Production of CETD transgenic mouse line allowing ablation of any type of specific cell population

Intravenous Delivery of piggyBac Transposons as a Useful Tool for Liver-Specific Gene-Switching

Hydrodynamics-based gene delivery (HGD) is an efficient method for transfecting plasmid DNA into hepatocytes in vivo. However, the resulting gene expression is transient, and occurs in a non-tissue specific manner. The piggyBac (PB) transposon system allows chromosomal integration of a transgene in vitro. This study aimed to achieve long-term in vivo expression of a transgene by performing hepatocyte-specific chromosomal integration of the transgene using PB and HGD. Using this approach, we generated a novel mouse model for a hepatic disorder. A distinct signal from the reporter plasmid DNA was discernible in the murine liver approximately two months after the administration of PB transposons carrying a reporter gene. Then, to induce the hepatic disorder, we first administered mice with a PB transposon carrying a CETD unit (loxP-flanked stop cassette, diphtheria toxin-A chain gene, and poly(A) sites), and then with a plasmid expressing the Cre recombinase under the control of a liver-specific promoter. We showed that this system can be used for in situ manipulation and analysis of hepatocyte function in vivo in non-transgenic (Tg) animals.

Yao HH, Jensen P. A new mouse line for cell ablation by diphtheria toxin subunit A controlled by a Cre-dependent FLEx switch

Recombinase responsive mouse lines expressing diphtheria toxin subunit A (DTA) are well established tools for targeted ablation of genetically defined cell populations. Here we describe a new knock-in allele at the Gt(Rosa)26Sor locus that retains the best features of previously described DTA alleles-including a CAG promoter, attenuated mutant DTA cDNA, and ubiquitous EGFP labeling-with the addition of a Cre-dependent FLEx switch for tight control of expression. The FLEx switch consists of two pairs of antiparallel lox sites requiring Cre-mediated recombination for inversion of the DTA to the proper orientation for transcription. We demonstrate its utility by Cre-dependent ablation of both a broad domain in the embryonic nervous system and a discrete population of cells in the fetal gonads. We conclude that this new DTA line is useful for targeted ablation of genetically-defined cell populations.

A new mouse line for cell ablation by diphtheria toxin subunit A controlled by a Cre-dependent FLEx switch

Recombinase responsive mouse lines expressing diphtheria toxin subunit A (DTA) are well established tools for targeted ablation of genetically defined cell populations. Here we describe a new knock-in allele at the Gt(Rosa)26Sor locus that retains the best features of previously described DTA alleles-including a CAG promoter, attenuated mutant DTA cDNA, and ubiquitous EGFP labeling-with the addition of a Cre-dependent FLEx switch for tight control of expression. The FLEx switch consists of two pairs of antiparallel lox sites requiring Cre-mediated recombination for inversion of the DTA to the proper orientation for transcription. We demonstrate its utility by Cre-dependent ablation of both a broad domain in the embryonic nervous system and a discrete population of cells in the fetal gonads. We conclude that this new DTA line is useful for targeted ablation of genetically-defined cell populations.

RIPK1 protects from TNF-α-mediated liver damage during hepatitis

Cell death of hepatocytes is a prominent characteristic in the pathogenesis of liver disease, while hepatolysis is a starting point of inflammation in hepatitis and loss of hepatic function. However, the precise molecular mechanisms of hepatocyte cell death, the role of the cytokines of hepatic microenvironment and the involvement of intracellular kinases, remain unclear. Tumor necrosis factor alpha (TNF-α) is a key cytokine involved in cell death or survival pathways and the role of RIPK1 has been associated to the TNF-α-dependent signaling pathway. We took advantage of two different deficient mouse lines, the RIPK1 kinase dead knock-in mice (Ripk1^K45A) and the conditional knockout mice lacking RIPK1 only in liver parenchymal cells (Ripk1^LPC-KO), to characterize the role of RIPK1 and TNF-α in hepatitis induced by concanavalin A (ConA). Our results show that RIPK1 is dispensable for liver homeostasis under steady-state conditions but in contrast, RIPK1 kinase activity contributes to caspase-independent cell death induction following ConA injection and RIPK1 also serves as a scaffold, protecting hepatocytes from massive apoptotic cell death in this model. In the Ripk1^LPC-KO mice challenged with ConA, TNF-α triggers apoptosis, responsible for the observed severe hepatitis. Mechanism potentially involves both TNF-independent canonical NF-κB activation, as well as TNF-dependent, but canonical NF-κB-independent mechanisms. In conclusion, our results suggest that RIPK1 kinase activity is a pertinent therapeutic target to protect liver against excessive cell death in liver diseases.

Genetic cell ablation

Targeted cell ablation has proven to be a valuable approach to study in vivo cell functions during organogenesis, tissue homeostasis, and regeneration. Over the last two decades, various approaches have been developed to refine the control of cell ablation. In this review, we give an overview of the distinct genetic tools available for targeted cell ablation, with a particular emphasis on their respective specificity.

