The mifepristone-inducible gene regulatory system in mouse models of disease and gene therapy

Pulsed Electric Fields Alter Expression of NF-κB Promoter-Controlled Gene

The possibility to artificially adjust and fine-tune gene expression is one of the key milestones in bioengineering, synthetic biology, and advanced medicine. Since the effects of proteins or other transgene products depend on the dosage, controlled gene expression is required for any applications, where even slight fluctuations of the transgene product impact its function or other critical cell parameters. In this context, physical techniques demonstrate optimistic perspectives, and pulsed electric field technology is a potential candidate for a noninvasive, biophysical gene regulator, exploiting an easily adjustable pulse generating device. We exposed mammalian cells, transfected with a NF-κB pathway-controlled transcription system, to a range of microsecond-duration pulsed electric field parameters. To prevent toxicity, we used protocols that would generate relatively mild physical stimulation. The present study, for the first time, proves the principle that microsecond-duration pulsed electric fields can alter single-gene expression in plasmid context in mammalian cells without significant damage to cell integrity or viability. Gene expression might be upregulated or downregulated depending on the cell line and parameters applied. This noninvasive, ligand-, cofactor-, nanoparticle-free approach enables easily controlled direct electrostimulation of the construct carrying the gene of interest; the discovery may contribute towards the path of simplification of the complexity of physical systems in gene regulation and create further synergies between electronics, synthetic biology, and medicine.

Generation and characterization of U937-TR: a platform cell line for inducible gene expression in human macrophages

Monocytes and macrophages are involved in a wide range of biological processes and parasitic diseases. The characterization of the molecular mechanisms governing such processes usually requires precise control of the expression of genes of interest. We implemented a tetracycline-controlled gene expression system in the U937 cell line, one of the most used in vitro models for the research of human monocytes and macrophages. Here we characterized U937-derived cell lines in terms of phenotypic (morphology and marker expression) and functional (capacity for phagocytosis and for Leishmania parasite hosting) changes induced by phorbol-12-myristate-13-acetate (PMA). Finally, we provide evidence of tetracycline-inducible and reversible Lamin-A gene silencing of the PMA-differentiated U937-derived cells.

Photorearrangement of Quinoline-Protected Dialkylanilines and the Photorelease of Aniline-Containing Biological Effectors

The direct release of dialkylanilines was achieved by controlling the outcome of a photorearrangement reaction promoted by the (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ) photoremovable protecting group. The substrate scope was investigated to obtain structure-activity relationships and to propose a reaction mechanism. Introducing a methyl substituent at the 2-methyl position of the CyHQ core enabled the bypass of the photorearrangement and significantly improved the aniline release efficiency. We successfully applied the strategy to the photoactivation of mifepristone (RU-486), an antiprogestin drug that is also used to induce the LexPR gene expression system in zebrafish and the gene-switch regulatory system based on the pGL-VP chimeric regulator in mammals.

Mifepristone-inducible recombinant adenovirus attenuates paraquat-induced lung injury in rats

Objective:

To investigate the effects of overexpression of nuclear factor E2-related factor-2 (NRF2) on lung injury in rats exposed to paraquat (PQ) poisoning.

Methods:

A mifepristone (RU486)-inducible recombinant adenoviral vector carrying the human NRF2 gene (Ad-RUNRF2) was constructed and transfected via airway into the rats 7 days before the administration of RU486. Rats were orally challenged with PQ at 20 mg/kg 24 h after the injection of RU486. On days 0.5, 3 and 21 after PQ poisoning, the expressions of NRF2 and cytokines related to inflammation and oxidation in lung tissue were examined.

