Cryptococcus neoformans: historical curiosity to modern pathogen

Environmental Factors That Contribute to the Maintenance of Cryptococcus neoformans Pathogenesis

The ability of microorganisms to colonise and display an intracellular lifestyle within a host body increases their fitness to survive and avoid extinction. This host–pathogen association drives microbial evolution, as such organisms are under selective pressure and can become more pathogenic. Some of these microorganisms can quickly spread through the environment via transmission. The non-transmittable fungal pathogens, such as Cryptococcus, probably return into the environment upon decomposition of the infected host. This review analyses whether re-entry of the pathogen into the environment causes restoration of its non-pathogenic state or whether environmental factors and parameters assist them in maintaining pathogenesis. Cryptococcus (C.) neoformans is therefore used as a model organism to evaluate the impact of environmental stress factors that aid the survival and pathogenesis of C. neoformans intracellularly and extracellularly.

Invasive Pulmonary Cryptococcal Infection Masquerading as Lung Cancer with Brain Metastases: A Case Report

Cryptococcosis, a global disease problem, seen frequently in the immuno-suppressed, also affects patients without apparent immuno-suppression. Pulmonary cryptococcosis patients often present as cryptococcal pneumonia, whereas intracranial cryptococcosis presents with meningitis. We present a 33-year-old immunocompetent man, diagnosed with invasive pulmonary cryptococcal disease with spread to the brain. This case is unique because the patient was previously treated for tuberculosis and presented with typical bronchopulmonary thoracic, extra-thoracic as well as computed tomography (CT) scan features suggestive of lung cancer. Cryptococcosis was diagnosed by identification of oval thick-walled yeast on histology of lung biopsy specimen. The patient was treated with flucytosine and fluconazole initially and subsequently with Amphotericin B and fluconazole. He made clinical improvement with the resolution of symptoms but had residual radiological features. Invasive cryptococcosis affecting the lung and brain may present with a clinical picture similar to metastatic lung cancer. We recommend routine fungal stains and fungal culture in suspected cases.

Overcoming the Brain Barriers: From Immune Cells to Nanoparticles

Nanoparticulate carriers, often referred to as nanoparticles (NPs), represent an important pharmacological advance for drug protection and tissue-specific drug delivery. Accessing the central nervous system (CNS), however, is a complex process regulated by mainly three brain barriers. While some leukocyte (i.e., immune cell) subsets are equipped with the adequate molecular machinery to infiltrate the CNS in physiological and/or pathological contexts, the successful delivery of NPs into the CNS remains hindered by the tightness of the brain barriers. Here, we present an overview of the three major brain barriers and the mechanisms allowing leukocytes to migrate across each of them. We subsequently review different immune-inspired and -mediated strategies to deliver NPs into the CNS. Finally, we discuss the prospect of exploiting leukocyte trafficking mechanisms for further progress.

Diversity, Virulence Factors, and Antifungal Susceptibility Patterns of Pathogenic and Opportunistic Yeast Species in Rock Pigeon (Columba livia) Fecal Droppings in Western Saudi Arabia

Bird fecal matter is considered a potential source of pathogenic microbes such as yeast species that contaminate the environment. Therefore, it needs to be scrutinized to assess potential environmental health risks. The aim of this study was to investigate the diversity of the yeasts in pigeon fecal droppings, their antifungal susceptibility patterns, and virulence factors. We used culturing techniques to detect the yeasts in pigeon fecal droppings. The isolates were then characterized based on colony morphologies, microscopic examinations, and biochemical reactions. The molecular identification of all yeast isolates was performed by sequencing of the amplified ITS gene. Genes encoding virulence factors CAP1, CAP59, and PLB were also detected. Antifungal susceptibility patterns were examined by the disk diffusion method. A total of 46 yeast-like isolates were recovered, and they belonged to nine different genera, namely, Cryptococcus, Saccharomyces, Rhodotorula, Candida, Meyerozyma, Cyberlindnera, Rhodosporidium, Millerozyma, and Lodderomyces. The prevalence of two genera Cryptococcus and Rhodotorula was high. None of the yeast isolates exhibited any resistance to the antifungal drugs tested; however, all pathogenic Cryptococcus species were positive for virulence determinants like urease activity, growth at 37°C, melanin production, the PLB and CAP genes. This is the first report on the molecular diversity of yeast species, particularly, Cryptococcus species and their virulence attributes in pigeon fecal droppings in Saudi Arabia.

Bird fecal matter is considered a potential source of pathogenic microbes such as yeast species that contaminate the environment. Therefore, it needs to be scrutinized to assess potential environmental health risks. The aim of this study was to investigate the diversity of the yeasts in pigeon fecal droppings, their antifungal susceptibility patterns, and virulence factors. We used culturing techniques to detect the yeasts in pigeon fecal droppings. The isolates were then characterized based on colony morphologies, microscopic examinations, and biochemical reactions. The molecular identification of all yeast isolates was performed by sequencing of the amplified ITS gene. Genes encoding virulence factors CAP1, CAP59, and PLB were also detected. Antifungal susceptibility patterns were examined by the disk diffusion method. A total of 46 yeast-like isolates were recovered, and they belonged to nine different genera, namely, Cryptococcus, Saccharomyces, Rhodotorula, Candida, Meyerozyma, Cyberlindnera, Rhodosporidium, Millerozyma, and Lodderomyces. The prevalence of two genera Cryptococcus and Rhodotorula was high. None of the yeast isolates exhibited any resistance to the antifungal drugs tested; however, all pathogenic Cryptococcus species were positive for virulence determinants like urease activity, growth at 37°C, melanin production, the PLB and CAP genes. This is the first report on the molecular diversity of yeast species, particularly, Cryptococcus species and their virulence attributes in pigeon fecal droppings in Saudi Arabia.

