Efficient and scheduled production of pseudopregnant female mice for embryo transfer by estrous cycle synchronization

Naturally sterile Mus spretus hybrids are suitable for the generation of pseudopregnant embryo transfer recipients

For the preparation of embryo transfer recipients, surgically vasectomized mice are commonly used, generated by procedures associated with pain and discomfort. Sterile transgenic strains provide a nonsurgical replacement, but their maintenance requires breeding and genotyping procedures. We have previously reported the use of naturally sterile STUSB6F1 hybrids for the production of embryo transfer recipients and found the behavior of these recipients to be indistinguishable from those generated by vasectomized males. The method provides two substantial 3R impacts: refinement (when compared with surgical vasectomy) and reduction in breeding procedures (compared with sterile transgenic lines). Despite initial promise, the 3Rs impact of this innovation was limited by difficulties in breeding the parental STUS/Fore strain, which precluded the wider distribution of the sterile hybrid. The value of a 3R initiative is only as good as the uptake in the community. Here we, thus, select a different naturally sterile hybrid, generated from strains that are widely available: the B6SPRTF1 hybrid between C57BL/6J and Mus spretus. We first confirmed its sterility by sperm counting and testes weight and then trialed the recovery of cryopreserved embryos and germplasm within three UK facilities. Distribution of sperm for the generation of these hybrids by in vitro fertilization was found to be the most robust distribution method and avoided the need to maintain a live M. spretus colony. We then tested the suitability of B6SPRTF1 sterile hybrids for the generation of embryo transfer recipients at these same three UK facilities and found the hybrids to be suitable when compared with surgical vasectomized mice and a sterile transgenic strain. In conclusion, the potential 3Rs impact of this method was confirmed by the ease of distribution and the utility of sterile B6SPRTF1 hybrids at independent production facilities.

Swollen Ampulla as an Indicator of Successful Pregnancy in B6C3F1 Recipient Mice used for Assisted Reproduction

In vitro fertilization (IVF), embryo cryopreservation, and embryo transfer (ET) are assisted reproductive technologies (ARTs) that are used extensively for the maintenance of mouse models in animal research. Inbred mouse strains with different genetic backgrounds vary in their reproductive performance. Cryopreservation can affect embryo quality and viability, and the genetic background of ET recipients can influence the ET result. In this retrospective study, we analyzed the out- comes of ETs performed in our facility during the last 6 y. We found that B6C3F1 mice with swollen ampullae show almost 3-fold higher pregnancy rates than mice with nonswollen ampullae when either freshly isolated or frozen-thawed embryos are implanted. Implantation of freshly collected embryos in recipients with swollen ampullae led to significantly higher pregnancy rates in comparison to implantation of frozen-thawed embryos, regardless of whether the latter were fertilized in vivo or in vitro. Moreover, we found a significant effect of genetic background on the birth rate; C57BL/6J mice and mice with a mixed genetic background had 34% higher birth rates than did C57BL/6N mice. Within the C57BL/6J group, the birth rates were significantly higher when using fresh in vivo-fertilized embryos, and cryopreservation negatively affected both in vivo- and in vitro-fertilized embryos. The success rate of obtaining one living pup was not significantly different between frozen-thawed and fresh embryos. Overall, a swollen ampulla is a strong indicator for a successful pregnancy, together with the embryo manipulation and genetic background. A better understanding of the factors that affect the reproductive outcome might lead to optimization of the ART protocols and contribute to a reduction in the number of mice used for these procedures.

Intraperitoneal administration of mouse kisspeptin-10 to mice during estrus stage induces pseudopregnancy

A simple method for producing pseudopregnant mice supports pup production. In this study, pregnant ICR were obtained mice without mating with vasectomized mice via administration of mouse Kisspeptin-10 (mKp-10) and transferring blastocysts to the uterus. Blastocyst transfer after mKp-10 administration to mice with gapping and reddish pink vagina resulted in 65.2% (15/23) pregnancies, and 39.1% (34/87) of the transferred blastocysts showed full-term growth. Vaginal smears were observed for accurate estrus cycle determination, and subsequent administration of mKp10 to mice during the estrus stage and blastocyst transfer resulted in 95.2% (20/21) pregnancies and 50.7% (104/205) birth rates. Regarding 2-cell transfer after administration of mKp-10, 100% (8/8) of the mice became pregnant, and 45.0% (36/80) of the embryos were born. Administration of mKp-10 to mice during the estrus stage is a convenient way to generate pseudopregnant mice.

