Msb2 shedding protects Candida albicans against antimicrobial peptides

Development of a novel target-based cell assay, reporter of the activity of Mycobacterium tuberculosis protein-O-mannosyltransferase

The Protein-O-mannosyltransferase is crucial for the virulence of Mycobacterium tuberculosis, the etiological agent of tuberculosis. This enzyme, called MtPMT (Rv1002c), is responsible for the post-translational O-mannosylation of mycobacterial proteins. It catalyzes the transfer of a single mannose residue from a polyprenol phospho-mannosyl lipidic donor to the hydroxyl groups of selected Ser/Thr residues in acceptor proteins during their translocation across the membrane. Previously, we provided evidence that the loss of MtPMT activity causes the absence of mannoproteins in Mycobacterium tuberculosis, severely impacting its intracellular growth, as well as a strong attenuation of its pathogenicity in immunocompromised mice. Therefore, it is of interest to develop specific inhibitors of this enzyme to better understand mycobacterial infectious diseases. Here we report the development of a "target-based" phenotypic assay for this enzyme, assessing its O-mannosyltransferase activity in bacteria, in the non-pathogenic Mycobacterium smegmatis strain. Robustness of the quantitative contribution of this assay was evaluated by intact protein mass spectrometry, using a panel of control strains, overexpressing the MtPMT gene, carrying different key point-mutations. Then, screening of a limited library of 30 compounds rationally chosen allowed us to identify 2 compounds containing pyrrole analogous rings, as significant inhibitors of MtPMT activity, affecting neither the growth of the mycobacterium nor its secretion of mannoproteins. These molecular cores could therefore serve as scaffold for the design of new pharmaceutical agents that could improve treatment of mycobacterial diseases. We report here the implementation of a miniaturized phenotypic activity assay for a glycosyltransferase of the C superfamily.

Mucin Glycans: A Target for Cancer Therapy

Mucin glycans are an important component of the mucus barrier and a vital defence against physical and chemical damage as well as pathogens. There are 20 mucins in the human body, which can be classified into secreted mucins and transmembrane mucins according to their distributions. The major difference between them is that secreted mucins do not have transmembrane structural domains, and the expression of each mucin is organ and cell-specific. Under physiological conditions, mucin glycans are involved in the composition of the mucus barrier and thus protect the body from infection and injury. However, abnormal expression of mucin glycans can lead to the occurrence of diseases, especially cancer, through various mechanisms. Therefore, targeting mucin glycans for the diagnosis and treatment of cancer has always been a promising research direction. Here, we first summarize the main types of glycosylation (O-GalNAc glycosylation and N-glycosylation) on mucins and the mechanisms by which abnormal mucin glycans occur. Next, how abnormal mucin glycans contribute to cancer development is described. Finally, we summarize MUC1-based antibodies, vaccines, radio-pharmaceuticals, and CAR-T therapies using the best characterized MUC1 as an example. In this section, we specifically elaborate on the recent new cancer therapy CAR-M, which may bring new hope to cancer patients.

Fungal resilience and host-pathogen interactions: Future perspectives and opportunities

We are constantly exposed to the threat of fungal infection. The outcome-clearance, commensalism or infection-depends largely on the ability of our innate immune defences to clear infecting fungal cells versus the success of the fungus in mounting compensatory adaptive responses. As each seeks to gain advantage during these skirmishes, the interactions between host and fungal pathogen are complex and dynamic. Nevertheless, simply compromising the physiological robustness of fungal pathogens reduces their ability to evade antifungal immunity, their virulence, and their tolerance against antifungal therapy. In this article I argue that this physiological robustness is based on a 'Resilience Network' which mechanistically links and controls fungal growth, metabolism, stress resistance and drug tolerance. The elasticity of this network probably underlies the phenotypic variability of fungal isolates and the heterogeneity of individual cells within clonal populations. Consequently, I suggest that the definition of the fungal Resilience Network represents an important goal for the future which offers the clear potential to reveal drug targets that compromise drug tolerance and synergise with current antifungal therapies.

Rationally designed antifungal protein chimeras reveal new insights into structure-activity relationship

Antifungal proteins (AFPs) are promising antimicrobial compounds that represent a feasible alternative to fungicides. Penicillium expansum encodes three phylogenetically distinct AFPs (PeAfpA, PeAfpB and PeAfpC) which show different antifungal profiles and fruit protection effects. To gain knowledge about the structural determinants governing their activity, we solved the crystal structure of PeAfpB and rationally designed five PeAfpA::PeAfpB chimeras (chPeAFPV1-V5). Chimeras showed significant differences in their antifungal activity. chPeAFPV1 and chPeAFPV2 improved the parental PeAfpB potency, and it was very similar to that of PeAfpA. chPeAFPV4 and chPeAFPV5 showed an intermediate profile of activity compared to the parental proteins while chPeAFPV3 was inactive towards most of the fungi tested. Structural analysis of the chimeras evidenced an identical scaffold to PeAfpB, suggesting that the differences in activity are due to the contributions of specific residues and not to induced conformational changes or structural rearrangements. Results suggest that mannoproteins determine protein interaction with the cell wall and its antifungal activity while there is not a direct correlation between binding to membrane phospholipids and activity. This work provides new insights about the relevance of sequence motifs and the feasibility of modifying protein specificity, opening the door to the rational design of chimeras with biotechnological applicability.

Candida albicans can foster gut dysbiosis and systemic inflammation during HIV infection

Candida albicans (C. albicans) is a ubiquitous fungal commensal component of the human microbiota, and under certain circumstances, such as during an immunocompromised state, it may initiate different types of infection. Moreover, C. albicans continuously and reciprocally interacts with the host immune system as well as with other elements of the gut microbiota, thus contributing significantly to both gut homeostasis and host immunity. People living with HIV (PLWH), including those receiving antiretroviral therapy, are characterized by a depletion of CD4 + T-cells and dysbiosis in their gut. C. albicans colonization is frequent in PLWH, causing both a high prevalence and high morbidity. Gut barrier damage and elevated levels of microbial translocation are also fairly common in this population. Herein, we take a closer look at the reciprocity among C. albicans, gut microbiota, HIV, and the host immune system, thus throwing some light on this complex interplay.

