Molecular interactions between Plasmodium and its insect vectors

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.

Susceptibility of Anopheles aquasalis and An. darlingi to Plasmodium vivax VK210 and VK247

The susceptibility of Anopheles aquasalis (F3 generation) and An. darlingi (F1 generation) to Plasmodium vivax circumsporozoite protein phenotypes from a limited number of blood samples of malaria patients in Belém, state of Pará, Brazil, was examined A polymerase chain reaction was used to determine the P. vivax phenotypes in blood samples and the blood-fed infected mosquitoes were dissected and tested by ELISA. In all patient infections, more infected An. aquasalis and An. darlingi were positive for VK210 compared with VK247.

The effects of blood feeding and exogenous supply of tryptophan on the quantities of xanthurenic acid in the salivary glands of Anopheles stephensi (Diptera: Culicidae)

Xanthurenic acid (XA), produced as a byproduct during the biosynthesis of insect eye pigment (ommochromes), is a strong inducer of Plasmodium gametogenesis at very low concentrations. In previous studies, it was shown that XA is present in Anopheles stephensi (Diptera: Culicidae) mosquito salivary glands and that during blood feeding the mosquitoes ingested their own saliva into the midgut. Considering these two facts together, it is therefore likely that XA is discharged with saliva during blood feeding and is swallowed into the midgut where it exerts its effect on Plasmodium gametocytes. However, the quantities of XA in the salivary glands and midgut are unknown. In this study, we used high performance liquid chromatography with electrochemical detection to detect and quantify XA in the salivary glands and midgut. Based on the results of this study, we found 0.28+/-0.05 ng of XA in the salivary glands of the mosquitoes, accounting for 10% of the total XA content in the mosquito whole body. The amounts of XA in the salivary glands reduced to 0.13+/-0.06 ng after mosquitoes ingested a blood meal. Approximately 0.05+/-0.01 ng of XA was detected in the midgut of nonblood fed An. stephensi mosquitoes. By adding synthetic tryptophan as a source of XA into larval rearing water (2 mM) or in sugar meals (10 mM), we evaluated whether XA levels in the mosquito (salivary glands, midgut, and whole body) were boosted and the subsequent effect on infectivity of Plasmodium berghei in the treated mosquito groups. A female specific increase in XA content was observed in the whole body and in the midgut of mosquito groups where tryptophan was added either in the larval water or sugar meals. However, XA in the salivary glands was not affected by tryptophan addition to larval water, and surprisingly it reduced when tryptophan was added to sugar meals. The P. berghei oocyst loads in the mosquito midguts were lower in mosquitoes fed tryptophan treated sugar meals than in mosquitoes reared on tryptophan treated larval water. Our results suggest that mosquito nutrition may have a significant impact on whole body and midgut XA levels in mosquitoes. We discuss the observed parasite infectivity results in relation to XA's relationship with malaria parasite development in mosquitoes.

Transgenic Plasmodium berghei sporozoites expressing beta-galactosidase for quantification of sporozoite transmission

Malaria transmission occurs during a blood-meal of an infected Anopheles mosquito. Visualization and quantification of sporozoites along the journey from the mosquito midgut, where they develop, to the vertebrate liver, their final target organ, is important for understanding many aspects of sporozoite biology. Here we describe the generation of Plasmodium berghei parasites that express the reporter gene lacZ as a stable transgene, under the control of the sporozoite-specific CSP promoter. Transgenic sporozoites expressing beta-galactosidase can be simply visualized and quantified in an enzymatic assay. In addition, these sporozoites can be used to quantify sporozoites deposited in subcutaneous tissue during natural infection.

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.

Protein kinases as targets for antimalarial intervention: Kinomics, structure-based design, transmission-blockade, and targeting host cell enzymes

The surge of interest in protein kinases as targets for chemotherapeutic intervention in a number of diseases such as cancer and neurodegenerative disorders has stimulated research aimed at determining whether enzymes of this class might also be considered as targets in the context of diseases caused by parasitic protists. Here, we present an overview of recent developments in this field, concentrating (i) on the benefits gained from the availability of genomic databases for a number of parasitic protozoa, (ii) on the emerging field of structure-aided design of inhibitors targeting protein kinases of parasitic protists, (iii) on the concept known as transmission-blockade, whereby kinases implicated in the development of the parasite in their arthropod vector might be targeted to interfere with disease transmission, and (iv) on the possibility of controlling parasitic diseases through the inhibition of host cell protein kinases that are required for the establishment of infection by the parasites.

