New diterpene pyrone-type compounds, metarhizins A and B, isolated from entomopathogenic fungus, Metarhizium flavoviride and their inhibitory effects on cellular proliferation

A novel partitivirus orchestrates conidiation, stress response, pathogenicity, and secondary metabolism of the entomopathogenic fungus Metarhizium majus

Mycoviruses are widely present in all major groups of fungi but those in entomopathogenic Metarhizium spp. remain understudied. In this investigation, a novel double-stranded (ds) RNA virus is isolated from Metarhizium majus and named Metarhizium majus partitivirus 1 (MmPV1). The complete genome sequence of MmPV1 comprises two monocistronic dsRNA segments (dsRNA 1 and dsRNA 2), which encode an RNA-dependent RNA polymerase (RdRp) and a capsid protein (CP), respectively. MmPV1 is classified as a new member of the genus Gammapartitivirus in the family Partitiviridae based on phylogenetic analysis. As compared to an MmPV1-free strain, two isogenic MmPV1-infected single-spore isolates were compromised in terms of conidiation, and tolerance to heat shock and UV-B irradiation, while these phenotypes were accompanied by transcriptional suppression of multiple genes involved in conidiation, heat shock response and DNA damage repair. MmPV1 attenuated fungal virulence since infection resulted in reduced conidiation, hydrophobicity, adhesion, and cuticular penetration. Additionally, secondary metabolites were significantly altered by MmPV1 infection, including reduced production of triterpenoids, and metarhizins A and B, and increased production of nitrogen and phosphorus compounds. However, expression of individual MmPV1 proteins in M. majus had no impact on the host phenotype, suggesting insubstantive links between defective phenotypes and a single viral protein. These findings indicate that MmPV1 infection decreases M. majus fitness to its environment and its insect-pathogenic lifestyle and environment through the orchestration of the host conidiation, stress tolerance, pathogenicity, and secondary metabolism.

Characterization of a new strain of Metarhizium novozealandicum with potential to be developed as a biopesticide

The fungal species Metarhizium novozealandicum, that occurs only in New Zealand and Australia has been poorly studied.  In this work, a new strain of M. novozealandicum isolated from a larva of Wiseana sp. is described based on morphology, genomic multilocus (ITS, EF-1α and β-tubulin) phylogeny, growth in different culture media and insecticidal activity. The isolate AgR-F177 was clustered in the same clade with M. novozealandicum. AgR-F177 colonies developed faster on Sabouraud Dextrose Agar (SDA) than on Potato Dextrose Agar (PDA) when incubated at 25°C, with no growth observed at 30°C on either media. Conidia yield on an oat-based medium in semisolid fermentation was 7.41 x 10⁸conidia/g of substrate and a higher yield of 1.68 x 10⁹conidia/g of substrate was obtained using solid fermentation on cooked rice. AgR-F177 formed microsclerotia (MS) in liquid fermentation after 7 days reaching the maximum yield of 3.3 × 10³ MS/mL after 10 days. AgR-F177 caused mortality in Wiseana copularis, Costelytra giveni and Plutella xylostella larvae with efficacies up to 100%, 69.2%, and 45.7%, respectively. The ease of production of AgR-F177 with different fermentation systems and its pathogenicity against different insect pests reveal its potential as a new biopesticide.

Molecular and Computational Bases for Spirofuranone Formation in Setosusin Biosynthesis

The 3(2H)-furanone unit is observed in many biologically active natural products, as represented by the antifungal medication griseofulvin. Setosusin (1) is a fungal meroditerpenoid featuring a unique spiro-fused 3(2H)-furanone moiety; however, the biosynthetic basis for spirofuranone formation has not been investigated since its isolation. Therefore, in this study we identified the biosynthetic gene cluster of 1 in the fungus Aspergillus duricaulis CBS 481.65 and elucidated its biosynthetic pathway by heterologous reconstitution of related enzyme activities in Aspergillus oryzae. To understand the reaction mechanism to afford spirofuranone, we subsequently performed a series of in vivo and in vitro isotope-incorporation experiments and theoretical calculations. The results indicated that SetF, the cytochrome P450 enzyme that is critical for spirofuranone synthesis, not only performs the epoxidation of the polyketide portion of the substrate but also facilitates the protonation-initiated structural rearrangement to yield 1. Finally, a mutagenesis experiment using SetF identified Lys303 as one of the potential catalytic residues that are important for spirofuranone synthesis.

