Kohtaro Morita

In vitro fertilization (IVF) and artificial insemination (AI) using frozen–thawed rat sperm have been established to enable the production of viable offspring. However, the current standard cryopreservation solution, which contains chicken egg yolk and Orvus ES Paste (OEP; sodium lauryl sulfate and other undefined components), poses the risk of microbial contamination and limits the experimental reproducibility of IVF and AI. In this study, we developed a chemically defined cryopreservation solution that does not contain chicken egg yolk or OEP and evaluated its efficacy in IVF. After examining the effects of various cryoprotective agents on sperm motility, we found that OptiPrep (a chemically defined reagent) and ethylene glycol in a lactose solution best preserved sperm motility after freezing and thawing. In addition, sericin, a cell membrane protective agent, effectively reduced damage to the plasma and acrosomal membranes of the sperm head. Although the fertilization rate of the sperm cryopreserved in a chemically defined solution (71.4%) was significantly lower than that of fresh sperm (90.5%), fertilized oocytes were successfully obtained. Approximately 60% of the fertilized oocytes developed to the blastocyst stage in vitro. Furthermore, when two-cell embryos were transferred to the oviducts of pseudopregnant females, 36% developed into offspring. Taken together, our results suggest a new strategy for the cryopreservation of rat strains using a chemically defined sperm-freezing solution, enabling the safe and reproducible production of offspring.

In the conventional method of producing transgenic (Tg) animals, donor DNA is microinjected into the pronuclei of zygotes using a sharp glass needle. However, this approach is generally inefficient as it requires highly skilled microinjection techniques to ensure zygote survival and the transgene is incorporated into only a small proportion of the offspring. In contrast, methods based on piggyBac transposase (PBase) enables more efficient insertion of DNA into the genome and generation of Tg animals. The use of piggyBac transposase have also been examined in rats→ However, this method has not yet been fully optimized or properly characterized. In this study, we examined the microinjection of PBase mRNA and donor plasmid DNA into the pronuclei of rat zygotes using piezo-assisted microinjection. This approach resulted in high survival rates and enabled the efficient generation of Tg rats, even with long donor DNA. When the zygotes were microinjected using Piezo, over 70% were viable, and after embryo transfer, over 80% of the pups carried the transgene. Furthermore, we confirmed germline transmission to the F1 and F2 generations. We also attempted to generate a rat model of Alzheimer’s using this method→ However, the protein was not detected despite mRNA expression, and the phenotype was not observed in behavioral tests. Although the generation of Alzheimer’s disease model remains a challenge, our findings show that piggyBac transposase mRNA combined with piezo-assisted microinjection represents a simple and efficient method for producing Tg rats, even with long donor DNA.