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

An efficient gene targeting system using Δku80 and functional analysis of Cyp51A in Trichophyton rubrum

Trichophyton rubrum is one of the most frequently isolated fungi in patients with dermatophytosis. Despite its clinical significance, the molecular mechanisms of drug resistance and pathogenicity of T. rubrum remain to be elucidated because of the lack of genetic tools, such as efficient gene targeting systems. In this study, we generated a T. rubrum strain that lacks the nonhomologous end-joining-related gene ku80 (Δku80) and then developed a highly efficient genetic recombination system with gene targeting efficiency that was 46 times higher than that using the wild-type strain. Cyp51A and Cyp51B are 14-α-lanosterol demethylase isozymes in T. rubrum that promote ergosterol biosynthesis and are the targets of azole antifungal drugs. The expression of cyp51A mRNA was induced by the addition of the azole antifungal drug efinaconazole, whereas no such induction was detected for cyp51B, suggesting that Cyp51A functions as an azole-responsive Cyp51 isozyme. To explore the contribution of Cyp51A to susceptibility to azole drugs, the neomycin phosphotransferase (nptII) gene cassette was inserted into the cyp51A 3'-untranslated region of Δku80 to destabilize the mRNA of cyp51A. In this mutant, the induction of cyp51A mRNA expression by efinaconazole was diminished. The minimum inhibitory concentration for several azole drugs of this strain was reduced, suggesting that dermatophyte Cyp51A contributes to the tolerance for azole drugs. These findings suggest that an efficient gene targeting system using Δku80 in T. rubrum is applicable for analyzing genes encoding drug targets.

Targeting dermatophyte Cdc42 and Rac GTPase signaling to hinder hyphal elongation and virulence

The development of antifungal drugs requires novel molecular targets due to limited treatment options and drug resistance. Through chemical screening and establishment of a novel genetic technique to repress gene expression in Trichophyton rubrum, the primary causal fungus of dermatophytosis, we demonstrated that fungal Cdc42 and Rac GTPases are promising antifungal drug targets. Chemical inhibitors of these GTPases impair hyphal formation, which is crucial for growth and virulence in T. rubrum. Conditional repression of Cdc24, a guanine nucleotide exchange factor for Cdc42 and Rac, led to hyphal growth defects, abnormal cell morphology, and cell death. EHop-016 inhibited the promotion of the guanine nucleotide exchange reaction in Cdc42 and Rac by Cdc24 as well as germination and growth on the nail fragments of T. rubrum and improved animal survival in an invertebrate infection model of T. rubrum. Our results provide a novel antifungal therapeutic target and a potential lead compound.

TrCla4 promotes actin polymerization at the hyphal tip and mycelial growth in Trichophyton rubrum

IMPORTANCE

Superficial fungal infections, such as athlete's foot, affect more than 10% of the world's population and have a significant impact on quality of life. Despite the fact that treatment-resistant fungi are a concern, there are just a few antifungal drug targets accessible, as opposed to the wide range of therapeutic targets found in bacterial infections. As a result, additional alternatives are sought. In this study, we generated a PAK TrCla4 deletion strain (∆Trcla4) of Trichophyton rubrum. The ∆Trcla4 strain exhibited deficiencies in mycelial growth, hyphal morphology, and polarized actin localization at the hyphal tip. IPA-3 and FRAX486, small chemical inhibitors of mammalian PAK, were discovered to limit fungal mycelial proliferation. According to our findings, fungal PAKs are interesting therapeutic targets for the development of new antifungal medicines.

A Dual-Plasmid-Based CRISPR/Cas9-Mediated Strategy Enables Targeted Editing of pH Regulatory Gene pacC in a Clinical Isolate of Trichophyton rubrum

Trichophyton rubrum is the most prevalent causative agent responsible for 80-90% of all known superficial fungal infections in humans, worldwide. Limited available methods for genetic manipulations have been one of the major bottlenecks in understanding relevant molecular mechanisms of disease pathogenesis in T. rubrum. Here, a dual-plasmid-based CRISPR/Cas9 strategy to edit pH regulatory transcription factor, pacC, of a clinical isolate of T. rubrum by non-homologous end joining (NHEJ) repair is presented. A cas9-eGFP fusion that aids pre-screening of primary transformants through detection of GFP fluorescence is expressed from one plasmid while target-specific sgRNA from the other brings about mutagenesis of pacC with an overall efficiency of 33.8-37.3%. The mutants had reduced transcript levels of pacC at both acidic and alkaline pH with several morphological abnormalities. We believe this dual-plasmid-based CRISPR/Cas9 strategy will aid functional genomics studies, especially in non-lab-adapted clinical strains of T. rubrum.

Assessment of the mechanism of drug resistance in Trichophyton mentagrophytes in response to various substances

BACKGROUND

Trichophyton mentagrophyte (TM), a zoonotic pathogen, has been endangering public health due to emerging drug resistance. Although increased attention is paid to this issue, there is very limited research available on drug resistance in TM. In this study, we studied the gene and proteomic changes, morphological changes, cellular fat localization, fat content changes, and biofilm of TM treated with different substances.

