Optimized expression of Plasmodium falciparum erythrocyte membrane protein 1 domains in Escherichia coli

Transforming parasites into their own foes: parasitic extracellular vesicles as a vaccine platform

Parasitic diseases continue to afflict millions of people globally. However, traditional vaccine development strategies are often difficult to apply to parasites, leaving an immense unmet need for new effective vaccines for the prevention and control of parasitic infections. As parasites commonly use extracellular vesicles (EVs) to interact with, interfere with, or modulate the host immune response from a distance, parasite-derived EVs may provide promising vaccine agents that induce immunity against parasitic infections. We here present achievements to date and the challenges and limitations associated with using parasitic EVs in a clinical context. Despite the many difficulties that need to be overcome, we believe this direction could offer a new and reliable source of therapeutics for various neglected parasitic diseases.

Optimization of the heterologous expression and purification of Plasmodium falciparum generative cell specific 1 in Escherichia coli

Generative cell specific 1 (GCS1) or Hapless2 (Hap2) is a main transmission-blocking vaccine (TBV) candidate against malaria. Experience has shown that this protein is difficult to express in heterologous hosts. In a study, Plasmodium falciparum GCS1 (PfGCS1) could be expressed in fusion with Glutathione S Transferase (GST). Since the large fusions could influence the immunogenicity of the recombinant antigens, in the current study we tried to express PfGCS1 protein without large fusion tags with an appropriate yield and purity in E. coli. To this end, pfgcs1 gene was codon-optimized and cloned in pET23a plasmid. The expression was evaluated in different E. coli hosts [E. coli BL21(DE3), E. coli BL21(DE3) pLysS, E. coli Rosetta(DE3), and E. coli Rosettagami(DE3)] and media cultures. In addition, the effect of post-induction times, inducer concentration, temperature, and supplementation of glucose and ethanol to culture media were evaluated. The obtained results revealed that rPfGCS1 protein was expressed in all examined E. coli hosts and media cultures with different yields, with the best yield in E. coli BL21(DE3), and E. coli Rosetta(DE3) hosts in TB medium, 16 h post-induction. The expression of rPfGCS1 was confirmed by western blotting using anti-His antibodies. Expression in low temperature at 20 °C and addition of glucose and ethanol to TB media could improve the expression of rPfGCS1. We could express and purify rPfGCS1 without a large fusion protein with an appropriate yield and purity in E. coli Rosetta(DE3). We will evaluate this antigen as TBV candidate against P. falciparum transmission in the future.

A Novel Thioredoxin-Like Protein of Babesia microti Involved in Parasite Pathogenicity

Babesiosis poses a serious threat to immunocompromised individuals and the major etiological species of Babesia for human babesiosis is Babesia microti. Merozoites are a critical stage in the life cycle of Babesia microti. Several merozoite proteins have been demonstrated to play important roles in this process; however, most of the merozoite proteins of B. microti remain unknown. In the present study, we identified a novel merozoite protein of B. microti with similar structure to the thioredoxin (Trx)-like domain of the Trx family, which was named as B. microti Trx-like protein (BmTLP). Western blot assays demonstrated that this protein was expressed by B. microti during the erythrocytic infection process, and its expression peaked on day 7 post-infection in vivo. Immunofluorescence assay further showed that this protein is mainly expressed in B. microti merozoites. BmTLP hold both heparin- and erythrocyte-binding properties, which are critical functions of invasion-related proteins. Immunization with recombinant BmTLP imparted significant protection against B. microti infection in mice. Taken together, these results suggest that the novel merozoite protein, BmTLP, is an important pathogenic molecule of B. microti and may be a possible target for the design of babesiosis control strategy.

How can we develop an effective subunit vaccine to achieve successful malaria eradication?

Malaria, a mosquito-borne infection, is the most widespread parasitic disease. Despite numerous efforts to eradicate malaria, this disease is still a health concern worldwide. Owing to insecticide-resistant vectors and drug-resistant parasites, available controlling measures are insufficient to achieve a malaria-free world. Thus, there is an urgent need for new intervention tools such as efficient malaria vaccines. Subunit vaccines are the most promising malaria vaccines under development. However, one of the major drawbacks of subunit vaccines is the lack of efficient and durable immune responses including antigen-specific antibody, CD4⁺, and CD8⁺ T-cell responses, long-lived plasma cells, memory cells, and functional antibodies for parasite neutralization or inhibition of parasite invasion. These types of responses could be induced by whole organism vaccines, but eliciting these responses with subunit vaccines has been proven to be more challenging. Consequently, subunit vaccines require several policies to overcome these challenges. In this review, we address common approaches that can improve the efficacy of subunit vaccines against malaria.

