Variegation associated with lacZ in transgenic animals: a warning note

Delivering on the promise of gene editing for cystic fibrosis

In this review, we describe a path for translation of gene editing into therapy for cystic fibrosis (CF). Cystic fibrosis results from mutations in the CFTR gene, with one allele predominant in patient populations. This simple, genetic etiology makes gene editing appealing for treatment of this disease. There already have been success in applying this approach to cystic fibrosis in cell and animal models, although these advances have been modest in comparison to advances for other disease.

Less than six years after its first demonstration in animals, CRISPR/Cas gene editing is in early clinical trials for several disorders. Most clinical trials, thus far, attempt to edit genes in cells of the blood lineages. The advantage of the blood is that the stem cells are known, can be isolated, edited, selected, expanded, and returned to the body. The likely next trials will be in the liver, which is accessible to many delivery methods. For cystic fibrosis, the biggest hurdle is to deliver editors to other, less accessible organs. We outline a path by which delivery can be improved.

The translation of new therapies doesn't occur in isolation, and the development of gene editors is occurring as advances in gene therapy and small molecule therapeutics are being made. The advances made in gene therapy may help develop delivery vehicles for gene editing, although major improvements are needed. Conversely, the approval of effective small molecule therapies for many patients with cystic fibrosis will raise the bar for translation of gene editing.

Ift172 conditional knock-out mice exhibit rapid retinal degeneration and protein trafficking defects

Intraflagellar transport (IFT) is a bidirectional transport process that occurs along primary cilia and specialized sensory cilia, such as photoreceptor outersegments. Genes coding for various IFT components are associated with ciliopathies. Mutations in IFT172 lead to diseases ranging from isolated retinal degeneration to severe syndromic ciliopathies. In this study, we created a mouse model of IFT172-associated retinal degeneration to investigate the ocular disease mechanism. We found that depletion of IFT172 in rod photoreceptors leads to a rapid degeneration of the retina, with severely reduced electroretinography (ERG) responses by 1 month and complete outer-nuclear layer (ONL) degeneration by 2 months. We investigated molecular mechanisms of degeneration and show that IFT172 protein reduction leads to mislocalization of specific photoreceptor outersegment (OS) proteins (RHO, RP1, IFT139), aberrant light-driven translocation of alpha transducin and altered localization of glioma-associated oncogene family member 1 (GLI1). This mouse model exhibits key features of the retinal phenotype observed in patients with IFT172-associated blindness and can be used for in vivo testing of ciliopathy therapies.

Transcriptome Analysis of Targeted Mouse Mutations Reveals the Topography of Local Changes in Gene Expression

The unintended consequences of gene targeting in mouse models have not been thoroughly studied and a more systematic analysis is needed to understand the frequency and characteristics of off-target effects. Using RNA-seq, we evaluated targeted and neighboring gene expression in tissues from 44 homozygous mutants compared with C57BL/6N control mice. Two allele types were evaluated: 15 targeted trap mutations (TRAP); and 29 deletion alleles (DEL), usually a deletion between the translational start and the 3' UTR. Both targeting strategies insert a bacterial beta-galactosidase reporter (LacZ) and a neomycin resistance selection cassette. Evaluating transcription of genes in +/- 500 kb of flanking DNA around the targeted gene, we found up-regulated genes more frequently around DEL compared with TRAP alleles, however the frequency of alleles with local down-regulated genes flanking DEL and TRAP targets was similar. Down-regulated genes around both DEL and TRAP targets were found at a higher frequency than expected from a genome-wide survey. However, only around DEL targets were up-regulated genes found with a significantly higher frequency compared with genome-wide sampling. Transcriptome analysis confirms targeting in 97% of DEL alleles, but in only 47% of TRAP alleles probably due to non-functional splice variants, and some splicing around the gene trap. Local effects on gene expression are likely due to a number of factors including compensatory regulation, loss or disruption of intragenic regulatory elements, the exogenous promoter in the neo selection cassette, removal of insulating DNA in the DEL mutants, and local silencing due to disruption of normal chromatin organization or presence of exogenous DNA. An understanding of local position effects is important for understanding and interpreting any phenotype attributed to targeted gene mutations, or to spontaneous indels.

