Stable mammalian producer cell lines for structural biology

SiMPl-GS: Advancing Cell Line Development via Synthetic Selection Marker for Next-Generation Biopharmaceutical Production

Rapid and efficient cell line development (CLD) process is essential to expedite therapeutic protein development. However, the performance of widely used glutamine-based selection systems is limited by low selection efficiency, stringency, and the inability to select multiple genes. Therefore, an AND-gate synthetic selection system is rationally designed using split intein-mediated protein ligation of glutamine synthetase (GS) (SiMPl-GS). Split sites of the GS are selected using a computational approach and validated with GS-knockout Chinese hamster ovary cells for their potential to enable cell survival in a glutamine-free medium. In CLD, SiMPl-GS outperforms the wild-type GS by selectively enriching high producers. Unlike wild-type GS, SiMPl-GS results in cell pools in which most cells produce high levels of therapeutic proteins. Harnessing orthogonal split intein pairs further enables the selection of four plasmids with a single selection, streamlining multispecific antibody-producing CLD. Taken together, SiMPl-GS is a simple yet effective means to expedite CLD for therapeutic protein production.

C11-hydroxy and C11-oxo C19 and C21 Steroids: Pre-Receptor Regulation and Interaction with Androgen and Progesterone Steroid Receptors

C11-oxy C19 and C11-oxy C21 steroids have been identified as novel steroids but their function remains unclear. This study aimed to investigate the pre-receptor regulation of C11-oxy steroids by 11β-hydroxysteroid dehydrogenase (11βHSD) interconversion and potential agonist and antagonist activity associated with the androgen (AR) and progesterone receptors (PRA and PRB). Steroid conversions were investigated in transiently transfected HEK293 cells expressing 11βHSD1 and 11βHSD2, while CV1 cells were utilised for agonist and antagonist assays. The conversion of C11-hydroxy steroids to C11-oxo steroids by 11βHSD2 occurred more readily than the reverse reaction catalysed by 11βHSD1, while the interconversion of C11-oxy C19 steroids was more efficient than C11-oxy C21 steroids. Furthermore, 11-ketodihydrotestosterone (11KDHT), 11-ketotestosterone (11KT) and 11β-hydroxydihydrotestosterone (11OHDHT) were AR agonists, while only progestogens, 11β-hydroxyprogesterone (11βOHP4), 11β-hydroxydihydroprogesterone (11βOHDHP4), 11α-hydroxyprogesterone (11αOHP4), 11α-hydroxydihydroprogesterone (11αOHDHP4), 11-ketoprogesterone (11KP4), 5α-pregnan-17α-diol-3,11,20-trione (11KPdione) and 21-deoxycortisone (21dE) exhibited antagonist activity. C11-hydroxy C21 steroids, 11βOHP4, 11βOHDHP4 and 11αOHP4 exhibited PRA and PRB agonistic activity, while only C11-oxo steroids, 11KP4 and 11-ketoandrostanediol (11K3αdiol) demonstrated PRB agonism. While no steroids antagonised the PRA, 11OHA4, 11β-hydroxytestosterone (11OHT), 11KT and 11KDHT exhibited PRB antagonism. The regulatory role of 11βHSD isozymes impacting receptor activation is clear-C11-oxo androgens exhibit AR agonist activity; only C11-hydroxy progestogens exhibit PRA and PRB agonist activity. Regulation by the downstream metabolites of active C11-oxy steroids at the receptor level is apparent-C11-hydroxy and C11-oxo metabolites antagonize the AR and PRB, progestogens the former, androgens the latter. The findings highlight the intricate interplay between receptors and active as well as "inactive" C11-oxy steroids, suggesting novel regulatory tiers.

Protein production from HEK293 cell line-derived stable pools with high protein quality and quantity to support discovery research

Antibody-based therapeutics and recombinant protein reagents are often produced in mammalian expression systems, which provide human-like post-translational modifications. Among the available mammalian cell lines used for recombinant protein expression, Chinese hamster ovary (CHO)-derived suspension cells are generally utilized because they are easy to culture and tend to produce proteins in high yield. However, some proteins purified from CHO cell overexpression suffer from clipping and display undesired non-human post translational modifications (PTMs). In addition, CHO cell lines are often not suitable for producing proteins with many glycosylation motifs for structural biology studies, as N-linked glycosylation of proteins poses challenges for structure determination by X-ray crystallography. Hence, alternative and complementary cell lines are required to address these issues. Here, we present a robust method for expressing proteins in human embryonic kidney 293 (HEK293)-derived stable pools, leading to recombinant protein products with much less clipped species compared to those expressed in CHO cells and with higher yield compared to those expressed in transiently-transfected HEK293 cells. Importantly, the stable pool generation protocol is also applicable to HEK293S GnTI- (N-acetylglucosaminyltransferase I-negative) and Expi293F GnTI- suspension cells, facilitating production of high yields of proteins with less complex glycans for use in structural biology projects. Compared to HEK293S GnTI- stable pools, Expi293F GnTI- stable pools consistently produce proteins with similar or higher expression levels. HEK293-derived stable pools can lead to a significant cost reduction and greatly promote the production of high-quality proteins for diverse research projects.

