The mode of action of hirsutellin A on eukaryotic cells

Toxicity of crude toxin protein produced by Cordyceps fumosorosea IF-1106 against Myzus persicae (Sulze)

The entomopathogenic fungus Cordyceps fumosorosea IF-1106 is a potential biocontrol agent with high pathogenicity to the aphid Myzus persicae. We extracted the crude toxin protein from a liquid culture broth of an isolated C. fumosorosea strain using the ammonium sulfate precipitation method, and its toxicity to Myzus persicae was measured by injection, oral exposure, and topical exposure. The crude toxin protein of C. fumosorosea had insecticidal activity against M. persicae. Body cavity injection and oral exposure had significantly higher insecticidal activity against adults than contact sprays. The highest cumulative corrected mortality of adults after injection was 81.85 ± 13.45 %, and the highest cumulative corrected mortality of adults after ingestion was 85.45 ± 11.88 %. The proportion of plasmatocytes in adult blood lymphocytes reached the highest at 3 days after injection and feeding, and the proportion of granulocytes was the highest at 2 days after injection and feeding. These data confirmed the toxicity of the crude toxin protein of C. fumosorosea toxin to M. persicae and helped clarify the pathogenic mechanism of the strain. Population management of M. persicae may be possible by using a natural toxic compound produced by C. fumosorosea that is selective to this pest species.

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.

Fungal extracellular ribotoxins as insecticidal agents

Fungal ribotoxins were discovered almost 50 years ago as extracellular ribonucleases (RNases) with antitumoral properties. However, the biological function of these toxic proteins has remained elusive. The discovery of the ribotoxin HtA, produced by the invertebrates pathogen Hirsutella thompsonii, revived the old proposal that insecticidal activity would be their long searched function. Unfortunately, HtA is rather singular among all ribotoxins known in terms of sequence and structure similarities. Thus, it was intriguing to answer the question of whether HtA is just an exception or, on the contrary, the paradigmatic example of the ribotoxins function. The work presented uses HtA and α-sarcin, the most representative member of the ribotoxins family, to show their strong toxic action against insect larvae and cells.

Implication of an Asp residue in the ribonucleolytic activity of hirsutellin A reveals new electrostatic interactions at the active site of ribotoxins

Ribotoxins are fungal extracellular ribonucleases that specifically cleave ribosomes leading to cell-death via apoptosis. α-Sarcin is the ribotoxin studied in deepest detail, and therefore constitutes the referential protein for the whole family. It has been demonstrated that ribotoxin activity depends on a very precise structural microenvironment in which electrostatic interactions among residues in the active site are of the highest importance. Hirsutellin A (HtA) has been recently described as the smallest ribotoxin known to date, encompassing all the abilities of previously characterized members of this family into a shorter sequence. Comparison of HtA and α-sarcin three-dimensional structures suggested that residues presumably forming the catalytic triad of HtA would be His 42, Glu 66, and His 113. Within this same idea, the presence of an Asp residue (Asp 40) in a position equivalent to α-sarcin Tyr 48 is highlighted as a novelty in this field. In this work, substitution mutants H42Q, E66Q and H113Q, as well as double and triple mutants in all possible combinations, are studied regarding their ribonucleolytic activity and cytotoxicity. Implication of these three residues in the ribotoxin activity of HtA is confirmed, though none of them is strictly essential for ribosomal cleavage. Studies with mutants D40N and D40N/E66Q demonstrate an important role for Asp 40 in the activity of HtA and establish a new set of electrostatic interactions different from the one described for already known ribotoxins.

The insecticidal protein hirsutellin A from the mite fungal pathogen Hirsutella thompsonii is a ribotoxin

The mite fungal pathogen Hirsutella thompsonii produces a single polypeptide chain, insecticidal protein named hirsutellin A (HtA) that is composed of 130 amino acid residues. This protein has been purified from its natural source and produced as a recombinant protein in Escherichia coli. Spectroscopic analysis has determined that the two protein forms are indistinguishable. HtA specifically inactivates ribosomes and produces the alpha-fragment characteristic of ribotoxin activity on rRNA. Behaving as a cyclizing ribonuclease, HtA specifically cleaves oligonucleotides that mimick the sarcin/ricin loop of the ribosome, as well as selected polynucleotides and dinucleosides. HtA interacts with phospholipid membranes as do other ribotoxins. As a consequence of its ribonuclease activity and its ability to interact with cell membranes, HtA exhibits cytotoxic activity on human tumor cells. On the basis of these results, HtA is considered to be a member of the ribotoxin group of proteins, although it is significantly smaller (130 aa) than all known ribotoxins that are composed of 149/150 amino acids. Ribotoxins are members of a larger family of fungal ribonucleases whose members of smaller size (100/110 aa) are not cytotoxic. Thus, the characterization of the fungal ribotoxin HtA represents an important milestone in the study of the diversity and the function of fungal ribonucleases.

