Spatial distribution of factors that determine sporogonic development of malaria parasites in mosquitoes

Genomic advances in the study of the mosquito vector during avian malaria infection

Invertebrate host–parasite associations are one of the keystones in order to understand vector-borne diseases. The study of these specific interactions provides information not only about how the vector is affected by the parasite at the gene-expression level, but might also reveal mosquito strategies for blocking the transmission of the parasites. A very well-known vector for human malaria is Anopheles gambiae. This mosquito species has been the main focus for genomics studies determining essential key genes and pathways over the course of a malaria infection. However, to-date there is an important knowledge gap concerning other non-mammophilic mosquito species, for example some species from the Culex genera which may transmit avian malaria but also zoonotic pathogens such as West Nile virus. From an evolutionary perspective, these 2 mosquito genera diverged 170 million years ago, hence allowing studies in both species determining evolutionary conserved genes essential during malaria infections, which in turn might help to find key genes for blocking malaria cycle inside the mosquito. Here, we extensively review the current knowledge on key genes and pathways expressed in Anopheles over the course of malaria infections and highlight the importance of conducting genomic investigations for detecting pathways in Culex mosquitoes linked to infection of avian malaria. By pooling this information, we underline the need to increase genomic studies in mosquito–parasite associations, such as the one in Culex–Plasmodium, that can provide a better understanding of the infection dynamics in wildlife and reduce the negative impact on ecosystems.

Novel Asaia bogorensis Signal Sequences for Plasmodium Inhibition in Anopheles stephensi

Mosquitoes vector many pathogens that cause human disease, such as malaria that is caused by parasites in the genus Plasmodium. Current strategies to control vector-transmitted diseases are hindered by mosquito and pathogen resistance, so research has turned to altering the microbiota of the vectors. In this strategy, called paratransgenesis, symbiotic bacteria are genetically modified to affect the mosquito's phenotype by engineering them to deliver antiplasmodial effector molecules into the midgut to kill parasites. One paratransgenesis candidate is Asaia bogorensis, a Gram-negative, rod-shaped bacterium colonizing the midgut, ovaries, and salivary glands of Anopheles sp. mosquitoes. However, common secretion signals from E. coli and closely related species do not function in Asaia. Here, we report evaluation of 20 native Asaia N-terminal signal sequences predicted from bioinformatics for their ability to mediate increased levels of antiplasmodial effector molecules directed to the periplasm and ultimately outside the cell. We tested the hypothesis that by increasing the amount of antiplasmodials released from the cell we would also increase parasite killing power. We scanned the Asaia bogorensis SF2.1 genome to identify signal sequences from extra-cytoplasmic proteins and fused these to the reporter protein alkaline phosphatase. Six signals resulted in significant levels of protein released from the Asaia bacterium. Three signals were successfully used to drive the release of the antimicrobial peptide, scorpine. Further testing in mosquitoes demonstrated that these three Asaia strains were able to suppress the number of oocysts formed after a blood meal containing P. berghei to a significantly greater degree than wild-type Asaia, although prevalence was not decreased beyond levels obtained with a previously isolated siderophore receptor signal sequence. We interpret these results to indicate that there is a maximum level of suppression that can be achieved when the effectors are constitutively driven due to stress on the symbionts. This suggests that simply increasing the amount of antiplasmodial effector molecules in the midgut is insufficient to create superior paratransgenic bacterial strains and that symbiont fitness must be considered as well.

Analysis of Stage-Specific Protein Expression during Babesia Bovis Development within Female Rhipicephalus Microplus

Arthropod-borne protozoan pathogens have a complex life cycle that includes asexual reproduction of haploid stages in mammalian hosts and the development of diploid stages in invertebrate hosts. The ability of pathogens to invade, survive, and replicate within distinct cell types is required to maintain their life cycle. In this study, we describe a comparative proteomic analysis of a cattle pathogen, Babesia bovis, during its development within the mammalian and tick hosts with the goal of identifying cell-surface proteins expressed by B. bovis kinetes as potential targets for the development of a transmission blocking vaccine. To determine parasite tick-stage-specific cell-surface proteins, CyDye labeling was performed with B. bovis blood stages from the bovine host and kinetes from the tick vector. Cell-surface kinete-stage-specific proteins were identified using 2D difference in gel electrophoresis and analyzed by mass spectrometry. Ten proteins were identified as kinete-stage-specific, with orthologs found in closely related Apicomplexan pathogens. Transcriptional analysis revealed two genes were highly expressed by kinetes as compared with blood stages. Immunofluorescence using antibodies against the two proteins confirmed kinete-stage-specific expression. The identified cell-surface kinete proteins are potential candidates for the development of a B. bovis transmission blocking vaccine.

