Gene annotation and drug target discovery in Candida albicans with a tagged transposon mutant collection

Application of the Mutant Libraries for Candida albicans Functional Genomics

Candida albicans is a typical opportunistic pathogen in humans that causes serious health risks in clinical fungal infections. The construction of mutant libraries has made remarkable developments in the study of C. albicans molecular and cellular biology with the ongoing advancements of gene editing, which include the application of CRISPR-Cas9 and novel high-efficient transposon. Large-scale genetic screens and genome-wide functional analysis accelerated the investigation of new genetic regulatory mechanisms associated with the pathogenicity and resistance to environmental stress in C. albicans. More importantly, sensitivity screening based on C. albicans mutant libraries is critical for the target identification of novel antifungal compounds, which leads to the discovery of Sec7p, Tfp1p, Gwt1p, Gln4p, and Erg11p. This review summarizes the main types of C. albicans mutant libraries and interprets their applications in morphogenesis, biofilm formation, fungus-host interactions, antifungal drug resistance, and target identification.

High-Throughput Chemical Screen Identifies a 2,5-Disubstituted Pyridine as an Inhibitor of Candida albicans Erg11

Fungal infections contribute to over 1.5 million deaths annually, with Candida albicans representing one of the most concerning human fungal pathogens. While normally commensal in nature, compromise of host immunity can result in C. albicans disseminating into the human bloodstream, causing infections with mortality rates of up to 40%. A contributing factor to this high mortality rate is the limited arsenal of antifungals approved to treat systemic infections. The most widely used antifungal class, the azoles, inhibits ergosterol biosynthesis by targeting Erg11. The rise of drug resistance among C. albicans clinical isolates, particularly against the azoles, has escalated the need to explore novel antifungal strategies. To address this challenge, we screened a 9,600-compound subset of the University of Tokyo Core Chemical Library to identify molecules with novel antifungal activity against C. albicans. The most potent hit molecule was CpdLC-6888, a 2,5-disubstituted pyridine compound, which inhibited growth of C. albicans and closely-related species. Chemical-genetic, biochemical, and modeling analyses suggest that CpdLC-6888 inhibits Erg11 in a manner similar to the azoles despite lacking the canonical five-membered nitrogen-containing azole ring. This work characterizes the antifungal activity of a 2,5-disubstituted pyridine against C. albicans, supporting the mining of existing chemical collections to identify compounds with novel antifungal activity.

IMPORTANCE Pathogenic fungi represent a serious but underacknowledged threat to human health. The treatment and management of these infections relies heavily on the use of azole antifungals, a class of molecules that contain a five-membered nitrogen-containing ring and inhibit the biosynthesis of the key membrane sterol ergosterol. By employing a high-throughput chemical screen, we identified a 2,5-disubstituted pyridine, termed CpdLC-6888, as possessing antifungal activity against the prominent human fungal pathogen Candida albicans. Upon further investigation, we determined this molecule exhibits azole-like activity despite being structurally divergent. Specifically, transcriptional repression of the azole target gene ERG11 resulted in hypersensitivity to CpdLC-6888, and treatment of C. albicans with this molecule blocked the production of the key membrane sterol ergosterol. Therefore, this work describes a chemical scaffold with novel antifungal activity against a prevalent and threatening fungal pathogen affecting human health, expanding the repertoire of compounds that can inhibit this useful antifungal drug target.

Candida albicans Potassium Transporters

Potassium is basic for life. All living organisms require high amounts of intracellular potassium, which fulfils multiple functions. To reach efficient potassium homeostasis, eukaryotic cells have developed a complex and tightly regulated system of transporters present both in the plasma membrane and in the membranes of internal organelles that allow correct intracellular potassium content and distribution. We review the information available on the pathogenic yeast Candida albicans. While some of the plasma membrane potassium transporters are relatively well known and experimental data about their nature, function or regulation have been published, in the case of most of the transporters present in intracellular membranes, their existence and even function have just been deduced because of their homology with those present in other yeasts, such as Saccharomyces cerevisiae. Finally, we analyse the possible links between pathogenicity and potassium homeostasis. We comment on the possibility of using some of these transporters as tentative targets in the search for new antifungal drugs.

A data library of Candida albicans functional genomic screens

Functional genomic screening of genetic mutant libraries enables the characterization of gene function in diverse organisms. For the fungal pathogen Candida albicans, several genetic mutant libraries have been generated and screened for diverse phenotypes, including tolerance to environmental stressors and antifungal drugs, and pathogenic traits such as cellular morphogenesis, biofilm formation and host-pathogen interactions. Here, we compile and organize C. albicans functional genomic screening data from ∼400 screens, to generate a data library of genetic mutant strains analyzed under diverse conditions. For quantitative screening data, we normalized these results to enable quantitative and comparative analysis of different genes across different phenotypes. Together, this provides a unique C. albicans genetic database, summarizing abundant phenotypic data from functional genomic screens in this critical fungal pathogen.

CRISPR-Based Genetic Manipulation of Candida Species: Historical Perspectives and Current Approaches

The Candida genus encompasses a diverse group of ascomycete fungi that have captured the attention of the scientific community, due to both their role in pathogenesis and emerging applications in biotechnology; the development of gene editing tools such as CRISPR, to analyze fungal genetics and perform functional genomic studies in these organisms, is essential to fully understand and exploit this genus, to further advance antifungal drug discovery and industrial value. However, genetic manipulation of Candida species has been met with several distinctive barriers to progress, such as unconventional codon usage in some species, as well as the absence of a complete sexual cycle in its diploid members. Despite these challenges, the last few decades have witnessed an expansion of the Candida genetic toolbox, allowing for diverse genome editing applications that range from introducing a single point mutation to generating large-scale mutant libraries for functional genomic studies. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology is among the most recent of these advancements, bringing unparalleled versatility and precision to genetic manipulation of Candida species. Since its initial applications in Candida albicans, CRISPR-Cas9 platforms are rapidly evolving to permit efficient gene editing in other members of the genus. The technology has proven useful in elucidating the pathogenesis and host-pathogen interactions of medically relevant Candida species, and has led to novel insights on antifungal drug susceptibility and resistance, as well as innovative treatment strategies. CRISPR-Cas9 tools have also been exploited to uncover potential applications of Candida species in industrial contexts. This review is intended to provide a historical overview of genetic approaches used to study the Candida genus and to discuss the state of the art of CRISPR-based genetic manipulation of Candida species, highlighting its contributions to deciphering the biology of this genus, as well as providing perspectives for the future of Candida genetics.

DeORFanizing Candida albicans Genes using Coexpression

Candida albicans is a common and deadly fungal pathogen of humans, yet the genome of this organism contains many genes of unknown function. By determining gene function, we can help identify essential genes, new virulence factors, or new regulators of drug resistance, and thereby give new targets for antifungal development.

Functional characterization of open reading frames in nonmodel organisms, such as the common opportunistic fungal pathogen Candida albicans, can be labor-intensive. To meet this challenge, we built a comprehensive and unbiased coexpression network for C. albicans, which we call CalCEN, from data collected from 853 RNA sequencing runs from 18 large-scale studies deposited in the NCBI Sequence Read Archive. Retrospectively, CalCEN is highly predictive of known gene function annotations and can be synergistically combined with sequence similarity and interaction networks in Saccharomyces cerevisiae through orthology for additional accuracy in gene function prediction. To prospectively demonstrate the utility of the coexpression network in C. albicans, we predicted the function of underannotated open reading frames (ORFs) and identified CCJ1 as a novel cell cycle regulator in C. albicans. This study provides a tool for future systems biology analyses of gene function in C. albicans. We provide a computational pipeline for building and analyzing the coexpression network and CalCEN itself at http://github.com/momeara/CalCEN.

