The second capsule gene of cryptococcus neoformans, CAP64, is essential for virulence

Sac1 links phosphoinositide turnover to cryptococcal virulence

Cryptococcus neoformans is an environmentally acquired fungal pathogen that causes over 140,000 deaths per year. Cryptococcal infection occurs when infectious particles are deposited into the lung, where they encounter host phagocytic cells. C. neoformans may be engulfed by these phagocytes, an important step of infection that leads to outcomes ranging from termination of infection to cryptococcal dissemination. To study this critical process, we screened approximately 4,700 cryptococcal gene deletion mutants for altered uptake, using primary mouse and human phagocytic cells. Among the hits of these two screens, we identified 93 mutants with perturbed uptake in both systems, as well as others with differences in uptake by only one cell type. We further screened the hits for changes in thickness of the capsule, a protective polysaccharide layer around the cell which is an important cryptococcal virulence factor. The combination of our three screens yielded 45 mutants, including one lacking the phosphatidylinositol-4-phosphate phosphatase Sac1. In this work, we implicate Sac1 in both host cell uptake and capsule production. We found that sac1 mutants exhibit lipid trafficking defects, reductions in secretory system function, and changes in capsule size and composition. Many of these changes occur specifically in tissue culture media, highlighting the role of Sac1 phosphatase activity in responding to the stress of host-like conditions. Overall, these findings show how genome-scale screening can identify cellular factors that contribute to our understanding of cryptococcal biology and demonstrate the role of Sac1 in determining fungal virulence.IMPORTANCECryptococcus neoformans is a fungal pathogen with significant impact on global health. Cryptococcal cells inhaled from the environment are deposited into the lungs, where they first contact the human immune system. The interaction between C. neoformans and host cells is critical because this step of infection can determine whether the fungal cells die or proliferate within the human host. Despite the importance of this stage of infection, we have limited knowledge of cryptococcal factors that influence its outcome. In this study, we identify cryptococcal genes that affect uptake by both human and mouse cells. We also identify mutants with altered capsule, a protective coating that surrounds the cells to shield them from the host immune system. Finally, we characterize the role of one gene, SAC1, in these processes. Overall, this study contributes to our understanding of how C. neoformans interacts with and protects itself from host cells.

Sac1 links phosphoinositide turnover to cryptococcal virulence

Cryptococcus neoformans is an environmentally-acquired fungal pathogen that causes over 140,000 deaths per year. Cryptococcal infection occurs when infectious particles are deposited into the lung, where they encounter host phagocytic cells. C. neoformans may be engulfed by these phagocytes, an important step of infection that leads to outcomes ranging from termination of infection to cryptococcal dissemination. To study this critical process, we screened approximately 4,700 cryptococcal gene deletion mutants for altered uptake, using primary mouse and human phagocytic cells. Among the hits of these two screens, we identified 93 mutants with perturbed uptake in both systems, as well as others with differences in uptake by only one cell type. We further screened the hits for changes in thickness of the capsule, a protective polysaccharide layer around the cell which is an important cryptococcal virulence factor. The combination of our three screens yielded 45 mutants, including one lacking the phosphatidylinositol-4-phosphate phosphatase Sac1. In this work, we implicate Sac1 in both host cell uptake and capsule production. We found that sac1 mutants exhibit lipid trafficking defects, reductions in secretory system function, and changes in capsule size and composition. Many of these changes occur specifically in tissue culture media, highlighting the role of Sac1 phosphatase activity in responding to the stress of host-like conditions. Overall, these findings show how genome-scale screening can identify cellular factors that contribute to our understanding of cryptococcal biology and demonstrate the role of Sac1 in determining fungal virulence.

IMPORTANCE

Cryptococcus neoformans is a fungal pathogen with significant impact on global health. Cryptococcal cells inhaled from the environment are deposited into the lungs, where they first contact the human immune system. The interaction between C. neoformans and host cells is critical because this step of infection can determine whether the fungal cells die or proliferate within the human host. Despite the importance of this stage of infection, we have limited knowledge of cryptococcal factors that influence its outcome. In this study, we identify cryptococcal genes that affect uptake by both human and mouse cells. We also identify mutants with altered capsule, a protective coating that surrounds the cells to shield them from the host immune system. Finally, we characterize the role of one gene, SAC1 , in these processes. Overall, this study contributes to our understanding of how C. neoformans interacts with and protects itself from host cells.

A Quick reCAP: Discovering Cryptococcus neoformans Capsule Mutants

Cryptococcus neoformans is an opportunistic fungal pathogen that can cause severe meningoencephalitis in immunocompromised hosts and is a leading cause of death in HIV/AIDS patients. This pathogenic yeast is surrounded by a polysaccharide capsule that is critical for virulence and plays important roles in host-pathogen interactions. Understanding capsule biosynthesis is therefore key to defining the biology of C. neoformans and potentially discovering novel therapeutic targets. By exploiting methods to identify mutants deficient in capsule, June Kwon-Chung and other investigators have discovered numerous genes involved in capsule biosynthesis and regulation. Successful approaches have incorporated combinations of techniques including mutagenesis and systematic gene deletion; complementation and genetic screens; morphological examination, physical separation, and antibody binding; and computational modeling based on gene expression analysis. In this review, we discuss these methods and how they have been used to identify capsule mutants.

Antifungal activity of eumelanin-inspired indoylenepheyleneethynylene against Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that causes meningitis in >152,000 immunocompromised individuals annually, leading to 112,000 yearly deaths. The four classes of existing antifungal agents target plasma membrane sterols (ergosterol), nucleic acid synthesis, and cell wall synthesis. Existing drugs are not highly effective against Cryptococcus, and antifungal drug resistance is an increasing problem. A novel antimicrobial compound, a eumelanin-inspired indoylenepheyleneethynylene, EIPE-1, was synthesized and has antimicrobial activity against Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MSRA), but not towards Gram-negative organisms. Based on EIPE-1's antibacterial activity, we hypothesized that EIPE-1 could have antifungal activity. For these studies, we tested EIPE-1 against C. neoformans strain H99 and 6 additional cryptococcal clinical isolates. We examined antifungal activity, cytotoxicity, effects on fungal gene expression, and mechanism of action of EIPE-1. Results showed that EIPE-1 has fungicidal effects on seven cryptococcal strains with MICs ranging from 1.56 to 3.125 μg/mL depending on the strain, and it is non-toxic to mammalian cells. We conducted scanning and transmission electron microscopy on the exposed cells to examine structural changes to the organism following EIPE-1 treatment. Cells exposed displayed structural changes to their cell wall and membranes, with internal contents leaking out of the cells. To understand the effect of EIPE-1 on fungal gene expression, RNA sequencing was conducted. Results showed that EIPE-1 affects several processes involved stress response, ergosterol biosynthesis, capsule biosynthesis, and cell wall attachment and remodeling. Therefore, our studies demonstrate that EIPE-1 has antifungal activity against C. neoformans, which affects both cellular structure and gene expression of multiple fungal pathways involved in cell membrane stability and viability.

