BiP and PDI cooperate in the oxidative folding of antibodies in vitro

Quantitative proteomics reveals cellular responses to individual mAb expression and tunicamycin in CHO cells

Chinese hamster ovary (CHO) cells are popular in the pharmaceutical industry for their ability to produce high concentrations of antibodies and their resemblance to human cells in terms of protein glycosylation patterns. Current data indicate the relevance of CHO cells in the biopharmaceutical industry, with a high number of product commendations and a significant market share for monoclonal antibodies. To enhance the production capabilities of CHO cells, a deep understanding of their cellular and molecular composition is crucial. Genome sequencing and proteomic analysis have provided valuable insights into the impact of the bioprocessing conditions, productivity, and product quality. In our investigation, we conducted a comparative analysis of proteomic profiles in high and low monoclonal antibody-producing cell lines and studied the impact of tunicamycin (TM)-induced endoplasmic reticulum (ER) stress. We examined the expression levels of different proteins including unfolded protein response (UPR) target genes by using label-free quantification techniques for protein abundance. Our results show the upregulation of proteins associated with protein folding mechanisms in low producer vs. high producer cell line suggesting a form of ER stress related to specific protein production. Further, Hspa9 and Dnaja3 are notable candidates activated by the mitochondria UPR and play important roles in protein folding processes in mitochondria. We identified significant upregulation of Nedd8 and Lgmn proteins in similar levels which may contribute to UPR stress. Interestingly, the downregulation of Hspa5/Bip and Pdia4 in response to tunicamycin treatment suggests a low-level UPR activation. KEY POINTS: • Proteome profiling of recombinant CHO cells under mild TM treatment. • Identified protein clusters are associated with the unfolded protein response (UPR). • The compared cell lines revealed noticeable disparities in protein expression levels.

Animal-free production of hen egg ovalbumin in engineered Saccharomyces cerevisiae via precision fermentation

To enable the sustainable production of ovalbumin (OVA) without relying on animal sources, the generally recognized as safe (GRAS) host Saccharomyces cerevisiae was used for the precision fermentation-based production of recombinant OVA. For this purpose, a signal peptide derived from EPX1, the most abundant extracellular protein produced by Pichia pastoris, was identified as a novel signal peptide for the efficient secretion of OVA in S. cerevisiae. To improve OVA secretion and cell growth, three helper proteins (PDI1, KAR2, and HAC1) present in the endoplasmic reticulum were expressed individually or in combination. Notably, the +P1/K2 strain coexpressing PDI1 and KAR2 with OVA produced 2 mg/L of OVA in the medium fraction; this value was 2.6-fold higher than the corresponding value for the control strain without helper proteins. Finally, a glucose-limited fed-batch fermentation process using the +P1/K2 strain yielded 132 mg/L of total OVA with 8 mg/L of extracellular OVA.

Gene copy number, gene configuration and LC/HC mRNA ratio impact on antibody productivity and product quality in targeted integration CHO cell lines

The augmentation of transgene copy numbers is a prevalent approach presumed to enhance transcriptional activity and product yield. CHO cell lines engineered via targeted integration (TI) offer an advantageous platform for investigating the interplay between gene copy number, mRNA abundance, product yield, and product quality. Our investigation revealed that incrementally elevating the gene copy numbers of both IgG heavy chain (HC) and light chain (LC) concurrently resulted in the attainment of plateaus in mRNA levels and product titers, notably occurring beyond four to five gene copies integrated at the same TI site. Furthermore, maintaining a fixed gene copy number while varying the position of genes within the vector influenced the LC/HC mRNA ratio, which subsequently exerted a substantial impact on product titer. Moreover, manipulation of the LC/HC gene ratio through the introduction of surplus LC gene copies led to heightened LC mRNA expression and a reduction in the levels of high molecular weight species. It is noteworthy that the effects of excess LC on product titer were dependent on the specific molecule under consideration. The strategic utilization of PCR tags enabled precise quantification of transcription from each expression slot within the vector, facilitating the identification of highly expressive and less expressive slots. Collectively, these findings significantly enhance our understanding of stable antibody production in TI CHO cell lines.

Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here, we clarify the mechanisms underlying Vg1 retention, processing, secretion, and signaling and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: Chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as an in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers.

1,25(OH)2 D3 induced vitamin D receptor signaling negatively regulates endoplasmic reticulum-associated degradation (ERAD) and androgen receptor signaling in human prostate cancer cells

Steroid hormone signaling is critical in the tumor progression and the regulation of physiological mechanisms such as endoplasmic reticulum-associated degradation (ERAD) and unfolded protein response (UPR) in prostate cancer. 1,25(OH)₂ D₃ is an active metabolite of vitamin D classified as a steroid hormone. It exhibits anti-tumor effects, including angiogenesis and suppression of cell cycle progression. Moreover, progressively reducing expression levels of vitamin D receptor (VDR) are observed in many cancer types, including the prostate. In the present study, we investigated the molecular action of 1,25(OH)₂ D₃ on ERAD, UPR and androgenic signaling. We found that 1,25(OH)₂ D₃ negatively regulated the expression level of ERAD components and divergently controlled the inositol-requiring enzyme 1⍺ (IRE1⍺) and protein kinase RNA-like ER kinase (PERK) branches of UPR in LNCaP human prostate cancer cells. Also, similar results were obtained with another human prostate cancer cell line, 22Rv1. More strikingly, we found that androgenic signaling is negatively regulated by VDR signaling. Also, molecular docking supported the inhibitory effect of 1,25(OH)₂ D₃ on AR signaling. Moreover, we found VDR signaling suppressed tumor progression by decreasing c-Myc expression and reducing the epithelial-mesenchymal transition (EMT). Additionally, 1,25(OH)₂ D₃ treatment significantly inhibited the 3D-tumor formation of LNCaP cells. Our results suggest that further molecular characterization of the action of VDR signaling in other cancer types such as estrogenic signal in breast cancer will provide important contributions to a better understanding of the roles of steroid hormone receptors in carcinogenesis processes.