IMPORTANCECandida albicans is a common and deadly fungal pathogen of humans, yet the genome of this organism contains many genes of unknown function. By determining gene function, we can help identify essential genes, new virulence factors, or new regulators of drug resistance, and thereby give new targets for antifungal development. Here, we use information from large-scale RNA sequencing (RNAseq) studies and generate a C. albicans coexpression network (CalCEN) that is robust and able to predict gene function. We demonstrate the utility of this network in both retrospective and prospective testing and use CalCEN to predict a role for C4_06590W/CCJ1 in cell cycle. This tool will allow for a better characterization of underannotated genes in pathogenic yeasts.

Loss of Arp1, a putative actin-related protein, triggers filamentous and invasive growth and impairs pathogenicity in Candida albicans

The polymorphous cellular shape of Candida albicans, in particular the transition from a yeast to a filamentous form, is crucial for either commensalism or life-threatening infections of the host. Various external or internal stimuli, including serum and nutrition starvation, have been shown to regulate filamentous growth primarily through two classical signaling pathways, the cAMP-PKA and the MAPK pathways. Genotoxic stress also induces filamentous growth, but through independent pathways, and little is known about negative regulation during this reversible morphological transition. In this study, we established that ARP1 in C. albicans, similar to its homolog in S. cerevisiae, has a role in nuclei separation and spindle orientation. Deletion of ARP1 generated filamentous and invasive growth as well as increased biofilm formation, accompanied by up-regulation of hyphae specific genes, such as HWP1, UME6 and ALS3. The filamentous and invasive growth of the ARP1 deletion strain was independent of transcription factors Efg1, Cph1 and Ume6, but was suppressed by deleting checkpoint BUB2 or overexpressing NRG1. Deletion of ARP1 impaired the colonization of Candida cells in mice and also attenuated virulence in a mouse model. All the data suggest that loss of ARP1 activates filamentous and invasive growth in vitro, and that it positively regulates virulence in vivo, which provides insight into actin-related morphology and pathogenicity in C. albicans.

Advances in fungal chemical genomics for the discovery of new antifungal agents

Invasive fungal infections have escalated from a rare curiosity to a major cause of human mortality around the globe. This is in part due to a scarcity in the number of antifungal drugs available to combat mycotic disease, making the discovery of novel bioactive compounds and determining their mode of action of utmost importance. The development and application of chemical genomic assays using the model yeast Saccharomyces cerevisiae has provided powerful methods to identify the mechanism of action of diverse molecules in a living cell. Furthermore, complementary assays are continually being developed in fungal pathogens, most notably Candida albicans and Cryptococcus neoformans, to elucidate compound mechanism of action directly in the pathogen of interest. Collectively, the suite of chemical genetic assays that have been developed in multiple fungal species enables the identification of candidate drug target genes, as well as genes involved in buffering drug target pathways, and genes involved in general cellular responses to small molecules. In this review, we examine current yeast chemical genomic assays and highlight how such resources provide powerful tools that can be utilized to bolster the antifungal pipeline.

Model-driven analysis of mutant fitness experiments improves genome-scale metabolic models of Zymomonas mobilis ZM4

Genome-scale metabolic models have been utilized extensively in the study and engineering of the organisms they describe. Here we present the analysis of a published dataset from pooled transposon mutant fitness experiments as an approach for improving the accuracy and gene-reaction associations of a metabolic model for Zymomonas mobilis ZM4, an industrially relevant ethanologenic organism with extremely high glycolytic flux and low biomass yield. Gene essentiality predictions made by the draft model were compared to data from individual pooled mutant experiments to identify areas of the model requiring deeper validation. Subsequent experiments showed that some of the discrepancies between the model and dataset were caused by polar effects, mis-mapped barcodes, or mutants carrying both wild-type and transposon disrupted gene copies-highlighting potential limitations inherent to data from individual mutants in these high-throughput datasets. Therefore, we analyzed correlations in fitness scores across all 492 experiments in the dataset in the context of functionally related metabolic reaction modules identified within the model via flux coupling analysis. These correlations were used to identify candidate genes for a reaction in histidine biosynthesis lacking an annotated gene and highlight metabolic modules with poorly correlated gene fitness scores. Additional genes for reactions involved in biotin, ubiquinone, and pyridoxine biosynthesis in Z. mobilis were identified and confirmed using mutant complementation experiments. These discovered genes, were incorporated into the final model, iZM4_478, which contains 747 metabolic and transport reactions (of which 612 have gene-protein-reaction associations), 478 genes, and 616 unique metabolites, making it one of the most complete models of Z. mobilis ZM4 to date. The methods of analysis that we applied here with the Z. mobilis transposon mutant dataset, could easily be utilized to improve future genome-scale metabolic reconstructions for organisms where these, or similar, high-throughput datasets are available.

Advances in Molecular Tools and In Vivo Models for the Study of Human Fungal Pathogenesis

Pathogenic fungi represent an increasing infectious disease threat to humans, especially with an increasing challenge of antifungal drug resistance. Over the decades, numerous tools have been developed to expedite the study of pathogenicity, initiation of disease, drug resistance and host-pathogen interactions. In this review, we highlight advances that have been made in the use of molecular tools using CRISPR technologies, RNA interference and transposon targeted mutagenesis. We also discuss the use of animal models in modelling disease of human fungal pathogens, focusing on zebrafish, the silkworm, Galleria mellonella and the murine model.

Broad-spectrum antifungal activities and mechanism of drimane sesquiterpenoids

Eight drimane sesquiterpenoids including (-)-drimenol and (+)-albicanol were synthesized from (+)-sclareolide and evaluated for their antifungal activities. Three compounds, (-)-drimenol, (+)-albicanol, and (1R,2R,4aS,8aS)-2-hydroxy-2,5,5,8a-tetramethyl-decahydronaphthalene-1-carbaldehyde (4) showed strong activity against C. albicans. (-)-Drimenol, the strongest inhibitor of the three, (at concentrations of 8 – 64 µg/ml, causing 100% death of various fungi), acts not only against C. albicans in a fungicidal manner, but also inhibits other fungi such as Aspergillus, Cryptococcus, Pneumocystis, Blastomyces, Saksenaea and fluconazole resistant strains of C. albicans, C. glabrata, C. krusei, C. parapsilosis and C. auris. These observations suggest that drimenol is a broad-spectrum antifungal agent. At a high concentration (100 μg/ml) drimenol caused rupture of the fungal cell wall/membrane. In a nematode model of C. albicans infection, drimenol rescued the worms from C. albicans-mediated death, indicating drimenol is tolerable and bioactive in metazoans. Genome-wide fitness profiling assays of both S. cerevisiae (nonessential homozygous and essential heterozygous) and C. albicans (Tn-insertion mutants) collections revealed putative genes and pathways affected by drimenol. Using a C. albicans mutant spot assay, the Crk1 kinase associated gene products, Ret2, Cdc37, and orf19.759, orf19.1672, and orf19.4382 were revealed to be involved in drimenol's mechanism of action. The three orfs identified in this study are novel and appear to be linked with Crk1 function. Further, computational modeling results suggest possible modifications of the structure of drimenol, including the A ring, for improving the antifungal activity.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Overcoming Fungal Echinocandin Resistance through Inhibition of the Non-essential Stress Kinase Yck2

New strategies are urgently needed to counter the threat to human health posed by drug-resistant fungi. To explore an as-yet unexploited target space for antifungals, we screened a library of protein kinase inhibitors for the ability to reverse resistance of the most common human fungal pathogen, Candida albicans, to caspofungin, a widely used antifungal. This screen identified multiple 2,3-aryl-pyrazolopyridine scaffold compounds capable of restoring caspofungin sensitivity. Using chemical genomic, biochemical, and structural approaches, we established the target for our most potent compound as Yck2, a casein kinase 1 family member. Combination of this compound with caspofungin eradicated drug-resistant C. albicans infection while sparing co-cultured human cells. In mice, genetic depletion of YCK2 caused an ∼3-log₁₀ decline in fungal burden in a model of systemic caspofungin-resistant C. albicans infection. Structural insights and our tool compound's profile in culture support targeting the Yck2 kinase function as a broadly active antifungal strategy.

