Biological function and molecular mapping of M antigen in yeast phase of Histoplasma capsulatum

Histoplasma antigens as novel players for the development of new enzyme immunoassays for the serodiagnosis of histoplasmosis: A comparative study of their analytical performance

Definitive diagnosis of histoplasmosis relies on culture and/or cytology/histopathology; however, these procedures have limited sensitivity and cultures are time-consuming. Antibodies detection by immunodiffusion has low sensitivity in immunocompromised individuals and uses histoplasmin (HMN), a crude antigenic extract, as reagent. Novel protein antigen candidates have been recently identified and produced by DNA-recombinant techniques to obtain standardized and specific reagents for diagnosing histoplasmosis. To compare the analytical performance of novel enzyme-linked immunosorbent assays (ELISAs) for antibodies testing for diagnosing histoplasmosis using different Histoplasma capsulatum antigens as reagents. The H. capsulatum 100 kDa protein (Hcp100), the M antigen and its immunoreactive fragment F1 were produced by DNA-recombinant techniques. Galactomannan was purified from both the yeast and mycelial cell walls (yGM and mGM, respectively). The analytical performance of the ELISA tests for the serological detection of antibodies against these antigens was evaluated and compared with those obtained using HMN as reagent. Antibodies detection by the Hcp100 ELISA demonstrated 90.0% sensitivity and 92.0% specificity, versus 43.3% sensitivity and 95.0% specificity of the M ELISA, 33.3% sensitivity and 84.0% specificity of the F1 ELISA, 96.7% sensitivity and 94.0% specificity of the yGM ELISA, 83.3% sensitivity and 88.0% specificity of the mGM ELISA, and 70.0% sensitivity and 86.0% specificity for the HMN ELISA. In summary, Hcp100 is proposed as the most promising candidate for the serodiagnosis of histoplasmosis. The primary immunoreactive element in HMN proved to be GM rather than the M antigen. Nevertheless, a higher incidence of cross-reactions was noted with GM compared to M.

In vitro and in vivo efficacies of Dectin-1-Fc(IgG)(s) fusion proteins against invasive fungal infections

Fungal infections have increased in the last years, particularly associated to an increment in the number of immunocompromised individuals and the emergence of known or new resistant species, despite the difficulties in the often time-consuming diagnosis. The controversial efficacy of the currently available strategies for their clinical management, apart from their high toxicity and severe side effects, have renewed the interest in the research and development of new broad antifungal alternatives. These encompass vaccines and passive immunization strategies with monoclonal antibodies (mAbs), recognizing ubiquitous fungal targets, such as fungal cell wall β-1,3-glucan polysaccharides, which could be used in early therapeutic intervention without the need for the diagnosis at species-level. As additional alternatives, based on the Dectin-1 great affinity to β-1,3-glucan, our group developed broad antibody-like Dectin1-Fc(IgG)(s) from distinct subclasses (IgG2a and IgG2b) and compared their antifungal in vitro and passive immunizations in vivo performances. Dectin1-Fc(IgG2a) and Dectin1-Fc(IgG2b) demonstrated high affinity to laminarin and the fungal cell wall by ELISA, flow cytometry and microscopy. Both Dectin-1-Fc(IgG)(s) inhibited H. capsulatum and C. neoformans growth in a dose-dependent fashion. For C. albicans, such inhibitory effect was observed with concentrations as low as 0.098 and 0.049 µg/mL, respectively, which correlated with the impairment of the kinetics and lengths of germ tubes in comparison to controls. Previous opsonization with Dectin-1-Fc(IgG)(s) enhanced considerably the macrophage antifungal effector functions, increasing the fungi macrophages-interactions and significantly reducing the intraphagosome fungal survival, as lower CFUs were observed. The administration of both Dectin1-Fc(IgG)(s) reduced the fungal burden and mortality in murine histoplasmosis and candidiasis models, in accordance with previous evaluations in aspergillosis model. These results altogether strongly suggested that therapeutic interventions with Dectin-1-Fc(IgG)(s) fusion proteins could directly impact the innate immunity and disease outcome in favor of the host, by direct neutralization, opsonization, phagocytosis, and fungal elimination, providing interesting information on the potential of these new strategies for the control of invasive fungal infections.

Challenges in Serologic Diagnostics of Neglected Human Systemic Mycoses: An Overview on Characterization of New Targets

Systemic mycoses have been viewed as neglected diseases and they are responsible for deaths and disabilities around the world. Rapid, low-cost, simple, highly-specific and sensitive diagnostic tests are critical components of patient care, disease control and active surveillance. However, the diagnosis of fungal infections represents a great challenge because of the decline in the expertise needed for identifying fungi, and a reduced number of instruments and assays specific to fungal identification. Unfortunately, time of diagnosis is one of the most important risk factors for mortality rates from many of the systemic mycoses. In addition, phenotypic and biochemical identification methods are often time-consuming, which has created an increasing demand for new methods of fungal identification. In this review, we discuss the current context of the diagnosis of the main systemic mycoses and propose alternative approaches for the identification of new targets for fungal pathogens, which can help in the development of new diagnostic tests.

