Cre-stimulated recombination at loxP-containing DNA sequences placed into the mammalian genome

Unlocking Genetic Mysteries during the Epic Sperm Journey toward Fertilization: Further Expanding Cre Mouse Lines

The spatiotemporal expression patterns of genes are crucial for maintaining normal physiological functions in animals. Conditional gene knockout using the cyclization recombination enzyme (Cre)/locus of crossover of P1 (Cre/LoxP) strategy has been extensively employed for functional assays at specific tissue or developmental stages. This approach aids in uncovering the associations between phenotypes and gene regulation while minimizing interference among distinct tissues. Various Cre-engineered mouse models have been utilized in the male reproductive system, including Dppa3-MERCre for primordial germ cells, Ddx4-Cre and Stra8-Cre for spermatogonia, Prm1-Cre and Acrv1-iCre for haploid spermatids, Cyp17a1-iCre for the Leydig cell, Sox9-Cre for the Sertoli cell, and Lcn5/8/9-Cre for differentiated segments of the epididymis. Notably, the specificity and functioning stage of Cre recombinases vary, and the efficiency of recombination driven by Cre depends on endogenous promoters with different sequences as well as the constructed Cre vectors, even when controlled by an identical promoter. Cre mouse models generated via traditional recombination or CRISPR/Cas9 also exhibit distinct knockout properties. This review focuses on Cre-engineered mouse models applied to the male reproductive system, including Cre-targeting strategies, mouse model screening, and practical challenges encountered, particularly with novel mouse strains over the past decade. It aims to provide valuable references for studies conducted on the male reproductive system.

Conditional splicing system for tight control of viral overlapping genes

Viral genomes frequently harbor overlapping genes, complicating the development of virus-vectored vaccines and gene therapies. This study introduces a novel conditional splicing system to precisely control the expression of such overlapping genes through recombinase-mediated conditional splicing. We refined site-specific recombinase (SSR) conditional splicing systems and explored their mechanisms. The systems demonstrated exceptional inducibility (116,700-fold increase) with negligible background expression, facilitating the conditional expression of overlapping genes in adenovirus-associated virus (AAV) and human immunodeficiency virus type 1. Notably, this approach enabled the establishment of stable AAV producer cell lines, encapsulating all necessary packaging genes. Our findings underscore the potential of the SSR-conditional splicing system to significantly advance vector engineering, enhancing the efficacy and scalability of viral-vector-based therapies and vaccines.

IMPORTANCE

Regulating overlapping genes is vital for gene therapy and vaccine development using viral vectors. The regulation of overlapping genes presents challenges, including cytotoxicity and impacts on vector capacity and genome stability, which restrict stable packaging cell line development and broad application. To address these challenges, we present a "loxp-splice-loxp"-based conditional splicing system, offering a novel solution for conditional expression of overlapping genes and stable cell line establishment. This system may also regulate other cytotoxic genes, representing a significant advancement in cell engineering and gene therapy as well as biomass production.

Suggestion of creatine as a new neurotransmitter by approaches ranging from chemical analysis and biochemistry to electrophysiology

The discovery of a new neurotransmitter, especially one in the central nervous system, is both important and difficult. We have been searching for new neurotransmitters for 12 y. We detected creatine (Cr) in synaptic vesicles (SVs) at a level lower than glutamate and gamma-aminobutyric acid but higher than acetylcholine and 5-hydroxytryptamine. SV Cr was reduced in mice lacking either arginine:glycine amidinotransferase (a Cr synthetase) or SLC6A8, a Cr transporter with mutations among the most common causes of intellectual disability in men. Calcium-dependent release of Cr was detected after stimulation in brain slices. Cr release was reduced in Slc6a8 and Agat mutants. Cr inhibited neocortical pyramidal neurons. SLC6A8 was necessary for Cr uptake into synaptosomes. Cr was found by us to be taken up into SVs in an ATP-dependent manner. Our biochemical, chemical, genetic, and electrophysiological results are consistent with the possibility of Cr as a neurotransmitter, though not yet reaching the level of proof for the now classic transmitters. Our novel approach to discover neurotransmitters is to begin with analysis of contents in SVs before defining their function and physiology.

The Past, Present, and Future of Genetically Engineered Mouse Models for Skeletal Biology

The ability to create genetically engineered mouse models (GEMMs) has exponentially increased our understanding of many areas of biology. Musculoskeletal biology is no exception. In this review, we will first discuss the historical development of GEMMs and how these developments have influenced musculoskeletal disease research. This review will also update our 2008 review that appeared in BONEKey, a journal that is no longer readily available online. We will first review the historical development of GEMMs in general, followed by a particular emphasis on the ability to perform tissue-specific (conditional) knockouts focusing on musculoskeletal tissues. We will then discuss how the development of CRISPR/Cas-based technologies during the last decade has revolutionized the generation of GEMMs.

Unclasping potentials of genomics and gene editing in chickpea to fight climate change and global hunger threat

Genomics and genome editing promise enormous opportunities for crop improvement and elementary research. Precise modification in the specific targeted location of a genome has profited over the unplanned insertional events which are generally accomplished employing unadventurous means of genetic modifications. The advent of new genome editing procedures viz; zinc finger nucleases (ZFNs), homing endonucleases, transcription activator like effector nucleases (TALENs), Base Editors (BEs), and Primer Editors (PEs) enable molecular scientists to modulate gene expressions or create novel genes with high precision and efficiency. However, all these techniques are exorbitant and tedious since their prerequisites are difficult processes that necessitate protein engineering. Contrary to first generation genome modifying methods, CRISPR/Cas9 is simple to construct, and clones can hypothetically target several locations in the genome with different guide RNAs. Following the model of the application in crop with the help of the CRISPR/Cas9 module, various customized Cas9 cassettes have been cast off to advance mark discrimination and diminish random cuts. The present study discusses the progression in genome editing apparatuses, and their applications in chickpea crop development, scientific limitations, and future perspectives for biofortifying cytokinin dehydrogenase, nitrate reductase, superoxide dismutase to induce drought resistance, heat tolerance and higher yield in chickpea to encounter global climate change, hunger and nutritional threats.

