MicroPET imaging of Cre-loxP-mediated conditional activation of a herpes simplex virus type 1 thymidine kinase reporter gene

AKT1 Regulates Endoplasmic Reticulum Stress and Mediates the Adaptive Response of Pancreatic β Cells

Isoforms of protein kinase B (also known as AKT) play important roles in mediating insulin and growth factor signals. Previous studies have suggested that the AKT2 isoform is critical for insulin-regulated glucose metabolism, while the role of the AKT1 isoform remains less clear. This study focuses on the effects of AKT1 on the adaptive response of pancreatic β cells. Using a mouse model with inducible β-cell-specific deletion of the Akt1 gene (βA1KO mice), we showed that AKT1 is involved in high-fat-diet (HFD)-induced growth and survival of β cells but is unnecessary for them to maintain a population in the absence of metabolic stress. When unchallenged, βA1KO mice presented the same metabolic profile and β-cell phenotype as the control mice with an intact Akt1 gene. When metabolic stress was induced by HFD, β cells in control mice with intact Akt1 proliferated as a compensatory mechanism for metabolic overload. Similar effects were not observed in βA1KO mice. We further demonstrated that AKT1 protein deficiency caused endoplasmic reticulum (ER) stress and potentiated β cells to undergo apoptosis. Our results revealed that AKT1 protein loss led to the induction of eukaryotic initiation factor 2 α subunit (eIF2α) signaling and ER stress markers under normal-chow-fed conditions, indicating chronic low-level ER stress. Together, these data established a role for AKT1 as a growth and survival factor for adaptive β-cell response and suggest that ER stress induction is responsible for this effect of AKT1.

Cre/lox-assisted non-invasive in vivo tracking of specific cell populations by positron emission tomography

Many pathophysiological processes are associated with proliferation, migration or death of distinct cell populations. Monitoring specific cell types and their progeny in a non-invasive, longitudinal and quantitative manner is still challenging. Here we show a novel cell-tracking system that combines Cre/lox-assisted cell fate mapping with a thymidine kinase (sr39tk) reporter gene for cell detection by positron emission tomography (PET). We generate Rosa26-mT/sr39tk PET reporter mice and induce sr39tk expression in platelets, T lymphocytes or cardiomyocytes. As proof of concept, we demonstrate that our mouse model permits longitudinal PET imaging and quantification of T-cell homing during inflammation and cardiomyocyte viability after myocardial infarction. Moreover, Rosa26-mT/sr39tk mice are useful for whole-body characterization of transgenic Cre mice and to detect previously unknown Cre activity. We anticipate that the Cre-switchable PET reporter mice will be broadly applicable for non-invasive long-term tracking of selected cell populations in vivo.Non-invasive cell tracking is a powerful method to visualize cells in vivo under physiological and pathophysiological conditions. Here Thunemann et al. generate a mouse model for in vivo tracking and quantification of specific cell types by combining a PET reporter gene with Cre-dependent activation that can be exploited for any cell population for which a Cre mouse line is available.

NONINVASIVE OPTICAL IMAGING FOR TRACKING GENE DELIVERY AND RECOMBINATION IN TUMOR

Here, we report the generation of optical imaging reporter breast tumor cells that allow the longitudinal, in vivo, noninvasive imaging of gene recombination in tumor. Tumor-gene targeting is a promising approach of treating cancers, and a suitable gene delivery method is the criteria for success. By using the cre lox genetic engineering tool, we targeted stable green fluorescent protein expression in metastatic-prone human breast cancer MDA-MB231 cells that switch to express firefly luciferase upon the exogenous delivery and expression of cre DNA recombinase. We tested this model in vivo by intratumor injection of cre adenovirus and demonstrated the usefulness of this model to achieve longitudinal bioluminescence imaging of DNA recombination in tumor. This optical imaging vector and tumor model will facilitate the research for biomaterial solutions for carriers in gene therapy, and in studies on tumor targeting, tracking for tumor metastasis and migration of tumor stem cells, and for determining the anticancer drug efficacy.

