Cultivation of Plasmodium falciparum parasites in a serum-free medium

Stearoyl-CoA desaturase regulates organelle biogenesis and hepatic merozoite formation in Plasmodium berghei

Plasmodium is an obligate intracellular parasite that requires intense lipid synthesis for membrane biogenesis and survival. One of the principal membrane components is oleic acid, which is needed to maintain the membrane's biophysical properties and fluidity. The malaria parasite can modify fatty acids, and stearoyl-CoA Δ9-desaturase (Scd) is an enzyme that catalyzes the synthesis of oleic acid by desaturation of stearic acid. Scd is dispensable in P. falciparum blood stages; however, its role in mosquito and liver stages remains unknown. We show that P. berghei Scd localizes to the ER in the blood and liver stages. Disruption of Scd in the rodent malaria parasite P. berghei did not affect parasite blood stage propagation, mosquito stage development, or early liver-stage development. However, when Scd KO sporozoites were inoculated intravenously or by mosquito bite into mice, they failed to initiate blood-stage infection. Immunofluorescence analysis revealed that organelle biogenesis was impaired and merozoite formation was abolished, which initiates blood-stage infections. Genetic complementation of the KO parasites restored merozoite formation to a level similar to that of WT parasites. Mice immunized with Scd KO sporozoites confer long-lasting sterile protection against infectious sporozoite challenge. Thus, the Scd KO parasite is an appealing candidate for inducing protective pre-erythrocytic immunity and hence its utility as a GAP.

In vitro infection of bovine erythrocytes with Theileria annulata merozoites as a key step in completing the T. annulata life cycle in vitro

Theileria annulata is a protozoan parasite with a complex life cycle involving a bovine host and a tick vector. It is transmitted by Hyalomma ticks and is the causative agent of tropical theileriosis, a debilitating and often fatal disease in southern Europe, northern Africa and large parts of Asia. Understanding the biology of different life cycle stages is critical for the control of tropical theileriosis and requires the use of experimental animals which poses an ethical concern. We present for the first time the in vitro infection of red blood cells (RBCs) with T. annulata differentiated schizonts. The Ankara cell line of T. annulata was cultured at 41 °C for nine days to induce merogony and subsequently incubated with purified RBCs for one to three days. Percentage of parasitized erythrocyte (PPE) over the short culture period was estimated by Giemsa staining (0.007-0.01%), Flow cytometry activated sorting (FACS) (0.02-1.1%) and observation of FACS sorted cells by confocal microscopy (0.05-0.4%). There was a significant difference in the PPE between FACS and the two other techniques (one-way ANOVA followed by Tukey test, P = 0.004) but no significant difference was observed between the confocal imaging and Giemsa staining methods (ANOVA one-way followed by Tukey test, P = 0.06). Importantly, all three complementary methods confirmed the invasion of RBCs by T. annulata merozoites in vitro. Although the experimental conditions will require further optimization to increase the PPE, the in vitro infection of RBCs by T. annulata merozoites is pivotal in paving the way for the eventual completion of the T. annulata life cycle in vitro when combined with artificial tick feeding.

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

Pharmacological activation of PIEZO1 in human red blood cells prevents Plasmodium falciparum invasion

An inherited gain-of-function variant (E756del) in the mechanosensitive cationic channel PIEZO1 was shown to confer a significant protection against severe malaria. Here, we demonstrate in vitro that human red blood cell (RBC) infection by Plasmodium falciparum is prevented by the pharmacological activation of PIEZO1. Yoda1 causes an increase in intracellular calcium associated with rapid echinocytosis that inhibits RBC invasion, without affecting parasite intraerythrocytic growth, division or egress. Notably, Yoda1 treatment significantly decreases merozoite attachment and subsequent RBC deformation. Intracellular Na⁺/K⁺ imbalance is unrelated to the mechanism of protection, although delayed RBC dehydration observed in the standard parasite culture medium RPMI/albumax further enhances the resistance to malaria conferred by Yoda1. The chemically unrelated Jedi2 PIEZO1 activator similarly causes echinocytosis and RBC dehydration associated with resistance to malaria invasion. Spiky outward membrane projections are anticipated to reduce the effective surface area required for both merozoite attachment and internalization upon pharmacological activation of PIEZO1. Globally, our findings indicate that the loss of the typical biconcave discoid shape of RBCs, together with an altered optimal surface to volume ratio, induced by PIEZO1 pharmacological activation prevent efficient P. falciparum invasion.

