Parasite suppression of the oxidations of eumelanin precursors in Drosophila melanogaster

How to eliminate pathogen without killing oneself? Immunometabolism of encapsulation and melanization in Drosophila

Cellular encapsulation associated with melanization is a crucial component of the immune response in insects, particularly against larger pathogens. The infection of a Drosophila larva by parasitoid wasps, like Leptopilina boulardi, is the most extensively studied example. In this case, the encapsulation and melanization of the parasitoid embryo is linked to the activation of plasmatocytes that attach to the surface of the parasitoid. Additionally, the differentiation of lamellocytes that encapsulate the parasitoid, along with crystal cells, is accountable for the melanization process. Encapsulation and melanization lead to the production of toxic molecules that are concentrated in the capsule around the parasitoid and, at the same time, protect the host from this toxic immune response. Thus, cellular encapsulation and melanization represent primarily a metabolic process involving the metabolism of immune cell activation and differentiation, the production of toxic radicals, but also the production of melanin and antioxidants. As such, it has significant implications for host physiology and systemic metabolism. Proper regulation of metabolism within immune cells, as well as at the level of the entire organism, is therefore essential for an efficient immune response and also impacts the health and overall fitness of the organism that survives. The purpose of this "perspective" article is to map what we know about the metabolism of this type of immune response, place it in the context of possible implications for host physiology, and highlight open questions related to the metabolism of this important insect immune response.

Cellular and humoral immune interactions between Drosophila and its parasitoids

The immune interactions occurring between parasitoids and their host insects, especially in Drosophila-wasp models, have long been the research focus of insect immunology and parasitology. Parasitoid infestation in Drosophila is counteracted by its multiple natural immune defense systems, which include cellular and humoral immunity. Occurring in the hemocoel, cellular immune responses involve the proliferation, differentiation, migration and spreading of host hemocytes and parasitoid encapsulation by them. Contrastingly, humoral immune responses rely more heavily on melanization and on the Toll, Imd and Jak/Stat immune pathways associated with antimicrobial peptides along with stress factors. On the wasps' side, successful development is achieved by introducing various virulence factors to counteract immune responses of Drosophila. Some or all of these factors manipulate the host's immunity for successful parasitism. Here we review current knowledge of the cellular and humoral immune interactions between Drosophila and its parasitoids, focusing on the defense mechanisms used by Drosophila and the strategies evolved by parasitic wasps to outwit it.

Melanization induced by Heliothis virescens ascovirus 3h promotes viral replication

Melanization is an important innate immune defense mechanism of insects, which can kill invading pathogens. Most pathogens, for their survival and reproduction, inhibit the melanization of the host. Interestingly, our results suggested that after infection with Heliothis virescens ascovirus 3h (HvAV-3h), the speed of melanization in infected Spodoptera exigua larval hemolymph was accelerated and that the phenoloxidase (PO) activity of hemolymph in larvae infected with HvAV-3h increased significantly (1.20-fold at 96 hpi, 1.52-fold at 120 hpi, 1.23-fold at 144 hpi, 1.12-fold at 168 hpi). The transcription level of the gene encoding S. exigua prophenoloxidase-1 (SePPO-1 gene) was upregulated dramatically in the fat body during the middle stage of infection. In addition, when melanization was inhibited or promoted, the replication of HvAV-3h was inhibited or promoted, respectively. In conclusion, infection with HvAV-3h can markedly induce melanization in the middle stage of infection, and melanization is helpful for HvAV-3h viral replication.