Pharmaceutical interest of in-silico approaches

The virtual environment within the computer using software performed on the computer is known as in-silico studies. These drugs designing software play a vital task in discovering new drugs in the field of pharmaceuticals. These designing programs and software are employed in gene sequencing, molecular modeling, and in assessing the three-dimensional structure of the molecule, which can further be used in drug designing and development. Drug development and discovery is not only a powerful, extensive, and an interdisciplinary system but also a very complex and time-consuming method. This book chapter mainly focused on different types of in-silico approaches along with their pharmaceutical applications in numerous diseases.

Deletion of the Stress Response Gene DDR48 from Histoplasma capsulatum Increases Sensitivity to Oxidative Stress, Increases Susceptibility to Antifungals, and Decreases Fitness in Macrophages

The stress response gene DDR48 has been characterized in Saccharomyces cerevisiae and Candida albicans to be involved in combating various cellular stressors, from oxidative agents to antifungal compounds. Surprisingly, the biological function of DDR48 has yet to be identified, though it is likely an important part of the stress response. To gain insight into its function, we characterized DDR48 in the dimorphic fungal pathogen Histoplasma capsulatum. Transcriptional analyses showed preferential expression of DDR48 in the mycelial phase. Induction of DDR48 in Histoplasma yeasts developed after treatment with various cellular stress compounds. We generated a ddr48∆ deletion mutant to further characterize DDR48 function. Loss of DDR48 alters the transcriptional profile of the oxidative stress response and membrane synthesis pathways. Treatment with ROS or antifungal compounds reduced survival of ddr48∆ yeasts compared to controls, consistent with an aberrant cellular stress response. In addition, we infected RAW 264.7 macrophages with DDR48-expressing and ddr48∆ yeasts and observed a 50% decrease in recovery of ddr48∆ yeasts compared to wild-type yeasts. Loss of DDR48 function results in numerous negative effects in Histoplasma yeasts, highlighting its role as a key player in the global sensing and response to cellular stress by fungi.

Comparative Proteomic Analysis of Histoplasma capsulatum Yeast and Mycelium Reveals Differential Metabolic Shifts and Cell Wall Remodeling Processes in the Different Morphotypes

Histoplasma capsulatum is a thermally dimorphic fungus distributed worldwide, but with the highest incidence in the Americas within specific geographic areas, such as the Mississippi River Valley and regions in Latin America. This fungus is the etiologic agent of histoplasmosis, an important life-threatening systemic mycosis. Dimorphism is an important feature for fungal survival in different environments and is related to the virulence of H. capsulatum, and essential to the establishment of infection. Proteomic profiles have made important contributions to the knowledge of metabolism and pathogenicity in several biological models. However, H. capsulatum proteome studies have been underexplored. In the present study, we report the first proteomic comparison between the mycelium and the yeast cells of H. capsulatum. Liquid chromatography coupled to mass spectrometry was used to evaluate the proteomic profile of the two phases of H. capsulatum growth, mycelium, and yeast. In summary, 214 and 225 proteins were only detected/or preferentially abundant in mycelium or yeast cells, respectively. In mycelium, enzymes related to the glycolytic pathway and to the alcoholic fermentation occurred in greater abundance, suggesting a higher use of anaerobic pathways for energy production. In yeast cells, proteins related to the tricarboxylic acid cycle and response to temperature stress were in high abundance. Proteins related to oxidative stress response or involved with cell wall metabolism were identified with differential abundance in both conditions. Proteomic data validation was performed by enzymatic activity determination, Western blot assays, or immunofluorescence microscopy. These experiments corroborated, directly or indirectly, the abundance of isocitrate lyase, 2-methylcitrate synthase, catalase B, and mannosyl-oligosaccharide-1,2-alpha-mannosidase in the mycelium and heat shock protein (HSP) 30, HSP60, glucosamine-fructose-6-phosphate aminotransferase, glucosamine-6-phosphate deaminase, and N-acetylglucosamine-phosphate mutase in yeast cells. The proteomic profile-associated functional classification analyses of proteins provided new and interesting information regarding the differences in metabolism between the two distinct growth forms of H. capsulatum.

Immunoproteomics Reveals Pathogen's Antigens Involved in Homo sapiens-Histoplasma capsulatum Interaction and Specific Linear B-Cell Epitopes in Histoplasmosis

Histoplasmosis is one of the most frequent systemic mycosis in HIV patients. In these patients, histoplasmosis has high rates of morbidity/mortality if diagnosis and treatment are delayed. Despite its relevance, there is a paucity of information concerning the interaction between Histoplasma capsulatum and the human host, especially regarding the B-cell response, which has a direct impact on the diagnosis. Culture-based “gold-standard” methods have limitations, making immunodiagnostic tests an attractive option for clinical decisions. Despite the continuous development of those tests, improving serological parameters is necessary to make these methods efficient tools for definitive diagnosis of histoplasmosis. This includes the determination of more specific and immunogenic antigens to improve specificity and sensitivity of assays. In this study, we performed a co-immunoprecipitation assay between a protein extract from the yeast form of H. capsulatum and pooled sera from patients with proven histoplasmosis, followed by shotgun mass spectrometry identification of antigenic targets. Sera from patients with other pulmonary infections or from healthy individuals living in endemic areas of histoplasmosis were also assayed to determine potentially cross-reactive proteins. The primary structures of H. capsulatum immunoprecipitated proteins were evaluated using the DNAStar Protean 7.0 software. In parallel, the online epitope prediction server, BCPREDS, was used to complement the B-epitope prediction analysis. Our approach detected 132 reactive proteins to antibodies present in histoplasmosis patients’ sera. Among these antigens, 127 were recognized also by antibodies in heterologous patients’ and/or normal healthy donors’ sera. Therefore, the only three antigens specifically recognized by antibodies of histoplasmosis patients were mapped as potential antigenic targets: the M antigen, previously demonstrated in the diagnosis of histoplasmosis, and the catalase P and YPS-3 proteins, characterized as virulence factors of H. capsulatum, with antigenic properties still unclear. The other two proteins were fragments of the YPS-3 and M antigen. Overlapping results obtained from the two aforementioned bioinformatic tools, 16 regions from these three proteins are proposed as putative B-cell epitopes exclusive to H. capsulatum. These data reveal a new role for these proteins on H. capsulatum interactions with the immune system and indicate their possible use in new methods for the diagnosis of histoplasmosis.

