Nitric oxide synthase activity from a hematophagous insect salivary gland

G6PDH as a key immunometabolic and redox trigger in arthropods

The enzyme glucose-6-phosphate dehydrogenase (G6PDH) plays crucial roles in glucose homeostasis and the pentose phosphate pathway (PPP), being also involved in redox metabolism. The PPP is an important metabolic pathway that produces ribose and nicotinamide adenine dinucleotide phosphate (NADPH), which are essential for several physiologic and biochemical processes, such as the synthesis of fatty acids and nucleic acids. As a rate-limiting step in PPP, G6PDH is a highly conserved enzyme and its deficiency can lead to severe consequences for the organism, in particular for cell growth. Insufficient G6PDH activity can lead to cell growth arrest, impaired embryonic development, as well as a reduction in insulin sensitivity, inflammation, diabetes, and hypertension. While research on G6PDH and PPP has historically focused on mammalian models, particularly human disorders, recent studies have shed light on the regulation of this enzyme in arthropods, where new functions were discovered. This review will discuss the role of arthropod G6PDH in regulating redox homeostasis and immunometabolism and explore potential avenues for further research on this enzyme in various metabolic adaptations.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Nitric oxide effects on Rhodnius prolixus's immune responses, gut microbiota and Trypanosoma cruzi development

The immune system of Rhodnius prolixus comprehends the synthesis of different effectors that modulate the intestinal microbiota population and the life cycle of the parasite Trypanosoma cruzi inside the vector midgut. One of these immune responses is the production of reactive nitrogen species (RNS) derived by the action of nitric oxide synthase (NOS). Therefore, we investigated the effects of L-arginine, the substrate for nitric oxide (NO) production and N_ω-Nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NOS, added in the insect blood meal. We analyzed the impact of these treatments on the immune responses and development of intestinal bacteria and parasites on R. prolixus nymphs. The L-arginine treatment in R. prolixus nymphs induced a higher NOS gene expression in the fat body and increased NO production, but reduced catalase and antimicrobial activities in the midgut. As expected, L-NAME treatment reduced NOS gene expression in the fat body. In addition, L-NAME treatment diminished catalase activity in the hemolymph and posterior midgut reduced phenoloxidase activity in the anterior midgut and increased the antimicrobial activity in the hemolymph. Both treatments caused a reduction in the cultivatable intestinal microbiota, especially in insects treated with L-NAME. However, T. cruzi development in the insect's digestive tract was suppressed after L-arginine treatment and the opposite was observed with L-NAME, which resulted in higher parasite counts. Therefore, we conclude that induction and inhibition of NOS and NO production are associated with other R. prolixus humoral immune responses, such as catalase, phenoloxidase, and antibacterial activities in different insect organs. These alterations reflect on intestinal microbiota and T. cruzi development.

Functional aspects of salivary nitric oxide synthase of Rhodnius prolixus (Hemiptera, Reduviidae) and nitric oxide trafficking at the vector-host interface

Rhodnius prolixus expresses nitric oxide synthase (NOS) in the cytosol of the salivary gland (SG) cells. The NO produced is stored in the SG lumen bound to NO-carrier haemeproteins called nitrophorins (NPs). NPs bind tightly to NO in the acidic SG lumen, but release NO when the pH becomes high, e.g., at the host skin (pH~7.4). NO elicits potent and transient relaxation of vascular smooth muscle. Here, we investigated the role of salivary NO in the R. prolixus feeding behaviour and the salivary vasodilator activity of the host microcirculation. NOS knockdown in R. prolixus changed the SG colour, decreased the number of NO-loaded NPs and caused impairment of feeding performance. When salivary gland extracts (SGEs) were obtained from NOS- and NPs-knockdown insects and prepared in pH 5.0 solution and injected (i.v.) into mice via the tail vein, no vasodilation was observed, whereas SGEs from control insects caused long-term venodilation in the mouse skin. SGs disrupted directly in PBS (pH 7.4) containing BSA produced long-term vasodilation compared to the controls without BSA due to the possible formation of nitroso-albumin, suggesting that host serum albumin extends the NO half-life when NO is injected into the host skin by triatomine during their blood-feeding.

Jaburetox-induced toxic effects on the hemocytes of Rhodnius prolixus (Hemiptera: Reduviidae)

Jaburetox is a recombinant peptide derived from a Canavalia ensiformis urease that presents toxic effects upon several species of insects, phytopathogenic fungi and yeasts of medical importance. So far, no toxicity of Jaburetox to mammals has been shown. Previous reports have identified biochemical targets of this toxic peptide in insect models, although its mechanism of action is not completely understood. In this work, we aimed to characterize the effects of Jaburetox in hemolymphatic insect cells. For this purpose, the model insect and Chagas' disease vector Rhodnius prolixus was used. In vivo and in vitro experiments indicated that Jaburetox interacts with a subset of hemocytes and it can be found in various subcellular compartments. In insects injected with Jaburetox there was an increase in the gene expression of the enzymes UDP-N-acetylglucosamine pyrophosphorylase (UAP), chitin synthase and nitric oxide synthase (NOS). Nevertheless, the expression of NOS protein, the enzyme activities of UAP and acid phosphatase (a possible link between UAP and NOS) as well as the phosphorylation state of proteins remained unchanged upon the in vivo Jaburetox treatment. Nitric oxide (NO) imaging using fluorescent probes showed that Jaburetox augmented NO production in the hemocyte aggregates when compared to controls. Even though Jaburetox activated the hemocytes, as demonstrated by wheat germ agglutinin binding assays, the peptide did not lead to an increase of their phagocytic behavior. Taken together, these findings contribute to our understanding of toxic effects of Jaburetox, a peptide with biotechnological applications and a prospective tool for rational insect control.

Jaburetox affects gene expression and enzyme activities in Rhodnius prolixus, a Chagas' disease vector

Jaburetox, a recombinant peptide of ∼11kDa derived from one of the Canavalia ensiformis (Jack Bean) urease isoforms, is toxic and lethal to insects belonging to different orders when administered orally or via injection. Previous findings indicated that Jaburetox acts on insects in a complex fashion, inhibiting diuresis and the transmembrane potential of Malpighian tubules, interfering with muscle contractility and affecting the immune system. In vitro, Jaburetox forms ionic channels and alters permeability of artificial lipid membranes. Moreover, recent data suggested that the central nervous system (CNS) is a target organ for ureases and Jaburetox. In this work, we employed biochemical, molecular and cellular approaches to explore the mode of action of Jaburetox using Rhodnius prolixus, one of the main Chagas' disease vectors, as experimental model. In vitro incubations with fluorescently labeled Jaburetox indicated a high affinity of the peptide for the CNS but not for salivary glands (SG). The in vitro treatment of CNS or SG homogenates with Jaburetox partially inhibited the activity of nitric oxide synthase (NOS), thus disrupting nitrinergic signaling. This inhibitory effect was also observed in vivo (by feeding) for CNS but not for SG, implying differential modulation of NOS in these organs. The inhibition of NOS activity did not correlate to a decrease in expression of its mRNA, as assessed by qPCR. UDP-N-acetylglucosamine pyrophosphorylase (UAP), a key enzyme in chitin synthesis and glycosylation pathways and a known target of Jaburetox in insect CNS, was also affected in SG, with activation of the enzyme seen after both in vivo or in vitro treatments with the peptide. Unexpectedly, incubation of Jaburetox with a recombinant R. prolixus UAP had no effect on its activity, implying that the enzyme's modulation by the peptide requires the participation of other factor(s) present in CNS or SG homogenates. Feeding Jaburetox to R. prolixus decreased the mRNA levels of UAP and chitin synthase, indicating a complex regulation exerted by the peptide on these enzymes. No changes were observed upon Jaburetox treatment in vivo and in vitro on the activity of the enzyme acid phosphatase, a possible link between UAP and NOS. Here we have demonstrated for the first time that the Jaburetox induces changes in gene expression and that SG are another target for the toxic action of the peptide. Taken together, these findings contribute to a better understanding of the mechanism of action of Jaburetox as well as to the knowledge on basic aspects of the biochemistry and neurophysiology of insects, and might help in the development of optimized strategies for insect control.

