In vivo imaging of beta-galactosidase activity using far red fluorescent switch

Aggregation-Induced Emission: Lighting up Cells, Revealing Life!

Understanding metabolism and dynamic biological events in cells, as well as physiological functions and pathological changes in organisms, is the major goal of biological investigations. It will improve our capability to diagnose and treat diseases, and will enhance personalized medicine. Fluorescence imaging is a powerful tool that plays an essential role in acquiring the comprehensive knowledge necessary to help reach this goal. Fluorescent molecules are crucial factors for obtaining high quality images. In contrast to conventional fluorogens with aggregation-caused quenching (ACQ) effect, molecules that show aggregation-induced emission (AIE) effect open up new avenues for fluorescence imaging. So far, a large variety of AIE probes have been developed and applied to bioimaging because of their outstanding characteristics, such as high fluorescence efficiency, excellent photostability and high signal-to-noise ratio (SNR). In this review, recent advances in AIE-based probes for biomedical imaging of intracellular microenvironments, natural macromolecules, subcellular organelles, intracellular processes, living tissues, and diagnosis and therapeutic evaluation of diseases in vivo are summarized. It is hoped that this review generates great research enthusiasm for AIE-based bioimaging, in order to promote the development of promising AIE probes and guide us to a better understanding of the biological essence of life.

Diamagnetic Imaging Agents with a Modular Chemical Design for Quantitative Detection of β-Galactosidase and β-Glucuronidase Activities with CatalyCEST MRI

Imaging agents for the noninvasive in vivo detection of enzyme activity in preclinical and clinical settings could have fundamental implications in the field of drug discovery. Furthermore, a new class of targeted prodrug treatments takes advantage of high enzyme activity to tailor therapy and improve treatment outcomes. Herein, we report the design and synthesis of new magnetic resonance imaging (MRI) agents that quantitatively detect β-galactosidase and β-glucuronidase activities by measuring changes in chemical exchange saturation transfer (CEST). Based on a modular approach, we incorporated the enzymes' respective substrates to a salicylate moiety with a chromogenic spacer via a carbamate linkage. This furnished highly selective diamagnetic CEST agents that detected and quantified enzyme activities of glycoside hydrolase enzymes. Michaelis-Menten enzyme kinetics studies were performed by monitoring catalyCEST MRI signals, which were validated with UV-vis assays.

In vivo imaging of tumour xenografts with an antibody targeting the potassium channel Kv10.1

The K_v10.1 (Eag1) voltage-gated potassium channel represents a promising molecular target for novel cancer therapies or diagnostic purposes. Physiologically, it is only expressed in the brain, but it was found overexpressed in more than 70 % of tumours of diverse origin. Furthermore, as a plasma membrane protein, it is easily accessible to extracellular interventions. In this study we analysed the feasibility of the anti-K_v10.1 monoclonal antibody mAb62 to target tumour cells in vitro and in vivo and to deliver therapeutics to the tumour. Using time-domain near infrared fluorescence (NIRF) imaging in a subcutaneous MDA-MB-435S tumour model in nude mice, we showed that mAb62-Cy5.5 specifically accumulates at the tumour for at least 1 week in vivo with a maximum intensity at 48 h. Blocking experiments with an excess of unlabelled mAb62 and application of the free Cy5.5 fluorophore demonstrate specific binding to the tumour. Ex vivo NIRF imaging of whole tumours as well as NIRF imaging and microscopy of tumour slices confirmed the accumulation of the mAb62-Cy5.5 in tumours but not in brain tissue. Moreover, mAb62 was conjugated to the prodrug-activating enzyme β-D-galactosidase (β-gal; mAb62-β-gal). The β-gal activity of the mAb62-β-gal conjugate was analysed in vitro on K_v10.1-expressing MDA-MB-435S cells in comparison to control AsPC-1 cells. We show that the mAb62-β-gal conjugate possesses high β-gal activity when bound to K_v10.1-expressing MDA-MB-435S cells. Moreover, using the β-gal activatable NIRF probe DDAOG, we detected mAb62-β-gal activity in vivo over the tumour area. In summary, we could show that the anti-K_v10.1 antibody is a promising tool for the development of novel concepts of targeted cancer therapy.

