In vivo molecular and cellular imaging with quantum dots

Exploring dynamics in living cells by tracking single particles

In the last years, significant advances in microscopy techniques and the introduction of a novel technology to label living cells with genetically encoded fluorescent proteins revolutionized the field of Cell Biology. Our understanding on cell dynamics built from snapshots on fixed specimens has evolved thanks to our actual capability to monitor in real time the evolution of processes in living cells. Among these new tools, single particle tracking techniques were developed to observe and follow individual particles. Hence, we are starting to unravel the mechanisms driving the motion of a wide variety of cellular components ranging from organelles to protein molecules by following their way through the cell. In this review, we introduce the single particle tracking technology to new users. We briefly describe the instrumentation and explain some of the algorithms commonly used to locate and track particles. Also, we present some common tools used to analyze trajectories and illustrate with some examples the applications of single particle tracking to study dynamics in living cells.

Quantitative single particle tracking of NGF-receptor complexes: transport is bidirectional but biased by longer retrograde run lengths

The retrograde transport of nerve growth factor (NGF) in neurite-like processes of living differentiated PC12 cells was studied using streptavidin-quantum dots (QDs) coupled to monobiotin-NGF. These reagents were active in differentiation, binding, internalization, and transport. Ten-35% of the QD-NGF-receptor complexes were mobile. Quantitative single particle tracking revealed a bidirectional step-like motion, requiring intact microtubules, with a net retrograde velocity of 0.054+/-0.020 microm/s. Individual runs had a mean velocity of approximately 0.15 microm/s at room temperature, and the run times were exponentially distributed. The photostability and brightness of QDs permit extended real-time analysis of individual QDbNGF- receptor complexes trafficking within neurites.

Phage display evolution of a peptide substrate for yeast biotin ligase and application to two-color quantum dot labeling of cell surface proteins

Site-specific protein labeling with Escherichia coli biotin ligase (BirA) has been used to introduce fluorophores, quantum dots (QDs), and photocross-linkers onto recombinant proteins fused to a 15-amino acid acceptor peptide (AP) substrate for BirA and expressed on the surface of living mammalian cells. Here, we used phage display to engineer a new and orthogonal biotin ligase-AP pair for site-specific protein labeling. Yeast biotin ligase (yBL) does not recognize the AP, but we discovered a new 15-amino acid substrate for yBL called the yeast acceptor peptide (yAP), using two generations of phage display selection from 15-mer peptide libraries. The yAP is not recognized by BirA, and thus, we were able to specifically label AP and yAP fusion proteins coexpressed in the same cell with differently colored QDs. We fused the yAP to a variety of recombinant proteins and demonstrated biotinylation by yBL at the N-terminus, C-terminus, and within a flexible internal region. yBL is extremely sequence-specific, as endogenous proteins on the surface of yeast and HeLa cells are not biotinylated. This new methodology expands the scope of biotin ligase labeling to two-color imaging and yeast-based applications.

A Quantum-Dot Nanocrystal Study of GABAA Receptor Subunits in Living Cerebellar Granule Cells in Culture

Quantum dots (QDs) are semiconductor nanocrystals emerging as a new class of fluorescent labels with large brightness, multi color fluorescence emission and resistance against photobleaching. Here we have used QDs as biological markers in an immunofluorescence approach. In this work GABA(A )receptors of rat cerebellar granule cells have been studied and in particular we have visualized the beta(2/3) and delta subunits in live cells. The results obtained were compared to those gathered with conventional probes. The images of the delta subunit in living cells appear to correspond to those expected for a subunit part of GABA(A )receptors mediating tonic inhibition in the granules cell bodies.

Investigating membrane protein dynamics in living cells

Live cell imaging is a powerful tool for understanding the function and regulation of membrane proteins. In this review, we briefly discuss 4 fluorescence-microscopy-based techniques for studying the transport dynamics of membrane proteins: fluorescence-correlation spectroscopy, image-correlation spectroscopy, fluorescence recovery after photobleaching, and single-particle and (or) molecule tracking. The advantages and limitations of each approach are illustrated using recent studies of an ion channel and cell adhesion molecules.

