Anopheles gambiae laminin interacts with the P25 surface protein of Plasmodium berghei ookinetes

The first complete genome of the simian malaria parasite Plasmodium brasilianum

Naturally occurring human infections by zoonotic Plasmodium species have been documented for P. knowlesi, P. cynomolgi, P. simium, P. simiovale, P. inui, P. inui-like, P. coatneyi, and P. brasilianum. Accurate detection of each species is complicated by their morphological similarities with other Plasmodium species. PCR-based assays offer a solution but require prior knowledge of adequate genomic targets that can distinguish the species. While whole genomes have been published for P. knowlesi, P. cynomolgi, P. simium, and P. inui, no complete genome for P. brasilianum has been available. Previously, we reported a draft genome for P. brasilianum, and here we report the completed genome for P. brasilianum. The genome is 31.4 Mb in size and comprises 14 chromosomes, the mitochondrial genome, the apicoplast genome, and 29 unplaced contigs. The chromosomes consist of 98.4% nucleotide sites that are identical to the P. malariae genome, the closest evolutionarily related species hypothesized to be the same species as P. brasilianum, with 41,125 non-synonymous SNPs (0.0722% of genome) identified between the two genomes. Furthermore, P. brasilianum had 4864 (82.1%) genes that share 80% or higher sequence similarity with 4970 (75.5%) P. malariae genes. This was demonstrated by the nearly identical genomic organization and multiple sequence alignments for the merozoite surface proteins msp3 and msp7. We observed a distinction in the repeat lengths of the circumsporozoite protein (CSP) gene sequences between P. brasilianum and P. malariae. Our results demonstrate a 97.3% pairwise identity between the P. brasilianum and the P. malariae genomes. These findings highlight the phylogenetic proximity of these two species, suggesting that P. malariae and P. brasilianum are strains of the same species, but this could not be fully evaluated with only a single genomic sequence for each species.

Structural analysis of Plasmodium falciparum ookinete surface antigen Pfs28 relevant for malaria vaccine design

Pfs28 is a Plasmodium falciparum malaria transmission-blocking vaccine candidate that is anchored to the parasite surface through a C-terminal glycosylphosphatidylinositol (GPI) moiety, and plays a role in parasite survival in the mosquito midgut. Pfs28 contains epidermal growth factor (EGF)-like domains and is part of a family of sexual stage malaria proteins that includes the related vaccine antigen Pfs25. The lack of structural definition of Pfs28 and the immune response to this candidate has limited further malaria vaccine development for this antigen. Here, we present the crystal structure of Pfs28, examine its conservation with P. vivax Pvs28, and evaluate the cross-reactivity of Pfs28 to antibodies that recognize Pfs25. Pfs28 is comprised of four EGF-like domains stabilized by ten disulfide bridges with an overall architecture that highly resembles Pfs25. Despite the high sequence and structural similarity between these antigens, no cross reactivity of Pfs28 to anti-Pfs25 monoclonal antibodies could be demonstrated.

Evolutionary insights into the microneme-secreted, chitinase-containing high molecular weight protein complexes involved in Plasmodium invasion of the mosquito midgut

While general mechanisms by which Plasmodium ookinetes invade the mosquito midgut have been studied, details remain to be understood regarding the interface of the ookinete, specifically its barriers to invasion, such as the proteolytic milieu, the chitin-containing, protein cross-linked peritrophic matrix, and the midgut epithelium. Here we review knowledge of Plasmodium chitinases and the mechanisms by which they mediate the ookinete crossing the peritrophic matrix. The integration of new genomic insights into previous findings advances our understanding of Plasmodium evolution. Recently obtained Plasmodium spp. genomic data enable identification of the conserved residues in the experimentally demonstrated hetero-multimeric, high molecular weight complex comprised of a short chitinase covalently linked to binding partners, von Willebrand factor A domain-related protein (WARP) and secreted ookinete adhesive protein (SOAP). Artificial intelligence-based high-resolution structural modeling using the DeepMind AlphaFold algorithm yielded highly informative 3D structures and insights into how short chitinases, WARP, and SOAP may interact at the atomic level to form the ookinete-secreted peritrophic matrix invasion complex. Elucidating the significance of the divergence of ookinete-secreted micronemal proteins among Plasmodium species could lead to a better understanding of ookinete invasion machinery and the co-evolution of Plasmodium-mosquito interactions.

Affinity purification of Plasmodium ookinetes from in vitro cultures using extracellular matrix gel

Malaria transmission depends on the parasites' successful invasion of the mosquito. This is achieved by the ookinete, a motile zygote that forms in the blood bolus after the mosquito takes an infectious blood meal. The ookinete invades the midgut epithelium and strongly attaches to the basal lamina, differentiating into an oocyst that produces the vertebrate-invasive sporozoites. Despite their importance, the ookinete and the oocyst are the least studied stages of the parasite. Much of what we know about the ookinete comes from in vitro experiments, which are hindered by the concomitant contamination with blood cells and other parasite stages. Although methods to purify them exist, they vary in terms of yield, costs, and difficulty to perform. A method for ookinete purification taking advantage of their adhesive properties was herein developed. The method consists of covering any culture-suitable surface with extracellular matrix gel, after which the ookinete culture is incubated on the gel to allow for ookinete attachment. The contaminant cells are then simply washed away. This procedure results in purer and less stressed ookinete preparations, which, by the nature of the method, are ready for oocyst production. Furthermore, it allows for micro-purifications using only 1 μl of blood, opening the possibility to make axenic ookinete cultures without sacrificing mice.

Functional Characterization and Comparison of Plasmodium falciparum Proteins as Targets of Transmission-blocking Antibodies

Plasmodium falciparum malaria continues to evade control efforts, utilizing highly specialized sexual-stages to transmit infection between the human host and mosquito vector. In a vaccination model, antibodies directed to sexual-stage antigens, when ingested in the mosquito blood meal, can inhibit parasite growth in the midgut and consequently arrest transmission. Despite multiple datasets for the Plasmodium sexual-stage transcriptome and proteome, there have been no rational screens to identify candidate antigens for transmission-blocking vaccine (TBV) development. This study characterizes 12 proteins from across the P. falciparum sexual-stages as possible TBV targets. Recombinant proteins are heterologously expressed as full-length ectodomains in a mammalian HEK293 cell system. The proteins recapitulate native parasite epitopes as assessed by indirect fluorescence assay and a proportion exhibits immunoreactivity when tested against sera from individuals living in malaria-endemic Burkina Faso and Mali. Purified IgG generated to the mosquito-stage parasite antigen enolase demonstrates moderate inhibition of parasite development in the mosquito midgut by the ex vivo standard membrane feeding assay. The findings support the use of rational screens and comparative functional assessments in identifying proteins of the P. falciparum transmission pathway and establishing a robust pre-clinical TBV pipeline.

