151
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Marty AJ, Thompson JK, Duffy MF, Voss TS, Cowman AF, Crabb BS. Evidence that Plasmodium falciparum chromosome end clusters are cross-linked by protein and are the sites of both virulence gene silencing and activation. Mol Microbiol 2006; 62:72-83. [PMID: 16942599 DOI: 10.1111/j.1365-2958.2006.05364.x] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The malaria parasite Plasmodium falciparum undergoes antigenic variation through allelic exclusion and variant expression of surface proteins encoded by the var gene family. Regulation of var genes is under epigenetic control and involves reversible silencing and activation that requires the physical repositioning of a var locus into a transcriptionally permissive zone of the nuclear periphery. P. falciparum chromosome ends appear to aggregate into large perinuclear clusters which house both subtelomeric and chromosome central var genes. In this study we further define the composition of telomeric clusters using fluorescent in situ hybridization, and provide evidence that chromosome end clusters are formed by cross-linking protein. In addition, we demonstrate that a subtelomeric reporter gene and a var gene remain within clusters regardless of their transcriptional status. Our findings support a model whereby a highly localized structure dedicated to the activation of a single var gene can be housed within a gene dense chromosome end cluster that is otherwise transcriptionally silent.
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Affiliation(s)
- Allison J Marty
- The Walter and Eliza Hall Institute of Medical Research, Melbourne, Victoria 3050, Australia
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152
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Duffy MF, Caragounis A, Noviyanti R, Kyriacou HM, Choong EK, Boysen K, Healer J, Rowe JA, Molyneux ME, Brown GV, Rogerson SJ. Transcribed var genes associated with placental malaria in Malawian women. Infect Immun 2006; 74:4875-83. [PMID: 16861676 PMCID: PMC1539630 DOI: 10.1128/iai.01978-05] [Citation(s) in RCA: 76] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Determining the diversity of PfEMP1 sequences expressed by Plasmodium falciparum-infected erythrocytes isolated from placentas is important for attempts to develop a pregnancy-specific malaria vaccine. The DBLgamma and var2csa DBL3x domains of PfEMP1 molecules are believed to mediate placental sequestration of infected erythrocytes, so the sequences encoding these domains were amplified from the cDNAs of placental parasites by using degenerate oligonucleotides. The levels of specific var cDNAs were then determined by quantitative reverse transcription-PCR. Homologues of var2csa DBL3x were the predominant sequences amplified from the cDNAs of most placental but not most children's parasites. There was 56% identity between all placental var2csa sequences. Many different DBLgamma domains were amplified from the cDNAs of placental and children's isolates. var2csa transcripts were the most abundant var transcripts of those tested in 11 of 12 placental isolates and 1 of 6 children's isolates. Gravidity did not affect the levels of var2csa transcripts. We concluded that placental malaria is frequently associated with transcription of var2csa but that other var genes are also expressed, and parasites expressing high levels of var2csa are not restricted to pregnant women. The diversity of var2csa sequences may be important for understanding immunity and for the development of vaccines for malaria during pregnancy.
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Affiliation(s)
- Michael F Duffy
- Department of Medicine (RMH), University of Melbourne, Post Office, Royal Melbourne Hospital, Victoria 3050, Australia.
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153
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Matuschewski K. Vaccine development against malaria. Curr Opin Immunol 2006; 18:449-57. [PMID: 16765576 DOI: 10.1016/j.coi.2006.05.004] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2006] [Accepted: 05/30/2006] [Indexed: 11/24/2022]
Abstract
Malaria vaccine development aims to significantly reduce mortality and morbidity in the two high-risk groups: young children and pregnant women in sub-Saharan Africa. A pre-erythrocytic subunit vaccine, RTS,S, was recently evaluated in a first Phase IIb clinical trial in young children that live in a mesoendemic area of malaria transmission. The identification of a major parasite-encoded ligand that was found to be involved in pregnancy-associated malaria allows for the rational development of a tailor-made subunit vaccine for young women. The generation of transgenic yellow fever viruses and defined attenuated vaccine lines by reverse genetics are complementary approaches that might further boost the development of safe vaccines that elicit protective immune responses in this population.
