A role for complement receptor-like molecules in iron acquisition by Candida albicans

Genetic Analysis of Candida albicans Filamentation by the Iron Chelator BPS Reveals a Role for a Conserved Kinase-WD40 Protein Pair

Candida albicans is a major human pathogenic fungus that is distinguished by its capability to switch from a yeast to a hyphal morphology under different conditions. Here, we analyze the cellular effects of high concentrations of the iron chelator bathophenanthroline disulfonate (BPS). BPS inhibits cellular growth by withholding iron, but when iron chelation is overcome by the addition of hemoglobin as an iron source, the cells resume growth as hyphae. The BPS hyphal induction pathway was characterized by identifying the hyphal-specific transcription factors that it requires and by a forward genetic screen for mutants that fail to form hyphae in BPS using a transposon library generated in a haploid strain. Among the mutants identified are the DYRK1-like kinase Yak1 and Orf19.384, a homolog of the DYRK1-associated protein WDR68/DCAF7. Orf19.384 nuclear localization depends on Yak1, similar to their mammalian counterparts. We identified the hyphal suppressor transcription factor Sfl1 as a candidate target of Yak1-Orf19.384 and show that Sfl1 modification is similarly affected in the yak1 and orf19.384 mutant strains. These results suggest that DYRK1/Yak1 and WDR68/Orf19.384 represent a conserved protein pair that regulates cell differentiation from fungi to animals.

The interactions of Candida albicans with gut bacteria: a new strategy to prevent and treat invasive intestinal candidiasis

BACKGROUND

The gut microbiota plays an important role in human health, as it can affect host immunity and susceptibility to infectious diseases. Invasive intestinal candidiasis is strongly associated with gut microbiota homeostasis. However, the nature of the interaction between Candida albicans and gut bacteria remains unclear.

OBJECTIVE

This review aimed to determine the nature of interaction and the effects of gut bacteria on C. albicans so as to comprehend an approach to reducing intestinal invasive infection by C. albicans.

METHODS

This review examined 11 common gut bacteria's interactions with C. albicans, including Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, Enterococcus faecalis, Staphylococcus aureus, Salmonella spp., Helicobacter pylori, Lactobacillus spp., Bacteroides spp., Clostridium difficile, and Streptococcus spp.

RESULTS

Most of the studied bacteria demonstrated both synergistic and antagonistic effects with C. albicans, and just a few bacteria such as P. aeruginosa, Salmonella spp., and Lactobacillus spp. demonstrated only antagonism against C. albicans.

CONCLUSIONS

Based on the nature of interactions reported so far by the literature between gut bacteria and C. albicans, it is expected to provide new ideas for the prevention and treatment of invasive intestinal candidiasis.

Candidalysin Is the Hemolytic Factor of Candida albicans

Candida albicans produces an important virulence factor, the hypha-associated Ece1-derived secreted peptide toxin candidalysin, which is crucial for the establishment of mucosal and systemic infections. C. albicans has also long been known to be hemolytic, yet the hemolytic factor has not been clearly identified. Here, we show that candidalysin is the hemolytic factor of C. albicans. Its hemolytic activity is modulated by fragments of another Ece1 peptide, P7. Hemolysis by candidalysin can be neutralized by the purinergic receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). PPADS also affects candidalysin's ability to intercalate into synthetic membranes. We also describe the neutralization potential of two anti-candidalysin nanobodies, which are promising candidates for future anti-Candida therapy. This work provides evidence that the historically proposed hemolytic factor of C. albicans is in fact candidalysin and sheds more light on the complex roles of this toxin in C. albicans biology and pathogenicity.

Regulation of heme utilization and homeostasis in Candida albicans

Heme (iron-protoporphyrin IX) is an essential but potentially toxic cellular cofactor. While most organisms are heme prototrophs, many microorganisms can utilize environmental heme as iron source. The pathogenic yeast Candida albicans can utilize host heme in the iron-poor host environment, using an extracellular cascade of soluble and anchored hemophores, and plasma membrane ferric reductase-like proteins. To gain additional insight into the C. albicans heme uptake pathway, we performed an unbiased genetic selection for mutants resistant to the toxic heme analog Ga³⁺-protoporphyrin IX at neutral pH, and a secondary screen for inability to utilize heme as iron source. Among the mutants isolated were the genes of the pH-responsive RIM pathway, and a zinc finger transcription factor related to S. cerevisiae HAP1. In the presence of hemin in the medium, C. albicans HAP1 is induced, the Hap1 protein is stabilized and Hap1-GFP localizes to the nucleus. In the hap1 mutant, cytoplasmic heme levels are elevated, while influx of extracellular heme is lower. Gene expression analysis indicated that in the presence of extracellular hemin, Hap1 activates the heme oxygenase HMX1, which breaks down excess cytoplasmic heme, while at the same time it also activates all the known heme uptake genes. These results indicate that Hap1 is a heme-responsive transcription factor that plays a role both in cytoplasmic heme homeostasis and in utilization of extracellular heme. The induction of heme uptake genes by C. albicans Hap1 under iron satiety indicates that preferential utilization of host heme can be a dietary strategy in a heme prototroph.

The yeast Candida albicans is a human commensal organism, as well as an important opportunistic systemic pathogen. During tissue invasion, systemic pathogens are confronted with iron scarcity, which they can overcome by scavenging host heme as iron source. It was however not known whether C. albicans can sense the presence of host heme independently of iron sensing. Using a forward genetics approach, we identified a transcription factor that regulates both homeostasis of internal heme and uptake of external heme. This transcription factor allows activation of the heme uptake pathway even in iron-rich medium, suggesting that heme can be a preferred iron source over elemental iron, and that heme prototrophs can scavenge host heme as a source for cellular heme.

