On the dual effect of glucose during production of pBAD/AraC-based minicircles

Real-Time PCR Method for Assessment of ParA-Mediated Recombination Efficiency in Minicircle Production

The in vivo intramolecular recombination of a parental plasmid allows excising prokaryotic backbone from the eukaryotic cassette of interest, leading to the formation of, respectively, a miniplasmid and a minicircle. Here we describe a real-time PCR protocol suitable for the determination of recombination efficiency of parental plasmids with multimer resolution sites (MRS). The protocol was successfully applied to purified DNA samples obtained from E. coli cultures, allowing a more reproducible determination of recombination efficiency than densitometry analysis of agarose gels.

Maximization of the Minicircle DNA Vaccine Production Expressing SARS-CoV-2 RBD

Nucleic acid vaccines have been proven to be a revolutionary technology to induce an efficient, safe and rapid response against pandemics, like the coronavirus disease (COVID-19). Minicircle DNA (mcDNA) is an innovative vector more stable than messenger RNA and more efficient in cell transfection and transgene expression than conventional plasmid DNA. This work describes the construction of a parental plasmid (PP) vector encoding the receptor-binding domain (RBD) of the S protein from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and the use of the Design of Experiments (DoE) to optimize PP recombination into mcDNA vector in an orbital shaker. First, the results revealed that host cells should be grown at 42 °C and the Terrific Broth (TB) medium should be replaced by Luria Broth (LB) medium containing 0.01% L-arabinose for the induction step. The antibiotic concentration, the induction time, and the induction temperature were used as DoE inputs to maximize the % of recombined mcDNA. The quadratic model was statistically significant (p-value < 0.05) and presented a non-significant lack of fit (p-value > 0.05) with a suitable coefficient of determination. The optimal point was validated using 1 h of induction, at 30 °C, without the presence of antibiotics, obtaining 93.87% of recombined mcDNA. Based on these conditions, the production of mcDNA was then maximized in a mini-bioreactor platform. The most favorable condition obtained in the bioreactor was obtained by applying 60% pO2 in the fermentation step during 5 h and 30% pO2 in the induction step, with 0.01% L-arabinose throughout 5 h. The yield of mcDNA-RBD was increased to a concentration of 1.15 g/L, when compared to the orbital shaker studies (16.48 mg/L). These data revealed that the bioreactor application strongly incremented the host biomass yield and simultaneously improved the recombination levels of PP into mcDNA. Altogether, these results contributed to improving mcDNA-RBD biosynthesis to make the scale-up of mcDNA manufacture simpler, cost-effective, and attractive for the biotechnology industry.

Recombination efficiency measurement by real-time PCR: A strategy to evaluate ParA-mediated minicircle production

Minicircles (MCs) are DNA molecules that are produced in Escherichia coli by replicating a parental plasmid (PP) and inducing its site-specific intramolecular recombination into miniplasmid (MP; containing the prokaryotic backbone) and MC molecules (comprised by the eukaryotic cassette). The determination of the recombination efficiency and the monitoring of PP, MC and MP species during processing and in the final product are critical aspects of MC manufacturing. This work describes a real-time PCR method for the specific identification of PP, MP or MC that uses sets of primers specific for each species. The method was evaluated using artificial mixtures of (i) PP and MP, (ii) PP and MC and (iii) MP and MC that were probed for all three DNA molecules. The ratio of molecules of each DNA species in these mixtures were determined with differences lower than 10% relatively to the expected ratio of the species in 90% of the mixtures. Next, the recombination efficiency was successfully estimated by analysing pre-purified DNA samples obtained from cell cultures. A standard deviation < 2% was obtained between replicas and results closely correlated with those obtained by densitometry analysis of agarose gels. Further optimization is required to determine recombination efficiency directly from whole cells.

Minicircle-based expression of vascular endothelial growth factor in mesenchymal stromal cells from diverse human tissues

BACKGROUND

Mesenchymal stromal cells (MSC) have been exploited for the treatment of ischemic diseases given their angiogenic potential. Despite bone marrow (BM) being the most studied tissue source, cells with similar intrinsic properties can be isolated from adipose tissue (AT) and umbilical cord matrix (UCM). The present study aims to compare the angiogenic potential of MSC obtained from BM, AT and UCM that were genetically modified with vascular endothelial growth factor (VEGF)-encoding minicircle (MC) vectors. The overexpression of VEGF combined with the intrinsic properties of MSC could represent a promising strategy towards angiogenic therapies.

METHODS

We established a microporation-based protocol to transfect human MSC using VEGF-encoding MC (MC-VEGF). VEGF production levels were measured by an enzyme-linked immunosorbent assay and a quantitative polymerase chain reaction. The in vitro angiogenic potential of transfected cells was quantified using cell tube formation and migration functional studies.

RESULTS

MSC isolated from BM, AT or UCM showed similar levels of VEGF secretion after transfection with MC-VEGF. Those values were significantly higher when compared to non-transfected cells, indicating an effective enhancement of VEGF production. Transfected cells displayed higher in vitro angiogenic potential than non-transfected controls, as demonstrated by functional in vitro assays. No significant differences were observed among cells from different sources.

CONCLUSIONS

Minicircles can be successfully used to transiently overexpress VEGF in human MSC, regardless of the cell tissue source, representing an important advantage in a clinical context (i.e., angiogenic therapy) because a standard protocol might be applied to MSC of different tissue sources, which can be differentially selected according to the application (e.g., autologous versus allogeneic settings).

