The biology of the Cryptococcus neoformans species complex

Aspartyl peptidase May1 induces host inflammatory response by altering cell wall composition in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans causes cryptococcal meningoencephalitis, a disease that kills more than 180,000 people annually. Contributing to its success as a fungal pathogen is its cell wall surrounded by a capsule. When the cryptococcal cell wall is compromised, exposed pathogen-associated molecular pattern molecules (PAMPs) could trigger host recognition and initiate attack against this fungus. Thus, cell wall composition and structure are tightly regulated. The cryptococcal cell wall is unusual in that chitosan, the acetylated form of chitin, is predominant over chitin and is essential for virulence. Recently, it was shown that acidic pH weakens the cell wall and increases exposure of PAMPs partly due to decreased chitosan levels. However, the molecular mechanism responsible for the cell wall remodeling in acidic pH is unknown. In this study, by screening for genes involved in cryptococcal tolerance to high levels of CO2, we serendipitously discovered that the aspartyl peptidase May1 contributes to cryptococcal sensitivity to high levels of CO2 due to acidification of unbuffered media. Overexpression of MAY1 increases the cryptococcal cell size and elevates PAMP exposure, causing a hyper-inflammatory response in the host while MAY1 deletion does the opposite. We discovered that May1 weakens the cell wall and reduces the chitosan level, partly due to its involvement in the degradation of Chs3, the sole chitin synthase that supplies chitin to be converted to chitosan. Consistently, overexpression of CHS3 largely rescues the phenotype of MAY1oe in acidic media. Collectively, we demonstrate that May1 remodels the cryptococcal cell wall in acidic pH by reducing chitosan levels through its influence on Chs3.

IMPORTANCE

The fungal cell wall is a dynamic structure, monitoring and responding to internal and external stimuli. It provides a formidable armor to the fungus. However, in a weakened state, the cell wall also triggers host immune attack when PAMPs, including glucan, chitin, and mannoproteins, are exposed. In this work, we found that the aspartyl peptidase May1 impairs the cell wall of Cryptococcus neoformans and increases the exposure of PAMPs in the acidic environment by reducing the chitosan level. Under acidic conditions, May1 is involved in the degradation of the chitin synthase Chs3, which supplies chitin to be deacetylated to chitosan. Consistently, the severe deficiency of chitosan in acidic pH can be rescued by overexpressing CHS3. These findings improve our understanding of cell wall remodeling and reveal a potential target to compromise the cell wall integrity in this important fungal pathogen.

Molecular characterization of clinical and environmental isolates from the Cryptococcus neoformans/C. Gattii species complexes of Maceió, Alagoas, Brazil

Cryptococcosis is one of the major life-threatening opportunistic/systemic fungal diseases of worldwide occurrence, which can be asymptomatic or establish pneumonia and meningoencephalitis mainly in immunosuppressed patients, caused by the Cryptococcus neoformans and C. gattii species complexes. Acquisition is by inhaling fungal propagules from avian droppings, tree hollows and decaying wood, and the association of the molecular types with geographic origin, virulence and antifungal resistance have epidemiological importance. Since data on cryptococcosis in Alagoas are limited, we sought to determine the molecular types of etiological agents collected from clinical and environmental sources. We evaluated 21 isolates previously collected from cerebrospinal fluid and from environment sources (pigeon droppings and tree hollows) in Maceió-Alagoas (Brazil). Restriction fragment length polymorphism of URA5 gene was performed to characterize among the eight standard molecular types (VNI-VNIV and VGI-VGIV). Among isolates, 66.67% (14) were assigned to C. neoformans VNI - 12 of them (12/14) recovered from liquor and 2 from a tree hollow (2/14). One isolate from pigeon droppings (4.76%) corresponded to C. neoformans VNIV, while five strains from tree hollows and one from pigeon droppings (6, 28.57%) to C. gattii VGII. VNI-type was present in clinical and environmental samples and most C. neoformans infections were observed in HIV-positive patients, while types VNIV and VGII were prevalent in environmental sources in Alagoas. This is the first molecular characterization of Cryptococcus spp. in Alagoas, our study provides additional information on the ecoepidemiology of Cryptococcus spp. in Brazil, contributing to a closer view of the endemic species.

Fungicide-tolerant persister formation during cryptococcal pulmonary infection

Bacterial persisters, a subpopulation of genetically susceptible cells that are normally dormant and tolerant to bactericides, have been studied extensively because of their clinical importance. In comparison, much less is known about the determinants underlying fungicide-tolerant fungal persister formation in vivo. Here, we report that during mouse lung infection, Cryptococcus neoformans forms persisters that are highly tolerant to amphotericin B (AmB), the standard of care for treating cryptococcosis. By exploring stationary-phase indicator molecules and developing single-cell tracking strategies, we show that in the lung, AmB persisters are enriched in cryptococcal cells that abundantly produce stationary-phase molecules. The antioxidant ergothioneine plays a specific and key role in AmB persistence, which is conserved in phylogenetically distant fungi. Furthermore, the antidepressant sertraline (SRT) shows potent activity specifically against cryptococcal AmB persisters. Our results provide evidence for and the determinant of AmB-tolerant persister formation in pulmonary cryptococcosis, which has potential clinical significance.

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

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

Antifungal Resistance in Cryptococcal Infections

Antifungal therapy, especially with the azoles, could promote the incidence of less susceptible isolates of Cryptococcus neoformans and C. gattii species complexes (SC), mostly in developing countries. Given that these species affect mostly the immunocompromised host, the infections are severe and difficult to treat. This review encompasses the following topics: 1. infecting species and their virulence, 2. treatment, 3. antifungal susceptibility methods and available categorical endpoints, 4. genetic mechanisms of resistance, 5. clinical resistance, 6. fluconazole minimal inhibitory concentrations (MICs), clinical outcome, 7. environmental influences, and 8. the relevance of host factors, including pharmacokinetic/pharmacodynamic (PK/PD) parameters, in predicting the clinical outcome to therapy. As of now, epidemiologic cutoff endpoints (ECVs/ECOFFs) are the most reliable antifungal resistance detectors for these species, as only one clinical breakpoint (amphotericin B and C. neoformans VNI) is available.

Evaluation of proline-rich antimicrobial peptides as potential lead structures for novel antimycotics against Cryptococcus neoformans

Background

Cryptococcosis and cryptococcal meningitis, caused by Cryptococcus neoformans infections, lead to approximately 180,000 deaths per year, primarily in developing countries. Individuals with compromised immune systems, e.g., due to HIV infection (AIDS) or chemotherapy, are particularly vulnerable. Conventional treatment options are often limited and can cause severe side effects. Therefore, this study aimed to investigate the antifungal effect of insect-derived proline-rich antimicrobial peptides (PrAMPs) against C. neoformans. These peptides are known for their low toxicity and their high efficacy in murine infection models, making them a promising alternative for treatment.

Results

A preliminary screening of the minimal inhibitory concentrations (MICs) of 20 AMPs, including the well-known PrAMPs Onc112, Api137, and Chex1Arg20 as well as the cathelicidin CRAMP against the C. neoformans strains 1841, H99, and KN99α revealed promising results, with MICs as low as 1.6 μmol/L. Subsequent investigations of selected peptides, determining their influence on fungal colony-forming units, confirmed their strong activity. The antifungal activity was affected by factors such as peptide net charge and sequence, with stronger effects at higher net charges probably due to better intracellular uptake confirmed by confocal laser scanning microscopy. Inactive scrambled peptides suggest a specific intracellular target, although scanning electron microscopy showed that PrAMPs also damaged the cell exterior for a low proportion of the cells. Possible pore formation could facilitate entry into the cytosol.

Measuring Stress Phenotypes in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic human fungal pathogen capable of surviving in a wide range of environments and hosts. It has been developed as a model organism to study fungal pathogenesis due to its fully sequenced haploid genome and optimized gene deletion and mutagenesis protocols. These methods have greatly aided in determining the relationship between Cryptococcus genotype and phenotype. Furthermore, the presence of congenic mata and matα strains associated with a defined sexual cycle has helped further understand cryptococcal biology. Several in vitro stress conditions have been optimized to closely mimic the stress that yeast encounter in the environment or within the infected host. These conditions have proven to be extremely useful in elucidating the role of several genes in allowing yeast to adapt and survive in hostile external environments. This chapter describes various in vitro stress conditions that could be used to test the sensitivity of different mutant strains, as well as the protocol for preparing them. We have also included a list of mutants that could be used as a positive control strain when testing the sensitivity of the desired strain to a specific stress.