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.

Exit of Plasmodium sporozoites from oocysts is an active process that involves the circumsporozoite protein

Plasmodium sporozoites develop within oocysts residing in the mosquito midgut. Mature sporozoites exit the oocysts, enter the hemolymph, and invade the salivary glands. The circumsporozoite (CS) protein is the major surface protein of salivary gland and oocyst sporozoites. It is also found on the oocyst plasma membrane and on the inner surface of the oocyst capsule. CS protein contains a conserved motif of positively charged amino acids: region II-plus, which has been implicated in the initial stages of sporozoite invasion of hepatocytes. We investigated the function of region II-plus by generating mutant parasites in which the region had been substituted with alanines. Mutant parasites produced normal numbers of sporozoites in the oocysts, but the sporozoites were unable to exit the oocysts. In in vitro as well, there was a profound delay, upon trypsin treatment, in the release of mutant sporozoites from oocysts. We conclude that the exit of sporozoites from oocysts is an active process that involves the region II-plus of CS protein. In addition, the mutant sporozoites were not infective to young rats. These findings provide a new target for developing reagents that interfere with the transmission of malaria.

Malaria affects hundreds of millions of people, and kills at least 1 million children per year. The infective stages of the malaria parasites, named “sporozoites,” are found in the salivary gland of Anopheles mosquitoes, and are injected along with the saliva during blood feeding. From the skin, sporozoites enter the blood circulation and invade liver cells where the parasites multiply. When they exit the liver, these parasites infect blood cells and can cause severe symptoms. If ingested by mosquitoes, the blood-stage parasites continue their lifecycle in the insect stomach. Thousands of sporozoites are formed within a cyst-like structure (oocyst). The sporozoites come out of the oocyst and infect the salivary gland, where they remain until injected back into humans. Malaria parasites are increasingly resistant to drugs, mosquitoes are difficult to eliminate, and effective vaccines are not yet available. New tools to combat malaria are urgently needed. One exciting approach, although the application is in the distant future, is to release in endemic areas genetically modified mosquitoes that are resistant to parasite growth. This paper provides a new target for generating these “transmission-block” mosquitoes and shows that the exit of sporozoites from the oocysts is an active process that requires the enzymatic digestion of components of the oocyst wall. If these enzymes are inhibited in transgenic mosquitoes, sporozoites will never reach the salivary gland.

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.

A malarial cysteine protease is necessary for Plasmodium sporozoite egress from oocysts

The Plasmodium life cycle is a sequence of alternating invasive and replicative stages within the vertebrate and invertebrate hosts. How malarial parasites exit their host cells after completion of reproduction remains largely unsolved. Inhibitor studies indicated a role of Plasmodium cysteine proteases in merozoite release from host erythrocytes. To validate a vital function of malarial cysteine proteases in active parasite egress, we searched for target genes that can be analyzed functionally by reverse genetics. Herein, we describe a complete arrest of Plasmodium sporozoite egress from Anopheles midgut oocysts by targeted disruption of a stage-specific cysteine protease. Our findings show that sporozoites exit oocysts by parasite-dependent proteolysis rather than by passive oocyst rupture resulting from parasite growth. We provide genetic proof that malarial cysteine proteases are necessary for egress of invasive stages from their intracellular compartment and propose that similar cysteine protease–dependent mechanisms occur during egress from liver-stage and blood-stage schizonts.

Prediction of the general transcription factors associated with RNA polymerase II in Plasmodium falciparum: conserved features and differences relative to other eukaryotes

BACKGROUND

To date, only a few transcription factors have been identified in the genome of the parasite Plasmodium falciparum, the causative agent of malaria. Moreover, no detailed molecular analysis of its basal transcription machinery, which is otherwise well-conserved in the crown group of eukaryotes, has yet been reported. In this study, we have used a combination of sensitive sequence analysis methods to predict the existence of several parasite encoded general transcription factors associated with RNA polymerase II.