Back to BAC: the use of infectious clone technologies for viral mutagenesis

Bacterial artificial chromosome (BAC) vectors were first developed to facilitate the propagation and manipulation of large DNA fragments in molecular biology studies for uses such as genome sequencing projects and genetic disease models. To facilitate these studies, methodologies have been developed to introduce specific mutations that can be directly applied to the mutagenesis of infectious clones (icBAC) using BAC technologies. This has resulted in rapid identification of gene function and expression at unprecedented rates. Here we review the major developments in BAC mutagenesis in vitro. This review summarises the technologies used to construct and introduce mutations into herpesvirus icBAC. It also explores developing technologies likely to provide the next leap in understanding these important viruses.

Molecular imaging-guided gene therapy of gliomas

Gene therapy of patients with glioblastoma using viral and non-viral vectors, which are applied by direct injection or convection-enhanced delivery (CED), appear to be satisfactorily safe. Up to date, only single patients show a significant therapeutic benefit as deduced from single long-term survivors. Non-invasive imaging by PET for the identification of viable target tissue and for assessment of transduction efficiency shall help to identify patients which might benefit from gene therapy, while non-invasive follow-up on treatment responses allows early and dynamic adaptations of treatment options. Therefore, molecular imaging has a critical impact on the development of standardised gene therapy protocols and on efficient and safe vector applications in humans.

PET imaging of herpes simplex virus type 1 thymidine kinase (HSV1-tk) or mutant HSV1-sr39tk reporter gene expression in mice and humans using [18F]FHBG

The herpes simplex virus type 1 thymidine kinase (HSV1-tk) positron emission tomography (PET) reporter gene (PRG) or its mutant HSV1-sr39tk are used to investigate intracellular molecular events in cultured cells and to image intracellular molecular events and cell trafficking in living subjects. The expression of these PRGs can be imaged using 18F- or 124I-radiolabeled acycloguanosine or pyrimidine analog PET reporter probes (PRPs). This protocol describes the procedures for imaging HSV1-tk or HSV1-sr39tk PRG expression in living subjects with the acycloguanosine analog 9-4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine ([18F]FHBG). [18F]FHBG is a high-affinity substrate for the HSV1-sr39TK enzyme with relatively low affinity for mammalian TK enzymes, resulting in improved detection sensitivity. Furthermore, [18F]FHBG is approved by the US Food and Drug Administration as an investigational new imaging agent and has been shown to detect HSV1-tk transgene expression in the liver tumors of patients. MicroPET imaging of each small animal can be completed in approximately 1.5 h, and each patient imaging session takes approximately 3 h.

Measuring herpes simplex virus thymidine kinase reporter gene expression in vitro

The herpes simplex 1 virus thymidine kinase (HSV1-tk) positron emission tomography (PET) reporter gene (PRG) or its mutant HSV1-sr39tk are used to investigate intracellular molecular events in cultured cells and for imaging intracellular molecular events and cell trafficking in living subjects. Two in vitro methods are available to assay gene expression of HSV1-tk or HSV1-sr39tk in cells or tissues. One method determines the level of HSV1-TK or HSV1-sr39TK enzyme activity in cell or tissue lysates by measuring the amount of the radiolabeled substrates that have been phosphorylated by these enzymes in a fixed amount of cell lysate protein after a fixed incubation time. The other method, called the 'cell-uptake assay', takes into account the natural uptake and efflux characteristics of the radiolabeled substrate by specific cells, in addition to the level of HSV1-TK or HSV1-sr39TK activity. Both of these assays can be used to validate molecular models in cultured cells, prior to studying them in living research subjects. Each of these assays can be completed in one day.