AcanCreER lacks specificity to chondrocytes and targets periosteal progenitors in the fractured callus

Aggrecan (Acan) is a large proteoglycan molecule constituting the extracellular matrix of cartilage, secreted by chondrocytes. To specifically target the chondrocyte lineage, researchers have widely used the AcanCreER mouse model. Evaluation of specificity and efficiency of recombination, requires Cre animals to be crossed with reporter mice. In order to accurately interpret data from Cre models, it is imperative to consider A) the amount of recombination occurring in cells/tissues that are not intended for targeting (i.e., non-specific expression), B) the efficiency of Cre recombination, which can depend on dose and duration of tamoxifen treatment, and C) the activation of CreER without tamoxifen induction, known as "Cre leakage." Using a highly sensitive reporter mouse (Ai9, tdTomato), we performed a comprehensive analysis of the AcanCreER system. Surprisingly, we observed expression in cells within the periosteum. These cells expand at a stage when chondrocytes are not yet present within the forming callus tissue (Acan/Ai9+ cells). In pulse-chase experiments, we confirmed that fibroblastic Acan/Ai9+ cells within the periosteum can directly give rise to osteoblasts. Our results show that Acan/Ai9+ is not specific for the chondrocyte lineage in the fracture callus or with the tibial holes. The expression of AcanCreER in periosteal progenitor cells complicates the interpretation of studies evaluating the transition of chondrocytes to osteoblasts (termed transdifferentiation). Awareness of these issues and the limitations of the system will lead to better data interpretation.

Seamless Gene Correction in the Human Cystic Fibrosis Transmembrane Conductance Regulator Locus by Vector Replacement and Vector Insertion Events

Background: Gene correction via homology directed repair (HDR) in patient-derived induced pluripotent stem (iPS) cells for regenerative medicine are becoming a more realistic approach to develop personalized and mutation-specific therapeutic strategies due to current developments in gene editing and iPSC technology. Cystic fibrosis (CF) is the most common inherited disease in the Caucasian population, caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. Since CF causes significant multi-organ damage and with over 2,000 reported CFTR mutations, CF patients could be one prominent population benefiting from gene and cell therapies. When considering gene-editing techniques for clinical applications, seamless gene corrections of the responsible mutations, restoring native "wildtype" DNA sequence without remnants of drug selectable markers or unwanted DNA sequence changes, would be the most desirable approach. Result: The studies reported here describe the seamless correction of the W1282X CFTR mutation using CRISPR/Cas9 nickases (Cas9n) in iPS cells derived from a CF patient homozygous for the W1282X Class I CFTR mutation. In addition to the expected HDR vector replacement product, we discovered another class of HDR products resulting from vector insertion events that created partial duplications of the CFTR exon 23 region. These vector insertion events were removed via intrachromosomal homologous recombination (IHR) enhanced by double nicking with CRISPR/Cas9n which resulted in the seamless correction of CFTR exon 23 in CF-iPS cells. Conclusion: We show here the removal of the drug resistance cassette and generation of seamless gene corrected cell lines by two independent processes: by treatment with the PiggyBac (PB) transposase in vector replacements or by IHR between the tandemly duplicated CFTR gene sequences.

Cre-mediated, loxP independent sequential recombination of a tripartite transcriptional stop cassette allows for partial read-through transcription

One of the widely used applications of the popular Cre-loxP method for targeted recombination is the permanent activation of marker genes, such as reporter genes or antibiotic resistance genes, by excision of a preceding transcriptional stop signal. The STOP cassette consists of three identical SV40-derived poly(A) signal repeats and is flanked by two loxP sites. We found that in addition to complete loxP-mediated recombination, limiting levels of the Cre recombinase also cause incomplete recombination of the STOP cassette. Partial recombination leads to the loss of only one or two of the three identical poly(A) repeats with recombination breakpoints always precisely matching the end/start of each poly(A) signal repeat without any relevant similarity to the canonical or known cryptic loxP sequences, suggesting that this type of Cre-mediated recombination is loxP-independent. Incomplete deletion of the STOP cassette results in partial read-through transcription, explaining at least some of the variability often observed in marker gene expression from an otherwise identical locus.

Site-specific recombinase genome engineering toolkit in maize

Site-specific recombinase enzymes function in heterologous cellular environments to initiate strand-switching reactions between unique DNA sequences termed recombinase binding sites. Depending on binding site position and orientation, reactions result in integrations, excisions, or inversions of targeted DNA sequences in a precise and predictable manner. Here, we established five different stable recombinase expression lines in maize through Agrobacterium-mediated transformation of T-DNA molecules that contain coding sequences for Cre, R, FLPe, phiC31 Integrase, and phiC31 excisionase. Through the bombardment of recombinase activated DsRed transient expression constructs, we have determined that all five recombinases are functional in maize plants. These recombinase expression lines could be utilized for a variety of genetic engineering applications, including selectable marker removal, targeted transgene integration into predetermined locations, and gene stacking.

Applications of Genome Editing Technology in Research on Chromosome Aneuploidy Disorders

Chromosomal segregation errors in germ cells and early embryonic development underlie aneuploidies, which are numerical chromosomal abnormalities causing fetal absorption, developmental anomalies, and carcinogenesis. It has been considered that human aneuploidy disorders cannot be resolved by radical treatment. However, recent studies have demonstrated that aneuploidies can be rescued to a normal diploid state using genetic engineering in cultured cells. Here, we summarize a series of studies mainly applying genome editing to eliminate an extra copy of human chromosome 21, the cause of the most common constitutional aneuploidy disorder Down syndrome. We also present findings on induced pluripotent stem cell reprogramming, which has been shown to be one of the most promising technologies for converting aneuploidies into normal diploidy without the risk of genetic alterations such as genome editing-mediated off-target effects.

Deliverable transgenics & gene therapy possibilities for the testes

Male infertility and hypogonadism are clinically prevalent conditions with a high socioeconomic burden and are both linked to an increased risk in cardiovascular-metabolic diseases and earlier mortality. Therefore, there is an urgent need to better understand the causes and develop new treatments for these conditions that affect millions of men. The accelerating advancement in gene editing and delivery technologies promises improvements in both diagnosis as well as affording the opportunity to develop bespoke treatment options which would both prove beneficial for the millions of individuals afflicted with these reproductive disorders. In this review, we summarise the systems developed and utilised for the delivery of gene therapy and discuss how each of these systems could be applied for the development of a gene therapy system in the testis and how they could be of use for the future diagnosis and repair of common male reproductive disorders.

Overview of Genetically Engineered Mouse Models of Prostate Cancer and Their Applications in Drug Discovery

Prostate cancer (PCa) is the most common malignant visceral neoplasm in males in Western countries. Despite progress made in the early treatment of localized malignancies, there remains a need for therapies effective against advanced forms of the disease. Genetically engineered mouse (GEM) models are valuable tools for addressing this issue, particularly in defining the cellular and molecular mechanisms responsible for tumor initiation and progression. While cell and tissue culture systems are important models for this purpose as well, they cannot recapitulate the complex interactions within heterotypic cells and the tumor microenvironment that are crucial in the initiation and progression of prostate tumors. Limitations of GEM models include resistance to developing invasive and metastatic tumors that resemble the advanced stages of human PCa. Nonetheless, because genetic models provide valuable information on the human condition that would otherwise be impossible to obtain, they are increasingly employed to identify molecular targets and to examine the efficacy of cancer therapeutics. The aim of this overview is to provide a brief but comprehensive summary of GEM models for PCa, with particular emphasis on the strengths and weaknesses of this experimental approach. © 2018 by John Wiley & Sons, Inc.

