Two-component signal transduction proteins as potential drug targets in medically important fungi

The Hog1-Nmd5 signaling pathway regulates asexual development, lipid metabolism, stress response, trap morphogenesis, and secondary metabolism of Arthrobotrys oligospora

The high-osmolarity glycerol (HOG) signalling pathway, comprising Ste11/Ssk2/Ssk22 (MAPKKK), Pbs2 (MAPKK), and Hog1 (MAPK), is an important and conserved pathway in fungi. However, the functions and downstream regulatory factors of Hog1 in nematode-trapping (NT) fungi remain poorly understood. Here, three proteins (AoNmd5, AoPyp1, and AoPtp) interacting with Hog1 were screened in a representative NT fungus Arthrobotrys oligospora using yeast screening library and verified using yeast two-hybrid (Y2H) assay. The function of AoNmd5 was furtherly characterized by phenotypic comparison, staining technique, and multi-omics analyses. AoNmd5 was essential for vegetative growth, conidial development, trap morphogenesis, and nematode predation ability. In addition, AoNmd5 played crucial roles in endocytosis, lipid metabolism, reactive oxygen species, stress response, autophagy, and other metabolic processes. Furthermore, we constructed an AoNmd5 interaction network based on transcriptomic analysis and Y2H, revealing its significant role in the respiratory chain and redox processes as well as its interaction with the small GTPase Ran1, which mediates Hog1 nucleocytoplasmic shuttling. These findings suggest that the Hog1-Nmd5 signalling pathway has pleiotropic roles in A. oligospora. This study deepens our understanding of the HOG pathway and its interaction with importins in NT fungi.

Backbone and side‑chain 1H, 13C and 15N resonance assignments and secondary structure determination of the rhizobial FixJ

The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum (B.japonicum) enables high soybean yields with little or no nitrogen fertiliser. A two component regulatory system comprising FixL, a histidine kinase with O2-sensing activity, and FixJ, a response regulator, controls the expression of genes involved in nitrogen fixation, such as fixK and nifA. Only under anaerobic conditions, the monophosphate group is transferred from FixL to the N-terminal receiver domain of FixJ (FixJN), which eventually promote the association of the C-terminal effector domain (FixJC) to the promoter regions of the nitrogen-fixation-related genes. Structural biological analyses carried out so far for rhizobial FixJ molecules have proposed a solution structure for FixJ that differs from the crystal structures, in which the two domains are extended. To understand the FixJ activation caused by phosphorylation of the N-terminal domain, which presumably regulates through the interactions between FixJN and FixJC, here we have performed backbone and sidechain resonance assignments of the unphosphorylated state of B. japonicum FixJ.

Polypeptides Targeting Paracoccidioides brasiliensis Drk1

Considering the toxicity of conventional therapeutic approaches and the importance of precise mechanistic targets, it is important to explore signaling pathways implicated in fungal pathobiology. Moreover, treatment of paracoccidioidomycosis, a systemic mycosis caused by a dimorphic fungus, requires prolonged therapeutic regimens. Among the numerous factors underpinning the establishment of Paracoccidioides spp. infection, the capacity to transition from the mycelial to the yeast form is of pivotal importance. The Drk1 protein of Paracoccidioides brasiliensis likely plays a decisive role in this morphological shift and subsequent virulence. We identified peptides with affinity for the PbDrk1 protein using the phage-display method and assessed the effects of these peptides on P. brasiliensis. The peptides were found to inhibit the phase transition of P. brasiliensis. Furthermore, a substantial proportion of these peptides prevented adhesion to pneumocytes. Although these peptides may not possess inherent antifungal properties, they can augment the effects of certain antifungal agents. Notably, the cell wall architecture of P. brasiliensis appears to be modulated by peptide intervention, resulting in a reduced abundance of glycosylated proteins and lipids. These peptides were also evaluated for their efficacy in a Galleria mellonella model and shown to contribute to enhanced larval survival rates. The role of PbDrk1, which is notably absent in mammals, should be further investigated to improve the understanding of its functional role in P. brasiliensis, which may be helpful for designing novel therapeutic modalities.

Proteomics of Two Thermotolerant Isolates of Trichoderma under High-Temperature Stress

Several species of the soil borne fungus of the genus Trichoderma are known to be versatile, opportunistic plant symbionts and are the most successful biocontrol agents used in today's agriculture. To be successful in field conditions, the fungus must endure varying climatic conditions. Studies have indicated that a high atmospheric temperature coupled with low humidity is a major factor in the inconsistent performance of Trichoderma under field conditions. Understanding the molecular modulations associated with Trichoderma that persist and deliver under abiotic stress conditions will aid in exploiting the value of these organisms for such uses. In this study, a comparative proteomic analysis, using two-dimensional gel electrophoresis (2DE) and matrix-assisted laser desorption/time-of-flight (MALDI-TOF-TOF) mass spectrometry, was used to identify proteins associated with thermotolerance in two thermotolerant isolates of Trichoderma: T. longibrachiatum 673, TaDOR673 and T. asperellum 7316, TaDOR7316; with 32 differentially expressed proteins being identified. Sequence homology and conserved domains were used to identify these proteins and to assign a probable function to them. The thermotolerant isolate, TaDOR673, seemed to employ the stress signaling MAPK pathways and heat shock response pathways to combat the stress condition, whereas the moderately tolerant isolate, TaDOR7316, seemed to adapt to high-temperature conditions by reducing the accumulation of misfolded proteins through an unfolded protein response pathway and autophagy. In addition, there were unique, as well as common, proteins that were differentially expressed in the two isolates studied.

