Gene overexpression/suppression analysis of candidate virulence factors of Candida albicans

Comparative evaluation of biologically and chemically synthesized zinc oxide nanoparticles for preventing Candida auris biofilm

Candidozyma auris (formerly Candida auris) is a multidrug-resistant yeast that poses a significant global health threat due to its ability to form biofilms and resist various antifungal treatments. This study evaluates and compares the antifungal efficacy of biologically synthesized zinc oxide nanoparticles (ZnO-NP-B) and chemically synthesized ZnO nanoparticles (ZnO-NP-C1 and ZnO-NP-C2), developed using the dry-wet chemical method and sol-gel method, respectively. ZnO-NP-B was synthesized using Lactobacillus gasseri. The nanoparticles were characterized for size, charge, and morphology using Particle Size Analyzer, photon correlation spectroscopy with a 90 Plus Zetasizer, and scanning electron microscopy (SEM), respectively. The antifungal activity was assessed through minimum inhibitory concentration (MIC50) determination, biofilm inhibition assays by XTT assay, and gene expression analysis. ZnO-NP-C1 exhibited the highest antifungal activity against C. auris planktonic cells, with a MIC50 value of 61.9 ± 3.3 µg/ml, followed by ZnO-NP-C2 (151 ± 7.83 µg/ml), whereas ZnO-NP-B showed limited efficacy (MIC50 = 1 mg/ml). Chemically synthesized ZnO-NPs, particularly ZnO-NP-C2, did not induce overexpression of resistance genes (CDR1, MDR1, ERG2, ERG11, FKS1, CHS1), whereas ZnO-NP-B triggered their upregulation, potentially promoting resistance. ZnO-NP-C1 was the most effective in preventing biofilm formation, reducing C. auris adhesion by 67.9 ± 2.35% at 150 µg/ml, while ZnO-NP-B exhibited negligible inhibition. Gene expression analysis further confirmed that ZnO-NP-C1 significantly downregulated adhesive genes (ALS5, IFF4, CSA1) by up to 0.37 ± 0.006, 0.043 ± 0.002, and 0.06 ± 0.0004, respectively. These findings highlight the potential of ZnO-NP-C1 as a promising antifungal agent for preventing C. auris biofilms, emphasizing the critical role of synthesis methods in optimizing nanoparticle properties for antifungal applications.

Parallel expansion and divergence of an adhesin family in pathogenic yeasts

Opportunistic yeast pathogens arose multiple times in the Saccharomycetes class, including the recently emerged, multidrug-resistant (MDR) Candida auris. We show that homologs of a known yeast adhesin family in Candida albicans, the Hyr/Iff-like (Hil) family, are enriched in distinct clades of Candida species as a result of multiple, independent expansions. Following gene duplication, the tandem repeat-rich region in these proteins diverged extremely rapidly and generated large variations in length and β-aggregation potential, both of which are known to directly affect adhesion. The conserved N-terminal effector domain was predicted to adopt a β-helical fold followed by an α-crystallin domain, making it structurally similar to a group of unrelated bacterial adhesins. Evolutionary analyses of the effector domain in C. auris revealed relaxed selective constraint combined with signatures of positive selection, suggesting functional diversification after gene duplication. Lastly, we found the Hil family genes to be enriched at chromosomal ends, which likely contributed to their expansion via ectopic recombination and break-induced replication. Combined, these results suggest that the expansion and diversification of adhesin families generate variation in adhesion and virulence within and between species and are a key step toward the emergence of fungal pathogens.

Development and applications of a CRISPR activation system for facile genetic overexpression in Candida albicans

For the fungal pathogen Candida albicans, genetic overexpression readily occurs via a diversity of genomic alterations, such as aneuploidy and gain-of-function mutations, with important consequences for host adaptation, virulence, and evolution of antifungal drug resistance. Given the important role of overexpression on C. albicans biology, it is critical to develop and harness tools that enable the analysis of genes expressed at high levels in the fungal cell. Here, we describe the development, optimization, and application of a novel, single-plasmid-based CRISPR activation (CRISPRa) platform for targeted genetic overexpression in C. albicans, which employs a guide RNA to target an activator complex to the promoter region of a gene of interest, thus driving transcriptional expression of that gene. Using this system, we demonstrate the ability of CRISPRa to drive high levels of gene expression in C. albicans, and we assess optimal guide RNA targeting for robust and constitutive overexpression. We further demonstrate the specificity of the system via RNA sequencing. We highlight the application of CRISPR activation to overexpress genes involved in pathogenesis and drug susceptibility, and contribute toward the identification of novel phenotypes. Consequently, this tool will facilitate a broad range of applications for the study of C. albicans genetic overexpression.

The Role of Host and Fungal Factors in the Commensal-to-Pathogen Transition of Candida albicans

Abstract

Purpose of Review

The fungus Candida albicans has evolved to live in close association with warm-blooded hosts and is found frequently on mucosal surfaces of healthy humans. As an opportunistic pathogen, C. albicans can also cause mucosal and disseminated infections (candidiasis). This review describes the features that differentiate the fungus in the commensal versus pathogenic state and the main factors underlying C. albicans commensal-to-pathogen transition.

Recent Findings

Adhesion, invasion, and tissue damage are critical steps in the infection process. Especially invasion and damage require transcriptional and morphological changes that differentiate C. albicans in the pathogenic from the commensal state. While the commensal-to-pathogen transition has some conserved causes and features in the oral cavity, the female urogenital tract, and the gut, site-specific differences have been identified in recent years.

Summary

This review highlights how specific factors in the different mucosal niches affect development of candidiasis. Recent evidence suggests that colonization of the gut is not only a risk factor for systemic candidiasis but might also provide beneficial effects to the host.

