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Articles by Roger Y. Tsien in JoVE

 JoVE Editorial

The 2009 Lindau Nobel Laureate Meeting: Roger Y. Tsien, Chemistry 2008


JoVE 1575

American biochemist Roger Tsien shared the 2008 Nobel Prize in Chemistry with Martin Chalfie and Osamu Shimomura for their discovery and development of the Green Fluorescent Protein (GFP). Tsien dramatically improved the wild-type GFP resulting in increased fluorescence, increased photostability, and a shift in the major excitation peak to 488 nm (matching FITC).

Other articles by Roger Y. Tsien on PubMed

Multicolor and Electron Microscopic Imaging of Connexin Trafficking

Recombinant proteins containing tetracysteine tags can be successively labeled in living cells with different colors of biarsenical fluorophores so that older and younger protein molecules can be sharply distinguished by both fluorescence and electron microscopy. Here we used this approach to show that newly synthesized connexin43 was transported predominantly in 100- to 150-nanometer vesicles to the plasma membrane and incorporated at the periphery of existing gap junctions, whereas older connexins were removed from the center of the plaques into pleiomorphic vesicles of widely varying sizes. Selective imaging by correlated optical and electron microscopy of protein molecules of known ages will clarify fundamental processes of protein trafficking in situ.

Partitioning of Lipid-modified Monomeric GFPs into Membrane Microdomains of Live Cells

Many proteins associated with the plasma membrane are known to partition into submicroscopic sphingolipid- and cholesterol-rich domains called lipid rafts, but the determinants dictating this segregation of proteins in the membrane are poorly understood. We suppressed the tendency of Aequorea fluorescent proteins to dimerize and targeted these variants to the plasma membrane using several different types of lipid anchors. Fluorescence resonance energy transfer measurements in living cells revealed that acyl but not prenyl modifications promote clustering in lipid rafts. Thus the nature of the lipid anchor on a protein is sufficient to determine submicroscopic localization within the plasma membrane.

New Biarsenical Ligands and Tetracysteine Motifs for Protein Labeling in Vitro and in Vivo: Synthesis and Biological Applications

We recently introduced a method (Griffin, B. A.; Adams, S. R.; Tsien, R. Y. Science 1998, 281, 269-272 and Griffin, B. A.; Adams, S. R.; Jones, J.; Tsien, R. Y. Methods Enzymol. 2000, 327, 565-578) for site-specific fluorescent labeling of recombinant proteins in living cells. The sequence Cys-Cys-Xaa-Xaa-Cys-Cys, where Xaa is an noncysteine amino acid, is genetically fused to or inserted within the protein, where it can be specifically recognized by a membrane-permeant fluorescein derivative with two As(III) substituents, FlAsH, which fluoresces only after the arsenics bind to the cysteine thiols. We now report kinetics and dissociation constants ( approximately 10(-11) M) for FlAsH binding to model tetracysteine peptides. Affinities in vitro and detection limits in living cells are optimized with Xaa-Xaa = Pro-Gly, suggesting that the preferred peptide conformation is a hairpin rather than the previously proposed alpha-helix. Many analogues of FlAsH have been synthesized, including ReAsH, a resorufin derivative excitable at 590 nm and fluorescing in the red. Analogous biarsenicals enable affinity chromatography, fluorescence anisotropy measurements, and electron-microscopic localization of tetracysteine-tagged proteins.

A New Form of Cerebellar Long-term Potentiation is Postsynaptic and Depends on Nitric Oxide but Not CAMP

Long-term depression (LTD) at cerebellar parallel fiber (PF)-Purkinje cell synapses must be balanced by long-term potentiation (LTP) to prevent saturation and allow reversal of motor learning. The only previously analyzed form of cerebellar LTP is induced by 4-8 Hz PF stimulation and requires cAMP but not nitric oxide. It is a poor candidate to reverse LTD because it is presynaptically expressed whereas LTD is postsynaptic. We now characterize a new form of LTP induced by 1 Hz PF stimulation for at least 300 s. This LTP is postsynaptically expressed, enhanced by chelating postsynaptic Ca(2+), and depends on nitric oxide but not cAMP or cGMP, making it a plausible anti-Hebbian counterpart to Hebbian LTD.

A Monomeric Red Fluorescent Protein

All coelenterate fluorescent proteins cloned to date display some form of quaternary structure, including the weak tendency of Aequorea green fluorescent protein (GFP) to dimerize, the obligate dimerization of Renilla GFP, and the obligate tetramerization of the red fluorescent protein from Discosoma (DsRed). Although the weak dimerization of Aequorea GFP has not impeded its acceptance as an indispensable tool of cell biology, the obligate tetramerization of DsRed has greatly hindered its use as a genetically encoded fusion tag. We present here the stepwise evolution of DsRed to a dimer and then either to a genetic fusion of two copies of the protein, i.e., a tandem dimer, or to a true monomer designated mRFP1 (monomeric red fluorescent protein). Each subunit interface was disrupted by insertion of arginines, which initially crippled the resulting protein, but red fluorescence could be rescued by random and directed mutagenesis totaling 17 substitutions in the dimer and 33 in mRFP1. Fusions of the gap junction protein connexin43 to mRFP1 formed fully functional junctions, whereas analogous fusions to the tetramer and dimer failed. Although mRFP1 has somewhat lower extinction coefficient, quantum yield, and photostability than DsRed, mRFP1 matures >10 times faster, so that it shows similar brightness in living cells. In addition, the excitation and emission peaks of mRFP1, 584 and 607 nm, are approximately 25 nm red-shifted from DsRed, which should confer greater tissue penetration and spectral separation from autofluorescence and other fluorescent proteins.

Creating New Fluorescent Probes for Cell Biology

Fluorescent probes are one of the cornerstones of real-time imaging of live cells and a powerful tool for cell biologists. They provide high sensitivity and great versatility while minimally perturbing the cell under investigation. Genetically-encoded reporter constructs that are derived from fluorescent proteins are leading a revolution in the real-time visualization and tracking of various cellular events. Recent advances include the continued development of 'passive' markers for the measurement of biomolecule expression and localization in live cells, and 'active' indicators for monitoring more complex cellular processes such as small-molecule-messenger dynamics, enzyme activation and protein-protein interactions.

Overview of the Alliance for Cellular Signaling

The Alliance for Cellular Signaling is a large-scale collaboration designed to answer global questions about signalling networks. Pathways will be studied intensively in two cells--B lymphocytes (the cells of the immune system) and cardiac myocytes--to facilitate quantitative modelling. One goal is to catalyse complementary research in individual laboratories; to facilitate this, all alliance data are freely available for use by the entire research community.

Detection of Calcium Transients in Drosophila Mushroom Body Neurons with Camgaroo Reporters

Camgaroos are yellow fluorescent protein derivatives that hold promise as transgenically encoded calcium sensors in behaving animals. We expressed two versions of camgaroo in Drosophila mushroom bodies using the galactosidase-4 (GAL4) system. Potassium depolarization of brains expressing the reporters produces a robust increase in fluorescence that is blocked by removing extracellular calcium or by antagonists of voltage-dependent calcium channels. The fluorescence increase is not attributable to cytoplasmic alkalization; depolarization induces a slight acidification of the cytoplasm of mushroom body neurons. Acetylcholine applied near the dendrites of the mushroom body neurons induces a rapid and ipsilateral-specific fluorescence increase in the mushroom body axons that is blocked by antagonists of calcium channels or nicotinic acetylcholine receptors. Fluorescence was observed to increase in all three classes of mushroom body neurons, indicating that all types respond to cholinergic innervation.

A Genetically Encoded Fluorescent Reporter Reveals Oscillatory Phosphorylation by Protein Kinase C

Signals transduced by kinases depend on the extent and duration of substrate phosphorylation. We generated genetically encoded fluorescent reporters for PKC activity that reversibly respond to stimuli activating PKC. Specifically, phosphorylation of the reporter expressed in mammalian cells causes changes in fluorescence resonance energy transfer (FRET), allowing real time imaging of phosphorylation resulting from PKC activation. Targeting of the reporter to the plasma membrane, where PKC is activated, reveals oscillatory phosphorylation in HeLa cells in response to histamine. Each oscillation in substrate phosphorylation follows a calcium oscillation with a lag of approximately 10 s. Novel FRET-based reporters for PKC translocation, phosphoinositide bisphosphate conversion to IP3, and diacylglycerol show that in HeLa cells the oscillatory phosphorylations correlate with Ca2+-controlled translocation of conventional PKC to the membrane without oscillations of PLC activity or diacylglycerol. However, in MDCK cells stimulated with ATP, PLC and diacylglycerol fluctuate together with Ca2+ and phosphorylation. Thus, specificity of PKC signaling depends on the local second messenger-controlled equilibrium between kinase and phosphatase activities to result in strict calcium-controlled temporal regulation of substrate phosphorylation.

All-optical Histology Using Ultrashort Laser Pulses

As a means to automate the three-dimensional histological analysis of brain tissue, we demonstrate the use of femtosecond laser pulses to iteratively cut and image fixed as well as fresh tissue. Cuts are accomplished with 1 to 10 microJ pulses to ablate tissue with micron precision. We show that the permeability, immunoreactivity, and optical clarity of the tissue is retained after pulsed laser cutting. Further, samples from transgenic mice that express fluorescent proteins retained their fluorescence to within microns of the cut surface. Imaging of exogenous or endogenous fluorescent labels down to 100 microm or more below the cut surface is accomplished with 0.1 to 1 nJ pulses and conventional two-photon laser scanning microscopy. In one example, labeled projection neurons within the full extent of a neocortical column were visualized with micron resolution. In a second example, the microvasculature within a block of neocortex was measured and reconstructed with micron resolution.

Imagining Imaging's Future

Imaging specific molecules and their interactions in space and time will be essential to understand how genomes create cells, how cells constitute organisms and how errant cells cause disease. Molecular imaging must be extended and applied from nanometre to metre scales and from milliseconds to days. This quest will require input from physics, chemistry, and the genetics and biochemistry of diverse organisms with useful talents.

