Transgenic animals: current and alternative strategies

Historical DNA Manipulation Overview

The history of DNA manipulation for the creation of genetically modified animals began in the 1970s, using viruses as the first DNA molecules microinjected into mouse embryos at different preimplantation stages. Subsequently, simple DNA plasmids were used to microinject into the pronuclei of fertilized mouse oocytes and that method became the reference for many years. The isolation of embryonic stem cells together with advances in genetics allowed the generation of gene-specific knockout mice, later on improved with conditional mutations. Cloning procedures expanded the gene inactivation to livestock and other non-model mammalian species. Lentiviruses, artificial chromosomes, and intracytoplasmic sperm injections expanded the toolbox for DNA manipulation. The last chapter of this short but intense history belongs to programmable nucleases, particularly CRISPR-Cas systems, triggering the development of genomic-editing techniques, the current revolution we are living in.

Simple Recommendations for Improving Efficiency in Generating Genome-Edited Mice

The generation of transgenic model organisms (primarily mice) is an integral part of modern fundamental and applied research. Simple techniques based on the biology of these laboratory rodents can often increase efficiency when generating genome-edited mouse strains. In this study, we share our three years of experience in the optimization of mouse genome editing based on microinjection of CRISPR/Cas9 components into ca. 10,000 zygotes. We tested a number of techniques meant to improve efficiency in generating knockout mice, such as optimization of the superovulation method and choosing the optimal mouse strains to be used as zygote donors and foster mothers. The presented results might be useful to laboratories aiming to quickly and efficiently create new mouse strains with tailored genome editing.

Efficient delivery of DNA into bovine preimplantation embryos by multiwall carbon nanotubes

The pellucid zone (PZ) is a protective embryonic cells barrier against chemical, physical or biological substances. This put, usual transfection methods are not efficient for mammal oocytes and embryos as they are exclusively for somatic cells. Carbon nanotubes have emerged as a new method for gene delivery, and they can be an alternative for embryos transfection, however its ability to cross the PZ and mediated gene transfer is unknown. Our data confirm that multiwall carbon nanotubes (MWNTs) can cross the PZ and delivery of pDNA into in vitro-fertilized bovine embryos. The degeneration rate and the expression of genes associated to cell viability were not affected in embryos exposed to MWNTs. Those embryos, however, had lower cell number and higher apoptotic cell index, but this did not impair the embryonic development. This study shows the potential utility of the MWNT for the development of new method for delivery of DNA into bovine embryos.

Transgenic mouse offspring generated by ROSI

The production of transgenic animals is an important tool for experimental and applied biology. Over the years, many approaches for the production of transgenic animals have been tried, including pronuclear microinjection, sperm-mediated gene transfer, transfection of male germ cells, somatic cell nuclear transfer and the use of lentiviral vectors. In the present study, we developed a new transgene delivery approach, and we report for the first time the production of transgenic animals by co-injection of DNA and round spermatid nuclei into non-fertilized mouse oocytes (ROSI). The transgene used was a construct containing the human CMV immediate early promoter and the enhanced GFP gene. With this procedure, 12% of the live offspring we obtained carried the transgene. This efficiency of transgenic production by ROSI was similar to the efficiency by pronuclear injection or intracytoplasmic injection of male gamete nuclei (ICSI). However, ICSI required fewer embryos to produce the same number of transgenic animals. The expression of Egfp mRNA and fluorescence of EGFP were found in the majority of the organs examined in 4 transgenic lines generated by ROSI. Tissue morphology and transgene expression were not distinguishable between transgenic animals produced by ROSI or pronuclear injection. Furthermore, our results are of particular interest because they indicate that the transgene incorporation mediated by intracytoplasmic injection of male gamete nuclei is not an exclusive property of mature sperm cell nuclei with compact chromatin but it can be accomplished with immature sperm cell nuclei with decondensed chromatin as well. The present study also provides alternative procedures for transgene delivery into embryos or reconstituted oocytes.

Efficient Generation of Myostatin Knock-Out Sheep Using CRISPR/Cas9 Technology and Microinjection into Zygotes

While CRISPR/Cas9 technology has proven to be a valuable system to generate gene-targeted modified animals in several species, this tool has been scarcely reported in farm animals. Myostatin is encoded by MSTN gene involved in the inhibition of muscle differentiation and growth. We determined the efficiency of the CRISPR/Cas9 system to edit MSTN in sheep and generate knock-out (KO) animals with the aim to promote muscle development and body growth. We generated CRISPR/Cas9 mRNAs specific for ovine MSTN and microinjected them into the cytoplasm of ovine zygotes. When embryo development of CRISPR/Cas9 microinjected zygotes (n = 216) was compared with buffer injected embryos (n = 183) and non microinjected embryos (n = 173), cleavage rate was lower for both microinjected groups (P<0.05) and neither was affected by CRISPR/Cas9 content in the injected medium. Embryo development to blastocyst was not affected by microinjection and was similar among the experimental groups. From 20 embryos analyzed by Sanger sequencing, ten were mutant (heterozygous or mosaic; 50% efficiency). To obtain live MSTN KO lambs, 53 blastocysts produced after zygote CRISPR/Cas9 microinjection were transferred to 29 recipient females resulting in 65.5% (19/29) of pregnant ewes and 41.5% (22/53) of newborns. From 22 born lambs analyzed by T7EI and Sanger sequencing, ten showed indel mutations at MSTN gene. Eight showed mutations in both alleles and five of them were homozygous for indels generating out-of frame mutations that resulted in premature stop codons. Western blot analysis of homozygous KO founders confirmed the absence of myostatin, showing heavier body weight than wild type counterparts. In conclusion, our results demonstrate that CRISPR/Cas9 system was a very efficient tool to generate gene KO sheep. This technology is quick and easy to perform and less expensive than previous techniques, and can be applied to obtain genetically modified animal models of interest for biomedicine and livestock.

