A survey to establish performance standards for the production of transgenic mice

ASIS-Seq: Transgene Insertion Site Mapping by Nanopore Adaptive Sampling

Generation of transgenic mice by direct microinjection of foreign DNA into fertilized ova has become a routine technique in biomedical research. It remains an essential tool for studying gene expression, developmental biology, genetic disease models, and their therapies. However, the random integration of foreign DNA into the host genome that is inherent to this technology can lead to confounding effects associated with insertional mutagenesis and transgene silencing. Locations of most transgenic lines remain unknown because the methods are often burdensome (Nicholls et al., G3: Genes Genomes Genetics 9:1481-1486, 2019) or have limitations (Goodwin et al., Genome Research 29:494-505, 2019). Here, we present a method that we call Adaptive Sampling Insertion Site Sequencing (ASIS-Seq) to locate transgene integration sites using targeted sequencing on Oxford Nanopore Technologies' (ONT) sequencers. ASIS-Seq requires only about 3 ug of genomic DNA, 3 hours of hands-on sample preparation time, and 3 days of sequencing time to locate transgenes in a host genome.

Microinjection of Zygotes for CRISPR/Cas9-Mediated Insertion of Transgenes into the Murine Rosa26 Safe Harbor

Genetically modified (GM) mice are widely used in biomedical research because they can address complex questions in an in-vivo setting that could not otherwise be addressed in-vitro. Microinjection of zygotes remains the most common technique to generate GM animals to date. Here, we describe the targeted insertion (knock-in) of transgenes by microinjection of 1-cell or 2-cell stage embryos into the murine Rosa26 safe harbor.

The History of Transgenesis

A transgenic mouse carries within its genome an artificial DNA construct (transgene) that is deliberately introduced by an experimentalist. These animals are widely used to understand gene function and protein function. When addressing the history of transgenic mouse technology, it is apparent that a number of basic science research areas laid the groundwork for success. These include reproductive science, genetics and molecular biology, and micromanipulation and microscopy equipment. From reproductive physiology came applications on how to optimize mouse breeding, how to superovulate mice to produce zygotes for DNA microinjection or preimplantation embryos for combination with embryonic stem (ES) cells, and how to return zygotes and embryos to a pseudopregnant surrogate dam for gestation and birth. From developmental biology, it was learned how to micromanipulate embryos for morula aggregation and blastocyst microinjection and how to establish germline competent ES cells. From genetics came the foundational principles governing the inheritance of genes, the interactions of gene products, and an understanding of the phenotypic consequences of genetic mutations. From molecular biology came a panoply of tools and reagents that are used to clone DNA transgenes, to detect the presence of transgenes, to assess gene expression by measuring transcription, and to detect proteins in cells and tissues. Technical advances in light microscopes, micromanipulators, micropipette pullers, and ancillary equipment made it possible for experimentalists to insert thin glass needles into zygotes or embryos under controlled conditions to inject DNA solutions or ES cells. To fully discuss the breadth of contributions of these numerous scientific disciplines to a comprehensive history of transgenic science is beyond the scope of this work. Examples will be used to illustrate scientific developments central to the foundation of transgenic technology and that are in use today.

Pronuclear Injection-Based Targeted Transgenesis

Microinjection of DNA expression cassettes into fertilized zygotes has been a standard method for generating transgenic animal models. While efficient, the injected DNA integrates randomly into the genome, leading to potential disruption of endogenous genes or regulatory elements, variation in copy number, or integration into heterochromatic regions that inhibit transgene expression. A recently developed method addresses such pitfalls of traditional transgenesis by targeting the transgene to predetermined sites in the genome that can safely harbor exogenous DNA. This method, called Pronuclear Injection-based Targeted Transgenesis (PITT), employs an enzymatic transfer of exogenous DNA from a donor vector to a previously created landing-pad site in the mouse genome. DNA transfer is achieved using molecular tools such as the Cre-LoxP recombinase and PhiC31-attB/P integrase systems. Here, we provide protocols for performing PITT and an overview of the current PITT tools available to the research community. © 2016 by John Wiley & Sons, Inc.

