The role of redox-sensitive antioxidants in oxidative stress signalling in Candida albicans

Abstract

Candida albicans is the major systemic fungal pathogen of humans. Consequently, much effort is directed at understanding how this medically relevant fungus evades the antimicrobial mechanisms mounted by the host’s immune system. An important defence mechanism employed by phagocytic cells involves the generation of highly toxic reactive oxygen species (ROS). However, although the ability of C. albicans to sense and respond to ROS is essential for virulence, the intracellular signalling mechanisms underlying such responses are poorly understood. C. albicans mounts a robust oxidative stress response when exposed to H2O2 in vitro or following phagocytosis, which involves a significant remodelling of the transcriptome and proteome. This is regulated by the Cap1 AP-1 like transcription factor and, to a lesser extent, the Hog1 stress activated protein kinase (SAPK). In addition, exposure of C. albicans to H2O2 stimulates filamentous growth. However, despite strong relationships between oxidative stress responses, filamentous growth, and virulence, very little is known about the intracellular signalling mechanisms that regulate H2O2-responsive signalling pathways in C. albicans. Redox sensitive antioxidant proteins, such as 2-Cys peroxiredoxins and thioredoxins, have recently been demonstrated in model yeasts and mammalian cells to have additional oxidative stress signalling functions. Hence, the aim of this thesis was to investigate the potential roles of thioredoxins and 2-Cys peroxiredoxins in oxidative stress sensing and signalling in C. albicans. Thioredoxins are conserved oxidoreductases which regulate the catalytic reduction of diverse proteins including 2- Cys peroxiredoxins which become oxidized upon reducing H2O2. Significantly, data is presented which illustrates that Trx1 is the major thioredoxin protein in C. albicans and, in addition to functioning as an antioxidant, plays a central role in oxidative stress signalling by regulating distinct pathways. For example, Trx1 is required for the H2O2– induced activation of the Hog1 SAPK, as either deletion of TRX1 or mutation of the catalytic cysteine residues of Trx1 significantly impairs H2O2-induced phosphorylation of Hog1. Notably, the sole 2-Cys peroxiredoxin in C. albicans, Tsa1, also positively regulates H2O2-induced Hog1 activation in C. albicans. As Tsa1 is a major substrate of Trx1, this is consistent with a model in which Trx1 regulates Hog1 through regulating the redox status of Tsa1. Evidence is also presented that Trx1 negatively regulates H2O2-induced filamentation in a mechanism that is independent of Tsa1 and instead involves activation of the Rad53 DNA checkpoint kinase. For example, either inactivation of TRX1, or treatment of cells with H2O2, results in significant hyperphosphorylation of Rad53 and the formation of hyperpolarized buds. In addition, deleting RAD53 completely abolishes H2O2-induced filamentous growth. These findings are consistent with a model in which oxidation and thus inactivation of Trx1 in response to H2O2 is a key step in stimulating Rad53 activation and hyperpolarized bud growth. In agreement with its key roles in responses to ROS, cells lacking Trx1 display attenuated virulence in both a macrophage killing model and a murine model of C. albicans systemic infection. This may be dependent on its signalling rather than antioxidant functions, as a previous study reported that inactivation of Tsa1 does not attenuate virulence. Collectively, the data presented in this thesis indicates that both Tsa1 and Trx1 have important roles in H2O2 signalling and that Trx1 promotes C. albicans survival in the host.