3-hydroxy-3-methyl glutaryl coenzyme A reductase: an essential actor in the biosynthesis of cantharidin in the blister beetle Epicauta chinensis Laporte

Functional studies of McSTE24, McCYP305a1, and McJHEH, three essential genes act in cantharidin biosynthesis in the blister beetle (Coleoptera: Meloidae)

Cantharidin is a toxic defensive substance secreted by most blister beetles when attacked. It has been used to treat many complex diseases since ancient times and has recently regained popularity as an anticancer agent. However, the detailed mechanism of the cantharidin biosynthesis has not been completely addressed. In this study, we cloned McSTE24 (encoding STE24 endopeptidase) from terpenoid backbone pathway, McCYP305a1 (encoding cytochrome P450, family 305) and McJHEH [encoding subfamily A, polypeptide 1 and juvenile hormone (JH) epoxide hydrolase] associated to JH synthesis/degradation in the blister beetle Mylabris cichorii (Linnaeus, 1758, Coleoptera: Meloidae). Expression pattern analyses across developmental stages in adult males revealed that the expressions of 3 transcripts were closely linked to cantharidin titer exclusively during the peak period of cantharidin synthesis (20-25 days old). In contrast, at other stages, these genes may primarily regulate different biological processes. When RNA interference with double-stranded RNA suppressed the expressions of the 3 genes individually, significant reductions in cantharidin production were observed in males and also in females following McJHEH knockdown, indicating that these 3 genes might primarily contribute to cantharidin biosynthesis in males, but not in females, while females could self-synthesis a small amount of cantharidin. These findings support the previously hypothesized sexual dimorphism in cantharidin biosynthesis during the adult phase. McCYP305a1 collaborates with its upstream gene McSTE24 in cantharidin biosynthesis, while McJHEH independently regulates cantharidin biosynthesis in males.

A Farnesyl Pyrophosphate Synthase Gene Is Expressed in Fat Body Regulates Cantharidin Synthesis in Male Epicauta impressicornis Blister Beetle

Blister beetles of Epicauta impressicornis have attracted attention because they contain a large amount of cantharidin (CTD). To date, however, the synthesis and transfer of CTD in adults of E. impressicornis are largely unknown. Here, we showed that the larvae E. impressicornis are capable of synthesizing CTD and they consume CTD during pupation. Before sexual maturity, both male and female adults synthesized a small amount of CTD, while after sexual maturity, males produced larger amounts of CTD, but females did not. The newly synthesized CTD in males first appeared in the hemolymph and then accumulated in the reproductive system. During the mating, the males transferred CTD to the reproductive system of females. In addition, a farnesyl pyrophosphate synthase (FPPS) gene was identified in male E. impressicornis. RNA-seq analysis, quantitative RT-PCR, and RNA interference analyses were conducted to investigate expression patterns and the functional roles of E. impressicornis FPPS (EiFPPS). Our results indicate that EiFPPS is highly expressed in the fat body of males. Moreover, the knock-down of EiFPPS led to a significant decrease in CTD synthesis. The current study indicates that EiFPPS is expressed in the fat body to regulate CTD synthesis in male E. impressicornis blister beetles.

A minireview of the medicinal and edible insects from the traditional Chinese medicine (TCM)

Entomoceuticals define a subset of pharmaceuticals derived from insects. The therapeutic effect of insect-derived drugs has been empirically validated by the direct use of various folk medicines originating from three sources in particular: the glandular secretions of insects (e.g., silk, honey, venom), the body parts of the insect or the whole used live or by various processing (e.g., cooked, toasted, ground), and active ingredients extracted from insects or insect-microbe symbiosis. Insects have been widely exploited in traditional Chinese medicine (TCM) relative to other ethnomedicines, especially in the prospect of insect species for medicinal uses. It is noticeable that most of these entomoceuticals are also exploited as health food for improving immune function. In addition, some edible insects are rich in animal protein and have high nutritional value, which are used in the food field, such as insect wine, health supplements and so on. In this review, we focused on 12 insect species that have been widely used in traditional Chinese herbal formulae but have remained less investigated for their biological properties in previous studies. We also combined the entomoceutical knowledge with recent advances in insect omics. This review specifies the underexplored medicinal insects from ethnomedicine and shows their specific medicinal and nutritional roles in traditional medicine.