Endoplasmic reticulum stress and ubiquitin-proteasome system impairment in natural scrapie

Chronic accumulation of misfolded proteins such as PrP^Sc can alter the endoplasmic reticulum homeostasis triggering the unfolded protein response (UPR). In this pathogenic event, the molecular chaperones play an important role. Several reports in humans and animals have suggested that neurodegeneration is related to endoplasmic reticulum stress in diseases caused by the accumulation of misfolded proteins. In this study, we investigated the expression of three endoplasmic reticulum stress markers: PERK (protein kinase R-like endoplasmic reticulum kinase), BiP (binding immunoglobulin protein), and PDI (Protein Disulfide Isomerase). In addition, we evaluated the accumulation of ubiquitin as a marker for protein degradation mediated by the proteasome. These proteins were studied in brain tissues of sheep affected by scrapie in clinical and preclinical stages of the disease. Results were compared with those observed in healthy controls. Scrapie-infected sheep showed significant higher levels of PERK, BiP/Grp78 and PDI than healthy animals. As we observed before in models of spontaneous prion disease, PDI was the most altered ER stress marker between scrapie-infected and healthy sheep. Significantly increased intraneuronal and neuropil ubiquitinated deposits were observed in certain brain areas in scrapie-affected animals compared to controls. Our results suggest that the neuropathological and neuroinflammatory phenomena that develop in prion diseases cause endoplasmic reticulum stress in brain cells triggering the UPR. In addition, the significantly higher accumulation of ubiquitin aggregates in scrapie-affected animals suggests an impairment of the ubiquitin-proteasome system in natural scrapie. Therefore, these proteins may contribute as biomarkers and/or therapeutic targets for prion diseases.

Proteinopathies: Deciphering Physiology and Mechanisms to Develop Effective Therapies for Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a cluster of diseases marked by progressive neuronal loss, axonal transport blockage, mitochondrial dysfunction, oxidative stress, neuroinflammation, and aggregation of misfolded proteins. NDs are more prevalent beyond the age of 50, and their symptoms often include motor and cognitive impairment. Even though various proteins are involved in different NDs, the mechanisms of protein misfolding and aggregation are very similar. Recently, several studies have discovered that, like prions, these misfolded proteins have the inherent capability of translocation from one neuron to another, thus having far-reaching implications for understanding the processes involved in the onset and progression of NDs, as well as the development of innovative therapy and diagnostic options. These misfolded proteins can also influence the transcription of other proteins and form aggregates, tangles, plaques, and inclusion bodies, which then accumulate in the CNS, leading to neuronal dysfunction and neurodegeneration. This review demonstrates protein misfolding and aggregation in NDs, and similarities and differences between different protein aggregates have been discussed. Furthermore, we have also reviewed the disposal of protein aggregates, the various molecular machinery involved in the process, their regulation, and how these molecular mechanisms are targeted to build innovative therapeutic and diagnostic procedures. In addition, the landscape of various therapeutic interventions for targeting protein aggregation for the effective prevention or treatment of NDs has also been discussed.

Vitexin Mitigates Staphylococcus aureus-Induced Mastitis via Regulation of ROS/ER Stress/NF-κB/MAPK Pathway

Mastitis, caused by a variety of pathogenic microorganisms, seriously threatens the safety and economic benefits of the dairy industry. Vitexin, a flavone glucoside found in many plant species, has been widely reported to have antioxidant, anti-inflammatory, antiviral, anticancer, neuroprotective, and cardioprotective effects. However, few studies have explored the effect of vitexin on mastitis. This study is aimed at exploring whether the antioxidant and anti-inflammatory functions of vitexin can improve Staphylococcus aureus-induced mastitis and its possible molecular mechanism. The expression profiles of S. aureus-infected bovine mammary epithelial cells and gland tissues from the GEO data set (GSE94056 and GSE139612) were analyzed and found that DEGs were mainly involved in immune signaling pathways, apoptosis, and ER stress through GO and KEGG enrichment. Vitexin blocked the production of ROS and increased the activity of antioxidant enzymes (SOD, GSH-PX, and CAT) via activation of PPARγ in vivo and in vitro. In addition, vitexin reduced the production of inflammatory cytokines (TNF-α, IL-1β, and IL-6) and inhibited apoptosis in MAC-T cells and mouse mammary tissues infected with Staphylococcus aureus. Moreover, vitexin decreased the expression of PDI, Ero1-Lα, p-IRE1α, PERK, p-eIF2α, and CHOP protein but increased BiP in both mammary gland cells and tissues challenged by S. aureus. Western blot results also found that the phosphorylation levels of JNK, ERK, p38, and p65 were reduced in vitexin-treated tissues and cells. Vitexin inhibited the production of ROS through promoting PPARγ, increased the activity of antioxidant enzymes, and reduced inflammatory cytokines and apoptosis by alleviating ER stress and inactivation MAPKs and NF-κB signaling pathway. Vitexin maybe have great potential to be a preventive and therapeutic agent for mastitis.

The secretory pathway - the key for unlocking the potential of Chinese hamster ovary cell factories for manufacturing therapeutic proteins

Mammalian cell factories (in particular the CHO cell system) have been crucial in the rise of biopharmaceuticals. Mammalian cells have compartmentalized organelles where intricate networks of proteins manufacture highly sophisticated biopharmaceuticals in a specialized production pipeline - the secretory pathway. In the bioproduction context, the secretory pathway functioning is key for the effectiveness of cell factories to manufacture these life-changing medicines. This review describes the molecular components and events involved in the secretory pathway, and provides a comprehensive summary of the intracellular steps limiting the production of therapeutic proteins as well as the achievements in engineering CHO cell secretory machinery. We also consider antibody-producing plasma cells (so called "professional" secretory cells) to explore the mechanisms underpinning their unique secretory function/features. Such understandings offer the potential to further enhancement of the current CHO cell production platforms for manufacturing next generation of biopharmaceuticals.

Identifying Interaction Partners of Yeast Protein Disulfide Isomerases Using a Small Thiol-Reactive Cross-Linker: Implications for Secretory Pathway Proteostasis

Protein disulfide isomerases (PDIs) function in forming the correct disulfide bonds in client proteins, thereby aiding the folding of proteins that enter the secretory pathway. Recently, several PDIs have been identified as targets of organic electrophiles, yet the client proteins of specific PDIs remain largely undefined. Here, we report that PDIs expressed in Saccharomyces cerevisiae are targets of divinyl sulfone (DVSF) and other thiol-reactive protein cross-linkers. Using DVSF, we identified the interaction partners that were cross-linked to Pdi1 and Eug1, finding that both proteins form cross-linked complexes with other PDIs, as well as vacuolar hydrolases, proteins involved in cell wall biosynthesis and maintenance, and many ER proteostasis factors involved ER stress signaling and ER-associated protein degradation (ERAD). The latter discovery prompted us to examine the effects of DVSF on ER quality control, where we found that DVSF inhibits the degradation of the ERAD substrate CPY*, in addition to covalently modifying Ire1 and blocking the activation of the unfolded protein response. Our results reveal that DVSF targets many proteins within the ER proteostasis network and suggest that these proteins may be suitable targets for covalent therapeutic development in the future.