An interferon gamma release assay specific for Histoplasma capsulatum to detect asymptomatic infected individuals: A proof of concept study

Histoplasmosis is the most common endemic mycosis in the Americas. Currently, there is no laboratory test capable to detect subclinical or latent infections by Histoplasma capsulatum (Hc), which might develop as severe infections in immunocompromised individuals. For the first time to our knowledge, we explore the suitability of an interferon gamma release assay (IGRA) to detect latent Hc infection in asymptomatic individuals. A cohort of 126 volunteers was enrolled in the study, 13 of which underwent a Hc infection in the past, and 93 of them showing risk factors for this infection. The remaining 20 participants did not refer any risk factors of Hc infection, but eight of them showed evidences of infection with Mycobacterium tuberculosis. All participants were recruited in Medellin, Colombia, between January 2014 and December 2017. Whole blood samples were cultured with four different Hc crude antigens and phytohemaglutinin as positive control. The interferon (IFN)-γ released by T lymphocytes upon antigen stimulation was quantified by ELISA. A defined cutoff value of 20 pg/ml for the IFN-γ concentration allowed us to distinguish between the group with documented past infections and the group of noninfected individuals with high sensitivity (70-92%) and specificity (85-95%), for the four tested antigens. Positive 82-95% and negative 77-92% predictive values were also very high, comparable to those reported for commercially available IGRAs. The new test constitutes a promising screening method to detect individuals with latent Hc infection, even decades after the primary infection, as evidenced in this study.

Immunotherapy against Systemic Fungal Infections Based on Monoclonal Antibodies

The increasing incidence in systemic fungal infections in humans has increased focus for the development of fungal vaccines and use of monoclonal antibodies. Invasive mycoses are generally difficult to treat, as most occur in vulnerable individuals, with compromised innate and adaptive immune responses. Mortality rates in the setting of our current antifungal drugs remain excessively high. Moreover, systemic mycoses require prolonged durations of antifungal treatment and side effects frequently occur, particularly drug-induced liver and/or kidney injury. The use of monoclonal antibodies with or without concomitant administration of antifungal drugs emerges as a potentially efficient treatment modality to improve outcomes and reduce chemotherapy toxicities. In this review, we focus on the use of monoclonal antibodies with experimental evidence on the reduction of fungal burden and prolongation of survival in in vivo disease models. Presently, there are no licensed monoclonal antibodies for use in the treatment of systemic mycoses, although the potential of such a vaccine is very high as indicated by the substantial promising results from several experimental models.

Identifying molecularly defined antigens for a Histoplasma capsulatum-specific interferon gamma release assay

BACKGROUND

In a previous work we showed the feasibility of an interferon gamma release assay (IGRA) for detecting latent infection by Histoplasma capsulatum. While in that proof-of-concept study we used crude fungal extracts as antigens, the newest IGRAs developed for other infections are based on molecularly defined antigens, mostly on mixtures of immunogenic peptides.

AIMS

To identify proteins in H. capsulatum that might serve as molecularly defined antigens for an IGRA test.

METHODS

We surveyed the literature looking for known H. capsulatum-immunogenic proteins and assayed two of them as antigens in an IGRA test, in a study that involved 80 volunteers. Furthermore, we used several bioinformatics tools to identify specific H. capsulatum proteins and to analyze possible strategies for the design of H. capsulatum-specific immunogenic peptides.

RESULTS

Seven H. capsulatum-immunogenic proteins were retrieved from the literature. IGRA tests using either the heat shock protein 60 or the M antigen showed high sensitivities but low specificities, most likely due to the high sequence similarity with the corresponding orthologs in other pathogenic microorganisms. We identified around 2000 H. capsulatum-specific proteins, most of which remain unannotated. Class II T-cell epitope predictions for a small number of these proteins showed a great variability among different alleles, prompting for a "brute force" approach for peptide design.

CONCLUSIONS

The H. capsulatum genome encodes a large number of distinctive proteins, which represent a valuable source of potential specific antigens for an IGRA test. Among them, the Cfp4 protein stands out as a very attractive candidate.

