Three-dimensional study of the intact cytoskeleton of coccidian sporozoites

Refractile bodies of Eimeria tenella are proteinaceous membrane-less organelles that undergo dynamic changes during infection

Introduction

Refractile bodies (RB) are large membrane-less organelles (MLO) of unknown function found as a prominent mismatched pair within the sporozoite stages of all species of Eimeria, parasitic coccidian protozoa.

Methods

High resolution imaging methods including time-lapse live confocal microscopy and serial block face-scanning electron microscopy (SBF-SEM) were used to investigate the morphology of RB and other intracellular organelles before and after sporozoite invasion of host cells.

Results

Live cell imaging of MDBK cells infected with E. tenella sporozoites confirmed previous reports that RB reduce from two to one post-infection and showed that reduction in RB number occurs via merger of the anterior RB with the posterior RB, a process that lasts 20-40 seconds and takes place between 2- and 5-hours post-infection. Ultrastructural studies using SBF-SEM on whole individual sporozoites, both pre- and post-host cell invasion, confirmed the live cell imaging observations and showed also that changes to the overall sporozoite cell shape accompanied RB merger. Furthermore, the single RB post-merger was found to be larger in volume than the two RB pre-merger. Actin inhibitors were used to investigate a potential role for actin in RB merger, Cytochalasin D significantly inhibited both RB merger and the accompanying changes in sporozoite cell shape.

Discussion

MLOs in eukaryotic organisms are characterised by their lack of a membrane and ability to undergo liquid-liquid phase separation (LLPS) and fusion, usually in an actin-mediated fashion. Based on the changes in sporozoite cell shape observed at the time of RB merger together with a potential role for actin in this process, we propose that RB are classed as an MLO and recognised as one of the largest MLOs so far characterised.

Life cycle stages, specific organelles and invasion mechanisms of Eimeria species

Apicomplexans, including species of Eimeria, pose a real threat to the health and wellbeing of animals and humans. Eimeria parasites do not infect humans but cause an important economic impact on livestock, in particular on the poultry industry. Despite its high prevalence and financial costs, little is known about the cell biology of these 'cosmopolitan' parasites found all over the world. In this review, we discuss different aspects of the life cycle and stages of Eimeria species, focusing on cellular structures and organelles typical of the coccidian family as well as genus-specific features, complementing some 'unknowns' with what is described in the closely related coccidian Toxoplasma gondii.

Toxoplasma F-box protein 1 is required for daughter cell scaffold function during parasite replication

By binding to the adaptor protein SKP1 and serving as substrate receptors for the SKP1 Cullin, F-box E3 ubiquitin ligase complex, F-box proteins regulate critical cellular processes including cell cycle progression and membrane trafficking. While F-box proteins are conserved throughout eukaryotes and are well studied in yeast, plants, and animals, studies in parasitic protozoa are lagging. We have identified eighteen putative F-box proteins in the Toxoplasma genome of which four have predicted homologs in Plasmodium. Two of the conserved F-box proteins were demonstrated to be important for Toxoplasma fitness and here we focus on an F-box protein, named TgFBXO1, because it is the most highly expressed by replicative tachyzoites and was also identified in an interactome screen as a Toxoplasma SKP1 binding protein. TgFBXO1 interacts with Toxoplasma SKP1 confirming it as a bona fide F-box protein. In interphase parasites, TgFBXO1 is a component of the Inner Membrane Complex (IMC), which is an organelle that underlies the plasma membrane. Early during replication, TgFBXO1 localizes to the developing daughter cell scaffold, which is the site where the daughter cell IMC and microtubules form and extend from. TgFBXO1 localization to the daughter cell scaffold required centrosome duplication but before kinetochore separation was completed. Daughter cell scaffold localization required TgFBXO1 N-myristoylation and was dependent on the small molecular weight GTPase, TgRab11b. Finally, we demonstrate that TgFBXO1 is required for parasite growth due to its function as a daughter cell scaffold effector. TgFBXO1 is the first F-box protein to be studied in apicomplexan parasites and represents the first protein demonstrated to be important for daughter cell scaffold function.