Modulation of therapy-induced senescence by reactive lipid aldehydes

Current understanding points to unrepairable chromosomal damage as the critical determinant of accelerated senescence in cancer cells treated with radiation or chemotherapy. Nonetheless, the potent senescence inducer etoposide not only targets topoisomerase II to induce DNA damage but also produces abundant free radicals, increasing cellular reactive oxygen species (ROS). Toward examining roles for DNA damage and oxidative stress in therapy-induced senescence, we developed a quantitative flow cytometric senescence assay and screened 36 redox-active agents as enhancers of an otherwise ineffective dose of radiation. While senescence failed to correlate with total ROS, the radiation enhancers, etoposide and the other effective topoisomerase inhibitors each produced high levels of lipid peroxidation. The reactive aldehyde 4-hydroxy-2-nonenal, a lipid peroxidation end product, was sufficient to induce senescence in irradiated cells. In turn, sequestering aldehydes with hydralazine blocked effects of etoposide and other senescence inducers. These results suggest that lipid peroxidation potentiates DNA damage from radiation and chemotherapy to drive therapy-induced senescence.

Peripheral Lymphoid Volume Expansion and Maintenance Are Controlled by Gut Microbiota via RALDH+ Dendritic Cells

Lymphocyte homing to draining lymph nodes is critical for the initiation of immune responses. Secondary lymphoid organs of germ-free mice are underdeveloped. How gut commensal microbes remotely regulate cellularity and volume of secondary lymphoid organs remains unknown. We report here that, driven by commensal fungi, a wave of CD45(+)CD103(+)RALDH(+) cells migrates to the peripheral lymph nodes after birth. The arrival of these cells introduces high amounts of retinoic acid, mediates the neonatal to adult addressin switch on endothelial cells, and directs the homing of lymphocytes to both gut-associated lymphoid tissues and peripheral lymph nodes. In adult mice, a small number of these RALDH(+) cells might serve to maintain the volume of secondary lymphoid organs. Homing deficiency of these cells was associated with lymph node attrition in vitamin-A-deficient mice, suggesting a perpetual dependence on retinoic acid signaling for structural and functional maintenance of peripheral immune organs.

A versatile two-photon fluorescent probe for ratiometric imaging E. coliβ-galactosidase in live cells and in vivo

We have described the design, synthesis, spectroscopy and biological applications of NI-βGal, a versatile fluorescent probe to detect E. coliβ-galactosidase in live cells and mice sensitively and directly, which holds great promise for its application in biomedical research such as gene therapy for cancer in the future.

MRI Reporter Genes for Noninvasive Molecular Imaging

Magnetic resonance imaging (MRI) is one of the most important imaging technologies used in clinical diagnosis. Reporter genes for MRI can be applied to accurately track the delivery of cell in cell therapy, evaluate the therapy effect of gene delivery, and monitor tissue/cell-specific microenvironments. Commonly used reporter genes for MRI usually include genes encoding the enzyme (e.g., tyrosinase and β-galactosidase), the receptor on the cells (e.g., transferrin receptor), and endogenous reporter genes (e.g., ferritin reporter gene). However, low sensitivity limits the application of MRI and reporter gene-based multimodal imaging strategies are common including optical imaging and radionuclide imaging. These can significantly improve diagnostic efficiency and accelerate the development of new therapies.

Synthesis of a far-red fluorophore and its use as an esterase probe in living cells

A new esterase-activatable probe expands the toolkit for staining living cells with a far-red fluorescent carbazine.

Theoretical investigation and design of two-photon fluorescent probes for visualizing β-galactosidase activity in living cells

The two-photon fluorescent probes show dual signal for β-gal bio-imaging.

Recent advances in the development of synthetic chemical probes for glycosidase enzymes

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

Fluoro-glycosyl acridinones are ultra-sensitive active site titrating agents for retaining β-glycosidases

Novel fluorogenic 2-deoxy-2-fluoroglycosyl acridinone active site titrating reagents were synthesised and kinetic parameters determined for their inactivation of two retaining β-glucosidases, a β-galactosidase, a β-xylosidase and several cellulases. Fluorescence-monitored active site titration using this class of reagents reliably measured active enzyme concentrations down to 3 nM.

Development of a reporter peptide that catalytically produces a fluorescent signal through α-complementation

In α-complementation, inactive N-terminal (α-domain) and C-terminal (ω-domain) fragments of β-galactosidase associate to reconstitute the active protein. To date, the effect of α-domain size on α-complementation activity has not been systematically investigated. In this study, we compared the complementation activities of α-domains of various sizes using an in vitro system. We found that the complementation activities are similar for α-domains comprising between 45 and 229 N-terminal residues but are significantly decreased for those containing less than 37 residues. However, these smaller α-domains (15 and 25 residues) exhibited sufficient α-complementation activity for application as reporters.

