GCY-35/GCY-36-TAX-2/TAX-4 Signalling in O2 Sensory Neurons Mediates Acute Functional Ethanol Tolerance in Caenorhabditis elegans

Synaptic Mechanisms of Ethanol Tolerance and Neuroplasticity: Insights from Invertebrate Models

Alcohol tolerance is a neuroadaptive response that leads to a reduction in the effects of alcohol caused by previous exposure. Tolerance plays a critical role in the development of alcohol use disorder (AUD) because it leads to the escalation of drinking and dependence. Understanding the molecular mechanisms underlying alcohol tolerance is therefore important for the development of effective therapeutics and for understanding addiction in general. This review explores the molecular basis of alcohol tolerance in invertebrate models, Drosophila and C. elegans, focusing on synaptic transmission. Both organisms exhibit biphasic responses to ethanol and develop tolerance similar to that of mammals. Furthermore, the availability of several genetic tools makes them a great candidate to study the molecular basis of ethanol response. Studies in invertebrate models show that tolerance involves conserved changes in the neurotransmitter systems, ion channels, and synaptic proteins. These neuroadaptive changes lead to a change in neuronal excitability, most likely to compensate for the enhanced inhibition by ethanol.

Natural allelic variation modifies acute ethanol response phenotypes in wild strains of C. elegans

BACKGROUND

Genetic variation contributes to the likelihood that an individual will develop an alcohol use disorder (AUD). Traditional laboratory studies in animal models have elucidated the molecular pharmacology of ethanol, but laboratory-derived genetic manipulations rarely model the naturally occurring genetic variation observed in wild populations. Rather, these manipulations are biased toward identifying genes of central importance in the phenotypes. Because changes in such genes can confer selective disadvantages, they are not ideal candidates for carrying AUD risk alleles in humans. We sought to exploit Caenorhabditis elegans to identify allelic variation existing in the wild that modulates ethanol response behaviors.

METHODS

We tested the acute ethanol responses of four strains recently isolated from the wild (JU1511, JU1926, JU1931, and JU1941) and 41 multiparental recombinant inbred lines (mpRILs) derived from them. We assessed locomotion at 10, 30, and 50 min on low and high ethanol concentrations. We performed principal component analyses (PCA) on the different phenotypes, tested for transgressive behavior, calculated heritability, and determined the correlations between behavioral responses.

RESULTS

We observed a range of responses to ethanol across the strains. We detected a low-concentration locomotor activation effect in some of the mpRILs not seen in the laboratory wild-type strain. PCA showed different ethanol response behaviors to be independent. We observed transgressive behavior for many of the measured phenotypes and found that multiple behaviors were uncorrelated. The average broad-sense heritability for all phenotypes was 23.2%.

CONCLUSIONS

Genetic variation significantly affects multiple acute ethanol response behaviors, many of which are independent of one another. This suggests that the genetic variation captured by these strains likely affects multiple biological mechanisms through which ethanol acts. Further study of these strains may allow these distinct mechanisms to be identified.

Alteration in expressions of ion channels in Caenorhabditis elegans exposed to polystyrene nanoparticles

Ion channels on cytoplasmic membrane function to sense various environmental stimuli. We here determined the changes of genes encoding ion channels in Caenorhabditis elegans after exposure to polystyrene nanoparticles (PS-NPs). Exposure to 1-1000 μg/L PS-NPs could increase expressions of egl-19, mec-10, trp-4, trp-2, tax-4, cca-1, unc-2, and unc-93, and decrease the expressions of cng-3, mec-6, ocr-2, deg-1, exc-4, kvs-1, and eat-2. Among these 15 ion channel genes, RNAi knockdown of cng-3 or eat-2 caused resistance to PS-NPs toxicity and RNAi knockdown of egl-19, cca-1, tax-4, or unc-93 induced susceptibility to PS-NPs toxicity, suggesting that cng-3, eat-2, egl-19, cca-1, tax-4, and unc-93 were involved in the control of PS-NPs toxicity. EGL-19 and CCA-1 functioned in intestinal cells to control PS-NPs toxicity, and CNG-3, EAT-2, EGL-19, TAX-4, and UNC-93 functioned in neuronal cells to control PS-NPs. Moreover, in intestinal cells of PS-NPs exposed worms, cca-1 RNAi knockdown decreased elt-2 expression, and egl-19 RNAi knockdown decreased daf-16 and elt-2 expressions. In neuronal cells of PS-NPs exposed worms, eat-2 RNAi knockdown increased jnk-1, mpk-1, and dbl-1 expressions, unc-93 RNAi knockdown decreased mpk-1 and daf-7 expressions, and tax-4 RNAi knockdown decreased jnk-1 and daf-7 expressions. Therefore, two molecular networks mediated by ion channels in intestinal cells and neuronal cells were dysregulated by PS-NPs exposure in C. elegans. Our data suggested that the dysregulation in expressions of these ion channels mediated a protective response to PS-NPs in the range of μg/L in worms.

Transcriptional analysis of the response of C. elegans to ethanol exposure

Ethanol-induced transcriptional changes underlie important physiological responses to ethanol that are likely to contribute to the addictive properties of the drug. We examined the transcriptional responses of Caenorhabditis elegans across a timecourse of ethanol exposure, between 30 min and 8 h, to determine what genes and genetic pathways are regulated in response to ethanol in this model. We found that short exposures to ethanol (up to 2 h) induced expression of metabolic enzymes involved in metabolizing ethanol and retinol, while longer exposure (8 h) had much more profound effects on the transcriptome. Several genes that are known to be involved in the physiological response to ethanol, including direct ethanol targets, were regulated at 8 h of exposure. This longer exposure to ethanol also resulted in the regulation of genes involved in cilia function, which is consistent with an important role for the effects of ethanol on cilia in the deleterious effects of chronic ethanol consumption in humans. Finally, we found that food deprivation for an 8-h period induced gene expression changes that were somewhat ameliorated by the presence of ethanol, supporting previous observations that worms can use ethanol as a calorie source.