Novel reporter and deleter mouse strains generated using VCre/VloxP and SCre/SloxP systems, and their system specificity in mice

DNA site-specific recombination by Cre/loxP is a powerful tool for gene manipulation in experimental animals. VCre/VloxP and SCre/SloxP are novel site-specific recombination systems, consisting of a recombinase and its specific recognition sequences, which function in a manner similar to Cre/loxP. Previous reports using Escherichia coli and Oryzias latipes demonstrated the existence of stringent specificity between each recombinase and its target sites; VCre/VloxP, SCre/SloxP, and Cre/loxP have no cross-reactivity with each other. In this study, we established four novel knock-in (KI) mouse strains in which VloxP-EGFP, SloxP-tdTomato, CAG-VCre, and CAG-SCre genes were inserted into the ROSA26 locus. VloxP-EGFP and SloxP-tdTomato KI mice were reporter mice carrying EGFP or tdTomato genes posterior to the stop codon, which was floxed by VloxP or SloxP fragments, respectively. CAG-VCre and CAG-SCre KI mice carried VCre or SCre genes that were expressed ubiquitously. These two reporter mice were crossed with three different deleter mice, CAG-VCre KI, CAG-SCre KI, and Cre-expressing transgenic mice. Through these matings, we found that VCre/VloxP and SCre/SloxP systems were functional in mice similar to Cre/loxP, and that the recombinases showed tight specificity for their recognition sequences. Our results suggest that these novel recombination systems allow highly sophisticated genome manipulations and will be useful for tracing the fates of multiple cell lineages or elucidating complex spatiotemporal regulations of gene expression.

The TGF-β Family in the Reproductive Tract

The transforming growth factor β (TGF-β) family has a profound impact on the reproductive function of various organisms. In this review, we discuss how highly conserved members of the TGF-β family influence the reproductive function across several species. We briefly discuss how TGF-β-related proteins balance germ-cell proliferation and differentiation as well as dauer entry and exit in Caenorhabditis elegans. In Drosophila melanogaster, TGF-β-related proteins maintain germ stem-cell identity and eggshell patterning. We then provide an in-depth analysis of landmark studies performed using transgenic mouse models and discuss how these data have uncovered basic developmental aspects of male and female reproductive development. In particular, we discuss the roles of the various TGF-β family ligands and receptors in primordial germ-cell development, sexual differentiation, and gonadal cell development. We also discuss how mutant mouse studies showed the contribution of TGF-β family signaling to embryonic and postnatal testis and ovarian development. We conclude the review by describing data obtained from human studies, which highlight the importance of the TGF-β family in normal female reproductive function during pregnancy and in various gynecologic pathologies.

Programmable Genome Editing Tools and their Regulation for Efficient Genome Engineering

Targeted genome editing has become a powerful genetic tool for studying gene function or for modifying genomes by correcting defective genes or introducing genes. A variety of reagents have been developed in recent years that can generate targeted double-stranded DNA cuts which can be repaired by the error-prone, non-homologous end joining repair system or via the homologous recombination-based double-strand break repair pathway provided a suitable template is available. These genome editing reagents require components for recognizing a specific DNA target site and for DNA-cleavage that generates the double-stranded break. In order to reduce potential toxic effects of genome editing reagents, it might be desirable to control the in vitro or in vivo activity of these reagents by incorporating regulatory switches that can reduce off-target activities and/or allow for these reagents to be turned on or off. This review will outline the various genome editing tools that are currently available and describe the strategies that have so far been employed for regulating these editing reagents. In addition, this review will examine potential regulatory switches/strategies that can be employed in the future in order to provide temporal control for these reagents.

Use of the DICE (Dual Integrase Cassette Exchange) System

When constructing transgenic cell lines via plasmid DNA integration, precise targeting to a desired genomic location is advantageous. It is also often advantageous to remove the bacterial backbone, since bacterial elements can lead to the epigenetic silencing of neighboring DNA. The least cumbersome method to remove the plasmid backbone is recombinase-mediated cassette exchange (RMCE). RMCE is accomplished by arranging recombinase sites in the genome and in a donor plasmid such that a recombinase can both integrate the donor plasmid and excise its bacterial backbone. When a single recombinase is used for RMCE, recombination between undesired pairings of the sites can lead to a significant number of unwanted cell lines. To reduce the frequency with which these side products occur, several variants of RMCE that increase desired outcomes have been developed. Nevertheless, an important feature lacking from these improved RMCE methods is that none have fully utilized the recombinases that have been demonstrated to be the most robust and stringent at performing genomic integration in plants and animals, i.e., the phiC31 and Bxb1 phage integrases. To address this need, we have developed an RMCE protocol using these two serine integrases that we call dual integrase cassette exchange (DICE). Our DICE system provides a means to achieve high-precision DNA integration at a desired location and is especially well suited for repeated recombination into the same locus. In this chapter, we provide our most current protocols for using DICE in feeder-free human-induced pluripotent stem cells .

Quantitative Analysis of Supporting Cell Subtype Labeling Among CreER Lines in the Neonatal Mouse Cochlea

Four CreER lines that are commonly used in the auditory field to label cochlear supporting cells (SCs) are expressed in multiple SC subtypes, with some lines also showing reporter expression in hair cells (HCs). We hypothesized that altering the tamoxifen dose would modify CreER expression and target subsets of SCs. We also used two different reporter lines, ROSA26 ^tdTomato and CAG-eGFP, to achieve the same goal. Our results confirm previous reports that Sox2 ^CreERT2 and Fgfr3-iCreER ^T2 are not only expressed in neonatal SCs but also in HCs. Decreasing the tamoxifen dose did not reduce HC expression for Sox2 ^CreERT2 , but changing to the CAG-eGFP reporter decreased reporter-positive HCs sevenfold. However, there was also a significant decrease in the number of reporter-positive SCs. In contrast, there was a large reduction in reporter-positive HCs in Fgfr3-iCreER ^T2 mice with the lowest tamoxifen dose tested yet only limited reduction in SC labeling. The targeting of reporter expression to inner phalangeal and border cells was increased when Plp-CreER ^T2 was paired with the CAG-eGFP reporter; however, the total number of labeled cells decreased. Changes to the tamoxifen dose or reporter line with Prox1 ^CreERT2 caused minimal changes. Our data demonstrate that modifications to the tamoxifen dose or the use of different reporter lines may be successful in narrowing the numbers and/or types of cells labeled, but each CreER line responded differently. When the ROSA26 ^tdTomato reporter was combined with any of the four CreER lines, there was no difference in the number of tdTomato-positive cells after one or two injections of tamoxifen given at birth. Thus, tamoxifen-mediated toxicity could be reduced by only giving one injection. While the CAG-eGFP reporter consistently labeled fewer cells, both reporter lines are valuable depending on the goal of the study.

