Targeting cell surface receptors with ligand-conjugated nanocrystals

Quantum dot hybrid gel blotting: a technique for identifying quantum dot-protein/protein-protein interactions

We describe an alternative to the molecular biology technique of polyacrylamide gel electrophoresis-based Western blotting and immunoprecipitation, which is an extensively used method for separating target proteins from complex cellular mixtures and for identification of protein expression and protein-protein interactions. This novel method, called quantum dot (QD) hybrid gel blotting, allows the purification and analysis of the action of QD bioconjugate-protein complexes in live cells. Moreover, these identified interactions can be correlated with spatial location in cells. QD hybrid gel blotting will be useful in the growing fields of molecular biology/proteomics and nanobiotechnology development in several respects: (1) as a method for identifying specific QD-protein interactions in cells, (2) as a method for correlating QD-protein interactions with their spatial location in live cells, (3) as a means to study the size and composition of QD bioconjugate probes/complexes; and, finally, (4) as an improvement over traditional bead-based immunoprecipitation methods for directly isolating and visualizing proteins from complex mixtures.

Application of semiconductor and metal nanostructures in biology and medicine

Advances in nanotechnology research have led to the creation of new generation of contrast agents, therapeutics, and delivery systems. These applications are expected to significantly improve the diagnosis and treatment of a variety of diseases. Two nanotechnologies—semiconductor and metallic nanostructures—are the most advanced in this young field and have been extensively investigated for clinical use. These nanostructures are currently the “model” for the developments of many novel nanostructures. This review describes their chemical design, tunable properties, and utility in medicine. Furthermore, we will describe the current understanding of their toxicity, which could be barriers to their use for human.

Enhanced fluorescence images for labeled cells on silver island films

Silver island films (SIFs) were deposited on glass substrates to serve as supports. T-Lymphocytic (PM1) cell lines were labeled by Alexa Fluor 680-dextran conjugates on the membranes or by YOYO in the nuclei. The fluorescence images of the cell lines were recorded in the emission intensity and lifetime using scanning confocal microscopy. The fluorescence signals by the fluorophores bound on the cell membranes were enhanced significantly by SIF supports as compared with those on the glass. In addition to the increase in the intensity, there was a dramatic shortening of the emission lifetime. In contrast to the Alexa Fluor 680 fluorophores on the membranes, the YOYO fluorophores intercalated in the cell nuclei were not influenced significantly by the silver islands. This result can be interpreted by an effect of the distance on coupling between the fluorophores and metal particles: the fluorophores on the cell membranes are localized within, but the fluorophores in the cell nuclei are beyond the region of metal-enhanced fluorescence. Thus, the metal supports can be used to improve the detection sensitivity for target molecules on cell surfaces when they are fluorescently labeled.

Luminescent passive-oxidized silicon quantum dots as biological staining labels and their cytotoxicity effects at high concentration

Semiconductor quantum dots (QDs) hold some advantages over conventional organic fluorescent dyes. Due to these advantages, they are becoming increasingly popular in the field of bioimaging. However, recent work suggests that cadmium based QDs affect cellular activity. As a substitute for cadmium based QDs, we have developed photoluminescent stable silicon quantum dots (Si-QDs) with a passive-oxidation technique. Si-QDs (size: 6.5 ± 1.5 nm) emit green light, and they have been used as biological labels for living cell imaging. In order to determine the minimum concentration for cytotoxicity, we investigated the response of HeLa cells. We have shown that the toxicity of Si-QDs was not observed at 112 µg ml(-1) and that Si-QDs were less toxic than CdSe-QDs at high concentration in mitochondrial assays and with lactate dehydrogenase (LDH) assays. Especially under UV exposure, Si-QDs were more than ten times safer than CdSe-QDs. We suggest that one mechanism for the cytotoxicity is that Si-QDs can generate oxygen radicals and these radicals are associated with membrane damages. This work has demonstrated the suitability of Si-QDs for bioimaging in lower concentration, and their cytotoxicity and one toxicity mechanism at high concentration.

Semiconductor quantum dots and metal nanoparticles: syntheses, optical properties, and biological applications

We review the syntheses, optical properties, and biological applications of cadmium selenide (CdSe) and cadmium selenide-zinc sulfide (CdSe-ZnS) quantum dots (QDs) and gold (Au) and silver (Ag) nanoparticles (NPs). Specifically, we selected the syntheses of QDs and Au and Ag NPs in aqueous and organic phases, size- and shape-dependent photoluminescence (PL) of QDs and plasmon of metal NPs, and their bioimaging applications. The PL properties of QDs are discussed with reference to their band gap structure and various electronic transitions, relations of PL and photoactivated PL with surface defects, and blinking of single QDs. Optical properties of Ag and Au NPs are discussed with reference to their size- and shape-dependent surface plasmon bands, electron dynamics and relaxation, and surface-enhanced Raman scattering (SERS). The bioimaging applications are discussed with reference to in vitro and in vivo imaging of live cells, and in vivo imaging of cancers, tumor vasculature, and lymph nodes. Other aspects of the review are in vivo deep tissue imaging, multiphoton excitation, NIR fluorescence and SERS imaging, and toxic effects of NPs and their clearance from the body.

