Cryptococcus neoformans growth and protection from innate immunity are dependent on expression of a virulence-associated DEAD-box protein, Vad1

The Cryptococcus neoformans Flc1 Homologue Controls Calcium Homeostasis and Confers Fungal Pathogenicity in the Infected Hosts

Cryptococcus neoformans, an opportunistic yeast pathogen, relies on a complex network of stress response pathways that allow for proliferation in the host. In Saccharomyces cerevisiae, stress responses are regulated by integral membrane proteins containing a transient receptor potential (TRP) domain, including the flavin carrier protein 1 (Flc1), which regulates calcium homeostasis and flavin transport. Here, we report that deletion of C. neoformans FLC1 results in cytosolic calcium elevation and increased nuclear content of calcineurin-dependent transcription factor Crz1, which is associated with an aberrant cell wall chitin overaccumulation observed in the flc1Δ mutant. Absence of Flc1 or inhibition of calcineurin with cyclosporine A prevents vacuolar fusion under conditions of combined osmotic and temperature stress, which is reversed in the flc1Δ mutant by the inhibition of TORC1 kinase with rapamycin. Flc1-deficient yeasts exhibit compromised vacuolar fusion under starvation conditions, including conditions that stimulate formation of carbohydrate capsule. Consequently, the flc1Δ mutant fails to proliferate under low nutrient conditions and displays a defect in capsule formation. Consistent with the previously uncharacterized role of Flc1 in vacuolar biogenesis, we find that Flc1 localizes to the vacuole. The flc1Δ mutant presents a survival defect in J774A.1 macrophage cell-line and profound virulence attenuation in both the Galleria mellonella and mouse pulmonary infection models, demonstrating that Flc1 is essential for pathogenicity. Thus, cryptococcal Flc1 functions in calcium homeostasis and links calcineurin and TOR signaling with vacuolar biogenesis to promote survival under conditions associated with vacuolar fusion required for this pathogen's fitness and virulence. IMPORTANCE Cryptococcosis is a highly lethal infection with limited drug choices, most of which are highly toxic or complicated by emerging antifungal resistance. There is a great need for new drug targets that are unique to the fungus. Here, we identify such a potential target, the Flc1 protein, which we show is crucial for C. neoformans stress response and virulence. Importantly, homologues of Flc1 exist in other fungal pathogens, such as Candida albicans and Aspergillus fumigatus, and are poorly conserved in humans, which could translate into wider spectrum therapy associated with minimal toxicity. Thus, Flc1 could be an "Achille's heel" of C. neoformans to be leveraged therapeutically in cryptococcosis and possibly other fungal infections.

Molecular characterization of Enterobacter aerogenes isolated from urinary tract infections in Iran

The prevalence of multidrug-resistant Enterobacter aerogenes strains in UTIs is increasing. Therefore, the purpose of this study was to examine the mechanisms of resistance in Enterobacter aerogenes strains isolated from the urinary tract of infected patients. To achieve this goal, 786 urine samples from Shahrekord, Iran, were collected from June 2019 to February 2020. After isolating and identifying E. aerogenes samples, antibiotic susceptibility testing was done on the strains using Kirby-Bauer's disk diffusion method. The biofilm formation assays were performed to study the link between antibiotic resistance and biofilm formation and virulence genes. As a result, amongst the 786 urine samples, 50 strains were identified as E. aerogenes. The lowest rate of resistance was observed with imipenem (30%). This study also reports that all the strains of E. aerogenes are biofilm producers, with 50% of isolates producing a large amount, 30% a moderate amount, and 20% a small amount of biofilm. 42% were identified in the phenotypic study of ESBLs. In the PCR test, (64%) produced broad-spectrum beta-lactamases. Prevalence of qnrC, qnrB, qnrA, tetA, tet B, acc(3)IIa, acc(2)IIa, ant(2)Ia and Sul1 in strong producing isolates reported 100%, 80.95%,% 58.14, 87.5%, 81.58%, 86.67%, 82.14, 81.48% and 90% respectively. In the statistical analysis based on the chi-square test, a statistically significant relationship was reported between qnrA, qnrB, tetA, tetB, Sul1, ant(2)Ia, ant(3)I, aac(3)II, and biofilm formation. Resistance to cephalothin, ceftriaxone, cefotaxime and ceftazidime were reported 40%, 34%, 30% and 30%, respectively. Out of 50 Enterobacter aerogenes, 32 isolates (64%) were identified in the phenotypic study of ESBLS, prevalence of bla_CTX-M, bla_TEM and bla_SHV reported 30%, 20% and 14% respectively. There is a significant relationship between resistance to ceftriaxone and bla_CTX-M. Prevalence of csgA, ybtS, markD, rmpA, csgD and fimH in strong biofilm formation isolates reported 84%, 83.33%, 80%, 80%, 80% and 66% respectively. The chi-square test showed a statistically significant relationship between biofilm production and resistance genes fimH, csgA, csgD, ybtS, and mrkD. The findings of this study indicate that the ability to produce biofilms is associated with the increase of antibiotic resistance and virulence genes. These agents enable bacteria to produce biofilms that ultimately lead to colonization and bacterial survival in the body.