Development of a process for large scale production of PfRH5 in E. coli expression system

The Plasmodium falciparum reticulocyte binding protein homologue 5 (PfRH5) has recently shown great promise to be developed as a vaccine candidate to prevent blood-stage malaria. However, because of its molecular complexity, most previous efforts were focused on expressing PfRH5 in its native and soluble form. Here, we describe the E. coli expression of full-length PfRH5 as inclusion bodies (IBs), followed by its high cell density fermentation at 1, 5 and 30 L scale. Denatured full-length PfRH5 was purified using a two-step chromatography process before being refolded using design of experiments (DoE). Refolded PfRH5 was further purified using size exclusion chromatography (SEC), recovering high purity antigen with an overall yield of 102 mg/L from fermentation cell harvest. Purified PfRH5 was further characterized using orthogonal analytical methods, and a short-term stability study revealed -80 °C as an optimum storage temperature. Moreover, refolded, and purified PfRH5, when formulated with adjuvant Glucopyranosyl A lipid stable emulsion (GLA-SE), elicited high antibody titers in BALB/c mice, proving its potential to neutralize the blood-stage malarial parasite. Here, we establish an E. coli-based process platform for the large-scale cGMP production of full-length PfRH5, enabling global malaria vaccine development efforts.

An open-label phase 1/2a trial of a genetically modified rodent malaria parasite for immunization against Plasmodium falciparum malaria

For some diseases, successful vaccines have been developed using a nonpathogenic counterpart of the causative microorganism of choice. The nonpathogenicity of the rodent Plasmodium berghei (Pb) parasite in humans prompted us to evaluate its potential as a platform for vaccination against human infection by Plasmodium falciparum (Pf), a causative agent of malaria. We hypothesized that the genetic insertion of a leading protein target for clinical development of a malaria vaccine, Pf circumsporozoite protein (CSP), in its natural pre-erythrocytic environment, would enhance Pb's capacity to induce protective immunity against Pf infection. Hence, we recently generated a transgenic Pb sporozoite immunization platform expressing PfCSP (PbVac), and we now report the clinical evaluation of its biological activity against controlled human malaria infection (CHMI). This first-in-human trial shows that PbVac is safe and well tolerated, when administered by a total of ~300 PbVac-infected mosquitoes per volunteer. Although protective efficacy evaluated by CHMI showed no sterile protection at the tested dose, significant delays in patency (2.2 days, P = 0.03) and decreased parasite density were observed after immunization, corresponding to an estimated 95% reduction in Pf liver parasite burden (confidence interval, 56 to 99%; P = 0.010). PbVac elicits dose-dependent cross-species cellular immune responses and functional PfCSP-dependent antibody responses that efficiently block Pf sporozoite invasion of liver cells in vitro. This study demonstrates that PbVac immunization elicits a marked biological effect, inhibiting a subsequent infection by the human Pf parasite, and establishes the clinical validation of a new paradigm in malaria vaccination.

Recombinant DBL2β-PfEMP1 of the Indonesian Plasmodium falciparum induces immune responses in Wistar rats

OBJECTIVES

The Duffy binding-like (DBL) domain of the Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) is reportedly responsible for the pathophysiology of cerebral malaria. People living in endemic malaria areas possess specific antibodies against PfEMP1 and elicit immune responses to control the severity of malaria infection. Therefore, PfEMP1 may be a potential protein-based vaccine candidate. This study aimed to explore the humoral and cellular immune responses induced by the recombinant DBL2β-PfEMP1 obtained from the Indonesian P. falciparum isolate.

METHODS

The recombinant protein was expressed in Escherichia coli BL21(DE3) as soluble and insoluble fractions, and this protein was purified using affinity chromatography before administration as a subcutaneous injection in Wistar rats on days 1, 21, and 42. Sera were harvested 14 days after the second and third injections to determine the titre of IgG and the concentration of CD4+ cells using the enzyme linked immunosorbent assay (ELISA).

RESULTS

The IgG titre and the CD4+ cell concentration were found to be increased after the second and third injections. The Mann-Whitney test results showed a significant difference between the control and treatment groups for both the IgG and CD4+ cells (p = 0.001 and p = 0.000, respectively). Western blotting results indicated the presence of a specific antibody against the recombinant DBL2β-PfEMP1.

CONCLUSIONS

The recombinant DBL2β-PfEMP1 of the Indonesian P. falciparum isolate could induce humoral and cellular immune responses. Further studies on IgG exerting inhibitory effects and the role of CD4+ cells and their association with other effector cells are essential to determine the efficacy of DBL2β-PfEMP1 and its potential application as a peptide-based malaria vaccine candidate.

A Sialic Acid-Binding Protein SABP1 of Toxoplasma gondii Mediates Host Cell Attachment and Invasion

Many obligate intracellular apicomplexan parasites have adapted a distinct invasion mechanism involving a close interaction between the parasite ligands and the sialic acid (SA) receptor. We found that sialic acid binding protein-1 (SABP1), localized on the outer membrane of the zoonotic parasite Toxoplasma gondii, readily binds to sialic acid on the host cell surface. The binding was sensitive to neuraminidase treatment. Cells preincubated with recombinant SABP1 protein resisted parasite invasion in vitro. The parasite lost its invasion capacity and animal infectivity after the SABP1 gene was deleted, whereas complementation of the SABP1 gene restored the virulence of the knockout strain. These data establish the critical role of SABP1 in the invasion process of T. gondii. The previously uncharacterized protein, SABP1, facilitated T. gondii attachment and invasion via sialic acid receptors.

