Variegated gene expression in mice

In Vivo Function of Flow-Responsive Cis-DNA Elements of eNOS Gene: A Role for Chromatin-Based Mechanisms

BACKGROUND

eNOS (endothelial nitric oxide synthase) is an endothelial cell (EC)-specific gene predominantly expressed in medium- to large-sized arteries where ECs experience atheroprotective laminar flow with high shear stress. Disturbed flow with lower average shear stress decreases eNOS transcription, which leads to the development of atherosclerosis, especially at bifurcations and curvatures of arteries. This prototypic arterial EC gene contains 2 distinct flow-responsive cis-DNA elements in the promoter, the shear stress response element (SSRE) and the KLF (Krüppel-like factor) element. Previous in vitro studies suggested their positive regulatory functions on flow-induced transcription of EC genes including eNOS. However, the in vivo function of these cis-DNA elements remains unknown.

METHODS

Insertional transgenic mice with a mutation at each flow-responsive cis-DNA element were generated using a murine eNOS promoter-β-galactosidase reporter by linker-scanning mutagenesis and compared with episomal-based mutations in vitro. DNA methylation at the eNOS proximal promoter in mouse ECs was assessed by bisulfite sequencing or pyrosequencing.

RESULTS

Wild type mice with a functional eNOS promoter-reporter transgene exhibited reduced endothelial reporter expression in the atheroprone regions of disturbed flow (n=5). It is surprising that the SSRE mutation abrogated reporter expression in ECs and was associated with aberrant hypermethylation at the eNOS proximal promoter (n=7). Reporter gene silencing was independent of transgene copy number and integration position, indicating that the SSRE is a critical cis-element necessary for eNOS transcription in vivo. The KLF mutation demonstrated an integration site-specific decrease in eNOS transcription, again with marked promoter methylation (n=8), suggesting that the SSRE alone is not sufficient for eNOS transcription in vivo. In wild type mice, the native eNOS promoter was significantly hypermethylated in ECs from the atheroprone regions where eNOS expression was markedly repressed by chronic disturbed flow, demonstrating that eNOS expression is regulated by flow-dependent DNA methylation that is region-specific in the arterial endothelium in vivo.

CONCLUSIONS

We report, for the first time, that the SSRE and KLF elements are critical flow sensors necessary for a transcriptionally permissive, hypomethylated eNOS promoter in ECs under chronic shear stress in vivo. Moreover, eNOS expression is regulated by flow-dependent epigenetic mechanisms, which offers novel mechanistic insight on eNOS gene regulation in atherogenesis.

Partial Rescue of Ocular Pigment Cells and Structure by Inducible Ectopic Expression of Mitf-M in MITF-Deficient Mice

Purpose

Complete deficiency of microphthalmia transcription factor (MITF) in Mitfmi-vga9/mi-vga9 mice is associated with microphthalmia, retinal dysplasia, and albinism. We investigated the ability of dopachrome tautomerase (DCT) promoter-mediated inducible ectopic expression of Mitf-M to rescue these phenotypic abnormalities.

Methods

A new mouse line was created with doxycycline-inducible ectopic Mitf-M expression on an Mitf-deficient Mitfmi-vga9 background (DMV mouse). Adult DMV mice were phenotypically characterized and tissues were collected for histology, immunohistochemistry, and evaluation of Mitf, pigmentary genes, and retinal pigment epithelium (RPE) gene expression.

Results

Ectopic Mitf-M expression was specifically induced in the eyes, but was not detected in the skin of DMV mice. Inducible expression of Mitf-M partially rescued the microphthalmia, RPE structure, and pigmentation as well as a subset of the choroidal and iris melanocytes but not cutaneous melanocytes. RPE function and vision were not restored in the DMV mice.

Conclusions

Ectopic expression of Mitf-M during development of Mitf-deficient mice is capable of partially rescuing ocular and retinal structures and uveal melanocytes. These findings provide novel information about the roles of Mitf isoforms in the development of mouse eyes.

Linking a cell-division gene and a suicide gene to define and improve cell therapy safety

Human pluripotent cell lines hold enormous promise for the development of cell-based therapies. Safety, however, is a crucial prerequisite condition for clinical applications. Numerous groups have attempted to eliminate potentially harmful cells through the use of suicide genes¹, but none has quantitatively defined the safety level of transplant therapies. Here, using genome-engineering strategies, we demonstrate the protection of a suicide system from inactivation in dividing cells. We created a transcriptional link between the suicide gene herpes simplex virus thymidine kinase (HSV-TK) and a cell-division gene (CDK1); this combination is designated the safe-cell system. Furthermore, we used a mathematical model to quantify the safety level of the cell therapy as a function of the number of cells that is needed for the therapy and the type of genome editing that is performed. Even with the highly conservative estimates described here, we anticipate that our solution will rapidly accelerate the entry of cell-based medicine into the clinic.

Bmi1+ Progenitor Cell Dynamics in Murine Cornea During Homeostasis and Wound Healing

The outermost layer of the eye, the cornea, is renewed continuously throughout life. Stem cells of the corneal epithelium reside in the limbus at the corneal periphery and ensure homeostasis of the central epithelium. However, in young mice, homeostasis relies on cells located in the basal layer of the central corneal epithelium. Here, we first studied corneal growth during the transition from newborn to adult and assessed Keratin 19 (Krt19) expression as a hallmark of corneal maturation. Next, we set out to identify a novel marker of murine corneal epithelial progenitor cells before, during and after maturation, and we found that Bmi1 is expressed in the basal epithelium of the central cornea and limbus. Furthermore, we demonstrated that Bmi1+ cells participated in tissue replenishment in the central cornea. These Bmi1+ cells did not maintain homeostasis of the cornea for more than 3 months, reflecting their status as progenitor rather than stem cells. Finally, after injury, Bmi1+ cells fueled homeostatic maintenance, whereas wound closure occurred via epithelial reorganization. Stem Cells 2018;36:562-573.

Lessons from mouse chimaera experiments with a reiterated transgene marker: revised marker criteria and a review of chimaera markers

Recent reports of a new generation of ubiquitous transgenic chimaera markers prompted us to consider the criteria used to evaluate new chimaera markers and develop more objective assessment methods. To investigate this experimentally we used several series of fetal and adult chimaeras, carrying an older, multi-copy transgenic marker. We used two additional independent markers and objective, quantitative criteria for cell selection and cell mixing to investigate quantitative and spatial aspects of developmental neutrality. We also suggest how the quantitative analysis we used could be simplified for future use with other markers. As a result, we recommend a five-step procedure for investigators to evaluate new chimaera markers based partly on criteria proposed previously but with a greater emphasis on examining the developmental neutrality of prospective new markers. These five steps comprise (1) review of published information, (2) evaluation of marker detection, (3) genetic crosses to check for effects on viability and growth, (4) comparisons of chimaeras with and without the marker and (5) analysis of chimaeras with both cell populations labelled. Finally, we review a number of different chimaera markers and evaluate them using the extended set of criteria. These comparisons indicate that, although the new generation of ubiquitous fluorescent markers are the best of those currently available and fulfil most of the criteria required of a chimaera marker, further work is required to determine whether they are developmentally neutral.

