Whole-body optical imaging of green fluorescent protein-expressing tumors and metastases

Biological characteristics of fluorescent protein-expressing human hepatocellular carcinoma xenograft model in nude mice

OBJECTIVES

To study biological characteristics of stable red fluorescent protein (RFP)-expressing or green fluorescent protein (GFP)-expressing HCCLM3 cell lines and those of their relevant xenograft models in nude mice.

METHODS

HCCLM3, a human hepatocellular carcinoma cell line with high metastatic potential was infected with RFP or GFP full-length cDNA via lentivirus. Stable RFP-expressing or GFP-expressing HCCLM3 cells, namely HCCLM3-R and HCCLM3-G, were subcutaneously injected and two patient-like metastatic models of HCCLM3-R and HCCLM3-G in nude mice were established using surgical orthotopic implantation from subcutaneous tumor tissues. Cell proliferation, karyotype, biomarker expression, tumor growth, and metastasis of HCCLM3-R and HCCLM3-G were analyzed in vitro and in vivo.

RESULTS

RFP and GFP genes were integrated in genomic DNA of HCCLM3. HCCLM3-R and HCCLM3-G expressed red and green fluorescence, stable and intense, 300 days after 60 consecutive passages, and also positively expressed CK8+, P16+, AFP+ and negatively expressed HBsAg-. Their biomarker expression and karyotype were found to be similar to those of the parental HCCLM3, and their tumorigenesis occurred in 10 nude mice without exception after a subcutaneous injection and did the same in 20 nude mice after an orthotopic implantation. The results showed that the rate of spontaneous metastasis to the liver and lung and peritoneal seeding was 100, 100, and 90%, respectively.

CONCLUSION

Stable fluorescent protein-expressing HCCLM3-R and HCCLM3-G xenografts in nude mice could be of two useful models for studying mechanisms of hepatocellular carcinoma growth and metastasis in real time.

Real-time in vivo green fluorescent protein imaging of a murine leishmaniasis model as a new tool for Leishmania vaccine and drug discovery

Leishmania species are obligate intracellular protozoan parasites that cause a broad spectrum of clinical diseases in mammalian hosts. The most frequently used approach to quantify parasites in murine model systems is based on thickness measurements of the footpad or ear after experimental infection. To overcome the limitations of this method, we used a Leishmania mutant episomally transfected with enhanced green fluorescent protein, enabling in vivo real-time whole-body fluorescence imaging, to follow the progression of Leishmania infection in parasitized tissues. Fluorescence correlated with the number of Leishmania parasites in the tissue and demonstrated the real-time efficacy of a therapeutic vaccine. This approach provides several substantial advantages over currently available animal model systems for the in vivo study of immunopathogenesis, prevention, and therapy of leishmaniasis. These include improvements in sensitivity and the ability to acquire real-time data on progression and spread of the infection.

Noninvasive photoacoustic imaging of the thoracic cavity and the kidney in small and large animals

The internal organs of rats and rabbits were clearly imaged noninvasively using a deeply penetrating reflection-mode photoacoustic imaging system. This imaging system had previously been found to provide an imaging depth limit of approximately 38 mm. In the thoracic cavity, major blood vessels connecting to the heart were imaged, and the right atrium was imaged as deeply as approximately 8 mm. In the abdominal cavities, the kidney and vena cava inferior were also imaged in situ. The vena cava inferior approximately 13.7 mm deep was imaged. The kidney of a New Zealand white rabbit was also imaged. This study shows the deep internal organ imaging capability of the system in animals. This technology can potentially be used to study tumors in internal organs, and be adapted to clinical diagnosis.

Imaging cancer dynamics in vivo at the tumor and cellular level with fluorescent proteins

Whole-body imaging with fluorescent proteins has been shown to be a powerful technology to follow the dynamics of metastatic cancer. Whole-body imaging of fluorescent protein-expressing-cancer cells enables the facile determination of efficacy of candidate anti-tumor and anti-metastatic agents in mouse models. GFP-expressing transgenic mice transplanted with the RFP-expressing cancer cells enable the distinction of cancer and host cells and the efficacy of drugs on each type of cell. This is particularly useful for imaging tumor angiogenesis. Cancer-cell trafficking through the cardiovascular and lymphatic systems is the critical means of spread of cancer. The use of fluorescent proteins to differentially label cancer calls in the nucleus and cytoplasm and high-powered imaging technology are used to visualize the nuclear-cytoplasmic dynamics of cancer-cell trafficking in both blood vessels and lymphatic vessels in the live animal. This technology has furthered our understanding of the spread of cancer at the subcellular level in the live mouse. Fluorescent proteins thus enable both macro and micro imaging technology and thereby provide the basis for the new field of in vivo cell biology.

Molecular imaging reveals a correlation between 2'-CH3-MPTP-induced neonatal neurotoxicity and dopaminergic neurodegeneration in adult transgenic mice

We previously reported that a single subcutaneous (s.c.) injection of the neurotoxicant, 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine or 2'-CH(3)-MPTP, to postnatal day 4 (PD4) mice caused acute and transient gliosis in the brain, which can be noninvasively monitored during a course of 8 h immediately after the dosing [Ho, G., Zhang, C.Y., Zhuo, L., 2007. Non-invasive fluorescent imaging of gliosis in transgenic mice for profiling developmental neurotoxicity. Toxicol. Appl. Pharmacol. 221, 76-85]. In the current study, we examined the consequence of PD4 mice receiving multiple injections (4 x 8 mg/kg, s.c. in 2 h intervals) of the same neurotoxicant 24-72 h after the last injection. Here we showed that the multiple dosing scheme (with a higher cumulative dose) triggered a severe gliosis not only in the striatum and substantia nigra pars compacta (SNpc), but also in hippocampus and cerebellum when examined by noninvasive in vivo imaging and by immunohistochemistry (IHC), respectively, in the PD5 to PD7 mice. When neonates treated with the neurotoxicant at PD4 were allowed to develop to 10 weeks of age and examined with IHC, a majority of the dopaminergic (DA) neurons were found to be permanently depleted from the adult SNpc. Our findings suggest that neurotoxicant-elicited neonatal gliosis can be used as an early molecular signature to predict the permanent loss of DA neurons in the developed brain. Since 2'-CH(3)-MPTP is an inducer of Parkinsonism in mice, the molecular imaging method described here is a relatively simple and powerful tool for longitudinally studying the developmental aspect of Parkinsonism.

