Multiplex gene regulation: a two-tiered approach to transgene regulation in transgenic mice

Identification of a Chondrocyte-Specific Enhancer in the Hoxc8 Gene

Hox genes encode transcription factors whose roles in patterning animal body plans during embryonic development are well-documented. Multiple studies demonstrate that Hox genes continue to act in adult cells, in normal differentiation, in regenerative processes, and, with abnormal expression, in diverse types of cancers. However, surprisingly little is known about the regulatory mechanisms that govern Hox gene expression in specific cell types, as they differentiate during late embryonic development, and in the adult organism. The murine Hoxc8 gene determines the identity of multiple skeletal elements in the lower thoracic and lumbar region and continues to play a role in the proliferation and differentiation of cells in cartilage as the skeleton matures. This study was undertaken to identify regulatory elements in the Hoxc8 gene that control transcriptional activity, specifically in cartilage-producing chondrocytes. We report that an enhancer comprising two 416 and 224 bps long interacting DNA elements produces reporter gene activity when assayed on a heterologous transcriptional promoter in transgenic mice. This enhancer is distinct in spatial, temporal, and molecular regulation from previously identified regulatory sequences in the Hoxc8 gene that control its expression in early development. The identification of a tissue-specific Hox gene regulatory element now allows mechanistic investigations into Hox transcription factor expression and function in differentiating cell types and adult tissues and to specifically target these cells during repair processes and regeneration.

Bringing Light to Transcription: The Optogenetics Repertoire

The ability to manipulate expression of exogenous genes in particular regions of living organisms has profoundly transformed the way we study biomolecular processes involved in both normal development and disease. Unfortunately, most of the classical inducible systems lack fine spatial and temporal accuracy, thereby limiting the study of molecular events that strongly depend on time, duration of activation, or cellular localization. By exploiting genetically engineered photo sensing proteins that respond to specific wavelengths, we can now provide acute control of numerous molecular activities with unprecedented precision. In this review, we present a comprehensive breakdown of all of the current optogenetic systems adapted to regulate gene expression in both unicellular and multicellular organisms. We focus on the advantages and disadvantages of these different tools and discuss current and future challenges in the successful translation to more complex organisms.

New MiniPromoter Ple345 (NEFL) Drives Strong and Specific Expression in Retinal Ganglion Cells of Mouse and Primate Retina

Retinal gene therapy is leading the neurological gene therapy field, with 32 ongoing clinical trials of recombinant adeno-associated virus (rAAV)–based therapies. Importantly, over 50% of those trials are using restricted promoters from human genes. Promoters that restrict expression have demonstrated increased efficacy and can limit the therapeutic to the target cells thereby reducing unwanted off-target effects. Retinal ganglion cells are a critical target in ocular gene therapy; they are involved in common diseases such as glaucoma, rare diseases such as Leber's hereditary optic neuropathy, and in revolutionary optogenetic treatments. Here, we used computational biology and mined the human genome for the best genes from which to develop a novel minimal promoter element(s) designed for expression in restricted cell types (MiniPromoter) to improve the safety and efficacy of retinal ganglion cell gene therapy. Gene selection included the use of the first available droplet-based single-cell RNA sequencing (Drop-seq) dataset, and promoter design was bioinformatically driven and informed by a wide range of genomics datasets. We tested seven promoter designs from four genes in rAAV for specificity and quantified expression strength in retinal ganglion cells in mouse, and then the single best in nonhuman primate retina. Thus, we developed a new human-DNA MiniPromoter, Ple345 (NEFL), which in combination with intravitreal delivery in rAAV9 showed specific and robust expression in the retinal ganglion cells of the nonhuman-primate rhesus macaque retina. In mouse, we also developed MiniPromoters expressing in retinal ganglion cells, the hippocampus of the brain, a pan neuronal pattern in the brain, and peripheral nerves. As single-cell transcriptomics such as Drop-seq become available for other cell types, many new opportunities for additional novel restricted MiniPromoters will present.

From adjacent activation in Escherichia coli and DNA cyclization to eukaryotic enhancers: the elements of a puzzle

Deoxyribonucleic acid cyclization, Escherichia coli lac repressor binding to two spaced lac operators and repression enhancement can be successfully used for a better understanding of the conditions required for interaction between eukaryotic enhancers and the machinery of transcription initiation. Chronologically, the DNA looping model has first accounted for the properties initially defining enhancers, i.e., independence of action with distance or orientation with respect to the start of transcription. It has also predicted enhancer activity or its disruption at short distance (site orientation, alignment between promoter and enhancer sites), with high-order complexes of protein, or with transcription factor concentrations close or different from the wild-type situation. In another step, histones have been introduced into the model to further adapt it to eukaryotes. They in fact favor DNA cyclization in vitro. The resulting DNA compaction might explain the difference counted in base pairs in the distance of action between eukaryotic transcription enhancers and prokaryotic repression enhancers. The lac looping system provides a potential tool for analysis of this discrepancy and of chromatin state directly in situ. Furthermore, as predicted by the model, the contribution of operators O2 and O3 to repression of the lac operon clearly depends on the lac repressor level in the cell and is prevented in strains overproducing lac repressor. By extension, gene regulation especially that linked to cell fate, should also depend on transcription factor levels, providing a potential tool for cellular therapy. In parallel, a new function of the O1–O3 loop completes the picture of lac repression. The O1–O3 loop would at the same time ensure high efficiency of repression, inducibility through the low-affinity sites and limitation of the level of repressor through self-repression of the lac repressor. Last, the DNA looping model can be successfully adapted to the enhancer auxiliary elements known as insulators.

