Developing efficient strategies for the generation of transgenic cattle which produce biopharmaceuticals in milk

Expression of a recombinant anti-programed cell death 1 antibody in the mammary gland of transgenic mice

Nivolumab, a fully human IgG4 anti-programed cell death 1（PD-1）antibody, is recently one of the most popular and successful therapeutic monoclonal antibodies in clinical use. With the increasing demands for Nivolumab and other therapeutic monoclonal antibodies, the mammary gland bioreactor has been regarded as another choice for the production of recombinant monoclonal antibodies besides mammalian cell culture. Here, we expressed a recombinant human anti-PD-1 antibody in the mammary glands of transgenic mice. Two expression vectors were constructed bearing the heavy and light chains of anti-PD-1 antibody respectively under the control of bovine α_s1-casein promoter. Transgenic mice were then generated by co-microinjection of the two expression cassettes. Three F0 founders with both heavy chain and light chain positive were obtained. Transgenes of both chains were detected to be stably transmitted to the offspring. The recombinant antibody was detected in the milk of transgenic mice with the highest expression level up to 80.52 ± 0.82 mg/L and could specifically binds to the human PD-1 antigen. Therefore, our results suggest the feasibility of anti-PD-1 antibody production in the milk of transgenic animals.

Identifying Biomarkers of Autophagy and Apoptosis in Transfected Nuclear Donor Cells and Transgenic Cloned Pig Embryos

In this study, we first investigated the effects of 3-methyladenine (3-MA), an autophagy inhibitor, and the inducer – rapamycin (RAPA) on the incidence of programmed cell death (PCD) symptoms during in vitro development of porcine somatic cell nuclear transfer (SCNT)-derived embryos. The expression of autophagy inhibitor mTOR protein was decreased in porcine SCNT blastocysts treated with 3MA. The abundance of the autophagy marker LC3 increased in blastocysts following RAPA treatment. Exposure of porcine SCNT-derived embryos to 3-MA suppressed their developmental abilities to reach the blastocyst stage. No significant difference in the expression pattern of PCD-related proteins was found between non-transfected dermal cell and transfected dermal cell groups. Additionally, the pattern of PCD in SCNT-derived blastocysts generated using SC and TSC was not significantly different, and in terms of porcine SCNT-derived embryo development rates and total blastocyst cell numbers, there was no significant difference between non-transfected cells and transfected cells. In conclusion, regulation of autophagy affected the development of porcine SCNT embryos. Regardless of the type of nuclear donor cells (transfected or non-transfected dermal cells) used for SCNT, there was no difference in the developmental potential and quantitative profiles of autophagy/apoptosis biomarkers between porcine transgenic and non-transgenic cloned embryos. These results led us to conclude that PCD is important for controlling porcine SCNT-derived embryo development, and that transfected dermal cells can be utilized as a source of nuclear donors for the production of transgenic cloned progeny in pigs.

Improvement of transgenic cloning efficiencies by culturing recipient oocytes and donor cells with antioxidant vitamins in cattle

The present study was conducted to investigate effects of antioxidants during maturation culture of recipient oocytes and/or culture of gene-transfected donor cells on the meiotic competence of recipient oocytes, and the developmental competence and quality of the reconstructed embryos after nuclear transfer (NT) in cattle. Gene-transfected donor cells had negative effects on the proportions of blastocyst formation, total cell numbers, and DNA fragmentation indices of reconstructed embryos. Supplementation of either vitamin E (alpha-tocopherol: 100 microM) or vitamin C (ascorbic acid: 100 microM) during maturation culture significantly enhanced the cytoplasmic maturation of oocytes and subsequent development of embryos reconstructed with the oocytes and gene-transfected donor cells, but did not have synergistic effects. The supplementation of vitamin E during maturation culture of recipient oocytes increased the proportions of fusion and blastocyst formation of gene-transfected NT embryos, in which the proportions were similar to those of nontransfected NT embryos. When the gene-transfected donor cells that had been cultured with 0, 50, or 100 microM of vitamin E were transferred into recipient oocytes matured with vitamin E (100 microM), 50 microM of vitamin E increased the proportion of blastocyst formation and reduced the index of DNA fragmentation of blastocysts. In conclusion, gene-transfected donor cells have negatively influenced the NT outcome. Supplementation of vitamin E during both recipient oocyte maturation and donor cell culture enhanced the blastocyst formation and efficiently blocked DNA damage in transgenic NT embryos.

