A unique profilin-actin interface is important for malaria parasite motility

Profilin is an actin monomer binding protein that provides ATP-actin for incorporation into actin filaments. In contrast to higher eukaryotic cells with their large filamentous actin structures, apicomplexan parasites typically contain only short and highly dynamic microfilaments. In apicomplexans, profilin appears to be the main monomer-sequestering protein. Compared to classical profilins, apicomplexan profilins contain an additional arm-like β-hairpin motif, which we show here to be critically involved in actin binding. Through comparative analysis using two profilin mutants, we reveal this motif to be implicated in gliding motility of Plasmodium berghei sporozoites, the rapidly migrating forms of a rodent malaria parasite transmitted by mosquitoes. Force measurements on migrating sporozoites and molecular dynamics simulations indicate that the interaction between actin and profilin fine-tunes gliding motility. Our data suggest that evolutionary pressure to achieve efficient high-speed gliding has resulted in a unique profilin-actin interface in these parasites.

The malaria parasite Plasmodium has two invasive forms that migrate across different tissue barriers, the ookinete and the very rapidly migrating sporozoite. Previous work has shown that the motility of these and related parasites (e.g. Toxoplasma gondii) depends on a highly dynamic actin cytoskeleton and retrograde flow of surface adhesins. These unusual actin dynamics are due to the divergent structure of protozoan actins and the actions of actin-binding proteins, which can have non-canonical functions in these parasites. Profilin is one of the most important and most investigated actin-binding proteins, which binds ADP-actin and catalyzes ADP-ATP exchange to then promote actin polymerization. Parasite profilins bind monomeric actin and contain an additional domain compared to canonical profilins. Here we show that this additional domain of profilin is critical for actin binding and rapid sporozoite motility but has little impact on the slower ookinete. Sporozoites of a parasite line carrying mutations in this domain cannot translate force production and retrograde flow into optimal parasite motility. Using molecular dynamics simulations, we find that differences between mutant parasites in their capacity to migrate can be traced back to a single hydrogen bond at the actin-profilin interface.

P. berghei telomerase subunit TERT is essential for parasite survival

Telomeres define the ends of chromosomes protecting eukaryotic cells from chromosome instability and eventual cell death. The complex regulation of telomeres involves various proteins including telomerase, which is a specialized ribonucleoprotein responsible for telomere maintenance. Telomeres of chromosomes of malaria parasites are kept at a constant length during blood stage proliferation. The 7-bp telomere repeat sequence is universal across different Plasmodium species (GGGTTT/CA), though the average telomere length varies. The catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), is present in all sequenced Plasmodium species and is approximately three times larger than other eukaryotic TERTs. The Plasmodium RNA component of TERT has recently been identified in silico. A strategy to delete the gene encoding TERT via double cross-over (DXO) homologous recombination was undertaken to study the telomerase function in P. berghei. Expression of both TERT and the RNA component (TR) in P. berghei blood stages was analysed by Western blotting and Northern analysis. Average telomere length was measured in several Plasmodium species using Telomere Restriction Fragment (TRF) analysis. TERT and TR were detected in blood stages and an average telomere length of ∼950 bp established. Deletion of the tert gene was performed using standard transfection methodologies and we show the presence of tert⁻ mutants in the transfected parasite populations. Cloning of tert- mutants has been attempted multiple times without success. Thorough analysis of the transfected parasite populations and the parasite obtained from extensive parasite cloning from these populations provide evidence for a so called delayed death phenotype as observed in different organisms lacking TERT. The findings indicate that TERT is essential for P. berghei cell survival. The study extends our current knowledge on telomere biology in malaria parasites and validates further investigations to identify telomerase inhibitors to induce parasite cell death.

The ETRAMP family member SEP2 is expressed throughout Plasmodium berghei life cycle and is released during sporozoite gliding motility

The early transcribed membrane proteins ETRAMPs belong to a family of small, transmembrane molecules unique to Plasmodium parasite, which share a signal peptide followed by a short lysine-rich stretch, a transmembrane domain and a variable, highly charged C-terminal region. ETRAMPs are usually expressed in a stage-specific manner. In the blood stages they localize to the parasitophorous vacuole membrane and, in described cases, to vesicle-like structures exported to the host erythrocyte cytosol. Two family members of the rodent parasite Plasmodium berghei, uis3 and uis4, localize to secretory organelles of sporozoites and to the parasitophorous membrane vacuole of the liver stages. By the use of specific antibodies and the generation of transgenic lines, we showed that the P. berghei ETRAMP family member SEP2 is abundantly expressed in gametocytes as well as in mosquito and liver stages. In intracellular parasite stages, SEP2 is routed to the parasitophorous vacuole membrane while, in invasive ookinete and sporozoite stages, it localizes to the parasite surface. To date SEP2 is the only ETRAMP protein detected throughout the parasite life cycle. Furthermore, SEP2 is also released during gliding motility of salivary gland sporozoites. A limited number of proteins are known to be involved in this key function and the best characterized, the CSP and TRAP, are both promising transmission-blocking candidates. Our results suggest that ETRAMP members may be viewed as new potential candidates for malaria control.

Metabolite extract of Streptomyces hygroscopicus Hygroscopicus inhibit the growth of Plasmodium berghei through inhibition of ubiquitin - proteasome system

Streptomyces hygroscopicus Hygroscopicus, a member of family of Actinomycetes produces eponemycin a proteasome inhibitor that can inhibit Ubiquitin-Proteasome System (UPS) function in eukaryotic cell. Previous study showed that coronamycin, an active substrate isolated from Streptomyces sp. can act as anti-plasmodial, antibacterial, and antifungal, however the research did not show the mechanism of coronamycin in inhibiting the growth of Plasmodium. This research was done to reveal if eponemycin that is contained in metabolite extract of S. hygroscopicus can inhibit UPS function of Plasmodium berghei. This study was an experimental study using P. berghei infected Balb/C mice as malaria model. Samples were divided into 1 control group (group infected with P. berghei without treatment) and 3 treatment groups (mice infected with P. berghei and treated intra-peritoneal with metabolite extract of S. hygroscopicus dose 130 μg/kgBW, 580 μg/kgBW, and 2600 μg/kgBW for 5 days). The degree of parasitemia and morphology of the parasite were measured from the first day of malaria induction until the last treatment. The accumulation level of polyubiquitin was measured using Western blot and ELISA method. The degree of parasitemia on day 6 showed significant differences among treatment groups and control (p=0,000). Percentage of inhibition showed significant differences between control and group treated with metabolite extract of S. hygroscopicus 2600 μg/kgBW. An increasing dose of extract of S. hygroscopicus followed by an increasing of inhibition in parasite growth (r=0,850). Probit analysis showed that ED50 was 9.418 μg/kgBW. There was a change in morphology of the parasite after treatment. Parasite morphology became crisis form. There was an accumulation of polyubiquitinated protein in the group treated with metabolite extract of S. hygroscopicus 2600 μg/kgBW. It can be concluded that analog eponemycin in metabolite of S. hygroscopicus is a potential candidate for new malarial drug by inhibiting UPS function of the parasite and cause stress and dead of the parasite.

