Stimulation of NO synthase activity in the immune-competent lepidopteran Estigmene acraea hemocyte line

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Resisting an invasion: A review of the triatomine vector (Kissing bug) defense strategies against a Trypanosoma sp infection

Triatomines are an important group of insects in the Americas. They serve as transmission vectors for Trypanosoma cruzi, the etiologic agent responsible for the deadly Chagas disease in humans. The digenetic parasite has a complex life cycle, alternating between mammalian and insect hosts, facing different environments. In the insect vector, the metacyclic trypomastigote (non-replicative) and epimastigote (replicative) stages face a set of insect-mediated environmental changes, such as intestinal pH, body temperature, nutrient availability, and vector immune response. These insects have the ability to differentiate between self and non-self-particles using their innate immune system. This immune system comprises physical barriers, cellular responses (phagocytosis, nodules and encapsulation), humoral factors, including effector mechanisms (antimicrobial peptides and prophenoloxidase cascade) and the intestinal microbiota. Here, we consolidate and synthesize the available literature to describe the defense mechanisms deployed by the triatomine vector against the parasite, as documented in recent years, the possible mechanisms developed by the parasite to protect against the insect's specific microenvironment and innate immune responses, and future perspectives on the Triatomine-Trypanosome interaction.

Nitric Oxide Synthase Regulates Gut Microbiota Homeostasis by ERK-NF-κB Pathway in Shrimp

The gut microbiota is a complex group of microorganisms that is not only closely related to intestinal immunity but also affects the whole immune system of the body. Antimicrobial peptides and reactive oxygen species participate in the regulation of gut microbiota homeostasis in invertebrates. However, it is unclear whether nitric oxide, as a key mediator of immunity that plays important roles in antipathogen activity and immune regulation, participates in the regulation of gut microbiota homeostasis. In this study, we identified a nitric oxide synthase responsible for NO production in the shrimp Marsupenaeus japonicus. The expression of Nos and the NO concentration in the gastrointestinal tract were increased significantly in shrimp orally infected with Vibrio anguillarum. After RNA interference of Nos or treatment with an inhibitor of NOS, L-NMMA, NO production decreased and the gut bacterial load increased significantly in shrimp. Treatment with the NO donor, sodium nitroprusside, increased the NO level and reduced the bacterial load significantly in the shrimp gastrointestinal tract. Mechanistically, V. anguillarum infection increased NO level via upregulation of NOS and induced phosphorylation of ERK. The activated ERK phosphorylated the NF-κB-like transcription factor, dorsal, and caused nuclear translocation of dorsal to increase expression of antimicrobial peptides (AMPs) responsible for bacterial clearance. In summary, as a signaling molecule, NOS-produced NO regulates intestinal microbiota homeostasis by promoting AMP expression against infected pathogens via the ERK-dorsal pathway in shrimp.

Storage of Carotenoids in Crustaceans as an Adaptation to Modulate Immunopathology and Optimize Immunological and Life-History Strategies

Why do some invertebrates store so much carotenoids in their tissues? Storage of carotenoids may not simply be passive and dependent on their environmental availability, as storage variation exists at various taxonomic scales, including among individuals within species. While the strong antioxidant and sometimes immune-stimulating properties of carotenoids may be beneficial enough to cause the evolution of features improving their assimilation and storage, they may also have fitness downsides explaining why massive carotenoid storage is not universal. Here, the functional and ecological implications of carotenoid storage for the evolution of invertebrate innate immune defenses are examined, especially in crustaceans, which massively store carotenoids for unclear reasons. Three testable hypotheses about the role of carotenoid storage in immunological (resistance and tolerance) and life-history strategies (with a focus on aging) are proposed, which may ultimately explain the storage of large amounts of these pigments in a context of host-pathogen interactions.

Role of Endosymbionts in Insect-Parasitic Nematode Interactions

Endosymbiotic bacteria exist in many animals where they develop relationships that affect certain physiological processes in the host. Insects and their nematode parasites form great models for understanding the genetic and molecular basis of immune and parasitic processes. Both organisms contain endosymbionts that possess the ability to interfere with certain mechanisms of immune function and pathogenicity. This review summarizes recent information on the involvement of insect endosymbionts in the response to parasitic nematode infections, and the influence of nematode endosymbionts on specific aspects of the insect immune system. Analyzing this information will be particularly useful for devising endosymbiont-based strategies to intervene in insect immunity or nematode parasitism for the efficient management of noxious insects in the field.

Dietary Supplementation of Honey Bee Larvae with Arginine and Abscisic Acid Enhances Nitric Oxide and Granulocyte Immune Responses after Trauma

Many biotic and abiotic stressors impact bees' health, acting as immunosupressors and contribute to colony losses. Thus, the importance of studying the immune response of honey bees is central to develop new strategies aiming to enhance bees' fitness to confront the threats affecting them. If a pathogen breaches the physical and chemical barriers, honey bees can protect themselves from infection with cellular and humoral immune responses which represent a second line of defense. Through a series of correlative studies we have previously reported that abscisic acid (ABA) and nitric oxide (NO) share roles in the same immune defenses of Apis mellifera (A. mellifera). Here we show results supporting that the supplementation of bee larvae's diet reared in vitro with l-Arginine (precursor of NO) or ABA enhanced the immune activation of the granulocytes in response to wounding and lipopolysaccharide (LPS) injection.

