Plasmodium berghei: infectivity of mice to Anopheles stephensi mosquitoes

Use of a selective inhibitor to define the chemotherapeutic potential of the plasmodial hexose transporter in different stages of the parasite's life cycle

During blood infection, malarial parasites use D-glucose as their main energy source. The Plasmodium falciparum hexose transporter (PfHT), which mediates the uptake of D-glucose into parasites, is essential for survival of asexual blood-stage parasites. Recently, genetic studies in the rodent malaria model, Plasmodium berghei, found that the orthologous hexose transporter (PbHT) is expressed throughout the parasite's development within the mosquito vector, in addition to being essential during intraerythrocytic development. Here, using a D-glucose-derived specific inhibitor of plasmodial hexose transporters, compound 3361, we have investigated the importance of D-glucose uptake during liver and transmission stages of P. berghei. Initially, we confirmed the expression of PbHT during liver stage development, using a green fluorescent protein (GFP) tagging strategy. Compound 3361 inhibited liver-stage parasite development, with a 50% inhibitory concentration (IC₅₀) of 11 μM. This process was insensitive to the external D-glucose concentration. In addition, compound 3361 inhibited ookinete development and microgametogenesis, with IC₅₀s in the region of 250 μM (the latter in a D-glucose-sensitive manner). Consistent with our findings for the effect of compound 3361 on vector parasite stages, 1 mM compound 3361 demonstrated transmission blocking activity. These data indicate that novel chemotherapeutic interventions that target PfHT may be active against liver and, to a lesser extent, transmission stages, in addition to blood stages.

Key factors regulating Plasmodium berghei sporozoite survival and transformation revealed by an automated visual assay

Malaria is transmitted to the host when Plasmodium sporozoites are injected by a mosquito vector. Sporozoites eventually enter hepatocytes, where they differentiate into liver-stage parasites. During the first hours after hepatocyte invasion, the crescent-shaped sporozoites transform into spherical intracellular exoerythrocytic parasites. This process, which precedes genome replication, can be mimicked in vitro in the absence of host cells. Here, we developed an automated method to follow transformation and cell death of sporozoites in vitro. This assay provides a rapid tool to test sporozoite survival and to screen for antiparasitic drugs. We found that extracellular bicarbonate and high temperature trigger transformation, whereas physiological serum albumin concentrations and media lacking bicarbonate delayed sporozoite death. Because bicarbonate also triggers ookinete transformation and exflagellation of gametocytes, we suggest that a common molecular mechanism regulates similar aspects of stage conversion in Plasmodium.

Anopheles gambiae collagen IV genes: cloning, phylogeny and midgut expression associated with blood feeding and Plasmodium infection

A prerequisite for understanding the role that mosquito midgut extracellular matrix molecules play in malaria parasite development is proper isolation and characterisation of the genes coding for components of the basal lamina. Here we have identified genes coding for alpha1 and alpha2 chains of collagen IV from the major malaria vector, Anopheles gambiae. Conserved sequences in the terminal NC1 domain were used to obtain partial gene sequences of this functional region, and full sequence was isolated from a pupal cDNA library. In a DNA-derived phylogeny, the alpha1 and alpha2 chains cluster with dipteran orthologs, and the alpha2 is ancestral. The expression of collagen alpha1(IV) peaked during the pupal stage of mosquito development, and was expressed continuously in the adult female following a blood meal with a further rise detected in older mosquitoes. Collagen alpha1(IV) is also upregulated when the early oocyst of Plasmodium yoelii was developing within the mosquito midgut and may contribute to a larger wound healing response. A model describing the expression of basal lamina proteins during oocyst development is presented, and we hypothesise that the development of new basal lamina between the oocyst and midgut epithelium is akin to a wound healing process.

Interaction between host complement and mosquito-midgut-stage Plasmodium berghei

After ingestion by mosquitoes, gametocytes of malaria parasites become activated and form extracellular gametes that are no longer protected by the red blood cell membrane against immune effectors of host blood. We have studied the action of complement on Plasmodium developmental stages in the mosquito blood meal using the rodent malaria parasite Plasmodium berghei and rat complement as a model. We have shown that in the mosquito midgut, rat complement components necessary to initiate the alternative pathway (factor B, factor D, and C3) as well as C5 are present for several hours following ingestion of P. berghei-infected rat blood. In culture, 30 to 50% of mosquito midgut stages of P. berghei survived complement exposure during the first 3 h of development. Subsequently, parasites became increasingly sensitive to complement lysis. To investigate the mechanisms involved in their protection, we tested for C3 deposition on parasite surfaces and whether host CD59 (a potent inhibitor of the complement membrane attack complex present on red blood cells) was taken up by gametes while emerging from the host cell. Between 0.5 and 22 h, 90% of Pbs21-positive parasites were positive for C3. While rat red and white blood cells stained positive for CD59, Pbs21-positive parasites were negative for CD59. In addition, exposure of parasites to rat complement in the presence of anti-rat CD59 antibodies did not increase lysis. These data suggest that parasite or host molecules other than CD59 are responsible for the protection of malaria parasites against complement-mediated lysis. Ongoing research aims to identify these molecules.