Cultivation, cryopreservation and resuscitation of Theileria annulata transformed cells in serum-free media

Introduction

Tropical theileriosis is a protozoan disease caused by Theileria annulata that affects cattle in Northern Africa, the Middle East and Asia where vector ticks of the genus Hyalomma occur. Various measures are applied to control the disease, including vaccination with attenuated T. annulata schizonts. Cultivation of T. annulata schizonts is mainly conducted in media containing Fetal Bovine Serum (FBS), which has some disadvantages such as costs, batch- to-batch variation and ethical concerns.

Methods

In this study, we conducted three experiments to evaluate the ability of (1) T. annulata strains grown in RPMI with 10% FBS (RPMI-FBS) to adapt and grow in serum-free media (i.e., HL-1, RPMI without FBS supplementation, ISF-1, and M199), (2) a T. annulata strain grown in ISF-1 and subsequently frozen in this medium to grow in ISF-1 again after long-term storage in liquid nitrogen, and (3) a T. annulata strain freshly isolated from infected bovine lymphocytes to growin ISF-1, also after cryopreservation. Cell numbers, schizont index, the viability and generation doubling time were calculated in all experiments.

Results and discussion

In the first experiment, the Hessiene and Beja cell lines from Tunisia previously cultivated in RPMI-FBS and adapted to serum-free media continued to grow significantly better in RPMI-FBS compared to the serum-freemedia. In the second experiment, a Tunisian cell line (Hessiene) cryopreserved in ISF-1 with 5%[v/v] dimethylsulfoxide (DMSO) grewbetter after thawing in RPMI-FBS compared to ISF-1 with a highly significant difference in cell growth (p < 0.001), whereas the third experiment showed that the Ankara cell line had similar growth characteristics in both RPMI-FBS and ISF-1 before and after thawing, with a shorter generation doubling time in ISF-1 than in RPMI-FBS (p = 0.23). Our findings suggest that freshly isolated cells can be propagated, frozen and thawed in serum-free media such as ISF-1, but once cells are adapted to cultivation in the presence of FBS or resuscitated from frozen storage, propagation in serum-free media may not perform as well as cultivation in RPMI-FBS.

Development of a continuous assay for high throughput screening to identify inhibitors of the purine salvage pathway in Plasmodium falciparum

Malaria, an infectious disease caused by protozoan parasites from the genus Plasmodium, represents a serious global health threat. The continued emergence of drug resistant strains has severely decreased current antimalarial drug efficacy and led to a perpetual race for drug discovery. Most protozoan parasites, including Plasmodium spp., are unable to synthesize purines de novo and instead rely on an essential purine salvage pathway for acquisition of purines from the infected host. Because purines are essential for Plasmodium growth and survival, the enzymes of the purine salvage pathway represent promising targets for drug discovery. Target-based high-throughput screening (HTS) assays traditionally focus on a single target, which severely limits the screening power of this type of approach. To circumvent this limitation, we have reconstituted the purine salvage pathway from Plasmodium falciparum in an assay combining four drug targets. This assay was developed for HTS and optimized to detect partial inhibition of any of the four enzymes in the pathway. Inhibitors of several enzymes in the pathway were identified in a pilot screen, with several compounds exhibiting effective inhibition when provided in micromolar amounts.

From marginal to essential: the golden thread between nutrient sensing, medium composition and Plasmodium vivax maturation in in vitro culture

Historically neglected, due to its biological peculiarities, the absence of a continuous long-term in vitro blood stage culture system and a propensity towards high morbidity rather than mortality, Plasmodium vivax was put back on the agenda during the last decade by the paradigm shift in the fight against malaria from malaria control to malaria eradication. While the incidence of the deadliest form of malaria, Plasmodium falciparum malaria, has declined since this paradigm shift took hold, the prospects of eradication are now threatened by the increase in the incidence of other human malaria parasite species. Plasmodium vivax is geographically the most widely distributed human malaria parasite, characterized by millions of clinical cases every year and responsible for a massive economic burden. The urgent need to tackle the unique biological challenges posed by this parasite led to renewed efforts aimed at establishing a continuous, long-term in vitro P. vivax blood stage culture. Based on recent discoveries on the role of nutrient sensing in Plasmodium’s pathophysiology, this review article critically assesses the extensive body of literature concerning Plasmodium culture conditions with a specific focus on culture media used in attempts to culture different Plasmodium spp. Hereby, the effect of specific media components on the parasite’s in vitro fitness and the maturation of the parasite’s host cell, the reticulocyte, is analysed. Challenging the wide-held belief that it is sufficient to find the right parasite isolate and give it the right type of cells to invade for P. vivax to grow in vitro, this review contends that a healthy side-by-side maturation of both the parasite and its host cell, the reticulocyte, is necessary in the adaptation of P. vivax to in vitro growth and argues that culture conditions and the media in particular play an essential role in this maturation process.

