Microinjection of bovine embryos with a foreign gene and its detection at the blastocyst stage

Birth of large calves that developed from in vitro-derived bovine embryos

High birth weights were observed in calves that developed from bovine embryos produced by in vitro maturation (IVM) and in vitro fertilization (IVF) procedures. After IVM and IVF, embryos were either co-cultured in vitro with oviductal epithelial cells or transferred into the sheep oviduct for development to the blastocyst stage. Blastocysts were transferred to the reproductive tracts of recipient heifers and cows for development to term. Birth weights and gestation periods were compared between calves that developed from in vitro-derived embryos and calves born after artificial insemination (AI) of cows in the herd from which recipient females were selected. Gestation periods were not different among the groups (P > 0.05), but calves that developed from IVM/IVF-derived embryos co-cultured in vitro were larger at birth than calves born from IVM/IVF-derived embryos that developed into blastocysts in the sheep oviduct and calves born from AI (P < 0.001). Dystocia and calf mortality were associated with large calf size at birth. These data were collected from an experiment designed for other purposes, and confounding variables and small sample size could have influenced the observed differences in birth weights. Nevertheless, the extreme birth weights of some calves suggest that abnormal prenatal growth occurs in some IVM/IVF-derived bovine embryos and that conditions for co-culture to the blastocyst stage may exacerbate the problem.

The effect of coating single- and double-stranded DNA with the recombinase A protein of Escherichia coli on transgene integration in mice

Embryo survival and transgene integration rates are two major factors that influence the efficiency of transgenic animal production by pronuclear microinjection. Recombinase A protein-coated transgenes were compared for transgene integration and embryo survival with their non-coated counterparts in both single- and double-stranded forms. Murine zygotes were microinjected with a large 30 kb alpha(S1)-casein/human lysozyme DNA construct and a small 5.5 kb beta-lactoglobulin/desaturase DNA construct using four different construct preparations for each gene. The preparations included recombinase A protein-coated, single- and double-stranded DNA constructs and non-coated, single- and double-stranded DNA constructs. Using conventional non-coated, double-stranded DNA constructs, we obtained a transgene integration efficiency of 1.5% (1352 embryos transferred produced 20 transgenic pups). The same double-stranded DNA constructs coated with recombinase A protein yielded a similar percentage of transgene integration (1.1%, 18/1697). Using single-stranded DNA, non-coated constructs produced a transgene integration rate of 0.5%, while none of the 1040 zygotes injected with recombinase A-coated constructs produced transgenic pups. While recombinase A protein coating produced no effect on embryo survival, litter size or pregnancy rate with double-stranded constructs, a detrimental effect was observed on embryo survival (P < 0.001) and pregnancy rate (P < 0.005) with recombinase A protein coating of single-stranded human lysozyme DNA constructs. A trend toward increased embryo survival (P = 0.054) with no difference in pregnancy rate (P > 0.05) was observed with the recombinase A protein coating of single-stranded desaturase constructs. These results suggest that recombinase A protein coating of single- and double-stranded DNA constructs produced no significant differences (P > 0.05) in the efficiency of generating transgenic mice with respect to the percentage of transgenic animals born.

Transgenic production from in vivo-derived embryos: effect on calf birth weight and sex ratio

We examined transgenic-cattle production by DNA microinjection into 1-, 2-, and 4-cell embryos, analyzing the impact on calf size and subsequent viability. Embryos were either collected at an abattoir by flushing oviducts from superovulated and artificially inseminated cows (in vivo-derived) or obtained by in vitro maturation and in vitro fertilization of oocytes aspirated from excised ovaries (in vitro-derived). A human serum albumin (hSA) milk-expression DNA construct was microinjected, either in one of the visible pronuclei of in vitro- and in vivo-derived 1-cell embryos or in the nuclei of two blastomeres of 2- and 4-cell in vivo-derived embryos. Microinjection-induced mortality (lysis and developmental block) was equivalent ( approximately 40%) for all microinjected embryos. Embryos were co-cultured with BRL cells in B-2 medium containing 10% fetal calf serum (FSC). Overall, embryo development to morulae/blastocysts was significantly greater for in vivo-derived ova (15.5%) than for in vitro-derived oocytes (9.3%). All morulae and blastocysts were transferred to synchronized recipient females on Days 6-8 post-fertilization. A total of 189 calves were delivered. Birth weights were significantly greater for calves generated from in vitro-derived oocytes compared with those generated from in vivo-derived oocytes. One transgenic bull calf was obtained from the microinjection of a 2-cell embryo. Fluorescence in situ hybridization (FISH) analysis of lymphocytes detected one transgenic integration site in all cells. Transmission frequency of the hSA transgene in embryos obtained through IVM/IVF/IVC utilizing the semen of the transgenic calf confirmed that it was not mosaic.