Targeted enzyme gene re-positioning: A computational approach for discovering alternative bacterial enzymes for the synthesis of plant-specific secondary metabolites

Plant-biosynthesised secondary metabolites are unique sources of pharmaceuticals, food additives, and flavourings, among other industrial uses. However, industrial production of these metabolites is difficult because of their structural complexity, dangerousness and unfriendliness to natural environment, so the development of new methods to synthesise them is required. In this study, we developed a novel approach to identifying alternative bacterial enzyme to produce plant-biosynthesised secondary metabolites. Based on the similarity of enzymatic reactions, we searched for candidate bacterial genes encoding enzymes that could potentially replace the enzymes in plant-specific secondary metabolism reactions that are contained in the KEGG database (enzyme re-positioning). As a result, we discovered candidate bacterial alternative enzyme genes for 447 plant-specific secondary metabolic reaction. To validate our approach, we focused on the ability of an enzyme from Streptomyces coelicolor strain A3(2) strain to convert valencene to the grapefruit metabolite nootkatone, and confirmed its enzymatic activity by gas chromatography-mass spectrometry. This enzyme re-positioning approach may offer an entirely new way of screening enzymes that cannot be achieved by most of other conventional methods, and it is applicable to various other metabolites and may enable microbial production of compounds that are currently difficult to produce industrially.

Systematic Comparison of Strategies to Achieve Soluble Expression of Plasmodium falciparum Recombinant Proteins in E. coli

Constructs containing partial coding sequences of myosin A, myosin B, and glideosome-associated protein (50 kDa) of Plasmodium falciparum were used to challenge several strategies designed in order to improve the production and solubility of recombinant proteins in Escherichia coli. Assays were carried out inducing expression in a late log phase culture, optimizing the inductor concentration, reducing the growth temperature for induced cultures, and supplementing additives in the lysis buffer. In addition, recombinant proteins were expressed as fusion proteins with three different tags (6His, GST, and MBP) in four different E. coli strains. We found that the only condition that consistently produced soluble proteins was the use of MBP as a fusion tag, which became a valuable tool for detecting the proteins used in this study and did not caused any interference in protein-protein interaction assays (Far Western Blot). Besides, we found that BL21-pG-KJE8 strain did not improve the solubility of any of the recombinant protein produced, while the BL21-CodonPlus(DE3)-RIL strain improved the expression of some of them independent of the rare codon content. Proteins with rare codons occurring at high frequencies (» 10%) were expressed efficiently in strains that do not supplement tRNAs for these triplets.

A Thioredoxin Homologous Protein of Plasmodium falciparum Participates in Erythrocyte Invasion

Invasion of erythrocytes by merozoites is required in the life cycle of malarial parasites. Proteins derived from the invasive merozoites are essential ligands for erythrocyte recognition and penetration.

Invasion of erythrocytes by merozoites is required in the life cycle of malarial parasites. Proteins derived from the invasive merozoites are essential ligands for erythrocyte recognition and penetration. In this study, we report a novel protein that possesses a Trx domain-like structure of the thioredoxin family and is expressed on the surface of merozoites of the malaria parasite Plasmodium falciparum. This protein, namely, PfTrx-mero protein, displayed a mutated sequence character at the Trx domain, but with a specific binding activity to human erythrocytes. Specific antibodies to the protein inhibited merozoite invasion into human erythrocytes. Immunization with a homologous protein of Plasmodium berghei strain ANKA also showed significant protection against lethal infection in mice. These results suggested that the novel PfTrx-like-mero protein expressed on the surface of merozoites is an important ligand participating in erythrocyte invasion and a potential vaccine candidate.

Characterization of the Catalytic Subunits of the RNA Exosome-like Complex in Plasmodium falciparum

The eukaryotic ribonucleic acid (RNA) exosome is a versatile multiribonuclease complex that mediates the processing, surveillance, and degradation of virtually all classes of RNA in both the nucleus and cytoplasm. The complex, composed of 10 to 11 subunits, has been widely described in many organisms. Bioinformatic analyses revealed that there may be also an exosome‐like complex in Plasmodium falciparum, a parasite of great importance in public health, with eight predicted subunits having high sequence similarity to their counterparts in yeast and human. In this work, the putative RNA catalytic components, designated as PfRrp4, PfRrp41, PfDis3, and PfRrp6, were identified and systematically analyzed. Quantitative polymerase chain reaction (QPCR) analyses suggested that all of them were transcribed steadily throughout the asexual stage. The expression of these proteins was determined by Western blot, and their localization narrowed to the cytoplasm of the parasite by indirect immunofluorescence. The recombinant proteins of PfRrp41, PfDis3, and PfRrp6 exhibited catalytic activity for single‐stranded RNA (ssRNA), whereas PfRrp4 showed no processing activity of both ssRNA and dsRNA. The identification of these putative components of the RNA exosome complex opens up new perspectives for a deep understanding of RNA metabolism in the malarial parasite P. falciparum.