Fucci2a: a bicistronic cell cycle reporter that allows Cre mediated tissue specific expression in mice

Markers of cell cycle stage allow estimation of cell cycle dynamics in cell culture and during embryonic development. The Fucci system incorporates genetically encoded probes that highlight G1 and S/G2/M phases of the cell cycle allowing live imaging. However the available mouse models that incorporate Fucci are beset by problems with transgene inactivation, varying expression level, lack of conditional potential and/or the need to maintain separate transgenes-there is no transgenic mouse model that solves all these problems. To address these shortfalls we re-engineered the Fucci system to create 2 bicistronic Fucci variants incorporating both probes fused using the Thosea asigna virus 2A (T2A) self cleaving peptide. We characterize these variants in stable 3T3 cell lines. One of the variants (termed Fucci2a) faithfully recapitulated the nuclear localization and cell cycle stage specific florescence of the original Fucci system. We go on to develop a conditional mouse allele (R26Fucci2aR) carefully designed for high, inducible, ubiquitous expression allowing investigation of cell cycle status in single cell lineages within the developing embryo. We demonstrate the utility of R26Fucci2aR for live imaging by using high resolution confocal microscopy of ex vivo lung, kidney and neural crest development. Using our 3T3 system we describe and validate a method to estimate cell cycle times from relatively short time-lapse sequences that we then apply to our neural crest data. The Fucci2a system and the R26Fucci2aR mouse model are compelling new tools for the investigation of cell cycle dynamics in cell culture and during mouse embryonic development.

Design and use of transgenic reporter strains for detecting activity of signaling pathways in Xenopus

Embryos and larvae of vertebrate species with external development are ideal subjects for investigating the dynamic spatiotemporal activity of developmental signaling pathways. The availability of efficient transgene technologies in Xenopus and zebrafish and the translucency and/or transparency of their embryos and larvae make these two species attractive for direct in vivo imaging of reporter gene expression. In this article we describe the design of efficient signaling reporters, using the Wnt/β-catenin pathway as a representative example. We define methods for validating the reporter constructs and describe how they can be used to generate stable transgenic lines in Xenopus. We provide efficient methods used in our laboratory for raising the tadpoles and froglets rapidly to sexual maturity. We further discuss how the reporter lines can be used for delineating the dynamic activity of a signaling pathway and how modulators of the pathway can be scrutinized via chemical intervention and the micro-injection of synthetic RNAs or morpholinos. The strategic outline discussed in this paper provides a template for studying other developmental signaling pathways in Xenopus.

Genetic background affects human glial fibrillary acidic protein promoter activity

The human glial fibrillary acidic protein (hGFAP) promoter has been used to generate numerous transgenic mouse lines, which has facilitated the analysis of astrocyte function in health and disease. Here, we evaluated the expression levels of various hGFAP transgenes at different ages in the two most commonly used inbred mouse strains, FVB/N (FVB) and C57BL/6N (B6N). In general, transgenic mice maintained on the B6N background displayed weaker transgene expression compared with transgenic FVB mice. Higher level of transgene expression in B6N mice could be regained by crossbreeding to FVB wild type mice. However, the endogenous murine GFAP expression was equivalent in both strains. In addition, we found that endogenous GFAP expression was increased in transgenic mice in comparison to wild type mice. The activities of the hGFAP transgenes were not age-dependently regulated. Our data highlight the importance of proper expression analysis when non-homologous recombination transgenesis is used.

Gene transfer and mutagenesis mediated by Sleeping Beauty transposon in Nile tilapia (Oreochromis niloticus)

The success of gene transfer has been demonstrated in many of vertebrate species, whereas the efficiency of producing transgenic animals remains pretty low due to the random integration of foreign genes into a recipient genome. The Sleeping Beauty (SB) transposon is able to improve the efficiency of gene transfer in zebrafish and mouse, but its activity in tilapia (Oreochromis niloticus) has yet to be characterized. Herein, we demonstrate the potential of using the SB transposon system as an effective tool for gene transfer and insertional mutagenesis in tilapia. A transgenic construct pT2/tiHsp70-SB11 was generated by subcloning the promoter of tilapia heat shock protein 70 (tiHsp70) gene, the SB11 transposase gene and the carp β-actin gene polyadenylation signal into the second generation of SB transposon. Transgenic tilapia was produced by microinjection of this construct with in vitro synthesized capped SB11 mRNA. SB11 transposon was detected in 28.89 % of founders, 12.9 % of F1 and 43.75 % of F2. Analysis of genomic sequences flanking integrated transposons indicates that this transgenic tilapia line carries two copies of SB transposon, which landed into two different endogenous genes. Induced expression of SB11 gene after heat shock was detected using reverse transcription PCR in F2 transgenic individuals. In addition, the Cre/loxP system was introduced to delete the SB11 cassette for stabilization of gene interruption and bio-safety. These findings suggest that the SB transposon system is active and can be used for efficient gene transfer and insertional mutagenesis in tilapia.

Conditional gene expression systems in the transgenic rat brain

BACKGROUND

Turning gene expression on and off at will is one of the most powerful tools for the study of gene function in vivo. While several conditional systems were successful in invertebrates, in mice the Cre/loxP recombination system and the tet-controlled transcription activation system are predominant. Both expression systems allow for spatial and temporal control of gene activities, and, in the case of tet regulation, even for the reversible activation/inactivation of gene expression. Although the rat is the principal experimental model in biomedical research, in particular in studies of neuroscience, conditional rat transgenic systems are exceptionally rare in this species.

RESULTS

We addressed this lack of technology, and established and thoroughly characterized CreERT2 and tTA transgenic rats with forebrain-specific transgene expression, controlled by the CaMKII alpha promoter. In addition, we developed new universal rat reporter lines for both transcription control systems and established inducible and efficient reporter gene expression in forebrain neurons.

CONCLUSIONS

We demonstrate that conditional genetic manipulations in the rat brain are both feasible and practicable and outline advantages and limitations of the Tet and Cre/loxP system in the rat brain.

