Imaging into the future: visualizing gene expression and protein interactions with fluorescent proteins

SOCS-1 localizes to the microtubule organizing complex-associated 20S proteasome

The regulation of cytokine signaling is critical for controlling cellular proliferation and activation during an immune response. SOCS-1 is a potent inhibitor of Jak kinase activity and of signaling initiated by several cytokines. SOCS-1 protein levels are tightly regulated, and recent data suggest that SOCS-1 may regulate the protein levels of some signaling proteins by the ubiquitin proteasome pathway; however, the cellular mechanism by which SOCS-1 directs proteins for degradation is unknown. In this report, SOCS-1 is found to colocalize and biochemically copurify with the microtubule organizing complex (MTOC) and its associated 20S proteasome. The SOCS-1 SH2 domain is required for the localization of SOCS-1 to the MTOC. Overexpression of SOCS-1 targets Jak1 in an SH2-dependent manner to a perinuclear distribution resembling the MTOC-associated 20S proteasome. Analysis of MTOCs fractionated from SOCS-1-deficient cells demonstrates that SOCS-1 may function redundantly to regulate the localization of Jak1 to the MTOC. Nocodazole inhibits the protein turnover of SOCS-1, demonstrating that the minus-end transport of SOCS-1 to the MTOC-associated 20S proteasome is required to regulate SOCS-1 protein levels. These data link SOCS-1 directly with the proteasome pathway and suggest another function for the SH2 domain of SOCS-1 in the regulation of Jak/STAT signaling.

Subnuclear distribution of the largest subunit of the human origin recognition complex during the cell cycle

In eukaryotes, initiation of DNA replication requires the activity of the origin recognition complex (ORC). The largest subunit of this complex, Orc1p, has a critical role in this activity. Here we have studied the subnuclear distribution of the overexpressed human Orc1p during the cell cycle. Orc1p is progressively degraded during S-phase according to a spatio-temporal program and it never colocalizes with replication factories. Orc1p is resynthesized in G1. In early G1, the protein is distributed throughout the cell nucleus, but successively it preferentially associates with heterochromatin. This association requires a functional ATP binding site and a protein region partially overlapping the bromo-adjacent homology domain at the N-terminus of Orc1p. The same N-terminal region mediates the in vitro interaction with heterochromatin protein 1 (HP1). Fluorescence resonance energy transfer (FRET) experiments demonstrate the interaction of human Orc1p and HP1 in vivo. Our data suggest a role of HP1 in the recruitment but not in the stable association of Orc1p with heterochromatin. Indeed, the subnuclear distribution of Orc1p is not affected by treatments that trigger the dispersal of HP1.

Real-time analysis of ureteric bud branching morphogenesis in vitro

While it is clear that the normal branching morphogenesis of the ureteric bud (UB) is critical for development of the metanephric kidney, the specific patterns of branching and growth have heretofore only been inferred from static images. Here, we present a systematic time-lapse analysis of UB branching morphogenesis during the early development of the mouse kidney in organ culture. Metanephric primordia from Hoxb7/GFP transgenic embryos were cultured for 3-4 days, and GFP images of the UB taken every 30 min were assembled into movies. Analysis of these movies (available as )revealed that the UB is a highly plastic structure, which can branch in a variety of complex patterns, including terminal bifid, terminal trifid, and lateral branching. To examine kinetic parameters of branching and elongation, skeletal representations of the UB were used to measure the number of segments and branch points and the length of each segment as a function of time and of branch generation. These measurements provide a baseline for future studies on mutant kidneys with defects in renal development. To illustrate how these quantitative methods can be applied to the analysis of abnormal kidney development, we examined the effects of the MEK1 inhibitor PD98059 on renal organ cultures and confirmed a previous report that the drug has a specific inhibitory effect on UB branching as opposed to elongation.

Baculovirus expression system for magnetic sorting of infected cells and enhanced titer determination

Recombinant baculoviruses derived from the Autographa californica nuclear polyhedrosis virus (AcNPV) are widely used to express heterologous genes in insect cells, but the use of the baculovirus expression vector system (BEVS) is hampered by slow and tedious procedures for the selection and separation of baculovirus-infected insect cells and for titer determination. Here we developed a new technology based on the bicistronic vector with a fusion protein of the human integral plasma membrane glycoprotein CD4 and green fluorescent protein (GFP) for concomitant expression of target proteins in insect Sf21 cells. Magnetic cell sorting (MACS) technology with anti-CD4 antibody-labeled superparamagnetic beads was used to separate the baculovirus-infected from the noninfected insect cells and therefore to increase the virus titer and to reduce process time. With the herein described use of the MACS-improved baculovirus expression plasmid MACS in baculovirus expression (pMACSiBac-1), we have been able to select the baculovirus-infected insect cells at an early time point of the infection cycle and therefore enrich the virus titer dramatically. Furthermore, simple end point dilution and GFP fluorescence detection can be used for early and facile detection of recombinant viruses and simplified titer determinations. We show that the bicistronic pMACSiBac-1 with an additional multiple cloning site under the control of the very late promoter polyhedrin (PPH) allows for the expression of target proteins in high amounts, less workloads, and shorter timelines.

Visualisation of Zebrafish infection by GFP-labelled Vibrio anguillarum

Vibrio anguillarum is an invasive pathogen of fish causing a septicaemia called vibriosis. In this work, transparent zebrafish were immersed in water containing green fluorescent protein labelled V. anguillarum. The infection was visualised at the whole fish and single bacterium levels using microscopy. The gastrointestinal tract was the first site where the pathogen was detected. This enteric localisation occurred independently of the flagellum or motility. On the other hand, chemotactic motility was essential for association of the pathogen with the fish surface. In conclusion, the zebrafish infection model provides evidence that the intestine and skin represent sites of infection by V. anguillarum and suggests a host site where chemotaxis may function in virulence.