Knock-in of Enhanced Green Fluorescent Protein or/and Human Fibroblast Growth Factor 2 Gene into β-Casein Gene Locus in the Porcine Fibroblasts to Produce Therapeutic Protein

Transgenic animals have become important tools for the production of therapeutic proteins in the domestic animal. Production efficiencies of transgenic animals by conventional methods as microinjection and retrovirus vector methods are low, and the foreign gene expression levels are also low because of their random integration in the host genome. In this study, we investigated the homologous recombination on the porcine β-casein gene locus using a knock-in vector for the β-casein gene locus. We developed the knock-in vector on the porcine β-casein gene locus and isolated knock-in fibroblast for nuclear transfer. The knock-in vector consisted of the neomycin resistance gene (neo) as a positive selectable marker gene, diphtheria toxin-A gene as negative selection marker, and 5′ arm and 3′ arm from the porcine β-casein gene. The secretion of enhanced green fluorescent protein (EGFP) was more easily detected in the cell culture media than it was by western blot analysis of cell extract of the HC11 mouse mammary epithelial cells transfected with EGFP knock-in vector. These results indicated that a knock-in system using β-casein gene induced high expression of transgene by the gene regulatory sequence of endogenous β-casein gene. These fibroblasts may be used to produce transgenic pigs for the production of therapeutic proteins via the mammary glands.

Production of transgenic mice expressing tumor virus A under ovarian‑specific promoter 1 control using testis‑mediated gene transfer

The aim of the present study was to produce transgenic mice expressing tumor virus A (TVA) in the ovary under ovarian specific promoter 1 (OSP1) control. A transgenic mouse model was established in which TVA, an avian retroviral receptor gene driven by OSP1, was selectively expressed in the ovary. A recombinant plasmid containing TVA cDNA and an OSP1 promoter was constructed. The DNA fragment was repeatedly injected into male mouse testes at multiple sites. At 4‑7, 7‑10 and 10‑13 weeks following the final injection, two DNA‑injected male mice were mated with four wild‑type female mice to produce transgenic mice. The transgenic positive rate in mouse F1 offspring was 39.69%. When the positive F1 individuals were mated with wild‑type Imprinting Control Region mice (PxW) or with positive F1 individuals (PxP), the F2 individuals had a transgenic rate of 12.44%. The transgenic rates in the F1 offspring, produced following mating at the three time intervals, were 55.71 (39/70), 30.77 (4/13) and 18.75% (9/48), respectively. The transgenic rates of the F2 offspring decreased with the age of the F1 offspring, from 26.67% when PxP were mated at 6‑8 weeks of age to 6.52% when PxW were mated at 5‑6 months of age. The results indicate a high efficiency of gene transfer to F1 offspring using testis‑mediated gene transfer (TMGT). The transgenic rate in the F2 offspring was lower than that in the F1 offspring. The results reveal that TMGT is suitable for creating transgenic animals among F1 offspring. Semi‑quantitative reverse transcription-polymerase chain reaction results showed that TVA was expressed in the mice ovaries. The results demonstrate the importance of using the replication‑competent avian sarcoma‑leukosis virus long terminal repeat with a splice acceptor‑TVA system in ovarian tumorigenesis research.

Blood pressure and renin-angiotensin system resetting in transgenic rats with elevated plasma Val5-angiotensinogen

OBJECTIVE

Increases in plasma angiotensinogen (Ang-N) due to genetic polymorphisms or pharmacological stimuli like estrogen have been associated with a blood pressure (BP) rise, increased salt sensitivity and cardiovascular risk. The relationship between Ang-N, the resetting of the renin-angiotensin system, and BP still remains unclear. Angiotensin (Ang) II-induced genetic hypertension should respond to lisinopril treatment.

METHODS

A new transgenic rat line (TGR) with hepatic overexpression of native (rat) Ang-N was established to study high plasma Ang-N. The transgene contained a mutation producing Val(5)-Ang-II, which was measured separately from nontransgenic Ile-Ang-II in plasma and renal tissue.

RESULTS

Male homozygous TGR had increased plasma Ang-N (~20-fold), systolic BP (ΔBP+26 mmHg), renin activity (~2-fold), renin activity/concentration (5-fold), total Ang-II (~2-fold, kidney 1.7-fold) but decreased plasma renin concentrations (-46%, kidney -85%) and Ile(5)-Ang-I and II (-93%, -94%) vs. controls. Heterozygous TGR exhibited ~10-fold higher plasma Ang-N and 17 mmHg ΔBP. Lisinopril decreased their SBP (-23 vs. -13 mmHg in controls), kidney Ang-II/I (~3-fold vs. ~2-fold) and Ile(5)-Ang-II (-70 vs. -40%), and increased kidney renin and Ile(5)-Ang-I (>2.5-fold vs. <2.5-fold). Kidney Ang-II remained higher and renin lower in TGR compared with controls.