Revisiting gametocyte biology in malaria parasites

Gametocytes are the only form of the malaria parasite that is transmissible to the mosquito vector. They are present at low levels in blood circulation and significant knowledge gaps exist in their biology. Recent reductions in the global malaria burden have brought the possibility of elimination and eradication, with renewed focus on malaria transmission biology as a basis for interventions. This review discusses recent insights into gametocyte biology in the major human malaria parasite, Plasmodium falciparum and related species.

Plasmodium falciparum GPCR-like receptor SR25 mediates extracellular K+ sensing coupled to Ca2+ signaling and stress survival

The malaria parasite Plasmodium falciparum is exposed, during its development, to major changes of ionic composition in its surrounding medium. We demonstrate that the P. falciparum serpentine-like receptor PfSR25 is a monovalent cation sensor capable of modulating Ca²⁺ signaling in the parasites. Changing from high (140 mM) to low (5.4 mM) KCl concentration triggers [Ca²⁺]_cyt increase in isolated parasites and this Ca²⁺ rise is blocked either by phospholipase C (PLC) inhibition or by depleting the parasite’s internal Ca²⁺ pools. This response persists even in the absence of free extracellular Ca²⁺ and cannot be elicited by addition of Na⁺, Mg²⁺ or Ca²⁺. However, when the PfSR25 gene was deleted, no effect on [Ca²⁺]_cyt was observed in response to changing KCl concentration in the knocked out (PfSR25⁻) parasite. Finally, we also demonstrate that: i) PfSR25 plays a role in parasite volume regulation, as hyperosmotic stress induces a significant decrease in parasite volume in wild type (wt), but not in PfSR25⁻ parasites; ii) parasites lacking PfSR25 show decreased parasitemia and metacaspase gene expression on exposure to the nitric oxide donor sodium nitroprusside (SNP) and iii), compared to PfSR25⁻ parasites, wt parasites showed a better survival in albumax-deprived condition.

In vitro adaptation of Plasmodium falciparum reveal variations in cultivability

BACKGROUND

Culture-adapted Plasmodium falciparum parasites can offer deeper understanding of geographic variations in drug resistance, pathogenesis and immune evasion. To help ground population-based calculations and inferences from culture-adapted parasites, the complete range of parasites from a study area must be well represented in any collection. To this end, standardized adaptation methods and determinants of successful in vitro adaption were sought.

METHODS

Venous blood was collected from 33 P. falciparum-infected individuals at Goa Medical College and Hospital (Bambolim, Goa, India). Culture variables such as whole blood versus washed blood, heat-inactivated plasma versus Albumax, and different starting haematocrit levels were tested on fresh blood samples from patients. In vitro adaptation was considered successful when two four-fold or greater increases in parasitaemia were observed within, at most, 33 days of attempted culture. Subsequently, parasites from the same patients, which were originally cryopreserved following blood draw, were retested for adaptability for 45 days using identical host red blood cells (RBCs) and culture media.

RESULTS

At a new endemic area research site, ~65% of tested patient samples, with varied patient history and clinical presentation, were successfully culture-adapted immediately after blood collection. Cultures set up at 1% haematocrit and 0.5% Albumax adapted most rapidly, but no single test condition was uniformly fatal to culture adaptation. Success was not limited by low patient parasitaemia nor by patient age. Some parasites emerged even after significant delays in sample processing and even after initiation of treatment with anti-malarials. When 'day 0' cryopreserved samples were retested in parallel many months later using identical host RBCs and media, speed to adaptation appeared to be an intrinsic property of the parasites collected from individual patients.

