KRE5 gene null mutant strains of Candida albicans are avirulent and have altered cell wall composition and hypha formation properties

Recent advances in the biosynthesis of fungal glucan structural diversity

Glucans are the most abundant class of macromolecule polymers in fungi, which are commonly found in Ascomycota and Basidiomycota. Fungal glucans are not only essential for cell integrity and function but also crucial for the immense industrial interest in high value applications. They present a variety of structural characteristics at the nanoscale due to the high regulation of genes and the involvement of stochastic processes in synthesis. However, although recent findings have demonstrated the genes of glucans synthesis are relatively conserved across diverse fungi, the formation and organization of diverse glucan structures is still unclear in fungi. Here, we summarize the structural features of fungal glucans and the recent developments in the mechanisms of glucans biosynthesis. Furthermore, we propose the engineering strategies of targeted glucan synthesis and point out the remaining challenges in the synthetic process. Understanding the synthesis process of diverse glucans is necessary for tailoring high value glucan towards specific applications. This engineering strategy contributes to enable the sustainable and efficient production of glucan diversity.

Identifying genetic variants associated with amphotericin B (AMB) resistance in Aspergillus fumigatus via k-mer-based GWAS

Aspergillus fumigatus is one of the most common pathogenic fungi, which results in high morbidity and mortality in immunocompromised patients. Amphotericin B (AMB) is used as the core drug for the treatment of triazole-resistant A. fumigatus. Following the usage of amphotericin B drugs, the number of amphotericin B-resistant A. fumigatus isolates showed an increasing trend over the years, but the mechanism and mutations associated with amphotericin B sensitivity are not fully understood. In this study, we performed a k-mer-based genome-wide association study (GWAS) in 98 A. fumigatus isolates from public databases. Associations identified with k-mers not only recapitulate those with SNPs but also discover new associations with insertion/deletion (indel). Compared to SNP sites, the indel showed a stronger association with amphotericin B resistance, and a significant correlated indel is present in the exon region of AFUA_7G05160, encoding a fumarylacetoacetate hydrolase (FAH) family protein. Enrichment analysis revealed sphingolipid synthesis and transmembrane transport may be related to the resistance of A. fumigatus to amphotericin B. The expansion of variant types detected by the k-mer method increases opportunities to identify and exploit complex genetic variants that drive amphotericin B resistance, and these candidate variants help accelerate the selection of prospective gene markers for amphotericin B resistance screening in A. fumigatus.

Mucosal Infection with Unmasked Candida albicans Cells Impacts Disease Progression in a Host Niche-Specific Manner

Shielding the immunogenic cell wall epitope β(1, 3)-glucan under an outer layer of mannosylated glycoproteins is an essential virulence factor deployed by Candida albicans during systemic infection. Accordingly, mutants with increased β(1, 3)-glucan exposure (unmasking) display increased immunostimulatory capabilities in vitro and attenuated virulence during systemic infection in mice. However, little work has been done to assess the impact of increased unmasking during the two most common manifestations of candidiasis, namely, oropharyngeal candidiasis (OPC) and vulvovaginal candidiasis (VVC). We have shown previously that the expression of a single hyperactive allele of the MAP3K STE11^ΔN467 induces unmasking via the Cek1 MAPK pathway, attenuates fungal burden, and prolongs survival during systemic infection in mice. Here, we expand on these findings and show that infection with an unmasked STE11^ΔN467 mutant also impacts disease progression during OPC and VVC murine infection models. Male mice sublingually infected with the STE11^ΔN467 mutant showed a significant reduction in tongue fungal burden at 2 days postinfection and a modest reduction at 5 days postinfection. However, we find that selection for STE11^ΔN467 suppressor mutants that no longer display increased unmasking occurs within the oral cavity and is likely responsible for the restoration of fungal burden trends to wild-type levels later in the infection. In the VVC infection model, no attenuation in fungal burden was observed. However, polymorphonuclear cell recruitment and interleukin-1β (IL-1β) levels within the vaginal lumen, markers of immunopathogenesis, were increased in mice infected with unmasked STE11^ΔN467 cells. Thus, our data suggest a niche-specific impact for unmasking on disease progression.

Stress combined with loss of the Candida albicans SUMO protease Ulp2 triggers selection of aneuploidy via a two-step process

A delicate balance between genome stability and instability ensures genome integrity while generating genetic diversity, a critical step for evolution. Indeed, while excessive genome instability is harmful, moderated genome instability can drive adaptation to novel environments by maximising genetic variation. Candida albicans, a human fungal pathogen that colonises different parts of the human body, adapts rapidly and frequently to different hostile host microenvironments. In this organism, the ability to generate large-scale genomic variation is a key adaptative mechanism triggering dangerous infections even in the presence of antifungal drugs. Understanding how fitter novel karyotypes are selected is key to determining how C. albicans and other microbial pathogens establish infections. Here, we identified the SUMO protease Ulp2 as a regulator of C. albicans genome integrity through genetic screening. Deletion of ULP2 leads to increased genome instability, enhanced genome variation and reduced fitness in the absence of additional stress. The combined stress caused by the lack of ULP2 and antifungal drug treatment leads to the selection of adaptive segmental aneuploidies that partially rescue the fitness defects of ulp2Δ/Δ cells. Short and long-read genomic sequencing demonstrates that these novel genotypes are selected via a two-step process leading to the formation of novel chromosomal fragments with breakpoints at microhomology regions and DNA repeats.

Caspofungin resistance in Candida albicans: genetic factors and synergistic compounds for combination therapies

Caspofungin and other echinocandins have been used for the treatment of human infections by the opportunistic yeast pathogen, Candida albicans. There has been an increase in infections by non-albicans Candida species such as Candida glabrata, Candida parapsilosis, Candida tropicalis, Candida krusei, and Candida auris in clinical or hospital settings. This is problematic to public health due to the increasing prevalence of echinocandin resistant species/strains. This review will present a summary on various studies that investigated the inhibitory action of caspofungin on 1,3-β-D-glucan synthesis, on cell wall structure, and biofilm formation of C. albicans. It will highlight some of the issues linked to caspofungin resistance or reduced caspofungin sensitivity in various Candida species and the potential benefits of antimicrobial peptides and other compounds in synergy with caspofungin.

When Is It Appropriate to Take Off the Mask? Signaling Pathways That Regulate ß(1,3)-Glucan Exposure in Candida albicans

Candida spp. are an important source of systemic and mucosal infections in immune compromised populations. However, drug resistance or toxicity has put limits on the efficacy of current antifungals. The C. albicans cell wall is considered a good therapeutic target due to its roles in viability and fungal pathogenicity. One potential method for improving antifungal strategies could be to enhance the detection of fungal cell wall antigens by host immune cells. ß(1,3)-glucan, which is an important component of fungal cell walls, is a highly immunogenic epitope. Consequently, multiple host pattern recognition receptors, such as dectin-1, complement receptor 3 (CR3), and the ephrin type A receptor A (EphA2) are capable of recognizing exposed (unmasked) ß(1,3)-glucan moieties on the cell surface to initiate an anti-fungal immune response. However, ß(1,3)-glucan is normally covered (masked) by a layer of glycosylated proteins on the outer surface of the cell wall, hiding it from immune detection. In order to better understand possible mechanisms of unmasking ß(1,3)-glucan, we must develop a deeper comprehension of the pathways driving this phenotype. In this review, we describe the medical importance of ß(1,3)-glucan exposure in anti-fungal immunity, and highlight environmental stimuli and stressors encountered within the host that are capable of inducing changes in the levels of surface exposed ß(1,3)-glucan. Furthermore, particular focus is placed on how signal transduction cascades regulate changes in ß(1,3)-glucan exposure, as understanding the role that these pathways have in mediating this phenotype will be critical for future therapeutic development.