The chemistry and biology of fungal meroterpenoids (2009-2019)

Fungal meroterpenoids are secondary metabolites from mixed terpene-biosynthetic origins. Their intriguing chemical structural diversification and complexity, potential bioactivities, and pharmacological significance make them attractive targets in natural product chemistry, organic synthesis, and biosynthesis. This review provides a systematic overview of the isolation, chemical structural features, biological activities, and fungal biodiversity of 1585 novel meroterpenoids from 79 genera terrestrial and marine-derived fungi including macrofungi, Basidiomycetes, in 441 research papers in 2009-2019. Based on the nonterpenoid starting moiety in their biosynthesis pathway, meroterpenoids were classified into four categories (polyketide-terpenoid, indole-, shikimate-, and miscellaneous-) with polyketide-terpenoids (mainly tetraketide-) and shikimate-terpenoids as the primary source. Basidiomycota produced 37.5% of meroterpenoids, mostly shikimate-terpenoids. The genera of Ganoderma, Penicillium, Aspergillus, and Stachybotrys are the four dominant producers. Moreover, about 56% of meroterpenoids display various pronounced bioactivities, including cytotoxicity, enzyme inhibition, antibacterial, anti-inflammatory, antiviral, antifungal activities. It's exciting that several meroterpenoids including antroquinonol and 4-acetyl antroquinonol B were developed into phase II clinically used drugs. We assume that the chemical diversity and therapeutic potential of these fungal meroterpenoids will provide biologists and medicinal chemists with a large promising sustainable treasure-trove for drug discovery.

Asperginols A and B, Diterpene Pyrones, from an Aspergillus sp. and the Structure Revision of Previously Reported Analogues

Two new diterpene pyrones, asperginols A (1) and B (2), and four known analogues (3-6) were isolated from the endophytic fungus Aspergillus sp. HAB10R12. The structures and absolute configurations of these compounds were elucidated based on the analysis of their NMR, MS, and X-ray diffraction data. The revision of the absolute configurations at C-10, C-11, and C-14 of the known diterpene pyrones (3-6) and the determination of the configuration at the polyene side chain for compounds (4-6) were made using chemical methods and vibrational circular dichroism analysis. This group of diterpene pyrone compounds showed unique structural features including a 7/6/6 tricyclic diterpene moiety with an unusual trans-syn-trans stereochemical arrangement. Compound 6 showed moderate activity against the HT-29 colon cancer cell line.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

Synthetic biology based construction of biological activity-related library of fungal decalin-containing diterpenoid pyrones

A synthetic biology method based on heterologous biosynthesis coupled with genome mining is a promising approach for increasing the opportunities to rationally access natural product with novel structures and biological activities through total biosynthesis and combinatorial biosynthesis. Here, we demonstrate the advantage of the synthetic biology method to explore biological activity-related chemical space through the comprehensive heterologous biosynthesis of fungal decalin-containing diterpenoid pyrones (DDPs). Genome mining reveals putative DDP biosynthetic gene clusters distributed in five fungal genera. In addition, we design extended DDP pathways by combinatorial biosynthesis. In total, ten DDP pathways, including five native pathways, four extended pathways and one shunt pathway, are heterologously reconstituted in a genetically tractable heterologous host, Aspergillus oryzae, resulting in the production of 22 DDPs, including 15 new analogues. We also demonstrate the advantage of expanding the diversity of DDPs to probe various bioactive molecules through a wide range of biological evaluations.

