Lathen, D. R. | bioRxiv Science

bioRxiv2023

Large genetic analysis of alcohol resistance and tolerance reveals an inverse correlation and suggests "true" tolerance mutants.

Tolerance occurs when, following an initial experience with a substance, more of the substance is required subsequently to induce the same behavioral effects. Tolerance is historically not well-understood, and numerous researchers have turned to model organisms, particularly Drosophila melanogaster, to unravel its mechanisms. Flies have high translational relevance for human alcohol responses, and there is substantial overlap in disease-causing genes between flies and humans, including those associated with Alcohol Use Disorder. Numerous Drosophila tolerance mutants have been described; however, approaches used to identify and characterize these mutants have varied across time and between labs and have mostly disregarded any impact of initial resistance/sensitivity to ethanol on subsequent tolerance development. Here, we have analyzed a large amount of data - our own published and unpublished data and data published by other labs - to uncover an inverse correlation between initial ethanol resistance and tolerance phenotypes. This inverse correlation suggests that initial resistance phenotypes can explain many perceived tolerance phenotypes. Additionally, we show that tolerance should be measured as a relative increase in time to sedation between an initial and second exposure rather than an absolute change in time to sedation. Finally, based on our analysis, we provide a method for using a linear regression equation to assess the residuals of potential tolerance mutants. We show that these residuals provide predictive insight into the likelihood of a mutant being a true tolerance mutant, and we offer a framework for understanding the relationship between initial resistance and tolerance.

neuroscience↗

bioRxiv2022

One-carbon Pathways and Methylation Potential in Glutamatergic Neurons Regulate Behavioral Alcohol Responses

Despite the enormous harms of alcohol use disorders (AUDs), many mechanisms, as well as effective prevention or treatment strategies remain elusive. Genetic factors dictate a majority of AUD risk. These risk factors can manifest as reduced naive sensitivity to alcohols intoxicating effects and increased functional tolerance, i.e., brain-mediated decreases in sensitivity upon repeat exposure. The underlying neurobiology of how AUD-associated genes alter these endophenotypes remains poorly understood. Genes implicated in AUDs include epigenetic modifiers, such as histone demethylases, including Kdm3. We previously showed that whole-body and neuronal Kdm3 strongly affect ethanol sensitivity and tolerance in Drosophila. Here, we investigate the mechanisms of these effects, and, by extension, mechanisms of sensitivity and tolerance. RNA-seq and pathway analysis on Kdm3KO flies revealed disproportionate upregulation of genes involved in amino acid metabolism, including 1-carbon pathways. We show that acute amino acid feeding modulates sensitivity and tolerance in a Kdm3-dependent manner. Global manipulation of 1-carbon genes, especially glycine N-methyltransferase (Gnmt), glycine decarboxylase (Gldc), and sarcosine dehydrogenase (Sardh), alters alcohol sensitivity and tolerance. These changes in alcohol responses are likely mediated by global glycine levels (a substrate of these enzymes) rather than by 1-carbon input. Conversely, neuronal manipulations of 1-carbon pathways change alcohol sensitivity and tolerance in a pattern that suggests a mechanism through S-adenosyl methionine (SAM), a 1-carbon metabolite that is the universal methyl donor required for epigenetic methylation. Increasing SAM production specifically in glutamatergic neurons increases sensitivity and tolerance. Together, these findings reveal distinct mechanisms affecting alcohol sensitivity and tolerance globally (via glycine) and neuronally (via SAM), thus revealing an important and complex role of 1-carbon metabolism in mediating AUD phenotypes.

neuroscience↗