Diffusion of microinjected markers across the parasitophorous vacuole membrane in cells infected with Eimeria nieschulzi (Coccidia, Apicomplexa)

Novel Ion Channel Genes in Malaria Parasites

Ion channels serve many cellular functions including ion homeostasis, volume regulation, signaling, nutrient acquisition, and developmental progression. Although the complex life cycles of malaria parasites necessitate ion and solute flux across membranes, the whole-genome sequencing of the human pathogen Plasmodium falciparum revealed remarkably few orthologs of known ion channel genes. Contrasting with this, biochemical studies have implicated the channel-mediated flux of ions and nutritive solutes across several membranes in infected erythrocytes. Here, I review advances in the cellular and molecular biology of ion channels in malaria parasites. These studies have implicated novel parasite genes in the formation of at least two ion channels, with additional ion channels likely present in various membranes and parasite stages. Computational approaches that rely on homology to known channel genes from higher organisms will not be very helpful in identifying the molecular determinants of these activities. Given their unusual properties, novel molecular and structural features, and essential roles in pathogen survival and development, parasite channels should be promising targets for therapy development.

Comprehensive Characterization of Toxoplasma Acyl Coenzyme A-Binding Protein TgACBP2 and Its Critical Role in Parasite Cardiolipin Metabolism

Toxoplasma gondii is one of the most successful human parasites, infecting nearly one-third of the total world population. T. gondii tachyzoites residing within parasitophorous vacuoles (PVs) can acquire fatty acids both via salvage from host cells and via de novo synthesis pathways for membrane biogenesis. However, although fatty acid fluxes are known to exist in this parasite, how fatty acids flow through Toxoplasma lipid metabolic organelles, especially mitochondria, remains unknown. In this study, we demonstrated that Toxoplasma expresses an active ankyrin repeat containing protein TgACBP2 to coordinate cardiolipin metabolism. Specifically, HMA acquisition resulting from heterologous functional expression of MAF1 rescued growth and lipid metabolism defects in ACBP2-deficient type II parasites, manifesting the complementary role of host mitochondria in parasite cardiolipin metabolism. This work highlights the importance of TgACBP2 in parasite cardiolipin metabolism and provides evidence for metabolic association of host mitochondria with T. gondii.

Acyl coenzyme A (CoA)-binding protein (ACBP) can bind acyl-CoAs with high specificity and affinity, thus playing multiple roles in cellular functions. Mitochondria of the apicomplexan parasite Toxoplasma gondii have emerged as key organelles for lipid metabolism and signaling transduction. However, the rationale for how this parasite utilizes acyl-CoA-binding protein to regulate mitochondrial lipid metabolism remains unclear. Here, we show that an ankyrin repeat-containing protein, TgACBP2, is localized to mitochondria and displays active acyl-CoA-binding activities. Dephosphorylation of TgACBP2 is associated with relocation from the plasma membrane to the mitochondria under conditions of regulation of environmental [K⁺]. Under high [K⁺] conditions, loss of ACBP2 induced mitochondrial dysfunction and apoptosis-like cell death. Disruption of ACBP2 caused growth and virulence defects in the type II strain but not in type I parasites. Interestingly, mitochondrial association factor-1 (MAF1)-mediated host mitochondrial association (HMA) restored the growth ability of ACBP2-deficient type II parasites. Lipidomics analysis indicated that ACBP2 plays key roles in the cardiolipin metabolism of type II parasites and that MAF1 expression complemented the lipid metabolism defects of ACBP2-deficient type II parasites. In addition, disruption of ACBP2 caused attenuated virulence of Prugniuad (Pru) parasites for mice. Taking the results collectively, these data indicate that ACBP2 is critical for the growth and virulence of type II parasites and for the growth of type I parasites under high [K⁺] conditions.

Host cell remodeling by pathogens: the exomembrane system in Plasmodium-infected erythrocytes

Malaria is caused by infection of erythrocytes by parasites of the genus Plasmodium. To survive inside erythrocytes, these parasites induce sweeping changes within the host cell, one of the most dramatic of which is the formation of multiple membranous compartments, collectively referred to as the exomembrane system. As an uninfected mammalian erythrocyte is devoid of internal membranes, the parasite must be the force and the source behind the formation of these compartments. Even though the first evidence of the presence these of internal compartments was obtained over a century ago, their functions remain mostly unclear, and in some cases completely unknown, and the mechanisms underlying their formation are still mysterious. In this review, we provide an overview of the different parts of the exomembrane system, describing the parasitophorous vacuole, the tubovesicular network, Maurer's clefts, the caveola-vesicle complex, J dots and other mobile compartments, and the small vesicles that have been observed in Plasmodium-infected cells. Finally, we combine the data into a simplified view of the exomembrane system and its relation to the alterations of the host erythrocyte.

Plasmodium parasites remodel the host erythrocyte in various ways, including the formation of several membranous compartments, together referred to as the exomembrane system, within the erythrocyte cytosol that together are key to the sweeping changes in the host cell.