Depot formation of doxycycline impairs Tet-regulated gene expression in vivo

The tetracycline (Tet) system is widely used for regulation of gene expression in vitro and in vivo. We constructed C57BL/6 transgenic mice (rtTA-CM2) with strong and ubiquitous reverse transactivator (rtTA2(S)-M2) gene expression. rtTA-CM2 mice were crossed to Tet-responsive reporter mice (LC-1) conditionally expressing the firefly luciferase (FLuc) gene under control of a Tet-responsive element, which allowed sensitive quantification of the transactivator activity by bioluminescent imaging. Following doxycycline (dox) application, up to 10(5)-fold increase in BL signal was measured. rtTA activity was inducible in most analyzed organs. After dox withdrawal the BL signal decreased significantly but did not disappear completely, most likely due to a dox depot formation in vivo. The residual dox was sufficient to partly down-regulate a Tet-off controlled oncogene in a tumor transplantation experiment, resulting in reduced tumor growth. rtTA-CM2 mice may be a useful tool to analyze the function of genes in various organs but also reveal that down-regulation of gene expression is not complete.

Transition from organogenesis to stem cell maintenance in the mouse adrenal cortex

Mice showing mosaic expression of an appropriate marker gene that is activated during development provide simple tools for investigating cell lineages. We used the mosaic β-galactosidase staining patterns in adrenal cortices of 21OH/ LacZ transgenic mice to study both organogenesis and maintenance of the adult tissue. Randomly orientated mosaic patterns present in embryonic day 14.5 (E14.5) adrenals changed progressively during the perinatal period from discrete spots, via patches and radial arrays, to radial stripes, which first emerged between postnatal days 0 and 7 (P0 and P7). The mosaic radial stripe pattern was fully established by P21 and remained unchanged throughout the adult period (8-52 weeks). The mouse adrenal gland grew continuously between E14.5 and P21, including the period during which stripes emerge. Ki67-positive, proliferative cells in the adrenal cortex were mainly localized to the outer cell layers between E18.5 and P3. By P10, cell proliferation had increased, and the proliferative region had expanded but was still mainly confined to the outer cortex. Correlation of changes in mosaic patterns in 21OH/LacZ adrenal cortices with the locations of adrenocortical cell proliferation suggest that the radial stripes arise by edge-biased growth during the perinatal period, even if they are maintained by stem cells in adults. The stability of the adult stripe pattern suggests that stem cell function is unchanged between 8 and 52 weeks.

Rare corneal clones in mice suggest an age-related decrease of stem cell activity and support the limbal epithelial stem cell hypothesis

The anterior ocular surface comprises the cornea, conjunctiva and a narrow intermediate region called the limbus. It is widely accepted that the corneal epithelium is maintained by stem cells but different hypotheses propose that the stem cells that maintain the mouse corneal epithelium during normal homeostasis are located either in the basal limbal epithelium or throughout the basal corneal epithelium. There are no specific markers to help test these alternatives and new methods are required to distinguish between them. We observed that KRT5(LacZ/-) transgenic mice produced rare β-galactosidase (β-gal)-positive radial stripes in the corneal epithelium. These stripes are likely to be clonal lineages of cells derived from stem cells, so they provide a lineage marker for actively proliferating stem cells. The distributions of the β-gal-positive radial stripes suggested they extended centripetally from the limbus, supporting the limbal epithelial stem cell (LESC) hypothesis. Stripe frequency declined between 15 and 30 weeks, which predicts a reduction in stem cell function with age. Pax6(+/-), KRT5(LacZ/-) corneas had small patches rather than stripes, which confirms that corneal maintenance is abnormal in Pax6(+/-) mice.

A previously functional tetracycline-regulated transactivator fails to target gene expression to the bone

Background

The tetracycline-controlled transactivator system is a powerful tool to control gene expression in vitro and to generate consistent and conditional transgenic in vivo model organisms. It has been widely used to study gene function and to explore pathological mechanisms involved in human diseases. The system permits the regulation of the expression of a target gene, both temporally and quantitatively, by the application of tetracycline or its derivative, doxycycline. In addition, it offers the possibility to restrict gene expression in a spatial fashion by utilizing tissue-specific promoters to drive the transactivator.

Findings

In this study, we report our problems using a reverse tetracycline-regulated transactivator (rtTA) in a transgenic mouse model system for the bone-specific expression of the Hutchinson-Gilford progeria syndrome mutation. Even though prior studies have been successful utilizing the same rtTA, expression analysis of the transactivator revealed insufficient activity for regulating the transgene expression in our system. The absence of transactivator could not be ascribed to differences in genetic background because mice in a mixed genetic background and in congenic mouse lines showed similar results.

Conclusions

The purpose of this study is to report our negative experience with previously functional transactivator mice, to raise caution in the use of tet-based transgenic mouse lines and to reinforce the need for controls to ensure the stable functionality of generated tetracycline-controlled transactivators over time.

Recombinase technology: applications and possibilities

The use of recombinases for genomic engineering is no longer a new technology. In fact, this technology has entered its third decade since the initial discovery that recombinases function in heterologous systems (Sauer in Mol Cell Biol 7(6):2087-2096, 1987). The random insertion of a transgene into a plant genome by traditional methods generates unpredictable expression patterns. This feature of transgenesis makes screening for functional lines with predictable expression labor intensive and time consuming. Furthermore, an antibiotic resistance gene is often left in the final product and the potential escape of such resistance markers into the environment and their potential consumption raises consumer concern. The use of site-specific recombination technology in plant genome manipulation has been demonstrated to effectively resolve complex transgene insertions to single copy, remove unwanted DNA, and precisely insert DNA into known genomic target sites. Recombinases have also been demonstrated capable of site-specific recombination within non-nuclear targets, such as the plastid genome of tobacco. Here, we review multiple uses of site-specific recombination and their application toward plant genomic engineering. We also provide alternative strategies for the combined use of multiple site-specific recombinase systems for genome engineering to precisely insert transgenes into a pre-determined locus, and removal of unwanted selectable marker genes.

Repeat-induced gene silencing of L1 transgenes is correlated with differential promoter methylation

Recent transgenic studies on L1 retrotransposons have afforded exciting insights into L1 biology, and a unique opportunity to model their function and regulation in vivo. Thus far, the majority of the transgenic L1 mouse lines are constructed via pronuclear microinjection, a procedure that typically results in the integration of tandem arrayed transgenes. Transgene arrays are susceptible to repeat-induced gene silencing (RIGS) in both plants and animals. In order to examine the potential impact of RIGS on L1 retrotransposition, we derived a cohort of animals carrying reduced copies of ORFeus transgene at the same genomic locus by Cre-mediated recombination. The copy number reduction of ORFeus transgenes did not decrease the overall retrotransposition activity. Using a sensitive and reproducible quantitative PCR assay, an average frequency of 0.45 insertions per cell was observed for animals carrying the donor transgene at a single copy, representing a 9-fold increase of retrotransposition frequency on a per-copy basis. DNA methylation analyses revealed that the observed retrotransposition activity was correlated with differential CpG methylation at the heterologous promoter: the promoter region was largely methylated in animals with the high-copy array but significantly hypomethylated in animals with the single-copy counterpart. In contrast, the ORF2 region, which represents the body of the ORFeus transgene, and the 3' end of the transgene showed high level of methylation in both high-copy and single-copy samples. The observed methylation patterns were metastable across generations. In summary, our data suggest that tandem arrayed L1 transgenes are subject to RIGS, and transgenes present at a single copy in the genome are thus recommended for modeling L1 in animals.

