Pivotal Role of TLR4 Receptors in Alcohol-Induced Neuroinflammation and Brain Damage

Abstract

Toll-like receptors play an important role in the innate immune response, although emerging evidence indicates their role in brain injury and neurodegeneration. Alcohol abuse induces brain damage and can sometimes lead to neurodegeneration. We recently found that ethanol can promote TLR4 signaling in glial cells by triggering the induction of inflammatory mediators and causing cell death, suggesting that the TLR4 response could be an important mechanism of ethanol-induced neuroinflammation. This study aims to establish the potential role of TLR4 in both ethanol-induced glial activation and brain damage. Here we report that TLR4 is critical for ethanol-induced inflammatory signaling in glial cells since the knockdown of TLR4, by using both small interfering RNA or cells from TLR4-deficient mice, abolished the activation of microtubule-associated protein kinase and nuclear factor-B pathways and the production of inflammatory mediators by astrocytes. Our results demonstrate, for the first time, that whereas chronic ethanol intake upregulates the immunoreactive levels of CD11b (microglial marker) and glial fibrillary acidic protein (astrocyte marker), and also increases caspase-3 activity and inducible nitric oxide synthase, COX-2, and cytokine levels [interleukin (IL)-1␤, tumor necrosis factor-␣, IL-6] in the cerebral cortex of female wild-type mice, TLR4 deficiency protects against ethanol-induced glial activation, induction of inflammatory mediators, and apoptosis. Our findings support the critical role of the TLR4 response in the neuroinflammation, brain injury, and possibly in the neurodegeneration induced by chronic ethanol intake.

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

“…Several members of the receptor complex-TLR4, MD-2, and CD14-are critical for alcohol-induced activation of astrocytes in culture, as small interfering RNA (siRNA) to knockdown expression of any of these three targets prevented nuclear translocation of the NF-κB-p65 subunit in response to alcohol (Alfonso-Loeches et al, 2010). As predicted from these results, TLR4-deficient mice, unlike their wild-type counterparts, show no evidence of alcohol-induced effects on NF-κB-p65 activation, proinflammatory cytokine production, or caspase-3 cleavage, a marker for apoptosis (Alfonso-Loeches et al, 2010). Null mutations in CD14 can broadly disrupt many of the LPS-or alcohol-induced changes in the electrophysiological properties of GABA neurons in mouse central amygdala, supporting the notion that TLR signaling contributes to alcohol-induced changes in neuronal activity (Bajo et al, 2014).…”

The Neuroimmune Transcriptome and Alcohol Dependence: Potential for Targeted Therapies

“…Furthermore, ethanol-stimulated astrocytes produced IL-1R1 by activating the same inflammatory mediators and signaling cascades observed in the cerebral cortex of ethanol-fed animals [62]. AlfonsoLoeches et al [63] linked astrocyte-mediated inflammatory responses to the TLR pathway by showing that TLR4 is critical for ethanol-induced inflammatory signaling. This group demonstrated that siRNAmediated knockdown of TLR4 abolished alcohol activation of MAPK and NF-κB responses, which was also observed in TLR4-deficient cultures [63].…”

“…AlfonsoLoeches et al [63] linked astrocyte-mediated inflammatory responses to the TLR pathway by showing that TLR4 is critical for ethanol-induced inflammatory signaling. This group demonstrated that siRNAmediated knockdown of TLR4 abolished alcohol activation of MAPK and NF-κB responses, which was also observed in TLR4-deficient cultures [63]. The studies above support the pivotal role of TLRs in astrocyte and microglia activation in ethanol-induced inflammation.…”

“…One potential drug already being investigated is (+)-naloxone, which inhibits TLR4-mediated microglial activation and decreases proinflammatory cytokine expression [63]. Another microglia-related drug is minocycline, which reduces microglial and neuroimmune activation [64].…”

Opportunities for the Development of Neuroimmune Therapies in Addiction

“…The activation of TLR4 predominantly contributes to glial activation and the subsequent release of numerous proinflammatory cytokines (Mayfield et al, 2013). Importantly, these TLR4-related processes are involved in the behavioral and neuroinflammatory effects of drugs of abuse (Mayfield et al, 2013), as TLR4 activation has been shown to be integral to alcohol-induced glial activation and proinflammatory signaling (Alfonso-Loeches, Pascual-Lucas, Blanco, Sanchez-Vera, & Guerri, 2010;Blanco, Pascual, Valles, & Guerri, 2004;Blanco, Valles, Pascual, & Guerri, 2005;Fernandez-Lizarbe, Pascual, & Guerri, 2009), as well as alcohol's behavioral effects in rodents (Wu et al, 2012). Furthermore, in rodents, naltrexone attenuates proinflammatory TLR4-related signaling (Hutchinson et al, 2011) and blocks ethanolinduced glial activation and neuronal death (Qin & Crews, 2012), while (+) naloxone reduces acute alcohol-induced sedation and motor impairment (Wu et al, 2012).…”