Intraerythrocytic development of Plasmodium knowlesi: structure, temperature- and Ca2+-response of the host and parasite membranes

Distinct lipid compositions of parasite and host cell plasma membranes from Plasmodium chabaudi-infected erythrocytes

Mouse erythrocytes infected with early or late trophozoites of the malaria parasite Plasmodium chabaudi were fractionated into free parasites and host cell plasma membranes, and both fractions were analyzed for cholesterol content and the composition of phospholipids and total fatty acids. The major results are: (i) parasites contain only a very low level of cholesterol which is about one-tenth of that of host cell plasma membranes. (ii) Parasites also contain less sphingomyelin and phosphatidylserine as well as more phosphatidylcholine than host cell plasma membranes. (iii) Parasites contain less 18:0 and 18:1 and more 18:2 and 20:4 fatty acids than host cell plasma membranes. (iv) During intraerythrocytic growth of parasites from early to late trophozoites, the relative proportions of cholesterol and phospholipids remain largely unchanged in both parasites and host cell plasma membranes. However, significant changes occur in the fatty composition of both compartments. There is an increase in the 20:4 and a decrease in the 18:0 and 18:1 fatty acids. (v) Plasma membranes of infected and non-infected erythrocytes exhibit about the same cholesterol content and phospholipid composition, but differ in the total fatty acid composition. Our data suggest the existence of distinct mechanisms controlling the different lipid compositions of parasites and host cell plasma membranes in whole Plasmodium chabaudi-infected erythrocytes during intraerythrocytic development of parasites, though both compartments are known to depend on the supply of various lipids from the host.

The role of lipids in Plasmodium falciparum invasion of erythrocytes: a coordinated biochemical and microscopic analysis

The role of lipids in Plasmodium falciparum invasion of erythrocytes was investigated by biochemical and fluorescent microscopic analysis. Metabolic incorporation of radioactive oleate or palmitate and fractionation of radiolabeled phospholipids by thin-layer chromatography revealed no difference in the major phospholipid classes of schizonts and early ring forms after merozoite invasion. Fluorescent anthroyloxy derivatives of oleate and palmitate were also metabolically incorporated into parasite phospholipids. By microscopic analysis, the fluorescent phospholipids were seen localized in the plasma membrane and, within the merozoite, concentrated near the apical end. During invasion fluorescent phospholipid appeared to be injected from the apical end of the merozoite into the host membrane, both within and outside the parasite-host membrane junctions. After invasion fluorescent lipid was only found in the parasite plasma membrane and/or parasitophorous vacuole membrane. Parallel experiments with a fluorescent cholesterol derivative, incorporated into parasite membranes by exchange, revealed neither heterogeneous distribution of label within the parasite nor evidence for cholesterol transfer from merozoite to host cell membrane. Results suggest that during invasion no major covalent alteration of parasite lipids, such as lysophospholipid formation, occurs. However, invasion and formation of the parasitophorous vacuolar membrane apparently involves insertion of parasite phospholipids into the host membrane.

Ultrastructure of the erythrocytic stages of Plasmodium malariae

This report describes the fine structure of the erythrocytic stages of Plasmodium malariae. Erythrocytic parasites from a naturally acquired human infection and an experimentally infected chimpanzee were morphologically indistinguishable and structurally similar to other primate malarias. New findings included observations of highly structured arrays of merozoite surface coat proteins in the cytoplasm of early schizonts and on the surface of budding merozoites and the presence of knobs in the membranes of Maurer's clefts. Morphological evidence is presented suggesting that proteins are transported between the erythrocyte surface and intracellular parasites via two routes: one associated with Maurer's clefts for transport of membrane-associated knob material and a second associated with caveolae in the host cell membrane for the import or export of host- or parasite-derived substances through the erythrocyte cytoplasm.

Isolation and characterization of parasites and host cell ghosts from erythrocytes infected with Plasmodium chabaudi

A new procedure has been developed which allows the concomitant isolation of viable parasites and host cell plasma membranes from erythrocytes infected with Plasmodium chabaudi trophozoites. The average final yield of parasites is 56%. Free parasites reveal a well preserved ultrastructure, incorporate [14C]isoleucine for at least 3 h, and synthesize about the same proteins as parasites within erythrocytes as monitored by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE)-autoradiography. The host cell plasma membranes can be isolated in the form of ghosts with an average yield of 27%. The ghosts possess a structurally intact plasma membrane as revealed by freeze-etch electron microscopy. The ghosts are regularly associated with seven neo-proteins as identified by SDS-PAGE and isoelectric focusing (IEF)/SDS-PAGE. These neo-proteins have the following apparent molecular masses: 154 kDa, 145 kDa, 90 kDa, 72 kDa (pI 4.5), 67 kDa, 52 kDa, and 33 kDa (pI 5.7), respectively. The contamination of ghosts by parasite material and, conversely, the contamination of parasites by host cell plasma membranes is very low as demonstrated by light and electron microscopy, lactoperoxidase-mediated radioiodination and the distribution of the typical parasite marker enzymes such as choline kinase, cholinephosphotransferase and ethanolaminephosphotransferase.