Collection of tubal stage bovine embryos by means of endoscopy. A technique report

Here we describe the development and optimization of endoscopy-mediated transvaginal access for collecting ova and embryos from the bovine oviduct. The novel technique was developed in three experimental setups: In Experiment 1 embryos were collected unilaterally from nonstimulated heifers. We flushed the oviducts of superovulated heifers unilaterally (Experiment 2) and bilaterally (Experiment 3). In Experiment 1 the oviducts of 18 heifers were successfully cannulated, which resulted in the collection of twelve 1-cell to 8-cell embryos and one empty zona. Unilateral flushing of 13 animals (Experiment 2) resulted in 84 ova with 6.3 +/- 3.2 observed ovulation sites. Bilateral flushing of 25 animals (Experiment 3) resulted in 293 ova plus 10 empty zonae from 11.8 +/- 5.4 ovulation sites. Given our experience from these studies we optimized the technical equipment by improving the flushing metal catheter (Experiment 4). The novel catheter hermetically sealed the lumen of the ampulla at the moment, the medium was flushed through the oviduct. This resulted in a visible flow of medium via oviducts toward the embryo filter connected to an embryo flushing catheter that was fixed in the uterine horns. Our endoscopy-guided method is minimally invasive and facilitates the flushing of tubal stage embryos.

Identification of homologous binding proteins in porcine and bovine gametes

The purpose of this study was to determine if sperm and oocyte proteins that mediate plasma membrane interaction during mammalian fertilization are conserved among porcine and bovine gametes. We examined homologous and heterologous sperm and zona-free oocyte interactions to determine the extent of cross-reactivity between the gametes of these two ungulate species. First, the numbers of ejaculated porcine and bovine sperm bound to the oocyte plasma membrane of intact porcine and bovine oocytes were determined in vitro. There was no significant difference between the number of porcine or bovine sperm that bound to porcine or bovine oocytes (P > 0.25). Second, individual porcine and bovine sperm plasma membrane proteins were identified by binding of homologous or heterologous oocyte plasma membrane to whole sperm plasma membrane on Western ligand blots. The relative amount of labeled oocyte plasma membrane bound to individual sperm plasma membrane proteins was analyzed by laser densitometry. Eight porcine sperm plasma membrane proteins and seven bovine sperm plasma membrane proteins were bound by both porcine and bovine oocyte plasma membrane. A significantly greater relative amount of porcine oocyte plasma membrane than bovine oocyte plasma membrane was bound to the 14- and 10-kD porcine sperm plasma membrane proteins (P < 0.001 and P < 0.01, respectively). A 27-kD bovine sperm plasma membrane protein bound proportionally more bovine oocyte plasma membrane probe than porcine oocyte plasma membrane probe (P < 0.04). These results are consistent with conservation of similar receptor ligand interactions at the gamete plasma membrane among porcine and bovine gametes.

Differential expression of two genes located on the X chromosome between male and female in vitro-produced bovine embryos at the blastocyst stage

The potentially unbalanced expression at preimplantation developmental stages of X-linked genes might be responsible of the faster development of male than female embryos in vitro. Two genes located on the X chromosome, glucose-6-phosphate dehydrogenase (G6PD) and hypoxanthine phosphoribosyl transferase (HPRT), are involved in controlling the amount of oxygen radicals, and hence they might have influence in embryo development. We have quantified mRNA expression of these two genes, using in vitro fertilized-in vitro cultured male and female bovine embryos. In vitro-produced early blastocysts obtained at days 7 and 8 were collected and biopsied for gender determination, and the remaining embryos were kept in LN(2) until RNA purification. After sex determination, embryos were pooled in groups of 3 males or 3 females, and mRNA was purified. Using a semiquantitative sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we detected G6PD and HPRT mRNA expression at the early blastocyst stage in all bovine embryos analyzed. Moreover, mRNA expression of both genes studied was significantly higher in female embryos than in male embryos. The differential expression of G6PD and HPRT at these early stages confirm that sex differences are evident prior to gonadal differentiation and that preimplantation bovine embryos have sexually dimorphic gene expression at least with respect to G6PD and HPRT transcripts. These differences might be responsible of the faster development in culture of in vitro-produced male bovine that has been reported. Mol. Reprod. Dev. 55:146-151, 2000.