RESULTS

Several orthologs of general transcription factors associated with RNA polymerase II can be predicted among the hypothetical proteins of the P. falciparum genome using the two-dimensional Hydrophobic Cluster Analysis (HCA) together with profile-based search methods (PSI-BLAST). These predicted orthologous genes encoding putative transcription factors include the large subunit of TFIIA and two candidates for its small subunit, the TFIIE beta-subunit, which would associate with the previously known TFIIE alpha-subunit, the TFIIF beta-subunit, as well as the p62/TFB1 subunit of the TFIIH core. Within TFIID, the putative orthologs of TAF1, TAF2, TAF7 and TAF10 were also predicted. However, no candidates for TAFs with classical histone fold domain (HFD) were found, suggesting an unusual architecture of TFIID complex of RNA polymerase II in the parasite.

CONCLUSION

Taken together, these results suggest that more general transcription factors may be present in the P. falciparum proteome than initially thought. The prediction of these orthologous general transcription factors opens the way for further studies dealing with transcriptional regulation in P. falciparum. These alternative and sensitive sequence analysis methods can help to identify candidates for other transcriptional regulatory factors in P. falciparum. They will also facilitate the prediction of biological functions for several orphan proteins from other apicomplexan parasites such as Toxoplasma gondii, Cryptosporidium parvum and Eimeria.

Functional Genomic Analysis of Midgut Epithelial Responses in Anopheles during Plasmodium Invasion

BACKGROUND

The malaria parasite Plasmodium must complete a complex developmental life cycle within Anopheles mosquitoes before it can be transmitted into the human host. One day after mosquito infection, motile ookinetes traverse the midgut epithelium and, after exiting to its basal site facing the hemolymph, develop into oocysts. Previously, we have identified hemolymph factors that can antagonize or promote parasite development.

RESULTS

We profiled on a genomic scale the transcriptional responses of the A. gambiae midgut to P. berghei and showed that more than 7% of the assessed mosquito transcriptome is differentially regulated during invasion. The profiles suggested that actin- and microtubule-cytoskeleton remodeling is a major response of the epithelium to ookinete penetration. Other responses encompass components of innate immunity, extracellular-matrix remodeling, and apoptosis. RNAi-dependent gene silencing identified both parasite antagonists and agonists among regulators of actin dynamics and revealed that actin polymerization is inhibitory to the invading parasite. Combined transcriptional and reverse-genetic analysis further identified an unexpected dual role of the lipid-trafficking machinery of the hemolymph for both parasite and mosquito-egg development.

CONCLUSIONS

We conclude that the determinants of malaria-parasite development in Anopheles include components not only of systemic humoral immunity but also of intracellular, local epithelial reactions. These results provide novel mechanistic insights for understanding malaria transmission in the mosquito vector.

The Plasmodium parasite--a 'new' challenge for insect innate immunity

Though lacking adaptive immunity, insects possess a powerful innate immune system, a genome-encoded defence machinery used to confront infections. Studies in the fruit fly Drosophila melanogaster revealed a remarkable capacity of the innate immune system to differentiate between and subsequently respond to different bacteria and fungi. However, hematophagous compared to non-hematophagous insects encounter additional blood-borne infectious agents, such as parasites and viruses, during their lifetime. Anopheles mosquitoes become infected with the malaria parasite Plasmodium during feeding on infected human hosts and may then transmit the parasite to new hosts during subsequent bites. Whether Anopheles has developed mechanisms to confront these infections is the subject of this review. Initially, we review our current understanding of innate immune reactions and give an overview of the Anopheles immune system as revealed through comparative genomic analyses. Then, we examine and discuss the capacity of mosquitoes to recognize and respond to infections, especially to Plasmodium, and finally, we explore approaches to investigate and potentially utilize the vector immune competence to prevent pathogen transmission. Such approaches constitute a new challenge for insect immunity research, a challenge for global health.

Comparative and functional genomics of the innate immune system in the malaria vector Anopheles gambiae