Interaction Between Beauveria bassiana (Hypocreales: Cordycipitaceae) and Coptera haywardi (Hymenoptera: Diapriidae) for the Management of Anastrepha obliqua (Diptera: Tephritidae)

The interaction between the entomopathogenic fungus Beauveria bassiana (Balsamo) and the parasitoid Coptera haywardi (Oglobin), as potential biological control agents for Anastrepha obliqua (Macquart) fruit flies, was evaluated under laboratory and semi-protected field cage conditions. The effects of the parasitoids and fungus were individually and jointly assessed in Plexiglas cages. Application of B. bassiana dry conidia to soil produced 40% mortality in A. obliqua adults. However, mortality was lower (21.2%) on evaluation under field cage conditions. According to the multiple decrement life table analysis, the probability of death of A. obliqua was 88% when C. haywardi parasitoids and B. bassiana conidia were used in conjunction, 89% when only C. haywardi parasitoids were released and 23% when only B. bassiana conidia were applied. These results demonstrate that no synergistic, additive or antagonistic interaction took place with the simultaneous use of these natural enemies, since the presence of B. bassiana had no effect on the C. haywardi parasitism. These results indicate that the parasitoid is a better natural enemy for the control of A. obliqua, and show that, although the two biological control agents can be used simultaneously, their joint application will not produce increased control.

Effect of the insect phenoloxidase on the metabolism of l-DOPA

Insect prophenoloxidase (PPO) induces melanization around pathogens. Before melanization, PPO is cleaved into phenoloxidase (PO) by serine proteases. Insect PPO can also be activated by exogenous proteases secreted by pathogens as well as by other compounds, such as ethanol and cetylpyridinium chloride (CPC). However, the effect of these activators on the activity of PO is unclear. In this study, the insect endogenous serine protease AMM1, α-chymotrypsin, and ethanol were used to activate recombinant Drosophila PPO1 (rPPO1), and the PO activity differed depending on the activator applied. The PO-induced intermediates during melanization also varied markedly in their numbers and abundances. Therefore, this study indicates that the mechanism of PPO activation influences PO activity. It also suggests that PO-induced different intermediates may affect the antibacterial activity during melanization due to their toxicity.

Eumelanin and pheomelanin are predominant pigments in bumblebee (Apidae: Bombus) pubescence

BACKGROUND

Bumblebees (Hymenoptera: Apidae: Bombus) are well known for their important inter- and intra-specific variation in hair (or pubescence) color patterns, but the chemical nature of the pigments associated with these patterns is not fully understood. For example, though melanization is believed to provide darker colors, it still unknown which types of melanin are responsible for each color, and no conclusive data are available for the lighter colors, including white.

METHODS

By using dispersive Raman spectroscopy analysis on 12 species/subspecies of bumblebees from seven subgenera, we tested the hypothesis that eumelanin and pheomelanin, the two main melanin types occurring in animals, are largely responsible for bumblebee pubescence coloration.

RESULTS

Eumelanin and pheomelanin occur in bumblebee pubescence. Black pigmentation is due to prevalent eumelanin, with visible signals of additional pheomelanin, while the yellow, orange, red and brown hairs clearly include pheomelanin. On the other hand, white hairs reward very weak Raman signals, suggesting that they are depigmented. Additional non-melanic pigments in yellow hair cannot be excluded but need other techniques to be detected. Raman spectra were more similar across similarly colored hairs, with no apparent effect of phylogeny and both melanin types appeared to be already used at the beginning of bumblebee radiation.

DISCUSSION

We suggest that the two main melanin forms, at variable amounts and/or vibrational states, are sufficient in giving almost the whole color range of bumblebee pubescence, allowing these insects to use a single precursor instead of synthesizing a variety of chemically different pigments. This would agree with commonly seen color interchanges between body segments across Bombus species.

Cotesia vestalis parasitization suppresses expression of a Plutella xylostella thioredoxin

Thioredoxins (Trxs) are a family of small, highly conserved and ubiquitous proteins involved in protecting organisms against toxic reactive oxygen species. In this study, a typical thioredoxin gene, PxTrx, was isolated from Plutella xylostella. The full-length cDNA sequence is composed of 959 bp containing a 321 bp open reading frame that encodes a predicted protein of 106 amino acids, a predicted molecular weight of 11.7 kDa and an isoelectric point of 5.03. PxTrx was mainly expressed in larval Malpighian tubules and the fat body. An enriched recombinant PxTrx had insulin disulphide reductase activity and stimulated Human Embryonic Kidney 293 (HEK293) cell proliferation. It also protected supercoiled DNA and living HEK293 cells from H₂ O₂ -induced damage. Parasitization by Cotesia vestalis and injections of 0.05 and 0.01 equivalents of C. vestalis Bracovirus (CvBv), the symbiotic virus carried by the parasitoid, led to down-regulation of PxTrx expression in host fat body. Taken together, our results indicate that PxTrx contributes to the maintenance of P. xylostella cellular haemostasis. Host fat body expression of PxTrx is strongly attenuated by parasitization and by injections of CvBv.