Role of the ESCRT Pathway in Laccase Trafficking and Virulence of Cryptococcus neoformans

The endosomal sorting complex required for transport (ESCRT) plays a crucial role in the transportation and degradation of proteins. We determined that Vps27, a key protein of the ESCRT-0 complex, is required for the transport of the virulence factor laccase to the cell wall in Cryptococcus neoformans Laccase activity was perturbed, as was melanin production, in vps27Δ strains. In the absence of VPS27, there was an accumulation of multivesicular bodies with vacuolar fragmentation and mistargeting of the vacuolar carboxypeptidase CPY/Prc1, resulting in an extracellular localization. In addition, deletion of VPS27 resulted in a defect in laccase targeting of a Lac1-green fluorescent protein (GFP) fusion to the cell wall with trapping within intracellular puncta; this deletion was accompanied by reduced virulence in a mouse model. However, the actin cytoskeleton remained intact, suggesting that the trafficking defect is not due to defects in actin-related localization. Extracellular vesicle maturation was also defective in the vps27Δ mutant, which had a larger vesicle size as measured by dynamic light scattering. Our data identify cryptococcal VPS27 as a required gene for laccase trafficking and attenuates virulence of C. neoformans in a mouse intravenous (i.v.) meningitis model.

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.

Role of dendritic cell-pathogen interactions in the immune response to pulmonary cryptococcal infection

This review discusses the unique contributions of dendritic cells (DCs) to T-cell priming and the generation of effective host defenses against Cryptococcus neoformans (C.neo) infection. We highlight DC subsets involved in the early and later stages of anticryptococcal immune responses, interactions between C.neo pathogen-associated molecular patterns and pattern recognition receptors expressed by DC, and the influence of DC on adaptive immunity. We emphasize recent studies in mouse models of cryptococcosis that illustrate the importance of DC-derived cytokines and costimulatory molecules and the potential role of DC epigenetic modifications that support maintenance of these signals throughout the immune response to C.neo. Lastly, we stipulate where these advances can be developed into new, immune-based therapeutics for treatment of this global pathogen.

Cryptococcus neoformans isolates from Yaoundé human immunodeficiency virus-infected patients exhibited intra-individual genetic diversity and variation in antifungal susceptibility profiles between isolates from the same patient