Results:

RU486 remarkably enhanced NRF2 mRNA and NRF2 protein levels in Ad-RUNRF2-transfected rats in a dose-dependent manner ( p < 0.01). PQ stimulated compensatory overexpression of NRF2, heme oxygenase 1 (HO-1) and NAD(P)H quinone oxidoreductase 1 (NQO-1) in lungs on days 0.5 and 3 after exposure ( p < 0.05), but depleted the expression of catalase (CAT), glutathione peroxidase (GSH-Px) and glutathione (GSH), with an increased malondialdehyde (MDA) ( p < 0.05). However, pretreatment with Ad-RUNRF2 and RU486 strongly enhanced the expression levels of NRF2, HO-1, NQO-1, CAT and GSH-Px in the lungs of PQ intoxicated rats, with increased GSH and decreased MDA ( p < 0.05). Pretreatment with Ad-RUNRF2 and RU486 also strongly suppressed the PQ-induced activation of nuclear factor κB (NF-κB) and decreased the levels of tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6). In addition, Ad-RUNRF2 and RU486 induction significantly reduced PQ-induced pathological changes in lungs and attenuated lung oedema and protein leakage caused by PQ ( p < 0.05).

Conclusion:

RU486-induced overexpression of NRF2 in lungs transfected with Ad-RUNRF2 can ameliorate PQ-induced lung injury by the activation of the NRF2-antioxidant response element (ARE) pathway.

An independent construct for conditional expression of atonal homolog-1

The mammalian homolog of the basic helix-loop-helix transcription factor atonal-1 (Atoh1 or Math1) is required for development of cochlear hair cells that function as the mechanosensory cells required for audition. Forced expression of Atoh1 in cochlear-supporting cells may provide a way to regenerate hair cells and provide for a therapy for hearing loss. Additionally, Atoh1 is an inhibitor of proliferation and has further clinical applications in anticancer therapies. The goal of these experiments was to improve the method for Atoh1 expression by engineering a genetic construct that may be used in future translational applications. To address the poor control of Atoh1 expression in standard gene expression systems where Atoh1 is expressed constitutively at abnormally elevated levels, our aim was to engineer an inducible system whereby Atoh1 was upregulated by an inducer and downregulated once the inducer was removed. A further aim was to engineer a single genetic construct that allowed for conditional expression of Atoh1 independent of secondary regulatory elements. Here we describe a stand-alone genetic construct that utilizes the tamoxifen sensitivity of a mutated estrogen receptor (ER) ligand-binding domain for the conditional expression of Atoh1. The Atoh1-ER-DsRed construct is translated into an ATOH1-ER-DSRED fusion protein that remains sequestered in the cytoplasm and therefore rendered inactive because it cannot enter the nucleus to activate Atoh1 signaling pathways. However, application of 4-hydroxytamoxifen results in translocation of the fusion protein to the nucleus, where it binds to the Atoh1 enhancer, upregulates transcription and translation of endogenous ATOH1 and activates downstream Atoh1 signaling such as upregulation of the hair cell protein MYOSIN 7A. Removal of tamoxifen reverses the upregulation of endogenous Atoh1 signaling. This construct serves as an independent genetic construct that allows for the conditional upregulation and downregulation of Atoh1, and may prove useful for manipulating Atoh1 expression in vivo.

Recombinant adenovirus Ad-RUNrf2 reduces paraquat-induced A549 injury

OBJECTIVE

An RU486-inducible recombinant adenovirus-Nrf2 construct (Ad-RUNrf2) was constructed and expressed in H460 cells to determine whether Nrf2 gene expression can be regulated and to observe the effect of the adenovirus Ad-RUNrf2 on inflammatory cytokines, oxidative stress and apoptotic factors that mediate paraquat (PQ)-induced A549 cell injury.

METHODS

The Nrf2 gene within the RU486 (mifepristone)-inducible system was introduced into an adenovirus vector. A549 cells were transfected with Ad-RUNrf2, and Nrf2 expression was detected using Western blotting and real time-polymerase chain reaction (RT-PCR). RT-PCR, Western blots and enzyme-linked immunosorbent assay were used for observing the effect of RU486-induced Nrf2 expression on the inflammatory cytokines (interleukin-6 (IL-6), IL-10 and tumor necrosis factor-α (TNF-α)), oxidative stress factors (catalase (CAT) and malondialdehyde (MDA)) and apoptosis factors (caspase-3, caspase-9 and cytochrome C) that mediated PQ-induced A549 cell injury.