Fungal evolution: major ecological adaptations and evolutionary transitions

Fungi are a highly diverse group of heterotrophic eukaryotes characterized by the absence of phagotrophy and the presence of a chitinous cell wall. While unicellular fungi are far from rare, part of the evolutionary success of the group resides in their ability to grow indefinitely as a cylindrical multinucleated cell (hypha). Armed with these morphological traits and with an extremely high metabolical diversity, fungi have conquered numerous ecological niches and have shaped a whole world of interactions with other living organisms. Herein we survey the main evolutionary and ecological processes that have guided fungal diversity. We will first review the ecology and evolution of the zoosporic lineages and the process of terrestrialization, as one of the major evolutionary transitions in this kingdom. Several plausible scenarios have been proposed for fungal terrestralization and we here propose a new scenario, which considers icy environments as a transitory niche between water and emerged land. We then focus on exploring the main ecological relationships of Fungi with other organisms (other fungi, protozoans, animals and plants), as well as the origin of adaptations to certain specialized ecological niches within the group (lichens, black fungi and yeasts). Throughout this review we use an evolutionary and comparative-genomics perspective to understand fungal ecological diversity. Finally, we highlight the importance of genome-enabled inferences to envision plausible narratives and scenarios for important transitions.

Chitosan-Based Drug Delivery Systems for Optimization of Photodynamic Therapy: a Review

Drug delivery systems (DDS) can be designed to enrich the pharmacological and therapeutic properties of several drugs. Many of the initial obstacles that impeded the clinical applications of conventional DDS have been overcome with nanotechnology-based DDS, especially those formed by chitosan (CS). CS is a linear polysaccharide obtained by the deacetylation of chitin, which has potential properties such as biocompatibility, hydrophilicity, biodegradability, non-toxicity, high bioavailability, simplicity of modification, aqueous solubility, and excellent chemical resistance. Furthermore, CS can prepare several DDS as films, gels, nanoparticles, and microparticles to improve delivery of drugs, such as photosensitizers (PS). Thus, CS-based DDS are broadly investigated for photodynamic therapy (PDT) of cancer and fungal and bacterial diseases. In PDT, a PS is activated by light of a specific wavelength, which provokes selective damage to the target tissue and its surrounding vasculature, but most PS have low water solubility and cutaneous photosensitivity impairing the clinical use of PDT. Based on this, the application of nanotechnology using chitosan-based DDS in PDT may offer great possibilities in the treatment of diseases. Therefore, this review presents numerous applications of chitosan-based DDS in order to improve the PDT for cancer and fungal and bacterial diseases.

Toward a clinical antifungal peptoid: Investigations into the therapeutic potential of AEC5

Cryptococcus neoformans is a fungal pathogen that causes cryptococcal meningitis in immunocompromised individuals. Existing antifungal treatment plans have high mammalian toxicity and increasing drug resistance, demonstrating the dire need for new, nontoxic therapeutics. Antimicrobial peptoids are one alternative to combat this issue. Our lab has recently identified a tripeptoid, AEC5, with promising efficacy and selectivity against C. neoformans. Here, we report studies into the broad-spectrum efficacy, killing kinetics, mechanism of action, in vivo half-life, and subchronic toxicity of this compound. Most notably, these studies have demonstrated that AEC5 rapidly reduces fungal burden, killing all viable fungi within 3 hours. Additionally, AEC5 has an in vivo half-life of 20+ hours and no observable in vivo toxicity following 28 days of daily injections. This research represents an important step in the characterization of AEC5 as a practical treatment option against C. neoformans infections.

An In Vitro Brain Endothelial Model for Studies of Cryptococcal Transmigration into the Central Nervous System

Cryptococcus neoformans is an environmental yeast found worldwide that causes lethal brain infections, particularly in immunocompromised hosts. In 2016, there were 280,000 cases of cryptococcal meningitis in the HIV+ population, two-thirds of them fatal; other immunocompromised patients are also affected. The burden of cryptococcal disease and the limits of current chemotherapy create a pressing need for improved treatment. One hindrance to the development of new therapies is lack of understanding of how this pathogen breaches the barriers protecting the brain. Here we describe a tool for investigating this process. This simple in vitro blood-brain-barrier (BBB) model, based on a human brain endothelial cell line grown on a permeable membrane, may be used to assay the BBB transmigration of C. neoformans or other neurotropic pathogens. © 2019 by John Wiley & Sons, Inc.

Spatio-temporal regulation of nuclear division by Aurora B kinase Ipl1 in Cryptococcus neoformans

The nuclear division takes place in the daughter cell in the basidiomycetous budding yeast Cryptococcus neoformans. Unclustered kinetochores gradually cluster and the nucleus moves to the daughter bud as cells enter mitosis. Here, we show that the evolutionarily conserved Aurora B kinase Ipl1 localizes to the nucleus upon the breakdown of the nuclear envelope during mitosis in C. neoformans. Ipl1 is shown to be required for timely breakdown of the nuclear envelope as well. Ipl1 is essential for viability and regulates structural integrity of microtubules. The compromised stability of cytoplasmic microtubules upon Ipl1 depletion results in a significant delay in kinetochore clustering and nuclear migration. By generating an in silico model of mitosis, we previously proposed that cytoplasmic microtubules and cortical dyneins promote atypical nuclear division in C. neoformans. Improving the previous in silico model by introducing additional parameters, here we predict that an effective cortical bias generated by cytosolic Bim1 and dynein regulates dynamics of kinetochore clustering and nuclear migration. Indeed, in vivo alterations of Bim1 or dynein cellular levels delay nuclear migration. Results from in silico model and localization dynamics by live cell imaging suggests that Ipl1 spatio-temporally influences Bim1 or/and dynein activity along with microtubule stability to ensure timely onset of nuclear division. Together, we propose that the timely breakdown of the nuclear envelope by Ipl1 allows its own nuclear entry that helps in spatio-temporal regulation of nuclear division during semi-open mitosis in C. neoformans.

Unlike the model ascomycetous budding yeast Saccharomyces cerevisiae, microtubule organizing centers (MTOCs) coalesce to form the spindle pole body (SPB) in C. neoformans. This process also ensures unclustered kinetochores to gradually cluster in this organism. As C. neoformans cells enter mitosis, the nuclear envelope ruptures and the nucleus eventually moves to the daughter bud before division. Here, we combine cell and systems biology techniques to understand the key determinants of nuclear division in C. neoformans. We show that the evolutionarily conserved Aurora B kinase Ipl1 enters the nucleus during the mitotic phase as cells undergo semi-open mitosis. Ipl1 regulates dynamics of cytoplasmic microtubules, cytosolic proteins such as Bim1 and dynein-mediated cortical forces and integrity of the nuclear envelope to ensure timely kinetochore clustering and nuclear division in this medically relevant human pathogenic budding yeast.

