Efficient processing and export of human growth hormone by heat labile enterotoxin chain B signal sequence

Growth hormone receptor agonists and antagonists: From protein expression and purification to long-acting formulations

Recombinant human growth hormone (rhGH) and GH receptor antagonists (GHAs) are used clinically to treat a range of disorders associated with GH deficiency or hypersecretion, respectively. However, these biotherapeutics can be difficult and expensive to manufacture with multiple challenges from recombinant protein generation through to the development of long-acting formulations required to improve the circulating half-life of the drug. In this review, we summarize methodologies and approaches used for making and purifying recombinant GH and GHA proteins, and strategies to improve pharmacokinetic and pharmacodynamic properties, including PEGylation and fusion proteins. Therapeutics that are in clinical use or are currently under development are also discussed.

Immunization with recombinant fusion of LTB and linear epitope (40-62) of epsilon toxin elicits protective immune response against the epsilon toxin of Clostridium perfringens type D

Epsilon toxin (Etx) produced by Clostridium perfringens types B and D, a major causative agent of enterotoxaemia causes significant economic losses to animal industry. Conventional vaccines against these pathogens generally employ formalin-inactivated culture supernatants. However, immunization with the culture supernatant and full length toxin subjects the animal to antigenic load and often have adverse effect due to incomplete inactivation of the toxins. In the present study, an epitope-based vaccine against Clostridium perfringens Etx, comprising 40-62 amino acid residues of the toxin in translational fusion with heat labile enterotoxin B subunit (LTB) of E. coli, was evaluated for its protective potential. The ability of the fusion protein rLTB.Etx_40-62 to form pentamers and biologically active holotoxin with LTA of E. coli indicated that the LTB present in the fusion protein retained its biological activity. Antigenicity of both the components in the fusion protein was retained as anti-fusion protein antisera detected both the wild type Etx and LTB in Western blot analysis. Immunization of BALB/c mice with the fusion protein resulted in a significant increase in all isotypes, predominantly IgG1, IgG2a and IgG2b. Anti-fusion protein antisera neutralized the cytotoxicity of epsilon toxin both in vitro and in vivo. Thus, the results demonstrate the potential of rLTB.Etx_40-62 as a candidate vaccine against C. perfringens.

Periplasmic production via the pET expression system of soluble, bioactive human growth hormone

A pET based expression system for the production of recombinant human growth hormone (hGH) directed to the Escherichia coli periplasmic space was developed. The pET22b plasmid was used as a template for creating vectors that encode hGH fused to either a pelB or ompA secretion signal under control of the strong bacteriophage T7 promoter. The pelB- and ompA-hGH constructs expressed in BL21 (λDE3)-RIPL E. coli are secreted into the periplasm which facilitates isolation of soluble hGH by selective disruption of the outer membrane. A carboxy-terminal poly-histidine tag enabled purification by Ni(2+) affinity chromatography with an average yield of 1.4 mg/L culture of purified hGH, independent of secretion signal. Purified pelB- and ompA-hGH are monomeric based on size exclusion chromatography with an intact mass corresponding to mature hGH indicating proper cleavage of the signal peptide and folding in the periplasm. Both pelB- and ompA-hGH bind the hGH receptor with high affinity and potently stimulate Nb2 cell growth. These results demonstrate that the pET expression system is suitable for the rapid and simple isolation of bioactive, soluble hGH from E. coli.

Mononuclear phagocyte differentiation, activation, and viral infection regulate matrix metalloproteinase expression: implications for human immunodeficiency virus type 1-associated dementia

The pathogenesis of human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) is mediated mainly by mononuclear phagocyte (MP) secretory products and their interactions with neural cells. Viral infection and MP immune activation may affect leukocyte entry into the brain. One factor that influences central nervous system (CNS) monocyte migration is matrix metalloproteinases (MMPs). In the CNS, MMPs are synthesized by resident glial cells and affect the integrity of the neuropil extracellular matrix (ECM). To ascertain how MMPs influence HAD pathogenesis, we studied their secretion following MP differentiation, viral infection, and cellular activation. HIV-1-infected and/or immune-activated monocyte-derived macrophages (MDM) and human fetal microglia were examined for production of MMP-1, -2, -3, and -9. MMP expression increased significantly with MP differentiation. Microglia secreted high levels of MMPs de novo that were further elevated following CD40 ligand-mediated cell activation. Surprisingly, HIV-1 infection of MDM led to the down-regulation of MMP-9. In encephalitic brain tissue, MMPs were expressed within perivascular and parenchymal MP, multinucleated giant cells, and microglial nodules. These data suggest that MMP production in MP is dependent on cell type, differentiation, activation, and/or viral infection. Regulation of MMP expression by these factors may contribute to neuropil ECM degradation and leukocyte migration during HAD.

N-terminus of mature heat-labile enterotoxin chain B is critical for its extracellular secretion in Vibrio cholerae

The effects of addition of a few amino acids to the amino- and carboxy-terminal regions of the mature portion of the heat-labile enterotoxin chain B (LTB) of Escherichia coli on protein export, secretion and assembly were investigated. In E. coli, LTB (secretory protein) with or without the extension at the N- or C-terminus accumulated in the periplasmic fraction. For Vibrio cholerae, LTB with the extension at the C-terminus was exported to the periplasm followed by secretion to the extracellular milieu. However, LTB with the N-terminus extension was exported to the periplasm only. Our findings suggest that in the case of V. cholerae, the N-terminus of the mature LTB plays an important role in its secretion to the extracellular milieu.

