Analysis of cell-specific promoters for viral gene therapy targeted at the vascular endothelium

Advances and Obstacles in Using CRISPR/Cas9 Technology for Non-Coding RNA Gene Knockout in Human Mesenchymal Stromal Cells

Non-coding RNA (ncRNAs) genes have attracted increasing attention in recent years due to their widespread involvement in physiological and pathological processes and regulatory networks. The study of the function and molecular partners of ncRNAs opens up opportunities for the early diagnosis and treatment of previously incurable diseases. However, the classical "loss-of-function" approach in ncRNA function analysis is challenged due to some specific issues. Here, we have studied the potency of two CRISPR/Cas9 variants, wild-type (SpCas9wt) and nickase (SpCas9D10A) programmable nucleases, for the editing of extended DNA sequences in human mesenchymal stromal cells (MSCs). Editing the genes of fibrosis-related hsa-miR-21-5p and hsa-miR-29c-3p, we have shown that a pair of SpCas9D10A molecules can effectively disrupt miRNA genes within the genomes of MSCs. This leads not only to a decrease in the level of knockout miRNA in MSCs and MSC-produced extracellular vesicles, but also to a change in cell physiology and the antifibrotic properties of the cell secretome. These changes correlate well with previously published data for the knockdown of certain miRNAs. The proposed approach can be used to knock out ncRNA genes within the genomes of MSCs or similar cell types in order to study their function in biological processes.

Endothelial cell-specific expression of serine/threonine kinase 11 modulates dendritic cell differentiation

In the bone marrow, classical and plasmacytoid dendritic cells (DC) develop from the macrophage-DC precursor (MDP) through a common DC precursor (CDP) step. This developmental process receives essential input from the niche in which it takes place, containing endothelial cells (EC) among other cell types. Here we show that targeted deletion of serine/threonine kinase 11 (Stk11) encoding tumor suppressor liver kinase b1 (Lkb1) in mouse ECs but not DCs, results in disrupted differentiation of MDPs to CDPs, severe reduction in mature DC numbers and spontaneous tumorigenesis. In wild type ECs, Lkb1 phosphorylates polypyrimidine tract binding protein 1 (Ptbp1) at threonine 138, which regulates stem cell factor (Scf) pre-mRNA splicing. In the absence of Lkb1, exon 6 of Scf is spliced out, leading to the loss of Scf secretion. Adeno-associated-virus-mediated delivery of genes encoding either soluble Scf or the phosphomimetic mutant Ptbp1T138E proteins rescued the defects of MDP to CDP differentiation and DC shortage in the endothelium specific Stk11 knockout mice. In summary, endothelial Stk11 expression regulates DC differentiation via modulation of Scf splicing, marking the Stk11-soluble-Scf axis as a potential cause of DC deficiency syndromes.

Deciphering the Ets-1/2-mediated transcriptional regulation of F8 gene identifies a minimal F8 promoter for hemophilia A gene therapy

Amajor challenge in the development of a gene therapy for hemophilia A is the selection of cell type- or tissue-specific promoters to ensure factor VIII (FVIII) expression without eliciting an immune response. As liver sinusoidal endothelial cells are the major FVIII source, understanding the transcriptional F8 regulation in these cells would help to optimize the minimal F8 promoter (pF8) to efficiently drive FVIII expression. In silico analyses predicted several binding sites (BS) for the E26 transformation-specific (Ets) transcription factors Ets-1 and Ets-2 in the pF8. Reporter assays demonstrated a significant up-regulation of pF8 activity by Ets-1 or Ets- 1/Est-2 combination, while Ets-2 alone was ineffective. Moreover, Ets-1/Ets- 2-DNA binding domain mutants (DBD) abolished promoter activation only when the Ets-1 DBD was removed, suggesting that pF8 up-regulation may occur through Ets-1/Ets-2 interaction with Ets-1 bound to DNA. pF8 carrying Ets-BS deletions unveiled two Ets-BS essential for pF8 activity and response to Ets overexpression. Lentivirus-mediated delivery of green fluorescent protein (GFP) or FVIII cassettes driven by the shortened promoters, led to GFP expression mainly in endothelial cells in the liver and to longterm FVIII activity without inhibitor formation in HA mice. These data strongly support the potential application of these promoters in hemophilia A gene therapy.

MicroRNAs Regulating Autophagy in Neurodegeneration

Social and economic impacts of neurodegenerative diseases (NDs) become more prominent in our constantly aging population. Currently, due to the lack of knowledge about the aetiology of most NDs, only symptomatic treatment is available for patients. Hence, researchers and clinicians are in need of solid studies on pathological mechanisms of NDs. Autophagy promotes degradation of pathogenic proteins in NDs, while microRNAs post-transcriptionally regulate multiple signalling networks including autophagy. This chapter will critically discuss current research advancements in the area of microRNAs regulating autophagy in NDs. Moreover, we will introduce basic strategies and techniques used in microRNA research. Delineation of the mechanisms contributing to NDs will result in development of better approaches for their early diagnosis and effective treatment.

Bioactive Silk Hydrogels with Tunable Mechanical Properties

Developing bioactive hydrogels with potential to guide the differentiation behavior of stem cells has become increasingly important in the biomaterials field. Here, silk hydrogels with tunable mechanical properties were developed by introducing inert silk fibroin nanofibers (SNF) within an enzyme crosslinked system of regenerated silk fibroin (RSF). After the crosslinking reaction of RSF, the inert SNF was embedded into the RSF hydrogel matrix, resulting in improved mechanical properties. Tunable stiffness in the range of 9-60 KPa was achieved by adjusting the amount of the added NSF, significantly higher than SNF-free hydrogels formed under same conditions (about 1 KPa). In addition, the proliferation of rat bone marrow derived mesenchymal stem cells cultured on the composite hydrogels and differentiated into endothelial cells, myoblast and osteoblast cells was improved, putatively due to the control of stiffness of the hydrogels. Bioactive and tunable silk-based hydrogels were prepared via a composite SNF and crosslinked RSF system, providing a new strategy to design silk biomaterials with tunable mechanical and biological performance.