Unraveling the role of male reproductive tract and haemolymph in cantharidin-exuding Lydus trimaculatus and Mylabris variabilis (Coleoptera: Meloidae): a comparative transcriptomics approach

Background

Meloidae (blister beetles) are known to synthetize cantharidin (CA), a toxic and defensive terpene mainly stored in male accessory glands (MAG) and emitted outward through reflex-bleeding. Recent progresses in understanding CA biosynthesis and production organ(s) in Meloidae have been made, but the way in which self-protection is achieved from the hazardous accumulation and release of CA in blister beetles has been experimentally neglected. To provide hints on this pending question, a comparative de novo assembly transcriptomic approach was performed by targeting two tissues where CA is largely accumulated and regularly circulates in Meloidae: the male reproductive tract (MRT) and the haemolymph. Differential gene expression profiles in these tissues were examined in two blister beetle species, Lydus trimaculatus (Fabricius, 1775) (tribe Lyttini) and Mylabris variabilis (Pallas, 1781) (tribe Mylabrini). Upregulated transcripts were compared between the two species to identify conserved genes possibly involved in CA detoxification and transport.

Results

Based on our results, we hypothesize that, to avoid auto-intoxication, ABC, MFS or other solute transporters might sequester purported glycosylated CA precursors into MAG, and lipocalins could bind CA and mitigate its reactivity when released into the haemolymph during the autohaemorrhaging response. We also found an over-representation in haemolymph of protein-domains related to coagulation and integument repairing mechanisms that likely reflects the need to limit fluid loss during reflex-bleeding.

Conclusions

The de novo assembled transcriptomes of L. trimaculatus and M. variabilis here provided represent valuable genetic resources to further explore the mechanisms employed to cope with toxicity of CA in blister beetle tissues. These, if revealed, might help conceiving safe and effective drug-delivery approaches to enhance the use of CA in medicine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08118-8.

Draft genome of the blister beetle, Epicauta chinensis

Existence of cantharidin (CTD) in blister beetles is a significant ecological adaptive mechanism that defends against predators and regulates courtship and mating behaviors. To better understand CTD biosynthetic information as well as its biology and pharmacology, we assembled a genome of 151.88 Mb for Epicauta chinensis using PacBio sequencing technology. Gene annotation yielded 249,238 repeats, 527 non-coding RNAs and 12,520 protein-coding genes. Compared to other 11 insects, expansions of gene families in E. chinensis for most core gene families likely associated with environmental adaptation, such as chemoreception, immunity, and detoxification. We further annotated P450s and immune-related genes, a total of 117 putative P450s comprising 7 CYP2, 67 CYP3, 36 CYP4, and 7 mitochondrial P450s and 281 immune-related genes were identified. Comparative analysis of the insect immune repertoires indicated presence of immune genes detected only from Coleopteran insects such as MD2-like. This suggested a lineage-specific gene evolution for Coleopteran insects. Based on the gene family evolution analysis, we identified two probable candidate genes including CYP4TT1 and phytanoyl-CoA dioxygenase for CTD biosynthesis. The high-quality reference genome of E. chinensis provides the genetic basis for further investigation of CTD biosynthesis and in-depth studies of the development and evolution of blister beetles.

RNAi silencing of the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) gene inhibits vitellogenesis in Chinese mitten crab Eriocheir sinensis

The sesquiterpenoid methyl farnesoate (MF), a de-epoxide form of insect juvenile hormone III (JH III), plays an essential role in regulating many crucial physiological processes in crustaceans including vitellogenesis and reproduction. 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is an important rate-limiting enzyme in the mevalonate pathway, which is critical for the synthesis of JH III and MF. In the present study, a full-length cDNA encoding HMGR (EsHMGR) in Eriocheir sinensis was isolated and characterised. Sequence analysis of EsHMGR revealed that it belongs to Class I HMGR family proteins with HMG-CoA-binding and NADPH-binding domains, both important for HMGR activity. In addition to its ubiquitous tissue expression, expression of EsHMGR was highly specific to the ovary, the main site of Vg synthesis. During ovarian development, EsHMGR expression in ovary displayed a stage-specific pattern, and was correlated with expression of vitellogenin (EsVg) in hepatopancreas, which suggests that EsHMGR possibly involved in vitellogenesis. To further investigate the functional role of EsHMGR in vitellogenin biosynthesis in E. sinensis, RNA interference-mediated gene silencing was carried out both in vitro and in vivo. Quantitative PCR results showed that injection of EsHMGR double-stranded RNA (dsRNA) led to a significant decrease in EsVg expression levels in ovary and hepatopancreas both in vitro and in vivo. Taken together, the results suggest that EsHMGR is involved in vitellogenin biosynthesis in female E. sinensis, which may provide a new resource for HMGR enzymes participating in reproduction in crustaceans.