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Affiliation(s)
- Kai Matuschewski
- Department of Parasitology, Heidelberg University School of Medicine, Germany.
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154
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Abstract
Pathogens of the genus Plasmodium are unicellular parasites that infect a variety of animals, including reptiles, birds and mammals. All Plasmodium species target host erythrocytes and replicate asexually within this niche. In humans, proliferation within erythrocytes causes disease symptoms ranging from asymtomatic infection to severe disease, including mild to severe febrile and respiratory symptoms, profound anaemia and obstruction of blood flow. The most serious form of human malaria is caused by Plasmodium falciparum, a pathogen that is responsible for several million deaths annually throughout the developing world. Malaria parasites succeed in evading the host immune response to establish long-term, persistent infections, thus increasing the efficiency by which they are transmitted to the mosquito vector. The ability to evade the host immune system, in particular the avoidance of antibody-mediated immunity against parasite-encoded surface proteins, is the result of amplification of extensive repertoires of multicopy, hypervariable gene families that encode infected erythrocyte or merozoite surface proteins. Via switching between antigenically diverse genes within these large families, populations of parasites have the capacity for rapid variation in antigenicity and virulence over the course of an infection. Here we review the amplification and generation of antigenic diversity within the Plasmodium variant gene families, as well as discuss the mechanisms underlying their tightly controlled gene expression and antigenic switching.
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Affiliation(s)
- Ron Dzikowski
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021, USA
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155
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Abstract
Immune evasion by the malaria parasite Plasmodium falciparum is mediated by the mutually exclusive expression of a single member of the var family of genes, which encode variant surface antigens. In a recent paper in Nature, demonstrate that a construct carrying a transcriptionally active var promoter is sufficient to promote allelic exclusion of the parasite's endogenous var gene.
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Affiliation(s)
- Artur Scherf
- Unité de Biologie des Interactions Hôte-Parasite CNRS-URA 2581, Institut Pasteur, Paris, France.
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156
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Dzikowski R, Frank M, Deitsch K. Mutually exclusive expression of virulence genes by malaria parasites is regulated independently of antigen production. PLoS Pathog 2006; 2:e22. [PMID: 16518466 PMCID: PMC1386720 DOI: 10.1371/journal.ppat.0020022] [Citation(s) in RCA: 160] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2005] [Accepted: 01/31/2006] [Indexed: 12/04/2022] Open
Abstract
The primary virulence determinant of Plasmodium falciparum malaria parasite–infected cells is a family of heterogeneous surface receptors collectively referred to as PfEMP1. These proteins are encoded by a large, polymorphic gene family called var. The family contains approximately 60 individual genes, which are subject to strict, mutually exclusive expression, with the single expressed var gene determining the antigenic, cytoadherent, and virulence phenotype of the infected cell. The mutually exclusive expression pattern of var genes is imperative for the parasite's ability to evade the host's immune response and is similar to the process of “allelic exclusion” described for mammalian Ig and odorant receptor genes. In mammalian systems, mutually exclusive expression is ensured by negative feedback inhibition mediated by production of a functional protein. To investigate how expression of the var gene family is regulated, we have created transgenic lines of parasites in which expression of individual var loci can be manipulated. Here we show that no such negative feedback system exists in P. falciparum and that this process is dependent solely on the transcriptional regulatory elements immediately adjacent to each gene. Transgenic parasites that are selected to express a var gene in which the PfEMP1 coding region has been replaced by a drug-selectable marker silence all other var genes in the genome, thus effectively knocking out all PfEMP1 expression and indicating that the modified gene is still recognized as a member of the var gene family. Mutually exclusive expression in P. falciparum is therefore regulated exclusively at the level of transcription, and a functional PfEMP1 protein is not necessary for viability or for proper gene regulation in cultured parasites. Mutually exclusive gene expression refers to the ability of an organism to select one member of a large, multicopy gene family for expression while simultaneously silencing all other members of the family. Human malaria parasites utilize this process in regulating the expression of the major antigenic and virulence-determining proteins encoded by a multicopy gene family called var. In any given parasite, only a single var gene is expressed at a time, while all other members of the family are transcriptionally silenced. The mechanism that regulates this tightly controlled process and coordinates switches in gene expression is largely unknown. Here Dzikowski and colleagues show that this process is regulated entirely at the level of transcription, and that protein production and chromosomal context of the genes are not involved. In addition, they identify the DNA elements required for a var gene promoter to be recognized and co-regulated along with the rest of the family. This knowledge has enabled the authors to create transgenic parasites in which they can manipulate expression of the entire var gene family through selection for expression of specific, modified var genes, thus knocking out expression of the main virulence factor of malaria.
