Cuticular pro-phenoloxidase of the silkworm, Bombyx mori. Purification and demonstration of its transport from hemolymph

Lack of signal peptide in insect prophenoloxidase to avoid glycosylation to damage the zymogen activity

Insect prophenoloxidases (PPOs) are important immunity proteins for defending against the invading pathogens and parasites. As a Type-Ⅲ copper-containing proteins, unlike Homo sapiens tyrosinases, the insect PPOs and most bacterial tyrosinases contain no signal peptides for unknown reason, however they can still be released. To this end, we fused different signal peptides to Drosophila melanogaster PPOs for in vitro and in vivo expression, respectively. We demonstrate that an artificial signal peptide can help PPO secretion in vitro. The secreted PPO appeared larger than wild-type PPO on molecular weight sizes due to glycosylation when expressed in S2 cells. Two asparagine residues for potential glycosylation in PPO1 were identified when a signal peptide was fused. After purification, the glycosylated PPO1 lost zymogen activity. When PPO1 containing a signal peptide was over-expressed in Drosophila larvae, the glycosylation and secretion of PPO1 was detected in vivo. Unlike insect PPO, human tyrosinase needs a signal peptide for protein expression and maintaining enzyme activity. An artificial signal peptide fused to bacterial tyrosinase had no influence on the protein expression and enzyme activity. These Type-Ⅲ copper-containing proteins from different organisms may evolve to perform their specific functions. Intriguingly, our study revealed that the addition of calcium inhibits PPO secretion from the transiently cultured larval hindguts in vitro, indicating that the calcium concentration may regulate PPO secretion. Taken together, insect PPOs can maintain enzyme activities without any signal peptide.

Biochemical and toxicological characteristics of polyphenol oxidase from red palm weevil Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae)

Red palm weevil (RPW) Rhynchophorus ferrugineus is the most destructive insect pests of numerous palm species in the world. The introduction of botanical extract(s) as integral part of an integrated pest management (IPM) programs against RPW will reduce the use of chemical insecticides. Polyphenol oxidase (PPO) is one of the RPW innate immune mechanisms and inhibition of such enzyme could result in a disorder of the insect's immune system. A one single PO isoenzyme has been purified from the hemolymph of the 12th instar larvae of RPW. Using L-DOPA as substrate, R. ferrugineus PPO exhibited specific activity 428 Units/mg proteins with 8.3-fold purification, optimum pH and temperature for activity at 7.5 and 40 °C, respectively and is enhanced by Cu2+ with 1.76-fold. The rank order for oxidizing R. ferrugineus PPO different substrates is catechol > pyrogallol > L-DOPA > pyrocatechuic acid and not tyrosine. The kinetic parameters Km, Vmax and Vmax/Km for L-DOPA are 3.3 mM, 1.3 μmol/ml/min, and 0.39, respectively. The catalytic efficiency of the enzyme towards catechol is 5.3-fold higher than that for L-DOPA. The enzyme completely inhibited by thiourea, ascorbic acid, dithiothreitol, and SDS. R. ferrugineus PPO is a catechol oxidase di-phenol: O2 oxidoreductase. Based on the toxicological studies of various botanical extracts, the IC50 ranged from 20 to 90 mg/ml. The enzyme completely inhibited by 50 mg/ml Cinnamomum camphora. Gallic acid, the major phenolic compound, has IC50 0.8 mM and competitively inhibited the enzyme with Ki 0.54 mM. C. camphora could be a useful natural RPW-controlling agent and used as integral part in IPM programs. This interpretation can be validated in future through an in vivo investigation.

A novel exoskeletal-derived C-type lectin facilitates phagocytosis of hemocytes in the oriental river prawn Macrobrachium nipponense

C-type lectins (CTLs) execute critical functions in multiple immune responses of crustaceans as a member of pattern recognition receptors (PRRs) family. In this study, a novel CTL was identified from the exoskeleton of the oriental river prawn Macrobrachium nipponense (MnLec3). The full-length cDNA of MnLec3 was 1150 bp with an open reading frame of 723 bp, encoding 240 amino acids. MnLec3 protein contained a signal peptide and one single carbohydrate-recognition domain (CRD). MnLec3 transcripts were widely distributed at the exoskeleton all over the body. Significant up-regulation of MnLec3 in exoskeleton after Aeromonas hydrophila challenged suggested the involvement of MnLec3 as well as the possible function of the exoskeleton in immune response. In vitro tests with recombinant MnLec3 protein (rMnLec3) manifested that it had polysaccharide binding activity, a wide spectrum of bacterial binding activity and agglutination activity only for tested Gram-negative bacteria (Escherichia coli, Vibrio anguillarum and A. hydrophila). Moreover, rMnLec3 significantly promoted phagocytic ability of hemocytes against A. hydrophila in vivo. What's more, MnLec3 interference remarkably impaired the survivability of the prawns when infected with A. hydrophila. Collectively, these results ascertained that MnLec3 derived from exoskeleton took an essential part in immune defense of the prawns against invading bacteria as a PRR.

Multicopper oxidase-2 mediated cuticle formation: Its contribution to evolution and success of insects as terrestrial organisms

The insect cuticle is a non-cellular matrix composed of polysaccharide chitins and proteins. The cuticle covers most of the body surface, including the trachea, foregut, and hindgut, and it is the body structure that separates the intraluminal environment from the external environment. The cuticle is essential to sustain their lives, both as a physical barrier to maintain homeostasis and as an exoskeleton that mechanically supports body shape and movement. Previously, we proposed a theory about the possibility that the cuticle-forming system contributes to the "evolution and success of insects." The main points of our theory are that 1) insects evolved an insect-specific system of cuticle formation and 2) the presence of this system may have provided insects with a competitive advantage in the early land ecosystems. The key to this theory is that insects utilize molecular oxygen abundant in the atmosphere, which differs from closely related crustaceans that form their cuticles with calcium ions. With newly obtained knowledge, this review revisits the significance of the insect-specific system for insects to adapt to terrestrial environments and also discusses the long-standing question in entomology as to why, despite their great success in terrestrial environments, they poorly adapt to marine environments.

Detection and substrate portrayal on the serum phenoloxidase activity from the grub of rhinoceros beetle, Oryctes rhinoceros

Phenoloxidase (PO) is a significant biomolecule involved in humoral defence mechanism of invertebrates. Spontaneous melanization of insect haemolymph is the major hinderance for studying PO activity, as haemolymph was collected devoid of phenylthiourea. In the study, no visible melanization was observed in crude serum from the grub of Oryctes rhinoceros up to 30 min of incubation amongst crude haemolymph, diluted haemolymph, crude serum and diluted serum that were subjected to visual observation for spontaneous melanization reaction. Accordingly, crude serum was taken for evaluating PO activity. At the same time, as PO substrates tend to auto-oxidize and provide false optical density value, tris-buffered saline devoid of any substrates were used as blank for PO assays. The ideal wavelength at which maximum PO activity occurred for each substrate, namely, tyrosine, tyramine, dopamine, L-dopa, DL-dopa, catechol, protocatechuic acid and pyrogallol was determined as 407, 410, 429, 465, 403, 466, 428 and 400 nm, respectively. Additionally, time course of oxidation for each phenolic substrate by the serum PO were examined and DL-dopa was identified as the specific substrate for serum PO in the grub of O. rhinoceros. Furthermore, maximum PO activity was observed at 5 min of incubation for 10 mM of DL-dopa that was considered as optimum concentration. The ideal pH and temperature for serum PO activity was observed as 7.5 and 20°C, respectively. These results suggested that standardizing a suitable substrate is an essential prerequisite to evaluate the real PO activity of serum which might significantly fluctuate in each insect model.

