Candida albicans scavenges host zinc via Pra1 during endothelial invasion

Adaptations of the Secretome of Candida albicans in Response to Host-Related Environmental Conditions

The wall proteome and the secretome of the fungal pathogen Candida albicans help it to thrive in multiple niches of the human body. Mass spectrometry has allowed researchers to study the dynamics of both subproteomes. Here, we discuss some major responses of the secretome to host-related environmental conditions. Three β-1,3-glucan-modifying enzymes, Mp65, Sun41, and Tos1, are consistently found in large amounts in culture supernatants, suggesting that they are needed for construction and expansion of the cell wall β-1,3-glucan layer and thus correlate with growth and might serve as diagnostic biomarkers. The genes ENG1, CHT3, and SCW11, which encode an endoglucanase, the major chitinase, and a β-1,3-glucan-modifying enzyme, respectively, are periodically expressed and peak in M/G₁. The corresponding protein abundances in the medium correlate with the degree of cell separation during single-yeast-cell, pseudohyphal, and hyphal growth. We also discuss the observation that cells treated with fluconazole, or other agents causing cell surface stress, form pseudohyphal aggregates. Fluconazole-treated cells secrete abundant amounts of the transglucosylase Phr1, which is involved in the accumulation of β-1,3-glucan in biofilms, raising the question whether this is a general response to cell surface stress. Other abundant secretome proteins also contribute to biofilm formation, emphasizing the important role of secretome proteins in this mode of growth. Finally, we discuss the relevance of these observations to therapeutic intervention. Together, these data illustrate that C. albicans actively adapts its secretome to environmental conditions, thus promoting its survival in widely divergent niches of the human body.

The Dynamic Genome and Transcriptome of the Human Fungal Pathogen Blastomyces and Close Relative Emmonsia

Three closely related thermally dimorphic pathogens are causal agents of major fungal diseases affecting humans in the Americas: blastomycosis, histoplasmosis and paracoccidioidomycosis. Here we report the genome sequence and analysis of four strains of the etiological agent of blastomycosis, Blastomyces, and two species of the related genus Emmonsia, typically pathogens of small mammals. Compared to related species, Blastomyces genomes are highly expanded, with long, often sharply demarcated tracts of low GC-content sequence. These GC-poor isochore-like regions are enriched for gypsy elements, are variable in total size between isolates, and are least expanded in the avirulent B. dermatitidis strain ER-3 as compared with the virulent B. gilchristii strain SLH14081. The lack of similar regions in related species suggests these isochore-like regions originated recently in the ancestor of the Blastomyces lineage. While gene content is highly conserved between Blastomyces and related fungi, we identified changes in copy number of genes potentially involved in host interaction, including proteases and characterized antigens. In addition, we studied gene expression changes of B. dermatitidis during the interaction of the infectious yeast form with macrophages and in a mouse model. Both experiments highlight a strong antioxidant defense response in Blastomyces, and upregulation of dioxygenases in vivo suggests that dioxide produced by antioxidants may be further utilized for amino acid metabolism. We identify a number of functional categories upregulated exclusively in vivo, such as secreted proteins, zinc acquisition proteins, and cysteine and tryptophan metabolism, which may include critical virulence factors missed before in in vitro studies. Across the dimorphic fungi, loss of certain zinc acquisition genes and differences in amino acid metabolism suggest unique adaptations of Blastomyces to its host environment. These results reveal the dynamics of genome evolution and of factors contributing to virulence in Blastomyces.

Dimorphic fungal pathogens including Blastomyces are the cause of major fungal diseases in North and South America. The genus Emmonsia includes species infecting small mammals as well as a newly emerging pathogenic species recently reported in HIV-positive patients in South Africa. Here, we synthesize both genome sequencing of four isolates of Blastomyces and two species of Emmonsia as well as deep sequencing of Blastomyces RNA to draw major new insights into the evolution of this group and the pathogen response to infection. We investigate the trajectory of genome evolution of this group, characterizing the phylogenetic relationships of these species, a remarkable genome expansion that formed large isochore-like regions of low GC content in Blastomyces, and variation of gene content, related to host interaction, among the dimorphic fungal pathogens. Using RNA-Seq, we profile the response of Blastomyces to macrophage and mouse pulmonary infection, identifying key pathways and novel virulence factors. The identification of key fungal genes involved in adaptation to the host suggests targets for further study and therapeutic intervention in Blastomyces and related dimorphic fungal pathogens.

