A mosquito salivary protein inhibits activation of the plasma contact system by binding to factor XII and high molecular weight kininogen

Circadian rhythms mediate malaria transmission potential

Malaria transmission begins when infected female Anopheles mosquitos deposit Plasmodium parasites into the mammalian host's skin during a bloodmeal. The salivary gland-resident sporozoite parasites migrate to the bloodstream, subsequently invading and replicating within hepatocytes. As Anopheles mosquitos are more active at night, with a 24-hour rhythm, we investigated whether their salivary glands are under circadian control, anticipating bloodmeals and modulating sporozoite biology for host encounters. Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian rhythmic expression. Furthermore, we demonstrate that mosquitoes prefer to feed during nighttime, with the amount of blood ingested varying cyclically throughout the day. Notably, we show a substantial subset of the sporozoite transcriptome cycling throughout the day. These include genes involved in parasite motility, potentially modulating the ability to initiate infection at different times of day. Thus, although sporozoites are typically considered quiescent, our results demonstrate their transcriptional activity, revealing robust daily rhythms of gene expression. Our findings suggest a circadian evolutionary relationship between the vector, parasite and mammalian host that together modulate malaria transmission.

Proteomic Analysis of Salivary Secretions from the Tea Green Leafhopper, Empoasca flavescens Fabrecius

Saliva plays a crucial role in shaping the compatibility of piercing-sucking insects with their host plants. Understanding the complex composition of leafhopper saliva is important for developing effective and eco-friendly control strategies for the tea green leafhopper, Empoasca flavescens Fabrecius, a major piercing-sucking pest in Chinese tea plantations. This study explored the saliva proteins of tea green leafhopper adults using a custom collection device, consisting of two layers of Parafilm stretched over a sucrose diet. A total of 152 proteins were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS) following the filter-aided sample preparation (FASP). These proteins were categorized into six groups based on their functions, including enzymes, transport proteins, regulatory proteins, cell structure proteins, other proteins, and unknown proteins. Bioinformatics analyses predicted 16 secreted proteins, which were successfully cloned and transcriptionally analyzed across various tissues and developmental stages. Genes encoding putative salivary secretory proteins, including Efmucin1, EfOBP1, EfOBP2, EfOBP3, Efmucin2, low-density lipoprotein receptor-related protein (EfLRP), EFVg1, and EFVg2, exhibited high expressions in salivary gland (SG) tissues and feeding-associated expressions at different developmental stages. These findings shed light on the potential elicitors or effectors mediating the leafhopper feeding and defense responses in tea plants, providing insights into the coevolution of tea plants and leafhoppers. The study's conclusions open avenues for the development of innovative leafhopper control technologies that reduce the reliance on pesticides in the tea industry.

Recognition of Aedes aegypti Mosquito Saliva Protein LTRIN by the Human Receptor LTβR for Controlling the Immune Response

Salivary proteins from mosquitoes have received significant attention lately due to their potential to develop therapeutic treatments or vaccines for mosquito-borne diseases. Here, we report the characterization of LTRIN (lymphotoxin beta receptor inhibitor), a salivary protein known to enhance the pathogenicity of ZIKV by interrupting the LTβR-initiated NF-κB signaling pathway and, therefore, diminish the immune responses. We demonstrated that the truncated C-terminal LTRIN (ΔLTRIN) is a dimeric protein with a stable alpha helix-dominant secondary structure, which possibly aids in withstanding the temperature fluctuations during blood-feeding events. ΔLTRIN possesses two Ca2+ binding EF-hand domains, with the second EF-hand motif playing a more significant role in interacting with LTβR. Additionally, we mapped the primary binding regions of ΔLTRIN on LTβR using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and identified that 91QEKAHIAEHMDVPIDTSKMSEQELQFHY118 from the N-terminal of ΔLTRIN is the major interacting region. Together, our studies provide insight into the recognition of LTRIN by LTβR. This finding may aid in a future therapeutic and transmission-blocking vaccine development against ZIKV.

Mosquito allergy: Immunological aspects and clinical management

Mosquito allergy has been conceived as the cutaneous reactions that appears during and after mosquito biting process; a perception that is supported by several scientific research. Additional data have led to conceive that other manifestations of allergic responses may occur as a cause of the exposure to somatic mosquito allergens. Two main phenotypes of mosquito allergy are identifiable: the cutaneous allergic reactions, induced by salivary allergens, and other manifestations of the allergic responses such as asthma and allergic rhino conjunctivitis that are caused by somatic allergens. The cutaneous reactions have kept the focus of attention of the scientific community. It appears as skin lesions that resembles the phenotype of papular urticaria with a defined natural history of the disease. Although these two phenotypes of mosquito allergy seem to be well differentiated in terms of the allergens that are involved and the routes of exposures, other factors such as geographical distribution, may participate. Mosquitoes have adapted to the host immune response against bites, producing immunomodulatory molecules that counteract such defensive response. The role that the immunomodulatory molecules have on the allergic immune response has not been studied yet and it is still not known if affects all mosquito allergy phenotypes. Only a few studies of allergen specific immunotherapy for cutaneous allergic reactions induced by mosquito bites have been done, and none for respiratory allergic responses. The clinical practice focuses on symptom management and avoiding mosquito bites as much as possible. Avoiding mosquitoes, using different well described methods, is still the best option to limit contact with these insects. The lack of knowledge of mosquito allergy have raised several questions that affects the clinical management of this allergic disease, from its diagnosis, prevention and immunotherapy.

Mosquito Salivary Proteins and Arbovirus Infection: From Viral Enhancers to Potential Targets for Vaccines

Arthropod-borne viruses present important public health challenges worldwide. Viruses such as DENV, ZIKV, and WNV are of current concern due to an increasing incidence and an expanding geographic range, generating explosive outbreaks even in non-endemic areas. The clinical signs associated with infection from these arboviruses are often inapparent, mild, or nonspecific, but occasionally develop into serious complications marked by rapid onset, tremors, paralysis, hemorrhagic fever, neurological alterations, or death. They are predominately transmitted to humans through mosquito bite, during which saliva is inoculated into the skin to facilitate blood feeding. A new approach to prevent arboviral diseases has been proposed by the observation that arthropod saliva facilitates transmission of pathogens. Viruses released within mosquito saliva may more easily initiate host invasion by taking advantage of the host's innate and adaptive immune responses to saliva. This provides a rationale for creating vaccines against mosquito salivary proteins, especially because of the lack of licensed vaccines against most of these viruses. This review aims to provide an overview of the effects on the host immune response by the mosquito salivary proteins and how these phenomena alter the infection outcome for different arboviruses, recent attempts to generate mosquito salivary-based vaccines against flavivirus including DENV, ZIKV, and WNV, and the potential benefits and pitfalls that this strategy involves.

