Candida albicans CHT3 encodes the functional homolog of the Cts1 chitinase of Saccharomyces cerevisiae

Candida albicans chitinase 3 with potential as a vaccine antigen: production, purification, and characterisation

Chitinases are widely studied enzymes that have already found widespread application. Their continued development and valorisation will be driven by the identification of new and improved variants and/or novel applications bringing benefits to industry and society. We previously identified a novel application for chitinases wherein the Candida albicans cell wall surface chitinase 3 (Cht3) was shown to have potential in vaccine applications as a subunit antigen against fungal infections. In the present study, this enzyme was investigated further, developing production and purification protocols, enriching our understanding of its properties, and advancing its application potential. Cht3 was heterologously expressed in Pichia pastoris and a 4-step purification protocol developed and optimised: this involves activated carbon treatment, hydrophobic interaction chromatography, ammonium sulphate precipitation, and gel filtration chromatography. The recombinant enzyme was shown to be mainly O-glycosylated and to retain the epitopes of the native protein. Functional studies showed it to be highly specific, displaying activity on chitin, chitosan, and chito-oligosaccharides larger than chitotriose only. Furthermore, it was shown to be a stable enzyme, exhibiting activity, and stability over broad pH and temperature ranges. This study represents an important step forward in our understanding of Cht3 and contributes to its development for application.

Cloning and biochemical characterization of a recombinant chitinase encoded by the CHT4 gene from Candida albicans

As one of the major components in the fungal cell wall, chitin is a polymer of β-1,4-linked N-acetylglucosamine. Chitinases are hydrolytic enzymes that break down glycosidic bonds in the chitin. The human fungal pathogen Candida albicans has three chitinase-encoding genes, CaCHT1, CaCHT2 and CaCHT3. The CaCHT4 gene encodes a protein with the glycoside hydrolase family GH18 domain, Glyco_18, which suggests that CaCht4 might be a chitinase. In the present study, we have cloned, expressed and purified the N-terminally His6-tagged CaCht4 protein from bacterial cells. Further biochemical characterization has shown that this recombinant CaCht4 protein shows both exochitinase (chitobiosidase) and endochitinase activities, but has no N-acetylglucosaminase activity. The optimal temperature for the exochitinase activity of CaCht4 is 55 °C. Taken together, these data support that the CaCHT4 gene encodes a chitinase. Our finding provides a basis for us to understand the biological functions of the CaCHT4 gene in C. albicans.

Traversing the Cell Wall: The Chitinolytic Activity of Histoplasma capsulatum Extracellular Vesicles Facilitates Their Release

Histoplasma capsulatum is the causative agent of histoplasmosis. Treating this fungal infection conventionally has significant limitations, prompting the search for alternative therapies. In this context, fungal extracellular vesicles (EVs) hold relevant potential as both therapeutic agents and targets for the treatment of fungal infections. To explore this further, we conducted a study using pharmacological inhibitors of chitinase (methylxanthines) to investigate their potential to reduce EV release and its subsequent impact on fungal virulence in an in vivo invertebrate model. Our findings revealed that a subinhibitory concentration of the methylxanthine, caffeine, effectively reduces EV release, leading to a modulation of H. capsulatum virulence. To the best of our knowledge, this is the first reported instance of a pharmacological inhibitor that reduces fungal EV release without any observed fungicidal effects.

Design of a lipid nano-delivery system containing recombinant Candida albicans chitinase 3 as a potential vaccine against fungal infections

Opportunistic fungi cause lethal systemic infections and impose high medical costs to health systems. The World Health Organization has recognized the importance of fungal infections, including them in its global priority list guiding research, development, and discovery of new therapeutic approaches. Fungal vaccine development has been proposed as one of the treatment and prevention strategies in the last decade. In this study, we present the design of a lipid antigen delivery system based on Dioctadecyldimethylammonium bromide: Monoolein (DODAB: MO) containing recombinant Candida albicans Chitinase 3 (Cht3) for modulation the immune response against fungal infections. Several DODAB:MO liposomes containing Cht3 were prepared and those prepared by the incubation method and containing 5 µg/mL Cht3 were selected due to their favorable size, ζ-potential and stability, suited for antigen delivery applications. The encapsulation of Cht3 in these liposomes resulted in a significant increase in cellular uptake compared to empty liposomes, demonstrating their efficacy in delivering the antigen. Moreover, the liposomes proved to be safe for use in immunization procedures. Subcutaneous administration of Cht3 liposomes elicited a Th1/Th17 immune response profile, associated with the production of high levels of antibodies against Cht3. These antibodies recognized both the native and the recombinant forms of the protein, opsonizing mother-yeast at the cell scars, which has the potential to disrupt cell separation and hinder yeast growth. The findings suggest that the designed lipid antigen delivery system shows promise as a potential candidate for enhancing immune responses against fungal infections, offering a valuable strategy for future fungal vaccine development.

Cloning, expression, purification and biochemical characterization of the recombinant chitinase enzyme encoded by CTS2 in the budding yeast

Chitin is a polymer of β-1,4-linked N-acetylglucosamine (GlcNAc) and plays a central role in the assembly of the fungal cell wall. Chitinases are hydrolytic enzymes that break down glycosidic bonds in the chitin. Chitinases are classified into three categories, endochitinases, exochitinases and N-acetylglucosaminases, according to the manner in which the enzyme cleaves the chitin polymer. Saccharomyces cerevisiae has two chitinase-encoding genes, CTS1 and CTS2. However, whether Cts2p shows a chitinase activity remains unknown. In this study, we have cloned, expressed and purified the recombinant Cts2p protein from bacterial cells. We have demonstrated that Cts2p has a higher chitobiosidase (exochitinase) activity than endochitinase activity, but no N-acetylglucosaminase activity. The optimal temperature for the chitobiosidase activity of Cts2p is 37 °C.

The Landscape of Gene Expression during Hyperfilamentous Biofilm Development in Oral Candida albicans Isolated from a Lung Cancer Patient

The filamentation ability of Candida albicans represents one of the main virulence factors allowing for host tissue penetration and biofilm formation. The aim of this paper was to study the genetic background of the hyperfilamentous biofilm development in vitro in C. albicans isolated from the oral cavity of a lung cancer patient. Analyzed C. albicans isolates (CA1, CA2, CA3) were chosen based on their different structures of mature biofilm. The CA3 isolate possessing hyperfilamentation properties and forming high biofilm was compared with CA1 and CA2 isolates exhibiting low or average biofilm-forming ability, respectively. The detailed biofilm organization was studied with the use of confocal scanning laser microscopy. The whole transcriptome analysis was conducted during three stages of biofilm development (24 h, 48 h, 72 h). In contrast to CA1 and/or CA2 isolate, the CA3 isolate was characterized by a significant upregulation of genes encoding for cell wall proteins (HWP1, PGA13, PGA44, ALS3) and candidalysin (ECE1), as well as being involved in iron metabolism (FRE1, ALS3), sulfur metabolism (HAL21), the degradation of aromatic compounds (HQD2), and membrane transport (DIP5, PHO89, TNA1). In contrast, some genes (SCW11, FGR41, RBE1) in the CA3 were downregulated. We also observed the overexpression of a few genes over time-mainly FRE1, ATX1, CSA2 involved in iron metabolism. This is the first insight into the potential function of multiple genes in the hyperfilamentous biofilm formation in C. albicans, primarily isolated from host tissue, which may have an important clinical impact on cancer patients. Moreover, the presented data can lay the foundation for further research on novel pathogen-specific targets for antifungal drugs.