Functional dissection of individual domains in group III histidine kinase Sshk1p from the phytopathogenic fungus Sclerotinia sclerotiorum

A conserved kinase domain and phosphoryl group receiver domain at the C-terminus and poly-HAMP domains at the N-terminus comprise the structural components of the group III HK which was considered as a potential antifungal target. However, the roles of individual domains in the function of group III HKs have rarely been dissected in fungi. In this study, we dissected the roles of individual domains to better understand the function of Sshk1p, a group III HK from Sclerotinia sclerotiorum. The results suggest that individual domains play different roles in the functionality of Sshk1p and are implicated in the regulation of mycelial growth, sclerotia formation, pathogenicity. And the mutants of each domain in Sshk1 showed significantly increased sensitivity to hyperosmotic stress. However, the mutants of each domain in Sshk1 showed high resistance to fludioxonil and dimethachlon which suggested that all nine domains of Sshk1p were indispensable for susceptibility to fludioxonil and dimethachlon. Moreover, deletion of each individual domain in Sshk1 cancelled intracellular glycerol accumulation and increased SsHog1p phosphorylation level triggered by NaCl and fludioxonil, suggesting that all the domains of Sshk1 were essential for Sshk1-mediated SsHog1p phosphorylation and subsequent polyol accumulation in response to fludioxonil and hyperosmotic stress.

Candida albicans CHK1 gene from two-component system is essential for its pathogenicity in oral candidiasis

The roles of Candida albicans CHK1, a key gene from two-component system, in oral mucosal infection are not clear. This study evaluated the key roles of CHK1 gene in vitro and in vivo. The expression of CHK1 and its regulated virulence factors were tested during the oral epithelial cell infection. The production of lactate dehydrogenase, ROS, and IL-1α combined with the confocal and scanning electron microscope observation was employed to identify the capability of CHK1 in damaging the epithelial cells. Both immunocompetent and immunodeficient mice oropharyngeal infection models were involved to confirm the roles of CHK1 gene in vivo. The expression of CHK1 gene was significantly increased during the oral epithelial cell infection. The chk1Δ/Δ mutant failed to damage the epithelial cells or induce IL-α and ROS production. Interestingly, chk1Δ/Δ can also form the similar hyphae with WT and complementary strains. Accordingly, chk1Δ/Δ did not affect the adhesion and invasion rates of C. albicans to oral epithelial cells. However, chk1Δ/Δ significantly decreased the expression levels of the virulence factors, including ALS2, SAP6, and YWP1. The chk1Δ/Δ also failed to cause oral candidiasis in both immunocompetent and immunodeficient mice indicating that CHK1 gene from the two-component system is essential for the pathogenicity of C. albicans. KEY POINTS: • CHK1gene is essential for C. albicans in oral candidiasis • C. albicans without CHK1 gene can form "non-pathogenic" hyphae. • CHK1 gene regulates the virulence of C. albicans.

F. R. Ferreira I, D. Soković M. Methanolic Extract of the Herb Ononis spinosa L. Is an Antifungal Agent with no Cytotoxicity to Primary Human Cells

Ononis spinosa L. is a plant traditionally used as folk remedy. There are numerous studies regarding chemical constituents and health beneficial properties of Ononidis Radix. The following study was designed to investigate chemical composition and antifungal potential of the methanolic extract obtained from the O. spinosa L. herb. Chemical analyses regarding phenolic compounds of O. spinosa were performed by liquid chromatography with mass spectrometry (LC-DAD-ESI/MSn). Antifungal activity, antibiofilm properties and antifungal mode of action of the extract were evaluated, as well as cytotoxicity. Chemical analyses revealed the presence of flavonoids, isoflavonoids and phenolic acids in O. spinosa, with kaempherol-O-hexoside-pentoside being the most abundant compound (5.1 mg/g extract). Methanolic extract was active against all of the tested microfungi with Penicillium aurantiogriseum being the most sensitive to the extract inhibitory effect at 0.02 mg/mL; and effectively inhibited biofilms formed by Candida strains. Minimum fungicidal concentrations of extract rose in the presence of ergosterol and leakage of cellular components was detected. The extract showed no cytotoxicity to human gingival fibroblast (HGF-1) cells. This study significantly contributes to overall knowledge about medicinal potential of O. spinosa herbal extract and enlightens previously unrevealed properties. O. spinosa aerial parts seem to be an interesting candidate for the development of antifungal preparations, non-toxic to human cells.

Virulence and biofilms as promising targets in developing antipathogenic drugs against candidiasis

Candida albicans has remained the main etiological agent of candidiasis, challenges clinicians with high mortality and morbidity. The emergence of resistance to antifungal drugs, toxicity and lower efficacy have all contributed to an urgent need to develop alternative drugs aiming at novel targets in C. albicans. Targeting the production of virulence factors, which are essential processes for infectious agents, represents an attractive substitute for the development of newer anti-infectives. The present review highlights the recent developments made in the understanding of the pathogenicity of C. albicans. Production of hydrolytic enzymes, morphogenesis and biofilm formation, along with their molecular and metabolic regulation in Candida are discussed with regard to the development of novel antipathogenic drugs against candidiasis.

Over the last decade, candidiasis has remained a major problematic disease worldwide. In spite of the existence of many antifungal drugs, the treatment of such diseases has still remained unsuccessful due to drug inefficacy. Therefore, there is a need to discover antifungals with different modes of action, such as antipathogenic drugs against Candida albicans. Here, we describe how various types of virulence factors such as proteinase, phospholipase, hemolysin, adhesion, morphogenesis and biofilm formation, could be targeted to develop novel therapeutics. We can inhibit production of these virulence factors by controlling their molecular/metabolic regulation.

