Production of transgenic rats using cryopreserved pronuclear-stage zygotes

Genome editing of rodents by electroporation of CRISPR/Cas9 into frozen-warmed pronuclear-stage embryos

Genome edited animals can now be easily produced using the clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated protein 9 (Cas9) system. Traditionally, these animals have been produced by the introduction of endonucleases into pronuclear-stage embryos. Recently, a novel electroporation method, the "Technique for Animal Knockout system by Electroporation (TAKE)," has been established as a simple and highly efficient tool to introduce endonucleases into embryos instead of methods such as microinjection. Use of frozen-warmed pronuclear-stage embryos in this method has further contributed to efficient production of genome edited animals. However, early developmental stage embryos, including pronuclear-stage embryos, especially those of rats, sometimes show low resistance to physical damage by vitrification and introduction of endonucleases during microinjection. In this study, we propose an ethanol-free, slow-freezing method to reduce physical damage to pronuclear-stage embryos followed by the TAKE method. All mouse and rat frozen embryos were survived after electroporation, and 18% and 100% of offspring were edited target gene, respectively. The resulting protocol is an efficient method for producing genome edited animals.

Protocols for Cryopreservation and Rederivation of Rat Gametes

New genome-editing tools, such as ZFNs, TALEN, and CRISPR/Cas9, have enabled the generation of gene-modified models effectively in mammals. These technologies are a powerful tool for studying gene function and creating animal models for human diseases. On the other hand, such gene-modified animals are raised in numerous experimental animal facilities, which puts pressure on breeding space and maintenance costs. Embryo and sperm cryopreservation is not only the most simple and cost-effective method available for most gene-modified strains but also the most reliable method to preserve strains to avoid breeding problems and contamination. We have established a reliable, high quality embryo and sperm cryopreservation system for rat strains, ensuring the longevity of these valuable resources for the scientific community. These cryopreserved resources have been successfully used to rederive next generation pups using embryo transfer and intracytoplasmic sperm injection (ICSI). In this chapter, we describe in detail protocols for rat embryo vitrification and sperm cryopreservation followed by pup rederivation using the ICSI procedure and embryo transfer.

Reproductive technologies for the generation and maintenance of valuable animal strains

Many types of mutant and genetically engineered strains have been produced in various animal species. Their numbers have dramatically increased in recent years, with new strains being rapidly produced using genome editing techniques. In the rat, it has been difficult to produce knockout and knock-in strains because the establishment of stem cells has been insufficient. However, a large number of knockout and knock-in strains can currently be produced using genome editing techniques, including zinc-finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) system. Microinjection technique has also contributed widely to the production of various kinds of genome edited animal strains. A novel electroporation method, the “Technique for Animal Knockout system by Electroporation (TAKE)” method, is a simple and highly efficient tool that has accelerated the production of new strains. Gamete preservation is extremely useful for maintaining large numbers of these valuable strains as genetic resources in the long term. These reproductive technologies, including microinjection, TAKE method, and gamete preservation, strongly support biomedical research and the bio-resource banking of animal models. In this review, we introduce the latest reproductive technologies used for the production of genetically engineered animals, especially rats, using genome editing techniques and the efficient maintenance of valuable strains as genetic resources. These technologies can also be applied to other laboratory animals, including mice, and domestic and wild animal species.

Targeted expression of step-function opsins in transgenic rats for optogenetic studies

Rats are excellent animal models for experimental neuroscience. However, the application of optogenetics in rats has been hindered because of the limited number of established transgenic rat strains. To accomplish cell-type specific targeting of an optimized optogenetic molecular tool, we generated ROSA26/CAG-floxed STOP-ChRFR(C167A)-Venus BAC rats that conditionally express the step-function mutant channelrhodopsin ChRFR(C167A) under the control of extrinsic Cre recombinase. In primary cultured cortical neurons derived from this reporter rat, only Cre-positive cells expressing ChRFR(C167A) became bi-stable, that is, their excitability was enhanced by blue light and returned to the baseline by yellow~red light. In bigenic pups carrying the Phox2B-Cre driver, ChRFR(C167A) was specifically expressed in the rostral parafacial respiratory group (pFRG) in the medulla, where endogenous Phox2b immunoreactivity was detected. These neurons were sensitive to blue light with an increase in the firing frequency. Thus, this transgenic rat actuator/reporter system should facilitate optogenetic studies involving the effective in vivo manipulation of the activities of specific cell fractions using light of minimal intensity.

Interspecies organogenesis generates autologous functional islets

Islet transplantation is an established therapy for diabetes. We have previously shown that rat pancreata can be created from rat pluripotent stem cells (PSCs) in mice through interspecies blastocyst complementation. Although they were functional and composed of rat-derived cells, the resulting pancreata were of mouse size, rendering them insufficient for isolating the numbers of islets required to treat diabetes in a rat model. Here, by performing the reverse experiment, injecting mouse PSCs into Pdx-1-deficient rat blastocysts, we generated rat-sized pancreata composed of mouse-PSC-derived cells. Islets subsequently prepared from these mouse-rat chimaeric pancreata were transplanted into mice with streptozotocin-induced diabetes. The transplanted islets successfully normalized and maintained host blood glucose levels for over 370 days in the absence of immunosuppression (excluding the first 5 days after transplant). These data provide proof-of-principle evidence for the therapeutic potential of PSC-derived islets generated by blastocyst complementation in a xenogeneic host.