Application of the Mutant Libraries for Candida albicans Functional Genomics

Candida albicans is a typical opportunistic pathogen in humans that causes serious health risks in clinical fungal infections. The construction of mutant libraries has made remarkable developments in the study of C. albicans molecular and cellular biology with the ongoing advancements of gene editing, which include the application of CRISPR-Cas9 and novel high-efficient transposon. Large-scale genetic screens and genome-wide functional analysis accelerated the investigation of new genetic regulatory mechanisms associated with the pathogenicity and resistance to environmental stress in C. albicans. More importantly, sensitivity screening based on C. albicans mutant libraries is critical for the target identification of novel antifungal compounds, which leads to the discovery of Sec7p, Tfp1p, Gwt1p, Gln4p, and Erg11p. This review summarizes the main types of C. albicans mutant libraries and interprets their applications in morphogenesis, biofilm formation, fungus-host interactions, antifungal drug resistance, and target identification.

Pharmacodynamics, Mechanisms of Action and Resistance, and Spectrum of Activity of New Antifungal Agents

Several new antifungals are currently in late-stage development, including those with novel pharmacodynamics/mechanisms of action that represent new antifungal classes (manogepix, olorofim, ATI-2307, GR-2397). Others include new agents within established classes or with mechanisms of action similar to clinically available antifungals (ibrexafungerp, rezafungin, oteseconazole, opelconazole, MAT2203) that have been modified in order to improve certain characteristics, including enhanced pharmacokinetics and greater specificity for fungal targets. Many of the antifungals under development also have activity against Candida and Aspergillus strains that have reduced susceptibility or acquired resistance to azoles and echinocandins, whereas others demonstrate activity against species that are intrinsically resistant to most clinically available antifungals. The tolerability and drug–drug interaction profiles of these new agents also appear to be promising, although the number of human subjects that have been exposed to many of these agents remains relatively small. Overall, these agents have the potential for expanding our antifungal armamentarium and improving clinical outcomes in patients with invasive mycoses.

Novel agents in the treatment of invasive fungal infections in solid organ transplant recipients

PURPOSE OF REVIEW

Recipients of solid organ transplants (SOTs) suffer a significant burden of invasive fungal infections (IFIs). The emergence of drug-resistant fungi and toxicities of currently used antifungal agents as well as drug-drug interactions with immunosuppressants make their treatment challenging. This review discusses selected novel antifungal agents in the development pipeline that can currently be used through clinical trials or may be commercially available in the near future.

RECENT FINDINGS

These agents in development have novel pharmacokinetics and pharmacodynamics, expanded spectra of activity and excellent safety profiles.

SUMMARY

The properties of novel antifungal agents have the potential to expand the therapeutic options for IFIs in recipients of SOTs.

A novel class of Candida glabrata cell wall proteins with β-helix fold mediates adhesion in clinical isolates

Candida glabrata is an opportunistic pathogenic yeast frequently causing infections in humans. Though it lacks typical virulence factors such as hyphal development, C. glabrata contains a remarkably large and diverse set of putative wall adhesins that is crucial for its success as pathogen. Here, we present an analysis of putative adhesins from the homology clusters V and VI. First, sequence similarity network analysis revealed relationships between cluster V and VI adhesins and S. cerevisiae haze protective factors (Hpf). Crystal structures of A-regions from cluster VI adhesins Awp1 and Awp3b reveal a parallel right-handed β-helix domain that is linked to a C-terminal β-sandwich. Structure solution of the A-region of Awp3b via single wavelength anomalous diffraction phasing revealed the largest known lanthanide cluster with 21 Gd³⁺ ions. Awp1-A and Awp3b-A show structural similarity to pectate lyases but binding to neither carbohydrates nor Ca²⁺ was observed. Phenotypic analysis of awp1Δ, awp3Δ, and awp1,3Δ double mutants did also not confirm their role as adhesins. In contrast, deletion mutants of the cluster V adhesin Awp2 in the hyperadhesive clinical isolate PEU382 demonstrated its importance for adhesion to polystyrene or glass, biofilm formation, cell aggregation and other cell surface-related phenotypes. Together with cluster III and VII adhesins our study shows that C. glabrata CBS138 can rely on a set of 42 Awp1-related adhesins with β-helix/α-crystallin domain architecture for modifying the surface characteristics of its cell wall.

Adhesion to host cells and abiotic, often hydrophobic surfaces, e.g. that of medical equipment like catheters, is an indispensable virulence factor for many pathogenic fungi. Among the latter, the yeast Candida glabrata excels by encoding in its genome large sets of surface-exposed cell wall proteins. Here, we show that in the clinical isolate PEU382 of C. glabrata, hyper-adhesiveness to plastics and the tendency to biofilm formation is conferred by a single adhesin, Awp2. An integrative bioinformatic and structural analysis of this and the related Awp1 and Awp3 adhesins unifies four, so far separately assigned Awp clusters—III, V, VI and VII–into one consisting of 42 Awp1-related adhesins. These adhesins commonly present an N-terminal module consisting of a right-handed β-helix and an α-crystallin domain on the yeast surface and use a calcium-independent mode for adhesion. Their sheer number contrasts to the 20 members of the well characterized Epa and 7 members of the Pwp family of surface proteins. Given these findings we suggest that C. glabrata utilizes just two structurally distinct motifs for colonizing different host niches by adhesion: the β-helix/α-crystallin module of Awp1-related adhesins and the C-type lectin PA14-domain for Epa and Pwp proteins.