New insights into the mechanism of action of the cyclopalladated complex - CP2 in Leishmania: Calcium Dysregulation, Mitochondrial Dysfunction and Cell Death

The current treatment of leishmaniasis is based on few drugs that present several drawbacks such as high toxicity, difficult administration route, and low efficacy. These disadvantages raise the necessity to develop novel antileishmanial compounds allied to a comprehensive understanding of their mechanisms of action. Here, we elucidate the probably mechanism of action of the antileishmanial binuclear cyclopalladated complex [Pd(dmba)(μ-N₃)]₂ (CP2) in Leishmania amazonensis. CP2 causes oxidative stress in the parasite resulting in disruption of mitochondrial Ca²⁺ homeostasis, cell cycle arrest at S-phase, increasing the ROS production and overexpression of stress-related and cell detoxification proteins, collapsing the Leishmania mitochondrial membrane potential and promotes apoptotic-like features in promastigotes leading to necrosis or directs programmed cell death (PCD)-committed cells toward necrotic-like destruction. Moreover, CP2 is able to reduce the parasite load in both liver and spleen in Leishmania infantum-infected hamsters when treated for 15 days with 1.5 mg/Kg/day CP2, expanding its potential application in addition to the already known effectiveness on cutaneous leishmaniasis for the treatment of visceral leishmaniasis, showing the broad spectrum of action of this cyclopalladated complex. The data herein presented bring new insights into the CP2 molecular mechanisms of action, assisting to promote its rational modification to improve both safety and efficacy.

Production of antibodies in SHuffle Escherichia coli strains

Antibodies are globally important macromolecules, used for research, diagnostics, and as therapeutics. The common mammalian antibody immunoglobulin G (IgG) is a complex glycosylated macromolecule, composed of two heavy chains and two light chains held together by multiple disulfide bonds. For this reason, IgG and related antibody fragments are usually produced through secretion from mammalian cell lines, such as Chinese Hamster Ovary cells. However, there is growing interest in production of antibodies in prokaryotic systems due to the potential for rapid and cheap production in a highly genetically manipulable system. Research on oxidative protein folding in prokaryotes has enabled engineering of Escherichia coli strains capable of producing IgG and other disulfide bonded proteins in the cytoplasm, known as SHuffle. In this protocol, we provide a review of research on prokaryotic antibody production, guidelines on cloning of antibody expression constructs, conditions for an initial expression and purification experiment, and parameters which may be optimized for increased purification yields. Last, we discuss the limitations of prokaryotic antibody production, and highlight potential future avenues for research on antibody expression and folding.

Heavy chain dimers stabilized by disulfide bonds are required to promote in vitro assembly of trastuzumab

BACKGROUND

Monoclonal antibodies (mAbs) and their derivatives have become one of the most important classes of therapeutic drugs. Their multiple applications increased the interest for understanding their complex structure. In vivo, animal cells are able to fold mAbs correctly (Song et al, J Biosci Bioeng 110:135-40, 2010), whereas previous in vitro approaches were scarce and mostly unsuccessful.

RESULTS

In this work, we compared in vitro assembly characteristics of trastuzumab, produced either by A) physical separation and refolding of its sub-units or B) direct joining of individually produced heavy and light chains. Native and denatured structures of trastuzumab were determined by SEC-HPLC, HIC-HPLC and SDS-PAGE.

CONCLUSIONS

Our results demonstrate the requirement of correctly folded HC, forming disulfide-bonded dimers, in order to form a fully functional mAb. Otherwise, the unfolded HC tend to precipitate. We were able to assemble trastuzumab in this fashion by only mixing them to LC in pH-buffered conditions, while monomeric HC structure was too unstable to render a functional mAb. This approach has been used in the generation of homogeneous ADC, with results pending to be published.

Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges

: Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.

Functional principles and regulation of molecular chaperones

To be able to perform their biological function, a protein needs to be correctly folded into its three dimensional structure. The protein folding process is spontaneous and does not require the input of energy. However, in the crowded cellular environment where there is high risk of inter-molecular interactions that may lead to protein molecules sticking to each other, hence forming aggregates, protein folding is assisted. Cells have evolved robust machinery called molecular chaperones to deal with the protein folding problem and to maintain proteins in their functional state. Molecular chaperones promote efficient folding of newly synthesized proteins, prevent their aggregation and ensure protein homeostasis in cells. There are different classes of molecular chaperones functioning in a complex interplay. In this review, we discuss the principal characteristics of different classes of molecular chaperones, their structure-function relationships, their mode of regulation and their involvement in human disorders.

Characterization of an A-Site Selective Protein Disulfide Isomerase A1 Inhibitor

Protein disulfide isomerase A1 (PDIA1) is an endoplasmic reticulum (ER)-localized thiol-disulfide oxidoreductase that is an important folding catalyst for secretory pathway proteins. PDIA1 contains two active-site domains (a and a'), each containing a Cys-Gly-His-Cys (CGHC) active-site motif. The two active-site domains share 37% sequence identity and function independently to perform disulfide-bond reduction, oxidation, and isomerization. Numerous inhibitors for PDIA1 have been reported, yet the selectivity of these inhibitors toward the a and a' sites is poorly characterized. Here, we identify a potent and selective PDIA1 inhibitor, KSC-34, with 30-fold selectivity for the a site over the a' site. KSC-34 displays time-dependent inhibition of PDIA1 reductase activity in vitro with a k_inact/ K_I of 9.66 × 10³ M^-1 s^-1 and is selective for PDIA1 over other members of the PDI family, and other cellular cysteine-containing proteins. We provide the first cellular characterization of an a-site selective PDIA1 inhibitor and demonstrate that KSC-34 has minimal sustained effects on the cellular unfolded protein response, indicating that a-site inhibition does not induce global protein folding-associated ER stress. KSC-34 treatment significantly decreases the rate of secretion of a destabilized, amyloidogenic antibody light chain, thereby minimizing pathogenic amyloidogenic extracellular proteins that rely on high PDIA1 activity for proper folding and secretion. Given the poor understanding of the contribution of each PDIA1 active site to the (patho)physiological functions of PDIA1, site selective inhibitors like KSC-34 provide useful tools for delineating the pathological role and therapeutic potential of PDIA1.