Transgenic studies on homeobox genes in nervous system development: spina bifida in Isl1 transgenic mice

To develop in vivo assays for homeobox gene function in neural development, we generated transgenic mice in which the expression of a homeobox gene is altered only within the nervous system, in neurons or neuronal precursor cells. Transgenic expression of Hoxc8 did not result in gross abnormalities, while a Hoxd4 transgene caused death shortly after birth. In neural progenitor cells, the motorneuron-specific homeodomain transcription factor Isl1 induced early developmental defects, including absence of anterior neural structures, profound defects in the neuroepithelium and defective neural tube closure. A fraction of Isl1 transgenic mice exhibited spina bifida. Isl1 transgene expression was also associated with decreased proliferation and increased Pbx1 expression in the ventral neural tube. Our results suggest a function for some homeobox genes in development of the nervous system, and that cell-type- and region-specific transgenic models will be useful to identify the cellular and molecular targets of homeobox transcription factors in nervous system development.

Genetic Manipulation of Mammary Gland Development and Lactation

The mammalian genome is believed to contain some 30,000 to 40,000 different genes. Of these an estimated 42% have no known function. Genetically engineered mouse models (GEMM) have been a powerful tool available for determining gene function in vivo. In the mammary gland, a variety of genetic engineering approaches have been applied successfully to understanding the importance of specific gene products to mammary gland development and lactation. Our own laboratory has applied genetically engineered mice to facilitate understanding of the regulation of mammary gland development and lactation by insulin-like growth factors (IGF) and by the transcription factor, upstream stimulatory factor (USF-2). Our studies on transgenic mice that overexpress IGF-I have demonstrated the importance of IGF-dependent signaling pathways to maintenance of mammary epithelial cells during the declining phase of lactation. Our analysis of early developmental processes in mammary tissue from mice that carry a targeted mutation in the IGF-I receptor gene suggests that IGF-dependent stimulation of cell cycle progression is more important to early mammary gland development than potential antiapoptotic effects. Lastly, our studies on mice that carry a targeted mutation of the Usf2 gene have demonstrated that this gene is necessary for normal lactation and have highlighted the importance of this gene to the maintenance of protein synthesis. These studies, as well as studies of others, have highlighted both the strengths and limitations inherent in the use of GEMM. Limitations serve as the driving force behind development of new experimental strategies and genetic engineering schemes that will allow for a full understanding of gene function within the mammary gland.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Genetic Models in Applied Physiology. Functional genomics in the mouse: powerful techniques for unraveling the basis of human development and disease

Now that near-complete DNA sequences of both the mouse and human genomes are available, the next major challenge will be to determine how each of these genes functions, both alone and in combination with other genes in the genome. The mouse has a long and rich history in biological research, and many consider it a model organism for the study of human development and disease. Over the past few years, exciting progress has been made in developing techniques for chromosome engineering, mutagenesis, mapping and maintenance of mutations, and identification of mutant genes in the mouse. In this mini-review, many of these powerful techniques will be presented along with their application to the study of development, physiology, and disease.

Recent advances in inducible expression in transgenic mice

In order to accurately analyze gene function in transgenic mice, as well as to generate credible murine models of human diseases, the ability to regulate temporal- and spatial-specific expression of target genes is absolutely critical. Pioneering work in inducible transgenics, begun in the 1980s and continuing to the present, has led to the development of a variety of different inducible systems dedicated to this goal, the shared basis of which is the accurate conditional expression of a given transgene. Recent advances in inducible transgene expression in mice are discussed.

Use of genetically engineered mice in drug discovery and development: wielding Occam's razor to prune the product portfolio

Genetically engineered mice (GEMs) that either overexpress (transgenic) or lack (gene-targeted, or "knock-out") genes are used increasingly in industry to investigate molecular mechanisms of disease, to evaluate innovative therapeutic targets, and to screen agents for efficacy and/or toxicity. High throughput GEM construction in drug discovery and development (DDD) serves two main purposes: to test whether a given gene participates in a disease condition, or to determine the function(s) of a protein that is encoded by an expressed sequence tag (EST, an mRNA fragment for a previously uncharacterized protein). In some instances, phenotypes induced by such novel GEMs also may yield clues regarding potential target organs and toxic effects of potential therapeutic molecules. The battery of tests used in phenotypic analysis of GEMs varies between companies, but the goal is to define one or more easily measured endpoints that can be used to monitor the disease course--especially during in vivo treatment with novel drug candidates. In many DDD projects, overt phenotypes are subtle or absent even in GEMs in which high-level expression or total ablation of an engineered gene can be confirmed. This outcome presents a major quandary for biotechnology and pharmaceutical firms: given the significant expense and labor required to generate GEMs, what should be done with "negative" constructs? The 14th century philosophical principle known as Occam's razor-that the simplest explanation for a phenomenon is likely the truth-provides a reasonable basis for pruning potential therapeutic molecules and targets. In the context of DDD, Occam's razor may be construed to mean that correctly engineered GEMs lacking obvious functional or structural phenotypes have none because the affected gene is not uniquely essential to normal homeostasis or disease progression. Thus, a "negative" GEM construct suggests that the gene under investigation encodes a ligand or target molecule without significant therapeutic potential. This interpretation indicates that, at least in a market-driven industrial setting, such "negative" projects should be pruned aggressively so that resources may be redirected to more promising DDD ventures.

