The structure of a C. neoformans polysaccharide motif recognized by protective antibodies: A combined NMR and MD study

Cryptococcus neoformans is a fungal pathogen responsible for cryptococcosis and cryptococcal meningitis. The C. neoformans' capsular polysaccharide and its shed exopolysaccharide function both as key virulence factors and to protect the fungal cell from phagocytosis. Currently, a glycoconjugate of these polysaccharides is being explored as a vaccine to protect against C. neoformans infection. In this study, NOE and J-coupling values from NMR experiments were consistent with a converged structure of the synthetic decasaccharide, GXM10-Ac3, calculated from MD simulations. GXM10-Ac3 was designed as an extension of glucuronoxylomannan (GXM) polysaccharide motif (M2) which is common in the clinically predominant serotype A strains and is recognized by protective forms of GXM-specific monoclonal antibodies. The M2 motif is a hexasaccharide with a three-residue α-mannan backbone, modified by β-(1→2)-xyloses (Xyl) on the first two mannoses (Man) and a β-(1→2)-glucuronic acid (GlcA) on the third Man. Combined NMR and MD analyses reveal that GXM10-Ac3 adopts an extended structure, with Xyl/GlcA branches alternating sides along the α-mannan backbone. O-acetyl esters also alternate sides and are grouped in pairs. MD analysis of a twelve M2-repeating unit polymer supports the notion that the GXM10-Ac3 structure is uniformly represented throughout the polysaccharide. This derived GXM model displays high flexibility while maintaining a structural identity, yielding insights to further explore intermolecular interactions between polysaccharides, interactions with anti-GXM mAbs, and the cryptococcal polysaccharide architecture.

GXMR-CAR containing distinct GXM-specific single-chain variable fragment (scFv) mediated the cell activation against Cryptococcus spp. And had difference in the strength of tonic signaling

Cryptococcus spp. has a polysaccharide capsule composed of glucuronoxylomannan-GXM, a major virulence factor that can prevent the recognition of fungi by immune cells. Chimeric Antigen Receptor (CAR) redirects T cells to target Cryptococcus spp. as previously demonstrated by a CAR specific to GXM, GXMR-CAR. The current study evaluated the strength of the signal transduction triggered by GXMR-CAR, composed of a distinct antigen-binding domain sourced from a single-chain variable fragment (scFv). GXM-specific scFv derived from mAbs 2H1 and 18B7, 2H1-GXMR-CAR and 18B7-GXMR-CAR, respectively, were designed to express CD8 molecule as hinge/transmembrane, and the costimulatory molecule CD137 (4-1BB) coupled to CD3ζ. The 2H1-GXMR-CAR or 18B7-GXMR-CAR Jurkat cells recognized soluble GXM from C. gattii and C. neoformans, and the levels of IL-2 released by the modified cells did not differ between the GXMR-CAR constructs after exposure to Cryptococcus spp. 18B7-GXMR-CAR triggered tonic signaling was more pronounced in modified Jurkat cells, and a protein kinase inhibitor of the Src family (dasatinib) significantly reduced GXMR-CAR tonic signaling and inhibited cell activation against ligands. 18B7 scFv showed a structural modification of the variable heavy (VH) chain that clarified the difference in the strength of tonic signaling and the level of cell activation between 2H1-GXMR-CAR and 18B7-GXMR-CAR. GXMR-CAR constructs induced T-cell activation against clinical isolates of Cryptococcus spp. and serum from patients with cryptococcosis induced high levels of IL-2, mainly in cells modified with 18B7-GXMR-CAR. Thus, 18B7-GXMR-CAR and 2H1-GXMR-CAR mediated T cell activation against Cryptococcus spp. and 18B7 and 2H1 scFv influenced the strength of tonic signaling.

Efficient splicing of the CPE intein derived from directed evolution of the Cryptococcus neoformans PRP8 intein

Intein-mediated protein splicing has been widely used in protein engineering; however, the splicing efficiency and extein specificity usually limit its further application. Thus, there is a demand for more general inteins that can overcome these limitations. Here, we study the trans-splicing of CPE intein obtained from the directed evolution of Cne PRP8, which shows that its splicing rate is ~29- fold higher than that of the wild-type. When the +1 residue of C-extein is changed to cysteine, CPE also shows high splicing activity. Faster association and higher affinity may contribute to the high splicing rate compared with wild-type intein. These findings have important implications for the future engineering of inteins and provide clues for fundamental studies of protein structure and folding.

An improved staining method of cell cycle analysis with Sybr Green I for fungi: Cryptococcus neoformans and Saccharomyces cerevisiae

Cryptococcus neoformans is a pathogenic fungus which causes millions of deaths and infections, especially threatening immunocompromised individuals. During the development of new drugs, the ubiquitination has been found to play an important role in the regulation of the virulence and cell cycle of this fungus. Based on this mechanism, ubiquitination-related mutant strains exhibiting cell cycle arrest have been established for drug development for the fungus. However, flow cytometry detection of the cell cycle in fungi is generally difficult because the thick cell wall and capsule of fungi generally contribute to a nonspecific signal of cytometry. In this study, an improved method, derived from Saccharomyces cerevisiae assays, is developed to specifically stain C. neoformans, in whose cell cycle the G1 and G2 peaks are separated enough to be allowed for cell cycle analysis. As a result, the improved method facilitates the detection of the alterations in the cell cycle of C. neoformans with a mutation that results in cell cycle arrest, which distinctly delays the cell division of C. neoformans. Thus, the improved method reported here provides detailed technical information regarding assays on C. neoformans and, more importantly, offers a solution for assessing the cell cycle in other fungi in the future. Abbreviation: PI: propidium iodide.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Cryptococcus neoformans Capsular GXM Conformation and Epitope Presentation: A Molecular Modelling Study

The pathogenic encapsulated Cryptococcus neoformans fungus causes serious disease in immunosuppressed hosts. The capsule, a key virulence factor, consists primarily of the glucuronoxylomannan polysaccharide (GXM) that varies in composition according to serotype. While GXM is a potential vaccine target, vaccine development has been confounded by the existence of epitopes that elicit non-protective antibodies. Although there is evidence for protective antibodies binding conformational epitopes, the secondary structure of GXM remains an unsolved problem. Here an array of molecular dynamics simulations reveal that the GXM mannan backbone is consistently extended and relatively inflexible in both C. neoformans serotypes A and D. Backbone substitution does not alter the secondary structure, but rather adds structural motifs: β DGlcA and β DXyl side chains decorate the mannan backbone in two hydrophillic fringes, with mannose-6-O-acetylation forming a hydrophobic ridge between them. This work provides mechanistic rationales for clinical observations-the importance of O-acetylation for antibody binding; the lack of binding of protective antibodies to short GXM fragments; the existence of epitopes that elicit non-protective antibodies; and the self-aggregation of GXM chains-indicating that molecular modelling can play a role in the rational design of conjugate vaccines.

Exploring Cryptococcus neoformans capsule structure and assembly with a hydroxylamine-armed fluorescent probe

Chemical biology is an emerging field that enables the study and manipulation of biological systems with probes whose reactivities provide structural insights. The opportunistic fungal pathogen Cryptococcus neoformans possesses a polysaccharide capsule that is a major virulence factor, but is challenging to study. We report here the synthesis of a hydroxylamine-armed fluorescent probe that reacts with reducing glycans and its application to study the architecture of the C. neoformans capsule under a variety of conditions. The probe signal localized intracellularly and at the cell wall-membrane interface, implying the presence of reducing-end glycans at this location where the capsule is attached to the cell body. In contrast, no fluorescence signal was detected in the capsule body. We observed vesicle-like structures containing the reducing-end probe, both intra- and extracellularly, consistent with the importance of vesicles in capsular assembly. Disrupting the capsule with DMSO, ultrasound, or mechanical shear stress resulted in capsule alterations that affected the binding of the probe, as reducing ends were exposed and cell membrane integrity was compromised. Unlike the polysaccharides in the assembled capsule, isolated exopolysaccharides contained reducing ends. The reactivity of the hydroxylamine-armed fluorescent probe suggests a model for capsule assembly whereby reducing ends localize to the cell wall surface, supporting previous findings suggesting that this is an initiation point for capsular assembly. We propose that chemical biology is a promising approach for studying the C. neoformans capsule and its associated polysaccharides to unravel their roles in fungal virulence.