Imaging single events at the cell membrane

The ability to sense and respond to the environment is a hallmark of living systems. These processes occur at the levels of the organism, cells and individual molecules. Sensing of extracellular changes could result in a structural or chemical alteration in a molecule, which could in turn trigger a cascade of intracellular signals or regulated trafficking of molecules at the cell surface. These and other such processes allow cells to sense and respond to environmental changes. Often, these changes and the responses to them are spatially and/or temporally localized, and visualization of such events necessitates the use of high-resolution imaging approaches. Here we discuss optical imaging approaches and tools for imaging individual events at the cell surface with improved speed and resolution.

Quantum dots are phagocytized by macrophages and colocalize with experimental gliomas

OBJECTIVE

The identification of neoplastic tissue within normal brain during biopsy and tumor resection remains a problem in the operative management of gliomas. A variety of nanoparticles are phagocytized by macrophages in vivo. This feature may allow optical nanoparticles, such as quantum dots, to colocalize with brain tumors and serve as an optical aid in the surgical resection or biopsy of brain tumors.

METHODS

Male Fisher rats (Charles River Labs, Wilmington, MA) were implanted intracranially with C6 gliosarcoma cell lines to establish tumors. Two weeks after the implantation of tumors, 705-nm emission Qdot ITK Amino(PEG) Quantum Dots (Quantum Dot Corp., Hayward, CA) were injected via the tail vein at doses of 3 to 17 nmol. The animals were sacrificed 24 hours after the injection of quantum dots and their tissues were examined.

RESULTS

Quantum dots are avidly phagocytized by macrophages and are taken up by the liver, spleen, and lymph nodes. A dose-response relationship was noted. At low doses, the majority of the quantum dots are sequestered in the liver, spleen, and lymph nodes. At higher doses, increasing quantities of quantum dots are noted within the experimental brain tumors. Macrophages and microglia colocalize with glioma cells, carrying the quantum dot and thereby optically outlining the tumor. Excitation with blue or ultraviolet wavelengths stimulates the quantum dots, which give off a deep red fluorescence detectable with charge-coupled device cameras, optical spectroscopy units, and in dark-field fluorescence microscopy.

CONCLUSION

Quantum dots are optical nanoparticles that, when delivered in nanomole doses, are phagocytized by the macrophages and microglia that infiltrate experimental gliomas. The optical signal may be detected, allowing for improved identification and visualization of tumors, potentially augmenting brain tumor biopsy and resection.

Quantum dots for molecular pathology: their time has arrived

Assessing malignant tumors for expression of multiple biomarkers provides data that are critical for patient management. Quantum dot-conjugated probes to specific biomarkers are powerful tools that can be applied in a multiplex manner to single tissue sections of biopsies to measure expression levels of multiple biomarkers.

In vivo optical imaging using quantum dots for the management of brain tumors

The surgical management of brain tumors requires the precise localization of tumor tissues within normal brain parenchyma in order to achieve accurate diagnostic biopsy and complete surgical resection. Quantum dots are optical semiconductor nanocrystals that exhibit stable, bright fluorescence. The intravenous injection of quantum dots is accompanied by reticuloendothelial system and macrophage sequestration. Macrophages infiltrate brain tumors and phagocytize intravenously injected quantum dots, optically labeling the tumors. Macrophage-mediated delivery of quantum dots to brain tumors may represent a novel technique to label tumors preoperatively. Quantum dots within tumors may be detected with optical imaging and optical spectroscopy tools, providing the surgeon with real-time optical feedback during the resection and biopsy of brain tumors.

Long-term exposure to CdTe quantum dots causes functional impairments in live cells

Several studies suggested that the cytotoxic effects of quantum dots (QDs) may be mediated by cadmium ions (Cd2+) released from the QDs cores. The objective of this work was to assess the intracellular Cd2+ concentration in human breast cancer MCF-7 cells treated with cadmium telluride (CdTe) and core/shell cadmium selenide/zinc sulfide (CdSe/ZnS) nanoparticles capped with mercaptopropionic acid (MPA), cysteamine (Cys), or N-acetylcysteine (NAC) conjugated to cysteamine. The Cd2+ concentration determined by a Cd2+-specific cellular assay was below the assay detection limit (<5 nM) in cells treated with CdSe/ZnS QDs, while in cells incubated with CdTe QDs, it ranged from approximately 30 to 150 nM, depending on the capping molecule. A cell viability assay revealed that CdSe/ZnS QDs were nontoxic, whereas the CdTe QDs were cytotoxic. However, for the various CdTe QD samples, there was no dose-dependent correlation between cell viability and intracellular [Cd2+], implying that their cytotoxicity cannot be attributed solely to the toxic effect of free Cd2+. Confocal laser scanning microscopy of CdTe QDs-treated cells imaged with organelle-specific dyes revealed significant lysosomal damage attributable to the presence of Cd2+ and of reactive oxygen species (ROS), which can be formed via Cd2+-specific cellular pathways and/or via CdTe-triggered photoxidative processes involving singlet oxygen or electron transfer from excited QDs to oxygen. In summary, CdTe QDs induce cell death via mechanisms involving both Cd2+ and ROS accompanied by lysosomal enlargement and intracellular redistribution.