A Novel Reporter Rat Strain That Conditionally Expresses the Bright Red Fluorescent Protein tdTomato

Despite the strength of the Cre/loxP recombination system in animal models, its application in rats trails that in mice because of the lack of relevant reporter strains. Here, we generated a floxed STOP tdTomato rat that conditionally expresses a red fluorescent protein variant (tdTomato) in the presence of exogenous Cre recombinase. The tdTomato signal vividly visualizes neurons including their projection fibers and spines without any histological enhancement. In addition, a transgenic rat line (FLAME) that ubiquitously expresses tdTomato was successfully established by injecting intracytoplasmic Cre mRNA into fertilized ova. Our rat reporter system will facilitate connectome studies as well as the visualization of the fine structures of genetically identified cells for long periods both in vivo and ex vivo. Furthermore, FLAME is an ideal model for organ transplantation research owing to improved traceability of cells/tissues.

Transgenic rat model of childhood-onset dermatitis by overexpressing telomerase reverse transcriptase (TERT)

Childhood-onset dermatitis is one of the most common skin disorders in children. Although various mouse models that mirror aspects of dermatitis have become available, there is still a need for an animal model that develops dermatitis in childhood and is more suitable for performing tissue transplantation experiments. There is emerging evidence that peripheral blood T lymphocytes from patients with dermatitis have significantly increased telomerase activity. Here, we developed telomerase reverse transcriptase (TERT)-expressing transgenic (Tg) rats that spontaneously developed eczematous skin inflammation in childhood. Newborn TERT-Tg rats developed visible dermatitis in 56 % of cases, and the skin lesions microscopically showed spongiosis and acanthosis with infiltration of lymphocytes, eosinophils and mast cells. TERT-Tg rats with dermatitis exhibited increased CD4 (2.5-fold) and CD8 (fivefold) T cell numbers compared with dermatitis-free TERT-Tg rats. Stronger TERT activity was observed in the peripheral lymphocytes of dermatitis-positive TERT-Tg rats than those of dermatitis-free TERT-Tg rats. RT-PCR analysis revealed that IL-4 was markedly elevated in the spleen of dermatitis-positive TERT-Tg rats, and that interferon-gamma was increased in the dermatitis lesions. Moreover, skin grafting of TERT-Tg rats with dermatitis onto T cell-deficient nude rats demonstrated that the inflamed skin lesions could not be maintained. Taken together, the results suggest that TERT activation in T lymphocytes is one of the potential predisposing factors for dermatitis. Moreover, our results demonstrated that the TERT-Tg rats mirror aspects of human childhood-onset dermatitis and that these animals represent a potential animal model system for studying childhood-onset dermatitis.

The vestibulo- and preposito-cerebellar cholinergic neurons of a ChAT-tdTomato transgenic rat exhibit heterogeneous firing properties and the expression of various neurotransmitter receptors

Cerebellar function is regulated by cholinergic mossy fiber inputs that are primarily derived from the medial vestibular nucleus (MVN) and prepositus hypoglossi nucleus (PHN). In contrast to the growing evidence surrounding cholinergic transmission and its functional significance in the cerebellum, the intrinsic and synaptic properties of cholinergic projection neurons (ChPNs) have not been clarified. In this study, we generated choline acetyltransferase (ChAT)-tdTomato transgenic rats, which specifically express the fluorescent protein tdTomato in cholinergic neurons, and used them to investigate the response properties of ChPNs identified via retrograde labeling using whole-cell recordings in brainstem slices. In response to current pulses, ChPNs exhibited two afterhyperpolarisation (AHP) profiles and three firing patterns; the predominant AHP and firing properties differed between the MVN and PHN. Morphologically, the ChPNs were separated into two types based on their soma size and dendritic extensions. Analyses of the firing responses to time-varying sinusoidal current stimuli revealed that ChPNs exhibited different firing modes depending on the input frequencies. The maximum frequencies in which each firing mode was observed were different between the neurons that exhibited distinct firing patterns. Analyses of the current responses to the application of neurotransmitter receptor agonists revealed that the ChPNs expressed (i) AMPA- and NMDA-type glutamate receptors, (ii) GABAA and glycine receptors, and (iii) muscarinic and nicotinic acetylcholine receptors. The current responses mediated by these receptors of MVN ChPNs were not different from those of PHN ChPNs. These findings suggest that ChPNs receive various synaptic inputs and encode those inputs appropriately across different frequencies.