Genome Editing Techniques and Their Therapeutic Applications

Fueled by advances in the field of genetics, the methods available to edit DNA sequences in living cells have continued to develop steadily. These technologies directly impact the fields of gene and cell therapy, where changes in the DNA sequence of target cells offer a route to correct genetic diseases and manipulate disorders like cancer. We review here the expanding menu of genome editing techniques and how they are being applied to therapeutic targets. The methods encompass a myriad of approaches to modify the covalent structure of DNA, including the targeted creation of double-strand breaks that can catalyze genomic changes, as well as the use of retroviruses and transposons to mediate gene addition, recombinases for sequence-specific gene addition and deletion, and base repair for direct sequence changes. The continued growth of the exciting field of genome editing is opening new possibilities for therapeutic intervention.

λ Recombination and Recombineering

The bacteriophage λ Red homologous recombination system has been studied over the past 50 years as a model system to define the mechanistic details of how organisms exchange DNA segments that share extended regions of homology. The λ Red system proved useful as a system to study because recombinants could be easily generated by co-infection of genetically marked phages. What emerged from these studies was the recognition that replication of phage DNA was required for substantial Red-promoted recombination in vivo, and the critical role that double-stranded DNA ends play in allowing the Red proteins access to the phage DNA chromosomes. In the past 16 years, however, the λ Red recombination system has gained a new notoriety. When expressed independently of other λ functions, the Red system is able to promote recombination of linear DNA containing limited regions of homology (∼50 bp) with the Escherichia coli chromosome, a process known as recombineering. This review explains how the Red system works during a phage infection, and how it is utilized to make chromosomal modifications of E. coli with such efficiency that it changed the nature and number of genetic manipulations possible, leading to advances in bacterial genomics, metabolic engineering, and eukaryotic genetics.

Development and Characterization of Cell-Specific Androgen Receptor Knockout Mice

Conditional gene targeting has revolutionized molecular genetic analysis of nuclear receptor proteins, however development and analysis of such conditional knockouts is far from simple, with many caveats and pitfalls waiting to snare the novice or unprepared. In this chapter, we describe our experience of generating and analyzing mouse models with conditional ablation of the androgen receptor (AR) from tissues of the reproductive system and other organs. The guidance, suggestions, and protocols outlined in the chapter provide the key starting point for analyses of conditional-ARKO mice, completing them as described provides an excellent framework for further focussed project-specific analyses, and applies equally well to analysis of reproductive tissues from any mouse model generated through conditional gene targeting.

Drug discovery in prostate cancer mouse models

INTRODUCTION

The mouse is an important, though imperfect, organism with which to model human disease and to discover and test novel drugs in a preclinical setting. Many experimental strategies have been used to discover new biological and molecular targets in the mouse, with the hopes of translating these discoveries into novel drugs to treat prostate cancer in humans. Modeling prostate cancer in the mouse, however, has been challenging, and often drugs that work in mice have failed in human trials.

AREAS COVERED

The authors discuss the similarities and differences between mice and men; the types of mouse models that exist to model prostate cancer; practical questions one must ask when using a mouse as a model; and potential reasons that drugs do not often translate to humans. They also discuss the current value in using mouse models for drug discovery to treat prostate cancer and what needs are still unmet in field.

EXPERT OPINION

With proper planning and following practical guidelines by the researcher, the mouse is a powerful experimental tool. The field lacks genetically engineered metastatic models, and xenograft models do not allow for the study of the immune system during the metastatic process. There remain several important limitations to discovering and testing novel drugs in mice for eventual human use, but these can often be overcome. Overall, mouse modeling is an essential part of prostate cancer research and drug discovery. Emerging technologies and better and ever-increasing forms of communication are moving the field in a hopeful direction.

Genetic lineage tracing analysis of anterior Hox expressing cells

Cell lineage studies have been widely used in developmental biology to establish which cells, and how many cells, in the early embryo will give rise to a specific structure and its derivatives. Several methods have been developed to label progenitor cells in the early embryo. Here, we describe the genetic tracing approach that relies on the use of the recombinase to genetically and permanently label progenitor cells as well as their progeny through specific activation of a conditional reporter gene, the ROSA26 reporter mouse. Labeling of progenitor cells is spatially controlled by the use of a tissue-specific promoter driving Cre, the Hoxb1 (IRES-Cre/+)  and the Hoxa1-enhIII-cre. ROSA26R mice and Hoxb1 (IRES-Cre/+) or Hoxa1-enhIII-cremice are crossed together to generate embryos at different stages of development. Embryos are collected and dissected at a specific stage of development and fixed in paraformaldehyde. To follow Hoxb1 (+) and Hoxa1 (+) progeny, X-gal staining is performed to detect β-galactosidase activity in embryos or developing organ such as the heart. Finally, X-gal-positive cells are observed on whole-mount embryos or dissected organ to determine the lineage contribution of Hoxa1 and Hoxb1 during development.

Genetically engineered mouse models to study prostate cancer

Genetically engineered mouse models have become fundamental tools in the basic and translational research of prostate cancer. There is a plethora of models available to dissect the genetic alterations and aberrant signaling events associated with human prostate cancer and, furthermore, to investigate new and "personalized" therapies to treat the disease. In this chapter, we discuss some of the models recently and currently used to study prostate cancer in vivo, and some considerations when selecting an appropriate model to investigate particular aspects of the disease. We describe the methods required to isolate prostate tumors and conduct basic characterization of the tumor to determine tumor load and histopathology. We also discuss important aspects to be considered when processing samples for further analysis.

