Targeted Disruption of Transcriptional Regulators in Chaetomium globosum Activates Biosynthetic Pathways and Reveals Transcriptional Regulator-Like Behavior of Aureonitol

Activation of secondary metabolite gene clusters in Chaetomium olivaceum via the deletion of a histone deacetylase

Histone acetylation modifications in filamentous fungi play a crucial role in epigenetic gene regulation and are closely linked to the transcription of secondary metabolite (SM) biosynthetic gene clusters (BGCs). Histone deacetylases (HDACs) play a pivotal role in determining the extent of histone acetylation modifications and act as triggers for the expression activity of target BGCs. The genus Chaetomium is widely recognized as a rich source of novel and bioactive SMs. Deletion of a class I HDAC gene of Chaetomium olivaceum SD-80A, g7489, induces a substantial pleiotropic effect on the expression of SM BGCs. The C. olivaceum SD-80A ∆g7489 strain exhibited significant changes in morphology, sporulation ability, and secondary metabolic profile, resulting in the emergence of new compound peaks. Notably, three polyketides (A1-A3) and one asterriquinone (A4) were isolated from this mutant strain. Furthermore, our study explored the BGCs of A1-A4, confirming the function of two polyketide synthases (PKSs). Collectively, our findings highlight the promising potential of molecular epigenetic approaches for the elucidation of novel active compounds and their biosynthetic elements in Chaetomium species. This finding holds great significance for the exploration and utilization of Chaetomium resources. KEY POINTS: • Deletion of a class I histone deacetylase activated secondary metabolite gene clusters. • Three polyketides and one asterriquinone were isolated from HDAC deleted strain. • Two different PKSs were reported in C. olivaceum SD-80A.

Cryptic piperazine derivatives activated by knocking out the global regulator LaeA in Aspergillus flavipes

Genome sequencing on an intertidal zone-derived Aspergillus flavipes strain revealed its great potential to produce secondary metabolites. To activate the cryptic compounds of A. flavipes, the global regulator flLaeA was knocked out, leading to substantial up-regulation of the expression of two NRPS-like biosynthetic gene clusters in the ΔflLaeA mutant. With a scaled-up fermentation of the ΔflLaeA strain, five compounds, including two previously undescribed piperazine derivatives flavipamides A and B (1 and 2), along with three known compounds (3-5), were obtained by LC-MS guided isolation. The new compounds were elucidated by spectroscopic analysis and electronic circular dichroism (ECD) calculations, and the biosynthetic pathway was proposed on the bias of bioinformatic analysis and 13C isotope labeling evidence. This is the first report to access cryptic fungi secondary metabolites by inactivating global regulator LaeA and may provide a new approach to discovering new secondary metabolites by such genetic manipulation.

Chemical constituents from the medicinal herb-derived fungus Chaetomium globosum Km1226

BACKGROUND

Endophytic fungi have proven to be a rich source of novel natural products with a wide-array of biological activities and higher levels of structural diversity.

RESULTS

Chemical investigation on the liquid- and solid-state fermented products of Chaetomium globosum Km1226 isolated from the littoral medicinal herb Atriplex maximowicziana Makino resulted in the isolation of compounds 1-14. Their structures were determined by spectroscopic analysis as three previously undescribed C13-polyketides, namely aureonitol C (1), mollipilins G (2), and H (3), along with eleven known compounds 4-14. Among these, mollipilin A (5) exhibited significant nitric oxide production inhibitory activity in LPS-induced BV-2 microglial cells with an IC50 value of 0.7 ± 0.1 µM, and chaetoglobosin D (10) displayed potent anti-angiogenesis property in human endothelial progenitor cells (EPCs) with an IC50 value of 0.8 ± 0.3 µM.

CONCLUSIONS

Three previously unreported compounds 1-3 were isolated and identified. Mollipilin A (5) and chaetoglobosin D (10) could possibly be developed as anti-inflammatory and anti-angiogenic lead drugs, respectively.

Daldipyrenones A-C: Caged [6,6,6,6,6] Polyketides Derived from an Endolichenic Fungus Daldinia pyrenaica 047188

Daldipyrenones A-C (1-3), three unprecedented caged xanthone [6,6,6,6,6] polyketides featuring a spiro-azaphilone unit, were discovered from an endolichenic fungus, Daldinia pyrenaica 047188. The structures of 1-3 were determined by using spectroscopic analysis and chemical derivatization. Daldipyrenones are likely derived by combining a chromane biosynthesis intermediate, 1-(2,6-dihydroxyphenyl)but-2-en-2-one, and a spiro-azaphilone, pestafolide A, via radical coupling or Michael addition to form a bicyclo[2.2.2]octane ring. Genome sequencing of the strain revealed two separate biosynthetic gene clusters responsible for forming two biosynthetic intermediates, suggesting a proposed biosynthetic pathway. Daldipyrenone A (1) exhibited significant antimelanogenic activity with lower EC50's than positive controls and moderate adiponectin-secretion promoting activity.

