Sexual development of malarial parasites in their mosquito vectors

Dynamics of DNA Replication during Male Gametogenesis in the Malaria Parasite Plasmodium Falciparum

Malaria parasites undergo a single phase of sexual reproduction in their complex lifecycle. It involves specialised, sexually committed cells called gametocytes, which develop rapidly into mature gametes and mate upon entering the mosquito midgut. Gamete development is unique, involving unprecedentedly fast replication to produce male gametes. Within ~15 minutes a male gametocyte replicates its ~23 Mb genome three times to produce 8 genomes, segregates these into newly-assembled flagellated gametes and releases them to seek female gametes. Here, for the first time, we use fluorescent labelling of de novo DNA synthesis to follow this process at the whole-cell and single-molecule levels. We make several novel observations, including characterising the origin recognition complex protein Orc1 for the first time in gametocytes, finding that cytokinesis is uncoupled from DNA replication (implying a lack of cell cycle checkpoints), and that the single-molecule dynamics of DNA replication are entirely different from the dynamics in asexual schizogony.

The Dynamic Roles of the Inner Membrane Complex in the Multiple Stages of the Malaria Parasite

Apicomplexan parasites, such as human malaria parasites, have complex lifecycles encompassing multiple and diverse environmental niches. Invading, replicating, and escaping from different cell types, along with exploiting each intracellular niche, necessitate large and dynamic changes in parasite morphology and cellular architecture. The inner membrane complex (IMC) is a unique structural element that is intricately involved with these distinct morphological changes. The IMC is a double membrane organelle that forms de novo and is located beneath the plasma membrane of these single-celled organisms. In Plasmodium spp. parasites it has three major purposes: it confers stability and shape to the cell, functions as an important scaffolding compartment during the formation of daughter cells, and plays a major role in motility and invasion. Recent years have revealed greater insights into the architecture, protein composition and function of the IMC. Here, we discuss the multiple roles of the IMC in each parasite lifecycle stage as well as insights into its sub-compartmentalization, biogenesis, disassembly and regulation during stage conversion of P. falciparum.

Kinase signalling in Plasmodium sexual stages and interventions to stop malaria transmission

The symptoms of malaria, one of the infectious diseases with the highest mortality and morbidity world-wide, are caused by asexual parasites replicating inside red blood cells. Disease transmission, however, is effected by non-replicating cells which have differentiated into male or female gametocytes. These are the forms infectious to mosquito vectors and the insects are the only hosts where parasite sexual reproduction can take place. Malaria is thus a complex infection in which pharmacological treatment of symptoms may still allow transmission for long periods, while pharmacological blockage of infectivity may not cure symptoms. The process of parasite sexual differentiation and development is still being revealed but it is clear that kinase-mediated signalling mechanisms play a significant role. This review attempts to summarise our limited current knowledge on the signalling mechanisms involved in the transition from asexual replication to sexual differentiation and reproduction, with a brief mention to the effects of current treatments on the sexual stages and to some of the difficulties inherent in developing pharmacological interventions to curtail disease transmission.

Optimized in vitro production of Plasmodium vivax ookinetes

Previous reports have described obtaining mature Plasmodium vivax ookinetes in vitro using blood from infected patients using a simplified, field-based protocol. Here, we report protocols that produce improved P. vivax ookinete yields and morphological development. Optimal conditions included induction of gametogenesis using 10 mM Tris, 170 mM NaCl, 10 mM glucose, 25 mM NaHCO(3), and 100 μM xanthurenic acid for 90 minutes at pH 8.0-8.2, followed by culture in RPMI-1640, 50 mg/mL hypoxanthine, 25 mM HEPES, 29 mM NaHCO(3), 2 mM L-glutamine, and 20% fetal bovine serum at pH 8.4 for 36 hours. Ookinetes were produced in 86% (18/21) of optimized in vitro cultures; yields ranged from 6.5 × 10(4) to 2.8 × 10(6); percent gametocyte conversion ranged from 1.4% to 4.7%. This improved method is suitable for preparation of P. vivax ookinetes in quantities sufficient for biochemical, molecular, and cell biological analysis where basic laboratory facilities are in proximity to patients with vivax malaria.

