Techniques: Reporter mice – a new way to look at drug action

Design and validation of a reporter mouse to study the dynamic regulation of TFEB and TFE3 activity through in vivo imaging techniques

TFEB and TFE3 belong to the MiT/TFE family of transcription factors that bind identical DNA responsive elements in the regulatory regions of target genes. They are involved in regulating lysosomal biogenesis, function, exocytosis, autophagy, and lipid catabolism. Precise control of TFEB and TFE3 activity is crucial for processes such as senescence, stress response, energy metabolism, and cellular catabolism. Dysregulation of these factors is implicated in various diseases, thus researchers have explored pharmacological approaches to modulate MiT/TFE activity, considering these transcription factors as potential therapeutic targets. However, the physiological complexity of their functions and the lack of suitable in vivo tools have limited the development of selective MiT/TFE modulating agents. Here, we have created a reporter-based biosensor, named CLEARoptimized, facilitating the pharmacological profiling of TFEB- and TFE3-mediated transcription. This innovative tool enables the measurement of TFEB and TFE3 activity in living cells and mice through imaging and biochemical techniques. CLEARoptimized consists of a promoter with six coordinated lysosomal expression and regulation motifs identified through an in-depth bioinformatic analysis of the promoters of 128 TFEB-target genes. The biosensor drives the expression of luciferase and tdTomato reporter genes, allowing the quantification of TFEB and TFE3 activity in cells and in animals through optical imaging and biochemical assays. The biosensor's validity was confirmed by modulating MiT/TFE activity in both cell culture and reporter mice using physiological and pharmacological stimuli. Overall, this study introduces an innovative tool for studying autophagy and lysosomal pathway modulation at various biological levels, from individual cells to the entire organism.Abbreviations: CLEAR: coordinated lysosomal expression and regulation; MAR: matrix attachment regions; MiT: microphthalmia-associated transcription factor; ROI: region of interest; TBS: tris-buffered saline; TF: transcription factor; TFE3: transcription factor binding to IGHM enhancer 3; TFEB: transcription factor EB; TH: tyrosine hydroxylase; TK: thymidine kinase; TSS: transcription start site.

Tactile cues are important to environmental novelty during repeated open field tests

The open field test (OFT) is a commonly used protocol to measure anxiety-like behaviors in rodents. Exploration in the central area of the open field and rearing frequency are often readouts of anxiety measurement. However, concerns about carry-over effects associated with repeated assessments limit its application, with the underlying mechanisms of this phenomenon still to be fully described. Here, we showed that repeated OFTs in the same mice led to reductions in the percentage of time spent in the central area and frequency of rearing. This effect reduced with an increase in the intervals between test. The decay caused by repeated OFTs was due to habituation, rather than frequent handling of the experimenter, since novel environments could prevent decay from repeated OFTs. Our results also indicated that tactile cues of the environment played important roles in the habituation of repeated OFTs. Furthermore, the decay of central area activity and rearing behavior during repeated OFTs would be blocked if the hippocampal CA1 was lesioned, suggesting that CA1 is a crucial region for habituation of the OFT in mice. Taken together, our study uncovers the important roles of tactile cues and hippocampal CA1 during repeated OFTs in mice.

A new bioluminescence-based tool for modulating target proteins in live cells

We have developed a new genetically encoded tool designed to generate reactive oxygen species (ROS) at target proteins in cultured cells; it is designed using firefly luciferase and photosensitiser protein KillerRed. Targeting this fusion protein, KillerFirefly, to F-actin in live cells and treatment with luciferin induced a characteristic structure, previously reported as a cofilin-actin rod, which is seen in patients with Alzheimer's disease. This structural change is considered to be elicited by the consistent generation of very low-level ROS by KillerFirefly in the vicinity of F-actin. Moreover, our results suggest the presence of an actin-regulating system, controlled by very low levels of endogenously generated ROS.