A Multi-Stage Plasmodium vivax Malaria Vaccine Candidate Able to Induce Long-Lived Antibody Responses Against Blood Stage Parasites and Robust Transmission-Blocking Activity

Malaria control and interventions including long-lasting insecticide-treated nets, indoor residual spraying, and intermittent preventative treatment in pregnancy have resulted in a significant reduction in the number of Plasmodium falciparum cases. Considerable efforts have been devoted to P. falciparum vaccines development with much less to P. vivax. Transmission-blocking vaccines, which can elicit antibodies targeting Plasmodium antigens expressed during sexual stage development and interrupt transmission, offer an alternative strategy to achieve malaria control. The post-fertilization antigen P25 mediates several functions essential to ookinete survival but is poorly immunogenic in humans. Previous clinical trials targeting this antigen have suggested that conjugation to a carrier protein could improve the immunogenicity of P25. Here we report the production, and characterization of a vaccine candidate composed of a chimeric P. vivax Merozoite Surface Protein 1 (cPvMSP1) genetically fused to P. vivax P25 (Pvs25) designed to enhance CD4+ T cell responses and its assessment in a murine model. We demonstrate that antibodies elicited by immunization with this chimeric protein recognize both the erythrocytic and sexual stages and are able to block the transmission of P. vivax field isolates in direct membrane-feeding assays. These findings provide support for the continued development of multi-stage transmission blocking vaccines targeting the life-cycle stage responsible for clinical disease and the sexual-stage development accountable for disease transmission simultaneously.

The Development of Whole Sporozoite Vaccines for Plasmodium falciparum Malaria

Each year malaria kills hundreds of thousands of people and infects hundreds of millions of people despite current control measures. An effective malaria vaccine will likely be necessary to aid in malaria eradication. Vaccination using whole sporozoites provides an increased repertoire of immunogens compared to subunit vaccines across at least two life cycle stages of the parasite, the extracellular sporozoite, and intracellular liver stage. Three potential whole sporozoite vaccine approaches are under development and include genetically attenuated parasites, radiation attenuated sporozoites, and wild-type sporozoites administered in combination with chemoprophylaxis. Pre-clinical and clinical studies have demonstrated whole sporozoite vaccine immunogenicity, including humoral and cellular immunity and a range of vaccine efficacy that depends on the pre-exposure of vaccinated individuals. While whole sporozoite vaccines can provide protection against malaria in some cases, more recent studies in malaria-endemic regions demonstrate the need for improvements. Moreover, challenges remain in manufacturing large quantities of sporozoites for vaccine commercialization. A promising solution to the whole sporozoite manufacturing challenge is in vitro culturing methodology, which has been described for several Plasmodium species, including the major disease-causing human malaria parasite, Plasmodium falciparum. Here, we review whole sporozoite vaccine immunogenicity and in vitro culturing platforms for sporozoite production.

PfCap380 as a marker for Plasmodium falciparum oocyst development in vivo and in vitro

Background

Despite the importance of the Plasmodium berghei oocyst capsule protein (PbCap380) in parasite survival, very little is known about the orthologous Plasmodium falciparum capsule protein (PfCap380). The goal of this work was to study the growth of P. falciparum oocysts using PfCap380 as a developmental marker.

Methods

To study P. falciparum oocyst development using both in vivo (mosquito-derived) and in vitro (culture-derived) growth conditions, antibodies (polyclonal antisera) were raised against PfCap380. For studies on in vivo oocysts, mature P. falciparum gametocytes were fed to Anopheles stephensi mosquitoes. For studies on in vitro parasites, P. falciparum gametocytes were induced and matured for subsequent ookinete production. Ookinetes were purified and then tested for binding affinity to basal lamina components and transformation into early oocysts, which were grown on reconstituted basal lamia coated wells with novel oocyst media. To monitor in vivo oocyst development, immunofluorescence assays (IFA) were performed using anti-PfCap380 antisera on Pf-infected mosquito midguts. IFA were also performed on culture-derived oocysts to follow in vitro oocyst development.

Results

The anti-PfCap380 antisera allowed detection of early midgut oocysts starting at 2 days after gametocyte infection, while circumsporozoite protein was definitively observed on day 6. For in vitro culture, significant transformation of gametocytes to ookinetes (24%) and of ookinetes to early oocysts (85%) was observed. After screening several basal lamina components, collagen IV provided greatest binding of ookinetes and transformation into early oocysts. Finally, PfCap380 expression was observed on the surface of culture-derived oocysts but not on gametocytes or ookinetes.

Conclusions

This study presents developmental monitoring of P. falciparum oocysts produced in vivo and in vitro. The anti-PfCap380 antisera serves as an important reagent for developmental studies of oocysts from the mosquito midgut and also from oocyst culture using in vitro methodology. The present data demonstrate that PfCap380 is a useful marker to follow the development and maturation of in vivo and in vitro produced oocysts as early as 2 days after zygote formation. Further in vitro studies focused on oocyst and sporozoite maturation will support the manufacturing of whole sporozoites for malaria vaccines.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2277-6) contains supplementary material, which is available to authorized users.

Comparative assessment of An. gambiae and An. stephensi mosquitoes to determine transmission-reducing activity of antibodies against P. falciparum sexual stage antigens

Background

With the increasing interest in vaccines to interrupt malaria transmission, there is a demand for harmonization of current methods to assess Plasmodium transmission in laboratory settings. Potential vaccine candidates are currently tested in the standard membrane feeding assay (SMFA) that commonly relies on Anopheles stephensi mosquitoes. Other mosquito species including Anopheles gambiae are the dominant malaria vectors for Plasmodium falciparum in sub-Saharan Africa.