Genetically Targeted Chromophore-assisted Light Inactivation

Studies of protein function would be facilitated by a general method to inactivate selected proteins in living cells noninvasively with high spatial and temporal precision. Chromophore-assisted light inactivation (CALI) uses photochemically generated, reactive oxygen species to inactivate proteins acutely, but its use has been limited by the need to microinject dye-labeled nonfunction-blocking antibodies. We now demonstrate CALI of connexin43 (Cx43) and alpha1C L-type calcium channels, each tagged with one or two small tetracysteine (TC) motifs that specifically bind the membrane-permeant, red biarsenical dye, ReAsH. ReAsH-based CALI is genetically targeted, requires no antibodies or microinjection, and inactivates each protein by approximately 90% in <30 s of widefield illumination. Similar light doses applied to Cx43 or alpha1C tagged with green fluorescent protein (GFP) had negligible to slight effects with or without ReAsH exposure, showing the expected molecular specificity. ReAsH-mediated CALI acts largely via singlet oxygen because quenchers or enhancers of singlet oxygen respectively inhibit or enhance CALI.

Aptamers Switch on Fluorescence of Triphenylmethane Dyes

MG and SRB aptamers, which are short RNA sequences originally selected only for binding to malachite green or sulforhodamine B, can greatly enhance the fluorescence of normally nonfluorescent triphenylmethane dyes. MG aptamer enhances the quantum yields of malachite green (MG) and a novel rigidized derivative, indolinyl malachite green (IMG) by >2000-fold. SRB aptamer brightens patent blue V and VF by >90-fold. These enhancements are specific because MG aptamer has no effect on patent blue dyes and SRB aptamer has little or no effect on MG and IMG. Such sequence-specific fluorescence labeling of short RNA motifs is a first step toward genetically encodable fusion tags for imaging selected RNAs in vitro and in cells.

Imaging Tetrahymena Ribozyme Splicing Activity in Single Live Mammalian Cells

Tetrahymena ribozymes hold promise for repairing genetic disorders but are largely limited by their modest splicing efficiency and low production of final therapeutic proteins. Ribozyme evolution in intact living mammalian cells would greatly facilitate the discovery of new ribozyme variants with high in vivo activity, but no such strategies have been reported. Here we present a study using a new reporter enzyme, beta-lactamase, to report splicing activity in single living cells and perform high-throughput screening with flow cytometry. The reporter ribozyme constructs consist of the self-splicing Tetrahymena thermophila group I intron ribozyme that is inserted into the ORF of the mRNA of beta-lactamase. The splicing activity in single living cells can be readily detected quantitatively and visualized. Individual cells have shown considerable heterogeneity in ribozyme activity. Screening of Tetrahymena ribozymes with insertions in the middle of the L1 loop led to identification of better variants with at least 4-fold more final in vivo activity than the native sequence. Our work has provided a new reporter system that allows high-throughput screening with flow cytometry of single living mammalian cells for a direct and facile in vivo selection of desired ribozyme variants.

Reversing Cerebellar Long-term Depression

The discovery of a postsynaptically expressed form of cerebellar parallel fiber-Purkinje cell long-term potentiation (LTP) raises the question whether this is the long-sought resetting mechanism for long-term depression (LTD). Extracellular monitoring of PC spikes enables stable prolonged recordings of parallel fiber-Purkinje cell synaptic efficacy. LTD, saturated by repeated induction protocols, can be reversed by a single round of postsynaptic LTP or nitric oxide (NO), enabling LTD to be reinduced. Conversely, after postsynaptic LTP has been saturated, one round of LTD permits fresh postsynaptic LTP. By contrast, after saturation of LTD, induction of presynaptic LTP or application of forskolin leaves LTD still saturated. Likewise, presynaptic LTP cannot be reversed by LTD. Therefore postsynaptic LTP mediated by NO without postsynaptic Ca2+ elevation, unlike presynaptic LTP mediated by cAMP, is a true counterbalance to LTD mediated by coincidence of NO plus postsynaptic Ca2+

Tetracysteine Genetic Tags Complexed with Biarsenical Ligands As a Tool for Investigating Gap Junction Structure and Dynamics

Gap junctions (GJ) are defined as contact regions between two adjacent cells containing tens to thousands of closely packed membrane channels. Cells dynamically modulate communication through GJ by regulating the synthesis, transport and turnover of these channels. Previously, we engineered a recombinant connexin43 (Cx43) by genetically appending a small tetracysteine peptide motif containing the sequence -Cys-Cys-Xaa-Xaa-Cys-Cys- to the carboxy terminus of Cx43 (Cx43-TC) (3). Cx43-TC was stably expressed in HeLa cells and was specifically labeled by exposing the cells to membrane-permeant non-fluorescent ligands, such as FlAsH (a fluorescein derivative) and ReAsH (a resorufin derivative). Direct correlation of live cell images with high resolution EM detection was possible because bound ReAsH not only becomes fluorescent, but can also be used to initiate the photoconversion of diaminobenzidine (DAB) that causes the localized polymerization of an insoluble osmiophilic precipitate then visible by EM. Cx43-TC GJ's could be labeled with ReAsH and photooxidized to give selectively stained channels. Here, how the development of these tetracysteine tags complexed with appropriate ligands are useful for experiments spanning resolution ranges from light microscopy to electron tomography to molecular purification and detection is described.

Breeding Molecules to Spy on Cells

Novel Fluorogenic Substrates for Imaging Beta-lactamase Gene Expression

A new class of small nonfluorescent fluorogenic substrates becomes brightly fluorescent after beta-lactamase hydrolysis with up to 153-fold enhancement in the fluorescence intensity. Less than 500 fM of beta-lactamase in cell lysates can be readily detected, and beta-lactamase expression in living cells can be imaged with a red fluorescence derivative. These new fluorogenic substrates should find uses in clinical diagnostics and facilitate the applications of beta-lactamase as a biosensor.

Investigating Mitochondrial Redox Potential with Redox-sensitive Green Fluorescent Protein Indicators

Current methods for determining ambient redox potential in cells are labor-intensive and generally require destruction of tissue. This precludes single cell or real time studies of changes in redox poise that result from metabolic processes or environmental influences. By substitution of surface-exposed residues on the Aequorea victoria green fluorescent protein (GFP) with cysteines in appropriate positions to form disulfide bonds, reduction-oxidation-sensitive GFPs (roGFPs) have been created. roGFPs have two fluorescence excitation maxima at about 400 and 490 nm and display rapid and reversible ratiometric changes in fluorescence in response to changes in ambient redox potential in vitro and in vivo. Crystal structure analyses of reduced and oxidized crystals of roGFP2 at 2.0- and 1.9-A resolution, respectively, reveal in the oxidized state a highly strained disulfide and localized main chain structural changes that presumably account for the state-dependent spectral changes. roGFP1 has been targeted to the mitochondria in HeLa cells. Fluorometric measurements on these cells using a fluorescence microscope or in cell suspension using a fluorometer reveal that the roGFP1 probe is in dynamic equilibrium with the mitochondrial redox status and responds to membrane-permeable reductants and oxidants. The roGFP1 probe reports that the matrix space in HeLa cell mitochondria is highly reducing, with a midpoint potential near -360 mV (assuming mitochondrial pH approximately 8.0 at 37 degrees C). In other work (C. T. Dooley, T. M. Dore, G. Hanson, W. C. Jackson, S. J. Remington, and R. Y. Tsien, submitted for publication), it is shown that the cytosol of HeLa cells is also unusually reducing but somewhat less so than the mitochondrial matrix.

Activity-dependent Regulation of Dendritic Synthesis and Trafficking of AMPA Receptors

Regulation of AMPA receptor (AMPAR) trafficking is important for neural plasticity. Here we examined the trafficking and synthesis of the GluR1 and GluR2 subunits using ReAsH-EDT(2) and FlAsH-EDT(2) staining. Activity blockade of rat cultured neurons increased dendritic GluR1, but not GluR2, levels. Examination of transected dendrites revealed that both AMPAR subunits were synthesized in dendrites and that activity blockade enhanced dendritic synthesis of GluR1 but not GluR2. In contrast, acute pharmacological manipulations increased dendritic synthesis of both subunits. AMPARs synthesized in dendrites were inserted into synaptic plasma membranes and, after activity blockade, the electrophysiological properties of native synaptic AMPARs changed in the manner predicted by the imaging experiments. In addition to providing a novel mechanism for synaptic modifications, these results point out the advantages of using FlAsH-EDT(2) and ReAsH-EDT(2) for studying the trafficking of newly synthesized proteins in local cellular compartments such as dendrites.

Imaging Tri-fusion Multimodality Reporter Gene Expression in Living Subjects

Imaging reporter gene expression in living subjects with various imaging modalities is a rapidly accelerating area of research. Applications of these technologies to cancer research, gene therapy, and transgenic models are rapidly expanding. We report construction and testing of several triple fusion reporter genes compatible with bioluminescence, fluorescence and positron emission tomography (PET) imaging. A triple fusion reporter vector harboring a bioluminescence synthetic Renilla luciferase (hrl) reporter gene, a reporter gene encoding the monomeric red fluorescence protein (mrfp1), and a mutant herpes simplex virus type 1 sr39 thymidine kinase [HSV1-truncated sr39tk (ttk); a PET reporter gene] was found to preserve the most activity for each protein component and was therefore investigated in detail. After validating the activities of all three proteins encoded by the fusion gene in cell culture, we imaged living mice bearing 293T cells transiently expressing the hrl-mrfp-ttk vector by microPET and using a highly sensitive cooled charge-coupled device camera compatible with both bioluminescence and fluorescence imaging. A lentiviral vector carrying the triple fusion reporter gene was constructed and used to isolate stable expressers by fluorescence-activated cell sorting. These stable 293T cells were further used to show good correlation (R(2) approximately 0.74-0.85) of signal from each component by imaging tumor xenografts in living mice with all three modalities. Furthermore, metastases of a human melanoma cell line (A375M) stably expressing the triple fusion were imaged by microPET and optical technologies over a 40-50-day time period in living mice. Imaging of reporter gene expression from single cells to living animals with the help of a single tri-fusion reporter gene will have the potential to accelerate translational cancer research.

