Plasmodium genomics: latest milestone

Recombinant Pvs48/45 antigen expressed in E. coli generates antibodies that block malaria transmission in Anopheles albimanus mosquitoes

Transmission of malaria parasites from humans to Anopheles mosquitoes can be inhibited by specific antibodies elicited during malaria infection, which target surface Plasmodium gametocyte/gamete proteins. Some of these proteins may have potential for vaccine development. Pvs48/45 is a P. vivax gametocyte surface antigen orthologous to Pfs48/45, which may play a role during parasite fertilization and thus has potential for transmission blocking (TB) activity. Here we describe the expression of a recombinant Pvs48/45 protein expressed in Escherichia coli as a ∼60kDa construct which we tested for antigenicity using human sera and for its immunogenicity and transmission blocking activity of specific anti-mouse and anti-monkey Pvs48/45 antibodies. The protein reacted with sera of individuals from malaria-endemic areas and in addition induced specific IgG antibody responses in BALB/c mice and Aotus l. griseimembra monkeys. Sera from both immunized animal species recognized native P. vivax protein in Western blot (WB) and immunofluorescence assays. Moreover, sera from immunized mice and monkeys produced significant inhibition of parasite transmission to An. Albimanus mosquitoes as shown by membrane feeding assays. Results indicate the presence of reactive epitopes in the Pvs48/45 recombinant product that induce antibodies with TB activity. Further testing of this protein is ongoing to determine its vaccine potential.

The Plasmodium translocon of exported proteins (PTEX) component thioredoxin-2 is important for maintaining normal blood-stage growth

Plasmodium parasites remodel their vertebrate host cells by translocating hundreds of proteins across an encasing membrane into the host cell cytosol via a putative export machinery termed PTEX. Previously PTEX150, HSP101 and EXP2 have been shown to be bona fide members of PTEX. Here we validate that PTEX88 and TRX2 are also genuine members of PTEX and provide evidence that expression of PTEX components are also expressed in early gametocytes, mosquito and liver stages, consistent with observations that protein export is not restricted to asexual stages. Although amenable to genetic tagging, HSP101, PTEX150, EXP2 and PTEX88 could not be genetically deleted in Plasmodium berghei, in keeping with the obligatory role this complex is postulated to have in maintaining normal blood-stage growth. In contrast, the putative thioredoxin-like protein TRX2 could be deleted, with knockout parasites displaying reduced grow-rates, both in vivo and in vitro, and reduced capacity to cause severe disease in a cerebral malaria model. Thus, while not essential for parasite survival, TRX2 may help to optimize PTEX activity. Importantly, the generation of TRX2 knockout parasites that display altered phenotypes provides a much-needed tool to dissect PTEX function.

Proteomic and genetic analyses demonstrate that Plasmodium berghei blood stages export a large and diverse repertoire of proteins

Malaria parasites actively remodel the infected red blood cell (irbc) by exporting proteins into the host cell cytoplasm. The human parasite Plasmodium falciparum exports particularly large numbers of proteins, including proteins that establish a vesicular network allowing the trafficking of proteins onto the surface of irbcs that are responsible for tissue sequestration. Like P. falciparum, the rodent parasite P. berghei ANKA sequesters via irbc interactions with the host receptor CD36. We have applied proteomic, genomic, and reverse-genetic approaches to identify P. berghei proteins potentially involved in the transport of proteins to the irbc surface. A comparative proteomics analysis of P. berghei non-sequestering and sequestering parasites was used to determine changes in the irbc membrane associated with sequestration. Subsequent tagging experiments identified 13 proteins (Plasmodium export element (PEXEL)-positive as well as PEXEL-negative) that are exported into the irbc cytoplasm and have distinct localization patterns: a dispersed and/or patchy distribution, a punctate vesicle-like pattern in the cytoplasm, or a distinct location at the irbc membrane. Members of the PEXEL-negative BIR and PEXEL-positive Pb-fam-3 show a dispersed localization in the irbc cytoplasm, but not at the irbc surface. Two of the identified exported proteins are transported to the irbc membrane and were named erythrocyte membrane associated proteins. EMAP1 is a member of the PEXEL-negative Pb-fam-1 family, and EMAP2 is a PEXEL-positive protein encoded by a single copy gene; neither protein plays a direct role in sequestration. Our observations clearly indicate that P. berghei traffics a diverse range of proteins to different cellular locations via mechanisms that are analogous to those employed by P. falciparum. This information can be exploited to generate transgenic humanized rodent P. berghei parasites expressing chimeric P. berghei/P. falciparum proteins on the surface of rodent irbc, thereby opening new avenues for in vivo screening adjunct therapies that block sequestration.