A combined treatment with progesterone, anti-inhibin serum, and equine chorionic gonadotropin improves number of ovulated oocytes in young C57BL/6J mice

Superovulation procedures are routinely and widely used in mouse reproductive technology. Previous studies have shown that a large number of oocytes can be obtained from adult mice (> 10 weeks old) using a combined treatment with progesterone (P4) and anti-inhibin serum (AIS). However, these effects have not been fully investigated in young (4 weeks) C57BL/6J mice. Here, we found that a modified superovulation protocol (combined treatment with P4, AIS, eCG (equine chorionic gonadotropin), and hCG (human chorionic gonadotropin); P4D2-Ae-h) improved the number of oocytes compared to the control (eCG and hCG) (39.7 vs. 21.3 oocytes/mouse). After in vitro fertilization, pronuclear formation rates were 69.3% (P4D2-Ae-h group) and 66.2% (control group). After embryo transfer, 46.4% (116/250) of the embryos in the P4D2-Ae-h group successfully developed to term, which was comparable to the control group (42.9%; 123/287 embryos). In conclusion, our protocol (P4D2-Ae-h) was effective for superovulation in young C57BL/6J mice.

Inducing Pseudopregnancy in Female Mice Without the Need for Vasectomized Males Prior to Non-Surgical Embryo Transfer or Artificial Insemination

For successfully maintaining pregnancy with embryo transfer or artificial insemination, female recipient mice must be induced into a pseudopregnant state. Female mice are traditionally paired overnight with vasectomized males, and the following morning, the presence of a copulation plug is assessed. To increase the efficiency of producing pseudopregnant females, a cervical manipulation technique has been standardized to be used in combination with non-surgical embryo transfer or artificial insemination techniques in mice. The blunt end of a small plastic rod is inserted vaginally to contact the cervix and is vibrated for 30 s by contact with a trimmer. The procedure is quick and does not require anesthesia or analgesia. This technique increases the reliability and predictability of producing pseudopregnant females and entirely eliminates the requirement for vasectomized males. For CD1 mice, the efficiency of pseudopregnancy induction using cervical manipulation was 83% for females in estrus (N = 76) but only 38% of females in estrus were plugged by vasectomized males (N = 24). Artificial insemination in CD1 mice was performed by estrus synchronization with hormones, cervical manipulation, and the uterine transfer of sperm. Artificial insemination recipients receiving cervical manipulation (N = 76) had a pregnancy rate of 72% and an average litter size of 8.3 pups. This method can also be used to produce pseudopregnant females for non-surgical embryo transfer. Therefore, inducing pseudopregnancy by cervical manipulation is a convenient and efficient alternative to mating with a vasectomized male when performing non-surgical assisted reproduction techniques. Using cervical manipulation provides 3Rs (replacement, reduction, and refinement) benefits for assisted reproduction techniques by reducing the number of animals required and eliminating the necessity for surgically altered males.

Effects of perinatal exposure to bisphenol A or S in EAE model of multiple sclerosis

Epidemiological studies support the idea that multiple sclerosis (MS) is a multifactorial disease, overlapping genetic, epigenetic, and environmental factors. A better definition of environmental risks is critical to understand both etiology and the sex-related differences of MS. Exposure to endocrine-disrupting compounds (EDCs) fully represents one of these risks. EDCs are natural or synthetic exogenous substances (or mixtures) that alter the functions of the endocrine system. Among synthetic EDCs, exposure to bisphenol A (BPA) has been implicated in the etiology of MS, but to date, controversial data has emerged. Furthermore, nothing is known about bisphenol S (BPS), one of the most widely used substitutes for BPA. As exposure to bisphenols will not disappear soon, it is necessary to clarify their role also in this pathological condition defining their role in disease onset and course in both sexes. In this study, we examined, in both sexes, the effects of perinatal exposure to BPA and BPS in one of the most widely used mouse models of MS, experimental autoimmune encephalomyelitis (EAE). Exposure to bisphenols seemed to be particularly deleterious in males. In fact, both BPA- and BPS-treated males showed anticipation of the disease onset and an increased motoneuron loss in the spinal cord. Overall, BPA-treated males also displayed an exacerbation of EAE course and an increase in inflammation markers in the spinal cord. Analyzing the consequences of bisphenol exposure on EAE will help to better understand the role of both xenoestrogens and endogenous estrogens on the sexually dimorphic characteristics of MS.