Pheomelanin in the secondary sexual characters of male parasitoid wasps (Hymenoptera: Pteromalidae)

The occurrence and distribution of eumelanin and pheomelanin, the most prevalent biological pigments, has been rarely investigated in insects. Particularly yellowish to brownish body parts, which in many vertebrates are associated with pheomelanin, are visible in many insects but their chemical nature was rarely examined to a similar detail. Here, by using Dispersive Raman spectroscopy analysis, we found both eumelanin and pheomelanin in different body parts of male parasitoid wasps of three species of the genus Mesopolobus (Hymenoptera: Pteromalidae), which are known to have species-specific spots and coloured stripes on the legs and/or antennae which are displayed to females during courtship. We found a strong eumelanin signal in the antennal clava of all studied Mesopolobus species and in the circular black spot or callosity and the triangular black projection on the outer apical angle of the typically expanded middle tibia of Mesopolobus tibialis and Mesopolobus xanthocerus. Eumelanin was also the predominant pigment in the black thorax of Mesopolobus and other members of the family. Pheomelanin, on the other hand, was detected as predominant only in certain body parts of M. tibialis and M. xanthocerus, precisely in a very narrow, longitudinal brownish stripe on the middle femur and, only in M. tibialis, in a brownish oval-longitudinal stripe on the middle tibia. The two melanin types co-occurred in most pigmented areas, but more often one is clearly predominant relative to the other, according to the variation of Raman signal intensity of their signature peaks. A further tibial yellowish-orange stripe present in both these species did not include melanins of any type. Pheomelanin, could be more widespread than previously known in insects. A convergent evolution of melanin-based male sexual ornaments between vertebrates (e.g. bird feathers) and wasps can be suggested, opening to a new line of comparative evolutionary studies.

Parasitization by Scleroderma guani influences expression of superoxide dismutase genes in Tenebrio molitor

Superoxide dismutase (SOD) is an antioxidant enzyme involved in detoxifying reactive oxygen species. In this study, we identified genes encoding the extracellular and intracellular copper-zinc SODs (ecCuZnSOD and icCuZnSOD) and a manganese SOD (MnSOD) in the yellow mealworm beetle, Tenebrio molitor. The cDNAs for ecCuZnSOD, icCuZnSOD, and MnSOD, respectively, encode 24.55, 15.81, and 23.14 kDa polypeptides, which possess structural features typical of other insect SODs. They showed 20-94% identity to other known SOD sequences from Bombyx mori, Musca domestica, Nasonia vitripennis, Pediculus humanus corporis, and Tribolium castaneum. Expression of these genes was analyzed in selected tissues and developmental stages, and following exposure to Escherichia coli and parasitization by Scleroderma guani. We recorded expression of all three SODs in cuticle, fat body, and hemocytes and in the major developmental stages. Relatively higher expressions were detected in late-instar larvae and pupae, compared to other developmental stages. Transcriptional levels were upregulated following bacterial infection. Analysis of pupae parasitized by S. guani revealed that expression of T. molitor SOD genes was significantly induced following parasitization. We infer that these genes act in immune response and in host-parasitoid interactions.