Multimodal molecular imaging system for pathway-specific reporter gene expression

Preclinical imaging modalities represent an essential tool to develop a modern and translational biomedical research. To date, Optical Imaging (OI) and Magnetic Resonance Imaging (MRI) are used principally in separate studies for molecular imaging studies. We decided to combine OI and MRI together through the development of a lentiviral vector to monitor the Wnt pathway response to Lithium Chloride (LiCl) treatment. The construct was stably infected in glioblastoma cells and, after intracranial transplantation in mice, serial MRI and OI imaging sessions were performed to detect human ferritin heavy chain protein (hFTH) and firefly luciferase enzyme (FLuc) respectively. The system allowed also ex vivo analysis using a constitutive fluorescence protein expression. In mice, LiCl administration has shown significantly increment of luminescence signal and a lower signal of T2 values (P<0.05), recorded noninvasively with OI and a 7 Tesla MRI scanner. This study indicates that OI and MRI can be performed in a single in vivo experiment, providing an in vivo proof-of-concept for drug discovery projects in preclinical phase.

In vivo dynamics of estrogen receptor activity: the ERE-Luc model

In recent years several studies demonstrated the presence of estrogen receptors in mammalian tissues and significantly improved our understanding of their ability to control biological processes in reproductive as well as non-reproductive organs. Considering the manifold mechanisms and organs that are involved in estrogen action and the implication of estrogens in human female physiology, innovative approaches are required to shed light on the widespread activities of estrogen receptors in woman physiology. This is particularly relevant for the definition of novel, more efficacious hormonal replacement therapies or for the evaluation of the risk associated with the exposure to endocrine disruptors. The introduction of genetic engineering and the development and application of in vivo imaging techniques offer new tools for pre-clinical studies. The generation of the ERE-Luc mouse, a reporter animal developed for in vivo studies of the estrogen receptor activity, allows assessing the activity state of the ER signaling pathway in all target tissues and organs at once, under physiological stimuli or as a result of a pharmacological treatment. This review summarizes the main steps in the generation and appraisal of the estrogen receptor reporter mouse ERE-Luc, designed for in vivo molecular imaging studies, and describes examples demonstrating the suitability of the ERE-Luc model for drug development and for the investigation of the effects of endogenous, environmental, and dietary estrogens in vivo. This article is part of a Special Issue entitled 'Phytoestrogens'.

The CRE luc bioluminescence transgenic mouse model for detecting ligand activation of GPCRs

Numerous assays have been developed to investigate the interactions between G-protein-coupled receptors (GPCRs) and their ligands since GPCRs are key therapeutic targets. Reporter-based assays using the cAMP response element (CRE) coupled with bioluminescence from a luciferase reporter have been used extensively in vitro with high-throughput screens (HTS) of large chemical compound libraries. We have generated a transgenic mouse model (CRE luc) with a luciferase reporter under the control of a synthetic promoter that contains several CREs, which supports real-time bioimaging of GPCR ligand activity in whole animals, tissues, or primary cells. In the CRE luc model, GPCR signaling through the cAMP pathway can be detected from the target GPCR that is in a native cellular environment with a full complement of associated receptors and membrane constituents. Multiple independent lines have been produced by random integration of the transgene, resulting in tissue expression profiles covering the major organs. The goal of the CRE luc model is to accelerate the transition from HTS to profiling of GPCR small-molecule leads in preclinical animal disease models, as well as define the mechanism of action of GPCR drugs in three experimental formats: primary cells, tissue homogenates, and whole animals.