De novo transcriptome assemblies of Epicauta tibialis provide insights into the sexual dimorphism in the production of cantharidin

Blister beetles have medicinal uses for their defensive secretion cantharidin, which has curative effects on many cancers and other diseases. It was demonstrated that sexual dimorphism exists in the production of cantharidin between male and female adults. This study performed a de novo assembly of Epicauta tibialis transcriptomes and analyzed the differentially expressed genes (DEGs) between male and female adults to help to find genes and pathways associated with cantharidin biosynthesis. A total of 99,295,624 paired reads were generated, and more than 7 Gb transcriptome data for each sample were obtained after trimming. The clean data were used to de novo assemble and then cluster into 27,355 unigenes, with a mean length of 1442 bp and an N50 of 2725 bp. Of these, 14,314 (52.33%) unigenes were annotated by protein databases. Differential expression analysis identified 284 differentially expressed genes (DEGs) between male and female adults. Nearly 239 DEGs were up-regulated in male adults than in female adults, while 45 DEGs were down-regulated. The Kyoto Encyclopedia of Gene and Genomes pathway enrichment manifested that seven up-regulated DEGs in male adults were assigned to the terpenoid biosynthesis pathway, to which 19 unigenes were annotated. The DEGs in the terpenoid biosynthesis pathway between male and female adults may be responsible for the sexual dimorphism in cantharidin production. The up-regulated genes assigned to the pathway in male adults may play a significant role in cantharidin biosynthesis, and its biosynthesis process is probably via the mevalonate pathway. The results would be helpful to better understand and reveal the complicated mechanism of the cantharidin biosynthesis.

Isolation and expression of five genes in the mevalonate pathway of the Chinese white pine beetle, Dendroctonus armandi (Curculionidae: Scolytinae)

The Chinese white pine beetle Dendroctonus armandi (Tsai and Li) is a significant pest of the Qinling and Bashan Mountains pine forests of China. The Chinese white pine beetle can overcome the defences of Chinese white pine Pinus armandi (Franch) through pheromone-assisted aggregation that results in a mass attack of host trees. We isolated five full-length complementary DNAs encoding mevalonate pathway-related enzyme genes from the Chinese white pine beetle (D. armandi), which are acetoacetyl-CoA thiolase (AACT), geranylgeranyl diphosphate synthase (GGPPS), mevalonate kinase (MK), mevalonate diphosphate decarboxylase (MPDC), and phosphomevalonate kinase (PMK). Bioinformatic analyses were performed on the full-length deduced amino acid sequences. Differential expression of these five genes was observed between sexes, and within these significant differences among topically applied juvenile hormone III (JH III), fed on phloem of P. armandi, tissue distribution, and development stage. Mevalonate pathway genes expression were induced by JH III and feeding.

What happens after a blood meal? A transcriptome analysis of the main tissues involved in egg production in Rhodnius prolixus, an insect vector of Chagas disease

The blood-sucking hemipteran Rhodnius prolixus is a vector of Chagas disease, one of the most neglected tropical diseases affecting several million people, mostly in Latin America. The blood meal is an event with a high epidemiological impact since adult mated females feed several times, with each meal resulting in a bout of egg laying, and thereby the production of hundreds of offspring. By means of RNA-Sequencing (RNA-Seq) we have examined how a blood meal influences mRNA expression in the central nervous system (CNS), fat body and ovaries in order to promote egg production, focusing on tissue-specific responses under controlled nutritional conditions. We illustrate the cross talk between reproduction and a) lipids, proteins and trehalose metabolism, b) neuropeptide and neurohormonal signaling, and c) the immune system. Overall, our molecular evaluation confirms and supports previous studies and provides an invaluable molecular resource for future investigations on different tissues involved in successful reproductive events. These analyses serve as a starting point for new investigations, increasing the chances of developing novel strategies for vector population control by translational research, with less impact on the environment and more specificity for a particular organism.