Heterologous expression of Plasmodium vivax apical membrane antigen 1 (PvAMA1) for binding peptide selection

Background

Plasmodium is an obligate intracellular parasite. Apical membrane antigen 1 (AMA1) is the most prominent and well characterized malarial surface antigen that is essential for parasite-host cell invasion, i.e., for sporozoite to invade and replicate within hepatocytes in the liver stage and merozoite to penetrate and replicate within erythrocytes in the blood stage. AMA1 has long served as a potent antimalarial drug target and is a pivotal vaccine candidate. A good understanding of the structure and molecular function of this Plasmodium protein, particularly its involvement in host-cell adhesion and invasion, is of great interest and hence it offers an attractive target for the development of novel therapeutics. The present study aims to heterologous express recombinant Plasmodium AMA1 ectodomain of P. vivax (rPvAMA1) for the selection of binding peptides.

Methods

The rPvAMA1 protein was heterologous expressed using a tag-free Profinity eXact^TM system and codon optimized BL21-Codon Plus (DE3)-RIL Escherichia coli strain and further refolded by dialysis for renaturation. Binding peptides toward refolded rPvAMA1 were panned using a Ph.D.-12 random phage display library.

Results

The rPvAMA1 was successfully expressed and refolded with three phage-displayed dodecapeptides designated as PdV1 (DLTFTVNPLSKA), PdV2 (WHWSWWNPNQLT), and PdV3 (TSVSYINNRHNL) with affinity towards rPvAMA1 identified. All of them exhibited positive binding signal to rPvAMA1 in both direct phage assays, i.e., phage ELISA binding assay and Western blot binding assay.

Discussion

Phage display technology enables the mapping of protein-protein interactions based on a simple principle that a library of phage particles displaying peptides is used and the phage clones that bind to the target protein are selected and identified. The binding sites of each selected peptides toward PvAMA1 (Protein Data Bank, PDB ID: 1W8K) were in silico predicted using CABS-dock web server. In this case, the binding peptides provide a valuable starting point for the development of peptidomimetic as antimalarial antagonists directed at PvAMA1.

Production of recombinant proteins from Plasmodium falciparum in Escherichia coli

INTRODUCTION

The production of recombinant proteins is essential for the characterization and functional study of proteins from Plasmodium falciparum. However, the proteins of P. falciparum are among the most challenging to express, and when expression is achieved, the recombinant proteins usually fold incorrectly and lead to the formation of inclusion bodies. 

OBJECTIVE

To obtain and purify four recombinant proteins and to use them as antigens to produce polyclonal antibodies. The production efficiency and solubility were evaluated as the proteins were expressed in two genetically modified strains of Escherichia coli to favor the production of heterologous proteins (BL21-CodonPlus (DE3)-RIL and BL21-pG-KJE8). 

MATERIALS AND METHODS

The four recombinant P. falciparum proteins corresponding to partial sequences of PfMyoA (Myosin A) and PfGAP50 (gliding associated protein 50), and the complete sequences of PfMTIP (myosin tail interacting protein) and PfGAP45 (gliding associated protein 45), were produced as glutathione S-transferase-fusion proteins, purified and used for immunizing mice. 

RESULTS

The protein expression was much more efficient in BL21-CodonPlus, the strain that contains tRNAs that are rare in wild-type E. coli, compared to the expression in BL21-pG-KJE8. In spite of the fact that BL21-pG-KJE8 overexpresses chaperones, this strain did not minimize the formation of inclusion bodies. 

CONCLUSION

The use of genetically modified strains of E. coli was essential to achieve high expression levels of the four evaluated P. falciparum proteins and lead to improved solubility of two of them. The approach used here allowed us to obtain and purify four P. falciparum proteins in enough quantity to produce polyclonal antibodies in mice, and a fair amount of two pure and soluble recombinant proteins for future assays.

Production of a recombinant membrane protein in an Escherichia coli strain for the whole cell biosynthesis of phenylacetic acids

The styrene oxide isomerase (SOI) represents a membrane-bound enzyme of the microbial styrene degradation pathway and has been discussed as promising biocatalyst. It catalyzes the isomerization of styrene oxide to phenylacetaldehyde. In this study a styC gene, which encodes the SOI of Rhodococcus opacus 1CP, was optimized for optimal expression in Escherichia coli BL21(DE3) pLysS. The expression of this synthetic styC was investigated and subsequently optimized. Highly active biomass was obtained yielding an SOI activity of 44.5 ± 8.7 U mg⁻¹ after 10 h. This represents the highest SOI activity reported for crude cell extracts of SOI-containing bacterial strains. Remarkably, this biomass can be applied as whole cell biocatalyst for the production of phenylacetic acids from styrene oxides. In the case of non-substituted styrene oxide, nearly 730 mg l⁻¹ phenylacetic acid (∼85% yield) was formed over a period of 20 days.

