Structural and functional characterization of recombinant human serum transferrin secreted from Pichia pastoris

Production of Recombinant Human Transferrin in Eukaryotic Pichia pastoris Expression System

The development and manufacturing of serum-free culture media allowing reducing the costs of preparations and standardizing the biotechnological process are important trends in biotechnology. Substitution of protein compounds in the serum-free media with recombinant analogues reduces the risk of contamination with various infectious agents. Human transferrin is a protein component of serum-free media responsible for the transport of Fe³⁺ ions into cells. We generated a producing strain P. pastoris secreting human transferrin to the culture medium. The use of constitutive GAP promoter and maintenance of medium pH at 6.5 allows attaining maximum level of transferrin expression (20 mg/liter).

Expression and Purification of Human Membrane Progestin Receptor α (mPRα)

Membrane progestin receptors (mPRs) are responsible for mediating the rapid, nongenomic activity of progestins and belong to the G protein-coupled receptor (GPCR) family. mPRs are also considered as attractive proteins to draw a new medicinal approach. In this study, we optimized a procedure for the expression and purification of recombinant human mPRα protein (hmPRα) by a methylotropic yeast, Pichia pastoris, expression system. The protein expressed in crude membrane fractions exhibited a binding affinity of Kd = 3.8 nM and Bmax = 288.8 fmol/mg for progesterone. These results indicated that the hmPRα expressed in yeast was active. Solubilized hmPRα was purified through three column chromatography steps. A nickel-nitrilotriacetic acid (Ni-NTA) column was first used, and the mPRα proteins were then bound to cellulose resin with free amino groups (Cellufine Amino) and finally passed through an SP-Sepharose column. The optimization of expression and purification conditions resulted in a high yield of purified hmPRα (1.3–1.5 mg from 1 L culture). The purified hmPRα protein demonstrated progesterone binding (Kd = 5.2 nM and Bmax = 111.6 fmol/mg). The results indicated that we succeeded in solubilizing and purifying hmPRα in an active form. Sufficient amount of active hmPRα protein will support the establishment of applications for the screening of ligands for mPRα.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

High-throughput plasmid construction using homologous recombination in yeast: its mechanisms and application to protein production for X-ray crystallography

Homologous recombination is a system for repairing the broken genomes of living organisms by connecting two DNA strands at their homologous sequences. Today, homologous recombination in yeast is used for plasmid construction as a substitute for traditional methods using restriction enzymes and ligases. This method has various advantages over the traditional method, including flexibility in the position of DNA insertion and ease of manipulation. Recently, the author of this review reported the construction of plasmids by homologous recombination in the methanol-utilizing yeast Pichia pastoris, which is known to be an excellent expression host for secretory proteins and membrane proteins. The method enabled high-throughput construction of expression systems of proteins using P. pastoris; the constructed expression systems were used to investigate the expression conditions of membrane proteins and to perform X-ray crystallography of secretory proteins. This review discusses the mechanisms and applications of homologous recombination, including the production of proteins for X-ray crystallography.

Characterization of transferrin receptor-mediated endocytosis and cellular iron delivery of recombinant human serum transferrin from rice (Oryza sativa L.)

Background

Transferrin (TF) plays a critical physiological role in cellular iron delivery via the transferrin receptor (TFR)-mediated endocytosis pathway in nearly all eukaryotic organisms. Human serum TF (hTF) is extensively used as an iron-delivery vehicle in various mammalian cell cultures for production of therapeutic proteins, and is also being explored for use as a drug carrier to treat a number of diseases by employing its unique TFR-mediated endocytosis pathway. With the increasing concerns over the risk of transmission of infectious pathogenic agents of human plasma-derived TF, recombinant hTF is preferred to use for these applications. Here, we carry out comparative studies of the TFR binding, TFR-mediated endocytosis and cellular iron delivery of recombinant hTF from rice (rhTF), and evaluate its suitability for biopharmaceutical applications.

Result

Through a TFR competition binding affinity assay with HeLa human cervic carcinoma cells (CCL-2) and Caco-2 human colon carcinoma cells (HTB-37), we show that rhTF competes similarly as hTF to bind TFR, and both the TFR binding capacity and dissociation constant of rhTF are comparable to that of hTF. The endocytosis assay confirms that rhTF behaves similarly as hTF in the slow accumulation in enterocyte-like Caco-2 cells and the rapid recycling pathway in HeLa cells. The pulse-chase assay of rhTF in Caco-2 and HeLa cells further illustrates that rice-derived rhTF possesses the similar endocytosis and intracellular processing compared to hTF. The cell culture assays show that rhTF is functionally similar to hTF in the delivery of iron to two diverse mammalian cell lines, HL-60 human promyelocytic leukemia cells (CCL-240) and murine hybridoma cells derived from a Sp2/0-Ag14 myeloma fusion partner (HB-72), for supporting their proliferation, differentiation, and physiological function of antibody production.