Imaging Dynamic Redox Changes in Mammalian Cells with Green Fluorescent Protein Indicators

Changes in the redox equilibrium of cells influence a host of cell functions. Alterations in the redox equilibrium are precipitated by changing either the glutathione/glutathione-disulfide ratio (GSH/GSSG) and/or the reduced/oxidized thioredoxin ratio. Redox-sensitive green fluorescent proteins (GFP) allow real time visualization of the oxidation state of the indicator. Ratios of fluorescence from excitation at 400 and 490 nm indicate the extent of oxidation and thus the redox potential while canceling out the amount of indicator and the absolute optical sensitivity. Because the indicator is genetically encoded, it can be targeted to specific proteins or organelles of interest and expressed in a wide variety of cells and organisms. We evaluated roGFP1 (GFP with mutations C48S, S147C, and Q204C) and roGFP2 (the same plus S65T) with physiologically or toxicologically relevant oxidants both in vitro and in living mammalian cells. Furthermore, we investigated the response of the redox probes under physiological redox changes during superoxide bursts in macrophage cells, hyperoxic and hypoxic conditions, and in responses to H(2)O(2)-stimulating agents, e.g. epidermal growth factor and lysophosphatidic acid.

Functional Fluorescent Ca2+ Indicator Proteins in Transgenic Mice Under TET Control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes-for the measurement of population activity, in particular-have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.

Bhc-cNMPs As Either Water-soluble or Membrane-permeant Photoreleasable Cyclic Nucleotides for Both One- and Two-photon Excitation

Cyclic nucleoside monophosphates (cNMPs) play key roles in many cellular regulatory processes, such as growth, differentiation, motility, and gene expression. Caged derivatives that can be activated by irradiation could be powerful tools for studying such diverse functions of intracellular second messengers, since the spatiotemporal dynamics of these molecules can be controlled by irradiation with appropriately focused light. Here we report the synthesis, photochemistry, and biological testing of 6-bromo-7-hydroxycoumarin-4-ylmethyl esters of cNMP (Bhc-cNMP) and their acetyl derivatives (Bhc-cNMP/Ac) as new caged second messengers. Irradiation of Bhc-cNMPs quantitatively produced the parent cNMPs with one-photon uncaging efficiencies (Phiepsilon) of up to one order of magnitude better than those of 2-nitrophenethyl (NPE) cNMPs. In addition, two-photon induced photochemical release of cNMP from Bhc-cNMPs (7 and 8) can be observed with the two-photon uncaging action cross-sections (delta(u)) of up to 2.28 GM (1 GM=10(-50) cm(4) s photon(-1)), which is the largest value among those of the reported Bhc-caged compounds. The wavelength dependence of the delta(u) values of 7 revealed that the peak wavelength was twice that of the one-photon absorption maximum. Bhc-cNMPs showed practically useful water solubility (nearly 500 microM), whereas 7-acetylated derivatives (Bhc-cNMPs/Ac) were expected to have a certain membrane permeability. Their advantages were demonstrated in two types of biological systems: the opening of cAMP-mediated transduction channels in newt olfactory receptor cells and cAMP-mediated motility responses in epidermal melanophores in scales from medaka fish. Both examples showed that Bhc and Bhc/Ac caged compounds have great potential for use in many cell biological applications.

BI-1 Regulates an Apoptosis Pathway Linked to Endoplasmic Reticulum Stress

Bax inhibitor-1 (BI-1) is an evolutionarily conserved endoplasmic reticulum (ER) protein that suppresses cell death in both animal and plant cells. We characterized mice in which the bi-1 gene was ablated. Cells from BI-1-deficient mice, including fibroblasts, hepatocytes, and neurons, display selective hypersensitivity to apoptosis induced by ER stress agents (thapsigargin, tunicamycin, brefeldin A), but not to stimulators of mitochondrial or TNF/Fas-death receptor apoptosis pathways. Conversely, BI-1 overexpression protects against apoptosis induced by ER stress. BI-1-mediated protection from apoptosis induced by ER stress correlated with inhibition of Bax activation and translocation to mitochondria, preservation of mitochondrial membrane potential, and suppression of caspase activation. BI-1 overexpression also reduces releasable Ca(2+) from the ER. In vivo, bi-1(-/-) mice exhibit increased sensitivity to tissue damage induced by stimuli that trigger ER stress, including stroke and tunicamycin injection. Thus, BI-1 regulates a cell death pathway important for cytopreservation during ER stress.

Evolution of New Nonantibody Proteins Via Iterative Somatic Hypermutation

B lymphocytes use somatic hypermutation (SHM) to optimize immunoglobulins. Although SHM can rescue single point mutations deliberately introduced into nonimmunoglobulin genes, such experiments do not show whether SHM can efficiently evolve challenging novel phenotypes requiring multiple unforeseeable mutations in nonantibody proteins. We have now iterated SHM over 23 rounds of fluorescence-activated cell sorting to create monomeric red fluorescent proteins with increased photostability and far-red emissions (e.g., 649 nm), surpassing the best efforts of structure-based design. SHM offers a strategy to evolve nonantibody proteins with desirable properties for which a high-throughput selection or viable single-cell screen can be devised.

Improved Monomeric Red, Orange and Yellow Fluorescent Proteins Derived from Discosoma Sp. Red Fluorescent Protein

Fluorescent proteins are genetically encoded, easily imaged reporters crucial in biology and biotechnology. When a protein is tagged by fusion to a fluorescent protein, interactions between fluorescent proteins can undesirably disturb targeting or function. Unfortunately, all wild-type yellow-to-red fluorescent proteins reported so far are obligately tetrameric and often toxic or disruptive. The first true monomer was mRFP1, derived from the Discosoma sp. fluorescent protein "DsRed" by directed evolution first to increase the speed of maturation, then to break each subunit interface while restoring fluorescence, which cumulatively required 33 substitutions. Although mRFP1 has already proven widely useful, several properties could bear improvement and more colors would be welcome. We report the next generation of monomers. The latest red version matures more completely, is more tolerant of N-terminal fusions and is over tenfold more photostable than mRFP1. Three monomers with distinguishable hues from yellow-orange to red-orange have higher quantum efficiencies.

Bcl-2-mediated Alterations in Endoplasmic Reticulum Ca2+ Analyzed with an Improved Genetically Encoded Fluorescent Sensor

The endoplasmic reticulum (ER) serves as a cellular storehouse for Ca(2+), and Ca(2+) released from the ER plays a role in a host of critical signaling reactions, including exocytosis, contraction, metabolism, regulation of transcription, fertilization, and apoptosis. Given the central role played by the ER, our understanding of these signaling processes could be greatly enhanced by the ability to image [Ca(2+)](ER) directly in individual cells. We created a genetically encoded Ca(2+) indicator by redesigning the binding interface of calmodulin and a calmodulin-binding peptide. The sensor has improved reaction kinetics and a K(d) ideal for imaging Ca(2+) in the ER and is no longer perturbed by large excesses of native calmodulin. Importantly, it provides a significant improvement over all previous methods for monitoring [Ca(2+)](ER) and has been used to directly show that, in MCF-7 breast cancer cells, the antiapoptotic protein B cell lymphoma 2 (Bcl-2) (i) lowers [Ca(2+)](ER) by increasing Ca(2+) leakage under resting conditions and (ii) alters Ca(2+) oscillations induced by ATP, and that acute inhibition of Bcl-2 by the green tea compound epigallocatechin gallate results in an increase in [Ca(2+)](ER) due to inhibition of Bcl-2-mediated Ca(2+) leakage.

Tumor Imaging by Means of Proteolytic Activation of Cell-penetrating Peptides

We have devised and tested a new strategy for selectively delivering molecules to tumor cells. Cellular association of polyarginine-based, cell-penetrating peptides (CPPs) is effectively blocked when they are fused to an inhibitory domain made up of negatively charged residues. We call these fusions activatable CPPs (ACPPs) because cleavage of the linker between the polycationic and polyanionic domains, typically by a protease, releases the CPP portion and its attached cargo to bind to and enter cells. Association with cultured cells typically increases 10-fold or more upon linker cleavage. In mice xenografted with human tumor cells secreting matrix metalloproteinases 2 and 9, ACPPs bearing a far-red-fluorescent cargo show in vivo contrast ratios of 2-3 and a 3.1-fold increase in standard uptake value for tumors relative to contralateral normal tissue or control peptides with scrambled linkers. Ex vivo slices of freshly resected human squamous cell carcinomas give similar or better contrast ratios. Because CPPs are known to import a wide variety of nonoptical contrast and therapeutic agents, ACPPs offer a general strategy toward imaging and treating disease processes associated with linker-cleaving activities such as extracellular proteases.

Spatio-temporal Dynamics of Protein Kinase B/Akt Signaling Revealed by a Genetically Encoded Fluorescent Reporter

The serine/threonine kinase protein kinase B (PKB)/Akt is a critical regulator of insulin signaling, cell survival, and oncogenesis. The activation mechanisms of this key kinase are well characterized. In contrast, inactivation of PKB signaling by phosphatases is less well understood. To study the dynamics of PKB signaling in live cells, we generated a genetically encoded fluorescent reporter for PKB activity that reversibly responds to stimuli activating phosphatidylinositol 3-kinase. Specifically, phosphorylation of the reporter expressed in mammalian cells causes changes in fluorescence resonance energy transfer, allowing real-time imaging of phosphorylation catalyzed by PKB. Because of its reversibility, the reporter also allows termination of PKB signaling by phosphatases to be monitored. We found that PKB signaling in the cytosol was more rapid and more transient compared with that in the nucleus, suggesting the presence of differentially regulated phosphatase activity in these two compartments. Furthermore, targeting of the reporter to the plasma membrane, where PKB is activated, resulted in accelerated and prolonged response compared with the response in the cytosol, suggesting that release of PKB or its substrates from the membrane is required for desensitization of PKB signaling. These data reveal spatio-temporal gradients of both signal propagation and signal termination in PKB signaling.

Building and Breeding Molecules to Spy on Cells and Tumors

Imaging of biochemical processes in living cells and organisms is essential for understanding how genes and gene products work together in space and time and in health and disease. Such imaging depends crucially on indicator molecules designed to maximize sensitivity and specificity. These molecules can be entirely synthetic, entirely genetically encoded macromolecules, or hybrid combinations, each approach having its own pros and cons. Recent examples from the author's laboratory include peptides whose uptake into cells is triggered by proteases typical of tumors, monomeric red fluorescent proteins and biarsenical-tetracysteine systems for determining the age and electron-microscopic location of proteins.