β-1,2-Mannosyltransferases 1 and 3 Participate in Yeast and Hyphae O- and N-Linked Mannosylation and Alter Candida albicans Fitness During Infection

β-1,2-mannosylation of Candida albicans glycoconjugates has been investigated through the identification of enzymes involved in the addition of β-1,2-oligomannosides (β-Mans) to phosphopeptidomannan and phospholipomannan. β-1,2-oligomannosides are supposed to have virulence properties that they confer to these glycoconjugates. In a previous study, we showed that cell wall mannoproteins (CWMPs) harbor β-Mans in their O-mannosides; therefore, we analyzed their biosynthesis and impact on virulence. In this study, we demonstrate that O-mannans are heterogeneous and that α-mannosylated O-mannosides, which are biosynthesized by Mnt1 and Mnt2 α-1,2-mannosyltransferases, can be modified with β-Mans but only at the nonreducing end of α-1,2-mannotriose. β-1,2-mannosylation of this O-mannotriose depends on growth conditions, and it involves 2 β-1,2-mannosyltransferases, Bmt1 and Bmt3. These Bmts are essential for β-1,2-mannosylation of CWMPs and expression of β-Mans on germ tubes. A bmt1Δ mutant and a mutant expressing no β-Mans unexpectedly disseminated more in BALB/c mice, whereas they had neither attenuated nor enhanced virulence in C57BL/6 mice. In galectin (Gal)3 knockout mice, the reference strain was more virulent than in C57BL/6 mice, suggesting that the β-Mans innate receptor Gal3 is involved in C. albicans fitness during infection.

Broadly Conserved Fungal Effector BEC1019 Suppresses Host Cell Death and Enhances Pathogen Virulence in Powdery Mildew of Barley (Hordeum vulgare L.)

The interaction of barley, Hordeum vulgare L., with the powdery mildew fungus Blumeria graminis f. sp. hordei is a well-developed model to investigate resistance and susceptibility to obligate biotrophic pathogens. The 130-Mb Blumeria genome encodes approximately 540 predicted effectors that are hypothesized to suppress or induce host processes to promote colonization. Blumeria effector candidate (BEC)1019, a single-copy gene encoding a putative, secreted metalloprotease, is expressed in haustorial feeding structures, and host-induced gene silencing of BEC1019 restricts haustorial development in compatible interactions. Here, we show that Barley stripe mosaic virus-induced gene silencing of BEC1019 significantly reduces fungal colonization of barley epidermal cells, demonstrating that BEC1019 plays a central role in virulence. In addition, delivery of BEC1019 to the host cytoplasm via Xanthomonas type III secretion suppresses cultivar nonspecific hypersensitive reaction (HR) induced by Xanthomonas oryzae pv. oryzicola, as well as cultivar-specific HR induced by AvrPphB from Pseudomonas syringae pv. phaseolicola. BEC1019 homologs are present in 96 of 241 sequenced fungal genomes, including plant pathogens, human pathogens, and free-living nonpathogens. Comparative analysis revealed variation at several amino acid positions that correlate with fungal lifestyle and several highly conserved, noncorrelated motifs. Site-directed mutagenesis of one of these, ETVIC, compromises the HR-suppressing activity of BEC1019. We postulate that BEC1019 represents an ancient, broadly important fungal protein family, members of which have evolved to function as effectors in plant and animal hosts.

The Zinc Transport Systems and Their Regulation in Pathogenic Fungi

Zinc is an essential micronutrient required for many enzymes that play essential roles in a cell. It was estimated that approximately 3% of the total cellular proteins are required for zinc for their functions. Zinc has long been considered as one of the key players in host-pathogen interactions. The host sequesters intracellular zinc by utilizing multiple cellular zinc importers and exporters as a means of nutritional immunity. To overcome extreme zinc limitation within the host environment, pathogenic microbes have successfully evolved a number of mechanisms to secure sufficient concentrations of zinc for their survival and pathogenesis. In this review, we briefly discuss the zinc uptake systems and their regulation in the model fungus Saccharomyces cerevisiae and in major human pathogenic fungi such as Aspergillus fumigatus, Candida albicans, and Cryptococcus gattii.

Essential metals at the host-pathogen interface: nutritional immunity and micronutrient assimilation by human fungal pathogens

The ability of pathogenic microorganisms to assimilate sufficient nutrients for growth within their hosts is a fundamental requirement for pathogenicity. However, certain trace nutrients, including iron, zinc and manganese, are actively withheld from invading pathogens in a process called nutritional immunity. Therefore, successful pathogenic species must have evolved specialized mechanisms in order to adapt to the nutritionally restrictive environment of the host and cause disease. In this review, we discuss recent advances which have been made in our understanding of fungal iron and zinc acquisition strategies and nutritional immunity against fungal infections, and explore the mechanisms of micronutrient uptake by human pathogenic fungi.

The human body tightly sequesters essential micronutrients, restricting their access to invading microorganisms, and pathogenic species must counteract this action of ‘nutritional immunity’.