Silkworm as an experimental animal for research on fungal infections

Silkworm, Bombyx mori, has various advantages as an experimental animal, such as the low cost for rearing and fewer ethical problems. Models utilizing silkworms of infection with pathogenic bacteria have been established for identification of genes encoding virulence factors by large-scale in vivo screening. In this review, we describe recent progress in the study of silkworm infection models for elucidating the mechanisms of fungi infection. Silkworm infection models have been established for Candida albicans, Candida tropicalis, Candida glabrata and Cryptococcus neoformans, which are yeast type fungi, and Aspergillus fumigatus, Arthroderma vanbreuseghemii, Arthroderma benhamiae, Microsporum canis, Trichophyton rubrum, and Rhizopus oryzae, which are filamentous fungi. Novel genes encoding virulence factors in C. albicans and C. glabrata have been identified by using the silkworm infection models. We here outline the benefits of using silkworm infection models and a strategy for identifying the genes responsible for pathogenicity of microorganisms such as fungi. © 2019 The Authors. Microbiology and Immunology Published by The Societies and John Wiley & Sons Australia, Ltd.

The Diagnostic Challenge of an Infrequent Spectrum of Cryptococcus Infection

Cryptococcal infection results from inhalation of fungal spores and usually is confined to the lungs, but may disseminate systemically. Radiologically, cryptococcal infection has multiple forms of presentation. The diagnosis is usually based on fungal isolation from cultured clinical specimens. Long term antifungal therapy is recommended, but surgical procedures may eventually be necessary when large thoracic symptomatic masses are present. We report a case of a 41-year-old male, immunocompetent, investigating a palpable mass in the left supraclavicular region associated with unintentional weight loss over the last three months. He also reported chest pain in this period. Chest X-ray, ultrasonography, and computed tomography were performed, which diagnosed a mediastinal and left supraclavicular mass, interpreted as lymph node conglomerates of unknown etiology. He also underwent a biopsy of the left supraclavicular mass for etiological determination by histopathology, which confirmed cryptococcosis infection. Although very infrequent, mediastinal cryptococcal infection (simulating masses) is a challenging but important differential diagnosis of benign and malignant lesions, since its treatment is usually clinical.

A Wor1-Like Transcription Factor Is Essential for Virulence of Cryptococcus neoformans

Gti1/Pac2 transcription factors occur exclusively in fungi and their roles vary according to species, including regulating morphological transition and virulence, mating and secondary metabolism. Many of these functions are important for fungal pathogenesis. We therefore hypothesized that one of the two proteins of this family in Cryptococcus neoformans, a major pathogen of humans, would also control virulence-associated cellular processes. Elimination of this protein in C. neoformans results in reduced polysaccharide capsule expression and defective cytokinesis and growth at 37°C. The mutant loses virulence in a mouse model of cryptococcal infection and retains only partial virulence in the Galleria mellonella alternative model at 30°C. We performed RNA-Seq experiments on the mutant and found abolished transcription of genes that, in combination, are known to account for all the observed phenotypes. The protein has been named Required for cytokinesis and virulence 1 (Rcv1).

Functional Characterization of Cryptococcal Genes: Then and Now

Site-directed mutagenesis enables researchers to switch a gene of interest off for functional characterization of the gene. In the pathogenic yeasts, Cryptococcus neoformans and sister species C. deneoformans, this is almost exclusively achieved by introducing DNA into cells through either biolistic transformation or electroporation. The targeted gene is then disrupted by homologous recombination (HR) between the gene and the transforming DNA. Both techniques have downsides; biolistic transformation equipment is very expensive, limiting the use thereof to well-resourced laboratories, and HR occurs at extremely low frequencies in electroporated cryptococcal cells, making this method unappealing for gene targeting when not making use of additional modifications or methods to enhance HR in these cells. One approach to increase the frequency of HR in electroporated cryptococcal cells have recently been described. In this approach, CRISPR-Cas9 technology is utilized to form a double strand break in the targeted gene where after the occurrence of HR seems to be higher. The less expensive electroporation technique can therefore be used to deliver the CRISPR-Cas9 components into cells to disrupt a gene of interest, but only if the CRISPR components can be maintained for long enough in cells to enable their expression. Maintenance of episomal DNA occurs readily in C. deneoformans, but only under certain conditions in C. neoformans. In addition, CRISPR-Cas9 allows for gene complementation in order to fulfill Falkow’s molecular Koch’s postulates and adds other novel methods for studying genes as well, such as the addition of a fluorophore to an inactive Cas9 enzyme to highlight the location of a gene in a chromosome. These developments add less expensive alternatives to current methods, which could lead to more research on this yeast in developing countries where cryptococcal infections are more prevalent and researchers have access to more clinical isolates.

An audit of the screen-and-treat intervention to reduce cryptococcal meningitis in HIV-positive patients with low CD4 count

BACKGROUND

 HIV-associated cryptococcal meningitis (CCM) and related mortality may be prevented by the effective implementation of a screen-and-treat intervention. Aim: The aim of this study was to assess the effectiveness of the screen-and-treat intervention at a regional hospital in KwaZulu-Natal province, South Africa. Method: This was a descriptive study in which the records of patients seen in 2015 and 2016 with a CD4 count ≤ 100 cell/mm3 were retrieved from National Health Laboratory Service (NHLS) records and matched against patients admitted for HIV-associated CCM. Results: A total of 5.1% (190 out of 3702) patients with CD4 count ≤ 100 cell/mm3 were cryptococcal antigen positive (CrAg +ve), of whom 22.6% (43 out of 190) were admitted with CCM. Patients who were CrAg +ve had significantly lower CD4 counts (mean CD4 = 38.9 ± 28.5) when compared to CrAg -ve patients (mean CD4 = 49.9 ± 37.4) with p = 0.0001. Only 2.6% (5 out of 190) of patients were referred for a lumbar puncture (LP) as part of the screen-and-treat intervention, whilst 38 who were CrAg +ve self-presented with CCM. Eighty-eight patients were admitted for suspected CCM: eight because of the screen-and-treat-intervention (none of whom had meningitis based on cerebrospinal fluid results) and 80 of whom self-presented and had confirmed CCM. The overall mortality of patients admitted with CCM was 30% (24 out of 80). Conclusion: The current ad-hoc screen-and-treat intervention was ineffective in detecting patients at risk of developing CCM. Systems need to be put in place to ensure that all CrAg +ve patients have an LP to detect subclinical CCM to improve the outcome for those with HIV-associated CCM.

