The cytochrome P450 reductase CprA is a rate-limiting factor for Cyp51A-mediated azole resistance in Aspergillus fumigatus.

Azole antifungals remain the "gold standard" therapy for invasive aspergillosis. The world-wide emergence of isolates resistant to this drug class, however, developed into a steadily increasing threat to human health over the past years. In Aspergillus fumigatus, major mechanisms of resistance involve increased expression of cyp51A encoding one of two isoenzymes targeted by azoles. Yet, the level of resistance caused by cyp51A upregulation, driven by either clinically relevant tandem repeat mutations within its promoter or the use of high expressing heterologous promoters, is limited. Cytochrome P450 enzymes such as Cyp51A rely on redox partners that provide electrons for their activity. A. fumigatus harbors several genes encoding putative candidate proteins including two paralogous cytochrome P450 reductases, CprA and CprB, and the cytochrome b 5 CybE. In this work, we investigated the contribution of each cprA, cprB, and cybE overexpression to cyp51A-mediated resistance to different medical and agricultural azoles. Using the bidirectional promoter PxylP, we conditionally expressed these genes in combination with cyp51A, revealing cprA as the main limiting factor. Similar to this approach, we overexpressed cprA in an azole-resistant background strain carrying a cyp51A allele with TR34 in its promoter, which led to a further increase in its resistance. Employing sterol measurements, we demonstrate an enhanced eburicol turnover during upregulation of either cprA or cyp51A, which was even more pronounced during their simultaneous overexpression. In summary, our work suggests that mutations leading to increased Cyp51A activity through increased electron supply could be key factors that elevate azole resistance.

Quantifying Isoprenoids in the Ergosterol Biosynthesis by Gas Chromatography-Mass Spectrometry.

The ergosterol pathway is a promising target for the development of new antifungals since its enzymes are essential for fungal cell growth. Appropriate screening assays are therefore needed that allow the identification of potential inhibitors. We developed a whole-cell screening method, which can be used to identify compounds interacting with the enzymes of isoprenoid biosynthesis, an important part of the ergosterol biosynthesis pathway. The method was validated according to the EMEA guideline on bioanalytical method validation. Aspergillus fumigatus hyphae and Saccharomyces cerevisiae cells were lysed mechanically in an aqueous buffer optimized for the enzymatic deconjugation of isoprenoid pyrophosphates. The residual alcohols were extracted, silylated and analyzed by GC-MS. The obtained isoprenoid pattern provides an indication of the inhibited enzyme, due to the accumulation of specific substrates. By analyzing terbinafine-treated A. fumigatus and mutant strains containing tunable gene copies of erg9 or erg1, respectively, the method was verified. Downregulation of erg9 resulted in a high accumulation of intracellular farnesol as well as elevated levels of geranylgeraniol and isoprenol. The decreased expression of erg1 as well as terbinafine treatment led to an increased squalene content. Additional analysis of growth medium revealed high farnesyl pyrophosphate levels extruded during erg9 downregulation.

Modular Inducible Multigene Expression System for Filamentous Fungi.

Inducible promoters are indispensable elements when considering the possibility to modulate gene expression on demand. Desirable traits of conditional expression systems include their capacity for tight downregulation, high overexpression, and in some instances for fine-tuning, to achieve a desired product's stoichiometry. Although the number of inducible systems is slowly increasing, suitable promoters comprising these features are rare. To date, the concomitant use of multiple regulatable promoter platforms for controlled multigene expression has been poorly explored. This work provides pioneer work in the human pathogenic fungus Aspergillus fumigatus, wherein we investigated different inducible systems, elucidated three candidate promoters, and proved for the first time that up to three systems can be used simultaneously without interfering with each other. Proof of concept was obtained by conditionally expressing three antifungal drug targets within the ergosterol biosynthetic pathway under the control of the xylose-inducible PxylP system, the tetracycline-dependent Tet-On system, and the thiamine-repressible PthiA system. IMPORTANCE In recent years, inducible promoters have gained increasing interest for industrial or laboratory use and have become key instruments for protein expression, synthetic biology, and metabolic engineering. Constitutive, high-expressing promoters can be used to achieve high expression yields; however, the continuous overexpression of specific proteins can lead to an unpredictable metabolic burden. To prevent undesirable effects on the expression host's metabolism, the utilization of tunable systems that allow expression of a gene product on demand is indispensable. Here, we elucidated several excellent tunable promoter systems and verified that each can be independently induced in a single strain to ultimately develop a unique conditional multigene expression system. This highly efficient, modular toolbox has the potential to significantly advance applications in fundamental as well as applied research in which regulatable expression of several genes is a key requirement.

Azole Resistance-Associated Regulatory Motifs within the Promoter of cyp51A in Aspergillus fumigatus.