Unique subsite specificity and potential natural function of a chitosan deacetylase from the human pathogen Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that infects ∼280,000 people every year, causing >180,000 deaths. The human immune system recognizes chitin as one of the major cell-wall components of invading fungi, but C. neoformans can circumvent this immunosurveillance mechanism by instead exposing chitosan, the partly or fully deacetylated form of chitin. The natural production of chitosans involves the sequential action of chitin synthases (CHSs) and chitin deacetylases (CDAs). C. neoformans expresses four putative CDAs, three of which have been confirmed as functional enzymes that act on chitin in the cell wall. The fourth (CnCda4/Fpd1) is a secreted enzyme with exceptional specificity for d-glucosamine at its -1 subsite, thus preferring chitosan over chitin as a substrate. We used site-specific mutagenesis to reduce the subsite specificity of CnCda4 by converting an atypical isoleucine residue in a flexible loop region to the bulkier or charged residues tyrosine, histidine, and glutamic acid. We also investigated the effect of CnCda4 deacetylation products on human peripheral blood-derived macrophages, leading to a model explaining the function of CnCda4 during infection. We propose that CnCda4 is used for the further deacetylation of chitosans already exposed on the C. neoformans cell wall (originally produced by CnChs3 and CnCda1 to 3) or released from the cell wall as elicitors by human chitinases, thus making the fungus less susceptible to host immunosurveillance. The absence of CnCda4 during infection could therefore promote the faster recognition and elimination of this pathogen.

A Heat-Killed Cryptococcus Mutant Strain Induces Host Protection against Multiple Invasive Mycoses in a Murine Vaccine Model

Cryptococcus neoformans is a fungal pathogen that infects the lungs and then often disseminates to the central nervous system, causing meningitis. How Cryptococcus is able to suppress host immunity and escape the antifungal activity of macrophages remains incompletely understood. We reported that the F-box protein Fbp1, a subunit of the SCF(Fbp1) E3 ligase, promotes Cryptococcus virulence by regulating host-Cryptococcus interactions. Our recent studies demonstrated that the fbp1Δ mutant elicited superior protective Th1 host immunity in the lungs and that the enhanced immunogenicity of heat-killed fbp1Δ yeast cells can be harnessed to confer protection against a subsequent infection with the virulent parental strain. We therefore examined the use of heat-killed fbp1Δ cells in several vaccination strategies. Interestingly, the vaccine protection remains effective even in mice depleted of CD4+ T cells. This finding is particularly important in the context of HIV/AIDS-induced immune deficiency. Moreover, we observed that vaccinating mice with heat-killed fbp1Δ induces significant cross-protection against challenge with diverse invasive fungal pathogens, including C. neoformans, C. gattii, and Aspergillus fumigatus, as well as partial protection against Candida albicans Thus, our data suggest that the heat-killed fbp1Δ strain has the potential to be a suitable vaccine candidate against cryptococcosis and other invasive fungal infections in both immunocompetent and immunocompromised populations.IMPORTANCE Invasive fungal infections kill more than 1.5 million people each year, with limited treatment options. There is no vaccine available in clinical use to prevent and control fungal infections. Our recent studies showed that a mutant of the F-box protein Fbp1, a subunit of the SCF(Fbp1) E3 ligase in Cryptococcus neoformans, elicited superior protective Th1 host immunity. Here, we demonstrate that the heat-killed fbp1Δ cells (HK-fbp1) can be harnessed to confer protection against a challenge by the virulent parental strain, even in animals depleted of CD4+ T cells. This finding is particularly important in the context of HIV/AIDS-induced immune deficiency. Moreover, we observed that HK-fbp1 vaccination induces significant cross-protection against challenge with diverse invasive fungal pathogens. Thus, our data suggest that HK-fbp1 has the potential to be a broad-spectrum vaccine candidate against invasive fungal infections in both immunocompetent and immunocompromised populations.

A Glucuronoxylomannan Epitope Exhibits Serotype-Specific Accessibility and Redistributes towards the Capsule Surface during Titanization of the Fungal Pathogen Cryptococcus neoformans

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, Cryptococcus gattii are an important cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigens. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterization of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodeling during the process of fungal titanization. We show that Crp127 recognizes a GXM epitope in an O-acetylation-dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and C. gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains, and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in titan cells, a recently characterized morphotype where haploid 5-μm cells convert to highly polyploid cells of >10 μm with distinct but poorly understood capsular characteristics. Titan cells are produced in the host lung and critical for successful infection. Crp127 therefore advances our understanding of cryptococcal morphological change and may hold significant potential as a tool to differentially identify cryptococcal strains and subtypes.

Clinical features and cause analysis of false positive results of Aspergillus galactomannan assay in pulmonary cryptococcosis patients

There have been conflicting reports of false positive galactomannan assay results in patients with systemic cryptococcosis. We sought to determine the frequency of GM positivity in patients with pulmonary cryptococcosis and confirm the source of this cross-reactivity in vitro. We conducted a retrospective study to elucidate the rate of galactomannan (GM) false positivity and cause in a cohort of 29 patients with pulmonary cryptococcal disease. The production of GM cross-reacting substances by clinical isolates and laboratory isolates of C. neoformans was tested in vitro. The mean serum GM index (Platelia Aspergillus) in patients with pulmonary cryptococcosis was 1.06, with 16 (55.2%) of patients having values above the positive cutoff value of 0.5. GM index values significantly decreased after treatment of cryptococcosis. There was no significant correlation between galactomannan and cryptococcal glucuronoxylomannan antigen (Eiken Latex test) results. Culture supernatants from clinical isolates and wild-type C. neoformans did not react in the GM assay; however, growth in the presence of 6% sodium chloride induced the production of cross-reacting GM antigens in culture supernatants from clinical isolates, wild type and a glucuronoxylomannan-deficient mutant of C. neoformans, but not in culture supernatants from a galactoxylomannan-deficient strain. Our results support the cross-reactivity of cryptococcal galactoxylomannan with the serum GM assay in vitro and in patients with pulmonary cryptococcal infection.

The melanization road more traveled by: Precursor substrate effects on melanin synthesis in cell-free and fungal cell systems

Natural brown-black eumelanin pigments confer structural coloration in animals and potently block ionizing radiation and antifungal drugs. These functions also make them attractive for bioinspired materials design, including coating materials for drug-delivery vehicles, strengthening agents for adhesive hydrogel materials, and free-radical scavengers for soil remediation. Nonetheless, the molecular determinants of the melanin "developmental road traveled" and the resulting architectural features have remained uncertain because of the insoluble, heterogeneous, and amorphous characteristics of these complex polymeric assemblies. Here, we used 2D solid-state NMR, EPR, and dynamic nuclear polarization spectroscopic techniques, assisted in some instances by the use of isotopically enriched precursors, to address several open questions regarding the molecular structures and associated functions of eumelanin. Our findings uncovered: 1) that the identity of the available catecholamine precursor alters the structure of melanin pigments produced either in Cryptococcus neoformans fungal cells or under cell-free conditions; 2) that the identity of the available precursor alters the scaffold organization and membrane lipid content of melanized fungal cells; 3) that the fungal cells are melanized preferentially by an l-DOPA precursor; and 4) that the macromolecular carbon- and nitrogen-based architecture of cell-free and fungal eumelanins includes indole, pyrrole, indolequinone, and open-chain building blocks that develop depending on reaction time. In conclusion, the availability of catecholamine precursors plays an important role in eumelanin development by affecting the efficacy of pigment formation, the melanin molecular structure, and its underlying scaffold in fungal systems.