Lectin-carbohydrate recognition mechanism of Plasmodium berghei in the midgut of malaria vector Anopheles stephensi using quantum dot as a new approach

Potential targets of Plasmodium ookinetes at the mosquito midgut walls were investigated in relation to interfering malarial transmission. In this study, the essential application of Quantum Dots (QDs) was used to examine the interaction between Plasmodium berghei ookinetes and the Anopheles stephensi midgut, based on lectin-carbohydrate recognition. Two significant lectins were utilized to determine this interaction. Two QDs, cadmium telluride (CdTe)/CdS and cadmium selenide (CdSe)/CdS, were employed in staining Plasmodium ookinete to study its interaction in the midgut of the mosquito vector in vivo. Concurrently, two lectins, wheat germ agglutinin (WGA) and concanavalin A (Con A), were inadvertently exploited to mask lectin binding sites between ookinetes and mosquito midgut cells. The numbers of ookinetes in both lumen and epithelial cells were eventually counted, following adequate preparation of wax sections extracted from whole midgut, and subsequent examination using a differential interference contrast a fluorescence microscopic technique. Interestingly, we detected that neither of the QDs mutated ookinete invasion into the midgut cells of the investigated mosquitoes. QD staining of ookinetes remained permanent despite the effective embedding procedure. The massive binding potency of ookinetes to midgut cells of the cross-examined mosquitoes undoubtedly revealed that Con A did not interrupt ookinete penetration into the midgut wall. In contrast, WGA inhibited ookinete invasion into the midgut cells. The results proved that QD nanoparticles are biocompatible, non-toxic to P. berghei and stable to photobleaching. The QDs staining, which was successfully implemented for ookinete labelling, is a simple and effective tool which plays a crucial role in bioimaging including the study of parasite-vector interactions.

Expression of active trypsin-like serine peptidases in the midgut of sugar-feeding female Anopheles aquasalis

BACKGROUND

Anopheles aquasalis is a dipteran of the family Culicidae that is widely distributed in the coastal regions of South and Central America. This species acts as a vector of Plasmodium vivax, an important etiological agent of malaria, which represents a serious public health problem. In mosquitoes, trypsin-like serine proteases are important in blood meal digestion, immune responses and reproductive functions. The study of peptidases expressed in the mosquito midgut is essential to understanding the mechanisms of parasite-host interaction and the physiological process of nutrient digestion.

METHODS

Our study aimed to identify and characterize the proteolytic activities in the midgut of sugar-fed An. aquasalis females using zymographic analyses (substrate-SDS-PAGE), in-solution assays and mass spectrometry.

RESULTS

Here, we used a zymographic analysis to further biochemically characterize the proteolytic profile of the midgut of sugar-feeding An. aquasalis females. The trypsin peptidases migrated between ~17 and ~76 kDa and displayed higher proteolytic activities between pH 7.5 and 10 and at temperatures between 37 °C and 50 °C. Four putative trypsin-like serine peptidases were identified using mass spectrometry and data mining. The molecular masses of these peptidases were similar to those observed using zymography, which suggested that these peptidases could be responsible for some of the observed proteolytic bands.

CONCLUSIONS

Taken together, our results contribute to the gene annotation of the unknown genome of this species, to the tissue location of these peptidases, and to the functional prediction of these crucial enzymes, which all impact further studies of this species.

Comparative susceptibility of different biological forms of Anopheles stephensi to Plasmodium berghei ANKA strain

Background

There are varying degrees of compatibility between malaria parasite-mosquito species, and understanding this compatibility may be crucial for developing effective transmission-blocking vaccines. This study investigates the compatibility of different biological forms of a malaria vector, Anopheles stephensi, to Plasmodium berghei ANKA strain.

