Itr4: The Sensor a Killer Fungus Uses to Find Your Brain
Review written by: Roshni Kadam
Fungal infections are a major but often overlooked global health crisis. Of the millions of fungal species on Earth, only a few hundred infect humans yet those few are linked to roughly 3.8 million deaths each year, with about 2.5 million directly attributable to fungal disease. The World Health Organization has named a critical priority group of the most dangerous fungi, and Cryptococcus neoformans is one of the pathogens classified in the critical priority group. C. neoformans causes fatal meningoencephalitis and accounts for approximately 15% of AIDS-related deaths worldwide. Previous work showed that C. neoformans exploits the abundant inositol in the human brain to support its growth and virulence, but how the fungus senses inositol was a major question in the field yet to be answered. Researchers in the Xue lab at Rutgers New Jersey Medical School have now found the answer. Previous work from the lab showed that inositol controls both the size and the structure of the fungus’s polysaccharide capsule that is one of its most important virulence factors. C. neoformans carries an unusually expanded family of ten inositol transporters (ITRs), but three of them, the group II transporters, had never been characterized. One of those three stood out because a genome-wide association study of clinical isolates from the COAT trial in Uganda had linked variants in ITR4 to differences in patient immune response and survival.
To test these genes, the Xue lab team deleted each group II transporter individually, that lead them to determine, ITR4 serves as a critical sensor. Cells lacking ITR4 made smaller capsule and lost the ability to mate with decreased inositol uptake. Heterologous expression in a Saccharomyces cerevisiae system, combined with ³H-inositol uptake and binding assays, established that Itr4 binds inositol at the plasma membrane without mediating its transport demonstrating that ITR4 is an inositol specific transceptor which senses and regulates inositol uptake in C. neoformans. Structure-guided mutagenesis, informed by AlphaFold 2 modeling, identified two functional determinants of Itr4: an unusual 120-residue cytosolic N-terminal leader required for membrane localization and function, and two inositol-contacting glutamines, Q388 and Q389, that act as gatekeepers restricting transport. On substituting both increased inositol uptake and yielded a dominant allele capable of sensing and transporting inositol, with molecular dynamics simulation confirming that the mutation destabilizes inositol in the binding pocket and widens the channel. In an in vitro human blood–brain barrier model, itr4Δ transmigration fell roughly 50%, and while supplemental inositol increased wild-type crossing 3- to 5-fold, the mutant showed no response, establishing Itr4 as the sensor coupling inositol availability to central nervous system invasion. RNA-seq of wild-type and itr4Δ cells extended its reach beyond inositol itself: biosynthesis genes were upregulated while uptake and catabolism genes including MIO1 were downregulated, and unexpectedly the glucose transceptor HXS1 was repressed over 125-fold, with MIG1 downregulated and SNF1 induced, implicating Itr4 in carbon catabolite repression. Together these findings identify Itr4 as the first inositol sensor described in a eukaryote and position it as a regulatory node linking inositol availability to glucose metabolism, capsule development, and brain invasion with an adaptation that likely arose for survival among plants and amoebal predators, and that now equips C. neoformans to exploit the inositol-rich human brain.
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READ MORE: Wang Y, Tancer RJ, Wear M, Jackson KM, Zhang Y, Li K, et al. (2026) An inositol receptor Itr4 coordinates carbon catabolite regulation and virulence in Cryptococcus neoformans. PLoS Biol 24(10): e3003709. https://doi.org/10.1371/journal.pbio.3003709
