Volume 21, Number 3, November 2010
The nervous and immune systems are key sensors of the environment. Neuro-immune communication improves survival through multiple complex mechanisms that are poorly understood. Neural-immune communication involves the central and peripheral nervous systems, the endocrine system and innate immune signaling first discovered in white blood cells, particularly monocyte-like cells. Infections that activate immune responses also change hormone release and brain activity-modifying behavior. For example, multiple forms of illness that activate innate immune signaling cause “sickness behavior,” a syndrome that includes lethargy, depression, anxiety, loss of appetite, loss of energy and motivation, hyperalgesia, and difficulty concentrating.
Dr. Fulton Crews, John Andrews Distinguished Professor, Professor of Pharmacology and Psychiatry, and Director of UNC’s Bowles Center for Alcohol Studies, has advanced the understanding of the role of innate immune signaling in the brain including the discovery that ethanol induction of innate immune signals contributes to the neurobiology of addiction. In a series of studies, Crews first found an important role for chemokines, cytokines, proteases and oxidase enzymes that contribute to alcohol-induced neurodegeneration. Although neurodegeneration is most often associated with Alzheimer’s Disease, dementia and neurodegeneration also occur with chronic alcoholism. Crews found neuro-immune activation by infections and/or alcohol create signaling cascades that continue through multiple loops of self-sustaining activation (Fig. 1). Crews and his laboratory discovered that these positive loops of activation of proinflammatory signals contribute to a slow, progressive degeneration that changes the brain and behavior.
Crews Research Team: (Left to Right) Leon Coleman, PhD, Qian Jiang, Tonya Hurst, Liya Qin, Ph.D., Fulton Crews, PhD, Shriya Soora
Jian Zou, PhD., and Wen Liu, PhD.
In the brain, cells known as microglia sense their surroundings, producing innate immune-signaling molecules. In an animal model of binge alcohol drinking, Crews’ group found that alcohol activated microglia and increased brain levels of the proinflammatory chemokines-cytokines, TNFa, IL-1b, and MCP-1(CCL2), as well as oxidases and proteases involved in innate immune activation. The alcohol-induced innate immune response was associated with cortical brain regions that show binge drinking-induced brain damage in rats. Crews hypothesized that the innate immune gene induction was not due to the brain damage, but rather, it was the cause of the binge drinking-induced brain damage. Crews’ group first extended previous studies suggesting that alcohol activated a key transcription factor, NF-kB, in brain. Crews linked binge drinking levels of alcohol in brain with activation of brain NF-kB and increased expression of chemokines-cytokines, oxidases and proteases. Indeed, Crews demonstrated that alcohol exposure increased NF-kB DNA binding in rat brain, activated microglia, and caused brain damage. Blockade of NF-kB transcription protected against brain damage. The results suggest a crucial role of NF-kB in alcohol-induced brain damage and support the hypothesis that innate immune gene induction contributes to alcohol-induced frontal cortical damage that could underlie the loss of behavioral control associated with alcohol addiction (Fig. 2).
Figure 1: Mechanisms of Alcohol Induction of Brain Innate Immune Genes. Alcohol (ethanol, ETOH) directly activates NFkB transcription, likely through increased levels of reactive oxygen species (O2-) from NOX (NADPH oxidase), an enzyme that produces superoxide or through cytochrome P4502E1 (CYP2E1), an enzyme that metabolizes ethanol. Activation of NFkB transcription leads to increased synthesis of cytokine-chemokines, e.g. TNFa, MCP1, IL1 and IL6. Cytokine-chemokines act on cellular receptors to further increase NFkB transcription of innate immune genes in additional brain cells. The NFkB to chemokine to NFkB loop crosses various cell types in brain leading to persistent activation. Activation of NFkB also increases NOX transcription, particularly gp91, creating a NOX-oxidation loop. A third loop involves NFkB to protease-TLR receptor induction to NFkB. Together these loops cause persistent NFkB transcription that alters the neurobiology of the brain. Repeated exposure promotes loops that activate limbic anxiety and disrupt the frontal cortex leading to degeneration.