Methods

Several biologically different and allopatric forms of A. stephensi were studied. Three forms were isolated from different regions of southern Iran: the variety mysorensis, the intermediate form and the native type form, and an additional type form originated from India (Beech strain).The mosquitoes were experimentally infected with P. berghei to compare their susceptibility to parasitism. Anti-mosquito midgut antiserum was then raised in BALB/cs mice immunized against gut antigens from the most susceptible form of A. stephensi (Beech strain), and the efficacy of the antiserum was assessed in transmission-blocking assays conducted on the least susceptible mosquito biological form.

Results

The susceptibility of different biological forms of A. stephensi mosquito to P. berghei was specifically inter-type varied. The Beech strain and the intermediate form were both highly susceptible to infection, with higher oocyst and sporozoite infection rates than intermediate and mysorensis forms. The oocyst infection, and particularly sporozite infection, was lowest in the mysorensis strain. Antiserum raised against midgut proteins of the Indian Beech type form blocked infection in this mosquito population, but it was ineffective at blocking both oocyst and sporozoite development in the permissive but geographically distant intermediate form mosquitoes. This suggests that a strong degree of incompatibility exists between the mosquito strains in terms of midgut protein(s) acting as putative ookinete receptors.

Conclusions

The incompatibility in the midgut protein profiles between two biological forms of A. stephensi demonstrates a well-differentiated population structure according to geographical origin. Therefore, the design of potential transmission-blocking strategies should incorporate a more thorough understanding of intra-species variations in host-parasite interactions.

Description and molecular characterization of Plasmodium (Novyella) unalis sp. nov. from the Great Thrush (Turdus fuscater) in highland of Colombia

Plasmodium (Novyella) unalis sp. nov. was found in the Great Thrush, Turdus fuscater (Passeriformes, Turdidae) in Bogotá, Colombia, at 2,560 m above sea level where the active transmission occurs. This parasite is described based on the morphology of its blood stages and a fragment of the mitochondrial cytochrome b gene (lineage UN227). Illustrations of blood stages of new species are given, and the phylogenetic analysis identifies closely related species and lineages of avian malaria parasites. The new species is most similar to Plasmodium (Novyella) vaughani (lineage SYAT05), a cosmopolitan avian malaria parasite; these parasites are also closely related genetically, with a genetic difference of 3.2% between them. P. unalis can be readily distinguished from the latter species morphologically, primarily due to the (1) presence of a single large, circular shaped pigment granule in the erythrocytic trophozoites and meronts; (2) presence of prominent vacuoles in trophozoites and growing meronts; and (3) presence of predominantly fan-like shaped erythrocytic meronts. Cytochrome b lineages with high similarity to the new species have been reported in Costa Rica, Brazil, Chile, and USA. It is probable that the new species of malaria parasite is widely distributed in the New World. This parasite has been reported only in the Great Thrush at the study site and might have a narrow range of avian hosts. Records of P. unalis are of particular theoretical interest due to its active transmission at highlands in Andes. Possible influence of urbanization on transmission of this malaria parasite in Bogotá is discussed.

Trypsin-like serine peptidase profiles in the egg, larval, and pupal stages of Aedes albopictus

BACKGROUND

Aedes albopictus, a ubiquitous mosquito, is one of the main vectors of dengue and yellow fever, representing an important threat to public health worldwide. Peptidases play key roles in processes such as digestion, oogenesis, and metamorphosis of insects. However, most of the information on the proteolytic enzymes of mosquitoes is derived from insects in the adult stages and is often directed towards the understanding of blood digestion. The aim of this study was to investigate the expression of active peptidases from the preimaginal stages of Ae. albopictus.

METHODS

Ae. albopictus eggs, larvae, and pupae were analyzed using zymography with susbtrate-SDS-PAGE. The pH, temperature and peptidase inhibitor sensitivity was evaluated. In addition, the proteolytic activities of larval instars were assayed using the fluorogenic substrate Z-Phe-Arg-AMC.