Phenotypic dissection of a Plasmodium-refractory strain of malaria vector Anopheles stephensi: the reduced susceptibility to P. berghei and P. yoelii

Anopheline mosquitoes are the major vectors of human malaria. Parasite-mosquito interactions are a critical aspect of disease transmission and a potential target for malaria control. Current investigations into parasite-mosquito interactions frequently assume that genetically resistant and susceptible mosquitoes exist in nature. Therefore, comparisons between the Plasmodium susceptibility profiles of different mosquito species may contribute to a better understanding of vectorial capacity. Anopheles stephensi is an important malaria vector in central and southern Asia and is widely used as a laboratory model of parasite transmission due to its high susceptibility to Plasmodium infection. In the present study, we identified a rodent malaria-refractory strain of A. stephensi mysorensis (Ehime) by comparative study of infection susceptibility. A very low number of oocysts develop in Ehime mosquitoes infected with P. berghei and P. yoelii, as determined by evaluation of developed oocysts on the basal lamina. A stage-specific study revealed that this reduced susceptibility was due to the impaired formation of ookinetes of both Plasmodium species in the midgut lumen and incomplete crossing of the midgut epithelium. There were no apparent abnormalities in the exflagellation of male parasites in the ingested blood or the maturation of oocysts after the rounding up of the ookinetes. Overall, these results suggest that invasive-stage parasites are eliminated in both the midgut lumen and epithelium in Ehime mosquitoes by strain-specific factors that remain unknown. The refractory strain newly identified in this report would be an excellent study system for investigations into novel parasite-mosquito interactions in the mosquito midgut.

Plasmodium berghei MAPK1 displays differential and dynamic subcellular localizations during liver stage development

Mitogen-activated protein kinases (MAPKs) regulate key signaling events in eukaryotic cells. In the genomes of protozoan Plasmodium parasites, the causative agents of malaria, two genes encoding kinases with significant homology to other eukaryotic MAPKs have been identified (mapk1, mapk2). In this work, we show that both genes are transcribed during Plasmodium berghei liver stage development, and analyze expression and subcellular localization of the PbMAPK1 protein in liver stage parasites. Live cell imaging of transgenic parasites expressing GFP-tagged PbMAPK1 revealed a nuclear localization of PbMAPK1 in the early schizont stage mediated by nuclear localization signals in the C-terminal domain. In contrast, a distinct localization of PbMAPK1 in comma/ring-shaped structures in proximity to the parasite's nuclei and the invaginating parasite membrane was observed during the cytomere stage of parasite development as well as in immature blood stage schizonts. The PbMAPK1 localization was found to be independent of integrity of a motif putatively involved in ATP binding, integrity of the putative activation motif and the presence of a predicted coiled-coil domain in the C-terminal domain. Although PbMAPK1 knock out parasites showed normal liver stage development, the kinase may still fulfill a dual function in both schizogony and merogony of liver stage parasites regulated by its dynamic and stage-dependent subcellular localization.

The alveolin IMC1h is required for normal ookinete and sporozoite motility behaviour and host colonisation in Plasmodium berghei

Alveolins, or inner membrane complex (IMC) proteins, are components of the subpellicular network that forms a structural part of the pellicle of malaria parasites. In Plasmodium berghei, deletions of three alveolins, IMC1a, b, and h, each resulted in reduced mechanical strength and gliding velocity of ookinetes or sporozoites. Using time lapse imaging, we show here that deletion of IMC1h (PBANKA_143660) also has an impact on the directionality and motility behaviour of both ookinetes and sporozoites. Despite their marked motility defects, sporozoites lacking IMC1h were able to invade mosquito salivary glands, allowing us to investigate the role of IMC1h in colonisation of the mammalian host. We show that IMC1h is essential for sporozoites to progress through the dermis in vivo but does not play a significant role in hepatoma cell transmigration and invasion in vitro. Colocalisation of IMC1h with the residual IMC in liver stages was detected up to 30 hours after infection and parasites lacking IMC1h showed developmental defects in vitro and a delayed onset of blood stage infection in vivo. Together, these results suggest that IMC1h is involved in maintaining the cellular architecture which supports normal motility behaviour, access of the sporozoites to the blood stream, and further colonisation of the mammalian host.

Resistance of a rodent malaria parasite to a thymidylate synthase inhibitor induces an apoptotic parasite death and imposes a huge cost of fitness

Background

The greatest impediment to effective malaria control is drug resistance in Plasmodium falciparum, and thus understanding how resistance impacts on the parasite's fitness and pathogenicity may aid in malaria control strategy.

Methodology/Principal Findings

To generate resistance, P. berghei NK65 was subjected to 5-fluoroorotate (FOA, an inhibitor of thymidylate synthase, TS) pressure in mice. After 15 generations of drug pressure, the 2% DT (the delay time for proliferation of parasites to 2% parasitaemia, relative to untreated wild-type controls) reduced from 8 days to 4, equalling the controls. Drug sensitivity studies confirmed that FOA-resistance was stable. During serial passaging in the absence of drug, resistant parasite maintained low growth rates (parasitaemia, 15.5%±2.9, 7 dpi) relative to the wild-type (45.6%±8.4), translating into resistance cost of fitness of 66.0%. The resistant parasite showed an apoptosis-like death, as confirmed by light and transmission electron microscopy and corroborated by oligonucleosomal DNA fragmentation.

Conclusions/Significance

The resistant parasite was less fit than the wild-type, which implies that in the absence of drug pressure in the field, the wild-type alleles may expand and allow drugs withdrawn due to resistance to be reintroduced. FOA resistance led to depleted dTTP pools, causing thymineless parasite death via apoptosis. This supports the tenet that unicellular eukaryotes, like metazoans, also undergo apoptosis. This is the first report where resistance to a chemical stimulus and not the stimulus itself is shown to induce apoptosis in a unicellular parasite. This finding is relevant in cancer therapy, since thymineless cell death induced by resistance to TS-inhibitors can further be optimized via inhibition of pyrimidine salvage enzymes, thus providing a synergistic impact. We conclude that since apoptosis is a process that can be pharmacologically modulated, the parasite's apoptotic machinery may be exploited as a novel drug target in malaria and other protozoan diseases of medical importance.

A new role for an old antimicrobial: lysozyme c-1 can function to protect malaria parasites in Anopheles mosquitoes

BACKGROUND

Plasmodium requires an obligatory life stage in its mosquito host. The parasites encounter a number of insults while journeying through this host and have developed mechanisms to avoid host defenses. Lysozymes are a family of important antimicrobial immune effectors produced by mosquitoes in response to microbial challenge.

METHODOLOGY/PRINCIPAL FINDINGS

A mosquito lysozyme was identified as a protective agonist for Plasmodium. Immunohistochemical analyses demonstrated that Anopheles gambiae lysozyme c-1 binds to oocysts of Plasmodium berghei and Plasmodium falciparum at 2 and 5 days after infection. Similar results were observed with Anopheles stephensi and P. falciparum, suggesting wide occurrence of this phenomenon across parasite and vector species. Lysozyme c-1 did not bind to cultured ookinetes nor did recombinant lysozyme c-1 affect ookinete viability. dsRNA-mediated silencing of LYSC-1 in Anopheles gambiae significantly reduced the intensity and the prevalence of Plasmodium berghei infection. We conclude that this host antibacterial protein directly interacts with and facilitates development of Plasmodium oocysts within the mosquito.