Recombinase-mediated reprogramming and dystrophin gene addition in mdx mouse induced pluripotent stem cells

A cell therapy strategy utilizing genetically-corrected induced pluripotent stem cells (iPSC) may be an attractive approach for genetic disorders such as muscular dystrophies. Methods for genetic engineering of iPSC that emphasize precision and minimize random integration would be beneficial. We demonstrate here an approach in the mdx mouse model of Duchenne muscular dystrophy that focuses on the use of site-specific recombinases to achieve genetic engineering. We employed non-viral, plasmid-mediated methods to reprogram mdx fibroblasts, using phiC31 integrase to insert a single copy of the reprogramming genes at a safe location in the genome. We next used Bxb1 integrase to add the therapeutic full-length dystrophin cDNA to the iPSC in a site-specific manner. Unwanted DNA sequences, including the reprogramming genes, were then precisely deleted with Cre resolvase. Pluripotency of the iPSC was analyzed before and after gene addition, and ability of the genetically corrected iPSC to differentiate into myogenic precursors was evaluated by morphology, immunohistochemistry, qRT-PCR, FACS analysis, and intramuscular engraftment. These data demonstrate a non-viral, reprogramming-plus-gene addition genetic engineering strategy utilizing site-specific recombinases that can be applied easily to mouse cells. This work introduces a significant level of precision in the genetic engineering of iPSC that can be built upon in future studies.

Heritable and inducible gene knockdown in astrocytes or neurons in vivo by a combined lentiviral and RNAi approach

Gene knockout by homologous recombination is a popular method to study gene functions in the mouse in vivo. However, its lack of temporal control has limited the interpretation of knockout studies because the complete elimination of a gene product often alters developmental processes, and can induce severe malformations or lethality. Conditional gene knockdown has emerged as a compelling alternative to gene knockout, an approach well-established in vitro but that remains challenging in vivo, especially in the adult brain. Here, we report a method for conditional and cell-specific gene knockdown in the mouse brain in vivo that combines Cre-mediated RNA interference (RNAi) with classical and lentivirus-mediated transgenesis. The method is based on the inducible expression of a silencing short hairpin RNA (shRNA) introduced in mice by lentivirus-mediated transgenesis, and on its activation by excision of a floxed stop EGFP reporter with an inducible Cre recombinase expressed in astrocytes or in neurons. This dual system should be of broad utility for comparative studies of gene functions in these two cell types in vivo.

Oct4 is required for lineage priming in the developing inner cell mass of the mouse blastocyst

The transcription factor Oct4 is required in vitro for establishment and maintenance of embryonic stem cells and for reprogramming somatic cells to pluripotency. In vivo, it prevents the ectopic differentiation of early embryos into trophoblast. Here, we further explore the role of Oct4 in blastocyst formation and specification of epiblast versus primitive endoderm lineages using conditional genetic deletion. Experiments involving mouse embryos deficient for both maternal and zygotic Oct4 suggest that it is dispensable for zygote formation, early cleavage and activation of Nanog expression. Nanog protein is significantly elevated in the presumptive inner cell mass of Oct4 null embryos, suggesting an unexpected role for Oct4 in attenuating the level of Nanog, which might be significant for priming differentiation during epiblast maturation. Induced deletion of Oct4 during the morula to blastocyst transition disrupts the ability of inner cell mass cells to adopt lineage-specific identity and acquire the molecular profile characteristic of either epiblast or primitive endoderm. Sox17, a marker of primitive endoderm, is not detected following prolonged culture of such embryos, but can be rescued by provision of exogenous FGF4. Interestingly, functional primitive endoderm can be rescued in Oct4-deficient embryos in embryonic stem cell complementation assays, but only if the host embryos are at the pre-blastocyst stage. We conclude that cell fate decisions within the inner cell mass are dependent upon Oct4 and that Oct4 is not cell-autonomously required for the differentiation of primitive endoderm derivatives, as long as an appropriate developmental environment is established.

Renal collecting duct NOS1 maintains fluid-electrolyte homeostasis and blood pressure

Nitric oxide is a pronatriuretic and prodiuretic factor. The highest renal NO synthase (NOS) activity is found in the inner medullary collecting duct. The collecting duct (CD) is the site of daily fine-tune regulation of sodium balance, and led us to hypothesize that a CD-specific deletion of NOS1 would result in an impaired ability to excrete a sodium load leading to a salt-sensitive blood pressure phenotype. We bred AQP2-CRE mice with NOS1 floxed mice to produce flox control and CD-specific NOS1 knockout (CDNOS1KO) littermates. CDs from CDNOS1KO mice produced 75% less nitrite, and urinary nitrite+nitrate (NOx) excretion was significantly blunted in the knockout genotype. When challenged with high dietary sodium, CDNOS1KO mice showed significantly reduced urine output, sodium, chloride, and NOx excretion, and increased mean arterial pressure relative to flox control mice. In humans, urinary NOx is a newly identified biomarker for the progression of hypertension. These findings reveal that NOS1 in the CD is critical in the regulation of fluid-electrolyte balance, and this new genetic model of CD NOS1 gene deletion will be a valuable tool to study salt-dependent blood pressure mechanisms.

Influence of loxP insertion upstream of the cis-acting packaging domain on adenovirus packaging efficiency

First-generation AdV enables efficient gene transduction, although its immunogenicity is an important problem in vivo. Helper-dependent AdV (HD-AdV) is one possible solution to this problem. The construction of HD-AdV requires a helper virus, in which the viral packaging domain is flanked by two inserted loxP to hamper its packaging in Cre-expressing 293 cells. Here, we constructed 19L viruses containing loxP at 191 nt from the left end of the genome upstream of the packaging domain, 15L viruses bearing loxP at 143 nt, and a control ΔL virus lacking loxP at these positions. The 19L position is used worldwide, and the 15L position has been reported to result in a lower titer than that of 19L. When the titers were compared for six pairs of 19L and 15L AdV, the 19L AdV produced titers similar to, or sometimes lower than, the 15L and ΔL AdV, unlike the results of previous reports. We next chose one pair of 15L and 19L AdV that produced titers similar to that of ΔL and a competitor AdV lacking loxP for use in a competition assay. When a small amount of the competitor AdV was co-infected, both the 15L and the 19L AdV, but not ΔL, gradually became minority components during subsequent viral passages. Therefore, the loxP insertions at 143 nt and 191 nt decreased the viral packaging efficiency.

Phage recombinases and their applications

The homologous recombination systems of linear double-stranded (ds)DNA bacteriophages are required for the generation of genetic diversity, the repair of dsDNA breaks, and the formation of concatemeric chromosomes, the immediate precursor to packaging. These systems have been studied for decades as a means to understand the basic principles of homologous recombination. From the beginning, it was recognized that these recombinases are linked intimately to the mechanisms of phage DNA replication. In the last decade, however, investigators have exploited these recombination systems as tools for genetic engineering of bacterial chromosomes, bacterial artificial chromosomes, and plasmids. This recombinational engineering technology has been termed "recombineering" and offers a new paradigm for the genetic manipulation of bacterial chromosomes, which is far more efficient than the classical use of nonreplicating integration vectors for gene replacement. The phage λ Red recombination system, in particular, has been used to construct gene replacements, deletions, insertions, inversions, duplications, and single base pair changes in the Escherichia coli chromosome. This chapter discusses the components of the recombination systems of λ, rac prophage, and phage P22 and properties of single-stranded DNA annealing proteins from these and other phage that have been instrumental for the development of this technology. The types of genetic manipulations that can be made are described, along with proposed mechanisms for both double-stranded DNA- and oligonucleotide-mediated recombineering events. Finally, the impact of this technology to such diverse fields as bacterial pathogenesis, metabolic engineering, and mouse genomics is discussed.