The Antifungal Pipeline: Fosmanogepix, Ibrexafungerp, Olorofim, Opelconazole, and Rezafungin

The epidemiology of invasive fungal infections is changing, with new populations at risk and the emergence of resistance caused by the selective pressure from increased usage of antifungal agents in prophylaxis, empiric therapy, and agriculture. Limited antifungal therapeutic options are further challenged by drug-drug interactions, toxicity, and constraints in administration routes. Despite the need for more antifungal drug options, no new classes of antifungal drugs have become available over the last 2 decades, and only one single new agent from a known antifungal class has been approved in the last decade. Nevertheless, there is hope on the horizon, with a number of new antifungal classes in late-stage clinical development. In this review, we describe the mechanisms of drug resistance employed by fungi and extensively discuss the most promising drugs in development, including fosmanogepix (a novel Gwt1 enzyme inhibitor), ibrexafungerp (a first-in-class triterpenoid), olorofim (a novel dihyroorotate dehydrogenase enzyme inhibitor), opelconazole (a novel triazole optimized for inhalation), and rezafungin (an echinocandin designed to be dosed once weekly). We focus on the mechanism of action and pharmacokinetics, as well as the spectrum of activity and stages of clinical development. We also highlight the potential future role of these drugs and unmet needs.

Overexpression approaches to advance understanding of Candida albicans

Candida albicans is an opportunistic fungal pathogen that is responsible for infections linked to high mortality. Loss‐of‐function approaches, taking advantage of gene knockouts or inducible down‐regulation, have been successfully used in this species in order to understand gene function. However, overexpression of a gene provides an alternative, powerful tool to elucidate gene function and identify novel phenotypes. Notably, overexpression can identify pathway components that might remain undetected using loss‐of‐function approaches. Several repressible or inducible promoters have been developed which allow to shut off or turn on the expression of a gene in C. albicans upon growth in the presence of a repressor or inducer. In this review, we summarize recent overexpression approaches used to study different aspects of C. albicans biology, including morphogenesis, biofilm formation, drug tolerance, and commensalism.

Metabolic flexibility and extensive adaptability governing multiple drug resistance and enhanced virulence in Candida albicans

Commensal fungus-Candida albicans turn pathogenic during the compromised immunity of the host, causing infections ranging from superficial mucosal to dreadful systemic ones. C. albicans has evolved various adaptive measures which collectively contribute towards its enhanced virulence. Among fitness attributes, metabolic flexibility and vigorous stress response are essential for its pathogenicity and virulence. Metabolic flexibility provides a means for nutrient assimilation and growth in diverse host microenvironments and reduces the vulnerability of the pathogen to various antifungals besides evading host immune response(s). Inside the host micro-environments, C. albicans efficiently utilizes the multiple fermentable and non-fermentable carbon sources to sustain and proliferate in glucose deficit conditions. The utilization of alternative carbon sources further highlights the importance of understanding these pathways as the attractive and potential therapeutic target. A thorough understanding of metabolic flexibility and adaptation to environmental stresses is warranted to decipher in-depth insights into virulence and molecular mechanisms of fungal pathogenicity. In this review, we have attempted to provide a detailed and recent understanding of some key aspects of fungal biology. Particular focus will be placed on processes like nutrient assimilation and utilization, metabolic adaptability, virulence factors, and host immune response in C. albicans leading to its enhanced pathogenicity.

Genetic Manipulation as a Tool to Unravel Candida parapsilosis Species Complex Virulence and Drug Resistance: State of the Art

An increase in the rate of isolation of Candida parapsilosis in the past decade, as well as increased identification of azole-resistant strains are concerning, and require better understanding of virulence-like factors and drug-resistant traits of these species. In this regard, the present review “draws a line” on the information acquired, thus far, on virulence determinants and molecular mechanisms of antifungal resistance in these opportunistic pathogens, mainly derived from genetic manipulation studies. This will provide better focus on where we stand in our understanding of the C. parapsilosis species complex–host interaction, and how far we are from defining potential novel targets or therapeutic strategies—key factors to pave the way for a more tailored management of fungal infections caused by these fungal pathogens.

A Candida parapsilosis Overexpression Collection Reveals Genes Required for Pathogenesis

Relative to the vast data regarding the virulence mechanisms of Candida albicans, there is limited knowledge on the emerging opportunistic human pathogen Candida parapsilosis. The aim of this study was to generate and characterize an overexpression mutant collection to identify and explore virulence factors in C. parapsilosis. With the obtained mutants, we investigated stress tolerance, morphology switch, biofilm formation, phagocytosis, and in vivo virulence in Galleria mellonella larvae and mouse models. In order to evaluate the results, we compared the data from the C. parapsilosis overexpression collection analysis to the results derived from previous deletion mutant library characterizations. Of the 37 overexpression C. parapsilosis mutants, we identified eight with altered phenotypes compared to the controls. This work is the first report to identify CPAR2_107240, CPAR2_108840, CPAR2_302400, CPAR2_406400, and CPAR2_602820 as contributors to C. parapsilosis virulence by regulating functions associated with host-pathogen interactions and biofilm formation. Our findings also confirmed the role of CPAR2_109520, CPAR2_200040, and CPAR2_500180 in pathogenesis. This study was the first attempt to use an overexpression strategy to systematically assess gene function in C. parapsilosis, and our results demonstrate that this approach is effective for such investigations.

A Fungal Immunotherapeutic Vaccine (NDV-3A) for Treatment of Recurrent Vulvovaginal Candidiasis-A Phase 2 Randomized, Double-Blind, Placebo-Controlled Trial

Background

Recurrent vulvovaginal candidiasis (RVVC) is a problematic form of mucosal Candida infection, characterized by repeated episodes per year. Candida albicans is the most common cause of RVVC. Currently, there are no immunotherapeutic treatments for RVVC.

Methods

This exploratory randomized, double-blind, placebo-controlled trial evaluated an immunotherapeutic vaccine (NDV-3A) containing a recombinant C. albicans adhesin/invasin protein for prevention of RVVC.