The Dynamics of the Defense Strategy of Pea Induced by Exogenous Nitric Oxide in Response to Aphid Infestation

The aim of this study was to investigate the effect of exogenous nitric oxide (NO), i.e., S-nitrosoglutathione (GSNO) and sodium nitroprusside (SNP), on the metabolic status of Pisum sativum L. cv. Cysterski leaves infested by Acyrthosiphon pisum Harris, population demographic parameters and A. pisum feeding activity. A reduction in the level of semiquinone radicals in pea seedling leaves pretreated with exogenous NO occurred 24 h after A. pisum infestation, which was earlier than in non-pretreated leaves. A decrease in the level of O₂•- was observed in leaves pretreated with GSNO and infested by aphids at 48 and 72 h post-infestation (hpi). Directly after the pretreatment with GSNO, an increase in the level of metal ions was recorded. NO considerably induced the relative mRNA levels for phenylalanine ammonia-lyase in 24-h leaves pretreated with NO donors, both non-infested and infested. NO stimulated the accumulation of pisatin in leaves until 24 h. The Electrical Penetration Graph revealed a reduction in the feeding activity of the pea aphid on leaves pretreated with NO. The present study showed that foliar application of NO donors induced sequentially defense reactions of pea against A. pisum and had a deterrent effect on aphid feeding and limited the population growth rate.

Molecular mechanisms of myocarditis caused by Trypanosoma cruzi

PURPOSE OF REVIEW

American trypanosomiasis, or Chagas disease, is a lifelong and persistent infection caused by the protozoan Trypanosoma cruzi and is the most significant cause of morbidity and mortality in South and Central America. Owing to immigration and additional risks from blood transfusion and organ transplantation, the number of reported cases of Chagas disease has increased recently in Europe and the USA. The disease is caused by a moderate to intense lasting inflammatory response that triggers local expression of inflammatory mediators and activates and recruits leukocytes to various tissues to eliminate the parasites.

RECENT FINDINGS

This long-term inflammatory process triggers biochemical, physiological and morphological alterations and clinical changes in the digestive, nervous and cardiac (e.g. myocarditis, arrhythmias, congestive heart failure, autonomic dysfunctions and microcirculatory disturbances) systems. Indeed, the pathogenesis of Chagas disease is intricate and multifactorial, and the roles of the parasite and the immune response in initiating and maintaining the disease are still controversial.

SUMMARY

In this review, we discuss the current knowledge of 'strategies' employed by the parasite to persist in the host and host defence mechanisms against Trypanosoma cruzi infection, which can result in equilibrium (absence of the disease) or disease development, mainly in the cardiac systems.

Dimerization of nitrophorin 4 at low pH and comparison to the K1A mutant of nitrophorin 1

Nitrophorin 4, one of the four NO-carrying heme proteins from the salivary glands of Rhodnius prolixus, forms a homodimer at pH 5.0 with a Kd of ∼8 μM. This dimer begins to dissociate at pH 5.5 and is completely dissociated to monomer at pH 7.3, even at 3.7 mM. The dimer is significantly stabilized by binding NO to the heme and at pH 7.3 would require dilution to well below 0.2 mM to completely dissociate the NP4-NO homodimer. The primary techniques used for investigating the homodimer and the monomer-dimer equilibrium were size-exclusion fast protein liquid chromatography at pH 5.0 and (1)H{(15)N} heteronuclear single-quantum coherence spectroscopy as a function of pH and concentration. Preparation of site-directed mutants of NP4 (A1K, D30A, D30N, V36A/D129A/L130A, K38A, R39A, K125A, K125E, D132A, L133V, and K38Q/R39Q/K125Q) showed that the N-terminus, D30, D129, D132, at least one heme propionate, and, by association, likely also E32 and D35 are involved in the dimerization. The "closed loop" form of the A-B and G-H flexible loops of monomeric NP4, which predominates in crystal structures of the monomeric protein reported at pH 5.6 but not at pH 7.5 and which involves all of the residues listed above except D132, is required for dimer formation. Wild-type NP1 does not form a homodimer, but NP1(K1A) and native N-terminal NP1 form dimers in the presence of NO. The homodimer of NP1, however, is considerably less stable than that of NP4 in the absence of NO. This suggests that additional aspartate or glutamate residues present in the C-terminal region of NP4, but not NP1, are also involved in stabilizing the dimer.

NMR investigations of nitrophorin 2 belt side chain effects on heme orientation and seating of native N-terminus NP2 and NP2(D1A)

Nitrophorin 2 (NP2), one of the four NO-storing and NO-releasing proteins found in the saliva of the blood-sucking bug Rhodnius prolixus, has a more ruffled heme and a high preference for a particular heme orientation (B) compared with nitrophorin 1 and nitrophorin 4, which show not a preference (A to B ratio of approximately 1:1), suggesting that it fits more tightly in the β-barrel protein. In this work we have prepared a series of "belt" mutants of NP2(D1A) and (ΔM0)NP2 aimed at reducing the size of aromatic or other residues that surround the heme, and investigated them as the high-spin aqua and low-spin N-methylimidazole complexes. The belt mutants included Y38A, Y38F, F42A, F66A, Y85A, Y85F, Y104A, I120T, and a triple mutant of NP2(D1A), the F42L, L106F, I120T mutant. Although I120 has been mainly considered to be a distal pocket residue, CδH3 of I120 lies directly above the heme 3-methyl, at 2.67 Å, of heme orientation B, or the 2-vinyl of A, and it thus plays a role as a belt mutant, a role that turns out to be extremely important in creating the strong favoring of the B heme orientation [A to B ratio of 1:14 for NP2(D1A) or 1:12 for (ΔM0)NP2]. The results show that the 1D (1)H NMR spectra of the high-spin forms are quite sensitive to changes in the shape of the heme binding cavity. The single mutation I120T eliminates the favorability of the B heme orientation by producing a heme A to B orientation ratio of 1:1, whereas the single mutation F42A reverses the heme orientation from an A to B ratio of 1:14 seen for NP2(D1A) to 10:1 for NP2(D1A,F42A). The most extreme ratio was found for the triple mutant of NP2(D1A), NP2(D1A,F42L,L105F,I120T), in which the A to B ratio is approximately 25:1, a ΔG change of about -3.5 kcal/mol or -14.1 kJ/mol with respect to NP2(D1A). The seating of the heme is modified as well in that mutant and in several others, by rotations of the heme by up to 4° from the seating observed in NP2(D1A), in order to relieve steric interactions between a vinyl β-carbon and a protein side chain, or to fill a cavity created by replacing a large protein side chain by a much smaller one; the latter was observed for all tyrosine to alanine mutants. These relatively small changes in seating have a measurable effect on the NMR spectra of the mutants, but are indeed minor in terms of overall seating and reactivity of the NP2(D1A) protein. The (1)H NMR resonances of the hemin substituents of the low-spin N-methylimidazole complexes of NP2(D1A,F42L,L105F,I120T) as well as NP2(D1A,I120T), NP2(D1A,Y104A), and NP2(D1A,F42A) have been assigned using natural abundance (1)H{(13)C} heteronuclear multiple quantum correlation and (1)H-(1)H nuclear Overhauser effect spectroscopy spectra.

