Redox Regulation, Rather than Stress-Induced Phosphorylation, of a Hog1 Mitogen-Activated Protein Kinase Modulates Its Nitrosative-Stress-Specific Outputs.

In all eukaryotic kingdoms, mitogen-activated protein kinases (MAPKs) play critical roles in cellular responses to environmental cues. These MAPKs are activated by phosphorylation at highly conserved threonine and tyrosine residues in response to specific inputs, leading to their accumulation in the nucleus and the activation of their downstream targets. A specific MAP kinase can regulate different downstream targets depending on the nature of the input signal, thereby raising a key question: what defines the stress-specific outputs of MAP kinases? We find that the Hog1 MAPK contributes to nitrosative-stress resistance in Candida albicans even though it displays minimal stress-induced phosphorylation under these conditions. We show that Hog1 becomes oxidized in response to nitrosative stress, accumulates in the nucleus, and regulates the nitrosative stress-induced transcriptome. Mutation of specific cysteine residues revealed that C156 and C161 function together to promote stress resistance, Hog1-mediated nitrosative-stress-induced gene expression, resistance to phagocytic killing, and C. albicans virulence. We propose that the oxidation of Hog1, rather than its phosphorylation, contributes to the nitrosative-stress-specific responses of this MAP kinase.IMPORTANCE Mitogen-activated protein kinases play key roles in the responses of eukaryotic cells to extracellular signals and are critical for environmental-stress resistance. The widely accepted paradigm is that MAP kinases are activated by phosphorylation, which then triggers their nuclear accumulation and the activation of target proteins and genes that promote cellular adaptation. Our data suggest that alternative forms of posttranslational modification can modulate MAP kinase functionality in Candida albicans We demonstrate that Hog1 is not significantly phosphorylated in response to nitrosative stress, yet it displays nuclear accumulation and contributes to the global transcriptional response to this stress, as well as promoting nitrosative-stress resistance. Instead, nitrosative stress triggers changes in the redox status of Hog1. We also show that specific Hog1 cysteine residues influence its activation of stress genes. Therefore, alternative posttranslational modifications appear to regulate the stress-specific outputs of MAP kinases.

Integrative Model of Oxidative Stress Adaptation in the Fungal Pathogen Candida albicans.

The major fungal pathogen of humans, Candida albicans, mounts robust responses to oxidative stress that are critical for its virulence. These responses counteract the reactive oxygen species (ROS) that are generated by host immune cells in an attempt to kill the invading fungus. Knowledge of the dynamical processes that instigate C. albicans oxidative stress responses is required for a proper understanding of fungus-host interactions. Therefore, we have adopted an interdisciplinary approach to explore the dynamical responses of C. albicans to hydrogen peroxide (H2O2). Our deterministic mathematical model integrates two major oxidative stress signalling pathways (Cap1 and Hog1 pathways) with the three major antioxidant systems (catalase, glutathione and thioredoxin systems) and the pentose phosphate pathway, which provides reducing equivalents required for oxidative stress adaptation. The model encapsulates existing knowledge of these systems with new genomic, proteomic, transcriptomic, molecular and cellular datasets. Our integrative approach predicts the existence of alternative states for the key regulators Cap1 and Hog1, thereby suggesting novel regulatory behaviours during oxidative stress. The model reproduces both existing and new experimental observations under a variety of scenarios. Time- and dose-dependent predictions of the oxidative stress responses for both wild type and mutant cells have highlighted the different temporal contributions of the various antioxidant systems during oxidative stress adaptation, indicating that catalase plays a critical role immediately following stress imposition. This is the first model to encapsulate the dynamics of the transcriptional response alongside the redox kinetics of the major antioxidant systems during H2O2 stress in C. albicans.

Contribution of Fdh3 and Glr1 to Glutathione Redox State, Stress Adaptation and Virulence in Candida albicans.

The major fungal pathogen of humans, Candida albicans, is exposed to reactive nitrogen and oxygen species following phagocytosis by host immune cells. In response to these toxins, this fungus activates potent anti-stress responses that include scavenging of reactive nitrosative and oxidative species via the glutathione system. Here we examine the differential roles of two glutathione recycling enzymes in redox homeostasis, stress adaptation and virulence in C. albicans: glutathione reductase (Glr1) and the S-nitrosoglutathione reductase (GSNOR), Fdh3. We show that the NADPH-dependent Glr1 recycles GSSG to GSH, is induced in response to oxidative stress and is required for resistance to macrophage killing. GLR1 deletion increases the sensitivity of C. albicans cells to H2O2, but not to formaldehyde or NO. In contrast, Fdh3 detoxifies GSNO to GSSG and NH3, and FDH3 inactivation delays NO adaptation and increases NO sensitivity. C. albicans fdh3⎔ cells are also sensitive to formaldehyde, suggesting that Fdh3 also contributes to formaldehyde detoxification. FDH3 is induced in response to nitrosative, oxidative and formaldehyde stress, and fdh3Δ cells are more sensitive to killing by macrophages. Both Glr1 and Fdh3 contribute to virulence in the Galleria mellonella and mouse models of systemic infection. We conclude that Glr1 and Fdh3 play differential roles during the adaptation of C. albicans cells to oxidative, nitrosative and formaldehyde stress, and hence during the colonisation of the host. Our findings emphasise the importance of the glutathione system and the maintenance of intracellular redox homeostasis in this major pathogen.