Conditional control of gene expression in the mouse

One of the most powerful tools that the molecular biology revolution has given us is the ability to turn genes on and off at our discretion. In the mouse, this has been accomplished by using binary systems in which gene expression is dependent on the interaction of two components, resulting in either transcriptional transactivation or DNA recombination. During recent years, these systems have been used to analyse complex and multi-staged biological processes, such as embryogenesis and cancer, with unprecedented precision. Here, I review these systems and discuss certain studies that exemplify the advantages and limitations of each system.

Targeted gene regulation and gene ablation

Investigations of the mechanisms involved in appropriate, developmentally regulated tissue-specific gene transcription have laid the foundations for transgenic and gene-therapy technologies directing specific induction or ablation of genes of interest in a tissue-restricted manner. This technology has further evolved to allow for temporal control of gene expression and ablation. Genes can now be switched on and off or be ablated by administering exogenous compounds. These technologies are based on the development of ligand-inducible transcription factors or recombinases that regulate gene expression or ablation by the administration of specific ligands and should lead to animal models that are better suited for investigating the molecular basis of human disease. This review describes the evolution, components and applications of systems that are currently being employed in transgenic and mutant-mouse technology for the conditional regulation of gene expression and ablation.

Retrograde axonal transport of herpes simplex virus: evidence for a single mechanism and a role for tegument

Herpes simplex virus type I (HSV) typically enters peripheral nerve terminals and then travels back along the nerve to reach the neuronal cell body, where it replicates or enters latency. To monitor axoplasmic transport of HSV, we used the giant axon of the squid, Loligo pealei, a well known system for the study of axoplasmic transport. To deliver HSV into the axoplasm, viral particles stripped of their envelopes by detergent were injected into the giant axon, thereby bypassing the infective process. Labeling the viral tegument protein, VP16, with green fluorescent protein allowed viral particles moving inside the axon to be imaged by confocal microscopy. Viral particles moved 2.2 +/- 0.26 micrometer/sec in the retrograde direction, a rate comparable to that of the transport of endogenous organelles and of virus in mammalian neurons in culture. Electron microscopy confirmed that 96% of motile (stripped) viral particles had lost their envelope but retained tegument, and Western blot analysis revealed that these particles had retained protein from capsid but not envelope. We conclude that (i) HSV recruits the squid retrograde transport machinery; (ii) viral tegument and capsid but not envelope are sufficient for this recruitment; and (iii) the giant axon of the squid provides a unique system to dissect the viral components required for transport and to identify the cellular transport mechanisms they recruit.

Developmental characterization and chromosomal mapping of a LacZ transgene expressed in the mouse apical ectodermal ridge

The apical ectodermal ridge (AER) has an essential role in limb morphogenesis involving the specification of the proximal-distal axis of the limb. During the analysis of transgenic mice that harbor a LacZ transgene, we detected strong expression of beta-galactosidase within the AER of developing embryos. In this mouse line, called Z16, the bacterial LacZ gene is linked to a Herpes simplex virus immediate early promoter that is normally silent in mice. Embryos from other independent mouse lines harboring the same DNA construct exhibited no AER specific staining. Thus, it appears that the LacZ transgene in the Z16 line is expressed in the AER in response to regulatory influences from genomic DNA flanking the integration site. By fluorescent in situ hybridization, the transgene insertion site was mapped to chromosome 12. Hemizygous and homozygous transgenic mice appear normal and are fertile. AER specific beta-galactosidase staining was detected by 9.5 days post coitum in the forelimb and hindlimb bud. beta-galactosidase staining could be seen throughout the development of the limbs up to 14.5 days post coitum when expression was restricted to the distal-most regions of the digits of the hindlimbs. The loss of beta-galactosidase staining between digits correlated with the onset of programmed cell death, or apoptosis, in the digit interzones. LacZ expression in this transgenic line represents a useful marker for studying AER function in limb specification during mouse embryogenesis.

Sustained mammary gland-directed, ponasterone A-inducible expression in transgenic mice

The ability to regulate temporal- and spatial-specific expression of target genes in transgenic mice will facilitate analysis of gene function and enable the generation of murine models of human diseases. The genetic analysis of mammary gland tumorigenesis requires the development of mammary gland-specific transgenics, which are tightly regulated throughout the adult mammary epithelium. Analysis of genes implicated in mammary gland tumorigenesis has been hampered by mosaic transgene expression and the findings that homozygous deletion of several candidate genes (cyclin D1, Stat5A, prolactin receptor) abrogates normal mammary gland development. We describe the development of transgenic mouse lines in which sustained transgene expression was inducibly regulated, both specifically and homogeneously, in the adult mammary gland epithelium. Transgenes encoding RXRalpha and a chimeric ecdysone receptor under control of a modified MMTV-LTR, which targets mammary gland expression, were used. These transgenic 'receptor' lines were crossed with transgenic 'enhancer' lines in which the ecdysone/RXR binding site induced ligand-dependent expression of transgenic beta-galactosidase. Pharmacokinetic analysis of a highly bioactive ligand (ponasterone A), identified through screening ecdysteroids from local plants, demonstrated sustained release and transgene expression in vivo. This transgenic model with both tightly regulated and homogeneous transgene expression, which was sustained in vivo using ligands readily extracted from local flora, has broad practical applicability for genetic analysis of mammary gland disease.