Variegation and silencing in a lentiviral-based murine transgenic model

Lentiviral based constructs represent a recent development in the generation of transgenic animals. The ease of use, and the fact that the same backbone vectors can be used to down-modulate endogenous gene expression and to produce transgenic animals overexpressing a gene of interest, have fuelled growing interest in this technology. In this study, we have used a lentiviral delivery system to generate transgenic mice expressing altered levels (up or downregulated) of a gene of interest. Although this lentiviral-based approach led to high levels of transgenesis and germ line transmission, a wide variation in transgene expression was observed in most first and second generation mouse lines. In particular, despite the segregation of integrants into single-copy expressing mouse lines, transgene expression appeared to be the target of epigenetic regulatory mechanism, often causing the coexistence of high and low transgene expressing cells within a given tissue such as blood peripheral lymphocytes. The establishment and analysis of large number of mouse lines may therefore be required to select a stable transgenic line with pancellular expression of a gene of interest using this lentiviral-based approach.

Identifying promoter elements necessary for enamelin tissue-specific expression

Enamel development requires the strictly regulated spatiotemporal expression of genes encoding enamel matrix proteins. The mechanisms orchestrating the initiation and termination of gene transcription at each specific stage of amelogenesis are unknown. In this study, we identify cis- regulatory regions necessary for normal enamelin (Enam) expression. Sequence analysis of the Enam promoter 5'-noncoding region identified potentially important cis-regulatory elements located within 5.2 kb upstream of the Enam translation initiation site. DNA constructs containing 5.2 or 3.9 kb upstream of the Enam translation initiation site were linked to an LacZ reporter gene and used to generate transgenic mice. The 3.9-kb Enam-LacZ transgenic lines showed no expression in ameloblasts, but ectopic LacZ staining was detected in osteoblasts. In contrast, the 5.2-kb Enam-LacZ construct was sufficient to mimic the endogenous Enam ameloblast-specific expression pattern. Our study provides new insights into the molecular control of Enam cell- and stage-specific expression.

Distal cis-regulatory elements are required for tissue-specific expression of enamelin (Enam)

Enamel formation is orchestrated by the sequential expression of genes encoding enamel matrix proteins; however, the mechanisms sustaining the spatio-temporal order of gene transcription during amelogenesis are poorly understood. The aim of this study was to characterize the cis-regulatory sequences necessary for normal expression of enamelin (Enam). Several enamelin transcription regulatory regions, showing high sequence homology among species, were identified. DNA constructs containing 5.2 or 3.9 kb regions upstream of the enamelin translation initiation site were linked to a LacZ reporter and used to generate transgenic mice. Only the 5.2-Enam-LacZ construct was sufficient to recapitulate the endogenous pattern of enamelin tooth-specific expression. The 3.9-Enam-LacZ transgenic lines showed no expression in dental cells, but ectopic beta-galactosidase activity was detected in osteoblasts. Potential transcription factor-binding sites were identified that may be important in controlling enamelin basal promoter activity and in conferring enamelin tissue-specific expression. Our study provides new insights into regulatory mechanisms governing enamelin expression.

Chicken beta-globin insulator overcomes variegation of transgenes in Xenopus embryos

Chromatin structure and gene transcription regulation are intimately linked, and mosaic expression of randomly integrated transgenes into the genome is frequently observed. This variegation of transgene expression is likely due to the genomic integration site, which can affect the behavior of the integrated DNA sequence in a positive or a negative way. Insulators are a class of DNA elements that can protect genes from inappropriate signals emanating from their environment by acting as boundaries that prevent the spreading of nearby condensed chromatin that may otherwise silence expression. Here we show that transgenes escape this silencing in Xenopus laevis and Xenopus tropicalis embryos and that a stable, uniform, and heritable expression pattern is obtained when transgenes are flanked with tandem copies of the chicken beta-globin 5'HS4 insulator. Our data also indicate that the insulator confers copy-number-dependent transgene expression and can increase transgene expression from weak regulatory elements. Hence, it will be an invaluable tool for generating stable lines expressing different levels of a particular coding sequence.

Synaptic destabilization by neuronal Nogo-A

Formation and maintenance of a neuronal network is based on a balance between plasticity and stability of synaptic connections. Several molecules have been found to regulate the maintenance of excitatory synapses but nothing is known about the molecular mechanisms involved in synaptic stabilization versus disassembly at inhibitory synapses. Here, we demonstrate that Nogo-A, which is well known to be present in myelin and inhibit growth in the adult CNS, is present in inhibitory presynaptic terminals in cerebellar Purkinje cells at the time of Purkinje cell-Deep Cerebellar Nuclei (DCN) inhibitory synapse formation and is then downregulated during synapse maturation. We addressed the role of neuronal Nogo-A in synapse maturation by generating several mouse lines overexpressing Nogo-A, starting at postnatal ages and throughout adult life, specifically in cerebellar Purkinje cells and their terminals. The overexpression of Nogo-A induced a progressive disassembly, retraction and loss of the inhibitory Purkinje cell terminals. This led to deficits in motor learning and coordination in the transgenic mice. Prior to synapse disassembly, the overexpression of neuronal Nogo-A led to the downregulation of the synaptic scaffold proteins spectrin, spectrin-E and beta-catenin in the postsynaptic neurons. Our data suggest that neuronal Nogo-A might play a role in the maintenance of inhibitory synapses by modulating the expression of synaptic anchoring molecules.

Conditional somatic mutagenesis in the mouse using site-specific recombinases

In the last decade, site-specific recombinases (SSRs), such as Cre and Flp, have emerged as indispensable tools for the precise in vivo manipulation of the mouse genome. It is now feasible to control, in space and time, the onset of gene knockouts in almost any tissue of the mouse, thus greatly facilitating the creation of sophisticated animal models for human disease and drug development. This review describes the basic principles and current status of the SSR technology, with a focus on strategies for conditional somatic mutagenesis using the Cre/lox system and ligand-activated Cre recombinases. Practical hints for generating and analysing conditional mouse mutants will be given and exciting novel applications of the SSR technology will be discussed, such as cell fate mapping and the combined use of Cre, Flp and other biotechnological tools. It will be shown how genetic manipulation of the mouse by site-specific recombination can provide new solutions to old problems in the analysis of human physiology and pathophysiology and how it can be employed for drug discovery and development.

