The level of synthesis and secretion of Gaussia princeps luciferase in transfected CHO cells is heavily dependent on the choice of signal peptide

Optimization of heavy chain and light chain signal peptides for high level expression of therapeutic antibodies in CHO cells

Translocation of a nascent protein from the cytosol into the ER mediated by its signal peptide is a critical step in protein secretion. The aim of this work was to develop a platform technology to optimize the signal peptides for high level production of therapeutic antibodies in CHO cells. A database of signal peptides from a large number of human immunoglobulin (Ig) heavy chain (HC) and kappa light chain (LC) was generated. Most of the HC signal peptides contain 19 amino acids which can be divided into three domains and the LC signal peptides contain 22 amino acids. The signal peptides were then clustered according to sequence similarity. Based on the clustering, 8 HC and 2 LC signal peptides were analyzed for their impacts on the production of 5-top selling antibody therapeutics, namely, Herceptin, Avastin, Remicade, Rituxan, and Humira. The best HC and LC signal peptides for producing these 5 antibodies were identified. The optimized signal peptides for Rituxan is 2-fold better compared to its native signal peptides which are available in the public database. Substitution of a single amino acid in the optimized HC signal peptide for Avastin reduced its production significantly. Mass spectrometry analyses revealed that all optimized signal peptides are accurately removed in the mature antibodies. The results presented in this report are particularly important for the production of these 5 antibodies as biosimilar drugs. They also have the potential to be the best signal peptides for the production of new antibodies in CHO cells.

Application of Gaussia luciferase in bicistronic and non-conventional secretion reporter constructs

Background

Secreted luciferases are highly useful bioluminescent reporters for cell-based assays and drug discovery. A variety of secreted luciferases from marine organisms have been described that harbor an N-terminal signal peptide for release along the classical secretory pathway. Here, we have characterized the secretion of Gaussia luciferase in more detail.

Results

We describe three basic mechanisms by which GLUC can be released from cells: first, classical secretion by virtue of the N-terminal signal peptide; second, internal signal peptide-mediated secretion and third, non-conventional secretion in the absence of an N-terminal signal peptide. Non-conventional release of dNGLUC is not stress-induced, does not require autophagy and can be enhanced by growth factor stimulation. Furthermore, we have identified the golgi-associated, gamma adaptin ear containing, ARF binding protein 1 (GGA1) as a suppressor of release of dNGLUC.

Conclusions

Due to its secretion via multiple secretion pathways GLUC can find multiple applications as a research tool to study classical and non-conventional secretion. As GLUC can also be released from a reporter construct by internal signal peptide-mediated secretion it can be incorporated in a novel bicistronic secretion system.

A dual host vector for Fab phage display and expression of native IgG in mammalian cells

A significant bottleneck in antibody discovery by phage display is the transfer of immunoglobulin variable regions from phage clones to vectors that express immunoglobulin G (IgG) in mammalian cells for screening. Here, we describe a novel phagemid vector for Fab phage display that allows expression of native IgG in mammalian cells without sub-cloning. The vector uses an optimized mammalian signal sequence that drives robust expression of Fab fragments fused to an M13 phage coat protein in Escherichia coli and IgG expression in mammalian cells. To allow the expression of Fab fragments fused to a phage coat protein in E.coli and full-length IgG in mammalian cells from the same vector without sub-cloning, the sequence encoding the phage coat protein was embedded in an optimized synthetic intron within the immunoglobulin heavy chain gene. This intron is removed from transcripts in mammalian cells by RNA splicing. Using this vector, we constructed a synthetic Fab phage display library with diversity in the heavy chain only and selected for clones binding different antigens. Co-transfection of mammalian cells with DNA from individual phage clones and a plasmid expressing the invariant light chain resulted in the expression of native IgG that was used to assay affinity, ligand blocking activity and specificity.