Cellular Composition and Viability of Cloned Bovine Embryos Using Exogene-Transfected Somatic Cells

The present study compared the efficiency of transgenic (TG) cloned embryo production by somatic cell nuclear transfer (SCNT) with fetal-derived fibroblast cells (FFCs) which were transfected with pEGFP-N1 to in vitro-fertilized (IVF), parthenogenetic and SCNT counterparts by evaluating the rates of cleavage and blastocyst formation, apoptosis rate at different developmental stages, cell number, ploidy and gene expression in blastocysts. In SCNT and TG embryos, the rates of cleavage and blastocyst formation were significantly lower (p < 0.05) than those of IVF controls, but it did not differ between SCNT and TG embryos. In IVF control, 86.7% embryos displayed diploid chromosomal complements and the rates were significantly (p < 0.05) higher than those of SCNT and TG embryos. Most TG embryos (79%) with FFCs expressed the gene by both PCR and under fluorescence microscopy. The expression of apoptosis by TUNEL was first detected at six to eight cell stages in all embryos of IVF, SCNT and TG groups, but the expression rate at each developmental stages was significantly higher (p < 0.05) in SCNT and TG embryos than in IVF counterparts. The expression rate in inner cell mass (ICM) of TG embryos was significantly higher (p < 0.05) than in SCNT and IVF embryos. These results indicate that the high occurrence of apoptosis observed in SCNT and TG embryos compared with IVF counterparts might influence the developmental competence. Moreover, the SCNT embryos derived using non-transfected donor cells exhibited a lower apoptosis expression in ICM cells than in TG embryos derived using pEGP-N1-transfected donor cells suggesting a possible role of negative gene effect in TG embryos.

Cloning cattle: the methods in the madness

Somatic cell nuclear transfer (SCNT) is much more widely and efficiently practiced in cattle than in any other species, making this arguably the most important mammal cloned to date. While the initial objective behind cattle cloning was commercially driven--in particular to multiply genetically superior animals with desired phenotypic traits and to produce genetically modified animals-researchers have now started to use bovine SCNT as a tool to address diverse questions in developmental and cell biology. In this paper, we review current cattle cloning methodologies and their potential technical or biological pitfalls at any step of the procedure. In doing so, we focus on one methodological parameter, namely donor cell selection. We emphasize the impact of epigenetic and genetic differences between embryonic, germ, and somatic donor cell types on cloning efficiency. Lastly, we discuss adult phenotypes and fitness of cloned cattle and their offspring and illustrate some of the more imminent commercial cattle cloning applications.

Designer milk

Dairy biotechnology is fast gaining ground in the area of altering milk composition for processing and/or animal and human health by employing nutritional and genetic approaches. Modification of the primary structure of casein, alteration in the lipid profile, increased protein recovery, milk containing nutraceuticals, and replacement for infant formula offer several advantages in the area of processing. Less fat in milk, altered fatty acid profiles to include more healthy fatty acids such as CLA and ω‐fats, improved amino acid profiles, more protein, less lactose, and absence of β‐lactoglobulin (β‐LG) are some opportunities of “designing” milk for human health benefits. Transgenic technology has also produced farm animals that secrete in their milk, human lactoferrin, lysozyme, and lipase so as to simulate human milk in terms of quality and quantity of these elements that are protective to infants. Cow milk allergenicity in children could be reduced by eliminating the β‐LG gene from bovines. Animals that produce milk containing therapeutic agents such as insulin, plasma proteins, drugs, and vaccines for human health have been genetically engineered. In order to cater to animal health, transgenic animals that express in their mammary glands, various components that work against mastitis have been generated. The ultimate acceptability of the “designer” products will depend on ethical issues such as animal welfare and safety, besides better health benefits and increased profitability of products manufactured by the novel techniques.