Positron emission tomography imaging of conditional gene activation in the heart

The Cre-loxP system has been routinely used for conditional activation and deletion of gene expression. However, the spatiotemporal manner of these events in the heart has not yet been defined by in vivo imaging. Adenovirus (1 x 10(9 )pfu) carrying the silent positron emission tomography (PET) reporter gene, herpes simplex virus type 1 thymidine kinase (HSV1-tk), was injected into the left ventricular wall of male transgenic mice (n=15) or FVB controls (n=8). Transgenic mice expressed Cre recombinase driven by a cardiac-specific alpha-myosin heavy chain (alpha-MHC) promoter. Following injection of the 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ([18F]-FHBG; 137+/-25 microCi) reporter probe, microPET imaging was used to assess the expression of HSV1-tk reporter gene in the myocardium. Two days following adenoviral injection, cardiac HSV1-tk gene activation resulted in tracer uptake of 3.20+/-0.51% ID/g for alpha-MHC-Cre and 0.05+/-0.02%ID/g for control mice (P<0.01). The in vivo results were confirmed by RT-PCR and Western blot analysis. Similar transfections were evaluated in both cardiac-specific and non-cardiac-specific cell lines. Enzyme activity showed a robust correlation (r2=0.82) between in vivo molecular imaging technique and traditional in vitro enzyme assays. With further development and validation, PET imaging will likely play an important role in the noninvasive, repetitive, and quantitative measurement of conditional gene activation in the future.

Evaluation of Firefly Luciferase Bioluminescence Mediated Photodynamic Toxicity in Cancer Cells

PURPOSE

This work investigated whether fLuc-catalyzed oxidation of D-luciferin generates sufficient light to induce photodynamic toxicity in cancer cells.

PROCEDURES

Light emission was assessed via cooled CCD (charge-coupled device) camera. Parental and fLuc expressing cancer cells were exposed to subtoxic concentrations of photosensitizers (Rose Bengal or hypericin) and D-luciferin, sunlight, or lamplight. Toxicity was assessed by MTT assay.

RESULTS

fLuc expressing cells emitted up to 500-fold higher levels of photons than parental cell lines. Although exposure to photosensitizer and sunlight reduced survival of various cell lines, survival of fLuc expressing cells incubated with photosensitizer and D-luciferin, or photosensitizer and lamplight, did not differ significantly from parental or untreated cells.

CONCLUSIONS

Contesting recent reports, fLuc bioluminescence does not generate sufficient photons to induce Rose Bengal or hypericin photodynamic toxicity in a range of malignant and nonmalignant cell lines, and is not suitable as a generalizable approach to antineoplastic therapy.

Human gene therapy and imaging in neurological diseases

Molecular imaging aims to assess non-invasively disease-specific biological and molecular processes in animal models and humans in vivo. Apart from precise anatomical localisation and quantification, the most intriguing advantage of such imaging is the opportunity it provides to investigate the time course (dynamics) of disease-specific molecular events in the intact organism. Further, molecular imaging can be used to address basic scientific questions, e.g. transcriptional regulation, signal transduction or protein/protein interaction, and will be essential in developing treatment strategies based on gene therapy. Most importantly, molecular imaging is a key technology in translational research, helping to develop experimental protocols which may later be applied to human patients. Over the past 20 years, imaging based on positron emission tomography (PET) and magnetic resonance imaging (MRI) has been employed for the assessment and "phenotyping" of various neurological diseases, including cerebral ischaemia, neurodegeneration and brain gliomas. While in the past neuro-anatomical studies had to be performed post mortem, molecular imaging has ushered in the era of in vivo functional neuro-anatomy by allowing neuroscience to image structure, function, metabolism and molecular processes of the central nervous system in vivo in both health and disease. Recently, PET and MRI have been successfully utilised together in the non-invasive assessment of gene transfer and gene therapy in humans. To assess the efficiency of gene transfer, the same markers are being used in animals and humans, and have been applied for phenotyping human disease. Here, we review the imaging hallmarks of focal and disseminated neurological diseases, such as cerebral ischaemia, neurodegeneration and glioblastoma multiforme, as well as the attempts to translate gene therapy's experimental knowledge into clinical applications and the way in which this process is being promoted through the use of novel imaging approaches.

