A metabolomic study of the effect of Candida albicans glutamate dehydrogenase (GDH2 or GDH3) deletion on growth and morphogenesis

Abstract

There are two glutamate dehydrogenases in the pathogenic fungus Candida albicans. One is a NAD+-dependent glutamate dehydrogenase (encoded by GDH2) responsible for the catabolism of glutamate into α-ketoglutarate and ammonium. The other is a NADPH-dependent glutamate dehydrogenase (encoded by GDH3) that catalyses the biosynthesis of glutamate from α-ketoglutarate. These two enzymes are part of the nitrogen and nicotinate/nicotinamide metabolic pathways, which have been identified in our previous studies as potentially playing an important role in C. albicans morphogenesis. In this study, we created single gene knockout mutants of both dehydrogenases in order to investigate whether or not they affect the morphogenesis of C. albicans. The GDH genes were deleted using the SAT1 flipper technique and the phenotypes of the knockout mutants were studied by growth characterisation, metabolomics, isotope labelling experiments, and by quantifying cofactors under various hyphae-inducing conditions. We found that the gdh2/gdh2 mutant was unable to grow on either arginine or proline as sole carbon and nitrogen source. While the gdh3/gdh3 mutant could grow on these carbon and nitrogen sources, the strain was locked in the yeast morphology in proline-containing medium. We detected different concentrations of ATP, NAD+, NADH, NAPD+, NADPH, as well as 62 other metabolites, and 19 isotopically labelled metabolites between the mutant and the wild type strains. These differences were associated with 44 known metabolic pathways. It appears that the disequilibrium of cofactors in the gdh3/gdh3 mutant leads to characteristic proline degradation in the central carbon metabolism. The analysis of the gdh2/gdh2 and the gdh3/gdh3 mutants confirmed our hypothesis that redox potential and nitrogen metabolism are related to filament formation and identified these metabolic pathways as potential drug targets to inhibit morphogenesis.