An Overview of D7 Protein Structure and Physiological Roles in Blood-Feeding Nematocera

Each time an insect bites a vertebrate host, skin and vascular injury caused by piercing triggers a series of responses including hemostasis, inflammation and immunity. In place, this set of redundant and interconnected responses would ultimately cause blood coagulation, itching and pain leading to host awareness, resulting in feeding interruption in the best-case scenario. Nevertheless, hematophagous arthropod saliva contains a complex cocktail of molecules that are crucial to the success of blood-feeding. Among important protein families described so far in the saliva of blood sucking arthropods, is the D7, abundantly expressed in blood feeding Nematocera. D7 proteins are distantly related to insect Odorant-Binding Proteins (OBP), and despite low sequence identity, observation of structural similarity led to the suggestion that like OBPs, they should bind/sequester small hydrophobic compounds. Members belonging to this family are divided in short forms and long forms, containing one or two OBP-like domains, respectively. Here, we provide a review of D7 proteins structure and function, discussing how gene duplication and some modifications in their OBP-like domains during the course of evolution lead to gain and loss of function among different hematophagous Diptera species.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Ayaconin, a novel inhibitor of the plasma contact system from the sand fly Lutzomyia ayacuchensis, a vector of Andean-type cutaneous leishmaniasis

Transcriptome analysis of the salivary gland cDNA library from a phlebotomine sand fly, Lutzomyia ayacuchensis, identified a transcript coding for the PpSP15/SL1 family protein as the second most abundant salivary component. In the present study, a recombinant protein of the PpSP15/SL1 family protein, designated ayaconin, was expressed in Escherichia coli, and its biological activity was characterized. The recombinant ayaconin purified from the soluble fraction of E. coli lysate efficiently inhibited the intrinsic but not extrinsic blood coagulation pathway. When the target of ayaconin was evaluated using fluorescent substrates of coagulation factors, ayaconin inhibited factor XIIa (FXIIa) activity more efficiently in a dose-dependent manner, suggesting that FXII is the primary target of ayaconin. In addition, incubation of ayaconin with FXII prior to activation effectively inhibited FXIIa activity, whereas such inhibition was not observed when ayaconin was mixed after the production of FXIIa, indicating that ayaconin inhibits the activation process of FXII to produce FXIIa, but not the enzymatic activity of FXIIa. Moreover, ayaconin was shown to bind to FXII, suggesting that the binding of ayaconin to FXII is involved in the inhibitory mechanism against FXII activation. These results suggest that ayaconin plays an important role in the blood-sucking of Lu. ayacuchensis.

Biting Innovations of Mosquito-Based Biomaterials and Medical Devices

Mosquitoes are commonly viewed as pests and deadly predators by humans. Despite this perception, investigations of their survival-based behaviors, select anatomical features, and biological composition have led to the creation of several beneficial technologies for medical applications. In this review, we briefly explore these mosquito-based innovations by discussing how unique characteristics and behaviors of mosquitoes drive the development of select biomaterials and medical devices. Mosquito-inspired microneedles have been fabricated from a variety of materials, including biocompatible metals and polymers, to mimic of the mouthparts that some mosquitoes use to bite a host with minimal injury during blood collection. The salivary components that these mosquitoes use to reduce the clotting of blood extracted during the biting process provide a rich source of anticoagulants that could potentially be integrated into blood-contacting biomaterials or administered in therapeutics to reduce the risk of thrombosis. Mosquito movement, vision, and olfaction are other behaviors that also have the potential for inspiring the development of medically relevant technologies. For instance, viscoelastic proteins that facilitate mosquito movement are being investigated for use in tissue engineering and drug delivery applications. Even the non-wetting nanostructure of a mosquito eye has inspired the creation of a robust superhydrophobic surface coating that shows promise for biomaterial and drug delivery applications. Additionally, biosensors incorporating mosquito olfactory receptors have been built to detect disease-specific volatile organic compounds. Advanced technologies derived from mosquitoes, and insects in general, form a research area that is ripe for exploration and can uncover potential in further dissecting mosquito features for the continued development of novel medical innovations.

Novel salivary antihemostatic activities of long-form D7 proteins from the malaria vector Anopheles gambiae facilitate hematophagy

To successfully feed on blood, hematophagous arthropods must combat the host's natural hemostatic and inflammatory responses. Salivary proteins of blood-feeding insects such as mosquitoes contain compounds that inhibit these common host defenses against blood loss, including vasoconstriction, platelet aggregation, blood clotting, pain, and itching. The D7 proteins are some of the most abundantly expressed proteins in female mosquito salivary glands and have been implicated in inhibiting host hemostatic and inflammatory responses. Anopheles gambiae, the primary vector of malaria, expresses three D7 long-form and five D7 short-form proteins. Previous studies have characterized the AngaD7 short-forms, but the D7 long-form proteins have not yet been characterized in detail. Here, we characterized the A. gambiae D7 long-forms by first determining their binding kinetics to hemostatic agonists such as leukotrienes and serotonin, which are potent activators of vasoconstriction, edema formation, and postcapillary venule leakage, followed by ex vivo functional assays. We found that AngaD7L1 binds leukotriene C4 and thromboxane A2 analog U-46619; AngaD7L2 weakly binds leukotrienes B4 and D4; and AngaD7L3 binds serotonin. Subsequent functional assays confirmed AngaD7L1 inhibits U-46619-induced platelet aggregation and vasoconstriction, and AngaD7L3 inhibits serotonin-induced platelet aggregation and vasoconstriction. It is therefore possible that AngaD7L proteins counteract host hemostasis by scavenging these mediators. Finally, we demonstrate that AngaD7L2 had a dose-dependent anticoagulant effect via the intrinsic coagulation pathway by interacting with factors XII, XIIa, and XI. The uncovering of these interactions in the present study will be essential for comprehensive understanding of the vector-host biochemical interface.

Identification of Candidate Olfactory Genes in Scolytus schevyrewi Based on Transcriptomic Analysis

The bark beetle, Scolytus schevyrewi (S. schevyrewi), is an economically important pest in China that causes serious damage to the fruit industry, particularly, in Xinjiang Province. Chemical signals play an important role in the behavior of most insects, accordingly, ecofriendly traps can be used to monitor and control the target pests in agriculture. In order to lay a foundation for future research on chemical communication mechanisms at the molecular level, we generate antennal transcriptome databases for male and female S. schevyrewi using RNA sequencing (RNA-seq) analysis. By assembling and analyzing the adult male and female antennal transcriptomes, we identified 47 odorant receptors (ORs), 22 ionotropic receptors (IRs), 22 odorant-binding proteins (OBPs), and 11 chemosensory proteins (CSPs). Furthermore, expression levels of all the candidate OBPs and CSPs were validated in different tissues of male and female adults by semiquantitative reverse transcription PCR (RT-PCR). ScosOBP2 and ScosOBP18 were highly expressed in female antennae. ScosCSP2, ScosCSP3, and ScosCSP5 were specifically expressed in the antennae of both males and females. These results provide new potential molecular targets to inform and improve future management strategies of S. schevyrewi.

Neurochemical regulation of Aedes aegypti salivary gland function

The salivary gland of hematophagous arthropods is critical for blood meal acquisition, blood vessel localization, and secretion of digestive enzymes. Thus, there is significant interest in the regulation of salivary gland function and mechanisms driving the secretion of saliva and digestive proteins. We aimed to gain a broader understanding of the regulatory role of aminergic, cholinergic, and octopaminergic neuromodulators to saliva and protein secretion from the female A. aegypti salivary gland. Quantification of saliva after injection with neuromodulators showed that dopamine, serotonin, and pilocarpine increased the secretory activity of the salivary gland with potency rankings dopamine = serotonin > pilocarpine. No change in saliva secretion was observed with octopamine or ergonovine, which indicates the A. aegypti salivary gland may be regulated by dopaminergic, serotonergic, and cholinergic systems, but are not likely regulated by octopaminergic or tryptaminergic systems. Next, we studied the regulatory control of dopamine-mediated salivation. Data indicate extracellular calcium flux, but not neural function, is critical for dopamine-mediated salivation, which suggests epithelial transport of ions and not neuronal control is responsible for dopamine-mediated salivation. For regulation of protein secretion, data indicate dopamine or serotonin exposure facilitates amylase secretion, whereas serotonin but not dopamine exposure increased apyrase concentrations in the secreted saliva. General immunoreactivity to anti-rat D1-dopamine receptor antibody was observed, yet immunoreactivity to the anti-rat D2-receptor antibody was identified in the proximal regions of the lateral lobes and slight immunoreactivity in the distal portion of the lateral lobe, with no expression in the medial lobe.

