Molecular parasitology at Woods Hole.
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Animals↗
Biomedical subjects
V Nussenzweig
Publications and source records attributed to V Nussenzweig.
The basolateral domain of the hepatocyte plasma membrane bears receptors for the circumsporozoite protein of Plasmodium falciparum sporozoites.
Minutes after injection into the circulation, malaria sporozoites enter hepatocytes. The speed and specificity of the invasion process suggest that it is receptor mediated. We show here that recombinant Plasmodium falciparum circumsporozoite protein (CS) binds specifically to regions of the plasma membrane of hepatocytes exposed to circulating blood in the Disse space. No binding has been detected in other organs, or even in other regions of the hepatocyte membrane. The interaction of CS with hepatocytes, as well as sporozoite invasion of HepG2 cells, is inhibited by synthetic peptides representing the evolutionarily conserved region II of CS. We conclude that region II is a sporozoite ligand for hepatocyte receptors localized to the basolateral domain of the plasma membrane. Our findings provide a rational explanation for the target cell specificity of malaria sporozoites.
Amino Acid Sequence↗
Evidence for the participation of the Ssp-3 antigen in the invasion of nonphagocytic mammalian cells by Trypanosoma cruzi.
Trypomastigotes of Trypanosoma cruzi have to invade mammalian cells in order to multiply. They bear on their plasma membrane a sialic acid-containing epitope (Ssp-3) defined by a series of monoclonal antibodies (mAbs). Previous investigations have shown that Fab fragments of these mAbs inhibit the attachment of trypomastigotes to 3T3 fibroblasts. To further define the role of Ssp-3 in invasion, here we use, as targets for infection, L cells and CHO cells stably transfected with cDNA coding for the mouse Fc receptors genes. When the trypomastigotes are incubated with small, nonagglutinating amounts of antibodies to Ssp-3, their attachment to the transfected cells is greatly enhanced, without a parallel increase in invasion. The enhancement in attachment is Fc mediated, since it is abolished by treatment of the transfected cells with mAbs to Fc receptors. In contrast, both attachment to, and invasion of, the transfected cells are increased if the parasites are incubated with polyclonal or monoclonal antibodies against T. cruzi surface membrane antigens other than Ssp-3. If, however, antibodies to Ssp-3 are added to the incubation mixtures containing any of the other anti-T. cruzi antibodies, the enhancement of invasion (but not of attachment) is reversed. These results suggest that Ssp-3-bearing molecules participate in the process of parasite internalization.
3T3 Cells↗
Trypanosoma cruzi trans-sialidase and neuraminidase activities can be mediated by the same enzymes.
Trans-sialidase and neuraminidase activities have been detected on the surface membrane of trypomastigotes of Trypanosoma cruzi, and both have been implicated in the parasite's invasion of host cells. We show here that these enzymes are structurally related. They are recognized by two independently derived monoclonal antibodies, are anchored to the membrane by glycosylphosphatidylinositol, copurify by ion exchange, molecular sieving, and hydrophobic chromatography, have maximal activities between pH 6.5 and 7.5, and are inactivated by heating at 56 degrees C. Furthermore, the neuraminidase and trans-sialidase reactions are coupled. An increase of the concentration of acceptors of the transfer reaction decreases the amount of free sialic acid released through the neuraminidase reaction. We conclude that a single enzyme can catalyze the transfer or the hydrolysis of macromolecular-bound sialic acid. The predominant direction of the reaction will depend on the availability of appropriate oligosaccharide acceptors of sialic acid.
Animals↗
Only some members of a gene family in Trypanosoma cruzi encode proteins that express both trans-sialidase and neuraminidase activities.
Trypomastigotes, the blood stage form of the human parasite Trypanosoma cruzi, contain an enzyme on their surface, trans-sialidase, which catalyses the transfer of sialic acid from host glycoconjugates to acceptors on its own cell surface. At least a subset of the sialic acid-bearing acceptor molecules are involved in parasite invasion of host cells, an essential step in the life cycle of the parasite. Another trypomastigote surface enzyme that affects host cell invasion is neuraminidase and recent evidence suggests that both trans-sialidase and neuraminidase activities may be expressed by the same proteins on the parasite surface. We describe here the isolation and expression of several members of a trans-sialidase--neuraminidase gene family from T.cruzi. One of the isolated genes does indeed encode a protein with both trans-sialidase and neuraminidase activities, while other members of the gene family encode closely related proteins that express neither enzymatic activity. Chimeric protein constructs combining different portions of active and inactive genes identified a region of the gene necessary for enzymatic activity. Sequence analysis of this portion of the gene revealed a limited number of amino acid differences between the predicted active and inactive gene products.