De novo generation of adipocytes from circulating progenitor cells in mouse and human adipose tissue

White adipocytes in adults are typically derived from tissue resident mesenchymal progenitors. The recent identification of de novo production of adipocytes from bone marrow progenitor-derived cells in mice challenges this paradigm and indicates an alternative lineage specification that adipocytes exist. We hypothesized that alternative lineage specification of white adipocytes is also present in human adipose tissue. Bone marrow from transgenic mice in which luciferase expression is governed by the adipocyte-restricted adiponectin gene promoter was adoptively transferred to wild-type recipient mice. Light emission was quantitated in recipients by in vivo imaging and direct enzyme assay. Adipocytes were also obtained from human recipients of hematopoietic stem cell transplantation. DNA was isolated, and microsatellite polymorphisms were exploited to quantify donor/recipient chimerism. Luciferase emission was detected from major fat depots of transplanted mice. No light emission was observed from intestines, liver, or lungs. Up to 35% of adipocytes in humans were generated from donor marrow cells in the absence of cell fusion. Nontransplanted mice and stromal-vascular fraction samples were used as negative and positive controls for the mouse and human experiments, respectively. This study provides evidence for a nontissue resident origin of an adipocyte subpopulation in both mice and humans.

Rapid and pervasive changes in genome-wide enhancer usage during mammalian development

Enhancers are distal regulatory elements that can activate tissue-specific gene expression and are abundant throughout mammalian genomes. Although substantial progress has been made toward genome-wide annotation of mammalian enhancers, their temporal activity patterns and global contributions in the context of developmental in vivo processes remain poorly explored. Here we used epigenomic profiling for H3K27ac, a mark of active enhancers, coupled to transgenic mouse assays to examine the genome-wide utilization of enhancers in three different mouse tissues across seven developmental stages. The majority of the ∼90,000 enhancers identified exhibited tightly temporally restricted predicted activity windows and were associated with stage-specific biological functions and regulatory pathways in individual tissues. Comparative genomic analysis revealed that evolutionary conservation of enhancers decreases following midgestation across all tissues examined. The dynamic enhancer activities uncovered in this study illuminate rapid and pervasive temporal in vivo changes in enhancer usage that underlie processes central to development and disease.

TGF-β signaling protects retinal neurons from programmed cell death during the development of the mammalian eye

We investigated the influence of transforming growth factor-β (TGF-β) signaling on developmental programmed cell death in the mouse retina by direct and specific molecular targeting of TGF-β type II receptor (TβRII) and Smad7 in retinal progenitor cells. Mice were generated carrying a conditional deletion of the TβRII in cells that originate from the inner layer of the optic cup. The animals showed a significant decrease of phosphorylated Smad3 in both the central and peripheral retina, which indicates the diminished activity of TGF-β signaling. TβRII deficiency significantly increased the apoptotic death of retinal neurons during embryonic and postnatal development without affecting their proliferation. In contrast, treatment with TGF-β2 inhibited cell death of retinal ganglion cells in dissociated retinal cell cultures, an effect that was blocked by inhibiting the phosphorylation of Smad3. The increase in apoptosis during development resulted in a significant reduction in the number of neurons in adult TβRII-deficient mice. The effect was most pronounced in the inner retina neurons and resulted in functional deficits as determined by electroretinography. In contrast, a conditional deletion of TGF-β-inhibiting Smad7 in retinal neurons significantly enhanced Smad3 phosphorylation and significantly decreased apoptosis of retinal neurons in embryos and pups. Moreover, the number of retinal ganglion cells was significantly higher in Smad7-deficient mice compared with control littermates. TβRII-deficient pups showed a lower level of nerve growth factor (NGF) in its mRNA; however, higher levels were observed in Smad7-deficient pups, which strongly suggests that the protective effects of TGF-β signaling on developmental cell death are mediated through NGF.

In vivo fate mapping identifies pre-TCRα expression as an intra- and extrathymic, but not prethymic, marker of T lymphopoiesis

A novel pre-TCRα (pTα) reporter mouse reveals that expression of pTα is confined to the T lineage and does not occur on prethymic progenitors.