Biolistic Transformation of Cryptococcus neoformans

Biolistic transformation of Cryptococcus neoformans is used as a molecular tool to genetically alter or delete targeted genes. The DNA is introduced into the yeast on DNA-coated gold beads by a helium shock wave produced using a biolistic particle system. The procedure often involves insertion of a dominant selectable marker into the desired site by homologous recombination. To increase the likelihood of homologous recombination, large fragments of overlapping DNA are used. The two most used dominant selectable markers are nourseothricin and Geneticin. With the need to generate multiple gene deletions in the same strain, there are recyclable marker systems, such as the bacteriophage P1 Cre-loxP system or CRISPR that provide additional useful molecular tools. While newer strategies exist to generate deletions and introduce markers and other gene modifications, biolistic transformation has remained a viable tool to facilitate the construction of genetically modified yeast strains. This chapter provides a working protocol on how to delete and restore a gene in C. neoformans.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Antifungal Peptide SP1 Damages Polysaccharide Capsule of Cryptococcus neoformans and Enhances Phagocytosis of Macrophages

Cryptococcus neoformans is a fungal pathogen which causes nearly half a million deaths worldwide each year. Under host-relevant conditions, it produces a characteristic polysaccharide capsule. The polysaccharide capsule is one of the main virulence factors of C. neoformans, which involves antiphagocytosis and immune responses of the host to cause a lack of an immune. Meanwhile, the polysaccharide capsule is a promising drug target because of the absence of analogs in the host. Here, we demonstrate that antifungal peptide SP1, which is derived from the N terminus of Saccharomyces cerevisiae GAPDH (glyceraldehyde-3-phosphate dehydrogenase), disrupts the polysaccharide capsule of C. neoformans H99. The mechanism is possibly due to the interaction of SP1 with glucuronoxylomannan (GXM). Disruption of the polysaccharide capsule enhances the adhesion and phagocytosis of C. neoformans H99 by macrophages and reduces the replication of C. neoformans H99 within macrophages. Additionally, SP1 exhibits antifungal activity against cryptococcal biofilms associated with the capsular polysaccharides. These findings suggest the potential of SP1 as a drug candidate for the treatment of cryptococcosis. IMPORTANCE C. neoformans is an opportunistic pathogen that causes invasive infections with a high mortality rate. Currently, the clinical drugs available for the treatment of cryptococcosis are limited to amphotericin B, azoles, and flucytosine. Amphotericin is nephrotoxic, and the widespread use of azoles and 5-flucytosine has led to a rapid development of drug resistance in C. neoformans. There is an urgent need to develop new and effective anticryptococcal drugs. Targeting virulence factors is a novel strategy for developing antifungal drugs. The antifungal peptide SP1 is capable of disrupting the polysaccharide capsule, which is a principal virulence factor of C. neoformans. Studying the mechanism by which SP1 damages the polysaccharide capsule and investigating the potential benefits of SP1 in removing C. neoformans from the host provides baseline data to develop a therapeutic strategy against refractory cryptococcal infections. This strategy would involve both inhibiting virulence factors and directly killing C. neoformans cells.

Secretions from Serratia marcescens Inhibit the Growth and Biofilm Formation of Candida spp. and Cryptococcus neoformans

Candida spp. and Cryptococcus are conditional pathogenic fungi that commonly infect immunocompromised patients. Over the past few decades, the increase in antifungal resistance has prompted the development of new antifungal agents. In this study, we explored the potential antifungal effects of secretions from Serratia marcescens on Candida spp. and Cryptococcus neoformans. We confirmed that the supernatant of S. marcescens inhibited fungal growth, suppressed hyphal and biofilm formation, and downregulated the expression of hyphae-specific genes and virulence-related genes in Candida spp. and C. neoformans. Furthermore, the S. marcescens supernatant retained biological stability after heat, pH, and protease K treatment. The chemical profile of the S. marcescens supernatant was characterized by ultra-high-performance liquid chromatography-linear ion trap/orbitrap high resolution mass spectrometry analysis and a total of 61 compounds with an mzCloud best match of greater than 70 were identified. In vivo, treatment with the S. marcescens supernatant reduced the mortality of fungi-infected Galleria mellonella. Taken together, our results revealed that the stable antifungal substances in the supernatant of S. marcescens have promising potential applications in the development of new antifungal agents.

The Cryptococcus gattii species complex: Unique pathogenic yeasts with understudied virulence mechanisms

Members of Cryptococcus gattii/neoformans species complex are the etiological agents of the potentially fatal human fungal infection cryptococcosis. C. gattii and its sister species cause disease in both immunocompetent and immunocompromised hosts, while the closely related species C. neoformans and C. deneoformans predominantly infect immunocompromised hosts. To date, most studies have focused on similarities in pathogenesis between these two groups, but over recent years, important differences have become apparent. In this review paper, we highlight some of the major phenotypic differences between the C. gattii and neoformans species complexes and justify the need to study the virulence and pathogenicity of the C. gattii species complex as a distinct cryptococcal group.

Simplified All-In-One CRISPR-Cas9 Construction for Efficient Genome Editing in Cryptococcus Species

Cryptococcus neoformans and Cryptococcus deneoformans are opportunistic fungal pathogens found worldwide that are utilized to reveal mechanisms of fungal pathogenesis. However, their low homologous recombination frequency has greatly encumbered genetic studies. In preliminary work, we described a ‘suicide’ CRISPR-Cas9 system for use in the efficient gene editing of C. deneoformans, but this has not yet been used in the C. neoformans strain. The procedures involved in constructing vectors are time-consuming, whether they involve restriction enzyme-based cloning of donor DNA or the introduction of a target sequence into the gRNA expression cassette via overlap PCR, as are sophisticated, thus impeding their widespread application. Here, we report the optimized and simplified construction method for all-in-one CRISPR-Cas9 vectors that can be used in C. neoformans and C. deneoformans strains respectively, named pNK003 (Genbank: MW938321) and pRH003 (Genbank: KX977486). Taking several gene manipulations as examples, we also demonstrate the accuracy and efficiency of the new simplified all-in-one CRISPR-Cas9 genome editing tools in both Serotype A and Serotype D strains, as well as their ability to eliminate Cas9 and gDNA cassettes after gene editing. We anticipate that the availability of new vectors that can simplify and streamline the technical steps for all-in-one CRISPR-Cas9 construction could accelerate genetic studies of the Cryptococcus species.

The Repurposing of Acetylsalicylic Acid as a Photosensitiser to Inactivate the Growth of Cryptococcal Cells

Photodynamic treatment (PDT) is often successful when used against aerobic microbes, given their natural susceptibility to oxidative damage. To this end, the current study aimed to explore the photodynamic action of acetylsalicylic acid (ASA; aspirin, which is commonly used to treat non-infectious ailments), when administered to respiring cryptococcal cells. The treatment of cryptococcal cells, i.e., exposure to 0.5 or 1 mM of ASA in the presence of ultraviolet light (UVL) for 10 min, resulted in a significant (p < 0.05) reduction in the growth of tested cells when compared to non-treated (non-Rx) cells, i.e., no ASA and no UVL. The treated cells were also characterised by diseased mitochondria, which is crucial for the survival of respiring cells, as observed by a significant (p < 0.05) loss of mitochondrial membrane potential (ΔΨM) and significant (p < 0.05) accumulation of reactive oxygen species (ROS) when compared to non-Rx cells. Moreover, the photolytic products of acetylsalicylic acid altered the ultrastructural appearance of treated cells as well as limited the expression levels of the capsular-associated gene, CAP64, when compared to non-Rx cells. The results of the study highlight the potential use of ASA as a photosensitiser that is effective for controlling the growth of cryptococcal cells. Potentially, this treatment can also be used as an adjuvant, to complement and support the usage of current anti-microbial agents.

Associations between Cryptococcus Genotypes, Phenotypes, and Clinical Parameters of Human Disease: A Review

The genus Cryptococcus contains two primary species complexes that are significant opportunistic human fungal pathogens: C. neoformans and C. gattii. In humans, cryptococcosis can manifest in many ways, but most often results in either pulmonary or central nervous system disease. Patients with cryptococcosis can display a variety of symptoms on a spectrum of severity because of the interaction between yeast and host. The bulk of our knowledge regarding Cryptococcus and the mechanisms of disease stem from in vitro experiments and in vivo animal models that make a fair attempt, but do not recapitulate the conditions inside the human host. To better understand the dynamics of initiation and progression in cryptococcal disease, it is important to study the genetic and phenotypic differences in the context of human infection to identify the human and fungal risk factors that contribute to pathogenesis and poor clinical outcomes. In this review, we summarize the current understanding of the different clinical presentations and health outcomes that are associated with pathogenicity and virulence of cryptococcal strains with respect to specific genotypes and phenotypes.