Extracellular superoxide dismutase in insects: characterization, function, and interspecific variation in parasitoid wasp venom

Endoparasitoid wasps inject venom proteins with their eggs to protect them from the host immune response and ensure successful parasitism. Here we report identification of Cu,Zn superoxide dismutase (SOD) transcripts for both intracellular SOD1 and extracellular SOD3 in the venom apparatus of two Leptopilina species, parasitoids of Drosophila. Leptopilina SODs show sequence and structure similarity to human SODs, but phylogenetic analyses indicate that the extracellular SODs are more related to cytoplasmic vertebrate SODs than to extracellular SODs, a feature shared by predicted insect extracellular SODs. We demonstrate that L. boulardi SOD3 is indeed secreted and active as monomeric glycosylated forms in venom. Our results also evidence quantitative variation in SOD3 venom contents between closely related parasitoid species, as sod3 is 100-fold less expressed in Leptopilina heterotoma venom apparatus and no protein and SOD activity are detected in its venom. Leptopilina recombinant SOD3s as well as a mammalian SOD in vitro inhibit the Drosophila phenoloxidase activity in a dose-dependent manner, demonstrating that SODs may interfere with the Drosophila melanization process and, therefore, with production of cytotoxic compounds. Although the recombinant L. boulardi SOD3 quantity needed to observe this effect precludes a systemic effect of the wasp venom SOD3, it is still consistent with a local action at oviposition. This work provides the first demonstration that insect extracellular SODs are indeed secreted and active in an insect fluid and can be used as virulence factors to counteract the host immune response, a strategy largely used by bacterial and fungal pathogens but also protozoan parasites during infection.

Antiviral, anti-parasitic, and cytotoxic effects of 5,6-dihydroxyindole (DHI), a reactive compound generated by phenoloxidase during insect immune response

Phenoloxidase (PO) and its activation system are implicated in several defense responses of insects. Upon wounding or infection, inactive prophenoloxidase (proPO) is converted to active PO through a cascade of serine proteases and their homologs. PO generates reactive compounds such as 5,6-dihydroxyindole (DHI), which have a broad-spectrum antibacterial and antifungal activity. Here we report that DHI and its spontaneous oxidation products are also active against viruses and parasitic wasps. Preincubation of a baculovirus stock with 1.25 mM DHI for 3 h near fully disabled recombinant protein production. The LC₅₀ for lambda bacteriophage and eggs of the wasp Microplitis demolitor were 5.6 ± 2.2 and 111.0 ± 1.6 μM, respectively. The toxicity of DHI and related compounds also extended to cells derived from insects that serve as hosts for several of the aforementioned pathogens. Pretreatment of Sf9 cells with 1.0 mM DHI for 4 h resulted in 97% mortality, and LC₅₀ values of 20.3 ± 1.2 μM in buffer and 131.8 ± 1.1 μM in a culture medium. Symptoms of DHI toxicity in Sf9 cells included DNA polymerization, protein crosslinking, and lysis. Taken together, these data showed that proPO activation and DHI production is strongly toxic against various pathogens but can also damage host tissues and cells if not properly controlled.

Recognition of microbial molecular patterns and stimulation of prophenoloxidase activation by a β-1,3-glucanase-related protein in Manduca sexta larval plasma

Detection of pathogenic invaders is the essential first step of a successful defense response in multicellular organisms. In this study, we have identified a new member of the β-1,3-glucanase-related protein superfamily from the tobacco hornworm Manduca sexta. This protein, designated microbe binding protein (MBP), is 61% identical in sequence to Bombyx mori Gram-negative bacteria binding protein, but only 34-36% identical to M. sexta β-1,3-glucan recognition protein-1 and 2. Its mRNA levels were strongly up-regulated in hemocytes and fat body of immune challenged larvae, along with an increase in concentration of the plasma protein. We expressed M. sexta MBP in a baculovirus-insect cell system. The purified protein associated with intact bacteria and fungi. It specifically bound to lipoteichoic acid, lipopolysaccharide, diaminopimelic acid-type peptidoglycans (DAP-PGs) from Escherichia coli and Bacillus subtilis, but less so to laminarin or Lys-type PG from Staphylococcus aureus. The complex binding pattern was influenced by other plasma factors and additional microbial surface molecules. After different amounts of MBP had been incubated with larval plasma on ice, a concentration-dependent increase in phenoloxidase (PO) activity occurred in the absence of any microbial elicitor. The activity increase was also observed in the mixture of plasma and a bacterial or fungal cell wall component. The prophenoloxidase (proPO) activation became more prominent when DAP-PGs, Micrococcus luteus Lys-PG, or lipoteichoic acid was included in the mixture of MBP and plasma. Statistic analysis suggested that a synergistic enhancement of proPO activation was caused by an interaction between MBP and these elicitors, but not S. aureus Lys-PG, lipopolysaccharide, curdlan, or laminarin. These data indicate that M. sexta MBP is a component of the surveillance mechanism and, by working together with other pattern recognition molecules and serine proteinases, triggers the proPO activation system.

