Multicolor and electron microscopic imaging of connexin trafficking

Is autism due to brain desynchronization?

The hypothesis is presented that a disruption in brain synchronization contributes to autism by destroying the coherence of brain rhythms and slowing overall cognitive processing speed. Particular focus is on the inferior olive, a precerebellar structure that is reliably disrupted in autism and which normally generates a coherent 5-13 Hz rhythmic output. New electrophysiological data reveal that the continuity of the rhythmical oscillation in membrane potential generated by inferior olive neurons requires the formation of neuronal assemblies by the connexin36 protein that mediates electrical synapses and promotes neuronal synchrony. An experiment with classical eyeblink conditioning is presented to demonstrate that the inferior olive is necessary to learn about sequences of stimuli presented at intervals in the range of 250-500 ms, but not at 700 ms, revealing that a disruption of the inferior olive slows stimulus processing speed on the time scale that is lost in autistic children. A model is presented in which the voltage oscillation generated by populations of electrically synchronized inferior olivary neurons permits the utilization of sequences of stimuli given at, or faster than, 2 per second. It is expected that the disturbance in inferior olive structure in autism disrupts the ability of inferior olive neurons to become electrically synchronized and to generate coherent rhythmic output, thereby impairing the ability to use rapid sequences of cues for the development of normal language skill. Future directions to test the hypothesis are presented.

Dynamic fluorescent imaging of human immunodeficiency virus type 1 gag in live cells by biarsenical labeling

Human immunodeficiency virus type 1 (HIV-1) Gag is the primary structural protein of the virus and is sufficient for particle formation. We utilized the recently developed biarsenical-labeling method to dynamically observe HIV-1 Gag within live cells by adding a tetracysteine tag (C-C-P-G-C-C) to the C terminus of Gag in both Pr55Gag expression and full-length proviral constructs. Membrane-permeable biarsenical compounds FlAsH and ReAsH covalently bond to this tetracysteine sequence and specifically fluoresce, effectively labeling Gag in the cell. Biarsenical labeling readily and specifically detected a tetracysteine-tagged HIV-1 Gag protein (Gag-TC) in HeLa, Mel JuSo, and Jurkat T cells by deconvolution fluorescence microscopy. Gag-TC was localized primarily at or near the plasma membrane in all cell types examined. Fluorescent two-color analysis of Gag-TC in HeLa cells revealed that nascent Gag was present mostly at the plasma membrane in distinct regions. Intracellular imaging of a Gag-TC myristylation mutant observed a diffuse signal throughout the cell, consistent with the role of myristylation in Gag localization to the plasma membrane. In contrast, mutation of the L-domain core sequence did not appreciably alter the localization of Gag, suggesting that the PTAP L domain functions at the site of budding rather than as a targeting signal. Taken together, our results show that Gag concentrates in specific plasma membrane areas rapidly after translation and demonstrate the utility of biarsenical labeling for visualizing the dynamic localization of Gag.

ZO-1 alters the plasma membrane localization and function of Cx43 in osteoblastic cells

ZO-1 is the major connexin-interacting protein in ROS 17/2.8 (ROS) osteoblastic cells. We examined the role of ZO-1 in Cx43-mediated gap junction formation and function in ROS cells that expressed the connexin-interacting fragment of ZO-1 (ROS/ZO-1dn) cells. Expression of this ZO-1(7-444) fusion protein in ROS cells disrupted the Cx43/ZO-1 interaction and decreased dye transfer by 85%, although Cx43 was retained on the plasma membrane as assessed by surface biotinylation. Fractionation of lysates derived from ROS/ZO-1dn cells on a 5-30% sucrose flotation gradient showed that 40% of the Cx43 floated into these sucrose gradients, whereas none of the Cx43 in ROS cell lysates entered the gradients, suggesting that more Cx43 is associated with lipid rafts in the transfected ROS cells than in lysates derived from untransfected ROS cells. In contrast to the ROS/ZO-1dn cells, ROS cells that over-expressed ZO-1 protein (ROS/ZO-1myc cells) exhibited increased gap junctional permeability and appositional membrane staining for Cx43. These data demonstrate that ZO-1 regulates Cx43-mediated gap junctional communication in osteoblastic cells and alters the membrane localization of Cx43. They suggest that ZO-1-mediated delivery of Cx43 from a lipid raft domain to gap junctional plaques may be an important regulatory step in gap junction formation.