In much of Africa, the mosquito Anopheles gambiae is the major vector of human malaria, a devastating infectious disease caused by Plasmodium parasites. Vector and parasite interact at multiple stages and locations, and the nature and effectiveness of this reciprocal interaction determines the success of transmission. Many of the interactions engage the mosquito's innate immunity, a primitive but very effective defense system. In some cases, the mosquito kills the parasite, thus blocking the transmission cycle. However, not all interactions are antagonistic; some represent immune evasion. The sequence of the A. gambiae genome revealed numerous potential components of the innate immune system, and it established that they evolve rapidly, as summarized in the present review. Their rapid evolution by gene family expansion diversification as well as the prevalence of haplotype alleles in the best-studied families may reflect selective adaptation of the immune system to the exigencies of multiple immune challenges in a variety of ecologic niches. As a follow-up to the comparative genomic analysis, the development of functional genomic methodologies has provided novel opportunities for understanding the immune system and the nature of its interactions with the parasite. In this context, identification of both Plasmodium antagonists and protectors in the mosquito represents a significant conceptual advance. In addition to providing fundamental understanding of primitive immune systems, studies of mosquito interactions with the parasite open unprecedented opportunities for novel interventions against malaria transmission. The generation of transgenic mosquitoes that resist malaria infection in the wild and the development of antimalarial 'smart sprays' capable of disrupting interactions that are protective of the parasite, or reinforcing others that are antagonistic, represent technical challenges but also immense opportunities for improvement of global health.

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

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

Mosquito immune responses against malaria parasites

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

Mosquito midgut barriers to malaria parasite development

Malaria is one of the deadliest infectious diseases and kills more than one million people every year. For transmission to occur, the malaria parasite has to complete an elaborate developmental program in hostile mosquito environment. Thus, understanding the molecular mechanisms by which mosquitoes limit the parasite development may lead to new methods for controlling malaria. There has been considerable progress during the last decade in this research area. This review focuses on the mosquito response to midgut invasion of the malaria parasite and examines the role of mosquito digestive enzymes, peritrophic matrix and microvillar proteins as barriers to parasite development.

Immune responses in Anopheles gambiae

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

Real-time, in vivo analysis of malaria ookinete locomotion and mosquito midgut invasion

Invasion of the Anopheles mosquito midgut by the Plasmodium ookinete is a critical step in the malaria transmission cycle. We have generated a fluorescent P. berghei transgenic line that expresses GFP in the ookinete and oocyst stages, and used it to perform the first real-time analysis of midgut invasion in the living mosquito as well as in explanted intact midguts whose basolateral plasma membranes were vitally stained. These studies permitted detailed analysis of parasite motile behaviour in the midgut and cell biological analysis of the invasion process. Throughout its journey, the ookinete displays distinct modes of motility: stationary rotation, translocational spiralling and straight-segment motility. Spiralling is based on rotational motility combined with translocation steps and changes in direction, which are achieved by transient attachments of the ookinete's trailing end. As it moves from the apical to the basal side of the midgut epithelium, the ookinete uses a predominant intracellular route and appears to glide on the membrane in foldings of the basolateral domain. However, it traverses serially the cytoplasm of several midgut cells before entering and migrating through the basolateral intercellular space to access the basal lamina. The invaded cells commit apoptosis, and their expulsion from the epithelium invokes wound repair mechanisms including extensive lamellipodia crawling. A 'hood' of lamellipodial origin, provided by the invaded cell, covers the ookinete during its egress from the epithelium. The flexible ookinete undergoes shape changes and temporary constrictions associated with passage through the plasma membranes. Similar observations were made in both A. gambiae and A. stephensi, demonstrating the conservation of P. berghei interactions with these vectors.

Effects of Mosquito Genes on Plasmodium Development

Malaria parasites must complete a complex developmental cycle in an Anopheles mosquito vector before transmission to a vertebrate host. Sexual development of the parasite in the midgut is initiated in the lumen immediately after the mosquito ingests infected blood, and the resulting ookinetes must traverse the surrounding epithelial layer before transforming into oocysts. The innate immune system of the mosquito is activated during midgut invasion, but to date, no evidence has been published identifying mosquito immune genes that affect parasite development. Here, we show by gene silencing that an Anopheles gambiae leucine rich-repeat protein acts as an antagonist and two C-type lectines act as protective agonists on the development of Plasmodium ookinetes to oocysts.

Plasmodium Biology

The highly A+T rich genomes of human and rodent malarial parasites offer unprecedented glimpses of a lineage that is distinct from other model organisms. Plasmodium is distinguished by the presence of numerous low complexity inserts within globular domains of proteins. It displays several peculiarities in its transcription apparatus, and its DNA repair system appears to favor a certain innate level of mutability. Plasmodium possesses many cell surface molecules with "animal-like" adhesion modules. Potential genetic footprints of the ancestral eukaryotic algal precursor of the apicoplast are also detectable in its genome.

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

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