Expression of the pre–T cell receptor α (pTα) gene has been exploited in previous studies as a molecular marker to identify tiny cell populations in bone marrow (BM) and blood that were suggested to contain physiologically relevant thymus settling progenitors (TSPs). But to what extent these cells genuinely contribute to thymopoiesis has remained obscure. We have generated a novel pTα^iCre knockin mouse line and performed lineage-tracing experiments to precisely quantitate the contribution of pTα-expressing progenitors to distinct differentiation pathways and to the genealogy of mature hematopoietic cells under physiological in vivo conditions. Using these mice in combination with fluorescent reporter strains, we observe highly consistent labeling patterns that identify pTα expression as a faithful molecular marker of T lineage commitment. Specifically, the fate of pTα-expressing progenitors was found to include all αβ and most γδ T cells but, in contrast to previous assumptions, to exclude B, NK, and thymic dendritic cells. Although we could detect small numbers of T cell progenitors with a history of pTα expression in BM and blood, our data clearly exclude these populations as physiologically important precursors of thymopoiesis and indicate that they instead belong to a pathway of T cell maturation previously defined as extrathymic.

Strategies for designing transgenic DNA constructs

Generation and characterization of transgenic mice are important elements of biomedical research. In recent years, transgenic technology has become more versatile and sophisticated, mainly because of the incorporation of recombinase-mediated conditional expression and targeted insertion, site-specific endonuclease-mediated genome editing, siRNA-mediated gene knockdown, various inducible gene expression systems, and fluorescent protein marking and tracking techniques. Site-specific recombinases (such as PhiC31) and engineered endonucleases (such as ZFN and Talen) have significantly enhanced our ability to target transgenes into specific genomic loci, but currently a great majority of transgenic mouse lines are continuingly being created using the conventional random insertion method. A major challenge for using this conventional method is that the genomic environment at the integration site has a substantial influence on the expression of the transgene. Although our understanding of such chromosomal position effects and our means to combat them are still primitive, adhering to some general guidelines can significantly increase the odds of successful transgene expression. This chapter first discusses the major problems associated with transgene expression, and then describes some of the principles for using plasmid and bacterial artificial chromosomes (BACs) for generating transgenic constructs. Finally, the strategies for conducting each of the major types of transgenic research are discussed, including gene overexpression, promoter characterization, cell-lineage tracing, mutant complementation, expression of double or multiple transgenes, siRNA knockdown, and conditional and inducible systems.

A matter of packaging: influence of nucleosome positioning on heterologous gene expression

The organization of DNA into the various levels of chromatin compaction is the main obstacle that restricts the access of transcriptional machinery to genes. Genome-wide chromatin analyses have shown that there are common chromatin organization patterns for most genes but have also revealed important differences in nucleosome positioning throughout the genome. Such chromatin heterogeneity is one of the reasons why recombinant gene expression is highly dependent on integration sites. Different solutions have been tested for this problem, including artificial targeting of chromatin-modifying factors or the addition of DNA elements, which efficiently counteract the influence of the chromatin environment.An influence of the chromatin configuration of the recombinant gene itself on its transcriptional behavior has also been established. This view is especially important for heterologous genes since the general parameters of chromatin organization change from one species to another. The chromatin organization of bacterial DNA proves particularly dramatic when introduced into eukaryotes. The nucleosome positioning of recombinant genes is the result of the interaction between the machinery of the hosting cell and the sequences of both the recombinant genes and the promoter regions. We discuss the key aspects of this phenomenon from the heterologous gene expression perspective.

Site-specific integrase-mediated transgenesis in mice via pronuclear injection

Microinjection of recombinant DNA into zygotic pronuclei has been widely used for producing transgenic mice. However, with this method, the insertion site, integrity, and copy number of the transgene cannot be controlled. Here, we present an integrase-based approach to produce transgenic mice via pronuclear injection, whereby an intact single-copy transgene can be inserted into predetermined chromosomal loci with high efficiency (up to 40%), and faithfully transmitted through generations. We show that neighboring transgenic elements and bacterial DNA within the transgene cause profound silencing and expression variability of the transgenic marker. Removal of these undesirable elements leads to global high-level marker expression from transgenes driven by a ubiquitous promoter. We also obtained faithful marker expression from a tissue-specific promoter. The technique presented here will greatly facilitate murine transgenesis and precise structure/function dissection of mammalian gene function and regulation in vivo.

Characterization of a heat resistant beta-glucosidase as a new reporter in cells and mice

BACKGROUND

Reporter genes are widely used in biology and only a limited number are available. We present a new reporter gene for the localization of mammalian cells and transgenic tissues based on detection of the bglA (SYNbglA) gene of Caldocellum saccharolyticum that encodes a thermophilic beta-glucosidase.