Megaesophagus in a line of transgenic rats: a model of achalasia

Megaesophagus is defined as the abnormal enlargement or dilatation of the esophagus, characterized by a lack of normal contraction of the esophageal walls. This is called achalasia when associated with reduced or no relaxation of the lower esophageal sphincter (LES). To date, there are few naturally occurring models for this disease. A colony of transgenic (Pvrl3-Cre) rats presented with megaesophagus at 3 to 4 months of age; further breeding studies revealed a prevalence of 90% of transgene-positive animals having megaesophagus. Affected rats could be maintained on a total liquid diet long term and were shown to display the classic features of dilated esophagus, closed lower esophageal sphincter, and abnormal contractions on contrast radiography and fluoroscopy. Histologically, the findings of muscle degeneration, inflammation, and a reduced number of myenteric ganglia in the esophagus combined with ultrastructural lesions of muscle fiber disarray and mitochondrial changes in the striated muscle of these animals closely mimic that seen in the human condition. Muscle contractile studies looking at the response of the lower esophageal sphincter and fundus to electrical field stimulation, sodium nitroprusside, and L-nitro-L-arginine methyl ester also demonstrate the similarity between megaesophagus in the transgenic rats and patients with achalasia. No primary cause for megaesophagus was found, but the close parallel to the human form of the disease, as well as ease of care and manipulation of these rats, makes this a suitable model to better understand the etiology of achalasia as well as study new management and treatment options for this incurable condition.

Efficient collection and cryopreservation of embryos in F344 strain inbred rats

In rats, it is now possible to produce genetically engineered strains, not only as transgenic animals but also using gene knockout techniques. Reproductive technologies have been used as indispensable tools to produce and maintain these novel valuable strains. Although studies for collecting and cryopreserving embryos have been reported using outbred rats, efficient methods have not been established in inbred strains. The F344 inbred strain is important in rat breeding and has been used for the production of transgenic/knockout strains and for genome sequencing. Here we studied the optimal conditions for oocyte collection by induction of superovulation, and the development of embryos after cryopreservation in F344 rats. The response to pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) was examined by injection of 150 IU/kg PMSG + 75 IU/kg hCG or 300 IU/kg PMSG + 300 IU/kg hCG. Superovulation was achieved at high efficiency by an injection of 150 IU/kg PMSG + 75 IU/kg hCG. Furthermore, superovulation in this strain showed similar high response as Wistar rats. Of 2-cell embryos cryopreserved by vitrification in a solution containing 10% propylene glycol, 30% ethylene glycol, 20% Percoll and 0.3 M sucrose, more than 90% survived after warming and 32% developed to offspring. However, the freezability of pronuclear stage embryos was extremely low. This study demonstrated that sufficient unfertilized oocytes and embryos can be collected from F344 rats by the induction of superovulation with 150 IU/kg PMSG + 75 IU/kg hCG. Furthermore, cryopreservation of 2-cell embryos using this vitrification protocol can now be applied to maintaining valuable rat strains derived from the F344 inbred strain as genetic resources.

Effects of transgenic Pit-1 overexpression on calcium phosphate and bone metabolism

The type III inorganic phosphate (Pi) transporter Pit-1 was previously found to be preferentially expressed in developing long bones. Several studies also described a regulation of its expression in cultured bone cells by osteotropic factors, suggesting a role of this transporter in bone metabolism. In the present study, we investigated the effects of the transgenic overexpression of Pit-1 in Wistar male rats on calcium phosphate and bone metabolism. A threefold increase and doubling of Pi transport activity were recorded in primary cultured osteoblastic cells derived from calvaria of two transgenic (Tg) lines compared with wild-type littermates (WT), respectively. Skeletal development was not affected by the transgene, and bone mass, analyzed by DXA, was slightly decreased in Tg compared with WT. Enhanced Pi uptake in calvaria-derived osteoblasts from Pit-1 Tg was associated with a significantly decreased expression of alkaline phosphatase activity and a normal deposition and calcification of the collagenous matrix. In 4-month-old adult Tg rats, serum Pi and renal Pi transport were increased compared with WT. The decrease of serum Ca concentration was associated with increased serum parathyroid hormone levels. Variations in serum Pi in Pit-1 Tg rats were negatively correlated with serum fibroblast growth factor-23, whereas 1,25-dihydroxyvitamin D(3) was not affected by Pit-1 overexpression. In conclusion, transgenic Pit-1 overexpression in rats affected bone and calcium phosphate metabolism. It also decreased alkaline phosphatase activity in osteoblasts without influencing bone matrix mineralization as well as skeletal development.

Visual properties of transgenic rats harboring the channelrhodopsin-2 gene regulated by the thy-1.2 promoter

Channelrhodopsin-2 (ChR2), one of the archea-type rhodopsins from green algae, is a potentially useful optogenetic tool for restoring vision in patients with photoreceptor degeneration, such as retinitis pigmentosa. If the ChR2 gene is transferred to retinal ganglion cells (RGCs), which send visual information to the brain, the RGCs may be repurposed to act as photoreceptors. In this study, by using a transgenic rat expressing ChR2 specifically in the RGCs under the regulation of a Thy-1.2 promoter, we tested the possibility that direct photoactivation of RGCs could restore effective vision. Although the contrast sensitivities of the optomotor responses of transgenic rats were similar to those observed in the wild-type rats, they were enhanced for visual stimuli of low-spatial frequency after the degeneration of native photoreceptors. This result suggests that the visual signals derived from the ChR2-expressing RGCs were reinterpreted by the brain to form behavior-related vision.

