Expression of mammalian proteins in Pseudomonas fluorescens

The invention is a process for improved production of a recombinant mammalian protein by expression in a Pseudomonad, particularly in a Pseudomonas fluorescens organism. The process improves production of mammalian proteins, particularly human or human-derived proteins, over known expression systems such as E. coli in comparable circumstances Processes for improved production of isolated mammalian, particularly human, proteins are provided.

This application claims priority to U.S. Provisional Application Nos. 60/564,798, entitled “Expression of Mammalian Proteins in Pseudomonas fluorescens,” filed Apr. 22, 2004, and 60/537,148, entitled “Protein Expression Systems,” filed Jan. 16, 2004. This application is also a continuation-in-part of U.S. application Ser. No. 10/681,540, entitled “Amended Recombinant Cells for the Production and Delivery of Gamma Interferon as an Antiviral Agent, Adjuvant And Vaccine Accelerant,” filed Oct. 7, 2003, which claims priority to U.S. Provisional Application No. 60/417,124, filed Oct. 8, 2002.

The invention is a process for improved production of a recombinant mammalian protein by expression in a Pseudomonad, particularly in a Pseudomonas fluorescens organism. The process improves production of mammalian protein expression over known expression systems.

BACKGROUND OF THE INVENTION

More than 325 million people worldwide have been helped by the more than 155 biotechnology drugs and vaccines approved by the U.S. Food and Drug Administration (FDA). In addition, there are more than 370 biotech drug products and vaccines currently in clinical trials targeting more than 200 diseases, including various cancers, Alzheimer's disease, heart disease, diabetes, multiple sclerosis, AIDS and arthritis. Unlike traditional small molecule therapeutics that are produced through classical chemical synthesis, proteins are ususally produced in living cells inefficiently and at high cost. Due to the high cost and complexity, there is a shortage of manufacturing capacity for protein-based therapeutics.

The use of microbial cells to produce products has a very long history. As early as 1897, Buchner discovered that enzymes extracted from yeast are effective in converting sugar into alcohol, leading to the production of key industrial chemicals using microorganisms. By the 1940s, large-scale production of penicillin via fermentation was achieved. Techniques for the insertion of foreign genes into bacteria were first developed in the early 1970s. Bacterial production of commercially viable recombinant mammalian protein was first exploited in the production of human insulin. Today fermentation and cell culture underlie the bulk of the industry's production of alcohol, antibiotics, biochemicals and therapeutic proteins. However, development and manufacturing of therapeutically useful proteins has been hampered due, in large part, to the limitations of the current organisms used to express these exogenous proteins.

Prokaryotic vs. Eukaryotic Protein Expression

Although bacterial expression system are often used to produce recombinant eukaryotic proteins, typically the proteins yielded differ significantly from their original counterparts. In general, it is a challenge to reproduce the eukaryotic secondary and tertiary structures in E. coli expression systems. At the same time, while the eukaryotic expression systems currently are better able to form the secondary and tertiary structures of recombinant eukaryotic proteins, the capacity of these systems to produce recombinant proteins in large quantity is limited.

Post-translational modifications represent the most significant differences between prokaryotic and eukaryotic protein expression. Prokaryotes (i.e., bacteria) have a very simply cellular structure and no membrane-bound organelles. In eukaryotes, a protein is often modified after it is intially produced. These modifications, in many cases, are necessary to convert the peptide into a functional form. Thus, even when exisiting bacterial expression systems produce a protein with the correct primary structure, the protein may not be post-translationally modified and is therefore often nonfunctional. Common modifications include disulfide bond formation, glycosylation, acetylation, acylation, phosphorylation, and gamma-carboxylation, all of which can regulate protein folding and biological activity. Bacterial expression systems generally do not properly glycosylate, acetylate, acylate, phosphorylate, or gamma-carboxylate eukaryotic proteins.

Bacteria, such as E. coli, can form disulfide bonds, but the bonds are often formed in the incorrect configuration required for biological activity; therefore, denaturation and refolding is usually required to produce active eukaryotic proteins. Molecular chaperone proteins are present in both prokaryotes and eukaryotes that facilitate the folding of other proteins. In the absence of such chaperones, unfolded or partially folded polypeptide chains are unstable within the cell, frequently folding incorrectly or aggregating into insoluble complexes. The binding of chaperones stabilizes these unfolded polypeptides, thereby preventing incorrect folding or aggregation and allowing the polypeptide chain to fold into its correct conformation. However, chaperones differ in each type of cell, and can be differentially expressed based on extracellular conditions.

Problems With Current Expression Systems

Escherichia coli (E. coli) is the most widely and routinely used protein expression system. Production in E. coli is inexpensive, fast, and well characterized. Further, scale-up and harvesting is possible and cGMP production is well established. However, there are significant limitations to the use of E. coli, which often prove difficult to overcome, particularly when expressing recombinant mammalian proteins.

Along with the limitations described above, the high-level expression of recombinant gene products in E. coli often results in the misfolding of the protein of interest and its subsequent degradation by cellular proteases or deposition into biologically inactive aggregates known as inclusion bodies. Protein found in inclusion bodies typically must be extracted and renautred for activity, adding time and expense to the process. Typical renaturation methods involve attempts to dissolve the aggregate in concentrated denaturant, and subsequent removal of the denaturant by dilution. Some of the factors which have been suggested to be involved in inclusion body formation include the high local concentration of protein; a reducing environment in the cytoplasm (E. coli cytoplasm has a high level of glutathione) preventing formation of disulfide bonds; lack of post-translational modifications, which can increase the protein solubility; improper interactions with chaperones and other enzymes involved in in vivo folding; intermolecular cross-linking via disulfide or other covalent bonds; and increased aggregation of folding intermediates due to their limited solubility. It is probably a combination of these factors, as well as a limited availability of chaperones, which most commonly lead to the formation of inclusion bodies.

Yeast expression systems, such as Saccharomyces cerevisiae or Pichia pastoris, are also commonly used to produce proteins. These systems are well characterized, provide good expression levels and are relatively fast and inexpensive compared to other eukaryotic ecpression systems. However, yeast can accomplish only limited post-translational protein modifications, the protein may need refolding, and harvesting of the protein can be a problem due to the characteristics of the cell wall.

Insect cell expression systems have also emerged as an attractive, but expensive, alternative as a protein expression system. Correctly folded proteins that are generally post-translationally modified can sometimes be produced and extracellular expression has been achieved. However, it is not as rapid as bacteria and yeast, and scale-up is generally challenging.

Mammalian cell expression systems, such as Chinese hamster ovary cells, are often used for complex protein expression. This system usually produces correctly folded proteins with the appropriate post-translational modifications and the proteins can be expressed extracellularly. However, the system is very expensive, scale-up is slow and often not feasible, and protein yields are lower than in any other system.