Deciphering integration loci of CHO manufacturing cell lines using long read nanopore sequencing

Despite advances in genetic characterization of Chinese hamster ovary (CHO) cell lines regarding identification of integration sites using next generation sequencing, e.g. targeted locus amplification sequencing (TLA-seq), the concatemer structure of the integrated vectors remains elusive. Here, the entire integration locus of two CHO manufacturing cell lines was reconstructed combining CRISPR/Cas9 target enrichment, nanopore sequencing and the Canu de novo assembly pipeline. An IgG producing CHO cell line integrated 3 vector copies, which were near full-length and contained all relevant vector elements such as transgenes and their promoters on each of the vector copies. In contrast, a second CHO cell line producing a bivalent bispecific antibody integrated 7 highly fragmented vector copies in different orientations leading to head-to-head and tail-to-tail fusions. The size of the vector fragments ranged from 3.0 to 11.4 kbp each carrying 1-3 transgenes. The breakpoints of the genome-vector and vector-vector junctions were validated using Sanger sequencing and Southern blotting. A comparison to TLA-seq data confirmed the genomic breakpoints, but most of the breakpoints of the vector-vector fusions were missed by TLA-seq. For the first time, the complete transgene locus of CHO manufacturing cell lines could be deciphered. Strikingly, the application of the nanopore long-read sequencing technology led to novel insights into the complexity of genomic transgene integrations of CHO manufacturing cell lines generated via random integration.

Expression systems for bovine rhodopsin: a review of the progress made in the Khorana laboratory

Here I will review the development of gene expression systems for production of bovine rhodopsin in the Khorana laboratory with particular focus on stable mammalian cell lines made using human embryonic kidney cells (HEK293S). The synthesis of a gene encoding bovine rhodopsin was completed in 1986. This gene was expertly designed with the built-in capacity for DNA duplex cassette replacement mutagenesis which made site-directed mutagenesis relatively straightforward. Intense effort was expended over several years in order to identify a gene expression system capable of producing rhodopsin in milligram amounts as required for biophysical studies. Mammalian expression systems, both transient and stable, were found to be the most favourable based on several criteria including receptor expression levels, correct folding and post translational processing, and capacity for purification of fully functional receptor. Transient expression using COS-1 cells was preferred for routine small-scale production of rhodopsin mutants, while HEK293S stable cell lines were used when milligram amounts of rhodopsin mutants were needed; for example, when conducting NMR studies.

Study of Calcium Signaling in Astrocytes with a Novel Endoplasmic Reticulum-Targeted GCaMP Sensor

The endoplasmic reticulum (ER), the major organelle for the storage of Ca²⁺ , maintains a concentration of Ca²⁺ much higher than in the cytosol or other subcellular organelles, such as the mitochondria. A variety of tools have been developed for measuring Ca²⁺ activity in neuronal and glial cells, but most of these sensors target either the plasma membrane (PM) or the cytosol. Though these sensors are important for measuring Ca²⁺ transients, they lack the capability to measure activity in the periphery of the ER or to measure low-amplitude events resulting from Ca²⁺ exchange between the ER and other organelles, such as the mitochondria. We recently developed an ER-targeted GCaMP6f anchored to the cytosolic side of the ER that can measure minute calcium exchange occurring in this region. In this article, we discuss detailed methods to characterize the ER-GCaMP6f sensor, utilize it for calcium imaging in cultured astrocytes, and assess its expression and calcium imaging in astrocytes in rodent brains. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Expression and characterization of ER-GCaMP6f Support Protocol 1: ER-GCaMP6f-expressing stable cell line generation Basic Protocol 2: In vitro calcium imaging with ER-GCaMP6f Support Protocol 2: Imaging of drug-induced calcium activity Alternate Protocol 1: Transduction of astrocytes with ER-GCaMP6f AAV Alternate Protocol 2: Calcium imaging of astrocytes with Fluo-4 AM Basic Protocol 3: In vivo ER-GCaMP6f expression and slice calcium imaging Support Protocol 3: Pharmacological studies with 2-APB in brain slices.

Development of HEK-293 Cell Lines Constitutively Expressing Flaviviral Antigens for Use in Diagnostics

Flaviviruses are important human pathogens worldwide. Diagnostic testing for these viruses is difficult because many of the pathogens require specialized biocontainment. To address this issue, we generated 39 virus-like particle (VLP)- and nonstructural protein 1 (NS1)-secreting stable cell lines in HEK-293 cells of 13 different flaviviruses, including dengue, yellow fever, Japanese encephalitis, West Nile, St. Louis encephalitis, Zika, Rocio, Ilheus, Usutu, and Powassan viruses. Antigen secretion was stable for at least 10 cell passages, as measured by enzyme-linked immunosorbent assays and immunofluorescence assays. Thirty-five cell lines (90%) had stable antigen expression over 10 passages, with three of these cell lines (7%) increasing in antigen expression and one cell line (3%) decreasing in antigen expression. Antigen secretion in the HEK-293 cell lines was higher than in previously developed COS-1 cell line counterparts. These antigens can replace current antigens derived from live or inactivated virus for safer use in diagnostic testing.

IMPORTANCE Serological diagnostic testing for flaviviral infections is hindered by the need for specialized biocontainment for preparation of reagents and assay implementation. The use of previously developed COS-1 cell lines secreting noninfectious recombinant viral antigen is limited due to diminished antigen secretion over time. Here, we describe the generation of 39 flaviviral virus-like particle (VLP)- and nonstructural protein 1 (NS1)-secreting stable cell lines in HEK-293 cells representing 13 medically important flaviviruses. Antigen production was more stable and statistically higher in these newly developed cell lines than in their COS-1 cell line counterparts. The use of these cell lines for production of flaviviral antigens will expand serological diagnostic testing of flaviviruses worldwide.

Next generation live-attenuated influenza vaccine platforms

INTRODUCTION

Influenza virus is a major cause of seasonal epidemics and intermittent pandemics. Despite the current molecular biology and vaccine development, influenza virus infection is a significant burden. Vaccines are considered an essential countermeasure for effective control and prevention of influenza virus infection. Even though current influenza virus vaccines provide efficient protection against seasonal influenza outbreaks, the efficacy of these vaccines is not suitable due to antigenic changes of the viruses.

AREAS COVERED

This review focuses on different live-attenuated platforms for influenza virus vaccine development and proposes essential considerations for a rational universal influenza virus vaccine design.