Peeling the onion: the outer layers of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is ubiquitous in the environment. It causes a deadly meningitis that is responsible for over 180,000 deaths worldwide each year, including 15% of all AIDS-related deaths. The high mortality rates for this infection, even with treatment, suggest a need for improved therapy. Unique characteristics of C. neoformans may suggest directions for drug discovery. These include features of three structures that surround the cell: the plasma membrane, the cell wall around it, and the outermost polysaccharide capsule. We review current knowledge of the fundamental biology of these fascinating structures and highlight open questions in the field, with the goal of stimulating further investigation that will advance basic knowledge and human health.

[A Silkworm Infection Model to Evaluate Antifungal Drugs for Cryptococcosis]

The development of effective drugs against fungal diseases involves performing infection experiments in animals to evaluate candidate therapeutic compounds. Cryptococcus neoformans is a pathogenic fungus that causes deep mycosis, resulting in respiratory illness and meningitis. Here we describe a silkworm system established to evaluate the safety and efficacy of therapeutic drugs against infection by Cryptococcus neoformans and the advantages of this system over other animal models. The silkworm assay system has two major advantages: 1) silkworms are less expensive to rear and their use is less problematic than that of mammals in terms of animal welfare, and 2) in vivo screenings for identifying candidate drugs can be easily performed using a large number of silkworms. The pharmacokinetics of compounds are consistent between silkworms and mammals. Moreover, the ED₅₀ values of antibiotics are concordant between mammalian and silkworm infection models. Furthermore, the body size of silkworms makes them easy to handle in experimental procedures compared with other invertebrate infectious experimental systems, and accurate amounts of pathogens and chemicals can be injected fairly easily. These advantages of silkworms as a host animal make them useful for screening candidate drugs for cryptococcosis.

Unintended Side Effects of Transformation Are Very Rare in Cryptococcus neoformans

Received wisdom in the field of fungal biology holds that the process of editing a genome by transformation and homologous recombination is inherently mutagenic. However, that belief is based on circumstantial evidence. We provide the first direct measurement of the effects of transformation on a fungal genome by sequencing the genomes of 29 transformants and 30 untransformed controls with high coverage. Contrary to the received wisdom, our results show that transformation of DNA segments flanked by long targeting sequences, followed by homologous recombination and selection for a drug marker, is extremely safe. If a transformation deletes a gene, that may create selective pressure for a few compensatory mutations, but even when we deleted a gene, we found fewer than two point mutations per deletion strain, on average. We also tested these strains for changes in gene expression and found only a few genes that were consistently differentially expressed between the wild type and strains modified by genomic insertion of a drug resistance marker. As part of our report, we provide the assembled genome sequence of the commonly used laboratory strain Cryptococcus neoformans var. grubii strain KN99α.

Parasitic and fungal infections

Parasitic infections of the central nervous system (CNS) comprise a plethora of infectious agents leading to a multitude of different disease courses and thus diagnostic and therapeutic challenges. The prevalence of different pathogens is basically dependent on geographic and ethnic backgrounds, its infectious route frequently involving a third party, such as flies or domestic animals. The present review focuses on cerebral malaria due to Plasmodium falciparum infection, and Toxoplasma gondii encephalitis. Fungi produce a large variety of inflammatory conditions of the CNS with a variegated spectrum of signs and symptoms, which may involve the meninges and the brain parenchyma, where they produce cerebritis or abscesses and granulomatous lesions, respectively. Fungal CNS lesions are increasingly prevalent and diagnostically relevant due to increasing numbers of human immunodeficiency virus-positive patients, increasing numbers of patients reaching old age suffering from malignant tumors or decreased immunity, and finally the increasing use of established and new immunosuppressive treatments, which increase the susceptibility of patients to develop invasive mycoses. Fungi appear with characteristic morphotypes comprising hyphae, yeasts, and pseudohyphae. The mode by which fungi penetrate into the CNS, and the host/immune requirements are incompletely understood and remain a challenge for research.

Overview of selected virulence attributes in Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton rubrum, and Exophiala dermatitidis

The incidence of fungal diseases has been increasing since 1980, and is associated with excessive morbidity and mortality, particularly among immunosuppressed patients. Of the known 625 pathogenic fungal species, infections caused by the genera Aspergillus, Candida, Cryptococcus, and Trichophyton are responsible for more than 300 million estimated episodes of acute or chronic infections worldwide. In addition, a rather neglected group of opportunistic fungi known as black yeasts and their filamentous relatives cause a wide variety of recalcitrant infections in both immunocompetent and immunosuppressed hosts. This article provides an overview of selected virulence factors that are known to suppress host immunity and enhance the infectivity of these fungi.

Glucosamine stimulates pheromone-independent dimorphic transition in Cryptococcus neoformans by promoting Crz1 nuclear translocation

Morphotype switch is a cellular response to external and internal cues. The Cryptococcus neoformans species complex can undergo morphological transitions between the yeast and the hypha form, and such morphological changes profoundly affect cryptococcal interaction with various hosts. Filamentation in Cryptococcus was historically considered a mating response towards pheromone. Recent studies indicate the existence of pheromone-independent signaling pathways but their identity or the effectors remain unknown. Here, we demonstrated that glucosamine stimulated the C. neoformans species complex to undergo self-filamentation. Glucosamine-stimulated filamentation was independent of the key components of the pheromone pathway, which is distinct from pheromone-elicited filamentation. Glucosamine stimulated self-filamentation in H99, a highly virulent serotype A clinical isolate and a widely used reference strain. Through a genetic screen of the deletion sets made in the H99 background, we found that Crz1, a transcription factor downstream of calcineurin, was essential for glucosamine-stimulated filamentation despite its dispensability for pheromone-mediated filamentation. Glucosamine promoted Crz1 translocation from the cytoplasm to the nucleus. Interestingly, multiple components of the high osmolality glycerol response (HOG) pathway, consisting of the phosphorelay system and some of the Hog1 MAPK module, acted as repressors of glucosamine-elicited filamentation through their calcineurin-opposing effect on Crz1’s nuclear translocation. Surprisingly, glucosamine-stimulated filamentation did not require Hog1 itself and was distinct from the conventional general stress response. The results demonstrate that Cryptococcus can resort to multiple genetic pathways for morphological transition in response to different stimuli. Given that the filamentous form attenuates cryptococcal virulence and is immune-stimulatory in mammalian models, the findings suggest that morphogenesis is a fertile ground for future investigation into novel means to compromise cryptococcal pathogenesis.

