UDP-glucose 4, 6-dehydratase Activity Plays an Important Role in Maintaining Cell Wall Integrity and Virulence of Candida albicans

Abstract

Candida albicans, a human fungal pathogen, undergoes morphogenetic changes that are associated with virulence. We report here that GAL102 in C. albicans encodes a homolog of dTDP-glucose 4,6-dehydratase, an enzyme that affects cell wall properties as well as virulence of many pathogenic bacteria. We found that GAL102 deletion leads to greater sensitivity to antifungal drugs and cell wall destabilizing agents like Calcofluor white and Congo red. The mutant also formed biofilms consisting mainly of hyphal cells that show less turgor. The NMR analysis of cell wall mannans of gal102 deletion strain revealed that a major constituent of mannan is missing and the phosphomannan component known to affect virulence is greatly reduced. We also observed that there was a substantial reduction in the expression of genes involved in biofilm formation but increase in the expression of genes encoding glycosylphosphatidylinositol-anchored proteins in the mutant. These, along with altered mannosylation of cell wall proteins together might be responsible for multiple phenotypes displayed by the mutant. Finally, the mutant was unable to grow in the presence of resident peritoneal macrophages and elicited a weak pro-inflammatory cytokine response in vitro. Similarly, this mutant elicited a poor serum pro-inflammatory cytokine response as judged by IFNγ and TNFα levels and showed reduced virulence in a mouse model of systemic candidiasis. Importantly, an Ala substitution for a conserved Lys residue in the active site motif YXXXK, that abrogates the enzyme activity also showed reduced virulence and increased filamentation similar to the gal102 deletion strain. Since inactivating the enzyme encoded by GAL102 makes the cells sensitive to antifungal drugs and reduces its virulence, it can serve as a potential drug target in combination therapies for C. albicans and related pathogens. Candida albicans is an opportunistic fungal pathogen which infects individuals with debilitated immune system either due to old age, diseases such as AIDS or immune suppressive treatments. The cell wall of C. albicans, like most pathogens, mediates interaction of the pathogen with the host and determines the outcome of the host-pathogen interaction. We discovered that inactivation of GAL102 encoded UDP-glucose 4,6-dehydratase activity in C. albicans causes altered mannosylation of cell wall proteins and loss of cell wall integrity. The mutant cells thus show increased sensitivity to antifungal drugs that target cell wall. Importantly, these mutant cells show significantly lower virulence and reduced ability to elicit inflammatory cytokine responses from the host. Hence, inactivating the enzyme could significantly aid in controlling the infections by C. albicans. Since, the gene encoding the UDP glucose 4, 6-dehydratase is also present in many other fungal genomes, inhibitors of this activity could be useful in effective treatment of candidiasis and other fungal infections.