CONCLUSIONS

Culture adaptation of P. falciparum in a field setting is formally shown to be robust. Parasites were found to have intrinsic variations in adaptability to culture conditions, with some lines requiring longer attempt periods for successful adaptation. Quantitative approaches described here can help describe phenotypic diversity of field parasite collections with precision. This is expected to improve population-based extrapolations of findings from field-derived fresh culture-adapted parasites to broader questions of public health importance.

Brain endothelial cells increase the proliferation of Plasmodium falciparum through production of soluble factors

We here describe the novel finding that brain endothelial cells in vitro can stimulate the growth of Plasmodium falciparum through the production of low molecular weight growth factors. By using a conditioned medium approach, we show that the brain endothelial cells continued to release these factors over time. If this mirrors the in vivo situation, these growth factors potentially would provide an advantage, in terms of enhanced growth, for sequestered parasitised red blood cells in the brain microvasculature. We observed this phenomenon with brain endothelial cells from several sources as well as a second P. falciparum strain. The characteristics of the growth factors included: <3 kDa molecular weight, heat stable, and in part chloroform soluble. Future efforts should be directed at identifying these growth factors, since blocking their production or actions might be of benefit for reducing parasite load and, hence, malaria pathology.

Modulation of Lipoprotein Cholesterol Levels in Plasmodium berghei Malarial Infection by Crude Aqueous Extract of Ganoderma lucidum

In this study, attempt is made to establish changes in serum and liver lipoprotein cholesterols accompanying Plasmodium berghei malarial infection in mice treated with aqueous extract of Ganoderma lucidum at 100, 250, and 500 mg/kg body weight in comparison with 15 mg/kg chloroquine (CQ). Significant increases in all the lipoprotein fractions were observed in infected untreated mice compared with normal control mice. Treatment with 100 and 250 mg/kg G. lucidum extract produced significant reduction in serum total cholesterol (TC) and low-density cholesterol (LDL-C) contents compared with 500 mg/kg G. lucidum and CQ. Treatment with CQ, however, produced significant reduction in hepatic TC and LDL-C compared with the extract. A dose-dependent significant increase in serum high-density lipoprotein cholesterol (HDL-C) was observed in the G. lucidum treated mice compared with normal control but significantly lower compared with CQ-treated mice. Liver HDL-C level was significantly higher in CQ-treated mice compared with normal control and significantly lower compared with G. lucidum-treated and infected untreated mice. A dose-dependent effect of the extract was observed in both serum and liver very-low density lipoprotein cholesterol (VLDL-C). The implication of these results is discussed with respect to the parasite survival and proliferation in the serum and liver.

Recent insights into fatty acid acquisition and metabolism in malarial parasites

The malarial parasite has a tremendous requirement for fatty acids during the replicative stages that take place in the mammalian host. A series of recent papers, discussed below, have revealed some of the mechanisms employed by the parasite to meet these demands.

Redefining the role of de novo fatty acid synthesis in Plasmodium parasites

Fatty acids are essential components of membranes, and are also involved in cell signalling. Plasmodium, the parasite that causes malaria, scavenges fatty acids from its hosts. However, Plasmodium also possesses enzymes for a prokaryotic-like de novo fatty acid synthesis pathway, which resides in the apicoplast. Recent research has demonstrated that Plasmodium parasites depend on de novo fatty acid synthesis only for liver-stage development. This finding demonstrates that basic anabolic functions of Plasmodium parasites are not necessary for the growth and replication of every life cycle stage. We discuss the role of fatty acid metabolism in Plasmodium and why we believe that de novo fatty acid synthesis is only required for parasite late liver-stage development.

Plasmodium falciparum: Chemically defined medium for continuous intraerythrocytic growth using lipids and recombinant albumin

Dioleoylphosphatidylcholine and other phosphatidylcholines containing different fatty acid moieties were found to increase the ability of nonesterified fatty acids (NEFA) to sustain continuous intraerythrocytic growth of Plasmodium falciparum in the presence of specific proteins. Other phospholipids, including phosphatidylethanolamine, phosphatidylserine, and phosphatidic acid, were beneficial to parasite growth. Different combinations and concentrations of NEFA tested in the presence of phospholipids and bovine albumin had variable effects on parasite growth. The most effective combination for promoting parasite growth consisted of 30 microg/ml cis-9-octadecenoic acid (oleic acid) plus 15 microg/ml hexadecanoic acid (palmitic acid). Recombinant human albumin could replace bovine or human albumin in culture media enriched with structurally defined lipids. This study therefore established a chemically defined culture medium suitable for sustaining the growth of P. falciparum.