Cultivation Conditions for Phytase Production from Recombinant Escherichia coli DH5α

Response surface methodology (RSM) was used to optimize the cultivation conditions for the production of phytase by recombinant Escherichia coli DH5α. The optimum predicted cultivation conditions for phytase production were at 3 hours seed age, a 2.5% inoculum level, an L-arabinose concentration of 0.20%, a cell concentration of 0.3 (as measured at 600 nm) and 17 hours post-induction time with a predicted phytase activity of 4194.45 U/mL. The model was validated and the results showed no significant difference between the experimental and the predicted phytase activity (P = 0.305). Under optimum cultivation conditions, the phytase activity of the recombinant E. coli DH5α was 364 times higher compared to the phytase activity of the wild-type producer, Enterobacter sakazakii ASUIA279. Hence, optimization of the cultivation conditions using RSM positively increased phytase production from recombinant E. coli DH5α.

Engineering of an elastic scaffolding polyprotein based on an SH3-binding intrinsically disordered titin PEVK module

Titin is a large elastic protein found in muscle that maintains the elasticity and structural integrity of the sarcomere. The PEVK region of titin is intrinsically disordered, highly elastic and serves as a hub to bind signaling proteins. Systematic investigation of the structure and affinity profile of the PEVK region will provide important information about the functions of titin. Since PEVK is highly heterogeneous due to extensive differential splicing from more than one hundred exons, we engineered and expressed polyproteins that consist of a defined number of identical single exon modules. These customized polyproteins reduce heterogeneity, amplify interactions of less dominant modules, and most importantly, provide tags for atomic force microscopy and allow more readily interpretable data from single-molecule techniques. Expression and purification of recombinant polyprotein with repeat regions presented many technical challenges: recombination events in tandem repeats of identical DNA sequences exacerbated by high GC content, toxicity of polymer plasmid and expressed protein to the bacteria; early truncation of proteins expressed with different numbers of modules; and extreme sensitivity to proteolysis. We have investigated a number of in vitro and in vivo bacterial and yeast expression systems, as well as baculoviral systems as potential solutions to these problems. We successfully expressed and purified in gram quantities a polyprotein derived from human titin exon 172 using Pichia pastoris yeast. This study provides valuable insights into the technical challenges regarding the engineering and purification of a tandem repeat sequence of an intrinsically disordered biopolymer.

Molecular characterization of Plasmodium falciparum putative polynucleotide kinase/phosphatase

Polynucleotide kinase/phosphatase (PNKP) is a bifunctional enzyme that can phosphorylate the 5'-OH termini and dephosphorylate the 3'-phosphate termini of DNA. It is a DNA repair enzyme involved in the processing of strand break termini, which permits subsequent repair proteins to replace missing nucleotides and rejoin broken strands. Little is known about DNA repair in Plasmodium falciparum, including the roles of PNKP in repairing parasite DNA. We identified a P. falciparum gene encoding a protein with 24% homology to human PNKP and thus suggestive of a putative PNKP. In this study, the PNKP gene of P. falciparum strain K1 (PfPNKP) was successfully cloned and expressed in E. coli as a GST-PfPNKP recombinant protein. MALDI-TOF/TOF analysis of the protein confirmed the identity of PfPNKP. Assays for enzymatic activity were carried out with a variety of single- and double-stranded substrates. Although 3'-phosphatase activity was detected, PfPNKP was observed to dephosphorylate single-stranded substrates or double-stranded substrates with a short 3'-single-stranded overhang, but not double-stranded substrates that mimicked single-strand breaks. We hypothesize that unlike human PNKP, PfPNKP may not be involved in single-strand break repair, since alternative terminal processing mechanisms can substitute for PfPNKP, and that PfPNKP DNA repair actions may be confined to overhanging termini of double-strand breaks.

Point mutations (Q19P and N23K) increase the operational solubility of a 2alpha-O-benzoyltransferase that conveys various acyl groups from CoA to a taxane acceptor