NF-κB/Rel, not STAT5, regulates nitric oxide synthase transcription in Apostichopus japonicus

Nitric oxide (NO) is an important signaling molecular in the immune system of all vertebrates and invertebrates for pathologic and physiologic process, and it is largely produced by inducible nitric oxide synthase (iNOS). To uncover key mechanisms regulating NOS expression in sea cucumber Apostichopus japonicus, we amplified a fragment of the NOS promoter by genome walking approach and characterized putative transcription factor binding motifs using luciferase assay. Transient transfection of EPC cells using 5'-deletion constructs linked to luciferase reporter revealed that the region -614/+39 contributed importantly to expression of the AjNOS gene, and the -614 bp of the 5'-flanking region of the AjNOS gene responded well to LPS. Analysis of the functional promoter region revealed the presence of two potential NF-κB (-375 bp to -366 bp, -76 bp to -67 bp) and three STAT binding sites (-284 bp to -276 bp, -95 bp to 87 bp, -81 bp to -73 bp). When luciferase reporter vector and expression vector co-transfected revealed that NF-κB/Rel, but not STAT5, activate the AjNOS promoter fragment. Furthermore, two truncated reporter vectors co-transfected with vector expressing NF-κB/Rel revealed that the first NF-κB binding site (-375 bp to -366 bp) was essential for the ability of this promoter to induce AjNOS transcription. In addition, blocking the AjRel by SN50 (NF-κB inhibitory peptide) depressed the AjNOS expression and NO production both in vivo and in vitro, respectively, revealing that AjRel might directly modulate AjNOS. All our findings confirmed that NF-κB dependent mechanisms regulating expression of AjNOS and suggested a means of linking NO production to the immune response.

The competence of hemocyte immunity in the armyworm Mythimna separata larvae to sublethal hexaflumuron exposure

Hemocytes circulating in the hemolymph are essential for the insect immunity to protect insects against infections. The effects of sublethal hexaflumuron exposure on the competence of hemocyte immunity of fifth-instar larvae of Mythimna separata were investigated. In this insect, the sublethal exposure could cause plasmatocyte filopodia to contract and shorten, and granulocytes to compact with a loss of cytoplasmic projections in vitro, and induce granulocytes to swell and expand in vivo. The mean number of total hemocytes was significantly declined in feed-thru larvae by 5.0μgmL(-1) hexaflumuron. Changes in proportional counts of hemocytes showed that sublethal hexaflumuron exposure caused a decrease of granulocytes and an increase of plasmatocytes in a concentration-dependant manner, but these changes were time-dependently reduced. Few effects of the sublethal exposure were revealed on the proportional counts of spherulocytes, oenocytoids, and prohemocytes. The exposure at 24h showed strong inhibition on phenoloxidase activity in plasma and hemocytes, but this inhibition was time-dependently weakened. The NADPH-diaphorase staining assays showed that a positive immune response of nitric oxide synthase (NOS) in hemocytes was incited by the sublethal exposure, and the longer-time exposure to the higher concentrations of hexaflumuron caused a heavier loss of NOS activity. Phagocytosis rates revealed the inhibitory effect of sublethal hexaflumuron exposure on the phagocytic ability of granulocytes and plasmatocytes that was significantly greater than the effect of chlorpyrifos at the same concentrations. These results show that sublethal hexaflumuron exposure reduces M. separata larval survival by depressing the competence of hemocyte-mediated immune responses.

The first description of complete invertebrate arginine metabolism pathways implies dose-dependent pathogen regulation in Apostichopus japonicus

In this study, three typical members representative of different arginine metabolic pathways were firstly identified from Apostichopus japonicus, including nitric oxide synthase (NOS), arginase, and agmatinase. Spatial expression analysis revealed that the AjNOS transcript presented negative expression patterns relative to those of Ajarginase or Ajagmatinase in most detected tissues. Furthermore, Vibrio splendidus-challenged coelomocytes and intestine, and LPS-exposed primary coelomocytes could significantly induce AjNOS expression, followed by obviously inhibited Arginase and AjAgmatinase transcripts at the most detected time points. Silencing the three members with two specific siRNAs in vivo and in vitro collectively indicated that AjNOS not only compete with Ajarginase but also with Ajagmatinase in arginine metabolism. Interestingly, Ajarginase and Ajagmatinase displayed cooperative expression profiles in arginine utilization. More importantly, live pathogens of V. splendidus and Vibrio parahaemolyticus co-incubated with primary cells also induced NO production and suppressed arginase activity in a time-dependent at an appropriate multiplicity of infection (MOI) of 10, without non-pathogen Escherichia coli. When increasing the pathogen dose (MOI = 100), arginase activity was significantly elevated, and NO production was depressed, with a larger magnitude in V. splendidus co-incubation. The present study expands our understanding of the connection between arginine's metabolic and immune responses in non-model invertebrates.