Cryptococcal meningitis claims half a million lives each year. There is no clinically available vaccine and the current antifungal therapies have serious limitations. Thus identifying cryptococcal specific programs that can be targeted for antifungal or vaccine development is of great value. We have shown previously that switching from the yeast to the hypha form drastically attenuates/abolishes cryptococcal virulence. Cryptococcal cells in the filamentous form also trigger host immune responses that can protect the host from a subsequent lethal challenge. However, self-filamentation is rarely observed in serotype A isolates that are responsible for the vast majority of cryptococcosis cases. In this study, we found that glucosamine stimulated self-filamentation in genetically distinct strains of the Cryptococcus species complex, including the most commonly used serotype A reference strain H99. We demonstrated that filamentation elicited by glucosamine did not depend on the pheromone pathway, but it requires the calcineurin transcription factor Crz1. Glucosamine promotes nuclear translocation of Crz1, which is positively controlled by the phosphatase calcineurin and is suppressed by the HOG pathway. These findings raise the possibility of manipulating genetic pathways controlling fungal morphogenesis against diseases caused by the Cryptococcus species complex.

Fungal Biofilms: Targets for the Development of Novel Strategies in Plant Disease Management

The global food supply has been facing increasing challenges during the first decades of the 21st century. Disease in plants is an important constraint to worldwide crop production, accounting for 20-40% of its annual harvest loss. Although the use of resistant varieties, good water management and agronomic practices are valid management tools in counteracting plant diseases, there are still many pathosystems where fungicides are widely used for disease management. However, restrictive regulations and increasing concern regarding the risk to human health and the environment, along with the incidence of fungicide resistance, have discouraged their use and have prompted for a search for new efficient, ecologically friendly and sustainable disease management strategies. The recent evidence of biofilm formation by fungal phytopathogens provides the scientific framework for designing and adapting methods and concepts developed by biofilm research that could be integrated in IPM practices. In this perspective paper, we provide evidence to support the view that the biofilm lifestyle plays a critical role in the pathogenesis of plant diseases. We describe the main factors limiting the durability of single-site fungicides, and we assemble the current knowledge on pesticide resistance in the specific context of the biofilm lifestyle. Finally, we illustrate the potential of antibiofilm compounds at sub-lethal concentrations for the development of an innovative, eco-sustainable strategy to counteract phytopathogenic fungi. Such fungicide-free solutions will be instrumental in reducing disease severity, and will permit more prudent use of fungicides decreasing thus the selection of resistant forms and safeguarding the environment.

Trojan Horse Transit Contributes to Blood-Brain Barrier Crossing of a Eukaryotic Pathogen

The blood-brain barrier (BBB) protects the central nervous system (CNS) by restricting the passage of molecules and microorganisms. Despite this barrier, however, the fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that is estimated to kill over 600,000 people annually. Cryptococcal infection begins in the lung, and experimental evidence suggests that host phagocytes play a role in subsequent dissemination, although this role remains ill defined. Additionally, the disparate experimental approaches that have been used to probe various potential routes of BBB transit make it impossible to assess their relative contributions, confounding any integrated understanding of cryptococcal brain entry. Here we used an in vitro model BBB to show that a “Trojan horse” mechanism contributes significantly to fungal barrier crossing and that host factors regulate this process independently of free fungal transit. We also, for the first time, directly imaged C. neoformans-containing phagocytes crossing the BBB, showing that they do so via transendothelial pores. Finally, we found that Trojan horse crossing enables CNS entry of fungal mutants that cannot otherwise traverse the BBB, and we demonstrate additional intercellular interactions that may contribute to brain entry. Our work elucidates the mechanism of cryptococcal brain invasion and offers approaches to study other neuropathogens.

The fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that kills hundreds of thousands of people each year. One route that has been proposed for this brain entry is a Trojan horse mechanism, whereby the fungus crosses the blood-brain barrier (BBB) as a passenger inside host phagocytes. Although indirect experimental evidence supports this intriguing mechanism, it has never been directly visualized. Here we directly image Trojan horse transit and show that it is regulated independently of free fungal entry, contributes to cryptococcal BBB crossing, and allows mutant fungi that cannot enter alone to invade the brain.

The role of macrophages in the susceptibility of Fc gamma receptor IIb deficient mice to Cryptococcus neoformans

Dysfunctional polymorphisms of FcγRIIb, an inhibitory receptor, are associated with Systemic Lupus Erythaematosus (SLE). Cryptococcosis is an invasive fungal infection in SLE, perhaps due to the de novo immune defect. We investigated cryptococcosis in the FcγRIIb-/- mouse-lupus-model. Mortality, after intravenous C. neoformans-induced cryptococcosis, in young (8-week-old) and older (24-week-old) FcγRIIb-/- mice, was higher than in age-matched wild-types. Severe cryptococcosis in the FcγRIIb-/- mice was demonstrated by high fungal burdens in the internal organs with histological cryptococcoma-like lesions and high levels of TNF-α and IL-6, but not IL-10. Interestingly, FcγRIIb-/- macrophages demonstrated more prominent phagocytosis but did not differ in killing activity in vitro and the striking TNF-α, IL-6 and IL-10 levels, compared to wild-type cells. Indeed, in vivo macrophage depletion with liposomal clodronate attenuated the fungal burdens in FcγRIIb-/- mice, but not wild-type mice. When administered to wild-type mice, FcγRIIb-/- macrophages with phagocytosed Cryptococcus resulted in higher fungal burdens than FcγRIIb+/+ macrophages with phagocytosed Cryptococcus. These results support, at least in part, a model whereby, in FcγRIIb-/- mice, enhanced C. neoformans transmigration occurs through infected macrophages. In summary, prominent phagocytosis, with limited effective killing activity, and high pro-inflammatory cytokine production by FcγRIIb-/- macrophages were correlated with more severe cryptococcosis in FcγRIIb-/- mice.