Reassessing the mechanics of parasite motility and host-cell invasion

The capacity to migrate is fundamental to multicellular and single-celled life. Apicomplexan parasites, an ancient protozoan clade that includes malaria parasites (Plasmodium) and Toxoplasma, achieve remarkable speeds of directional cell movement. This rapidity is achieved via a divergent actomyosin motor system, housed within a narrow compartment that lies underneath the length of the parasite plasma membrane. How this motor functions at a mechanistic level during motility and host cell invasion is a matter of debate. Here, we integrate old and new insights toward refining the current model for the function of this motor with the aim of revitalizing interest in the mechanics of how these deadly pathogens move.

Progress in imaging methods: insights gained into Plasmodium biology

Over the past decade, major advances in imaging techniques have enhanced our understanding of Plasmodium spp. parasites and their interplay with mammalian hosts and mosquito vectors. Cryoelectron tomography, cryo-X-ray tomography and super-resolution microscopy have shifted paradigms of sporozoite and gametocyte structure, the process of erythrocyte invasion by merozoites, and the architecture of Maurer's clefts. Intravital time-lapse imaging has been revolutionary for our understanding of pre-erythrocytic stages of rodent Plasmodium parasites. Furthermore, high-speed imaging has revealed the link between sporozoite structure and motility, and improvements in time-lapse microscopy have enabled imaging of the entire Plasmodium falciparum erythrocytic cycle and the complete Plasmodium berghei pre-erythrocytic stages for the first time. In this Review, we discuss the contribution of key imaging tools to these and other discoveries in the malaria field over the past 10 years.

A cyclic GMP signalling module that regulates gliding motility in a malaria parasite

The ookinete is a motile stage in the malaria life cycle which forms in the mosquito blood meal from the zygote. Ookinetes use an acto-myosin motor to glide towards and penetrate the midgut wall to establish infection in the vector. The regulation of gliding motility is poorly understood. Through genetic interaction studies we here describe a signalling module that identifies guanosine 3′, 5′-cyclic monophosphate (cGMP) as an important second messenger regulating ookinete differentiation and motility. In ookinetes lacking the cyclic nucleotide degrading phosphodiesterase δ (PDEδ), unregulated signalling through cGMP results in rounding up of the normally banana-shaped cells. This phenotype is suppressed in a double mutant additionally lacking guanylyl cyclase β (GCβ), showing that in ookinetes GCβ is an important source for cGMP, and that PDEδ is the relevant cGMP degrading enzyme. Inhibition of the cGMP-dependent protein kinase, PKG, blocks gliding, whereas enhanced signalling through cGMP restores normal gliding speed in a mutant lacking calcium dependent protein kinase 3, suggesting at least a partial overlap between calcium and cGMP dependent pathways. These data demonstrate an important function for signalling through cGMP, and most likely PKG, in dynamically regulating ookinete gliding during the transmission of malaria to the mosquito.

Malaria parasites are single celled organisms, which must alternate between vertebrate and mosquito hosts to survive and spread. In both hosts, certain parasite stages can glide through tissues and invade cells. Many components of the molecular motor that powers gliding and invasion are known and we have a good idea how these may interact to generate force. It is less well understood how the motor is assembled and how its component parts are regulated to switch it on and off. We have begun to address these questions in the ookinete, a parasite stage, which forms in the blood meal of a mosquito and relies on gliding to penetrate the gut wall. Using a malaria parasite of rodents, we have examined the effect of deleting candidate genes involved in controlling levels of the intracellular signalling molecule cyclic guanosine monophosphate (cGMP). We show that the right balance between cGMP production and degradation is important for ookinetes to glide, while also maintaining their typical cell shape. Overall levels of cGMP are not much affected in the mutants, though, and we therefore believe the messenger exerts its effect either locally within the cell or only while the parasite is gliding.

Organellar dynamics during the cell cycle of Toxoplasma gondii

The protozoan phylum Apicomplexa encompasses approximately 5000 species of obligate intracellular parasites, including those responsible for malaria and toxoplasmosis. Rather than dividing by binary fission, apicomplexans use a remarkable mechanism for replication, assembling daughters de novo within the cytoplasm. Here, we exploit time-lapse microscopy of fluorescent markers targeted to various subcellular structures in Toxoplasma gondii tachyzoites to determine how these unicellular eukaryotes efficiently package a complete set of organelles, maintaining the highly polarized organization necessary for host cell invasion and pathogenesis. Golgi division and elongation of the apicoplast are among the first morphologically observable events, associated with an unusual pattern of centriolar migration. Daughter parasites are assembled on cytoskeletal scaffolding, whose growth proceeds from the apical end, first encapsulating the divided Golgi. Further extension of the cytoskeletal scaffold results in partitioning of the apicoplast, nucleus, endoplasmic reticulum, and finally the mitochondrion, which enters the developing daughters rapidly, but only very late during the division cycle. The specialized secretory organelles (micronemes and rhoptries) form de novo. This distinctive pattern of replication -- in which organellar segregation spans approximately 75% of the cell cycle, completely encompassing S phase -- suggests an unusual mechanism of cell cycle regulation.