Cation Transporters of Candida albicans-New Targets to Fight Candidiasis?

Candidiasis is the wide-spread fungal infection caused by numerous strains of yeast, with the prevalence of Candida albicans. The current treatment of candidiasis is becoming rather ineffective and costly owing to the emergence of resistant strains; hence, the exploration of new possible drug targets is necessary. The most promising route is the development of novel antibiotics targeting this pathogen. In this review, we summarize such candidates found in C. albicans and those involved in the transport of (metal) cations, as the latter are essential for numerous processes within the cell; hence, disruption of their fluxes can be fatal for C. albicans.

Using genetically encoded heme sensors to probe the mechanisms of heme uptake and homeostasis in Candida albicans

Candida albicans is a major fungal pathogen that can utilise hemin and haemoglobin as iron sources in the iron-scarce host environment. While C. albicans is a heme prototroph, we show here that it can also efficiently utilise external heme as a cellular heme source. Using genetically encoded ratiometric fluorescent heme sensors, we show that heme extracted from haemoglobin and free hemin enter the cells with different kinetics. Heme supplied as haemoglobin is taken up via the Common in Fungal Extracellular Membrane (CFEM) hemophore cascade, and reaches the cytoplasm over several hours, whereas entry of free hemin via CFEM-dependent and independent pathways is much faster, less than an hour. To prevent an influx of extracellular heme from reaching toxic levels in the cytoplasm, the cells deploy Hmx1, a heme oxygenase. Hmx1 was previously suggested to be involved in utilisation of haemoglobin and hemin as iron sources, but we find that it is primarily required to prevent heme toxicity. Taken together, the combination of novel heme sensors with genetic analysis revealed new details of the fungal mechanisms of heme import and homeostasis, necessary to balance the uses of heme as essential cofactor and potential iron source against its toxicity.

Pathways of heme utilization in fungi

Iron acquisition is challenging in most environments. As an alternative to elemental iron, organisms can take up iron-protoporphyrin IX, or heme. Heme can be found in decaying organic matter and is particularly prevalent in animal hosts. Fungi have evolved at least three distinct endocytosis-mediated heme uptake systems, which have been studied in detail in the organisms Candida albicans, Cryptococcus neoformans and Schizosaccharomyces pombe. Here we summarize the known molecular details of these three uptake systems that enable parasitic and saprophytic fungi to take advantage of external heme as either cellular iron or heme sources.

Human Serum Albumin Facilitates Heme-Iron Utilization by Fungi

Heme constitutes a major iron source for microorganisms and particularly for pathogenic microbes; to overcome the iron scarcity in the animal host, many pathogenic bacteria and fungi have developed systems to extract and take up heme from host proteins such as hemoglobin. Microbial heme uptake mechanisms are usually studied using growth media containing free heme or hemoglobin as a sole iron source. However, the animal host contains heme-scavenging proteins that could prevent this uptake. In the human host in particular, the most abundant serum heme-binding protein is albumin. Surprisingly, however, we found that in the case of fungi of the Candida species family, albumin promoted rather than prevented heme utilization. Albumin thus constitutes a human-specific factor that can affect heme-iron utilization and could serve as target for preventing heme-iron utilization by fungal pathogens. As a proof of principle, we identify two drugs that can inhibit albumin-stimulated heme utilization.

A large portion of biological iron is found in the form of an iron-protoporphyrin IX complex, or heme. In the human host environment, which is exceptionally poor in free iron, heme iron, particularly from hemoglobin, constitutes a major source of iron for invading microbial pathogens. Several fungi were shown to utilize free heme, and Candida albicans, a major opportunistic pathogen, is able both to capture free heme and to extract heme from hemoglobin using a network of extracellular hemophores. Human serum albumin (HSA) is the most abundant host heme-scavenging protein. Tight binding of heme by HSA restricts its toxic chemical reactivity and could diminish its availability as an iron source for pathogenic microbes. We found, however, that rather than inhibiting heme utilization, HSA greatly increases availability of heme as an iron source for C. albicans and other fungi. In contrast, hemopexin, a low-abundance but high-affinity heme-scavenging serum protein, does inhibit heme utilization by C. albicans. However, inhibition by hemopexin is mitigated in the presence of HSA. Utilization of albumin-bound heme requires the same hemophore cascade as that which mediates hemoglobin-iron utilization. Accordingly, we found that the C. albicans hemophores are able to extract heme bound to HSA in vitro. Since many common drugs are known to bind to HSA, we tested whether they could interfere with heme-iron utilization. We show that utilization of albumin-bound heme by C. albicans can be inhibited by the anti-inflammatory drugs naproxen and salicylic acid.

Mechanisms of Pathogenic Candida Species to Evade the Host Complement Attack

Candida species are common colonizers of the human skin, vagina, and the gut. As human commensals, Candida species do not cause any notable damage in healthy individuals; however, in certain conditions they can initiate a wide range of diseases such as chronic disseminated candidiasis, endocarditis, vaginitis, meningitis, and endophthalmitis. The incidence of Candida caused infections has increased worldwide, with mortality rates exceeding 70% in certain patient populations. C. albicans, C. glabrata, C. tropicalis, C. parapsilosis, and C. krusei are responsible for more than 90% of Candida-related infections. Interestingly, the host immune response against these closely related fungi varies. As part of the innate immune system, complement proteins play a crucial role in host defense, protecting the host by lysing pathogens or by increasing their phagocytosis by phagocytes through opsonization. This review summarizes interactions of host complement proteins with pathogenic Candida species, including C. albicans and non-albicans Candida species such as C. parapsilosis. We will also highlight the various ways of complement activation, describe the antifungal effects of complement cascades and explore the mechanisms adopted by members of pathogenic Candida species for evading complement attack.