Methods

Using human serum and monoclonal pre-fertilization (anti-Pfs48/45) and post-fertilization (anti-Pfs25) antibodies known to effectively inhibit sporogony, we directly compared SMFA based estimates of transmission-reducing activity (TRA) for An. stephensi and An. gambiae mosquitoes.

Results

In the absence of transmission-reducing antibodies, average numbers of oocysts were similar between An. gambiae and An. stephensi. Antibody-mediated TRA was strongly correlated between both mosquito species, and absolute TRA estimates for pre-fertilisation monoclonal antibodies (mAb) showed no significant difference between the two species. TRA estimates for IgG of naturally exposed individuals and partially effective concentrations of anti-Pfs25 mAb were higher for An. stephensi than for An. gambiae.

Conclusion

Our findings support the use of An. stephensi in the SMFA for target prioritization. As a vaccine moves through product development, better estimates of TRA and transmission-blocking activity (TBA) may need to be obtained in epidemiologically relevant parasite-species combination.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2414-z) contains supplementary material, which is available to authorized users.

Plasmodium Oocysts: Overlooked Targets of Mosquito Immunity

Although the ability of mosquitoes to limit Plasmodium infection is well documented, many questions remain as to how malaria parasites are recognized and killed by the mosquito host. Recent evidence suggests that anti-Plasmodium immunity is multimodal, with different immune mechanisms regulating ookinete and oocyst survival. However, most experiments determine the number of mature oocysts, without considering that different immune mechanisms may target different developmental stages of the parasite. Complement-like proteins have emerged as important determinants of early immunity targeting the ookinete stage, yet the mechanisms by which the mosquito late-phase immune response limits oocyst survival are less understood. Here, we describe the known components of the mosquito immune system that limit oocyst development, and provide insight into their possible mechanisms of action.

Toward the development of effective transmission-blocking vaccines for malaria

The continued global burden of malaria can in part be attributed to a complex lifecycle, with both human hosts and mosquito vectors serving as transmission reservoirs. In preclinical models of vaccine-induced immunity, antibodies to parasite sexual-stage antigens, ingested in the mosquito blood meal, can inhibit parasite survival in the insect midgut as judged by ex vivo functional studies such as the membrane feeding assay. In an era of renewed political momentum for malaria elimination and eradication campaigns, such observations have fueled support for the development and implementation of so-called transmission-blocking vaccines. While leading candidates are being evaluated using a variety of promising vaccine platforms, the field is also beginning to capitalize on global '-omics' data for the rational genome-based selection and unbiased characterization of parasite and mosquito proteins to expand the candidate list. This review covers the progress and prospects of these recent developments.

Functional characterization of Anopheles matrix metalloprotease 1 reveals its agonistic role during sporogonic development of malaria parasites

The ability to invade tissues is a unique characteristic of the malaria stages that develop/differentiate within the mosquitoes (ookinetes and sporozoites). On the other hand, tissue invasion by many pathogens has often been associated with increased matrix metalloprotease (MMP) activity in the invaded tissues. By employing cell biology and reverse genetics, we studied the expression and explored putative functions of one of the three MMPs encoded in the genome of the malaria vector Anopheles gambiae, namely, the Anopheles gambiae MMP1 (AgMMP1) gene, during the processes of blood digestion, midgut epithelium invasion by Plasmodium ookinetes, and oocyst development. We show that AgMMP1 exists in two alternative isoforms resulting from alternative splicing; one secreted (S-MMP1) and associated with hemocytes, and one membrane type (MT-MMP1) enriched in the cell attachment sites of the midgut epithelium. MT-MMP1 showed a remarkable response to ookinete midgut invasion manifested by increased expression, enhanced zymogen maturation, and subcellular redistribution, all indicative of an implication in the midgut epithelial healing that accompanies ookinete invasion. Importantly, RNA interference (RNAi)-mediated silencing of the AgMMP1 gene revealed a postinvasion protective function of AgMMP1 during oocyst development. The combined results link for the first time an MMP with vector competence and mosquito-Plasmodium interactions.

Molecular characterization of calreticulin from Anopheles stephensi midgut cells and functional assay of the recombinant calreticulin with Plasmodium berghei ookinetes

Transmission blocking vaccines (TBVs) that target the antigens on the midgut epithelium of Anopheles mosquitoes are among the promising tools for the elimination of the malaria parasite. Characterization and analysis of effective antigens is the first step to design TBVs. Calreticulin (CRT), a lectin-like protein, from Anopheles albimanus midgut, has shown antigenic features, suggesting a promising and novel TBV target. CRT is a highly conserved protein with similar features in vertebrates and invertebrates including anopheline. We cloned the full-length crt gene from malaria vector, Anopheles stephensi (AsCrt) and explored the interaction of recombinant AsCrt protein, expressed in a prokaryotic system (pGEX-6p-1), with surface proteins of Plasmodium berghei ookinetes by immunofluorescence assay. The cellular localization of AsCrt was determined using the baculovirus expression system. Sequence analysis of the whole cDNA of AsCrt revealed that AsCrt contains an ORF of 1221 bp. The amino acid sequence of AsCrt protein obtained in this study showed 64% homology with similar protein in human. The AsCrt shares the most common features of CRTs from other species. This gene encodes a 406 amino-acid protein with a molecular mass of 46 kDa, which contains a predicted 16 amino-acid signal peptides, conserved cysteine residues, a proline-rich region, and highly acidic C-terminal domain with endoplasmic reticulum retrieval sequence HDEL. The production of GST-AsCrt recombinant protein was confirmed by Western blot analysis using an antibody against the GST protein. The FITC-labeled GST-AsCrt exhibited a significant interaction with P. berghei ookinete surface proteins. Purified recombinant GST-AsCrt, labeled with FITC, displayed specific binding to the surface of P. berghei ookinetes in comparison with control. Moreover, the expression of AsCrt in baculovirus expression system indicated that AsCrt was localized on the surface of Sf9 cells. Our results suggest that AsCrt could be utilized as a potential target for future studies in TBV area for malaria control.