Novel rab GAP-like protein, CIP85, interacts with connexin43 and induces its degradation

Gap junctions play critical roles in tissue function and homeostasis. Connexin43 (Cx43) is a major gap junction protein expressed in the mammalian heart and other tissues and may be regulated by its interaction with other cellular proteins. Using the yeast two-hybrid screen, we identified a novel Cx43-interacting protein of 85-kDa, CIP85, which contains a single TBC, SH3, and RUN domain, in addition to a short coiled coil region. Homologues containing this unique combination of domains were found in human, D. melanogaster, and C. elegans. CIP85 mRNA is expressed ubiquitously in mouse and human tissues. In vitro interaction assays and in vivo co-immunoprecipitation experiments confirmed the interaction of endogenous CIP85 with Cx43. In vitro interaction experiments using CIP85 mutants with in-frame deletions of the TBC, SH3, and RUN domains indicated that the SH3 domain of CIP85 is involved in its interaction with Cx43. Conversely, analysis of Cx43 mutants with proline to alanine substitutions in the two proline-rich regions of Cx43 revealed that the P(253)LSP(256) motif is an important determinant of the ability of Cx43 to interact with CIP85. Laser-scanning confocal microscopy showed that CIP85 colocalized with Cx43 at the cell periphery, particularly in areas reminiscent of gap junction plaques. The functional importance of the interaction between CIP85 and Cx43 was suggested by the observation that CIP85 appears to induce the turnover of Cx43 through the lysosomal pathway.

One-step, non-denaturing isolation of an RNA polymerase enzyme complex using an improved multi-use affinity probe resin

The rapid isolation of protein complexes is critical to the goal of establishing protein interaction networks. High-throughput methods for identifying protein binding partners in a way suitable for mass spectrometric identification and structural analysis are required and small molecule/peptide interactions provide the key. We have now shown that a redesigned resin derivatized with a bisarsenical dye can be used to isolate the Shewanella oneidensis RNA polymerase core enzyme with a tetracysteine-tagged RNA polymerase A as bait protein. A critical advantage of this method is the ability to release the intact complex using a mild, one-step procedure with a competing dithiol. In addition to the identification of the core complex, additional interaction partners, including universal stress protein, were identified. These results provide a path forward to identifying how changes in critical protein complexes over time modulate cell function.

Connexins: functions without junctions

Emerging studies indicate that connexins have activities completely unrelated to gap junctions and, conversely, that non-connexin proteins can form gap junction channels.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.

From proteomic inventory to architecture

Electron tomography can provide three-dimensional reconstructions of large pleomorphic structures at molecular resolution. While the principles of electron tomography have been known for decades, its use has gathered momentum only in recent years. Technological advances have made it possible to apply it to ice-embedded biological material (cryotomography), thereby ensuring a close-to-life preservation of the samples. In combination with advanced computational methods, such as molecular identification based on pattern recognition, it is a promising approach to comprehensively map macromolecular architecture inside organelles and cells and to visualize macromolecules at work in their natural environment.

Connexin-based gap junction hemichannels: gating mechanisms

Connexins (Cxs) form hemichannels and gap junction channels. Each gap junction channel is composed of two hemichannels, also termed connexons, one from each of the coupled cells. Hemichannels are hexamers assembled in the ER, the Golgi, or a post Golgi compartment. They are transported to the cell surface in vesicles and inserted by vesicle fusion, and then dock with a hemichannel in an apposed membrane to form a cell-cell channel. It was thought that hemichannels should remain closed until docking with another hemichannel because of the leak they would provide if their permeability and conductance were like those of their corresponding cell-cell channels. Now it is clear that hemichannels formed by a number of different connexins can open in at least some cells with a finite if low probability, and that their opening can be modulated under various physiological and pathological conditions. Hemichannels open in different kinds of cells in culture with conductance and permeability properties predictable from those of the corresponding gap junction channels. Cx43 hemichannels are preferentially closed in cultured cells under resting conditions, but their open probability can be increased by the application of positive voltages and by changes in protein phosphorylation and/or redox state. In addition, increased activity can result from the recruitment of hemichannels to the plasma membrane as seen in metabolically inhibited astrocytes. Mutations of connexins that increase hemichannel open probability may explain cellular degeneration in several hereditary diseases. Taken together, the data indicate that hemichannels are gated by multiple mechanisms that independently or cooperatively affect their open probability under physiological as well as pathological conditions.