Specific micro RNA-regulated TetR-KRAB transcriptional control of transgene expression in viral vector-transduced cells

Precise control of transgene expression in a tissue-specific and temporally regulated manner is desirable for many basic and applied investigations gene therapy applications. This is important to regulate dose of transgene products and minimize unwanted effects. Previously described methods have employed tissue specific promoters, miRNA-based transgene silencing or tetR-KRAB-mediated suppression of transgene promoters. To improve on versatility of transgene expression control, we have developed expression systems that use combinations of a tetR-KRAB artificial transgene-repressor, endogenous miRNA silencing machinery and tissue specific promoters. Precise control of transgene expression was demonstrated in liver-, macrophage- and muscle-derived cells. Efficiency was also demonstrated in vivo in murine muscle. This multicomponent and modular regulatory system provides a robust and easily adaptable method for achieving regulated transgene expression in different tissue types. The improved precision of regulation will be useful for many gene therapy applications requiring specific spatiotemporal transgene regulation.

Optimizing adenoviral transduction of endothelial cells under flow conditions

PURPOSE

To target adenoviral vectors to cells of the vasculature and shielding vectors from inactivation by the immune system.

METHODS

Complexes of reporter gene expressing adenoviral vectors with positively charged magnetic nanoparticles were formed by electrostatic interaction in presence or absence of additional negatively charged poly(ethylene glycol)-based polymer. Transduction of HUVEC was analyzed in vitro under flow. Protection from inactivation by the immune system was analyzed by pre-incubation of AdV and complexes with neutralizing antibodies and subsequent reporter protein analysis of infected cells.

RESULTS

Physical association of AdV with MNP and polymers was demonstrated by radioactive labelling of components and co-sedimentation in a magnetic field. Ad-MNP+/-polymer resulted in efficient transduction of HUVEC, depending on MOI and flow rate in presence of magnetic field, whereas no transduction was observed without complex formation with MNP or in absence of magnetic field. Association with MNP did result in protection from neutralizing antibodies, with slightly increased protection provided by the polymer.

CONCLUSIONS

Complex formation of AdV with MNP is a viable means for targeting of vectors to areas of magnetic field gradient. Additional coating with polymer might proof useful in protection from inactivation by the immune system.

Gene therapy for pulmonary hypertension: prospects and challenges

INTRODUCTION

Recent evidence shows that pulmonary arterial hypertension (PAH) remains a fatal disease despite the introduction of new pharmacological treatments. New options are therefore needed and gene therapy approaches are a rational consideration based on emerging understanding of the genetic basis of PAH.

AREAS COVERED

This review briefly discusses the recent developments in clinical management of PAH and the investigation of gene delivery techniques for pulmonary vascular disease from 1997 to 2010, relating this to improved understanding of disease pathogenesis during this period. There is a focus on bone morphogenetic protein receptor type 2, as mutations in this gene are clearly linked to disease pathogenesis and outcomes. The reader will gain insight into the gene vector strategies being used, the target cells and the specific genes being delivered as candidate therapeutic approaches for PAH.

EXPERT OPINION

Various genes and strategies for delivery have achieved improvements in PAH in animal models, which is encouraging for the development of this technology for human application. The main limiting factor for clinical progress relates to gene delivery vector technology.

Construction of a novel expression cassette for increasing transgene expression in vivo in endothelial cells of large blood vessels

The success of gene therapy hinges on achievement of adequate transgene expression. To ensure high transgene expression, many gene-therapy vectors include highly active virus-derived transcriptional elements. Other vectors include tissue-specific eukaryotic transcriptional elements, intended to limit transgene expression to specific cell types, avoid toxicity and prevent immune responses. Unfortunately, tissue specificity is often accompanied by lower transgene expression. Here, we use eukaryotic (murine) transcriptional elements and a virus-derived posttranscriptional element to build cassettes designed to express a potentially therapeutic gene (interleukin (IL)-10) in large-vessel endothelial cells (ECs) at levels as high as obtained with the cytomegalovirus (CMV) immediate early promoter, while retaining EC specificity. The cassettes were tested by incorporation into helper-dependent adenoviral vectors, and transduction into bovine aortic EC in vitro and rabbit carotid EC in vivo. The murine endothelin-1 promoter showed EC specificity, but expressed only 3% as much IL-10 mRNA as CMV. Inclusion of precisely four copies of an EC-specific enhancer and a posttranscriptional regulatory element increased IL-10 expression to a level at or above the CMV promoter in vivo, while retaining--and possibly enhancing--EC specificity, as measured in vitro. The cassette reported here will likely be useful for maximizing transgene expression in large-vessel EC, while minimizing systemic effects.

Viruses in pharmaceutical research: pulmonary vascular disease

The management and understanding of pulmonary arterial hypertension (PAH) has undergone something of a revolution in the last 10 years, with new pharmacological agents entering routine clinical practice and significantly improving outcomes. Nevertheless many patients ultimately progress, and additional new treatment approaches are needed. There is now greater understanding of the molecular and genetic basis for the development of PAH, specifically in regard to the role of bone morphogenetic protein receptor 2 (BMPR2) signaling and related pathways. The challenge is to determine whether these new discoveries can be exploited for new therapies. In this article the role of viruses as tools for gene delivery for pulmonary vascular disease is discussed. Gene delivery of BMPR2 has now been shown to ameliorate the development and progression of PAH in animal models, thereby identifying this approach as a therapeutic target.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

Tropism-modification strategies for targeted gene delivery using adenoviral vectors

Achieving high efficiency, targeted gene delivery with adenoviral vectors is a long-standing goal in the field of clinical gene therapy. To achieve this, platform vectors must combine efficient retargeting strategies with detargeting modifications to ablate native receptor binding (i.e. CAR/integrins/heparan sulfate proteoglycans) and “bridging” interactions. “Bridging” interactions refer to coagulation factor binding, namely coagulation factor X (FX), which bridges hepatocyte transduction in vivo through engagement with surface expressed heparan sulfate proteoglycans (HSPGs). These interactions can contribute to the off-target sequestration of Ad5 in the liver and its characteristic dose-limiting hepatotoxicity, thereby significantly limiting the in vivo targeting efficiency and clinical potential of Ad5-based therapeutics. To date, various approaches to retargeting adenoviruses (Ad) have been described. These include genetic modification strategies to incorporate peptide ligands (within fiber knob domain, fiber shaft, penton base, pIX or hexon), pseudotyping of capsid proteins to include whole fiber substitutions or fiber knob chimeras, pseudotyping with non-human Ad species or with capsid proteins derived from other viral families, hexon hypervariable region (HVR) substitutions and adapter-based conjugation/crosslinking of scFv, growth factors or monoclonal antibodies directed against surface-expressed target antigens. In order to maximize retargeting, strategies which permit detargeting from undesirable interactions between the Ad capsid and components of the circulatory system (e.g. coagulation factors, erythrocytes, pre-existing neutralizing antibodies), can be employed simultaneously. Detargeting can be achieved by genetic ablation of native receptor-binding determinants, ablation of “bridging interactions” such as those which occur between the hexon of Ad5 and coagulation factor X (FX), or alternatively, through the use of polymer-coated “stealth” vectors which avoid these interactions. Simultaneous retargeting and detargeting can be achieved by combining multiple genetic and/or chemical modifications.