Malaria adhesins: structure and function

The malaria parasite Plasmodium utilizes specialized proteins for adherence to cellular receptors in its mosquito vector and human host. Adherence is critical for parasite development, host cell traversal and invasion, and protection from vector and host immune mechanisms. These vital roles have identified several adhesins as vaccine candidates. A deficiency in current adhesin-based vaccines is induction of antibodies targeting non-conserved, non-functional and decoy epitopes due to the use of full length proteins or binding domains. To alleviate the elicitation of non-inhibitory antibodies, conserved functional regions of proteins must be identified and exploited. Structural biology provides the tools necessary to achieve this goal, and has succeeded in defining biologically functional receptor binding and oligomerization interfaces for a number of promising malaria vaccine candidates. We describe here the current knowledge of Plasmodium adhesin structure and function, and how it has illuminated elements of parasite biology and defined interactions at the host/vector and parasite interface.

No evidence for positive selection at two potential targets for malaria transmission-blocking vaccines in Anopheles gambiae s.s

Human malaria causes nearly a million deaths in sub-Saharan Africa each year. The evolution of drug-resistance in the parasite and insecticide resistance in the mosquito vector has complicated control measures and made the need for new control strategies more urgent. Anopheles gambiae s.s. is one of the primary vectors of human malaria in Africa, and parasite-transmission-blocking vaccines targeting Anopheles proteins have been proposed as a possible strategy to control the spread of the disease. However, the success of these hypothetical technologies would depend on the successful ability to broadly target mosquito populations that may be genetically heterogeneous. Understanding the evolutionary pressures shaping genetic variation among candidate target molecules offers a first step towards evaluating the prospects of successfully deploying such technologies. We studied the population genetics of genes encoding two candidate target proteins, the salivary gland protein saglin and the basal lamina structural protein laminin, in wild populations of the M and S molecular forms of A. gambiae in Mali. Through analysis of intraspecific genetic variation and interspecific comparisons, we found no evidence of positive natural selection at the genes encoding these proteins. On the contrary, we found evidence for particularly strong purifying selection at the laminin gene. These results provide insight into the patterns of genetic diversity of saglin and laminin, and we discuss these findings in relation to the potential development of these molecules as vaccine targets.

Evidence that a laminin-like insect protein mediates early events in the interaction of a Phytoparasite with its vector's salivary gland

Phytomonas species are plant parasites of the family Trypanosomatidae, which are transmitted by phytophagous insects. Some Phytomonas species cause major agricultural damages. The hemipteran Oncopeltus fasciatus is natural and experimental host for several species of trypanosomatids, including Phytomonas spp. The invasion of the insect vectors' salivary glands is one of the most important events for the life cycle of Phytomonas species. In the present study, we show the binding of Phytomonas serpens at the external face of O. fasciatus salivary glands by means of scanning electron microscopy and the in vitro interaction of living parasites with total proteins from the salivary glands in ligand blotting assays. This binding occurs primarily through an interaction with a 130 kDa salivary gland protein. The mass spectrometry of the trypsin-digest of this protein matched 23% of human laminin-5 β3 chain precursor sequence by 16 digested peptides. A protein sequence search through the transcriptome of O. fasciatus embryo showed a partial sequence with 51% similarity to human laminin β3 subunit. Anti-human laminin-5 β3 chain polyclonal antibodies recognized the 130 kDa protein by immunoblotting. The association of parasites with the salivary glands was strongly inhibited by human laminin-5, by the purified 130 kDa insect protein, and by polyclonal antibodies raised against the human laminin-5 β3 chain. This is the first report demonstrating that a laminin-like molecule from the salivary gland of O. fasciatus acts as a receptor for Phytomonas binding. The results presented in this investigation are important findings that will support further studies that aim at developing new approaches to prevent the transmission of Phytomonas species from insects to plants and vice-versa.

Conserved peptide sequences bind to actin and enolase on the surface of Plasmodium berghei ookinetes

The description of Plasmodium ookinete surface proteins and their participation in the complex process of mosquito midgut invasion is still incomplete. In this study, using phage display, a consensus peptide sequence (PWWP) was identified in phages that bound to the Plasmodium berghei ookinete surface and, in selected phages, bound to actin and enolase in overlay assays with ookinete protein extracts. Actin was localized on the surface of fresh live ookinetes by immunofluorescence and electron microscopy using specific antibodies. The overall results indicated that enolase and actin can be located on the surface of ookinetes, and suggest that they could participate in Plasmodium invasion of the mosquito midgut.

A new role for an old antimicrobial: lysozyme c-1 can function to protect malaria parasites in Anopheles mosquitoes

BACKGROUND

Plasmodium requires an obligatory life stage in its mosquito host. The parasites encounter a number of insults while journeying through this host and have developed mechanisms to avoid host defenses. Lysozymes are a family of important antimicrobial immune effectors produced by mosquitoes in response to microbial challenge.

METHODOLOGY/PRINCIPAL FINDINGS

A mosquito lysozyme was identified as a protective agonist for Plasmodium. Immunohistochemical analyses demonstrated that Anopheles gambiae lysozyme c-1 binds to oocysts of Plasmodium berghei and Plasmodium falciparum at 2 and 5 days after infection. Similar results were observed with Anopheles stephensi and P. falciparum, suggesting wide occurrence of this phenomenon across parasite and vector species. Lysozyme c-1 did not bind to cultured ookinetes nor did recombinant lysozyme c-1 affect ookinete viability. dsRNA-mediated silencing of LYSC-1 in Anopheles gambiae significantly reduced the intensity and the prevalence of Plasmodium berghei infection. We conclude that this host antibacterial protein directly interacts with and facilitates development of Plasmodium oocysts within the mosquito.

CONCLUSIONS/SIGNIFICANCE

This work identifies mosquito lysozyme c-1 as a positive mediator of Plasmodium development as its reduction reduces parasite load in the mosquito host. These findings improve our understanding of parasite development and provide a novel target to interrupt parasite transmission to human hosts.