Histone acetyltransferase Gcn5-mediated histone H3 acetylation facilitates cryptococcal morphogenesis and sexual reproduction

IMPORTANCE

Eukaryotic gene transcription is typically regulated by a series of histone modifications, which play a crucial role in adapting to complex environmental stresses. In the ubiquitous human fungal pathogen Cryptococcus neoformans, sexual life cycle is a continuous intracellular differentiation process that strictly occurs in response to mating stimulation. Despite the comprehensive identification of the regulatory factors and genetic pathways involved in its sexual cycle, understanding of the epigenetic modifications involved in this process remains quite limited. In this research, we found that histone acetyltransferase Gcn5-mediated histone H3 acetylation plays a crucial role in completing the cryptococcal sexual cycle, including yeast-hyphae morphogenesis and the subsequent sexual reproduction. Furthermore, we demonstrated that Gcn5 participates in this process primarily through regulating the key morphogenesis regulator Znf2 and its targets. This study thus provided a comprehensive understanding of how histone acetylation modification impacts sexual life cycle in a high-risk human pathogenic fungus.

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

Cryptococcus neoformans is a fungal pathogen responsible for cryptococcosis and cryptococcal meningitis. The C. neoformans capsular polysaccharide and shed exopolysaccharide functions both as a key virulence factor and to protect the fungal cell from phagocytosis. Currently, a glycoconjugate of these polysaccharides is being explored as a vaccine to protect against C. neoformans infection. In this combined NMR and MD study, experimentally determined NOEs and J-couplings support a structure of the synthetic decasaccharide, GXM10-Ac3, obtained by MD. GXM10-Ac3 was designed as an extension of glucuronoxylomannan (GXM) polysaccharide motif (M2) which is common in the clinically predominant serotype A strains and is recognized by protective forms of GXM-specific monoclonal antibodies. The M2 motif is characterized by a 6-residue α-mannan backbone repeating unit, consisting of a triad of α-(1→3)-mannoses, modified by β-(1→2)-xyloses on the first two mannoses and a β-(1→2)-glucuronic acid on the third mannose. The combined NMR and MD analyses reveal that GXM10-Ac3 adopts an extended structure, with xylose/glucuronic acid branches alternating sides along the α-mannan backbone. O-acetyl esters also alternate sides and are grouped in pairs. MD analysis of a twelve M2-repeating unit polymer supports the notion that the GXM10-Ac3 structure is uniformly represented throughout the polysaccharide. This experimentally consistent GXM model displays high flexibility while maintaining a structural identity, yielding new insights to further explore intermolecular interactions between polysaccharides, interactions with anti-GXM mAbs, and the cryptococcal polysaccharide architecture.

Connecting Cryptococcal Meningitis and Gut Microbiome

Fungal pathogens of the Cryptococcus neoformans species complex (C. neoformans SC) are a major cause of fungal meningitis in immunocompromised individuals. As with other melanotic microorganisms associated with human diseases, the cell-wall-associated melanin of C. neoformans SC is a major virulence factor that contributes to its ability to evade host immune responses. The levels of melanin substrate and the regulation of melanin formation could be influenced by the microbiota-gut-brain axis. Moreover, recent studies show that C. neoformans infections cause dysbiosis in the human gut microbiome. In this review, we discuss the potential association between cryptococcal meningitis and the gut microbiome. Additionally, the significant potential of targeting the gut microbiome in the diagnosis and treatment of this debilitating disease is emphasized.

In vivo assessment of differences in fungal cell density in cerebral cryptococcomas of mice infected with Cryptococcus neoformans or Cryptococcus gattii

In cerebral cryptococcomas caused by Cryptococcus neoformans or Cryptococcus gattii, the density of fungal cells within lesions can contribute to the overall brain fungal burden. In cultures, cell density is inversely related to the size of the cryptococcal capsule, a dynamic polysaccharide layer surrounding the cell. Methods to investigate cell density or related capsule size within fungal lesions of a living host are currently unavailable, precluding in vivo studies on longitudinal changes. Here, we assessed whether intravital microscopy and quantitative magnetic resonance imaging techniques (diffusion MRI and MR relaxometry) would enable non-invasive investigation of fungal cell density in cerebral cryptococcomas in mice. We compared lesions caused by type strains C. neoformans H99 and C. gattii R265 and evaluated potential relations between observed imaging properties, fungal cell density, total cell and capsule size. The observed inverse correlation between apparent diffusion coefficient and cell density permitted longitudinal investigation of cell density changes. Using these imaging methods, we were able to study the multicellular organization and cell density within brain cryptococcomas in the intact host environment of living mice. Since the MRI techniques are also clinically available, the same approach could be used to assess fungal cell density in brain lesions of patients.

Ubiquitin Degradation of the AICAR Transformylase/IMP Cyclohydrolase Ade16 Regulates the Sexual Reproduction of Cryptococcus neoformans

F-box protein is a key protein of the SCF E3 ubiquitin ligase complex, responsible for substrate recognition and degradation through specific interactions. Previous studies have shown that F-box proteins play crucial roles in Cryptococcus sexual reproduction. However, the molecular mechanism by which F-box proteins regulate sexual reproduction in C. neoformans is unclear. In the study, we discovered the AICAR transformylase/IMP cyclohydrolase Ade16 as a substrate of Fbp1. Through protein interaction and stability experiments, we demonstrated that Ade16 is a substrate for Fbp1. To examine the role of ADE16 in C. neoformans, we constructed the iADE16 strains and ADE16^OE strains to analyze the function of Ade16. Our results revealed that the iADE16 strains had a smaller capsule and showed growth defects under NaCl, while the ADE16^OE strains were sensitive to SDS but not to Congo red, which is consistent with the stress phenotype of the fbp1Δ strains, indicating that the intracellular protein expression level after ADE16 overexpression was similar to that after FBP1 deletion. Interestingly, although iADE16 strains can produce basidiospores normally, ADE16^OE strains can produce mating mycelia but not basidiospores after mating, which is consistent with the fbp1Δmutant strains, suggesting that Fbp1 is likely to regulate the sexual reproduction of C. neoformans through the modulation of Ade16. A fungal nuclei development assay showed that the nuclei of the ADE16^OE strains failed to fuse in the bilateral mating, indicating that Ade16 plays a crucial role in the regulation of meiosis during mating. In summary, our findings have revealed a new determinant factor involved in fungal development related to the post-translational regulation of AICAR transformylase/IMP cyclohydrolase.

Vaccination with a ZNF2oe Strain of Cryptococcus Provides Long-Lasting Protection against Cryptococcosis and Is Effective in Immunocompromised Hosts

Systemic cryptococcosis is fatal without treatment. Even with the current antifungal therapies, this disease kills 180,000 of 225,000 infected people annually. Exposure to the causative environmental fungus Cryptococcus neoformans is universal. Either reactivation of a latent infection or an acute infection after high exposure to cryptococcal cells can result in cryptococcosis. Currently, there is no vaccine to prevent cryptococcosis. Previously, we discovered that Znf2, a transcription factor that directs Cryptococcus yeast-to-hypha transition, profoundly affects cryptococcal interaction with the host. Overexpression of ZNF2 drives filamentous growth, attenuates cryptococcal virulence, and elicits protective host immune responses. Importantly, immunization with cryptococcal cells overexpressing ZNF2, in either live or heat-inactivated form, offers significant protection to the host from a subsequent challenge by the otherwise lethal clinical isolate H99. In this study, we found that the heat-inactivated ZNF2^oe vaccine offered long-lasting protection with no relapse upon challenge with the wild-type H99. Vaccination with heat-inactivated ZNF2^oe cells provides partial protection in hosts with preexisting asymptomatic cryptococcal infection. Importantly, once animals have been vaccinated with heat-inactivated or live short-lived ZNF2^oe cells, they are protected against cryptococcosis even when their CD4⁺ T cells are depleted at the time of fungal challenge. Remarkably, vaccination with live, short-lived ZNF2^oe cells in CD4-depleted hosts still provides strong protection to these hosts with preexisting immunodeficiency at the time of vaccination. This work raises hope for developing effective vaccines with long-lasting protection for individuals who are immunocompromised or could become immunocompromised later in life.