EXPERT OPINION

Despite the recent efforts for universal influenza virus vaccines, there is a lack of broadly reactive antibodies' induction that can confer broad and long-lasting protection. Various strategies using live-attenuated influenza virus vaccines (LAIVs) are investigated to induce broadly reactive, durable, and cross-protective immune responses. LAIVs based on NS segment truncation prevent influenza virus infection and have shown to be effective vaccine candidates among other vaccine platforms. Although many approaches have been used for LAIVs generation, there is still a need to focus on the LAIVs development platforms to generate a universal influenza virus vaccine candidate.

Repeated cell sorting ensures the homogeneity of ocular cell populations expressing a transgenic protein

Transgenic proteins can be routinely expressed in various mammalian cell types via different transgenic systems, but the efficiency of transgene expression is constrained by the complex interplay among factors such as the temporal consistency of expression and compatibility with specific cell types, including ocular cells. Here, we report a more efficient way to express an enhanced green fluorescent protein (EGFP) in human corneal fibroblasts, corneal epithelial cells, and conjunctival epithelial cells through a lentiviral expression system. The relative transducing unit criterion for EGFP-expressing pseudovirions was first determined in HEK-293T cells. Homogeneous populations of EGFP-positive and EGFP-negative cells could be isolated by cell sorting. The half-maximal inhibitory concentration (IC₅₀) value for puromycin was calculated according to viability curves for each cell type. The results revealed that cell types differed with respect to EGFP expression efficiency after transduction with the same amount of EGFP-encoding pseudovirions. Using a cell sorter, the homogeneity of EGFP-positive cells reached >95%. In the initial sorting stage, however, the efficiency of EGFP expression in the sorted cells was noticeably reduced after two rounds of sequential culture, but repeated sorting for up to four rounds yielded homogeneous EGFP-positive human corneal fibroblasts that could be maintained in continuous culture in vitro. The sorted EGFP-positive cells retained their proper morphology and cell type-specific protein expression patterns. Puromycin resistance was found to depend on cell type, indicating that the IC₅₀ for puromycin must be determined for each cell type to ensure the isolation of homogeneous EGFP-positive cells. Taken together, repeated cell sorting is an efficient means of obtaining homogeneous populations of ocular cells expressing a transgenic protein during continuous culture without the potential confounding effects of antibiotics.

CelloSelect - A synthetic cellobiose metabolic pathway for selection of stable transgenic CHO cell lines

Current protocols for generating stable transgenic cell lines mostly rely on antibiotic selection or the use of specialized cell lines lacking an essential part of their metabolic machinery, but these approaches require working with either toxic chemicals or knockout cell lines, which can reduce productivity. Since most mammalian cells cannot utilize cellobiose, a disaccharide consisting of two β-1,4-linked glucose molecules, we designed an antibiotic-free selection system, CelloSelect, which consists of a selection cassette encoding Neurospora crassa cellodextrin transporter CDT1 and β-glucosidase GH1-1. When cultivated in glucose-free culture medium containing cellobiose, CelloSelect-transfected cells proliferate by metabolizing cellobiose as a primary energy source, and are protected from glucose starvation. We show that the combination of CelloSelect with a PiggyBac transposase-based integration strategy provides a platform for the swift and efficient generation of stable transgenic cell lines. Growth rate analysis of metabolically engineered cells in cellobiose medium confirmed the expansion of cells stably expressing high levels of a cargo fluorescent marker protein. We further validated this strategy by applying the CelloSelect system for stable integration of sequences encoding two biopharmaceutical proteins, erythropoietin and the monoclonal antibody rituximab, and confirmed that the proteins are efficiently produced in either cellobiose- or glucose-containing medium in suspension-adapted CHO cells cultured in chemically defined media. We believe coupling heterologous metabolic pathways additively to the endogenous metabolism of mammalian cells has the potential to complement or to replace current cell-line selection systems.

Generation and Characterization of Single-Cycle Infectious Canine Influenza A Virus (sciCIV) and Its Use as Vaccine Platform

Influenza A viruses (IAVs) infect a broad range of hosts, including multiple avian and mammalian species. The frequent emergence of novel IAV strains in different hosts, including in humans, results in the need for vigilance and ongoing development of new approaches to fighting or prevent those infections. Canine influenza is a contagious respiratory disease in dogs caused by two subtypes of IAV, the equine-origin H3N8 canine influenza virus (CIV), and the avian-origin H3N2 CIV. A novel approach to influenza vaccination involves single-cycle infectious influenza A viruses (sciIAVs), which are defective for an essential viral gene. They are propagated in complementing cell lines which provide the missing gene in trans. As sciIAV cannot complete their replication cycle in regular cells they are limited to a single round of viral replication. Because of their safety profile and ability to express foreign antigens inside infected cells, sciIAVs have served both as live-attenuated vaccines and as vaccine vectors for the expression of heterologous antigens. Here, we describe experimental procedures for the generation of a single-cycle infectious CIV (sciCIV), where the viral hemagglutinin (HA) gene was exchanged for the gene for green fluorescent protein (GFP). Complementation of the viral HA protein is provided in trans by stable HA-expressing cell lines. Methods for the in vitro characterization of HA deficient but GFP-expressing sciCIV (sciCIV ΔHA/GFP) are described, as well as its use as a potential vaccine.