Nutritional flavonoids impact on nuclear and extranuclear estrogen receptor activities

Flavonoids are a large group of nonnutrient compounds naturally produced from plants as part of their defence mechanisms against stresses of different origins. They emerged from being considered an agricultural oddity only after it was observed that these compounds possess a potential protective function against several human degenerative diseases. This has led to recommending the consumption of food containing high concentrations of flavonoids, which at present, especially as soy isoflavones, are even available as overthecounter nutraceuticals. The increased use of flavonoids has occurred even though their mechanisms are not completely understood, in particular those involving the flavonoid impact on estrogen signals. In fact, most of the human health protective effects of flavonoids are described either as estrogenmimetic, or as antiestrogenic, while others do not involve estrogen signaling at all. Thus, the same molecule is reported as an endocrine disruptor, an estrogen mimetic or as an antioxidant without estrogenic effects. This is due in part to the complexity of the estrogen mechanism, which is conducted by different pathways and involves two different receptor isoforms. These pathways can be modulated by flavonoids and should be considered for a reliable evaluation of flavonoid, both estrogenicity and antiestrogenicity, and for a correct prediction of their effects on human health.

Protein reporter bioassay systems for the phenotypic screening of candidate drugs: a mouse platform for anti-aging drug screening

Recent drug discovery efforts have utilized high throughput screening (HTS) of large chemical libraries to identify compounds that modify the activity of discrete molecular targets. The molecular target approach to drug screening is widely used in the pharmaceutical and biotechnology industries, because of the amount of knowledge now available regarding protein structure that has been obtained by computer simulation. The molecular target approach requires that the structure of target molecules, and an understanding of their physiological functions, is known. This approach to drug discovery may, however, limit the identification of novel drugs. As an alternative, the phenotypic- or pathway-screening approach to drug discovery is gaining popularity, particularly in the academic sector. This approach not only provides the opportunity to identify promising drug candidates, but also enables novel information regarding biological pathways to be unveiled. Reporter assays are a powerful tool for the phenotypic screening of compound libraries. Of the various reporter genes that can be used in such assays, those encoding secreted proteins enable the screening of hit molecules in both living cells and animals. Cell- and animal-based screens enable simultaneous evaluation of drug metabolism or toxicity with biological activity. Therefore, drug candidates identified in these screens may have increased biological efficacy and a lower risk of side effects in humans. In this article, we review the reporter bioassay systems available for phenotypic drug discovery.

Non-invasive monitoring of hepatocellular carcinoma in transgenic mouse with bioluminescent imaging

A small animal imaging system for hepatocellular carcinoma (HCC)-specific reporter gene expression will enable monitoring of carcinogenesis or therapeutic intervention in vivo. Transgenic mouse was developed in which firefly luciferase (fLuc) expression was controlled by the AFP enhancer/promoter. The bioluminescent signals of the transgenic neonates were strong at their liver region and decreased after birth. Bioluminescent imaging (BLI) of a transgenic mouse treated with N-nitrosodiethylamine revealed distinct fLuc activity in the liver and an increased pattern with time. The transgenic mouse model can be used to monitor AFP producing HCC by a chemical carcinogen in a live animal by BLI.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

The minipig as a platform for new technologies in toxicology

The potential of the minipig as a platform for future developments in genomics, high density biology, transgenic technology, in vitro toxicology and related emerging technologies was reviewed. Commercial interests in the pig as an agricultural production species have driven scientific progress in these areas. There is no equivalent economic driver for progress in the dog or the monkey. As a result the available knowledge-bases are much greater for pigs (than for dogs or monkeys) in many areas (physiology, disease, genetics, immunology etc). Fundamental genomic knowledge and phenotypic characterization in regard to the pig is well in advance of the dog or the monkey and basic knowledge of the pig is therefore likely to stay ahead of the other two species. While the emerging technologies are essentially "species neutral" and can in principle be applied to all species, for all the technologies that we examined, basic knowledge and technical capabilities are greater for the pig than the dog or monkey. In concrete terms, in application to safety testing we have seen that: (i) The Göttingen minipig is well positioned for the performance of toxicogenomics studies, (ii) The close sequence homology between pigs and humans suggest that minipigs will be useful for the testing of biotechnology products (and possibly for in silico toxicology) and (iii) the minipig is the only non-rodent toxicology model where transgenic animals can be readily generated, and reproductive technologies are well developed in the pig. These properties should also make the minipig an interesting model for the testing of biotechnology products. These factors all support the idea that the minipig is well placed to meet the challenges of the emerging technologies and the toxicology of the future; it also seems likely that the minipig can be an advantageous model for the testing of biotechnology products.