Discovery and Characterization of a Peptoid with Antifungal Activity against Cryptococcus neoformans

Studies show there is an increasing rate of fungal infections, especially in immunocompromised patients and treatments for fungal genera, such as Aspergillus, Candida, and Cryptococcus, carry significant cytotoxicity with an increasing prevalence of antifungal resistance. We have previously reported a high-throughput assay for identifying peptoids with antimicrobial properties from combinatorial libraries. Here we report the application of this assay in identifying a peptoid with antifungal properties against Cryptococcus neoformans. Termed AEC5, this peptoid has comparable potency to existing clinical antifungal agents, excellent stability, and minimal cytotoxicity in mammalian cells.

Amino Acid Permeases and Virulence in Cryptococcus neoformans

Fungal opportunistic pathogens colonize various environments, from plants and wood to human and animal tissue. Regarding human pathogens, one great challenge during contrasting niche occupation is the adaptation to different conditions, such as temperature, osmolarity, salinity, pressure, oxidative stress and nutritional availability, which may constitute sources of stress that need to be tolerated and overcome. As an opportunistic pathogen, C. neoformans faces exactly these situations during the transition from the environment to the human host, encountering nutritional constraints. Our previous and current research on amino acid biosynthetic pathways indicates that amino acid permeases are regulated by the presence of the amino acids, nitrogen and temperature. Saccharomyces cerevisiae and Candida albicans have twenty-four and twenty-seven genes encoding amino acid permeases, respectively; conversely, they are scarce in number in Basidiomycetes (C. neoformans, Coprinopsis cinerea and Ustilago maydis), where nine to ten permease genes can be found depending on the species. In this study, we have demonstrated that two amino acid permeases are essential for virulence in C. neoformans. Our data showed that C. neoformans uses two global and redundant amino acid permeases, Aap4 and Aap5 to respond correctly to thermal and oxidative stress. Double deletion of these permeases causes growth arrest in C. neoformans at 37°C and in the presence of hydrogen peroxide. The inability to uptake amino acid at a higher temperature and under oxidative stress also led to virulence attenuation in vivo. Our data showed that thermosensitivity caused by the lack of permeases Aap4 and Aap5 can be remedied by alkaline conditions (higher pH) and salinity. Permeases Aap4 and Aap5 are also required during fluconazole stress and they are the target of the plant secondary metabolite eugenol, a potent antifungal inhibitor that targets amino acid permeases. In summary, our work unravels (i) interesting physiological property of C. neoformans regarding its amino acid uptake system; (ii) an important aspect of virulence, which is the need for amino acid permeases during thermal and oxidative stress resistance and, hence, host invasion and colonization; and (iii) provides a convenient prototype for antifungal development, which are the amino acid permeases Aap4/Aap5 and their inhibitor.

Geographic distribution of patients affected by Cryptococcus neoformans/Cryptococcus gattii species complexes meningitis, pigeon and tree populations in Southern Brazil

Cryptococcal meningitis is mainly caused by members of the C. neoformans/C. gattii species complexes. The ecological niches of Cryptococcus species have extensively been studied, but its epidemiological relationship with meningitis cases is still unknown. In this study, we estimate the relationship between cryptococcal meningitis cases and tree and pigeon populations, the classical niches of members of C. neoformans/C. gattii sensu lato. We analysed the records of every patient whose cerebrospinal fluid culture yielded Cryptococcus spp. during the last 30 years at Clinical Hospital of Curitiba. Data about Curitiba's pigeon and tree distribution were obtained from Curitiba's Secretaries of Zoonosis and Environment archives. We used ArcGis9 software to plot the distribution of the pigeon and tree populations in this city as well as cryptococcal meningitis cases, distinguishing them according to the causal agent in C. neoformans or C. gattii s.l. In total, 489 cryptococcal cultures were documented, with 140 corresponding to patients eligible for this study (134 affected by C. neoformans s.l. and 6 by C. gattii s.l.). The map showed a relationship between C. neoformans s.l. patients and pigeon population. C. gattii s.l. patients were associated with neither tree nor pigeon populations, but lived close to large unbuilt, unforested areas.

Computational Analysis Reveals a Key Regulator of Cryptococcal Virulence and Determinant of Host Response

Cryptococcus neoformans is a ubiquitous, opportunistic fungal pathogen that kills over 600,000 people annually. Here, we report integrated computational and experimental investigations of the role and mechanisms of transcriptional regulation in cryptococcal infection. Major cryptococcal virulence traits include melanin production and the development of a large polysaccharide capsule upon host entry; shed capsule polysaccharides also impair host defenses. We found that both transcription and translation are required for capsule growth and that Usv101 is a master regulator of pathogenesis, regulating melanin production, capsule growth, and capsule shedding. It does this by directly regulating genes encoding glycoactive enzymes and genes encoding three other transcription factors that are essential for capsule growth: GAT201, RIM101, and SP1. Murine infection with cryptococci lacking Usv101 significantly alters the kinetics and pathogenesis of disease, with extended survival and, unexpectedly, death by pneumonia rather than meningitis. Our approaches and findings will inform studies of other pathogenic microbes.

Cryptococcus neoformans causes fatal meningitis in immunocompromised individuals, mainly HIV positive, killing over 600,000 each year. A unique feature of this yeast, which makes it particularly virulent, is its polysaccharide capsule; this structure impedes host efforts to combat infection. Capsule size and structure respond to environmental conditions, such as those encountered in an infected host. We have combined computational and experimental tools to elucidate capsule regulation, which we show primarily occurs at the transcriptional level. We also demonstrate that loss of a novel transcription factor alters virulence factor expression and host cell interactions, changing the lethal condition from meningitis to pneumonia with an exacerbated host response. We further demonstrate the relevant targets of regulation and kinetically map key regulatory and host interactions. Our work elucidates mechanisms of capsule regulation, provides methods and resources to the research community, and demonstrates an altered pathogenic outcome that resembles some human conditions.

