Hsp90‐dependent regulatory circuitry controlling temperature‐dependent fungal development and virulence

The pathogenic fungi Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans are an increasing cause of human mortality, especially in immunocompromised populations. During colonization and adaptation to various host environments, these fungi undergo morphogenetic alterations that allow for survival within the host. One key environmental cue driving morphological changes is external temperature. The Hsp90 chaperone protein provides one mechanism to link temperature with the signalling cascades that regulate morphogenesis, fungal development and virulence. Candida albicans is a model system for understanding the connections between morphogenesis and Hsp90. Due to the high degree of conservation in Hsp90, many of the connections in C. albicans may be extrapolated to other fungal pathogens or parasites. Examining the role of Hsp90 during development and morphogenesis in these three major fungal pathogens may provide insight into key aspects of adaptation to the host, leading to additional avenues for therapy.     

Neutrophil activation by Candida glabrata but not Candida albicans promotes fungal uptake by monocytes

Candida albicans and Candida glabrata account for the majority of candidiasis cases worldwide. Although both species are in the same genus, they differ in key virulence attributes. Within this work, live cell imaging was used to examine the dynamics of neutrophil activation after confrontation with either C. albicans or C. glabrata. Analyses revealed higher phagocytosis rates of C. albicans than C. glabrata that resulted in stronger PMN (polymorphonuclear cells) activation by C. albicans. Furthermore, we observed differences in the secretion of chemokines, indicating chemotactic differences in PMN signalling towards recruitment of further immune cells upon confrontation with Candida spp. Supernatants from co‐incubations of neutrophils with C. glabrata primarily attracted monocytes and increased the phagocytosis of C. glabrata by monocytes. In contrast, PMN activation by C. albicans resulted in recruitment of more neutrophils. Two complex infection models confirmed distinct targeting of immune cell populations by the two Candida spp.: In a human whole blood infection model, C. glabrata was more effectively taken up by monocytes than C. albicans and histopathological analyses of murine model infections confirmed primarily monocytic infiltrates in C. glabrata kidney infection in contrast to PMN‐dominated infiltrates in C. albicans infection. Taken together, our data demonstrate that the human opportunistic fungi C. albicans and C. glabrata are differentially recognized by neutrophils and one outcome of this differential recognition is the preferential uptake of C. glabrata by monocytes.     

Candida albicans and Candida tropicalis in oral candidosis: quantitative analysis, exoenzyme activity, and antifungal drug sensitivity

Candida albicans and C. tropicalis obtained from whole saliva of patients presenting signs of oral candidosis were assayed for quantification of colony forming units, exoenzyme activity (phospholipase and proteinase) and antifungal drug sensitivity (amphotericin B, fluconazole and itraconazole) by the reference method of the Clinical and Laboratory Standards Institute. The number of colony forming units per milliliter varied according to the Candida species involved and whether a single or mixed infection was present. Proteinase activity was observed in both C. albicans and C. tropicalis, but phospholipase activity was noted only in C. albicans. In vitro resistance to antifungals was verified in both species, but C. tropicalis appears to be more resistant to the tested antifungals than C. albicans.     

Candida albicans colonization and dissemination from the murine gastrointestinal tract: the influence of morphology and Th17 immunity

The ability of Candida albicans to cause disease is associated with its capacity to undergo morphological transition between yeast and filamentous forms, but the role of morphology in colonization and dissemination from the gastrointestinal (GI) tract remains poorly defined. To explore this, we made use of wild‐type and morphological mutants of C. albicans in an established model of GI tract colonization, induced following antibiotic treatment of mice. Our data reveal that GI tract colonization favours the yeast form of C. albicans, that there is constitutive low level systemic dissemination in colonized mice that occurs irrespective of fungal morphology, and that colonization is not controlled by Th17 immunity in otherwise immunocompetent animals. These data provide new insights into the mechanisms of pathogenesis and commensalism of C. albicans, and have implications for our understanding of human disease.     

N-acetylglucosamine (GlcNAc) Triggers a Rapid,Temperature-Responsive Morphogenetic Program in Thermally Dimorphic Fungi

The monosaccharide N-acetylglucosamine (GlcNAc) is a major component of microbial cell walls and is ubiquitous in the environment. GlcNAc stimulates developmental pathways in the fungal pathogen Candida albicans, which is a commensal organism that colonizes the mammalian gut and causes disease in the setting of host immunodeficiency. Here we investigate GlcNAc signaling in thermally dimorphic human fungal pathogens, a group of fungi that are highly evolutionarily diverged from C. albicans and cause disease even in healthy individuals. These soil organisms grow as polarized, multicellular hyphal filaments that transition into a unicellular, pathogenic yeast form when inhaled by a human host. Temperature is the primary environmental cue that promotes reversible cellular differentiation into either yeast or filaments; however, a shift to a lower temperature in vitro induces filamentous growth in an inefficient and asynchronous manner. We found GlcNAc to be a potent and specific inducer of the yeast-to-filament transition in two thermally dimorphic fungi, Histoplasma capsulatum and Blastomyces dermatitidis. In addition to increasing the rate of filamentous growth, micromolar concentrations of GlcNAc induced a robust morphological transition of H. capsulatum after temperature shift that was independent of GlcNAc catabolism, indicating that fungal cells sense GlcNAc to promote filamentation. Whole-genome expression profiling to identify candidate genes involved in establishing the filamentous growth program uncovered two genes encoding GlcNAc transporters, NGT1 and NGT2, that were necessary for H. capsulatum cells to robustly filament in response to GlcNAc. Unexpectedly, NGT1 and NGT2 were important for efficient H. capsulatum yeast-to-filament conversion in standard glucose medium, suggesting that Ngt1 and Ngt2 monitor endogenous levels of GlcNAc to control multicellular filamentous growth in response to temperature. Overall, our work indicates that GlcNAc functions as a highly conserved cue of morphogenesis in fungi, which further enhances the significance of this ubiquitous sugar in cellular signaling in eukaryotes.     

Carbon source‐induced reprogramming of the cell wall proteome and secretome modulates the adherence and drug resistance of the fungal pathogen Candida albicans

The major fungal pathogen Candida albicans can occupy diverse microenvironments in its human host. During colonization of the gastrointestinal or urogenital tracts, mucosal surfaces, bloodstream, and internal organs, C. albicans thrives in niches that differ with respect to available nutrients and local environmental stresses. Although most studies are performed on glucose‐grown cells, changes in carbon source dramatically affect cell wall architecture, stress responses, and drug resistance. We show that growth on the physiologically relevant carboxylic acid, lactate, has a significant impact on the C. albicans cell wall proteome and secretome. The regulation of cell wall structural proteins (e.g. Cht1, Phr1, Phr2, Pir1) correlated with extensive cell wall remodeling in lactate‐grown cells and with their increased resistance to stresses and antifungal drugs, compared with glucose‐grown cells. Moreover, changes in other proteins (e.g. Als2, Gca1, Phr1, Sap9) correlated with the increased adherence and biofilm formation of lactate‐grown cells. We identified mating and pheromone‐regulated proteins that were exclusive to lactate‐grown cells (e.g. Op4, Pga31, Pry1, Scw4, Yps7) as well as mucosa‐specific and other niche‐specific factors such as Lip4, Pga4, Plb5, and Sap7. The analysis of the corresponding null mutants confirmed that many of these proteins contribute to C. albicans adherence, stress, and antifungal drug resistance. Therefore, the cell wall proteome and secretome display considerable plasticity in response to carbon source. This plasticity influences important fitness and virulence attributes known to modulate the behavior of C. albicans in different host microenvironments during infection.