RESULTS

SYNbglA was generated by introducing codon substitutions to remove CpG motifs as these are associated with gene silencing in mammalian cells. SYNbglA expression can be localized in situ or detected quantitatively in colorimetric assays and can be co-localized with E. coli beta-galactosidase. Further, we have generated a Cre-reporter mouse in which SYNbglA is expressed following recombination to demonstrate the general utility of SYNbglA for in vivo analyses. SYNbglA can be detected in tissue wholemounts and in frozen and wax embedded sections.

CONCLUSIONS

SYNbglA will have general applicability to developmental and molecular studies in vitro and in vivo.

Hoxb13 regulatory elements mediate transgene expression during prostate organogenesis and carcinogenesis

The prostate requires androgens for development and homeostasis. Prostate cancer shares this dependence, however progression to androgen-independence is common after androgen deprivation. There is considerable interest in achieving therapeutic gene expression after androgen ablation using prostate-specific promoters. Paradoxically, known prostate-restricted cis-regulatory elements are androgen dependent. Hoxb13 expression is restricted in adults to the prostate and colon, and robust Hoxb13 expression persists after castration. To locate regulatory elements conferring this expression pattern, a lacZ reporter was inserted into the Hoxb13 locus on a mouse genomic bacterial artificial chromosome. In transgenic mice, this construct recapitulated the Hoxb13 expression pattern, including expression after castration. Reporter gene activity was maintained during carcinogenesis in a prostate cancer model. Hoxb13 cis-regulatory elements provide a powerful tool to achieve androgen-independent transgene expression in the prostate and distal colon-specific expression in the gastrointestinal tract. These data establish a framework for high-resolution analyses of factors regulating Hoxb13.

Position effect variegation and imprinting of transgenes in lymphocytes

Sequences proximal to transgene integration sites are able to deregulate transgene expression resulting in complex position effect phenotypes. In addition, transgenes integrated as repeated arrays are susceptible to repeat-induced gene silencing. Using a Cre recombinase-based system we have addressed the influence of transgene copy number (CN) on expression of hCD2 transgenes. CN reduction resulted in a decrease, increase or no effect on variegation depending upon the site of integration. This finding argues that repeat-induced gene silencing is not the principle cause of hCD2 transgene variegation. These results also suggest that having more transgene copies can be beneficial at some integration sites. The transgenic lines examined in this report also exhibited a form of imprinting, which was manifested by decreased levels of expression and increased levels of variegation, upon maternal transmission; and this correlated with DNA hypermethylation and a reduction in epigenetic chromatin modifications normally associated with active genes.

Adopting the good reFLEXes when generating conditional alterations in the mouse genome

Major advances have been made in the use of the Cre/loxP system for conditional gene targeting in the mouse. By combining the ability of Cre recombinase to invert or excise a DNA fragment, depending upon the orientation of the flanking loxP sites, and the use of wild-type loxP and variant lox511 sites, we devised an efficient and reliable Cre-mediated genetic switch, called FLEX, through which expression of a given gene can be turned off, while expression of another one can be simultaneously turned on. We discuss how this innovative, flexible and powerful approach, which virtually adapts to any kind of site-specific recombinase (e.g., Cre and Flp recombinases), can be used to easily generate, even at high throughput and genome wide scale, many genetic modifications in a conditional manner, including those which were considered as difficult or impossible to achieve.

Faithful activation of an extra-bright red fluorescent protein in "knock-in" Cre-reporter mice ideally suited for lineage tracing studies

The considerable potential of Cre recombinase as a tool for in vivo fate-mapping studies depends on the availability of reliable reporter mice. By targeting a tandem-dimer red fluorescent protein (tdRFP) with advanced spectral and biological properties into the ubiquitously expressed ROSA26 locus of C57BL/6-ES cells, we have generated a novel inbred Cre-reporter mouse with several unique characteristics. We directly demonstrate the usefulness of our reporter strain in inter-crosses with a "universal Cre-deleter" strain and with mice expressing Cre recombinase in a T lineage-specific manner. Cytofluorometric and histological analyses illustrate: (i) non-toxicity and extraordinary brightness of the fluorescent reporter, allowing quantitative detection and purification of labeled cells with highest accuracy, (ii) reliable Cre-mediated activation of tdRFP from an antisense orientation relative to ROSA26 transcription, effectively excluding "leaky" reporter expression, (iii) absence of gene expression variegation effects, (iv) quantitative detection of tdRFP-expressing cells even in paraformaldehyde-fixed tissue sections, and (v) full compatibility with GFP/YFP-based fluorescent markers in multicolor experiments. Taken together, the data show that our C57BL/6-inbred reporter mice are ideally suited for sophisticated lineage-tracing experiments requiring sensitive and quantitative detection/purification of live Cre-expressing cells and their progeny.

