Production, characterization, and crystallization of truncated forms of pneumococcal surface protein A from Escherichia coli

A Jack of All Trades: The Role of Pneumococcal Surface Protein A in the Pathogenesis of Streptococcus pneumoniae

Streptococcus pneumoniae (Spn), or the pneumococcus, is a Gram-positive bacterium that colonizes the upper airway. Spn is an opportunistic pathogen capable of life-threatening disease should it become established in the lungs, gain access to the bloodstream, or disseminate to vital organs including the central nervous system. Spn is encapsulated, allowing it to avoid phagocytosis, and current preventative measures against infection include polyvalent vaccines composed of capsular polysaccharide corresponding to its most prevalent serotypes. The pneumococcus also has a plethora of surface components that allow the bacteria to adhere to host cells, facilitate the evasion of the immune system, and obtain vital nutrients; one family of these are the choline-binding proteins (CBPs). Pneumococcal surface protein A (PspA) is one of the most abundant CBPs and confers protection against the host by inhibiting recognition by C-reactive protein and neutralizing the antimicrobial peptide lactoferricin. Recently our group has identified two new roles for PspA: binding to dying host cells via host-cell bound glyceraldehyde 3-phosphate dehydrogenase and co-opting of host lactate dehydrogenase to enhance lactate availability. These properties have been shown to influence Spn localization and enhance virulence in the lower airway, respectively. Herein, we review the impact of CBPs, and in particular PspA, on pneumococcal pathogenesis. We discuss the potential and limitations of using PspA as a conserved vaccine antigen in a conjugate vaccine formulation. PspA is a vital component of the pneumococcal virulence arsenal - therefore, understanding the molecular aspects of this protein is essential in understanding pneumococcal pathogenesis and utilizing PspA as a target for treating or preventing pneumococcal pneumonia.

ClearColi as a platform for untagged pneumococcal surface protein A production: cultivation strategy, bioreactor culture, and purification

Abstract

Several studies have searched for new antigens to produce pneumococcal vaccines that are more effective and could provide broader coverage, given the great number of serotypes causing pneumococcal diseases. One of the promising subunit vaccine candidates is untagged recombinant pneumococcal surface protein A (PspA4Pro), obtainable in high quantities using recombinant Escherichia coli as a microbial factory. However, lipopolysaccharides (LPS) present in E. coli cell extracts must be removed, in order to obtain the target protein at the required purity, which makes the downstream process more complex and expensive. Endotoxin-free E. coli strains, which synthesize a nontoxic mutant LPS, may offer a cost-effective alternative way to produce recombinant proteins for application as therapeutics. This paper presents an investigation of PspA4Pro production employing the endotoxin-free recombinant strain ClearColi® BL21(DE3) with different media (defined, auto-induction, and other complex media), temperatures (27, 32, and 37 °C), and inducers. In comparison to conventional E. coli cells in a defined medium, ClearColi presented similar PspA4Pro yields, with lower productivities. Complex medium formulations supplemented with salts favored PspA4Pro yields, titers, and ClearColi growth rates. Induction with isopropyl-β-d-thiogalactopyranoside (0.5 mM) and lactose (2.5 g/L) together in a defined medium at 32 °C, which appeared to be a promising cultivation strategy, was reproduced in 5 L bioreactor culture, leading to a yield of 146.0 mg PspA4Pro/g dry cell weight. After purification, the cell extract generated from ClearColi led to 98% purity PspA4Pro, which maintained secondary structure and biological function. ClearColi is a potential host for industrial recombinant protein production.

Key points

• ClearColi can produce as much PspA4Pro as conventional E. coli BL21(DE3) cells.

• 10.5 g PspA4Pro produced in ClearColi bioreactor culture using a defined medium.

• Functional PspA4Pro (98% of purity) was obtained in ClearColi bioreactor culture.

Expression, purification, and characterization of pneumococcal PsaA-PspA fusion protein

Streptococcus pneumoniae is a gram-positive bacterial pathogen causing invasive pneumonia, meningitis, otitis media, and bacteremia. Owing to the current pitfalls of polysaccharide and polysaccharide-conjugate vaccines, protein vaccines are considered promising candidates against pneumonia. Pneumococcal surface protein A (PspA) and pneumococcal surface adhesin A (PsaA) are virulence proteins showing good immunogenicity and protective effects against S. pneumoniae strains in mice. In this study, we expressed the fusion protein PsaA-PspA, which consists of PsaA and the N-terminal region of PspA family 1 and 2, in Escherichia coli. We describe a novel and effective method to purify PsaA-PspA using hydroxyapatite and two-step chromatography. After determining the optimal induction conditions and a series of purification steps, we obtained PsaA-PspA fusion protein with over 95% purity at a final yield of 22.44% from the starting cell lysate. The molecular weight of PsaA-PspA was approximately 83.6 kDa and its secondary structure was evaluated by circular dichroism. Immunization with the purified protein induced high levels of IgG antibodies in mice. Collectively, these results demonstrate that our purification method can effectively produce high-purity PsaA-PspA fusion protein with biological activity and chemical integrity, which can be widely applied to the purification of other PspA subclass proteins.