Nitric oxide synthase gene therapy: progress and prospects

NOS gene therapy has been the focus of extensive research as dysfunction of this enzyme has been implicated in several cardiovascular diseases. Research has concentrated on comparing the effect of gene delivery of NOS isoforms (eNOS, iNOS and nNOS) in healthy and diseased animal models on intimal hyperplasia, restenosis, vascular tone and ischemia-reperfusion injury. Most results demonstrate therapeutic benefits following vascular gene delivery of all NOS in pre-clinical models of cardiovascular disease. eNOS has been shown to have particular promise as it promotes re-endothelialisation and inhibits intimal hyperplasia in injured blood vessels. The ultimate goal is to translate the benefit of NOS gene therapy in animal models into clinical practise. To develop NOS gene therapy for clinical use further work needs to be undertaken to improve delivery systems and vectors to minimise detrimental side-effects and enhance positive treatment outcomes. This review focuses on current research on NOS gene therapy in cardiovascular disease and identifies the next steps that would be necessary to lead to clinical trials.

Hypoxia-specific gene expression for ischemic disease gene therapy

Gene therapy for ischemic diseases has been developed with various growth factors and anti-apoptotic genes. However, non-specific expression of therapeutic genes may induce deleterious side effects such as tumor formation. Hypoxia-specific regulatory systems can be used to regulate transgene expression in hypoxic tissues, in which gene expression is induced in ischemic tissues, but reduced in normal tissues by transcriptional, translational or post-translational regulation. Since hypoxia-inducible factor 1 (HIF-1) activates transcription of genes in hypoxic tissues, it can play an important role in the prevention of myocardial and cerebral ischemia. Hypoxia-specific promoters including HIF-1 binding sites have been used for transcriptional regulation of therapeutic genes. Also, hypoxia-specific untranslated regions (UTRs) and oxygen dependent degradation (ODD) domains have been investigated for translational and post-translational regulations, respectively. Hypoxia-specific gene expression systems have been applied to various ischemic disease models, including ischemic myocardium, stroke, and injured spinal cord. This review examines the current status and future challenges of hypoxia-specific systems for safe and effective gene therapy of ischemic diseases.

Transcriptional targeting of tumor endothelial cells for gene therapy

It is well known that angiogenesis plays a critical role in the pathobiology of tumors. Recent clinical trials have shown that inhibition of angiogenesis can be an effective therapeutic strategy for patients with cancer. However, one of the outstanding issues in anti-angiogenic treatment for cancer is the development of toxicities related to off-target effects of drugs. Transcriptional targeting of tumor endothelial cells involves the use of specific promoters for selective expression of therapeutic genes in the endothelial cells lining the blood vessels of tumors. Recently, several genes that are expressed specifically in tumor-associated endothelial cells have been identified and characterized. These discoveries have enhanced the prospectus of transcriptionally targeting tumor endothelial cells for cancer gene therapy. In this manuscript, we review the promoters, vectors, and therapeutic genes that have been used for transcriptional targeting of tumor endothelial cells, and discuss the prospects of such approaches for cancer gene therapy.

CRAdRGDflt-IL24 virotherapy in combination with chemotherapy of experimental glioma

Malignant forms of glioma, the most common primary brain tumors, remain poorly responsive to multimodality therapeutic interventions, including chemotherapy. Suppressed apoptosis and extraordinary invasiveness are important distinctive features that contribute to the malignant phenotype of glioma. We have developed the vascular endothelial growth factor receptor 1 (VEGFR-1/flt-1) conditional replicating adenoviral vector (CRAdRGDflt-IL24) encoding the interleukin-24 (IL-24) gene. We investigated whether a combination of CRAdRGDflt-IL24-mediated oncolytic virotherapy and chemotherapy using temozolomide (TMZ) produces increased cytotoxicity against human glioma cells in comparison with these agents alone. Combination of CRAdRGDflt-IL24 and TMZ significantly enhanced cytotoxicity in vitro, inhibited D54MG tumor growth and prolonged survival of mice harboring intracranial human glioma xenografts in comparison with CRAdRGDflt-IL24 or TMZ alone. These data indicate that combined treatment with CRAdRGDflt-IL24-mediated oncolytic virotherapy and TMZ chemotherapy provides a promising approach for glioma therapy.

Spinal cord injury reduces the efficacy of pseudorabies virus labeling of sympathetic preganglionic neurons

The retrograde transsynaptic tracer pseudorabies virus (PRV) is used as a marker for synaptic connectivity in the spinal cord. Using PRV, we sought to document putative synaptic plasticity below a high thoracic (T) spinal cord transection. This lesion has been linked to the development of a number of debilitating conditions, including autonomic dysreflexia. Two weeks after injury, complete T4-transected and/or T4-hemisected and sham rats were injected with PRV-expressing enhanced green fluorescent protein (EGFP) or monomeric red fluorescent protein (mRFP1) into the kidneys. We expected greater PRV labeling after injury because of the plasticity of spinal circuitry, but 96 hours post-PRV-EGFP inoculation, we found fewer EGFP+ cells in the thoracolumbar gray matter of T4-transected compared with sham rats (p < 0.01); Western blot analysis corroborated decreased EGFP protein levels (p < 0.01). Moreover, viral glycoproteins that are critical for cell adsorption and entry were also reduced in the thoracolumbar spinal cord of injured versus sham rats (p < 0.01). Pseudorabies virus labeling of sympathetic postganglionic neurons in the celiac ganglia innervating the kidneys was also significantly reduced in injured versus sham rats (p < 0.01). By contrast, the numbers and distribution of Fluoro-Gold-labeled (intraperitoneal injection) sympathetic preganglionic neurons throughout the sampled regions appeared similar in injured and sham rats. These results question whether spinal cord injury exclusively retards PRV expression and/or transport or whether this injury broadly affects host cell-viral interactions.

