Effect of Larval Food Availability on Adult Aedes Aegypti (Diptera: Culicidae) Fitness and Susceptibility to Zika Infection

Aedes (Stegomyia) aegypti (Linnaeus, 1762) is a mosquito species of significant medical importance. The use of this vector in research studies usually requires a large number of mosquitoes as well as rearing and maintenance in a laboratory-controlled environment. However, laboratory conditions may be different from field environments, presenting stressful challenges such as low food concentration, especially during larval stages, which may, in turn, impair vector biology. Therefore, we tested herein if larval food availability (0.004, 0.009, 0.020, and 0.070% diets) would affect overall adult insect fitness. We observed slower development in mosquitoes fed a 0.004% diet 15 d post-eclosion (DPE) and shorter mean time in mosquitoes fed a 0.020% diet (7 DPE). Larval diet and adult mosquito weight were positively correlated, and heavier females fed higher larval diets exhibited greater blood feeding capacity and oviposition. In addition, larval diet concentrations led to median adult lifespan variations (male/female in days-0.004%: 30 ± 1.41, 45 ± 1.3; 0.009%: 31.5 ± 1.33, 41 ± 1.43; 0.020%: 26 ± 1.18, 41 ± 1.45; 0.070%: 29 ± 1.07, 44 ± 1.34), reduced tolerance to deltamethrin (1 mg/m2) and changes in detoxification enzyme activities. Moreover, in the larval 0.070% diet, females presented higher Zika susceptibility (plaque-forming unit [PFU]: 1.218 × 106) compared with other diets (0.004%: 1.31 × 105; 0.009%: 2.0 × 105; 0.020%: 1.25 × 105 PFU). Altogether, our study demonstrates that larval diet restriction results not only in larval developmental arrest but also in adult fitness impairment, which must be considered in future assessments.

Insecticide resistance, fitness and susceptibility to Zika infection of an interbred Aedes aegypti population from Rio de Janeiro, Brazil

Background

Aedes aegypti is a vector of high relevance, since it transmits several arboviruses, including dengue, chikungunya and Zika. Studies on vector biology are usually conducted with laboratory strains presenting a divergent genetic composition from field populations. This may impair vector control policies that were based on laboratory observations employing only long maintained laboratory strains. In the present study we characterized a laboratory strain interbreed with Ae. aegypti collected from five different localities in Rio de Janeiro (Aedes Rio), for insecticide resistance (IR), IR mechanisms, fitness and Zika virus infection.

Methods

We compared the recently established Aedes Rio with the laboratory reference strain Rockefeller. Insecticide resistance (deltamethrin, malathion and temephos), activity of metabolic resistance enzymes and kdr mutation frequency were determined. Some life table parameters (longevity, blood-feeding, number and egg viability) and Zika virus susceptibility was also determined.

Results

Aedes Rio showed resistance to deltamethrin (resistance ratio, RR₅₀ = 32.6) and temephos (RR₅₀ = 7.0) and elevated activity of glutathione S-transferase (GST) and esterases (α-EST and pNPA-EST), but not acetylcholinesterase (AChE). In total, 92.1% of males genotyped for kdr presented a “resistant” genotype. Weekly blood-fed females from both strains, presented reduced mortality compared to sucrose-fed mosquitoes; however, Aedes Rio blood-fed females did not live as long (mean lifespan: Rockefeller = 70 ± 3.07; Aedes Rio = 53.5 ± 2.16 days). There were no differences between strains in relation to blood-feeding and number of eggs, but Aedes Rio eggs presented reduced viability (mean hatch: Rockefeller = 77.79 ± 1.4%; Aedes Rio = 58.57 ± 1.77%). Zika virus infection (plaque-forming unit, PFU) was similar in both strains (mean PFU ± SE: Aedes Rio: 4.53 × 10⁴ ± 1.14 × 10⁴ PFU; Rockefeller: 2.02 × 10⁴ ± 0.71 × 10⁴ PFU).

Conclusion

Selected conditions in the field, such as IR mechanisms, may result in pleiotropic effects that interfere in general physiology of the insect. Therefore, it is important to well characterize field populations to be tested in parallel with laboratory reference strains. This practice would improve the significance of laboratory tests for vector control methods.