MaCts1, an Endochitinase, Is Involved in Conidial Germination, Conidial Yield, Stress Tolerances and Microcycle Conidiation in Metarhizium acridum

Entomopathogenic fungi are promising biocontrol agents of insect-mediated crop damage. Microcycle conidiation has shown great potential in enhancing the conidial yield and quality of entomopathogenic fungi. Homologs of Cts1, an endochitinase of Saccharomyces cerevisiae, participate in cell separation in several fungal spp. and may contribute to the morphological differences that occur during the shift to microcycle conidiation. However, the precise functions of Cts1 in entomopathogenic fungi remain unclear. Herein, the endochitinase gene, MaCts1, was characterized in the model entomopathogen, Metarhizium acridum. A loss of function line for MaCts1 led to a delay of 1 h in the median germination time, a 28% reduction in conidial yield and significant defects in fungal resistances to UV-irradiation (18%) and heat-shock (15%), while fungal tolerances to cell wall stressors, oxidative and hyperosmotic stresses and virulence remained unchanged. The MaCts1-disruption strain displayed typical conidiation on the microcycle conidiation induction medium, SYA. In contrast, deletion of key genes in the morphogenesis-related NDR kinase network (MOR pathway)/regulation of Ace2 and morphogenesis (RAM pathway) did not affect the SYA-induction of microcycle conidiation. This indicates that MaCts1 makes contributions to the microcycle conidiation, which may not be dependent on the MOR/RAM pathway in M. acridum.

MaNsdD regulates conidiation negatively by inhibiting the AbaA expression required for normal conidiation in Metarhizium acridum

Conidiation necessary for filamentous fungal survival and dispersal, proceeds in two fashions, namely normal conidiation through conidiophores differentiated from hyphae, and microcycle conidiation through conidial budding. Normal conidiation has been well studied whereas mechanisms underlying microcycle conidiation are still largely unknown. Here, we report that a gene (MaNsdD) homologous to NsdD in Aspergillus nidulans serves as a suppressor of normal conidiation but a positive regulator of hyphal development in Metarhizium acridum. Disruption of MaNsdD (ΔMaNsdD) resulted in microcycle conidiation and significantly descended in conidial resistance to heat while improved to UV irradiation. Transcriptomic analysis revealed that many genes involved in conidiation, cell division and cell wall formation were differentially expressed in ΔMaNsdD, and likely associated with the conidiation process. We found that a gene (MaAbaA) homologous to the core asexual development regulator AbaA in A. nidulans, was negatively controlled by MaNsdD. Disruption of MaAbaA led to the abolition of the conidiation process of M. acridum. These findings unravel a novel regulatory mechanism of microcycle conidiation, and add a knowledge to the asexual conidiation pathway of filamentous fungi.

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.

Forward and reverse genetic dissection of morphogenesis identifies filament-competent Candida auris strains

Candida auris is an emerging healthcare-associated pathogen of global concern. Recent reports have identified C. auris isolates that grow in cellular aggregates or filaments, often without a clear genetic explanation. To investigate the regulation of C. auris morphogenesis, we applied an Agrobacterium-mediated transformation system to all four C. auris clades. We identified aggregating mutants associated with disruption of chitin regulation, while disruption of ELM1 produced a polarized, filamentous growth morphology. We developed a transiently expressed Cas9 and sgRNA system for C. auris that significantly increased targeted transformation efficiency across the four C. auris clades. Using this system, we confirmed the roles of C. auris morphogenesis regulators. Morphogenic mutants showed dysregulated chitinase expression, attenuated virulence, and altered antifungal susceptibility. Our findings provide insights into the genetic regulation of aggregating and filamentous morphogenesis in C. auris. Furthermore, the genetic tools described here will allow for efficient manipulation of the C. auris genome.

Some isolates of the emerging fungal pathogen Candida auris can form cellular aggregates or filaments. Here, Santana and O’Meara use Agrobacterium-mediated transformation and a CRISPR-Cas9 system to identify several genes that regulate C. auris morphogenesis.

Virulence Vs. Immunomodulation: Roles of the Paracoccin Chitinase and Carbohydrate-Binding Sites in Paracoccidioides brasiliensis Infection

Paracoccin (PCN) is a bifunctional protein primarily present in the cell wall of Paracoccidioides brasiliensis, a human pathogenic dimorphic fungus. PCN has one chitinase region and four potential lectin sites and acts as both a fungal virulence factor and an immunomodulator of the host response. The PCN activity on fungal virulence, mediated by the chitinase site, was discovered by infecting mice with yeast overexpressing PCN (PCN-ov). PCN-ov are characterized by increased chitin hydrolysis, a narrow cell wall, and augmented resistance to phagocytes' fungicidal activity. Compared to wild-type (wt) yeast, infection with PCN-ov yeast causes a more severe disease, which is attributed to the increased PCN chitinase activity. In turn, immunomodulation of the host response was demonstrated by injecting, subcutaneously, recombinant PCN in mice infected with wt-P. brasiliensis. Through its carbohydrate binding site, the injected recombinant PCN interacts with Toll-like receptor 2 (TLR2) and Toll-like receptor 4 (TLR4) N-glycans on macrophages, triggers M1 polarization, and stimulates protective Th1 immunity against the fungus. The PCN-treatment of wt yeast-infected mice results in mild paracoccidioidomycosis. Therefore, PCN paradoxically influences the course of murine paracoccidioidomycosis. The disease is severe when caused by yeast that overexpress endogenous PCN, which exerts a robust local chitinase activity, followed by architectural changes of the cell wall and release of low size chito-oligomers. However, the disease is mild when exogenous PCN is injected, which recognizes N-glycans on systemic macrophages resulting in immunomodulation.

Diversification of Fungal Chitinases and Their Functional Differentiation in Histoplasma capsulatum

Chitinases enzymatically hydrolyze chitin, a highly abundant and utilized polymer of N-acetyl-glucosamine. Fungi are a rich source of chitinases; however, the phylogenetic and functional diversity of fungal chitinases are not well understood. We surveyed fungal chitinases from 373 publicly available genomes, characterized domain architecture, and conducted phylogenetic analyses of the glycoside hydrolase (GH18) domain. This large-scale analysis does not support the previous division of fungal chitinases into three major clades (A, B, C) as chitinases previously assigned to the “C” clade are not resolved as distinct from the “A” clade. Fungal chitinase diversity was partly shaped by horizontal gene transfer, and at least one clade of bacterial origin occurs among chitinases previously assigned to the “B” clade. Furthermore, chitin-binding domains (including the LysM domain) do not define specific clades, but instead are found more broadly across clades of chitinases. To gain insight into biological function diversity, we characterized all eight chitinases (Cts) from the thermally dimorphic fungus, Histoplasma capsulatum: six A clade, one B clade, and one formerly classified C clade chitinases. Expression analyses showed variable induction of chitinase genes in the presence of chitin but preferential expression of CTS3 in the mycelial stage. Activity assays demonstrated that Cts1 (B-I), Cts2 (A-V), Cts3 (A-V), Cts4 (A-V) have endochitinase activities with varying degrees of chitobiosidase function. Cts6 (C-I) has activity consistent with N-acetyl-glucosaminidase exochitinase function and Cts8 (A-II) has chitobiase activity. These results suggest chitinase activity is variable even within subclades and that predictions of functionality require more sophisticated models.