Current state and recent advances in biopharmaceutical production in Escherichia coli, yeasts and mammalian cells

Almost all of the 200 or so approved biopharmaceuticals have been produced in one of three host systems: the bacterium Escherichia coli, yeasts (Saccharomyces cerevisiae, Pichia pastoris) and mammalian cells. We describe the most widely used methods for the expression of recombinant proteins in the cytoplasm or periplasm of E. coli, as well as strategies for secreting the product to the growth medium. Recombinant expression in E. coli influences the cell physiology and triggers a stress response, which has to be considered in process development. Increased expression of a functional protein can be achieved by optimizing the gene, plasmid, host cell, and fermentation process. Relevant properties of two yeast expression systems, S. cerevisiae and P. pastoris, are summarized. Optimization of expression in S. cerevisiae has focused mainly on increasing the secretion, which is otherwise limiting. P. pastoris was recently approved as a host for biopharmaceutical production for the first time. It enables high-level protein production and secretion. Additionally, genetic engineering has resulted in its ability to produce recombinant proteins with humanized glycosylation patterns. Several mammalian cell lines of either rodent or human origin are also used in biopharmaceutical production. Optimization of their expression has focused on clonal selection, interference with epigenetic factors and genetic engineering. Systemic optimization approaches are applied to all cell expression systems. They feature parallel high-throughput techniques, such as DNA microarray, next-generation sequencing and proteomics, and enable simultaneous monitoring of multiple parameters. Systemic approaches, together with technological advances such as disposable bioreactors and microbioreactors, are expected to lead to increased quality and quantity of biopharmaceuticals, as well as to reduced product development times.

Albumin 3'untranslated region facilitates increased recombinant protein production from Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are used for recombinant protein production in the pharmaceutical industry but there is a need to improve expression levels. In the present work experiments were carried out to test the effectiveness of different 3'untranslated regions (3'UTRs) in promoting production of a naturally secreted luciferase. Seamless cloning was used to produce expression vectors in which Gaussia princeps luciferase coding sequences were linked to the human albumin, immunoglobulin or chymotrypsinogen 3'UTR. Stably transfected CHO cells expressing these constructs were selected. Luciferase activity in the culture medium was increased 2-3-fold by replacing the endogenous 3'UTR with the albumin 3'UTR and decreased by replacement with immunoglobulin or chymotrypsinogen 3'UTR. Replacement of the native 3'UTR with the albumin 3'UTR led to a 10-fold increase in luciferase mRNA levels. Deletion analysis of the albumin 3'UTR showed that loss of nucleotides 1-50, which removed an AU-rich complex stem loop region, caused significant reductions in both luciferase protein expression and luciferase mRNA levels. The results suggest that recombinant protein expression and yield could be improved by the careful selection of appropriate 3'UTR sequences.

Evaluation of the potency of the anti-idiotypic antibody Ab2/3H6 mimicking gp41 as an HIV-1 vaccine in a rabbit prime/boost study

The HIV-1 envelope protein harbors several conserved epitopes that are recognized by broadly neutralizing antibodies. One of these neutralizing sites, the MPER region of gp41, is targeted by one of the most potent and broadly neutralizing monoclonal antibody, 2F5. Different vaccination strategies and a lot of efforts have been undertaken to induce MPER neutralizing antibodies but little success has been achieved so far. We tried to consider the alternative anti-idiotypic vaccination approach for induction of 2F5-like antibodies. The previously developed and characterized anti-idiotypic antibody Ab2/3H6 was expressed as antibody fragment fusion protein with C-terminally attached immune-modulators and used for immunization of rabbits to induce antibodies specific for HIV-1. Only those rabbits immunized with immunogens fused with the immune-modulators developed HIV-1 specific antibodies. Anti-anti-idiotypic antibodies were affinity purified using a two-step affinity purification protocol which revealed that only little amount of the total rabbit IgG fraction contained HIV-1 specific antibodies. The characterization of the induced anti-anti-idiotypic antibodies showed specificity for the linear epitope of 2F5 GGGELDKWASL and the HIV-1 envelope protein gp140. Despite specificity for the linear epitope and the truncated HIV-1 envelope protein these antibodies were not able to exhibit virus neutralization activities. These results suggest that Ab2/3H6 alone might not be suitable as a vaccine.