A human osteosarcoma cell line expressing herpes simplex type-1 thymidine kinase: studies with radiolabeled (E)-5-(2-iodovinyl)-2'-fluoro-2'-deoxyuridine

INTRODUCTION

(E)-5-(2-Iodovinyl)-2'-fluoro-2'-deoxyuridine (IVFRU) is a pyrimidine nucleoside analogue that accumulates selectively in murine cells expressing herpes simplex type-1 thymidine kinase (HSV-1 TK). The uptake of [(125)I]IVFRU in human 143B osteosarcoma cells transduced with a retroviral vector bearing the HSV-1 TK gene (143B-LTK cells) is now reported.

METHODS

HSV-1 TK gene expression in 143B-LTK cells was confirmed by Western blotting and reverse transcriptase (RT)-PCR. Cell and subcellular uptake of [(125)I]IVFRU was determined in cell culture, and whole body biodistribution after intravenous injection of [(125)I]IVFRU was determined using nude mice bearing implanted 143B or 143B-LTK tumors.

RESULTS

Although IVFRU was less toxic to the human cell line expressing HSV-1 TK (143B-LTK) than ganciclovir, both IVFRU and ganciclovir were not toxic to the cell line not expressing HSV-1 TK (143B). When cells were exposed to [(125)I]IVFRU in vitro, only the 143B-LTK cells accumulated radioactivity. The acid-soluble fraction from 143B-LTK cell lysates contained 8-fold greater activity than the acid-insoluble fraction after an 8-h exposure to [(125)I]IVFRU. Biodistribution of [(125)I]IVFRU in nude mice bearing subcutaneous 143B and 143B-LTK tumors revealed widespread distribution of the nucleoside in vivo but with specific localization in 143B-LTK tumors.

CONCLUSION

The underlying biochemical process of metabolic entrapment of IVFRU in human osteosarcoma cells expressing HSV-1 TK is responsible for selective localization in these cells. The differences in subcellular distribution into the nucleic acid fraction, and in cytotoxicity, reflect the importance of cell type and lineage as determinants of the performance of gene imaging radiopharmaceuticals.

Spying on cancer: molecular imaging in vivo with genetically encoded reporters

Genetically encoded imaging reporters introduced into cells and transgenic animals enable noninvasive, longitudinal studies of dynamic biological processes in vivo. The most common reporters include firefly luciferase (bioluminescence imaging), green fluorescence protein (fluorescence imaging), herpes simplex virus-1 thymidine kinase (positron emission tomography), and variants with enhanced spectral and kinetic properties. When cloned into promoter/enhancer sequences or engineered into fusion proteins, imaging reporters allow transcriptional regulation, signal transduction, protein-protein interactions, oncogenic transformation, cell trafficking, and targeted drug action to be spatiotemporally resolved in vivo. Spying on cancer with genetically encoded imaging reporters provides insight into cancer-specific molecular machinery within the context of the whole animal.

Advances in imaging mouse tumour modelsin vivo

Significant progress has been made recently in the variety of ways that cancer can be non-invasively imaged in murine tumour models. The development and continued refinement of specialized hardware for an array of small animal imaging methodologies are only partly responsible. So too has been the development of new imaging techniques and materials that enable specific, highly sensitive and quantitative measurement of a wide range of tumour-related parameters. Included amongst these new materials are imaging probes that selectively accumulate in tumours, or that become activated by tumour-specific molecules in vivo. Other tumour imaging strategies have been developed that rely upon the detection of reporter transgene expression in vivo, and these too have made a significant impact on both the versatility and the specificity of tumour imaging in living mice. The biological implications resulting from these latest advances are presented here, with particular emphasis on those associated with MRI, PET, SPECT, BLI, and fluorescence-based imaging modalities. Taken together, these advances in tumour imaging are set to have a profound impact on our basic understanding of in vivo tumour biology and will radically alter the application of mouse tumour models in the laboratory.