Eca1, a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase, is involved in stress tolerance and virulence in Cryptococcus neoformans

Transcriptional and Post-Translational Roles of Calcineurin in Cationic Stress and Glycerol Biosynthesis in Cryptococcus neoformans

Stress management is an adaptive advantage for survival in adverse environments. Pathogens face this challenge during host colonization, requiring an appropriate stress response to establish infection. The fungal pathogen Cryptococcus neoformans undergoes thermal, oxidative, and osmotic stresses in the environment and animal host. Signaling systems controlled by Ras1, Hog1, and calcineurin respond to high temperatures and osmotic stress. Cationic stress caused by Na+, K+, and Li+ can be overcome with glycerol, the preferred osmolyte. Deleting the glycerol phosphate phosphatase gene (GPP2) prevents cells from accumulating glycerol due to a block in the last step of its biosynthetic pathway. Gpp2 accumulates in a phosphorylated form in a cna1Δ strain, and a physical interaction between Gpp2 and Cna1 was found; moreover, the gpp2Δ strain undergoes slow growth and has attenuated virulence in animal models of infection. We provide biochemical evidence that growth in 1 M NaCl increases glycerol content in the wild type, whereas gpp2Δ, cna1Δ, and cnb1Δ mutants fail to accumulate it. The deletion of cnb1Δ or cna1Δ renders yeast cells sensitive to cationic stress, and the Gfp-Gpp2 protein assumes an abnormal localization. We suggest a mechanism in which calcineurin controls Gpp2 at the post-translational level, affecting its localization and activity, leading to glycerol biosynthesis. Also, we showed the transcriptional profile of glycerol-deficient mutants and established the cationic stress response mediated by calcineurin; among the biological processes differentially expressed are carbon utilization, translation, transmembrane transport, glutathione metabolism, oxidative stress response, and transcription regulation. To our knowledge, this is the first time that this transcriptional profile has been described. These results have implications for pathogen stress adaptability.

Multiple Roles of the Low-Affinity Calcium Uptake System in Drechslerella dactyloides, a Nematode-Trapping Fungus That Forms Constricting Rings

(1) Background: the low-affinity calcium uptake system (LACS) has been shown to play a crucial role in the conidiation and formation of adhesive nets and knobs by nematode-trapping fungi (NTF), but its involvement in the formation of constricting rings (CRs), mechanical traps to capture free-living nematodes, remains unexplored. (2) Methods: we investigated the function of two LACS genes (DdaFIG_1 and DdaFIG_2) in Drechslerella dactyloides, an NTF that forms CRs. We generated single (DdaFIG_1Ri and DdaFIG_2Ri) and double (DdaFIG_1,2Ri) knockdown mutants via the use of RNA interference (RNAi). (3) Results: suppression of these genes significantly affected conidiation, trap formation, vegetative growth, and response to diverse abiotic stresses. The number of CRs formed by DdaFIG_1Ri, DdaFIG_2Ri, and DdaFIG_1,2Ri decreased to 58.5%, 59.1%, and 38.9% of the wild-type (WT) level, respectively. The ring cell inflation rate also decreased to 73.6%, 60.6%, and 48.8% of the WT level, respectively. (4) Conclusions: the LACS plays multiple critical roles in diverse NTF.

New nanotechnological formulation based on amiodarone-loaded lipid core nanocapsules displays anticryptococcal effect

Cryptococcus neoformans is the etiological agent of cryptococcal meningoencephalitis. The recommended available treatment has low efficiency, with high toxicity and resistance as recurrent problems. In the search of new treatment protocols, the proposal of new pharmacological approaches is considered an innovative strategy, mainly nanotechnological systems considering fungal diseases. The antiarrhythmic drug amiodarone has demonstrated antifungal activity against a range of fungi, including C. neoformans. Here, considering the importance of calcium storage mediated by transporters on cryptococcal virulence, we evaluated the use of the calcium channel blocker amiodarone as an alternative therapy for cryptococcosis. C. neoformans displayed high sensitivity to amiodarone, which was also synergistic with fluconazole. Amiodarone treatment influenced some virulence factors, interrupting the calcium-calcineurin signaling pathway. Experiments with murine cryptococcosis models revealed that amiodarone treatment increased the fungal burden in the lungs, while its combination with fluconazole did not improve treatment compared to fluconazole alone. In addition, we have developed different innovative nanotechnological formulations, one of which combining two drugs with different mechanisms of action. Lipid-core nanocapsules (LNC) loaded with amiodarone (LNC_AMD), fluconazole (LNC_FLU) and both (LNC_AMD+FLU) were produced to achieve a better efficacy in vivo. In an intranasal model of treatment, all the LNC formulations had an antifungal effect. In an intraperitoneal treatment, LNC_AMD showed an enhanced anticryptococcal effect compared to the free drug, whereas LNC_FLU or LNC_AMD+FLU displayed no differences from the free drugs. In this way, nanotechnology using amiodarone formulations could be an effective therapy for cryptococcal infections.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Natural alkaloid tryptanthrin exhibits novel anticryptococcal activity

Cryptococcal meningitis is a prevalent invasive fungal infection that causes around 180 000 deaths annually. Currently, treatment for cryptococcal meningitis is limited and new therapeutic options are needed. Historically, medicinal plants are used to treat infectious and inflammatory skin infections. Tryptanthrin is a natural product commonly found in these plants. In this study, we demonstrated that tryptanthrin had antifungal activity with minimum inhibitory concentration (MIC) of 2 μg/ml against Cryptococcus species and of 8 μg/ml against Trichophyton rubrum. Further analysis demonstrated that tryptanthrin exerted fungistatic and potent antifungal activity at elevated temperature. In addition, tryptanthrin exhibited a synergistic effect with the calcineurin inhibitors FK506 and cyclosporine A against Cryptococcus neoformans. Furthermore, our data showed that tryptanthrin induced cell cycle arrest at the G1/S phase by regulating the expression of genes encoding cyclins and the SBF/MBF complex (CLN1, MBS1, PCL1, and WHI5) in C. neoformans. Screening of a C. neoformans mutant library further revealed that tryptanthrin was associated with various transporters and signaling pathways such as the calcium transporter (Pmc1) and protein kinase A signaling pathway. In conclusion, tryptanthrin exerted novel antifungal activity against Cryptococcus species through a mechanism that interferes with the cell cycle and signaling pathways.

LAY SUMMARY

The natural product tryptanthrin had antifungal activity against Cryptococcus species by interfering cell cycle and exerted synergistic effects with immunosuppressants FK506 and cyclosporine A. Our findings suggest that tryptanthrin may be a potential drug or adjuvant for the treatment of cryptococcosis.

