Regulation of gene expression by acidic pH in an opportunistic human fungal pathogen Cryptococcus neoformans: modulating antifungal susceptibility and iron Uptake
2020-04-20T23:21:31Z (GMT) by
Cryptococcus neoformans is an opportunistic human fungal pathogen, but its infection is militated by phagocytosis and phagolysosomes in the host immune system. It has been reported that the pH of phagolysosomes containing C. neoformans is approximately 5.3. In general, an acidic pH condition, in comparison with neutral pH, is known to alter several physiological characteristics of fungi, including susceptibility to azole antifungal drugs. Indeed, azole antifungal susceptibility of another well know human fungal pathogen, Candida albicans, is modulated by pH, although the underlying mechanism is not clear yet. Therefore, we investigated whether the environmental pH influences the azole antifungal susceptibility of C. neoformans and how the fungus responds to acidic pH. We found that the minimal inhibitory concentration (MIC) of C. neoformans against fluconazole was increased under an acidic pH condition, and our GC-MS analysis revealed that the ergosterol content in C. neoformans that was grown under an acidic condition was greatly increased in comparison to that in cells grown at a neutral pH level. Moreover, a mutant strain lacking CFO1, which is the major component in the high-affinity reductive iron uptake system in C. neoformans, displayed significantly reduced sensitivity to fluconazole at an acidic pH level. This implies that a different iron uptake pathway governs the transport of iron under such a condition. Considering that a number of the proteins involved in ergosterol biosynthesis require iron as a cofactor, our data implied the involvement of a yet unknown iron uptake pathway, which is independent of the CFO1 function, in azole antifungal susceptibility of C. neoformans under an acidic pH condition. In addition, we investigated the underlying molecular regulatory mechanism to understand how C. neoformans responds to an acidic pH condition through transcriptome analysis as well as phenotypic and biochemical analysis of the series mutant strains lacking the genes involved in the major iron uptake pathways.