The adaptive response to iron involves changes in energetic strategies in the pathogen Candida albicans

Candida albicans is an opportunist pathogen responsible for a large spectrum of infections, from superficial mycosis to systemic diseases known as candidiasis. Its ability to grow in different morphological forms, such as yeasts or filamentous hyphae, contributes to its survival in diverse microenvironments. Iron uptake has been associated with virulence, and C. albicans has developed elaborate strategies for acquiring iron from its host. In this work, we analyze the metabolic changes in response to changes in iron content in the growth medium and compare C. albicans adaptation to the presence or absence of iron. Functional and morphological studies, correlated to a quantitative proteomic analysis, were performed to assess the specific pathways underlying the response to iron, both in the yeast and filamentous forms. Overall, the results show that the adaptive response to iron is associated with a metabolic remodeling affecting the energetic pathways of the pathogen. This includes changes in the thiol-dependent redox status, the activity of key mitochondrial enzymes and the respiratory chain. Iron deficiency stimulates bioenergetic pathways, whereas iron-rich condition is associated with greater biosynthetic needs, particularly in filamentous forms. Moreover, we found that C. albicans yeast cells have an extraordinary capability to adapt to changes in environmental conditions.

Oriented Assembly of Cell-Mimicking Nanoparticles via a Molecular Affinity Strategy for Targeted Drug Delivery

Cell membrane cloaking is an emerging field in drug delivery in which specific functions of parent cells are conferred to newly formed biomimetic vehicles. A growing variety of delivery systems with diverse surface properties have been utilized for this strategy, but it is unclear whether the affinity of membrane-core pairs could guarantee effective and proper camouflaging. In this study, we propose a concise and effective "molecular affinity" strategy using the intracellular domain of transmembrane receptors as "grippers" during membrane coating. Red blood cell (RBC) membranes and cationic liposomes were adopted for fabrication, and a peptide ligand derived from the cytoplasmic protein P4.2 was prepared to specifically recognize the cytoplasmic domain of band 3, a key transmembrane receptor of erythrocytes. Once anchored onto the liposome surface, the P4.2-derived peptide would interact with the isolated RBC membrane, forming a "hidden peptide button", which ensures the right-side-out orientation. The membrane-coated liposomes exhibited an appropriate size distribution around 100 nm and high stability, with superior circulation durations compared with those of conventional PEGylated liposomes. Importantly, they possessed the ability to target Candida albicans by the interaction between the pathogenic fungus and host erythrocytes and to neutralize hemotoxin secreted by the pathogenic fungi. The curative effect of the model drug was thus substantially improved. In summary, the "molecular affinity" strategy may provide a powerful and universal approach for the construction of cell membrane-coated biomaterials and nanomedicines at both laboratory and industrial scales.

Candida albicans Interactions with Mucosal Surfaces during Health and Disease

Flexible adaptation to the host environment is a critical trait that underpins the success of numerous microbes. The polymorphic fungus Candida albicans has evolved to persist in the numerous challenging niches of the human body. The interaction of C. albicans with a mucosal surface is an essential prerequisite for fungal colonisation and epitomises the complex interface between microbe and host. C. albicans exhibits numerous adaptations to a healthy host that permit commensal colonisation of mucosal surfaces without provoking an overt immune response that may lead to clearance. Conversely, fungal adaptation to impaired immune fitness at mucosal surfaces enables pathogenic infiltration into underlying tissues, often with devastating consequences. This review will summarise our current understanding of the complex interactions that occur between C. albicans and the mucosal surfaces of the human body.

Iron at the Centre of Candida albicans Interactions

Iron is an absolute requirement for both the host and most pathogens alike and is needed for normal cellular growth. The acquisition of iron by biological systems is regulated to circumvent toxicity of iron overload, as well as the growth deficits imposed by iron deficiency. In addition, hosts, such as humans, need to limit the availability of iron to pathogens. However, opportunistic pathogens such as Candida albicans are able to adapt to extremes of iron availability, such as the iron replete environment of the gastrointestinal tract and iron deficiency during systemic infection. C. albicans has developed a complex and effective regulatory circuit for iron acquisition and storage to circumvent iron limitation within the human host. As C. albicans can form complex interactions with both commensal and pathogenic co-inhabitants, it can be speculated that iron may play an important role in these interactions. In this review, we highlight host iron regulation as well as regulation of iron homeostasis in C. albicans. In addition, the review argues for the need for further research into the role of iron in polymicrobial interactions. Lastly, the role of iron in treatment of C. albicans infection is discussed.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

Candida albicans Hap43 Domains Are Required under Iron Starvation but Not Excess

Iron availability is a central factor in infections, since iron is a critical micronutrient for all living organisms. The host employs both iron limitation and toxicity strategies to control microbial growth, and successful pathogens are able to tightly coordinate iron homeostasis in response to changing iron levels. As a commensal and opportunistic pathogen, Candida albicans copes with both iron deficiency and excess via the precise regulation of iron acquisition, consumption and storage. The C. albicans transcription factor Hap43 is known to be required for the iron starvation response, while specific domains of its ortholog, HapX, in Aspergillus fumigatus, were recently shown to regulate iron uptake and consumptions genes under both low and high iron levels. Therefore, we investigated the contribution of C. albicans Hap43 domains in response to changing iron levels. We found the C-terminus of Hap43 to be essential for the activation of iron uptake genes during iron starvation, whereas, in contrast to A. fumigatus, Hap43 was not required in mediating adaptation to iron resistance. These data indicate that the generally conserved metal acquisition systems in fungal pathogens can show individual adaptations to the host environment.