Stability and homogeneity of transgene expression in isogenic cells

Genetically engineered cells are an important tool not only for basic research applications, but also for biotechnology and molecular medicine. Among the issues yet to be solved for this technology are the consequences of randomly integrating DNA into a cell's genome. The problems encountered range from unpredictable expression levels to safety concerns. Recombinase-mediated chromosome engineering is a popular tool for generating stably transfected isogenic cell lines. Using this approach, single-copy integration of foreign DNA fragments can be achieved at predetermined chromosomal loci in the genome. We used such a technology based on the Flp/Flp recombinase target (FRT) recombination system in human 293 cells for comparative promoter studies. The expected integration patterns were obtained with high frequency. In contrast, the phenotypic characterization of expression of the integrated reporter transgene showed remarkable differences between isogenic cell lines, ranging from homogenous expression to mosaic to even complete expression silencing. As long as cell clones with homogenous expression were kept under selective conditions, their expression characteristics could be maintained over a long period. However, this desirable phenotype was progressively lost upon withdrawal of selective pressure. These results were also reflected by the expression instability of the reporter cassette originally inserted in the recipient cell line. Thus, selection for appropriate integration events that ensure reproducible and long-term gene expression has to go beyond the verification of isogenicity.

Human High Mobility Group Box Transcription Factor 1 Affects Thymocyte Development and Transgene Variegation

It has been shown previously that a human CD2 (hCD2) disabled locus control region (LCR) transgene is unable to establish an open chromatin configuration in all the T cells, and this leads to position effect variegation of the transgene. In this study we show that thymus-specific overexpression of human high mobility group box transcription factor 1 (HBP1), a transcription factor that binds a specific sequence within the hCD2 LCR, affects thymus cellularity as well as the number of CD8(+) thymocytes in two independent transgenic mouse lines and increases the proportion of T cells that fully activate the transgenic locus in hCD2 variegating mice in a sequence-specific dependent manner. This finding suggests that overexpression of HBP1 can affect lineage commitment and can relieve the suppressive influence of heterochromatin, allowing thymocytes to express the variegating target locus more efficiently. These effects could be the result of direct HBP1 action on LCR activity. Alternatively, the extra HBP1 molecules may sequester repressive elements away from the LCR, thus allowing transcription permissive states to form on the transgene locus.

A Transgenic Mouse Model with Inducible Tyrosinase Gene Expression Using the Tetracycline (Tet-on) System Allows Regulated Rescue of Abnormal Chiasmatic Projections Found in Albinism

Congenital defects in retinal pigmentation, as in oculocutaneous albinism Type I (OCA1), where tyrosinase is defective, result in visual abnormalities affecting the retina and pathways into the brain. Transgenic animals expressing a functional tyrosinase gene on an albino genetic background display a correction of all these abnormalities, implicating a functional role for tyrosinase in normal retinal development. To address the function of tyrosinase in the development of the mammalian visual system, we have generated a transgenic mouse model with inducible expression of the tyrosinase gene using the tetracycline (TET-ON) system. We have produced two types of transgenic mice: first, mice expressing the transactivator rtTA chimeric protein under the control of mouse tyrosinase promoter and its locus control region (LCR), and; second, transgenic mice expressing a mouse tyrosinase cDNA construct driven by a minimal promoter inducible by rtTA in the presence of doxycycline. Inducible experiments have been carried out with selected double transgenic mouse lines. Tyrosinase expression has been induced from early embryo development and its impact assessed with histological and biochemical methods in heterozygous and homozygous double transgenic individuals. We have found an increase of tyrosinase activity in the eyes of induced animals, compared with littermate controls. However, there was significant variability in the activation of this gene, as reported in analogous experiments. In spite of this, we could observe corrected uncrossed chiasmatic pathways, decreased in albinism, in animals induced from their first gestational week. These mice could be instrumental in revealing the role of tyrosinase in mammalian visual development.