CONCLUSIONS/SIGNIFICANCE

This work identifies mosquito lysozyme c-1 as a positive mediator of Plasmodium development as its reduction reduces parasite load in the mosquito host. These findings improve our understanding of parasite development and provide a novel target to interrupt parasite transmission to human hosts.

Molecular genetics evidence for the in vivo roles of the two major NADPH-dependent disulfide reductases in the malaria parasite

Malaria-associated pathology is caused by the continuous expansion of Plasmodium parasites inside host erythrocytes. To maintain a reducing intracellular milieu in an oxygen-rich environment, malaria parasites have evolved a complex antioxidative network based on two central electron donors, glutathione and thioredoxin. Here, we dissected the in vivo roles of both redox pathways by gene targeting of the respective NADPH-dependent disulfide reductases. We show that Plasmodium berghei glutathione reductase and thioredoxin reductase are dispensable for proliferation of the pathogenic blood stages. Intriguingly, glutathione reductase is vital for extracellular parasite development inside the insect vector, whereas thioredoxin reductase is dispensable during the entire parasite life cycle. Our findings suggest that glutathione reductase is the central player of the parasite redox network, whereas thioredoxin reductase fulfils a specialized and dispensable role for P. berghei. These results also indicate redundant roles of the Plasmodium redox pathways during the pathogenic blood phase and query their suitability as promising drug targets for antimalarial intervention strategies.

Naturally occurring triggers that induce apoptosis-like programmed cell death in Plasmodium berghei ookinetes

Several protozoan parasites have been shown to undergo a form of programmed cell death that exhibits morphological features associated with metazoan apoptosis. These include the rodent malaria parasite, Plasmodium berghei. Malaria zygotes develop in the mosquito midgut lumen, forming motile ookinetes. Up to 50% of these exhibit phenotypic markers of apoptosis; as do those grown in culture. We hypothesised that naturally occurring signals induce many ookinetes to undergo apoptosis before midgut traversal. To determine whether nitric oxide and reactive oxygen species act as such triggers, ookinetes were cultured with donors of these molecules. Exposure to the nitric oxide donor SNP induced a significant increase in ookinetes with condensed nuclear chromatin, activated caspase-like molecules and translocation of phosphatidylserine that was dose and time related. Results from an assay that detects the potential-dependent accumulation of aggregates of JC-1 in mitochondria suggested that nitric oxide does not operate via loss of mitochondrial membrane potential. L-DOPA (reactive oxygen species donor) also caused apoptosis in a dose and time dependent manner. Removal of white blood cells significantly decreased ookinetes exhibiting a marker of apoptosis in vitro. Inhibition of the activity of nitric oxide synthase in the mosquito midgut epithelium using L-NAME significantly decreased the proportion of apoptotic ookinetes and increased the number of oocysts that developed. Introduction of a nitric oxide donor into the blood meal had no effect on mosquito longevity but did reduce prevalence and intensity of infection. Thus, nitric oxide and reactive oxygen species are triggers of apoptosis in Plasmodium ookinetes. They occur naturally in the mosquito midgut lumen, sourced from infected blood and mosquito tissue. Up regulation of mosquito nitric oxide synthase activity has potential as a transmission blocking strategy.

Synergistic and additive effects of epigallocatechin gallate and digitonin on Plasmodium sporozoite survival and motility

BACKGROUND

Most medicinal plants contain a mixture of bioactive compounds, including chemicals that interact with intracellular targets and others that can act as adjuvants to facilitate absorption of polar agents across cellular membranes. However, little is known about synergistic effects between such potential drug candidates and adjuvants. To probe for such effects, we tested the green tea compound epigallocatechin gallate (EGCG) and the membrane permeabilising digitonin on Plasmodium sporozoite motility and viability.

METHODOLOGY/PRINCIPAL FINDINGS

Green fluorescent P. berghei sporozoites were imaged using a recently developed visual screening methodology. Motility and viability parameters were automatically analyzed and IC50 values were calculated, and the synergism of drug and adjuvant was assessed by the fractional inhibitory concentration index. Validating our visual screening procedure, we showed that sporozoite motility and liver cell infection is inhibited by EGCG at nontoxic concentrations. Digitonin synergistically increases the cytotoxicity of EGCG on sporozoite survival, but shows an additive effect on sporozoite motility.

CONCLUSIONS/SIGNIFICANCE

We proved the feasibility of performing highly reliable visual screens for compounds against Plasmodium sporozoites. We thereby could show an advantage of administering mixtures of plant metabolites on inhibition of cell motility and survival. Although the effective concentration of both drugs is too high for use in malaria prophylaxis, the demonstration of a synergistic effect between two plant compounds could lead to new avenues in drug discovery.

Direct microscopic quantification of dynamics of Plasmodium berghei sporozoite transmission from mosquitoes to mice

The number of malaria sporozoites delivered to a host by mosquitoes is thought to have a significant influence on the subsequent course of the infection in the mammalian host. We did studies with Anopheles stephensi mosquitoes with salivary gland infections of Plasmodium berghei sporozoites expressing a red fluorescent protein. After individual mosquitoes fed on an ear pinna or the ventral abdomen of a mouse, fluorescence microscopy was used to count numbers of sporozoites. Mosquitoes allowed to feed on the ear for periods of 3 versus 15 min deposited means of 281 versus 452 sporozoites, respectively, into the skin; this may have epidemiological implications because mosquitoes can feed for longer periods of time on sleeping hosts. Mosquitoes feeding on the ventral abdomen injected sporozoites not only into the skin but also into the underlying peritoneal musculature. Although mosquitoes injected fewer sporozoites into the abdominal tissues, more of these were reingested into the mosquito midgut, probably a consequence of easier access to blood intake from the abdominal area. The most consistent parameter of sporozoite transmission dynamics under all conditions of mosquito probing and feeding was the relatively slow release rate of sporozoites (approximately 1 to 2.5 per second) from the mosquito proboscis. The numbers of sporozoites introduced into the host by mosquitoes and the transmission efficiencies of sporozoite delivery are multifactorial phenomena that vary with length of probing time, skin site being fed upon, and numbers of sporozoites within the salivary glands.

In vivo imaging of malaria parasites in the murine liver

The form of the malaria parasite inoculated by the mosquito, called the sporozoite, transforms inside the host liver into thousands of a new form of the parasite, called the merozoite, which infects erythrocytes. We present here a protocol to visualize in vivo the behavior of Plasmodium berghei parasites in the hepatic tissue of the murine host. The use of GFP-expressing parasites and a high-speed spinning disk confocal microscope allows for the acquisition of four-dimensional images, which provide a time lapse view of parasite displacement and development in tissue volumes. These data can be analyzed to give information on the early events of sporozoite penetration of the hepatic tissue, that is, sporozoite gliding in the liver sinusoids, crossing the sinusoidal barrier, gliding in the parenchyma and traversal of hepatocytes, and invasion of a final hepatocyte, as well as the terminal events of merosome and merozoite release from infected hepatocytes. Combined with the use of mice expressing fluorescent cell types or cell markers, the system will provide useful information not only on the primary infection process, but also on parasite interactions with the host immune cells in the liver.