Transgenic cows that produce recombinant human lactoferrin in milk are not protected from experimental Escherichia coli intramammary infection

This is the first study describing an experimental mastitis model using transgenic cows expressing recombinant human lactoferrin (rhLf) in their milk. The aim of the study was to investigate the concentrations in milk and protective effects of bovine and recombinant human lactoferrin in experimental Escherichia coli mastitis. Experimental intramammary infection was induced in one udder quarter of seven first-lactating rhLf-transgenic cows and six normal cows, using an E. coli strain isolated from cows with clinical mastitis and known to be susceptible to Lf in vitro. Clinical signs were recorded during the experimental period, concentrations of human and bovine Lf and indicators of inflammation and bacterial counts were determined for milk, and concentrations of acute-phase proteins and tumor necrosis factor alpha were determined for sera and milk. Serum cortisol and blood hematological and biochemical parameters were also determined. Expression levels of rhLf in the milk of transgenic cows remained constant throughout the experiment (mean, 2.9 mg/ml). The high Lf concentrations in the milk of transgenic cows did not protect them from intramammary infection. All cows became infected and developed clinical mastitis. The rhLf-transgenic cows showed milder systemic signs and lower serum cortisol and haptoglobin concentrations than did controls. This may be explained by lipopolysaccharide-neutralizing and immunomodulatory effects of the high Lf concentrations in their milk. However, Lf does not seem to be a very efficient protein for genetic engineering to enhance the mastitis resistance of dairy cows.

Human and bovine lactoferrins in the milk of recombinant human lactoferrin-transgenic dairy cows during lactation

Seven Friesian human lactoferrin (hLf)-transgenic primiparous dairy cows expressing recombinant hLf (rhLf) in their milk were included in the study. After calving, concentrations of rhLf and bovine LF (bLf) in the milk, somatic cell count and milk yield were determined. The concentration of rhLf was found to be constant, about 2.9 mg/mL, throughout the early lactation period of 3 months. The concentration of bLf in colostrum was higher after calving, but decreased rapidly during the first days of lactation. The mean concentration of bLf was 0.15 mg/mL, but concentrations varied between cows from 0.07 mg/mL to 0.26 mg/mL. Based on that, it may be possible to improve the non-specific host defence mechanism in the mammary gland of dairy cows by enhancing the content of rhLf in the milk.

Human lactoferrin transgenic rabbits produced efficiently using dimethylsulfoxide - sperm-mediated gene transfer

Transgenic animal mammary gland bioreactors are used to produce recombinant proteins. However, it is difficult to validate whether these transgenic domestic animals are able to express the recombinant protein efficiently in their mammary glands before the birth of transgenic offspring. In the present study, a simple and efficient method was established to evaluate the functionality of animal mammary gland tissue-expressed cassettes. The gene transfer vector pGBC2LF was constructed, and the expression of human lactoferrin (LF) gene was controlled by the goat β-casein gene 5′ flanking sequence. To obtain the most efficient transfection, the influence of DNA concentration, dimethylsulfoxide (DMSO) concentration, and the ratio of linear-to-circular DNA required for associating DNA with spermatozoa were evaluated. Transfection of exogenous DNA into rabbit spermatozoa was found to be efficient using 30 μg mL–1 DNA, DMSO at a final concentration of 3%, and a 3 : 1 ratio of linear-to-circular DNA, with 29 of 85 (34.1%) in vitro-fertilised embryos being transgenic. Using DMSO–sperm-mediated gene transfer (DMSO-SMGT), 89 rabbit offspring were produced, with 46 of these (57.1%) being transgenic. As mammary gland bioreactor models, 17 of 21 (81%) transgenic female rabbits could express human LF protein in their glands. During lactation of the transgenic rabbits, the highest level of human LF protein expressed was 153 ± 31 μg mL–1, and the mean expression level in all of the transgenic rabbits was 103 ± 20 μg mL–1 in the third week, declining gradually after this time. Our results demonstrate that transgenic rabbits produced by DMSO–SMGT were able to express human LF protein in the correct tissue.