Cryptococcal meningitis is a dreadful opportunistic fungal infection amongst human immunodeficiency virus (HIV)-infected patients. One complication in the management of the disease is the possible infection of a patient by two or more different strains of Cryptococcus neoformans. This study investigated the intra-individual genetic diversity and antifungal susceptibility of C. neoformans isolates from Yaoundé (Cameroon) HIV-infected patients with cryptococcal meningitis. Twenty-five clinical isolates were obtained during a prospective study. Five colonies were randomly collected from each initial sample. The 150 isolates obtained (125 colonies and 25 initial samples) were submitted to serotyping by multiplex PCR. Genotyping analyses were achieved using RFLP, and minisatellite- and microsatellite-length polymorphism. The antifungal susceptibility testing was carried out using a Sensititre YeastOne kit. Seven antifungals were tested: itraconazole, fluconazole, amphotericin B, ketoconazole, 5-fluorocytosine, posaconazole and voriconazole. The 150 isolates were identified as C. neoformans serotype A and genotype VNI. The microsatellite and minisatellite sequence analyses generated 15 genotypes. Six out of 25 (24 %) patients were found to be infected by two different genotypes. Antifungal susceptibility showed several profiles: posaconazole (0.015-0.25 µg ml-1), amphotericin B (0.06-1 µg ml-1), fluconazole (0.5-16 µg ml-1), itraconazole (0.008-0.12 µg ml-1), ketoconazole (0.008-0.12 µg ml-1), 5-fluorocytosine (0.25-16 µg ml-1) and voriconazole (0.008-0.12 µg ml-1). It was noted that isolates from the same patient might present different susceptibility profiles to an antifungal drug with differences of more than four dilutions. The results achieved highlighted the possible presence of isolates with different genotypes in a patient with dissimilar antifungal susceptibility profiles during a single episode of cryptococcal meningitis.

Transcriptional control of sexual development in Cryptococcus neoformans

Developmental processes are essential for the normal life cycles of many pathogenic fungi, and they can facilitate survival in challenging environments, including the human host. Sexual development of the human fungal pathogen Cryptococcus neoformans not only produces infectious particles (spores) but has also enabled the evolution of new disease-related traits such as drug resistance. Transcription factor networks are essential to the development and pathogenesis of C. neoformans, and a variety of sequence-specific DNA-binding proteins control both key developmental transitions and virulence by regulating the expression of their target genes. In this review we discuss the roles of known transcription factors that harbor important connections to both development and virulence. Recent studies of these transcription factors have identified a common theme in which metabolic, stress, and other responses that are required for sexual development appear to have been co-opted for survival in the human host, thus facilitating pathogenesis. Future work elucidating the connection between development and pathogenesis will provide vital insights into the evolution of complex traits in eukaryotes as well as mechanisms that may be used to combat fungal pathogens.

Cryptococcal heat shock protein 70 homolog Ssa1 contributes to pulmonary expansion of Cryptococcus neoformans during the afferent phase of the immune response by promoting macrophage M2 polarization

Numerous virulence factors expressed by Cryptococcus neoformans modulate host defenses by promoting nonprotective Th2-biased adaptive immune responses. Prior studies demonstrate that the heat shock protein 70 homolog, Ssa1, significantly contributes to serotype D C. neoformans virulence through the induction of laccase, a Th2-skewing and CNS tropic factor. In the present study, we sought to determine whether Ssa1 modulates host defenses in mice infected with a highly virulent serotype A strain of C. neoformans (H99). To investigate this, we assessed pulmonary fungal growth, CNS dissemination, and survival in mice infected with either H99, an SSA1-deleted H99 strain (Δssa1), and a complement strain with restored SSA1 expression (Δssa1::SSA1). Mice infected with the Δssa1 strain displayed substantial reductions in lung fungal burden during the innate phase (days 3 and 7) of the host response, whereas less pronounced reductions were observed during the adaptive phase (day 14) and mouse survival increased only by 5 d. Surprisingly, laccase activity assays revealed that Δssa1 was not laccase deficient, demonstrating that H99 does not require Ssa1 for laccase expression, which explains the CNS tropism we still observed in the Ssa1-deficient strain. Lastly, our immunophenotyping studies showed that Ssa1 directly promotes early M2 skewing of lung mononuclear phagocytes during the innate phase, but not the adaptive phase, of the immune response. We conclude that Ssa1's virulence mechanism in H99 is distinct and laccase-independent. Ssa1 directly interferes with early macrophage polarization, limiting innate control of C. neoformans, but ultimately has no effect on cryptococcal control by adaptive immunity.