RESULTS

The proteolytic profile of the larval stage was composed of 8 bands ranging from 17 to 130 kDa. These enzymes displayed activity in a broad range of pH values, from 5.5 to 10.0. The enzymatic profile of the eggs was similar to that of the larvae, although the proteolytic bands of the eggs showed lower intensities. The pupal stage showed a complex proteolytic pattern, with at least 6 bands with apparent molecular masses ranging from 30 to 150 kDa and optimal activity at pH 7.5. Peptidases from larval instars were active from 10°C to 60°C, with optimal activity at temperatures between 37°C and 50°C. The proteolytic profile of both the larval and pupal stages was inhibited by phenyl-methyl sulfonyl-fluoride (PMSF) and Nα-Tosyl L-lysine chloromethyl ketone hydrochloride (TLCK), indicating that the main peptidases expressed during these developmental stages are trypsin-like serine peptidases.

CONCLUSION

The preimaginal stages of Ae. albopictus exhibited a complex profile of trypsin-like serine peptidase activities. A comparative analysis of the active peptidase profiles revealed differential expression of trypsin-like isoforms among the preimaginal stages, suggesting that some of these enzymes are stage specific. Additionally, a comparison of the peptidase expression between larvae from eggs collected in the natural environment and larvae obtained from the eggs of female mosquitoes maintained in colonies for a long period of time demonstrated that the proteolytic profile is invariable under such conditions.

Proteolytic profiling and comparative analyses of active trypsin-like serine peptidases in preimaginal stages of Culex quinquefasciatus

Background

The mosquito Culex quinquefasciatu s, a widespread insect in tropical and sub-tropical regions of the world, is a vector of multiple arboviruses and parasites, and is considered an important risk to human and veterinary health. Proteolytic enzymes play crucial roles in the insect physiology including the modulation of embryonic development and food digestion. Therefore, these enzymes represent important targets for the development of new control strategies. This study presents zymographic characterization and comparative analysis of the proteolytic activity found in eggs, larval instars and pupae of Culex quinquefasciatus.

Methods

The proteolytic profiles of eggs, larvae and pupa of Cx. quinquefasciatus were characterized by SDS-PAGE co-polymerized with 0.1% gelatin, according to the pH, temperature and peptidase inhibitor sensitivity. In addition, the proteolytic activities were characterized in solution using 100 μM of the fluorogenic substrate Z-Phe-Arg-AMC.

Results

Comparison of the proteolytic profiles by substrate-SDS-PAGE from all preimaginal stages of the insect revealed qualitative and quantitative differences in the peptidase expression among eggs, larvae and pupae. Use of specific inhibitors revealed that the proteolytic activity from preimaginal stages is mostly due to trypsin-like serine peptidases that display optimal activity at alkaline pH. In-solution, proteolytic assays of the four larval instars using the fluorogenic substrate Z-Phe-Arg-AMC in the presence or absence of a trypsin-like serine peptidase inhibitor confirmed the results obtained by substrate-SDS-PAGE analysis. The trypsin-like serine peptidases of the four larval instars were functional over a wide range of temperatures, showing activities at 25°C and 65°C, with an optimal activity between 37°C and 50°C.

Conclusion

The combined use of zymography and in-solution assays, as performed in this study, allowed for a more detailed analysis of the repertoire of proteolytic enzymes in preimaginal stages of the insect. Finally, differences in the trypsin-like serine peptidase profile of preimaginal stages were observed, suggesting that such enzymes exert specific functions during the different stages of the life cycle of the insect.

A dengue receptor as possible genetic marker of vector competence in Aedes aegypti

Background

Vector competence refers to the intrinsic permissiveness of an arthropod vector for infection, replication and transmission of a virus. Notwithstanding studies of Quantitative Trait Loci (QTL) that influence the ability of Aedes aegypti midgut (MG) to become infected with dengue virus (DENV), no study to date has been undertaken to identify genetic markers of vector competence. Furthermore, it is known that mosquito populations differ greatly in their susceptibility to flaviviruses. Differences in vector competence may, at least in part, be due to the presence of specific midgut epithelial receptors and their identification would be a significant step forward in understanding the interaction of the virus with the mosquito. The first interaction of DENV with the insect is through proteins in the apical membrane of the midgut epithelium resulting in binding and receptor-mediated endocytosis of the virus, and this determines cell permissiveness to infection. The susceptibility of mosquitoes to infection may therefore depend on their specific virus receptors. To study this interaction in Ae. aegypti strains that differ in their vector competence for DENV, we investigated the DS3 strain (susceptible to DENV), the IBO-11 strain (refractory to infection) and the membrane escape barrier strain, DMEB, which is infected exclusively in the midgut epithelial cells.