The Impact of Gene Dosage and Heterozygosity on The Diploid Pathobiont Candida albicans

Candida albicans is a fungal species that can colonize multiple niches in the human host where it can grow either as a commensal or as an opportunistic pathogen. The genome of C. albicans has long been of considerable interest, given that it is highly plastic and can undergo a wide variety of alterations. These changes play a fundamental role in determining C. albicans traits and have been shown to enable adaptation both to the host and to antifungal drugs. C. albicans isolates contain a heterozygous diploid genome that displays variation from the level of single nucleotides to largescale rearrangements and aneuploidy. The heterozygous nature of the genome is now increasingly recognized as being central to C. albicans biology, as the relative fitness of isolates has been shown to correlate with higher levels of overall heterozygosity. Moreover, loss of heterozygosity (LOH) events can arise frequently, either at single polymorphisms or at a chromosomal level, and both can alter the behavior of C. albicans cells during infection or can modulate drug resistance. In this review, we examine genome plasticity in this pathobiont focusing on how gene dosage variation and loss of heterozygosity events can arise and how these modulate C. albicans behavior.

Iterative screening methodology enables isolation of strains with improved properties for a FACS-based screen and increased L-DOPA production

Optimizing microbial hosts for the large-scale production of valuable metabolites often requires multiple mutations and modifications to the host's genome. We describe a three-round screen for increased L-DOPA production in S. cerevisiae using FACS enrichment of an enzyme-coupled biosensor for L-DOPA. Multiple rounds of screening were enabled by a single build of a barcoded in vitro transposon-mediated disruption library. New background strains for screening were built for each iteration using results from previous iterations. The same in vitro transposon-mediated disruption library was integrated by homologous recombination into new background strains in each round of screening. Compared with creating new transposon insertions in each round, this method takes less time and saves the cost of additional sequencing to characterize transposon insertion sites. In the first two rounds of screening, we identified deletions that improved biosensor compartmentalization and, consequently, improved our ability to screen for L-DOPA production. In a final round, we discovered that deletion of heme oxygenase (HMX1) increases total heme concentration and increases L-DOPA production, using dopamine measurement as a proxy. We further demonstrated that deleting HMX1 may represent a general strategy for P450 function improvement by improving activity of a second P450 enzyme, BM3, which performs a distinct reaction.

Transposon insertional mutagenesis in Saccharomyces uvarum reveals trans-acting effects influencing species-dependent essential genes

To understand how complex genetic networks perform and regulate diverse cellular processes, the function of each individual component must be defined. Comprehensive phenotypic studies of mutant alleles have been successful in model organisms in determining what processes depend on the normal function of a gene. These results are often ported to newly sequenced genomes by using sequence homology. However, sequence similarity does not always mean identical function or phenotype, suggesting that new methods are required to functionally annotate newly sequenced species. We have implemented comparative analysis by high-throughput experimental testing of gene dispensability in Saccharomyces uvarum, a sister species of Saccharomyces cerevisiae. We created haploid and heterozygous diploid Tn7 insertional mutagenesis libraries in S. uvarum to identify species-dependent essential genes, with the goal of detecting genes with divergent functions and/or different genetic interactions. Comprehensive gene dispensability comparisons with S. cerevisiae predicted diverged dispensability at 12% of conserved orthologs, and validation experiments confirmed 22 differentially essential genes. Despite their differences in essentiality, these genes were capable of cross-species complementation, demonstrating that trans-acting factors that are background-dependent contribute to differential gene essentiality. This study shows that direct experimental testing of gene disruption phenotypes across species can inform comparative genomic analyses and improve gene annotations. Our method can be widely applied in microorganisms to further our understanding of genome evolution.

Gene Essentiality Analyzed by In Vivo Transposon Mutagenesis and Machine Learning in a Stable Haploid Isolate of Candida albicans

Comprehensive understanding of an organism requires that we understand the contributions of most, if not all, of its genes. Classical genetic approaches to this issue have involved systematic deletion of each gene in the genome, with comprehensive sets of mutants available only for very-well-studied model organisms. We took a different approach, harnessing the power of in vivo transposition coupled with deep sequencing to identify >500,000 different mutations, one per cell, in the prevalent human fungal pathogen Candida albicans and to map their positions across the genome. The transposition approach is efficient and less labor-intensive than classic approaches. Here, we describe the production and analysis (aided by machine learning) of a large collection of mutants and the comprehensive identification of 1,610 C. albicans genes that are essential for growth under standard laboratory conditions. Among these C. albicans essential genes, we identify those that are also essential in two distantly related model yeasts as well as those that are conserved in all four major human fungal pathogens and that are not conserved in the human genome. This list of genes with functions important for the survival of the pathogen provides a good starting point for the development of new antifungal drugs, which are greatly needed because of the emergence of fungal pathogens with elevated resistance and/or tolerance of the currently limited set of available antifungal drugs.

Knowing the full set of essential genes for a given organism provides important information about ways to promote, and to limit, its growth and survival. For many non-model organisms, the lack of a stable haploid state and low transformation efficiencies impede the use of conventional approaches to generate a genome-wide comprehensive set of mutant strains and the identification of the genes essential for growth. Here we report on the isolation and utilization of a highly stable haploid derivative of the human pathogenic fungus Candida albicans, together with a modified heterologous transposon and machine learning (ML) analysis method, to predict the degree to which all of the open reading frames are required for growth under standard laboratory conditions. We identified 1,610 C. albicans essential genes, including 1,195 with high “essentiality confidence” scores, thereby increasing the number of essential genes (currently 66 in the Candida Genome Database) by >20-fold and providing an unbiased approach to determine the degree of confidence in the determination of essentiality. Among the genes essential in C. albicans were 602 genes also essential in the model budding and fission yeasts analyzed by both deletion and transposon mutagenesis. We also identified essential genes conserved among the four major human pathogens C. albicans, Aspergillus fumigatus, Cryptococcus neoformans, and Histoplasma capsulatum and highlight those that lack homologs in humans and that thus could serve as potential targets for the design of antifungal therapies.

Candida albicans gains azole resistance by altering sphingolipid composition

Fungal infections by drug-resistant Candida albicans pose a global public health threat. However, the pathogen’s diploid genome greatly hinders genome-wide investigations of resistance mechanisms. Here, we develop an efficient piggyBac transposon-mediated mutagenesis system using stable haploid C. albicans to conduct genome-wide genetic screens. We find that null mutants in either gene FEN1 or FEN12 (encoding enzymes for the synthesis of very-long-chain fatty acids as precursors of sphingolipids) exhibit resistance to fluconazole, a first-line antifungal drug. Mass-spectrometry analyses demonstrate changes in cellular sphingolipid composition in both mutants, including substantially increased levels of several mannosylinositolphosphoceramides with shorter fatty-acid chains. Treatment with fluconazole induces similar changes in wild-type cells, suggesting a natural response mechanism. Furthermore, the resistance relies on a robust upregulation of sphingolipid biosynthesis genes. Our results shed light into the mechanisms underlying azole resistance, and the new transposon-mediated mutagenesis system should facilitate future genome-wide studies of C. albicans.