In vitro and in vivo studies of the Yrp1 protease from Yersinia ruckeri and its role in protective immunity against enteric red mouth disease of salmonids

Yersinia ruckeri, the etiological agent of the enteric red mouth disease (ERM) of salmonids, produces Yrp1, a serralysin metalloprotease involved in pathogenesis. We describe here the hydrolytic and immunogenic properties of Yrp1. The protease was able to hydrolyze different matrix and muscle proteins as laminin, fibrinogen, gelatine, actin, and myosin but not type II and IV collagens. In addition, the Yrp1 protein, when inactivated by heat and used as an immunogen, was able to elicit a strong protection against the development of ERM. The analysis of different Y. ruckeri strains with (Azo+) or without (Azo-) Yrp1 activity showed that all of them contained the yrp1 operon. By using yrp1::lacZ operon fusions, protease production analysis, and complementation studies, it was possible to show that an Azo- strain was blocked at the transcription level. The transcriptional study of the yrp1 operon under different environmental conditions showed that it was regulated by osmolarity and temperature, without pH influence. Finally, when beta-galactosidase activity was used as a probe in vivo, the progression of the disease in the fish could be visualized, and the tropism of the bacterium and affected organs could be defined. This system opens a vast field of study not only with regard to fish disease progression but also in pathogen interactions, temporal gene expression, carrier stages, antibiotic resistance selection, etc.

[Experimental pilot study on surface activation of implants with liposomal vectors]

BACKGROUND

Surface coating with mitogenic or morphogenic proteins can improve the healing of bone adjacent to implants and increase the bone-implant interface. Clinical surveys have shown liposome-mediated gene transfer to be a promising and safe new therapeutic method. The aim of our study was to evaluate an experimental model of new approaches for topical treatment of the implant surface and of periimplant defects by using DNA liposomes encoding for BMP-2 (bone morphogenetic protein).

MATERIAL AND METHODS

A total of 27 implants (3.5 x 14 mm) were placed in critically sized defects of the frontal skull bone of adult pigs (n=3). The bottom of the implant was placed in the base of the defect which guaranteed primary stability, whereas the superior part of the implant (10 mm) represented an implant in a defect area. Liposomes containing DNA encoding for BMP-2 and GFP (green fluorescence protein) were used. In a first trial GFP-DNA liposomes on a collagen matrix were directly applied to the periimplant defect. In a second stage, the surface of the implants was encoded with BMP-2 DNA liposomes. Subsequently, these implants were inserted in the manner described. The resulting bone samples were prepared for immunohistochemical staining. Staining for GFP was performed in the area of the defect and for BMP-2 on the bone-implant interface.

RESULTS

Immunohistochemical staining on day 3 postoperatively revealed an increased GFP expression in the periimplant defect. Therefore, the effectiveness of the liposomal vector was verified for the chosen animal model. On the surface of the implants encoded with BMP-2 DNA liposomes an increased BMP-2 expression was found. Thus, the liposomal vector system was validated also for BMP-2 DNA transfer in the chosen animal model. Further, the established system allows a sustainable and delayed release of BMP-2 in the area of the bone-implant interface.

CONCLUSIONS

As a result of the study we were able to collect data concerning the influence of implant surface conditioning on the bone-implant interface and on therapeutically relevant options for the treatment of periimplant defects. These approaches are currently being evaluated in a long-term study.

RalA activation at nascent lamellipodia of epidermal growth factor-stimulated Cos7 cells and migrating Madin-Darby canine kidney cells

RalA, a member of the Ras-family GTPases, regulates various cellular functions such as filopodia formation, endocytosis, and exocytosis. On epidermal growth factor (EGF) stimulation, activated Ras recruits guanine nucleotide exchange factors (GEFs) for RalA, followed by RalA activation. By using fluorescence resonance energy transfer-based probes for RalA activity, we found that the EGF-induced RalA activation in Cos7 cells was restricted at the EGF-induced nascent lamellipodia, whereas under a similar condition both Ras activation and Ras-dependent translocation of Ral GEFs occurred more diffusely at the plasma membrane. This EGF-induced RalA activation was not observed when lamellipodial protrusion was suppressed by a dominant negative mutant of Rac1, a GTPase-activating protein for Cdc42, inhibitors of phosphatidylinositol 3-kinase, or inhibitors of actin polymerization. On the other hand, EGF-induced lamellipodial protrusion was inhibited by microinjection of the RalA-binding domains of RalBP1 and Sec5. Furthermore, we found that RalA activity was high at the lamellipodia of migrating Madin-Darby canine kidney cells and that the migration of Madin-Darby canine kidney cells was perturbed by the microinjection of RalBP1-RalA-binding domain. Thus, RalA activation is required for the induction of lamellipodia, and conversely, lamellipodial protrusion seems to be required for the RalA activation, suggesting the presence of a positive feedback loop between RalA activation and lamellipodial protrusion. Our observation also demonstrates that the spatial regulation of RalA is conducted by a mechanism distinct from the temporal regulation conducted by Ras-dependent plasma membrane recruitment of Ral guanine nucleotide exchange factors.

Green fluorescent protein causes mitochondria to aggregate in the presence of the Bcl-2 family proteins

Green fluorescent protein (GFP) has been widely used in a variety of experiments in cell biology. When cells were co-transfected with the GFP gene and the bcl-2 family genes bcl-2, bcl-x(L), and bax, mitochondria appeared to aggregate at the periphery of the nucleus specifically where GFP was expressed. Little aggregation was seen in the presence of other members of the GFP family, EGFP (enhanced GFP), ECFP (enhanced cyan variant), and EYFP (enhanced yellow-green variant). GFP but not EGFP seemed to promote cell death induced by pro-apoptotic Bax. Thus, GFP specifically promotes the aggregation of mitochondria when co-expressed with a member of the Bcl-2 family in association with apoptosis.

Nuclear organization and the control of HIV-1 transcription

The regulation of transcription of the human immunodeficiency virus (HIV) is a complex event of significant pathological relevance, which recapitulates general concepts of cellular transcription with some peculiarities. The viral promoter is embedded in a chromatin structure that exerts powerful repression on transcription; activation of gene expression relies on the combined activity of a series of cellular factors that respond to different external stimuli, and on the function of a single viral regulatory protein, the Tat transactivator. Transcriptional activation is consequent to both chromatin remodeling and to the recruitment of elongation-competent RNA polymerase II complexes onto the integrated promoter, two events that require the coordinate, but transient, assembly of different protein complexes. Application of optical imaging techniques now allows us to appreciate the spatial and temporal evolvement of these reactions in vivo. The picture that is emerging is not only descriptive, but also relevant to the understanding of the regulation of the process. In particular, it appears that the confinement of biomolecules within specific subcellular compartments represents a way to control and coordinate the assembly of functional complexes that regulate viral gene expression.