The role of metacaspase 1 in Plasmodium berghei development and apoptosis

The malaria parasite encodes a wide range of proteases necessary to facilitate its many developmental transitions in vertebrate and insect hosts. Amongst these is a predicted cysteine protease structurally related to caspases, named Plasmodium metacaspase 1 (PxMC1). We have generated Plasmodium berghei parasites in which the PbMC1coding sequence is removed and replaced with a green fluorescent reporter gene to investigate the expression of PbMC1, its contribution to parasite development, and its involvement in previously reported apoptosis-like cell death of P. berghei ookinetes. Our results show that the pbmc1 gene is expressed in female gametocytes and all downstream mosquito stages including sporozoites, but not in asexual blood stages. We failed to detect an apparent loss-of-function phenotype, suggesting that PbMC1 constitutes a functionally redundant gene. We discuss these findings in the context of two other putative Plasmodium metacaspases that we describe here.

Set regulation in asexual and sexual Plasmodium parasites reveals a novel mechanism of stage-specific expression

Transmission of the malaria parasite depends on specialized gamete precursors (gametocytes) that develop in the bloodstream of a vertebrate host. Gametocyte/gamete differentiation requires controlled patterns of gene expression and regulation not only of stage and gender-specific genes but also of genes associated with DNA replication and mitosis. Once taken up by mosquito, male gametocytes undergo three mitotic cycles within few minutes to produce eight motile gametes. Here we analysed, in two Plasmodium species, the expression of SET, a conserved nuclear protein involved in chromatin dynamics. SET is expressed in both asexual and sexual blood stages but strongly accumulates in male gametocytes. We demonstrated functionally the presence of two distinct promoters upstream of the set open reading frame, the one active in all blood stage parasites while the other active only in gametocytes and in a fraction of schizonts possibly committed to sexual differentiation. In ookinetes both promoters exhibit a basal activity, while in the oocysts the gametocyte-specific promoter is silent and the reporter gene is only transcribed from the constitutive promoter. This transcriptional control, described for the first time in Plasmodium, provides a mechanism by which single-copy genes can be differently modulated during parasite development. In male gametocytes an overexpression of SET might contribute to a prompt entry and execution of S/M phases within mosquito vector.

Plasmodium berghei alpha-tubulin II: a role in both male gamete formation and asexual blood stages

Plasmodium falciparum contains two genes encoding different isotypes of alpha-tubulin, alpha-tubulin I and alpha-tubulin II. alpha-Tubulin II is highly expressed in male gametocytes and forms part of the microtubules of the axoneme of male gametes. Here we present the characterization of Plasmodium berghei alpha-tubulin I and alpha-tubulin II that encode proteins of 453 and 450 amino acids, respectively. alpha-Tubulin II lacks the well-conserved three amino acid C-terminal extension including a terminal tyrosine residue present in alpha-tubulin I. Investigation of transcription by Northern analysis and RT-PCR and analysis of promoter activity by GFP tagging showed that alpha-tubulin I is expressed in all blood and mosquito stages. As expected, alpha-tubulin II was highly expressed in the male gametocytes, but transcription was also observed in the asexual blood stages, female gametocytes, ookinetes and oocysts. Gene disruption experiments using standard transfection technologies did not produce viable parasites indicating that both alpha-tubulin isotypes are essential for the asexual blood stages. Targeted modification of alpha-tubulin II by the addition of the three C-terminal amino acids of alpha-tubulin I did not affect either blood stage development nor male gamete formation. Attempts to modify the C-terminal region by adding a TAP tag to the endogenous alpha-tubulin II gene were not successful. Introduction of a transgene, expressing TAP-tagged alpha-tubulin II, next to the endogenous alpha-tubulin II gene, had no effect on the asexual blood stages but strongly impaired formation of male gametes. These results show that alpha-tubulin II not only plays an important role in the male gamete but is also expressed in and essential for asexual blood stage development.

A malaria membrane skeletal protein is essential for normal morphogenesis, motility, and infectivity of sporozoites

Membrane skeletons are structural elements that provide mechanical support to the plasma membrane and define cell shape. Here, we identify and characterize a putative protein component of the membrane skeleton of the malaria parasite. The protein, named PbIMC1a, is the structural orthologue of the Toxoplasma gondii inner membrane complex protein 1 (TgIMC1), a component of the membrane skeleton in tachyzoites. Using targeted gene disruption in the rodent malaria species Plasmodium berghei, we show that PbIMC1a is involved in sporozoite development, is necessary for providing normal sporozoite cell shape and mechanical stability, and is essential for sporozoite infectivity in insect and vertebrate hosts. Knockout of PbIMC1a protein expression reduces, but does not abolish, sporozoite gliding locomotion. We identify a family of proteins related to PbIMC1a in Plasmodium and other apicomplexan parasites. These results provide new functional insight in the role of membrane skeletons in apicomplexan parasite biology.

PTRAMP; a conserved Plasmodium thrombospondin-related apical merozoite protein

A gene encoding a 352 amino acid protein with a putative signal sequence, transmembrane domain and thrombospondin structural homology repeat was identified in the genome of the human malaria parasite, Plasmodium falciparum and the rodent malaria parasite, Plasmodium berghei. The protein localises in the apical organelles of P. falciparum and P. berghei merozoites within intraerythrocytic schizonts and has, therefore, been termed the Plasmodium thrombospondin-related apical merozoite protein (PTRAMP). PTRAMP co-localises with the Apical Merozoite Antigen-1 (AMA-1) in developing micronemes and subsequently relocates onto the merozoite surface. Although the gene appears to be specific to the Plasmodium genus, orthologues are present in the genomes of all malaria parasite species examined suggesting a conserved function in host-cell invasion. PTRAMP, therefore, has all the features to merit further evaluation as a malaria vaccine candidate.

Complement-Like Protein TEP1 Is a Determinant of Vectorial Capacity in the Malaria Vector Anopheles gambiae

Anopheles mosquitoes are major vectors of human malaria in Africa. Large variation exists in the ability of mosquitoes to serve as vectors and to transmit malaria parasites, but the molecular mechanisms that determine vectorial capacity remain poorly understood. We report that the hemocyte-specific complement-like protein TEP1 from the mosquito Anopheles gambiae binds to and mediates killing of midgut stages of the rodent malaria parasite Plasmodium berghei. The dsRNA knockdown of TEP1 in adults completely abolishes melanotic refractoriness in a genetically selected refractory strain. Moreover, in susceptible mosquitoes this knockdown increases the number of developing parasites. Our results suggest that the TEP1-dependent parasite killing is followed by a TEP1-independent clearance of dead parasites by lysis and/or melanization. Further elucidation of the molecular mechanisms of TEP1-mediated parasite killing will be of great importance for our understanding of the principles of vectorial capacity in insects.