Ligand-bound quantum dot probes for studying the molecular scale dynamics of receptor endocytic trafficking in live cells

Endocytic receptor trafficking is a complex, dynamic process underlying fundamental cell function. An integrated understanding of endocytosis at the level of single or small numbers of ligand bound-receptor complexes inside live cells is currently hampered by technical limitations. Here, we develop and test ligand nerve growth factor-bound quantum dot (NGF-QD) bioconjugates for imaging discrete receptor endocytic events inside live NGF-responsive PC12 cells. Using single particle tracking, QD hybrid gel coimmunoprecipitation, and immuno-colocalization, we illustrate and validate the use of QD-receptor complexes for imaging receptor trafficking at synchronized time points after QD-ligand-receptor binding and internalization (t = 15-150 min). The unique value of these probes is illustrated by new dynamic observations: (1) that endocytosis proceeds at strikingly regulated fashion, and (2) that diffusive and active forms of transport inside cells are rapid and efficient. QDs are powerful intracellular probes that can provide biologists with new capabilities and fresh insight for studying endocytic receptor signaling events, in real time, and at the resolution of single or small numbers of receptors in live cells.

Intracellular delivery of nanoparticles via the HIV-1 tat peptide

Functionalized nanoparticles are heralded as part of the future with regards to targeted cell and nuclear delivery. However, direct intracellular and intranuclear delivery has, until recently, been difficult to achieve owing to the impermeable nature of the plasma and nuclear membranes. During the past 15 years, a range of peptides, termed cell-penetrating peptides (CPPs), which have the ability to translocate into living cells, have been discovered. Thus, in more recent years, the combination of CPPs with nanoparticles, enabling CPP-mediated cell delivery, has opened up many avenues of research. This review discusses the use of various CPPs, focusing on tat peptide, to functionalize nanoparticles and the possible move from the laboratory to the clinic.

Nanoparticles as fluorescence labels: is size all that matters?

Fluorescent labels are often used in bioassays as a means to detect and characterize ligand-receptor binding. This is due in part to the inherently high sensitivity of fluorescence-based technology and the relative accessibility of the technique. There is often little concern raised as to whether or not the fluorescent label itself affects the ligand-receptor binding dynamics and equilibrium. This may be particularly important when considering nanoparticle labels. In this study, we examine the affects of nanoparticle (quantum dots and polymer nanospheres) fluorescent labels on the streptavidin-biotin binding system. Since the nanoparticle labels are larger than the species they tag, one could anticipate significant perturbation of the binding equilibrium. We demonstrate, using fluorescence cross-correlation spectroscopy, that although the binding equilibria do change, the relative changes are largely predictable. We suggest that the nanoparticles' mesoscopic size and surface tension effects can be used to explain changes in streptavidin-biotin binding.

Single-cell fluorescence imaging using metal plasmon-coupled probe 2: single-molecule counting on lifetime image

Multiple Alexa Fluor 647-conjugated concanavalin A (conA) molecules were covalently bound to a single 20 nm silver particle to synthesize metal plasmon-coupled probes (PCPs). The fluorescence images were recorded by scanning confocal microscopy in both intensity and lifetime. The brightness of PCPs was 30-fold brighter than those of free conA and the lifetime of PCPs was shortened dramatically. PCPs were used to label T-lymphocytic ( PM1) cells. The emission spots by PCPs bound on the cell surfaces were separated clearly from the cell images by autofluorescence due to the brighter signal and shorter lifetime of PCPs. The emission spots by PCPs were also scanned in three dimensions to count the distribution of bound fluorophores on the cell surfaces. The metal-associated fluorophores thus are suggested using as novel molecular imaging agents to quantify the components and describe their distributions on the cell surfaces.