Painting protein misfolding in the cell in real time with an atomic-scale brush

The direct observation of specific biochemical events in living cells is now possible as a result of combined advances in molecular biology and fluorescence microscopy. By genetically encoding the source of a unique spectroscopic signal, target proteins can be selectively detected within the complex cellular environment, with limited interference from background signals. A recent study takes advantage of arsenical reagent-based methodologies to monitor in vivo protein misfolding and inclusion body formation in real time. This approach promises to yield important information on the kinetics of aggregate formation in living cells and its relation to the time-course of protein expression and post-translational processing. The ability to follow protein self-association in real time accurately from its early stages is unique to this method, and has far-reaching implications for both biotechnology and misfolding-based disease.

Pharmacological properties of homomeric and heteromeric pannexin hemichannels expressed in Xenopus oocytes

Several new findings have emphasized the role of neuron-specific gap junction proteins (connexins) and electrical synapses in processing sensory information and in synchronizing the activity of neuronal networks. We have recently shown that pannexins constitute an additional family of proteins that can form gap junction channels in a heterologous expression system and are also widely expressed in distinct neuronal populations in the brain, where they may represent a novel class of electrical synapses. In this study, we have exploited the hemichannel-forming properties of pannexins to investigate their sensitivity to well-known connexin blockers. By combining biochemical and electrophysiological approaches, we report here further evidence for the interaction of pannexin1 (Px1) with Px2 and demonstrate that the pharmacological sensitivity of heteromeric Px1/Px2 is similar to that of homomeric Px1 channels. In contrast to most connexins, both Px1 and Px1/Px2 hemichannels were not gated by external Ca2+. In addition, they exhibited a remarkable sensitivity to blockade by carbenoxolone (with an IC50 of approximately 5 microm), whereas flufenamic acid exerted only a modest inhibitory effect. The opposite was true in the case of connexin46 (Cx46), thus indicating that gap junction blockers are able to selectively modulate pannexin and connexin channels.

A FlAsH-based FRET approach to determine G protein–coupled receptor activation in living cells

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

A new protein conformation indicator based on biarsenical fluorescein with an extended benzoic acid moiety

We demonstrate herein a new protein conformation indicator based on biarsenical fluorescein with an extended benzoic acid moiety. The present indicator is reactive to a genetically introduced tetracysteine motif (Cys-Cys-Xaa-Xaa-Cys-Cys, where Xaa is a noncysteine amino acid) of proteins. Compared to the original biarsenical fluorescein (FlAsH) and the biarsenical Nile red analogue (BArNile), the present indicator exhibited larger fluorescence intensity changes in response to Ca(2+)-induced conformational rearrangements of calmodulin. A calculation of the highest occupied molecular orbital (HOMO) level of the benzoic acid moiety of the indicator molecule supports possible involvement of a photoinduced electron transfer (PET) process. These results indicate that the present indicator is useful for sensitive detection of protein conformational changes.

Selective chemical labeling of proteins in living cells

Labeling proteins with fluorophores, affinity labels or other chemically or optically active species is immensely useful for studying protein function in living cells or tissue. The use of genetically encoded green fluorescent protein and its variants has been particularly valuable in this regard. In an effort to increase the diversity of available protein labels, various efforts to append small molecules to selected proteins in vivo have been reported. This review discusses recent advances in selective, in vivo protein labeling based on small molecule ligand-receptor interactions, intein-mediated processes, and enzyme-catalyzed protein modifications.

Chemical biology on PINs and NeeDLes

Systematic studies of the organization of biochemical networks that make up the living cell can be defined by studying the organization and dynamics of protein interaction networks (PINs). Here, we describe recent conceptual and experimental advances that can achieve this aim and how chemical perturbations of interactions can be used to define the organization of biochemical networks. Resulting perturbation profiles and subcellular locations of interactions allow us to 'place' each gene product at its relevant point in a network. We discuss how experimental strategies can be used in conjunction with other genome-wide analyses of physical and genetic protein interactions and gene transcription profiles to determine network dynamic linkage (NDL) in the living cell. It is through such dynamic studies that the intricate networks that make up the chemical machinery of the cell will be revealed.

Site-specific labeling of proteins with small molecules in live cells

The principal bottleneck for the utilization of small-molecule probes in live cells is the shortage of methodologies for targeting them with very high specificity to biological molecules or compartments of interest. Recently developed methods for labeling proteins with small-molecule probes in cells employ special protein or peptide handles that recruit small-molecule ligands, harness enzymes to catalyze small-molecule conjugation or hijack the cell's protein translation machinery.

