Real-time in vivo molecular detection of primary tumors and metastases with ratiometric activatable cell-penetrating peptides

Management of metastatic disease is integral to cancer treatment. Evaluation of metastases often requires surgical removal of all anatomically susceptible lymph nodes for ex vivo pathologic examination. We report a family of novel ratiometric activatable cell-penetrating peptides, which contain Cy5 as far red fluorescent donor and Cy7 as near-infrared fluorescent acceptor. Cy5 is quenched in favor of Cy7 re-emission until the intervening linker is cut by tumor-associated matrix metalloproteinases-2 and 9 (MMP2,9) or elastases. Such cleavage increases the Cy5:Cy7 emission ratio 40-fold and triggers tissue retention of the Cy5-containing fragment. This ratiometric increase provides an accelerated and quantifiable metric to identify primary tumors and metastases to liver and lymph nodes with increased sensitivity and specificity. This technique represents a significant advance over existing nonratiometric protease sensors and sentinel lymph node detection methods, which give no information about cancer invasion.

A structural basis for the assembly and functions of a viral polymer that inactivates multiple tumor suppressors

Evolution of minimal DNA tumor virus' genomes has selected for small viral oncoproteins that hijack critical cellular protein interaction networks. The structural basis for the multiple and dominant functions of adenovirus oncoproteins has remained elusive. E4-ORF3 forms a nuclear polymer and simultaneously inactivates p53, PML, TRIM24, and MRE11/RAD50/NBS1 (MRN) tumor suppressors. We identify oligomerization mutants and solve the crystal structure of E4-ORF3. E4-ORF3 forms a dimer with a central β core, and its structure is unrelated to known polymers or oncogenes. E4-ORF3 dimer units coassemble through reciprocal and nonreciprocal exchanges of their C-terminal tails. This results in linear and branched oligomer chains that further assemble in variable arrangements to form a polymer network that partitions the nuclear volume. E4-ORF3 assembly creates avidity-driven interactions with PML and an emergent MRN binding interface. This reveals an elegant structural solution whereby a small protein forms a multivalent matrix that traps disparate tumor suppressors.

Improving FRET dynamic range with bright green and red fluorescent proteins

A variety of genetically encoded reporters use changes in fluorescence (or Förster) resonance energy transfer (FRET) to report on biochemical processes in living cells. The standard genetically encoded FRET pair consists of CFPs and YFPs, but many CFP-YFP reporters suffer from low FRET dynamic range, phototoxicity from the CFP excitation light and complex photokinetic events such as reversible photobleaching and photoconversion. We engineered two fluorescent proteins, Clover and mRuby2, which are the brightest green and red fluorescent proteins to date and have the highest Förster radius of any ratiometric FRET pair yet described. Replacement of CFP and YFP with these two proteins in reporters of kinase activity, small GTPase activity and transmembrane voltage significantly improves photostability, FRET dynamic range and emission ratio changes. These improvements enhance detection of transient biochemical events such as neuronal action-potential firing and RhoA activation in growth cones.

In vivo fluorescence imaging of atherosclerotic plaques with activatable cell-penetrating peptides targeting thrombin activity

Thrombin and other coagulation enzymes have been shown to be important during atherosclerotic disease development. Study of these proteases is currently limited because of lack of robust molecular imaging agents for imaging protease activity in vivo. Activatable cell penetrating peptides (ACPPs) have been used to monitor MMP activity in tumors and, in principle, can be modified to detect other proteases. We have developed a probe that incorporates the peptide sequence DPRSFL from the proteinase activated receptor 1 (PAR-1) into an ACPP and shown that it is preferentially cleaved by purified thrombin. Active thrombin in serum cleaves DPRSFL-ACPP with >90% inhibition by lepirudin or argatroban. The DPRSFL-ACPP cleavage product accumulated in advanced atherosclerotic lesions in living mice, with 85% reduction in retention upon pre-injection of mice with hirudin. Uptake of the ACPP cleavage product was highest in plaques with histological features associated with more severe disease. Freshly resected human atheromas bathed in DPRSFL-ACPP retained 63% greater cleavage product compared to control ACPP. In conclusion, DPRSFL-ACPP can be used to study thrombin activity in coagulation and atherosclerosis with good spatial and temporal resolution. Thrombin-sensitive ACPPs may be developed into probes for early detection and intraoperative imaging of high risk atherosclerotic plaques.

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.