Electron spin density on the axial His ligand of high-spin and low-spin nitrophorin 2 probed by heteronuclear NMR spectroscopy

The electronic structure of heme proteins is exquisitely tuned by the interaction of the iron center with the axial ligands. NMR studies of paramagnetic heme systems have been focused on the heme signals, but signals from the axial ligands have been rather difficult to detect and assign. We report an extensive assignment of the (1)H, (13)C and (15)N resonances of the axial His ligand in the NO-carrying protein nitrophorin 2 (NP2) in the paramagnetic high-spin and low-spin forms, as well as in the diamagnetic NO complex. We find that the high-spin protein has σ spin delocalization to all atoms in the axial His57, which decreases in size as the number of bonds between Fe(III) and the atom in question increases, except that within the His57 imidazole ring the contact shifts are a balance between positive σ and negative π contributions. In contrast, the low-spin protein has π spin delocalization to all atoms of the imidazole ring. Our strategy, adequately combined with a selective residue labeling scheme, represents a straightforward characterization of the electron spin density in heme axial ligands.

Native N-terminus nitrophorin 2 from the kissing bug: similarities to and differences from NP2(D1A)

The first amino acid of mature native nitrophorin 2 is aspartic acid, and when expressed in E. coli, the wild-type gene of the mature protein retains the methionine-0, which is produced by translation of the start codon. This form of NP2, (M0)NP2, has been found to have different properties from its D1A mutant, for which the Met0 is cleaved by the methionine aminopeptidase of E. coli (R. E. Berry, T. K. Shokhireva, I. Filippov, M. N. Shokhirev, H. Zhang, F. A. Walker, Biochemistry 2007, 46, 6830). Native N-terminus nitrophorin 2 ((ΔM0)NP2) has been prepared by employing periplasmic expression of NP2 in E. coli using the pelB leader sequence from Erwinia carotovora, which is present in the pET-26b expression plasmid (Novagen). This paper details the similarities and differences between the three different N-terminal forms of nitrophorin 2, (M0)NP2, NP2(D1A), and (ΔM0)NP2. It is found that the NMR spectra of high- and low-spin (ΔM0)NP2 are essentially identical to those of NP2(D1A), but the rate and equilibrium constants for histamine and NO dissociation/association of the two are different.

Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin: formation of a [(heme)(2) (α(1)-Microglobulin)](3) complex

α(1)-Microglobulin (α(1)m) is a 26 kDa plasma and tissue protein belonging to the lipocalin protein family. Previous investigations indicate that the protein interacts with heme and suggest that it has a function in heme metabolism. However, detailed characterizations of the α(1)m-heme interactions are lacking. Here, we report for the first time the preparation and analysis of a stable α(1)m-heme complex upon carbamidomethylation of the reactive Cys34 by using recombinantly expressed human α(1)m. Analytical size-exclusion chromatography coupled with a diode-array absorbance spectrophotometry demonstrates that at first an α(1)m-heme monomer is formed. Subsequently, a second heme triggers oligomerization that leads to trimerization. The resulting (α(1)m[heme](2))(3) complex was characterized by resonance Raman and EPR spectroscopy, which support the presence of two ferrihemes, thus indicating an unusual spin-state admixed ground state with S=(3)/(2), (5)/(2).

Glycoinositolphospholipids from Trypanosomatids subvert nitric oxide production in Rhodnius prolixus salivary glands

Background

Rhodnius prolixus is a blood-sucking bug vector of Trypanosoma cruzi and T. rangeli. T. cruzi is transmitted by vector feces deposited close to the wound produced by insect mouthparts, whereas T. rangeli invades salivary glands and is inoculated into the host skin. Bug saliva contains a set of nitric oxide-binding proteins, called nitrophorins, which deliver NO to host vessels and ensure vasodilation and blood feeding. NO is generated by nitric oxide synthases (NOS) present in the epithelium of bug salivary glands. Thus, T. rangeli is in close contact with NO while in the salivary glands.

Methodology/Principal Findings

Here we show by immunohistochemical, biochemical and molecular techniques that inositolphosphate-containing glycolipids from trypanosomatids downregulate NO synthesis in the salivary glands of R. prolixus. Injecting insects with T. rangeli-derived glycoinositolphospholipids (Tr GIPL) or T. cruzi-derived glycoinositolphospholipids (Tc GIPL) specifically decreased NO production. Salivary gland treatment with Tc GIPL blocks NO production without greatly affecting NOS mRNA levels. NOS protein is virtually absent from either Tr GIPL- or Tc GIPL-treated salivary glands. Evaluation of NO synthesis by using a fluorescent NO probe showed that T. rangeli-infected or Tc GIPL-treated glands do not show extensive labeling. The same effect is readily obtained by treatment of salivary glands with the classical protein tyrosine phosphatase (PTP) inhibitor, sodium orthovanadate (SO). This suggests that parasite GIPLs induce the inhibition of a salivary gland PTP. GIPLs specifically suppressed NO production and did not affect other anti-hemostatic properties of saliva, such as the anti-clotting and anti-platelet activities.

Conclusions/Significance

Taken together, these data suggest that trypanosomatids have overcome NO generation using their surface GIPLs. Therefore, these molecules ensure parasite survival and may ultimately enhance parasite transmission.

Assignment of the 1H NMR resonances of protein residues in close proximity to the heme of the nitrophorins: similarities and differences among the four proteins from the saliva of the adult blood-sucking insect Rhodnius prolixus

The nuclear Overhauser effects (NOEs) observed between heme substituent protons and a small number of nearby protein side chain protons in the water-elimination Fourier transform NOE spectroscopy (WEFT-NOESY) spectra of high- and low-spin wild-type nitrophorin (NP) 2 and its ligand complexes have been analyzed and compared with those observed for the same complexes of wild-type NP3. These assignments were made on naturally abundant isotope samples, with the most useful protein side chains being those of Ile120, Leu122, and Leu132 for NP2 and NP3, and Thr121, Leu123, and Leu133 for NP1 and NP4. It is found that the NOEs observed are identical, with extremely similar protein side chain proton chemical shifts. This is strong evidence that the structure of NP3, for which no X-ray crystal structures are available, is essentially identical to that of NP2, at least near the heme binding pocket. Similarly, the NOEs observed between heme substituents and protein side chains for NP1 and NP4 also indicate that the structures of the protein having both A and B heme orientations are very similar to each other, as well as to the proteins with major B heme orientation of NP2 and NP3. These A and B connectivities can be seen, even though the two heme orientations have similar populations in NP1 and NP4, which complicates the analysis of the NOESY spectra. The histamine complex of wild-type NP2 shows significant shifts of the Leu132 side chain protons relative to all other ligand complexes of NP1-NP4 because of the perturbation of the structure near Leu132 caused by the histamine's side chain ammonium hydrogen bond to the Asp29 side chain carboxylate.