Family members stick together: multi-protein complexes of malaria parasites

Malaria parasites express a broad repertoire of proteins whose expression is tightly regulated depending on the life-cycle stage of the parasite and the environment of target organs in the respective host. Transmission of malaria parasites from the human to the anopheline mosquito is mediated by intraerythrocytic sexual stages, termed gametocytes, which circulate in the peripheral blood and are essential for the spread of the tropical disease. In Plasmodium falciparum, gametocytes express numerous extracellular proteins with adhesive motifs, which might mediate important interactions during transmission. Among these is a family of six secreted proteins with adhesive modules, termed PfCCp proteins, which are highly conserved throughout the apicomplexan clade. In P. falciparum, the proteins are expressed in the parasitophorous vacuole of gametocytes and are subsequently exposed on the surface of macrogametes during parasite reproduction in the mosquito midgut. One characteristic of the family is a co-dependent expression, such that loss of all six proteins occurs if expression of one member is disrupted via gene knockout. The six PfCCp proteins interact by adhesion domain-mediated binding and thus form complexes on the sexual stage surface having adhesive properties. To date, the PfCCp proteins represent the only protein family of the malaria parasite sexual stages that assembles to multimeric complexes, and only a small number of such protein complexes have so far been identified in other life-cycle stages of the parasite.

Symbiont genomics, our new tangled bank

Microbial symbionts inhabit the soma and surfaces of most multicellular species and instigate both beneficial and harmful infections. Despite their ubiquity, we are only beginning to resolve major patterns of symbiont ecology and evolution. Here, we summarize the history, current progress, and projected future of the study of microbial symbiont evolution throughout the tree of life. We focus on the recent surge of data that whole-genome sequencing has introduced into the field, in particular the links that are now being made between symbiotic lifestyle and molecular evolution. Post-genomic and systems biology approaches are also emerging as powerful techniques to investigate host-microbe interactions, both at the molecular level of the species interface and at the global scale. In parallel, next-generation sequencing technologies are allowing new questions to be addressed by providing access to population genomic data, as well as the much larger genomes of microbial eukaryotic symbionts and hosts. Throughout we describe the questions that these techniques are tackling and we conclude by listing a series of unanswered questions in microbial symbiosis that can potentially be addressed with the new technologies.

Malaria parasite development in the mosquito and infection of the mammalian host

Plasmodium sporozoites are the product of a complex developmental process in the mosquito vector and are destined to infect the mammalian liver. Attention has been drawn to the mosquito stages and pre-erythrocytic stages owing to recognition that these are bottlenecks in the parasite life cycle and that intervention at these stages can block transmission and prevent infection. Parasite progression in the Anopheles mosquito, sporozoite transmission to the mammalian host by mosquito bite, and subsequent infection of the liver are characterized by extensive migration of invasive stages, cell invasion, and developmental changes. Preparation for the liver phase in the mammalian host begins in the mosquito with an extensive reprogramming of the sporozoite to support efficient infection and survival. Here, we discuss what is known about the molecular and cellular basis of the developmental progression of parasites and their interactions with host tissues in the mosquito and during the early phase of mammalian infection.

Localisation of laminin within Plasmodium berghei oocysts and the midgut epithelial cells of Anopheles stephensi

Background

Oocysts of the malaria parasite form and develop in close proximity to the mosquito midgut basal lamina and it has been proposed that components of this structure play a crucial role in the development and maturation of oocysts that produce infective sporozoites. It is further suggested that oocysts incorporate basal lamina proteins into their capsule and that this provides them with a means to evade recognition by the mosquito's immune system. The site of production of basal lamina proteins in insects is controversial and it is still unclear whether haemocytes or midgut epithelial cells are the main source of components of the mosquito midgut basal lamina. Of the multiple molecules that compose the basal lamina, laminin is known to interact with a number of Plasmodium proteins. In this study, the localisation of mosquito laminin within the capsule and cytoplasm of Plasmodium berghei oocysts and in the midgut epithelial cells of Anopheles stephensi was investigated.

Results

An ultrastructural examination of midgut sections from infected and uninfected An. stephensi was performed. Post-embedded immunogold labelling demonstrated the presence of laminin within the mosquito basal lamina. Laminin was also detected on the outer surface of the oocyst capsule, incorporated within the capsule and associated with sporozoites forming within the oocysts. Laminin was also found within cells of the midgut epithelium, providing support for the hypothesis that these cells contribute towards the formation of the midgut basal lamina.

Conclusion

We suggest that ookinetes may become coated in laminin as they pass through the midgut epithelium. Thereafter, laminin secreted by midgut epithelial cells and/or haemocytes, binds to the outer surface of the oocyst capsule and that some passes through and is incorporated into the developing oocysts. The localisation of laminin on sporozoites was unexpected and the importance of this observation is less clear.

The microneme proteins CTRP and SOAP are not essential for Plasmodium berghei ookinete to oocyst transformation in vitro in a cell free system

BACKGROUND

Two Plasmodium berghei ookinete micronemal proteins, circumsporozoite and TRAP related protein (CTRP) and secreted ookinete adhesive protein (SOAP) both interact with the basal lamina component laminin. Following gene disruption studies it has been proposed that, apart from their role in motility, these proteins may be required for interactions leading to ookinete-to-oocyst transformation.

METHODS

CTRP and SOAP null mutant P. berghei ookinetes were compared to P. berghei ANKA wild-type for their ability to transform and grow in vitro. To confirm in vitro findings for P. berghei CTRP-KO ookinetes were injected into the haemocoel of Anopheles gambiae female mosquitoes.

RESULTS

Transformation, growth, and viability were comparable for the gene disrupted and wild-type parasites. P. berghei CTRP-KO ookinetes were able to transform into oocysts in the haemocoel of An. gambiae mosquitoes.

CONCLUSION

Neither CTRP nor SOAP is required for parasite transformation in vitro. By-passing the midgut lumen allows for the transformation of P. berghei CTRP-KO ookinetes suggesting that it is not required for transformation in vivo.

PbCap380, a novel oocyst capsule protein, is essential for malaria parasite survival in the mosquito

An essential requisite for transmission of Plasmodium, the causative agent of malaria, is the successful completion of a complex developmental cycle in its mosquito vector. Of hundreds of ookinetes that form in the mosquito midgut, only few transform into oocysts, a loss attributed to the action of the mosquito immune system. However, once oocysts form, they appear to be resistant to mosquito defences. During oocyst development, a thick capsule forms around the parasite and appears to function as a protective cover. Little information is available about the composition of this capsule. Here we report on the identification and partial characterization of the first Plasmodium oocyst capsule protein (PbCap380). Genetic analysis indicates that the gene is essential and that PbCap380(-) mutant parasites form oocysts in normal numbers but are gradually eliminated. As a result, mosquitoes infected with PbCap380(-) parasites do not transmit malaria. Targeting of the oocyst capsule may provide a new strategy for malaria control.

