Atf2 transcription factor binds to the APP1 promoter in Cryptococcus neoformans: stimulatory effect of diacylglycerol

Antiphagocytic protein 1 increases the susceptibility of Cryptococcus neoformans to amphotericin B and fluconazole

Cryptococcus neoformans is a facultative intracellular pathogen responsible for the most common cause of fungal meningioencephalitis, occurring primarily in immunocompromised individuals. Antiphagocytic protein 1 (App1) is a virulence factor produced by C. neoformans that inhibits phagocytosis of the yeast by host macrophages. Treatment of cryptococcosis includes amphotericin B, fluconazole, and flucytosine. Virulence factors have been shown to affect the susceptibility of the pathogen to antifungal drugs. In this study, we aimed to examine the relationship between App1 and antifungal drugs. We found that short-term exposure to amphotericin B downregulates APP1 expression while exposure to fluconazole upregulates APP1. In addition, App1 was found to increase the susceptibility of the yeast to amphotericin B and fluconazole. This study provides evidence of an intricate relationship between App1 and antifungal drugs.

Cryptococcus neoformans: historical curiosity to modern pathogen

The importance of the Basidiomycete Cryptococcus neoformans to human health has stimulated its development as an experimental model for both basic physiology and pathogenesis. We briefly review the history of this fascinating and versatile fungus, some notable aspects of its biology that contribute to virulence, and current tools available for its study.

Fbp1-mediated ubiquitin-proteasome pathway controls Cryptococcus neoformans virulence by regulating fungal intracellular growth in macrophages

Cryptococcus neoformans is a human fungal pathogen that often causes lung and brain infections in immunocompromised patients, with a high fatality rate. Our previous results showed that an F-box protein, Fbp1, is essential for Cryptococcus virulence independent of the classical virulence factors, suggesting a novel virulence control mechanism. In this study, we show that Fbp1 is part of the ubiquitin-proteasome system, and we further investigated the mechanism of Fbp1 function during infection. Time course studies revealed that the fbp1Δ mutant causes little damage in the infected lung and that the fungal burden in the lung remains at a low but persistent level throughout infection. The fbp1Δ mutant cannot disseminate to other organs following pulmonary infection in the murine inhalation model of cryptococcosis but still causes brain infection in a murine intravenous injection model, suggesting that the block of dissemination of the fbp1Δ mutant is due to its inability to leave the lung. The fbp1Δ mutant showed a defect in intracellular proliferation after phagocytosis in a Cryptococcus-macrophage interaction assay, which likely contributes to its virulence attenuation. To elucidate the molecular basis of the SCF(Fbp1) E3 ligase function, we analyzed potential Fbp1 substrates based on proteomic approaches combined with phenotypic analysis. One substrate, the inositol phosphosphingolipid-phospholipase C1 (Isc1), is required for fungal survival inside macrophage cells, which is consistent with the role of Fbp1 in regulating Cryptococcus-macrophage interaction and fungal virulence. Our results thus reveal a new determinant of fungal virulence that involves the posttranslational regulation of inositol sphingolipid biosynthesis.

Analysis of Cryptococcus neoformans sexual development reveals rewiring of the pheromone-response network by a change in transcription factor identity

The fundamental mechanisms that control eukaryotic development include extensive regulation at the level of transcription. Gene regulatory networks, composed of transcription factors, their binding sites in DNA, and their target genes, are responsible for executing transcriptional programs. While divergence of these control networks drives species-specific gene expression that contributes to biological diversity, little is known about the mechanisms by which these networks evolve. To investigate how network evolution has occurred in fungi, we used a combination of microarray expression profiling, cis-element identification, and transcription-factor characterization during sexual development of the human fungal pathogen Cryptococcus neoformans. We first defined the major gene expression changes that occur over time throughout sexual development. Through subsequent bioinformatic and molecular genetic analyses, we identified and functionally characterized the C. neoformans pheromone-response element (PRE). We then discovered that transcriptional activation via the PRE requires direct binding of the high-mobility transcription factor Mat2, which we conclude functions as the elusive C. neoformans pheromone-response factor. This function of Mat2 distinguishes the mechanism of regulation through the PRE of C. neoformans from all other fungal systems studied to date and reveals species-specific adaptations of a fungal transcription factor that defies predictions on the basis of sequence alone. Overall, our findings reveal that pheromone-response network rewiring has occurred at the level of transcription factor identity, despite the strong conservation of upstream and downstream components, and serve as a model for how selection pressures act differently on signaling vs. gene regulatory components during eukaryotic evolution.

