Immuno-electron microscopic observation of Plasmodium berghei CTRP localization in the midgut of the vector mosquito Anopheles stephensi

Ookinete-Specific Genes and 18S SSU rRNA Evidenced in Plasmodium vivax Selection and Adaptation by Sympatric Vectors

In the southern Pacific coast of Chiapas, Mexico (SM), the two most abundant vector species, Nyssorhynchus albimanus and Anopheles pseudopunctipennis, were susceptible to different Plasmodium vivax Pvs25/28 haplotypes. To broaden our understanding of the existing P. vivax in the area, genes encoding proteins relevant for ookinete development and the 18S rRNA were studied. P. vivax infectivity (percentage of infected mosquitoes and oocyst numbers) was evaluated by simultaneously feeding infected blood samples from patients to Ny. albimanus and An. pseudopunctipennis female mosquitoes. Three infectivity patterns were identified: one group of parasites were more infective to An. pseudopunctipennis than to Ny. albimanus, another group was more infective to Ny. albimanus, while a third group infected both vectors similarly. In 29 parasite isolates, the molecular variations of ookinete-specific genes and the 18S rRNA-type S were analyzed. Using concatenated sequences, phylogenetic trees, and Structure analysis, parasite clustering within SM isolates and between these and those from other geographical origins were investigated. A ML phylogenetic tree resolved two parasite lineages: PvSM-A and PvSM-B. They were associated to a different 18S rRNA variant. PvSM-A parasites had 18S rRNA variant rV2 and correspond to parasites causing high oocyst infection in Ny. albimanus. A new ML tree and Structure analysis, both comprising global sequences, showed PvSM-A clustered with Latin American parasites. Meanwhile, all isolates of PvSM-B had 18S rRNA variant rV1 and remained as unique genetic cluster comprising two subgroups: PvSM-Ba, producing high infection in An. pseudopunctipennis, and PvSM-Bb, causing similar oocyst infection in both vector species. PvSM-A parasites were genetically similar to parasites from South America. Meanwhile, PvSM-B were exclusive to southern Mexico and share ancestry with Asian parasites. The results suggest that these lineages evolved separately, likely by geographic and vector restriction.

New ultrastructural analysis of the invasive apparatus of the Plasmodium ookinete

Invasion of the mosquito midgut by the Plasmodium ookinete determines the success of transmission of malaria parasites from humans to mosquitoes and therefore, is a potential target for molecular intervention. Here, we show higher-resolution ultrastructural details of developing and mature P. gallinaceum ookinetes than previously available. Improved fixation and processing methods yielded substantially improved transmission electron micrographs of ookinetes, particularly with regard to visualization of subcellular secretory and other organelles. These new images provide new insights into the synthesis and function of vital invasive machinery focused on the following features: apical membrane protrusions presumptively used for attachment and protein secretion, dark spherical bodies at the apical end of the mature ookinete, and the presence of a dense array of micronemes apposed to microtubules at the apical end of the ookinete involved in constitutive secretion. This work advances understanding of the molecular and cellular details of the Plasmodium ookinete and provides the basis of future, more detailed mechanistic experimentation on the biology of the Plasmodium ookinete.

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

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

Vital functions of the malarial ookinete protein, CTRP, reside in the A domains

The transformation of malaria ookinetes into oocysts occurs in the mosquito midgut and is a major bottleneck for parasite transmission. The secreted ookinete surface protein, circumsporozoite- and thrombospondin-related adhesive protein (TRAP)-related protein (CTRP), is essential for this transition and hence constitutes a potential target for malaria transmission blockade. CTRP is a modular multidomain protein containing six tandem von Willebrand factor A-like (A) domains and seven tandem thrombospondin type I repeat-like (TS) domains. Here we present, to our knowledge, the first structure-function analysis of CTRP using genetically modified Plasmodium berghei parasites expressing mutant versions of the ctrp gene. Our data show that the A domains of CTRP are critical for ookinete gliding motility and oocyst formation whilst, unexpectedly, its TS domains are fully redundant. These results may have important implications for the design of CTRP-based transmission blocking strategies.