Disparity between prostate tumor interior versus peripheral vasculature in response to verteporfin-mediated vascular-targeting therapy

Photodynamic therapy (PDT) is a light-based cancer treatment modality. Here we employed both in vivo and ex vivo fluorescence imaging to visualize vascular response and tumor cell survival after verteporfin-mediated PDT designed to target tumor vasculature. EGFP-MatLyLu prostate tumor cells, transduced with EGFP using lentivirus vectors, were implanted in athymic nude mice. Immediately after PDT with different doses of verteporfin, tumor-bearing animals were injected with a fluorochrome-labeled albumin. The extravasation of fluorescent albumin along with tumor EGFP fluorescence was monitored noninvasively with a whole-body fluorescence imaging system. Ex vivo fluorescence microscopy was performed on frozen sections of tumor tissues taken at different times after treatment. Both in vivo and ex vivo imaging demonstrated that vascular-targeting PDT with verteporfin significantly increased the extravasation of fluorochrome-labeled albumin in the tumor tissue, especially in the tumor periphery. Although PDT induced substantial vascular shutdown in interior blood vessels, some peripheral tumor vessels were able to maintain perfusion function up to 24 hr after treatment. As a result, viable tumor cells were typically detected in the tumor periphery in spite of extensive tumor cell death. Our results demonstrate that vascular-targeting PDT with verteporfin causes a dose- and time-dependent increase in vascular permeability and decrease in blood perfusion. However, compared to the interior blood vessels, peripheral tumor blood vessels were found less sensitive to PDT-induced vascular shutdown, which was associated with subsequent tumor recurrence in the tumor periphery.

Noninvasive magnetic resonance imaging of the development of individual colon cancer tumors in rat liver

Monitoring tumor development is essential for the understanding of mechanisms involved in tumor progression and to determine efficacy of therapy. One of the evolving approaches is longitudinal noninvasive magnetic resonance imaging (MRI) of tumors in experimental models. We applied high-resolution MRI at 7 Tesla to study the development of colon cancer tumors in rat liver. MRI acquisition was triggered to the respiratory cycle to minimize motion artifacts. A special radio frequency (RF) coil was designed to acquire detailed T1-weighted and T2-weighted images of the liver. T2-weighted images identified hyperintense lesions representing tumors with a minimum diameter of 2 mm, enabling the determination of growth rates and morphological aspects of individual tumors. It is concluded that high-resolution MRI using a dedicated RF coil and triggering to the respiratory cycle is an excellent tool for quantitative and morphological analysis of individual diffusely distributed tumors throughout the liver. However, at present, MRI requires expensive equipment and expertise and is a time-consuming methodology. Therefore, it should preferably be used for dedicated applications rather than for high-throughput assessment of total tumor load in animals.

Performance of the red-shifted fluorescent proteins in deep-tissue molecular imaging applications

The discovery of new fluorescent proteins (FPs) that emit in the far-red part of the spectrum, where light absorption from tissue is significantly lower than in the visible, offers the possibility for noninvasive biological interrogation at the entire organ or small animal level in vivo. The performance of FPs in deep-tissue imaging depends not only on their optical characteristics, but also on the wavelength-dependent tissue absorption and the depth of the fluorescence activity. To determine the optimal choice of FP and illumination wavelength, we compared the performance of five of the most promising FPs: tdTomato, mCherry, mRaspberry, mPlum, and Katushka. We experimentally measured the signal strength through mice and employed theoretical predictions to obtain an understanding of the performance of different illumination scenarios, especially as they pertain to tomographic imaging. It was found that the appropriate combination of red-shifted proteins and illumination wavelengths can improve detection sensitivity in small animals by at least two orders of magnitude compared with green FP. It is also shown that the steep attenuation change of the hemoglobin spectrum around the 600-nm range may significantly affect the detection sensitivity and, therefore, necessitates the careful selection of illumination wavelengths for optimal imaging performance.

The challenges for molecular nutrition research 2: quantification of the nutritional phenotype

In quantifying the beneficial effect of dietary interventions in healthy subjects, nutrition research meets a number of new challenges. Inter individual variation in biomarker values often is larger than the effect related to the intervention. Healthy subjects have a remarkable capacity to maintain homeostasis, both through direct metabolic regulation, metabolic compensation of altered diets, and effective defence and repair mechanisms in oxidative and inflammatory stress. Processes involved in these regulatory activities essentially different from processes involved in early onset of diet related diseases. So, new concepts and approaches are needed to better quantify the subtle effects possibly achieved by dietary interventions in healthy subjects. Apart from quantification of the genotype and food intake (these are discussed in separate reviews in this series), four major areas of innovation are discussed: the biomarker profile concept, perturbation of homeostasis combined with omics analysis, imaging, modelling and fluxes. All of these areas contribute to a better understanding and quantification of the nutritional phenotype.

Comparison of fracture healing among different inbred mouse strains

Quantitative trait locus analysis can be used to identify genes critically involved in biological processes. No such analysis has been applied to identifying genes that control bone fracture healing. To determine the feasibility of such an approach, healing of femur fractures was measured between C57BL/6, DBA/2, and C3H inbred strains of mice. Healing was assessed by radiography and histology and measured by histomorphometry and biomechanical testing. In all strains, radiographic bridging of the fracture was apparent after 3 weeks of healing. Histology showed that healing occurred through endochondral ossification in all strains. Histomorphometric measurements found more bone in the C57BL/6 fracture calluses 7 and 10 days after fracture. In contrast, more cartilage was present after 7 days in the C3H callus, which rapidly declined to levels less than those of C57BL/6 or DBA/2 mice by 14 days after fracture. An endochondral ossification index was calculated by multiplying the callus percent cartilage and bone areas as a measure of endochondral ossification. At 7 and 10 days after fracture, this value was higher in C57BL/6 mice. Using torsional mechanical testing, normalized structural and material properties of the C57BL/6 healing femurs were higher than values from the DBA/2 or C3H mice 4 weeks after fracture. The data indicate that fracture healing proceeds more rapidly in C57BL/6 mice and demonstrate that genetic variability significantly contributes to the process of bone regeneration. Large enough differences exist between C57BL/6 and DBA/2 or C3H mice to permit a quantitative trait locus analysis to identify genes controlling bone regeneration.