Antifungal drugs: New insights in research & development

The need for better antifungal therapy is commonly accepted in view of the high mortality rates associated with systemic infections, the low number of available antifungal classes, their associated toxicity and the increasing number of infections caused by strains with natural or acquired resistance. The urgency to expand the range of therapeutic options for the treatment of fungal infections has led researchers in recent decades to seek alternative antifungal targets when compared to the conventional ones currently used. Although new potential targets are reported, translating the discoveries from bench to bedside is a long process and most of these drugs fail to reach the patients. In this review, we discuss the development of antifungal drugs focusing on the approach of drug repurposing and the search for novel drugs for classical targets, the most recently described gene targets for drug development, the possibilities of immunotherapy using antibodies, cytokines, therapeutic vaccines and antimicrobial peptides.

Architecture of the complete oxygen-sensing FixL-FixJ two-component signal transduction system

The symbiotic nitrogen-fixing bacterium Bradyrhizobium japonicum is critical to the agro-industrial production of soybean because it enables the production of high yields of soybeans with little use of nitrogenous fertilizers. The FixL and FixJ two-component system (TCS) of this bacterium ensures that nitrogen fixation is only stimulated under conditions of low oxygen. When it is not bound to oxygen, the histidine kinase FixL undergoes autophosphorylation and transfers phosphate from adenosine triphosphate (ATP) to the response regulator FixJ, which, in turn, stimulates the expression of genes required for nitrogen fixation. We purified full-length B. japonicum FixL and FixJ proteins and defined their structures individually and in complex using small-angle x-ray scattering, crystallographic, and in silico modeling techniques. Comparison of active and inactive forms of FixL suggests that intramolecular signal transduction is driven by local changes in the sensor domain and in the coiled-coil region connecting the sensor and histidine kinase domains. We also found that FixJ exhibits conformational plasticity not only in the monomeric state but also in tetrameric complexes with FixL during phosphotransfer. This structural characterization of a complete TCS contributes both a mechanistic and evolutionary understanding to TCS signal relay, specifically in the context of the control of nitrogen fixation in root nodules.

Novel Agents and Drug Targets to Meet the Challenges of Resistant Fungi

The emergence of drug-resistant fungi poses a major threat to human health. Despite advances in preventive, diagnostic, and therapeutic interventions, resistant fungal infections continue to cause significant morbidity and mortality in patients with compromised immunity, underscoring the urgent need for new antifungal agents. In this article, we review the challenges associated with identifying broad-spectrum antifungal drugs and highlight novel targets that could enhance the armamentarium of agents available to treat drug-resistant invasive fungal infections.

Adaptation to macrophage killing by Talaromyces marneffei

Talaromyces (Penicillium) marneffei is an important opportunistic fungal pathogen. It causes disseminated infection in immunocompromised patients especially in Southeast Asian countries. The pathogenicity of T. marneffei depends on the ability of the fungus to survive the killing process and replicate inside the macrophage. Major stresses inside the phagosome of macrophages are heat, oxidative substances and nutrient deprivation. The coping strategies of this pathogen with these stresses are under investigation. This paper summarizes factors relating to the stress responses that contribute to the intracellular survival of T. marneffei. These include molecules in the MAP signal transduction cascade, heat shock proteins, antioxidant enzymes and enzymes responsible in nutrient retrieval. There is speculation that the ability of T. marneffei to withstand these defenses plays an important role in its pathogenicity.

Talaromyces marneffei is an important dimorphic fungus that causes disease in immunocompromised patients. The pathogenicity of T. marneffei depends on the ability of the fungus to survive the killing process and replicate inside the host macrophage cells. This paper summarizes factors relating to the stress responses that contribute to the intracellular survival of T. marneffei. There is speculation that the ability of T. marneffei to withstand these defenses plays an important role in its pathogenicity.

The yeasts phosphorelay systems: a comparative view

Cells contain signal transduction pathways that mediate communication between the extracellular environment and the cell interior. These pathways control transcriptional programs and posttranscriptional processes that modify cell metabolism in order to maintain homeostasis. One type of these signal transduction systems are the so-called Two Component Systems (TCS), which conduct the transfer of phosphate groups between specific and conserved histidine and aspartate residues present in at least two proteins; the first protein is a sensor kinase which autophosphorylates a histidine residue in response to a stimulus, this phosphate is then transferred to an aspartic residue located in a response regulator protein. There are classical and hybrid TCS, whose difference consists in the number of proteins and functional domains involved in the phosphorelay. The TCS are widespread in bacteria where the sensor and its response regulator are mostly specific for a given stimulus. In eukaryotic organisms such as fungi, slime molds, and plants, TCS are present as hybrid multistep phosphorelays, with a variety of arrangements (Stock et al. in Annu Rev Biochem 69:183-215, 2000; Wuichet et al. in Curr Opin Microbiol 292:1039-1050, 2010). In these multistep phosphorelay systems, several phosphotransfer events take place between different histidine and aspartate residues localized in specific domains present in more than two proteins (Thomason and Kay, in J Cell Sci 113:3141-3150, 2000; Robinson et al. in Nat Struct Biol 7:626-633, 2000). This review presents a brief and succinct description of the Two-component systems of model yeasts, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, Cryptococcus neoformans and Kluyveromyces lactis. We have focused on the comparison of domain organization and functions of each component present in these phosphorelay systems.