Amino Acid Sequence↗
Binding of malarial circumsporozoite protein to sulfatides [Gal(3-SO4)beta 1-Cer] and cholesterol-3-sulfate and its dependence on disulfide bond formation between cysteines in region II.
Region II of the malaria circumsporozoite (CS) protein is highly conserved between the CS proteins of different species of malaria. Amino acid sequences homologous to that of region II are found in thrombospondin, properdin, von Willebrand factor and a few other proteins. We show here that the native CS protein from the rodent parasite Plasmodium berghei, and recombinant Plasmodium vivax and Plasmodium falciparum CS proteins containing region II, but not recombinant proteins lacking region II, specifically bind to sulfatides and cholesterol-3-sulfate. The binding is abolished following reduction and alkylation of the proteins. Region II contains 2 cysteines separated by only 3 amino acids, S(N), V, T, and these are the only cysteines present in our recombinant proteins. Therefore, our findings strongly suggest that the region II cysteines are linked by a disulfide bond forming a small peptide loop. We also present evidence that the recognition of sulfatides, cholesterol-3-sulfate, or other cross-reactive sulfated macromolecules by region II may be required during sporozoite invasion of liver cells. Antibodies to a peptide representing region II react with live sporozoites and with sporozoites fixed with glutaraldehyde, indicating that this region is exposed on the surface of the parasites. Furthermore, we have found that the sulfatide and cholesterol-3-sulfate recognition by the CS proteins, and the invasion of hepatocytes by P. berghei sporozoites, are specifically inhibited by dextran sulfate.
Amino Acid Sequence↗
Substrate specificity of the Trypanosoma cruzi trans-sialidase.
Trypanosoma cruzi trypomastigotes acquire sialic acid (SA) from host glycoconjugates by means of a plasma membrane-associated trans-sialidase (TS). Here we study the substrate specificity of TS, which differs from all known sialyltransferases in that it does not require cytidine monophosphate (CMP)-SA as donor. The T. cruzi TS reversibly transfers SA to saccharides with terminal beta-Gal (but not alpha-Gal) residues. Donors are saccharides with SA linked to terminal beta-Gal residues by (alpha 2-3), but not (alpha 2-6) bonds. The type of beta-linkage of the terminal Gal residue is of minor importance (beta 1-4 and beta 1-6 are slightly better than beta 1-3), whereas chain length and the structure of additional vicinal sugar residues are not relevant. SA on the surface of living trypomastigotes of T. cruzi is transferred back and forth between the parasite surface and acceptor molecules with terminal beta-Gal, either in solution or on the surface of neighbouring mammalian cells. Addition of fucose residue on or close to the terminal galactose impairs TS activity. As a consequence, the enzyme acts poorly on the E-selectin ligand sialyl-Lewisx and its precursor Lewisx, and in vitro adhesion of TS-treated neutrophils to L-cells expressing L-selectin is not affected. Modifications in the structure of the (alpha 2-3)-linked N-acetyl-neuraminic acid (Neu5Ac) (deoxy or methoxy) of the donor molecules do not impair transfer if the changes are at C9, whereas changes at C4, C7 and C8 impair the ability to donate the modified SA.(ABSTRACT TRUNCATED AT 250 WORDS)
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Resialylation of sialidase-treated sheep and human erythrocytes by Trypanosoma cruzi trans-sialidase: restoration of complement resistance of desialylated sheep erythrocytes.
Trypanosoma cruzi trans-sialidase (TS) is a recently described enzyme which transfers alpha(2-3)-linked sialic acid from host-derived sialylated glycoconjugates to parasite surface molecules [Schenkman et al. (1991) Cell, 65, 1117]. We report here on the ability of TS to transfer sialic acid from donor sialyl-alpha(2-3)lactose to sialidase-treated sheep and human erythrocytes. Up to approximately 50% resialylation of both desialylated red cells could be attained. Resialylation of desialylated sheep erythrocytes restores their resistance to lysis by human complement. This ascribes a possible biological role for T. cruzi TS and demonstrates directly that sialic acid is solely responsible for preventing alternative pathway activation of human complement by sheep erythrocytes.
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Stage-specific expression and intracellular shedding of the cell surface trans-sialidase of Trypanosoma cruzi.