Quantum dot ex vivo labeling of neuromuscular synapses

Nicotinic receptors (nAchRs) are responsible for fast excitatory signaling by the neurotransmitter acetylcholine (Ach). They are present on the postsynaptic membrane at neuromuscular junctions (NMJs) and also at brain synapses. Alpha-bungarotoxin (alpha-BTX), a high-affinity nAchR antagonist, inhibits Ach binding and neurotransmission. Here we demonstrate biotinylated alpha-BTX, bound to native mouse diaphragm nAchRs, can be quantified and visualized ex vivo using streptavidin-conjugated quantum dots. This approach provides a novel methodology for the direct assessment of the presence and mobility of neurotransmitter receptors in native tissue.

In vivo quantum dot labeling of mammalian stem and progenitor cells

Fluorescent semiconductor nanocrystal quantum dots (QDs) are a class of multifunctional inorganic fluorophores that hold great promise for clinical applications and biomedical research. Because no methods currently exist for directed QD-labeling of mammalian cells in the nervous system in vivo, we developed novel in utero electroporation and ultrasound-guided in vivo delivery techniques to efficiently and directly label neural stem and progenitor cells (NSPCs) of the developing mammalian central nervous system with QDs. Our initial safety and proof of concept studies of one and two-cell QD-labeled mouse embryos reveal that QDs are compatible with early mammalian embryonic development. Our in vivo experiments further show that in utero labeled NSPCs continue to develop in an apparent normal manner. These studies reveal that QDs can be effectively used to label mammalian NSPCs in vivo and will be useful for studies of in vivo fate mapping, cellular migration, and NSPC differentiation during mammalian development.

Targeting the human serotonin transporter (hSERT) with quantum dots

In this paper we report our work on the development of a human serotonin transporter (hSERT) antagonist that can be conjugated to quantum dots. This approach has been used to target and visualize the human serotonin transporter protein (hSERT). We demonstrate that labeling is blocked by the addition of high affinity hSERT antagonists such as paroxetine. This approach may be useful for the development of fluorescent assays to study the location and temporal dynamics of biogenic amine transporters and also holds promise for the development of plate-based high throughput assays used to identify novel transporter antagonists.

Quantum dots – Nano-sized probes for the exploration of cellular and intracellular targeting

Nanoparticles emerged as promising tool in drug targeting, since, after appropriate modification, they are able to deliver their payload to specific sites, like tissues, cells, or even certain cellular organelles. In this context, the delivery of nanoparticles from the circulation into the target cells represents a crucial step. Here, model drug delivery systems such as quantum dots are ideal candidates to elucidate this process in more detail, since they provide outstanding features like a small and uniform size, unique optical properties for most sensitive detection and modifiable surfaces. Recent progress in the surface chemistry of quantum dots expanded their use in biological applications, reduced their cytotoxicity and rendered quantum dots a powerful tool for the investigation of distinct cellular processes, like uptake, receptor trafficking and intracellular delivery. In this review, we will not only describe the ideal attributes of QDs for biological applications and imaging but also their distinct specific and non-specific pathways into the cells as well as their intracellular fate.

Validation of a fluorescence-based high-throughput assay for the measurement of neurotransmitter transporter uptake activity

Pre-synaptic dopamine, norepinephrine and serotonin transporters (DAT, NET and SERT) terminate synaptic catecholamine transmission through reuptake of released neurotransmitter. Common approaches for studying these transporters involve radiolabeled substrates or inhibitors which, however, have several limitations. In this study we have used a novel neurotransmitter transporter uptake assay kit. The assay employs a fluorescent substrate that mimics the biogenic amine neurotransmitters and is taken up by the cell through the specific transporters, resulting in increased fluorescence intensity. In order to validate the assay, a variety of reference and proprietary neurotransmitter transporter ligands from a number of chemical and pharmacological classes were tested. The ability of these compounds to inhibit the selective transporter-mediated uptake demonstrated a similar rank order of potency and IC(50) values close to those obtained in radiolabeled neurotransmitter uptake assays. The described assay enables monitoring of dynamic transport activity of DAT, NET and SERT and is amenable for high-throughput screening and compound characterization.

Cholera toxin B conjugated quantum dots for live cell labeling

Cholera toxin subunit B (CTB)--quantum dot conjugates were developed for labeling mammalian cells. The conjugates were internalized by all tested cell lines into small vesicles dispersed throughout the cytoplasm, while commercially available polyarginine conjugates rapidly accumulated in large perinuclear endosomes. Although a large proportion of CTB conjugates eventually also accumulated in perinuclear endosomes, this accumulation required several days, and even then many CTB conjugated quantum dots remained in small vesicles dispersed throughout the cytoplasm. Thus CTB conjugates are a practical alternative to polyarginine conjugates for the general labeling of mammalian cells.