A FlAsH-based FRET Approach to Determine G Protein-coupled Receptor Activation in Living Cells

Fluorescence resonance energy transfer (FRET) from cyan to yellow fluorescent proteins (CFP/YFP) is a well-established method to monitor protein-protein interactions or conformational changes of individual proteins. But protein functions can be perturbed by fusion of large tags such as CFP and YFP. Here we use G protein-coupled receptor (GPCR) activation in living cells as a model system to compare YFP with the small, membrane-permeant fluorescein derivative with two arsen-(III) substituents (fluorescein arsenical hairpin binder; FlAsH) targeted to a short tetracysteine sequence. Insertion of CFP and YFP into human adenosine A(2A) receptors allowed us to use FRET to monitor receptor activation but eliminated coupling to adenylyl cyclase. The CFP/FlAsH-tetracysteine system gave fivefold greater agonist-induced FRET signals, similar kinetics (time constant of 66-88 ms) and perfectly normal downstream signaling. Similar results were obtained for the mouse alpha(2A)-adrenergic receptor. Thus, FRET from CFP to FlAsH reports GPCR activation in living cells without disturbing receptor function and shows that the small size of the tetracysteine-biarsenical tag can be decisively advantageous.

Visualizing the Mechanical Activation of Src

The mechanical environment crucially influences many cell functions. However, it remains largely mysterious how mechanical stimuli are transmitted into biochemical signals. Src is known to regulate the integrin-cytoskeleton interaction, which is essential for the transduction of mechanical stimuli. Using fluorescent resonance energy transfer (FRET), here we develop a genetically encoded Src reporter that enables the imaging and quantification of spatio-temporal activation of Src in live cells. We introduced a local mechanical stimulation to human umbilical vein endothelial cells (HUVECs) by applying laser-tweezer traction on fibronectin-coated beads adhering to the cells. Using the Src reporter, we observed a rapid distal Src activation and a slower directional wave propagation of Src activation along the plasma membrane. This wave propagated away from the stimulation site with a speed (mean +/- s.e.m.) of 18.1 +/- 1.7 nm s(-1). This force-induced directional and long-range activation of Src was abolished by the disruption of actin filaments or microtubules. Our reporter has thus made it possible to monitor mechanotransduction in live cells with spatio-temporal characterization. We find that the transmission of mechanically induced Src activation is a dynamic process that directs signals via the cytoskeleton to spatial destinations.

Mammalian Cell-based Optimization of the Biarsenical-binding Tetracysteine Motif for Improved Fluorescence and Affinity

Membrane-permeant biarsenical dyes such as FlAsH and ReAsH fluoresce upon binding to genetically encoded tetracysteine motifs expressed in living cells, yet spontaneous nonspecific background staining can prevent detection of weakly expressed or dilute proteins. If the affinity of the tetracysteine peptide could be increased, more stringent dithiol washes should increase the contrast between specific and nonspecific staining. Residues surrounding the tetracysteine motif were randomized and fused to GFP, retrovirally transduced into mammalian cells and iteratively sorted by fluorescence-activated cell sorting for high FRET from GFP to ReAsH in the presence of increasing concentrations of dithiol competitors. The selected sequences show higher fluorescence quantum yields and markedly improved dithiol resistance, culminating in a >20-fold increase in contrast. The selected tetracysteine sequences, HRWCCPGCCKTF and FLNCCPGCCMEP, maintain their enhanced properties as fusions to either terminus of GFP or directly to beta-actin. These improved biarsenical-tetracysteine motifs should enable detection of a much broader spectrum of cellular proteins.

Insulin Disrupts Beta-adrenergic Signalling to Protein Kinase A in Adipocytes

Hormones mobilize intracellular second messengers and initiate signalling cascades involving protein kinases and phosphatases, which are often spatially compartmentalized by anchoring proteins to increase signalling specificity. These scaffold proteins may themselves be modulated by hormones. In adipocytes, stimulation of beta-adrenergic receptors increases cyclic AMP levels and activates protein kinase A (PKA), which stimulates lipolysis by phosphorylating hormone-sensitive lipase and perilipin. Acute insulin treatment activates phosphodiesterase 3B, reduces cAMP levels and quenches beta-adrenergic receptor signalling. In contrast, chronic hyperinsulinaemic conditions (typical of type 2 diabetes) enhance beta-adrenergic receptor-mediated cAMP production. This amplification of cAMP signalling is paradoxical because it should enhance lipolysis, the opposite of the known short-term effect of hyperinsulinaemia. Here we show that in adipocytes, chronically high insulin levels inhibit beta-adrenergic receptors (but not other cAMP-elevating stimuli) from activating PKA. We measured this using an improved fluorescent reporter and by phosphorylation of endogenous cAMP-response-element binding protein (CREB). Disruption of PKA scaffolding mimics the interference of insulin with beta-adrenergic receptor signalling. Chronically high insulin levels may disrupt the close apposition of beta-adrenergic receptors and PKA, identifying a new mechanism for crosstalk between heterologous signal transduction pathways.

A Guide to Choosing Fluorescent Proteins

The recent explosion in the diversity of available fluorescent proteins (FPs) promises a wide variety of new tools for biological imaging. With no unified standard for assessing these tools, however, a researcher is faced with difficult questions. Which FPs are best for general use? Which are the brightest? What additional factors determine which are best for a given experiment? Although in many cases, a trial-and-error approach may still be necessary in determining the answers to these questions, a unified characterization of the best available FPs provides a useful guide in narrowing down the options.

Molecular Biology and Mutation of Green Fluorescent Protein

Dynamics of the Upper 50-kDa Domain of Myosin V Examined with Fluorescence Resonance Energy Transfer

The upper 50-kDa region of myosin may be critical for coupling between the nucleotide- and actin-binding regions. We introduced a tetracysteine motif in the upper 50-kDa domain (residues 292-297) of myosin V containing a single IQ domain (MV 1IQ), allowing us to label this site with the fluorescein biarscenical hairpin-binding dye (FlAsH) (MV 1IQ FlAsH). The enzymatic properties of MV 1IQ FlAsH were similar to those of unlabeled MV 1IQ except for a 3-fold reduced ADP-release rate. MV 1IQ FlAsH was also capable of moving actin filaments in the in vitro motility assay. To examine rotation of the upper 50-kDa region, we determined the difference in the degree of energy transfer from N-methylanthraniloyl (mant)-labeled nucleotides to FlAsH in both steady-state and transient kinetic experiments. The energy transfer efficiency was higher with mant-ATP (0.65 +/- 0.02) compared with mant-ADP (0.55 +/- 0.02) in the absence of actin. Stopped-flow measurements suggested that the energy transfer efficiency decreased with phosphate release (0.04 s(-1)) in the absence of actin. In contrast, upon mixing MV 1IQ FlAsH in the ADP.P(i) state with actin, a decrease in the energy transfer signal was observed at a rate of 13 s(-1), similar to the ADP release rate. Our results demonstrate there was no change in the energy transfer signal upon actin-activated phosphate release and suggest that actin binding alters the dynamics of the upper 50-kDa region, which may be critical for the ability of myosin to bind tightly to both ADP and actin.

Control of Mammalian Translation by MRNA Structure Near Caps

The scanning model of RNA translation proposes that highly stable secondary structures within mRNAs can inhibit translation, while structures of lower thermal stability also affect translation if close enough to the 5' methyl G cap. However, only fragmentary information is available about the dependence of translation efficiency in live mammalian cells on the thermodynamic stability, location, and GC content of RNA structures in the 5'-untranslated region. We devised a two-color fluorescence assay for translation efficiency in single live cells and compared a wide range of hairpins with predicted thermal stabilities ranging from -10 to -50 kcal/mol and 5' G cap-to-hairpin distances of 1-46 bases. Translation efficiency decreased abruptly as hairpin stabilities increased from deltaG = -25 to -35 kcal/mol. Shifting a hairpin as little as nine bases relative to the 5' cap could modulate translation more than 50-fold. Increasing GC content diminished translation efficiency when predicted thermal stability and cap-to-hairpin distances were held constant. We additionally found naturally occurring 5'-untranslated regions affected translation differently in live cells compared with translation in in vitro lysates. Our study will assist scientists in designing experiments that deliberately modulate mammalian translation with designed 5' UTRs.

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.

Ca2+ Indicators Based on Computationally Redesigned Calmodulin-peptide Pairs

The binding interface of calmodulin and a calmodulin binding peptide were reengineered by computationally designing complementary bumps and holes. This redesign led to the development of sensitive and specific pairs of mutant proteins used to sense Ca(2+) in a second generation of genetically encoded Ca(2+) indicators (cameleons). These cameleons are no longer perturbed by large excesses of native calmodulin, and they display Ca(2+) sensitivities tuned over a 100-fold range (0.6-160 microM). Incorporation of circularly permuted Venus in place of Citrine results in a 3- to 5-fold increase in the dynamic range. These redesigned cameleons show significant improvements over previous versions in the ability to monitor Ca(2+) in the cytoplasm as well as distinct subcellular localizations, such as the plasma membrane of neurons and the mitochondria.

Novel Chromophores and Buried Charges Control Color in MFruits

mFruits are second-generation monomeric red fluorescent proteins (mRFPs) that have improved brightness and photostability compared to the first-generation mRFP1. The emission and excitation maxima are distributed over the remarkably large ranges of about 550-650 and 540-590 nm, respectively; however, the variations in the spectra can be traced to a few key amino acids. Spectroscopic and atomic resolution crystallographic analyses of three representatives, mOrange, mStrawberry, and mCherry, reveal that different mechanisms operate to establish the excitation and emission maxima. Evidently, they all undergo the second oxidation step to produce an acylimine linkage in the polypeptide backbone. In comparison to the progenitor DsRed, direct covalent modification to this linkage (mOrange) and indirect modification of the chromophore environment (mStrawberry and mCherry) produce strong blue- and red-shifted variants. The blue shift of mOrange is induced by an unprecedented covalent modification of the protein backbone. The electron-density map indicates the formation of a third heterocycle, 2-hydroxy-dihydrooxazole, upon the reaction of Thr 66 Ogamma with the polypeptide backbone, which in turn reduces the conjugation of the carbonyl at position 65 with the rest of the chromophore. In mStrawberry and mCherry, the movement of charged Lys 70 and protonation of Glu 215 are proposed to modify the chromophore electron-density distribution, inducing the red shift. pH-dependent spectral shifts of mCherry and mStrawberry appear to result from the titration of Glu 215, although, for mStrawberry, partial cyclization of Thr 66 may contribute at high pH.