The entomopathogenic bacterial endosymbionts Xenorhabdus and Photorhabdus: convergent lifestyles from divergent genomes

Members of the genus Xenorhabdus are entomopathogenic bacteria that associate with nematodes. The nematode-bacteria pair infects and kills insects, with both partners contributing to insect pathogenesis and the bacteria providing nutrition to the nematode from available insect-derived nutrients. The nematode provides the bacteria with protection from predators, access to nutrients, and a mechanism of dispersal. Members of the bacterial genus Photorhabdus also associate with nematodes to kill insects, and both genera of bacteria provide similar services to their different nematode hosts through unique physiological and metabolic mechanisms. We posited that these differences would be reflected in their respective genomes. To test this, we sequenced to completion the genomes of Xenorhabdus nematophila ATCC 19061 and Xenorhabdus bovienii SS-2004. As expected, both Xenorhabdus genomes encode many anti-insecticidal compounds, commensurate with their entomopathogenic lifestyle. Despite the similarities in lifestyle between Xenorhabdus and Photorhabdus bacteria, a comparative analysis of the Xenorhabdus, Photorhabdus luminescens, and P. asymbiotica genomes suggests genomic divergence. These findings indicate that evolutionary changes shaped by symbiotic interactions can follow different routes to achieve similar end points.

A calcium-insensitive attenuated nitrosative stress response contributes significantly in the radioresistance of Sf9 insect cells

Lepidopteran insects/insect cells display 50-100 times higher radioresistance than humans, and are evolutionarily closest to mammals amongst all radioresistant organisms known. Compared to mammalian cells, Lepidopteran cells (TN-368, Sf9) display more efficient antioxidant system and DNA repair and suffer considerably less radiation-induced DNA/cytogenetic damage and apoptosis. Recent studies indicate that a considerably lower radiation-induced oxidative stress may significantly reduce macromolecular damage in Lepidopteran cells. Since nitrosative stress contributes in radiation-induced cellular damage, we investigated its nature in the γ-irradiated Sf9 cells (derived from Spodoptera frugiperda; order Lepidoptera; family Noctuidae) and compared with BMG-1 human cell line having significant NOS expression. Radiation induced considerably less ROS/RNS in Sf9 cells, which remained unchanged on treatment with NOS inhibitor l-NMMA. Surprisingly, growth of Sf9 cultures or irradiation could not induce NO or its metabolites, indicating negligible basal/radiation-induced NOS activity that remained unchanged even after supplementation with arginine. Cytosolic calcium release following high-dose (1000-2000Gy at 61.1cGys(-1)) γ-irradiation or H(2)O(2) (250μM) treatment also failed to generate NO in Sf9 cells having high constitutive levels of calmodulin, whereas BMG-1 cells displayed considerable calcium-dependent NO generation even following 10Gy dose. These results strongly imply the lack of calcium-mediated NOS activity in Sf9 cells. Addition of exogenous NO from GSH-NO caused considerable increase in radiation-induced apoptosis, indicating significant contribution of constitutively attenuated nitrosative stress response into the radioresistance of Lepidopteran cells. Our study demonstrates for the first time that a calcium-insensitive, attenuated nitrosative stress response may contribute significantly in the unusual radioresistance displayed by Lepidopteran insect cells.

A proteomic view into infection of greyback canegrubs (Dermolepida albohirtum) by Metarhizium anisopliae

Metarhizium anisopliae is a naturally occurring cosmopolitan fungus infecting greyback canegrubs (Dermolepida albohirtum). The main molecular factors involved in the complex interactions occurring between the greyback canegrubs and M. anisopliae (FI-1045) were investigated by comparing the proteomes of healthy canegrubs, canegrubs infected with Metarhizium and fungus only. Differentially expressed proteins from the infected canegrubs were subjected to mass spectrometry to search for pathogenicity related proteins. Immune-related proteins of canegrubs identified in this study include cytoskeletal proteins (actin), cell communication proteins, proteases and peptidases. Fungal proteins identified include metalloproteins, acyl-CoA, cyclin proteins and chorismate mutase. Comparative proteome analysis provided a view into the cellular reactions triggered in the canegrub in response to the fungal infection at the onset of biological control.

Induced nitric oxide synthesis in the gut of Manduca sexta protects against oral infection by the bacterial pathogen Photorhabdus luminescens

Injecting the insect pathogenic bacterium Photorhabdus luminescens into the blood system of the model lepidopteran insect Manduca sexta induces nitric oxide synthase (NOS) expression in the fat body and blood cells (haemocytes), whereas following oral ingestion of bacteria NOS expression is limited to the gut. We used RNA interference to knock-down expression of NOS throughout the insect. Preventing NOS induction in this way adversely affected the survival of orally infected insects and caused a significant increase in the number of bacteria crossing into the haemolymph. By contrast, knock-down of NOS had no effect on the mortality rate of insects infected with P. luminescens by injection. Pharmacological inhibition of NOS decreased both nitric oxide (NO) levels in the gut wall and survival of orally infected insects, whereas elevation of gut wall NO using an NO donor increased survival of NOS silenced caterpillars. Together, our results imply that induced synthesis of NO is important in mediating insect immune defence against the pathogen by inhibiting transfer of bacteria across the gut wall.

Toxoplasma gondii: molecular cloning and characterization of a nitric oxide synthase-like protein

Toxoplasma gondii has a nitrite production and a putative nitric oxide synthase (NOS) motif genomic sequence. In order to demonstrate that this sequence is functional and could be involved in the metabolism of l-arginine derivatives, we constructed a baculovirus carrying the previously identified Toxoplasma NOS-like DNA sequence. The recombinant protein was expressed into insect Sf9 cells and his activity was tested in serial microplate colorimetric assays. The protein produced 21 nmol/min/ml nitrites per microgram of protein and followed Michaelis-Menten kinetics, with a K(m) for L-arginine of 2.3mM. Furthermore, the optimal pH, temperature and incubation time for the recombinant Toxoplasma NOS-like protein were established. Toxoplasma NOS runs as a band of 11.6 kDa on tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results indicate that the recombinant protein derived from the putative genomic sequence, at the chromosome 1b of T. gondii, is able to produce nitrites from L-arginine as substrate.

