Nerve growth factor somatic mosaicism produced by herpes virus-directed expression of cre recombinase

Defining functional gene-circuit interfaces in the mouse nervous system

Complexity in the nervous system is established by developmental genetic programs, maintained by differential genetic profiles and sculpted by experiential and environmental influence over gene expression. Determining how specific genes define neuronal phenotypes, shape circuit connectivity and regulate circuit function is essential for understanding how the brain processes information, directs behavior and adapts to changing environments. Mouse genetics has contributed greatly to current percepts of gene-circuit interfaces in behavior, but considerable work remains. Large-scale initiatives to map gene expression and connectivity in the brain, together with advanced techniques in molecular genetics, now allow detailed exploration of the genetic basis of nervous system function at the level of specific circuit connections. In this review, we highlight several key advances for defining the function of specific genes within a neural network.

Cardiac-specific inducible and conditional gene targeting in mice

Mouse genetic engineering has revolutionized our understanding of the molecular and genetic basis of heart development and disease. This technology involves conditional tissue-specific and temporal transgenic and gene targeting approaches, as well as introduction of polymorphisms into the mouse genome. These approaches are increasingly used to elucidate the genetic pathways underlying tissue homeostasis, physiology, and pathophysiology of adult heart. They have also led to the development of clinically relevant models of human cardiac diseases. Here, we review the technologies and their limitations in general and the cardiovascular research community in particular.

Assessment of CA1 injury after global ischemia using supervised 2D analyses of nuclear pyknosis

Selective neuronal vulnerability is a common theme in both acute and chronic diseases affecting the nervous system. This phenomenon is particularly conspicuous after global cerebral ischemia wherein CA1 pyramidal neurons undergo delayed death while surrounding hippocampal regions are relatively spared. While injury in this model can be easily demonstrated using either histological or immunological stains, current methods used to assess the cellular injury present in these biological images lack the precision required to adequately compare treatment effects. To address this shortcoming, we devised a supervised work-flow that can be used to quantify ischemia-induced nuclear condensation using microscopic images. And while we demonstrate the utility of this technique using models of ischemic brain injury, the approach can be readily applied to other paradigms in which programmed cell death is a major component.

Generating conditional knockout mice

Gene targeting in ES cells is extensively used to generate designed mouse mutants and to study gene function in vivo. Knockout mice that harbor a null allele in their germline provide appropriate genetic models of inherited diseases and often exhibit embryonic or early postnatal lethality. To study gene function in adult mice and in selected cell types, a refined strategy for conditional gene inactivation has been developed that relies on the DNA recombinase Cre and its recognition (loxP) sites. For conditional mutagenesis, a target gene is modified by the insertion of two loxP sites that enable to excise the flanked (floxed) gene segment through Cre-mediated recombination. Conditional mutant mice are obtained by crossing the floxed strain with a Cre transgenic line such that the target gene becomes inactivated in vivo within the expression domain of Cre. A large collection of Cre transgenic lines has been generated over time and can be used in a combinatorial manner to achieve gene inactivation in many different cell types. A growing number of CreER(T2) transgenic mice further allows for inducible inactivation of floxed alleles in adult mice upon administration of tamoxifen. This chapter covers the design and construction of loxP flanked alleles and refers to the vectors, ES cells, and mice generated by the European conditional mouse mutagenesis (EUCOMM) project. We further describe the design and use of Cre and CreER(T2) transgenic mice and a convenient breeding strategy to raise conditional mutants and controls for phenotype analysis.

Optimized adeno-associated virus 8 produces hepatocyte-specific Cre-mediated recombination without toxicity or affecting liver regeneration

Engineering viral vectors to produce liver-specific protein expression may help advance understanding of hepatic regeneration and disease states. In addition to introducing genes of interest to the liver, these vectors can be adapted for gene deletion when designed to express Cre recombinase. The ability to use this system requires high, liver-restricted expression, low toxicity, and no effect on the process of interest. We developed an adeno-associated virus 8 (AAV8) with a codon-optimized Cre recombinase under a hepatocyte-specific major urinary protein (MUP) promoter (MUP-iCre-AAV8) that fulfills these requirements. A single intravenous injection of ROSA26R reporter mice, which express lacZ after Cre-mediated recombination, demonstrated homogeneous beta-galactosidase expression limited to hepatocytes after only 7 days. Cre protein expression remained strong for at least 31 days. Serum liver function tests and histology demonstrated minimal liver toxicity. The presence of MUP-iCre-AAV8 did not affect hepatocyte proliferation after partial hepatectomy as measured by Ki67 staining.