Functional Coupling between the Unfolded Protein Response and Endoplasmic Reticulum/Golgi Ca2+-ATPases Promotes Stress Tolerance, Cell Wall Biosynthesis, and Virulence of Aspergillus fumigatus

Many species of pathogenic fungi deploy the unfolded protein response (UPR) to expand the folding capacity of the endoplasmic reticulum (ER) in proportion to the demand for virulence-related proteins that traffic through the secretory pathway. Although Ca²⁺ plays a pivotal role in ER function, the mechanism by which transcriptional upregulation of the protein folding machinery is coordinated with Ca²⁺ homeostasis is incompletely understood. In this study, we investigated the link between the UPR and genes encoding P-type Ca²⁺-ATPases in the human-pathogenic mold Aspergillus fumigatus We demonstrate that acute ER stress increases transcription of the srcA gene, encoding a member of the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) family, as well as that of pmrA, encoding a secretory pathway Ca²⁺-ATPase (SPCA) in the Golgi membrane. Loss of the UPR transcription factor HacA prevented the induction of srcA and pmrA transcription during ER stress, defining these ER/Golgi Ca²⁺ pumps as novel downstream targets of this pathway. While deletion of srcA alone caused no major deficiencies, a ΔsrcA/ΔpmrA mutant displayed a severe polarity defect, was hypersensitive to ER stress, and showed attenuated virulence. In addition, cell wall analyses revealed a striking reduction in mannose levels in the absence of both Ca²⁺ pumps. The ΔhacA mutant was hypersensitive to agents that block calcineurin-dependent signaling, consistent with a functional coupling between the UPR and Ca²⁺ homeostasis. Together, these findings demonstrate that the UPR integrates the need for increased levels of chaperone and folding enzymes with an influx of Ca²⁺ into the secretory pathway to support fungal growth, stress adaptation, and pathogenicity.IMPORTANCE The UPR is an intracellular signal transduction pathway that maintains homeostasis of the ER. The pathway is also tightly linked to the expression of virulence-related traits in diverse species of human-pathogenic and plant-pathogenic fungal species, including the predominant mold pathogen infecting humans, Aspergillus fumigatus Despite advances in the understanding of UPR signaling, the linkages and networks that are governed by this pathway are not well defined. In this study, we revealed that the UPR is a major driving force for stimulating Ca²⁺ influx at the ER and Golgi membranes and that the coupling between the UPR and Ca²⁺ import is important for virulence, cell wall biosynthesis, and resistance to antifungal compounds that inhibit Ca²⁺ signaling.

Calcium Transport Proteins in Fungi: The Phylogenetic Diversity of Their Relevance for Growth, Virulence, and Stress Resistance

The key players of calcium (Ca²⁺) homeostasis and Ca²⁺ signal generation, which are Ca²⁺ channels, Ca²⁺/H⁺ antiporters, and Ca²⁺-ATPases, are present in all fungi. Their coordinated action maintains a low Ca²⁺ baseline, allows a fast increase in free Ca²⁺ concentration upon a stimulus, and terminates this Ca²⁺ elevation by an exponential decrease – hence forming a Ca²⁺ signal. In this respect, the Ca²⁺ signaling machinery is conserved in different fungi. However, does the similarity of the genetic inventory that shapes the Ca²⁺ peak imply that if “you’ve seen one, you’ve seen them all” in terms of physiological relevance? Individual studies have focused mostly on a single species, and mechanisms elucidated in few model organisms are usually extrapolated to other species. This mini-review focuses on the physiological relevance of the machinery that maintains Ca²⁺ homeostasis for growth, virulence, and stress responses. It reveals common and divergent functions of homologous proteins in different fungal species. In conclusion, for the physiological role of these Ca²⁺ transport proteins, “seen one,” in many cases, does not mean: “seen them all.”

The Glycerol Phosphatase Gpp2: A Link to Osmotic Stress, Sulfur Assimilation and Virulence in Cryptococcus neoformans

Cryptococcus neoformans is an opportunist fungal pathogen that causes meningoencephalitis in immunocompromised patients. During infection, this basidiomycete yeast has to adapt to several adverse conditions, especially nutrient availability. The interruption on various amino acid biosynthetic pathways and on amino acid uptake causes reduced viability, inability to cope with various stresses, failure in virulence factors expression and avirulence in animal model of infection. The sulfur amino acid biosynthesis and uptake is an important feature for pathogen survival in vivo and in vitro. Our previous work demonstrates that C. neoformans Cys3 BZip transcription factor controls the gene expression in several steps of the sulfur assimilation and sulfur amino acid biosynthesis. Also, we have shown that Gpp2 phosphatase modulates Cys3 activity. In Saccharomyces cerevisiae Gpp2 is induced in response to hyper osmotic or oxidative stress and during diauxic shift. In this work, we will show that, in C. neoformans, Gpp2 is required to respond to stresses, mainly osmotic stress; also its transcription is induced during exposure to NaCl. Global transcriptional profile of gpp2Δ by RNAseq shows that CYS3 and other genes in the sulfur assimilation pathway are up regulated, which is consistent with our previous report, in which Gpp2 acts by avoiding Cys3 accumulation and nuclear localization. In addition, several transporters genes, especially amino acid permeases and oxidative stress genes are induced in the gpp2Δ strain; on the contrary, genes involved in glucose and tricarboxylic acid metabolism are down regulated. gpp2Δ strain fails to express virulence factors, as melanin, phospholipase, urease and has virulence attenuation in Galleria mellonella. Our data suggest that Gpp2 is an important factor for general pathogen adaptation to various stresses and also to the host, and perhaps it could be an interesting target for therapeutic use.

Genomic Insights into Nematicidal Activity of a Bacterial Endophyte, Raoultella ornithinolytica MG against Pine Wilt Nematode

Pine wilt disease, caused by the nematode Bursaphelenchus xylophilus, is one of the most devastating conifer diseases decimating several species of pine trees on a global scale. Here, we report the draft genome of Raoultella ornithinolytica MG, which is isolated from mountain-cultivated ginseng plant as an bacterial endophyte and shows nematicidal activity against B. xylophilus. Our analysis of R. ornithinolytica MG genome showed that it possesses many genes encoding potential nematicidal factors in addition to some secondary metabolite biosynthetic gene clusters that may contribute to the observed nematicidal activity of the strain. Furthermore, the genome was lacking key components of avermectin gene cluster, suggesting that nematicidal activity of the bacterium is not likely due to the famous anthelmintic agent of wide-spread use, avermectin. This genomic information of R. ornithinolytica will provide basis for identification and engineering of genes and their products toward control of pine wilt disease.