Light scattering sensing detection of pathogens based on the molecular recognition of immunoglobulin with cell wall-associated protein A

In this contribution, we report a rapid optical detection method of pathogens using Staphylococcus aureus (S. aureus) as the model analyte based on the molecular recognition of immunoglobulin with cell wall-associated Protein A (SpA). It was found that the molecular recognition of human immunoglobulin (IgG) with protein A on the cell wall of S. aureus on glass slide sensing area could result in strong surface enhanced light scattering (SELS) signals, and the SELS intensity (deltaI) increases proportionally with the concentration of S. aureus over the range of 2.5x10(5)-1.0x10(8) CFU mL(-1) with right angle light scattering (RALS) signals detection mode. In order to identify the solid support based molecular recognition between IgG with SpA, we also employed water-soluble CdS quantum dots (CdS-QDs) as a fluorescent marker for IgG by immobilizing the IgG onto the surfaces of CdS-QDs through covalent binding in order to generate recognition probes for SpA on the cell wall of S. aureus. Consequently, the fluorescent method also showed that the detection for pathogens with solid supports is reliable based on the molecular recognition of IgG with SpA.

Homogeneously alloyed CdSxSe1-x nanocrystals: synthesis, characterization, and composition/size-dependent band gap

Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions. The nanocrystals are characterized using UV-visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods.

Synthesis and characterization of a pegylated derivative of 3-(1,2,3,6-tetrahydro-pyridin-4yl)-1H-indole (IDT199): a high affinity SERT ligand for conjugation to quantum dots

Quantum dots consisting of a cadmium selenide core encapsulated in a shell of cadmium doped zinc sulfide have the potential to revolutionize fluorescent imaging of live cell cultures. In order to utilize these fluorescent probes it is necessary to functionalize them with biologically active ligands. In this paper we report the design and synthesis of a ligand that has a high affinity for the serotonin transporter (SERT) that may be conjugated to quantum dots.

Silica-coated Ln3+-Doped LaF3 nanoparticles as robust down- and upconverting biolabels

The preparation of nearly monodisperse (40 nm), silica-coated LaF(3):Ln(3+) nanoparticles and their bioconjugation to FITC-avidin (FITC=fluorescein isothiocyanate) is described in this report. Doping of the LaF(3) core with selected luminescent Ln(3+) ions allows the particles to display a range of emission lines from the visible to the near-infrared region (lambda=450-1650 nm). First, the use of Tb(3+) and Eu(3+) ions resulted in green (lambda=541 nm) and red (lambda=591 and 612 nm) emissions, respectively, by energy downconversion processes. Second, the use of Nd(3+) gave emission lines at lambda=870, 1070 and 1350 nm and Er(3+) gave an emission line at lambda=1540 nm by energy downconversion processes. Additionally, the Er(3+) ions gave green and red emissions and Tm(3+) ions gave an emission at lambda=800 nm by upconversion processes when codoped with Yb(3+) (lambda(ex)=980 nm). Bioconjugation of avidin, which has a bound fluorophore (FITC) as the reporter, was carried out by means of surface modification of the silica particles with 3-aminopropyltrimethoxysilane, followed by reaction with the biotin-N-hydroxysuccinimide activated ester to form an amide bond, imparting biological activity to the particles. A 25-fold or better increase in the FITC signal relative to the non-biotinylated silica particles indicated that there is minimal nonspecific binding of FITC-avidin to the silica particles.

Magnetic Field Switching of Nanoparticles between Orthogonal Microfluidic Channels

This paper reports on the manipulation of magnetic nanoparticles between microfluidic channels by the application of an external magnet. Two orthogonal channels were prepared using standard PDMS techniques with pressure-driven flow used to deliver the mobile phase. To study the ability to control magnetic nanoparticles within micrometer-sized channels, Fe2O3, MnFe2O4, and Au nanoparticle samples were compared. For the magnetic particles, transfer between flow streams is greatly increased by placing a permanent magnet beneath the intersection of the channels, but no change is observed for the nonmagnetic Au particles. More nanoparticles are magnetically transferred into the orthogonal channel as the solvent flow rate decreases. We demonstrate the ability to use this technique to perform multiple injections of plugs of magnetic particles by periodic application of a magnetic field.

Are quantum dots ready for in vivo imaging in human subjects?