In vivo gene silencing in Plasmodium berghei--a mouse malaria model

RNA interference (RNAi) has emerged as a specific and efficient tool to silence gene expression in a variety of organisms and cell lines. An important prospect for RNAi technology is its possible application in the treatment of diseases using short interfering RNAs (siRNAs). However, the effect of siRNAs in adult animals and their potential to treat or prevent diseases are yet to be fully investigated. The main goal of the present study is to find out whether it was possible to carry out RNAi on circulating malaria parasite in vivo. To trigger RNAi in mouse malaria parasite, we used siRNAs corresponding to cysteine protease genes of Plasmodium berghei (berghepain-1 & 2). Intravenous injections of berghepains' siRNAs in infected animal resulted in characteristic enlargement of food vacuole in circulating parasites. Protein analysis of these treated parasites showed substantial accumulation of hemoglobin, which is reminiscent of the effect observed upon treating Plasmodium falciparum with different cysteine protease inhibitors. Parasites treated with berghepain 1 & 2 siRNAs showed marked reduction in the levels of their cognate mRNAs, thereby suggesting specific inhibition of berghepains' gene expression in vivo. We also observed the generation of approximately 25 nt RNA species from berghepains' mRNAs in the treated parasites, which is a characteristic of an RNAi phenomenon. These results thus provide evidence that beyond its value for validation of gene functions, RNAi may provide a new approach for disease therapy.

SOAP, a novel malaria ookinete protein involved in mosquito midgut invasion and oocyst development

An essential, but poorly understood part of malaria transmission by mosquitoes is the development of the ookinetes into the sporozoite-producing oocysts on the mosquito midgut wall. For successful oocyst formation newly formed ookinetes in the midgut lumen must enter, traverse, and exit the midgut epithelium to reach the midgut basal lamina, processes collectively known as midgut invasion. After invasion ookinete-to-oocyst transition must occur, a process believed to require ookinete interactions with basal lamina components. Here, we report on a novel extracellular malaria protein expressed in ookinetes and young oocysts, named secreted ookinete adhesive protein (SOAP). The SOAP gene is highly conserved amongst Plasmodium species and appears to be unique to this genus. It encodes a predicted secreted and soluble protein with a modular structure composed of two unique cysteine-rich domains. Using the rodent malaria parasite Plasmodium berghei we show that SOAP is targeted to the micronemes and forms high molecular mass complexes via disulphide bonds. Moreover, SOAP interacts strongly with mosquito laminin in yeast-two-hybrid assays. Targeted disruption of the SOAP gene gives rise to ookinetes that are markedly impaired in their ability to invade the mosquito midgut and form oocysts. These results identify SOAP as a key molecule for ookinete-to-oocyst differentiation in mosquitoes.

Apoptosis in the malaria protozoan, Plasmodium berghei: a possible mechanism for limiting intensity of infection in the mosquito

Death by apoptosis regulates cell numbers in metazoan tissues and it is mediated by activation of caspases and results in characteristic morphological and biochemical changes. We report here that the malaria protozoan, Plasmodium berghei, exhibits features typical of metazoan apoptotic cells including condensation of chromatin, fragmentation of the nuclear DNA and movement of phosphatidylserine from the inner to the outer lamellae of the cell membrane. In addition, proteins with caspase-like activity were identified in the cytoplasm of the ookinete suggesting that the cellular mechanism of cell death may be similar to that of multicellular eukaryotes. Our data show that more than 50% of the mosquito midgut stages of the parasite die naturally by apoptosis before gut invasion. Cell death was prevented by a caspase inhibitor, treatment resulting in a doubling of parasite intensity. All these features also occur in vitro. Cell suicide thus plays a major and hitherto unrecognised role in controlling parasite populations and could be a novel target for malaria control strategies.

Complete development of mosquito phases of the malaria parasite in vitro

Methods for reproducible in vitro development of the mosquito stages of malaria parasites to produce infective sporozoites have been elusive for over 40 years. We have cultured gametocytes of Plasmodium berghei through to infectious sporozoites with efficiencies similar to those recorded in vivo and without the need for salivary gland invasion. Oocysts developed extracellularly in a system whose essential elements include co-cultured Drosophila S2 cells, basement membrane matrix, and insect tissue culture medium. Sporozoite production required the presence of para-aminobenzoic acid. The entire life cycle of P. berghei, a useful model malaria parasite, can now be achieved in vitro.

Interspecies conservation of gene order and intron-exon structure in a genomic locus of high gene density and complexity in Plasmodium

A 13.6 kb contig of chromosome 5 of Plasmodium berghei, a rodent malaria parasite, has been sequenced and analysed for its coding potential. Assembly and comparison of this genomic locus with the orthologous locus on chromosome 10 of the human malaria Plasmodium falciparum revealed an unexpectedly high level of conservation of the gene organisation and complexity, only partially predicted by current gene-finder algorithms. Adjacent putative genes, transcribed from complementary strands, overlap in their untranslated regions, introns and exons, resulting in a tight clustering of both regulatory and coding sequences, which is unprecedented for genome organisation of PLASMODIUM: In total, six putative genes were identified, three of which are transcribed in gametocytes, the precursor cells of gametes. At least in the case of two multiple exon genes, alternative splicing and alternative transcription initiation sites contribute to a flexible use of the dense information content of this locus. The data of the small sample presented here indicate the value of a comparative approach for Plasmodium to elucidate structure, organisation and gene content of complex genomic loci and emphasise the need to integrate biological data of all Plasmodium species into the P.falciparum genome database and associated projects such as PlasmodB to further improve their annotation.

Fluorescent Plasmodium berghei sporozoites and pre-erythrocytic stages: a new tool to study mosquito and mammalian host interactions with malaria parasites

To track malaria parasites for biological studies within the mosquito and mammalian hosts, we constructed a stably transformed clonal line of Plasmodium berghei, PbFluspo, in which sporogonic and pre-erythrocytic liver-stage parasites are autonomously fluorescent. A cassette containing the structural gene for the FACS-adapted green fluorescent protein mutant 2 (GFPmut2), expressed from the 5' and 3' flanking sequences of the circumsporozoite (CS) protein gene, was integrated and expressed at the endogenous CS locus. Recombinant parasites, which bear a wild-type copy of CS, generated highly fluorescent oocysts and sporozoites that invaded mosquito salivary glands and were transmitted normally to rodent hosts. The parasites infected cultured hepatocytes in vitro, where they developed into fluorescent pre-erythrocytic forms. Mammalian cells infected by these parasites can be separated from non-infected cells by fluorescence activated cell sorter (FACS) analysis. These fluorescent insect and mammalian stages of P. berghei should be useful for phenotypic studies in their respective hosts, as well as for identification of new genes expressed in these parasite stages.