The fungal pathogen Candida albicans is diploid, which hinders genome-wide studies. Here, Gao et al. present a piggyBac transposon-mediated mutagenesis system using stable haploid C. albicans strains, and use it to identify genes and mechanisms underlying azole resistance.

Candida albicans Sfl1/Sfl2 regulatory network drives the formation of pathogenic microcolonies

Candida albicans is an opportunistic fungal pathogen that can infect oral mucosal surfaces while being under continuous flow from saliva. Under specific conditions, C. albicans will form microcolonies that more closely resemble the biofilms formed in vivo than standard in vitro biofilm models. However, very little is known about these microcolonies, particularly genomic differences between these specialized biofilm structures and the traditional in vitro biofilms. In this study, we used a novel flow system, in which C. albicans spontaneously forms microcolonies, to further characterize the architecture of fungal microcolonies and their genomics compared to non-microcolony conditions. Fungal microcolonies arose from radially branching filamentous hyphae that increasingly intertwined with one another to form extremely dense biofilms, and closely resembled the architecture of in vivo oropharyngeal candidiasis. We identified 20 core microcolony genes that were differentially regulated in flow-induced microcolonies using RNA-seq. These genes included HWP1, ECE1, IHD1, PLB1, HYR1, PGA10, and SAP5. A predictive algorithm was utilized to identify ten transcriptional regulators potentially involved in microcolony formation. Of these transcription factors, we found that Rob1, Ndt80, Sfl1 and Sfl2, played a key role in microcolony formation under both flow and static conditions and to epithelial surfaces. Expression of core microcolony genes were highly up-regulated in Δsfl1 cells and down-regulated in both Δsfl2 and Δrob1 strains. Microcolonies formed on oral epithelium using C. albicans Δsfl1, Δsfl2 and Δrob1 deletion strains all had altered adhesion, invasion and cytotoxicity. Furthermore, epithelial cells infected with deletion mutants had reduced (SFL2, NDT80, and ROB1) or enhanced (SFL2) immune responses, evidenced by phosphorylation of MKP1 and c-Fos activation, key signal transducers in the hyphal invasion response. This profile of microcolony transcriptional regulators more closely reflects Sfl1 and Sfl2 hyphal regulatory networks than static biofilm regulatory networks, suggesting that microcolonies are a specialized pathogenic form of biofilm.

Maize Transposable Elements Ac/Ds as Insertion Mutagenesis Tools in Candida albicans

In nonmodel systems, genetic research is often limited by the lack of techniques for the generation and identification of gene mutations. One approach to overcome this bottleneck is the application of transposons for gene tagging. We have established a two-element transposon tagging system, based on the transposable elements Activator (Ac)/Dissociation (Ds) from maize, for in vivo insertion mutagenesis in the fungal human pathogen Candida albicans A nonautonomous Ds transposon carrying a selectable marker was constructed into the ADE2 promoter on chromosome 3 and a codon usage-adapted Ac transposase gene was inserted into the neutral NEUT5L locus on chromosome 5. In C. albicans cells expressing the transposase, the Ds element efficiently excised and reintegrated elsewhere in the genome, which makes the Ac/Ds transposons promising tools for saturating insertion mutagenesis in clinical strains of C. albicans.

Systematic Complex Haploinsufficiency-Based Genetic Analysis of Candida albicans Transcription Factors: Tools and Applications to Virulence-Associated Phenotypes

Genetic interaction analysis is a powerful approach to the study of complex biological processes that are dependent on multiple genes. Because of the largely diploid nature of the human fungal pathogen Candida albicans, genetic interaction analysis has been limited to a small number of large-scale screens and a handful for gene-by-gene studies. Complex haploinsufficiency, which occurs when a strain containing two heterozygous mutations at distinct loci shows a phenotype that is distinct from either of the corresponding single heterozygous mutants, is an expedient approach to genetic interactions analysis in diploid organisms. Here, we describe the construction of a barcoded-library of 133 heterozygous TF deletion mutants and deletion cassettes for designed to facilitate complex haploinsufficiency-based genetic interaction studies of the TF networks in C. albicans. We have characterized the phenotypes of these heterozygous mutants under a broad range of in vitro conditions using both agar-plate and pooled signature tag-based assays. Consistent with previous studies, haploinsufficiency is relative uncommon. In contrast, a set of 12 TFs enriched in mutants with a role in adhesion were found to have altered competitive fitness at early time points in a murine model of disseminated candidiasis. Finally, we characterized the genetic interactions of a set of biofilm related TFs in the first two steps of biofilm formation, adherence and filamentation of adherent cells. The genetic interaction networks at each stage of biofilm formation are significantly different indicating that the network is not static but dynamic.

Chemogenomic Profiling of the Fungal Pathogen Candida albicans

There is currently a small number of classes of antifungal drugs, and these drugs are known to target a very limited set of cellular functions. We derived a set of approximately 900 nonessential, transactivator-defective disruption strains from the tetracycline-regulated GRACE collection of strains of the fungal pathogen Candida albicans This strain set was screened against classic antifungal drugs to identify gene inactivations that conferred either enhanced sensitivity or increased resistance to the compounds. We examined two azoles, fluconazole and posaconazole; two echinocandins, caspofungin and anidulafungin; and a polyene, amphotericin B. Overall, the chemogenomic profiles within drug classes were highly similar, but there was little overlap between classes, suggesting that the different drug classes interacted with discrete networks of genes in C. albicans We also tested two pyridine amides, designated GPI-LY7 and GPI-C107; these drugs gave very similar profiles that were distinct from those of the echinocandins, azoles, or polyenes, supporting the idea that they target a distinct cellular function. Intriguingly, in cases where these gene sets can be compared to genetic disruptions conferring drug sensitivity in other fungi, we find very little correspondence in genes. Thus, even though the drug targets are the same in the different species, the specific genetic profiles that can lead to drug sensitivity are distinct. This implies that chemogenomic screens of one organism may be poorly predictive of the profiles found in other organisms and that drug sensitivity and resistance profiles can differ significantly among organisms even when the apparent target of the drug is the same.

Antifungals discovery: an insight into new strategies to combat antifungal resistance

Undeniably, new antifungal treatments are necessary against pathogenic fungi. Fungal infections have significantly increased in recent decades, being highlighted as important causes of morbidity and mortality, particularly in immunocompromised patients. Five main antifungal classes are used: (i) azoles, (ii) echinocandins, (iii) polyenes, (iv) allylamines and (v) pyrimidine analogues. Moreover, the treatment of mycoses has several limitations, such as undesirable side effects, narrow activity spectrum, a small number of targets and fungal resistance, which are still of major concern in clinical practice. The discovery of new antifungals is mostly achieved by the screening of natural or synthetic/semisynthetic chemical compounds. The most recent discoveries in drug resistance mechanism and their avoidance were explored in a review, focusing on different antifungal targets, as well as new agents or strategies, such as combination therapy, that could improve antifungal therapy.

SIGNIFICANCE AND IMPACT OF THE STUDY

The failure to respond to antifungal therapy is complex and is associated with microbiological resistance and increased expression of virulence in fungal pathogens. Thus, this review offers an overview of current challenges in the treatment of fungal infections associated with increased antifungal drug resistance and the formation of biofilms in these opportunistic pathogens. Furthermore, the most recent and potential strategies to combat fungal pathogens are explored here, focusing on new agents as well as innovative approaches, such as combination therapy between antifungal drugs or with natural compounds.

