Lectins of distinct specificity in Rhodnius prolixus interact selectively with Trypanosoma cruzi

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

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

The innate immune response of triatomines against Trypanosoma cruzi and Trypanosoma rangeli with an unresolved question: Do triatomines have immune memory?

The present work aimed to review the immune response from different triatomines against Trypanosoma cruzi and Trypanosoma rangeli and propose the study of immune memory in such insects. Trypanosoma use triatomines as vectors to reach and infect mammals. A key question to be answered about vector-parasite interaction is why the immune defense and resistance of the insect against the parasites vary. Up to date data shows that the defense of triatomines against parasites includes cellular (phagocytosis, nodulation and encapsulation) and humoral (antimicrobial peptides, phenoloxidase and reactive oxygen and nitrogen species) responses. The immune response varies depending on the triatomine species, the trypanosome strain and species, and the insect intestinal microbiota. Despite significant advances to understand parasite-insect interaction, it is still unknown if triatomines have immune memory against parasites and if this memory may derive from tolerance to parasites attack. Therefore, a closer study of such interaction could contribute and establish new proposals to control the parasite at the vector level to reduce parasite transmission to mammals, including men. For instance, if immune memory exists in the triatomines, it would be interesting to induce weak infections in insects to find out if subsequent infections are less intense and if the insects succeed in eliminating the parasites.

A rhamnose-binding lectin from Rhodnius prolixus and the impact of its silencing on gut bacterial microbiota and Trypanosoma cruzi

Lectins are ubiquitous proteins involved in the immune defenses of different organisms and mainly responsible for non-self-recognition and agglutination reactions. This work describes molecular and biological characterization of a rhamnose-binding lectin (RBL) from Rhodnius prolixus, which possesses a 21 amino acid signal peptide and a mature protein of 34.6 kDa. The in-silico analysis of the primary and secondary structures of RpLec revealed a lectin domain fully conserved among previous insects studied. The three-dimensional homology model of RpLec was similar to other RBL-lectins. Docking predictions with the monosaccharides showed rhamnose and galactose-binding sites comparable to Latrophilin-1 and N-Acetylgalactosamine-binding in a different site. The effects of RpLec gene silencing on levels of infecting Trypanosoma cruzi Dm 28c and intestinal bacterial populations in the R. prolixus midgut were studied by injecting RpLec dsRNA into the R. prolixus hemocoel. Whereas T. cruzi numbers remained unchanged compared with the controls, numbers of bacteria increased significantly. The silencing also induced the up regulation of the R. prolixus defC (defensin) expression gene. These results with RpLec reveal the potential importance of this little studied molecule in the insect vector immune response and homeostasis of the gut bacterial microbiota.

Roles and Biomedical Applications of Haemolymph Lectin

BACKGROUND

Lectins are class of proteins characterized by their ability to selectively bind carbohydrate moieties of glycoproteins. Many invertebrate lectins, especially derived from hemolymph, are being purified, and yet their functions and medical applications are subjects of major interest.

METHODS

Hemolymph lectins in invertebrates play a major role in protecting against many pathogens and microbes. Further, many hemolymph lectins show anticancer properties towards various cancer cell lines, which expresses globotriaosyl ceramides on their cell surface.

RESULTS

These vast repertoires of hemolymph lectins in recognizing and inhibiting the growth of various harmful microbes and cancerous cells have spurred the biochemist to use them in histochemical and cytochemical studies.

CONCLUSION

The present review will address the biological roles and biomedical applications of hemolymph lectin.

Glycosylation on proteins of the intestine and perimicrovillar membrane of Triatoma (Meccus) pallidipennis, under different feeding conditions

Trypanosoma cruzi, the causative agent of Chagas disease, interacts with molecules in the midgut of its insect vector to multiply and reach the infective stage. Many studies suggest that the parasite binds to midgut-specific glycans. We identified several glycoproteins expressed in the intestine and perimicrovillar membrane (PMM) of Triatoma (Meccus) pallidipennis under different feeding conditions. In order to assess changes in protein-linked glycans, we performed lectin and immunoblot analyses on glycoprotein extracts from these intestinal tissues using well-characterized lectins, and an antibody, which collectively recognize a wide range of different glycans epitopes. We observed that the amount and composition of proteins and glycoproteins associated with different glycans structures changed over time in the intestines and PMM under different physiological conditions. PMM extracts contained a wide variety of glycoproteins with different sugar residues, including abundant high-mannose and complex sialylated glycans. We propose that these molecules could be involved in the process of parasite-vector interactions.

Trypanosoma cruzi: A review of biological and methodological factors in Mexican strains

Trypanosoma cruzi, responsible for Chagas disease, is a serious public health problem in Latin America with eight million people infected in the world. Clinical manifestations observed in humans due to T. cruzi infection are largely associated with the wide biological and genetic heterogeneity of the parasite. This review presents an overview of the parasitological aspects of various strains of T. cruzi isolated mainly in Mexico, as well as an analysis of the methodological processes used to determine their virulence that could be influencing their biological characterization. We emphasize the importance of using uniform protocols to study T. cruzi virulence, taking into account factors related to: strain (i.e. developmental stage, lineage, biological origin, genetic variability), animal model used (i.e. role of hormones, host immune response, age) and methodology (i.e. inoculum size, inoculation route, and laboratory conditions used during strain maintenance). These uniform protocols will then allow proposing elements for understanding clinical evolution and management of the disease, for providing adequate treatment, and for developing tools for future vaccines against Chagas disease.

Trypanosoma cruzi surface mucins are involved in the attachment to the Triatoma infestans rectal ampoule

Background

Trypanosoma cruzi, the agent of Chagas disease, is a protozoan parasite transmitted to humans by blood-sucking triatomine vectors. However, and despite its utmost biological and epidemiological relevance, T. cruzi development inside the digestive tract of the insect remains a poorly understood process.

Methods/Principle findings

Here we showed that Gp35/50 kDa mucins, the major surface glycoproteins from T. cruzi insect-dwelling forms, are involved in parasite attachment to the internal cuticle of the triatomine rectal ampoule, a critical step leading to its differentiation into mammal-infective forms. Experimental evidence supporting this conclusion could be summarized as follows: i) native and recombinant Gp35/50 kDa mucins directly interacted with hindgut tissues from Triatoma infestans, as assessed by indirect immunofluorescence assays; ii) transgenic epimastigotes over-expressing Gp35/50 kDa mucins on their surface coat exhibited improved attachment rates (~2–3 fold) to such tissues as compared to appropriate transgenic controls and/or wild-type counterparts; and iii) certain chemically synthesized compounds derived from Gp35/50 kDa mucins were able to specifically interfere with epimastigote attachment to the inner lining of T. infestans rectal ampoules in ex vivo binding assays, most likely by competing with or directly blocking insect receptor(s). A solvent-exposed peptide (smugS peptide) from the Gp35/50 kDa mucins protein scaffolds and a branched, Galf-containing trisaccharide (Galfβ1–4[Galpβ1–6]GlcNAcα) from their O-linked glycans were identified as main adhesion determinants for these molecules. Interestingly, exogenous addition of a synthetic Galfβ1–4[Galpβ1–6]GlcNAcα derivative or of oligosaccharides containing this structure impaired the attachment of Dm28c but not of CL Brener epimastigotes to triatomine hindgut tissues; which correlates with the presence of Galf residues on the Gp35/50 kDa mucins’ O-glycans on the former but not the latter parasite clone.

