IRF) signalling, and triggers pro-inflammatory cytokine release. Type-1 interferons (IFNs) are master regulators of the pro-inflammatory cytokine response, however, their CNS function remains largely unclear. Type-1 IFNs bind their cognate receptor IFNAR1, activating the JAK-STAT signalling pathway. Significantly, this cascade has been implicated as a mediator soluble Aβ1-42-induced toxicity [1]. We have previously demonstrated that removal of IFNAR1, contributes to neuro-protection following Aβ1-42 insult, decreasing type-1 IFN production and apoptosis. This study investigated a role for type-1 IFNs in an AD mouse model and utilised an in vitro approach to analyse TLR signalling as a potential production pathway.
Materials and methods
Total protein brain extracts were prepared from wild-type and B6C3-TgAPPswe, PSEN1ΔE9 (APP/PS1) mice (9 months) with IFNα levels measured by ELISA and phosphorylated Stat-3 (p-Stat-3) by Western blot. Immunohistochemistry was performed on aged brains (13 months) using p-Stat-3, FOX-3a and GFAP antibodies. Primary wild-type and Myd88-/- murine neurons were treated with 2.5µM Aβ1-42 for 24-72 hours and Q-PCR analysed IFNα, IFNβ, IL-1β and IL-6 expression. BE(2)-M17 human neuroblastoma cells were transfected stably with an IRF7 or transiently with an IRF3 knockdown (KD) shRNA plasmid or negative control (NC) plasmid and subjected to 7.5µM Aβ1-42 for 24-96 hours. Q-PCR analysed IFNα and IFNβ expression, Western blot determined p-Stat-3 expression, and an MTS assay assessed cell viability.
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Results
APP/PS1 brains (9 months) demonstrated a significant 2-fold increase in IFNα protein levels compared to aged matched controls (n=4, P<0.05). Western blot analysis confirmed robust p-Stat-3 in these APP/PS1 brains (n=4). Immunohistochemistry of 13 month brains confirmed co-localisation of p-Stat-3 with the neuronal marker, FOX-3a, not the glial marker, GFAP (n=5). Following Aβ1-42 treatment Myd88-/- neurons showed reduced IFNα (5.3-fold), IFNβ (2.2-fold), IL-1β (2.1-fold) and IL-6 (27.2-fold) expression (n=2, 24hrs). IRF7 KD in M17 cells did not alter the Aβ1-42-induced type-1 IFN response with no change in Aβ1-42 toxicity compared to NC cells (n=3). However, M17 IRF3 KD cells demonstrated reduced IFNα (3.7-fold) and IFNβ (2.3-fold) expression following Aβ1-42 insult (n=3, P<0.05, 24hrs). This correlated with reduced Aβ-induced toxicity compared to NC cells (97.7% vs. 65.2%, 96hrs, n=5, P<0.05).
Conclusions
This study supports a role for type-1 IFN signalling in the pro-inflammatory response generated by Aβ. The data suggests that Aβ-induced type-1 IFN production is TLR-regulated and occurs through an IRF3-dependent mechanism. Modulating these pathways may be beneficial in reducing type-1 IFN production and neuro-inflammation, consequently limiting neuronal damage in AD.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.