Noninvasive quantification of tumor volume in preclinical liver metastasis models using contrast-enhanced x-ray computed tomography

OBJECTIVES

To determine a timepoint after contrast injection that yields equal liver parenchymal and vascular enhancement in micro-computed tomography images. To evaluate the utility of images acquired during this time period for the noninvasive measurement of liver-tumor volume.

MATERIALS AND METHODS

The imaging timepoint was determined by quantifying the enhancement kinetics of Fenestra VC (0.015 mL/g) in NIH III mice. In respiratory-gated images of tumor bearing mice, the ability to measure tumor volume was evaluated with a measurement variability study, and by comparing in vivo and histologically measured tumor volume.

RESULTS

Eight hours after contrast injection the liver parenchyma and vasculature were equally enhanced allowing for clear delineation of the unenhanced tumors. The smallest tumor detected in this study was 1.1 mm in diameter. The coefficient of variation for tumor-volume measurement ranged from 3.6% to 12.9% and from 6.3% to 25.8% for intra and interobserver variability, respectively. In vivo and histologic tumor-volume measurements were closely correlated (r = 0.98, P < 0.0001).

CONCLUSIONS

Imaging at a time period of equal liver parenchyma and vascular enhancement after contrast injection allows for clear delineation of liver-tumor borders, thereby enabling quantitative tumor-volume monitoring.

Large Field of View Scanning Fluorescence Lifetime Imaging System for Multimode Optical Imaging of Small Animals

We describe a scanning fluorescence lifetime imaging (SFLIM) system that provides a large field of view (LFOV), using a femtosecond (fs) pulsed laser, for multi-mode optical imaging of small animals. Fluorescence lifetime imaging (FLIM) can be a useful optical method to distinguish between fluorophores inside small animals. However, difficulty arises when LFOV is required in FLIM using a fs pulsed laser for the excitation of the fluorophores at low wavelengths (<500nm), primarily because the field of view of the pulsed blue excitation light generated from the second harmonic of the fs pulsed light is limited to about a centimeter in diameter due to the severe scattering and absorption of the light inside tissues. Here, we choose a scanning method in order to acquire a FLIM image with LFOV as one alternative. In the SFLIM system, we used a conventional cooled CCD camera coupled to an ultra-fast time-gated intensifier, a tunable femtosecond laser for the excitation of fluorophores, and an x-y moving stage for scanning. Images acquired through scanning were combined into a single image and then this reconstructed image was compared with images obtained by spectral imaging. The resulting SFLIM system is promising as an alternative method for the FLIM imaging of small animals, containing fluorophores exited by blue light, for LFOV applications such as whole animal imaging.

Lycopene inhibits experimental metastasis of human hepatoma SK-Hep-1 cells in athymic nude mice

Lycopene has been shown to inhibit tumor metastasis in vitro, but it is unclear whether lycopene is antimetastatic in vivo. Here, nude mice were orally supplemented 2 times per week for 12 wk with a low or high dose of lycopene [1 or 20 mg/kg body weight (BW)] or with beta-carotene (20 mg/kg BW). Two weeks after the beginning of supplementation, mice were injected once with human hepatoma SK-Hep-1 cells via the tail vein. Plasma levels of matrix metalloproteinase (MMP)-2 and vascular endothelial growth factor (VEGF) increased gradually in tumor-injected mice (tumor controls) following tumor injection but were markedly lowered by lycopene or beta-carotene supplementation. Ten weeks after tumor injection, mice were killed and tumor metastasis was found to be confined to the lungs. Compared with the tumor controls, high-lycopene supplementation lowered the mean number of tumors from 14 +/- 8 to 3 +/- 5 (P < 0.05) and decreased tumor cross-sectional areas by 62% (P < 0.05). High-lycopene supplementation also decreased the positive rate of proliferating cellular nuclear antigen (PCNA), the level of VEGF, and protein expressions of PCNA, MMP-9, and VEGF in lung tissues. However, high-lycopene increased the protein expression of nm23-H1 (an antimetastatic gene) by 133% (P < 0.001). For most variables measured, effects of lycopene were dose dependent and the effect of beta-carotene was between those of high-dose and low-dose lycopene. These results show that lycopene supplementation reduces experimental tumor metastasis in vivo and suggest that such an action is associated with attenuation of tumor invasion, proliferation, and angiogenesis.

Imaging In Mice With Fluorescent Proteins: From Macro To Subcellular

Whole-body imaging with fluorescent proteins has been shown to be a powerful technology with many applications in small animals. Brighter, red-shifted proteins can make whole-body imaging even more sensitive due to reduced absorption by tissues and less scatter. For example, a new protein called Katushka has been isolated that is the brightest known protein with emission at wavelengths longer than 620 nm. This new protein offers potential for noninvasive whole-body macro imaging such as of tumor growth. For subcellular imaging, to observe cytoplasmic and nuclear dynamics in the living mouse, cancer cells were labeled in the nucleus with green fluorescent protein and with red fluorescent protein in the cytoplasm. The nuclear and cytoplasmic behavior of cancer cells in real time in blood vessels was imaged as they trafficked by various means or adhered to the vessel surface in the abdominal skin flap. During extravasation, real-time dual-color imaging showed that cytoplasmic processes of the cancer cells exited the vessels first, with nuclei following along the cytoplasmic projections. Both cytoplasm and nuclei underwent deformation during extravasation. Cancer cells trafficking in lymphatic vessels was also imaged. To noninvasively image cancer cell/stromal cell interaction in the tumor microenvironment as well as drug response at the cellular level in live animals in real time, we developed a new imageable three-color animal model. The model consists of GFP-expressing mice transplanted with the dual-color cancer cells. With the dual-color cancer cells and a highly sensitive small animal imaging system, subcellular dynamics can now be observed in live mice in real time. Fluorescent proteins thus enable both macro and micro imaging technology and thereby provide the basis for the new field of in vivo cell biology.