Conclusion/Significance

These results provide novel insights into the mechanisms underlying T. cruzi-triatomine interplay, and indicate that inter-strain variations in the O-glycosylation of Gp35/50 kDa mucins may lead to differences in parasite differentiation and hence, in parasite transmissibility to the mammalian host. Most importantly, our findings point to Gp35/50 kDa mucins and/or the Galf biosynthetic pathway, which is absent in mammals and insects, as appealing targets for the development of T. cruzi transmission-blocking strategies.

Chagas disease, caused by the protozoan Trypanosoma cruzi, is a life-long and debilitating neglected illness of major significance to Latin America public health, for which no vaccine or adequate drugs are yet available. In this scenario, identification of novel drug targets and/or strategies aimed at controlling parasite transmission are urgently needed. By using ex vivo binding assays together with different biochemical and genetic approaches, we herein show that Gp35/50 kDa mucins, the major T. cruzi epimastigote surface glycoproteins, specifically adhere to the internal cuticle of the rectal ampoule of the triatomine vector, a critical step leading to their differentiation into mammal-infective metacyclic forms. Ex vivo binding assays in the presence of chemically synthesized analogs allowed the identification of a solvent-exposed peptide and a branched, galactofuranose (Galf)-containing trisaccharide (Galfβ1–4[Galpβ1–6]GlcNAcα) as major Gp35/50 kDa mucins adhesion determinants. Overall, these results provide novel insights into the mechanisms underlying the complex T. cruzi-triatomine interplay. In addition, and since the presence of Galf-based glycotopes on the O-glycans of Gp35/50 kDa mucins is restricted to certain parasite strains/clones, they also indicate that the Galfβ1–4[Galpβ1–6]GlcNAcα motif may contribute to the well-established phenotypic variability among T. cruzi isolates. Most importantly, and taking into account that Galf residues are not found in mammals, we propose Gp35/50 kDa mucins and/or Galf biosynthesis as appealing and novel targets for the development of T. cruzi transmission-blocking strategies.

What makes an effective Chagas disease vector? Factors underlying Trypanosoma cruzi-triatomine interactions

The Chagas disease is caused by the parasite Trypanosoma cruzi, which infect blood-feeding triatomine bugs to finally reach mammal hosts. Chagas disease is endemic in Latin America, and is ranked among the 13 neglected tropical diseases worldwide. Currently, an estimate of 7 million people is infected by T. cruzi, leading to about 22 000 deaths per year throughout the Americas. As occurs with other vectors, a major question towards control programs is what makes a susceptible bug. In this review, we focus on findings linked to insect gut structure and microbiota, immunity, genetics, blood sources, abiotic factors (with special reference to ambient temperature and altitude) to understand the interactions occurring between T. cruzi and triatomine bugs, under a co-evolutionary scenario. These factors lead to varying fitness benefits and costs for bugs, explaining why infection in the insect takes place and how it varies in time and space. Our analysis highlights that major factors are gut components and microbiota, blood sources and temperature. Although their close interaction has never been clarified, knowledge reviewed here may help to boost the success of triatomine control programs, reducing the use of insecticides.

Effect of temperature and vector nutrition on the development and multiplication of Trypanosoma rangeli in Rhodnius prolixus

Trypanosoma rangeli is a protozoan parasite that infects mammals and triatomines, causing different levels of pathogenicity in its invertebrate vectors, particularly those from the genus Rhodnius. We have recently shown that temperature can modulate T. rangeli growth during in vitro culture, as well as its in vivo pathogenicity to R. prolixus. In the present study, we investigated colonization of R. prolixus by T. rangeli and assessed the role of temperature and vector nutrition on parasite development and multiplication. We infected nymphs and either assessed parasite density in the first hours after the ingestion of the infected blood or maintained the nymphs for up to 60 days at different temperatures (21, 24, 27, and 30 °C) and under different blood-feeding schedules (either every 15 days, or on day 30 post infection only), with parasite development and multiplication measured on days 15, 30, and 60 post infection. In the first hours after ingesting infected blood, epimastigogenesis not only occurred in the anterior midgut, but a stable parasite population also established in this intestinal region. T. rangeli subsequently colonized all intestinal regions examined, but with fewer parasites being found in the rectum. The number of parasites was only affected by higher temperatures (27 and 30 °C) during the beginning of the infection (15 days post infection). Nutritional status of the vector also had a significant effect on parasite development, as reduced blood-feeding decreased infection rates by approximately 30%.

Triatomine physiology in the context of trypanosome infection

Triatomines are hematophagous insects that feed on the blood of vertebrates from different taxa, but can occasionally also take fluids from invertebrate hosts, including other insects. During the blood ingestion process, these insects can acquire diverse parasites that can later be transmitted to susceptible vertebrates if they complete their development inside bugs. Trypanosoma cruzi, the etiological agent of Chagas disease, and Trypanosoma rangeli are protozoan parasites transmitted by triatomines, the latter only transmitted by Rhodnius spp. The present work makes an extensive revision of studies evaluating triatomine-trypanosome interaction, with special focus on Rhodnius prolixus interacting with the two parasites. The sequences of events encompassing the development of these trypanosomes inside bugs and the consequent responses of insects to this infection, as well as many pathological effects produced by the parasites are discussed.

Rhodnius prolixus: from physiology by Wigglesworth to recent studies of immune system modulation by Trypanosoma cruzi and Trypanosoma rangeli

This review is dedicated to the memory of Professor Sir Vincent B. Wigglesworth (VW) in recognition of his many pioneering contributions to insect physiology which, even today, form the basis of modern-day research in this field. Insects not only make vital contributions to our everyday lives by their roles in pollination, balancing eco-systems and provision of honey and silk products, but they are also outstanding models for studying the pathogenicity of microorganisms and the functioning of innate immunity in humans. In this overview, the immune system of the triatomine bug, Rhodnius prolixus, is considered which is most appropriate to this dedication as this insect species was the favourite subject of VW's research. Herein are described recent developments in knowledge of the functioning of the R. prolixus immune system. Thus, the roles of the cellular defences, such as phagocytosis and nodule formation, as well as the role of eicosanoids, ecdysone, antimicrobial peptides, reactive oxygen and nitrogen radicals, and the gut microbiota in the immune response of R. prolixus are described. The details of many of these were unknown to VW although his work gives indications of his awareness of the importance to R. prolixus of cellular immunity, antibacterial activity, prophenoloxidase and the gut microbiota. This description of R. prolixus immunity forms a backdrop to studies on the interaction of the parasitic flagellates, Trypanosoma cruzi and Trypanosoma rangeli, with the host defences of this important insect vector. These parasites remarkably utilize different strategies to avoid/modulate the triatomine immune response in order to survive in the extremely hostile host environments present in the vector gut and haemocoel. Much recent information has also been gleaned on the remarkable diversity of the immune system in the R. prolixus gut and its interaction with trypanosome parasites. This new data is reviewed and gaps in our knowledge of R. prolixus immunity are identified as subjects for future endeavours. Finally, the publication of the T. cruzi, T. rangeli and R. prolixus genomes, together with the use of modern molecular techniques, should lead to the enhanced identification of the determinants of infection derived from both the vector and the parasites which, in turn, could form targets for new molecular-based control strategies.