Apical surface expression of aspartic protease Plasmepsin 4, a potential transmission-blocking target of the plasmodium ookinete

To invade its definitive host, the mosquito, the malaria parasite must cross the midgut peritrophic matrix that is composed of chitin cross-linked by chitin-binding proteins and then develop into an oocyst on the midgut basal lamina. Previous evidence indicates that Plasmodium ookinete-secreted chitinase is important in midgut invasion. The mechanistic role of other ookinete-secreted enzymes in midgut invasion has not been previously examined. De novo mass spectrometry sequencing of a protein obtained by benzamidine affinity column of Plasmodium gallinaceum ookinete axenic culture supernatant demonstrated the presence of an ookinete-secreted plasmepsin, an aspartic protease previously only known to be present in the digestive vacuole of asexual stage malaria parasites. This plasmepsin, the ortholog of Plasmodium falciparum plasmepsin 4, was designated PgPM4. PgPM4 and PgCHT2 (the P. gallinaceum ortholog of P. falciparum chitinase PfCHT1) are both localized on the ookinete apical surface, and both are present in micronemes. Aspartic protease inhibitors (peptidomimetic and natural product), calpain inhibitors, and anti-PgPM4 monoclonal antibodies significantly reduced parasite infectivity for mosquitoes. These results suggest that plasmepsin 4, previously known only to function in the digestive vacuole of asexual blood stage Plasmodium, plays a role in how the ookinete interacts with the mosquito midgut interactions as it becomes an oocyst. These data are the first to delineate a role for an aspartic protease in mediating Plasmodium invasion of the mosquito and demonstrate the potential for plasmepsin 4 as a malaria transmission-blocking vaccine target.

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

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

No TRAP, no invasion

Host-cell invasion by apicomplexan parasites is a unique process that is powered by the gliding motility motor and requires a transmembrane link between the parasite cytoskeleton and the host cell. The thrombospondin-related anonymous protein (TRAP) from Plasmodium plays such a part during sporozoite invasion by linking to actin through its cytoplasmic tail while binding to hepatocytes via its extracellular portion. In recent years, there have been major advances in the identification and characterization of TRAP-family proteins in the other invasive stages of Plasmodium as well as other Apicomplexa. This review summarizes the recent experimental data on these TRAP-family proteins, focusing on their structure and function.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

The developmental migration of Plasmodium in mosquitoes

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

Plasmodium ookinete invasion of the mosquito midgut

The Plasmodium ookinete is the developmental stage of the malaria parasite that invades the mosquito midgut. The ookinete faces two physical barriers in the midgut which it must traverse to become an oocyst: the chitin- and protein-containing peritrophic matrix; and the midgut epithelial cell. This chapter will consider basic aspects of ookinete biology, molecules known to be involved in midgut invasion, and cellular processes of the ookinete that facilitate parasite invasion. Detailed knowledge of these mechanisms may be exploitable in the future towards developing novel strategies of blocking malaria transmission.

Interactions between malaria and mosquitoes: the role of apoptosis in parasite establishment and vector response to infection