Timing of DNA integration, transgenic mosaicism, and pronuclear microinjection

Selection of transgenic embryos prior to embryo transfer is a means to increase the efficiency of transgenic livestock production. Among transgenic reporters, cytoplasmic expression of green fluorescent protein (GFP) has features that make it ideal for transgenic embryo selection. The primary objective of this study was to assess cytoplasmic expression of a specially designed GFP reporter as a tool for transgenic bovine embryo selection. A second objective was to evaluate this reporter for studying transgenic mosaicism related to timing of integration of pronuclear microinjected DNA. Transgenic embryos produced by pronuclear injection showed a discrete pattern of GFP expression with clusters at 25, 50, and 100% of blastomeres expressing GFP. This pattern of mosaicism is interpreted to indicate that the integration of microinjected DNA occurred, not only at the pronuclear stage, but also in the subsequent cell divisions. Among the GFP-positive transgenic embryos, only in 21% did all the blastomeres show the green fluorescence. Using the fraction of positive blastomeres within an embryo, the timing of integration of microinjected DNA was estimated. The frequency of nonmosaic embryos expressing GFP is consistent with published germline transmission success rates of transgenic cattle derived from pronuclear microinjected embryos. These results indicate the possible application of GFP as a marker of transgenic embryos and graphically illustrate underlying complexities in DNA integration in embryos subjected to pronuclear microinjection.

Prediction of transgene integration by noninvasive bioluminescent screening of microinjected bovine embryos

Transgenesis in domestic species, as a research tool and in biotechnological applications, has been limited by the expense of producing transgenic offspring by standard microinjection techniques. A major factor is the inefficiency of maintaining large numbers of recipient females, when a high percentage of these carry nontransgenic fetuses. There are two approaches to reduce this cost, the fusion of transfected fetal fibroblasts with enucleated oocytes, and the screening of microinjected embryos for transgene integration in blastocysts, prior to transfer. Here, we develop a luminescent screening system to select transgenic bovine embryos. A transgene with scaffold attachment regions flanking the murine HSP70.1 promoter linked to firefly luciferase cDNA, was microinjected into pronuclei of in vitro produced zygotes. At the blastocyst stage, the transgene was induced by heat shock (45 degrees C, 15 min) and 4-6 h later, luciferase expression was analyzed by photon counting imaging. Screened blastocysts were transferred to recipients and day 50 fetuses or calves were analyzed by PCR and Southern blot for transgene integration. When nonluminescent blastocysts were transferred, transgene integration was never observed. Of 13 fetuses derived from luminescent blastocysts, 3 contained integrated transgenes that were functional in all tissues examined. Image analysis of the signal emitted by positive blastocysts revealed that 9 nontransgenic fetuses were obtained from blastocysts that exhibited a localized luminescent signal. On the other hand, 3 of 4 fetuses derived from blastocysts that emitted light over more than 70% of their surface were transgenic. Thus, by selecting luminescent blastocysts on the basis of both signal intensity and distribution, the number of recipient females required to produce transgenic offspring can be greatly reduced. Using this technique it should also be possible to improve the efficiency of transgenesis by microinjection through studies in which vector design and integration conditions are examined at the blastocyst stage.

Quantitative analysis of luciferase activity of viral and hybrid promoters in bovine preimplantation embryos

This study was conducted to investigate quantitatively the luciferase activity of gene constructs with viral and hybrid enhancers and promoters in bovine preimplantation embryos by using firefly luciferase reporter genes. In Experiment I, to examine the stability of the luciferase, bioluminescence intensity of bovine embryos injected with the luciferase gene driven by the SV40 early promoter and enhancer (SVEluc) was measured with a luminometer at 2 days after microinjection. The results indicated that the bioluminescence could be analysed at any time within 30 min because the luciferase activity was constant during the measurement period from 5 to 30 min. In Experiment II, the luciferase expression of fertilized oocytes injected with four gene constructs (TKEluc, TK6WEluc, SVEluc, and Miwluc) was analysed by using a photon imaging system at 2 or 6 days following microinjection. The results from Experiment II indicated that the reporter gene governed by the Miw promoter (RSV LTR and chicken beta-actin promoter) was expressed more intensively in bovine morulae and blastocysts than three other gene constructs. In Experiment III, the effect of SV40 enhancer was investigated when fused downstream to the luciferase cDNA of the Miwluc vector. The results showed that SV40 enhancer further activated the luciferase activity of the Miw promoter in bovine preimplantation embryos. It was concluded, therefore, that the Miw promoter together with the SV40 enhancer would confer the strongest expression of the firefly luciferase reporter gene among the gene constructs tested in preimplantation bovine embryos.