Direct comparison between ICSI-mediated DNA transfer and pronuclear DNA microinjection for producing transgenic rats

Production efficiency of transgenic rats was compared directly between the routine pronuclear microinjection of exogenous DNA solution (PNMI-Tg method) and the ooplasmic injection of sperm cells exposed to exogenous DNA solution (ICSI-Tg method) using six DNA constructs. The overall production efficiency per treated oocyte in the ICSI-Tg method (mean 1.1%, range 0.2 to 3.1%) was similar to that in the PNMI-Tg method (mean 1.1%, range 0 to 2.4%). An advantage of the ICSI-Tg method in the production of transgenic rats is noted in cases in which a low yield of pronuclear zygotes is an inevitable fate of the rat strain.

Dendritic Ih ensures high-fidelity dendritic spike responses of motion-sensitive neurons in rat superior colliculus

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that generate I(h) currents are widely distributed in the brain and have been shown to contribute to various neuronal functions. In the present study, we investigated the functions of I(h) in the motion-sensitive projection neurons [wide field vertical (WFV) cells] of the superior colliculus, a pivotal visual center for detection of and orientating to salient objects. Combination of whole cell recordings and immunohistochemical investigations suggested that HCN1 channels dominantly contribute to the I(h) in WFV cells among HCN isoforms expressed in the superficial superior colliculus and mainly located on their expansive dendritic trees. We found that blocking I(h) suppressed the initiation of short- and fixed-latency dendritic spike responses and led instead to long- and fluctuating-latency somatic spike responses to optic fiber stimulations. These results suggest that the dendritic I(h) facilitates the dendritic initiation and/or propagation of action potentials and ensures that WFV cells generate spike responses to distal synaptic inputs in a sensitive and robustly time-locked manner, probably by acting as continuous depolarizing drive and fixing dendritic membrane potentials close to the spike threshold. These functions are different from known functions of dendritic I(h) revealed in hippocampal and neocortical pyramidal cells, where they spatiotemporally limit the propagations of synaptic inputs along the apical dendrites by reducing dendritic membrane resistance. Thus we have revealed new functional aspects of I(h), and these dendritic properties are likely critical for visual motion processing in these neurons.

Quantitative Chemical Composition of Cortical GABAergic Neurons Revealed in Transgenic Venus-Expressing Rats

Although neocortical GABAergic (gamma-aminobutyric acidergic) interneurons have been the focus of intense study, especially in the rat, a consensus view of the functional diversity and organization of inhibitory cortical neurons has not yet been achieved. To better analyze GABAergic neurons in the rat, we used a bacterial artificial chromosome (BAC) construct and established 2 lines of transgenic rats that coexpress Venus, a yellow fluorescent protein, with the vesicular GABA transporter. The brain GABA content from both transgenic lines was similar to the level found in wild-type rats. In the frontal cortex, Venus was expressed in >95% of GABAergic neurons, most of which also expressed at least one of 6 biochemical markers, including alpha-actitin-2, which preferentially labeled late-spiking neurogliaform cells. Taking advantage of the fact that Venus expression allows for targeted recording from all classes of nonpyramidal cells, irrespective of their somatic morphologies, we demonstrated that fast-spiking neurons, which were heterogeneous in somatic size as well as vertical dendritic projection, had relatively uniform horizontal dimensions, suggesting a cell type-specific columnar input territory. Our data demonstrate the benefits of VGAT-Venus rats for investigating GABAergic circuits, as well as the feasibility of using BAC technology in rats to label subsets of specific, genetically defined neurons.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.

No Effect of Recombinase-Mediated DNA Transfer on Production Efficiency of Transgenic Rats

It was reported that recombinase-A protein (RecA)-coated exogenous DNA was more likely to be integrated into mouse, goat and pig genomes. The objective of this study was to investigate whether integration of exogenous DNA into the rat genome is improved by the recombinase-mediated DNA transfer. Pronuclear microinjection of RecA-coated EGFP or OAMB DNA resulted in a production efficiency of transgenic rats of 1.4-2.9%, comparable with 0.9-2.6% when non-coated control DNA was used. Intracytoplasmic injection of the sperm heads exposed to RecA-coated EGFP DNA did not produce any transgenic rats (0 vs. 0-2.8% in control groups). Thus, the recombinase-mediated DNA transfer contributed very little to the production of transgenic rats by means of pronuclear microinjection and intracytoplasmic sperm injection.