Getting infectious: formation and maturation of Plasmodium sporozoites in the Anopheles vector

Research on Plasmodium sporozoite biology aims at understanding the developmental program steering the formation of mature infectious sporozoites - the transmission stage of the malaria parasite. The recent identification of genes that are vital for sporozoite egress from oocysts and subsequent targeting and transmigration of the mosquito salivary glands allows the identification of mosquito factors required for life cycle completion. Mature sporozoites appear to be equipped with the entire molecular repertoire for successful transmission and subsequent initiation of liver stage development. Innovative malaria intervention strategies that target the early, non-pathogenic phases of the life cycle will crucially depend on our insights into sporozoite biology and the underlying molecular mechanisms that lead the parasite from the mosquito midgut to the liver.

Laminin and a Plasmodium ookinete surface protein inhibit melanotic encapsulation of Sephadex beads in the hemocoel of mosquitoes

In refractory mosquitoes, melanotic encapsulation of Plasmodium ookinetes and oocysts is a commonly observed immune response. However, in susceptible mosquitoes, Plasmodium oocysts develop extracellularly in the body cavity without being recognized by the immune system. Like Plasmodium gallinaceum oocysts, negatively charged carboxymethyl (CM)-Sephadex beads implanted in the hemocoel of Aedes aegypti female mosquitoes were not usually melanized, but were coated with mosquito-derived laminin. Conversely, electrically neutral G-Sephadex beads were routinely melanized. Since mosquito laminin coated both CM-Sephadex beads and P. gallinaceum oocysts, we hypothesized that laminin prevents melanization of both. To test this hypothesis, we coated cyanogen-bromide-activated G-Sephadex beads with laminin, recombinant P. gallinaceum ookinete surface protein (PgS28) or bovine serum albumin (BSA). Beads were implanted into the abdominal body cavity of female Aedes aegypti and retrieved 4 days later. Uncoated controls as well as BSA-coated G-Sephadex beads were melanized in a normal manner. However, melanization of beads coated with mouse laminin, Drosophila L2-secreted proteins or PgS28 was markedly reduced. Fluorescent antibody labeling showed that PgS28-coated beads had adsorbed mosquito laminin on their surface. Thus, mosquito laminin interacting with Plasmodium surface proteins probably masks oocysts from the mosquito's immune system, thereby facilitating their development in the body cavity.

The developmental migration of Plasmodium in mosquitoes

Migration of the protozoan parasite Plasmodium through the mosquito is a complex and delicate process, the outcome of which determines the success of malaria transmission. The mosquito is not simply the vector of Plasmodium but, in terms of the life cycle, its definitive host: there, the parasite undergoes its sexual development, which results in colonization of the mosquito salivary glands. Two of the parasite's developmental stages in the mosquito, the ookinete and the sporozoite, are invasive and depend on gliding motility to access, penetrate and traverse their host cells. Recent advances in the field have included the identification of numerous Plasmodium molecules that are essential for parasite migration in the mosquito vector.

Functional proteome and expression analysis of sporozoites and hepatic stages of malaria development

An evolution in modern malaria research occurred with the completion of the Plasmodium falciparum genome project and the onset and application of novel post-genomic technologies. Corresponding with these technological achievements are improvements in accessing and purifying parasite material from 'hard-to-reach' stages of malaria development. Characterization of gene and protein expression in the infectious sporozoite and subsequent liver-stage parasite development is critical to identify novel pre-erythrocytic drug and vaccine targets as well as to understand the basic biology of this deadly parasite. Both transcriptional and proteomic analyses on these stages and the remaining stages of development will assist in the 'credentialing process' of the complete malaria genome.

The complex interplay between mosquito positive and negative regulators of Plasmodium development

The malaria parasite, Plasmodium, requires sexual development in the mosquito before it can be transmitted to the vertebrate host. Mosquito genes are able to substantially modulate this process, which can result in major decreases in parasite numbers. Even in susceptible mosquitoes, haemolymph proteins implicated in systemic immune reactions, together with local epithelial responses, cause lysis of more than 80% of the ookinetes that cross the mosquito midgut. In a refractory mosquito strain, immune responses lead to melanisation of virtually all parasites. Conversely, certain mosquito genes have an opposite effect: they are used by the parasite to evade defence reactions. Detailed understanding of the interplay between positive and negative regulators of parasite development could lead to the generation of novel approaches for malaria control through the vector.

Laminin and the malaria parasite's journey through the mosquito midgut

During the invasion of the mosquito midgut epithelium, Plasmodium ookinetes come to rest on the basal lamina, where they transform into the sporozoite-producing oocysts. Laminin, one of the basal lamina's major components, has previously been shown to bind several surface proteins of Plasmodium ookinetes. Here, using the recently developed RNAi technique in mosquitoes, we used a specific dsRNA construct targeted against the LANB2 gene (laminin gamma1) of Anopheles gambiae to reduce its mRNA levels, leading to a substantial reduction in the number of successfully developed oocysts in the mosquito midgut. Moreover, this molecular relationship is corroborated by the intimate association of developing P. berghei parasites and laminin in the gut, as observed using confocal microscopy. Our data support the notion of laminin playing a functional role in the development of the malaria parasite within the mosquito midgut.

Close association of invading Plasmodium berghei and beta integrin in the Anopheles gambiae midgut

We have used confocal microscopy and an antibody against Anopheles gambiae beta integrin to study this protein's distribution in the mosquito midgut and its relationship to invading Plasmodium berghei parasites. An extensive reorganization of integrin is seen to take place in the midgut epithelial cells following the uptake of either non-infected or parasite-infected blood meal, probably reflecting the reshaping of the gut due to the presence of the food bolus and the peritrophic membrane that surrounds it. Furthermore, malaria parasites are coated with beta integrin immediately upon entry into the epithelium, independent of whether they develop intra- or extracellularly. Although this coat is shed a few days after the invasion, beta integrin remains concentrated in the cells surrounding the maturing oocyst for several days. Finally, the antibody detects a structural change in the midgut epithelial cells in the immediate vicinity of the invading ookinete, which is consistent with Plasmodium-induced apoptosis followed by wound healing. This intimate association suggests a specific role of beta integrin in the invasion process.