Biochemical characterization of AeD7L2 and its physiological relevance in blood feeding in the dengue mosquito vector, Aedes aegypti

Aedes aegypti saliva facilitates blood meal acquisition through pharmacologically active compounds that prevent host hemostasis. Among these salivary proteins are the D7s, which are highly abundant and have been shown to act as scavengers of biogenic amines and eicosanoids. In this work, we performed comparative structural modeling, characterized the binding capabilities, and assessed the physiological functions of the Ae. aegypti salivary protein AeD7L2 compared to the well-characterized AeD7L1. AeD7L1 and AeD7L2 show different binding affinities to several biogenic amines and biolipids involved in host hemostasis. Interestingly, AeD7L2 tightly binds U-46619, the stable analog of thromboxane A₂ (K_D  = 69.4 nm), which is an important platelet aggregation mediator, while AeD7L1 shows no binding. We tested the ability of these proteins to interfere with the three branches of hemostasis: vasoconstriction, platelet aggregation, and blood coagulation. Pressure myography experiments showed these two proteins reversed isolated resistance artery vasoconstriction induced by either norepinephrine or U-46619. These proteins also inhibited platelet aggregation induced by low doses of collagen or U-46619. However, D7 long proteins did not affect blood coagulation. The different ligand specificity and affinities of AeD7L1 and AeD7L2 matched our experimental observations from studying their effects on vasoconstriction and platelet aggregation, which confirm their role in preventing host hemostasis. This work highlights the complex yet highly specific biological activities of mosquito salivary proteins and serves as another example of the sophisticated biology underlying arthropod blood feeding.

Kinins and Their Receptors in Infectious Diseases

Kinins and their receptors have been implicated in a series of pathological alterations, representing attractive pharmacological targets for several diseases. The present review article aims to discuss the role of the kinin system in infectious diseases. Literature data provides compelling evidence about the participation of kinins in infections caused by diverse agents, including viral, bacterial, fungal, protozoan, and helminth-related ills. It is tempting to propose that modulation of kinin actions and production might be an adjuvant strategy for management of infection-related complications.

Sergentomyia schwetzi: Salivary gland transcriptome, proteome and enzymatic activities in two lineages adapted to different blood sources

During the blood feeding, sand fly females inject saliva containing immunomodulatory and anti-haemostatic molecules into their vertebrate hosts. The saliva composition is species-specific, likely due to an adaptation to particular haemostatic pathways of their preferred host. Research on sand fly saliva is limited to the representatives of two best-studied genera, Phlebotomus and Lutzomyia. Although the members of the genus Sergentomyia are highly abundant in many areas in the Old World, their role in human disease transmission remains uncertain. Most Sergentomyia spp. preferentially attack various species of reptiles, but feeding on warm-blooded vertebrates, including humans and domestic animals, has been repeatedly described, especially for Sergentomyia schwetzi, of which salivary gland transcriptome and proteome is analyzed in the current study. Illumina RNA sequencing and de novo assembly of the reads and their annotation revealed 17,293 sequences homologous to other arthropods’ proteins. In the sialome, all proteins typical for sand fly saliva were identified–antigen 5-related, lufaxin, yellow-related, PpSP15-like, D7-related, ParSP25-like, and silk proteins, as well as less frequent salivary proteins included 71kDa-like, ParSP80-like, SP16-like, and ParSP17-like proteins. Salivary enzymes include apyrase, hyaluronidase, endonuclease, amylase, lipase A2, adenosine deaminase, pyrophosphatase, 5’nucleotidase, and ribonuclease. Proteomics analysis of salivary glands identified 631 proteins, 81 of which are likely secreted into the saliva. We also compared two S. schwetzi lineages derived from the same origin. These lineages were adapted for over 40 generations for blood feeding either on mice (S-M) or geckos (S-G), two vertebrate hosts with different haemostatic mechanisms. Altogether, 20 and 40 annotated salivary transcripts were up-regulated in the S-M and S-G lineage, respectively. Proteomic comparison revealed ten salivary proteins more abundant in the lineage S-M, whereas 66 salivary proteins were enriched in the lineage S-G. No difference between lineages was found for apyrase activity; contrarily the hyaluronidase activity was significantly higher in the lineage feeding on mice.

Aedes aegypti Odorant Binding Protein 22 selectively binds fatty acids through a conformational change in its C-terminal tail

Aedes aegypti is the primary vector for transmission of Dengue, Zika and chikungunya viruses. Previously it was shown that Dengue virus infection of the mosquito led to an in increased expression of the odorant binding protein 22 (AeOBP22) within the mosquito salivary gland and that siRNA mediated knockdown of AeOBP22 led to reduced mosquito feeding behaviors. Insect OBPs are implicated in the perception, storage and transport of chemosensory signaling molecules including air-borne odorants and pheromones. AeOBP22 is unusual as it is additionally expressed in multiple tissues, including the antenna, the male reproductive glands and is transferred to females during reproduction, indicating multiple roles in the mosquito life cycle. However, it is unclear what role it plays in these tissues and what ligands it interacts with. Here we present solution and X-ray crystallographic studies that indicate a potential role of AeOBP22 binding to fatty acids, and that the specificity for longer chain fatty acids is regulated by a conformational change in the C-terminal tail that leads to creation of an enlarged binding cavity that enhances binding affinity. This study sheds light onto the native ligands for AeOBP22 and provides insight into its potential functions in different tissues.

An insight into the sialotranscriptome and virome of Amazonian anophelines

Background

Saliva of mosquitoes contains anti-platelet, anti-clotting, vasodilatory, anti-complement and anti-inflammatory substances that help the blood feeding process. The salivary polypeptides are at a fast pace of evolution possibly due to their relative lack of structural constraint and possibly also by positive selection on their genes leading to evasion of host immune pressure.

Results

In this study, we used deep mRNA sequence to uncover for the first time the sialomes of four Amazonian anophelines species (Anopheles braziliensis, A. marajorara, A. nuneztovari and A. triannulatus) and extend the knowledge of the A. darlingi sialome. Two libraries were generated from A. darlingi mosquitoes, sampled from two localities separated ~ 1100 km apart. A total of 60,016 sequences were submitted to GenBank, which will help discovery of novel pharmacologically active polypeptides and the design of specific immunological markers of mosquito exposure. Additionally, in these analyses we identified and characterized novel phasmaviruses and anpheviruses associated to the sialomes of A. triannulatus, A. marajorara and A. darlingi species.

Conclusions

Besides their pharmacological properties, which may be exploited for the development of new drugs (e.g. anti-thrombotics), salivary proteins of blood feeding arthropods may be turned into tools to prevent and/or better control vector borne diseases; for example, through the development of vaccines or biomarkers to evaluate human exposure to vector bites. The sialotranscriptome study reported here provided novel data on four New World anopheline species and allowed to extend our knowledge on the salivary repertoire of A. darlingi. Additionally, we discovered novel viruses following analysis of the transcriptomes, a procedure that should become standard within future RNAseq studies.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5545-0) contains supplementary material, which is available to authorized users.

Saliva of hematophagous insects: a multifaceted toolkit

Transcriptomic, proteomic and genomic studies significantly improved our understanding of the complexity of blood feeding insect saliva providing unparalleled evolutionary insights. Salivary genes appeared to be under strong selective pressure with gene duplication and functional diversification being a powerful driver in the evolution of novel salivary genes/functions. The first insect salivary proteins responsible for complement inhibition were identified and a widespread mechanism of action shared by unrelated salivary protein families was recognized and named kratagonism. microRNAs were for the first time described in the saliva of a few blood feeding arthropods raising intriguing questions on their possible contribution to vertebrate host manipulation and pathogen transmission and further emphasizing how much we still have to learn on blood feeding insect saliva.