The Role of Candida albicans Virulence Factors in the Formation of Multispecies Biofilms With Bacterial Periodontal Pathogens

Periodontal disease depends on the presence of different microorganisms in the oral cavity that during the colonization of periodontal tissues form a multispecies biofilm community, thus allowing them to survive under adverse conditions or facilitate further colonization of host tissues. Not only numerous bacterial species participate in the development of biofilm complex structure but also fungi, especially Candida albicans, that often commensally inhabits the oral cavity. C. albicans employs an extensive armory of various virulence factors supporting its coexistence with bacteria resulting in successful host colonization and propagation of infection. In this article, we highlight various aspects of individual fungal virulence factors that may facilitate the collaboration with the associated bacterial representatives of the early colonizers of the oral cavity, the bridging species, and the late colonizers directly involved in the development of periodontitis, including the “red complex” species. In particular, we discuss the involvement of candidal cell surface proteins—typical fungal adhesins as well as originally cytosolic “moonlighting” proteins that perform a new function on the cell surface and are also present within the biofilm structures. Another group of virulence factors considered includes secreted aspartic proteases (Sap) and other secreted hydrolytic enzymes. The specific structure of the candidal cell wall, dynamically changing during morphological transitions of the fungus that favor the biofilm formation, is equally important and discussed. The non-protein biofilm-composing factors also show dynamic variability upon the contact with bacteria, and their biosynthesis processes could be involved in the stability of mixed biofilms. Biofilm-associated changes in the microbe communication system using different quorum sensing molecules of both fungal and bacterial cells are also emphasized in this review. All discussed virulence factors involved in the formation of mixed biofilm pose new challenges and influence the successful design of new diagnostic methods and the application of appropriate therapies in periodontal diseases.

Our pursuit for effective antifungal agents targeting fungal cell wall components, where are we?

Invasive mycotic infections account for an unacceptably high mortality rates in humans. These infections are initiated by the fungal cell wall which mediates host-fungi interactions. The cell wall is fused to the physiology of fungi, and it is involved in essential functions in the entire cell functionality. Components of the cell wall are synthesised and modified in the cell wall space by the activities of cell wall proteins through a range of signalling pathways that have only been described in many fungi, therefore making them suitable drug targets. The echinocandins class of cell wall-active drugs block cell wall β-1,3-glucan biosynthesis through inhibiting the catalytic subunit of the synthetic protein complex. Resistance to echinocandins can be through the acquisition of single nucleotide polymorphisms and/or through activation of cell wall signalling pathways resulting in altered cell wall proteome and elevated chitin content in the cell wall. Countering the cell wall remodelling process will enhance the effectiveness of β-1,3-glucan-active antifungal agents. Cell surface proteins are also important antifungal targets which can be used to develop rapid and robust diagnostics and more effective therapeutics. The cell wall remains a crucial target in fungi that needs to be harnessed to combat mycotic infections.

Defeated by the nines: nine extracellular strategies to avoid microbe-associated molecular patterns recognition in plants

Recognition of microbe-associated molecular patterns (MAMPs) by cell-surface receptors is pivotal in host-microbe interactions. Both pathogens and symbionts establish plant-microbe interactions using fascinating intricate extracellular strategies to avoid recognition. Here we distinguish nine different extracellular strategies to avoid recognition by the host, acting at three different levels. To avoid the accumulation of MAMP precursors (Level 1), microbes take advantage of polymorphisms in both MAMP proteins and glycans, or downregulate MAMP production. To reduce hydrolytic MAMP release (Level 2), microbes shield MAMP precursors with proteins or glycans and inhibit or degrade host-derived hydrolases. And to prevent MAMP perception directly (Level 3), microbes degrade or sequester MAMPs before they are perceived. We discuss examples of these nine strategies and envisage three additional extracellular strategies to avoid MAMP perception in plants.

Activation of Cph1 causes ß(1,3)-glucan unmasking in Candida albicans and attenuates virulence in mice in a neutrophil-dependent manner

Masking the immunogenic cell wall epitope ß(1,3)-glucan under an outer layer of mannosylated glycoproteins is an important virulence factor deployed by Candida albicans during infection. Consequently, increased ß(1,3)-glucan exposure (unmasking) reveals C. albicans to the host's immune system and attenuates its virulence. We have previously shown that activation of the Cek1 MAPK pathway via expression of a hyperactive allele of an upstream kinase (STE11ΔN467) induced unmasking. It also increased survival of mice in a murine disseminated candidiasis model and attenuated kidney fungal burden by ≥33 fold. In this communication, we utilized cyclophosphamide-induced immunosuppression to test if the clearance of the unmasked STE11ΔN467 mutant was dependent on the host immune system. Suppression of the immune response by cyclophosphamide reduced the attenuation in fungal burden caused by the STE11ΔN467 allele. Moreover, specific depletion of neutrophils via 1A8 antibody treatment also reduced STE11ΔN467-dependent fungal burden attenuation, but to a lesser extent than cyclophosphamide, demonstrating an important role for neutrophils in mediating fungal clearance of unmasked STE11ΔN467 cells. In an effort to understand the mechanism by which Ste11ΔN467 causes unmasking, transcriptomics were used to reveal that several components in the Cek1 MAPK pathway were upregulated, including the transcription factor CPH1 and the cell wall sensor DFI1. In this report we show that a cph1ΔΔ mutation restored ß(1,3)-glucan exposure to wild-type levels in the STE11ΔN467 strain, confirming that Cph1 is the transcription factor mediating Ste11ΔN467-induced unmasking. Furthermore, Cph1 is shown to induce a positive feedback loop that increases Cek1 activation. In addition, full unmasking by STE11ΔN467 is dependent on the upstream cell wall sensor DFI1. However, while deletion of DFI1 significantly reduced Ste11ΔN467-induced unmasking, it did not impact activation of the downstream kinase Cek1. Thus, it appears that once stimulated by Ste11ΔN467, Dfi1 activates a parallel signaling pathway that is involved in Ste11ΔN467-induced unmasking.