CONCLUSION

High plasma Ang-N increases plasma and kidney Ang-II levels, and amplifies the plasma and renal Ang-II response to a given change in renal renin secretion. This enzyme-kinetic amplification dominates over the Ang-II mediated feedback reduction of renin secretion. High Ang-N levels thus facilitate hypertension via small increases of Ang II and may influence the effectiveness of renin-angiotensin system inhibitors.

Human germline genetic modification: scientific and bioethical perspectives

The latest mammalian genetic modification technology offers efficient and reliable targeting of genomic sequences, in the guise of designer genetic recombination tools. These and other improvements in genetic engineering technology suggest that human germline genetic modification (HGGM) will become a safe and effective prospect in the relatively near future. Several substantive ethical objections have been raised against HGGM including claims of unacceptably high levels of risk, damage to the status of future persons, and violations of justice and autonomy. This paper critically reviews the latest GM science and discusses the key ethical objections to HGGM. We conclude that major benefits are likely to accrue through the use of safe and effective HGGM and that it would thus be unethical to take a precautionary stance against HGGM.

Generation of transgenic monkeys with human inherited genetic disease

Modeling human diseases using nonhuman primates including chimpanzee, rhesus, cynomolgus, marmoset and squirrel monkeys has been reported in the past decades. Due to the high similarity between nonhuman primates and humans, including genome constitution, cognitive behavioral functions, anatomical structure, metabolic, reproductive, and brain functions; nonhuman primates have played an important role in understanding physiological functions of the human body, clarifying the underlying mechanism of human diseases, and the development of novel treatments for human diseases. However, nonhuman primate research has been restricted to cognitive, behavioral, biochemical and pharmacological approaches of human diseases due to the limitation of gene transfer technology in nonhuman primates. The recent advancement in transgenic technology that has led to the generation of the first transgenic monkey in 2001 and a transgenic monkey model of Huntington's disease (HD) in 2008 has changed that focus. The creation of transgenic HD monkeys that replicate key pathological features of human HD patients further suggests the crucial role of nonhuman primates in the future development of biomedicine. These successes have opened the door to genetic manipulation in nonhuman primates and a new era in modeling human inherited genetic disorders. We focused on the procedures in creating transgenic Huntington's disease monkeys, but our work can be applied to transgenesis in other nonhuman primate species.

Progress in gene transfer by germ cells in mammals

Use of germ cells as vectors for transgenesis in mammals has been well developed and offers exciting prospects for experimental and applied biology, agricultural and medical sciences. Such approach is referred to as either male germ cell mediated gene transfer (MGCMGT) or female germ cell mediated gene transfer (FGCMGT) technique. Sperm-mediated gene transfer (SMGT), including its alternative method, testis-mediated gene transfer (TMGT), becomes an established and reliable method for transgenesis. They have been extensively used for producing transgenic animals. The newly developed approach of FGCMGT, ovary-mediated gene transfer (OMGT) is also a novel and useful tool for efficient transgenesis. This review highlights an overview of the recent progress in germ cell mediated gene transfer techniques, methods developed and mechanisms of nucleic acid uptake by germ cells.

Transient transgene transmission to piglets by intrauterine insemination of spermatozoa incubated with DNA fragments

An efficient and low-cost production of transgenic pigs has significant applications to the pig industry and biomedical science. Generation of transgenic pig by sperm-mediated gene transfer (SMGT) was inexpensive and convenient, and reported with high efficiency. To test the method of SMGT in pigs, we employed deep post-cervical intrauterine insemination of incubated spermatozoa in this study. A test of sperm motility of semen from nine Landrace boars after incubation with radioactively labeled DNA construct indicated that DNA uptake of the sperm was highly correlated with sperm motility at the time of collection. DNA concentration of 50 and 300 microg per one billion sperm was incubated with washed high-motility sperm at 17 degrees C for 2 hr. Twenty one hybrid gilts and sows of Meishan crossed with Large White were inseminated with transgene-incubated sperm and produced 156 piglets. Transgene DNA sequences were identified in 31 piglets by PCR amplification of genomic DNA isolated from piglet ears at the age of 3 days. The deep intrauterine insemination had a higher rate of positive transgenic piglets than regular insemination (29.6% of 98 piglets vs. 3.4% of 58 piglets). However, the exogenous transgene DNA was not detected in any piglets at the age of 70-100 days. Therefore, the results further demonstrated that transgene through incubation with spermatozoa was mostly transiently transmitted to the offspring at early growing stage and lost in adulthood, which may result from episomal DNA replications during cell divisions only at the early stage of development.