Optical imaging of targeted β-galactosidase in brain tumors to detect EGFR levels

A current limitation in molecular imaging is that it often requires genetic manipulation of cancer cells for noninvasive imaging. Other methods to detect tumor cells in vivo using exogenously delivered and functionally active reporters, such as β-gal, are required. We report the development of a platform system for linking β-gal to any number of different ligands or antibodies for in vivo targeting to tissue or cells, without the requirement for genetic engineering of the target cells prior to imaging. Our studies demonstrate significant uptake in vitro and in vivo of an EGFR-targeted β-gal complex. We were then able to image orthotopic brain tumor accumulation and localization of the targeted enzyme when a fluorophore was added to the complex, as well as validate the internalization of the intravenously administered β-gal reporter complex ex vivo. After fluorescence imaging localized the β-gal complexes to the brain tumor, we topically applied a bioluminescent β-gal substrate to serial sections of the brain to evaluate the delivery and integrity of the enzyme. Finally, robust bioluminescence of the EGFR-targeted β-gal complex was captured within the tumor during noninvasive in vivo imaging.

Far-red fluorogenic probes for esterase and lipase detection

Fluorogenic enzyme probes go from a dark to a bright state following hydrolysis and can provide a sensitive, real-time readout of enzyme activity. They are useful for examining enzymatic activity in bacteria, including the human pathogen Mycobacterium tuberculosis. Herein, we describe two fluorogenic esterase probes derived from the far-red fluorophore 7-hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one) (DDAO). These probes offer enhanced optical properties compared to existing esterase probes because the hydrolysis product, DDAO, excites above 600 nm while retaining a good quantum yield (ϕ=0.40). We validated both probes with a panel of commercially available enzymes alongside known resorufin- and fluorescein-derived esterase substrates. Furthermore, we used these probes to reveal esterase activity in protein gel-resolved mycobacterial lysates. These probes represent new tools for esterase detection and characterization and should find use in a variety of applications.

In vivo NIRF imaging-guided delivery of a novel NGR-VEGI fusion protein for targeting tumor vasculature

Pathological angiogenesis is crucial in tumor growth, invasion and metastasis. Previous studies demonstrated that the vascular endothelial growth inhibitor (VEGI), a member of the tumor necrosis factor superfamily, can be used as a potent endogenous inhibitor of tumor angiogenesis. Molecular probes containing the asparagine-glycine-arginine (NGR) sequence can specifically bind to CD13 receptor which is overexpressed on neovasculature and several tumor cells. Near-infrared fluorescence (NIRF) optical imaging for targeting tumor vasculature offers a noninvasive method for early detection of tumor angiogenesis and efficient monitoring of response to anti-tumor vasculature therapy. The aim of this study was to develop a new NIRF imaging probe on the basis of an NGR-VEGI protein for the visualization of tumor vasculature. The NGR-VEGI fusion protein was prepared from prokaryotic expression, and its function was characterized in vitro. The NGR-VEGI protein was then labeled with a Cy5.5 fluorophore to afford Cy5.5-NGR-VEGI probe. Using the NIRF imaging technique, we visualized and quantified the specific delivery of Cy5.5-NGR-VEGI protein to subcutaneous HT-1080 fibrosarcoma tumors in mouse xenografts. The Cy5.5-NGR-VEGI probe exhibited rapid HT-1080 tumor targeting, and highest tumor-to-background contrast at 8 h post-injection (pi). Tumor specificity of Cy5.5-NGR-VEGI was confirmed by effective blocking of tumor uptake in the presence of unlabeled NGR-VEGI (20 mg/kg). Ex vivo NIRF imaging further confirmed in vivo imaging findings, demonstrating that Cy5.5-NGR-VEGI displayed an excellent tumor-to-muscle ratio (18.93 ± 2.88) at 8 h pi for the non-blocking group and significantly reduced ratio (4.92 ± 0.75) for the blocking group. In conclusion, Cy5.5-NGR-VEGI provided highly sensitive, target-specific, and longitudinal imaging of HT-1080 tumors. As a novel theranostic protein, Cy5.5-NGR-VEGI has the potential to improve cancer treatment by targeting tumor vasculature.