Nutritional Immunity and Fungal Pathogenesis: The Struggle for Micronutrients at the Host-Pathogen Interface

All living organisms require certain micronutrients such as iron, zinc, manganese and copper for cellular function and growth. For human pathogens however, the maintenance of metal ion homeostasis is particularly challenging. This is because the mammalian host actively enforces extremes of micronutrient availability on potential microbial invaders-processes collectively termed nutritional immunity. The role of iron sequestration in controlling microbial infections is well established and, more recently, the importance of other metals including zinc, manganese and copper has been recognised. In this chapter, we explore the nutritional immune mechanisms that defend the human body against fungal infections and the strategies that these important pathogens exploit to counteract nutritional immunity and thrive in the infected host.

Synthesis, characterization, optical and electrochemical properties and antifungal and anticancer activities of ferrocenyl conjugated novel dendrimers

A new class of triazoloferrocenyl conjugates was prepared by copper(i) catalyzed click chemistry, which shows good antifungal activity against fungal pathogens, and also shows excellent anticancer activity against MCF-7 cells.

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’.

Comparison of the hemolytic activity between C. albicans and non-albicans Candida species

The ability to produce enzymes, such as hemolysins, is an important virulence factor for the genus Candida.The objective of this study was to compare the hemolytic activity between C. albicansand non-albicans Candida species. Fifty strains of Candida species, isolated from the oral cavity of patients infected with HIV were studied. The isolates included the following species: C. albicans, C. dubliniensis, C. glabrata, C. tropicalis, C. krusei, C. parapsilosis, C. dubliniensis, C. norvegensis, C. lusitaniae, and C. guilliermondii. Hemolysin production was evaluated on Sabouraud dextrose agar containing chloramphenicol, blood, and glucose. A loop-full of pure Candidaculture was spot-inoculated onto plates and incubated at 37 ºC for 24 h in a 5% CO2 atmosphere. Hemolytic activity was defined as the formation of a translucent halo around the colonies. All C. albicansstrains that were studied produced hemolysins. Among the non-albicans Candidaspecies, 86% exhibited hemolytic activity. Only C. guilliermondiiand some C. parapsilosis isolates were negative for this enzyme. In conclusion, most non-albicans Candidaspecies had a similar ability to produce hemolysins when compared to C. albicans.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

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.

Cryptococcus neoformans requires the ESCRT protein Vps23 for iron acquisition from heme, for capsule formation, and for virulence

Iron availability is a key regulator of virulence factor elaboration in Cryptococcus neoformans, the causative agent of fungal meningoencephalitis in HIV/AIDS patients. In addition, iron is an essential nutrient for pathogen proliferation in mammalian hosts but little is known about the mechanisms of iron sensing and uptake in fungal pathogens that attack humans. In this study, we mutagenized C. neoformans by Agrobacterium-mediated T-DNA insertion and screened for mutants with reduced growth on heme as the sole iron source. Among 34 mutants, we identified a subset with insertions in the gene for the ESCRT-I (endosomal sorting complex required for transport) protein Vps23 that resulted in a growth defect on heme, presumably due to a defect in uptake via endocytosis or misregulation of iron acquisition from heme. Remarkably, vps23 mutants were also defective in the elaboration of the cell-associated capsular polysaccharide that is a major virulence factor, while overexpression of Vps23 resulted in cells with a slightly enlarged capsule. These phenotypes were mirrored by a virulence defect in the vps23 mutant in a mouse model of cryptococcosis and by hypervirulence of the overexpression strain. Overall, these results reveal an important role for trafficking via ESCRT functions in both heme uptake and capsule formation, and they further reinforce the connection between iron and virulence factor deployment in C. neoformans.

Ferric reductase genes involved in high-affinity iron uptake are differentially regulated in yeast and hyphae of Candida albicans

The pathogenic yeast Candida albicans possesses a reductive iron uptake system which is active in iron-restricted conditions. The sequestration of iron by this mechanism initially requires the reduction of free iron to the soluble ferrous form, which is catalysed by ferric reductase proteins. Reduced iron is then taken up into the cell by a complex of a multicopper oxidase protein and an iron transport protein. Multicopper oxidase proteins require copper to function and so reductive iron and copper uptake are inextricably linked. It has previously been established that Fre10 is the major cell surface ferric reductase in C. albicans and that transcription of FRE10 is regulated in response to iron levels. We demonstrate here that Fre10 is also a cupric reductase and that Fre7 also makes a significant contribution to cell surface ferric and cupric reductase activity. It is also shown, for the first time, that transcription of FRE10 and FRE7 is lower in hyphae compared to yeast and that this leads to a corresponding decrease in cell surface ferric, but not cupric, reductase activity. This demonstrates that the regulation of two virulence determinants, the reductive iron uptake system and the morphological form of C. albicans, are linked.

Regulatory network modelling of iron acquisition by a fungal pathogen in contact with epithelial cells

BACKGROUND

Reverse engineering of gene regulatory networks can be used to predict regulatory interactions of an organism faced with environmental changes, but can prove problematic, especially when focusing on complicated multi-factorial processes. Candida albicans is a major human fungal pathogen. During the infection process, this fungus is able to adapt to conditions of very low iron availability. Such adaptation is an important virulence attribute of virtually all pathogenic microbes. Understanding the regulation of iron acquisition genes will extend our knowledge of the complex regulatory changes during the infection process and might identify new potential drug targets. Thus, there is a need for efficient modelling approaches predicting key regulatory events of iron acquisition genes during the infection process.