Putative Drug and Vaccine Target Identification in Leishmania donovani Membrane Proteins Using Naïve Bayes Probabilistic Classifier

Predicting the role of protein is one of the most challenging problems. There are few approaches available for the prediction of role of unknown protein in terms of drug target or vaccine candidate. We propose here Naïve Bayes probabilistic classifier, a promising method for reliable predictions. This method is tested on the proteins identified in our mass spectrometry based membrane protemics study of Leishmania donovani parasite that causes a fatal disease (Visceral Leishmaniasis) in humans all around the world. Most of the vaccine/drug targets belonging to membrane proteins are represented as key players in the pathogenesis of Leishmania infection. Analyses of our previous results, using Naïve Bayes probabilistic classifier, indicate that this method predicts the role of unknown/hypothetical protein (as drug target/vaccine candidate) significantly with higher precision. We have employed this method in order to provide probabilistic predictions of unknown/hypothetical proteins as targets. This study reports the unknown/hypothetical proteins of Leishmania membrane fraction as a potential drug targets and vaccine candidate which is vital information for this parasite. Future molecular studies and characterization of these potent targets may produce a recombinant therapeutic/prophylactic tool against Visceral Leishmaniasis. These unknown/hypothetical proteins may open a vast research field to be exploited for novel treatment strategies.

A conserved dimorphism-regulating histidine kinase controls the dimorphic switching in Paracoccidioides brasiliensis

Paracoccidioides brasiliensis and P. lutzii, thermally dimorphic fungi, are the causative agents of paracoccidioidomycosis (PCM). Paracoccidioides infection occurs when conidia or mycelium fragments are inhaled by the host, which causes the Paracoccidioides cells to transition to the yeast form. The development of disease requires conidia inside the host alveoli to differentiate into yeast cells in a temperature-dependent manner. We describe the presence of a two-component signal transduction system in P. brasiliensis, which we investigated by expression analysis of a hypothetical protein gene (PADG_07579) that showed high similarity with the dimorphism-regulating histidine kinase (DRK1) gene of Blastomyces dermatitidis and Histoplasma capsulatum This gene was sensitive to environmental redox changes, which was demonstrated by a dose-dependent decrease in transcript levels after peroxide stimulation and a subtler decrease in transcript levels after NO stimulation. Furthermore, the higher PbDRK1 levels after treatment with increasing NaCl concentrations suggest that this histidine kinase can play a role as osmosensing. In the mycelium-yeast (M→Y) transition, PbDRK1 mRNA expression increased 14-fold after 24 h incubation at 37°C, consistent with similar observations in other virulent fungi. These results demonstrate that the PbDRK1 gene is differentially expressed during the dimorphic M→Y transition. Finally, when P. brasiliensis mycelium cells were exposed to a histidine kinase inhibitor and incubated at 37°C, there was a delay in the dimorphic M→Y transition, suggesting that histidine kinases could be targets of interest for PCM therapy.

Management of Candida biofilms: state of knowledge and new options for prevention and eradication

Biofilms formed by Candida species (spp.) on medical devices represent a potential health risk. The focus of current research is searching for new options for the treatment and prevention of biofilm-associated infections using different approaches including modern nanotechnology. This review summarizes current information concerning the most relevant resistance/tolerance mechanisms to conventional drugs and a role of additional factors contributing to these phenomena in Candida spp. (mostly Candida albicans). Additionally, it provides an information update in prevention and eradication of a Candida biofilm including experiences with 'lock' therapy, potential utilization of small molecules in biomedical applications, and perspectives of using photodynamic inactivation in the control of a Candida biofilm.

Regulation of Candida albicans Interaction with Macrophages through the Activation of HOG Pathway by Genistein

The severity of infections caused by Candida albicans, the most common opportunistic human fungal pathogen, needs rapid and effective antifungal treatments. One of the effective ways is to control the virulence factors of the pathogen. Therefore, the current study examined the effects of genistein, a natural isoflavone present in soybeans, on C. albicans. The genistein-treated C. albicans cells were then exposed to macrophages. Although no inhibition effect on the growth rates of C. albicans was noted an enhancement of the immune response to macrophages has been observed, indicated by phagocytosis and release of cytokines TNF-α and IL-10. The effect of genistein on the enhanced phagocytosis can be mimicked by the fungicides fludioxonil or iprodione, which inhibit the histidine kinase Cos1p and lead to activation of HOG pathway. The western blot results showed a clear phosphorylation of Hog1p in the wild type strain of C. albicans after incubation with genistein. In addition, effects of genistein on the phosphorylation of Hog1p in the histidine kinase mutants Δcos1 and Δsln1 were also observed. Our results thus indicate a new bio-activity of genistein on C. albicans by activation of the HOG pathway of the human pathogen C. albicans.

Potential Targets for Antifungal Drug Discovery Based on Growth and Virulence in Candida albicans

Fungal infections, especially infections caused by Candida albicans, remain a challenging problem in clinical settings. Despite the development of more-effective antifungal drugs, their application is limited for various reasons. Thus, alternative treatments with drugs aimed at novel targets in C. albicans are needed. Knowledge of growth and virulence in fungal cells is essential not only to understand their pathogenic mechanisms but also to identify potential antifungal targets. This article reviews the current knowledge of the mechanisms of growth and virulence in C. albicans and examines potential targets for the development of new antifungal drugs.

Hitting the caspofungin salvage pathway of human-pathogenic fungi with the novel lasso peptide humidimycin (MDN-0010)

Fungal infections have increased dramatically in the last 2 decades, and fighting infectious diseases requires innovative approaches such as the combination of two drugs acting on different targets or even targeting a salvage pathway of one of the drugs. The fungal cell wall biosynthesis is inhibited by the clinically used antifungal drug caspofungin. This antifungal activity has been found to be potentiated by humidimycin, a new natural product identified from the screening of a collection of 20,000 microbial extracts, which has no major effect when used alone. An analysis of transcriptomes and selected Aspergillus fumigatus mutants indicated that humidimycin affects the high osmolarity glycerol response pathway. By combining humidimycin and caspofungin, a strong increase in caspofungin efficacy was achieved, demonstrating that targeting different signaling pathways provides an excellent basis to develop novel anti-infective strategies.

