A binding site for highly sulfated heparan sulfate is identified in the N terminus of the circumsporozoite protein: significance for malarial sporozoite attachment to hepatocytes

Plasmodium sporozoite passage across the sinusoidal cell layer

Malaria sporozoites must cross at least two cell barriers to reach their initial site of replication in the mammalian host. After transmission into the skin by an infected mosquito, they migrate towards small dermal capillaries, traverse the vascular endothelial layer, and rapidly home to the liver. To infect hepatocytes, the parasites must cross the sinusoidal cell layer, composed of specialized highly fenestrated sinusoidal endothelia and Kupffer cells, the resident macrophages of the liver (Fig. 1). The exact route Plasmodium sporozoites take to hepatocytes has been subject of controversial discussions for many years. Recent cell biological, microscopic, and genetic approaches have considerably enhanced our understanding of the initial events leading to the establishment of a malaria infection in the liver.

Liposomes incorporating a Plasmodium amino acid sequence target heparan sulfate binding sites in liver

Previous studies demonstrated that intravenously administered liposomes, incorporating a peptide from the Plasmodium circumsporozoite protein, accumulate rapidly and selectively in mouse liver. The present investigation was designed to determine the molecular components in liver responsible for liposome targeting. Studies of liver tissue slices demonstrated that immunoreactivity for heparan sulfate proteoglycan (HSPG), but not other tested proteoglycans, was distributed along sinusoidal borders of liver; this immunoreactivity appeared associated with nonparenchymal cells of the sinusoids and with the basolateral portion of hepatocytes. Peptide-containing liposomes bound to liver tissue in a pattern similar to the distribution of heparan sulfate immunoreactivity, either after intravenous injection of liposomes in vivo or after incubation of liposomes with liver slices in vitro. Control liposomes, without the peptide, displayed very light binding without a pattern. Pretreatment of liver slices with heparinase, but not chondroitinase or hyaluronidase, eliminated peptide-containing liposome binding, but did not affect binding of control liposomes. Coincubation of peptide-containing liposomes with heparin, but not with other glycosaminoglycans, markedly inhibited liposome binding to liver slices. N-desulfated and O-desulfated heparins individually were less effective inhibitors of liposome binding than was heparin. These results indicate that liposomes containing a peptide from Plasmodium target liver tissue by binding to HSPGs in the extracellular matrix.

Exoerythrocytic development of Plasmodium gallinaceum in the White Leghorn chicken

Plasmodium gallinaceum typically causes sub-clinical disease with low mortality in its primary host, the Indian jungle fowl Gallus sonnerati. Domestic chickens of European origin, however, are highly susceptible to this avian malaria parasite. Here we describe the development of P. gallinaceum in young White Leghorn chicks with emphasis on the primary exoerythrocytic phase of the infection. Using various regimens for infection, we found that P. gallinaceum induced a transient primary exoerythrocytic infection followed by a fulminant lethal erythrocytic phase. Prerequisite for the appearance of secondary exoerythrocytic stages was the development of a certain level of parasitaemia. Once established, secondary exoerythrocytic stages could be propagated from bird to bird for several generations without causing fatalities. Infected brains contained large secondary exoerythrocytic stages in capillary endothelia, while in the liver primary and secondary erythrocytic stages developed primarily in Kupffer cells and remained smaller. At later stages, livers exhibited focal hepatocyte necrosis, Kupffer cell hyperplasia, stellate cell proliferation, inflammatory cell infiltration and granuloma formation. Because P. gallinaceum selectively infected Kupffer cells in the liver and caused a histopathology strikingly similar to mammalian species, this avian Plasmodium species represents an evolutionarily closely related model for studies on the hepatic phase of mammalian malaria.