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Affiliation(s)
- Ron Dzikowski
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, New York, United States of America
| | - Matthias Frank
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, New York, United States of America
- Division of International Health and Infectious Diseases, Weill Medical College of Cornell University, New York, New York, United States of America
| | - Kirk Deitsch
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, New York, United States of America
- * To whom correspondence should be addressed. E-mail:
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157
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Frank M, Dzikowski R, Costantini D, Amulic B, Berdougo E, Deitsch K. Strict pairing of var promoters and introns is required for var gene silencing in the malaria parasite Plasmodium falciparum. J Biol Chem 2006; 281:9942-52. [PMID: 16455655 PMCID: PMC3941977 DOI: 10.1074/jbc.m513067200] [Citation(s) in RCA: 83] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The human malaria parasite, Plasmodium falciparum, maintains a persistent infection altering the proteins expressed on the surface of the infected red blood cells, thus avoiding the host immune response. The primary surface antigen, a protein called PfEMP1, is encoded by a multicopy gene family called var. Each individual parasite only expresses a single var gene at a time, maintaining all other members of the family in a transcriptionally silent state. Previous work using reporter genes in transiently transfected plasmid constructs implicated a conserved intron found in all var genes in the silencing process. Here we have utilized episomal recombination within stably transformed parasites to generate different var promoter and intron arrangements and show that loss of the intron results in var promoter activation. Further, in multicopy plasmid concatamers, each intron could only silence a single promoter, suggesting a one-to-one pairing requirement for silencing. Transcriptionally active, "unpaired" promoters remained active after integration into a chromosome; however, they were not recognized by the pathway that maintains mutually exclusive var gene expression. The data indicate that intron/promoter pairing is responsible for silencing each individual var gene and that disruption of silencing of one gene does not affect the transcriptional activity of neighboring var promoters. This suggests that silencing is regulated at the level of individual genes rather than by assembly of silent chromatin throughout a chromosomal region, thus providing a possible explanation of how a var gene can be maintained in a silent state while the immediately adjacent var gene is transcriptionally active.
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Affiliation(s)
- Matthias Frank
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
- Division of International Health and Infectious Diseases, Weill Medical College of Cornell University, New York, NY 10021
| | - Ron Dzikowski
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
| | - Daniel Costantini
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
| | - Borko Amulic
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
| | - Eli Berdougo
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
| | - Kirk Deitsch
- Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10021
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158
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Viebig NK, Nunes MC, Scherf A, Gamain B. The human placental derived BeWo cell line: A useful model for selecting Plasmodium falciparum CSA-binding parasites. Exp Parasitol 2006; 112:121-5. [PMID: 16274691 DOI: 10.1016/j.exppara.2005.09.007] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2005] [Revised: 09/19/2005] [Accepted: 09/20/2005] [Indexed: 10/25/2022]
Abstract
Chondroitin sulfate A (CSA) present in the placental intervillous blood spaces has been described as the main receptor involved in the massive sequestration of Plasmodium falciparum parasitized erythrocytes to the placenta. Placental parasite isolates are functionally distinct from isolates that sequester in other organs, because they do not cytoadhere to CD36 but instead bind to CSA. To investigate for the parasites molecules associated with the CSA adhesion phenotype, different methodologies have been developed to select for CSA-binding lines in vitro mainly using non-placental sources of CSA that differ in their sulfation pattern. In this study, we show that the human trophoblastic BeWo cell line is a very efficient alternative to select for the CSA-binding phenotype in parasitized erythrocytes.