GpMyoF, a WD40 repeat-containing myosin associated with the myonemes of Gregarina polymorpha

This study presents the first characterization of a WD40 repeat-containing myosin identified in the apicomplexan parasite Gregarina polymorpha. This 222.7 kDa myosin, GpMyoF, contains a canonical myosin motor domain, a neck domain with 6 IQ motifs, a tail domain containing short regions of predicted coiled-coil structure, and, most notably, multiple WD40 repeats at the C-terminus. In other proteins such repeats assemble into a beta-propeller structure implicated in mediating protein-protein interactions. Confocal microscopy suggests that GpMyoF is localized to the annular myonemes that gird the parasite cortex. Extraction studies indicate that this myosin shows an unusually tight association with the cytoskeletal fraction and can be solubilized only by treatment with high pH (11.5) or the anionic detergent sarkosyl. This novel myosin and its homologs, which have been identified in several related genera, appear to be unique to the Apicomplexa and represent the only myosins known to contain the WD40 domain. The function of this myosin in G. polymorpha or any of the other apicomplexan parasites remains uncertain.

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

Unusual properties of Plasmodium falciparum actin: new insights into microfilament dynamics of apicomplexan parasites

Plasmodium falciparum, the etiologic agent of malaria, is a facultative intracellular parasite of the phylum Apicomplexa. A limited turnover of microfilaments takes place beneath the parasite plasma membrane, but the cytoplasm of apicomplexans is virtually devoid of F-actin. We produced Plasmodium actin in yeast. Purified recombinant Plasmodium actin polymerized inefficiently unless both gelsolin and phalloidin were added. The resulting actin polymers appeared fragmented in the fluorescence microscope. Plasmodium actin bound DNaseI about 200 times weaker than bovine non-muscle actin. Our findings suggest that the unique properties of Plasmodium actin can explain some of the unusual features of apicomplexan parasite microfilaments.

Molecular characterization of a gene encoding a 29-kDa cytoplasmic protein of Babesia gibsoni and evaluation of its diagnostic potentiality

A cDNA expression library prepared from Babesia gibsoni merozoite mRNA was screened with B. gibsoni-infected dog serum. cDNA encoding 29-kDa protein was cloned and designated as the P29 gene. The complete nucleotide sequence of the P29 gene was 792 bp. Computer analysis suggested that the sequence of the P29 gene contained an open reading frame of 597 bp with a coding capacity of approximately 23.4 kDa and a single intron of 250 bp. The P29 protein had homology to Toxoplasma gondii cytoskeletal protein IMC1. Southern blot analysis indicated that the P29 gene was present as a single copy in the B. gibsoni genome. The native P29 protein of B. gibsoni with a molecular mass of 29 kDa was identified by Western blotting with anti-recombinant P29 mouse serum. Confocal laser microscopic analysis showed that the P29 protein was located on the cytoplasma of B. gibsoni merozoites. The recombinant P29 protein expressed in E. coli was used as an antigen in an enzyme-linked immunosorbent assay (ELISA). The ELISA was able to differentiate between B. gibsoni-infected dog serum and B. canis subspecies-infected dog serum or normal dog serum. Furthermore, the antibody response against the P29 protein was maintained during the chronic stage of infection in an experimentally infected dog, indicating that the recombinant P29 protein might be a useful diagnostic reagent for the detection of antibodies to B. gibsoni in dogs.

Plasmodium falciparum apical membrane antigen 1 (PfAMA-1) is translocated within micronemes along subpellicular microtubules during merozoite development

During the assembly of Plasmodium falciparum merozoites within the schizont stage, the parasite synthesizes and positions three sets of secretory vesicles (rhoptries, micronemes and dense granules) that are active during red cell invasion. There are up to 40 micronemes per merozoite, shaped like long-necked bottles, about 160 nm long and 65 nm at their widest diameter. On their external surfaces, they bear bristle-like filaments, each 3-4 nm thick and 25 nm long. Micronemes are translocated from a single Golgi-like cisterna near the nucleus along a band of two or three subpellicular microtubules to the merozoite apex, where they dock with the rhoptry tips. Dense granules are also formed around the periphery of the Golgi cisternae but their distribution is unrelated to microtubules. Three polyclonal antibodies raised against the recombinant PfAMA-1 ectodomain sequence recognizing both the 83 kDa and processed 66 kDa molecules label the peripheries of translocating and mature micronemes but do not label rhoptries significantly at any stage of merozoite development within schizonts. This result confirms that PfAMA-1 is a micronemal protein, and indicates that within the microneme it is located near or inserted into this organelle's boundary membrane.