Evaluation of the mouse TgTP6.3 tauGFP transgene as a lineage marker in chimeras

The mouse TgTP6.3 transgene, encoding a tauGFP fusion protein, is becoming widely used but has yet to be fully characterized and evaluated as suitable lineage marker. The aim of the present study was to investigate the phenotype of TgTP6.3(+/+) homozygotes and TgTP6.3(+/-) hemizygotes, characterize the expression of the TgTP6.3 transgene in different tissues and critically evaluate its use as a lineage marker. TgTP6.3(+/+) homozygotes died between embryonic day 14.5 and weaning, whereas TgTP6.3(+/-) hemizygotes were mostly viable and fertile but smaller than non-transgenic siblings. TgTP6.3 expression began in the late two-cell stage, persisted in most fetal and adult tissues and was uniformly expressed in many (but not all) tissues. TgTP6.3(+/-) cells were readily identified in many chimeric tissues and their contribution appeared to be quantitatively and spatially normal. Overall, tauGFP expression in hemizygous TgTP6.3(+/-) cells fulfils the main criteria of a good lineage marker for many tissues. It provides a useful lineage marker, which should be particularly suitable for axons, blood vessels and pre-implantation embryos.

Quantitative determination of mosaic GFP gene expression in tobacco

A specific form of gene silencing that was observed visually as a mosaic distribution of fluorescent and non-fluorescent cells apparently dispersed at random within tissues was found in a few green fluorescent protein (GFP)-transformed tobacco lines. To characterize this event quantitatively, we studied flow cytometric measurements in GFP-expressing and -silenced cells in T1 and T2 progeny of four selected plants. The proportion of silenced cells varied considerably among the T1 lines but with notable genotype differences. Mosaic expression was inherited into the T2 generation in which the majority of progenies tested exhibited a level of silencing similar to that of their T1 parental plants. However, in some T2 progenies segregation, evident as a decrease or increase in the proportion of fluorescent cells, was observed. We discuss several factors, such as copy number, promoter activity or polyploidy, that may be the possible causes of the gene silencing, but none sufficiently explain the appearance of the mosaic distribution.

Regulation of gene expression by the CYP27B1 promoter-study of a transgenic mouse model

The enzyme 25-hydroxyvitamin D 1-hydroxylase (CYP27B1) is the rate limiting enzyme in the two-step activation process of Vitamin D to its active form 1,25-dihydroxyvitamin D (1,25D) and is located in the mitochondrial fraction of the proximal tubular cells of the kidney. More recently CYP27B1 activity and expression have also been identified in a number of non-renal cells, which is suggestive of new, previously unidentified roles for Vitamin D in the human body. Although the regulation of CYP27B1 activity and expression has been a major focus of interest over the past decades, the exact molecular mechanism behind the regulation of CYP27B1 activity and expression and the role of the CYP27B1 promoter, herein, are still poorly understood. In this study, we created a transgenic mouse model that expresses the luciferase reporter gene under the control of the full-length, 1.5kb, human CYP27B1 promoter. This animal model allows us to study in vivo the tissue-specific, CYP27B1 promoter-controlled, regulation of the expression of the CYP27B1 gene.

Intermingling versus clonal coherence during skeletal muscle development: Mosaicism in eGFP/nLacZ-labeled mouse chimeras

To study the relative contributions of clonal coherence vs. myoblast intermingling to the formation of mammalian skeletal muscles, enhanced green fluorescent protein (eGFP) and nLacZ labels were used to analyze neonatal and adult mouse embryo aggregation chimeras. The eGFP marker allowed us to estimate absolute levels of the eGFP transgenic parental contributions as well as variances of levels within a chimeric individual; nLacZ served as a counter-label in neonatal chimeras. In mature chimeric muscle, free intracellular diffusion of eGFP led to an averaging of the eGFP level in individual myofibers. Chimerism, i.e., differing eGFP levels, was evident between corresponding right and left muscles, between adjacent back, body wall, limb (shank, foot), and extraocular muscles and among myofibers within these muscles. Inhomogeneities in muscles far from their somitic origin indicated unbalanced sampling from small founder pools. The results obtained with adult eGFP<-->0 chimeras were corroborated by neonatal chimeras with complementary eGFP<-->nLacZ labels.

Transgene manipulation in zebrafish by using recombinases

Although much remains to be done, our results to date suggest that efficient and precise genome engineering in zebrafish will be possible in the future by using Cre recombinase and SB transposase in combination with their respective target sites. In this study, we provide the first evidence that Cre recombinase can mediate effective site-specific deletion of transgenes in zebrafish. We found that the efficiency of target site utilization could approach 100%, independent of whether the target site was provided transiently by injection or stably within an integrated transgene. Microinjection of Cre mRNA appeared to be slightly more effective for this purpose than microinjection of Cre-expressing plasmid DNA. Our work has not yet progressed to the point where SB-mediated mobilization of our transgene constructs would be observed. However, a recent report has demonstrated that SB can enhance transgenesis rates sixfold over conventional methods by efficiently mediating multiple single-copy insertion of transgenes into the zebrafish genome (Davidson et al., 2003). Therefore, it seems likely that a combined system should eventually allow both SB-mediated transgene mobilization and Cre-mediated transgene modification. Our goal is to validate methods for the precise reengineering of the zebrafish genome by using a combination of Cre-loxP and SB transposon systems. These methods can be used to delete, replace, or mobilize large pieces of DNA or to modify the genome only when and where required by the investigator. For example, it should be possible to deliver particular RNAi genes to well-expressed chromosomal loci and then exchange them easily with alternative RNAi genes for the specific suppression of alternative targets. As a nonviral vector for gene therapy, the transposon component allows for the possibility of highly efficient integration, whereas the Cre-loxP component can target the integration and/or exchange of foreign DNA into specific sites within the genome. The specificity and efficiency of this system also make it ideal for applications in which precise genome modifications are required (e.g., stock improvement). Future work should establish whether alternative recombination systems (e.g., phiC31 integrase) can improve the utility of this system. After the fish system is fully established, it would be interesting to explore its application to genome engineering in other organisms.

Phage integrases for the construction and manipulation of transgenic mammals

Phage integrases catalyze site-specific, unidirectional recombination between two short att recognition sites. Recombination results in integration when the att sites are present on two different DNA molecules and deletion or inversion when the att sites are on the same molecule. Here we demonstrate the ability of the phiC31 integrase to integrate DNA into endogenous sequences in the mouse genome following microinjection of donor plasmid and integrase mRNA into mouse single-cell embryos. Transgenic early embryos and a mid-gestation mouse are reported. We also demonstrate the ability of the phiC31, R4, and TP901-1 phage integrases to recombine two introduced att sites on the same chromosome in human cells, resulting in deletion of the intervening material. We compare the frequencies of mammalian chromosomal deletion catalyzed by these three integrases in different chromosomal locations. The results reviewed here introduce these bacteriophage integrases as tools for site-specific modification of the genome for the creation and manipulation of transgenic mammals.