Exploiting the tumor microenvironment in the development of targeted cancer gene therapy

The future success of cancer gene therapy is critically dependent upon the development of safe, practical and effective targeting strategies. In this study, we describe a novel and broadly applicable targeting approach in which the induction of apoptotic tumor cell death is linked to the differential expression within the tumor microenvironment of elevated levels of the pro-angiogenic cytokine vascular endothelial growth factor (VEGF). As VEGF is generally absent or produced at only low levels in most normal tissues, undesirable toxicity will not result even if the therapeutic gene in question is inadvertently expressed in non-targeted tissue sites. The basic approach makes use of a chimeric cell-surface protein in which the membrane-spanning and cytoplasmic 'death domain' of the pro-apoptotic protein Fas are fused in frame to the extracellular ligand-binding domain of the VEGF receptor Flk-1/KDR/VEGFR2 (Flk-1/Fas). The resultant chimeric Flk-1/Fas receptor was found to be stable and capable of inducing a rapid apoptotic response when expressed in tumor cells that produce endogenous VEGF. Importantly, in the absence of VEGF, transduced tumor cells remain viable although they can be triggered to die by the addition of recombinant VEGF. Given the key role played by VEGF in tumor development and progression, it is proposed that the Flk-1/Fas chimera may have great potential in the context of tumor cell-targeted cancer gene therapy.

Endothelial dysfunction: from molecular mechanisms to measurement, clinical implications, and therapeutic opportunities

Endothelial dysfunction has been implicated as a key factor in the development of a wide range of cardiovascular diseases, but its definition and mechanisms vary greatly between different disease processes. This review combines evidence from cell-culture experiments, in vitro and in vivo animal models, and clinical studies to identify the variety of mechanisms involved in endothelial dysfunction in its broadest sense. Several prominent disease states, including hypertension, heart failure, and atherosclerosis, are used to illustrate the different manifestations of endothelial dysfunction and to establish its clinical implications in the context of the range of mechanisms involved in its development. The size of the literature relating to this subject precludes a comprehensive survey; this review aims to cover the key elements of endothelial dysfunction in cardiovascular disease and to highlight the importance of the process across many different conditions.

Efficient targeted transduction of primary human endothelial cells with dual-targeted lentiviral vectors

Angiogenesis is a rate-limiting factor for numerous human diseases. Angiogenic vessels and also the endothelium of certain organs such as the lung display molecular addresses that can be exploited for the selective delivery of gene therapeutics. Lentiviral vectors (LVs) are powerful tools for stable gene delivery but their integration and expression in undesired cell types poses a serious safety concern. We have developed a dual-targeted LV that can specifically target primary endothelial cells (ECs). Cell selectivity is achieved during entry, using a modified Sindbis virus envelope, and during transcription, with an EC-specific promoter. We evaluated four surface markers for EC targeting and seven promoter sequences from genes preferentially expressed in ECs. The efficiency and specificity of the double targeted vector were assayed in a panel of human primary cultures and tumor cell lines. A vector targeted to CD146, an endothelial adhesion molecule, and carrying a derivative of the EC tyrosine kinase Tie2 promoter, increased specificity of transduction up to 50 times and was also effective at selectively transducing ECs in a mixed coculture with human fibroblasts. The vector presented here is a potentially powerful tool that could be used in a variety of human diseases.

In vitro and in vivo analysis of expression cassettes designed for vascular gene transfer

Increasing the level and duration of transgene expression and restricting expression to vascular cells are important goals for clinically useful gene therapy vectors. We evaluated several promoters, enhancers and introns in endothelial, smooth muscle and liver cells in tissue culture and in vivo, comparing local delivery to the carotid artery with intravenous delivery to the liver. A 1800-bp fragment of the oxidized LDL receptor (LOX-1) promoter showed highest in vivo activity in the carotid artery, achieving 39% the activity of the reference cytomegalovirus promoter, with 188-fold greater specificity for carotid artery over liver. An enhancer from the Tie2 gene in combination with the intracellular adhesion molecule-2 promoter improved endothelial specificity of plasmid vectors, increased the expression from adenoviral vectors in cultured endothelial cells and doubled the specificity for carotid artery over liver in vivo. Adding a short intron to expression cassettes increased expression in both endothelial and smooth muscle cells in vitro; however, the eNOS enhancer failed to consistently increase the expression or endothelial specificity of the vector. In conclusion, elements from the LOX-1 promoter and Tie2 enhancer together with an intron can be used to improve vectors for vascular gene transfer.

In vivo transcriptional targeting into the retinal vasculature using recombinant baculovirus carrying the human flt-1 promoter

Background

Endothelial cells are a target for gene therapy because they are implicated in a number of vascular diseases. Recombinant baculovirus have emerged as novel gene delivery vectors. However, there is no information available concerning the use of endothelial-specific promoters in the context of the baculovirus genome. In the present study, we have generated a recombinant baculovirus containing the human flt-1 promoter (BacFLT-GFP) driving the expression of the green fluorescent protein. Transcriptional gene targeting was analyzed in vitro in different mammalian cell lines and in vivo in adult rat retinal vasculature.

Results

BacFLT-GFP evoked the highest levels of expression in the endothelial cell line BUVEC-E6E7-1, similar to those reached by recombinant baculovirus carrying the CMV promoter (112% relative to BacCMV-GFP, n = 4). Interestingly, BacFLT-GFP directed high levels of expression in rat glioma C6 and in human glioblastoma CH235 cells (34.78% and 47.86% relative to BacCMV-GFP, respectively). Histone deacetylase inhibitors such as butyrate or trichostatin A enhanced the transcriptional activity of both BacCMV-GFP and BacFLT-GFP. Thus, in this study histone deacetylation appears to be a central mechanism for the silencing of baculovirus, independently of the promoter utilized. In vivo transcriptional targeting was demonstrated in adult rat retinal vasculature by intravitreal delivery of BacFLT-GFP and immunohistochemical staining with von Willebrand factor (vWF). Analysis by fluorescence microscopy and deconvolved three-dimensional confocal microscopy of retinal whole mounts obtained after 3 days of baculovirus injection showed that most GFP-expressing cells localized to the inner limiting membrane (ILM) and ganglion cell layer (GCL) and colocalize with vWF (70%, n = 10) in blood vessels, confirming the endothelial phenotype of the transduced cells.