Cryptococcal dissemination to the central nervous system requires the vacuolar calcium transporter Pmc1

Cryptococcus neoformans is a basidiomycetous yeast and the cause of cryptococcosis in immunocompromised individuals. The most severe form of the disease is meningoencephalitis, which is one of the leading causes of death in HIV/AIDS patients. In order to access the central nervous system, C. neoformans relies on the activity of certain virulence factors such as urease, which allows transmigration through the blood-brain barrier. In this study, we demonstrate that the calcium transporter Pmc1 enables C. neoformans to penetrate the central nervous system, because the pmc1 null mutant failed to infect and to survive within the brain parenchyma in a murine systemic infection model. To investigate potential alterations in transmigration pathways in these mutants, global expression profiling of the pmc1 mutant strain was undertaken, and genes associated with urease, the Ca²⁺ -calcineurin pathway, and capsule assembly were identified as being differentially expressed. Also, a decrease in urease activity was observed in the calcium transporter null mutants. Finally, we demonstrate that the transcription factor Crz1 regulates urease activity and that the Ca²⁺ -calcineurin signalling pathway positively controls the transcription of calcium transporter genes and factors related to transmigration.

Comparative transcriptome analysis of the calcium signaling and expression analysis of sodium/calcium exchanger in Aspergillus cristatus

Aspergillus cristatus develops into various stages under different Na concentrations: the sexual stage in 0.5 M NaCl and asexual development stage in 3 M NaCl. In order to explore whether the Ca²⁺ signaling pathway in A. cristatus responded to the changes in the salt stress, we analyzed the gene expression levels in A. cristatus respectively cultured in 0.5 M NaCl and 3 M NaCl. According to the BLAST analysis results, we identified 25 Ca²⁺ -signaling proteins in A. cristatus. The expression levels of most genes involved in the Ca²⁺ -signaling pathway in A. cristatus cultured in different salt concentrations showed significant differences, indicating that the Ca²⁺ signaling pathway was involved in the response to the changes in the salt stress. In yeasts, only calcium ion influx proteins were reported to be involved in the response to the changes in the salt stress. So far, the protein for the exchanger of calcium/sodium ions has not been reported. Therefore, we obtained the sodium/calcium exchanger (termed NCX) proteins from the KEGG Database. The ncx gene of A. cristatus was cloned and characterized. The full length of ncx gene is 3055 bp, including a 2994-bp open reading frame encoding 994 amino acids. The expression levels of ncx in the sexual development stage and asexual development stage were respectively ∼8.94 times and ∼2.57 times of that in the hyphal formation stage. Therefore, we suggested that ncx gene was up-regulated to resist the sodium stress. The study results provide the basis for further exploring the Ca²⁺ -signaling mechanism and ion exchanger mechanism.

Differential Roles for Six P-Type Calcium ATPases in Sustaining Intracellular Ca2+ Homeostasis, Asexual Cycle and Environmental Fitness of Beauveria bassiana

A global insight into the roles of multiple P-type calcium ATPase (CA) pumps in sustaining the life of a filamentous fungal pathogen is lacking. Here we elucidated the functions of five CA pumps (Eca1, Spf1 and PmcA/B/C) following previous characterization of Pmr1 in Beauveria bassiana, a fungal insect pathogen. The fungal CA pumps interacted at transcriptional level, at which singular deletions of five CA genes depressed eca1 expression by 76–98% and deletion of spf1 resulted in drastic upregulation of four CA genes by 36–50-fold. Intracellular Ca²⁺ concentration increased differentially in most deletion mutants exposed to the stresses of Ca²⁺, EDTA chelator, and/or endoplasmic reticulum and calcineurin inhibitors, accompanied with their changed sensitivities to not only the mentioned agents but also Fe²⁺, Cu²⁺ and Zn²⁺. Liquid culture acidification was delayed in the Δspf1, Δpmr1 and ΔpmcA mutants, coinciding well with altered levels of their extracellular lactic and oxalic acids. Moreover, all deletion mutants showed differential defects in conidial germination, vegetative growth, conidiation capacity, antioxidant activity, cell wall integrity, conidial UV-B resistance and/or virulence. Our results provide the first global insight into differential roles for six CA pumps in sustaining intracellular Ca²⁺ level, asexual cycle and environmental fitness of B. bassiana.

Complete genome sequence of Fictibacillus arsenicus G25-54, a strain with toxicity to nematodes

Root-knot nematodes (RKNs) can infect almost all crops and cause huge economic losses in agriculture worldwide. An in-depth understanding of bacteria with nematicidal activity is essential for an effective and environmentally friendly control of RKNs. Fictibacillus arsenicus G25-54, a gram-positive and spore-forming bacterium isolated from a submerged sand bank, shows nematicidal activity against free-living Caenorhabditis elegans and RKNs. Here, we report the complete genome of F. arsenicus G25-54, which contains a circular chromosome and encodes ten potential nematicidal factors with twelve secondary metabolite gene clusters. Additionally, it encodes five arsenic resistance and transformation related proteins, which may provide the potential arsenic-resistance activity.

Complete genome sequence of Fictibacillus phosphorivorans G25-29, a strain toxic to nematodes

Root-knot nematodes (RKNs) which can infect almost all crops lead to huge economic losses in agriculture around the world. Unavailability of effective and environmentally friendly control of RKNs provides an opportunity to nematicidal bacteria. Fictibacillus phosphorivorans G25-29 is a gram-positive and spore-forming bacterium with nematicidal capability against root-knot nematodes and free-living nematode Caenorhabditis elegans. Here, we report the complete genome of F. phosphorivorans G25-29, containing a circular chromosome and encoding nine potential nematicidal factors which may contribute to its nematicidal activity.

Nematicidal spore-forming Bacilli share similar virulence factors and mechanisms

In the soil environment, Bacilli can affect nematode development, fecundity and survival. However, although many Bacillus species can kill nematodes, the virulence mechanisms Bacilli utilize remain unknown. In this study, we collected 120 strains comprising 30 species across the Bacillaceae and Paenibacillaceae families of the Bacillales order and measured their nematicidal activities in vitro. Comparison of these strains’ nematicidal capacities revealed that nine species, including Bacillus thuringiensis, B. cereus, B. subtilis, B. pumilus, B. firmus, B. toyonensis, Lysinibacillus sphaericus, Brevibacillus laterosporus and B. brevis, were highly nematicidal, the first of which showed the highest activity. Genome sequencing and analysis identified many potential virulence factors, which grouped into five types. At least four possible mechanisms were deduced on the basis of the combination of these factors and the bacterial nematicidal activity, including a pore-forming mechanism of crystal proteins, an inhibition-like mechanism of thuringiensin and a degradation mechanism of proteases and/or chitinases. Our results demonstrate that 120 spore-forming Bacilli across different families share virulence factors that may contribute to their nematicidal capacity.