Scheduled simple production method of pseudopregnant female mice for embryo transfer using the luteinizing hormone-releasing hormone agonist

The use of mice as experimental animal models has been a practice since the development of genetically engineered mouse models (GEMMs) in the early 1980s. New technologies, including genome editing, have helped in the time- and cost-efficient generation of GEMMs. However, methods for preparing pseudopregnant females, essential for the generation of GEMMs, remain less advanced. This study proposes a new method to achieve simple production of pseudopregnant female mice using a luteinizing hormone-releasing hormone agonist (LHRHa). A 20 µg LHRHa/mouse was identified as the best dose for inducing estrus synchronization. However, the frequency of copulation was 40% on a single injection. With sequential injections of 20 µg LHRHa/mouse on Days-1 and -2, followed by pairing on Day-5, 74% of LHRHa-treated females copulated with male mice. Moreover, LHRHa treatment did not affect fetal and postnatal development. Eventually, successful generation of offspring via embryo transfer was attained using LHRHa-treated pseudopregnant females. LHRHa administration method is efficient in producing pseudopregnant female mice for the generation of GEMMs, and we expect that it will contribute towards advancing the clinical research.

Exposure to either Bisphenol A or S Represents a Risk for Crucial Behaviors for Pup Survival, Such as Spontaneous Maternal Behavior in Mice

INTRODUCTION

Maternal behavior depends on a multitude of factors, including environmental ones, such as Endocrine Disrupting Chemicals (EDCs), which are increasingly attracting attention. Bisphenol A (BPA), an EDC present in plastic, is known to exert negative effects on maternal behavior. Bisphenol S (BPS), a BPA substitute, seems to share some endocrine disrupting properties. In this study, we focused on the analysis of the effects of low-dose (i.e., 4 µg/kg body weight/day, EFSA TDI for BPA) BPA or BPS exposure throughout pregnancy and lactation in mice.

METHODS

We administered adult C57BL/6 J females orally BPA, BPS, or vehicle from mating to offspring weaning. We assessed the number of pups at birth, the sex ratio, and the percentage of dead pups in each litter, and during the first postnatal week, we observed spontaneous maternal behavior. At the weaning of the pups, we sacrificed the dams and analyzed the oxytocin system, known to be involved in the control of maternal care, in the hypothalamic magnocellular nuclei.

RESULTS

At birth, pups from BPA-treated dams tended to have a lower male-to-female ratio compared to controls, while the opposite was observed among BPS-treated dams' litters. During the first postnatal week, offspring mortality impacted differentially in the BPA and BPS litters, with more female dead pups among the BPA litters, while more male dead pups in the BPS litters, sharpening the difference in the sex ratio. BPA- and BPS-treated dams spent significantly less time in pup-related behaviors than controls. Oxytocin immunoreactivity in the paraventricular and supraoptic nuclei was increased only in the BPA-treated dams.

DISCUSSION/CONCLUSIONS

Alterations in maternal care, along with the treatment itself, may affect, later in life, the offspring's physiology and behavior. Exposure to BPs during sensitive developmental periods represents a risk for both dams and offspring, even at low environmentally relevant doses, through the functional alteration of neural circuits controlling fundamental behaviors for pup survival, such as maternal behaviors.

Use of anti-inhibin monoclonal antibody for increasing the litter size of mouse strains and its application to i-GONAD

The litter size of mouse strains is determined by the number of oocytes naturally ovulated. Many attempts have been made to increase litter sizes by conventional superovulation regimens (e.g., using equine or human gonadotropins, eCG/hCG but had limited success because of unexpected decreases in the numbers of embryos surviving to term. Here, we examined whether rat-derived anti-inhibin monoclonal antibodies (AIMAs) could be used for this purpose. When C57BL/6 female mice were treated with an AIMA and mated, the number of healthy offspring per mouse increased by 1.4-fold (11.9 vs. 8.6 in controls). By contrast, treatment with eCG/hCG or anti-inhibin serum resulted in fewer offspring than in nontreated controls. The overall efficiency of production based on all females treated (including nonpregnant ones) was improved 2.4 times with AIMA compared with nontreated controls. The AIMA treatment was also effective in ICR mice, increasing the litter size from 15.3 to 21.2 pups. We then applied this technique to an in vivo genome-editing method (improved genome-editing via oviductal nucleic acid delivery, i-GONAD) to produce C57BL/6 mice deficient for tyrosinase. The mean litter size following i-GONAD increased from 4.8 to 7.3 after the AIMA treatment and genetic modifications were confirmed in 80/88 (91%) of the offspring. Thus, AIMA treatment is a promising method for increasing the litter size of mice and may be applied for the easy proliferation of mouse colonies as well as in vivo genetic manipulation, especially when the mouse strains are sensitive to handling.