Histoplasma capsulatum Glycans From Distinct Genotypes Share Structural and Serological Similarities to Cryptococcus neoformans Glucuronoxylomannan

The cell wall is a ubiquitous structure in the fungal kingdom, with some features varying depending on the species. Additional external structures can be present, such as the capsule of Cryptococcus neoformans (Cn), its major virulence factor, mainly composed of glucuronoxylomannan (GXM), with anti-phagocytic and anti-inflammatory properties. The literature shows that other cryptococcal species and even more evolutionarily distant species, such as the Trichosporon asahii, T. mucoides, and Paracoccidioides brasiliensis can produce GXM-like polysaccharides displaying serological reactivity to GXM-specific monoclonal antibodies (mAbs), and these complex polysaccharides have similar composition and anti-phagocytic properties to cryptococcal GXM. Previously, we demonstrated that the fungus Histoplasma capsulatum (Hc) incorporates, surface/secreted GXM of Cn and the surface accumulation of the polysaccharide enhances Hc virulence in vitro and in vivo. In this work, we characterized the ability of Hc to produce cellular-attached (C-gly-Hc) and secreted (E-gly) glycans with reactivity to GXM mAbs. These C-gly-Hc are readily incorporated on the surface of acapsular Cn cap59; however, in contrast to Cn GXM, C-gly-Hc had no xylose and glucuronic acid in its composition. Mapping of recognized Cn GXM synthesis/export proteins confirmed the presence of orthologs in the Hc database. Evaluation of C-gly and E-gly of Hc from strains of distinct monophyletic clades showed serological reactivity to GXM mAbs, despite slight differences in their molecular dimensions. These C-gly-Hc and E-gly-Hc also reacted with sera of cryptococcosis patients. In turn, sera from histoplasmosis patients recognized Cn glycans, suggesting immunogenicity and the presence of cross-reacting antibodies. Additionally, C-gly-Hc and E-gly-Hc coated Cn cap59 were more resistant to phagocytosis and macrophage killing. C-gly-Hc and E-gly-Hc coated Cn cap59 were also able to kill larvae of Galleria mellonella. These GXM-like Hc glycans, as well as those produced by other pathogenic fungi, may also be important during host-pathogen interactions, and factors associated with their regulation are potentially important targets for the management of histoplasmosis.

Deletion of pksA attenuates the melanogenesis, growth and sporulation ability and causes increased sensitivity to stress response and antifungal drugs in the human pathogenic fungus Fonsecaea monophora

Fonsecaea monophora, which is very similar to Fonsecaea pedrosoi in morphological features, has been commonly misdiagnosed as F. pedrosoi. Like F. pedrosoi, F. monophora has been also identified as a predominant pathogen of Chromoblastomycosis (CBM). Melanin has been recognized as a virulence factor in several fungi, however, it is still largely unknown about the biological role of melanin and how melanin is synthesized in F. monophora. In this study, we identified two putative polyketide synthase genes (pks), AYO21_03016 (pksA) and AYO21_10638, by searching against the genome of F. monophora. AYO21_03016 and AYO21_10638 were further targeted disrupted by Agrobacterium tumefaciens-mediated transformation (ATMT). We discovered that pksA gene was the major polyketide synthase required for melanin synthesis in F. monophora, rather than AYO21_10638. Phenotypic analysis showed that, knocking out of the pksA gene attenuated melanogenesis, growth rate, sporulation ability and virulence of F. monophora, as compared with wild-type and complementation strain (pksA-C). Furthermore, the ΔpksA mutant was confirmed to be more sensitive to the oxidative stress, extreme pH environment, and antifungal drugs including itraconazole (ITC), terbinafine (TER), and amphotericin B (AMB). Taken together, these findings enabled us to comprehend the role of pksA in regulating DHN-melanin pathway and its effect on the biological function of F. monophora.

Population diversity and virulence characteristics of Cryptococcus neoformans/C. gattii species complexes isolated during the pre-HIV-pandemic era

Cryptococcosis has become a major global health problem since the advent of the HIV pandemic in 1980s. Although its molecular epidemiology is well-defined, using isolates recovered since then, no pre-HIV-pandemic era epidemiological data exist. We conducted a molecular epidemiological study using 228 isolates of the C. neoformans/C. gattii species complexes isolated before 1975. Genotypes were determined by URA5 restriction fragment length polymorphism analysis and multi-locus sequence typing. Population genetics were defined by nucleotide diversity measurements, neutrality tests, and recombination analysis. Growth at 37°C, melanin synthesis, capsule production, and urease activity as virulence factors were quantified. The pre-HIV-pandemic isolates consisted of 186 (81.5%) clinical, 35 (15.4%) environmental, and 7 (3.1%) veterinary isolates. Of those, 204 (89.5%) belonged to C. neoformans VNI (64.0%), VNII (14.9%) and VNIV (10.5%) while 24 (10.5%) belonged to C. gattii VGIII (7.5%), VGI (2.6%) and VGII (0.5%). Among the 47 sequence types (STs) identified, one of VNII and 8 of VNIV were novel. ST5/VNI (23.0%) in C. neoformans and ST75/VGIII (25.0%) in C. gattii were the most common STs in both species complexes. Among C. neoformans, VNIV had the highest genetic diversity (Hd = 0.926) and the minimum recombination events (Rm = 10), and clinical isolates had less genetic diversity (Hd = 0.866) than environmental (Hd = 0.889) and veterinary isolates (Hd = 0.900). Among C. gattii, VGI had a higher nucleotide diversity (π = 0.01436) than in VGIII (π = 0.00328). The high-virulence genotypes (ST5/VNI and VGIIIa/serotype B) did not produce higher virulence factors levels than other genotypes. Overall, high genetic variability and recombination rates were found for the pre-HIV-pandemic era among strains of the C. neoformans/C. gattii species complexes. Whole genome analysis and in vivo virulence studies would clarify the evolution of the genetic diversity and/or virulence of isolates of the C. neoformans/C. gattii species complexes during the pre- and post-HIV-pandemic eras.

Since the beginning of the HIV pandemic in 1980, infections due to isolates of the Cryptococcus neoformans/C. gattii species complexes have caused many deaths worldwide, especially in the HIV-infected population. Annually, approximately one-third, of all AIDS-related deaths,—representing more than 1,000,000 cases,—are caused by cryptococcosis. Since 1980, extensive molecular epidemiological surveys have been conducted, and the VNI molecular type has been found to be responsible for more than 90% of cryptococcosis in HIV patients. Whether the high VNI prevalence is associated with the HIV pandemic remains controversial as information on the isolates of the pre-HIV pandemic era is lacking. Therefore, this study of the molecular epidemiology and in vitro characteristics of the strains from the pre-HIV-pandemic era was undertaken. We found that only 64% of cryptococcosis was caused by VNI, and 9 sequence types existed only in the pre-HIV pandemic era. Unlike what was already known about the strains collected during the HIV pandemic era, ST5 and VGIIIa,—supposedly high virulence genotypes,—did not express higher virulence factors than other genotypes. These results implied that the HIV pandemic altered both the molecular epidemiology and virulence of Cryptococcus neoformans/C. gattii species complexes have been altered during HIV pandemic. However, detailed mechanism of these alteration remains to be deciphered further.

Transformation of Cryptococcus neoformans by electroporation using a transient CRISPR-Cas9 expression (TRACE) system

The basidiomycete Cryptococcus neoformans is not only a clinically important pathogen, but also a model organism for studying microbial pathogenesis and eukaryotic biology. One key factor behind its rise as a model organism is its genetic amenability. The widely used methods for transforming the C. neoformans species complex are Agrobacterium-mediated transformation (AMT) for random insertional mutagenesis and biolistic transformation for targeted mutagenesis. Electroporation was introduced to C. neoformans in early 1990s. Although electroporation is economic and yields a large number of transformants, introduced DNA rarely integrates into cryptococcal genome, which limits its use. Biolistic transformation, although costly and inefficient, has been the only method used in targeted mutagenesis in the past two decades. Several modifications, including the use of a donor DNA with split markers, a drug-resistant selection marker, and a recipient strain deficient in non-homologous end joining (NHEJ), have since modestly increased the frequency of genome integration and the rate of homologous replacement of the DNA introduced by electroporation. However, electroporation was not the method of choice for transformation until the recent adoption of CRISPR-Cas9 systems. We have developed a Transient CRISPR-Cas9 coupled with Electroporation System (TRACE), which dramatically facilitates targeted mutagenesis in the Cryptococcus species complex. TRACE combines the high transformation efficiency of electroporation with the high rates of DNA integration due to the transiently expressed CRISPR-Cas9. Here, we briefly discussed the history of electroporation for Cryptococcus transformation and provided detailed procedures for electroporation and the cassettes construction of the TRACE system for various genetic manipulations.