Mechanistic Insights into the Anticandidal Action of Vanillin Reveal Disruption of Cell Surface Integrity and Mitochondrial Functioning

BACKGROUND

Considering the emergence of multidrug resistance (MDR) in prevalent human fungal pathogen, Candida albicans, there is a parallel spurt in the development of novel strategies aimed to disrupt MDR. Compounds from natural resources could be exploited as efficient antifungal drugs owing to their structural diversity, cost effectiveness and negligible side effects.

OBJECTIVE

The present study elucidates the antifungal mechanisms of Vanillin (Van), a natural food flavoring agent against Candida albicans.

METHODS

Antifungal activities were assessed by broth microdilution and spot assays. Membrane and cell wall perturbations were studied by PI uptake, electron microscopy, plasma membrane H+ extrusion activity and estimation of ergosterol and chitin contents. Mitochondrial functioning was studied by growth on non-fermentable carbon sources, rhodamine B labeling and using retrograde signaling mutants. Gene expressions were validated by semi-quantitative RT-PCR.

RESULTS

We observed that the antifungal activity of Van was not only limited to clinical isolates of C. albicans but also against non-albicans species of Candida. Mechanistic insights revealed the effect of Van on cell surface integrity as evident from hypersensitivity against membrane perturbing agent SDS, depleted ergosterol levels, transmission electron micrographs and diminished plasma membrane H+ extrusion activity. In addition, spot assays with cell wall perturbing agents, scanning electron micrographs, delayed sedimentation rate and lower chitin content further substantiate cell wall damage by Van. Furthermore, Van treated cells underwent mitochondrial dysfunctioning via impaired retrograde signaling leading to abrogated iron homeostasis and DNA damage. All the perturbed phenotypes were also validated by RT-PCR depicting differential regulation of genes (NPC2, KRE62, FTR2 and CSM3) in response to Van.

CONCLUSION

Together, our results suggested that Van is promising antifungal agent that may be advocated for further investigation in therapeutic strategies to treat Candida infections.

Recording of DNA-binding events reveals the importance of a repurposed Candida albicans regulatory network for gut commensalism

Candida albicans is a fungal component of the human gut microbiota and an opportunistic pathogen. C. albicans transcription factors (TFs), Wor1 and Efg1, are master regulators of an epigenetic switch required for fungal mating that also control colonization of the mammalian gut. We show that additional mating regulators, WOR2, WOR3, WOR4, AHR1, CZF1, and SSN6, also influence gut commensalism. Using Calling Card-seq to record Candida TF DNA-binding events in the host, we examine the role and relationships of these regulators during murine gut colonization. By comparing in-host transcriptomes of regulatory mutants with enhanced versus diminished commensal fitness, we also identify a set of candidate commensalism effectors. These include Cht2, a GPI-linked chitinase whose gene is bound by Wor1, Czf1, and Efg1 in vivo, that we show promotes commensalism. Thus, the network required for a C. albicans sexual switch is biochemically active in the host intestine and repurposed to direct commensalism.

Als3-mediated attachment of enolase on the surface of Candida albicans cells regulates their interactions with host proteins

The multifunctional protein enolase has repeatedly been identified on the surface of numerous cell types, including a variety of pathogenic microorganisms. In Candida albicans-one of the most common fungal pathogens in humans-a surface-exposed enolase form has been previously demonstrated to play an important role in candidal pathogenicity. In our current study, the presence of enolase at the fungal cell surface under different growth conditions was examined, and a higher abundance of enolase at the surface of C. albicans hyphal forms compared to yeast-like cells was found. Affinity chromatography and chemical cross-linking indicated a member of the agglutinin-like sequence protein family-Als3-as an important potential partner required for the surface display of enolase. Analysis of Saccharomyces cerevisiae cells overexpressing Als3 with site-specific deletions showed that the Ig-like N-terminal region of Als3 (aa 166-225; aa 218-285; aa 270-305; aa 277-286) and the central repeat domain (aa 434-830) are essential for the interaction of this adhesin with enolase. In addition, binding between enolase and Als3 influenced subsequent docking of host plasma proteins-high molecular mass kininogen and plasminogen-on the candidal cell surface, thus supporting the hypothesis that C. albicans can modulate plasma proteolytic cascades to affect homeostasis within the host and propagate inflammation during infection.

Evolutionary Overview of Molecular Interactions and Enzymatic Activities in the Yeast Cell Walls

Fungal cell walls are composed of a polysaccharide network that serves as a scaffold in which different glycoproteins are embedded. Investigation of fungal cell walls, besides simple identification and characterization of the main cell wall building blocks, covers the pathways and regulations of synthesis of each individual component of the wall and biochemical reactions by which they are cross-linked and remodeled in response to different growth phase and environmental signals. In this review, a survey of composition and organization of so far identified and characterized cell wall components of different yeast genera including Saccharomyces, Candida, Kluyveromyces, Yarrowia, and Schizosaccharomyces are presented with the focus on their cell wall proteomes.

Molecular Mechanisms Involved in the Multicellular Growth of Ustilaginomycetes

Multicellularity is defined as the developmental process by which unicellular organisms became pluricellular during the evolution of complex organisms on Earth. This process requires the convergence of genetic, ecological, and environmental factors. In fungi, mycelial and pseudomycelium growth, snowflake phenotype (where daughter cells remain attached to their stem cells after mitosis), and fruiting bodies have been described as models of multicellular structures. Ustilaginomycetes are Basidiomycota fungi, many of which are pathogens of economically important plant species. These fungi usually grow unicellularly as yeasts (sporidia), but also as simple multicellular forms, such as pseudomycelium, multicellular clusters, or mycelium during plant infection and under different environmental conditions: Nitrogen starvation, nutrient starvation, acid culture media, or with fatty acids as a carbon source. Even under specific conditions, Ustilago maydis can form basidiocarps or fruiting bodies that are complex multicellular structures. These fungi conserve an important set of genes and molecular mechanisms involved in their multicellular growth. In this review, we will discuss in-depth the signaling pathways, epigenetic regulation, required polyamines, cell wall synthesis/degradation, polarized cell growth, and other cellular-genetic processes involved in the different types of Ustilaginomycetes multicellular growth. Finally, considering their short life cycle, easy handling in the laboratory and great morphological plasticity, Ustilaginomycetes can be considered as model organisms for studying fungal multicellularity.