Histoplasma Capsulatum: Mechanisms for Pathogenesis

Histoplasmosis, caused by the dimorphic environmental fungus Histoplasma capsulatum, is a major mycosis on the global stage. Acquisition of the fungus by mammalian hosts can be clinically silent or it can lead to life-threatening systemic disease, which can occur in immunologically intact or deficient hosts, albeit severe disease is more likely in the setting of compromised cellular immunity. H. capsulatum yeast cells are highly adapted to the mammalian host as they can effectively survive within intracellular niches in select phagocytic cells. Understanding the biological response by both the host and H. capsulatum will facilitate improved approaches to prevent and/or modify disease. This review presents our current understanding of the major pathogenic mechanisms involved in histoplasmosis.

Host membrane glycosphingolipids and lipid microdomains facilitate Histoplasma capsulatum internalisation by macrophages

Recognition and internalisation of intracellular pathogens by host cells is a multifactorial process, involving both stable and transient interactions. The plasticity of the host cell plasma membrane is fundamental in this infectious process. Here, the participation of macrophage lipid microdomains during adhesion and internalisation of the fungal pathogen Histoplasma capsulatum (Hc) was investigated. An increase in membrane lateral organisation, which is a characteristic of lipid microdomains, was observed during the first steps of Hc-macrophage interaction. Cholesterol enrichment in macrophage membranes around Hc contact regions and reduced levels of Hc-macrophage association after cholesterol removal also suggested the participation of lipid microdomains during Hc-macrophage interaction. Using optical tweezers to study cell-to-cell interactions, we showed that cholesterol depletion increased the time required for Hc adhesion. Additionally, fungal internalisation was significantly reduced under these conditions. Moreover, macrophages treated with the ceramide-glucosyltransferase inhibitor (P4r) and macrophages with altered ganglioside synthesis (from B4galnt1^-/- mice) showed a deficient ability to interact with Hc. Coincubation of oligo-GM1 and treatment with Cholera toxin Subunit B, which recognises the ganglioside GM1, also reduced Hc association. Although purified GM1 did not alter Hc binding, treatment with P4 significantly increased the time required for Hc binding to macrophages. The content of CD18 was displaced from lipid microdomains in B4galnt1^-/- macrophages. In addition, macrophages with reduced CD18 expression (CD18^low ) were associated with Hc at levels similar to wild-type cells. Finally, CD11b and CD18 colocalised with GM1 during Hc-macrophage interaction. Our results indicate that lipid rafts and particularly complex gangliosides that reside in lipid rafts stabilise Hc-macrophage adhesion and mediate efficient internalisation during histoplasmosis.

Computational methods in drug discovery

The process for drug discovery and development is challenging, time consuming and expensive. Computer-aided drug discovery (CADD) tools can act as a virtual shortcut, assisting in the expedition of this long process and potentially reducing the cost of research and development. Today CADD has become an effective and indispensable tool in therapeutic development. The human genome project has made available a substantial amount of sequence data that can be used in various drug discovery projects. Additionally, increasing knowledge of biological structures, as well as increasing computer power have made it possible to use computational methods effectively in various phases of the drug discovery and development pipeline. The importance of in silico tools is greater than ever before and has advanced pharmaceutical research. Here we present an overview of computational methods used in different facets of drug discovery and highlight some of the recent successes. In this review, both structure-based and ligand-based drug discovery methods are discussed. Advances in virtual high-throughput screening, protein structure prediction methods, protein-ligand docking, pharmacophore modeling and QSAR techniques are reviewed.

RYP1 gene as a target for molecular diagnosis of histoplasmosis

This study analyzed the RYP1 gene as a target for the molecular diagnosis of histoplasmosis. This assay detected fungal DNA in 13/13 blood samples from HIV/AIDS-patients with histoplasmosis. Therefore, the detection of RYP1 gene in whole blood sample is a quick and sensitive test to diagnose histoplasmosis.

Histoplasma capsulatum surmounts obstacles to intracellular pathogenesis

The fungal pathogen Histoplasma capsulatum causes respiratory and disseminated disease, even in immunocompetent hosts. In contrast to opportunistic pathogens, which are readily controlled by phagocytic cells, H. capsulatum yeasts are able to infect macrophages, survive antimicrobial defenses, and proliferate as an intracellular pathogen. In this review, we discuss some of the molecular mechanisms that enable H. capsulatum yeasts to overcome obstacles to intracellular pathogenesis. H. capsulatum yeasts gain refuge from extracellular obstacles such as antimicrobial lung surfactant proteins by engaging the β-integrin family of phagocytic receptors to promote entry into macrophages. In addition, H. capsulatum yeasts conceal immunostimulatory β-glucans to avoid triggering signaling receptors such as the β-glucan receptor Dectin-1. H. capsulatum yeasts counteract phagocyte-produced reactive oxygen species by expression of oxidative stress defense enzymes including an extracellular superoxide dismutase and an extracellular catalase. Within the phagosome, H. capsulatum yeasts block phagosome acidification, acquire essential metals such as iron and zinc, and utilize de novo biosynthesis pathways to overcome nutritional limitations. These mechanisms explain how H. capsulatum yeasts avoid and negate macrophage defense strategies and establish a hospitable intracellular niche, making H. capsulatum a successful intracellular pathogen of macrophages.