Natural products targeting the synthesis of β(1,3)-D-glucan and chitin of the fungal cell wall. Existing drugs and recent findings

BACKGROUND

During the last three decades systemic fungal infections associated to immunosuppressive therapies have become a serious healthcare problem. Clinical development of new antifungals is an urgent requirement. Since fungal but not mammalian cells are encased in a carbohydrate-containing cell wall, which is required for the growth and viability of fungi, the inhibition of cell wall synthesizing machinery, such as β(1,3)-D-glucan synthases (GS) and chitin synthases (CS) that catalyze the synthesis of β(1-3)-D-glucan and chitin, respectively, represent an ideal mode of action of antifungal agents. Although the echinocandins anidulafungin, caspofungin and micafungin are clinically well-established GS inhibitors for the treatment of invasive fungal infections, much effort must still be made to identify inhibitors of other enzymes and processes involved in the synthesis of the fungal cell wall.

PURPOSE

Since natural products (NPs) have been the source of several antifungals in clinical use and also have provided important scaffolds for the development of semisynthetic analogues, this review was devoted to investigate the advances made to date in the discovery of NPs from plants that showed capacity of inhibiting cell wall synthesis targets. The chemical characterization, specific target, discovery process, along with the stage of development are provided here.

METHODS

An extensive systematic search for NPs against the cell wall was performed considering all the articles published until the end of 2020 through the following scientific databases: NCBI PubMed, Scopus and Google Scholar and using the combination of the terms "natural antifungals" and "plant extracts" with "fungal cell wall".

RESULTS

The first part of this review introduces the state of the art of the structure and biosynthesis of the fungal cell wall and considers exclusively those naturally produced GS antifungals that have given rise to both existing semisynthetic approved drugs and those derivatives currently in clinical trials. According to their chemical structure, natural GS inhibitors can be classified as 1) cyclic lipopeptides, 2) glycolipids and 3) acidic terpenoids. We also included nikkomycins and polyoxins, NPs that inhibit the CS, which have traditionally been considered good candidates for antifungal drug development but have finally been discarded after enduring unsuccessful clinical trials. Finally, the review focuses in the most recent findings about the growing field of plant-derived molecules and extracts that exhibit activity against the fungal cell wall. Thus, this search yielded sixteen articles, nine of which deal with pure compounds and seven with plant extracts or fractions with proven activity against the fungal cell wall. Regarding the mechanism of action, seven (44%) produced GS inhibition while five (31%) inhibited CS. Some of them (56%) interfered with other components of the cell wall. Most of the analyzed articles refer to tests carried out in vitro and therefore are in early stages of development.

CONCLUSION

This report delivers an overview about both existing natural antifungals targeting GS and CS activities and their mechanisms of action. It also presents recent discoveries on natural products that may be used as starting points for the development of potential selective and non-toxic antifungal drugs.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

The Role of RodA-Conserved Cysteine Residues in the Aspergillus fumigatus Conidial Surface Organization

Immune inertness of Aspergillusfumigatus conidia is attributed to its surface rodlet-layer made up of RodAp, characterized by eight conserved cysteine residues forming four disulfide bonds. Earlier, we showed that the conserved cysteine residue point (ccrp) mutations result in conidia devoid of the rodlet layer. Here, we extended our study comparing the surface organization and immunoreactivity of conidia carrying ccrp-mutations with the RODA deletion mutant (∆rodA). Western blot analysis using anti-RodAp antibodies indicated the absence of RodAp in the cytoplasm of ccrp-mutant conidia. Immunolabeling revealed differential reactivity to conidial surface glucans, the ccrp-mutant conidia preferentially binding to α-(1,3)-glucan, ∆rodA conidia selectively bound to β-(1,3)-glucan; the parental strain conidia showed negative labeling. However, permeability of ccrp-mutants and ∆rodA was similar to the parental strain conidia. Proteomic analyses of the conidial surface exposed proteins of the ccrp-mutants showed more similarities with the parental strain, but were significantly different from the ∆rodA. Ccrp-mutant conidia were less immunostimulatory compared to ∆rodA conidia. Our data suggest that (i) the conserved cysteine residues are essential for the trafficking of RodAp and the organization of the rodlet layer on the conidial surface, and (ii) targeted point mutation could be an alternative approach to study the role of fungal cell-wall genes in host–fungal interaction.

Mannan Molecular Substructures Control Nanoscale Glucan Exposure in Candida

Cell wall mannans of Candida albicans mask β-(1,3)-glucan from recognition by Dectin-1, contributing to innate immune evasion. Glucan exposures are predominantly single receptor-ligand interaction sites of nanoscale dimensions. Candida species vary in basal glucan exposure and molecular complexity of mannans. We used super-resolution fluorescence imaging and a series of protein mannosylation mutants in C. albicans and C. glabrata to investigate the role of specific N-mannan features in regulating the nanoscale geometry of glucan exposure. Decreasing acid labile mannan abundance and α-(1,6)-mannan backbone length correlated most strongly with increased density and nanoscopic size of glucan exposures in C. albicans and C. glabrata, respectively. Additionally, a C. albicans clinical isolate with high glucan exposure produced similarly perturbed N-mannan structures and elevated glucan exposure geometry. Thus, acid labile mannan structure influences the nanoscale features of glucan exposure, impacting the nature of the pathogenic surface that triggers immunoreceptor engagement, aggregation, and signaling.

Candida albicans rvs161Δ and rvs167Δ Endocytosis Mutants Are Defective in Invasion into the Oral Cavity

Oropharyngeal candidiasis (OPC) is a common fungal infection that is associated with severe morbidity. Another concern is that patients at risk for developing OPC often take long courses of antifungal drugs, which can lead to the emergence of drug-resistant C. albicans strains. We therefore identified nine mutants with defects in undergoing invasive hyphal growth in the oral cavity, increasing the number of genes known to be involved in OPC by more than 30%. The two strongest mutants, rvs161Δ and rvs167Δ, have defects in endocytosis. The rvsΔ mutants appear to have a specific defect in initiating invasive growth, as preinducing the cells to form hyphae prior to infection restored their ability to cause OPC. These results indicate that blocking endocytosis could have therapeutic value in preventing the initiation of OPC without leading to development of resistance against drugs currently used to treat fungal infections.

Invasive growth in tissues by the human fungal pathogen Candida albicans is promoted by a switch from budding to hyphal morphogenesis that is stimulated by multiple environmental factors that can vary at different sites of infection. To identify genes that promote invasive growth in the oral cavity to cause oropharyngeal candidiasis (OPC), we first identified C. albicans mutants that failed to invade agar medium. Analysis of nine severely defective mutants in a mouse model of OPC revealed that the strongest defects were seen for the rvs161Δ and rvs167Δ mutants, which lack amphiphysin proteins needed for endocytosis. The rvsΔ mutants initially adhered to the tongue but failed to invade efficiently and were lost from the oral cavity. Previous studies indicated that rvsΔ mutants formed filamentous hyphae in the kidney albeit with morphological abnormalities, suggesting that the rvsΔ mutants were influenced by factors that vary at different sites of infection. Consistent with this, increasing concentrations of CO₂, an inducer of hyphal growth that is more abundant in internal organs than air, partially rescued the invasive-growth defects of the rvsΔ mutants in vitro. Interestingly, preinduction of the rvsΔ mutants to form hyphae prior to introduction into the oral cavity restored their ability to cause OPC, identifying a key role for endocytosis in initiating invasive hyphal growth. These results highlight the influence of distinct environmental factors in promoting invasive hyphal growth in the oral cavity and indicate that blocking endocytosis could have therapeutic value in preventing the initiation of OPC.

