Advances and applications of genetically modified pig models in biomedical and agricultural field.

Compared with rodents, pigs are closer to humans in terms of anatomy, metabolism and physiology, so they are ideal animal models of human diseases and xenotransplantation donors. In addition, as one of the most important livestock in China, pigs are closely related to our lives in terms of breeding improvement, disease prevention and animal welfare. In this review, we mainly summarize the research progress and future application of genetically modified pig models in the fields of xenotransplantation, molecular breeding and human disease models. We wish to take this opportunity to raise the awareness of researchers in related fields on cutting-edge technologies such as gene editing and understand the significance of genetically modified pig models in life science research.

[Expression analysis of green fluorescent protein in tissues and organs in α-1,3 galactosyltransferase knockout pigs].

The pig is an ideal source to provide organs because its organ size and physiology are similar to humans. However, an acute rejection will ensue after pig-to-human xenotransplantation. The α-1,3 galactosyltransferase gene knockout (GTKO) pigs were generated in recent years, and could solve the problem of hyperacute rejection. But due to lack of reporting genes, the rejection status of cells and organs post pig-to-human xenotransplantation cannot be visualized. In this study, we introduced the enhanced green fluorescent protein (EGFP) gene driven by the CAG promoter into GTKO porcine ear fibroblasts. Then we produced transgenic pigs expressing the EGFP gene by nuclear transfer technology. Expression levels of EGFP in different tissues and organs of the cloned pig were investigated by Nightsea DFP-1 Fluorescent Protein Flashlight, fluorescence microscope and quantitative PCR assays. The results showed that the protein and transcript of EGFP were expressed in all tissues and organs of the GTKO pig, but the expression was weak in the liver and central nervous system. In conclusion, we have successfully produced the transgenic GTKO pigs expressing EGFP in all tested tissues and organs, which builds up a good basis to track transplanted cells or tissues.

[Production of myostatin gene knockout Wuzhishan miniature pig fibroblasts with zinc-finger nucleases].

Disruption of myostatin (MSTN) gene in pigs may improve porcine lean meat percentage (LMP), and create an animal model for certain human diseases. Using zinc-finger nucleases (ZFNs) technology, MSTN gene was deleted in Wuzhishan miniature pig fibroblasts by transfection of either ZFNs plasmids or ZFNs mRNA in high efficiency. Strikingly, ZFNs encoding mRNA could produce MSTN+/-and MSTN-/- cell colonies with single or double allele deletion by single transfection. Sequencing results demonstrated that 92.18% of the mutations were short fragment deletions or insertions (≤10 bp). Prediction of amino acids sequences indicated that more than half of the mutations cause premature transla-tional-termination codon. MSTN+/+, MSTN+/-, and MSTN-/- cell colonies were used as nuclear donor for somatic cell nuclear transfer (SCNT), and developmental potential of SCNT embryos were measured by the blastocyst rate. The results revealed no significant difference in development competence among the three kinds of reconstructed embryos (14.29% vs. 19.64% vs. 16.13%), which provides the possibility of making myostatin knock out pigs in the future.

[Molecular cloning, expression profile analysis and construction of adipose tissue specific expression vector of pig Gli1 gene].

The Hedgehog (Hh) signaling pathway inhibits fat accumulation, which is conserved in a wide variety of organisms from Drosophila to vertebrates, but few reports about its effect on pigs are available. In this study, pig Gli1 gene was cloned for the first time by rapid amplification of cDNA ends (RACE) and RT-PCR. Pig Gli1 expression profiles were then studied in different tissues and in different developmental stages of the adipose tissue of pigs using real-time PCR. Finally, the eukaryotic expression vector and the adipose tissue specific expression vector were constructed. The results showed that the full pig Gli1 cDNA length was 3 576 bp, the genomic sequence contained 10 715 bp with 12 exons, and 1 106 amino acids were encoded. Pig Gli1 was predicted as an unstable hydrophilic protein without a tans-membrane structure or a signal peptide. The C2H2 zinc finger domain and a nuclear localization sequence were found in pig Gli1. A homology analysis of the Gli1 amino acids and the genomic sequences among seven species showed that the identities were all greater than 80%, which indicates that Gli1 is highly conserved among different species. Tissue expression profile analysis showed that pig Gli1 was only expressed in the tone tissue of adult pigs. Analysis of the pig adipose tissue developmental process showed that Gli1 was detected in the adipose tissue of one-week-old pigs, but not in one-month-old and three-month-old pigs. Finally, a pig Gli1 eukaryotic expression vector was constructed and properly expressed with cell transfection. An adipose tissue specific expression vector was constructed for transgenic animal studies.