Early transcription of the SRY gene by bovine preimplantation embryos

We have examined mRNA expression of two genes located on the Y chromosome, the sex-determining region Y gene (SRY) and the linked zinc finger gene (ZFY), using in vitro fertilized-in vitro cultured bovine embryos. Expression of the SRY gene, implicated in sex determination in mammals, has been reported to occur both for a short time at the sex-determining stage of development around the period of the primitive undifferentiated gonad and in the adult testis. In this study, using a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay, we detected SRY but not ZFY mRNA expression as early as the 4- to 8-cell stage and through to the blastocyst stage in bovine embryos. The expression of SRY at these early stages and the previous observation that in vitro-produced male bovine embryos develop faster in culture than female embryos suggest that sex differences are evident prior to gonadal differentiation and that preimplantation bovine embryos have sexually dimorphic gene expression at least with respect to SRY transcripts.

Relationship between stage of development and sex of bovine IVM-IVF embryos cultured in vitro versus in the sheep oviduct

We have confirmed more rapid development of male compared with female in vitro-cultured bovine embryos during the first 7 d after in vitro fertilization. The male-to-female sex ratio of expanded blastocysts after 10 d of in vitro culture was 1.37:1.00, which was significantly different from the expected 1:1 ratio, but no deviation from a 1:1 ratio was observed for male and female expanded blastocysts derived from culture of bovine embryos in the sheep oviduct (1.11:1.00). When embryos that developed only to the morula stage were analyzed for sex, a greater number of female than male bovine embryos was observed from in vitro culture but not after culture in the sheep oviduct. Possible causes of these sex-related differences in development of cultured bovine embryos are discussed.

Bovine oocyte plasma membrane binding sites for sperm plasma membrane during in vitro oocyte maturation and fertilisation

The experimental objective was to determine whether the capability of bovine oocyte plasma membrane to bind sperm changes during in vitro oocyte maturation and fertilisation. Binding was quantified by the intensity of tetramethylrhodamine isothiocyanate (TRITC) fluorescence at the periphery of oocytes following incubation with biotinylated sperm plasma membrane proteins and subsequent incubation with TRITC-avidin. Bovine oocytes were matured in vitro. Sample groups were removed after 0,6 and 22 h, or inseminated and further cultured for 24 or 48 h. Oocytes were denuded of cumulus cells and zona pellucida and co-incubated with 56 micrograms biotinylated bovine sperm plasma membrane protein for 45 min in 150 microliters drops of saline-BSA. Controls were incubated for the same time period in the absence of sperm plasma membrane proteins. All oocytes were rinsed, incubated with TRITC-avidin and subsequently fixed and transferred to mounting medium. Oocytes were scanned with a confocal microscope and analysed using ImageQuant software. The binding of sperm plasma membrane was quantified by integrated fluorescent intensity in standardised ellipses spaced around the plasma membrane of the oocyte. Values are expressed as mean intensity units per 320 pixel ellipse. Binding of sperm plasma membrane continued to increase throughout in vitro oocyte maturation and fertilisation (9051, 24318 and 49953 for 0 and 22 h in vitro matured oocytes and fertilised oocytes, respectively; p = 0.0001). A dramatic decrease in sperm plasma membrane binding to the oocyte plasma membrane was observed in 2-cell embryos (mean intensity = 24477, p = 0.0001). The observed binding was primarily due to the binding of sperm plasma membrane proteins, as control oocytes incubated with TRITC-avidin only were barely visible (integrated fluorescence intensity values ranged from 8 to 3757.

Use of PCR-based methods for selection of integrated transgenes in preimplantation embryos

The production of transgenic animals from ungulate species is an inefficient and expensive procedure. The development of selection methods to identify the small number of transgenic preimplantation embryos produced following DNA microinjection of one-cell embryos would greatly reduce both the cost and effort of these procedures. This study has examined the fate of the ovine beta-lactoglobulin-human alpha 1-antitrypsin (AATB) minigene construct or a subfragment of this following microinjection into one-cell mouse embryos. It has examined two PCR-based methods that were designed to identify a biochemical difference between microinjected DNA constructs to select preimplantation stage embryos in which chromosomal integration of exogenous DNA has occurred. The two methods involved the modification of the AATB DNA construct either by dam methylation or the substitution of dTTP by dUTP. The dam-sensitive DNA endonuclease DpnI, that was used to digest nonintegrated AATB sequences at sites located between PCR oligonucleotide sequences, was found to interfere with the activity of the subsequent PCR reaction. Analyses of the fate of dUTP-DNA indicated that either repair or replication of microinjected DNA interfered with the ability to distinguish between integrated and nonintegrated DNA constructs in the mid-preimplantation stage embryo. The distribution of microinjected AATB DNA between the blastomeres of individual four and eight-cell stage embryos was also examined by the PCR reaction. Microinjected DNA was not found to be evenly distributed between all the blastomeres of individual embryos.