We have used antibodies to the Trypanosoma cruzi trans-sialidase and to its product, the host cell invasion-related Ssp-3 epitope, to study the expression of the corresponding antigens during the intracellular development of the parasite and in the extracellular trypomastigotes. As soon as 2 h after host cell invasion, trans-sialidase was no longer detected, whereas the Ssp-3 epitope was still present on intracellular parasites. The amastigotes which subsequently developed remained nonreactive with the antibodies. Expression of enzymatically active T. cruzi trans-sialidase started again only after transformation of the amastigotes into trypomastigotes 72 h after host cell invasion. trans-Sialidase was shed from the trypanosomes into the host cell cytoplasm, where the enzyme accumulated until release of the parasites. All released trypomastigotes expressed trans-sialidase on their surfaces and in the flagellar pockets, but stumpy trypomastigotes were stained more intensely than slender trypomastigotes. Ssp-3, the sialylated reaction product of trans-sialidase, was assembled only after rupture of the host cell membrane and was detected on the plasma membranes and in the flagellar pockets of all trypomastigotes.
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An improved polymerase chain reaction assay to detect Trypanosoma cruzi in blood.
Amplification by the polymerase chain reaction of Trypanosoma cruzi satellite DNA was used to enhance sensitivity in the detection of the parasite in blood, with the ultimate goal of improving diagnosis of the chronic phase of Chagas' disease. Two contiguous oligonucleotides were synthesized corresponding to the most conserved region of the 195-basepair repeated sequence and used as primers for the amplification reaction. Nineteen femtograms of parasite DNA that was amplified in the presence of 15 micrograms of human or mouse DNA produced a visible band upon electrophoresis in agarose gels and staining with ethidium bromide. In reconstitution experiments, one parasite in 10 ml of blood could be unambiguously determined when the DNA was isolated from nuclei after the blood was treated with NP40 and centrifuged. Polymerase chain reaction assays were carried out to detect T. cruzi in chronically infected mice. Most mice were parasite-positive when organs or tissues were tested, but all were negative when total blood was tested.
Animals↗
Membrane vesiculation protects erythrocytes from destruction by complement.
Nucleated cells can resist attack by C by exocytosis or endocytosis of the terminal C components C5b-9 (membrane attack complex) (MAC), but it is generally accepted that formation of a single MAC channel on E leads to lysis (one-hit theory). We find that human and guinea pig E, but not SRBC, can eliminate the MAC from the membrane in the form of microvesicles and escape destruction. When guinea pig or human E are incubated with C5b-9, vesiculation proceeds without a lag and is detected at nonlytic doses of C9. Continuous Ca2+ influx is required for vesiculation. The amount of released vesicles is in direct relation to Ca2+ concentration, and the increase in vesiculation is associated with a parallel decrease in lysis. SRBC, which do not vesiculate when Ca2+ loaded, are lysed by C5b-9 with the same efficiency in the presence or absence of Ca2+. Vesicles released from guinea pig RBC under C5b-9 attack are enriched in C9 by a factor of 10, compared with the unlysed cells, and by a factor of 3 to 4, compared with ghosts. We conclude that E are protected from lysis not only by CD59 and C8bp/HRF, which prevent MAC assembly, but also by selective elimination of the MAC.
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A novel cell surface trans-sialidase of Trypanosoma cruzi generates a stage-specific epitope required for invasion of mammalian cells.
When trypomastigotes of T. cruzi emerge from cells of the mammalian host, they contain little or no sialic acids on their surfaces. However, rapidly upon entering the circulation, they express a unique cell surface trans-sialidase activity. This enzyme specifically transfers alpha (2-3)-linked sialic acid from extrinsic host-derived macromolecules to parasite surface molecules, leading to the assembly of Ssp-3, a trypomastigote-specific epitope. The T. cruzi trans-sialidase does not utilize cytidine 5' monophospho-N-acetylneuraminic acid as a donor substrate, but readily transfers sialic acid from exogenously supplied alpha (2-3)-sialyllactose. Monoclonal antibodies that recognize sialic acid residues of Ssp-3 inhibit attachment of trypomastigotes to host cells, suggesting that the unusual trans-sialidase provides Ssp-3 with structural features required for target cell recognition.
Animals↗
Progress toward malaria preerythrocytic vaccines.
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Amino Acid Sequence↗
Attachment of Trypanosoma cruzi trypomastigotes to receptors at restricted cell surface domains.