Recent advances on in vivo imaging with fluorescent proteins

In vivo imaging with green fluorescent protein (GFP) and other fluorescent proteins is revolutionizing cancer biology and other fields of in vivo biology (Hoffman, 2005; Hoffman and Yang, 2006a,b,c). Our laboratory pioneered the use of GFP for in vivo imaging in 1997 (Chishima et al., 1997). This chapter highlights recent developments from our laboratory on both macro and micro in vivo imaging by using fluorescent proteins.

Comparison of mouse mammary gland imaging techniques and applications: reflectance confocal microscopy, GFP imaging, and ultrasound

BACKGROUND

Genetically engineered mouse models of mammary gland cancer enable the in vivo study of molecular mechanisms and signaling during development and cancer pathophysiology. However, traditional whole mount and histological imaging modalities are only applicable to non-viable tissue.

METHODS

We evaluated three techniques that can be quickly applied to living tissue for imaging normal and cancerous mammary gland: reflectance confocal microscopy, green fluorescent protein imaging, and ultrasound imaging.

RESULTS

In the current study, reflectance confocal imaging offered the highest resolution and was used to optically section mammary ductal structures in the whole mammary gland. Glands remained viable in mammary gland whole organ culture when 1% acetic acid was used as a contrast agent. Our application of using green fluorescent protein expressing transgenic mice in our study allowed for whole mammary gland ductal structures imaging and enabled straightforward serial imaging of mammary gland ducts in whole organ culture to visualize the growth and differentiation process. Ultrasound imaging showed the lowest resolution. However, ultrasound was able to detect mammary preneoplastic lesions 0.2 mm in size and was used to follow cancer growth with serial imaging in living mice.

CONCLUSION

In conclusion, each technique enabled serial imaging of living mammary tissue and visualization of growth and development, quickly and with minimal tissue preparation. The use of the higher resolution reflectance confocal and green fluorescent protein imaging techniques and lower resolution ultrasound were complementary.

A comparison between a time domain and continuous wave small animal optical imaging system

We present a phantom study to evaluate the performance of the eXplore Optix (Advanced Research Technologies-GE Healthcare), the first commercially available time-domain tomography system for small animal fluorescence imaging, and compare its capabilities with the widely used IVIS 200 (Xenogen Corporation-Caliper) continuous wave planar imaging system. The eXplore Optix, based on point-wise illumination and collection scheme, is found to be a log order more sensitive with significantly higher detection depth and spatial resolution as compared with the wide-area illumination IVIS 200 under the conditions tested. A time-resolved detection system allows the eXplore Optix to measure the arrival time distribution of fluorescence photons. This enables fluorescence lifetime measurement, absorption mapping, and estimation of fluorescent inclusion depth, which in turn is used by a reconstruction algorithm to calculate the volumetric distribution of the fluorophore concentration. An increased acquisition time and lack of ability to image multiple animals simultaneously are the main drawbacks of the eXplore Optix as compared with the IVIS 200.

Bioluminescence: imaging modality for in vitro and in vivo gene expression

Molecular imaging offers many unique opportunities to study biological processes in intact organisms. Bioluminescence is the emission of light from biochemical reactions that occur within a living organism. Luciferase has been used as a reporter gene in transgenic mice but, until bioluminescence imaging was described, the detection of luciferase activity required either sectioning of the animal or excision of tissue and homogenization to measure enzyme activities in a conventional luminometer. Bioluminescence imaging (BLI) is based on the idea that biological light sources can be incorporated into cells and animal models artificially that does not naturally express the luminescent genes. This imaging modality has proven to be a very powerful methodology to detect luciferase reporter activity in intact animal models. This form of optical imaging is low cost and noninvasive and facilitates real-time analysis of disease processes at the molecular level in living organisms. Bioluminescence provides a noninvasive method to monitor gene expression in vivo and has enormous potential to elucidate the pathobiology of lung diseases in intact mouse models, including models of inflammation/injury, infection, and cancer.

A better fluorescent protein for whole-body imaging

Whole-body imaging with fluorescent proteins is a powerful technology with many applications in small animals. Brighter, red-shifted proteins can make whole-body imaging more sensitive owing to reduced absorption by tissues and less scatter. A new protein called Katushka has been isolated. It is the brightest known protein with emission at wavelengths longer than 620 nm. This new protein offers the potential for noninvasive whole-body imaging of numerous cellular and molecular processes in live animals.

Molecular pathology of malignant gliomas

Malignant gliomas, the most common type of primary brain tumor, are a spectrum of tumors of varying differentiation and malignancy grades. These tumors may arise from neural stem cells and appear to contain tumor stem cells. Early genetic events differ between astrocytic and oligodendroglial tumors, but all tumors have an initially invasive phenotype, which complicates therapy. Progression-associated genetic alterations are common to different tumor types, targeting growth-promoting and cell cycle control pathways and resulting in focal hypoxia, necrosis, and angiogenesis. Knowledge of malignant glioma genetics has already impacted clinical management of these tumors, and researchers hope that further knowledge of the molecular pathology of malignant gliomas will result in novel therapies.

Validity of bioluminescence measurements for noninvasive in vivo imaging of tumor load in small animals

A relatively new strategy to longitudinally monitor tumor load in intact animals and the effects of therapy is noninvasive bioluminescence imaging (BLI). The validity of BLIf or quantitative assessment of tumor load in small animals is critically evaluated in the present review. Cancer cells are grafted in mice or rats after transfection with a luciferase gene--usually that of a firefly. To determine tumor load, animals receive the substrate agent luciferin intraperitoneally, which luciferase converts into oxyluciferin in an ATP-dependent manner Light emitted by oxyluciferin in viable cancer cells is captured noninvasively with a highly sensitive charge-coupled device (CCD) camera. Validation studies indicate that BLI is useful to determine tumor load in the course of time, with each animal serving as its own reference. BLI is rapid, easy to perform, and sensitive. It can detect tumor load shortly after inoculation, even when relatively few cancer cells (2500-10,000) are used. BLI is less suited for the determination of absolute tumor mass in an animal because of quenching of bioluminescence by tissue components and the exact location of tumors because its spatial resolution is limited. Nevertheless, BLI is a powerful tool for high-throughput longitudinal monitoring of tumor load in small animals and allows the implementation of more advanced orthotopic tumor models in therapy intervention studies with almost the same simplicity as when measuring traditional ectopic subcutaneous models in combination with calipers.

