Calcium-Free and Cytochalasin B Treatment Inhibits Blastomere Fusion in 2-Cell Stage Embryos for the Generation of Floxed Mice via Sequential Electroporation

Efficient Detection of Flox Mice Using In Vitro Cre Recombination

Advances in CRISPR/Cas9 genome editing technologies have allowed for the rapid generation of Cre-loxP conditional knockout mice. However, current strategies for genotyping flox mice, typically based on Sanger sequencing following cloning of target sequences from dozens of pups, are time-consuming. Here, we describe a rapid screening method for flox mice, using in vitro Cre recombination that can be performed using simple enzymatic reactions and enables detection of functional flox mouse within 1 day. In addition, we introduce an efficient strategy for subsequent sequence analysis by cloning of floxed regions using the In-Fusion system. Our genotyping pipeline reduces laborious tasks and thus contributes to the rapid selection of accurately edited flox mice.

Generation of Floxed Mice by Sequential Electroporation

Generation of conditional knockout mice using the Cre-loxP system is essential for the analysis of gene functions. The use of CRISPR-Cas9 in combination with two sets of guide RNAs and single-stranded oligonucleotides including loxP sites enables simultaneous insertion of two loxP sequences. Unfortunately, this method induces double-strand breaks at two sites in the same chromosome, which causes an undesirable large chromosomal deletion and reduces the flanked loxP (flox) rate. To overcome this problem, we have developed a method that sequentially introduces each loxP sequence by electroporation at the one- and two-cell embryonic stages, respectively. This sequential electroporation method improves the floxing efficiency compared with the conventional simultaneous method, leading to a high yield of offspring with floxed alleles.

Introduction of loxP sites by electroporation in the mouse genome; a simple approach for conditional allele generation in complex targeting loci

Background

The discovery of the CRISPR-Cas9 system and its applicability in mammalian embryos has revolutionized the way we generate genetically engineered animal models. To date, models harbouring conditional alleles (i.e. two loxP sites flanking an exon or a critical DNA sequence of interest) are amongst the most widely requested project type that are challenging to generate as they require simultaneous cleavage of the genome using two guides in order to properly integrate the repair template. An approach, using embryo sequential electroporation has been reported in the literature to successfully introduce loxP sites on the same allele. Here, we describe a modification of this sequential electroporation procedure that demonstrated the production of conditional allele mouse models for eight different genes via one of two possible strategies: either by consecutive sequential electroporation (strategy A) or non-consecutive sequential electroporation (strategy B). This latest strategy originated from using the by-product produced when using consecutive sequential electroporation (i.e. mice with a single targeted loxP site) to complete the project.

Results

By using strategy A, we demonstrated successful generation of conditional allele models for three different genes (Icam1, Lox, and Sar1b), with targeting efficiencies varying between 5 and 13%. By using strategy B, we generated five conditional allele models (Loxl1, Pard6a, Pard6g, Clcf1, and Mapkapk5), with targeting efficiencies varying between 3 and 25%.

Conclusion

Our modified electroporation-based approach, involving one of the two alternative strategies, allowed the production of conditional allele models for eight different genes via two different possible paths. This reproducible method will serve as another reliable approach in addition to other well-established methodologies in the literature for conditional allele mouse model generation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-022-00744-8.

Progress of genome editing technology and developmental biology useful for radiation research

Following the development of genome editing technology, it has become more feasible to create genetically modified animals such as knockout (KO), knock-in, and point-mutated animals. The genome-edited animals are useful to investigate the roles of various functional genes in many fields of biological science including radiation research. Nevertheless, some researchers may experience difficulty in generating genome-edited animals, probably due to the requirement for equipment and techniques for embryo manipulation and handling. Furthermore, after obtaining F0 generation, genome-edited animals generally need to be expanded and maintained for analyzing the target gene function. To investigate genes essential for normal birth and growth, the generation of conditional KO (cKO) animals in which a tissue- or stage-specific gene mutation can be introduced is often required. Here, we describe the basic principle and application of genome editing technology including zinc-finger nuclease, transcription-activator-like effector nuclease, and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR associated protein (Cas) systems. Recently advanced developmental biology methods have enabled application of the technology, especially CRISPR/Cas, to zygotes, leading to the prompt production of genome-edited animals. For pre-implantation embryos, genome editing via oviductal nucleic acid delivery has been developed as an embryo manipulation- or handling-free method. Examining the gene function at F0 generation is becoming possible by employing triple-target CRISPR technology. This technology, in combination with a blastocyst complementation method enables investigation of even birth- and growth-responsible genes without establishing cKO strains. We hope that this review is helpful for understanding and expanding genome editing-related technology and for progressing radiation research.