RESULTS

The TM growth curve showed a positive correlation with the concentration of Fenarimol (FE), genistein (GE), clotrimazole (KM), and Miconazole nitrate salt (MK). The morphology of TM cells changed in different degrees after treatment with different substances as observed by TEM and SEM. The results showed that under KM and berberine hydrochloride (BB) treatment, a total of 3305 differentially expressed genes were detected, with the highest number in the KM-treated group (578 up-regulated and 615 down-regulated). A total of 847 proteins and 1850 peptides were identified in TM proteomics. Nile red staining showed that the fat content of TM was significantly higher in the BB-, ethidium bromide- (EB), FE-, KM-, Adriamycin hydrochloride- (YA), and MK-treated group compared to the control group. Results of the biofilm thickness showed that it gradually increased under treatment with specific concentrations of KM or BB, which may be related to the up-regulation of ERG25 and CYP related gene proteins.

CONCLUSIONS

It is suggested that in order to effectively deal with dermatomycosis caused by TM, it is necessary to inhibit the expression of ERG25 and CYP related genes and fat metabolism, which can result in the inhibition of the production of biofilm by the fungus and solve the problem of fungal drug resistance in clinical settings.

Deletion of pksA attenuates the melanogenesis, growth and sporulation ability and causes increased sensitivity to stress response and antifungal drugs in the human pathogenic fungus Fonsecaea monophora

Fonsecaea monophora, which is very similar to Fonsecaea pedrosoi in morphological features, has been commonly misdiagnosed as F. pedrosoi. Like F. pedrosoi, F. monophora has been also identified as a predominant pathogen of Chromoblastomycosis (CBM). Melanin has been recognized as a virulence factor in several fungi, however, it is still largely unknown about the biological role of melanin and how melanin is synthesized in F. monophora. In this study, we identified two putative polyketide synthase genes (pks), AYO21_03016 (pksA) and AYO21_10638, by searching against the genome of F. monophora. AYO21_03016 and AYO21_10638 were further targeted disrupted by Agrobacterium tumefaciens-mediated transformation (ATMT). We discovered that pksA gene was the major polyketide synthase required for melanin synthesis in F. monophora, rather than AYO21_10638. Phenotypic analysis showed that, knocking out of the pksA gene attenuated melanogenesis, growth rate, sporulation ability and virulence of F. monophora, as compared with wild-type and complementation strain (pksA-C). Furthermore, the ΔpksA mutant was confirmed to be more sensitive to the oxidative stress, extreme pH environment, and antifungal drugs including itraconazole (ITC), terbinafine (TER), and amphotericin B (AMB). Taken together, these findings enabled us to comprehend the role of pksA in regulating DHN-melanin pathway and its effect on the biological function of F. monophora.

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.

Flippase (FLP) recombinase-mediated marker recycling in the dermatophyte Arthroderma vanbreuseghemii

Biological processes can be elucidated by investigating complex networks of relevant factors and genes. However, this is not possible in species for which dominant selectable markers for genetic studies are unavailable. To overcome the limitation in selectable markers for the dermatophyte Arthroderma vanbreuseghemii (anamorph: Trichophyton mentagrophytes), we adapted the flippase (FLP) recombinase-recombination target (FRT) site-specific recombination system from the yeast Saccharomyces cerevisiae as a selectable marker recycling system for this fungus. Taking into account practical applicability, we designed FLP/FRT modules carrying two FRT sequences as well as the flp gene adapted to the pathogenic yeast Candida albicans (caflp) or a synthetic codon-optimized flp (avflp) gene with neomycin resistance (nptII) cassette for one-step marker excision. Both flp genes were under control of the Trichophyton rubrum copper-repressible promoter (PCTR4). Molecular analyses of resultant transformants showed that only the avflp-harbouring module was functional in A. vanbreuseghemii. Applying this system, we successfully produced the Ku80 recessive mutant strain devoid of any selectable markers. This strain was subsequently used as the recipient for sequential multiple disruptions of secreted metalloprotease (fungalysin) (MEP) or serine protease (SUB) genes, producing mutant strains with double MEP or triple SUB gene deletions. These results confirmed the feasibility of this system for broad-scale genetic manipulation of dermatophytes, advancing our understanding of functions and networks of individual genes in these fungi.

Keratin degradation by dermatophytes relies on cysteine dioxygenase and a sulfite efflux pump

Millions of people suffer from superficial infections caused by dermatophytes. Intriguingly, these filamentous fungi exclusively infect keratin-rich host structures such as hair, nails, and skin. Keratin is a hard, compact protein, and its utilization by dermatophytes for growth has long been discussed as a major virulence attribute. Here, we provide strong support for the hypothesis that keratin degradation is facilitated by the secretion of the reducing agent sulfite, which can cleave keratin-stabilizing cystine bonds. We discovered that sulfite is produced by dermatophytes from environmental cysteine, which at elevated concentrations is toxic for microbes and humans. We found that sulfite formation from cysteine relies on the key enzyme cysteine dioxygenase Cdo1. Sulfite secretion is supported by the sulfite efflux pump Ssu1. Targeted mutagenesis proved that dermatophyte mutants in either Cdo1 or Ssu1 were highly growth-sensitive to cysteine, and mutants in Ssu1 were specifically sensitive to sulfite. Most notably, dermatophyte mutants in Cdo1 and Ssu1 were specifically growth-defective on hair and nails. As keratin is rich in cysteine, our identified mechanism of cysteine conversion and sulfite efflux supports both cysteine and sulfite tolerance per se and progression of keratin degradation. These in vitro findings have implications for dermatophyte infection pathogenesis.

