The process of fertilization is critically dependent on the mutual recognition of gametes and in Plasmodium, the male gamete surface protein P48/45 is vital to this process. This protein belongs to a family of 10 structurally related proteins, the so called 6-cys family. To identify the role of additional members of this family in Plasmodium fertilisation, we performed genetic and functional analysis on the five members of the 6-cys family that are transcribed during the gametocyte stage of P. berghei. This analysis revealed that in addition to P48/45, two members (P230 and P47) also play an essential role in the process of parasite fertilization. Mating studies between parasites lacking P230, P48/45 or P47 demonstrate that P230, like P48/45, is a male fertility factor, consistent with the previous demonstration of a protein complex containing both P48/45 and P230. In contrast, disruption of P47 results in a strong reduction of female fertility, while males remain unaffected. Further analysis revealed that gametes of mutants lacking expression of p48/45 or p230 or p47 are unable to either recognise or attach to each other. Disruption of the paralog of p230, p230p, also specifically expressed in gametocytes, had no observable effect on fertilization. These results indicate that the P. berghei 6-cys family contains a number of proteins that are either male or female specific ligands that play an important role in gamete recognition and/or attachment. The implications of low levels of fertilisation that exist even in the absence of these proteins, indicating alternative pathways of fertilisation, as well as positive selection acting on these proteins, are discussed in the context of targeting these proteins as transmission blocking vaccine candidates.

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.

SummaryMalaria parasites invade erythrocytes of their host both for asexual multiplication and for differentiation to male and female gametocytes -the precursor cells of Plasmodium gametes. For further development the parasite is dependent on efficient release of the asexual daughter cells and of the gametes from the host erythrocyte. How malarial parasites exit their host cells remains largely unknown. We here report the characterization of a Plasmodium berghei protein that is involved in egress of both male and female gametes from the host erythrocyte. Protein MDV-1/PEG3, like its Plasmodium falciparum orthologue, is present in gametocytes of both sexes, but more abundant in the female, where it is associated with dense granular organelles, the osmiophilic bodies. Dmdv-1/peg3 parasites in which MDV-1/PEG3 production was abolished by gene disruption had a strongly reduced capacity to form zygotes resulting from a reduced capability of both the male and female gametes to disrupt the surrounding parasitophorous vacuole and to egress from the host erythrocyte. These data demonstrate that emergence from the host cell of male and female gametes relies on a common, MDV-1/PEG3-dependent mechanism that is distinct from mechanisms used by asexual parasites.

SummaryIn the apicomplexan protozoans motility and cell invasion are mediated by the TRAP/MIC2 family of transmembrane proteins, members of which link extracellular adhesion to the intracellular actomyosin motor complex. Here we characterize a new member of the TRAP/MIC2 family, named TRAP-Like Protein (TLP), that is highly conserved within the Plasmodium genus. Similar to the Plasmodium sporozoite protein, TRAP, and the ookinete protein, CTRP, TLP possesses an extracellular domain architecture that is comprised of von Willebrand factor A (vWA) and thrombospondin type 1 (TSP1) domains, plus a short cytoplasmic domain. Comparison of the vWA domain of TLP genes from multiple Plasmodium falciparum isolates showed relative low sequence diversity, suggesting that the protein is not under selective pressures of the host immune system. Analysis of transcript levels by quantitative reverse transcription polymerase chain reaction (RT-PCR) showed that TLP is predominantly expressed in salivary gland sporozoites of P. falciparum and P. berghei. Targeted disruption of P. berghei TLP resulted in a decreased capacity for cell traversal by sporozoites, and reduced infectivity of sporozoites in vivo, whereas in vitro sporozoite motility and hepatocyte invasion were unaffected. These results indicate a role of TLP in cell traversal by sporozoites.

A limitation of transfection of malaria parasites is the availability of only a low number of positive selectable markers for selection of transformed mutants. This is exacerbated for the rodent parasite Plasmodium berghei as selection of mutants is performed in vivo in laboratory rodents. We here report the development and application of a negative selection system based upon transgenic expression of a bifunctional protein (yFCU) combining yeast cytosine deaminase and uridyl phosphoribosyl transferase (UPRT) activity in P.berghei followed by in vivo selection with the prodrug 5-fluorocytosine (5-FC). The combination of yfcu and a positive selectable marker was used to first achieve positive selection of mutant parasites with a disrupted gene in a conventional manner. Thereafter through negative selection using 5-FC, mutants were selected where the disrupted gene had been restored to its original configuration as a result of the excision of the selectable markers from the genome through homologous recombination. This procedure was carried out for a Plasmodium gene (p48/45) encoding a protein involved in fertilization, the function of which had been previously implied through gene disruption alone. Such reversible recombination can therefore be employed for both the rapid analysis of the phenotype by targeted disruption of a gene and further associate phenotype and function by genotype restoration through the use of a single plasmid and a single positive selectable marker. Furthermore the negative selection system may also be adapted to facilitate other procedures such as ‘Hit and Run’ and ‘vector recycling’ which in principle will allow unlimited manipulation of a single parasite clone. This is the first demonstration of the general use of yFCU in combination with a positive selectable marker in reverse genetics approaches and it should be possible to adapt its use to many other biological systems.