Severe cryptococcal-associated neurological immune reconstitution inflammatory syndrome in a renal transplant recipient treated with adalimumab

Cryptococcosis is a major concern in organ transplant recipients. A decrease in immunosuppressants following the initiation of antifungal therapy is currently recommended, but can occasionally be complicated by the onset of immune reconstitution inflammatory syndrome (IRIS). We report on a case of cryptococcosis in a kidney transplant recipient, compounded by severe neurological IRIS, the outcome of which was unfavorable despite the use of anti-tumor necrosis factor-alpha monoclonal antibodies.

All about that fat: Lipid modification of proteins in Cryptococcus neoformans

Lipid modification of proteins is a widespread, essential process whereby fatty acids, cholesterol, isoprenoids, phospholipids, or glycosylphospholipids are attached to polypeptides. These hydrophobic groups may affect protein structure, function, localization, and/or stability; as a consequence such modifications play critical regulatory roles in cellular systems. Recent advances in chemical biology and proteomics have allowed the profiling of modified proteins, enabling dissection of the functional consequences of lipid addition. The enzymes that mediate lipid modification are specific for both the lipid and protein substrates, and are conserved from fungi to humans. In this article we review these enzymes, their substrates, and the processes involved in eukaryotic lipid modification of proteins. We further focus on its occurrence in the fungal pathogen Cryptococcus neoformans, highlighting unique features that are both relevant for the biology of the organism and potentially important in the search for new therapies.

The Zinc Finger Protein Mig1 Regulates Mitochondrial Function and Azole Drug Susceptibility in the Pathogenic Fungus Cryptococcus neoformans

Fungal pathogens of humans are difficult to treat, and there is a pressing need to identify new targets for antifungal drugs and to obtain a detailed understanding of fungal proliferation in vertebrate hosts. In this study, we examined the roles of the regulatory proteins Mig1 and HapX in mitochondrial function and antifungal drug susceptibility in the fungus Cryptococcus neoformans. This pathogen is a particular threat to the large population of individuals infected with human immunodeficiency virus (HIV). Our analysis revealed regulatory interactions between Mig1 and HapX, and a role for Mig1 in mitochondrial functions, including respiration, tolerance for reactive oxygen species, and expression of genes for iron consumption and iron acquisition functions. Importantly, loss of Mig1 increased susceptibility to the antifungal drug fluconazole, which is commonly used to treat cryptococcal disease. These studies highlight an association between mitochondrial dysfunction and drug susceptibility that may provide new targets for the development of antifungal drugs.

The opportunistic pathogen Cryptococcus neoformans causes fungal meningoencephalitis in immunocompromised individuals. In previous studies, we found that the Hap complex in this pathogen represses genes encoding mitochondrial respiratory functions and tricarboxylic acid (TCA) cycle components under low-iron conditions. The orthologous Hap2/3/4/5 complex in Saccharomyces cerevisiae exerts a regulatory influence on mitochondrial functions, and Hap4 is subject to glucose repression via the carbon catabolite repressor Mig1. In this study, we explored the regulatory link between a candidate ortholog of the Mig1 protein and the HapX component of the Hap complex in C. neoformans. This analysis revealed repression of MIG1 by HapX and activation of HAPX by Mig1 under low-iron conditions and Mig1 regulation of mitochondrial functions, including respiration, tolerance for reactive oxygen species, and expression of genes for iron consumption and iron acquisition functions. Consistently with these regulatory functions, a mig1Δ mutant had impaired growth on inhibitors of mitochondrial respiration and inducers of ROS. Furthermore, deletion of MIG1 provoked a dysregulation in nutrient sensing via the TOR pathway and impacted the pathway for cell wall remodeling. Importantly, loss of Mig1 increased susceptibility to fluconazole, thus further establishing a link between azole antifungal drugs and mitochondrial function. Mig1 and HapX were also required together for survival in macrophages, but Mig1 alone had a minimal impact on virulence in mice. Overall, these studies provide novel insights into a HapX/Mig1 regulatory network and reinforce an association between mitochondrial dysfunction and drug susceptibility that may provide new targets for the development of antifungal drugs.

IMPORTANCE Fungal pathogens of humans are difficult to treat, and there is a pressing need to identify new targets for antifungal drugs and to obtain a detailed understanding of fungal proliferation in vertebrate hosts. In this study, we examined the roles of the regulatory proteins Mig1 and HapX in mitochondrial function and antifungal drug susceptibility in the fungus Cryptococcus neoformans. This pathogen is a particular threat to the large population of individuals infected with human immunodeficiency virus (HIV). Our analysis revealed regulatory interactions between Mig1 and HapX, and a role for Mig1 in mitochondrial functions, including respiration, tolerance for reactive oxygen species, and expression of genes for iron consumption and iron acquisition functions. Importantly, loss of Mig1 increased susceptibility to the antifungal drug fluconazole, which is commonly used to treat cryptococcal disease. These studies highlight an association between mitochondrial dysfunction and drug susceptibility that may provide new targets for the development of antifungal drugs.

Validation and clinical application of a molecular method for the identification of Cryptococcus neoformans/Cryptococcus gattii complex DNA in human clinical specimens

The diagnosis of cryptococcosis is usually performed based on cultures of tissue or body fluids and isolation of the fungus, but this method may require several days. Direct microscopic examination, although rapid, is relatively insensitive. Biochemical and immunodiagnostic rapid tests are also used. However, all of these methods have limitations that may hinder final diagnosis. The increasing incidence of fungal infections has focused attention on tools for rapid and accurate diagnosis using molecular biological techniques. Currently, PCR-based methods, particularly nested, multiplex and real-time PCR, provide both high sensitivity and specificity.