Nanotechnological applications in medicine

Nanotechnology-based tools and techniques are rapidly emerging in the fields of medical imaging and targeted drug delivery. Employing constructs such as dendrimers, liposomes, nanoshells, nanotubes, emulsions and quantum dots, these advances lead toward the concept of personalized medicine and the potential for very early, even pre-symptomatic, diagnoses coupled with highly-effective targeted therapy. Highlighting clinically available and preclinical applications, this review explores the opportunities and issues surrounding nanomedicine.

Nanoparticles and cells: good companions and doomed partnerships

Engineered nanoparticles are emerging as useful tools for different purposes in life sciences, medicine and agriculture. Nanomedicine, an emerging discipline, involves the application of nanotechnology (usually regarded within the size range of 1-1000 nm) in the design of systems and devices that can facilitate our understanding of disease pathophysiology, nano-imaging, nanomedicines and nano-diagnostics. Among the different nanomaterials used to construct nanoparticles, are organic polymers, co-polymers and metals. Some of these materials can self assemble, and depending on the conditions under which the self-assembly process occurs, a vast array of shapes can be formed. Frequently, the nanoparticle morphology is spherical or tubular, mimicking the shape, but thus far, not the functions of subcellular organelles. We discuss here several representative nanoparticles, made of block copolymers and metals, highlighting some of their current uses, advantages and limitations in medicine. Nano-oncology and nano-neurosciences will also be discussed in more detail in the context of the intracellular fate of nanoparticles and possible long-term consequences on cell functions.

Luminescent chemosensors based on semiconductor quantum dots

Semiconductor quantum dots are inorganic nanoparticles with unique photophysical properties. In particular, their huge one- and two-photon absorption cross sections, tunable emission bands and excellent photobleaching resistances are stimulating the development of luminescent probes for biomedical imaging and sensing applications. Indeed, electron and energy transfer processes can be designed to switch the luminescence of semiconductor quantum dots in response to molecular recognition events. On the basis of these operating principles, the presence of target analytes can be transduced into detectable luminescence signals. In fact, luminescent chemosensors based on semiconductor quantum dots are starting to be developed to detect small molecules, monitor DNA hybridization, assess protein-ligand complementarities, test enzymatic activity and probe pH distributions. Although fundamental research is still very much needed to understand further the fundamental factors regulating the behavior of these systems and refine their performance, it is becoming apparent that sensitive probes based on semiconductor quantum dots will become invaluable analytical tools for a diversity of applications in biomedical research.

Quantum dots for cancer molecular imaging

Quantum dots (QDs), tiny light-emitting particles on the nanometer scale, are emerging as a new class of fluorescent probes for biomolecular and cellular imaging. In comparison with organic dyes and fluorescent proteins, quantum dots have unique optical and electronic properties such as size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors. These properties are most promising for improving the sensitivity of molecular imaging and quantitative cellular analysis by 1-2 orders of magnitude. Recent advances have led to multifunctional nanoparticle probes that are highly bright and stable under complex in-vivo conditions. A new structural design involves encapsulating luminescent QDs with amphiphilic block copolymers, and linking the polymer coating to tumor-targeting ligands and drug-delivery functionalities. Polymer-encapsulated QDs are essentially nontoxic to cells and small animals, but their long-term in-vivo toxicity and degradation need more careful studies. Nonetheless, bioconjugated QDs have raised new possibilities for ultrasensitive and multiplexed imaging of molecular targets in living cells and animal models.