Lrg1 Regulates β (1,3)-Glucan Masking in Candida albicans through the Cek1 MAP Kinase Pathway

Candida albicans is among the most prevalent opportunistic human fungal pathogens. The ability to mask the immunogenic polysaccharide β (1,3)-glucan from immune detection via a layer of mannosylated proteins is a key virulence factor of C. albicans We previously reported that hyperactivation of the Cek1 mitogen-activated protein (MAP) kinase pathway promotes β (1,3)-glucan exposure. In this communication, we report a novel upstream regulator of Cek1 activation and characterize the impact of Cek1 activity on fungal virulence. Lrg1 encodes a GTPase-activating protein (GAP) that has been suggested to inhibit the GTPase Rho1. We found that disruption of LRG1 causes Cek1 hyperactivation and β (1,3)-glucan unmasking. However, when GTPase activation was measured for a panel of GTPases, the lrg1ΔΔ mutant exhibited increased activation of Cdc42 and Ras1 but not Rho1 or Rac1. Unmasking and Cek1 activation in the lrg1ΔΔ mutant can be blocked by inhibition of the Ste11 MAP kinase kinase kinase (MAPKKK), indicating that the lrg1ΔΔ mutant acts through the canonical Cek1 MAP kinase cascade. In order to determine how Cek1 hyperactivation specifically impacts virulence, a doxycycline-repressible hyperactive STE11ΔN467 allele was expressed in C. albicans In the absence of doxycycline, this allele overexpressed STE11ΔN467 , which induced production of proinflammatory tumor necrosis factor alpha (TNF-α) from murine macrophages. This in vitro phenotype correlates with decreased colonization and virulence in a mouse model of systemic infection. The mechanism by which Ste11ΔN467 causes unmasking was explored with RNA sequencing (RNA-Seq) analysis. Overexpression of Ste11ΔN467 caused upregulation of the Cph1 transcription factor and of a group of cell wall-modifying proteins which are predicted to impact cell wall architecture.IMPORTANCECandida albicans is an important source of systemic infections in humans. The ability to mask the immunogenic cell wall polymer β (1,3)-glucan from host immune surveillance contributes to fungal virulence. We previously reported that the hyperactivation of the Cek1 MAP kinase cascade promotes cell wall unmasking, thus increasing strain immunogenicity. In this study, we identified a novel regulator of the Cek1 pathway called Lrg1. Lrg1 is a predicted GTPase-activating protein (GAP) that represses Cek1 activity by downregulating the GTPase Cdc42 and its downstream MAPKKK, Ste11. Upregulation of Cek1 activity diminished fungal virulence in the mouse model of infection, and this correlates with increased cytokine responses from macrophages. We also analyzed the transcriptional profile determined during β (1,3)-glucan exposure driven by Cek1 hyperactivation. Our report provides a model where Cek1 hyperactivation causes β (1,3)-glucan exposure by upregulation of cell wall proteins and leads to more robust immune detection in vivo, promoting more effective clearance.

Contributions of Candida albicans Dimorphism, Adhesive Interactions, and Extracellular Matrix to the Formation of Dual-Species Biofilms with Streptococcus gordonii

Microbial communities have a great impact in health and disease. C. albicans interacts with multiple microorganisms in the oral cavity, frequently forming polymicrobial biofilms. We report on the synergistic interactions between C. albicans and the Gram-positive bacterium S. gordonii, for which we have examined the different contributions of adhesive interactions, filamentation, and the extracellular matrix to the formation of dual-species biofilms. Our results demonstrate that growth in the presence of the bacterium can restore the biofilm-forming ability of different C. albicans mutant strains with defects in adhesion and filamentation. The mixed-species biofilms also show high levels of resistance to antibacterial and antifungal antibiotics, and our results indicate that the fungal biofilm matrix protects bacterial cells within these mixed-species biofilms. Our observations add to a growing body of evidence indicating a high level of complexity in the reciprocal interactions and consortial behavior of fungal/bacterial biofilms.

Fungal and bacterial populations coexist in the oral cavity, frequently forming mixed-species biofilms that complicate treatment against polymicrobial infections. However, despite relevance to oral health, the bidirectional interactions between these microbial populations are poorly understood. In this study, we aimed to elucidate the mechanisms underlying the interactions between the fungal species Candida albicans and the bacterial species Streptococcus gordonii as they coexist in mixed-species biofilms. Specifically, the interactions of different C. albicans mutant strains deficient in filamentation (efg1Δ/Δ and brg1Δ/Δ), adhesive interactions (als3Δ/Δ and bcr1Δ/Δ), and production of matrix exopolymeric substances (EPS) (kre5Δ/Δ, mnn9Δ/Δ, rlm1Δ/Δ, and zap1Δ/Δ) were evaluated with S. gordonii under different conditions mimicking the environment in the oral cavity. Interestingly, our results revealed that growth of the biofilm-deficient C. albicansals3Δ/Δ and bcr1Δ/Δ mutant strains in synthetic saliva or with S. gordonii restored their biofilm-forming ability. Moreover, challenging previous observations indicating an important role of morphogenetic conversions in the interactions between these two species, our results indicated a highly synergistic interaction between S. gordonii and the C. albicans filamentation-deficient efg1Δ/Δ and brg1Δ/Δ deletion mutants, which was particularly noticeable when the mixed biofilms were grown in synthetic saliva. Importantly, dual-species biofilms were found to exhibit increase in antimicrobial resistance, indicating that components of the fungal exopolymeric material confer protection to streptococcal cells against antibacterial treatment. Collectively, these findings unravel a high degree of complexity in the interactions between C. albicans and S. gordonii in mixed-species biofilms, which may impact homeostasis in the oral cavity.

The fungal cell wall as a target for the development of new antifungal therapies

In the past three decades invasive mycoses have globally emerged as a persistent source of healthcare-associated infections. The cell wall surrounding the fungal cell opposes the turgor pressure that otherwise could produce cell lysis. Thus, the cell wall is essential for maintaining fungal cell shape and integrity. Given that this structure is absent in host mammalian cells, it stands as an important target when developing selective compounds for the treatment of fungal infections. Consequently, treatment with echinocandins, a family of antifungal agents that specifically inhibits the biosynthesis of cell wall (1-3)β-D-glucan, has been established as an alternative and effective antifungal therapy. However, the existence of many pathogenic fungi resistant to single or multiple antifungal families, together with the limited arsenal of available antifungal compounds, critically affects the effectiveness of treatments against these life-threatening infections. Thus, new antifungal therapies are required. Here we review the fungal cell wall and its relevance in biotechnology as a target for the development of new antifungal compounds, disclosing the most promising cell wall inhibitors that are currently in experimental or clinical development for the treatment of some invasive mycoses.

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.