Improvement of cis,cis-Muconic Acid Production in Saccharomyces cerevisiae through Biosensor-Aided Genome Engineering

Muconic acid is a potential platform chemical for the production of nylon, polyurethanes, and terephthalic acid. It is also an attractive functional copolymer in plastics due to its two double bonds. At this time, no economically viable process for the production of muconic acid exists. To harness novel genetic targets for improved production of cis,cis-muconic acid (CCM) in the yeast Saccharomyces cerevisiae, we employed a CCM-biosensor coupled to GFP expression with a broad dynamic response to screen UV-mutagenesis libraries of CCM-producing yeast. Via fluorescence activated cell sorting we identified a clone Mut131 with a 49.7% higher CCM titer and 164% higher titer of biosynthetic intermediate-protocatechuic acid (PCA). Genome resequencing of the Mut131 and reverse engineering identified seven causal missense mutations of the native genes (PWP2, EST2, ATG1, DIT1, CDC15, CTS2, and MNE1) and a duplication of two CCM biosynthetic genes, encoding dehydroshikimate dehydratase and catechol 1,2-dioxygenase, which were not recognized as flux controlling before. The Mut131 strain was further rationally engineered by overexpression of the genes encoding for PCA decarboxylase and AROM protein without shikimate dehydrogenase domain (Aro1p^ΔE), and by restoring URA3 prototrophy. The resulting engineered strain produced 20.8 g/L CCM in controlled fed-batch fermentation, with a yield of 66.2 mg/g glucose and a productivity of 139 mg/L/h, representing the highest reported performance metrics in a yeast for de novo CCM production to date and the highest production of an aromatic compound in yeast. The study illustrates the benefit of biosensor-based selection and brings closer the prospect of biobased muconic acid.

The Glucan-Remodeling Enzyme Phr1p and the Chitin Synthase Chs1p Cooperate to Maintain Proper Nuclear Segregation and Cell Integrity in Candida albicans

GH72 family of β-(1,3)-glucanosyltransferases is unique to fungi and is required for cell wall biogenesis, morphogenesis, virulence, and in some species is essential for life. Candida albicans PHR1 and PHR2 are pH-regulated genes that encode GH72 enzymes highly similar to Gas1p of Saccharomyces cerevisiae. PHR1 is expressed at pH ≥ 5.5 while PHR2 is transcribed at pH ≤ 5.5. Both are essential for C. albicans morphogenesis and virulence. During growth at neutral-alkaline pH, Phr1p-GFP preferentially localizes to sites of active cell wall formation as the incipient bud, the mother-daughter neck, the bud periphery, and concentrates in the septum at cytokinesis. We further investigated this latter localization. In chs3Δ cells, lacking the chitin of the chitin ring and lateral cell wall, Phr1p-GFP still concentrated along the thin line of the primary septum formed by chitin deposited by chitin synthase I (whose catalytic subunit is Chs1p) suggesting that it plays a role during formation of the secondary septa. RO-09-3143, a highly specific inhibitor of Chs1p activity, inhibits septum formation and blocks cell division. However, alternative septa are produced and are crucial for cell survival. Phr1p-GFP is excluded from such aberrant septa. Finally, we determined the effects of RO-09-3143 in cells lacking Phr1p. PHR1 null mutant was more susceptible to the drug than the wild type. The phr1Δ cells were larger, devoid of septa, and underwent endomitosis and cell death. Phr1p and Chs1p cooperate in maintaining cell integrity and in coupling morphogenesis with nuclear division in C. albicans.

Glucanases and Chitinases

In many yeast and fungi, β-(1,3)-glucan and chitin are essential components of the cell wall, an important structure that surrounds cells and which is responsible for their mechanical protection and necessary for maintaining the cellular shape. In addition, the cell wall is a dynamic structure that needs to be remodelled along with the different phases of the fungal life cycle or in response to extracellular stimuli. Since β-(1,3)-glucan and chitin perform a central structural role in the assembly of the cell wall, it has been postulated that β-(1,3)-glucanases and chitinases should perform an important function in cell wall softening and remodelling. This review focusses on fungal glucanases and chitinases and their role during fungal morphogenesis.

Identification of genome-wide binding sites of heat shock factor 1, Hsf1, under basal conditions in the human pathogenic yeast, Candida albicans

The master regulator of thermal stress response, Hsf1, is also an essential determinant for viability and virulence in Candida albicans. Our recent studies highlighted that apart from ubiquitous roles of Hsf1 at higher temperatures, it also has myriad non-heat shock responsive roles essential under iron deprivation and drug defense. Here, we further explored its implications in the normal cellular functioning, by profiling its genome-wide occupancy using chromatin immuno-precipitation coupled to high-density tiling arrays under basal and iron deprived conditions. Hsf1 recruitment profiles revealed that it binds to promoters of 660 genes of varied functions, under both the conditions, however, elicited variability in intensity of binding. For instance, Hsf1 binding was observed on several genes of oxidative and osmotic stress response, cell wall integrity, iron homeostasis, mitochondrial, hyphal and multidrug transporters. Additionally, the present study divulged a novel motif under basal conditions comprising, -GTGn₃GTGn₃GTG- where, Hsf1 displays strong occupancy at significant number of sites on several promoters distinct from the heat induced motif. Hence, by binding to and regulating major chaperones, stress responsive genes and drug resistance regulators, Hsf1 is imperative in regulating various cellular machineries. The current study provides a framework for understanding novel aspects of how Hsf1 coordinates diverse cellular functions.

Ceratocystis cacaofunesta genome analysis reveals a large expansion of extracellular phosphatidylinositol-specific phospholipase-C genes (PI-PLC)

BACKGROUND

The Ceratocystis genus harbors a large number of phytopathogenic fungi that cause xylem parenchyma degradation and vascular destruction on a broad range of economically important plants. Ceratocystis cacaofunesta is a necrotrophic fungus responsible for lethal wilt disease in cacao. The aim of this work is to analyze the genome of C. cacaofunesta through a comparative approach with genomes of other Sordariomycetes in order to better understand the molecular basis of pathogenicity in the Ceratocystis genus.

RESULTS

We present an analysis of the C. cacaofunesta genome focusing on secreted proteins that might constitute pathogenicity factors. Comparative genome analyses among five Ceratocystidaceae species and 23 other Sordariomycetes fungi showed a strong reduction in gene content of the Ceratocystis genus. However, some gene families displayed a remarkable expansion, in particular, the Phosphatidylinositol specific phospholipases-C (PI-PLC) family. Also, evolutionary rate calculations suggest that the evolution process of this family was guided by positive selection. Interestingly, among the 82 PI-PLCs genes identified in the C. cacaofunesta genome, 70 genes encoding extracellular PI-PLCs are grouped in eight small scaffolds surrounded by transposon fragments and scars that could be involved in the rapid evolution of the PI-PLC family. Experimental secretome using LC-MS/MS validated 24% (86 proteins) of the total predicted secretome (342 proteins), including four PI-PLCs and other important pathogenicity factors.