Labeling of fusion proteins of O6-alkylguanine-DNA alkyltransferase with small molecules in vivo and in vitro

The in vivo and in vitro labeling of fusion proteins with synthetic molecules capable of probing and controlling protein function has the potential to become an important method in functional genomics and proteomics. We have recently introduced an approach for the specific labeling of fusion proteins, which is based on the generation of fusion proteins with the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (hAGT) and the irreversible reaction of hAGT with O6-benzylguanine derivatives. Here, we report optimized protocols for the synthesis of O6-benzylguanine derivatives and the use of such derivatives for the labeling of different hAGT fusion proteins in vivo and in vitro.

Nanometre localization of single ReAsH molecules

ReAsH is a red-emitting dye that binds to the unique sequence Cys-Cys-Xaa-Xaa-Cys-Cys (where Xaa is a noncysteine amino acid) in the protein. We attached a single ReAsH to a calmodulin with an inserted tetracysteine motif and immobilized individual calmodulins to a glass surface at low density. Total internal reflection fluorescence microscopy was used to image individual ReAsH molecules. We determined the centre of the distribution of photons in the image of a single molecule in order to determine the position of the dye within 5 nm precision and with an image integration time of 0.5 s. The photostability of ReAsH was also characterized and observation times ranging from several seconds to over a minute were observed. We found that 2-mercaptoethanesulphonic acid increased the number of collected photons from ReAsH molecules by a factor of two. Individual ReAsH molecules were then moved via a nanometric stage in 25 or 40 nm steps, either at a constant rate or at a Poisson-distributed rate. Individual steps were clearly seen, indicating that the observation of translational motion on this scale, which is relevant for many biomolecular motors, is possible with ReAsH.

Automated most-probable loss tomography of thick selectively stained biological specimens with quantitative measurement of resolution improvement

We describe the technique and application of energy filtering, automated most-probable loss (MPL) tomography to intermediate voltage electron microscopy (IVEM). We show that for thick, selectively stained biological specimens, this method produces a dramatic increase in resolution of the projections and the computed volumes versus standard unfiltered transmission electron microscopy (TEM) methods. This improvement in resolution is attributed to the reduction of chromatic aberration, which results from the large percentage of inelastic electron-scattering events for thick specimens. These improvements are particularly evident at the large tilt angles required to improve tomographic resolution in the z-direction. This method effectively increases the usable thickness of selectively stained samples that can be imaged at a given accelerating voltage by dramatically improving resolution versus unfiltered TEM and increasing signal-to-noise versus zero-loss imaging, thereby expanding the utility of the IVEM to deliver information from within specimens up to 3 microm thick.

Building and breeding molecules to spy on cells and tumors

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

Fusion of GFP to the carboxyl terminus of connexin43 increases gap junction size in HeLa cells

The pattern of gap junctional coupling between cells is thought to be important for the proper function of many types of tissues. At present, little is known about the molecular mechanisms that control the size and distribution of gap junctions. We addressed this issue by expressing connexin43 (Cx43) constructs in HeLa cells, a connexin-deficient cell line. HeLa cells expressing exogenously introduced wild-type Cx43 formed small, punctate gap junctions. By contrast, cells expressing Cx43-GFP formed large, sheet-like gap junctions. These results suggest that the GFP tag, which is fused to the carboxyl terminus of Cx43, alters gap junction size by masking the carboxyl terminal amino acids of Cx43 that comprise a zonula occludins-1 (ZO-1) binding site. We are currently testing this hypothesis using deletion and dominant-negative constructs that directly target the interaction between Cx43 and ZO-1.

Tetracysteine genetic tags complexed with biarsenical ligands as a tool for investigating gap junction structure and dynamics

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

A Fluorogenic 1,3-Dipolar Cycloaddition Reaction of 3-Azidocoumarins and Acetylenes†

Copper(I)-catalyzed 1,3-dipolar cycloaddition reaction of nonfluorescent 3-azidocoumarins and terminal alkynes afforded intense fluorescent 1,2,3-triazole products. The mild condition of this reaction allowed us to construct a large library of pure fluorescent coumarin dyes. Since both azide and alkyne are quite inert to biological systems, this reaction has potential in bioconjugation and bioimaging applications. [reaction: see text]