Evidence for astrocyte heterogeneity: a distinct subpopulation of protoplasmic-like glial cells is detected in transgenic mice expressing Lmo1-lacZ

The adult mammalian central nervous system (CNS) contains a large number of different cell types, which arise from the ventricular (VZ) and subventricular zones during embryonic development. In this study, we used a transgenic mouse expressing Lmo1-LacZ from a randomly inserted promoter/reporter gene construct to identify a glial subpopulation. LMO1 is an LIM domain-containing protein, thought to act in protein-protein interactions. We found first that in the adult transgenic CNS, beta-galactosidase (beta-gal) was expressed in a specific subpopulation of protoplasmic-like cells, which did not express detectable levels of glial fibrilary acidic protein unless a lesion was produced. Secondly, during development, beta-gal(+) cells were found arising from discrete regions of the VZ. Taken together, these results identify a subpopulation of protoplasmic glial cells in the adult CNS and suggest that they arise from a restricted VZ region during CNS development.

Tetracycline-inducible system for regulation of skeletal muscle-specific gene expression in transgenic mice

Tightly regulated control of over-expression is often necessary to study one aspect or time point of gene function and, in transgenesis, may help to avoid lethal effects and complications caused by ubiquitous over-expression. We have utilized the benefits of an optimized tet-on system and a modified muscle creatine kinase (MCK) promoter to generate a skeletal muscle-specific, doxycycline (Dox) controlled over-expression system in transgenic mice. A DNA construct was generated in which the codon optimized reverse tetracycline transactivator (rtTA) was placed under control of a skeletal muscle-specific version of the mouse MCK promoter. Transgenic mice containing this construct expressed rtTA almost exclusively in skeletal muscles. These mice were crossed to a second transgenic line containing a bi-directional promoter centered on a tet responder element driving both a luciferase reporter gene and a tagged gene of interest; in this case the calpain inhibitor calpastatin. Compound hemizygous mice showed high level, Dox dependent muscle-specific luciferase activity often exceeding 10,000-fold over non-muscle tissues of the same mouse. Western and immunocytochemical analysis demonstrated similar Dox dependent muscle-specific induction of the tagged calpastatin protein. These findings demonstrate the effectiveness and flexibility of the tet-on system to provide a tightly regulated over-expression system in adult skeletal muscle. The MCKrtTA transgenic lines can be combined with other transgenic responder lines for skeletal muscle-specific over-expression of any target gene of interest.

Of mice and models: improved animal models for biomedical research

The ability to engineer the mouse genome has profoundly transformed biomedical research. During the last decade, conventional transgenic and gene knockout technologies have become invaluable experimental tools for modeling genetic disorders, assigning functions to genes, evaluating drugs and toxins, and by and large helping to answer fundamental questions in basic and applied research. In addition, the growing demand for more sophisticated murine models has also become increasingly evident. Good state-of-principle knowledge about the enormous potential of second-generation conditional mouse technology will be beneficial for any researcher interested in using these experimental tools. In this review we will focus on practice, pivotal principles, and progress in the rapidly expanding area of conditional mouse technology. The review will also present an internet compilation of available tetracycline-inducible mouse models as tools for biomedical research (http://www.zmg.uni-mainz.de/tetmouse/).

When reverse genetics meets physiology: the use of site-specific recombinases in mice

The use of site-specific recombinases enables the precise introduction of defined genetic mutations into the mouse genome. In theory, any deletion, point mutation, inversion or translocation can be modeled in mice. Because gene targeting is controlled both spatially and temporally, the function of a given gene can be studied in the desired cell types and at a specific time point. This 'genetic dissection' allows to define gene function in development, physiology or behavior. In this review, we focus on the technical possibilities of Cre and other site-specific recombinases but also discuss their limitations.

Position-independent and tissue-specific expression of porcine whey acidic protein gene from a bacterial artificial chromosome in transgenic mice

Silencing of transgenes is a frequent event after the random integration of foreign DNA in the host genome following microinjection. Long genomic fragments are expected to contain all the regulatory elements necessary to induce an appropriate expression of transgenes. A bacterial artificial chromosome containing the porcine wap gene with approximately 145 and 5 kb of 5'- and 3'-flanking sequences, respectively, was microinjected into fertilized mouse ovocytes. In the six transgenic lines studied, expression was strictly specific to the mammary gland of lactating animals and was position-independent. Levels of exogenous porcine wap mRNA per copy compared favorably with the porcine wap mRNA yield in the mammary gland of a 9-day lactating pig. These findings suggest that this insert contained most if not all of the cis-acting elements involved in the full specific expression of the porcine wap gene. These elements constitute good candidates for directing the optimized expression of protein recombinant-encoding genes in the mammary gland of lactating animals.

Prosaposin: threshold rescue and analysis of the "neuritogenic" region in transgenic mice

Prosaposin is the precursor of four glycoprotein activators (saposins) for lysosomal hydrolases. Intact prosaposin also has lipid transfer properties in vitro as well as neuritogenic effects ex vivo and in vivo. Such "neuritogenic" effects of saposin C were evaluated in vivo using transgenic mice with prosaposin cDNAs having normal (PS-N) or mutated neuritogenic region. The mutant prosaposin cDNA (PS-CBC) encoded a chimeric saposin C that contained the non-neuritogenic sequence of saposin B, but retained acid beta-glucosidase (GCase) activation effects. When driven by the PGK (3-phosphoglycerate kinase) promoter, transgene expression was highest in the cerebrum for any of the transgenes (range from 15% to 42% of wild-type). Low levels were in visceral tissues. Prosaposin knock-out (PS-/-) mice expressing N or CBC transgenes, even at low levels, had delayed onset of neurologic signs and neuropathology, and significant lengthening of life span (from 1.7- to 7-fold) with age dependent partial correction of GlcCer and LacCer accumulation in the brain. Neuropathologic progression and neuronal glycosphingolipid storage were related directly to the transgene expression levels in the brain. Purkinje cell loss was age dependent. Gross brain and neuronal organizations were indistinguishable in PS-/- mice with or without the various transgenes, albeit the phenotype appeared later in the mice with transgenes. These studies show the degree of neuropathologic manifestations in each transgenic line depended on expression level rather than on the nature of the transgene. These studies also show in vivo localization of the GCase activation region to the carboxy terminal half of saposin C and the lack of a significant gross trophic effect of saposin C on CNS organization in vivo.

Multiple effects of genetic background on variegated transgene expression in mice

BLG/7 transgenic mice express an ovine beta-lactoglobulin transgene during lactation. Unusually, transgene expression levels in milk differ between siblings. This variable expression is due to variegated transgene expression in the mammary gland and is reminiscent of position-effect variegation. The BLG/7 line was created and maintained on a mixed CBA x C57BL/6 background. We have investigated the effect on transgene expression of backcrossing for 13 generations into these backgrounds. Variable transgene expression was observed in all populations examined, confirming that it is an inherent property of the transgene array at its site of integration. There were also strain-specific effects on transgene expression that appear to be independent of the inherent variegation. The transgene, compared to endogenous milk protein genes, is specifically susceptible to inbreeding depression. Outcrossing restored transgene expression levels to that of the parental population; thus suppression was not inherited. Finally, no generation-dependent decrease in mean expression levels was observed in the parental population. Thus, although the BLG/7 transgene is expressed in a variegated manner, there was no generation-associated accumulated silencing of transgene expression.