Results

(1) We determined the MG proteins that bind DENV by an overlay protein binding assay (VOPBA) in Ae. aegypti mosquitoes of the DS3, DMEB and IBO-11 strains. The main protein identified had an apparent molecular weight of 67 kDa, although the protein identified in the IBO-11 strain showed a lower mass (64 kDa). (2) The midgut proteins recognized by DENV were also determined by VOPBA after two-dimensional gel electrophoresis. (3) To determine whether the same proteins were identified in all three strains, we obtained polyclonal antibodies against R67 and R64 and tested them against the three strains by immunoblotting; both antibodies recognized the 67 and 64 kDa proteins, corroborating the VOPBA results. (4) Specific antibodies against both proteins were used for immunofluorescent location by confocal microscopy; the antibodies recognized the basal lamina all along the MG, and cell membranes and intercellular spaces from the middle to the end of the posterior midgut (pPMG) in the neighborhood of the hindgut. (5) Quantitative analysis showed more intense fluorescence in DS3 and DMEB than in IBO-11. (6) The viral envelope antigen was not homogeneously distributed during MG infection but correlated with MG density and the distribution of R67/R64.

Conclusion

In this paper we provide evidence that the 67 kDa protein (R67/R64), described previously as a DENV receptor, is related to vector competence in Ae. aegypti. Consequently, our results strongly suggest that this protein may be a marker of vector competence for DENV in Ae. aegypti mosquitoes.

The Gram-negative bacteria-binding protein gene family: its role in the innate immune system of anopheles gambiae and in anti-Plasmodium defence

Gram-negative bacteria-binding proteins (GNBPs) are pattern recognition receptors which contribute to the defensive response against Plasmodium infection in Anopheles. We have characterized the GNBP gene family in Anopheles gambiae at the molecular level, and show that they are functionally diverse components of the A. gambiae innate immune system. GNBPB4 is a major factor in the defence against a broad range of pathogens, while the other GNBPs have narrower defence specificities. GNBPB4 is associated with the regulation of immune signalling pathways and was found to interact with the Gram-negative Escherichia coli and weakly co-localized with Plasmodium berghei ookinetes in the mosquito midgut epithelium.

Role of heat-labile serum factor or host complement in the inhibition of Plasmodium falciparum sporogonic stages in Anopheles stephensi by gametocyte carriers' serological factors

This study investigated the significance of serum complement on transmission-reducing activity (TRA) of field sera from 24 infected Plasmodium falciparum gametocyte carriers (from Cameroon) against cultured NF54 P. falciparum. Laboratory-reared Anopheles stephensi were given infectious blood meals prepared either with sera from naïve Dutch donor (AB type) or pair-matched field serum samples, both with and without active complement. TRA of serum factors and host complement on mosquito infection rate and oocyst intensity were divided into the various components involved in the early stages of sporogony. The majority (>80%) of sera tested showed positive antibody titres to Pfs230, the relevant complement-dependent target of transmission-reducing mechanisms. Regardless of the presence of active complement, bloodmeals with field sera exhibited significantly lower infection rates and oocyst intensity than the control group. Serological reactivity in Capture-ELISA against Pfs230 was significantly correlated with the reduction of parasite infectivity. Contrary to our expectation, the presence of active complement in the mosquito bloodmeal did not increase parasite losses and therefore the magnitude of transmission reduction by individual immune sera. Our findings on P. falciparum are consistent with previous studies on animal hosts of Plasmodium, indicating that early P. falciparum sporogonic stages may be insensitive to the antibody-dependent pathways of complement in human serum.