Origin, evolution and function of the hemipteran perimicrovillar membrane with emphasis on Reduviidae that transmit Chagas disease

The peritrophic matrix is a chitin-protein structure that envelops the food bolus in the midgut of the majority of insects, but is absent in some groups which have, instead, an unusual extra-cellular lipoprotein membrane named the perimicrovillar membrane. The presence of the perimicrovillar membrane (PMM) allows these insects to exploit restricted ecological niches during all life stages. It is found only in some members of the superorder Paraneoptera and many of these species are of medical and economic importance. In this review we present an overview of the midgut and the digestive system of insects with an emphasis on the order Paraneoptera and differences found across phylogenetic groups. We discuss the importance of the PMM in Hemiptera and the apparent conservation of this structure among hemipteran groups, suggesting that the basic mechanism of PMM production is the same for different hemipteran species. We propose that the PMM is intimately involved in the interaction with parasites and as such should be a target for biological and chemical control of hemipteran insects of economic and medical importance.

Colonization of Rhodnius prolixus gut by Trypanosoma cruzi involves an extensive parasite killing

Trypanosoma cruzi, the etiological agent of Chagas disease, is ingested by triatomines during their bloodmeal on an infected mammal. Aiming to investigate the development and differentiation of T. cruzi inside the intestinal tract of Rhodnius prolixus at the beginning of infection we fed insects with cultured epimastigotes and blood trypomastigotes from infected mice to determine the amount of recovered parasites after ingestion. Approximately 20% of the ingested parasites was found in the insect anterior midgut (AM) 3 h after feeding. Interestingly, a significant reduction (80%) in the numbers of trypomastigotes was observed after 24 h of infection suggesting that parasites were killed in the AM. Moreover, few parasites were found in that intestinal portion after 96 h of infection. The evaluation of the numbers of parasites in the posterior midgut (PM) at the same periods showed a reduced parasite load, indicating that parasites were not moving from the AM. Additionally, incubation of blood trypomastigotes with extracts from R. prolixus AMs revealed that components of this tissue could induce significant death of T. cruzi. Finally, we observed that differentiation from trypomastigotes to epimastigotes is not completed in the AM; instead we suggest that trypomastigotes change to intermediary forms before their migration to the PM, where differentiation to epimastigotes takes place. The present work clarifies controversial points concerning T. cruzi development in insect vector, showing that parasite suffers a drastic decrease in population size before epimastigonesis accomplishment in PM.

Monitoring of the Parasite Load in the Digestive Tract of Rhodnius prolixus by Combined qPCR Analysis and Imaging Techniques Provides New Insights into the Trypanosome Life Cycle

Background

Here we report the monitoring of the digestive tract colonization of Rhodnius prolixus by Trypanosoma cruzi using an accurate determination of the parasite load by qPCR coupled with fluorescence and bioluminescence imaging (BLI). These complementary methods revealed critical steps necessary for the parasite population to colonize the insect gut and establish vector infection.

Methodology/Principal Findings

qPCR analysis of the parasite load in the insect gut showed several limitations due mainly to the presence of digestive-derived products that are thought to degrade DNA and inhibit further the PCR reaction. We developed a real-time PCR strategy targeting the T. cruzi repetitive satellite DNA sequence using as internal standard for normalization, an exogenous heterologous DNA spiked into insect samples extract, to precisely quantify the parasite load in each segment of the insect gut (anterior midgut, AM, posterior midgut, PM, and hindgut, H). Using combined fluorescence microscopy and BLI imaging as well as qPCR analysis, we showed that during their journey through the insect digestive tract, most of the parasites are lysed in the AM during the first 24 hours independently of the gut microbiota. During this short period, live parasites move through the PM to establish the onset of infection. At days 3–4 post-infection (p.i.), the parasite population begins to colonize the H to reach a climax at day 7 p.i., which is maintained during the next two weeks. Remarkably, the fluctuation of the parasite number in H remains relatively stable over the two weeks after refeeding, while the populations residing in the AM and PM increases slightly and probably constitutes the reservoirs of dividing epimastigotes.

Conclusions/Significance

These data show that a tuned dynamic control of the population operates in the insect gut to maintain an equilibrium between non-dividing infective trypomastigote forms and dividing epimastigote forms of the parasite, which is crucial for vector competence.

Although the key aspects of the T. cruzi life cycle were described more than one century ago, the development and interactions of T. cruzi with its vector are poorly characterized. By dissection of different compartments of the triatomine gut (prototype Rhodnius prolixus) (i.e., AM, PM and H) at regular time intervals, we evaluated trypanosome development within the insect using an accurate qPCR assay. qPCR analysis of trypanosomal colonization and clearance dynamics in real-time were confirmed in vivo using both fluorescence and bioluminescence imaging, which revealed massive parasite lysis during the first 24 hours post-feeding (p.f.). After one week, the parasite succeeded in establishing a resident population in each compartment of the gut, albeit at varying levels. From one week after the onset of infection in the AM and PM, some resident forms agglomerated into rosettes, clustering in close association with the vector tissue and constituting potential parasite reservoirs of the bug. For the first time, we have described a methodology to accurately quantify parasites in the insect gut that would be a useful tool for evaluating the impact of RNAi silencing of insect genes during the course of infection by T. cruzi.