Molecular docking and molecular dynamics studies reveal structural basis of inhibition and selectivity of inhibitors EGCG and OSU-03012 toward glucose regulated protein-78 (GRP78) overexpressed in glioblastoma

Glioblastoma (GBM), a malignant form of brain tumor, has a high mortality rate. GRP78, one of the HSP70 protein family members, is overexpressed in GBM. GRP78 is the key chaperone protein involved in the unfolded protein response. Upregulated GRP78 expression in cancer cells inhibits apoptosis and promotes chemoresistance. GRP78 has an ATPase domain, a substrate-binding domain, and a linker region. ATP-competitive inhibitors such as EGCG and OSU-03012 inhibit GRP78 activity and reduce its expression in GBM. However, there is a lack of structural data on the binding modes of these inhibitors to GRP78 ATPase domain. Further, the mode of selectivity of these inhibitors toward GRP78 also is unknown. Toward this end, molecular docking was performed with AutoDock Vina and confirmation obtained by docking using ROSIE. The stability and MM-PBSA binding energy of GRP78-inhibitor complexes as well as energetic contribution of individual residues was analyzed by 50 ns molecular dynamics run with GROMACS. MSA by ClustalW2 identified unique amino acid residues in the ATPase domain of GRP78 which were different from the residues present in other HSP70 proteins. Important and unique amino acid residues of GRP78 such as Ile61, Glu293, Arg297, and Arg367 played a major role in the intermolecular interactions with these inhibitors. The interactions with unique residues of GRP78 as compared with those of HSP70-1A provided the basis for selectivity. It was found that the binding affinity and specificity/selectivity of EGCG toward GRP78 was higher than that toward HSP70-1A, and selectivity was even better than OSU-03012. OSU-03012 was predicted to bind to GRP78. Analyses from MD runs showed tight binding and stability of complexes, and the highest number of hydrogen bonds during the trajectory runs were comparable to those found in the docking studies. Energetic contribution of individual inhibitor-interacting residues showed that energy values of Ile61 and Glu293 were among the most negative. These studies are, to the best of our knowledge, the first studies characterizing EGCG and OSU-03012 interactions with GRP78 on a structural basis and provide a significant insight into their binding modes, selectivity, and structural stability.

Overexpression of Candida rugosa lipase Lip1 via combined strategies in Pichia pastoris

In this study, combined strategies were employed to heterologously overexpress Candida rugosa lipase Lip1 (CRL1) in a Pichia pastoris system. The LIP1 gene was systematically codon-optimized and synthesized in vitro. The Lip1 activity of a recombinant strain harboring three copies of the codon-optimized LIP1 gene reached 1200 U/mL in a shake flask culture. Higher lipase activity, 1450 U/mL, was obtained using a five copy number construct. Co-expressing one copy of the ERO1p and BiP chaperones with Lip1p, the CRL1 lipase yield further reached 1758 U/mL, which was significantly higher than that achieved by expressing Lip1p alone or only co-expressing one molecular chaperone. When cultivated in a 3 L fermenter under optimal conditions, the recombinant strain GS115/87-ZA-ERO1p-BiP #7, expressing the molecular chaperones Ero1p and BiP, produced 13,490 U/mL of lipase activity at 130 h, which was greater than the 11,400 U/mL of activity for the recombinant strain GS115/pAO815-α-mCRL1 #87, which did not express a molecular chaperone. This study indicates that a strategy of combining codon optimization with co-expression of molecular chaperones has great potential for the industrial-scale production of pure CRL1.

The nucleotide exchange factor SIL1 is required for glucose-stimulated insulin secretion from mouse pancreatic beta cells in vivo

AIMS/HYPOTHESIS

Regulation of insulin secretion along the secretory pathway is incompletely understood. We addressed the expression of SIL1, a nucleotide exchange factor for the endoplasmic reticulum (ER) chaperone glucose-regulated protein 78 kD (GRP78), in pancreatic beta cells and investigated whether or not SIL1 is involved in beta cell function.

METHODS

SIL1 expression was analysed by immunoblotting and immunofluorescence. Metabolic and islet variables, including glucose tolerance, beta cell mass, insulin secretion, islet ultrastructure, insulin content and levels of ER stress marker proteins, were addressed in Sil1 knockout (Sil1 (-/-)) mice. Insulin, proinsulin and C-peptide release was addressed in Sil1 (-/-) islets, and SIL1 overexpression or knockdown was explored in MIN6 cells in vitro. Models of type 1 diabetes and insulin resistance were induced in Sil1 (-/-) mice by administration of streptozotocin (STZ) and a high-fat diet (HFD), respectively.

RESULTS

We show that SIL1 is expressed in pancreatic beta cells and is required for islet insulin content, islet sizing, glucose tolerance and glucose-stimulated insulin secretion in vivo. Levels of pancreatic ER stress markers are increased in Sil1 (-/-) mice, and Sil1 (-/-) beta cell ER is ultrastructurally compromised. Isolated Sil1 (-/-) islets show lower proinsulin and insulin content and impaired glucose-stimulated insulin secretion. Modulation of SIL1 protein levels in MIN6 cells correlates with changes in insulin content and secreted insulin. Furthermore, Sil1 (-/-) mice are more susceptible to STZ-induced type 1 diabetes with increased apoptosis. Upon HFD feeding, Sil1 (-/-) mice show markedly lower insulin secretion and exacerbated glucose intolerance compared with control mice. Surprisingly, however, HFD-fed Sil1 (-/-) mice display pronounced islet hyperplasia with low amounts of insulin in total pancreas.

CONCLUSIONS/INTERPRETATION

These results reveal a novel role for the nucleotide exchange factor SIL1 in pancreatic beta cell function under physiological and disease conditions such as diabetes and the metabolic syndrome.

Principles and engineering of antibody folding and assembly

Antibodies are uniquely suited to serve essential roles in the human immune defense as they combine several specific functions in one hetero-oligomeric protein. Their constant regions activate effector functions and their variable domains provide a stable framework that allows incorporation of highly diverse loop sequences. The combination of non-germline DNA recombination and mutation together with heavy and light chain assembly allows developing variable regions that specifically recognize essentially any antigen they may encounter. However, this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully controlled before the protein is secreted from a plasma cell. Accordingly, the generic immunoglobulin fold β-barrel structure of antibody domains has been fine-tuned during evolution to fit the different requirements. Work over the past decades has identified important aspects of the folding and assembly of antibody domains and chains revealing domain specific variations of a general scheme. The most striking is the folding of an intrinsically disordered antibody domain in the context of its partner domain as the basis for antibody assembly and its control on the molecular level in the cell. These insights have not only allowed a better understanding of the antibody folding process but also provide a wealth of opportunities for rational optimization of antibody molecules. In this review, we summarize current concepts of antibody folding and assembly and discuss how they can be utilized to engineer antibodies with improved performance for different applications. This article is part of a Special Issue entitled: Recent advances in the molecular engineering of antibodies.