Hyphal Growth in Trichosporon asahii Is Accelerated by the Addition of Magnesium

Fungal dimorphism involves two morphologies: a unicellular yeast cell and a multicellular hyphal form. Invasion of hyphae into human cells causes severe opportunistic infections. The transition between yeast and hyphal forms is associated with the virulence of fungi; however, the mechanism is poorly understood. Therefore, we aimed to identify factors that induce hyphal growth of Trichosporon asahii, a dimorphic basidiomycete that causes trichosporonosis. T. asahii showed poor growth and formed small cells containing large lipid droplets and fragmented mitochondria when cultivated for 16 h in a nutrient-deficient liquid medium. However, these phenotypes were suppressed via the addition of yeast nitrogen base. When T. asahii cells were cultivated in the presence of different compounds present in the yeast nitrogen base, we found that magnesium sulfate was a key factor for inducing cell elongation, and its addition dramatically restored hyphal growth in T. asahii. In T. asahii hyphae, vacuoles were enlarged, the size of lipid droplets was decreased, and mitochondria were distributed throughout the cell cytoplasm and adjacent to the cell walls. Additionally, hyphal growth was disrupted due to treatment with an actin inhibitor. The actin inhibitor latrunculin A disrupted the mitochondrial distribution even in hyphal cells. Furthermore, magnesium sulfate treatment accelerated hyphal growth in T. asahii for 72 h when the cells were cultivated in a nutrient-deficient liquid medium. Collectively, our results suggest that an increase in magnesium levels triggers the transition from the yeast to hyphal form in T. asahii. These findings will support studies on the pathogenesis of fungi and aid in developing treatments. IMPORTANCE Understanding the mechanism underlying fungal dimorphism is crucial to discern its invasion into human cells. Invasion is caused by the hyphal form rather than the yeast form; therefore, it is important to understand the mechanism of transition from the yeast to hyphal form. To study the transition mechanism, we utilized Trichosporon asahii, a dimorphic basidiomycete that causes severe trichosporonosis since there are fewer studies on T. asahii than on ascomycetes. This study suggests that an increase in Mg2+, the most abundant mineral in living cells, triggers growth of filamentous hyphae and increases the distribution of mitochondria throughout the cell cytoplasm and adjacent to the cell walls in T. asahii. Understanding the mechanism of hyphal growth triggered by Mg2+ increase will provide a model system to explore fungal pathogenicity in the future.

Cell wall composition in Cryptococcus neoformans is media dependent and alters host response, inducing protective immunity

Introduction

Cryptococcus neoformans is a basidiomycete fungus that can cause meningoencephalitis, especially in immunocompromised patients. Cryptococcus grows in many different media, although little attention has been paid to the role of growth conditions on the cryptococcal cell wall or on virulence.

Objective

The purpose of this study was to determine how different media influenced the amount of chitin and chitosan in the cell wall, which in turn impacted the cell wall architecture and host response.

Methods

Yeast extract, peptone, and dextrose (YPD) and yeast nitrogen base (YNB) are two commonly used media for growing Cryptococcus before use in in vitro or in vivo experiments. As a result, C. neoformans was grown in either YPD or YNB, which were either left unbuffered or buffered to pH 7 with MOPS. These cells were then labeled with cell wall-specific fluorescent probes to determine the amounts of various cell wall components. In addition, these cells were employed in animal virulence studies using the murine inhalation model of infection.

Results

We observed that the growth of wild-type C. neoformans KN99 significantly changes the pH of unbuffered media during growth. It raises the pH to 8.0 when grown in unbuffered YPD but lowers the pH to 2.0 when grown in unbuffered YNB (YNB-U). Importantly, the composition of the cell wall was substantially impacted by growth in different media. Cells grown in YNB-U exhibited a 90% reduction in chitosan, the deacetylated form of chitin, compared with cells grown in YPD. The decrease in pH and chitosan in the YNB-U-grown cells was associated with a significant increase in some pathogen-associated molecular patterns on the surface of cells compared with cells grown in YPD or YNB, pH 7. This altered cell wall architecture resulted in a significant reduction in virulence when tested using a murine model of infection. Furthermore, when heat-killed cells were used as the inoculum, KN99 cells grown in YNB-U caused an aberrant hyper-inflammatory response in the lungs, resulting in rapid animal death. In contrast, heat-killed KN99 cells grown in YNB, pH 7, caused little to no inflammatory response in the host lung, but, when used as a vaccine, they conferred a robust protective response against a subsequent challenge infection with the virulent KN99 cells.

Conclusion

These findings emphasize the importance of culture media and pH during growth in shaping the content and organization of the C. neoformans cell wall, as well as their impact on fungal virulence and the host response.

Oral delivery of brain-targeted miltefosine-loaded alginate nanoparticles functionalized with polysorbate 80 for the treatment of cryptococcal meningitis

OBJECTIVES

To develop alginate nanoparticles functionalized with polysorbate 80 (P80) as miltefosine carriers for brain targeting in the oral treatment of cryptococcal meningitis.

METHODS

Miltefosine-loaded alginate nanoparticles functionalized or not with P80 were produced by an emulsification/external gelation method and the physicochemical characteristics were determined. The haemolytic activity and cytotoxic and antifungal effects of nanoparticles were assessed in an in vitro model of the blood-brain barrier (BBB). A murine model of disseminated cryptococcosis was used for testing the efficacy of oral treatment with the nanoparticles. In addition, serum biomarkers were measured for toxicity evaluation and the nanoparticle biodistribution was analysed.

RESULTS

P80-functionalized nanoparticles had a mean size of ∼300 nm, a polydispersity index of ∼0.4 and zeta potential around -50 mV, and they promoted a sustained drug release. Both nanoparticles were effective in decreasing the infection process across the BBB model and reduced drug cytotoxicity and haemolysis. In in vivo cryptococcosis, the oral treatment with two doses of P80 nanoparticles reduced the fungal burden in the brain and lungs, while the non-functionalized nanoparticles reduced fungal amount only in the lungs, and the free miltefosine was not effective. In addition, the P80-functionalization improved the nanoparticle distribution in several organs, especially in the brain. Finally, treatment with nanoparticles did not cause any toxicity in animals.

CONCLUSIONS

These results support the potential use of P80-functionalized alginate nanoparticles as miltefosine carriers for non-toxic and effective alternative oral treatment, enabling BBB translocation and reduction of fungal infection in the brain.

The small heat shock protein Hsp12.1 has a major role in the stress response and virulence of Cryptococcus gattii

Cryptococcus gattii is one of the etiological agents of cryptococcosis. To achieve a successful infection, C. gattii cells must overcome the inhospitable host environment and deal with the highly specialized immune system and poor nutrients availability. Inside the host, C. gattii uses a diversified set of tools to maintain homeostasis and establish infection, such as the expression of remarkable and diverse heat shock proteins (Hsps). Grouped by molecular weight, little is known about the Hsp12 subset in pathogenic fungi. In this study, the function of the C. gattii HSP12.1 and HSP12.2 genes was characterized. Both genes were upregulated during murine infection and heat shock. The hsp12.1 Δ null mutant cells were sensitive to plasma membrane and oxidative stressors. Moreover, HSP12 deletion induced C. gattii reactive oxygen species (ROS) accumulation associated with a differential expression pattern of oxidative stress-responsive genes compared to the wild type strain. Apart from these findings, the deletion of the paralog gene HSP12.2 did not lead to any detectable phenotype. Additionally, the double-deletion mutant strain hsp12.1 Δ /hsp12.2 Δ presented a similar phenotype to the single-deletion mutant hsp12.1 Δ, suggesting a minor participation of Hsp12.2 in these processes. Furthermore, HSP12.1 disruption remarkably affected C. gattii virulence and phagocytosis by macrophages in an invertebrate model of infection, demonstrating its importance for C. gattii pathogenicity.

Bioluminescence Imaging, a Powerful Tool to Assess Fungal Burden in Live Mouse Models of Infection

Aspergillus fumigatus and Cryptococcus neoformans species infections are two of the most common life-threatening fungal infections in the immunocompromised population. Acute invasive pulmonary aspergillosis (IPA) and meningeal cryptococcosis are the most severe forms affecting patients with elevated associated mortality rates despite current treatments. As many unanswered questions remain concerning these fungal infections, additional research is greatly needed not only in clinical scenarios but also under controlled preclinical experimental settings to increase our understanding concerning their virulence, host-pathogen interactions, infection development, and treatments. Preclinical animal models are powerful tools to gain more insight into some of these needs. However, assessment of disease severity and fungal burden in mouse models of infection are often limited to less sensitive, single-time, invasive, and variability-prone techniques such as colony-forming unit counting. These issues can be overcome by in vivo bioluminescence imaging (BLI). BLI is a noninvasive tool that provides longitudinal dynamic visual and quantitative information on the fungal burden from the onset of infection and potential dissemination to different organs throughout the development of disease in individual animals. Hereby, we describe an entire experimental pipeline from mouse infection to BLI acquisition and quantification, readily available to researchers to provide a noninvasive, longitudinal readout of fungal burden and dissemination throughout the course of infection development, which can be applied for preclinical studies into pathophysiology and treatment of IPA and cryptococcosis in vivo.