Tyrosine phosphorylation and invasion mechanisms in Eimeria tenella

The role of signal transduction mechanisms with regard to the host cell invasion mechanics used by apicomplexans appears to have been overlooked: indeed, it is obvious that a signal must be transduced from the surface of an invading parasite to an intracellular location within it once the parasite makes contact with a host cell for the invasion process to be initiated. Data outlined in this communication show strong evidence for the role of tyrosine phosphorylation in the molecular mechanics and control of invasion of host cells when set within the context of the available literature, as detailed in this study.

Proteolytic processing of TgIMC1 during maturation of the membrane skeleton of Toxoplasma gondii

Membrane skeletons play an important role in the maintenance of cell shape and integrity in many cell types. In the protozoan parasite Toxoplasma gondii this function is performed by the subpellicular network, a resilient structure composed of tightly interwoven 10-nm filaments. We report here that this network is assembled at an early stage in the development of daughter parasites. The networks of immature and mature parasites differ dramatically with respect to their stability. Although in immature parasites the network is completely solubilized by detergent, the network in mature parasites is entirely detergent-resistant. Conversion of the detergent-labile to the detergent-resistant network occurs late in daughter cell development and appears to be coupled to proteolytic processing of the carboxyl terminus of TgIMC1, the major subunit of the network filaments. A single cysteine residue in the TgIMC1 carboxyl terminus was found to be essential for this processing event. The dramatic change in resistance to detergent extraction probably reflects an overall change in structural stability of the subpellicular network that accompanies maturation of daughter parasites and allows a switch from an assembly-competent but loose structure to one that is rigid and offers mechanical strength to the mature parasite.

Disruption of microtubules uncouples budding and nuclear division inToxoplasma gondii

The tachyzoite stage of the protozoan parasite Toxoplasma gondiihas two populations of microtubules: spindle microtubules and subpellicular microtubules. To determine how these two microtubule populations are regulated, we investigated microtubule behavior during the cell cycle following treatment with microtubule-disrupting drugs. Previous work had established that the microtubule populations are individually nucleated by two distinct microtubule-organizing centers (MTOCs): the apical polar ring for the subpellicular microtubules and spindle pole plaques/centrioles for the spindle microtubules. When replicating tachyzoites were treated with 0.5 μM oryzalin or 1.0 mM colchicine they retained the capacity to form a spindle and undergo nuclear division. Although these parasites could complete budding,they lost the bulk of their subpellicular microtubules and the ability to reinvade host cells. Both nascent spindle and subpellicular microtubules were disrupted in 2.5 μM oryzalin or 5.0 mM colchicine. Under these conditions,parasites grew in size and replicated their genome but were incapable of nuclear division. After removal from 0.5 μM oryzalin, Toxoplasmatachyzoites were able to restore normal subpellicular microtubules and a fully invasive phenotype. When oryzalin was removed from Toxoplasmatachyzoites treated with 2.5 μM drug, the parasites attempted to bud as crescent-shaped tachyzoites. Because the polyploid nuclear mass could not be correctly segregated, many daughter parasites lacked nuclei altogether although budding and scission from the maternal mass was able to be completed. Multiple MTOCs permit Toxoplasma tachyzoites to control nuclear division independently from cell polarity and cytokinesis. This unusual situation grants greater cell cycle flexibility to these parasites but abolishes the checks for coregulation of nuclear division and cytokinesis found in other eukaryotes.

