Expression of a reporter gene after microinjection of mammalian artificial chromosomes into pronuclei of bovine zygotes

Laser-assisted Cytoplasmic Microinjection in Livestock Zygotes

Cytoplasmic microinjection into one-cell embryos is a very powerful technique. As an example, it enables the delivery of genome editing tools that can create genetic modifications that will be present in every cell of an adult organism. It can also be used to deliver siRNA, mRNAs or blocking antibodies to study gene function in preimplantation embryos. The conventional technique for microinjecting embryos used in rodents consists of a very thin micropipette that directly penetrates the plasma membrane when advanced into the embryo. When this technique is applied to livestock animals it usually results in low efficiency. This is mainly because in contrast to mice and rats, bovine, ovine, and porcine zygotes have a very dark cytoplasm and a highly elastic plasma membrane that makes visualization during injection and penetration of the plasma membrane hard to achieve. In this protocol, we describe a suitable microinjection method for the delivery of solutions into the cytoplasm of cattle zygotes that has proved to be successful for sheep and pig embryos as well. First, a laser is used to create a hole in the zona pellucida. Then a blunt-end glass micropipette is introduced through the hole and advanced until the tip of the needle reaches about 3/4 into the embryo. Then, the plasma membrane is broken by aspiration of cytoplasmic content inside the needle. Finally, the aspirated cytoplasmic content followed by the solution of interest is injected back into the embryonic cytoplasm. This protocol has been successfully used for the delivery of different solutions into bovine and ovine zygotes with 100% efficiency, minimal lysis, and normal blastocysts development rates.

Developmental rate and allocation of transgenic cells in rabbit chimeric embryos

The objective of this study was to compare developmental capacity of rabbit chimeric embryos and the allocation of the EGFP gene expression to the embryoblast (ICM) or embryonic shield. We produced chimeric embryos (TR<>N) by synchronous transfer of two or three blastomeres at the 16-cell stage from transgenic (TR) into normal host embryos (N) at the same stage. In the control group, two to three non-transgenic blastomeres were used to produce chimeric embryos. The TR embryos were produced by microinjection of EGFP into both pronuclei of fertilized rabbit eggs. The developmental rate and allocation of EGFP-positive cells of the reconstructed chimeric embryos was controlled at blastocyst (96 h PC) and embryonic shield (day 6) stage.

All chimeric embryos (120/120, 100%) developed up to blastocyst stage. Using fluorescent microscope, we detected green signal (EGFP expression). In 90 chimeric (TR<>N) embryos (75%). Average total number of cells in chimeric embryos at blastocyst stage was 175 ± 13.10, of which 58 ± 2.76 cells were found in the ICM area. The number of EGFP-positive cells in the ICM area was 24 ± 5.02 (35%). After the transfer of 50 chimeric rabbit embryos at the 16-cell stage, 20 embryos (40%) were flushed from five recipients on day 6 of pregnancy, of which five embryos (25%) were EGFP positive at the embryonic shield stage.

Our results demonstrate that transgenic blastomeres in synchronous chimeric embryos reconstructed from TR embryos have an ability to develop and colonize ICM and embryonic shield area.

Engineering chromosomes for delivery of therapeutic genes

The ability to create fully functional human chromosome vectors represents a potentially exciting gene-delivery system for the correction of human genetic disorders with several advantages over viral delivery systems. However, for the full potential of chromosome-based gene-delivery vectors to be realized, several key obstacles must be overcome. Methods must be developed to insert therapeutic genes reliably and efficiently and to enable the stable transfer of the resulting chromosomal vectors to different therapeutic cell types. Research to achieve these outcomes continues to encounter major challenges; however recent developments have reiterated the potential of chromosome-based vectors for therapeutic gene delivery. Here we review the different strategies under development and discuss the advantages and problems associated with each.

A mammalian artificial chromosome engineering system (ACE System) applicable to biopharmaceutical protein production, transgenesis and gene-based cell therapy

Mammalian artificial chromosomes (MACs) provide a means to introduce large payloads of genetic information into the cell in an autonomously replicating, non-integrating format. Unique among MACs, the mammalian satellite DNA-based Artificial Chromosome Expression (ACE) can be reproducibly generated de novo in cell lines of different species and readily purified from the host cells' chromosomes. Purified mammalian ACEs can then be re-introduced into a variety of recipient cell lines where they have been stably maintained for extended periods in the absence of selective pressure. In order to extend the utility of ACEs, we have established the ACE System, a versatile and flexible platform for the reliable engineering of ACEs. The ACE System includes a Platform ACE, containing >50 recombination acceptor sites, that can carry single or multiple copies of genes of interest using specially designed targeting vectors (ATV) and a site-specific integrase (ACE Integrase). Using this approach, specific loading of one or two gene targets has been achieved in LMTK(-) and CHO cells. The use of the ACE System for biological engineering of eukaryotic cells, including mammalian cells, with applications in biopharmaceutical production, transgenesis and gene-based cell therapy is discussed.

Phage integrases for the construction and manipulation of transgenic mammals

Phage integrases catalyze site-specific, unidirectional recombination between two short att recognition sites. Recombination results in integration when the att sites are present on two different DNA molecules and deletion or inversion when the att sites are on the same molecule. Here we demonstrate the ability of the phiC31 integrase to integrate DNA into endogenous sequences in the mouse genome following microinjection of donor plasmid and integrase mRNA into mouse single-cell embryos. Transgenic early embryos and a mid-gestation mouse are reported. We also demonstrate the ability of the phiC31, R4, and TP901-1 phage integrases to recombine two introduced att sites on the same chromosome in human cells, resulting in deletion of the intervening material. We compare the frequencies of mammalian chromosomal deletion catalyzed by these three integrases in different chromosomal locations. The results reviewed here introduce these bacteriophage integrases as tools for site-specific modification of the genome for the creation and manipulation of transgenic mammals.

High level expression of the bioactive human interleukin-10 in milk of transgenic mice

Human interleukin-10 (hIL-10) has wide spectrum of anti-inflammatory activities and has shown a potential to be used for treatment of inflammatory or immune illness. In this study, transgenic mice that over-express human interleukin-10 (IL-10) in their milk were generated using a bovine beta-casein/human IL-10 hybrid gene. After cloning of the IL-10 gene, a 22 kb hybrid gene was constructed by linking a 10 kb promoter sequence of the bovine beta-casein gene to the cloned 12 kb IL-10 gene. In six of the eight transgenic mice, the transgene RNA was expressed only in the mammary gland and in the other two mice, it was also slightly expressed in the lung. The highest human IL-10 level in milk was 1620 microg x ml(-1). Notably, transgenes in all the eight transgenic mice were expressed regardless of the integration site even though no correlation was shown between the copy numbers of the transgene and expression level. These results suggest that the genomic sequence of the human IL-10 gene can induce the IL-10 expression at high levels under the control of the bovine beta-casein promoter.