Luminescent lanthanide complexes as probes for the determination of enzyme activities

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

Luminescent Lanthanide Probes for Inorganic and Organic Phosphates

Inorganic and organic phosphates-including orthophosphate, nucleotides, and DNA-are some of the most fundamental anions in cellular biology, regulating numerous processes of both medical and environmental significance. The characteristic long lifetimes of emitting lanthanides, including the brighter europium(III) and terbium(III), make them ideally suited for the development of molecular probes for the detection of phosphates directly in complex aqueous media. Moreover, given their high oxophilicity and the exquisite sensitivity of their quantum yields to their hydration number, those luminescent lanthanides are perfect for the detection of phosphates. Herein we discuss the principles that have guided the recent developments of molecular probes selective for inorganic or organic phosphates and how these lanthanide complexes facilitate the study of numerous biological processes.

A Structure-Activity Relationship Study of Bimodal BODIPY-Labeled PSMA-Targeting Bioconjugates

The aim of this study was to identify a high-affinity BODIPY peptidomimetic that targets the prostate-specific membrane antigen (PSMA) as a potential bimodal imaging probe for prostate cancer. For the structure-activity study, several BODIPY (difluoroboron dipyrromethene) derivatives with varying spacers between the BODIPY dye and the PSMA Glu-CO-Lys binding motif were prepared. Corresponding affinities were determined by competitive binding assays in PSMA-positive LNCaP cells. One compound was identified with comparable affinity (IC50 =21.5±0.1 nM) to Glu-CO-Lys-Ahx-HBED-CC (PSMA-11) (IC50 =18.4±0.2 nM). Radiolabeling was achieved by Lewis-acid-mediated 19 F/18 F exchange in moderate molar activities (∼0.7 MBq nmol-1 ) and high radiochemical purities (>99 %) with mean radiochemical yields of 20-30 %. Cell internalization of the 18 F-labeled high-affinity conjugate was demonstrated in LNCaP cells showing gradual increasing PSMA-mediated internalization over time. By fluorescence microscopy, localization of the high-affinity BODIPY-PSMA conjugate was found in the cell membrane at early time points and also in subcellular compartments at later time points. In summary, a high-affinity BODIPY-PSMA conjugate has been identified as a suitable candidate for the development of PSMA-specific dual-imaging agents.

Discovery of catalytic-site-fluorescent probes for tracing phosphodiesterase 5 in living cells

Small molecule fluorescent probes provide a powerful labelling technology to enhance our understanding of particular proteins. However, the discovery of a proper fluorescent probe for detecting PDE5 is still a challenge due to the highly conservative structure of the catalytic domain in the phosphodiesterase (PDE) families. Herein, we identified probes based on the key amino residues in the ligand binding pocket of PDE5 and catalytic-site-fluorescent probes PCO2001–PCO2003 were well designed and synthesized. Among them, PCO2003 exhibited extraordinary fluorescence properties and the ability to be applied to PDE5 visualization in live cells as well as in pulmonary tissue slices, demonstrating the location and expression level of PDE5 proteins. Overall, the environment-sensitive “turn-on” probe is economical, convenient and rapid for PDE5 imaging, implying that the catalytic-site-fluorescent probe will have a variety of future applications in pathological diagnosis as well as drug screening.

To enhance the understanding of PDE5 as the drug target. Herein, we designed catalytic-site-fluorescent probes that can be applied to PDE5 visualization in live cells and tissue slices, implying the potential in diagnosis and drug screening.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

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.

Fluorescent imaging of cancerous tissues for targeted surgery

To maximize tumor excision and minimize collateral damage are the primary goals of cancer surgery. Emerging molecular imaging techniques have made "image-guided surgery" developed into "molecular imaging-guided surgery", which is termed as "targeted surgery" in this review. Consequently, the precision of surgery can be advanced from tissue-scale to molecule-scale, enabling "targeted surgery" to be a component of "targeted therapy". Evidence from numerous experimental and clinical studies has demonstrated significant benefits of fluorescent imaging in targeted surgery with preoperative molecular diagnostic screening. Fluorescent imaging can help to improve intraoperative staging and enable more radical cytoreduction, detect obscure tumor lesions in special organs, highlight tumor margins, better map lymph node metastases, and identify important normal structures intraoperatively. Though limited tissue penetration of fluorescent imaging and tumor heterogeneity are two major hurdles for current targeted surgery, multimodality imaging and multiplex imaging may provide potential solutions to overcome these issues, respectively. Moreover, though many fluorescent imaging techniques and probes have been investigated, targeted surgery remains at a proof-of-principle stage. The impact of fluorescent imaging on cancer surgery will likely be realized through persistent interdisciplinary amalgamation of research in diverse fields.