A computational model for regulation of nanoscale glucan exposure in Candida albicans

Candida albicans is a virulent human opportunistic pathogen. It evades innate immune surveillance by masking an immunogenic cell wall polysaccharide, β-glucan, from recognition by the immunoreceptor Dectin-1. Glucan unmasking by the antifungal drug caspofungin leads to changes in the nanostructure of glucan exposure accessible to Dectin-1. The physical mechanism that regulates glucan exposure is poorly understood, but it controls the nanobiology of fungal pathogen recognition. We created computational models to simulate hypothetical physical processes of unmasking glucan in a biologically realistic distribution of cell wall glucan fibrils. We tested the predicted glucan exposure nanostructural features arising from these models against experimentally measured values. A completely spatially random unmasking process, reflective of random environmental damage to the cell wall, cannot account for experimental observations of glucan unmasking. However, the introduction of partially edge biased unmasking processes, consistent with an unmasking contribution from active, local remodeling at glucan exposure sites, produces markedly more accurate predictions of experimentally observed glucan nanoexposures in untreated and caspofungin-treated yeast. These findings suggest a model of glucan unmasking wherein cell wall remodeling processes in the local nanoscale neighborhood of glucan exposure sites are an important contributor to the physical process of drug-induced glucan unmasking in C. albicans.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

Single-particle electron microscopy structure of UDP-glucose:glycoprotein glucosyltransferase suggests a selectivity mechanism for misfolded proteins

The enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) mediates quality control of glycoproteins in the endoplasmic reticulum by attaching glucose to N-linked glycan of misfolded proteins. As a sensor, UGGT ensures that misfolded proteins are recognized by the lectin chaperones and do not leave the secretory pathway. The structure of UGGT and the mechanism of its selectivity for misfolded proteins have been unknown for 25 years. Here, we used negative-stain electron microscopy and small-angle X-ray scattering to determine the structure of UGGT from Drosophila melanogaster at 18-Å resolution. Three-dimensional reconstructions revealed a cage-like structure with a large central cavity. Particle classification revealed flexibility that precluded determination of a high-resolution structure. Introduction of biotinylation sites into a fungal UGGT expressed in Escherichia coli allowed identification of the catalytic and first thioredoxin-like domains. We also used hydrogen-deuterium exchange mass spectrometry to map the binding site of an accessory protein, Sep15, to the first thioredoxin-like domain. The UGGT structural features identified suggest that the central cavity contains the catalytic site and is lined with hydrophobic surfaces. This enhances the binding of misfolded substrates with exposed hydrophobic residues and excludes folded proteins with hydrophilic surfaces. In conclusion, we have determined the UGGT structure, which enabled us to develop a plausible functional model of the mechanism for UGGT's selectivity for misfolded glycoproteins.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

β-(1,3)-Glucan Unmasking in Some Candida albicans Mutants Correlates with Increases in Cell Wall Surface Roughness and Decreases in Cell Wall Elasticity

Candida albicans is among the most common human fungal pathogens, causing a broad range of infections, including life-threatening systemic infections. The cell wall of C. albicans is the interface between the fungus and the innate immune system. The cell wall is composed of an outer layer enriched in mannosylated glycoproteins (mannan) and an inner layer enriched in β-(1,3)-glucan and chitin. Detection of C. albicans by Dectin-1, a C-type signaling lectin specific for β-(1,3)-glucan, is important for the innate immune system to recognize systemic fungal infections. Increased exposure of β-(1,3)-glucan to the immune system occurs when the mannan layer is altered or removed in a process called unmasking. Nanoscale changes to the cell wall during unmasking were explored in live cells with atomic force microscopy (AFM). Two mutants, the cho1Δ/Δ and kre5Δ/Δ mutants, were selected as representatives that exhibit modest and strong unmasking, respectively. Comparisons of the cho1Δ/Δ and kre5Δ/Δ mutants to the wild type reveal morphological changes in their cell walls that correlate with decreases in cell wall elasticity. In addition, AFM tips functionalized with Dectin-1 revealed that the forces of binding of Dectin-1 to all of the strains were similar, but the frequency of binding was highest for the kre5Δ/Δ mutant, decreased for the cho1Δ/Δ mutant, and rare for the wild type. These data show that nanoscale changes in surface topology are correlated with increased Dectin-1 adhesion and decreased cell wall elasticity. AFM, using tips functionalized with immunologically relevant molecules, can map epitopes of the cell wall and increase our understanding of pathogen recognition by the immune system.

KRE5 Suppression Induces Cell Wall Stress and Alternative ER Stress Response Required for Maintaining Cell Wall Integrity in Candida glabrata

The maintenance of cell wall integrity in fungi is required for normal cell growth, division, hyphae formation, and antifungal tolerance. We observed that endoplasmic reticulum stress regulated cell wall integrity in Candida glabrata, which possesses uniquely evolved mechanisms for unfolded protein response mechanisms. Tetracycline-mediated suppression of KRE5, which encodes a predicted UDP-glucose:glycoprotein glucosyltransferase localized in the endoplasmic reticulum, significantly increased cell wall chitin content and decreased cell wall β-1,6-glucan content. KRE5 repression induced endoplasmic reticulum stress-related gene expression and MAP kinase pathway activation, including Slt2p and Hog1p phosphorylation, through the cell wall integrity signaling pathway. Moreover, the calcineurin pathway negatively regulated cell wall integrity, but not the reduction of β-1,6-glucan content. These results indicate that KRE5 is required for maintaining both endoplasmic reticulum homeostasis and cell wall integrity, and that the calcineurin pathway acts as a regulator of chitin-glucan balance in the cell wall and as an alternative mediator of endoplasmic reticulum stress in C. glabrata.

Attenuation of PAMP-triggered immunity in maize requires down-regulation of the key β-1,6-glucan synthesis genes KRE5 and KRE6 in biotrophic hyphae of Colletotrichum graminicola

In plants, pathogen defense is initiated by recognition of pathogen-associated molecular patterns (PAMPs) via plasma membrane-localized pattern-recognition receptors (PRRs). Fungal structural cell wall polymers such as branched β-glucans are essential for infection structure rigidity and pathogenicity, but at the same time represent PAMPs. Kre5 and Kre6 are key enzymes in β-1,6-glucan synthesis and formation of branch points of the β-glucan network. In spite of the importance of branched β-glucan for hyphal rigidity and plant-fungus interactions, neither the role of KRE5 and KRE6 in pathogenesis nor mechanisms allowing circumventing branched β-glucan-triggered immune responses are known. We functionally characterized KRE5 and KRE6 of the ascomycete Colletotrichum graminicola, a hemibiotroph that infects maize (Zea mays). After appressorial plant invasion, this fungus sequentially differentiates biotrophic and highly destructive necrotrophic hyphae. RNAi-mediated reduction of KRE5 and KRE6 transcript abundance caused appressoria to burst and swelling of necrotrophic hyphae, indicating that β-1,6-glucosidic bonds are essential in these cells. Live cell imaging employing KRE5:mCherry and KRE6:mCherry knock-in strains and probing of infection structures with a YFP-conjugated β-1,6-glucan-binding protein showed expression of these genes and exposure of β-1,6-glucan in conidia, appressoria and necrotrophic, but not in biotrophic hyphae. Overexpression of KRE5 and KRE6 in biotrophic hyphae led to activation of broad-spectrum plant defense responses, including papilla and H2 O2 formation, as well as transcriptional activation of several defense-related genes. Collectively, our results strongly suggest that down-regulation of synthesis and avoidance of exposure of branched β-1,3-β-1,6-glucan in biotrophic hyphae is required for attenuation of plant immune responses.