The Einstein-Brazil Fogarty: A decade of synergy

A rich, collaborative program funded by the US NIH Fogarty program in 2004 has provided for a decade of remarkable opportunities for scientific advancement through the training of Brazilian undergraduate, graduate and postdoctoral students from the Federal University and Oswaldo Cruz Foundation systems at Albert Einstein College of Medicine. The focus of the program has been on the development of trainees in the broad field of Infectious Diseases, with a particular focus on diseases of importance to the Brazilian population. Talented trainees from various regions in Brazil came to Einstein to learn techniques and study fungal, parasitic and bacterial pathogens. In total, 43 trainees enthusiastically participated in the program. In addition to laboratory work, these students took a variety of courses at Einstein, presented their results at local, national and international meetings, and productively published their findings. This program has led to a remarkable synergy of scientific discovery for the participants during a time of rapid acceleration of the scientific growth in Brazil. This collaboration between Brazilian and US scientists has benefitted both countries and serves as a model for future training programs between these countries.

N-acetylated α-linked acidic dipeptidase is identified as an antigen of Histoplasma capsulatum

Histoplasmosis, one of the most important mycoses, needs to be diagnosed rapidly and accurately. The main method used to diagnose histoplasmosis is serological detection of antibodies to the Histoplasma capsulatum H and M antigens. Several other protein antigens have been reported in H. capsulatum; however, they have not been used for diagnosis. In this study, we explored novel antigens that were detected during H. capsulatum infection. We obtained a protein mixture from H. capsulatum yeast cells after vigorous mixing in a 0.1% Triton X-100 solution. From the resultant pool, we detected nine spots that reacted with sera from patients with histoplasmosis and identified eight seroactive proteins with mass spectrometry. The seroactive proteins were purified, and their antigenicities were tested with an enzyme-linked immunosorbent assay (ELISA). ELISA revealed that the titer of the patients' sera to N-acetylated α-linked acidic dipeptidase was significantly higher than those of healthy volunteers (P < 0.01). These data indicate that N-acetylated α-linked acidic dipeptidase of H. capsulatum is recognized as a major antigen during histoplasmosis.

Tolerance of pentose utilising yeast to hydrogen peroxide-induced oxidative stress

Background

Bioethanol fermentations follow traditional beverage fermentations where the yeast is exposed to adverse conditions such as oxidative stress. Lignocellulosic bioethanol fermentations involve the conversion of pentose and hexose sugars into ethanol. Environmental stress conditions such as osmotic stress and ethanol stress may affect the fermentation performance; however, oxidative stress as a consequence of metabolic output can also occur. However, the effect of oxidative stress on yeast with pentose utilising capabilities has yet to be investigated.

Results

Assaying for the effect of hydrogen peroxide-induced oxidative stress on Candida, Pichia and Scheffersomyces spp. has demonstrated that these yeast tolerate hydrogen peroxide-induced oxidative stress in a manner consistent with that demonstrated by Saccharomyces cerevisiae. Pichia guillermondii appears to be more tolerant to hydrogen peroxide-induced oxidative stress when compared to Candida shehatae, Candida succiphila or Scheffersomyces stipitis.

Conclusions

Sensitivity to hydrogen peroxide-induced oxidative stress increased in the presence of minimal media; however, addition of amino acids and nucleobases was observed to increase tolerance. In particular adenine increased tolerance and methionine reduced tolerance to hydrogen peroxide-induced oxidative stress.

Redundant catalases detoxify phagocyte reactive oxygen and facilitate Histoplasma capsulatum pathogenesis

Histoplasma capsulatum is a respiratory pathogen that infects phagocytic cells. The mechanisms allowing Histoplasma to overcome toxic reactive oxygen molecules produced by the innate immune system are an integral part of Histoplasma's ability to survive during infection. To probe the contribution of Histoplasma catalases in oxidative stress defense, we created and analyzed the virulence defects of mutants lacking CatB and CatP, which are responsible for extracellular and intracellular catalase activities, respectively. Both CatB and CatP protected Histoplasma from peroxide challenge in vitro and from antimicrobial reactive oxygen produced by human neutrophils and activated macrophages. Optimal protection required both catalases, as the survival of a double mutant lacking both CatB and CatP was lower than that of single-catalase-deficient cells. Although CatB contributed to reactive oxygen species defenses in vitro, CatB was dispensable for lung infection and extrapulmonary dissemination in vivo. Loss of CatB from a strain also lacking superoxide dismutase (Sod3) did not further reduce the survival of Histoplasma yeasts. Nevertheless, some catalase function was required for pathogenesis since simultaneous loss of both CatB and CatP attenuated Histoplasma virulence in vivo. These results demonstrate that Histoplasma's dual catalases comprise a system that enables Histoplasma to efficiently overcome the reactive oxygen produced by the innate immune system.

Dynamic changes in the secondary structure of ECE-1 and XCE account for their different substrate specificities

Background

X-converting enzyme (XCE) involved in nervous control of respiration, is a member of the M13 family of zinc peptidases, for which no natural substrate has been identified yet. In contrast, it’s well characterized homologue endothelin-converting enzyme-1 (ECE-1) showed broad substrate specificity and acts as endopeptidase as well as dipeptidase. To explore the structural differences between XCE and ECE-1, homology model of XCE was built using the complex structure of ECE-1 with phosphoramidon (pdb-id: 3DWB) as template. Phosphoramidon was docked into the binding site of XCE whereas phosphate oxygen of the inhibitor was used as water molecule to design the apo forms of both enzymes. Molecular dynamics simulation of both enzymes was performed to analyze the dynamic nature of their active site residues in the absence and presence of the inhibitor.

