Signal transduction pathways are biochemical mechanisms by which cells are able to sense the environment and develop and appropriate adaptive response. Some of these pathways are controlled by MAP kinases, a set of dual specificity kinases that play a central role in several of physiological conditions. In the model organism S. cerevisiae, there have been described at least five of these cascades, each of them responding to a specific physiological stimulus. In this organism, the HOG pathway has been shown to be involved in the response against external high osmolarity and its activation results in the accumulation of glycerol. For pathogenic organisms, signal transduction pathways provide a mechanism for the cell to respond to a continuously changing environment such as the human body, where both natural and acquired defenses continuously challenge its survival. We are analyzing the role of the HOG pathway in C. albicans by the analysis of strains deficient in the Hog1p MAP kinase. Although it has been shown that hog1 mutants are more sensitive to high osmolarity, we have also shown that they are more sensitive to oxidants such as hydrogen peroxide and the superoxide generator menadione. In order to characterize the mechanism by which this process takes place, we have performed some epistatic experiments with CAP1, the homologous to the S. cerevisiae transcription factor YAP1, a major mediator of the oxidative stress response in this yeast. C. albicans cap1 mutants were constructed either in a wild type or a hog1 background using the strategy already developed by our group that used two different nutritional for disruption of both copies of the gene. In this work we have addressed the role of Hog1 in mediating the oxidative stress response in this organism using combined biochemical and genetic analysis. We show here that 1) Hog1 is activated not only in response to high osmolarity (NaCl) but also in response to oxidants such as peroxides (H2O2); 2) The activation of Hog1 is fast (less than 2 minutes) and it can be induced in the 2 mM to 100 mM range; 3) Activation of Hog1 is Cap1-independent and, consistent with this, translocation of Cap1-GFP in response to hydrogen peroxide was Hog1-independent. Perhaps surprisingly, 4) we have observed nuclear translocation of a GFP tagged version of Hog1 in response to NaCl but not to hydrogen peroxide. All these data support a role for the HOG pathway in sensing oxidative stress in this microorganism, thus opening the possibility of using it a therapeutic target in antifungal therapy.
Full conference title:
The 15 th Congress of the International Society for Human and Animal Mycology
- ISHAM 15th (2003)