Trypanosoma cruzi heparin-binding proteins and the nature of the host cell heparan sulfate-binding domain

Trypanosoma cruzi invasion is mediated by receptor-ligand recognition between the surfaces of both parasite and target cell. We have previously demonstrated the role of heparan sulfate proteoglycan in the attachment and invasion of T. cruzi in cardiomyocytes. Herein, we have isolated the T. cruzi heparin-binding proteins (HBP-Tc) and investigated the nature of cardiomyocyte heparan sulfate (HS)-binding site to the parasite surface ligand. Two major heparin-binding proteins with molecular masses of 65.8 and 59 kDa were observed in total extract of amastigote and trypomastigote forms of T. cruzi. Hydrophobic [S(35)]methionine labeled proteins eluted from heparin-sepharose affinity chromatography also revealed both proteins in trypomastigotes but only the 59 kDa is strongly recognized by biotin-conjugated glycosaminoglycans. Competition assays were performed to analyze the role of sulfated proteoglycans, including heparin, keratan sulfate and both acetylated and highly sulfated domains of heparan sulfate, in the recognition and invasion process of T. cruzi. Significant inhibitions of 84% and 35% in the percentage of infection were revealed after treatment of the parasites with heparin and the N-acetylated/ N-sulfated heparan sulfate domain, respectively, suggesting the important role of the glycuronic acid and NS glucosamine domain of the HS chain in the recognition of the HBP-Tc during the T. cruzi-cardiomyocyte interaction.

Malaria circumsporozoite protein inhibits the respiratory burst in Kupffer cells

After transmission by infected mosquitoes, malaria sporozoites rapidly travel to the liver. To infect hepatocytes, sporozoites traverse Kupffer cells, but surprisingly, the parasites are not killed by these resident macrophages of the liver. Here we show that Plasmodium sporozoites and recombinant circumsporozoite protein (CSP) suppress the respiratory burst in Kupffer cells. Sporozoites and CSP increased the intracellular concentration of cyclic adenosyl mono-phosphate (cAMP) and inositol 1,4,5-triphosphate in Kupffer cells, but not in hepatocytes or liver endothelia. Preincubation with cAMP analogues or inhibition of phosphodiesterase also inhibited the respiratory burst. By contrast, adenylyl cyclase inhibition abrogated the suppressive effect of sporozoites. Selective protein kinase A (PKA) inhibitors failed to reverse the CSP-mediated blockage and stimulation of the exchange protein directly activated by cAMP (EPAC), but not PKA inhibited the respiratory burst. Both blockage of the low-density lipoprotein receptor-related protein (LRP-1) with receptor-associated protein and elimination of cell surface proteoglycans inhibited the cAMP increase in Kupffer cells. We propose that by binding of CSP to LRP-1 and cell surface proteoglycans, malaria sporozoites induce a cAMP/EPAC-dependent, but PKA-independent signal transduction pathway that suppresses defence mechanisms in Kupffer cells. This allows the sporozoites to safely pass through these professional phagocytes and to develop inside neighbouring hepatocytes.

Developmental biology of sporozoite-host interactions in Plasmodium falciparum malaria: implications for vaccine design

The Plasmodium falciparum sporozoite infects different types of cells in a mosquito's salivary glands and human epithelial and Kuppfer cells and hepatocytes. These become differentiated later on, transforming themselves into the invasive red blood cell form, the merozoite. The ability of sporozoites to interact with different types of cells requires a wide variety of mechanisms allowing them to survive in both hosts: mobility, receptor-ligand interactions with different cellular receptors, and transformation and development into other invasive parasite forms, which are vitally important for parasite survival. Sporozoite complexity is reflected in the large quantity of proteins that can be expressed. Some of them have been extensively studied, such as CSP, TRAP, STARP, LSA-1, LSA-3, SALSA, SPECT1, SPECT2, MAEBL, and SPATR, due to their importance in infection and their potential use as vaccines. Our work has been focused on the search for the molecular mechanisms of parasite-host cellular receptor-ligand interactions by identifying amino acid sequences and the critical binding residues from these proteins relevant to parasite invasion. Once such sequences have been identified, it will be possible to modify them to induce a strong immune response against P. falciparum in the experimental Aotus monkey model. This all leads towards developing multistage, multicomponent, subunit-based vaccines that will be effective in eradicating or controlling malaria caused by P. falciparum.