NMR studies of nitrophorin distal pocket side chain effects on the heme orientation and seating of NP2 as compared to NP1

The nitrophorins (NP) of the adult blood-sucking insect Rhodnius prolixus fall into two pairs based on sequence identity (NP1,4 (90%) and NP2,3 (79%)), which differ significantly in the size of side chains of residues which contact the heme. These residues include those in the distal pocket of NP2 (I120) and NP1 (T121) and the "belt" that surrounds the heme of NP2 (S40, F42), and NP1(A42, L44). To determine the importance of these residues and others conserved or very similar for the two pairs, including L122(123), L132(133), appropriate mutants of NP2 and NP1 have been prepared and studied by (1)H NMR spectroscopy. Wild-type NP2 has heme orientation ratio (A:B) of 1:8 at equilibrium, while wild-type NP1 has A:B ~1:1 at equilibrium. Another difference between NP2 and NP1 is in the heme seating with regard to His57(59). It is found that among the distal pocket residues investigated, the residue most responsible for heme orientation and seating is I120(T121). F42(L44) and L106(F107) may also be important, but must be investigated in greater detail.

Nitrophorins: nitrite disproportionation reaction and other novel functionalities of insect heme-based nitric oxide transport proteins

Nitrophorins (NPs) comprise a unique class of heme proteins used by the blood-sucking insect Rhodnius prolixus to deliver the signaling gas molecule NO into the blood vessel of a host during feeding. Upon NO release, histamine can be scavenged by coordination to the heme iron. Although the protein is of similar size as the mammalian globin monomers and shares the same cofactor and proximal histidine coordination, nitrophorin structure, in contrast, is almost entirely composed of a β-barrel. Comparison of the NO and histamine association constants with the concentrations of both compounds invivo raises concerns about the very simple ligand release model in case of at least some of the NPs. Therefore, novel functionalities of the NPs were sought. As a result, catalysis of the nitrite disproportionation reaction was found, which leads to the formation of NO with nitrite as the sole substrate. This is the first example of a ferriheme protein that can perform this reaction. Furthermore, although NPs stabilize the ferriheme state, a peroxidase reactivity of the cofactor involving the higher oxidation state iron (Compound I/II) was studied with the potential to catalyze the oxidation of histamine and norepinephrine. In contrast to many other heme proteins including the globins, the ferroheme state was found to be extremely sensitive to O(2) , which is a consequence of the much lower reduction potential of the NPs, so that the 1-electron reduction of O(2) to O (•-)(2) becomes a thermodynamically favored process. Altogether, the detailed study of the NPs gives insight into the structure-function relationships required for the targeted delivery of diatomic gas molecules in biology. Moreover, the comparison of the structure-function relationships of the NPs (NO transporters) with those of the globins (O(2) transporters) will help to elucidate the architectural requirement for the respective tasks.

Ultrafast dynamics of diatomic ligand binding to nitrophorin 4

Nitrophorin 4 (NP4) is a heme protein that stores and delivers nitric oxide (NO) through pH-sensitive conformational change. This protein uses the ferric state of a highly ruffled heme to bind NO tightly at low pH and release it at high pH. In this work, the rebinding kinetics of NO and CO to NP4 are investigated as a function of iron oxidation state and the acidity of the environment. The geminate recombination process of NO to ferrous NP4 at both pH 5 and pH 7 is dominated by a single approximately 7 ps kinetic phase that we attribute to the rebinding of NO directly from the distal pocket. The lack of pH dependence explains in part why NP4 cannot use the ferrous state to fulfill its function. The kinetic response of ferric NP4NO shows two distinct phases. The relative geminate amplitude of the slower phase increases dramatically as the pH is raised from 5 to 8. We assign the fast phase of NO rebinding to a conformation of the ferric protein with a closed hydrophobic pocket. The slow phase is assigned to the protein in an open conformation with a more hydrophilic heme pocket environment. Analysis of the ultrafast kinetics finds the equilibrium off-rate of NO to be proportional to the open state population as well as the pH-dependent amplitude of escape from the open pocket. When both factors are considered, the off-rate increases by more than an order of magnitude as the pH changes from 5 to 8. The recombination of CO to ferrous NP4 is observed to have a large nonexponential geminate amplitude with rebinding time scales of approximately 10(-11)-10(-9) s at pH 5 and approximately 10(-10)-10(-8) s at pH 7. The nonexponential CO rebinding kinetics at both pH 5 and pH 7 are accounted for using a simple model that has proven effective for understanding CO binding in a variety of other heme systems (Ye, X.; et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 14682).

1H and 13C NMR spectroscopic studies of the ferriheme resonances of three low-spin complexes of wild-type nitrophorin 2 and nitrophorin 2(V24E) as a function of pH

The ferriheme resonances of the low-spin (S = 1/2) complexes of wild-type (wt) nitrophorin 2 (NP2) and its heme pocket mutant NP2(V24E) with imidazole (ImH), histamine (Hm), and cyanide (CN⁻) as the sixth ligand have been investigated by NMR spectroscopy as a function of pH (4.0–7.5). For the three wt NP2 complexes, the ratio of the two possible heme orientational isomers, A and B, remains almost unchanged (ratio of A:B approximately 1:6 to 1:5) over this wide pH range. However, strong chemical exchange cross peaks appear in the nuclear Overhauser effect spectroscopy/exchange spectroscopy (NOESY/EXSY) spectra for the heme methyl resonances at low pH (pH* 4.0–5.5), which indicate chemical exchange between two species. We have shown these to be two different exogenous ImH or Hm orientations that are denoted B and B′, with the ImH plane nearly parallel and perpendicular to the ImH plane of the protein-provided His57, respectively. The wt NP2–CN complex also shows EXSY cross peaks due to chemical exchange, which is shown to be a result of interchange between two ruffling distortions of the heme. The same ruffling distortion interchange is also responsible for the ImH and Hm chemical exchange. For the three NP2(V24E) ligand complexes, no EXSY cross peaks are observed, but the A:B ratios change dramatically with pH. The fact that heme favors the A orientation highly for NP2(V24E) at low pH as compared with wt NP2 is believed to be due to the steric effect of the V24E mutation. The existence of the B′ species at lower pH for wt NP2 complexes and the increase in A heme orientation at lower pH for NP2(V24E) are believed to be a result of a change in structure near Glu53 when it is protonated at low pH. ¹H{¹³C} heteronuclear multiple quantum coherence (HMQC) spectra are very helpful for the assignment of heme and nearby protein side chain resonances.

pH-dependent mechanism of nitric oxide release in nitrophorins 2 and 4

Nitrophorins are NO carrier proteins that transport and release NO through a pH-dependent conformational change. They bind NO tightly in a low pH environment and release it in a higher pH environment. Experimental evidence shows that the increase in the NO dissociation equilibrium constant, K(d), is due mainly to an increase in the NO release rate. Structural and kinetic data strongly suggest that NPs control NO escape by modulating its migration from the active site to the solvent through a pH-dependent conformational change. NP2 and NP4 are two representative proteins of the family displaying a 39% overall sequence identity, and interestingly, NP2 releases NO slower than NP4. The proposal that NPs' NO release relies mainly on the NO escape rate makes NPs a very peculiar case among typical heme proteins. The connection between the pH-dependent conformational change and ligand release mechanism is not fully understood and the structural basis for the pH induced structural transition and the different NO release patterns in NPs are unresolved, yet interesting issues. In this work, we have used state of the art molecular dynamics simulations to study the NO escape process in NP2 and NP4 in both the low and high pH states. Our results show that both NPs modulate NO release by switching between a "closed" conformation in a low pH environment and an "open" conformation at higher pH. In both proteins, the change is caused by the differential protonation of a common residue Asp30 in NP4 and Asp29 in NP2, and the NO escape route is conserved. Finally, our results show that, in NP2, the conformational change to the "open" conformation is smaller than that for NP4 which results in a higher barrier for NO release.

