Regulation of restrictocin production in Aspergillus restrictus

Aspergillus fumigatus and Its Allergenic Ribotoxin Asp f I: Old Enemies but New Opportunities for Urine-Based Detection of Invasive Pulmonary Aspergillosis Using Lateral-Flow Technology

Invasive pulmonary aspergillosis (IPA) caused by Aspergillus fumigatus is a life-threatening lung disease of immunocompromised patients. Diagnosis currently relies on non-specific chest CT, culture of the fungus from invasive lung biopsy, and detection of the cell wall carbohydrate galactomannan (GM) in serum or in BAL fluids recovered during invasive bronchoscopy. Urine provides an ideal bodily fluid for the non-invasive detection of pathogen biomarkers, with current urine-based immunodiagnostics for IPA focused on GM. Surrogate protein biomarkers might serve to improve disease detection. Here, we report the development of a monoclonal antibody (mAb), PD7, which is specific to A. fumigatus and related species in the section Fumigati, and which binds to its 18 kDa ribotoxin Asp f I. Using PD7, we show that the protein is secreted during hyphal development, and so represents an ideal candidate for detecting invasive growth. We have developed a lateral-flow device (Afu-LFD®) incorporating the mAb which has a limit of detection of ~15 ng Asp f I/mL urine. Preliminary evidence of the test's diagnostic potential is demonstrated with urine from a patient with acute lymphoid leukaemia with probable IPA. The Afu-LFD® therefore provides a potential novel opportunity for non-invasive urine-based detection of IPA caused by A. fumigatus.

Phylogeny of xerophilic aspergilli (subgenus Aspergillus) and taxonomic revision of section Restricti

Aspergillus section Restricti together with sister section Aspergillus (formerly Eurotium) comprises xerophilic species, that are able to grow on substrates with low water activity and in extreme environments. We adressed the monophyly of both sections within subgenus Aspergillus and applied a multidisciplinary approach for definition of species boundaries in sect. Restricti. The monophyly of sections Aspergillus and Restricti was tested on a set of 102 isolates comprising all currently accepted species and was strongly supported by Maximum likelihood (ML) and Bayesian inferrence (BI) analysis based on β-tubulin (benA), calmodulin (CaM) and RNA polymerase II second largest subunit (RPB2) loci. More than 300 strains belonging to sect. Restricti from various isolation sources and four continents were characterized by DNA sequencing, and 193 isolates were selected for phylogenetic analyses and phenotypic studies. Species delimitation methods based on multispecies coalescent model were employed on DNA sequences from four loci, i.e., ID region of rDNA (ITS + 28S), CaM, benA and RPB2, and supported recognition of 21 species, including 14 new. All these species were also strongly supported in ML and BI analyses. All recognised species can be reliably identified by all four examined genetic loci. Phenotype analysis was performed to support the delimitation of new species and includes colony characteristics on seven cultivation media incubated at several temperatures, growth on an osmotic gradient (six media with NaCl concentration from 0 to 25 %) and analysis of morphology including scanning electron microscopy. The micromorphology of conidial heads, vesicle dimensions, temperature profiles and growth parameters in osmotic gradient were useful criteria for species identification.

The vast majority of species in sect. Restricti produce asperglaucide, asperphenamate or both in contrast to species in sect. Aspergillus. Mycophenolic acid was detected for the first time in at least six members of the section. The ascomata of A. halophilicus do not contain auroglaucin, epiheveadride or flavoglaucin which are common in sect. Aspergillus, but shares the echinulins with sect. Aspergillus.

Fungal Ribotoxins: A Review of Potential Biotechnological Applications

Fungi establish a complex network of biological interactions with other organisms in nature. In many cases, these involve the production of toxins for survival or colonization purposes. Among these toxins, ribotoxins stand out as promising candidates for their use in biotechnological applications. They constitute a group of highly specific extracellular ribonucleases that target a universally conserved sequence of RNA in the ribosome, the sarcin-ricin loop. The detailed molecular study of this family of toxic proteins over the past decades has highlighted their potential in applied research. Remarkable examples would be the recent studies in the field of cancer research with promising results involving ribotoxin-based immunotoxins. On the other hand, some ribotoxin-producer fungi have already been studied in the control of insect pests. The recent role of ribotoxins as insecticides could allow their employment in formulas and even as baculovirus-based biopesticides. Moreover, considering the important role of their target in the ribosome, they can be used as tools to study how ribosome biogenesis is regulated and, eventually, may contribute to a better understanding of some ribosomopathies.

