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Aaron Carman

Advisor: Mike Lorenz, Ph.D.

Accepted post-doctoral position at Cornell University after receiving PhD
Now employed as Program Manager, Aging and Alzheimer’s Prevention, Alzheimer’s Drug Discovery Foundation, New York, NY

Candida albicans is the most common opportunistic fungal pathogen of humans. The balance between commensal and pathogenic C. albicans is maintained largely by phagocytes of the innate immune system. Analysis of transcriptional changes after macrophage phagocytosis indicates the C. albicans response is broadly similar to starvation, including up-regulation of alternate carbon metabolism. Systems known and suspected to be part of acetate/acetyl-CoA metabolism were also up-regulated, importantly the ACH and ACS genes, which manage acetate/acetyl-CoA interconversion, and the nine-member ATO gene family, thought to participate in transmembrane acetate transport and also linked to the process of environmental alkalinization.
Studies into the roles of Ach, Acs1 and Acs2 function in alternate carbon metabolism revealed a substantial role for Acs2 and lesser, but distinct roles, for Ach and Acs1. Deletion mutants were made in C. albicans and were phenotypically evaluated both in vitro and in vivo. Loss of Ach function resulted in mild growth defects on ethanol and acetate and no significant attenuation in virulence in a disseminated mouse model of infection. While loss of Acs1 did not produce any significant phenotypes, loss of Acs2 greatly impaired growth on multiple carbon sources, including glucose, ethanol and acetate. We also concluded that ACS1 and ACS2 likely comprise an essential gene pair. Expression analyses indicated that ACS2 is the predominant form under most growth conditions.
While the ATO gene family was uniformly up-regulated after macrophage phagocytosis, their function in yeast remained unclear. Previous studies had indicated their involvement in acetate transport and had also identified a conserved N-terminal domain that, when mutated by one amino acid and over-expressed, resulted in a dominant phenotype that cannot grow on acetate. By recapitulating this point-mutation in C. albicans ATO1 and S. cerevisiae ATO1, we provided evidence that the point mutation does not prevent growth by blocking acetate import but by creating toxicity. pH and concentration studies with numerous small carboxylic acids including lactate, formate, acetate and propionate strongly suggested that ATO gene function served a vital role during weak acid stress, possibly by controlling the efflux of these weak acids from the cell.
ATO gene function also had been linked to the process of environmental alkalinization, an ammonium-mediated phenomenon described here for the first time in C. albicans. During growth in glucose-poor, amino acid-rich conditions C. albicans can rapidly change its extracellular pH, e.g. from 4 to 7 in 24 hours. This process was glucose-repressible and was accompanied by hyphal formation and changes in colony morphology. We showed that introduction of the ATO1G53D point mutant to C. albicans blocked alkalinization, as did over-expression of C. albicans ATO2, the only C. albicans ATO gene to lack the conserved N-terminal domain. A screen for alkalinization-deficient mutants revealed that ACH1 is essential for alkalinization. However, addition of acetate to the media restored alkalinization to the ach1 mutant. We proposed a model of ATO function in which Atos regulated the cellular co-export of ammonium and acetate.

Search pubmed for papers by A. Carman and M. Lorenz

Research Info

Acetate metabolism and control of environmental pH in Candida albicans and Saccharomyces cerevisiae