Cryptococcus neoformans modulates extracellular killing by neutrophils

We recently established a key role for host sphingomyelin synthase (SMS) in regulating the killing activity of neutrophils against Cryptococcus neoformans. In this paper, we studied the effect of C. neoformans on the killing activity of neutrophils and whether SMS would still be a player against C. neoformans in immunocompromised mice lacking T and natural killer (NK) cells (Tgε26 mice). To this end, we analyzed whether C. neoformans would have any effect on neutrophil survival and killing in vitro and in vivo. We show that unlike Candida albicans, neither the presence nor the capsule size of C. neoformans cells have any effect on neutrophil viability. Interestingly, melanized C. neoformans cells totally abrogated the killing activity of neutrophils. We monitored how exposure of neutrophils to C. neoformans cells would interfere with any further killing activity of the conditioned medium and found that pre-incubation with live but not “heat-killed” fungal cells significantly inhibits further killing activity of the medium. We then studied whether activation of SMS at the site of C. neoformans infection is dependent on T and NK cells. Using matrix-assisted laser desorption–ionization tissue imaging in infected lung we found that similar to previous observations in the isogenic wild-type CBA/J mice, SM 16:0 levels are significantly elevated at the site of infection in mice lacking T and NK cells, but only at early time points. This study highlights that C. neoformans may negatively regulate the killing activity of neutrophils and that SMS activation in neutrophils appears to be partially independent of T and/or NK cells.

Characterization of inositol phospho-sphingolipid-phospholipase C 1 (Isc1) in Cryptococcus neoformans reveals unique biochemical features

In this work, we biochemically characterized inositol phosphosphingolipid-phospholipase C (Isc1) from the pathogenic fungus Cryptococcus neoformans. Unlike Isc1 from other fungi and parasites which hydrolyze both fungal complex sphingolipids (IPC-PLC) and mammalian sphingomyelin (SM-PLC), C. neoformans Isc1 only exerts IPC-PLC activity. Genetic mutations thought to regulate substrate recognition in other Isc1 proteins do not restore SM-PLC activity of the cryptococcal enzyme. C. neoformans Isc1 regulates the level of complex sphingolipids and certain species of phytoceramide, especially when fungal cells are exposed to acidic stress. Since growth in acidic environments is required for C. neoformans to cause disease, this study has important implications for understanding of C. neoformans pathogenicity.

Role of glucose in the expression of Cryptococcus neoformans antiphagocytic protein 1, App1

The cryptococcus-specific protein antiphagocytic protein 1 (App1) regulates Cryptococcus neoformans virulence by controlling macrophage-driven fungal phagocytosis. This is accomplished through complement receptors (CR), specifically CR3. When inhaled, C. neoformans can cause a life-threatening meningoencephalitis in immunocompromised patients. Because glucose starvation can significantly change the gene expression and virulence of C. neoformans and because App1 is critical for phagocytosis in the lung-a low-glucose environment-we investigated the role of glucose in App1 expression. We found that App1 was upregulated dramatically under low-glucose conditions, and it was upregulated when C. neoformans cells were incubated in bronchoalveolar lavage (BAL) fluid, serum, and cerebrospinal fluid, which are low-glucose environments. Characterization of App1's regulation based on mammalian lung physiology revealed that App1 is upregulated via both increases in transcription and increases in mRNA stability. Our data provide new insights regarding C. neoformans adaptations to low-glucose environments.