A mosquito-specific protein family includes candidate receptors for malaria sporozoite invasion of salivary glands

We describe a previously unrecognized protein family from Aedes and Anopheles mosquitoes, here named SGS proteins. There are no SGS homologues in Drosophila or other eukaryotes, but SGS presence in two mosquito genera suggests that the protein family is widespread among mosquitoes. Ae. aegypti aaSGS1 mRNA and protein are salivary gland specific, and protein is localized in the basal lamina covering the anatomical regions that are preferentially invaded by malaria sporozoites. Anti-aaSGS1 antibodies inhibited sporozoite invasion into the salivary glands in vivo, confirming aaSGS1 as a candidate sporozoite receptor. By homology to aaSGS1 we identified the complete complement of four SGS genes in An. gambiae, which were not recognized in the genome annotation. Two An. gambiae SGS genes display salivary gland specific expression like aaSGS1. Bioinformatic analysis predicts that SGS proteins possess heparin-binding domains, and have among the highest density of tyrosine sulphation sites of all An. gambiae proteins. The major sporozoite surface proteins (CS and TRAP) also bind heparin, and interact with sulphoconjugates during liver cell invasion. Thus, we speculate that sporozoite invasion of mosquito salivary glands and subsequently the vertebrate liver may share similar mechanisms based on sulphation. Phylogenomic analysis suggests that an SGS ancestor was involved in a lateral gene transfer.

Effective targeting of liposomes to liver and hepatocytes in vivo by incorporation of a Plasmodium amino acid sequence

PURPOSE

Several species of the protozoan Plasmodium effectively target mammalian liver during the initial phase of host invasion. The purpose of this study was to demonstrate that a Plasmodium targeting amino acid sequence can be engineered into therapeutic nanoparticle delivery systems.

METHODS

A 19-amino peptide from the circumsporozoite protein of Plasmodium berghei was prepared containing the conserved region I as well as a consensus heparan sulfate proteoglycan binding sequence. This peptide was attached to the distal end of a lipid-polyethylene glycol bioconjugate. The bioconjugate was incorporated into phosphatidylcholine liposomes containing fluorescently labeled lipids to follow blood clearance and organ distribution in vivo.

RESULTS

When administered intravenously into mice, the peptide-containing liposomes were rapidly cleared from the circulation and were recovered almost entirely in the liver. Fluorescence and electron microscopy demonstrated that the liposomes were accumulated both by nonparenchymal cells and hepatocytes, with the majority of the liposomal material associated with hepatocytes. Accumulation of liposomes in the liver was several hundredfold higher compared to heart, lung, and kidney, and more than 10-fold higher compared to spleen. In liver slice experiments, liposome binding was specific to sites sensitive to heparinase.

CONCLUSIONS

Incorporation of amino acid sequences that recognize glycosaminoglycans is an effective strategy for the development of targeted drug delivery systems.

Safety and elicitation of humoral and cellular responses in colombian malaria-naive volunteers by a Plasmodium vivax circumsporozoite protein-derived synthetic vaccine

Substantial experimental evidence indicates that the Plasmodium circumsporozoite (CS) protein has great potential as a vaccine candidate. We tested the safety and immunogenicity of vaccines composed of P. vivax CS-derived synthetic peptides. Sixty-nine healthy, malaria-naive volunteers were randomized to receive three injections of placebo or synthetic proteins N, R, or C (10, 30, or 100 microg/dose) in a double-blinded fashion. Vaccines were well tolerated and no serious adverse events were observed. Peptides N and R elicited humoral responses at all doses; peptide C elicicted these responses only at doses of 30 and 100 microg. The N peptide at a dose of 100 microg elicited the greatest antibody response. Antibodies to the three peptides recognized P. vivax sporozoites in an immunofluorescent antibody test. Peripheral blood mononuclear cells from most immunized volunteers also produced interferon-gamma upon peptide in vitro stimulation. These vaccines appear safe, well tolerated, and immunogenic in malaria-naive volunteers. Further optimization and development of this vaccine is being attempted to conduct phase II clinical trials.

Motility and infectivity of Plasmodium berghei sporozoites expressing avian Plasmodium gallinaceum circumsporozoite protein

Avian and rodent malaria sporozoites selectively invade different vertebrate cell types, namely macrophages and hepatocytes, and develop in distantly related vector species. To investigate the role of the circumsporozoite (CS) protein in determining parasite survival in different vector species and vertebrate host cell types, we replaced the endogenous CS protein gene of the rodent malaria parasite Plasmodium berghei with that of the avian parasite P. gallinaceum and control rodent parasite P. yoelii. In anopheline mosquitoes, P. berghei parasites carrying P. gallinaceum and rodent parasite P. yoelii CS protein gene developed into oocysts and sporozoites. Plasmodium gallinaceum CS expressing transgenic sporozoites, although motile, failed to invade mosquito salivary glands and to infect mice, which suggests that motility alone is not sufficient for invasion. Notably, a percentage of infected Anopheles stephensi mosquitoes showed melanotic encapsulation of late stage oocysts. This was not observed in control infections or in A. gambiae infections. These findings shed new light on the role of the CS protein in the interaction of the parasite with both the mosquito vector and the rodent host.