The Buoyancy of Cryptococcus neoformans Is Affected by Capsule Size

Cryptococcus neoformans is an environmental pathogenic fungus with a worldwide geographical distribution that is responsible for hundreds of thousands of human cryptococcosis cases each year. During infection, the yeast undergoes a morphological transformation involving capsular enlargement that increases microbial volume. To understand the factors that play a role in environmental dispersal of C. neoformans and C. gattii, we evaluated the cell density of Cryptococcus using Percoll isopycnic gradients. We found differences in the cell densities of strains belonging to C. neoformans and C. gattii species complexes. The buoyancy of C. neoformans strains varied depending on growth medium. In minimal medium, the cryptococcal capsule made a major contribution to the cell density such that cells with larger capsules had lower density than those with smaller capsules. Removing the capsule, by chemical or mechanical methods, increased the C. neoformans cell density and reduced buoyancy. Melanization of the C. neoformans cell wall, which also contributes to virulence, produced a small but consistent increase in cell density. Encapsulated C. neoformans sedimented much more slowly in seawater as its density approached the density of water. Our results suggest a new function for the capsule whereby it can function as a flotation device to facilitate transport and dispersion in aqueous fluids.IMPORTANCE The buoyancy of a microbial cell is an important physical characteristic that may affect its transportability in fluids and interactions with tissues during infection. The polysaccharide capsule surrounding C. neoformans is required for infection and dissemination in the host. Our results indicate that the capsule has a significant effect on reducing cryptococcal cell density, altering its sedimentation in seawater. Modulation of microbial cell density via encapsulation may facilitate dispersal for other important encapsulated pathogens.

The Viscoelastic Properties of the Fungal Cell Wall Allow Traffic of AmBisome as Intact Liposome Vesicles

The fungal cell wall is a critically important structure that represents a permeability barrier and protective shield. We probed Candida albicans and Cryptococcus neoformans with liposomes containing amphotericin B (AmBisome), with or without 15-nm colloidal gold particles. The liposomes have a diameter of 60 to 80 nm, and yet their mode of action requires them to penetrate the fungal cell wall to deliver amphotericin B to the cell membrane, where it binds to ergosterol. Surprisingly, using cryofixation techniques with electron microscopy, we observed that the liposomes remained intact during transit through the cell wall of both yeast species, even though the predicted porosity of the cell wall (pore size, ~5.8 nm) is theoretically too small to allow these liposomes to pass through intact. C. albicans mutants with altered cell wall thickness and composition were similar in both their in vitro AmBisome susceptibility and the ability of liposomes to penetrate the cell wall. AmBisome exposed to ergosterol-deficient C. albicans failed to penetrate beyond the mannoprotein-rich outer cell wall layer. Melanization of C. neoformans and the absence of amphotericin B in the liposomes were also associated with a significant reduction in liposome penetration. Therefore, AmBisome can reach cell membranes intact, implying that fungal cell wall viscoelastic properties are permissive to vesicular structures. The fact that AmBisome can transit through chemically diverse cell wall matrices when these liposomes are larger than the theoretical cell wall porosity suggests that the wall is capable of rapid remodeling, which may also be the mechanism for release of extracellular vesicles.IMPORTANCE AmBisome is a broad-spectrum fungicidal antifungal agent in which the hydrophobic polyene antibiotic amphotericin B is packaged within a 60- to 80-nm liposome. The mode of action involves perturbation of the fungal cell membrane by selectively binding to ergosterol, thereby disrupting membrane function. We report that the AmBisome liposome transits through the cell walls of both Candida albicans and Cryptococcus neoformans intact, despite the fact that the liposome is larger than the theoretical cell wall porosity. This implies that the cell wall has deformable, viscoelastic properties that are permissive to transwall vesicular traffic. These observations help explain the low toxicity of AmBisome, which can deliver its payload directly to the cell membrane without unloading the polyene in the cell wall. In addition, these findings suggest that extracellular vesicles may also be able to pass through the cell wall to deliver soluble and membrane-bound effectors and other molecules to the extracellular space.

Changes in glucosylceramide structure affect virulence and membrane biophysical properties of Cryptococcus neoformans

Fungal glucosylceramide (GlcCer) is a plasma membrane sphingolipid in which the sphingosine backbone is unsaturated in carbon position 8 (C8) and methylated in carbon position 9 (C9). Studies in the fungal pathogen, Cryptococcus neoformans, have shown that loss of GlcCer synthase activity results in complete loss of virulence in the mouse model. However, whether the loss of virulence is due to the lack of the enzyme or to the loss of the sphingolipid is not known. In this study, we used genetic engineering to alter the chemical structure of fungal GlcCer and studied its effect on fungal growth and pathogenicity. Here we show that unsaturation in C8 and methylation in C9 is required for virulence in the mouse model without affecting fungal growth in vitro or common virulence factors. However, changes in GlcCer structure led to a dramatic susceptibility to membrane stressors resulting in increased cell membrane permeability and rendering the fungal mutant unable to grow within host macrophages. Biophysical studies using synthetic vesicles containing GlcCer revealed that the saturated and unmethylated sphingolipid formed vesicles with higher lipid order that were more likely to phase separate into ordered domains. Taken together, these studies show for the first time that a specific structure of GlcCer is a major regulator of membrane permeability required for fungal pathogenicity.

Synthesis of part structures of Cryptococcus neoformans serotype C capsular polysaccharide

Cryptococcus neoformans is a fungal pathogen that can cause life-threatening infections in immunocompromised patients. The development of a vaccine based on the capsular polysaccharide of C. neoformans is still an open challenge due to the heterogeneity of the capsular polysaccharide and the difficulty of identifying protective epitopes. Therefore, construction of structurally defined part structures of the C. neoformans GXM capsule is in great demand. Herein is presented the synthesis of a 3-O-naphthalenylmethyl protected trisaccharide thioglycoside building block which is present in C. neoformans serotype C polysaccharide. Its property as a donor in a glycosylation reaction with a model acceptor has been evaluated together with its behaviour as an acceptor following removal of the temporary protecting group. The heavily branched hexasaccharide was obtained in good yields and excellent α-selectivity. The frame shifted octasaccharide structural triad motif for serotype C was also prepared following the same building block strategy. For the first time this structural motif, which is the most substituted amongst the four C. neoformans serotypes, was prepared. Three synthesized C. neoformans serotype C fragments of varying size, from penta-up to octasaccharide, were deprotected and will be included in unique glycoarrays to further investigate the possibility to develop a synthetic vaccine against this pathogen.

Secretome profiling of Cryptococcus neoformans reveals regulation of a subset of virulence-associated proteins and potential biomarkers by protein kinase A

BACKGROUND

The pathogenic yeast Cryptococcus neoformans causes life-threatening meningoencephalitis in individuals suffering from HIV/AIDS. The cyclic-AMP/protein kinase A (PKA) signal transduction pathway regulates the production of extracellular virulence factors in C. neoformans, but the influence of the pathway on the secretome has not been investigated. In this study, we performed quantitative proteomics using galactose-inducible and glucose-repressible expression of the PKA1 gene encoding the catalytic subunit of PKA to identify regulated proteins in the secretome.

METHODS

The proteins in the supernatants of cultures of C. neoformans were precipitated and identified using liquid chromatography-coupled tandem mass spectrometry. We also employed multiple reaction monitoring in a targeted approach to identify fungal proteins in samples from macrophages after phagocytosis of C. neoformans cells, as well as from the blood and bronchoalveolar fluid of infected mice.

RESULTS

We identified 61 secreted proteins and found that changes in PKA1 expression influenced the extracellular abundance of five proteins, including the Cig1 and Aph1 proteins with known roles in virulence. We also observed a change in the secretome profile upon induction of Pka1 from proteins primarily involved in catabolic and metabolic processes to an expanded set that included proteins for translational regulation and the response to stress. We further characterized the secretome data using enrichment analysis and by predicting conventional versus non-conventional secretion. Targeted proteomics of the Pka1-regulated proteins allowed us to identify the secreted proteins in lysates of phagocytic cells containing C. neoformans, and in samples from infected mice. This analysis also revealed that modulation of PKA1 expression influences the intracellular survival of cryptococcal cells upon phagocytosis.