InXy and SeXy, compact heterologous reporter proteins for mammalian cells

Mammalian reporter proteins are essential for gene-function analysis, drugscreening initiatives and as model product proteins for biopharmaceutical manufacturing. Bacillus subtilis can maintain its metabolism by secreting Xylanase A (XynA), which converts xylan into shorter xylose oligosaccharides. XynA is a family 11 xylanase monospecific for D-xylose containing substrates. Mammalian cells transgenic for constitutive expression of wild-type xynA showed substantial secretion of this prokaryotic enzyme. Deletion analysis confirmed that a prokaryotic signal sequence encoded within the first 81 nucleotides was compatible with the secretory pathway of mammalian cells. Codon optimization combined with elimination of the prokaryotic signal sequence resulted in an exclusively intracellular mammalian Xylanase A variant (InXy) while replacement by an immunoglobulin-derived secretion signal created an optimal secreted Xylanase A derivative (SeXy). A variety of chromogenic and fluorescence-based assays adapted for use with mammalian cells detected InXy and SeXy with high sensitivity and showed that both reporter proteins resisted repeated freeze/thaw cycles, remained active over wide temperature and pH ranges, were extremely stable in human serum stored at room temperature and could independently be quantified in samples also containing other prominent reporter proteins such as the human placental alkaline phosphatase (SEAP) and the Bacillus stearothermophilus-derived secreted alpha-amylase (SAMY). Glycoprofiling revealed that SeXy produced in mammalian cells was N- glycosylated at four different sites, mutation of which resulted in impaired secretion. SeXy was successfully expressed in a variety of mammalian cell lines and primary cells following transient transfection and transduction with adeno-associated virus particles (AAV) engineered for constitutive SeXy expression. Intramuscular injection of transgenic AAVs into mice showed significant SeXy levels in the bloodstream. InXy and SeXy are highly sensitive, compact and robust reporter proteins, fully compatible with pre-existing marker genes and can be assayed in high-throughput formats using very small sample volumes.

Advances in optical imaging and novel model systems for cancer metastasis research

Research into the genetic and physiological interactions of tumours with their host environment requires in vivo assays to address molecular expression patterns and function. In recent years much of this work has been performed using bioluminescent and fluorescent imaging techniques that allow real-time and non-invasive imaging of gene expression and (tumour) tissue development. Luminescence imaging has until now been more or less the only tool that allows the imaging of intra-osseous breast cancer cells and indeed this technique has been pioneered in our laboratory. Here we summarise some recent innovations and developments using cancer cells and some of the first imaging models of multimodal dual luminescence and luminescence combined with fluorescence of intra-osseous tumours. We further engineered our models to incorporate a specific insertion site in the genome and will discuss some of the possible applications. These include the insertion of signalling pathway-specific reporters and studying the fate of multiple injected populations in a single mouse. We conclude that recent improvements in luminescence- and fluorescence-detection platforms now clearly allow multimodal imaging which will greatly enhance our ability to assess gene function and for the first time to visualise multiple gene- and cellular interactions in real time and in vivo.

Whole-body optical imaging in animal models to assess cancer development and progression

Different optical-based imaging models were used to investigate tumor progression and metastasis with particular emphasis on metastasis to bone and bone marrow. We describe how optical imaging can be used to follow important processes in tumor development and treatment response, including angiogenesis, apoptosis, and proteolysis. Finally, we discuss the translation of one optical imaging modality, near-IR fluorescence, from animal validation studies to applications in the clinic related to cancer management.

Current issues and perspectives in small rodent magnetic resonance imaging using clinical MRI scanners

Small rodents such as mice and rats are frequently used in animal experiments for several reasons. In the past, animal experiments were frequently associated with invasive methods and groups of animals had to be killed to perform longitudinal studies. Today's modern imaging techniques such as magnetic resonance imaging (MRI) allow non-invasive longitudinal monitoring of multiple parameters. Although only a few institutions have access to dedicated small animal MR scanners, most institutions carrying out animal experiments have access to clinical MR scanners. Technological advances and the increasing field strength of clinical scanners make MRI a broadly available and viable technique in preclinical in vivo research. This review provides an overview of current concepts, limitations, and recent studies dealing with small animal imaging using clinical MR scanners.

Whole-body imaging with fluorescent proteins

The intrinsic brightness of fluorescent proteins has been taken advantage of to develop a technology of whole-body imaging of tumors and gene expression in mouse internal organs. Stable transformation with fluorescent protein genes can be effected using retroviral vectors containing a selectable marker such as neomycin resistance. The cells that stably express fluorescent proteins can then be transplanted into appropriate mouse models. For whole-body imaging, nude mice are very appropriate. If wild-type mice are used, then hair must be removed by shaving or depilation. The instruments used can range from a simple LED flashlight and appropriate excitation and emission filters to sophisticated equipment such as the Olympus OV100 with a wide range of magnification, enabling both macroimaging and microimaging. It is crucial that proper filters be used such that background autofluorescence is minimal. Fluorescent protein-based imaging technology can be used for whole-body imaging of fluorescent cells on essentially all organs. The timeline for these experiments varies from 2 days to 2 months.

Garlic-derived S-allylmercaptocysteine is a novel in vivo antimetastatic agent for androgen-independent prostate cancer

PURPOSE

There is epidemiologic evidence that high garlic consumption decreases the incidence of prostate cancer, and compounds isolated from garlic have been shown to have cancer-preventive and tumor-suppressive effects. Recent in vitro studies in our laboratory have shown that garlic-derived organosulfur compound S-allylmercaptocysteine suppresses invasion and cell motility of androgen-independent prostate cancer cells via the up-regulation of cell-adhesion molecule E-cadherin. S-allylmercaptocysteine is therefore a potential antimetastatic drug with broad clinical applications that we tested in vivo for the first time in this study.