Asymmetric Total Synthesis of Chaetoglobin A

An asymmetric total synthesis of chaetoglobin A was achieved. Atroposelective oxidative coupling of a phenol incorporating all but one carbon of the final product was used as a key step to generate axial chirality. The stereochemical outcome of the catalytic oxidative phenolic with the highly substituted phenol used herein was found to be opposite that of the simpler congeners reported previously, providing a cautionary tale about extrapolating asymmetric processes from simple to more complex substrates. Optimization of the postphenolic coupling steps including formylation, oxidative dearomatization, and selective deprotection steps are outlined. The tertiary acetates of chaetoglobin A were exceptionally labile due to activation by the adjacent keto groups, which complicated each of these steps. In contrast, the final oxygen to nitrogen exchange proceeded readily and the spectroscopic data from the synthetic material matches that of the isolated natural product in all respects.

Investigation of chetomin as a lead compound and its biosynthetic pathway

Chaetomium fungi produce a diversity of bioactive compounds. Chaetomium cochliodes SD-280 possesses 91 secondary metabolite gene clusters and exhibits strong antibacterial activity. One of the active compounds responsible for that activity, chetomin, has a minimum inhibitory concentration (MIC) for anti-methicillin-resistant Staphylococcus aureus (MRSA) of 0.05 μg/mL (vancomycin: 0.625 μg/mL). This study demonstrated that the addition of glutathione (GSH) can enhance chetomin yield dramatically, increasing its production 15.43-fold. Following genome sequencing, cluster prediction, and transcriptome and proteome analyses of the fungus were carried out. Furthermore, a relatively complete chetomin biosynthetic gene cluster was proposed, and the coding sequences were acquired. In the cluster of GSH-treated cells, proteome analysis revealed two up-regulated proteins that are critical enzymes for chetomin biosynthesis. One of these enzymes, a nonribosomal peptide synthetase (NRPS), was heterologously expressed in Aspergillus nidulans, and one of its metabolites was determined to be an intermediate in the chetomin biosynthetic pathway. We present here, to our knowledge, the first experimental evidence that chetomin exhibits strong bioactivity against MRSA. Our work also provides extensive insights into the biosynthetic pathway of chetomin, in particular identifying two key enzymes (glutathione S-transferase (CheG) and NRPS (CheP)) that substantially up-regulate chetomin. These mechanistic insights into chetomin biosynthesis will provide the foundation for further investigation into the anti-pathogenic properties and applications of chetomin. KEY POINTS: • Chetomin exhibits strong anti-MRSA activity with MIC of 0.05 μg/mL. • Addition of glutathione improved the yield of chetomin by 15.43-fold. • CheG and CheP involved in the chetomin biosynthesis were revealed for the first time.

Diversity of biologically active secondary metabolites in the ascomycete order Sordariales

Ascomycetes belonging to the order Sordariales are a well-known reservoir of secondary metabolites with potential beneficial applications. Species of the Sordariales are ubiquitous, and they are commonly found in soils and in lignicolous, herbicolous, and coprophilous habitats. Some of their species have been used as model organisms in modern fungal biology or were found to be prolific producers of potentially useful secondary metabolites. However, the majority of sordarialean species are poorly studied. Traditionally, the classification of the Sordariales has been mainly based on morphology of the ascomata, ascospores, and asexual states, characters that have been demonstrated to be homoplastic by modern taxonomic studies based on multi-locus phylogeny. Herein, we summarize for the first time relevant information about the available knowledge on the secondary metabolites and the biological activities exerted by representatives of this fungal order, as well as a current outlook of the potential opportunities that the recent advances in omic tools could bring for the discovery of secondary metabolites in this order.

A Homologous Recombination System to Generate Epitope-Tagged Target Genes in Chaetomium thermophilum: A Genetic Approach to Investigate Native Thermostable Proteins

Chaetomium thermophilum is an attractive eukaryotic model organism which, due to its unusually high temperature tolerance (optimal growth at 50-52 °C), has a thermostable proteome that can be exploited for biochemical, structural and biotechnological applications. Site directed gene manipulation for the expression of labeled target genes is a desirable approach to study the structure and function of thermostable proteins and their organization in complexes, which has not been established for this thermophile yet. Here, we describe the development of a homologous recombination system to epitope-tag chromosomal genes of interest in Chaetomium thermophilum with the goal to exploit the derived thermostable fusion proteins for tandem-affinity purification. This genetic approach was facilitated by the engineering of suitable strains, in which factors of the non-homologous end-joining pathway were deleted, thereby improving the efficiency of homologous integration at specific gene loci. Following this strategy, we could demonstrate that gene tagging via homologous recombination improved the yield of purified bait proteins and co-precipitated factors, paving the way for related studies in fundamental research and industrial applications.