Ferritin as a reporter gene for MRI: chronic liver over expression of H-ferritin during dietary iron supplementation and aging

The iron storage protein, ferritin, provides an important endogenous MRI contrast that can be used to determine the level of tissue iron. In recent years the impact of modulating ferritin expression on MRI contrast and relaxation rates was evaluated by several groups, using genetically modified cells, viral gene transfer and transgenic animals. This paper reports the follow-up of transgenic mice that chronically over-expressed the heavy chain of ferritin (h-ferritin) in liver hepatocytes (liver-hfer mice) over a period of 2 years, with the aim of investigating the long-term effects of elevated level of h-ferritin on MR signal and on the well-being of the mice. Analysis revealed that aging liver-hfer mice, exposed to chronic elevated expression of h-ferritin, have increased R(2) values compared to WT. As expected for ferritin, R(2) difference was strongly enhanced at high magnetic field. Histological analysis of these mice did not reveal liver changes with prolonged over expression of ferritin, and no differences could be detected in other organs. Furthermore, dietary iron supplementation significantly affected MRI contrast, without affecting animal wellbeing, for both wildtype and ferritin over expressing transgenic mice. These results suggest the safety of ferritin over-expression, and support the use of h-ferritin as a reporter gene for MRI.

Noninvasive imaging of bone-specific collagen I expression in a luciferase transgenic mouse model

Luciferase transgenic mice are a very promising tool for noninvasive, quantitative, and longitudinal evaluation of gene expression. The aim of this study was to validate the Col(I)-Luc transgenic mouse model in which the luciferase gene is driven by bone-specific regulatory elements from the mouse collagen α1(I) gene for bioluminescent imaging of bone development and remodeling. We observed strong luciferase activity in skeletal tissues of Col(I)-Luc mice, and observed that the light intensity declined with postnatal bone development. Luciferase activity was enhanced in a tail bone repair model and we were able to monitor the process of ectopic bone formation induced by recombinant human bone morphogenetic protein 2 using bioluminescent imaging. We conclude that Col(I)-Luc transgenic mice can be applied in the field of bone tissue engineering for monitoring bone repair processes and for investigating osteoinductive molecules or scaffolds.

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.

Molecular imaging study on in vivo distribution and pharmacokinetics of modified small interfering RNAs (siRNAs)

Molecular imaging was used to study the biodistribution, pharmacokinetics, and activity of naked small interfering RNAs (siRNAs). siRNAs with riboses chemically modified in the 2' position were compared with unmodified siRNA. In vitro, replacement of the 2'-hydroxyl (2'OH) group of certain nucleotides in an siRNA sequence by a fluorine atom (2'F) on both antisense (AS) and sense (S) strands [2'F(AS/S)], or by a methoxy group (2'OMe) on the S strand [2'OH(AS)/2'OMe(S)], was compatible with RNA interference. Different siRNAs [2'F(AS/S), 2'OH(AS)/2'OMe(S), and 2'OH(AS/S)] were labeled with fluorine-18 (conjugation with [(18)F]FPyBrA), and comparative dynamic and quantitative imaging was performed with positron emission tomography. After intravenous injections of [(18)F]siRNAs in rodents, total radioactivity was rapidly eliminated by the kidneys and the liver. Tissue distribution of the different siRNAs were similar, and their bioavailability (as judged from blood persistence and stability) increased in the order 2'OH(AS/S) = 2'OH(AS)/2'OMe(S) < 2'F(AS/S). However, in our in vivo model, the 2'F(AS/S) siRNA, despite its higher bioavailability, was not able to induce a higher interference effect with respect to the 2'OH(AS/S) siRNA. Molecular imaging approaches, applied in the present work to both natural and chemically modified siRNAs, can contribute to the development of these macromolecules as therapeutic agents.