An expanded set of fluorogenic sulfatase activity probes

Fluorogenic probes that are activated by an enzymatic transformation are ideally suited for profiling enzyme activities in biological systems. Here, we describe two fluorogenic enzyme probes, 3-O-methylfluorescein-sulfate and resorufin-sulfate, that can be used to detect sulfatases in mycobacterial lysates. Both probes were validated with a set of commercial sulfatases and used to reveal species-specific sulfatase banding patterns in a gel-resolved assay of mycobacterial lysates. The fluorogenic probes described here are suitable for various assays and provide a starting point for creating new sulfatase probes with improved selectivity for mycobacterial sulfatases.

Senescence bypass in mesenchymal stem cells: a potential pathogenesis and implications of pro-senescence therapy in sarcomas

Cellular senescence is a mechanism that limits the lifespan of somatic cells as the results of replicative proliferation and response to stresses, and that prevents undesired oncogenic changes constituting a barrier against immortalization and tumorigenesis. Mesenchymal stem cells (MSCs) reside in a variety of tissues, and participates in tissue maintenance with their multipotent differentiation ability. MSCs are also considered to be as cells of origin for certain type of sarcomas. We reviewed the mechanisms of cellular senescence in MSCs and hypothesized senescence bypass as the potential pathogenesis for sarcoma development, and proposed the possibility of senescence induction therapy for an alternative treatment strategy against sarcomas, especially cells with the resistance to conventional chemo and radiotherapy including sarcoma stem cells.

Recent Advance of Biological Molecular Imaging Based on Lanthanide-Doped Upconversion-Luminescent Nanomaterials

Lanthanide-doped upconversion-luminescent nanoparticles (UCNPs), which can be excited by near-infrared (NIR) laser irradiation to emit multiplex light, have been proven to be very useful for in vitro and in vivo molecular imaging studies. In comparison with the conventionally used down-conversion fluorescence imaging strategies, the NIR light excited luminescence of UCNPs displays high photostability, low cytotoxicity, little background auto-fluorescence, which allows for deep tissue penetration, making them attractive as contrast agents for biomedical imaging applications. In this review, we will mainly focus on the latest development of a new type of lanthanide-doped UCNP material and its main applications for in vitro and in vivo molecular imaging and we will also discuss the challenges and future perspectives.

A fluorogenic probe for β-galactosidase activity imaging in living cells

A cell permeable fluorescence turn-on probe, AcGQCy7, was developed to image β-galactosidase activity in living cells. Once internalized by β-galactosidase-expressing cells, the probe was hydrolyzed into a highly fluorescent molecule, and the fluorescent signal was retained in mitochondria for several days. This resulted in a long-lasting and strong β-galactosidase-dependent intracellular fluorescent signal with little background fluorescence in the culture media.

Sulfatase-activated fluorophores for rapid discrimination of mycobacterial species and strains

Most current diagnostic tests for tuberculosis do not reveal the species or strain of pathogen causing pulmonary infection, which can lead to inappropriate treatment regimens and the spread of disease. Here, we report an assay for mycobacterial strain assignment based on genetically conserved mycobacterial sulfatases. We developed a sulfatase-activated probe, 7-hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one)-sulfate, that detects enzyme activity in native protein gels, allowing the rapid detection of sulfatases in mycobacterial lysates. This assay revealed that mycobacterial strains have distinct sulfatase fingerprints that can be used to judge both the species and lineage. Our results demonstrate the potential of enzyme-activated probes for rapid pathogen discrimination for infectious diseases.

Novel S-Gal(®) analogs as (1)H MRI reporters for in vivo detection of β-galactosidase

The quantitative assessment of gene expression and related enzyme activity in vivo could be important for the characterization of gene altering diseases and therapy. The development of imaging techniques, based on specific reporter molecules may enable routine non-invasive assessment of enzyme activity and gene expression in vivo. We recently reported the use of commercially available S-Gal(®) as a β-galactosidase reporter for (1)H MRI, and the synthesis of several S-Gal(®) analogs with enhanced response to β-galactosidase activity. We have now compared these analogs in vitro and have identified the optimal analog, C3-GD, based on strong T1 and T2 response to enzyme presence (ΔR1 and ΔR2~1.8 times S-Gal(®)). Moreover, application is demonstrated in vivo in human breast tumor xenografts. MRI studies in MCF7-lacZ tumors implanted subcutaneously in athymic nude mice (n=6), showed significant reduction in T1 and T2 values (each~13%) 2h after intra-tumoral injection of C3-GD, whereas the MCF7 (wild type) tumors showed slight increase. Thus, C3-GD successfully detects β-galactosidase activity in vivo and shows promise as a lacZ gene (1)H MR reporter molecule.