CONCLUSION

AAV8 with the MUP promoter, by virtue of its lack of hepatic toxicity or effect on liver regeneration, may be an efficient alternative to complex transgenic methodologies for studies of the mouse liver.

Neuronal specificity of HSV/sleeping beauty amplicon transduction in utero is driven primarily by tropism and cell type composition

A novel bipartite vector system consisting of the herpes simplex virus (HSV) amplicon and the Sleeping Beauty(SB) transposon was previously shown to efficiently deliver a "transgenon" (integrating transgene) in utero. This vector platform facilitated long-term transgenon expression specifically within neurons and neuronal precursor cells of the rodent brain. However, the mechanism underlying the neurospecificity of the HSV/SB amplicon in the setting of mouse embryogenesis is unknown. We find that embryonic cells expressing the Sox1 "neurocompetence" transcription factor represent the primary targets for HSV amplicon transduction in utero. These cells, which comprise the ependymal and subventricular zones (SVZs), express significant levels of high-mobility-group protein B1 (HMGB1), a co-factor shown to facilitate SB-mediated transposition. Using a conventional, non-integrating amplicon expressing Cre recombinase to "tag" transduced cells embryonically in ROSA26 Cre indicator mice in utero, we found transduced cells were exclusively of the neuronal lineage but that in comparison to HSV/SB-mediated in utero delivery, staining patterns were less widespread and "tagged" neuroprogenitor cells were absent. Our findings demonstrate that in utero HSV/SB amplicon gene transfer is primarily neurospecific owing to viral tropism and target cell populations present embryonically, where multi-potent cells of the developing embryo are supportive of SB-driven transposition.

Sustained hippocampal IL-1 beta overexpression mediates chronic neuroinflammation and ameliorates Alzheimer plaque pathology

Neuroinflammation is a conspicuous feature of Alzheimer disease (AD) pathology and is thought to contribute to the ultimate neurodegeneration that ensues. IL-1 beta has emerged as a prime candidate underlying this response. Here we describe a transgenic mouse model of sustained IL-1 beta overexpression that was capable of driving robust neuroinflammation lasting months after transgene activation. This response was characterized by astrocytic and microglial activation in addition to induction of proinflammatory cytokines. Surprisingly, when triggered in the hippocampus of the APPswe/PS1dE9 mouse model of AD, 4 weeks of IL-1 beta overexpression led to a reduction in amyloid pathology. Congophilic plaque area fraction and frequency as well as insoluble amyloid beta 40 (A beta 40) and A beta 42 decreased significantly. These results demonstrate a possible adaptive role for IL-1 beta-driven neuroinflammation in AD and may help explain recent failures of antiinflammatory therapeutics for this disease.

Molecular and genetic approaches for behavioral analysis of protein function

Behavioral neuroscientists traditionally have relied on pharmacological and lesion methods to explore the neural basis of complex behaviors. In the past decade, molecular and genetic approaches have been added to the repertoire, but their benefits are often severely constrained by a lack of temporal and spatial control over target protein modulation. Recent advances have allowed investigators to manipulate neuronal protein levels with far greater control over target selectivity, regional and cellular specificity, and duration of action than previously available. This article compares strengths and weaknesses in current molecular and genetic approaches for studying neuronal protein function in complex behaviors.

Transcriptional and posttranslational regulation of cre recombinase by ru486 as the basis for an enhanced inducible expression system

Genetic studies often require the employment of an inducible expression system, whereby the expression of a particular gene can be regulated by the exogenous administration of an inert ligand. Cre/loxP-based systems have been previously described as the basis for inducible expression systems by exerting site-specific DNA recombination. In our effort to enhance the properties of the RU486-responsive CrePr1 construct, we have developed the dual GLVP/CrePr system, in which RU486 confers activity control at both the transcriptional and the posttranslational level of CrePr1. This was achieved by placing CrePr1 transcriptional regulation under the control of the RU486-sensitive chimeric regulator GLVP. Stable cell lines harboring the dual GLVP/CrePr as well as the single CrePr1 system were developed. Our results indicate that the dually regulated system is highly inducible by RU486 while maintaining minimal basal activity ("leakage"), characteristics that can be employed in the development of transgenic mice in which genetic pathways can be turned on or turned off after exogenous administration of RU486 at physiologically inert doses.