Minicircle Biopharmaceuticals–An Overview of Purification Strategies

Minicircles are non-viral delivery vectors with promising features for biopharmaceutical applications. These vectors are plasmid-derived circular DNA molecules that are obtained in vivo in Escherichia coli by the intramolecular recombination of a parental plasmid, which generates a minicircle containing the eukaryotic therapeutic cassette of interest and a miniplasmid containing the prokaryotic backbone. The production process results thus in a complex mixture, which hinders the isolation of minicircle molecules from other DNA molecules. Several strategies have been proposed over the years to meet the challenge of purifying and obtaining high quality minicircles in compliance with the regulatory guidelines for therapeutic use. In minicircle purification, the characteristics of the strain and parental plasmid used have a high impact and strongly affect the purification strategy that can be applied. This review summarizes the different methods developed so far, focusing not only on the purification method itself but also on its dependence on the upstream production strategy used.

Engineering of Human Mesenchymal Stem/Stromal Cells with Vascular Endothelial Growth Factor-Encoding Minicircles for Angiogenic Ex Vivo Gene Therapy

Peripheral artery disease (PAD) is a debilitating and prevalent condition characterized by blockage of the arteries, leading to limb amputation in more severe cases. Mesenchymal stem/stromal cells (MSC) are known to have intrinsic regenerative properties that can be potentiated by the introduction of pro-angiogenic genes such as the vascular endothelial growth factor (VEGF). Herein, the use of human bone marrow MSC transiently transfected with minicircles encoding for VEGF is proposed as an ex vivo gene therapy strategy to enhance angiogenesis in PAD patients. The VEGF gene was cloned in minicircle and conventional plasmid vectors and used to transfect bone marrow-derived MSC ex vivo. VEGF expression was evaluated both by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay. The number of VEGF transcripts following MSC transfection with minicircles increased 130-fold relative to the expression in non-transfected MSC, whereas for the plasmid (pVAX1)-based transfection, the increase was 50-fold. Compared to the VEGF basal levels secreted by MSC (11.1 ± 3.4 pg/1,000 cells/day), significantly higher values were detected by enzyme-linked immunosorbent assay after both minicircle and pVAX1 transfection (644.8 ± 82.5 and 508.3 ± 164.0 pg/1,000 cells/day, respectively). The VEGF overexpression improved the angiogenic potential of MSC in vitro, as confirmed by endothelial cell tube formation and cell migration assays, without affecting the expansion potential ex vivo, as well as multilineage differentiation capacity or immunophenotype of MSC. Although preclinical in vivo studies are required, these results suggest that minicircle-mediated VEGF gene delivery, combined with the unique properties of human MSC, could represent a promising ex vivo gene therapy approach for an improved angiogenesis in the context of PAD.

Autoinducer-2 Quorum Sensing Contributes to Regulation of Microcin PDI in Escherichia coli

The Escherichia coli quorum sensing (QS) signal molecule, autoinducer-2 (AI-2), reaches its maximum concentration during mid-to-late growth phase after which it quickly degrades during stationary phase. This pattern of AI-2 concentration coincides with the up- then down-regulation of a recently described microcin PDI (mccPDI) effector protein (McpM). To determine if there is a functional relationship between these systems, a prototypical mccPDI-expressing strain of E. coli 25 was used to generate ΔluxS, ΔlsrACDBFG (Δlsr), and ΔlsrR mutant strains that are deficient in AI-2 production, transportation, and AI-2 transport regulation, respectively. Trans-complementation, RT-qPCR, and western blot assays were used to detect changes of microcin expression and synthesis under co-culture and monoculture conditions. Compared to the wild-type strain, the AI-2-deficient strain (ΔluxS) and -uptake negative strain (Δlsr) were >1,000-fold less inhibitory to susceptible bacteria (P < 0.05). With in trans complementation of luxS, the AI-2 deficient mutant reduced the susceptible E. coli population by 4-log, which was within 1-log of the wild-type phenotype. RT-qPCR and western blot results for the AI-2 deficient E. coli 25 showed a 5-fold reduction in mcpM transcription with an average 2-h delay in McpM synthesis. Furthermore, overexpression of sRNA micC and micF (both involved in porin protein regulation) was correlated with mcpM regulation, consistent with a possible link between QS and mcpM regulation. This is the direct first evidence that microcin regulation can be linked to quorum sensing in a Gram-negative bacterium.

Minicircle DNA vectors for gene therapy: advances and applications

INTRODUCTION

Nucleic-acid-based biopharmaceuticals enclose a remarkable potential for treating debilitating or life-threatening diseases that currently remain incurable. This promising area of research envisages the creation of state-of-the-art DNA vaccines, pluripotent cells or gene-based therapies, which can be used to overcome current issues. To achieve this goal, DNA minicircles are emerging as ideal nonviral vectors due to their safety and persistent transgene expression in either quiescent or actively dividing cells.

AREAS COVERED

This review focuses on the characteristics of minicircle DNA (mcDNA) technology and the current advances in their production. The possible modifications to further improve minicircle efficacy are also emphasized and discussed in light of recent advances. As a final point, the main therapeutic applications of mcDNA are summarized, with a special focus on pluripotent stem cells production and cancer therapy.

EXPERT OPINION

Achieving in-target and persistent transgene expression is a challenging issue that is of critical importance for a successful therapeutic outcome. The use of miniaturized mcDNA cassettes with additional modifications that increase and prolong expression may contribute to an improved generation of biopharmaceuticals. The unique features of mcDNA render it an attractive alternative to overcome current technical issues and to bridge the significant gap that exists between basic research and clinical applications.