CONCLUSION

Analysis of the Ceratocystis cacaofunesta genome provides evidence that PI-PLCs may play a role in pathogenicity. Subsequent functional studies will be aimed at evaluating this hypothesis. The observed genetic arsenals, together with the analysis of the PI-PLC family shown in this work, reveal significant differences in the Ceratocystis genome compared to the classical vascular fungi, Verticillium and Fusarium. Altogether, our analyses provide new insights into the evolution and the molecular basis of plant pathogenicity.

Paracoccin distribution supports its role in Paracoccidioides brasiliensis growth and dimorphic transformation

Paracoccidioides brasiliensis yeast was reported to express paracoccin, a GlcNAc-binding protein that displays N-acetyl-β-d-glucosaminidase (NAGase) activity. Highly specific anti-paracoccin antibodies have been previously used to examine the localization of paracoccin in yeast and inhibit its growth in vitro. In the present study, anti-paracoccin antibodies were used to characterize, by scanning confocal microscopy, the distribution of paracoccin in P. brasiliensis hyphae, transition forms from hyphae to yeast, and mature yeast. In the mycelial phase, paracoccin was detected mainly in the hyphae tips, where it demonstrated a punctate distribution, and was associated with the cell wall. During the first 48 hours after a temperature shift from 26°C to 37°C, paracoccin expression in the differentiating hyphae was mainly detected in the budding regions, i.e. lateral protrusions, and inside the new daughter cells. There was an increased number of chlamydoconidia that expressed a high concentration of paracoccin on their surfaces and/or in their interiors 72–96 hours after the temperature shift. After 120 hours, yeast cells were the predominant form and their cytoplasm stained extensively for paracoccin, whereas Wheat Germ Agglutinin (WGA) staining was predominant on their exterior walls. After 10 days at 37°C, the interior of both mother and daughter yeast cells, as well as the budding regions, stained intensely for paracoccin. The comparison of mRNA-expression in the different fungal forms showed that PCN transcripts, although detected in all evaluated morphological forms, were higher in hypha and yeast-to-hypha transition forms. In conclusion, the pattern of paracoccin distribution in all P. brasiliensis morphotypes supports prevalent beliefs that it plays important roles in fungal growth and dimorphic transformation.

Impact of Paracoccin Gene Silencing on Paracoccidioides brasiliensis Virulence

Among the endemic deep mycoses in Latin America, paracoccidioidomycosis (PCM), caused by thermodimorphic fungi of the Paracoccidioides genus, is a major cause of morbidity. Disease development and its manifestations are associated with both host and fungal factors. Concerning the latter, several recent studies have employed the methodology of gene modulation in P. brasiliensis using antisense RNA (AsRNA) and Agrobacterium tumefaciens-mediated transformation (ATMT) to identify proteins that influence fungus virulence. Our previous observations suggested that paracoccin (PCN), a multidomain fungal protein with both lectin and enzymatic activities, may be a potential P. brasiliensis virulence factor. To explore this, we used AsRNA and ATMT methodology to obtain three independent PCN-silenced P. brasiliensis yeast strains (AsPCN1, AsPCN2, and AsPCN3) and characterized them with regard to P. brasiliensis biology and pathogenicity. AsPCN1, AsPCN2, and AsPCN3 showed relative PCN expression levels that were 60%, 40%, and 60% of that of the wild-type (WT) strain, respectively. PCN silencing led to the aggregation of fungal cells, blocked the morphological yeast-to-mycelium transition, and rendered the yeast less resistant to macrophage fungicidal activity. In addition, mice infected with AsPCN1, AsPCN2, and AsPCN3 showed a reduction in fungal burden of approximately 96% compared with those inoculated with the WT strain, which displayed a more extensive destruction of lung tissue. Finally, mice infected with the PCN-silenced yeast strains had lower mortality than those infected with the WT strain. These data demonstrate that PCN acts as a P. brasiliensis contributory virulence factor directly affecting fungal pathogenesis.

The nonexistence of efficient genetic transformation systems has hampered studies in the dimorphic fungus Paracoccidioides brasiliensis, the etiological agent of the most frequent systemic mycosis in Latin America. The recent development of a method for gene expression knockdown by antisense RNA technology, associated with an Agrobacterium tumefaciens-mediated transformation system, provides new strategies for studying P. brasiliensis. Through this technology, we generated yeasts that were silenced for paracoccin (PCN), a P. brasiliensis component that has lectin and enzymatic properties. By comparing the phenotypes of PCN-silenced and wild-type strains of P. brasiliensis, we identified PCN as a virulence factor whose absence renders the yeasts unable to undergo the transition to mycelium and causes a milder pulmonary disease in mice, with a lower mortality rate. Our report highlights the importance of the technology used for P. brasiliensis transformation and demonstrates that paracoccin is a virulence factor acting on fungal biology and pathogenesis.

Adaptation of Candida albicans to environmental pH induces cell wall remodelling and enhances innate immune recognition

Candida albicans is able to proliferate in environments that vary dramatically in ambient pH, a trait required for colonising niches such as the stomach, vaginal mucosal and the GI tract. Here we show that growth in acidic environments involves cell wall remodelling which results in enhanced chitin and β-glucan exposure at the cell wall periphery. Unmasking of the underlying immuno-stimulatory β-glucan in acidic environments enhanced innate immune recognition of C. albicans by macrophages and neutrophils, and induced a stronger proinflammatory cytokine response, driven through the C-type lectin-like receptor, Dectin-1. This enhanced inflammatory response resulted in significant recruitment of neutrophils in an intraperitoneal model of infection, a hallmark of symptomatic vaginal colonisation. Enhanced chitin exposure resulted from reduced expression of the cell wall chitinase Cht2, via a Bcr1-Rim101 dependent signalling cascade, while increased β-glucan exposure was regulated via a non-canonical signalling pathway. We propose that this "unmasking" of the cell wall may induce non-protective hyper activation of the immune system during growth in acidic niches, and may attribute to symptomatic vaginal infection.

Time course of global gene expression alterations in Candida albicans during infection of HeLa cells

Candida albicans (C. albicans) is an opportunistic fungus that quickly adapts to various microniches. It causes candidiasis, a common fungal infection for which the pathogenic mechanism has not been elucidated yet. To explore the pathogenic mechanism of candidiasis we used several methods, including microscopic observation of morphological changes of HeLa cells and fungus, analysis of differentially expressed genes using gene chips, and a series of biological and bioinformatic analyses to explore genes that are possibly involved in the pathogenesis of C. albicans. During the C. albicans infection, significant morphological changes of the fungus were observed, and the HeLa cells were gradually destroyed. The gene chip experiments showed upregulated expression of 120 genes and downregulated expression of 178 genes. Further analysis showed that some genes may play an important role in the pathogenesis of C. albicans. Overall, morphological variation and adaptive gene expression within a particular microniche may exert important effects during C. albicans infections.

