Highly efficient reprogrammable mouse lines with integrated reporters to track the route to pluripotency.

Revealing the molecular events associated with reprogramming different somatic cell types to pluripotency is critical for understanding the characteristics of induced pluripotent stem cell (iPSC) therapeutic derivatives. Inducible reprogramming factor transgenic cells or animals-designated as secondary (2°) reprogramming systems-not only provide excellent experimental tools for such studies but also offer a strategy to study the variances in cellular reprogramming outcomes due to different in vitro and in vivo environments. To make such studies less cumbersome, it is desirable to have a variety of efficient reprogrammable mouse systems to induce successful mass reprogramming in somatic cell types. Here, we report the development of two transgenic mouse lines from which 2° cells reprogram with unprecedented efficiency. These systems were derived by exposing primary reprogramming cells containing doxycycline-inducible Yamanaka factor expression to a transient interruption in transgene expression, resulting in selection for a subset of clones with robust transgene response. These systems also include reporter genes enabling easy readout of endogenous Oct4 activation (GFP), indicative of pluripotency, and reprogramming transgene expression (mCherry). Notably, somatic cells derived from various fetal and adult tissues from these 2° mouse lines gave rise to highly efficient and rapid reprogramming, with transgene-independent iPSC colonies emerging as early as 1 wk after induction. These mouse lines serve as a powerful tool to explore sources of variability in reprogramming and the mechanistic underpinnings of efficient reprogramming systems.

Production of knockout mouse lines with Cas9.

Knockout mice are used extensively to explore the phenotypic effects of mammalian gene dysfunction. With the application of RNA-guided Cas9 nuclease technology for the production of knockout mouse lines, the time, as well as the resources needed, to progress from identification of a gene of interest to production of a knockout line is significantly reduced. Here we present our standard methodology to produce knockout mouse lines by the electroporation of Cas9 ribonucleoprotein (RNP) into mouse zygotes. Using this protocol, we have obtained an 80% success rate in the generation of founders for null alleles with a subsequent 93% germline transmission rate. These methods rely on equipment already present in the majority of transgenic facilities and should be straightforward to implement where appropriate embryo handling expertise exists.

Inhibition of mTOR induces a paused pluripotent state.

Cultured pluripotent stem cells are a cornerstone of regenerative medicine owing to their ability to give rise to all cell types of the body. Although pluripotent stem cells can be propagated indefinitely in vitro, pluripotency is paradoxically a transient state in vivo, lasting 2-3 days around the time of blastocyst implantation. The exception to this rule is embryonic diapause, a reversible state of suspended development triggered by unfavourable conditions. Diapause is a physiological reproductive strategy widely employed across the animal kingdom, including in mammals, but its regulation remains poorly understood. Here we report that the partial inhibition of mechanistic target of rapamycin (mTOR), a major nutrient sensor and promoter of growth, induces reversible pausing of mouse blastocyst development and allows their prolonged culture ex vivo. Paused blastocysts remain pluripotent and competent-able to give rise to embryonic stem (ES) cells and live, fertile mice. We show that both naturally diapaused blastocysts in vivo and paused blastocysts ex vivo display pronounced reductions in mTOR activity, translation, histone modifications associated with gene activity and transcription. Pausing can be induced directly in cultured ES cells and sustained for weeks without appreciable cell death or deviations from cell cycle distributions. We show that paused ES cells display a remarkable global suppression of transcription, maintain a gene expression signature of diapaused blastocysts and remain pluripotent. These results uncover a new pluripotent stem cell state corresponding to the epiblast of the diapaused blastocyst and indicate that mTOR regulates developmental timing at the peri-implantation stage. Our findings have implications in the fields of assisted reproduction, regenerative medicine, cancer, metabolic disorders and ageing.

Transcription factor ASCL2 is required for development of the glycogen trophoblast cell lineage.

The basic helix-loop-helix (bHLH) transcription factor ASCL2 plays essential roles in diploid multipotent trophoblast progenitors, intestinal stem cells, follicular T-helper cells, as well as during epidermal development and myogenesis. During early development, Ascl2 expression is regulated by genomic imprinting and only the maternally inherited allele is transcriptionally active in trophoblast. The paternal allele-specific silencing of Ascl2 requires expression of the long non-coding RNA Kcnq1ot1 in cis and the deposition of repressive histone marks. Here we show that Del7AI, a 280-kb deletion allele neighboring Ascl2, interferes with this process in cis and leads to a partial loss of silencing at Ascl2. Genetic rescue experiments show that the low level of Ascl2 expression from the paternal Del7AI allele can rescue the embryonic lethality associated with maternally inherited Ascl2 mutations, in a level-dependent manner. Despite their ability to support development to term, the rescued placentae have a pronounced phenotype characterized by severe hypoplasia of the junctional zone, expansion of the parietal trophoblast giant cell layer, and complete absence of invasive glycogen trophoblast cells. Transcriptome analysis of ectoplacental cones at E7.5 and differentiation assays of Ascl2 mutant trophoblast stem cells show that ASCL2 is required for the emergence or early maintenance of glycogen trophoblast cells during development. Our work identifies a new cis-acting mutation interfering with Kcnq1ot1 silencing function and establishes a novel critical developmental role for the transcription factor ASCL2.

piggyBac transposition reprograms fibroblasts to induced pluripotent stem cells.