RESULTS

After infection of H460 cells by Ad-RUNrf2, RT-PCR and Western blot analyses showed that Nrf2 expression increased with additional RU486 doses. IL-6 and TNF-α protein and gene expression levels were significantly reduced, and IL-10 protein levels were significantly increased. Although IL-10 expression increased, it remained significantly lower than that of noninduced adenovirus infection and the simple virus exposure group. RU486 induced a significant reduction in MDA expression and increased CAT protein levels. Caspase-9 and caspase-3 protein and gene expression levels decreased in the RU486 induction group (p < 0.05). Cytochrome C protein levels were not significantly reduced, but its gene expression was significantly decreased (p < 0.05).

CONCLUSION

Ad-RUNrf2 adenovirus was successfully constructed and can be stably expressed and regulated in cells. Ad-RUNrf2 can reduce PQ-induced inflammation, oxidative stress and apoptosis in A549 cells.

Genetic aspects of cell line development from a synthetic biology perspective

Animal cells can be regarded as factories for the production of relevant proteins. The advances described in this chapter towards the development of cell lines with higher productivity capacities, certain metabolic and proliferation properties, reduced apoptosis and other features must be regarded in an integrative perspective. The systematic application of systems biology approaches in combination with a synthetic arsenal for targeted modification of endogenous networks are proposed to lead towards the achievement of a predictable and technologically advanced cell system with high biotechnological impact.

Atypical induction of the unfolded protein response by mifepristone

Mifepristone is a synthetic progesterone antagonist that is being used widely for the treatment of various conditions such as endometriosis, glaucoma, meningiomas, breast, ovarian and prostate cancer, as well as for research purposes, in the conditional induction of gene expression by using artificial plasmid-based systems. Here, we report that exposure of A549 human lung cancer cells to mifepristone caused an atypical induction of the cellular unfolded protein response, as evidenced by the time-dependent stimulation of RNA levels of the chaperone Grp94 and PDIa, as well as the endoplasmic reticulum stress-associated receptors ATF6, PERK and eIF2 but not of their downstream target, transcription factor ATF4. This profile was very different from that of progesterone, which at the same dose as mifepristone, failed to induce all of the ER-stress-related genes examined, apart from PERK. Furthermore, XBP1, a transcription factor that is regulated predominantly by alternative splicing by the IRE1 receptor, remains unspliced and therefore inactive either by mifepristone or progesterone treatment. Finally, the pro-apoptotic molecules CHOP and BIM are only induced in the presence of tunicamycin in the culture medium. Tunicamycin, the most commonly used pharmacologic inducer of ER stress that triggers the canonical ER stress response, was used for comparison purposes. Our results suggest that mifepristone can elicit an atypical ER stress response when used at different doses and for different time points. The subsequent induction of UPR should be taken into consideration when this agent is being used either for therapeutic or for experimental uses.

Genetically engineered mouse models in cancer research

Mouse models of human cancer have played a vital role in understanding tumorigenesis and answering experimental questions that other systems cannot address. Advances continue to be made that allow better understanding of the mechanisms of tumor development, and therefore the identification of better therapeutic and diagnostic strategies. We review major advances that have been made in modeling cancer in the mouse and specific areas of research that have been explored with mouse models. For example, although there are differences between mice and humans, new models are able to more accurately model sporadic human cancers by specifically controlling timing and location of mutations, even within single cells. As hypotheses are developed in human and cell culture systems, engineered mice provide the most tractable and accurate test of their validity in vivo. For example, largely through the use of these models, the microenvironment has been established to play a critical role in tumorigenesis, since tumor development and the interaction with surrounding stroma can be studied as both evolve. These mouse models have specifically fueled our understanding of cancer initiation, immune system roles, tumor angiogenesis, invasion, and metastasis, and the relevance of molecular diversity observed among human cancers. Currently, these models are being designed to facilitate in vivo imaging to track both primary and metastatic tumor development from much earlier stages than previously possible. Finally, the approaches developed in this field to achieve basic understanding are emerging as effective tools to guide much needed development of treatment strategies, diagnostic strategies, and patient stratification strategies in clinical research.