The PHR Family: The Role of Extracellular Transglycosylases in Shaping Candida albicans Cells

Candida albicans is an opportunistic microorganism that can become a pathogen causing mild superficial mycosis or more severe invasive infections that can be life-threatening for debilitated patients. In the etiology of invasive infections, key factors are the adaptability of C. albicans to the different niches of the human body and the transition from a yeast form to hypha. Hyphal morphology confers high adhesiveness to the host cells, as well as the ability to penetrate into organs. The cell wall plays a crucial role in the morphological changes C. albicans undergoes in response to specific environmental cues. Among the different categories of enzymes involved in the formation of the fungal cell wall, the GH72 family of transglycosylases plays an important assembly role. These enzymes cut and religate β-(1,3)-glucan, the major determinant of cell shape. In C. albicans, the PHR family encodes GH72 enzymes, some of which work in specific environmental conditions. In this review, we will summarize the work from the initial discovery of PHR genes to the study of the pH-dependent expression of PHR1 and PHR2, from the characterization of the gene products to the recent findings concerning the stress response generated by the lack of GH72 activity in C. albicans hyphae.

Genome-wide functional analysis in Candida albicans

Candida albicans is an important etiological agent of superficial and life-threatening infections in individuals with compromised immune systems. To date, we know of several overlapping genetic networks that govern virulence attributes in this fungal pathogen. Classical use of deletion mutants has led to the discovery of numerous virulence factors over the years, and genome-wide functional analysis has propelled gene discovery at an even faster pace. Indeed, a number of recent studies using large-scale genetic screens followed by genome-wide functional analysis has allowed for the unbiased discovery of many new genes involved in C. albicans biology. Here we share our perspectives on the role of these studies in analyzing fundamental aspects of C. albicans virulence properties.

Mitochondrial Complex V α Subunit Is Critical for Candida albicans Pathogenicity through Modulating Multiple Virulence Properties

The α subunit (ATP1) is a vital component of mitochondrial complex V which counts for the majority of cellular ATP production in a living organism. Nevertheless, how the α subunit influences other cellular processes such as pathogenicity in Candida albicans remains poorly understood. To address this question, ATP1 mutant (atp1Δ/Δ) and the gene-reconstituted strain (atp1Δ/ATP1) have been constructed in this study and their pathogenicity-related traits are compared to those of wild type (WT). In a murine model of disseminated candidiasis, atp1Δ/Δ infected mice have a significantly higher survival rate and experience a lower fungal burden in tissues. In in vitro studies atp1Δ/Δ lose a capability to damage or destroy macrophages and endothelial cells. Furthermore, atp1Δ/Δ is not able to grow under either glucose-denial conditions or high H₂O₂ conditions, both of which are associated with the potency of the macrophages to kill C. albicans. Defects in filamentation and biofilm formation may impair the ability of atp1Δ/Δ to penetrate host cells and establish robust colonies in the host tissues. In concert with these pathogenic features, intracellular ATP levels of atp1Δ/Δ can drop to 1/3 of WT level. These results indicate that the α subunit of Complex V play important roles in C. albicans pathogenicity.

System-Wide Adaptations of Desulfovibrio alaskensis G20 to Phosphate-Limited Conditions

The prevalence of lipids devoid of phosphorus suggests that the availability of phosphorus limits microbial growth and activity in many anoxic, stratified environments. To better understand the response of anaerobic bacteria to phosphate limitation and starvation, this study combines microscopic and lipid analyses with the measurements of fitness of pooled barcoded transposon mutants of the model sulfate reducing bacterium Desulfovibrio alaskensis G20. Phosphate-limited G20 has lower growth rates and replaces more than 90% of its membrane phospholipids by a mixture of monoglycosyl diacylglycerol (MGDG), glycuronic acid diacylglycerol (GADG) and ornithine lipids, lacks polyphosphate granules, and synthesizes other cellular inclusions. Analyses of pooled and individual mutants reveal the importance of the high-affinity phosphate transport system (the Pst system), PhoR, and glycolipid and ornithine lipid synthases during phosphate limitation. The phosphate-dependent synthesis of MGDG in G20 and the widespread occurrence of the MGDG/GADG synthase among sulfate reducing ∂-Proteobacteria implicate these microbes in the production of abundant MGDG in anaerobic environments where the concentrations of phosphate are lower than 10 μM. Numerous predicted changes in the composition of the cell envelope and systems involved in transport, maintenance of cytoplasmic redox potential, central metabolism and regulatory pathways also suggest an impact of phosphate limitation on the susceptibility of sulfate reducing bacteria to other anthropogenic or environmental stresses.

Competitive Growth Enhances Conditional Growth Mutant Sensitivity to Antibiotics and Exposes a Two-Component System as an Emerging Antibacterial Target in Burkholderia cenocepacia

Chemogenetic approaches to profile an antibiotic mode of action are based on detecting differential sensitivities of engineered bacterial strains in which the antibacterial target (usually encoded by an essential gene) or an associated process is regulated. We previously developed an essential-gene knockdown mutant library in the multidrug-resistant Burkholderia cenocepacia by transposon delivery of a rhamnose-inducible promoter. In this work, we used Illumina sequencing of multiplex-PCR-amplified transposon junctions to track individual mutants during pooled growth in the presence of antibiotics. We found that competition from nontarget mutants magnified the hypersensitivity of a clone underexpressing gyrB to novobiocin by 8-fold compared with hypersensitivity measured during clonal growth. Additional profiling of various antibiotics against a pilot library representing most categories of essential genes revealed a two-component system with unknown function, which, upon depletion of the response regulator, sensitized B. cenocepacia to novobiocin, ciprofloxacin, tetracycline, chloramphenicol, kanamycin, meropenem, and carbonyl cyanide 3-chlorophenylhydrazone, but not to colistin, hydrogen peroxide, and dimethyl sulfoxide. We named the gene cluster esaSR for enhanced sensitivity to antibiotics sensor and response regulator. Mutational analysis and efflux activity assays revealed that while esaS is not essential and is involved in antibiotic-induced efflux, esaR is an essential gene and regulates efflux independently of antibiotic-mediated induction. Furthermore, microscopic analysis of cells stained with propidium iodide provided evidence that depletion of EsaR has a profound effect on the integrity of cell membranes. In summary, we unraveled a previously uncharacterized two-component system that can be targeted to reduce antibiotic resistance in B. cenocepacia.

Yeast: a microbe with macro-implications to antimicrobial drug discovery

Paramount to any rational discovery of new antibiotics displaying novel mechanisms of action is a deep knowledge of the genetic basis of microbial growth, division and virulence. The bakers' yeast,Saccharomyces cerevisiae, illustrates the highest understanding of the genetic underpinnings of microbial life, and from this framework, a systems biology paradigm has evolved, begging to be emulated in antibacterial discovery. Here, we review landmark events in the history of yeast genomics that provide this new foundation for antibacterial drug discovery.

Budding off: bringing functional genomics to Candida albicans

Candida species are the most prevalent human fungal pathogens, with Candida albicans being the most clinically relevant species. Candida albicans resides as a commensal of the human gastrointestinal tract but is a frequent cause of opportunistic mucosal and systemic infections. Investigation of C. albicans virulence has traditionally relied on candidate gene approaches, but recent advances in functional genomics have now facilitated global, unbiased studies of gene function. Such studies include comparative genomics (both between and within Candida species), analysis of total RNA expression, and regulation and delineation of protein-DNA interactions. Additionally, large collections of mutant strains have begun to aid systematic screening of clinically relevant phenotypes. Here, we will highlight the development of functional genomics in C. albicans and discuss the use of these approaches to addressing both commensalism and pathogenesis in this species.

