Transcriptional control of environmental neutralization
Candida albicans and other Candida spp. grown on non-preferred carbon sources available in the host can actively neutralize the environmental pH from a very wide range of starting pHs (Vylkova et al., MBio 2011). In C. albicans neutral pH is a potent stimulator of hyphal morphogenesis, a well-recognized virulence factor, and this fungal species is able to switch morphologies during transition from pH 4 to neutral.
Stp2p, a transcription factor that regulates amino acid permease (AAP) genes, is required for environmental alkalinization in vitro, during macrophage phagocytosis, and for full virulence (Vylkova and Lorenz, PLoS Pathog 2014). Recently, we identified that another transcription factor, Ahr1p, is essential for pH modulation (Vylkova and Lorenz, submitted in Infect and Immun). Ahr1p plays role in adherence to biofilm and is predicted, but not verified, to control the expression of genes required for amino acid utilization, including AAPs (Askew et al., Mol Microbiol 2011). Thus, this project will investigate the connection between nutrient catabolic pathways and modulation of pH controlled by Ahr1 and Stp2. In addition, we seek to identify the factors controlling hyphal morphogenesis during the process of environmental neutralization in vitro and within the host.
C. albicans utilizes amino acids to promote neutralization of the phagosomal pH, hyphal morphogenesis, and escape from macrophages. C. albicans neutralizes the macrophage phagosome in a Stp2p and Ahr1p-dependent manner, since the majority of the wild type cells fail to co-localize with the acidophilic dye Lysotracker Red (red staining in panel B), while the tested neutralization mutants were located in acidic phagosomes (A; B). The ability of the mutant cells to form hyphae and escape from phagosomes neutralized with the vATPase inhibitor Bafilomycin A (Baf A) was restored, indicating that neutral pH is sufficient for macrophage damage. In the future, we will try to delineate the role of Ahr1p in the regulation of amino acid utilization (C).