Results

Homology model of XCE explained the role of non-conserved residues of its S2’ subsite. Molecular dynamics (MD) simulations identified the flexible transitions of F149/I150, N566/N571, W714/W719, and R145/R723 residues of ECE-1/XCE for the strong binding of the inhibitor. Secondary structure calculations using DSSP method reveals the folding of R145/R723 residue of ECE-1/XCE into β-sheet structure while unfolding of the S2’ subsite residues in aECE-1 and sustained compact folding of that of aXCE. The results evaluated are in good agreement with available experimental data, thus providing detailed molecular models which can explain the structural and specificities differences between both zinc peptidases.

Conclusions

Secondary structure changes of both enzymes during the simulation time revealed the importance of β-sheet structure of R145/R723 for its binding with the terminal carboxylate group of the inhibitor. Unfolding of the α-helix comprising the S2’ subsite residues in aECE-1 correlate well with its endopeptidase activity while their compact folding in aXCE may account for the inactivity of the enzyme towards large C-terminal containing substrates.

Extracellular superoxide dismutase protects Histoplasma yeast cells from host-derived oxidative stress

In order to establish infections within the mammalian host, pathogens must protect themselves against toxic reactive oxygen species produced by phagocytes of the immune system. The fungal pathogen Histoplasma capsulatum infects both neutrophils and macrophages but the mechanisms enabling Histoplasma yeasts to survive in these phagocytes have not been fully elucidated. We show that Histoplasma yeasts produce a superoxide dismutase (Sod3) and direct it to the extracellular environment via N-terminal and C-terminal signals which promote its secretion and association with the yeast cell surface. This localization permits Sod3 to protect yeasts specifically from exogenous superoxide whereas amelioration of endogenous reactive oxygen depends on intracellular dismutases such as Sod1. While infection of resting macrophages by Histoplasma does not stimulate the phagocyte oxidative burst, interaction with polymorphonuclear leukocytes (PMNs) and cytokine-activated macrophages triggers production of reactive oxygen species (ROS). Histoplasma yeasts producing Sod3 survive co-incubation with these phagocytes but yeasts lacking Sod3 are rapidly eliminated through oxidative killing similar to the effect of phagocytes on Candida albicans yeasts. The protection provided by Sod3 against host-derived ROS extends in vivo. Without Sod3, Histoplasma yeasts are attenuated in their ability to establish respiratory infections and are rapidly cleared with the onset of adaptive immunity. The virulence of Sod3-deficient yeasts is restored in murine hosts unable to produce superoxide due to loss of the NADPH-oxidase function. These results demonstrate that phagocyte-produced ROS contributes to the immune response to Histoplasma and that Sod3 facilitates Histoplasma pathogenesis by detoxifying host-derived reactive oxygen thereby enabling Histoplasma survival.

Histoplasma capsulatum is a fungal pathogen that is endemic to the Mississippi and Ohio River valleys. An estimated 200,000 infections occur annually in the United States. Histoplasma is adept at surviving within both neutrophils and macrophages, which normally kill fungal cells by producing reactive oxygen molecules that are toxic to microbes. In this study, we demonstrate the role of a superoxide dismutase enzyme (Sod3) produced by Histoplasma cells and we show that it enables Histoplasma to survive these reactive oxidative molecules produced by the host. We show that Histoplasma directs the Sod3 protein to the surface of yeast cells and into the extracellular environment, positioning it to destroy extracellular superoxide produced by neutrophils and macrophages. Our results highlight the importance of reactive oxygen produced by immune cells and define the mechanism by which Histoplasma survives these immune defenses and establishes infections in its host.

Antibody therapy for histoplasmosis

The endemic human pathogenic fungus Histoplasma capsulatum is a major fungal pathogen with a broad variety of clinical presentations, ranging from mild, focal pulmonary disease to life-threatening systemic infections. Although azoles, such as itraconazole and voriconazole, and amphotericin B have significant activity against H. capsulatum, about 1 in 10 patients hospitalized due to histoplasmosis die. Hence, new approaches for managing disease are being sought. Over the past 10 years, studies have demonstrated that monoclonal antibodies (mAbs) can modify the pathogenesis of histoplasmosis. Disease has been shown to be impacted by mAbs targeting either fungal cell surface proteins or host co-stimulatory molecules. This review will detail our current knowledge regarding the impact of antibody therapy on histoplasmosis.

Surface architecture of histoplasma capsulatum

The dimorphic fungal pathogen Histoplasma capsulatum is the most frequent cause of clinically significant fungal pneumonia in humans. H. capsulatum virulence is achieved, in part, through diverse and dynamic alterations to the fungal cell surface. Surface components associated with H. capsulatum pathogenicity include carbohydrates, lipids, proteins, and melanins. Here, we describe the various structures comprising the cell surface of H. capsulatum that have been associated with virulence and discuss their involvement in the pathobiology of disease.

Histoplasma mechanisms of pathogenesis--one portfolio doesn't fit all

Histoplasma capsulatum is the leading cause of endemic mycosis in the world. Analyses of clinical isolates from different endemic regions show important diversity within the species. Recent molecular studies of two isolates, the Chemotype I NAm2 strain G217B and the Chemotype II Panamanian strain G186A, reveal significant genetic, structural, and molecular differences between these representative Histoplasma strains. Some of these variations have functional consequences, representing distinct molecular mechanisms that facilitate Histoplasma pathogenesis. The realization of Histoplasma strain diversity highlights the importance of characterizing Histoplasma virulence factors in the context of specific clinical strain isolates.