Antagonistic effects of the endophytic fungus Meira geulakonigii on the citrus rust mite Phyllocoptruta oleivora

AIMS

The fungus Meira geulakonigii has been shown to reduce populations of citrus rust mite (CRM; Phyllocoptruta oleivora) on citrus leaves and fruits, in both the field and laboratory. However, attempts to isolate the fungus from leaves and fruits have been unsuccessful. The aims of this study were therefore to determine whether M. geulakonigii is a citrus endophyte, and to assess possible mechanisms involved in its mite-antagonist activity.

METHODS AND RESULTS

A quantitative real-time PCR and regular PCR approaches were developed to detect M. geulakonigii in both the field and laboratory. The fungus was detected throughout. Different methods revealed that M. geulakonigii is an endophyte, which colonizes both the peel of grapefruits. Applications of conidia protected the grapefruits against CRM, and fungal secretions extracted from growth media caused 100% CRM mortality.

CONCLUSIONS

Meira geulakonigii is a beneficial endophyte of grapefruits that colonizes the fruit's peel, and protects it from CRM.

SIGNIFICANCE AND IMPACT OF THE STUDY

Findings from this study demonstrate the endophytic nature of M. geulakonigii in its interaction with grapefruits. In addition, a molecular approach was developed to specifically detect the fungus inside the grapefruit peel. This approach can be used to assess the natural occurrence of M. geulakonigii in grapefruit.

Cytotoxicity of fungal metabolites to lepidopteran (Spodoptera frugiperda) cell line (SF-9)

The toxicity of sixteen fungal metabolites produced by some entomopathogenic fungi or biological control fungi agents was evaluated on lepidopteran Spodoptera frugiperda (SF-9) cell line by Trypan blue dye exclusion and MTT-colorimetric assay, after 48 h of incubation. No statistical difference was found between IC50values (50% Inhibiting Concentration) and CC50 values (50% Cytotoxicity Concentration) obtained by MTT test and Trypan blue dye exclusion for each fungal metabolite. By MTT assay, the cytotoxicity ranking was fusarenon X (IC50 0.3 microM) = diacetoxyscirpenol (IC50 0.5 microM) = beauvericin (IC50 2.5 microM) = nivalenol (IC50 5.3 microM) = enniatin (IC50 6.6 microM) > or = gliotoxin (IC50 7.5 microM) > zearalenone (IC50 17.5 microM) > deoxynivalenol (IC50 47.6 microM). By Trypan blue dye exclusion the cytotoxicity ranking was fusarenon X (CC50 0.4 microM) = diacetoxyscirpenol (CC50 1.1 microM) beauvericin = (CC50 3.0 microM)=gliotoxin (CC50 4.0 microM) = enniatin (CC50 6.7 microM) > or = nivalenol (CC50 9.5 microM) > zearalenone (CC50 18.3 microM) > deoxynivalenol (CC50 45.0 microM). The comparison with other bioassays showed that the SF-9 insect cell line could represent a further tool to screen for the toxic effects of fungal metabolites especially for beauvericin, gliotoxin, and zearalenone.

Bassiacridin, a protein toxic for locusts secreted by the entomopathogenic fungus Beauveria bassiana

A toxic protein, bassiacridin, was purified from a strain of the entomopathogenic fungus Beauveria bassiana isolated from a locust, using chromatographic methods. The final toxic fraction contained between 0.1 and 0.3% of the proteins of the crude extract. Bassiacridin showed no affinity for ion exchangers, was characterised as a monomer with a mol. wt of 60 kDa and an isoelectric point of 9.5, and exhibited beta-glucosidase, beta-galactosidase and N-acetylglucosaminidase activities. Injection of fourth instar nymphs of Locusta migratoria with the pure protein at relatively low dosage (3.3 microg toxin g body wt(-1)) caused a rate of mortality near to 50%. The effects of the crude and pure fractions were characterized at tissular and cellular levels. The formation of melanised spots on tracheae and air sacs and of melanised nodules in contact with the fat body was observed in injected locusts. Alterations of the fine structure of epithelial cells of tracheae, air bags, and integument were also revealed. The insecticidal protein showed a specific activity against locusts. Bassiacridin is different from the other macromolecular toxins of entomopathogenic fungi already described. Microsequencing of peptides generated by trypsic digestion of bassiacridin confirmed that it is a novel molecule and showed that it exhibits a probably limited similarity with a chitin binding protein from yeast.

