The Hog1 mitogen-activated protein (MAP) kinase mediates an adaptive response to both osmotic and oxidative stress in the fungal pathogen Candida albicans. This protein also participates in two distinct morphogenetic processes, namely the yeast-to-hypha transition (as a repressor) and chlamydospore formation (as an inducer). We show here that repression of filamentous growth occurs both under serum limitation and under other partially inducing conditions, such as low temperature, low pH, or nitrogen starvation. To understand the relationship of the HOG pathway to other MAP kinase cascades that also play a role in morphological transitions, we have constructed and characterized a set of double mutants in which we deleted both the HOG1 gene and other signaling elements (the CST20, CLA4, and HST7 kinases, the CPH1 and EFG1 transcription factors, and the CPP1 protein phosphatase). We also show that Hog1 prevents the yeast-to-hypha switch independent of all the elements analyzed and that the inability of the hog1 mutants to form chlamydospores is suppressed when additional elements of the CEK1 pathway (CST20 or HST7) are altered. Finally, we report that Hog1 represses the activation of the Cek1 MAP kinase under basal conditions and that Cek1 activation correlates with resistance to certain cell wall inhibitors (such as Congo red), demonstrating a role for this pathway in cell wall biogenesis.Polymorphism, that is, the ability to acquire different morphologies, has long been considered a major virulence factor in the human fungal pathogen Candida albicans. This fungus is present on the skin and mucosal surfaces of many organisms, including humans, acquiring mainly a unicellular yeast-like form, while in infected tissues, different morphologies (yeast, mycelia, and even chlamydospores) have been observed (9, 13). These types of morphologies have distinct abilities to adhere, proliferate, invade, or escape phagocytic cells and, therefore, contribute by different degrees to the pathogenesis of the infection. The transfer from the yeast form to the filamentous form of growth is induced by certain chemicals (14,18,20,48), a temperature close to 37°C (30), and a neutral pH (49), while chlamydospore formation is induced in vitro under special conditions, such as a low concentration of glucose, darkness, low temperature (24 to 28°C) and microaerophilia.The molecular mechanisms involved in the regulation of polymorphism in C. albicans are very complex. Genetic analysis has shown the implication of several genes and regulatory cascades in this process (31,37,54,56). These include, among others, the cyclic AMP (cAMP)-dependent protein kinase pathway and the mitogen-activated protein (MAP) kinase pathway. The cAMP pathway leads to an increase in intracellular cAMP (44) and controls the Efg1 transcription factor (16, 51, 52). C. albicans efg1 mutants are defective in both filamentation and chlamydospore formation (50, 51) and have a reduced virulence in certain models of experimental infection (33). Other pathways involved in fila...

The human fungal pathogen Candida albicans responds to stress by phosphorylation of the Hog1 MAP kinase. PBS2 was cloned and shown to encode the MAP kinase kinase that is involved in this activation, as determined by immunoblot analyses using antibodies that recognize the active form of the target Hog1 protein. Characterization of pbs2 mutants revealed that they were sensitive to both osmotic and oxidative stress and that they, interestingly, displayed differential behaviour from that of hog1 mutants, losing viability when exposed to an oxidative challenge more rapidly than the hog1 strain. Hog1 and Pbs2 were also shown to be involved in the mechanism of adaptation to oxidative stress, as evidenced by the enhanced susceptibility to oxidants of pbs2 and hog1 mutants, compared with the wild-type strain, when cells were previously exposed to a low, sub-lethal concentration of hydrogen peroxide and by the PBS2-dependent diminished activation of Hog1 MAP kinase in the adaptive process. Studies with a chimaeric Hog1-green fluorescent protein fusion revealed that this protein was localized throughout the cell (being excluded from the vacuole), but concentrated in the nucleus in response to NaCl stress, a process that was dependent on the Pbs2 protein. Both Hog1 and Pbs2 also play a role in controlling the phosphorylation state of the other MAP kinases Mkc1 and Cek1, involved respectively in cell-wall integrity and invasive growth. Furthermore, it is demonstrated that PBS2 plays a role in cell-wall biogenesis in this fungal pathogen, as its deletion renders cells with an altered susceptibility to certain cell wall-interfering compounds. INTRODUCTIONEukaryotic cells respond to environmental changes through signal-transduction pathways. MAPK (mitogen-activated protein kinase) routes consist of a three-kinase module: the MAP kinase kinase kinase, the MAP kinase kinase and the MAP kinase, which are activated by sequential phosphorylation in response to different extracellular signals. These pathways are conserved through evolution (Kültz & Burg, 1998) and their functionality is well-documented in some model organisms, such as Saccharomyces cerevisiae and Schizosaccharomyces pombe (Banuett, 1998;Gustin et al., 1998). In budding yeast, six MAPK pathways have been described that perform essential functions in fungal physiology, such as mating, pseudohyphal/invasive growth, cellwall integrity, STE vegetative growth (SVG), spore-wall assembly and adaptation to high osmolarity (HOG) (Gustin et al., 1998;Posas et al., 1998). This last route is activated in response to osmotic and -as described recently -oxidative stress (Haghnazari & Heyer, 2004;Singh, 2000) and mutant cells in some elements of the pathway are sensitive to both kinds of stresses. Osmosensitivity is partially explained by the inability to increase intracellular glycerol under restrictive conditions in order to counteract the extracellular turgor (Albertyn et al., 1994). Under hyperosmotic conditions, hog1 and pbs2 mutant strains display different alterations, suc...