Insect phenoloxidase and its diverse roles: melanogenesis and beyond

Insect life on earth is greatly diversified despite being exposed to several infectious agents due to their diverse habitats and ecological niche. One of the major factors responsible for their successful establishment is having a powerful innate immune system. The most common and effective method used by insects in recognizing pathogen and non-self-substances is the melanization process among others. The key enzyme involved in melanin biosynthesis is the copper containing humoral defense enzyme, phenoloxidase (PO). This review focused on understanding about PO and that had been in research for nearly a century. The review elaborates about evolutionary significance of PO in arthropods, its relationship with mammalian tyrosinases, various substrates, activators and inhibitors involved in the activation of phenoloxidase cascade, as it requires an integrated system of activation that vary among insect species. The enzyme also plays a vital role in insect immunity by involving in several other immune functions like sclerotization, wound healing, opsonization, encapsulation and nodule formation. Further, gene knock down or knock out of PO genes and inhibition of PO-melanization cascade by several mechanisms can also be considered as promising future alternative to control serious pests by making them highly susceptible to any targeted attack.

Prophenoloxidase of Odontotermes formosanus (Shiraki) (Blattodea: Termitidae) Is a Key Gene in Melanization and Has a Defensive Role during Bacterial Infection

Melanization mediated by the prophenoloxidase (PPO)-activating system is an important innate immunity to fight pathogens in insects. In this study, the in vitro time-dependent increase in the intensity of melanization and phenoloxidase (PO) activity from the hemolymph of Odontotermes formosanus (Shiraki) challenged by pathogenic bacteria was detected. PPO is one of the key genes in melanization pathway, whereas the molecular characteristics and functions of O. formosanus PPO are unclear. The OfPPO gene was cloned and characterized. The open reading frame of OfPPO is 2085 bp in length and encodes a 79.497 kDa protein with 694 amino acids. A BLASTx search and phylogenetic analyses revealed that OfPPO shares a high degree of homology to the Blattodea PPOs. Moreover, real-time fluorescent quantitative PCR analysis showed that OfPPO is ubiquitously expressed in all castes and tissues examined, with the highest expression in workers and variable expression patterns in tissues of different termite castes. Furthermore, the expression of OfPPO was significantly induced in O. formosanus infected by pathogenic bacteria. Intriguingly, in combination with silencing of OfPPO expression, pathogenic bacteria challenge caused greatly increased mortality of O. formosanus. These results suggest that OfPPO plays a role in defense against bacteria and highlight the novel termite control strategy combining pathogenic bacteria application with termite PPO silencing.

Crucial roles of a novel exoskeletal-derived lectin in innate immunity of the oriental river prawn, Macrobrachium nipponense

As important pattern recognition receptors (PRRs), C-type lectins play crucial roles in the crustacean innate immune system. In this study, a novel C-type lectin, designated as MnLec1, was obtained from the exoskeleton of the oriental river prawn Macrobrachium nipponense for the first time. The full-length cDNA of MnLec1 was 1329 bp with an open reading frame of 774 bp. The predicted MnLec1 protein contains a single carbohydrate-recognition domain with an EPN/LND motif and one Ca²⁺ binding site-2. MnLec1 transcripts were widely detected in the tested tissues of M. nipponense and significantly up-regulated after Aeromonas hydrophila challenge. The recombinant MnLec1 protein was found to have a wide spectrum of binding activities towards various microorganisms, agglutinate two kinds of Gram-negative bacteria (Escherichia coli and A. hydrophila) in a Ca²⁺ -independent manner. What's more, the survivability of prawns was significantly down-regulated after RNAi of MnLec1 when infected with A. hydrophila. Collectively, these findings suggest that MnLec1 from the exoskeleton might function as a PRR and play a crucial role in immune defense against invading pathogens in M. nipponense.

Disruption of insect immunity using analogs of the pleiotropic insect peptide hormone Neb-colloostatin: a nanotech approach for pest control II

This work continues our studies on the pleiotropic activity of the insect peptide Neb-colloostatin in insects. In vivo immunological bioassays demonstrated that hemocytotoxic analogs of Neb-colloostatin injected into Tenebrio molitor significantly reduced the number of hemocytes in the hemolymph and impaired phagocytosis, nodulation and phenoloxidase activities in the insects. Among the analogs tested, [Ala¹]-,[Val¹]-, [Hyp⁴]- and [Ach⁴]-colloostatin were particularly potent in disrupting cellular immunity in larvae, pupae and adult insects. This result suggests that the most effective analogs showed increases in the bioactivity period in the hemolymph of insects when compared to Neb-colloostatin. Recently, we demonstrated that it is possible to introduce Neb-colloostatin through the cuticle of an insect into the hemolymph when the peptide is coupled with nanodiamonds. In this study, we showed that [Ala¹]-, [Val¹]-, [Hyp⁴]- and [Ach⁴]-colloostatin, when complexed with nanodiamonds, may also pass through the cuticle into the hemolymph and induce long-term impairments of immunity in T. molitor at all developmental stages. Studies on the tissue selectivity and effectiveness of Neb-colloostatin analogs and efficient methods for their introduction into insects may contribute to the development of eco-friendly pest control methods based on bioactive peptidomimetics.

Lepidopteran wing scales contain abundant cross-linked film-forming histidine-rich cuticular proteins

Scales are symbolic characteristic of Lepidoptera; however, nothing is known about the contribution of cuticular proteins (CPs) to the complex patterning of lepidopteran scales. This is because scales are resistant to solubilization, thus hindering molecular studies. Here we succeeded in dissolving developing wing scales from Bombyx mori, allowing analysis of their protein composition. We identified a distinctive class of histidine rich (His-rich) CPs (6%-45%) from developing lepidopteran scales by LC-MS/MS. Functional studies using RNAi revealed CPs with different histidine content play distinct and critical roles in constructing the microstructure of the scale surface. Moreover, we successfully synthesized films in vitro by crosslinking a 45% His-rich CP (BmorCPR152) with laccase2 using N-acetyl- dopamine or N-β-alanyl-dopamine as the substrate. This molecular study of scales provides fundamental information about how such a fine microstructure is constructed and insights into the potential application of CPs as new biomaterials.

