Analysis of the Plasmodium and Anopheles transcriptomes during oocyst differentiation

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.

PfMORC protein regulates chromatin accessibility and transcriptional repression in the human malaria parasite, P. falciparum

The environmental challenges the human malaria parasite, Plasmodium falciparum, faces during its progression into its various lifecycle stages warrant the use of effective and highly regulated access to chromatin for transcriptional regulation. Microrchidia (MORC) proteins have been implicated in DNA compaction and gene silencing across plant and animal kingdoms. Accumulating evidence has shed light into the role MORC protein plays as a transcriptional switch in apicomplexan parasites. In this study, using CRISPR/Cas9 genome editing tool along with complementary molecular and genomics approaches, we demonstrate that PfMORC not only modulates chromatin structure and heterochromatin formation throughout the parasite erythrocytic cycle, but is also essential to the parasite survival. Chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) experiments suggest that PfMORC binds to not only sub-telomeric regions and genes involved in antigenic variation but is also most likely a key modulator of stage transition. Protein knockdown experiments followed by chromatin conformation capture (Hi-C) studies indicate that downregulation of PfMORC induces the collapse of the parasite heterochromatin structure leading to its death. All together these findings confirm that PfMORC plays a crucial role in chromatin structure and gene regulation, validating this factor as a strong candidate for novel antimalarial strategies.

Preliminary characterization of Plasmodium vivax sporozoite antigens as pre-erythrocytic vaccine candidates

Plasmodium vivax pre-erythrocytic (PE) vaccine research has lagged far behind efforts to develop Plasmodium falciparum vaccines. There is a critical gap in our knowledge of PE antigen targets that can induce functionally inhibitory neutralizing antibody responses. To overcome this gap and guide the selection of potential PE vaccine candidates, we considered key characteristics such as surface exposure, essentiality to infectivity and liver stage development, expression as recombinant proteins, and functional immunogenicity. Selected P. vivax sporozoite antigens were surface sporozoite protein 3 (SSP3), sporozoite microneme protein essential for cell traversal (SPECT1), sporozoite surface protein essential for liver-stage development (SPELD), and M2 domain of MAEBL. Sequence analysis revealed little variation occurred in putative B-cell and T-cell epitopes of the PE candidates. Each antigen was tested for expression as refolded recombinant proteins using an established bacterial expression platform and only SPELD failed. The successfully expressed antigens were immunogenic in vaccinated laboratory mice and were positively reactive with serum antibodies of P. vivax-exposed residents living in an endemic region in Thailand. Vaccine immune antisera were tested for reactivity to native sporozoite proteins and for their potential vaccine efficacy using an in vitro inhibition of liver stage development assay in primary human hepatocytes quantified on day 6 post-infection by high content imaging analysis. The anti-PE sera produced significant inhibition of P. vivax sporozoite invasion and liver stage development. This report provides an initial characterization of potential new PE candidates for a future P. vivax vaccine.

MAEBL Contributes to Plasmodium Sporozoite Adhesiveness

The sole currently approved malaria vaccine targets the circumsporozoite protein-the protein that densely coats the surface of sporozoites, the parasite stage deposited in the skin of the mammalian host by infected mosquitoes. However, this vaccine only confers moderate protection against clinical diseases in children, impelling a continuous search for novel candidates. In this work, we studied the importance of the membrane-associated erythrocyte binding-like protein (MAEBL) for infection by Plasmodium sporozoites. Using transgenic parasites and live imaging in mice, we show that the absence of MAEBL reduces Plasmodium berghei hemolymph sporozoite infectivity to mice. Moreover, we found that maebl knockout (maebl-) sporozoites display reduced adhesion, including to cultured hepatocytes, which could contribute to the defects in multiple biological processes, such as in gliding motility, hepatocyte wounding, and invasion. The maebl- defective phenotypes in mosquito salivary gland and liver infection were reverted by genetic complementation. Using a parasite line expressing a C-terminal myc-tagged MAEBL, we found that MAEBL levels peak in midgut and hemolymph parasites but drop after sporozoite entry into the salivary glands, where the labeling was found to be heterogeneous among sporozoites. MAEBL was found associated, not only with micronemes, but also with the surface of mature sporozoites. Overall, our data provide further insight into the role of MAEBL in sporozoite infectivity and may contribute to the design of future immune interventions.

Plasmodium sporozoites on the move: Switching from cell traversal to productive invasion of hepatocytes

Parasites of the genus Plasmodium, the etiological agent of malaria, are transmitted through the bite of anopheline mosquitoes, which deposit sporozoites into the host skin. Sporozoites migrate through the dermis, enter the bloodstream, and rapidly traffic to the liver. They cross the liver sinusoidal barrier and traverse several hepatocytes before switching to productive invasion of a final one for replication inside a parasitophorous vacuole. Cell traversal and productive invasion are functionally independent processes that require proteins secreted from specialized secretory organelles known as micronemes. In this review, we summarize the current understanding of how sporozoites traverse through cells and productively invade hepatocytes, and discuss the role of environmental sensing in switching from a migratory to an invasive state. We propose that timely controlled secretion of distinct microneme subsets could play a key role in successful migration and infection of hepatocytes. A better understanding of these essential biological features of the Plasmodium sporozoite may contribute to the development of new strategies to fight against the very first and asymptomatic stage of malaria.

Transcriptomics and proteomics reveal two waves of translational repression during the maturation of malaria parasite sporozoites

Plasmodium sporozoites are transmitted from infected mosquitoes to mammals, and must navigate the host skin and vasculature to infect the liver. This journey requires distinct proteomes. Here, we report the dynamic transcriptomes and proteomes of both oocyst sporozoites and salivary gland sporozoites in both rodent-infectious Plasmodium yoelii parasites and human-infectious Plasmodium falciparum parasites. The data robustly define mRNAs and proteins that are upregulated in oocyst sporozoites (UOS) or upregulated in infectious sporozoites (UIS) within the salivary glands, including many that are essential for sporozoite functions in the vector and host. Moreover, we find that malaria parasites use two overlapping, extensive, and independent programs of translational repression across sporozoite maturation to temporally regulate protein expression. Together with gene-specific validation experiments, these data indicate that two waves of translational repression are implemented and relieved at different times during sporozoite maturation, migration and infection, thus promoting their successful development and vector-to-host transition.