Novel Immunoinformatics Approaches to Design Multi-epitope Subunit Vaccine for Malaria by Investigating Anopheles Salivary Protein

Malaria fever has been pervasive for quite a while in tropical developing regions causing high morbidity and mortality. The causal organism is a protozoan parasite of genus Plasmodium which spreads to the human host by the bite of hitherto infected female Anopheles mosquito. In the course of biting, a salivary protein of Anopheles helps in blood feeding behavior and having the ability to elicit the host immune response. This study represents a series of immunoinformatics approaches to design multi-epitope subunit vaccine using Anopheles mosquito salivary proteins. Designed subunit vaccine was evaluated for its immunogenicity, allergenicity and physiochemical parameters. To enhance the stability of vaccine protein, disulfide engineering was performed in a region of high mobility. Codon adaptation and in silico cloning was also performed to ensure the higher expression of designed subunit vaccine in E. coli K12 expression system. Finally, molecular docking and simulation study was performed for the vaccine protein and TLR-4 receptor, to determine the binding free energy and complex stability. Moreover, the designed subunit vaccine was found to induce anti-salivary immunity which may have the ability to prevent the entry of Plasmodium sporozoites into the human host.

Insights into the sand fly saliva: Blood-feeding and immune interactions between sand flies, hosts, and Leishmania

Background

Leishmaniases are parasitic diseases present worldwide that are transmitted to the vertebrate host by the bite of an infected sand fly during a blood feeding. Phlebotomine sand flies inoculate into the mammalian host Leishmania parasites embedded in promastigote secretory gel (PSG) with saliva, which is composed of a diverse group of molecules with pharmacological and immunomodulatory properties.

Methods and findings

In this review, we focus on 3 main aspects of sand fly salivary molecules: (1) structure and composition of salivary glands, including the properties of salivary molecules related to hemostasis and blood feeding, (2) immunomodulatory properties of salivary molecules and the diverse impacts of these molecules on leishmaniasis, ranging from disease exacerbation to vaccine development, and (3) use of salivary molecules for field applications, including monitoring host exposure to sand flies and the risk of Leishmania transmission. Studies showed interesting differences between salivary proteins of Phlebotomus and Lutzomyia species, however, no data were ever published on salivary proteins of Sergentomyia species.

Conclusions

In the last 15 years, numerous studies have characterized sand fly salivary proteins and, in parallel, have addressed the impact of such molecules on the biology of the host–sand fly–parasite interaction. The results obtained shall pave the way for the development of field-application tools that could contribute to the management of leishmaniasis in endemic areas.

Anopheline salivary protein genes and gene families: an evolutionary overview after the whole genome sequence of sixteen Anopheles species

BACKGROUND

Mosquito saliva is a complex cocktail whose pharmacological properties play an essential role in blood feeding by counteracting host physiological response to tissue injury. Moreover, vector borne pathogens are transmitted to vertebrates and exposed to their immune system in the context of mosquito saliva which, in virtue of its immunomodulatory properties, can modify the local environment at the feeding site and eventually affect pathogen transmission. In addition, the host antibody response to salivary proteins may be used to assess human exposure to mosquito vectors. Even though the role of quite a few mosquito salivary proteins has been clarified in the last decade, we still completely ignore the physiological role of many of them as well as the extent of their involvement in the complex interactions taking place between the mosquito vectors, the pathogens they transmit and the vertebrate host. The recent release of the genomes of 16 Anopheles species offered the opportunity to get insights into function and evolution of salivary protein families in anopheline mosquitoes.

RESULTS

Orthologues of fifty three Anopheles gambiae salivary proteins were retrieved and annotated from 18 additional anopheline species belonging to the three subgenera Cellia, Anopheles, and Nyssorhynchus. Our analysis included 824 full-length salivary proteins from 24 different families and allowed the identification of 79 novel salivary genes and re-annotation of 379 wrong predictions. The comparative, structural and phylogenetic analyses yielded an unprecedented view of the anopheline salivary repertoires and of their evolution over 100 million years of anopheline radiation shedding light on mechanisms and evolutionary forces that contributed shaping the anopheline sialomes.

CONCLUSIONS

We provide here a comprehensive description, classification and evolutionary overview of the main anopheline salivary protein families and identify two novel candidate markers of human exposure to malaria vectors worldwide. This anopheline sialome catalogue, which is easily accessible as hyperlinked spreadsheet, is expected to be useful to the vector biology community and to improve the capacity to gain a deeper understanding of mosquito salivary proteins facilitating their possible exploitation for epidemiological and/or pathogen-vector-host interaction studies.

Molecular Diversity between Salivary Proteins from New World and Old World Sand Flies with Emphasis on Bichromomyia olmeca, the Sand Fly Vector of Leishmania mexicana in Mesoamerica

Background

Sand fly saliva has been shown to have proteins with potent biological activities, salivary proteins that can be used as biomarkers of vector exposure, and salivary proteins that are candidate vaccines against different forms of leishmaniasis. Sand fly salivary gland transcriptomic approach has contributed significantly to the identification and characterization of many of these salivary proteins from important Leishmania vectors; however, sand fly vectors in some regions of the world are still neglected, as Bichromomyia olmeca (formerly known as Lutzomyia olmeca olmeca), a proven vector of Leishmania mexicana in Mexico and Central America. Despite the importance of this vector in transmitting Leishmania parasite in Mesoamerica there is no information on the repertoire of B. olmeca salivary proteins and their relationship to salivary proteins from other sand fly species.

Methods and Findings

A cDNA library of the salivary glands of wild-caught B. olmeca was constructed, sequenced, and analyzed. We identified transcripts encoding for novel salivary proteins from this sand fly species and performed a comparative analysis between B. olmeca salivary proteins and those from other sand fly species. With this new information we present an updated catalog of the salivary proteins specific to New World sand flies and salivary proteins common to all sand fly species. We also report in this work the anti-Factor Xa activity of Lofaxin, a salivary anticoagulant protein present in this sand fly species.

Conclusions

This study provides information on the first transcriptome of a sand fly from Mesoamerica and adds information to the limited repertoire of salivary transcriptomes from the Americas. This comparative analysis also shows a fast degree of evolution in salivary proteins from New World sand flies as compared with Old World sand flies.

Leishmaniasis is a neglected disease caused by a parasite transmitted to the host by the bite of an infected sand fly. Sand fly saliva contains biologically active components that allow the sand fly to take a blood meal and also the parasite to spread in the host by countering the host immune mechanisms that fights the parasite. Research on sand fly saliva has allowed us to understand the biological functions of some of these proteins, to identify salivary proteins producing an immune response in different hosts and to select potential salivary vaccine that could be used to protect the host against the parasite. However, vectors transmitting different species of Leishmania in diverse regions of the world are still neglected. The present work focuses on the identification of the secreted proteins from the saliva of B. olmeca, a vector of Leishmania mexicana in North and Central America. We catalogued these proteins with those previously identified in other sand fly species from Old and New World. We showed here how conserved or divergent are these proteins families when comparing different sand fly species. We also report the anti-Factor Xa activity of Lofaxin, a salivary anticoagulant protein identified in the saliva of this sand fly species.