Neuron-specific deletion of peroxisome proliferator-activated receptor delta (PPARδ) in mice leads to increased susceptibility to diet-induced obesity

Central nervous system (CNS) lipid accumulation, inflammation and resistance to adipo-regulatory hormones, such as insulin and leptin, are implicated in the pathogenesis of diet-induced obesity (DIO). Peroxisome proliferator-activated receptors (PPAR α, δ, γ) are nuclear transcription factors that act as environmental fatty acid sensors and regulate genes involved in lipid metabolism and inflammation in response to dietary and endogenous fatty acid ligands. All three PPAR isoforms are expressed in the CNS at different levels. Recent evidence suggests that activation of CNS PPARα and/or PPARγ may contribute to weight gain and obesity. PPARδ is the most abundant isoform in the CNS and is enriched in the hypothalamus, a region of the brain involved in energy homeostasis regulation. Because in peripheral tissues, expression of PPARδ increases lipid oxidative genes and opposes inflammation, we hypothesized that CNS PPARδ protects against the development of DIO. Indeed, genetic neuronal deletion using Nes-Cre loxP technology led to elevated fat mass and decreased lean mass on low-fat diet (LFD), accompanied by leptin resistance and hypothalamic inflammation. Impaired regulation of neuropeptide expression, as well as uncoupling protein 2, and abnormal responses to a metabolic challenge, such as fasting, also occur in the absence of neuronal PPARδ. Consistent with our hypothesis, KO mice gain significantly more fat mass on a high-fat diet (HFD), yet are surprisingly resistant to diet-induced elevations in CNS inflammation and lipid accumulation. We detected evidence of upregulation of PPARγ and target genes of both PPARα and PPARγ, as well as genes of fatty acid oxidation. Thus, our data reveal a previously underappreciated role for neuronal PPARδ in the regulation of body composition, feeding responses, and in the regulation of hypothalamic gene expression.

Partial promoter substitutions generating transcriptional sentinels of diverse signaling pathways in embryonic stem cells and mice

Extracellular signals in development, physiology, homeostasis and disease often act by regulating transcription. Herein we describe a general method and specific resources for determining where and when such signaling occurs in live animals and for systematically comparing the timing and extent of different signals in different cellular contexts. We used recombinase-mediated cassette exchange (RMCE) to test the effect of successively deleting conserved genomic regions of the ubiquitously active Rosa26 promoter and substituting the deleted regions for regulatory sequences that respond to diverse extracellular signals. We thereby created an allelic series of embryonic stem cells and mice, each containing a signal-responsive sentinel with different fluorescent reporters that respond with sensitivity and specificity to retinoic acids, bone morphogenic proteins, activin A, Wnts or Notch, and that can be adapted to any pathway that acts via DNA elements.

Utility of genetically modified mice for understanding the neurobiology of substance use disorders

Advances in our ability to modify the mouse genome have enhanced our understanding of the genetic and neurobiological mechanisms contributing to addiction-related behaviors underlying substance use and abuse. These experimentally induced manipulations permit greater spatial and temporal specificity for modification of gene expression within specific cellular populations and during select developmental time periods. In this review, we consider the current mouse genetic model systems that have been employed to understand aspects of addiction and highlight significant conceptual advances achieved related to substance use and abuse. The mouse models reviewed herein include conventional knock-out and knock-in, conditional knockout, transgenic, inducible transgenic, mice suitable for optogenetic control of discrete neuronal populations, and phenotype-selected mice. By establishing a reciprocal investigatory relationship between genetic findings in humans and genomic manipulations in mice, a far better understanding of the discrete neuromechanisms underlying addiction can be achieved, which is likely to provide a strong foundation for developing and validating novel therapeutics for the treatment of substance abuse disorders.

Conditional gene expression in the mouse inner ear using Cre-loxP

In recent years, there has been significant progress in the use of Cre-loxP technology for conditional gene expression in the inner ear. Here, we introduce the basic concepts of this powerful technology, emphasizing the differences between Cre and CreER. We describe the creation and Cre expression pattern of each Cre and CreER mouse line that has been reported to have expression in auditory and vestibular organs. We compare the Cre expression patterns between Atoh1-CreER(TM) and Atoh1-CreER(T2) and report a new line, Fgfr3-iCreER(T2), which displays inducible Cre activity in cochlear supporting cells. We also explain how results can vary when transgenic vs. knock-in Cre/CreER alleles are used to alter gene expression. We discuss practical issues that arise when using the Cre-loxP system, such as the use of proper controls, Cre efficiency, reporter expression efficiency, and Cre leakiness. Finally, we introduce other methods for conditional gene expression, including Flp recombinase and the tetracycline-inducible system, which can be combined with Cre-loxP mouse models to investigate conditional expression of more than one gene.

A resource of vectors and ES cells for targeted deletion of microRNAs in mice

The 21-23 nucleotide, single-stranded RNAs classified as microRNAs (miRNA) perform fundamental roles in diverse cellular and developmental processes. In contrast to the situation for protein-coding genes, no public resource of miRNA mouse mutant alleles exists. Here we describe a collection of 428 miRNA targeting vectors covering 476 of the miRNA genes annotated in the miRBase registry. Using these vectors, we generated a library of highly germline-transmissible C57BL/6N mouse embryonic stem (ES) cell clones harboring targeted deletions for 392 miRNA genes. For most of these targeted clones, chimerism and germline transmission can be scored through a coat color marker. The targeted alleles have been designed to be adaptable research tools that can be efficiently altered by recombinase-mediated cassette exchange to create reporter, conditional and other allelic variants. This miRNA knockout (mirKO) resource can be searched electronically and is available from ES cell repositories for distribution to the scientific community.

Controlled somatic and germline copy number variation in the mouse model

Changes in the number of chromosomes, but also variations in the copy number of chromosomal regions have been described in various pathological conditions, such as cancer and aneuploidy, but also in normal physiological condition. Our classical view of DNA replication and mitotic preservation of the chromosomal integrity is now challenged as new technologies allow us to observe such mosaic somatic changes in copy number affecting regions of chromosomes with various sizes. In order to go further in the understanding of copy number influence in normal condition we could take advantage of the novel strategy called Targeted Asymmetric Sister Chromatin Event of Recombination (TASCER) to induce recombination during the G2 phase so that we can generate deletions and duplications of regions of interest prior to mitosis. Using this approach in the mouse we could address the effects of copy number variation and segmental aneuploidy in daughter cells and allow us to explore somatic mosaics for large region of interest in the mouse.