Here, the authors report transcriptomes and proteomes of oocyst sporozoite and salivary gland sporozoite stages in rodent-infectious Plasmodium yoelii parasites and human infectious Plasmodium falciparum parasites and define two waves of translational repression during sporozoite maturation.

Characterization of a protozoan Phosducin-like protein-3 (PhLP-3) reveals conserved redox activity

We recently identified three novel thioredoxin-like genes in the genome of the protozoan parasite Plasmodium that belong to the Phosducin-like family of proteins (PhLP). PhLPs are small cytosolic proteins hypothesized to function in G-protein signaling and protein folding. Although PhLPs are highly conserved in eukaryotes from yeast to mammals, only a few representatives have been experimentally characterized to date. In addition, while PhLPs contain a thioredoxin domain, they lack a CXXC motif, a strong indicator for redox activity, and it is unclear whether members of the PhLP family are enzymatically active. Here, we describe PbPhLP-3 as the first phosducin-like protein of a protozoan organism, Plasmodium berghei. Initial transcription analysis revealed continuous low-level expression of pbphlp-3 throughout the complex Plasmodium life cycle. Attempts to knockout pbphlp-3 in P. berghei did not yield live parasites, suggesting an essential role for the gene in Plasmodium. We cloned, expressed and purified PbPhLP-3 and determined that the recombinant protein is redox active in vitro in a thioredoxin-coupled redox assay. It also has the capacity to reduce the organic compound tert-Butyl hydroperoxide (TBHP) in vitro, albeit at low efficiency. Sequence analysis, structural modeling, and site-directed mutagenesis revealed a conserved cysteine in the thioredoxin domain to be the redox active residue. Lastly, we provide evidence that recombinant human PhLP-3 exhibits redox activity similar to that of PbPhLP-3 and suggest that redox activity may be conserved in PhLP-3 homologs of other species. Our data provide new insight into the function of PhLP-3, which is hypothesized to act as co-chaperones in the folding and regulation of cytoskeletal proteins. We discuss the potential implications of PhLP-3 as a thioredoxin-target protein and possible links between the cellular redox network and the eukaryotic protein folding machinery.

Systems Biology-Based Investigation of Host-Plasmodium Interactions

Malaria is a serious, complex disease caused by parasites of the genus Plasmodium. Plasmodium parasites affect multiple tissues as they evade immune responses, replicate, sexually reproduce, and transmit between vertebrate and invertebrate hosts. The explosion of omics technologies has enabled large-scale collection of Plasmodium infection data, revealing systems-scale patterns, mechanisms of pathogenesis, and the ways that host and pathogen affect each other. Here, we provide an overview of recent efforts using systems biology approaches to study host-Plasmodium interactions and the biological themes that have emerged from these efforts. We discuss some of the challenges in using systems biology for this goal, key research efforts needed to address those issues, and promising future malaria applications of systems biology.

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.

Recent advances in recombinant protein-based malaria vaccines

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Protein-based vaccines remain the cornerstone approach for B cell and antibody induction against leading target malaria antigens.

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Advances in antigen selection, immunogen design and epitope-focusing are advancing the field.

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New heterologous expression platforms are enabling cGMP production of next-generation protein vaccines.

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Next-generation antigens, protein-based immunogens and virus-like particle (VLP) delivery platforms are in clinical development.

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Protein-based vaccines will form part of a highly effective multi-component/multi-stage/multi-antigen subunit formulation against malaria.

Plasmodium parasites are the causative agent of human malaria, and the development of a highly effective vaccine against infection, disease and transmission remains a key priority. It is widely established that multiple stages of the parasite's complex lifecycle within the human host and mosquito vector are susceptible to vaccine-induced antibodies. The mainstay approach to antibody induction by subunit vaccination has been the delivery of protein antigen formulated in adjuvant. Extensive efforts have been made in this endeavor with respect to malaria vaccine development, especially with regard to target antigen discovery, protein expression platforms, adjuvant testing, and development of soluble and virus-like particle (VLP) delivery platforms. The breadth of approaches to protein-based vaccines is continuing to expand as innovative new concepts in next-generation subunit design are explored, with the prospects for the development of a highly effective multi-component/multi-stage/multi-antigen formulation seeming ever more likely. This review will focus on recent progress in protein vaccine design, development and/or clinical testing for a number of leading malaria antigens from the sporozoite-, merozoite- and sexual-stages of the parasite's lifecycle–including PfCelTOS, PfMSP1, PfAMA1, PfRH5, PfSERA5, PfGLURP, PfMSP3, Pfs48/45 and Pfs25. Future prospects and challenges for the development, production, human delivery and assessment of protein-based malaria vaccines are discussed.

CD81 is required for rhoptry discharge during host cell invasion by Plasmodium yoelii sporozoites

Plasmodium sporozoites are transmitted by Anopheles mosquitoes and first infect the liver of their mammalian host, where they develop as liver stages before the onset of erythrocytic infection and malaria symptoms. Sporozoite entry into hepatocytes is an attractive target for anti-malarial prophylactic strategies but remains poorly understood at the molecular level. Apicomplexan parasites invade host cells by forming a parasitophorous vacuole that is essential for parasite development, a process that involves secretion of apical organelles called rhoptries. We previously reported that the host membrane protein CD81 is required for infection by Plasmodium falciparum and Plasmodium yoelii sporozoites. CD81 acts at an early stage of infection, possibly at the entry step, but the mechanisms involved are still unknown. To investigate the role of CD81 during sporozoite entry, we generated transgenic P. yoelii parasites expressing fluorescent versions of three known rhoptry proteins, RON2, RON4 and RAP2/3. We observed that RON2 and RON4 are lost following rhoptry discharge during merozoite and sporozoite entry. In contrast, our data indicate that RAP2/3 is secreted into the parasitophorous vacuole during infection. We further show that sporozoite rhoptry discharge occurs only in the presence of CD81, providing the first direct evidence for a role of CD81 during sporozoite productive invasion.