Development and glycoprotein composition of the perimicrovillar membrane in Triatoma (Meccus) pallidipennis (Hemiptera: Reduviidae)

Hemipterans and thysanopterans (Paneoptera: Condylognatha) differ from other insects by having an intestinal perimicrovillar membrane (PMM) which extends from the base of the microvilli to the intestinal lumen. The development and composition of the PMM in hematophagous Reduviidae depend on factors related to diet. The PMM may also allow the human parasite Trypanosoma cruzi, the etiological agent of human Chagas Disease, to establish and develop in this insect vector. We studied the PMM development in the Mexican vector of Chagas Disease, Triatoma (Meccus) pallidipennis. We describe changes in the midgut epithelial cells of insects in response to starvation, and at different times (10, 15 and 20 days) after bloodfeeding. In starved insects, the midguts showed epithelial cells closely connected to each other but apparently free of PMM with some regions being periodic acid-Schiff (PAS-Schiff) positive. In contrast, the PMM was evident and fully developed in the midgut region of insects 15 days after feeding. After this time, the PMM completely covered the microvilli and reached the midgut lumen. At 15 days following feeding the labeled PAS-Schiff increased in the epithelial apex, suggesting an increase in carbohydrates. Lectins as histochemical reagents show the presence of a variety of glycoconjugates including mannose, glucose, galactosamine, N-acetyl-galactosamine. Also present were N-acetyl-glucosamine and sialic acid which contribute to the successful establishment and replication or T. cruzi in its insect vectors. By means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the formation and structure of the PMM is confirmed at 15 days post feeding. Our results confirmed the importance of the feeding processes in the formation of the PMM and showed the nature of the biochemical composition of the vectors' intestine in this important Mexican vector of Chagas disease.

An insight into the transcriptome of the digestive tract of the bloodsucking bug, Rhodnius prolixus

The bloodsucking hemipteran Rhodnius prolixus is a vector of Chagas' disease, which affects 7-8 million people today in Latin America. In contrast to other hematophagous insects, the triatomine gut is compartmentalized into three segments that perform different functions during blood digestion. Here we report analysis of transcriptomes for each of the segments using pyrosequencing technology. Comparison of transcript frequency in digestive libraries with a whole-body library was used to evaluate expression levels. All classes of digestive enzymes were highly expressed, with a predominance of cysteine and aspartic proteinases, the latter showing a significant expansion through gene duplication. Although no protein digestion is known to occur in the anterior midgut (AM), protease transcripts were found, suggesting secretion as pro-enzymes, being possibly activated in the posterior midgut (PM). As expected, genes related to cytoskeleton, protein synthesis apparatus, protein traffic, and secretion were abundantly transcribed. Despite the absence of a chitinous peritrophic membrane in hemipterans - which have instead a lipidic perimicrovillar membrane lining over midgut epithelia - several gut-specific peritrophin transcripts were found, suggesting that these proteins perform functions other than being a structural component of the peritrophic membrane. Among immunity-related transcripts, while lysozymes and lectins were the most highly expressed, several genes belonging to the Toll pathway - found at low levels in the gut of most insects - were identified, contrasting with a low abundance of transcripts from IMD and STAT pathways. Analysis of transcripts related to lipid metabolism indicates that lipids play multiple roles, being a major energy source, a substrate for perimicrovillar membrane formation, and a source for hydrocarbons possibly to produce the wax layer of the hindgut. Transcripts related to amino acid metabolism showed an unanticipated priority for degradation of tyrosine, phenylalanine, and tryptophan. Analysis of transcripts related to signaling pathways suggested a role for MAP kinases, GTPases, and LKBP1/AMP kinases related to control of cell shape and polarity, possibly in connection with regulation of cell survival, response of pathogens and nutrients. Together, our findings present a new view of the triatomine digestive apparatus and will help us understand trypanosome interaction and allow insights into hemipteran metabolic adaptations to a blood-based diet.

Involvement of sulfated glycosaminoglycans on the development and attachment of Trypanosoma cruzi to the luminal midgut surface in the vector, Rhodnius prolixus

In the present study, we investigated the involvement of sulfated glycosaminoglycans in both the in vivo development and adhesion of T. cruzi epimastigotes to the luminal surface of the digestive tract of the insect vector, Rhodnius prolixus. Pre-incubation of T. cruzi, Dm 28c epimastigotes with heparin, chondroitin 4-sulfate, chondroitin 6-sulfate or protamine chloridrate inhibited in vitro attachment of parasites to the insect midgut. Enzymatic removal of heparan sulfate moieties by heparinase I or of chondroitin sulfate moieties by chondroitinase AC from the insect posterior midgut abolished epimastigote attachment in vitro. These treatments also reduced the labelling of anionic sites exposed at the luminal surface of the perimicrovillar membranes in the triatomine midgut epithelial cells. Inclusion of chondroitin 4-sulfate or chondroitin 6-sulfate and to a lesser extent, heparin, in the T. cruzi-infected bloodmeal inhibited the establishment of parasites in R. prolixus. These observations indicate that sulfated glycosaminoglycans are one of the determinants for both adhesion of the T. cruzi epimastigotes to the posterior midgut epithelial cells of the triatomine and the parasite infection in the insect vector, R. prolixus.

Genetics and evolution of triatomines: from phylogeny to vector control

Triatomines are hemipteran bugs acting as vectors of the protozoan parasite Trypanosoma cruzi. This parasite causes Chagas disease, one of the major parasitic diseases in the Americas. Studies of triatomine genetics and evolution have been particularly useful in the design of rational vector control strategies, and are reviewed here. The phylogeography of several triatomine species is now slowly emerging, and the struggle to reconcile the phenotypic, phylogenetic, ecological and epidemiological species concepts makes for a very dynamic field. Population genetic studies using different markers indicate a wide range of population structures, depending on the triatomine species, ranging from highly fragmented to mobile, interbreeding populations. Triatomines transmit T. cruzi in the context of complex interactions between the insect vectors, their bacterial symbionts and the parasites; however, an integrated view of the significance of these interactions in triatomine biology, evolution and in disease transmission is still lacking. The development of novel genetic markers, together with the ongoing sequencing of the Rhodnius prolixus genome and more integrative studies, will provide key tools to expanding our understanding of these important insect vectors and allow the design of improved vector control strategies.

Effect of intestinal erythrocyte agglutination on the feeding performance of Triatoma brasiliensis (Hemiptera: Reduviidae)

Triatoma brasiliensis is an important vector of Trypanosoma cruzi in Brazil. The feeding efficiency on its hosts depends on several parameters including the maintenance of the ingested blood at low viscosity, which could be modulated by the anterior midgut (crop) anticoagulant and haemagglutinant activities. In the present study, we characterized T. brasiliensis crop haemagglutination activity and evaluated its importance in the feeding process. Soluble crop contents (SCC) of T. brasiliensis were able to agglutinate rat, mouse and rabbit eryhtrocytes, but had no activity on cattle and Thrichomys apereoides, a rodent species commonly associated with T. brasiliensis in the wild. The haemagglutination was characterized by the immediate formation of several clusters of erythrocytes connected by flexible elastic-like fibers. The feeding efficiency of T. brasiliensis on rat (agglutinated by SCC) was almost double that from T. apereoides (not agglutinated by SCC). The influence of haemagglutination on feeding was confirmed by artificially feeding bugs on a diet composed of cattle or rat erythrocytes. The bugs fed on cattle erythrocytes had lower ingestion rates in comparison to those fed on rats. The results indicate that, in addition to other parameters, haemagglutination brought about by SCC has an important role in the feeding efficiency of T. brasiliensis.

