Feasibility, sensitivity, and reliability of laser-induced fluorescence imaging of green fluorescent protein-expressing tumors in vivo

Reversible and irreversible fluorescence activity of the Enhanced Green Fluorescent Protein in pH: Insights for the development of pH-biosensors

Enhanced Green Fluorescent Protein (EGFP) is a biomolecule with intense and natural fluorescence, with biological and medical applications. Although widely used as a biomarker in research, its application as a biosensor is limited by the lack of in-depth knowledge regarding its structure and behavior in adverse conditions. This study is focused on addressing this need by evaluating EGFP activity and structure at different pH using three-dimensional fluorescence, circular dichroism and small-angle X-ray scattering. The focus was on the reversibility of the process to gain insights for the development of biocompatible pH-biosensors. EGFP was highly stable at alkaline pH and quenched from neutral-to-acidic pH. Above pH 6.0, the fluorescence loss was almost completely reversible on return to neutral pH, but only partially reversible from pH 5.0 to 2.0. This work updates the knowledge regarding EGFP behavior in pH by accounting for the recent data on its structure. Hence, it is evident that EGFP presents the required properties for use as natural, biocompatible and environmentally friendly neutral to acidic pH-biosensors.

Emission shaping in fluorescent proteins: role of electrostatics and π-stacking

We obtained the fluorescence spectrum of the GFP with trajectory simulations, and revealed the role of the protein sidechains in emission shifts.

Red fluorescent proteins for imaging Zymoseptoria tritici during invasion of wheat

The use of fluorescent proteins (FPs) in plant pathogenic fungi provides valuable insight into their intracellular dynamics, cell organization and invasion mechanisms. Compared with green-fluorescent proteins, their red-fluorescent "cousins" show generally lower fluorescent signal intensity and increased photo-bleaching. However, the combined usage of red and green fluorescent proteins allows powerful insight in co-localization studies. Efficient signal detection requires a bright red-fluorescent protein (RFP), combined with a suitable corresponding filter set. We provide a set of four vectors, suitable for yeast recombination-based cloning that carries mRFP, TagRFP, mCherry and tdTomato. These vectors confer carboxin resistance after targeted single-copy integration into the sdi1 locus of Zymoseptoria tritici. Expression of the RFPs does not affect virulence of this wheat pathogen. We tested all four RFPs in combination with four epi-fluorescence filter sets and in confocal laser scanning microscopy, both in and ex planta. Our data reveal that mCherry is the RFP of choice for investigation in Z. tritici, showing highest signal intensity in epi-fluorescence, when used with a Cy3 filter set, and laser scanning confocal microscopy. However, mCherry bleached significantly faster than mRFP, which favors this red tag in long-term observation experiments. Finally, we used dual-color imaging of eGFP and mCherry expressing wild-type strains in planta and show that pycnidia are formed by single strains. This demonstrates the strength of this method in tracking the course of Z. tritici infection in wheat.

Illuminating cancer systems with genetically engineered mouse models and coupled luciferase reporters in vivo

Bioluminescent imaging (BLI) is a powerful noninvasive tool that has dramatically accelerated the in vivo interrogation of cancer systems and longitudinal analysis of mouse models of cancer over the past decade. Various luciferase enzymes have been genetically engineered into mouse models (GEMM) of cancer, which permit investigation of cellular and molecular events associated with oncogenic transcription, posttranslational processing, protein-protein interactions, transformation, and oncogene addiction in live cells and animals. Luciferase-coupled GEMMs ultimately serve as a noninvasive, repetitive, longitudinal, and physiologic means by which cancer systems and therapeutic responses can be investigated accurately within the autochthonous context of a living animal.

In vivo optical imaging and dynamic contrast methods for biomedical research

This paper provides an overview of optical imaging methods commonly applied to basic research applications. Optical imaging is well suited for non-clinical use, since it can exploit an enormous range of endogenous and exogenous forms of contrast that provide information about the structure and function of tissues ranging from single cells to entire organisms. An additional benefit of optical imaging that is often under-exploited is its ability to acquire data at high speeds; a feature that enables it to not only observe static distributions of contrast, but to probe and characterize dynamic events related to physiology, disease progression and acute interventions in real time. The benefits and limitations of in vivo optical imaging for biomedical research applications are described, followed by a perspective on future applications of optical imaging for basic research centred on a recently introduced real-time imaging technique called dynamic contrast-enhanced small animal molecular imaging (DyCE).