Maintenance of divergent lineages of the Rice Blast Fungus Pyricularia oryzae through niche separation, loss of sex and post-mating genetic incompatibilities

Many species of fungal plant pathogens coexist as multiple lineages on the same host, but the factors underlying the origin and maintenance of population structure remain largely unknown. The rice blast fungus Pyricularia oryzae is a widespread model plant pathogen displaying population subdivision. However, most studies of natural variation in P. oryzae have been limited in genomic or geographic resolution, and host adaptation is the only factor that has been investigated extensively as a contributor to population subdivision. In an effort to complement previous studies, we analyzed genetic and phenotypic diversity in isolates of the rice blast fungus covering a broad geographical range. Using single-nucleotide polymorphism genotyping data for 886 isolates sampled from 152 sites in 51 countries, we showed that population subdivision of P. oryzae in one recombining and three clonal lineages with broad distributions persisted with deeper sampling. We also extended previous findings by showing further population subdivision of the recombining lineage into one international and three Asian clusters, and by providing evidence that the three clonal lineages of P. oryzae were found in areas with different prevailing environmental conditions, indicating niche separation. Pathogenicity tests and bioinformatic analyses using an extended set of isolates and rice varieties indicated that partial specialization to rice subgroups contributed to niche separation between lineages, and differences in repertoires of putative virulence effectors were consistent with differences in host range. Experimental crosses revealed that female sterility and early post-mating genetic incompatibilities acted as strong additional barriers to gene flow between clonal lineages. Our results demonstrate that the spread of a fungal pathogen across heterogeneous habitats and divergent populations of a crop species can lead to niche separation and reproductive isolation between distinct, widely distributed, lineages.

The Fungal Protein Mes1 Is Required for Morphogenesis and Virulence in the Dimorphic Phytopathogen Ustilago maydis

Polarized growth is a defining property of filamentous fungi, which plays an important role in different aspects of their biology, including virulence. However, little information is available about the determinants of cell surface organization and their role in polarized growth. The fungal protein MesA was identified in a genetic screen in Aspergillus nidulans and is involved in the stabilization of the polarity axes, but it has no evident role in budding yeast. In this work, I present evidence that in the dimorphic fungal phytopathogen Ustilago maydis MesA/Mes1 is involved in cell wall stability and polarized growth. mes1 mutants were more sensitive to drugs provoking cell wall stress, and they displayed a temperature-sensitive phenotype. Actin cytoskeleton was disorganized in a mes1 mutant, suggesting that there is a connection between Mes1, the actin cytoskeleton and polarized morphogenesis. The septin ring was also absent from the bud tip, but not the bud neck. Deletion of mes1 provoked defects in endocytosis and vacuolar organization in the cells. Mes1 was essential for strong polarized growth in the hyphal form, but it was dispensable during low or moderate polarized growth in the yeast form in U. maydis at a permissive temperature. Consistently, mes1 mutants showed delayed mating and they were avirulent.

Functions and applications of glycolipid-hydrolyzing microbial glycosidases

Glycolipids are important components of cell membranes in several organisms. The major glycolipids in mammals are glycosphingolipids (GSLs), which are composed of ceramides. In mammals, GSLs are degraded stepwise from the non-reducing end of the oligosaccharides via exo-type glycosidases. However, endoglycoceramidase (EGCase), an endo-type glycosidase found in actinomycetes, is a unique enzyme that directly acts on the glycosidic linkage between oligosaccharides and ceramides to generate intact oligosaccharides and ceramides. Three molecular species of EGCase, namely EGCase I, EGCase II, and endogalactosylceramidase, have been identified based on their substrate specificity. EGCrP1 and EGCrP2, which are homologs of EGCase in pathogenic fungi, were identified as the first fungal glucosylceramide- and sterylglucoside-hydrolyzing glycosidases, respectively. These enzymes are promising targets for antifungal drugs against pathogenic fungi. This review describes the functions and properties of these microbial glycolipid-degrading enzymes, the molecular basis of their differential substrate specificity, and their applications.

Cryptococcus: History, Epidemiology and Immune Evasion

Cryptococcosis is a disease caused by the pathogenic fungi Cryptococcus neoformans and Cryptococcus gattii, both environmental fungi that cause severe pneumonia and may even lead to cryptococcal meningoencephalitis. Although C. neoformans affects more fragile individuals, such as immunocompromised hosts through opportunistic infections, C. gattii causes a serious indiscriminate primary infection in immunocompetent individuals. Typically seen in tropical and subtropical environments, C. gattii has increased its endemic area over recent years, largely due to climatic factors that favor contagion in warmer climates. It is important to point out that not only C. gattii, but the Cryptococcus species complex produces a polysaccharidic capsule with immunomodulatory properties, enabling the pathogenic species of Cryptococccus to subvert the host immune response during the establishment of cryptococcosis, facilitating its dissemination in the infected organism. C. gattii causes a more severe and difficult-to-treat infection, with few antifungals eliciting an effective response during chronic treatment. Much of the immunopathology of this cryptococcosis is still poorly understood, with most studies focusing on cryptococcosis caused by the species C. neoformans. C. gattii became more important in the epidemiological scenario with the outbreaks in the Pacific Northwest of the United States, which resulted in phylogenetic studies of the virulent variant responsible for the severe infection in the region. Since then, the study of cryptococcosis caused by C. gattii has helped researchers understand the immunopathological aspects of different variants of this pathogen.

Faster Cryptococcus Melanization Increases Virulence in Experimental and Human Cryptococcosis

Cryptococcus spp. are human pathogens that cause 181,000 deaths per year. In this work, we systematically investigated the virulence attributes of Cryptococcus spp. clinical isolates and correlated them with patient data to better understand cryptococcosis. We collected 66 C. neoformans and 19 C. gattii clinical isolates and analyzed multiple virulence phenotypes and host-pathogen interaction outcomes. C. neoformans isolates tended to melanize faster and more intensely and produce thinner capsules in comparison with C. gattii. We also observed correlations that match previous studies, such as that between secreted laccase and disease outcome in patients. We measured Cryptococcus colony melanization kinetics, which followed a sigmoidal curve for most isolates, and showed that faster melanization correlated positively with LC3-associated phagocytosis evasion, virulence in Galleria mellonella and worse prognosis in humans. These results suggest that the speed of melanization, more than the total amount of melanin Cryptococcus spp. produces, is crucial for virulence.

Role of F-box Protein Cdc4 in Fungal Virulence and Sexual Reproduction of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic yeast-like pathogen that mainly infects immunocompromised individuals and causes fatal meningitis. Sexual reproduction can promote the exchange of genetic material between different strains of C. neoformans, which is one of the reasons leading to the emergence of highly pathogenic and drug-resistant strains of C. neoformans. Although much research has been done on the regulation mechanism of Cryptococcus sexual reproduction, there are few studies on the sexual reproduction regulation of Cryptococcus by the ubiquitin-proteasome system. This study identified an F-box protein, Cdc4, which contains a putative F-box domain and eight WD40 domains. The expression pattern analysis showed that the CDC4 gene was expressed in various developmental stages of C. neoformans, and the Cdc4 protein was localized in the nucleus of cryptococcal cells. In vitro stress responses assays showed that the CDC4 overexpression strains are sensitive to SDS and MMS but not Congo red, implying that Cdc4 may regulate the cell membrane integrity and repair of DNA damage of C. neoformans. Fungal virulence assay showed that although the cdc4Δ mutant grows normally and can produce typical virulence factors such as capsule and melanin, the cdc4Δ mutant completely loses its pathogenicity in a mouse systemic-infection model. Fungal mating assays showed that Cdc4 is also essential for fungal sexual reproduction in C. neoformans. Although normal mating hyphae were observed during mating, the basidiospores' production was blocked in bilateral mating between cdc4Δ mutants. Fungal nuclei development assay showed that the nuclei failed to undergo meiosis after fusion inside the basidia during the bilateral mating of cdc4Δ mutants, indicating that Cdc4 is critical to regulating meiosis during cryptococcal mating. In summary, our study revealed that the F-box protein Cdc4 is critical for fungal virulence and sexual reproduction in C. neoformans.

