Limited use of the Cre/loxP recombination system in efficient production of loxP-containing minicircles in vivo

Txnip C247S mutation protects the heart against acute myocardial infarction

RATIONALE

Thioredoxin-interacting protein (Txnip) is a novel molecular target with translational potential in diverse human diseases. Txnip has several established cellular actions including binding to thioredoxin, a scavenger of reactive oxygen species (ROS). It has been long recognized from in vitro evidence that Txnip forms a disulfide bridge through cysteine 247 (C247) with reduced thioredoxin to inhibit the anti-oxidative properties of thioredoxin. However, the physiological significance of the Txnip-thioredoxin interaction remains largely undefined in vivo.

OBJECTIVE

A single mutation of Txnip, C247S, abolishes the binding of Txnip with thioredoxin. Using a conditional and inducible approach with a mouse model of a mutant Txnip that does not bind thioredoxin, we tested whether the interaction of thioredoxin with Txnip is required for Txnip's pro-oxidative or cytotoxic effects in the heart.

METHODS AND RESULTS

Overexpression of Txnip C247S in cells resulted in a reduction in ROS, due to an inability to inhibit thioredoxin. Hypoxia (1% O₂, 24 h)-induced killing effects of Txnip were decreased by lower levels of cellular ROS in Txnip C247S-expressing cells compared with wild-type Txnip-expressing cells. Then, myocardial ischemic injuries were assessed in the animal model. Cardiomyocyte-specific Txnip C247S knock-in mice had better survival with smaller infarct size following myocardial infarction (MI) compared to control animals. The absence of Txnip's inhibition of thioredoxin promoted mitochondrial anti-oxidative capacities in cardiomyocytes, thereby protecting the heart from oxidative damage induced by MI. Furthermore, an unbiased RNA sequencing screen identified that hypoxia-inducible factor 1 signaling pathway was involved in Txnip C247S-mediated cardioprotective mechanisms.

CONCLUSION

Txnip is a cysteine-containing redox protein that robustly regulates the thioredoxin system via a disulfide bond-switching mechanism in adult cardiomyocytes. Our results provide the direct in vivo evidence that regulation of redox state by Txnip is a crucial component for myocardial homeostasis under ischemic stress.

A Practical Guide to Recombineering in Photorhabdus and Xenorhabdus

Fluent genetic manipulation of prokaryote genomes is still limited to only a few commonly used hosts. Ideally the advanced technologies available for cloning into recombinant Escherichia coli should also be applicable in other prokaryotes. In particular, 'recombineering' is mediated by the lambda Red operon that permits fluent and precise engineering of the E. coli genome and associated recombinant DNA. The major limitation is that host-specific phage-derived recombination systems are also required in more distant species. Recently, an endogenous Red-like operon Pluγβα has been reported to be effective in both Photorhabdus and Xenorhabdus bacteria. The Pluγβα recombineering system is based on three host-specific phage proteins from Photorhabdus luminescens, Plu2935, Plu2936, and Plu2934, which are functional analogs of Redβ, Redα, and Redγ, respectively. In this chapter, we provide a comprehensive and up-to-date method for P. luminescens and Xenorhabdus stockiae genome engineering via the Pluγβα recombineering system. In order to facilitate the rapid construction of knock-in vectors, recET-mediated recombineering is incorporated in the pipeline. Concerted recET system in E. coli with Pluγβα system in Photorhabdus and Xenorhabdus could promote reverse genetics, functional genomics, and bioprospecting research for these two genera.

Markerless Deletion System for Escherichia coli Using Short Homologous Sequences and Positive-Negative Selectable Cassette

Red homologous recombination has been extensively used in recombineering. Because foreign sequences, such as antibiotic resistance genes, FRT-sites, or loxP-sites, are often unwanted in mutant Escherichia coli, we established a markerless deletion system containing short homologous sequences, a positive-selectable marker (kan), and a negative-selectable marker (sacB) for E. coli. For markerless deletion of a specific region of the E. coli genome, a two-step recombination procedure using two different PCR fragments, which were amplified from pUC57-kan-sacB and pUC57-298, was performed. The generation of a pheA-tyrA deficient mutant demonstrated that this markerless deletion system was a simple and efficient method to generate markerless chromosomal deletions in E. coli.

RecET driven chromosomal gene targeting to generate a RecA deficient Escherichia coli strain for Cre mediated production of minicircle DNA

BACKGROUND

Minicircle DNA is the non-replicating product of intramolecular site-specific recombination within a bacterial minicircle producer plasmid. Minicircle DNA can be engineered to contain predominantly human sequences which have a low content of CpG dinucleotides and thus reduced immunotoxicity for humans, whilst the immunogenic bacterial origin and antibiotic resistance marker gene sequences are entirely removed by site-specific recombination. This property makes minicircle DNA an excellent vector for non-viral gene therapy. Large-scale production of minicircle DNA requires a bacterial strain expressing tightly controlled site-specific recombinase, such as Cre recombinase. As recombinant plasmids tend to be more stable in RecA-deficient strains, we aimed to construct a recA- bacterial strain for generation of minicircle vector DNA with less chance of unwanted deletions.

RESULTS

We describe here the construction of the RecA-deficient minicircle DNA producer Escherichia coli HB101Cre with a chromosomally located Cre recombinase gene under the tight control of the araC regulon. The Cre gene expression cassette was inserted into the chromosomal lacZ gene by creating transient homologous recombination proficiency in the recA- strain HB101 using plasmid-born recET genes and homology-mediated chromosomal "pop-in, pop-out" of the plasmid pBAD75Cre containing the Cre gene and a temperature sensitive replication origin. Favourably for the Cre gene placement, at the "pop-out" step, the observed frequency of RecET-led recombination between the proximal regions of homology was 10 times higher than between the distal regions. Using the minicircle producing plasmid pFIXluc containing mutant loxP66 and loxP71 sites, we isolated pure minicircle DNA from the obtained recA- producer strain HB101Cre. The minicircle DNA preparation consisted of monomeric and, unexpectedly, also multimeric minicircle DNA forms, all containing the hybrid loxP66/71 site 5'-TACCGTTCGT ATAATGTATG CTATACGAAC GGTA-3', which was previously shown to be an inefficient partner in Cre-mediated recombination.

CONCLUSION

Using transient RecET-driven recombination we inserted a single copy of the araC controlled Cre gene into the lacZ gene on the chromosome of E. coli recA- strain HB101. The resultant recA- minicircle DNA producer strain HB101Cre was used to obtain pure minicircle DNA, consisting of monomeric and multimeric minicircle forms. The obtained recA- minicircle DNA producer strain is expected to decrease the risk of undesired deletions within minicircle producer plasmids and, therefore, to improve production of the therapeutic minicircle vectors.