Advances in livestock nuclear transfer

Establishment and characterization of fetal fibroblast cell lines for generating human lysozyme transgenic goats by somatic cell nuclear transfer

This study was performed to qualify goat fetal fibroblast (GFF) cell lines for genetic modification and somatic cell nuclear transfer (SCNT) to produce human lysozyme (hLYZ) transgenic goats. Nine GFF cell lines were established from different fetuses, and the proliferative lifespan and chromosomal stability were analyzed. The results suggested that cell lines with a longer lifespan had stable chromosomes compared with those of cells lines with a shorter lifespan. According to the proliferative lifespan, we divided GFF cell lines into two groups: cell lines with a long lifespan (GFF1/2/7/8/9; group L) and cell lines with a short lifespan (GFF3/4/5/6; group S). Next, a hLYZ expression vector was introduced into these cell lines by electroporation. The efficiencies of colony formation, expansion in culture, and the quality of transgenic clonal cell lines were significant higher in group L than those in group S. The mean fusion rate and blastocyst rate in group L were higher than those in group S (80.3 ± 1.7 vs. 65.1 ± 4.2 % and 19.5 ± 0.6 vs. 15.1 ± 1.1 %, respectively, P < 0.05). After transferring cloned embryos into the oviducts of recipient goats, three live kids were born. PCR and Southern blot analyses confirmed integration of the transgene in cloned goats. In conclusion, the lifespan of GFF cell lines has a major effect on the efficiency to produce transgenic cloned goats. Therefore, the proliferative lifespan of primary cells may be used as a criterion to characterize the quality of cell lines for genetic modification and SCNT.

Effect of donor cell types on developmental potential of cattle (Bos taurus) and swamp buffalo (Bubalus bubalis) cloned embryos

This study investigated the effect of donor cell types on the developmental potential and quality of cloned swamp buffalo embryos in comparison with cloned cattle embryos. Fetal fibroblasts (FFs), ear fibroblasts (EFs), granulosa cells (GCs) and cumulus cells (CCs) were used as the donor cells in both buffalo and cattle. The cloned cattle or buffalo embryos were produced by fusion of the individual donor cells with enucleated cattle or buffalo oocytes, respectively. The reconstructed (cloned) embryos and in vitro matured oocytes without enucleation were parthenogenetically activated (PA) and cultured for 7 days. Their developmental ability to the blastocyst stage was evaluated. The total number of trophectoderm (TE) and inner cell mass (ICM) cells and the ICM ratio in each blastocyst was determined by differential staining as an indicator of embryo quality. The fusion rate of CCs with enucleated oocytes was significantly lower than for those of other donor cell types both in cattle and buffalo. The rates of cleavage and development to the 8-cell, morula and blastocyst stages of cloned embryos derived from all donor cell types did not significantly differ within the same species. However, the cleavage rate of cloned cattle embryos derived from FFs was significantly higher than those of cattle PA and cloned buffalo embryos. The blastocyst rates of cloned cattle embryos, except for the ones derived from CCs, were significantly higher than those of cloned buffalo embryos. In buffalo, only cloned embryos derived from CCs showed a significantly higher blastocyst rate than that of PA embryos. In contrast, all the cloned cattle embryos showed significantly higher blastocyst rates than that of PA embryos. There was no difference in ICM ratio among any of the blastocysts derived from any of the donor cell types and PA embryos in both species. FFs, EFs, GCs and CCs had similar potentials to support development of cloned cattle and buffalo embryos to the blastocyst stage with the same quality.