RESULTS

This study deals with the regulation of C. albicans iron uptake genes during adhesion to and invasion into human oral epithelial cells. A reverse engineering strategy is presented, which is able to infer regulatory networks on the basis of gene expression data, making use of relevant selection criteria such as sparseness and robustness. An exhaustive use of available knowledge from different data sources improved the network prediction. The predicted regulatory network proposes a number of new target genes for the transcriptional regulators Rim101, Hap3, Sef1 and Tup1. Furthermore, the molecular mode of action for Tup1 is clarified. Finally, regulatory interactions between the transcription factors themselves are proposed. This study presents a model describing how C. albicans may regulate iron acquisition during contact with and invasion of human oral epithelial cells. There is evidence that some of the proposed regulatory interactions might also occur during oral infection.

CONCLUSIONS

This study focuses on a typical problem in Systems Biology where an interesting biological phenomenon is studied using a small number of available experimental data points. To overcome this limitation, a special modelling strategy was used which identifies sparse and robust networks. The data is augmented by an exhaustive search for additional data sources, helping to make proposals on regulatory interactions and to guide the modelling approach. The proposed modelling strategy is capable of finding known regulatory interactions and predicts a number of yet unknown biologically relevant regulatory interactions.

The iC3b receptor of Candida albicans and its roles in pathogenesis

On the basis of biochemical and immunologic studies, a receptor for iC3b with some activities reminiscent of the integrins CD11b and CD11c was defined on the cell wall of clinical and laboratory isolates of Candida albicans. The INT1 gene encodes a protein of 1659 amino acids; the Int1 protein participates in adhesion to epithelial cells in vitro and in vivo. Int1 is essential for hyphal morphogenesis and virulence in a murine model. Recent evidence points to the amino terminus of Int1 as the source of a peptide, Pep263, with superantigen-like activities.

Complement and fungal pathogens: an update

Fungal infections are a serious complication in immunocompromised patients such as human immunodeficiency virus-infected individuals, patients with organ transplantations or with haematological neoplasia. The lethality of opportunistic fungal infection is high despite a growing arsenal of antimycotic drugs, implying the urgent need for supportive immunological therapies to strengthen the current inefficient antimicrobial defences of the immunocompromised host. Therefore, increasing effort has been directed to investigating the interplay between fungi and the host immunity and thus to find starting points for additional therapeutic approaches. In this article, we review the actual state of the art concerning the role of complement in the pathogenesis of fungal infections. Important aspects include the activation of the complement system by the fungal pathogen, the efficiency of the complement-associated antimicrobial functions and the arsenal of immune evasion strategies applied by the fungi. The twin functions of complement as an interactive player of the innate immunity and at the same time as a modulator of the adaptive immunity make this defence weapon a particularly interesting therapeutic candidate to mobilise a more effective immune response and to strengthen in one fell swoop a broad spectrum of different immune reactions. However, we also mention the 'Yin-Yang' nature of the complement system in fungal infections, as growing evidence assigns to complement a contributory part in the pathogenesis of fungus-induced allergic manifestations.

Infection-associated genes of Candida albicans

Advances in the medical treatment of life-threatening disorders have increased the population of patients that are more susceptible to opportunistic microbial infections, such as those caused by the Candida species, in particular Candida albicans. This fungus normally belongs to the microbial flora but may cause a range of diseases from superficial to disseminated. What exactly causes the transition from commensalism to pathogenesis is not clear and how this fungus switches from a commensal mode of growth to a parasitic lifestyle remains unknown. Identifying the genes and factors essential for the different stages of C. albicans infections will not only help understanding of the infection process but also provide information about those fungal factors that have to be inhibited, and those parts of the immune system that have to be stimulated, in order to control or prevent infections. Furthermore, knowledge of those genes whose expression is associated with infection but not commensalism may provide valuable information to improve our diagnostic tools. A number of methodologies and models have already been used to identify infection-associated genes. In addition to genes encoding classical virulence determinants, such as those involved in interactions with the immune system and immune evasion, scientists have monitored the expression of genes involved in nutrient acquisition, metabolism, stress response, physical interaction and hyphal formation in infection models and have begun to elucidate the roles of these genes.

Vulvovaginal candidosis

Despite therapeutic advances, vulvovaginal candidosis remains a common problem worldwide, affecting all strata of society. Understanding of anti-candida host defence mechanisms in the vagina has developed slowly and, despite a growing list of recognised risk factors, a fundamental grasp of pathogenic mechanisms continues to elude us. The absence of rapid, simple, and inexpensive diagnostic tests continues to result in both overdiagnosis and underdiagnosis of vulvovaginal candidosis. I review the epidemiology and pathogenesis of this infection, and also discuss management strategies.

The serum levels of calcium, magnesium, iron and zinc in patients with recurrent vulvovaginal candidosis during attack, remission and in healthy controls

The real cause of recurrent vulvovaginal candidosis (RVVC) is concealed and the etiopathogenesis of this disease remains to be determined. In a cohort study, concentrations of metals in 44 patients with RVVC and 30 healthy age-matched women were measured and compared. The concentrations of serum calcium (Ca), magnesium (Mg) and iron (Fe) were measured photometrically, the zinc (Zn) levels were determined using flame atomic absorption spectrometry. For statistical analysis were used the Student's t-tests (paired analysis for attack vs. remission; non-paired analysis for patient vs. control). Although all measured metals were within normal ranges the patients with RVVC had in contrast to the healthy controls significantly lower levels of serum Ca, Mg and Zn and insignificantly higher levels of Fe. These relative changes may contribute to the development of attacks in patients with RVVC.