The antifungal and Cryptococcus neoformans virulence attenuating activity of Pelargonium sidoides extracts

ETHNOPHARMACOLOGICAL RELEVANCE

Limitations of clinical antifungal treatments and drug-resistance are drivers of the search for novel antifungal strategies. Extracts prepared from the tubers of the medicinal plant, Pelargonium sidoides, are known for their antiviral and antibacterial activities and are used in ethnomedicine for the treatment of acute respiratory infections. Their impact on fungi has not been well characterised. Here, we provide a first report on the antifungal activity of a P. sidoides aerial tissue extract against Cryptococcus neoformans as well as the effects of both tuber and aerial tissue extracts on selected virulence factors.

AIM OF THE STUDY

Novel antimicrobial strategies that target multiple cellular pathways or make use of anti-pathogenic compounds that inhibit virulence factors have been proposed. This work aimed to evaluate P. sidoides plant parts for their anticryptococcal activity and antipathogenic properties on selected virulence factors.

MATERIALS AND METHODS

The antifungal activity of crude P. sidoides tuber and aerial tissue extracts (15% m/m ethanol) were compared using a modified colourimetric antifungal susceptibility test. Fungicidal activity of the extracts was confirmed by plate counts. To test yeast resistance to the extracts, it was conditioned by multiple passages in sub-lethal doses followed by antifungal susceptibility testing. Cytotoxicity of the extracts was tested with a blood agar haemolysis assay. Extracts were evaluated for the presence of multiple bioactive compounds by solid-phase fractionation and visualisation by thin-layer chromatography in combination with bioassays. The influence of extracts on the production of the polysaccharide capsule, ergosterol content as well as laccase and urease activities were also evaluated. Cell surface variations after extract exposure were visualised by scanning electron microscopy (SEM).

RESULTS

Both tuber and aerial tissue extracts were fungicidal and contained multiple bioactive compounds which constrained the development of antifungal resistance. No haemolytic activity was observed, and the extracts did not appear to target ergosterol biosynthesis. However, the extracts displayed anti-pathogenic potential by significantly inhibiting laccase and urease activity while also significantly reducing capsule size. SEM revealed notable cell surface variations and provided support for the observed reduction in capsule size.

CONCLUSIONS

Our results provide support to the exploration of medicinal plants as sources of alternative antifungal therapies and the potential use of multicomponent inhibition and or virulence attenuation for next-generation treatment strategies. Our data also provide relevant information that may support the further use of P. sidoides in traditional medicines as well as in commercialised phytopharmaceuticals.

The Spectrum of Interactions between Cryptococcus neoformans and Bacteria

Cryptococcus neoformans is a major fungal pathogen that infects immunocompromised people and causes life-threatening meningoencephalitis. C. neoformans does not occur in isolation either in the environment or in the human host, but is surrounded by other microorganisms. Bacteria are ubiquitously distributed in nature, including soil, and make up the dominant part of the human microbiota. Pioneering studies in the 1950s demonstrated antifungal activity of environmental bacteria against C. neoformans. However, the mechanisms and implications of these interactions remain largely unknown. Recently, interest in polymicrobial interaction studies has been reignited by the development of improved sequencing methodologies, and by the realization that such interactions may have a huge impact on ecology and human health. In this review, we summarize our current understanding of the interaction of bacteria with C. neoformans.

The Zebrafish as a Model Host for Invasive Fungal Infections

The zebrafish has become a widely accepted model host for studies of infectious disease, including fungal infections. The species is genetically tractable, and the larvae are transparent and amenable to prolonged in vivo imaging and small molecule screening. The aim of this review is to provide a thorough introduction into the published studies of fungal infection in the zebrafish and the specific ways in which this model has benefited the field. In doing so, we hope to provide potential new zebrafish researchers with a snapshot of the current toolbox and prior results, while illustrating how the model has been used well and where the unfulfilled potential of this model can be found.

A chemical genetic screen reveals a role for proteostasis in capsule and biofilm formation by Cryptococcus neoformans

Pathogenic microorganisms employ specialized virulence factors to cause disease. Biofilm formation and the production of a polysaccharide capsule are two important virulence factors in Cryptococcus neoformans, the fungal pathogen that causes meningoencephalitis. Here, we show that the bipolar disorder drug lithium inhibits formation of both virulence factors by a mechanism involving dysregulation of the ubiquitin/proteasome system. By using a chemical genetics approach and bioinformatic analyses, we describe the cellular landscape affected by lithium treatment. We demonstrate that lithium affects many different pathways in C. neoformans, including the cAMP/protein kinase A, inositol biosynthesis, and ubiquitin/proteasome pathways. By analyzing mutants with defects in the ubiquitin/proteasome system, we uncover a role for proteostasis in both capsule and biofilm formation. Moreover, we demonstrate an additive influence of lithium and the proteasome inhibitor bortezomib in inhibiting capsule production, thus establishing a link between lithium activity and the proteasome system. Finally, we show that the lithium-mimetic drug ebselen potently blocks capsule and biofilm formation, and has additive activity with lithium or bortezomib. In summary, our results illuminate the impact of lithium on C. neoformans, and link dysregulation of the proteasome to capsule and biofilm inhibition in this important fungal pathogen.

The capsule of Cryptococcus neoformans

The capsule of Cryptococcus neoformans is its dominant virulence factor and plays a key role in the biology of this fungus. In this essay, we focus on the capsule as a cellular structure and note the limitations inherent in the current methodologies available for its study. Given that no single method can provide the structure of the capsule, our notions of what is the cryptococcal capsule must be arrived at by synthesizing information gathered from very different methodological approaches including microscopy, polysaccharide chemistry and physical chemistry of macromolecules. The emerging picture is one of a carefully regulated dynamic structure that is constantly rearranged as a response to environmental stimulation and cellular replication. In the environment, the capsule protects the fungus against desiccation and phagocytic predators. In animal hosts the capsule functions in both offensive and defensive modes, such that it interferes with immune responses while providing the fungal cell with a defensive shield that is both antiphagocytic and capable of absorbing microbicidal oxidative bursts from phagocytic cells. Finally, we delineate a set of unsolved problems in the cryptococcal capsule field that could provide fertile ground for future investigations.

Comparative Ecology of Capsular Exophiala Species Causing Disseminated Infection in Humans

Exophiala spinifera and Exophiala dermatitidis (Fungi: Chaetothyriales) are black yeast agents potentially causing disseminated infection in apparently healthy humans. They are the only Exophiala species producing extracellular polysaccharides around yeast cells. In order to gain understanding of eventual differences in intrinsic virulence of the species, their clinical profiles were compared and found to be different, suggesting pathogenic strategies rather than coincidental opportunism. Ecologically relevant factors were compared in a model set of strains of both species, and significant differences were found in clinical and environmental preferences, but virulence, tested in Galleria mellonella larvae, yielded nearly identical results. Virulence factors, i.e., melanin, capsule and muriform cells responded in opposite direction under hydrogen peroxide and temperature stress and thus were inconsistent with their hypothesized role in survival of phagocytosis. On the basis of physiological profiles, possible natural habitats of both species were extrapolated, which proved to be environmental rather than animal-associated. Using comparative genomic analyses we found differences in gene content related to lipid metabolism, cell wall modification and polysaccharide capsule production. Despite the fact that both species cause disseminated infections in apparently healthy humans, it is concluded that they are opportunists rather than pathogens.