Structure and development of the surface coat of erythrocytic merozoites of Plasmodium knowlesi

The surface of extracellular merozoites of P. knowlesi is covered with a coat 15-20 nm thick, made up of clusters of filaments standing erect on the plasma membrane. Filaments have stems 2 nm thick, the peripheral ends of which are complex, branching or ending in long trailing threads. Coat filaments occur on the surface of the parasite in regular rows at an early schizont stage, and persist until well after merozoite release. They are sensitive to trypsin and papain, and bind ethanolic phosphotungstate, indicating a proteinaceous nature. They are also removed by exposure to phosphate-buffered saline. Filaments bear negative charges, binding cationised ferritin throughout the depth of the coat and staining with ruthenium red. They cover the whole merozoite surface and mediate intercellular adhesion at distances of 15-150 nm, membrane to membrane. It is suggested that these filaments correspond to a major merozoite surface protein, and are important in the initial capture of red cells.

Receptors for the malarial parasite

During the erythrocytic cycle of Plasmodium, the parasite develops within an enclosed space, the parasitophorous vacuole, formed by endocytosis of an invasive stage, the merozoite. Among the erythrocyte membrane proteins possibly acting as a receptor for the attachment of P. falciparum merozoites to human erythrocytes is glycophorin A. Isolated glycophorin inhibits merozoite entry in a competitive manner, perhaps via association with a 155 kDa surface protein. Another protein that competitively inhibits merozoite invasion, is band 3, the erythrocyte anion transport protein. The protein bearing Duffy blood group antigens may act to modulate invasion, but does not behave as a receptor.

Membrane structure and function of malaria parasites and the infected erythrocyte

According to the World Health Organization the global estimate of malaria is over 200 million infections, the majority of which are caused by the most life-threatening species,Plasmodium falciparum(Report of the Steering Committees of the Scientific Working Groups on Malaria, World Health Organization, June 1983). The causative agent of the disease, the malarial parasite, requires two hosts: a blood-sucking mosquito and a blood-containing vertebrate. Commonly, infection of the vertebrate begins when an infected mosquito bites a suitable vertebrate and injects minute sporozoites into the bloodstream. Within 30 mm the introduced sporozoites leave the bloodstream and enter parenchymal cells of the liver (mammals) or endothelial cells (birds). In these sites the parasite undergoes asexual multiplication (= exo-erythrocytic schizogony) producing daughter progeny called merozoites. The exo-erythrocytic merozoites are released from the tissues into the circulation where they invade red blood cells. Within an erythrocyte the merozoite undergoes asexual multiplication (= erythrocytic schizogony) producing a substantial number of merozoites. The erythrocyte lyses, merozoites are released, and invasion of another erythrocyte may then take place. The synchronous rupture of the red cell and merozoite release is marked by the periodic fever–chill cycles so characteristic of the malarial infection. Some merozoites continue to reinvade other erythrocytes and multiply by asexual means, whereas others enter erythrocytes and differentiate into sexual stages, male or female gametocytes. When a suitable mosquito feeds on an infected vertebrate gametocytes are ingested and the sexual cycle of development is initiated. In the mosquito stomach the gametocytes transform into gametes, fertilization takes place, the resultant worm-like zygote penetrates the cells of the mosquito gut and comes to lie on the outer surface of the stomach. Here each zygote forms a cyst-like body, the oocyst, within which thousands of sporozoites are produced by asexual multiplication. When the swollen oocysts burst, sporozoites are freed and these make their way to the salivary gland. At the next blood feeding the mosquito injects the infective sporozoites and the life-cycle is completed.

Fractionation of Plasmodium chabaudi-infected erythrocytes into parasites and ghosts

We have developed a new procedure for subfractionation of Plasmodium chabaudi-infected erythrocytes into parasites and ghosts. Trophozoite-infected erythrocytes enriched over a percoll-step (:1.10 g/cm3) are subjected to a glycerol-enhanced osmotic shock. This induces the release of parasites and the emergence of erythrocyte ghosts, which can be separated on a continuous percoll gradient (:1.02----1.10 g/cm3). The parasites are intact in terms of ultrastructure and incorporation of 14C-isoleucine. The erythrocyte ghosts are purified over a two-step percoll gradient (:1.01-1.02 g/cm3). Our method recovers about 40%-50% of the initial ghosts and 70%-95% of the initially freed parasites.