Carbohydrate phenotyping of human and animal milk glycoproteins

Breast-milk has a well-known anti-microbial effect, which is in part due to the many different carbohydrate structures expressed. This renders it a position as a potential therapeutic for treatment of infection by different pathogens, thus avoiding the drawbacks of many antibiotics. The plethora of carbohydrate epitopes in breast-milk is known to differ between species, with human milk expressing the most complex one. We have investigated the expression of protein-bound carbohydrate epitopes in milk from man, cow, goat, sheep, pig, horse, dromedary and rabbit. Proteins were separated by SDS-PAGE and the presence of carbohydrate epitopes on milk proteins were analysed by Western blotting using different lectins and carbohydrate-specific antibodies. We show that ABH, Lewis (Le)x, sialyl-Lex, Lea, sialyl-Lea and Leb carbohydrate epitopes are expressed mainly on man, pig and horse milk proteins. The blood group precursor structure H type 1 is expressed in all species investigated, while only pig, dromedary and rabbit milk proteins carry H type 2 epitopes. These epitopes are receptors for Helicobacter pylori (Leb and sialyl-Lex), enteropathogenic (H type 1, Lea and Lex) and enterotoxic Escherichia coli (heat-stable toxin; H type 1 and 2), and Campylobacter jejuni (H type 2). Thus, milk from these animals or their genetically modified descendants could have a therapeutic effect by inhibiting pathogen colonization and infection.

Ultrasonographic Monitoring of Nuclear Transferred Fetal Weight during the Final Stage of Gestation in Holstein Cows

Dystocia or stillbirth accompanied by Large Offspring Syndrome (LOS) occurs rather frequently in Holstein nuclear transferred calves. In regard to prophylaxes, nuclear transferred Holstein fetuses were monitored with ultrasonography during the final stage of gestation. Fetal weight was estimated weekly based on the fetal metacarpal width using ultrasonography. Fourteen Holstein cows pregnant with Holstein nuclear transferred fetuses were the subjects of this experiment. The fetal weight was estimated by measuring the fetal metacarpal width during the last month of gestation according to the expected date of parturition. Measurements were performed on a weekly basis. The ultrasound-estimated metacarpal width and body weight of 13 of the fetuses in the last week of gestation (30.2+/-2.2 mm, 50.0+/-4.7 kg) were similar to the actual measurement immediately after birth (30.0+/-2.1 mm, 51.2+/-5.5 kg). These results indicate that ultrasonographic monitoring within a week of parturition to is accurate for estimating fetal weight. Prediction of LOS with ultrasonography contributes to reliable a diagnostic method that minimizes syndrome-related gyneco-obstetric complications at parturition with the aid of appropriate treatments.

Efficiency of gene transfection into donor cells for nuclear transfer of bovine embryos

The production of transgenic (TG) animals by somatic cell nuclear transfer (SCNT) has proven to be a more efficient method than other methods, such as gene injection or sperm mediation. The present study was intended to evaluate the efficiency of gene transfection by Effectene (Qiagen, Inc.), a lipid-based reagent compared to electroporation in fetal-derived fibroblast cells (FFC), cumulus-derived fibroblast cells (CFC), and adult ear skin-derived fibroblast cells (AEFC). Parameters compared were factors such as chromosome abnormality, gene expression, and the incidence of apoptosis. Further, the TG embryos with transfected donor cells generated by electroporation or Effectene were compared to IVF and SCNT embryos in terms of rates of cleavage, blastocyst formation, and blastocyst cell number. Most of the cells (>80%) at confluence were at G0/G1 and considered to be suitable nuclear donors for cloning. Transfection with a plasmid containing the enhanced green fluorescent protein (pEGFP-N1) gene into FFC did not increase the incidence of chromosomal abnormalities. The rates of apoptosis in different cell types transfected with pEGFP-N1 were 3.3%-5.0%, and the values did not differ among groups. In addition, the rates of apoptosis in various cells between 5-7 and 20-22 cell passages did not differ. However, the efficiency of gene transfecton into FFC by Effectene reagent (14.2 +/- 1.7) was significantly (P < 0.05) higher than that obtained by electroporation (5.1 +/- 1.0). Among various cell types, the efficiency of gene transfection by Effectene and eletroporation of FFC (14.2 +/- 1.7 and 5.1 +/- 1.0, respectively) was significantly (P < 0.05) higher than transfection of CFC and AEFC by either method (9.4 +/- 1.5 and 3.3 +/- 0.8, 8.8 +/- 0.7, and 2.1 +/- 0.4, respectively). In TG embryos produced by SCNT with electroporation and Effectene, the rates of cleavage and blastocyst formation were significantly lower (P < 0.05) than those of IVF controls, but rates did not differ between SCNT and TG embryos. Similarly, significantly higher (P < 0.05) total cell numbers in day-8 blastocysts were observed in IVF controls than those in SCNT and TG embryos, but did not differ between SCNT and TG (136 vs. approximately 110, respectively). The results demonstrated that, though there were no difference in the rates of chromosomal aneuploidy and the incidence of apoptosis among various cell types, transfected with or without pEGFP-N1, FFC were the cell type most effectively transfected and Effectene was a suitable agent for transfection.