CONCLUSIONS

Overall, we found that the cAMP/PKA pathway regulates specific components of the secretome including proteins that affect the virulence of C. neoformans. The detection of secreted cryptococcal proteins from infected phagocytic cells and tissue samples suggests their potential utility as biomarkers of infection. The proteomics data are available via ProteomeXchange with identifiers PXD002731 and PASS00736.

Structural Characterization of Melanin Pigments from Commercial Preparations of the Edible Mushroom Auricularia auricula

Many of the most widely consumed edible mushrooms are pigmented, and these have been associated with some beneficial health effects. Nevertheless, the majority of the reported compounds associated with these desirable properties are non-pigmented. We have previously reported that melanin pigment from the edible mushroom Auricularia auricula can protect mice against ionizing radiation, although no physicochemical characterization was reported. Consequently, in this study we have characterized commercial A. auricula mushroom preparations for melanin content and carried out structural characterization of isolated insoluble melanin materials using a panel of sophisticated spectroscopic and physical/imaging techniques. Our results show that approximately 10% of the dry mass of A. auricula is melanin and that the pigment has physicochemical properties consistent with those of eumelanins, including hosting a stable free radical population. Electron microscopy studies show that melanin is associated with the mushroom cell wall in a manner similar to that of melanin from the model fungus C. neoformans. Elemental analysis of melanin indicated C, H, and N ratios consistent with 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and 1,8-dihydroxynaphthalene eumelanin. Validation of the identity of the isolated product as melanin was achieved by EPR analysis. A. auricula melanin manifested structural differences, relative to the C. neoformans melanin, with regard to the variable proportions of alkyl chains or oxygenated carbons. Given the necessity for new oral and inexpensive radioprotective materials coupled with the commercial availability of A. auricula mushrooms, this product may represent an excellent source of edible melanin.

Biochemical and kinetic characterization of xylulose 5-phosphate/fructose 6-phosphate phosphoketolase 2 (Xfp2) from Cryptococcus neoformans

Xylulose 5-phosphate/fructose 6-phosphate phosphoketolase (Xfp), previously thought to be present only in bacteria but recently found in fungi, catalyzes the formation of acetyl phosphate from xylulose 5-phosphate or fructose 6-phosphate. Here, we describe the first biochemical and kinetic characterization of a eukaryotic Xfp, from the opportunistic fungal pathogen Cryptococcus neoformans, which has two XFP genes (designated XFP1 and XFP2). Our kinetic characterization of C. neoformans Xfp2 indicated the existence of both substrate cooperativity for all three substrates and allosteric regulation through the binding of effector molecules at sites separate from the active site. Prior to this study, Xfp enzymes from two bacterial genera had been characterized and were determined to follow Michaelis-Menten kinetics. C. neoformans Xfp2 is inhibited by ATP, phosphoenolpyruvate (PEP), and oxaloacetic acid (OAA) and activated by AMP. ATP is the strongest inhibitor, with a half-maximal inhibitory concentration (IC50) of 0.6 mM. PEP and OAA were found to share the same or have overlapping allosteric binding sites, while ATP binds at a separate site. AMP acts as a very potent activator; as little as 20 μM AMP is capable of increasing Xfp2 activity by 24.8% ± 1.0% (mean ± standard error of the mean), while 50 μM prevented inhibition caused by 0.6 mM ATP. AMP and PEP/OAA operated independently, with AMP activating Xfp2 and PEP/OAA inhibiting the activated enzyme. This study provides valuable insight into the metabolic role of Xfp within fungi, specifically the fungal pathogen Cryptococcus neoformans, and suggests that at least some Xfps display substrate cooperative binding and allosteric regulation.

Opsonizing properties of rat ficolin-A in the defence against Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus causing life threatening infections in humans. The present in vitro study aimed to investigate the opsonizing properties of a well characterized serum ficolin (rat ficolin-A), a member of carbohydrate-recognition molecules of the innate immune system, in the defence against this fungal pathogen. Using flow cytometric analysis we have been able to demonstrate that ficolin-A readily binds to two different acapsular C. neoformans serotypes (representative of the primary infectious form of this fungus) whereas the encapsulated forms are not being recognized. The ficolin-A binding was concentration dependent and inhibited by the acetylated sugars N-acetyleglucosamine and N-acetylegalactosamine but less so by galactose, glucose and mannan. The binding was enhanced at acidic pHs (5.7 and 4.7) compared to physiological pH (7.4) which may indicate that the carbohydrate recognizing fibrinogen-like domains of ficolins undergo conformational changes providing more efficient binding at sites of inflammation where the pH is much lower than normal. We further assessed the biological consequence of the ficolin-A recognition of acapsular C. neoformans by investigating their interaction with lung epithelial cells (type II pneumocytes cell line A549). Flow cytometric analysis demonstrated that ficolin-A opsonized acapsular C. neoformans showed significantly increased adherence to A549 cells when exposed to acidic conditions compared to the unopsonized controls (p=0.04). We conclude that ficolin-A binds acapsular C. neoformans via their carbohydrate recognizing fibrinogen-like domains leading to enhanced uptake by lung epithelial cells in vitro.

The microbial capsular polysaccharide galactoxylomannan inhibits IL-17A production in circulating T cells from rheumatoid arthritis patients

The persistence of activated T cells in rheumatoid arthritis (RA) synovium may be attributable to increased homing, increased retention or a possible imbalance between cell proliferation and programmed cell death. Induction of apoptosis may represent a potential therapeutic approach. Galactoxylomannan (GalXM) from the opportunistic fungus Cryptococcus neoformans can interact with T cells and induce T-cell apoptosis through the inhibition of CD45 phosphatase activity. The aim of this study was to determine the effect of GalXM on circulating T cells from patients with RA and the underlying mechanisms. GalXM immunomodulating effect on apoptosis and signal transduction pathway involved in IL-17A production was evaluated on T cells. RA T-cell apoptosis, higher than that of control T cells, was further increased by GalXM through induction of caspase-3 activation. Activated T cells expressing the CD45RO molecule and producing IL-17A were the main target of GalXM-induced apoptosis. GalXM induced consistent impairment of IL-17A production and inhibition of STAT3, which was hyperactivated in RA. In conclusion, GalXM triggered apoptosis of activated memory T cells and interfered with IL-17A production in RA. These data suggest therapeutic targeting of deleterious Th17 cells in RA and other autoimmune diseases.

Fungal melanins differ in planar stacking distances

Melanins are notoriously difficult to study because they are amorphous, insoluble and often associated with other biological materials. Consequently, there is a dearth of structural techniques to study this enigmatic pigment. Current models of melanin structure envision the stacking of planar structures. X ray diffraction has historically been used to deduce stacking parameters. In this study we used X ray diffraction to analyze melanins derived from Cryptococcus neoformans, Aspergillus niger, Wangiella dermatitides and Coprinus comatus. Analysis of melanin in melanized C. neoformans encapsulated cells was precluded by the fortuitous finding that the capsular polysaccharide had a diffraction spectrum that was similar to that of isolated melanin. The capsular polysaccharide spectrum was dominated by a broad non-Bragg feature consistent with origin from a repeating structural motif that may arise from inter-molecular interactions and/or possibly gel organization. Hence, we isolated melanin from each fungal species and compared diffraction parameters. The results show that the inferred stacking distances of fungal melanins differ from that reported for synthetic melanin and neuromelanin, occupying intermediate position between these other melanins. These results suggest that all melanins have a fundamental diffracting unit composed of planar graphitic assemblies that can differ in stacking distance. The stacking peak appears to be a distinguishing universal feature of melanins that may be of use in characterizing these enigmatic pigments.