Comparative Analysis of Protein Glycosylation Pathways in Humans and the Fungal Pathogen Candida albicans

Protein glycosylation pathways are present in all kingdoms of life and are metabolic pathways found in all the life kingdoms. Despite sharing commonalities in their synthesis, glycans attached to glycoproteins have species-specific structures generated by the presence of different sets of enzymes and acceptor substrates in each organism. In this review, we present a comparative analysis of the main glycosylation pathways shared by humans and the fungal pathogen Candida albicans: N-linked glycosylation, O-linked mannosylation and glycosylphosphatidylinositol-anchorage. The knowledge of similarities and divergences between these metabolic pathways could help find new pharmacological targets for C. albicans infection.

Participation of Candida albicans transcription factor RLM1 in cell wall biogenesis and virulence

Candida albicans cell wall is important for growth and interaction with the environment. RLM1 is one of the putative transcription factors involved in the cell wall integrity pathway, which plays an important role in the maintenance of the cell wall integrity. In this work we investigated the involvement of RLM1 in the cell wall biogenesis and in virulence. Newly constructed C. albicans Δ/Δrlm1 mutants showed typical cell wall weakening phenotypes, such as hypersensitivity to Congo Red, Calcofluor White, and caspofungin (phenotype reverted in the presence of sorbitol), confirming the involvement of RLM1 in the cell wall integrity. Additionally, the cell wall of C. albicans Δ/Δrlm1 showed a significant increase in chitin (213%) and reduction in mannans (60%), in comparison with the wild-type, results that are consistent with cell wall remodelling. Microarray analysis in the absence of any stress showed that deletion of RLM1 in C. albicans significantly down-regulated genes involved in carbohydrate catabolism such as DAK2, GLK4, NHT1 and TPS1, up-regulated genes involved in the utilization of alternative carbon sources, like AGP2, SOU1, SAP6, CIT1 or GAL4, and genes involved in cell adhesion like ECE1, ALS1, ALS3, HWP1 or RBT1. In agreement with the microarray results adhesion assays showed an increased amount of adhering cells and total biomass in the mutant strain, in comparison with the wild-type. C. albicans mutant Δ/Δrlm1 strain was also found to be less virulent than the wild-type and complemented strains in the murine model of disseminated candidiasis. Overall, we showed that in the absence of RLM1 the modifications in the cell wall composition alter yeast interaction with the environment, with consequences in adhesion ability and virulence. The gene expression findings suggest that this gene participates in the cell wall biogenesis, with the mutant rearranging its metabolic pathways to allow the use of alternative carbon sources.

The cellular roles of Ccr4-NOT in model and pathogenic fungi-implications for fungal virulence

The fungal Ccr4-NOT complex has been implicated in orchestrating gene expression networks that impact on pathways key for virulence in pathogenic species. The activity of Ccr4-NOT regulates cell wall integrity, antifungal drug susceptibility, adaptation to host temperature, and the developmental switches that enable the formation of pathogenic structures, such as filamentous hyphae. Moreover, Ccr4-NOT impacts on DNA repair pathways and genome stability, opening the possibility that this gene regulator could control adaptive responses in pathogens that are driven by chromosomal alterations. Here we provide a synthesis of the cellular roles of the fungal Ccr4-NOT, focusing on pathways important for virulence toward animals. Our review is based on studies in models yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, and two species that cause serious human infections, Candida albicans and Cryptococcus neoformans. We hypothesize that the activity of Ccr4-NOT could be targeted for future antifungal drug discovery, a proposition supported by the fact that inactivation of the genes encoding subunits of Ccr4-NOT in C. albicans and C. neoformans reduces virulence in the mouse infection model. We performed bioinformatics analysis to identify similarities and differences between Ccr4-NOT subunits in fungi and animals, and discuss this knowledge in the context of future antifungal strategies.

Global transcriptomic profiling of aspen trees under elevated [CO2] to identify potential molecular mechanisms responsible for enhanced radial growth

Aspen (Populus tremuloides) trees growing under elevated [CO(2)] at a free-air CO(2) enrichment (FACE) site produced significantly more biomass than control trees. We investigated the molecular mechanisms underlying the observed increase in biomass by producing transcriptomic profiles of the vascular cambium zone (VCZ) and leaves, and then performed a comparative study to identify significantly changed genes and pathways after 12 years exposure to elevated [CO(2)]. In leaves, elevated [CO(2)] enhanced expression of genes related to Calvin cycle activity and linked pathways. In the VCZ, the pathways involved in cell growth, cell division, hormone metabolism, and secondary cell wall formation were altered while auxin conjugation, ABA synthesis, and cytokinin glucosylation and degradation were inhibited. Similarly, the genes involved in hemicellulose and pectin biosynthesis were enhanced, but some genes that catalyze important steps in lignin biosynthesis pathway were inhibited. Evidence from systemic analysis supported the functioning of multiple molecular mechanisms that underpin the enhanced radial growth in response to elevated [CO(2)].

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Action of multiple endoplasmic reticulum chaperon-like proteins is required for proper folding and polarized localization of Kre6 protein essential in yeast cell wall β-1,6-glucan synthesis

Saccharomyces cerevisiae Kre6 is a type II membrane protein essential for cell wall β-1,6-glucan synthesis. Recently we reported that the majority of Kre6 is in the endoplasmic reticulum (ER), but a significant portion of Kre6 is found in the plasma membrane of buds, and this polarized appearance of Kre6 is required for β-1,6-glucan synthesis. An essential membrane protein, Keg1, and ER chaperon Rot1 bind to Kre6. In this study we found that in mutant keg1-1 cells, accumulation of Kre6 at the buds is diminished, binding of Kre6 to Keg1 is decreased, and Kre6 becomes susceptible to ER-associated degradation (ERAD), which suggests Keg1 participates in folding and transport of Kre6. All mutants of the calnexin cycle member homologues (cwh41, rot2, kre5, and cne1) showed defects in β-1,6-glucan synthesis, although the calnexin chaperon system is considered not functional in yeast. We found synthetic defects between them and keg1-1, and Cne1 co-immunoprecipitated with Keg1 and Kre6. A stronger binding of Cne1 to Kre6 was detected when two glucosidases (Cwh41 and Rot2) that remove glucose on N-glycan were functional. Skn1, a Kre6 homologue, was not detected by immunofluorescence in the wild type yeast, but in kre6Δ cells it became detectable and behaved like Kre6. In conclusion, the action of multiple ER chaperon-like proteins is required for proper folding and localization of Kre6 and probably Skn1 to function in β-1,6-glucan synthesis.

New β-glucan inhibitors as antifungal drugs

INTRODUCTION

New classes of synthetic and semi-synthetic β-glucan inhibitors have recently emerged, providing analogs that, in some cases, have been proven to have a high degree of activity against fungi, offering the prospect of alternatives to the commercially available lipopeptide/echinocandin agents caspofungin, micafungin and anidulafungin.