A forkhead transcription factor contributes to the regulatory differences of pathogenicity in closely related fungal pathogens

Cryptococcus neoformans and its sister species Cryptococcus deuterogattii are important human fungal pathogens. Despite their phylogenetically close relationship, these two Cryptococcus pathogens are greatly different in their clinical characteristics. However, the determinants underlying the regulatory differences of their pathogenicity remain largely unknown. Here, we show that the forkhead transcription factor Hcm1 promotes infection in C. neoformans but not in C. deuterogattii. Monitoring in vitro and in vivo fitness outcomes of multiple clinical isolates from the two pathogens indicates that Hcm1 mediates pathogenicity in C. neoformans through its key involvement in oxidative stress defense. By comparison, Hcm1 is not critical for antioxidation in C. deuterogattii. Furthermore, we identified SRX1, which encodes the antioxidant sulfiredoxin, as a conserved target of Hcm1 in two Cryptococcus pathogens. Like HCM1, SRX1 had a greater role in antioxidation in C. neoformans than in C. deuterogattii. Significantly, overexpression of SRX1 can largely rescue the defective pathogenicity caused by the absence of Hcm1 in C. neoformans. Conversely, Srx1 is dispensable for virulence in C. deuterogattii. Overall, our findings demonstrate that the difference in the contribution of the antioxidant sulfiredoxin to oxidative stress defense underlies the Hcm1-mediated regulatory differences of pathogenicity in two closely related pathogens.

Deciphering the Association among Pathogenicity, Production and Polymorphisms of Capsule/Melanin in Clinical Isolates of Cryptococcus neoformans var. grubii VNI

Background: Cryptococcus neoformans is an opportunistic fungal pathogen that can cause meningitis in immunocompromised individuals. The objective of this work was to study the relationship between the phenotypes and genotypes of isolates of clinical origin from different cities in Colombia. Methods: Genome classification of 29 clinical isolates of C. neoformans var. grubii was performed using multilocus sequence typing (MLST), and genomic sequencing was used to genotype protein-coding genes. Pathogenicity was assessed in a larval model, and melanin production and capsule size were evaluated in vitro and in vivo. Results: Eleven MLST sequence types (STs) were found, the most frequent being ST69 (n = 9), ST2, ST93, and ST377 (each with n = 4). In the 29 isolates, different levels of pigmentation, capsule size and pathogenicity were observed. Isolates classified as highly pathogenic showed a tendency to exhibit larger increases in capsule size. In the analysis of polymorphisms, 48 non-synonymous variants located in the predicted functional domains of 39 genes were found to be associated with capsule size change, melanin, or pathogenicity. Conclusions: No clear patterns were found in the analysis of the phenotype and genotype of Cryptococcus. However, the data suggest that the increase in capsule size is a key variable for the differentiation of pathogenic isolates, regardless of the method used for its induction.

Identification and Characterization of an Intergenic “Safe Haven” Region in Human Fungal Pathogen Cryptococcus gattii

Cryptococcus gattii is a primary fungal pathogen, which causes pulmonary and brain infections in healthy as well as immunocompromised individuals. Genetic manipulations in this pathogen are widely employed to study its biology and pathogenesis, and require integration of foreign DNA into the genome. Thus, identification of gene free regions where integrated foreign DNA can be expressed without influencing, or being influenced by, nearby genes would be extremely valuable. To achieve this goal, we examined publicly available genomes and transcriptomes of C. gattii, and identified two intergenic regions in the reference strain R265 as potential “safe haven” regions, named as CgSH1 and CgSH2. We found that insertion of a fluorescent reporter gene and a selection marker at these two intergenic regions did not affect the expression of their neighboring genes and were also expressed efficiently, as expected. Furthermore, DNA integration at CgSH1 or CgSH2 had no apparent effect on the growth of C. gattii, its response to various stresses, or phagocytosis by macrophages. Thus, the identified safe haven regions in C. gattii provide an effective tool for researchers to reduce variation and increase reproducibility in genetic experiments.

Early clinical and microbiological predictors of outcome in hospitalized patients with cryptococcal meningitis

Background

Cryptococcal meningitis causes high mortality in immunocompromised and immunocompetent patients. The objective of this study was to identify early predictors of clinical outcome, available at the first days of hospitalization, in patients with cryptococcal meningitis in a tertiary center in Brazil.

Methods

Ninety-six cases of cryptococcal meningitis with clinical, epidemiological and laboratory data, and identification and antifungal susceptibility of the strains were analyzed. Quantitative CSF yeast counts were performed by direct microscopic exam with a Fuchs-Rosenthal cell counting chamber using an institutional protocol. Univariable and multiple analyses using logistic regression were performed to identify predictors, available at the beginning of hospitalization, of in-hospital mortality. Moreover, we performed a secondary analysis for a composite outcome defined by hospital mortality and intensive care unit transfer.

Results

The species and the antifungal susceptibility were not associated with the outcomes evaluated. The variables significantly associated with the mortality were age (OR = 1.08, 95% CI 1.02–1.15), the cerebrospinal fluid (CSF) yeasts count (OR = 1.65, 95% CI 1.20–2.27), systemic arterial hypertension (OR = 22.63, 95% CI 1.64–312.91) and neurological impairment identified by computed tomography (OR = 41.73, 95% CI 3.10–561.65). At the secondary analysis, CSF yeast count was also associated with the composite outcome, in addition to the culture of Cryptococcus spp. from bloodstream and cerebral toxoplasmosis. The associations were consistent with survival models evaluated.

Conclusions

Age and CSF yeast count were independently associated with in-hospital mortality of patients with cryptococcal meningitis but Cryptococcus species identification and antifungal susceptibility were not associated with the outcomes. Quantitative CSF yeast counts used in this study can be evaluated and implemented in other low and middle-income settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-022-07118-7.

Ophthalmic manifestations of Cryptococcus gattii species complex: a case series and review of the literature

AIM

To report 4 cases of Cryptococcus gattii (C. gattii) species complex infection with diverse ophthalmic manifestations, and to review the literature to examine pathobiology of disease, classical ophthalmic presentations and outcomes, and treatment modalities for this emerging pathogen.

METHODS

Cases of C. gattii meningoencephalitis with ophthalmic manifestations were identified via chart review at two institutions in Australia and one institution in the mid-west region of the United States and are reported as a case series. Additionally, a MEDLINE literature review was conducted to identify all reported cases of C. gattii with ophthalmic manifestations from 1990-2020. Cases were reviewed and tabulated, together with our series of patients, in this report.

RESULTS

Four cases of C. gattii with ophthalmic manifestations are presented; three from Australia and one from the USA. A literature review identified a total of 331 cases of C. gattii with visual sequelae. The majority of cases occurred in immunocompetent individuals. Blurred vision and diplopia were the most common presenting symptoms, with papilloedema the most common sign, reported in 10%-50% of cases. Visual loss was reported in 10%-53% of cases, as compared to rates of visual loss of 1%-9% in C. neoformans infection. Elevated intracranial pressure, cerebrospinal fluid (CSF) fungal burden, and abnormal neurological exam at presentation correlated with poor visual outcomes. The mainstays of treatment are anti-fungal agents and aggressive management of intracranial hypertension with serial lumbar punctures. CSF diversion procedures should be considered for refractory cases. Acetazolamide and mannitol are associated with high complication rates, and adjuvant corticosteroids have demonstrated higher mortality rates; these treatments should be avoided.

CONCLUSION

Permanent visual loss represents a devastating yet potentially preventable sequelae of C. gattii infection. Intracranial hypertension needs to be recognised early and aggressively managed. Referral to an ophthalmologist/neuro-ophthalmologist in all cases of cryptococcal infection independent of visual symptoms at time of diagnosis is recommended.

