Rous sarcoma virus is integrated but not expressed in chicken early embryonic cells

Transgenic animals: current and alternative strategies

Transgenic animal technology is one of the most fascinating technologies developed in the last two decades. It allows us to address questions in life sciences that no other methods have achieved. The impact on biomedical and biological research, as well as commercial interests are overwhelming. The questions accompanying this fast growing technology and its diversified applications attract the attention from a variety of entities. Still, one of the most fundamental problems remaining is the search for an efficient and reliable gene delivery system for creating transgenic animals. The traditional method of pronuclear microinjection has displayed great variability in success among species. While an acceptable efficiency in the production of transgenic mice has been attained, the relative low efficiency (<1%) in creating transgenic livestock has become one of the barriers for its application. In the past decades, improvements in producing transgenic livestock have made a slow progression, however, the recent advancement in cloning technology and the ability to create transgenic livestock in a highly efficient manner, have opened the gate to a new era in transgenic technology. Discoveries of new gene delivery systems have created an enthusiastic atmosphere that has made this technology so unique. This review focuses on gene delivery strategies as well as various approaches that may assist the advancement of transgenic efficiency in large animals.

High efficiency gene transfer into the embryonic chicken CNS using B-subgroup retroviruses

Attempts to use replication-competent retroviruses to target genes to the chick CNS have met with limited success for injections performed prior to stage 14 using A- or E-subgroup viruses. This study was aimed at improving CNS infection by varying the stage of injection, viral envelope subgroup, viral titer, and the presence or absence of a transgene and/or the polycation polybrene in the inoculum. RCASBP vectors were injected into the neural tube of stages 3-13 embryos and protein expression was determined 9-48 hr later for forebrain, hindbrain, retina, and inner ear. Optimal injection parameters were defined which balanced good survival rates with high levels of transgene expression at early stages. The results demonstrate nearly complete expression of virus-mediated transgenes in neural tissues at stages 15-21 following injection of B-envelope RCASBP with polybrene at stages 7.5-12. This technique can now be applied to study the roles of genes in cell-autonomous events such as cell connectivity, physiology, and differentiation, as well as neural patterning and regional identity.

Replication-competent retroviral vectors encoding alkaline phosphatase reveal spatial restriction of viral gene expression/transduction in the chick embryo

Replication-competent avian retroviruses, capable of transducing and expressing up to 2 kb of nonviral sequences, are now available to effect widespread gene transfer in chicken (chick) embryos (S. H. Hughes, J. J. Greenhouse, C. J. Petropoulos, and P. Sutrave, J. Virol. 61:3004-3012, 1987). We have constructed novel avian retroviral vectors that encode human placental alkaline phosphatase as a marker whose expression can be histochemically monitored. These vectors have been tested for expression by introducing them into the embryonic chick nervous system. They have revealed that the expression of retrovirally transduced genes can be spatially and temporally limited without the need for tissue-specific promoters. By varying the site and time of infection, targeted gene transfer can be confined to selected populations of neural cells over the course of several days, a time window that is sufficient for many key developmental processes. The capability of differentially infecting specific target populations may avoid confounding variables such as detrimental effects of a transduced gene on processes unrelated to the cells or tissue of interest. These vectors and methods thus should be useful in studies of the effect of transduced genes on the development of various organs and tissues during avian embryogenesis. In addition, the vectors will facilitate studies aimed at an understanding of viral infection and expression patterns.

Replication-competent retroviral vectors encoding alkaline phosphatase reveal spatial restriction of viral gene expression/transduction in the chick embryo

Replication-competent avian retroviruses, capable of transducing and expressing up to 2 kb of nonviral sequences, are now available to effect widespread gene transfer in chicken (chick) embryos (S. H. Hughes, J. J. Greenhouse, C. J. Petropoulos, and P. Sutrave, J. Virol. 61:3004-3012, 1987). We have constructed novel avian retroviral vectors that encode human placental alkaline phosphatase as a marker whose expression can be histochemically monitored. These vectors have been tested for expression by introducing them into the embryonic chick nervous system. They have revealed that the expression of retrovirally transduced genes can be spatially and temporally limited without the need for tissue-specific promoters. By varying the site and time of infection, targeted gene transfer can be confined to selected populations of neural cells over the course of several days, a time window that is sufficient for many key developmental processes. The capability of differentially infecting specific target populations may avoid confounding variables such as detrimental effects of a transduced gene on processes unrelated to the cells or tissue of interest. These vectors and methods thus should be useful in studies of the effect of transduced genes on the development of various organs and tissues during avian embryogenesis. In addition, the vectors will facilitate studies aimed at an understanding of viral infection and expression patterns.