Malaria-infected erythrocytes serve as biological standards to ensure reliable and consistent scoring of micronucleated erythrocytes by flow cytometry

A procedure for optimizing the configuration of flow cytometers for enumerating micronucleated erythrocytes is described. The method is based on the use of a biological model for micronucleated erythrocytes, the malaria parasite Plasmodium berghei. P. berghei endows target cells of interest (erythrocytes) with a micronucleus-like DNA content. Unlike micronuclei, parasitized red blood cells have a homogenous DNA content, and can be very prevalent in circulation. These characteristics make malaria-infected erythrocytes extremely well suited for optimizing instrument setup on a daily basis. The experiment described herein was designed to test the hypothesis that malaria-infected erythrocytes can greatly enhance the consistency with which flow cytometers are configured for micronucleus analyses, and thereby minimize intra- and interexperimental variation. Data collected over the course of several months, on two different flow cytometers, supports the premise that malaria-infected blood represents a useful biological standard which helps ensure reliable and consistent flow cytometric enumeration of rare micronucleated erythrocytes.

The biosynthesis and post-translational modification of Pbs21 an ookinete-surface protein of Plasmodium berghei

Radiolabelled methionine incorporation into synchronised Plasmodium berghei gametocytes or ookinete cultures, showed that Pbs21 is not synthesised in bloodstage parasites; synthesis was detected within three hours of induction of gametogenesis; synthesis was triggered at gametogenesis, not by fertilisation. We show native Pbs21 to be a hydrophobic membrane protein that was insensitive to cleavage by phosphatidylinositol phospholipase C (PI-PLC), but sensitive to alkaline hydroxylamine, and partially sensitive to glycosylphosphatidylinositol-dependent phospholipase D (GPI-PLD) and HNO2. 3H-myristic and palmitic acid, 3H-glucosamine and mannose incorporation indicated Pbs21 was acylated and glycosylated. Linkage of the acyl group was sensitive to HNO2, which released an acyl-phosphatidylinositol more hydrophobic than that released from P3 of Trypanosoma brucei. All these properties are consistent with the presence of a malaria-specific glycosylphosphatidylinositol (GPI) anchor. In contrast recombinant Pbs21 (rPbs21), expressed in Spodoptera frugiperda cells, was sensitive to both PI-PLC and GPI-PLD, consistent with the protein being modified by a different (S. frugiperda) GPI anchor. Brefeldin A blocked secretion of rPbs21 within a cytoplasmic reticular compartment. Following deletion of the putative GPI anchor addition site (amino acids 189 213), the protein was transported to the cell surface and secreted directly into the aqueous phase of the culture medium. Deletion of amino acids 205-213 disrupted Pbs21 processing, transport through the ER and distribution onto the cell surface. Deletion of amino acids 1-28 prevented transport of Pbs21 into the ER. This suggests that correct processing of the GPI anchor in the ER-Golgi network is essential for the successful secretion of the recombinant protein, which is additionally dependent upon an N-terminal secretory signal sequence.

Characterization of the effector mechanisms of a transmission-blocking antibody upon differentiation of Plasmodium berghei gametocytes into ookinetes in vitro

The transmission-blocking monoclonal antibody 13.1, which recognizes the ookinete surface antigen Pbs21 of Plasmodium berghei, and an IgG2a isotype control antibody 26.37 were purified by caprylic acid and ammonium sulphate precipitation. Fab fragments were prepared by papain digestion. IgG but not Fab from antibody 13.1 reduced ookinete formation by P. berghei in culture by as much as 94% at a concentration of 100 micrograms/ml. There was little difference in antibody efficacy in the range 6.25-400 micrograms/ml in this assay. The parasite was most sensitive to antibody activity in the first 6-9 h of culture, i.e. the gamete/zygote and early retort stages. Peripheral blood leucocytes (PBL) were essential to achieve maximal inhibition by mAb 13.1 (activity was abrogated totally if PBL were removed). Together the data suggest that one of the mechanisms of action of this antibody is antibody-mediated PBL killing. Phagocytosis of parasites was noted in these experiments in all cultures. We have not attempted in this study to distinguish between Fc-mediated opsonization, as opposed to antibody-dependent cellular cytotoxicity.

Plasmodium berghei: in vivo generation and selection of karyotype mutants and non-gametocyte producer mutants

We previously reported that karyotype and gametocyte-producer mutants spontaneously arose during in vivo asexual multiplication of Plasmodium berghei. Here we studied the rate of selection of these mutants in vivo. Gametocyte production and karyotype pattern were established at regular intervals during prolonged periods of asexual multiplication of clone 8417 of P. berghei. We found that karyotype mutants and mutants which do not produce gametocytes can replace the original high-producer parasites of clone 8417 within several weeks. The time at which mutants became predominant in the population in different experiments, however, differed greatly. Mutants with intermediate or low gametocyte production were not found. In experimentally mixed infections, containing parasites from two clones from different strains (clone 8417 of the ANKA strain; clone 1 of the K173 strain), high-producer parasites of clone 8417 were overgrown by parasites of the nonproducer clone. Nonproducer mutants from the originally high-producer clone 8417, however, were able to coexist with parasites of the nonproducer clone. These results demonstrate that in our experiments nonproducer parasites had a strong selective advantage during asexual multiplication compared to high producers. All karyotype mutants which became predominant in our experiments were nonproducers. In two experiments a change in karyotype coincided with the loss of gametocyte production which may suggest a causal relationship between these events.

Malaria sporozoites release circumsporozoite protein from their apical end and translocate it along their surface

Plasmodium sporozoites, the causative agents of malaria, release circumsporozoite (CS) protein into medium when under conditions simulating those that the parasites encounter in the bloodstream of the vertebrate host. CS protein of the rodent parasite, Plasmodium berghei, is released as the lower molecular weight form, Pb44. This release is substratum- and antibody-independent. Previous studies show that CS protein is released at the trailing, posterior end of motile sporozoites. Video and electron microscopic studies now demonstrate that CS protein is released at the apical end of cytochalasin b-immobilized sporozoites. We propose that CS protein released from the apical end, the leading end of gliding sporozoites, adheres to the sporozoite surface and is translocated posteriorly by a cytochalasin-sensitive and apparently actin-mediated surface motor, which drives gliding motility. This model explains the mechanism of both the circumsporozoite precipitation (CSP) reaction and formation of the CS protein trail by gliding sporozoites.

The development and routine application of high-density exoerythrocytic-stage cultures of Plasmodium berghei

Methods are reviewed for the culture of the exoerythrocytic stages of Plasmodium berghei wherein development reproducibly reflects growth observed in vivo in laboratory rodents. The combination of these methods with the culture of both asexual and sexual blood stages has allowed the completion of the entire vertebrate phase of malaria development in vitro. The development of new methods for high-density exoerythrocytic-stage culture combined with robust statistical analysis of parasite growth by morphological (light microscopy), or DNA probe methods now allows the critical and precise evaluation of chemotherapeutic or immunological treatments. These methods are illustrated by data obtained on pyrimethamine, primaquine and a hydroxynaphthoquinone. Some of the new avenues of research made feasible by the high-density cultures, e.g., direct immunization to produce monoclonal antibodies and biochemical studies are discussed.