In vitro cultivation of Plasmodium falciparum: studies with modified medium supplemented with ALBUMAX II and various animal sera

RPNI, a combination of three commercially available growth media (RPMI-1640, NCTC-135 and IMDM) has been found to support long term continuous cultivation of 3D7 strain of Plasmodium falciparum in the presence of 10% bovine calf serum. During the present study, the suitability of this medium was evaluated for the development of P. falciparum in the presence of horse, goat and rabbit sera as well as various concentrations of ALBUMAX II. RPNI medium supplemented with 10% bovine calf serum (RPNI-BCS) was used as control. The cultures were maintained in candle jars protocol and parasitaemia was monitored daily up to day 7. Horse, goat and rabbit sera all supported the development of P. falciparum. Horse serum gave best results in RPNI medium and supported continuous culture up to day 100. The parasitaemia in the presence of ALBUMAX was significantly higher in RPNI than in RPMI-1640. Addition of hypoxanthine in RPMI-1640 caused an increase in parasitaemia whereas no obvious advantage could be observed in RPNI. The findings exhibited that medium RPNI has an edge over conventional RPMI-1640 medium for in vitro cultivation of P. falciparum.

IntraerythrocyticPlasmodium falciparumutilize a broad range of serum-derived fatty acids with limited modification for their growth

Plasmodium falciparumcauses the most severe form of malaria. Utilization of fatty acids in serum is thought to be necessary for survival of this parasite in erythrocytes, and thus characterization of the parasite fatty acid metabolism is important in developing a new strategy for controlling malaria. Here, we examined which combinations of fatty acids present in human serum support the continuous culture ofP.falciparumin serum-free medium. Metabolic labelling and gas chromatography analyses revealed that, despite the need for particular fatty acids for the growth of intraerythrocyticP.falciparum, it can metabolize a broad range of serum-derived fatty acids into the major lipid species of their membranes and lipid bodies. In addition, these analyses showed that the parasite's overall fatty acid composition reflects that of the medium, although the parasite has a limited capacity to desaturate and elongate serum-derived fatty acids. These results indicate that thePlasmodiumparasite is distinct from most cells, which maintain their fatty acid composition by coordinatingde novobiosynthesis, scavenging, and modification (desaturation and elongation).

Cultivation of plasmodium spp

Cultivation of both human and non-human species of Plasmodium spp., the causal agent of malaria, has been a major research success, leading to a greater understanding of the parasite. Efforts at cultivating the organisms in vitro are complicated by the parasites' alternating between a human host and an arthropod vector, each having its own set of physiological, metabolic, and nutritional parameters. Life cycle stages of the four species that infect humans have been established in vitro. Of these four, P. falciparum remains the only species for which all stages have been cultured in vitro; different degrees of success have been achieved with the other human Plasmodium spp. The life cycle includes the exoerythrocytic stage (within liver cells), the erythrocytic stage (within erythrocytes or precursor reticulocytes), and the sporogonic stage (within the vector). Culture media generally consist of a basic tissue culture medium (e.g., minimal essential medium or RPMI 1640) to which serum and erythrocytes are added. Most of the efforts have been directed toward the stage found in the erythrocyte. This stage has been cultivated in petri plates or other growth vessels in a candle jar to generate elevated CO(2) levels or in a more controlled CO(2) atmosphere. Later developments have employed continuous-flow systems to reduce the labor-intensive nature of medium changing. The exoerythrocytic and sporogonic life cycle stages have also been cultivated in vitro. A number of avian, rodent, and simian malarial parasites have also been established in vitro. Although cultivation is of great help in understanding the biology of Plasmodium, it does not lend itself to use for diagnostic purposes.

Simple isoquinoline and benzylisoquinoline alkaloids as potential antimicrobial, antimalarial, cytotoxic, and anti-HIV agents

Twenty-six simple isoquinolines and 21 benzylisoquinolines were tested for antimicrobial, antimalarial, cytotoxic, and anti-HIV activities. Some simple isoquinoline alkaloids were significantly active in each assay, and may be useful as lead compounds for developing potential chemotherapeutic agents. These compounds include 13 (antimicrobial), 25, 26, and 42 (antimalarial), 13 and 25 (cytotoxic), and 28 and 29 (anti-HIV). A quaternary nitrogen atom of isoquinolium or dihydroisoquinolinium type may contribute to enhanced potency in the first three types of activities. In contrast, anti-HIV activity was found with tetrahydroisoquinoline and 6,7-dihydroxyisoquinolium salts.