Quantification of factors influencing fluorescent protein expression using RMCE to generate an allelic series in the ROSA26 locus in mice

Fluorescent proteins (FPs) have great utility in identifying specific cell populations and in studying cellular dynamics in the mouse. To quantify the factors that determine both the expression and relative brightness of FPs in mouse embryonic stem cells (mESCs) and in mice, we generated eight different FP-expressing ROSA26 alleles using recombinase-mediated cassette exchange (RMCE). These alleles enabled us to analyze the effects on FP expression of a translational enhancer and different 3'-intronic and/or polyadenylation sequences, as well as the relative brightness of five different FPs, without the confounding position and copy number effects that are typically associated with randomly inserted transgenes. We found that the expression of a given FP can vary threefold or more depending on the genetic features present in the allele. The optimal FP expression cassette contained both a translational enhancer sequence in the 5'-untranslated region (UTR) and an intron-containing rabbit β-globin sequence within the 3'-UTR. The relative expressed brightness of individual FPs varied up to tenfold. Of the five different monomeric FPs tested, Citrine (YFP) was the brightest, followed by Apple, eGFP, Cerulean (CFP) and Cherry. Generation of a line of Cherry-expressing mice showed that there was a 30-fold variation of Cherry expression among different tissues and that there was a punctate expression pattern within cells of all tissues examined. This study should help investigators make better-informed design choices when expressing FPs in mESCs and mice.

MicroRNA therapeutics

MicroRNAs (miRNAs) provide new therapeutic targets for many diseases, while their myriad roles in development and cellular processes make them fascinating to study. We still do not fully understand the molecular mechanisms by which miRNAs regulate gene expression nor do we know the complete repertoire of mRNAs each miRNA regulates. However, recent progress in the development of effective strategies to block miRNAs suggests that anti-miRNA drugs may soon be used in the clinic.

AAV-mediated gene targeting methods for human cells

Gene targeting with adeno-associated virus (AAV) vectors has been demonstrated in multiple human cell types, with targeting frequencies ranging from 10(-5) to 10(-2) per infected cell. These targeting frequencies are 1-4 logs higher than those obtained by conventional transfection or electroporation approaches. A wide variety of different types of mutations can be introduced into chromosomal loci with high fidelity and without genotoxicity. Here we provide a detailed protocol for gene targeting in human cells with AAV vectors. We describe methods for vector design, stock preparation and titration. Optimized transduction protocols are provided for human pluripotent stem cells, mesenchymal stem cells, fibroblasts and transformed cell lines, as well as a method for identifying targeted clones by Southern blots. This protocol (from vector design through a single round of targeting and screening) can be completed in ∼10 weeks; each subsequent round of targeting and screening should take an additional 7 weeks.

Beyond knockout rats: new insights into finer genome manipulation in rats

The ability to "knockout" specific genes in mice via embryonic stem (ES) cell-based gene-targeting technology has significantly enriched our understanding of gene function in normal and disease phenotypes. Improvements on this original strategy have been developed to enable the manipulation of genomes in a more sophisticated fashion with unprecedented precision. The rat is the model of choice in many areas of scientific investigation despite the lack of rat genetic toolboxes. Most Recent advances of zinc finger nucleases (ZFNs) and rat ES cells are diminishing the gap between rat and mouse with respect to reverse genetic approaches. Importantly, the establishment of rat ES cell-based gene targeting technology, in combination with the unique advantages of using rats, provides new, exciting opportunities to create animal models that mimic human diseases more faithfully. We hereby report our recent results concerning finer genetic modifications in the rat, and propose their potential applications in addressing biological questions.

Characterization and Validation of Cre-Driver Mouse Lines

Conditional gene manipulations in mice are increasingly popular strategies in biomedical research. These approaches rely on the production of conditional genetically engineered mutant mouse (GEMM) lines with mutations in protein-encoding genes. These conditional GEMMs are then bred with one or several transgenic mouse lines expressing a site-specific recombinase, most often the Cre recombinase, in a tissue-specific manner. Conditional GEMMs can only be exploited if Cre transgenic mouse lines are available to generate somatic mutations, and thus the number of Cre transgenic lines has significantly increased over the last 15 years. Once produced, these transgenic lines must be validated for reliable, efficient, and specific Cre expression and Cre-mediated recombination. In this overview, the minimum level of information that is ideally required to validate a Cre-driver transgenic line is first discussed. The vagaries associated with validation procedures are considered next, and some solutions are proposed to assess the expression and activity of constitutive or inducible Cre recombinase before undertaking extensive breeding experiments and exhaustive phenotyping. Curr. Protoc. Mouse Biol. 1:1-15. © 2011 by John Wiley & Sons, Inc.

A method for producing transgenic cells using a multi-integrase system on a human artificial chromosome vector

The production of cells capable of expressing gene(s) of interest is important for a variety of applications in biomedicine and biotechnology, including gene therapy and animal transgenesis. The ability to insert transgenes at a precise location in the genome, using site-specific recombinases such as Cre, FLP, and ΦC31, has major benefits for the efficiency of transgenesis. Recent work on integrases from ΦC31, R4, TP901-1 and Bxb1 phages demonstrated that these recombinases catalyze site-specific recombination in mammalian cells. In the present study, we examined the activities of integrases on site-specific recombination and gene expression in mammalian cells. We designed a human artificial chromosome (HAC) vector containing five recombination sites (ΦC31 attP, R4 attP, TP901-1 attP, Bxb1 attP and FRT; multi-integrase HAC vector) and de novo mammalian codon-optimized integrases. The multi-integrase HAC vector has several functions, including gene integration in a precise locus and avoiding genomic position effects; therefore, it was used as a platform to investigate integrase activities. Integrases carried out site-specific recombination at frequencies ranging from 39.3-96.8%. Additionally, we observed homogenous gene expression in 77.3-87.5% of colonies obtained using the multi-integrase HAC vector. This vector is also transferable to another cell line, and is capable of accepting genes of interest in this environment. These data suggest that integrases have high DNA recombination efficiencies in mammalian cells. The multi-integrase HAC vector enables us to produce transgene-expressing cells efficiently and create platform cell lines for gene expression.

