Role of an Expanded Inositol Transporter Repertoire in Cryptococcus neoformans Sexual Reproduction and Virulence

Abstract

Cryptococcus neoformans and Cryptococcus gattii are globally distributed human fungal pathogens and the leading causes of fungal meningitis. Recent studies reveal that myo-inositol is an important factor for fungal sexual reproduction. That C. neoformans can utilize myo-inositol as a sole carbon source and the existence of abundant inositol in the human central nervous system suggest that inositol is important for Cryptococcus development and virulence. In accord with this central importance of inositol, an expanded myo-inositol transporter (ITR) gene family has been identified in Cryptococcus. This gene family contains two phylogenetically distinct groups, with a total of 10 or more members in C. neoformans and at least six members in the sibling species C. gattii. These inositol transporter genes are differentially expressed under inositol-inducing conditions based on quantitative real-time PCR analyses. Expression of ITR genes in a Saccharomyces cerevisiae itr1 itr2 mutant lacking inositol transport can complement the slow-growth phenotype of this strain, confirming that ITR genes are bona fide inositol transporters. Gene mutagenesis studies reveal that the Itr1 and Itr1A transporters are important for myo-inositol stimulation of mating and that functional redundancies among the myo-inositol transporters likely exist. Deletion of the inositol 1-phosphate synthase gene INO1 in an itr1 or itr1a mutant background compromised virulence in a murine inhalation model, indicating the importance of inositol sensing and acquisition for fungal infectivity. Our study provides a platform for further understanding the roles of inositol in fungal physiology and virulence.

Two Major Inositol Transporters and Their Role in Cryptococcal Virulence

“…Our previous functional studies on the seven group I ITR genes revealed that individual single mutations had no clear impact on fungal virulence and that this may be due to functional redundancy, but the ino1 itr1a double mutant did show virulence attenuation. This result suggests that inositol acquisition is important for the pathogenicity of C. neoformans (64).…”

“…It is known that C. neoformans is one of a few yeast organisms that can utilize inositol as a sole carbon source (6,23). Recently, we identified inositol as one important compound for stimulating the sexual reproduction of this yeast (64,65). Furthermore, an unusually large ITR gene family has been identified in C. neoformans based on the sequence homology with known ITR genes in other organisms.…”

“…Furthermore, an unusually large ITR gene family has been identified in C. neoformans based on the sequence homology with known ITR genes in other organisms. Functional characterization revealed that this ITR gene family is important for both sexual reproduction and fungal virulence (64,65). Unlike most fungi that contain one or two inositol transporters, the H99 strain carries 10 genes that are considered ITR candidates, and most of them are located in the telomere regions of the chromosomes (64).…”

“…Inositol can also be imported from the extracellular environment via inositol transporters. The inositol transporter (ITR) gene family is part of the sugar transporter superfamily and plays an important role in inositol sensing in fungi, including Saccharomyces cerevisiae (30,31,(42)(43)(44)51), Candida albicans (12,28,50), C. neoformans (64), and Schizosaccharomyces pombe (40). Studying ITR genes in fungi has clinical significance as ITR genes identified in lower eukaryotes, such as fungi (28) and protozoa (15,39,52), are proton coupled and differ both kinetically and pharmacologically from the sodium-dependent inositol transporter system (SMIT) in humans.…”

Cryptococcus inositol utilization modulates the host protective immune response during brain infection

“…Recently, we have shown that Cryptococcus contains an unusually large ITR gene family [15,16]. Based on our studies on two major ITR, Itr1a and Itr3c, we found that ITRs are required for full virulence of Cryptococcus, particularly during the fungal penetration of the BBB to cause infection of the CNS [9].…”