UCP1 expression in the mouse adrenal gland is not upregulated by thermogenic conditions

The uncoupling protein 1 (UCP1) gene is known to be highly expressed in brown adipose tissue (BAT) that functions in thermogenesis. It has been shown that UCP1 mRNA is localized to the mouse adrenal gland, but its significance remains elusive. To explore how UCP1 expression in the adrenal gland is regulated, we generated a reporter knock-in mouse in which the GFP gene was inserted into the UCP1 locus using CRISPR-Cas9 system. Firstly, we confirmed by Western blot analysis UCP1-driven GFP protein expression in interscapular BAT of the knock-in mice kept at 4 °C. Immunohistochemistry showed that GFP protein was detected in the adrenal gland of the knock-in mice. More intense GFP expression was observed in the adrenal medulla than in the cortex of the reporter mice irrespectively of cold exposure. Immunohistochemistry using anti-UCP1 antibody, as well as Western blot analysis verified UCP1 protein expression in the wild-type adrenal medulla. These results suggest that the mouse adrenal gland is a novel organ expressing UCP1 protein and its expression is not upregulated by cold exposure.

Easy and quick (EQ) sperm freezing method for urgent preservation of mouse strains

Cryopreservation of mouse spermatozoa is widely used for the efficient preservation and safe transport of valuable mouse strains. However, the current cryopreservation method requires special containers (plastic straws), undefined chemicals (e.g., skim milk), liquid nitrogen, and expertise when handling sperm suspensions. Here, we report an easy and quick (EQ) sperm freezing method. The main procedure consists of only one step: dissecting a single cauda epididymis in a microtube containing 20% raffinose solution, which is then stored in a -80 °C freezer. The frozen-thawed spermatozoa retain practical fertilization rates after 1 (51%) or even 3 months (25%) with the C57BL/6 J strain, the most sensitive strain for sperm freezing. More than half of the embryos thus obtained developed into offspring after embryo transfer. Importantly, spermatozoa stored at -80 °C can be transferred into liquid nitrogen for indefinite storage. As far as we know, our EQ method is the easiest and quickest method for mouse sperm freezing and should be applicable in all laboratories without expertise in sperm cryopreservation. This technique can help avoid the loss of irreplaceable strains because of closure of animal rooms in emergency situations such as unexpected microbiological contamination or social emergencies such as the COVID-19 threat.

C57BL/6J and B6129F1 Embryo Transfer: Unilateral and Bilateral Transfer, Embryo Number and Recipient Female Background Control for the Optimization of Embryo Survival and Litter Size

Simple Summary

Embryo transfer is a common procedure in rodent facilities related to rederivation protocols, recovery of cryopreserved embryos and production of genetically engineered animals. This procedure consists of the transfer of mouse embryos into the oviduct of a pseudopregnant recipient female in order to obtain live pups. The aim of this study is to further characterize the optimal conditions to perform embryo transfer using wild type strains and particularly the bilateral transfer. C57BL/6J and B6129F1 embryos were freshly collected and transferred to recipient females, after overnight culture to a 2-cell stage and tested for different conditions (unilateral and bilateral surgical procedures, variable number of embryos and reciprocity between recipient mother and embryo’s genetic background). The results achieved show that C57BL/6J transfers with a low number of embryos provide higher success rates when using unilateral transfers, but for bilateral transfers a minimum number of embryos seems to be necessary. B6129F1 presented similar results, but bilateral transfers were more effective with low number of embryos. These results allow a better planning of the embryo transfer procedure, considering low number of embryos and the choice of unilateral transfers as the ideal condition for an optimal outcome. This optimization has a positive impact on the 3R’s application: it can help to reduce the number of recipient and donor females and to improve recipient female’s welfare through the use of a less invasive technique.