Functional and structural similarities of D7 proteins in the independently-evolved salivary secretions of sand flies and mosquitoes

The habit of blood feeding evolved independently in many insect orders of families. Sand flies and mosquitoes belong to separate lineages of blood-feeding Diptera and are thus considered to have evolved the trait independently. Because of this, sand fly salivary proteins differ structurally from those of mosquitoes, and orthologous groups are nearly impossible to define. An exception is the long-form D7-like proteins that show conservation with their mosquito counterparts of numerous residues associated with the N-terminal domain binding pocket. In mosquitoes, this pocket is responsible for the scavenging of proinflammatory cysteinyl leukotrienes and thromboxanes at the feeding site. Here we show that long-form D7 proteins AGE83092 and ABI15936 from the sand fly species, Phlebotomus papatasi and P. duboscqi, respectively, inhibit the activation of platelets by collagen and the thromboxane A₂ analog U46619. Using isothermal titration calorimetry, we also demonstrate direct binding of U46619 and cysteinyl leukotrienes C₄, D₄ and E₄ to the P. papatasi protein. The crystal structure of P. duboscqi ABI15936 was determined and found to contain two domains oriented similarly to those of the mosquito proteins. The N-terminal domain contains an apparent eicosanoid binding pocket. The C-terminal domain is smaller in overall size than in the mosquito D7s and is missing some helical elements. Consequently, it does not contain an obvious internal binding pocket for small-molecule ligands that bind to many mosquito D7s. Structural similarities indicate that mosquito and sand fly D7 proteins have evolved from similar progenitors, but phylogenetics and differences in intron/exon structure suggest that they may have acquired the ability to bind vertebrate eicosanoids independently, indicating a convergent evolution scenario.

A mosquito hemolymph odorant-binding protein family member specifically binds juvenile hormone

Juvenile hormone (JH) is a key regulator of insect development and reproduction. In adult mosquitoes, it is essential for maturation of the ovary and normal male reproductive behavior, but how JH distribution and activity is regulated after secretion is unclear. Here, we report a new type of specific JH-binding protein, given the name mosquito juvenile hormone-binding protein (mJHBP), which circulates in the hemolymph of pupal and adult Aedes aegypti males and females. mJHBP is a member of the odorant-binding protein (OBP) family, and orthologs are present in the genomes of Aedes, Culex, and Anopheles mosquito species. Using isothermal titration calorimetry, we show that mJHBP specifically binds JH II and JH III but not eicosanoids or JH derivatives. mJHBP was crystallized in the presence of JH III and found to have a double OBP domain structure reminiscent of salivary "long" D7 proteins of mosquitoes. We observed that a single JH III molecule is contained in the N-terminal domain binding pocket that is closed in an apparent conformational change by a C-terminal domain-derived α-helix. The electron density for the ligand indicated a high occupancy of the natural 10R enantiomer of JH III. Of note, mJHBP is structurally unrelated to hemolymph JHBP from lepidopteran insects. A low level of expression of mJHBP in Ae. aegypti larvae suggests that it is primarily active during the adult stage where it could potentially influence the effects of JH on egg development, mating behavior, feeding, or other processes.

Structure and Ligand-Binding Mechanism of a Cysteinyl Leukotriene-Binding Protein from a Blood-Feeding Disease Vector

Blood-feeding disease vectors mitigate the negative effects of hemostasis and inflammation through the binding of small-molecule agonists of these processes by salivary proteins. In this study, a lipocalin protein family member (LTBP1) from the saliva of Rhodnius prolixus, a vector of the pathogen Trypanosoma cruzi, is shown to sequester cysteinyl leukotrienes during feeding to inhibit immediate inflammatory responses. Calorimetric binding experiments showed that LTBP1 binds leukotrienes C4 (LTC4), D4 (LTD4), and E4 (LTE4) but not biogenic amines, adenosine diphosphate, or other eicosanoid compounds. Crystal structures of ligand-free LTBP1 and its complexes with LTC4 and LTD4 reveal a conformational change during binding that brings Tyr114 into close contact with the ligand. LTC4 is cleaved in the complex, leaving free glutathione and a C20 fatty acid. Chromatographic analysis of bound ligands showed only intact LTC4, suggesting that cleavage could be radiation-mediated.