Nanotechnology has the potential to profoundly transform the nature of cancer diagnosis and cancer patient management in the future. Over the past decade, quantum dots (QDs) have become one of the fastest growing areas of research in nanotechnology. QDs are fluorescent semiconductor nanoparticles suitable for multiplexed in vitro and in vivo imaging. Numerous studies on QDs have resulted in major advancements in QD surface modification, coating, biocompatibility, sensitivity, multiplexing, targeting specificity, as well as important findings regarding toxicity and applicability. For in vitro applications, QDs can be used in place of traditional organic fluorescent dyes in virtually any system, outperforming organic dyes in the majority of cases. In vivo targeted tumor imaging with biocompatible QDs has recently become possible in mouse models. With new advances in QD technology such as bioluminescence resonance energy transfer, synthesis of smaller size non-Cd based QDs, improved surface coating and conjugation, and multifunctional probes for multimodality imaging, it is likely that human applications of QDs will soon be possible in a clinical setting.

Single cell fluorescence imaging using metal plasmon-coupled probe

This work constitutes the first fluorescent imaging of cells using metal plasmon-coupled probes (PCPs) at single cell resolution. N-(2-Mercapto-propionyl)glycine-coated silver nanoparticles were synthesized by reduction of silver nitrate using sodium borohyride and then succinimidylated via ligand exchange. Alexa Fluor 647-labeled concanavalin A (con A) was chemically bound to the silver particles to make the fluorescent metal plasmon-coupled probes. The fluorescence images were collected using a scanning confocal microscopy. The fluorescence intensity was observed to enhance 7-fold when binding the labeled con A on a single silver particle. PCPs were conjugated on HEK 293 A cells. Imaging results demonstrate that cells labeled by PCPs were 20-fold brighter than those by free labeled con A.

Identification of quantum dot bioconjugates and cellular protein co-localization by hybrid gel blotting

New approaches are needed to address the interaction of nanoparticles and cellular proteins at the molecular level. We present a modification of PAGE co-immunoprecipitation, QD-based PA-AGE electrophoresis blotting, and apply this to identify quantum dot (QD) bioconjugate-cellular protein association. This method provides the capability to isolate and evaluate the action of QD bioconjugate-protein complexes in intact cells and to correlate these identified interactions with their location in cells.

Detection of single bacterial pathogens with semiconductor quantum dots

Semiconductor quantum dots (QDs) have been used in a simple fluorometric assay to detect single cells of the pathogenic Escherichia coli O157:H7 serotype. Composed of CdSe/ZnS core/shell QDs conjugated to streptavidin, this system exhibits 2 orders of magnitude more sensitivity than a similar assay using a common organic dye. Selectivity for this pathogenic bacterial strain over a common lab strain (E. coli DH5alpha), which is gained from the use of specific biotinylated antibodies, is also demonstrated for QD labeling. Under continuous excitation, these QDs retain high fluorescence intensities for hours, whereas a typical organic dye bleaches within seconds, allowing for more rapid and accurate identification of E. coli O157:H7 in single-cell fluorescence-based assays. This indirect QD labeling method, based on antibody-antigen and streptavidin-biotin interactions, is flexible enough to expand to other systems and has great potential for use in simultaneous multicolor detection schemes.

Binding of muscimol-conjugated quantum dots to GABAC receptors

Functionalization of highly fluorescent CdSe/ZnS core-shell nanocrystals (quantum dots, qdots) is an emerging technology for labeling cell surface proteins. We have synthesized a conjugate consisting of approximately 150-200 muscimols (a GABA receptor agonist) covalently joined to the qdot via a poly(ethylene glycol) (PEG) linker (approximately 78 ethylene glycol units) and investigated the binding of this muscimol-PEG-qdot conjugate to homomeric rho1 GABAC receptors expressed in Xenopus oocytes. GABAC receptors mediate inhibitory synaptic signaling at multiple locations in the central nervous system (CNS). Binding of the conjugate was analyzed quantitatively by determining the fluorescence intensity of the oocyte surface membrane in relation to that of the surrounding incubation medium. Upon 5- to 10-min incubation with muscimol-PEG-qdots (34 nM in qdot concentration), GABAC-expressing oocytes exhibited a fluorescent halo at the surface membrane that significantly exceeded the fluorescence of the incubation medium. This halo was absent following muscimol-PEG-qdot treatment of oocytes lacking GABAC receptors. Incubation of the oocyte with free muscimol (100 microM-5 mM), PEG-muscimol (500 microM), or GABA (100 microM - 5 mM) substantially reduced or eliminated the fluorescence halo produced by muscimol-PEG-qdots, and the removal of GABA or free muscimol led to a recovery of muscimol-PEG-qdot binding. Unconjugated qdots and PEG-qdots that lacked conjugated muscimol neither exhibited significant binding activity nor diminished the subsequent binding of muscimol-PEG-qdots. The results indicate that muscimol joined to qdots via a long-chain PEG linker exhibits specific binding activity at the ligand-binding pocket of expressed GABAC receptors, despite the presence of both the long PEG linker and the sterically bulky qdot.