Genome editing of a hybridoma cell line via the CRISPR/Cas9 system: A new approach for constitutive high-level expression of heterologous proteins in eukaryotic system

The power of the CRISPR/Cas9 system has revolutionized genome editing in many fields of biology. These applications have expanded exponentially over recent years, including those regarding protein expression technologies. The CRISPR/Cas9 system avoids random integration of the gene of interest and due to this characteristic can be exploited to obtain a stable cell line for the high-yield expression of recombinant proteins. Here we propose a method to edit a hybridoma cell line for the constitutive expression of proteins of interest using the CRISPR/Cas9 system. First, with the scope of optimizing the method, we replaced part of the light chain of immunoglobulin with the Green Fluorescent Protein (GFP) gene, obtaining a precise knock-in in the hybridoma genome. We confirmed the expression and secretion of GFP into the culture medium via fluorimetric analysis, as well as correct genome editing by RNA sequencing. Then, using the same approach, we included the gene encoding a protein of diagnostic interest, the Bovine Herpesvirus 1 glycoprotein E, in the donor DNA. We obtained a stable clone able to secrete gE protein in fusion with GFP into the culture medium. This result was confirmed by ELISA and Western Blot analysis. This study confirms the suitability of this cell line for the production of proteins of diagnostic interest by stable gene expression in a mammalian system. These experiments will enable the technique to be developed from its proof of concept to more specific applications in the field of infectious disease diagnostics.

STEP® vectors for rapid generation of stable transfected CHO cell pools and clones with high expression levels and product quality homogeneity of difficult-to-express proteins

Many proteins produced in CHO cells need evaluation for their clinical and commercial potential. Traditional methods based on stable clone generation are slow and unsuitable for screening larger numbers of proteins, while transient expression technologies are fast but unpredictable regarding product quality and lacking an optional path to subcloning. The STEP® vector technology introduced here combines the best properties of both methods. STEP® vectors contain a strong transcriptional cassette driving expression of a bicistronic mRNA. The gene-of-interest (GOI) is cloned upstream of a functionally impaired zeocin resistance gene (FI-Zeo) whose translation is coupled to that of the GOI through an IRES. Stable transfected cells surviving zeocin selection produce high levels of FI-Zeo and thus, high levels of the GOI-encoded protein. By using different spacers, the translational coupling efficiency and selection strength can be controlled allowing maximization of expression of any GOI. Production of laronidase and factor VII (FVII) is presented as examples of unrelated, difficult-to-express (DTE) proteins. First step is rapid generation of transfected pools with the STEP® vectors. All high expressing surviving pools showed high product quality homogeneity as did monoclonal cell lines obtained from the top pools. Up to 500 μg/mL laronidase was obtained with virtually identical glycosylation profile as reference product. For FVII, cell specific productivity of 0.45 pg/cell/day with 50 IU/μg protein matched highest reported levels of reference product even before process development. Taken together, STEP® vector technology is ideally suited for rapid, small to large-scale production of DTE proteins compared to traditional methods.

Special Focus Issue Part II: Recruitment of solid lipid nanoparticles for the delivery of CRISPR/Cas9: primary evaluation of anticancer gene editing

Aim: The CRISPR/Cas9 system is a promising gene-editing tool for various anticancer therapies; however, development of a biocompatible, nonviral and efficient delivery of CRISPR/Cas9 expression systems remains a challenge. Materials & methods: Solid lipid nanoparticles (SLNs) were produced based on pseudo and 3D ternary plots. Obtained SLNs and their complexes with PX458 plasmid DNA were characterized and evaluated in terms of cytotoxicity and transfection efficiency. Results: SLNs were found to be nanosized, monodispersed, stable and nontoxic. Furthermore, they revealed similar transfection efficiency as the positive control. Conclusion: Overall, we have achieved a good SLN basis for CRISPR/Cas9 delivery and have the potential to produce SLNs with targeted anticancer properties by modifying production parameters and components to facilitate translating CRISPR/Cas9 into preclinical studies.

A high-throughput cell-based gaussia luciferase reporter assay for measurement of CYP1A1, CYP2B6, and CYP3A4 induction

The induction of cytochrome P450s can result in reduced drug efficacy and lead to potential drug-drug interactions. The xenoreceptors-aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR)-play key roles in CYP induction by xenobiotics. In order to be able to rapidly screen for the induction of three enzymes (CYP1A1, CYP2B6, and CYP3A4), we generated a stable AhR-responsive HepG2 cell line, a stable CAR-responsive HepG2 cell line, and a stable PXR-responsive HepG2 cell line.To validate these stable xenoreceptor-responsive HepG2 cell lines, we evaluated the induction of the different Gaussia reporter activities, as well as the mRNA and protein expression levels of endogenous CYPs in response to different inducers.The induction of luciferase activity in the stable xenoreceptor-responsive HepG2 cell lines by specific inducers occurred in a concentration dependent manner. There was a positive correlation between the induction of luciferase activities and the induction endogenous CYP mRNA expression levels. These xenoreceptor-responsive HepG2 cell lines were further validated with known CYP1A1, CYP2B6, and CYP3A4 inducers.These stable xenoreceptor-responsive HepG2 cell lines may be used in preclinical research for the rapid and sensitive detection of AhR, CAR, and PXR ligands that induce CYP450 isoforms.

A Simple and Efficient Method to Generate Gene-Knockout and Transgenic Cell Lines

Knockout (KO) or exogenous expression of a gene of interest in cultured cells is one of the most important ways to study the function of the gene. Compared with transient transfection, stable cell lines possess great advantages such as excellent cell homogeneity and feasibility for long-term use. However, technical challenges in generating stable cell lines still exist in many laboratories using conventional techniques like limiting dilution or cloning cylinders. In this study we describe an optimized method to efficiently create stable cell lines for functional studies. This method was successfully used to generate a PIEZO1 gene-KO cell line with the CRISPR/Cas9 technology, and TRPC5/GCaMP6f-mCherry-coexpressing cell lines without antibiotic selection. Monoclonal cell lines can be obtained in 2-4 weeks after transfection. This method does not require any special equipment or consumables and can be conducted in all laboratories with general cell-culture facility.

