Anchoring of an immunogenic Plasmodium falciparum circumsporozoite protein on the surface of Dictyostelium discoideum

Comparative genomics of two protozoans Dictyostelium discoideum and Plasmodium falciparum reveals conserved as well as distinct regulatory pathways crucial for exploring novel therapeutic targets for Malaria

Plasmodium falciparum, which causes life-threatening cerebral malaria has rapidly gained resistance against most frontline anti-malarial drugs, thereby generating an urgent need to develop novel therapeutic approaches. Conducting in-depth investigations on Plasmodium in its native form is challenging, thereby necessitating the requirement of an efficient model system. In line, mounting evidence suggests that Dictyostelium discoideum retains both conformational and functional properties of Plasmodium proteins, however, the true potential of Dictyostelium as a host system is not fully explored. In the present study, we have exploited comparative genomics as a tool to extract, compare, and curate the extensive data available on the organism-specific databases to evaluate if D. discoideum can be established as a prime model system for functional characterization of P. falciparum genes. Through comprehensive in silico analysis, we report that despite the presence of adaptation-specific genes, the two display noteworthy conservation in the housekeeping genes, signaling pathway components, transcription regulators, and post-translational modulators. Furthermore, through orthologue analysis, the known, potential, and novel drug target genes of P. falciparum were found to be significantly conserved in D. discoideum. Our findings advocate that D. discoideum can be employed to express and functionally characterize difficult-to-express P. falciparum genes.

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Production of recombinant proteins from protozoan parasites

Although the past decade has witnessed sequencing from an increasing number of parasites, modern high-throughput DNA sequencing technologies have the potential to generate complete genome sequences at even higher rates. Along with the discovery of genes that might constitute potential targets for chemotherapy or vaccination, the need for novel protein expression platforms has become a pressing matter. In addition to reviewing the advantages and limitations of the currently available and emerging expression systems, we discuss novel approaches that could overcome current limitations, including the 'pseudoparasite' concept, an expression platform in which the choice of the surrogate organism is based on its phylogenetic affinity to the target parasite, while taking advantage of the whole engineered organism as a vaccination adjuvant.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

Efficient expression and primary purification of 6-his tagged human Fas ligand in Dictyostelium discoideum

Human Fas ligand (hFasL) is a member of the tumor necrosis factor (TNF) family with many medical interests. To produce this protein efficiently, an improved vector which could express the recombinant hFasL protein with a 6-his tag at its C-terminal was constructed. The new vector was transformed into Dictyostelium discoideum AX3 which then produced 157 microg hFasL l(-1). Using one-step Ni-affinity chromatography, it was purified with a recovery of 92% and purity of 91%.

Production of the soluble human Fas ligand by Dictyostelium discoideum cultivated on a synthetic medium

Human Fas ligand (hFasL) is of considerable interest since it is a type II transmembrane glycoprotein that induces programmed cell death, or apoptosis. In this study Dictyostelium discoideum was used to produce a soluble form of the human Fas ligand. The recombinant cells were adapted to a modified synthetic FM medium, called SIH medium. Cells adapted to the SIH medium reached about 2 times higher cell densities and hFasL concentrations on this medium compared with cells growing on the standard complex medium HL-5C. Even higher values were achieved by a dissolved oxygen-controlled fed-batch cultivation in a conventional stirred bioreactor on SIH medium. Cell densities of up to 5.5 x 10(7) ml(-1) and a maximum hFasL concentration of 148 microgl(-1) were obtained. These results were further improved by means of continuous cultivation of D. discoideum in a bioreactor equipped with cell retention by microfiltration. At low space velocity very high cell densities of up to 2.4 x 10(8) ml(-1) and hFasL concentrations of up to 205 microgl(-1) were achieved.

The circumsporozoite protein of Plasmodium falciparum is expressed and localized to the cell surface in the free-living ciliate Tetrahymena thermophila

Heterologous expression is an important tool for characterization of protein function, structural studies, and production of antigen. While many different host systems have been utilized for the expression of Plasmodium falciparum proteins, the extreme AT-richness of its genome represents an obstacle to efficient expression. In addition, primary sequence motifs such as glycosyl phosphatidyl-inositol (GPI) cleavage/attachment sites of P. falciparum are not recognized in currently used expression hosts. Recently, DNA-mediated transformation has been used for expression of heterologous genes in the ciliated protozoan Tetrahymena thermophila. We report the stable expression of full-length P. falciparum circumsporozoite (CS) protein in T. thermophila. The expressed gene utilized the native CS protein N-terminal secretory signal sequence and the C-terminal GPI anchoring signal. Immunofluorescence imaging demonstrated that the CS protein was localized to the cell surface of Tetrahymena. Metabolic labeling with tritiated myristate resulted in incorporation of label into the recombinant CS protein, indicating that the protein was bound to the cell surface via a GPI anchor. This is the first report of the recognition of targeting and GPI anchoring signals of the P. falciparum CS protein in a heterologous expression host.

Plasmodium falciparum: glycosylation status of Plasmodium falciparum circumsporozoite protein expressed in the baculovirus system

We expressed the main surface antigen of Plasmodium falciparum sporozoites, the circumsporozoite protein (CSP), in High Five (Trichoplusia ni) insect cells using the baculovirus system. Significant amounts of the recombinant protein could be obtained, as judged by SDS-PAGE, Western blot, and immunofluorescence analysis. The cellular localization for recombinant CSP was determined by immunofluorescence. The high fluorescence signal of the permeabilized cells, relative to that of fixed nonpermeabilized cells, revealed a clear intracellular localization of this surface antigen. Analysis of possible posttranslational modifications of CSP showed that this recombinant protein is only N-glycosylated in the baculovirus system. Although DNA-sequence analysis revealed a GPI-cleavage/attachment site, no GPI anchor could be demonstrated. These analyses show that the glycosylation status of this recombinant protein may not reflect its native form in P. falciparum. The impact of these findings on vaccine development will be discussed.