EXPERIMENTAL DESIGN

We used a newly established fluorescent orthotopic androgen-independent prostate cancer mouse model to assess the ability of S-allylmercaptocysteine to inhibit tumor growth and dissemination.

RESULTS

We showed that oral S-allylmercaptocysteine not only inhibited the growth of primary tumors by up to 71% (P < 0.001) but also reduced the number of lung and adrenal metastases by as much as 85.5% (P = 0.001) without causing notable toxicity. This metastatic suppression was accompanied by a 91% reduction of viable circulating tumor cells (P = 0.041), suggesting that S-allylmercaptocysteine prevents dissemination by decreasing tumor cell intravasation.

CONCLUSIONS

Our results provide in vivo evidence supporting the potential use of S-allylmercaptocysteine as an E-cadherin up-regulating antimetastatic agent for the treatment of androgen-independent prostate cancer. This is the first report of the in vivo antimetastatic properties of garlic, which may also apply to other cancer types.

Whole-body subcellular multicolor imaging of tumor-host interaction and drug response in real time

To noninvasively image cancer cell/stromal cell interaction in the tumor microenvironment and drug response at the cellular level in live animals in real time, we developed a new imageable three-color animal model. The model consists of green fluorescent protein (GFP)-expressing mice transplanted with dual-color cancer cells labeled with GFP in the nucleus and red fluorescent protein in the cytoplasm. The Olympus IV100 Laser Scanning Microscope, with ultra-narrow microscope objectives ("stick objectives"), is used for three-color whole-body imaging of the two-color cancer cells interacting with the GFP-expressing stromal cells. In this model, drug response of both cancer and stromal cells in the intact live animal is also imaged in real time. Various in vivo phenomena of tumor-host interaction and cellular dynamics were imaged, including mitotic and apoptotic tumor cells, stromal cells interacting with the tumor cells, tumor vasculature, and tumor blood flow. This new model system enables the first cellular and subcellular images of unperturbed tumors in the live intact animal. New visible real-time targets for novel anticancer agents are provided in this model, including the color-coded interacting cancer and stromal cells, tumor vasculature, and blood flow. This imageable model should lead to many new insights of in vivo cancer cell biology and to novel drug discovery.

Detection of myeloma in skeleton of mice by whole-body optical fluorescence imaging

Development of new therapies for myeloma has been hindered by the lack of suitable preclinical animal models of the disease in which widespread tumor foci in the skeleton can be detected reliably. Traditional means of detecting skeletal tumor infiltration such as histopathology are cumbersome and labor-intensive and do not allow temporal monitoring of tumor progression or regression in response to therapy. To resolve this problem, we modified the Radl 5TGM1 model of myeloma bone disease such that fluorescent myeloma tumors can be optically imaged in situ. Here, we show that murine myeloma 5TGM1 tumor cells, engineered to express enhanced green fluorescent protein (eGFP; 5TGM1-eGFP cells), can be imaged in a temporal fashion using a fluorescence illuminator and a charge-coupled device camera in skeletons of live C57BL/KaLwRij mice. High-resolution, whole-body images of tumor-bearing mice revealed that myeloma cells homed almost exclusively to the skeleton, with multiple focal tumor foci in the axial skeleton, consistent with myeloma tumor distribution in humans. Finally, the tested antitumor treatment effect of Velcade (bortezomib), a proteasome inhibitor used clinically in myeloma, was readily detected by GFP imaging, suggesting the power of the technique in combination with the Radl 5TGM1-eGFP model for rapid preclinical assessment and sensitive monitoring of novel and potential therapeutics. Whole-body GFP imaging is practical, convenient, inexpensive, and rapid, and these advantages should enable a high throughput when evaluating in vivo efficacy of new potential antimyeloma therapeutics and assessing response to treatment.

Developing MR reporter genes: promises and pitfalls

MR reporter genes have the potential to monitor transgene expression non-invasively in real time at high resolution. These genes can be applied to interrogate the efficacy of gene therapy, to assess cellular differentiation, cell trafficking, and specific metabolic activity, and also assess changes in the microenvironment. Efforts toward the development of MR reporter genes have been made for at least a decade, but, despite these efforts, the field is still in its early developmental stage. This reflects the fact that there are potential pitfalls, caused by the low sensitivity of detection, the need for substrates with their associated undesirable pharmacokinetics, and/or the difficult and, in some cases, delayed interpretation of signal changes. Nevertheless, significant progress has been made during the last few years. Whereas enzyme-based reporters were initially applied to NMR spectroscopic monitoring of changes in phosphor and fluorine metabolism, MRI-based approaches are now emerging that rely on: (1) enzyme-based cleavage of functional groups that block water (proton) exchange or protein binding of MR contrast agents; (2) expression of surface receptors that enable binding of specific MR contrast agents; (3) expression of para- and anti-ferromagnetic (metallo)proteins involved with iron metabolism, such as tyrosinase, transferrin receptor, and ferritin. After an introduction to the basic principles of designing promoters, expression vectors, and cloning of transgenes, a fresh look is provided on the use of reporter genes for optical (including bioluminescent) and nuclear imaging, with which MR reporter genes compete. Although progress in the use of MR reporter genes has been slow, newer strategies that use metalloproteins or alternative contrast mechanisms, with no need for substrates, promise rapid growth potential for this field.

Gene expression imaging by enzymatic catalysis of a fluorescent probe via membrane-anchored beta-glucuronidase

Development of nonimmunogenic and specific reporter genes to monitor gene expression in vivo is important for the optimization of gene therapy protocols. We developed a membrane-anchored form of mouse beta-glucuronidase (mbetaG) as a reporter gene to hydrolyze a nonfluorescent glucuronide probe (fluorescein di-beta-D-glucuronide, (FDGlcU) to a highly fluorescent reporter to assess the location and persistence of gene expression. A functional beta-glucuronidase (betaG) was stably expressed on the surface of murine CT26 colon adenocarcinoma cells where it selectively hydrolyzed the cell-impermeable FDGlcU probe. FDGlcU was also preferentially converted to fluorescent probe by (betaG) on CT26 tumors. The fluorescent intensity in betaG-expressing CT26 tumors was 240 times greater than the intensity in control tumors. Selective imaging of gene expression was also observed after intratumoral injection of adenoviral betaG vector into carcinoma xenografts. Importantly, mbetaG did not induce an antibody response after hydrodynamic plasmid immunization of Balb/c mice, indicating that the reporter gene product displayed low immunogenicity. A membrane-anchored form of human betaG also allowed in vivo imaging, demonstrating that human betaG can be employed for imaging. This imaging system therefore, displays good selectivity with low immunogenicity and may help assess the location, magnitude and duration of gene expression in living animals and humans.