Cytochemical characterization of microvillar and perimicrovillar membranes in the posterior midgut epithelium of Rhodnius prolixus

Perimicrovillar membranes (PMM) are structures present on the surface of midgut epithelial cells of the hematophagous insect, Rhodnius prolixus. They cover the microvilli and are especially evident 10 days after blood meal, providing the compartmentalization of the enzymatic processes in the intestinal microenvironment. Using an enzyme cytochemical approach, Mg2+-ATPase and ouabain-sensitive Na+K+-ATPase activities were observed in the plasma (or microvillar) membrane (MM) of midgut cells and in the PMM. In contrast, alkaline phosphatase was only detected in MM. Using cationized ferritin and colloidal iron hydroxide particles, anionic sites were found only on the luminal surface of the PMM. Using fluorescein isothiocyanate (FITC)-labeled lectins, residues of alpha-d-galactose, mannose, N-acetyl-neuraminic acid, N-acetyl-d-galactosamine and N-acetyl-galactosamine-alpha-1,3-galactose were detected on the apical surface of posterior midgut epithelial cells. On the other hand, using FITC-labeled neoglycoproteins (NGP) it was possible to detect the presence of carbohydrate binding molecules (CBM) recognizing N-acetyl-d-galactosamine, alpha-d-mannose, alpha-l-fucose and alpha-d-glucose in the posterior midgut epithelium. The use of digitonin showed the presence of sterols in the MM and PMM. These results have led the authors to suggest that for some components the PMM resembles the MM lining the midgut cells of R. prolixus, composing a system which covers the microvilli and stretches to the luminal space.

Purification and characterisation of a natural lectin from the serum of the shrimp Litopenaeus vannamei

A natural lectin from the serum of the shrimp Litopenaeus vannamei was purified to homogeneity by a single-step affinity chromatography using fetuin-coupled agarose. The purified serum lectin (named LVL) showed a strong affinity for human A/B/O erythrocytes (RBC), mouse RBC, chicken RBC and its haemagglutinating (HA) activity was specifically dependent on Ca2+ and reversibly sensitive to EDTA. LVL inactive form had a molecular mass estimate of 172 kDa and was composed of two non-identical subunits (32 and 38 kDa) cross-linked by interchain disulphide bonds. Significant LVL activity was observed between pH 7 and 11. In HA-inhibition assays performed with several carbohydrates and glycoproteins, LVL showed a distinct and unique specificity for GalNAc/GluNAc/NeuAc which had an acetyl group, while glycoproteins fetuin and bovine submaxillary mucin (BSM) had sialic acid. Moreover, this agglutinin appeared to recognise the terminal N- and O-acetyl groups in the oligosaccharide chain of glycoconjugates. The HA activity of L. vannamei lectin was also susceptible to inhibition by lipopolysaccharides from diverse Gram-negative bacteria, which might indicate a significant in vivo role of this humoral agglutinin in the host immune response against bacterial infections.

Trypanosoma cruzi: attachment to perimicrovillar membrane glycoproteins of Rhodnius prolixus

Studies were carried out to identify proteins involved in the interface of Trypanosoma cruzi with the perimicrovillar membranes (PMM) of Rhodnius prolixus. Video microscopy experiments demonstrated high level of adhesion of T. cruzi Dm 28c epimastigotes to the surface of posterior midgut cells of non-treated R. prolixus. The parasites however were unable to attach to gut cells obtained from decapitated or azadirachtin-treated insects. The influence of carbohydrates on the adhesion to insect midgut was confirmed by inhibition of parasite attachment after midgut incubation with N-acetylgalactosamine, N-acetylmannosamine, N-acetylglucosamine, D-galactose, D-mannose or sialic acid. We observed that hydrophobic proteins in the surface of epimastigotes bind to polypeptides with 47.7, 45.5, 44, 43, 40.5, 36, 31 and 13kDa from R. prolixus PMM and that pre-incubation of lectins specifically inhibited binding to 31, 40.5, 44 and 45.5kDa proteins. We suggest that glycoproteins from PMM and hydrophobic proteins from epimastigotes are important for the adhesion of the parasite to the posterior midgut cells of the vector.

Rhodnius prolixus: identification of immune-related genes up-regulated in response to pathogens and parasites using suppressive subtractive hybridization

We report the identification of immune-related molecules from the fat body, and intestine of Rhodnius prolixus, an important vector of Chagas disease. Insects were challenged by introducing pathogens or Trypanosoma cruzi, the parasite that causes Chagas disease, into the hemocoel. RNA from intestines, or fat body were isolated 24h after stimulation. We used suppressive subtractive hybridization to identify immune-related genes, generated three subtracted libraries, sequenced the clones and assembled the sequences. The functional annotation revealed expressed sequence tags (ESTs) generated in response to various stimuli in all tissues, and included pathogen recognition molecules, regulatory molecules, and effector molecules.

Tick lectins: structural and functional properties

Few papers have been published on tick lectins so far, and therefore more data are needed to complete the mosaic of knowledge of their structural and functional properties. Tissue-specific lectin/haemagglutinin activities of both soft and hard ticks have been investigated. Some tick lectins are proteins with binding affinity for sialic acid, various derivatives of hexosamines and different glycoconjugates. Most tick lectin/haemagglutinin activities are blood meal enhanced, and could serve as molecular factors of self/non-self recognition in defence reactions against bacteria or fungi, as well as in pathogen/parasite transmission. Dorin M, the plasma lectin of Ornithodoros moubata, is the first tick lectin purified so far from tick haemolymph, and the first that has been fully characterized. Partial characterization of other tick lectins/haemagglutinins has been performed mainly with respect to their carbohydrate binding specificities and immunochemical features.

Platelet-activating factor-like activity isolated from Trypanosoma cruzi

Platelet-activating factor is a phospholipid mediator that exhibits a wide variety of physiological and pathophysiological effects, including induction of inflammatory response, chemotaxis and cellular differentiation. Trypanosoma cruzi, the etiological agent of Chagas' disease, is transmitted by triatomine insects and while in the triatomine midgut the parasite differentiates from a non-infective epimastigote stage into the pathogenic trypomastigote metacyclic form. We have previously demonstrated that platelet activating factor triggers in vitro cell differentiation of T. cruzi. Here we show a platelet activating factor-like activity isolated from lipid extract of T. cruzi epimastigotes incubated in the presence of [14C]acetate. Trypanosoma cruzi-platelet activating factor-like lipid induced the aggregation of rabbit platelets, which was prevented by platelet activating factor-acetylhydrolase. Mouse macrophage infection by T. cruzi was stimulated when epimastigotes were kept for 5 days in the presence of T. cruzi-platelet activating factor, before interacting with the macrophages. The differentiation of epimastigotes into metacyclic trypomastigotes was also triggered by T. cruzi-platelet activating factor. These effects were abrogated by a platelet activating factor antagonist, WEB 2086. Polyclonal antibody raised against mouse platelet activating factor receptor showed labelling for T. cruzi epimastigotes using immunoblotting and immunofluorescence assays. These data suggest that T. cruzi contain the components of an autocrine platelet activating factor-like ligand-receptor system that modulates cell differentiation towards the infectious stage.