Seawater fungi-derived compound screening to identify novel small molecules against dengue virus NS5 methyltransferase and NS2B/NS3 protease

Dengue fever is a virus spread by mosquitoes that has no effective treatment or vaccination. Several dengue cases combined with the current COVID-19 pandemic, exacerbates this problem. Two proteins, NS5 methyltransferase and NS2B/NS3 primary protease complexes, are crucial for dengue viral replication and are the target sites for antiviral development. Thus, this study screened published literature and identified 162 marine fungus-derived compounds with active bioavailability. Following Lipinski's rules and antiviral property prediction, 41 compounds were selected for docking with NS5 methyltransferase and NS2B/NS3 protease (PDB ID: 6IZZ and 2FOM) to evaluate compounds that could stop the action of dengue viral protein complexes. To find the best candidates, computational ADME, toxicity, and drug target prediction were performed to estimate the potential of the multi-targeting fungal-derived natural compounds. Analyzing the result from 41 compounds, Chevalone E (−13.5 kcal/mol), Sterolic acid (−10.3 kcal/mol) showed higher binding energy against dengue NS2B/NS3 protease; meanwhile, Chevalone E (−12.0 kcal/mol), Brevione K (−7.4 kcal/mol), had greater binding affinity against NS5 methyltransferase. Consequently, this study suggests that Chevalone E is an effective inhibitor of NS5 methyltransferase and NS2B/NS3 protease. Ligand-based virtual screening from DrugBank was utilized to predict biologically active small compounds against dengue virus NS2B/NS3 major protease and NS5 methyltransferase. Both licensed medications, estramustine (DB01196) and quinestrol (DB04575), were found to be similar to Chevalone E, with prediction scores of 0.818 and 0.856, respectively. In addition, cholic acid (DB02659), acitretin (DB00459), and mupirocin (DB00410) are similar to Sterolic acid, zidovudine (DB00495), imipenem (DB01598), and nadolol (DB01203) are similar to Brocazine A, and budesonide (DB01222) and colchicine (DB01394) are related to Brevione K. These findings suggest that these could be feasible dengue virus treatment options, meaning that more research is needed.

Transformation of Chaetomium thermophilum and Affinity Purification of Native Thermostable Protein Complexes

Chaetomium thermophilum, a eukaryotic thermophile, is an aspiring organism holding great potential for various biochemical and biotechnological applications. Prerequisite for genetic manipulation is a reliable transformation system for target genes combined with selection markers operating at the high growth temperatures of the fungus. Here, we present a detailed protocol for Chaetomium thermophilum protoplast transformation to allow stable chromosomal integration of constructs into its genome, rendering this eukaryotic thermophile a valuable resource for affinity purification of native thermostable protein complexes, like nuclear pore subcomplexes.

Core Steps to the Azaphilone Family of Fungal Natural Products

Azaphilones are a family of polyketide-based fungal natural products that exhibit interesting and useful bioactivities. This minireview explores the literature on various characterised azaphilone biosynthetic pathways, which allows for a proposed consensus scheme for the production of the core azaphilone structure, as well as identifying early diversification steps during azaphilone biosynthesis. A consensus understanding of the core enzymatic steps towards a particular family of fungal natural products can aid in genome-mining experiments. Genome mining for novel fungal natural products is a powerful technique for both exploring chemical space and providing new insights into fungal natural product pathways.

Cytochalasans and azaphilones: suitable chemotaxonomic markers for the Chaetomium species

The accurate taxonomic concept of the fungal Chaetomium species has been a hard work due to morphological similarity. Chemotaxonomy based on secondary metabolites is a powerful tool for taxonomical purposes, which could be used as an auxiliary reference to solve the problems encountered in the classification of Chaetomium. Among secondary metabolites produced by Chaetomium, cytochalasans and azaphilones exhibited a pattern of distribution and frequency of occurrence that establish them as chemotaxonomic markers for the Chaetomium species. This review attempted to elucidate the composition of the Chaetomium species and its relationship with classical taxonomy by summarizing the pattern of cytochalasans and azaphilones distribution and biosynthesis in the Chaetomium species. KEY POINTS: • Secondary metabolites from the genus Chaetomium are summarized. • Cytochalasans and azaphilones could be characteristic metabolites of the Chaetomium species. • Cytochalasans and azaphilones could be used to analyze for taxonomical purposes.

A Putative C2H2 Transcription Factor CgTF6, Controlled by CgTF1, Negatively Regulates Chaetoglobosin A Biosynthesis in Chaetomium globosum

Gα signaling pathway as well as the global regulator LaeA were demonstrated to positively regulate the biosynthesis of chaetoglobosin A (ChA), a promising biotic pesticide produced by Chaetomium globosum. Recently, the regulatory function of Zn2Cys6 binuclear finger transcription factor CgcheR that lies within the ChA biosynthesis gene cluster has been confirmed. However, CgcheR was not merely a pathway specific regulator. In this study, we showed that the homologs gene of CgcheR (designated as Cgtf1) regulate ChA biosynthesis and sporulation in C. globosum NK102. More importantly, RNA-seq profiling demonstrated that 1,388 genes were significant differentially expressed as Cgtf1 deleted. Among them, a putative C2H2 transcription factor, named Cgtf6, showed the highest gene expression variation in zinc-binding proteins encoding genes as Cgtf1 deleted. qRT-PCR analysis confirmed that expression of Cgtf6 was significantly reduced in CgTF1 null mutants. Whereas, deletion of Cgtf6 resulted in the transcriptional activation and consequent increase in the expression of ChA biosynthesis gene cluster and ChA production in C. globosum. These data suggested that CgTF6 probably acted as an end product feedback effector, and interacted with CgTF1 to maintain a tolerable concentration of ChA for cell survival.