Giordano R, Sargo CR, Horta AC, Zangirolami TC. Cost analysis based on bioreactor cultivation conditions: Production of a soluble recombinant protein using Escherichia coli BL21(DE3)

The impact of cultivation strategy on the cost of recombinant protein production is crucial for defining cost-effective bioreactor operation conditions. This paper presents a methodology to estimate and compare cost impacts related to utilities as well as medium composition, using simple design equations and accessible data. Data from batch bioreactor cultures were used as case study involving the production of pneumococcal surface protein A, a soluble recombinant protein, employing E. coli BL21(DE3). Cultivation strategies and corresponding process costs covered a wide range of operational conditions, including different media, inducers, and temperatures. The core expenses were related to the medium and cooling. When the price of peptone was above the threshold value of US$ 30/kg, defined medium became the best choice. IPTG and temperatures around 32 °C led to shorter cultures and lower PspA4Pro production costs. The procedure offers a simple, accessible theoretical tool to identify cost-effective production strategies using bioreactors.

Production and purification of an untagged recombinant pneumococcal surface protein A (PspA4Pro) with high-purity and low endotoxin content

Streptococcus pneumoniae is the main cause of pneumonia, meningitis, and other conditions that kill thousands of children every year worldwide. The replacement of pneumococcal serotypes among the vaccinated population has evidenced the need for new vaccines with broader coverage and driven the research for protein-based vaccines. Pneumococcal surface protein A (PspA) protects S. pneumoniae from the bactericidal effect of human apolactoferrin and prevents complement deposition. Several studies indicate that PspA is a very promising target for novel vaccine formulations. Here we describe a production and purification process for an untagged recombinant fragment of PspA from clade 4 (PspA4Pro), which has been shown to be cross-reactive with several PspA variants. PspA4Pro was obtained using lactose as inducer in Phytone auto-induction batch or glycerol limited fed-batch in 5-L bioreactor. The purification process includes two novel steps: (i) clarification using a cationic detergent to precipitate contaminant proteins, nucleic acids, and other negatively charged molecules as the lipopolysaccharide, which is the major endotoxin; and (ii) cryoprecipitation that eliminates aggregates and contaminants, which precipitate at -20 °C and pH 4.0, leaving PspA4Pro in the supernatant. The final process consisted of cell rupture in a continuous high-pressure homogenizer, clarification, anion exchange chromatography, cryoprecipitation, and cation exchange chromatography. This process avoided costly tag removal steps and recovered 35.3 ± 2.5% of PspA4Pro with 97.8 ± 0.36% purity and reduced endotoxin concentration by >99.9%. Circular dichroism and lactoferrin binding assay showed that PspA4Pro secondary structure and biological activity were preserved after purification and remained stable in a wide range of temperatures and pH values.

Optimizing expression of Streptococcus pneumoniae surface protein a, PspA: serocross-reactivity within families of antisera induced against clades 1 and 3

Streptococcus pneumoniae is the agent responsible for infections such as pneumonia, otitis media, and meningitis. Among virulence factors, the Pneumococcal surface protein A (PspA) has been shown to be immunogenic and protective in mice, and is thus a good vaccine candidate. PspA has been classified into 6 clades and 3 families. Initially, pspA fragments, clades 1 and 3, were cloned into the pAE-6His expression vector. Proteins were expressed in Escherichia coli BL21(DE3) and purified by affinity and anion exchange chromatographies, with a yield of 11 mg/l of culture. Due to plasmid instability in E. coli, another construct using pspA1 was obtained based on pET-37b(+), which was shown to be stable in E. coli and increased the yield approximately 3-fold. Our results show good conditions for scale-up. Sera from immunized mice recognized PspA in total extracts of S. pneumoniae strains: anti-rPspA1p sera recognized native PspA clades 1 (+++), 2 (++) and 4 (+) and anti-rPspA3p sera recognized PspA clades 1 (+), 2 (+), 3 (+++) and 4 (+). The cross-reactivity pattern obtained confirms the notion that proteins from both families should be included for development of a broad-coverage vaccine; lower-cross reactivity between rPspAs of family 2 indicates that it may be necessary to include 2 proteins from this family.

