Cryptococcus neoformans hyperfilamentous strain is hypervirulent in a murine model of cryptococcal meningoencephalitis

Antifungal Effect of Vitamin D3 against Cryptococcus neoformans Coincides with Reduced Biofilm Formation, Compromised Cell Wall Integrity, and Increased Generation of Reactive Oxygen Species

Cryptococcus neoformans is an invasive fungus that causes both acute and chronic infections, especially in immunocompromised patients. Owing to the increase in the prevalence of drug-resistant pathogenic fungi and the limitations of current treatment strategies, drug repositioning has become a feasible strategy to accelerate the development of new drugs. In this study, the minimum inhibitory concentration of vitamin D₃ (VD₃) against C. neoformans was found to be 0.4 mg/mL by broth microdilution assay. The antifungal activities of VD₃ were further verified by solid dilution assays and "time-kill" curves. The results showed that VD₃ reduced fungal cell adhesion and hydrophobicity and inhibited biofilm formation at various developmental stages, as confirmed by crystal violet staining and the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide assay. Fluorescence staining of cellular components and a stress susceptibility assay indicated that VD₃ compromised cell integrity. Reverse transcription quantitative PCR demonstrated that VD₃ treatment upregulated the expression of fungal genes related to cell wall synthesis (i.e., CDA3, CHS3, FKS1, and AGS1). Moreover, VD₃ enhanced cell membrane permeability and caused the accumulation of intracellular reactive oxygen species. Finally, VD₃ significantly reduced the tissue fungal burden and prolonged the survival of Galleria mellonella larvae infected with C. neoformans. These results showed that VD₃ could exert significant antifungal activities both in vitro and in vivo, demonstrating its potential application in the treatment of cryptococcal infections.

Pleiotropy and epistasis within and between signaling pathways defines the genetic architecture of fungal virulence

Cryptococcal disease is estimated to affect nearly a quarter of a million people annually. Environmental isolates of Cryptococcus deneoformans, which make up 15 to 30% of clinical infections in temperate climates such as Europe, vary in their pathogenicity, ranging from benign to hyper-virulent. Key traits that contribute to virulence, such as the production of the pigment melanin, an extracellular polysaccharide capsule, and the ability to grow at human body temperature have been identified, yet little is known about the genetic basis of variation in such traits. Here we investigate the genetic basis of melanization, capsule size, thermal tolerance, oxidative stress resistance, and antifungal drug sensitivity using quantitative trait locus (QTL) mapping in progeny derived from a cross between two divergent C. deneoformans strains. Using a "function-valued" QTL analysis framework that exploits both time-series information and growth differences across multiple environments, we identified QTL for each of these virulence traits and drug susceptibility. For three QTL we identified the underlying genes and nucleotide differences that govern variation in virulence traits. One of these genes, RIC8, which encodes a regulator of cAMP-PKA signaling, contributes to variation in four virulence traits: melanization, capsule size, thermal tolerance, and resistance to oxidative stress. Two major effect QTL for amphotericin B resistance map to the genes SSK1 and SSK2, which encode key components of the HOG pathway, a fungal-specific signal transduction network that orchestrates cellular responses to osmotic and other stresses. We also discovered complex epistatic interactions within and between genes in the HOG and cAMP-PKA pathways that regulate antifungal drug resistance and resistance to oxidative stress. Our findings advance the understanding of virulence traits among diverse lineages of Cryptococcus, and highlight the role of genetic variation in key stress-responsive signaling pathways as a major contributor to phenotypic variation.

