Stress adaptation in a pathogenic fungus

Abstract

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus-host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

Keywords: Candida albicans; Carbon metabolism; Cationic stress; Fungal pathogenicity; Heat shock; Nitrosative stress; Osmotic stress; Oxidative stress; Stress adaptation.

Fig. 1.

Conserved stress regulators in Candida albicans. Evolutionarily conserved mitogen-activated protein kinase (MAPK) signalling molecules (red) and transcription factors (blue) contribute to the regulation of stress functions in C. albicans (see ‘Overview of stress adaptation mechanisms in C. albicans’). Hsf1 and Hsp90 operate in an autoregulatory circuit, whereby synthesis of the biological transistor Hsp90 (green) is activated by Hsf1 in response to heat shock, and Hsp90 then downregulates Hsf1 (see ‘Overview of stress adaptation mechanisms in C. albicans’). These pathways are represented as linear pathways (for simplicity), but most probably operate in an integrated network. Heat shock pathway: Hsp90, heat shock protein 90; Hsf1, heat shock transcription factor. Nitrosative stress pathway: Cta4, zinc cluster transcription factor; Yhb1, nitric oxide dioxygenase. Oxidative stress pathway: Cap1, AP-1 bZIP transcription factor; Skn7; putative response regulator; SODs, superoxide dismutases. Hog1 signalling pathway: Ssk2, MAPK kinase kinase (MAPKKK); Pbs2, MAPK kinase (MAPKK); Hog1, MAPK/stress-activated protein kinase (SAPK). Cell integrity pathway: Bck1, MAPKKK; Mkk1, MAPKK; Mkc1, MAPK. Mating/invasive growth pathway: Ste11, MAPKKK; Hst7, MAPKK; Cek1, MAPK.

Fig. 2.

Acquired stress tolerance and stress cross-protection in yeasts. (A) Prior exposure to a stress can protect C. albicans cells against subsequent exposure to that stress (acquired stress tolerance) (upper panel). This indicates the existence of a molecular memory (see ‘Adaptation to sequential stresses’). However, the molecules that represent this memory have biological half-lives. Therefore, this molecular memory is transient, and will be lost during protracted time intervals between stresses (lower panel). (B) In some yeasts, some stresses (stress 1; blue) activate a core transcriptional response (purple) that includes genes that protect against another stress (stress 2; red). In this case, prior exposure to stress 1 often activates a molecular memory that confers protection against stress 2 (upper panel). However, if this core transcriptional response does not include genes that protect against a third stress (stress 3; green), then prior exposure to stress 1 does not activate a relevant molecular memory and does not confer protection against stress 3 (lower panel).

Fig. 3.

Core stress responses in yeasts. The yeasts C. albicans, Saccharomyces cerevisiae, Candida glabrata and Schizosaccharomyces pombe are evolutionarily separated by many millions of years and occupy contrasting niches: green, environmental niches; red, pathogens. Three of these yeasts display core transcriptional responses to stress in which relatively large numbers of genes are commonly induced in response to different stresses. In S. cerevisiae and C. glabrata the zinc finger transcription factors Msn2 and Msn4 contribute significantly to the core stress response, whereas this response in S. pombe is driven by the Sty1 SAPK. The core transcriptional response has diverged significantly in C. albicans, in which there is a relatively small number of core stress genes (see ‘Adaptation to sequential stresses’).

Fig. 4.

Anticipatory prediction in C. albicans and S. cerevisiae. (A) As described by Mitchell and co-workers, microbes often display adaptive prediction, whereby exposure to one environmental input can lead to the anticipatory induction of the response to a second environmental input (Mitchell et al., 2009). The authors argue that this provides an evolutionary advantage to the microbe because the first input is often followed by the second input in its normal environmental niche. Anticipatory responses can be asymmetric or symmetric. (B) Saccharomyces cerevisiae displays asymmetric anticipatory adaptive prediction by activating oxidative stress genes in response to elevated temperatures. Candida albicans displays an analogous asymmetric anticipatory adaptive response (Mitchell et al., 2009). This pathogen also displays symmetric anticipatory adaptive prediction by activating oxidative stress genes in response to glucose exposure and by activating carbohydrate metabolism in response to oxidative stress (see ‘Adaptation to sequential stresses’).

Fig. 5.

Mechanisms underlying combinatorial stress effects in C. albicans. Several distinct mechanisms contribute to combinatorial stress effects in C. albicans (see ‘Adaptation to combinatorial stresses’). (A) Classical cross-talk occurs between the MAPK signalling pathways (Alonso Monge et al., 2006). Hog1 signalling pathway: Ssk2, MAPKKK; Pbs2, MAPKK; Hog1, MAPK/SAPK. Cell integrity pathway: Bck1, MAPKKK; Mkk1, MAPKK; Mkc1, MAPK. Mating/invasive growth pathway: Ste11, MAPKKK; Hst7, MAPKK; Cek1, MAPK. (B) Hsp90 acts as a biological transistor, modulating the activities of the transcription factor Hsf1 and the MAPKs in response to thermal fluctuations (Leach et al., 2012a; Leach et al., 2012b). (C) Combinatorial cationic plus oxidative stress leads to stress pathway interference, whereby Hog1 and Cap1 signalling are affected by oxidative and cationic stress, respectively (D.K., M.D.J., A.T. and A.J.P.B., unpublished). (D) There is cross-talk at the chemical level, whereby different reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive chlorine species (RCS) can be generated spontaneously and by enzymatic catalysis (Brown et al., 2009; Brown et al., 2011), presumably leading to the activation of different subsets of stress genes.

Fig. 6.

Impact of carbon source on C. albicans. Changes in carbon source affect the proteome, architecture and biophysical properties of the C. albicans cell wall. This affects stress adaptation, immune recognition and virulence (Ene et al., 2012a; Ene et al., 2012b; Ene et al., 2013). Transmission electron micrographs of cell walls from C. albicans cells grown on glucose or lactate are shown on the right.