Annotated differentially expressed salivary proteins of susceptible and insecticide-resistant mosquitoes of Anopheles stephensi

Vector control is one of the major global strategies for control of malaria. However, the major obstacle for vector control is the development of multiple resistances to organochlorine, organophosphorus insecticides and pyrethroids that are currently being used in public health for spraying and in bednets. Salivary glands of vectors are the first target organ for human-vector contact during biting and parasite-vector contact prior to parasite development in the mosquito midguts. The salivary glands secrete anti-haemostatic, anti-inflammatory biologically active molecules to facilitate blood feeding from the host and also inadvertently inject malaria parasites into the vertebrate host. The Anopheles stephensi mosquito, an urban vector of malaria to both human and rodent species has been identified as a reference laboratory model to study mosquito—parasite interactions. In this study, we adopted a conventional proteomic approach of 2D-electrophoresis coupled with MALDI-TOF mass spectrometry and bioinformatics to identify putative differentially expressed annotated functional salivary proteins between An. stephensi susceptible and multiresistant strains with same genetic background. Our results show 2D gel profile and MALDI-TOF comparisons that identified 31 differentially expressed putative modulated proteins in deltamethrin/DDT resistant strains of An. stephensi. Among these 15 proteins were found to be upregulated and 16 proteins were downregulated. Our studies interpret that An. stephensi (multiresistant) caused an upregulated expression of proteins and enzymes like cytochrome 450, short chain dehyrdogenase reductase, phosphodiesterase etc that may have an impact in insecticide resistance and xenobiotic detoxification. Our study elucidates a proteomic response of salivary glands differentially regulated proteins in response to insecticide resistance development which include structural, redox and regulatory enzymes of several pathways. These identified proteins may play a role in regulating mosquito biting behavior patterns and may have implications in the development of malaria parasites in resistant mosquitoes during parasite transmission.

New agents for thromboprotection. A role for factor XII and XIIa inhibition

Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Mosquito genomics. Highly evolvable malaria vectors: the genomes of 16 Anopheles mosquitoes

Variation in vectorial capacity for human malaria among Anopheles mosquito species is determined by many factors, including behavior, immunity, and life history. To investigate the genomic basis of vectorial capacity and explore new avenues for vector control, we sequenced the genomes of 16 anopheline mosquito species from diverse locations spanning ~100 million years of evolution. Comparative analyses show faster rates of gene gain and loss, elevated gene shuffling on the X chromosome, and more intron losses, relative to Drosophila. Some determinants of vectorial capacity, such as chemosensory genes, do not show elevated turnover but instead diversify through protein-sequence changes. This dynamism of anopheline genes and genomes may contribute to their flexible capacity to take advantage of new ecological niches, including adapting to humans as primary hosts.

Detection of heparin in the salivary gland and midgut of Aedes togoi

Mosquitoes secrete saliva that contains biological substances, including anticoagulants that counteract a host's hemostatic response and prevent blood clotting during blood feeding. This study aimed to detect heparin, an anticoagulant in Aedes togoi using an immunohistochemical detection method, in the salivary canal, salivary gland, and midgut of male and female mosquitoes. Comparisons showed that female mosquitoes contained higher concentrations of heparin than male mosquitoes. On average, the level of heparin was higher in blood-fed female mosquitoes than in non-blood-fed female mosquitoes. Heparin concentrations were higher in the midgut than in the salivary gland. This indicates presence of heparin in tissues of A. togoi.

Comparative analysis of salivary gland transcriptomes of Phlebotomus orientalis sand flies from endemic and non-endemic foci of visceral leishmaniasis

Background

In East Africa, Phlebotomus orientalis serves as the main vector of Leishmania donovani, the causative agent of visceral leishmaniasis (VL). Phlebotomus orientalis is present at two distant localities in Ethiopia; Addis Zemen where VL is endemic and Melka Werer where transmission of VL does not occur. To find out whether the difference in epidemiology of VL is due to distant compositions of P. orientalis saliva we established colonies from Addis Zemen and Melka Werer, analyzed and compared the transcriptomes, proteomes and enzymatic activity of the salivary glands.

Methodology/Principal Findings

Two cDNA libraries were constructed from the female salivary glands of P. orientalis from Addis Zemen and Melka Werer. Clones of each P. orientalis library were randomly selected, sequenced and analyzed. In P. orientalis transcriptomes, we identified members of 13 main protein families. Phylogenetic analysis and multiple sequence alignments were performed to evaluate differences between the P. orientalis colonies and to show the relationship with other sand fly species from the subgenus Larroussius. To further compare both colonies, we investigated the humoral antigenicity and cross-reactivity of the salivary proteins and the activity of salivary apyrase and hyaluronidase.

Conclusions

This is the first report of the salivary components of P. orientalis, an important vector sand fly. Our study expanded the knowledge of salivary gland compounds of sand fly species in the subgenus Larroussius. Based on the phylogenetic analysis, we showed that P. orientalis is closely related to Phlebotomus tobbi and Phlebotomus perniciosus, whereas Phlebotomus ariasi is evolutionarily more distinct species. We also demonstrated that there is no significant difference between the transcriptomes, proteomes or enzymatic properties of the salivary components of Addis Zemen (endemic area) and Melka Werer (non-endemic area) P. orientalis colonies. Thus, the different epidemiology of VL in these Ethiopian foci cannot be attributed to the salivary gland composition.

Phlebotomus orientalis is the vector of visceral leishmaniasis (VL) caused by Leishmania donovani in Northeast Africa. Immunization with sand fly saliva or with individual salivary proteins has been shown to protect against leishmaniasis in different hosts, warranting the intensive study of salivary proteins of sand fly vectors. In our study, we characterize the salivary compounds of P. orientalis, thereby broadening the repertoire of salivary proteins of sand fly species belonging to the subgenus Larroussius. In order to find out whether there is any connection between the composition of P. orientalis saliva and the epidemiology of VL in two distinct Ethiopian foci, Addis Zemen and Melka Werer, we studied the transcriptomes, proteomes, enzymatic activities, and the main salivary antigens in two P. orientalis colonies originating from these areas. We did not detect any significant difference between the saliva of female sand flies originating in Addis Zemen (endemic area) and Melka Werer (non-endemic area). Therefore, the different epidemiology of VL in these Ethiopian foci cannot be related to the distant salivary gland protein composition. Identifying the sand fly salivary gland compounds will be useful for future research focused on characterizing suitable salivary proteins as potential anti-Leishmania vaccine candidates.

Novel family of insect salivary inhibitors blocks contact pathway activation by binding to polyphosphate, heparin, and dextran sulfate

OBJECTIVE

Polyphosphate and heparin are anionic polymers released by activated mast cells and platelets that are known to stimulate the contact pathway of coagulation. These polymers promote both the autoactivation of factor XII and the assembly of complexes containing factor XI, prekallikrein, and high-molecular-weight kininogen. We are searching for salivary proteins from blood-feeding insects that counteract the effect of procoagulant and proinflammatory factors in the host, including elements of the contact pathway.

APPROACH AND RESULTS

Here, we evaluate the ability of the sand fly salivary proteins, PdSP15a and PdSP15b, to inhibit the contact pathway by disrupting binding of its components to anionic polymers. We attempt to demonstrate binding of the proteins to polyphosphate, heparin, and dextran sulfate. We also evaluate the effect of this binding on contact pathway reactions. We also set out to determine the x-ray crystal structure of PdSP15b and examine the determinants of relevant molecular interactions. Both proteins bind polyphosphate, heparin, and dextran sulfate with high affinity. Through this mechanism they inhibit the autoactivation of factor XII and factor XI, the reciprocal activation of factor XII and prekallikrein, the activation of factor XI by thrombin and factor XIIa, the cleavage of high-molecular-weight kininogen in plasma, and plasma extravasation induced by polyphosphate. The crystal structure of PdSP15b contains an amphipathic helix studded with basic side chains that forms the likely interaction surface.

CONCLUSIONS

The results of these studies indicate that the binding of anionic polymers by salivary proteins is used by blood feeders as an antihemostatic/anti-inflammatory mechanism.

Immunization of mice with recombinant mosquito salivary protein D7 enhances mortality from subsequent West Nile virus infection via mosquito bite

BACKGROUND

Mosquito salivary proteins (MSPs) modulate the host immune response, leading to enhancement of arboviral infections. Identification of proteins in saliva responsible for immunomodulation and counteracting their effects on host immune response is a potential strategy to protect against arboviral disease. We selected a member of the D7 protein family, which are among the most abundant and immunogenic in mosquito saliva, as a vaccine candidate with the aim of neutralizing effects on the mammalian immune response normally elicited by mosquito saliva components during arbovirus transmission.