Ookinete antigens of Plasmodium berghei: a light and electron-microscope immunogold study of expression of the 21 kDa determinant recognized by a transmission-blocking antibody

The expression of a 21 kDa transmission-blocking determinant on the malarial parasite Plasmodium berghei was studied by using the immunogold method at the light, scanning-electron and transmission-electron microscope levels. The determinant was shown to be expressed exclusively on the macrogamete and its immediate progeny the zygote, ookinete and oocyst. It is first detected on the plasmalemma two hours after the escape of the parasite from the red blood cell, reaches a maximal density on the young ookinete some ten hours later, and is still found on the oocyst after six days. The antigen is distributed evenly over the entire surface of the zygote and ookinete, but is readily shed from the parasite surface. The general applicability of the silver-enhanced immunogold method in parasitological research is emphasized.

Organization of the exoerythrocytic stage of the rodent malaria parasite Plasmodium berghei. A cytochemical study

With the osmium tetroxide-zinc iodide impregnation technique the visualization of the internal organization of the exoerythrocytic form of the rodent malaria parasite Plasmodium berghei was improved. Osmium impregnation leached the ground matrix of the parasite thereby displaying a system of intermediate-sized filaments. Because microtubules are only present as part of the mitotic spindle and as remnants of the sporozoite cytoskeleton, the observed intermediate-sized filaments comprise most of the cytoskeletal organization of the liver stage malaria parasite.

Characterization of monoclonal antibodies directed against erythrocytic stage antigens of Plasmodium berghei

Monoclonal antibodies recognizing various facets of the malaria parasite Plasmodium berghei and of the infected erythrocyte were obtained after generation of hybridomas between spleen cells from immunized mice and myeloma cells. The monoclonal antibodies were characterized by enzyme-linked immunosorbent assay, indirect immunofluorescence, immunoprecipitation of [35S]methionine-labeled proteins and immunoblotting. The most readily identified antigen was a parasite surface-associated protein of 230 kDa which is similar to the polymorphic schizont antigen described in a number of malarial species. In addition, three distinct antigens of 13, 31 and 120 kDa, which are external to the parasite, but within the infected erythrocyte were identified.

Plasmodium berghei sporozoite invasion is blocked in vitro by sporozoite-immobilizing antibodies

A monoclonal antibody directed against the circumsporozoite protein on the surface of Plasmodium berghei sporozoites inhibited sporozoite motility in vitro. These immobilized sporozoites could adhere to but not invade target cultured cells. Other sporozoite-immobilizing agents also inhibited sporozoite invasion into cultured cells and did not prevent sporozoite adherence. These results indicate that sporozoite invasiveness is associated with sporozoite motility. Thus, the immobilizing effect of this antibody could explain its functional activity against sporozoite invasion in vivo.

Effect of macrophage activation on phagocyte-Plasmodium interaction

We investigated the effect of both immune and normal sera on the binding of free Plasmodium berghei by resident and activated macrophages. Resident macrophages bound plasmodia to a greater extent than did activated macrophages, regardless of treatment. Resident macrophages bound free plasmodia, predominantly trophozoites, in the presence of normal serum by a mechanism inhibited by N-acetylglucosamine and N-acetylmannosamine. Macrophages activated through treatment with Propionibacterium acnes ("Corynebacterium parvum"), on the other hand, did not bind free plasmodia in the presence of normal serum through systems inhibited by N-acetylmannosamine or N-acetylglucosamine. The binding of free plasmodia by activated macrophages was greatest in the presence of immune serum and could be inhibited by immune complexes but not by N-acetylmannosamine or N-acetylglucosamine. These results suggest that a receptor for a carbohydrate component of a normal serum opsonin mediates initial adherence of plasmodial antigen onto resident macrophages, triggering both the immunological cascade and macrophage activation. After activation, the macrophages no longer have the carbohydrate-specific receptor but do have functional Fc receptors which mediate the adherence of immune-serum-opsonized plasmodia.

Identification of the meiotic division of malarial parasites

Zygotes of Plasmodium berghei were cultured 15-25 h in vitro to yield mature infective ookinetes. Samples taken in the first 5 h of culture were examined by electron microscopy. Meiotic figures were detected in the nuclei of the zygotes. Threadlike leptotene chromatids (chromosomes) condensed from attachment plaques on the nuclear envelope; chromatid pairing followed (zygotene), with synaptonemal complexes subsequently appearing (pachytene). These complexes persisted into metaphase but dissociated when the chromatids rapidly decondensed during anaphase. At telophase of the first meiotic division the kinetochores were retracted toward two small spindle complexes, which were found at widely separated poles in the nuclear envelope. The observations are consistent with a haploid genome of 8-10 chromosomes.

The development of Plasmodium ookinetes in vitro: an ultrastructural study including a description of meiotic division

Ookinetes have been cultured in vitro using modifications to the method of Weiss & Vanderberg (1977). Significant improvements in technique were produced by culture in medium at pH 8.4 and at a blood dilution at or over 1/10. Ookinetes produced were infective to mosquitoes by membrane feeding techniques. Ultrastructural analyses were made of nuclear, cytoskeletal, crystalloid and microneme development. The first intranuclear division in the zygote has been recognized as meiosis. Chromosome condensation during prophase follows the classical stages of leptotene, zygotene and pachytene. Diplotene and diakinesis are not present - the synaptonemal complexes persist into metaphase I. Chromosomes separate at anaphase and rapidly de-condense prior to telophase. We have not recognized a second meiotic division in the ookinete. The implication of these findings to the molecular and Mendelian organization of the parasite genome are discussed.

Transformation of sporozoites of Plasmodium berghei into exoerythrocytic forms in the liver of its mammalian host

Intrahepatocytic transformation in vivo of the rodent malaria sporozoite of Plasmodium berghei, into the young trophic exoerythrocytic tissue stage was studied by immunofluorescence, light- and electron microscopy. The first 20 h of intracellular life were involved entirely in dedifferentiation with limited proliferation of organelles. From about 20 h onwards nuclear division commenced, rough endoplasmic reticulum became markedly expanded, and mitochondria increased in numbers. However, remains of the sporozoite pellicle (i.e., inner membranes and subpellicular microtubules) persisted for at least 28 h, which correlates with the persisting reaction of young exoerythrocytic forms with antisporozoite antibodies. In general, the basic mechanism of transformation resembles that of the ookinete into oocyst and that of the merozoite into erythrocytic trophozoite.

In vitro formation of ookinetes and functional maturity of Plasmodium berghei gametocytes

In vitro formation of Plasmodium berghei ookinetes was studied. Gametocytes produced in vitro were obtained from heart and tail blood of Swiss mice and from blood removed from mosquitoes directly after feeding on these mice. In vitro produced gametocytes were obtained from short-term cultures of the erythrocytic stages of P. berghei. Reproducible ookinete production was obtained in medium RPMI 1640, pH 7.8-8.0, using in vivo and in vitro produced gametocytes. The morphology of developmental stages of ookinetes and degenerate forms at the light microscope level is described. More ookinetes were produced in medium RPMI 1640 compared to MEM and ookinete yield--defined as the ratio between the number of in vitro produced ookinetes/10(5) erythrocytes and the number of exflagellations/10(5) erythrocytes in the infected blood--increased with lower erythrocyte densities in the cultures within the range of dilutions tested. A linear relationship existed between gametocytaemia and the number of ookinetes produced. The methods for in vitro ookinete formation and for estimating ookinete yields enabled us to study aspects of functional maturity of gametocytes independent of mosquitoes. The numbers of exflagellating gametocytes and in vitro ookinete yields in tail blood corresponded with those in heart blood and blood ingested by mosquitoes, suggesting a random distribution of functionally mature gametocytes within the vertebrate host.