Increased optical contrast in imaging of epidermal growth factor receptor using magnetically actuated hybrid gold/iron oxide nanoparticles

We describe a new approach for optical imaging that combines the advantages of molecularly targeted plasmonic nanoparticles and magnetic actuation. This combination is achieved through hybrid nanoparticles with an iron oxide core surrounded by a gold layer. The nanoparticles are targeted in-vitro to epidermal growth factor receptor, a common cancer biomarker. The gold portion resonantly scatters visible light giving a strong optical signal and the superparamagnetic core provides a means to externally modulate the optical signal. The combination of bright plasmon resonance scattering and magnetic actuation produces a dramatic increase in contrast in optical imaging of cells labeled with hybrid gold/iron oxide nanoparticles.

Reduction in nonfluorescence state of quantum dots on an immunofluorescence staining

Fluorescence quantum dots are widely used in immunofluorescence staining because of their intense and stable fluorescence. However, the nonfluorescence state of the quantum dots is their disadvantage. Here, the nonfluorescence state of the dots labeled to cells and tissues was suppressed. Cells and tissues where the receptor HER2 had been overexpressed were fixed and then labeled with anti-HER2 crosslinked with the dots. The intensity of the dots increased with the illumination time. The majority of the single dots were in the nonfluorescence state at beginning of the illumination period and the number of fluorescence dots observed increased with the illumination time. Living cells were also labeled with the anti-HER2-Qdots. Blinking and bleaching of the Qdots was effectively suppressed by adding beta-mercaptoethanol and glutathione. Therefore, the movement of the Qdots bound to cell membrane could be observed for long periods of time.

Nanoparticles for bioimaging

The emergence of synthesis strategies for the fabrication of nanosized contrast agents is anticipated to lead to advancements in understanding biological processes at the molecular level in addition to progress in the development of diagnostic tools and innovative therapies. Imaging agents such as fluorescent dye-doped silica nanoparticles, quantum dots and gold nanoparticles have overcome many of the limitations of conventional contrast agents (organic dyes) such as poor photostability, low quantum yield, insufficient in vitro and in vivo stability, etc. Such particulates are now being developed for absorbance and emission in the near infrared region, which is expected to allow for real time and deep tissue imaging via optical routes. Other efforts to facilitate deep tissue imaging with pre-existing technologies have lead to the development of multimodal nanoparticles which are both optical and MRI active. The main focus of this article is to provide an overview of properties and design of contrast agents such as dye-doped silica nanoparticles, quantum dots and gold nanoparticles for non-invasive bioimaging.

Inorganic phosphate nanorods are a novel fluorescent label in cell biology

We report the first use of inorganic fluorescent lanthanide (europium and terbium) ortho phosphate [LnPO4.H2O, Ln = Eu and Tb] nanorods as a novel fluorescent label in cell biology. These nanorods, synthesized by the microwave technique, retain their fluorescent properties after internalization into human umbilical vein endothelial cells (HUVEC), 786-O cells, or renal carcinoma cells (RCC). The cellular internalization of these nanorods and their fluorescence properties were characterized by fluorescence spectroscopy (FS), differential interference contrast (DIC) microscopy, confocal microscopy, and transmission electron microscopy (TEM). At concentrations up to 50 microg/ml, the use of [3H]-thymidine incorporation assays, apoptosis assays (TUNEL), and trypan blue exclusion illustrated the non-toxic nature of these nanorods, a major advantage over traditional organic dyes.

Characterization of the coupling of quantum dots and immunoglobulin antibodies

Water-soluble quantum dots (QDs) were used to label goat anti-human immunoglobulin antibodies (Abs), and the labeling process was characterized by column purification. The QDs obtained in organic solvent were modified with mercaptoacetic acid (MAA) and became water-soluble. These water-soluble QDs were linked to the antibodies using the coupling reagents ethyl-3-(dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The linking process was shown to be effective by ultra-filter centrifugation and column purification. After comparing the quantities of Abs and water-soluble QDs involved in the linking reaction via column purification, it was found that a molar Abs:QD ratio of >1.2 resulted in most of the water-soluble QDs becoming covalently linked to the Abs. The circular dichroism (CD) spectra of Abs and QD-Ab conjugates were very similar to each other, indicating that the secondary structure of Abs remained largely intact after the conjugation. Finally, antigen (Ag)-antibody (Ab) recognition reactions perfomed on the surface of a glass slide showed that the conjugate retained the activity of Abs. This work lends support to the idea of linking biomolecules to QDs, and thus should aid the application of QDs to the life sciences.