Strategies for identifying genes that play a role in spinal cord regeneration

A search for genes that promote or block CNS regeneration requires numerous approaches; for example, tests can be made on individual candidate molecules. Here, however, we describe methods for comprehensive identification of genes up- and down-regulated in neurons that can and cannot regenerate after injury. One problem concerns identification of low-abundance genes out of the 30,000 or so genes expressed by neurons. Another difficulty is knowing whether a single gene or multiple genes are necessary. When microchips and subtractive differential display are used to identify genes turned on or off, the numbers are still too great to test which molecules are actually important for regeneration. Candidates are genes coding for trophic, inhibitory, receptor and extracellular matrix molecules, as well as unknown genes. A preparation useful for narrowing the search is the neonatal opossum. The spinal cord and optic nerve can regenerate after injury at 9 days but cannot at 12 days after birth. This narrow window allows genes responsible for the turning off of regeneration to be identified. As a next step, sites at which they are expressed (forebrain, midbrain, spinal cord, neurons or glia, intracellular or extracellular) must be determined. An essential step is to characterize proteins, their levels of expression, and their importance for regeneration. Comprehensive searches for molecular mechanisms represent a lengthy series of experiments that could help in devising strategies for repairing injured spinal cord.

Fluorescent probes for proteolysis: tools for drug discovery

Our body contains many different protease and proteolytic systems that are involved in the recycling of proteins into amino acids, and also in a multitude of regulatory events inside and outside cells. Proteases are prominent drug targets because of their well-defined chemistry and their implication in a large number of diseases, such as cancer, neurodegeneration, arteriosclerosis, inflammation and infection.

Fluorescent reporter substrates can be used to directly probe the activities of proteases in their natural environment — that is, in cells and organisms. Conceptually different strategies have been used for this purpose depending on the location and the nature of the protease of interest.

Fluorescent reporters for the ubiquitin–proteasome system have been generated by linking constitutively active degradation signals to green fluorescent protein (GFP). These GFP-based substrates can be used for functional analysis of the ubiquitin–proteasome system in cells and transgenic animals.

A collection of different proteases is involved in degradation of small peptide fragments. This process can be followed in real time in living cells by confocal laser scanning microscopy after microinjection of internally quenched peptide substrates.

Extracellular and lysosomal proteases have the advantage that they are accessible for membrane-impermeable reporter substrates. Near-infrared fluorescence (NIRF) substrates are quenched fluorescent peptides that, because of their near-infrared excitation, can be readily detected in living animals and used for in vivo monitoring of, for example, lysosomal cathepsins or surface matrix metalloproteinases.

By combining specific pairs of fluorescent proteins (GFP and its variants with shifted excitation and emission spectra) in fusion proteins, fluorescence energy transfer (FRET) reporter substrates have been generated for initiator and effector caspases.

A fluorescent intracellular reporter for human immunodeficiency virus (HIV-1) protease activity was constructed by fusing a protease precursor protein composed of HIV-1 protease and GFP. Cells will only survive and emit fluorescence when the toxic protease activity is sufficiently blocked by drugs.

The diffusion rate of the endoplasmic reticulum-resident peptide transporter complex TAP correlates with activity and thus cytosolic peptide levels. By measuring the diffusion of TAP–GFP fusions with fluorescence recovery after photobleaching (FRAP), the kinetics of peptide generation can be followed in living cells.

Cells contain numerous proteases, which are found at many different locations. These proteases recognize an even larger number of different substrates and are involved in almost every process in the cell. Aberrations in proteolysis are linked to a plethora of diseases, such as cancer, inflammation, arteriosclerosis, neurodegeneration and infection. Because of their well-defined chemistry and key role in pathologies, proteases have been important targets for drug development. Recent progress in the development of fluorescent probes has opened up the possibility of visualizing protease activities in the natural environment of the cell. We will describe various strategies to follow protease activities in cells and organisms.

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.

Nuclear Export Inhibitors and Kinase Inhibitors Identified Using a MAPK-Activated Protein Kinase 2 Redistribution®Screen

Redistribution (BioImage) A/S, Søborg, Denmark) is a novel high-throughput screening technology that monitors translocation of specific protein components of intracellular signaling pathways within intact mammalian cells, using green fluorescent protein as a tag. A single Redistribution assay can be used to identify multiple classes of compounds that act at, or upstream of, the level of the protein target used in the primary screening assay. Such compounds may include both conventional and allosteric enzyme inhibitors, as well as protein-protein interaction modulators. We have developed a series of Redistribution assays to discover and characterize compounds that inhibit tumor necrosis factor-alpha biosynthesis via modulation of the p38 mitogen-activated protein kinase (MAPK) pathway. A primary assay was designed to identify low-molecular-weight compounds that inhibit the activation-dependent nuclear export of the p38 kinase substrate MAPK-activated protein kinase 2 (MK2). Hits from the primary screen were categorized, using secondary assays, either as direct inhibitors of MK2 nuclear export, or as inhibitors of the upstream p38 MAPK pathway. Activity profiles are presented for a nuclear export inhibitor, and a compound that structurally and functionally resembles a known p38 kinase inhibitor. These results demonstrate the utility of Redistribution technology as a pathway screening method for the identification of diverse and novel compounds that are active within therapeutically important signaling pathways.

Characterization ofDrosophila fruitless-gal4 transgenes reveals expression in male-specificfruitless neurons and innervation of male reproductive structures

The fruitless (fru) gene acts in the central nervous system (CNS) of Drosophila melanogaster to establish male sexual behavior. Genetic dissection of the locus has shown that one of the fru gene's promoter, P1, controls the spatial and temporal expression of male-specific FruM proteins critical to determining stereotypical male sexual behavior. By using the Gal4-expression system, we show that a 16-kb fragment of the fru P1 promoter's 5' regulatory region drives the expression of Gal4 in a subset of FruM-expressing neurons within both the pupal and adult CNS. Colocalization of FruM and a Gal4-responsive reporter shows that the fru(P1)-gal4 fusion construct generates expression in both previously characterized FruM-expressing neurons as well as within cells of both the CNS and the peripheral nervous system that have not been demonstrated as FruM-expressing. Gal4-expressing neurons are shown to innervate abdominal organs directly relevant to fru function; specifically, the muscle of Lawrence (MOL) and the male internal reproductive organs. Innervations of the latter are shown to originate from identified FruM-serotonergic neurons. Furthermore, we show that the MOL neuromuscular junction is sexually dimorphic. Finally, we describe Gal4 expression in neurites innervating male reproductive structures that are hypothesized to be targets of fru function. Isolation of the regulatory sequences controlling the expression of fru in the CNS, therefore, provides a potent tool for the manipulation of FruM-expressing neurons and for understanding the cellular basis of Drosophila reproductive behavior.