Candida albicans Kinesin Kar3 Depends on a Cik1-Like Regulatory Partner Protein for Its Roles in Mating, Cell Morphogenesis, and Bipolar Spindle Formation

Candida albicans is a major fungal pathogen whose virulence is associated with its ability to transition from a budding yeast form to invasive hyphal filaments. The kinesin-14 family member CaKar3 is required for transition between these morphological states, as well as for mitotic progression and karyogamy. While kinesin-14 proteins are ubiquitous, CaKar3 homologs in hemiascomycete fungi are unique because they form heterodimers with noncatalytic kinesin-like proteins. Thus, CaKar3-based motors may represent a novel antifungal drug target. We have identified and examined the roles of a kinesin-like regulator of CaKar3. We show that orf19.306 (dubbed CaCIK1) encodes a protein that forms a heterodimer with CaKar3, localizes CaKar3 to spindle pole bodies, and can bind microtubules and influence CaKar3 mechanochemistry despite lacking an ATPase activity of its own. Similar to CaKar3 depletion, loss of CaCik1 results in cell cycle arrest, filamentation defects, and an inability to undergo karyogamy. Furthermore, an examination of the spindle structure in cells lacking either of these proteins shows that a large proportion have a monopolar spindle or two dissociated half-spindles, a phenotype unique to the C. albicans kinesin-14 homolog. These findings provide new insights into mitotic spindle structure and kinesin motor function in C. albicans and identify a potentially vulnerable target for antifungal drug development.

Rapid quantification of mutant fitness in diverse bacteria by sequencing randomly bar-coded transposons

Transposon mutagenesis with next-generation sequencing (TnSeq) is a powerful approach to annotate gene function in bacteria, but existing protocols for TnSeq require laborious preparation of every sample before sequencing. Thus, the existing protocols are not amenable to the throughput necessary to identify phenotypes and functions for the majority of genes in diverse bacteria. Here, we present a method, random bar code transposon-site sequencing (RB-TnSeq), which increases the throughput of mutant fitness profiling by incorporating random DNA bar codes into Tn5 and mariner transposons and by using bar code sequencing (BarSeq) to assay mutant fitness. RB-TnSeq can be used with any transposon, and TnSeq is performed once per organism instead of once per sample. Each BarSeq assay requires only a simple PCR, and 48 to 96 samples can be sequenced on one lane of an Illumina HiSeq system. We demonstrate the reproducibility and biological significance of RB-TnSeq with Escherichia coli, Phaeobacter inhibens, Pseudomonas stutzeri, Shewanella amazonensis, and Shewanella oneidensis. To demonstrate the increased throughput of RB-TnSeq, we performed 387 successful genome-wide mutant fitness assays representing 130 different bacterium-carbon source combinations and identified 5,196 genes with significant phenotypes across the five bacteria. In P. inhibens, we used our mutant fitness data to identify genes important for the utilization of diverse carbon substrates, including a putative d-mannose isomerase that is required for mannitol catabolism. RB-TnSeq will enable the cost-effective functional annotation of diverse bacteria using mutant fitness profiling.

A large challenge in microbiology is the functional assessment of the millions of uncharacterized genes identified by genome sequencing. Transposon mutagenesis coupled to next-generation sequencing (TnSeq) is a powerful approach to assign phenotypes and functions to genes. However, the current strategies for TnSeq are too laborious to be applied to hundreds of experimental conditions across multiple bacteria. Here, we describe an approach, random bar code transposon-site sequencing (RB-TnSeq), which greatly simplifies the measurement of gene fitness by using bar code sequencing (BarSeq) to monitor the abundance of mutants. We performed 387 genome-wide fitness assays across five bacteria and identified phenotypes for over 5,000 genes. RB-TnSeq can be applied to diverse bacteria and is a powerful tool to annotate uncharacterized genes using phenotype data.

Essential gene discovery in the basidiomycete Cryptococcus neoformans for antifungal drug target prioritization

Fungal diseases represent a major burden to health care globally. As with other pathogenic microbes, there is a limited number of agents suitable for use in treating fungal diseases, and resistance to these agents can develop rapidly. Cryptococcus neoformans is a basidiomycete fungus that causes cryptococcosis worldwide in both immunocompromised and healthy individuals. As a basidiomycete, it diverged from other common pathogenic or model ascomycete fungi more than 500 million years ago. Here, we report C. neoformans genes that are essential for viability as identified through forward and reverse genetic approaches, using an engineered diploid strain and genetic segregation after meiosis. The forward genetic approach generated random insertional mutants in the diploid strain, the induction of meiosis and sporulation, and selection for haploid cells with counterselection of the insertion event. More than 2,500 mutants were analyzed, and transfer DNA (T-DNA) insertions in several genes required for viability were identified. The genes include those encoding the thioredoxin reductase (Trr1), a ribosome assembly factor (Rsa4), an mRNA-capping component (Cet1), and others. For targeted gene replacement, the C. neoformans homologs of 35 genes required for viability in ascomycete fungi were disrupted, meiosis and sporulation were induced, and haploid progeny were evaluated for their ability to grow on selective media. Twenty-one (60%) were found to be required for viability in C. neoformans. These genes are involved in mitochondrial translation, ergosterol biosynthesis, and RNA-related functions. The heterozygous diploid mutants were evaluated for haploinsufficiency on a number of perturbing agents and drugs, revealing phenotypes due to the loss of one copy of an essential gene in C. neoformans. This study expands the knowledge of the essential genes in fungi using a basidiomycete as a model organism. Genes that have no mammalian homologs and are essential in both Cryptococcus and ascomycete human pathogens would be ideal for the development of antifungal drugs with broad-spectrum activity.

IMPORTANCE

Fungal infections are very common in humans but may be neglected due to misdiagnosis and inattention. Cryptococcus neoformans is a yeast that infects mainly immunocompromised people, causing high mortality rates in developing countries. The fungus infects the lungs, crosses the blood-brain barrier, and invades the cerebrospinal fluid, causing fatal meningitis. C. neoformans infections are treated with amphotericin B, flucytosine, and azoles, all developed decades ago. However, problems with antifungal agents highlight the urgent need for more-effective drugs to treat C. neoformans and other invasive fungal infections. These issues include the negative side effects of amphotericin B, the spontaneous resistance of C. neoformans to azoles, and the inefficacy of the echinocandin antifungals. In this study, we report the identification of C. neoformans essential genes as targets for the development of novel antifungals. Because of the level of evolutionary divergence between C. neoformans and the ascomycetes, a subset of these genes is likely essential in all fungi. Genes identified in this study represent an excellent starting point for the future development of new antifungals by pharmaceutical companies.

Overcoming antifungal resistance

Fungal infections have become one of the major causes of morbidity and mortality in immunocompromised patients. Despite increased awareness and improved treatment strategies, the frequent development of resistance to the antifungal drugs used in clinical settings contributes to the increasing toll of mycoses. Although a natural phenomenon, antifungal drug resistance can compromise advances in the development of effective diagnostic techniques and novel antifungals. In this review, we will discuss the advent of cellular-micro- arrays, microfluidics, genomics, proteomics and other state-of-the art technologies in conquering antifungal drug resistance.