Fiber-optic fluorescence correlation spectrometer

Fluorescence correlation spectroscopy (FCS) is a sensitive technique used to probe size, concentration, flow velocity, and reaction kinetics in a dilute solution. Conventional FCS spectrometers achieve this sensitivity at the cost of using bulky optics. We demonstrate a technique that utilizes a single-mode optical fiber of 3.3 microm mode field diameter to perform FCS measurements. We demonstrate that the technique has adequate sensitivity to perform FCS measurements on fluorescent beads of 13 nm radius, and that the results agree with theoretical predictions. Our method potentially allows FCS to be extended to remote and in vivo applications.

Emerging implications of nanotechnology on cancer diagnostics and therapeutics

Nanotechnology is multidisciplinary field that involves the design and engineering of objects <500 nanometers (nm) in size. The National Cancer Institute has recognized that nanotechnology offers an extraordinary, paradigm-changing opportunity to make significant advances in cancer diagnosis and treatment. In the last several decades, nanotechnology has been studied and developed primarily for use in novel drug-delivery systems (e.g. liposomes, gelatin nanoparticles, micelles). A recent explosion in engineering and technology has led to 1) the development of many new nanoscale platforms, including quantum dots, nanoshells, gold nanoparticles, paramagnetic nanoparticles, and carbon nanotubes, and 2) improvements in traditional, lipid-based nanoscale platforms. The emerging implications of these platforms for advances in cancer diagnostics and therapeutics form the basis of this review. A widespread understanding of these new technologies is important, because they currently are being integrated into the clinical practice of oncology.

Overview of stem cells and imaging modalities for cardiovascular diseases

Stem cell therapy is emerging as a promising approach to treat heart diseases. Considerable evidence from experimental studies and initial clinical trials suggests that stem cell transplantation promotes systolic function and prevent ventricular remodeling. However, the specific mechanisms by which stem cells improve heart function remain largely unknown. In addition, interpreting the long-term effects of stem cell therapy is difficult because of the limitations of conventional techniques. The recent development of molecular imaging techniques offers great potential to address these critical issues by noninvasively tracking the fate of the transplanted cells. This review offers a focused discussion on the use of stem cell therapy and imaging in the context of cardiology.

Use of lipophilic near-infrared dye in whole-body optical imaging of hematopoietic cell homing

We develop an optical whole-body imaging technique for monitoring normal and leukemic hematopoietic cell homing in vivo. A recently developed near-infrared (NIR) lipophilic carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindotricarbocyanine iodide (DiR) is used to safely and directly label the membranes of human leukemic Pre-B ALL G2 cell lines as well as primary murine lymphocytes and erythrocytes. DiR has absorption and fluorescence maxima at 750 and 782 nm, respectively, which corresponds to low light absorption and autofluorescence in living tissues. This allows us to obtain a significant signal with very low background level. A charge-coupled device (CCD)-based imager is used for noninvasive whole-body imaging of DiR-labeled cell homing in intact animals. This powerful technique can potentially visualize any cell type without use of specific antibodies conjugated with NIR fluorescent tag or loading cells with transporter-delivered NIR fluorophores. Thus, in vivo imaging based on NIR lipophilic carbocyanine dyes in combination with advanced optical techniques may serve as a powerful alternative or complementation to other small animal imaging methods.

Multispectral imaging of clinically relevant cellular targets in tonsil and lymphoid tissue using semiconductor quantum dots

Determination of the expression and spatial distribution of molecular epitopes, or antigens, in patient tissue specimens has substantially improved the pathologist's ability to classify disease processes. Certain disease pathophysiologies are marked by characteristic increased or decreased expression of developmentally controlled antigens, defined as Cluster of Differentiation markers, that currently form the foundation for understanding lymphoid malignancies. While chromogens and organic fluorophores have been utilitized for some time in immunohistochemical analyses, developments in synthetic, inorganic fluorophore semiconductors, namely quantum dots, offer a versatile alternative reporter system. Quantum dots are stable fluorophores, are resistant to photobleaching, and are attributed with wide excitation ranges and narrow emission spectra. To date, routinely processed, formalin-fixed tissues have only been probed with two quantum dot reporters simultaneously. In the present study, streptavidin-conjugated quantum dots with distinct emission spectra were tested for their utility in identifying a variety of differentially expressed antigens (surface, cytoplasmic, and nuclear). Slides were analyzed using confocal laser scanning microscopy, which enabled with a single excitation wavelength (488 nm argon laser) the detection of up to seven signals (streptavidin-conjugated quantum dots 525, 565, 585, 605, 655, 705 and 805 nm) plus the detection of 4'6-DiAmidino-2-PhenylIndole with an infra-red laser tuned to 760 nm for two photon excitation. Each of these signals was specific for the intended morphologic immunohistochemical target. In addition, five of the seven streptavidin-conjugated quantum dots tested (not streptavidin-conjugated quantum dots 585 or 805 nm) were used on the same tissue section and could be analyzed simultaneously on routinely processed formalin-fixed, paraffin-embedded sections. Application of this multiplexing method will enable investigators to explore the clinically relevant multidimensional cellular interactions that underlie diseases, simultaneously.