Abstract

Embryo transfer (ET) is a common procedure in rodent facilities. Optimizing this technique may help to reduce the number of animals, but little information is available regarding wild type strains and the conditions that affect embryo transfer. To explore this theme, 2-cell C57BL/6J embryos were transferred after overnight culture of freshly collected zygotes using different conditions: unilateral transfers using a total of 6, 8, 12, 15, 20 and 25 embryos were performed initially; then, this strain was also used for bilateral transfers using a total of 6, 12 and 20 embryos equally divided by the two oviducts. Groups of 25 embryos were not tested for the bilateral technique, since this condition produced the lower success rate when using the unilateral technique and 20 embryos would still represent a large number of embryos. A group of 2-cell B6129F1 embryos was also transferred using unilateral and bilateral ET with 6, 12 and 20 embryos. Crl:CD1(ICR) were used as recipient females for non-reciprocal transfers and C57BL/6J were used to test reciprocal transfers (only tested for six C57BL/6J unilateral transfers). Unilateral transfers using C57BL/6J mice produced higher success rates using six embryos, compared to the other groups transferred unilaterally (p-values between 0.0001 and 0.0267), but the mean number of pups per litter was not different among groups. Bilateral transfer produced higher number of pups when 20 embryos were divided by the two oviducts compared to six (p = 0.0012) or 12 (p = 0.0148) embryos, but with no differences in success rates. No statistical differences were found between the groups of B6129F1, but better results were obtained on bilateral transfers using a total of six embryos. For the strain tested (C57BL/6J), the uterine environment (Crl:CD1(ICR) or C57BL/6J recipient) does not impact the outcome of the technique. These results complement previous work published using genetically engineered mice strains and show that unilateral transfers using low number of embryos (6), produce better outcomes when compared to bilateral or unilateral transfers using more embryos. It also highlights differences between the outcome of bilateral transfers in the two strains tested. A set of historical data of genetically engineered mice at a C57BL/6J background was also included, confirming that lower embryo numbers are related to higher success rates. Together, the outcome of these experiments can be important to reduce the number of recipient and donor females, optimize embryo transfers and improve animal welfare discouraging the use of a more invasive technique.

Development of assisted reproductive technologies in small animal species for their efficient preservation and production

Assisted reproductive technologies (ARTs) are widely used in the animal industry, human clinics, and for basic research. In small laboratory animal species such as mice, ARTs are essential for the production of animals for experiments, the preservation of genetic resources, and for the generation of new strains of genetically modified animals. The RIKEN BioResource Research Center (BRC) is one of the largest repositories of such animal bioresources, and maintains approximately 9,500 strains of mice with a variety of genetic backgrounds. We have sought to devise ARTs specific to the reproductive and physiological characteristics of each strain. Such ARTs include superovulation, in vitro fertilization (IVF), the cryopreservation of embryos and spermatozoa, transportation of cryopreserved materials and embryo transfer (ET). Of these, superovulation likely has the most influence on animal production because it determines the quantity of starting material for other ARTs. Superovulation using anti-inhibin serum combined with estrous synchronization has resulted in approximately a three-fold increase in production efficiency with IVF-ET in the C57BL/6J strain. Wild-derived strains are important as genetically diverse resources for murine rodents (Genus Mus), and many are unique to the BRC. We have also successfully developed ARTs for more than 50 wild-derived strains, which have been cryopreserved for future use. Our work to improve and develop ARTs for mice and other small laboratory species will contribute to the cost-effectiveness of routine operations at repository centers, and to the provision of high quality animals for research use.

Endometrial Axin2+ Cells Drive Epithelial Homeostasis, Regeneration, and Cancer following Oncogenic Transformation

The remarkable regenerative capacity of the endometrium (the inner lining of the uterus) is essential for the sustenance of mammalian life. Over the years, the role of stem cells in endometrial functions and their pathologies has been suggested; however, the identity and location of such stem cells remain unclear. Here, we used in vivo lineage tracing to show that endometrial epithelium self-renews during development, growth, and regeneration and identified Axin2, a classical Wnt reporter gene, as a marker of long-lived bipotent epithelial progenitors that reside in endometrial glands. Axin2-expressing cells are responsible for epithelial regeneration in vivo and for endometrial organoid development in vitro. Ablation of Axin2⁺ cells severely impairs endometrial homeostasis and compromises its regeneration. More important, upon oncogenic transformation, these cells can lead to endometrial cancer. These findings provide valuable insights into the cellular basis of endometrial functions and diseases.

Extracellular vesicles derived from donor oviduct fluid improved birth rates after embryo transfer in mice

Embryo transfer (ET) is an important procedure for assisted reproduction. However, the relatively lower success rate of ET hampers its application potential. In this study we aimed to elucidate the effects of extracellular vesicles derived from donor oviduct fluid (EDOF) on embryo development after ET. Extracellular vesicles from the oviduct were isolated and purified using ultracentrifugation and identified using transmission electron microscopy, NanoSight, bicinchoninic acid (BCA) protein assay and western blotting. The results revealed that extracellular vesicles were present in donor oviduct fluid in higher concentrations (P<0.05) and contained more proteins (P<0.05) than extracellular vesicles derived from recipient oviduct fluid (EROF). EDOF or EROF were supplemented in an ET medium (ETM) and the results showed that EDOF significantly improved birth rate via resisting apoptosis and promoting differentiation. In conclusion, our study indicated that there are differences in EDOF and EROF and that supplementing EDOF to ETM can improve the efficiency of ET; improved ET efficiency promotes the use of gene editing and benefits assisted reproductive technology and animal welfare.