Focal deletion of the adenosine A1 receptor in adult mice using an adeno-associated viral vector

Adenosine is a ubiquitous neuromodulator that increases sleep, inhibits seizures, and promotes neuroprotection. Many of these effects are mediated by A1 receptors, but A1 receptors are expressed in most brain regions, and distinguishing the precise site of action of adenosine is challenging. To test the role of adenosine in different hippocampal regions, we have used the Cre-loxP system and an adeno-associated viral (AAV) vector to focally delete endogenous adenosine A1 receptors in the hippocampus. Microinjection of an AAV vector containing the gene for Cre recombinase induced intense, focal, neuron-specific recombination in reporter mice. In a separate line of mice with loxP sites flanking the major coding exon for the adenosine A1 receptor, this AAV-Cre markedly reduced A1 receptor mRNA and focally abolished the postsynaptic response to adenosine without any change in basic electrophysiologic properties. Adenosine inhibits signaling between CA3 and CA1 neurons, but it is unclear from pharmacologic studies whether this response is caused by presynaptic or postsynaptic effects. Deletion of A1 receptors from CA3 neurons abolished this response to adenosine, but deletion of A1 receptors from CA1 neurons had no effect, demonstrating a presynaptic site of action. This transduction knock-out technique holds enormous potential for dissecting the functions of different CNS pathways.

Induction of cre recombinase activity using modified androgen receptor ligand binding domains: a sensitive assay for ligand-receptor interactions

Novel systems of inducible gene expression are presented in which CRE-M, an altered form of cre recombinase (cre), is fused to and activated by ligand binding to two forms of the androgen receptor (AR) ligand binding domain (LBD). Selective activation or inactivation of gene transcription is induced upon the addition of appropriate ligand. The coupling of this cre-LBD system with our previously reported highly sensitive assay to measure cre activity in vitro using a dual fluorescent gene switch reporter provides a novel, high-throughput assay system for identifying compounds that bind to and activate various forms of the LBD of androgen receptor. This method can similarly be applied to screen compounds for their activating properties on other steroid hormone LBDs. Three different forms of the AR-LBD were fused to CRE-M, including the wild-type AR-LBD (wt), a non-ligand binding truncated form, LBD (T), and a mutated form (Thr-->Ala substitution) identified in the LNCaP prostate cancer cell line, LBD (LNCaP). We demonstrate a 10-fold induction of cre activity by the addition of androgen agonists to the CRE-M-AR-LBD(wt) fusion protein, but not in the presence of the anti-androgen, flutamide. However, cre activity can be induced by flutamide with the CRE-M-AR-LBD(LNCaP) fusion protein. Similar activation properties were obtained when these fusion proteins were expressed using adenoviral vectors. When combined with our previously reported cre-lox gene switch system, the CRE-M-AR-LBD system can be utilized in gene therapy systems in which a therapeutic product may be initially expressed, replaced by a second product, or turned-off following exposure to ligand. This provides an important, additional level of regulation to gene therapy systems.

Application of adeno-associated viral vectors in behavioral research

Viral vectors afford the capability of genetically manipulating the expression of neurotransmitters, neuropeptides, hormones, and their receptors in specific brain sites of adult animals of any species. Hence, they are a powerful tool for investigating the neurochemistry underlying complex cognitive processes and behaviors. Here we discuss how the recombinant adeno-associated virus (rAAV) can be engineered for use in neurobehavioral studies, techniques for site-specific delivery of vector into the brain, characterization of expression profiles, and biosafety issues. Finally, we discuss issues of experimental design and interpretation of behavioral results in viral vector studies.

Adeno-associated virus effectively mediates conditional gene modification in the brain

The Cre/loxP system is increasingly showing promise for investigating genes involved in neural function. Here, we demonstrate that in vivo modification of genes in the mouse brain can be accomplished in a spatial- and temporal-specific manner by targeted delivery of an adeno-associated virus (AAV) encoding a green fluorescent protein/Cre recombinase (GFP/Cre) fusion protein. By using a reporter mouse, in which Cre recombinase activates beta-galactosidase expression, we demonstrate long-term recombination of neurons in the hippocampus, striatum, and septum as early as 7 days after stereotaxic injection of virus. Recombined cells were observed for at least 6 months postinjection without evidence of cell loss or neural damage. AAV-mediated delivery of GFP/Cre provides a valuable approach to alter the mouse genome, as AAV delivers genes efficiently to neurons with low toxicity. This approach will greatly facilitate the study of genetic modifications in the mouse brain.