Animal transgenesis: an overview

Transgenic animals are extensively used to study in vivo gene function as well as to model human diseases. The technology for producing transgenic animals exists for a variety of vertebrate and invertebrate species. The mouse is the most utilized organism for research in neurodegenerative diseases. The most commonly used techniques for producing transgenic mice involves either the pronuclear injection of transgenes into fertilized oocytes or embryonic stem cell-mediated gene targeting. Embryonic stem cell technology has been most often used to produce null mutants (gene knockouts) but may also be used to introduce subtle genetic modifications down to the level of making single nucleotide changes in endogenous mouse genes. Methods are also available for inducing conditional gene knockouts as well as inducible control of transgene expression. Here, we review the main strategies for introducing genetic modifications into the mouse, as well as in other vertebrate and invertebrate species. We also review a number of recent methodologies for the production of transgenic animals including retrovirus-mediated gene transfer, RNAi-mediated gene knockdown and somatic cell mutagenesis combined with nuclear transfer, methods that may be more broadly applicable to species where both pronuclear injection and ES cell technology have proven less practical.

A novel estrogen receptor intramolecular folding-based titratable transgene expression system

The use of regulated gene expression systems is important for successful gene therapy applications. In this study, ligand-induced structural change in the estrogen receptor (ER) was used to develop a novel ER intramolecular folding-based transcriptional activation system. The system was studied using ER-variants of different lengths, flanked on either side by the GAL4-DNA-binding domain and the VP16-transactivation domain (GAL4(DBD)-ER-VP16). The ER ligands of different types showed efficient ligand-regulated transactivation. We also characterized a bidirectional transactivation system based on the ER and demonstrated its utility in titrating both reporter and therapeutic gene expression. The ligand-regulated transactivation system developed by using a mutant form of the ER (G521T, lacking affinity for the endogenous ligand 17beta-estradiol, whereas maintaining affinity for other ligands) showed efficient activation by the ligand raloxifene in living mice without significant interference from the circulating endogenous ligand. The ligand-regulated transactivation system was used to test the therapeutic efficiency of the tumor suppressor protein p53 in HepG2 (p53(+/+)) and SKBr3 (p53(-/-)/mutant-p53(+/+)) cells in culture and tumor xenografts in living mice. The multifunctional capabilities of this system should be useful for gene therapy applications, to study ER biology, to evaluate gene regulation, ER ligand screening, and ER ligand biocharacterization in cells and living animals.

Double-inducible gene activation system for caspase 3 and 9 in epidermis

Expression of genes with tight and precise temporal and spatial control is desired in a wide variety of applications ranging from cultured cells and transgenic animals to gene therapy. While current inducible systems, such as RU486 and chemical inducers of dimerization (CID), have improved earlier inducible models (Gossen et al., 1995, Science. 268:1766-1769; Wang et al., 1994, Proc Natl Acad Sci USA 91:8180-8184), no single system is perfect at present. One potential drawback of these systems is leakage of transgene expression, causing limitations of each system. We have developed an inducible model containing both RU486 and CID systems, which in addition to inducing caspase activation, has potential applicability specifically to other genes encoding proteins that require a dimerization event for activation. This Double-Inducible Gene Activation System generates two barriers for the target gene expression and protein activation thereby minimizing leakage.

Regulatable gene expression systems for gene therapy

It is feasible to restrict transgene expression to a tissue or region in need of therapy by using promoters that respond to focusable physical stimuli. The most extensively investigated promoters of this type are radiation-inducible promoters and heat shock protein gene promoters that can be activated by directed, transient heat. Temporal regulation of transgenes can be achieved by various two- or three-component gene switches that are triggered by an appropriate small molecule inducer. The most commonly considered gene switches that are reviewed herein are based on small molecule-responsive transactivators derived from bacterial tetracycline repressor, insect or mammalian steroid receptors, or mammalian FKBP12/FRAP. A new generation of gene switches combines a heat shock protein gene promoter and a small molecule-responsive gene switch and can provide for both spatial and temporal regulation of transgene activity.