Post-transcriptional regulatory elements for enhancing transient gene expression levels in mammalian cells

Low yield from transient gene expression in mammalian cells limits its application to areas where large amount of proteins are needed. One effective approach to enhance transient gene expression levels is to use post-transcriptional regulatory elements (PTREs). We have evaluated the effect of five PTREs on the transient gene expression of three proteins in two cell lines. Most of the elements increased expression but exhibited cell-specific and gene-specific effects. The tripartite leader sequence of human adenovirus mRNA linked with a major late promoter enhancer gave the most universal and highest enhancement of gene expression levels. It increased the expression of all three proteins in HEK293 cells and two proteins in CHO K1 cells by 3.6- to 7.6-fold. Combinations of multiple PTREs increased protein expression as much as 10.5-fold.

Evolution of poliovirus defective interfering particles expressing Gaussia luciferase

Polioviruses (PVs) carrying a reporter gene are useful tools for studies of virus replication, particularly if the viral chimeras contain the polyprotein that provides all of the proteins necessary for a complete replication cycle. Replication in HeLa cells of a previously constructed poliovirus expressing the gene for Renilla luciferase (RLuc) fused to the N terminus of the polyprotein H(2)N-RLuc-P1-P2-P3-COOH (P1, structural domain; P2 and P3, nonstructural domains) led to the deletion of RLuc after only one passage. Here we describe a novel poliovirus chimera that expresses Gaussia luciferase (GLuc) inserted into the polyprotein between P1 and P2 (N(2)H-P1-GLuc-P2-P3-COOH). This chimera, termed PV-GLuc, replicated to 10% of wild-type yield. The reporter signal was fully retained for three passages and then gradually lost. After six passages the signal was barely detectable. On further passages, however, the GLuc signal reappeared, and after eight passages it had reached the same levels observed with the original PV-GLuc at the first passage. We demonstrated that this surprising observation was due to coevolution of defective interfering (DI) particles that had lost part or all of the capsid coding sequence (ΔP1-GLuc-P2-P3) and wild-type-like viruses that had lost the GLuc sequence (P1-P2-P3). When used at low passage, PV-GLuc is an excellent tool for studying aspects of genome replication and morphogenesis. The GLuc protein was secreted from mammalian cells but, in agreement with published data, was not secreted from PV-GLuc-infected cells due to poliovirus-induced inhibition of cellular protein secretion. Published evidence indicates that individual expression of enterovirus polypeptide 3A, 2B, or 2BC in COS-1 cells strongly inhibits host protein secretion. In HeLa cells, however, expression of none of the poliovirus polypeptides, either singly or in pairs, inhibited GLuc secretion. Thus, inhibition of GLuc secretion in PV-infected HeLa cells is likely a result of the interaction between several viral and cellular proteins that are different from those in COS-1 cells.

Application of bioluminescence imaging for in vivo monitoring of fungal infections

Fungi can cause severe invasive infections especially in the immunocompromised host. Patient populations at risk are increasing due to ongoing developments in cancer treatment and transplantation medicine. Only limited diagnostic tools and few antifungals are available, rendering a significant number of invasive fungal infections life threatening. To reduce mortality rates, a better understanding of the infection processes is urgently required. Bioluminescence imaging (BLI) is a powerful tool for such purposes, since it allows visualisation of temporal and spatial progression of infections in real time. BLI has been successfully used to monitor infections caused by various microorganisms, in particular bacteria. However, first studies have also been performed on the fungi Candida albicans and Aspergillus fumigatus. Although BLI was, in principle, suitable to study the infection process, some limitations remained. Here, different luciferase systems are introduced, and current approaches are summarised. Finally, suggestions for further improvements of BLI to monitor fungal infections are provided.