METHODOLOGY/PRINCIPAL FINDINGS

We identified D7 salivary proteins of Culex tarsalis, a West Nile virus (WNV) vector in North America, and expressed 36 kDa recombinant D7 (rD7) protein for use as a vaccine. Vaccinated mice exhibited enhanced interferon-γ and decreased interleukin-10 expression after uninfected mosquito bite; however, we found unexpectedly that rD7 vaccination resulted in enhanced pathogenesis from mosquito-transmitted WNV infection. Passive transfer of vaccinated mice sera to naïve mice also resulted in increased mortality rates from subsequent mosquito-transmitted WNV infection, implicating the humoral immune response to the vaccine in enhancement of viral pathogenesis. Vaccinated mice showed decreases in interferon-γ and increases in splenocytes producing the regulatory cytokine IL-10 after WNV infection by mosquito bite.

CONCLUSIONS/SIGNIFICANCE

Vector saliva vaccines have successfully protected against other blood-feeding arthropod-transmitted diseases. Nevertheless, the rD7 salivary protein vaccine was not a good candidate for protection against WNV disease since immunized mice infected via an infected mosquito bite exhibited enhanced mortality. Selection of salivary protein vaccines on the bases of abundance and immunogenicity does not predict efficacy.

Analysis of salivary gland transcripts of the sand fly Lutzomyia ayacuchensis, a vector of Andean-type cutaneous leishmaniasis

The saliva of blood sucking insects contains potent pharmacologically active components that assist them in counteracting the host hemostatic and inflammatory systems during blood feeding. In addition, sand fly salivary proteins affect host immunity and have the potential to be a vaccine against Leishmania infection. In the present study, the salivary gland transcripts of Lutzomyia ayacuchensis, a vector of cutaneous leishmaniasis in Ecuadorian and Peruvian Andes, were analyzed by sequencing randomly selected clones of the salivary gland cDNA library of this sand fly. This resulted in the identification of the most abundant transcripts coding for secreted proteins. These proteins were homologous to the salivary molecules present in other sand flies including the RGD-containing peptide, PpSP15/SL1 family protein, yellow-related protein, putative apyrase, antigen 5-related protein, D7 family protein, and 27 kDa salivary protein. Of note, homologues of maxadilan, an active vasodilator abundantly present in saliva of Lutzomyia longipalpis, were not identified. This analysis is the first description of salivary proteins from a sand fly of the subgenus Helcocyrtomyia and from vector of cutaneous leishmaniasis in the New World. The present analysis will provide further insights into the evolution of salivary components in blood sucking arthropods.

Implication of haematophagous arthropod salivary proteins in host-vector interactions

The saliva of haematophagous arthropods contains an array of anti-haemostatic, anti-inflammatory and immunomodulatory molecules that contribute to the success of the blood meal. The saliva of haematophagous arthropods is also involved in the transmission and the establishment of pathogens in the host and in allergic responses. This survey provides a comprehensive overview of the pharmacological activity and immunogenic properties of the main salivary proteins characterised in various haematophagous arthropod species. The potential biological and epidemiological applications of these immunogenic salivary molecules will be discussed with an emphasis on their use as biomarkers of exposure to haematophagous arthropod bites or vaccine candidates that are liable to improve host protection against vector-borne diseases.

The function and three-dimensional structure of a thromboxane A2/cysteinyl leukotriene-binding protein from the saliva of a mosquito vector of the malaria parasite

A salivary protein from a malaria-transmitting mosquito uses a single domain to bind to thromboxane A₂ and cysteinyl leukotrienes and prevent blood clotting and inflammation in the host on which it feeds.

The highly expressed D7 protein family of mosquito saliva has previously been shown to act as an anti-inflammatory mediator by binding host biogenic amines and cysteinyl leukotrienes (CysLTs). In this study we demonstrate that AnSt-D7L1, a two-domain member of this group from Anopheles stephensi, retains the CysLT binding function seen in the homolog AeD7 from Aedes aegypti but has lost the ability to bind biogenic amines. Unlike any previously characterized members of the D7 family, AnSt-D7L1 has acquired the important function of binding thromboxane A₂ (TXA₂) and its analogs with high affinity. When administered to tissue preparations, AnSt-D7L1 abrogated Leukotriene C₄ (LTC₄)-induced contraction of guinea pig ileum and contraction of rat aorta by the TXA₂ analog U46619. The protein also inhibited platelet aggregation induced by both collagen and U46619 when administered to stirred platelets. The crystal structure of AnSt-D7L1 contains two OBP-like domains and has a structure similar to AeD7. In AnSt-D7L1, the binding pocket of the C-terminal domain has been rearranged relative to AeD7, making the protein unable to bind biogenic amines. Structures of the ligand complexes show that CysLTs and TXA₂ analogs both bind in the same hydrophobic pocket of the N-terminal domain. The TXA₂ analog U46619 is stabilized by hydrogen bonding interactions of the ω-5 hydroxyl group with the phenolic hydroxyl group of Tyr 52. LTC₄ and occupies a very similar position to LTE₄ in the previously determined structure of its complex with AeD7. As yet, it is not known what, if any, new function has been acquired by the rearranged C-terminal domain. This article presents, to our knowledge, the first structural characterization of a protein from mosquito saliva that inhibits collagen mediated platelet activation.

When feeding, a female mosquito must inhibit the blood clotting and inflammatory responses of the host. To do this, the insect produces salivary proteins that neutralize key host molecules participating in clotting and inflammation. Here, we describe a salivary protein AnSt-D7L1 that scavenges both thomboxane A₂ and cysteinyl leukotrienes, two substances involved in blood vessel constriction, platelet aggregation, and inflammatory responses to an insect bite. We produced this protein in bacteria and showed that it tightly binds both these molecules, inhibiting the processes in which they are involved. We then determined its structure using X-ray crystallography and showed that there is a single binding site in one domain of the protein, accommodating both thromboxane A₂ and cysteinyl leukotrienes, and that this site is responsible for the scavenging effect of the protein. These studies reveal the structural features of proteins needed to bind to key molecules of potential pharmacological importance and add to our understanding of the process of mosquito blood feeding, which is essential for transmission of the malaria parasite.

Transcriptomic and functional analysis of the Anopheles gambiae salivary gland in relation to blood feeding

Background

The Anopheles gambiae salivary glands play a major role in malaria transmission and express a variety of bioactive components that facilitate blood-feeding by preventing platelet aggregation, blood clotting, vasodilatation, and inflammatory and other reactions at the probing site on the vertebrate host.

Results

We have performed a global transcriptome analysis of the A. gambiae salivary gland response to blood-feeding, to identify candidate genes that are involved in hematophagy. A total of 4,978 genes were found to be transcribed in this tissue. A comparison of salivary gland transcriptomes prior to and after blood-feeding identified 52 and 41 transcripts that were significantly up-regulated and down-regulated, respectively. Ten genes were further selected to assess their role in the blood-feeding process using RNAi-mediated gene silencing methodology. Depletion of the salivary gland genes encoding D7L2, anophelin, peroxidase, the SG2 precursor, and a 5'nucleotidase gene significantly increased probing time of A. gambiae mosquitoes and thereby their capacity to blood-feed.

Conclusions

The salivary gland transcriptome comprises approximately 38% of the total mosquito transcriptome and a small proportion of it is dynamically changing already at two hours in response to blood feeding. A better understanding of the salivary gland transcriptome and its function can contribute to the development of pathogen transmission control strategies and the identification of medically relevant bioactive compounds.