Preliminary studies of application of CdTe nanocrystals and dextran–Fe3O4 magnetic nanoparticles in sandwich immunoassay

BACKGROUND

The favorable properties of water-soluble CdTe nanocrystals as novel biological luminescent label over conventional fluorescent probes have attracted considerable interest. The magnetic separation technique has widely been applied to various aspects in biotechnology in recent years. In this paper, we made use of CdTe nanocrystals and dextran-Fe3O4 magnetic nanoparticles for fluorescence immunoassay.

METHODS

The CdTe nanocrystals and dextran-Fe3O4 magnetic nanoparticles were applied to immunoassay for the determination of human immunoglobulin G (HIgG). A rabbit anti-HIgG antibody (primary antibody) was immobilized on magnetic nanoparticles, which was used as a solid support. A sheep anti-HIgG antibody (secondary antibody) was attached to the surface of the CdTe nanocrystals via electrostatic interaction. The immunoassay was based on a sandwich immunoreaction of primary antibody on the magnetic nanoparticles, HIgG (or serum sample), and the secondary antibody labeled with CdTe nanocrystals.

RESULTS

The CdTe label was compared with a fluorescein isothiocyanate (FITC) label. The CdTe is an order of magnitude more sensitive than the FITC. The immunoassay method was applied to determining the HIgG in practical samples and the results obtained are in good agreement with those obtained by nephelometry.

CONCLUSION

This technique may be applied in many types of antibody-antigen system.

Tissue and species differences in the application of quantum dots as probes for biomolecular targets in the inner ear and kidney

Quantum dots (QDs) are useful biological probes because of the increased photostability and quantum efficiency they offer over organic fluorophores. However, toxicity concerns arise because the QD core is composed of cadmium and selenium, metals known to be unsafe for humans and animals. We investigated the feasibility of quantum dots as biological labels for imaging studies of inner ear and kidney, tissues that share a polarized epithelial arrangement and drug susceptibility. We found that methods for labeling the actin cytoskeleton of monolayers of cultured amphibian kidney cells (Xenopus A6) with 565 nm QD conjugates were not feasible with large Xenopus inner ear organs. We then compared the uptake of 565 nm cationic peptide-targeted and nontargeted QDs in live kidney cell lines (amphibian, A6 and XLK-WG; human, HEK-293). Results showed that targeted QDs are internalized by all three kidney cell lines, and that nontargeted CdSe nanocrystals are sequestered only by human kidney cells. CellTracker Red CMTPX confirmed the membrane integrity and viability of HEK-293 cells that internalized QDs. Our results demonstrate species and tissue differences in QD uptake and labeling, and underscore the need for long-term studies of QD toxicity and fate in cells.

Fluorescence correlation spectroscopy using quantum dots: advances, challenges and opportunities

Semiconductor nanocrystals (quantum dots) have been increasingly employed in measuring the dynamic behavior of biomacromolecules using fluorescence correlation spectroscopy. This poses a challenge, because quantum dots display their own dynamic behavior in the form of intermittent photoluminescence, also known as blinking. In this review, the manifestation of blinking in correlation spectroscopy will be explored, preceded by an examination of quantum dot blinking in general.

Evidence for significant contribution of a newly identified monoamine transporter (PMAT) to serotonin uptake in the human brain

The high affinity serotonin transporter (SERT) constitutes the principal pathway for removal of serotonin (5-HT) from extracellular fluid of brain, but evidence indicates that other transporters may also be involved in this process. We recently reported the cloning of a novel plasma membrane monoamine transporter (PMAT), which is abundantly expressed in the human brain and avidly transports 5-HT [Engel K, Zhou M, Wang J. Identification and characterization of a novel monoamine transporter in the human brain. J Biol Chem 2004;279:50042-9]. In this study, we evaluated whether PMAT contributes to total human brain uptake of 5-HT using a hybrid depletion approach in Xenopus laevis oocytes. We also examined whether PMAT interacts with selective serotonin reuptake inhibitors (SSRIs) using MDCK cells stably expressing recombinant human PMAT. Microinjection of total human brain poly(A)(+) mRNA into oocytes elicited approximately 2.5-3-fold increase in 5-HT uptake. Pre-hybridization of poly(A)(+) mRNA with PMAT or SERT antisense oligonucleotides significantly reduced mRNA-induced 5-HT uptake. An additive inhibitory effect was observed when poly(A)(+) mRNA was co-hybridized with both PMAT and SERT antisense oligonucleotides. In contrast, mRNA-induced 5-HT uptake was not affected by pre-hybridization with sense oligonucleotides. These data suggest that functional transcripts of PMAT are present in the human brain, and the PMAT transporter may be significantly involved in brain uptake of 5-HT. All five tested SSRIs inhibited PMAT with IC(50) values ranging from 11 to 116 microM, which are much greater than clinically encountered concentrations, suggesting that PMAT activity is minimally affected by SSRI therapies.