Neural precursor cells for delivery of replication-conditional HSV-1 vectors to intracerebral gliomas

Cellular delivery of a replication-conditional herpes simplex virus type 1 (HSV-1) vector provides a means for gene therapy of invasive tumor cells. LacZ-bearing neural precursor cells, which can migrate and differentiate in the brain, were infected with a ribonucleotide reductase-deficient HSV-1 mutant virus (rRp450) that replicates only in dividing cells. Replication of rRp450 in neural precursor cells was blocked prior to implantation into the tumor by growth arrest in late G1 phase through treatment with mimosine. Viral titers in the medium of mimosine-treated, rRp450-infected neural precursor cells were below detection levels 3 days after infection. In culture, after removal of mimosine and passaging, cells resumed growth and replication of rRp450 so that, 7 days later, virus was present in the medium and cell death was evident. Mimosine-treated neural precursor cells injected into established intracerebral CNS-1 gliomas in nude mice migrated extensively throughout the tumor and into the surrounding parenchyma beyond the tumor over 3 days. Mimosine-treated neural precursor cells, infected with rRp450 and injected into intracerebral CNS-1 tumors, also migrated within the tumor with the appearance of foci of HSV-thymidine kinase-positive (TK+) cells, presumably including tumor cells, distributed throughout the tumor and in the surrounding parenchyma over a similar period. This migratory cell delivery method has the potential to expand the range of delivery of HSV-1 vectors to tumor cells in the brain.

Herpes simplex virus transcriptional activator VP16 is detrimental to preimplantation development in mice

The herpes simplex virus transactivator protein VP16 is frequently used to regulate gene expression in several experimental systems, including transgenic mice. It has been suggested that high levels of VP16 expression in mice may be lethal. In order to systematically address this issue, we linked the VP16 gene to promoters that are active early and in a variety of tissues throughout development, such as the human beta-actin promoter or the rat nestin gene enhancer. VP16 expression was assayed using a LacZ reporter gene linked to a VP16-responsive immediate early gene promoter. We show here that expression of VP16 at high levels is detrimental to pre-implantation development. By culturing embryos in vitro, we demonstrate that this effect is exerted at the transition from the 2-cell to the 4-cell stage, reducing survival to the blastocyst stage dramatically. On the other hand, transgenic mice expressing VP16 transgenes at postimplantation stages are viable. These results suggest a differential sensitivity to VP16 expression in different cell types and stages of development. The reduction of embryo survival by VP16 implicates herpes virus infection as a potential cause of infertility.

Evidence for regulation of cartilage differentiation by the homeobox gene Hoxc-8

Homeobox genes of the Hox class are required for proper patterning of skeletal elements, but how they regulate the differentiation of specific tissues is unclear. We show here that overexpression of a Hoxc-8 transgene causes cartilage defects whose severity depends on transgene dosage. The abnormal cartilage is characterized by an accumulation of proliferating chondrocytes and reduced maturation. Since Hoxc-8 is normally expressed in chondrocytes, these results suggest that Hoxc-8 continues to regulate skeletal development well beyond pattern formation in a tissue-specific manner, presumably by controlling the progression of cells along the chondrocyte differentiation pathway. The comparison to Hoxd-4 and Isl-1 indicates that this role in chondrogenesis is specific to proteins of the Hox class. Their capacity for regulation of cartilage differentiation suggests that Hox genes could also be involved in human chondrodysplasias or other cartilage disorders.

Transactivation of Hox gene expression in a VP16-dependent binary transgenic mouse system

Mice with aberrant expression of Hox genes have provided valuable insight into the role of Hox class transcription factors in patterning the developing skeleton and the nervous system. However, a recurrent problem is the lethality of mice expressing a Hox-transgene. To circumvent premature death frequently associated with transgenes that interfere with development, we have established a binary transgenic mouse system. Transactivator mice harbor the VP16 gene regulated by a promoter of interest while transresponder mice contain the VP16-responsive immediate early (IE) promoter linked to the gene to be expressed [G.W. Byrne, F.H. Ruddle, Multiplex gene regulation: a two-tiered approach to transgene regulation in transgenic mice, Proc. Natl. Acad. Sci. U.S.A., 86 (1989) 5473-5477]. Here, we report the generation of transresponder mouse strains that harbor murine homeobox genes linked to the IE promoter. We provide evidence that these transgenes are transcriptionally activated in progeny that inherit both a transactivator and transresponder transgene. By microdissection of mouse embryos and reverse transcription polymerase chain reaction (RT-PCR) analysis, we demonstrate that the expression of the Hox-transgenes is restricted to those regions of the mouse embryos where VP16 is present. The ability to activate stable Hox-transgenes in a reproducible fashion now permits a detailed in vivo dissection of the molecular mechanisms that lead to developmental abnormalities caused by deregulated Hox-gene expression.

A novel RU486 inducible system for the activation and repression of genes

We have developed an inducible system that consists of a transactivator and a target gene. The transactivator encodes a chimeric regulator that is responsive to RU486 (mifepristone, a progesterone receptor antagonist) but not to progestins and other hormones or endogenous ligands for activation. The target gene can be any gene under the control of Gal4 DNA binding sites. When the regulator is activated by RU486, it induces target gene expression by binding to the Gal4 recognition sequences upstream of the target. To verify this concept, we have successfully demonstrated the functionality of this system in tissue culture and in transgenic mice. Furthermore, for applications that require higher levels of a target gene, we also have generated regulators that can induce greater target gene expression. In addition, we also have constructed a modified regulator which can repress gene expression. The versatility of our system should prove useful for many applications in biology and gene therapy.