Recent Progress in Functional Genomic Research in Plasmodium falciparum

With the completion and near completion of many malaria parasite genome-sequencing projects, efforts are now being directed to a better understanding of gene functions and to the discovery of vaccine and drug targets. Inter- and intraspecies comparisons of the parasite genomes will provide invaluable insights into parasite evolution, virulence, drug resistance, and immune invasion. Genome-wide searches for loci under various selection pressures may lead to discovery of genes conferring drug resistance or encoding for protective antigens. In addition, the Plasmodium falciparum genome sequence provides the basis for the development of various microarrays to monitor gene expression and to detect nucleotide substitution and deletion/amplification. Genome-wide profiling of the parasite proteome, chromatin modification, and nucleosome position also depend on availability of the parasite genome. In this brief review, we will highlight some recent advances and studies in characterizing gene function and related phenotype in P. falciparum that were made possible by the genome sequence, particularly the development of a genome-wide diversity map and various high-throughput genotyping methods for genome-wide association studies (GWAS).

Identification of a highly antigenic linear B cell epitope within Plasmodium vivax apical membrane antigen 1 (AMA-1)

Apical membrane antigen 1 (AMA-1) is considered to be a major candidate antigen for a malaria vaccine. Previous immunoepidemiological studies of naturally acquired immunity to Plasmodium vivax AMA-1 (PvAMA-1) have shown a higher prevalence of specific antibodies to domain II (DII) of AMA-1. In the present study, we confirmed that specific antibody responses from naturally infected individuals were highly reactive to both full-length AMA-1 and DII. Also, we demonstrated a strong association between AMA-1 and DII IgG and IgG subclass responses. We analyzed the primary sequence of PvAMA-1 for B cell linear epitopes co-occurring with intrinsically unstructured/disordered regions (IURs). The B cell epitope comprising the amino acid sequence 290–307 of PvAMA-1 (SASDQPTQYEEEMTDYQK), with the highest prediction scores, was identified in domain II and further selected for chemical synthesis and immunological testing. The antigenicity of the synthetic peptide was identified by serological analysis using sera from P. vivax-infected individuals who were knowingly reactive to the PvAMA-1 ectodomain only, domain II only, or reactive to both antigens. Although the synthetic peptide was recognized by all serum samples specific to domain II, serum with reactivity only to the full-length protein presented 58.3% positivity. Moreover, IgG reactivity against PvAMA-1 and domain II after depletion of specific synthetic peptide antibodies was reduced by 18% and 33% (P = 0.0001 for both), respectively. These results suggest that the linear epitope SASDQPTQYEEEMTDYQK is highly antigenic during natural human infections and is an important antigenic region of the domain II of PvAMA-1, suggesting its possible future use in pre-clinical studies.

NMR and crystallographic structures of the FK506 binding domain of human malarial parasite Plasmodium vivax FKBP35

The emergence of drug-resistant malaria parasites is the major threat to effective malaria control, prompting a search for novel compounds with mechanisms of action that are different from the traditionally used drugs. The immunosuppressive drug FK506 shows an antimalarial activity. The mechanism of the drug action involves the molecular interaction with the parasite target proteins PfFKBP35 and PvFKBP35, which are novel FK506 binding protein family (FKBP) members from Plasmodium falciparum and Plasmodium vivax, respectively. Currently, molecular mechanisms of the FKBP family proteins in the parasites still remain elusive. To understand their functions, here we have determined the structures of the FK506 binding domain of Plasmodium vivax (PvFKBD) in unliganded form by NMR spectroscopy and in complex with FK506 by X-ray crystallography. We found out that PvFKBP35 exhibits a canonical FKBD fold and shares kinetic profiles similar to those of PfFKBP35, the homologous protein in P. falciparum, indicating that the parasite FKBP family members play similar biological roles in their life cycles. Despite the similarity, differences were observed in the ligand binding modes between PvFKBD and HsFKBP12, a human FKBP homolog, which could provide insightful information into designing selective antimalarial drug against the parasites.

The malaria merozoite, forty years on

The invasive blood stage of malaria parasites, merozoites, are complex entities specialized for the capture and entry of red blood cells. Their potential for vaccination and other anti-malaria strategies have attracted much research attention over the last 40 years, and there is now a considerable body of data relating to their biology. In this article some of the major advances over this period and remaining challenges are reviewed.