Intravital observation of Plasmodium berghei sporozoite infection of the liver

Plasmodium sporozoite invasion of liver cells has been an extremely elusive event to study. In the prevailing model, sporozoites enter the liver by passing through Kupffer cells, but this model was based solely on incidental observations in fixed specimens and on biochemical and physiological data. To obtain direct information on the dynamics of sporozoite infection of the liver, we infected live mice with red or green fluorescent Plasmodium berghei sporozoites and monitored their behavior using intravital microscopy. Digital recordings show that sporozoites entering a liver lobule abruptly adhere to the sinusoidal cell layer, suggesting a high-affinity interaction. They glide along the sinusoid, with or against the bloodstream, to a Kupffer cell, and, by slowly pushing through a constriction, traverse across the space of Disse. Once inside the liver parenchyma, sporozoites move rapidly for many minutes, traversing several hepatocytes, until ultimately settling within a final one. Migration damage to hepatocytes was confirmed in liver sections, revealing clusters of necrotic hepatocytes adjacent to structurally intact, sporozoite-infected hepatocytes, and by elevated serum alanine aminotransferase activity. In summary, malaria sporozoites bind tightly to the sinusoidal cell layer, cross Kupffer cells, and leave behind a trail of dead hepatocytes when migrating to their final destination in the liver.

Cellular response of cardiac fibroblasts to amyloidogenic light chains

Amyloidoses are a group of disorders characterized by abnormal folding of proteins that impair organ function. We investigated the cellular response of primary cardiac fibroblasts to amyloidogenic light chains and determined the corresponding change in proteoglycan expression and localization. The cellular response to 11 urinary immunoglobulin light chains of kappa1, lambda6, and lambda 3 subtypes was evaluated. The localization of the light chains was monitored by conjugating them to Oregon Green 488 and performing live cell confocal microscopy. Sulfation of the proteoglycans was determined after elution over Q1-columns with a single-step salt gradient (1.5 mol/L NaCl) via dimethylmethylene blue. Light chains were detected inside cells within 4 hours and demonstrated perinuclear localization. Over 80% of the cells showed intracellular localization of the amyloid light chains. The light chains induced sulfation of the secreted glycosaminoglycans, but the cell fraction possessed only minimal sulfation. Furthermore, the light chains caused a translocation of heparan sulfate proteoglycan to the nucleus. The conformation and thermal stability of light chains was altered when they were incubated in the presence of heparan sulfate and destabilization of the amyloid light chains was detected. These studies indicate that internalization of the light chains mediates the expression and localization of heparan sulfate proteoglycans.

Survey of the year 2004: literature on applications of isothermal titration calorimetry

The market for commercially available isothermal titration calorimeters continues to grow as new applications and methodologies are developed. Concomitantly the number of users (and abusers) increases dramatically, resulting in a steady increase in the number of publications in which isothermal titration calorimetry (ITC) plays a role. In the present review, we will focus on areas where ITC is making a significant contribution and will highlight some interesting applications of the technique. This overview of papers published in 2004 also discusses current issues of interest in the development of ITC as a tool of choice in the determination of the thermodynamics of molecular recognition and interaction.

Sneaking in through the back entrance: the biology of malaria liver stages

Malaria infection is caused by sporozoites, the life cycle stage of Plasmodium that is transmitted by female anopheline mosquitoes. The inoculated sporozoites migrate in the skin, enter a capillary and use the bloodstream for the long haul to the liver. Here, the parasites invade hepatocytes and differentiate to thousands of merozoites that specifically infect red blood cells. Hepatocytes, however, are not directly accessible to sporozoites entering the liver sinusoid. The liver phase of the malaria life cycle can occur only if the parasites first cross the layer of sinusoidal cells that line the liver capillaries. Experimental observations show that sporozoite entry into the liver parenchyma involves a complex cascade of events, from binding to extracellular matrix proteoglycans via passage through Kupffer cells and transmigration through several hepatocytes, until the final host cell is found. By choosing the liver as their initial site of replication, Plasmodium sporozoites can exploit the tolerogenic properties of this unique immune organ to evade the host's immune response.