Tracing of islet graft survival by way of in vivo fluorescence imaging

BACKGROUND

To increase the success rate in xenogeneic islet transplantation, proper assessment of graft mass is required following transplantation. For this reason, we aimed to develop a suitable fluorescence imaging system to monitor islet xenograft survival in diabetic mice.

METHODS

Adenovirus vector encoding enhanced green fluorescent protein-transduced rat pancreatic islets were transplanted under the renal capsule of streptozotocin-induced diabetic mice and the fluorescence signal was quantified over time using a cooled charge-coupled device. Non-fasting blood glucose levels were recorded during the same period. Insulin release from transduced and control islets was detected via enzyme-linked immunosorbent assay.

RESULTS

Adenovirus vector encoding enhanced green fluorescent protein infection did not alter the function or survival of pancreatic islets post transduction. A direct correlation was found between the number of islets (250-750) transplanted under the kidney capsule and the blood glucose recovery.

CONCLUSIONS

Fluorescence imaging appears to be a useful tool for quantitative assessment of islet cell viability post transplantation and could permit earlier detection of graft rejection.

Multimeric small interfering ribonucleic acid for highly efficient sequence-specific gene silencing

Small interfering RNA (siRNA) with 19-21 base pairs has been recently recognized as a new therapeutic agent for effectively silencing a specific gene on a post-transcription level. For siRNA therapeutics, safe and efficient delivery issues are significant hurdles to clinical applications. Here we present a new class of biologically active siRNA structure based on chemically self-crosslinked and multimerized siRNA through cleavable disulphide linkages. The multimerized siRNA can produce more stable and compact polyelectrolyte complexes with less cytotoxic cationic carriers than naked siRNA because of substantially increased charge densities and the presence of flexible chemical linkers in the backbone. The cleavable and multimerized siRNA shows greatly enhanced gene-silencing efficiencies in vitro and in vivo through a target-messenger-RNA-specific RNA interference processing without significantly eliciting immune induction. This study demonstrates that the multimerized siRNA structure complexed with selected cationic condensing agents can serve as potential gene-silencing therapeutics for treating various diseases.

Real Time Imaging of Biomarkers in the Parkinson's Brain Using Mini-Implantable Biosensors. II. Pharmaceutical Therapy with Bromocriptine

We used Neuromolecular Imaging (NMI) and trademarked BRODERICK PROBE^® mini-implantable biosensors, to selectively and separately detect neurotransmitters in vivo, on line, within seconds in the dorsal striatal brain of the Parkinson's Disease (PD) animal model. We directly compared our results derived from PD to the normal striatal brain of the non-Parkinson's Disease (non-PD) animal. This advanced biotechnology enabled the imaging of dopamine (DA), serotonin (5-HT), homovanillic acid (HVA) a metabolite of DA, L-tryptophan (L-TP) a precursor to 5-HT and peptides, dynorphin A 1-17 (Dyn A) and somatostatin (somatostatin releasing inhibitory factor) (SRIF). Each neurotransmitter and neurochemical was imaged at a signature electroactive oxidation/half-wave potential in dorsal striatum of the PD as compared with the non-PD animal. Both endogenous and bromocriptine-treated neurochemical profiles in PD and non-PD were imaged using the same experimental paradigm and detection sensitivities. Results showed that we have found significant neurotransmitter peptide biomarkers in the dorsal striatal brain of endogenous and bromocriptine-treated PD animals. The peptide biomarkers were not imaged in dorsal striatal brain of non-PD animals, either endogenously or bromocriptine-treated. These findings provide new pharmacotherapeutic strategies for PD patients. Thus, our findings are highly applicable to the clinical treatment of PD.