Identification and Mechanistic Analysis of a Novel Tick-Derived Inhibitor of Thrombin

A group of peptides from the salivary gland of the tick Hyalomma marginatum rufipes, a vector of Crimean Congo hemorrhagic fever show weak similarity to the madanins, a group of thrombin-inhibitory peptides from a second tick species, Haemaphysalis longicornis. We have evaluated the anti-serine protease activity of one of these H. marginatum peptides that has been given the name hyalomin-1. Hyalomin-1 was found to be a selective inhibitor of thrombin, blocking coagulation of plasma and inhibiting S2238 hydrolysis in a competitive manner with an inhibition constant (Ki) of 12 nM at an ionic strength of 150 mM. It also blocks the thrombin-mediated activation of coagulation factor XI, thrombin-mediated platelet aggregation, and the activation of coagulation factor V by thrombin. Hyalomin-1 is cleaved at a canonical thrombin cleavage site but the cleaved products do not inhibit coagulation. However, the C-terminal cleavage product showed non-competitive inhibition of S2238 hydrolysis. A peptide combining the N-terminal parts of the molecule with the cleavage region did not interact strongly with thrombin, but a 24-residue fragment containing the cleavage region and the C-terminal fragment inhibited the enzyme in a competitive manner and also inhibited coagulation of plasma. These results suggest that the peptide acts by binding to the active site as well as exosite I or the autolysis loop of thrombin. Injection of 2.5 mg/kg of hyalomin-1 increased arterial occlusion time in a mouse model of thrombosis, suggesting this peptide could be a candidate for clinical use as an antithrombotic.

Molecular analysis of Aedes aegypti classical protein tyrosine phosphatases uncovers an ortholog of mammalian PTP-1B implicated in the control of egg production in mosquitoes

Background

Protein Tyrosine Phosphatases (PTPs) are enzymes that catalyze phosphotyrosine dephosphorylation and modulate cell differentiation, growth and metabolism. In mammals, PTPs play a key role in the modulation of canonical pathways involved in metabolism and immunity. PTP1B is the prototype member of classical PTPs and a major target for treating human diseases, such as cancer, obesity and diabetes. These signaling enzymes are, hence, targets of a wide array of inhibitors. Anautogenous mosquitoes rely on blood meals to lay eggs and are vectors of the most prevalent human diseases. Identifying the mosquito ortholog of PTP1B and determining its involvement in egg production is, therefore, important in the search for a novel and crucial target for vector control.

Methodology/Principal Findings

We conducted an analysis to identify the ortholog of mammalian PTP1B in the Aedes aegypti genome. We identified eight genes coding for classical PTPs. In silico structural and functional analyses of proteins coded by such genes revealed that four of these code for catalytically active enzymes. Among the four genes coding for active PTPs, AAEL001919 exhibits the greatest degree of homology with the mammalian PTP1B. Next, we evaluated the role of this enzyme in egg formation. Blood feeding largely affects AAEL001919 expression, especially in the fat body and ovaries. These tissues are critically involved in the synthesis and storage of vitellogenin, the major yolk protein. Including the classical PTP inhibitor sodium orthovanadate or the PTP substrate DiFMUP in the blood meal decreased vitellogenin synthesis and egg production. Similarly, silencing AAEL001919 using RNA interference (RNAi) assays resulted in 30% suppression of egg production.

Conclusions/Significance

The data reported herein implicate, for the first time, a gene that codes for a classical PTP in mosquito egg formation. These findings raise the possibility that this class of enzymes may be used as novel targets to block egg formation in mosquitoes.

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.