Malaria parasites of the genus Plasmodium are transmitted from host to host by mosquitoes. Sexual reproduction occurs in the blood meal and the resultant motile zygote, the ookinete, migrates through the midgut epithelium and transforms to an oocyst under the basal lamina. After sporogony, sporozoites are released into the mosquito haemocoel and invade the salivary gland before injection when next the mosquito feeds on a host. Interactions between parasite and vector occur at all stages of the establishment and development of the parasite and some of these result in the death of parasite and host cells by apoptosis. Infection-induced programmed cell death occurs in patches of follicular epithelial cells in the ovary, resulting in follicle resorption and thus a reduction in egg production. We argue that fecundity reduction will result in a change in resource partitioning that may benefit the parasite. Apoptosis also occurs in cells of the midgut epithelium that have been invaded by the parasite and are subsequently expelled into the midgut. In addition, the parasite itself dies by a process of programmed cell death (PCD) in the lumen of the midgut before invasion has occurred. Caspase-like activity has been detected in the cytoplasm of the ookinetes, despite the absence of genes homologous to caspases in the genome of this, or any, unicellular eukaryote. The putative involvement of other cysteine proteases in ancient apoptotic pathways is discussed. Potential signal pathways for induction of apoptosis in the host and parasite are reviewed and we consider the evidence that nitric oxide may play a role in this induction. Finally, we consider the hypothesis that death of some parasites in the midgut will limit infection and thus prevent vector death before the parasites have developed into mature sporozoites.

Gene expression in Plasmodium: from gametocytes to sporozoites

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

Cloning and expression of a Na+, K+-ATPase α-subunit from Taenia solium (TNaK1α)

The Na(+), K(+)-ATPase are membrane-associated enzymes that transport Na(+) and K(+) across the membrane generating chemical and electrical gradients, essential to maintain the resting potential for the excitation of myocytons and neurons and for transport of nutrients. The cDNA encoding a full-length isoform of Taenia solium Na(+), K(+)-ATPase alpha-subunit (TNaK1alpha) was isolated from a cysticercal cDNA library. TNaK1alpha has 1014 amino acids and a predicted molecular mass of 111,989Da. The protein displays strong sequence homology and conserved motifs typical of Na(+), K(+)-ATPase alpha-subunits. Northern and Southern hybridizations reveal a TNaK1alpha mRNA of about 3.7kb, which is encoded by a single gene. Polyclonal antibodies raised against a synthetic peptide corresponding to the NH(2)-terminal sequence of TNaK1alpha recognized a 100-kDa polypeptide in the membrane fraction of adult and larval stages of T. solium and other Taenia species. Immunolocalization studies using the same antibodies revealed that the TNaK1 is preferentially localized in muscle cells and protonephridial ducts, and in small quantities in the tegument of T. solium cysticerci.

Plasmodium ookinete-secreted proteins secreted through a common micronemal pathway are targets of blocking malaria transmission

The mosquito midgut ookinete stage of the malaria parasite, Plasmodium, possesses microneme secretory organelles that mediate locomotion and midgut wall egress to establish sporogonic stages and subsequent transmission. The purpose of this study was 2-fold: 1) to determine whether there exists a single micronemal population with respect to soluble and membrane-associated secreted proteins; and 2) to evaluate the ookinete micronemal proteins chitinase (PgCHT1), circumsporozoite and TRAP-related protein (CTRP), and von Willebrand factor A domain-related protein (WARP) as immunological targets eliciting sera-blocking malaria parasite infectivity to mosquitoes. Indirect immunofluorescence localization studies in Plasmodium gallinaceum using specific antisera showed that all three proteins are distributed intracellularly with a similar granular cytoplasmic appearance and with focal concentration of PgCHT1 and PgCTRP, but not PgWARP, at the ookinete apical end. Immunogold double-labeling electron microscopy, using antisera against the membrane-associated protein CTRP and the soluble WARP, showed that these two proteins co-localized to the same micronemal population. Within the microneme CTRP was associated peripherally at the microneme membrane, whereas PgCHT1 and WARP were diffuse within the micronemal lumen. Sera produced against Plasmodium falciparum WARP significantly reduced the infectivity of P. gallinaceum to Aedes aegypti and P. falciparum to Anopheles mosquitoes. Antisera against PgCTRP and PgCHT1 also significantly reduced the infectivity of P. gallinaceum for A. aegypti. These results support the concept that ookinete micronemal proteins may constitute a general class of malaria transmission-blocking vaccine candidates.

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

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