An insight into the sialotranscriptome of the West Nile mosquito vector, Culex tarsalis

BACKGROUND

Saliva of adult female mosquitoes help sugar and blood feeding by providing enzymes and polypeptides that help sugar digestion, control microbial growth and counteract their vertebrate host hemostasis and inflammation. Mosquito saliva also potentiates the transmission of vector borne pathogens, including arboviruses. Culex tarsalis is a bird feeding mosquito vector of West Nile Virus closely related to C. quinquefasciatus, a mosquito relatively recently adapted to feed on humans, and the only mosquito of the genus Culex to have its sialotranscriptome so far described.

RESULTS

A total of 1,753 clones randomly selected from an adult female C. tarsalis salivary glands (SG) cDNA library were sequenced and used to assemble a database that yielded 809 clusters of related sequences, 675 of which were singletons. Primer extension experiments were performed in selected clones to further extend sequence coverage, allowing for the identification of 283 protein sequences, 80 of which code for putative secreted proteins.

CONCLUSION

Comparison of the C. tarsalis sialotranscriptome with that of C. quinquefasciatus reveals accelerated evolution of salivary proteins as compared to housekeeping proteins. The average amino acid identity among salivary proteins is 70.1%, while that for housekeeping proteins is 91.2% (P < 0.05), and the codon volatility of secreted proteins is significantly higher than those of housekeeping proteins. Several protein families previously found exclusive of mosquitoes, including only in the Aedes genus have been identified in C. tarsalis. Interestingly, a protein family so far unique to C. quinquefasciatus, with 30 genes, is also found in C. tarsalis, indicating it was not a specific C. quinquefasciatus acquisition in its evolution to optimize mammal blood feeding.

Structure and mechanism in salivary proteins from blood-feeding arthropods

The saliva of blood-feeding arthropods contains rich mixtures of ligand binding proteins targeted at inhibiting hemostasis and inflammation in the host. Since blood feeding has evolved many times, different taxonomic groups utilize completely different families of proteins to perform similar tasks. Structural studies performed on a number of these proteins have revealed biologically novel and sophisticated mechanisms used to perform their functions. Here, the results of these structural and mechanistic studies are reviewed.

Insight into the sialome of the Black Fly, Simulium vittatum

Adaptation to vertebrate blood feeding includes development of a salivary "magic potion" that can disarm host hemostasis and inflammatory reactions. Within the lower Diptera, a vertebrate blood-sucking mode evolved in the Psychodidae (sand flies), Culicidae (mosquitoes), Ceratopogonidae (biting midges), Simuliidae (black flies), and the frog-feeding Corethrellidae. Sialotranscriptome analyses from several species of mosquitoes and sand flies and from one biting midge indicate divergence in the evolution of the blood-sucking salivary potion, manifested in the finding of many unique proteins within each insect family, and even genus. Gene duplication and divergence events are highly prevalent, possibly driven by vertebrate host immune pressure. Within this framework, we describe the sialome (from Greek sialo, saliva) of the black fly Simulium vittatum and discuss the findings within the context of the protein families found in other blood-sucking Diptera. Sequences and results of Blast searches against several protein family databases are given in Supplemental Tables S1 and S2, which can be obtained from http://exon.niaid.nih.gov/transcriptome/S_vittatum/T1/SV-tb1.zip and http://exon.niaid.nih.gov/transcriptome/S_vittatum/T2/SV-tb2.zip .

The Anopheles gambiae salivary protein gSG6: an anopheline-specific protein with a blood-feeding role

The Anopheles gambiae salivary gland protein 6 (gSG6) is a small protein specifically found in the salivary glands of adult female mosquitoes. We report here the expression of a recombinant form of the protein and we show that in vivo gSG6 is expressed in distal-lateral lobes and is secreted with the saliva while the female mosquito probes for feeding. Injection of gSG6 dsRNA into adult A. gambiae females results in decreased gSG6 protein levels, increased probing time and reduced blood feeding ability. gSG6 orthologs have been found so far only in the salivary glands of Anopheles stephensi and Anopheles funestus, both members of the Cellia subgenus. We report here the gSG6 sequence from five additional anophelines, four species of the A. gambiae complex and Anopheles freeborni, a member of the subgenus Anopheles. We conclude that gSG6 plays some essential blood feeding role and was recruited in the anopheline subfamily most probably after the separation of the lineage which gave origin to Cellia and Anopheles subgenera.

The salivary gland transcriptome of the neotropical malaria vector Anopheles darlingi reveals accelerated evolution of genes relevant to hematophagy

BACKGROUND

Mosquito saliva, consisting of a mixture of dozens of proteins affecting vertebrate hemostasis and having sugar digestive and antimicrobial properties, helps both blood and sugar meal feeding. Culicine and anopheline mosquitoes diverged ~150 MYA, and within the anophelines, the New World species diverged from those of the Old World ~95 MYA. While the sialotranscriptome (from the Greek sialo, saliva) of several species of the Cellia subgenus of Anopheles has been described thoroughly, no detailed analysis of any New World anopheline has been done to date. Here we present and analyze data from a comprehensive salivary gland (SG) transcriptome of the neotropical malaria vector Anopheles darlingi (subgenus Nyssorhynchus).

RESULTS

A total of 2,371 clones randomly selected from an adult female An. darlingi SG cDNA library were sequenced and used to assemble a database that yielded 966 clusters of related sequences, 739 of which were singletons. Primer extension experiments were performed in selected clones to further extend sequence coverage, allowing for the identification of 183 protein sequences, 114 of which code for putative secreted proteins.

CONCLUSION

Comparative analysis of sialotranscriptomes of An. darlingi and An. gambiae reveals significant divergence of salivary proteins. On average, salivary proteins are only 53% identical, while housekeeping proteins are 86% identical between the two species. Furthermore, An. darlingi proteins were found that match culicine but not anopheline proteins, indicating loss or rapid evolution of these proteins in the old world Cellia subgenus. On the other hand, several well represented salivary protein families in old world anophelines are not expressed in An. darlingi.

Salivary gland transcriptome analysis during Plasmodium infection in malaria vector Anopheles stephensi

BACKGROUND

Understanding the tissue-specific molecular cross-talk mechanism during the mosquito-parasite interaction is of prime importance in the design of new strategies for malaria control. Because mosquito salivary glands are the final destination for the parasite maturation and transmission of vector-borne diseases, identification and characterization of salivary genes and their products are equally important in order to access their effect on the infectivity of the parasite. During the last five years there have been several studies on the sialomes of Anopheles mosquitoes, however very limited information is available on the changes in the salivary gland transcriptome in the presence of Plasmodium, and this information is limited to the mosquito Anopheles gambiae.

METHODS

In this study we aimed to explore and identify parasite-induced transcripts from the salivary glands of Anopheles stephensi, using a subtractive hybridization protocol.

RESULTS

Ninety-four percent of expressed sequence tags (ESTs) showed close homology to previously known families of mosquito salivary gland secretary proteins, representing the induced expression of alternative splicing and/or additional new members of the protein family. The remaining 6% of ESTs did not yield significant homology to any known proteins in the non-redundant database and thus may represent a class of unknown/novel salivary proteins. Primary analysis of the ESTs also revealed identification of several novel immune-related transcripts, including defensin and cecropins, probably involved in counter-activation of the antagonistic defense system. A comprehensive description of each family of proteins has been discussed in relation to the tissue-specific mosquito-parasite interaction.

CONCLUSION

This is the first report on the identification of new putative salivary genes, presumably activated during parasite infection.