Conclusion

Taken together, our results indicate that the restricted expression in endothelial cells mediated by the flt-1 promoter is not affected by the context of the baculovirus genome and demonstrate the potential of using recombinant baculovirus for transcriptional targeted gene expression into the eye vasculature.

Targeted gene-delivery strategies for angiostatic cancer treatment

Gene therapy is one of the promising strategies in cancer treatment. Recent studies identified molecular targets on angiogenically activated endothelial cells that can be used to deliver gene-transfer vehicles to the tumor site specifically. Furthermore, non-viral vehicles are emerging as an alternative for traditional viral gene-therapy approaches. Here, we describe how viral and non-viral gene-transfer vehicles have been and can be modified to target tumor endothelial cells for anti-angiogenesis gene therapy. Improving the specificity and safety of existing gene-therapy vehicles will make angiogenesis-targeted cancer gene therapy a valuable tool in the clinical setting.

New vectors and strategies for cardiovascular gene therapy

Cardiovascular diseases are the major cause of morbidity and mortality in both men and women in industrially developed countries. These disorders may result from impaired angiogenesis, particularly in response to hypoxia. Despite many limitations, gene therapy is still emerging as a potential alternative for patients who are not candidates for traditional revascularization procedures, like angioplasty or vein grafts. This review focuses on recent approaches in the development of new gene delivery vectors, with great respect to newly discovered AAV serotypes and their modified forms. Moreover, some new cardiovascular gene therapy strategies have been highlighted, such as combination of different angiogenic growth factors or simultaneous application of genes and progenitor cells in order to obtain stable and functional blood vessels in ischemic tissue.

In vivo analysis of a genetically modified adenoviral vector targeted to human CD40 using a novel transient transgenic model

BACKGROUND

Retargeting is necessary to overcome the limitations of adenovirus (Ad)-based gene therapy vectors. To this end, we previously constructed an adenovirus with the fiber knob domain replaced by a fibritin trimerization motif fused to the CD40 ligand (Ad5Luc.FF/CD40L). We demonstrated the utility of this fiber replacement strategy for targeting CD40 (hCD40) on human dendritic cells in vitro. The in vivo targeting capacity of this virus, however, is unknown, and there is a limited repertoire of animal models that present hCD40 at an accessible site. Therefore, a new animal model for evaluating CD40-targeted vectors is required.

METHODS

We constructed a recombinant adenovirus that expresses hCD40 under transcriptional control of the flt-1 promoter (AdflthCD40). Expression of hCD40 was validated both in vitro and in transgenic mice expressing the human coxsackie adenovirus receptor (hCAR mice). We then evaluated the targeting efficiency of Ad5Luc.FF/CD40L to hCD40 expressed in the pulmonary vasculature of the hCAR mice.

RESULTS

Infection of flt-1-positive cells with AdflthCD40 resulted in abundant hCD40 expression in vitro, which could subsequently be targeted by Ad5Luc.FF/CD40L. In vivo administration of AdflthCD40 to hCAR mice resulted in hCD40 expression in the pulmonary vasculature, which was successfully targeted with systemically administered Ad5Luc.FF/CD40L.

CONCLUSIONS

This is the first data showing that genetically modified Ad5Luc.FF/CD40L can successfully target hCD40 in vivo. Our data also establishes the utility of transcriptionally targeted, Ad-mediated transient expression of human target molecules in the pulmonary vasculature of hCAR mice as models for in vivo analysis of targeted gene therapy vectors.

Plasmid engineering for controlled and sustained gene expression for nonviral gene therapy

Gene therapy requires the introduction of genetic material in diseased cells with the aim of treating or ultimately curing a disease. Since the start of gene therapy clinical trials in 1990, gene therapy has proven to be possible, but studies to date have highlighted the difficulty of achieving efficient, specific, and long-term transgene expression. Efforts to improve gene therapy strategies over the past years were mainly aimed at solving the problem of delivery, without paying much attention to the optimization of the expression cassette. With the current understanding of the eukaryotic transcription machinery and advanced molecular biology techniques at our disposition, it has now become possible to create custom-made transgene expression cassettes optimized for gene therapy applications. In this review, we will discuss several strategies that have been explored to improve the level and duration of transgene expression, to increase control over expression, or to restrict transgene expression to specific cell types or tissues. Although still in its infancy, such strategies will eventually lead to improvement of nonviral gene therapy and expansion of the range of possible therapeutic applications.

Selective induction of tumor-associated antigens in murine pulmonary vasculature using double-targeted adenoviral vectors

Targeted therapies directed to tumor-associated antigens are being investigated for the treatment of cancer. However, there are few suitable animal models for testing the ability to target these tumor markers. Therefore, we have exploited mice transgenic for the human coxsackie and adenovirus receptor (hCAR) to establish a new model for transient expression of human tumor-associated antigens in the pulmonary vasculature. Systemic administration of Ad in hCAR mice resulted in an increase in transgene expression in the lungs compared to wild-type mice, as determined using a luciferase reporter gene. To reduce transgene expression in the liver, the predominant organ of ectopic Ad localization and transgene expression following systemic administration, we utilized the endothelial-specific flt-1 promoter, which resulted in a further increased lung-to-liver ratio of luciferase expression. Administration of an adenoviral vector encoding the tumor-associated antigen carcinoembryonic antigen (CEA) under transcriptional control of the flt-1 promoter resulted in selective expression of this antigen in the pulmonary vasculature of hCAR mice. Feasibility of targeting to expressed CEA was subsequently demonstrated using adenoviral vectors preincubated with a bifunctional adapter molecule recognizing this tumor-associated antigen, thus demonstrating utility of this transient transgenic animal model.

Genetic Targeting Strategies for Adenovirus

Adenovirus serotype 5 (Ad5) continues to be regarded as a gene delivery vehicle of high utility for a variety of clinical applications. However, targeting of the virus to alternate, non-native receptors has become a mandate for many gene therapy approaches, as inefficient viral transduction of target tissues has proven detrimental to the utility of Ad5. Thus, various targeting strategies have been endeavored to the end of highly specific cellular transduction, including that of genetic manipulation of the viral capsid. Modification of the tropism-determining fiber protein and other capsid locales has allowed vectorologists to develop vectors that are highly superior to the first-generation adenoviruses employed for gene therapy. Herein, the various genetic targeting strategies for Ad5 are reviewed, and the various schemas in which targeted transduction has been achieved with tropism-modified vectors are outlined.