Combining genome mining and heterologous expression in a genetically tractable host can lead to bioactive natural products discovery and production. Here, the authors employ this strategy for new decalin-containing diterpenoid pyrenes production by expressing native, extended, and shunt pathways in Aspergillus oryzae.

Natural diterpene pyrones: chemistry and biology

Diterpene pyrones (DTPs) are a group of well-known, mainly fungal, natural products, first isolated in 1966. As the name indicates, they are composed of two main structural features: a diterpenyl moiety and a pyrone ring. Various names have been given to this class of metabolites; however, biogenetic evidence indicates that they originate through the same metabolic pathway. Based on their biosynthesis, which leads to differences in their structural architecture, the DTPs can be classified into three main types. In addition to their intriguing chemistry, these compounds demonstrate a wide range of biological activities rendering them a desirable target for total synthesis. To date, sixty-seven DTPs have been isolated from various fungal species, with one example originating from the plant kingdom. This review aims at unifying the classification of these compounds, in addition to presenting a detailed description of their isolation, bioactivities, biosynthesis, and total synthesis.

Structure and biological activity of Metarhizin C, a stereoisomer of BR-050 from Tolypocladium album RK17-F0007

Metarhizin C, a stereoisomer of BR-050 was isolated from a fungus Tolypocladium album RK17-F0007 through a screening program to search for new antitumor compounds. A structure of the isomer was determined by spectroscopic methods including detailed analysis of NOESY correlation and mass spectrometry, and found to be identical to that of 3-desacylmetarhizin A with the absolute structure. Previously, it had been isolated by Kikuchi et al and proposed as BR-050 including the stereo-structure. However, detailed analysis for the newly isolated isomer confirmed that 3-desacylmetarhizin A was not identical to BR-050. Therefore, we assigned it metarhizin C as a new BR-050 isomer. Metarhizin C showed selective cytotoxicity against osteosarcoma MG-63 cells in a glucose independent condition with IC₅₀ value of 0.79 µg/ml, while > 30 µg/ml of IC₅₀ value in a normal condition, and inhibited a mitochondrial respiration.

Proteomics as a tool for tapping potential of entomopathogens as microbial insecticides

Biopesticides are collective pest control harnessing the knowledge of the target pest and its natural enemies that minimize the risks of synthetic pesticides. A subset of biopesticides; bioinsecticides, are specifically used in controlling insect pests. Entomopathogens (EPMs) are micro-organisms sought after as subject for bioinsecticide development. However, lack of understanding of EPM mechanism of toxicity and pathogenicity slowed the progress of bioinsecticide development. Proteomics is a useful tool in elucidating the interaction of entomopathogenic fungi, entomopathogenic bacteria, and entomopathogenic virus with their target host. Collectively, proteomics shed light onto insect host response to EPM infection, mechanism of action of EPM's toxic proteins and secondary metabolites besides characterizing secreted and membrane-bound proteins of EPM that more precisely describe relevant proteins for host recognition and mediating pathogenesis. However, proteomics requires optimized protein extraction methods to maximize the number of proteins for analysis and availability of organism's genome for a more precise protein identification.

Divergent Synthesis of Pyrone Diterpenes via Radical Cross Coupling

A divergent strategy for assembling pyrone diterpenes is presented. Capitalizing on the unique stereo- and chemoselectivity features of radical-based chemistry, the core decalin of these structures is efficiently forged using an electrochemically assisted oxidative radical polycyclization while key peripheral substituents are appended using decarboxylative radical cross couplings. In this way, access to four natural products (subglutinols A/B, higginsianin A, and sesquicillin A) is achieved in a concise and stereocontrolled fashion that is modular and amenable to future medicinal chemistry explorations.

Synthetic efforts towards the stereoselective synthesis of NF00659B1

NF00659B₁ is a novel α-pyrone diterpenoid natural product with potent anti-colon cancer activity. A stereoselective approach to the 2,2-dimethyl oxepanol core of NF00659B₁ is described enlisting a sequence of olefinic ester ring-closing metathesis, epoxidation, and Grignard addition. This strategy paves the way to a total synthesis of NF00659B₁ for further biological studies.