Serum factors governing intraerythrocytic development and cell cycle progression of Plasmodium falciparum

Malaria is clinically manifested only when the human malaria parasites in the genus Plasmodium enter the obligatory intraerythrocytic life cycle. Elucidation for the roles of the serum, the key nutrient, and its components is then deemed essential for thorough understanding of the proliferation of Plasmodium cells at the erythrocytic stage. Fractionation and analysis of serum and its components was performed by chromatography, solvent extraction, and subsequent reconstitution experiments. Only fractions containing serum albumin (SA) from the serum and purified intact bovine serum albumin (BSA) showed comparable growth promoting activity with human serum (HS). Delipidated BSA can only effect parasite growth after reconstitution with lipids extracted from intact BSA. Fatty acid (FA) species in the neutral lipid fraction from intact BSA proved likewise when reconstituted with delipidated BSA. Furthermore, the involved FA species have to come in a pair of one saturated and one unsaturated, with palmitic and oleic acids as the best combination. The results were further substantiated by morphological analysis as well as biochemical analysis of the DNA synthesis during the intraerythrocytic development. This study can be a basis to explore the molecular mechanism of lipid traffic within the parasitized red blood cell (RBC), which can be an important adjunct to the development of drugs for malaria therapy.

In vitro culture and drug sensitivity assay of Plasmodium falciparum with nonserum substitute and acute-phase sera

The short-term in vitro growth of Plasmodium falciparum parasites in the asexual erythrocytic stage and the in vitro activities of eight standard antimalarial drugs were assessed and compared by using RPMI 1640 medium supplemented with 10% nonimmune human serum, 10% autologous or homologous acute-phase serum, or 0.5% Albumax I (lipid-enriched bovine serum albumin). In general, parasite growth was maximal with autologous (or homologous) serum, followed by Albumax I and nonimmune serum. The 50% inhibitory concentrations (IC50s) varied widely, depending on the serum or serum substitute. The comparison of IC50s between assays with autologous and nonimmune sera showed that monodesethylamodiaquine, halofantrine, pyrimethamine, and cycloguanil had similar IC50s. Although the IC50s of chloroquine, monodesethylamodiaquine, and dihydroartemisinin were similar with Albumax I and autologous sera, the IC50s of all test compounds obtained with Albumax I differed considerably from the corresponding values obtained with nonimmune serum. Our results suggest that Albumax I and autologous and homologous sera from symptomatic, malaria-infected patients may be useful alternative sources of serum for in vitro culture of P. falciparum isolates in the field. However, autologous sera and Albumax I do not seem to be suitable for the standardization of isotopic in vitro assays for all antimalarial drugs.

Metabolic changes of the malaria parasite during the transition from the human to the mosquito host

Plasmodium falciparum is an obligate human parasite that is the causative agent of the most lethal form of human malaria. Transmission of P. falciparum to a new human host requires a mosquito vector within which sexual replication occurs. P. falciparum replicates as an intracellular parasite in man and as an extracellular parasite in the mosquito, and it undergoes multiple developmental changes in both hosts. Changes in the environment and the activities of parasites in these various life-cycle stages are likely to be reflected in changes in the metabolic needs and capabilities of the parasite. Most of our knowledge of the metabolic capabilities of P. falciparum is derived from studies of the asexual erythrocytic cycle of the parasite, the portion of the parasite life cycle found in infected humans that is responsible for malarial symptoms. Efforts to control transmission and to understand the sometimes unique biology of this parasite have led to information about the metabolic capabilities of sexual and/or sporogonic stages of these parasites. This review focuses on comparing and contrasting the carbohydrate, nucleic acid, and protein synthetic capabilities of asexual erythrocytic stages and sexual stages of P. falciparum.