Deletion of a Rare Fungal PKS CgPKS11 Promotes Chaetoglobosin A Biosynthesis, Yet Defers the Growth and Development of Chaetomium globosum

We previously reported that chaetoglobosin A (ChA) exhibits a great potential in the biocontrol of nematodes and pathogenic fungi. To improve the production of ChA, a CRISPR-Cas9 system was created and applied for eliminating potential competitive polyketide products. One of the polyketide synthase encoding genes, Cgpks11, which is putatively involved in the biosynthesis of chaetoglocin A, was disrupted. Cgpks11 deletion led to the overexpression of the CgcheA gene cluster, which is responsible for ChA biosynthesis, and a 1.6-fold increase of ChA. Transcription of pks-1, a melanin PKS, was simultaneously upregulated. Conversely, the transcription of genes for chaetoglocin A biosynthesis, e.g., CHGG_10646 and CHGG_10649, were significantly downregulated. The deletion also led to growth retardation and seriously impaired ascospore development. This study found a novel regulatory means on the biosynthesis of ChA by CgPKS11. CgPKS11 affects chaetoglobosin A biosynthesis, growth, and development in Chaetomium globosum.

Investigation of biological activity of soil fungal extracts and LC/MS-QTOF based metabolite profiling

Soil is considered an extensively explored ecological niche for microorganisms that produce useful biologically active natural products suitable for pharmaceutical applications. The current study aimed at investigating biological activities and metabolic profiles of three fungal strains identified from different desert sites in Saudi Arabia. Soil fungal isolates were collected from AlQasab, Tabuk, and Almuzahimiyah in Saudi Arabia and identified. Furthermore, their antibacterial activity was investigated against Staphylococcus aureus, Enterococcus faecalis, Klebsiella pneumonia, and Escherichia coli in blood, nutrient, and Sabouraud dextrose agars. Moreover, fungal extracts were evaluated on cell viability/proliferation against human breast carcinoma and colorectal adenocarcinoma cells. To identify the biomolecules of the fungal extracts, High-performance liquid chromatography HPLC-DAD coupled to analytical LC-QTOF-MS method was employed for fungal ethyl acetate crude extract. Identified fungal isolates, Chaetomium sp. Bipolaris sp. and Fusarium venenatum showed varied inhibitory activity against tested microbes in relation to crude extract, microbial strain tested, and growth media. F. venenatum showed higher anticancer activity compared to Chaetomium sp. and Bipolaris sp. extracts against four of the tested cancer cell lines. Screening by HPLC and LC/MS-QTOF identified nine compounds from Chaetomium sp. and three from Bipolaris sp. however, for F. venenatum extracts compounds were not fully identified. In light of the present findings, some biological activities of fungal extracts were approved in vitro, suggesting that such extracts could be a useful starting point to find compounds that possess promising agents for medical applications. Further investigations to identify exact biomolecules from F. venenatum extracts are needed.

Biosynthesis of azaphilones: a review

Covering up to 2020 Azaphilones are fungal polyketide pigments bearing a highly oxygenated pyranoquinone bicyclic core; they are receiving a great deal of increasing research interest for their applications in the agroalimentary, dyeing, cosmetic, printing and pharmaceutical industries. Their biosynthetic pathways are not fully elucidated; however, thanks to recent genomic approaches combined with the increasing genome sequencing of fungi, some of these pathways have been recently unveiled. This is the first review on the biosynthesis of azaphilonoids adressed from a genomic point of view.

Genomics-directed activation of cryptic natural product pathways deciphers codes for biosynthesis and molecular function

Natural products, which can be isolated from living organisms worldwide, have played a pivotal role in drug discovery since ancient times. However, it has become more challenging to identify a structurally novel molecule with promising biological activity for pharmaceutical development, mainly due to the limited methodologies for their acquisition. In this review, we summarize our recent studies that activate the biosynthetic potential of filamentous fungi by genetic engineering to harness the metabolic flow for the efficient production of unprecedented natural products. The recent revolution in genome sequencing technology enables the accumulation of vast amounts of information on biosynthetic genes, the blueprint of the molecular construction. Utilizing the established heterologous expression system, activation of the pathway-specific transcription factor coupled with a knockout strategy, and manipulating the global regulatory gene, the biosynthetic genes were exploited to activate biosynthetic pathways and decipher the encoded enzyme functions. We show that this methodology was beneficial for acquiring fungal treasures for drug discovery. These studies also enabled the investigation of the molecular function of natural products in fungal development.

Decaturenol A and known oxalicine related meroterpenoids isolated from Penicillium decaturense RO050, and their new biological activities

Decaturenol A (1), a new oxalicine related meroterpenoid, has been isolated from Penicillium decaturense RO050 along with seven known compounds (2-8). The structure of 1 was elucidated by spectroscopic data. The effects of isolated compounds (1-8) on endoplasmic reticulum (ER) stress-induced cell death in HT22 hippocampal nerve cells and on the interleukin 10 (IL-10)-induced expression of CD163, a M2 phenotype marker, in human monocyte-derived macrophages were evaluated. While decaturenol A (1) exhibited a protective effect on ER stress-induced cell death in HT22 cells at 10 µM, on the other hand oxalicine A (7) showed cytotoxic activity (IC₅₀ = 5.9 µM). Additionally, decaturenol A (1), decaturins D (2), E (3), and B (4) inhibited the IL-10-induced expression of CD163 each at a concentration of 20 µg/mL.