Human cytomegalovirus infection of endothelial cells triggers platelet adhesion and aggregation

Human cytomegalovirus (HCMV) is an opportunistic pathogen that has been implicated in the pathogenesis of vascular diseases. HCMV infection of endothelial cells may lead to vascular damage in vitro, and acute-phase HCMV infection has been associated with thrombosis. We hypothesized that viral infection of endothelial cells activates coagulation cascades and contributes to thrombus formation and acute vascular catastrophes in patients with atherosclerotic disease. To assess the effects of HCMV on thrombogenesis, we examined the adhesion and aggregation of blood platelets to uninfected and HCMV-infected endothelial cells. At 7 days after infection, platelet adherence and aggregation were greater in infected than in uninfected cultures (2,000 platelets/100 cells and 225 +/- 15 [mean +/- standard error of the mean] aggregates/five microscopic fields versus 100 platelets/100 cells and no aggregates). von Willebrand factor (vWF), ICAM-1, and VCAM-1 but not collagen IV, E-selectin, P-selectin, CD13, and CD31 were expressed at higher levels on infected cells than on uninfected cells. Platelet aggregation was inhibited by blocking of platelet GPIb (with blocking antibodies) or GPIIb/IIIa (with ReoPro) or by blocking of vWF (with polyclonal antibodies to vWF). Furthermore, blocking of vWF, platelet GPIb, and ICAM-1 but not of the endothelial cell marker CD13, alpha(5)beta(3)-integrin, or HCMV glycoprotein B reduced platelet adherence to infected cells by 75% +/- 5%, 74% +/- 5%, or 18% +/- 5%, respectively. The increased thrombogenicity was dependent on active virus replication and could be inhibited by foscarnet and ganciclovir; these results suggest that a late viral gene may be mediating this phenomenon, which may contribute to vascular catastrophes in patients with atherosclerotic disease.

Vascular-targeted cancer gene therapy

As a consequence of the dramatic progress that has been made in recent years towards elucidating the diverse molecular events involved in the development and pathogenesis of malignant disease, there is now no shortage of genes that can be exploited or targeted in the context of cancer gene therapy. Many of these have been shown to be effective both in vitro and in various animal models, and a number have progressed to the clinic. The results of these later studies, although generally encouraging, are perhaps less dramatic than one might have hoped. Although a number of factors undoubtedly contribute to this finding, it is evident that a major reason relates to the difficulties implicit in achieving efficient in vivo gene transfer, particularly in a clinical context. Targeting gene therapy, not to the malignant population, but instead to the vasculature upon which the survival and growth of a tumour depends constitutes an alternative approach that overcomes some of the delivery problems associated with established tumour cell-directed strategies.

Adenoviruses for treatment of cancer

Most cases of cancer, when detected at an advanced stage, cannot be cured with conventional therapeutic modalities. Therefore, novel targeted approaches such as gene therapy are needed. Nevertheless, while the safety record of gene therapy for cancer has been excellent with more than a thousand patients treated without mortality related to the therapy, clinical efficacy has so far been limited. Moreover, it has become evident that clinical efficacy is partly determined by efficacy of gene delivery. Most adenoviruses used for gene therapy have been based on serotype 5 (Ad5). Unfortunately, recent data suggest that the primary receptor, the coxsackie-adenovirus receptor (CAR) expression in tumors may be highly variable resulting in resistance to adenovirus infection. Consequently, various strategies have been evaluated to modify adenovirus tropism in order to circumvent CAR deficiency, including retargeting complexes or genetic capsid modifications. To further improve tumor penetration and local amplification of the anti-tumor effect, selectively oncolytic agents, e.g. conditionally replicating adenoviruses (CRAds), have been constructed. Infection of tumor cells results in replication, oncolysis, and subsequent release of the virus progeny. Normal tissue is spared due to lack of replication. This review will focus on a discussion of various modifications of adenovirus to achieve efficient anti-tumor effect, and special emphasis will be placed on CRAds in multimodality treatments.

Adenovirus-mediated FLT1-targeted proapoptotic gene therapy of human prostate cancer

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) is of particular interest in the development of prostate carcinoma therapeutics as it preferentially induces apoptosis of tumor cells. To employ adenoviral vectors for highly efficient and specific TRAIL gene transfer into cancer cells could overcome some potential problems for recombinant TRAIL. The vascular endothelial growth factor receptor FLT-1 is involved in regulation of angiogenesis and tumor growth, invasion, and metastasis of prostate carcinoma. FLT-1 expression is observed in both tumor endothelial cells and prostate cancer cells. We developed an adenoviral vector encoding the TRAIL gene under control of the FLT1 promoter (AdFlt-TRAIL), which produced endothelial and prostate cancer cell death. The combination of ionizing radiation and adenovirus-driven TRAIL expression overcame human prostate cancer cell resistance to TRAIL. Furthermore, in vivo administration of AdFlt-TRAIL at the site of tumor growth in combination with radiation treatment produced significant suppression of the growth of DU145 human prostate tumor xenografts in athymic nude mice. Our results suggest that specific TRAIL delivery employing the FLT1 promoter can effectively inhibit tumor growth and demonstrate the advantage of combination radiotherapy and gene therapy for the treatment of prostate cancer.

Dual targeting of gene delivery by genetic modification of adenovirus serotype 5 fibers and cell-selective transcriptional control

Adenovirus (Ad)-mediated gene delivery is a promising approach for genetic manipulation of the vasculature and is being used in both preclinical models and clinical trials. However, safety concerns relating to infection of nontarget tissue and the poor infectivity of vascular cells compared to other cell types necessitates Ad vector refinement. Here, we combine a transductional targeting approach to improve vascular cell infectivity through RGD peptide insertion into adenovirus fibers, combined with transcriptional targeting to endothelial cells using a approximately 1 kb fragment of the fms-like tyrosine kinase receptor-1 (FLT-1) promoter. Single- and double-modified vectors were characterized in human cell lines that either support or have silenced FLT-1 expression. In rat hepatocytes and endothelial cells, the double modification substantially shifted transduction profiles toward vascular endothelial cells. Furthermore, in intact aortae derived from spontaneously hypertensive rats that display enhanced alphav integrin expression on dysfunctional endothelium, enhanced levels of transduction were observed using the double-modified vector but not in aortae derived from normotensive control rats. Our data indicate that Ad-mediated transduction can be beneficially modified in vitro and in vivo by combining fiber modification and a cell-selective promoter within a single-component vector system.