Histatin 5 Metallopeptides and Their Potential against Candida albicans Pathogenicity and Drug Resistance

Usually caused by Candida albicans, buccal candidiasis begins with the morphological transition between yeast and hyphal cells. Over time and without the correct treatment, it can be disseminated through the bloodstream becoming a systemic infection with high mortality rates. C. albicans already shows resistance against antifungals commonly used in treatments. Therefore, the search for new drugs capable of overcoming antifungal resistance is essential. Histatin 5 (Hst5) is an antimicrobial peptide of the Histatin family, that can be found naturally in human saliva. This peptide presents high antifungal activity against C. albicans. However, Hst5 action can be decreased for interaction with enzymes and metal ions present in the oral cavity. The current work aims to bring a brief review of relevant aspects of the pathogenesis and resistance mechanisms already reported for C. albicans. In addition, are also reported here the main immune responses of the human body and the most common antifungal drugs. Finally, the most important aspects regarding Histatin 5 and the benefits of its interaction with metals are highlighted. The intention of this review is to show the promising use of Hst5 metallopeptides in the development of effective drugs.

Virulence Factors in Sporothrix schenckii, One of the Causative Agents of Sporotrichosis

Sporothrix schenckii is one of the etiological agents of sporotrichosis, a fungal infection distributed worldwide. Both, the causative organism and the disease have currently received limited attention by the medical mycology community, most likely because of the low mortality rates associated with it. Nonetheless, morbidity is high in endemic regions and the versatility of S. schenckii to cause zoonosis and sapronosis has attracted attention. Thus far, virulence factors associated with this organism are poorly described. Here, comparing the S. schenckii genome sequence with other medically relevant fungi, genes involved in morphological change, cell wall synthesis, immune evasion, thermotolerance, adhesion, biofilm formation, melanin production, nutrient uptake, response to stress, extracellular vesicle formation, and toxin production are predicted and discussed as putative virulence factors in S. schenckii.

Yeast biofilm in food realms: occurrence and control

In natural environments, microorganisms form microbial aggregates called biofilms able to adhere to a multitude of different surfaces. Yeasts make no exception to this rule, being able to form biofilms in a plethora of environmental niches. In food realms, yeast biofilms may cause major problems due to their alterative activities. In addition, yeast biofilms are tenacious structures difficult to eradicate or treat with the current arsenal of antifungal agents. Thus, much effort is being made to develop novel approaches to prevent and disrupt yeast biofilms, for example through the use of natural antimicrobials or small molecules with both inhibiting and dispersing properties. The aim of this review is to provide a synopsis of the most recent literature on yeast biofilms regarding: (i) biofilm formation mechanisms; (ii) occurrence in food and in food-related environments; and (iii) inhibition and dispersal using natural compounds, in particular.

Bio- and Nanotechnology as the Key for Clinical Application of Salivary Peptide Histatin: A Necessary Advance

Candida albicans is a common microorganism of human’s microbiota and can be easily found in both respiratory and gastrointestinal tracts as well as in the genitourinary tract. Approximately 30% of people will be infected by C. albicans during their lifetime. Due to its easy adaptation, this microorganism started to present high resistance to antifungal agents which is associated with their indiscriminate use. There are several reports of adaptive mechanisms that this species can present. Some of them are intrinsic alteration in drug targets, secretion of extracellular enzymes to promote host protein degradation and efflux receptors that lead to a diminished action of common antifungal and host’s innate immune response. The current review aims to bring promising alternatives for the treatment of candidiasis caused mainly by C. albicans. One of these alternatives is the use of antifungal peptides (AFPs) from the Histatin family, like histatin-5. Besides that, our focus is to show how nanotechnology can allow the application of these peptides for treatment of this microorganism. In addition, our intention is to show the importance of nanoparticles (NPs) for this purpose, which may be essential in the near future.

Expression of genes containing tandem repeat patterns involved in the fungal-host interaction and in the response to antifungals in Trichophyton rubrum

BACKGROUND

Trichophyton rubrum is the most common aetiological agent of human dermatophytoses. These infections mainly occur in keratinised layers such as skin, hair and nails because the fungus uses keratin as a nutrient source. Fluconazole and amphotericin are antifungal agents most commonly used to treat dermatophytoses and acts on cell membrane ergosterol. Despite the clinical importance of T rubrum, the mechanisms underlying the fungal-host relationship have not yet been clarified. Tandem repeats (TRs) are short DNA sequences that are involved in a variety of adaptive functions, including the process of fungal infection. It is known that the larger the number of TRs in the genome, the greater the capacity of cell-cell junction and surface adhesion, especially when these repeats are present in regions encoding cell surface proteins.

OBJECTIVES

To identify in silico T rubrum genes containing TR patterns and to analyse the modulation of these genes in culture medium containing keratin (a model simulating skin infection) and antifungal drugs.

METHODS

The Dermatophyte Tandem Repeats Database (DTRDB) and the FaaPred tool were used to identify four T rubrum genes containing TR patterns. Quantitative real-time (RT) PCR was used to evaluate the gene expression during the growth of T rubrum on keratin and in the presence of fluconazole, amphotericin B and Congo red (acts in the cell wall).

RESULTS

The expression of these genes was found to be induced in culture medium containing keratin. In addition, these genes were induced in the presence of antifungal agents, especially fluconazole, indicating an adaptive response to the stress caused by this drug.

CONCLUSION

The results suggest an important role of genes containing TRs in the fungal-host interaction and in the susceptibility to inhibitory compounds, indicating these sequences as new potential targets for the development of antifungal agents.

New Insights in Candida albicans Innate Immunity at the Mucosa: Toxins, Epithelium, Metabolism, and Beyond

The mucosal surfaces of the human body are challenged by millions of microbes on a daily basis. Co-evolution with these microbes has led to the development of plastic mechanisms in both host and microorganisms that regulate the balance between preserving beneficial microbes and clearing pathogens. Candida albicans is a fungal pathobiont present in most healthy individuals that, under certain circumstances, can become pathogenic and cause everything from mild mucosal infections to life-threatening systemic diseases. As an essential part of the innate immunity in mucosae, epithelial cells elaborate complex immune responses that discriminate between commensal and pathogenic microbes, including C. albicans. Recently, several significant advances have been made identifying new pieces in the puzzle of host-microbe interactions. This review will summarize these advances in the context of our current knowledge of anti-Candida mucosal immunity, and their impact on epithelial immune responses to this fungal pathogen.