Development Potential of Transgenic Somatic Cell Nuclear Transfer Embryos According to Various Factors of Donor Cell

The present study was conducted to establish an efficient production system for bovine transgenic somatic cell nuclear transfer (SCNT) embryos, the effect of various conditions of donor cells including cell type, size, and passage number on the developmental competence of transgenic SCNT embryos were examined with their expression rates of a marker gene. An expression plasmid for human prourokinase was constructed by inserting a bovine beta-casein promoter, a green fluorescent protein (GFP) marker gene, and a human prourokinase target gene into a pcDNA3 plasmid. Three types of bovine somatic cells including two adult cells (cumulus cells and ear fibroblasts) and fetal fibroblasts were prepared and transfected with the expression plasmid using a liposomal transfection reagent, Fugene6, as a carrier. In Experiment 1, three types of bovine cells were transfected at passages 2 to 4, and then trypsinized and GFP-expressing cells were randomly selected and used for SCNT. Developmental competence and rates of GFP expression in bovine transgenic SCNT embryos reconstructed with cumulus cells were significantly higher than those from fetal and ear fibroblasts. In all cell types used, GFP expression rates of SCNT embryos gradually decreased with the progression of embryo development. In Experiment 2, the effect of passage number of cumulus cells in early (2 to 4) and late (8 to 12) passages was investigated. No significant differences in the development of transgenic SCNT embryos were observed, but significantly higher GFP expression was shown in blastocysts reconstructed with cumulus cells at early passage. In Experiment 3, different sizes of GFP-expressing transfected cumulus cells [large (>30 microm) or small cell (<30 microm)] at passages 2 to 4 were used for SCNT. A significant improvement in embryo development and GFP expression was observed when small cumulus cells were used for SCNT. Taken together, these results demonstrate that (1) adult somatic cells as well as fetal cells could serve as donor cells in transgenic SCNT embryo production and cumulus cells with small size at early passage were the optimal cell type, and (2) transgenic SCNT embryos derived from adult somatic cells have embryonic development potential.

The potential for improving the growth and development of cultured farm animal oocytes

Previous predictions that the technologies for producing genetically engineered large animal embryos containing genes for faster growth rates, leaner carcasses, greater disease resistance and improved lactational performance would be available early in the twenty-first century have been, for the most part, realized. The animal industries have been slow to adopt these technological advances and it cannot be said that any of them are currently having great impact on animal agriculture worldwide. A major reason for this is the inefficiencies of the techniques for superovulation, ovum recovery, in vitro fertilization, nuclear transfer, cloning and embryo transfer. Although improvements in these techniques can be expected, the best hope for increasing the impact of embryo transfer technologies on the animal industries lies in developing ways to mature, harvest, store and fertilize in vitro the large numbers of primordial oocytes present in the ovaries of all farm animals. Although limited progress has been made in the culture of bovine primordial oocytes, it is clear that much more research is needed to achieve success in this important area.

Differential cytoplast requirement for embryonic and somatic cell nuclear transfer in cattle

Effective activation of a recipient oocyte and its compatibility with the nuclear donor are critical to the successful nuclear reprogramming during nuclear transfer. We designed a series of experiments using various activation methods to determine the optimum activation efficiency of bovine oocytes. We then performed nuclear transfer (NT) of embryonic and somatic cells into cytoplasts presumably at G1/S phase (with prior activation) or at metaphase II (MII, without prior activation). Oocytes at 24 hr of maturation in vitro were activated with various combinations of calcium ionophore A23187 (A187) (5 microM, 5 min), electric pulse (EP), ethanol (7%, 7 min), cycloheximide (CHX) (10 micro g/ml, 6 hr), and then cultured in cytochalasin D (CD) for a total of 18 hr. Through a series of experiments (Exp. 1-4), an improved activation protocol (A187/EP/CHX/CD) was identified and used for comparison of NT efficiency of embryonic versus somatic donor cells (Exp. 5). When embryonic cells from morula and blastocysts (BL) were used as nuclear donors, a significantly higher rate of blastocyst development from cloned embryos was obtained with G1/S phase cytoplasts than with MII-phase cytoplasts (36 vs. 11%, P < 0.05). In contrast, when skin fibroblasts were used as donor cells, the use of an MII cytoplast (vs. G1/S phase) was imperative for blastocyst development (30 vs. 6%, P < 0.05). Differential staining showed that parthenogenetic, embryonic, and somatic cloned BL contained 26, 29, and 33% presumptive inner cell mass (ICM) cells, respectively, which is similar to that of frozen-thawed in vivo embryos at a comparable developmental stage (23%). These data indicate that embryonic and somatic nuclei require different recipient cytoplast environment for remodeling/ reprogramming, and this is likely due to the different cell cycle stage and profiles of molecular differentiation of the transferred donor nuclei.