Effects of mouse strain, position of integration and tetracycline analogue on the tetracycline conditional system in transgenic mice

The tetracycline conditional system is a very powerful method for achieving control of gene expression in transgenic mice, allowing one to turn expression both off and on in the same animal. We have used it to make a tissue-specific transgenic mouse model of Charcot-Marie-Tooth disease type 1A. This disease is most commonly caused by overexpression of peripheral myelin protein 22 (PMP22) in Schwann cells of the peripheral nervous system. Here we describe the effects of position of integration of the transgene, tetracycline analogue and mouse strain in this model. The small transgenes used to express tTA, the LacZ reporter and the pmp22 cDNA were all very dependent on the position of integration with few of the transgenic lines working successfully. In contrast, the single transgenic made with the 560 kb yeast artificial chromosome construct containing the tTA open reading frame worked well. Tetracycline was found to be cleared from mice relatively fast in comparison with doxycycline and is thus useful if one wants to switch on gene expression after extended periods of administration. Finally, the initial litters were on a mixed genetic background and the level of LacZ or pmp22 expression was very variable between mice. We found that expression became uniform between mice, and occurred in a higher proportion of cells, when the transgenes were crossed onto the CBA/Ca background in comparison with the C57BL/6J background.

Site- and time-specific gene targeting in the mouse

The efficient introduction of somatic mutations in a given gene, at a given time, in a specific cell type, will facilitate studies of gene function and the generation of animal models for human diseases. We have established a conditional site-specific recombination system in mice using a new version of the Cre/lox system. The Cre recombinase has been fused to a mutated ligand binding domain of the human estrogen receptor (ER), resulting in a tamoxifen-dependent Cre recombinase, Cre-ER(T), that is activated by tamoxifen, but not by estradiol. Transgenic mice were generated expressing Cre-ER(T) under the control of a cytomegalovirus promoter. Administration of tamoxifen to these transgenic mice induced excision of a chromosomally integrated gene flanked by loxP sites in a number of tissues, whereas no excision could be detected in untreated animals. However, the efficiency of excision varied between tissues, and the highest level (approximately 40%) was obtained in the skin. To determine the efficiency of excision mediated by Cre-ER(T) in a given cell type, Cre-ER(T)-expressing mice were crossed with reporter mice in which expression of Escherichia coli beta-galactosidase can be induced through Cre-mediated recombination. The efficiency and kinetics of this recombination were analyzed at the cellular level in the epidermis of 6- to 8-week-old double transgenic mice. Site-specific excision occurred within a few days of tamoxifen treatment in essentially all epidermis cells expressing Cre-ER(T). These results indicate that cell-specific expression of Cre-ER(T) in transgenic mice can be used for efficient tamoxifen-dependent Cre-mediated recombination at loci containing loxP sites, to generate site-specific somatic mutations in a spatiotemporally controlled manner. This conditional site-specific recombination system should allow the analysis of knockout phenotypes that cannot be addressed by conventional gene targeting.

Position effects are influenced by the orientation of a transgene with respect to flanking chromatin

We have inserted two expression cassettes at tagged reference chromosomal sites by using recombinase-mediated cassette exchange in mammalian cells. The three sites of integration displayed either stable or silencing position effects that were dominant over the different enhancers present in the cassettes. These position effects were strongly dependent on the orientation of the construct within the locus, with one orientation being permissive for expression and the other being nonpermissive. Orientation-specific silencing, which was observed at two of the three site tested, was associated with hypermethylation but not with changes in chromatin structure, as judged by DNase I hypersensitivity assays. Using CRE recombinase, we were able to switch in vivo the orientation of the transgenes from the permissive to the nonpermissive orientation and vice versa. Switching from the permissive to the nonpermissive orientation led to silencing, but switching from the nonpermissive to the permissive orientation did not lead to reactivation of the transgene. Instead, transgene expression occurred dynamically by transcriptional oscillations, with 10 to 20% of the cells expressing at any given time. This result suggested that the cassette had been imprinted (epigenetically tagged) while it was in the nonpermissive orientation. Methylation analysis revealed that the methylation state of the inverted cassettes resembled that of silenced cassettes except that the enhancer had selectively lost some of its methylation. Sorting of the expressing and nonexpressing cell populations provided evidence that the transcriptional oscillations of the epigenetically tagged cassette are associated with changes in the methylation status of regulatory elements in the transgene. This suggests that transgene methylation is more dynamic than was previously assumed.

Copy number related transgene expression and mosaic somatic expression in hemizygous and homozygous transgenic tilapia (Oreochromis niloticus)

Three lines of transgenic tilapia (Oreochromis niloticus) fish were generated with a construct containing a lacZ reporter gene spliced to a 4.7 kb 5' regulatory region of a carp beta actin gene. All these three lines contain different copy numbers of transgenes and the levels of lacZ expression were found to be related to transgene copy number. Mosaic patterns of somatic lacZ expression were observed in these three lines which differed between lines but were consistent within a line. We also observed that expression of the reporter gene in homozygous transgenic fish was approximately two-fold greater than in the hemizygous transgenics. Analysis of expression of the reporter gene on a tissue-to-tissue basis demonstrated that lacZ expression of the reporter gene in stably transformed fish occured with variable intensity in different organs and tissues and was also sometimes variable in different cells of the same tissue in Gland G2 generations of the transgenic lines.

Temporally controlled somatic mutagenesis in smooth muscle

Ligand-dependent site-specific recombinases are powerful tools to engineer the mouse genome in specific somatic cell types at selected times during pre- and postnatal development. Current efforts are primarily directed towards increasing the efficiency of this recombination system in mice. We have generated transgenic mouse lines expressing a tamoxifen-activated Cre recombinase, CreER(T2), under the control of the smooth muscle-specific SM22 promoter. Both a randomly integrated transgene [SM-CreER(T2)(tg)] and a transgene that has been "knocked in" into the endogenous SM22 locus [SM-CreER(T2)(ki)] were expressed in smooth muscle-containing tissues. The level of CreER(T2) expression and tamoxifen-induced recombination was lower in SM-CreER(T2)(tg) mice compared with SM-CreER(T2)(ki) mice. Whereas no recombinase activity could be detected in vehicle-treated SM-CreER(T2)(ki) mice, administration of tamoxifen induced the excision of a loxP-flanked reporter transgene in up to 100% of smooth muscle cells. The recombined genome persisted for at least four months after tamoxifen treatment. SM-CreER(T2)(ki) transgenic mice should be useful to study the effects of various somatic mutations in smooth muscle.