Pre-clinical whole-body fluorescence imaging: Review of instruments, methods and applications

Fluorescence sampling of cellular function is widely used in all aspects of biology, allowing the visualization of cellular and sub-cellular biological processes with spatial resolutions in the range from nanometers up to centimeters. Imaging of fluorescence in vivo has become the most commonly used radiological tool in all pre-clinical work. In the last decade, full-body pre-clinical imaging systems have emerged with a wide range of utilities and niche application areas. The range of fluorescent probes that can be excited in the visible to near-infrared part of the electromagnetic spectrum continues to expand, with the most value for in vivo use being beyond the 630 nm wavelength, because the absorption of light sharply decreases. Whole-body in vivo fluorescence imaging has not yet reached a state of maturity that allows its routine use in the scope of large-scale pre-clinical studies. This is in part due to an incomplete understanding of what the actual fundamental capabilities and limitations of this imaging modality are. However, progress is continuously being made in research laboratories pushing the limits of the approach to consistently improve its performance in terms of spatial resolution, sensitivity and quantification. This paper reviews this imaging technology with a particular emphasis on its potential uses and limitations, the required instrumentation, and the possible imaging geometries and applications. A detailed account of the main commercially available systems is provided as well as some perspective relating to the future of the technology development. Although the vast majority of applications of in vivo small animal imaging are based on epi-illumination planar imaging, the future success of the method relies heavily on the design of novel imaging systems based on state-of-the-art optical technology used in conjunction with high spatial resolution structural modalities such as MRI, CT or ultrasound.

In vivo direct molecular imaging of early tumorigenesis and malignant progression induced by transgenic expression of GFP-Met

The tyrosine kinase receptor Met and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), play an important role in normal developmental processes, as well as in tumorigenicity and metastasis. We constructed a green fluorescent protein (GFP) Met chimeric molecule that functions similarly to the wild-type Met receptor and generated GFP-Met transgenic mice. These mice ubiquitously expressed GFP-Met in specific epithelial and endothelial cells and displayed enhanced GFP-Met fluorescence in sebaceous glands. Thirty-two percent of males spontaneously developed adenomas, adenocarcinomas, and angiosarcomas in their lower abdominal sebaceous glands. Approximately 70% of adenocarcinoma tumors metastasized to the kidneys, lungs, or liver. Quantitative subcellular-resolution intravital imaging revealed very high levels of GFP-Met in tumor lesions and in single isolated cells surrounding them, relative to normal sebaceous glands. These single cells preceded the formation of local and distal metastases. Higher GFP-Met levels correlated with earlier tumor onset and aggressiveness, further demonstrating the role of Met-HGF/SF signaling in cellular transformation and acquisition of invasive and metastatic phenotypes. Our novel mouse model and high-resolution intravital molecular imaging create a powerful tool that enables direct real-time molecular imaging of receptor expression and localization during primary events of tumorigenicity and metastasis at single-cell resolution.

The multiple uses of fluorescent proteins to visualize cancer in vivo

Naturally fluorescent proteins have revolutionized biology by enabling what was formerly invisible to be seen clearly. These proteins have allowed us to visualize, in real time, important aspects of cancer in living animals, including tumour cell mobility, invasion, metastasis and angiogenesis. These multicoloured proteins have allowed the colour-coding of cancer cells growing in vivo and enabled the distinction of host from tumour with single-cell resolution. Visualization of many aspects of cancer initiation and progression in vivo should be possible with fluorescent proteins.

Advantages of multi-color fluorescent proteins for whole-body and in vivo cellular imaging

The revolution of in vivo cancer biology enabled by fluorescent proteins is described. The high extinction coefficients, quantum yields, and unique spectral properties of fluorescent proteins have been taken advantage of in order to visualize, in real time, the important aspects of cancer in living animals, including tumor cell trafficking, invasion, metastasis, and angiogenesis. Fluorescent proteins enable whole-body imaging of tumors on internal organs. These multicolored proteins have allowed the color-coding of cancer cells growing in vivo with distinction of different cell types, including host from tumor, with single-cell resolution.