Transgenic analyses reveal developmentally regulated neuron- and muscle-specific elements in the murine neurofilament light chain gene promoter

We report here the developmental activity of regulatory elements that reside within 1.7 kilobases of the murine neurofilament light chain (NF-L) gene promoter. NF-L promoter activity is first detected at embryonic day 8.5 in neuroepithelial cells. Neuron-specific gene expression is maintained in the spinal cord until embryonic day 12.5 and at later developmental stages in the brain and sensory neuroepithelia. After day 14.5, the promoter becomes active in myogenic cells. Transgene expression in both neurons and muscle is consistent with the detection of endogenous NF-L transcript in both neuronal and myogenic tissues of neonates by reverse transcriptase-polymerase chain reaction. Neuron- and muscle-specific activities of the NF-L promoter decrease and are nearly undetectable after birth. Thus, the 1.7-kilobase NF-L promoter contains regulatory elements for initiation but not maintenance of transcription from the NF-L locus. Deletion analyses reveal that independent regulatory elements control the observed tissue-specific activities and implicate a potential MyoD binding site as the muscle-specific enhancer. Our results demonstrate that the NF-L promoter contains distinct regulatory elements for both neuron- and muscle-specific gene expression and that these activities are temporally separated during embryogenesis.

Transactivation domains facilitate promoter occupancy for the dioxin-inducible CYP1A1 gene in vivo

We have studied the transcriptional regulation of the dioxin-inducible mouse CYP1A1 gene in its native chromosomal setting. We analyzed the ability of aromatic hydrocarbon receptor (AhR) mutants and AhR chimeras to restore dioxin responsiveness to the CYP1A1 gene in AhR-defective mouse hepatoma cells. Our data reveal that transactivation domains in AhR's C-terminal half mediate occupancy of the nuclear factor 1 site and TATA box for the CYP1A1 promoter in vivo. Transactivation domains of VP16 and AhR nuclear translocator, but not Sp1, can substitute for AhR's C-terminal half in facilitating protein binding at the promoter. Our data also reveal an apparent linear relationship between promoter occupancy and CYP1A1 gene expression in chromatin. These findings provide new insights into the in vivo mechanism of transcriptional activation for an interesting mammalian gene.

Doxycycline-mediated quantitative and tissue-specific control of gene expression in transgenic mice

The tet regulatory system in which doxycycline (dox) acts as an inducer of specifically engineered RNA polymerase II promoters was transferred into transgenic mice. Tight control and a broad range of regulation spanning up to five orders of magnitude were monitored dependent on the dox concentration in the water supply of the animals. Administration of dox rapidly induces the synthesis of the indicator enzyme luciferase whose activity rises over several orders of magnitude within the first 4 h in some organs. Induction is complete after 24 h in most organs analyzed. A comparable regulatory potential was revealed with the tet regulatory system where dox prevents transcription activation. Directing the synthesis of the tetracycline-controlled transactivator (tTA) to the liver led to highly specific regulation in hepatocytes where, in presence of dox, less than one molecule of luciferase was detected per cell. By contrast, a more than 10(5)-fold activation of the luciferase gene was observed in the absence of the antibiotic. This regulation was homogeneous throughout but stringently restricted to hepatocytes. These results demonstrate that both tetracycline-controlled transcriptional activation systems provide genetic switches that permit the quantitative control of gene activities in transgenic mice in a tissue-specific manner and, thus, suggest possibilities for the generation of a novel type of conditional mutants.

Characterization of DNase I hypersensitive sites in the mouse 68-kDa neurofilament gene

Four brain-specific DNase I hypersensitive sites (HSS) have previously been identified flanking the mouse 68-kDa neurofilament gene within a 1.7 kb upstream sequence which confers neuronal specificity of expression of this gene in transgenic mice. Previously several DNA-binding factors were detected at the HSS closest to the transcription start site (HSS1). However, no major brain-specific factors were identified, suggesting a possible role for the three remaining HSS in conferring tissue-specificity to the NF-L gene. Sequence analysis of the NF-L promoter region demonstrated the presence of an extensive CT repeat and several potential binding sites which are also found in other neurofilament promoters. Gel mobility shift assays revealed a similar but not identical banding pattern with brain and liver nuclear extracts at HSS2, and HSS3, however the banding pattern for HSS4 was predominantly brain-specific. DNase I footprinting revealed several factors binding to the upstream HSS regions in brain and liver nuclear extracts. These include a CCAAT box at HSS2, a novel brain-specific footprint near an adenovirus promoter element E2aE-C beta and a single liver-specific footprint associated with an POU/octamer binding site at HSS4. The presence of brain-specific gel shift bands and tissue-specific footprints associated with HSS4, suggest that this region may play an important role in the regulation of the NF-L gene.

The use of null mutant mice to study complex learning and memory processes

A number of neural substrates have been proposed to mediate complex learning and memory processes in mammalian organisms. One strategy for testing the involvement of a particular gene in learning and memory is to create a mouse line with a null mutation in that gene. Recently, embryonic stem cell-based gene-targeted homologous recombination techniques have been employed to create a number of such mutant mouse lines that do not express interesting candidate genes. These animals have been examined for impairments in several complex learning paradigms which are known to depend on the integrity of the hippocampus. In this review several complex learning and memory paradigms are described, the techniques to create null mutants are reviewed, and the results of recent studies with null mutants are described. Finally, the limitations for interpretation of behavioral data using null mutants are discussed.