Disarming Fungal Pathogens: Bacillus safensis Inhibits Virulence Factor Production and Biofilm Formation by Cryptococcus neoformans and Candida albicans

Bacteria interact with each other in nature and often compete for limited nutrient and space resources. However, it is largely unknown whether and how bacteria also interact with human fungal pathogens naturally found in the environment. Here, we identified a soil bacterium, Bacillus safensis, which potently blocked several key Cryptococcus neoformans virulence factors, including formation of the antioxidant pigment melanin and production of the antiphagocytic polysaccharide capsule. The bacterium also inhibited de novo cryptococcal biofilm formation but had only modest inhibitory effects on already formed biofilms or planktonic cell growth. The inhibition of fungal melanization was dependent on direct cell contact and live bacteria. B. safensis also had anti-virulence factor activity against another major human-associated fungal pathogen, Candida albicans. Specifically, dual-species interaction studies revealed that the bacterium strongly inhibited C. albicans filamentation and biofilm formation. In particular, B. safensis physically attached to and degraded candidal filaments. Through genetic and phenotypic analyses, we demonstrated that bacterial chitinase activity against fungal cell wall chitin is a factor contributing to the antipathogen effect of B. safensis.

Pathogenic fungi are estimated to contribute to as many human deaths as tuberculosis or malaria. Two of the most common fungal pathogens, Cryptococcus neoformans and Candida albicans, account for up to 1.4 million infections per year with very high mortality rates. Few antifungal drugs are available for treatment, and development of novel therapies is complicated by the need for pathogen-specific targets. Therefore, there is an urgent need to identify novel drug targets and new drugs. Pathogens use virulence factors during infection, and it has recently been proposed that targeting these factors instead of the pathogen itself may represent a new approach to develop antimicrobials. Here, we identified a soil bacterium that specifically blocked virulence factor production and biofilm formation by C. neoformans and C. albicans. We demonstrate that the bacterial antipathogen mechanism is based in part on targeting the fungal cell wall, a structure not found in human cells.

A 'suicide' CRISPR-Cas9 system to promote gene deletion and restoration by electroporation in Cryptococcus neoformans

Loss-of-function mutagenesis is an important tool used to characterize gene functions, and the CRISPR-Cas9 system is a powerful method for performing targeted mutagenesis in organisms that present low recombination frequencies, such as the serotype D strains of Cryptococcus neoformans. However, when the CRISPR-Cas9 system persists in the host cells, off-target effects and Cas9 cytotoxicity may occur, which might block subsequent genetic manipulation. Here, we report a method of spontaneously eliminating the CRISPR-Cas9 system without impairing its robust editing function. We successfully expressed single guide RNA under the driver of an endogenous U6 promoter and the human codon-optimized Cas9 endonuclease with an ACT1 promoter. This system can effectively generate an indel mutation and efficiently perform targeted gene disruption via homology-directed repair by electroporation in yeast. We then demonstrated the spontaneous elimination of the system via a cis arrangement of the CRISPR-Cas9 expression cassettes to the recombination construct. After a system-mediated double crossover, the CRISPR-Cas9 cassettes were cleaved and degraded, which was validated by Southern blotting. This 'suicide' CRISPR-Cas9 system enables the validation of gene functions by subsequent complementation and has the potential to minimize off-target effects. Thus, this technique has the potential for use in functional genomics studies of C. neoformans.

Intracellular Action of a Secreted Peptide Required for Fungal Virulence

Quorum sensing (QS) is a bacterial communication mechanism in which secreted signaling molecules impact population function and gene expression. QS-like phenomena have been reported in eukaryotes with largely unknown contributing molecules, functions, and mechanisms. We identify Qsp1, a secreted peptide, as a central signaling molecule that regulates virulence in the fungal pathogen Cryptococcus neoformans. QSP1 is a direct target of three transcription factors required for virulence, and qsp1Δ mutants exhibit attenuated infection, slowed tissue accumulation, and greater control by primary macrophages. Qsp1 mediates autoregulatory signaling that modulates secreted protease activity and promotes cell wall function at high cell densities. Peptide production requires release from a secreted precursor, proQsp1, by a cell-associated protease, Pqp1. Qsp1 sensing requires an oligopeptide transporter, Opt1, and remarkably, cytoplasmic expression of mature Qsp1 complements multiple phenotypes of qsp1Δ. Thus, C. neoformans produces an autoregulatory peptide that matures extracellularly but functions intracellularly to regulate virulence.

Magnesium Ion Acts as a Signal for Capsule Induction in Cryptococcus neoformans

Cryptococcal meningitis caused by Cryptococcus neoformans, is a common opportunistic neural infection in immunocompromised individuals. Cryptococcus meningitis is associated with fungal burden with larger capsule size in cerebrospinal fluid (CSF). To understand the role of CSF constituents in capsule enlargement, we have evaluated the effect of artificial CSF on capsule induction in comparison with various other capsule inducing media. Two different strains of C. neoformans, an environmental and a clinical isolates were used in the present study. While comparing the various capsule inducing media for the two different strains of C. neoformans, it was observed that the capsule growth was significantly increased when grown in artificial CSF at pH 5.5, temperature 34°C for ATCC C. neoformans and 37°C for Clinical C. neoformans and with an incubation period of 72 h. In addition, artificial CSF supports biofilm formation in C. neoformans. While investigating the individual components of artificial CSF, we found that Mg²⁺ ions influence the capsule growth in both environmental and clinical strains of C. neoformans. To confirm our results we studied the expression of four major CAP genes namely, CAP10, CAP59, CAP60, and CAP64 in various capsule inducing media and in different concentrations of Mg²⁺ and Ca²⁺. Our results on gene expression suggest that, Mg²⁺ does have an effect on CAP gene expression, which are important for capsule biosynthesis and virulence. Our findings on the role of Mg²⁺ ion as a signal for capsule induction will promote a way to elucidate the control mechanisms for capsule biosynthesis in C. neoformans.

Virulence of Cryptococcus sp. Biofilms In Vitro and In Vivo using Galleria mellonella as an Alternative Model

Cryptococcus neoformans and C. gattii are fungal pathogens that are most commonly found in infections of the central nervous system, which cause life-threatening meningoencephalitis and can grow as a biofilm. Biofilms are structures conferring protection and resistance of microorganism to the antifungal drugs. This study compared the virulence of planktonic and biofilm cells of C. neoformans and C. gattii in Galleria mellonella model, as well as, the quantification of gene transcripts LAC1, URE1, and CAP59 by real time PCR. All three of the genes showed significantly increased expressions in the biofilm conditions for two species of Cryptococcus, when compared to planktonic cells. C. neoformans and C. gattii cells in the biofilm forms were more virulent than the planktonic cells in G. mellonella. This suggests that the biofilm conditions may contribute to the virulence profile. Our results contribute to a better understanding of the agents of cryptococcosis in the host-yeast aspects of the interaction.

Eucalyptus Tree: A Potential Source of Cryptococcus neoformans in Egyptian Environment

In Egypt, the River Red Gum (Eucalyptus camaldulensis) is a well-known tree and is highly appreciated by the rural and urban dwellers. The role of Eucalyptus trees in the ecology of Cryptococcus neoformans is documented worldwide. The aim of this survey was to show the prevalence of C. neoformans during the flowering season of E. camaldulensis at the Delta region in Egypt. Three hundred and eleven samples out of two hundred Eucalyptus trees, including leaves, flowers, and woody trunks, were collected from four governorates in the Delta region. Thirteen isolates of C. neoformans were recovered from Eucalyptus tree samples (4.2%). Molecular identification of C. neoformans was done by capsular gene specific primer CAP64 and serotype identification was done depending on LAC1 gene. This study represents an update on the ecology of C. neoformans associated with Eucalyptus tree in Egyptian environment.

