The Candida albicans Rgd1 is a RhoGAP protein involved in the control of filamentous growth

p21-activated kinase is involved in the sporulation, pathogenicity, and stress response of Arthrobotrys oligospora under the indirect regulation of Rho GTPase-activating protein

The p21-GTPase-activated protein kinases (PAKs) participate in signal transduction downstream of Rho GTPases, which are regulated by Rho GTPase-activating proteins (Rho-GAP). Herein, we characterized two orthologous Rho-GAPs (AoRga1 and AoRga2) and two PAKs (AoPak1 and AoPak2) through bioinformatics analysis and reverse genetics in Arthrobotrys oligospora, a typical nematode-trapping (NT) fungus. The transcription analyses performed at different development stages suggested that Aopaks and Aorga1 play a crucial role during sporulation and trap formation, respectively. In addition, we successfully deleted Aopak1 and Aorga1 via the homologous recombination method. The disruption of Aopak1 and Aorga1 caused a remarkable reduction in spore yield and the number of nuclei per cell, but did not affect mycelial growth. In ∆Aopak1 mutants, the trap number was decreased at 48 h after the introduction of nematodes, but nematode predatory efficiency was not affected because the extracellular proteolytic activity was increased. On the contrary, the number of traps in ∆Aorga1 mutants was significantly increased at 36 h and 48 h. In addition, Aopak1 and Aorga1 had different effects on the sensitivity to cell-wall-disturbing reagent and oxidant. A yeast two-hybrid assay revealed that AoPak1 and AoRga1 both interacted with AoRac, and AoPak1 also interacted with AoCdc42. Furthermore, the Aopaks were up-regulated in ∆Aorga1 mutants, and Aorga1 was down-regulated in ∆Aopak1 mutants. These results reveal that AoRga1 indirectly regulated AoPAKs by regulating small GTPases.

Mediator Subunit Med15 Regulates Cell Morphology and Mating in Candida lusitaniae

Candida lusitaniae is an emerging opportunistic pathogenic yeast capable of shifting from yeast to pseudohyphae form, and it is one of the few Candida species with the ability to reproduce sexually. In this study, we showed that a dpp3Δ mutant, inactivated for a putative pyrophosphatase, is impaired in cell separation, pseudohyphal growth and mating. The defective phenotypes were not restored after the reconstruction of a wild-type DPP3 locus, reinforcing the hypothesis of the presence of an additional mutation that we suspected in our previous study. Genetic crosses and genome sequencing identified an additional mutation in MED15, encoding a subunit of the mediator complex that functions as a general transcriptional co-activator in Eukaryotes. We confirmed that inactivation of MED15 was responsible for the defective phenotypes by rescuing the dpp3Δ mutant with a wild-type copy of MED15 and constructing a med15Δ knockout mutant that mimics the phenotypes of dpp3Δ in vitro. Proteomic analyses revealed the biological processes under the control of Med15 and involved in hyphal growth, cell separation and mating. This is the first description of the functions of MED15 in the regulation of hyphal growth, cell separation and mating, and the pathways involved in C. lusitaniae.

New antifungal 4-chloro-3-nitrophenyldifluoroiodomethyl sulfone reduces the Candida albicans pathogenicity in the Galleria mellonella model organism

Candida albicans represents an interesting microorganism to study complex host-pathogen interactions and for the development of effective antifungals. Our goal was to assess the efficacy of 4-chloro-3-nitrophenyldifluoroiodomethyl sulfone (named Sulfone) against the C. albicans infections in the Galleria mellonella host model. We assessed invasiveness of CAI4 parental strain and mutants: kex2Δ/KEX2 and kex2Δ/kex2Δ in G. mellonella treated with Sulfone. We determined that KEX2 expression was altered following Sulfone treatment in G. mellonella-C. albicans infection model. Infection with kex2Δ/kex2Δ induced decreased inflammation and minimal fault in fitness of larvae vs CAI4. Fifty percent of larvae died within 4–5 days (P value < 0.0001) when infected with CAI4 and kex2Δ/KEX2 at 10⁹ CFU/mL; survival reached 100% in those injected with kex2Δ/kex2Δ. Larvae treated with Sulfone at 0.01 mg/kg 30 min before infection with all C. albicans tested survived infection at 90–100% vs C. albicans infected-PBS-treated larvae. Hypersensitive to Sulfone, kex2Δ/kex2Δ reduced virulence in survival. KEX2 was down-regulated when larvae were treated with Sulfone: 30 min before and 2 h post-SC5314-wild-type infection respectively. kex2Δ/kex2Δ was able to infect larvae, but failed to kill host when treated with Sulfone. Sulfone can be used to prevent or treat candidiasis. G. mellonella facilitates studding of host-pathogen interactions, i.e., testing host vs panel of C. albicans mutants when antifungal is dosed.