Towards an understanding of the interactions of Trypanosoma cruzi and Trypanosoma rangeli within the reduviid insect host Rhodnius prolixus

This review outlines aspects on the developmental stages of Trypanosoma cruzi and Trypanosoma rangeli in the invertebrate host, Rhodnius prolixus. Special attention is given to the interactions of these parasites with gut and hemolymph molecules and the effects of the organization of midgut epithelial cells on the parasite development. The vector insect's permissiveness to T. cruzi, which develops in the vector gut, largely depends on the host nutritional state, the parasite strain and the molecular interactions with trypanolytic compounds, lectins and resident bacteria in the gut. T. rangeli invades the hemocoel and once in the hemolymph, can be recognized and activates the defense system of its insect vector, i.e., the prophenoloxidase system, phagocytosis, hemocyte microaggregation, superoxide and nitric oxide activity and the eicosanoid biosynthesis pathway. Taken together, these findings not only provide a better understanding of the interactions parasite-insect vector, but also offer new insights into basic physiological processes involved in the parasites transmission.

Immune mechanisms in insects

Understanding immune evasion by parasites in their insect vectors requires some understanding of the insect immune system. Until fairly recently, technical difficulties in handling cells and plasma hampered laboratory investigations into insect immunology, but modern techniques combined with a sound knowledge of insect physiology are now permitting rapid advances. Rather than discussing the many controversies, this review aims to point out current areas of research into cellular and 'humoral' mechanisms that might be followed up by parasitologists.

TsetseEP, a gut protein from the tsetse Glossina morsitans, is related to a major surface glycoprotein of trypanosomes transmitted by the fly and to the products of a Drosophila gene family

African trypanosomes live in the lumen of the gut of tsetse (Glossina) and may have to face an immune response. As yet, it is unclear whether they are sensitive to antimicrobial peptides in vivo, but for some years there has been indirect evidence that one or more lectins can influence the infection. We have purified a protein complex from midgut extracts that, by SDS-PAGE, is a doublet of 37 and 38 kDa in a ratio of 3:1. Through prediction from corresponding cDNA clones, the full-length protein (tsetseEP) contains 320 amino acids, including a signal peptide. There is apparently only one gene encoding this protein. Towards the C terminus, the protein contains a run of 59 (EP) repeats, which surprisingly is what comprises almost the entire mature EP procyclin molecule present on the surface of trypanosomes in the tsetse gut. Drosophila contains a number of genes encoding proteins, of unknown function, with the same cysteine pattern as tsetseEP; this pattern is not reported for any other protein. Immunoblotting with a monoclonal antibody against (EP) repeats reveals expression in the gut, but not salivary glands, of female and male flies, whether or not fed. Immunoelectron microscopy shows the presence in vesicles in midgut cells and in the lumen of the gut. Attempts to demonstrate lectin activity were thwarted by limited availability of the protein complex.

Effects of eicosanoid biosynthesis inhibitors on the prophenoloxidase-activating system and microaggregation reactions in the hemolymph of Rhodnius prolixus infected with Trypanosoma rangeli

Investigations on the effects of eicosanoid biosynthesis inhibitors on the hemocyte microaggregation and prophenoloxidase (proPO)-activating system in the hemolymph, parasitemia and mortality of Rhodnius prolixus infected with Trypanosoma rangeli were performed. Hemocoelic injection of live T. rangeli epimastigotes into fifth-instar larvae of R. prolixus that previously fed on blood containing an inhibitor of phospholipase A(2) (dexamethasone), a specific inhibitor of the cyclooxygenase pathway (indomethacin), and a non-selective lipoxygenase inhibitor (NDGA) (i) reduced the hemocyte microaggregation, (ii) attenuated the proPO system in the hemolymph and (iii) enhanced parasitemia and mortality induced by the parasite challenge in these insects. The effects obtained by dexamethasone administered orally were counteracted by inoculation of the insects with arachidonic acid. We suggest that the infectivity of T. rangeli can be increased by interference with the R. prolixus immune system. This is the first demonstration that the triatomine's immune responses to a parasite infection are modulated by a physiological system that includes eicosanoid biosynthesis.

Purification and characterization of a sialic acid specific lectin from the hemolymph of the freshwater crab Paratelphusa jacquemontii

A naturally occurring hemagglutinin was detected in the serum of the freshwater crab, Paratelphusa jacquemontii (Rathbun). Hemagglutination activity with different mammalian erythrocytes suggested a strong affinity of the serum agglutinin for horse and rabbit erythrocytes. The most potent inhibitor of hemagglutination proved to be bovine submaxillary mucin. The lectin was purified by affinity chromatography using bovine submaxillary mucin-coupled agarose. The molecular mass of the purified lectin was 34 kDa as determined by SDS/PAGE. The hemagglutination of purified lectin was inhibited by N-acetylneuraminic acid but not by N-glycolylneuraminic acid, even at a concentration of 100 mm. Bovine submaxillary mucin, which contains mainly 9-O-acetyl- and 8,9 di-O-acety-N-acetyl neuraminic acid was the most potent inhibitor of the lectin. Sialidase treatment and de-O-acetylation of bovine submaxillary mucin abolished its inhibitory capacity completely. Also, asialo-rabbit erythrocytes lost there binding specificity towards the lectin. The findings indicated an O-acetyl neuraminic acid specificity of the lectin.

Suppression of the prophenoloxidase system in Rhodnius prolixus orally infected with Trypanosoma rangeli

Investigations were carried out to compare aspects of the prophenoloxidase (proPO)-activating pathway in Rhodnius prolixus hemolymph in response to oral infection and inoculation of the insects with two developmental forms of Trypanosoma rangeli epimastigotes strain H14. In vivo experiments demonstrated that in control insects fed on uninfected blood, inoculation challenge with short epimastigotes resulted in high phenoloxidase (PO) activity. In contrast, previous feeding on blood containing either short or long epimastigotes was able to suppress the proPO activation induced by thoracic inoculation of the short forms. In vitro assays in the presence of short epimastigotes demonstrated that control hemolymph or hemolymph provided by insects previously fed on blood containing epimastigotes incubated with fat body homogenates from control insects significantly increased the PO activity. However, fat body homogenates from insects previously fed on blood containing epimastigotes, incubated with hemolymph taken from insects fed on control blood or blood infected with epimastigotes, drastically reduced the proPO activation. The proteolytic activity in the fat body homogenates of control insects was significantly higher than in those obtained from fat body extracts of insects previously fed on blood containing epimastigotes. These findings indicate that the reduction of the proteolytic activities in the fat body from insects fed on infected blood no longer allows a significant response of the proPO system against parasite challenge. It also provides a better understanding of T. rangeli infection in the vector and offer novel insights into basic immune processes in their invertebrate hosts.