Diastereoselective Synthesis of 2,3,4-Trisubstituted Tetrahydrofurans via Thermally Reactive 1,5-Diene-tert-butyl Carbonates

We report that 3,3-dicyano-1,5-dienes bearing tert-butyl carbonates can be thermally converted to 2,3,4-trisubstituted tetrahydrofurans. The transformation relies on two thermally reactive functional groups, a 1,5-diene and a tert-butyl carbonate, that react cooperatively to yield the furan scaffolds by thermal Cope rearrangement, Boc deprotection, and oxy-Michael addition. Described herein is background related to the discovery, optimization, and scope of the key transformation and representative functional group interconversion chemistry for the tetrahydrofuran scaffolds.

Recent advances in the chemistry and biology of azaphilones

Recent advances in the chemistry and biology of structurally diverse azaphilones from 2012 to 2019.

Integrative Activity of Mating Loci, Environmentally Responsive Genes, and Secondary Metabolism Pathways during Sexual Development of Chaetomium globosum

Fungal diversity has amazed evolutionary biologists for decades. One societally important aspect of this diversity manifests in traits that enable pathogenicity. The opportunistic pathogen Chaetomium globosum is well adapted to a high-humidity environment and produces numerous secondary metabolites that defend it from predation. Many of these chemicals can threaten human health. Understanding the phases of the C. globosum life cycle in which these products are made enables better control and even utilization of this fungus. Among its intriguing traits is that it both is self-fertile and lacks any means of propagule-based asexual reproduction. By profiling genome-wide gene expression across the process of sexual reproduction in C. globosum and comparing it to genome-wide gene expression in the model filamentous fungus N. crassa and other closely related fungi, we revealed associations among mating-type genes, sexual developmental genes, sexual incompatibility regulators, environmentally responsive genes, and secondary metabolic pathways.

The origins and maintenance of the rich fungal diversity have been longstanding issues in evolutionary biology. To investigate how differences in expression regulation contribute to divergences in development and ecology among closely related species, transcriptomes were compared between Chaetomium globosum, a homothallic pathogenic fungus thriving in highly humid ecologies, and Neurospora crassa, a heterothallic postfire saprotroph. Gene expression was quantified in perithecia at nine distinct morphological stages during nearly synchronous sexual development. Unlike N. crassa, expression of all mating loci in C. globosum was highly correlated. Key regulators of the initiation of sexual development in response to light stimuli—including orthologs of N. crassasub-1, sub-1-dependent gene NCU00309, and asl-1—showed regulatory dynamics matching between C. globosum and N. crassa. Among 24 secondary metabolism gene clusters in C. globosum, 11—including the cochliodones biosynthesis cluster—exhibited highly coordinated expression across perithecial development. C. globosum exhibited coordinately upregulated expression of histidine kinases in hyperosmotic response pathways—consistent with gene expression responses to high humidity we identified in fellow pathogen Fusarium graminearum. Bayesian networks indicated that gene interactions during sexual development have diverged in concert with the capacities both to reproduce asexually and to live a self-compatible versus self-incompatible life cycle, shifting the hierarchical roles of genes associated with conidiation and heterokaryon incompatibility in N. crassa and C. globosum. This divergence supports an evolutionary history of loss of conidiation due to unfavorable combinations of heterokaryon incompatibility in homothallic species.

Regulation of the Gα-cAMP/PKA signaling pathway in cellulose utilization of Chaetomium globosum

BACKGROUND

The canonical heterotrimeric G protein-cAMP/PKA pathway regulates numerous cellular processes in filamentous fungi. Chaetomium globosum, a saprophytic fungus, is known for producing many secondary metabolites, including cytotoxic chaetoglobosin A (ChA), as well as abundant cellulase and xylanase.

RESULTS

Here we report on the functional characterization of this signaling pathway in C. globosum. We blocked the pathway by knocking down the putative Gα-encoding gene gna1 (in the pG14 mutant). This led to impaired cellulase production and significantly decreased transcription of the major cellulase and xylanase genes. Almost all the glycohydrolase family genes involved in cellulose degradation were downregulated, including the major cellulase genes, cel7a, cel6a, egl1, and egl2. Importantly, the expression of transcription factors was also found to be regulated by gna1, especially Ace1, Clr1/2 and Hap2/3/5 complex. Additionally, carbon metabolic processes including the starch and sucrose metabolism pathway were substantially diminished, as evidenced by RNA-Seq profiling and quantitative reverse transcription (qRT)-PCR. Interestingly, these defects could be restored by simultaneous knockdown of the pkaR gene encoding the regulatory subunit of cAMP-dependent PKA (in the pGP6 mutant) or supplement of the cAMP analog, 8-Br-cAMP. Moreover, the Gα-cAMP/PKA pathway regulating cellulase production is modulated by environmental signals including carbon sources and light, in which VelB/VeA/LaeA complex and ENVOY probably work as downstream effectors.

CONCLUSION

These results revealed, for the first time, the positive role of the heterotrimeric Gα-cAMP/PKA pathway in the regulation of cellulase and xylanase utilization in C. globosum.

