Green fluorescent protein (GFP) as a vital marker for pathogenic development of the dermatophyte Trichophyton mentagrophytes

Auxotrophic mutations of Trichophyton rubrum created by in vitro synthesized Cas9 ribonucleoprotein

BACKGROUND

Trichophyton rubrum is an obligate human parasitic fungus and responsible for approximately 80-90% of dermatomycosis in human. Molecular genetic manipulations of this pathogen are challenging and available tools and protocols are only rudimentary. We adapt molecular genetics methods of well established fungal model organism, to knock out genes in T. rubrum. For the adaptation, crucial modifications are necessary. With the implementation of in vitro synthesized Cas9-sgRNA ribonucleoprotein complex, it is possible to adapt molecular genetic methods, to knock out genes in T. rubrum.

RESULTS

The gene knock-out method is based on integration of a selection marker into the target site, to interrupt the gene translation. The target gene gets preassigned by the homologous sequence of the in vitro synthesized Cas9-sgRNA ribonucleoprotein complex. To develop the method, we first isolated and characterized a T. rubrum strain with a high amount of microconidia. Next, we developed a transformation protocol, whereby the Cas9-sgRNA ribonucleoprotein gets delivered into the fungal protoplast by the PEG method. We knocked out the URA3 gene and resulted, as predicted, uracil auxotrophic strains. These strains can be used for specific gene knock-outs by reintegrating the URA3 fragment and selection on uracil lacking cultivation media. Exemplary, we knocked out the TRP3 gene and got the predicted phenotype, tryptophan auxotrophic strains. The mutation had been verified by sequencing.

CONCLUSIONS

We developed a method, based on in vitro synthesized Cas9-sgRNA ribonucleoprotein complex, for target specific gene knock-outs in T. rubrum. We knocked out the Ura3 gene and resulted uracil auxotrophic strains. These strains were used for target specific gene knock-outs by reintegrating the Ura3 fragment into the target gene site to interrupt the gene transcription. The developed method allows to adapt sophisticate gene manipulation methods of model fungal species to non-model species.

Diagnosis of dermatophytosis using single fungus endogenous fluorescence spectrometry

We propose to use a single fungus endogenous fluorescence spectrometry base on a hyperspectral fluorescence microscope for the diagnosis of dermatophytosis. Dermatophyte samples, including Aspergillus, Trichophyton rubrum, Microsporum gypseum, and Microsporum canis were imaged, and the endogenous fluorescence spectrum of a single fungus was calculated. High contrast fluorescence images and endogenous fluorescence spectrum of the single fungus were used to identify the type of dermatophyte. Morphologically similar Microsporum gypseum and Microsporum canis can be distinguished using an endogenous fluorescence spectrum of the single fungus. Meanwhile, our result showed that the sensitivity and specificity of identifying Microsporum gypseum were 95% and 93%, and the sensitivity and specificity of identifying Microsporum canis were 94% and 93%.

Application of the red fluorescent protein mCherry in mycelial labeling and organelle tracing in the dermatophyte Trichophyton mentagrophytes

Trichophyton mentagrophytes is a fungus that causes skin disease in humans and other animals worldwide. Studies on molecular biology and fluorescent labeling of the fungus are limited. Here, we applied mCherry for the first time in T. mentagrophytes to label the fungus and its organelles. We constructed four expression vectors of mCherry or mCherry fusions containing a variety of resistance markers and promoters, which were then integrated, together with two previous mCherry expression vectors, in T. mentagrophytes via Agrobacterium tumefaciens-mediated transformation (AtMT). The resulting transformants emitted bright red fluorescence. We used the histone protein H2B and the peroxisome targeting signal 1 (PTS1) peptide to target the nucleus and peroxisomes, respectively, in T. mentagrophytes. In the transformants expressing mCherry-fused H2B, the fluorescence was distinctly localized to the nuclei in hyphae, spores and the fungal cells in infected animal tissue. In the T. mentagrophytes transformants where the peroxisome was targeted, the mCherry was present as small dots (0.2-1 μm diameter) throughout the spores and the hyphae. We also constructed a T. mentagrophytes AtMT library containing more than 1000 hygromycin-resistant transformants that were genetically stable. Our results provide useful tools for further investigations on molecular pathogenesis of T. mentagrophytes.