Development of reverse-transcription PCR techniques to analyse the density and sex ratio of gametocytes in genetically diverse Plasmodium chabaudi infections

We have developed cross-genotype and genotype-specific quantitative reverse-transcription PCR (qRT-PCR) assays to detect and quantify the number of parasites, transmission stages (gametocytes) and male gametocytes in blood stage Plasmodium chabaudi infections. Our cross-genotype assays are reliable, repeatable and generate counts that correlate strongly (R(2)s>90%) with counts expected from blood smears. Our genotype-specific assays can distinguish and quantify different stages of genetically distinct parasite clones (genotypes) in mixed infections and are as sensitive as our cross-genotype assays. Using these assays we show that gametocyte density and gametocyte sex ratios vary during infections for two genetically distinct parasite lines (genotypes) and present the first data to reveal how sex ratio is affected when each genotype experiences competition in mixed-genotype infections. Successful infection of mosquito vectors depends on both gametocyte density and their sex ratio and we discuss the implications of competition in genetically diverse infections for transmission success.

Babesia bigemina sexual stages are induced in vitro and are specifically recognized by antibodies in the midgut of infected Boophilus microplus ticks

Babesia bigemina, a causative agent of bovine babesiosis, is transmitted from one bovine to another only by infected ticks. The life cycle of B. bigemina includes a sexual phase in the tick host; however, molecules from sexual stages of any Babesia species have not been characterized. This is the first report of the induction of sexual stages of any Babesia species in vitro, free of tick antigens. Intraerythrocytic parasites were cultured in vitro for 20h using an induction medium. Extraerythrocytic parasites were first seen 3h post induction; elongated stages with long projections appeared at 6h post induction and by 9h they paired and fused to form larger stages. Round zygotes appeared 20h post induction. Moreover, by using Percoll gradients, sexual stages were purified free of contaminating intraerythrocytic stages. Purified parasites were used to generate polyclonal antibodies, which specifically bound to antigens expressed in sexual stages induced in vitro, but not to antigens expressed in intraerythrocytic stages. Importantly, these antibodies specifically identified sexual stages from midguts of female Boophilus microplus ticks fed on infected cattle.

A malaria scavenger receptor-like protein essential for parasite development

Malaria parasites suffer severe losses in the mosquito as they cross the midgut, haemolymph and salivary gland tissues, in part caused by immune responses of the insect. The parasite compensates for these losses by multiplying during the oocyst stage to form the infectious sporozoites. Upon human infection, malaria parasites are again attenuated by sustained immune attack. Here, we report a single copy gene that is highly conserved amongst Plasmodium species that encodes a secreted protein named PxSR. The predicted protein is composed of a unique combination of metazoan protein domains that have been previously associated with immune recognition/activation and lipid/protein adhesion interactions at the cell surface, namely: (i) scavenger receptor cysteine rich (SRCR); (ii) pentraxin (PTX); (iii) polycystine-1, lipoxygenase, alpha toxin (LH2/PLAT); (iv) Limulus clotting factor C, Coch-5b2 and Lgl1 (LCCL). In our assessment the PxSR molecule is completely novel in biology and is only found in Apicomplexa parasites. We show that PxSR is expressed in sporozoites of both human and rodent malaria species. Disruption of the PbSR gene in the rodent malaria parasite P. berghei results in parasites that form normal numbers of oocysts, but fail to produce any sporozoites. We suggest that, in addition to a role in sporogonic development, PxSR may have a multiplicity of functions.

Plasmodium ookinete development in the mosquito midgut: a case of reciprocal manipulation

The ookinete is one of the most important stages of Plasmodium development in the mosquito. It is morphologically and biochemically distinct from the earlier sexual stages--gametocytes and zygote, and from the later stages--oocyst and sporozoites. Development to ookinete allows the parasite to escape from the tightly packed blood bolus, to cross the sturdy peritrophic matrix (PM), to be protected from the digestive environment of the midgut lumen, and to invade the gut epithelium. The success of each of these activities may depend on the degree of the biochemical and physical barriers in the mosquito (such as density of blood bolus, thickness of peritrophic matrix, proteolytic activities in the gut lumen etc.) and the ability of the ookinete to overcome these barriers. Ookinete motility, secretion of chitinase, resistance to the digestive enzymes, and recognition/invasion of the midgut epithelium all may play crucial roles in the transformation to oocyst. The overall sporogonic development of Plasmodium, therefore, depends on the results of the two-way manipulations between the parasite and the vector mosquito. Study of ookinete development and of the cellular and biochemical complexities of the mosquito gut may therefore lead to the design of novel strategies to block the transmission of malaria. This article reviews the intricate interactions between the parasite and the mosquito midgut in the context of development and transmission of Plasmodium parasites.