The antioxidants dimethylsulfoxide and dimethylthiourea affect the immediate adhesion responses of larval haemocytes from 3 lepidopteran insect species

Antioxidants, dimethylsulfoxide (DMSO) and dimethylthiourea (DMTU), at concentrations not affecting the viability of blood cells (haemocytes) from the larval stage of 3 lepidopteran insects - Galleria mellonella, Lymantria dispar, and Malacosoma disstria - differed in their influence on the innate binding of haemocytes to glass, bacteria to haemocytes, and on humoral responses to alien materials. In vitro DMSO had little effect, whereas DMTU substantially impaired the adhesion of the haemocyte types, the plasmatocytes and granular cells, to slides as well as the attachment of Bacillus subtilis to these haemocytes. Although both antioxidants increased lysozyme and phenoloxidase activities, there was no correlation of enzyme activity and haemocyte adhesion responses, possibly reflecting sequestered radicals. Nitric oxide and hydroxyl radicals offset the DMTU effect. In the absence of antioxidants, inactivate protein kinases A (PKA) and C (PKC) enhanced haemocyte aggregation. In general, DMSO, as opposed to DMTU, did not alter the effects of PKA and PKC activators and inhibitors on haemocyte aggregation or of PKC and PKA activities. High concentrations of DMSO and all levels of DMTU, although inhibiting PKA and PKC, inhibited haemocyte adhesion to slides. Comparable results occurred for DMTU-treated haemocytes incubated with B. subtilis. In vivo DMSO, unlike DMTU, did not impair plasmatocyte or granular cell responses to foreign materials, including bacterial removal from the haemolymph and nodulation.

The HcaR regulatory protein of Photorhabdus luminescens affects the production of proteins involved in oxidative stress and toxemia

Comparison of the proteomes of wild-type Photorhabdus luminescens and its hcaR derivative, grown in insect hemolymph, showed that hcaR disruption decreased the production of toxins (tcdA1, mcf, and pirAB) and proteins involved in oxidative stress response (SodA, AhpC, Gor). The disruption of hcaR did not affect growth rate in insects, but did delay the virulence of P. luminescens in Bombyx mori and Spodoptera littoralis larvae. This delayed virulence was associated with a lower toxemia rather than delay in bacteremia. The disruption of hcaR also increased bacterial sensitivity to hydrogen peroxide. A sodA mutant and an hcaR mutant had similar phenotypes in terms of sensitivity to hydrogen peroxide, virulence, toxin gene expression, and growth rate in insects. Thus, the two processes affected by hcaR disruption - toxemia and oxidative stress response - appear to be related. Besides, expression of toxin genes tcdA1, mcf, and pirAB was decreased by paraquat challenge. We provide here the first demonstration of the importance of toxemia for P. luminescens virulence. Our results also highlight the power of proteomic analysis for detecting unexpected links between different, concomitant processes in bacteria.

Parasite suppression of the oxidations of eumelanin precursors in Drosophila melanogaster

In insects, eukaryotic endoparasites encounter a series of innate immune effector responses mediated in large part by circulating blood cells (hemocytes) that rapidly form multilayer capsules around foreign organisms. Critical components of the encapsulation response are chemical and enzyme-catalyzed oxidations involving phenolic and catecholic substrates that lead to synthesis of eumelanin. These responses are initiated immediately upon infection and are very site-specific, provoking no undesirable systemic responses in the host. In this study, we were interested to learn if the principal oxidation pathways leading to the synthesis of eumelanin in larvae of Drosophila melanogaster were targets for inhibition by immune suppressive factors (ISF) derived from a virulent strain of the endoparasitic wasp Leptopilina boulardi. Comparative in vitro assays monitored by sensitive electrochemical detection methods showed that ISF derived from female reproductive tissues significantly diminished the oxidations of the two diphenol eumelanin precursors, dopamine and 5,6-dihydroxyindole (DHI). The oxidations of the monophenol tyrosine, and two other related diphenols, dopa and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), were not significantly inhibited by ISF. The data suggest that melanogenesis represents at least one of the host responses suppressed by L. boulardi ISF, and that the oxidation pathways selectively targeted for inhibition are those synthesizing decarboxylated pigment precursors derived from DHI. These observations, together with previous reports of adverse effects of ISF on the ability of hemocytes to adhere to foreign surfaces, suggest a multifaceted approach by the parasitoid to circumvent the innate immune response of D. melanogaster.

