Researchers at the Perelman School of Medicine at the University of Pennsylvania, and the Icahn School of Medicine at Mount Sinai, have shown how acetate that is produced when the liver breaks down alcohol travels to the brain, where it is involved in epigenetic processes that impact on the expression of genes involved in learning and memory. The results of studies in mice indicated that acetate’s activity in the brain’s memory center played a role in governing how neuronal genes are expressed and, ultimately, how previously conditioned animals behaved in the face of environmental cues to consume alcohol. The findings also demonstrated that the acetate resulting from alcohol breakdown in pregnant rats is involved with histone acetylation in the developing fetal brain.
“It was a huge surprise to us that metabolized alcohol is directly used by the body to add chemicals called acetyl groups to the proteins that package DNA, called histones,” said research lead Shelley Berger, Ph.D., the Daniel S. Och university professor and director of the Penn Epigenetics Program. “To our knowledge, this data provides the first empirical evidence indicating that a portion of acetate derived from alcohol metabolism directly influences epigenetic regulation in the brain.” Berger, together with colleagues including first authors Philipp Mews, PhD, who is now a postdoctoral fellow at the Friedman Brain Institute at the Icahn School of Medicine at Mount Sinai in New York, and Gabor Egervari, PhD, a postdoctoral fellow in the Berger lab, reported their studies in Nature, in a paper titled, “Alcohol metabolism contributes to brain histone acetylation.”
The National Institute on Drug Abuse (NIAID) estimates that between 40% and 60% of people who have gone through treatment for alcohol addiction will experience relapse. Triggers for relapse may be everyday events—perhaps walking past a once-familiar bar, or meeting up with friends and former drinking partners. Research has focused on trying to understand the biological mechanisms that trigger addiction cravings, and how it might be possible to develop strategies for helping people to overcome these triggers.
Emerging evidence suggests that epigenetic regulation is dependent on metabolic state, “… and implicates specific metabolic factors in neural functions that drive behavior,” the authors explained. In the nucleus of brain cells, DNA is tightly wrapped around histone proteins to form the complex, chromatin. Acetylation of histone proteins effectively opens up the chromatin structure at a specific place on the genome in neurons, so that the genes involved in memory formation are available for transcription. “In neurons, acetylation of histones relies on the metabolite acetyl-CoA, which is produced from acetate by chromatin-bound acetyl-CoA synthetase 2 (ACSS2),” the researchers further explained. “Our team in the Berger lab had previously discovered that ACSS2 ‘fuels’ a whole new machinery of gene expression ‘on-site’ in the nucleus of brain cells to turn on key memory genes after learning,” said Mews. The team’s work describing these findings were published back in 2017. “We learned then that the metabolic factor ACSS2 is needed to lay down new memories.”
For their current study the team was interested to discover whether acetate produced from the breakdown of alcohol directly contributes to acetylation of histones in the brain. “Notably, the breakdown of alcohol in the liver leads to a rapid increase in levels of blood acetate, and alcohol is therefore a major source of acetate in the body,” the investigators wrote. Histone acetylation in neurons may thus be under the influence of acetate that is derived from alcohol, with potential effects on alcohol-induced gene expression in the brain, and on behavior.”
The team used isotopically labeled alcohol and advanced mass spectrometry to track where the alcohol and its breakdown products were transported to in the body and brain. Their results showed that alcohol metabolism rapidly impacted histone acetylation in the hippocampus—the learning and memory center of the brain—by directly depositing alcohol-derived acetyl groups onto histones via the ACSS2 enzyme. “Here, using in vivo stable-isotope labeling in mice, we show that the metabolism of alcohol contributes to rapid acetylation of histones in the brain, and that this occurs in part through the direct deposition of acetyl groups that are derived from alcohol onto histones in an ACSS2-dependent manner,” the investigators noted.
This finding represents a newly discovered route by which alcohol affects the brain and contributes to histone acetylation in neurons via ACSS2 enzyme, which plays an important role not only in gene expression but also in alcohol-related learning. To better understand how the alcohol-induced changes in gene expression might ultimately affect behavior, mice were exposed to neutral and alcohol rewards in distinct compartments of their living environment, distinguished by environmental cues. After a conditioning period, the researchers allowed the mice free access to either compartment, and determined the animal’s preferences by recording the relative amount of time spent in the chamber that offered alcohol. They found that animals with normal ACSS2 activity in their brains spent more time in the alcohol compartment. When they reduced ACSS2 protein levels in the brain, the animals didn’t display any preference for the alcohol compartment over the neutral compartment. “The data indicates to us that that alcohol-related memory formation requires ACSS2,” noted Egervari.
“This is significant because in alcohol use disorders, memory of alcohol-associated cues is a primary driver of craving and relapse, even after prolonged periods of abstinence,” said Mews. “Our findings establish a direct link between alcohol metabolism and histone acetylation in the hippocampus, indicating that translational treatment strategies that target this metabolic-epigenetic nexus may pave the way for novel therapeutic interventions for alcohol use and other neuropsychiatric disorders.”
Interestingly, when the team studied the effects of acetate on isolated hippocampal neurons grown in the lab, they found that extracellular acetate triggered patterns of transcription that related to learning and memory, and which were also sensitive to ACSS2 inhibition (ACSS2i). “In primary hippocampal neurons, supplementation with acetate induced the expression of 3,613 genes …” the authors stated. “…we found that these genes are involved in nervous system processes, including signal transduction and learning and memory … In the presence of ACSS2i, 2,107 of the genes that were induced by acetate were no longer upregulated, indicating that acetate-induced transcription relies heavily on the catalytic activity of ACSS2.” They suggest that their combined results “establish ACSS2 as a promising candidate for therapeutic intervention in alcohol-use disorders, in which the memory of alcohol-associated environmental cues is a primary driver of craving and relapse.”
Alcohol exposure not only disrupts epigenetic and transcriptional processes in the adult brain, the team continued. It is also linked with epigenetic dysregulation in the fetus during pregnancy. “In utero, alcohol is an environmental teratogen that affects the expression of neurodevelopmental genes and can elicit numerous alcohol-associated postnatal disease phenotypes, which together are categorized as FASD [fetal alcohol spectrum disorders].” In a separate series of experiments the team investigated the effects of alcohol on histone acetylation in the developing fetal midbrain and forebrain. Using mass spectrometry analyses they saw that in pregnant mice, the labeled alcohol-derived acetyl groups were incorporated into the brain cells of developing mice in utero. The fetal brains showed that “binge drinking-like” alcohol exposure, in parallel with maternal labeling of neuronal histone acetylation, was associated with alcohol-derived acetate being deposited onto histones in the fetal mouse forebrain and midbrain areas during early neuronal development. This showed an unanticipated potential mechanism for how fetal alcohol spectrum disorders work, and could have implications for combating fetal alcohol syndrome.
The researches say their findings indicate that in the hippocampus, “the incorporation of acetyl groups that are derived from alcohol may be critical for alcohol-related associative learning, which encodes environmental cues associated with alcohol that drive craving, seeking, and consumption even after protracted periods of abstinence.” The findings also throw up the question of whether other sources of acetate may be involved in histone acetylation in the brain, and hint at potential strategies for addressing alcohol addiction and potentially other neuropsychiatric disorders. “Notably, our findings suggest that other peripheral sources of physiological acetate—primarily the gut microbiome—may affect central histone acetylation and brain function in a similar manner, which may either control or foster other metabolic syndromes. Translational treatment strategies that target this nexus between peripheral metabolic activity and neuroepigenetic regulation may pave the way for therapeutic interventions for alcohol use and other neuropsychiatric disorders.”