Abstract IA1: Building molecules to guide anticancer therapy

For clinical imaging, we need synthetic molecules with novel amplifying mechanisms for homing to diseased tissues. Activatable cell penetrating peptides (ACPPs) are polycationic cell penetrating peptides (CPPs) whose cellular uptake is minimized by a polyanionic inhibitory domain and then restored upon proteolysis of the peptide linker connecting the polyanionic and polycationic domains. Local activity of proteases able to cut the linker causes amplified retention in tissues and uptake into cells. ACPPs on dendrimers labeled with Cy5 and Gd-DOTA enable whole body magnetic resonance imaging (Olson et al (2010) PNAS 107: 4311–4316) followed by fluorescenceguided surgery. Such fluorescence guidance improves tumor-free survival in two animal models (Nguyen et al (2010) PNAS 107: 4317–4322). Contrast for tumor over normal tissues is amplified and accelerated when the polyanionic domain includes an acceptor of fluorescence resonance energy transfer (FRET), because loss of FRET (monitored either by multispectral emission or donor excited-state lifetime) instantly signals proteolysis without waiting for uncleaved substrate to wash out of normal tissues. Thrombin-cleavable ACPPs accumulate in atherosclerotic plaques, and their labeling intensity seems to correlate with progression towards rupture. Separately, we have developed fluorescent peptides that light up peripheral nerves to show surgeons where not to cut (Whitney et al (2011) Nature Biotech. 29: 352–356). Thus we believe that basic tumor biology and molecular design can help improve early detection and surgical resection, not just chemotherapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr IA1.

Abstract 5279: Imaging of pancreatic cancer with activated cell penetrating peptide dendrimers

We have recently described novel activatable cell penetrating peptides attached to dendrimers (ACPPDs) which can selectively label tumors for real-time imaging during surgery. The aim of this study was to evaluate the ability of Cy5-labeled ACPPDs to visualize primary tumors and their margins in an orthotopic mouse model of pancreatic cancer. BxPC-3 or BxPC-3-RFP-expressing human pancreatic cancer cells were injected into the pancreas of 6-week old athymic mice to establish the orthotopic mouse models. ACPPs were synthesized on an automated peptide synthesizer by using standard protocols for fluorenylmethoxycarbonyl solid-phase synthesis. Peptides were cleaved off the resin, purified by HPLC, and conjugated to a generation 5 PAMAM dendrimer. Cy5 labeling was performed directly on the dendrimer. Dendrimers were then capped completely with Peg4 NHS ester. Tumor-bearing mice were injected with 20 nmoles of ACPPDs via tail vein 48 hours prior to imaging. Mice were first imaged non-invasively using OV-100 (Olympus) or Maestro (CRI) small animal imaging systems. Mice then underwent laparotomy. Resection of the primary pancreatic tumor using fluorescence guidance was performed using a MVX-10 fluorescence-dissecting microscope (Olympus) customized to allow the surgeon to see white-light reflectance, fluorescence, or the two superimposed in real time. At the conclusion of the experiments, H&E staining was used to confirm the pathological diagnosis of cancer in the resected specimens. We found that in the orthotopic BxPC3 mouse model of pancreatic cancer, intravenous injection of ACPPD showed bright fluorescence labeling of tumors compared to surrounding normal tissue. Co-localization studies showed a good correlation between ACPPD uptake and RFP tumor expression. Tumor margins were more clearly delineated with fluorescence imaging using ACPPDs compared to with white light reflectance alone. Post resection, the tumor bed was examined and imaged for residual tumor. With fluorescence-guided resection, the tumor bed was free of residual tumor, which was confirmed histologically. In this study, we have shown that ACPPDs can accurately and effectively label primary pancreatic tumors in an orthotopic mouse model of pancreatic cancer. This technology has the potential to improve surgical outcomes in patients with pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5279. doi:10.1158/1538-7445.AM2011-5279

Spiers Memorial Lecture. Breeding and building molecular spies

To circumvent the limited spatial resolution of fluorescent protein imaging, we are developing genetically encoded tags for electron microscopy (EM).

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.

Electron microscopy (EM) once revolutionized cell biology by revealing subcellular anatomy at resolutions of tens of nanometers, well below the diffraction limit of light microscopy. Over the past two decades, light microscopy has been revitalized by the development of spontaneously fluorescent proteins, which allow nearly any protein of interest to be specifically tagged by genetic fusion. EM has lacked comparable genetic tags that are generally applicable. Here, we introduce “miniSOG”, a small (106-residue) fluorescent flavoprotein that efficiently generates singlet oxygen when illuminated by blue light. In fixed tissue, photogenerated singlet oxygen locally polymerizes diaminobenzidine into a precipitate that is stainable with osmium and therefore can be readily imaged at high resolution by EM. Thus miniSOG is a versatile label for correlated light and electron microscopy of genetically tagged proteins in cells, tissues, and organisms including intact nematodes and mice. As a demonstration of miniSOG's capabilities, controversies about the localization of synaptic cell adhesion molecules are resolved by EM of miniSOG fusions in neuronal culture and intact mouse brain.

Improved facial nerve identification with novel fluorescently labeled probe

OBJECTIVES/HYPOTHESIS

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.

STUDY DESIGN

Forty-seven wild-type mice were studied prospectively. One surgeon performed the nerve transection, reanastomosis, and monitoring of functional recovery.