Systems Analysis of PKA-mediated Phosphorylation Gradients in Live Cardiac Myocytes

Compartmentation and dynamics of cAMP and PKA signaling are important determinants of specificity among cAMP's myriad cellular roles. Both cardiac inotropy and the progression of heart disease are affected by spatiotemporal variations in cAMP/PKA signaling, yet the dynamic patterns of PKA-mediated phosphorylation that influence differential responses to agonists have not been characterized. We performed live-cell imaging and systems modeling of PKA-mediated phosphorylation in neonatal cardiac myocytes in response to G-protein coupled receptor stimuli and UV photolysis of "caged" cAMP. cAMP accumulation was rate-limiting in PKA-mediated phosphorylation downstream of the beta-adrenergic receptor. Prostaglandin E1 stimulated higher PKA activity in the cytosol than at the sarcolemma, whereas isoproterenol triggered faster sarcolemmal responses than cytosolic, likely due to restricted cAMP diffusion from submembrane compartments. Localized UV photolysis of caged cAMP triggered gradients of PKA-mediated phosphorylation, enhanced by phosphodiesterase activity and PKA-mediated buffering of cAMP. These findings indicate that combining live-cell FRET imaging and mechanistic computational models can provide quantitative understanding of spatiotemporal signaling.

Golgi Twins in Late Mitosis Revealed by Genetically Encoded Tags for Live Cell Imaging and Correlated Electron Microscopy

Combinations of molecular tags visible in light and electron microscopes become particularly advantageous in the analysis of dynamic cellular components like the Golgi apparatus. This organelle disassembles at the onset of mitosis and, after a sequence of poorly understood events, reassembles after cytokinesis. The precise location of Golgi membranes and resident proteins during mitosis remains unclear, partly due to limitations of molecular markers and the resolution of light microscopy. We generated a fusion consisting of the first 117 residues of alpha-mannosidase II tagged with a fluorescent protein and a tetracysteine motif. The mannosidase component guarantees docking into the Golgi membrane, with the tags exposed in the lumen. The fluorescent protein is optically visible without further treatment, whereas the tetracysteine tag can be reduced acutely with a membrane-permeant phosphine, labeled with ReAsH, monitored in the light microscope, and used to trigger the photoconversion of diaminobenzidine, allowing 4D optical recording on live cells and correlated ultrastructural analysis by electron microscopy. These methods reveal that Golgi reassembly is preceded by the formation of four colinear clusters at telophase, two per daughter cell. Within each daughter, the smaller cluster near the midbody gradually migrates to rejoin the major cluster on the far side of the nucleus and asymmetrically reconstitutes a single Golgi apparatus, first in one daughter cell and then in the other. Our studies provide previously undescribed insights into Golgi disassociation and reassembly during mitosis and offer a powerful approach to follow recombinant protein distribution in 4D imaging and correlated high-resolution analysis.

Imaging of CAMP Levels and Protein Kinase A Activity Reveals That Retinal Waves Drive Oscillations in Second-messenger Cascades

Recent evidence demonstrates that low-frequency oscillations of intracellular calcium on timescales of seconds to minutes drive distinct aspects of neuronal development, but the mechanisms by which these calcium transients are coupled to signaling cascades are not well understood. Here we test the hypothesis that spontaneous electrical activity activates protein kinase A (PKA). We use live-cell indicators to observe spontaneous and evoked changes in cAMP levels and PKA activity in developing retinal neurons. Expression of cAMP and PKA indicators in neonatal rat retinal explants reveals spontaneous oscillations in PKA activity that are temporally correlated with spontaneous depolarizations associated with retinal waves. In response to short applications of forskolin, dopamine, or high-potassium concentration, we image an increase in cAMP levels and PKA activity, indicating that this second-messenger pathway can be activated quickly by neural activity. Depolarization-evoked increases in PKA activity were blocked by the removal of extracellular calcium, indicating that they are mediated by a calcium-dependent mechanism. These findings demonstrate for the first time that spontaneous activity in developing circuits is correlated with activation of the cAMP/PKA pathway and that PKA activity is turned on and off on the timescale of tens of seconds. These results show a link between neural activity and an intracellular biochemical cascade associated with plasticity, axon guidance, and neural differentiation.

Breeding and Building Molecules to Spy on Cells and Tumors

Measuring Calcium Signaling Using Genetically Targetable Fluorescent Indicators

Genetically encoded Ca2+ indicators allow researchers to quantitatively measure Ca2+ dynamics in a variety of experimental systems. This protocol summarizes the indicators that are available, and highlights those that are most appropriate for a number of experimental conditions, such as measuring Ca2+ in specific organelles and localizations in mammalian tissue-culture cells. The protocol itself focuses on the use of a cameleon, which is a fluorescence resonance-energy transfer (FRET)-based indicator comprising two fluorescent proteins and two Ca2+-responsive elements (a variant of calmodulin (CaM) and a CaM-binding peptide). This protocol details how to set up and conduct a Ca2+-imaging experiment, accomplish offline data processing (such as background correction) and convert the observed FRET ratio changes to Ca2+ concentrations. Additionally, we highlight some of the challenges in observing organellar Ca2+ and the alternative strategies researchers can employ for effectively calibrating the genetically encoded Ca2+ indicators in these locations. Setting up and conducting an initial calibration of the microscope system is estimated to take approximately 1 week, assuming that all the component parts are readily available. Cell culture and transfection is estimated to take approximately 3 d (from the time of plating cells on imaging dishes). An experiment and calibration will probably take a few hours. Finally, the offline data workup can take approximately 1 d depending on the extent of analysis.

Evolving Proteins in Mammalian Cells Using Somatic Hypermutation

We describe a new method to mutate target genes through somatic hypermutation (SHM) and to evolve proteins directly in living mammalian cells. Target genes are expressed under the control of an inducible promoter in a B-cell line that hypermutates its immunoglobulin (Ig) V genes constitutively. Mutations can be introduced into the target gene through SHM upon transcription. Mutant genes are then expressed and selected or screened for desired properties in cells. Identified cells are subjected to another round of mutation and selection or screening. This process can be iterated easily for numerous rounds, and multiple reinforcing mutations can be accumulated to produce desirable phenotypes. This approach bypasses labor-intensive in vitro mutagenesis and samples a large protein sequence space. In this protocol a monomeric red fluorescent protein (mRFP1.2) was evolved in Ramos cells to afford a mutant (mPlum) with far-red emission. This method can be adapted to evolve other eukaryotic proteins and to be used in other cells able to perform SHM. For each round of evolution, it takes approximately 1 d to mutate the target gene, approximately 0.5-1 d to select or screen, and 2-4 d to propagate the cells for the next round depending on how many cells are collected.

Calcium-dependent Regulation of Protein Kinase D Revealed by a Genetically Encoded Kinase Activity Reporter

Protein kinase D (PKD) regulates many diverse cellular functions in response to diacylglycerol. To monitor PKD signaling in live cells, we generated a genetically encoded fluorescent reporter for PKD activity, DKAR (D kinase activity reporter). DKAR expressed in mammalian cells undergoes reversible fluorescence resonance energy transfer changes upon activation and inhibition of endogenous PKD. Surprisingly, we find that agonist-evoked activation of PKD is driven not only by diacylglycerol production, but by Ca(2+). Furthermore, elevation of intracellular Ca(2+), in the absence of any other stimulus, is sufficient to activate PKD. Concurrent imaging of Ca(2+), diacylglycerol, and PKD activity reveals that thapsigargin-mediated elevation of intracellular Ca(2+) is closely followed by a robust increase in diacylglycerol production, in turn followed by PKD activation. The Ca(2+)-induced production of diacylglycerol and accompanying PKD activation is dependent on phospholipase C activity. These data reveal that Ca(2+) is a major contributor to the initiation of PKD signaling through positive feedback regulation of diacylglycerol production, unveiling a new mechanism in PKD activation.

A Hexahistidine-Zn2+-dye Label Reveals STIM1 Surface Exposure

Site-specific fluorescent labeling of proteins in vivo remains one of the most powerful techniques for imaging complex processes in live cells. Although fluorescent proteins in many colors are useful tools for tracking expression and localization of fusion proteins in cells, these relatively large tags (>220 aa) can perturb protein folding, trafficking and function. Much smaller genetically encodable domains (<15 aa) offer complementary advantages. We introduce a small fluorescent chelator whose membrane-impermeant complex with nontoxic Zn(2+) ions binds tightly but reversibly to hexahistidine (His(6)) motifs on surface-exposed proteins. This live-cell label helps to resolve a current controversy concerning externalization of the stromal interaction molecule STIM1 upon depletion of Ca(2+) from the endoplasmic reticulum. Whereas N-terminal fluorescent protein fusions interfere with surface exposure of STIM1, short His(6) tags are accessible to the dye or antibodies, demonstrating externalization.

Exploration of New Chromophore Structures Leads to the Identification of Improved Blue Fluorescent Proteins

The variant of Aequorea green fluorescent protein (GFP) known as blue fluorescent protein (BFP) was originally engineered by substituting histidine for tyrosine in the chromophore precursor sequence. Herein we report improved versions of BFP along with a variety of engineered fluorescent protein variants with novel and distinct chromophore structures that all share the property of a blue fluorescent hue. The two most intriguing of the new variants are a version of GFP in which the chromophore does not undergo excited-state proton transfer and a version of mCherry with a phenylalanine-derived chromophore. All of the new blue fluorescing proteins have been critically assessed for their utility in live cell fluorescent imaging. These new variants should greatly facilitate multicolor fluorescent imaging by legitimizing blue fluorescing proteins as practical and robust members of the fluorescent protein "toolkit".