Endocytosis of connexin protein in adrenal cells

The ability of adrenocorticotropin (ACTH) to affect gap junctions was examined in adrenal cells in vivo and in vitro. Treatment with ACTH increased the size and number of gap junction plaques on the cell membranes in hypophysectomized animals and in adrenal culture. Intracellular (cytoplasmic) annular gap junctions were observed in cells of the inner adrenal cortical zones and in adrenal cell cultures. To investigate the relationship of annular gap junctions to surface junctions, adrenal cells in culture were transfected with cDNA encoding a green fluorescent protein tagged connexin 43 construct (Cx43-GFP), and subsequently studied by time-lapse video microscopy, immunocytochemistry, and transmission electron microscopy (TEM). Internalization of part or all of a surface gap junction plaque resulted in annular gap junction formation. These studies support the hypothesis that cytoplasmic vesicles, initially described with TEM methods, can result from removal of gap junction plaques from the cell surface. It is suggested that hormones can play a regulatory role in cell-cell communication by influencing the availability of gap junction protein at the cell surface and that hormonally-sensitive processes might serve as a means of altering intercellular communication.

Connexin phosphorylation as a regulatory event linked to gap junction channel assembly

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells and are important in development and maintenance of cell homeostasis. Phosphorylation has been implicated in the regulation of gap junctional communication at several stages of the cell cycle and the connexin "lifecycle", such as trafficking, assembly/disassembly, degradation, as well as in the gating of "hemi" channels or intact gap junction channels. This review focuses on how phosphorylation can regulate the early stages of the connexin life cycle through assembly of functional gap junctional channels. The availability of sequences from the human genome databases has indicated that the number of connexins in the gene family is approximately 20, but we know mostly about how connexin43 (Cx43) is regulated. Recent technologies and investigations of interacting proteins have shown that activation of several kinases including protein kinase A, protein kinase C (PKC), p34(cdc2)/cyclin B kinase, casein kinase 1 (CK1), mitogen-activated protein kinase (MAPK) and pp60(src) kinase can lead to phosphorylation of the majority of the 21 serine and two of the tyrosine residues in the C-terminal region of Cx43. While many studies have correlated changes in kinase activity with changes in gap junctional communication, further research is needed to directly link specific phosphorylation events with changes in connexin oligomerization and gap junction assembly.

Short tetracysteine tags to beta-tubulin demonstrate the significance of small labels for live cell imaging

Genetically encoded tags are of fundamental importance for live cell imaging. We show that small tetracysteine (TetCys) tags can be highly advantageous for the functionality of the host protein compared with large fluorescent protein tags. One to three concatenated small TetCys tags as well as the large green fluorescent protein (GFP) were fused by integrative epitope tagging to the C terminus of beta-tubulin (Tub2) in the budding yeast Saccharomyces cerevisiae. The increasing tag size correlated with functional interference to the host protein. Tub2 tagged with either 1 x TetCys (10 amino acids [aa]) or 2 x TetCys (20 aa) was able to substitute Tub2 in haploid cells. In contrast, C-terminal tagging of Tub2 with 3 x TetCys (29 aa) or with GFP (244 aa) resulted in nonviable haploid cells. Cells expressing Tub2-1 x TetCys or Tub2-2 x TetCys were stained with FlAsH, which selectively binds to the TetCys-tag. The stained cells displayed dynamic FlAsH-labeled microtubules and low cellular background fluorescence. The presented approach to tag open reading frames (ORFs) at their native loci with very small TetCys-tags and the subsequent visualization of the tagged proteins in vivo can be extended in principle to any ORF in S. cerevisiae.

Quantitative three-dimensional microscopy approaches with applications in breast cancer biology including measurement of genomic instability

Understanding tissue development, tissue homeostasis and what goes wrong in these processes during tumorigenesis, requires knowledge of the kinetics of multiple, molecular pathways in individual cells while cells are in their tissue context. This review outlines progress and future directions necessary in quantitative microscopy for gaining this knowledge, using the mammary gland as a model system.

Novel techniques in electron microscopy

Emerging techniques in electron microscopy promise to yield a wide range of new information about the nervous system. Aided by the development of detectors, electron optics, energy filters, computer automation and preparative methods, electron tomography now provides three-dimensional structures over a scale ranging from single receptor molecules to synapses and neurons. To relate structure to function, a variety of correlative methods are being developed, including protein tags observable both by light microscopy of living cells and, subsequently, by electron microscopy. It is also becoming possible to measure functionally important ions like Ca(2+) in cellular compartments at a scale of about 10 nm by exploiting new advances in electron energy loss and X-ray spectroscopic imaging.

A tribute to Shinya Inoue and innovation in light microscopy

The 2003 International Prize for Biology was awarded to Shinya Inoue for his pioneering work in visualizing dynamic processes within living cells using the light microscope. He and his scientific descendants are now pushing light microscopy even further by developing new techniques such as imaging single molecules, visualizing processes in living animals, and correlating results from light and electron microscopy.

