Role of the Heat Shock Transcription Factor in the Fungal Pathogen Candida albicans

Microbes must respond to changes in their environmental if they are to survive, and many of these environmental changes are perceived as a stress. For example, many cells respond to a sudden increase in temperature (a heat shock) by making new proteins that repair the damage caused by the heat shock. This type of response is conserved from bacteria to humans. The fungus Candida albicans lives in mammals. (No niches have been found for Candida albicans other than in mammals.) Most of the time, this fungus exists relatively harmlessly on mucous (damp) membranes in the mouth, intestines or genitalia, for example. However, it can cause human disease. Candida causes oral and vaginal thrush, and it can cause life-threatening bloodstream infections in hospital patients who are too ill to combat microbial infections effectively. Given the sites that it occupies in humans, Candida will rarely be exposed to large temperature fluctuations, because our body temperature is maintained within physiological limits (close to 37C). Why then has Candida retained a heat shock response during its evolution within its mammalian host? We do not exclude the possibility that Candida might be exposed to infrequent heat shocks. However, we propose that the heat shock apparatus protects Candida against other environmental stresses. In this project we will test this idea by exploring whether Candida mutants that lack a normal heat shock response are more sensitive to the types of stress that it encounters in the human host. These include exposure to the oxidative stresses that human immunological defences use to combat microbial infections. We will also define how Candida responds to heat shock, establish what other types of stress induce a heat shock-like response, and test whether this contributes to the virulence of this fungus. This project will provide valuable new insights into the ways in which the fungus interacts with its human host during an infection, as well as insights into the evolution of stress responses in fungi.

Most cells respond to a rapid temperature increase (heat shock) by inducing the expression of heat shock proteins, many of which are molecular chaperones. We have shown that this is also the case in Candida albicans, although this major pathogen of humans thrives in niches where exposure to a significant heat shock is probably a rare event. In S. cerevisiae and S. pombe, transcriptional responses to heat shock are dependent on the Heat Shock Transcription Factor and regulators of the General Stress Response. We have shown that the regulation of these responses has diverged significantly in C. albicans. In this pathogen, heat shock responses are not mediated via a General Stress Response. However, C. albicans does express a well-conserved Heat Shock Transcription Factor. In S. cerevisiae and S. pombe, this factor is essential for viability even at normal growth temperatures, and we have shown that this is also the case in C. albicans. Based on the behaviour of its homologues in budding and fission yeasts, we hypothesise that the Heat Shock Transcription Factor is required for the expression of essential functions during normal growth of C. albicans in the absence of a stress. We predict that this factor is also required for the responses of this fungus to the environmental stresses that it encounters during disease progression in its human host. The overall aim of this project is to test this hypothesis. A series of well-defined conditional C. albicans mutants will be generated to examine the various cellular and molecular roles of the Heat Shock Transcription Factor (encoded by HSF1). These mutants will allow us to distinguish experimentally between the roles of this transcription factor in the absence and presence of stress in vitro. A combination of biochemical and genomic approaches will allow us to establish which stress conditions activate the Heat Shock Transcription Factor, and which C. albicans genes are regulated by this factor. This definition of HSF1 regulons will reveal the global cellular roles of the Heat Shock Transcription Factor under different environmental conditions. We will then use state-of-the-art genomic and cellular approaches to determine the roles of the Heat Shock Transcription Factor during disease progression in vivo. This project will provide major new insights into the evolution of fungal stress responses, and the relevance of these responses to fungus-host interactions during fungal infections.