Mini-review an insect-specific system for terrestrialization: Laccase-mediated cuticle formation

Insects are often regarded as the most successful group of animals in the terrestrial environment. Their success can be represented by their huge biomass and large impact on ecosystems. Among the factors suggested to be responsible for their success, we focus on the possibility that the cuticle might have affected the process of insects' evolution. The cuticle of insects, like that of other arthropods, is composed mainly of chitin and structural cuticle proteins. However, insects seem to have evolved a specific system for cuticle formation. Oxidation reaction of catecholamines catalyzed by a copper enzyme, laccase, is the key step in the metabolic pathway for hardening of the insect cuticle. Molecular phylogenetic analysis indicates that laccase functioning in cuticle sclerotization has evolved only in insects. In this review, we discuss a theory on how the insect-specific "laccase" function has been advantageous for establishing their current ecological position as terrestrial animals.

Purification and characterization of phenoloxidase from the hemolymph of healthy and diseased Antheraea assamensis Helfer (Lepidoptera: Saturniidae): Effects of certain biological components and chemical agents on enzyme activity

In the current study, a dimeric phenoloxidase (PO) from the hemolymph of healthy and diseased (pebrine infected) larvae of Antheraea assamensis Helfer was extracted and purified. The protein was subjected to purification using Sephacryl S-100 and CM Sepharose chromatography. The enzyme comprised of two subunits of ~76.8 and 76 kDa that showed PO activity in 6 mM l-3,4-dihydroxyphenylalanine (L-DOPA) and 8 mM catechol but not in hydroquinone. Optimum temperature for PO activity was 30°C in l-DOPA and 37°C in catechol. Optimum pH ranged from 6.8 to 7.0 in L-DOPA and 7.0-7.2 in catechol. Specific activity of the purified PO from healthy larvae was 53.9 µM/min per mg of protein per ml in L-DOPA and 50.77 µM/min per mg of protein per ml in catechol. Specific activity of PO from diseased larvae was 30.0 µM/min per mg of protein per ml in L-DOPA and 28.55 µM/min per mg of protein per ml in catechol. Purification fold was 3.27-4.21 for healthy and 2.38-2.56 for diseased fractions. The enzyme showed the Michaelis constant (K_m ) of 2.46-2.85 mM for healthy and diseased fractions in L-DOPA. In catechol K_m of 9.23-17.71 mM was observed. Peptidoglycan was the best activator of purified PO from both healthy and diseased fractions. Interactions between controls and activators appeared statistically significant (F = 767.5; df = 3; P < 0.0001). Na⁺ , K⁺ , and Cu²⁺ increased, whereas Ca²⁺ , Zn²⁺ , Mg²⁺ , and Co²⁺ decreased PO activity. The overall interactions appeared highly significant (F = 217.0; df = 27; P < 0.0001). Kojic acid, dithiothreitol, thiourea, phenylthiourea, carbendazim, N-bromosuccinimide, N,N,N',N'-tetraacetic acid, and diethyldithiocarbamate inhibited PO activity.

Dual role of the serine protease homolog BmSPH-1 in the development and immunity of the silkworm Bombyx mori

Serine proteases and serine protease homologs are involved in the prophenoloxidase (proPO)-activating system leading to melanization. The Bombyx mori serine protease homolog BmSPH-1 regulates nodule melanization. Here, we show the dual role of BmSPH-1 in the development and immunity of B. mori. BmSPH-1 was expressed in hemocytes after molting and during the larval-pupal transformation in normal development. In contrast, following infection, BmSPH-1 was expressed in hemocytes and cleaved in the hemolymph, which resulted in the induction of PO activity. Moreover, BmSPH-1 was cleaved in the cuticle during the larval-pupal transformation and early pupal stages. In BmSPH-1 RNAi-treated silkworms, the reduced BmSPH-1 mRNA levels during the spinning stage or the prepupal stage resulted in the arrest of pupation or pupal cuticular melanization, respectively. The binding assays revealed that BmSPH-1 interacts with B. mori immulectin, proPO, and proPO-activating enzyme. Our findings demonstrate that BmSPH-1 paticipates larval-pupal transformation, pupal cuticular melanization and innate immunity of silkworms, illustrating the dual role of BmSPH-1 in development and immunity.

Innate immune responses in the Chinese oak silkworm, Antheraea pernyi

Innate immunity, the evolutionarily conserved defense system, has been extensively analyzed in insect models over recent decades. The significant progress in this area has formed our dominant conceptual framework of the innate immune system, but critical advances in other insects have had a profound impact on our insights into the mystery of innate immunity. In recent years, we focused on the immune responses in Antheraea pernyi, an important commercial silkworm species reared in China. Here, we review the immune responses of A. pernyi based on immune-related gene-encoded proteins that are divided into five categories, namely pattern recognition receptors, hemolymph proteinases and their inhibitors, prophenoloxidase, Toll pathway factors and antimicrobial peptides, and others. Although the summarized information is limited since the research on A. pernyi immunity is in its infancy, we hope to provide evidence for further exploration of innate immune mechanisms.

Chitin synthase 1 gene is crucial to antifungal host defense of the model beetle, Tribolium castaneum

The importance of the insect cuticle as a primary protective barrier against entomopathogens has long been noted. In the present study, we addressed this issue by utilizing an experimental infection system composed of the model beetle T. castaneum and two entomopathogenic fungal species, Beauveria bassiana and Metarhizium anisopliae. The pupae were relatively susceptible to these fungi by the natural route of infection, with some refractoriness developed with age, while the adults exhibited much higher refractoriness. Whereas M. anisopliae exhibited seemingly higher infectivity to the pupae compared to B. bassiana when the natural conidium infection was employed, direct inoculation of cultured hyphal body cells into the hemocoel was found highly and equally virulent in the pupae for the both fungal species. These results collectively suggest an important role of the cuticular integument in antifungal host defense, and we subsequently conducted the knockdown of chitin synthase 1 gene (CHS1). We targeted the prepupal and mid-pupal peaks of its expression respectively by using injection of the dsRNA at very low dosages to avoid lethality. The resulting pupae looked normal, but the adults showed a mild phenotype with dimpled/wrinkled elytra. The CHS1 gene knockdown compromised significantly host defense against the fungal infection via the natural route, except the configuration of knockdown pupae and M. anisopliae, suggesting an indispensable role of CHS1.

The structure of a prophenoloxidase (PPO) from Anopheles gambiae provides new insights into the mechanism of PPO activation

BACKGROUND

Phenoloxidase (PO)-catalyzed melanization is a universal defense mechanism of insects against pathogenic and parasitic infections. In mosquitos such as Anopheles gambiae, melanotic encapsulation is a resistance mechanism against certain parasites that cause malaria and filariasis. PO is initially synthesized by hemocytes and released into hemolymph as inactive prophenoloxidase (PPO), which is activated by a serine protease cascade upon recognition of foreign invaders. The mechanisms of PPO activation and PO catalysis have been elusive.

RESULTS

Herein, we report the crystal structure of PPO8 from A. gambiae at 2.6 Å resolution. PPO8 forms a homodimer with each subunit displaying a classical type III di-copper active center. Our molecular docking and mutagenesis studies revealed a new substrate-binding site with Glu364 as the catalytic residue responsible for the deprotonation of mono- and di-phenolic substrates. Mutation of Glu364 severely impaired both the monophenol hydroxylase and diphenoloxidase activities of AgPPO8. Our data suggested that the newly identified substrate-binding pocket is the actual site for catalysis, and PPO activation could be achieved without withdrawing the conserved phenylalanine residue that was previously deemed as the substrate 'placeholder'.