Definition of the extracellular proteome of pathogenic-phase Histoplasma capsulatum

The dimorphic fungal pathogen Histoplasma capsulatum causes respiratory and systemic disease. Within the mammalian host, pathogenic Histoplasma yeast infect, replicate within, and ultimately kill host phagocytes. Surprisingly, few factors have been identified that contribute to Histoplasma virulence. To address this deficiency, we have defined the constituents of the extracellular proteome using LC-MS/MS analysis of the proteins in pathogenic-phase culture filtrates of Histoplasma. In addition to secreted Cbp1, the extracellular proteome of pathogenic Histoplasma yeast consists of 33 deduced proteins. The proteins include glycanases, extracellular enzymes related to oxidative stress defense, dehydrogenase enzymes, chaperone-like factors, and five novel culture filtrate proteins (Cfp's). For independent verification of proteomics-derived identities, we employed RNA interference (RNAi)-based depletion of candidate factors and showed loss of specific proteins from the cell-free culture filtrate. Quantitative RT-PCR revealed the expression of 10 of the extracellular factors was particularly enriched in pathogenic yeast cells as compared to nonpathogenic Histoplasma mycelia, suggesting that these proteins are linked to Histoplasma pathogenesis. In addition, Histoplasma yeast express these factors within macrophages and during infection of murine lungs. As extracellular proteins are positioned at the interface between host and pathogen, the definition of the pathogenic-phase extracellular proteome provides a foundation for the molecular dissection of how Histoplasma alters the host-pathogen interaction to its advantage.

Passive administration of monoclonal antibodies against H. capsulatum and other fungal pathogens

The purpose of the use of this methodology is 1) to advance our capacity to protect individuals with antibody or vaccine for preventing or treating histoplasmosis caused by the fungus Histoplasma capsulatum and 2) to examine the role of virulence factors as target for therapy. To generate mAbs, mice are immunized, the immune responses are assessed using a solid phase ELISA system developed in our laboratory, and the best responder mice are selected for isolation of splenocytes for fusion with hybridoma cells. C57BL/6 mice have been extensively used to study H. capsulatum pathogenesis and provide the best model for obtaining the data required. In order to assess the role of the mAbs in infection, mice are intraperitoneally administered with either mAb to H. capsulatum or isotype matched control mAb and then infected by either intravenous (i.v.), intraperitoneal (i.p.), or intranasal (i.n.) routes. In the scientific literature, efficacy of mAbs for fungal infections in mice relies on mortality as an end point, in conjunction with colony forming units (CFU) assessments at earlier time points. Survival (time to death) studies are necessary as they best represent human disease. Thus, efficacy of our intervention would not adequately be established without survival curves. This is also true for establishing efficacy of vaccine or testing of mutants for virulence. With histoplasmosis, the mice often go from being energetic to dead over several hours. The capacity of an intervention such as the administration of a mAb may initially protect an animal from disease, but the disease can relapse which would not be realized in short CFU experiments. In addition to survival and fungal burden assays, we examine the inflammatory responses to infection (histology, cellular recruitment, cytokine responses). For survival/time to death experiments, the mice are infected and monitored at least twice daily for signs of morbidity. To assess fungal burden, histopathology, and cytokine responses, the mice are euthanized at various times after infection. Animal experiments are performed according to the guidelines of the Institute for Animal Studies of the Albert Einstein College of Medicine.

Histoplasma capsulatum heat-shock 60 orchestrates the adaptation of the fungus to temperature stress

Heat shock proteins (Hsps) are among the most widely distributed and evolutionary conserved proteins. Hsps are essential regulators of diverse constitutive metabolic processes and are markedly upregulated during stress. A 62 kDa Hsp (Hsp60) of Histoplasma capsulatum (Hc) is an immunodominant antigen and the major surface ligand to CR3 receptors on macrophages. However little is known about the function of this protein within the fungus. We characterized Hc Hsp60-protein interactions under different temperature to gain insights of its additional functions oncell wall dynamism, heat stress and pathogenesis. We conducted co-immunoprecipitations with antibodies to Hc Hsp60 using cytoplasmic and cell wall extracts. Interacting proteins were identified by shotgun proteomics. For the cell wall, 84 common interactions were identified among the 3 growth conditions, including proteins involved in heat-shock response, sugar and amino acid/protein metabolism and cell signaling. Unique interactions were found at each temperature [30°C (81 proteins), 37°C (14) and 37/40°C (47)]. There were fewer unique interactions in cytoplasm [30°C (6), 37°C (25) and 37/40°C (39)] and four common interactions, including additional Hsps and other known virulence factors. These results show the complexity of Hsp60 function and provide insights into Hc biology, which may lead to new avenues for the management of histoplasmosis.