Genetic Manipulations in Dermatophytes

Dermatophytes are a group of closely related fungi that nourish on keratinized materials for their survival. They infect stratum corneum, nails, and hair of human and animals, accounting the largest portion of fungi causing superficial mycoses. Huge populations are suffering from dermatophytoses, though the biology of these fungi is largely unknown yet. Reasons are partially attributed to the poor amenability of dermatophytes to genetic manipulation. However, advancements in this field over the last decade made it possible to conduct genetic studies to satisfying extents. These included genetic transformation methods, indispensable molecular tools, i.e., dominant selectable markers, inducible promoter, and marker recycling system, along with improving homologous recombination frequency and gene silencing. Furthermore, annotated genome sequences of several dermatophytic species have recently been available, ensuring an optimal recruitment of the molecular tools to expand our knowledge on these fungi. In conclusion, the establishment of basic molecular tools and the availability of genomic data will open a new era that might change our understanding on the biology and pathogenicity of this fungal group.

Dermatophyte virulence factors: identifying and analyzing genes that may contribute to chronic or acute skin infections

Dermatophytes are prevalent causes of cutaneous mycoses and, unlike many other fungal pathogens, are able to cause disease in immunocompetent individuals. They infect keratinized tissue such as skin, hair, and nails, resulting in tinea infections, including ringworm. Little is known about the molecular mechanisms that underlie the ability of these organisms to establish and maintain infection. The recent availability of genome sequence information and improved genetic manipulation have enabled researchers to begin to identify and study the role of virulence factors of dermatophytes. This paper will summarize our current understanding of dermatophyte virulence factors and discuss future directions for identifying and testing virulence factors.

Analysis of the differentially expressed genes in Microsporum canis in inducing smooth skin and scalp tissue conditions

Microsporum canis is a common zoophilic dermatophyte, which causes a range of infections. To explore the pathogenic mechanism of tinea capitis, we used the suppression subtractive hybridization (SSH) technique to investigate the differences in gene expression between different cultures of Microsporum canis incubated on three different types of mineral media containing child glabrous skin, child scalp tissue and adult scalp tissue. Using dot-blot hybridization and real-time PCR technique, we successfully screened and identified a pair of genes that had expression levels 44.6 and 117 times higher in culture 1 (M. canis cultured in mineral medium with child scalp tissue) than in culture 2 (M. canis cultured in mineral medium with glabrous skin tissue), and another pair of genes with expression levels 78.2 and 9.8 times higher in culture 1 than in culture 3 (M. canis cultured in mineral medium with adult scalp tissue). These four genes were found to have 41%, 53%, 40% and 94% homology to those encoding a hypothetical protein [family of serine hydrolases 1; (FSH1)], PQ loop repeat protein (PQ-LRP), a predicted protein [porphyrin galactose 4; (P-GAL4)] and NADH dehydrogenase subunit (NADH)1, respectively. The upregulation of the FSH1, PQ-LRP, P-GAL4 and NADH1 genes in cultures of child scalp tissue indicates that they are essential in the pathogenesis of tinea capitis caused by M. canis.

Development of efficient tools for genetic manipulation of dermatophytes

Molecular biological approaches have recently begun to be applied to molecular genetics studies of dermatophytes. High-throughput gene analysis methodologies, such as EST sequencing, differential cDNA screening, and cDNA-based microarray analysis have been used to obtain information on many dermatophyte genes and their expression profiles under different experimental conditions. In addition, whole genome sequencing projects are underway for several important dermatophytes, such as Trichophyton rubrum and Microsporum canis. These studies will provide large amounts of valuable information for elucidating the molecular basis of host invasion by dermatophytes and their virulence. Targeted gene disruption by homologous recombination is one of the most common approaches for determining the functions and roles of numerous genes isolated from pathogenic fungi. However, the difficulty of genetic manipulation due to low transformation frequency of dermatophytes may limit the successful production of null mutants by targeted gene disruption via homologous recombination. To overcome these problems, our group has developed useful genetic manipulation systems for dermatophytes using the clinically important dermatophyte, T. mentagrophytes.

Agrobacterium tumefaciens-mediated transformation of the dermatophyte, Trichophyton mentagrophytes: an efficient tool for gene transfer

Agrobacterium tumefaciens-mediated transformation (ATMT) was used to facilitate gene transfer into the clinically important dermatophyte, Trichophyton mentagrophytes (teleomorph: Arthroderma vanbreuseghemii). A binary vector containing a hygromycin B resistance cassette was introduced into A. tumefaciens, and the resultant strain was co-cultivated with fungal small conidia. Transformation yielded a large number of hygromycin B-resistant transformants. Hybridization analysis showed that most of the transformants harboured a single copy of T-DNA randomly integrated into the genome. Transformation frequency was increased to more than 200 per 10(7) conidia by optimizing the co-cultivation time and temperature. ATMT was then used for targeted gene disruption mediated by homologous recombination. Using a PCR-based strategy, we isolated the areA/nit-2-like nitrogen regulatory gene (tnr:Trichophytonnitrogen regulator) from T. mentagrophytes. A binary vector containing two regions of the tnr locus flanking the hygromycin B resistance cassette was constructed and introduced into T. mentagrophytesvia ATMT. Transformants with disruption of the areA/nit-2-like gene (tnr) were obtained in three of four independent disruption experiments, most of which showed homologous recombination via double crossover without additional ectopic integration of the disruption construct.