We have used glutaraldehyde-fixed target cells to study the attachment phase of cell invasion by live trypomastigotes of Trypanosoma cruzi, and determined that attachment is polarized and receptor-mediated. T. cruzi trypomastigotes bind much less efficiently to confluent epithelial cells, which are polarized, than to sparse epithelial cells. When the tight junctions of confluent epithelial cells are disrupted by removing Ca2+ from the incubation medium before glutaraldehyde fixation, binding of T. cruzi increases. T. cruzi also shows preference for attachment underneath cells or to the edges of cells. The binding occurs within a few minutes, is saturable, and is influenced by the parasite developmental stage. Fab fragment derived from monoclonal antibodies that immunoprecipitate a 160-kDa molecule present only on the surface of trypomastigotes inhibit adhesion to fixed and live cells. Future characterization of the target cell receptors for this molecule and the use of fixed target cells should facilitate studies of the mechanisms involved in the initial interaction of T. cruzi with its host cells.
Animals↗
Attachment of Trypanosoma cruzi to mammalian cells requires parasite energy, and invasion can be independent of the target cell cytoskeleton.
We have previously shown that the binding of Trypanosoma cruzi trypomastigotes to glutaraldehyde-fixed mammalian cells has the characteristics of a receptor-mediated process and that it mimics the attachment step of the invasion of live cells by this parasite. In this study we examined the metabolic requirements for the attachment of trypomastigotes to glutaraldehyde-fixed fibroblasts. The attachment of trypomastigotes to fixed cells is prevented when the energy conservation mechanisms are inhibited with the drugs 2-deoxyglucose, sodium azide, antimycin, crystal violet, oligomycin, N,N'-dicyclohexylcarbodiimide, and carbonyl cyanide 3-chlorophenylhydrazone. However, under the same experimental conditions, the movement of parasites is not significantly affected. Several of these drugs totally inhibit the penetration of the parasite into live target cells. We conclude that the attachment of trypomastigotes to mammalian cells is an active process that requires trypomastigote energy. In addition, we present evidence that penetration into nonphagocytic cells can also be an active process. Trypomastigotes can be seen in scanning electron micrographs traversing extended lamellipodia and entering paraformaldehyde-fixed epithelial cells. Cytochalasin D, a drug that disrupts microfilaments and prevents the formation of plasma membrane extensions mediated by actin, had little or no effect on trypomastigote invasion, while it inhibited Salmonella entry into epithelial cells.
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Progress toward a malaria vaccine.
In initial human trials, synthetic vaccines have induced humoral immunity sufficient to prevent clinical infection in some cases and delay it in others. Progress in induction of cellular immunity is also noteworthy with identification of determinants recognized by T cells. Antigenic variation and consequent blunting of immunogenicity may not be as troublesome as feared.
Amino Acid Sequence↗
The exit of Trypanosoma cruzi from the phagosome is inhibited by raising the pH of acidic compartments.
The protozoan parasite Trypanosoma cruzi can infect many distinct mammalian cell types. The parasites enter cells through the formation of phagocytic vacuoles, but later are found free in the cytosol, where they multiply as amastigotes. Using transmission electron microscopy we found that within 2 h after infection 70% of the parasites, including examples of both mammalian forms (trypomastigotes and amastigotes), were inside partially disrupted vacuoles or free in the cytosol. We demonstrated that the pH of vacuoles containing recently interiorized parasites is acidic, through immunocytochemical localization of the acidotropic compound DAMP (18) in their interior. Increasing the vacuolar pH with chloroquine, ammonium chloride, methylamine, or monensin significantly inhibited the escape of the parasites into the cytosol. These results are compatible with the hypothesis that an acid-active hemolysin of T. cruzi (15) might be involved in the escape mechanism.
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Incorporation of T and B epitopes of the circumsporozoite protein in a chemically defined synthetic vaccine against malaria.
We show here an effective and novel approach to engineer peptide-based vaccines using a chemically defined system, known as multiple peptide antigen systems (MAPs), to protect an inbred mouse strain from infection against rodent malaria. 10 mono- and di-epitope MAP models containing different arrangements and stoichiometry of functional B and/or T helper cell epitopes from the circumsporozoite protein of Plasmodium berghei were used to immunize A/J mice. While these mice did not respond to the mono-epitope MAP bearing only the B or T epitope, very high titers of antibody and protective immunity against sporozoite challenge were elicited by di-epitope MAPs, particularly those with the B and T epitopes in tandem and present in equimolar amounts. These results, obtained in a well-defined rodent malaria model, indicate that MAPs may overcome some of the difficulties in the development of synthetic vaccines, not only for malaria but also for other infectious diseases.
Amino Acid Sequence↗