Mucins and Pathogenic Mucin-Like Molecules Are Immunomodulators During Infection and Targets for Diagnostics and Vaccines

Mucins and mucin-like molecules are highly O-glycosylated proteins present on the cell surface of mammals and other organisms. These glycoproteins are highly diverse in the apoprotein and glycan cores and play a central role in many biological processes and diseases. Mucins are the most abundant macromolecules in mucus and are responsible for its biochemical and biophysical properties. Mucin-like molecules cover various protozoan parasites, fungi and viruses. In humans, modifications in mucin glycosylation are associated with tumors in epithelial tissue. These modifications allow the distinction between normal and abnormal cell conditions and represent important targets for vaccine development against some cancers. Mucins and mucin-like molecules derived from pathogens are potential diagnostic markers and targets for therapeutic agents. In this review, we summarize the distribution, structure, role as immunomodulators, and the correlation of human mucins with diseases and perform a comparative analysis of mucins with mucin-like molecules present in human pathogens. Furthermore, we review the methods to produce pathogenic and human mucins using chemical synthesis and expression systems. Finally, we present applications of mucin-like molecules in diagnosis and prevention of relevant human diseases.

The administration of L-cysteine and L-arginine inhibits biofilm formation in wild-type biofilm-forming yeast by modulating FLO11 gene expression

Microbial biofilms are undesired in food manufacturing, drinking water distribution systems, and clinical realms. Yeast biofilms are particularly problematic because of the strong capacity of yeast cells to adhere to abiotic surfaces, cells, and tissues. Novel approaches have been developed over recent years to prevent the establishment of microbial biofilms, such as through the use of small molecules with inhibiting and dispersing properties. Here, we studied the inhibitory activity of 11 different amino acids on the biofilm formation ability of three wild-type Saccharomyces cerevisiae strains and the reference strain ∑1278b. Subsequent evaluation of different concentrations of the two most effective amino acids, namely, arginine and cysteine, revealed that they acted in different ways. Arginine prevented biofilm formation by reducing FLO11 gene expression; its addition did not affect cell viability and was even found to enhance cell metabolism (vitality marker) as determined by phenotype microarray (PM) analysis. On the contrary, the addition of cysteine reduced both cell viability and vitality as well as FLO11 expression. Thus, the use of cysteine and arginine as agents against biofilm formation can be diversified depending on the most desired action towards yeast growth.

The Aspergillus fumigatus Mucin MsbA Regulates the Cell Wall Integrity Pathway and Controls Recognition of the Fungus by the Immune System

Aspergillus fumigatus is a filamentous fungus which causes invasive pulmonary aspergillosis in immunocompromised individuals. In fungi, cell signaling and cell wall plasticity are crucial for maintaining physiologic processes. In this context, Msb2 is an important signaling mucin responsible for activation of a variety of mitogen-activated protein kinase (MAPK)-dependent signaling pathways that regulate cell growth in several organisms, such as the cell wall integrity (CWI) pathway. Here, we aimed to characterize the MSB2 homologue in A. fumigatus Our results showed that MsbA plays a role in the vegetative and reproductive development of the fungus, in stress adaptation, and in resistance to antifungal drugs by modulating the CWI pathway gene expression. Importantly, cell wall composition is also responsible for activation of diverse receptors of the host immune system, thus leading to a proper immune response. In a model of acute Aspergillus pulmonary infection, results demonstrate that the ΔmsbA mutant strain induced less inflammation with diminished cell influx into the lungs and lower cytokine production, culminating in increased lethality rate. These results characterize for the first time the role of the signaling mucin MsbA in the pathogen A. fumigatus, as a core sensor for cell wall morphogenesis and an important regulator of virulence.IMPORTANCE Aspergillus fumigatus is an opportunistic fungus with great medical importance. During infection, Aspergillus grows, forming hyphae that colonize the lung tissue and invade and spread over the mammal host, resulting in high mortality rates. The knowledge of the mechanisms responsible for regulation of fungal growth and virulence comprises an important point to better understand fungal physiology and host-pathogen interactions. Msb2 is a mucin that acts as a sensor and an upstream regulator of the MAPK pathway responsible for fungal development in Candida albicans and Aspergillus nidulans Here, we show the role of the signaling mucin MsbA in the pathogen A. fumigatus, as a core sensor for cell wall morphogenesis, fungal growth, and virulence. Moreover, we show that cell wall composition, controlled by MsbA, is detrimental for fungal recognition and clearance by immune cells. Our findings are important for the understanding of how fungal sensors modulate cell physiology.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Candida albicans at Host Barrier Sites: Pattern Recognition Receptors and Beyond

Over the last decades, fungal infections have emerged as a growing threat to human health. Although the human body is at potential risk, various body sites host several commensal fungal species, including Candida albicans. In healthy individuals, C. albicans colonizes different mucosal surfaces without causing harm, while under diverse circumstances the fungus can proliferate and cause disease. In this context, the understanding of host⁻C. albicans interactions in health and during infection may lead to novel therapeutic approaches. Importantly, host cells express pattern recognition receptors (PRRs), which sense conserved fungal structures and orchestrate innate immune responses. Herein, important findings on the topic of the recognition of C. albicans at host barrier sites are discussed. This review briefly summarizes the importance and functions of myeloid PRRs, reviews the fungal recognition and biology of stromal cells, and highlights important C. albicans virulence attributes during site-specific proliferation and invasion.

Nutrient and Stress Sensing in Pathogenic Yeasts

More than 1.5 million fungal species are estimated to live in vastly different environmental niches. Despite each unique host environment, fungal cells sense certain fundamentally conserved elements, such as nutrients, pheromones and stress, for adaptation to their niches. Sensing these extracellular signals is critical for pathogens to adapt to the hostile host environment and cause disease. Hence, dissecting the complex extracellular signal-sensing mechanisms that aid in this is pivotal and may facilitate the development of new therapeutic approaches to control fungal infections. In this review, we summarize the current knowledge on how two important pathogenic yeasts, Candida albicans and Cryptococcus neoformans, sense nutrient availability, such as carbon sources, amino acids, and ammonium, and different stress signals to regulate their morphogenesis and pathogenicity in comparison with the non-pathogenic model yeast Saccharomyces cerevisiae. The molecular interactions between extracellular signals and their respective sensory systems are described in detail. The potential implication of analyzing nutrient and stress-sensing systems in antifungal drug development is also discussed.