Protein localization in living cells and tissues using FRET and FLIM

Interacting proteins assemble into molecular machines that control cellular homeostasis in living cells. While the in vitro screening methods have the advantage of providing direct access to the genetic information encoding unknown protein partners, they do not allow direct access to interactions of these protein partners in their natural environment inside the living cell. Using wide-field, confocal, or two-photon (2p) fluorescence resonance energy transfer (FRET) microscopy, this information can be obtained from living cells and tissues with nanometer resolution. One of the important conditions for FRET to occur is the overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. As a result of spectral overlap, the FRET signal is always contaminated by donor emission into the acceptor channel and by the excitation of acceptor molecules by the donor excitation wavelength. Mathematical algorithms are required to correct the spectral bleed-through signal in wide-field, confocal, and two-photon FRET microscopy. In contrast, spectral bleed-through is not an issue in FRET/FLIM imaging because only the donor fluorophore lifetime is measured; also, fluorescence lifetime imaging microscopy (FLIM) measurements are independent of excitation intensity or fluorophore concentration. The combination of FRET and FLIM provides high spatial (nanometer) and temporal (nanosecond) resolution when compared to intensity-based FRET imaging. In this paper, we describe various FRET microscopy techniques and its application to protein-protein interactions.

Optical imaging: bacteria, viruses, and mammalian cells encoding light-emitting proteins reveal the locations of primary tumors and metastases in animals

Early detection of tumors and their metastases is crucial for the prognosis of cancer treatment. Traditionally, tumor detection is achieved by various methods, including magnetic resonance imaging and computerized tomography. With the recent cloning, cellular expression, and real-time imaging of light-emitting proteins, such as Renilla luciferase (Ruc), bacterial luciferase (Lux), firefly luciferase (Luc), green fluorescent protein (GFP), or Ruc-GFP fusion protein, significant efforts have been focused on using these marker proteins for tumor detection. It has also been demonstrated that certain bacteria, viruses, and mammalian cells (BVMC), when administered systemically, are able to gain entry and replicate selectively in tumors. In addition, many tissue/tumor specific promoters have been cloned which allow transgene expression specifically in tumor tissues. Therefore, when light-emitting protein encoded BVMC are injected systemically into rodents, tumor-specific marker gene expression is achieved and is detected in real time based on light emission. Consequently, the locations of primary tumors and previously unknown metastases in animals are revealed in vivo. In the future it will likely be feasible to use engineered light-emitting BVMC as probes for tumor detection and as gene-delivery vehicles in vivo for cancer therapy.

Fluorescent ligands, antibodies, and proteins for the study of receptors

Fluorescent molecules bound to receptors can show their location and, if binding is reversible, can provide pharmacological information such as affinity and proximity between interacting molecules. The spatial precision offered by visualisation transcends the diverse localisation and low molecular concentration of receptor molecules. Consequently, the relationships between receptor location and function and life cycles of receptors have become better understood as a result of fluorescent labeling. Each of these aspects contributes new insights to drug action and potential new targets. The relationships between spatial distribution of receptor and function are largely unknown. This is particularly apparent for native receptors expressed in their normal host tissues where communication between heterogeneous cell types influences receptor distribution and function. In cultured cell systems, particularly for G-protein-coupled receptors (GPCR), fluorescence-based methods have enabled the visualisation of the cycle of agonist-stimulated receptor clustering, endocytic internalisation to the perinuclear region, degradation of the receptor-ligand complex, and recycling back to the surface membrane. Using variant forms of green fluorescent protein (GFP), antibodies, or fluorescent ligands, it is possible to detect or visualise the formation of oligomeric receptor complexes. Careful selection of fluorescent molecules based on their spectral properties enables resonance energy transfer and multilabel visualisation with colocalisation studies. Fluorescent agonist and antagonist ligands are now being used in parallel with GFP to study receptor cycling in live cells. This review covers how labeling and visualisation technologies have been applied to the study of major pharmacologically important receptors and illustrates this by giving examples of recent techniques that have relied on GFP, antibodies, or fluorescent ligands alone or in combination for the purpose of studying GPCR.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Cytoplasmic calcium measurement in rotavirus enterotoxin-enhanced green fluorescent protein (NSP4-EGFP) expressing cells loaded with Fura-2

The green fluorescent protein (GFP) and its analogs are standard markers of protein expression and intracellular localization of proteins. The fluorescent properties of GFP complicate accurate measurement of intracellular calcium using calcium sensitive fluorophores, which show a great degree of spectral overlap with GFP, or their K(d) values are too high for accurate measurement of subtle changes in cytoplasmic calcium concentrations. Here we describe a simple modification of the standard microscope-based Fura-2 calcium-imaging technique which permits the quantitative measurement of intracellular calcium levels in cells expressing enhanced green fluorescent protein (EGFP) fusion proteins. Longpass emission filtering of the Fura-2 signal in cells expressing an EGFP fusion protein is sufficient to eliminate the EGFP-Fura-2 emission spectra overlap and allows quantitative calibration of intracellular calcium. To validate this technique, we investigated the ability of rotavirus enterotoxin NSP4-EGFP to elevate intracellular calcium levels in mammalian HEK 293 cells. We show here that inducible intracellular expression of NSP4-EGFP fusion protein elevates basal intracellular calcium more than two-fold by a phospholipase C (PLC) independent mechanism.