Results

The study in 188 women with RVVC (n = 178 evaluable) showed that 1 intramuscular dose of NDV-3A was safe and generated rapid and robust B- and T-cell immune responses. Post hoc exploratory analyses revealed a statistically significant increase in the percentage of symptom-free patients at 12 months after vaccination (42% vaccinated vs 22% placebo; P = .03) and a doubling in median time to first symptomatic episode (210 days vaccinated vs 105 days placebo) for the subset of patients aged <40 years (n = 137). The analysis of evaluable patients, which combined patients aged <40 years (77%) and ≥40 years (23%), trended toward a positive impact of NDV-3A versus placebo (P = .099).

Conclusions

In this unprecedented study of the effectiveness of a fungal vaccine in humans, NDV-3A administered to women with RVVC was safe and highly immunogenic and reduced the frequency of symptomatic episodes of vulvovaginal candidiasis for up to 12 months in women aged <40 years. These results support further development of NDV-3A vaccine and provide guidance for meaningful clinical endpoints for immunotherapeutic management of RVVC.

Clinical Trials Registration

NCT01926028.

Transcriptome Assembly and Profiling of Candida auris Reveals Novel Insights into Biofilm-Mediated Resistance

Candida auris has emerged as a significant global nosocomial pathogen. This is primarily due to its antifungal resistance profile but also its capacity to form adherent biofilm communities on a range of clinically important substrates. While we have a comprehensive understanding of how other Candida species resist and respond to antifungal challenge within the sessile phenotype, our current understanding of C. auris biofilm-mediated resistance is lacking. In this study, we are the first to perform transcriptomic analysis of temporally developing C. auris biofilms, which were shown to exhibit phase- and antifungal class-dependent resistance profiles. A de novo transcriptome assembly was performed, where sequenced sample reads were assembled into an ~11.5-Mb transcriptome consisting of 5,848 genes. Differential expression (DE) analysis demonstrated that 791 and 464 genes were upregulated in biofilm formation and planktonic cells, respectively, with a minimum 2-fold change. Adhesin-related glycosylphosphatidylinositol (GPI)-anchored cell wall genes were upregulated at all time points of biofilm formation. As the biofilm developed into intermediate and mature stages, a number of genes encoding efflux pumps were upregulated, including ATP-binding cassette (ABC) and major facilitator superfamily (MFS) transporters. When we assessed efflux pump activity biochemically, biofilm efflux was greater than that of planktonic cells at 12 and 24 h. When these were inhibited, fluconazole sensitivity was enhanced 4- to 16-fold. This study demonstrates the importance of efflux-mediated resistance within complex C. auris communities and may explain the resistance of C. auris to a range of antimicrobial agents within the hospital environment.IMPORTANCE Fungal infections represent an important cause of human morbidity and mortality, particularly if the fungi adhere to and grow on both biological and inanimate surfaces as communities of cells (biofilms). Recently, a previously unrecognized yeast, Candida auris, has emerged globally that has led to widespread concern due to the difficulty in treating it with existing antifungal agents. Alarmingly, it is also able to grow as a biofilm that is highly resistant to antifungal agents, yet we are unclear about how it does this. Here, we used a molecular approach to investigate the genes that are important in causing the cells to be resistant within the biofilm. The work provides significant insights into the importance of efflux pumps, which actively pump out toxic antifungal drugs and therefore enhance fungal survival within a variety of harsh environments.

Candida innate immunity at the mucosa

The tremendous diversity in microbial species that colonise the mucosal surfaces of the human body is only now beginning to be fully appreciated. Distinguishing between the behaviour of commensal microbes and harmful pathogens that reside at mucosal sites in the body is a complex, and exquisitely fine-tuned process central to mucosal health. The fungal pathobiont Candida albicans is frequently isolated from mucosal surfaces with an asymptomatic carriage rate of approximately 60% in the human population. While normally a benign member of the microbiota, overgrowth of C. albicans often results in localised mucosal infection causing morbidity in otherwise healthy individuals, and invasive infection that often causes death in the absence of effective immune defence. C. albicans triggers numerous innate immune responses at mucosal surfaces, and detection of C. albicans hyphae in particular, stimulates the production of antimicrobial peptides, danger-associated molecular patterns and cytokines that function to reduce fungal burdens during infection. This review will summarise our current understanding of innate immune responses to C. albicans at mucosal surfaces.

Candida-Epithelial Interactions

A plethora of intricate and dynamic molecular interactions occur between microbes and the epithelial cells that form the mucosal surfaces of the human body. Fungi, particularly species of Candida, are commensal members of our microbiota, continuously interacting with epithelial cells. Transient and localised perturbations to the mucosal environment can facilitate the overgrowth of fungi, causing infection. This minireview will examine the direct and indirect mechanisms by which Candida species and epithelial cells interact with each other, and explore the factors involved in the central processes of adhesion, invasion, and destruction of host mucosal surfaces.

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

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

Assessment of Post-Vaccination Phagocytic Activation Using Candida albicans Killing Assays

Candida albicans is an important opportunistic fungal pathogen. It is now the fourth leading cause of nosocomial bloodstream infections and a great threat to the immuncompromised patients attributed to the disseminated candidiasis with the mortality up to 40%. Phagocytic cells are the first line of defense against Candida infections. Antibodies induced by vaccination can effectively enhance the capacities of phagocytosis and killing of neutrophils and macrophages. In this chapter, flow cytometric analysis (FACS) and killing assay by plate culture methods are introduced to evaluate the phagocytosis and killing of strains of Candida albicans opsonized with immune serum obtained from mice vaccinated with yeast and recombinant enolase.