Two site-directed mutations within the wild-type 2-O-benzoyltransferase (tbt) cDNA, from Taxus cuspidata plants, yielded an encoded protein containing replacement amino acids at Q19P and N23K that map to a solvent-exposed loop region. The likely significant changes in the biophysical properties invoked by these mutations caused the overexpressed, modified TBT (mTBT) to partition into the soluble enzyme fraction about 5-fold greater than the wild-type enzyme. Sufficient protein could now be acquired to examine the scope of the substrate specificity of mTBT by incubation with 7,13-O,O-diacetyl-2-O-debenzoylbaccatin III that was mixed individually with various substituted benzoyls, alkanoyls, and (E)-butenoyl CoA donors. The mTBT catalyzed the conversion of each 7,13-O,O-diacetyl-2-O-debenzoylbaccatin III to several 7,13-O,O-diacetyl-2-O-acyl-2-O-debenzoylbaccatin III analogues. The relative catalytic efficiency of mTBT with the 7,13-O,O-diacetyl-2-O-debenzoyl surrogate substrate and heterole carbonyl CoA substrates was slightly greater than with the natural aroyl substrate benzoyl CoA, while substituted benzoyl CoA thioesters were less productive. Short-chain hydrocarbon carbonyl and cyclohexanoyl CoA thioesters were also productive, where C(4) substrates were transferred by mTBT with approximately 10- to 17-fold greater catalytic efficiency compared to the transfer of benzoyl. The described broad specificity of mTBT suggests that a plethora of 2-O-acyl variants of the antimitotic paclitaxel can be assembled through biocatalytic sequences.

In vitro selection of RNA aptamers against a conserved region of the Plasmodium falciparum erythrocyte membrane protein 1

The var-gene encoding Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is known to play a major role in the pathogenicity of the P. falciparum parasite. The protein enables the parasite to adhere to the endothelial linings of small blood vessels (cytoadherence) as well as to non-infected erythrocytes (rosetting), thus preventing clearance from the bloodstream. The development and spread of resistance towards most anti-malarial drugs used for treatment and prevention of the most severe form of malaria truly emphasise the importance of a continuous research and development of new drugs. In this study we use Systematic Evolution of Ligands by EXponential enrichment (SELEX) methodology to isolate high-affinity ligands (aptamers). To validate the results from the SELEX in vitro selection, different aptamers have been selected against PfEMP1 in a live cell assay of P. falciparum strain FCR3S1.2, a highly rosetting strain. We have been able to show the rosette disrupting capacity of these SELEX-aptamers at concentrations of 33 nM and with 100% disruption at 387 nM. The described results show that RNA aptamers are promising candidates for adjunct therapy in severe malaria.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

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.

Production and comprehensive quality control of recombinant human Interleukin-1beta: a case study for a process development strategy

We describe an efficient strategy to produce high-quality proteins by using a single large IMAC chromatography column and enzymatic His-tag removal via the TAGZyme system in pilot scale. Numerous quality assays demonstrated a high purity of the final product, the human cytokine Interleukin-1beta (IL-1beta). The protein preparation was apparently free of host cell proteins, endotoxins, protease, and aggregates. The N-terminal amino acid sequence of IL-1beta was in full agreement with the natural mature form of IL-1beta. The homogeneity of the product was further shown by X-ray structure determination which confirmed the previously solved structure of the protein. We propose the applied workflow as a strategy for industrial production of protein-based biopharmaceuticals.

A molecular link between malaria and Epstein-Barr virus reactivation

Although malaria and Epstein–Barr (EBV) infection are recognized cofactors in the genesis of endemic Burkitt lymphoma (BL), their relative contribution is not understood. BL, the most common paediatric cancer in equatorial Africa, is a high-grade B cell lymphoma characterized by c-myc translocation. EBV is a ubiquitous B lymphotropic virus that persists in a latent state after primary infection, and in Africa, most children have sero-converted by 3 y of age. Malaria infection profoundly affects the B cell compartment, inducing polyclonal activation and hyper-gammaglobulinemia. We recently identified the cystein-rich inter-domain region 1α (CIDR1α) of the Plasmodium falciparum membrane protein 1 as a polyclonal B cell activator that preferentially activates the memory compartment, where EBV is known to persist. Here, we have addressed the mechanisms of interaction between CIDR1α and EBV in the context of B cells. We show that CIDR1α binds to the EBV-positive B cell line Akata and increases the number of cells switching to the viral lytic cycle as measured by green fluorescent protein (GFP) expression driven by a lytic promoter. The virus production in CIDR1α-exposed cultures was directly proportional to the number of GFP-positive Akata cells (lytic EBV) and to the increased expression of the EBV lytic promoter BZLF1. Furthermore, CIDR1α stimulated the production of EBV in peripheral blood mononuclear cells derived from healthy donors and children with BL. Our results suggest that P. falciparum antigens such as CIDR1α can directly induce EBV reactivation during malaria infection that may increase the risk of BL development for children living in malaria-endemic areas. To our knowledge, this is the first report to show that a microbial protein can drive a latently infected B cell into EBV replication.