Type VI Secretion System Transports Zn2+ to Combat Multiple Stresses and Host Immunity

Type VI secretion systems (T6SSs) are widespread multi-component machineries that translocate effectors into either eukaryotic or prokaryotic cells, for virulence or for interbacterial competition. Herein, we report that the T6SS-4 from Yersinia pseudotuberculosis displays an unexpected function in the transportation of Zn²⁺ to combat diverse stresses and host immunity. Environmental insults such as oxidative stress induce the expression of T6SS-4 via OxyR, the transcriptional factor that also regulates many oxidative response genes. Zinc transportation is achieved by T6SS-4-mediated translocation of a novel Zn²⁺-binding protein substrate YezP (YPK_3549), which has the capacity to rescue the sensitivity to oxidative stress exhibited by T6SS-4 mutants when added to extracellular milieu. Disruption of the classic zinc transporter ZnuABC together with T6SS-4 or yezP results in mutants that almost completely lost virulence against mice, further highlighting the importance of T6SS-4 in resistance to host immunity. These results assigned an unconventional role to T6SSs, which will lay the foundation for studying novel mechanisms of metal ion uptake by bacteria and the role of this process in their resistance to host immunity and survival in harmful environments.

One unique feature of type VI secretion system is the presence of multiple distinct systems in certain bacterial species. It is well established that some of these systems function to compete for their living niches among diverse bacterial species, whilst the activity of many such transporters remains unknown. Because metal ions are essential components to virtually all forms of life including bacteria, eukaryotic hosts have evolved complicated strategies to sequester metal ions, which constitute a major branch of their nutritional immunity. Therefore the ability to acquire metal ions is critical for bacterial virulence. This study reveals that the T6SS-4 of Yersinia pseudotuberculosis (Yptb) functions to import Zn²⁺ from the environment to mitigate the detrimental effects such as hydroxyl radicals induced by diverse stresses. Expression of the transporter is activated by multiple regulatory proteins, including OxyR and OmpR that sense diverse environmental cues. Zinc ion acquisition is achieved by translocating a Zn²⁺-binding substrate YezP, which is co-regulated with T6SS-4 by OxyR. Our results reveal a novel role for type VI secretion system, which is important in the study of the mechanism of metal ion acquisition by bacteria and the role of this process in bacterial pathogenesis and survival in detrimental environments.

Csr1/Zap1 Maintains Zinc Homeostasis and Influences Virulence in Candida dubliniensis but Is Not Coupled to Morphogenesis

The supply and intracellular homeostasis of trace metals are essential for every living organism. Therefore, the struggle for micronutrients between a pathogen and its host is an important determinant in the infection process. In this work, we focus on the acquisition of zinc by Candida dubliniensis, an emerging pathogen closely related to Candida albicans. We show that the transcription factor Csr1 is essential for C. dubliniensis to regulate zinc uptake mechanisms under zinc limitation: it governs the expression of the zinc transporter genes ZRT1, ZRT2, and ZRT3 and of the zincophore gene PRA1. Exclusively, artificial overexpression of ZRT2 partially rescued the growth defect of a csr1Δ/Δ mutant in a zinc-restricted environment. Importantly, we found that, in contrast to what is seen in C. albicans, Csr1 (also called Zap1) is not a major regulator of dimorphism in C. dubliniensis. However, although a csr1Δ/Δ strain showed normal germ tube formation, we detected a clear attenuation in virulence using an embryonated chicken egg infection model. We conclude that, unlike in C. albicans, Csr1 seems to be a virulence factor of C. dubliniensis that is not coupled to filamentation but is strongly linked to zinc acquisition during pathogenesis.

Effects of zinc transporters on Cryptococcus gattii virulence

Zinc is an essential nutrient for all living organisms because it is a co-factor of several important proteins. Furthermore, zinc may play an essential role in the infectiousness of microorganisms. Previously, we determined that functional zinc metabolism is associated with Cryptococcus gattii virulence. Here, we characterized the ZIP zinc transporters in this human pathogen. Transcriptional profiling revealed that zinc levels regulated the expression of the ZIP1, ZIP2 and ZIP3 genes, although only the C. gattii zinc transporter Zip1 was required for yeast growth under zinc-limiting conditions. To associate zinc uptake defects with virulence, the most studied cryptococcal virulence factors (i.e., capsule, melanin and growth at 37 °C) were assessed in ZIP mutant strains; however, no differences were detected in these classical virulence-associated traits among the mutant and WT strains. Interestingly, higher levels of reactive oxygen species were detected in the zip1Δ and in the zip1Δ zip2Δ double mutants. In line with these phenotypic alterations, the zip1Δ zip2Δ double mutant displayed attenuated virulence in a murine model of cryptococcosis. Together, these results indicate that adequate zinc uptake is necessary for cryptococcal fitness and virulence.

Candida survival strategies

Only few Candida species, e.g., Candida albicans, Candida glabrata, Candida dubliniensis, and Candida parapsilosis, are successful colonizers of a human host. Under certain circumstances these species can cause infections ranging from superficial to life-threatening disseminated candidiasis. The success of C. albicans, the most prevalent and best studied Candida species, as both commensal and human pathogen depends on its genetic, biochemical, and morphological flexibility which facilitates adaptation to a wide range of host niches. In addition, formation of biofilms provides additional protection from adverse environmental conditions. Furthermore, in many host niches Candida cells coexist with members of the human microbiome. The resulting fungal-bacterial interactions have a major influence on the success of C. albicans as commensal and also influence disease development and outcome. In this chapter, we review the current knowledge of important survival strategies of Candida spp., focusing on fundamental fitness and virulence traits of C. albicans.