Development of a tightly regulatable copper-mediated gene switch system in dermatophytes

Targeted gene deletion is now available for molecular genetic research of dermatophytes, and the physiological roles of several genes have been elucidated. However, this method cannot be applied to essential genes, which can be potential drug targets. To overcome this limitation, we have developed a conditional gene knockdown system using a copper-responsive promoter. The promoter sequence of the copper transporter gene CTR4 (P(CTR4)) and that of the copper efflux pump gene CRP1 (P(CRP1)) derived from Trichophyton rubrum were examined for their response to copper in Arthroderma vanbreuseghemii. P(CTR4) was demonstrated to repress expression of a reporter gene in the presence of copper, while the activity of P(CRP1) was induced by addition of copper. Importantly, P(CTR4) regulated the gene expression more tightly. Furthermore, when P(CTR4) was applied to regulate the expression of the endogenous genes ERG1 and TRP5, their conditional mutants exhibited decreased growth activity under the repressive conditions. These results suggest that the P(CTR4)-based gene regulation system represents a powerful tool for identification and characterization of a broad range of genes, including essential genes, in dermatophytes.

Insights into a nonhomologous integration pathway in the dermatophyte Trichophyton mentagrophytes: efficient targeted gene disruption by use of mutants lacking ligase IV

Targeted gene disruption experiments in Trichophyton mentagrophytes are impeded by the dominant of repair of DNA double strand breaks through a nonhomologous end joining pathway (NHEJ). Inactivation of human DNA ligase IV homologs, which is involved in the final step of the NHEJ pathway, has been shown to enhance homologous recombination (HR) frequency in filamentous fungi. To improve the frequency of HR in T. mentagrophytes, the lig4 homolog (TmLIG4) was disrupted. T. mentagrophytes lacking TmLIG4 showed no discernable phenotypic differences when compared to wild-type controls. Both mutant and parent strains had almost identical growth ability, sporulation rate and sensitivity to DNA damaging agents. When four different loci were disrupted in the TMLIG4-deficient mutant, HR frequencies reached as high as 93% depending on the locus, whereas they ranged from 0%-40% in the wild-type. These results suggest that studies in strains lacking TmLIG4 would help to improve our understanding of dermatophytosis by facilitating the genetic manipulation of dermatophytes.

Sequenced dermatophyte strains: growth rate, conidiation, drug susceptibilities, and virulence in an invertebrate model

Although dermatophytes are the most common cause of fungal infections in the world, their basic biology is not well understood. The recent sequencing and annotation of the genomes of five representative dermatophyte species allows for the creation of hypotheses as to how they cause disease and have adapted to their distinct environments. An understanding of the microbiology of these strains will be essential for testing these hypotheses. This study is the first to generally characterize these five sequenced strains of dermatophytes for their microbiological aspects. We measured the growth rate on solid medium and found differences between species, with Microsporum gypseum CBS118893 having the fastest growth and Trichophyton rubrum CBS118892 the slowest. We also compared different media for conidia production and found that the highest numbers of conidia were produced when dermatophytes were grown on MAT agar. We determined the Minimum Inhibitory Concentration (MIC) of nine antifungal agents and confirmed susceptibility to antifungals commonly used as selectable markers. Finally, we tested virulence in the Galleria mellonella (wax moth) larvae model but found the results variable. These results increase our understanding of the microbiology and molecular biology of these dermatophyte strains and will be of use in advancing hypothesis-driven research about dermatophytes.

Nourseothricin acetyltransferase: a new dominant selectable marker for the dermatophyte Trichophyton mentagrophytes

Dermatophytes are filamentous fungi that colonize the keratinized layer of human and animal skin. The availability of several selectable markers for dermatophyte gene manipulation would provide important tools to understand the genetic properties of this fungal group. In this study, we report the nourseothricin resistance gene nat1 that confers resistance to the aminoglycoside antibiotic nourseothricin as a dominant marker in Trichophyton mentagrophytes. The NAT cassette was introduced into T. mentagrophytes by the Agrobacterium tumefaciens-mediated transformation (ATMA) method. Transformation occurred at a frequency of 78 transformants per 1 x 10(7) cells. Molecular analysis showed integration of the NAT cassette into the genomic DNA of T. mentagrophytes. This study presents the nourseothricin resistance gene nat1 as a useful selectable marker for selection of T. mentagrophytes.

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.