Two-component phosphorelays in fungal mitochondria and beyond

Prokaryotes, eukaryotic microorganisms and plants utilize two-component signal transduction pathways to detect and respond to various environmental cues. These signaling cascades were acquired by eukaryotes via horizontal gene transfer events from ancestral bacteria. Recent exciting discoveries have identified two-component signaling systems in mitochondria and chloroplasts of several eukaryotic microorganisms and plants, therefore providing important clues to the evolutionary transition of these signaling cascades from prokaryotes to eukaryotes. This review will focus on the role of two-component signal transduction pathways in fungal pathogenesis and also discuss key new discoveries of presence of proteins participating in these signaling pathways in mitochondrion. Before addressing these issues, I first briefly describe the magnitude and the economic impact of the healthcare problems caused by fungal pathogens.

Molecular mechanisms of action of herbal antifungal alkaloid berberine, in Candida albicans

Candida albicans causes superficial to systemic infections in immuno-compromised individuals. The concomitant use of fungistatic drugs and the lack of cidal drugs frequently result in strains that could withstand commonly used antifungals, and display multidrug resistance (MDR). In search of novel fungicidals, in this study, we have explored a plant alkaloid berberine (BER) for its antifungal potential. For this, we screened an in-house transcription factor (TF) mutant library of C. albicans strains towards their susceptibility to BER. Our screen of TF mutant strains identified a heat shock factor (HSF1), which has a central role in thermal adaptation, to be most responsive to BER treatment. Interestingly, HSF1 mutant was not only highly susceptible to BER but also displayed collateral susceptibility towards drugs targeting cell wall (CW) and ergosterol biosynthesis. Notably, BER treatment alone could affect the CW integrity as was evident from the growth retardation of MAP kinase and calcineurin pathway null mutant strains and transmission electron microscopy. However, unlike BER, HSF1 effect on CW appeared to be independent of MAP kinase and Calcineurin pathway genes. Additionally, unlike hsf1 null strain, BER treatment of Candida cells resulted in dysfunctional mitochondria, which was evident from its slow growth in non-fermentative carbon source and poor labeling with mitochondrial membrane potential sensitive probe. This phenotype was reinforced with an enhanced ROS levels coinciding with the up-regulated oxidative stress genes in BER-treated cells. Together, our study not only describes the molecular mechanism of BER fungicidal activity but also unravels a new role of evolutionary conserved HSF1, in MDR of Candida.

Two-component histidine phosphotransfer protein Ypd1 is not essential for viability in Candida albicans

Prokaryotes and lower eukaryotes, such as yeasts, utilize two-component signal transduction pathways to adapt cells to environmental stress and to regulate the expression of genes associated with virulence. One of the central proteins in this type of signaling mechanism is the phosphohistidine intermediate protein Ypd1. Ypd1 is reported to be essential for viability in the model yeast Saccharomyces cerevisiae. We present data here showing that this is not the case for Candida albicans. Disruption of YPD1 causes cells to flocculate and filament constitutively under conditions that favor growth in yeast form. To determine the function of Ypd1 in the Hog1 mitogen-activated protein kinase (MAPK) pathway, we measured phosphorylation of Hog1 MAPK in ypd1Δ/Δ and wild-type strains of C. albicans. Constitutive phosphorylation of Hog1 was observed in the ypd1Δ/Δ strain compared to the wild-type strain. Furthermore, fluorescence microscopy revealed that green fluorescent protein (GFP)-tagged Ypd1 is localized to both the nucleus and the cytoplasm. The subcellular segregation of GFP-tagged Ypd1 hints at an important role(s) of Ypd1 in regulation of Ssk1 (cytosolic) and Skn7 (nuclear) response regulator proteins via phosphorylation in C. albicans. Overall, our findings have profound implications for a mechanistic understanding of two-component signaling pathways in C. albicans, and perhaps in other pathogenic fungi.

NikA/TcsC histidine kinase is involved in conidiation, hyphal morphology, and responses to osmotic stress and antifungal chemicals in Aspergillus fumigatus

The fungal high osmolarity glycerol (HOG) pathway is composed of a two-component system (TCS) and Hog1-type mitogen-activated protein kinase (MAPK) cascade. A group III (Nik1-type) histidine kinase plays a major role in the HOG pathway of several filamentous fungi. In this study, we characterized a group III histidine kinase, NikA/TcsC, in the life-threatening pathogenic fungus, Aspergillus fumigatus. A deletion mutant of nikA showed low conidia production, abnormal hyphae, marked sensitivity to high osmolarity stresses, and resistance to cell wall perturbing reagents such as congo red and calcofluor white, as well as to fungicides such as fludioxonil, iprodione, and pyrrolnitrin. None of these phenotypes were observed in mutants of the SskA response regulator and SakA MAPK, which were thought to be downstream components of NikA. In contrast, in response to fludioxonil treatment, NikA was implicated in the phosphorylation of SakA MAPK and the transcriptional upregulation of catA, dprA, and dprB, which are regulated under the control of SakA. We then tested the idea that not only NikA, but also the other 13 histidine kinases play certain roles in the regulation of the HOG pathway. Interestingly, the expression of fos1, phkA, phkB, fhk5, and fhk6 increased by osmotic shock or fludioxonil treatment in a SakA-dependent manner. However, deletion mutants of the histidine kinases showed no significant defects in growth under the tested conditions. Collectively, although the signal transduction network related to NikA seems complicated, NikA plays a crucial role in several aspects of A. fumigatus physiology and, to a certain extent, modulates the HOG pathway.