S32826, a nanomolar inhibitor of autotaxin: discovery, synthesis and applications as a pharmacological tool

Autotaxin catalyzes the transformation of lyso-phosphatidylcholine in lyso-phosphatidic acid (LPA). LPA is a phospholipid possessing a large panel of activity, in particular as a motility factor or as a growth signal, through its G-protein coupled seven transmembrane receptors. Indirect evidence strongly suggests that autotaxin is the main, if not the only source of circulating LPA. Because of its central role in pathologic conditions, such as oncology and diabetes/obesity, the biochemical properties of autotaxin has attracted a lot of attention, but confirmation of its role in pathology remains elusive. One way to validate and/or confirm its central role, is to find potent and selective inhibitors. A systematic screening of several thousand compounds using a colorimetric assay and taking advantage of the phosphodiesterase activity of autotaxin that requires the enzymatic site than for LPA generation, led to the discovery of a potent nanomolar inhibitor, [4-(tetradecanoylamino)benzyl]phosphonic acid (S32826). This compound was inhibitory toward the various autotaxin isoforms, using an assay measuring the [(14)C]lyso-phosphatidylcholine conversion into [(14)C]LPA. We also evaluated the activity of S32826 in cellular models of diabesity and oncology. Nevertheless, the poor in vivo stability and/or bioavailability of the compound did not permit to use it in animals. S32826 is the first reported inhibitor of autotaxin with an IC(50) in the nanomolar range that can be used to validate the role of autotaxin in various pathologies in cellular models.

Molecular Imaging Provides Novel Insights on Estrogen Receptor Activity in Mouse Brain

Estrogen receptors have long been known to be expressed in several brain areas in addition to those directly involved in the control of reproductive functions. Investigations in humans and in animal models suggest a strong influence of estrogens on limbic and motor functions, yet the complexity and heterogeneity of neural tissue have limited our approaches to the full understanding of estrogen activity in the central nervous system. The aim of this study was to examine the transcriptional activity of estrogen receptors in the brain of male and female mice. Exploiting the ERE-Luc reporter mouse, we set up a novel, bioluminescence-based technique to study brain estrogen receptor transcriptional activity. Here we show, for the first time, that estrogen receptors are similarly active in male and female brains and that the estrous cycle affects estrogen receptor activity in regions of the central nervous system not known to be associated with reproductive functions. Because of its reproducibility and sensitivity, this novel bioluminescence application stands as a candidate as an innovative methodology for the study and development of drugs targeting brain estrogen receptors.

Molecular-genetic imaging based on reporter gene expression

Molecular imaging includes proteomic, metabolic, cellular biologic process, and genetic imaging. In a narrow sense, molecular imaging means genetic imaging and can be called molecular-genetic imaging. Imaging reporter genes play a leading role in molecular-genetic imaging. There are 3 major methods of molecular-genetic imaging, based on optical, MRI, and nuclear medicine modalities. For each of these modalities, various reporter genes and probes have been developed, and these have resulted in successful transitions from bench to bedside applications. Each of these imaging modalities has its unique advantages and disadvantages. Fluorescent and bioluminescent optical imaging modalities are simple, less expensive, more convenient, and more user friendly than other imaging modalities. Another advantage, especially of bioluminescence imaging, is its ability to detect low levels of gene expression. MRI has the advantage of high spatial resolution, whereas nuclear medicine methods are highly sensitive and allow data from small-animal imaging studies to be translated to clinical practice. Moreover, multimodality imaging reporter genes will allow us to choose the imaging technologies that are most appropriate for the biologic problem at hand and facilitate the clinical application of reporter gene technologies. Reporter genes can be used to visualize the levels of expression of particular exogenous and endogenous genes and several intracellular biologic phenomena, including specific signal transduction pathways, nuclear receptor activities, and protein-protein interactions. This technique provides a straightforward means of monitoring tumor mass and can visualize the in vivo distributions of target cells, such as immune cells and stem cells. Molecular imaging has gradually evolved into an important tool for drug discovery and development, and transgenic mice with an imaging reporter gene can be useful during drug and stem cell therapy development. Moreover, instrumentation improvements, the identification of novel targets and genes, and imaging probe developments suggest that molecular-genetic imaging is likely to play an increasingly important role in the diagnosis and therapy of cancer.