Enterobacter-activated mosquito immune responses to Plasmodium involve activation of SRPN6 in Anopheles stephensi

Successful development of Plasmodium in the mosquito is essential for the transmission of malaria. A major bottleneck in parasite numbers occurs during midgut invasion, partly as a consequence of the complex interactions between the endogenous microbiota and the mosquito immune response. We previously identified SRPN6 as an immune component which restricts Plasmodium berghei development in the mosquito. Here we demonstrate that SRPN6 is differentially activated by bacteria in Anopheles stephensi, but only when bacteria exposure occurs on the lumenal surface of the midgut epithelium. Our data indicate that AsSRPN6 is strongly induced following exposure to Enterobacter cloacae, a common component of the mosquito midgut microbiota. We conclude that AsSRPN6 is a vital component of the E. cloacae-mediated immune response that restricts Plasmodium development in the mosquito An. stephensi.

[Plasmodium sporozoite entry pathways during malaria liver infection]

Plasmodium parasites, the causative agents of malaria, are transmitted by female Anopheles mosquitoes, which inject sporozoites into the skin of the host. The motile sporozoites enter the blood stream and, upon reaching the liver, transform into liver stages inside hepatocytes. The parasites enter host cells actively, using their actomyosin motor machinery to propel themselves through a specialized structure called junction. Penetration inside an invagination of the host cell plasma membrane results in the formation of the parasitophorous vacuole, which is essential for parasite further development. The mechanisms of sporozoite entry into host cells remain poorly understood at the molecular level. We reported for the first time a host factor required for infection of hepatocytes by Plasmodium sporozoites, the tetraspanin CD81, which also serves as a receptor for the hepatitis C virus. CD81 is involved at an early step of the infection, however no evidence for a direct interaction between CD81 and the parasite could be found. Although sporozoites can use several independent pathways to enter hepatocytes, depending on the parasite species and the host cell type, we showed that P. falciparum, the deadliest human malaria parasite, depends on CD81 to infect hepatocytes. We identified structural determinants in the CD81 large extracellular domain, and demonstrated that CD81 function is regulated by its molecular environment in specialized tetraspanin-enriched membrane microdomains. Based on these data we propose that CD81 acts indirectly during malaria infection, by interacting with other essential but still unidentified factor(s), possibly a receptor for the sporozoites, within specific microdomains of the hepatocyte plasma membrane.

Stage-specific depletion of myosin A supports an essential role in motility of malarial ookinetes

Functional analysis of Plasmodium genes by classical reverse genetics is currently limited to mutants that are viable during erythrocytic schizogony, the pathogenic phase of the malaria parasite where transfection is performed. Here, we describe a conceptually simple experimental approach to study the function of genes essential to the asexual blood stages in a subsequent life cycle stage by a promoter-swap approach. As a proof of concept we targeted the unconventional class XIV myosin MyoA, which is known to be required for Toxoplasma gondii tachyzoite locomotion and host cell invasion. By placing the corresponding Plasmodium berghei gene, PbMyoA, under the control of the apical membrane antigen 1 (AMA1) promoter, expression in blood stages is maintained but switched off during transmission to the insect vector, i.e. ookinetes. In those mutant ookinetes gliding motility is entirely abolished resulting in a complete block of life cycle progression in Anopheles mosquitoes. Similar approaches should permit the analysis of gene function in the mosquito forms that are shared with the erythrocytic stages of the malaria parasite.

In search of the origin of FREPs: characterization of Aplysia californica fibrinogen-related proteins

All haemolymph lectins with uniquely juxtaposed N-terminal domain similar to the immunoglobulin superfamily (IgSF) and C-terminal fibrinogen (FBG) termed FBG-related proteins (FREP) are documented till now only in the pulmonate mollusc Biomphalaria glabrata. Using genomic WGS database we have found two FREP genes from marine opistobranch Aplysia californica named AcFREP1 and AcFREP2. The AcFREP1 and AcFREP2 mRNA molecules have been subsequently isolated from cDNA of sea hare larvae as well as adult mollusc tissues. These genes encode proteins (504 and 510aa respectively) with domain architecture typical for FREPs with two N-terminal IgSF domains and C-terminal FBG domain. Although cDNA sequences of AcFREP1 and AcFREP2 are 81% identical, their genomic structure is entirely different: AcFREP1 is intronless and AcFREP2 is encoded in four exons. These genes are paralogous pair in which AcFREP2 is a parental gene and AcFREP1 is the new transposed copy that has lost the introns (retrogene). Using RT-PCR analysis, expression of AcFREP1 and AcFREP2 was shown to be developmentally and tissue-specific and no constitutive expression in haemocytes was found. The overall frequency of nucleotide substitutions in genomic DNA trace sequences of coding region of the AcFREP1 and AcFREP2 is not higher than in the sequences of control conserved genes (actin, FMRFamide). Thus, previously reported high diversification of Biomphalaria FREP gene, BgFREP3, is not detected in Aplysia FREPs. A search for FREP homologs in other available complete genome of mollusc, Lottia gigantea (Patellogastropoda), a representative of the evolutionary earliest gastropod clade, did not reveal any DNA sequences coding for similar lectins. We suggest that unique domain architecture of FREPs is an evolutionary novelty that appeared and evolved only within one branch of Protostomata species, exclusively in heterobranch molluscs (Pulmonata and Opistobranchia).

Anopheles aquasalis Infected by Plasmodium vivax displays unique gene expression profiles when compared to other malaria vectors and plasmodia

Malaria affects 300 million people worldwide every year and is endemic in 22 countries in the Americas where transmission occurs mainly in the Amazon Region. Most malaria cases in the Americas are caused by Plasmodium vivax, a parasite that is almost impossible to cultivate in vitro, and Anopheles aquasalis is an important malaria vector. Understanding the interactions between this vector and its parasite will provide important information for development of disease control strategies. To this end, we performed mRNA subtraction experiments using A. aquasalis 2 and 24 hours after feeding on blood and blood from malaria patients infected with P. vivax to identify changes in the mosquito vector gene induction that could be important during the initial steps of infection. A total of 2,138 clones of differentially expressed genes were sequenced and 496 high quality unique sequences were obtained. Annotation revealed 36% of sequences unrelated to genes in any database, suggesting that they were specific to A. aquasalis. A high number of sequences (59%) with no matches in any databases were found 24 h after infection. Genes related to embryogenesis were down-regulated in insects infected by P. vivax. Only a handful of genes related to immune responses were detected in our subtraction experiment. This apparent weak immune response of A. aquasalis to P. vivax infection could be related to the susceptibility of this vector to this important human malaria parasite. Analysis of some genes by real time PCR corroborated and expanded the subtraction results. Taken together, these data provide important new information about this poorly studied American malaria vector by revealing differences between the responses of A. aquasalis to P. vivax infection, in relation to better studied mosquito-Plasmodium pairs. These differences may be important for the development of malaria transmission-blocking strategies in the Americas.