Deletion of the HAMP domains from the histidine kinase CaNik1p of Candida albicans or treatment with fungicides activates the MAP kinase Hog1p in S. cerevisiae transformants

Background

Microorganisms use two-component signal transduction (TCST) systems to regulate the response of the organism to changes of environmental conditions. Such systems are absent from mammalian cells and are thus of interest as drug targets. Fungal TCST systems are usually composed of a hybrid histidine kinase, comprising the histidine kinase (HisKA) domain and a receiver domain, a histidine phosphotransfer protein and a response regulator. Among the 11 groups of fungal histidine kinases, group III histidine kinases are of particular relevance as they are essential for the activity of different groups of fungicides. A characteristic feature is the N-terminal amino acid repeat domain comprising multiple HAMP domains, of which the function is still largely unknown. In Candida albicans, a fungal human pathogen, three histidine kinases were identified, of which CaNik1p is a group III histidine kinase. Heterologous expression of this protein in Sacchromyces cerevisiae conferred susceptibility to different fungicides. Fungicide activity was associated with phosphorylation of the mitogen activated protein kinase Hog1p.

Results

We have constructed mutated versions of CaNik1p, from which either all HAMP domains were deleted (CaNik1pΔHAMP) or in which the histidine kinase or the receiver domains were not-functional. Expression of CaNIK1ΔHAMP in S. cerevisiae led to severe growth inhibition. Normal growth could be restored by either replacing the phosphate-accepting histidine residue in CaNik1pΔHAMP or by expressing CaNIK1ΔHAMP in S. cerevisiae mutants, in which single genes encoding several components of the HOG pathway were deleted. Expression of proteins with non-functional histidine kinase or receiver domains resulted in complete loss of susceptibility to antifungals, such as fludioxonil. Conditions leading to growth inhibition of transformants also led to phosphorylation of the MAP kinase Hog1p.

Conclusion

Our results show that functional histidine kinase and receiver domains of CaNik1p were essential for antifungal susceptibility and for activation of the Hog1p. Moreover, for the first time we show that deletion of all HAMP domains from CaNik1p led to activation of Hog1p without an external stimulus. This phenotype was similar to the effects obtained upon treatment with fungicides, as in both cases growth inhibition correlated with Hog1p activation and was dependent on the functionality of the conserved phosphate-accepting histidine residue.

The sixth HAMP domain negatively regulates the activity of the group III HHK containing seven HAMP domains

In fungi, the group III hybrid histidine kinases (HHK) act as important sensors to regulate osmoadaptation, hyphal growth, morphogenesis, conidia formation and virulence. They are molecular targets for antifungal agent fludioxonil. They typically have HAMP domain repeats at the NH2-terminus that are important for their activity. Interestingly, the numbers of HAMP domain vary among the orthologs from different genera. The orthologs from basidiomycetes harbor seven HAMP domains whereas those from yeast contain five HAMP domains. In order to understand the functioning of a seven-HAMP module, we have constructed a yeast-like chimera DhNik1-Tco1 containing seven HAMP domains. The functional characterization of this chimera in yeast Saccharomyces cerevisiae showed that the sixth HAMP domain played important regulatory role. Our results indicated that the negative regulation of histidine kinase activity by the penultimate HAMP domain could possibly be an evolutionarily conserved theme in the group III HHK containing different lengths of poly HAMP module.

Mitochondrial two-component signaling systems in Candida albicans

Two-component signal transduction pathways are one of the primary means by which microorganisms respond to environmental signals. These signaling cascades originated in prokaryotes and were inherited by eukaryotes via endosymbiotic lateral gene transfer from ancestral cyanobacteria. We report here that the nuclear genome of the pathogenic fungus Candida albicans contains elements of a two-component signaling pathway that seem to be targeted to the mitochondria. The C. albicans two-component response regulator protein Srr1 (stress response regulator 1) contains a mitochondrial targeting sequence at the N terminus, and fluorescence microscopy reveals mitochondrial localization of green fluorescent protein-tagged Srr1. Moreover, phylogenetic analysis indicates that C. albicans Srr1 is more closely related to histidine kinases and response regulators found in marine bacteria than are other two-component proteins present in the fungi. These data suggest conservation of this protein during the evolutionary transition from endosymbiont to a subcellular organelle. We used microarray analysis to determine whether the phenotypes observed with a srr1Δ/Δ mutant could be correlated with gene transcriptional changes. The expression of mitochondrial genes was altered in the srr1Δ/Δ null mutant in comparison to their expression in the wild type. Furthermore, apoptosis increased significantly in the srr1Δ/Δ mutant strain compared to the level of apoptosis in the wild type, suggesting the activation of a mitochondrion-dependent apoptotic cell death pathway in the srr1Δ/Δ mutant. Collectively, this study shows for the first time that a lower eukaryote like C. albicans possesses a two-component response regulator protein that has survived in mitochondria and regulates a subset of genes whose functions are associated with the oxidative stress response and programmed cell death (apoptosis).