Transgenic expression of just four defined transcription factors (c-Myc, Klf4, Oct4 and Sox2) is sufficient to reprogram somatic cells to a pluripotent state. The resulting induced pluripotent stem (iPS) cells resemble embryonic stem cells in their properties and potential to differentiate into a spectrum of adult cell types. Current reprogramming strategies involve retroviral, lentiviral, adenoviral and plasmid transfection to deliver reprogramming factor transgenes. Although the latter two methods are transient and minimize the potential for insertion mutagenesis, they are currently limited by diminished reprogramming efficiencies. piggyBac (PB) transposition is host-factor independent, and has recently been demonstrated to be functional in various human and mouse cell lines. The PB transposon/transposase system requires only the inverted terminal repeats flanking a transgene and transient expression of the transposase enzyme to catalyse insertion or excision events. Here we demonstrate successful and efficient reprogramming of murine and human embryonic fibroblasts using doxycycline-inducible transcription factors delivered by PB transposition. Stable iPS cells thus generated express characteristic pluripotency markers and succeed in a series of rigorous differentiation assays. By taking advantage of the natural propensity of the PB system for seamless excision, we show that the individual PB insertions can be removed from established iPS cell lines, providing an invaluable tool for discovery. In addition, we have demonstrated the traceless removal of reprogramming factors joined with viral 2A sequences delivered by a single transposon from murine iPS lines. We anticipate that the unique properties of this virus-independent simplification of iPS cell production will accelerate this field further towards full exploration of the reprogramming process and future cell-based therapies.

The development and characterization of a CRISPR/Cas9-mediated PD-1 functional knockout rat as a tool to study idiosyncratic drug reactions.

Idiosyncratic drug reactions are rare but serious adverse drug reactions unrelated to the known therapeutic properties of the drug and manifest in only a small percentage of the treated population. Animal models play an important role in advancing mechanistic studies examining idiosyncratic drug reactions. However, to be useful, they must possess similarities to those seen clinically. Although mice currently represent the dominant mammalian genetic model, rats are advantageous in many areas of pharmacologic study where their physiology can be examined in greater detail and is more akin to that seen in humans. In the area of immunology, this includes autoimmune responses and susceptibility to diabetes, in which rats more accurately mimic disease states in humans compared with mice. For example, oral nevirapine treatment can induce an immune-mediated skin rash in humans and rats, but not in mice due to the absence of the sulfotransferase required to form reactive metabolites of nevirapine within the skin. Using CRISPR-mediated gene editing, we developed a modified line of transgenic rats in which a segment of IgG-like ectodomain containing the core PD-1 interaction motif containing the native ligand and therapeutic antibody domain in exon 2 was deleted. Removal of this region critical for mediating PD-1/PD-L1 interactions resulted in animals with an increased immune response resulting in liver injury when treated with amodiaquine.

Phenotypic annotation of the mouse X chromosome.

Mutational screens are an effective means used in the functional annotation of a genome. We present a method for a mutational screen of the mouse X chromosome using gene trap technologies. This method has the potential to screen all of the genes on the X chromosome without establishing mutant animals, as all gene-trapped embryonic stem (ES) cell lines are hemizygous null for mutations on the X chromosome. Based on this method, embryonic morphological phenotypes and expression patterns for 58 genes were assessed, approximately 10% of all human and mouse syntenic genes on the X chromosome. Of these, 17 are novel embryonic lethal mutations and nine are mutant mouse models of genes associated with genetic disease in humans, including BCOR and PORCN. The rate of lethal mutations is similar to previous mutagenic screens of the autosomes. Interestingly, some genes associated with X-linked mental retardation (XLMR) in humans show lethal phenotypes in mice, suggesting that null mutations cannot be responsible for all cases of XLMR. The entire data set is available via the publicly accessible website (http://xlinkedgenes.ibme.utoronto.ca/).

Zonadhesin is essential for species specificity of sperm adhesion to the egg zona pellucida.