Designing gene delivery vectors for cardiovascular gene therapy

Genetic therapy in the cardiovascular system has been proposed for a variety of diseases ranging from prevention of vein graft failure to hypertension. Such diversity in pathogenesis requires the delivery of therapeutic genes to diverse cell types in vivo for varying lengths of time if efficient clinical therapies are to be developed. Data from extensive preclinical studies have been compiled and a certain areas have seen translation into large-scale clinical trials, with some encouraging reports. It is clear that progress within a number of disease areas is limited by a lack of suitable gene delivery vector systems through which to deliver therapeutic genes to the target site in an efficient, non-toxic manner. In general, currently available systems, including non-viral systems and viral vectors such as adenovirus (Ad) or adeno-associated virus (AAV), have a propensity to transduce non-vascular tissue with greater ease than vascular cells thereby limiting their application in cardiovascular disease. This problem has led to the development and testing of improved vector systems for cardiovascular gene delivery. Traditional viral and non-viral systems are being engineered to increase their efficiency of vascular cell transduction and diminish their affinity for other cell types through manipulation of vector:cell binding and the use of cell-selective promoters. It is envisaged that future use of such technology will substantially increase the efficacy of cardiovascular gene therapy.

Evaluation of tissue-specific promoters in carcinomas of the cervix uteri

BACKGROUND

Gene therapy is a novel approach for treatment of patients with advanced, recurrent, or metastatic cervical cancer. One effective way to direct transgene expression to specific tissues or tumors is the use of tissue-specific-promoters (TSPs). In the context of adenovirus (Ad)-mediated cancer gene therapy it is rational to choose a TSP which is highly expressed in the tumor but has potentially low activity in non-tumor cells, especially the liver. In this study, we have investigated several promoters which fulfill these criteria. Candidate cervical cancer specific TSPs include promoters of the genes for secretory leukoprotease inhibitor (SLPI), cyclooxygenase-2 (COX-2), Midkine (MK), vascular endothelial growth factor receptor type 1 (flt-1), vascular endothelial growth factor (VEGF), Survivin and the receptor for chemokine SDS-1 (CXCR4).

METHODS

To evaluate the specific gene expression of the different promoters in the context of cervical cancer, we constructed a panel of E1-deleted Ads that express luciferase under the control of the promoters of interest. We investigated various established cervical cancer cell lines, as well as purified primary cancer cells and normal control cells from the cervix uteri.

RESULTS

In all cell lines tested, promoters for MK, VEGF and CXCR4 showed the highest activity. Both MK and VEGF promoters also resulted in a high activity in primary cervical cancer cells. Interestingly, gene expression profiles correlate with luciferase activity in both cell lines and primary cancer samples.

CONCLUSIONS

Our study demonstrates that the promoters for MK and VEGF are active in cervical cancer. We believe that both promoters can be successfully employed as TSPs for gene therapy targeted to cervical cancer.

Transductional and transcriptional targeting of cancer cells using genetically engineered viral vectors

Gene delivery vectors, including adenovirus (Ad) and adeno-associated virus (AAV), are inefficient and non-selective for cancer due to low levels of viral receptors with high levels on other tissues, including liver. We tested Ads and AAVs with the SIGYPLP-targeting peptide inserted into virus capsids for transduction in a panel of cancer cells. Six of twelve lines (C8161, PC-3, G-CCM, MKN-45, LnCAP and A549) were transduced, independently of native viral tropism. Furthermore the candidate cancer gene therapy promoter FLT-1 was active in three of these six cell lines. This offers the potential for dual targeting of selected cancer cells.

Therapeutic effect of endostatin-vascular endothelial growth inhibitor recombinant adenoviruses on gastric carcinoma in nude mice

AIM

To investigate the effects of recombinant adenoviruses carrying the endostatin-VEGI151 fusion gene AIerapy on gastric carcinoma in nude mice.

METHODS

Human gastric carcinoma SGC7901 cells (5×10⁶) were injected sc into the dorsal midline of nude mice (3 weeks old), and 500 μL (10¹² TCID₅₀/L) of recombinant adenoviruses AdCA13-hENDO-VEGI₁₅₁ or AdLacZ was given respectively sc every other day for ten times. Twenty-four hours after the last injection, tumor size, inhibition rate, expression of target gene, proliferating cell nuclear antigen label index (PCNA LI), apoptotic index (AI), and intratumoral microvessel density (MVD) were evaluated respectively after the mice were sacrificed.

RESULTS

Compared with the control group AdLacZ (2 356±1 140 mm³), the average tumor size of group AdCA13-hENDO-VEGI₁₅₁ was reduced in size(328156 mm³, F =12.42, P =0.0125) with an inhibition rate of 86.1%. The adenoviruses carriyring the target gene could be expressed in gastric carcinoma. The PCNA LI of the treated mice (0.13±0.09%) was lower than the contol (1.400.53%, F =22.30, P =0.0 033). The AI was higher in the treated mice (5.09±0.25%) than in the control (0.61±0.67%, F =155.13, P =0.0 001). The MVD was lower in the treated mice (0.06±0.03% ) than in the control (1.09±0.76%, F =7.38, P =0.0 348).

CONCLUSION

The recombinant adenovirus carrying fusion gene of hENDO-VEGI₁₅₁ can express the fuion protein in gastric carcinoma in nude mice and inhibit the tumor growth.