Production of transgenic-clone pigs by the combination of ICSI-mediated gene transfer with somatic cell nuclear transfer

The objective of this study was to examine whether the ICSI-mediated gene transfer method using in vitro matured oocytes and frozen sperm head could actually produce transgenic pigs. We also aimed at examining whether transgenic pigs can be cloned from somatic cells of a transgenic pig generated by the ICSI-mediated method. A bicistronic gene constituted of the human albumin (hALB) and enhanced green fluorescent protein (EGFP) genes was introduced into pig oocytes by the ICSI-mediated method. Transfer of 702 embryos produced by the ICSI-mediated method into five gilts resulted in 4 pregnancies. When three of the recipients, which had received total 312 of the embryos were autopsied, 32 including 1 transgenic fetuses were obtained. One of the recipients gave birth to three live piglets including one transgenic pig, showing a strong green fluorescence in the eyeballs, oral mucous membrane and subcutaneous tissues. Fluorescent microscopy revealed uniform GFP expression in all cell lines established from kidney, lung and muscle of the founder transgenic pig obtained. Nuclear transfer of these cells resulted in stable in vitro development of cloned embryos into the blastocyst stage, ranging from 12.9 to 19.8%. When 767 of the nuclear transfer embryos were transferred to 5 recipients, all became pregnant and gave birth to a total of six live transgenic-clones. The transgene copy number and integrity in the founder pig were maintained in the primary culture cells established from the founder as well as in the clones produced from these cells. Our study demonstrates that the ICSI-mediated gene transfer is an efficient and practical method to produce transgenic pigs, using frozen sperm heads and in vitro matured oocytes. It was also shown that combination of ICSI-mediated transgenesis and nuclear transfer is a feasible technology of great potential in transgenic pig production.

Lentiviral vectors: are they the future of animal transgenesis?

Lentiviral vectors have become a promising new tool for the establishment of transgenic animals and the manipulation of the mammalian genome. While conventional microinjection-based methods for transgenesis have been successful in generating small and large transgenic animals, their relatively low transgenic efficiency has opened the door for alternative approaches, including lentiviral vectors. Lentiviral vectors are an appealing tool for transgenesis in part because of their ability to incorporate into genomic DNA with high efficiency, especially in cells that are not actively dividing. Lentiviral vector-mediated transgene expression can also be maintained for long periods of time. Recent studies have documented high efficiencies for lentiviral transgenesis, even in animal species and strains, such as NOD/ scid and C57Bl/6 mouse, that are very difficult to manipulate using the standard transgenic techniques. These advantages of the lentiviral vector system have broadened its use as a gene therapy vector to additional applications that include transgenesis and knockdown functional genetics. This review will address the components of the lentiviral vector system and recent successes in lentiviral transgenesis using both male- and female-derived pluripotent cells. The advantages and disadvantages of lentiviral transgenesis vs. other approaches to produce transgenic animals will be compared with regard to efficiency, the ability to promote persistent transgene expression, and the time necessary to generate a sufficient number of animals for phenotyping.

Genomic DNA damage in mouse transgenesis

Creating transgenic mammals is currently a very inefficient process. In addition to problems with transgene integration and unpredictable expression patterns of the inserted gene, embryo loss occurs at various developmental stages. In the present study, we demonstrate that this loss is due to chromosomal damage. We examined the integrity of chromosomes in embryos produced by microinjection of pronuclei, intracytoplasmic sperm injection (ICSI), and in vitro fertilization (IVF)-mediated transgenesis, and correlated these findings with the abilities of embryos to develop in vitro and yield transgenic morulas/blastocysts. Chromosomal analysis was performed after microinjection of the pronuclei in zygotes, as well as in parthenogenetic and androgenetic embryos. In all the pronuclei injection groups, significant oocyte arrest and increased incidence of chromosome breaks were observed after both transgenic DNA injection and sham injection. This indicates that the DNA damage is a transgene-independent effect. In ICSI-mediated transgenesis, there was no significant oocyte arrest. The observed chromosomal damage was lower than that after pronuclei microinjection in zygotes and was dependent upon the presence of exogenous DNA. The occurrence of DNA breaks, as measured by comet assay performed on the sperm prior to ICSI, showed that DNA damage was present in the sperm before fertilization. Embryonic development in vitro and transgene expression at the morula/blastocyst stage were higher in ICSI-mediated transgenesis than after microinjection of pronuclei into zygotes. Sperm-mediated gene transfer via IVF did not affect chromosome integrity, allowed good embryo development, but did not yield any transgenic embryos. The present study demonstrates that DNA damage occurs after both the microinjection of pronuclei and ICSI-mediated transgenesis, albeit through different mechanisms.

Magnetic resonance imaging and spectroscopy of transgenic models of cancer

The complexity of cancer, where a single genetic alteration can have multiple functional effects, makes it a fascinating but humbling disease to study, and the necessity of investigating it in its entirety is more imperative than ever before. Advances in transgene technology have made it possible to create cancer cells, or mice with specific genetic alterations, and the application of an array of both functional and molecular non-invasive MR methods to these transgenic cancer cells and mice to characterize their phenotypic traits is revolutionizing our understanding of cancer. With the establishment of multi-modality molecular imaging centers within barrier or pathogen-free facilities, multi-parametric and multi-modality imaging of transgenic mouse models of human cancer are becoming increasingly prevalent. In this review, we outline some of the methods currently available for generating transgenic mice and cancer cell lines. We also present examples of the application of MR methods to transgenic models that are providing novel insights into the molecular and functional characteristics of cancer and are leading to an era of "non-invasive phenotyping" of the effects of specific molecular alterations in cancer.