Sequence variations and protein expression levels of the two immune evasion proteins Gpm1 and Pra1 influence virulence of clinical Candida albicans isolates

Candida albicans, the important human fungal pathogen uses multiple evasion strategies to control, modulate and inhibit host complement and innate immune attack. Clinical C. albicans strains vary in pathogenicity and in serum resistance, in this work we analyzed sequence polymorphisms and variations in the expression levels of two central fungal complement evasion proteins, Gpm1 (phosphoglycerate mutase 1) and Pra1 (pH-regulated antigen 1) in thirteen clinical C. albicans isolates. Four nucleotide (nt) exchanges, all representing synonymous exchanges, were identified within the 747-nt long GPM1 gene. For the 900-nt long PRA1 gene, sixteen nucleotide exchanges were identified, which represented synonymous, as well as non-synonymous exchanges. All thirteen clinical isolates had a homozygous exchange (A to G) at position 73 of the PRA1 gene. Surface levels of Gpm1 varied by 8.2, and Pra1 levels by 3.3 fold in thirteen tested isolates and these differences influenced fungal immune fitness. The high Gpm1/Pra1 expressing candida strains bound the three human immune regulators more efficiently, than the low expression strains. The difference was 44% for Factor H binding, 51% for C4BP binding and 23% for plasminogen binding. This higher Gpm1/Pra1 expressing strains result in enhanced survival upon challenge with complement active, Factor H depleted human serum (difference 40%). In addition adhesion to and infection of human endothelial cells was increased (difference 60%), and C3b surface deposition was less effective (difference 27%). Thus, variable expression levels of central immune evasion protein influences immune fitness of the human fungal pathogen C. albicans and thus contribute to fungal virulence.

Dermatomycoses and inflammation: The adaptive balance between growth, damage, and survival

Dermatomycosis is characterized by both superficial and subcutaneous infections of keratinous tissues and mucous membranes caused by a variety of fungal agents, the two most common classes being dermatophytes and yeasts. Overall, the stepwise process of host infection is similar among the main dermatomycotic species; however, the species-specific ability to elicit a host reaction upon infection is distinct. Yeasts such as Candida albicans elicit a relatively low level of host tissue damage and inflammation during pathogenic infection, while dermatophytes may induce a higher level of tissue damage and inflammatory reaction. Both pathogens can, however, manipulate the host's immune response, ensuring survival and prolonging chronic infection. One common element of most dermatomycotic infections is the disease burden caused by inflammation and associated signs and symptoms, such as erythema, burning and pruritus. There is a strong clinical rationale for the addition of a topical corticosteroid agent to an effective antimycotic therapy, especially in patients who present with inflammatory dermatomycoses (e.g., tinea inguinalis). In this review, we aim to compare the pathogenesis of common dermatomycotic species, including Candida yeasts (Candida albicans), dermatophytes (Trichophyton, Epidermophyton or Microsporum species), and other pathogenic yeasts (Malassezia), with a special focus on unique species-specific aspects of the respective infection processes, the interaction between essential aspects of pathogenic infection, the different roles of the host inflammatory response, and the clinical consequences of the infection-related tissue damage and inflammation. We hope that a broader understanding of the various mechanisms of dermatomycoses may contribute to more effective management of affected patients.

The spectrum of fungi that infects humans

Few among the millions of fungal species fulfill four basic conditions necessary to infect humans: high temperature tolerance, ability to invade the human host, lysis and absorption of human tissue, and resistance to the human immune system. In previously healthy individuals, invasive fungal disease is rare because animals' sophisticated immune systems evolved in constant response to fungal challenges. In contrast, fungal diseases occur frequently in immunocompromised patients. Paradoxically, successes of modern medicine have put increasing numbers of patients at risk for invasive fungal infections. Uncontrolled HIV infection additionally makes millions vulnerable to lethal fungal diseases. A concerted scientific and social effort is needed to meet these challenges.

Candida albicans VPS4 contributes differentially to epithelial and mucosal pathogenesis