Production of cloned pigs from cultured fetal fibroblast cells

Somatic cell nuclear transfer was used to produce live piglets from cultured fetal fibroblast cells. This was achieved by exposing donor cell nuclei to oocyte cytoplasm for approximately 3 h before activation by chemical means. Initially, an experiment was performed to optimize a cell fusion system that prevented concurrent activation in the majority of recipient cytoplasts. Cultured fibroblast cells were fused in medium with or without calcium into enucleated oocytes flushed from superovulated gilts. Cybrids fused in the presence of calcium cleaved at a significantly (P < 0.05) greater rate (69%, 37 out of 54) after 2 days of culture compared with those fused without calcium (10%, 7 out of 73), suggesting that calcium-free conditions are needed to avoid activation in the majority of recipient cytoplasts during fusion. In the second experiment, cybrids fused in calcium-free medium were activated approximately 3 h later with ionomycin, followed by incubation in 6-dimethylaminopurine to determine development in vitro. Following 2 days of culture, cleavage rates of chemically activated and unactivated cybrids (fusion without activation control) were 93% (100 out of 108) and 7% (2 out of 27), respectively. After an additional 5 days of culture, activated cloned embryos formed blastocysts at a rate of 23% (25 out of 108) with an average inner cell mass and trophectoderm cell number of 10 (range, 3 to 38) and 31 (range, 16 to 58), respectively. In the third experiment, activated nuclear transfer embryos were transferred to the uteri of synchronized recipients after 3 days of culture to assess their development in vivo. Of 10 recipients receiving an average of 80 cleaved embryos (range, 40 to 107), 5 became pregnant (50%) as determined by ultrasound between Day 25 and Day 35 of gestation. Of the five pregnant recipients, two subsequently farrowed one piglet per litter originating from two different cell culture lines. In this study, efficient reprogramming of porcine donor nuclei by fusing cells in the absence of calcium followed by chemical activation of recipient cytoplasts was reflected in high rates of development to blastocyst and pregnancy initiation leading to full term development.

Clonal Lines of Transgenic Fibroblast Cells Derived from the Same Fetus Result in Different Development When Used for Nuclear Transfer in Pigs1

Different factors are believed to influence the outcome of nuclear transfer (NT) experiments. Besides the cell cycle stage of both recipient cytoplast and donor karyoplast, the origin of the donor cells (embryonic, fetal, and adult) is of interest. We compared in vitro development of NT embryos derived from small serum-starved (G0) or small cycling (G1) porcine fetal fibroblast cells. Serum starvation did not have a positive effect on cleavage rate or the percentage of embryos that developed to the morula and blastocyst stages. Next, we investigated the development of porcine NT embryos derived from different transgenic clonal cell lines that had originated from the same fetus. When different clonal lines of fetal fibroblasts were fused to enucleated metaphase II oocytes, differences in fusion rates as well as in development to the morula and blastocyst stages were observed (P < 0.05). When oocytes derived from sow ovaries were used as recipient cytoplasts, significantly better cleavage (P = 0.03) and blastocyst formation (P < 0.014) was obtained when compared with oocytes derived from gilts. Our data indicate that not only different cell lines, but also different clones derived from one primary cell line, result in different development when used for NT. In addition, the use of sow oocytes as a cytoplast source also improves the efficiency of NT experiments.