Inhibition of HIV entry by extracellular glucuronoxylomannan of Cryptococcus neoformans

Cryptococcosis, caused by Cryptococcus neoformans, is the most common opportunistic fungal disease in HIV/AIDS patients. The prognosis of AIDS patients with Cryptococcus infection is very poor. One of the major characteristics in cryptococcosis patients is the presence of high concentrations of the cryptococcal capsule polysaccharide (CCP) in the serum and cerebrospinal fluid. CCP enhances HIV replication in H9 T-cells, but the mechanism is unknown. In this study, we tested whether extracellular glucuronoxylomannan (GXM), a major component of CCP, enhances HIV entry using replication-incompetent HIV and a cell line which expresses a stable amount of CD4 and both of the HIV co-receptors. Extracellular GXM had no effect on cell-cell fusion however; viral entry surprisingly was inhibited by GXM. Hence, any enhancement of replication must be due to an effect that occurs post-entry.

Gamma radiation interacts with melanin to alter its oxidation-reduction potential and results in electric current production

The presence of melanin pigments in organisms is implicated in radioprotection and in some cases, enhanced growth in the presence of high levels of ionizing radiation. An understanding of this phenomenon will be useful in the design of radioprotective materials. However, the protective mechanism of microbial melanin in ionizing radiation fields has not yet been elucidated. Here we demonstrate through the electrochemical techniques of chronoamperometry, chronopotentiometry and cyclic voltammetry that microbial melanin is continuously oxidized in the presence of gamma radiation. Our findings establish that ionizing radiation interacts with melanin to alter its oxidation-reduction potential. Sustained oxidation resulted in electric current production and was most pronounced in the presence of a reductant, which extended the redox cycling capacity of melanin. This work is the first to establish that gamma radiation alters the oxidation-reduction behavior of melanin, resulting in electric current production. The significance of the work is that it provides the first step in understanding the initial interactions between melanin and ionizing radiation taking place and offers some insight for production of biomimetic radioprotective materials.

Ctr2 links copper homeostasis to polysaccharide capsule formation and phagocytosis inhibition in the human fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is a human opportunistic fungal pathogen responsible for approximately 1/3 of HIV/AIDS deaths worldwide. This budding yeast expresses a polysaccharide capsule necessary for virulence. Capsule production inhibits phagocytosis by macrophages. Here we describe results that link copper homeostasis to capsule production and the inhibition of phagocytosis. Specifically, using Agrobacterium-mediated insertional mutagenesis, we identified an insertion in the promoter region of the putative copper transporter-encoding gene CTR2 that results in reduced expression of CTR2 and increased phagocytosis by murine RAW264.7 macrophages. The mutant also displayed sensitivity to copper starvation and defects in polysaccharide capsule production and melanization. These defects were all reversed by genetic correction of the promoter insertion by homologous targeting. Several melanization-defective mutants identified previously, those in the RIM20, RIM101, and VPS25 genes, also display sensitivity to copper starvation, reduced capsule production and increased phagocytosis. Together these results indicate a previously undescribed link between copper homeostasis to polysaccharide capsule production and phagocytosis inhibition in Cryptococcus neoformans.

The structure of Cryptococcus neoformans galactoxylomannan contains beta-D-glucuronic acid

The structure of galactoxylomannan, a capsular polysaccharide from the opportunistic yeast Cryptococcus neoformans, was re-examined by NMR spectroscopy and GC-MS. The residue that is 3-linked to the side chain galactose and was previously assigned as beta-D-xylose [Vaishnav, V. V.; Bacon, B. E.; O'Neill, M.; Cherniak, R. Carbohydr. Res.1998, 306, 315-330] was determined to be beta-D-glucuronic acid. A revised structure for this polymer is presented, along with a proposal that this compound be termed glucuronoxylomannogalactan (GXMGal).

Polarization optical analysis of the surface structures of various fungi

Vicinal hydroxyl groups of the sugar compounds sialic acid and 9-O-acyl sialic acid can be visualised for polarization optical analysis on the surface of different fungi using several topo-optical reactions. We investigated the presence of these molecules in cultures of Cryptococcus neoformans (heterogeneous form), Saccharomyces cerevisiae, Candida albicans, Candida glabrata, Candida krusei and Candida tropicalis by topo-optical reactions. Additionally, we examined brain and stomach tissues of patients with infections by C. neoformans and C. albicans, respectively. The results suggest a highly fashioned orientation of the sugar chains on the fungal surface. Terminal sialic- and O-acyl sialic acid residues are permanently present and orientated in a highly specific way in the cell wall of fungi. Based on the polarization optical analysis after the ABT-r (anisotropic PAS-r), the linear oriented hydroxyl groups of the sugar molecules are localized either perpendicular or parallel to the surface coat, depending on the species. According to the orientation of the vicinal hydroxyl groups, the oligosaccharide chains are orientated vertically. The capsule of the heterogeneous form of C. neoformans presented an especially strong metachromatic reaction and anisotropy. It is especially remarkable that the sterical orientation of sugar chains, and the terminal sialic acid and 9-O-acyl sialic acid molecules, was opposite in the inner and outer layer of the capsule.

Cryptococcal xylosyltransferase 1 (Cxt1p) from Cryptococcus neoformans plays a direct role in the synthesis of capsule polysaccharides

The opportunistic yeast Cryptococcus neoformans causes serious disease in humans and expresses a prominent polysaccharide capsule that is required for its virulence. Little is known about how this capsule is synthesized. We previously identified a beta1,2-xylosyltransferase (Cxt1p) with in vitro enzymatic activity appropriate for involvement in capsule synthesis. Here, we investigate C. neoformans strains in which the corresponding gene has been deleted (cxt1Delta). Loss of CXT1 does not affect in vitro growth of the mutant cells or the general morphology of their capsules. However, NMR structural analysis of the two main capsule polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), showed that both were missing beta1,2-xylose residues. There was an approximately 30% reduction in the abundance of this residue in GXM in mutant compared with wild-type strains, and mutant GalXM was almost completely devoid of beta1,2-linked xylose. The GalXM from the mutant strain was also missing a beta1,3-linked xylose residue. Furthermore, deletion of CXT1 led to attenuation of cryptococcal growth in a mouse model of infection, suggesting that the affected xylose residues are important for normal host-pathogen interactions. Cxt1p is the first glycosyltransferase with a defined role in C. neoformans capsule biosynthesis, and cxt1Delta is the only strain identified to date with structural alterations of the capsule polysaccharide GalXM.

Cooperative stimulation of dendritic cells by Cryptococcus neoformans mannoproteins and CpG oligodeoxynucleotides

While mannosylation targets antigens to mannose receptors on dendritic cells (DC), the resultant immune response is suboptimal. We hypothesized that the addition of toll-like receptor (TLR) ligands would enhance the DC response to mannosylated antigens. Cryptococcus neoformans mannoproteins (MP) synergized with CpG-containing oligodeoxynucleotides to stimulate enhanced production of proinflammatory cytokines and chemokines from murine conventional and plasmacytoid DC. Synergistic stimulation required the interaction of mannose residues on MP with the macrophage mannose receptor (MR), CD206. Moreover, synergy with MP was observed with other TLR ligands, including tripalmitoylated lipopeptide (Pam3CSK4), polyinosine-polycytidylic acid (pI:C), and imiquimod. Finally, CpG enhanced MP-specific MHC II-restricted CD4⁺ T-cell responses by a mechanism dependent upon DC expression of CD206 and TLR9. These data suggest a rationale for vaccination strategies that combine mannosylated antigens with TLR ligands and imply that immune responses to naturally mannosylated antigens on pathogens may be greatly augmented if TLR and MR are cooperatively stimulated.

3-hydroxy fatty acids found in capsules of Cryptococcus neoformans

Using immunofluorescence confocal laser scanning microscopy, immunogold transmission electron microscopy and gas chromatography--mass spectrometry, we demonstrated the presence of 3-hydroxy fatty acids in Cryptococcus neoformans. Our results suggest that these oxylipins accumulate in capsules where they are released as hydrophobic droplets through tubular protuberances into the surrounding medium.