Lipid signalling in pathogenic fungi

In recent years, the study of lipid signalling networks has significantly increased. Although best studied in mammalian cells, lipid signalling is now appreciated also in microbial cells, particularly in yeasts and moulds. For instance, microbial sphingolipids and their metabolizing enzymes play a key role in the regulation of fungal pathogenicity, especially in Cryptococcus neoformans, through the modulation of different microbial pathways and virulence factors. Another example is the quorum sensing molecule (QSM) farnesol. In fact, this QSM is involved not only in mycelial growth and biofilm formation of Candida albicans, but also in many stress related responses. In moulds, such as Aspergillus fumigatus, QSM and sphingolipids are important for maintaining cell wall integrity and virulence. Finally, fungal cells make oxylipins to increase their virulence attributes and to counteract the host immune defences. In this review, we discuss these aspects in details.

Sphingolipid signaling in fungal pathogens

Sphingolipid involvement in infectious disease is a new and exciting branch of research. Various microbial pathogens have been shown to synthesize their own sphingolipids and some have evolved methods to "hijack" host sphingolipids for their own use. For instance, Sphingomonas species are bacterial pathogens that lack the lipopolysaccharide component typical but instead contain glycosphingolipids (Kawahara 1991, 2006). In terms of sphingolipid signaling and function, perhaps the best-studied group of microbes is the pathogenic fungi. Pathogenic fungi still represent significant problems in human disease, despite treatments that have been used for decades. Because fungi are eukaryotic, drug targets in fungi can have many similarities to mammalian processes. This often leads to significant side effects of antifungal drugs that can be dose limiting in many patient populations. The search for fungal-specific drugs and the need for better understanding of cellular processes of pathogenic fungi has led to a large body of research on fungal signaling. One particularly interesting and rapidly growing field in this research is the involvement of fungal sphingolipid pathways in signaling and virulence. In this chapter, the research relating to sphingolipid signaling pathogenic fungi will be reviewed and summarized, in addition to highlighting pathways that show promise for future research.

Lipid signaling in pathogenic fungi

Lipid signaling in pathogenic fungi has been studied to determine the role of these pathways in fungal biology and human infections. Owing to their unique nature, they may represent targets for future antifungal treatments. Farnesol signaling was characterized as a quorum-sensing molecule, with exposure inhibiting filamentation. Research has shown involvement in both the Ras1-adenylate cyclase and MAP kinase pathways. In species of Aspergillus, farnesol exposure induces apoptosis-like changes and alterations in ergosterol synthesis. Eicosanoid production has been characterized in several pathogenic fungi, utilizing host lipids in some cases. The role in virulence is not known yet, but it may involve modulation of host lipids. Sphingolipid signaling pathways seem to center around the production of diacylglycerol in the formation of inositol phosphorylceramide. Diacylglycerol activates both melanin production through laccase and transcription of antiphagocytic protein, both of which are involved in virulence.

App1: an antiphagocytic protein that binds to complement receptors 3 and 2

In previous studies, we showed that the pathogenic fungus Cryptococcus neoformans (Cn) produces a specific and unique protein called antiphagocytic protein 1 (App1), which inhibits phagocytosis of Cn by alveolar macrophages (AMs). Phagocytosis of Cn by AMs occurs mainly through a complement- or Ab-mediated mechanism. Among AM receptors, complement receptor 3 (CR3) and FcRgamma are the most common receptors involved in the phagocytic process. Because App1 inhibits phagocytosis of complement- but not Ab-coated erythrocytes, we investigated the role of CR3 in App1-macrophage interactions. We found that App1 binds to CR3 and if CR3 is absent from the surface of AMs, its antiphagocytic action is lost. When we investigated whether App1 would also bind to other complement receptor(s), we found that App1 does bind to complement receptor 2 (CR2) in a dose-dependent manner. In certain lymphoma cell lines, cellular proliferation is stimulated by complement through CR2, providing a potential use of App1 as a proliferation inhibitor of these cells. Initially discovered as an antiphagocytic protein regulating CR3-mediated innate immunity, App1 may also play a key role in the regulation of acquired immunity, because CR2 is mainly localized on B cells.