Emerging use of nanoparticles in diagnosis and treatment of breast cancer

The biological application of nanoparticles is a rapidly developing area of nanotechnology that raises new possibilities in the diagnosis and treatment of human cancers. In cancer diagnostics, fluorescent nanoparticles can be used for multiplex simultaneous profiling of tumour biomarkers and for detection of multiple genes and matrix RNA with fluorescent in-situ hybridisation. In breast cancer, three crucial biomarkers can be detected and accurately quantified in single tumour sections by use of nanoparticles conjugated to antibodies. In the near future, the use of conjugated nanoparticles will allow at least ten cancer-related proteins to be detected on tiny tumour sections, providing a new method of analysing the proteome of an individual tumour. Supermagnetic nanoparticles have exciting possibilities as contrast agents for cancer detection in vivo, and for monitoring the response to treatment. Several chemotherapy agents are available as nanoparticle formulations, and have at least equivalent efficacy and fewer toxic effects compared with conventional formulations. Ultimately, the use of nanoparticles will allow simultaneous tumour targeting and drug delivery in a unique manner. In this review, we give an overview of the use of clinically applicable nanoparticles in oncology, with particular focus on the diagnosis and treatment of breast cancer.

Dynamic imaging of the immune system: progress, pitfalls and promise

Both innate and adaptive immunity are dependent on the migratory capacity of myeloid and lymphoid cells. Effector cells of the innate immune system rapidly enter infected tissues, whereas sentinel dendritic cells in these sites mobilize and transit to lymph nodes. In these and other secondary lymphoid tissues, interactions among various cell types promote adaptive humoral and cell-mediated immune responses. Recent advances in light microscopy have allowed direct visualization of these events in living animals and tissue explants, which allows a new appreciation of the dynamics of immune-cell behaviour. In this article, we review the basic techniques and the tools used for in situ imaging, as well as the limitations and potential artefacts of these methods.

A mechanism to signal receptor-substrate interactions with luminescent quantum dots

Semiconductor quantum dots are becoming valuable analytical tools for biomedical applications. Indeed, their unique photophysical properties offer the opportunity to design luminescent probes for imaging and sensing with unprecedented performance. In this context, we have identified operating principles to transduce the supramolecular association of complementary receptor-substrate pairs into an enhancement in the luminescence of sensitive quantum dots. Our mechanism is based on the electrostatic adsorption of cationic quenchers on the surface of anionic quantum dots. The adsorbed quenchers suppress efficiently the emission character of the associated nanoparticles on the basis of photoinduced electron transfer. In the presence of target receptors able to bind the quenchers and prevent electron transfer, however, the luminescence of the quantum dots is restored. Thus, complementary receptor-substrate pairs can be identified with luminescence measurements relying on our design logic. In fact, we have demonstrated with a representative example that our protocol can be adapted to signal protein-ligand interactions.

Chemical labeling strategies for cell biology

Methods to visualize, track, measure and perturb proteins in living cells are central to biomedicine's efforts to characterize and understand the spatial and temporal underpinnings of life inside cells. Although fluorescent proteins have revolutionized such studies, they have shortcomings, which have spurred the creation of alternative approaches to chemically label proteins in live cells. In this review we highlight research questions that can be addressed using site-specific chemical labeling and present a comparison of the various labeling techniques that have been developed. We also provide a 'roadmap' for selection of appropriate labeling techniques(s) and outline generalized strategies to validate and troubleshoot chemical labeling experiments.

