Volatile anesthetics affect nutrient availability in yeast

Volatile anesthetics affect all cells and tissues tested, but their mechanisms and sites of action remain unknown. To gain insight into the cellular activities of anesthetics, we have isolated genes that, when overexpressed, render Saccharomyces cerevisiae resistant to the volatile anesthetic isoflurane. One of these genes, WAK3/TAT1, encodes a permease that transports amino acids including leucine and tryptophan, for which our wild-type strain is auxotrophic. This suggests that availability of amino acids may play a key role in anesthetic response. Multiple lines of evidence support this proposal: (i) Deletion or overexpression of permeases that transport leucine and/or tryptophan alters anesthetic response; (ii) prototrophic strains are anesthetic resistant; (iii) altered concentrations of leucine and tryptophan in the medium affect anesthetic response; and (iv) uptake of leucine and tryptophan is inhibited during anesthetic exposure. Not all amino acids are critical for this response since we find that overexpression of the lysine permease does not affect anesthetic sensitivity. These findings are consistent with models in which anesthetics have a physiologically important effect on availability of specific amino acids by altering function of their permeases. In addition, we show that there is a relationship between nutrient availability and ubiquitin metabolism in this response.

Ubiquitin metabolism affects cellular response to volatile anesthetics in yeast

To investigate the mechanism of action of volatile anesthetics, we are studying mutants of the yeast Saccharomyces cerevisiae that have altered sensitivity to isoflurane, a widely used clinical anesthetic. Several lines of evidence from these studies implicate a role for ubiquitin metabolism in cellular response to volatile anesthetics: (i) mutations in the ZZZ1 gene render cells resistant to isoflurane, and the ZZZ1 gene is identical to BUL1 (binds ubiquitin ligase), which appears to be involved in the ubiquitination pathway; (ii) ZZZ4, which we previously found is involved in anesthetic response, is identical to the DOA1/UFD3 gene, which was identified based on altered degradation of ubiquitinated proteins; (iii) analysis of zzz1Delta zzz4Delta double mutants suggests that these genes encode products involved in the same pathway for anesthetic response since the double mutant is no more resistant to anesthetic than either of the single mutant parents; (iv) ubiquitin ligase (MDP1/RSP5) mutants are altered in their response to isoflurane; and (v) mutants with decreased proteasome activity are resistant to isoflurane. The ZZZ1 and MDP1/RSP5 gene products appear to play important roles in determining effective anesthetic dose in yeast since increased levels of either gene increases isoflurane sensitivity whereas decreased activity decreases sensitivity. Like zzz4 strains, zzz1 mutants are resistant to all five volatile anesthetics tested, suggesting there are similarities in the mechanisms of action of a variety of volatile anesthetics in yeast and that ubiquitin metabolism affects response to all the agents examined.