Cryptococcus neoformans adapts to host CO2 concentrations via metabolic and stress-response remodeling

Cryptococcus neoformans is an environmental pathogen that remodels its cellular physiology to survive within mammals and, in susceptible hosts, cause life-threatening meningoencephalitis. Of the many distinctions between the external environment and mammalian tissues, CO 2 concentration in the host is two orders of magnitude higher than in the environment and represents a critical stress for C. neoformans . C. neoformans strains that do not replicate at host CO 2 concentrations are less virulent in mouse models of infection, further supporting CO 2 tolerance as a virulence trait. To further understand the genetic determinants of C. neoformans CO 2 tolerance, we performed a near genome-wide screen for deletion mutants with altered CO 2 fitness using a competitive growth assay. A total of 301 of 4,692 deletion mutants showed altered CO 2 tolerance (245 reduced fitness; 56 increased fitness) demonstrating the global effect of host CO 2 on C. neoformans physiology. Based on this data set as well as a metabolomic analysis of C. neoformans adaptation to host CO 2 , we show that remodeling of central carbon metabolism, oxidative stress buffering, and membrane homeostasis represent an integrated response to CO 2 stress that is mediated in part by the TOR-Ypk1 signaling axis. We propose that CO 2 -induced capsule formation leads to reduced cellular glucose which, in turn, triggers remodeling of central carbon metabolism toward utilization of alternative carbon sources and increased mitochondrial respiration/reactive oxygen generation. Thus, these data provide a near genome-wide profile of the genetic determinants of C. neoformans CO 2 tolerance as well as a model for how this important environmental human fungal pathogen alters its physiology to proliferate in the host.