Viral vectors as part of an integrated functional genomics program

Over the past decade, viral vectors have slowly gained mainstream acceptance in the neuroscience and genetics communities for the in vivo study of gene function [1]. Using stereotactic techniques, it is possible to characterize neuroanatomical relationships through the delivery of neurotropic viral vectors to specific brain regions. More sophisticated studies combine viral vectors with other methods of genetic manipulation such as germline transgenic mice. As more is learned about the properties of different viral vectors, it has become possible to use viral vectors to test hypotheses about the function of genes, through targeted in vivo delivery to the central nervous system (CNS). The effects of gene expression in the brain can be measured on the molecular, biochemical, electrophysiological, morphological, and behavioral levels. We propose that viral vectors should be considered as part of an integrated functional genomics platform in the CNS.

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.

Delivery of the Cre recombinase by a self-deleting lentiviral vector: efficient gene targeting in vivo

The Cre recombinase (Cre) from bacteriophage P1 is an important tool for genetic engineering in mammalian cells. We constructed lentiviral vectors that efficiently deliver Cre in vitro and in vivo. Surprisingly, we found a significant reduction in proliferation and an accumulation in the G(2)/M phase of Cre-expressing cells. To minimize the toxic effect of Cre, we designed a lentiviral vector that integrates into the host genome, expresses Cre in the target cell, and is subsequently deleted from the genome in a Cre-dependent manner. Thus, the activity of Cre terminates its own expression (self-deleting). We showed efficient modification of target genes in vitro and in the brain after transduction with the self-deleting vectors. In contrast to sustained Cre expression, transient expression of Cre from the self-deleting vector induced significantly less cytotoxicity. Such a self-deleting Cre vector is a promising tool for the induction of conditional gene modifications with minimal Cre toxicity in vivo.

Gene-experience interaction alters the cholinergic septohippocampal pathway of mice

Spatial learning requires the septohippocampal pathway. The interaction of learning experience with gene products to modulate the function of a pathway may underlie use-dependent plasticity. The regulated release of nerve growth factor (NGF) from hippocampal cultures and hippocampus, as well as its actions on cholinergic septal neurons, suggest it as a candidate protein to interact with a learning experience. A method was used to evaluate NGF gene-experience interaction on the septohippocampal neural circuitry in mice. The method permits brain region-specific expression of a new gene by using a two-component approach: a virus vector directing expression of cre recombinase; and transgenic mice carrying genomic recombination substrates rendered transcriptionally inactive by a "floxed" stop cassette. Cre recombinase vector delivery into transgenic mouse hippocampus resulted in recombination in 30% of infected cells and the expression of a new gene in those cells. To examine the interaction of the NGF gene and experience, adult mice carrying a NGF transgene with a floxed stop cassette (NGFXAT) received a cre recombinase vector to produce localized unilateral hippocampal NGF gene expression, so-called "activated" mice. Activated and control nonactivated NGFXAT mice were subjected to different experiences: repeated spatial learning, repeated rote performance, or standard vivarium housing. Latency, the time to complete the learning task, declined in the repeated spatial learning groups. The measurement of interaction between NGF gene expression and experience on the septohippocampal circuitry was assessed by counting retrogradely labeled basal forebrain cholinergic neurons projecting to the hippocampal site of NGF gene activation. Comparison of all NGF activated groups revealed a graded effect of experience on the septohippocampal pathway, with the largest change occurring in activated mice provided with repeated learning experience. These data demonstrate that plasticity of the adult spatial learning circuitry can be robustly modulated by experience-dependent interactions with a specific hippocampal gene product.

Genetic engineering of herpes simplex virus and vector genomes carrying loxP sites in cells expressing Cre recombinase

The prokaryotic Cre-loxP recombination system is a powerful tool that enables in vitro and in vivo site-specific manipulations of the genome of eukaryotic cells as well as of DNA viruses and their derived vectors. This system, however, has not yet been exploited in the context of herpes simplex virus type 1 (HSV-1) infected cells, perhaps because this virus encodes several functions that induce a strong shutoff of cellular protein synthesis, a fact that could preclude expression of cellular-encoded Cre recombinase. In the present study, we show that efficient site-specific recombination can take place in cell lines expressing Cre, even in the context of HSV-1 infection, as evidenced by the engineering of an HSV-1 recombinant virus and several viral vectors carrying one or two loxP sequences. More precisely, we have used this system to induce an irreversible switch in the expression of a viral complex transcription unit encoding two different open reading frames and allowing consecutive expression of two reporter genes. Furthermore Cre recombinations were also used to induce the decatenation of the genomic concatemers harbored by amplicon particles upon infection of cells under nonreplicative conditions, thus enabling the rescue of many independent plasmids corresponding to the original amplicon plasmid used to generate the vectors. Thus the Cre-loxP recombination system can successfully be used for engineering the genome of HSV-1 or HSV-1-based vectors in cultured cells.