In vivo targeting of tumor endothelial cells by systemic delivery of lentiviral vectors

Tumor angiogenesis is a rate-limiting factor for tumor growth, and the endothelial cells of tumor vessels display specific features that can be exploited for the selective delivery of cancer therapeutics. To specifically target exogenous genes to angiogenic tumor vessels, we generated a panel of vesicular stomatitis virus-pseudotyped lentiviral vectors (LVs) engineered for endothelial cell (EC)-specific expression. We cloned a wide repertoire of transcription regulatory sequences from genes preferentially expressed in ECs (Tie1, Tie2, Flk-1, VE-Cad, and ICAM-2) into self-inactivating LVs to drive expression of the marker gene encoding green fluorescent protein (GFP) or of the conditionally toxic gene encoding nitroreductase, and compared them with the ubiquitously expressing phosphoglycerate kinase (PGK) and cytomegalovirus (CMV) promoters. We evaluated the efficiency and specificity of vector expression in vitro in a panel of human primary cultures, including ECs, fibroblasts, neurons, lymphocytes, and hematopoietic progenitors, and in tumor cell lines. We found that vectors containing promoter and enhancer sequences from the Tie2 gene achieved remarkable specificity of expression in ECs in vitro and in vivo. On intravenous delivery into tumor-bearing mice, the Tie2 vector targeted expression to the ECs of tumor vessels. In contrast, LVs carrying the PGK or CMV promoter gave widespread GFP marking in ECs and non-ECs of tumors and other organs. The previously reported upregulation of the Tie2 gene in ECs activated for angiogenesis may explain the remarkable selectivity of expression of the Tie2 vector in ECs of tumor vessels. The new vector provides the means for selective delivery of gene therapy to tumor sites in vivo.

Targeting gene therapy vectors to the vascular endothelium

The ability to deliver genes, and hence therapeutic gene over-expression site-specifically in vivo remains the major challenge for research in the field. The obligate need to target transgene expression safely, efficiently, and selectively has become increasingly evident as a result of recent events in the clinical setting. The endothelium represents an important target for gene delivery given its fundamental role in the physiology and pathophysiology of many diseases. Recently, studies demonstrating the ability to target viral vectors to the endothelium have been reported. In this review, we discuss progress to date and highlight those areas still requiring further investigation and validation.

Targeting the cell cycle machinery for the treatment of cardiovascular disease

Cardiovascular disease represents a major clinical problem affecting a significant proportion of the world's population and remains the main cause of death in the UK. The majority of therapies currently available for the treatment of cardiovascular disease do not cure the problem but merely treat the symptoms. Furthermore, many cardioactive drugs have serious side effects and have narrow therapeutic windows that can limit their usefulness in the clinic. Thus, the development of more selective and highly effective therapeutic strategies that could cure specific cardiovascular diseases would be of enormous benefit both to the patient and to those countries where healthcare systems are responsible for an increasing number of patients. In this review, we discuss the evidence that suggests that targeting the cell cycle machinery in cardiovascular cells provides a novel strategy for the treatment of certain cardiovascular diseases. Those cell cycle molecules that are important for regulating terminal differentiation of cardiac myocytes and whether they can be targeted to reinitiate cell division and myocardial repair will be discussed as will the molecules that control vascular smooth muscle cell (VSMC) and endothelial cell proliferation in disorders such as atherosclerosis and restenosis. The main approaches currently used to target the cell cycle machinery in cardiovascular disease have employed gene therapy techniques. We will overview the different methods and routes of gene delivery to the cardiovascular system and describe possible future drug therapies for these disorders. Although the majority of the published data comes from animal studies, there are several instances where potential therapies have moved into the clinical setting with promising results.

Gene transfer strategies to inhibit neointima formation

Vascular smooth muscle cell (VSMC) proliferation after arterial injury results in neointima formation and plays an important role in the pathogenesis of restenosis after angioplasty, in-stent restenosis, vascular bypass graft occlusion, and allograft vasculopathy. Major progress has been made recently in elucidating the cellular and molecular mechanisms underlying neointima formation. However, no known curative treatment currently exists. In cases in which pharmacologic and surgical interventions have had limited success, gene therapy remains a potential strategy for the treatment of such vascular proliferative diseases. To date, recombinant adenoviral vectors continue to be the most efficient methods of gene transfer into the arterial wall. However, concerns over the safety of using viral vectors in a clinical situation have inspired the considerable progress that has been made in improving both viral and nonviral modes of gene transfer. This review discusses some of the recent insights and outstanding progress in vascular gene therapeutic approaches to inhibit neointima both from a biologic and therapeutic perspective.

Development and use of gene transfer for treatment of cardiovascular disease

Gene therapy holds promise for the treatment of cardiovascular diseases for which effective pharmacological therapies are insufficient or unavailable. Recent studies have suggested that modification of current gene delivery systems combined with the use of efficacious therapeutic genes may ultimately be successful for clinical vascular gene therapy. Although certain applications such as vein-graft failure may be best suited for short-term transient overexpression of therapeutic genes, other disorders including human essential hypertension and atherosclerosis require sustained overexpression of genes. Hence, design and use of vector systems for delivery of genes to the required site in vivo requires careful consideration. Both viral and nonviral gene therapy vectors show low efficiency for gene transfer into vascular cells and demonstrate a lack of selectivity, as vectors have natural tropism for other cells and tissues. Recent work has focused on the design, development, and utility of vascular cell-selective gene therapy vectors for use in distinct and diverse vascular gene therapy scenarios. Using phage display technology we have isolated small peptide ligands that mediate selective binding to either vascular endothelial cells or vascular smooth muscle cells. When engineered into either adenoviral (Ad) or adeno-associated viral (AAV) vectors, candidate peptides enabled the virus to selectively bind to the desired cell type thus generating novel vascular cell-selective gene transfer. As preclinical studies have highlighted both the potential for vascular gene therapy as well as defining the potential pitfalls, the development of disease-selective gene therapeutics will increase safety and efficiency of gene therapy for future clinical use.

Inducible and selective transgene expression in murine vascular endothelium

We have developed a system utilizing the murine Tie2 promoter/enhancer coupled with the "tetracycline-on" regulatory elements to create a model that allows regulated and selective expression of a beta-galactosidase (betaGal) reporter transgene in the adult murine vascular endothelium. Two independent lines of viable and fertile mice were characterized, and they exhibit minimal betaGal expression under basal conditions. In response to exogenous doxycycline (Dox), selective expression of betaGal was demonstrated in the vascular endothelium of all tissues examined. En face analyses of the aorta and its principle branches indicate that the vast majority of lumenal endothelial cells express the transgene. Inducible betaGal expression also extends to the endocardium and the microvasculature of all organs. There is no evidence of specific transgene expression in nonendothelial cell types. Induction of the betaGal was effectively achieved after 3 days of oral Dox treatment and persisted for over 3 mo with continuous administration. This model can now be widely applied to study the role of specific genes in the phenotype of adult murine vasculature.