Draft Genome of a Blister Beetle Mylabris aulica

Mylabris aulica is a widely distributed blister beetle of the Meloidae family. It has the ability to synthesize a potent defensive secretion that includes cantharidin, a toxic compound used to treat many major illnesses. However, owing to the lack of genetic studies on cantharidin biosynthesis in M. aulica, the commercial use of this species is less extensive than that of other blister beetle species in China. This study reports a draft assembly and possible genes and pathways related to cantharidin biosynthesis for the M. aulica blister beetle using nanopore sequencing data. The draft genome assembly size was 288.5 Mb with a 467.8 Kb N50, and a repeat content of 50.62%. An integrated gene finding pipeline performed for assembly obtained 16,500 protein coding genes. Benchmarking universal single-copy orthologs assessment showed that this gene set included 94.4% complete Insecta universal single-copy orthologs. Over 99% of these genes were assigned functional annotations in the gene ontology, Kyoto Encyclopedia of Genes and Genomes, or Genbank non-redundant databases. Comparative genomic analysis showed that the completeness and continuity of our assembly was better than those of Hycleus cichorii and Hycleus phaleratus blister beetle genomes. The analysis of homologous orthologous genes and inference from evolutionary history imply that the Mylabris and Hycleus genera are genetically close, have a similar genetic background, and have differentiated within one million years. This M. aulica genome assembly provides a valuable resource for future blister beetle studies and will contribute to cantharidin biosynthesis.

Characterized cantharidin distribution and related gene expression patterns in tissues of blister beetles, Epicauta chinensis

Cantharidin (CTD), a terpenoid defensive toxin mainly produced by blister beetles, is widely known by its toxicity to both cancer cells and pests. However, little information is known about its biosynthesis in blister beetles. In this study, first we determined the CTD content in various tissues of adult blister beetles on different days after mating, and then detected the temporal and spatial expression patterns of genes related to CTD biosynthesis in Epicauta chinensis. Results revealed that the accessory gland is the source of the highest CTD production. The second highest level was in the fat body in male blister beetles after mating. In females, the highest CTD content was in the reproductive system except the ovary after mating. As revealed by messenger RNA expression level analysis, the highest levels of 3-hydroxy-3-methylglutary-CoA reductase (HMGR) and juvenile hormone epoxide hydrolase (JHEH) transcripts of E. chinensis were observed in the fat body in males after mating. However, the highest transcript level of EcHMGR was in the ovary and EcJHEH was maintained at a nearly similar level in females. The transcript level of methyl-farnesoate epoxide was significantly higher in the head and that of CYP4BM1 in the midgut in both male and female E. chinensis. We speculate that the fat body may play a more important role than other tissues on the CTD biosynthesis in male E. chinensis after mating. There may be multiple tissues involved in the process of CTD biosynthesis. These four genes probably play regulatory roles in different tissues in males.

Both farnesyl diphosphate synthase genes are involved in the production of alarm pheromone in the green peach aphid Myzus persicae

Farnesyl diphosphate synthase (FPPS) catalyzes the formation of FPP, providing the precursor for the biosynthesis of (E)-β-farnesene (EβF) in plants, but it is unknown if FPPS supplies the precursor for the biosynthesis of EβF, the major component of aphid alarm pheromone, though our previous studies support the hypothesis that EβF is synthesized by the aphid itself. Here, we used two cohorts of the green peach aphid Myzus persicae separately, reared on pepper plant and artificial diet to test the correlations among droplet emission, EβF quantity, and FPPS gene expression. It was found that the proportion of aphids emitting cornicle droplets and the quantity of EβF per milligram of aphid were both significantly different between the two cohorts, which were positively correlated with the expression of the two FPPS genes ( MpFPPS1/ 2) in M. persicae. These results were further confirmed by RNAi-mediated knockdown of MpFPPS1/ 2. Specifically, knockdown of MpFPPS1/ 2 imposed no significant cost on the survival of aphid but remarkably increased the number of offspring per aphid; most importantly, knockdown of MpFPPS1/ 2 significantly reduced the proportion of aphids emitting droplets and the quantity of EβF calculated as per the weight of aphid. Our results suggest that both FPPS genes are involved in the production of EβF in M. persicae and cornicle droplet emission is closely associated with the EβF release in the aphid.