On the History and Applications of Congenic Strains in Cryptococcus Research

Congenic strains have been utilized in numerous model organisms to determine the genetic underpinning of various phenotypic traits. Congenic strains are usually derived after 10 backcrosses to a recipient parent, at which point they are 99.95% genetically identical to the parental strain. In recent decades, congenic pairs have provided an invaluable tool for genetics and molecular biology research in the Cryptococcus neoformans species complex. Here, we summarize the history of Cryptococcus congenic pairs and their application in Cryptococcus research on topics including the impact of the mating type locus on unisexual reproduction, virulence, tissue tropism, uniparental mitochondrial inheritance, and the genetic underpinning of other various traits. We also discuss the limitations of these approaches and other biological questions, which could be explored by employing congenic pairs.

Antifungal activity of peptide MSI-1 against Cryptococcus neoformans infection in vitro and in murine cryptococcal meningoencephalitis

The development of novel antifungal agents with high efficacy, low drug tolerance and few side effects is urgent. MSI-1 (GIWKFLKKAKKFWK-NH₂), a cationic antimicrobial peptide, may be an attractive antifungal agent because of its structural characteristics, perfect stability against pH and high-temperature/salt, low toxicity towards mammalian cells and low potential for emergence of drug tolerance. In this study, the antifungal activity of MSI-1 in vitro and in a murine model of cryptococcal meningoencephalitis was evaluated. Zeta potential assay, flow cytometry, fluorescence microscope, transmission electron microscopy and microscale thermophoresis were performed to clarify the mechanisms underlying MSI-1 against C. neoformans. The results showed that MSI-1 exerted effective anti-cryptococcal activity in vitro, with MICs of 8-16 μg/mL and MFCs of 8-32 μg/mL, and in a C neoformans-infected mouse model, with significantly improved animal survival, decreased production of pro-inflammatory cytokines and alleviated lung injury, because the potent and rapid fungicidal activity of MSI-1 could effectively eliminate fungal counts in mouse organs. We confirmed that the positively charged peptide bound to C. neoformans by electrostatic attraction after interacting with glucuronoxylomannan (the primary component of C. neoformans capsule). Subsequently, MSI-1 increased the membrane fluidity of fungal cells and the cell membrane permeability, causing destabilized membrane integrity and leading to the final death of fungi. Collectively, MSI-1 possessed potent anti-cryptococcal activity via its notable membrane disruption effect and may be a potential candidate for use in antifungal infection induced by C. neoformans, especially azole-resistant cryptococcus.

Transposon mobilization in the human fungal pathogen Cryptococcus is mutagenic during infection and promotes drug resistance in vitro

When transitioning from the environment, pathogenic microorganisms must adapt rapidly to survive in hostile host conditions. This is especially true for environmental fungi that cause opportunistic infections in immunocompromised patients since these microbes are not well adapted human pathogens. Cryptococcus species are yeastlike fungi that cause lethal infections, especially in HIV-infected patients. Using Cryptococcus deneoformans in a murine model of infection, we examined contributors to drug resistance and demonstrated that transposon mutagenesis drives the development of 5-fluoroorotic acid (5FOA) resistance. Inactivation of target genes URA3 or URA5 primarily reflected the insertion of two transposable elements (TEs): the T1 DNA transposon and the TCN12 retrotransposon. Consistent with in vivo results, increased rates of mutagenesis and resistance to 5FOA and the antifungal drugs rapamycin/FK506 (rap/FK506) and 5-fluorocytosine (5FC) were found when Cryptococcus was incubated at 37° compared to 30° in vitro, a condition that mimics the temperature shift that occurs during the environment-to-host transition. Inactivation of the RNA interference (RNAi) pathway, which suppresses TE movement in many organisms, was not sufficient to elevate TE movement at 30° to the level observed at 37°. We propose that temperature-dependent TE mobilization in Cryptococcus is an important mechanism that enhances microbial adaptation and promotes pathogenesis and drug resistance in the human host.