Nuclear localization of the Hermes transposase depends on basic amino acid residues at the N-terminus of the protein

For the Hermes transposable element to be mobilized in its eukaryotic host, the transposase, encoded by the element, must make contact with its DNA. After synthesis in the cytoplasm, the transposase has to be actively imported into the nucleus because its size of 70.1 kDa prevents passive diffusion through the nuclear pore. Studies in vitro using transient expression of a Hermes-EGFP fusion protein in Drosophila melanogaster Schneider 2 cells showed the transposase was located predominantly in the nucleus. In silico sequence analysis, however, did not reveal any nuclear localization signal (NLS). To identify the sequence(s) responsible for localization of Hermes transposase in the nucleus, truncated or mutated forms of the transposase were examined for their influence on sub-cellular localization of marker proteins fused to the transposase. Using the same expression system and a GFP-GUS fusion double marker, residues 1-110 were recognized as sufficient, and residues 1-32 as necessary, for nuclear localization. Amino acid K25 greatly facilitated nuclear localization, indicating that at least this basic amino acid plays a significant role in this process. This sequence overlaps the proposed DNA binding region of the Hermes transposase and is not necessarily conserved in all members of the hAT transposable element family.

Effective expression of the green fluorescent fusion proteins in cultured Aplysia neurons

The green fluorescent fusion protein and its isoforms are extensively used to monitor gene expression, protein localisation and their dynamics in relations to fundamental cellular processes. However, it has not yet been effectively applied to Aplysia neurons that serve as a powerful model to study the mechanisms underlying neuroplasticity. We report here the development of a procedure combining in vitro transcription of mRNA encoding fluorescent-tagged proteins and its subsequent injection into the cytoplasm to image, in real-time, protein dynamics in cultured Aplysia neurones. To illustrate the efficiency of the procedure we report here the visualisation of actin, microtubules and vesicle trafficking. The results presented here introduce a reliable and effective method to express green fluorescent protein (GFP) fusion proteins in cultured Aplysia neurons.

Single HSV-amplicon vector mediates drug-induced gene expression via dimerizer system

A variety of viral vectors have been used to deliver genes into various tissues. Most have typically relied on either viral or cell-specific mammalian promoters to express transgenes. More recently, regulated promoter systems have been developed to fine-tune gene expression. Due to limited transgene capacity in most viral vectors, regulatory elements are typically subcloned into two separate vectors, which must be delivered simultaneously to a target cell. Here, we have cloned all the components of the rapamycin-based "dimerizer" system into the pantropic HSV-amplicon vector and used it to deliver and regulate red fluorescent protein (RFP) expression in cultured cells in a drug-dose-dependent manner. 293T/17 cells infected at an m.o.i. of 1 transducing unit/cell and induced with 20 nM rapamycin resulted in a 25-fold increase in RFP mRNA levels after 24 h as assessed by quantitative RT-PCR. However, due to a reduced ability to detect RFP optically, only a 5-fold induction in the number of RFP-expressing cells was noted by FACS analysis 48 h after infection. Further, there was at least 100-fold variation in the levels of RFP in individual, infected cells in the induced state. Gene induction in several neuronal models, including primary cell culture and organotypic cultures, as well as in rodent brain, was observed.

The Dynamics of Chromosome Organization and Gene Regulation

With the sequence of the human genome now complete, studies must focus on how the genome is functionally organized within the confines of the cell nucleus and the dynamic interplay between the genome and its regulatory factors to effectively control gene expression and silencing. In this review I describe our current state of knowledge with regard to the organization of chromosomes within the nucleus and the positioning of active versus inactive genes. In addition, I discuss studies on the dynamics of chromosomes and specific genetic loci within living cells and its relationship to gene activity and the cell cycle. Furthermore, our current understanding of the distribution and dynamics of RNA polymerase II transcription factors is discussed in relation to chromosomal loci and other nuclear domains.

Break ins and break outs: viral interactions with the cytoskeleton of Mammalian cells

The host cytoskeleton plays important roles in the entry, replication, and egress of viruses. An assortment of viruses hijack cellular motor proteins to move on microtubules toward the cell interior during the entry process; others reverse this transport during egress to move assembling virus particles toward the plasma membrane. Polymerization of actin filaments is sometimes used to propel viruses from cell to cell, while many viruses induce the destruction of select cytoskeletal filaments apparently to effect efficient egress. Indeed, the tactics used by any given virus to achieve its infectious life cycle are certain to involve multiple cytoskeletal interactions. Understanding these interactions, and their orchestration during viral infections, is providing unexpected insights into basic virology, viral pathogenesis, and the biology of the cytoskeleton.

Recruitment of human cyclin T1 to nuclear bodies through direct interaction with the PML protein

Human cyclin T1, the cyclin partner of Cdk9 kinase in the positive transcription elongation factor b (P-TEFb), is an essential cellular cofactor that is recruited by the human immunodeficiency virus type 1 (HIV-1) Tat transactivator to promote transcriptional elongation from the HIV-1 long terminal repeat (LTR). Here we exploit fluorescence resonance energy transfer (FRET) to demonstrate that cyclin T1 physically interacts in vivo with the promyelocytic leukaemia (PML) protein within specific subnuclear compartments that are coincident with PML nuclear bodies. Deletion mutants at the C-terminal region of cyclin T1 are negative for FRET with PML and fail to localize to nuclear bodies. Cyclin T1 and PML are also found associated outside of nuclear bodies, and both proteins are present at the chromatinized HIV-1 LTR promoter upon Tat transactivation. Taken together these results suggest that PML proteins regulate Tat- mediated transcriptional activation by modulating the availability of cyclin T1 and other essential cofactors to the transcription machinery.

A fluorogenic dye activated by the staudinger ligation

Specific labeling of biomolecules with biochemical and biophysical probes is a central element of proteomics research. Here we describe a coumarin-phosphine dye that undergoes activation of coumarin fluorescence upon Staudinger ligation with azides. Since azides can be metabolically incorporated into cellular proteins and oligosaccharides, this dye may be a useful tool for profiling proteins and their posttranslational modifications.

Development and use of fluorescent protein markers in living cells

The ability to visualize, track, and quantify molecules and events in living cells with high spatial and temporal resolution is essential for understanding biological systems. Only recently has it become feasible to carry out these tasks due to the advent of fluorescent protein technology. Here, we trace the development of highly visible and minimally perturbing fluorescent proteins that, together with updated fluorescent imaging techniques, are providing unprecedented insights into the movement of proteins and their interactions with cellular components in living cells.