Usefulness of silkworm as a host animal for understanding pathogenicity of Cryptococcus neoformans

We propose Cryptococcus neoformans infection model using silkworm for understanding cryptococcosis and screening of therapeutically effective antibiotics. Silkworm is an insect whose rearing methods were established through a long history of the sericulture industry. Silkworm facilitates experiments using a large number of individuals because of low cost for rearing and few ethical problems caused by killing animals. Silkworm can be reared at 37˚C to perform infection experiments at same temperature to human body. Injection of accurate amounts of samples into hemolymph of silkworm by usual syringes is easy to be done since silkworm has an appropriate size to handle. Moreover two injection methods, injection into hemolymph and intestine, are distinguishable for silkworms. The former is correspondent to intravenous injection, and the latter is to oral administration in humans. Taking these advantages of silkworms as host animals, it is possible to evaluate the virulence factors in C. neoformans and the therapeutic efficacy of antifungal agents.

Complete Genome Sequence of a High Lipid-Producing Strain of Mucor circinelloides WJ11 and Comparative Genome Analysis with a Low Lipid-Producing Strain CBS 277.49

The genome of a high lipid-producing fungus Mucor circinelloides WJ11 (36% w/w lipid, cell dry weight, CDW) was sequenced and compared with that of the low lipid-producing strain, CBS 277.49 (15% w/w lipid, CDW), which had been sequenced by Joint Genome Institute. The WJ11 genome assembly size was 35.4 Mb with a G+C content of 39.7%. The general features of WJ11 and CBS 277.49 indicated that they have close similarity at the level of gene order and gene identity. Whole genome alignments with MAUVE revealed the presence of numerous blocks of homologous regions and MUMmer analysis showed that the genomes of these two strains were mostly co-linear. The central carbon and lipid metabolism pathways of these two strains were reconstructed and the numbers of genes encoding the enzymes related to lipid accumulation were compared. Many unique genes coding for proteins involved in cell growth, carbohydrate metabolism and lipid metabolism were identified for each strain. In conclusion, our study on the genome sequence of WJ11 and the comparative genomic analysis between WJ11 and CBS 277.49 elucidated the general features of the genome and the potential mechanism of high lipid accumulation in strain WJ11 at the genomic level. The different numbers of genes and unique genes involved in lipid accumulation may play a role in the high oleaginicity of strain WJ11.

In silico analysis of the core signaling proteome from the barley powdery mildew pathogen (Blumeria graminis f.sp. hordei)

BACKGROUND

Compared to other ascomycetes, the barley powdery mildew pathogen Blumeria graminis f.sp. hordei (Bgh) has a large genome (ca. 120 Mbp) that harbors a relatively small number of protein-coding genes (ca. 6500). This genomic assemblage is thought to be the result of numerous gene losses, which likely represent an evolutionary adaptation to a parasitic lifestyle in close association with its host plant, barley (Hordeum vulgare). Approximately 8% of the Bgh genes are predicted to encode virulence effectors that are secreted into host tissue and/or cells to promote pathogenesis; the remaining proteome is largely uncharacterized at present.

RESULTS

We provide a comparative analysis of the conceptual Bgh proteome, with an emphasis on proteins with known roles in fungal development and pathogenicity, for example heterotrimeric G proteins and G protein coupled receptors; components of calcium and cAMP signaling; small monomeric GTPases; mitogen-activated protein cascades and transcription factors. The predicted Bgh proteome lacks a number of proteins that are otherwise conserved in filamentous fungi, including two proteins that are required for the formation of anastomoses (somatic hyphal connections). By contrast, apart from minor modifications, all major canonical signaling pathways are retained in Bgh. A family of kinases that preferentially occur in pathogenic species of the fungal clade Leotiomyceta is unusually expanded in Bgh and its close relative, Blumeria graminis f.sp. tritici.

CONCLUSIONS

Our analysis reveals characteristic features of the proteome of a fungal phytopathogen that occupies an extreme habitat: the living plant cell.

Inhibition of x-box binding protein 1 reduces tunicamycin-induced apoptosis in aged murine macrophages

Endoplasmic reticulum (ER) stress is induced by the accumulation of unfolded and misfolded proteins in the ER. Although apoptosis induced by ER stress has been implicated in several aging-associated diseases, such as atherosclerosis, it is unclear how aging modifies ER stress response in macrophages. To decipher this relationship, we assessed apoptosis in macrophages isolated from young (1.5-2 months) and aged (16-18 months) mice and exposed the cells to the ER stress inducer tunicamycin. We found that aged macrophages exhibited more apoptosis than young macrophages, which was accompanied by reduced activation of phosphorylated inositol-requiring enzyme-1 (p-IRE1α), one of the three key ER stress signal transducers. Reduced gene expression of x-box binding protein 1 (XBP1), a downstream effector of IRE1α, enhanced p-IRE1α levels and reduced apoptosis in aged, but not young macrophages treated with tunicamycin. These findings delineate a novel, age-dependent interaction by which macrophages undergo apoptosis upon ER stress, and suggest an important protective role of IRE1α in aging-associated ER stress-induced apoptosis. This novel pathway may not only be important in our understanding of longevity, but may also have important implications for pathogenesis and potential treatment of aging-associated diseases in general.

The calcium transporter Pmc1 provides Ca2+ tolerance and influences the progression of murine cryptococcal infection

The Ca(2+)-calcineurin signaling pathway in the human fungal pathogen Cryptococcus neoformans is essential for adaptation to the host environment during infection. Calcium transporters regulate cytosolic calcium concentrations, providing Ca(2+) loading into storage organelles. The three calcium transporters that have been characterized in C. neoformans, Cch1, Eca1 and Vcx1, are required for fungal virulence, supporting a role for calcium-mediated signaling in cryptococcal pathogenesis. In the present study, we report the functional characterization of the putative vacuolar calcium ATPase Pmc1 in C. neoformans. Our results demonstrate that Pmc1 provides tolerance to high Ca(2+) concentrations. The double knockout of C. neoformans PMC1 and VCX1 genes impaired the intracellular calcium transport, resulting in a significant increase in cytosolic calcium levels. Furthermore, Pmc1 was essential for both the progression of pulmonary infection and brain colonization in mice, emphasizing the crucial role of calcium signaling and transport for cryptococcal pathogenesis.