Total Synthesis of Chaetoglobin A via Catalytic, Atroposelective Oxidative Phenol Coupling

The first total synthesis of chaetoglobin A (1), which features a chiral axis between two identical highly oxygenated bicyclic cores, was successfully completed in 12 steps from 2,6-dimethoxytoluene. Vanadium-catalyzed oxidative phenol coupling, as a key step, enabled generation of the axial chirality.

Gα-cAMP/PKA pathway positively regulates pigmentation, chaetoglobosin A biosynthesis and sexual development in Chaetomium globosum

Sensing the environmental signals, the canonical Gα-cAMP/PKA pathway modulates mycelial growth and development, and negatively regulates some secondary metabolism in filamentous fungi, e.g. aflatoxin in Aspergillus nidulans. Here we report the characterization of this signaling pathway in Chaetomium globosum, a widely spread fungus known for synthesizing abundant secondary metabolites, e.g. chaetoglobosin A (ChA). RNAi-mediated knockdown of a putative Gα-encoding gene gna-1, led to plural changes in phenotype, e.g. albino mycelium, significant restriction on perithecium development and decreased production of ChA. RNA-seq profiling and qRT-PCR verified significantly fall in expression of corresponding genes, e.g. pks-1 and CgcheA. These defects could be restored by simultaneous knock-down of the pkaR gene encoding a regulatory subunit of cAMP-dependent protein kinase A (PKA), suggesting that pkaR had a negative effect on the above mentioned traits. Confirmatively, the intracellular level of cAMP in wild-type strain was about 3.4-fold to that in gna-1 silenced mutant pG14, and addition of a cAMP analog, 8-Br-cAMP, restored the same defects, e.g., the expression of CgcheA. Furthermore, the intracellular cAMP in gna-1 and pkaR double silenced mutant was approaching the normal level. The following activity inhibition experiment proved that the expression of CgcheA was indeed regulated by PKA. Down-regulation of LaeA/VeA/SptJ expression in gna-1 mutant was also observed, implying that Gα signaling may crosstalk to other regulatory pathways. Taken together, this study proposes that the heterotrimeric Gα protein-cAMP/PKA signaling pathway positively mediates the sexual development, melanin biosynthesis, and secondary metabolism in C. globosum.

Elucidation of Biosynthetic Pathways of Natural Products

During the last decade, we have revealed biosynthetic pathways responsible for the formation of important and chemically complex natural products isolated from various organisms through genetic manipulation. Detailed in vivo and in vitro characterizations enabled elucidation of unexpected mechanisms of secondary metabolite biosynthesis. This personal account focuses on our recent efforts in identifying the genes responsible for the biosynthesis of spirotryprostatin, aspoquinolone, Sch 210972, pyranonigrin, fumagillin and pseurotin. We exploit heterologous reconstitution of biosynthetic pathways of interest in our study. In particular, extensive involvement of oxidation reactions is discussed. Heterologous hosts employed here are Saccharomyces cerevisiae, Aspergillus nidulans and A. niger that can also be used to prepare biosynthetic intermediates and product analogs by engineering the biosynthetic pathways using the knowledge obtained by detailed characterizations of the enzymes. (998 char.).

Trienylfuranol A and trienylfuranone A-B: metabolites isolated from an endophytic fungus, Hypoxylon submoniticulosum, in the raspberry Rubus idaeus

A strain of Hypoxylon submonticulosum was isolated as an endophyte from a surface-sterilized leaf of a cultivated raspberry (Rubus idaeus). The liquid culture extract displayed growth inhibition activity against Saccharomyces cerevisiae using a disc diffusion assay. The extract's major component was identified as a new natural product, trienylfuranol A (1S,2S,4R)-1-((1'E,3'E)-hexa-1',3',5'-trienyl)-tetrahydro-4-methylfuran-2-ol (1), by high-resolution LC-MS and 1D and 2D NMR spectroscopy. Two additional new metabolites, trienylfuranones A (2) and B (3), were isolated as minor components of the extract and their structure elucidation revealed that they were biosynthetically related to 1. Absolute stereochemical configurations of compounds 1-3 were confirmed by NOE NMR experiments and by the preparation of Mosher esters. Complete hydrogenation of 1 yielded tetrahydrofuran 7 that was used for stereochemical characterization and assessment of antifungal activity.

Identification of two mutations that cause defects in the ligninolytic system through an efficient forward genetics in the white-rot agaricomycete Pleurotus ostreatus

White-rot fungi play an important role in the global carbon cycle because they are the species that almost exclusively biodegrade wood lignin in nature. Lignin peroxidases (LiPs), manganese peroxidases (MnPs) and versatile peroxidases (VPs) are considered key players in the ligninolytic system. Apart from LiPs, MnPs and VPs, however, only few other factors involved in the ligninolytic system have been investigated using molecular genetics, implying the existence of unidentified elements. By combining classical genetic techniques with next-generation sequencing technology, they successfully showed an efficient forward genetics approach to identify mutations causing defects in the ligninolytic system of the white-rot fungus Pleurotus ostreatus. In this study, they identified two genes - chd1 and wtr1 - mutations in which cause an almost complete loss of Mn²⁺ -dependent peroxidase activity. The chd1 gene encodes a putative chromatin modifier, and wtr1 encodes an agaricomycete-specific protein with a putative DNA-binding domain. The chd1-1 mutation and targeted disruption of wtr1 hamper the ability of P. ostreatus to biodegrade wood lignin. Examination of the effects of the aforementioned mutation and disruption on the expression of certain MnP/VP genes suggests that a complex mechanism underlies the ligninolytic system in P. ostreatus.