In the present study, we evaluated a nested PCR targeting the gene encoding the ITS-1 and ITS-2 regions of rDNA in samples from a cohort of patients diagnosed with cryptococcosis. The results showed that in our hands, this Cryptococcus nested PCR assay has 100% specificity and 100% sensitivity and was able to detect until 2 femtograms of Cryptococcus DNA.

Rising to the challenge of multiple Cryptococcus species and the diseases they cause

Cryptococcus neoformans and Cryptococcus gattii are well-studied basidiomyceteous yeasts that are capable of causing disease in healthy and immunocompromised people. The Conference on Cryptococcus and Cryptococcosis (ICCC) is held every three years: the accompanying Special Issue stems from the 9th ICCC and covers a subset of the topics related to these fungi in detail. This conference started with a revised and reduced estimate of disease burden globally, in part due to improved treatment for HIV(+) people. However, mortality from cryptococcosis remains consistently high for those unfortunate to have limited access to therapies or without underlying immunodeficiencies. As such, there are yet still great distances to be covered to address antifungal drug availability, the need for new antifungal agents and the timing and doses of these agents in conjunction with antiviral therapy, underscoring the importance of continued research. A notable point from the 9th ICCC was the research addressing the variation in the pathogen and host populations. Analysis of cryptococcal strain variability, particularly at the molecular level, has resolved distinct lineages with the consequence of a taxonomic revision that divides C. neoformans and C. gattii into seven Cryptococcus species. Similarly, analysis of host factors in so called "immune-competent" individuals revealed previously unrecognized risk factors. Research on these species has established them as important model organisms to understand gene evolution and function in other fungi and eukaryotes. The stage is set for the refinement of research directions, leading ultimately to better treatment of this monophyletic clade of pathogens in the genus Cryptococcus.

Characterization of Lipids and Proteins Associated to the Cell Wall of the Acapsular Mutant Cryptococcus neoformans Cap 67

Cryptococcus neoformans is an opportunistic human pathogen that causes life-threatening meningitis. In this fungus, the cell wall is exceptionally not the outermost structure due to the presence of a surrounding polysaccharide capsule, which has been highly studied. Considering that there is little information about C. neoformans cell wall composition, we aimed at describing proteins and lipids extractable from this organelle, using as model the acapsular mutant C. neoformans cap 67. Purified cell wall preparations were extracted with either chloroform/methanol or hot sodium dodecyl sulfate. Total lipids fractionated in silica gel 60 were analyzed by electrospray ionization tandem mass spectrometry (ESI-MS/MS), while trypsin digested proteins were analyzed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). We detected 25 phospholipid species among phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and phosphatidic acid. Two glycolipid species were identified as monohexosyl ceramides. We identified 192 noncovalently linked proteins belonging to different metabolic processes. Most proteins were classified as secretory, mainly via nonclassical mechanisms, suggesting a role for extracellular vesicles (EV) in transwall transportation. In concert with that, orthologs from 86% of these proteins have previously been reported both in fungal cell wall and/or in EV. The possible role of the presently described structures in fungal-host relationship is discussed.

Characterization of metabolically quiescent Leishmania parasites in murine lesions using heavy water labeling

Information on the growth rate and metabolism of microbial pathogens that cause long-term chronic infections is limited, reflecting the absence of suitable tools for measuring these parameters in vivo. Here, we have measured the replication and physiological state of Leishmania mexicana parasites in murine inflammatory lesions using ²H₂O labeling. Infected BALB/c mice were labeled with ²H₂O for up to 4 months, and the turnover of parasite DNA, RNA, protein and membrane lipids estimated from the rate of deuterium enrichment in constituent pentose sugars, amino acids, and fatty acids, respectively. We show that the replication rate of parasite stages in these tissues is very slow (doubling time of ~12 days), but remarkably constant throughout lesion development. Lesion parasites also exhibit markedly lower rates of RNA synthesis, protein turnover and membrane lipid synthesis than parasite stages isolated from ex vivo infected macrophages or cultured in vitro, suggesting that formation of lesions induces parasites to enter a semi-quiescent physiological state. Significantly, the determined parasite growth rate accounts for the overall increase in parasite burden indicating that parasite death and turnover of infected host cells in these lesions is minimal. We propose that the Leishmania response to lesion formation is an important adaptive strategy that minimizes macrophage activation, providing a permissive environment that supports progressive expansion of parasite burden. This labeling approach can be used to measure the dynamics of other host-microbe interactions in situ.

Microbial pathogens can adapt to changing conditions in their hosts by switching between different growth and physiological states. However, current methods for measuring microbial physiology in vivo are limited, hampering detailed dissection of host-pathogen interactions. Here we have used heavy water labeling to measure the growth rate and physiological state of Leishmania parasites in murine lesions. Based on the rate of in situ labeling of parasite DNA, RNA, protein, and lipids, we show that the growth rate of intracellular parasite stages is very slow, and that these stages enter a semi-quiescent state characterized by very low rates of RNA, protein, and membrane turnover. These changes in parasite growth and physiology are more pronounced than in in vitro differentiated parasites, suggesting that they are induced in part by the lesion environment. Despite their slow growth, the parasite burden in these lesions progressively increases as a result of low rates of parasite death and host cell turnover. We propose that these changes in Leishmania growth and physiology contribute to the development of a relatively benign tissue environment that is permissive for long term parasite expansion. This approach is suitable for studying the dynamics of other host-pathogen systems.

Model-driven mapping of transcriptional networks reveals the circuitry and dynamics of virulence regulation

Key steps in understanding a biological process include identifying genes that are involved and determining how they are regulated. We developed a novel method for identifying transcription factors (TFs) involved in a specific process and used it to map regulation of the key virulence factor of a deadly fungus—its capsule. The map, built from expression profiles of 41 TF mutants, includes 20 TFs not previously known to regulate virulence attributes. It also reveals a hierarchy comprising executive, midlevel, and “foreman” TFs. When grouped by temporal expression pattern, these TFs explain much of the transcriptional dynamics of capsule induction. Phenotypic analysis of TF deletion mutants revealed complex relationships among virulence factors and virulence in mice. These resources and analyses provide the first integrated, systems-level view of capsule regulation and biosynthesis. Our methods dramatically improve the efficiency with which transcriptional networks can be analyzed, making genomic approaches accessible to laboratories focused on specific physiological processes.