Hyperthermia sensitizes Rhizopus oryzae to posaconazole and itraconazole action through apoptosis

The high mortality rate of mucormycosis with currently available monotherapy has created interest in studying novel strategies for antifungal agents. With the exception of amphotericin B (AMB), the triazoles (posaconazole [PCZ] and itraconazole [ICZ]) are fungistatic in vitro against Rhizopus oryzae . We hypothesized that growth at a high temperature (42°C) results in fungicidal activity of PCZ and ICZ that is mediated through apoptosis. R. oryzae had high MIC values for PCZ and ICZ (16 to 64 μg/ml) at 25°C; in contrast, the MICs for PCZ and ICZ were significantly lower at 37°C (8 to 16 μg/ml) and 42°C (0.25 to 1 μg/ml). Furthermore, PCZ and ICZ dose-dependent inhibition of germination was more pronounced at 42°C than at 37°C. In addition, intracellular reactive oxygen species (ROS) increased significantly when fungi were exposed to antifungals at 42°C. Characteristic cellular changes of apoptosis in R. oryzae were induced by the accumulation of intracellular reactive oxygen species. Cells treated with PCZ or ICZ in combination with hyperthermia (42°C) exhibited characteristic markers of early apoptosis: phosphatidylserine externalization visualized by annexin V staining, membrane depolarization visualized by bis-[1,3-dibutylbarbituric acid] trimethine oxonol (DiBAC) staining, and increased metacaspase activity. Moreover, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assay and DAPI (4',6-diamidino-2-phenylindole) staining demonstrated DNA fragmentation and condensation, respectively. The addition of N-acetylcysteine increased fungal survival, prevented apoptosis, reduced ROS accumulation, and decreased metacaspase activation. We concluded that hyperthermia, either alone or in the presence of PCZ or ICZ, induces apoptosis in R. oryzae. Local thermal delivery could be a therapeutically useful adjunct strategy for these refractory infections.

Chaperone insufficiency links TLR4 protein signaling to endoplasmic reticulum stress

Inflammation plays an important pathogenic role in a number of metabolic diseases such as obesity, type 2 diabetes, and atherosclerosis. The activation of inflammation in these diseases depends at least in part on the combined actions of TLR4 signaling and endoplasmic reticulum stress, which by acting in concert can boost the inflammatory response. Defining the mechanisms involved in this phenomenon may unveil potential targets for the treatment of metabolic/inflammatory diseases. Here we used LPS to induce endoplasmic reticulum stress in the human monocyte cell-line, THP-1. The unfolded protein response, produced after LPS, was dependent on CD14 activity but not on RNA-dependent protein kinase and could be inhibited by an exogenous chemical chaperone. The induction of the endoplasmic reticulum resident chaperones, GRP94 and GRP78, by LPS was of a much lower magnitude than the effect of LPS on TLR4 and MD-2 expression. In face of this apparent insufficiency of chaperone expression, we induced the expression of GRP94 and GRP78 by glucose deprivation. This approach completely reverted endoplasmic reticulum stress. The inhibition of either GRP94 or GRP78 with siRNA was sufficient to rescue the protective effect of glucose deprivation on LPS-induced endoplasmic reticulum stress. Thus, insufficient LPS-induced chaperone expression links TLR4 signaling to endoplasmic reticulum stress.

A proteomic interrogation of Cryptococcus neoformans: interaction networks for calcineurin in a heated environment

Calcineurin (CN) is a calcium- and calmodulin-dependent protein phosphatase that consists of a catalytic subunit (calcineurin A [CnA]) and a calcium-binding, regulatory subunit (calcineurin B [CnB]). Calcineurin has been shown to be involved in a number of cellular processes, and aberrant signaling has been linked to multiple human diseases, such as cardiac hypertrophy and diabetes. Recent studies demonstrated that CN was involved in the survival of Cryptococcus neoformans, a fungal pathogen that infects humans, especially patients who are immunocompromised. CN appears to be essential for the survival and virulence of C. neoformans; however, the underlying mechanisms remain largely unknown. The Heitman laboratory recently identified a group of potential CnA-interacting proteins in C. neoformans during heat stress, and demonstrated an interaction of CnA with Sec28 and Sec13, which represent COPI and COPII protein complex members, respectively. The COP protein complexes are key proteins involved in intracellular endoplasmic reticulum and golgi protein trafficking. The results from the Heitman group suggest that CN interacts with components of the endoplasmic reticulum and the golgi during heat stress in C. neoformans and could highlight potential mechanisms by which these microbes could be targeted.

Calcium signaling components in the human pathogen: Cryptococcus neoformans

Calcium signaling through calmodulin and the phosphatase calcineurin are required for key events of the biology of the human pathogen Cryptococcus neoformans, including mating, morphogenesis, growth at 37°C and virulence. In a recent work we described the functional characterization of a new component of this calcium signaling network: the vacuolar calcium exchanger Vcx1. This transporter is involved in calcium tolerance and virulence in C. neoformans. Two other uncharacterized calcium transporters which are putative orthologs of Saccharomyces cerevisiae PMC1 (a vacuolar calcium ATPase) and PMR1 (a Golgi calcium ATPase) are also functional in C. neoformans. No ortholog of CRZ1, the target of calcineurin in other fungi, has been identified in C. neoformans, indicating a high complexity in cryptococcal calcium-related pathways. Future studies are necessary for the complete understanding of calcium signaling regulation in C. neoformans.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

The marine sponge-derived polyketide endoperoxide plakortide F acid mediates its antifungal activity by interfering with calcium homeostasis

Plakortide F acid (PFA), a marine-derived polyketide endoperoxide, exhibits strong inhibitory activity against the opportunistic fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. In the present study, transcriptional profiling coupled with mutant and biochemical analyses were conducted using the model organism Saccharomyces cerevisiae to investigate the mechanism of action of this compound. PFA elicited a transcriptome response indicative of a Ca(2+) imbalance, affecting the expression of genes known to be responsive to altered cellular calcium levels. Several additional lines of evidence obtained supported a role for Ca(2+) in PFA's activity. First, mutants lacking calcineurin and various Ca(2+) transporters, including pumps (Pmr1 and Pmc1) and channels (Cch1 and Mid1), showed increased sensitivity to PFA. In addition, the calcineurin inhibitors FK506 and cyclosporine strongly enhanced PFA activity in wild-type cells. Furthermore, PFA activated the transcription of a lacZ reporter gene driven by the calcineurin-dependent response element. Finally, elemental analysis indicated a significant increase in intracellular calcium levels in PFA-treated cells. Collectively, our results demonstrate that PFA mediates its antifungal activity by perturbing Ca(2+) homeostasis, thus representing a potentially novel mechanism distinct from that of currently used antifungal agents.

Response to environmental stresses, cell-wall integrity, and virulence are orchestrated through the calcineurin pathway in Ustilago hordei

In eukaryotes, several biological processes are regulated through calcium signaling. Calcineurin is a calcium-calmodulin-regulated serine/threonine phosphatase consisting of catalytic subunit A and regulatory subunit B. Phosphatase activity resides in the catalytic subunit, which activates by dephosphorylation downstream components such as transcription factor Crz1. The importance of this pathway to respond to environmental stress has been explored in several fungal pathogens. The basidiomycete Ustilago hordei causes covered smut of barley. We addressed the role of the Ca(2+)-calcineurin activated pathway by deleting UhCna1 and UhCnb1. These genes were not essential in U. hordei but the corresponding mutants displayed a variety of phenotypes when applying environmental stress such as sensitivity to pH, temperature, H₂O₂, mono- and divalent cations; and to genotoxic, acid, or oxidative stresses. Cell-wall integrity was compromised and mutants displayed altered cell morphologies. Mating was delayed but not abolished, and combined sensitivities likely explained a severely reduced virulence toward barley plants. Expression analyses revealed that response to salt stress involved the induction of membrane ATPase genes UhEna1 and UhEna2, which were regulated through the calcineurin pathway. Upregulation of UhFKS1, a 1,3-β-d-glucan synthase gene, correlated with the increased amount of 1,3-β-d-glucan in the calcineurin mutants grown under salt stress.