Ubiquitination and down-regulation of gap junction protein connexin-43 in response to 12-O-tetradecanoylphorbol 13-acetate treatment

Gap junctions are specialized plasma membrane domains enriched in connexin proteins that form channels between adjacent cells. Gap junctions are highly dynamic, and modulation of the connexin turnover rate is considered to play an important role in the regulation of gap junctional intercellular communication. In the present study, we show that the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) induces ubiquitination of connexin-43 (Cx43) in IAR20 rat liver epithelial cells. The accelerated ubiquitination of Cx43 in response to TPA occurred concomitantly with Cx43 hyperphosphorylation and inhibition of cell-cell communication via gap junctions. The TPA-induced ubiquitination of Cx43 was mediated via protein kinase C and partly involved the mitogen-activated protein kinase pathway. Following ubiquitination, Cx43 was internalized and degraded. The loss of Cx43 protein was counteracted by ammonium chloride, indicating that acidification of internalized Cx43 gap junctions is a prerequisite for its degradation. Furthermore, the Cx43 degradation was partly counteracted by leupeptin, an inhibitor of cathepsin B, H, and L. Cx43 internalization and subsequent degradation were blocked by inhibitors of the proteasome. Evidence is provided that Cx43 is modified by multiple monoubiquitins rather than a polyubiquitin chain in response to TPA. Moreover, the TPA-induced ubiquitination of Cx43 was blocked by proteasomal inhibitors. Taken together, the data indicate that Cx43 ubiquitination is a highly regulated process. Moreover, the results suggest that the proteasome might play an indirect role in Cx43 degradation by affecting the level of monoubiquitin conjugation and trafficking of Cx43 to endosomal compartments.

Retroviruses 2004: review of the 2004 Cold Spring Harbor Retroviruses Conference

For the past several decades, retrovirologists from around the world have gathered in late May at the Cold Spring Harbor Laboratories in New York to present their studies in formal talks and posters, and to discuss their ongoing research informally at the bar or on the beach. As organizers of the 2004 Cold Spring Harbor Retroviruses Conference, we have been asked by the editors of Retrovirology to prepare a review of the meeting for publication on-line. Our goal in this review is not to provide a detailed description of data presented at the meeting but rather to highlight some of the significant developments reported this year. The review is structured in a manner that parallels the organization of the meeting; beginning with the entry phase of the replication cycle, proceeding with post-entry events, assembly and release, integration, reverse transcription, pathogenesis/host factors, RNA-related events (transcription, processing, export, and packaging) and finishing with antivirals. While the most striking developments this year involved post-entry events and assembly/release, significant progress was made towards elucidating a number of aspects of the retroviral replication cycle.

Protease-sensitive signalling by chemically engineered intramolecular fluorescent resonance energy transfer mutants of green fluorescent protein

The native cysteine residues of green fluorescent protein (GFP) at positions 48 and 70 were replaced by non-thiolic amino acids, and new cysteine sites were introduced at specific, surface positions. Based on molecular modeling of the GFP structure, the sites chosen for mutagenesis to Cys were glutamic acid at position 6 and isoleucine at position 229. These new, unique cysteine sites provided reactive thiol groups suitable for site-specific chemical modification by eosin-based fluorescence labels. The new constructs were designed to serve as the basis of proof of principle for fluorescence resonance energy transfer (FRET) using an enzyme-activated (trypsin) intervening sequence between native and chemically conjugated fluorophores. These eosin moieties provided chemical FRET partners for the native GFP chromophore. On excitation, these GFP-eosin constructs exhibited strong intramolecular FRET, with quenching of the native GFP (511 nm) fluorophore emission and emission around 540 nm, corresponding to eosin. GFP mutants engineered with trypsin-sensitive sequences close to the eosin site, so that on trypsinolysis FRET was destroyed, the emission wavelength switching from that of the chemical FRET partner back to that of the native GFP fluorophore, providing efficient, ratio-based detection. This protein engineering provides the basis for novel bioprobes for enzymatic triggering using intramolecular FRET between GFP and carefully sited chemical labels.

Labeling of fusion proteins with synthetic fluorophores in live cells

A general approach for the sequential labeling of fusion proteins of O(6)-alkylguanine-DNA alkyltransferase (AGT) with different fluorophores in mammalian cells is presented. AGT fusion proteins with different localizations in the cell can be labeled specifically with different fluorophores, and the fluorescence labeling can be used for applications such as multicolor analysis of dynamic processes and fluorescence resonance energy transfer measurements. The facile access to a variety of different AGT substrates as well as the specificity of the labeling reaction should make the approach an important tool to study protein function in live cells.