Hirsutellin A: A Paradigmatic Example of the Insecticidal Function of Fungal Ribotoxins

The fungal pathogen Hirsutella thompsonii produces an insecticidal protein named hirsutellin A (HtA), which has been described to be toxic to several species of mites, insect larvae, and cells. On the other hand, on the basis of an extensive biochemical and structural characterization, HtA has been considered to be a member of the ribotoxins family. Ribotoxins are fungal extracellular ribonucleases, which inactivate ribosomes by specifically cleaving a single phosphodiester bond located at the large rRNA. Although ribotoxins were brought to light in the 1960s as antitumor agents, their biological function has remained elusive. Thus, the consideration of hirsutellin A, an insecticidal protein, as a singular ribotoxin recalled the idea of the biological activity of these toxins as insecticidal agents. Further studies have demonstrated that the most representative member of the ribotoxin family, α-sarcin, also shows strong toxic action against insect cells. The determination of high resolution structures, the characterization of a large number of mutants, and the toxicity assays against different cell lines have been the tools used for the study of the mechanism of action of ribotoxins at the molecular level. The aim of this review is to serve as a compilation of the facts that allow identification of HtA as a paradigmatic example of the insecticidal function of fungal ribotoxins.

Metabolomics of Aspergillus fumigatus

Aspergillus fumigatus is the most important species in Aspergillus causing infective lung diseases. This species has been reported to produce a large number of extrolites, including secondary metabolites, acids, and proteins such as hydrophobins and extracellular enzymes. At least 226 potentially bioactive secondary metabolites have been reported from A. fumigatus that can be ordered into 24 biosynthetic families. Of these families we have detected representatives from the following families of secondary metabolites: fumigatins, fumigaclavines, fumiquinazolines, trypacidin and monomethylsulochrin, fumagillins, gliotoxins, pseurotins, chloroanthraquinones, fumitremorgins, verruculogen, helvolic acids, and pyripyropenes by HPLC with diode array detection and mass spectrometric detection. There is still doubt whether A. fumigatus can produce tryptoquivalins, but all isolates produce the related fumiquinazolines. We also tentatively detected sphingofungins in A. fumigatus Af293 and in an isolate of A. lentulus. The sphingofungins may have a similar role as the toxic fumonisins, found in A. niger. A further number of mycotoxins, including ochratoxin A, and other secondary metabolites have been reported from A. fumigatus, but in those cases either the fungus or its metabolite appear to be misidentified.

Ribotoxin genes in isolates of Aspergillus section Clavati

Ribotoxins are ribosome inactivator proteins with high specificity against the sarcin/ricin domain of the 28S ribosomal RNA. We examined the presence of ribotoxin genes in isolates of species recently assigned to Aspergillus section Clavati using specific primer pairs. All species assigned to this section have been found to carry ribotoxin genes. Phylogenetic analysis of the sequences of the amplified gene fragments allowed us to classify the genes to different groups including the α-sarcin, gigantin, c-sarcin and mitogillin/restrictocin families. Two species, A. longivesica and N. acanthosporus produced ribotoxins which were only distantly related to gigantins and c-sarcins, respectively. Comparison of the protein sequences of the genes to known ribotoxin sequences revealed that all of them carry the presumed catalytic residues of ribotoxins, the cystein residues, and also the two Trp residues of α-sarcin conserved in all ribotoxins known so far. These data indicate that these genes probably encode active ribotoxins. Further studies are in progress to examine the secretion and activities of these new ribotoxins.

Fungal ribotoxins: molecular dissection of a family of natural killers

RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium species. Ribotoxins stand out among them by their cytotoxic character. They exert their toxic action by first entering the cells and then cleaving a single phosphodiester bond located within a universally conserved sequence of the large rRNA gene, known as the sarcin-ricin loop. This cleavage leads to inhibition of protein biosynthesis, followed by cellular death by apoptosis. Although no protein receptor has been found for ribotoxins, they preferentially kill cells showing altered membrane permeability, such as those that are infected with virus or transformed. Many steps of the cytotoxic process have been elucidated at the molecular level by means of a variety of methodological approaches and the construction and purification of different mutant versions of these ribotoxins. Ribotoxins have been used for the construction of immunotoxins, because of their cytotoxicity. Besides this activity, Aspf1, a ribotoxin produced by Aspergillus fumigatus, has been shown to be one of the major allergens involved in allergic aspergillosis-related pathologies. Protein engineering and peptide synthesis have been used in order to understand the basis of these pathogenic mechanisms as well as to produce hypoallergenic proteins with potential diagnostic and immunotherapeutic applications.

