No abstract

Fungal pathogens like Candida albicans exhibit several survival mechanisms to evade attack by antifungals and colonize host tissues. Rta3, a member of the Rta1-like family of lipid-translocating exporters has a 7-transmembrane domain (7TMD) topology, similar to the G-protein-coupled receptors (GPCR) and is unique to the fungal kingdom. Our findings point towards a role for the plasma membrane localized Rta3 in providing tolerance to miltefosine, an analog of alkylphosphocholine, by maintaining mitochondrial energetics. Concurrent with miltefosine susceptibility, the rta3Δ/Δ strain displays increased inward translocation (flip) of fluorophore-labelled phosphatidylcholine (PC) across the plasma membrane attributed to enhanced PC-specific flippase activity. We also assign a novel role to Rta3 in the Bcr1-regulated pathway for in vivo biofilm development. Transcriptome analysis reveals that Rta3 regulates expression of Bcr1 target genes involved in cell surface properties, adhesion, and hyphal growth. We show that rta3Δ/Δ mutant is biofilm-defective in a rat venous catheter model of infection and that BCR1 overexpression rescues this defect, indicating that Bcr1 functions downstream of Rta3 to mediate biofilm formation in C. albicans. The identification of this novel Rta3-dependent regulatory network that governs biofilm formation and PC asymmetry across the plasma membrane will provide important insights into C. albicans pathogenesis.

The pathogenicity of Candida albicans, an opportunistic human fungal pathogen, is attributed to several virulence factors. β-citronellol is a monoterpenoid present in several plant essential oils. The present study explores the antifungal potential and mode of action of β-citronellol against C. albicans ATCC 90028 (standard), C. albicans D-27 (FLC-sensitive), and C. albicans S-1 (FLC-resistant). Anti-Candida potential was studied by performing MIC, MFC, growth curves, disc diffusion, spot assay, and WST1 cytotoxic assay. Morphological transition was monitored microscopically in both solid and liquid hyphae inducing media. β-citronellol inhibits yeast to hyphal transition in both liquid and solid hyphae inducing media. It had a significant inhibitory effect on biofilm formation and secretion of extracellular proteinases and phospholipases. We showed that it has an adverse effect on membrane ergosterol levels and modulates expression of related ERG genes. Expression profiles of selected genes associated with C. albicans pathogenicity displayed reduced expression in treated cells. This work suggests that β-citronellol inhibits morphological transition in C. albicans and decreases the secretion of hydrolytic enzymes involved in the early stage of infection as well as modulates the expression of associated genes. Pleiotropic phenotype shown by β-citronellol treated Candida cells suggests various modes of action. Further studies will assess the clinical application of β-citronellol in the treatment of fungal infections.

Hypoxic adaptation pathways, essential for Candida albicans pathogenesis, are tied to its transition from a commensal to a pathogen. Herein, we identify a WW domaincontaining protein, Ifu5, as a determinant of hypoxic adaptation that also impacts normoxic responses in this fungus. Ifu5 activity supports glycosylation homeostasis via the Cek1 mitogen-activated protein kinase-dependent up-regulation of PMT1, under normoxia. Transcriptome analysis of ifu5Δ/Δ under normoxia shows a significant up-regulation of the hypoxic regulator EFG1 and EFG1-dependent genes. We demonstrate physical interaction between Ifu5 by virtue of its WW domain and Efg1 that represses EFG1 expression under normoxia. This interaction is lost under hypoxic growth conditions, relieving EFG1 repression. Hypoxic adaptation processes such as filamentation and biofilm formation are affected in ifu5Δ/Δ cells revealing the role of Ifu5 in hypoxic signalling and modulating pathogenicity traits of C. albicans under varied oxygen conditions. Additionally, the WW domain of Ifu5 facilitates its role in hypoxic adaptation, revealing the importance of this domain in providing a platform to integrate various cellular processes. These data forge a relationship between Efg1 and Ifu5 that fosters the role of Ifu5 in hypoxic adaptation thus illuminating novel strategies to undermine the growth of C. albicans.