An empirical modeling platform to evaluate the relative control discrete CHO cell synthetic processes exert over recombinant monoclonal antibody production process titer

In this study we have combined empirically derived mathematical models of intracellular Mab synthesis to quantitatively compare the degree to which individual cellular processes limit recombinant IgG(4) monoclonal antibody production by GS-CHO cells throughout a state-of-the-art industrial fed-batch culture process. Based on the calculation of a production process control coefficient for each stage of the intracellular Mab synthesis and secretion pathway, we identified the major cellular restrictions on Mab production throughout the entire culture process to be recombinant heavy chain gene transcription and heavy chain mRNA translation. Surprisingly, despite a substantial decline in the rate of cellular biomass synthesis during culture, with a concomitant decline in the calculated rate constants for energy-intensive Mab synthetic processes (Mab folding/assembly and secretion), these did not exert significant control of Mab synthesis at any stage of production. Instead, cell-specific Mab production was maintained by increased Mab gene transcription which offset the decline in cellular biosynthetic rates. Importantly, this study shows that application of this whole-process predictive modeling strategy should rationally precede and inform cell engineering approaches to increase production of a recombinant protein by a mammalian host cell--where control of productivity is inherently protein product and cell line specific.

Signal peptide replacements enhance expression and secretion of hepatitis C virus envelope glycoproteins

A large number of researches focused on glycoproteins E1 and E2 of hepatitis C virus (HCV) aimed at the development of anti-HCV vaccines and inhibitors. Enhancement of E1/E2 expression and secretion is critical for the characterization of these glycoproteins and thus for subunit vaccine development. In this study, we designed and synthesized three signal peptide sequences based on online programs SignalP, TargetP, and PSORT, then removed and replaced the signal peptide preceding E1/E2 by overlapping the polymerase chain reaction method. We assessed the effect of this alteration on E1/E2 expression and secretion in mammalian cells, using western blot analysis, dot blot, and Galanthus nivalis agglutinin lectin capture enzyme immunoassay. Replacing the peptides preceding E1 and E2 with the signal peptides of the tissue plasminogen activator and Gaussia luciferase resulted in maximum enhancement of E1/E2 expression and secretion of E1 in mammalian cells, without altering glycosylation. Such an advance would help to facilitate both the research of E1/E2 biology and the development of an effective HCV subunit vaccine. The strategy used in this study could be applied to the expression and production of other glycoproteins in mammalian cell line-based systems.

Transient expression of human TorsinA enhances secretion of two functionally distinct proteins in cultured Chinese hamster ovary (CHO) cells

Cultured mammalian cells, particularly Chinese hamster ovary (CHO) cells, are widely exploited as hosts for the production of recombinant proteins, but often yields are limiting. Such limitations may be due in part to the misfolding and subsequent degradation of the heterologous proteins. Consequently we have determined whether transiently co-expressing yeast and/or mammalian chaperones that act to disaggregate proteins, in CHO cell lines, improve the levels of either a cytoplasmic (Fluc) or secreted (Gluc) form of luciferase or an immunoglobulin IgG4 molecule. Over-expression of the yeast 'protein disaggregase' Hsp104 in a CHO cell line increased the levels of Fluc more significantly than for Gluc although levels were not further elevated by over-expression of the yeast or mammalian Hsp70/40 chaperones. Over-expression of TorsinA, a mammalian protein related in sequence to yeast Hsp104, but located in the ER, significantly increased the level of secreted Gluc from CHO cells by 2.5-fold and to a lesser extent the secreted levels of a recombinant IgG4 molecule. These observations indicate that the over-expression of yeast Hsp104 in mammalian cells can improve recombinant protein yield and that over-expression of TorsinA in the ER can promote secretion of heterologous proteins from mammalian cells.

The Feynman trajectories: determining the path of a protein using fixed-endpoint assays

Richard Feynman postulated in 1948 that the path of an electron can be best described by the sum or functional integral of all possible trajectories rather than by the notion of a single, unique trajectory. As a consequence, the position of an electron does not harbor any information about the paths that contributed to this position. This observation constitutes a classical endpoint observation. The endpoint assay is the desired type of experiment for high-throughput screening applications, mainly because of limitations in data acquisition and handling. Quite contrary to electrons, it is possible to extract information about the path of a protein using endpoint assays, and these types of applications are reviewed in this article.