What's new in antifungals: an update on the in-vitro activity and in-vivo efficacy of new and investigational antifungal agents

PURPOSE OF REVIEW

Despite the availability of new antifungals and improved antifungal formulations, there is a continued need for the development of new drugs to treat invasive fungal infections. In high-risk populations, including heavily immunocompromised patients, those with multiple comorbidities, and patients in intensive care settings, invasive fungal infections remain a significant problem, and are associated with high morbidity and mortality. In addition, concerns of increasing antifungal resistance to available agents further highlight the need for new drugs to treat these infections.

RECENT FINDINGS

Recent studies have reported potent in-vitro activity for several investigational agents, including both yeasts and moulds. This in-vitro activity has also translated into in-vivo efficacy in animal models of various invasive fungal infections, including those caused by isolates that are resistant to clinically available agents. These agents include those with mechanisms of action similar to available agents and those that target fungi by novel means.

SUMMARY

Several new antifungal agents are currently in various stages of development. This is promising, as there is a continued need for new agents to treat invasive fungal infections. Which ones will receive approval for clinical use and their impact in patients with these infections remain unknown.

Candida albicans-epithelial interactions and pathogenicity mechanisms: scratching the surface

Until recently, epithelial cells have been a largely ignored component of host responses to microbes. However, this has been largely overturned over the last decade as an ever increasing number of studies have highlighted the key role that these cells play in many of our interactions with our microbiota and pathogens. Interactions of these cells with Candida albicans have been shown to be critical not just in host responses, but also in fungal cell responses, regulating fungal morphology and gene expression profile. In this review, we will explore the interactions between C. albicans and epithelial cells, and discuss how these interactions affect our relationship with this fungus.

Vaccination with Recombinant Non-transmembrane Domain of Protein Mannosyltransferase 4 Improves Survival during Murine Disseminated Candidiasis

Candida albicans is the most common cause of invasive fungal infections in humans. The C. albicans cell wall proteins play an important role in crucial host-fungus interactions and might be ideal vaccine targets to induce protective immune response in host. Meanwhile, protein that is specific to C. albicans is also an ideal target of vaccine. In this study, 11 proteins involving cell wall biosynthesis, yeast-to-hypha formation, or specific to C. albicans were chosen and were successfully cloned, purified and verified. The immune protection of vaccination with each recombinant protein respectively in preventing systemic candidiasis in BALB/c mice was assessed. The injection of rPmt4p vaccination significantly increased survival rate, decreased fungal burdens in the heart, liver, brain, and kidneys, and increased serum levels of both immunoglobulin G (IgG) and IgM against rPmt4p in the immunized mice. Histopathological assessment demonstrated that rPmt4p vaccination protected the tissue structure, and decreased the infiltration of inflammatory cells. Passive transfer of the rPmt4p immunized serum increased survival rate against murine systemic candidiasis and significantly reduced organ fungal burden. The immune serum enhanced mouse neutrophil killing activity by directly neutralizing rPmt4p effects in vitro. Levels of interleukin (IL)-4, IL-10, IL-12p70, IL-17A and tumor necrosis factor (TNF)-α in serum were higher in the immunized mice compared to those in the adjuvant control group. In conclusion, our results suggested that rPmt4p vaccination may be considered as a potential vaccine candidate against systemic candidiasis.

Targeted changes of the cell wall proteome influence Candida albicans ability to form single- and multi-strain biofilms

Biofilm formation is an important virulence trait of the pathogenic yeast Candida albicans. We have combined gene overexpression, strain barcoding and microarray profiling to screen a library of 531 C. albicans conditional overexpression strains (∼10% of the genome) for genes affecting biofilm development in mixed-population experiments. The overexpression of 16 genes increased strain occupancy within a multi-strain biofilm, whereas overexpression of 4 genes decreased it. The set of 16 genes was significantly enriched for those encoding predicted glycosylphosphatidylinositol (GPI)-modified proteins, namely Ihd1/Pga36, Phr2, Pga15, Pga19, Pga22, Pga32, Pga37, Pga42 and Pga59; eight of which have been classified as pathogen-specific. Validation experiments using either individually- or competitively-grown overexpression strains revealed that the contribution of these genes to biofilm formation was variable and stage-specific. Deeper functional analysis of PGA59 and PGA22 at a single-cell resolution using atomic force microscopy showed that overexpression of either gene increased C. albicans ability to adhere to an abiotic substrate. However, unlike PGA59, PGA22 overexpression led to cell cluster formation that resulted in increased sensitivity to shear forces and decreased ability to form a single-strain biofilm. Within the multi-strain environment provided by the PGA22-non overexpressing cells, PGA22-overexpressing cells were protected from shear forces and fitter for biofilm development. Ultrastructural analysis, genome-wide transcript profiling and phenotypic analyses in a heterologous context suggested that PGA22 affects cell adherence through alteration of cell wall structure and/or function. Taken together, our findings reveal that several novel predicted GPI-modified proteins contribute to the cooperative behaviour between biofilm cells and are important participants during C. albicans biofilm formation. Moreover, they illustrate the power of using signature tagging in conjunction with gene overexpression for the identification of novel genes involved in processes pertaining to C. albicans virulence.

Candida albicans is the most prevalent human fungal pathogen. Its ability to cause disease relies, in part, on the formation of biofilms, a protective structure of highly adherent cells tolerant to antifungal agents and the host immune response. The biofilm is considered as a persistent root of infection, disseminating infectious cells to other locations. In this study, we performed large-scale phenotypic analyses aimed at identifying genes whose overexpression affects biofilm development in C. albicans. Our screen relied on a collection of 531 C. albicans strains, each conditionally overexpressing one given gene and carrying one specific molecular tag allowing the quantification of strain abundance in mixed-population experiments. Our results strikingly revealed the enrichment of strains overproducing poorly-characterized surface proteins called Pgas (Putative GPI-Anchored proteins), within a 531-strain-containing biofilm model. We show that these PGA genes differentially contribute to single-strain and multi-strain biofilm formation and are involved in specific stages of the biofilm developmental process. Taken together, our results reveal the importance of C. albicans cell surface proteins during biofilm formation and reflect the powerful use of strain barcoding in combination with gene overexpression to identify genes and/or pathways involved in processes pertaining to virulence of pathogenic microbes.