CYTOTOXICITY INDUCED BY NIVALENOL, DEOXYNIVALENOL, AND FUMONISIN B1 IN THE SF-9 INSECT CELL LINE

The toxicity of the mycotoxins nivalenol (NIV), deoxynivalenol (DON), and fumonisin B1 (FB1) was studied in the lepidopteran Spodoptera frugiperda (SF-9) cells, by the trypan blue dye-exclusion and 3-(4,5-dimethylthiozole-2-yl)-2,5-biphenyl tetrazolium bromide (MTT) tests, uptake analyses of cytotoxicity, and cell metabolism, respectively. Deoxyribonucleic acid analysis by flow cytometry was used to identify apoptosis and cell cycle distribution. After 48 h of exposure, the MTT and trypan blue dye-exclusion tests indicated that NIV was significantly more toxic than DON, and both were significantly more toxic than FB1. The IC50 (mycotoxin concentration resulting in 50% inhibition of proliferation) values for NIV and DON were 4.5 and 41 microM, and the CC50 (mycotoxin concentration that caused 50% cytotoxicity) values were 9.5 and 45 microM, respectively. At the highest concentration of FB1 (100 microM), there was 80% viability. With the same incubation time, cell cycle distribution showed an arrest of cells in the G0/G1 phase in the presence of NIV (up to 0.3 microM), DON (up to 3 microM), and FB1 (up to 10 microM). Morphological evidence of apoptosis was related to the toxicity of the substances in that the more toxic NIV induced late apoptosis, whereas DON and FB1 produced less-severe morphological changes characteristic of early apoptosis. This study suggests that NIV is more toxic than DON, which in turn is more toxic than FB1. These mycotoxins can modify the normal progression of the cell cycle and induce an apoptotic process.

Detection of the toxin Hirsutellin A from Hirsutella thompsonii

A total of 162 strains of Hirsutella thompsonii, isolated from infected mites collected worldwide, were examined for the production of Hirsutellin A (HtA). More than half of the broth filtrates exhibited mortality rates superior to 50% when assayed against Galleria mellonella. The presence of the gene coding for HtA, a previously characterized H. thompsonii protein exotoxin, was determined by PCR amplification using gene-specific primers. Most isolates (100 out of 162) were shown to possess the HtA gene. However, the presence of the gene could not be associated with enhanced insecticidal activity. Both isolate groups (with or without an amplifiable HtA gene) produced filtrates that caused the same average mortality rate (65%) when assayed against G. mellonella. The production and secretion of the HtA toxin were estimated by probing broth filtrates with an anti-HtA monoclonal antibody. Again, the detection of the HtA protein was poorly correlated with subsequent mortality rates induced by the broth filtrates of the various H. thompsonii strains. This study suggests that HtA is requisite for neither survival nor pathogenicity, and that H. thompsonii strains are likely to secrete other toxins that have yet to be characterized. Sequencing of a limited number of HtA genes showed that, when present, the gene is highly conserved, and it displays an interesting intronic polymorphism.

Toxicity of purified fungal toxin hirsutellin A to the citrus rust mite phyllocoptruta oleivora (Ash.)

A toxic protein, hirsutellin A, isolated from culture filtrate during liquid fermentation of the fungal mycelia of Hirsutella thompsonii, was tested using contact/residual and residual leaf bioassay methodologies at concentrations of 0, 10, 32, 56, and 100 &mgr;g/ml against adult citrus rust mite, Phyllocoptruta oleivora. P. oleivora is the natural host of the parasitic fungus, H. thompsonii. Mite mortality increased with an increase in hirsutellin A concentration, reaching virtually 100% at 100 &mgr;g/ml using both leaf assay methods. The number of eggs found on leaf disk bioassays within a 3-day period decreased significantly with an increase in hirsutellin A concentration, suggesting that fecundity was affected prior to the death of the host. Copyright 1998 Academic Press.

Cloning and sequencing of cDNA of the insecticidal toxin hirsutellin A

The hyphomycete Hirsutella thompsonii produces an extracellular insecticidal protein, Hirsutellin A. This basic protein, cytolytic against insect cells and capable of inhibiting protein translation, possesses biological features similar to the well-characterized ribosomal-inhibiting proteins (RIPs) alpha-sarcin, mitogellin, and restrictocin. Cloning and DNA sequencing analysis of the 3' and 5' RACE products of HtA cDNA identifies a consensus DNA sequence which encompasses the complete open reading frame of the HtA gene. This gene codes for a precursor of 164 aa which includes a 34-aa leader sequence. The leader sequence of HtA, like those found in RIPs, contains a signal and a pro sequence. The mature 130-aa HtA, having a calculated Mr = 14,159 and pI = 9.21, is considered a stable hydrophilic protein. HtA does not possess the characteristic RNase motif of fungal RIPs but does possess a series of consensus phosphorylation and myristoylation sites and a putative ATP/GTP binding site. The sequence of HtA is unique and does not produce the secondary or tertiary structures characteristic of other fungal RIPs. Copyright 1998 Academic Press.