Production of bioproducts through the use of transgenic animal models

Transgenic livestock that produce recombinant proteins in their milk can provide an economic and safe system for production of valuable proteins, such as pharmaceutical proteins for treatment or prevention of human disease or biomaterials for medical use. This method of production is frequently referred to as biopharming. The promise of biopharming, that is the actual commercial production of pharmaceuticals and other bioproducts, is nearing fulfillment. Improvements in molecular and reproductive techniques and strong economic incentives have continued to drive the implementation of transgenic technology to domestic animals. Nuclear transfer using transgenic donor cells is rapidly becoming the predominant technique used in the production of transgenic livestock, replacing the direct injection of DNA into the zygotic pronuclei. Production of transgenic founder animals by nuclear transfer in combination with traditional reproductive technologies can result in the propagation of transgenic herds of sufficient size to meet market demands for commercially important proteins. While some of the companies that have established transgenic programs have run into setbacks owing to a combination of economic, scientific and regulatory difficulties, other companies are continuing to make significant advances. While further improvements are needed to increase efficiencies of production, economically viable production of recombinant proteins using livestock species is not only possible but should be a commercial reality in the very near future.

Cloning: questions answered and unsolved

Cloning by the transfer of adult somatic cell nuclei to oocytes has produced viable offspring in a variety of mammalian species. The technology is still in its initial stages of development. Studies to date have answered several basic questions related to such issues as genome potency, life expectancy of clones, mitochondrial fates, and feasibility of inter-species nuclear transfer. They have also raised new questions related to the control of nuclear reprogramming and function. These questions are reviewed here.

Commercial aspects of cloning and genetic modification in cattle

A range of potential commercial applications of cloning and genetic modification in cattle has been suggested over the last decade. It includes the rapid multiplication of elite genotypes, production of valuable human proteins, altered production characteristics, increased disease resistance and milk with improved nutritional value and processing capabilities. However, an economic return from the sale of product is far from reality in any of these areas. One impediment to achieving economic sustainability is the extremely low efficiency in producing healthy offspring from transferred cloned embryos. Other significant impediments are societal concerns surrounding such technologies, animal welfare issues and regulatory requirements. This review will focus on current biological limitations and technical capabilities in commercial settings, the changes required to allow the production and sale of products at economically sustainable levels, cryopreservation and the progress towards automation of cloning techniques.

The current status and future of commercial embryo transfer in cattle

A commercially viable cattle embryo transfer (ET) industry was established in North America during the early 1970s, approximately 80 years after the first successful embryo transfer was reported in a mammal. Initially, techniques for recovering and transferring cattle embryos were exclusively surgical. However, by the late 1970s, most embryos were recovered and transferred nonsurgically. Successful cryopreservation of embryos was widespread by the early 1980s, followed by the introduction of embryo splitting, in vitro procedures, direct transfer of frozen embryos and sexing of embryos. The wide spread adoption of ethylene glycol as a cryoprotectant has simplified the thaw-transfer procedures for frozen embryos. The number of embryos recovered annually has not grown appreciably over the last 10 years in North America and Europe; however, there has been significant growth of commercial ET in South America. Within North America, ET activity has been relatively constant in Holstein cattle, whereas there has been a large ET increase in the Angus breed and a concomitant ET decrease in some other beef breeds. Although a number of new technologies have been adopted within the ET industry in the last decade, the basic procedure of superovulation of donor cattle has undergone little improvement over the last 20 years. The export-import of frozen cattle embryos has become a well-established industry, governed by specific health regulations. The international movement of embryos is subject to sudden and dramatic disturbances, as exemplified by the 2001 outbreak of foot and mouth disease in Great Britain. It is probable that there will be an increased influence of animal rights issues on the ET industry in the future. Several companies in North America are currently commercially producing cloned cattle. The sexing of bovine semen with the use of flow cytometry is extremely accurate and moderate pregnancy rates in heifers have been achieved in field trials, but sexed semen currently is available in only a few countries and on an extremely limited basis. As of yet, all programs involving the production of transgenic cattle are experimental in nature.