Aspergillus fumigatus is one of the deadliest fungal species, causing hundreds of thousands of deaths each year. Because azoles provide the preferred first-line option for treatment of aspergillosis, the increase in rates of resistance and the poor therapeutic outcomes for patients infected with a resistant isolate constitute a serious global health threat. Azole resistance is frequently associated with specific tandem repeat duplications of a promoter element upstream of cyp51A, the gene that encodes the target for this drug class in A. fumigatus. This promoter element is recognized by the activating transcription factors SrbA and AtrR. This region also provides a docking platform for the CCAAT-binding complex (CBC) and HapX, which cooperate in the regulation of genes involved in iron-consuming pathways, including cyp51A. Here, we studied the regulatory contributions of SrbA, AtrR, CBC, and HapX binding sites to cyp51A expression and azole resistance under different iron availability employing promoter mutational analysis and protein-DNA interaction analysis. This strategy revealed iron status-dependent and -independent roles of these regulatory elements. We show that promoter occupation by both AtrR and SrbA is required for iron-independent steady-state transcriptional activation of cyp51A and its induction during short-term iron exposure relies on HapX binding. We further reveal the HapX binding site as a repressor element, disruption of which increases cyp51A expression and azole resistance regardless of iron availability. IMPORTANCE First-line treatment of aspergillosis typically involves the use of azole antifungals. Worryingly, their future clinical use is challenged by an alarming increase in resistance. Therapeutic outcomes for such patients are poor due to delays in switching to alternative treatments and reduced efficacy of salvage therapeutics. Our lack of understanding of the molecular mechanisms that underpin resistance hampers our ability to develop novel therapeutic interventions. In this work, we dissect the regulatory motifs associated with azole resistance in the promoter of the gene that encodes the azole drug target Cyp51A. These motifs include binding platforms for SrbA and AtrR, as well as the CCAAT-binding complex and HapX. Employing mutational analyses, we uncovered crucial cyp51A-activating and -repressing functions of the binding sites. Remarkably, disrupting binding of the iron regulator HapX increased cyp51A expression and azole resistance in an iron-independent manner.

Inducible Selectable Marker Genes to Improve Aspergillus fumigatus Genetic Manipulation.

The hygromycin B phosphotransferase gene from Escherichia coli and the pyrithiamine resistance gene from Aspergillus oryzae are two dominant selectable marker genes widely used to genetically manipulate several fungal species. Despite the recent development of CRISPR/Cas9 and marker-free systems, in vitro molecular tools to study Aspergillus fumigatus, which is a saprophytic fungus causing life-threatening diseases in immunocompromised hosts, still rely extensively on the use of dominant selectable markers. The limited number of drug selectable markers is already a critical aspect, but the possibility that their introduction into a microorganism could induce enhanced virulence or undesired effects on metabolic behavior constitutes another problem. In this context, here, we demonstrate that the use of ptrA in A. fumigatus leads to the secretion of a compound that allows the recovery of thiamine auxotrophy. In this study, we developed a simple modification of the two commonly used dominant markers in which the development of resistance can be controlled by the xylose-inducible promoter PxylP from Penicillium chrysogenum. This strategy provides an easy solution to avoid undesired side effects, since the marker expression can be readily silenced when not required.

RcLS2F - A Novel Fungal Class 1 KDAC Co-repressor Complex in Aspergillus nidulans.

The fungal class 1 lysine deacetylase (KDAC) RpdA is a promising target for prevention and treatment of invasive fungal infection. RpdA is essential for survival of the most common air-borne mold pathogen Aspergillus fumigatus and the model organism Aspergillus nidulans. In A. nidulans, RpdA depletion induced production of previously unknown small bioactive substances. As known from yeasts and mammals, class 1 KDACs act as components of multimeric protein complexes, which previously was indicated also for A. nidulans. Composition of these complexes, however, remained obscure. In this study, we used tandem affinity purification to characterize different RpdA complexes and their composition in A. nidulans. In addition to known class 1 KDAC interactors, we identified a novel RpdA complex, which was termed RcLS2F. It contains ScrC, previously described as suppressor of the transcription factor CrzA, as well as the uncharacterized protein FscA. We show that recruitment of FscA depends on ScrC and we provide clear evidence that ΔcrzA suppression by ScrC depletion is due to a lack of transcriptional repression caused by loss of the novel RcLS2F complex. Moreover, RcLS2F is essential for sexual development and engaged in an autoregulatory feed-back loop.

Type I and II PRMTs regulate catabolic as well as detoxifying processes in Aspergillus nidulans.