Neuron-specific expression of a chicken gicerin cDNA in transient transgenic zebrafish

Gicerin, a novel cell adhesion molecule which belongs to the immunoglobulin superfamily, is expressed temporally and spatially in the developing chick brain and retina. The previous in vitro experiments using transfected cells showed that gicerin can function as a cell adhesion molecule which has both homophilic and heterophilic binding activities. For the in vivo analyses of gicerin in neural development, we tried to utilize a zebrafish system, a vertebrate suitable for studying early development. We generated transient transgenic animals by microinjecting DNA constructs into zebrafish embryos. Chicken gicerin, under control of the neurofilament gene promoter, was preferentially expressed in neuronal cells and gicerin-expressing neurons exhibited a fasciculation formation with neighboring gicerin-positive axons, which may be partly due to homophilic cell adhesion activity of gicerin. These experimental results suggest that this fast and efficient transgenic animal system is useful for studying the functional roles of neuron-specific genes during the development.

Spatial and temporal regulation of a lacZ reporter transgene in a binary transgenic mouse system

The transgenic mouse system is a powerful tool for the study of gene function. However, when the analysis involves genes that are critical for the normal developmental process, the usefulness of transgenic mouse systems is limited (for review see Hanahan, 1989; Westphal and Gruss, 1989; Byrne et al., 1991). This is due to potential transgene interference with development in case of ectopic or high level expression. As a result, establishing permanent transgenic mouse lines expressing these types of genes has proven difficult. To circumvent these difficulties, a binary transgenic mouse system has been established, termed the Multiplex System (Byrne and Ruddle, 1989). This is a two-tiered gene activation system in which expression of the gene of interest occurs only in offspring carrying transgenes encoding both components: transactivator and transresponder. Transactivator lines contain the gene encoding the VP16 protein of herpes simplex virus. Transresponder lines harbour the gene of interest linked to the IE promoter which includes recognition sequences for the VP16 transactivator. Previously, the inducibility of a chloramphenicol acetyltransferase reporter gene in newborn offspring that carried both a transactivator and transresponder transgene (Byrne and Ruddle, 1989) has been shown. Moreover, it has been demonstrated that expression of the VP16 protein was not detrimental to development and that transactivation appeared to be tissue specific. Here, the potential of the system for the expression of transgenes in early mouse embryogenesis was examined, using the Escherichia coli beta-galactosidase gene as a reporter in the transresponder mouse strain. To direct expression of VP16, the murine Hoxc-8 promoter, which is known to be active during early development, was used. Embryos from crosses of transactivators to transresponders were isolated at different stages of development and stained for beta-galactosidase activity. Transactivation, as demonstrated by strong beta-galactosidase staining, could be detected as early as eight days of development. At all stages examined, the pattern of lacZ transresponder gene expression accurately reflected the activity of the Hoxc-8 promoter controlling VP16 expression. It is demonstrated that the Multiplex System can be used to express transresponder transgenes in a spatially and temporally defined manner in multiple cell types early during mouse embryogenesis.

Differentiation is induced in three-dimensional cultures of brain cells immortalized by the LAP mammalian regulatory system

Immortalized neuroectodermal precursor cell lines were generated from mouse brain by the SV40 large T antigen expressed under the control of the LAP (lac activating protein) mammalian regulatory system. The LAP system permits the reversible expression of T antigen as a function of the exogenous inducer, isopropyl-beta-D-thiogalactopyranoside. Immortalized cells can be stably maintained in an undifferentiated state in monolayer cultures. Cell lines expressed the early neurofilament-like protein nestin, but not markers characteristic for mature cells such as the neurofilament light protein and glial fibrillary acidic protein. Downregulating the LAP-controlled T antigen with isopropyl-beta-D-thiogalactopyranoside was not sufficient to induce differentiation. However, when cells formed three-dimensional aggregates, differentiation to a neuronal phenotype occurred, indicating that cell-cell interaction plays an important role in their differentiation. Cells in aggregates did not proliferate, even in the presence of T antigen, suggesting that an aggregation-induced signal to cease growth was dominant over the growth signal of T antigen. Further morphological differentiation was induced by basic fibroblast growth factor. These immortalized cells should facilitate molecular and cellular studies concerned with the mechanism of commitment, fate determination, and mitotic arrest of neuronal precursor cells in the developing mammalian CNS.

The Alzheimer's A beta peptide induces neurodegeneration and apoptotic cell death in transgenic mice

To test whether the hypothesis that the Alzheimer's A beta peptide is neurotoxic, we introduced a transgene into mice to direct expression of this peptide to neurons. We show that the transgene is expressed in brain regions which are severely affected in Alzheimer's disease resulting in extensive neuronal degeneration. Morphological and biochemical evidence indicates that the eventual death of these cells occurs by apoptosis. Coincident with the cell degeneration and cell death is the presence of a striking reactive gliosis. Over 50% of the transgenic mice die by 12 months of age, half the normal life span of control mice. These data show that A beta is neurotoxic in vivo and suggest that apoptosis may be responsible for the accompanying neuronal loss, the principal underlying cellular feature of Alzheimer's disease.