Histochemical observations on the exoerythrocytic malaria parasite Plasmodium berghei in rat liver

Enzyme histochemical methods were performed on sporozoite infected liver tissue of rats in order to gain insight into the nutrition and metabolism of exoerythrocytic forms of Plasmodium berghei. The following enzymes were demonstrated in the hepatocytic stages of the parasites, obtained 41 and 48 h after inoculation of sporozoites: acid phosphatase, cytochrome oxidase, NADH-tetrazolium reductase, succinate dehydrogenase, NAD+ and NADP+ dependent isocitrate dehydrogenase, NADP+-dependent malate dehydrogenase, lactate dehydrogenases, 6-phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenases and alpha-glycerol-phosphate dehydrogenase. The results suggest that a conventional Embden-Meyerhoff pathway, pentose phosphate pathway and Krebs' citric acid cycle may in part be present in these exoerythrocytic parasites. Alkaline phosphatase, nucleoside polyphosphatase, 5' nucleotidase, glucose-6-phosphatase, alpha-glucan phosphorylase, NAD+ dependent malate dehydrogenase, amino-peptidase M and non-specific esterases were not detected by our techniques in the parasite. The enzyme distribution of this intrahepatocytic malaria parasite revealed by histochemistry is compared with the enzyme distribution in the other phases of the parasite's life cycle.

An ultrastructural study on the role of Kupffer cells in the process of infection by Plasmodium berghei sporozoites in rats

The interactions in vivo between Plasmodium berghei sporozoites and Kupffer cells in rat livers were studied by transmission electron microscopy. By 10 and 15 min after inoculation, sporozoites were both free in the liver sinusoids and inside endocytotic vacuoles of the Kupffer cells. The latter cells were very active in phagocytosing sporozoites, bacteria and red blood cells. The sporozoites retained their integrity inside the endocytotic vacuoles and no signs of lysosomal digestion were observed. Sporozoites seen within endocytotic vacuoles 1 h after inoculation were still morphologically intact, although bristle-coated vesicles fused with the vacuole membrane. Evidence is presented which suggests that Kupffer cells transport sporozoites towards the space of Disse and adjacent hepatocytes. No sporozoites were seen to penetrate an endothelial cell or its narrow fenestrae. It is proposed that Kupffer cell passage, rather than gaps in the sinusoidal lining, represents the normal route that sporozoites take to circumvent the endothelial barrier. The localization of exo-erythrocytic forms was made easier by the use of Brown Norway rats in which many more parasites develop than in the Wistar rats. The distribution pattern of the parasites was found to be mainly around the 'periportal' zones of the acini of liver tissue.

Free-flow electrophoretic separation of Plasmodium berghei sporozoites

Sporozoites of the rodent malaria, Plasmodium berghei, were obtained from infected Anopheles stephensi by grinding mosquitoes, prepurifying the material in a discontinuous Hypaque gradient and further purifying by means of continuous free-flow electrophoresis. Bacteria, debris, mitochondria, mitoplasts, and other contaminants were removed in the electric field. The isolated sporozoites were morphologically intact and were positive in indirect immunofluorescence assay. They were infective to mice prior to and following free-flow electrophoretic separation. The surface of the sporozoites exhibited a polysaccharide-rich layer. The predominant surface protein labelled after surface iodination had a molecular weight between 42,000 and 46,000 daltons.

A simple method for separation of uninfected erythrocytes from those infected with Plasmodium berghei and for isolation of artificially released parasites

Rat erythrocytes infected with Plasmodium berghei were disrupted by gentle passage of Concanavalin A (Con A) agglutinated cells through a 100 mesh stainless steel grid. The free parasites were separated from cell debris, unbroken infected cells, and from uninfected rat erythrocytes on a Percoll gradient. The parasites remained morphologically intact, metabolically active, and infective to mice. The parasites were observed by light and electron microscopy. The incorporation of 3H-isoleucine and 3H-hypoxanthine was compared in intact and infected cells, and the infectivity was measured by the injection of parasites into susceptible mice. It seems that the combination of the two techniques used, produces a high yield of intact free parasites suitable for physiological or immunological studies.

[Changes in Plasmodium berghei berghei in mice maintained at high temperatures]

The study of the evolution of Plasmodium berghei berghei is made in mice kept in a high temperature (35 degrees C) throughout the experiment. Some of these mouse parasites (less than 30%) show a gigantic atypical morphology. In the parasite growing in animals kept at 35 degrees C, the amount of DNA is higher than DNA rate of the parasites growing in control mice (20-22 degrees C). There is no evidence of any relation between the increase of DNA amount and the morphological modification of these parasites.

Studies on the invasive ability of malarial merozoites (Plasmodium berghei)

Studies were performed to evaluate several methods for the artificial removal of Plasmodium berghei merozoites from infected mouse erythrocytes. These methods, many of which have been reported to yield free parasites capable of establishing a patent infection when injected into a suitable host, included NH4C1-mediated lysis, complement-mediated immune lysis, pressure filtration, and multiple-burst and continuous-flow sonication. Free parasites isolated from infected mouse blood were examined in vitro under conditions known to support merozoite invasion, and were found to be noninvasive, irrespective of the method used for their isolation. Although all methods tested achieved high degrees of lysis, none removed all intact parasitized erythrocytes. Using multiple-burst and continuous-flow sonication, the infective potential of free parasite preparations could be accounted for solely on the basis of the intact parasitized cells contaminating the free parasite preparations.

Isolation and analysis of nucleotides from erythrocyte-free malarial parasites (Plasmodium berghei) and potential relevance to malaria chemotherapy

Studies using erythrocyte-free preparations of P. berghei were conducted with a view to improving knowledge of parasite metabolism, particularly nucleotide metabolism. The free parasites employed in these studies were prepared by saponin lysis of parasitized mouse erythrocytes in isotonic glucose solutions. A comparative study of post-lytic metabolic activity of free parasites prepared by saponin, ammonium chloride, or osmotic lysis indicated a significantly greater retention of metabolic activity in the saponin-lysis preparations. Separations of nucleoside mono-, di-, and triphosphates extracted from free parasites were performed by means of high pressure liquid chromatography (HPLC), and ATP was additionally measured by luciferin-luciferase assay. Studies designed to differentiate among uptake, phosphorylation, and subsequent incorporation of (3)H-adenosine into nucleic acids of the free parasite strongly suggested that adenosine is metabolized either outside or on the parasite membrane, being first deaminated to inosine and then deribosylated to hypoxanthine. Observations from HPLC and radioisotope precursor studies support a hypothesis in which hypoxanthine may be proposed as being a pivotal substrate for purine salvage by malarial parasites. Some of the key steps in purine salvage and pyrimidine biosynthesis were investigated, using radiolabel uptake studies and HPLC analysis of nucleotides of the free malarial parasite. These studies suggest that hypoxanthine uptake may constitute an important new basis for chemotherapeutic attack on the malarial parasite.