We have previously demonstrated that the C. albicans pre-vacuolar protein sorting gene VPS4 is required for extracellular secretion of the secreted aspartyl proteases Sap2p and Saps4-6p. Furthermore, the vps4Δ null mutant has been shown to be markedly hypovirulent in a murine tail vein model of disseminated candidiasis. In these experiments, we sought to further define the role of the pre-vacuolar secretion pathway mediated by the pre-vacuolar sorting gene VPS4 in the pathogenesis of epithelial and mucosal infection using a broad range of virulence models. The C. albicans vps4Δ mutant demonstrates reduced tolerance of cell wall stresses compared to its isogenic, complemented control strain. In an in vitro oral epithelial model (OEM) of tissue invasion, the vps4Δ mutant caused reduced tissue damage compared to controls. Further, the vps4Δ mutant was defective in macrophage killing in vitro, and was attenuated in virulence in an in vivo Caenorhabditis elegans model representative of intestinal epithelial infection. In contrast, the vps4Δ mutant caused a similar degree of tissue damage in an in vitro uroepithelial model of Candida infection compared with controls. Furthermore, in an in vivo murine model of vaginal candidiasis there was no reduction in fungal colony burden and no differences in vaginal histopathology compared to wild-type and complemented controls. These results suggest that VPS4 contributes to several key aspects of oral epithelial but not uroepithelial infection, and in contrast to systemic infection, plays no major role in the pathogenesis of Candida vaginitis. By using a wide range of virulence models, we demonstrate that C. albicans VPS4 contributes to virulence according to the specific tissue that is infected. Thus, in order to gain a full understanding of C. albicans virulence in relation to a particular gene or pathway of interest, a selected range of infection models may need to be utilized.

Metal limitation and toxicity at the interface between host and pathogen

Metals are required cofactors for numerous fundamental processes that are essential to both pathogen and host. They are coordinated in enzymes responsible for DNA replication and transcription, relief from oxidative stress, and cellular respiration. However, excess transition metals can be toxic due to their ability to cause spontaneous, redox cycling and disrupt normal metabolic processes. Vertebrates have evolved intricate mechanisms to limit the availability of some crucial metals while concurrently flooding sites of infection with antimicrobial concentrations of other metals. To compete for limited metal within the host while simultaneously preventing metal toxicity, pathogens have developed a series of metal regulatory, acquisition, and efflux systems. This review will cover the mechanisms by which pathogenic bacteria recognize and respond to host-induced metal scarcity and toxicity.

Metabolism in fungal pathogenesis

Fungal pathogens must assimilate local nutrients to establish an infection in their mammalian host. We focus on carbon, nitrogen, and micronutrient assimilation mechanisms, discussing how these influence host-fungus interactions during infection. We highlight several emerging trends based on the available data. First, the perturbation of carbon, nitrogen, or micronutrient assimilation attenuates fungal pathogenicity. Second, the contrasting evolutionary pressures exerted on facultative versus obligatory pathogens have led to contemporary pathogenic fungal species that display differing degrees of metabolic flexibility. The evolutionarily ancient metabolic pathways are conserved in most fungal pathogen, but interesting gaps exist in some species (e.g., Candida glabrata). Third, metabolic flexibility is generally essential for fungal pathogenicity, and in particular, for the adaptation to contrasting host microenvironments such as the gastrointestinal tract, mucosal surfaces, bloodstream, and internal organs. Fourth, this metabolic flexibility relies on complex regulatory networks, some of which are conserved across lineages, whereas others have undergone significant evolutionary rewiring. Fifth, metabolic adaptation affects fungal susceptibility to antifungal drugs and also presents exciting opportunities for the development of novel therapies.

Stress adaptation in a pathogenic fungus

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

Metabolism impacts upon Candida immunogenicity and pathogenicity at multiple levels

Metabolism is integral to the pathogenicity of Candida albicans, a major fungal pathogen of humans. As well as providing the platform for nutrient assimilation and growth in diverse host niches, metabolic adaptation affects the susceptibility of C. albicans to host-imposed stresses and antifungal drugs, the expression of key virulence factors, and fungal vulnerability to innate immune defences. These effects, which are driven by complex regulatory networks linking metabolism, morphogenesis, stress adaptation, and cell wall remodelling, influence commensalism and infection. Therefore, current concepts of Candida-host interactions must be extended to include the impact of metabolic adaptation upon pathogenicity and immunogenicity.

The Yersinia pestis siderophore, yersiniabactin, and the ZnuABC system both contribute to zinc acquisition and the development of lethal septicaemic plague in mice

Bacterial pathogens must overcome host sequestration of zinc (Zn(2+) ), an essential micronutrient, during the infectious disease process. While the mechanisms to acquire chelated Zn(2+) by bacteria are largely undefined, many pathogens rely upon the ZnuABC family of ABC transporters. Here we show that in Yersinia pestis, irp2, a gene encoding the synthetase (HMWP2) for the siderophore yersiniabactin (Ybt) is required for growth under Zn(2+) -deficient conditions in a strain lacking ZnuABC. Moreover, growth stimulation with exogenous, purified apo-Ybt provides evidence that Ybt may serve as a zincophore for Zn(2+) acquisition. Studies with the Zn(2+) -dependent transcriptional reporter znuA::lacZ indicate that the ability to synthesize Ybt affects the levels of intracellular Zn(2+) . However, the outer membrane receptor Psn and TonB as well as the inner membrane (IM) ABC transporter YbtPQ, which are required for Fe(3+) acquisition by Ybt, are not needed for Ybt-dependent Zn(2+) uptake. In contrast, the predicted IM protein YbtX, a member of the Major Facilitator Superfamily, was essential for Ybt-dependent Zn(2+) uptake. Finally, we show that the ZnuABC system and the Ybt synthetase HMWP2, presumably by Ybt synthesis, both contribute to the development of a lethal infection in a septicaemic plague mouse model.