Age-enhanced endoplasmic reticulum stress contributes to increased Atg9A inhibition of STING-mediated IFN-β production during Streptococcus pneumoniae infection

Pneumococcal infections remain a leading cause of death in persons ≥ 65 y of age. Recent reports have illustrated detrimental changes in the endoplasmic reticulum stress response or unfolded protein response in aging and age-related diseases; however, the relationship between aging, the unfolded protein response, and innate immune responses to Streptococcus pneumoniae has not been fully elucidated. Our results illustrate that stimulator of IFN genes-mediated production of IFN-β during S. pneumoniae infection is decreased in aged hosts. Enhanced endoplasmic reticulum stress in response to S. pneumoniae augmented inositol-requiring protein 1/X-box binding protein 1-mediated production of autophagy-related gene 9 (Atg9a). Knockdown of Atg9a or treatment with gemcitabine HCl resulted in enhanced stimulator of IFN genes-mediated production of IFN-β by aged macrophages. Consecutive treatments with gemcitabine during in vivo S. pneumoniae infection decreased morbidity and mortality in aged hosts, which was associated with decreased Atg9a expression, increased IFN-β production, and improved bacterial clearance from lung tissue. Taken together, data presented in this study provide new evidence as to why older persons are more susceptible to S. pneumoniae, and provide a possible mechanism to enhance these responses, thereby decreasing morbidity and mortality in this population.

Engineering toward a bacterial "endoplasmic reticulum" for the rapid expression of immunoglobulin proteins

Antibodies are well-established as therapeutics, and the preclinical and clinical pipeline of these important biologics is growing rapidly. Consequently, there is considerable interest in technologies to engineer and manufacture them. Mammalian cell culture is commonly used for production because eukaryotic expression systems have evolved complex and efficient chaperone systems for the folding of antibodies. However, given the ease and manipulability of bacteria, antibody discovery efforts often employ bacterial expression systems despite their limitations in generating high titers of functional antibody. Open-Cell Free Synthesis (OCFS) is a coupled transcription-translation system that has the advantages of prokaryotic systems while achieving high titers of antibody expression. Due to the open nature of OCFS, it is easily modified by chemical or protein additives to improve the folding of select proteins. As such, we undertook a protein additive screen to identify chaperone proteins that improve the folding and assembly of trastuzumab in OCFS. From the screen, we identified the disulfide isomerase DsbC and the prolyl isomerase FkpA as important positive effectors of IgG folding. These periplasmic chaperones function synergistically for the folding and assembly of IgG, and, when present in sufficient quantities, gram per liter IgG titers can be produced. This technological advancement allows the rapid development and manufacturing of immunoglobulin proteins and pushes OCFS to the forefront of production technologies for biologics.

Modulation of conotoxin structure and function is achieved through a multienzyme complex in the venom glands of cone snails

The oxidative folding of large polypeptides has been investigated in detail; however, comparatively little is known about the enzyme-assisted folding of small, disulfide-containing peptide substrates. To investigate the concerted effect of multiple enzymes on the folding of small disulfide-rich peptides, we sequenced and expressed protein-disulfide isomerase (PDI), peptidyl-prolyl cis-trans isomerase, and immunoglobulin-binding protein (BiP) from Conus venom glands. Conus PDI was shown to catalyze the oxidation and reduction of disulfide bonds in two conotoxins, α-GI and α-ImI. Oxidative folding rates were further increased in the presence of Conus PPI with the maximum effect observed in the presence of both enzymes. In contrast, Conus BiP was only observed to assist folding in the presence of microsomes, suggesting that additional co-factors were involved. The identification of a complex between BiP, PDI, and nascent conotoxins further suggests that the folding and assembly of conotoxins is a highly regulated multienzyme-assisted process. Unexpectedly, all three enzymes contributed to the folding of the ribbon isomer of α-ImI. Here, we identify this alternative disulfide-linked species in the venom of Conus imperialis, providing the first evidence for the existence of a "non-native" peptide isomer in the venom of cone snails. Thus, ER-resident enzymes act in concert to accelerate the oxidative folding of conotoxins and modulate their conformation and function by reconfiguring disulfide connectivities. This study has evaluated the role of a number of ER-resident enzymes in the folding of conotoxins, providing novel insights into the enzyme-guided assembly of these small, disulfide-rich peptides.

A peptidic unconjugated GRP78/BiP ligand modulates the unfolded protein response and induces prostate cancer cell death

The molecular chaperone GRP78/BiP is a key regulator of protein folding in the endoplasmic reticulum, and it plays a pivotal role in cancer cell survival and chemoresistance. Inhibition of its function has therefore been an important strategy for inhibiting tumor cell growth in cancer therapy. Previous efforts to achieve this goal have used peptides that bind to GRP78/BiP conjugated to pro-drugs or cell-death-inducing sequences. Here, we describe a peptide that induces prostate tumor cell death without the need of any conjugating sequences. This peptide is a sequence derived from the cochaperone Bag-1. We have shown that this sequence interacts with and inhibits the refolding activity of GRP78/BiP. Furthermore, we have demonstrated that it modulates the unfolded protein response in ER stress resulting in PARP and caspase-4 cleavage. Prostate cancer cells stably expressing this peptide showed reduced growth and increased apoptosis in in vivo xenograft tumor models. Amino acid substitutions that destroyed binding of the Bag-1 peptide to GRP78/BiP or downregulation of the expression of GRP78 compromised the inhibitory effect of this peptide. This sequence therefore represents a candidate lead peptide for anti-tumor therapy.

Sub-proteomic study on macrophage response to Candida albicans unravels new proteins involved in the host defense against the fungus

In previous proteomic studies on the response of murine macrophages against Candida albicans, many differentially expressed proteins involved in processes like inflammation, cytoskeletal rearrangement, stress response and metabolism were identified. In order to look for proteins important for the macrophage response, but in a lower concentration in the cell, 3 sub-cellular extracts were analyzed: cytosol, organelle/membrane and nucleus enriched fractions from RAW 264.7 macrophages exposed or not to C. albicans SC5314 for 3 h. The samples were studied using DIGE technology, and 17 new differentially expressed proteins were identified. This sub-cellular fractionation permitted the identification of 2 mitochondrion proteins, a membrane receptor, Galectin-3, and some ER related proteins, that are not easily detected in total cell extracts. Besides, the study of different fractions allowed us to detect, not only total increase in Galectin-3 protein amount, but its distinct allocation along the interaction. The identified proteins are involved in the pro-inflammatory and oxidative responses, immune response, unfolded protein response and apoptosis. Some of these processes increase the host response and others could be the effect of C. albicans resistance to phagocytosis. Thus, the sub-proteomic approach has been a very useful tool to identify new proteins involved in macrophage-fungus interaction. This article is part of a Special Issue entitled: Translational Proteomics.