On the use of post-transcriptional processing elements in transgenes

RNA processing events modulate final productivity of a given transgene. We have evaluated a series of RNA elements for their ability to enhance α1-antitrypsin production in mammary cells. Our results indicate the need for a case-by-case assessment of each construct design and the occurrence of gene silencing events in vivo.

The upstream regulatory region of the gene for the human homologue of the adhesion molecule TAG-1 contains elements driving neural specific expression in vivo

Cell adhesion molecules (CAMs) of the immunoglobulin superfamily (IgSF) exhibit restricted spatial and temporal expression profiles requiring a tight regulatory program during development. The rodent glycoprotein TAG-1 and its orthologs TAX-1 in the human and axonin-1 in chick are cell adhesion molecules belonging to the contactin/F3 subgroup of the IgSF. TAG-1 is expressed in restricted subsets of central and peripheral neurons, not only during development but also in adulthood, and is implicated in neurite outgrowth, axon guidance and fasciculation, as well as neuronal migration. In an attempt to identify the regulatory elements that guide the neuronal expression of TAG-1, we have isolated genomic clones containing 4 kb of the TAX-1 upstream sequence and used them to drive the expression of the LacZ reporter gene in transgenic mice. We demonstrate that this sequence includes elements not only sufficient to restrict expression to the nervous system, but also to recapitulate to a great extent the endogenous pattern of the TAG-1 expression in the developing CNS.

Tyrosinase gene expression is not detected in mouse brain outside the retinal pigment epithelium cells

Tyrosinase is the rate-limiting enzyme for melanin synthesis. Its gene is expressed in two cell types: melanocytes, derived from migrating neural crest cells, and, in the CNS, retinal pigment epithelium cells, derived from the optic cup. Its absence from the eye results in profound pathway selection errors of optic fibres at the chiasm and, hence, it has been implicated as a developmental regulator of CNS pathway selection. Recently, it has been proposed that tyrosinase can also be expressed in the developing and adult brain, although the methods used were indirect. Its presence in the brain could be very significant in terms of a potentially wider role in pathway finding. Here, we have evaluated the presence of tyrosinase expression in mouse developing, perinatal and adult brain by in situ hybridization in whole-mount embryos and histological sections and by real-time reverse transcription-polymerase chain reaction. We find no evidence for tyrosinase gene expression in the CNS outside the retinal pigment epithelium cells.

Use of the tetracycline system for inducible protein synthesis in the kidney

The great advantage of the tetracycline-inducible system lies in its ability to address a large variety of biological questions in a time-dependent and tissue-specific manner. This study describes a transgenic mouse line, rTA(LAP)-1, which produces the reverse tetracycline transactivator under control of the liver activator protein (LAP) promoter. Two reporter lines with luciferase and LacZ reporter genes were used to demonstrate predominant expression in the kidney and liver when doxycycline was added to the drinking water. In the kidney, transgene expression was found primarily in cortical proximal tubules. No luciferase and beta-galactosidase activity was detected in mice without doxycycline in the drinking water, which attests to the tight control of this system. One of the advantages of the tet system lies in its reversibility, and indeed, a virtually complete remission of transgene activity in both the kidney and liver was observed when doxycycline was withdrawn. Also examined was transactivator activity during development by exposing the mothers producing the reverse transactivator to doxycycline before mating. Transgene activity was detected in newborn kidneys and liver, indicating that sufficient amounts of doxycycline had crossed the placental barrier. During nephron development, the LAP promoter appeared to be only active in the more mature proximal tubules. Finally, the rTA(LAP)-1 line was used to inducibly express the human PKD2 cDNA in proximal tubules of transgenic mice, but no cystic changes were detected, even after 6 mo of induction.

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

A directional strategy for monitoring Cre-mediated recombination at the cellular level in the mouse