Multiple impacts of zinc on immune function

Even though zinc is essential for virtually all processes in the human body, observations during zinc deficiency indicate that the absence of this trace element most severely affects the immune response. Numerous investigations of the cellular and molecular requirements for zinc in the immune system have indicated that there is not just one single function of zinc underlying this essentiality. In fact, there is a wide range of different roles of zinc in immunity. This review summarizes the recent developments in three of the major fields: the role of zinc as a second messenger in signal transduction, the importance of zinc for immune cell function, and the competition for zinc between the host and the pathogen, a concept known as nutritional immunity.

Candida albicans and cancer: Can this yeast induce cancer development or progression?

There is currently increasing concern about the relation between microbial infections and cancer. More and more studies support the view that there is an association, above all, when the causal agents are bacteria or viruses. This review adds to this, summarizing evidence that the opportunistic fungus Candida albicans increases the risk of carcinogenesis and metastasis. Until recent years, Candida spp. had fundamentally been linked to cancerous processes as it is an opportunist pathogen that takes advantage of the immunosuppressed state of patients particularly due to chemotherapy. In contrast, the most recent findings demonstrate that C. albicans is capable of promoting cancer by several mechanisms, as described in the review: production of carcinogenic byproducts, triggering of inflammation, induction of Th17 response and molecular mimicry. We underline the need not only to control this type of infection during cancer treatment, especially given the major role of this yeast species in nosocomial infections, but also to find new therapeutic approaches to avoid the pro-tumor effect of this fungal species.

Iron and copper as virulence modulators in human fungal pathogens

Fungal pathogens have evolved sophisticated machinery to precisely balance the fine line between acquiring essential metals and defending against metal toxicity. Iron and copper are essential metals for many processes in both fungal pathogens and their mammalian hosts, but reduce viability when present in excess. However, during infection, the host uses these two metals differently. Fe has a long-standing history of influencing virulence in pathogenic fungi, mostly in regards to Fe acquisition. Numerous studies demonstrate the requirement of the Fe acquisition pathway of Candida, Cryptococcus and Aspergillus for successful systemic infection. Fe is not free in the host, but is associated with Fe-binding proteins, leading fungi to develop mechanisms to interact with and to acquire Fe from these Fe-bound proteins. Cu is also essential for cell growth and development. Essential Cu-binding proteins include Fe transporters, superoxide dismutase (SOD) and cytochrome c oxidase. Although Cu acquisition plays critical roles in fungal survival in the host, recent work has revealed that Cu detoxification is extremely important. Here, we review fungal responses to altered metal conditions presented by the host, contrast the roles of Fe and Cu during infection, and outline the critical roles of fungal metal homeostasis machinery at the host-pathogen axis.

Histidine degradation via an aminotransferase increases the nutritional flexibility of Candida glabrata

The ability to acquire nutrients during infections is an important attribute in microbial pathogenesis. Amino acids are a valuable source of nitrogen if they can be degraded by the infecting organism. In this work, we analyzed histidine utilization in the fungal pathogen of humans Candida glabrata. Hemiascomycete fungi, like C. glabrata or Saccharomyces cerevisiae, possess no gene coding for a histidine ammonia-lyase, which catalyzes the first step of a major histidine degradation pathway in most other organisms. We show that C. glabrata instead initializes histidine degradation via the aromatic amino acid aminotransferase Aro8. Although ARO8 is also present in S. cerevisiae and is induced by extracellular histidine, the yeast cannot use histidine as its sole nitrogen source, possibly due to growth inhibition by a downstream degradation product. Furthermore, C. glabrata relies only on Aro8 for phenylalanine and tryptophan utilization, since ARO8, but not its homologue ARO9, was transcriptionally activated in the presence of these amino acids. Accordingly, an ARO9 deletion had no effect on growth with aromatic amino acids. In contrast, in S. cerevisiae, ARO9 is strongly induced by tryptophan and is known to support growth on aromatic amino acids. Differences in the genomic structure of the ARO9 gene between C. glabrata and S. cerevisiae indicate a possible disruption in the regulatory upstream region. Thus, we show that, in contrast to S. cerevisiae, C. glabrata has adapted to use histidine as a sole source of nitrogen and that the aromatic amino acid aminotransferase Aro8, but not Aro9, is the enzyme required for this process.