Regulation of Neurospora crassa cell wall remodeling via the cot-1 pathway is mediated by gul-1

Impairment of the Neurospora crassa Nuclear DBF2-related kinase-encoding gene cot-1 results in pleiotropic effects, including abnormally thick hyphal cell walls and septa. An increase in the transcript abundance of genes encoding chitin and glucan synthases and the chitinase gh18-5, but not the cell wall integrity pathway transcription factor rlm-1, accompany the phenotypic changes observed. Deletion of chs-5 or chs-7 in a cot-1 background results in a reduction of hyperbranching frequency characteristic of the cot-1 parent. gul-1 (a homologue of the yeast SSD1 gene) encodes a translational regulator and has been shown to partially suppress cot-1. We demonstrate that the high expression levels of the cell wall remodeling genes analyzed is curbed, and reaches near wild type levels, when gul-1 is inactivated. This is accompanied by morphological changes that include reduced cell wall thickness and restoration of normal chitin levels. We conclude that gul-1 is a mediator of cell wall remodeling within the cot-1 pathway.

Recent advances in the understanding of the Aspergillus fumigatus cell wall

Over the past several decades, research on the synthesis and organization of the cell wall polysaccharides of Aspergillus fumigatus has expanded our knowledge of this important fungal structure. Besides protecting the fungus from environmental stresses and maintaining structural integrity of the organism, the cell wall is also the primary site for interaction with host tissues during infection. Cell wall polysaccharides are important ligands for the recognition of fungi by the innate immune system and they can mediate potent immunomodulatory effects. The synthesis of cell wall polysaccharides is a complicated process that requires coordinated regulation of many biosynthetic and metabolic pathways. Continuous synthesis and remodeling of the polysaccharides of the cell wall is essential for the survival of the fungus during development, reproduction, colonization and invasion. As these polysaccharides are absent from the human host, these biosynthetic pathways are attractive targets for antifungal development. In this review, we present recent advances in our understanding of Aspergillus fumigatus cell wall polysaccharides, including the emerging role of cell wall polysaccharides in the host-pathogen interaction.

Fungal chitinases: function, regulation, and potential roles in plant/pathogen interactions

In the past decades our knowledge about fungal cell wall architecture increased tremendously and led to the identification of many enzymes involved in polysaccharide synthesis and remodeling, which are also of biotechnological interest. Fungal cell walls play an important role in conferring mechanic stability during cell division and polar growth. Additionally, in phytopathogenic fungi the cell wall is the first structure that gets into intimate contact with the host plant. A major constituent of fungal cell walls is chitin, a homopolymer of N-acetylglucosamine units. To ensure plasticity, polymeric chitin needs continuous remodeling which is maintained by chitinolytic enzymes, including lytic polysaccharide monooxygenases N-acetylglucosaminidases, and chitinases. Depending on the species and lifestyle of fungi, there is great variation in the number of encoded chitinases and their function. Chitinases can have housekeeping function in plasticizing the cell wall or can act more specifically during cell separation, nutritional chitin acquisition, or competitive interaction with other fungi. Although chitinase research made huge progress in the last decades, our knowledge about their role in phytopathogenic fungi is still scarce. Recent findings in the dimorphic basidiomycete Ustilago maydis show that chitinases play different physiological functions throughout the life cycle and raise questions about their role during plant-fungus interactions. In this work we summarize these functions, mechanisms of chitinase regulation and their putative role during pathogen/host interactions.

Chitinases Are Essential for Cell Separation in Ustilago maydis

Chitin is an essential component of the fungal cell wall, providing rigidity and stability. Its degradation is mediated by chitinases and supposedly ensures the dynamic plasticity of the cell wall during growth and morphogenesis. Hence, chitinases should be particularly important for fungi with dramatic morphological changes, such as Ustilago maydis. This smut fungus switches from yeast to filamentous growth for plant infection, proliferates as a mycelium in planta, and forms teliospores for spreading. Here, we investigate the contribution of its four chitinolytic enzymes to the different morphological changes during the complete life cycle in a comprehensive study of deletion strains combined with biochemical and cell biological approaches. Interestingly, two chitinases act redundantly in cell separation during yeast growth. They mediate the degradation of remnant chitin in the fragmentation zone between mother and daughter cell. In contrast, even the complete lack of chitinolytic activity does not affect formation of the infectious filament, infection, biotrophic growth, or teliospore germination. Thus, unexpectedly we can exclude a major role for chitinolytic enzymes in morphogenesis or pathogenicity of U. maydis. Nevertheless, redundant activity of even two chitinases is essential for cell separation during saprophytic growth, possibly to improve nutrient access or spreading of yeast cells by wind or rain.

A single nucleotide polymorphism uncovers a novel function for the transcription factor Ace2 during Candida albicans hyphal development

Candida albicans is a major invasive fungal pathogen in humans. An important virulence factor is its ability to switch between the yeast and hyphal forms, and these filamentous forms are important in tissue penetration and invasion. A common feature for filamentous growth is the ability to inhibit cell separation after cytokinesis, although it is poorly understood how this process is regulated developmentally. In C. albicans, the formation of filaments during hyphal growth requires changes in septin ring dynamics. In this work, we studied the functional relationship between septins and the transcription factor Ace2, which controls the expression of enzymes that catalyze septum degradation. We found that alternative translation initiation produces two Ace2 isoforms. While full-length Ace2, Ace2^L, influences septin dynamics in a transcription-independent manner in hyphal cells but not in yeast cells, the use of methionine-55 as the initiation codon gives rise to Ace2^S, which functions as the nuclear transcription factor required for the expression of cell separation genes. Genetic evidence indicates that Ace2^L influences the incorporation of the Sep7 septin to hyphal septin rings in order to avoid inappropriate activation of cell separation during filamentous growth. Interestingly, a natural single nucleotide polymorphism (SNP) present in the C. albicans WO-1 background and other C. albicans commensal and clinical isolates generates a stop codon in the ninth codon of Ace2^L that mimics the phenotype of cells lacking Ace2^L. Finally, we report that Ace2^L and Ace2^S interact with the NDR kinase Cbk1 and that impairing activity of this kinase results in a defect in septin dynamics similar to that of hyphal cells lacking Ace2^L. Together, our findings identify Ace2^L and the NDR kinase Cbk1 as new elements of the signaling system that modify septin ring dynamics in hyphae to allow cell-chain formation, a feature that appears to have evolved in specific C. albicans lineages.

Candida albicans is a major fungal pathogen in immunologically compromised patients. A key virulence trait is its ability to switch between the yeast and hyphal forms. Whereas yeast cells are required for dissemination, the filamentous forms are important in tissue penetration and invasion. In order to make a hypha, cell separation must be inhibited after cytokinesis, although the full extent of its regulation remains unknown. Previously, we have shown that the inhibition of cell separation in hyphae requires a modification of the dynamic properties of septins, a conserved family of GTPases that normally form a ring at the site of cytokinesis. Here we describe new factors regulating septin dynamics during hyphal development. We have discovered that an alternative translation initiation of ACE2 mRNAs gives rise to an Ace2 protein, Ace2^L, with an extra 54 aa at the N-terminus that exhibits a localization and function different from Ace2 transcription factor. This Ace2^L protein is upregulated upon hyphal induction and regulates the incorporation of the Sep7 septin into the septin rings to avoid inappropriate activation of cell separation in hyphae. Finally, we present evidence suggesting that the NDR kinase Cbk1 interacts with Ace2^L to regulate this process.