CONCLUSIONS

We present the structural and functional data from a mosquito PPO. Our results revealed a novel substrate-binding site with Glu364 identified as the key catalytic residue for PO enzymatic activities. Our data offered a new model for PPO activation at the molecular level, which differs from the canonical mechanism that demands withdrawing a blocking phenylalanine residue from the previously deemed substrate-binding site. This study provides new insights into the mechanisms of PPO activation and enzymatic catalysis of PO.

Proteomic Analysis of Silkworm Antennae

The silkworm Bombyx mori is an oligophagous insect that feeds mainly on mulberry leaves. The olfactory system of silkworm is a good model to study olfaction in Lepidoptera. Here, we carried out shotgun proteomic analysis and MS sequencing of the silkmoth antennae. A total of 364 proteins were detected, 77 were female specific, 143 were male specific, and 144 were expressed in both male and female antennae. Five odorant-binding proteins, two chemosensory proteins, and one olfactory receptor were identified. They may play a major role in the perception of odorants. An esterase and an aldehyde dehydrogenase were found only in male antennae. Glutathione S-transferases (GSTs) and cytochrome P450s, also found in silkworm antennae, may be involved in the degradation of xenobiotics. Additionally, antioxidation proteins and immunity proteins were identified. Juvenile hormone binding proteins (JHBP), juvenile hormone resistance protein II, and juvenile hormone episode hydrolase (JHEH) were found in the proteomic analysis, which suggests that the antennae are a target for juvenile hormone in the silkworm. Our results provide insight into the expression of proteins in the antennae of silkworm and will facilitate the future functional analysis of silkworm antennae.

The Involvement of Hemocyte Prophenoloxidase in the Shell-Hardening Process of the Blue Crab, Callinectes sapidus

Cuticular structures of arthropods undergo dramatic molt-related changes from being soft to becoming hard. The shell-hardening process of decapod crustaceans includes sclerotization and mineralization. Hemocyte PPO plays a central role in melanization and sclerotization particularly in wound healing in crustaceans. However, little is known about its role in the crustacean initial shell-hardening process. The earlier findings of the aggregation of heavily granulated hemocytes beneath the hypodermis during ecdysis imply that the hemocytes may be involved in the shell-hardening process. In order to determine if hemocytes and hemocyte PPO have a role in the shell-hardening of crustaceans, a knockdown study using specific CasPPO-hemo-dsRNA was carried out with juvenile blue crabs, Callinectes sapidus. Multiple injections of CasPPO-hemo-dsRNA reduce specifically the levels of CasPPO-hemo expression by 57% and PO activity by 54% in hemocyte lysate at the postmolt, while they have no effect on the total hemocyte numbers. Immunocytochemistry and flow cytometry analysis using a specific antiserum generated against CasPPO show granulocytes, semigranulocytes and hyaline cells as the cellular sources for PPO at the postmolt. Interestingly, the type of hemocytes, as the cellular sources of PPO, varies by molt stage. The granulocytes always contain PPO throughout the molt cycle. However, semigranulocytes and hyaline cells become CasPPO immune-positive only at early premolt and postmolt, indicating that PPO expression in these cells may be involved in the shell-hardening process of C. sapidus.

Antimicrobial activity in the cuticle of the American lobster, Homarus americanus

American lobster, Homarus americanus, continues to be an ecologically and socioeconomically important species despite a severe decline in catches from Southern New England and Long Island Sound (USA) and a high prevalence of epizootic shell disease in these populations. A better understanding of lobster immune defenses remains necessary. Cuticle material collected from Long Island Sound lobsters was found to be active against a broad spectrum of bacteria, including Gram-negative and -positive species. The antimicrobial activity was characterized by boiling, muffling, and size fractioning. Boiling did not significantly reduce activity, while muffling did have a significant effect, suggesting that the active component is organic and heat stable. Size fractioning with 3 and 10 kDa filters did not significantly affect activity. Fast protein liquid chromatography fractions were also tested for antimicrobial activity, and fractions exhibiting protein peaks remained active. MALDI mass spectrometry revealed peptide peaks at 1.6, 2.8, 4.6, and 5.6 kDa. The data presented suggest that one or several antimicrobial peptides contribute to antimicrobial activity present in the American lobster cuticle.

Cloning, expression, and characterization of prophenoloxidase from Antheraea pernyi

Prophenoloxidase (PPO) is an essential enzyme in insect innate immunity because of its role in humoral defense. In this study, we have cloned a full-length cDNA of Antheraea pernyi prophenoloxidase (ApPPO) with an open-reading frame encoding 683 amino acids, and the deduced amino acid sequence of ApPPO exhibited a high similarity with those of lepidoptera. The expression of ApPPO was inducible so that the mRNA level was significantly upregulated in the microbial challenged tissues, including fat body, hemocytes, and midgut. To better investigate the enzymatic and immunological properties of ApPPO, recombinant ApPPO (rApPPO) was produced in Escherichia coli. Several functional verification experiments were performed after studying the enzymatic properties. It was found that rApPPO could be stimulated by the microbial challenged larvae hemolymph and then killed bacteria in the radial diffusion assay. Furthermore, rApPPO also induced the transcription of cecropins after injected into the larvae 24 h later.

Re-examination of a α-chymotrypsin-solubilized laccase in the pupal cuticle of the silkworm, Bombyx mori: Insights into the regulation system for laccase activation during the ecdysis process

The laccase in the pupal cuticle of the silkworm, Bombyx mori, is thought to accumulate as an inactive precursor that can be activated stage-dependently. In this study we isolated an 81-kDa laccase from cuticular extract of B. mori that was prepared by digestion of the pupal cuticles with α-chymotrypsin. The mass spectrometric analysis of the purified protein indicates that this 81-kDa laccase is a product of the Bombyx laccase2 gene. The purified 81-kDa laccase (α-chymotrypsin-solubilized Bombyx laccase2: Bm-clac2) has an N-terminal sequence of RNPADS that corresponds to Arg146 to Ser151 of the deduced protein sequence of Bmlaccase2 cDNA, indicating that Bm-clac2 lacks the N-terminal part upstream from residue Arg146. Bm-clac2 shows enzymatic activity, but its specific activity is increased around 17-fold after treatment with trypsin, which involves cleavage of peptide bonds at the C-terminal region. We also found that the activity of Bm-clac2 is increased in the presence of isopropanol. In previous reports, proteolytic processing has been hypothesized as a system for laccase activation in vivo, but the present result implies that this type of processing is not the only way to convert Bm-clac2 to the high-activity enzyme.