In vivo cell biology of cancer cells visualized with fluorescent proteins

This chapter describes a new cell biology where the behavior of individual cells can be visualized in the living animal. Previously it has been demonstrated that fluorescent proteins can be used for whole-body imaging of metastatic tumor growth, bacterial infection, and gene expression. An example of the new cell biology is dual-color fluorescence imaging using red fluorescent protein (RFP)-expressing tumors transplanted in green fluorescent protein (GFP)-expressing transgenic mice. These models show with great clarity the details of tumor-stroma interactions and especially tumor-induced angiogenesis, tumor-infiltrating lymphocytes, stromal fibroblasts, and macrophages. Another example is the color coding of cells with RFP or GFP such that both cell types can be simultaneously visualized in vivo. Stem cells can also be visualized and tracked in vivo. Mice in which the regulatory elements of the stem cell marker nestin drive GFP expression enable nascent vasculature to be visualized interacting with transplanted RFP-expressing cancer cells. Nestin-driven GFP expression can also be used to visualize hair follicle stem cells. Dual-color cells expressing GFP in the nucleus and RFP in the cytoplasm enable real-time visualization of nuclear-cytoplasm dynamics including cell cycle events and apoptosis. Highly elongated cancer cells in capillaries in living mice were observed within skin flaps. The migration velocities of the cancer cells in the capillaries were measured by capturing images of the dual-color fluorescent cells over time. The cells in the capillaries elongated to fit the width of these vessels. The use of the dual-color cancer cells differentially labeled in the cytoplasm and nucleus and associated fluorescent imaging provide a powerful tool to understand the mechanism of cancer cell migration and deformation in small vessels.

Strategies of Conditional Gene Expression in Myocardium

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

Optical imaging of in vivo gene expression: a critical assessment of the methodology and associated technologies

Following and quantifying the expression of reporter gene expression in vivo is very important to monitor the expression of therapeutic genes in targeted tissues in disease models and/or to assess the effectiveness of systems of gene therapy delivery. Gene expression of luminescent or fluorescent proteins can be detected directly on living animals by simply observing the associated optical signals by means of a cooled charged-coupled device camera. More accurate resolution can be obtained with more sophisticated technologies. Time-course and quasi-quantitative monitoring of the expression can be obtained on a given animal and followed on a large time window. The present paper describes the physical and technological methodologies and associated problems of in vivo optical imaging. Several examples of in vivo detection of gene delivery are described.

Combined suicide gene therapy for pancreatic peritoneal carcinomatosis using BGTC liposomes

Peritoneal dissemination is a common end-stage complication of pancreatic cancer for which novel therapeutic modalities are actively investigated, as there is no current effective therapy. Thus, we evaluated, in a mouse model of pancreatic peritoneal carcinomatosis, the therapeutic potential of a novel nonviral gene therapy approach consisting of bis-guanidinium-tren-cholesterol (BGTC)-mediated lipofection of a combined suicide gene system. Human BxPC-3 pancreatic cells secreting the carcinoembryonic antigen (CEA) tumor marker were injected into the peritoneal cavity of nude mice. After 8 days, intraperitoneal (i.p.) lipofection was performed using BGTC/DOPE cationic liposomes complexed with plasmids encoding the two prodrug-activating enzymes Herpes Simplex Virus thymidine kinase and Escherichia coli cytosine deaminase, the latter being expressed from a bicistronic cassette also encoding E. coli uracil phosphoribosyltransferase. Administration of the lipoplexes was followed by treatment with the corresponding prodrugs ganciclovir and 5-fluorocytosine. The results presented herein demonstrate that BGTC/DOPE liposomes can efficiently mediate gene transfection into peritoneal tumor nodules. Indeed, HSV-TK mRNA was detected in tumor nodule tissues by semiquantitative reverse transcription-polymerase chain reaction analysis. In addition, green fluorescent protein (GFP) fluorescence and X-gal staining were observed in the peritoneal tumor foci following lipofection of the corresponding EGFP and LacZ reporter genes. These expression analyses also showed that transgene expression lasted for about 2 weeks and was preferential for the tumor nodules, this tumor preference being in good agreement with the absence of obvious treatment-related toxicity. Most importantly, mice receiving the full treatment scheme (BGTC liposomes, suicide genes and prodrugs) had significantly lower serum CEA levels than those of the various control groups, a finding indicating that peritoneal carcinomatosis progression was strongly reduced in these mice. In conclusion, our results demonstrate the therapeutic efficiency of BGTC-mediated i.p. lipofection of a combined suicide gene system in a mouse peritoneal carcinomatosis model and suggest that BGTC-based prodrug-activating gene therapy approaches may constitute a potential treatment modality for patients with peritoneal carcinomatosis and minimal residual disease.