Cryptococcus gattii Capsule Blocks Surface Recognition Required for Dendritic Cell Maturation Independent of Internalization and Antigen Processing

Cryptococcus gattii is an emerging fungal pathogen on the west coast of Canada and the United States that causes a potentially fatal infection in otherwise healthy individuals. In previous investigations of the mechanisms by which C. gattii might subvert cell-mediated immunity, we found that C. gattii failed to induce dendritic cell (DC) maturation, leading to defective T cell responses. However, the virulence factor and the mechanisms of evasion of DC maturation remain unknown. The cryptococcal polysaccharide capsule is a leading candidate because of its antiphagocytic properties. Consequently, we asked if the capsule of C. gattii was involved in evasion of DC maturation. We constructed an acapsular strain of C. gattii through CAP59 gene deletion by homologous integration. Encapsulated C. gattii failed to induce human monocyte-derived DC maturation and T cell proliferation, whereas the acapsular mutant induced both processes. Surprisingly, encapsulation impaired DC maturation independent of its effect on phagocytosis. Indeed, DC maturation required extracellular receptor signaling that was dependent on TNF-α and p38 MAPK, but not ERK activation, and the cryptococcal capsule blocked this extracellular recognition. Although the capsule impaired phagocytosis that led to pH-dependent serine-, threonine-, and cysteine-sensitive protease-dependent Ag processing, it was insufficient to impair T cell responses. In summary, C. gattii affects two independent processes, leading to DC maturation and Ag processing. The polysaccharide capsule masked extracellular detection and reduced phagocytosis that was required for DC maturation and Ag processing, respectively. However, the T cell response was fully restored by inducing DC maturation.

Masking the Pathogen: Evolutionary Strategies of Fungi and Their Bacterial Counterparts

Pathogens reduce immune recognition of their cell surfaces using a variety of inert structural polysaccharides. For example, capsular polysaccharides play critical roles in microbial survival strategies. Capsules are widely distributed among bacterial species, but relatively rare in eukaryotic microorganisms, where they have evolved considerable complexity in structure and regulation and are exemplified by that of the HIV/AIDS-related fungus Cryptococcus neoformans. Endemic fungi that affect normal hosts such as Histoplasma capsulatum and Blastomyces dermatitidis have also evolved protective polysaccharide coverings in the form of immunologically inert α-(1,3)-glucan polysaccharides to protect their more immunogenic β-(1,3)-glucan-containing cell walls. In this review we provide a comparative update on bacterial and fungal capsular structures and immunogenic properties as well as the polysaccharide masking strategies of endemic fungal pathogens.

Virulence-Associated Enzymes of Cryptococcus neoformans

Enzymes play key roles in fungal pathogenesis. Manipulation of enzyme expression or activity can significantly alter the infection process, and enzyme expression profiles can be a hallmark of disease. Hence, enzymes are worthy targets for better understanding pathogenesis and identifying new options for combatting fungal infections. Advances in genomics, proteomics, transcriptomics, and mass spectrometry have enabled the identification and characterization of new fungal enzymes. This review focuses on recent developments in the virulence-associated enzymes from Cryptococcus neoformans. The enzymatic suite of C. neoformans has evolved for environmental survival, but several of these enzymes play a dual role in colonizing the mammalian host. We also discuss new therapeutic and diagnostic strategies that could be based on the underlying enzymology.

How Cryptococcus interacts with the blood-brain barrier

Cryptococcus demonstrates predilection for invasion of the brain, but the mechanism by which Cryptococcus crosses the blood-brain barrier (BBB) to cause brain invasion is largely unknown. In order for Cryptococcus to cross the BBB, there must be a way to either cross human brain microvascular endothelial cells, which are the main constitute of the BBB, or go in between tight junctions. Recent evidence of human brain microvascular endothelial cell responses to transcellular brain invasions includes membrane rearrangements, intracellular signaling pathways and cytoskeletal activations. Several Cryptococcal genes related to the traversal of BBB have been identified, including CPS1, ITR1a, ITR3c, PLB1, MPR1, FNX1 and RUB1. In addition, Cryptococcus neoformans-derived microvesicles may contribute to cryptococcal brain invasion. Paracellularly, Cryptococcus may traverse across BBB using either routes utilizing plasmin, ammonia or macrophages in a Trojan horse mechanism.

Live Imaging of Host-Parasite Interactions in a Zebrafish Infection Model Reveals Cryptococcal Determinants of Virulence and Central Nervous System Invasion

The human fungal pathogen Cryptococcus neoformans is capable of infecting a broad range of hosts, from invertebrates like amoebas and nematodes to standard vertebrate models such as mice and rabbits. Here we have taken advantage of a zebrafish model to investigate host-pathogen interactions of Cryptococcus with the zebrafish innate immune system, which shares a highly conserved framework with that of mammals. Through live-imaging observations and genetic knockdown, we establish that macrophages are the primary immune cells responsible for responding to and containing acute cryptococcal infections. By interrogating survival and cryptococcal burden following infection with a panel of Cryptococcus mutants, we find that virulence factors initially identified as important in causing disease in mice are also necessary for pathogenesis in zebrafish larvae. Live imaging of the cranial blood vessels of infected larvae reveals that C. neoformans is able to penetrate the zebrafish brain following intravenous infection. By studying a C. neoformans FNX1 gene mutant, we find that blood-brain barrier invasion is dependent on a known cryptococcal invasion-promoting pathway previously identified in a murine model of central nervous system invasion. The zebrafish-C. neoformans platform provides a visually and genetically accessible vertebrate model system for cryptococcal pathogenesis with many of the advantages of small invertebrates. This model is well suited for higher-throughput screening of mutants, mechanistic dissection of cryptococcal pathogenesis in live animals, and use in the evaluation of therapeutic agents.

IMPORTANCE

Cryptococcus neoformans is an important opportunistic pathogen that is estimated to be responsible for more than 600,000 deaths worldwide annually. Existing mammalian models of cryptococcal pathogenesis are costly, and the analysis of important pathogenic processes such as meningitis is laborious and remains a challenge to visualize. Conversely, although invertebrate models of cryptococcal infection allow high-throughput assays, they fail to replicate the anatomical complexity found in vertebrates and, specifically, cryptococcal stages of disease. Here we have utilized larval zebrafish as a platform that overcomes many of these limitations. We demonstrate that the pathogenesis of C. neoformans infection in zebrafish involves factors identical to those in mammalian and invertebrate infections. We then utilize the live-imaging capacity of zebrafish larvae to follow the progression of cryptococcal infection in real time and establish a relevant model of the critical central nervous system infection phase of disease in a nonmammalian model.

Positional cloning in Cryptococcus neoformans and its application for identification and cloning of the gene encoding methylenetetrahydrofolate reductase

Cryptococcus neoformans, a basidiomycetous human pathogenic yeast, has been widely used in research fields in medical mycology as well as basic biology. Gene cloning or identification of the gene responsible for a mutation of interest is a key step for functional analysis of a particular gene. The availability therefore, of the multiple methods for cloning is desirable. In this study, we proposed a method for a mapping-based gene identification/cloning (positional cloning) method in C. neoformans. To this end, we constructed a series of tester strains, one of whose chromosomes was labeled with the URA5 gene. A heterozygous diploid constructed by crossing one of the tester strains to a mutant strain of interest loses a chromosome(s) spontaneously, which is the basis for assigning a recessive mutant gene to a particular chromosome in the mitotic mapping method. Once the gene of interest is mapped to one of the 14 chromosomes, classical genetic crosses can then be performed to determine its more precise location. The positional information thus obtained can then be used to significantly narrow down candidate genes by referring to the Cryptococcus genome database. Each candidate gene is then examined whether it would complement the mutation. We successfully applied this method to identify CNA07390 encoding methylenetetrahydrofolate reductase as the gene responsible for a methionine-requiring mutant in our mutant collection.