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Innate immunity to Candida albicans depends upon the recognition of molecular patterns on the fungal cell wall. However, the masking of major components such as ␤-glucan seems to be a mechanism that fungi have evolved to avoid immune cell recognition through the dectin-1 receptor. Although the role of C. albicans mitogen-activated protein kinase (MAPK) pathways as virulence determinants has been established previously with animal models, the mechanism involved in this behavior is largely unknown. In this study we demonstrate that a disruption of the C. albicans extracellular signal-regulated kinase (ERK)-like 1 (CEK1)-mediated MAPK pathway causes enhanced cell wall ␤-glucan exposure, triggering immune responses more efficiently than the wild type, as measured by dectin-1-mediated specific binding and human dendritic cell (hDC)-and macrophage-mediated phagocytosis, killing, and activation of intracellular signaling pathways. At the molecular level, the disruption of CEK1 resulted in altered spleen tyrosine kinase (Syk), Raf-1, and ERK1/2 activations together with IB degradation on hDCs and increased dectin-1-dependent activator protein 1 (AP-1) activation on transfected cells. In addition, concurring with these altered pathways, we detected increased reactive oxygen species production and cytokine secretion. In conclusion, the CEK1-mediated MAPK pathway is involved in ␤-glucan exposure in a fungal pathogen, hence influencing dectin-1-dependent immune cell recognition, thus establishing this fungal intracellular signaling route as a promising novel therapeutic target.

SummaryThe role of four mitogen-activated protein (MAP) kinase pathways in the survival of Candida albicans following infection of human phagocytes has been addressed through the analysis of mutants defective in their respective MAP kinase. While the contribution of the cell integrity (Mkc1-mediated) or mating (Cek2-mediated) pathways is relatively minor to survival, clear and opposite effects were observed for cek1 and hog1 mutants, despite the fact that these two MAP kinases are important virulence determinants in the mouse model of experimental infection. The Cek1-mediated pathway is involved in sensitivity to phagocyte-mediated killing, while the HOG pathway contributes to the survival of the fungal cells in this interaction. Furthermore, reporter genes have been developed to quantify oxidative and nitrosative stress. hog1 mutants show an oxidative and nitrosative stress response augmented -albeit nonprotective -when challenged with oxidants and NO donors in vitro or phagocytic cells (macrophages, neutrophils and the myelomonocytic cell line HL-60), suggesting this as the cause of their reduced virulence in the murine model of infection. These data have important consequences for the development of novel antifungal therapies to combat against fungal infection.

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SummaryFungal infections are a serious health problem. In recent years, basic research is focusing on the identification of fungal virulence factors as promising targets for the development of novel antifungals. The wall, as the most external cellular component, plays a crucial role in the interaction with host cells mediating processes such as adhesion or phagocytosis that are essential during infection. Specific components of the cell wall (called PAMPs) interact with specific receptors in the immune cell (called PRRs), triggering responses whose molecular mechanisms are being elucidated. We review here the main structural carbohydrate components of the fungal wall (glucan, mannan and chitin), how their biogenesis takes place in fungi and the specific receptors that they interact with. Different model fungal pathogens are chosen to illustrate the functional consequences of this interaction. Finally, the identification of the key components will have important consequences in the future and will allow better approaches to treat fungal infections.

The maintenance of T-cell homeostasis must be tightly regulated. Here, we have identified a coordinated role of Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 in maintaining T-lymphocyte number and function. Mice bearing a T-cell specific deficiency of PARP-2 in a PARP-1-deficient background showed defective thymocyte maturation and diminished numbers of peripheral CD4+ and CD8+ T-cells. Meanwhile, peripheral T-cell number was not affected in single PARP-1 or PARP-2-deficient mice. T-cell lymphopenia was associated with dampened in vivo immune responses to synthetic T-dependent antigens and virus, increased DNA damage and T-cell death. Moreover, double-deficiency in PARP-1/PARP-2 in T-cells led to highly aggressive T-cell lymphomas with long latency. Our findings establish a coordinated role of PARP-1 and PARP-2 in T-cell homeostasis that might impact on the development of PARP-centred therapies.