Effects of the timing of electroporation during in vitro maturation on triple gene editing in porcine embryos using CRISPR/Cas9 system

CRISPR-based genome editing of a diurnal rodent, Nile grass rat (Arvicanthis niloticus)

BACKGROUND

Diurnal and nocturnal mammals have evolved distinct pathways to optimize survival for their chronotype-specific lifestyles. Conventional rodent models, being nocturnal, may not sufficiently recapitulate the biology of diurnal humans in health and disease. Although diurnal rodents are potentially advantageous for translational research, until recently, they have not been genetically tractable. The present study aims to address this major limitation by developing experimental procedures necessary for genome editing in a well-established diurnal rodent model, the Nile grass rat (Arvicanthis niloticus).

RESULTS

A superovulation protocol was established, which yielded nearly 30 eggs per female grass rat. Fertilized eggs were cultured in a modified rat 1-cell embryo culture medium (mR1ECM), in which grass rat embryos developed from the 1-cell stage into blastocysts. A CRISPR-based approach was then used for gene editing in vivo and in vitro, targeting Retinoic acid-induced 1 (Rai1), the causal gene for Smith-Magenis Syndrome, a neurodevelopmental disorder. The CRISPR reagents were delivered in vivo by electroporation using an improved Genome-editing via Oviductal Nucleic Acids Delivery (i-GONAD) method. The in vivo approach produced several edited founder grass rats with Rai1 null mutations, which showed stable transmission of the targeted allele to the next generation. CRISPR reagents were also microinjected into 2-cell embryos in vitro. Large deletion of the Rai1 gene was confirmed in 70% of the embryos injected, demonstrating high-efficiency genome editing in vitro.

CONCLUSION

We have established a set of methods that enabled the first successful CRISPR-based genome editing in Nile grass rats. The methods developed will guide future genome editing of this and other diurnal rodent species, which will promote greater utility of these models in basic and translational research.

Optimizing oocyte electroporation for genetic modification of porcine embryos: Evaluation of the parthenogenetic activation

In reproductive biology, understanding the effects of novel techniques on early embryo development is of paramount importance. To date, the effects of electrical activation on oocytes prior to in vitro fertilization (IVF) are not well understood. The aim of this study was to investigate the effects of oocyte electroporation prior to IVF on embryo development and to differentiate between true embryos and parthenotes by using a TPCN2 knock-out (KO) male to evaluate the presence of the KO allele in the resulting blastocysts. The study consisted of three experiments. The first one examined oocyte electroporation with and without subsequent IVF and found that electroporated oocytes had higher activation rates, increased occurrence of a single pronucleus, and no effect on sperm penetration. Cleavage rates improved in electroporated oocytes, but blastocyst rates remained constant. Genotype analysis revealed a significant increase in the proportion of parthenotes in the electroporated groups compared to the IVF control (30.2 % vs. 6.8 %). The second experiment compared two electroporation media, Opti-MEM and Nuclease-Free Duplex Buffer (DB). DB induced higher oocyte degeneration rates, and lower cleavage and blastocyst rates than Opti-MEM, while parthenogenetic formation remained consistent (60.0 and 48.5 %). In the third experiment, the timing of electroporation relative to IVF was evaluated (1 h before IVF, immediately before IVF and 7 h after IVF). Electroporation immediately before IVF resulted in higher activation rates and different pronuclear proportions compared to the other timing groups. The penetration rate was higher in the immediate electroporation group, and cleavage rate improved in all electroporated groups compared to the control. Blastocyst rates remained constant. Genotyping revealed no significant differences in parthenote proportions among the timing groups, but these were higher than the control (56.25 %, 63.89 %, 51.61 %, 2.44 %, respectively), and showed higher mutation rates when electroporation was performed 7 h after IVF. Overall, this comprehensive study sheds light on the potential of electroporation for creating genetically modified embryos and the importance of media selection and timing in the process, the best media being the Opti-MEM and the more efficient timing regarding mutation rate, 7 h post-IVF, even when the parthenote formation did not differ among electroporated groups. Further studies are needed to reduce the parthenogenetic activation while maintaining high mutation rates to optimize the use of this procedure for the generation of gene-edited pig embryos by oocyte/zygote electroporation.