The vacuolar Ca²(+) exchanger Vcx1 is involved in calcineurin-dependent Ca²(+) tolerance and virulence in Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated yeast that causes a life-threatening meningoencephalitis in immunocompromised individuals. The ability to survive and proliferate at the human body temperature is an essential virulence attribute of this pathogen. This trait is controlled in part by the Ca²(+)-calcineurin pathway, which senses and utilizes cytosolic calcium for signaling. In the present study, the identification of the C. neoformans gene VCX1, which encodes a vacuolar calcium exchanger, is reported. The VCX1 knockout results in hypersensitivity to the calcineurin inhibitor cyclosporine A at 35°C, but not at 30°C. Furthermore, high concentrations of CaCl₂ lead to growth inhibition of the vcx1 mutant strain only in the presence of cyclosporine A, indicating that Vcx1 acts in parallel with calcineurin. The loss of VCX1 does not influence cell wall integrity or capsule size but decreases secretion of the major capsular polysaccharide glucuronoxylomannan (GXM) in culture supernatants.Vcx1 also influences C. neoformans phagocytosis by murine macrophages and is required for full virulence in mice. Analysis of cellular distribution by confocal microscopy confirmed the vacuolar localization of Vcx1 in C. neoformans cells.

Aspergillus fumigatus AcuM regulates both iron acquisition and gluconeogenesis

Relatively few transcription factors that govern the virulence of Aspergillus fumigatus are known. We constructed 11 A. fumigatus transcription factor mutants and screened them for altered virulence in Galleria mellonella larvae. We discovered that the zinc cluster transcription factor, AcuM, is essential for maximal virulence in this model, as well as in murine models of haematogenously disseminated and invasive pulmonary aspergillosis. Transcriptional profiling experiments suggested that AcuM suppresses sreA and induces hapX to stimulate expression of genes involved in both reductive iron assimilation and siderophore-mediated iron uptake. Consistent with these results, a ΔacuM mutant had reduced iron incorporation, decreased extracellular siderophore production and impaired capacity to grow under iron-limited conditions. Interestingly, an Aspergillus nidulansΔacuM mutant had normal extracellular siderophore production and growth under iron-limited conditions, indicating that AcuM does not govern iron acquisition in this organism. A. fumigatus AcuM also regulated genes involved in gluconeogenesis, and the ΔacuM mutant had impaired growth on gluconeogenic carbon sources. Deletion of sreA in the ΔacuM mutant restored iron uptake, extracellular siderophore production and virulence, but not the defect in gluconeogenesis. Thus, AcuM represses SreA and thereby induces iron acquisition, a process that is essential for the maximal virulence of A. fumigatus.

Survival defects of Cryptococcus neoformans mutants exposed to human cerebrospinal fluid result in attenuated virulence in an experimental model of meningitis

Cryptococcus neoformans is a fungal pathogen that encounters various microenvironments during growth in the mammalian host, including intracellular vacuoles, blood, and cerebrospinal fluid (CSF). Because the CSF is isolated by the blood-brain barrier, we hypothesize that CSF presents unique stresses that C. neoformans must overcome to establish an infection. We assayed 1,201 mutants for survival defects in growth media, saline, and human CSF. We assessed CSF-specific mutants for (i) mutant survival in both human bronchoalveolar lavage (BAL) fluid and fetal bovine serum (FBS), (ii) survival in macrophages, and (iii) virulence using both Caenorhabditis elegans and rabbit models of cryptococcosis. Thirteen mutants exhibited significant survival defects unique to CSF. The mutations of three of these mutants were recreated in the clinical serotype A strain H99: deletions of the genes for a cation ATPase transporter (ena1Δ), a putative NEDD8 ubiquitin-like protein (rub1Δ), and a phosphatidylinositol 4-kinase (pik1Δ). Mutant survival rates in yeast media, saline, and BAL fluid were similar to those of the wild type; however, survival in FBS was reduced but not to the levels in CSF. These mutant strains also exhibited decreased intracellular survival in macrophages, various degrees of virulence in nematodes, and severe attenuation of survival in a rabbit meningitis model. We analyzed the CSF by mass spectrometry for candidate compounds responsible for the survival defect. Our findings indicate that the genes required for C. neoformans survival in CSF ex vivo are necessary for survival and infection in this unique host environment.

Phagocytosis of Cryptococcus neoformans by, and nonlytic exocytosis from, Acanthamoeba castellanii

Cryptococcus neoformans, an encapsulated, pathogenic yeast, is endowed with a variety of virulence factors, including a polysaccharide capsule. During mammalian infection, the outcome of the interaction between C. neoformans and macrophages is central to determining the fate of the host. Previous studies have shown similarities between the interaction of C. neoformans with macrophages and with amoebae, resulting in the proposal that fungal virulence for mammals originated from selection by amoeboid predators. In this study, we investigated the interaction of C. neoformans with the soil amoeba Acanthamoeba castellanii. Comparison of phagocytic efficiency of the wild type, nonencapsulated mutants, and complemented strains showed that the capsule was antiphagocytic for amoebae. Capsular enlargement was associated with a significant reduction in phagocytosis, suggesting that this phenomenon protects against ingestion by phagocytic predators. C. neoformans var. neoformans cells were observed to exit amoebae several hours after ingestion, in a process similar to the recently described nonlytic exocytosis from macrophages. Cryptococcal exocytosis from amoebae was dependent on the strain and on actin and required fungal viability. Additionally, the presence of a capsule was inversely correlated with the likelihood of extrusion in certain strains. In summary, nonlytic exocytosis from amoebae provide another parallel to observations in fungus-macrophage interactions. These results provide additional support for the notion that some mechanisms of virulence observed during mammalian infection originated, and were selected for, by environmental interactions.