Generation of stable mutants and targeted gene deletion strains in Cryptococcus neoformans through electroporation

Cryptococcus neoformans is the etiologic agent of cryptococcal meningitis that causes more than half a million deaths worldwide each year. This capsulated basidiomycetous yeast also serves as a model for micropathogenic studies. The ability to make stable mutants, either via ectopic integration or homologous recombination, has been accomplished using biolistic transformation. This technical advance has greatly facilitated the research on the basic biology and pathogenic mechanisms of this pathogen in the past two decades. However, biolistic transformation is costly, and its reproducibility varies widely. Here we found that stable ectopic integration or targeted gene deletion via homologous replacement could be accomplished through electroporative transformation. The stability of the transformants obtained through electroporation and the frequency of homologous replacement is highly dependent on the selective marker. A frequency of homologous recombination among the stable transformants obtained by electroporation is comparable to those obtained by biolistic transformation (∼10%) when dominant drug selection markers are used, which is much higher than what has been previously reported for electroporation when auxotrophic markers were used (0.001% to 0.1%). Furthermore, disruption of the KU80 gene or generation of gene deletion constructs using the split marker strategy, two approaches known to increase homologous replacement among transformants obtained through biolistic transformation, also increase the frequency of homologous replacement among transformants obtained through electroporation. Therefore, electroporation provides a low cost alternative for mutagenesis in Cryptococcus.

Fluconazole alters the polysaccharide capsule of Cryptococcus gattii and leads to distinct behaviors in murine Cryptococcosis

Cryptococcus gattii is an emergent human pathogen. Fluconazole is commonly used for treatment of cryptococcosis, but the emergence of less susceptible strains to this azole is a global problem and also the data regarding fluconazole-resistant cryptococcosis are scarce. We evaluate the influence of fluconazole on murine cryptococcosis and whether this azole alters the polysaccharide (PS) from cryptococcal cells. L27/01 strain of C. gattii was cultivated in high fluconazole concentrations and developed decreased drug susceptibility. This phenotype was named L27/01F, that was less virulent than L27/01 in mice. The physical, structural and electrophoretic properties of the PS capsule of L27/01F were altered by fluconazole. L27/01F presented lower antiphagocytic properties and reduced survival inside macrophages. The L27/01F did not affect the central nervous system, while the effect in brain caused by L27/01 strain began after only 12 hours. Mice infected with L27/01F presented lower production of the pro-inflammatory cytokines, with increased cellular recruitment in the lungs and severe pulmonary disease. The behavioral alterations were affected by L27/01, but no effects were detected after infection with L27/01F. Our results suggest that stress to fluconazole alters the capsule of C. gattii and influences the clinical manifestations of cryptococcosis.

The role of the de novo pyrimidine biosynthetic pathway in Cryptococcus neoformans high temperature growth and virulence

Fungal infections are often difficult to treat due to the inherent similarities between fungal and animal cells and the resulting host toxicity from many antifungal compounds. Cryptococcus neoformans is an opportunistic fungal pathogen of humans that causes life-threatening disease, primarily in immunocompromised patients. Since antifungal therapy for this microorganism is limited, many investigators have explored novel drug targets aim at virulence factors, such as the ability to grow at mammalian physiological temperature (37°C). To address this issue, we used the Agrobacterium tumefaciens gene delivery system to create a random insertion mutagenesis library that was screened for altered growth at elevated temperatures. Among several mutants unable to grow at 37°C, we explored one bearing an interruption in the URA4 gene. This gene encodes dihydroorotase (DHOase) that is involved in the de novo synthesis of pyrimidine ribonucleotides. Loss of the C. neoformans Ura4 protein, by targeted gene interruption, resulted in an expected uracil/uridine auxotrophy and an unexpected high temperature growth defect. In addition, the ura4 mutant displayed phenotypic defects in other prominent virulence factors (melanin, capsule and phospholipase) and reduced stress response compared to wild type and reconstituted strains. Accordingly, this mutant had a decreased survival rate in macrophages and attenuated virulence in a murine model of cryptococcal infection. Quantitative PCR analysis suggests that this biosynthetic pathway is induced during the transition from 30°C to 37°C, and that transcriptional regulation of de novo and salvage pyrimidine pathway are under the control of the Ura4 protein.

Pbx proteins in Cryptococcus neoformans cell wall remodeling and capsule assembly

The cryptococcal capsule is a critical virulence factor of an important pathogen, but little is known about how it is associated with the cell or released into the environment. Two mutants lacking PBX1 and PBX2 were found to shed reduced amounts of the capsule polysaccharide glucuronoxylomannan (GXM). Nuclear magnetic resonance, composition, and physical analyses showed that the shed material was of normal mass but was slightly enriched in xylose. In contrast to previous reports, this material contained no glucose. Notably, the capsule fibers of pbxΔ mutant cells grown under capsule-inducing conditions were present at a lower than usual density and were loosely attached to the cell wall. Mutant cell walls were also defective, as indicated by phenotypes including abnormal cell morphology, reduced mating filamentation, and altered cell integrity. All observed phenotypes were shared between the two mutants and exacerbated in a double mutant. Consistent with a role in surface glycan synthesis, the Pbx proteins localized to detergent-resistant membrane domains. These results, together with the sequence motifs in the Pbx proteins, suggest that Pbx1 and Pbx2 are redundant proteins that act in remodeling the cell wall to maintain normal cell morphology and precursor availability for other glycan synthetic processes. Their absence results in aberrant cell wall growth and metabolic imbalance, which together impact cell wall and capsule synthesis, cell morphology, and capsule association. The surface changes also lead to increased engulfment by host phagocytes, consistent with the lack of virulence of pbx mutants in animal models.

Role of the Apt1 protein in polysaccharide secretion by Cryptococcus neoformans

Flippases are key regulators of membrane asymmetry and secretory mechanisms. Vesicular polysaccharide secretion is essential for the pathogenic mechanisms of Cryptococcus neoformans. On the basis of the observations that flippases are required for polysaccharide secretion in plants and the putative Apt1 flippase is required for cryptococcal virulence, we analyzed the role of this enzyme in polysaccharide release by C. neoformans, using a previously characterized apt1Δ mutant. Mutant and wild-type (WT) cells shared important phenotypic characteristics, including capsule morphology and dimensions, glucuronoxylomannan (GXM) composition, molecular size, and serological properties. The apt1Δ mutant, however, produced extracellular vesicles (EVs) with a lower GXM content and different size distribution in comparison with those of WT cells. Our data also suggested a defective intracellular GXM synthesis in mutant cells, in addition to changes in the architecture of the Golgi apparatus. These findings were correlated with diminished GXM production during in vitro growth, macrophage infection, and lung colonization. This phenotype was associated with decreased survival of the mutant in the lungs of infected mice, reduced induction of interleukin-6 (IL-6) cytokine levels, and inefficacy in colonization of the brain. Taken together, our results indicate that the lack of APT1 caused defects in both GXM synthesis and vesicular export to the extracellular milieu by C. neoformans via processes that are apparently related to the pathogenic mechanisms used by this fungus during animal infection.

The Cryptococcus neoformans Rim101 transcription factor directly regulates genes required for adaptation to the host

The Rim101 protein is a conserved pH-responsive transcription factor that mediates important interactions between several fungal pathogens and the infected host. In the human fungal pathogen Cryptococcus neoformans, the Rim101 protein retains conserved functions to allow the microorganism to respond to changes in pH and other host stresses. This coordinated cellular response enables this fungus to effectively evade the host immune response. Preliminary studies suggest that this conserved transcription factor is uniquely regulated in C. neoformans both by the canonical pH-sensing pathway and by the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. Here we present comparative transcriptional data that demonstrate a strong concordance between the downstream effectors of PKA and Rim101. To define Rim101-dependent gene expression during a murine lung infection, we used nanoString profiling of lung tissue infected with a wild-type or rim101Δ mutant strain. In this setting, we demonstrated that Rim101 controls the expression of multiple cell wall-biosynthetic genes, likely explaining the enhanced immunogenicity of the rim101Δ mutant. Despite its divergent upstream regulation, the C. neoformans Rim101 protein recognizes a conserved DNA binding motif. Using these data, we identified direct targets of this transcription factor, including genes involved in cell wall regulation. Therefore, the Rim101 protein directly controls cell wall changes required for the adaptation of C. neoformans to its host environment. Moreover, we propose that integration of the cAMP/PKA and pH-sensing pathways allows C. neoformans to respond to a broad range of host-specific signals.