Global analysis of genetic circuitry and adaptive mechanisms enabling resistance to the azole antifungal drugs

Invasive fungal infections caused by the pathogen Candida albicans have transitioned from a rare curiosity to a major cause of human mortality. This is in part due to the emergence of resistance to the limited number of antifungals available to treat fungal infections. Azoles function by targeting the biosynthesis of ergosterol, a key component of the fungal cell membrane. Loss-of-function mutations in the ergosterol biosynthetic gene ERG3 mitigate azole toxicity and enable resistance that depends upon fungal stress responses. Here, we performed a genome-wide synthetic genetic array screen in Saccharomyces cerevisiae to map ERG3 genetic interactors and uncover novel circuitry important for azole resistance. We identified nine genes that enabled erg3-mediated azole resistance in the model yeast and found that only two of these genes had a conserved impact on resistance in C. albicans. Further, we screened a C. albicans homozygous deletion mutant library and identified 13 genes for which deletion enhances azole susceptibility. Two of the genes, RGD1 and PEP8, were also important for azole resistance acquired by diverse mechanisms. We discovered that loss of function of retrograde transport protein Pep8 overwhelms the functional capacity of the stress response regulator calcineurin, thereby abrogating azole resistance. To identify the mechanism through which the GTPase activator protein Rgd1 enables azole resistance, we selected for mutations that restore resistance in strains lacking Rgd1. Whole genome sequencing uncovered parallel adaptive mechanisms involving amplification of both chromosome 7 and a large segment of chromosome 3. Overexpression of a transporter gene on the right portion of chromosome 3, NPR2, was sufficient to enable azole resistance in the absence of Rgd1. Thus, we establish a novel mechanism of adaptation to drug-induced stress, define genetic circuitry underpinning azole resistance, and illustrate divergence in resistance circuitry over evolutionary time.

Fungal infections caused by the pathogen Candida albicans pose a serious threat to human health. Treating these infections relies heavily on the azole antifungals, however, resistance to these drugs develops readily demanding novel therapeutic strategies. We performed large-scale systematic screens in both C. albicans and the model yeast Saccharomyces cerevisiae to identify genes that enable azole resistance. Our genome-wide screen in S. cerevisiae identified nine determinants of azole resistance, only two of which were important for resistance in C. albicans. Our screen of C. albicans mutants identified 13 genes for which deletion enhances susceptibility to azoles, including RGD1 and PEP8. We found that loss of Pep8 overwhelms the functional capacity of a key stress response regulator, calcineurin. In contrast, amplification of chromosome 7 and the right portion of chromosome 3 can restore resistance in strains lacking Rgd1, suggesting that Rgd1 may enable azole resistance by inducing genes in these amplified regions. Specifically, overexpression of a gene involved in transport on chromosome 3, NPR2, was sufficient to restore azole resistance in the absence of Rgd1. Thus, we establish novel circuitry important for antifungal drug resistance, and uncover adaptive mechanisms involving genomic plasticity that occur in response to drug induced stress.

Effect of serine protease KEX2 on Candida albicans virulence under halogenated methyl sulfones

Aim: The effect of KEX2 mutations on C. albicans virulence and resistance to halogenated methyl sulfones was assessed. Materials & methods: The mechanism of action of sulfones was studied using flow cytometry and microscopy. Expression of KEX2 and SAP5 was assessed using quantitative Real-Time-PCR. 2,3-Bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide and lactate dehydrogenase assays were elaborated to study, respectively, metabolism of Candida treated with sulfones and their cytotoxicity against tissues. Inflammatory response was detected by ELISA. Results: Lysosome permeabilization and dose-dependent programmed cell death under sulfones were noted. KEX2 induction depended on halogenomethylsulfonyl groups, which affected cell wall biosynthesis and adhesion. Conclusion: Sulfones treatment reduced Candida pathogenicity in Galleria mellonella. Sulfones are an alternative for antifungal therapies due to their safety profile and antibiofilm activity.

