TRAIL Mediates Neuronal Death in AUD: A Link between Neuroinflammation and Neurodegeneration

Abstract

Although the cause of progressive neurodegeneration is often unclear, neuronal death can occur through several mechanisms. In conditions such as Alzheimer’s or alcohol use disorder (AUD), Toll-like receptor (TLR) induction is observed with neurodegeneration. However, links between TLR activation and neurodegeneration are lacking. We report a role of apoptotic neuronal death in AUD through TLR7-mediated induction of death receptor signaling. In postmortem human cortex, a two-fold increase in apoptotic terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining in neurons was found in AUD versus controls. This occurred with the increased expression of TLR7 and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) death receptors. Binge ethanol treatment in C57BL/6 mice increased TLR7 and induced neuronal apoptosis in cortical regions that was blocked by TLR7 antagonism. Mechanistic studies in primary organotypic brain slice culture (OBSC) found that the inhibition of TLR7 and its endogenous ligand let-7b blocked ethanol-induced neuronal cell death. Both IMQ and ethanol induced the expression of TRAIL and its death receptor. In addition, TRAIL-neutralizing monoclonal antibodies blocked both imiquimod (IMQ) and ethanol induced neuronal death. These findings implicate TRAIL as a mediator of neuronal apoptosis downstream of TLR7 activation. TLR7 and neuronal apoptosis are implicated in other neurodegenerative diseases, including Alzheimer’s disease. Therefore, TRAIL may represent a therapeutic target to slow neurodegeneration in multiple diseases.

Increased Toll‐like Receptor‐MyD88‐NFκB‐Proinflammatory neuroimmune signaling in the orbitofrontal cortex of humans with alcohol use disorder

“…and RELA in the OFC, consistent with TLR coupling to MyD88 activating the proinflammatory nuclear factor NFκB and downstream transcription of proinflammatory signaling molecules (Kawai & Akira, 2007;Pahl, 1999;Schmitz & Baeuerle, 1991). Indeed, we previously reported that AUD increases activated pRELA+IR in the human OFC (Qin et al, 2021), consistent with activation of NFκB.…”

“…NFKB2, RELB, and REL were unchanged between groups. Increases of NFKB1 and RELA are consistent with activation of NFκB transcription, and we previously reported increased activated pRELA+IR in the postmortem human AUD OFC relative to CONs (Qin et al, 2021). We extend those findings with colocalization of pRELA+IR with Iba-1+IR microglia, GFAP+IR astrocytes (Figure S1C,D), and NeuN+IR neurons (Figure 2D), consistent with NFκB activation across cell types within the AUD OFC.…”

“…Toll-like receptors, originally characterized as receptors for exogenous pathogens, have emerged as neuroimmune signaling receptors in the sterile brain that respond to endogenous agonists, thereby inducing self-expression and amplifying signals across cells through autocrine and paracrine mechanisms. In previous studies, we reported significantly increased protein levels of TLR2, TLR3, TLR4, and TLR7 as well as the endogenous TLR agonist HMGB1 in the AUD brain (Coleman et al, 2017;Crews et al, 2013;Qin et al, 2021). In the present investigation, we assessed induction of the universal TLR agonist HMGB1 and its association with levels of TLRs 1-9 in the postmortem human OFC of individuals diagnosed with AUD (n = 10) relative to age-matched CONs (n = 10).…”

“…HMGB1 is a highly conserved, ubiquitously expressed nuclear protein released from cells in response to injury whereupon it acts as a universal TLR agonist through either direct activation (i.e., TLR4) or indirect activation through formation of heteromers with TLR agonists (e.g., TLR7 and TLR9; Yanai et al, 2009;Andersson & Tracey, 2011;Ivanov et al, 2007). In preclinical studies, alcohol releases HMGB1-let7 miRNA heteromers in microglial vesicles, activating TLR7-induced gene expression and neurodegeneration (Coleman et al, 2017;Qin et al, 2021). Studies in bone marrow-derived DCs find that HMGB1 release with CpG-DNA, which is a TLR9 ligand, enhances TLR9 proinflammatory gene induction (Ivanov et al, 2007).…”

Increased alcohol self-administration following repeated Toll-like receptor 3 agonist treatment in male and female rats

“…An area that has gained considerable attention is the role that the innate immune or neuroimmune system plays in the development and maintenance of alcohol use and abuse (Blednov et al, 2012;Crews et al, 2017a;Crews et al, 2013;Erickson et al, 2019;Mayfield et al, 2013). Studies in postmortem human brain tissue find that proinflammatory signaling molecules are increased in AUD (Coleman et al, 2017;Crews et al, 2013;Crews and Vetreno, 2014;Crews et al, 2016;Crews et al, 2017b;Qin and Crews, 2012;Qin et al, 2021).…”