Human signal peptide had advantage over mouse in secretory expression

The signal peptide is a critical component in the secretory expression of protein in eukaryotic cells. It has been verified that the signal peptide of mouse nerve growth factor could mediate the secretory expression of beta-endorphin in cultured non-neuronal cells. Although there is a counterpart of nerve growth factor in human genome, no research about the signal sequence from human genome has been reported. The function of mediating secretory expression is affected by many factors. We assumed that the counterpart from human genome could function as the signal peptide from mouse nerve growth factor does and these two signal sequences had different efficiency in mediating secretory expression of beta-endorphin, but we could not figure out which one had a better function. To validate our hypothesis and give an answer to the question, we constructed two eukaryotic vectors, pcDNA3.1-hEP and pcDNA3.1-mEP, containing human and mouse signal sequences in fusion genes, respectively. RT-PCR showed that the constructed fusion genes were expressed in NIH3T3 cells. We also found that the detected beta-endorphin by the immunofluorescent technique was mainly in the cytoplasm of NIH3T3 cells. The concentration of beta-endorphin in the culture medium by RIA is 280.33 +/- 24.16 (pg/ml) and 191.04 +/- 7.96 (pg/ml) from pcDNA3.1-hEP and pcDNA3.1-mEP, respectively, and there was a significant statistical difference between them (P < 0.05). A difference existed between them and that from blank vector individually (P < 0.01). These findings suggest that our constructed fusion gene containing the signal sequence of human nerve growth factor can be secretorily expressed and the efficiency of the signal peptide from human nerve growth factor is higher than that of mouse signal peptide.

A cell-based luciferase assay amenable to high-throughput screening of inhibitors of arenavirus budding

Several arenaviruses cause hemorrhagic fever (HF) disease in humans for which there are no licensed vaccines, and current therapy is limited to the use of ribavirin (Rib) that is only partially effective and associated with significant side effects. In addition, compelling evidence indicates that the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected human pathogen of clinical significance. Therefore, it is important to develop novel and effective anti-arenaviral drugs. The arenavirus Z protein is the driving force of arenavirus budding, and PPPY and PTAP late (L) domain motifs within Z are critical for Z-mediated budding, which involves the interaction of Z with a variety of host cellular factors. Compounds capable of inhibiting these virus-host cell interactions represent candidate anti-arenaviral drugs. The identification of these candidate compounds would be facilitated by the availability of a Z budding assay amenable to high-throughput screens (HTS). To this end, we have developed a novel assay that allows for rapid and quantitative assessment of Z-mediated budding. We provide evidence that this novel assay is amenable to HTS to identify small molecule inhibitors of Z-mediated budding, as well as to uncover cellular genes contributing to arenavirus budding.

The absence of effect of gene copy number and mRNA level on the amount of mAb secretion from mammalian cells

Recombinant human antibody production represents a major growing class of biopharmaceuticals based on the technological progress within the last decades especially in CHO cells. The HIV neutralizing human monoclonal antibody 2F5 was developed as hybridoma from human lymphocyte preparations. In order to estimate the potential of recombinant 2F5-expressing CHO cells, we generated different recombinant CHO cell lines by varying regulatory sequences, the codon usage, the signal peptides, and the transfection technique. These 2F5-expressing cell lines were developed by selection of the best producer, clone homogeneity, and clone stability. The gene copy number of the clones differed significantly due to methotrexate amplification. In one cell line, we identified only one copy of heavy chain and two copies of light chain. Neither the gene copy number nor the promoter was found to influence the amount of transcript exclusively emphasizing the positioning effect of the transgene. Messenger RNA levels were highest in 2F5/CO and may have resulted from a combination of the promoter and codon-optimized sequences, but unexpectedly, the amount of secreted product was not elevated in this configuration. In our example, translational and post-translational limitations are responsible for decreased antibody secretion.

Quantitation of autophagy by luciferase release assay

Autophagy is a cellular process that has been defined and analyzed almost entirely by qualitative measures. In no small part, this is attributable to the absence of robust quantitative assays that can easily and reliably permit the progress of key steps in autophagy to be assessed. We have recently developed a cell-based assay that specifically measures proteolytic cleavage of a tripartite sensor protein by the autophagy protease ATG4B. Activation of ATG4B results in release of Gaussia luciferase from cells that can be non-invasively harvested from cellular supernatants. Here, we compare this technique to existing methods and propose that this type of assay will be suitable for genome-wide functional screens and in vivo analysis of autophagy.