Upregulation of two actin genes and redistribution of actin during diapause and cold stress in the northern house mosquito, Culex pipiens

Two actin genes cloned from Culex pipiens L. are upregulated during adult diapause. Though actins 1 and 2 were expressed throughout diapause, both genes were most highly expressed early in diapause. These changes in gene expression were accompanied by a conspicuous redistribution of polymerized actin that was most pronounced in the midguts of diapausing mosquitoes that were exposed to low temperature. In nondiapausing mosquitoes reared at 25 degrees C and in diapausing mosquitoes reared at 18 degrees C, polymerized actin was clustered at high concentrations at the intersections of the muscle fibers that form the midgut musculature. When adults 7-10 days post-eclosion were exposed to low temperature (-5 degrees C for 12 h), the polymerized actin was evenly distributed along the muscle fibers in both nondiapausing and diapausing mosquitoes. Exposure of older adults (1 month post-eclosion) to low temperature (-5 degrees C for 12 h) elicited an even greater distribution of polymerized actin, an effect that was especially pronounced in diapausing mosquitoes. These changes in gene expression and actin distribution suggest a role for actins in enhancing survival of diapausing adults during the low temperatures of winter by fortification of the cytoskeleton.

PbGCβ Is Essential for Plasmodium Ookinete Motility to Invade Midgut Cell and for Successful Completion of Parasite Life Cycle in Mosquitoes

When malaria parasites enter to mosquitoes, they fertilize and differentiate to zygotes and ookinetes. The motile ookinetes cross the midgut cells and arrive to the basement membranes where they differentiate into oocysts. The midgut epithelium is thus a barrier for ookinetes to complete their life cycle in the mosquitoes. The ookinetes develop gliding motility to invade midgut cells successfully, but the molecular mechanisms behind are poorly understood. Here, we identified a single molecule with guanylate cyclase domain and N-terminal P-type ATPase like domain in the rodent malaria parasite Plasmodium berghei and named it PbGCbeta. We demonstrated that transgenic parasites in which the PbGCbeta gene was disrupted formed normal ookinetes but failed to produce oocyst. Confocal microscopic analysis showed that the disruptant ookinetes remained on the surface of the microvilli. The disruptant ookinetes showed severe defect in motility, resulting in failure of parasite invasion of the midgut epithelium. When the disruptant ookinetes were cultured in vitro, they transformed into oocysts and sporozoites. These results demonstrate that PbGCbeta is essential for ookinete motility when passing through the midgut cells, but not for further development of the parasites.

Mosquito midguts and malaria: cell biology, compartmentalization and immunology

The malaria parasite Plasmodium has an absolute requirement for both a vertebrate and a mosquito host in order to complete its life cycle, and its interactions with the latter provide the focus for this review. The mosquito midgut represents one of the most challenging environments for the survival and development of Plasmodium, and is thus also one of the most attractive sites for novel targeted malaria control strategies. During their attempts to cross the midgut epithelium en route to the salivary glands, motile ookinetes are swiftly detected and labelled by mosquito recognition factors and targeted for destruction by a variety of immune responses that recruit killing factors both from the midgut and from other tissues in the surrounding body cavity. The exact interplay between these factors and the parasite is highly species- and strain-specific, as are the timing and the route of parasite invasion. These features are paramount to determining the success of the infection and the vector competence of the mosquito. Here we discuss recent advances in genomic analyses, coupled with detailed microscopical investigations, which are helping to unravel the identity and roles of the major players of these complex systems.

Anopheles infection responses; laboratory models versus field malaria transmission systems

The molecular biology of disease vectors, particularly mosquitoes, has experienced a remarkable progress in the past two decades. This is mainly attributed to methodological advances and the emerging genome sequences of vector species, which have brought experimental biology to an unprecedented level. It is now possible to determine the entire transcriptome of Anopheles gambiae at a variety of conditions, with a low per-gene effort and cost. Proteomic profiles can be generated for as small samples as the hemolymph, and transient reverse genetic and stable germ line based transgenic analyses can be performed to analyze gene function. High throughput screening for receptors and ligands can be used to characterize interactions between vectors and pathogens. At the current breathtaking rates of data production it is essential to question and evaluate the relevance of laboratory infection models to the real disease transmission systems. The majority of scientific discoveries in mosquito molecular biology have been based on highly inbred laboratory strains and rodent malaria parasite infection models, which may differ substantially to their counterparts that transmit human malaria in the field. This review addresses the recent advances in high throughput transcription analyses of Anopheles responses to infection, and discusses considerations for the use of laboratory malaria infection models.