Sho1 and Msb2 Play Complementary but Distinct Roles in Stress Responses, Sexual Differentiation, and Pathogenicity of Cryptococcus neoformans

The high-osmolarity glycerol response (HOG) pathway is pivotal in environmental stress response, differentiation, and virulence of Cryptococcus neoformans, which causes fatal meningoencephalitis. A putative membrane sensor protein, Sho1, has been postulated to regulate HOG pathway, but its regulatory mechanism remains elusive. In this study, we characterized the function of Sho1 with relation to the HOG pathway in C. neoformans. Sho1 played minor roles in osmoresistance, thermotolerance, and maintenance of membrane integrity mainly in a HOG-independent manner. However, it was dispensable for cryostress resistance, primarily mediated through the HOG pathway. A mucin-like transmembrane (TM) protein, Msb2, which interacts with Sho1 in Saccharomyces cerevisiae, was identified in C. neoformans, but found not to interact with Sho1. MSB2 codeletion with SHO1 further decreased osmoresistance and membrane integrity, but not thermotolerance, of sho1Δ mutant, indicating that both factors play to some level redundant but also discrete roles in C. neoformans. Sho1 and Msb2 played redundant roles in promoting the filamentous growth in sexual differentiation in a Cpk1-independent manner, in contrast to the inhibitory effect of the HOG pathway in the process. However, both factors contributed independently to Cpk1 phosphorylation during vegetative growth and endoplasmic reticulum (ER) stress response. Finally, Sho1 and Msb2 play distinct but complementary roles in the pulmonary virulence of C. neoformans. Overall, Sho1 and Msb2 play complementary but distinct roles in stress response, differentiation, and pathogenicity of C. neoformans.

Propeptide genesis by Kex2-dependent cleavage of yeast wall protein 1 (Ywp1) of Candida albicans

Candida albicans is a prevalent fungal resident and opportunistic pathogen of humans, exhibiting a variety of ovoid and filamentous morphologies. Anchored within the cell wall of the ovoid yeast form of C. albicans is an abundant glycoprotein termed yeast wall protein 1 (Ywp1). Ywp1 has an antiadhesive effect that may facilitate yeast cell dispersal; it also contributes to the masking of the glucan matrix of the yeast cell wall, potentially providing shielding from recognition by the human immune system. Mature Ywp1 consists of an O-glycosylated core of 378 amino acids associated with an N-glycosylated propeptide that originates from an N-terminal segment of Ywp1. A tribasic (-RRR-) sequence in the immature Ywp1 polypeptide is separated by 8 amino acids from a dibasic (-KR-) sequence that is a canonical site for cleavage by the intracellular endopeptidase Kex2, and cleavage occurs at both of these sites to generate an 11 kilodalton (kDa) propeptide that remains strongly associated with the mature core of Ywp1. Previous studies demonstrated an absence of the 11 kDa propeptide in strains lacking Kex2, but the presence of lesser amounts of a 12 kDa propeptide ostensibly (and paradoxically) arising from cleavage at the dibasic site. Subsequent studies of wild type strains, however, suggested that post-secretion cleavages were carried out in vitro by acid proteases in unbuffered cultures to generate the 12 kDa propeptide. Here, intact and Gfp-tagged Ywp1 are utilized to show that neither of the two multibasic sites is normally cleaved in the absence of Kex2, but that uncleaved Ywp1 is still N-glycosylated and subsequently anchored to the cell wall. This furthers our understanding of the multistep cleavage of this highly conserved sequence, as well as the possible contributions of the cleaved propeptide to the maturation and functioning of Ywp1.

A Genome-Wide Screen of Deletion Mutants in the Filamentous Saccharomyces cerevisiae Background Identifies Ergosterol as a Direct Trigger of Macrophage Pyroptosis

Phagocytic cells such as macrophages play an important role in the host defense mechanisms mounted in response to the common human fungal pathogen Candida albicans. In vitro, C. albicans triggers macrophage NLRP3-Casp1/11-mediated pyroptosis, an inflammatory programmed cell death pathway. Here, we provide evidence that Casp1/11-dependent pyroptosis occurs in the kidney of infected mice during the early stages of infection. We have also used a genome-wide screen of nonessential Σ1278b Saccharomyces cerevisiae genes to identify genes required for yeast-triggered macrophage pyroptosis. The set of genes identified by this screen was enriched for those with functions in lipid and sterol homeostasis and trafficking. These observations led us to discover that cell surface localization and/or total levels of ergosterol correlate with the ability of S. cerevisiae, C. albicans, and Cryptococcus neoformans to trigger pyroptosis. Since the mammalian sterol cholesterol triggers NLRP3-mediated pyroptosis, we hypothesized that ergosterol may also do so. Consistent with that hypothesis, ergosterol-containing liposomes but not ergosterol-free liposomes induce pyroptosis. Cell wall mannoproteins directly bind ergosterol, and we found that Dan1, an ergosterol receptor mannoprotein, as well as specific mannosyltransferases, is required for pyroptosis, suggesting that cell wall-associated ergosterol may mediate the process. Taken together, these data indicate that ergosterol, like mammalian cholesterol, plays a direct role in yeast-mediated pyroptosis.

Innate immune cells such as macrophages are key components of the host response to the human fungal pathogen Candida albicans. Macrophages undergo pyroptosis, an inflammatory, programmed cell death, in response to some species of pathogenic yeast. Prior to the work described in this report, yeast-triggered pyroptosis has been observed only in vitro; here, we show that pyroptosis occurs in the initial stages of murine kidney infection, suggesting that it plays an important role in the initial response of the innate immune system to invasive yeast infection. We also show that a key component of the fungal plasma membrane, ergosterol, directly triggers pyroptosis. Ergosterol is also present in the fungal cell wall, most likely associated with mannoproteins, and is increased in hyphal cells compared to yeast cells. Our data indicate that specific mannoproteins are required for pyroptosis. This is consistent with a potential mechanism whereby ergosterol present in the outer mannoprotein layer of the cell wall is accessible to the macrophage-mediated process. Taken together, our data provide the first evidence that ergosterol plays a direct role in the host-pathogen interactions of fungi.