The effect of equilibration time on survival and development rates of mouse pronuclear-stage embryos vitrified in solid surface (SSV) and convential straws: In vitro and In vivo evaluations

The objective of this study was to improve the efficiency of cryopreservation of pronuclear-stage (PN) mouse embryos. A novel vitrification technique (solid surface vitrification, SSV) was compared with a convential one in straws both for cryosurvival and obtaining progeny from cryopreserved PN mouse embryos. In the SSV method, 15-20 PN embryos were exposed to vitrification solutions for approximately 20 sec after equilibration, and then they were dropped in 2 microl drops onto a pre-cooled (-150 to -180 degrees C) metal surface. In the straws method, groups of 5-10 PN embryos were loaded in a single straw after equilibration. In experiment I, it was compared the effect of the vitrification solutions alone, without vitrification. No reduction was detected in survival, cleavage and blastocysts rates and the lowest development rate was obtained from hatched blastocyst for 20 min equilibration (24.5%). In experiment II, SSV method resulted in significantly higher survival and cleavage rates than that of in-straw vitrified 15-20 min group (87% vs. 60%, 83% vs. 67%, respectively; P < 0.05). There were no statistical differences among any of the blastocyts groups. However, there was a statistical difference in hatched blastocysts between 15 to 5, 10, and 20 min (P < 0.05). In experiment III, it was found no major effect among equilibration time periods in toxicity groups according to the mean cell number of blastocysts developed from PN embryos. But, there was a significant differences between 15 min SSV and 10 min in straw vitrified according to the mean cell number of blastocysts developed from PN embryos following vitrification (P < 0.05). The good results were obtained from 15 min equilibration group for SSV and 10 min equilibration group for straw vitrification. In the last experiment, embryo transfer after vitrification and toxicity was investigated. There were significant differences between SSV and straw just on the rate of pups born (30% and 20.5% respectively; P < 0.05). In conclusion, vitrification of PN mouse embryos by SSV can result in high rates of in vitro development to expanded and hatched blastocyst stage and in vivo development to live pups.

Strain Differences in Superovulatory Response, Embryo Development and Efficiency of Transgenic Rat Production

The differences between rat strains in superovulation response, in vitro and in vivo development of preimplantation embryos and overall transgenic efficiency was studied. The protocols for induction of superovulation using single injections of pregnant mare's serum gonadotropin (PMSG) or minipumps with follicle stimulating hormone (FSH) were compared in Lewis (LEW), Wistar-Kyoto (WKY), and stroke-prone spontaneously hypertensive rats (SHRSP) or Sprague-Dawley (SD) and Wistar rats as representative inbred or outbred strains, respectively. The percentage of mated animals with positive superovulatory response was similar in all strains (60.0-100%). The mean number of ova per donor was not dependent on the kind of hormonal treatment used within each rat strain. In general, females from outbred SD and Wistar rats were more responsive to hormonal treatments than animals from inbred rat strains. In addition, SD female rats produced a significantly higher number of embryos per female in response to PMSG-treatment compared to all other strains. Between the inbred strains, SHRSP was the most effective for superovulation. In vitro development of intact zygotes to the blastocyst stage was not different between SD, Wistar and SHRSP rats. In contrast, in vitro development of WKY zygotes was significantly less efficient than in other strains. However, 2-cell stage embryos in vivo produced from SD, SD x Wistar and WKY animals showed no difference in competence to develop to blastocyst stage in vitro. The proportion of offspring developing after oviduct transfer of intact zygotes was similar in all strains (44.0-56.4%) with the exception of WKY rats (35.9%). We also compared the survival rate after injection, ability of manipulated zygotes to develop to term and overall transgenic efficiency in various rat strains. SD and SHRSP zygotes survived after microinjection better than the WKY and Lewis zygotes. No differences were found in the efficiency of transgene integration per newborn in different strains ranging from 5.7 to 16.7%. The results of this study demonstrate that different rat strains have varying responses to superovulation, sensitivity to microinjection, capability to develop in vitro until blastocyst stage or in vivo to term after transfer to foster mothers. Despite these differences all studied strains can be used for efficient transgenic rat production.

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.

Japanese Society for Animal Reproduction: award for outstanding research 2002. Cryopreservation of follicular oocytes and preimplantation embryos in cattle and horses

