Conservation of the Sterol Regulatory Element-Binding Protein Pathway and Its Pathobiological Importance in Cryptococcus neoformans

Abstract

The mammalian sterol regulatory element-binding protein (SREBP) homolog, Sre1, is important for adaptation and growth of Cryptococcus neoformans in the mouse brain, where oxygen concentration and nutritional conditions are suboptimal for fungal growth. The extent of conservation of the SREBP pathway in C. neoformans or in any other fungi, however, has not been investigated. We generated mutants susceptible to low oxygen and identified six genes that play a role in the SREBP pathway. Three of these genes (SFB2, KAP123, and GSK3) are not known to be involved in the SREBP pathway in other fungi. Furthermore, we show that C. neoformans contains an additional gene, DAM1, which functions in the SREBP pathway but is yet to be described. Mutants associated with the steps prior to formation of the nuclear Sre1 form dramatically reduced accumulation of the nuclear form under low-oxygen conditions. Concurrently, two mutant strains, scp1⌬ and stp1⌬, and the previously isolated sre1⌬ strain showed reduction in ergosterol levels, hypersensitivity to several chemical agents, including azole antifungals, CoCl 2 , and compounds producing reactive oxygen or nitrogen species, and most importantly, reduced virulence in mice. Mutants affecting genes involved in later steps of the Sre1 pathway, such as those required for import and phosphorylation of proteins in the nucleus, showed less compelling phenotypes. These findings suggest that the SREBP pathway is highly conserved in C. neoformans and it serves as an important link between sterol biosynthesis, oxygen sensing, CoCl 2 sensitivity, and virulence in C. neoformans.

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

“…Studies have also shown that sre1 and stp1 mutants cause itraconazole to act in a fungicidal manner (56). Recently, six additional genes that play a role in the SREBP pathway in C. neoformans were identified: SFB2, STP1, SCP1, KAP123, GSK3, and DAM1 (97). Interestingly, SFB2, KAP123, and GSK3 were not known to be involved in the SREBP pathway in other fungi, suggesting a divergence of this pathway in C. neoformans.…”

“…Interestingly, SFB2, KAP123, and GSK3 were not known to be involved in the SREBP pathway in other fungi, suggesting a divergence of this pathway in C. neoformans. Furthermore, all novel mutants discovered in that study exhibited hypersensitivity to fluconazole, and all but KAP123 showed defects in mouse virulence models (97).…”

The Cryptococcus neoformans Capsule: a Sword and a Shield

“…In C. neoformans, the Scp1 protein (CNAG_01580) processes the Sre1 transcription factor (CNAG_04804) (29,41). Processed Sre1 is then imported into the nucleus by the Kap123 protein (CNAG_05884) and phosphorylated by the Gsk3 kinase (CNAG_06730) (30). The Dam1 protein regulates the turnover of Sre1, thus regulating the transcriptional response to low oxygen.…”

Structural Requirements for Sterol Regulatory Element-binding Protein (SREBP) Cleavage in Fission Yeast

“…This motif is broadly conserved across ascomycete fungi, whose members lack homologs of the intramembrane-cleaving Site-2 protease present in mammals. These findings demonstrate a role for this conserved glycineleucine motif in fungal SREBP cleavage activation, provide tools for dissecting the mechanism of SREBP cleavage, and identify the SREBP C terminus as a target for antifungal therapy for pathogenic fungi that contain a relevant conserved SREBP pathway (5,7,9,14,15).…”