Actin in the parasite Toxoplasma gondii is encoded by a single copy gene, ACT1 and exists primarily in a globular form

Actin is a highly conserved microfilament protein that plays an important role in the invasion of host cells by the protozoan parasite Toxoplasma gondii. We have characterized the ACT1 gene and localized the conventional isoform of actin that it encodes within T. gondii. The predicted amino acid sequence of ACT1 was most similar to two other parasite actins, Plasmodium falciparum Pfact-1 (93.1% identical) and Cryptosporidium parvum actin (88.1%): among vertebrate actins, ACT1 was most closely related to the mammalian beta and gamma (83%) actin isoforms. Actin-specific antibodies and fluorescently labeled DNAse I were used to localize actin in T. gondii tachyzoites by immunofluorescence and immunoelectron microscopy. Actin was detected beneath the parasite cell membrane and in clusters scattered within the cytosol of T. gondii tachyzoites. Actin filaments were not detected in detergent-solubilized parasites separated by high speed centrifugation, indicating that actin exists primarily in a globular form in T. gondii.

Subpellicular microtubules associate with an intramembranous particle lattice in the protozoan parasite Toxoplasma gondii

Application of Fourier analysis techniques to images of isolated, frozen-hydrated subpellicular microtubules from the protozoan parasite Toxoplasma gondii demonstrates a distinctive 32 nm periodicity along the length of the microtubules. A 32 nm longitudinal repeat is also observed in the double rows of intramembranous particles seen in freeze-fracture images of the parasite's pellicle; these rows are thought to overlie the subpellicular microtubules. Remarkably, the 32 nm intramembranous particle periodicity is carried over laterally to the single rows of particles that lie between the microtubule-associated double rows. This creates a two-dimensional particle lattice, with the second dimension at an angle of approximately 75 degrees to the longitudinal rows (depending on position along the length of the parasite). Drugs that disrupt known cytoskeletal components fail to destroy the integrity of the particle lattice. This intramembranous particle organization suggests the existence of multiple cytoskeletal filaments of unknown identity. Filaments associated with the particle lattice provide a possible mechanism for motility and shape change in Toxoplasma: distortion of the lattice may mediate the twirling motility seen upon host-cell lysis, and morphological changes observed during invasion.

Ca(2+)-dependence of conoid extrusion in Toxoplasma gondii tachyzoites

The role of Ca2+ in conoid extrusion was investigated in isolated Toxoplasma gondii tachyzoites by treatment with Ca(2+)-ionophores, Ca(2+)-chelating agents and an inhibitor of the Ca(2+)-ATPase at the endoplasmic reticulum. The results were evaluated by light phase-contrast microscopy and electron microscopy. Ionomycin (0.5-1 microM) caused an immediate and sustained extrusion of the conoid in up to 80% of the tachyzoites, depending on the concentrations of ionophore and Ca2+ in the medium. However, over 50% of the tachyzoites extruded the conoid when treated with ionomycin in Ca(2+)-free saline complemented with EGTA. The effect of ionomycin was reversible and could be induced a second time in about half of the responsive population. Similar results were obtained with A23187. Conoid extrusion induced by ionomycin in Ca(2+)-free medium was almost completely abolished when the tachyzoites were previously loaded with a permeable compound known to chelate intracellular Ca2+ (BAPTA/AM; 25 microM). On the other hand, exposure of tachyzoites to the Ca(2+)-ATPase inhibitor thapsigargin (0.5-1 microM) produced significant extrusion of the conoid. Tachyzoites loaded with BAPTA/AM as well as those treated with ionomycin, i.e. with conoids paralyzed in opposite positions, had a diminished capacity to invade cultured epithelial cells. A substantial reduction in the response to stimulation by ionomycin was found also in parasites treated with cytochalasin-D, a drug that depolymerizes actin-filaments. The results suggest that Ca(2+)-release from internal stores may act as a key signal to activate a mechanism of conoid extrusion probably mediated, at least in part, by actin-filaments.

Kinematic analysis of Toxoplasma gondii motility

Toxoplasma gondii tachyzoites execute a complex and little understood combination of rapid movements to reach and penetrate human or other animals cells. In the present study, computer-assisted simulation was used to quantitatively analyze the motility of these parasites in three-dimensional space with spatial and temporal resolutions in the micrometer and subsecond ranges. A digital model based on electron-micrographs of a serially sectioned tachyzoite was animated according to a videomicrographed sequence of a characteristic repetitive movement. Keyframe animation defined over 150 frames by a total of 36 kinematic parameters for specific motions, of both the whole model and particular domains, resulted in a real-time life-like simulation of the videorecorded tachyzoite movement. The kinematic values indicate that a full revolution of the model is composed of three half-turns accomplished in nearly 5 s with two phases: a relatively slow 180 degrees tilting with regard to the substratum plane, followed by fast (over 200 degrees/s) spinning almost simultaneous with pivoting around the posterior end, each clockwise and for about 180 degrees. Maximal flexing of the body, as well as bowing and retraction of its anterior end, occur at midway during the tilting phase. An estimated 70 degrees. clockwise torsion of the body seems to precede the spinning-pivoting phase. The results suggest the operation of two basic forces in the motility of T. gondii tachyzoites: (1) a clockwise torque that causes torsion, spinning, and pivoting; and (2) a longitudinal pull that contracts, bends and tilts the parasite. We discuss the possibility that both of these forces might result from the action of an actin-myosin system enveloping the twisted framework of microtubules characteristic of these organisms.