Melanization reaction products of shrimp display antimicrobial properties against their major bacterial and fungal pathogens

Melanization is a rapid defense mechanism in invertebrates. The substrate specificity of phenoloxidases (POs) and the role of melanization reaction products were investigated in the black tiger shrimp, Penaeus monodon. Two PmPOs (PmproPO1 and PmproPO2) were found to display a substrate specificity towards monophenols and diphenols, and exhibit relatively weak activity against 5,6-dihydroxyindole (DHI). Systemic infection of the PmproPO1/2 co-silenced shrimp with the fungus, Fusarium solani, led to a significantly increased mortality, suggesting an important role of PmproPOs in shrimp's defense against fungal infection. Using L-DOPA, dopamine or DHI as a substrate, the melanization reaction products exhibited in vitro antimicrobial activities towards Gram-negative bacteria (Vibrio harveyi and Vibrioparahaemolyticus) and Gram-positive bacteria (Bacillus subtilis), whereas the lower effect was detected against the fungus (F. solani). SEM analysis revealed the morphological changes and damage of cell membranes of V. harveyi and F. solani after treatment with shrimp melanization reaction products. Together, these findings demonstrate the crucial functions of the proPO system and the importance of melanization reaction products in the shrimp's immune defense.

Origins and activation of prophenoloxidases in the digestive tract of the cricket, Gryllus bimaculatus

The function of Phenoloxidases (POs) in sclerotization and defense in insects is well understood, but little is known concerning their occurrence, origins, and function in the digestive tract. In Gyrllus bimaculatus gut all of the PO activity is found in the lumen of the digestive tract, and no detectible activity is found in homogenates of the gut epithelium or secretions from incubated epithelial tissues. Prophenoloxidases (PPOs) are synthesized in the hemocytes of  Bombyx mori and are transported into the cuticle. It is suggested that the PPOs in the caecal lumen of G. bimaculatus likewise are synthesized in hemocytes and are transported by unknown means into the caecal lumen, where they are activated to POs by trypsin. Peristalsis transports the POs both forward into the crop and posterior within the peritrophic membrane into the hind gut. The PPOs in the hemolymph consist of a trimer (270-280 kDa) and a tetramer (340-370 kDa). The active POs in the gut lumen consist of a monomer (85-95 kDa) in addition to an activated trimer and tetramer.

Thermoregulatory strategy may shape immune investment in Drosophila melanogaster

As temperatures change, insects alter the amount of melanin in their cuticle to improve thermoregulation. However, melanin is also central to insect immunity, suggesting that thermoregulatory strategy may indirectly impact immune defense by altering the abundance of melanin pathway components (a hypothesis we refer to as thermoregulatory-dependent immune investment). This may be the case in the cricket Allonemobius socius, where warm environments (both seasonal and geographical) produced crickets with lighter cuticles and increased pathogen susceptibility. Unfortunately, the potential for thermoregulatory strategy to influence insect immunity has not been widely explored. Here we address the relationships between temperature, thermoregulatory strategy and immunity in the fruit fly Drosophila melanogaster. To this end, flies from two separate Canadian populations were reared in either a summer- or autumn-like environment. Shortly after adult eclosion, flies were moved to a common environment where their cuticle color and susceptibility to a bacterial pathogen (Pseudomonas aeruginosa) were measured. As with A. socius, individuals from summer-like environments exhibited lighter cuticles and increased pathogen susceptibility, suggesting that the thermoregulatory-immunity relationship is evolutionarily conserved across the hemimetabolous and holometabolous clades. If global temperatures continue to rise as expected, then thermoregulation might play an important role in host infection and mortality rates in systems that provide critical ecosystem services (e.g. pollination), or influence the prevalence of insect-vectored disease (e.g. malaria).

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

Factors functioning in nodule melanization of insects and their mechanisms of accumulation in nodules

Nodules consisting of hemocytes and trapped microorganisms are important targets for melanization, which is best known in the insect immune system. We investigated factors functioning in nodule melanization and the mechanism by which these factors congregate in the nodule. BmHP21, BmSPH1 and BmSPH2, Bombyx mori orthologs of Manduca sexta serine protease HP21, serine protease homologs (SPH1 and SPH2), and a prophenoloxidase, BmPO1 were observed as inactive forms in the plasma, but as putatively active forms in the nodule. Production of prophenoloxidase-activating proteinases, BmPAP1 and BmPAP3/PPAE and BmPO1 were confirmed in hemocytes. BmSPH1 and BmSPH2 were observed on trapped bacterial cells in the nodule and were isolated from the surface of bacterial cells incubated with plasma. BmSPH1 and BmSPH2 were found in plasma in complex with a pattern recognition receptor, BmLBP. These data suggest that melanization-regulating factors congregate in nodules through a combination of microorganism-dependent and hemocyte-dependent routes.

Cloning of prophenoloxidase from hemocytes of the blue crab, Callinectes sapidus and its expression and enzyme activity during the molt cycle

The arthropods cuticle undergoes dramatic morphological and biochemical changes from being soft to hardness through each molting process. Prophenoloxidase (PPO) known as a key enzyme in the arthropod innate immune system involved in the melanization reaction, has been related with the initial shell-hardening process, specifically in the sclerotization of the protein matrix in the new cuticle. Since hemocytes have been reported as the main PPO source in arthropods, the transport of hemocyte PPO into the newly laid, soft cuticle has been proposed for shell-hardening occurring during and immediately after ecdysis. In order to define the role of hemocyte PPO in the shell-hardening of crustaceans, the full-length cDNA sequence (2806 nt) of hemocytes PPO of the blue crab Callinectes sapidus (CasPPO-hemo) is isolated using degenerate PCR and 5'-3' RACE. CasPPO-hemo encodes a putative PPO (672 aa) showing three hemocyanin domains: N, M, and C in order and two copper binding sites (CuA & CuB). The sequence analysis identifies the putative CasPPO-hemo as zymogen which requires the cleavage at the N-terminus for its activation. Hemocyte extract (CasHLS) contains the PO, the activity of which depends on the in vitro activation of trypsin. The expression levels of CasPPO-hemo are kept constant during the molt cycle. The increase in the number of hemocytes at early premolt correlates with the elevated PO activity, while at late premolt, the increment in hemocyte numbers does not reflect on the PO activity. The functional importance of the changes in the levels of CasHLS-PO activity during molt cycle is discussed in relation to cuticle hardening process.

Gene cloning and expression patterns of two prophenoloxidases from Catantops pinguis (Orthoptera: Catantopidae)

In insect, fat body plays major roles in insect innate immunity. Phenoloxidase (PO) is an important component in insect innate immunity and is necessary for acclimatization. In our study, two prophenoloxidase (PPO) subunits were obtained from fat body of Catantops pinguis (Stål). The full-length cDNA sequence of one PPO (CpPPO1) consisted of 2347 bp with an open reading frame (ORF) of 2187 bp encoding 728 amino acids, while the other subunit (CpPPO2) had a full length of 2445 bp, encoding 691 amino acids. Both the PPO gene products are predicted to possess all the structural features of other PPO members, including two putative tyrosinase copper-binding motifs with six highly conserved histidine residues and a thiolester-like motif. Tissue distribution analysis showed that both PPO mRNAs were abundantly expressed in the fat body among 11 tissues examined, and they were transiently up-regulated after Escherichia coli infection, consistent with them being immune-responsive genes. Total levels of CpPPO1 and CpPPO2 mRNA transcripts were much higher in first instar larvae and adults. A much higher transcript level of CpPPO1 was detected in several months, while there were extremely high mRNA expression levels of CpPPO2 in January, July, October, and December. The above results suggested that PPO from fat body might also bring significant function during the processes of development and acclimatization for C. pinguis.