Protein disulfide isomerase-2 of Arabidopsis mediates protein folding and localizes to both the secretory pathway and nucleus, where it interacts with maternal effect embryo arrest factor

Protein disulfide isomerase (PDI) is a thiodisulfide oxidoreductase that catalyzes the formation, reduction and rearrangement of disulfide bonds in proteins of eukaryotes. The classical PDI has a signal peptide, two CXXC-containing thioredoxin catalytic sites (a,a'), two noncatalytic thioredoxin fold domains (b,b'), an acidic domain (c) and a C-terminal endoplasmic reticulum (ER) retention signal. Although PDI resides in the ER where it mediates the folding of nascent polypeptides of the secretory pathway, we recently showed that PDI5 of Arabidopsis thaliana chaperones and inhibits cysteine proteases during trafficking to vacuoles prior to programmed cell death of the endothelium in developing seeds. Here we describe Arabidopsis PDI2, which shares a primary structure similar to that of classical PDI. Recombinant PDI2 is imported into ER-derived microsomes and complements the E. coli protein-folding mutant, dsbA. PDI2 interacted with proteins in both the ER and nucleus, including ER-resident protein folding chaperone, BiP1, and nuclear embryo transcription factor, MEE8. The PDI2-MEE8 interaction was confirmed to occur in vitro and in vivo. Transient expression of PDI2-GFP fusions in mesophyll protoplasts resulted in labeling of the ER, nucleus and vacuole. PDI2 is expressed in multiple tissues, with relatively high expression in seeds and root tips. Immunoelectron microscopy with GFP- and PDI2-specific antisera on transgenic seeds (PDI2-GFP) and wild type roots demonstrated that PDI2 was found in the secretory pathway (ER, Golgi, vacuole, cell wall) and the nuclei. Our results indicate that PDI2 mediates protein folding in the ER and has new functional roles in the nucleus.

Targeting antibodies to the cytoplasm

A growing number of research consortia are now focused on generating antibodies and recombinant antibody fragments that target the human proteome. A particularly valuable application for these binding molecules would be their use inside a living cell, e.g., for imaging or functional intervention. Animal-derived antibodies must be brought into the cell through the membrane, whereas the availability of the antibody genes from phage display systems allows intracellular expression. Here, the various technologies to target intracellular proteins with antibodies are reviewed.

Comprehensive engineering of Escherichia coli for enhanced expression of IgG antibodies

The expression of IgG antibodies in Escherichia coli is of increasing interest for analytical and therapeutic applications. In this work, we describe a comprehensive and systematic approach to the development of a dicistronic expression system for enhanced IgG expression in E. coli encompassing: (i) random mutagenesis and high-throughput screening for the isolation of over-expressing strains using flow cytometry and (ii) optimization of translation initiation via the screening of libraries of synonymous codons in the 5' region of the second cistron (heavy chain). The effects of different promoters and co-expression of molecular chaperones on full-length IgG production were also investigated. The optimized system resulted in reliable expression of fully assembled IgG at yields between 1 and 4 mg/L of shake flask culture for different antibodies.

Diselenides as universal oxidative folding catalysts of diverse proteins

Small-molecule diselenides show considerable potential as catalysts of oxidative protein folding. To explore their scope, diselenide-containing redox buffers were used to promote the folding of proteins that varied in properties such as size, overall tertiary structure, number of disulfide bonds, pI value, and difficulty of in vitro folding. Diselenides are able to catalyze the oxidative folding of all proteins tested, providing significant increases in both rate and yield relative to analogous disulfides. Compared to the disulfide-linked dimer of glutathione (the most commonly used oxidant for in vitro protein folding), selenoglutathione provided markedly improved efficiencies in the folding of biotechnologically important proteins such as hirudin, lysozyme, human epidermal growth factor and interferon α-2a. Selenoglutathione also enhances the renaturation of more challenging targets such as bovine serum albumin, whose native state contains 17 disulfide bonds, and the Fab fragment of an antibody. In the latter case, micromolar amounts of selenoglutathione are able to match the modest yield provided by a previously optimized redox buffer, which contains millimolar levels of glutathione. Taken together, the folding reactions of these diverse proteins exemplify the advantages and limitations of diselenide catalysts.

Antibodies to the endoplasmic reticulum-resident chaperones calnexin, BiP and Grp94 in patients with rheumatoid arthritis and systemic lupus erythematosus

OBJECTIVES

To investigate the presence of autoantibodies against mammalian chaperones of the endoplasmic reticulum (ER) in patients with RA and other immune-mediated diseases.

METHODS

Sera from healthy donors, from early RA patients with two follow-up samples, patients with SLE, SSc and IBD were collected and analysed for anti-ER chaperone antibodies. Detection of serum IgG antibodies against immunoglobulin heavy chain binding protein (BiP), glucose-regulated protein 94 (Grp94) and calnexin was carried out using ELISA. The specificity of sera positive for individual ER chaperones was confirmed by immunoblotting. Statistical analysis was performed using Welch's t-test, Mann-Whitney U-test, partial correlation and Pearson's correlation.

RESULTS

In patients with RA and SLE, autoantibody titres against BiP, Grp94 and calnexin were significantly higher than those in healthy controls. These autoantibodies were detectable in patients with early RA and titres remained stable for at least 6-12 months. Also several SSc and IBD patients exhibited autoantibodies against these ER chaperones; however, titres and frequencies were lower than in RA or SLE patients. Furthermore, anti-calnexin antibodies correlated significantly with the presence of BiP and Grp94 autoantibodies in patients with RA and SLE.

CONCLUSION

Calnexin and Grp94 were identified as novel autoantigens in RA and calnexin in SLE. Since calnexin, Grp94 and BiP are ER-resident proteins of eukaryotic cells, our data suggest that autoantibody generation against ER chaperones is independent of initial exposure to the corresponding bacterial chaperones; rather, ER chaperones may represent genuine autoantigens.

Comparing the functional properties of the Hsp70 chaperones, DnaK and BiP

The Hsp70 family of molecular chaperones is an essential class of chaperones that is present in many different cell types and cellular compartments. We have compared the bioactivities of the prokaryotic cytosolic Hsp70, DnaK, to that of the eukaryotic Hsp70, BiP, located in the endoplasmic reticulum (ER). Both chaperones helped to prevent protein aggregation. However, only DnaK provided enhanced refolding of denatured proteins. We also tested chaperone folding assistance during translation in the context of cell-free protein synthesis reactions for several protein targets and show that both DnaK and BiP can provide folding assistance under these conditions. Our results support previous reports suggesting that DnaK provides both post-translational and co-translational folding assistance while BiP predominantly provides folding assistance that is contemporaneous with translation.