FRET imaging

Förster (or Fluorescence) Resonance Energy Transfer (FRET) is unique in generating fluorescence signals sensitive to molecular conformation, association, and separation in the 1-10 nm range. We introduce a revised photophysical framework for the phenomenon and provide a systematic catalog of FRET techniques adapted to imaging systems, including new approaches proposed as suitable prospects for implementation. Applications extending from a single molecule to live cells will benefit from multidimensional microscopy techniques, particularly those adapted for optical sectioning and incorporating new algorithms for resolving the component contributions to images of complex molecular systems.

Effects of mechanical forces and mediators of hypertrophy on remodeling of gap junctions in the heart

This review article focuses on remodeling of gap junctions in response to chemical mediators of ventricular hypertrophy, mechanical forces, and alterations in cell-to-cell adhesion. Signaling mediated by mechanical forces is likely to be involved in the upregulation of cardiac gap junctions during the early phase of cardiac hypertrophy and the subsequent downregulation in cardiac failure. Several signaling pathways involving cAMP, angiotensin II, transforming growth factor-beta, vascular endothelial growth factor, and integrin-mediated regulators have been shown to affect expression of gap junction proteins. However, a comprehensive view of regulation of gap junction trafficking, synthesis, and degradation is still lacking. In addition to gap junction regulation by extracellular mechanical forces, there is a close relation between gap junctions and adhesion junctions and their linkage to the cytoskeleton. This can be inferred from experiments on neoformation of cell-to-cell coupling, concomitant upregulation of adherens and gap junctions after mechanical stretch, and human cardiomyopathies caused by genetic defects in cell-cell adhesion junction proteins. The molecular mechanisms responsible for the interaction between mechanical and functional cell-to-cell coupling remain to be elucidated.

Regulation of connexin biosynthesis, assembly, gap junction formation, and removal

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.

Gap junction channel gating

Over the last two decades, the view of gap junction (GJ) channel gating has changed from one with GJs having a single transjunctional voltage-sensitive (V(j)-sensitive) gating mechanism to one with each hemichannel of a formed GJ channel, as well as unapposed hemichannels, containing two, molecularly distinct gating mechanisms. These mechanisms are termed fast gating and slow or 'loop' gating. It appears that the fast gating mechanism is solely sensitive to V(j) and induces fast gating transitions between the open state and a particular substate, termed the residual conductance state. The slow gating mechanism is also sensitive to V(j), but there is evidence that this gate may mediate gating by transmembrane voltage (V(m)), intracellular Ca(2+) and pH, chemical uncouplers and GJ channel opening during de novo channel formation. A distinguishing feature of the slow gate is that the gating transitions appear to be slow, consisting of a series of transient substates en route to opening and closing. Published reports suggest that both sensorial and gating elements of the fast gating mechanism are formed by transmembrane and cytoplamic components of connexins among which the N terminus is most essential and which determines gating polarity. We propose that the gating element of the slow gating mechanism is located closer to the central region of the channel pore and serves as a 'common' gate linked to several sensing elements that are responsive to different factors and located in different regions of the channel.

The cell-centered database: a database for multiscale structural and protein localization data from light and electron microscopy

The creation of structured shared data repositories for molecular data in the form of web-accessible databases like GenBank has been a driving force behind the genomic revolution. These resources serve not only to organize and manage molecular data being created by researchers around the globe, but also provide the starting point for data mining operations to uncover interesting information present in the large amount of sequence and structural data. To realize the full impact of the genomic and proteomic efforts of the last decade, similar resources are needed for structural and biochemical complexity in biological systems beyond the molecular level, where proteins and macromolecular complexes are situated within their cellular and tissue environments. In this review, we discuss our efforts in the development of neuroinformatics resources for managing and mining cell level imaging data derived from light and electron microscopy. We describe the main features of our web-accessible database, the Cell Centered Database (CCDB; http://ncmir.ucsd.edu/CCDB/), designed for structural and protein localization information at scales ranging from large expanses of tissue to cellular microdomains with their associated macromolecular constituents. The CCDB was created to make 3D microscopic imaging data available to the scientific community and to serve as a resource for investigating structural and macromolecular complexity of cells and tissues, particularly in the rodent nervous system.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Proteomics in living cells

The study of protein dynamics in vivo is central to understanding the chemical machinery that makes up the process of life: in determining how, when, where and under what conditions proteins are created, form complexes, are modified and disintegrate. Until recently, facile methods to study these processes in intact living cells did not exist. This review describes promising experimental strategies that have been devised to visualize the dynamic nature of proteins and protein complex formation occurring in living cells. The approaches described are based largely on genetically encoded fusions to proteins of interest combined with labelling strategies. The feasibility of employing such strategies in genome-wide biochemical pathway mapping efforts is discussed.