Contribution of glycosylation to T cell responses stimulated by recombinant Cryptococcus neoformans mannoprotein

Mannoproteins are major antigens driving T cell responses to the opportunistic fungus Cryptococcus neoformans. To investigate the role played by mannosylation, an immunoreactive cryptococcal mannoprotein was expressed recombinantly in E. coli and Pichia pastoris, resulting in unglycosylated and mannosylated proteins, respectively. The Pichia-derived antigen stimulated stronger major histocompatibility class II-restricted T cell responses. Moreover, responses were potently inhibited if the antigen was chemically deglycosylated or if mannose receptors were blocked with mannans. Thus, mannosylation is critical for optimal T cell responses to a fungal antigen and should be taken into account when vaccines to protect against mycoses are designed.

Capsule structural heterogeneity and antigenic variation in Cryptococcus neoformans

Cryptococcus neoformans is a human pathogenic fungus with a capsule composed primarily of glucuronoxylomannan (GXM) that is important for virulence. Current views of GXM structure postulate a polymer composed of repeating mannose trisaccharide motifs bearing a single beta(1,2) glucuronic acid with variable xylose and O-acetyl substitutions to form six triads. GXM from different strains is notoriously variable in triad composition, but it is not known if the polymer consists of one or more motif-repeating units. We investigated the polymeric organization of GXM by using mass spectrometry to determine if its compositional motif arrangement was similar to that of bacterial capsular polysaccharides, namely, a polymer of a single repeating unit. The results were consistent with, and confirmatory for, the current view that the basic unit of GXM is a repeating mannose trisaccharide motif, but we also found evidence for the copolymerization of different GXM repeating units in one polysaccharide molecule. Analysis of GXM from isogenic phenotypic switch variants suggested structural differences caused by glucuronic acid positional effects, which implied flexibility in the synthetic pathway. Our results suggest that cryptococcal capsule synthesis is fundamentally different from that observed in prokaryotes and employs a unique eukaryotic approach, which theoretically could synthesize an infinite number of structural combinations. The biological significance of this capsule construction scheme is that it is likely to confer a powerful avoidance strategy for interactions with the immune system and phagocytic environmental predators. Consistent with this premise, the antigenic variation of a capsular epitope recognized by a nonprotective antibody was observed under different growth conditions.

Self-aggregation of Cryptococcus neoformans capsular glucuronoxylomannan is dependent on divalent cations

The capsular components of the human pathogen Cryptococcus neoformans are transported to the extracellular space and then used for capsule enlargement by distal growth. It is not clear, however, how the glucuronoxylomannan (GXM) fibers are incorporated into the capsule. In the present study, we show that concentration of C. neoformans culture supernatants by ultrafiltration results in the formation of highly viscous films containing pure polysaccharide, providing a novel, nondenaturing, and extremely rapid method to isolate extracellular GXM. The weight-averaged molecular mass of GXM in the film, determined using multiangle laser light scattering, was ninefold smaller than that of GXM purified from culture supernatants by differential precipitation with cetyl trimethyl ammonium bromide (CTAB). Polysaccharides obtained either by ultrafiltration or by CTAB-mediated precipitation showed different reactivities with GXM-specific monoclonal antibodies. Viscosity analysis associated with inductively coupled plasma mass spectrometry and measurements of zeta potential in the presence of different ions implied that polysaccharide aggregation was a consequence of the interaction between the carboxyl groups of glucuronic acid and divalent cations. Consistent with this observation, capsule enlargement in living C. neoformans cells was influenced by Ca(2+) in the culture medium. These results suggest that capsular assembly in C. neoformans results from divalent cation-mediated self-aggregation of extracellularly accumulated GXM molecules.

Cryptococcus neoformans biofilm formation depends on surface support and carbon source and reduces fungal cell susceptibility to heat, cold, and UV light

The fungus Cryptococcus neoformans possesses a polysaccharide capsule and can form biofilms on medical devices. We describe the characteristics of C. neoformans biofilm development using a microtiter plate model, microscopic examinations, and a colorimetric 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium-hydroxide (XTT) reduction assay to observe the metabolic activity of cryptococci within a biofilm. A strong correlation between XTT and CFU assays was demonstrated. Chemical analysis of the exopolymeric material revealed sugar composition consisting predominantly of xylose, mannose, and glucose, indicating the presence of other polysaccharides in addition to glucurunoxylomannan. Biofilm formation was affected by surface support differences, conditioning films on the surface, characteristics of the medium, and properties of the microbial cell. A specific antibody to the capsular polysaccharide of this fungus was used to stain the extracellular polysaccharide matrix of the fungal biofilms using light and confocal microscopy. Additionally, the susceptibility of C. neoformans biofilms and planktonic cells to environmental stress was investigated using XTT reduction and CFU assays. Biofilms were less susceptible to heat, cold, and UV light exposition than their planktonic counterparts. Our findings demonstrate that fungal biofilm formation is dependent on support surface characteristics and that growth in the biofilm state makes fungal cells less susceptible to potential environmental stresses.

Microreview: capsule-associated genes of Cryptococcus neoformans

Cryptococcosis, caused by Cryptococcus neoformans is a common systemic mycosis in man and animals, particularly immunocompromised patients. This pathogenic fungus produces a thick extracellular polysaccharide capsule. Four capsule-associated genes (CAP10, CAP59, CAP60, CAP64) were cloned and sequenced, and proved to be essential for capsule synthesis. However biochemical functions of CAP gene products have not been clarified yet. Recently, the relatedness of the polysaccharide capsule and four capsule-associated genes has partly been elucidated. Nucleotide sequence of four CAP gene fragments was analyzed for phylogenetic relationships, and they were in agreement with the conventional classification of varieties and serotypes within C. neoformans. Expression of four CAP genes and capsule size were examined using two media containing different amount of glucose, and the results indicated that CAP genes might play important roles in elaboration of extracellular polysaccharide capsule. Furthermore, analyses of CAP genes in various clinical samples would give the useful information to diagnose cryptococcosis in human and animals.

Characterization of glycoinositolphosphoryl ceramide structure mutant strains of Cryptococcus neoformans

In fungi, glycoinositolphosphoryl ceramide (GIPC) biosynthetic pathway produces essential molecules for growth, viability, and virulence. In previous studies, we demonstrated that the opportunistic fungus Cryptococcus neoformans synthesizes a complex family of xylose-(Xyl) branched GIPCs, all of which have not been previously reported in fungi. As an effort to understand the biosynthesis of these sphingolipids, we have now characterized the structures of GIPCs from C. neoformans wild-type (KN99alpha) and mutant strains that lack UDP-Xyl, by disruption of either UDP-glucose dehydrogenase (NE321) or UDP-glucuronic acid decarboxylase (NE178). The structures of GIPCs were determined by a combination of nuclear magnetic resonance (NMR) spectroscopy, tandem mass spectrometry (MS), and gas chromatography-MS. The main and largest GIPC from wild-type strain was identified as an alpha-Manp(1 --> 6)alpha-Manp(1 --> 3)alpha-Manp[beta-Xylp(1 --> 2)]alpha-Manp(1 --> 4)beta-Galp(1 --> 6)alpha-Manp(1 --> 2) Ins-1-P-Ceramide, whereas the most abundant GIPC from both mutant strains was found to be an alpha-Manp(1 --> 3)alpha-Manp(1 --> 4)beta-Galp(1 --> 6)alpha-Manp(1 --> 2)Ins-1-P-Ceramide. The ceramide moieties of C. neoformans wild-type and mutant strains were composed of a C(18) phytosphingosine, which was N-acylated with 2-hydroxy tetra-, or hexacosanoic acid, and 2,3-dihydroxy-tetracosanoic acid. Our structural analysis results indicate that the C. neoformans mutant strains are unable to complete the assembly of the GIPC-oligosaccharide moiety due the absence of Xyl side chain.