The Toxoplasma Dense Granule Proteins GRA17 and GRA23 Mediate the Movement of Small Molecules between the Host and the Parasitophorous Vacuole

Toxoplasma gondii is a protozoan pathogen in the phylum Apicomplexa that resides within an intracellular parasitophorous vacuole (PV) that is selectively permeable to small molecules through unidentified mechanisms. We have identified GRA17 as a Toxoplasma-secreted protein that localizes to the parasitophorous vacuole membrane (PVM) and mediates passive transport of small molecules across the PVM. GRA17 is related to the putative Plasmodium translocon protein EXP2 and conserved across PV-residing Apicomplexa. The PVs of GRA17-deficient parasites have aberrant morphology, reduced permeability to small molecules, and structural instability. GRA17-deficient parasites proliferate slowly and are avirulent in mice. These GRA17-deficient phenotypes are rescued by complementation with Plasmodium EXP2. GRA17 functions synergistically with a related protein, GRA23. Exogenous expression of GRA17 or GRA23 alters the membrane conductance properties of Xenopus oocytes in a manner consistent with a large non-selective pore. Thus, GRA17 and GRA23 provide a molecular basis for PVM permeability and nutrient access.

Molecular make-up of the Plasmodium parasitophorous vacuolar membrane

Plasmodium, the causative agent of malaria, is an obligate, intracellular, eukaryotic cell that invades, replicates, and differentiates within hepatocytes and erythrocytes. Inside a host cell, a second membrane delineates the developing pathogen in addition to the parasite plasma membrane, resulting in a distinct cellular compartment, termed parasitophorous vacuole (PV). The PV membrane (PVM) constitutes the parasite-host cell interface and is likely central to nutrient acquisition, host cell remodeling, waste disposal, environmental sensing, and protection from innate defense. Over the past two decades, a number of parasite-encoded PVM proteins have been identified. They include multigene families and protein complexes, such as early-transcribed membrane proteins (ETRAMPs) and the Plasmodium translocon for exported proteins (PTEX). Nearly all Plasmodium PVM proteins are restricted to this genus and display transient and stage-specific expression. Here, we provide an overview of the PVM proteins of Plasmodium blood and liver stages. Biochemical and experimental genetics data suggest that some PVM proteins are ideal targets for novel anti-malarial intervention strategies.

The nascent parasitophorous vacuole membrane of Encephalitozoon cuniculi is formed by host cell lipids and contains pores which allow nutrient uptake

Microsporidia are obligate intracellular pathogens which enter host cells by the discharge of a hollow tube through which the sporoplasma is extruded into the host cell. Since this invasion mechanism is very different from common entry strategies, the formation of the parasitophorous vacuole (PV) in Encephalitozoon species is likely to be distinct from known principles. We investigated the origin of the nascent Encephalitozoon cuniculi PV membrane with the aid of fluorescent lipid probes. When Bodipy 500/510-C(12)-HPC-labeled spores were used for infection, the emerging PV membrane was unlabeled, suggesting that sporoplasma-derived lipids do not significantly contribute to the formation of the PV membrane. In contrast, when raft and nonraft microdomains of the host cell plasma membrane were selectively labeled with DiIC(16) and Speedy DiO, both tracers were detectable in the nascent PV membrane shortly after infection, indicating that the bulk lipids of the PV membrane are host cell derived. Time-lapse fluorescence microscopy revealed that the formation of the PV membrane is a fast event (<1.3 s), which occurred simultaneously with the extrusion of the sporoplasma. The portion of the discharged tube which is in contact with the host cell was found to be coated with labeled host cell lipids, which might be an indication for a plasma membrane invagination at the contact site. To investigate the presence of pores in the E. cuniculi PV membrane, we microinjected fluorescent dyes of different sizes into infected host cells. A 0.5-kDa dextran as well as 0.8- to 1.1-kDa peptides could rapidly enter the PV, while a 10-kDa dextran was stably excluded from the PV lumen, indicating that the PV membrane possesses pores with an exclusion size of <10 kDa, which should allow metabolite exchange.

Cellular interactions of Plasmodium liver stage with its host mammalian cell

The Plasmodium liver forms are bridgehead stages between the mosquito sporozoite stages and mammalian blood stages that instigate the malaria disease. In hepatocytes, Plasmodium achieves one of the fastest growth rates among eukaryotic cells. However, nothing is known about host hepatic cell interactions, e.g. nutrient scavenging and/or subversion of cellular functions necessary for Plasmodium development and replication. Plasmodium usually invades hepatocytes by establishing a parasitophorous vacuole wherein it undergoes multiple nuclear division cycles. We show that Plasmodium preferentially develops in the host juxtanuclear region. By comparison with the parasitophorous vacuole of other apicomplexan parasites which associate with diverse host organelles, the Plasmodium parasitophorous vacuole only forms an association with the host endoplasmic reticulum. Intrahepatic Plasmodium actively modifies the permeability of its vacuole to allow the transfer of a large variety of molecules from the host cytosol to the vacuolar space through open channels. In contrast with malaria blood stages, the pores within the parasitophorous vacuole membrane of the liver stage display a smaller size as they restrict the passage of solutes to less than 855Da. These pores are stably maintained during parasite karyokinesis until complete cellularisation. Host-derived cholesterol accumulated at the parasitophorous vacuole membrane may modulate the channel activity. These observations define the parasitophorous vacuole of the Plasmodium liver stage as a dynamic and highly permeable compartment that can ensure the sustained supply of host molecules to support parasite growth in the nutrient-rich environment of liver cells.