The Evolution of Sexual Reproduction and the Mating-Type Locus: Links to Pathogenesis of Cryptococcus Human Pathogenic Fungi

Cryptococcus species utilize a variety of sexual reproduction mechanisms, which generate genetic diversity, purge deleterious mutations, and contribute to their ability to occupy myriad environmental niches and exhibit a range of pathogenic potential. The bisexual and unisexual cycles of pathogenic Cryptococcus species are stimulated by properties associated with their environmental niches and proceed through well-characterized signaling pathways and corresponding morphological changes. Genes governing mating are encoded by the mating-type (MAT) loci and influence pathogenesis, population dynamics, and lineage divergence in Cryptococcus. MAT has undergone significant evolutionary changes within the Cryptococcus genus, including transition from the ancestral tetrapolar state in nonpathogenic species to a bipolar mating system in pathogenic species, as well as several internal reconfigurations. Owing to the variety of established sexual reproduction mechanisms and the robust characterization of the evolution of mating and MAT in this genus, Cryptococcus species provide key insights into the evolution of sexual reproduction.

Blood neutrophil counts in HIV-infected patients with cryptococcal meningitis: Association with mortality

BACKGROUND

The mortality from cryptococcal meningitis remains high, despite the availability of antiretroviral therapy (ART) and amphotericin-based fungal regimens. The role of neutrophils in cryptococcosis is controversial. Our objective was to examine the association between blood neutrophil counts and outcomes in terms of mortality, the incidence of bacterial infections (including Mycobacterium tuberculosis) and hospitalization among HIV-infected patients presenting with cryptococcal meningitis.

METHODS

We used data from participants from the Cryptococcal Optimal ART Timing (COAT) trial (2010-2012; Uganda and South Africa) and the Adjunctive Sertraline for Treatment of Cryptococcal Meningitis (ASTRO-CM) trial (2013-2017; Uganda). We estimated 30-day mortality risk with Cox proportional hazards models by baseline neutrophil counts (a) on a continuous scale and (b) with indicators for both relatively high (> 3,500 cells/mm3) and low (≤ 1,000 cells/mm3) counts. Follow-up neutrophil counts from the COAT trial were used to examine the time-dependent association of neutrophil counts with 12-month mortality and rehospitalization.

RESULTS

801 participants had an absolute neutrophil value at meningitis diagnosis. The median baseline absolute neutrophil count was 2100 cells/mm3 (IQR, 1400 to 3300 cells/mm3). Baseline neutrophil count was positively associated with 30-day mortality (adjusted hazard ratio = 1.09, 95%CI, 1.04-1.13, per 1000 cells/mm3 increase; p<0.001). Baseline absolute neutrophil counts ≤ 1000 cells/mm3 did not have increased risk of 30-day mortality compared to those with baseline neutrophils of 1001-3500 cells/mm3; however, baseline >3500 cells/mm3 had significantly increased risk, with an adjusted hazard ratio of 1.85(95%CI, 1.40-2.44; p<0.001). Among the COAT participants with follow-up neutrophil data, there was a strong association between time-updated neutrophil count and 12-month mortality (adjusted hazard ratio = 1.16, 95% CI 1.09-1.24; p<0.001.

CONCLUSION

Higher blood neutrophil counts in HIV-infected patients with cryptococcal meningitis were associated with mortality. Neutrophils role requires further investigation as to whether this may be a mediator directly contributing to mortality or merely a marker of underlying pathologies that increase mortality risk.

Involvement of the capsular GalXM-induced IL-17 cytokine in the control of Cryptococcus neoformans infection

Cryptococcus neoformans is an opportunistic fungus that can cause lethal brain infections in immunosuppressed individuals. Infection usually occurs via the inhalation of a spore or desiccated yeast which can then disseminate from the lung to the brain and other tissues. Dissemination and disease is largely influence by the production of copious amounts of cryptococcal polysaccharides, both which are secreted to the extracellular environment or assembled into a thick capsule surrounding the cell body. There are two important polysaccharides: glucuronoxylomannan (GXM) and galactoxylomannan, also called as glucuronoxylomanogalactan (GXMGal or GalXM). Although GXM is more abundant, GalXM has a more potent modulatory effect. In the present study, we show that GalXM is a potent activator of murine dendritic cells, and when co-cultured with T cells, induces a Th17 cytokine response. We also demonstrated that treating mice with GalXM prior to infection with C. neoformans protects from infection, and this phenomenon is dependent on IL-6 and IL-17. These findings help us understand the immune biology of capsular polysaccharides in fungal pathogenesis.