Mouse models of prostate cancer

The development and optimization of high-throughput screening methods has identified a multitude of genetic changes associated with human disease. The use of immunodeficient and genetically engineered mouse models that mimic the human disease has been crucial in validating the importance of these genetic pathways in prostate cancer. These models provide a platform for finding novel therapies to treat human patients afflicted with prostate cancer as well as those who have debilitating bone metastases. In this paper, we focus on the historical development and phenotypic descriptions of mouse models used to study prostate cancer. We also comment on how closely each model recapitulates human prostate cancer.

Good planning and serendipity: exploiting the Cre/Lox system in the testis

Over the past 20 years, genetic manipulation has revolutionised our understanding of male reproductive development and function. The advent of transgenic mouse lines has permitted elegant dissection of previously intractable issues. The development of the Cre/Lox system, which has permitted spatial and temporal localisation of genetic manipulation, has expanded upon this, and now makes up one of the primary approaches underpinning our increasing understanding of testis development and function. The success of conditional gene targeting is largely reliant upon the choice of Cre recombinase expressing mouse line, which is required to specifically target the correct cell type at the correct time. Presupposition that Cre lines will behave as expected has been one of the main oversights in the design of Cre/Lox experiments, as in practice, many Cre lines are prone to ectopic expression (both temporal and spatial), transgene silencing or genetic background effects. Empirical validation of the spatiotemporal profile of Cre expression prior to undertaking conditional gene targeting studies is essential and can be achieved through a combination of molecular and immunohistochemical approaches, along with in vivo examination of reporter gene expression in targeted tissues. This paper details the key considerations associated with exploitation of the Cre/Lox system and highlights a variety of validated Cre lines that have utility for conditional gene targeting within the testis.

Postnatal male germ-cell expression of cre recombinase in Tex101-iCre transgenic mice

We have generated a transgenic mouse line that expresses improved Cre recombinase (iCre) under the control of the testis-expressed gene 101 (Tex101) promoter. This transgenic mouse line was named Tex101-iCre. Using the floxed ROSA reporter mice, we found that robust Cre recombinase activity was detected in postnatal testes with weak or no activity in other tissues. Within the testis, Cre recombinase was active in spermatogenic cells as early as the prospermatogonia stage at day 1 after birth. In 30- and 60-day-old mice, positive Cre recombinase activity was detected not only in prospermatogonia but also in spermatogenic cells at later stages of spermatogenesis. There was little or no Cre activity in interstitial cells. Breeding wild-type females with homozygous floxed fibroblast growth factor receptor 2 (Fgfr2) males carrying the Tex101-iCre transgene did not produce any progeny with the floxed Fgfr2 allele. All the progeny inherited a recombined Fgfr2 allele, indicating that complete deletion of the floxed Fgfr2 allele by Tex101-iCre can be achieved in the male germline. Furthermore, FGFR2 protein was not detected in spermatocytes and spermatids of adult Fgfr2(fl/fl) ;Tex101-iCre mice. Taken together, our results suggest that the Tex101-iCre mouse line allows the inactivation of a floxed gene in spermatogenic cells in adult mice, which will facilitate the functional characterization of genes in normal spermatogenesis and male fertility.

Tamoxifen-independent recombination in the RIP-CreER mouse

BACKGROUND

The inducible Cre-lox system is a valuable tool to study gene function in a spatial and time restricted fashion in mouse models. This strategy relies on the limited background activity of the modified Cre recombinase (CreER) in the absence of its inducer, the competitive estrogen receptor ligand, tamoxifen. The RIP-CreER mouse (Tg (Ins2-cre/Esr1) 1Dam) is among the few available β-cell specific CreER mouse lines and thus it has been often used to manipulate gene expression in the insulin-producing cells of the endocrine pancreas.

PRINCIPAL FINDINGS

Here, we report the detection of tamoxifen-independent Cre activity as early as 2 months of age in RIP-CreER mice crossed with three distinct reporter strains.

SIGNIFICANCE

Evidence of Cre-mediated recombination of floxed alleles even in the absence of tamoxifen administration should warrant cautious use of this mouse for the study of pancreatic β-cells.

Nestin-Cre mice are affected by hypopituitarism, which is not due to significant activity of the transgene in the pituitary gland

Nestin-Cre mice express Cre recombinase under control of the rat nestin promoter and central nervous system (CNS) enhancer. While endogenous Nestin is expressed in some other tissues including the pituitary gland, Nestin-Cre mice induce recombination predominantly in the CNS. For this reason, they have been widely used to explore gene function or cell fate in the latter. Pituitary hormonal deficiencies, or hypopituitarism, are associated with a wide range of symptoms and with a significant morbidity. These can have a neural and/or a pituitary origin as the gland's secretions are controlled by the hypothalamus. We report here that Nestin-Cre mice themselves are affected by mild hypopituitarism. Hence, physiological consequences are expected, especially in combination with defects resulting from Cre mediated deletion of any gene under investigation. To further investigate the origin of this phenotype, we re-examined the activity of the transgene. We compared it with expression of Nestin itself in the context of the hypothalamo-pituitary axis, especially in the light of a recent report showing pituitary Nestin-Cre activity, which contrasts with previous data. Our results disagree with those of this recent study and do not support the claim that Nestin positive cells are present in the pituitary anlagen, the Rathke's pouch (RP). Moreover we did not observe any significant activity in the post-natal pituitary, in agreement with the initial report.

Engineering the mouse genome to model human disease for drug discovery

Genetically engineered mice (GEM) have become invaluable tools for human disease modeling and drug development. Completion of the mouse genome sequence in combination with transgenesis and gene targeting in embryonal stem cells have opened up unprecedented opportunities. Advanced technologies for derivation of GEM models will be introduced and discussed.

TAT-phiC31 integrase mediates DNA recombination in mammalian cells

Streptomyces phage integrase phiC31 is capable of mediating site-specific insertions in mammalian genomes. To avoid potential toxicity of long-term expression of phiC31 in host cells, we developed a method employing a cell-permeable TAT-phiC31 integrase. His6-tagged phiC31 proteins with or without an HIV TAT intercellular transducing peptide were generated and purified. Both of them retained integrase activity in vitro. However, TAT-phiC31 but not phiC31 was able to mediate a specific integration between two att sites in the genome of 293-PB [EGFP] report cell line. Transduced TAT-phiC31 was mainly localized in the cytoplasm that is similar to the localization of phiC31 when expressed through cDNA transfection. Adding a nuclear localization signal (NLS) peptide to the C-terminus of TAT-phiC31 facilitated nuclear localization of the integrase with an increased efficiency of recombination in the reporter cell line. These results demonstrated that TAT can mediate a cell membrane entry of phiC31 protein to perform a site-specific integration in mammalian cells. This is a simple and possibly safer method of site-specific recombination for gene delivery.