Temporal control of gene expression in transgenic mice by a tetracycline-responsive promoter

Promoters whose temporal activity can be directly manipulated in transgenic animals provide a tool for the study of gene functions in vivo. We have evaluated a tetracycline-responsive binary system for its ability to temporally control gene expression in transgenic mice. In this system, a tetracycline-controlled trans-activator protein (tTA), composed of the repressor of the tetracycline-resistance operon (tet from Escherichia coli transposon Tn10) and the activating domain of viral protein VP16 of herpes simplex virus, induces transcription from a minimal promoter (PhCMV*-1; see below) fused to seven tet operator sequences in the absence of tetracycline but not in its presence. Transgenic mice were generated that carried either a luciferase or a beta-galactosidase reporter gene under the control of PhCMV*-1 or a transgene containing the tTA coding sequence under the control of the human cytomegalovirus immediate early gene 1 (hCMV IE1) promoter/enhancer. Whereas little luciferase or beta-galactosidase activity was observed in tissues of mice carrying only the reporter genes, the presence of tTA in double-transgenic mice induced expression of the reporter genes up to several thousand-fold. This induction was abrogated to basal levels upon administration of tetracycline. These findings can be used, for example, to design dominant gain-of-function experiments in which temporal control of transgene expression is required.

Targeted gene expression as a means of altering cell fates and generating dominant phenotypes

We have designed a system for targeted gene expression that allows the selective activation of any cloned gene in a wide variety of tissue- and cell-specific patterns. The gene encoding the yeast transcriptional activator GAL4 is inserted randomly into the Drosophila genome to drive GAL4 expression from one of a diverse array of genomic enhancers. It is then possible to introduce a gene containing GAL4 binding sites within its promoter, to activate it in those cells where GAL4 is expressed, and to observe the effect of this directed misexpression on development. We have used GAL4-directed transcription to expand the domain of embryonic expression of the homeobox protein even-skipped. We show that even-skipped represses wingless and transforms cells that would normally secrete naked cuticle into denticle secreting cells. The GAL4 system can thus be used to study regulatory interactions during embryonic development. In adults, targeted expression can be used to generate dominant phenotypes for use in genetic screens. We have directed expression of an activated form of the Dras2 protein, resulting in dominant eye and wing defects that can be used in screens to identify other members of the Dras2 signal transduction pathway.

Targeted oncogene activation by site-specific recombination in transgenic mice

An efficient and accurate method for controlled in vivo transgene modulation by site-directed recombination is described. Seven transgenic mouse founder lines were produced carrying the murine lens-specific alpha A-crystallin promoter and the simian virus 40 large tumor-antigen gene sequence, separated by a 1.3-kilobase-pair Stop sequence that contains elements preventing expression of the large tumor-antigen gene and Cre recombinase recognition sites. Progeny from two of these lines were mated with transgenic mice expressing the Cre recombinase under control of either the murine alpha A-crystallin promoter or the human cytomegalovirus promoter. All double-transgenic offspring developed lens tumors. Subsequent analysis confirmed that tumor formation resulted from large tumor-antigen activation via site-specific, Cre-mediated deletion of Stop sequences.

Interaction of multiple nuclear proteins with the promoter region of the mouse 68-kDa neurofilament gene

Four brain-specific, DNase I hypersensitive sites (HSS) have been mapped to the 5' flanking region of the mouse 68-kDa neurofilament gene. These sites are contained within a 1.7-kb sequence that confers neuronal specificity of expression of this gene in transgenic mice. To identify DNA sequences that might be involved in gene regulation, the HSS situated near the promoter region has been analyzed by gel mobility shift assays and DNase I footprinting to investigate protein binding sequences. Of particular interest are two footprints localized to a 9-nucleotide sequence that flanks both the light and medium neurofilament gene in mouse and to a sequence that demonstrates partial homology to several promoter regions, including element-1, a motif required for neuron specificity in Drosophila. A prominent footprint was also detected at a sequence that contains a near-perfect palindrome centered at a PstI restriction site.

Repression of the beta-amyloid gene in a Hox-3.1-producing cell line

Mammalian homeobox genes are widely expressed in the developing central nervous system and are postulated to control developmental processes by regulating gene expression at the transcriptional level. In vitro studies have identified consensus DNA sequences that contain an ATTA core as sites for interaction with homeodomain proteins. Such elements have been found in the upstream regulatory region of the gene encoding beta-amyloid precursor protein, which is associated with the neurological disorder Alzheimer disease. As the beta-amyloid precursor protein gene is also expressed in the developing central nervous system and appears to play a role in cellular regulatory processes, we have examined the possibility that a homeobox gene product can regulate its transcription. We demonstrate by Northern blot analyses and transfection experiments that the expression of the beta-amyloid precursor protein gene is decreased in cultured cells expressing the mouse homeobox gene Hox-3.1.

Neuroantibodies: molecular cloning of a monoclonal antibody against substance P for expression in the central nervous system

We present a strategy to study functional and/or developmental processes occurring in the nervous system, as well as in other systems, of mice. This strategy is based on the local expression of specific monoclonal antibodies (mAbs) by cells of the nervous system. As an application of this strategy, we report the cloning of the anti-substance P rat mAb NC1/34HL. Functional substance P-binding antibodies were reconstituted from the cloned variable domains by using vectors for expression in myeloma cells. With these and other vectors a general system for the cloning and expression of mAbs under a series of promoters (of the rat VGF8a gene, the neurofilament light-chain gene, and the methallothionein gene) has been created. The activity of these plasmids was confirmed by expressing the recombinant NC1/34HL mAb in GH3 pituitary cells, PC12 pheochromocytoma cells, and COS cells. DNA from the described constructs can be used to target the expression of the NC1/34HL mAb to the central nervous system of transgenic mice. This procedure will allow us to perturb substance P activity in a controlled way in order to dissect its multiple roles.