Cytochemical characterization of Triatoma infestans and Panstrongylus megistus salivary gland cells (Hemiptera, Reduviidae, Triatominae)

The Triatominae subfamily has medical sanitary importance, since these insects are vectors of Trypanosoma cruzi, etiologic agent of Chagas Disease, and Trypanosoma rangeli, which develops in the salivary glands and it is frequently found in mixed infections with T. cruzi. Triatomines of Triatoma and Panstrongylus genera possess a salivary gland complex composed of a pair with three well differentiated units: the anterior (D1), median (D2) and posterior (D3). Saliva is secreted during blood meal and antagonizes hemostatic, inflammatory and immunological systems imposed by the host, which facilitate the hematophagy. In order to identify nuclear structures, we studied interphase nuclei of salivary gland cells of adult insects, males and females, of Triatoma infestans and Panstrongylus megistus. The glands were removed from insects, fixed in acetic acid (45%) and lactic acid (50%), squashed and submitted to different cytochemical methods: lacto-acetic orcein, silver ion impregnation, Feulgen reaction, Toluidine Blue, Variant method of critical electrolyte concentration and C-banding. The preparations were examined with a Zeiss Jenaval photomicroscope and photographed. The results evidenced predominance of binucleated cells in D1 and D2 glands and mononucleated ones in D3. In all salivary glands were observed bulky and polyploid nucleus, a clear association between nucleolar and heterochromatic corpuscles, cytoplasmatic metachromasy and many pre-secretion vesicles in cytoplasm. Such characteristics were associated with intense synthesis activity to produce the saliva. Species were mainly differentiated by a larger heterochromatic corpuscle observed only in T. infestans (called as chromocenter), while P. megistus showed a predominance of euchromatin, with some heterochromatic corpuscles just in males. Females of both species showed a smaller quantity of heterochromatin, which could be related to the high metabolism because of the oviposition.

Assignment of the ferriheme resonances of high- and low-spin forms of the symmetrical hemin-reconstituted nitrophorins 1-4 by 1H and 13C NMR spectroscopy: the dynamics of heme ruffling deformations

The four major nitrophorins (NPs) of the adult blood-sucking insect Rhodnius prolixus have been reconstituted with the "symmetrical hemin" 2,4-dimethyldeuterohemin, and their NMR spectra have been investigated as the high-spin (S=5/2) aqua and low-spin (S=1/2) N-methylimidazole (NMeIm) and cyanide complexes. The NMeIm complexes allow assignment of the high-spin hemin resonances by saturation transfer difference spectroscopy. The cyanide complexes were investigated as paramagnetic analogues of the NO complexes. It is shown that the hemin ring is highly distorted from planarity, much more so for NP2 than for NP1 and NP4 (with ruffling being the major distortion mode), for both high- and low-spin forms. For the cyanide complexes, the conformation of the distorted ring changes on the NMR timescale to yield chemical exchange (exchange spectroscopy, EXSY) cross peaks for NP1sym(CN), NP3sym(CN) and NP4sym(CN) but not for NP2sym(CN). These changes in nonplanar conformation are visualized as a "rolling" of the ruffled macrocycle ridges through some number of degrees, the lowest-energy ruffling mode. This probably occurs in response to slow protein dynamics that cause the I120 and L132 side chains in the distal heme pocket to move in opposite directions (up and away vs. down and toward the hemin ring). This in turn changes the out-of-plane displacements of the 2M and 3M of the symmetrical hemin on the NMR timescale. Two other types of dynamics, i.e., changes in heme seating and NMeIm rotation, are also observed. The highly distorted heme and the dynamics it causes are unique to the NPs and a few other heme proteins with highly distorted macrocycles.

Assignment of Ferriheme Resonances for High- and Low-Spin Forms of Nitrophorin 3 by H and C NMR Spectroscopy and Comparison to Nitrophorin 2: Heme Pocket Structural Similarities and Differences

Nitrophorin 3 (NP3) is the only one of the four major NO-binding heme proteins found in the saliva of the blood-sucking insect Rhodnius prolixus (also called the Kissing Bug) for which it has not been possible to obtain crystals of diffraction quality for structure determination by X-ray crystallography. Thus we have used NMR spectroscopy, mainly of the hyperfine-shifted ferriheme substituent resonances, to learn about the similarities and differences in the heme pocket and the iron active site of NP3 as compared to NP2, which has previously been well-characterized by both X-ray crystallography and NMR spectroscopy. Only one residue in the heme pocket differs between the two, F27 of NP2 is Y27 for NP3; in both cases this residue is expected to interact strongly with the 2-vinyl side chain of the B heme rotational isomer or the 4-vinyl of the A heme rotational isomer. Both the high-spin (S = 5/2) aquo complex, NP3-H(2)O, and the low-spin (S = 1/2) N-methylimidazole (NMeIm) complex of NP3 have been studied. It is found that the chemical shifts of the protons of both forms are similar to those of the corresponding NP2 complexes, but with minor differences that indicate a slightly different angle for the proximal histidine (H57) ligand plane. The B heme rotational isomer is preferred by both NP3 and NP2 in both spin states, but to a greater extent when phenylalanine is present at position 27 (A:B = 1:8 for NP2, 1:6 for NP3-Y27F, 1:4 for NP3, and 1:3 for NP2-F27Y). Careful analysis of the 5Me and 8Me shifts of the A and B isomers of the two high-spin nitrophorins leads to the conclusion that the heme environment for the two isomers differs in some way that cannot be explained at the present time. The kinetics of deprotonation of the high-spin complexes of NP2 and NP3 are very different, with NP2 giving well-resolved high-spin aquo and "low-spin" hydroxo proton NMR spectra until close to the end of the titration, while NP3 exhibits broadened (1)H NMR spectra indicative of an intermediate rate of exchange on the NMR timescale between the two forms throughout the titration. The heme methyl shifts of NP2-OH are similar in magnitude and spread to those of NP2-CN, while those of metmyoglobin-hydroxo complexes are much larger in magnitude but not spread. It is concluded that the hydroxo complex of NP2 is likely S = 1/2 with a mixed (d(XY))(2)(d(XZ),d(YZ))(3)/(d(xy))(1)(d(xz),d(yz))(4) electron configuration, while those of met-Mb-OH are likely S = 1/2,3/2 mixed spin systems.