Harnessing the power of transposon mutagenesis for antibacterial target identification and evaluation

Determining the mechanism of action of bacterial growth inhibitors can be a formidable challenge in the progression of small molecules into antibacterial therapies. To help address this bottleneck, we have developed a robust transposon mutagenesis system using a suite of outward facing promoters in order to generate a comprehensive range of expression genotypes in Staphylococcus aureus from which to select defined compound-resistant transposon insertion mutants. Resistance stemming from either gene or operon over/under-expression, in addition to deletion, provides insight into multiple factors that contribute to a compound's observed activity, including means of cell envelope penetration and susceptibility to efflux. By profiling the entire resistome, the suitability of an antibacterial target itself is also evaluated, sometimes with unanticipated results. We herein show that for the staphylococcal signal peptidase (SpsB) inhibitors, modulating expression of lipoteichoic acid synthase (LtaS) confers up to a 100-fold increase in the minimal inhibitory concentration. As similarly efficient transposition systems are or will become established in other bacteria and cell types, we discuss the utility, limitations and future promise of Tnp mutagenesis for determining both a compound's mechanism of action and in the evaluation of novel targets.

Evolutionary analysis of multidrug resistance genes in fungi - impact of gene duplication and family conservation

Although the emergence of bacterial drug resistance is of great concern to the scientific community, few studies have evaluated this phenomenon systematically in fungi by using genome-wide datasets. In the present study, we assembled a large compendium of Saccharomyces cerevisiae chemical genetic data to study the evolution of multidrug resistance genes (MDRs) in the fungal lineage. We found that MDRs typically emerge in widely conserved families, most of which containing homologs from pathogenic fungi, such as Candida albicans and Coccidioides immitis, which could favor the evolution of drug resistance in those species. By integrating data from chemical genetics with protein family conservation, genetic and protein interactions, we found that gene families rarely have more than one MDR, indicating that paralogs evolve asymmetrically with regard to multidrug resistance roles. Furthermore, MDRs have more genetic and protein interaction partners than non-MDRs, supporting their participation in complex biochemical systems underlying the tolerance to multiple bioactive molecules. MDRs share more chemical genetic interactions with other MDRs than with non-MDRs, regardless of their evolutionary affinity. These results suggest the existence of an intricate system involved in the global drug tolerance phenotypes. Finally, MDRs are more likely to be hit repeatedly by mutations in laboratory evolution experiments, indicating that they have great adaptive potential. The results presented here not only reveal the main genomic features underlying the evolution of MDRs, but also shed light on the gene families from which drug resistance is more likely to emerge in fungi.

Towards an informative mutant phenotype for every bacterial gene

Mutant phenotypes provide strong clues to the functions of the underlying genes and could allow annotation of the millions of sequenced yet uncharacterized bacterial genes. However, it is not known how many genes have a phenotype under laboratory conditions, how many phenotypes are biologically interpretable for predicting gene function, and what experimental conditions are optimal to maximize the number of genes with a phenotype. To address these issues, we measured the mutant fitness of 1,586 genes of the ethanol-producing bacterium Zymomonas mobilis ZM4 across 492 diverse experiments and found statistically significant phenotypes for 89% of all assayed genes. Thus, in Z. mobilis, most genes have a functional consequence under laboratory conditions. We demonstrate that 41% of Z. mobilis genes have both a strong phenotype and a similar fitness pattern (cofitness) to another gene, and are therefore good candidates for functional annotation using mutant fitness. Among 502 poorly characterized Z. mobilis genes, we identified a significant cofitness relationship for 174. For 57 of these genes without a specific functional annotation, we found additional evidence to support the biological significance of these gene-gene associations, and in 33 instances, we were able to predict specific physiological or biochemical roles for the poorly characterized genes. Last, we identified a set of 79 diverse mutant fitness experiments in Z. mobilis that are nearly as biologically informative as the entire set of 492 experiments. Therefore, our work provides a blueprint for the functional annotation of diverse bacteria using mutant fitness.

Functional toxicology: tools to advance the future of toxicity testing

The increased presence of chemical contaminants in the environment is an undeniable concern to human health and ecosystems. Historically, by relying heavily upon costly and laborious animal-based toxicity assays, the field of toxicology has often neglected examinations of the cellular and molecular mechanisms of toxicity for the majority of compounds—information that, if available, would strengthen risk assessment analyses. Functional toxicology, where cells or organisms with gene deletions or depleted proteins are used to assess genetic requirements for chemical tolerance, can advance the field of toxicity testing by contributing data regarding chemical mechanisms of toxicity. Functional toxicology can be accomplished using available genetic tools in yeasts, other fungi and bacteria, and eukaryotes of increased complexity, including zebrafish, fruit flies, rodents, and human cell lines. Underscored is the value of using less complex systems such as yeasts to direct further studies in more complex systems such as human cell lines. Functional techniques can yield (1) novel insights into chemical toxicity; (2) pathways and mechanisms deserving of further study; and (3) candidate human toxicant susceptibility or resistance genes.

Molecular genetic techniques for gene manipulation in Candida albicans

Candida albicans is one of the most common fungal pathogen in humans due to its high frequency as an opportunistic and pathogenic fungus causing superficial as well as invasive infections in immunocompromised patients. An understanding of gene function in C. albicans is necessary to study the molecular basis of its pathogenesis, virulence and drug resistance. Several manipulation techniques have been used for investigation of gene function in C. albicans, including gene disruption, controlled gene expression, protein tagging, gene reintegration, and overexpression. In this review, the main cassettes containing selectable markers used for gene manipulation in C. albicans are summarized; the advantages and limitations of these cassettes are discussed concerning the influences on the target gene expression and the virulence of the mutant strains.

The HOG pathway is critical for the colonization of the mouse gastrointestinal tract by Candida albicans

The opportunistic pathogen Candida albicans is a frequent inhabitant of the human gastrointestinal tract where it usually behaves as a harmless commensal. In this particular niche, it needs to adapt to the different micro environments that challenge its survival within the host. In order to determine those factors involved in gut adaptation, we have used a gastrointestinal model of colonization in mouse to trace the behaviour of fungal cells. We have developed a genetic labelling system based on the complementary spectral properties of the fluorescent proteins GFP and a new C. albicans codon-adapted RFP (dTOM2) that allow a precise quantification of the fungal population in the gut via standard in vitro cultures or flow cytometry. This methodology has allowed us to determine the role of the three MAP kinase pathways of C. albicans (mediated by the MAPK Mkc1, Cek1 or Hog1) in mouse gut colonization via competitive assays with MAPK pathway mutants and their isogenic wild type strain. This approach reveals the signalling through HOG pathway as a critical factor influencing the establishment of C. albicans in the mouse gut. Less pronounced effects for mkc1 or cek1 mutants were found, only evident after 2-3 weeks of colonization. We have also seen that hog1 mutants is defective in adhesion to the gut mucosa and sensitive to bile salts. Finally, we have developed a genetic strategy for the in vivo excision (tetracycline-dependent) of any specific gene during the course of colonization in this particular niche, allowing the analysis of its role during gut colonization.

Antifungal drug discovery: the process and outcomes

New data suggest that the global incidence of several types of fungal diseases have traditionally been under-documented. Of these, mortality caused by invasive fungal infections remains disturbingly high, equal to or exceeding deaths caused by drug-resistant tuberculosis and malaria. It is clear that basic research on new antifungal drugs, vaccines and diagnostic tools is needed. In this review, we focus upon antifungal drug discovery including in vitro assays, compound libraries and approaches to target identification. Genome mining has made it possible to identify fungal-specific targets; however, new compounds to these targets are apparently not in the antimicrobial pipeline. We suggest that 'repurposing' compounds (off patent) might be a more immediate starting point. Furthermore, we examine the dogma on antifungal discovery and suggest that a major thrust in technologies such as structural biology, homology modeling and virtual imaging is needed to drive discovery.