Dimerization of the quorum sensing regulator RhlR: development of a method using EGFP fluorescence anisotropy

Of considerable interest in the biology of pathogenic bacteria are the mechanisms of intercellular signalling that can lead to the formation of persistent infections. In this article, we have examined the intracellular behaviour of a Pseudomonas aeruginosa quorum sensing regulator RhlR believed to be important in this process. We have further examined the modulation of this behaviour in response to various auto-inducers. For these measurements, we have developed an assay based on the fluorescence anisotropy of EGFP fusion proteins that we use to measure protein-protein interactions in vivo. We show that the transcriptional regulator, RhlR, expressed as an EGFP fusion protein in Escherichia coli, forms a homodimer. This homodimer can be dissociated into monomers by the auto-inducer N-(3-oxododecanoyl)-l-homoserine lactone (3O-C12-HSL) whereas N-(butanoyl)-l-homoserine lactone (C4-HSL) has little effect. These observations are of particular interest as RhlR modulation of gene expression depends on the presence of C4-HSL, whereas 3O-C12-HSL modulates the expression of genes regulated by LasR. These observations thus provide a framework for understanding the regulatory network that links the various different QS regulators in P. aeruginosa. Furthermore, the technique we have developed should permit the study of numerous protein/protein or protein/nucleic acid interactions in vivo and so shed light on natural protein function.

Fluorescence resonance energy transfer (FRET) microscopy imaging of live cell protein localizations

The current advances in fluorescence microscopy, coupled with the development of new fluorescent probes, make fluorescence resonance energy transfer (FRET) a powerful technique for studying molecular interactions inside living cells with improved spatial (angstrom) and temporal (nanosecond) resolution, distance range, and sensitivity and a broader range of biological applications.

Molecular quantification of genes encoding for green-fluorescent proteins

A quantitative PCR approach is presented to analyze the amount of recombinant green fluorescent protein (gfp) genes in environmental DNA samples. The quantification assay is a combination of specific PCR amplification and temperature gradient gel electrophoresis (TGGE). Gene quantification is provided by a competitively coamplified DNA standard constructed by point mutation PCR. A single base difference was introduced to achieve a suitable migration difference in TGGE between the original target DNA and the modified standard without altering the PCR amplification efficiency. This competitive PCR strategy is a highly specific and sensitive way to monitor recombinant DNA in environments like the efflux of a biotechnological plant.

MR molecular imaging of the Her-2/neu receptor in breast cancer cells using targeted iron oxide nanoparticles

MR molecular imaging is an exciting new frontier in the biomedical applications of MR. One of the clinically relevant targets is the tyrosine kinase Her-2/neu receptor, which has a significant role in staging and treating breast cancer. In this study Her-2/neu receptors were imaged in a panel of breast cancer cells expressing different numbers of the receptors on the cell membrane. Commercially available streptavidin-conjugated superparamagnetic nanoparticles were used as targeted MR contrast agent. The nanoparticles were directed to receptors prelabeled with a biotinylated monoclonal antibody and generated strong T(2) MR contrast in Her-2/neu-expressing cells. The contrast observed in MR images was proportional to the expression level of Her-2/neu receptors determined independently with FACS analysis. In these experiments, iron oxide nanoparticles were attached to the cell surface and were not internalized into the cells, which is a major advantage for in vivo applications of the method.

Visualization of identified GFP-expressing cells by light and electron microscopy

We have developed a procedure for visualizing GFP expression in fixed tissue after embedding in LR White. We find that GFP fluorescence survives fixation in 4% paraformaldehyde/0.1% glutaraldehyde and can be visualized directly by fluorescence microscopy in unstained, 1 microm sections of LR White-embedded material. The antigenicity of the GFP is retained in these preparations, so that GFP localization can be visualized in the electron microscope after immunogold labeling with anti-GFP antibodies. The ultrastructural morphology of tissue fixed and embedded by this protocol is of quality sufficient for subcellular localization of GFP. Thus, expression of GFP constructs can be visualized in living tissue and the same cells relocated in semithin sections. Furthermore, semithin sections can be used to locate GFP-expressing cells for examination by immunoelectron microscopy of the same material after thin sectioning.

Visualization of the spatial and temporal dynamics of intracellular signaling

In recent years, our ability to unravel the finer details of intracellular signaling has improved remarkably. Technological innovations resulting from the introduction of green fluorescent protein (GFP) have played a significant role in these advances. Fluorescent indicators allow us to visualize events within a cell in real-time and space. This review focuses on indicators that use GFP-based fluorescence resonance energy transfer (FRET) technologies and discusses how these methodologies have given insights into biological questions relating to the spatiotemporal patterns of signaling by cAMP, calcium, receptor tyrosine kinases, and other molecules.

Differential expression of L- and S-MAG upon cAMP stimulated differentiation in oligodendroglial cells

Myelin-associated glycoprotein (MAG), an immunoglobulin-like cell signaling protein involved in axon-glial interactions, displays two intracellular C-termini as a result of alternative mRNA splicing. During brain development, the two MAG mRNAs that encode L-MAG and S-MAG differ in their relative abundance. We have investigated the differential expression of L- and S-MAG upon cAMP treatment in the oligodendroglial cell line Oli-neu, a cell line able to differentiate in vitro. We have engineered GFP and VSVG fusions by small insertions into the alternatively spliced exons of the cloned MAG gene and reintroduced them into Oli-neu cells. The individually tagged MAG isoforms were expressed under the control of the MAG promoter and regulatory region. In this system, L-MAG was the predominant isoform before the stimulation of cells with cAMP, whereas upon cAMP treatment the S-MAG isoform was predominantly expressed in cells with a high degree of morphological differentiation. We suggest that the regulation of the MAG alternative splicing and the morphological differentiation in oligodendrocytes are controlled both by the same cAMP-responsive differentiation step.