Chaephilones A and B, Two New Azaphilone Derivatives Isolated from Chaetomium globosum

Two new azaphilone derivatives, chaephilones A (1) and B (2), were isolated from the fungus Chaetomium globosum, together with four structurally related analogs 3 - 6. The structures of 1 and 2 were elucidated by comprehensive spectroscopic analyses including HR-ESI-MS and NMR. The known compounds were identified as chaetomugilin Q (3), chaetomugilin D (4), 11-epichaetomugilin A (5), and chaetomugilin S (6) by comparing their NMR data and optical rotation values with those reported. Compound 2 represents the first example of azaphilone with an open furan ring. Compounds 1 and 2 were evaluated for cytotoxic activities against five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480) by the MTS method.

Overexpression of the Global Regulator LaeA in Chaetomium globosum Leads to the Biosynthesis of Chaetoglobosin Z

Overexpression of laeA in Chaetomium globosum CBS148.51 up-regulated expression of the chaetoglobosin gene cluster and resulted in the isolation of a new cytochalasan, chaetoglobosin Z (1), together with six known analogues, chaetoglobosins A (2), B (3), D (4), E (5), O (6), and V (7). RT-PCR analysis confirmed that the key genes in the chaetoglobosin gene cluster were significantly up-regulated. The structure of the new compound chaetoglobosin Z (1) was elucidated using NMR data. The relative and absolute configurations were determined by NOESY and electronic circular dichroism combined with quantum-chemical calculations adopting time-dependent density functional theory methods, respectively. These compounds displayed strong biological effects against the HepG 2 cell line compared with the positive control. The results further supported that LaeA is a global regulator that could up-regulate and/or activate cryptic gene clusters to produce new secondary metabolites.

Epigenetic stimulation of polyketide production in Chaetomium cancroideum by an NAD(+)-dependent HDAC inhibitor

Exposure of the fungus Chaetomium cancroideum to an NAD(+)-dependent HDAC inhibitor, nicotinamide, enhanced the production of aromatic and branched aliphatic polyketides, which allowed us to isolate new secondary metabolites, chaetophenol G and cancrolides A and B. Their structures were determined using spectroscopic analyses, and their absolute configuration was elucidated by electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and chemical transformations. Biosynthesis of the branched aliphatic polyketide skeletons in cancrolides A and B was evidenced by conducting a feeding experiment using compounds labeled with a (13)C stable isotope.

Marker recycling via 5-fluoroorotic acid and 5-fluorocytosine counter-selection in the white-rot agaricomycete Pleurotus ostreatus

Of all of the natural polymers, lignin, an aromatic heteropolymer in plant secondary cell walls, is the most resistant to biological degradation. White-rot fungi are the only known organisms that can depolymerize or modify wood lignin. Investigating the mechanisms underlying lignin biodegradation by white-rot fungi would contribute to the ecofriendly utilization of woody biomass as renewable resources in the future. Efficient gene disruption, which is generally very challenging in the white-rot fungi, was established in Pleurotus ostreatus (the oyster mushroom). Some of the genes encoding manganese peroxidases, enzymes that are considered to be involved in lignin biodegradation, were disrupted separately, and the phenotype of each single-gene disruptant was analysed. However, it remains difficult to generate multi-gene disruptants in this fungus. Here we developed a new genetic transformation marker in P. ostreatus and demonstrated two marker recycling methods that use counter-selection to generate a multigene disruptant. This study will enable future genetic studies of white-rot fungi, and it will increase our understanding of the complicated mechanisms, which involve various enzymes, including lignin-degrading enzymes, underlying lignin biodegradation by these fungi.

Combinatorial Generation of Chemical Diversity by Redox Enzymes in Chaetoviridin Biosynthesis

Chaetoviridins constitute a large family of structurally related secondary metabolites isolated from Chaetomium fungi. To elucidate the biosynthesis pathway and understand how the chemical diversity of chaetoviridins is generated, gene deletion and in vitro characterization of the four post-PKS modifications enzymes were undertaken. CazL and CazP were identified to have substrate promiscuity that facilitates the formation of nonchlorinated analogues. In addition, enzymatic oxidation and reduction combined with spontaneous dehydration and lactonization of the intermediates further expand the chemical diversity.

Aureonitol, a Fungi-Derived Tetrahydrofuran, Inhibits Influenza Replication by Targeting Its Surface Glycoprotein Hemagglutinin

The influenza virus causes acute respiratory infections, leading to high morbidity and mortality in groups of patients at higher risk. Antiviral drugs represent the first line of defense against influenza, both for seasonal infections and pandemic outbreaks. Two main classes of drugs against influenza are in clinical use: M2-channel blockers and neuraminidase inhibitors. Nevertheless, because influenza strains that are resistant to these antivirals have been described, the search for novel compounds with different mechanisms of action is necessary. Here, we investigated the anti-influenza activity of a fungi-derived natural product, aureonitol. This compound inhibited influenza A and B virus replication. This compound was more effective against influenza A(H3N2), with an EC₅₀ of 100 nM. Aureonitol cytoxicity was also very low, with a CC₅₀ value of 1426 μM. Aureonitol inhibited influenza hemagglutination and, consequently, significantly impaired virus adsorption. Molecular modeling studies revealed that aureonitol docked in the sialic acid binding site of hemagglutinin, forming hydrogen bonds with highly conserved residues. Altogether, our results indicate that the chemical structure of aureonitol is promising for future anti-influenza drug design.