Stable and full rescue of the pigmentation in a medaka albino mutant by transfer of a 17 kb genomic clone containing the medaka tyrosinase gene

In the medaka Oryzias latipes, several albino strains have mutations in the tyrosinase gene that have been fully characterized at the molecular level. A genomic clone from wild-type medaka containing the 5 kb tyrosinase gene with its five exons, 10 kb of upstream sequences and 2 kb downstream sequences was introduced into fertilized eggs from a tyrosinase-negative albino strain. We show that the injection of this genomic clone predominantly conferred mosaic expression ending before the hatching stage. A minority of juveniles retained a variable number of pigmented cells, including four individuals keeping one pigmented eye through adulthood. Two of these could be mated, and one of these transmitted the transgene resulting in complete rescue of pigmentation to 16% of its offspring. The resulting transgenic line harbors a single copy of the wild-type tyrosinase gene and all fish are wild-type with respect to pigmentation. These experiments suggest that the tyrosinase genomic clone, or a future shorter version of it, can be used in fish to routinely detect transgenic lines. The apparent faithful and systematic expression of the tyrosinase transgene is most probably due to the presence of a locus control region (LCR) in the injected clone.

The use of yeast artificial chromosomes in transgenic animals: expression studies of the tyrosinase gene in transgenic mice

Variegation and inherited somatic mosaicism has been observed in transgenic mice carrying yeast artificial chromosomes (YACs) in which a DNAse I hypersensitive site (HS) located -12 kb upstream of the mouse tyrosinase gene had been deleted. At present, we are generating new transgenic animals with minor deletions of the HS.

Position-independent expression of transgenes in zebrafish

The variability in expression patterns of transgenes, caused by the influence of neighboring chromatin, is called 'position effect'. Border elements are DNA sequences, which have the ability to alleviate position effects. The abilities of two types of border elements, scs/scs' from the D. melanogaster 87A7 heat shock locus and the A-element from the chicken lysozyme gene, to protect transgenes from position effects were quantified in developing zebrafish embryos. The transgenic construct used was FV3CAT, which consists of the carp beta-actin transcriptional regulatory region, the chloramphenicol acetyltransferase (CAT) gene and the 3'-untranslated region from the Chinook salmon growth hormone gene. FV3CAT constructs flanked by either scs/scs'-elements or A-elements were introduced into zebrafish chromosomes and the spatial and temporal expression patterns of the transgenes were quantified in multiple generations of transgenic zebrafish. Levels of transgene expression were uniform in the pre-differentiated and fully differentiated populations of cells present during embryonic development. Levels of transgene expression were proportional to the numbers of integrated transgenes. Expression of transgenes per cell varied less than two-fold in different transgenic lines. Both types of border elements were able to prevent the influences of neighboring chromatin on transgene expression through three generations of fish. The results are consistent with the ability of border elements to function with equal efficiencies in the many cell types found in vertebrates. Thus, inclusion of border elements in genetic constructs can provide reliable and reproducible levels of gene expression in multiple lines of fish.

Heterochromatic silencing of Drosophila heat shock genes acts at the level of promoter potentiation

In a variety of organisms, genes placed near heterochromatin are transcriptionally silenced. In order to understand the molecular mechanisms responsible for this block in transcription, high resolution in vivo chromatin structure analysis was performed on two heat shock genes, hsp26 and hsp70. These genes normally reside in euchromatin where GAGA factor and RNA Pol II are present on the promoter prior to heat shock induction. P-element transformation experiments led to the identification of stocks in which these two genes were inserted within heterochromatin of the chromosome 4 telomeric region. These transgenes exhibit silencing that is partially suppressed by mutations in the gene encoding HP1. Micrococcal nuclease analysis revealed that the heterochromatic transgenes were packaged in a more regular nucleosome array than when located in euchromatin. High resolution DNase I analysis demonstrated that GAGA factor and TFIID were not associated with these promoters in heterochromatin; potassium permanganate experiments showed a loss of Pol II association. Taken together, these data suggest that occlusion of trans-acting factors from their cis- acting regulatory elements leading to a block in promoter potentiation is a mechanism for heterochromatin gene silencing.

Cystic malformation of the posterior cerebellar vermis in transgenic mice that ectopically express Engrailed-1, a homeodomain transcription factor

In WEXPZ-En-1 transgenic mice, Engrailed-1, a homeodomain-containing transcription factor, is ectopically expressed in the developing brain under control of the Wnt-1 enhancer. En-1 is a developmental regulatory control gene which has an essential role in the formation of the midbrain and cerebellum. Approximately 28% of WEXPZ-En-1 + mice develop cystic malformations of the posterior lobe of the cerebellar vermis, fourth ventricular dilatation, and postnatal hydrocephalus. These anatomic features are also found among the spectrum of posterior fossa malformations in humans. Expression characteristics of the WEXP transgene suggest that the neuropathology observed in WEXPZ-En-1+ mice stems from overexpression of En-1 during fetal and neonatal phases of cerebellar development. These observations raise the possibility that abnormal regulation of Engrailed genes, or targets of Engrailed, may be involved in the pathogenesis of cystic central nervous system malformations of the posterior fossa in humans.

In vivo and in vitro expression of human serum albumin genomic sequences in mammary epithelial cells with beta-lactoglobulin and whey acidic protein promoters

The expression pattern of human serum albumin (HSA) in transgenic mice carrying various HSA genomic sequences driven either by the mouse whey acidic protein (WAP) or the sheep beta-lactoglobulin (BLG) promoters, was compared. The pattern of HSA expression in both WAP/HSA and BLG/HSA transgenic lines was copy number independent, and the major site of ectopic expression was the skeletal muscle. Although an equal proportion of expressors was determined in both sets of mice (approximately 25% secreting >0.1 mg/ml), the highest level of HSA secreted into the milk in the WAP/HSA transgenic lines was one order of magnitude lower than in the BLG/HSA lines. Despite this difference, the HSA expression patterns in the mammary gland were similar and consisted of two levels of variegated expression. Studies using mammary explant cultures revealed a comparable responsiveness to the lactogenic hormones insulin, hydrocortisone, and prolactin, although the WAP/HSA gene constructs were more sensitive to the hydrocortisone effect than were the BLG/HSA vectors. When HSA vectors were stably transfected into the mouse mammary cell line CID-9, they displayed a hierarchy of expression, dependent upon the specific complement of HSA introns included. Nevertheless, the expression of HSA in four out of five WAP/HSA constructs was similar to their BLG/HSA counterparts. This construct-dependent, and promoter-independent, hierarchy was also found following transfection into the newly established Golda-1 ovine mammary epithelial cell line.