The immunoglobulin heavy chain constant region affects kinetic and thermodynamic parameters of antibody variable region interactions with antigen

A central dogma in immunology is that antibody specificity is a function of the variable (V) region. However serological analysis of IgG(1), IgG(2a), and IgG(2b) switch variants of murine monoclonal antibody (mAb) 3E5 IgG(3) with identical V domains revealed apparent specificity differences for Cryptococcus neoformans glucuronoxylomannan (GXM). Kinetic and thermodynamic binding properties of mAbs 3E5 to a 12-mer peptide mimetic of GXM revealed differences in the affinity of these mAbs for a monovalent ligand, a result that implied that the constant (C) region affects the secondary structure of the antigen binding site, thus accounting for variations in specificity. Structural models of mAbs 3E5 suggested that isotype-related differences in binding resulted from amino acid sequence polymorphisms in the C region. This study implies that isotype switching is another mechanism for generating diversity in antigen binding and that isotype restriction of certain antibody responses may reflect structural constraints imposed by C region on V region binding. Furthermore, isotype affected the polyreactivity of V region identical antibodies, implying a role for C region in determining self-reactivity.

Parallel beta-helix proteins required for accurate capsule polysaccharide synthesis and virulence in the yeast Cryptococcus neoformans

The principal capsular polysaccharide of the opportunistic fungal pathogen Cryptococcus neoformans consists of an alpha-1,3-linked mannose backbone decorated with a repeating pattern of glucuronyl and xylosyl side groups. This structure is critical for virulence, yet little is known about how the polymer, called glucuronoxylomannan (GXM), is faithfully synthesized and assembled. We have generated deletions in two genes encoding predicted parallel beta-helix repeat proteins, which we have designated PBX1 and PBX2. Deletion of either gene results in a dry-colony morphology, clumpy cells, and decreased capsule integrity. Two-dimensional nuclear magnetic resonance spectroscopy of purified GXM from the mutants indicated that both the wild-type GXM structure and novel, aberrant linkages were present. Carbohydrate composition and linkage analysis determined that these aberrant structures are correlated with the incorporation of terminal glucose residues that are not found in wild-type capsule polysaccharide. We conclude that Pbx1 and Pbx2 are required for the fidelity of GXM synthesis and may be involved in editing incorrectly added glucose residues. PBX1 and PBX2 knockout mutants showed severely attenuated virulence in a murine inhalation model of cryptococcosis. Unlike acapsular strains, these mutant strains induced delayed symptoms of cryptococcosis, though the infected animals eventually contained the infection and recovered.

Extracellular glycosylphosphatidylinositol-anchored mannoproteins and proteases of Cryptococcus neoformans

Extracellular proteins of Cryptococcus neoformans are involved in the pathogenesis of cryptococcosis, and some are immunoreactive antigens that may potentially serve as candidates for vaccine development. To further study the extracellular proteome of the human fungal pathogen Cry. neoformans, we conducted a proteomic analysis of secreted and cell wall-bound proteins with an acapsular strain of Cry. neoformans. Proteins were identified from both intact cells and cell walls. In both cases, extracellular proteins were removed with trypsin or beta-glucanase, and then all proteins/peptides were purified by solid-phase extraction, spin dialysis, and HPLC, and identified by liquid chromatography-mass spectrometry. This study identified 29 extracellular proteins with a predicted N-terminal signal sequence and also a predicted glycosylphosphatidylinositol anchor motif in more than half. Among the novel proteins identified were five glycosylphosphatidylinositol-anchored proteins with extensive Ser/Thr-rich regions but no apparent functional domains, a glycosylphosphatidylinositol-anchored aspartic protease, and a metalloprotease with structural similarity to an elastinolytic metalloprotease of Aspergillus fumigatus. This study suggests that Cry. neoformans has the machinery required to target glycosylphosphatidylinositol-anchored proteins to the cell wall, and it confirms the extracellular proteolytic ability of Cry. neoformans.

Phagosome extrusion and host-cell survival after Cryptococcus neoformans phagocytosis by macrophages

Cryptococcus neoformans (Cn) is an encapsulated yeast that is a facultative intracellular pathogen and a frequent cause of human disease. The interaction of Cn with alveolar macrophages is critical for containing the infection , but Cn can also replicate intracellularly and lyse macrophages . Cn has a unique intracellular pathogenic strategy that involves cytoplasmic accumulation of polysaccharide-containing vesicles and intracellular replication leading to the formation of spacious phagosomes in which multiple cryptococcal cells are present . The Cn intracellular pathogenic strategy in macrophages and amoebas is similar, leading to the proposal that it originated as a mechanism for survival against phagocytic predators in the environment . Here, we report that under certain conditions, including phagosomal maturation, possible actin depolymerization, and homotypic phagosome fusion, Cn can exit the macrophage host through an extrusion of the phagosome, while both the released pathogen and host remain alive and able to propagate. The phenomenon of "phagosomal extrusion" indicates the existence of a previously unrecognized mechanism whereby a fungal pathogen can escape the intracellular confines of mammalian macrophages to continue propagation and, possibly, dissemination.

Heat killed cells of Cryptococcus neoformans var. grubii induces protective immunity in rats: immunological and histopathological parameters

Different clinical parameters which included cell-mediated immune (CMI) response, were evaluated in a model of disseminated cryptococcosis in rats. The experimental animals were pretreated four days prior to their exposure to Cryptococcus neoformans var. grubii with either heat killed cells of this yeastlike pathogen (HKC) or capsular polysaccharide (CPS) emulsified in complete Freund adjuvant (CFA). Rats treated with HKC-CFA and intraperitoneally infected with C. neoformans var. grubii had significantly better clearance of yeasts from tissues, a lower concentration of the cryptococcal capsular polysaccharide, glucuronoxylomannan (GXM), in serum and tissues, and better histopathological parameters compared to unpretreated infected rats. In contrast, rats treated with CPS-CFA presented an exacerbation of infection with a significantly higher fungal burden in tissues, a higher concentration of GXM in serum, and worse histopathological parameters compared to similar unpretreated infected rats. In addition, HKC-CFA treatment produced a T helper 1 (Th1) profile with improvements in the spleen cell proliferative response, in the level of INFgamma production by CD4 T cells, and in the nitric oxide (NO) production by peritoneal cells. On the other hand, rats treated with CPS-CFA showed an increased level of the immunoregulatory cytokine IL10 production by CD4 T cells, but no modification in the NO production by peritoneal cells.

Application of proton nuclear magnetic resonance spectroscopy to the study of Cryptococcus and cryptococcosis

Proton nuclear magnetic resonance spectroscopy is a nondestructive technique that identifies chemicals in solution and in living cells. It has been used in cryptococcal research to identify the primary structure of capsular glucuronoxylomannans, link cellular apoptosis susceptibility (CAS) genes to positioning of residues on the mannose backbone of glucuronoxylomannan, and verify that the cryptococcal virulence determinant, phospholipase B, is elaborated in vivo. Promising clinical applications include speciation (Cryptococcus neoformans and Cryptococcus gattii), with preliminary evidence that varieties neoformans and grubii can also be distinguished, non-invasive diagnosis of cerebral cryptococcomas, and, in cases of meningitis, monitoring therapeutic response by analysis of cerebrospinal fluid.

Monoclonal antibodies can affect complement deposition on the capsule of the pathogenic fungus Cryptococcus neoformans by both classical pathway activation and steric hindrance

The capsule of the human pathogenic fungus Cryptococcus neoformans presents the immune system with a formidable problem for phagocytosis. Capsule-mediated activation of the alternative complement (C) pathway results in component 3 (particularly, C3) binding to the capsule near the cell wall surface. Hence, for cells with large capsule, C3 cannot interact with the complement receptor (CR) and is not opsonic. However, C activation in either immune serum or in the presence of monoclonal antibody (mAb) to capsular polysaccharide localizes C3 to the capsular edge. When C. neoformans cells were coated with both C and antibody (Ab) opsonins, Ab bound first and promoted C3 deposition at the edge of the capsule. The mechanism for the Ab-mediated change in C3 localization to the capsule edge involved both classical C pathway activation and steric hindrance preventing C3 penetration into the capsule. The change in C3 localization changed the mode of phagocytosis in macrophages, such that localizing C3 at the edge of the capsule allowed phagocytosis through C3-CR3 and C3-CR4 interactions, which did not occur in serum without Ab. These findings reveal a new mechanism of Ab action whereby Abs affect the location of C3 and its interaction with its receptor in macrophages depending on the immunoglobulin concentration.