Venom proteins from endoparasitoid wasps and their role in host-parasite interactions

Endoparasitoids introduce a variety of factors into their host during oviposition to ensure successful parasitism. These include ovarian and venom fluids that may be accompanied by viruses and virus-like particles. An overwhelming number of venom components are enzymes with similarities to insect metabolic enzymes, suggesting their recruitment for expression in venom glands with modified functions. Other components include protease inhibitors, paralytic factors, and constituents that facilitate/enhance entry and expression of genes from symbiotic viruses or virus-like particles. In addition, the venom gland may itself support replication/production of some viruses or virus-like entities. Overlapping functions and structural similarities of some venom, ovarian, and virus-encoded proteins suggest coevolution of molecules recruited by endoparasitoids to maintain their fitness relative to their host.

The role of melanization and cytotoxic by-products in the cellular immune responses of Drosophila against parasitic wasps

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

The insect immune response and other putative defenses as effective predictors of parasitism

Parasitic wasps and flies (parasitoids) exert high mortality on caterpillars, and previous studies have demonstrated that most primary and secondary defenses do not protect caterpillars against parasitoids. We investigated the efficacy of tertiary defenses (i.e., immune responses) against parasitoids. Using a bead injection technique to measure the immune response and a 15-year database to measure parasitism, we compared the immune response for 16 species of caterpillars in nine different families. We found that caterpillar species with a strong immune response had the lowest incidence of parasitism, and when statistically compared to other defensive traits, the immune response was the best predictor of parasitism. Parasitoids either avoid attacking caterpillar species with a capacity for high levels of melanization or are killed once they have parasitized. In either case, the immune response is clearly one of the most effective defenses that caterpillars have against parasitism, and elucidating consistent predictors of variation in encapsulation could improve understanding of parasitism patterns in time and space and could enhance biological control efforts.

A serpin from the parasitoid wasp Leptopilina boulardi targets the Drosophila phenoloxidase cascade

The insect phenoloxidase (PO) cascade is known to be tightly regulated by serine proteases and serine protease inhibitors of the serpin family. As a key component of the insect immune system, it is also suspected to be inhibited by several endoparasitoid wasps, insects that develop inside other arthropods as hosts. However, the underlying mechanisms of this inhibition are largely undescribed. Here, we report the characterization of a gene encoding a serpin, LbSPNy, highly expressed in the venom of the wasp Leptopilina boulardi (IS(y) type), and we show that either the venom or the recombinant LbSPNy inhibit the PO cascade in the hemolymph of Drosophila yakuba host larva. Altogether, our results identify the first serpin used as a virulence factor by a parasitoid wasp and show that it disrupts the activation pathway of the PO in the Drosophila host.

Virulence strategies in parasitoid Hymenoptera as an example of adaptive diversity

Parasitoids are mostly insects that develop at the expense of other arthropods, which will die as a result of the interaction. Their reproductive success thus totally depends on their ability to successfully infest their host whose reproductive success relies on its own ability to avoid or overcome parasitism. Such intense selective pressures have resulted in extremely diverse adaptations in parasitoid strategies that ensure parasitism success. For instance, wasp-specific viruses (polydnaviruses) are injected into the host by parasitoid females to modulate its physiology and immunity. This article synthesizes available physiological and molecular data on parasitoid virulence strategies and discusses the evolutionary processes at work.