Development of transformation system for Trichophyton rubrum by electroporation of germinated conidia

Dermatophytes are the fungi that can cause infections of skin, hair, and nails due to their ability to utilize keratin. The genetic transformation systems of dermatophytes were successfully applied to Trichophyton mentagrophytes and Microsporum canis. Here we describe the procedure for genetic transformation of Trichophyton rubrum by electroporation of their germinated conidia. A linearized transformation vector (pCHSH75-Pch/GFP/TtrpC) containing bacterial hygromycin B phosphotransferase gene (hph) and green fluorescent protein gene (egfp) was introduced into the germinated conidia of T. rubrum by electroporation. PCR reaction analysis showed that egfp gene was integrated randomly and Southern blotting analysis demonstrated a single integration of hph gene into the chromosomal DNA of randomly selected transformant. In this work we report the efficient transformation and selection of the stable T. rubrum transformants.

Enhanced gene replacements in Ku80 disruption mutants of the dermatophyte, Trichophyton mentagrophytes

The frequency of targeted gene disruption via homologous recombination is low in the clinically important dermatophyte, Trichophyton mentagrophytes. The Ku genes, Ku70 and Ku80, encode key components of the nonhomologous end-joining pathway involved in DNA double-strand break repair. Their deletion increases the homologous recombination frequency, facilitating targeted gene disruption. To improve the homologous recombination frequency in T. mentagrophytes, the Ku80 ortholog was inactivated. The nucleotide sequence of the Ku80 locus containing a 2788-bp ORF encoding a predicted product of 728 amino acids was identified, and designated as TmKu80. The predicted TmKu80 product showed a high degree of amino acid sequence similarity to known fungal Ku80 proteins. Ku80 disruption mutant strains of T. mentagrophytes were constructed by Agrobacterium tumefaciens-mediated genetic transformation. The average homologous recombination frequency was 73.3 +/- 25.2% for the areA/nit-2-like nitrogen regulatory gene (tnr) in Ku80(-) mutants, about 33-fold higher than that in wild-type controls. A high frequency (c. 67%) was also obtained for the Tri m4 gene encoding a putative serine protease. Ku80(-) mutant strains will be useful for large-scale reverse genetics studies of dermatophytes, including T. mentagrophytes, providing valuable information on the basic mechanisms of host invasion.

[Molecular approach to pathology of and immunity against dermatophytes]

Molecular biological studies of the host invasion mechanisms and possible virulence-related factors of dermatophytes have just begun. The identification of individual genes and large-scale investigations of transcripts expressed under different experimental culture conditions have provided useful information on the structure, expression, and regulation of the genes of major dermatophyte species such as Trichophyton rubrum. The next goal of dermatophytosis research will be to elucidate the functions and roles of the identified fungal genes during the infection process. It will also be necessary to investigate the host immune responses to fungal gene expression and regulation during infection. For such research, genetic manipulation techniques for dermatophytes, such as exogenous gene transfer into fungal cells and targeted gene disruption, are indispensable. However, such methods are not yet well established. We have developed an efficient dermatophyte genetic manipulation system using T. mentagrophytes. Here, we present our current research findings, mainly with regard to the system for exogenous gene transfer into T. mentagrophytes cells.

Pathogenesis of dermatophytosis

Despite the superficial localization of most dermatophytosis, host-fungus relationship in these infections is complex and still poorly elucidated. Though many efforts have been accomplished to characterize secreted dermatophytic proteases at the molecular level, only punctual insights have been afforded into other aspects of the pathogenesis of dermatophytosis, such as fungal adhesion, regulation of gene expression during the infection process, and immunomodulation by fungal factors. However, new genetic tools were recently developed, allowing a more rapid and high-throughput functional investigation of dermatophyte genes and the identification of new putative virulence factors. In addition, sophisticated in vitro infection models are now used and will open the way to a more comprehensive view of the interactions between these fungi and host epidermal cells, especially keratinocytes.