Protein quality control in the ER: the recognition of misfolded proteins

The mechanism, in molecular terms of protein quality control, specifically of how the cell recognizes and discriminates misfolded proteins, remains a challenge. In the secretory pathway the folding status of glycoproteins passing through the endoplasmic reticulum is marked by the composition of the N-glycan. The different glycoforms are recognized by specialized lectins. The folding sensor UGGT acts as an unusual molecular chaperone and covalently modifies the Man9 N-glycan of a misfolded protein by adding a glucose moiety and converts it to Glc1Man9 that rebinds the lectin calnexin. However, further links between the folding status of a glycoprotein and the composition of the N-glycan are unclear. There is little unequivocal evidence for other proteins in the ER recognizing the N-glycan and also acting as molecular chaperones. Nevertheless, based upon a few examples, we suggest that this function is carried out by individual proteins in several different complexes. Thus, calnexin binds the protein disulfide isomerase ERp57, that acts upon Glc1Man9 glycoproteins. In another example the protein disulfide isomerase ERdj5 binds specifically to EDEM (which is probably a mannosidase) and a lectin OS9, and reduces the disulfide bonds of bound glycoproteins destined for ERAD. Thus the glycan recognition is performed by a lectin and the chaperone function performed by a specific partner protein that can recognize misfolded proteins. We predict that this will be a common arrangement of proteins in the ER and that members of protein foldase families such as PDI and PPI will bind specifically to lectins in the ER. Molecular chaperones BiP and GRp94 will assist in the folding of proteins bound in these complexes as well as in the folding of non-glycoproteins.

pERp1 is significantly up-regulated during plasma cell differentiation and contributes to the oxidative folding of immunoglobulin

Plasma cells can synthesize and secrete thousands of Ig molecules per second, which are folded and assembled in the endoplasmic reticulum (ER) and are likely to place unusually high demands on the resident chaperones and folding enzymes. We have discovered a new resident ER protein (pERp1) that is a component of the BiP chaperone complex. PERp1 is substantially up-regulated during B to plasma cell differentiation and can be induced in B cell lines by some UPR activators, arguing that it represents a potentially new class of conditional UPR targets. In LPS-stimulated murine splenocytes, pERp1 interacted covalently via a disulfide bond with IgM monomers and noncovalently with other Ig assembly intermediates. Knockdown and overexpression experiments revealed that pERp1 promoted correct oxidative folding of Ig heavy chains and prevented off-pathway assembly intermediates. Although pERp1 has no homology with known chaperones or folding enzymes, it possesses a thioredoxin-like active site motif (CXXC), which is the signature of oxidoreductases. Mutation of this sequence did not affect its in vivo activity, suggesting that pERp1 is either a unique type of oxidoreductase or a previously unidentified class of molecular chaperone that is dedicated to enhancing the oxidative folding of Ig precursors.

Efficient production of human bivalent and trivalent anti-MUC1 Fab-scFv antibodies in Pichia pastoris

Background

Tumour associated antigens on the surface of tumour cells, such as MUC1, are being used as specific antibody targets for immunotherapy of human malignancies. In order to address the poor penetration of full sized monoclonal antibodies in tumours, intermediate sized antibodies are being developed. The cost-effective and efficient production of these molecules is however crucial for their further success as anti-cancer therapeutics. The methylotropic P. pastoris yeast grows in cheap mineral media and is known for its short process times and the efficient production of recombinant antibody fragments like scFvs, bivalent scFvs and Fabs.

Results

Based on the anti-MUC1 PH1 Fab, we have developed bivalent PH1 bibodies and trivalent PH1 tribodies of intermediate molecular mass by adding PH1 scFvs to the C-terminus of the Fab chains using flexible peptide linkers. These recombinant antibody derivatives were efficiently expressed in both mammalian and P. pastoris cells. Stable production in NS0 cells produced 130.5 mg pure bibody and 27 mg pure tribody per litre. This high yield is achieved as a result of the high overall purification efficiency of 77%. Expression and purification of PH1 bibodies and tribodies from Pichia supernatant yielded predominantly correctly heterodimerised products, free of light chain homodimers. The yeast-produced bi- and tribodies retained the same specific activity as their mammalian-produced counterparts. Additionally, the yields of 36.8 mg pure bibody and 12 mg pure tribody per litre supernatant make the production of these molecules in Pichia more efficient than most other previously described trispecific or trivalent molecules produced in E. coli.

Conclusion

Bi- and tribody molecules are efficiently produced in P. pastoris. Furthermore, the yeast produced molecules retain the same specific affinity for their antigen. These results establish the value of P. pastoris as an efficient alternative expression system for the production of recombinant multivalent Fab-scFv antibody derivatives.

An unfolded CH1 domain controls the assembly and secretion of IgG antibodies

A prerequisite for antibody secretion and function is their assembly into a defined quaternary structure, composed of two heavy and two light chains for IgG. Unassembled heavy chains are actively retained in the endoplasmic reticulum (ER). Here, we show that the C(H)1 domain of the heavy chain is intrinsically disordered in vitro, which sets it apart from other antibody domains. It folds only upon interaction with the light-chain C(L) domain. Structure formation proceeds via a trapped intermediate and can be accelerated by the ER-specific peptidyl-prolyl isomerase cyclophilin B. The molecular chaperone BiP recognizes incompletely folded states of the C(H)1 domain and competes for binding to the C(L) domain. In vivo experiments demonstrate that requirements identified for folding the C(H)1 domain in vitro, including association with a folded C(L) domain and isomerization of a conserved proline residue, are essential for antibody assembly and secretion in the cell.

On the Optimal Ratio of Heavy to Light Chain Genes for Efficient Recombinant Antibody Production by CHO Cells

Monoclonal antibodies (Mab) are heterotetramers consisting of an equimolar ratio of heavy chain (HC) and light chain (LC) polypeptides. Accordingly, most recombinant Mab expression systems utilize an equimolar ratio of heavy chain (hc) to light chain (lc) genes encoded on either one or two plasmids. However, there is no evidence to suggest that this gene ratio is optimal for stable or transient production of recombinant Mab. In this study we have determined the optimal ratio of hc:lc genes for production of a recombinant IgG4 Mab, cB72.3, by Chinese hamster ovary (CHO) cells using both empirical and mathematical modeling approaches. Polyethyleneimine-mediated transient expression of cB72.3 at varying ratios of hc:lc genes encoded on separate plasmids yielded an optimal Mab titer at a hc:lc gene ratio of 3:2; a conclusion confirmed by separate mathematical modeling of the Mab folding and assembly process using transient expression data. On the basis of this information, we hypothesized that utilization of hc genes at low hc:lc gene ratios is more efficient. To confirm this, cB72.3 Mab was transiently produced by CHO cells at constant hc and varying lc gene dose. Under these conditions, Mab yield was increased with a concomitant increase in lc gene dose. To determine if the above findings also apply to stably transfected CHO cells producing recombinant Mab, we compared the intra- and extracellular ratios of HC and LC polypeptides for three GS-CHO cells lines transfected with a 1:1 ratio of hc:lc genes and selected for stable expression of the same recombinant Mab, cB72.3. Intra- and extracellular HC:LC polypeptide ratios ranged from 1:2 to 1:5, less than that observed on transient expression of the same Mab in parental CHO cells using the same vector. In conclusion, our data suggest that the optimal ratio of hc:lc genes used for transient and stable expression of Mab differ. In the case of the latter, we infer that optimal Mab production by stably transfected cells represents a compromise between HC abundance limiting productivity and the requirement for excess LC to render Mab folding and assembly more efficient.