Factors affecting sensitivity of preimplantation embryos and follicular oocytes to cryopreservation were analyzed in the equine and bovine species. (1) Survival of equine blastocysts after two-step freezing in the presence of glycerol as the cryoprotective agent (CPA) was influenced by development of the embryonic capsule. The use of ethylene glycol (EG) with sucrose as CPAs improved the post-thaw survival of blastocysts and made it possible to transfer the embryos into recipient mares without removing the CPAs. In addition, early blastocysts cryopreserved by vitrification could develop both in vitro and in vivo when the embryos were exposed to vitrification solution in a stepwise manner. The vitrification procedure was also applied to the relatively large expanded blastocysts. (2) Bovine embryos produced in vitro have been considered to be highly sensitive to the process of cryopreservation. To solve this problem, Day-7 blastocysts produced in a serum-free system were cooled at 0.3 C/min rather than 0.6 C/min before being plunged into liquid nitrogen, resulting in no loss of the post-thaw viability. The supplementation of LAA in IVM/IVF media or IVC medium was effective in producing pronuclear-stage zygotes or morula-stage embryos relatively tolerable to freezing, respectively. (3) Transmission electron microscopic observation of immature equine oocytes showed that cellular injury occurred near the sites of gap-junctions between cumulus cells and the oocyte. In cattle, higher fertilization rates of oocytes were obtained when the oocytes were subjected to cryopreservation at an intermediate stage during IVM (GVBD for freezing, Met-I for vitrification). Vitrification of bovine Met-II oocytes in open-pulled glass capillaries, characterized by an ultra-rapid cooling rate (3,000-5,000 C/min), was found to avoid any harmful influence of vitrification and warming.

Successful vitrification of pronuclear-stage rabbit zygotes by minimum volume cooling procedure

Rabbit zygotes at the pronuclear-stage were cryopreserved by vitrification using a gel-loading tip (GL-tip), Cryoloop or Cryotop. In GL-tip and Cryoloop methods, zygotes were first exposed to 10% ethylene glycol (EG)+10% DMSO in TCM199+20% fetal bovine serum (FBS) for 2 min, and then equilibrated for 30 s in a vitrification solution composed of 20% EG+20% DMSO+0.6 M sucrose in TCM199+20% FBS. In Cryotop method, zygotes were first exposed to 7.5% EG+7.5% DMSO+20% FBS in TCM199 for 3 min, and then equilibrated for 1 min in a vitrification solution composed of 15% EG+15% DMSO+0.5 M sucrose+20% FBS in TCM199. In vitro culture of vitrified-warmed zygotes using GL-tip and Cryoloop resulted in low cleavage rates (2 and 5%, respectively) and no development into blastocysts. In contrast, zygotes vitrified-warmed using Cryotop exhibited higher proportions of cleavage (58%) and development into blastocysts (24%). When compacted morulae or early blastocysts were vitrified by these three procedures, 80-93% of them exhibited blastocoele expansion or zona hatching during the subsequent 48 h of culture. Use of Cryotop instead of GL-tip or Cryoloop for zygote vitrification, without changing conditions of solutions and periods for exposure, equilibration and post-warm dilution, resulted in cleavage and blastocyst development rates of 88 and 45%, respectively. A longer exposure time (10 min) of zygotes to 7.5% EG+7.5% DMSO+20% FBS in TCM199 resulted in higher proportions of zygotes cleaving (94%) and developing into blastocysts (51%) after Cryotop vitrification. Proportions of post-warm zygotes (10-min exposure group) and fresh control zygotes developing into newborn offspring were 36 and 53%, respectively. Pronuclear-stage rabbit zygotes were successfully cryopreserved by vitrification using the Cryotop method.

Production of transgenic rats by ooplasmic injection of spermatogenic cells exposed to exogenous DNA: A preliminary study

The aim of the present study was to investigate the efficiencies of producing transgenic rats by the ooplasmic injection of sperm heads (intracytoplasmic sperm injection: ICSI) and elongating spermatids (elongating spermatid injection: ELSI) exposed to the EGFP DNA solution. A slightly lower proportion of ICSI oocytes using sperm heads exposed to a concentration of 0.5 microg/ml DNA solution for 1 min developed into offspring (13.3%, 48/361) when compared to that of oocytes injected with nontreated sperm heads (19.4%, 32/165). Eight ICSI offspring were found to be EGFP-carrying transgenic rats (16.7% per offspring; 2.2% per embryo). After a 1-min exposure of the elongating spermatids to 5 microg/ml of DNA solution, 8.8% (45/511) of the ELSI oocytes developed into offspring while 12.7% (22/173) of the ELSI oocytes using nontreated spermatids developed. Six ELSI offspring carried the EGFP DNA (13.3% per offspring; 1.2% per embryo). The conventional pronuclear microinjection of 5 microg/ml of DNA solution resulted in the higher production of offspring (29.7%, 104/350) and the birth of three transgenic rats (2.9% per offspring; 0.9% per embryo). Thus, sperm heads and elongating spermatids were practically useful as the vector of exogenous DNA if the DNA-exposed spermatogenic cells were microinseminated into rat oocytes.