Quantum dot-mediated detection of gamma-aminobutyric acid binding sites on the surface of living pollen protoplasts in tobacco

gamma-Aminobutyric acid (GABA) is an inhibitory transmitter in the central nervous system of mammals. Recent investigations showed that it also plays an important role in regulating pollen tube growth and orientation in plants. To determine whether GABA receptors are also present on the membrane of pollen protoplasts, a fluorescence probe of quantum dots (QDs) was constructed and applied. The water-soluble CdSe-ZnS (core-shell) QDs were first synthesized and verified to possess good optical properties. GABA was then bioconjugated to the QDs in the presence of 1-ethyl-3-(3)-dimethylaminopropyl carbodiimide (EDC) and N-hydroxysuccinimide (NHS) to make the fluorescence probe. Using the probe, GABA binding sites were detected on the protoplast membrane of both pollen and somatic cells. Both the fluorescent signals on the surface of the protoplasts and the Ca(2+) oscillation assayed via the Ca(2+) probe Fluo-3/AM inside the protoplasts provided evidence that the potential GABA(B) receptors are present on the plant protoplast membrane.

Membrane lateral diffusion and capture of CFTR within transient confinement zones

The cystic fibrosis transmembrane conductance regulator (CFTR) channel interacts with scaffolding and other proteins that are expected to restrict its lateral movement, yet previous studies have reported predominantly free diffusion. We examined the lateral mobility of CFTR channels on live baby hamster kidney cells using three complementary methods. Channels bearing an extracellular biotinylation target sequence were labeled with streptavidin conjugated with fluorescent dyes (Alexa Fluor 488 or 568) or quantum dots (qDot605). Fluorescence recovery after photobleaching and image correlation spectroscopy of the dye-labeled channels revealed a significant immobile population ( approximately 50%), which was confirmed by direct single particle tracking (SPT) of qDot605-labeled CFTR. Adding 10 histidine residues at the C-terminus of CFTR to mask the postsynaptic density 95, Discs large, ZO-1 (PDZ) binding motif abolished its association with EBP50/NHERF1, reduced the immobile fraction, and increased mobility. Other interactions that are not normally detected on this timescale became apparent when binding of PDZ domain proteins was disrupted. SPT revealed that CFTR(His-10) channels diffuse randomly, become immobilized for periods lasting up to 1 min, and in some instances are recaptured at the same location. The impact of transient confinement on the measured diffusion using the three fluorescence techniques were assessed using computer simulations of the biological experiments. Finally, the impact of endosomal CFTR on mobility measurements was assessed by fluorescence correlation spectroscopy. These results reveal unexpected features of CFTR dynamics which may influence its ion channel activity.

Singlet oxygen generation from water-soluble quantum dot-organic dye nanocomposites

Water-soluble quantum dot-organic dye nanocomposites have been prepared via electrostatic interaction. We used CdTe quantum dots with diameters up to 3.4 nm, 2-aminoethanethiol as a stabilizer, and meso-tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP) as an organic dye. The photophysical properties of the nanocomposite have been investigated. The fluorescence of the parent CdTe quantum dot is largely suppressed. Instead, indirect excitation of the TSPP moiety leads to production of singlet oxygen with a quantum yield of 0.43. The nanocomposite is sufficiently photostable for biological applications.

Labeling and intracellular tracking of functionally active plasmid DNA with semiconductor quantum dots

Semiconductor nanocrystal quantum dots (QDs) allow long-term imaging in the cellular environment with high photostability. QD biolabeling techniques have previously been developed for tagging proteins and peptides as well as oligonucleotides. In this contribution, QD-decorated plasmid DNA was utilized for the first time for long-term intracellular and intranuclear tracking studies. Conjugation of plasmid DNA with phospholipid-coated QDs was accomplished using a peptide nucleic acid (PNA)-N-succinimidyl-3-(2-pyridylthio) propionate linker. Gel electrophoresis and confocal and atomic force microscopy (AFM) were used to confirm the structure of QD-DNA conjugates. AFM imaging also revealed that multiple QDs were attached in a cluster at the PNA-reactive site of the plasmid DNA. These QD-DNA conjugates were capable of expressing the reporter protein, enhanced green fluorescent protein, following transfection in Chinese hamster ovary (CHO-K1) cells with an efficiency of ca. 62%, which was comparable to the control (unconjugated) plasmid DNA.

The fluorescent toolbox for assessing protein location and function

Advances in molecular biology, organic chemistry, and materials science have recently created several new classes of fluorescent probes for imaging in cell biology. Here we review the characteristic benefits and limitations of fluorescent probes to study proteins. The focus is on protein detection in live versus fixed cells: determination of protein expression, localization, activity state, and the possibility for combination of fluorescent light microscopy with electron microscopy. Small organic fluorescent dyes, nanocrystals ("quantum dots"), autofluorescent proteins, small genetic encoded tags that can be complexed with fluorochromes, and combinations of these probes are highlighted.