Genes and chromosomes: control of development

The past decade has witnessed immense progress in research into the molecular basis behind the developmental regulation of genes. Sets of genes functioning under hierarchical control have been identified, evolutionary conserved systems of genes effecting the cell-to-cell transmission of transmembrane signals and assigned a central role in morphogenesis have been intensively studied; the concept of genomic regulatory networks coordinating expression of many genes has been introduced, to mention some of the major breakthroughs. It should be noted that the temporal and tissue-specific parameters of gene expression are correctly regulated in development only in the context of the chromosome and that they are to a great extent dependent on the position of the gene on the chromosome or the interphase nucleus. Moreover epigenetic inheritance of the gene states through successive cell generations has been conducted exclusively at the chromosome level by virtue of cell or chromosome memory. The ontogenetic memory is an inherent property of the chromosome and cis-regulation has a crucial role in its maintenance.

Transgenic nonhuman primates for neurodegenerative diseases

Animal models that represent human diseases constitute an important tool in understanding the pathogenesis of the diseases, and in developing effective therapies. Neurodegenerative diseases are complex disorders involving neuropathologic and psychiatric alterations. Although transgenic and knock-in mouse models of Alzheimer's disease, (AD), Parkinson's disease (PD) and Huntington's disease (HD) have been created, limited representation in clinical aspects has been recognized and the rodent models lack true neurodegeneration. Chemical induction of HD and PD in nonhuman primates (NHP) has been reported, however, the role of intrinsic genetic factors in the development of the diseases is indeterminable. Nonhuman primates closely parallel humans with regard to genetic, neuroanatomic, and cognitive/behavioral characteristics. Accordingly, the development of NHP models for neurodegenerative diseases holds greater promise for success in the discovery of diagnoses, treatments, and cures than approaches using other animal species. Therefore, a transgenic NHP carrying a mutant gene similar to that of patients will help to clarify our understanding of disease onset and progression. Additionally, monitoring disease onset and development in the transgenic NHP by high resolution brain imaging technology such as MRI, and behavioral and cognitive testing can all be carried out simultaneously in the NHP but not in other animal models. Moreover, because of the similarity in motor repertoire between NHPs and humans, it will also be possible to compare the neurologic syndrome observed in the NHP model to that in patients. Understanding the correlation between genetic defects and physiologic changes (e.g. oxidative damage) will lead to a better understanding of disease progression and the development of patient treatments, medications and preventive approaches for high risk individuals. The impact of the transgenic NHP model in understanding the role which genetic disorders play in the development of efficacious interventions and medications is foreseeable.

Effect of transfection and passage number of ear fibroblasts on in vitro development of bovine transgenic nuclear transfer embryos

The objective of this study was to determine if the transfection of human prourokinase (ProU) gene and passage number of transfected ear fibroblasts affected in vitro development of bovine transgenic nuclear transfer (NT) embryos. An expression plasmid for human ProU was constructed by inserting a bovine beta-casein promoter, a green fluorescent protein (GFP) marker and human ProU gene into a pcDNA3 plasmid and transfected into bovine ear fibroblasts using a lipid mediated method. Abattoir derived oocytes were enucleated at 18-20 hr post maturation and a single donor cell was transferred into the perivitelline space of a recipient oocyte. After fusion and activation, the couplets were cultured in modified synthetic oviductal fluid (mSOF) medium for 168 hr. In Experiment 1, significantly lower rate in blastocysts formation (10.3%) was observed in transfected donor cells at early passage than that in nontransfected counterparts (22.1%, P<0.05). In Experiment 2, development to blastocysts and GFP expression in blastocysts were not significantly different between early (3-7) and late (8-12) passage donor cells (10.3 vs. 11.3% and 54.5 vs. 41.7%, respectively). This study indicates that in vitro development of bovine transgenic NT embryos is negatively influenced by transfection of human ProU gene into donor fibroblasts. However, passage number of transfected ear fibroblasts does not affect in vitro development of bovine transgenic NT embryos.

Efficiency of transgenic rat production is independent of transgene-construct and overnight embryo culture

The aim of the present work was to study factors affecting the efficiency of transgenic technology in rats. We investigated the possible effects of pronuclear microinjection of buffer or different DNA-constructs on survival and development of rat zygotes in vitro and in vivo as well as the influence of overnight culture of these embryos before transfer into pseudopregnant foster mothers. The survival rate of zygotes and their development to the two-cell and morula stage was not affected by pronuclear microinjection with different DNA-constructs or buffer. However, the development to the blastocyst stage was impaired. Nevertheless, there was no difference in blastocyst development between zygotes injected with DNA-constructs or with buffer. Neither was there a difference in cell number in in vitro cultured blastocysts resulting from pronuclear microinjection of a transgene compared with non-injected controls. The survival rate to term was about 30% irrespective of whether microinjected embryos were transferred immediately after microinjection or after overnight culture in vitro. However, a reduction in the survival to term was observed for non-injected zygotes when they were developed in vitro to the two-cell stage before transfer to a pseudopregnant female. The percentage of transgenic rats that resulted from microinjected zygotes was similar in all groups regardless of the DNA-construct used (2.7-10.0%). In conclusion, the main detrimental factor in the microinjection of rat zygotes is the introduction of solution in the pronucleus. Overnight culture of zygotes between microinjection and oviduct transfer does not decrease the efficiency of transgenic rat generation.

