Production of genetically modified porcine blastocysts by somatic cell nuclear transfer: preliminary results toward production of xenograft-competent miniature pigs

A combination of targeted toxin technology and the piggyBac-mediated gene transfer system enables efficient isolation of stable transfectants in nonhuman mammalian cells

Isolation of cells harboring exogenous DNA is typically achieved by the introduction of plasmids, but its efficiency remains still low. In this study, we developed a novel strategy to obtain stable transfectants efficiently. Porcine embryonic fibroblasts were transfected with two plasmids: (i) pTransIEnd, which comprises the ubiquitous promoter, the piggyBac (PB) transposase gene, an internal ribosomal entry site, the Clostridium perfringens-derived endo-β-galactosidase C (EndoGalC) gene, and a poly(A) tail and (ii) a PB-based plasmid, termed pT-EGFP, which contains enhanced green fluorescent protein (EGFP) expression unit flanked by PB acceptor sites. The PB transposase can accelerate the chromosomal integration of transposon vectors. EndoGalC expression results in removal of a cell surface α-Gal epitope, which is specifically recognized by Bandeiraea simplicifolia isolectin-B4 (IB4). Four days after transfection, cells were treated with IB4SAP (IB4 conjugated to saporin, which eliminates any α-Gal epitope-expressing cells) for a short period, followed by standard culture for approximately 10 days. Several colonies emerged, most of which were positive for EGFP expression and lacked TransIEnd. These results indicated that the proposed approach is useful and efficient for obtaining stable transfectants without the use of drug-resistance genes, and offers a novel route for gene manipulation in cultured nonhuman mammalian cells.

Targeted toxin-based selectable drug-free enrichment of Mammalian cells with high transgene expression

Almost all transfection protocols for mammalian cells use a drug resistance gene for the selection of transfected cells. However, it always requires the characterization of each isolated clone regarding transgene expression, which is time-consuming and labor-intensive. In the current study, we developed a novel method to selectively isolate clones with high transgene expression without drug selection. Porcine embryonic fibroblasts were transfected with pCEIEnd, an expression vector that simultaneously expresses enhanced green fluorescent protein (EGFP) and endo-β-galactosidase C(EndoGalC; an enzyme capable of digesting cell surface α-Gal epitope) upon transfection. After transfection, the surviving cells were briefly treated with IB4SAP (α-Gal epitope-specific BS-I-B₄ lectin conjugated with a toxin saporin). The treated cells were then allowed to grow in normal medium, during which only cells strongly expressing EndoGalC and EGFP would survive because of the absence of α-Gal epitopes on their cell surface. Almost all the surviving colonies after IB4SAP treatment were in fact negative for BS-I-B₄ staining, and also strongly expressed EGFP. This system would be particularly valuable for researchers who wish to perform large-scale production of therapeutically important recombinant proteins.