Nitric oxide production and nitric oxide synthase immunoreactivity in Naegleria fowleri

Free-living ameba Naegleria fowleri produces an acute and fatal infectious disease called primary amebic meningoencephalitis (PAM), whose pathophysiological mechanism is largely unknown. The aim of this study was to investigate the role of nitric oxide (NO) in PAM. Although NO has a cytotoxic effect on various parasites, it is produced by others as part of the pathology, as is the case with Entamoeba histolytica. To test for the production of NO, we analyzed whether antibodies against mammalian NO synthase isoforms (neuronal, inducible, and endothelial) presented immunoreactivity to N. fowleri proteins. We found that the trophozoites produced NO in vitro. The Western blot results, which showed N. fowleri trophozoites, contained proteins that share epitopes with the three described mammalian NOS, but have relative molecular weights different than those described in the literature, suggesting that N. fowleri may contain undescribed NOS isoforms. Moreover, we found that trophozoites reacted to the NOS2 antibody, in amebic cultures as well as in the mouse brain infected with N. fowleri, suggesting that nitric oxide may participate in the pathogenesis of PAM. Further research aimed at determining whether N. fowleri contains active novel NOS isoforms could lead to the design of new therapies against this parasite.

Demonstration of nitric oxide synthase activity in crustacean hemocytes and anti-microbial activity of hemocyte-derived nitric oxide

We determined the biochemical characteristics of nitric oxide synthase (NOS) in hemocytes of the crayfish Procambarus clarkii and investigated the roles of hemocyte-derived NO in host defense. Biochemical analysis indicated the presence of a Ca2+ -independent NOS activity, which was elevated by lipopolysaccharide (LPS) treatment. When bacteria (Staphylococcus aureus) and hemocytes were co-incubated, adhesion of bacteria to hemocytes was observed. NO donor sodium nitroprusside (SNP) significantly increased the numbers of hemocytes to which bacteria adhered. Similarly, LPS elicited bacterial adhesion and the LPS-induced adhesion was prevented by NOS inhibitor NG-monomethyl-L-arginine (L-NMMA). Finally, plate count assay demonstrated that addition of LPS to the hemocytes/bacteria co-incubation resulted in a significant decrease in bacterial colony forming unit (CFU), and that L-NMMA reversed the decreasing effect of LPS on CFU. The combined results demonstrate the presence of a Ca2+ -independent LPS-inducible NOS activity in crayfish hemocytes and suggest that hemocyte-derived NO is involved in promoting bacterial adhesion to hemocytes and enhancing bactericidal activity of hemocytes.

Nitric oxide: an antiparasitic molecule of invertebrates

Since Furchgott, Ignarro and Murad won the Nobel prize in 1998 for their work on the role of nitric oxide (NO) as a signaling molecule, many reports have shown the seemingly limitless range of body functions controlled by this compound. In vertebrates, the role of NO as a defense against infection caused by viruses, bacteria, and protozoan and metazoan parasites has been known for several years. New evidence, however, shows that NO is also important in defending invertebrates against parasites. This discovery is a breakthrough in the understanding of how the invertebrate immune system works, and it has implications for the emerging field of invertebrate ecological immunology.

Studies on nitric oxide synthase activity in haemocytes of shrimps Fenneropenaeus chinensis and Marsupenaeus japonicus after white spot syndrome virus infection

The nitric oxide synthase (NOS) activity in the haemocytes of shrimps Fenneropenaeus chinensis (Osbeck) and Marsupenaeus japonicus (Bate) was studied after white spot syndrome virus (WSSV) infection to determine its characteristics in response to virus infection. First, the NOS activity in haemocytes of shrimps was determined by the means of NBT reduction and changes in cell conformation. And the variations of NOS activity in shrimps after challenge with WSSV intramuscularly were evaluated through the analysis of l-citrulline and total nitrite/nitrate (both as NO derivates) concentrations. The result showed that NOS activity in the haemocytes of F. chinensis increased slightly from 0 to 12 h postchallenge, indicated by the variations of l-citrulline (from 11.15 ± 0.10 to 12.08 ± 0.64 μM) and total nitrite/nitrate concentrations (from 10.45 ± 0.65 to 12.67 ± 0.52 μM). Then it decreased sharply till the end of the experiment (84 h postchallenge), the concentrations of l-citrulline and total nitrite/nitrate at 84 h were 1.58 ± 0.24 and 2.69 ± 0.70 μM, respectively. The LPS-stimulated NOS activity kept constant during the experiment. However, in M. japonicus, the NOS activity kept increasing during the first 72 h postchallenge, the concentrations of l-citrulline and total nitrite/nitrate increased from 7.82 ± 0.77 at 0 h to 10.79 ± 0.50 μM at 72 h, and from 8.98 ± 0.43 at 0 h to 11.20 ± 0.37 μM at 72 h, respectively. Then it decreased till the end of the experiment (216 h postchallenge), and the concentrations of l-citrulline and total nitrite/nitrate at 216 h were 5.66 ± 0.27 and 4.68 ± 0.16 μM, respectively. More importantly, an apparent increase of LPS-stimulated NOS activity was observed in M. japonicus at 48 h postchallenge, which was about 4 times higher than that in the control group of health shrimps. In correspondence with the difference of NOS activity between the two species of shrimps, the cumulative mortalities of the shrimps were also different. All shrimps of F. chinensis in the mortality experiment died in 66 h, much more quickly than M. japonicus, whose accumulative mortality reached 100% after 240 h. Data here reported let us hypothesize that NOS activity in the haemocytes of shrimps F. chinensis and M. japonicus responses to WSSV infection differently, and this might be one of the reasons for the different susceptibility of F. chinensis and M. japonicus to WSSV infection.

Nitric oxide production by hemocytes of larva and pharate prepupa of Galleria mellonella in response to bacterial lipopolysaccharide: Cytoprotective or cytotoxic?