Differential transcriptomic responses of Biomphalaria glabrata (Gastropoda, Mollusca) to bacteria and metazoan parasites, Schistosoma mansoni and Echinostoma paraensei (Digenea, Platyhelminthes)

A 70-mer-oligonucleotide-based microarray (1152 features) that emphasizes stress and immune responses factors was constructed to study transcriptomic responses of the snail Biomphalaria glabrata to different immune challenges. In addition to sequences with relevant putative ID and Gene Ontology (GO) annotation, the array features non-immune factors and unknown B. glabrata ESTs for functional gene discovery. The transcription profiles of B. glabrata (3 biological replicates, each a pool of 5 snails) were recorded at 12h post-wounding, exposure to Gram negative or Gram positive bacteria (Escherichia coli and Micrococcus luteus, respectively), or infection with compatible trematode parasites (Schistosoma mansoni or Echinostoma paraensei, 20 miracidia/snail), relative to controls, using universal reference RNA. The data were subjected to Significance Analysis for Microarrays (SAM), with a false positive rate (FPR) <or=10%. Wounding yielded a modest differential expression profile (27 up/21 down) with affected features mostly dissimilar from other treatments. Partially overlapping, yet distinct expression profiles were recorded from snails challenged with E. coli (83 up/20 down) or M. luteus (120 up/42 down), mostly showing up-regulation of defense and stress-related features. Significantly altered expression of selected immune features indicates that B. glabrata detects and responds differently to compatible trematodes. Echinostoma paraensei infection was associated mostly with down-regulation of many (immune-) transcripts (42 up/68 down), whereas S. mansoni exposure yielded a preponderance of up-regulated features (140 up/23 down), with only few known immune genes affected. These observations may reflect the divergent strategies developed by trematodes during their evolution as specialized pathogens of snails to negate host defense responses. Clearly, the immune defenses of B. glabrata distinguish and respond differently to various immune challenges.

Plasmodium sporozoite invasion of the mosquito salivary gland

About one to two million people die of malaria every year. Anopheline mosquitoes are the obligatory vectors of Plasmodium spp., the causative agent of malaria. For transmission to occur, the parasite has to undergo a complex developmental programme in the mosquito, culminating with sporozoite invasion of the salivary glands. Strong circumstantial evidence suggests that sporozoite invasion requires specific interactions and recognition between sporozoite and salivary gland proteins. Here we review recent progress towards the elucidation of invasion mechanisms.

Reality check for malaria proteomics

Post-translationally modified protein isoforms are common in red blood cell stages of the malaria parasite.

New studies highlight the wide diversity of post-translational protein modifications in the intra-erythrocytic stages of the malaria parasite, raising new avenues for inquiry.

A case for sequencing the genome of Musca domestica (Diptera: Muscidae)

House flies are carriers of >100 devastating diseases that have severe consequences for human and animal health. Despite the fact that it is a passive vector, a key bottleneck to progress in controlling the human diseases transmitted by house flies is lack of knowledge of the basic molecular biology of this species. Sequencing of the house fly genome will provide important inroads to the discovery of novel target sites for house fly control, understanding of the house fly immune response, rapid elucidation of insecticide resistance genes, and understanding of numerous aspects of the basic biology of this insect pest. The ability of the house fly to prosper in a remarkably septic environment motivates analysis of its innate immune system. Its polymorphic sex determination system, with male-determining factors on either the autosomes or the Y chromosome, is ripe for a genomic analysis. Sequencing of the house fly genome would allow the first opportunity to study the interactions between a pest insect and its parasitoid (Nasonia vitripennis) at the whole genome level. In addition, the house fly is well placed phylogenetically to leverage analysis of the multiple Dipteran genomes that have been sequenced (including several mosquito and Drosophila species). The community of researchers investigating Musca domestica are well prepared and highly motivated to apply genomic analyses to their widely varied research programs.

Distinct roles of Plasmodium rhomboid 1 in parasite development and malaria pathogenesis

Invasion of host cells by the malaria parasite involves recognition and interaction with cell-surface receptors. A wide variety of parasite surface proteins participate in this process, most of which are specific to the parasite's particular invasive form. Upon entry, the parasite has to dissociate itself from the host-cell receptors. One mechanism by which it does so is by shedding its surface ligands using specific enzymes. Rhomboid belongs to a family of serine proteases that cleave cell-surface proteins within their transmembrane domains. Here we identify and partially characterize a Plasmodium berghei rhomboid protease (PbROM1) that plays distinct roles during parasite development. PbROM1 localizes to the surface of sporozoites after salivary gland invasion. In blood stage merozoites, PbROM1 localizes to the apical end where proteins involved in invasion are also present. Our genetic analysis suggests that PbROM1 functions in the invasive stages of parasite development. Whereas wild-type P. berghei is lethal to mice, animals infected with PbROM1 null mutants clear the parasites efficiently and develop long-lasting protective immunity. The results indicate that P. berghei Rhomboid 1 plays a nonessential but important role during parasite development and identify rhomboid proteases as potential targets for disease control.