The sialotranscriptome of the blood-sucking bug Triatoma brasiliensis (Hemiptera, Triatominae)

Triatoma brasiliensis is the most important autochthon vector of Trypanosoma cruzi in Brazil, where it is widely distributed in the semiarid areas of the Northeast. In order to advance the knowledge of the salivary biomolecules of Triatominae, a salivary gland cDNA library of T. brasiliensis was mass sequenced and analyzed. Polypeptides were sequenced by HPLC/Edman degradation experiments. Then 1712 cDNA sequences were obtained and grouped in 786 clusters. The housekeeping category had 24.4% and 17.8% of the clusters and sequences, respectively. The putatively secreted category contained 47.1% of the clusters and 68.2% of the sequences. Finally, 28.5% of the clusters, containing 14% of all sequences, were classified as unknown. The sialoma of T. brasiliensis showed a high amount and great variety of different lipocalins (93.8% of secreted proteins). Remarkably, a great number of serine proteases that were not observed in previous blood-sucking sialotranscriptomes were found. Nine Kazal peptides were identified, among them one with high homology to the tabanid vasodilator vasotab, suggesting that the Triatoma vasodilator could be a Kazal protein.

Effect of the N-terminus on heme cavity structure, ligand equilibrium, rate constants, and reduction potentials of nitrophorin 2 from Rhodnius prolixus

The D1A mutant of recombinant NP2 has been prepared and shown to have the expression-initiation methionine-0 cleaved during expression in E. coli, as is the case for recombinant NP4, where Ala is the first amino acid for the recombinant protein as well as for the mature native protein. The heme substituent 1H NMR chemical shifts of NP2-D1A and those of its imidazole, N-methylimidazole, and cyanide complexes are rather different from those of NP2-M0D1. This difference is likely due to the much smaller size of the N-terminal amino acid (A) of NP2-D1A, which allows the formation of the closed loop form of this protein, as it does for NP4 (Weichsel, A., Andersen, J. F., Roberts, S. A., and Montfort, W. R. (2000) Nature Struct. Biol. 7, 551-554). The ratio of the two hemin rotational isomers A and B is different for the two proteins, and the rate at which the A:B ratio reaches equilibrium is strikingly different (NP2-M0D1 t1/2 for heme rotation approximately 2 h, NP2-D1A t1/2 approximately 43 h). This difference is consistent with the high stability of the closed loop form of the NP2-D1A protein and infrequent opening of the loops that could allow heme to at least partially exit the binding pocket in order to rotate about its alpha,gamma-meso axis. Consistent with this, the rates of histamine binding and release to/from NP2-D1A are significantly slower than those for NP2-M0D1 at pH 7.5. This work suggests that care must be taken in interpreting data obtained from proteins that carry the expression-initiation M0.

Distribution and characterization of nitric oxide synthase in the nervous system of Triatoma infestans (Insecta: Heteroptera)

The biochemical characterization of nitric oxide synthase (NOS) and its distribution in the central nervous system (CNS) were studied in the heteropteran bug Triatoma infestans. NOS-like immunoreactivity was found in the brain, subesophageal ganglion, and thoracic ganglia by using immunocytochemistry. In the protocerebrum, NOS-immunoreactive (IR) somata were detected in the anterior, lateral, and posterior soma rinds. In the optic lobe, numerous immunostained somata were observed at the level of the first optic chiasma, around the lobula, and in the proximal optic lobe. In the deutocerebrum, NOS-IR perikarya were mainly observed in the lateral soma rind, surrounding the sensory glomeruli, and a few cell bodies were seen in association with the antennal mechanosensory and motor neuropil. No immunostaining could be detected in the antennal nerve. The subesophageal and prothoracic ganglia contained scattered immunostained cell bodies. NOS-IR somata were present in all the neuromeres of the posterior ganglion. Western blotting showed that a universal NOS antiserum recognized a band at 134 kDa, in agreement with the expected molecular weight of the protein. Analysis of the kinetics of nitric oxide production revealed a fully active enzyme in tissue samples of the CNS of T. infestans.

Assignment of the ferriheme resonances of the high-spin forms of nitrophorins 1 and 4 by 1H NMR spectroscopy: comparison to structural data obtained from X-ray crystallography

In this work, we report the assignment of the majority of the ferriheme resonances of high-spin nitrophorins (NPs) 1 and 4 and compare them to those of NP2, published previously. It is found that the structures of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the histidine imidazole ligand, can be described with good accuracy by NMR techniques and that the angle plot proposed previously for the high-spin form of the NPs (Shokhireva, T. Kh.; Shokhirev, N. V.; Walker, F. A. Biochemistry 2003, 42, 679-693) describes the angle of the effective nodal plane of the axial histidine imidazole in solution. There is an equilibrium between the two heme orientations (A and B), which depends on the heme cavity shape, which can be altered by mutation of amino acids with side chains (phenyl vs tyrosyl) near the potential position where a heme vinyl group would be in one of the isomers. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.

Salivation pattern of Rhodnius prolixus (Reduviidae; Triatominae) in mouse skin

The objective of this work was to study the pattern of salivation of triatomines during feeding in mouse skin. Rhodnius prolixus was fed with a solution of the dye acridine orange or fluorescein. The saliva was efficiently labelled with acridine orange, probably due to the difference in pH between the salivary gland (6.0) and the hemolymph (6.5-7.0). This procedure was not effective at labelling the saliva of Triatoma infestans, however, fluorescent labelling of R. prolixus saliva allowed us to demonstrate that salivation occurs during entire feeding process. The saliva is released soon after the bite. In the probing phase, saliva is pumped continuously in the host skin, including around the blood vessels. During the engorgement phase, saliva is observed in a bolus within the blood vessel and some of it is sucked up by the insect, together with blood. The frequency of saliva emission inside the vessels was low (0.51+/-0.18 Hz). The saliva deposition in the microcirculation is continuous and modulated by the frequency of the cibarial pump because, when functioning at high frequency, cibarial pump sucks almost all saliva to the insect gut. This mechanism would determine the quantity of saliva deposited in the microcirculation as necessary, and consequently minimizing the host's immune response to salivary antigens.

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 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.

Nitrophorin synthesis is modulated by protein kinase CK2

Rhodnius prolixus is a blood-sucking bug whose saliva contains a family of nitric oxide-carrying proteins named nitrophorins (NPs). Saliva is injected into the host bloodstream during insect feeding. Nitric oxide is then released from NPs and will act on vascular smooth muscle, promoting vasodilation. Epithelial cells of salivary glands then undergo a massive synthesis of antihemostatics including NPs which produces saliva for the next blood meal. Here, we demonstrate the transient activation of a protein kinase in the salivary glands of R. prolixus after a blood meal. Biochemical, immunological, and pharmacological assays were used to identify this enzyme as protein kinase CK2. CK2 is activated after a blood meal and decreases to basal levels when salivary gland refilling is resumed. Inhibition of CK2 blocked [(35)S]methionine incorporation into newly synthesized salivary gland proteins in cultured tissue. Dissected salivary glands were then incubated with the heme fluorescent analog palladium (II) mesoporphyrin IX (Pd-MP) in the presence of a selective cell-permeable CK2 inhibitor, TBB (4,5,6,7-tetrabromobenzotriazole). NP synthesis was quantified based on fluorescence of the Pd-MP group bound to the NP heme pocket. TBB dramatically blocked NP synthesis. Altogether, these data are the first demonstration to show that antihemostatic synthesis in a blood-sucking arthropods is under protein phosphorylation control.