Vitrification of pronuclear-stage mouse embryos on solid surface (SSV) versus in cryotube: Comparison of the effect of equilibration time and different sugars in the vitrification solution

The cryopreservation of pronuclear-stage embryos has particular importance in transgenic technology and human assisted reproductive technology (ART). The objective of this study was to improve the efficiency of cryopreservation of pronuclear-stage mouse embryos. Two vitrification methods (solid surface vitrification (SSV) vs. vitrification in cryotube) have been compared with special emphasis on the effect of the exposure of the embryos to the solutions for various times and the sugar content (trehalose, sucrose, or raffinose) of the vitrification solutions. Pronuclear-stage embryos were either exposed to 1 M dimethyl sulfoxide (DMSO) + 1 M propylene-glycol (PG) solution for 2, 5, 10, or 15 min or not exposed to this "equilibration" solution. The vitrification solutions consisted of 2.75 M DMSO and 2.75 M PG in M2 medium supplemented with 1 M trehalose (DPT), 1 M sucrose (DPS), or 1 M raffinose (DPR). In the cryotube method, groups of 15-25 embryos were transferred into a 1.8 ml cryotube containing 30 microl of DPT, DPS, or DPR. After 30 sec, the cryotubes were directly plunged into liquid nitrogen (LN(2)) and stored for 1 day to 1 month. Vitrified samples were warmed by immersing the cryotubes in a 40 degrees C water bath and then immediately diluted with 300 microl of 0.3 M trehalose, sucrose, or raffinose in M2. In the SSV method, after equilibration 15-20 embryos were exposed to DPT, DPS, or DPR solutions for around 20 sec before being dropped in 2-microl drops onto a pre-cooled (-150 to -180 degrees C) metal surface. Vitrified droplets were stored in cryovials in LN(2). Warming was performed by transferring the vitrified droplets into 0.3 M solutions of trehalose, sucrose, or raffinose at 37 degrees C, respectively. Results showed that both SSV and cryotube vitrification methods can result in high rates of in vitro blastocyst development (up to 58.3 and 68.5% with DPR, respectively), not statistically different from that of the controls (58.3 and 64.4%). Even without the equilibration step prior to vitrification, relatively high-survival rates have been achieved, except for the DPS solution. In conclusion, vitrification of pronuclear-stage mouse embryos can result in high rates of in vitro development to blastocyst, and the use of raffinose in the vitrification solution is advantageous to improve cryosurvival.

Comparison between PMSG- and FSH-induced superovulation for the generation of transgenic rats

Superovulation protocols using single injections of pregnant mare's serum gonadotropin (PMSG) or minipumps with follicle-stimulating hormone (FSH) were compared in immature Sprague-Dawley (SD) rats. We used the following criteria: total number of ova, rate of fertilization, in vitro embryo development, sensitivity of zygotes to the microinjection of foreign DNA into the pronucleus, and their in-vivo development after transplantation into the oviduct of a recipient. Female SD rats were stimulated with 15 IU PMSG or 10 mg FSH followed by the injection of human chorionic gonadotropin (hCG) at doses of 20 and 30 IU per female. After hCG administration, they were mated with males of the same strain and sacrificed on day 1 of pregnancy. The percentage of mated animals and the fertilization rate was similar in all groups. In rats given PMSG, the number of ovulated zygotes was hCG dose-dependent. In contrast, the dose of hCG did not influence the efficiency of superovulation in rats given FSH, which was equal to PMSG-treated rats at the optimal dose of hCG. The rates of in vitro blastocyst development (31.4 and 23.3%) and the resistance to microinjection into the pronucleus did also not differ significantly between zygotes of both studied groups. The proportion of offspring developing from microinjected zygotes after oviduct transfer (26.2 and 26.8%, respectively) and the rate of transgene integration per newborns (7.3 and 4.9%, respectively) was similar in both experimental groups. The results of this study demonstrate that superovulation of immature SD rats by PMSG is equally effective as FSH treatment and, thus, preferable for transgenic rat technology due to the lower costs and easier handling.

Offspring derived from intracytoplasmic injection of transgenic rat sperm

The objective of the present study was to produce rat offspring by intracytoplasmic sperm injection (ICSI) using a Piezo-driven micromanipulator. Transgenic male rats carrying a green fluorescent protein gene (GFP: homozygous) were used as sperm donors. The epididymal spermatozoa were suspended and sonicated in m-KRB medium and were frozen in the same medium at -20 degrees C until use. When the sperm heads were aspirated into injection pipettes 7-10 microm in diameter and introduced into oocytes from the Wistar strain, no offspring resulted from the transfer of 59 eggs. In contrast, the sperm heads were hung on the tip of injection pipettes 2-4 microm in diameter and introduced into the oocytes, use of Piezo resulting in the production of 18 transgenic offspring carrying the GFP gene from 181 eggs transferred. The oocytes from the Sprague-Dawley strain also supported full-term development following ICSI with three offspring resulting from 163 transferred eggs. In an additional ICSI trial, spermatozoa from infertile transgenic rats carrying human lactalbumin with the thymidine kinase gene (LAC3: heterozygous) were used. The spermatozoa of the LAC3 transgenic rats appeared to be defective and immotile because of the expression of thymidine kinase in the testes, and no ICSI offspring resulted from 218 transferred eggs. These results suggest that ICSI is applicable in rats when Piezo-driven smaller pipettes are used to inject sperm heads together with a limited amount of the surrounding medium and that the ability of isolated sperm heads to participate in normal embryo development is maintained under the cryopreservation conditions employed.