Malaria is one of the major infectious diseases and is responsible for the death of more than a million people, mostly children under the age of five. Plasmodium, the causative agent of malaria, is transmitted by female Anopheles mosquitoes. Successful development of the parasite requires efficient recognition, attachment, and invasion of host cells. Several parasite cell-surface molecules have been implicated in these processes and may require proteolytic processing in order for the parasite to complete invasion. Rhomboid family proteins are serine proteases that cleave within the transmembrane region of their substrates. Here, we use a genetic approach to study the function of Plasmodium berghei rhomboid 1 (PbROM1). PbROM1 is expressed in both vertebrate and mosquito stages of parasite development, and the protein is present in secretory organelles that contain other parasite molecules required for invasion. We find that PbROM1 is required for efficient infection of both the mosquito and the vertebrate host. Interestingly, we also find that mice infected with ROM1(−) parasites clear the infection efficiently and are protected upon subsequent wild-type parasite challenge. Our study suggests a role for PbROM1 throughout parasite development and identifies ROM1 as a target for disease intervention.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Expression profiling and binding properties of fibrinogen-related proteins (FREPs), plasma proteins from the schistosome snail host Biomphalaria glabrata

A growing body of evidence suggests an important role for fibrinogen-like proteins in innate immunity in both vertebrates and invertebrates. It has been shown that fibrinogen-related proteins (FREPs), plasma proteins present in the freshwater snail Biomphalaria glabrata, the intermediate host for the human blood fluke Schistosoma mansoni, are diverse and involved in snail innate defense responses. To gain further insight into the functions of FREPs, recombinant FREP proteins (rFREPs) were produced in Escherichia coli and antibodies (Abs) were raised against the corresponding rFREPs. We first show that most FREP proteins exist in their native conformation in snail hemolymph as multimeric proteins. Western blot analyses reveal that expression of multiple FREPs including FREP4 in plasma from M line and BS-90 snails, which are susceptible and resistant to S. mansoni infection, respectively, is up-regulated significantly after infection with the trematode Echinostoma paraensei. Moreover, our assays demonstrate that FREPs are able to bind E. paraensei sporocysts and their secretory/excretory products (SEPs), and a variety of microbes (Gram-positive and Gram-negative bacteria and yeast). Furthermore, this binding capability shows evidence of specificity with respect to pathogen type; for example, 65-75-kDa FREPs (mainly FREP4) bind to E. paraensei sporocysts and their SEPs whereas 95-kDa and 125-kDa FREPs bind the microbes assayed. Our results suggest that FREPs can recognize a wide range of pathogens, from prokaryotes to eukaryotes, and different categories of FREPs seem to exhibit functional specialization with respect to the pathogen encountered.

Proteomic profiling of Plasmodium sporozoite maturation identifies new proteins essential for parasite development and infectivity

Plasmodium falciparum sporozoites that develop and mature inside an Anopheles mosquito initiate a malaria infection in humans. Here we report the first proteomic comparison of different parasite stages from the mosquito—early and late oocysts containing midgut sporozoites, and the mature, infectious salivary gland sporozoites. Despite the morphological similarity between midgut and salivary gland sporozoites, their proteomes are markedly different, in agreement with their increase in hepatocyte infectivity. The different sporozoite proteomes contain a large number of stage specific proteins whose annotation suggest an involvement in sporozoite maturation, motility, infection of the human host and associated metabolic adjustments. Analyses of proteins identified in the P. falciparum sporozoite proteomes by orthologous gene disruption in the rodent malaria parasite, P. berghei, revealed three previously uncharacterized Plasmodium proteins that appear to be essential for sporozoite development at distinct points of maturation in the mosquito. This study sheds light on the development and maturation of the malaria parasite in an Anopheles mosquito and also identifies proteins that may be essential for sporozoite infectivity to humans.

Human malaria is caused by Plasmodium falciparum, a unicellular protozoan parasite that is transmitted by Anopheles mosquitoes. An infectious mosquito injects saliva containing sporozoite forms of the parasite and these then migrate from the skin to the liver, where they establish an infection. Many intervention strategies are currently focused on preventing the establishment of infection by sporozoites. Clearly, an understanding of the biology of the sporozoite is essential for developing new intervention strategies. Sporozoites are produced within the oocyst, located on the outside wall of the mosquito midgut, and migrate after release from the oocysts to the salivary glands where they are stored as mature infectious forms. Comparison of the proteomes of sporozoites derived from either the oocyst or from the salivary gland reveals remarkable differences in the protein content of these stages despite their similar morphology. The changes in protein content reflect the very specific preparations the sporozoites make in order to establish an infection of the liver. Analysis of the function of several previously uncharacterized, conserved proteins revealed proteins essential for sporozoite development at distinct points of their maturation.

The transmembrane isoform of Plasmodium falciparum MAEBL is essential for the invasion of Anopheles salivary glands

Malaria transmission depends on infective stages in the mosquito salivary glands. Plasmodium sporozoites that mature in midgut oocysts must traverse the hemocoel and invade the mosquito salivary glands in a process thought to be mediated by parasite ligands. MAEBL, a homologue of the transmembrane EBP ligands essential in merozoite invasion, is expressed abundantly in midgut sporozoites. Alternative splicing generates different MAEBL isoforms and so it is unclear what form is functionally essential. To identify the MAEBL isoform required for P. falciparum (NF54) sporozoite invasion of salivary glands, we created knockout and allelic replacements each carrying CDS of a single MAEBL isoform. Only the transmembrane form of MAEBL is essential and is the first P. falciparum ligand validated as essential for invasion of Anopheles salivary glands. MAEBL is the first P. falciparum ligand experimentally determined to be essential for this important step in the life cycle where the vector becomes infectious for transmitting sporozoites to people. With an increasing emphasis on advancing vector-based transgenic methods for suppression of malaria, it is important that this type of study, using modern molecular genetic tools, is done with the agent of the human disease. Understanding what P. falciparum sporozoite ligands are critical for mosquito transmission will help validate targets for vector-based transmission-blocking strategies.

Determination of nitric oxide metabolites, nitrate and nitrite, in Anopheles culicifacies mosquito midgut and haemolymph by anion exchange high-performance liquid chromatography: plausible mechanism of refractoriness

Background

The diverse physiological and pathological role of nitric oxide in innate immune defenses against many intra and extracellular pathogens, have led to the development of various methods for determining nitric oxide (NO) synthesis. NO metabolites, nitrite (NO₂^-) and nitrate (NO₃^-) are produced by the action of an inducible Anopheles culicifacies NO synthase (AcNOS) in mosquito mid-guts and may be central to anti-parasitic arsenal of these mosquitoes.