Expression of Rous sarcoma virus-derived retroviral vectors in the avian blastoderm: potential as stable genetic markers

Retroviruses are valuable tools in studies of embryonic development, both as gene expression vectors and as cell lineage markers. In this study early chicken blastoderm cells are shown to be permissive for infection by Rous sarcoma virus and derivative replication-defective vectors, and, in contrast to previously published data, these cells will readily express viral genes. In cultured blastoderm cells, Rous sarcoma virus stably integrates and is transcribed efficiently, producing infectious virus particles. Using replication-defective vectors encoding the bacterial lacZ gene, we further show that blastoderms can be infected in culture and in ovo. In ovo, lacZ expression is seen within 24 hr of virus inoculation, and by 96 hr stably expressing clones of cells are observed in diverse tissues throughout the embryo, including epidermis, somites, and heart, as well as in extraembryonic membranes. Given the rapid onset of vector expression and the broad range of permissive cell types, it should be feasible to use Rous sarcoma virus-derived retroviruses as early lineage markers and expression vectors beginning at the blastoderm stage of avian embryogenesis.

Segregation, viral phenotype, and proviral structure of 23 avian leukosis virus inserts in the germ line of chickens

We have artificially introduced 23 avian leukosis virus (ALV) proviral inserts into the chicken germ line by injection of wild-type and recombinant subgroup A ALV near the blastoderm of fertile eggs just before incubation. Eight viremic males were identified as germline mosaics because they transmitted proviral DNA to their generation 1 (G-1) progeny at a low frequency. Eleven female and 9 male G-1 progeny carried 23 distinct proviruses that had typical major clonal proviral-host DNA junction fragments detectable after digestion of their DNA with SacI, Southern blotting and hybridization with a probe representing the complete ALV genome. These proviruses, identified by their typical proviral-host DNA junction fragments, were transmitted to approximately 50% of their G-2 progeny after mating the G-1 parents to a line of chickens lacking endogenous ALV proviral inserts. One G-1 female carried 2 proviruses and another 3. The proviruses appeared to be scattered throughout the genome. One of the 14 proviruses carried by females was on the sex (Z) chromosome. Two of the 3 proviruses carried by a single G-1 female were linked with a recombination frequency of about 0.20. Twenty-one of the proviruses coded for infectious ALV. Two proviruses coded for envelope glycoprotein, and cell cultures carrying them were relatively resistant to subgroup A sarcoma virus, but failed to produce infectious ALV. One of these proviruses coded for internal gag proteins, had a deletion in pol, but produced non-infectious virus particles. The other failed to code for gag proteins and had no detectable internal deletions nor did it produce virus particles. Thus, we have shown that replication-competent ALV can artificially infect germ-line cells and that spontaneous defects in the inherited proviruses occur at a rather low rate.

Episomal replication of cloned DNA injected into the fertilised ovum of the hen, Gallus domesticus

We describe preliminary experiments to analyse the fate of cloned DNA microinjected into the cytoplasm of the chick fertilised ovum. The reporter gene construct pRSVcat was injected into the germinal disc before the first cleavage division, and the chick embryos were cultured for up to 7 days using the method of Perry (Nature 331:70-72, 1988). Linear plasmid molecules ligated rapidly after injection to form high-molecular-weight DNA molecules consisting mainly of random concatemers of the injected plasmid. Recombination involving circular molecules resulted in head-to-tail multimers of the plasmid. Some of the DNA was lost after injection, but the remainder was replicated approximately 20-fold during the first 24 h of development. Between days 1 and 7 in culture, the DNA was gradually lost and diluted out as the embryos developed. By day 7 in culture plasmid DNA was detectable in only 30% of the cultures analysed. No evidence for chromosomal integration of the exogenous DNA was obtained, suggesting that the plasmid DNA persisted episomally. Expression of the reporter gene construct pRSVcat was detected in day 2 and day 7 embryos.

Transfer and expression of the bacterial NPT-II gene in chick embryos using a Schmidt-Ruppin retrovirus vector

In an effort to introduce foreign genes into chickens, the bacterial neomycin phosphotransferase (NPT-II) gene was cloned into an infectious avian retroviral vector derived from the Schmidt-Ruppin A strain of RSV. The NPT-II gene was stable in the vector during passage in vitro and infected cells were resistant to G418. Fertilized chicken embryos were inoculated with the recombinant virus on day 0 and screened on day 20 for the NPT-II gene in blood cell DNA. Approximately 12% of the embryos were positive for the NPT-II gene. Screening of DNA from the brain, muscle, liver and foot of the positive embryos indicated that the NPT-II gene copy number could vary in a single embryo. However, some embryos had nearly equal NPT-II copy number in each tissue examined. To determine the expression of the bacterial gene, tissue extracts from the positive embryos were assayed for NPT-II activity. The results indicated that NPT-II activity varied depending on the tissue, with activity being highest in muscle and foot regardless of NPT-II gene copy number.