GONAD: Genome-editing via Oviductal Nucleic Acids Delivery system: a novel microinjection independent genome engineering method in mice

Microinjection is considered the gold standard technique for delivery of nucleic acids (NAs; transgenes or genome editing tools such as CRISPR/Cas9 systems) into embryos, for creating genetically modified organisms. It requires sophisticated equipment as well as well-trained and highly skilled personnel to perform the micro-injection technique. Here, we describe a novel and simple microinjection-independent technique, called Genome-editing via Oviductal Nucleic Acids Delivery (GONAD). Using GONAD, we show that NAs (e.g., eGFP mRNA or Cas9 mRNA/sgRNAs) can be effectively delivered to pre-implantation embryos within the intact mouse oviduct by a simple electroporation method, and result in the desired genetic modification in the embryos. Thus GONAD can bypass many complex steps in transgenic technology such as isolation of zygotes, microinjection of NAs into them, and their subsequent transfer to pseudo-pregnant animals. Furthermore, this method can potentially be used for genome editing in species other than mice.

Mouse Genome Editing Using the CRISPR/Cas System

The availability of techniques to create desired genetic mutations has enabled the laboratory mouse as an extensively used model organism in biomedical research including human genetics. A new addition to this existing technical repertoire is the CRISPR/Cas system. Specifically, this system allows editing of the mouse genome much more quickly than the previously used techniques, and, more importantly, multiple mutations can be created in a single experiment. Here we provide protocols for preparation of CRISPR/Cas reagents and microinjection into one-cell mouse embryos to create knockout or knock-in mouse models.

Glucose transport by epithelia prepared from harvested enterocytes

Transformed and cultured cell lines have significant shortcomings for investigating the characteristics and responses of native villus enterocytes in situ. Interpretations of results from intact tissues are complicated by the presence of underlying tissues and the crypt compartment. We describe a simple, novel, and reproducible method for preparing functional epithelia using differentiated enterocytes harvested from the small intestine upper villus of adult mice and preterm pigs with and without necrotizing enterocolitis. Concentrative, rheogenic glucose uptake was used as an indicator of epithelial function and was demonstrated by cellular accumulation of tracer ¹⁴C d-glucose and Ussing chamber based short-circuit currents. Assessment of the epithelia by light and immunofluorescent microscopy revealed the harvested enterocytes remain differentiated and establish cell–cell connections to form polarized epithelia with distinct apical and basolateral domains. As with intact tissues, the epithelia exhibit glucose induced short-circuit currents that are increased by exposure to adenosine and adenosine 5′-monophosphate (AMP) and decreased by phloridzin to inhibit the apical glucose transporter SGLT-1. Similarly, accumulation of ¹⁴C d-glucose by the epithelia was inhibited by phloridzin, but not phloretin, and was stimulated by pre-exposure to AMP and adenosine, apparently by a microtubule-based mechanism that is disrupted by nocodazole, with the magnitudes of responses to adenosine, forskolin, and health status exceeding those we have measured using intact tissues. Our findings indicate that epithelia prepared from harvested enterocytes provide an alternative approach for comparative studies of the characteristics of nutrient transport by the upper villus epithelium and the responses to different conditions and stimuli.

9th Transgenic Technology Meeting (TT2010) in Berlin, Germany: a meeting report

The first Transgenic Technology (TT) Meeting was organized in 1999 by Johannes Wilbertz, Karolinska Institute, Stockholm, Sweden as a regional meeting. The TT Meetings continued in this way, constantly gathering additional practitioners of transgenic methodologies until the breakthrough in 2005 when the 6th TT Meeting in Barcelona, Spain, hosted by Lluis Montoliu (Centro Nacional de Biotecnologia, Madrid, Spain), generated the momentum to establish the International Society for Transgenic Technologies (ISTT). Since 2006, the ISTT has continued to promote the TT Meetings and provide its membership with a forum to discuss best practices and new methods in the field. The TT2010 Meeting was held at the Max Delbrück Center for Molecular Medicine (Berlin, Germany). Participation at the TT2010 Meeting exceeded the registration capacity and set a new attendance record. Session topics included methods for the generation of rat and mouse models of human disease, fundamental and advanced topics in rodent embryonic stem cells, and the newest transgenic technologies. Short presentations from selected abstracts were of especial interest. Roundtable discussions on transgenic facility establishment and cryoarchiving of mouse lines were favorably received. Students, technical staff, and professors participated in numerous discussions and came away with practical methods and new ideas for research.