Molecular imaging of novel cell- and viral-based therapies

Drugs, surgery, and radiation are the traditional modalities of therapy in medicine. To these are being added new therapies based on cells and viruses or their derivatives. In these novel therapies, a cell or viral vector acts as a drug in its own right, altering the host or a disease process to bring about healing. Most of these advances originate from the significant recent advances in molecular medicine, but some have been around for some time. Blood transfusions and cowpox vaccinations are part of the history of medicine...but nevertheless are examples of cell- and viral-based therapies. This article focuses on the modern molecular incarnations of these therapies, and specifically on how imaging is used to track and guide these novel agents. We survey the literature dealing with imaging these new cell and viral particle therapies and provide a framework for understanding publications in this area. Leading technology of gene modifications are the fundamental modifications applied to make these new therapies amenable to imaging.

Molecular imaging using visible light to reveal biological changes in the brain

Advances in imaging have enabled the study of cellular and molecular processes in the context of the living body that include cell migration patterns, location and extent of gene expression, degree of protein-protein interaction, and levels of enzyme activity. These tools, which operate over a range of scales, resolutions, and sensitivities, have opened up broad new areas of investigation where the influence of organ systems and functional circulation is intact. There are a myriad of imaging modalities available, each with its own advantages and disadvantages, depending on the specific application. Among these modalities, optical imaging techniques, including in vivo bioluminescence imaging and fluorescence imaging, use visible light to interrogate biology in the living body. Optimal imaging with these modalities require that the appropriate marker be used to tag the process of interest to make it uniquely visible using a particular imaging technology. For each optical modality, there are various labels to choose from that range from dyes that permit tissue contrast and dyes that can be activated by enzymatic activity, to gene-encoding proteins with optical signatures that can be engineered into specific biological processes. This article provides and overview of optical imaging technologies and commonly used labels, focusing on bioluminescence and fluorescence, and describes several examples of how these tools are applied to biological questions relating to the central nervous system.

Intravital imaging of fluorescent markers and FRET probes by DNA tattooing

Background

Advances in fluorescence microscopy and mouse transgenesis have made it possible to image molecular events in living animals. However, the generation of transgenic mice is a lengthy process and intravital imaging requires specialized knowledge and equipment. Here, we report a rapid and undemanding intravital imaging method using generally available equipment.

Results

By DNA tattooing we transfect keratinocytes of living mice with DNA encoding fluorescent biosensors. Subsequently, the behavior of individual cells expressing these biosensors can be visualized within hours and using conventional microscopy equipment. Using this "instant transgenic" model in combination with a corrected coordinate system, we followed the in vivo behavior of individual cells expressing either FRET- or location-based biosensors for several days. The utility of this approach was demonstrated by assessment of in vivo caspase-3 activation upon induction of apoptosis.

Conclusion

This "instant skin transgenic" model can be used to follow the in vivo behavior of individual cells expressing either FRET- or location-based probes for several days after tattooing and provides a rapid and inexpensive method for intravital imaging in murine skin.

Effects of residual fluorescence on time-resolved signals simulated with the finite element method in biological tissues

A computational model based on finite element method is derived to examine how the simulated time-dependent signals are related to the presence of residual fluorescence in biological media surrounding a fluorescent object. We apply a subtraction technique on recorded data when imperfect uptake of fluorescing agent into the tumor is considered. We show the limits of the subtracting method for low target: background fluorescent absorption contrast by extracting the time to reach the half maximum and analyzing the maximum of the time-resolved signals versus target depth.

In situ analysis of breast cancer progression in murine models using a macroscopic fluorescence imaging system

BACKGROUND AND OBJECTIVE

The goal of this study was to use an inexpensive macroscopic imaging system to monitor tumor progression in mouse models in real-time with minimal intervention.

STUDY DESIGN/MATERIALS AND METHODS

Illumination is provided via a xenon arc lamp and a fiber optic probe which delivers white light or quasi-monochromatic excitation via specific bandpass filters. Fluorescence emission from SCID and nude mice following mammary fat pad injection of red fluorescence protein (RFP)-expressing human breast cancer cell lines was recorded and quantified using a single lens reflex (SLR) digital camera.

RESULTS

This simple system enabled the verification of successful tumor take and temporal quantification of tumor progression in mouse models.

CONCLUSION

The macroscopic fluorescence imaging system represents an inexpensive and portable tool to facilitate non-invasive in situ cancer detection with the potential to monitor fluorescent tumor formation and investigation of the efficacy of potential cancer therapeutics.

Enhanced green fluorescent protein is a nearly ideal long-term expression tracer for hematopoietic stem cells, whereas DsRed-express fluorescent protein is not

Validated gene transfer and expression tracers are essential for elucidating functions of mammalian genes. Here, we have determined the suitability and unintended side effects of enhanced green fluorescent protein (EGFP) and DsRed-Express fluorescent protein as expression tracers in long-term hematopoietic stem cells (HSCs). Retrovirally transduced mouse bone marrow cells expressing either EGFP or DsRed-Express in single or mixed dual-color cell populations were clearly discerned by flow cytometry and fluorescence microscopy. The results from in vivo competitive repopulation assays demonstrated that EGFP-expressing HSCs were maintained nearly throughout the lifespan of the transplanted mice and retained long-term multilineage repopulating potential. All mice assessed at 15 months post-transplantation were EGFP positive, and, on average, 24% total peripheral white blood cells expressed EGFP. Most EGFP-expressing recipient mice lived at least 22 months. In contrast, Discosoma sp. red fluorescent protein (DsRed)-expressing donor cells dramatically declined in transplant-recipient mice over time, particularly in the competitive setting, in which mixed EGFP- and DsRed-expressing cells were cotransplanted. Moreover, under in vitro culture condition favoring preservation of HSCs, purified EGFP-expressing cells grew robustly, whereas DsRed-expressing cells did not. Therefore, EGFP has no detectable deteriorative effects on HSCs, and is nearly an ideal long-term expression tracer for hematopoietic cells; however, DsRed-Express fluorescent protein is not suitable for these cells.