A top-down approach to mechanistic biological modeling: application to the single-chain antibody folding pathway

A top-down approach to mechanistic modeling of biological systems is presented and exemplified with the development of a hypothesis-driven mathematical model for single-chain antibody fragment (scFv) folding in Saccharomyces cerevisiae by mediators BiP and PDI. In this approach, model development starts with construction of the most basic mathematical model--typically consisting of predetermined or newly-elucidated biological behavior motifs--capable of reproducing desired biological behaviors. From this point, mechanistic detail is added incrementally and systematically, and the effects of each addition are evaluated. This approach follows the typical progression of experimental data availability in that higher-order, lumped measurements are often more prevalent initially than specific, mechanistic ones. It also necessarily provides the modeler with insight into the structural requirements and performance capabilities of the resulting detailed mechanistic model, which facilitates further analysis. The top-down approach to mechanistic modeling identified three such requirements and a branched dependency-degradation competition motif critical for the scFv folding model to reproduce experimentally observed scFv folding dependencies on BiP and PDI and increased production when both species are overexpressed and promoted straightforward prediction of parameter dependencies. It also prescribed modification of the guiding hypothesis to capture BiP and PDI synergy.

Synthesis and characterization of a functional intact IgG in a prokaryotic cell-free expression system

Antibodies are an important component of the immune system of higher eukaryotes. Furthermore, they are effective tools in basic research, medical diagnostics and therapy. Recombinant expression of these heterotetrameric, disulfide-bridged proteins is usually performed in mammalian cells. Here, we describe the cell-free expression of a mouse monoclonal antibody, MAK33, in a coupled transcription/translation system, based on an Escherichia coli lysate. Both the heavy and the light chain can be produced efficiently in this setup. However, they fail to form functional antibodies. With a view to overcome folding and oxidation defects, we supplemented the system with the oxidoreductases PDI (protein disulfide isomerase) and DsbC and the ER-specific chaperones Grp94 and BiP; furthermore, we optimized the redox conditions. We found that functional antibodies can only be obtained in the presence of an oxidoreductase. In contrast, the addition of Grp94 and/or BiP had no influence on the productive folding reaction. The comparison of the antibody expressed in vitro with MAK33 expressed in cell culture showed that the in vitro expressed antibody is correctly assembled, disulfide-bridged and shows identical antigen affinity. The stability of the in vitro expressed non-glycosylated IgG is comparable to that of the authentic antibody.

Folding of conotoxins: formation of the native disulfide bridges during chemical synthesis and biosynthesis of Conus peptides

Conopeptides from >700 species of predatory marine Conus snails provide an impressive molecular diversity of cysteine-rich peptides. Most of the estimated 50,000-100,000 distinct conopeptides range in size from 10 to 50 amino acid residues, often with multiple posttranslational modifications. The great majority contain from two to four disulfide bridges. As the biosynthetic and chemical production of this impressive repertoire of disulfide-rich peptides has been investigated, particularly the formation of native disulfide bridges, differences between in vivo and in vitro oxidative folding have become increasingly evident. In this article, we provide an overview of the molecular diversity of conotoxins with an emphasis on the cysteine patterns and disulfide frameworks. The conotoxin folding studies reviewed include regioselective and direct oxidation strategies, recombinant expression, optimization of folding methods, mechanisms of in vitro folding, and preliminary data on the biosynthesis of conotoxins in venom ducts. Despite these studies, how the cone snails efficiently produce properly folded conotoxins remains unanswered. As chemists continue to master oxidative folding techniques, insights gleaned from how conotoxins are folded in vivo will likely lead to the development of the new folding methods, as well as shed some light on fundamental mechanisms relevant to the protein folding problem.

The activities and function of molecular chaperones in the endoplasmic reticulum

Most proteins in the secretory pathway are translated, folded, and subjected to quality control at the endoplasmic reticulum (ER). These processes must be flexible enough to process diverse protein conformations, yet specific enough to recognize when a protein should be degraded. Molecular chaperones are responsible for this decision making process. ER associated chaperones assist in polypeptide translocation, protein folding, and ER associated degradation (ERAD). Nevertheless, we are only beginning to understand how chaperones function, how they are recruited to specific substrates and assist in folding/degradation, and how unique chaperone classes make quality control "decisions".

Effect of doxycycline-regulated protein disulfide isomerase expression on the specific productivity of recombinant CHO cells: thrombopoietin and antibody

Protein disulfide isomerase (PDI), one of the ER-resident molecular chaperones, forms and isomerizes disulfide bonds. This study attempts to investigate the effect of PDI expression level on specific productivity (q) of recombinant Chinese hamster ovary (rCHO) cells producing thrombopoietin (TPO) and antibody (Ab). To regulate the PDI expression level, the Tet-Off system was introduced in TPO and Ab producing CHO cells, and stable Tet-Off cells (TPO-Tet-Off and Ab-Tet-Off) were screened using the luciferase assay. The doxycycline-regulated PDI expression system in Tet-Off rCHO cells (Tet-TPO-PDI and Tet-Ab-PDI) was established by the cotransfection of pTRE-PDI and pTK-Hyg expression vector into TPO-Tet-Off and Ab-Tet-Off cells, respectively. Subsequent screening was done by Western blot analysis of PDI and an enzyme-linked immunosorbent assay of the secreted TPO and antibody. We cultured two Tet-TPO-PDI and two Tet-Ab-PDI clones, and all these clones showed an average of 2.5-fold increase in PDI expression when compared to the basal level. In both these cell lines the PDI expression was tightly controlled by various concentrations of doxycycline. The q of TPO (q(TPO)) was unaffected but that of antibody producing cells was increased by 15-27% due to the PDI expression level.