Plant Homeodomain Genes Play Important Roles in Cryptococcal Yeast-Hypha Transition

Cryptococcus neoformans is a major opportunistic fungal pathogen. Like many dimorphic fungal pathogens, C. neoformans can undergo morphological transition from the yeast form to the hypha form, and its morphotype is tightly linked to its virulence. Although some genetic factors controlling morphogenesis have been identified, little is known about the epigenetic regulation in this process. Proteins with the plant homeodomain (PHD) finger, a structurally conserved domain in eukaryotes, were first identified in plants and are known to be involved in reading and effecting chromatin modification. Here, we investigated the role of the PHD finger family genes in Cryptococcus mating and yeast-hypha transition. We found 16 PHD finger domains distributed among 15 genes in the Cryptococcus genome, with two genes, ZNF1α and RUM1α, located in the mating type locus. We deleted these 15 PHD genes and examined the impact of their disruption on cryptococcal morphogenesis. The deletion of five PHD finger genes dramatically affected filamentation. The rum1αΔ and znf1αΔ mutants showed enhanced ability to initiate filamentation but impaired ability to maintain filamentous growth. The bye1Δ and the phd11Δ mutants exhibited enhanced filamentation, while the set302Δ mutants displayed reduced filamentation. Ectopic overexpression of these five genes in the corresponding null mutants partially or completely restored the defect in filamentation. Furthermore, we demonstrated that Phd11, a suppressor of filamentation, regulates the yeast-hypha transition through the known master regulator Znf2. The findings indicate the importance of epigenetic regulation in controlling dimorphic transition in C. neoformansIMPORTANCE Morphotype is known to have a profound impact on cryptococcal interaction with various hosts, including mammalian hosts. The yeast form of Cryptococcus neoformans is considered the virulent form, while its hyphal form is attenuated in mammalian models of cryptococcosis. Although some genetic regulators critical for cryptococcal morphogenesis have been identified, little is known about epigenetic regulation in this process. Given that plant homeodomain (PHD) finger proteins are involved in reading and effecting chromatin modification and their functions are unexplored in C. neoformans, we investigated the roles of the 15 PHD finger genes in Cryptococcus mating and yeast-hypha transition. Five of them profoundly affect filamentation as either a suppressor or an activator. Phd11, a suppressor of filamentation, regulates this process via Znf2, a known master regulator of morphogenesis. Thus, epigenetic regulation, coupled with genetic regulation, controls this yeast-hypha transition event.

Innate Immunity against Cryptococcus, from Recognition to Elimination

Cryptococcus species, the etiological agents of cryptococcosis, are encapsulated fungal yeasts that predominantly cause disease in immunocompromised individuals, and are responsible for 15% of AIDS-related deaths worldwide. Exposure follows the inhalation of the yeast into the lung alveoli, making it incumbent upon the pattern recognition receptors (PRRs) of pulmonary phagocytes to recognize highly conserved pathogen-associated molecular patterns (PAMPS) of fungi. The main challenges impeding the ability of pulmonary phagocytes to effectively recognize Cryptococcus include the presence of the yeast's large polysaccharide capsule, as well as other cryptococcal virulence factors that mask fungal PAMPs and help Cryptococcus evade detection and subsequent activation of the immune system. This review will highlight key phagocyte cell populations and the arsenal of PRRs present on these cells, such as the Toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs), and soluble receptors. Additionally, we will highlight critical cryptococcal PAMPs involved in the recognition of Cryptococcus. The question remains as to which PRR-ligand interaction is necessary for the recognition, phagocytosis, and subsequent killing of Cryptococcus.