Survival of protozoan intracellular parasites in host cells

The most common human diseases are caused by pathogens. Several of these microorganisms have developed efficient ways in which to exploit host molecules, along with molecular pathways to ensure their survival, differentiation and replication in host cells. Although the contribution of the host cell to the development of many intracellular pathogens (particularly viruses and bacteria) has been unequivocally established, the study of host-cell requirements during the life cycle of protozoan parasites is still in its infancy. In this review, we aim to provide some insight into the manipulation of the host cell by parasites through discussing the hurdles that are faced by the latter during infection.

Nutrient acquisition by intracellular apicomplexan parasites: staying in for dinner

The intracellular forms of the apicomplexan parasites Plasmodium, Toxoplasma and Eimeria reside within a parasitophorous vacuole. The nutrients required by these intracellular parasites to support their high rate of growth and replication originate from the host cell which, in turn, takes up such compounds from the extracellular milieu. Solutes moving from the external medium to the interior of the parasite, are confronted by a series of three membranes --the host cell membrane, the parasitophorous vacuole membrane and the parasite plasma membrane. Each constitutes a potential permeability barrier which must be either crossed or bypassed. It is the mechanisms by which this occurs that are the subject of this review.

The host-parasite relationship in neosporosis

Neospora caninum is an apicomplexan parasite which invades many different cell types and tissues. It causes neosporosis, namely stillbirth and abortion in cattle and neuromuscular disease in dogs, and has been found in several other animal species. N. caninum is closely related to Toxoplasma gondii, and controversial opinions exist with respect to its phylogenetical status. Initially, two stages of N. caninum had been identified, namely asexually proliferating tachyzoites and bradyzoites. The sexually produced stage of this parasite, oocysts containing sporozoites, has been found only recently. In order to answer the many open questions regarding its basic biology and its relationship with the host, a number of diagnostic tools have been developed. These techniques are based on the detection of antibodies against parasites in body fluids, the direct visualization of the parasite within tissue samples by immunohistochemistry, or the specific amplification of parasite DNA by PCR. Other studies have been aiming at the identification of specific antigenic components of N. caninum, and the molecular and functional characterization of these antigens with respect to the cell biology of the parasite. Clearly, molecular approaches will also be used increasingly to elucidate the immunological and pathogenetic events during infection, but also to prepare potential new immunotherapeutic tools for future vaccination against N. caninum infection.

Apical organelles and host-cell invasion by Apicomplexa

Host-cell invasion by apicomplexan parasites involves the successive exocytosis of three different secretory organelles; namely micronemes, rhoptries and dense granules. The findings of recent studies have extended the structural homologies of each set of organelles between most members of the phylum and suggest shared functions of each set. Micronemes are apparently used for host-cell recognition, binding, and possibly motility; rhoptries for parasitophorous vacuole formation; and dense granules for remodeling the vacuole into a metabolically active compartment. In addition, gene cloning and sequencing have demonstrated conserved domains, which are likely to serve similar functions in the invasion process. This is especially true for microneme proteins containing thrombospondin-like domains, which are likely to be involved in binding to sulphated glycoconjugates. One such protein was recently shown to be required for the motility of Plasmodium sporozoites. These molecules have been shown to be shed on the parasite and/or cell surfaces during the invasion process in Plasmodium, Toxoplasma and Eimeria. For rhoptries and dense granules, the association between exocytosed proteins and the parasitophorous vacuole membrane had been analyzed extensively in Toxoplasma, as these proteins are likely to play a crucial role in metabolic interactions between the parasites and their host cells. The development of parasite transformation by gene transfection has provided powerful tools to analyze the fate and function(s) of the corresponding proteins.

Structure and function of the parasitophorous vacuole in Eimeria species

The intracellular life-cycle of Eimeria are located in the host cell within a membrane-bound parasitophorous vacuole. The invasion process and the formation of the parasitophorous vacuole are mediated by characteristic organelles within the apical complex. During invasion, the parasitophorous-vacuole membrane is manipulated by the parasite and functions later in the development cycle as a molecular sieve, allowing the exchange of metabolites between parasite and host cell. Unlike the cyst-forming coccidia, there is little evidence of parasitophorous-vacuole membrane transformation in the later stages of the lifecycle of Eimeria species. Compared with the human pathogens Plasmodium and Toxoplasma, rather little is known about the parasitophorous vacuole and parasitophorous-vacuole membrane of animal pathogens of the genus Eimeria.