CRISPR/Cas9-based simple transgenesis in Xenopus laevis

Modelling human genetic disorders in Xenopus tropicalis

Recent progress in human disease genetics is leading to rapid advances in understanding pathobiological mechanisms. However, the sheer number of risk-conveying genetic variants being identified demands in vivo model systems that are amenable to functional analyses at scale. Here we provide a practical guide for using the diploid frog species Xenopus tropicalis to study many genes and variants to uncover conserved mechanisms of pathobiology relevant to human disease. We discuss key considerations in modelling human genetic disorders: genetic architecture, conservation, phenotyping strategy and rigour, as well as more complex topics, such as penetrance, expressivity, sex differences and current challenges in the field. As the patient-driven gene discovery field expands significantly, the cost-effective, rapid and higher throughput nature of Xenopus make it an essential member of the model organism armamentarium for understanding gene function in development and in relation to disease.

A CRISPR-Cas9-mediated versatile method for targeted integration of a fluorescent protein gene to visualize endogenous gene expression in Xenopus laevis

Xenopus laevis is a widely used model organism in developmental and regeneration studies. Despite several reports regarding targeted integration techniques in Xenopus, there is still room for improvement of them, especially in creating reporter lines that rely on endogenous regulatory enhancers/promoters. We developed a CRISPR-Cas9-based simple method to efficiently introduce a fluorescent protein gene into 5' untranslated regions (5'UTRs) of target genes in Xenopus laevis. A donor plasmid DNA encoding an enhanced green fluorescent protein (eGFP) flanked by a genomic fragment ranging from 66 bp to 878 bp including target 5'UTR was co-injected into fertilized eggs with a single guide RNA and Cas9 protein. Injections for krt12.2.L, myod1.S, sox2.L or brevican.S resulted in embryos expressing eGFP fluorescence in a tissue-specific manner, recapitulating endogenous expression of target genes. Integrations of the donor DNA into the target regions were examined by genotyping PCR for the eGFP-expressing embryos. The rate of embryos expressing the specific eGFP varied from 2.1% to 13.2% depending on the target locus and length of the genomic fragment in the donor plasmids. Germline transmission of an integrated DNA was observed. This simple method provides a powerful tool for exploring gene expression and function in developmental and regeneration research in X. laevis.

Protocols for transgenesis at a safe harbor site in the Xenopus laevis genome using CRISPR-Cas9

We have established a new transgenesis protocol based on CRISPR-Cas9, "New and Easy XenopusTransgenesis (NEXTrans)," and identified a novel safe harbor site in African clawed frogs, Xenopus laevis. We describe steps in detail for the construction of NEXTrans plasmid and guide RNA, CRISPR-Cas9-mediated NEXTrans plasmid integration into the locus, and its validation by genomic PCR. This improved strategy allows us to simply generate transgenic animals that stably express the transgene. For complete details on the use and execution of this protocol, please refer to Shibata et al. (2022).1.

CRISPR-Cas9-Based Functional Analysis in Amphibians: Xenopus laevis, Xenopus tropicalis, and Pleurodeles waltl

Amphibians have made many fundamental contributions to our knowledge, from basic biology to biomedical research on human diseases. Current genome editing tools based on the CRISPR-Cas system enable us to perform gene functional analysis in vivo, even in non-model organisms. We introduce here a highly efficient and easy protocol for gene knockout, which can be used in three different amphibians seamlessly: Xenopus laevis, Xenopus tropicalis, and Pleurodeles waltl. As it utilizes Cas9 ribonucleoprotein complex (RNP) for injection, this cloning-free method enables researchers to obtain founder embryos with a nearly complete knockout phenotype within a week. To evaluate somatic mutation rate and its correlation to the phenotype of a Cas9 RNP-injected embryo (crispant), we also present accurate and cost-effective genotyping methods using pooled amplicon-sequencing and a user-friendly web-based tool.