Thigmo Responses: The Fungal Sense of Touch

The growth and development of most fungi take place on a two-dimensional surface or within a three-dimensional matrix. The fungal sense of touch is therefore critical for fungi in the interpretation of their environment and often signals the switch to a new developmental state. Contact sensing, or thigmo-based responses, include thigmo differentiation, such as the induction of invasion structures by plant pathogens in response to topography; thigmonasty, where contact with a motile prey rapidly triggers its capture; and thigmotropism, where the direction of hyphal growth is guided by physical features in the environment. Like plants and some bacteria, fungi grow as walled cells. Despite the well-demonstrated importance of thigmo responses in numerous stages of fungal growth and development, it is not known how fungal cells sense contact through the relatively rigid structure of the cell wall. However, while sensing mechanisms at the molecular level are not entirely understood, the downstream signaling pathways that are activated by contact sensing are being elucidated. In the majority of cases, the response to contact is complemented by chemical cues and both are required, either sequentially or simultaneously, to elicit normal developmental responses. The importance of a sense of touch in the lifestyles and development of diverse fungi is highlighted in this review, and the candidate molecular mechanisms that may be involved in fungal contact sensing are discussed.

Innate Immunity to Mucosal Candida Infections

Mucosal epithelial tissues are exposed to high numbers of microbes, including commensal fungi, and are able to distinguish between those that are avirulent and those that cause disease. Epithelial cells have evolved multiple mechanisms to defend against colonization and invasion by Candida species. The interplay between mucosal epithelial tissues and immune cells is key for control and clearance of fungal infections. Our understanding of the mucosal innate host defense system has expanded recently with new studies bringing to light the importance of epithelial cell responses, innate T cells, neutrophils, and other phagocytes during Candida infections. Epithelial tissues release cytokines, host defense peptides, and alarmins during Candida invasion that act in concert to limit fungal proliferation and recruit immune effector cells. The innate T cell/IL-17 axis and recruitment of neutrophils are of central importance in controlling mucosal fungal infections. Here, we review current knowledge of the innate immunity at sites of mucosal Candida infection, with a focus on infections caused by C. albicans.

Development and regulation of single- and multi-species Candida albicans biofilms

Candida albicans is among the most prevalent fungal species of the human microbiota and asymptomatically colonizes healthy individuals. However, it is also an opportunistic pathogen that can cause severe, and often fatal, bloodstream infections. The medical impact of C. albicans typically depends on its ability to form biofilms, which are closely packed communities of cells that attach to surfaces, such as tissues and implanted medical devices. In this Review, we provide an overview of the processes involved in the formation of C. albicans biofilms and discuss the core transcriptional network that regulates biofilm development. We also consider some of the advantages that biofilms provide to C. albicans in comparison with planktonic growth and explore polymicrobial biofilms that are formed by C. albicans and certain bacterial species.

An acquired mechanism of antifungal drug resistance simultaneously enables Candida albicans to escape from intrinsic host defenses

The opportunistic fungal pathogen Candida albicans frequently produces genetically altered variants to adapt to environmental changes and new host niches in the course of its life-long association with the human host. Gain-of-function mutations in zinc cluster transcription factors, which result in the constitutive upregulation of their target genes, are a common cause of acquired resistance to the widely used antifungal drug fluconazole, especially during long-term therapy of oropharyngeal candidiasis. In this study, we investigated if C. albicans also can develop resistance to the antimicrobial peptide histatin 5, which is secreted in the saliva of humans to protect the oral mucosa from pathogenic microbes. As histatin 5 has been shown to be transported out of C. albicans cells by the Flu1 efflux pump, we screened a library of C. albicans strains that contain artificially activated forms of all zinc cluster transcription factors of this fungus for increased FLU1 expression. We found that a hyperactive Mrr1, which confers fluconazole resistance by upregulating the multidrug efflux pump MDR1 and other genes, also causes FLU1 overexpression. Similarly to the artificially activated Mrr1, naturally occurring gain-of-function mutations in this transcription factor also caused FLU1 upregulation and increased histatin 5 resistance. Surprisingly, however, Mrr1-mediated histatin 5 resistance was mainly caused by the upregulation of MDR1 instead of FLU1, revealing a previously unrecognized function of the Mdr1 efflux pump. Fluconazole-resistant clinical C. albicans isolates with different Mrr1 gain-of-function mutations were less efficiently killed by histatin 5, and this phenotype was reverted when MRR1 was deleted. Therefore, antimycotic therapy can promote the evolution of strains that, as a consequence of drug resistance mutations, simultaneously have acquired increased resistance against an innate host defense mechanism and are thereby better adapted to certain host niches.

The yeast Candida albicans is part of the normal microflora of most healthy persons, but it can also cause symptomatic infections when host defenses are compromised. C. albicans frequently generates genetically altered variants that are better adapted to changes in its environment during colonization and infection. We investigated if C. albicans can evolve resistance to histatin 5 (Hst 5), an antimicrobial peptide that is produced in the saliva of humans and protects the oral cavity against this pathogen. We found that activated forms of the transcription factor Mrr1 reduce the susceptibility of C. albicans to killing by Hst 5, a phenotype that was partially caused by Mrr1-mediated overexpression of the multidrug efflux pump MDR1. Gain-of-function (GOF) mutations in Mrr1 are a frequent cause of resistance to the antifungal drug fluconazole, especially during long-term treatment of oropharyngeal candidiasis in AIDS patients, but they may also reduce the fitness of the fungus in the absence of the drug. Fluconazole-resistant clinical C. albicans isolates containing GOF mutations in Mrr1 displayed enhanced Hst 5 resistance, demonstrating that antimycotic therapy can promote the evolution of strains that simultaneously have acquired increased resistance against an innate host defense mechanism and are thereby better adapted to specific host niches.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