Time-resolved fluorescence-based assay for rapid detection of Escherichia coli

Fast and simple detection of pathogens is of utmost importance in health care and the food industry. In this article, a novel technology for the detection of pathogenic bacteria is presented. The technology uses lytic-specific bacteriophages and a nonspecific interaction of cellular components with a luminescent lanthanide chelate. As a proof of principle, Escherichia coli-specific T4 bacteriophage was used to infect the bacteria, and the cell lysis was detected. In the absence of E. coli, luminescent Eu(3+)-chelate complex cannot be formed and low time-resolved luminescence signal is monitored. In the presence of E. coli, increased luminescence signal is observed as the cellular contents are leached to the surrounding medium. The luminescence signal is observed as a function of the number of bacteria in the sample. The homogeneous assay can detect living E. coli in bacterial cultures and simulated urine samples within 25 min with a detection limit of 1000 or 10,000 bacterial cells/ml in buffer or urine, respectively. The detection limit is at the clinically relevant level, which indicates that the method could also be applicable to clinical settings for fast detection of urine bacteria.

Terbium(iii)-cholate functionalized vesicles as luminescent indicators for the enzymatic conversion of dihydroxynaphthalene diesters

The phosphorescence intensity of unilamellar DOPC vesicles with embedded Tb³⁺-cholate complexes depends on the concentration of dihydroxynaphthalene (DHN) as sensitizer in solution.

Luminogenic "clickable" lanthanide complexes for protein labeling

Development of lanthanide-based luminescent "switch-on" systems via azide-alkyne [3+2] cycloaddition is described. We used these for non-specific protein labeling and as tags for specific and selective activity-based protein labeling.

Evaluation of triazole-chelated lanthanides as chemically stabile bioimaging agents

Europium (Eu), dysprosium (Dy), samarium (Sm), and terbium (Tb) complexes were prepared using the neutral tridentate chelator 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyridine and one equivalent of each lanthanide salt. The physicochemical, aerodynamic, and in vitro cellular properties of each lanthanide metal complex were studied to determine their viability as cell surface fluorescent probes. Each compound was characterized by electrospray ionization mass spectroscopy (ESI-MS), ultraperformance liquid chromatography (UPLC), differential scanning calorimetry (DSC), and thermogravimetic analysis (TGA). Upon excitation at 320 nm each complex displayed characteristic lanthanide-based fluorescence emission in the visible wavelength range with stokes shifts greater than 200 nm. Each complex was found to be chemically stable when exposed to pH range of 1-11 for 72 h and resistant to photobleaching. To simulate pulmonary administration of these fluorophores, the aerodynamic properties of the Eu and Tb complexes were determined using a next generation impactor (NGI). This measurement confirmed that the complexes retain their fluorescence emission properties after nebulization. Cellular cytotoxicity was determined on A-549 lung cancer cell line using methylthiazol tetrazolium (MTT) cytotoxicity assay at 24, 48, and 72 h postexposure to the complexes. The complexes showed a dose and time-dependent effect on the percent viability of the cells.

Quantum dot-Eu3+ conjugate as a luminescence turn-on sensor for ultrasensitive detection of nucleoside triphosphates

We report a conjugate of thioglycolic acid (TGA) capped CdTe quantum dot and Eu(3+) ion (TGA-CdTe QD-Eu(3+)) that can be used as an ultrasensitive luminescence turn-on sensor for nucleoside triphosphates (NTPs). The TGA-CdTe QD-Eu(3+) conjugate is a weakly luminescent species as a result of the strong quenching effect of Eu(3+) ion on the luminescence of TGA-CdTe QDs. The conjugate's luminescence can be readily restored by its reaction with adenosine triphosphate (ATP) and other NTPs, and thus gives an ultrasensitive detection of NTPs, with a detection limit of 2 nM. The sensing mechanism has also been explored, and the effective quenching of TGA-CdTe QDs emission by Eu(3+) ions has been attributed to photoinduced electron transfer (PET). ATP, as the representative of NTPs, can remove Eu(3+) from the surface of TGA-CdTe QDs, leading to restoration of the TGA-CdTe QDs luminescence.

Comparative cytotoxicity evaluation of lanthanide nanomaterials on mouse and human cell lines with metabolic and DNA-quantification assays

Lanthanide nanomaterials are considered a less toxic alternative to quantum dots for bioimaging applications. This study evaluated the cytotoxicity of terbium (Tb)-doped gadolinium oxide (Gd(2)O(3)) and dysprosium oxide (Dy(2)O(3)) nanoparticles exposed to human (BEAS-2B) and mouse (L929) cell lines at a concentration range of 200-2000 μg/ml for 48 h. Two assay methods were utilized-WST-8 assay (colorimetric) based on mitochondrial metabolic activity and Pico-Green assay (fluorescence), which measures total DNA content. The authors' data showed that Tb-doped Gd(2)O(3) nanoparticles were consistently more toxic than Tb-doped Dy(2)O(3) nanoparticles. However, exposure to these nanomaterials caused a decrease in proliferation rate for both cell lines rather than a net loss of viable cells after 48 h of exposure. Additionally, there was some degree of discrepancy observed with the two assay methods. For the mouse L929 cell line, the WST-8 assay yielded consistently lower proliferation rates compared to the Pico-Green assay, whereas the opposite trend was observed for the human BEAS-2B cell line. This could arise because of the differential effects of these nanoparticles on the metabolism of L929 and BEAS-2B cells, which in turn may translate to differences in their postexposure proliferation rates. Hence, the Pico-Green assay could have an advantage over the WST-8 assay because it is not skewed by the differential effects of nanomaterials on cellular metabolism.

