Molecular characterisation of fungal environmental carbon dioxide sensing and its integration with cAMP signalling and metabolism.

The fungal pathogen Candida albicans is non-virulent in healthy individuals where it is part of the normal skin and intestinal flora. On the contrary, in patients whose immune system is not functional as a consequence of cancer, age, or AIDS, this fungus is a major cause of death. C. albicans undergoes a series of characteristic morphological changes important for its ability to cause damage to the infected host. These changes are controlled by host environmental cues such as the elevated temperature inside the body, the slightly alkaline pH of blood as well as high carbon dioxide concentrations. Inside our body the concentration of carbon dioxide is approximately 150 fold higher than in the air surrounding us. When the fungal pathogen C. albicans senses the higher CO2 inside the host, it initiates a series of morphological changes, which enable it to infect that host. Recently we identified the enzyme from C. albicans responsible for these changes. Now we want to investigate precisely how this enzyme senses CO2. CO2 serves other functions important for the survival of C. albicans. For example it is a fundamental part of basic cellular metabolism. We also propose to study whether the CO2 sensing enzyme is related to C. albicans metabolism. Interfering with fungal CO2 sensing may open up possibilities to better manage C. albicans infection in the future.

The environmental CO2 determines fungal morphology and influences metabolism. Recently we showed that the Candida albicans adenylyl cyclase Cdc35 is a CO2 sensor. We also found that expression of the C. albicans carbonic anhydrase Nce103 is tightly controlled by environmental CO2. In addition we demonstrated that CO2/HCO3- equilibration by Nce103 is required for survival of C. albicans in host niches with low ambient CO2. Adenylyl cyclase is the most widely used effector protein in signalling pathways and Cdc35 is essential for morphological differentiation and virulence of C. albicans. Our overall aim is to understand the molecular basis of CO2/bicarbonate activation of Cdc35 and determine how CO2 signalling and metabolism are integrated. We will mutagenise Cdc35 to identify key amino acid residues required for CO2 chemosensing. Furthermore, we will investigate how NCE103 expression is controlled by environmental CO2 and characterise main metabolic pathways that require carbonic anhydrase activity. Finally, we will localise Cdc35 inside the yeast cell and determine if it interacts physically with Nce103. In doing so we will gain a general understanding on the mechanism of fungal adenylyl cyclase activation and establish the role of CO2 sensing and metabolism for fungal growth and virulence. The experiments proposed in this project application will provide major new insights into the dynamics of CO2 sensing and the interaction of C. albicans with its host as well as elucidate the integration of CO2-dependent cAMP signalling and metabolism. This knowledge may provide the basis for a better management of fungal disease.