Production of tissue plasminogen activators from animal cells

A pooled CRISPR/AsCpf1 screen using paired gRNAs to induce genomic deletions in Chinese hamster ovary cells

• Development of a small-scale CRISPR/AsCpf1 screen in CHO.

• Usage of paired gRNAs enables full deletion of coding or noncoding genomic regions.

• Growth perturbing paired gRNAs identified.

• Key points for considerations in future screens identified.

Chinese hamster ovary (CHO) cells are the most widely used host for the expression of therapeutic proteins. Recently, significant progress has been made due to advances in genome sequence and annotation quality to unravel the black box CHO. Nevertheless, in many cases the link between genotype and phenotype in the context of suspension cultivated production cell lines is still not fully understood. While frameshift approaches targeting coding genes are frequently used, the non-coding regions of the genome have received less attention with respect to such functional annotation. Importantly, for non-coding regions frameshift knock-out strategies are not feasible. In this study, we developed a CRISPR-mediated screening approach that performs full deletions of genomic regions to enable the functional study of both the translated and untranslated genome.

An in silico pipeline for the computational high-throughput design of paired guide RNAs (pgRNAs) directing CRISPR/AsCpf1 was established and used to generate a library tackling process-related genes and long non-coding RNAs. Next generation sequencing analysis of the plasmid library revealed a sufficient, but highly variable pgRNA composition. Recombinase-mediated cassette exchange was applied for pgRNA library integration rather than viral transduction to ensure single copy representation of pgRNAs per cell. After transient AsCpf1 expression, cells were cultivated over two sequential batches to identify pgRNAs which massively affected growth and survival. By comparing pgRNA abundance, depleted candidates were identified and individually validated to verify their effect.

Cloning and soluble expression of mature α-luffin from Luffa cylindrica in E. coli using SUMO fusion protein

α-Lufin, found in Luaf cylindrica seeds, is a type I ribosome inactivating proteins. Cytotoxic effects make it an appropriate candidate for the construction of immunotoxins and conjugates. Because of limited natural resources, recombinant technology is the best approach to achieve large-scale production of plant-based proteins. In the present study, α-lufin protein was expressed in E. coli and the effects of different temperature conditions, SUMO fusion tag, and cultivation strategies on total expression and solubility were investigated. Protein expression was evaluated at different intervals (0, 4, 6, 8, 24 h) postinduction. Our results showed that EnBase had higher eficiency than LB, and maximum solubility and total protein expression were achieved 24 h after induction at 30 °C and 25 °C, respectively. It was shown that SUMO tag is an effective strategy to improve protein solubility.

Expression of a novel chimeric truncated t-PA in CHO cells based on in silico experiments

Tissue plasminogen activator (t-PA) is one of the fibrin-specific serine proteases that play a crucial role in the fibrinolytic system. The rapid clearance of the drug from the circulation, caused by its active uptake in the liver, has lead to complicated clinical applications. Different forms of plasminogen activators have been developed to treat thrombotic disease. Deletion of the first three domains of t-PA by gene manipulation techniques has shown a significant increase in its plasma half life. In order to compensate the disadvantage of higher bleeding risk, a novel chimeric truncated form of t-PA with 394 amino acids and more fibrin affinity compared to the truncated form was designed to be expressed in Chinese Hamster Ovarian (CHO) cells. The recombinant chimeric plasminogen activator consists of kringle 2 and serine protease (K2S) domains of t-PA, namely GHRP-SYQ-K2S. The level of expression was found to be 752 IU/ml with 566,917 IU/mg specific activity, based on amidolytic activity. The fibrin binding of this novel chimeric truncated t-PA was 86% of the full length t-PA at a fibrinogen concentration of 0.2 mg/ml. This could be a promising approach with more desirable pharmacodynamic properties compared to existing commercial forms.

Cloning and expression of functional full-length human tissue plasminogen activator in Pichia pastoris

Human tissue plasminogen activator (t-PA) plays a pivotal role in the treatment of acute myocardial infarction, ischemic stroke, and deep vein thrombosis. It has the benefit of generating no adverse effects such as fibrinogen depletion, systemic hemorrhage, and immunologic reactions. Human t-PA is a serine-protease enzyme containing 527 amino acid residues in five structural domains. The correct folding of t-PA requires the correct pairing of 17 disulfide bridges in the molecule. A gene encoding full-length human t-PA was cloned into pPICZαA expression vector downstream of alcohol oxidase promoter and α-mating signal sequence from Saccharomyces cerevisiae and flush with the kex2 cleavage site to express the protein with a native N terminus. The methylotrophic yeast, Pichia pastoris GS115 strain, was transformed with this cassette, and methanol utilizing (mut+) transformants were selected for production and secretion of human t-PA into culture media. SDS-PAGE and Western blot analysis showed the expressed bands of t-PA protein. Zymography test indicated suitable folding and proper function of the expressed recombinant human t-PA in conversion of plasminogen to plasmin and gelatin lysis. Amidolytic activity test showed the amidolytic activity of 1,650 IU/ml. The results of this study concluded that P. pastoris methylotrophic yeast can be a suitable alternative for mammalian and prokaryotic expression systems to produce t-PA.