Regulatory networks affected by iron availability in Candida albicans

Iron, an essential element for almost every organism, serves as a regulatory signal for the expression of virulence determinants in many prokaryotic and eukaryotic pathogens. Using a custom Affymetrix GeneChip representing the entire Candida albicans genome, we examined the changes in genome-wide gene expression in this opportunistic pathogen as a function of alterations in environmental concentrations of iron. A total of 526 open reading frame (ORF) transcripts are more highly expressed when the levels of available iron are low, while 626 ORF transcripts are more highly expressed in high-iron conditions. The transcripts dominantly affected by iron concentration range from those associated with cell-surface properties to others which affect mitochondrial function, iron transport and virulence-related secreted hydrolases. Moreover gene expression as assayed in DNA microarrays confirms and extends reports of alterations in cell-surface antigens and drug sensitivity correlated with iron availability. To understand how these genes and pathways might be regulated, we isolated a gene designated SFU1 that encodes a homologue of the Ustilago maydis URBS1, a transcriptional repressor of siderophore uptake/biosynthesis. Comparisons between wild-type and SFU1-null mutant strains revealed 139 potential target genes of Sfu1p; many of which are iron-responsive. Together, these results not only expand our understanding of global iron regulation in C. albicans, but also provide insights into the potential role of iron availability in C. albicans virulence.

A family of Candida cell surface haem-binding proteins involved in haemin and haemoglobin-iron utilization

The ability to acquire iron from host tissues is a major virulence factor of pathogenic microorganisms. Candida albicans is an important fungal pathogen, responsible for an increasing proportion of systemic infections. C. albicans, like many pathogenic bacteria, is able to utilize haemin and haemoglobin as iron sources. However, the molecular basis of this pathway in pathogenic fungi is unknown. Here, we identify a conserved family of plasma membrane-anchored proteins as haem-binding proteins that are involved in haem-iron utilization. We isolated RBT51 as a gene that is sufficient by itself to confer to S. cerevisiae the ability to utilize haemoglobin iron. RBT51 is highly homologous to RBT5, which was previously identified as a gene negatively regulated by the transcriptional suppressor CaTup1. Rbt5 and Rbt51 are mannosylated proteins that carry the conserved CFEM domain. We find that RBT5 is strongly induced by starvation for iron, and that deletion of RBT5 is by itself sufficient to significantly reduce the ability of C. albicans to utilize haemin and haemoglobin as iron sources. Iron starvation-inducible, antigenically cross-reacting haem-binding proteins are also present in other Candida species that are able to utilize haem-iron, underscoring the conservation of this iron acquisition pathway among pathogenic fungi.

The CaCTR1 gene is required for high-affinity iron uptake and is transcriptionally controlled by a copper-sensing transactivator encoded by CaMAC1

The ability of Candida albicans to acquire iron from the hostile environment of the host is known to be necessary for virulence and appears to be achieved using a similar system to that described for Saccharomyces cerevisiae. In S. cerevisiae, high-affinity iron uptake is dependent upon the acquisition of copper. The authors have previously identified a C. albicans gene (CaCTR1) that encodes a copper transporter. Deletion of this gene results in a mutant strain that grows predominantly as pseudohyphae and displays aberrant morphology in low-copper conditions. This paper demonstrates that invasive growth by C. albicans is induced by low-copper conditions and that this is augmented in a Cactr1-null strain. It also shows that deletion of CaCTR1 results in defective iron uptake. In S. cerevisiae, genes that facilitate high-affinity copper uptake are controlled by a copper-sensing transactivator, ScMac1p. The authors have now identified a C. albicans gene (CaMAC1) that encodes a copper-sensing transactivator. A Camac1-null mutant displays phenotypes similar to those of a Cactr1-null mutant and has no detectable CaCTR1 transcripts in low-copper conditions. It is proposed that high-affinity copper uptake by C. albicans is necessary for reductive iron uptake and is transcriptionally controlled by CaMac1p in a similar manner to that in S. cerevisiae.

The role of complement in invasive fungal infections. Die Rolle des Komplements bei invasiven Pilzinfektionen

New therapeutic approaches enable organ transplantations and guarantee longer survival for AIDS patients or patients with haematological neoplasia. The price for these medical advances is immunosuppression and thus enhanced susceptibility to opportunistic fungal infections. As a consequence invasive fungal infections are on the march in modern medicine. Therapeutic limitations and difficulties strongly demand for a deeper understanding of the interaction between the various fungi and the hosts' innate and adaptive immune defence system. This understanding is the essential prerequisite for a potential therapeutic approach, which may support specifically the insufficient antifungal attack of the host. In the present article, we therefore review the current knowledge of the role of the complement system as a central part of innate immunity and as a fine tuner of adaptive immunity in the pathogenesis of invasive fungal infections, such as aspergillosis, candidosis, cryptococcosis, paracoccidioidomycosis, blastomycosis and histoplasmosis.

The Candida albicans CTR1 gene encodes a functional copper transporter

Copper and iron uptake in Saccharomyces cerevisiae are linked through a high-affinity ferric/cupric-reductive uptake system. Evidence suggests that a similar system operates in Candida albicans. The authors have identified a C. albicans gene that is able to rescue a S. cerevisiae ctr1/ctr3-null mutant defective in high-affinity copper uptake. The 756 bp ORF, designated CaCTR1, encodes a 251 amino acid protein with a molecular mass of 27.8 kDa. Comparisons between the deduced amino acid sequence of the C. albicans Ctr1p and S. cerevisiae Ctr1p indicated that they share 39.6 % similarity and 33.0 % identity over their entire length. Within the predicted protein product of CaCTR1 there are putative transmembrane regions and sequences that resemble copper-binding motifs. The promoter region of CaCTR1 contains four sequences with significant identity to S. cerevisiae copper response elements. CaCTR1 is transcriptionally regulated in S. cerevisiae in response to copper availability by the copper-sensing transactivator Mac1p. Transcription of CaCTR1 in C. albicans is also regulated in a copper-responsive manner. This raises the possibility that CaCTR1 may be regulated in C. albicans by a Mac1p-like transactivator. A C. albicans ctr1-null mutant displays phenotypes consistent with the lack of copper uptake including growth defects in low-copper and low-iron conditions, a respiratory deficiency and sensitivity to oxidative stress. Furthermore, changes in morphology were observed in the C. albicans ctr1-null mutant. It is proposed that CaCTR1 facilitates transport of copper into the cell.