Genetic transformation of the dermatophyte, Trichophyton mentagrophytes, based on the use of G418 resistance as a dominant selectable marker

BACKGROUND

Dermatophytes are closely related keratinophilic fungal pathogens and are the causative agents of a superficial cutaneous infection called dermatophytosis (ringworm). A lack of gene manipulation techniques has prevented detailed analyses of the mechanisms of host invasion by dermatophytes. We have introduced the tetracycline-regulatable (TR) gene expression system into dermatophytes to facilitate functional analyses of genes essential for growth and virulence. As the TR gene expression system consists of two plasmid vector components, two dominant selectable markers are required for genetic transformation. In dermatophytes, only the hygromycin B phosphotransferase gene (hph) is available as a selectable marker.

OBJECTIVE

We investigated the possibility of G418 resistance as a secondary selectable marker for genetic transformation in dermatophytes.

METHODS

A series of plasmid vectors carrying the neomycin phosphotransferase gene (nptII) were introduced into the protoplasts of Trichophyton mentagrophytes, one of the most clinically important dermatophyte species, by polyethylene glycol (PEG)-mediated transformation. Transformants were selected on selective medium containing G418 at 300-500 microg/ml.

RESULTS

Molecular biological analyses indicated that colonies appearing on the selective medium harbored nptII in their chromosomes. Colonies produced from protoplasts transformed with the enhanced green fluorescent protein (eGFP) gene-T. mentagrophytes cyclophilin cDNA (TmcypB) fusion vector also exhibited GFP fluorescence throughout their mycelia, but accumulation of the GFP-TmCYPB fusion protein in specific intracellular compartments was not observed.

CONCLUSIONS

This study has provided a new selectable marker for genetic transformation in dermatophytes.

Phylogenetic analysis of Trichophyton mentagrophytes human and animal isolates based on MnSOD and ITS sequence comparison

Dermatophytes are keratinophilic fungi able to infect keratinized tissues of human or animal origin. Among them, Trichophyton mentagrophytes is known to be a species complex composed of several species or variants, which occur in both human and animals. Since the T. mentagrophytes complex includes both anthropophilic and zoophilic pathogens, accurate molecular identification is a critical issue for comprehensive understanding of the clinical and epidemiological implications of the genetic heterogeneity of this complex. Here, 41 T. mentagrophytes isolates from either human patients (14 isolates) or animals (27 isolates) with dermatophytosis were prospectively isolated by culture and identified on morphological bases at the University Hospital Centres of Lille and Poitiers, and the Veterinary School of Alfort, respectively. The isolates were differentiated by DNA sequencing of the variable internal transcribed spacer (ITS) regions flanking the 5.8S rDNA, and of the housekeeping gene encoding the manganese-containing superoxide dismutase (MnSOD), an enzyme which is involved in defence against oxidative stress and has previously provided interesting insight into both fungal taxonomy and phylogeny. ITS1-ITS2 regions and MnSOD sequences successfully differentiate between members of the T. mentagrophytes complex and the related species Trichophyton rubrum. Whatever the phylogenetic marker used, members of this complex were classified into two major clades exhibiting a similar topology, with a higher variability when the ITS marker was used. Relationships between ITS/MnSOD sequences and host origin, clinical pattern and phenotypic characteristics (macroscopic and microscopic morphologies) were analysed.

Infection stages of the dermatophyte pathogen Trichophyton: microscopic characterization and proteolytic enzymes

Dermatophytes are pathogenic fungi that infect human skin, nails and hair and cause dermatophytosis. Trichophyton mentagrophytes is one of the most widespread species that belong to this group. Infection of the skin tissues include several stages, i.e., adhesion to the surface of the skin, invasion into the sublayers by the penetration of fungal elements and secretion of enzymes that degrade the skin components. In this study we have followed the morphology of the fungal elements, such as arthroconidia and hyphae, during the adhesion and invasion stages. Skin explants were inoculated with the dermatophyte and observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Skin explants were also inoculated with a transgenic isolate of T. mentagrophytes expressing the green fluorescent protein (GFP). The infected sublayers were investigated by confocal scanning laser microscopy (CSLM). As an adaptation to the tissue environment, the dermatophyte produced long fibrils when it is on the open surface of the stratum corneum, while short and thin fibrils are produced inside the dense sublayers. The short and long projections might have a role in adhesion. Invasion may be produced by mechanical and biochemical means. Invasion of the tissue showed hyphal branching and growth in multiple directions. The proteolytic profile was assayed by substrate gel and proteolytic activity. Two serine proteases of similar molecular weight were secreted during growth on the epidermal matrix components keratin and elastin. The dermatophyte may use the proteolytic enzymes to invade the surface and also the deep layer of the skin in immunocompromised patients. Dermatophytes, which are well adapted infectious agents, seem to use their mechanical and biochemical capabilities to invade the skin tissue effectively.