Interaction of rapidly evolving molecules imparts species specificity to sperm-egg recognition in marine invertebrates, but it is unclear whether comparable interactions occur during fertilization in any vertebrate species. In mammals, the sperm acrosomal protein zonadhesin is a rapidly evolving molecule with species-specific binding activity for the egg zona pellucida (ZP). Here we show using null mice produced by targeted disruption of Zan that zonadhesin confers species specificity to sperm-ZP adhesion. Sperm capacitation selectively exposed a partial von Willebrand D domain of mouse zonadhesin on the surface of living, motile cells. Antibodies to the exposed domain inhibited adhesion of wild-type spermatozoa to the mouse ZP but did not inhibit adhesion of spermatozoa lacking zonadhesin. Zan(-/-) males were fertile, and their spermatozoa readily fertilized mouse eggs in vitro. Remarkably, however, loss of zonadhesin increased adhesion of mouse spermatozoa to pig, cow, and rabbit ZP but not mouse ZP. We conclude that zonadhesin mediates species-specific ZP adhesion, and Zan(-/-) males are fertile because their spermatozoa retain adhesion capability that is not species-specific. Mammalian sperm-ZP adhesion is therefore molecularly robust, and species-specific egg recognition by a protein in the sperm acrosome is conserved between invertebrates and vertebrates, even though the adhesion molecules themselves are unrelated.

The interval between Ins2 and Ascl2 is dispensable for imprinting centre function in the murine Beckwith-Wiedemann region.

Imprinted genes are commonly clustered in domains across the mammalian genome, suggesting a degree of coregulation via long-range coordination of their monoallelic transcription. The distal end of mouse chromosome 7 (Chr 7) contains two clusters of imprinted genes within a approximately 1 Mb domain. This region is conserved on human 11p15.5 where it is implicated in the Beckwith-Wiedemann syndrome. In both species, imprinted regulation requires two critical cis-acting imprinting centres, carrying different germline epigenetic marks and mediating imprinted expression in the proximal and distal sub-domains. The clusters are separated by a region containing the gene for tyrosine hydroxylase (Th) as well as a high density of short repeats and retrotransposons in the mouse. We have used the Cre-loxP recombination system in vivo to engineer an interstitial deletion of this approximately 280-kb intervening region previously proposed to participate in the imprinting mechanism or to act as a boundary between the two sub-domains. The deletion allele, Del(7AI), is silent with respect to epigenetic marking at the two flanking imprinting centres. Reciprocal inheritance of Del(7AI) demonstrates that the deleted region, which represents more than a quarter of the previously defined imprinted domain, is associated with intrauterine growth restriction in maternal heterozygotes. In homozygotes, the deficiency behaves as a Th null allele and can be rescued pharmacologically by bypassing the metabolic requirement for TH in utero. Our results show that the deleted interval is not required for normal imprinting on distal Chr 7 and uncover a new imprinted growth phenotype.

Characterization of human embryonic stem cell lines by the International Stem Cell Initiative.

The International Stem Cell Initiative characterized 59 human embryonic stem cell lines from 17 laboratories worldwide. Despite diverse genotypes and different techniques used for derivation and maintenance, all lines exhibited similar expression patterns for several markers of human embryonic stem cells. They expressed the glycolipid antigens SSEA3 and SSEA4, the keratan sulfate antigens TRA-1-60, TRA-1-81, GCTM2 and GCT343, and the protein antigens CD9, Thy1 (also known as CD90), tissue-nonspecific alkaline phosphatase and class 1 HLA, as well as the strongly developmentally regulated genes NANOG, POU5F1 (formerly known as OCT4), TDGF1, DNMT3B, GABRB3 and GDF3. Nevertheless, the lines were not identical: differences in expression of several lineage markers were evident, and several imprinted genes showed generally similar allele-specific expression patterns, but some gene-dependent variation was observed. Also, some female lines expressed readily detectable levels of XIST whereas others did not. No significant contamination of the lines with mycoplasma, bacteria or cytopathic viruses was detected.

Efficient Generation of Large-Fragment Knock-In Mouse Models Using 2-Cell (2C)-Homologous Recombination (HR)-CRISPR.

Generating large-fragment knock-ins, such as reporters, conditional alleles, or humanized alleles, directly in mouse embryos is still a challenging feat. We have developed 2C-HR-CRISPR, a technology that allows highly efficient (10-50%) and rapid (generating founders in 2 months) targeting of large DNA fragments. Key to this strategy is the delivery of CRISPR reagents into 2-cell-stage mouse embryos, taking advantage of the high homologous recombination activity during the long G2 cell cycle phase at this stage. Furthermore, by exploiting a Cas9-monomeric streptavidin (Cas-mSA) and biotinylated PCR template (BioPCR) system to localize the repair template to specific double strand breaks, the efficiency can be further improved to up to 95%. Here we provide a procedure to generate large-fragment knock-in mouse models using 2C-HR-CRISPR. We first describe the principles for designing single guide RNAs and repair templates but refer to published manuscripts and protocols for molecular cloning methods or commercial sources for these reagents. We then describe two unique aspects of 2C-HR-CRISPR that are critical for success: (1) production of the CRISPR reagents for 2C-HR-CRISPR, particularly for applying the Cas9-mSA/BioPCR method, and (2) microinjection of mouse embryos at the 2-cell stage. © 2020 by John Wiley & Sons, Inc. Basic Protocol 1: Single guide RNA and repair template design Basic Protocol 2: Preparing reagents for 2C-HR-CRISPR Basic Protocol 3: Microinjecting 2-cell-stage mouse embryos.