Conclusion

The functional similarity between rice derived rhTF and native hTF in their cellular iron delivery, TFR binding, and TFR-mediated endocytosis and intracellular processing support that rice-derived rhTF can be used as a safe and animal-free alternative to serum hTF for bioprocessing and biopharmaceutical applications.

Biochemical and structural characterization of recombinant human serum transferrin from rice (Oryza sativa L.)

The Fe(3+) binding protein human serum transferrin (hTF) is well known for its role in cellular iron delivery via the transferrin receptor (TFR). A new application is the use of hTF as a therapy and targeted drug delivery system for a number of diseases. Recently, production of hTF in plants has been reported; such systems provide a relatively inexpensive, animal-free (eliminating potential contamination by animal pathogens) method to produce large amounts of recombinant proteins for such biopharmaceutical applications. Specifically, the production of Optiferrin (hTF produced in rice, Oryza sativa, from InVitria) has been shown to yield large amounts of functional protein for use in culture medium for cellular iron delivery to promote growth. In the present work we describe further purification (by gel filtration) and characterization of hTF produced in rice (purified Optiferrin) to determine its suitability in biopharmaceutical applications. The spectral, mass spectrometric, urea gel and kinetic analysis shows that purified Optiferrin is similar to recombinant nonglycosylated N-His tagged hTF expressed by baby hamster kidney cells and/or serum derived glycosylated hTF. Additionally, in a competitive immunoassay, iron-loaded Optiferrin is equivalent to iron-loaded N-His hTF in its ability to bind to the soluble portion of the TFR immobilized in an assay plate. As an essential requirement for any functional hTF, both lobes of purified Optiferrin bind Fe(3+) tightly yet reversibly. Although previously shown to be capable of delivering Fe(3+) to cells, the kinetics of iron release from iron-loaded Optiferrin™/sTFR and iron-loaded N-His hTF/sTFR complexes differ somewhat. We conclude that the purified Optiferrin might be suitable for consideration in biopharmaceutical applications.

Transferrin-mediated cellular iron delivery

Essential to iron homeostasis is the transport of iron by the bilobal protein human serum transferrin (hTF). Each lobe (N- and C-lobe) of hTF forms a deep cleft which binds a single Fe(3+). Iron-bearing hTF in the blood binds tightly to the specific transferrin receptor (TFR), a homodimeric transmembrane protein. After undergoing endocytosis, acidification of the endosome initiates the release of Fe(3+) from hTF in a TFR-mediated process. Iron-free hTF remains tightly bound to the TFR at acidic pH; following recycling back to the cell surface, it is released to sequester more iron. Efficient delivery of iron is critically dependent on hTF/TFR interactions. Therefore, identification of the pH-specific contacts between hTF and the TFR is crucial. Recombinant protein production has enabled deconvolution of this complex system. The studies reviewed herein support a model in which pH-induced interrelated events control receptor-stimulated iron release from each lobe of hTF.

Recombinant human transferrin: beyond iron binding and transport

Iron is indispensible for life and essential for such processes as oxygen transport, electron transfer and DNA synthesis. Transferrin (Tf) is a ubiquitous protein with a central role in iron transport and metabolism. There is evidence, however, that Tf has many other biological roles in addition to its primary function of facilitating iron transport and metabolism, such as its profound effect on mammalian cell growth and productivity. The multiple functions of Tf can be exploited to develop many novel applications. Indeed, over the past several years, considerable efforts have been directed towards exploring human serum Tf (hTf), especially the use of recombinant native hTf and recombinant Tf fusion proteins, for various applications within biotechnology and medicine. Here, we review some of the remarkable progress that has been made towards the application of hTf in these diverse areas and discuss some of the exciting future prospects for hTf.

High-level production of animal-free recombinant transferrin from Saccharomyces cerevisiae

Background

Animal-free recombinant proteins provide a safe and effective alternative to tissue or serum-derived products for both therapeutic and biomanufacturing applications. While recombinant insulin and albumin already exist to replace their human counterparts in cell culture media, until recently there has been no equivalent for serum transferrin.

Results

The first microbial system for the high-level secretion of a recombinant transferrin (rTf) has been developed from Saccharomyces cerevisiae strains originally engineered for the commercial production of recombinant human albumin (Novozymes' Recombumin^® USP-NF) and albumin fusion proteins (Novozymes' albufuse^®). A full-length non-N-linked glycosylated rTf was secreted at levels around ten-fold higher than from commonly used laboratory strains. Modification of the yeast 2 μm-based expression vector to allow overexpression of the ER chaperone, protein disulphide isomerase, further increased the secretion of rTf approximately twelve-fold in high cell density fermentation. The rTf produced was functionally equivalent to plasma-derived transferrin.

Conclusions

A Saccharomyces cerevisiae expression system has enabled the cGMP manufacture of an animal-free rTf for industrial cell culture application without the risk of prion and viral contamination, and provides a high-quality platform for the development of transferrin-based therapeutics.