Lipase-Catalyzed Kinetic Resolution of Novel Antifungal N-Substituted Benzimidazole Derivatives

A series of new N-substituted benzimidazole derivatives was synthesized and their antifungal activity against Candida albicans was evaluated. The chemical step included synthesis of appropriate ketones containing benzimidazole ring, reduction of ketones to the racemic alcohols, and acetylation of alcohols to the esters. All benzimidazole derivatives were obtained with satisfactory yields and in relatively short times. All synthesized compounds exhibit significant antifungal activity against Candida albicans 900028 ATCC (% cell inhibition at 0.25 μg concentration > 98%). Additionally, racemic mixtures of alcohols were separated by lipase-catalyzed kinetic resolution. In the enzymatic step a transesterification reaction was applied and the influence of a lipase type and solvent on the enantioselectivity of the reaction was studied. The most selective enzymes were Novozyme SP 435 and lipase Amano AK from Pseudomonas fluorescens (E > 100).

Design and studies of multiple mechanism of anti-Candida activity of a new potent Trp-rich peptide dendrimers

PURPOSE

Eight peptide dendrimers were designed as structural mimics of natural cationic amphiphilic peptides with antifungal activity and evaluated for their anti-Candida potential against the wild type strains and mutants.

METHODS

Dendrimer 14 containing four Trp residues and dodecyl tail and a slightly smaller dendrimer 9 decorated with four N-methylated Trp that displayed 100 and 99.7% of growth inhibition at 16 μg/mL respectively, were selected for evaluation against the Candida albicans mutants with disabled biosynthesis of aspartic proteases responsible for host tissue colonization and morphogenesis during biofilm formation (sessile model). Flow cytometry method was employed to detect apoptotic cells with membrane alterations (phosphatidylserine translocation), and differentiation of apoptotic from necrotic cells was also performed. Simultaneous staining of cell surface phosphatidylserine with Annexin-V-Fluorescein and necrotic cells with propidium iodide was conducted.

RESULTS

14 at 16 μg/mL caused C. albicans cells to undergo cellular apoptosis but its increasing concentrations induced necrosis. 14 influenced C. albicans biofilm viability as well as hyphal and cell wall morphology. Confocal microscopy and cell wall staining with calcofluor white revealed that in epithelial model the cell surface structure was perturbed at MIC of peptide dendrimer. It appears that tryptophan or 1-methyltryptophan groups displayed at the surface and positive charges hidden in the dendrimer tree along with hydrocarbon tail located at C-terminus are important for the anti-Candida activity since dendrimers containing tryptamine at C-terminus showed only a moderate activity.

CONCLUSIONS

Our results suggest that membranolytic dendrimer 14, targeting cellular apoptotic pathway and impairing the cell wall formation in mature biofilm, may be a potential multifunctional antifungal lead compound for the control of C. albicans infections.

The expression of the Candida albicans gene SAP4 during hyphal formation in human serum and in adhesion to monolayer cell culture of colorectal carcinoma Caco-2 (ATCC)

Candida albicans SAP4 gene encodes secretory aspartyl protease Sap4 which is involved in hyphae formation and virulence. Transcriptional factors Cph1 and Efg1 govern the expression of several C. albicans genes and contribute to morphogenesis. We investigated the expression of SAP4 in C. albicans clinical isolate and mutants lacking Efg1 or/ and Cph1 grown in human serum and during contact with Caco-2 cell line. mRNA was analyzed with the use of RT-PCR; relative quantification was normalized against an ACT1 in cells after 18-h growth either in serum or on monolayer as well as in their counterparts in YEPD medium. We assessed the role of Sap4, Efg1 and Cph1 in adhesion of C. albicans to epithelial cells. Additionally, adherence assay was performed with sap4/sap4. Adhesion was expressed as a percent of adherent cells to monolayer at 90 min vs. total cells added (100%). No differences were observed in adhesion of efg1/efg1 and sap4/sap4 compared with SC5314 (P≥0.05 statisitically insignificant). SAP4 expression indicated that it is not involved in adapting to the tested conditions. SAP4 expression can be strainspecific and is not solely controlled by the Efg1 pathway but also by the Cph1 pathway. Neither Efg1 nor Sap4 can influence adhesion.