Malaria and Epstein–Barr virus (EBV) infections are recognized cofactors in the genesis of endemic Burkitt lymphoma, the most common paediatric cancer in equatorial Africa. EBV is a ubiquitous virus residing in B lymphocytes that establishes a lifelong persistence in the host after primary infection. EBV has two lifestyles: latent infection (non-productive), and lytic replication (productive). Children living in malaria-endemic areas exhibit an elevated viral load, and acute malaria infection increases the levels of circulating EBV. The mechanisms leading to viral reactivation during Plasmodium falciparum malaria infection are not well understood. Cystein-rich inter-domain region 1α (CIDR1α) is a domain of a large protein expressed at the surface of P. falciparum–infected red blood cells. Based on previous findings showing that CIDR1α activates and expands the B cells compartment where EBV persists, we assessed the impact of CIDR1α on viral reactivation. Here, we identify CIDR1α as the first microbial protein able to drive a latently EBV-infected B cell (no virus production) into lytic replication (virus production). Our results suggest that P. falciparum–derived proteins can lead to a direct reactivation of EBV during acute malaria infection, increasing the risk of Burkitt lymphoma development for children living in malaria-endemic areas.

Expression and biochemical characterization of the Plasmodium falciparum DNA repair enzyme, flap endonuclease-1 (PfFEN-1)

Flap endonuclease-1 (FEN-1) is a structure-specific endonuclease that is critical for the resolution of single-stranded DNA flap intermediates that form during long patch DNA base excision repair (BER). This investigation reports that Plasmodium species encode FEN-1 homologs. Protein sequence analysis revealed the N and I domains of Plasmodium falciparum (PfFEN-1) and Plasmodium yoelii (PyFEN-1) to be homologous to FEN-1 from other species. However, each possessed an extended C domain which had limited homology to apicomplexan FEN-1s and no homology to eukaryotic FEN-1s. A conserved proliferating cell nuclear antigen (PCNA)-binding site was identified at an internal location rather than the extreme C-terminal location typically seen in FEN-1 from other organisms. The endonuclease and exonuclease activities of PfFEN-1 and PyFEN-1 were investigated using recombinant protein produced in Escherichia coli. Pf and PyFEN-1 possessed DNA structure-specific flap endonuclease and 5'-->3' exonuclease activities, similar to FEN-1s from other species. Endonuclease activity was stimulated by Mg(2+) or Mn(2+) and inhibited by monovalent ions (>20.0 mM). A PfFEN-1 C-terminal truncation mutant lacking the terminal 250 amino acids (PfFEN-1DeltaC) had endonuclease activity that was approximately 130-fold greater (k(cat)=1.2x10(-1)) than full-length PfFEN-1 (k(cat)=9.1x10(-4)) or approximately 240-fold greater than PyFEN-1 (k(cat)=4.9x10(-4)) in vitro. PfFEN-1 generated a nicked DNA substrate that was ligated by recombinant Pf DNA Ligase I (PfLigI) using an in vitro DNA repair assay. Plasmodium FEN-1s have enzymatic activities similar to other species but contain extended C-termini and a more internally located PCNA-binding site.

A novel DBL-domain of the P. falciparum 332 molecule possibly involved in erythrocyte adhesion

Plasmodium falciparum malaria is brought about by the asexual stages of the parasite residing in human red blood cells (RBC). Contact between the erythrocyte surface and the merozoite is the first step for successful invasion and proliferation of the parasite. A number of different pathways utilised by the parasite to adhere and invade the host RBC have been characterized, but the complete biology of this process remains elusive. We here report the identification of an open reading frame (ORF) representing a hitherto unknown second exon of the Pf332 gene that encodes a cysteine-rich polypeptide with a high degree of similarity to the Duffy-binding-like (DBL) domain of the erythrocyte-binding-ligand (EBL) family. The sequence of this DBL-domain is conserved and expressed in all parasite clones/strains investigated. In addition, the expression level of Pf332 correlates with proliferation efficiency of the parasites in vitro. Antibodies raised against the DBL-domain are able to reduce the invasion efficiency of different parasite clones/strains. Analysis of the DBL-domain revealed its ability to bind to uninfected human RBC, and moreover demonstrated association with the iRBC surface. Thus, Pf332 is a molecule with a potential role to support merozoite invasion. Due to the high level of conservation in sequence, the novel DBL-domain of Pf332 is of possible importance for development of novel anti-malaria drugs and vaccines.

Production of a recombinant carotenoid cleavage dioxygenase from grape and enzyme assay in water-miscible organic solvents

A recombinant carotenoid cleavage dioxygenase from Vitis vinifera L. was produced by Escherichia coli as a fusion with the glutathione-S-transferase (GST) protein under different bacterial growth conditions. The enzyme production was monitored by a GST assay. Addition of Triton X-100 prior to bacterial cell disruption doubled the release of soluble protein. A simple spectrophotometric enzyme assay was developed to measure carotenoid cleavage activity using lutein as substrate. Enzyme activity showed a 26-fold increase with the addition of 10% (v/v) acetone in the reaction mixture.