Novel molecular platform integrated iron chelation therapy for 1H-MRI detection of β-galactosidase activity

Targeting the increased Fe(3+) content in tumors, we propose a novel molecular platform integrated cancer iron chelation therapy for (1)H-magnetic resonance imaging (MRI) detection of β-galactosidase (β-gal) activity. Following this idea, we have designed, synthesized, and characterized a series of β-d-galactosides conjugated with various chelators and demonstrated the feasibility of this concept for assessing β-gal activity in solution by (1)H-MRI T1 and T2 relaxation mapping.

A smart (19) F and (1) H MRI probe with self-immolative linker as a versatile tool for detection of enzymes

Here we report on a dual-modal (19) F and (1) H MRI paramagnetic probe with a self-immolative linker, Gd-DOMF-Gal. The enzymatic conversion of this probe by β-galactosidase resulted in a simultaneous turning on of the fluorine signal and changed ability of the Gd(3+) complex to modulate the (1) H MR signal intensity of the surrounding water molecules. A versatile imaging platform for monitoring a variety of enzymes by (19) F and (1) H MRI using this molecular design is proposed.

Characterization of sensitivity and specificity in leaky droplet-based assays

This paper uses numerical methods to characterize the crosstalk of small fluorescent molecules and molecular probes among aqueous droplets immersed in a continuous phase of hydrocarbons or fluorocarbons in microfluidic systems. Droplet-based biochemical assays rely on the reagents to remain isolated in individual droplets. It has been observed, however, that small and hydrophobic fluorescent molecules can diffuse across the droplet boundary into other drops. The contents among droplets become mixed and homogenized over time. Such cross-contamination can have detrimental effects on the accuracy of droplet-based assays, especially those using fluorescent molecules and the corresponding number of fluorescent droplets for a quantitative readout. This work examines the competing dynamics of the generation of fluorescent molecules in "positive" drops (in response to the presence of molecules or cells of interest), against its leakage into "negative" drops, where such molecules or cells of interest are absent. In ideal droplet assays, the signal-to-noise ratio (SNR)--defined as the fluorescence signal from a positive drop to that from a negative drop--would increase and saturate with time. In a leaky droplet assay, the SNR tends to decay with time. Under certain conditions, however, the SNR from a leaky droplet assay could increase and reach a maximum value before it starts to diminish. This maximum value can be estimated from a dimensionless number relating the rate of leakage relative to the rate of generation of fluorescence signal in the drops. Beyond the time when the SNR peaks, the SNR value, as well as the accuracy of the leaky droplet assay continues to degrade. In the absence of immediate experimental remedies to completely eliminate the crosstalk of molecules among drops, performing detection at the optimal time point becomes critical to minimize errors in leaky droplet assays.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

Monogalactopyranosides of fluorescein and fluorescein methyl ester: synthesis, enzymatic hydrolysis by biotnylated β-galactosidase, and determination of translational diffusion coefficient

Fluorescein monoglycosides (D-galactopyranoside (FMG) and D-glucopyranoside) and their methyl ester (MFMG) have been prepared from acetobromoglucose/galactose and fluorescein methyl ester in good yields. Enzymatic hydrolysis experiments (using biotinylated β-galactosidase) of the galacto derivatives have been performed and kinetic parameters were calculated. A 15-20 times increase of the fluorescence intensity has been observed during the hydrolysis. A linear increase of fluorescence has been noted at short time and low concentration of substrate, making these compounds useful and sensitive probes for galactosidases. The magnitude of the Michaelis-Menten constant (K(m)) value for MFMG is higher than that of FMG suggesting a possible conformational change of the fluorogenic substrate. K(m) value for biotinylated β-Gal with FMG is lower than that for the native enzyme. This observation indicates higher substrate affinity of the biotinylated enzyme in comparison to the native enzyme. Translational diffusion coefficients have been measured, for both fluorogenic substrates and both the products, employing fluorescence correlation spectroscopy. Translational diffusion coefficients for fluorogenic substrates and the enzymatic hydrolysis products have been measured to be similar, in the range of 3.5-4.5×10(-10) m(2) s(-1). Thus an enhancement or retardation of the enzymatic kinetics due to difference in translational mobility of substrate and product is not that apparent.