A europium complex that selectively stains nucleoli of cells

A europium complex selectively staining the nucleolus of NIH 3T3, HeLa, and HDF cells is reported. This complex possesses not only the advantage of the long lifetime of europium emission (0.3 ms), but also a chromophore that allows excitation at a relatively long wavelength (lambda(max) = 384 nm) and gives rise to an acceptable quantum yield (9%). The complex can be used both in live cell and fixed cell imaging, giving an average intracellular concentration on the order of 0.5 microM. Strong binding to serum albumin has been demonstrated by examination of the analogous gadolinium complex, studying relaxivity changes with increasing protein concentration. The intracellular speciation of the complex has been examined by circularly polarized emission spectroscopy and is consistent with the presence of more than one europium species, possibly protein bound.

Optical technologies for the read out and quality control of DNA and protein microarrays

Microarray formats have become an important tool for parallel (or multiplexed) monitoring of biomolecular interactions. Surface-immobilized probes like oligonucleotides, cDNA, proteins, or antibodies can be used for the screening of their complementary targets, covering different applications like gene or protein expression profiling, analysis of point mutations, or immunodiagnostics. Numerous reviews have appeared on this topic in recent years, documenting the intriguing progress of these miniaturized assay formats. Most of them highlight all aspects of microarray preparation, surface chemistry, and patterning, and try to give a systematic survey of the different kinds of applications of this new technique. This review places the emphasis on optical technologies for microarray analysis. As the fluorescent read out of microarrays is dominating the field, this topic will be the focus of the review. Basic principles of labeling and signal amplification techniques will be introduced. Recent developments in total internal reflection fluorescence, resonance energy transfer assays, and time-resolved imaging are addressed, as well as non-fluorescent imaging methods. Finally, some label-free detection modes are discussed, such as surface plasmon microscopy or ellipsometry, since these are particularly interesting for microarray development and quality control purposes.

Cellular imaging in drug discovery

Traditional screening paradigms often focus on single targets. To facilitate drug discovery in the more complex physiological environment of a cell or organism, powerful cellular imaging systems have been developed. The emergence of these detection technologies allows the quantitative analysis of cellular events and visualization of relevant cellular phenotypes. Cellular imaging facilitates the integration of complex biology into the screening process, and addresses both high-content and high-throughput needs. This review describes how cellular imaging technologies contribute to the drug discovery process.

Advances in fluorescence imaging with quantum dot bio-probes

After much effort in surface chemistry development and optimization by several groups, fluorescent semiconductor nanocrystals probes, also known as quantum dots or qdots, are now entering the realm of biological applications with much to offer to biologists. The road to success has been paved with hurdles but from these efforts has stemmed a multitude of original surface chemistries that scientists in the biological fields can draw from for their specific biological applications. The ability to easily modulate the chemical nature of qdot surfaces by employing one or more of the recently developed qdot coatings, together with their exceptional photophysics have been key elements for qdots to acquire a status of revolutionary fluorescent bio-probes. Indeed, the unique properties of qdots not only give biologists the opportunity to explore advanced imaging techniques such as single molecule or lifetime imaging but also to revisit traditional fluorescence imaging methodologies and extract yet unobserved or inaccessible information in vitro or in vivo.

Multi-functional polymeric nanoparticles for tumour-targeted drug delivery

The use of nanoparticles as drug delivery vehicles for anticancer therapeutics has great potential to revolutionise the future of cancer therapy. As tumour architecture causes nanoparticles to preferentially accumulate at the tumour site, their use as drug delivery vectors results in the localisation of a greater amount of the drug load at the tumour site; thus improving cancer therapy and reducing the harmful nonspecific side effects of chemotherapeutics. In addition, formulation of these nanoparticles with imaging contrast agents provides a very efficient system for cancer diagnostics. Given the exhaustive possibilities available to polymeric nanoparticle chemistry, research has quickly been directed at multi-functional nanoparticles, combining tumour targeting, tumour therapy and tumour imaging in an all-in-one system, providing a useful multi-modal approach in the battle against cancer. This review will discuss the properties of nanoparticles that allow for such multiple functionality, as well as recent scientific advances in the area of multi-functional nanoparticles for cancer therapeutics.