Nitric oxide production by the hemocytes of the last instar larvae and sessile pharate prepupa of Galleria mellonella (Lepidoptera: Pyralidae) was demonstrated in vitro in response to preparations of bacterial lipopolysaccharide (LPS) from Escherichia coli using the Griess reaction. Augmented, dose dependent nitric oxide production was observed in the pharate prepupal hemocytes compared with larval hemocytes. This was partially reversed in a dose dependent manner with S-methyl thiourea (SMT), a specific inhibitor of inducible nitric oxide synthase (iNOS). A decrease in NO production was also observed when non-selective inhibitors such as N(G)-nitro-L-arginine (L-NAME) and N-omega-nitro-L-arginine (L-NNA) were used, albeit the inhibition was not to the extent of SMT. Challenge with the entomopathogenic Gram-negative bacterium Photorhabdus asymbiotica also enhanced NO production by hemocytes of both stages. SMT, alone or in combination with P. asymbiotica significantly decreased levels of NO production. However, it was observed that phenoloxidase activity (a cascade for innate immune responses) was independent of NO production stimulation. NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and diethylenetriamine NO adduct (DETA/NO) at various concentrations (100-500 microM) resulted in the lysis of hemocytes dose dependently. The nitrite production in these cases was however similar to LPS stimulation (10 microg/mL) and 1.5-3 fold lower than those observed upon P. asymbiotica (2.5 x 10(7) cfu/mL) stimulation. Survival analysis (Kaplan-Meier) following injection of P. asymbiotica alone or in combination with SMT revealed that only 12.5% (median survival 25.5 h) of co-injected larvae of G. mellonella survived in comparison to 28.6% (median survival 29 h) survivors in P. asymbiotica alone-injected groups till the end of the study. In contrast, co-injected pharate prepupa survived longer (median survival 28 h) than the P. asymbiotica alone-injected individuals (median survival 24 h); however, both co-injected and P. asymbiotica-injected groups showed 100% mortality at the end of the study. Based on the above, we propose that although NO production is involved in cellular immune responses of this insect to bacterial infection it does not appear to be a part of the signalling pathway that initiates the prophenoloxidase (PPO) cascade, and the extended NO production/overproduction by pharate prepupal hemocytes could result in cytotoxic rather than cytoprotective effects compared with larval hemocytes.

Nitric oxide production in blowfly hemolymph after yeast inoculation

Although insects lack the adaptive immune response of the mammalians, they manifest effective innate immune responses that include both cellular and humoral components. Cellular responses are mediated by hemocytes and humoral responses include the activation of proteolytic cascades that initiate many events, including NO production. In this work, we determined NO production in Chrysomya megacephala hemolymph and hemocytes after yeast inoculation. Assays were performed with non-infected controls (NIL), saline-injected larvae (SIL) or larvae injected with Saccharomyces cerevisiae (YIL). The hemolymph of injected groups was collected 0.5, 1, 2, 4, 12, 24 or 48h post-injection. NO levels in SIL were comparable to those measured in NIL until 12h, which might be considered the basal production, increasing at 24 and 48h post-injection, probably in response to the increased larval fragility after cuticle rupture. YIL exhibited significantly higher levels of NO than were found in other groups, peaking at 24h. l-NAME and EDTA caused a significant reduction of NO production in YIL at this time, suggesting the activity of a Ca(2+)-dependent NOS. Plasmatocytes and granular cells phagocytosed the yeasts. Plasmatocytes initiated the nodule formation and granular cells were the only hemocyte type to produce NO. These results permit us to conclude that yeasts induced augmented NO production in C. megacephala hemolymph and granular cells are the hemocyte type involved with the generation of this molecule.

Insect renal tubules constitute a cell-autonomous immune system that protects the organism against bacterial infection

Innate immunity is a widespread and important defence against microbial attack, which in insects is thought to originate mainly in the fat body. Here we demonstrate that the fluid-transporting Malpighian (renal) tubule of Drosophila melanogaster constitutes an autonomous immune-sensing tissue utilising the nitric oxide (NO) signalling pathway. Reverse transcriptase PCR (RT-PCR) shows that tubules express those genes encoding components of the Imd pathway. Furthermore, isolated tubules bind and respond to lipopolysaccharide (LPS), by upregulating anti-microbial peptide (diptericin) gene expression and increased bacterial killing. Excised, LPS-challenged tubules, as well as tubules from LPS-infected flies, display increased NO synthase (NOS) activity upon immune challenge. Targetted expression of a Drosophila NOS (dNOS) transgene to only principal cells of the tubule main segment using the GAL4/UAS system increases diptericin expression. In live flies, such targetted over-expression of dNOS to tubule principal cells confers increased survival of the whole animal upon E. coli challenge. Thus, we describe a novel role of Malpighian tubules in immune sensing and insect survival.