Stage-specific gene expression of Candida albicans in human blood

The pathogenic fungus Candida albicans commonly causes mucosal surface infections. In immunocompromised patients, C. albicans may penetrate into deeper tissue, enter the bloodstream and disseminate within the host causing life-threatening systemic infections. In order to elucidate how C. albicans responds to the challenge of a blood environment, we analysed the transcription profile of C. albicans cells exposed to human blood using genomic arrays and a cDNA subtraction protocol. By combining data obtained with these two methods, we were able to identify unique sets of different fungal genes specifically expressed at different stages of this model that mimics bloodstream infections. By removing host cells and incubation in plasma, we were also able to identify several genes in which the expression level was significantly influenced by the presence of these cells. Differentially expressed genes included those that are involved in the general stress response, antioxidative response, glyoxylate cycle as well as putative virulence attributes. These data point to possible mechanisms by which C. albicans ensures survival in the hostile environment of the blood and how the fungus may escape the bloodstream as an essential step in its systemic dissemination.

Haemin uptake and use as an iron source by Candida albicans: role of CaHMX1-encoded haem oxygenase

Candida albicans, unlike Saccharomyces cerevisiae, was able to use extracellular haemin as an iron source. Haemin uptake kinetics by C. albicans cells showed two phases: a rapid phase of haemin binding (with a K(d) of about 0.2 microM) followed by a slower uptake phase. Both phases were strongly induced in iron-deficient cells compared to iron-rich cells. Haemin uptake did not depend on the previously characterized reductive iron uptake system and siderophore uptake system. CaHMX1, encoding a putative haem oxygenase, was shown to be required for iron assimilation from haemin. A double DeltaCahmx1 mutant was constructed. This mutant could not grow with haemin as the sole iron source, although haemin uptake was not affected. The three different iron uptake systems (reductive, siderophore and haemin) were regulated independently and in a complex manner. CaHMX1 expression was induced by iron deprivation, by haemin and by a shift of temperature from 30 to 37 degrees C. CaHMX1 expression was strongly deregulated in a Deltaefg1 mutant but not in a Deltatup1 mutant. C. albicans colonies forming on agar plates with haemin as the sole iron source showed a very unusual morphology. Colonies were made up of tubular structures that were organized into a complex network. The effect of haemin on filamentation was increased in the double DeltaCahmx1 mutant. This study provides the first experimental evidence that haem oxygenase is required for iron assimilation from haem by a pathogenic fungus.

Characterization and functional analysis of the siderophore-iron transporter CaArn1p in Candida albicans

Siderophores are small organic compounds with high affinity for ferric iron. Microorganisms commonly acquire iron via siderophore secretion and uptake. Here we report the characterization of the siderophore transporter CaArn1p in the fungal pathogen Candida albicans. Deletion of CaARN1 reduced the ability of C. albicans to use iron bound to the hydroxamate-type siderophore ferrichrome and abolished it when two high-affinity iron permease genes (CaFTR1 and CaFTR2) were also deleted, indicating a role of CaArn1p as well as the permeases in ferrichrome-iron uptake. Caarn1Delta (but not Caftr1DeltaCaftr2Delta) assimilated iron from another hydroxamate-type siderophore, ferrioxamine B, suggesting that iron uptake from this compound depends on the permeases, but not on CaArn1p. Northern blot analysis revealed that the transcription repressor CaTup1p repressed CaARN1 expression under iron-replete conditions via the DNA-binding protein Rfg1p. Green fluorescent protein-tagged CaArn1p was observed predominantly in the plasma membrane, with some in the cytoplasm as distinct spots. The number of these spots increased with the increase in ferrichrome concentration, suggesting that CaArn1p internalization might be a mechanism for ferrichrome-iron uptake or for recycling the transporter. Caarn1Delta did not show reduced virulence when injected into the blood stream of mice, implying that CaArn1p is not required for iron uptake along this route of infection.

Deletion of the copper transporter CaCCC2 reveals two distinct pathways for iron acquisition in Candida albicans

Efficient iron acquisition is an essential requirement for growth of pathogenic organisms in the iron-poor host environment. In Saccharomyces cerevisiae, high-affinity iron import depends on the multicopper ferroxidase ScFet3. ScFet3 biogenesis in the trans-Golgi compartment requires a copper-transporting P-type ATPase, ScCcc2. Here, we describe the isolation by functional complementation of a Ccc2 homologue from the pathogenic yeast Candida albicans. CaCcc2 is functionally distinct from a previously described C. albicans copper-transporting P-type ATPase, CaCrp1, which appears to be specifically involved in copper detoxification. Regulation of CaCCC2 and the phenotype of the homozygous CaCCC2 deletion indicate that it is required for high-affinity iron import, making it the bona fide CCC2 homologue of C. albicans. Remarkably, in a mouse model of systemic infection, the Caccc2Delta strain displayed robust proliferation and no significant reduction in pathogenicity, suggesting the existence of alternative mechanisms of iron uptake from host tissues. We identify haemin and haemoglobin as potential iron sources that can be used by C. albicans in a CaCcc2-independent manner.