Ribotoxins are a more widespread group of proteins within the filamentous fungi than previously believed

Alpha-sarcin, restrictocin and mitogillin are the best known members of the family of fungal ribotoxins. In recent years, new members of this family have been discovered and characterised. In this work, we study the occurrence of ribotoxins among different species of fungi. The presence of ribotoxins has been identified in some new species by means of genetic studies, as well as expression and activity assays. The ribotoxin genes have been partially sequenced, and demonstrate a high degree of similarity. These studies demonstrate that these toxins are more widespread than previously considered. This is surprising, considering the ribotoxins are such specific and potent toxins, of unknown biological function. These studies confirm the hypothesis that these proteins are naturally engineered toxins derived from ribonucleases of broad substrate specificity.

Secretion of recombinant pro- and mature fungal alpha-sarcin ribotoxin by the methylotrophic yeast Pichia pastoris: the Lys-Arg motif is required for maturation

alpha-Sarcin is a ribosome-inactivating protein from the mold Aspergillus giganteus. The methylotrophic yeast Pichia pastoris has been transformed with two plasmids (pHILD2prealphaS and pHILS1prealphaS), which contain the complete alpha-sarcin cDNA, including its original fungal leader peptide, under the control of yeast alcohol oxidase promoter. The second one is indeed fused to the signal sequence of P. pastoris acid phosphatase. The transformed yeasts secreted both mature and pro-alpha-sarcin. The presence of this pro-alpha-sarcin in the yeast extracellular medium is due to an inefficient recognition of the pro-sequence by a putative Kex2p-like endopeptidase. A third plasmid accounting for a single mutation of the alpha-sarcin leader peptide was designed to produce a more efficient Kex2p recognition motif. This approach resulted in the extracellular production of only the mature protein, suggesting the existence of a two-step mechanism for processing its leader peptide. This recombinant alpha-sarcin is identical to the original fungal protein, according to activity and spectroscopic criteria. In addition, pro-alpha-sarcin, which has been characterized for the first time, also exhibits ribonucleolytic activity as the mature protein does. Therefore, protection of the producing cells against this kind of ribotoxins may depend on an efficient recognition of the signal sequence followed by translocation of the nascent polypeptide to the endoplasmic reticulum.

Fungal ribotoxins: a family of naturally engineered targeted toxins?

alpha-Sarcin, mitogillin, and restrictocin are small (approximately 17 kDa) basic robosome-inactivating proteins (RIPs) produced by the Aspergilli that catalytically inactivate the large ribosomal subunits of all organisms tested to date. These three fungal ribotoxins act as specific ribonucleases by hydrolyzing one single phosphodiester bond in the universally conserved alpha-sarcin domain of 23-28S rRNAs and are among the most potent inhibitors of protein synthesis known. Previous molecular studies of ribotoxins indicated that they belong to the superfamily of ribonucleases and analysis of the mitogillin gene employing PCR-mediated site-specific mutagenesis suggests that certain domains in ribotoxins, which share homologies with motifs in ribosome-related proteins, may be responsible for the targeting of ribotoxins to the ribosome. The applications of the ribotoxins as tools in research and their uses as therapeutic and diagnostic agents are also reviewed in this paper.

Refinement of physiological roles for cerato-ulmin by analogy with other hydrophobins

Cerato-ulmin, a hydrophobic protein produced by the Dutch elm disease fungus Ophiostoma ulmi, is toxic to susceptible elms. The physiological role of this protein may be related to its location on hyphal surfaces. Cerato-ulmin is a member of the hydrophobins, proteins used in the formation of structures involved in asexual reproduction (sporulation and conidiation).

Construction and pathogenicity of Aspergillus fumigatus mutants that do not produce the ribotoxin restrictocin

Aspergillus fumigatus, the most common cause of invasive pulmonary aspergillosis (IPA), produces a potent cytotoxin called restrictocin. To investigate the role of restrictocin in IPA, we have constructed fungal strains in which the res gene has been inactivated by gene disruption. These disruptants lack the specific extracellular ribonucleolytic activity associated with restrictocin, as measured by an in vitro rabbit reticulocyte lysate assay. Western blot analysis of one disruptant, using an anti-restrictocin monoclonal antibody, confirmed that the toxin is not produced. The growth characteristics of the disruptants could not be distinguished from those of their parental isolates on a variety of culture media. The pathogenicity of two disruptants was assessed in a murine model of IPA. There were no significant differences in mortality when these strains were compared with the parental isolates and an ectopic transformant. In addition, histological examination of infected lung tissue did not reveal any obvious differences in the number or size of fungal colonies or in the polymorphonuclear leucocyte response. Our results demonstrate that restrictocin is not an important virulence factor in this model of IPA.