Minimizing loss of sequence information in SAGE ditags by modulating the temperature dependent 3' --> 5' exonuclease activity of DNA polymerases on 3'-terminal isoheptyl amino groups

Numerous steps are required to prepare a sequencing library for serial analysis of gene expression (or SAGE) from an original mRNA sample. The presence of inefficiencies, however, can lead to a cumulative loss of sample during processing which can yield a library of short sequence tags (SSTs) that represents only a minute fraction of the original starting sample, potentially compromising the quality of the analysis and necessitating relatively large amounts of starting material. We show here that commonly observed higher molecular weight (HMW) amplification products observed following the PCR amplification of ditags are a direct result of the presence of HMW ligation products created during ditag formation. Using model tags, we demonstrate that the formation of these HMW ligation products becomes permissible following the release of the 3'-terminal isoheptyl amine (3'-IHA) from the SST during the fill-in reaction with the Klenow fragment (KF) of DNA polymerase (DNAP) I and is mediated by its 3' --> 5' exonuclease activity. We further show that the incorporation of SSTs into HMW ligation products can lead to a loss of sequence information from SAGE analysis, potentially skewing sequencing results away from the true distribution in the original sample. By modifying fill-in conditions through the use of Vent DNAP at 12 degrees C and by including terminal phosphorothioate linkages within the SAGE adaptors to specifically inhibit exonucleolytic removal of the 3'-terminal amine, we are able to maximize the yield of ditags and bypass the need for gel purification via PAGE following PCR. The modifications described here, combined with the modifications described previously by our group for adaptor ligation, ensure that the full sequence information content in SSTs derived from the transcriptome is preserved in the pool of amplified ditags prior to the creation of a SAGE library.

Specific antibody to Cryptococcus neoformans glucurunoxylomannan antagonizes antifungal drug action against cryptococcal biofilms in vitro

The fungus Cryptococcus neoformans possesses a polysaccharide capsule and can form biofilms on medical devices. We investigated the efficacy that the combination of a specific antibody to the capsular polysaccharide and antifungal therapy has against cryptococcal biofilms. The antibody enhanced the susceptibility of planktonic cells to antifungal agents, but an antagonistic effect was observed for combination therapy against C. neoformans biofilms. Our findings suggest that antibody therapies for infectious diseases that involve biofilms may antagonize certain antimicrobial therapies, and they also imply that products of the immune response may contribute to drug resistance of biofilms formed in vivo.

Killing of cryptococcus neoformans by Staphylococcus aureus: the role of cryptococcal capsular polysaccharide in the fungal-bacteria interaction

Microbes compete for the environmental niche which is their host. To investigate the effects of a pathogenic bacterium on invasion and colonization by a pathogenic yeast, Cryptococcus neoformans was co-cultured with Staphylococcus aureus. We found that the number of colony forming units of C. neoformans was decreased by Staphylococcus aureus. In contrast, the viability of Candida albicans was not affected. Under the microscope, wild-type C. neoformans cells were shown to be surrounded by S. aureus, while cells of a capsuleless mutant of C. neoformans were not. C. neoformans was not killed when a membrane separated it from S. aureus in co-culture. Killing was confirmed by staining with cyanoditolyl tetrazolium chloride: S. aureus stained red, indicating viability, while C. neojormans did not stain, indicating lethality. The in situ terminal deoxynucleotidyl transferase-mediated dUTR nick end labeling (TUNEL) assay indicated cell death with fragmentation of DNA of C. neoformans. Capsular polysaccharide from C. neoformans inhibited the killing. Treatment of the crude polysaccharide with protease increased the inhibition. The protective activity resided in the glucuronoxylomannan (GXM) fraction, although the concentration required for the inhibition was high. These results suggest that S. aureus kills C. neoformans by a process that involves attachment to the cryptococcal capsule.

Characterization of two novel cryptococcal mannoproteins recognized by immune sera

Host defenses against the encapsulated yeast Cryptococcus neoformans involve both humoral and cell-mediated immunity. Mannoproteins (MPs) are a heterogeneous class of immunodominant glycoproteins which have been only incompletely characterized. In this study, we report on the molecular features of two novel MPs that are recognized by serum antibodies during cryptococcosis. After fractionation of extracellular cryptococcal products, MPs reacted more strongly than other components with sera from C. neoformans-infected AIDS patients. Further fractionation and Western blot analysis of MPs evidenced the presence of highly reactive bands with molecular masses of 250, 125, 115, and 84 kDa. The 115- and 84-kDa bands contained significant amounts of N-linked oligosaccharides, as shown by decreased molecular mass after peptide-N-glycosidase F treatment. N-terminal amino acid sequences of the two bands were used to search C. neoformans nucleotide databases. Homologous genomic sequences were used to synthesize DNA probes and isolate cDNA clones containing the full-length genes, which were designated MP84 and MP115. Both genes showed the presence of a serine/threonine-rich region, a potential site for heavy glycosylation. MP84 and MP115 showed homology with, respectively, polysaccharide deacetylases and carboxylesterases from other organisms. Recombinant, deglycosylated proteins expressed in Escherichia coli still reacted with sera from patients, albeit more weakly than natural MPs, indicating that at least some of the reactive epitopes were retained in the recombinant forms. In conclusion, we identified two novel MPs that are important targets of antibody responses during cryptococcosis. These data may be useful to devise alternative immunity-based strategies to control the disease.

The physical properties of the capsular polysaccharides from Cryptococcus neoformans suggest features for capsule construction

The most distinctive feature of the human pathogenic fungus is a polysaccharide capsule that is essential for virulence and is composed primarily of glucuronoxylomannan (GXM) and galactoxylomannan (GalXM). GXM mediates multiple deleterious effects on host immune function, yet relatively little is known about its physical properties. The average mass of Cryptococcus neoformans GXM from four antigenically different strains ranged from 1.7 to 7 x 10(6) daltons as calculated from Zimm plots of light-scattering data. GalXM was significantly smaller than GXM, with an average mass of 1 x 10(5) daltons. These molecular masses imply that GalXM is the most numerous polysaccharide in the capsule on a molar basis. The radius of gyration of the capsular polysaccharides ranged between 68 and 208 nm. Viscosity measurements suggest that neither polysaccharide altered fluid dynamics during infection since GXM behaved in solution as a polyelectrolyte and GalXM did not increase solution viscosity. Immunoblot analysis indicated heterogeneity within GXM. In agreement with this, scanning transmission electron microscopy of GXM preparations revealed a tangled network of two different types of molecules. Mass per length measurements from light scattering and scanning transmission electron microscopy were consistent and suggested GXM molecules self-associate. A mechanism for capsule growth is proposed based on the extracellular release and entanglement of GXM molecules.

The cellular responses induced by the capsular polysaccharide of Cryptococcus neoformans differ depending on the presence or absence of specific protective antibodies

The capsule of Cryptococcus neoformans, the principal virulence factor of this fungus, is composed primarily of polysaccharide. The predominant component of the polysaccharide capsule is glucuronoxylomannan (GXM), a compound with potent immunoregulatory properties. GXM is bound and internalized by natural immune cells affecting innate and subsequent adaptive immune response. The cellular pattern recognition receptors involved in GXM binding include toll-like receptor (TLR)4, CD14, TLR2, CD18, Fc gamma receptor II (FcgammaRPi). This multiple cross-linking leads to a suppressive outcome that is arrested and even reversed by protective antibodies to GXM. This review analyzes the immunosuppressive effects induced by capsular material, considering its pattern recognition receptors, and dissects the mechanism of monoclonal antibody shifting to immunoactivation.