Effects of cryopreservation of pronuclear-stage rabbit zygotes on the morphological survival, blastocyst formation, and full-term development after DNA microinjection

The objectives of this study were to examine the freezing sensitivity of pronuclear-stage rabbit zygotes and to produce transgenic rabbits using the cryopreserved zygotes. Zygotes were cryopreserved either by one of two vitrification protocols or by one of the two conventional freezing protocols. The morphological survival rates of zygotes subjected to two-step freezing in 1.5 M ethylene glycol and 0.1 M sucrose (74%) or to vitrification in 7.2 M ethylene glycol and 1.0 M sucrose (81%) were higher than those subjected to freezing in 1.5 M DMSO (46%) or to vitrification in a mixture of 2.0 M DMSO, 1.0 M acetamide, and 3.0 M propylene glycol (41%). But the in vitro development into blastocysts of zygotes cryopreserved by vitrification (17%) or to a lesser extent by freezing (52%) was impaired, when compared to that of fresh control zygotes (89%). Next, a fusion gene composed from bovine aS1-casein promoter and a human GH structural gene (2.8 kb) was microinjected into the pronucleus of rabbit zygotes frozen-thawed in ethylene glycol and sucrose. Then, the presence of exogenous DNA in the genome of newborn offspring was determined by PCR. The post-injection survival of frozen zygotes (97%) was the same as that of fresh control zygotes (96%). However, of 18 offspring derived from 414 frozen-thawed and DNA-injected zygotes, no transgenic rabbits were produced. Of 52 offspring derived from 403 DNA-injected fresh zygotes, 3 transgenic rabbits were found. Here we report the first rabbit offspring resulting from zygotes cryopreserved at the pronuclear-stage, although the cryopreservation procedure employed must be improved if zygotes are to be used for systematic production of transgenic rabbits.

Production of transgenic rats using young Sprague-Dawley females treated with PMSG and hCG

The aim of this study was to examine the effects of gonadotrophin treatments on estrus synchronization and superovulation in young Sprague-Dawley (SD) rats that had not yet exhibited defined estrus cycles (5 to 7 weeks old), and to produce transgenic rats using these females as embryo donors and recipients. In Experiment 1, female rats were injected with PMSG and hCG (12.5, 25, 50 and 100 IU/kg each) and were mated with stud males. The reproductive performance of young rats were highest when PMSG and hCG at doses of 25 IU/kg each were injected (delivery rate 87.5%, nursing rate 92.9%). In Experiment 2, female rats were injected with PMSG and hCG (100, 150 and 300 IU/kg each) to induce superovulation. More eggs were recovered from the rats injected with PMSG and hCG at 150 and 300 IU/kg than from those treated with 100 IU/kg (33.4 and 41.3 vs. 13.3 eggs per female, respectively; p < 0.05). In Experiment 3, pronuclear-stage zygotes from 150 IU/kg PMSG/hCG-treated rats were used for microinjection of the fusion gene of bovine alpha S1-casein gene promoter and human growth hormone gene (2.8 kb), and the microinjected zygotes were transferred into the oviduct ampullae of the 25 IU/kg PMSG/hCG-treated rats. Seventeen transgenic rats were obtained from the 334 DNA-injected zygotes (5.1%). These results indicate that recipients and embryo donors for the production of transgenic rats can be prepared by the appropriate PMSG and hCG treatments of young SD rats, regardless of their estrus stages.

A comparative study on the integration of exogenous DNA into mouse, rat, rabbit, and pig genomes

Transgenic mammals, from small laboratory rodents to domestic animals, have been successfully produced to date, but their production efficiency within or across species has been variable. This is probably due to the differences in the type of injected DNA and/or technical procedures employed in each laboratory, as well as the reproductive characteristics of the species. Here we report the direct comparison of the efficiencies of producing transgenic mice, rats, rabbits and pigs by one technician using a fusion gene composed of the bovine alpha S1-casein promoter and human growth hormone (hGH) gene. Before the fusion gene was injected into the zygotes, high magnitude centrifugation to visualize the pronuclei was necessary for all of the pig zygotes and one-third of the rabbit zygotes, but not for mouse and rat zygotes. Post-injection survival of the mouse zygotes (67.1%) was lower than those of the rat, rabbit and pig zygotes (89.6 to 100%). The volume change of the pronucleus following DNA injection was the lowest in mice (50% increase), moderate in rabbits (148% increase), and the most prominent in rats (238% increase). The data from only 1 pig zygote indicated a 22% increase in the pronucleus volume by DNA injection. The PCR analyses of the tail DNA of new born offspring indicated that 0.8% (4/493), 4.8% (22/463), 0.8% (3/367) and 0.9% (2/221) of the injected eggs in mice, rats, rabbits and pigs, respectively, developed into transgenic offspring. Some of the founder animals in all four species expressed the transgene in the mammary gland which was confirmed in hGH mRNA by RT-PCR and/or hGH peptide in Witch's milk with ELISA. These results suggest that the maximum volume of DNA solution injectable into the pronucleus is a possible factor explaining the species differences in the production of transgenic animals.