Commercialization of animal biotechnology

Commercialization of animal biotechnology is a wide-ranging topic for discussion. In this paper, we will attempt to review embryo transfer (ET) and related technologies that relate to food-producing mammals. A brief review of the history of advances in biotechnology will provide a glimpse to present and future applications. Commercialization of animal biotechnology is presently taking two pathways. The first application involves the use of animals for biomedical purposes. Very few companies have developed all of the core competencies and intellectual properties to complete the bridge from lab bench to product. The second pathway of application is for the production of animals used for food. Artificial insemination (AI), embryo transfer, in vitro fertilization (IVF), cloning, transgenics, and genomics all are components of the toolbox for present and future applications. Individually, these are powerful tools capable of providing significant improvements in productivity. Combinations of these technologies coupled with information systems and data analysis, will provide even more significant change in the next decade. Any strategies for the commercial application of animal biotechnology must include a careful review of regulatory and social concerns. Careful review of industry infrastructure is also important. Our colleagues in plant biotechnology have helped highlight some of these pitfalls and provide us with a retrospective review. In summary, today we have core competencies that provide a wealth of opportunities for the members of this society, commercial companies, producers, and the general population. Successful commercialization will benefit all of the above stakeholders.

The recent history of somatic cloning in mammals

The history of somatic cell nuclear transfer (NT) in mammals is full of exciting experiments and findings regarding the technique and outcome of NT, despite only covering a period of 6 years. The production of Dolly, for the first time demonstrating cloning from an adult somatic cell, had a great impact on subsequent studies. However, the more progress we make, the more obvious it becomes how little we know about the processes during NT, specifically how reprogramming events occur. Therefore, it is certainly challenging to continue investigating every step of somatic cell NT more intensively, starting from the donor cell, (type, cell cycle, synchronization, population doublings) and continuing until the cloned offspring are born and even further, to see how and if NT has an influence on health, viability, quantitative traits, and reproduction of cloned individuals.

Functional homologs of cyanovirin-N amenable to mass production in prokaryotic and eukaryotic hosts

Cyanovirin-N (CV-N) is under development as a topical (vaginal or rectal) microbicide to prevent sexual transmission of human immunodeficiency virus (HIV), and an economically feasible means for very large-scale production of the protein is an urgent priority. We observed that N-glycosylation of CV-N in yeast eliminated the anti-HIV activity, and that dimeric forms and aggregates of CV-N occurred under certain conditions, potentially complicating the efficient, large-scale manufacture of pure monomeric CV-N. We therefore expressed and tested CV-N homologs in which the glycosylation-susceptible Asn residue at position 30 was replaced with Ala, Gln, or Val, and/or the Pro at position 51 was replaced by Gly to eliminate potential conformational heterogeneity. All homologs exhibited anti-HIV activity comparable to wild-type CV-N, and the Pro51Gly homologs were significantly more stable proteins. These glycosylation-resistant, functional cyanovirins should be amenable to large-scale production either in bacteria or in eukaryotic hosts.

Large scale production of recombinant human lactoferrin in the milk of transgenic cows

The limited capacity of current bioreactors has led the biopharmaceutical industry to investigate alternative protein expression systems. The milk of transgenic cattle may provide an attractive vehicle for large-scale production of biopharmaceuticals, but there have been no reports on the characteristics of such recombinant proteins. Here we describe the production of recombinant human lactoferrin (rhLF), an iron-binding glycoprotein involved in innate host defense, at gram per liter concentrations in bovine milk. Natural hLF from human milk and rhLF had identical iron-binding and -release properties. Although natural hLF and rhLF underwent differential N-linked glycosylation, they were equally effective in three different in vivo infection models employing immunocompetent and leukocytopenic mice, and showed similar localization at sites of infection. Taken together, the results illustrate the potential of transgenic cattle in the large-scale production of biopharmaceuticals.