Method

While exploring a plausible mechanism of refractoriness based on nitric oxide synthase physiology among the sibling species of An. culicifacies, a sensitive, specific and cost effective high performance liquid chromatography (HPLC) method was developed, which is not influenced by the presence of biogenic amines, for the determination of NO₂^- and NO₃^- from mosquito mid-guts and haemolymph.

Results

This method is based on extraction, efficiency, assay reproducibility and contaminant minimization. It entails de-proteinization by centrifugal ultra filtration through ultracel 3 K filter and analysis by high performance anion exchange liquid chromatography (Sphereclone, 5 μ SAX column) with UV detection at 214 nm. The lower detection limit of the assay procedure is 50 pmoles in all midgut and haemolymph samples. Retention times for NO₂^- and NO₃^- in standards and in mid-gut samples were 3.42 and 4.53 min. respectively. Assay linearity for standards ranged between 50 nM and 1 mM. Recoveries of NO₂^- and NO₃^- from spiked samples (1–100 μM) and from the extracted standards (1–100 μM) were calculated to be 100%. Intra-assay and inter assay variations and relative standard deviations (RSDs) for NO₂^- and NO₃^- in spiked and un-spiked midgut samples were 5.7% or less. Increased levels NO₂^- and NO₃^- in midguts and haemolymph of An. culicifacies sibling species B in comparison to species A reflect towards a mechanism of refractoriness based on AcNOS physiology.

Conclusion

HPLC is a sensitive and accurate technique for identification and quantifying pmole levels of NO metabolites in mosquito midguts and haemolymph samples that can be useful for clinical investigations of NO biochemistry, physiology and pharmacology in various biological samples.

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.

Perspectives in the control of infectious diseases by transgenic mosquitoes in the post-genomic era--a review

Arthropod-borne diseases caused by a variety of microorganisms such as dengue virus and malaria parasites afflict billions of people worldwide imposing major economic and social burdens. Despite many efforts, vaccines against diseases transmitted by mosquitoes, with the exception of yellow fever, are not available. Control of such infectious pathogens is mainly performed by vector management and treatment of affected individuals with drugs. However, the numbers of insecticide-resistant insects and drug-resistant parasites are increasing. Therefore, inspired in recent years by a lot of new data produced by genomics and post-genomics research, several scientific groups have been working on different strategies to control infectious arthropod-borne diseases. This review focuses on recent advances and perspectives towards construction of transgenic mosquitoes refractory to malaria parasites and dengue virus transmission.

Gene expression studies in mosquitoes

Research on gene expression in mosquitoes is motivated by both basic and applied interests. Studies of genes involved in hematophagy, reproduction, olfaction, and immune responses reveal an exquisite confluence of biological adaptations that result in these highly-successful life forms. The requirement of female mosquitoes for a bloodmeal for propagation has been exploited by a wide diversity of viral, protozoan and metazoan pathogens as part of their life cycles. Identifying genes involved in host-seeking, blood feeding and digestion, reproduction, insecticide resistance and susceptibility/refractoriness to pathogen development is expected to provide the bases for the development of novel methods to control mosquito-borne diseases. Advances in mosquito transgenesis technologies, the availability of whole genome sequence information, mass sequencing and analyses of transcriptomes and RNAi techniques will assist development of these tools as well as deepen the understanding of the underlying genetic components for biological phenomena characteristic of these insect species.

Fibrinogen-bearing protein genes in the snail Biomphalaria glabrata: characterization of two novel genes and expression studies during ontogenesis and trematode infection

All fibrinogen (FBG)-bearing proteins documented to date in the freshwater snail Biomphalaria glabrata, the intermediate host of the human blood fluke Schistosoma mansoni, possess the same molecular structure; one or two immunoglobin superfamily (IgSF) domains at the N-terminus and a FBG domain at the C-terminus (named as FBG-related protein (FREP)). Here we report two novel genes that encode FBG-bearing proteins from B. glabrata. Different from all known FREPs, the first gene encodes a protein (657 amino acids (aa)) composed of a long N-terminal region with no sequence homology to any known protein, a middle epidermal growth factor (EGF) repeat region and a C-terminal FBG domain, designated FBG-related molecule (FReM). Differential expression at 2 days post-exposure (dpe) to the trematode S. mansoni or Echinostoma paraensei was found in the S. mansoni susceptible M line and resistant BS-90 snail strains. The second gene is a new member of the FREP family, designated FREP14, which encodes a 399 aa putative secreted protein. FREP14 is different from known FREPs in that it is encoded by a single locus and is not upregulated in early or late stage S. mansoni exposure, but is upregulated in late stage E. paraensei infection. Furthermore, gene expression during the snail's ontogenesis and at a late stage of trematode infection (52 dpe) has been investigated in the two newly identified genes (FReM and FREP14) described in this paper and five representative members of known FREPs (FREPs 2, 3, 4, 12, and 13). A variety of expression patterns were observed, suggestive of functional diversity among the members of FBG-bearing proteins. Our findings further broaden our understanding of the diversity and function of the FBG-bearing protein encoded genes in B. glabrata.

Protocol for Plasmodium falciparum infections in mosquitoes and infection phenotype determination

Once a gene is identified as potentially refractory for malaria, it must be evaluated for its role in preventing Plasmodium infections within the mosquito. This protocol illustrates how the extent of plasmodium infections of mosquitoes can be assayed. The techniques for preparing the gametocyte culture, membrane feeding mosquitoes human blood, and assaying viral titers in the mosquito midgut are demonstrated.

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.

Developmental biology of sporozoite-host interactions in Plasmodium falciparum malaria: implications for vaccine design

The Plasmodium falciparum sporozoite infects different types of cells in a mosquito's salivary glands and human epithelial and Kuppfer cells and hepatocytes. These become differentiated later on, transforming themselves into the invasive red blood cell form, the merozoite. The ability of sporozoites to interact with different types of cells requires a wide variety of mechanisms allowing them to survive in both hosts: mobility, receptor-ligand interactions with different cellular receptors, and transformation and development into other invasive parasite forms, which are vitally important for parasite survival. Sporozoite complexity is reflected in the large quantity of proteins that can be expressed. Some of them have been extensively studied, such as CSP, TRAP, STARP, LSA-1, LSA-3, SALSA, SPECT1, SPECT2, MAEBL, and SPATR, due to their importance in infection and their potential use as vaccines. Our work has been focused on the search for the molecular mechanisms of parasite-host cellular receptor-ligand interactions by identifying amino acid sequences and the critical binding residues from these proteins relevant to parasite invasion. Once such sequences have been identified, it will be possible to modify them to induce a strong immune response against P. falciparum in the experimental Aotus monkey model. This all leads towards developing multistage, multicomponent, subunit-based vaccines that will be effective in eradicating or controlling malaria caused by P. falciparum.