Involvement of Lipocalin-like CghA in Decalin-Forming Stereoselective Intramolecular [4+2] Cycloaddition

Understanding enzymatic Diels-Alder (DA) reactions that can form complex natural product scaffolds is of considerable interest. Sch 210972 1, a potential anti-HIV fungal natural product, contains a decalin core that is proposed to form through a DA reaction. We identified the gene cluster responsible for the biosynthesis of 1 and heterologously reconstituted the biosynthetic pathway in Aspergillus nidulans to characterize the enzymes involved. Most notably, deletion of cghA resulted in a loss of stereoselective decalin core formation, yielding both an endo (1) and a diastereomeric exo adduct of the proposed DA reaction. Complementation with cghA restored the sole formation of 1. Density functional theory computation of the proposed DA reaction provided a plausible explanation of the observed pattern of product formation. Based on our study, we propose that lipocalin-like CghA is responsible for the stereoselective intramolecular [4+2] cycloaddition that forms the decalin core of 1.

Effective use of heterologous hosts for characterization of biosynthetic enzymes allows production of natural products and promotes new natural product discovery

In the past few years, there has been impressive progress in elucidating the mechanism of biosynthesis of various natural products accomplished through the use of genetic, molecular biological and biochemical techniques. Here, we present a comprehensive overview of the current results from our studies on fungal natural product biosynthetic enzymes, including nonribosomal peptide synthetase and polyketide synthase-nonribosomal peptide synthetase hybrid synthetase, as well as auxiliary enzymes, such as methyltransferases and oxygenases. Specifically, biosynthesis of the following compounds is described in detail: (i) Sch210972, potentially involving a Diels-Alder reaction that may be catalyzed by CghA, a functionally unknown protein identified by targeted gene disruption in the wild type fungus; (ii) chaetoglobosin A, formed via multi-step oxidations catalyzed by three redox enzymes, one flavin-containing monooxygenase and two cytochrome P450 oxygenases as characterized by in vivo biotransformation of relevant intermediates in our engineered Saccharomyces cerevisiae; (iii) (-)-ditryptophenaline, formed by a cytochrome P450, revealing the dimerization mechanism for the biosynthesis of diketopiperazine alkaloids; (iv) pseurotins, whose variations in the C- and O-methylations and the degree of oxidation are introduced combinatorially by multiple redox enzymes; and (v) spirotryprostatins, whose spiro-carbon moiety is formed by a flavin-containing monooxygenase or a cytochrome P450 as determined by heterologous de novo production of the biosynthetic intermediates and final products in Aspergillus niger. We close our discussion by summarizing some of the key techniques that have facilitated the discovery of new natural products, production of their analogs and identification of biosynthetic mechanisms in our study.

Armochaetoglobins A-J: Cytochalasan Alkaloids from Chaetomium globosum TW1-1, a Fungus Derived from the Terrestrial Arthropod Armadillidium vulgare

Ten new cytochalasan alkaloids, termed armochaetoglobins A-J (1-10), and four known chaetoglobosins (11-14) were isolated from a methanol extract of Chaetomium globosum TW1-1, a fungus isolated from the medicinal terrestrial arthropod Armadillidium vulgare. Their structures were elucidated by a combination of spectroscopy, single-crystal X-ray crystallography, and ECD calculations. Armochaetoglobins A-E (1-5) represented the first examples of seco-chaetoglobosins arising from an oxidative cleavage of C-19 and C-20. Among these compounds, armochaetoglobin A (1) features an unusual pyrrole ring. The cytotoxic activities of 2-10 were evaluated, and armochaetoglobin H (8) showed moderate inhibitory activities against five human cancer cell lines, with IC50 values ranging from 3.31 to 9.83 μM.

Elucidation of the shanorellin biosynthetic pathway and functional analysis of associated enzymes

Since fungal natural products biosynthesized by polyketide synthases frequently exhibit useful biological activities, identifying and understanding the mechanism of biosynthetic steps taken by PKSs are of great interest.

Thielavialides A-E, nor-spiro-azaphilones, and a bis-spiro-azaphilone from Thielavia sp. PA0001, an endophytic fungus isolated from aeroponically grown Physalis alkekengi

Four new nor-spiro-azaphilones, thielavialides A-D (1- 4), a new bis-spiro-azaphilone, thielavialide E (5), together with pestafolide A (6), were isolated from the endophytic fungal strain, Thielavia sp. PA0001, occurring in the healthy leaf tissue of aeroponically grown Physalis alkekengi. The structures and relative configurations of 1-5 were established on the basis of their MS and NMR data. Possible biosynthetic pathways to thielavialides A-E (1- 5) from pestafolide A (6), some involving a Favorskii-like rearrangement, are proposed.