Expression of plant flavor genes in Lactococcus lactis

Lactic acid bacteria, such as Lactococcus lactis, are attractive hosts for the production of plant-bioactive compounds because of their food grade status, efficient expression, and metabolic engineering tools. Two genes from strawberry (Fragaria x ananassa), encoding an alcohol acyltransferase (SAAT) and a linalool/nerolidol synthase (FaNES), were cloned in L. lactis and actively expressed using the nisin-induced expression system. The specific activity of SAAT could be improved threefold (up to 564 pmol octyl acetate h-1 mg protein-1) by increasing the concentration of tRNA1Arg, which is a rare tRNA molecule in L. lactis. Fermentation tests with GM17 medium and milk with recombinant L. lactis strains expressing SAAT or FaNES resulted in the production of octyl acetate (1.9 microM) and linalool (85 nM) to levels above their odor thresholds in water. The results illustrate the potential of the application of L. lactis as a food grade expression platform for the recombinant production of proteins and bioactive compounds from plants.

Integration and mining of malaria molecular, functional and pharmacological data: how far are we from a chemogenomic knowledge space?

The organization and mining of malaria genomic and post-genomic data is important to significantly increase the knowledge of the biology of its causative agents, and is motivated, on a longer term, by the necessity to predict and characterize new biological targets and new drugs. Biological targets are sought in a biological space designed from the genomic data from Plasmodium falciparum, but using also the millions of genomic data from other species. Drug candidates are sought in a chemical space containing the millions of small molecules stored in public and private chemolibraries. Data management should, therefore, be as reliable and versatile as possible. In this context, five aspects of the organization and mining of malaria genomic and post-genomic data were examined: 1) the comparison of protein sequences including compositionally atypical malaria sequences, 2) the high throughput reconstruction of molecular phylogenies, 3) the representation of biological processes, particularly metabolic pathways, 4) the versatile methods to integrate genomic data, biological representations and functional profiling obtained from X-omic experiments after drug treatments and 5) the determination and prediction of protein structures and their molecular docking with drug candidate structures. Recent progress towards a grid-enabled chemogenomic knowledge space is discussed.

Prediction of solubility on recombinant expression of Plasmodium falciparum erythrocyte membrane protein 1 domains in Escherichia coli

Background

Cellular interactions elicited by Plasmodium falciparum erythrocyte membrane protein antigen 1 (PfEMP1) are brought about by multiple DBL (Duffy binding like), CIDR (cysteine-rich interdomain region) and C2 domain types. Elucidation of the functional and structural characteristics of these domains is contingent on the abundant availability of recombinant protein in a soluble form. A priori prediction of PfEMP1 domains of the 3D7 genome strain, most likely to be expressed in the soluble form in Escherichia coli was computed and proven experimentally.

Methods

A computational analysis correlating sequence-dependent features to likelihood for expression in soluble form was computed and predictions were validated by the colony filtration blot method for rapid identification of soluble protein expression in E. coli.

Results

Solubility predictions for all constituent PfEMP1 domains in the decreasing order of their probability to be expressed in a soluble form (% mean solubility) are as follows: ATS (56.7%) > CIDR1α (46.8%) > CIDR2β (42.9%) > DBL2-4γ (31.7%) > DBL2β + C2 (30.6%) > DBL1α (24.9%) > DBL2-7ε (23.1%) > DBL2-5δ (14.8%). The length of the domains does not correlate to their probability for successful expression in the soluble form. Immunoblot analysis probing for soluble protein confirmed the differential in solubility predictions.

Conclusion

The acidic terminal segment (ATS) and CIDR α/β domain types are suitable for recombinant expression in E. coli while all DBL subtypes (α, β, γ, δ, ε) are a poor choice for obtaining soluble protein on recombinant expression in E. coli. This study has relevance for researchers pursuing functional and structural studies on PfEMP1 domains.

Induction of cross-reactive immune responses to NTS-DBL-1alpha/x of PfEMP1 and in vivo protection on challenge with Plasmodium falciparum

The interactions of Plasmodium falciparum infected erythrocytes parasitized red blood cells (pRBC) with endothelial receptors and erythrocytes are mediated by multiple Duffy-binding like (DBL) and cysteine-rich interdomain region (CIDR) domains harboured in the Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). The success of a subunit vaccine based on PfEMP1 depends on its ability to elicit cross-reactive responses to a substantial number of PfEMP1 variants. We have here evaluated serological PfEMP1 cross-reactivity by immunizing rats with phylogenetically diverse recombinant NTS-DBL-1alpha/x fusion domains from the 3D7 genome parasite emulsified in Montanide ISA 720. Cross-reactivity was elicited to these diverse DBL-1alpha/x domains as measured by ELISA and by immunoblotting. Employing a novel in vivo model of human infected erythrocyte sequestration, immunized animals were challenged with the FCR3S1.2 clone and cross-protection in terms of reduction in lung sequestration amounting to approximately 50% was demonstrated. Our results suggest that immunization with phylogenetically distant DBL-1alpha/x variants, can elicit partial cross-protection to challenge with the parasites harbouring a distant variant. These observations have implications for the design of multi-component vaccines against P. falciparum malaria.