The chemotherapy of rodent malaria. LIX. Drug combinations to impede the selection of drug resistance, Part 3: Observations on cyproheptadine, an antihistaminic agent, with chloroquine

Several compounds in current clinical use as antihistaminic agents, among them cyproheptadine (CYP), have been shown, in experimental models, to reverse resistance of the asexual, intra-erythrocytic stages of rodent or human malarial parasites to chloroquine (CQ). Although preliminary clinical trials with CYP failed to confirm such activity in subjects with naturally acquired infection with Plasmodium falciparum, Nigerian investigators have reported that another antihistaminic, chlorpheniramine, significantly restores the blood schizontocidal action of CQ in semi-immune patients with CQ-resistant P. falciparum, when the two compounds are administered together. The rates at which resistance to CYP can be produced, in mice infected either with CQ-resistant P. yoelii ssp. NS or CQ-resistant P. yoelii nigeriensis, when drug-selection pressure is exerted with this compound alone have now been compared with the rate and extent to which resistance develops in infected animals that are treated with various combinations of CYP and CQ. The data indicate that, in both parasites, stable resistance develops slowly to CYP alone and that exposure to a combination of CYP plus CQ significantly impedes the selection of resistance to CYP. Although the antimalarial action of CYP is reported to extend to the pre-erythrocytic hepatic stages, there was no evidence of gametocytocidal activity in the present study. The future implications of these observations are discussed in relation to the clinical potential of CYP + CQ and similar combinations and possible future research.

Cloning and characterization of a Na+-driven anion exchanger (NDAE1). A new bicarbonate transporter

Regulation of intra- and extracellular ion activities (e.g. H(+), Cl(-), Na(+)) is key to normal function of the central nervous system, digestive tract, respiratory tract, and urinary system. With our cloning of an electrogenic Na(+)/HCO(3)(-) cotransporter (NBC), we found that NBC and the anion exchangers form a bicarbonate transporter superfamily. Functionally three other HCO(3)(-) transporters are known: a neutral Na(+)/ HCO(3)(-) cotransporter, a K(+)/ HCO(3)(-) cotransporter, and a Na(+)-dependent Cl(-)-HCO(3)(-) exchanger. We report the cloning and characterization of a Na(+)-coupled Cl(-)-HCO(3)(-) exchanger and a physiologically unique bicarbonate transporter superfamily member. This Drosophila cDNA encodes a 1030-amino acid membrane protein with both sequence homology and predicted topology similar to the anion exchangers and NBCs. The mRNA is expressed throughout Drosophila development and is prominent in the central nervous system. When expressed in Xenopus oocytes, this membrane protein mediates the transport of Cl(-), Na(+), H(+), and HCO(3)(-) but does not require HCO(3)(-). Transport is blocked by the stilbene 4,4'-diisothiocyanodihydrostilbene- 2, 2'-disulfonates and may not be strictly electroneutral. Our functional data suggest this Na(+) driven anion exchanger (NDAE1) is responsible for the Na(+)-dependent Cl(-)-HCO(3)(-) exchange activity characterized in neurons, kidney, and fibroblasts. NDAE1 may be generally important for fly development, because disruption of this gene is apparently lethal to the Drosophila larva.

Inhibition of the mosquito transmission of Plasmodium berghei by Malarone (atovaquone-proguanil)

Sera from patients treated with atovaquone-proguanil (Malarone) have previously been shown to inhibit the mosquito transmission of Plasmodium falciparum, though the inhibition was not complete and the effect declined 2 weeks after treatment. In marked contrast, the inhibition of transmission of P. berghei by human sera (fed to mosquitoes, with P. berghei gametocytes, via membrane feeders) from volunteers treated with atovaquone-proguanil was total up to day 28 post-treatment and still very significant at day 56. In view of the short half-lives of atovaquone and proguanil, this was unexpected, and further experiments, reported here, were undertaken. In contrast to the incomplete blockade of infectivity of P. falciparum by serum taken 4 days post-treatment, such serum was totally inhibitory against P. berghei at a 1:10,000 dilution, indicating a remarkable sensitivity of P. berghei and demonstrating an unusual difference between the two Plasmodium species in response to a drug. The inhibitory effect on P. berghei after day 4 was caused by atovaquone and mainly through blockade of development from ookinete to oocyst. Despite previous information on the rapid elimination of atovaquone by patients, the present data indicate that low concentrations of this drug may persist in the plasma for some weeks after treatment.

The mosquito transmission of malaria: the effects of atovaquone-proguanil (Malarone) and chloroquine

Despite its recognized importance, the prevention of patients with malaria from continuing to infect mosquitoes after treatment is not always achieved in practice. An inevitable consequence of the prolonged life-span and relative metabolic stasis of the mature gametocytes of Plasmodium falciparum is that they are not cleared by most antimalarials, and few antimalarials block infection in the mosquito vector. Previous research on the constituents of Malarone, a new 'combined antimalarial', suggested that the active components, atovaquone and proguanil, might inhibit infectivity of gametocytes to mosquitoes. We contrast here the impact of atovaquone-proguanil and chloroquine on the transmission of P. falciparum and P. berghei. While chloroquine enhanced infectivity of P. falciparum, atovaquone-proguanil caused a significant reduction. Surprisingly, sporontocidal activity against the rodent parasite persisted long after the levels of the constituent drugs would have been expected to have fallen below effective plasma concentrations on the basis of the established pharmacokinetics of atovaquone and proguanil. The P. berghei model may thus have provided a sensitive bioassay, detecting drug(s) at levels below that normally found with the usual chemical assays.