Novel Fe3+-Based 1H MRI β-Galactosidase Reporter Molecules**

There is increasing interest in the development of reporter agents to reveal enzyme activity in vivo using small animal imaging. We have previously demonstrated the feasibility of detecting lacZ gene activity using the commercially available 3,4-cyclohexenoesculetin-β-D-galactopyranoside (S-Gal™) as a 1H MRI reporter. Specifically, β-galactosidase (β-gal) releases the aglycone, which forms an MR contrast-inducing paramagnetic precipitate in the presence of Fe3+. Contrast was primarily T2-weighted signal loss, but T1 effects were also observed. Since T1-contrast generally provides signal enhancement as opposed to loss, it appeared attractive to explore whether analogues could be generated with enhanced characteristics. We now report the design and successful synthesis of novel analogues together with characterization of 1H MRI contrast based on both T1 and T2 response to β-gal activity in vitro for the lead agent.

Near-infrared fluorescence probes for enzymes based on binding affinity modulation of squarylium dye scaffold

We present a novel design strategy for near-infrared (NIR) fluorescence probes utilizing dye-protein interaction as a trigger for fluorescence enhancement. The design principle involves modification of a polymethine dye with cleavable functional groups that reduce the dye's protein-binding affinity. When these functional groups are removed by specific interaction with the target enzymes, the dye's protein affinity is restored, protein binding occurs, and the dye's fluorescence is strongly enhanced. To validate this strategy, we first designed and synthesized an alkaline phosphatase (ALP) sensor by introducing phosphate into the squarylium dye scaffold; this sensor was able to detect ALP-labeled secondary antibodies in Western blotting analysis. Second, we synthesized a probe for β-galactosidase (widely used as a reporter of gene expression) by means of β-galactosyl substitution of the squarylium scaffold; this sensor was able to visualize β-galactosidase activity both in vitro and in vivo. Our strategy should be applicable to obtain NIR fluorescence probes for a wide range of target enzymes.

Dual 19F/1H MR gene reporter molecules for in vivo detection of β-galactosidase

Increased emphasis on personalized medicine and novel therapies requires the development of noninvasive strategies for assessing biochemistry in vivo. The detection of enzyme activity and gene expression in vivo is potentially important for the characterization of diseases and gene therapy. Magnetic resonance imaging (MRI) is a particularly promising tool, since it is noninvasive and has no associated radioactivity, yet penetrates deep tissue. We now demonstrate a novel class of dual (1)H/(19)F nuclear magnetic resonance (NMR) lacZ gene reporter molecule to specifically reveal enzyme activity in human tumor xenografts growing in mice. We report the design, synthesis, and characterization of six novel molecules and evaluation of the most effective reporter in mice in vivo. Substrates show a single (19)F NMR signal and exposure to β-galactosidase induces a large (19)F NMR chemical shift response. In the presence of ferric ions, the liberated aglycone generates intense proton MRI T(2) contrast. The dual modality approach allows both the detection of substrate and the imaging of product enhancing the confidence in enzyme detection.

Lanthanide(III) complexes that contain a self-immolative arm: potential enzyme responsive contrast agents for magnetic resonance imaging

Enzyme-responsive MRI-contrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln(III) complex, with a concomitant response in its MRI-contrast-enhancing properties. We synthesized and investigated a series of Gd(3+) and Yb(3+) complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for β-galactosidase; we synthesized complex LnL(1), its NH(2) amine derivatives formed after enzymatic cleavage, LnL(2), and two model compounds, LnL(3) and LnL(4). All of the Gd(3+) complexes synthesized have a single inner-sphere water molecule. The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mM(-1) s(-1) for complexes GdL(1) and GdL(2), respectively; 37 °C, 60 MHz), which prevents application of this system as an enzyme-responsive T(1) relaxation agent. Variable-temperature (17)O NMR spectroscopy and (1)H NMRD (nuclear magnetic relaxation dispersion) analysis were used to assess the parameters that determine proton relaxivity for the Gd(3+) complexes, including the water-exchange rate (k(ex)(298), varies in the range 1.5-3.9×10(6) s(-1)). Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiometric measurements (log K(H)=5.12(±0.01) and 5.99(±0.01) for GdL(2) and GdL(3), respectively). The Yb(3+) analogues show a PARACEST effect after enzymatic cleavage that can be exploited for the specific detection of enzymatic activity. The proton-exchange rates were determined at various pH values for the amine derivatives by using the dependency of the CEST effect on concentration, saturation time, and saturation power. A concentration-independent analysis of the saturation-power-dependency data was also applied. All these different methods showed that the exchange rate of the amine protons of the Yb(III) complexes decreases with increasing pH value (for YbL(3), k(ex)=1300 s(-1) at pH 8.4 vs. 6000 s(-1) at pH 6.4), thereby resulting in a diminution of the observed CEST effect.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Pro-senescence therapy for cancer treatment

Abundant evidence points to a crucial physiological role for cellular senescence in combating tumorigenesis. Thus, the engagement of senescence may represent a key component for therapeutic intervention in the eradication of cancer. In this Opinion article, we focus on concepts that are relevant to a pro-senescence approach to therapy and we propose potential therapeutic strategies that aim to enhance the pro-senescence response in tumours.