[DNA methylation patterns of mouse tetraploid embryos].

In the present study, the DNA methylation patterns of in vitro-derived mouse tetraploid embryos were investigated by immunofluorescence staining with an antibody against 5-methylcytosine (5MeC). Tetraploid embryos could be produced by electrofusion at the stage of two-cell embryos, which could develop to blastocysts followed by fusion of cytoplasm and nucleus and cleavage in vitro. During the fusion of cytoplasm, the DNA methylation levels of the fused embryos are as high as these of two-cell diploid embryos in vivo Then the embryos are rapidly demethylated when the nucleus begin to fuse, resulting in the lowest DNA methylation levels when the nucleus are fused completely. After that, the DNA methylation levels of the fused embryos are gradually increased until the morula stage. However, whereas an asymmetric distribution of DNA methylation is established in vivo-derived blastocysts with a higher methylation level in the inner cell mass (ICM) than that in the trophectoderm, we can not detect the asymmetric distribution in most in vitro-derived tetraploid blastocysts. So the DNA methylation patterns of mouse tetraploid embryos are aberrant, which may lead to subsequent developmental failure and embryo death. This is the first report on the methylation patterns of in vitro-derived mouse tetraploid embryos.

In vitro developmental competence of pig nuclear transferred embryos: effects of GFP transfection, refrigeration, cell cycle synchronization and shapes of donor cells.

The present study was designed to evaluate the feasibility of producing pig transgenic blastocysts expressing enhanced green fluorescent protein (GFP) and to examine the effects of shape and preparation methods of donor cells on in vitro developmental ability of pig nuclear transferred embryos (NTEs). In experiment 1, the effect of GFP transfection on development of pig NTEs was evaluated. The cleavage and blastocyst rates showed no significant difference between NTEs derived from transfected and non-transfected donors. In experiment 2, the effect of different nuclear donor preparation methods on in vitro development of NTEs was examined. The cleavage rate showed no statistically significant differences among three preparation methods. The blastocyst rates of donor cells treated once at -4 degrees C and those of freshly digested cells were similar to each other (26.3% vs 17.9%). The lowest blastocyst rates (5.88%) were observed when cells cryopreserved at -196 degrees C were used as donors. In experiment 3, the effect of different cell cycle synchronization methods on the in vitro development potential of pig NTEs was evaluated. The cleavage rate of NTEs derived from cycling cells was much better than that of NTEs derived from serum-starved cells (64.4% vs 50.5%, p < 0.05), but no significant difference was observed between the the blastocyst rates of the two groups. In experiment 4, the effect of different shapes of cultured fibroblast cells on the in vitro development of pig NTEs was examined. The fusion rate for couplets derived from rough cells was poorer than that observed in couplets derived from round smooth cells (47.8% vs 76.8%, p < 0.05). However, there were no significant differences observed in the cleavage rate and blastocyst rate. In conclusion, the present study indicated that (i) refrigerated pig GFP-transfected cells could be used as donors in nuclear transfer and these NTEs could be effectively developed to blastocyst stage; (ii) serum starvation of GFP-transfected cells is not required for preimplantation development of pig NTEs; and (iii) a rough surface of GFP-transfected donor cells affects fusion rate negatively but has no influence on the cleavage rate or blastocyst rate of pig NTEs.