The investigational agent E1210 is effective in treatment of experimental invasive candidiasis caused by resistant Candida albicans

The in vitro and in vivo activity of the inositol acyltransferase inhibitor E1210 was evaluated against echinocandin-resistant Candida albicans. E1210 demonstrated potent in vitro activity, and in mice with invasive candidiasis caused by echinocandin-resistant C. albicans, oral doses of 10 and 40 mg E1210/kg of body weight twice daily significantly improved survival and reduced fungal burden compared to those of controls and mice treated with caspofungin (10 mg/kg/day). These results demonstrate the potential use of E1210 against resistant C. albicans infections.

Adhesins in human fungal pathogens: glue with plenty of stick

Understanding the pathogenesis of an infectious disease is critical for developing new methods to prevent infection and diagnose or cure disease. Adherence of microorganisms to host tissue is a prerequisite for tissue invasion and infection. Fungal cell wall adhesins involved in adherence to host tissue or abiotic medical devices are critical for colonization leading to invasion and damage of host tissue. Here, with a main focus on pathogenic Candida species, we summarize recent progress made in the field of adhesins in human fungal pathogens and underscore the importance of these proteins in establishment of fungal diseases.

A versatile overexpression strategy in the pathogenic yeast Candida albicans: identification of regulators of morphogenesis and fitness

Candida albicans is the most frequently encountered human fungal pathogen, causing both superficial infections and life-threatening systemic diseases. Functional genomic studies performed in this organism have mainly used knock-out mutants and extensive collections of overexpression mutants are still lacking. Here, we report the development of a first generation C. albicans ORFeome, the improvement of overexpression systems and the construction of two new libraries of C. albicans strains overexpressing genes for components of signaling networks, in particular protein kinases, protein phosphatases and transcription factors. As a proof of concept, we screened these collections for genes whose overexpression impacts morphogenesis or growth rates in C. albicans. Our screens identified genes previously described for their role in these biological processes, demonstrating the functionality of our strategy, as well as genes that have not been previously associated to these processes. This article emphasizes the potential of systematic overexpression strategies to improve our knowledge of regulatory networks in C. albicans. The C. albicans plasmid and strain collections described here are available at the Fungal Genetics Stock Center. Their extension to a genome-wide scale will represent important resources for the C. albicans community.

Modular gene over-expression strategies for Candida albicans

Over-expression is a valid functional genomics approach to characterise genes of unknown function on a genome-wide scale. Strains are engineered to over-express a specific gene and the resulting gain-of-function phenotype assessed. Here, we describe the strategy we are adopting to synthesise a Candida albicans ORFeome collection and the options available to create over-expressing strains from this collection.

Hosting infection: experimental models to assay Candida virulence

Although normally commensals in humans, Candida albicans, Candida tropicalis, Candida parapsilosis, Candida glabrata, and Candida krusei are capable of causing opportunistic infections in individuals with altered physiological and/or immunological responses. These fungal species are linked with a variety of infections, including oral, vaginal, gastrointestinal, and systemic infections, with C. albicans the major cause of infection. To assess the ability of different Candida species and strains to cause infection and disease requires the use of experimental infection models. This paper discusses the mucosal and systemic models of infection available to assay Candida virulence and gives examples of some of the knowledge that has been gained to date from these models.

E1210, a new broad-spectrum antifungal, suppresses Candida albicans hyphal growth through inhibition of glycosylphosphatidylinositol biosynthesis

Continued research toward the development of new antifungals that act via inhibition of glycosylphosphatidylinositol (GPI) biosynthesis led to the design of E1210. In this study, we assessed the selectivity of the inhibitory activity of E1210 against Candida albicans GWT1 (Orf19.6884) protein, Aspergillus fumigatus GWT1 (AFUA_1G14870) protein, and human PIG-W protein, which can catalyze the inositol acylation of GPI early in the GPI biosynthesis pathway, and then we assessed the effects of E1210 on key C. albicans virulence factors. E1210 inhibited the inositol acylation activity of C. albicans Gwt1p and A. fumigatus Gwt1p with 50% inhibitory concentrations (IC(50)s) of 0.3 to 0.6 μM but had no inhibitory activity against human Pig-Wp even at concentrations as high as 100 μM. To confirm the inhibition of fungal GPI biosynthesis, expression of ALS1 protein, a GPI-anchored protein, on the surfaces of C. albicans cells treated with E1210 was studied and shown to be significantly lower than that on untreated cells. However, the ALS1 protein levels in the crude extract and the RHO1 protein levels on the cell surface were found to be almost the same. Furthermore, E1210 inhibited germ tube formation, adherence to polystyrene surfaces, and biofilm formation of C. albicans at concentrations above its MIC. These results suggested that E1210 selectively inhibited inositol acylation of fungus-specific GPI which would be catalyzed by Gwt1p, leading to the inhibition of GPI-anchored protein maturation, and also that E1210 suppressed the expression of some important virulence factors of C. albicans, through its GPI biosynthesis inhibition.

