Neuroimmune Function and the Consequences of Alcohol Exposure.

ALCOHOL RESEARCH: Current Reviews Neuroimmune Function and the Consequences of Alcohol Exposure Fulton T. Crews, Ph.D. is John Andrews Distinguished Professor of Pharmacology and Psychiatry and director; Liya Qin, Ph.D., is a research associate; Jian Zou, Ph.D., is a research associate; and Ryan P. Vetrano, Ph.D, is a postdoctoral research associate, all at the Bowles Center for Alcohol Studies, University of North Carolina, Chapel Hill, North Carolina. Dipak K. Sarkar, Ph.D., D.Phil., is Board of Governors and distinguished professor in the Department of Animal Sciences and director of the Endocrine Program, and Nadka Boyadjieva, M.D., Ph.D., D.Sci., is a research professor in the Department of Animal Sciences, both at Rutgers University, New Brunswick, New Jersey. To a large extent, signaling processes between neurons in the brain are distinct from signaling mechanisms between cells in the immune system and use different signaling molecules. However, some proteins first discovered within the immune system act as both peripheral immune-signaling molecules and brain-signaling molecules. These neuroimmune factors include various cytokines, Toll-like receptors (TLRs), and high-mobility group protein box 1 (HMGB1). In the brain, both neurons and supporting glial cells (both astrocytes and microglia) contribute to the release of and responses to these neuroimmune factors. Neuroimmune signaling in the brain not only is a part of the innate immune response, but its Fulton T. Crews, Ph.D.; Dipak K. Sarkar, Ph.D., D.Phil.; Liya Qin, Ph.D.; Jian Zou, Ph.D.; Nadka Boyadjieva, M.D., Ph.D., D.Sci.; and Ryan P. Vetreno, Ph.D. Induction of neuroimmune genes by binge drinking increases neuronal excitability and oxidative stress, contributing to the neurobiology of alcohol dependence and causing neurodegeneration. Ethanol exposure activates signaling pathways featuring highmobility group box 1 and Toll-like receptor 4 (TLR4), resulting in induction of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells, which regulates expression of several cytokine genes involved in innate immunity, and its target genes.This leads to persistent neuroimmune responses to ethanol that stimulate TLRs and/or certain glutamate receptors (i.e., N-methyl-d-aspartate receptors). Alcohol also alters stress responses, causing elevation of peripheral cytokines, which further sensitize neuroimmune responses to ethanol. Neuroimmune signaling and glutamate excitotoxicity are linked to alcoholic neurodegeneration. Models of alcohol abuse have identified significant frontal cortical degeneration and loss of hippocampal neurogenesis, consistent with neuroimmune activation pathology contributing to these alcoholinduced, long-lasting changes in the brain.These alcohol-induced long-lasting increases in brain neuroimmune-gene expression also may contribute to the neurobiology of alcohol use disorder. Key words: Alcohol use, abuse, and dependence; alcohol effects and consequences; alcohol exposure; binge drinking; immunity; neuroimmune responses; neuroimmune genes; neurodegeneration; brain; microglia; stress axis; stress responses; oxidative stress; glutamate receptors; Toll-like receptors; cytokines; high-mobility group box 1; nuclear factor-kappa B effects also persist for long periods and could contribute to long-lasting changes in neurobiology. Studies found that brain neuroimmune signaling is activated in models of binge drinking and neurodegeneration, suggesting another pathway through which alcohol may affect brain function. This review defines the roles of various cellular compartments and signaling molecules involved in neuroimmune activation, including the role of the stress axis in the communication between the central and peripheral immune systems and in sensitizing the neuroimmune response to alcohol. The article also will offer evidence from animal studies and postmortem human alcoholic brain studies that neuroimmune signaling may increase alcohol drinking and risky decision making and (in alcoholtreated animals) blunt the ability to change, decreasing behavioral flexibility. Neuroimmune Signaling in the Alcoholic Brain Monocytes and Innate Immune Genes Innate immune genes are associated with rapid first-line responses to infections that involve primarily immune cells called monocytes (e.g., the acute- Neuroimmune Function and the Consequences of Alcohol Exposure| 331 phase response). These responses include increases in multiple cytokines as well as in their cellular receptors. Together, these changes amplify expression of a large number of genes through kinase signaling pathways that converge on two transcription factors called nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and activa- tor protein-1 (AP-1). NF-κB and AP-1 promote expression of innate immune cytokines, such as tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β), as well as of TLRs and cytokine receptors (see figure 1). In addition, innate immune responses include the activation of proteases and oxidases, particularly Human Alcoholics Have Increased Levels of TLR2, TLR3, TLR4 & HMGB1 (Crews et al. 2013) cyclooxygenase and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase,1 as well as of major histocompatibility complex (MHC) signaling molecules, such as beta-2 microglobulin. 1 NADPH oxidase is an enzyme that produces reactive oxygen species (ROS)—for example, during ethanol metabolism—thereby increasing oxidative stress and contributing to cell damage. HMGB1 TLR/ IL1R Family LPS Rage LPS Causes a Persistent Increase in EtOH Drinking (Blednov et al. 2011) CD14 Mice Exposed to Chronic EtOH have Increased TLR2, TLR3, TLR4 & HMGB1 Expression (Crews et al. 2013) TLR4 siRNA in CeA Reduces Binge EtOH Drinking in Rats (Liu et al. 2011) Ethanol Activates NADPH Oxidase (Qin et al. 2011) MyD88 IPAK 1 TRAF6 CD14 KO Mice Do Not Drink EtOH (Blednov et al. 2011) TRAM TRIF Ras TAK1 NADPH oxidase MAPK PI3k 1κBα cFos AP-1 NFκB Inhibitor BHT Blocks EtOH Induction of Proinflammatory Genes & Inhibition of Neurogenesis (Crews et al. 2006b; Zou and Crews 2006, 2011) Erk1/2 JNK3 (+) Naltrexone Blocks TLR4 Activity (Hutchinson et al. 2008) Caspase 3 Minocycline & Naltreone Reduce EtOH & TLR Agonist-induced Inflammation & Neuronal Cell Death (Qin et al. 2012) ROS TLR4 KO Blocks EtOH Activation of NFκB & Proinflammatory Gene Induction (Blanco 2005) RIP1 NFκB Nucleus Proinflammatory Gene Expression NFκB Target Genes TLR, IL1β, TLRα, etc. Figure 1 Simplified schematic of the Toll-like receptor (TLR) and the receptor for advanced glycation end products (RAGE) signaling cascades. Stimulation of TLRs with high-mobility group box 1 protein (HMGB1) and other inflammation-inducing agents leads to the generation of reactive oxygen species (ROS) and downstream activation of the transcription factor nuclear factor kappa-light-chain-enhancer of activa (...truncated)