The Harris’ hawk (Parabuteo unicinctus) as a source of pathogenic human yeasts: a potential risk to human health

Aim: Invasive human fungal infections have been a serious public health problem among immunocompromised patients. Wild bird species are related to the eco-epidemiology of some infectious diseases, mainly Cryptococcosis, Histoplasmosis, Aspergillosis, Chlamydiosis, Salmonellosis and allergic diseases. Falconry is the art of training predators for hunting. Nowadays, birds of prey are used as pets, which brings new sources of infections to humans. Materials & methods: We identified fungal pathogenic yeasts, Candida parapsilosis, Debaryomyces hansenii and Rhodotorula mucilaginosa. Conclusion: Study new environmental niches of human pathogens is vitally important to establish preventive actions with the purpose of minimizing the risks of human contamination. Our work describes yeast microbiota from the excreta of Parabuteo unicinctus as a potential hazard for human disease.

Cryptococcus neoformans melanization incorporates multiple catecholamines to produce polytypic melanin

Melanin is a major virulence factor in pathogenic fungi that enhances the ability of fungal cells to resist immune clearance. Cryptococcus neoformans is an important human pathogenic fungus that synthesizes melanin from exogenous tissue catecholamine precursors during infection, but the type of melanin made in cryptococcal meningoencephalitis is unknown. We analyzed the efficacy of various catecholamines found in brain tissue in supporting melanization using animal brain tissue and synthetic catecholamine mixtures reflecting brain tissue proportions. Solid-state NMR spectra of the melanin pigment produced from such mixtures yielded more melanin than expected if only the preferred constituent dopamine had been incorporated, suggesting uptake of additional catecholamines. Probing the biosynthesis of melanin using radiolabeled catecholamines revealed that C. neoformans melanization simultaneously incorporated more than one catecholamine, implying that the pigment was polytypic in nature. Nonetheless, melanin derived from individual or mixed catecholamines had comparable ability to protect C. neoformans against ultraviolet light and oxidants. Our results indicate that melanin produced during infection differs depending on the catecholamine composition of tissue and that melanin pigment synthesized in vivo is likely to accrue from the polymerization of a mixture of precursors. From a practical standpoint, our results strongly suggest that using dopamine as a polymerization precursor is capable of producing melanin pigment comparable to that produced during infection. On a more fundamental level, our findings uncover additional structural complexity for natural cryptococcal melanin by demonstrating that pigment produced during human infection is likely to be composed of polymerized moieties derived from chemically different precursors.

Hsp90 interaction networks in fungi-tools and techniques

Heat-shock protein 90 (Hsp90) is a central regulator of cellular proteostasis. It stabilizes numerous proteins that are involved in fundamental processes of life, including cell growth, cell-cycle progression and the environmental response. In addition to stabilizing proteins, Hsp90 governs gene expression and controls the release of cryptic genetic variation. Given its central role in evolution and development, it is important to identify proteins and genes that interact with Hsp90. This requires sophisticated genetic and biochemical tools, including extensive mutant collections, suitable epitope tags, proteomics approaches and Hsp90-specific pharmacological inhibitors for chemogenomic screens. These usually only exist in model organisms, such as the yeast Saccharomyces cerevisiae. Yet, the importance of other fungal species, such as Candida albicans and Cryptococcus neoformans, as serious human pathogens accelerated the development of genetic tools to study their virulence and stress response pathways. These tools can also be exploited to map Hsp90 interaction networks. Here, we review tools and techniques for Hsp90 network mapping available in different fungi and provide a summary of existing mapping efforts. Mapping Hsp90 networks in fungal species spanning >500 million years of evolution provides a unique vantage point, allowing tracking of the evolutionary history of eukaryotic Hsp90 networks.

Functional Roles of Homologous Recombination and Non-Homologous End Joining in DNA Damage Response and Microevolution in Cryptococcus neoformans

DNA double-strand breaks (DSBs) are the most deleterious type of DNA lesions because they cause loss of genetic information if not properly repaired. In eukaryotes, homologous recombination (HR) and non-homologous end joining (NHEJ) are required for DSB repair. However, the relationship of HR and NHEJ in DNA damage stress is unknown in the radiation-resistant fungus Cryptococcus neoformans. In this study, we found that the expression levels of HR- and NHEJ-related genes were highly induced in a Rad53-Bdr1 pathway-dependent manner under genotoxic stress. Deletion of RAD51, which is one of the main components in the HR, resulted in growth under diverse types of DNA damage stress, whereas perturbations of KU70 and KU80, which belong to the NHEJ system, did not affect the genotoxic stresses except when bleomycin was used for treatment. Furthermore, deletion of both RAD51 and KU70/80 renders cells susceptible to oxidative stress. Notably, we found that deletion of RAD51 induced a hypermutator phenotype in the fluctuation assay. In contrast to the fluctuation assay, perturbation of KU70 or KU80 induced rapid microevolution similar to that induced by the deletion of RAD51. Collectively, Rad51-mediated HR and Ku70/Ku80-mediated NHEJ regulate the DNA damage response and maintain genome stability.

Population genomic analysis of Cryptococcus Brazilian isolates reveals an African type subclade distribution

The genomes of a large number of Cryptococcus neoformans isolates have been sequenced and analyzed in recent years. These genomes have been used to understand the global population structure of this opportunistic pathogen. However, only a small number of South American isolates have been considered in these studies, and the population structure of C. neoformans in this part of the world remains elusive. Here, we analyzed the genomic sequences of 53 Brazilian Cryptococcus isolates and deciphered the C. neoformans population structure in this country. Our data reveal an African-like structure that suggested repeated intercontinental transports from Africa to South America. We also identified a mutator phenotype in one VNBII Brazilian isolate, exemplifying how fast-evolving isolates can shape the Cryptococcus population structure. Finally, phenotypic analyses revealed wide diversity but not lineage specificity in the expression of classical virulence traits within the set of isolates.

Investigation of Antifungal Mechanisms of Thymol in the Human Fungal Pathogen, Cryptococcus neoformans

Due to lifespan extension and changes in global climate, the increase in mycoses caused by primary and opportunistic fungal pathogens is now a global concern. Despite increasing attention, limited options are available for the treatment of systematic and invasive mycoses, owing to the evolutionary similarity between humans and fungi. Although plants produce a diversity of chemicals to protect themselves from pathogens, the molecular targets and modes of action of these plant-derived chemicals have not been well characterized. Using a reverse genetics approach, the present study revealed that thymol, a monoterpene alcohol from Thymus vulgaris L., (Lamiaceae), exhibits antifungal activity against Cryptococcus neoformans by regulating multiple signaling pathways including calcineurin, unfolded protein response, and HOG (high-osmolarity glycerol) MAPK (mitogen-activated protein kinase) pathways. Thymol treatment reduced the intracellular concentration of Ca²⁺ by controlling the expression levels of calcium transporter genes in a calcineurin-dependent manner. We demonstrated that thymol decreased N-glycosylation by regulating the expression levels of genes involved in glycan-mediated post-translational modifications. Furthermore, thymol treatment reduced endogenous ergosterol content by decreasing the expression of ergosterol biosynthesis genes in a HOG MAPK pathway-dependent manner. Collectively, this study sheds light on the antifungal mechanisms of thymol against C. neoformans.

Cryptococcal Virulence in Humans: Learning From Translational Studies With Clinical Isolates

Cryptococcosis, an invasive mycosis caused by Cryptococcus spp, kills between 20% and 70% of the patients who develop it. There are no vaccines for prevention, and treatment is based on a limited number of antifungals. Studying fungal virulence and how the host responds to infection could lead to new therapies, improving outcomes for patients. The biggest challenge, however, is that experimental cryptococcosis models do not completely recapitulate human disease, while human experiments are limited due to ethical reasons. To overcome this challenge, one of the approaches used by researchers and clinicians is to: 1) collect cryptococcal clinical isolates and associated patient data; 2) study the set of isolates in the laboratory (virulence and host-pathogen interaction variables, molecular markers); 3) correlate the laboratory and patient data to understand the roles fungal attributes play in the human disease. Here we review studies that have shed light on the cryptococcosis pathophysiology using these approaches, with a special focus on human disease. Isolates that more effectively evade macrophage responses, that secrete more laccase, melanize faster and have larger capsules in the cerebrospinal fluid are associated with poorer patient outcomes. Additionally, molecular studies have also shown that cryptococcal clades vary in virulence, with clinical impact. Limitations of those studies include the use of a small number of isolates or retrospectively collected clinical data. The fact that they resulted in very important information is a reflection of the impact this strategy has in understanding cryptococcosis and calls for international collaboration that could boost our knowledge.