Fate of GFP-expressing Escherichia coli in the midgut and response to ingestion in a tick, Ornithodoros moubata (Acari: Argasidae)

Ticks are well-known vectors of various pathogens but migration of the pathogens in the tick midgut is not fully understood. In the present study, the fate of microbes in the midgut of Ornithodoros moubata was observed using green fluorescent protein (GFP)-expressing Escherichia coli. Fluctuations in the percentage of hemocytes in the hemolymph (Hc) and expression of an antimicrobial peptide, defensin, in the midgut was also investigated. Most E. coli gradually disappeared in the midgut after ingestion fluctuations in Hc coincided with the changes. Expression of defensin was also confirmed and slightly up-regulated after E. coli ingestion. Moreover, it was demonstrated that E. coli can not pass through the tick midgut epithelium after ingestion by the hemolymph cultures. It is known that various pathogens and host immunoglobulins ingested with a blood meal can enter into the hemocoel, which suggests the presence of unique and complex passage mechanisms for each molecule and organism. The results obtained here help to clarify that digestion enzymes is an important function of the tick midgut to protect against invading molecules and organisms.

Stage-specific effects of host plasma factors on the early sporogony of autologous Plasmodium falciparum isolates within Anopheles gambiae

Summary Quantitatively assessing the impact of naturally occurring transmission-blocking (TB) immunity on malaria parasite sporogonic development may provide a useful interpretation of the underlying mechanisms. Here, we compare the effects of plasma derived from 23 naturally infected gametocyte carriers (OWN) with plasma from donors without previous malaria exposure (AB) on the early sporogonic development of Plasmodium falciparum in Anopheles gambiae. Reduced parasite development efficiency was associated with mosquitoes taking a blood meal mixed with the gametocyte carriers' own plasma, whereas replacing autologous plasma with non-immune resulted in the highest level of parasite survival. Seven days after an infective blood meal, 39.1% of the gametocyte carriers' plasma tested showed TB activity as only a few macrogametocytes ingested along with immune plasma ended up as ookinetes but subsequent development was blocked in the presence of immune plasma. In other experiments (60.9%), the effective number of parasites declined dramatically from one developmental stage to the next, and resulted in an infection rate that was two-fold lower in OWN than in AB infection group. These findings are in agreement with those in other reports and go further by quantitatively examining at which transition stages TB immunity exerts its action. The transitions from macrogametocytes to gamete/zygote and from gamete/zygote to ookinete were identified as main targets. However, the net contribution of host plasma factors to these interstage parasite reductions was low (5-20%), suggesting that irrespective of the host plasma factors, mosquito factors might also lower the survival level of parasites during the early sporogonic development.

Mosquito immune responses against malaria parasites

Anopheline mosquitoes are the major vectors of human malaria. Mosquito-parasite interactions are a critical aspect of disease transmission and a potential target for malaria control. Mosquitoes vary in their innate ability to support development of the malaria parasite, but the molecular mechanisms that determine vector competence are poorly understood. This area of research has been revolutionized by recent advances in the mosquito genome characterization and by the development of new tools for functional gene analysis.

Immune responses in Anopheles gambiae

Transmission of human malaria requires a successful development of Plasmodium parasites in anopheline mosquitoes. Insects have developed efficient immune responses to oppose microbial and eukaryotic invaders. The completion of the sequencing of the Anopheles genome provides a wealth of information on putative immune genes that are homologous to components of the Drosophila and mammalian immune systems. In this review, we will summarize our present knowledge of immune responses in the mosquito Anopheles gambiae and attempt a comparative analysis of insect immune systems.

Monoclonal antibody MG96 completely blocks Plasmodium yoelii development in Anopheles stephensi