Purkinje cell protein-2 regulatory regions and transgene expression in cerebellar compartments

The Purkinje cell protein 2 (Pcp-2) is expressed in cerebellar Purkinje cells and retinal bipolar neurons. To illuminate how Pcp-2 expression is restricted to only two neuronal types and to derive tools to express heterologous genes in these neuronal subpopulations, genomic sequences of the mouse Pcp-2 gene have been cloned and flanking sequences have been evaluated as a source of neuron-specific regulatory elements. An upstream region with homology to other genes expressed in neurons was identified and a hybrid gene containing this sequence was constructed by ligating 0.4 kb of upstream and 0.3 kb of downstream Pcp-2-flanking DNA to lacZ. Transgenic mice bearing this construct exhibited beta-galactosidase in a wide array of neuron types, suggesting that this sequence may play an important role in specifying neuronal expression. Addition of a further 3.1 kb of Pcp-2 upstream sequences restricted expression of beta-galactosidase to a small number of neuron types and most notably to Purkinje cells within parasagitally oriented cerebellar compartments. The presence of elements lying within the 3.1-kb upstream region and acting to specifically restrict Pcp-2 expression is therefore suggested. Moreover, as beta-galactosidase was not expressed in the bipolar cells of these transgenic mice, retinal expression of the endogenous Pcp-2 gene must involve elements in addition to those conferring expression within Purkinje cells.

Binary system for regulating transgene expression in mice: targeting int-2 gene expression with yeast GAL4/UAS control elements

We have developed a binary transgenic system that activates an otherwise silent transgene in the progeny of a simple genetic cross. The system consists of two types of transgenic mouse strains, targets and transactivators. A target strain bears a transgene controlled by yeast regulatory sequences (UAS) that respond only to the yeast transcriptional activator GAL4. A transactivator strain expresses an active GAL4 gene that can be driven by any selected promoter. The current paradigm uses the murine growth factor int-2 cDNA as the target gene and the GAL4 gene driven by the mouse mammary tumor virus long terminal repeat as the transactivator. Both target and transactivator strains are phenotypically normal. By contrast, the bigenic offspring of these two strains express high levels of the target int-2 gene in each organ expressing the GAL4 transactivator. They also display a characteristic dominant int-2 phenotype that consists of epithelial hyperplasia in mammary and salivary glands, as well as prostatic and epididymal hypertrophy, which results in male sterility.

Expression of chicken liver cell adhesion molecule fusion genes in transgenic mice

The tissue-specific expression of the chicken liver cell adhesion molecule (L-CAM) was studied by generating transgenic mice. The rat insulin II promoter was fused to a chicken L-CAM cDNA or to chicken genomic L-CAM sequences. Mice carrying the cDNA showed no expression of L-CAM. Mice carrying L-CAM genomic sequences showed expression in the beta cells of the pancreas, suggesting that sequences in introns or in flanking regions are required for expression. Murine L-CAM was undetectable in the beta cells of the pancreas of those transgenic mice expressing chicken L-CAM and thus appeared to be down-regulated, but expression of the mouse protein was not altered at other sites. Chicken L-CAM was also found in extrapancreatic tissues such as skin, kidney, liver, lung, intestine, blood vessels, and the choroid plexus and leptomeninges of the central nervous system. These findings raised the possibility that the chicken L-CAM gene contains cis regulatory elements that interfere with the specificity of a tissue-specific promoter such as the rat insulin promoter. To test this hypothesis, transgenic mice were produced with a construct containing the murine neurofilament promoter fused to genomic chicken L-CAM sequences. Chicken L-CAM was expressed in the brain and spinal cord, where L-CAM is not normally found, but it was also found in some nonneural tissues (kidney, liver, intestine, lung) in which L-CAM is normally expressed. The combined results suggest that tissue-specific cis-acting elements in the chicken L-CAM gene, when combined with heterologous promoters/enhancers, can generate novel patterns of gene expression.

Expression of NF-L in both neuronal and nonneuronal cells of transgenic mice: increased neurofilament density in axons without affecting caliber

We have generated transgenic mice containing additional copies of the murine NF-L gene in order to examine the consequences of neurofilament-L overexpression on axonal morphology. Founder mice were constructed to carry a transgene in which the presumptive 5' promoter sequences of NF-L were replaced with the strong murine sarcoma virus long terminal repeat promoter. The transgenes were expressed prominently in several tissues, including skeletal muscle and kidney where NF-L accumulated to approximately 2% of cell protein. This was not accompanied by an overt phenotype, except that expression in lens led to cataract formation. In the brains of these animals, transgene RNA levels exceeded the endogenous NF-L RNAs by up to 20-fold, although no additional protein accumulated, indicating posttranscriptional regulation of NF-L expression. However, in peripheral neurons transgene RNA was approximately fourfold higher than endogenous NF-L mRNAs, and a corresponding increase in NF-L subunits was found in axons arising from these neurons. Myelinated nerve fibers of transgenic animals contained increased numbers of NFs, assembled predominantly of NF-L. This was reflected in an increase in the density of axonal NFs; axonal caliber was not affected.