Relationship between endoplasmic reticulum- and Golgi-associated calcium homeostasis and 4-NQO-induced DNA repair in Saccharomyces cerevisiae

Calcium (Ca(2+)) is an important ion that is necessary for the activation of different DNA repair mechanisms. However, the mechanism by which DNA repair and Ca(2+) homeostasis cooperate remains unclear. We undertook a systems biology approach to verify the relationship between proteins associated with Ca(2+) homeostasis and DNA repair for Saccharomyces cerevisiae. Our data indicate that Pmr1p, a Ca(2+) transporter of Golgi complex, interacts with Cod1p, which regulates Ca(2+) levels in the endoplasmic reticulum (ER), and with Rad4p, which is a nucleotide excision repair (NER) protein. This information was used to construct single and double mutants defective for Pmr1p, Cod1p, and Rad4p followed by cytotoxic, cytostatic, and cell cycle arrest analyses after cell exposure to different concentrations of 4-nitroquinoline 1-oxide (4-NQO). The results indicated that cod1Delta, cod1Deltarad4Delta, and cod1Deltapmr1Delta strains have an elevated sensitivity to 4-NQO when compared to its wild-type (WT) strain. Moreover, both cod1Deltapmr1Delta and cod1Deltarad4Delta strains have a strong arrest at G(2)/M phases of cell cycle after 4-NQO treatment, while pmr1Deltarad4Delta have a similar sensitivity and cell cycle arrest profile when compared to rad4Delta after 4-NQO exposure. Taken together, our results indicate that deletion in Golgi- and ER-associated Ca(2+) transporters affect the repair of 4-NQO-induced DNA damage.

Cch1 restores intracellular Ca2+ in fungal cells during endoplasmic reticulum stress

Pathogens endure and proliferate during infection by exquisitely coping with the many stresses imposed by the host to prevent pathogen survival. Recent evidence has shown that fungal pathogens and yeast respond to insults to the endoplasmic reticulum (ER) by initiating Ca(2+) influx across their plasma membrane. Although the high affinity Ca(2+) channel, Cch1, and its subunit Mid1, have been suggested as the protein complex responsible for mediating Ca(2+) influx, a direct demonstration of the gating mechanism of the Cch1 channel remains elusive. In this first mechanistic study of Cch1 channel activity we show that the Cch1 channel from the model human fungal pathogen, Cryptococcus neoformans, is directly activated by the depletion of intracellular Ca(2+) stores. Electrophysiological analysis revealed that agents that enable ER Ca(2+) store depletion promote the development of whole cell inward Ca(2+) currents through Cch1 that are effectively blocked by La(3+) and dependent on the presence of Mid1. Cch1 is permeable to both Ca(2+) and Ba(2+); however, unexpectedly, in contrast to Ca(2+) currents, Ba(2+) currents are steeply voltage-dependent. Cch1 maintains a strong Ca(2+) selectivity even in the presence of high concentrations of monovalent ions. Single channel analysis indicated that Cch1 channel conductance is small, similar to that reported for the Ca(2+) current I(CRAC). This study demonstrates that Cch1 functions as a store-operated Ca(2+)-selective channel that is gated by intracellular Ca(2+) depletion. The inability of cryptococcal cells that lacked the Cch1-Mid1 channel to survive ER stress suggests that Cch1 and its co-regulator, Mid1, are critical players in the restoration of Ca(2+) homeostasis.

A putative P-type ATPase, Apt1, is involved in stress tolerance and virulence in Cryptococcus neoformans

The export of virulence factors, such as the capsule polysaccharide, to the cell surface is a critical aspect of the pathogenicity of Cryptococcus neoformans. A view of capsule export via exocytosis and extracellular vesicles is emerging, but the molecular mechanisms underlying virulence factor transport pathways remain to be established. In this study, we characterized the APT1 gene, which encodes a predicted integral membrane P-type ATPase belonging to the type IV, Drs2 family of aminophospholipid translocases (flippases) (APTs). APTs maintain the phospholipid asymmetry that is critical in membrane fusion events for trafficking and in establishing cell polarity. Deletion of the APT1 gene resulted in phenotypes consistent with similar roles in C. neoformans. These included altered actin distribution, increased sensitivity to stress conditions (oxidative and nitrosative stress) and to trafficking inhibitors, such as brefeldin A and monensin, a reduction in exported acid phosphatase activity, and hypersensitivity to the antifungal drugs amphotericin B, fluconazole, and cinnamycin. However, there was no difference in growth, capsule size, or melanin production between the wild type and the apt1 mutant strains at either 30 degrees C or 37 degrees C. Despite the absence of an influence on these major virulence factors, Apt1 was required for survival during interactions with macrophages, and apt1 mutants exhibited attenuated virulence in a mouse inhalation model of cryptococcosis. Therefore, Apt1 contributes to virulence and the stress response in C. neoformans through apparent functions in membrane fusion and trafficking that do not influence the deposition of major virulence factors, such as capsule and melanin, outside the cell.

Efflux in fungi: la pièce de résistance

Pathogens must be able to overcome both host defenses and antimicrobial treatment in order to successfully infect and maintain colonization of the host. One way fungi accomplish this feat and overcome intercellular toxin accumulation is efflux pumps, in particular ATP-binding cassette transporters and transporters of the major facilitator superfamily. Members of these two superfamilies remove many toxic compounds by coupling transport with ATP hydrolysis or a proton gradient, respectively. Fungal genomes encode a plethora of members of these families of transporters compared to other organisms. In this review we discuss the role these two fungal superfamilies of transporters play in virulence and resistance to antifungal agents. These efflux transporters are responsible not only for export of compounds involved in pathogenesis such as secondary metabolites, but also export of host-derived antimicrobial compounds. In addition, we examine the current knowledge of these transporters in resistance of pathogens to clinically relevant antifungal agents.

First reported case of Cryptococcus gattii in the Southeastern USA: implications for travel-associated acquisition of an emerging pathogen

In 2007, the first confirmed case of Cryptococcus gattii was reported in the state of North Carolina, USA. An otherwise healthy HIV negative male patient presented with a large upper thigh cryptococcoma in February, which was surgically removed and the patient was started on long-term high-dose fluconazole treatment. In May of 2007, the patient presented to the Duke University hospital emergency room with seizures. Magnetic resonance imaging revealed two large CNS lesions found to be cryptococcomas based on brain biopsy. Prior chest CT imaging had revealed small lung nodules indicating that C. gattii spores or desiccated yeast were likely inhaled into the lungs and dissemination occurred to both the leg and CNS. The patient's travel history included a visit throughout the San Francisco, CA region in September through October of 2006, consistent with acquisition during this time period. Cultures from both the leg and brain biopsies were subjected to analysis. Based on phenotypic and molecular methods, both isolates were C. gattii, VGI molecular type, and distinct from the Vancouver Island outbreak isolates. Based on multilocus sequence typing of coding and noncoding regions and virulence in a heterologous host model, the leg and brain isolates are identical, but the two differed in mating fertility. Two clinical isolates, one from a transplant recipient in San Francisco and the other from Australia, were identical to the North Carolina clinical isolate at all markers tested. Closely related isolates that differ at only one or a few noncoding markers are present in the Australian environment. Taken together, these findings support a model in which C. gattii VGI was transferred from Australia to California, possibly though an association with its common host plant E. camaldulensis, and the patient was exposed in San Francisco and returned to present with disease in North Carolina.