Identification of a cuticle protein with unique repeated motifs in the silkworm, Bombyx mori

The insect cuticle is non-cellular matrix secreted from a monolayer of epidermal cells. After abrasion of the larval cuticle of the silkworm, Bombyx mori, a protein with molecular mass of 135 kDa is newly detected in the cuticle. Mass spectrometric analysis of the tryptic fragments from this protein revealed that the 135-kDa protein is encoded by the Cb10 gene. In the predicted amino acid sequence of Cb10, three repeated motifs with [YxGGFGGppG(L/V)L] sequence are found in the C-terminal region. In addition to the repeated motifs, Cb10 has seventeen CxxxxC motifs randomly distributed throughout the polypeptide chain and serine rich region at the N-terminal region. The Cb10 gene is strongly expressed in epidermal cells after pupal ecdysis, and its expression in the larval epidermal cells is induced not only by cuticular abrasion, but also by bacterial infection. These expression patterns suggest some specific roles of this protein in pupal cuticle formation and defense reactions.

Activation of autophagic programmed cell death and innate immune gene expression reveals immuno-competence of integumental epithelium in Bombyx mori infected by a dipteran parasitoid

In insects, the integument forms the primary barrier between the environment and internal milieu, but cellular and immune responses of the integumental epithelium to infection by micro- and macro-parasites are mostly unknown. We elucidated cellular and immune responses of the epithelium induced through infection by a dipteran endoparasitoid, Exorista bombycis in the economically important silkworm Bombyx mori. Degradative autophagic vacuoles, lamella-like bodies, a network of cytoplasmic channels with cellular cargo, and an RER network that opened to vacuoles were observed sequentially with increase in age after infection. This temporal sequence culminated in apoptosis, accompanied by the upregulation of the caspase gene and fragmentation of DNA. The infection significantly enhanced the tyrosine level and phenol oxidase activity in the integument. Proteomic analysis revealed enhanced expression of innate immunity components of toll and melanization pathways, cytokines, signaling molecules, chaperones, and proteolytic enzymes demonstrating diverse host responses. qPCR analysis revealed the upregulation of spatzle, BmToll, and NF kappa B transcription factors Dorsal and BmRel. NF kappa B inhibitor cactus showed diminished expression when Dorsal and BmRel were upregulated, revealing a negative correlation (R = (-)0.612). During melanization, prophenol oxidase 2 was expressed, a novel finding in integumental epithelium. The integument showed a low level of melanin metabolism and localized melanism in order to prevent the spreading of cytotoxic quinones. The gene-encoding proteolytic enzyme, beta-N-acetylglucosaminidase, was activated at 24 h post-infection, whereas chitinase, was activated at 96 h post-infection; however, most of the immune genes enhanced their expression in the early stages of infection. Thus the integument contributes to humoral immune responses that enhance resistance against macroparasite invasion.

New functions of arthropod bursicon: inducing deposition and thickening of new cuticle and hemocyte granulation in the blue crab, Callinectes sapidus

Arthropod growth requires molt-associated changes in softness and stiffness of the cuticle that protects from desiccation, infection and injury. Cuticle hardening in insects depends on the blood-borne hormone, bursicon (Burs), although it has never been determined in hemolymph. Whilst also having Burs, decapod crustaceans reiterate molting many more times during their longer life span and are encased in a calcified exoskeleton, which after molting undergoes similar initial cuticle hardening processes as in insects. We investigated the role of homologous crustacean Burs in cuticular changes and growth in the blue crab, Callinectes sapidus. We found dramatic increases in size and number of Burs cells during development in paired thoracic ganglion complex (TGC) neurons with pericardial organs (POs) as neurohemal release sites. A skewed expression of Burs β/Burs α mRNA in TGC corresponds to protein contents of identified Burs β homodimer and Burs heterodimer in POs. In hemolymph, Burs is consistently present at ∼21 pM throughout the molt cycle, showing a peak of ∼89 pM at ecdysis. Since initial cuticle hardness determines the degree of molt-associated somatic increment (MSI), we applied recombinant Burs in vitro to cuticle explants of late premolt or early ecdysis. Burs stimulates cuticle thickening and granulation of hemocytes. These findings demonstrate novel cuticle-associated functions of Burs during molting, while the unambiguous and constant presence of Burs in cells and hemolymph throughout the molt cycle and life stages may implicate further functions of its homo- and heterodimer hormone isoforms in immunoprotective defense systems of arthropods.

A novel type of prophenoloxidase from the kuruma prawn Marsupenaeus japonicus contributes to the melanization of plasma in crustaceans

Melanization is one of the major immune responses in arthropods. Prophenoloxidases (proPOs) catalyze the oxidation of mono- or o-diphenols, a reaction that is the key initial step of melanin formation. Well-characterized proPOs from crustaceans are synthesized in haemocytes and are released into plasma in response to microbial attack. However, PO activity does exist in the plasma of haemolymph without pathogenic infections. Here, we demonstrate that a novel type of proPO contributes to such PO activity in the plasma fraction of haemolymph of crustaceans. The novel enzyme, which was purified from the plasma of the kuruma prawn (Marsupenaeus japonicus), possessed strong and specific monophenol and o-diphenol oxidation activity compared with that of known haemocyte-type proPO. Amino acid sequence analyses indicated that this enzyme was distinct from the known proPO. The cDNA sequence and deduced amino acid sequence of this enzyme has a putative binuclear copper center, and showed approximately 30% and 20% identity with the primary structures of reported proPO and haemocyanin sequences of the kuruma prawn, respectively. Reverse transcription PCR analysis showed that this enzyme was synthesized in the hepatopancreas rather than in haemocytes. Although the primary structure and enzymatic properties of this novel enzyme suggested that it is a phenoloxidase, its biogenesis, tissue distribution, and oligomeric state resemble those of haemocyanin, which belongs to the same protein family (type III copper protein). This novel proPO enzyme may share a role with the already characterized version, itself a major component of the innate immune system in crustaceans.

Selectively enhanced expression of prophenoloxidase activating enzyme 1 (PPAE1) at a bacteria clearance site in the white shrimp, Litopenaeus vannamei

BACKGROUND

The prophenoloxidase-activating (PO activating) system plays an important role in the crustacean innate immunity, particularly in wound healing and pathogen defense. A key member of this system is prophenoloxidase-activating enzyme (PPAE), which is the direct activator of prophenoloxidase (proPO). Despite their importance in crustacean PO activating system, the studies on them remain limited.