Metabolic Conservation and Diversification of Metarhizium Species Correlate with Fungal Host-Specificity

The ascomycete genus Metarhizium contains several species of insect pathogenic fungi ranging from specialists with narrow host ranges to generalists that can infect diverse invertebrates. Genetic and metabolic conservations and diversifications of Metarhizium species are not well understood. In this study, using the genome information of seven Metarhizium species, we performed a comparative analysis of gene clusters involved in secondary metabolisms (SMs) in these species. The results revealed that the generalist species contain more SM gene clusters than the specialists, and that both conserved and divergent evolutions may have occurred in SM genes during fungal speciation. In particular, the loss/gain events, as well as gene mutagenesis, are evident for the gene cluster responsible for the biosynthesis of non-ribosomal cyclopeptide destruxins. The presence of conserved SM gene clusters in Metarhizium and other divergently evolved insect pathogenic fungi implies their link to fungal entomopathogenicity. Mass spectrometry based metabolomic analyses were also conducted to investigate the chemical diversities of seven Metarhizium species. Consistent with the evolutionary relationships of SM genes among the seven species, significant differences are observed in fungal metabolic profiles, whether the same or different metabolites are produced in different species. Clustering analysis based on the metabolome data revealed that Metarhizium species could be grouped based on their association to fungal host specificity. Our metabolomics-based methods also facilitate the identification of bioactive metabolites that have not been reported previously in Metarhizium. The results of this study will benefit future investigations of the chemical biology of insect-fungal interactions.

Unusual cyclic terpenoids with terminal pendant prenyl moieties: from occurrence to synthesis

The paper reviews the known examples of cyclic terpenoids produced from open chain polyenic precursors by an "unusual" biosynthetic pathway, involving selective electrophilic attack on an internal double bond followed by cyclization. The resulting compounds possess cyclic backbones with pendant terminal prenyl groups. Synthetic approaches applied for the synthesis of such specifically functionalized compounds are also discussed, as well as biological activity of reported representatives.

Metarhizin A suppresses cell proliferation by inhibiting cytochrome c oxidase activity

AIMS

Metarhizin A was originally isolated from Metarhizium flavoviride as a potent inhibitor of the growth of insect and mammalian cells. In this study, we aimed to understand the molecular targets of metarhizin A involved in its anti-proliferative activity against human cells.

MAIN METHODS

Cell cycle regulators and signaling molecules were examined by immunoblotting using specific antibodies. A mitochondria-enriched fraction was prepared from mouse liver, and mitochondrial activity was monitored using an oxygen electrode. Enzyme activity was measured using purified cytochrome c oxidase and permeabilized cells.

KEY FINDINGS

Metarhizin A inhibits the growth of MCF-7 cells with an IC50 value of ~0.2 μM and other cells in a similar manner; a cell cycle-dependent kinase inhibitor, p21, is selectively induced. Significant amounts of reactive oxygen species (ROS) are generated and ERK1/2 is activated in cells treated with metarhizin A. Metarhizin A completely suppresses oxygen consumption by mitochondria, and potently inhibits the activity of cytochrome c oxidase. It induces cell death when MCF-7 cells are cultured under limiting conditions.

SIGNIFICANCE

Metarhizin A is a potent inhibitor of cytochrome c oxidase and activates the MAPK pathway through the generation of ROS, which induces growth arrest of cells, and, under some conditions, enhances cell death. The cytochrome c oxidase system is a possible molecular target of metarhizin A.

Penicillipyrones A and B, meroterpenoids from a marine-derived Penicillium sp. fungus

Penicillipyrones A (1) and B (2), two novel meroterpenoids, were isolated from the marine-derived fungus Penicillium sp. On the basis of the results of combined spectroscopic analyses, these compounds were structurally elucidated to be sesquiterpene γ-pyrones from a new skeletal class derived from a unique linkage pattern between the drimane sesquiterpene and pyrone moieties. Compound 2 elicited significant induction of quinone reductase.