Recent developments of biological reporter technology for detecting gene expression

Reporter gene assay is an invaluable tool for both biomedical and pharmaceutical researches to monitor cellular events associated with gene expression, regulation and signal transduction. On the basis of the alternations in reporter gene activities mediated by attaching response elements to these reporter genes, one sensitive, reliable and convenient assay can be provided to efficiently report the activation of particular messenger cascades and their effects on gene expression and regulations inside cells or living subjects. In this review, we introduce the current status of several commonly used reporter genes such as chloramphenicol acetyltransferase (CAT), alkaline phosphatase (AP), β-galactosidase (β-gal), luciferases, green fluorescent protein (GFP), and β-lactamase. Their applications in monitoring gene expression and regulations in vitro and in vivo will be summarized. With the development of advanced technology in gene expression and optical imaging modalities, reporter genes will become increasingly important in real-time detection of the gene expression at the single-cell level. This synergy will make it possible to understand the molecular basis of diseases, track the effectiveness of pharmaceuticals, monitor the response to therapies and evaluate the development process of new drugs.

Optical probes and the applications in multimodality imaging

Optical imaging essentially refers to in vivo fluorescence imaging and bioluminescence imaging. These types of imaging are widely used visualization methods in biomedical research and are important in molecular imaging. A new generation of imaging agents called multimodal probes have emerged in the past few years. These probes can be detected by two or more imaging modalities, which harnesses the strengths of the different modalities and enables researchers to obtain more information than can be achieved using only one modality. Owing to its low cost and the large number of probes available, the optical method plays an important role in multimodality imaging. In this mini-review, we describe the available multimodal imaging probes for in vivo imaging that combine optical imaging with other modalities.

A Cy5.5-labeled phage-displayed peptide probe for near-infrared fluorescence imaging of tumor vasculature in living mice

Near-infrared (NIR) fluorescence optical imaging is an emerging imaging technique for studying diseases at the molecular level. Optical imaging with a NIR emitting fluorophore for targeting tumor vasculature offers a noninvasive method for early detection of tumor angiogenesis and efficient monitoring of response to anti-tumor vasculature therapy. The previous in vitro results demonstrated that the GX1 peptide, identified by phage-display technology, is a tumor vasculature endothelium-specific ligand. In this report, Cy5.5-conjugated GX1 peptide was evaluated in a subcutaneous U87MG glioblastoma xenograft model to investigate tumor-targeting efficacy. The in vitro flow cytometry results revealed dose-dependent binding of Cy5.5-GX1 peptide to U87MG glioma cells. In vivo optical imaging with the Cy5.5-GX1 probe exhibited rapid U87MG tumor targeting at 0.5 h p.i., and high tumor-to-background contrast at 4 h p.i. Tumor specificity of Cy5.5-GX1 was confirmed by effective blocking of tumor uptake in the presence of unlabeled GX1 peptide (20 mg/kg). Ex vivo imaging further confirmed in vivo imaging findings, and demonstrated that Cy5.5-GX1 has a tumor-to-muscle ratio (15.21 ± 0.84) at 24 h p.i. for the non-blocked group and significantly decreased ratio (6.95 ± 0.75) for the blocked group. In conclusion, our studies suggest that Cy5.5-GX1 is a promising molecular probe for optical imaging of tumor vasculature.

Enhanced tumor therapy using vaccinia virus strain GLV-1h68 in combination with a β-galactosidase-activatable prodrug seco-analog of duocarmycin SA

Breast cancer is the most common cause of cancer-related death worldwide, thus remaining a crucial health problem among women despite advances in conventional therapy. Therefore, new alternative strategies are needed for effective diagnosis and treatment. One approach is the use of oncolytic viruses for gene-directed enzyme prodrug therapy. Here, the lacZ-carrying vaccinia virus (VACV) strain GLV-1h68 was used in combination with a β-galactosidase-activatable prodrug derived from a seco-analog of the natural antibiotic duocarmycin SA. Tumor cell infection with the VACV strain GLV-1h68 led to production of β-galactosidase, essential for the conversion of the prodrug to the toxic compound. Furthermore, drug-dependent cell kill and induction of the intrinsic apoptosis pathway in tumor cells was also observed on combination therapy using the prodrug and the GLV-1h68 strain, despite the fact that VACV strains encode antiapoptotic proteins. Moreover, GI-101A breast cancer xenografts were effectively treated by the combination therapy. In conclusion, the combination of a β-galactosidase-activatable prodrug with a tumor-specific vaccinica virus strain encoding this enzyme, induced apoptosis in cultures of the human GI-101A breast cancer cells, in which a synergistic oncolytic effect was observed. Moreover, in vivo, additional prodrug treatment had beneficial effects on tumor regression in GLV-1h68-treated GI-101A-xenografted mice.