Functional analysis of the MAPK pathways in fungi

The Mitogen-Activated Protein Kinase (MAPK) signaling pathways constitute one of the most important and evolutionarily conserved mechanisms for the perception of extracellular information in all the eukaryotic organisms. The MAPK pathways are involved in the transfer to the cell of the information perceived from extracellular stimuli, with the final outcome of activation of different transcription factors that regulate gene expression in response to them. In all species of fungi, the MAPK pathways have important roles in their physiology and development; e.g. cell cycle control, mating, morphogenesis, response to different stresses, resistance to UV radiation and to temperature changes, cell wall assembly and integrity, degradation of cellular organelles, virulence, cell-cell signaling, fungus-plant interaction, and response to damage-associated molecular patterns (DAMPs). Considering the importance of the phylogenetically conserved MAPK pathways in fungi, an updated review of the knowledge on them is discussed in this article. This information reveals their importance, their distribution in fungal species evolutionarily distant and with different lifestyles, their organization and function, and the interactions occurring between different MAPK pathways, and with other signaling pathways, for the regulation of the most complex cellular processes.

Nutrient Sensing at the Plasma Membrane of Fungal Cells

To respond to the changing environment, cells must be able to sense external conditions. This is important for many processes including growth, mating, the expression of virulence factors, and several other regulatory effects. Nutrient sensing at the plasma membrane is mediated by different classes of membrane proteins that activate downstream signaling pathways: nontransporting receptors, transceptors, classical and nonclassical G-protein-coupled receptors, and the newly defined extracellular mucin receptors. Nontransporting receptors have the same structure as transport proteins, but have lost the capacity to transport while gaining a receptor function. Transceptors are transporters that also function as a receptor, because they can rapidly activate downstream signaling pathways. In this review, we focus on these four types of fungal membrane proteins. We mainly discuss the sensing mechanisms relating to sugars, ammonium, and amino acids. Mechanisms for other nutrients, such as phosphate and sulfate, are discussed briefly. Because the model yeast Saccharomyces cerevisiae has been the most studied, especially regarding these nutrient-sensing systems, each subsection will commence with what is known in this species.

Signalling mucin Msb2 Regulates adaptation to thermal stress in Candida albicans

Temperature is a potent inducer of fungal dimorphism. Multiple signalling pathways control the response to growth at high temperature, but the sensors that regulate these pathways are poorly defined. We show here that the signalling mucin Msb2 is a global regulator of temperature stress in the fungal pathogen Candida albicans. Msb2 was required for survival and hyphae formation at 42°C. The cytoplasmic signalling domain of Msb2 regulated temperature-dependent activation of the CEK mitogen activated proteins kinase (MAPK) pathway. The extracellular glycosylated domain of Msb2 (100-900 amino acid residues) had a new and unexpected role in regulating the protein kinase C (PKC) pathway. Msb2 also regulated temperature-dependent induction of genes encoding regulators and targets of the unfolded protein response (UPR), which is a protein quality control (QC) pathway in the endoplasmic reticulum that controls protein folding/degradation in response to high temperature and other stresses. The heat shock protein and cell wall component Ssa1 was also required for hyphae formation and survival at 42°C and regulated the CEK and PKC pathways.

Flor Yeast: New Perspectives Beyond Wine Aging

The most important dogma in white-wine production is the preservation of the wine aroma and the limitation of the oxidative action of oxygen. In contrast, the aging of Sherry and Sherry-like wines is an aerobic process that depends on the oxidative activity of flor strains of Saccharomyces cerevisiae. Under depletion of nitrogen and fermentable carbon sources, these yeast produce aggregates of floating cells and form an air–liquid biofilm on the wine surface, which is also known as velum or flor. This behavior is due to genetic and metabolic peculiarities that differentiate flor yeast from other wine yeast. This review will focus first on the most updated data obtained through the analysis of flor yeast with -omic tools. Comparative genomics, proteomics, and metabolomics of flor and wine yeast strains are shedding new light on several features of these special yeast, and in particular, they have revealed the extent of proteome remodeling imposed by the biofilm life-style. Finally, new insights in terms of promotion and inhibition of biofilm formation through small molecules, amino acids, and di/tri-peptides, and novel possibilities for the exploitation of biofilm immobilization within a fungal hyphae framework, will be discussed.

Adaptation of Candida albicans to commensalism in the gut

Candida albicans is a common resident of the oral cavity, GI tract and vagina in healthy humans where it establishes a commensal relationship with the host. Colonization of the gut, which is an important niche for the microbe, may lead to systemic dissemination and disease upon alteration of host defences. Understanding the mechanisms responsible for the adaptation of C. albicans to the gut is therefore important for the design of new ways of combating fungal diseases. In this review we discuss the available models to study commensalism of this yeast, the main mechanisms controlling the establishment of the fungus, such as microbiota, mucus layer and antimicrobial peptides, and the gene regulatory circuits that ensure its survival in this niche.

Complementary roles of the Cek1 and Cek2 MAP kinases in Candida albicans cell-wall biogenesis

AIMS

To investigate whether Cek2 (as Cek1) is involved in the biogenesis of the fungal cell wall and to uncover differences and similitudes between both proteins.

MATERIALS & METHODS

We used molecular genetics to characterize the role of Cek2 in MTL-heterozygous cells.

RESULTS

Deletion of CEK2 (similar to CEK1) renders cells sensitive to cell-wall-interfering drugs and, when overexpressed, Cek2 can become phosphorylated upon the same stimuli that activate Cek1. This is dependent on elements of the sterile-vegetative growth (SVG) pathway. Cek2, contrary to Cek1, mediates a transcriptional activity in a C. albicans-adapted two-hybrid system that is essential for Cek1-Cek2 interaction.

CONCLUSION

Cek2 has a cryptic role in cell-wall biogenesis and its role is not entirely redundant to Cek1.