Bovine viral diarrhea virus (BVDV) and anti-BVDV antibodies in pooled samples of follicular fluid

Bovine viral diarrhea virus (BVDV) can be found in cells and fluids from ovaries collected at the abattoir. On the other hand, immunoglobulins are also found in the fluid of ovarian follicles. Anti-BVDV antibodies in follicular fluid might reduce cross-contamination of COCs at the time of collection or hinder the use of virus isolation to test for the presence of virus. One objective of this study was to determine the frequency with which BVDV could be found in pooled follicular fluid collected during the periodic aspiration of COCs from abattoir-origin ovaries. A second objective was to determine the prevalence and neutralizing activity of anti-BVDV antibodies in these blended samples. We collected samples of pooled follicular fluid (n = 55) over a 20-month period as part of our routine oocyte collection activities. We assayed each sample for BVDV using virus isolation as well as reverse transcription nested polymerase chain reaction (RT-nPCR) procedures. We also tested follicular fluid for antibody that would neutralize four representative strains of BVDV (SD-1, a genotype 1a strain; NY-1, a genotype lb strain; CD-87, a genotype 2 strain, and PA-131, a divergent genotype 2 strain). We detected no BVDV by virus isolation, but we did identify the virus by RT-nPCR in one of the 55 samples of follicular fluid. Automated dye terminator nucleotide sequencing of the amplified portion of the viral genome indicated a genotype 1 strain that was distinct from any of our laboratory strains. In addition, each of the samples of follicular fluid contained sufficient antibody to neutralize large quantities of each of the four laboratory strains that were used. Finding BVDV in just 1 of 55 samples was consistent with reports of similar studies in which the occurrence of BVDV in abattoir-origin materials ranged from 0.9 to 12%. We presumed that failure to isolate the virus was due to neutralizing antibody in the sample. Thus, the incidence of BVDV contamination of our IVF system at the level of pooling of follicular fluid was low for the 20-month period. The presence of anti-BVDV antibody in pooled follicular fluid provided a coincidental means of neutralizing BVDV when it was introduced in fluid aspirated from infected ovaries.

Cloning: experience from the mouse and other animals

Cloning mammals has been successful for many years by splitting an early embryo or transferring embryonic cell nuclei into enucleated oocytes. Cloning is now possible with adult somatic cells. At present, cloning efficiency--as determined by the proportion of live offspring developed from all oocytes that received donor cell nuclei--is low regardless of the cell type (including, embryonic stem (ES) cells) and animal species used. In all animals, except of Japanese black beef cattle, the vast majority (>97%) of cloned embryos perish before reaching full term. Even in the Japanese cattle, less than 20% of cloned embryos reach the adulthood. This low efficiency of cloning seems to be due largely to faulty epigenetic reprogramming of donor cell nuclei after transfer into recipient oocytes. Cloned embryos with major epigenetic errors die before or soon after implantation. Those with relatively 'minor' epigenetic errors may survive birth and reach adulthood. We found that almost all fetuses of inbred mice die at birth from respiratory problems, while those of hybrid mice do not, suggesting that genomic heterogeneity masks-to some extent-faulty epigenetic errors. Thus far, the majority of cloned mice that survived birth, had a normal life span and were fertile. However, these animals may not be totally free of health problems. Postpubertal obesity in certain strains of mice is one example. A trial and error approach may discover better cells for cloning, but it would be wiser to understand the molecular mechanisms of epigenetic nuclear programming and reprogramming to find the way to make cloning safer and more efficient. The relatively high cloning success rate in the Japanese black cattle may provide us a clue of solving the problem of high mortality of cloned offspring.

Nuclear transfer in cattle with non-transfected and transfected fetal or cloned transgenic fetal and postnatal fibroblasts