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.

Regulatable gene expression systems for gene therapy applications: progress and future challenges

Gene therapy aims to revert diseased phenotypes by the use of both viral and nonviral gene delivery systems. Substantial progress has been made in making gene transfer vehicles more efficient, less toxic, and nonimmunogenic and in allowing long-term transgene expression. One of the key issues in successfully implementing gene therapies in the clinical setting is to be able to regulate gene expression very tightly and consistently as and when it is needed. The regulation ought to be achievable using a compound that should be nontoxic, be able to penetrate into the desired target tissue or organ, and have a half-life of a few hours (as opposed to minutes or days) so that when withdrawn or added (depending on the regulatable system used) gene expression can be turned "on" or "off" quickly and effectively. Also, the genetic switches employed should ideally be nonimmunogenic in the host. The ability to switch transgenes on and off would be of paramount importance not only when the therapy is no longer needed, but also in the case of the development of adverse side effects to the therapy. Many regulatable systems are currently under development and some, i.e., the tetracycline-dependent transcriptional switch, have been used successfully for in vivo preclinical applications. Despite this, there are no examples of switches that have been employed in a human clinical trial. In this review, we aim to highlight the main regulatable systems currently under development, the gene transfer systems employed for their expression, and also the preclinical models in which they have been used successfully. We also discuss the substantial challenges that still remain before these regulatable switches can be employed in the clinical setting.

Gas-inducible product gene expression in bioreactors

Inducible transgene expression technologies are of unmatched potential for biopharmaceutical manufacturing of unstable, growth-impairing and cytotoxic proteins as well as conditional metabolic engineering to improve desired cell phenotypes. Currently available transgene dosing modalities which rely on physical parameters or small-molecule drugs for transgene fine-tuning compromise downstream processing and/or are difficult to implement technologically. The recently designed gas-inducible acetaldehyde-inducible regulation (AIR) technology takes advantage of gaseous acetaldehyde to modulate product gene expression levels. At regulation effective concentrations gaseous acetaldehyde is physiologically inert and approved as food additive by the Federal Drug Administration (FDA). During standard bioreactor operation, gaseous acetaldehyde could simply be administered using standard/existing gas supply tubing and eventually eliminated by stripping with inducer-free air. We have determined key parameters controlling acetaldehyde transfer in three types of bioreactors and designed a mass balance-based model for optimal product gene expression fine-tuning using gaseous acetaldehyde. Operating a standard stirred-tank bioreactor set-up at 10 L scale we have validated AIR technology using CHO-K1-derived serum-free suspension cultures transgenic for gas-inducible production of human interferon-beta (IFN-beta). Gaseous acetaldehyde-inducible IFN-beta production management was fully reversible while maintaining cell viability at over 95% during the entire process. Compatible with standard bioreactor design and downstream processing procedures AIR-based technology will foster novel opportunities for pilot and large-scale manufacturing of difficult-to-produce protein pharmaceuticals.

Engineered Streptomyces quorum-sensing components enable inducible siRNA-mediated translation control in mammalian cells and adjustable transcription control in mice

BACKGROUND

Recent advances in functional genomics, gene therapy, tissue engineering, drug discovery and biopharmaceuticals production have been fostered by precise small-molecule-mediated fine-tuning of desired transgenes.

METHODS

Capitalizing on well-evolved quorum-sensing regulatory networks in Streptomyces coelicolor we have designed a mammalian regulation system inducible by the non-toxic butyrolactone SCB1. Fusion of the S. coelicolor SCB1 quorum-sensing receptor ScbR to the human Kox-1-derived transsilencing domain reconstituted a mammalian transsilencer (SCS) able to repress transcription from SCS-specific operator-containing promoters in a reverse SCB1-adjustable manner.

RESULTS

This quorum-sensing-derived mammalian transgene control system (Q-ON) enabled precise SCB1-specific fine-tuning of (i) desired transgene transcription in a variety of mammalian/human cell lines and human primary cells, (ii) small interfering RNA-mediated posttranscriptional knockdown (siRNA) in mammalian cells, and (iii) dosing of a human glycoprotein in mice.