Candida albicans Shed Msb2 and Host Mucins Affect the Candidacidal Activity of Salivary Hst 5

Salivary Histatin 5 (Hst 5) is an antimicrobial peptide that exhibits potent antifungal activity towards Candida albicans, the causative agent of oral candidiasis. However, it exhibits limited activity in vivo, largely due to inactivation by salivary components of both host and pathogen origin. Proteins secreted by C. albicans during infection such as secreted aspartyl proteases (Saps) and shed mucin Msb2 can reduce Hst 5 activity; and human salivary mucins, while suggested to protect Hst 5 from proteolytic degradation, can entrap peptides into mucin gels, thereby reducing bioavailability. We show here that Sap6 that is secreted during hyphal growth reduces Hst 5 activity, most likely a result of proteolytic degradation of Hst 5 since this effect is abrogated with heat inactivated Sap 6. We further show that just like C. albicans shedding Msb2, mammalian mucins, fetuin and porcine gut mucin (that is related to salivary mucins), also reduce Hst 5 activity. However, we identify mucin-like protein-induced changes in C. albicans cell morphology and aggregation patterns, suggesting that the effect of such proteins on Hst 5 cannot be interpreted independently of their effect on yeast cells.

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 Penicillium digitatum protein O-mannosyltransferase Pmt2 is required for cell wall integrity, conidiogenesis, virulence and sensitivity to the antifungal peptide PAF26

The activity of protein O-mannosyltransferases (Pmts) affects the morphogenesis and virulence of fungal pathogens. Recently, PMT genes have been shown to determine the sensitivity of Saccharomyces cerevisiae to the antifungal peptide PAF26. This study reports the identification and characterization of the three Pdpmt genes in the citrus post-harvest pathogen Penicillium digitatum. The Pdpmt genes are expressed during fungal growth and fruit infection, with the highest induction for Pdpmt2. Pdpmt2 complemented the growth defect of the S. cerevisiae Δpmt2 strain. The Pdpmt2 gene mutation in P. digitatum caused pleiotropic effects, including a reduction in fungal growth and virulence, whereas its constitutive expression had no phenotypic effect. The Pdpmt2 null mutants also showed a distinctive colourless phenotype with a strong reduction in the number of conidia, which was associated with severe alterations in the development of conidiophores. Additional effects of the Pdpmt2 mutation were hyphal morphological alterations, increased sensitivity to cell wall-interfering compounds and a blockage of invasive growth. In contrast, the Pdpmt2 mutation increased tolerance to oxidative stress and to the antifungal activity of PAF26. These data confirm the role of protein O-glycosylation in the PAF26-mediated antifungal mechanism present in distantly related fungal species. Important to future crop protection strategies, this study demonstrates that a mutation rendering fungi more resistant to an antifungal peptide results in severe deleterious effects on fungal growth and virulence.

Host-pathogen interactions between the human innate immune system and Candida albicans-understanding and modeling defense and evasion strategies

The diploid, polymorphic yeast Candida albicans is one of the most important human pathogenic fungi. C. albicans can grow, proliferate and coexist as a commensal on or within the human host for a long time. However, alterations in the host environment can render C. albicans virulent. In this review, we describe the immunological cross-talk between C. albicans and the human innate immune system. We give an overview in form of pairs of human defense strategies including immunological mechanisms as well as general stressors such as nutrient limitation, pH, fever etc. and the corresponding fungal response and evasion mechanisms. Furthermore, Computational Systems Biology approaches to model and investigate these complex interactions are highlighted with a special focus on game-theoretical methods and agent-based models. An outlook on interesting questions to be tackled by Systems Biology regarding entangled defense and evasion mechanisms is given.

Activation of the signalling mucin MoMsb2 and its functional relationship with Cbp1 in Magnaporthe oryzae

Various surface signals are recognized by Magnaporthe oryzae to activate the Pmk1 MAP kinase that is essential for appressorium formation and invasive growth. One of upstream sensors of the Pmk1 pathway is the MoMsb2 signalling mucin. However, the activation of MoMsb2 and its relationship with other sensors is not clear. In this study, we showed that the cleavage and transmembrane domains are essential for MoMsb2 functions. Cleavage of MoMsb2 was further confirmed by western blot analysis, and five putative cleavage sites were functionally characterized. Expression of the extracellular region alone partially rescued the defects of Momsb2 in appressorium formation and virulence. The cytoplasmic region of MoMsb2, although dispensable for appressorium formation, was more important for penetration and invasive growth. Interestingly, the Momsb2 cbp1 double mutant deleted of both mucin genes was blocked in Pmk1 activation. It failed to form appressoria on artificial surfaces and was non-pathogenic. In addition, we showed that MoMsb2 interacts with Ras2 but not with MoCdc42 in co-immunoprecipitation assays. Overall, results from this study indicated that the extracellular and cytoplasmic regions of MoMsb2 have distinct functions in appressorium formation, penetration and invasive growth, and MoMsb2 has overlapping functions with Cbp1 in recognizing environmental signals for Pmk1 activation.

Signaling domains of mucin Msb2 in Candida albicans

Candida albicans adapts to the human host by environmental sensing using the Msb2 signal mucin, which regulates fungal morphogenesis and resistance characteristics. Msb2 is anchored within the cytoplasmic membrane by a single transmembrane (TM) region dividing it into a large N-terminal exodomain, which is shed, and a small cytoplasmic domain. Analyses of strains carrying deleted Msb2 variants revealed an exodomain segment required for cleavage, shedding, and all functions of Msb2. Phosphorylation of the mitogen-activated protein kinase (MAP kinase) Cek1 was regulated by three distinct regions in Msb2: in unstressed cells, N-terminal sequences repressed phosphorylation, while its induction under cell wall stress required the cytoplasmic tail (C-tail) and sequences N-terminally flanking the TM region, downstream of the proposed cleavage site. Within the latter Msb2 region, overlapping but not identical sequences were also required for hyphal morphogenesis, basal resistance to antifungals, and, in unstressed cells, downregulation of the PMT1 transcript, encoding protein O-mannosyltransferase-1. Deletion of two-thirds of the exodomain generated a truncated Msb2 variant with a striking ability to induce hyperfilamentous growth, which depended on the presence of the Msb2-interacting protein Sho1, the MAP kinase Cek1, and the Efg1 transcription factor. Under cell wall stress, the cytoplasmic tail relocalized partially to the nucleus and contributed to regulation of 117 genes, as revealed by transcriptomic analyses. Genes regulated by the C-tail contained binding sites for the Ace2 and Azf1 transcription factors and included the ALS cell wall genes. We concluded that Msb2 fulfills its numerous functions by employing functional domains that are distributed over its entire length.