Expression and one-step purification of Plasmodium proteins in dictyostelium

Nearly full-length Circumsporozoite protein (CSP) from Plasmodium falciparum, the C-terminal fragments from both P. falciparm and P. yoelii CSP and a fragment comprising 351 amino acids of P.vivax MSPI were expressed in the slime mold Dictyostelium discoideum. Discoidin-tag expression vectors allowed both high yields of these proteins and their purification by a nearly single-step procedure. We exploited the galactose binding activity of Discoidin Ia to separate the fusion proteins by affinity chromatography on Sepharose-4B columns. Inclusion of a thrombin recognition site allowed cleavage of the Discoidin-tag from the fusion protein. Partial secretion of the protein was obtained via an ER independent pathway, whereas routing the recombinant proteins to the ER resulted in glycosylation and retention. Yields of proteins ranged from 0.08 to 3 mg l(-1) depending on the protein sequence and the purification conditions. The recognition of purified MSPI by sera from P. vivax malaria patients was used to confirm the native conformation of the protein expressed in Dictyostelium. The simple purification procedure described here, based on Sepharose-4B, should facilitate the expression and the large-scale purification of various Plasmodium polypeptides.

Glycosyl-phosphatidylinositol-anchor addition signals are processed in Nicotiana tabacum

Recent studies have demonstrated the existence of glycosyl-phosphatidylinositol (GPI)-anchored proteins in higher plants. In this study we tested whether GPI-addition signals from diverse evolutionary sources would function to link a GPI-anchor to a reporter protein in plant cells. Tobacco protoplasts were transiently transfected with a truncated form of the Clostridium thermocellum endoglucanase E reporter gene (celE') fused with a tobacco secretion signal (PR-1a) at the N-terminus and either a yeast (GAS1), mammalian (Thy-1) or putative plant (LeAGP-1) GPI-anchor addition signal at the C-terminus. The yeast and plant C-terminal signals were found to be capable of directing the addition of a GPI-anchor to the endoglucanase protein (EGE') as shown by the sensitivity of the lipid component of GPI to phosphatidylinositol-specific phospholipase C (PI-PLC) digestion. In contrast, the mammalian signal was poorly processed for anchor addition. When EGE' was fused to a truncated form of the LeAGP-1 signal (missing three amino acids predicted to be critical to signal cleavage and anchor addition), a GPI-anchor was not linked to the EGE' protein indicating the necessity for the missing amino acids. Our results show the conservation of the properties of GPI-signals in plant cells and that there may be some similar preferences in GPI-addition signal sequences for yeast and plant cells.

Processing and localisation of a GPI-anchored Plasmodium falciparum surface protein expressed by the baculovirus system

We describe the expression, in insect cells using the baculovirus system, of two protein fragments derived from the C-terminus of merozoite surface protein 1(MSP-1) of the human malaria parasite Plasmodium falciparum, and their glycosylation and intracellular location. The transport and intracellular localisation of the intact C-terminal MSP-1 fragment, modified by addition of a signal sequence for secretion, was compared with that of a similar control protein in which translation of the GPI-cleavage/attachment site was abolished by insertion of a stop codon into the DNA sequence. Both proteins could only be detected intracellularly, most likely in the endoplasmic reticulum. This lack of transport to the cell surface or beyond, was confirmed for both proteins by immunofluorescence with a specific antibody and characterisation of their N-glycans. The N-glycans had not been processed by enzymes localised in post-endoplasmic reticulum compartments. In contrast to MSP-1, the surface antigen SAG-1 of Toxoplasma gondii was efficiently transported out of the endoplasmic reticulum of insect cells and was located, at least in part, on the cell surface. No GPI-anchor could be detected for either of the MSP-1 constructs or SAG-1, showing that the difference in transport is a property of the individual proteins and cannot be attributed to the lack of a GPI-anchor. The different intracellular location and post-translational modification of recombinant proteins expressed in insect cells, as compared to the native proteins expressed in parasites, and the possible implications for vaccine development are discussed.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

From virus to vaccine: developments using the simple eukaryote, Dictyostelium discoideum

Mass vaccination compaigns against viral diseases, both human and anim al, depend on the availability of cheap viral antigens. The eukaryote Dictyostelium discoideum has simple growth requirements and rapid growth rates and forms stable cell lines. These features, together with the possibility of secreting recombinant (glyco)proteins into a defined buffer, make the D. discoideum expression system an attractive option for producing economical recombinant subunit vaccines.

Recombinant glycoprotein production in the slime mould Dictyostelium discoideum

Dictyostelium discoideum is a well known amoeboid organism, with unicellular and multicellular life-cycle stages, that is used for studying cell and developmental biology. With advances in gene-disruption technology and transformation of this organism, many homologous proteins have been expressed either to complement defective proteins or to study basic cell biology. Now, D. discoideum is being used to express heterologous proteins that are difficult to study in other systems, and its unique cell biology is being exploited to facilitate a wide range of protein modifications. In the past year, substantial progress has been made in expressing correctly folded forms of malarial circumsporozoite antigen and rotavirus surface glycoprotein VP7. Exciting developments have also been made in expressing human muscarinic receptors.