Secretion of active recombinant human tissue plasminogen activator derivatives in Escherichia coli

The DNA fragment coding for kringle 2 plus serine protease domains (K2S) of tissue plasminogen activator (tPA) was inserted into a phagemid vector, pComb3HSS. In the recombinant vector, pComb3H-K2S, the K2S gene was fused to gpIII of PhiM13 and linked to the OmpA signal sequence. The resulting gene, rK2S-gpIII, was inducibly expressed in Escherichia coli XL-1 Blue. The protein was presented on the phage particle. To stop the expression of gpIII, a stop codon between K2S and the gpIII gene was inserted by site-directed mutagenesis. This mutated vector, MpComb3H-K2S, was transformed in XL-1 Blue. After induction with IPTG (isopropyl-beta-D-thiogalactopyranoside), rK2S was found both in the periplasm as an inactive form of approximately 32% and in the culture supernatant as an active form of approximately 68%. The secreted form of rK2S was partially purified by ammonium sulfate (55%) precipitation. The periplasmic form was isolated from whole cells by chloroform extraction. The fibrin binding site of kringle 2 was demonstrated in all expressed versions (phage-bound, periplasmic, and secreted forms) using the monoclonal anti-kringle 2 antibody (16/B). Only the secreted form of rK2S revealed a fibrinogen-dependent amidolytic activity with the specific activity of 236 IU/microg. No amidolytic activity of rK2S was observed in either the periplasmic or the phage-bound form. The secretion of rK2S as an active enzyme offers a novel approach for the production of the active-domain deletion mutant tPA, rK2S, without any requirements for bacterial compartment preparation and in vitro refolding processes. This finding is an important technological advance in the development of large-scale, bacterium-based tPA production systems.

Tissue-type plasminogen activator: Characteristics, applications and production technology

Plasminogen activators have immense clinical significance as thrombolytic agents for management of stroke and myocardial infarction. Tissue-type plasminogen activator (tPA) is generally preferred as being effective and safer than either urokinase or streptokinase type activators. Large-scale production of tPA became possible through groundbreaking developments in cell lines and bioprocess technology. Nevertheless, at thousands of dollars per treatment, tPA remains expensive. Enhancing cellular productivity and downstream product recovery through new approaches continue to be major challenges as discussed in this review. Recent clinical experience suggests the need for yet better fibrinolytic agents and attempts are underway to modify the tPA molecule to second generation products. Emerging trends in this field are outlined.

Effector-assisted refolding of recombinant tissue-plasminogen activator produced in Escherichia coli

Recombinant tissue-plasminogen activator (r-tPA), expressed in Escherichia coli cells in an aggregated form, was solubilized with a strong chaotrope in the absence of any reducing agent. The solubilized molecule was reactivated by a procedure that was developed to mimic the physiological conditions optimal for the functional folding and activity of the native protein. The use of partially purified fibrinogen, as a source of fibrin (the effector), is shown to facilitate the reactivation process and increase its yield by at least a factor of two. The yield of the process is also shown to be particularly dependent on the recombinant protein concentration. At a concentration level of 3-3.7 mg r-tPA/L in the reactivation mixture, up to a 90% yield of activity was obtained. Purification of the activated form of r-tPA was achieved with a two-step column-chromatography scheme. This included a gel filtration step on a Sephadex G-50 column followed by an affinity chromatography step on a lysine-sepharose column. The product was composed of roughly equal amounts of one-chain and two-chain t-PA. The feasibility of using a two water-soluble polymeric phase system, with a centrifugal partition chromatography (CPC), in scaling up the reactivation process or the purification step was also evaluated.

A comparison of the effects of various stimulatory agents on t-PA secretion by normal and malignant cell lines

Succinyl con A and acetyl con A both stimulated epithelial cells to produce similar yields of tissue plasminogen activator (t-PA) to those previously obtained with native con A. However, unlike con A, the derivatized lectins did not adversely affect cell morphology and viability, and cells treated with succinyl con A could secrete t-PA for a prolonged period. Con A and the two derivatives produced similar morphological effects in Bowes melanoma cells, but t-PA production was not increased. Elevated cyclic nucleotide concentrations did not affect t-PA production from epithelial cells, but calcium ionophore treatment generated t-PA yields similar to those obtained with lectins. Azacytidine, which enhanced t-PA production from epithelial cells, did not increase yields from Bowes melanoma cells, and also sodium butyrate, reported to increase t-PA yields from human endothelial cells, had no effect on either cell line.