Functional characterization of extracellular chitinase encoded by the YlCTS1 gene in a dimorphic yeast Yarrowia lipolytica

The hemiascomycetes yeast Yarrowia lipolytica is a dimorphic yeast with alternating yeast and mycelia forms. Bioinformatic analysis revealed the presence of three putative chitinase genes, YlCTS1, YlCTS2, and YlCTS3, in the Y. lipolytica genome. Here, we demonstrated that the protein of YlCTS1 (YlCts1p), which contains an N-terminal secretion signal peptide, a long C-terminal Ser/Thr-rich domain, and a chitin-binding domain, is a homologue to Saccharomyces cerevisiae chitinase 1 (ScCts1p). Deletion of YlCTS1 remarkably reduced extracellular endochitinase activity in the culture supernatant of Y. lipolytica and enhanced cell aggregation, suggesting a role of YlCts1p in cell separation as ScCts1p does in S. cerevisiae. However, loss of YlCts1p function did not affect hyphal formation induced by fetal bovine serum addition. The mass of YlCts1p was dramatically decreased by jack bean α-mannosidase digestion but not by PNGase F treatment, indicating that YlCts1p is modified only by O-mannosylation without N-glycosylation. Moreover, the O-glycan profile of YlCts1p was identical to that of total cell wall mannoproteins, supporting the notion that YlCts1p can be used as a good model for studying O-glycosylation in this dimorphic yeast.

Mannosylation in Candida albicans: role in cell wall function and immune recognition

The fungal cell wall is a dynamic organelle required for cell shape, protection against the environment and, in pathogenic species, recognition by the innate immune system. The outer layer of the cell wall is comprised of glycosylated mannoproteins with the majority of these post‐translational modifications being the addition of O‐ and N‐linked mannosides. These polysaccharides are exposed on the outer surface of the fungal cell wall and are, therefore, the first point of contact between the fungus and the host immune system. This review focuses on O‐ and N‐linked mannan biosynthesis in the fungal pathogen Candida albicans and highlights new insights gained from the characterization of mannosylation mutants into the role of these cell wall components in host–fungus interactions. In addition, we discuss the use of fungal mannan as a diagnostic marker of fungal disease.

Dynamic transcript profiling of Candida albicans infection in zebrafish: a pathogen-host interaction study

Candida albicans is responsible for a number of life-threatening infections and causes considerable morbidity and mortality in immunocompromised patients. Previous studies of C. albicans pathogenesis have suggested several steps must occur before virulent infection, including early adhesion, invasion, and late tissue damage. However, the mechanism that triggers C. albicans transformation from yeast to hyphae form during infection has yet to be fully elucidated. This study used a systems biology approach to investigate C. albicans infection in zebrafish. The surviving fish were sampled at different post-infection time points to obtain time-lapsed, genome-wide transcriptomic data from both organisms, which were accompanied with in sync histological analyses. Principal component analysis (PCA) was used to analyze the dynamic gene expression profiles of significant variations in both C. albicans and zebrafish. The results categorized C. albicans infection into three progressing phases: adhesion, invasion, and damage. Such findings were highly supported by the corresponding histological analysis. Furthermore, the dynamic interspecies transcript profiling revealed that C. albicans activated its filamentous formation during invasion and the iron scavenging functions during the damage phases, whereas zebrafish ceased its iron homeostasis function following massive hemorrhage during the later stages of infection. Most of the immune related genes were expressed as the infection progressed from invasion to the damage phase. Such global, inter-species evidence of virulence-immune and iron competition dynamics during C. albicans infection could be crucial in understanding control fungal pathogenesis.

Effects of chitosan on Candida albicans: conditions for its antifungal activity

The effects of low molecular weight (96.5 KDa) chitosan on the pathogenic yeast Candida albicans were studied. Low concentrations of chitosan, around 2.5 to 10 μg·mL⁻¹ produced (a) an efflux of K⁺ and stimulation of extracellular acidification, (b) an inhibition of Rb⁺ uptake, (c) an increased transmembrane potential difference of the cells, and (d) an increased uptake of Ca²⁺. It is proposed that these effects are due to a decrease of the negative surface charge of the cells resulting from a strong binding of the polymer to the cells. At higher concentrations, besides the efflux of K⁺, it produced (a) a large efflux of phosphates and material absorbing at 260 nm, (b) a decreased uptake of Ca²⁺, (c) an inhibition of fermentation and respiration, and (d) the inhibition of growth. The effects depend on the medium used and the amount of cells, but in YPD high concentrations close to 1 mg·mL⁻¹ are required to produce the disruption of the cell membrane, the efflux of protein, and the growth inhibition. Besides the findings at low chitosan concentrations, this work provides an insight of the conditions required for chitosan to act as a fungistatic or antifungal and proposes a method for the permeabilization of yeast cells.

HgrA is necessary and sufficient to drive hyphal growth in the dimorphic pathogen Penicillium marneffei

Fungi produce multiple morphological forms as part of developmental programs or in response to changing, often stressful, environmental conditions. An opportunistic pathogen of humans, Penicillium marneffei displays multicellular hyphal growth and asexual development (conidiation) in the environment at 25°C and unicellular yeast growth in macrophages at 37°C. We characterized the transcription factor, hgrA, which contains a C(2)H(2) DNA binding domain closely related to that of the stress-response regulators Msn2/4 of Saccharomyces cerevisiae. Northern hybridization analysis demonstrated that hgrA expression is specific to hyphal growth, and its constitutive overexpression prevents conidiation and yeast growth, even in the presence of inductive cues, and causes apical hyperbranching during hyphal growth. Consistent with its expression pattern, deletion of hgrA causes defects in hyphal morphogenesis and the dimorphic transition from yeast cells to hyphae. Specifically, loss of HgrA causes cell wall defects, reduced expression of cell wall biosynthetic enzymes and increased sensitvity to cell wall, oxidative, but not osmotic stress agents. These data suggest that HgrA does not have a direct role in the response to stress but is an inducer of the hyphal growth program and its activity must be downregulated to allow alternative developmental programs, including the morphogenesis of yeast cells in macrophages.

Fungal cell wall organization and biosynthesis

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

Chitin and glucan, the yin and yang of the fungal cell wall, implications for antifungal drug discovery and therapy

The structural carbohydrate polymers glucan and chitin compliment and reinforce each other in a dynamic process to maintain the integrity and physical strength of the fungal cell wall. The assembly of chitin and glucan in the cell wall of the budding yeast Saccharomyces cerevisiae and the polymorphic human pathogen Candida albicans are essential processes that involve a range of fungal-specific enzymes and regulatory networks. The fungal cell wall is, therefore, an attractive target for novel therapies as host cells lack many cell wall-related proteins. The most recent class of antifungal drug approved for clinical use, the echinocandins, targets the synthesis of cell wall β(1-3)glucan. The echinocandins are effective at treating invasive and bloodstream Candida infections and are now widely used in the clinic. However, there have been sporadic reports of breakthrough infections in patients undergoing echinocandin therapy. The acquisition of point mutations in the FKS genes that encode the catalytic β(1-3)glucan synthase subunits, the target of the echinocandins, has emerged as a dominant resistance mechanism. Cells with elevated chitin levels are also less susceptible to echinocandins and in addition, treatment with sub-MIC echinocandin activates cell wall salvage pathways that increase chitin synthesis to compensate for reduced glucan production. The development of drugs targeting the cell wall has already proven to be beneficial in providing an alternative class of drug for use in the clinic. Other cell wall targets such as chitin synthesis still hold great potential for drug development but careful consideration should be given to the capacity of fungi to manipulate their walls in a dynamic response to cell wall perturbations.

