Physiology, Regulation, and Pathogenesis of Nitrogen Metabolism in the Opportunistic Fungal Pathogen Candida albicans

Candida albicans is an opportunistic polymorphic fungus that causes clinically important disease candidiasis to humans. Being polymorphic C. albicans can grow in yeast, hyphae, or pseudohyphae forms and the switch from one form to another is required for virulence. The morphological transitions from one phase to another are carefully orchestrated events which are regulated by several signal transduction pathways. Several environmental factors determine the morphology of the fungus C. albicans. For example growth at lower temperature ~30 oC, in preferred nitrogen sources cause the fungus to grow as yeasts while at higher temperature ~37 oC and in poor nitrogen sources, in the presence of serum, N-acetyl glucosamine, or high CO2, C. albicans cells grow as hyphae. Under several clinically relevant circumstances, including biofilms, C. albicans cells encounter poor or low nitrogen conditions. In this project utilization of different nitrogen sources by C. albicans was evaluated and their roles in pathogenesis were studied. The aromatic amino acids are metabolized when the C. albicans cells grow under poor nitrogen conditions, and the resulting carbon skeletons are secreted outside the cell. They are well known as fusel oils or aromatic alcohols. The aromatic alcohol biosynthesis is enhanced under anaerobic conditions compared to aerobic conditions, by the presence of precursor amino acids (phenylalanine, tyrosine, or tryptophan), and in alkaline conditions compared to acidic conditions, but it is reduced greatly in the presence of ammonia. Also, aromatic alcohol yield is dependent on the transcription regulators Aro80p and Rim101p. In another project the role of arginine metabolism in the yeast to hypha morphological switch was studied. When C. albicans cells enter the bloodstream, they first encounter macrophages and are engulfed by them. But in four to six hours C. albicans cells form hyphae, penetrate and kill the macrophages, and get out in the bloodstream again. In this series of events at the initial phase, right after engulfment, C. albicans up-regulates arginine biosynthesis. We found that arginine biosynthesis is critical for the fungus because it is metabolized and produces CO2 inside the cell, a signal important for the yeast to hypha switch. C. albicans mutants that either failed to make arginine (arginine auxotrophs) or could not metabolize arginine to CO2 (urea amidolyase mutants) were defective in making germ tubes inside the macrophages. However, wild type C. albicans and C. albicans auxotrophic for other amino acids than arginine can kill macrophages within four to six hours after phagocytosis. So, another project studied if macrophages can induce appropriate cytokines within that short time span before being killed by C. albicans. We found that after engulfing C. albicans, macrophages induce cytokines within one hour. Chief among them were IL-6, IL-23, and TGF-β, important for the development of the Th-17 subset of T cells. Finally, two major components of C. albicans, the quorum sensing molecule farnesol and the cell wall component zymosan, together induced TLR2, a pattern recognition receptor, and both were responsible for the induction of IL-6, IL-23, and TGF-β. Farnesol was ca. 100 times more effective than farnesoic acid at inducing these cytokines. Overall, this body of work has taken a major step towards elucidating farnesol‟s mode of action as a virulence factor and a lipid signaling molecule while at the same time highlighting the magnitude of the gaps remaining our knowledge.