Interaction of Leptomonas wallacei with the intestinal tract of its natural host Oncopeltus fasciatus (Hemiptera: Lygaeidae)

While investigating the distribution of Leptomonas wallacei in the intestine of the insect host Oncopeltus fasciatus, promastigotes and cyst-like forms of L. wallacei were observed only in the midgut ventricles V(3) and V(4) and the hindgut. In video-microscopy, once contact had occurred, the parasites remained attached to the midgut epithelium. Scanning electron microscopy revealed the adhesion of flagellates and cyst-like forms to the midgut wall and to the rectal pads of the hindgut. Using transmission electron microscopy, we observed that adhesion occurred mainly between the flagellum and the perimicrovillar membranes secreted by the midgut epithelium. No modifications were observed either in the parasite or in the epithelial cells. In the hindgut, adhesion to the superficial wax layer of the epithelial cells of the rectal pads was via flagellum. Host cell morphology appeared unaffected by L. wallacei.

Binding specificity of mannose-specific carbohydrate-binding protein from the cell surface of Trypanosoma cruzi

The sugar binding specificity of the recently described mannose-specific carbohydrate-binding proteins (CBP) isolated to homogeneity from both the epimastigote and trypomastigote stages of the pathogenic protozoa Trypanosoma cruzi has been studied by quantitative hapten inhibition of the biotinylated CBPs to immobilized thyroglobulin using model oligosaccharides. The results clearly show a differential specificity toward high-mannose glycans between the CBPs from the two developmental stages. Thus, the isolated CBP from epimastigotes exhibited stronger affinity for higher mannose oligomers containing the Manalpha1-2Manalpha1-6Manalpha1-6 structure. Its affinity decreased, as did the number of mannose residues on the oligomer or removal of the terminal Manalpha1-2-linked mannose. By contrast the CBP isolated from the trypomastigote stage showed about 400-fold lower avidity than the epimastigote form, and contrary to it, it was slightly more specific toward Man5GlcNAc than Man9GlcNAc. Analysis of the interaction of epimastigote-Man-CBP with its ligands by UV difference spectroscopy indicates the existence of an extended binding site in that protein with a large enthalpic contribution to the binding. The thermodynamic parameters of binding were obtained by isothermal titration calorimetry and been found that the DeltaH values to be in good agreement with the van't Hoff values. The binding reactions are mainly enthalpically driven and exhibit enthalpy-enthropy compensation. In addition, analysis of the high-mannose glycans from different parts of the digestive tract of the reduviid insect vector of T. cruzi suggest a role of the CBP in the retention of the epimastigote stage in the anterior portion of the gut.

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

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

Veneza zonata (Hemiptera: Coreidae)/Trypanosomatid Relationship: Action of Hemolymph in Vitro and Experimental Infection

The defense response of Veneza zonata (Hemiptera: Coreidae) against three different trypanosomatid infections was assessed: (1) strain 714TD, a Leptomonas which has V. zonata as vector of a plant trypanosomatid, (2) strain 563TD, a Leptomonas isolated from the digestive tract of Euchistus heros (Hemiptera: Pentatomidae), and (3) Leishmania (L.) amazonensis, a human parasite that cannot infect V. zonata. Experiments with V. zonata hemolymph showed agglutination only of L. (L.) amazonensis culture forms and hemocytic recognition was more intense with this strain. L. (L.) amazonensis also activated the prophenoloxidase system, whereas strains 714TD and 563TD did not activate this system but rather seemed to inhibit phenoloxidase activity. No flagellates were seen in the digestive tract, hemolymph, or salivary glands in insects infected with L. (L.) amazonensis. The digestive tract, the hemolymph, and the salivary glands of insects fed on tomatoes inoculated with 714TD are sequentially invaded by the flagellate, which is inoculated in plants together with saliva. Insects fed on tomatoes inoculated with 563TD exhibited culture forms in the digestive tract (6 days after) and hemocoel (three additional days); however, they died 12 to 14 days after exposure. The salivary glands in insects inoculated in the hemocoel with 714TD strain are rapidly invaded, whereas those with 563TD culture forms died approximately 24 h after infection. Bacterial proliferation in the hemocoel and hemocyte surface blebbing were seen in insects infected only with 563TD strain as the probable pathogenic mechanism of insect death.

Immune response of Drosophila melanogaster to infection with the flagellate parasite Crithidia spp

Insects are able to recognize invading microorganisms and to mount an immune response to bacterial and fungal infections. Recently, the fruitfly Drosophila melanogaster has emerged as a promising invertebrate model to investigate innate immunity because of its well-characterized genetics. Insects are also vectors of numerous parasites which can trigger an immune response. We have investigated the interaction of Drosophila melanogaster with the flagellate protozoan Crithidia spp. We show that a per os parasitic infection triggers the synthesis of several antimicrobial peptides. By reverse phase HPLC and mass spectrometry, peptides were shown to be present in the hemolymph and not in the gut tissue, suggesting the presence of immune messengers between the site of the infection, namely the gut, and the fat body, the main site of synthesis for antimicrobial peptides. Interestingly, we have identified one molecule which is specifically induced in the hemolymph after infection with Crithidia, but not with bacteria, suggesting that Drosophila can discriminate between pathogens. When flagellates were injected into the hemolymph, a low synthesis of antimicrobial peptides was observed together with phagocytosis of parasites by circulating hemocytes. The data presented here suggest that Drosophila-Crithidia spp. represents an interesting model to study host defense against protozoan parasites.

The development of Trypanosoma cruzi in triatominae

Trypanosoma cruzi multiplies and differentiates in the digestive tract of triatomine insects. These insects ingest an enormous amount of blood, with ingestion followed very rapidly by a strong diuresis, slow digestion and occasionally long periods of starvation. Resulting changes in the intestinal environment induce the development of dominant stages of T. cruzi--epimastigotes and metacyclic trypomastigotes--and can be correlated with the appearance of specific developmental stages--spheromastigotes and giant cells--which otherwise are only rarely seen. Here, Astrid Kollien and Günter Schaub outline recent research on these developmental steps of T. cruzi in the vector, and the effects of different compounds acting against the parasite in the vector.