Sequence variations in the Plasmodium vivax dihydrofolate reductase-thymidylate synthase gene and their relationship with pyrimethamine resistance

The gene encoding dihydrofolate reductase-thymidylate synthase of the human malaria parasite, Plasmodium vivax, was isolated by polymerase chain reaction from genomic DNA and cloned. The sequences of the dihydrofolate reductase domain of 30 clinical isolates originating from various geographic areas were compared. Interstrain analysis revealed several genotypic variations, including short tandem repeat arrays which produced length polymorphism between different parasite isolates and point mutations in the putative dihydrofolate reductase active site cavity corresponding to those associated with pyrimethamine resistance in P. falciparum and rodent malaria parasites. Amino acid substitutions Ser-->Asn-117 and Ser-->Arg-58 were associated with decreased level of in vitro pyrimethamine sensitivity. These findings suggest that the P. vivax dihydrofolate reductase domain is characterized by polymorphism that has not been observed in P. falciparum and may explain the resistance of some P. vivax isolates to pyrimethamine. Nucleotide sequence data reported in this paper are available in the EMBL, GenBank and DDJB databases under the accession numbers X98123 (isolate ARI/Pakistan), AJ003050 (isolate CNC/Thailand), AJ003051 (isolate COU/unknown geographic origin), AJ003052 (isolate DUF/French Guiana), AJ003053 (isolate GRO/Madagascar), AJ003054 (isolate HRT/Comoros Islands), AJ003071 (isolate LFT/Cambodia), AJ003072 (isolate LGF/'India), AJ003073 (isolate MAN/Comoros Islands), AJ003074 (isolate MAT/Surinam), AJ003075 (isolate PHI/Djibouti), AJ003076 (isolate PIT/Madagascar), AJ003077 (isolate YTZ/Indonesia), AJ222630 (isolate Burma-1), AJ222631 (isolate Burma-151), AJ222632 (isolate Burma-5), AJ222633 (isolate Burma-6), AJ222634 (isolate Burma-98).

Continuous culture of Plasmodium falciparum: its impact on malaria research

The methods developed by us in 1976 for the continuous culture of the erythrocytic stages of Plasmodium falciparum make this organism available to a large variety of scientists. As a result, much has been learned about P. falciparum during the past 20 years. Here we attempt to emphasize recent developments in the diverse aspects for which the culture method has been particularly useful: chemotherapy; drug resistance; vaccine development; pathogenesis; export of proteins into the host cell; cell biology, the mitochondrion and the plastid; innate resistance involving mutant human erythrocytes; gametocytogenesis; genetics, transfection; molecular biology; biochemistry; extracellular cultivation.

Hypoxanthine: a low molecular weight factor essential for growth of erythrocytic Plasmodium falciparum in a serum-free medium

A low molecular weight factor in a basal medium essential for erythrocytic Plasmodium falciparum development in a serum-free medium using a cell growth-promoting factor derived from adult bovine serum was detected. The factor was hypoxanthine. The optimal hypoxanthine concentration for parasite growth was between 15 and 120 microM. The contribution of hypoxanthine to increased parasite growth was clearly evident in cultures on day 4. Among various low molecular weight supplements tested, adenine, adenosine, AMP, ATP, cyclic AMP, guanine, guanosine, inosine, inosine monophosphate, xanthine, NAD, NADH, NADP, NADPH and deoxyguanosine triphosphate showed a similar effect to that of hypoxanthine in the serum-free culture system. On the other hand, the addition of uric acid, FAD, thymidine, uridine, orotic acid, deoxythymidine triphosphate, deoxycytidine triphosphate, deoxyadenosine triphosphate, ribose-1-phosphate, or ethanolamine was not beneficial to the parasite growth. The results presented here will not only be of practical value, but will provide important information about the developmental requirements of the parasite.

Continuous cultivation of intraerythrocytic Plasmodium falciparum in a serum-free medium with the use of a growth-promoting factor

Serum-free media were used to culture Plasmodium falciparum. A commercial preparation, Daigo's GF21 developed as a growth-promoting factor for many kinds of mammalian cells, and consisting of the 55-70% ammonium sulphate fraction of adult bovine serum, insulin, transferrin, ethanolamine and sodium selenite, was found to sustain growth of the parasite when Daigo's T was employed as a basal medium. The optimal Daigo's GF21 concentration for parasite growth was between 5 and 20% (v/v), with the best results at 10%. Differential counts indicated that Daigo's GF21 is essential for schizogony. Established serum-free medium, GIT, consisting of Daigo's T basal medium and Daigo's GF21, yielded good parasite growth without any supplementation. Growth-promoting factor derived from adult bovine serum in Daigo's GF21 was shown to be crucial to parasite growth. The results presented here will not only be of practical value, but will provide important information about the developmental requirements for the parasite.