Improvement of hydrodynamics-based gene transfer of nonviral DNA targeted to murine hepatocytes

The liver is an important organ for supporting the life of an individual. Gene transfer toward this organ has been attempted in many laboratories to date; however, there have been few reports on improved liver-targeted gene delivery by using a nonviral vector. In this study, we examined the effect of various types of gene delivery carriers on enhancing the uptake and gene expression of exogenous DNA in murine hepatocytes when a hydrodynamics-based gene delivery (HGD) is performed via tail-vein injection. Mice were singly injected with a large amount of phosphate-buffered saline containing reporter plasmid DNA and/or with a gene delivery carrier. One day after the gene delivery, the animals' livers were dissected and subjected to biochemical, histochemical, and molecular biological analyses. The strongest signal from the reporter plasmid DNA was observed when the DNA was mixed with a polyethylenimine- (PEI-) based reagent. Coinjection with pCRTEIL (a loxP-floxed reporter construct) and pTR/NCre (a liver-specific Cre expression vector) resulted in the liver-specific recombination of pCRTEIL. The combination of PEI with HGD would thus be a valuable tool for liver-specific manipulation to examine the function of a gene of interest in the liver and for creating liver disease models.

A Cre knock-in mouse line on the Sickle tail locus induces recombination in the notochord and intervertebral disks

Sickle tail (Skt) was originally identified by gene trap mutagenesis in mice, and the trapped gene is highly expressed in the notochord, intervertebral discs (IVD), and mesonephros. Here, we report the generation of Skt(cre) mice expressing Cre recombinase in the IVD due to target insertion of the cre gene into the Skt locus by recombinase-mediated cassette exchange. Crossing a conditional lacZ Reporter (R26R), Cre expression from the Skt(cre) allele specifically activates β-galactosidase expression in the whole notochord from E9.5 onwards. In E15.5 Skt(cre);R26R embryos, reporter activity was detected in the nucleus pulposus and in a portion of the annulus fibrosus, resulting in expansion of Cre-expressing cells in the adult IVD. Reporter activity was also seen in the Skt(cre);R26R mesonephros at E15.5. These results suggest that Skt(cre) mice are useful for exploring the fate specification of notochordal cells and creating models for IVD-related skeletal diseases.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

Inducible male infertility by targeted cell ablation in zebrafish testis

To generate a zebrafish model of inducible male sterility, we expressed an Escherichia coli nitroreductase (Ntr) gene in the male germ line of zebrafish. The Ntr gene encodes an enzyme that can convert prodrugs such as metronidazole (Met) to cytotoxins. A fusion protein eGFP:Ntr (fusing Ntr to eGFP) under control of approximately 2 kb putative promoters of the zebrafish testis-specific genes, A-kinase anchoring protein-associated protein (Asp), outer dense fibers (Odf), and sperm acrosomal membrane-associated protein (Sam) was expressed in the male germ line. Three independent and four compound transgenic zebrafish lines expressing eGFP:Ntr were established. Female carriers were fertile, while males exhibited different levels of sterility and appeared normal, otherwise. Developmental analysis shows that germ cells survived and testes were normal before Met treatment, but that the testes of all male transgenic zebrafish exhibited variously depleted prospermatogonia after Met treatment. Particularly in a triple-transgenic line, Tg(AOS-eGFP:Ntr)[Tg(Asp-eGFP:Ntr; Odf-eGFP:Ntr; Sam-eGFP:Ntr)], the transgenic males had very small testes that were virtually devoid of germ cells, and the residual germ cells had almost completely disappeared after 2 weeks of Met treatment. These zebrafish transgenic lines show the complete testis specificity of inducible male sterility after Met treatment and reveal a period of the Ntr/Met ablation activity just prior to formation of the definitive adult spermatogonial cell population. This study demonstrates that combined genetic and pharmacological methods for developing an "infertile breeding technology" have practical application in controlling genetically modified (GM) fish breeding and meet the standards of biological and environment safety for other GM species.

Analysis of mouse development with conditional mutagenesis

Explorations into the molecular embryology of the mouse have played a vital role in our understanding of the basic mechanisms of gene regulation that govern development and disease. In the last 15 years, these mechanisms have been analyzed with vastly greater precision and clarity with the advent of systems that allow the conditional control of gene expression. Typically, this control is achieved by silencing or activating the gene of interest with site-specific DNA recombination or transcriptional transactivation. In this review, I discuss the application of these technologies to mouse development, focusing on recent innovations and experimental designs that specifically aid the study of the mouse embryo.

In vivo genetic ablation by Cre-mediated expression of diphtheria toxin fragment A

We generated a ROSA26-eGFP-DTA mouse line by introducing an eGFP-DTA (enhanced green fluorescent protein -- diphtheria toxin fragment A) cassette into the ROSA26 locus by homologous recombination in ES cells. This mouse expresses eGFP ubiquitously, but DTA expression is prevented by the presence of eGFP, a Neo cassette, and a strong transcriptional stop sequence. Mice carrying this construct are normal and fertile, indicating the absence of DTA expression. However, upon Cre-mediated excision of the floxed region DTA expression is activated, resulting in the specific ablation of Cre-expressing cells. As an example of this approach, we ablated Nkx2.5 and Wnt1-expressing cells by using the Nkx2.5-Cre and Wnt1-Cre mouse lines, respectively. We observed loss of the precise tissues in which Nkx2.5 and Wnt1 are expressed. Apart from being a general GFP reporter, the ROSA26-GFP-DTA mouse line should provide a useful resource for genetic ablation of specific groups of cells.