Application of the systematic "DAmP" approach to create a partially defective C. albicans mutant

An understanding of gene function often relies upon creating multiple kinds of alleles. Functional analysis in Candida albicans, a major fungal pathogen, has generally included characterization of mutant strains with insertion or deletion alleles and over-expression alleles. Here we use in C. albicans another type of allele that has been employed effectively in the model yeast Saccharomyces cerevisiae, a "Decreased Abundance by mRNA Perturbation" (DAmP) allele (Yan et al., 2008). DAmP alleles are created systematically through replacement of 30 noncoding regions with nonfunctional heterologous sequences, and thus are broadly applicable. We used a DAmP allele to probe the function of Sun41, a surface protein with roles in cell wall integrity, cell-cell adherence, hyphal formation, and biofilm formation that has been suggested as a possible therapeutic target (Firon et al., 2007; Hiller et al., 2007; Norice et al., 2007). A SUN41-DAmP allele results in approximately 10-fold reduced levels of SUN41 RNA, and yields intermediate phenotypes in most assays. We report that a sun41Δ/Δ mutant is defective in biofilm formation in vivo, and that the SUN41-DAmP allele complements that defect. This finding argues that Sun41 may not be an ideal therapeutic target for biofilm inhibition, since a 90% decrease in activity has little effect on biofilm formation in vivo. We anticipate that DAmP alleles of C. albicans genes will be informative for analysis of other prospective drug targets, including essential genes.

Active and passive immunization with rHyr1p-N protects mice against hematogenously disseminated candidiasis

We previously reported that Candida albicans cell surface protein Hyr1 encodes a phagocyte killing resistance factor and active vaccination with a recombinant N-terminus of Hyr1 protein (rHyr1p-N), significantly protects immunocompetent mice from disseminated candidiasis. Here we report the marked efficacy of rHyr1p-N vaccine on improving the survival and reducing the fungal burden of disseminated candidiasis in both immunocompetent and immunocompromised mice using the FDA-approved adjuvant, alum. Importantly, we also show that pooled rabbit anti-Hyr1p polyclonal antibodies raised against 8 different peptide regions of rHyr1p-N protected mice in a hematogenously disseminated candidiasis model, raising the possibility of developing a successful passive immunotherapy strategy to treat this disease. Our data suggest that the rabbit anti-Hyr1p antibodies directly neutralized the Hyr1p virulence function, rather than enhanced opsonophagocytosis for subsequent killing by neutrophil in vitro. Finally, the rHyr1p-N vaccine was protective against non-albicans Candida spp. These preclinical data demonstrate that rHyr1p-N is likely to be a novel target for developing both active and passive immunization strategies against Candida infections.

Candida albicans interactions with epithelial cells and mucosal immunity

Candida albicans interactions with epithelial cells are critical for commensal growth, fungal pathogenicity and host defence. This review will outline our current understanding of C. albicans-epithelial interactions and will discuss how this may lead to the induction of a protective mucosal immune response.

Candida albicans virulence and drug-resistance requires the O-acyltransferase Gup1p

Background

GUP1 gene was primarily identified in Saccharomyces cerevisiae being connected with glycerol uptake defects in association with osmotic stress response. Soon after, Gup1p was implicated in a complex and extensive series of phenotypes involving major cellular processes. These include membrane and wall maintenance, lipid composition, bud-site selection, cytoskeleton orientation, vacuole morphology, secretory/endocytic pathway, GPI anchors remodelling, and lipid-ordered domains assembly, which is compatible with their inclusion in the Membrane Bound O-acyl transferases (MBOAT) family. In mammals, it has been described as a negative regulator of the Sonic hedgehog pathway involved in morphogenesis, differentiation, proliferation, among other processes.

Results

We show that Candida albicans Gup1p strongly interferes with the capacity of cells to develop hyphae, to adhere, to invade, and to form a biofilm, all of which are significant virulence factors. Furthermore, the mutant colonies exhibited an aberrant morphology/differentiation pattern. Identically to S. cerevisiae, Cagup1Δ null mutant was more resistant to antifungals like fluconazole, ketoconazole, and clotrimazole, and displayed an abnormal even sterol distribution at the plasma membrane.

Conclusions

This work is the first study in the opportunistic yeast Candida albicans, showing a role for the GUP1 gene in virulence as well as in the mechanisms underlying antifungal resistance. Moreover, its impact is even more significant since these results, taken together with all the knowledge about GUP1 gene (from S. cerevisiae and mammals) give consistence to the possibility that Gup1p may be part of a yeast morphogenic pathway parallel to the mammalian Hedgehog.

Candida albicans Hyr1p confers resistance to neutrophil killing and is a potential vaccine target

Candida albicans is the most common cause of invasive fungal infections in humans. It is unclear how C. albicans escapes from phagocytic attack and survives in the hostile blood environment during life-threatening systemic infections. Using a conditional overexpression or suppression genetic strategy, we discovered that HYR1 gene reduced phagocytic killing activity of C. albicans in vitro and increased tissue fungal burden in vivo. Concordant with its positive regulation by the transcription factor Bcr1p, autonomous expression of HYR1 complemented the hypersusceptibility to phagocyte-mediated killing of a bcr1 null mutant of C. albicans in vitro. As for C. albicans, heterologous expression of HYR1 in Candida glabrata rendered the organism more resistant to neutrophil killing activity. Vaccination with a recombinant Hyr1p significantly protected mice against hematogenously disseminated candidiasis (P = .001). Finally, anti-rHyr1p polyclonal antibodies enhanced mouse neutrophil killing activity by directly neutralizing rHyr1p effects in vitro. Thus, Hyr1 is an important virulence factor for C. albicans, mediating resistance to phagocyte killing. Hyr1p is a promising target for vaccine or other immunological or small molecule intervention to improve the outcomes of disseminated candidiasis.

Interactions of Candida albicans with epithelial cells

The fungus, Candida albicans, interacts with epithelial cells in the human host both as a normal commensal and as an invasive pathogen. It has evolved multiple complementary mechanisms to adhere to epithelial cells. Adherent C. albicans cells can invade epithelial surfaces both by penetrating into individual epithelial cells, and by degrading interepithelial cell junctions and passing between epithelial cells. Invasion into epithelial cells is mediated by both induced endocytosis and active penetration, whereas degradation of epithelial cell junction proteins, such as E-cadherin, occurs mainly via proteolysis by secreted aspartyl proteinases. C. albicans invasion of epithelial cells results in significant epithelial cell damage, which is probably induced by lytic enzymes, such as proteases and phospholipase secreted by the organism. Future challenges include identifying the epithelial cell targets of adhesins and invasins, and determining the mechanisms by which C. albicans actively penetrates epithelial cells and induces epithelial cell damage.