In vivo imaging reveals selective peroxisome proliferator activated receptor modulator activity of the synthetic ligand 3-(1-(4-chlorobenzyl)-3-t-butylthio-5-isopropylindol-2-yl)-2,2-dimethylpropanoic acid (MK-886)

We report here the finding of a new pharmacological activity of a well known antagonist of peroxisome proliferator-activated receptors (PPARs). PPARs belong to the family of nuclear receptors playing a relevant role in mammalian physiology and are currently believed to represent a major target for the development of innovative drugs for metabolic and inflammatory diseases. In the present study, the application of reporter animal technology was instrumental to obtain the global pharmacological profiling indispensable to unraveling 3-(1-(4-chlorobenzyl)-3-t-butylthio-5-isopropylindol-2-yl)-2,2-dimethylpropanoic acid (MK-886)-selective PPAR modulator (SPPARM) activity not underlined by previous traditional, cell-based studies. The results of this study, demonstrating the usefulness of reporter mice, may open new avenues for the development of innovative drugs for cardiovascular, endocrine, neural, and skeletal systems characterized by limited side effects.

Antioxidant activity in the newborn brain: a luciferase mouse model

INTRODUCTION

Oxidative stress in the newborn period may cause cell injury and inflammation if the antioxidant capacity is insufficient. To monitor antioxidant and inflammatory activity we examined by in vivo imaging various strains of luciferase reporter mice whose light-emitting properties were regulated by response elements or complete promoters related to oxidative stress and/or inflammation. The aim of this study is to present a model that can monitor genetic activity in vivo during pregnancy and the first 10 days of life.

METHODS

One mouse strain reports the activity of nuclear factor-kappaB (NF-kappaB) activity, a transcription factor essential for modulating inflammation, apoptosis, differentiation and cell growth. A second mouse strain reports on superoxide dismutase 1-promoter activity. A third strain reports the promoter activity of gamma-glutamylcysteine synthetase, the rate limiting enzyme in glutathione production, and the last strain reports on antioxidant responsive element (ARE)/electrophil responsive element. Wild-type female mice mated with NF-kappaB mice were imaged through pregnancy to monitor intrauterine NF-kappaB activation.

RESULTS

Intrauterine NF-kappaB activity increased dramatically from day 17 towards labor. During the first 4 days of life luminescence measured was intense in all mice with distinct strain differences. All strains had high luminescence levels at day 1 and a considerably lower level at day 10.

CONCLUSION

This model allows investigation of the transcriptional regulation of key proteins related to oxidative stress and inflammation in pregnancy and the first days of life. With very little stress to the newborn animals genetic activity can be monitored day by day.

Cancer modeling: modern imaging applications in the generation of novel animal model systems to study cancer progression and therapy

Cancer is the result of a series of genetic and epigenetic mutations that evolve over years even decades and lead to the transformed phenotype. Paradoxically, most methods developed to study these changes are static and do not provide insights on the dynamics of the sequela of steps involved in tumorigenesis. This major shortcoming now can be overcome with the application of reporter genes and imaging technologies, which are providing tools to examine specific molecular events and their role in the carcinogenic process in single cells. In the last decade reporter-based biosensors were created to study gene transcription, protein/protein interactions, sub-cellular trafficking and protease activities; this wealth of systems enable to monitor intracellular signaling pathways at several key check points specifically involved in cancer cell development. The challenge is now to extend cell-based models to the generation of reporter mice, where non-invasive in vivo imaging technologies allow to follow single molecular events. When combined with murine models of cancer, these technologies will give an unprecedented opportunity to spatio-temporally investigate the molecular events resulting in neoplasia. The aim of the present review is to highlight the major changes occurring in this rapidly evolving field and their potential for increasing our knowledge in cancer biology and for the research of novel and more efficacious therapies.