Homogeneously N-glycosylated proteins derived from the GlycoDelete HEK293 cell line enable diffraction-quality crystallogenesis

Structural studies of glycoproteins and their complexes provide critical insights into their roles in normal physiology and disease. Most glycoproteins contain N-linked glycosylation, a key post-translation modification that critically affects protein folding and stability and the binding kinetics underlying protein interactions. However, N-linked glycosylation is often an impediment to yielding homogeneous protein preparations for structure determination by X-ray crystallography or other methods. In particular, obtaining diffraction-quality crystals of such proteins and their complexes often requires modification of both the type of glycosylation patterns and their extent. Here, we demonstrate the benefits of producing target glycoproteins in the GlycoDelete human embryonic kidney 293 cell line that has been engineered to produce N-glycans as short glycan stumps comprising N-acetylglucosamine, galactose and sialic acid. Protein fragments of human Down syndrome cell-adhesion molecule and colony-stimulating factor 1 receptor were obtained from the GlycoDelete cell line for crystallization. The ensuing reduction in the extent and complexity of N-glycosylation in both protein molecules compared with alternative glycoengineering approaches enabled their productive deployment in structural studies by X-ray crystallography. Furthermore, a third successful implementation of the GlycoDelete technology focusing on murine IL-12B is shown to lead to N-glycosylation featuring an immature glycan in diffraction-quality crystals. It is proposed that the GlycoDelete cell line could serve as a valuable go-to option for the production of homogeneous glycoproteins and their complexes for structural studies by X-ray crystallography and cryo-electron microscopy.

Heterologous expression of plant glycosyltransferases for biochemistry and structural biology

Much of the carbon captured by photosynthesis is converted into the polysaccharides that constitute plant cell walls. These complex macrostructures are composed of cellulose, hemicellulose, and pectins, together with small amounts of structural proteins, minerals, and in many cases lignin. Wall components assemble and interact with one another to produce dynamic structures with many capabilities, including providing mechanical support to plant structures and determining plant cell shape and size. Despite their abundance, major gaps in our knowledge of the synthesis of the building blocks of these polymers remain, largely due to ineffective methods for expression and purification of active synthetic enzymes for in vitro biochemical analyses. The hemicellulosic polysaccharide, xyloglucan, comprises up to 25% of the dry weight of primary cell walls in plants. Most of the knowledge about the glycosyltransferases (GTs) involved in the xyloglucan biosynthetic pathway has been derived from the identification and carbohydrate analysis of knockout mutants, lending little information on how the catalytic biosynthesis of xyloglucan occurs in planta. In this chapter we describe methods for the heterologous expression of plant GTs using the HEK293 expression platform. As a demonstration of the utility of this platform, nine xyloglucan-relevant GTs from three different CAZy families were evaluated, and methods for expression, purification, and construct optimization are described for biochemical and structural characterization.

The Lumiptosome, an engineered luminescent form of the apoptosome can report cell death by using the same Apaf-1 dependent pathway

Detection of the apoptosis signature becomes central in understanding cell death modes. We present here a whole-cell biosensor that detects Apaf-1 association and apoptosome formation using a split-luciferase complementary assay. Fusion of N-terminal (Nluc) and C-terminal (Cluc)-fragments of firefly luciferase to the N-terminus of human Apaf-1 was performed in HEK293 cells by using CRISPR-Cas9 technology. This resulted in a luminescent form of the apoptosome that we named ‘Lumiptosome’. During Apaf-1 gene editing, a high number of knock-in events were observed without selection, suggesting that the Apaf-1 locus is important for the integration of exogenous transgenes. Since activation of caspase-9 is directly dependent on the apoptosome formation, measured reconstitution of luciferase activity should result from the cooperative association of Nluc-Apaf-1 and Cluc-Apaf-1. Time-response measurements also confirmed that formation of the apoptosome occurs prior to activation of caspase-3. Additionally, overexpression of the Bcl2 apoptosis regulator in transgenic and normal HEK293 cells confirmed that formation of the Lumiptosome depends on release of cytochrome c. Thus, HEK293 cells that stably express the Lumiptosome can be utilized to screen pro- and anti-apoptotic drugs, and to examine Apaf-1-dependent cellular pathways.

Development and evaluation of a gp85 protein-based subgroup-specific indirect enzyme-linked immunosorbent assay for the detection of anti-subgroup J avian leukosis virus antibodies

Avian leukosis virus subgroup J (ALV-J) is an important pathogen for various neoplasms and causes significant economic losses in the poultry industry. Serological detection of specific antibodies against ALV-J infection is important for successful clinical diagnosis. Here, a 293F stable cell line was established to stably express gp85 protein. In this cell line, gp85 protein was expressed at approximately 30 mg/L. A subgroup-specific indirect enzyme-linked immunosorbent assay (iELISA) was developed using ALV-J gp85 protein as coated antigen to detect antibodies against ALV-J. The sensitivity of the iELISA (1:51200 diluted in serum) was 16 times more than that of indirect immunofluorescence assay (IFA; 1:3200 diluted in serum). Moreover, there was no crossreactivity with antibodies against other common avian viruses and other avian leukosis virus subgroups, such as subgroups A and B. The practicality of the iELISA was further evaluated by experimental infection and clinical samples. The results from experimental infection indicated that anti-ALV-J antibodies were readily detected by iELISA as early as 4 weeks after ALV-J infection, and positive antibodies were detected until 20 weeks, with an antibody-positive rate of 11.1% to 33.3%. Moreover, analysis of clinical samples showed that 9.49% of samples were positive for anti-ALV-J antibodies, and the concordance rate of iELISA and IFA was 99.24%. Overall, these results suggested that the subgroup-specific iELISA developed in this study had good sensitivity, specificity, and feasibility. This iELISA will be very useful for epidemiological surveillance, diagnosis, and eradication of ALV-J in poultry farms.