Calcium Green FlAsH As a Genetically Targeted Small-molecule Calcium Indicator

Intracellular Ca(2+) regulates numerous proteins and cellular functions and can vary substantially over submicron and submillisecond scales, so precisely localized fast detection is desirable. We have created a approximately 1-kDa biarsenical Ca(2+) indicator, called Calcium Green FlAsH (CaGF, 1), to probe [Ca(2+)] surrounding genetically targeted proteins. CaGF attached to a tetracysteine motif becomes ten-fold more fluorescent upon binding Ca(2+), with a K(d) of approximately 100 microM, <1-ms kinetics and good Mg(2+) rejection. In HeLa cells expressing tetracysteine-tagged connexin 43, CaGF labels gap junctions and reports Ca(2+) waves after injury. Total internal reflection microscopy of tetracysteine-tagged, CaGF-labeled alpha(1C) L-type calcium channels shows fast-rising depolarization-evoked Ca(2+) transients, whose lateral nonuniformity suggests that the probability of channel opening varies greatly over micron dimensions. With moderate Ca(2+) buffering, these transients decay surprisingly slowly, probably because most of the CaGF signal comes from closed channels feeling Ca(2+) from a tiny minority of clustered open channels. With high Ca(2+) buffering, CaGF signals decay as rapidly as the calcium currents, as expected for submicron Ca(2+) domains immediately surrounding active channels. Thus CaGF can report highly localized, rapid [Ca(2+)] dynamics.

Isoform-specific PKA Dynamics Revealed by Dye-triggered Aggregation and DAKAP1alpha-mediated Localization in Living Cells

The tetracysteine sequence YRECCPGCCMWR fused to the N terminus of green fluorescent protein (GFP) self-aggregates upon biarsenical labeling in living cells or in vitro. Such dye-triggered aggregates form temperature-dependent morphologies and are dispersed by photobleaching. Fusion of the biarsenical aggregating GFP to the regulatory (R) or catalytic (C) subunit of PKA traps intact holoenzyme in compact fluorescent puncta upon biarsenical labeling. Contrary to the classical model of PKA activation, elevated cAMP does not allow RIalpha and Calpha to diffuse far apart unless the pseudosubstrate inhibitor PKI or locally concentrated substrate is coexpressed. However, RIIalpha releases Calpha upon elevated cAMP alone, dependent on autophosphorylation of the RIIalpha inhibitory domain. DAKAP1alpha overexpression induced R and C outer mitochondrial colocalization and showed similar regulation. Overall, effective separation of type I PKA is substrate dependent, whereas type II PKA dissociation relies on autophosphorylation.

Optical Measurement of Synaptic Glutamate Spillover and Reuptake by Linker Optimized Glutamate-sensitive Fluorescent Reporters

Genetically encoded sensors of glutamate concentration are based on FRET between cyan and yellow fluorescent proteins bracketing a bacterial glutamate-binding protein. Such sensors have yet to find quantitative applications in neurons, because of poor response amplitude in physiological buffers or when expressed on the neuronal cell surface. We have improved our glutamate-sensing fluorescent reporter (GluSnFR) by systematic optimization of linker sequences and glutamate affinities. Using SuperGluSnFR, which exhibits a 6.2-fold increase in response magnitude over the original GluSnFR, we demonstrate quantitative optical measurements of the time course of synaptic glutamate release, spillover, and reuptake in cultured hippocampal neurons with centisecond temporal and spine-sized spatial resolution. During burst firing, functionally significant spillover persists for hundreds of milliseconds. These glutamate levels appear sufficient to prime NMDA receptors, potentially affecting dendritic spike initiation and computation. Stimulation frequency-dependent modulation of spillover suggests a mechanism for nonsynaptic neuronal communication.

Improving the Photostability of Bright Monomeric Orange and Red Fluorescent Proteins

All organic fluorophores undergo irreversible photobleaching during prolonged illumination. Although fluorescent proteins typically bleach at a substantially slower rate than many small-molecule dyes, in many cases the lack of sufficient photostability remains an important limiting factor for experiments requiring large numbers of images of single cells. Screening methods focusing solely on brightness or wavelength are highly effective in optimizing both properties, but the absence of selective pressure for photostability in such screens leads to unpredictable photobleaching behavior in the resulting fluorescent proteins. Here we describe an assay for screening libraries of fluorescent proteins for enhanced photostability. With this assay, we developed highly photostable variants of mOrange (a wavelength-shifted monomeric derivative of DsRed from Discosoma sp.) and TagRFP (a monomeric derivative of eqFP578 from Entacmaea quadricolor) that maintain most of the beneficial qualities of the original proteins and perform as reliably as Aequorea victoria GFP derivatives in fusion constructs.

A Drug-controllable Tag for Visualizing Newly Synthesized Proteins in Cells and Whole Animals

Research on basic cellular processes involving local production or delivery of proteins, such as activity-dependent synaptic modification in neurons, would benefit greatly from a robust, nontoxic method to visualize selectively newly synthesized copies of proteins of interest within cells, tissues, or animals. We report a technique for covalent labeling of newly synthesized proteins of interest based on drug-dependent preservation of epitope tags. Epitope tags are removed from proteins of interest immediately after translation by the activity of a sequence-specific protease until the time a protease inhibitor is added, after which newly synthesized protein copies retain their tags. This method, which we call TimeSTAMP for time-specific tagging for the age measurement of proteins, allows sensitive and nonperturbative visualization and quantification of newly synthesized proteins of interest with exceptionally tight temporal control. We demonstrate applications of TimeSTAMP in retrospectively identifying growing synapses in cultured neurons and in visualizing the distribution of recently synthesized proteins in intact fly brains.

Preparation of the Membrane-permeant Biarsenicals FlAsH-EDT2 and ReAsH-EDT2 for Fluorescent Labeling of Tetracysteine-tagged Proteins

The membrane-permeant fluorogenic biarsenicals FlAsH-EDT(2) and ReAsH-EDT(2) can be prepared in good yields by a straightforward two-step procedure from the inexpensive precursor dyes fluorescein and resorufin, respectively. Handling of toxic reagents such as arsenic trichloride is minimized so the synthesis can be carried out in a typical chemistry laboratory, usually taking about 2-3 d. A wide range of other biarsenical reagents and intermediates that also bind to tetracysteine-tagged (CysCysProGlyCysCys) proteins can be prepared similarly using this general procedure.

Single-spike Detection in Vitro and in Vivo with a Genetic Ca2+ Sensor

Measurement of population activity with single-action-potential, single-neuron resolution is pivotal for understanding information representation and processing in the brain and how the brain's responses are altered by experience. Genetically encoded indicators of neuronal activity allow long-term, cell type-specific expression. Fluorescent Ca2+ indicator proteins (FCIPs), a main class of reporters of neural activity, initially suffered, in particular, from an inability to report single action potentials in vivo. Although suboptimal Ca2+-binding dynamics and Ca2+-induced fluorescence changes in FCIPs are important factors, low levels of expression also seem to play a role. Here we report that delivering D3cpv, an improved fluorescent resonance energy transfer-based FCIP, using a recombinant adeno-associated virus results in expression sufficient to detect the Ca2+ transients that accompany single action potentials. In upper-layer cortical neurons, we were able to detect transients associated with single action potentials firing at rates of <1 Hz, with high reliability, from in vivo recordings in living mice.

Characterization of Engineered Channelrhodopsin Variants with Improved Properties and Kinetics

Channelrhodopsin 2 (ChR2), a light-activated nonselective cationic channel from Chlamydomonas reinhardtii, has become a useful tool to excite neurons into which it is transfected. The other ChR from Chlamydomonas, ChR1, has attracted less attention because of its proton-selective permeability. By making chimeras of the transmembrane domains of ChR1 and ChR2, combined with site-directed mutagenesis, we developed a ChR variant, named ChEF, that exhibits significantly less inactivation during persistent light stimulation. ChEF undergoes only 33% inactivation, compared with 77% for ChR2. Point mutation of Ile(170) of ChEF to Val (yielding "ChIEF") accelerates the rate of channel closure while retaining reduced inactivation, leading to more consistent responses when stimulated above 25 Hz in both HEK293 cells and cultured hippocampal neurons. In addition, these variants have altered spectral responses, light sensitivity, and channel selectivity. ChEF and ChIEF allow more precise temporal control of depolarization, and can induce action potential trains that more closely resemble natural spiking patterns.

Hairpin Structure of a Biarsenical-tetracysteine Motif Determined by NMR Spectroscopy

The biarsenical-tetracysteine motif is a useful tag for genetic labeling of proteins with small molecules in living cells. The present study concerns the structure of a 12 amino acid peptide FLNCCPGCCMEP bound to the fluorophore ReAsH based on resorufin. (1)H NMR spectroscopy was used to determine the solution structure of the complex formed between the peptide and the ReAsH moiety. Structure calculations based on the NMR results showed that the backbone structure of the peptide is fairly well defined, with a hairpinlike turn, similar to a type-II beta-turn, formed by the central CPGC segment. The most stable complex was formed when As2 was bonded to C4 and C5 and As1 to C8 and C9. Two clear NOESY cross-peaks between the Phe1 side chain and ReAsH confirmed the close positioning of the phenyl ring of Phe1 and ReAsH. Phe1 was found to have an edge-face geometry relative to ReAsH. The close interaction between Phe1 and ReAsH may be highly significant for the fluorescence properties of the ReAsH complex.

Mammalian Expression of Infrared Fluorescent Proteins Engineered from a Bacterial Phytochrome

Visibly fluorescent proteins (FPs) from jellyfish and corals have revolutionized many areas of molecular and cell biology, but the use of FPs in intact animals, such as mice, has been handicapped by poor penetration of excitation light. We now show that a bacteriophytochrome from Deinococcus radiodurans, incorporating biliverdin as the chromophore, can be engineered into monomeric, infrared-fluorescent proteins (IFPs), with excitation and emission maxima of 684 and 708 nm, respectively; extinction coefficient >90,000 M(-1) cm(-1); and quantum yield of 0.07. IFPs express well in mammalian cells and mice and spontaneously incorporate biliverdin, which is ubiquitous as the initial intermediate in heme catabolism but has negligible fluorescence by itself. Because their wavelengths penetrate tissue well, IFPs are suitable for whole-body imaging. The IFPs developed here provide a scaffold for further engineering.

Constructing and Exploiting the Fluorescent Protein Paintbox (Nobel Lecture)

Autofluorescent Proteins with Excitation in the Optical Window for Intravital Imaging in Mammals

Fluorescent proteins have become valuable tools for biomedical research as protein tags, reporters of gene expression, biosensor components, and cell lineage tracers. However, applications of fluorescent proteins for deep tissue imaging in whole mammals have been constrained by the opacity of tissues to excitation light below 600 nm, because of absorbance by hemoglobin. Fluorescent proteins that excite efficiently in the "optical window" above 600 nm are therefore highly desirable. We report here the evolution of far-red fluorescent proteins with peak excitation at 600 nm or above. The brightest one of these, Neptune, performs well in imaging deep tissues in living mice. The crystal structure of Neptune reveals a novel mechanism for red-shifting involving the acquisition of a new hydrogen bond with the acylimine region of the chromophore.