High-level expression of ovine growth hormone in Escherichia coli: single-step purification and characterization

The gene for ovine growth hormone (oGH) was expressed without signal sequences in Escherichia coli. A recombinant plasmid expression vector has been constructed which directs the synthesis of a fusion protein containing a stretch of six histidine residues (His6) at the amino-terminus under the control of a T5 promoter. Upon induction with isopropyl-beta-D-thiogalactopyranoside, the recombinant protein was synthesized and accumulated in the cytoplasm in the form of inclusion bodies, at levels of approximately 18% of the total cellular protein. The recombinant ovine growth hormone containing His tag was recovered and purified to >95% homogeneity in a single step by immobilized metal-ion chromatography with a special affinity Ni2+.NTA resin that has selectivity for proteins with neighboring histidine residues. Characterization by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblotting and amino terminal analysis demonstrated the authenticity of the fusion protein. The purified RoGH after refolding was found to be functionally active in terms of its receptor binding and antigenicity as analyzed by radio receptor assay and radio immuno assay. Yields of the purified expressed protein were found to be 32 microg/ml at a shake-flask level. Thus, results indicate that a combination of E. coli expression and affinity purification by Ni2+.NTA chromatography promises to be a rapid method to produce oGH for use in structure-function studies.

Production of the beta-subunit of human chorionic gonadotropin in Escherichia coli and its export mediated by the heat-labile enterotoxin chain-B signal sequence

The PCR-amplified beta-subunit of the human chorionic gonadotropin structural gene (betahCG) was cloned under the control of the tac promoter and the heat-labile enterotoxin chain B (LTB) signal sequence (LTBss). BetahCG was successfully produced, processed and exported to the periplasmic space in Escherichia coli. Expression of betahCG was confirmed by immunoblot analysis using an anti-betahCG polyclonal antibody. The processing of the protein was very efficient, as only the processed band could be detected at all time points during the course of induction. Expression was evident soon after the addition of the lactose analogue, IPTG. These results demonstrate that E. coli cells can synthesize, process and export betahCG using the LTBss.

Efficient expression, processing and secretion of a biologically active mammalian protein by Vibrio cholerae

The use of Vibrio cholerae as a secretory expression system for the expression of a mammalian protein, namely human growth hormone, under the control of the heat labile enterotoxin chain B signal sequence is reported. The protein is efficiently expressed and processed. The mature protein is exported to the periplasm after which it is secreted to the extracellular milieu. The expressed and secreted hGH actively binds to its receptor as established by its receptor binding activity. The biological activity of the protein is demonstrated in vitro in a Nb2 proliferation assay.

Translational fusion of heat labile enterotoxin chain B and beta-subunit of human chorionic gonadotropin: periplasmic expression in Escherichia coli and its immunogenicity

A fusion gene was constructed consisting of heat labile enterotoxin chain B (LTB) of E. coli genetically linked at its C-terminus to the beta-subunit of human chorionic gonadotropin in translational fusion, under the control of tac promoter and LTB signal sequence. Expression of the fusion gene (about 5 microgram/ml) in E. coli was confirmed by immunoblot analysis using both anti-LTB and anti-betahCG polyclonal antibodies. The fusion protein was efficiently processed and exported to the periplasmic space. LTB in the fusion protein retained its ability to bind to GM1 ganglioside receptor. Mice immunized with the fusion protein produce antibodies that recognize recombinant betahCG and the native hCG suggesting its potential use as a contraceptive vaccine.

High-level production and one-step purification of biologically active human growth hormone in Escherichia coli

A plasmid has been constructed to direct the synthesis of recombinant human growth hormone (re-hGH) in Escherichia coli as a fusion protein containing a His6 tag at the N-terminus under the control of the T5 promoter. The re-hGH was synthesized in large amounts and accumulated in the form of inclusion bodies upon induction with IPTG. Inclusion bodies were solubilized in 6 M guanidine.HCl and the re-hGH was purified by single-step affinity chromatography on Ni(2+)-nitrilotriacetic acid (NTA) agarose. At the shake flask level, the purified re-hGH was obtained with a yield of 30 mg/l of culture. The re-hGH was biologically active in a node rat lymphoma (Nb2) cell bioassay.

Heterologous protein secretion and the versatile Escherichia coli haemolysin translocator

Heterologous proteins synthesized in the Gram-negative bacterium Escherichia coli in bioreactor culture may accumulate in one of three 'compartments':the cytoplasm, the periplasm, or the extracellular medium. Many overexpressed proteins from various origins have been purified from each of these locations. However, to date, each system has required specific tailoring to meet the stringent requirements for each protein product to ensure correct folding, activity and appropriate yield. The E. coli haemolysin secretion system appears to provide a flexible mechanism with which to secrete a wide variety of heterologous fusion proteins into the extracellular medium.

Protein secretion pathway in Escherichia coli

The export of proteins to the Escherichia coli periplasm is a well established system for heterologous protein production. With a better understanding of the protein export (SecA, Y-dependent) process and a greater awareness of the conditions necessary for correct folding of proteins in the periplasm, serious efforts are now being made to manipulate this system to achieve substantial increases in the yield of authentically folded proteins. Further advances in the development of methods for the recovery of recombinant proteins from the culture medium have made the use of fusion proteins secreted by the protein A or haemolysin pathways a more attractive option. Recent studies of the haemolysin system indicate its ability to secrete a wide range of polypeptides, including normally cytoplasmic proteins. As their features and potential applications become much clearer, a rapidly expanding number of protein-secretion mechanisms in Gram-negative bacteria are becoming available for heterologous protein expression. Most, if not all, of these systems can be successfully transplanted into E. coli, providing a wider choice of systems for the future.