Endogenous Fluorescence Tagging by CRISPR

Fluorescent proteins have revolutionized biomedical research as they are easy to use for protein tagging, cope without fixation or permeabilization, and thus, enable live cell imaging in various models. Current methods allow easy and quick integration of fluorescent markers to endogenous genes of interest. In this review, we introduce the three central methods, zinc finger nucleases (ZFNs), transcription activator-like effectors (TALENs), and CRISPR, that have been widely used to manipulate cells or organisms. Focusing on CRISPR technology, we give an overview on homology-directed repair (HDR)-, microhomology-mediated end joining (MMEJ)-, and nonhomologous end joining (NHEJ)-based strategies for the knock-in of markers, figure out recent developments of the technique for highly efficient knock-in, and demonstrate pros and cons. We highlight the unique aspects of fluorescent protein knock-ins and pinpoint specific improvements and perspectives, like the combination of editing with stem cell derived organoid development.

Nanofountain Probe Electroporation for Monoclonal Cell Line Generation

In the field of genetic engineering, the modification of genes to produce stable cell lines has a variety of applications ranging from the development of novel therapeutics to patient specific treatments. To successfully generate a cell line, the gene of interest must be delivered into the cell and integrated into the genome. The efficiency of cell line generation systems therefore depends on the efficiency of delivery of genetically modifying molecules such as plasmids and CRISPR/CAS9 complexes. In this work, we describe a localized electroporation-based system to generate stable monoclonal cell lines. By employing the nanofountain probe electroporation (NFP-E) system, single cells in patterned cultures are selectively transfected with plasmids, grown, and harvested to obtain stably expressing cell lines. Methods for microcontact printing, cell culture, electroporation, and harvesting are detailed in this chapter.

Novel Approaches for The Development of Live Attenuated Influenza Vaccines

Influenza virus still represents a considerable threat to global public health, despite the advances in the development and wide use of influenza vaccines. Vaccination with traditional inactivate influenza vaccines (IIV) or live-attenuated influenza vaccines (LAIV) remains the main strategy in the control of annual seasonal epidemics, but it does not offer protection against new influenza viruses with pandemic potential, those that have shifted. Moreover, the continual antigenic drift of seasonal circulating influenza viruses, causing an antigenic mismatch that requires yearly reformulation of seasonal influenza vaccines, seriously compromises vaccine efficacy. Therefore, the quick optimization of vaccine production for seasonal influenza and the development of new vaccine approaches for pandemic viruses is still a challenge for the prevention of influenza infections. Moreover, recent reports have questioned the effectiveness of the current LAIV because of limited protection, mainly against the influenza A virus (IAV) component of the vaccine. Although the reasons for the poor protection efficacy of the LAIV have not yet been elucidated, researchers are encouraged to develop new vaccination approaches that overcome the limitations that are associated with the current LAIV. The discovery and implementation of plasmid-based reverse genetics has been a key advance in the rapid generation of recombinant attenuated influenza viruses that can be used for the development of new and most effective LAIV. In this review, we provide an update regarding the progress that has been made during the last five years in the development of new LAIV and the innovative ways that are being explored as alternatives to the currently licensed LAIV. The safety, immunogenicity, and protection efficacy profile of these new LAIVs reveal their possible implementation in combating influenza infections. However, efforts by vaccine companies and government agencies will be needed for controlled testing and approving, respectively, these new vaccine methodologies for the control of influenza infections.

Monoclonal Cell Line Generation and CRISPR/Cas9 Manipulation via Single-Cell Electroporation

Stably transfected cell lines are widely used in drug discovery and biological research to produce recombinant proteins. Generation of these cell lines requires the isolation of multiple clones, using time-consuming dilution methods, to evaluate the expression levels of the gene of interest. A new and efficient method is described for the generation of monoclonal cell lines, without the need for dilution cloning. In this new method, arrays of patterned cell colonies and single cell transfection are employed to deliver a plasmid coding for a reporter gene and conferring resistance to an antibiotic. Using a nanofountain probe electroporation system, probe positioning is achieved through a micromanipulator with sub-micron resolution and resistance-based feedback control. The array of patterned cell colonies allows for rapid selection of numerous stably transfected clonal cell lines located on the same culture well, conferring a significant advantage over slower and labor-intensive traditional methods. In addition to plasmid integration, this methodology can be seamlessly combined with CRISPR/Cas9 gene editing, paving the way for advanced cell engineering.

Human Embryonic Kidney 293 Cells: A Vehicle for Biopharmaceutical Manufacturing, Structural Biology, and Electrophysiology

Mammalian cells, e.g., CHO, BHK, HEK293, HT-1080, and NS0 cells, represent important manufacturing platforms in bioengineering. They are widely used for the production of recombinant therapeutic proteins, vaccines, anticancer agents, and other clinically relevant drugs. HEK293 (human embryonic kidney 293) cells and their derived cell lines provide an attractive heterologous system for the development of recombinant proteins or adenovirus productions, not least due to their human-like posttranslational modification of protein molecules to provide the desired biological activity. Secondly, they also exhibit high transfection efficiency yielding high-quality recombinant proteins. They are easy to maintain and express with high fidelity membrane proteins, such as ion channels and transporters, and thus are attractive for structural biology and electrophysiology studies. In this article, we review the literature on HEK293 cells regarding their origins but also stress their advancements into the different cell lines engineered and discuss some significant aspects which make them versatile systems for biopharmaceutical manufacturing, drug screening, structural biology research, and electrophysiology applications.

Development of a CHO-Based Cell-Free Platform for Synthesis of Active Monoclonal Antibodies

Chinese Hamster Ovary (CHO) cells are routinely optimized to stably express monoclonal antibodies (mAbs) at high titers. At the early stages of lead isolation and optimization, hundreds of sequences for the target protein of interest are screened. Typically, cell-based transient expression technology platforms are used for expression screening, but these can be time- and resource-intensive. Here, we have developed a cell-free protein synthesis (CFPS) platform utilizing a commercially available CHO extract for the rapid in vitro synthesis of active, aglycosylated mAbs. Specifically, we optimized reaction conditions to maximize protein yields, established an oxidizing environment to enable disulfide bond formation, and demonstrated the importance of temporal addition of heavy chain and light chain plasmids for intact mAb production. Using our optimized platform, we demonstrate for the first time to our knowledge the cell-free synthesis of biologically active, intact mAb at >100 mg/L using a eukaryotic-based extract. We then explored the utility of our system as a tool for ranking yields of candidate antibodies. Unlike stable or transient transfection-based screening, which requires a minimum of 7 days for setup and execution, results using our CHO-based CFPS platform are attained within 2 days and it is well-suited for automation. Further development would provide a tool for rapid, high-throughput prediction of mAb expression ranking to accelerate design-build-test cycles required for antibody expression and engineering. Looking forward, the CHO-based CFPS platform could facilitate the synthesis of toxic proteins as well.