The F-Box Protein Fbp1 Shapes the Immunogenic Potential of Cryptococcus neoformans

Cryptococcus neoformans is the main etiologic agent of cryptococcal meningitis and causes a significant number of deadly infections per year. Although it is well appreciated that host immune responses are crucial for defense against cryptococcosis, our understanding of factors that control the development of effective immunity to this fungus remains incomplete. In previous studies, we identified the F-box protein Fbp1 as a novel determinant of C. neoformans virulence. In this study, we found that the hypovirulence of the fbp1Δ mutant is linked to the development of a robust host immune response. Infection with the fbp1Δ mutant induces a rapid influx of CCR2⁺ monocytes and their differentiation into monocyte-derived dendritic cells (mo-DCs). Depletion of CCR2⁺ monocytes and their derivative mo-DCs resulted in impaired activation of a protective inflammatory response and the rapid death of mice infected with the fbp1Δ mutant. Mice lacking B and T cells also developed fungal meningitis and succumbed to infection with the fbp1Δ mutant, demonstrating that adaptive immune responses to the fbp1Δ mutant help to maintain the long-term survival of the host. Adaptive immune responses to the fbp1Δ mutant were characterized by enhanced differentiation of Th1 and Th17 CD4⁺ T cells together with diminished Th2 responses compared to the H99 parental strain. Importantly, we found that the enhanced immunogenicity of fbp1Δ mutant yeast cells can be harnessed to confer protection against a subsequent infection with the virulent H99 parental strain. Altogether, our findings suggest that Fbp1 functions as a novel virulence factor that shapes the immunogenicity of C. neoformansIMPORTANCECryptococcus neoformans is the most common cause of deadly fungal meningitis, with over 270,000 infections per year. Immune responses are critically required for the prevention of cryptococcosis, and patients with impaired immunity and low CD4⁺ T cell numbers are at high risk of developing these deadly infections. Although it is well appreciated that the development of protective immunity is shaped by the interactions of the host immune system with fungal cells, our understanding of fungal products that influence this process remains poor. In this study, we found that the activity of F-box protein 1 (Fbp1) in highly virulent C. neoformans clinical strain H99 shapes its immunogenicity and thus affects the development of protective immune responses in the host. The identification of this new mechanism of virulence may facilitate the future development of therapeutic interventions aimed at boosting antifungal host immunity.

Systemic Approach to Virulence Gene Network Analysis for Gaining New Insight into Cryptococcal Virulence

Cryptococcus neoformans is pathogenic yeast, responsible for highly lethal infections in compromised patients around the globe. C. neoformans typically initiates infections in mammalian lung tissue and subsequently disseminates to the central nervous system where it causes significant pathologies. Virulence genes of C. neoformans are being characterized at an increasing rate, however, we are far from a comprehensive understanding of their roles and genetic interactions. Some of these reported virulence genes are scattered throughout different databases, while others are not yet included. This study gathered and analyzed 150 reported virulence associated factors (VAFs) of C. neoformans. Using the web resource STRING database, our study identified different interactions between the total VAFs and those involved specifically in lung and brain infections and identified a new strain specific virulence gene, SHO1, involved in the mitogen-activated protein kinase signaling pathway. As predicted by our analysis, SHO1 expression enhanced C. neoformans virulence in a mouse model of pulmonary infection, contributing to enhanced non-protective immune Th2 bias and progressively enhancing fungal growth in the infected lungs. Sequence analysis indicated 77.4% (116) of total studied VAFs are soluble proteins, and 22.7% (34) are transmembrane proteins. Motifs involved in regulation and signaling such as protein kinases and transcription factors are highly enriched in Cryptococcus VAFs. Altogether, this study represents a pioneering effort in analysis of the virulence composite network of C. neoformans using a systems biology approach.