Functional redundancies, compensatory mechanisms, and lethal phenotypes often prevent the full analysis of gene functions through generation of germline null mutations in the mouse. The use of site-specific recombinases, such as Cre, which catalyzes recombination between loxP sites, has allowed the engineering of mice harboring targeted somatic mutations, which are both temporally controlled and cell-type restricted. Many Cre-expressing mouse lines exist, but only a few transgenic lines are available that harbor a reporter gene whose expression is dependent on a Cre-mediated event. Moreover, their use to monitor gene ablation at the level of individual cells is often limited, as in some tissues the reporter gene may be silenced, be affected by position-effect variegation, or reside in a chromatin configuration inaccessible for recombination. Thus, one cannot validly extrapolate from the expression of a reporter transgene to an identical ablation pattern for the conditional allele of a given gene. By combining the ability of Cre recombinase to invert or excise a DNA fragment, depending on the orientation of the flanking loxP sites, and the availability of both wild-type (WT) and mutant loxP sites, we designed a Cre-dependent genetic switch (FLEx switch) through which the expression of a given gene is turned off, while the expression of another one is concomitantly turned on. We demonstrate the efficiency and reliability of this switch to readily detect, in the mouse, at the single cell level, Cre-mediated gene ablation. We discuss how this strategy can be used to generate genetic modifications in a conditional manner.

Transgenic tilapia and the tilapia genome

The tilapia fish (Oreochromis niloticus) has an important place in the aquaculture of the developing world. It is also a very useful laboratory animal, and readily lends itself to the transgenic technology. Through the use of reporter genes, a range of potential gene promoters have been tested in tilapia, both through transient and stable expression of the reporter construct. Using the transgenic technology, growth enhanced lines of tilapia have been produced. These fish have no abnormalities and offer a considerable growth advantage for future exploitation. It is however crucial that transgenic fish, to be exploited in aquaculture, be sterile, and various methods of achieving sterility are considered. These include triploidy, gene knock out of crucial hormone encoding genes via homologous recombination, and knock down of the function of the same genes via ribozyme or antisense technologies. Transgenic tilapia also offer the potential for exploitation as biofactories in the production of valuable pharmaceutical products, and this is also discussed.

The use of the uromodulin promoter to target production of recombinant proteins into urine of transgenic animals

A uromodulin promoter has been isolated, sequenced, and used to generate two sets of transgenic mice for expression of the lacZ marker gene and for production of the human recombinant erythropoietin (rhEPO) in urine. We demonstrated that the 5.6-kb fragment of the uromodulin gene containing the 3.7-kb promoter area and, both the first exon and part of the second exon, were sufficient to provide kidney-specific expression of the lacZ gene. Histological analysis of the lacZ expression pattern revealed beta-galactosidase activity specifically in the thick limb of Henle's loop. However, due to random integration of the transgene, ectopic expression was detected in some transgenic lines. Analysis of the EPO-transgenic mice showed that rhEPO was secreted into the urine of founder mice (up to 6 ng/ml). We were able to breed and analyze only two sublines with a very low expression level of rhEPO (up to 260 pg/ml). All of our transgenic mice expressing rhEPO in urine developed disease symptoms similar to polycythemia in humans. These included a considerable increase in red blood cell counts, hemoglobin concentration, and hematocrit concomitant with severe thrombocytopenia, all of which were detected in the rhEPO-expressing mice. Although our model did not prove to be beneficial for commercial production of rhEPO, we concluded that the uromodulin promoter could be useful for expression of other important therapeutic proteins into the urine of transgenic animals.

Artificial chromosome transgenesis in pigmentary research

Pigmentary genes were among the first mammalian genes to be studied, mostly because of the obvious phenotypes associated with their mutations. In 1990, tyrosinase, encoding the limiting enzyme in the melanin synthesis pathway, was eventually assigned to the c (albino) locus by classical rescue experiments driven by functional constructs in transgenic mice. These pioneer reports triggered the study of the regulation of endogenous tyrosinase gene expression by combining different amounts of upstream regulatory and promoter regions and testing their function in vivo in transgenic animals. However, faithful and reproducible transgenic expression was not achieved until the entire tyrosinase expression domain was transferred to the germ-line of mice using artificial-chromosome-type transgenes. The use of these large tyrosinase transgenic constructs and the ease with which they could be manipulated in vitro enabled the discovery of previously unknown but fundamental regulatory regions, such as the tyrosinase locus control region (LCR), whose presence was required in order to guarantee position-independent and copy-number-dependent expression of tyrosinase transgenes, with an expression level, per copy, comparable to that of an endogenous wild-type allele. Subsequently, functional dissection of elements present within this LCR through the generation of new artificial-chromosome type tyrosinase transgenes has revealed the existence of different regulatory activities. The existence of some of these units had been suggested previously by standard-type transgenic analyses. In this review, we will discuss both independent approaches and conclude that optimal tyrosinase transgene expression requires the use of its complete expression domain.