Ultrastructural observations of host-cell invasion by sporozoites of Eimeria papillata in vivo

Scanning and transmission electron microscopy were used to study the invasion of mouse small-intestinal epithelium by sporozoites of Eimeria papillata. Some mice received oocysts by gavage and others received either sporocysts or sporozoites by direct injection into the small intestine. The highest concentration of invaded cells were found in ligated intestinal tissues studied at 5-45 min after the inoculation of sporozoites. Sporozoites actively invaded anterior end first, which resulted in extensive damage to the host cell. Such cells showed disrupted microvilli; protuberances of cytoplasm into the lumen, apparently the result of a disrupted plasma membrane; vacuolization of the cytoplasm; and damage to the mitochondria. These damaged cells were rapidly vacated as the sporozoite moved laterally into one or more adjacent intact host cells without entering the lumen. It is suggested that the host cell initially entered from the lumen becomes so severely traumatized that the parasite of necessity enters an adjacent cell as a prelude to further development. Various aspects of host-cell invasion by coccidia and malarial parasites are reviewed.

Microtubular cytoskeletons of parasitic protozoa

Shape, motility and division of eukaryotic cells are all determined to some extent by intracellular microtubules. Parasites, particularly protozoan parasites, offer important models for the study of microtubule organization, which may also provide useful leads for novel chemotherapeutic agents against parasite-specific forms of these organelles. Research on the biology of microtubules has advanced rapidly in the last 2-3 years, from purely descriptive studies to on understanding of some of the genetic and biochemical aspects. In this article, David Russell and Jean-François Dubremetz summarize the progress and discuss the potential of tubulin as a drug target for antiprotozool chemotherapy.

Scanning electron microscopy of Toxoplasma gondii: parasite torsion and host-cell responses during invasion

Scanning electron microscopy confirmed our previous finding that toxoplasmas actively invade mouse peritoneal cells that are inhibited from phagocytosis. The parasites entered cells with the conoid end first and sometimes showed a counter-clockwise torsion of the body during invasion. Counter-clockwise torsion was also noted in free toxoplasmas. Host-cell responses to active invasion varied with experimental conditions and with the type of host cell. Under adverse culture conditions for phagocytosis, normal macrophages formed rudimentary filopodia or lamellipodia around the tips of invading toxoplasmas; macrophages subjected to hyperthermia before similar incubation with toxoplasmas showed little or no response to invasion. Normal and heat-treated lymphocytes showed little surface reaction to invasion, but occasionally a flocculent collar was seen around the tip of an invading toxoplasma. Scanning electron microscopy provides clues to possible mechanisms of toxoplasma locomotion and host-cell invasion.

Host cell invasion by Apicomplexa: an expression of the parasite's contractile system?

Recent studies on the motility of coccidian sporozoites have demonstrated a membrane-associated contractile system capable of moving certain intramembraneous components down the parasite surface propelling it forwards. The properties of this system resemble recorded observations on host cell invasion. In this study the invasive behaviour of Eimeria tenella and E. acervulina has been examined, with reference to the above findings, by light microscope and scanning and transmission electron microscopes. Known inhibitors of motility prevent invasion, though attachment appears unaffected. Invasion itself consists of 3 phases; attachment and orientation, induction of a parasitophorous vacuole and translocation of the parasite into the vacuole. Ultrastructural examination reveals a close membrane/membrane association maintained throughout invasion. From these results it is suggested that the parasite enters the parasitophorous vacuole by 'capping' the host/parasite junction down its body, so locomoting into the host cell. Such a model has two main advantages; it requires no additional modifications to either cell, and the specificity of membrane receptors would enable the one membrane-associated contractile system to be responsible for locomotion, antibody capping and host cell invasion.