Examination of potential virulence factors of Candida tropicalis clinical isolates from hospitalized patients

Candida tropicalis has been reported to be one of the Candida species which is most likely to cause bloodstream and urinary tract infections in hospitalized patients. Accordingly, the aim of this study was to characterize the virulence of C. tropicalis by assessing antifungal susceptibility and comparing the expression of several virulence factors. This study was conducted with seven isolates of C. tropicalis from urine and blood cultures and from central venous catheter. C. tropicalis ATCC 750 was used as reference strain. Yeasts adhered (2 h) to epithelial cells and silicone and 24 h biofilm biomass were determined by crystal violet staining. Pseudohyphae formation ability was determined after growth in fetal bovine serum. Enzymes production (hemolysins, proteases, phospholipases) was assessed by halo formation on agar plates. Susceptibility to antifungal agents was determined by E-test. Regarding adhesion, it can be highlighted that C. tropicalis strains adhered significantly more to epithelium than to silicone. Furthermore, all C. tropicalis strains were able to form biofilms and to express total hemolytic activity. However, protease was only produced by two isolates from urine and by the isolates from catheter and blood. Moreover, only one C. tropicalis (from catheter) was phospholipase positive. All isolates were susceptible to voriconazole, fluconazole and amphotericin B. Four strains were susceptible-dose dependent to itraconazole and one clinical isolate was found to be resistant.

Covalently linked cell wall proteins of Candida albicans and their role in fitness and virulence

The cell wall of Candida albicans consists of an internal skeletal layer and an external protein coat. This coat has a mosaic-like nature, containing c. 20 different protein species covalently linked to the skeletal layer. Most of them are GPI proteins. Coat proteins vary widely in function. Many of them are involved in the primary interactions between C. albicans and the host and mediate adhesive steps or invasion of host cells. Others are involved in biofilm formation and cell-cell aggregation. They further include iron acquisition proteins, superoxide dismutases, and yapsin-like aspartic proteases. In addition, several covalently linked carbohydrate-active enzymes are present, whose precise functions remain hitherto largely elusive. The expression levels of the genes that encode covalently linked cell wall proteins (CWPs) can vary enormously. They depend on the mode of growth and the combined inputs of several signaling pathways that sense environmental conditions. This is reflected in the unusually long intergenic regions of most of these genes. Finally, the precise location of several covalently linked CWPs is temporally and spatially regulated. We conclude that covalently linked CWPs of C. albicans play a crucial role in fitness and virulence and that their expression is tightly controlled.

Protection by anti-beta-glucan antibodies is associated with restricted beta-1,3 glucan binding specificity and inhibition of fungal growth and adherence

Anti-β-glucan antibodies elicited by a laminarin-conjugate vaccine confer cross-protection to mice challenged with major fungal pathogens such as Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. To gain insights into protective β-glucan epitope(s) and protection mechanisms, we studied two anti-β-glucan monoclonal antibodies (mAb) with identical complementarity-determining regions but different isotypes (mAb 2G8, IgG2b and mAb 1E12, IgM). C. albicans, the most relevant fungal pathogen for humans, was used as a model.

Both mAbs bound to fungal cell surface and to the β1,3-β1,6 glucan of the fungal cell wall skeleton, as shown by immunofluorescence, electron-microscopy and ELISA. They were also equally unable to opsonize fungal cells in a J774 macrophage phagocytosis and killing assay. However, only the IgG2b conferred substantial protection against mucosal and systemic candidiasis in passive vaccination experiments in rodents. Competition ELISA and microarray analyses using sequence-defined glucan oligosaccharides showed that the protective IgG2b selectively bound to β1,3-linked (laminarin-like) glucose sequences whereas the non-protective IgM bound to β1,6- and β1,4-linked glucose sequences in addition to β1,3-linked ones. Only the protective IgG2b recognized heterogeneous, polydisperse high molecular weight cell wall and secretory components of the fungus, two of which were identified as the GPI-anchored cell wall proteins Als3 and Hyr1. In addition, only the IgG2b inhibited in vitro two critical virulence attributes of the fungus, hyphal growth and adherence to human epithelial cells.

Our study demonstrates that the isotype of anti-β-glucan antibodies may affect details of the β-glucan epitopes recognized, and this may be associated with a differing ability to inhibit virulence attributes of the fungus and confer protection in vivo. Our data also suggest that the anti-virulence properties of the IgG2b mAb may be linked to its capacity to recognize β-glucan epitope(s) on some cell wall components that exert critical functions in fungal cell wall structure and adherence to host cells.

Disruption of the GPI protein-encoding gene IFF4 of Candida albicans results in decreased adherence and virulence

Candida albicans is the most important cause of systemic fungal infection in immunocompromised humans. Candidiasis is often initiated by the adherence and the colonization of inert surfaces such as peripheral venous catheters, central catheters, prosthetic cardiac valves, and other prostheses. We have studied the early stage of adherence and have shown that the disruption of C. albicans IFF4 gene encoding a GPI-anchor protein, led to a decrease of adherence of the germ tubes to plastic. Here, we demonstrated the role of the IFF4 gene in adherence to silicone catheter, as well as in virulence using a murine model of disseminated candidiasis. The iff4 Delta null mutant showed both a decrease of adherence to silicone catheter and a reduction of virulence. This work presents evidence for the importance of the IFF4 gene in host-fungal interaction.