Differential gene expression in the ookinete stage of the malaria parasite Plasmodium berghei

Plasmodium, the malaria parasite, undergoes a complex developmental program in its mosquito vector. The ookinete is the parasite form which invades the mosquito midgut and is an important stage for genetic mixing. To identify genes expressed during ookinete development and mosquito midgut invasion, purified zygotes and ookinetes of the rodent parasite Plasmodium berghei were used to construct a suppression subtractive hybridization cDNA library, enriched in sequences expressed in the ookinete stage. In addition to four genes coding for previously described major ookinete-secreted proteins, we isolated ookinete-expressed sequences representing 18 predicted genes. Their gene products include proteins involved in signal transduction and regulatory processes. For six of these genes our analysis provides the first evidence for expression in the ookinete stage. A majority of the genes are not expressed in the zygote, the preceding developmental stage. Furthermore, four of the genes are also transcribed in sporozoites, and one of these in merozoites, suggesting that they code for proteins with a function common to Plasmodium invasive stages.

Alternative invasion pathways for Plasmodium berghei sporozoites

Invasion of hepatocytes by Plasmodium sporozoites is a prerequisite for establishment of a natural malaria infection. The molecular mechanisms underlying sporozoite invasion are largely unknown. We have previously reported that infection by Plasmodium falciparum and Plasmodium yoelii sporozoites depends on CD81 and cholesterol-dependent tetraspanin-enriched microdomains (TEMs) on the hepatocyte surface. Here we have analyzed the role of CD81 and TEMs during infection by sporozoites from the rodent parasite Plasmodium berghei. We found that depending on the host cell type, P. berghei sporozoites can use several distinct pathways for invasion. Infection of human HepG2, HuH7 and HeLa cells by P. berghei does not depend on CD81 or host membrane cholesterol, whereas both CD81 and cholesterol are required for infection of mouse hepatoma Hepa1-6 cells. In primary mouse hepatocytes, both CD81-dependent and -independent mechanisms participate in P. berghei infection and the relative contribution of the different pathways varies, depending on mouse genetic background. The existence of distinct invasion pathways may explain why P. berghei sporozoites are capable of infecting a wide range of host cell types in vitro. It could also provide a means for human parasites to escape immune responses and face polymorphisms of host receptors. This may have implications for the development of an anti-malarial vaccine targeting sporozoites.

Apoptosis-like death as a feature of malaria infection in mosquitoes

Malaria parasites of the genusPlasmodiummake a hazardous journey through their mosquito vectors. The majority die in the process, many as a result of the action of mosquito defence mechanisms. The mosquito too is not unscathed by the encounter with these parasites. Tissue damage occurs as a result of mid-gut invasion and reproductive fitness is lost when many developing ovarian follicles are resorbed. Here we discuss some of the mechanisms that are involved in killing the parasite and in the self-defence mechanisms employed by the mosquito to repair the mid-gut epithelium and to manipulate resources altering the trade-off position that balances reproduction and survival. In all cases, cells die by apoptotic-like mechanisms. In the midgut cells, apoptosis-induction pathways are being elucidated, the molecules involved in apoptosis are being recognised andDrosophilahomologues sought. The death of ookinetes in the mosquito mid-gut lumen is associated with caspase-like activity and, although homologues of mammalian caspases are not present in the malaria genome, other cysteine proteases that are potential candidates have been discussed. In the ovary, apoptosis of patches of follicular epithelial cells is followed by resorption of the developing follicle and a subsequent loss of egg production in that follicle.

Plasmodium post-genomics: better the bug you know?

Since the publication of the sequence of the genome of Plasmodium falciparum, the major causative agent of human malaria, many post-genomic studies have been completed. Invaluably, these data can now be analysed comparatively owing to the availability of a significant amount of genome-sequence data from several closely related model species of Plasmodium and accompanying global proteome and transcriptome studies. This review summarizes our current knowledge and how this has already been--and will continue to be--exploited in the search for vaccines and drugs against this most significant infectious disease of the tropics.

A transcriptome analysis of the Aedes aegypti vitellogenic fat body

Aedes (Stegomyia) aegypti is an important dengue vector in tropical and subtropical zones throughout the world. A transcriptome of Ae. aegypti vitellogenic fat bodies is described here. The fat body is a dynamic tissue that participates in multiple biochemical functions of intermediate metabolism. A total of 589 randomly selected cDNAs were assembled into 262 clusters based on their primary sequence similarities. The putative translated proteins were classified into categories based on their function in accordance with significant similarity using the BlastX at NCBI FTP site and Pfam (Bateman et al. 2000) and SMART (Schultz et al. 2000) databases. The characterization of transcripts expressed in the fat body of Ae. aegypti at 24 hours post blood meal provides a basic tool for understanding the processes occurring in this organ and could identify putative new genes whose promoters can be used to specifically express transgenes in the fat bodies of Ae. aegypti.

Spreading the seeds of million-murdering death**This title and some subheadings are taken from lines in Ronald Ross' poem In Exile, Reply – What Ails the Solitude, written on 21 August 1897, the day after he made his Nobel-Prize-winning discovery of parasite stages in the mosquito. ‘This day relenting God hath placed within my hand a wondrous thing; and God be praised. At His command, seeking His secret deeds with tears and toiling breath I find thy cunning seeds, O million-murdering Death. I know this little thing a myriad men will save. O Death, where is thy sting, thy victory, O Grave!’: metamorphoses of malaria in the mosquito

Plasmodium spp. undergo a complex obligate developmental cycle within their invertebrate vectors that enables transmission between vertebrate hosts. This developmental cycle involves sexual reproduction and then asexual multiplication, separated by phases of invasion and colonization of distinct vector tissues. As with other stages in the Plasmodium life cycle, there is exquisite adaptation of the malaria parasite to its changing environment as it transforms within the blood of its vertebrate host, through the different tissues of its mosquito vector and onwards to infect a new vertebrate host. Despite the intricacies inherent in these successive transformations, malaria parasites remain staggeringly successful at disseminating through their vertebrate host populations.