Generation and characterization of a potentially applicable Vero cell line constitutively expressing the Schmallenberg virus nucleocapsid protein

Schmallenberg virus (SBV) is a Culicoides-transmitted orthobunyavirus that poses a threat to susceptible livestock species such as cattle, sheep and goats. The nucleocapsid (N) protein of SBV is an ideal diagnostic antigen for the detection of viral infection. In this study, a stable Vero cell line, Vero-EGFP-SBV-N, constitutively expressing the SBV-N protein was established using a lentivirus system combined with puromycin selection. This cell line spontaneously emitted green fluorescent signals distributed throughout the cytoplasm, in which the expression of SBV-N fusion protein was confirmed by western blot analysis. The expression of SBV-N protein in Vero-EGFP-SBV-N cells was stable for more than fifty passages without puromycin pressure. The SBV-N fusion protein contained both an N-terminal enhanced green fluorescent protein (EGFP) tag and a C-terminal hexa-histidine (6 × His) tag, by which the N protein was successfully purified using Ni-NTA affinity chromatography. The cell line was further demonstrated to be reactive with SBV antisera and an anti-SBV monoclonal antibody in indirect immunofluorescence assays. Taken together, our results demonstrate that the Vero-EGFP-SBV-N cell line has potential for application in the serological diagnosis of SBV infection.

Strategies and Considerations for Improving Expression of "Difficult to Express" Proteins in CHO Cells

Despite substantial advances in the field of mammalian expression, there are still proteins that are characterized as difficult to express. Determining the expression bottleneck requires troubleshooting techniques specific for the given molecule and host. The complex array of intracellular processes involved in protein expression includes transcription, protein folding, post-translation processing, and secretion. Challenges in any of these steps could result in low protein expression, while the inherent properties of the molecule itself may limit its production via mechanisms such as cytotoxicity or inherent instability. Strategies to identify the rate-limiting step and subsequently improve expression and production are discussed here.

Non-conventional expression systems for the production of vaccine proteins and immunotherapeutic molecules

The increasing demand for recombinant vaccine antigens or immunotherapeutic molecules calls into question the universality of current protein expression systems. Vaccine production can require relatively low amounts of expressed materials, but represents an extremely diverse category consisting of different target antigens with marked structural differences. In contrast, monoclonal antibodies, by definition share key molecular characteristics and require a production system capable of very large outputs, which drives the quest for highly efficient and cost-effective systems. In discussing expression systems, the primary assumption is that a universal production platform for vaccines and immunotherapeutics will unlikely exist. This review provides an overview of the evolution of traditional expression systems, including mammalian cells, yeast and E.coli, but also alternative systems such as other bacteria than E. coli, transgenic animals, insect cells, plants and microalgae, Tetrahymena thermophila, Leishmania tarentolae, filamentous fungi, cell free systems, and the incorporation of non-natural amino acids.

Efficient PRNP deletion in bovine genome using gene-editing technologies in bovine cells

Even though prion (encoded by the PRNP gene) diseases like bovine spongiform encephalopathy (BSE) are fatal neurodegenerative diseases in cattle, their study via gene deletion has been limited due to the absence of cell lines or mutant models. In this study, we aim to develop an immortalized fibroblast cell line in which genome-engineering technology can be readily applied to create gene-modified clones for studies. To this end, this study is designed to 1) investigate the induction of primary fibroblasts to immortalization by introducing Bmi-1 and hTert genes; 2) investigate the disruption of the PRNP in those cells; and 3) evaluate the gene expression and embryonic development using knockout (KO) cell lines. Primary cells from a male neonate were immortalized with Bmi-1and hTert. Immortalized cells were cultured for more than 180 days without any changes in their doubling time and morphology. Furthermore, to knockout the PRNP gene, plasmids that encode transcription activator-like effector nuclease (TALEN) pairs were transfected into the cells, and transfected single cells were propagated. Mutated clonal cell lines were confirmed by T7 endonuclease I assay and sequencing. Four knockout cell lines were used for somatic cell nuclear transfer (SCNT), and the resulting embryos were developed to the blastocyst stage. The genes (CSNK2A1, FAM64A, MPG and PRND) were affected after PRNP disruption in immortalized cells. In conclusion, we established immortalized cattle fibroblasts using Bmi-1 and hTert genes, and used TALENs to knockout the PRNP gene in these immortalized cells. The efficient PRNP KO is expected to be a useful technology to develop our understanding of in vitro prion protein functions in cattle.

Generating Recombinant Antibodies to Membrane Proteins through Phage Display

One of the most important classes of proteins in terms of drug targets is cell surface membrane proteins, and yet it is a challenging set of proteins for generating high-quality affinity reagents. In this review, we focus on the use of phage libraries, which display antibody fragments, for generating recombinant antibodies to membrane proteins. Such affinity reagents generally have high specificity and affinity for their targets. They have been used for cell staining, for promoting protein crystallization to solve three-dimensional structures, for diagnostics, and for treating diseases as therapeutics. We cover publications on this topic from the past 10 years, with a focus on the various formats of membrane proteins for affinity selection and the diverse affinity selection strategies used. Lastly, we discuss the challenges faced in this field and provide possible directions for future efforts.