Visualization of coupled protein folding and binding in bacteria and purification of the heterodimeric complex

During overexpression of recombinant proteins in Escherichia coli, misfolded proteins often aggregate and form inclusion bodies. If an aggregation-prone recombinant protein is fused upstream (as an N-terminal fusion) to GFP, aggregation of the recombinant protein domain also leads to misfolding of the downstream GFP domain, resulting in a decrease or loss of fluorescence. We investigated whether the GFP domain could fold correctly if aggregation of the upstream protein domain was prevented in vivo by a coupled protein folding and binding interaction. Such interaction has been previously shown to occur between the E. coli integration host factors alpha and beta, and between the domains of the general transcriptional coactivator cAMP response element binding protein (CREB)-binding protein and the activator for thyroid hormone and retinoid receptors. In this study, fusion of integration host factor beta or the CREB-binding protein domain upstream to GFP resulted in aggregation of the fusion protein. Coexpression of their respective partners, on the other hand, allowed soluble expression of the fusion protein and a dramatic increase in fluorescence. The study demonstrated that coupled protein folding and binding could be correlated to GFP fluorescence. A modified miniintein containing an affinity tag was inserted between the upstream protein domain and GFP to allow rapid purification and identification of the heterodimeric complex. The GFP coexpression fusion system may be used to identify novel protein-protein interactions that involve coupled folding and binding or protein partners that can solubilize aggregation-prone recombinant proteins.

Role of the promyelocytic leukemia body in the dynamic interaction between the androgen receptor and steroid receptor coactivator-1 in living cells

The dynamic interaction between the androgen receptor (AR) and steroid receptor coactivator-1 (SRC-1) was explored in living cells expressing chimeric forms of the receptor and the coactivator containing two spectral variants of jellyfish fluorescent protein. Laser scanning confocal imaging of transfected cells expressing fluorescently labeled SRC-1 revealed that in an unsynchronized cell population, the coactivator is distributed in approximately 40% cells as nuclear bodies of 0.2-1.0 microm in diameter. Immunostaining of cyan fluorescent protein-labeled SRC-1 (CFP-SRC1)-expressing cells with antibody to promyelocytic leukemia (PML) protein showed significant overlap of the CFP fluorescence with the antibody stain. Cotransfection of cells with a plasmid expressing the CFP conjugate of Sp100 (another marker protein for the PML nuclear body) also showed colocalization of the yellow fluorescent protein (YFP)-SRC1 containing nuclear foci with the PML bodies in living cells. Analysis of the three-dimensional structure revealed that the PML bodies are round to elliptical in shape with multiple satellite bodies on their surface. Some of these satellite bodies contain the SRC-1. Activation and nuclear import of CFP-AR by the agonistic ligand 5alpha-dihydrotestosterone, but not by the antagonist casodex, transferred YFP-SRC1 from the PML bodies to an interlacing filamentous structure. In a single living cell, agonist-activated AR caused a time-dependent movement of YFP-SRC1 from the PML bodies to this filamentous structure. Additionally, coexpression of a constitutively active mutant of AR (AR-deltaligand binding domain) also displaced YFP-SRC1 from the PML bodies to this intranuclear filamentous structure. The fluorescence recovery after photobleaching approach was used to examine changes in the kinetics of movement of YFP-SRC1 during its mobilization from the PML bodies to the intranuclear filamentous structure by the agonist-activated AR. Results of the relative half-times (t(1/2)) of replacement of YFP-SRC1 within the photobleached region of a single PML body from its surrounding nuclear space supported the conclusion that SRC-1 is actively transported from the PML bodies to the intranuclear filamentous structure by the ligand-activated AR. This observation also suggests an interaction between AR and SRC-1 before its binding to the target gene. The PML bodies have been implicated as a cross-road for multiple regulatory pathways that control cell proliferation, cellular senescence, and apoptosis. Our present results along with other recent reports expand the role of this subnuclear structure to include the regulation of steroid hormone action.

New technologies: bioluminescence resonance energy transfer (BRET) for the detection of real time interactions involving G-protein coupled receptors

The natural phenomenon of bioluminescence resonance energy transfer (BRET) has become an extremely useful tool for studying protein-protein interactions in the laboratory, including those involving G-protein coupled receptors (GPCRs). The technology involves fusion of donor and acceptor molecules to proteins of interest. Following assessment to ensure correct functionality, co-expression of fusion constructs in live cells enables their interaction to be studied in real time in a quantitative manner. Energy is transferred from the donor to the acceptor when in close proximity, resulting in fluorescence emission at a characteristic wavelength. The energy emitted by the acceptor relative to that emitted by the donor is termed the BRET signal. It is dependent upon the spectral properties, ratio, distance and relative orientation of the donor and acceptor molecules, as well as the strength and stability of the interaction between the proteins of interest. The ability to study interactions in live mammalian cells circumvents many of the problems associated with techniques such as co-immunoprecipitation and yeast two-hybrid screening. Furthermore, the high sensitivity of BRET enables the study of proteins at physiological concentrations, a significant advantage over techniques that require high levels of protein expression. BRET technology has already made a substantial contribution to our understanding of GPCRs and protein-protein interactions, in particular by providing strong evidence that GPCRs homo- and hetero-oligomerize. New BRET detection systems and the potential for novel high throughput screening applications means that BRET promises to play an important role in future research and drug discovery.

Applications of novel resonance energy transfer techniques to study dynamic hormone receptor interactions in living cells

Many aspects of hormone receptor function that are crucial for controlling signal transduction of endocrine pathways can be monitored more accurately with the use of non-invasive, live cell resonance energy transfer (RET) techniques. Fluorescent RET (FRET), and its variation, bioluminescent RET (BRET), can be used to assess the real-time responses to specific hormonal stimuli, whilst preserving the cellular protein network, compartmentalization and spatial arrangement. Both FRET and BRET can be readily adapted to the study of membrane proteins. Here, we focus on their applications to the analysis of interactions involving the superfamily of hormone G-protein-coupled receptors. RET is also emerging as a significant tool for the determination of protein function in general. Such techniques will undoubtedly be of value in determining the functional identities of the vast array of proteins that are encoded by the human genome.