Photoswitching Mechanism of Cyanine Dyes

Cyanine dyes have been shown to undergo reversible photoswitching, where the fluorophore can be switched between a fluorescent state and a dark state upon illumination at different wavelengths. The photochemical mechanism by which switching occurs has yet to be elucidated. In this study, we have determined the mechanism of photoswitching by characterizing the kinetics of dark state formation and the spectral and structural properties of the dark state. The rate of switching to the dark state depends on the concentration of the primary thiol in the solution and the solution pH in a manner quantitatively consistent with the formation of an encounter complex between the cyanine dye and ionized thiol prior to their conjugation. Mass spectrometry suggests that the photoconversion product is a thiol-cyanine adduct in which covalent attachment of the thiol to the polymethine bridge disrupts the original conjugated pi-electron system of the dye.

Systemic in Vivo Distribution of Activatable Cell Penetrating Peptides is Superior to That of Cell Penetrating Peptides

Cell penetrating peptides (CPPs) have been developed as vehicles for payload delivery into cells in culture and in animals. However several biologic features limit their usefulness in living animals. Activatable cell penetrating peptides (ACPPs) are polycationic CPPs whose adsorption and cellular uptake are minimized by a covalently attached polyanionic inhibitory domain. Cleavage of the linker connecting the polyanionic and polycationic domains by specific proteases (tumor associated matrix metalloproteases discussed herein) dissociates the polyanion and enables the cleaved ACPP to enter cells. In contrast to their CPP counterpart, ACPPs are relatively nonadherent and distributed uniformly to normal tissues. While nonaarginine (r(9)) CPP administered intravenously into mice initially bind to the local vasculature and redistribute to the liver, where >90% of the injected dose accumulates 30 min after injection. Regardless of the presence of the polyanionic inhibitory domain, confocal imaging of live tissues reveals that the majority of the ACPP and CPP remain in punctate organelles, presumably endosomes. Therefore further improvements in the efficiency of delivery to the cytosol and nucleus are necessary. In addition to improved target specificity, a major advantage of ACPPs over CPPs for potential clinical applications is reduced toxicity. Systemically administered r(9) CPP causes acute toxicity in mice at a dose 4-fold lower than the MMP cleavable ACPP, a complication not observed with an uncleavable ACPP presumably because the polycationic charge remains masked systemically. These data suggest that ACPPs have greater potential than CPPs for systemic delivery of imaging and therapeutic agents.

In Vivo Characterization of Activatable Cell Penetrating Peptides for Targeting Protease Activity in Cancer

Activatable cell penetrating peptides (ACPPs) are novel in vivo targeting agents comprised of a polycationic cell penetrating peptide (CPP) connected via a cleavable linker to a neutralizing polyanion (). Adsorption and uptake into cells are inhibited until the linker is proteolyzed. An ACPP cleavable by matrix metalloproteinase-2 (MMP-2) in vitro was the first one demonstrated to work in a tumor model in vivo, but only HT-1080 xenografts and resected human squamous cell carcinomas were tested. Generality to other cancer types, in vivo selectivity of ACPPs for MMPs, and spatial resolution require further characterization. We now show that ACPPs can target many xenograft tumor models from different cancer sites, as well as a thoroughly studied transgenic model of spontaneous breast cancer (mouse mammary tumor virus promoter driving polyoma middle T antigen, MMTV-PyMT). Pharmacological inhibitors and genetic knockouts indicate that current ACPPs are selective for MMP-2 and MMP-9 in the above in vivo models. In accord with the known local distribution of MMP activity, accumulation is strongest at the tumor-stromal interface in primary tumors and associated metastases, indicating better spatial resolution (<50 mum) than other currently available MMP-cleavable probes. We also find that background uptake of ACPPs into normal tissues such as cartilage can be decreased by appending inert macromolecules of 30-50 KDa to the polyanionic inhibitory domain. Our results validate an approach that should generally deliver imaging agents and chemotherapeutics to sites of invasion, tumor-promoting inflammation, and metastasis.

Indicators Based on Fluorescence Resonance Energy Transfer (FRET)

One of the major new trends in the design of indicators for optically imaging biochemical and physiological functions of living cells has been the exploitation of fluorescence resonance energy transfer (FRET). FRET is a well-known spectroscopic technique for monitoring changes in the proximity and mutual orientation of pairs of chromophores. It has long been used in biochemistry and cell biology to assess distances and orientations between specific labeling sites within a single macromolecule or between two separate molecules. More recently, macromolecules or molecular pairs have been engineered to change their FRET in response to biochemical and physiological signals such as membrane potential, cyclic AMP (cAMP), protease activity, free Ca(2+) and Ca(2+)-calmodulin (CaM) concentrations, protein-protein heterodimerization, phosphorylation, and reporter-gene expression. Because FRET is general, nondestructive, and easily imaged, it has proven to be one of the most versatile spectroscopic readouts available to the designer of new probes. FRET is particularly amenable to emission ratioing, which is more reliably quantifiable than single-wavelength monitoring and better suited than excitation ratioing to high-speed and laser-excited imaging. This article summarizes the photophysical principles of FRET and the types of indicators used.

TimeSTAMP Tagging of Newly Synthesized Proteins

The ability to quantify or visualize newly synthesized proteins has important uses in cell biology. For example, a researcher may wish to quantify basal or inducible rates of translation of a specific gene of interest, or detect subcellular locations of newly synthesized copies of a protein in order to study the role of new protein synthesis in the growth of specialized cellular structures. In this unit, the TimeSTAMP method for labeling of newly synthesized copies of a protein of interest is described. In the TimeSTAMP method, the experimenter expresses a protein of interest as a fusion with a cis-acting protease and an epitope tag, both of which are removed by default protease activity. Addition of a specific protease inhibitor then allows preservation of the tag on subsequently synthesized proteins. Finally, the tag is detected by immunological methods.

Activatable Cell Penetrating Peptides Linked to Nanoparticles As Dual Probes for in Vivo Fluorescence and MR Imaging of Proteases

High-resolution imaging of molecules intrinsically involved in malignancy and metastasis would be of great value for clinical detection and staging of tumors. We now report in vivo visualization of matrix metalloproteinase activities by MRI and fluorescence of dendrimeric nanoparticles coated with activatable cell penetrating peptides (ACPPs), labeled with Cy5, gadolinium, or both. Uptake of such nanoparticles in tumors is 4- to 15-fold higher than for unconjugated ACPPs. With fluorescent molecules, we are able to detect residual tumor and metastases as small as 200 microm, which can be resected under fluorescence guidance and analyzed histopathologically with fluorescence microscopy. We show that uptake via this mechanism is comparable to that of other near infrared protease sensors, with the added advantage that the approach is translatable to MRI. Once activated, the Gd-labeled nanoparticles deposit high levels (30-50 microM) of Gd in tumor parenchyma with even higher amounts deposited in regions of infiltrative tumor, resulting in useful T(1) contrast lasting several days after injection. These results should improve MRI-guided clinical staging, presurgical planning, and intraoperative fluorescence-guided surgery. The approach may be generalizable to deliver radiation-sensitizing and chemotherapeutic agents.

Surgery with Molecular Fluorescence Imaging Using Activatable Cell-penetrating Peptides Decreases Residual Cancer and Improves Survival

The completeness of tumor removal during surgery is dependent on the surgeon's ability to differentiate tumor from normal tissue using subjective criteria that are not easily quantifiable. A way to objectively assess tumor margins during surgery in patients would be of great value. We have developed a method to visualize tumors during surgery using activatable cell-penetrating peptides (ACPPs), in which the fluorescently labeled, polycationic cell-penetrating peptide (CPP) is coupled via a cleavable linker to a neutralizing peptide. Upon exposure to proteases characteristic of tumor tissue, the linker is cleaved, dissociating the inhibitory peptide and allowing the CPP to bind to and enter tumor cells. In mice, xenografts stably transfected with green fluorescent protein show colocalization with the Cy5-labeled ACPPs. In the same mouse models, Cy5-labeled free ACPPs and ACPPs conjugated to dendrimers (ACPPDs) delineate the margin between tumor and adjacent tissue, resulting in improved precision of tumor resection. Surgery guided by ACPPD resulted in fewer residual cancer cells left in the animal after surgery as measured by Alu PCR. A single injection of ACPPD dually labeled with Cy5 and gadolinium chelates enabled preoperative whole-body tumor detection by MRI, intraoperative guidance by real-time fluorescence, intraoperative histological analysis of margin status by fluorescence, and postoperative MRI tumor quantification. Animals whose tumors were resected with ACPPD guidance had better long-term tumor-free survival and overall survival than animals whose tumors were resected with traditional bright-field illumination only.

Simultaneous Visualization of Protumorigenic Src and MT1-MMP Activities with Fluorescence Resonance Energy Transfer

Both Src kinase and membrane type 1 matrix metalloproteinase (MT1-MMP) play critical roles in cancer invasion and metastasis. It is not clear, however, how the spatiotemporal activation of these two critical enzymes is coordinated in response to an oncogenic epithelial growth factor (EGF) stimulation. Here, we have visualized the activities of Src and MT1-MMP concurrently in a single live cell by combining two fluorescence resonance energy transfer (FRET) pairs with distinct spectra: (a) cyan fluorescent protein (CFP) and yellow FP (YFP), and (b) orange FP (mOrange2) and red FP (mCherry). The new FRET pair, mOrange2 and mCherry, was first characterized in vitro and in cultured mammalian cells. When integrated with the CFP/YFP pair, this new pair allowed the revelation of an immediate, rapid, and relatively dispersed Src activity. In contrast, the MT1-MMP activity displayed a slow increase at the cell periphery, although Src was shown to play a role upstream to MT1-MMP globally. This difference in the activation patterns of MT1-MMP and Src in response to EGF is further confirmed using an optimized MT1-MMP biosensor capable of being rapidly cleaved by MT1-MMP. The results indicate that although Src and MT1-MMP act globally in the same signaling pathway, their activations differ in space and time upon EGF stimulation, possibly mediated by different sets of intermediates at different subcellular locations. Our results also showed the potential of mOrange2/mCherry as a new FRET pair, together with the popular variants of CFP and YFP, for the simultaneous visualization of multiple molecular activities in a single live cell.