Development and applications of single-cycle infectious influenza A virus (sciIAV)

The diverse host range, high transmissibility, and rapid evolution of influenza A viruses justify the importance of containing pathogenic viruses studied in the laboratory. Other than physically or mechanically changing influenza A virus containment procedures, modifying the virus to only replicate for a single round of infection similarly ensures safety and consequently decreases the level of biosafety containment required to study highly pathogenic members in the virus family. This biological containment is more ideal because it is less apt to computer, machine, or human error. With many necessary proteins that can be deleted, generation of single-cycle infectious influenza A viruses (sciIAV) can be achieved using a variety of approaches. Here, we review the recent burst in sciIAV generation and summarize the applications and findings on this important human pathogen using biocontained viral mimics.

Algal autolysate medium to label proteins for NMR in mammalian cells

In-cell NMR provides structural and functional information on proteins directly inside living cells. At present, the high costs of the labeled media for mammalian cells represent a limiting factor for the development of this methodology. Here we report a protocol to prepare a homemade growth medium from Spirulina platensis autolysate, suitable to express uniformly labeled proteins inside mammalian cells at a reduced cost-per-sample. The human proteins SOD1 and Mia40 were overexpressed in human cells grown in (15)N-enriched S. platensis algal-derived medium, and high quality in-cell NMR spectra were obtained.

Development of Genetically Modified Chinese Hamster Ovary Host Cells for the Enhancement of Recombinant Tissue Plasminogen Activator Expression

BACKGROUND

Chinese hamster ovary (CHO) cells are the most commonly used host system for the expression of high quality recombinant proteins. However, the development of stable, high-yielding CHO cell lines is a major bottleneck in the industrial manufacturing of therapeutic proteins. Therefore, different strategies such as the generation of more efficient expression vectors and establishment of genetically engineered host cells have been employed to increase the efficiency of cell line development. In order to examine the possibility of generating improved CHO host cells, cell line engineering approaches were developed based on ceramide transfer protein (CERT), and X-box binding protein 1s (XBP1s).

METHODS

CHO cells were transfected with CERT S132A, a mutant variant of CERT which is resistant to phosphorylation, or XBP1s expression plasmids, and then stable cell pools were generated. Transient expression of t-PA was examined in engineered cell pools in comparison to un-modified CHO host cells.

RESULTS

Overexpression of CERT S132A led to the enhancement of recombinant tissue plasminogen activator (t-PA) expression in transient expression by 50%. On the other hand, it was observed that the ectopic expression of the XBP1s, did not improve the t-PA expression level.

CONCLUSION

The results obtained in this study indicate successful development of the improved CHO host cells through CERT S132A overexpression.

A Streamlined, Automated Protocol for the Production of Milligram Quantities of Untagged Recombinant Rat Lactate Dehydrogenase A Using ÄKTAxpressTM

We developed an efficient, automated 2-step purification protocol for the production of milligram quantities of untagged recombinant rat lactate dehydrogenase A (rLDHA) from E. coli, using the ÄKTAxpress™ chromatography system. Cation exchange followed by size exclusion results in average final purity in excess of 93% and yields ~ 14 milligrams per 50 ml of original cell culture in EnPresso B media, in under 8 hrs, including all primary sample processing and column equilibration steps. The protein is highly active and coherent biophysically and a viable alternative to the more problematic human homolog for structural and ligand-binding studies; an apo structure of untagged rLDHA was solved to a resolution 2.29 Å (PDB ID 5ES3). Our automated methodology uses generic commercially available pre-packed columns and simple buffers, and represents a robust standard method for the production of milligram amounts of untagged rLDHA, facilitating a novel fragment screening approach for new inhibitors.

Sample preparation of biological macromolecular assemblies for the determination of high-resolution structures by cryo-electron microscopy

Single particle cryo-EM has recently developed into a powerful tool to determine the 3D structure of macromolecular complexes at near-atomic resolution, which allows structural biologists to build atomic models of proteins. All technical aspects of cryo-EM technology have been considerably improved over the last two decades, including electron microscopic hardware, image processing software and the ever growing speed of computers. This leads to a more widespread use of the technique, and it can be anticipated that further automation of electron microscopes and image processing tools will soon fully shift the focus away from the technological aspects, onto biological questions that can be answered. In single particle cryo-EM, no crystals of a macromolecule are required. In contrast to X-ray crystallography, this significantly facilitates structure determination by cryo-EM. Nevertheless, a relatively high level of biochemical control is still essential to obtain high-resolution structures by cryo-EM, and it can be anticipated that the success of the cryo-EM technology goes hand in hand with further developments of sample purification and preparation techniques. This will allow routine high-resolution structure determination of the many macromolecular complexes of the cell that until now represent evasive targets for X-ray crystallographers. Here we discuss the various biochemical tools that are currently available and the existing sample purification and preparation techniques for cryo-EM grid preparation that are needed to obtain high-resolution images for structure determination.

Hijacked then lost in translation: the plight of the recombinant host cell in membrane protein structural biology projects

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Membrane protein structural biologists need high-quality protein for crystallisation.

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Recombinant proteins are central to the structural biology supply chain.

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Understanding quality control in protein production is an emerging trend.

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The roles of translation and protein folding in the host cell are examined.

Membrane protein structural biology is critically dependent upon the supply of high-quality protein. Over the last few years, the value of crystallising biochemically characterised, recombinant targets that incorporate stabilising mutations has been established. Nonetheless, obtaining sufficient yields of many recombinant membrane proteins is still a major challenge. Solutions are now emerging based on an improved understanding of recombinant host cells; as a ‘cell factory’ each cell is tasked with managing limited resources to simultaneously balance its own growth demands with those imposed by an expression plasmid. This review examines emerging insights into the role of translation and protein folding in defining high-yielding recombinant membrane protein production in a range of host cells.