A non-cytotoxic herpes simplex virus vector which expresses Cre recombinase directs efficient site specific recombination

The coding sequences for the bacteriophage P1 recombinase Cre were cloned into the genome of a herpes simplex virus type 1 (HSV-1) mutant which is severely impaired for the synthesis of immediate early (IE) proteins. The resulting recombinant, virus in1372, expressed functional Cre which mediated the excision in trans of loxP-flanked sequences located in the HSV-1 genome, both in tissue culture cells and in vivo in mouse sensory neurons. Infection with in1372 also resulted in recombination, at high efficiency, between loxP sequences in the cellular genome without causing detectable cytotoxicity. Mutant in1372 is a versatile vector for the delivery of Cre in tissue culture and in vivo.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part II. Vector systems and applications

Many properties of HSV-1 are especially suitable for using this virus as a vector to treat diseases affecting the central nervous system (CNS), such as Parkinson's disease or malignant gliomas. These advantageous properties include natural neurotropism, high transduction efficiency, large transgene capacity, and the ability of entering a latent state in neurons. Selective oncolysis in combination with modulation of the immune response mediated by replication-conditional HSV-1 vectors appears to be a highly promising approach in the battle against malignant glioma. Helper virus-free HSV/AAV hybrid amplicon vectors have great promise in mediating long-term gene expression in the PNS and CNS for the treatment of various neurodegenerative disorders or chronic pain. Current research focuses on the design of HSV-1-derived vectors which are targeted to certain cell types and support transcriptionally regulatable transgene expression. Here, we review the recent developments on HSV-1-based vector systems and their applications in experimental and clinical gene therapy protocols.

Timing is everything in life: conditional transgene expression in the cardiovascular system

Manipulation of the mouse genome by traditional transgenic approaches has facilitated studies of gene function within the context of the intact organism and allowed for the creation of useful animal models of human disease. However, the timing of gene activation or repression is a critical determinant of phenotype, and the ability to regulate the temporal profile of transgene expression remains an important experimental goal. In this Mini Review, we describe the current status of systems to tightly regulate target gene expression in vivo, focusing on binary systems using chimeric transcription factors. Although experimental difficulties persist, regulated expression systems are beginning to produce conditional phenotypes with exciting experimental implications. We review the experience to date and examine the potential utility of these approaches within the context of cardiovascular medicine.

Reproducible and efficient murine CNS gene delivery using a microprocessor-controlled injector

To develop a reproducible gene transfer method for the murine CNS we evaluated delivery of various gene vehicles using mechanical or manual stereotaxic intracranial inoculation. A microprocessor controlled microsyringe pump (The World Precision Instruments/UltraMicroPump) programmable for volume, rate and syringe size and designed to dispense nanoliter and picoliter volumes was compared to a standard manual deliver method. Gene transfer efficiency of two viral vectors, two synthetic cationic lipid molecules, and naked DNA were evaluated in mice injected unilaterally in two brain regions. Animals received 1 microl over 10 min. of either HSVlac (1 x 10(5) b.f.u), AdLac (1 x 10(5) p.f.u), Tfx-10 or Tfx-20 (2.6 microg DNA in 2.0 microl Tfx; 1:1 charge ratio of DNA to liposome), or naked DNA (HSVlac plasmid, 10 microg/microl). After 4 days, animals from each group were perfused and tissue prepared for X-gal histochemical detection of beta-galactosidase expression. Blue cells were observed in the HSV, Adenovirus, and Tfx-20 groups only at the injection site in animals injected using the UMP. Animals injected manually exhibited fewer blue cells and positive cells were not restricted to the injection site. To quantify expression, tissue punches harvested from the injection sites as well as other brain regions were analyzed using a chemiluminescent reporter assay to detect beta-galactosidase (Galacto-Light). These data indicated increased activity in all animals injected with a lacZ containing vector via the UMP as compared to manual delivery: A 41% increase in the expression levels of beta-gal in HSVlac infected animals (p = 0.0029); a 29% increase in Adlac infected animals (p = 0.01); a 56% increase in Tfx-10 transduced animals (p = 0.04); a 24% increase in Tfx-20 transduced animals (p = 0.01); and a 69% increase in naked DNA gene transfer (p = 0.05). Total beta-galactosidase activity was greatest in HSVlac infected mice followed by Adlac > Tfx-20 > Tfx-10 = naked DNA.