A comparison of the emission efficiency of four common green fluorescence dyes after internalization into cancer cells

In vivo optical imaging to enhance the detection of cancer during endoscopy or surgery requires a targeted fluorescent probe with high emission efficiency and high signal-to-background ratio. One strategy to accurately detect cancers is to have the fluorophore internalize within the cancer cells permitting nonbound fluorophores to be washed away or absorbed. The choice of fluorophores for this task must be carefully considered. For depth of penetration, near-infrared probes are ordinarily preferred but suffer from relatively low quantum efficiency. Although green fluorescent protein has been widely used to image tumors on internal organs in mice, green fluorescent probes are better suited for imaging the superficial tissues because of the short penetration distance of green light in tissue and the highly efficient production of signal. While the fluorescence properties of green fluorophores are well-known in vitro, less attention has been paid to their fluorescence once they are internalized within cells. In this study, the emission efficiency after cellular internalization of four common green fluorophores conjugated to avidin (Av-fluorescein, Av-Oregon green, Av-BODIPY-FL, and Av-rhodamine green) were compared after each conjugate was incubated with SHIN3 ovarian cancer cells. Using the lectin binding receptor system, the avidin-fluorophore conjugates were endocytosed, and their fluorescence was evaluated with fluorescence microscopy and flow cytometry. While fluorescein demonstrated the highest signal outside the cell, among the four fluorophores, internalized Av-rhodamine green emitted the most light from SHIN3 ovarian cancer cells both in vitro and in vivo. The internalized Av-rhodamine green complex appeared to localize to the endoplasmic vesicles. Thus, among the four common green fluorescent dyes, rhodamine green is the brightest green fluorescence probe after cellular internalization. This information could have implications for the design of tumor-targeted fluorescent probes that rely on cellular internalization for cancer detection.

EGFP fluorescence as an indicator of cancer cells response to methotrexate

Methotrexate action in viable cells was monitored by registering changes in EGFP (Enhanced Green Fluorescent Protein) fluorescence intensity. Treatment with 1 microM methotrexate for 48 h of human colorectal adenocarcinoma C85 cells, stably transfected to express EGFP, caused 5-fold increase in EGFP fluorescence assayed by flow cytometry with no distinct increase in EGFP protein level. This was correlated with morphological changes, including an increase of cell granularity and cell shape flattening, as well as cell cycle G1 phase arrest revealed by DNA content analysis. Methotrexate removal allowed the morphology of the cells in culture to revert in 10 days to normal. The cells that survived methotrexate exposure were propagated as C85r cell subline and displayed kinetics of methotrexate sensitivity parallel to that of the parental C85 line. As the increase in EGFP fluorescence could also be visualized by fluorescence microscopy, this reporter system may be employed to image methotrexate action in cancer cells in living models.

Fluorescence molecular imaging

There is a wealth of new fluorescent reporter technologies for tagging of many cellular and subcellular processes in vivo. This imposed contrast is now captured with an increasing number of available imaging methods that offer new ways to visualize and quantify fluorescent markers distributed in tissues. This is an evolving field of imaging sciences that has already achieved major advances but is also facing important challenges. It is nevertheless well poised to significantly impact the ways of biological research, drug discovery, and clinical practice in the years to come. Herein, the most pertinent technologies associated with in vivo noninvasive or minimally invasive fluorescence imaging of tissues are summarized. Focus is given to small-animal imaging. However, while a broad spectrum of fluorescence reporter technologies and imaging methods are outlined, as necessary for biomedical research, and clinical translation as well.

Molecular Imaging: A Primer for Interventionalists and Imagers

The characterization of human diseases by their underlying molecular and genomic aberrations has been the hallmark of molecular medicine. From this, molecular imaging has emerged as a potentially revolutionary discipline that aims to visually characterize normal and pathologic processes at the cellular and molecular levels within the milieu of living organisms. Molecular imaging holds promise to provide earlier and more precise disease diagnosis, improved disease characterization, and timely assessment of therapeutic response. This primer is intended to provide a broad overview of molecular imaging with specific focus on future clinical applications relevant to interventional radiology.

Minimally invasive localization of oncolytic herpes simplex viral therapy of metastatic pleural cancer

Herpes simplex virus-1 (HSV-1) oncolytic therapy and gene therapy are promising treatment modalities against cancer. NV1066, one such HSV-1 virus, carries a marker gene for enhanced green fluorescent protein (EGFP). The purpose of this study was to determine whether NV1066 is cytotoxic to lung cancer and whether EGFP is a detectable marker of viral infection in vitro and in vivo. We further investigated whether EGFP expression in infected cells can be used to localize the virus and to identify small metastatic tumor foci (<1 mm) in vivo by means of minimally invasive endoscopic systems equipped with fluorescent filters. In A549 human lung cancer cells, in vitro viral replication was determined by plaque assay, cell kill by LDH release assay, and EGFP expression by flow cytometry. In vivo, A549 cells were injected into the pleural cavity of athymic mice. Mice were treated with intrapleural injection of NV1066 or saline and examined for EGFP expression in tumor deposits using a stereomicroscope or a fluorescent thoracoscopic system. NV1066 replicated in, expressed EGFP in infected cells and killed tumor cells in vitro. In vivo, treatment with intrapleural NV1066 decreased pleural disease burden, as measured by chest wall nodule counts and organ weights. EGFP was easily visualized in tumor deposits, including microscopic foci, by fluorescent thoracoscopy. NV1066 has significant oncolytic activity against a human NSCLC cell line and is effective in limiting the progression of metastatic disease in an in vivo orthotopic model. By incorporating fluorescent filters into endoscopic systems, a minimally invasive means for diagnosing small metastatic pleural deposits and localization of viral therapy for thoracic malignancies may be developed using the EGFP marker gene inserted in oncolytic herpes simplex viruses.