Isolation of a Candida albicans gene, tightly linked to URA3, coding for a putative transcription factor that suppresses a Saccharomyces cerevisiae aft1 mutation

A pathogen such as C. albicans needs an efficient mechanism of iron uptake in an iron-restricted environment such as is the human body. A ferric-reductase activity regulated by iron and copper, and analogous to that in S. cerevisiae, has been described in C. albicans. We have developed an in-plate protocol for the isolation of clones that complement an aft1 mutation in S. cerevisiae that makes cells dependent on iron for growth. After transformation of S. cerevisiae aft1 with a C. albicans library, we have selected clones that grow in conditions of iron deficiency and share an identical plasmid, pIRO1, with a 4500 bp insert containing the URA3 gene and an ORF (IRO1) responsible for the suppression of the iron dependency. IRO1 does not show homology with AFT1 or with other sequences in the databases. Northern analysis demonstrates constitutive expression of IRO1. CAI4, a C. albicans strain isolated as Deltaura3, also has a deletion of the 3' half of IRO1, and displays in YNB medium similar phenotypic characteristics to S. cerevisiae aft1 mutant strains. Therefore, we consider IRO1 as a gene of C. albicans involved in the utilization of iron. However, in extreme conditions of iron deprivation, CAI4 seems to activate alternative mechanisms of iron uptake that allow a better growth than the wild strain SC5314. Analysis of its predicted protein sequence is in agreement with a role of Iro1p as a transcription factor.

Structural requirements for hemoglobin to induce fibronectin receptor expression in Candida albicans

Hemoglobin (Hb) is a host factor that induces expression of a promiscuous receptor on Candida albicans for fibronectin (FN) and several other extracellular matrix proteins. FN receptor expression was induced by ferric (Hb(+)Met and Hb(+)CN), ferrous (HbCO and HbO(2)), and cobalt-protoporphyrin derivatives of Hb, whereas globin was inactive. The Hb derivatives all exhibited saturable, dose-dependent kinetics of FN receptor induction, suggesting that Hb may be acting as a receptor ligand. Soluble Hb bound saturably to a low-affinity binding site [K(d) = (1.1+/-0.2) x 10(-6) M] on C. albicans blastospores. However, uptake of (55)FeHb revealed that heme or iron transport into the cell is not required for induction, since internalization of (55)Fe from Hb did not occur until after induction of FN binding. The serum Hb-binding protein, haptoglobin, specifically abrogated this response, indicating that protein structure rather than the heme ligand or iron is necessary for induction of this signaling pathway. C. albicans also adhered to immobilized Hb, which was sufficient to induce FN receptor expression, and to Hb polymers that formed in defined Hb liquid media in the presence of cells. Formation of Hb polymers in solution required metabolic energy, since the aggregation process was halted with azide addition. Collectively, these data demonstrate that C. albicans recognizes polymerized Hb through multivalent low-affinity interactions, and this may be a host environmental cue that triggers extracellular matrix receptor expression at a septic site.

Candida albicans CFL1 encodes a functional ferric reductase activity that can rescue a Saccharomyces cerevisiae fre1 mutant

Candida albicans, like other pathogens, has to compete with the host for a limited supply of available iron. Consequently, iron acquisition is likely to be an important factor for the growth, survival and virulence of this organism. It was previously demonstrated that C. albicans has a surface-associated ferric reductase similar to that of Saccharomyces cerevisiae. Therefore, functional rescue of a S. cerevisiae fre1 mutant was used to isolate a C. albicans ferric reductase gene (CFL1). This gene has been previously identified. However, the workers had not observed any functional reductase activity associated with the gene. The discrepancy with the findings in this report appears to be due to the clone previously reported carrying a non-contiguous piece of C. albicans DNA. Results shown in this paper demonstrate that CFL1 transcription is regulated in response to levels of iron and copper. This is the first demonstration of a functional ferric reductase gene from C. albicans.

Acquisition, transport, and storage of iron by pathogenic fungi

Iron is required by most living systems. A great variety of means of acquisition, avenues of uptake, and methods of storage are used by pathogenic fungi to ensure a supply of the essential metal. Solubilization of insoluble iron polymers is the first step in iron assimilation. The two methods most commonly used by microorganisms for solubilization of iron are reduction and chelation. Reduction of ferric iron to ferrous iron by enzymatic or nonenzymatic means is a common mechanism among pathogenic yeasts. Under conditions of iron starvation, many fungi synthesize iron chelators known as siderophores. Two classes of compounds that function in iron gathering are commonly observed: hydroxamates and polycarboxylates. Two major responses to iron stress in fungi are a high-affinity ferric iron reductase and siderophore synthesis. Regulation of these two mechanisms at the molecular level has received attention. Uptake of siderophores is a diverse process, which varies among the different classes of compounds. Since free iron is toxic, it must be stored for further metabolic use. Polyphosphates, ferritins, and siderophores themselves have been described as storage molecules. The iron-gathering mechanisms used by a pathogen in an infected host are largely unknown and can only be posited on the basis of in vitro studies at present.

The role of iron in protozoan and fungal infectious diseases

To survive and replicate in vertebrate hosts, protozoan and fungal invaders must be capable of securing host iron. Successful pathogens obtain the metal from either extraction of heme, binding of siderophilins, binding of siderophores, and/or iron pools within host cells. The actual strategy can vary with the availability of iron in the particular host milieu. As a corollary, hosts have developed an elaborate iron withholding defense system. Conditions that can compromise the system as well as procedures that can strengthen it are reviewed.