Agglutination of Histoplasma capsulatum by IgG monoclonal antibodies against Hsp60 impacts macrophage effector functions

Histoplasma capsulatum can efficiently survive within macrophages, facilitating H. capsulatum translocation from the lung into the lymphatics and bloodstream. We have recently generated monoclonal antibodies (MAbs) to an H. capsulatum surface-expressed heat shock protein of 60 kDa (Hsp60) that modify disease in a murine histoplasmosis model. Interestingly, the MAbs induced different degrees of yeast cell agglutination in vitro. In the present study, we characterized the agglutination effects of the antibodies to Hsp60 on H. capsulatum yeast cells by light microscopy, flow cytometry, dynamic light scattering, measuring zeta potential, and using optical tweezers. We found that immunoglobulin Gs (IgGs) to Hsp60 cause H. capsulatum aggregation dependent on the (i) concentration of MAbs, (ii) MAb binding constant, and (iii) IgG subclass. Furthermore, infection of macrophages using agglutinates of various sizes after incubation with different Hsp60-binding MAbs induced association to macrophages through distinct cellular receptors and differentially affected macrophage antifungal functions. Hence, the capacity of IgG MAbs to agglutinate H. capsulatum significantly impacted pathogenic mechanisms of H. capsulatum during macrophage infection, and the effect was dependent on the antibody subclass and antigen epitope.

Evaluation of an enzyme-linked immunosorbent assay using purified, deglycosylated histoplasmin for different clinical manifestations of histoplasmosis

Diagnosis of invasive fungal diseases remains problematic, especially in undeveloped countries. We have developed an enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to Histoplasma capsulatum using metaperiodate treated purified histoplasmin (ptHMIN). Our ELISA was validated comparing sera from patients with histoplasmosis, related mycoses, and healthy individuals. The overall test specificity was 96%, with sensitivities of 100% (8/8) in acute disease, 90% (9/10) in chronic disease, 89% (8/9) in disseminated infection in individuals without HIV infection, 86% (12/14) in disseminated disease in the setting of HIV infection and 100% (3/3) in mediastinal histoplasmosis. These parameters are superior to the use of untreated histoplasmin in diagnostic ELISAs. The high specificities, sensitivities, and simplicity of our ELISA support further development of a deglycosylated HMIN ELISA for clinical use and for monitoring the humoral immune response during therapy in patients with chronic and disseminated histoplasmosis.

Automated comparative protein structure modeling with SWISS-MODEL and Swiss-PdbViewer: a historical perspective

SWISS-MODEL pioneered the field of automated modeling as the first protein modeling service on the Internet. In combination with the visualization tool Swiss-PdbViewer, the Internet-based Workspace and the SWISS-MODEL Repository, it provides a fully integrated sequence to structure analysis and modeling platform. This computational environment is made freely available to the scientific community with the aim to hide the computational complexity of structural bioinformatics and encourage bench scientists to make use of the ever-increasing structural information available. Indeed, over the last decade, the availability of structural information has significantly increased for many organisms as a direct consequence of the complementary nature of comparative protein modeling and experimental structure determination. This has a very positive and enabling impact on many different applications in biomedical research as described in this paper.

Homology modeling in drug discovery: current trends and applications

As structural genomics (SG) projects continue to deposit representative 3D structures of proteins, homology modeling methods will play an increasing role in structure-based drug discovery. Although computational structure prediction methods provide a cost-effective alternative in the absence of experimental structures, developing accurate enough models still remains a big challenge. In this contribution, we report the current developments in this field, discuss in silico modeling limitations, and review the successful application of this technique to different stages of the drug discovery process.

Histoplasma capsulatum var. duboisii infection in a patient with AIDS: rapid diagnosis using polymerase chain reaction-sequencing

We describe an original case of disseminated infection with Histoplasma capsulatum (Hc) var. duboisii in an African patient with AIDS who migrated to Switzerland. The diagnosis of histoplasmosis was suggested using direct examination of tissues and confirmed in 24 h with a panfungal polymerase chain reaction assay. The variety duboisii of Hc was established using DNA sequencing of the polymorphic genomic region OLE. Molecular tools allow diagnosis of histoplasmosis in 24 h, which is drastically shorter than culture procedures.

Monoclonal antibodies to heat shock protein 60 alter the pathogenesis of Histoplasma capsulatum

Heat shock proteins with molecular masses of approximately 60 kDa (Hsp60) are widely distributed in nature and are highly conserved immunogenic molecules that can function as molecular chaperones and enhance cellular survival under physiological stress conditions. The fungus Histoplasma capsulatum displays an Hsp60 on its cell surface that is a key target of the cellular immune response during histoplasmosis, and immunization with this protein is protective. However, the role of humoral responses to Hsp60 has not been fully elucidated. We generated immunoglobulin G (IgG) isotype monoclonal antibodies (MAbs) to H. capsulatum Hsp60. IgG1 and IgG2a MAbs significantly prolonged the survival of mice infected with H. capsulatum. An IgG2b MAb was not protective. The protective MAbs reduced intracellular fungal survival and increased phagolysosomal fusion of macrophages in vitro. Histological examination of infected mice showed that protective MAbs reduced the fungal burden and organ damage. Organs of infected animals treated with protective MAbs had significantly increased levels of interleukin-2 (IL-2), IL-12, and tumor necrosis factor alpha and decreased levels of IL-4 and IL-10. Hence, IgG1 and IgG2a MAbs to Hsp60 can modify H. capsulatum pathogenesis in part by altering the intracellular fate of the fungus and inducing the production of Th1-associated cytokines.