In spite of research efforts to develop vaccines against the causative agent of human malaria, Plasmodium falciparum, effective control remains elusive. The predominant vaccine strategy focuses on targeting parasite blood stages in the vertebrate host. An alternative approach has been the development of transmission-blocking vaccines (TBVs). TBVs target antigens on parasite sexual stages that persist within the insect vector, anopheline mosquitoes, or target mosquito midgut proteins that are presumed to mediate parasite development. By blocking parasite development within the insect vector, TBVs effectively disrupt transmission and the resultant cascade of secondary infections. Using a mosquito midgut-specific mouse monoclonal antibody (MG96), we have partially characterized membrane-bound midgut glycoproteins in Anopheles gambiae and Anopheles stephensi. These proteins are present on the microvilli of midgut epithelial cells in both blood-fed and unfed mosquitoes, suggesting that the expression of the protein is not induced as a result of blood feeding. MG96 exhibits a dose-dependent blocking effect against Plasmodium yoelii development in An. stephensi. We achieved 100% blocking of parasite development in the mosquito midgut. Preliminary deglycosylation assays indicate that the epitope recognized by MG96 is a complex oligosaccharide. Future investigation of the carbohydrate epitope as well as gene identification should provide valuable insight into the possible mechanisms of ookinete attachment and invasion of mosquito midgut epithelial cells.

The dynamics of interactions between Plasmodium and the mosquito: a study of the infectivity of Plasmodium berghei and Plasmodium gallinaceum, and their transmission by Anopheles stephensi, Anopheles gambiae and Aedes aegypti

Knowledge of parasite-mosquito interactions is essential to develop strategies that will reduce malaria transmission through the mosquito vector. In this study we investigated the development of two model malaria parasites, Plasmodium berghei and Plasmodium gallinaceum, in three mosquito species Anopheles stephensi, Anopheles gambiae and Aedes aegypti. New methods to study gamete production in vivo in combination with GFP-expressing ookinetes were employed to measure the large losses incurred by the parasites during infection of mosquitoes. All three mosquito species transmitted P. gallinaceum; P. berghei was only transmitted by Anopheles spp. Plasmodium gallinaceum initiates gamete production with high efficiency equally in the three mosquito species. By contrast P. berghei is less efficiently activated to produce gametes, and in Ae. aegypti microgamete formation is almost totally suppressed. In all parasite/vector combinations ookinete development is inefficient, 500-100,000-fold losses were encountered. Losses during ookinete-to-oocyst transformation range from fivefold in compatible vector parasite combinations (P. berghei/An. stephensi), through >100-fold in poor vector/parasite combinations (P. gallinaceum/An. stephensi), to complete blockade (>1,500 fold) in others (P. berghei/Ae. aegypti). Plasmodium berghei ookinetes survive poorly in the bloodmeal of Ae. aegypti and are unable to invade the midgut epithelium. Cultured mature ookinetes of P. berghei injected directly into the mosquito haemocoele produced salivary gland sporozoites in An. stephensi, but not in Ae. aegypti, suggesting that further species-specific incompatibilities occur downstream of the midgut epithelium in Ae. aegypti. These results show that in these parasite-mosquito combinations the susceptibility to malarial infection is regulated at multiple steps during the development of the parasites. Understanding these at the molecular level may contribute to the development of rational strategies to reduce the vector competence of malarial vectors.

Synthetic propeptide inhibits mosquito midgut chitinase and blocks sporogonic development of malaria parasite

Incessant transmission of the parasite by mosquitoes makes most attempts to control malaria fail. Blocking of parasite transmission by mosquitoes therefore is a rational strategy to combat the disease. Upon ingestion of blood meal mosquitoes secrete chitinase into the midgut. This mosquito chitinase is a zymogen which is activated by the removal of a propeptide from the N-terminal. Since the midgut peritrophic matrix acts as a physical barrier, the activated chitinase is likely to contribute to the further development of the malaria parasite in the mosquito. Earlier it has been shown that inhibiting chitinase activity in the mosquito midgut blocked sporogonic development of the malaria parasite. Since synthetic propeptides of several zymogens have been found to be potent inhibitors of their respective enzymes, we tested propeptide of mosquito midgut chitinase as an inhibitor and found that the propeptide almost completely inhibited the recombinant or purified native Anopheles gambiae chitinase. We also examined the effect of the inhibitory peptide on malaria parasite development. The result showed that the synthetic propeptide blocked the development of human malaria parasite Plasmodium falciparum in the African malaria vector An. gambiae and avian malaria parasite Plasmodium gallinaceum in Aedes aegypti mosquitoes. This study implies that the expression of inhibitory mosquito midgut chitinase propeptide in response to blood meal may alter the mosquito's vectorial capacity. This may lead to developing novel strategies for controlling the spread of malaria.