Absorption spectroscopic probe to investigate the interaction between Nd(III) and calf-thymus DNA

The interaction between Nd(III) and Calf Thymus DNA (CT-DNA) in physiological buffer (pH 7.4) has been studied using absorption spectroscopy involving 4f-4f transition spectra in different aquated organic solvents. Complexation with CT-DNA is indicated by the changes in absorption intensity following the subsequent changes in the oscillator strengths of different 4f-4f bands and Judd-Ofelt intensity (T(λ)) parameters. The other spectral parameters namely Slator-Condon (F(k)'s), nephelauxetic effect (β), bonding (b(1/2)) and percent covalency (δ) parameters are computed to correlate with the binding of Nd(III) with DNA. The absorption spectra of Nd(III) exhibited hyperchromism and red shift in the presence of DNA. The binding constant, K(b) has been determined by absorption measurement. The relative viscosity of DNA decreased with the addition of Nd(III). Thermodynamic parameters have been calculated according to relevant absorption data and Van't Hoff equation. The characterisation of bonding mode has been studied in detail. The results suggested that the major interaction mode between Nd(III) and DNA was external electrostatic binding.

Recent progress toward the templated synthesis and directed evolution of sequence-defined synthetic polymers

Biological polymers such as nucleic acids and proteins are ubiquitous in living systems, but their ability to address problems beyond those found in nature is constrained by factors such as chemical or biological instability, limited building-block functionality, bioavailability, and immunogenicity. In principle, sequence-defined synthetic polymers based on nonbiological monomers and backbones might overcome these constraints; however, identifying the sequence of a synthetic polymer that possesses a specific desired functional property remains a major challenge. Molecular evolution can rapidly generate functional polymers but requires a means of translating amplifiable templates such as nucleic acids into the polymer being evolved. This review covers recent advances in the enzymatic and nonenzymatic templated polymerization of nonnatural polymers and their potential applications in the directed evolution of sequence-defined synthetic polymers.

Luminescent bimetallic lanthanide bioprobes for cellular imaging with excitation in the visible-light range

A series of homoditopic ligands H(2)L(CX) (X=4-6) has been designed to self-assemble with lanthanide ions (Ln(III)), resulting in neutral bimetallic helicates of overall composition [Ln(2)(L(CX))(3)] with the aim of testing the influence of substituents on the photophysical properties, particularly the excitation wavelength. The complex species are thermodynamically stable in water (log beta(23) in the range 26-28 at pH 7.4) and display a metal-ion environment with pseudo-D(3) symmetry and devoid of coordinated water molecules. The emission of Eu(III), Tb(III), and Yb(III) is sensitised to various extents, depending on the properties of the ligand donor levels. The best helicate is [Eu(2)(L(C5))(3)] with excitation maxima at 350 and 365 nm and a quantum yield of 9 %. The viability of cervix cancer HeLa cells is unaffected when incubated with up to 500 mum of the chelate during 24 h. The helicate permeates into the cells by endocytosis and locates into lysosomes, which co-localise with the endoplasmatic reticulum, as demonstrated by counterstaining experiments. The relatively long excitation wavelength allows easy recording of bright luminescent images on a confocal microscope (lambda(exc)=405 nm). The new lanthanide bioprobe remains undissociated in the cell medium, and is amenable to facile derivatisation. Examination of data for seven Eu(III) and Tb(III) bimetallic helicates point to shortcomings in the phenomenological rules of thumb between the energy gap DeltaE((3)pipi*-(5)D(J)) and the sensitisation efficiency of the ligands.

A fluorimetric assay for real-time monitoring of adenylyl cyclase activity based on terbium norfloxacin

Adenylyl cyclases catalyze the production of the second messenger cyclic AMP from ATP. Until now, there has been no fluorescent adenylyl cyclase assay known that is applicable to high-throughput screening and kinetic determinations that can directly monitor the turnover of the unmodified substrate ATP. In this study, a fluorescence-based assay is described using the Ca(II)- and calmodulin-dependent adenylyl cyclase edema factor (EF) from Bacillus anthracis and Tb(III)-norfloxacin as probe for the enzyme activity. This assay can be used to study enzyme regulators, allows real-time monitoring of adenylyl cyclase activity, and does not substitute ATP by fluorescent derivatives. These derivatives must be judged critically due to their interference on the activity of enzymes. Furthermore, the new assay makes redundant the application of radioactively labeled substrates such as [alpha-(32)P]ATP or fluorescently labeled antibodies such as anti-cyclic AMP. We determined the Michaelis-Menten constant (K(M)), the v(0)(max) value of ATP turnover, and the IC(50) values for three inhibitors of EF by this newly developed fluorescent method.