The efficiency of nuclear transfer (NT) using two primary cultures of fetal fibroblasts (FF1 and FF2) was compared vs. the same cultures transfected with an expression vector in which the bovine prochymosin coding sequence is placed under the control of the bovine alpha(S1)-casein promoter (TFF1 and TFF2). In addition, fibroblasts of a cloned transgenic fetus (TRFF1) derived from TFF1 and ear skin fibroblasts of a 1-month-old cloned transgenic calf (TRCF1) derived from TRFF1 were used as nuclear donors. Embryos reconstructed from FF1 (44%) and FF2 (52%) developed to the blastocyst stage at a significantly (P < 0.05) higher rate than those derived from TFF1 (24%) and TFF2 (27%). The proportions of cleaved embryos and blastocysts were significantly (P < 0.05) higher with TRFF1 than with TRCF1 used as nuclear donors (75 vs. 66% and 33 vs. 16%, respectively). Transfer of NT embryos derived from FF2 and TFF2 to recipients resulted in similar pregnancy rates on day 30 (52 and 48%, respectively). However, with TFF2 embryos, the majority of pregnancies (8/11; 73%) was lost in the first and second trimesters of gestation, whereas 4/11 (36%) pregnancies with FF2 embryos were lost during the full period of in vivo development. Of 11 FF2 and 6 TFF2 born calves (25 and 13% of transferred embryos, respectively), 6 and 3 survived including one oversized FF2 calf. After transfer of TRFF1 and TRCF1 NT embryos to recipients, initial pregnancy rate was as a tendency higher in the TRFF1 (49%) than in the TRCF1 group (30%). The majority (14/17) of TRFF1 pregnancies and all TRCF1 pregnancies were lost in the first and second trimester. A high proportion of TRFF1 calves (5/8) showed increased body weights, and only two calves which were also large survived. These findings demonstrate that (i) extended culture associated with transfection and selection procedures may induce changes of donor cells which markedly decrease the efficiency of nuclear transfer and (ii) these changes are not reversed by recloning.

Nuclear transfer technology in mammalian cloning

The past several years have witnessed remarkable progress in mammalian cloning using nuclear transfer (NT). Until 1997 and the announcement of the successful cloning of sheep from adult mammary gland or fetal fibroblast cells, our working assumption was that cloning by NT could only be accomplished with relatively undifferentiated embryonic cells. Indeed, live offspring were first produced by NT over 15 years ago from totipotent, embryonic blastomeres derived from early cleavage-stage embryos. However, once begun, the progression to somatic cell cloning or NT employing differentiated cells as the source of donor nuclei was meteoric, initially involving differentiated embryonic cell cultures in sheep in 1996 and quickly thereafter, fetal or adult somatic cells in sheep, cow, mouse, goat, and pig. Several recent reviews provide a background for and discussion of these successes. Here we will focus on the potential uses of reproductive cloning along with recent activities in the field and a discussion concerning current interests in human reproductive and therapeutic cloning.

Blastocyst viability and generation of transgenic cattle following freezing of in vitro produced, DNA-injected embryos

This study examined whether the viability, determined in vitro, of DNA-injected bovine embryos produced in vitro was affected by freezing, and if the frozen embryos developed to term following transfer to recipients. In vitro fertilized zygotes were injected with the pBL1 gene and then co-cultured with mouse embryonic fibroblasts (MEF) in CR1aa medium. Embryos were prepared for cryopreservation by exposure to a 10% (v/v) glycerol solution, loaded into 0.25 ml straws and then frozen by conventional slow freezing. Thawing was by rapid warming in water (37 degrees C) and embryos were rehydrated in PBS diluents of 6%, 3% and 0% (v/v) glycerol supplemented with 0.25 M sucrose and 0.5% (w/v) BSA. In Experiment 1, blastocysts that developed from DNA-injected embryos were individually classified into three morphological groups and three stages of development prior to freezing. DNA-injected blastocysts of excellent quality at freezing showed a higher survival rate (78.8+/-10.6%) after thawing than those of good (60. 9+/-16.4%) or fair (12.5+/-5.9%) quality (P<0.05). Post-thaw survival rate, judged in vitro, increased with more advanced stage of blastocyst development at freezing (early 48.8+/-15.9%, mid 52. 1+/-12.6% and expanded 71.2+/-1.1; P<0.05). In Experiment 2, the frozen/thawed embryos were transferred to recipients to examine in vivo viability. Following transfer of one or two embryos per recipient, pregnancy rates at 60 days of gestation were 13.6% (13/96) for frozen embryos and 26.5% (43/162) for fresh embryos (P<0. 05). Of the 12 live calves born from the frozen/thawed embryos, two males (18.3%) were transgenic. None of the live-born calves derived from fresh embryos exhibited the transgene. One of transgenic bulls did not produce transgenic sperm. Three out of 23 calves (13.0%) produced from cows inseminated with semen of the other bull were transgenic, suggesting that this animal was a germ-line mosaic. These studies indicated that the viability of in vitro produced, DNA-injected bovine blastocysts was affected by freezing and by both the quality and stage of development of the embryo prior to freezing. The generation of transgenic cattle demonstrates that it is feasible to freeze DNA-injected, in vitro produced embryos.