Surface stress induces a conserved cell wall stress response in the pathogenic fungus Candida albicans

The human fungal pathogen Candida albicans can grow at temperatures of up to 45°C. Here, we show that at 42°C substantially less biomass was formed than at 37°C. The cells also became more sensitive to wall-perturbing compounds, and the wall chitin levels increased, changes that are indicative of wall stress. Quantitative mass spectrometry of the wall proteome using (15)N metabolically labeled wall proteins as internal standards revealed that at 42°C the levels of the β-glucan transglycosylases Phr1 and Phr2, the predicted chitin transglycosylases Crh11 and Utr2, and the wall maintenance protein Ecm33 increased. Consistent with our previous results for fluconazole stress, this suggests that a wall-remodeling response is mounted to relieve wall stress. Thermal stress as well as different wall and membrane stressors led to an increased phosphorylation of the mitogen-activated protein (MAP) kinase Mkc1, suggesting activation of the cell wall integrity (CWI) pathway. Furthermore, all wall and membrane stresses tested resulted in diminished cell separation. This was accompanied by decreased secretion of the major chitinase Cht3 and the endoglucanase Eng1 into the medium. Consistent with this, cht3 cells showed a similar phenotype. When treated with exogenous chitinase, cell clusters both from stressed cells and mutant strains were dispersed, underlining the importance of Cht3 for cell separation. We propose that surface stresses lead to a conserved cell wall remodeling response that is mainly governed by Mkc1 and is characterized by chitin reinforcement of the wall and the expression of remedial wall remodeling enzymes.

Carbon source-induced reprogramming of the cell wall proteome and secretome modulates the adherence and drug resistance of the fungal pathogen Candida albicans

The major fungal pathogen Candida albicans can occupy diverse microenvironments in its human host. During colonization of the gastrointestinal or urogenital tracts, mucosal surfaces, bloodstream, and internal organs, C. albicans thrives in niches that differ with respect to available nutrients and local environmental stresses. Although most studies are performed on glucose-grown cells, changes in carbon source dramatically affect cell wall architecture, stress responses, and drug resistance. We show that growth on the physiologically relevant carboxylic acid, lactate, has a significant impact on the C. albicans cell wall proteome and secretome. The regulation of cell wall structural proteins (e.g. Cht1, Phr1, Phr2, Pir1) correlated with extensive cell wall remodeling in lactate-grown cells and with their increased resistance to stresses and antifungal drugs, compared with glucose-grown cells. Moreover, changes in other proteins (e.g. Als2, Gca1, Phr1, Sap9) correlated with the increased adherence and biofilm formation of lactate-grown cells. We identified mating and pheromone-regulated proteins that were exclusive to lactate-grown cells (e.g. Op4, Pga31, Pry1, Scw4, Yps7) as well as mucosa-specific and other niche-specific factors such as Lip4, Pga4, Plb5, and Sap7. The analysis of the corresponding null mutants confirmed that many of these proteins contribute to C. albicans adherence, stress, and antifungal drug resistance. Therefore, the cell wall proteome and secretome display considerable plasticity in response to carbon source. This plasticity influences important fitness and virulence attributes known to modulate the behavior of C. albicans in different host microenvironments during infection.

Methylxanthine inhibit fungal chitinases and exhibit antifungal activity

Chitinases are necessary for fungal cell wall remodeling and cell replication. Methylxanthines have been shown to competitively inhibit family 18 chitinases in vitro. We sought to determine the effects of methylxanthines on fungal chitinases. Fungi demonstrated variable chitinase activity and incubation with methylxanthines (0.5-10 mM) resulted in a dose-dependent decrease in this activity. All fungi tested, except for Candida spp., demonstrated growth inhibition in the presence of methylxanthines at a concentration of 10 mM. India ink staining demonstrated impaired budding and decreased cell size for methylxanthine-treated Cryptococcus neoformans. C. neoformans and Aspergillus fumigatus treated with pentoxifylline also exhibited abnormal cell morphology. In addition, pentoxifylline-treated C. neoformans exhibited increased susceptibility to calcofluor and a leaky melanin phenotype consistent with defective cell wall function. Our data suggest that a variety of fungi express chitinases and that methylxanthines have antifungal properties related to their inhibition of fungal chitinases. Our results highlight the potential utility of targeting chitinases in the development of novel antifungal therapies.

Fungal chitinases: diversity, mechanistic properties and biotechnological potential

Chitin derivatives, chitosan and substituted chito-oligosaccharides have a wide spectrum of applications ranging from medicine to cosmetics and dietary supplements. With advancing knowledge about the substrate-binding properties of chitinases, enzyme-based production of these biotechnologically relevant sugars from biological resources is becoming increasingly interesting. Fungi have high numbers of glycoside hydrolase family 18 chitinases with different substrate-binding site architectures. As presented in this review, the large diversity of fungal chitinases is an interesting starting point for protein engineering. In this review, recent data about the architecture of the substrate-binding clefts of fungal chitinases, in connection with their hydrolytic and transglycolytic abilities, and the development of chitinase inhibitors are summarized. Furthermore, the biological functions of chitinases, chitin and chitosan utilization by fungi, and the effects of these aspects on biotechnological applications, including protein overexpression and autolysis during industrial processes, are discussed in this review.

The RNA-binding protein Rrm4 is essential for efficient secretion of endochitinase Cts1

Long-distance transport of mRNAs is crucial in determining spatio-temporal gene expression in eukaryotes. The RNA-binding protein Rrm4 constitutes a key component of microtubule-dependent mRNA transport in filaments of Ustilago maydis. Although a number of potential target mRNAs could be identified, cellular processes that depend on Rrm4-mediated transport remain largely unknown. Here, we used differential proteomics to show that ribosomal, mitochondrial, and cell wall-remodeling proteins, including the bacterial-type endochitinase Cts1, are differentially regulated in rrm4Δ filaments. In vivo UV crosslinking and immunoprecipitation and fluorescence in situ hybridization revealed that cts1 mRNA represents a direct target of Rrm4. Filaments of cts1Δ mutants aggregate in liquid culture suggesting an altered cell surface. In wild type cells Cts1 localizes predominantly at the growth cone, whereas it accumulates at both poles in rrm4Δ filaments. The endochitinase is secreted and associates most likely with the cell wall of filaments. Secretion is drastically impaired in filaments lacking Rrm4 or conventional kinesin Kin1 as well as in filaments with disrupted microtubules. Thus, Rrm4-mediated mRNA transport appears to be essential for efficient export of active Cts1, uncovering a novel molecular link between mRNA transport and the mechanism of secretion.