Pharmacological regulation of factor XII activation may be a new target to control pathological coagulation

FXII was identified 50 years ago as a coagulation protein in the intrinsic pathway of blood coagulation as FXII deficient patients had marked prolongation of the in vitro surface-activated coagulation time. However, series of investigations have convincingly shown that FXII has no role in normal hemostasis. Recently, experimentally induced thrombosis in factor XII-knockout mice has provided evidence that factor XII (FXII) deficient mice are protected against ischemic brain injury after obstructive clot formation. Based on these experiments it has, therefore, been suggested, that blocking of FXII could be a unique target to prevent obstructive clot formation in arterial thrombosis without side effect of increased bleeding. FXII deficiency has, however, not convincingly been shown to protect against arterial thrombosis in humans. The target mentioned above may either be an inhibition of FXII activation or an inhibition of its proteolytic activity. FXII is a zymogen of the proteolytic enzyme, FXIIa, the substrates of which are factor XI and prekallikrein. Thus, FXIIa is not only involved in the activation of the coagulation system, but is also associated with the kallikrein/kinin system. The activation of the latter is deeply involved in inflammation and pain sensation. Furthermore, FXIIa binds to endothelial cells and to the extracellular matrix, indicating a role in vascular repair. Therefore, a complete evaluation of all these properties of FXII and FXIIa has to be considered when formulating a strategy for blocking FXII activation.

Identification and characterization of plasma kallikrein-kinin system inhibitors from salivary glands of the blood-sucking insectTriatoma infestans

Two plasma kallikrein-kinin system inhibitors in the salivary glands of the kissing bug Triatoma infestans, designated triafestin-1 and triafestin-2, have been identified and characterized. Reconstitution experiments showed that triafestin-1 and triafestin-2 inhibit the activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII and prekallikrein, and subsequent release of bradykinin. Binding analyses showed that triafestin-1 and triafestin-2 specifically interact with factor XII and high molecular weight kininogen in a Zn2+-dependent manner, suggesting that they specifically recognize Zn2+-induced conformational changes in factor XII and high molecular weight kininogen. Triafestin-1 and triafestin-2 also inhibit factor XII and high molecular weight kininogen binding to negatively charged surfaces. Furthermore, they interact with both the N-terminus of factor XII and domain D5 of high molecular weight kininogen, which are the binding domains for biological activating surfaces. These results suggest that triafestin-1 and triafestin-2 inhibit activation of the kallikrein-kinin system by interfering with the association of factor XII and high molecular weight kininogen with biological activating surfaces, resulting in the inhibition of bradykinin release in an animal host during insect blood-feeding.

GP35 ANOAL, an abundant acidic glycoprotein of female Anopheles albimanus saliva

Salivary glands of female mosquitoes produce proteins, not completely described yet, that participate in carbohydrate and blood feeding. Here, we report an acidic glycoprotein of 35 kDa (GP35 ANOAL) secreted in the saliva of the malaria vector mosquito Anopheles albimanus. GP35 ANOAL is produced exclusively in the distal lateral lobes of adult female salivary glands, it has a pI of 4.45 and is negatively stained by regular silver stain. An 888 bp cDNA clone encoding a predicted product of 240 amino acids has a signal peptide, potential post-translational modification sites, and a disintegrin signature RGD. The GP35 ANOAL sequence depicts high similarities with the 30 kDa saliva allergen of Aedes aegypti, 30 kDa allergen-like hypothetical proteins, and GE-rich proteins present in several Anopheles species, as well as in Ae. albopictus and Culex pipiens quinquefasciatus. The function of this protein family is still unknown.

An annotated catalogue of salivary gland transcripts in the adult female mosquito, Aedes aegypti

Background

Saliva of blood-sucking arthropods contains a cocktail of antihemostatic agents and immunomodulators that help blood feeding. Mosquitoes additionally feed on sugar meals and have specialized regions of their glands containing glycosidases and antimicrobials that might help control bacterial growth in the ingested meals. To expand our knowledge on the salivary cocktail of Ædes ægypti, a vector of dengue and yellow fevers, we analyzed a set of 4,232 expressed sequence tags from cDNA libraries of adult female mosquitoes.

Results

A nonredundant catalogue of 614 transcripts (573 of which are novel) is described, including 136 coding for proteins of a putative secretory nature. Additionally, a two-dimensional gel electrophoresis of salivary gland (SG) homogenates followed by tryptic digestion of selected protein bands and MS/MS analysis revealed the expression of 24 proteins. Analysis of tissue-specific transcription of a subset of these genes revealed at least 31 genes whose expression is specific or enriched in female SG, whereas 24 additional genes were expressed in female SG and in males but not in other female tissues. Most of the 55 proteins coded by these SG transcripts have no known function and represent high-priority candidates for expression and functional analysis as antihemostatic or antimicrobial agents. An unexpected finding is the occurrence of four protein families specific to SG that were probably a product of horizontal transfer from prokaryotic organisms to mosquitoes.

Conclusion

Overall, this paper contributes to the novel identification of 573 new transcripts, or near 3% of the Æ. ægypti proteome assuming a 20,000-protein set, and to the best-described sialome of any blood-feeding insect.

High degree of conservancy among secreted salivary gland proteins from two geographically distant Phlebotomus duboscqi sandflies populations (Mali and Kenya)

Background

Salivary proteins from sandflies are potential targets for exploitation as vaccines to control Leishmania infection; in this work we tested the hypothesis that salivary proteins from geographically distant Phlebotomus duboscqi sandfly populations are highly divergent due to the pressure exerted by the host immune response. Salivary gland cDNA libraries were prepared from wild-caught P. duboscqi from Mali and recently colonised flies of the same species from Kenya.

Results

Transcriptome and proteome analysis resulted in the identification of the most abundant salivary gland-secreted proteins. Orthologues of these salivary proteins were identified by phylogenetic tree analysis. Moreover, comparative analysis between the orthologues of these two different populations resulted in a high level of protein identity, including the predicted MHC class II T-cell epitopes from all these salivary proteins.

Conclusion

These data refute the hypothesis that salivary proteins from geographically distinct populations of the same Phlebotomus sandfly species are highly divergent. They also suggest the potential for using the same species-specific components in a potential vector saliva-based vaccine.

Toward silencing the burden of malaria: progress and prospects for RNAi-based approaches

The discovery of RNA interference (RNAi) is one of the most significant of recent years, with potential for application beyond the laboratory to the clinic. As a tool for functional genomics, RNAi has permitted the characterization of genes in organisms that had previously remained recalcitrant to targeted gene manipulation. Efforts to understand its mode of action have revealed a central role in gene regulation and host defense. Finally, as a therapeutic tool, it has shown enormous promise in the control of a large array of diseases. Here we examine how RNAi is revolutionizing malaria research in an organism, the Anopheles mosquito, that until recently was essentially resistant to genetic study, and show how its application in both the mosquito vector and the Plasmodium parasite might ultimately lead to new ways of controlling and perhaps even eradicating this devastating disease.

The immunomodulatory factors of arthropod saliva and the potential for these factors to serve as vaccine targets to prevent pathogen transmission

In general, attempts to develop vaccines for pathogens transmitted by arthropods have met with little or no success. It has been widely observed that the saliva of arthropods that transmit disease enhances the infectivity of pathogens the arthropod transmits to the vertebrate host. Indeed, it has been observed that vaccinating against components of the saliva of arthropods or against antigens expressed in the gut of arthropods can protect the host from infection and decrease the viability of the arthropod. These results suggest that multi-subunit vaccines that target the pathogen itself as well as arthropod salivary gland components and arthropod gut antigens may be the most effective at controlling arthropod-borne pathogens as these vaccines would target several facets of the lifecycle of the pathogen. This review covers known immunomodulators in arthropod salivary glands, instances when arthropod saliva has been shown to enhance infection and a limited number of examples of antiarthropod vaccines, with emphasis on three arthropods: sandflies, mosquitoes and hard ticks.