A transient increase in total head phosphotyrosine levels is observed upon the emergence of Aedes aegypti from the pupal stage

Phosphorylation and dephosphorylation of protein tyrosine residues constitutes a major biochemical regulatory mechanism for the cell. We report a transient increase in the total tyrosine phosphorylation of the Aedes aegypti head during the first days after emergence from the pupal stage. This correlates with an initial reduction in total head protein tyrosine phosphatase (PTP) activity. Similarly, phosphotyrosine (pTyr)-containing bands are seen in extracts prepared from both male and female heads and are spread among a variety of structures including the antennae, proboscis and the maxillary palps combined with the proboscis. Also, mosquitoes treated with sodium orthovanadate, a classical PTP inhibitor, show reduced blood-feeding activity and higher head tyrosine phosphorylation levels. These results suggest that pTyr-mediated signalling pathways may play a role in the initial days following the emergence of the adult mosquito from the pupal stage.

Deviation of carbohydrate metabolism by the SIT4 phosphatase in Saccharomyces cerevisiae

A prominent phenotype of the yeast sit4 mutant, which lacks the Ser-Thr phosphatase Sit4, is hyper-accumulation of glycogen and the failure to grow on respiratory substrates. We investigated whether these two phenotypes are linked by studying the metabolic response to SIT4 deletion. Although the sit4 mutant failed to grow on respiratory substrates, in the exponential growth, phase respiration was de-repressed; active respiration was confirmed by measuring oxygen consumption and NADH generation. However, the fermentation rate and the internal glucose 6-phosphate and pyruvate levels were reduced, while glycogen content was high. Respiro-fermentative and respiratory substrates such as galactose, glycerol and ethanol were directed toward glycogen synthesis, which indicates that sit4 mutant deviates metabolism to glycogenesis by activating a glycogen futile cycle and depleting cells of Krebs cycle intermediates. An important feature of the sit4 mutant was the lack of growth under anaerobic conditions, suggesting that respiration is necessary to meet the energy requirements of the cell. Addition of aspartic acid, which can restore Krebs cycle intermediates, partially restored growth on ethanol. Our findings suggest that inhibition of Sit4 activity may be essential for redirecting carbohydrate flux to gluconeogenesis and glycogen storage.

Low resolution structure of the human alpha4 protein (IgBP1) and studies on the stability of alpha4 and of its yeast ortholog Tap42

The yeast Tap42 and mammalian alpha4 proteins belong to a highly conserved family of regulators of the type 2A phosphatases, which participate in the rapamycin-sensitive signaling pathway, connecting nutrient availability to cell growth. The mechanism of regulation involves binding of Tap42 to Sit4 and PPH21/22 in yeast and binding of alpha4 to the catalytic subunits of type 2A-related phosphatases PP2A, PP4 and PP6 in mammals. Both recombinant proteins undergo partial proteolysis, generating stable N-terminal fragments. The full-length proteins and alpha4 C-terminal deletion mutants at amino acids 222 (alpha4Delta222), 236 (alpha4Delta236) and 254 (alpha4Delta254) were expressed in E. coli. alpha4Delta254 undergoes proteolysis, producing a fragment similar to the one generated by full-length alpha4, whereas alpha4Delta222 and alpha4Delta236 are highly stable proteins. alpha4 and Tap42 show alpha-helical circular dichroism spectra, as do their respective N-terminal proteolysis resistant products. The cloned truncated proteins alpha4Delta222 and alpha4Delta236, however, possess a higher content of alpha-helix, indicating that the C-terminal region is less structured, which is consistent with its higher sensitivity to proteolysis. In spite of their higher secondary structure content, alpha4Delta222 and alpha4Delta236 showed thermal unfolding kinetics similar to the full-length alpha4. Based on small angle X-ray scattering (SAXS), the calculated radius of gyration for alpha4 and Tap42 were 41.2 +/- 0.8 A and 42.8 +/- 0.7 A and their maximum dimension approximately 142 A and approximately 147 A, respectively. The radii of gyration for alpha4Delta222 and alpha4Delta236 were 21.6 +/- 0.3 A and 25.7 +/- 0.2 A, respectively. Kratky plots show that all studied proteins show variable degree of compactness. Calculation of model structures based on SAXS data showed that alpha4Delta222 and alpha4Delta236 proteins have globular conformation, whereas alpha4 and Tap42 exhibit elongated shapes.