Where do they come from and where do they go: candidates for regulating extracellular vesicle formation in fungi

In the past few years, extracellular vesicles (EVs) from at least eight fungal species were characterized. EV proteome in four fungal species indicated putative biogenesis pathways and suggested interesting similarities with mammalian exosomes. Moreover, as observed for mammalian exosomes, fungal EVs were demonstrated to be immunologically active. Here we review the seminal and most recent findings related to the production of EVs by fungi. Based on the current literature about secretion of fungal molecules and biogenesis of EVs in eukaryotes, we focus our discussion on a list of cellular proteins with the potential to regulate vesicle biogenesis in the fungi.

Binding of the wheat germ lectin to Cryptococcus neoformans chitooligomers affects multiple mechanisms required for fungal pathogenesis

The principal capsular component of Cryptococcus neoformans, glucuronoxylomannan (GXM), interacts with surface glycans, including chitin-like oligomers. Although the role of GXM in cryptococcal infection has been well explored, there is no information on how chitooligomers affect fungal pathogenesis. In this study, surface chitooligomers of C. neoformans were blocked through the use of the wheat germ lectin (WGA) and the effects on animal pathogenesis, interaction with host cells, fungal growth and capsule formation were analyzed. Treatment of C. neoformans cells with WGA followed by infection of mice delayed mortality relative to animals infected with untreated fungal cells. This observation was associated with reduced brain colonization by lectin-treated cryptococci. Blocking chitooligomers also rendered yeast cells less efficient in their ability to associate with phagocytes. WGA did not affect fungal viability, but inhibited GXM release to the extracellular space and capsule formation. In WGA-treated yeast cells, genes that are involved in capsule formation and GXM traffic had their transcription levels decreased in comparison with untreated cells. Our results suggest that cellular pathways required for capsule formation and pathogenic mechanisms are affected by blocking chitin-derived structures at the cell surface of C. neoformans. Targeting chitooligomers with specific ligands may reveal new therapeutic alternatives to control cryptococcosis.

The transcriptional response of Cryptococcus neoformans to ingestion by Acanthamoeba castellanii and macrophages provides insights into the evolutionary adaptation to the mammalian host

Virulence of Cryptococcus neoformans for mammals, and in particular its intracellular style, was proposed to emerge from evolutionary pressures on its natural environment by protozoan predation, which promoted the selection of strategies that allow intracellular survival in macrophages. In fact, Acanthamoeba castellanii ingests yeast cells, which then can replicate intracellularly. In addition, most fungal factors needed to establish infection in the mammalian host are also important for survival within the amoeba. To better understand the origin of C. neoformans virulence, we compared the transcriptional profile of yeast cells internalized by amoebae and murine macrophages after 6 h of infection. Our results showed 656 and 293 genes whose expression changed at least 2-fold in response to the intracellular environments of amoebae and macrophages, respectively. Among the genes that were found in both groups, we focused on open reading frame (ORF) CNAG_05662, which was potentially related to sugar transport but had no determined biological function. To characterize its function, we constructed a mutant strain and evaluated its ability to grow on various carbon sources. The results showed that this gene, named PTP1 (polyol transporter protein 1), is involved in the transport of 5- and 6-carbon polyols such as mannitol and sorbitol, but its presence or absence had no effect on cryptococcal virulence for mice or moth larvae. Overall, these results are consistent with the hypothesis that the capacity for mammalian virulence originated from fungus-protozoan interactions in the environment and provide a better understanding of how C. neoformans adapts to the mammalian host.

Cryptococcus neoformans Rim101 is associated with cell wall remodeling and evasion of the host immune responses

Infectious microorganisms often play a role in modulating the immune responses of their infected hosts. We demonstrate that Cryptococcus neoformans signals through the Rim101 transcription factor to regulate cell wall composition and the host-pathogen interface. In the absence of Rim101, C. neoformans exhibits an altered cell surface in response to host signals, generating an excessive and ineffective immune response that results in accelerated host death. This host immune response to the rim101Δ mutant strain is characterized by increased neutrophil influx into the infected lungs and an altered pattern of host cytokine expression compared to the response to wild-type cryptococcal infection. To identify genes associated with the observed phenotypes, we performed whole-genome RNA sequencing experiments under capsule-inducing conditions. We defined the downstream regulon of the Rim101 transcription factor and determined potential cell wall processes involved in the capsule attachment defects and altered mechanisms of virulence in the rim101Δ mutant. The cell wall generates structural stability for the cell and allows the attachment of surface molecules such as capsule polysaccharides. In turn, the capsule provides an effective mask for the immunogenic cell wall, shielding it from recognition by the host immune system.

Cryptococcus neoformans is an opportunistic human pathogen that is a significant cause of death in immunocompromised individuals. There are two major causes of death due to this pathogen: meningitis due to uncontrolled fungal proliferation in the brain in the face of a weakened immune system and immune reconstitution inflammatory syndrome characterized by an overactive immune response to subclinical levels of the pathogen. In this study, we examined how C. neoformans uses the conserved Rim101 transcription factor to specifically remodel the host-pathogen interface, thus regulating the host immune response. These studies explored the complex ways in which successful microbial pathogens induce phenotypes that ensure their own survival while simultaneously controlling the nature and degree of the associated host response.

Cryptococcus neoformans-derived microvesicles enhance the pathogenesis of fungal brain infection

Cryptococcal meningoencephalitis is the most common fungal disease in the central nervous system. The mechanisms by which Cryptococcus neoformans invades the brain are largely unknown. In this study, we found that C. neoformans-derived microvesicles (CnMVs) can enhance the traversal of the blood-brain barrier (BBB) by C. neoformans invitro. The immunofluorescence imaging demonstrates that CnMVs can fuse with human brain microvascular endothelial cells (HBMECs), the constituents of the BBB. This activity is presumably due to the ability of the CnMVs to activate HBMEC membrane rafts and induce cell fusogenic activity. CnMVs also enhanced C. neoformans infection of the brain, found in both infected brains and cerebrospinal fluid. In infected mouse brains, CnMVs are distributed inside and around C. neoformans-induced cystic lesions. GFAP (glial fibrillary acidic protein)-positive astrocytes were found surrounding the cystic lesions, overlapping with the 14-3-3-GFP (14-3-3-green fluorescence protein fusion) signals. Substantial changes could be observed in areas that have a high density of CnMV staining. This is the first demonstration that C. neoformans-derived microvesicles can facilitate cryptococcal traversal across the BBB and accumulate at lesion sites of C. neoformans-infected brains. Results of this study suggested that CnMVs play an important role in the pathogenesis of cryptococcal meningoencephalitis.

The Cryptococcus neoformans capsule: a sword and a shield

The human fungal pathogen Cryptococcus neoformans is characterized by its ability to induce a distinct polysaccharide capsule in response to a number of host-specific environmental stimuli. The induction of capsule is a complex biological process encompassing regulation at multiple steps, including the biosynthesis, transport, and maintenance of the polysaccharide at the cell surface. By precisely regulating the composition of its cell surface and secreted polysaccharides, C. neoformans has developed intricate ways to establish chronic infection and dormancy in the human host. The plasticity of the capsule structure in response to various host conditions also underscores the complex relationship between host and parasite. Much of this precise regulation of capsule is achieved through the transcriptional responses of multiple conserved signaling pathways that have been coopted to regulate this C. neoformans-specific virulence-associated phenotype. This review focuses on specific host stimuli that trigger the activation of the signal transduction cascades and on the downstream transcriptional responses that are required for robust encapsulation around the cell.