Two-component signal transduction in Agaricus bisporus: a comparative genomic analysis with other basidiomycetes through the web-based tool BASID2CS

Two-component systems (TCSs) are signal transduction mechanisms present in many eukaryotes, including fungi that play essential roles in the regulation of several cellular functions and responses. In this study, we carry out a genomic analysis of the TCS proteins in two varieties of the white button mushroom Agaricus bisporus. The genomes of both A. bisporus varieties contain eight genes coding for TCS proteins, which include four hybrid Histidine Kinases (HKs), a single histidine-containing phosphotransfer (HPt) protein and three Response Regulators (RRs). Comparison of the TCS proteins among A. bisporus and the sequenced basidiomycetes showed a conserved core complement of five TCS proteins including the Tco1/Nik1 hybrid HK, HPt protein and Ssk1, Skn7 and Rim15-like RRs. In addition, Dual-HKs, unusual hybrid HKs with 2 HK and 2 RR domains, are absent in A. bisporus and are limited to various species of basidiomycetes. Differential expression analysis showed no significant up- or down-regulation of the Agaricus TCS genes in the conditions/tissue analyzed with the exception of the Skn7-like RR gene (Agabi_varbisH97_2|198669) that is significantly up-regulated on compost compared to cultured mycelia. Furthermore, the pipeline web server BASID2CS (http://bioinformatics.unavarra.es:1000/B2CS/BASID2CS.htm) has been specifically designed for the identification, classification and functional annotation of putative TCS proteins from any predicted proteome of basidiomycetes using a combination of several bioinformatic approaches.

Analysis of bacterial biofilms using NMR-based metabolomics

Infectious diseases can be difficult to cure, especially if the pathogen forms a biofilm. After decades of extensive research into the morphology, physiology and genomics of biofilm formation, attention has recently been directed toward the analysis of the cellular metabolome in order to understand the transformation of a planktonic cell to a biofilm. Metabolomics can play an invaluable role in enhancing our understanding of the underlying biological processes related to the structure, formation and antibiotic resistance of biofilms. A systematic view of metabolic pathways or processes responsible for regulating this 'social structure' of microorganisms may provide critical insights into biofilm-related drug resistance and lead to novel treatments. This review will discuss the development of NMR-based metabolomics as a technology to study medically relevant biofilms. Recent advancements from case studies reviewed in this manuscript have shown the potential of metabolomics to shed light on numerous biological problems related to biofilms.

Candida albicans SRR1, a putative two-component response regulator gene, is required for stress adaptation, morphogenesis, and virulence

We report here the identification and characterization of a previously uncharacterized, two-component response regulator gene (orf19.5843) from Candida albicans. Because of its apparent functions in stress adaptation, we have named this gene SRR1 (stress response regulator 1). Disruption of SRR1 causes defects in hyphal development, reduced resistance to stress, and severe virulence attenuation in the mouse model of disseminated candidiasis.

The Pho regulon and the pathogenesis of Escherichia coli

During the course of infection, bacteria must coordinately regulate gene expression in response to environmental stimuli. The phosphate (Pho) regulon is controlled by the two component-regulatory system PhoBR. PhoBR is activated during starvation and regulates genes involved in phosphate homeostasis. Several studies have highlighted the importance of the Pho regulon in bacterial pathogenesis, showing how induction of PhoBR, in addition to regulating genes participating in phosphate metabolism, leads to modulation of many cellular processes. The pleiotropic effects of Pho regulon activation include attenuated virulence and alteration of many virulence traits, including adhesion to host cells and resistance to cationic antimicrobial peptides, acidity and oxidative stresses. This review provides an overview of the relationship between the Pho regulon and virulence in Escherichia coli and illustrates that, in addition to regulating phosphate homeostasis, the Pho regulon plays a key role in regulating stress responses and virulence.

Oxidative stress survival in a clinical Saccharomyces cerevisiae isolate is influenced by a major quantitative trait nucleotide

One of the major challenges in characterizing eukaryotic genetic diversity is the mapping of phenotypes that are the cumulative effect of multiple alleles. We have investigated tolerance of oxidative stress in the yeast Saccharomyces cerevisiae, a trait showing phenotypic variation in the population. Initial crosses identified that this is a quantitative trait. Microorganisms experience oxidative stress in many environments, including during infection of higher eukaryotes. Natural variation in oxidative stress tolerance is an important aspect of response to oxidative stress exerted by the human immune system and an important trait in microbial pathogens. A clinical isolate of the usually benign yeast S. cerevisiae was found to survive oxidative stress significantly better than the laboratory strain. We investigated the genetic basis of increased peroxide survival by crossing those strains, phenotyping 1500 segregants, and genotyping of high-survival segregants by hybridization of bulk and single segregant DNA to microarrays. This effort has led to the identification of an allele of the transcription factor Rds2 as contributing to stress response. Rds2 has not previously been associated with the survival of oxidative stress. The identification of its role in the oxidative stress response here is an example of a specific trait that appears to be beneficial to Saccharomyces cerevisiae when growing as a pathogen. Understanding the role of this fungal-specific transcription factor in pathogenicity will be important in deciphering how fungi infect and colonize the human host and could eventually lead to a novel drug target.

Characterization of the conserved phosphorylation site in the Aspergillus nidulans response regulator SrrA

Ssk1- and Skn7-type response regulators are widely conserved in fungal His-Asp phosphorelay (two-component) signaling systems. SrrA, a Skn7-type RR of Aspergillus nidulans, is implicated not only in oxidative stress responses but also in osmotic adaptation, conidia production (asexual development), inhibition by fungicides, and cell wall stress resistance. Here, we characterized SrrA, focusing on the role of the conserved aspartate residue in the receiver domain, which is essential for phosphorelay function. We constructed strains carrying an SrrA protein in which aspartate residue D385 was replaced with either asparagine (N) or alanine (A). These mutants exhibited normal conidiation and partial oxidative stress resistance. In osmotic adaptation, mutants with substitution at SrrA D385 showed as much sensitivity as ΔsrrA strains, suggesting that SrrA plays a role in osmotic stress adaptation in a phosphorelay-dependent manner. The SrrA D385 substitution mutants showed significant resistance to fungicides and cell wall stresses. These results together led us to conclude that the conserved aspartate residue has a substantial impact on SrrA function, and that SrrA plays a role in several aspects of cellular function via His-Asp phosphorelay circuitry in Aspergillus nidulans.