AREA COVERED

This review covers applications disclosing compound classes that include synthetic pyridazinone analogs, bicyclic heteroaryl ring compounds, aniline derivates, and semi-synthetic echinocandin and enfumafungin derivatives. MK-3118 is an analog of the natural product enfumafungin that, in particular, shows promise as it has a spectrum of activity comparable with caspofungin but has the advantageous property of oral bioavailability.

EXPERT OPINION

The diversity of chemical classes in the present review, which have demonstrable activity against β-glucan and the prospect of oral bioavailability, offers hope that safe and effective antifungal drugs will emerge and be commercialized. Of particular note, the Merck compound MK-3118, with solid evidence of efficacy based on preclinical data, has moved into clinical trials.

The MP65 gene is required for cell wall integrity, adherence to epithelial cells and biofilm formation in Candida albicans

Background

The MP65 gene of Candida albicans (orf19.1779) encodes a putative β-glucanase mannoprotein of 65 kDa, which plays a main role in a host-fungus relationship, morphogenesis and pathogenicity. In this study, we performed an extensive analysis of a mp65Δ mutant to assess the role of this protein in cell wall integrity, adherence to epithelial cells and biofilm formation.

Results

The mp65Δ mutant showed a high sensitivity to a range of cell wall-perturbing and degrading agents, especially Congo red, which induced morphological changes such as swelling, clumping and formation of hyphae. The mp65Δ mutant showed an activation of two MAPKs (Mkc1p and Cek1p), a high level of expression of two stress-related genes (DDR48 and SOD5), and a modulated expression of β-glucan epitopes, but no gross changes in cell wall polysaccharide composition. Interestingly, the mp65Δ mutant displayed a marked reduction in adhesion to BEC and Caco-2 cells and severe defects in biofilm formation when compared to the wild type. All of the mentioned properties were totally or partially recovered in a revertant strain, demonstrating the specificity of gene deletion.

Conclusions

We demonstrate that the MP65 gene of Candida albicans plays a significant role in maintaining cell wall integrity, as well as in adherence to epithelia and biofilm formation, which are major virulence attributes of this fungus.

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.

KRE genes are required for beta-1,6-glucan synthesis, maintenance of capsule architecture and cell wall protein anchoring in Cryptococcus neoformans

The polysaccharide beta-1,6-glucan is a major component of the cell wall of Cryptococcus neoformans, but its function has not been investigated in this fungal pathogen. We have identified and characterized seven genes, belonging to the KRE family, which are putatively involved in beta-1,6-glucan synthesis. The H99 deletion mutants kre5Delta and kre6Deltaskn1Delta contained less cell wall beta-1,6-glucan, grew slowly with an aberrant morphology, were highly sensitive to environmental and chemical stress and were avirulent in a mouse inhalation model of infection. These two mutants displayed alterations in cell wall chitosan and the exopolysaccharide capsule, a primary cryptococcal virulence determinant. The cell wall content of the GPI-anchored phospholipase B1 (Plb1) enzyme, which is required for cryptococcal cell wall integrity and virulence, was reduced in kre5Delta and kre6Deltaskn1Delta. Our results indicate that KRE5, KRE6 and SKN1 are involved in beta-1,6-glucan synthesis, maintenance of cell wall integrity and retention of mannoproteins and known cryptococcal virulence factors in the cell wall of C. neoformans. This study sets the stage for future investigations into the function of this abundant cell wall polymer.

Gibberellins regulate lateral root formation in Populus through interactions with auxin and other hormones

The role of gibberellins (GAs) in regulation of lateral root development is poorly understood. We show that GA-deficient (35S:PcGA2ox1) and GA-insensitive (35S:rgl1) transgenic Populus exhibited increased lateral root proliferation and elongation under in vitro and greenhouse conditions, and these effects were reversed by exogenous GA treatment. In addition, RNA interference suppression of two poplar GA 2-oxidases predominantly expressed in roots also decreased lateral root formation. GAs negatively affected lateral root formation by inhibiting lateral root primordium initiation. A whole-genome microarray analysis of root development in GA-modified transgenic plants revealed 2069 genes with significantly altered expression. The expression of 1178 genes, including genes that promote cell proliferation, growth, and cell wall loosening, corresponded to the phenotypic severity of the root traits when transgenic events with differential phenotypic expression were compared. The array data and direct hormone measurements suggested crosstalk of GA signaling with other hormone pathways, including auxin and abscisic acid. Transgenic modification of a differentially expressed gene encoding an auxin efflux carrier suggests that GA modulation of lateral root development is at least partly imparted by polar auxin transport modification. These results suggest a mechanism for GA-regulated modulation of lateral root proliferation associated with regulation of plant allometry during the stress response.

Gibberellins regulate lateral root formation in Populus through interactions with auxin and other hormones

The role of gibberellins (GAs) in regulation of lateral root development is poorly understood. We show that GA-deficient (35S:PcGA2ox1) and GA-insensitive (35S:rgl1) transgenic Populus exhibited increased lateral root proliferation and elongation under in vitro and greenhouse conditions, and these effects were reversed by exogenous GA treatment. In addition, RNA interference suppression of two poplar GA 2-oxidases predominantly expressed in roots also decreased lateral root formation. GAs negatively affected lateral root formation by inhibiting lateral root primordium initiation. A whole-genome microarray analysis of root development in GA-modified transgenic plants revealed 2069 genes with significantly altered expression. The expression of 1178 genes, including genes that promote cell proliferation, growth, and cell wall loosening, corresponded to the phenotypic severity of the root traits when transgenic events with differential phenotypic expression were compared. The array data and direct hormone measurements suggested crosstalk of GA signaling with other hormone pathways, including auxin and abscisic acid. Transgenic modification of a differentially expressed gene encoding an auxin efflux carrier suggests that GA modulation of lateral root development is at least partly imparted by polar auxin transport modification. These results suggest a mechanism for GA-regulated modulation of lateral root proliferation associated with regulation of plant allometry during the stress response.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

Effect of beta-1,6-glucan inhibitors on the invasion process of Candida albicans: potential mechanism of their in vivo efficacy

Beta-1,6-glucan is a fungus-specific cell wall component that is essential for the retention of many cell wall proteins. We recently reported the discovery of a small molecule inhibitor of beta-1,6-glucan biosynthesis in yeasts. In the course of our study of its derivatives, we found a unique feature in their antifungal profile. D21-6076, one of these compounds, exhibited potent in vitro and in vivo antifungal activities against Candida glabrata. Interestingly, although it only weakly reduced the growth of Candida albicans in conventional media, it significantly prolonged the survival of mice infected by the pathogen. Biochemical evaluation of D21-6076 indicated that it inhibited beta-1,6-glucan synthesis of C. albicans, leading the cell wall proteins, which play a critical role in its virulence, to be released from the cell. Correspondingly, adhesion of C. albicans cells to mammalian cells and their hyphal elongation were strongly reduced by the drug treatment. The results of the experiment using an in vitro model of vaginal candidiasis showed that D21-6076 strongly inhibited the invasion process of C. albicans without a significant reduction in its growth in the medium. These evidences suggested that D21-6076 probably exhibited in vivo efficacy against C. albicans by inhibiting its invasion process.

