Diazepam binding inhibitor and tolerance to ethanol in Drosophila melanogaster
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Tolerance to ethanol is an endophenotype of alcoholism, allowing the study of a complex psychiatric condition using animal models. To identify new genes involved in the acquisition of tolerance, I designed an automated and high-throughput tolerance assay and screened a collection of deficiency mutants for the inability to develop tolerance. The screen yielded several “regions of interest” where more than one overlapping deficiency failed to develop tolerance. One of these regions comprised nine genes, and testing the expression levels of each gene revealed that diazepam binding inhibitor (Dbi) showed grossly increased expression in the deficiency mutant compared to wild type. Another mutant stock, with a P-element transposon inserted downstream of the Dbi gene, both failed to develop tolerance and showed further increased expression of Dbi. There are two insulator binding sites flanking Dbi, and the P-element transposon also contains insulator binding sites. Based on these results, it was hypothesized that an insulator complex kept Dbi expression low in wild type flies and that disrupting the insulator complex allowed aberrantly high expression of Dbi in the mutants. Furthermore, we assumed that induction of Dbi blocked tolerance by making the mutants resistant prior to the first sedation. A UAS-DBI transgene was constructed to over-express Dbi. Induction of the UAS-DBI with a heat shock gal4 driver induced resistance to ethanol sedation; a similar response was observed in the parental control, but the effect was smaller. Although driving UAS-DBI with the neural elav-gal4 driver did not block tolerance, the experimental stock was resistant to ethanol sedation compared to the parental controls, indicating that increased Dbi expression produced “pre-tolerance.” To confirm the theory that insulator disruption was responsible for the increase in Dbi and the resulting no-tolerance phenotype, the P-element in the second mutant was mobilized by introducing a transposase source. These offspring lines were analyzed using qualitative PCR to determine whether the transposon excised precisely, left a portion of the transposon behind, or removed some of the flanking region. A precise excision mutant was identified, but this mutation did not rescue tolerance as predicted. This result might indicate that genetic background was the cause of the no-tolerance phenotype, or it might indicate that the excision was not exactly precise and removed the native insulator binding site, causing the insulator complex to remain disrupted.