Nitric oxide modulates peristaltic muscle activity associated with fluid circulation in the sea pansy Renilla koellikeri

Nitric oxide (NO) is a well-known regulator of vascular activities in vertebrates and it has also been implicated as a vasodilatatory agent in a cephalopod. In the sea pansy Renilla koellikeri, an octocorallian representative of the most basal animals with a nervous system, we investigated the role of NO in peristalsis, an activity that moves body fluids through the coelenteron (gastrovascular cavity) of the polyps across the colony. NO donors increased the amplitude of peristaltic contractions and increased tonic contractions in relaxed preparations, but caused a relaxation of basal tension in contracted preparations. The NO synthase (NOS) inhibitors L-NAME (N(ω)-nitro-L-arginine methyl ester) and 7-nitroindazole reduced the amplitude of peristaltic contractions and lowered basal tension. In contrast, aminoguanidine, a specific inhibitor of inducible NOS, increased the amplitude but reduced the rate of peristalsis. Zaprinast, a cGMP-specific phosphodiesterase inhibitor, decreased the amplitude of peristaltic contractions, a decrease that was amplified by dibutyryl cGMP. In contrast,the inhibitor of soluble guanylyl cyclase ODQ(1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one) enhanced peristalsis. Putative NOS-containing neurons, revealed by NADPH-diaphorase activity and citrulline immunohistochemistry, were observed in the basiectoderm at the base of the autozooid polyp tentacles and in a nerve-net around the oral disc. Their neurites ran up the tentacles and down to the polyp body wall, crossing from the ectoderm through the mesoglea and into the endoderm musculature where musculo-epithelial cells were also reactive. These data suggest that two distinct nitrergic pathways, one of which is mediated by cGMP, regulate peristalsis and muscle tone in the sea pansy and that these pathways may involve NOS-containing ectodermal neurons and musculo-epithelial cells.

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.

Axial ligand complexes of the Rhodnius nitrophorins: reduction potentials, binding constants, EPR spectra, and structures of the 4-iodopyrazole and imidazole complexes of NP4

Previously, we utilized 4-iodopyrazole (4IPzH) as a heavy atom derivative for the initial solution of the crystal structure of the nitrophorin from Rhodnius prolixus, NP1, where it was found to bind to the heme with the iodo group disordered in two positions. We have now determined the structure of the 4IPzH complex of NP4 at pH 7.5 and find that the geometry and bond lengths at the iron center are extremely similar to those of the imidazole (ImH) complex of the same protein (structure determined at pH 5.6), except that the G-H loop is not in the closed conformation. 4IPzH binds to the heme of NP4 in an ordered manner, with the iodo substituent pointed toward the opening of the heme pocket, near the surface of the protein. In order to understand the solution chemistry in terms of the relative binding abilities of 4IPzH, ImH, and histamine (Hm, a physiological ligand for the nitrophorins), we have also investigated the equilibrium binding constants and reduction potentials of these three ligand complexes of the four Rhodnius nitrophorins as a function of pH. We have found that, unlike the other Lewis bases, 4IPzH forms less stable complexes with the Fe(III) than the Fe(II) oxidation states of NP1 and NP4, and similar stability for the two oxidation states of NP2 and NP3, suggesting that this ligand is a softer base than ImH or Hm, for both of which the Fe(III) complexes are more stable than those of Fe(II) for all four nitrophorins. Surprisingly, in spite of this and the much lower basicity of 4IPzH than imidazole and histamine, the EPR g-values of all three ligand complexes are very similar.

Electrochemical and NMR spectroscopic studies of distal pocket mutants of nitrophorin 2: stability, structure, and dynamics of axial ligand complexes

WT and leucine --> valine distal pocket mutants of nitrophorin 2 (NP2) and their NO complexes have been investigated by spectroelectrochemistry. NO complexes of two of the mutants exhibit more positive reduction potential shifts than does the WT protein, thus indicating stabilization of the Fe(II)-NO state. This more positive reduction potential for NP2-L132V and the double mutant is consistent with the hypothesis that smaller valine residues may allow the heme to regain planarity instead of being significantly ruffled, as in WT NP2. Thus, ruffling may stabilize the Fe(III)-NO state, which is required for facile NO dissociation. NMR spectroscopic investigations show that the sterically demanding 2-methylimidazole ligand readily binds to all three distal pocket mutants to create low-spin Fe(III) complexes having axial ligands in nearly perpendicular planes; it also binds to the WT protein in the presence of higher concentrations of 2-methylimidazole, but yields a different ligand plane orientation than is present in any of the three distal pocket mutants. NOESY spectra of NP2-ImH mutants exhibit chemical exchange cross peaks, whereas WT NP2-ImH shows no chemical exchange. Chemical exchange in the case of the distal leucine --> valine mutants is caused by ImH ligand orientational dynamics. The two angular orientations of the ImH ligand could be determined from the (1)H chemical shifts of the heme methyls, and the rate of interconversion of the two forms could be estimated from the NOESY diagonal and cross peak intensities. K(eq) is 100 or larger and favors an orientation similar to that found for the WT NP2-ImH complex.

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.

Cloning and molecular characterization of two mosquito iron regulatory proteins

Iron regulatory proteins (IRPs) control the synthesis of various proteins at the translational level by binding to iron responsive elements (IREs) in the mRNAs. Iron, infection, and stress can alter IRP/IRE binding activity. Insect messenger RNAs for ferritin and succinate dehydrogenase subunit b have IREs that are active translational control sites. We have cloned and sequenced cDNAs encoding proteins from the IRP1 family for the mosquitoes, Aedes aegypti and Anopheles gambiae. Both deduced amino acid sequences show substantial similarity to human IRP1 and Drosophila IRP1A and IRP1B, and all of the residues thought to be involved in aconitase activity and iron-sulfur cluster formation are conserved. Recombinant A. aegypti IRP1 binds to transcripts of the IREs of mosquito or human ferritin subunit mRNAs. No significant change in A. gambiae IRP1 messenger RNA could be detected during the various developmental stages of the life cycle, following iron loading by blood feeding, or after bacterial or parasitic infections. These data suggest that there is no change in gene transcription. Furthermore, bacterial challenge of A. gambiae cells did not change IRP1 protein levels. In contrast, IRP1 binding activity for the IRE was elevated following immune induction. These data show that changes in IRP1/IRE binding activity occur as part of the insect immune response.

Nitrophorins and related antihemostatic lipocalins from Rhodnius prolixus and other blood-sucking arthropods

Recent gene sequence and crystal structure determinations of salivary proteins from several blood-sucking arthropods have revealed an unusual evolutionary relationship: many such proteins derive their functions from lipocalin protein folds. Many blood-sucking arthropods have independently evolved the ability to overcome a host organism's means of preventing blood loss (called hemostasis). Most blood feeders have proteins that induce vasodilation, inhibit blood coagulation, and reduce inflammation, but do so by distinctly different mechanisms. Despite this diversity, in many cases the antihemostatic activities in such organisms reside in proteins with lipocalin folds. Thirteen such lipocalins are described in this review, with a particular focus on the heme-containing nitrophorins from Rhodnius prolixus, which transport nitric oxide, sequester histamine, and disrupt blood coagulation. Also described are the antiplatelet compounds RPAI, moubatin, and pallidipin from R. prolixus, Ornithodoros moubata, and Triatoma pallidipennis; the antithrombin protein triabin from T. pallidipennis; and the tick histamine binding proteins from Rhipicephalus appendiculatus.