In filamentous fungi, arginine methylation has been implicated in morphogenesis, mycotoxin biosynthesis, pathogenicity, and stress response although the exact role of this posttranslational modification in these processes remains obscure. Here, we present the first genome-wide transcriptome analysis in filamentous fungi that compared expression levels of genes regulated by type I and type II protein arginine methyltransferases (PRMTs). In Aspergillus nidulans, three conserved type I and II PRMTs are present that catalyze asymmetric or symmetric dimethylation of arginines. We generated a double type I mutant (ΔrmtA/rmtB) and a combined type I and type II mutant (ΔrmtB/rmtC) to perform genome-wide comparison of their effects on gene expression, but also to monitor putative overlapping activities and reciprocal regulations of type I and type II PRMTs in Aspergillus. Our study demonstrates, that rmtA and rmtC as type I and type II representatives act together as repressors of proteins that are secreted into the extracellular region as the majority of up-regulated genes are mainly involved in catabolic pathways that constitute the secretome of Aspergillus. In addition to a strong up-regulation of secretory genes we found a significant enrichment of down-regulated genes involved in processes related to oxidation-reduction, transmembrane transport and secondary metabolite biosynthesis. Strikingly, nearly 50% of down-regulated genes in both double mutants correspond to redox reaction/oxidoreductase processes, a remarkable finding in light of our recently observed oxidative stress phenotypes of ΔrmtA and ΔrmtC. Finally, analysis of nuclear and cytoplasmic extracts for mono-methylated proteins revealed the presence of both, common and specific substrates of RmtA and RmtC. Thus, our data indicate that type I and II PRMTs in Aspergillus seem to co-regulate the same biological processes but also specifically affect other pathways in a non-redundant fashion.

Lipocalin-interacting-membrane-receptor (LIMR) mediates cellular internalization of beta-lactoglobulin.

beta-Lactoglobulin (BLG) is a member of the lipocalin protein family and a major food-borne allergen in humans. Numerous in vitro studies have suggested a role for BLG in molecular transport processes; however, its physiological role remains enigmatic. A cellular receptor for BLG has been proposed, but has not yet been identified. Here we show that human LIMR, known to act as an endocytic receptor for lipocalin-1, also binds bovine BLG and mediates its cellular uptake. The specificity of this interaction is corroborated by a complete block of cellular uptake of BLG in the presence of LIMR antibodies or LIMR downregulation by antisense RNA. Furthermore, heterologous expression of human LIMR in insect cells mediates cellular internalization of FITC-BLG. Since LIMR is highly expressed in the human intestine, it might also function in the uptake of food-borne BLG.

Development of a fast and reliable method for the assessment of microbial colonization and growth on textiles by DNA quantification.

There is a lack of relevant methods to assess the colonization of textiles by skin bacteria because present methods are mainly culture-based procedures. Therefore, the goal of this study was to develop a fast and sensitive culture-independent procedure for the quantification of microbial colonization and growth on textiles. We have established a suitable protocol to use DNA quantification as a reliable method for in vitroand in vivoinvestigations of textiles. For DNA extraction, a two-step procedure comprising treatment of the textile with a solution containing Triton X-100 and lysozyme for 1 h and a successive treatment by SDS and proteinase K for 2 h turned out to be most efficient. DNA extracted from textiles and fabrics was than quantified with the highly sensitive PicoGreen fluorescent dye. In vitrochallenge tests demonstrated a strong correlation between numbers of bacteria on textiles and amount of DNA extracted from textiles. Therefore, this method was used to compare different materials after in vivotrials for assessment of their susceptibility for microbial colonization and growth.

Oligomeric state of lipocalin-1 (LCN1) by multiangle laser light scattering and fluorescence anisotropy decay.

Multiangle laser light scattering and fluorescence anisotropy decay measurements clarified the oligomeric states of native and recombinant tear lipocalin (lipocalin-1, TL). Native TL is monomeric. Recombinant TL (5-68 microM) with or without the histidine tag shows less than 7% dimer formation that is not in equilibrium with the monomeric form. Fluorescence anisotropy decay showed a correlation time of 9-10 ns for TL (10 microM-1 mM). Hydrodynamic calculations based on the crystallographic structure of a monomeric TL mutant closely concur with the observed correlation time. The solution properties calculated with HYDROPRO and SOLPRO programs from the available crystallographic structure of a monomeric TL mutant concur closely with the observed fluorescence anisotropy decay. The resulting model shows that protein topology is the major determinant of rotational correlation time and accounts for deviation from the Stokes-Einstein relation. The data challenge previous gel filtration studies to show that native TL exists predominantly as a monomer in solution rather than as a dimer. Delipidation of TL results in a formation of a complex oligomeric state (up to 25%). These findings are important as the dynamic processes in the tear film are limited by diffusional, translational as well as rotational, properties of the protein.

Human tear lipocalin exhibits antimicrobial activity by scavenging microbial siderophores.

Human tear lipocalin (TL; also known as Lcn1) is a secretory protein present in large amounts in fluids that cover epithelial surfaces such as tears and respiratory secretions. It is supposed to act as a physiological scavenger of hydrophobic, potentially harmful molecules, but there is evidence that it also inhibits bacterial growth. In the present study, we reconsidered the possibility that TL might interfere with microbial growth by scavenging of siderophores, as described for human neutrophil gelatinase-associated lipocalin (NGAL). Indeed, our experiments revealed that TL binds to microbial siderophores with high affinities. In contrast to NGAL, which was shown to have some specificity for bacterial catecholate-type siderophores, TL binds to a broad array of siderophores, including bacterial catecholate-type enterobactin and hydroxamate-type desferrioxamine B, and all major classes of fungal siderophores. By adding exogenous TL, bacterial and fungal growth could be inhibited under iron-limiting conditions. Thus, TL might be a novel member of the innate immune system especially involved in mucosal defense against fungal infections.