Transgene manipulation in zebrafish by using recombinases

Although much remains to be done, our results to date suggest that efficient and precise genome engineering in zebrafish will be possible in the future by using Cre recombinase and SB transposase in combination with their respective target sites. In this study, we provide the first evidence that Cre recombinase can mediate effective site-specific deletion of transgenes in zebrafish. We found that the efficiency of target site utilization could approach 100%, independent of whether the target site was provided transiently by injection or stably within an integrated transgene. Microinjection of Cre mRNA appeared to be slightly more effective for this purpose than microinjection of Cre-expressing plasmid DNA. Our work has not yet progressed to the point where SB-mediated mobilization of our transgene constructs would be observed. However, a recent report has demonstrated that SB can enhance transgenesis rates sixfold over conventional methods by efficiently mediating multiple single-copy insertion of transgenes into the zebrafish genome (Davidson et al., 2003). Therefore, it seems likely that a combined system should eventually allow both SB-mediated transgene mobilization and Cre-mediated transgene modification. Our goal is to validate methods for the precise reengineering of the zebrafish genome by using a combination of Cre-loxP and SB transposon systems. These methods can be used to delete, replace, or mobilize large pieces of DNA or to modify the genome only when and where required by the investigator. For example, it should be possible to deliver particular RNAi genes to well-expressed chromosomal loci and then exchange them easily with alternative RNAi genes for the specific suppression of alternative targets. As a nonviral vector for gene therapy, the transposon component allows for the possibility of highly efficient integration, whereas the Cre-loxP component can target the integration and/or exchange of foreign DNA into specific sites within the genome. The specificity and efficiency of this system also make it ideal for applications in which precise genome modifications are required (e.g., stock improvement). Future work should establish whether alternative recombination systems (e.g., phiC31 integrase) can improve the utility of this system. After the fish system is fully established, it would be interesting to explore its application to genome engineering in other organisms.

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 making of "transgenic spermatozoa"

The processes of making transgenic animals by microinjecting DNA into the pronucleus of a fertilized oocyte or after the transfection of embryonic stem cells are now well established. However, attempts have also been made, with varying degrees of success, to use spermatozoa as a vector for transgenesis in mammals and other vertebrates during the last decade. A number of different approaches for making transgenic spermatozoa have been developed. These include directly incubating mature, isolated spermatozoa with DNA or pretreating mature, isolated spermatozoa before assisted fertilization. Microinjection procedures have also been established to transfect male germ cells directly in vivo within the seminiferous tubules or to reimplant previously isolated male germ cells submitted to in vitro transfection into a recipient testis. The latter two techniques present the advantage of being able to create transgenic progeny simply by mating with wild-type females, which avoids the possibility of interference or damage as a result of assisted fertilization or the manipulation of embryos. The different aspects of sperm-mediated transgenesis are presented.

Development of efficient strategies for the production of genetically modified pigs

Although pronuclear DNA micro-injection has long been the most reliable method to produce transgenic pigs, the efficiency of production of transgenic offspring is generally plagued by 1% of the DNA-injected embryos. Therefore, a problem with this method is the need for large numbers of pronuclear stage embryos. One great advancement would be the use of in vitro-matured (IVM) oocytes for the purpose of transgenic pig production. High developmental competence of IVM oocytes was proven by transfer of parthenogenetic IVM oocytes. A combined method of sperm vectors with the IVM of oocytes would make the production of transgenic pigs remarkably feasible. Rate of blastocyst formation following intracytoplasmic sperm injection (ICSI) by frozen sperm was over 20%, and transgene was expressed in approximately 50% of blastocysts generated. Somatic cell nuclear transfer would enable more efficient and sophisticated genetic modification of the pig. Simultaneous comparison between two nuclear transfer methods by electro-fusion and intracytoplasmic injection revealed clear differences in the pattern of nuclear remodeling and development of the reconstructed embryos. To specify the donor cell type that allows efficient genetic modification and easy reprogramming or to establish such cell lines is a critical issue in pig cloning. We tested pre-adipocytes from the subcutaneous adipose tissue of adult pigs for nuclear transfer. Cell cycle synchronization by differentiation induction is unique to the pre-adipocytes. Frequency of apoptosis was low in the cells synchronized by differentiation induction compared with other synchronization methods, including serum starvation, confluency, and chemical treatment. It would be of great worth if cryopreserved clone embryos were available. We have demonstrated that cryopreservation of in vitro-produced porcine embryos as well as clone blastocysts is possible by our unique method.

Efficient production by sperm-mediated gene transfer of human decay accelerating factor (hDAF) transgenic pigs for xenotransplantation

A large number of hDAF transgenic pigs to be used for xenotransplantation research were generated by using sperm-mediated gene transfer (SMGT). The efficiency of transgenesis obtained with SMGT was much greater than with any other method. In the experiments reported, up to 80% of pigs had the transgene integrated into the genome. Most of the pigs carrying the hDAF gene transcribed it in a stable manner (64%). The great majority of pigs that transcribed the gene expressed the protein (83%). The hDAF gene was transmitted to progeny. Expression was stable and found in caveolae as it is in human cells. The expressed gene was functional based on in vitro experiments performed on peripheral blood mononuclear cells. These results show that our SMGT approach to transgenesis provides an efficient procedure for studies involving large animal models.