An immune-responsive serpin, SRPN6, mediates mosquito defense against malaria parasites

We have functionally analyzed the orthologous SRPN6 genes from Anopheles stephensi and Anopheles gambiae using phylogenetic, molecular, reverse genetic, and cell biological tools. The results strongly implicate SRPN6 in the innate immune response against Plasmodium. This gene belongs to a mosquito-specific gene cluster including three additional Anopheles serpins. SRPN6 expression is induced by Escherichia coli and both rodent and human malaria parasites. The gene is specifically expressed in midgut cells invaded by Plasmodium ookinetes and in circulating and attached hemocytes. Knockdown of SRPN6 expression by RNA interference in susceptible An. stephensi leads to substantially increased parasite numbers, whereas depletion in susceptible An. gambiae delays progression of parasite lysis without affecting the number of developing parasites. However, the An. gambiae SRPN6 knockdown increases the number of melanized parasites in the L3-5 refractory strain and in susceptible G3 mosquitoes depleted of CTL4. These results indicate that AsSRPN6 is involved in the parasite-killing process, whereas AgSRPN6 acts on parasite clearance by inhibiting melanization and/or promoting parasite lysis. We propose that these observed phenotypic differences are due to changed roles of the respective target serine proteases in the two mosquito species.

Identification and characterization of the fibrinogen-like domain of fibrinogen-related proteins in the mosquito, Anopheles gambiae, and the fruitfly, Drosophila melanogaster, genomes

Background

The fibrinogen-like (FBG) domain, which consists of approximately 200 amino acid residues, has high sequence similarity to the C-terminal halves of fibrinogen β and γ chains. Fibrinogen-related proteins (FREPs), which contain FBG domains in their C-terminal region, are found universally in vertebrates and invertebrates. In invertebrates, FREPs are involved in immune responses and other aspects of physiology. To understand the complexity of this family in insects, we analyzed FREPs in the mosquito genome and made comparisons to FREPs in the fruitfly genome.

Results

By using the genome data of the mosquito, Anopheles gambiae, 53 FREPs were identified, whereas only 20 members were found in the Drosophila melanogaster genome. Using sequence profile analysis, we found that FBG domains have high sequence similarity and are highly conserved throughout the FBG domain region. By secondary structure analysis and comparison, the FBG domains of FREPs are predicted to function in recognition of carbohydrates and their derivatives on the surface of microorganisms in innate immunity.

Conclusion

Detailed sequence and structural analysis discloses that the FREP family contains FBG domains that have high sequence similarity in the A. gambiae genome. Expansion of the FREP family in mosquitoes during evolutionary history is mainly accounted for by a major expansion of the FBG domain architecture. The characterization of the FBG domains in the FREP family is likely to aid in the experimental analysis of the ability of mosquitoes to recognize parasites in innate immunity and physiologies associated with blood feeding.

Molecular cloning and comparative analysis of fibrinogen-related proteins from the soft tick Ornithodoros moubata and the hard tick Ixodes ricinus

Among disease-vectors, the evolution of the tick innate immune system is still lagging when compared to insects. Such an investigation, which was initiated, by first cloning and sequencing lectins associated in the innate immunity of invertebrates and having fibrinogen related domains, helped in the sequencing of cDNA encoding for OMFREP from the soft tick, Ornithodoros moubata. Also obtained were Ixoderin A and Ixoderin B cDNA sequences from the hard tick Ixodes ricinus. Tissue-specific expression of OMFREP showed that it was present primarily in the hemocytes and salivary glands. Ixoderin A besides sharing a similar expression profile was also expressed in the midgut. Both showed significantly high homology to the lectin Dorin M, from O. moubata. Further, phylogenetic comparisons between these molecules of the soft and hard ticks showed their relatedness to Tachylectins 5A and 5B, involved in the innate immunity of Tachypleus tridentatus and ficolins from both vertebrates and invertebrates. Ixoderin B showing tissue-specific expression only in the salivary glands and the sequence displaying certain motif differences in homology point towards a possible function different from the other two molecules. This is the first report of lectin-like sequences, with a fibrinogen-domain, from the hard tick I. ricinus and a preliminary phylogenetic study of these tick sequences with related fibrinogen-domain containing sequences highlights a possible role for them in the innate immunity of the ticks.

Gene expression in Plasmodium berghei ookinetes and early oocysts in a co-culture system with mosquito cells

Using an in vitro development system for Plasmodium berghei sporogonic stages and microarray technology we examined parasite gene expression during ookinete invasion of Aedes cells and the ensuing oocyst development. A number of genes were found to be differentially expressed. The most prominent class of up-regulated elements corresponded to products involved in protein synthesis and metabolism. Furthermore, several previously studied genes with a known in vivo developmental profile matched published data. A large number of genes with a hitherto unknown function during the life cycle stages studied also show a differential pattern of expression, indicating the involvement of their products in control and execution of active developmental processes.

Mosquito immunity against Plasmodium

Understanding the molecular mechanisms of the innate immune responses of Anopheles gambiae against Plasmodium parasites is of great importance for current efforts to develop novel strategies for malaria disease control. The parasite undergoes substantial stage-specific losses during its development in the mosquito, which in some cases lead to complete refractoriness of the mosquito against the parasite. The underlying genetics of refractoriness are complex and multifactorial. Completion of the genome sequence of An. gambiae 2 years ago, together with the development of DNA microarrays in this species and the extension of the RNAi technique to adult mosquitoes, has allowed comparative and functional genomic approaches of the mosquito innate immune system. A variety of factors were shown to negatively affect the development of Plasmodium parasites in the mosquito, in some cases leading to complete transmission blockage. In addition, mosquito factors have been identified that play positive roles and are required for successful transmission of the parasite. These findings indicate a highly complex interplay between parasite and vector. Research is continuing to identify new factors involved in this interaction and to decipher the interplay of these molecules and their regulation.