Targeting the calcineurin pathway enhances ergosterol biosynthesis inhibitors against Trichophyton mentagrophytes in vitro and in a human skin infection model

Fluconazole-FK506 or fluconazole-cyclosporine drug combinations were tested in an ex vivo Trichophyton mentagrophytes human skin infection model. Conidia colonization was monitored by scanning electron microscopy over a 7-day treatment period. The fluconazole-FK506 combination demonstrated the most obvious advantage over single-drug therapy by clearing conidia and protecting skin from damage at low drug concentrations.

Isolation, characterization, and disruption of dnr1, the areA/nit-2-like nitrogen regulatory gene of the zoophilic dermatophyte, Microsporum canis

A homolog of the major nitrogen regulatory genes areA from Aspergillus nidulans and nit-2 from Neurospora crassa was isolated from the zoophilic dermatophyte, Microsporum canis. This gene, dnr1, encodes a polypeptide of 761 amino acid residues containing a single zinc-finger DNA-binding domain, which is almost identical in amino acid sequence to the zinc-finger domains of AREA and NIT-2. The functional equivalence of dnr1 to areA was demonstrated by complementation of an areA loss-of-function mutant of A. nidulans with dnr1 cDNA. To further characterize this gene, dnr1 was disrupted by gene replacement based on homologous recombination. Of 100 transformants analyzed, two showed the results expected for replacement of dnr1. The growth properties of the two dnr1(-) mutant strains on various nitrogen sources were examined. Unlike the A. nidulansareA(-) mutant, these dnr1(-) mutants showed significantly reduced growth on ammonia, a preferred nitrogen source for fungi. These mutant strains were also able to utilize various amino acids for growth. In comparison with wild-type M. canis, the two dnr1(-) mutants showed reduced growth on medium containing keratin as the sole nitrogen source. This is the first report describing successful production of targeted gene-disrupted mutants by homologous recombination and their phenotypic analysis in dermatophytes.

Approaches to functional genomics in filamentous fungi

The study of gene function in filamentous fungi is a field of research that has made great advances in very recent years. A number of transformation and gene manipulation strategies have been developed and applied to a diverse and rapidly expanding list of economically important filamentous fungi and oomycetes. With the significant number of fungal genomes now sequenced or being sequenced, functional genomics promises to uncover a great deal of new information in coming years. This review discusses recent advances that have been made in examining gene function in filamentous fungi and describes the advantages and limitations of the different approaches.

Markers for host-induced gene expression in Trichophyton dermatophytosis

Dermatophytes are adapted to infect keratinized tissues by their ability to utilize keratin as a nutrient source. Although there have been numerous reports that dermatophytes like Trichophyton sp. secrete proteolytic enzymes, virtually nothing is known about the patterns of gene expression in the host or even when the organisms are cultured on protein substrates in the absence of a host. We characterized the expression of an aminopeptidase gene, the Trichophyton mentagrophytes homolog of the Trichophyton rubrum Tri r 4 gene. The T. rubrum gene was originally isolated based on the ability of the protein encoded by it to induce immediate and delayed-type hypersensitivity in skin tests. T. mentagrophytes Tri m 4 is closely related to Tri r 4 (almost 94% identity at the protein level). Tri m 4 resembles other protease-encoding genes thought to be virulence factors (for example, DPP V of Aspergillus fumigatus). The Tri m 4 protein was detected immunochemically both in fungal extracts and in the culture medium. Expression of the Tri m 4 gene was induced severalfold when T. mentagrophytes was grown on keratin and elastin. Ex vivo, strong induction was observed after culture on blood plasma, but the use of homogenized skin did not result in a significant increase in Tri m 4 transcript levels. In order to identify additional genes encoding putative virulence factors, differential cDNA screening was performed. By this method, a fungal thioredoxin and a cellulase homolog were identified, and both genes were found to be strongly induced by skin extracellular matrix proteins. Induction by superficial (keratin) and deep (elastin) skin elements suggests that the products of these genes may be important in both superficial and deep dermatophytosis, and models for their function are proposed. Upregulation of several newly identified T. mentagrophytes genes on protein substrates suggests that these genes encode proteins which are relevant to the dermatophyte-skin interaction.