Melanogenesis and associated cytotoxic reactions: applications to insect innate immunity

Insects transmit the causative agents for such debilitating diseases as malaria, lymphatic filariases, sleeping sickness, Chagas' disease, leishmaniasis, river blindness, Dengue, and yellow fever. The persistence of these diseases provides testimony to the genetic capacity of parasites to evolve strategies that ensure their successful development in two genetically diverse host species: insects and mammals. Current efforts to address the problems posed by insect-borne diseases benefit from a growing understanding of insect and mammalian immunity. Of considerable interest are recent genomic investigations that show several similarities in the innate immune effector responses and associated regulatory mechanisms manifested by insects and mammals. One notable exception, however, is the nearly universal presence of a brown-black pigment accompanying cellular innate immunity in insects. This response, which is unique to arthropods and certain other invertebrates, has focused attention on the elements involved in pigment synthesis as causing or contributing to the death of the parasite, and has even prompted speculation that the enzyme cascade mediating melanogenesis constitutes an ill-defined recognition mechanism. Experimental evidence defining the role of melanin and its precursors in insect innate immunity is severely lacking. A great deal of what is known about melanogenesis comes from studies of the process occurring in mammalian systems, where the pigment is synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain, and retinal epithelium. Fortunately, many of the components in the metabolic pathways leading to the formation of melanin have been found to be common to both insects and mammals. This review examines some of the factors that influence enzyme-mediated melanogenic responses, and how these responses likely contribute to blood cell-mediated, target-specific cytotoxicity in immune challenged insects.

A crustacean nitric oxide synthase expressed in nerve ganglia, Y-organ,gill and gonad of the tropical land crab,Gecarcinus lateralis

NO signaling is involved in many physiological processes in invertebrates. In crustaceans, it plays a role in the regulation of the nervous system and muscle contraction. Nested reverse transcription-polymerase chain reaction(RT-PCR) and 5′ and 3′ rapid amplification of cDNA ends (RACE) PCR generated a full-length cDNA sequence (3982 bp) of land crab NO synthase(Gl-NOS) from molting gland (Y-organ) and thoracic ganglion mRNA. The open reading frame encoded a protein of 1199 amino acids with an estimated mass of 135 624 Da. Gl-NOS had the highest sequence identity with insect NOS. The amino acid sequences for binding heme and tetrahydrobiopterin in the oxygenase domain, binding calmodulin and binding FMN, FAD and NADPH in the reductase domain were highly conserved. Gl-NOS had single amino acid differences in all three highly conserved FAD-binding sequences, which distinguished it from other NOS sequences. RT-PCR showed that the Gl-NOS mRNA was present in testis,ovary, gill, eyestalk neural ganglia, thoracic ganglion and Y-organ. NOS mRNA varied between preparations of Y-organ, thoracic ganglion and gill, while NOS mRNA was at consistently high levels in the ovary, testis and eyestalk ganglia. Immunohistochemistry confirmed that the Gl-NOS protein was expressed in Y-organ, ovary and gill. These results suggest that NOS has functions in addition to neuromodulation in adults, such as regulating or modulating ecdysteroid synthesis in the Y-organ.

Nitric oxide production by carpet shell clam (Ruditapes decussatus) hemocytes

We have demonstrated that carpet shell clam (Ruditapes decussatus) hemocytes produce nitric oxide (NO) in response to zymosan or bacterial lipopolysaccharide (LPS). This NO production was partially inhibited by the NO synthase inhibitor, N-omega-nitro-L-arginine (L-NAME). The capability of clam hemocytes to produce NO in response to the bacterial pathogen Vibrio tapetis was also studied. Incubation with bacteria induced a significant NO production by clam hemocytes, even though exogenous NO only slightly decreased the growth of V. tapetis. The effect of exogenous NO on the capability of clam hemocytes to phagocytose labeled Escherichia coli was studied using two different NO donors S-nitroso-N-acetyl-penicillamine (SNAP), and diethylenetriamine NO adduct (DETA/NO). Exogenous NO did not increase hemocyte phagocytosis, indicating that NO does not mediate phagocytosis in this species. These results are in accordance to those observed in other mollusk species, in which NO was independent of phagocytosis and constitutes an alternative method of killing invading pathogens.

cDNA cloning, characterization and gene expression of nitric oxide synthase from the silkworm, Bombyx mori

Molecular cloning and nucleotide sequencing of cDNA encoding Bombyx mori nitric oxide synthase (BmNOS) was conducted to analyse its possible role in insect immunity. The amino acid sequence deduced from the BmNOS cDNA showed 84%, 54% and 53% identity with those of NOSs from Manduca sexta, Drosophila melanogaster and Rhodonius prolixus. Recombinant BmNOS produced in insect cells using baculovirus was found to require NADPH, Ca2+, calmodulin and tetrahydrobiopterin (BH4) for its activity. The BmNOS gene was constitutively expressed at a low level in the larval fat body, haemocyte, Malpighian tubule and midgut, and adult antenna, and induced strongly in the fat body by lipopolysaccharide (LPS), suggesting that the BmNOS gene plays different physiological roles in different tissues. Injection of NO donors that produce NO in vivo induced the gene expression of an antibacterial peptide, cecropin B, strongly suggesting that NO produced by BmNOS following LPS stimulation is involved in signal transduction as a signalling molecule for immune gene expression.

Production of reactive oxygen species by hemocytes from the cattle tick Boophilus microplus

We have investigated the phagocytic activity and the production of reactive oxygen species (ROS) by hemocytes from the cattle tick Boophilus microplus. Two main types of hemocytes were detected in tick hemolymph: plasmatocytes and granulocytes. The plasmocytes were the most abundant cells, being responsible for the in vivo phagocytosis of yeast. ROS production was evaluated by luminol-amplified luminescence and phenol red oxidation. The luminescence increased when hemocytes were incubated with bacteria, zymosan, or phorbol 12-miristate 13-acetate (PMA). The luminescence was inhibited by superoxide dismutase and catalase, which are antioxidant enzymes that remove superoxide and hydrogen peroxide, respectively. The phenol red oxidation assay also showed an increase in the level of hydrogen peroxide produced by hemocytes stimulated with bacteria and PMA. Taken all together, our data indicate that tick hemocytes are able to produce ROS during the phagocytic process similarly to vertebrate phagocytes.