Cell-free metabolic engineering promotes high-level production of bioactive Gaussia princeps luciferase

Due to its small size and intense luminescent signal, Gaussia princeps luciferase (GLuc) is attractive as a potential imaging agent in both cell culture and small animal research models. However, recombinant GLuc production using in vivo techniques has only produced small quantities of active luciferase, likely due to five disulfide bonds being required for full activity. Cell-free biology provides the freedom to control both the catalyst and chemical compositions in biological reactions, and we capitalized on this to produce large amounts of highly active GLuc in cell-free reactions. Active yields were improved by mutating the cell extract source strain to reduce proteolysis, adjusting reaction conditions to enhance oxidative protein folding, further activating energy metabolism, and encouraging post-translational activation. This cell-free protein synthesis procedure produced 412mug/mL of purified GLuc, relative to 5mug/mL isolated for intracellular Escherichia coli expression. The cell-free product had a specific activity of 4.2x10(24)photons/s/mol, the highest reported activity for any characterized luciferase.

A pathway sensor for genome-wide screens of intracellular proteolytic cleavage

A new system based on non-conventional secretion of the luciferase from Gaussia princeps (GLUC) can be used to detect intracellular proteolysis in vivo.

Protein cleavage is a central event in many regulated biological processes. We describe a system for detecting intracellular proteolysis based on non-conventional secretion of Gaussia luciferase (GLUC). GLUC exits the cell without benefit of a secretory leader peptide, but can be anchored in the cell by fusion to β-actin. By including protease cleavage sites between GLUC and β-actin, proteolytic cleavage can be detected. Using this assay, we have identified regulators of autophagy, apoptosis and β-actin cleavage.

Vectors Encoding Seven Oikosin Signal Peptides Transfected into CHO Cells Differ Greatly in Mediating Gaussia luciferase and Human Endostatin Production although mRNA Levels are Largely Unaffected

The signal peptide of the luciferase secreted by the marine copepod Gaussia princeps has been shown to promote high-level protein synthesis/secretion of recombinant proteins, being far superior to mammalian counterparts. The main aim of the present study was to investigate the effects of seven selected signal peptides derived from oikosins, house proteins of the marine organism Oikopleura dioica, on synthesis/secretion of recombinant proteins. Vector constructs were made in which the coding regions of two naturally secreted proteins, Gaussia luciferase and human endostatin (hEndostatin), were “seamlessly” fused to the signal peptide coding sequences of interest. CHO cells were transfected with the plasmids and populations of stably transfected cells established. The amounts of reporter proteins in cell extract and medium samples were determined and the results compared to those obtained from cells stably transfected with a reference vector construct. In addition, the amounts of luciferase or hEndostatin encoding mRNAs in the cells were determined and related to the protein levels obtained. The levels of reporter protein produced varied greatly among the seven oikosin signal peptides tested. Whereas the oikosin 1 signal peptide resulted in about 40% production of Gaussia luciferase compared to the reference construct, oikosins 2–7 were extremely ineffective (<1%). mRNA levels were not dramatically affected such that inadequate availability of transcript for translation was not the underlying reason for the observations. The oikosin 1 signal peptide was also the most effective regarding synthesis/secretion of hEndostatin. No secreted product was observed using the oikosin 3 signal peptide. Interestingly, the molecular weight of hEndostatin in cell extracts prepared from cells transfected with oikosin 2 and 3 constructs was higher than that using the oikosin 1 signal peptide. The overall findings indicate that the signal peptide affects the efficiency of protein synthesis and secretion through a mechanism operating at the post-transcriptional level. The results described here provide substantial support to our previous observations which suggested that the choice of the signal peptide is imperative when aiming to achieve optimal synthesis and secretion of a recombinant protein using transfected mammalian cells.