What Are the Best Parents for Hybrid Progeny? An Investigation into the Human Pathogenic Fungus Cryptococcus

Hybridization between more divergent organisms is likely to generate progeny with more novel genetic interactions and genetic variations. However, the relationship between parental genetic divergence and progeny phenotypic variation remains largely unknown. Here, using strains of the human pathogenic Cryptococcus, we investigated the patterns of such a relationship. Twenty-two strains with up to 15% sequence divergence were mated. Progeny were genotyped at 16 loci. Parental strains and their progeny were phenotyped for growth ability at two temperatures, melanin production at seven conditions, and susceptibility to the antifungal drug fluconazole. We observed three patterns of relationships between parents and progeny for each phenotypic trait, including (i) similar to one of the parents, (ii) intermediate between the parents, and (iii) outside the parental phenotypic range. We found that as genetic distance increases between parental strains, progeny showed increased fluconazole resistance and growth at 37 °C but decreased melanin production under various oxidative and nitrosative stresses. Our findings demonstrate that, depending on the traits, both evolutionarily more similar strains and more divergent strains may be better parents to generate progeny with hybrid vigor. Together, the results indicate the enormous potential of Cryptococcus hybrids in their evolution and adaptation to diverse conditions.

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.

An analysis of the population of Cryptococcus neoformans strains isolated from animals in Poland, in the years 2015-2019

Fungi belonging to the Cryptococcus neoformans/C. gattii species complex (CNGSC) are pathogens causing severe infections in humans and animals, that for humans may result in a mortality rate ranging up to 70%. The CNGSC is divided into eight major molecular types, that may differ in their virulence and susceptibility. In order to fully understand the epidemiology of cryptococcosis, it is important to study the world distribution and population structure of these pathogens. The present study is the first presenting a population of strains isolated in Poland and one of the few using a multi-species animal group as a source of the specimen. The pathogen was present in 2.375% of the tested animals. The URA5-RFLP and MALDI-TOF MS analyses have revealed that the population consisted exclusively of C. neoformans strains, with a predominance of major molecular type VNIV (C. neoformans var. neoformans). The MALDI-TOF MS was used to perform the CNGSC strains identification on both the species and sub-species level. Despite the fact that the animals providing the specimens were not treated with 5-fluorocytosine, around 10% of the tested population presented MIC values exceeding 64 mg/L, indicating the existence of the 5-fluorocytosine-resistant strains in the environment.

Bridgin connects the outer kinetochore to centromeric chromatin

The microtubule-binding outer kinetochore is coupled to centromeric chromatin through CENP-C^Mif2, CENP-T^Cnn1, and CENP-U^Ame1 linker pathways originating from the constitutive centromere associated network (CCAN) of the inner kinetochore. Here, we demonstrate the recurrent loss of most CCAN components, including certain kinetochore linkers during the evolution of the fungal phylum of Basidiomycota. By kinetochore interactome analyses in a model basidiomycete and human pathogen Cryptococcus neoformans, a forkhead-associated domain containing protein “bridgin” was identified as a kinetochore component along with other predicted kinetochore proteins. In vivo and in vitro functional analyses of bridgin reveal its ability to connect the outer kinetochore with centromeric chromatin to ensure accurate chromosome segregation. Unlike established CCAN-based linkers, bridgin is recruited at the outer kinetochore establishing its role as a distinct family of kinetochore proteins. Presence of bridgin homologs in non-fungal lineages suggests an ancient divergent strategy exists to bridge the outer kinetochore with centromeric chromatin.

The kinetochore is a multi-complex structure that helps attach chromosomes to spindle microtubules, ensuring accurate chromosome segregation during cell division. Kinetochores are thought to be evolutionarily conserved, but which components are conserved is unclear. Here, the authors report that some members of the fungal phylum of Basidomycota lack many conventional kinetochore linker proteins. Instead, they possess a human Ki67-like protein that bridges the outer part of the kinetochore to centromere DNA, which may compensate for the loss of a conventional linker.

Cryptococcus neoformans Secretes Small Molecules That Inhibit IL-1β Inflammasome-Dependent Secretion

Cryptococcus neoformans is an encapsulated yeast that causes disease mainly in immunosuppressed hosts. It is considered a facultative intracellular pathogen because of its capacity to survive and replicate inside phagocytes, especially macrophages. This ability is heavily dependent on various virulence factors, particularly the glucuronoxylomannan (GXM) component of the polysaccharide capsule. Inflammasome activation in phagocytes is usually protective against fungal infections, including cryptococcosis. Nevertheless, recognition of C. neoformans by inflammasome receptors requires specific changes in morphology or the opsonization of the yeast, impairing proper inflammasome function. In this context, we analyzed the impact of molecules secreted by C. neoformans B3501 strain and its acapsular mutant Δcap67 in inflammasome activation in an in vitro model. Our results showed that conditioned media derived from B3501 was capable of inhibiting inflammasome-dependent events (i.e., IL-1β secretion and LDH release via pyroptosis) more strongly than conditioned media from Δcap67, regardless of GXM presence. We also demonstrated that macrophages treated with conditioned media were less responsive against infection with the virulent strain H99, exhibiting lower rates of phagocytosis, increased fungal burdens, and enhanced vomocytosis. Moreover, we showed that the aromatic metabolite DL-Indole-3-lactic acid (ILA) and DL-p-Hydroxyphenyllactic acid (HPLA) were present in B3501's conditioned media and that ILA alone or with HPLA is involved in the regulation of inflammasome activation by C. neoformans. These results were confirmed by in vivo experiments, where exposure to conditioned media led to higher fungal burdens in Acanthamoeba castellanii culture as well as in higher fungal loads in the lungs of infected mice. Overall, the results presented show that conditioned media from a wild-type strain can inhibit a vital recognition pathway and subsequent fungicidal functions of macrophages, contributing to fungal survival in vitro and in vivo and suggesting that secretion of aromatic metabolites, such as ILA, during cryptococcal infections fundamentally impacts pathogenesis.

The Role of Oxidoreductase-Like Protein Olp1 in Sexual Reproduction and Virulence of Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycete human fungal pathogen causing lethal meningoencephalitis, mainly in immunocompromised patients. Oxidoreductases are a class of enzymes that catalyze redox, playing a crucial role in biochemical reactions. In this study, we identified one Cryptococcus oxidoreductase-like protein-encoding gene OLP1 and investigated its role in the sexual reproduction and virulence of C. neoformans. Gene expression patterns analysis showed that the OLP1 gene was expressed in each developmental stage of Cryptococcus, and the Olp1 protein was located in the cytoplasm of Cryptococcus cells. Although it produced normal major virulence factors such as melanin and capsule, the olp1Δ mutants showed growth defects on the yeast extract peptone dextrose (YPD) medium supplemented with lithium chloride (LiCl) and 5-fluorocytosine (5-FC). The fungal mating analysis showed that Olp1 is also essential for fungal sexual reproduction, as olp1Δ mutants show significant defects in hyphae growth and basidiospores production during bisexual reproduction. The fungal nuclei imaging showed that during the bilateral mating of olp1Δ mutants, the nuclei failed to undergo meiosis after fusion in the basidia, indicating that Olp1 is crucial for regulating meiosis during mating. Moreover, Olp1 was also found to be required for fungal virulence in C. neoformans, as the olp1Δ mutants showed significant virulence attenuation in a murine inhalation model. In conclusion, our results showed that the oxidoreductase-like protein Olp1 is required for both fungal sexual reproduction and virulence in C. neoformans.

Identification and characterization of a sulfite reductase gene and new insights regarding the sulfur-containing amino acid metabolism in the basidiomycetous yeast Cryptococcus neoformans

The amino acid biosynthetic pathway of invasive pathogenic fungi has been studied as a potential antifungal drug target. Studies of the disruption of genes involved in amino acid biosynthesis have demonstrated the importance of this pathway in the virulence of Cryptococcus neoformans. Here, we identified the MET5 (CNL05500) and MET10 (CNG03990) genes in this pathway, both encoding sulfite reductase, which catalyzes the reduction of sulfite to sulfide. The MET14 (CNE03880) gene was also identified, which is responsible for the conversion of sulfate to sulfite. The use of cysteine as a sulfur source led to the production of methionine via hydrogen sulfide synthesis mediated by CYS4 (CNA06170), CYS3 (CNN01730), and MST1 (CND03690). MST1 exhibited high homology with the TUM1 gene of Saccharomyces cerevisiae, which has functional similarity with the 3-mercaptopyruvate sulfurtransferase (3-MST) gene in humans. Although the hypothesis that hydrogen sulfide is produced from cysteine via CYS4, CYS3, and MST1 warrants further study, the new insight into the metabolic pathway of sulfur-containing amino acids in C. neoformans provided here indicates the usefulness of this system in the development of screening tools for antifungal drug agents.

