Investigation of the functional significance of phytase activity in human fungal pathogen Candida albicans

Introduction and Project Objectives: Candida albicans is one of the most prevalent human fungal pathogens. In healthy individuals, C. albicans cohabits as a harmless commensal on the skin and mucosal surfaces of oral cavity, digestive tract and urogenital system. In cases of impaired immunity, C. albicans can become invasive and cause an infection (candidiasis). Mild superficial infection is not fatal, but disseminated candidiasis can be life-threatening. The ability of pathogens to colonize and proliferate in host tissues contributes to pathogenicity. Phosphorus is a building block of nucleic acids, ATP and is involved in phosphorylation and glycolysis. Myo-inositol plays key role in membrane formation, signal transduction and osmoregulation. More importantly, it is a precursor of cell surface glycosylphosphatidylinositol-anchored glycolipids, a C. albicans virulence trait through interactions with human macrophages. Phytate degradation by phytase liberates myo-inositol and inorganic phosphate, both are essential molecules for fungal growth and pathobiology. Our previous study demonstrated the presence of phytase activity in Candida fungi and suggested its potential involvement in host-pathogen interactions. The objectives of this study were: (1) To create C. albicans phytase null mutants and examine their phenotypic determinants; (2) To examine the effects of pH and transcription factors on phytase activity; and (3) To evaluate the importance of phytase activity in C. albicans fitness and virulence. Methods: C. albicans phytase null mutants were created using a PCR-based gene targeting method. The phenotypic properties of the mutants were evaluated, including phytase activity, fungal growth, yeast-to-hyphal morphogenesis, adhesion to buccal epithelial cells (BECs), and virulence. The effect of pH on phytase activity was evaluated by incubating the fungal cells at different pH (from 4.0 to pH 8.5). The effect of transcription factors on phytase activity was investigated using quantitative PCR. Results: C. albicans phytase null mutants have been created and verified. Biochemical analyses indicated that PHO112 contributed to the C. albicans phytase activity. C. albicans pho112Δ/pho112Δ possessed decreased phytase activity, reduced ability to form hyphae in the presence of fetal calf serum at 37oC, and attenuated adhesion to BECs and virulence. pH and transcription factors had no effect on phytase activity. Conclusions: The collective data of the present study suggest that PHO112 is responsible for the C. albicans phytase activity, which is not affected by pH and transcription factors; and virulence