Effective generation of transgenic pigs and mice by linker based sperm-mediated gene transfer

BACKGROUND

Transgenic animals have become valuable tools for both research and applied purposes. The current method of gene transfer, microinjection, which is widely used in transgenic mouse production, has only had limited success in producing transgenic animals of larger or higher species. Here, we report a linker based sperm-mediated gene transfer method (LB-SMGT) that greatly improves the production efficiency of large transgenic animals.

RESULTS

The linker protein, a monoclonal antibody (mAb C), is reactive to a surface antigen on sperm of all tested species including pig, mouse, chicken, cow, goat, sheep, and human. mAb C is a basic protein that binds to DNA through ionic interaction allowing exogenous DNA to be linked specifically to sperm. After fertilization of the egg, the DNA is shown to be successfully integrated into the genome of viable pig and mouse offspring with germ-line transfer to the F1 generation at a highly efficient rate: 37.5% of pigs and 33% of mice. The integration is demonstrated again by FISH analysis and F2 transmission in pigs. Furthermore, expression of the transgene is demonstrated in 61% (35/57) of transgenic pigs (F0 generation).

CONCLUSIONS

Our data suggests that LB-SMGT could be used to generate transgenic animals efficiently in many different species.

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.

Selection of in vitro produced, transgenic embryos by nested PCR for efficient production of transgenic goats

The production of valuable pharmaceutical proteins using transgenic animals as bioreactors has become one of the goals of biotechnology. However, the efficiency of producing transgenic animals by means of pronuclear microinjection is low. This may be attributed in part to the low integration rate of foreign DNA. Therefore, a large number of recipients are required to produce transgenic animals. We recently developed a transgenic procedure that combined the techniques of goat oocyte in vitro maturation (IVM), in vitro fertilization (IVF), microinjection, preimplantation selection of the transgenic embryos with nested PCR and transferring the transgenic embryos into the recipient goat uterus to produce transgenic goats. Thirty-seven transgenic embryos determined by nested PCR were transferred to thirty-two recipient goats. In the end, four live-born kids were produced. As predicted, all the live kids were transgenic as identified by PCR as well as Southern blot hybridization, The integration rate was 100% (4/4) which was completely in accordance with the results of embryo preimplantation detection. The results showed a significant decrease in the number of recipients required as only 8 recipients (32/4) were needed to obtain one live transgenic goat. We suggest that the transgenic system described herein may provide an improved way to efficiently produce transgenic goats on a large scale.

Use of cryopreserved pronuclear embryos for the production of transgenic mice

A series of experiments was conducted to test the hypothesis that an improved cryopreservation protocol for pronuclear stage mouse embryos will produce transgenic (Tg) mice by pronuclear gene injection at a rate not significantly different from noncryopreserved embryos. In the first experiment, three cryoprotective agents (CPAs) (dimethyl sulfoxide [DMSO], propylene glycol [PG], ethylene glycol [EG]) and two cryopreservation protocols, currently used for pronuclear embryos, were compared in regard to their ability to maintain post-thaw morphological integrity and in vitro developmental competence. In the second and third experiments, the optimal cryopreservation protocol determined from the first experiment was used to evaluate in vitro developmental competence of pronuclear embryos following green fluorescence protein gene injection and in vivo developmental competence as well as the gene integration rates. Survival (morphological integrity and development to two cells) of embryos cryopreserved in the presence of DMSO was higher (P < 0.05) than those cryopreserved with either PG or EG. Postinjection developmental competence (development to two cells) of cryopreserved CBA, C57B6/JxCBA-F1 and noncryopreserved (control) embryos was not different (P > 0.05). Postinjection blastocyst formation rate of cryopreserved and noncryopreserved C57B6/JxCBA-F1 embryos was similar (P > 0.05); however, noncryopreserved CBA embryos resulted in a higher blastocyst formation than controls (P < 0.05). While there was no difference in the percentage of transgenic fetuses between cryopreserved and control CBA embryos (P > 0.05), cryopreserved C57B6/JxCBA-F1 embryos resulted in lower transgenic fetuses than control (P < 0.05). These results indicate that the use of cryopreserved mouse pronuclear embryos can be a useful and efficient approach to the production of Tg mice.

Interindividual variability and correlation among morphological parameters of knee joint cartilage plates: analysis with three-dimensional MR imaging

OBJECTIVE

To determine the range and variability of the cartilage volume, thickness, and articular surface areas in the knee joints of healthy male subjects, the association of these parameters within and between the knee joint cartilage plates, and their correlation with anthropometric variables.

METHOD

The right knees of 27 individuals (age 23 to 64 years) without cartilage damage were examined. Sagittal magnetic resonance imaging was with a fat-suppressed gradient echo sequence (resolution 2 x 0.31 x 0.31 mm(3)), quantitative parameters being computed for all cartilage plates.