Parallel in Vivo and in Vitro Selection Using Phage Display Identifies Protease-dependent Tumor-targeting Peptides

We recently developed activatable cell-penetrating peptides (ACPPs) that target contrast agents to in vivo sites of matrix metalloproteinase activity, such as tumors. Here we use parallel in vivo and in vitro selection with phage display to identify novel tumor-homing ACPPs with no bias for primary sequence or target protease. Specifically, phage displaying a library of ACPPs were either injected into tumor-bearing mice, followed by isolation of cleaved phage from dissected tumor, or isolated based on selective cleavage by extracts of tumor versus normal tissue. Selected sequences were synthesized as fluorescently labeled peptides, and tumor-specific cleavage was confirmed by digestion with tissue extracts. The most efficiently cleaved peptide contained the substrate sequence RLQLKL and labeled tumors and metastases from several cancer models with up to 5-fold contrast. This uniquely identified ACPP was not cleaved by matrix metalloproteinases or various coagulation factors but was efficiently cleaved by plasmin and elastases, both of which have been shown to be aberrantly overexpressed in tumors. The identification of an ACPP that targets tumor expressed proteases without rational design highlights the value of unbiased selection schemes for the development of potential therapeutic agents.

Nobel Lecture: Constructing and Exploiting the Fluorescent Protein Paintbox

Neuroligin Trafficking Deficiencies Arising from Mutations in the Alpha/beta-hydrolase Fold Protein Family

Despite great functional diversity, characterization of the alpha/beta-hydrolase fold proteins that encompass a superfamily of hydrolases, heterophilic adhesion proteins, and chaperone domains reveals a common structural motif. By incorporating the R451C mutation found in neuroligin (NLGN) and associated with autism and the thyroglobulin G2320R (G221R in NLGN) mutation responsible for congenital hypothyroidism into NLGN3, we show that mutations in the alpha/beta-hydrolase fold domain influence folding and biosynthetic processing of neuroligin3 as determined by in vitro susceptibility to proteases, glycosylation processing, turnover, and processing rates. We also show altered interactions of the mutant proteins with chaperones in the endoplasmic reticulum and arrest of transport along the secretory pathway with diversion to the proteasome. Time-controlled expression of a fluorescently tagged neuroligin in hippocampal neurons shows that these mutations compromise neuronal trafficking of the protein, with the R451C mutation reducing and the G221R mutation virtually abolishing the export of NLGN3 from the soma to the dendritic spines. Although the R451C mutation causes a local folding defect, the G221R mutation appears responsible for more global misfolding of the protein, reflecting their sequence positions in the structure of the protein. Our results suggest that disease-related mutations in the alpha/beta-hydrolase fold domain share common trafficking deficiencies yet lead to discrete congenital disorders of differing severity in the endocrine and nervous systems.

Fast 18F Labeling of a Near-infrared Fluorophore Enables Positron Emission Tomography and Optical Imaging of Sentinel Lymph Nodes

We combine a novel boronate trap for F(-) with a near-infrared fluorophore into a single molecule. Attachment to targeting ligands enables localization by positron emission tomography (PET) and near-infrared fluorescence (NIRF). Our first application of this generic tag is to label Lymphoseek (tilmanocept), an agent designed for receptor-specific sentinel lymph node (SLN) mapping. The new conjugate incorporates (18)F(-) in a single, aqueous step, targets mouse SLN rapidly (1 h) with reduced distal lymph node accumulation, permits PET or scintigraphic imaging of SLN, and enables NIRF-guided excision and histological verification even after (18)F decay. This embodiment is superior to current SLN mapping agents such as nontargeted [(99m)Tc]sulfur colloids and Isosulfan Blue, as well as the phase III targeted ligand [(99m)Tc]SPECT Lymphoseek counterpart, species that are visible by SPECT or visible absorbance separately. Facile incorporation of (18)F into a NIRF probe should promote many synergistic PET and NIRF combinations.

Fluorescent Labeling of Tetracysteine-tagged Proteins in Intact Cells

In this paper, we provide a general protocol for labeling proteins with the membrane-permeant fluorogenic biarsenical dye fluorescein arsenical hairpin binder-ethanedithiol (FlAsH-EDT₂). Generation of the tetracysteine-tagged protein construct by itself is not described, as this is a protein-specific process. This method allows site-selective labeling of proteins in living cells and has been applied to a wide variety of proteins and biological problems. We provide here a generally applicable labeling procedure and discuss the problems that can occur as well as general considerations that must be taken into account when designing and implementing the procedure. The method can even be applied to proteins with expression below 1 pmol mg⁻¹ of protein, such as G protein-coupled receptors, and it can be used to study the intracellular localization of proteins as well as functional interactions in fluorescence resonance energy transfer experiments. The labeling procedure using FlAsH-EDT₂ as described takes 2-3 h, depending on the number of samples to be processed.

Fluorescent Peptides Highlight Peripheral Nerves During Surgery in Mice

Nerve preservation is an important goal during surgery because accidental transection or injury leads to significant morbidity, including numbness, pain, weakness or paralysis. Nerves are usually identified by their appearance and relationship to nearby structures or detected by local electrical stimulation (electromyography), but thin or buried nerves are sometimes overlooked. Here, we use phage display to select a peptide that binds preferentially to nerves. After systemic injection of a fluorescently labeled version of the peptide in mice, all peripheral nerves are clearly delineated within 2 h. Contrast between nerve and adjacent tissue is up to tenfold, and useful contrast lasts up to 8 h. No changes in behavior or activity are observed after treatment, indicating a lack of obvious toxicity. The fluorescent probe also labels nerves in human tissue samples. Fluorescence highlighting is independent of axonal integrity, suggesting that the probe could facilitate surgical repair of injured nerves and help prevent accidental transection.

Improved Facial Nerve Identification with Novel Fluorescently Labeled Probe

By phage display, we have developed a novel peptide (NP41) that binds selectively to nerves following systemic administration. We evaluated the pattern of facial nerve labeling with fluorescently-labeled NP41 (F-NP41). We also tested whether F-NP41 highlights facial nerves well enough to identify nerve stumps accurately several weeks after nerve transection.

A Genetically Encoded Tag for Correlated Light and Electron Microscopy of Intact Cells, Tissues, and Organisms

Electron microscopy (EM) achieves the highest spatial resolution in protein localization, but specific protein EM labeling has lacked generally applicable genetically encoded tags for in situ visualization in cells and tissues. Here we introduce "miniSOG" (for mini Singlet Oxygen Generator), a fluorescent flavoprotein engineered from Arabidopsis phototropin 2. MiniSOG contains 106 amino acids, less than half the size of Green Fluorescent Protein. Illumination of miniSOG generates sufficient singlet oxygen to locally catalyze the polymerization of diaminobenzidine into an osmiophilic reaction product resolvable by EM. MiniSOG fusions to many well-characterized proteins localize correctly in mammalian cells, intact nematodes, and rodents, enabling correlated fluorescence and EM from large volumes of tissue after strong aldehyde fixation, without the need for exogenous ligands, probes, or destructive permeabilizing detergents. MiniSOG permits high quality ultrastructural preservation and 3-dimensional protein localization via electron tomography or serial section block face scanning electron microscopy. EM shows that miniSOG-tagged SynCAM1 is presynaptic in cultured cortical neurons, whereas miniSOG-tagged SynCAM2 is postsynaptic in culture and in intact mice. Thus SynCAM1 and SynCAM2 could be heterophilic partners. MiniSOG may do for EM what Green Fluorescent Protein did for fluorescence microscopy.

Concurrent Imaging of Synaptic Vesicle Recycling and Calcium Dynamics

Synaptic transmission involves the calcium dependent release of neurotransmitter from synaptic vesicles. Genetically encoded optical probes emitting different wavelengths of fluorescent light in response to neuronal activity offer a powerful approach to understand the spatial and temporal relationship of calcium dynamics to the release of neurotransmitter in defined neuronal populations. To simultaneously image synaptic vesicle recycling and changes in cytosolic calcium, we developed a red-shifted reporter of vesicle recycling based on a vesicular glutamate transporter, VGLUT1-mOrange2 (VGLUT1-mOr2), and a presynaptically localized green calcium indicator, synaptophysin-GCaMP3 (SyGCaMP3) with a large dynamic range. The fluorescence of VGLUT1-mOr2 is quenched by the low pH of synaptic vesicles. Exocytosis upon electrical stimulation exposes the luminal mOr2 to the neutral extracellular pH and relieves fluorescence quenching. Reacidification of the vesicle upon endocytosis again reduces fluorescence intensity. Changes in fluorescence intensity thus monitor synaptic vesicle exo- and endocytosis, as demonstrated previously for the green VGLUT1-pHluorin. To monitor changes in calcium, we fused the synaptic vesicle protein synaptophysin to the recently improved calcium indicator GCaMP3. SyGCaMP3 is targeted to presynaptic varicosities, and exhibits changes in fluorescence in response to electrical stimulation consistent with changes in calcium concentration. Using real time imaging of both reporters expressed in the same synapses, we determine the time course of changes in VGLUT1 recycling in relation to changes in presynaptic calcium concentration. Inhibition of P/Q- and N-type calcium channels reduces calcium levels, as well as the rate of synaptic vesicle exocytosis and the fraction of vesicles released.

Optically Monitoring Voltage in Neurons by Photo-induced Electron Transfer Through Molecular Wires

Fluorescence imaging is an attractive method for monitoring neuronal activity. A key challenge for optically monitoring voltage is development of sensors that can give large and fast responses to changes in transmembrane potential. We now present fluorescent sensors that detect voltage changes in neurons by modulation of photo-induced electron transfer (PeT) from an electron donor through a synthetic molecular wire to a fluorophore. These dyes give bigger responses to voltage than electrochromic dyes, yet have much faster kinetics and much less added capacitance than existing sensors based on hydrophobic anions or voltage-sensitive ion channels. These features enable single-trial detection of synaptic and action potentials in cultured hippocampal neurons and intact leech ganglia. Voltage-dependent PeT should be amenable to much further optimization, but the existing probes are already valuable indicators of neuronal activity.

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