Evaluating the activity of the filamentous growth mitogen-activated protein kinase pathway in yeast

Mitogen-activated protein kinase (MAPK) pathways are evolutionarily conserved signaling pathways that regulate diverse processes in eukaryotes. One such pathway regulates filamentous growth, a nutrient limitation response in budding yeast and other fungal species. This protocol describes three assays used to measure the activity of the filamentous growth pathway. First, western blotting for phosphorylated (activated) MAPKs (P∼MAPKs; Slt2p, Kss1p, Fus3p, and Hog1p) provides a measure of MAPK activity in yeast and other fungal species. Second, the PGU1 gene is a transcriptional target of the filamentous growth pathway. Cells that undergo filamentous growth secrete Pgu1p, an endopolygalacturonase that degrades the plant-specific polysaccharide pectin. We describe an assay that measures secreted pectinase activity, which reflects an active filamentous growth pathway. Finally, in yeast, two mucin-like glycoproteins, Msb2 and Flo11, regulate filamentous growth. Secretion of the processed and shed glycodomain of Msb2 is an indicator of MAPK activity. Flo11, the major adhesion molecule that controls filamentous growth and biofilm/mat formation, is also shed from cells. Detecting shed mucins with epitope-tagged versions of the proteins (secretion profiling) provides information about the regulation of filamentous growth across fungal species.

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.

L-histidine inhibits biofilm formation and FLO11-associated phenotypes in Saccharomyces cerevisiae flor yeasts

Flor yeasts of Saccharomyces cerevisiae have an innate diversity of Flo11p which codes for a highly hydrophobic and anionic cell-wall glycoprotein with a fundamental role in biofilm formation. In this study, 380 nitrogen compounds were administered to three S. cerevisiae flor strains handling Flo11p alleles with different expression levels. S. cerevisiae strain S288c was used as the reference strain as it cannot produce Flo11p. The flor strains generally metabolized amino acids and dipeptides as the sole nitrogen source, although with some exceptions regarding L-histidine and histidine containing dipeptides. L-histidine completely inhibited growth and its effect on viability was inversely related to Flo11p expression. Accordingly, L-histidine did not affect the viability of the Δflo11 and S288c strains. Also, L-histidine dramatically decreased air-liquid biofilm formation and adhesion to polystyrene of the flor yeasts with no effect on the transcription level of the Flo11p gene. Moreover, L-histidine modified the chitin and glycans content on the cell-wall of flor yeasts. These findings reveal a novel biological activity of L-histidine in controlling the multicellular behavior of yeasts [corrected].

Profiling the secretome and extracellular proteome of the potato late blight pathogen Phytophthora infestans

Oomycetes are filamentous organisms that cause notorious diseases, several of which have a high economic impact. Well known is Phytophthora infestans, the causal agent of potato late blight. Previously, in silico analyses of the genome and transcriptome of P. infestans resulted in the annotation of a large number of genes encoding proteins with an N-terminal signal peptide. This set is collectively referred to as the secretome and comprises proteins involved in, for example, cell wall growth and modification, proteolytic processes, and the promotion of successful invasion of plant cells. So far, proteomic profiling in oomycetes was primarily focused on subcellular, intracellular or cell wall fractions; the extracellular proteome has not been studied systematically. Here we present the first comprehensive characterization of the in vivo secretome and extracellular proteome of P. infestans. We have used mass spectrometry to analyze P. infestans proteins present in seven different growth media with mycelial cultures and this resulted in the consistent identification of over two hundred proteins. Gene ontology classification pinpointed proteins involved in cell wall modifications, pathogenesis, defense responses, and proteolytic processes. Moreover, we found members of the RXLR and CRN effector families as well as several proteins lacking an obvious signal peptide. The latter were confirmed to be bona fide extracellular proteins and this suggests that, similar to other organisms, oomycetes exploit non-conventional secretion mechanisms to transfer certain proteins to the extracellular environment.

How does it kill?: understanding the candidacidal mechanism of salivary histatin 5

Histatins are salivary cationic peptides that provide the first line of defense against oral candidiasis caused by Candida albicans. This minireview presents a critical evaluation of our knowledge of the candidacidal mechanism of histatin 5 (Hst 5). Hst 5 is the most potent among all histatin family members with regard to its antifungal activity. The mode of action of Hst 5 has been a subject of intense debate. Unlike other classical host innate immune proteins, pore formation or membrane lysis by Hst 5 has largely been disproven, and it is now known that all targets of Hst 5 are intracellular. Hst 5 binds C. albicans cell wall proteins (Ssa1/2) and glycans and is taken up by the cells through fungal polyamine transporters in an energy-dependent manner. Once inside the fungal cells, Hst 5 may affect mitochondrial functions and cause oxidative stress; however, the ultimate cause of cell death is by volume dysregulation and ion imbalance triggered by osmotic stress. Besides these diverse targets, a novel mechanism based on the metal binding abilities of Hst 5 is discussed. Finally, translational approaches for Hst 5, based on peptide design and synergy with other known drugs, are considered a step forward for bench-to-bed application of Hst 5.

Interplay between Candida albicans and the antimicrobial peptide armory

Antimicrobial peptides (AMPs) are key elements of innate immunity, which can directly kill multiple bacterial, viral, and fungal pathogens. The medically important fungus Candida albicans colonizes different host niches as part of the normal human microbiota. Proliferation of C. albicans is regulated through a complex balance of host immune defense mechanisms and fungal responses. Expression of AMPs against pathogenic fungi is differentially regulated and initiated by interactions of a variety of fungal pathogen-associated molecular patterns (PAMPs) with pattern recognition receptors (PRRs) on human cells. Inflammatory signaling and other environmental stimuli are also essential to control fungal proliferation and to prevent parasitism. To persist in the host, C. albicans has developed a three-phase AMP evasion strategy, including secretion of peptide effectors, AMP efflux pumps, and regulation of signaling pathways. These mechanisms prevent C. albicans from the antifungal activity of the major AMP classes, including cathelicidins, histatins, and defensins leading to a basal resistance. This minireview summarizes human AMP attack and C. albicans resistance mechanisms and current developments in the use of AMPs as antifungal agents.