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Affiliation(s)
- Nicola K Viebig
- Unité de Biologie des Interactions Hôte-Parasite, CNRS URA 2581, Institut Pasteur, 25 rue du Dr Roux, F-75724 Paris Cedex 15, France
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159
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Abstract
Detailed analyses of the 5500 genes revealed by the complete Plasmodium genome sequence are yielding new candidate parasite antigens and strategies that may contribute to a successful vaccine against malaria in the coming decade.
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Affiliation(s)
- Andrew Waters
- Department of Parasitology, Leiden University Medical Centre, 2333ZA Leiden, The Netherlands.
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160
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Ralph SA, Bischoff E, Mattei D, Sismeiro O, Dillies MA, Guigon G, Coppee JY, David PH, Scherf A. Transcriptome analysis of antigenic variation in Plasmodium falciparum--var silencing is not dependent on antisense RNA. Genome Biol 2005; 6:R93. [PMID: 16277748 PMCID: PMC1297649 DOI: 10.1186/gb-2005-6-11-r93] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2005] [Revised: 07/12/2005] [Accepted: 09/21/2005] [Indexed: 11/10/2022] Open
Abstract
A microarray analysis of Plasmodium falciparum selected to express different var genes suggests that antisense transcripts are not responsible for the transcriptional silencing of non-expressed var genes. Background Plasmodium falciparum, the causative agent of the most severe form of malaria, undergoes antigenic variation through successive presentation of a family of antigens on the surface of parasitized erythrocytes. These antigens, known as Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins, are subject to a mutually exclusive expression system, and are encoded by the multigene var family. The mechanism whereby inactive var genes are silenced is poorly understood. To investigate transcriptional features of this mechanism, we conducted a microarray analysis of parasites that were selected to express different var genes by adhesion to chondroitin sulfate A (CSA) or CD36. Results In addition to oligonucleotides for all predicted protein-coding genes, oligonucleotide probes specific to each known var gene of the FCR3 background were designed and added to the microarray, as well as tiled sense and antisense probes for a subset of var genes. In parasites selected for adhesion to CSA, one full-length var gene (var2csa) was strongly upregulated, as were sense RNA molecules emanating from the 3' end of a limited subset of other var genes. No global relationship between sense and antisense production of var genes was observed, but notably, some var genes had coincident high levels of both antisense and sense transcript. Conclusion Mutually exclusive expression of PfEMP1 proteins results from transcriptional silencing of non-expressed var genes. The distribution of steady-state sense and antisense RNA at var loci are not consistent with a silencing mechanism based on antisense silencing of inactive var genes. Silencing of var loci is also associated with altered regulation of genes distal to var loci.
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Affiliation(s)
- Stuart A Ralph
- Institut Pasteur, Unit of Biology of Host-Parasite Interactions, Centre National de la Recherche Scientifique, Unité de Recherche Associée 2581, 25 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
- The Walter and Eliza Hall Institute of Medical Research, 1G Royal Parade, Parkville, Melbourne 3050, Victoria, Australia
| | - Emmanuel Bischoff
- Institut Pasteur, Plate-Forme 2 - Puces à ADN, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Denise Mattei
- Institut Pasteur, Unit of Biology of Host-Parasite Interactions, Centre National de la Recherche Scientifique, Unité de Recherche Associée 2581, 25 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Odile Sismeiro
- Institut Pasteur, Plate-Forme 2 - Puces à ADN, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Marie-Agnès Dillies
- Institut Pasteur, Plate-Forme 2 - Puces à ADN, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Ghislaine Guigon
- Institut Pasteur, Plate-Forme 2 - Puces à ADN, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
- Institut Pasteur, Plate-Forme 8 - CNR/Santé Publique, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Jean-Yves Coppee
- Institut Pasteur, Plate-Forme 2 - Puces à ADN, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Peter H David
- Institut Pasteur, Unité d'Immunologie Moléculaire des Parasites, 28 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
| | - Artur Scherf
- Institut Pasteur, Unit of Biology of Host-Parasite Interactions, Centre National de la Recherche Scientifique, Unité de Recherche Associée 2581, 25 Rue du Docteur Roux, F-75724 Paris Cedex 15, France
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