AU-rich elements in the 3'-untranslated region of a new mucin-type gene family of Trypanosoma cruzi confers mRNA instability and modulates translation efficiency

Trypanosoma cruzi has a complex mucin gene family of 500 members with hypervariable regions expressed preferentially in vertebrate associated stages of the parasite. In this work, a novel mucin-type gene family is reported, composed of two groups of genes organized in independent tandems and having very short open reading frames. The structures of deduced proteins share the N and C termini but differ in central regions. One group has repeats with the consensus Lys-Asn-Thr(7)-Ser-Thr(3)-Ser(Ser/Lys)-Ala-Pro and the other a Thr-rich sequence of the type Asp-Gln-Thr(17-20)-Asn-Ala-Pro-Ala-Lys-Asp-Thr(5-7)-Asn-Ala-Pro-Ala-L ys. In both cases, expected mature core proteins are around 7 kDa. Both groups, named L and S, respectively, differ in the structure of genomic loci and mRNA, with differential blocks in the 3'-untranslated region. The highest mRNA level for S and L groups are in the epimastigote stage but they show distinct developmentally regulated patterns. Transcripts are short lived and their steady-state abundance is regulated post-transcriptionally with increased mRNA stability in insect stage epimastigote. AU-rich sequences, similar to ARE motives known to cause mRNA instability in higher eukaryotes, are present in the 3'-untranslated region of the transcripts. In transfection experiments this sequence is shown to be functional for the L group destabilizing its mRNA in a stage-specific manner. Furthermore, an effect of this AU-rich region on translation efficiency is shown. To our knowledge, this is the first time that a functional ARE sequence-dependent post-transcriptional regulation mechanism is reported in a lower eukaryote.

Isolation and characterization of Dorin M, a lectin from plasma of the soft tick Ornithodoros moubata

A lectin with high hemagglutinating activity, which we have named Dorin M, was identified in the plasma of the soft tick Ornithodoros moubata. The activity of the plasma lectin could be efficiently inhibited by sialic acid, N-acetyl-D-hexosamines and sialoglycoproteins. Dorin M was purified to homogeneity using two different isolation systems: affinity chromatography on a column of bovine submaxillary mucin conjugated to Sepharose 4B with specific elution by N-acetyl-D-glucosamine and chromatography on Blue-Sepharose followed by anion exchange FPLC on a MonoQ column. The purified lectin is a glycoprotein which, in the native state, forms aggregates with molecular mass of about 640 kDa. Non-reducing SDS PAGE revealed that the lectin consists of two noncovalently bound subunits migrating closely around 37 kDa. Dorin M is a glycoprotein, probably modified by N-type glycosylation. After chemical deglycosylation, only one band of about 32 kDa was detected. Dorin M is the first lectin purified from ticks.

Tsetse-trypanosome interactions: rites of passage

Trypanosomes that cause sleeping sickness (Trypanosoma brucei rhodesiense and T. b. gambiense) are entirely dependent on tsetse for their transmission between hosts, but the flies are not easily infected. This situation has not arisen by chance - the tsetse has evolved an efficient defence system against trypanosome invasion. In this review, Susan Welburn and Ian Maudlin chart the progress of trypanosomes through the fly and identify some of the hazards faced by both parasite and fly that affect vector competence of tsetse.

Detection and characterization of a mannan-binding lectin from the mosquito, Anopheles stephensi (Liston)

Two lectins from the serum of the mosquito, Anopheles stephensi (Liston), with distinct characteristics, were detected by agglutination of various animal erythrocytes. The lectins were developmental stage-specific and/or sex-related. One adult female-specific lectin was identified as mannan-specific, and named mosquito mannan-binding lectin (MBL). MBL cross-reacted immunologically with antibodies against a previously characterized cockroach lectin, Blaberus discoidalis lectin (BDL1), and its activity was almost completely blocked by the antibodies. Mosquito MBL agglutinated erythrocytes from human, sheep, goat and rabbit, but not chicken or mouse, and agglutination was inhibited by mannan and nitrophenol-modified sugar derivatives, but not by simple sugars. Using affinity chromatography with immobilized mannan on Sepharose 6B, the mosquito MBL was partially purified. Purified mosquito MBL shared biochemical properties with BDL1, containing two subunits of molecular mass of 28 and 30 kDa under reducing conditions in SDS/PAGE. Its activity is dependent on Ca(2+), and it is stable at pH 7-9 and at temperatures less than 30 degrees C.

Studies on a haemolymph lectin isolated from Rhodnius prolixus and its interaction with Trypanosoma rangeli

We demonstrated that in Rhodnius prolixus haemocyte monolayers, both Trypanosoma cruzi and Trypanosoma rangeli are capable of inducing haemocyte/parasite clump formation. We also purified, by one-step affinity chromatography, a haemolymph galactoside-binding lectin from R. prolixus which we believe could play an important role in the development of T. rangeli in the haemocoel of the insect vector. This lectin markedly enhanced the activation of clump formation by T. rangeli in R. prolixus haemocyte monolayers, with an increase in clump size and haemocyte aggregation. The haemolymph lectin also significantly affected the motilitity and survival of T. rangeli culture short forms, but not the long forms, when they were incubated in vitro. This molecule is also one of the few described in insects with agglutination activity independent of calcium ions. The partial N-terminal amino acid sequence of this lectin demonstrated similarity to a bacterial xylulose kinase and in preliminary experiments the purified haemolymph lectin phosphorylated a tyrosine kinase substrate in a dose-dependent manner. The possible role of this haemolymph lectin in the life cycle of T. rangeli is discussed.

Genetic variation in arthropod vectors of disease-causing organisms: obstacles and opportunities

An overview of the genetic variation in arthropods that transmit pathogens to vertebrates is presented, emphasizing the genetics of vector-pathogen relationships and the biochemical genetics of vectors. Vector-pathogen interactions are reviewed briefly as a prelude to a discussion of the genetics of susceptibility and refractoriness in vectors. Susceptibility to pathogens is controlled by maternally inherited factors, sex-linked dominant alleles, and dominant and recessive autosomal genes. There is widespread interpopulation (including intercolony) and temporal variation in susceptibility to pathogens. The amount of biochemical genetic variation in vectors is similar to that found in other invertebrates. However, the amount varies widely among species, among populations within species, and temporally within populations. Biochemical genetic studies show that there is considerable genetic structuring of many vectors at the local, regional, and global levels. It is argued that genetic variation in vectors is critical in understanding vector-pathogen interactions and that genetic variation in vectors creates both obstacles to and opportunities for application of genetic techniques to the control of vectors.

Post-engorgement dynamics of haemagglutination activity in the midgut of phlebotomine sandflies

Haemagglutination activity (HA) was studied in gut extracts of both sexes of adults of six sandfly species. HA was sex dependent, with the activity in males more than 50 times lower than that of unfed females. In females, high HA was demonstrable in both the thoracic and abdominal midgut but not in the hindgut. In unfed flies the activity was similar in the midgut wall and the gut contents whereas, in fed females, a high increase was seen in the midgut contents. After blood-feeding, HA was elevated, reaching peak titres 2 days later and then falling to the base level or less immediately after defaecation. The magnitude of the HA response differed according to species, ranging from 2-fold in Lutzomyia carmelinoi up to 16-fold in Phlebotomus duboscqi. Quantitative differences between sandflies in their HA response may influence their ability to support the development of Leishmania spp.