Plasmodium berghei ookinetes bind to Anopheles gambiae and Drosophila melanogaster annexins

Using a proteomic approach we identified polypeptides from Anopheles gambiae and Drosophila melanogaster protein extracts that selectively bind purified Plasmodium berghei ookinetes in vitro; these were two and three distinct polypeptides, respectively, with an apparent molecular weight of about 36 kDa. Combining two-dimensional electrophoresis and MALDI-TOF (matrix-associated laser desorption ionization time of flight) mass spectrometry we determined that the polypeptides correspond to isomorphs of the annexin B11 protein of the fruit fly. When protein extracts derived from A. gambiae and D. melanogaster tissue culture cells were further fractionated, the binding activity matching the annexin protein could be localized in the fraction derived from cell membranes in both diptera. Antibody staining showed that annexin also binds to ookinetes during the invasion of the mosquito midgut. Finally, inclusion of antiannexin antisera in a mosquito blood meal impaired parasite development, suggesting a facilitating role for annexins in the infection of the mosquito by Plasmodium.

Do malaria ookinete surface proteins P25 and P28 mediate parasite entry into mosquito midgut epithelial cells?

Background

P25 and P28 are related ookinete surface proteins highly conserved throughout the Plasmodium genus that are under consideration as candidates for inclusion in transmission-blocking vaccines. Previous research using transgenic rodent malaria parasites lacking P25 and P28 has demonstrated that these proteins have multiple partially redundant functions during parasite infection of the mosquito vector, including an undefined role in ookinete traversal of the mosquito midgut epithelium, and it has been suggested that, unlike wild-type parasites, Dko P25/P28 parasites migrate across the midgut epithelium via an intercellular, rather than intracellular, route.

Presentation of the hypothesis

This paper presents an alternative interpretation for the previous observations of Dko P25/P28 parasites, based upon a recently published model of the route of ookinete invasion across the midgut epithelium. This model claims ookinete invasion is intracellular, with entry occurring through the lateral apical plasma membrane of midgut epithelial cells, and is associated with significant invagination of the midgut epithelium localised at the site of parasite penetration. Following this model, it is hypothesized that: (1) a sub-population of Dko P25/P28 ookinetes invaginate, but do not penetrate, the apical surface of the midgut epithelium and thus remain within the midgut lumen; and (2) another sub-population of Dko P25/P28 parasites successfully enters and migrates across the midgut epithelium via an intracellular route similar to wild-type parasites and subsequently develops into oocysts.

Testing the hypothesis

These hypotheses are tested by showing how they can account for previously published observations and incorporate them into a coherent and consistent explanatory framework. Based upon these hypotheses, several quantitative predictions are made, which can be experimentally tested, about the relationship between the densities of invading Dko P25/P28 ookinetes in different regions of the midgut epithelium and the number of oocyst stage parasites to which these mutant ookinetes give rise.

Implications of the hypothesis

The recently published model of ookinete invasion implies that Dko P25/P28 parasites are greatly, although not completely, impaired in their ability to enter the midgut epithelium. Therefore, P25 and/or P28 have a novel, previously unrecognized, function in mediating ookinete entry into midgut epithelial cells, suggesting that one mode of action of transmission-blocking antibodies to these ookinete surface proteins is to inhibit this function.

Gene expression in Plasmodium: from gametocytes to sporozoites

Completion of the complex developmental program of Plasmodium in the mosquito is essential for parasite transmission, yet this part of its life cycle is still poorly understood. In recent years, considerable progress has been made in the identification and characterization of genes expressed during parasite development in the mosquito. This line of investigation was greatly facilitated by the availability of the genome sequence of several Plasmodium, and by the application of approaches such as proteomics, microarrays, gene disruption by homologous recombination (gene knockout) and by use of subtraction libraries. Here, we review what is presently known about genes expressed in gametocytes and during the Plasmodium life cycle in the mosquito.

Interactions between malaria parasites and their mosquito hosts in the midgut

This review examines what is presently known of the molecular interactions between Plasmodium and Anopheles that take place in the latter's midgut upon ingestion of the parasites with an infectious blood meal. In order to become 'established' in the gut and to transform into a sporozoite-producing oocyst, the malaria parasite needs to undergo different developmental steps that are often characterized by the use of selected resources provided by the mosquito vector. Moreover, some of these resources may be used by the parasite in order to overcome the insect host's defence mechanisms. The molecular partners of this interplay are now in the process of being defined and analyzed for both Plasmodium and mosquito and, thus, understood; these will be presented here in some detail.

Anopheles gambiae collagen IV genes: cloning, phylogeny and midgut expression associated with blood feeding and Plasmodium infection

A prerequisite for understanding the role that mosquito midgut extracellular matrix molecules play in malaria parasite development is proper isolation and characterisation of the genes coding for components of the basal lamina. Here we have identified genes coding for alpha1 and alpha2 chains of collagen IV from the major malaria vector, Anopheles gambiae. Conserved sequences in the terminal NC1 domain were used to obtain partial gene sequences of this functional region, and full sequence was isolated from a pupal cDNA library. In a DNA-derived phylogeny, the alpha1 and alpha2 chains cluster with dipteran orthologs, and the alpha2 is ancestral. The expression of collagen alpha1(IV) peaked during the pupal stage of mosquito development, and was expressed continuously in the adult female following a blood meal with a further rise detected in older mosquitoes. Collagen alpha1(IV) is also upregulated when the early oocyst of Plasmodium yoelii was developing within the mosquito midgut and may contribute to a larger wound healing response. A model describing the expression of basal lamina proteins during oocyst development is presented, and we hypothesise that the development of new basal lamina between the oocyst and midgut epithelium is akin to a wound healing process.