Role of superoxide and reactive nitrogen intermediates in Rhodnius prolixus (Reduviidae)/Trypanosoma rangeli interactions

This study compares aspects of the superoxide, nitric oxide and prophenoloxidase pathways in Rhodnius prolixus hemolymph, measured in parallel, in response to Trypanosoma rangeli inoculation. Responses to two strains of T. rangeli, and two developmental forms, were studied, and the results obtained were correlated with the ability of the parasites to survive, multiply, and complete their life cycles in the hemolymph of the host. T. rangeli H14 strain parasites, which fail to complete their life cycle in Rhodnius by invading the salivary glands, stimulated high levels of superoxide and prophenoloxidase activity, which peaked 24 h after inoculation. Simultaneously, the concentration of hemolymph nitrites and nitrates increased, indicative of nitric oxide activity, but parasite numbers remained low. T. rangeli Choachi strain parasite inoculation also stimulated superoxide and prophenoloxidase activity, which, though significantly lower than the equivalent responses to the H14 strain, also peaked at 24 h. However, nitrate and nitrite levels in Choachi strain-inoculated hemolymph remained low, and this parasite strain multiplied rapidly, especially following peak superoxide activity, and eventually invaded the salivary glands for transmission to a vertebrate host. In both strains, short form epimastigotes stimulated greater superoxide and prophenoloxidase responses than long form epimastigotes. Injection of the NADPH oxidase inhibitor N-ethylmaleimide or the inducible nitric oxide synthase inhibitor S-methyl isothiourea sulfate caused significantly higher insect mortalities in groups of R. prolixus inoculated with either parasite strain compared with those of uninfected control insects. This indicates that both NADPH oxidase and nitric oxide synthase activity may be involved in the immune response of R. prolixus to infection by T. rangeli. Finally, Western blotting of R. prolixus hemocyte lysates revealed the presence of a protein immunologically related to the human NADPH oxidase complex, the initiator enzyme of the respiratory burst.

Nitric oxide induces apoptosis in the fat body cell line IPLB-LdFB from the insect Lymantria dispar

The presence of immunoreactive inducible nitric oxide synthase molecules (ir-iNOS) is demonstrated in the Lymantria dispar IPLB-LdFB cell line. The maximum ir-iNOS inducibility is observed 18 h after incubation with sodium nitroprusside (SNP). The increase in NO provoked by SNP in turn induces apoptosis. However, this phenomenon is observed only after 48 h. The NOS-inhibitors N(omega)-nitro-L-arginine methyl ester (L-NAME) and N-[3-(aminomethyl)-benzyl]acetamide (1400W) were both unable to block the SNP-induced apoptosis at all the concentrations used. Incubation with SNP plus N-acetyl-L-cysteine (NAC) further augmented the percentage of cell death with respect to SNP used alone, and this process is seen earlier, i.e. after 24 h. Moreover, the induction of apoptosis in the presence of NAC is time- and concentration-dependent. The high percentage of cell death with SNP+NAC suggests that NAC forms S-nitrosothiols with NO, resulting in an increase in the bioavailability of NO. In conclusion, these findings show the existence of a close relationship between mammalian and invertebrate cells with regards to SNP and NAC induction and the related NO response.

Insect immune activation by recombinant Galleria mellonella apolipophorin III(1)

Apolipophorin III (apoLp-III) is an exchangeable insect apolipoprotein. Its function, as currently understood, lies in the stabilization of low-density lipophorin particles (LDLp) crossing the hemocoel in phases of high energy consumption to deliver lipids from the fat body to the flight muscle cells. Recent studies with native Galleria mellonella-apoLp-III gave first indications of an unexpected role of that protein in insect immune activation. Here we report the immune activation by the recombinant protein, documenting a newly discovered correlation between lipid physiology and immune defense in insects. The complete cDNA sequence of G. mellonella-apoLp-III was identified by mixed oligonucleotide-primed amplification of cDNA (MOPAC), 3'-RACE-PCR, and cRACE-PCR. The sequence coding for the native protein was ligated into a pET-vector; this construct was transfected into Escherichia coli and overexpressed in the bacteria. Photometric turbidity assays with human low density lipoprotein (LDL) and transmission electron microscopy studies on apoLp-III-stabilized lipid discs revealed the full functionality of the isolated recombinant apoLp-III with regard to its lipid-association ability. For proving its immune-stimulating capacity, apoLp-III was injected into the hemocoel of last instar G. mellonella larvae and the antibacterial activity in cell-free hemolymph was determined 24 h later. As a result, the hemolymph samples of injected insects contained strongly increased antibacterial activities against E. coli as well as clearly enhanced lysozyme-like activities. From Northern blot analysis of total RNA from insects injected with apoLp-III or the bacterial immune provocator lipopolysaccharide, it could be concluded that the transcription rate of apoLp-III mRNA does not vary in comparison to untreated last instar larvae.