Effect of the rabbit alphas1-casein gene distal enhancer on the expression of a reporter gene in vitro and in vivo

Several gene constructs containing the firefly luciferase gene and the herpes simplex virus thymidine kinase gene promoter (TK) were used to evaluate the transcriptional activity of the distal enhancer (-3442, -3285) of the rabbit alphas1-casein gene. Six copies of the enhancer (6i) were added upstream of the TK-luciferase construct in the presence or absence of the chicken beta-globin 5'HS4 insulator. The activity of the constructs was tested by transient transfection in CHO cells and in rabbit primary mammary cell cultured on plastic or on floating collagen. Constructs were also tested in stably transfected mouse mammary HC11 cells. In all cell types the multimerized alphas1-casein enhancer strongly stimulated luciferase gene expression in the presence of lactogenic hormones. It was also sensitive to the extracellular matrix in rabbit primary mammary cells. The constructs were used to generate transgenic mice. The 6i TK transgenic animals expressed the luciferase gene at very low levels irrespectively of the physiological state. No preferential expression in the mammary gland was observed. Addition of 5'HS4 insulator to the 6i TK construct did not prevent silencing in most of the transgenic lines. However, two lines expressed high luciferase levels specifically in the mammary gland. Our data suggest that 6i may confer, when insulated properly, a higher and mammary-specific expression to the TK promoter.

Real-time imaging of beta-lactoglobulin-targeted luciferase activity in the mammary glands of transgenic mice

This study was aimed at establishing a new platform for real-time monitoring of milk-protein gene expression in the mammary glands. A transgenic reporter composed of the beta-lactoglobulin (BLG)/luciferase hybrid gene was targeted to the mammary glands of pregnant and lactating mice and luciferase activity was imaged in vivo with a low-light imaging system. The mammary glands of a 17-day pregnant mouse occupied an area comparable to that of a 6-day lactating mouse. Nevertheless, the intensity of the luciferase signal was much weaker and confined to regions in the inguinal and thoracic glands. A few small and defined locations of higher expression were also detected, indicating diversity in the initiation of this transgenic milk protein expression. In the lactating mice, high inter- and intra-heterogeneity among regions in a particular gland and among glands was demonstrated, and confirmed by ex vivo analysis of luciferase activity in mammary biopsies. The lack of correlation between luciferase activities and levels of beta-casein accumulation in these biopsies resulted, most probably, from the longer half-life of the native milk protein, compared to the activity of the transgenic marker in the tissue. Unilateral sealing of mammary glands for 4 hr resulted in complete abrogation of luciferase activity, establishing the BLG/luciferase transgene as a reliable tool to follow short-term stimuli. Dispersed mammary epithelial cells preserved luciferase activity in culture, and thus could be used for following mammary gland development after re-implantation. The bioluminescence-based methodology presented here eliminates averaging of heterogeneity in gene expression among glands, and misinterpretations resulting from sampling biopsies taken from inactive regions. Imaging luciferase expression in the mammary glands may enable an accurate monitoring of milk-protein gene expression during cyclic periods of development and apoptosis in a limited number of animals, and could be applied for reporting the consequences of selected drugs on milk-protein gene expression.

Tetracycline-inducible gene expression in cultured rat renal CD cells and in intact CD from transgenic mice

The renal collecting duct (CD) plays a key role in the control of ion and fluid homeostasis. Several genetic diseases that involve mutations in genes encoding for ion transporters or hormone receptors specifically expressed in CD have been described. Suitable cellular or transgenic animal models expressing such mutated genes in an inducible manner should represent attractive systems for structure-function relationship analyses and the generation of appropriate physiopathological models of related diseases. Our first goal was to develop a CD cell line that allows inducible gene expression using the tetracycline-inducible system (Tet-On). We designed several strategies aimed at the development of a tight and highly inducible system in RCCD1 cells, a rat cortical collecting duct (CCD) cell line exhibiting several properties of the native CCD. Analysis of reporter gene expression demonstrated that the Tet-On system is suitable for inducible gene expression in these cells. In a second step, we have tested whether transgenic Tet-On mice expressing the tetracycline transactivator under the control of the human cytomegalovirus promoter were suitable for inducible gene expression in tubule epithelial cells. The results indicate that, in vivo, the inducible expression of the lacZ reporter gene appeared to be restricted to the CD. This particular strain of transgenic mice may therefore be useful for the expression of genes of interest in an inducible manner in the collecting duct.