Optical imaging in vivo with a focus on paediatric disease: technical progress, current preclinical and clinical applications and future perspectives

To obtain information on the occurrence and location of molecular events as well as to track target-specific probes such as antibodies or peptides, drugs or even cells non-invasively over time, optical imaging (OI) technologies are increasingly applied. Although OI strongly contributes to the advances made in preclinical research, it is so far, with the exception of optical coherence tomography (OCT), only very sparingly applied in clinical settings. Nevertheless, as OI technologies evolve and improve continuously and represent relatively inexpensive and harmful methods, their implementation as clinical tools for the assessment of children disease is increasing. This review focuses on the current preclinical and clinical applications as well as on the future potential of OI in the clinical routine. Herein, we summarize the development of different fluorescence and bioluminescence imaging techniques for microscopic and macroscopic visualization of microstructures and biological processes. In addition, we discuss advantages and limitations of optical probes with distinct mechanisms of target-detection as well as of different bioluminescent reporter systems. Particular attention has been given to the use of near-infrared (NIR) fluorescent probes enabling observation of molecular events in deeper tissue.

Bioluminescence imaging of reporter mice for studies of infection and inflammation

In vivo bioluminescence imaging offers the opportunity to study biological processes in living animals, and the study of viral infections and host immune responses can be enhanced substantially through this imaging modality. For most studies of viral pathogenesis and effects of anti-viral therapies, investigators have used recombinant viruses engineered to express a luciferase enzyme. This strategy requires stable insertion of an imaging reporter gene into the viral genome, which is not feasible for many RNA viruses, and provides data on the viral component of pathogenesis but not on the host. Genetically engineered mice with luciferase reporters for specific viral or host genes provide opportunities to overcome these limitations and expand applications of bioluminescence imaging in viral infection and therapy. We review several different types of reporter mice for bioluminescence imaging, including animals that permit in vivo detection of viral replication, trafficking of immune cells, activation of key genes in host immunity to viral infection, and response to tissue damage. By utilizing luciferase enzymes with different emission spectra and/or substrates, it is possible to monitor two different biologic processes in the same animal, such as pathogen replication and sites of tissue injury. Combining imaging reporter viruses with genetically engineered reporter mice is expected to substantially enhance the power of bioluminescence imaging for quantitative studies of viral and host factors that control disease outcome and effects of established and new therapeutic agents.

Imaging beta-galactosidase activity in human tumor xenografts and transgenic mice using a chemiluminescent substrate

Background

Detection of enzyme activity or transgene expression offers potential insight into developmental biology, disease progression, and potentially personalized medicine. Historically, the lacZ gene encoding the enzyme β-galactosidase has been the most common reporter gene and many chromogenic and fluorogenic substrates are well established, but limited to histology or in vitro assays. We now present a novel approach for in vivo detection of β-galactosidase using optical imaging to detect light emission following administration of the chemiluminescent 1,2-dioxetane substrate Galacto-Light PlusTM.

Methodology and Principal Findings

B-gal activity was visualized in stably transfected human MCF7-lacZ tumors growing in mice. LacZ tumors were identified versus contralateral wild type tumors as controls, based on two- to tenfold greater light emission following direct intra tumoral or intravenous administration of reporter substrate. The 1,2-dioxetane substrate is commercially available as a kit for microplate-based assays for β-gal detection, and we have adapted it for in vivo application. Typically, 100 µl substrate mixture was administered intravenously and light emission was detected from the lacZ tumor immediately with gradual decrease over the next 20 mins. Imaging was also undertaken in transgenic ROSA26 mice following subcutaneous or intravenous injection of substrate mixture.

Conclusion and Significance

Light emission was detectable using standard instrumentation designed for more traditional bioluminescent imaging. Use of 1,2-dioxetane substrates to detect enzyme activity offers a new paradigm for non-invasive biochemistry in vivo.