Crews’ lab has also investigated the effects of binge levels of alcohol intoxication on stem cells in the hippocampus, a key part of the brain that encodes mood and memory. They found that ethanol inhibition of neurogenesis, the formation of new neurons in brain, was related to ethanol induction of innate immune genes. Loss of hippocampal neurogenesis is associated with bad feelings and depression. In collaboration with Bowles Center Professor Dr. Clyde Hodge, chronic alcohol drinking was found to inhibit neurogenesis and induce depression-like behavior, and both effects could be reversed by antidepressants. Other studies showed that stress-induced loss of neurogenesis is related to innate immune genes, and anti-depressants reverse stress-induced depression-like behavior and stress inhibition of neurogenesis. These and other studies are consistent with Crews’ hypothesis that the anxiety and depression-like feelings of alcohol withdrawal overlap with “sickness behaviors” and the psychopathology of many mental diseases and other addictions (Fig 2).
Figure 2: The Neurobiology of Addiction and Innate Immune Gene Induction. Addiction involves compulsive behaviors, due in part, to loss of frontal cortex behavioral control and attention. Frontal cortical disruption and degeneration due to innate immune gene induction alters neurotransmission and increases impulsivity and compulsivity. Innate Immune gene induction in the amygdala, hippocampus and other limbic brain regions alters peptides and other transmitter signaling. Innate immune gene induction increases anxiety, negative affect, craving and wanting - promoting impulsiveness. Innate immune genes induced by drugs and/or stress reduce hippocampal neurogenesis contributing to these symptoms. Thus, innate immune gene induction is hypothesized to create the neurobiology that leads to addictive behavior.
Binge drinking-induced frontal cortical damage is common in alcoholism and associated with cognitive deficits and compulsive behaviors that are characteristic of addiction. Crews’ group found that binge drinking induced long lasting reversal learning deficits in both rats and mice. Reversal learning deficits are a form of cognitive dysfunction that is common in addiction when patients perseverate upon an incorrect choice and can’t seem to relearn the correct choice in a task. This work prompted studies of human alcoholic brain. Crews undertook a postmortem study of the brains of alcoholics and found that human alcoholic brain showed increased innate immune molecules similar to those found in rat and mouse brain following binge alcohol treatment. Post-mortem human alcoholic brain showed increased innate immune gene expression, specifically increased the proinflammatory chemokine MCP-1 and increased microglial markers. In mice, binge alcohol treatment leads to a persistent increase in MCP-1, consistent with activation of the cytokine and chemokine loops that sustain innate immune activation (Fig.1). These studies suggest that innate immune genes contribute to frontal cortical neurodegeneration and the cognitive deficits associated with the poor decision-making common in alcoholism. Further, more recent studies in other laboratories have shown that innate immune genes regulate alcohol drinking behavior and may contribute to the genetics of alcoholism. Together, these studies suggest that many of the behavioral manifestations of abstinence that promote alcoholic relapse are also related to increases in innate immune gene expression in brain.
“We are interested not only in the neural correlates of excessive alcohol consumption but also the behavioral correlates as they relate to innate immune gene induction,” says Crews. “What does alcohol-induced brain damage mean for the functioning of the alcoholic? Our research, considered in the context of others’ findings, suggests that excessive alcohol consumption creates a neuroinflammatory response that mimics the neurobiological changes associated with addiction. One aspect involves a hyperglutamatergic-diffuse excitation state that contributes to confused decisions. Due to this decreased ability to focus and discriminate, the alcoholic defaults to automatic behaviors, to habits, and to anxious “wanting” that is often maladaptive. Defaulting to automatic behavior is a big part of addiction.
“We hope to use our knowledge of the molecular mechanisms of innate immune activation loops to work toward interventions and treatments that improve mental health and brain function for alcoholics and others suffering from brain diseases related to innate immune activation in brain.”