New technology and resources for cryptococcal research

Rapid advances in molecular biology and genome sequencing have enabled the generation of new technology and resources for cryptococcal research. RNAi-mediated specific gene knock down has become routine and more efficient by utilizing modified shRNA plasmids and convergent promoter RNAi constructs. This system was recently applied in a high-throughput screen to identify genes involved in host-pathogen interactions. Gene deletion efficiencies have also been improved by increasing rates of homologous recombination through a number of approaches, including a combination of double-joint PCR with split-marker transformation, the use of dominant selectable markers and the introduction of Cre-Loxp systems into Cryptococcus. Moreover, visualization of cryptococcal proteins has become more facile using fusions with codon-optimized fluorescent tags, such as green or red fluorescent proteins or, mCherry. Using recent genome-wide analytical tools, new transcriptional factors and regulatory proteins have been identified in novel virulence-related signaling pathways by employing microarray analysis, RNA-sequencing and proteomic analysis.

Cryptoccocal meningitis in Yaoundé (Cameroon) HIV infected patients: Diagnosis, frequency and Cryptococcus neoformans isolates susceptibility study to fluconazole

Cryptococcal meningitis is a mycosis encountered especially in patients with Acquired Immunodeficiency Syndrome and is fatal in the absence of treatment. Information on epidemiology, diagnosis and susceptibility profile to antifungal drugs, are scarce in Cameroon. Authors evaluated the diagnosis possibilities of the cryptococcal meningitis in Cameroon, and studied the antifungal susceptibility of isolated strains to fluconazole, used as first line treatment of the disease in Cameroon. Between December 2009 and July 2011, 146 cerebrospinal fluids obtained from HIV patients with suspicion of meningitis were analysed. The diagnosis procedure involved macroscopic and cyto-chemical analysis, India ink test, culture on Sabouraud chloramphenicol medium and antigen latex agglutination test. Antifungal susceptibility testing of isolated strains to fluconazole was done by the E-test(®) method. The diagnosis of cryptococcal meningitis gave 28.08% positive cases. Among these patients, 80% were at stages III and IV and 20% at stage I of the HIV infection, according to the WHO previous classification. Cyto-chemical analysis showed current findings in the case of cryptococcal meningitis. India ink test and latex agglutination test exhibited very high sensitivity and specificity (>94%). Fluconazole antifungal susceptibility testing gave MICs lower than 32μg/mL to 92.7% of isolated strains and MICs greater than this value to 7.3% of isolates. These results showed that cryptococcal meningitis remains a real problem among HIV infected patients in Yaoundé. The emergence of fluconazole reduced susceptibility strains is worrying. Nevertheless, efficacy of rapid detection tests is interesting because this will help in rapid diagnosis and treatment of patients.

The tools for virulence of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes almost half a million deaths each year. It is believed that most humans are infected with C. neoformans, possibly in a form that survives through latency in the lung and can reactivate to cause disease if the host becomes immunosuppressed. C. neoformans has a remarkably sophisticated intracellular survival capacities yet it is a free-living fungus with no requirement for mammalian virulence whatsoever. In this review, we discuss the tools that C. neoformans possesses to achieve survival, latency and virulence within its host. Some of these tools are mechanisms to withstand starvation and others aim to protect against microbicidal molecules produced by the immune system. Furthermore, we discuss how these tools were acquired through evolutionary pressures and perhaps accidental stochastic events, all of which combined to produce an organism with an unusual and unique intracellular pathogenic strategy.

Elucidating the immunological function of the Cryptococcus neoformans capsule

The encapsulated fungal pathogen Cryptococcus neoformans represents a significant agent of life-threatening infections in immunocompromised subjects. A unique characteristic of Cryptococcus species is the presence of a polysaccharide capsule, which is essential for virulence and endows Cryptococcus with potent immunoregulatory properties. This review provides an overview of the immunological properties of the principal components of C. neoformans capsule.