Comparison of ICSI, IVF, and in vivo derived embryos to produce CRISPR-Cas9 gene-edited pigs for xenotransplantation

Genome editing in pigs for xenotransplantation has seen significant advances in recent years. This study compared three methodologies to generate gene-edited embryos, including co-injection of sperm together with the CRISPR-Cas9 system into oocytes, named ICSI-MGE (mediated gene editing); microinjection of CRISPR-Cas9 components into oocytes followed by in vitro fertilization (IVF), and microinjection of in vivo fertilized zygotes with the CRISPR-Cas9 system. Our goal was to knock-out (KO) porcine genes involved in the biosynthesis of xenoantigens responsible for the hyperacute rejection of interspecific xenografts, namely GGTA1, CMAH, and β4GalNT2. Additionally, we attempted to KO the growth hormone receptor (GHR) gene with the aim of limiting the growth of porcine organs to a size that is physiologically suitable for human transplantation. Embryo development, pregnancy, and gene editing rates were evaluated. We found an efficient mutation of the GGTA1 gene following ICSI-MGE, comparable to the results obtained through the microinjection of oocytes followed by IVF. ICSI-MGE also showed higher rates of biallelic mutations compared to the other techniques. Five healthy piglets were born from in vivo-derived embryos, all of them exhibiting biallelic mutations in the GGTA1 gene, with three displaying mutations in the GHR gene. No mutations were observed in the CMAH and β4GalNT2 genes. In conclusion, in vitro methodologies showed high rates of gene-edited embryos. Specifically, ICSI-MGE proved to be an efficient technique for obtaining homozygous biallelic mutated embryos. Lastly, only live births were obtained from in vivo-derived embryos showing efficient multiple gene editing for GGTA1 and GHR.

CRISPR-based Genome Editing of a Diurnal Rodent, Nile Grass Rat ( Arvicanthis niloticus)

Diurnal and nocturnal mammals have evolved distinct pathways to optimize survival for their chronotype-specific lifestyles. Conventional rodent models, being nocturnal, may not sufficiently recapitulate the biology of diurnal humans in health and disease. Although diurnal rodents are potentially advantageous for translational research, until recently, they have not been genetically tractable. Here, we address this major limitation by demonstrating the first successful CRISPR genome editing of the Nile grass rat ( Arvicanthis niloticus ), a valuable diurnal rodent. We establish methods for superovulation; embryo development, manipulation, and culture; and pregnancy maintenance to guide future genome editing of this and other diurnal rodent species.

Efficient editing BMP15 in porcine oocytes through microinjection of CRISPR ctRNP

Bone morphogenetic protein 15 (BMP15) is an X-linked gene encoding an oocyte secreted factor, which plays varied functions in the female fertility between mono-ovulatory and poly-ovulatory mammalian species. We previously found that knockout of BMP15 completely blocked porcine follicular development at preantral stages. However, the specific function of BMP15 on porcine oocytes in vitro maturation remains largely unknown. Here, we injected the pre-assembled crRNA + tracrRNA + Cas9 ribonucleoprotein (ctRNP) complex into the cytoplasm of germinal vesicle stage porcine oocytes to disrupt BMP15. The ctRNP composed of Cas9 nuclease and crRNA-tracrRNA complex at 1.2/1 content ratio. The tested crRNA-tracrRNA complex concentration ranging from 50 to 200 ng/μL, all presented effective editing of BMP15 in porcine oocytes, and the 125 ng/μL crRNA-tracrRNA complex presented the highest editing efficiency (39.23 ± 3.33%). Surprisingly, we found approximately 95% edited oocytes presented monoallelic mutations, and only 5% edited oocytes harbored biallelic mutations. Interestingly, the coinjected two crRNAs guided the ctRNP complex to concurrently cut within a 10 bp window of the PAM (protospacer adjacent motif), resulting in a precise deletion within BMP15 in 85.9% edited oocytes, and additional deletion happened in 14.1% edited oocytes, which resulted in large fragment deletions in BMP15. Most deletions caused frameshift and introduced premature stop codon in BMP15, resulting in the disruption of BMP15 protein expression, which was confirmed by the Western blot analysis showing the reduced BMP15 protein expression in ctRNP injected oocytes. The disruption of BMP15 attenuated the activation of SMAD1/5/8 signaling, and impaired cumulus expansion of porcine cumulus cell-oocyte complexes (COCs). Our study proved that delivering CRISPR ctRNP into porcine oocytes by microinjection was able to edit BMP15 efficiently, providing a new strategy to investigate the functions of oocyte-specific secreted factors in oocyte in vitro maturation.