Aspergillus fumigatus calcipressin CbpA is involved in hyphal growth and calcium homeostasis

Calcineurin is a conserved protein phosphatase that plays a critical role in Ca(2+) signaling and stress responses. Previously, a new class of conserved calcineurin-binding proteins, the calcipressins, was identified. However, the role of these proteins remains controversial, and both inhibitory and stimulatory effects on calcineurin were observed. In this study, we investigate the role of CbpA, the Aspergillus fumigatus member of the calcipressin family, and report that deletion of the cbpA gene resulted in reduced hyphal growth and limited attenuated virulence. Interestingly, under high-calcium-level conditions, the DeltacbpA strain displayed improved Ca(2+) tolerance compared to the wild-type strain and revealed increased expression of vcxA, chsA, and cnaA, which encode the vacuolar Ca(2+)/H(+) exchanger VcxA, chitin synthase A, and the calcineurin catalytic subunit CnaA, respectively. The increased transcript levels of these three genes were reversed in the presence of the calcineurin inhibitor FK506, indicating a calcineurin-dependent mechanism. Overexpression of cbpA resulted in decreased transcription of vcxA, chsA, and cnaA, associated with wild-type sensitivity to Ca(2+). Taken together, our study highlights the importance of CbpA in the regulation of hyphal growth and calcium adaptation of A. fumigatus and provides evidence that CbpA may serve as a feedback inhibitor in some aspects of calcineurin functions.

Identification of ENA1 as a virulence gene of the human pathogenic fungus Cryptococcus neoformans through signature-tagged insertional mutagenesis

A library of more than 4,500 signature-tagged insertion mutants of the human pathogenic fungus Cryptococcus neoformans was generated, and a subset was screened in a murine inhalation model to identify genes required for virulence. New genes that regulate aspects of C. neoformans virulence were also identified by screening the entire library for in vitro phenotypes related to the ability to cause disease, including melanin production, growth at high temperature, and growth under conditions of nutrient limitation. A screen of 10% of the strain collection in mice identified an avirulent mutant strain with an insertion in the ENA1 gene, which is predicted to encode a fungus-specific sodium or potassium P-type ATPase. The results of the deletion of the gene and complementation experiments confirmed its key role in mammalian virulence. ena1 mutant strains exhibited no change in sensitivity to high salt concentrations but were sensitive to alkaline pH conditions, providing evidence that the fungus may have to survive at elevated pH during infection of the mammalian host. The mutation of the well-characterized virulence factor calcineurin (CNA1) also rendered C. neoformans strains sensitive to elevated pH. ENA1 transcripts in wild-type and cna1 mutant strains were upregulated in response to high pH, and cna1 ena1 double mutant strains exhibited increased sensitivity to elevated pH, indicating that at least two pathways in the fungus mediate survival under alkaline conditions. Signature-tagged mutagenesis is an effective strategy for the discovery of new virulence genes in fungal pathogens of animals.

Signalling pathways in the pathogenesis of Cryptococcus

Efficient communication with the environment is critical for all living organisms. Fungi utilize complex signalling systems to sense their environments and control proliferation, development and in some cases virulence. Well-studied signalling pathways include the protein kinase A/cyclic AMP (cAMP), protein kinase C (PKC)/mitogen-activated protein kinase (MAPK), lipid signalling cascades, and the calcium-calcineurin signalling pathway. The human pathogenic basidiomycetous fungus Cryptococcus neoformans deploys sensitive signalling systems to survive in the human host, leading to life-threatening meningoencephalitis. Known virulence traits of this fungus, including the antioxidant melanin production, the antiphagocytic polysaccharide capsule and the ability to grow at 37 degrees C, are orchestrated by complex signalling networks, whose understanding is crucial to better treat, diagnose and prevent cryptococcosis.

Aspergillus parasiticus crzA, which encodes calcineurin response zinc-finger protein, is required for aflatoxin production under calcium stress

Two morphologically different Aspergillus parasiticus strains, one producing aflatoxins, abundant conidia but few sclerotia (BN9) and the other producing O-methyl-sterimatocystin (OMST), copious sclerotia but a low number of conidia (RH), were used to assess the role of crzA which encodes a putative calcium-signaling pathway regulatory protein. Under standard culture conditions, BN9ΔcrzA mutants conidiated normally but decreased slightly in radial growth, regardless of illumination conditions. RHΔcrzA mutants produced only conidia under light and showed decreased conidiation and delayed sclerotial formation in the dark. Regulation of conidiation of both A. parasiticus strains by light was independent of crzA. Increased concentrations of lithium, sodium, and potassium impaired conidiation and sclerotial formation of the RHΔcrzA mutants but they did not affect conidiation of the BN9ΔcrzA mutants. Vegetative growth and asexual development of both ΔcrzA mutants were hypersensitive to increased calcium concentrations. Calcium supplementation (10 mM) resulted in 3-fold and 2-fold decreases in the relative expression of the endoplasmic reticulum calcium ATPase 2 gene in the BN9 and RH parental strains, respectively, but changes in both ΔcrzA mutants were less significant. Compared to the parental strains, the ΔcrzA mutants barely produced aflatoxins or OMST after the calcium supplementation. The relative expression levels of aflatoxin biosynthesis genes, nor1, ver1, and omtA, in both ΔcrzA mutants were decreased significantly, but the decreases in the parental strains were at much lower extents. CrzA is required for growth and development and for aflatoxin biosynthesis under calcium stress conditions.

Specificity of the innate immune system and diversity of C-type lectin domain (CTLD) proteins in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has become an important model for the study of innate immunity. Its immune system is based on several signaling cascades, including a Toll-like receptor, three mitogen-activated protein kinases (MAPK), one transforming growth factor-beta (TGF-beta), the insulin-like receptor (ILR), and the programmed cell death (PCD) pathway. Furthermore, it also involves C-type lectin domain- (CTLD) containing proteins as well as several classes of antimicrobial effectors such as lysozymes. Almost all components of the nematode immune system have homologs in other organisms, including humans, and are therefore likely of ancient evolutionary origin. At the same time, most of them are part of a general stress response, suggesting that they only provide unspecific defense. In the current article, we re-evaluate this suggestion and explore the level of specificity in C. elegans innate immunity, i.e. the nematode's ability to mount a distinct defense response towards different pathogens. We draw particular attention to the CTLD proteins, which are abundant in the nematode genome (278 genes) and many of which show a pathogen-specific response during infection. Specificity may also be achieved through the differential activation of antimicrobial genes, distinct functions of the immunity signaling cascades as well as signal integration across pathways. Taken together, our evaluation reveals high potential for immune specificity in C. elegans that may enhance the nematode's ability to fight off pathogens.