Quantum Dot Probes for Monitoring Dynamic Cellular Response: Reporters of T Cell Activation

Antibody-conjugated quantum dots (QDs) have been used to map the expression dynamics of the cytokine receptor interleukin-2 receptor-alpha (IL-2Ralpha) following Jurkat T cell activation. Maximal receptor expression was observed 48 h after activation, followed by a sharp decrease consistent with IL-2R internalization subsequent to IL-2 engagement. Verification of T cell activation and specificity of QD labeling were demonstrated using fluorescence microscopy, ELISA, and FACS analyses. These antibody conjugates provide a versatile means to rapidly determine cell state and interrogate membrane associated proteins involved in cell signaling pathways. Ultimately, incorporation with a microfluidic platform capable of simultaneously monitoring several cell signaling pathways will aid in toxin detection and discrimination.

Quantum dot nanocrystals for in vivo molecular and cellular imaging

Semiconductor quantum dots (QD) are nanometer-sized crystals with unique photochemical and photophysical properties that are not available from either isolated molecules or bulk solids. In comparison with organic dyes and fluorescent proteins, QD are emerging as a new class of fluorescent labels with improved brightness, resistance against photobleaching and multicolor fluorescence emission. These properties could improve the sensitivity of biological detection and imaging by at least 10- to 100-fold. Further development in high-quality near-infrared-emitting QD should allow ultrasensitive and multicolor imaging of molecular targets in deep tissue and living animals. Here, we discuss recent developments in QD synthesis and bioconjugation, applications in molecular and cellular imaging as well as promising directions for future research.

Use of quantum dot luminescent probes to achieve single-cell resolution of human oral bacteria in biofilms

Oral biofilms are multispecies communities, and in their nascent stages of development, numerous bacterial species engage in interspecies interactions. Better insight into the spatial relationship between different species and how species diversity increases over time can guide our understanding of the role of interspecies interactions in the development of the biofilms. Quantum dots (QD) are semiconductor nanocrystals and have emerged as a promising tool for labeling and detection of bacteria. We sought to apply QD-based primary immunofluorescence for labeling of bacterial cells with in vitro and in vivo biofilms and to compare this approach with the fluorophore-based primary immunofluorescence approach we have used previously. To investigate QD-based primary immunofluorescence as the means to detect distinct targets with single-cell resolution, we conjugated polyclonal and monoclonal antibodies to the QD surface. We also conducted simultaneous QD conjugate-based and fluorophore conjugate-based immunofluorescence and showed that these conjugates were complementary tools in immunofluorescence applications. Planktonic and biofilm cells were labeled effectively by considering two factors: the final nanomolar concentration of QD conjugate and the amount of antibody conjugated to the QD, which we define as the degree of labeling. These advances in the application of QD-based immunofluorescence for the study of biofilms in vitro and in vivo will help to define bacterial community architecture and to facilitate investigations of interactions between bacterial species in these communities.

The use of quantum dots for analysis of chick CAM vasculature

Quantum dots (QDs) are fluorescent semiconductor nanocrystals that possess a number of superior fluorescent properties compared to more established organic dyes and fluorescent proteins. As a result, QDs are being studied for use in a wide range of biological applications. We have examined QDs for one such application, visualization of blood vessels of the chick chorioallantoic membrane (CAM), a popular model for studying various aspects of blood vessel development including angiogenesis. Intravitally injected QDs were found to be biocompatible and were kept in circulation over the course of 4 days without any observed deleterious effects. QD vascular residence time was tunable through QD surface chemistry modification. We also found that use of QDs with higher emission wavelengths (>655 nm) virtually eliminated all chick-derived autofluorescence and improved depth-of-field imaging. QDs were compared to FITC-dextrans, a fluorescent dye commonly used for imaging CAM vessels. QDs were found to image vessels as well as or better than FITC-dextrans at 2-3 orders of magnitude lower concentration. We also demonstrated that QDs are fixable with low fluorescence loss and thus can be used in conjunction with histological processing for further sample analysis.