RESULTS

The total knee joint cartilage volume ranged from 16.6 to 31.4 ml, the size of the articular surfaces from 102 to 163 cm(2), and the mean cartilage thickness from 1.57 to 2.43 mm. The mean and maximal cartilage thickness were highest in the patella (2.76 and 5.72 mm). There was a significant correlation of the cartilage volume with the mean thickness (R=0.80) and with the joint surface areas (R=0.56), but not between the thickness and surface area (R=0.37). The association among the patella, tibia, and femur was 0.16 to 0.72 for volumes, 0.08 to 0.78 for thickness, and 0.24 to 0.62 for surfaces. The knee joint cartilage volume and the surface areas were significantly associated with the body height (R=0.51 and 0.57), but not the cartilage thickness (R=0.22).

CONCLUSION

There is a surprisingly high variability of the quantitative distribution of cartilage within the knee joint, with only moderate correlations between knee joint cartilage plates, and this variability cannot be adequately predicted based on anthropometric variables.

Genetic modification of mammalian genome at chromosome level

The review is concerned with a progress in genetic modification of a mammalian genome in vitro and in vivo at chromosomal level. Recently three new approaches for the chromosome biotechnology have been developed: Using Cre/loxP-system a researcher is able to produce targeted rearrangements of whole chromosomes or their segments or particular genes within the genome, and therefore to modify the set, position and copy number of the endogenous elements of the genome. Mammalian artificial chromosomes (MACs) provide a possibility to introduce into genome relatively large segments of alien chromosome material, either artificially constructed or derived from the genome of different species. Using ES-somatic cell hybrids allows to transfer whole chromosomes or their fragments between different genomes within and between species. Advantages and limitations of these approaches are discussed.

Advances in livestock nuclear transfer

Cloning and transgenic animal production have been greatly enhanced by the development of nuclear transfer technology. In the past, genetic modification in domestic animals was not tightly controlled. With the nuclear transfer technology one can now create some domestic animals with specific genetic modifications. An ever-expanding variety of cell types have been successfully used as donors to create the clones. Both cell fusion and microinjection are successfully being used to create these animals. However, it is still not clear which stage(s) of the cell cycle for donor and recipient cells yield the greatest degree of development. While for the most part gene expression is reprogrammed in nuclear transfer embryos, all structural changes may not be corrected as evidenced by the length of the telomeres in sheep resulting from nuclear transfer. Even after these animals are created the question of "are they really clones?" arises due to mitochondrial inheritance from the donor cell versus the recipient oocyte. This review discusses these issues as they relate to livestock.

TransgenICSI reviewed: foreign DNA transmission by intracytoplasmic sperm injection in rhesus monkey

This brief review considers the status of transgenesis by intracytoplasmic sperm injection (ICSI) with nonhuman primates. GFP expressing rhesus macaques embryos (mean = 34.6%; N = 81) were produced by ICSI using rhodamine-tagged DNA encoding the green fluorescence protein (GFP) gene bound on sperm. Rhodamine signal was lost at the egg surface during in vitro fertilization (IVF) but could be traced by dynamic imaging during ICSI within the egg cytoplasm. GFP gene was expressed as early as the 4-cell stage in ICSI embryos but not in embryos produced by in vitro fertilization (IVF). The percentage of GFP expressing blastomeres increased during embryogenesis to the blastocyst stage. Three offspring resulted from seven embryo transfers-a set of anatomically normal twins (a male and a female) stillborn 35 days premature, and a healthy male born at term. Although transgene was not detected in the offspring, the successful production of live primates using DNA bound sperm by ICSI suggests an alternative route to creating transgenic animals. It also raises concern regarding transmission of infectious material during ICSI.

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

At the close of the millennium, a revolution in the treatment of disease is taking shape due to the emergence of new therapies based on human recombinant proteins. The ever-growing demand for such pharmaceutical proteins is an important driving force for the development of safe and large-scale production platforms. Since the efficacy of a human protein is generally dependent on both its amino acid composition as well as various post-translational modifications, many recombinant human proteins can only be obtained in a biologically active conformation when produced in mammalian cells. Hence, mammalian cell culture systems are often used for expression. However, this approach is generally known for limited production capacity and high costs. In contrast, the production of (human) recombinant proteins in milk of transgenic farm animals, particularly cattle, presents a safe alternative without the constraint of limited protein output. Moreover, compared to cell culture, production in milk is very cost-effective. Although transgenic farm animal technology was still in its infancy a decade ago, today it is on the verge of fulfilling its potential of providing therapeutic proteins that can not be produced otherwise in sufficient quantities or at affordable cost. Since 1989, we have been at the forefront of this development, as illustrated by the birth of Herman, the first transgenic bull. In this communication, we will present an overview of approaches we have taken over the years to generate transgenic founder animals and production herds. Our initial strategies were based on microinjection; at the time the only viable option to generate transgenic cattle. Recently, we have adopted a more powerful approach founded on the application of nuclear transfer. As we will illustrate, this strategy presents a breakthrough in the overall efficiency of generating transgenic animals, product consistency, and time of product development.

Review: Development of Reproductive Biotechnologies in Domestic Animals from Artificial Insemination to Cloning: A Perspective

There has been a remarkable increase in knowledge resulting in the application of reproductive biotechnologies in animals, with profound implications for human beings. The major accomplishments in domestic animals, particularly with cattle, are reviewed here. An attempt is made to examine these, in perspective, to indicate the steps by which progress was made, and to appreciate that the explosion of new findings today would not have been possible without the careful studies of yesteryears.