Genetic and mechanistic analysis of carbon dioxide tolerance in Cryptococcus pathogenesis

PROJECT SUMMARY Cryptococcus neoformans is an important cause of human fungal infections and is one of the most common causes of infection-related deaths in people living with HIV/AIDS. As an environmental fungus, C. neoformans must be able to adapt to mammalian physiology in order to cause disease and only a minority of C. neoformans strains can do so. Three traits have been identified that distinguish many pathogenic C. neoformans strains from non-pathogenic strains: 1) capsule formation; 2) melanization; and 3) the ability to grow at mammalian body temperature. Recent studies of large collections of C. neoformans strains indicate that these three traits do not explain the observed variation in virulence because many closely related strains that express the three major traits still show significant differences in virulence. In the previous funding period, we have discovered that the ability to tolerate carbon dioxide (CO2) concentrations of mammalian host is required for C. neoformans virulence. We also identified protein kinase and transcription factor networks involved in CO2 tolerance. Finally, quantitative trait loci (QTL) mapping and in vitro evolution of low CO2 fitness isolates to identified modulators of CO2 fitness and revealed that multiple pathways and genes contribute to variation in CO2 fitness. In the next funding period, we propose to determine how these virulence-associated regulatory pathways interact to balance positive and negative responses to host CO2 stress (Aim 1). We will also identify and characterize the downstream physiologic and cell biologic processes by which the regulatory networks modulate CO2 tolerance (Aim 2). Our preliminary data indicate that CO2 tolerance may have independently emerged multiple times across the Cn genomic complexes. In Aim 3, we will identify and characterize the mechanisms through which this trait is expressed in clinical isolates with an initial focus on loss of Rim pathway function. Our mechanistic characterization of this emerging Cn virulence trait is expected to provide new insights into Cn pathogenesis and to identify novel pathways and targets for antifungal drug development.