RESULTS

Here we report on a PPAE of white shrimp, Litopenaeus vannamei (lvPPAE1), which showed 94% similarity to PPAE1 of Penaeus monodon. We found that lvPPAE1 in fluid hemocytes was down regulated after challenge by Vibrio harveyi but was enhanced when shrimps were exposed to a bacteria-rich environment for long-term. In vivo gene silence of lvPPAE1 by RNAi can significantly reduce the phenoloxidase activity (PO) and increase the susceptibility of shrimps to V. harveyi. Although lvPPAE1 was down-regulated in fluid hemocytes by Vibrio challenge, its expression increased significantly in gill after bacteria injection, which is the primary bacteria-clearance tissue.

CONCLUSION

Suppressed expression in fluid hemocytes and enhanced expression in gill indicates selectively enhanced expression at the bacterial clearance site. This is a novel feature for PPAE expression. The results will contribute to our understanding of the PO activating system in crustaceans.

Purification and characterization of phenoloxidase from brine shrimp Artemia sinica

Phenoloxidase from Artemia sinica (AsPO) was purified by Superdex 200 gel-filtration and Q Sepharose fast flow ion-exchange chromatography, and its properties were characterized biochemically and enzymatically by using L-dihydroxyphenylalanine (L-DOPA) as the specific substrate. Results showed that AsPO was isolated as a monomeric protein of 125.5 kDa in molecular mass. The optimal pH value and temperature are 7.0 and 50°C, respectively, for its PO activity. The AsPO had an apparent K(m) value of 4.2 mM on L-DOPA, and 10.9 mM on catechol, respectively. Oxidase inhibitor on PO activity showed that the AsPO was extremely sensitive to ascorbic acid, sodium sulfite, and citric acid; and was very sensitive to cysteine, benzoic acid, and 1-phenyl-2-thiourea. Combined with its specific enzyme activity on L-DOPA and catechol, it can be concluded that AsPO is most probably a typical catechol-type O-diphenoloxidase. Its PO activity was also sensitive to metal ions and chelators, and 20 mM DETC-inhibited PO activity was obviously recovered by 15 mM Cu(2+), indicating that AsPO is most probably a copper-containing metalloenzyme. All these data about specific substrate, sensitivity to oxidase inhibitor metal ions and chelators indicate that the AsPO has the properties of a catechol-type copper-containing O-diphenoloxidase that functions as a vital humoral factor in host defense via melaninization as in other Crustaceans.

Prophenoloxidase activating enzyme-III from giant freshwater prawn Macrobrachium rosenbergii: characterization, expression and specific enzyme activity

The prophenoloxidase activating system is an important innate immune response against microbial infections in invertebrates. The major enzyme, phenoloxidase, is synthesized as an inactive precursor and its activation to an active enzyme is mediated by a cascade of clip domain serine proteinases. In this study, a cDNA encoding a prophenoloxidase activating enzyme-III from the giant freshwater prawn Macrobrachium rosenbergii, designated as MrProAE-III, was identified and characterized. The full-length cDNA contains an open reading frame of 1110 base pair (bp) encoding a predicted protein of 370 amino acids including an 22 amino acid signal peptide. The MrProAE-III protein exhibits a characteristic sequence structure of a long serine proteases-trypsin domain and an N- and C-terminal serine proteases-trypsin family histidine active sites, respectively, which together are the characteristics of the clip-serin proteases. Sequence analysis showed that MrProAE-III exhibited the highest amino acid sequence similarity (63%) to a ProAE-III from Atlantic blue crab, Callinectes sapidus. MrProAE-III mRNA and enzyme activity of MrProAE-III were detectable in all examined tissues, including hepatopancreas, hemocytes, pleopods, walking legs, eye stalk, gill, stomach, intestine, brain and muscle with the highest level of both in hepatopancreas. This is regulated after systemic infectious hypodermal and hematopoietic necrosis virus infection supporting that it is an immune-responsive gene. These results indicate that MrProAE-III functions in the proPO system and is an important component in the prawn immune system.

Expression profile of prophenoloxidase-encoding (acppo6) gene of Plasmodium vivax-refractory strain of Anopheles culicifacies

Anopheles culicifacies is the main vector for transmission of Plasmodium vivax malaria in the Indian subcontinent. A strain of An. culicifacies isolated from its natural niche displayed complete refractoriness to P. vivax by melanotic encapsulation of ookinetes. Prophenoloxidases are key components of the phenoloxidase cascade that leads to recognition and melanization of invading organisms. We isolated and cloned prophenoloxidase-encoding acppo6 gene of An. culicifacies and analyzed its expression profile under various regimens of immune challenge. The acppo6 was differentially expressed during various stages of larval development. The acppo6 transcription was also up-regulated in response to bacteria and Plasmodium vinckei petteri challenge. The transcript levels of the acppo6 gene were higher in naive adult refractory female mosquitoes as compared with female susceptible mosquitoes. Furthermore, the induction of acppo6 in the susceptible strain upon Plasmodium infection was negligible as compared with that of the refractory strain. The observation is suggestive of the role of acppo6 in effectuating a melanotic response in Plasmodium-incompetent naturally occurring refractory An. culicifacies strain.

Porphyromonas gingivalis peptidoglycans induce excessive activation of the innate immune system in silkworm larvae

Porphyromonas gingivalis, a pathogen that causes inflammation in human periodontal tissue, killed silkworm (Bombyx mori, Lepidoptera) larvae when injected into the blood (hemolymph). Silkworm lethality was not rescued by antibiotic treatment, and heat-killed bacteria were also lethal. Heat-killed bacteria of mutant P. gingivalis strains lacking virulence factors also killed silkworms. Silkworms died after injection of peptidoglycans purified from P. gingivalis (pPG), and pPG toxicity was blocked by treatment with mutanolysin, a peptidoglycan-degrading enzyme. pPG induced silkworm hemolymph melanization at the same dose as that required to kill the animal. pPG injection increased caspase activity in silkworm tissues. pPG-induced silkworm death was delayed by injecting melanization-inhibiting reagents (a serine protease inhibitor and 1-phenyl-2-thiourea), antioxidants (N-acetyl-l-cysteine, glutathione, and catalase), and a caspase inhibitor (Ac-DEVD-CHO). Thus, pPG induces excessive activation of the innate immune response, which leads to the generation of reactive oxygen species and apoptotic cell death in the host tissue.

Trafficking through COPII stabilises cell polarity and drives secretion during Drosophila epidermal differentiation

Background

The differentiation of an extracellular matrix (ECM) at the apical side of epithelial cells implies massive polarised secretion and membrane trafficking. An epithelial cell is hence engaged in coordinating secretion and cell polarity for a correct and efficient ECM formation.

Principal Findings

We are studying the molecular mechanisms that Drosophila tracheal and epidermal cells deploy to form their specific apical ECM during differentiation. In this work we demonstrate that the two genetically identified factors haunted and ghost are essential for polarity maintenance, membrane topology as well as for secretion of the tracheal luminal matrix and the cuticle. We show that they code for the Drosophila COPII vesicle-coating components Sec23 and Sec24, respectively, that organise vesicle transport from the ER to the Golgi apparatus.