Identification and characterization of genes involving the early step of Juvenile Hormone pathway in Helicoverpa armigera

Juvenile hormones (JHs) are crucial regulators for multiple physiological processes in insects. In the current study, 10 genes in mevalonate pathway involved in JH biosynthesis were identified from Helicoverpa armigera. Tissue-specific expression analysis showed that six genes were highly expressed in the head which contained the JH biosynthetic gland (corpora allata). Temporal expression pattern showed that 10 of 12 genes were highly transcribed in the late 2^nd-instar when the in vivo JH titer reached the peak, indicating a tight correlation between JH titer and the transcription of JH synthetic pathway genes. Moreover, ingestion of methoprene, a JH analogue, significantly suppressed the transcription of nine JH biosynthetic genes and caused a feedback upregulation of the JH degradation enzyme. Particularly, the Acetoacetyl CoA thiolase (HaAce) and Farnesyl diphosphate synthase gene 4 (HaFpps4) showed high transcript abundance, and their temporal expressions keep pace with JH fluctuations. Further study by RNAi showed that knockdown of HaFpps4 caused the decrease of JH titer, led to a negative effect on the transcript levels of other genes in JH pathway, and resulted in molting disturbance in larvae. Altogether, these results contribute to our understanding of JH biosynthesis in H. armigera and provide target genes for pest control based on JH-dependent regulation.

Non-target effects on soil microbial parameters of the synthetic pesticide carbendazim with the biopesticides cantharidin and norcantharidin

Considering the fact that biopesticides are increasingly used to replace synthetic pesticides in pest control, it is necessary to assess their ecotoxicity and especially their non-target effects on soil microorganisms, which is largely unknown. In this study, the effects of the synthetic pesticide carbendazim and the biopesticides (cantharidin and norcantharidin) on soil microbial parameters in a silt loam soil were evaluated. By using commercial formulations at the recommended and higher rates, both cantharidin and norcantharidin induced adverse effects on soil invertase, phosphatase activities and fungal gene structure, but these changes were transient. After about two weeks, the harmful effects owing to the application of pesticides phased out and eventually became comparable with non-treated samples. The degradation of cantharidin and norcantharidin was rapid and can be completed within a few days in the soil. None of the three pesticides caused significant shifts in urease activity. This study provides a comprehensive assessment of the soil microbial toxicity of these biopesticides for reasonable and efficient usage.

Characterization of Juvenile Hormone Related Genes Regulating Cantharidin Biosynthesis in Epicauta chinensis

Cantharidin is a defensive toxin biosynthesized by blister beetles. It is well known for medical applications and toxicity. However, the biosynthesis process of cantharidin is still not well understood. In the present study, three genes (methyl farnesoate epoxidase (EcMFE), juvenile hormone acid O-methyltransferase (EcJHAMT) and juvenile hormone epoxide hydrolase (EcJHEH)) were identified from Epicauta chinensis. The temporal and spatial expression patterns of these three genes revealed that the expression levels of EcMFE and EcJHEH were high in the first instar larval stage of E. chinensis with EcJHEH transcripts highest in the fifth larval instar. The expression level of EcJHAMT was significantly higher in the 2^nd and 3^rd larval instars. The transcripts of EcMFE, EcJHEH and EcJHAMT showed a similar tendency with the cantharidin production in male blister beetles after mating. We verified the functions of these three genes in cantharidin biosynthesis using the RNA interference method. Interference of EcMFE and EcJHEH significantly inhibited the biosynthesis of cantharidin in male E. chinensis after mating, but interference of EcJHAMT has no apparent influence on cantharidin biosynthesis. We propose that EcMFE and EcJHEH may be involved in the biosynthesis of cantharidin, but JH III might not be the direct precursor of cantharidin.

The Potential Organ Involved in Cantharidin Biosynthesis in Epicauta chinensis Laporte (Coleoptera: Meloidae)

Cantharidin, a terpenoid defensive toxin mainly produced by blister beetles, is among the most widely known insect natural products in the world. However, little is known about the site of cantharidin biosynthesis in vivo. Our previous research showed that 3-hydroxy-3-methylglutary-CoA reductase (HMGR) is an essential enzyme in cantharidin biosynthesis. In this report, we further investigated cantharidin titer and HMGR mRNA expression levels in different tissues of male and female Epicauta chinensis, and performed a comparative analysis of HMGR transcript levels in male Tenebrio molitor, a Tenebrionidae beetle that cannot produce cantharidin. HMGR transcripts had a positive correlation with cantharidin production. Furthermore, the specifically high amounts of HMGR transcript and abundant cantharidin production in fat body of male E. chinensis indicated the process of cantharidin synthesis may occur in the fat body.