Engineering challenges of BioNEMS: the integration of microfluidics, micro- and nanodevices, models and external control for systems biology

Systems biology, i.e. quantitative, postgenomic, postproteomic, dynamic, multiscale physiology, addresses in an integrative, quantitative manner the shockwave of genetic and proteomic information using computer models that may eventually have 10(6) dynamic variables with non-linear interactions. Historically, single biological measurements are made over minutes, suggesting the challenge of specifying 10(6) model parameters. Except for fluorescence and micro-electrode recordings, most cellular measurements have inadequate bandwidth to discern the time course of critical intracellular biochemical events. Micro-array expression profiles of thousands of genes cannot determine quantitative dynamic cellular signalling and metabolic variables. Major gaps must be bridged between the computational vision and experimental reality. The analysis of cellular signalling dynamics and control requires, first, micro- and nano-instruments that measure simultaneously multiple extracellular and intracellular variables with sufficient bandwidth; secondly, the ability to open existing internal control and signalling loops; thirdly, external BioMEMS micro-actuators that provide high bandwidth feedback and externally addressable intracellular nano-actuators; and, fourthly, real-time, closed-loop, single-cell control algorithms. The unravelling of the nested and coupled nature of cellular control loops requires simultaneous recording of multiple single-cell signatures. Externally controlled nano-actuators, needed to effect changes in the biochemical, mechanical and electrical environment both outside and inside the cell, will provide a major impetus for nanoscience.

Universal polyethylene glycol linkers for attaching receptor ligands to quantum dots

Biologically active small molecule derivatives that can be conjugated to quantum dots have the promise of revolutionizing fluorescent imaging in biology. In order to achieve this several technical hurdles have to be surmounted, one of which is non-specific adsorption of quantum dots to cell membranes. Pegylating quantum dots has been shown to eliminate non-specific binding. Consequently it is necessary to develop a universal synthetic methodology to attach small molecule ligands to polyethylene glycol. These pegylated small molecules may then be conjugated to the surfaces of quantum dots. Ideally this universal strategy should be adaptable and be applicable to PEG chains of varying lengths. This paper describes the development of one such methodology and the synthesis of a pegylated derivative of the known 5HT(2) agonist 1-(2-aminopropyl)-2,5-dimethoxy benzene. This compound was tested and found to be an agonist for the 5HT(2A) and 5HT(2C) receptor having EC(50) values of 250 and 50 nM, respectively.

Quantum dots monitor TrkA receptor dynamics in the interior of neural PC12 cells

Can quantum dots (QDs) serve as physiologically relevant receptor probes in the interior of cells? We directly visualize endocytosis, redistribution, and shuttling of QD bound-TrkA receptors to PC12 neural processes and far-reaching growth cone tips. Internalized QDs are contained in microtubule-associated vesicles and possess transport properties that reflect TrkA receptor dynamics. This opens up new possibilities for the development of QD platforms as molecular tools to image biochemical signaling and transport cargo in the cell interior.

Atomic force microscopy-based cell nanostructure for ligand-conjugated quantum dot endocytosis

While it has been well demonstrated that quantum dots (QDs) play an important role in biological labeling both in vitro and in vivo, there is no report describing the cellular nanostructure basis of receptor-mediated endocytosis. Here, nanostructure evolution responses to the endocytosis of transferrin (Tf)-conjugated QDs were characterized by atomic force microscopy (AFM). AFM-based nanostructure analysis demonstrated that the Tf-conjugated QDs were specifically and tightly bound to the cell receptors and the nanostructure evolution is highly correlated with the cell membrane receptor-mediated transduction. Consistently, confocal microscopic and flow cytometry results have demonstrated the specificity and dynamic property of Tf-QD binding and internalization. We found that the internalization of Tf-QD is linearly related to time. Moreover, while the nanoparticles on the cell membrane increased, the endocytosis was still very active, suggesting that QD nanoparticles did not interfere sterically with the binding and function of receptors. Therefore, ligand-conjugated QDs are potentially useful in biological labeling of cells at a nanometer scale.

In vitro study of drug accumulation in cancer cells via specific association with CdS nanoparticles

We report a novel approach to enhance the efficient accumulation and utilization of anticancer drug daunorubicin on cancer cells through the combination with CdS nanoparticles. Our observations using confocal fluorescence scanning microscopy as well as electrochemical analysis methods demonstrate that CdS nanoparticles can readily bind with daunorubicin on the external membrane of the targeted cells and facilitate the uptake of drug molecules in the human leukemia K562 cells. Besides, our results also indicate that the competitive binding of CdS nanoparticles with accompanying anticancer drug to the membrane of leukemia K562 cells could efficiently prevent the drug release by the drug-sensitive and drug-resistant leukemia cells and thus inhibit the possible multidrug resistance of cancer cells, which could be further utilized to improve the future drug efficiency in respective tumor chemotherapies.