Regulatory networks controlling nitrogen sensing and uptake in Candida albicans

Nitrogen is one of the key nutrients for microbial growth. During infection, pathogenic fungi like C. albicans need to acquire nitrogen from a broad range of different and changing sources inside the host. Detecting the available nitrogen sources and adjusting the expression of genes for their uptake and degradation is therefore crucial for survival and growth as well as for establishing an infection. Here, we analyzed the transcriptional response of C. albicans to nitrogen starvation and feeding with the infection-relevant nitrogen sources arginine and bovine serum albumin (BSA), representing amino acids and proteins, respectively. The response to nitrogen starvation was marked by an immediate repression of protein synthesis and an up-regulation of general amino acid permeases, as well as an up-regulation of autophagal processes in its later stages. Feeding with arginine led to a fast reduction in expression of general permeases for amino acids and to resumption of protein synthesis. The response to BSA feeding was generally slower, and was additionally characterized by an up-regulation of oligopeptide transporter genes. From time-series data, we inferred network interaction models for genes relevant in nitrogen detection and uptake. Each individual network was found to be largely specific for the experimental condition (starvation or feeding with arginine or BSA). In addition, we detected several novel connections between regulator and effector genes, with putative roles in nitrogen uptake. We conclude that C. albicans adopts a particular nitrogen response network, defined by sets of specific gene-gene connections for each environmental condition. All together, they form a grid of possible gene regulatory networks, increasing the transcriptional flexibility of C. albicans.

The ZrfC alkaline zinc transporter is required for Aspergillus fumigatus virulence and its growth in the presence of the Zn/Mn-chelating protein calprotectin

Aspergillus fumigatus can invade the lungs of immunocompromised individuals causing a life-threatening disease called invasive pulmonary aspergillosis (IPA). To grow in the lungs, A. fumigatus obtains from the host all nutrients, including zinc. In living tissues, however, most zinc is tightly bound to zinc-binding proteins. Moreover, during infection the bioavailability of zinc can be further decreased by calprotectin, an antimicrobial Zn/Mn-chelating protein that is released by neutrophils in abscesses. Nevertheless, A. fumigatus manages to uptake zinc from and grow within the lungs of susceptible individuals. Thus, in this study we investigated the role of the zrfA, zrfB and zrfC genes, encoding plasma membrane zinc transporters, in A. fumigatus virulence. We showed that zrfC is essential for virulence in the absence of zrfA and zrfB, which contribute to fungal pathogenesis to a lesser extent than zrfC and are dispensable for virulence in the presence of zrfC. The special ability of ZrfC to scavenge and uptake zinc efficiently from lungtissue depended on its N-terminus, which is absent in the ZrfA and ZrfB transporters. In addition, under Zn- and/or Mn-limiting conditions zrfC enables A. fumigatus to grow in the presence of calprotectin, which is detected in fungal abscesses of non-leucopenic animals. This study extends our knowledge about the pathobiology of A. fumigatus and suggests that fungal zinc uptake could be a promising target for new antifungals.

Fungal zinc metabolism and its connections to virulence

Zinc is a ubiquitous metal in all life forms, as it is a structural component of the almost 10% of eukaryotic proteins, which are called zinc-binding proteins. In zinc-limiting conditions such as those found during infection, pathogenic fungi activate the expression of several systems to enhance the uptake of zinc. These systems include ZIP transporters (solute carrier 39 family) and secreted zincophores, which are proteins that are able to chelate zinc. The expression of some fungal zinc uptake systems are regulated by a master regulator (Zap1), first characterized in the yeast Saccharomyces cerevisiae. In this review, we highlight features of zinc uptake and metabolism in human fungal pathogens and aspects of the relationship between proper zinc metabolism and the expression of virulence factors and adaptation to the host habitat.

Candida albicans specializations for iron homeostasis: from commensalism to virulence

Candida albicans is a fungal commensal-pathogen that persistently associates with its mammalian hosts. Between the commensal and pathogenic lifestyles, this microorganism inhabits host niches that differ markedly in the levels of bioavailable iron. A number of recent studies have exposed C. albicans specializations for acquiring iron from specific host molecules in regions where iron is scarce, while also defending against iron-related toxicity in regions where iron occurs in surfeit. Together, these results point to a central role for iron homeostasis in the evolution of this important human pathogen.

Pathogenicity phenomena in three model systems: from network mining to emerging system-level properties

Understanding the interconnections of microbial pathogenicity phenomena, such as biofilm formation, quorum sensing and antimicrobial resistance, is a tremendous open challenge for biomedical research. Progress made by wet-lab researchers and bioinformaticians in understanding the underlying regulatory phenomena has been significant, with converging evidence from multiple high-throughput technologies. Notably, network reconstructions are already of considerable size and quality, tackling both intracellular regulation and signal mediation in microbial infection. Therefore, it stands to reason that in silico investigations would play a more active part in this research. Drug target identification and drug repurposing could take much advantage of the ability to simulate pathogen regulatory systems, host-pathogen interactions and pathogen cross-talking. Here, we review the bioinformatics resources and tools available for the study of the gram-negative bacterium Pseudomonas aeruginosa, the gram-positive bacterium Staphylococcus aureus and the fungal species Candida albicans. The choice of these three microorganisms fits the rationale of the review converging into pathogens of great clinical importance, which thrive in biofilm consortia and manifest growing antimicrobial resistance.

