A Link between Virulence and Homeostatic Responses to Hypoxia during Infection by the Human Fungal Pathogen Cryptococcus neoformans

Abstract

Fungal pathogens of humans require molecular oxygen for several essential biochemical reactions, yet virtually nothing is known about how they adapt to the relatively hypoxic environment of infected tissues. We isolated mutants defective in growth under hypoxic conditions, but normal for growth in normoxic conditions, in Cryptococcus neoformans, the most common cause of fungal meningitis. Two regulatory pathways were identified: one homologous to the mammalian sterol-response element binding protein (SREBP) cholesterol biosynthesis regulatory pathway, and the other a two-component-like pathway involving a fungal-specific hybrid histidine kinase family member, Tco1. We show that cleavage of the SREBP precursor homolog Sre1—which is predicted to release its DNA-binding domain from the membrane—occurs in response to hypoxia, and that Sre1 is required for hypoxic induction of genes encoding for oxygen-dependent enzymes involved in ergosterol synthesis. Importantly, mutants in either the SREBP pathway or the Tco1 pathway display defects in their ability to proliferate in host tissues and to cause disease in infected mice, linking for the first time to our knowledge hypoxic adaptation and pathogenesis by a eukaryotic aerobe. SREBP pathway mutants were found to be a hundred times more sensitive than wild-type to fluconazole, a widely used antifungal agent that inhibits ergosterol synthesis, suggesting that inhibitors of SREBP processing could substantially enhance the potency of current therapies.

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).…”

Evolutionary conservation and adaptation in the mechanism that regulates SREBP action: what a long, strange tRIP it's been

“…One of the rapamycin-insensitive effects is likely through the direct action of AKT on SREBP-1 maturation. However, AKT probably also stabilizes the nuclear form of SREBP-1 through inhibition of GSK3 (Cross et al 1995), which can phosphorylate nuclear SREBP-1 at two closely spaced residues close to its C terminus (Sundqvist et al 2005). Once phosphorylated by GSK3, SREBP-1 interacts with the E3 ubiquitin ligase Fbw7, resulting in ubiquitination and degradation by the proteasome.…”

“…Studies of the opportunistic pathogen Cryptococcus neoformans demonstrated that Sre1 is a hypoxic transcription factor that activates expression of genes required for ergosterol synthesis and iron acquisition (Chang et al 2007;Chun et al 2007). While it appears that S. pombe lacks a Site-2 protease, C. neoformans has a Site-2 protease that is required for Sre1 activation (Chun et al 2007;Bien et al 2009). Importantly, Sre1 is required for virulence and disease progression in two different mouse models of cryptococcosis, indicating that Sre1 is essential for adaptation to the host environment.…”

Regulation of hypoxia adaptation: an overlooked virulence attribute of pathogenic fungi?

“…The SREBP pathway in Cryptococcus neoformans: Orthologs of SREBP (SRE1), SCAP (SCP1) and a Site-2-protease (STP1) were identified and characterized in C. neoformans (100,102). C. neoformans appears to lack an identifiable homologue of Insig, the ER retentionprotein that controls ER-to-Golgi transport of SREBP-SCAP complex in mammalian cells.…”

“…The two-component like (Tco) system in Cryptococcus neoformans: Additional studies observed that the Sre1p pathway acts in parallel with a two-component signal transduction like pathway controlled by Tco1p in hypoxic adaptation of C. neoformans (100). Tco1p is a member of a highly conserved family of fungal-specific histidine kinases.…”