Cell type-selective expression of green fluorescent protein and the calcium indicating protein, yellow cameleon, in rat cortical primary cultures

A cell type-specific green fluorescent protein (GFP) expression system in rat cortical primary cultures has been developed for the fluorescence labeling of brain cells. Lipid-mediated transfection (lipofection) was employed, allowing the establishment of a convenient efficient system for the analysis of individual cells. To achieve cell type-specific labeling, GFP expression vectors containing the rat neuron-specific enolase (NSE) gene promoter, human glial fibril acidic protein (GFAP) gene promoter, human elongation factor (EF-1alpha) gene promoter, or human cytomegalovirus (CMV) immediate early promoter were constructed, and their specificities examined. Vectors containing the CMV or GFAP promoter resulted primarily in GFP expression in astrocytes, while those containing the EF1-alpha or NSE promoter resulted primarily in GFP expression in neurons. This labeling system was applied to the morphological analysis of living neurons and to cell type-selective calcium imaging. Confocal microscopy revealed that individual GFP-expressing neurons had processes, which were longer than 500 microm and bore spine-like protrusions. A calcium-indicating GFP variant, yellow cameleon (YC2.1), was expressed in the same system, and cell type-selective calcium imaging performed. On pharmacological stimulation, YC2.1-expressing neurons responded to depolarizing stimuli, but not to the metabotropic glutamate receptor agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (tACPD), while astrocytes responded only to tACPD.

Quantitation of GFP-fusion proteins in single living cells

The green fluorescent protein (GFP) has revolutionized cell biology. The ability to observe genetically encoded fluorescently tagged fusion proteins in intact cells has made virtually any biological process amenable to investigation in living cells. However, most in vivo imaging studies are qualitative and little information about the number of fluorescently labeled molecules observed in a cell or a cellular structure is available. This deficiency severely limits the interpretation of imaging experiments and it impedes the application of in vivo imaging methods for biophysical purposes. Here we describe a simple method for the quantitative determination of the number of GFP-tagged molecules in cellular structures in single living cells. The method is based on the use of rotavirus-like particles containing a known number of GFP molecules as an internal calibration standard during in vivo imaging. We have applied this method to estimate in single living cells the number of fluorescent transcription factor molecules on RNA polymerase I and polymerase II genes. In addition, we have estimated the number of molecules for several proteins in subnuclear compartments and in exocytic vesicles. VLP-GFP calibration is a simple, convenient, rapid, and noninvasive method for routine quantification of GFP-labeled molecules in single, living cells.

Investigation of nuclear architecture with a domain-presenting expression system

We have investigated the topogenic properties of the nucleus by ectopic expression of chimeric proteins consisting of a NLS-modified cytoplasmic filament-forming protein, Xenopus laevis vimentin, and domains of inner nuclear membrane proteins. Whereas the "carrier" without cargo, the NLS-vimentin alone, is deposited in a few nuclear body-type structures (J.M. Bridger, H. Herrmann, C. Münkel, P. Lichter, J. Cell Sci., 111, 1241-1253), the distribution is entirely changed upon coupling with the evolutionarily conserved domain of the lamin B tail, the entire lamin B tail, the amino-terminal nucleoplasmic segment of the lamin B receptor (LBR), and the LEM domain of emerin, respectively. Remarkably, every individual chimeric protein exhibits a completely different distribution. Therefore, we assume that the chimeric parts are specifically recognized by factors engaged in nucleus-specific topogenesis. Thus, the conserved domain of the lamin B tail results in the formation of many small accumulations spread all over the nucleus. The chimera with the complete lamin B tail is deposited in short fibrillar aggregates within the nucleus. It does not mediate the integration of the chimeric protein into the nuclear membrane in cultured cells, indicating that the lamin tail alone is not sufficient to direct the integration of a protein into the lamina in vivo. In contrast, in the nuclear assembly system of Xenopus laevis the recombinant NLS-vimentin-lamin tail protein is concentrated at the nuclear membrane. The LBR chimera is arranged in a "beaded-chain"-type fashion, quite different from the more random deposition of NLS-vimentin alone. To our surprise, the LEM domain of emerin induces the retention of most of the chimeric proteins within the cytoplasm. Hence, it appears to be engaged in a strong cytoplasmic interaction that overrides the nuclear localization signal. Finally, the lamin chimera with the conserved part of the lamin B tail is shown to recruit LBR to the nuclear vimentin bodies and, vice versa, the LBR chimera attracts lamin B in transfected cells, thereby demonstrating their bona fide interaction in vivo.

New developments in multiphoton microscopy

Multiphoton laser-scanning microscopy is still developing rapidly, both technologically and by broadening its range of application. Technical progress has been made in the optimization of fluorophores, in increasing the imaging depth of multiphoton microscopy, and in microscope miniaturization. These advances further facilitate the study of neuronal structure and signaling in living and even in behaving animals, in particular in combination with the expression of fluorescent proteins. In addition, nonlinear optical contrast mechanisms other than multiphoton excitation of fluorescence are being explored.

A functional link between glucokinase binding to insulin granules and conformational alterations in response to glucose and insulin

Glucokinase (GK) activity is essential for the physiological regulation of insulin secretion by glucose. Because the enzyme exerts nearly total control over glucose metabolism in the beta-cell, even small changes in GK activity exert effects on glucose-stimulated insulin secretion and, consequently, the blood glucose concentration. Using quantitative imaging of multicolor fluorescent proteins fused to GK, we found that the association of GK with insulin granules is regulated by glucose in the beta-cell. Glucose stimulation increased the rate of fluorescence recovery after photobleaching of GK to insulin granules, indicating that GK is released into the cytoplasm after glucose stimulation. Changes in fluorescence resonance energy transfer between two different fluorescent protein variants inserted on opposing ends of GK were observed after glucose stimulation and correlated with increased enzyme activity. Furthermore, glucose-stimulated changes in GK regulation were blocked by two inhibitors of insulin secretion. Insulin treatment restored GK regulation in inhibited cells and stimulated GK translocation and activation by itself. Together, these data support a model for post-translational regulation of GK whereby insulin regulates both the association of GK with secretory granules and the activity of the enzyme within the pancreatic beta-cell.

Genetic tags for labelling live cells: gap junctions and beyond

The availability of green fluorescent protein (GFP) as a tracer for observing proteins in living cells has revolutionized cell biology and spurred an intensive search for GFP variants with novel characteristics, additional autofluorescent proteins and alternative techniques of protein labelling. Two recent studies - one on tagging with tetracysteine motifs and labelling with biarsenic fluorophores of different colours, and the other on GFP tagging and fluorescence recovery after photobleaching (FRAP) - show how membrane channels are added and removed from gap junctions by using different fluorescent tags to distinguish between newly synthesized and older protein populations.