Crossover fungal pathogens: the biology and pathogenesis of fungi capable of crossing kingdoms to infect plants and humans

The outbreak of fungal meningitis associated with contaminated methylprednisolone acetate has thrust the importance of fungal infections into the public consciousness. The predominant pathogen isolated from clinical specimens, Exserohilum rostratum (teleomorph: Setosphaeria rostrata), is a dematiaceous fungus that infects grasses and rarely humans. This outbreak highlights the potential for fungal pathogens to infect both plants and humans. Most crossover or trans-kingdom pathogens are soil saprophytes and include fungi in Ascomycota and Mucormycotina phyla. To establish infection, crossover fungi must overcome disparate, host-specific barriers, including protective surfaces (e.g. cuticle, skin), elevated temperature, and immune defenses. This review illuminates the underlying mechanisms used by crossover fungi to cause infection in plants and mammals, and highlights critical events that lead to human infection by these pathogens. Several genes including veA, laeA, and hapX are important in regulating biological processes in fungi important for both invasive plant and animal infections.

Two unlike cousins: Candida albicans and C. glabrata infection strategies

Candida albicans and C. glabrata are the two most common pathogenic yeasts of humans, yet they are phylogenetically, genetically and phenotypically very different. In this review, we compare and contrast the strategies of C. albicans and C. glabrata to attach to and invade into the host, obtain nutrients and evade the host immune response. Although their strategies share some basic concepts, they differ greatly in their outcome. While C. albicans follows an aggressive strategy to subvert the host response and to obtain nutrients for its survival, C. glabrata seems to have evolved a strategy which is based on stealth, evasion and persistence, without causing severe damage in murine models. However, both fungi are successful as commensals and as pathogens of humans. Understanding these strategies will help in finding novel ways to fight Candida, and fungal infections in general.

Candida albicans pathogenicity mechanisms

The polymorphic fungus Candida albicans is a member of the normal human microbiome. In most individuals, C. albicans resides as a lifelong, harmless commensal. Under certain circumstances, however, C. albicans can cause infections that range from superficial infections of the skin to life-threatening systemic infections. Several factors and activities have been identified which contribute to the pathogenic potential of this fungus. Among them are molecules which mediate adhesion to and invasion into host cells, the secretion of hydrolases, the yeast-to-hypha transition, contact sensing and thigmotropism, biofilm formation, phenotypic switching and a range of fitness attributes. Our understanding of when and how these mechanisms and factors contribute to infection has significantly increased during the last years. In addition, novel virulence mechanisms have recently been discovered. In this review we present an update on our current understanding of the pathogenicity mechanisms of this important human pathogen.

Zinc transporter ZIP14 functions in hepatic zinc, iron and glucose homeostasis during the innate immune response (endotoxemia)

ZIP14 (slc39A14) is a zinc transporter induced in response to pro-inflammatory stimuli. ZIP14 induction accompanies the reduction in serum zinc (hypozincemia) of acute inflammation. ZIP14 can transport Zn²⁺ and non-transferrin-bound Fe²⁺ in vitro. Using a Zip14^−/− mouse model we demonstrated that ZIP14 was essential for control of phosphatase PTP1B activity and phosphorylation of c-Met during liver regeneration. In the current studies, a global screening of ZIP transporter gene expression in response to LPS-induced endotoxemia was conducted. Following LPS, Zip14 was the most highly up-regulated Zip transcript in liver, but also in white adipose tissue and muscle. Using ZIP14^−/− mice we show that ZIP14 contributes to zinc absorption from the gastrointestinal tract directly or indirectly as zinc absorption was decreased in the KOs. In contrast, Zip14^−/− mice absorbed more iron. The Zip14 KO mice did not exhibit hypozincemia following LPS, but do have hypoferremia. Livers of Zip14−/− mice had increased transcript abundance for hepcidin, divalent metal transporter-1, ferritin and transferrin receptor-1 and greater accumulation of iron. The Zip14^−/− phenotype included greater body fat, hypoglycemia and higher insulin levels, as well as increased liver glucose and greater phosphorylation of the insulin receptor and increased GLUT2, SREBP-1c and FASN expression. The Zip14 KO mice exhibited decreased circulating IL-6 with increased hepatic SOCS-3 following LPS, suggesting SOCS-3 inhibited insulin signaling which produced the hypoglycemia in this genotype. The results are consistent with ZIP14 ablation yielding abnormal labile zinc pools which lead to increased SOCS-3 production through G-coupled receptor activation and increased cAMP production as well as signaled by increased pSTAT3 via the IL-6 receptor, which inhibits IRS 1/2 phosphorylation. Our data show the role of ZIP14 in the hepatocyte is multi-functional since zinc and iron trafficking are altered in the Zip14^−/− mice and their phenotype shows defects in glucose homeostasis.