The Role of Phosphate Acquisition in Promoting Stress Resistance and Virulence in a Major Human Fungal Pathogen

Abstract

The ability of pathogenic fungi to obtain essential nutrients from the host is vital for virulence. In Candida albicans, acquisition of the macronutrient phosphate (Pi) is regulated by the Pho4 transcription factor, which is important for both virulence and resistance to diverse and physiologically important stresses. A key aim of this work was to investigate the regulation of Pho4, and the roles of Pho80-Pho85 cyclin dependent kinase (CDK) signalling and inositol polyphosphates were explored. As reported in the model yeast Saccharomyces cerevisiae, the Pho80 cyclin functions as a negative regulator of Pho4 in C. albicans. However, in contrast to S. cerevisiae, the CDK inhibitor Pho81 also negatively regulates C. albicans Pho4; Pho4 accumulates in the nucleus in pho81Δ cells and Pi acquisition strategies are activated under Pi replete conditions. With regard to inositol polyphosphates, in contrast to that reported in S. cerevisiae, IP7 synthesis by the Kcs1 inositol pyrophosphate synthase was found to be largely dispensable for Pi homeostasis with Vip1-derived IP7 synthesis playing a more prominent role in C. albicans. The synthesis of the Pi storage molecule polyphosphate (polyP) is also regulated by Pho4. Previous work found that mobilization of Pi from polyP is one of the first responses evoked in response to Pi starvation and precedes activation of the Pho4 transcription factor. A further aim of this thesis was to investigate the importance of polyP mobilisation in the pathobiology of C. albicans. It was found that two polyphosphatases, Ppn1 and Ppx1, function redundantly in C. albicans to release Pi from polyP. Strikingly, it was shown that polyP mobilisation plays a role in Pho4 activation and stress resistance in C. albicans. Blocking polyP mobilisation also resulted in significant morphological defects. Consistent with these findings, data is also presented illustrating that polyP mobilisation is important for the virulence of C. albicans. Given the links between Pi acquisition and virulence, a further aim was to explore whether Pi acquisition could be exploited as a novel antifungal strategy. High-throughput screening of compound libraries revealed potential candidates directly targeting the PHO pathway, which present an exciting avenue for future work. Taken together, the findings presented in this thesis reveal novel insight into Pi homeostasis mechanisms in C. albicans and the potential of targeting this important virulence trait in the development of future therapeutic strategies.