Nadph-diaphorase activity in corpus allatum cells of the cockroach, Diploptera punctata

Using the fixation insensitive NADPH-diaphorase reaction as a histochemical marker for the enzyme nitric oxide synthase (NOS), we investigated the possible sites of putatively NOS-related NADPH-diaphorase in the brain and retrocerebral complex of the cockroach, Diploptera punctata. In the cerebral ganglion, NADPH-diaphorase expression was localized in antennal lobes, optic lobes, mushroom bodies and neurosecretory cells. The highest NADPH activity was detected in the corpora allata (CA). Spectrophotometric quantitation indicated that NADPH-diaphorase activity first increased and then decreased (cycled) in the CA of mated females. In addition, during the first ovarian cycle, NADPH-diaphorase activity fluctuated concurrently with cyclic changes in the size of corpus allatum cells. In virgin females, NADPH-diaphorase activity remained at a low level, but it increased if the neural connectives between CA and brain were severed, indicating that the brain inhibited NADPH-diaphorase expression in the CA. Although nerve terminals were abundant in the CA, NADPH-diaphorase was clearly endogenous and synthesized by glandular cells, as was shown by histochemical staining of the cytosol in all dissociated cells of the CA. We have also demonstrated NADPH-diaphorase activity in the CA of the American cockroach Periplaneta americana, the house cricket Acheta domesticus, the lepidopteran Leucania loreyi, and the fruit fly Drosophila melanogaster, suggesting that NOS occurs in the CA of most, if not all insects. It is therefore possible that corpus allatum cells release NO, along with juvenile hormone, which presumably can function as a messenger molecule.

Nitric oxide synthase and cGMP activity in the salivary glands of the American dog tick Dermacentor variabilis

We colocalized nitric oxide synthase (NOS) activity in epithelial cells that surround the salivary gland duct in female Dermacentor variabilis with NADPH diaphorase histochemistry and immunohistochemistry using a polyclonal anti-endothelial NOS. Using size-exclusion chromatography, a fraction with a molecular mass of about 185 kDa that had diaphorase activity was eluted from tick salivary gland homogenate. This fraction converted arginine to citrulline with the production of nitric oxide (NO), which was detected by using electron spin resonance spectroscopy. The complete activity of the diaphorase fraction was dependent on NADPH, FAD, tetrahydrobiopterin, calmodulin, (CaM), and Ca(2+), but was not dependent on dithiothreitol. The arginine analog N(G)-monomethyl-L-arginine inhibited the activity of this fraction. NO and arginine activated soluble guanylate cyclase to produce cGMP in dopamine-stimulated isolated salivary glands. Dopamine-stimulated isolated salivary glands treated with tick saline containing either EDTA, the NOS inhibitor N(G)-nitro-L-arginine methyl ester, or the calcium/CaM binding inhibitor W-7 showed no increase in cGMP. The NO donor sodium nitroprusside significantly increased cGMP levels in unstimulated isolated salivary glands. A possible function for NO in salivation by this ixodid tick is discussed.

Gene structure and polymorphism of an invertebrate nitric oxide synthase gene

Nitric oxide synthases (NOSs) are ubiquitous in living organisms. However, little is known about the evolution of this large gene family. The first inducible NOS to be described from an invertebrate regulates malaria parasite (Plasmodium spp.) development in the mosquito Anopheles stephensi. This single copy gene shows the highest homology to the vertebrate neuronal isoforms, followed by decreasing homology to endothelial and inducible isoforms. The open reading frame of 1247 amino acids is encoded by 19 exons, which span approximately 33 kilobases. More than 50% of the mosquito exons, distributed around the putative heme, calmodulin, and FAD/NADPH cofactor-binding domains, are conserved with those of the three human genes. Repetitive elements identified within the larger introns include a polymorphic dinucleotide repeat, two tandem repeats, and a putative miniature inverted repeat transposable element. Sequence analysis and primer extension indicate that the upstream promoter is 'TATA-less' with multiple transcription start sites within approximately 250 base pairs of the initiation methionine. Transcription factor binding sites in the 5'-flanking sequence demonstrate a bipartite distribution of lipopolysaccharide- and inflammatory cytokine-responsive elements that is strikingly similar to that described for vertebrate inducible NOS gene promoters.

Expression of an inducible nitric oxide synthase gene in rainbow trout Oncorhynchus mykiss

Using oligonucleotide primers based on mammalian nitric oxide synthases (NOS), expression of an inducible NOS (iNOS) gene was detected in head kidney and gill tissue of bacterially-challenged rainbow trout. Three overlapping fragments were amplified by RT-PCR and used to construct a contiguous sequence of 1410bp, with high nucleotide homology to iNOS in birds (61%) and mammals (57-59%). The nucleotide sequence translated in one reading frame to produce a partial peptide containing 470 amino acids, with 69-71% amino acid homology with mammalian iNOS, 81% homology with chicken iNOS and 85% homology with a partial (492bp) goldfish iNOS sequence. In vitro stimulation of head kidney macrophages with LPS also induced expression of the trout iNOS RNA, with optimal expression seen using 20-50 microg/ml LPS at 2h to 6h post-stimulation. The evolutionary and functional significance of the trout iNOS sequence are discussed.

Malaria infection of the mosquito Anopheles gambiae activates immune-responsive genes during critical transition stages of the parasite life cycle

Six gene markers have been used to map the progress of the innate immune response of the mosquito vector, Anopheles gambiae, upon infection by the malaria parasite, Plasmodium berghei. In addition to four previously reported genes, the set of markers included NOS (a nitric oxide synthase gene fragment) and ICHIT (a gene encoding two putative chitin-binding domains separated by a polythreonine-rich mucin region). In the midgut, a robust response occurs at 24 h post-infection, at a time when malaria ookinetes traverse the midgut epithelium, but subsides at later phases of malaria development. In contrast, the salivary glands show no significant response at 24 h, but are activated in a prolonged late phase when sporozoites are released from the midgut into the haemolymph and invade the glands, between 10 and 25 days after blood feeding. Furthermore, the abdomen of the mosquito minus the midgut shows significant activation of immune markers, with complex kinetics that are distinct from those of both midgut and salivary glands. The parasite evidently elicits immune responses in multiple tissues of the mosquito, two of which are epithelia that the parasite must traverse to complete its development. The mechanisms of these responses and their significance for malaria transmission are discussed.

The mosquito Anopheles stephensi limits malaria parasite development with inducible synthesis of nitric oxide

We have discovered that the mosquito Anopheles stephensi, a natural vector of human malaria, limits parasite development with inducible synthesis of nitric oxide (NO). Elevated expression of A. stephensi NO synthase (NOS), which is highly homologous to characterized NOS genes, was detected in the midgut and carcass soon after invasion of the midgut by Plasmodium. Early induction is likely primed by bacterial growth in the blood meal. Later increases in A. stephensi NOS expression and enzyme activity occurred at the beginning of sporozoite release. Circulating levels of nitrite/nitrate, end-products of NO synthesis, were significantly higher in Plasmodium-infected mosquitoes. Dietary provision of the NOS substrate L-arginine reduced Plasmodium infections in A. stephensi. In contrast, dietary provision of a NOS inhibitor significantly increased parasite numbers in infected mosquitoes, confirming that A. stephensi limits Plasmodium development with NO.