CONCLUSIONS

As exemplified by Q-ON technology, bacterial quorum-sensing regulons may represent a near-infinite source for the design of mammalian gene control systems compatible with molecular interventions relevant to future gene therapy and tissue engineering scenarios.

New approaches for modelling cancer mechanisms in the mouse

Mouse models of human cancer are vital to our understanding of the neoplastic process, and to advances in both basic and clinical research. Indeed, models of many of the major human tumours are now available and are subject to constant revision to more faithfully recapitulate human disease. Despite these advances, it is important to recognize that limitations do exist to the current range of models. The principal approach to modelling has relied upon the use of constitutive gene knockouts, which can often result in embryonic lethality, can potentially be affected by developmental compensation, and which do not mimic the sporadic development of a tumour expanding from a single cell in an otherwise normal environment. Furthermore, simple knockouts are usually designed to lead to loss of protein function, whereas a subset of cancer-causing mutations clearly results in gain of function. These drawbacks are well recognized and this review describes some of the approaches used to address these issues. Key amongst these is the development of conditional alleles that precisely mimic the mutations found in vivo, and which can be spatially and tissue-specifically controlled using 'smart' systems such as the tetracycline system and Cre-Lox technology. Examples of genes being manipulated in this way include Ki-Ras, Myc, and p53. These new developments in modelling mean that any mutant allele can potentially be turned on or off, or over- or under-expressed, in any tissue at any stage of the life-cycle of the mouse. This will no doubt lead to ever more accurate and powerful mouse models to dissect the genetic pathways that lead to cancer.

Strategies of Conditional Gene Expression in Myocardium

The use of specialized reporter genes to monitor real-time, tissue-specific transgene expression in animal models offers an opportunity to circumvent current limitations associated with the establishment of transgenic mouse models. The Cre-loxP and the tetracycline (Tet)-inducible systems are useful methods of conditional gene expression that allow spatial (cell-type-specific) and temporal (inducer-dependent) control. Most often, the alpha-myosin heavy chain (alpha-MHC) promoter is used in these inducible systems to restrict expression of reporter genes and transgenes to the myocardium. An overview of each inducible system is described, along with suggested reporter genes for real-time, noninvasive imaging in the myocardium. Effective gene delivery of the inducible gene expression system is carried out by lentiviral vectors, which offer high transduction efficiency, long-term transgene expression, and low immunogenicity. This chapter outlines the packaging of myocardium-specific inducible expression systems into lentiviral vectors, in which a transgene and a reporter gene are transduced into cardiomyocytes. In doing so, transgene and reporter expression can be monitored/tracked with bioluminescence imaging (BLI) and positron emission tomography (PET).

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Conditional human VEGF-mediated vascularization in chicken embryos using a novel temperature-inducible gene regulation (TIGR) system

Advanced heterologous transcription control systems for adjusting desired transgene expression are essential for gene function assignments, drug discovery, manufacturing of difficult to produce protein pharmaceuticals and precise dosing of gene-based therapeutic interventions. Conversion of the Streptomyces albus heat shock response regulator (RheA) into an artificial eukaryotic transcription factor resulted in a vertebrate thermosensor (CTA; cold-inducible transactivator), which is able to adjust transcription initiation from chimeric target promoters (P(CTA)) in a low-temperature- inducible manner. Evaluation of the temperature-dependent CTA-P(CTA) interaction using a tailored ELISA-like cell-free assay correlated increased affinity of CTA for P(CTA) with temperature downshift. The temperature-inducible gene regulation (TIGR) system enabled tight repression in the chicken bursal B-cell line DT40 at 41 degrees C as well as precise titration of model product proteins up to maximum expression at or below 37 degrees C. Implantation of microencapsulated DT40 cells engineered for TIGR-controlled expression of the human vascular endothelial growth factor A (hVEGF121) provided low-temperature-induced VEGF-mediated vascularization in chicken embryos.