METHODS

Fluorescent labeling: F-NP41 was administered intravenously (20 mice). Nerve labeling was studied with fluorescence microscopy. Transection and reanastomosis: the right facial nerve was transected (25 mice). Three weeks after transection, F-NP41 was administered intravenously and fluorescence microscopy was used to identify the nerve stumps and reanastomosis in one group. Nerve identification and reanastomosis was performed with white light in another group without F-NP41. The control group underwent sham surgery. Time to nerve identification was recorded. Functional recovery was monitored for at least 8 weeks.

RESULTS

We found excellent labeling of intact and transected facial nerves following F-NP41 administration. Several weeks following nerve transection, F-NP41 provided accurate identification of the proximal and distal nerve stumps. Following reanastomosis, time to recovery and level of functional recovery was similar in the absence and presence of F-NP41.

CONCLUSIONS

We show improved visualization of facial nerves with a novel systemically applied fluorescently labeled probe. Use of F-NP41 resulted in accurate identification of facial nerve stumps several weeks following transection. Functional recovery was similar with and without the use of F-NP41.

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 ¹⁸F⁻ 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 ¹⁸F decay. This embodiment is superior to current SLN mapping agents such as nontargeted [^99mTc]sulfur colloids and Isosulfan Blue, as well as the phase III targeted ligand [^99mTc]SPECT Lymphoseek counterpart, species that are visible by SPECT or visible absorbance separately. Facile incorporation of ¹⁸F into a NIRF probe should promote many synergistic PET and NIRF combinations.

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.

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.

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.

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.

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.

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.

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

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, who was born in New York in 1952 and grew up in Livingston New Jersey, began to experiment in the basement of the family home at a young age. From growing silica gardens of colorful crystallized metal salts to attempting to synthesize aspirin, these early experiments fueled what would become Tsien's lifelong interest in chemistry and colors. Tsien's first official laboratory experience was an NSF-supported summer research program in which he used infrared spectroscopy to examine how metals bind to thiocyanate, for which he was awarded a $10,000 scholarship in the Westinghouse Science Talent Search. Following graduation from Harvard in 1972, Tsien attended Cambridge University in England under a Marshall Scholarship. There he learned organic chemistry --a subject he'd hated as an undergraduate-- and looked for a way to synthesize dyes for imaging neuronal activity, generating BAPTA based optical calcium indicator dyes. Following the completion of his postdoctoral training at Cambridge in 1982, Tsien accepted a faculty position at the University of California, Berkeley. There he and colleagues developed and improved numerous small molecule indicators, including indicators fura-2 and indo-1. In 1989, Tsien moved his laboratory to the University of California at San Diego, where he and his colleagues developed the enhanced mutant of GFP as a way to devise a cyclic AMP (cAMP) sensor for use in live cells. They initially engineered molecules to take advantage of the conformational change that occurs when cAMP binds to protein kinase A (PKA). By labeling one part of PKA with fluoroscein and another with a rhodamine, they hoped to detect Fluorescence Resonance Energy Transfer (FRET), which would occur when the two molecules were in close proximity. The initial experiments presented numerous difficulties due to the challenges of expressing PKA subunits in E. coli, labeling the protein without destroying its function, and delivering the protein to cells via microinjection. Eventually, Tsien sought a more elegant approach, hoping to use and modify a naturally fluorescent protein that could be expressed in the cell. GFP originally described by Davenport in 1955, extracted and purified by Shimomura in 1965, and cloned by Prasher in 1992 was an appealing candidate. To make the protein more useful for their FRET studies, Tsien and colleagues modified the amino acid structure of the protein (S65T). The improved protein had an excitation peak near that of fluoroscein, and was photostable. Tsien and colleagues also solved the protein's crystal structure, enabling them to generate additional colors with spectral properties suitable for FRET. However, when they attempted to use the GFP proteins in the detection of cAMP, they experienced further difficulties with PKA. Instead, their first successful use of GFP derivatives for FRET was in the detection of intracellular calcium using their engineered calmodulin-based calcium indicator, Cameleon. In a short time, Tsien's work has led to further technological developments and important scientific findings. GFP and its derivatives have been used in a wide range of biological applications, from the study of protein localization to understanding how HIV spreads from cell to cell. The need for such probes is highlighted by the abundance of research conducted using these fluorescent proteins, as well as the continued development of similar fluorescent proteins, such as the coral-derived dsRED. Tsien is currently developing genetically encoded Infrared Fluorescent Proteins (IFPs), which with their long emission wavelengths of >700 nm, have the ability to pass through living tissue and improve imaging in living organisms. He is also building synthetic molecules for use in humans. He cites team effort and the contributions of students and post-docs as key components of progress and success: "Even if I had the time, I couldn't have done the experiments, because I don't know how. It's very much a team effort."

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.