A novel peroxisome proliferator-activated receptor responsive element-luciferase reporter mouse reveals gender specificity of peroxisome proliferator-activated receptor activity in liver

There is a growing interest in peroxisome proliferator-activated receptors (PPARs) as major players in the regulation of lipid and carbohydrate metabolism. Drugs targeting PPARs were in fact shown to have major relevance for the treatment of diseases associated with aging, such as arteriosclerosis and diabetes. However, a variety of toxic effects associated with PPAR ligand administration has been documented, including hepatocarcinogenesis, which may severely limit its therapeutic use. A better comprehension of the multiplicity of PPAR physiological functions is therefore mandatory for the development of novel, safer drugs. We here describe the generation of a novel transgenic mouse for the detection of the generalized activities of PPARs, the PPAR responsive element-Luc reporter mouse. In this model luciferase expression is under the control of a PPAR-inducible promoter in all target organs. By optical imaging and ex vivo analysis, we were able to demonstrate the remarkable gender specificity of the PPAR transcriptional activity in liver. In fact, in the liver of female PPAR responsive element-Luc, the PPAR reporter transgene is more than one order of magnitude less expressed, thus leading to the conclusion that the signaling in females is much less activated than in males. Diet or hormonal manipulations as demonstrated here by treatments with high-fat diet or gonad removal and hormone replacement do not influence this low activation. The extent of the gender difference in PPAR transcriptional activity and the ineffectiveness of hormone treatments or diet to significantly elevate liver PPAR activity in females led us to hypothesize that gender-specific epigenetic events occurring during development may affect PPAR signaling in the liver. This study sets the ground for understanding the differential susceptibility of the two genders to metabolic disorders; furthermore, the model generated provides a novel opportunity for the molecular characterization of PPAR activity in pathophysiological conditions.

The dynamics of estrogen receptor activity

In the latest few years, the merging of imaging and animal engineering technologies has led to the generation of innovative tools that provide the opportunity to look into the dynamics of specific molecular events in living animals during their entire life under a completely renewed perspective. These tools will have a profound impact not only on basic research, but also on drug discovery and development allowing to depict the activity of any therapeutic agents in all their designed targets as well as in the organs where they may cause undesired effects. Along this research line, our laboratory has recently described the first animal model reporting the state of activity of estrogen receptors (ERs) in real time: the ERE-luc reporter mouse. The application of optical imaging to the ERE-luc has allowed an unprecedented in depth view of estrogen signaling in all of its target tissues. For example, the analysis of the state of activity of ERs in the physiological setting of the estrous cycle has provided compelling evidence that hormone-independent mechanisms are responsible for activating ERs in non-reproductive organs. This discovery may pave the way to a rational basis for the development of novel, more selective and effective treatments for menopause.

Bioluminescent imaging: Emerging technology for non-invasive imaging of bone tissue engineering

Bone tissue engineering is a multidisciplinary research area in which new strategies are developed to treat patients with large bone defects as occurring during e.g. hip revisions, upon trauma or in spinal fusions. In vivo evaluation of bone formation in animal models is highly relevant for graft evaluation but is time-consuming, invasive and difficult to quantify. As a consequence, most in vivo studies ignore the dynamic nature of bone regeneration and the molecular processes underlying it. In vivo bioluminescent imaging (BLI) is a relatively young research field with great potential that may overcome these problems. BLI encompasses non-invasive imaging of luciferase gene activity using cooled charge coupled device cameras in luciferase transgenic animals or in grafted, luciferase transgenic cells. The imaging procedure is technically simple and quantifiable. Because luciferase expression can be put under the control of tissue-specific regulatory elements, BLI allows non-invasive imaging of processes highly relevant to bone tissue engineering like differentiation, apoptosis, vasculogenesis and inflammation. In this review, we describe the basic principle of BLI and discuss transgenic animals and constructs currently available for application in bone tissue engineering. Furthermore, we reflect on technical developments that will make BLI even more promising for future application in bone tissue engineering research.