Deletion of the Candida albicans histidine kinase gene CHK1 improves recognition by phagocytes through an increased exposure of cell wall beta-1,3-glucans

The pathogenic fungus Candida albicans is able to cover its most potent proinflammatory cell wall molecules, the β-glucans, underneath a dense mannan layer, so that the pathogen becomes partly invisible for immune cells such as phagocytes. As the C. albicans histidine kinases Chk1p, Cos1p and CaSln1p had been reported to be involved in virulence and cell wall biosynthesis, we investigated whether deletion of the respective genes influences the activity of phagocytes against C. albicans. We found that among all histidine kinase genes, CHK1 plays a prominent role in phagocyte activation. Uptake of the deletion mutant Δchk1 as well as the acidification of Δchk1-carrying phagosomes was significantly increased compared with the parental strain. These improved activities could be correlated with an enhanced accessibility of the mutant β-1,3-glucans for immunolabelling. In addition, any inhibition of β-1,3-glucan-mediated phagocytosis resulted in a reduced uptake of Δchk1, while ingestion of the parental strain was hardly affected. Moreover, deletion of CHK1 caused an enhanced release of interleukins 6 and 10, indicating a stronger activation of the β-1,3-glucan receptor dectin-1. In conclusion, the Chk1p protein is likely to be involved in masking β-1,3-glucans from immune recognition. As there are no homologues of fungal histidine kinases in mammals, Chk1p has to be considered as a promising target for new antifungal agents.

Histidine kinases keep fungi safe and vigorous

Signal transduction in human pathogenic fungi, like in other microorganisms, regulates a number of adaptive transcriptional responses to a variety of environmental cues. Among signal relay proteins, sensor, histidine kinase proteins (HK) are auto-phosphorylated upon perception of an environmental cue, and initiate a phosphorelay that results in transcriptional regulation of genes associated with specific stress signals or multiple stress cues. Human pathogenic fungi utilize HK proteins to adapt to stress, grow, sporulate, undergo morphogenesis, mate, sense anti-fungal drugs, and cause disease. While much is known about HK and RR proteins functionally, the MAPK pathways and downstream transcription factors and gene targets are largely unstudied in human pathogenic fungi. However, as HK proteins are broadly conserved and not apparently in humans, we suggest that they offer exploitation as new targets in anti-fungal drug discovery.

Protein histidine kinases: assembly of active sites and their regulation in signaling pathways

Protein histidine kinases (PHKs) function in Two Component Signaling pathways utilized extensively by bacteria and archaea. Many PHKs participate in three distinct, but interrelated signaling reactions: autophoshorylation, phosphotransfer (to a partner Response Regulator (RR) protein), and dephosphorylation of this RR. Detailed biochemical and structural characterization of several PHKs has revealed how the domains of these proteins can interact to assemble the three active sites that promote the necessary chemistry and how these domain interactions might be regulated in response to sensory input: the relative orientation of helices in the PHK dimerization domain can reorient, via cogwheeling (rotation) and kinking (bending), to effect changes in PHK activities that probably involve sequestration/release of the PHK catalytic domain by the dimerization domain.

Two-component signal transduction as potential drug targets in pathogenic bacteria

Gene clusters contributing to processes such as cell growth and pathogenicity are often controlled by two-component signal transduction systems (TCSs). Specific inhibitors against TCS systems work differently from conventional antibiotics, and developing them into new drugs that are effective against various drug-resistant bacteria may be possible. Furthermore, inhibitors of TCSs that control virulence factors may reduce virulence without killing the pathogenic bacteria. Previous TCS inhibitors targeting the kinase domain of the histidine kinase sensor suffered from poor selectivity. Recent TCS inhibitors, however, target the sensory domains of the sensors blocking the quorum sensing system, or target the essential response regulator. These new targets are introduced, together with several specific TCSs that have the potential to serve as effective drug targets.

The Candida albicans histidine kinase Chk1p: signaling and cell wall mannan

Several published functions associated with the CHK1 histidine kinase of Candida albicans resemble those of the MAPK Cek1p and its cognate receptor Sho1p (SSU81). To explore this further, we have compared mutants lacking the proteins mentioned above and have constructed a double sho1/chk1Delta null mutant to determine relationships among these proteins. We observed that the sensitivity to Congo red (CR), calcofluor white (CW), as well as clumping of cells, was slightly increased in the double mutant compared to the single chk1Delta or sho1Delta mutants. However, Cek1p phosphorylation via Sho1p, which occurs during log phase growth in the presence or absence of CR in Wt cells, does not require Chk1p. These data suggest that Chk1p and Sho1p are components of parallel but independent signal pathways. In addition, bulk mannan of strains was analyzed by GLC/MS and GPC MALLS and NMR. Compared to Wt and a CHK1 gene-reconstituted strain (CHK23) that contained high, intermediate and low Mw mannan species, we found that the mannan of strains CHK21 (chk1Delta null), the cek1Delta null, and the double mutant consisted only of low Mw mannan. The sho1Delta null mutant only demonstrated a reduced intermediate type of mannan. Alcian blue binding was lower in cek1Delta, chk1Delta, and the double sho1/chk1Delta null mutant lacking high and intermediate Mw mannan than in the sho1Delta null which had a partial loss of intermediate Mw mannan only. We conclude that the Chk1p HK is part of a functionally similar but parallel pathway to the Sho1p-Cek1p pathway that confers resistance to the cell wall inhibitors CR and CW. However, a functional relationship in mannan biosynthesis of Chk1p and Cek1p exists that only partially requires Sho1p.