Cell wall beta-(1,6)-glucan of Saccharomyces cerevisiae: structural characterization and in situ synthesis

Despite its essential role in the yeast cell wall, the exact composition of the beta-(1,6)-glucan component is not well characterized. While solubilizing the cell wall alkali-insoluble fraction from a wild type strain of Saccharomyces cerevisiae using a recombinant beta-(1,3)-glucanase followed by chromatographic characterization of the digest on an anion exchange column, we observed a soluble polymer that eluted at the end of the solvent gradient run. Further characterization indicated this soluble polymer to have a molecular mass of approximately 38 kDa and could be hydrolyzed only by beta-(1,6)-glucanase. Gas chromatography mass spectrometry and NMR ((1)H and (13)C) analyses confirmed it to be a beta-(1,6)-glucan polymer with, on average, branching at every fifth residue with one or two beta-(1,3)-linked glucose units in the side chain. This polymer peak was significantly reduced in the corresponding digests from mutants of the kre genes (kre9 and kre5) that are known to play a crucial role in the beta-(1,6)-glucan biosynthesis. In the current study, we have developed a biochemical assay wherein incubation of UDP-[(14)C]glucose with permeabilized S. cerevisiae yeasts resulted in the synthesis of a polymer chemically identical to the branched beta-(1,6)-glucan isolated from the cell wall. Using this assay, parameters essential for beta-(1,6)-glucan synthetic activity were defined.

Discovery of a small-molecule inhibitor of {beta}-1,6-glucan synthesis

It is possible that antifungal drugs with novel modes of action will provide favorable options to treat fungal infections. In the course of our screening for antifungal compounds acting on the cell wall, a pyridobenzimidazole derivative with unique activities, named D75-4590, was discovered. During treatment of Saccharomyces cerevisiae with D75-4590, (i) incorporation of [(14)C]glucose into the beta-1,6-glucan component was selectively reduced, (ii) proteins released from the cell had lost the beta-1,6-glucan moiety, and (iii) cells tended to clump, resulting in impaired cell growth. Genetic analysis of a D75-4590-resistant mutant of S. cerevisiae indicated that its primary target was Kre6p, which is considered to be one of the beta-1,6-glucan synthases. These results strongly suggest that D75-4590 is a specific inhibitor of beta-1,6-glucan synthesis. D75-4590 showed potent activities against various Candida species. It inhibited hyphal elongation of C. albicans as well. KRE6 is conserved in various fungi, but no homologue has been found in mammalian cells. These lines of evidence indicate that D75-4590 is a promising lead compound for novel antifungal drugs. To our knowledge, this is the first report of a beta-1,6-glucan inhibitor.

Enzybiotics: A Look to the Future, Recalling the Past

The discovery and development of antibiotics was one of the greatest successes of Medicine in the 20th century and allowed the control of many diseases caused by microorganisms. Nevertheless, it is necessary to search constantly for new therapeutic tools in the continuing fight against disease-causing microorganisms and this probably leads us to today's concept of enzybiotics. Although microorganism-degrading enzymes have been known since the beginning of the last century, their use was soon forgotten because of the widespread use of antibiotics. The term enzybiotic is a hybrid word from "enzyme" and "antibiotic" and refers to phages: that is, viruses that attack and lyse bacteria and that can potentially help us to fight bacterial diseases. If the concept of enzybiotic is extended to antifungal enzymes, an enormous potential in the struggle against microorganism-due diseases may become available in the foreseeable future.

Morphology transition genes in the dimorphic fission yeast Schizosaccharomyces japonicus

The ability of the saprophytic fungus Schizosaccharomyces japonicus to alternate between unicellular yeast form and multicellular true mycelium makes this organism an attractive non-pathogenic model for the investigation of di- and polymorphism critical for pathogenicity in many pathogenic fungi. In a previous work we described three mutations that made the cells unable to form hyphae. Here we report on the isolation of additional mycelium-minus mutants and show that the mutations represent seven genes. Apart from the inhibition of the yeast-to-mycelium transition, the mutations also cause drastic changes in the yeast phase. Cells of myc3-34 and myc4-35 grow isotropically with apolar distribution of actin and randomised localisation of division planes. The mutants myc1-4, myc1-10, myc1-36, myc5-39 and myc6-43 form sep-like, branching microhyphae containing bipolarly growing cells. All mutants are defective in the polarisation of vacuole distribution, and myc3-34, myc4-35 and myc7-56 are also defective in vacuole fusion. The diversity of the mutant phenotypes indicates that several cellular processes must take place simultaneously for the transition from the yeast phase into the mycelial phase.

Putative role of beta-1,3 glucans in Candida albicans biofilm resistance

Biofilms are microbial communities, embedded in a polymeric matrix, growing attached to a surface. Nearly all device-associated infections involve growth in the biofilm life style. Biofilm communities have characteristic architecture and distinct phenotypic properties. The most clinically important phenotype involves extraordinary resistance to antimicrobial therapy, making biofilm infections very difficulty to cure without device removal. The current studies examine drug resistance in Candida albicans biofilms. Similar to previous reports, we observed marked fluconazole and amphotericin B resistance in a C. albicans biofilm both in vitro and in vivo. We identified biofilm-associated cell wall architectural changes and increased beta-1,3 glucan content in C. albicans cell walls from a biofilm compared to planktonic organisms. Elevated beta-1,3 glucan levels were also found in the surrounding biofilm milieu and as part of the matrix both from in vitro and in vivo biofilm models. We thus investigated the possible contribution of beta-glucans to antimicrobial resistance in Candida albicans biofilms. Initial studies examined the ability of cell wall and cell supernatant from biofilm and planktonic C. albicans to bind fluconazole. The cell walls from both environmental conditions bound fluconazole; however, four- to fivefold more compound was bound to the biofilm cell walls. Culture supernatant from the biofilm, but not planktonic cells, bound a measurable amount of this antifungal agent. We next investigated the effect of enzymatic modification of beta-1,3 glucans on biofilm cell viability and the susceptibility of biofilm cells to fluconazole and amphotericin B. We observed a dose-dependent killing of in vitro biofilm cells in the presence of three different beta-glucanase preparations. These same concentrations had no impact on planktonic cell viability. beta-1,3 Glucanase markedly enhanced the activity of both fluconazole and amphotericin B. These observations were corroborated with an in vivo biofilm model. Exogenous biofilm matrix and commercial beta-1,3 glucan reduced the activity of fluconazole against planktonic C. albicans in vitro. In sum, the current investigation identified glucan changes associated with C. albicans biofilm cells, demonstrated preferential binding of these biofilm cell components to antifungals, and showed a positive impact of the modification of biofilm beta-1,3 glucans on drug susceptibility. These results provide indirect evidence suggesting a role for glucans in biofilm resistance and present a strong rationale for further molecular dissection of this resistance mechanism to identify new drug targets to treat biofilm infections.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.