Conclusion

Taken together, epithelial differentiation during Drosophila embryogenesis is a concerted action of ECM formation, plasma membrane remodelling and maintenance of cell polarity that all three rely mainly, if not absolutely, on the canonical secretory pathway from the ER over the Golgi apparatus to the plasma membrane. Our results indicate that COPII vesicles constitute a central hub for these processes.

Dual function of a bee venom serine protease: prophenoloxidase-activating factor in arthropods and fibrin(ogen)olytic enzyme in mammals

Bee venom contains a variety of peptides and enzymes, including serine proteases. While the presence of serine proteases in bee venom has been demonstrated, the role of these proteins in bee venom has not been elucidated. Furthermore, there is currently no information available regarding the melanization response or the fibrin(ogen)olytic activity of bee venom serine protease, and the molecular mechanism of its action remains unknown. Here we show that bee venom serine protease (Bi-VSP) is a multifunctional enzyme. In insects, Bi-VSP acts as an arthropod prophenoloxidase (proPO)-activating factor (PPAF), thereby triggering the phenoloxidase (PO) cascade. Bi-VSP injected through the stinger induces a lethal melanization response in target insects by modulating the innate immune response. In mammals, Bi-VSP acts similarly to snake venom serine protease, which exhibits fibrin(ogen)olytic activity. Bi-VSP activates prothrombin and directly degrades fibrinogen into fibrin degradation products, defining roles for Bi-VSP as a prothrombin activator, a thrombin-like protease, and a plasmin-like protease. These findings provide a novel view of the mechanism of bee venom in which the bee venom serine protease kills target insects via a melanization strategy and exhibits fibrin(ogen)olytic activity.

Distinct melanization pathways in the mosquito Aedes aegypti

Serine protease cascades are involved in blood coagulation and immunity. In arthropods, they regulate melanization, which plays an important role in immune defense and wound healing. However, the mechanisms underlying melanization pathways are not completely characterized. We found that in the mosquito Aedes aegypti, there are two distinct melanization activation pathways carried out by different modules of serine proteases and their specific inhibitors serpins. Immune melanization proteases (IMP-1 and IMP-2) and Serpin-1 mediate hemolymph prophenoloxidase cleavage and immune response against the malaria parasite. Tissue melanization, exemplified by the formation of melanotic tumors, is controlled by tissue melanization protease (CLIPB8), IMP-1, and Serpin-2. In addition, serine proteases CLIPB5 and CLIPB29 are involved in activation of Toll pathway by fungal infection or by infection-independent manner, respectively. Serpin-2 is implicated in the latter activation of Toll pathway. This study revealed the complexity underlying melanization and Toll pathway in mosquitoes.

Purification, characterization and immunolocalization of a novel protease inhibitor from hemolymph of tasar silkworm, Antheraea mylitta

A novel serine protease inhibitor (AmPI) was purified from larval hemolymph of tasar silkworm, Antheraea mylitta by two-step process of trypsin-affinity and gel-filtration (FPLC) chromatography. AmPI was active against larval midgut and commercial bovine trypsin and chymotrypsin. The extent of purification was determined by SDS and Native PAGE. The protease inhibitor had an apparent molecular weight of approximately 14.5 kDa as determined by SDS-PAGE. Its activity was stable over a pH range of 4.5-9 and temperatures range of 4-65 degrees C. Molecular weight as determined by MALDITOF-MS was between 13241.63 and 13261.66 Da. MS profile of AmPI also suggests two isoforms of AmPI because of glycosylation by heptose (C(7)H(14)O(7)). This confirmed the result of Native PAGE showing two bands. N-terminal amino acid sequence of this protein did not show similarity to any known protease inhibitor. To study the functional implications of AmPI in insect, it was localized in insect body tissue of different larval instars by immunogold labeling technique using GAR-gold conjugate as secondary antibody. The pattern of localization suggests constitutive nature of AmPI, which may have role in insect's defense mechanism.

Tyrosine hydroxylase is required for cuticle sclerotization and pigmentation in Tribolium castaneum

Newly synthesized insect cuticle is soft and pale but becomes stronger (sclerotized) and often darker (pigmented) over several hours or days. The first step in the sclerotization and pigmentation pathways is the hydroxylation of tyrosine to produce 3,4-dihydroxyphenylalanine (DOPA). Tyrosine hydroxylase (TH) is known to catalyze this reaction during pigmentation, but a role for TH in sclerotization has not been documented. The goal of this study was to determine whether TH is required for cuticle sclerotization in the red flour beetle, Tribolium castaneum. We used quantitative RT-PCR to verify that TH expression occurs at the time of cuticle tanning and immunohistochemistry to confirm that TH is expressed in the epithelial cells underlying sclerotized cuticle. In addition, we found that a reduction in TH function (mediated by RNA interference) resulted in a decrease in cuticle pigmentation and a decrease in the hardness of both pigmented and colorless cuticle. These results demonstrate a requirement for TH in sclerotization as well as brown pigmentation of insect cuticle.

Insect cuticular sclerotization: a review

Different regions of an insect cuticle have different mechanical properties, partly due to different degrees of stabilization and hardening occurring during the process of sclerotization, whereby phenolic material is incorporated into the cuticular proteins. Our understanding of the chemistry of cuticular sclerotization has increased considerably since Mark Pryor in 1940 suggested that enzymatically generated ortho-quinones react with free amino groups, thereby crosslinking the cuticular proteins. The results obtained since then have confirmed the essential features of Pryor's suggestion, and the many observations and experiments, which have been obtained, have led to a detailed and rather complex picture of the sclerotization process, as described in this review. However, many important questions still remain unanswered, especially regarding the precise regional and temporal regulation of the various steps in the process.

Purification and characterization of phenoloxidase from Octopus ocellatus

Phenoloxidase (PO) from ink sacs of Octopus ocellatus was purified by gel-filtration and ion-exchange chromatography, and characterized in terms of its biochemical and enzymatic properties by using L-dihydroxyphenylalanine (L-DOPA) as the specific substrate. It was found that prophenoloxidase from O. ocellatus was isolated as a heterodimeric protein of 153.8 kDa, and two subunits of 75.6 and 73.0 kDa were often detected in preparations after SDS activation. The PO-like activity showed optimal pH of 7.0, optimal temperature of 40 degrees C, and an apparent Km value of 3.1 mM on L-DOPA, and 6.3 mM on catechol, respectively. The PO-like activity was extremely sensitive to 1-phenyl-2-thiourea and sodium sulfite, and very sensitive to ascorbic acid, thiourea, citric acid, and benzoic acid. Together with its specific enzyme activity on catechol and L-DOPA, it can be concluded that the Octopus PO is most probably a typical o-diphenoloxidase. The PO-like activity was also strongly inhibited by Cu(2+), Zn(2+), ethylenediaminetetraacetic acid and diethyldithiocarbamate (DETC), and the DETC-inhibited PO-like activity could be perfectly restored by Cu(2+). These results indicated that Octopus PO is most probably a copper-containing metalloenzyme. All these results implied that the PO from O. ocellatus has the properties of a catechol-type copper-containing o-diphenoloxidase which functions not only as a catalytic enzyme in melanin production in ink sacs but also as a humoral factor in host defense via melaninization as in other crustaceans.