Update on invasive fungal infections in the Middle Eastern and North African region

In the recent years, the epidemiology of invasive fungal infections (IFIs) has changed worldwide. This is remarkably noticed with the significant increase in high-risk populations. Although surveillance of such infections is essential, data in the Middle Eastern and North African (MENA) region remain scarce. In this paper, we reviewed the existing data on the epidemiology of different IFIs in the MENA region. Epidemiological surveillance is crucial to guide optimal healthcare practices. This study can help to guide appropriate interventions and to implement antimicrobial stewardship and infection prevention and control programs in countries.

Evaluation of antifungal activity of cinnamaldehyde against Cryptococcus neoformans var. grubii

Cryptococcosis is a potentially fatal fungal disease which has aggrandized with the emergence of AIDS and antifungal resistance. The currently used antifungals lack the broad-spectrum activity and result in several toxicities during long treatment regimens. Thus, the present study aims to evaluate the antifungal activity of cinnamaldehyde against Cryptococcus neoformans var. grubii, the etiological agent of the disease. Quantitative and qualitative in vitro fungal susceptibilities were carried out by minimum inhibitory concentration assay, flow cytometric analysis, and confocal microscopy. Micromorphological alterations were studied through scanning electron and light microscopies. "In vivo" antifungal efficacy of cinnamaldehyde was assessed. Cinnamaldehyde showed antifungal activity against C. neoformans in a dose-dependent manner. A concentration of 1.37 mg/mL of cinnamaldehyde was found to be inhibitory and fungicidal while the low concentration (0.68 mg/mL) was found to induce micromorphological changes and formation of giant/titan-like cells in this pathogen. The reparative activity of cinnamaldehyde and its ability to prolong the life even after the advent of cryptococcal meningitis in mice was also noticed. This study suggests potent anti-cryptococcal activity of cinnamaldehyde. Though, it has a couple of limitations like allergy and low bioavailability. However, these problems can be circumvented by developing suitable analogs of the compound. It, therefore, could be used as a therapeutic option against cryptococcosis and cryptococcal meningitis. Moreover, the evaluation of its pharmacokinetic and pharmacodynamic properties is desirable.

Autophagy Regulates Fungal Virulence and Sexual Reproduction in Cryptococcus neoformans

Autophagy (macroautophagy) is an evolutionarily conserved degradation pathway involved in bulk degradation of cytoplasmic organelles, old protein, and other macromolecules and nutrient recycling during starvation. Extensive studies on functions of autophagy-related genes have revealed that autophagy plays a role in cell differentiation and pathogenesis of pathogenic fungi. In this study, we identified and characterized 14 core autophagy machinery genes (ATGs) in C. neoformans. To understand the function of autophagy in virulence and fungal development in C. neoformans, we knocked out the 14 ATGs in both α and a mating type strain backgrounds in C. neoformans, respectively, by using biolistic transformation and in vivo homologous recombination. Fungal virulence assay showed that virulence of each atgΔ mutants was attenuated in a murine inhalation systemic-infection model, although virulence factor production was not dramatically impaired in vitro. Fungal mating assays showed that all the 14 ATGs are essential for fungal sexual reproduction as basidiospore production was blocked in bilateral mating between each atgΔ mutants. Fungal nuclei development assay showed that nuclei in the bilateral mating of each atgΔ mutants failed to undergo meiosis after fusion, indicating autophagy is essential for regulating meiosis during mating. Overall, our study showed that autophagy is essential for fungal virulence and sexual reproduction in C. neoformans, which likely represents a conserved novel virulence and sexual reproduction control mechanism that involves the autophagy-mediated proteolysis pathway.

Core N-Glycan Structures Are Critical for the Pathogenicity of Cryptococcus neoformans by Modulating Host Cell Death

Cryptococcus neoformans is a human-pathogenic fungal pathogen that causes life-threatening meningoencephalitis in immunocompromised individuals. To investigate the roles of N-glycan core structure in cryptococcal pathogenicity, we constructed mutant strains of C. neoformans with defects in the assembly of lipid-linked N-glycans in the luminal side of the endoplasmic reticulum (ER). Deletion of ALG3 (alg3Δ), which encodes dolichyl-phosphate-mannose (Dol-P-Man)-dependent α-1,3-mannosyltransferase, resulted in the production of truncated neutral N-glycans carrying five mannose residues as a major species. Despite moderate or nondetectable defects in virulence-associated phenotypes in vitro, the alg3Δ mutant was avirulent in a mouse model of systemic cryptococcosis. Notably, the mutant did not show defects in early stages of host cell interaction during infection, including attachment to lung epithelial cells, opsonic/nonopsonic phagocytosis, and manipulation of phagosome acidification. However, the ability to drive macrophage cell death was greatly decreased in this mutant, without loss of cell wall remodeling capacity. Furthermore, deletion of ALG9 and ALG12, encoding Dol-P-Man-dependent α-1,2-mannosyltransferases and α-1,6-mannosyltransferases, generating truncated core N-glycans with six and seven mannose residues, respectively, also displayed remarkably reduced macrophage cell death and in vivo virulence. However, secretion levels of interleukin-1β (IL-1β) were not reduced in the bone marrow-derived dendritic cells obtained from Asc- and Gsdmd-deficient mice infected with the alg3Δ mutant strain, excluding the possibility that pyroptosis is a main host cell death pathway dependent on intact core N-glycans. Our results demonstrated N-glycan structures as a critical feature in modulating death of host cells, which is exploited by as a strategy for host cell escape for dissemination of C. neoformansIMPORTANCE We previously reported that the outer mannose chains of N-glycans are dispensable for the virulence of C. neoformans, which is in stark contrast to findings for the other human-pathogenic yeast, Candida albicans Here, we present evidence that an intact core N-glycan structure is required for C. neoformans pathogenicity by systematically analyzing alg3Δ, alg9Δ, and alg12Δ strains that have defects in lipid-linked N-glycan assembly and in in vivo virulence. The alg null mutants producing truncated core N-glycans were defective in inducing host cell death after phagocytosis, which is triggered as a mechanism of pulmonary escape and dissemination of C. neoformans, thus becoming inactive in causing fatal infection. The results clearly demonstrated the critical features of the N-glycan structure in mediating the interaction with host cells during fungal infection. The delineation of the roles of protein glycosylation in fungal pathogenesis not only provides insight into the glycan-based fungal infection mechanism but also will aid in the development of novel antifungal agents.

New Approaches for Cryptococcosis Treatment

Cryptococcosis is an important opportunistic infection and a leading cause of meningitis in patients with HIV infection. The antifungal pharmacological treatment is limited to amphotericin B, fluconazole and 5- flucytosine. In addition to the limited pharmacological options, the high toxicity, increased resistance rate and difficulty of the currently available antifungal molecules to cross the blood-brain barrier hamper the treatment. Thus, the search for new alternatives for the treatment of cryptococcal meningitis is extremely necessary. In this review, we describe the therapeutic strategies currently available, discuss new molecules with antifungal potential in different phases of clinical trials and in advanced pre-clinical phase, and examine drug nanocarriers to improve delivery to the central nervous system.

Diverse transposable element landscapes in pathogenic and nonpathogenic yeast models: the value of a comparative perspective

Genomics and other large-scale analyses have drawn increasing attention to the potential impacts of transposable elements (TEs) on their host genomes. However, it remains challenging to transition from identifying potential roles to clearly demonstrating the level of impact TEs have on genome evolution and possible functions that they contribute to their host organisms. I summarize TE content and distribution in four well-characterized yeast model systems in this review: the pathogens Candida albicans and Cryptococcus neoformans, and the nonpathogenic species Saccharomyces cerevisiae and Schizosaccharomyces pombe. I compare and contrast their TE landscapes to their lifecycles, genomic features, as well as the presence and nature of RNA interference pathways in each species to highlight the valuable diversity represented by these models for functional studies of TEs. I then review the regulation and impacts of the Ty1 and Ty3 retrotransposons from Saccharomyces cerevisiae and Tf1 and Tf2 retrotransposons from Schizosaccharomyces pombe to emphasize parallels and distinctions between these well-studied elements. I propose that further characterization of TEs in the pathogenic yeasts would enable this set of four yeast species to become an excellent set of models for comparative functional studies to address outstanding questions about TE-host relationships.