Highly conserved gene arrangement of the mitochondrial genomes of 23 Plasmodium species.

Mitochondrial (mt) genomes from diverse phylogenetic groups vary considerably in size, structure and organization. The genus Plasmodium, the causative agent of malaria, has the smallest mt genome in the form of a tandemly repeated, linear element of 6 kb. The Plasmodium mt genome encodes only three protein genes (cox1, cox3 and cob) and large- and small-subunit ribosomal RNA (rRNA) genes, which are highly fragmented with 19 identified rRNA pieces. The complete mt genome sequences of 21 Plasmodium species have been published but a thorough investigation of the arrangement of rRNA gene fragments has been undertaken for only Plasmodium falciparum, the human malaria parasite. In this study, we determined the arrangement of mt rRNA gene fragments in 23 Plasmodium species, including two newly determined mt genome sequences from P. gallinaceum and P. vinckei vinckei, as well as Leucocytozoon caulleryi, an outgroup of Plasmodium. Comparative analysis reveals complete conservation of the arrangement of rRNA gene fragments in the mt genomes of all the 23 Plasmodium species and L. caulleryi. Surveys for a new rRNA gene fragment using hidden Markov models enriched with recent mt genome sequences led us to suggest the mtR-26 sequence as a novel candidate LSU rRNA fragment in the mt genomes of the 24 species. Additionally, we found 22-25 bp-inverted repeat sequences, which may be involved in the generation of lineage-specific mt genome arrangements after divergence from a common ancestor of the genera Eimeria and Plasmodium/Leucocytozoon.

Plasmodium berghei XAT: contribution of gammadelta T cells to host defense against infection with blood-stage nonlethal malaria parasite.

We examined a potential role of gammadelta T cells in protective immunity to blood-stage Plasmodium berghei XAT infection. Plasmodium berghei XAT is an attenuated variant of the lethal strain P. berghei NK65 and its infection is self-resolving in immune competent mice. To determine whether gammadelta T cells are essential for the resolution of P. berghei XAT malaria, mice were depleted of gammadelta T cells with anti-TCRgammadelta antibody treatment. Although mice that had received control antibody resolved infections, mice received anti-TCRgammadelta antibody could not control their infections and eventually died. Spleen cells from infected mice produced IFN-gamma and nitric oxide (NO) within the first week of infection, however, levels of IFN-gamma and NO in gammadelta T cell-depleted mice were significantly lower than in control mice. To examine whether gammadelta T cells are involved in the antibody production, malarial-specific antibodies of the various isotypes were measured in the sera of gammadelta T cell-depleted mice and control mice. Serum levels of IgG2a, which was known to be a protective antibody in P. berghei XAT malaria, were significantly lower in gammadelta T cell-depleted mice than in control mice, whereas levels of IgG1 were comparable to those in control mice. Our results indicated that the presence of the gammadelta T cell subset was essential for resolution of blood-stage P. berghei XAT malaria and played a modulatory role in the development of Th1 response and host defense against this malarial parasites.

Plasmodium berghei XAT: protective 155/160 kDa antigens are located in parasitophorous vacuoles of schizont-stage parasite.

Effective blood-stage malaria vaccine candidates have been mainly developed from the proteins in exposed locations on the parasite such as the surface of free merozoites or infected red blood cells. In the present study, we identified and localized novel protective antigens derived from the blood-stage of Plasmodium berghei XAT after establishment of hybridomas producing protective monoclonal antibodies (mAbs) against the parasites. The protective antigens were expressed in schizonts but not in trophozoites, and located in the parasitophorous vacuoles in the infected erythrocyte cytoplasm. The antigens, with molecular weight of 155/160 kDa, were not identical to any merozoite/schizont antigens that have been reported as target molecules recognized by mAbs developed to rodent malaria parasites. The characterization of new malarial antigenic targets of potentially protective antibody responses following infection would give us new insights for the selection of candidate antigens for malaria vaccine.

Parasite mitochondria as a target of chemotherapy: inhibitory effect of licochalcone A on the Plasmodium falciparum respiratory chain.

Parasites have exploited unique energy metabolic pathways as adaptations to the natural host habitat. In fact, the respiratory systems of parasites typically show greater diversity in electron transfer pathways than do those of host animals. These unique aspects of parasite mitochondria and related enzymes may represent promising targets for chemotherapy. Natural products have been recognized as a source of the candidates of the specific inhibitors for such parasite respiratory chains. Chalcones was recently evaluated for its antimalarial activity in vitro and in vivo. However, its target is still unclear in malaria parasites. In this study, we investigated that licochalcone A inhibited the bc1 complex (ubiquinol-cytochrome c reductase) as well as complex II (succinate ubiquinone reductase, SQR) of Plasmodium falciparum mitochondria. In particular, licochalcone A inhibits bc1 complex activity at very low concentrations. Because the property of the P. falciparum bc1 complex is different from that of the mammalian host, chalcones would be a promising candidate for a new antimalarial drug.