Putative RhoGAP proteins orchestrate vegetative growth, conidiogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

Rho GTPases, acting as molecular switches, are involved in the regulation of diverse cellular functions. Rho GTPase activating proteins (Rho GAPs) function as negative regulators of Rho GTPases and are required for a variety of signaling processes in cell development. But the mechanisms underlying Rho GAPs in Rho-mediated signaling pathways in fungi are still elusive. There are eight RhoGAP domain-containing genes annotated in the Magnaporthe oryzae genome. To understand the function of these RhoGAP genes, we generated knockout mutants of each of the RhoGAP genes through a homologous recombination-based method. Phenotypic analysis showed that growth rate of aerial hyphae of the Molrg1 deletion mutant decreased dramatically. The ΔMolrg1 mutant showed significantly reduced conidiation and appressorium formation by germ tubes. Moreover, it lost pathogenicity completely. Deletion of another Rho GAP (MoRga1) resulted in high percentage of larger or gherkin-shaped conidia and slight decrease in conidiation. Appressorial formation of the ΔMoRga1 mutant was delayed significantly on hydrophobic surface, while the development of mycelial growth and pathogenicity in plants was not affected. Confocal fluorescence microscopy imaging showed that MoRga1-GFP localizes to septal pore of the conidium, and this localization pattern requires both LIM and RhoGAP domains. Furthermore, either deleting the LIM or RhoGAP domain or introducing an inactivating R1032A mutation in the RhoGAP domain of MoRga1 caused similar defects as the Morga1 deletion mutant in terms of conidial morphology and appressorial formation, suggesting that MoRga1 is a stage-specific regulator of conidial differentiation by regulating some specific Rho GTPases. In this regard, MoRga1 and MoLrg1 physically interacted with both MoRac1-CA and MoCdc42-CA in the yeast two-hybrid and pull-down assays, suggesting that the actions of these two GAPs are involved in MoRac1 and MoCdc42 pathways. On the other hand, six other putative Rho GAPs (MoRga2 to MoRga7) were dispensable for conidiation, vegetative growth, appressorial formation and pathogenicity, suggesting that these Rho GAPs function redundantly during fungal development. Taking together, Rho GAP genes play important roles in M. oryzae development and infectious processes through coordination and modulation of Rho GTPases.

Candida albicans morphologies revealed by scanning electron microscopy analysis

Scanning electron microscope (SEM) observations were used to analyze particular morphologies of Candida albicans clinical isolate (strain 82) and mutants defective in hyphae-promoting genes EFG1 (strain HLC52) and/or CPH1 (strains HLC54 and Can16). Transcription factors Efg1 and Cph1 play role in regulating filamentation and adhesion of C. albicans’ morphologies. Comparative analysis of such mutants and clinical isolate showed that Efg1 is required for human serum-induced cell growth and morphological switching. In the study, distinct differences between ultrastructural patterns of clinical strain’s and null mutants’ morphologies were observed (spherical vs tube-like blastoconidia, or solid and fragile constricted septa vs only the latter observed in strains with EFG1 deleted). In addition, wild type strain displayed smooth colonies of cells in comparison to mutants which exhibited wrinkled phenotype. It was observed that blastoconidia of clinical strain exhibited either polarly or randomly located budding. Contrariwise, morphotypes of mutants showed either multiple polar budding or a centrally located single bud scar (mother-daughter cell junction) distinguishing tube-like yeast/pseudohyphal growth (the length-to-width ratios larger than 1.5). In their planktonic form of growth, blastoconidia of clinical bloodstream isolate formed constitutively true hyphae under undiluted human serum inducing conditions. It was found that true hyphae are essential elements for developing structural integrity of conglomerate, as mutants displaying defects in their flocculation and conglomerate-forming abilities in serum. While filamentation is an important virulence trait in C. albicans the true hyphae are the morphologies which may be expected to play a role in bloodstream infections.