Molecular imaging: a new way to study molecular processes in vivo

Non-invasive imaging of reporter gene expression using different imaging modalities is increasing its role for the in vivo assessment of molecular processes. Multimodality imaging protocols overcome limitations to a single imaging modality and provide a thorough view of specific processes, often allowing a quantitative measurement and direct visualization of the process in a specific target organ or tissue. The use of the right reporter gene for the development of animal models and the characterization of its expression in different conditions and tissues is fundamental for basic, translational and future pharmacological applications of a given model. This paper summarizes the major steps in the development and evaluation of a specific animal model for in vivo molecular imaging studies and describes the first example of an animal model designed for the in vivo assessment of a specific receptor activity and its possible evolution towards multimodality imaging analysis.

Reduction of astrogliosis by early treatment of pneumococcal meningitis measured by simultaneous imaging, in vivo, of the pathogen and host response

We developed a method for simultaneous in vivo biophotonic monitoring of pneumococcal meningitis and the accompanying neuronal injury in live transgenic mice. Streptococcus pneumoniae engineered for bioluminescence (lux) was used for direct visualization of disease progression and antibiotic treatment in a mouse model of meningitis. The host response was monitored in transgenic mice containing an inducible firefly luciferase (luc) reporter gene under transcriptional control of the mouse glial fibrillary acidic protein (GFAP) promoter. Based on the different spectra of light emission and substrate requirements for lux and luc, we were able to separately monitor the two reporters using a highly sensitive in vivo imaging system. The level of neuronal damage and recovery following antibiotic treatment was dependent on the time of treatment. This model has potential for simultaneous multiparameter monitoring and testing of therapies that target the pathogen or host response to prevent neuronal injury and recovery.

Visualizing Drug Efficacy In Vivo

Many enzymes are therapeutic targets for drug discovery, whereas other enzymes are important for understanding drug metabolism and pharmacokinetics during compound testing in animals. Testing of drug efficacy and metabolism in an animal model requires the measurement of disease endpoints as well as assays of enzyme activity in specific tissues at selected time points during treatment. This requires the removal of tissue and biochemical assays. Techniques to noninvasively assess drug effects on enzyme activity using imaging technology would facilitate understanding of drug efficacy, pharmacokinetics, and drug metabolism. Using a commercially available cytochrome P−450 3A substrate whose oxidized product is a luciferase substrate, we show for the first time that cytochrome P−450 enzyme activity can be measured in vivo in real time by bioluminescent imaging. This imaging approach could be applicable to study drug effects on therapeutic target enzymes, as well as drug metabolism enzymes.

Monitoring of ubiquitin-dependent proteolysis with green fluorescent protein substrates

A reliable and robust means of evaluating the functional status of ubiquitin-dependent proteolysis in living cells is to follow the turnover of readily detectable reporter substrates. During the past few years, several reporter substrates have been generated by use of the green fluorescent protein (GFP), which is converted for this purpose from a normally very stable protein into a short-lived substrate of the ubiquitin/proteasome system. These short-lived substrates are valuable tools providing researchers with unique information about the absence or presence of blockades in this system in living cells. We have recently generated the first transgenic mouse model for monitoring the ubiquitin/proteasome system based on the ubiquitous expression of a GFP-based proteasome substrate. Together these models can be used to study ubiquitin-dependent degradation in health and disease and for the identification of small synthetic compounds or proteins capable of modifying the activity of the system. In this chapter, we describe the basic principles of GFP-based reporter substrates, their strengths and weaknesses, and a number of protocols that can be used to study the ubiquitin/proteasome system in yeast, cell lines, and transgenic mice.