SOAP, a novel malaria ookinete protein involved in mosquito midgut invasion and oocyst development

An essential, but poorly understood part of malaria transmission by mosquitoes is the development of the ookinetes into the sporozoite-producing oocysts on the mosquito midgut wall. For successful oocyst formation newly formed ookinetes in the midgut lumen must enter, traverse, and exit the midgut epithelium to reach the midgut basal lamina, processes collectively known as midgut invasion. After invasion ookinete-to-oocyst transition must occur, a process believed to require ookinete interactions with basal lamina components. Here, we report on a novel extracellular malaria protein expressed in ookinetes and young oocysts, named secreted ookinete adhesive protein (SOAP). The SOAP gene is highly conserved amongst Plasmodium species and appears to be unique to this genus. It encodes a predicted secreted and soluble protein with a modular structure composed of two unique cysteine-rich domains. Using the rodent malaria parasite Plasmodium berghei we show that SOAP is targeted to the micronemes and forms high molecular mass complexes via disulphide bonds. Moreover, SOAP interacts strongly with mosquito laminin in yeast-two-hybrid assays. Targeted disruption of the SOAP gene gives rise to ookinetes that are markedly impaired in their ability to invade the mosquito midgut and form oocysts. These results identify SOAP as a key molecule for ookinete-to-oocyst differentiation in mosquitoes.

In vitro methods for culturing vertebrate and mosquito stages of Plasmodium

The development of in vitro culture systems for the vertebrate stages of Plasmodium led to major advancements in malaria research. Here we review both improvements made in these techniques and the recent achievement of the in vitro growth of mosquito stages from ookinete to infective sporozoite.

Molecular interactions between Plasmodium and its insect vectors

Our understanding of the intricate interactions between the malarial parasite and the mosquito vector is complicated both by the number and diversity of parasite and vector species, and by the experimental inaccessibility of phenomena under investigation. Steady developments in techniques to study the parasite in the mosquito have recently been augmented by methods to culture in their entirety the sporogonic stages of some parasite species. These, together with the new saturation technologies, and genetic transformation of both parasite and vector will permit penetrating studies into an exciting and largely unknown area of parasite-host interactions, an understanding of which must result in the development of new intervention strategies. This microreview highlights key areas of current basic molecular interest, and identifies numerous lacunae in our knowledge that must be filled if we are to make rational decisions for future control strategies. It will conclude by trying to explain why in the opinion of this reviewer understanding malaria-mosquito interactions may be critical to our future attempts to limit a disease of growing global importance.

Dressed for success: the surface coats of insect-borne protozoan parasites

Three major human diseases, malaria, sleeping sickness and leishmaniasis, are caused by protozoan parasites that are transmitted by blood-sucking insects. These insects are not mere 'flying syringes' that mechanically transfer parasites from one mammal to the next. Instead, they provide a specific environment--albeit not a particularly hospitable one--in which the parasites differentiate, proliferate and migrate to the correct tissues to ensure transmission to the next mammalian host. Recent studies on the role of parasite surface molecules in insect vectors have delivered some surprises and could provide insights on ways to interrupt transmission.

Complete development of mosquito phases of the malaria parasite in vitro

Methods for reproducible in vitro development of the mosquito stages of malaria parasites to produce infective sporozoites have been elusive for over 40 years. We have cultured gametocytes of Plasmodium berghei through to infectious sporozoites with efficiencies similar to those recorded in vivo and without the need for salivary gland invasion. Oocysts developed extracellularly in a system whose essential elements include co-cultured Drosophila S2 cells, basement membrane matrix, and insect tissue culture medium. Sporozoite production required the presence of para-aminobenzoic acid. The entire life cycle of P. berghei, a useful model malaria parasite, can now be achieved in vitro.

A comparative genomic analysis of two distant diptera, the fruit fly, Drosophila melanogaster, and the malaria mosquito, Anopheles gambiae

Genome evolution entails changes in the DNA sequence of genes and intergenic regions, changes in gene numbers, and also changes in gene order along the chromosomes. Genes are reshuffled by chromosomal rearrangements such as deletions/insertions, inversions, translocations, and transpositions. Here we report a comparative study of genome organization in the main African malaria vector, Anopheles gambiae, relative to the recently determined sequence of the Drosophila melanogaster genome. The ancestral lines of these two dipteran insects are thought to have separated approximately 250 Myr, a long period that makes this genome comparison especially interesting. Sequence comparisons have identified 113 pairs of putative orthologs of the two species. Chromosomal mapping of orthologous genes reveals that each polytene chromosome arm has a homolog in the other species. Between 41% and 73% of the known orthologous genes remain linked in the respective homologous chromosomal arms, with the remainder translocated to various nonhomologous arms. Within homologous arms, gene order is extensively reshuffled, but a limited degree of conserved local synteny (microsynteny) can be recognized.

The role of Plasmodium berghei ookinete proteins in binding to basal lamina components and transformation into oocysts

The ookinete is a motile form of the malaria parasite that travels from the midgut lumen of the mosquito, invades the epithelial cells and settles beneath the basal lamina. The events surrounding cessation of ookinete motility and its transformation into an oocyst are poorly understood, but interaction between components of the basal lamina and the parasite surface has been implicated. Here we report that interactions occur between basal lamina constituents and ookinete proteins and that these interactions inhibit motility and are likely to be involved in transformation to an oocyst. Plasmodium berghei ookinetes bound weakly to microtitre plate wells coated with fibronectin and much more strongly to wells coated with laminin and collagen IV. A 1:1 mixture of collagen and laminin significantly enhanced binding. Binding increased with time of incubation up to 10 h and different components showed different binding profiles with time. Two parasite molecules were shown to act as ligands for basal lamina components. Western blots demonstrated that the surface molecule Pbs21 bound strongly to laminin but not to collagen IV whereas a 215 kDa molecule (possibly PbCTRP) bound to both laminin and collagen IV. Furthermore up to 90% inhibition of binding of ookinetes to collagen IV/laminin combination occurred if parasites were pre-incubated with anti-Pbs21 monoclonal antibody 13.1. Some transformation of ookinetes to oocysts occurred in wells coated with laminin or laminin/collagen IV combinations but collagen IV alone did not trigger transformation. No binding or transformation occurred in uncoated wells. Our data support the suggestion that ookinete proteins Pbs21 and a 215 kDa protein may have multiple roles including interactions with midgut basal lamina components that cause binding, inhibit motility and trigger transformation.