Gymnemic acids inhibit hyphal growth and virulence in Candida albicans

Candida albicans is an opportunistic and polymorphic fungal pathogen that causes mucosal, disseminated and invasive infections in humans. Transition from the yeast form to the hyphal form is one of the key virulence factors in C. albicans contributing to macrophage evasion, tissue invasion and biofilm formation. Nontoxic small molecules that inhibit C. albicans yeast-to-hypha conversion and hyphal growth could represent a valuable source for understanding pathogenic fungal morphogenesis, identifying drug targets and serving as templates for the development of novel antifungal agents. Here, we have identified the triterpenoid saponin family of gymnemic acids (GAs) as inhibitor of C. albicans morphogenesis. GAs were isolated and purified from Gymnema sylvestre leaves, the Ayurvedic traditional medicinal plant used to treat diabetes. Purified GAs had no effect on the growth and viability of C. albicans yeast cells but inhibited its yeast-to-hypha conversion under several hypha-inducing conditions, including the presence of serum. Moreover, GAs promoted the conversion of C. albicans hyphae into yeast cells under hypha inducing conditions. They also inhibited conidial germination and hyphal growth of Aspergillus sp. Finally, GAs inhibited the formation of invasive hyphae from C. albicans-infected Caenorhabditis elegans worms and rescued them from killing by C. albicans. Hence, GAs could be useful for various antifungal applications due to their traditional use in herbal medicine.

Systems-level antimicrobial drug and drug synergy discovery

Here, we review the 'target-centric' genomic strategy to antimicrobial discovery and share our perspective on identification, validation and prioritization of potential antimicrobial drug targets in the context of emerging chemical biology, genomics and phenotypic screening strategies. We propose that coupling the dual processes of antimicrobial small-molecule screening and target identification in a whole-cell context is essential to empirically annotate 'druggable' targets and advance early stage antimicrobial discovery. We also advocate a systems-level approach to annotating synthetic-lethal genetic interactions comprehensively within yeast and bacteria models. The resulting genetic interaction networks provide a landscape to rationally predict and exploit drug synergy between cognate inhibitors. We posit that synergistic combination agents provide an important and largely unexploited strategy to 'repurpose' existing chemical space and simultaneously address issues of potency, spectrum, toxicity and drug resistance in early stages of antimicrobial drug discovery.

Candida parapsilosis is a significant neonatal pathogen: a systematic review and meta-analysis

BACKGROUND

Candida is the third most common cause of late-onset neonatal sepsis in infants born at <1500 g. Candida parapsilosis infections are increasingly reported in preterm neonates in association with indwelling catheters.

METHODS

We systematically reviewed neonatal literature and synthesized data pertaining to percentage of C. parapsilosis infections and mortality by meta-analyses. We also reviewed risk factors, virulence determinants, antimicrobial susceptibility patterns and outlined clinical management strategies.

RESULTS

C. parapsilosis infections comprised 33.47% (95% confidence interval [CI]: 30.02, 37.31) of all neonatal Candida infections. C. parapsilosis rates were similar in studies performed before the year 2000, 33.53% (95% CI: 30.06, 37.40) (28 studies), to those after 2000, 27.00% (95% CI: 8.25, 88.37) (8 studies). The mortality due to neonatal C. parapsilosis infections was 10.02% (95% CI: 7.66, 13.12). Geographical variations in C. parapsilosis infections included a low incidence in Europe and higher incidence in North America and Australia. Biofilm formation was a significant virulence determinant and predominant risk factors for C. parapsilosis infections were prematurity, prior colonization and catheterization. Amphotericin B remains the antifungal drug of choice and combination therapy with caspofungin or other echinocandins may be considered in resistant cases.

CONCLUSION

C. parapsilosis is a significant neonatal pathogen, comprises a third of all Candida infections and is associated with 10% mortality. Availability of tools for genetic manipulation of this organism will identify virulence determinants and organism characteristics that may explain predilection for preterm neonates. Strategies to prevent horizontal transmission in the neonatal unit are paramount in decreasing infection rates.

AGC protein kinases: from structural mechanism of regulation to allosteric drug development for the treatment of human diseases

The group of AGC protein kinases includes more than 60 protein kinases in the human genome, classified into 14 families: PDK1, AKT/PKB, SGK, PKA, PKG, PKC, PKN/PRK, RSK, NDR, MAST, YANK, DMPK, GRK and SGK494. This group is also widely represented in other eukaryotes, including causative organisms of human infectious diseases. AGC kinases are involved in diverse cellular functions and are potential targets for the treatment of human diseases such as cancer, diabetes, obesity, neurological disorders, inflammation and viral infections. Small molecule inhibitors of AGC kinases may also have potential as novel therapeutic approaches against infectious organisms. Fundamental in the regulation of many AGC kinases is a regulatory site termed the "PIF-pocket" that serves as a docking site for substrates of PDK1. This site is also essential to the mechanism of activation of AGC kinases by phosphorylation and is involved in the allosteric regulation of N-terminal domains of several AGC kinases, such as PKN/PRKs and atypical PKCs. In addition, the C-terminal tail and its interaction with the PIF-pocket are involved in the dimerization of the DMPK family of kinases and may explain the molecular mechanism of allosteric activation of GRKs by GPCR substrates. In this review, we briefly introduce the AGC kinases and their known roles in physiology and disease and the discovery of the PIF-pocket as a regulatory site in AGC kinases. Finally, we summarize the current status and future therapeutic potential of small molecules directed to the PIF-pocket; these molecules can allosterically activate or inhibit the kinase as well as act as substrate-selective inhibitors. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Exploitation of genomics in fungicide research: current status and future perspectives

Every year, fungicide use to control plant disease caused by pathogenic fungi increases. The global fungicide market is now worth more than £5.3 billion, second only to the herbicide market in importance. In the UK, over 5500 tonnes of fungicide were applied to crops in 2010 (The Food and Environment Research Agency, Pesticide Usage Statistics), with 95.5% of the wheat-growing area receiving three fungicide sprays. Although dependence on fungicides to produce food securely, reliably and cheaply may be moderated in the future by further developments in crop biotechnology, modern crop protection will continue to require a diversity of solutions, including effective and safe chemical control. Therefore, investment in exploiting the increasingly available genome sequences of the most devastating fungal and oomycete phytopathogenic species should bring an array of new opportunities for chemical intervention. To date, the impact of whole genome research on the development, introduction and stewardship of fungicides has been limited, but ongoing improvements in computational analysis, molecular biology, chemical genetics, genome sequencing and transcriptomics will facilitate the development and registration of the future suite of crop protection chemicals.

Shuttle vectors for facile gap repair cloning and integration into a neutral locus in Candida albicans

Candida albicans is the most prevalent fungal pathogen of humans. The current techniques used to construct C. albicans strains require integration of exogenous DNA at ectopic locations, which can exert position effects on gene expression that can confound the interpretation of data from critical experiments such as virulence assays. We have identified a large intergenic region, NEUT5L, which facilitates the integration and expression of ectopic genes. To construct and integrate inserts into this novel locus, we re-engineered yeast/bacterial shuttle vectors by incorporating 550 bp of homology to NEUT5L. These vectors allow rapid, facile cloning through in vivo recombination (gap repair) in Saccharomyces cerevisiae and efficient integration of the construct into the NEUT5L locus. Other useful features of these vectors include a choice of three selectable markers (URA3, the recyclable URA3-dpl200 or NAT1), and rare restriction enzyme recognition sites for releasing the insert from the vector prior to transformation into C. albicans, thereby reducing the insert size and preventing integration of non-C. albicans DNA. Importantly, unlike the commonly used RPS1/RP10 locus, integration at NEUT5L has no negative effect on growth rates and allows native-locus expression levels, making it an ideal genomic locus for the integration of exogenous DNA in C. albicans.