Generation of Large Fragment Knock-In Mouse Models by Microinjecting into 2-Cell Stage Embryos.

Large fragment knock-in mouse models such as reporters and conditional mutant mice are important models for biological research. Here we describe 2-cell (2C)-homologous recombination (HR)-CRISPR, a highly efficient method to generate large fragment knock-in mouse models by CRISPR-based genome engineering. Using this method, knock-in founders can be generated routinely in a time frame of about two months with high germline transmission efficiency. 2C-HR-CRISPR will significantly promote the advancement of basic and translational research using genetic mouse models.

Targeted Mutations in the Mouse via Embryonic Stem Cells.

Genetic modification of mouse embryonic stem (ES) cells is a powerful technology that enabled the generation of a plethora of mutant mouse lines to study gene function and mammalian biology. Here we describe ES cell culture and transfection techniques used to manipulate the ES cell genome to obtain targeted ES cell clones. We include the standard gene targeting approach as well as the application of the CRISPR/Cas9 system that can improve the efficiency of homologous recombination in ES cells by introducing a double-strand DNA break at the target site.

Preparation of Polymerase Chain Reaction Template DNA from Mouse Tail Tissue.

A simple cell or tissue lysate can provide a sufficient quality and amount of template DNA for polymerase chain reaction (PCR). In this protocol, a small piece from the tip of the tail is removed and processed using hot sodium hydroxide and Tris (HotSHOT).

Observation of Fluorescent Proteins in Living Cells.

Frequently, there is a need for fluorescent protein detection in mouse cell cultures, including embryonic stem cells or their differentiated derivatives, primary and transformed cells. Here, cells expressing green fluorescent protein-labeled proteins are observed using fluorescent microscopy.

Observation of Fluorescent Proteins in Fixed Cells.

Fluorescent proteins (FPs) are popular reporters available for gene expression detection and determination of cellular identities in the mouse. This protocol can be used to detect green fluorescent protein spectral variants and proteins labeled with the fusion tag dsRed in fixed cells.

Isolation of High-Molecular-Weight DNA from Mouse Yolk Sacs and the Like.

Often, genotyping of mouse embryos is required, and a small part, such as the yolk sac, can be used for this purpose. Here, DNA samples are prepared from extra-embryonic tissues by digestion with Proteinase K and subsequent extraction. The yolk sac of mid-gestation or later-stage embryos provides a sufficient amount of DNA for Southern analysis. Small tissues of a few hundred cells are used for genotyping early postimplantation- and preimplantation-stage embryos by polymerase chain reaction (PCR).

Isolation of High-Molecular-Weight DNA from Mouse Tail Tips.

DNA samples are prepared from mouse tail tips by Proteinase K digestion and subsequently extracted. The resulting preparation is suitable for use in Southern blotting or polymerase chain reaction (PCR).

Adhesion is prerequisite, but alone insufficient, to elicit stem cell pluripotency.

Primitive mammalian neural stem cells (NSCs), arising during the earliest stages of embryogenesis, possess pluripotency in embryo chimera assays in contrast to definitive NSCs found in the adult. We hypothesized that adhesive differences determine the association of stem cells with embryonic cells in chimera assays and hence their ability to contribute to later tissues. We show that primitive NSCs and definitive NSCs possess adhesive differences, resulting from differential cadherin expression, that lead to a double dissociation in outcomes after introduction into the early- versus midgestation embryo. Primitive NSCs are able to sort with the cells of the inner cell mass and thus contribute to early embryogenesis, in contrast to definitive NSCs, which cannot. Conversely, primitive NSCs sort away from cells of the embryonic day 9.5 telencephalon and are unable to contribute to neural tissues at midembryogenesis, in contrast to definitive NSCs, which can. Overcoming these adhesive differences by E-cadherin overexpression allows some definitive NSCs to integrate into the inner cell mass but is insufficient to allow them to contribute to later development. These adhesive differences suggest an evolving compartmentalization in multipotent NSCs during development and serve to illustrate the importance of cell-cell association for revealing cellular contribution.