Structure and molecular mechanism of Bacillus anthracis cofactor-independent phosphoglycerate mutase: a crucial enzyme for spores and growing cells of Bacillus species

Phosphoglycerate mutases (PGMs) catalyze the isomerization of 2- and 3-phosphoglycerates and are essential for glucose metabolism in most organisms. This study reports the production, structure, and molecular dynamics analysis of Bacillus anthracis cofactor-independent PGM (iPGM). The three-dimensional structure of B. anthracis PGM is composed of two structural and functional domains, the phosphatase and transferase. The structural relationship between these two domains is different than in the B. stearothermophilus iPGM structure determined previously. However, the structures of the two domains of B. anthracis iPGM show a high degree of similarity to those in B. stearothermophilus iPGM. The novel domain arrangement in B. anthracis iPGM and the dynamic property of these domains is directly linked to the mechanism of enzyme catalysis, in which substrate binding is proposed to result in close association of the two domains. The structure of B. anthracis iPGM and the molecular dynamics of this structure provide unique insight into the mechanism of iPGM catalysis, in particular the roles of changes in coordination geometry of the enzyme's two bivalent metal ions and the regulation of this enzyme's activity by changes in intracellular pH during spore formation and germination in Bacillus species.

Unveiling molecular mechanisms of pneumococcal surface protein A interactions with antibodies and lactoferrin

BACKGROUND

Streptococcus pneumoniae is a Gram-positive bacterium and a major human pathogen. The organism displays on its surface a variety of molecules that are involved in many essential processes including interactions with the tissues and molecules of its human host. A number of such surface molecules are essential virulence factors in disease processes and pathogenesis during all stages of bacterial life.

FOCUS

Here we introduce one such surface protein, pneumococcal surface protein A (PspA), and show its molecular and structural aspects, and underlying mechanism of function at the atomic level as currently understood. The basis of its anti-complementary properties and functional interactions with its ligand, lactoferrin, is discussed. The PspA antigen binding to lactoferrin prevents the bactericidal effect of this human molecule of many functions. This review is focused on new function characterization studies performed during this century (year 2001 and later). Earlier studies on PspA were reviewed by this author in 2001 and 2004 [Jedrzejas MJ. Pneumococcal virulence factors: structure and function. Microbiol Mol Biol Rev, 2001;65:187-207; Jedrzejas MJ. Extracellular virulence factors of Streptococcus pneumoniae. Front Biosci 2004;9:891-914].

CONCLUSIONS

The discovery and understanding of the molecular mechanisms of individual virulence factors, including PspA, are essential to the appreciation of S. pneumoniae function and mechanisms responsible for colonization and invasion of human tissues by this organism. The utilization of a microscopic view at the atomic level provided by structural biology is essential to this process of discovery. The development of new and better cures for the disease might follow as a result of such awareness.

Epitope mapping of pneumococcal surface protein A of strain Rx1 using monoclonal antibodies and molecular structure modelling

Pneumococcal surface protein A (PspA) is an antigenic variable vaccine candidate of Streptococcus pneumoniae. Epitope similarities between PspA from the American vaccine candidate strain Rx1 and Norwegian clinical isolates were studied using PspA specific monoclonal antibodies (mAbs) made against clinical Norwegian strains. Using recombinant PspA/Rx1 fragments and immunoblotting the epitopes for mAbs were mapped to two regions of amino acids, 1-67 and 67-236. The discovered epitopes were visualized by modelling of the PspA:Fab part of mAb in three dimensions. Flow cytometric analysis showed that the epitopes for majority of mAbs were accessible for antibody binding on live pneumococci. Also, the epitopes for majority of the mAbs are widely expressed among clinical Norwegian isolates.

Pneumococcal virulence factors: structure and function

The overall goal for this review is to summarize the current body of knowledge about the structure and function of major known antigens of Streptococcus pneumoniae, a major gram-positive bacterial pathogen of humans. This information is then related to the role of these proteins in pneumococcal pathogenesis and in the development of new vaccines and/or other antimicrobial agents. S. pneumoniae is the most common cause of fatal community-acquired pneumonia in the elderly and is also one of the most common causes of middle ear infections and meningitis in children. The present vaccine for the pneumococcus consists of a mixture of 23 different capsular polysaccharides. While this vaccine is very effective in young adults, who are normally at low risk of serious disease, it is only about 60% effective in the elderly. In children younger than 2 years the vaccine is ineffective and is not recommended due to the inability of this age group to mount an antibody response to the pneumococcal polysaccharides. Antimicrobial drugs such as penicillin have diminished the risk from pneumococcal disease. Several pneumococcal proteins including pneumococcal surface proteins A and C, hyaluronate lyase, pneumolysin, autolysin, pneumococcal surface antigen A, choline binding protein A, and two neuraminidase enzymes are being investigated as potential vaccine or drug targets. Essentially all of these antigens have been or are being investigated on a structural level in addition to being characterized biochemically. Recently, three-dimensional structures for hyaluronate lyase and pneumococcal surface antigen A became available from X-ray crystallography determinations. Also, modeling studies based on biophysical measurements provided more information about the structures of pneumolysin and pneumococcal surface protein A. Structural and biochemical studies of these pneumococcal virulence factors have facilitated the development of novel antibiotics or protein antigen-based vaccines as an alternative to polysaccharide-based vaccines for the treatment of pneumococcal disease.