Parasitophorous vacuole: morphofunctional diversity in different coccidian genera (a short insight into the problem)

We present a short insight into the problem of parasitophorous vacuole (PV) formation as a most peculiar kind of cell vacuolization occurring in the course of intracellular development of coccidian pathogens of the genera Eimeria, Isospora, Toxoplasma, Sarcocystis, Cryptosporidium, Epieimeria, and Karyolysus. The review focuses on the morpho-functional diversity of PVs in these parasites. By the present time, the PVs containing different parasite genera and species have been examined to different extent. The membrane of the PV (PVM) obviously derives from the host cell plasmalemma. But soon after parasite penetration, the morphofunctional organization and biochemical composition of the PVM drastically changes: its proteins are selectively excluded and those of the parasite are incorporated. As the result, the PV becomes not fusigenic for lysosomes or any other vacuoles or vesicles, because host cell surface markers necessary for membrane fusion are eliminated from the PVM during parasite invasion.The pattern of the PVs is parasite specific and demonstrates a broad diversity within the same genera and species and even at different stages of the endogenous development. The PV is far from being an indifferent membrane vesicle containing the parasite. Instead, it represents a dynamic system that reflects the innermost events of host-parasite relationships, thus promoting the accomplishing of the parasite life cycle, which, in its turn, is a necessary prerequisite of the parasite eventual survival as a species.     

Expression, purification, and biochemical characterization of a recombinant Iectin of Sarcocystis muris (Apicomplexa) cyst merozoites

The mature major microneme protein of Sarcocystis muris cyst merozoites, which is known as a dimeric lectin with high affinity to galactose and some of its derivatives, was expressed in Escherichia coli as a histidine-tagged fusion protein. The recombinant polypeptide, which was recognized by a monoclonal antibody directed against the native lectin, was purified from inclusion bodies after solubilization and refolding, using a combination of metal chelate and lactose affinity chromatography. The apparent molecular mass of the refolded polypeptide as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoreses was 16 kDa, whereas gel filtration chromatography clearly demonstrated that the recombinant protein, like its native counterpart, exists as a homodimer of two non-covalently associated subunits. Inhibition of haemagglutination suggests that the combining site of the recombinant lectin recognizes N-acetyl-galactosamine as the dominant sugar, thus confirming the correct folding of the monosaccharide combining site in the renatured lectin. To the best of our knowledge, this work represents the first reported detailed characterization of a recombinant lectin from apicomplexan parasites, and may contribute to a better understanding of the process of host cell recognition and invasion by these obligate intracellular protozoa.     

Ultrastructure of Sarcocystis booliati Dissanaike and Poopalachelvam, 1975 from the moonrat, Echinosorex gymnurus, in Malaysia

The ultrastructure of the cyst wall and zoites of Sarcocystis booliati from the moonrat Echinosorex gymnurus, was studied with the electron microscope. The primary cyst wall was thin, smooth and filled with a finely-granular, electron-dense material. The surface of the cyst wall had a row of vesicular invaginations. The ground substance beneath the primary cyst wall did not extend into the cyst to form septae. The zoites were covered with a double-layered membrane or pellicle and had an anterior conoid, 2 conoidal rings, 22 subpellicular microtubules, about 8 rhoptries, 50–60 micronemes, scattered lipid droplets, a micropore and a posteriorly situated nucleus, in front of which was a sac-like mitochondrion with vesicular internal cristae. The distinctive features in the ultrastructure of S. booliati were the thinness of the cyst wall, the absence of cytophaneres or trabeculae and the comparatively small number of micronemes in the zoites.     

The cytoplasmic domain of the Plasmodium falciparum ligand EBA-175 is essential for invasion but not protein trafficking

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.     

Protein Trafficking to Apical Organelles of Malaria Parasites – Building an Invasion Machine

Malaria is caused by four species of apicomplexan protozoa belonging to the genus Plasmodium. These parasites possess a specialized collection of secretory organelles called rhoptries, micronemes and dense granules (DGs) that in part facilitate invasion of host cells. The mechanism by which the parasite traffics proteins to these organelles as well as regulates their secretion has important implications for understanding the invasion process and may lead to development of novel intervention strategies. In this review, we focus on emerging data about trafficking signals, mechanisms of biogenesis and secretion. At least some of these are conserved in higher eukaryotes, suggesting that rhoptries, micronemes and DGs are related to organelles such as secretory lysosomes that are well known to mainstream cell biologists.     

Ultrastructure of In Vivo-Produced Caryocysts Containing the Coccidian Caryospora bigenetica (Apicomplexa: Eimerüdae)

A cotton rat was inoculated orally with oocysts of Caryospora bigenetica from the feces of a rattlesnake. Sixteen days later the rat was euthanized, and portions of the scrotum, foot pad and muzzle were processed for histological sections and transmission electron microscopy. Sporozoites within caryocysts had typical coccidian features such as an anterior and posterior refractile body, centrally located nucleus, micronemes, rhoptries, a conoid, a micropore near the anterior refractile body, a posterior pore, amylopectin granules, lipid bodies, a Golgi-like body, a mitochondrion and subpellicular microtubules. The infected host cell was spherical and surrounded by a fibrous wall-like covering, 0.35–1.00 μm thick. This outer covering, when viewed in stained histological sections, was periodic acid-Schiff (PAS)-positive.     

Fine Structure of the Endogenous Stages of Eimeria labbeana. I. The First Generation Merozoites

SYNOPSIS. The fine structure of the 1st generation merozoites of Eimeria labbeana from the ileal mucosa of artificially infected pigeons ( Columba livia ) was investigated and described. The 1st generation merozoites which appeared between 36-48 hr after infection averaged 4.4 × 2.1 μm in size. The 3-membraned pellicle was irregular in texture and harbored a single micropore, and many micropore-like invaginations. Closely apposed to the inner pellicular membrane were seen 22 microtubules, each 22–25 nm in diameter. An apical vesicle, 50 nm in diameter, seen at the anterior extremity, was connected with the common duct of the micronemes. The conoid consisted of 9 spiral elements, each 30 × 25 nm. The paired organelle (rhoptries) varied in length (1.4–2.2 μm), and the ductules (23 nm diameter) were composed of 2 inner tubules, each 6 nm in diameter. A unit membrane enveloped the partially alveolar and differentially osmiophilic interior of the bulbous regions of the rhoptries. The "rod-like structure"was found to be tubular and represented the common duct of the micronemes.     

Ultrastructure of Intraerythrocytic Babesia microti with Emphasis on the Feeding Mechanism*

SYNOPSIS. Babesia microti is a highly polymorphic organism. To unravel its fine structure and the function of organelles it was necessary to resort often to serial sections. A single plasma membrane covers the organism. In trophozoites approaching reproduction, segments of double membranes can be found below the plasma membrane. In electron micrographs of poor resolution these segments of double membranes look like pieces of thick membranes and they were often thought to be a thick 2nd membrane. Before the segments of double membranes appear 2 other organelles are formed in older trophozoites: micronemes and rhoptries. There are indications that these structures originate from vesicles of the Golgi apparatus. Large dense bodies of the same structure as the host cytoplasm are not food vacuoles but merely invaginations of host cytoplasm, as found in serial sections and in organisms removed from the host cell. Feeding in Babesia seems to take place by a special organelle composed of tightly coiled double membranes located partly inside and partly outside the parasite. It is assumed that extracellular digestion of host cytoplasm take place through this organelle. The nucleus remains undifferentiated throughout the whole intraerythrocytic stage. It becomes irregular, loboid, but does not divide and remains a single body until the late stage of reproduction when only a small portion, a bud, extends into the forming merozoite.