Nerve-independent formation of a topologically complex postsynaptic apparatus

As the mammalian neuromuscular junction matures, its acetylcholine receptor (AChR)–rich postsynaptic apparatus is transformed from an oval plaque into a pretzel-shaped array of branches that precisely mirrors the branching pattern of the motor nerve terminal. Although the nerve has been believed to direct postsynaptic maturation, we report here that myotubes cultured aneurally on matrix-coated substrates form elaborately branched AChR-rich domains remarkably similar to those seen in vivo. These domains share several characteristics with the mature postsynaptic apparatus, including colocalization of multiple postsynaptic markers, clustering of subjacent myonuclei, and dependence on the muscle-specific kinase and rapsyn for their formation. Time-lapse imaging showed that branched structures arise from plaques by formation and fusion of AChR-poor perforations through a series of steps mirroring that seen in vivo. Multiple fluorophore imaging showed that growth occurs by circumferential, asymmetric addition of AChRs. Analysis in vivo revealed similar patterns of AChR addition during normal development. These results reveal the sequence of steps by which a topologically complex domain forms on a cell and suggest an unexpected nerve-independent role for the postsynaptic cell in generating this topological complexity.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Epidermal growth factor regulates ubiquitination, internalization and proteasome-dependent degradation of connexin43

Connexins are membrane-spanning proteins that form gap junction channels between adjacent cells. Connexin43 (Cx43), the most widely expressed member of the connexin family in tissues and cell lines, has a rapid turnover rate and its degradation involves both the lysosomal and ubiquitin-proteasome pathway. It was previously shown that the proteasome is involved in regulating the number of functional gap junctions at the plasma membrane. However, little is known about how proteasome-dependent turnover of Cx43 is controlled. Epidermal growth factor (EGF) induces hyperphosphorylation of Cx43 and a rapid, transient decrease in gap junctional intercellular communication. In this study, we show that, along with inhibition of gap junctional intercellular communication, EGF induces disorganization, internalization and degradation of Cx43 gap junction plaques in IAR20 rat liver epithelial cells. These EGF-induced modifications of Cx43 were counteracted by the MEK1 inhibitor PD98059, indicating that the effects were mediated by the mitogen-activated protein kinase pathway. The EGF-induced destruction of Cx43 was proteasome-dependent, because the loss of Cx43 protein was counteracted by the proteasome inhibitor MG132 but not the lysosomal inhibitor leupeptin. Furthermore, EGF induced ubiquitination of Cx43, which was associated with the Cx43 hyperphosphorylation. The EGF-induced Cx43 ubiquitination was counteracted by PD98059. The EGF-induced internalization of Cx43 was blocked by hypertonic sucrose treatment, indicating that EGF mediates internalization of Cx43 via a clathrin-dependent mechanism. Our results indicate that ubiquitination of Cx43 occurs at the plasma membrane before Cx43 internalization. Taken together, these data provide the first evidence that EGF-induced phosphorylation of Cx43 induces binding of ubiquitin and targets Cx43 for internalization and degradation in a proteasome-dependent manner.

Binding of SycH chaperone to YscM1 and YscM2 activates effector yop expression in Yersinia enterocolitica

Yersinia enterocolitica transports YscM1 and YscM2 via the type III pathway, a mechanism that is required for the establishment of bacterial infections. Prior to host cell contact, YscM1 and YscM2 exert posttranscriptional regulation to inhibit expression of effector yop genes, which encode virulence factors that travel the type III pathway into the cytoplasm of macrophages. Relief from repression has been predicted to occur via the type III secretion of YscM1 and YscM2 into the extracellular medium, resulting in the depletion of regulatory molecules from the bacterial cytoplasm. Using digitonin fractionation and fluorescence microscopy of FlAsH-labeled polypeptides in Yersinia-infected cells, we have localized YscM1 and YscM2 within the host cell cytoplasm. Type III injection of YscM1 and YscM2 required the SycH chaperone. Expression of C-terminal fusions of YscM1 and YscM2 to the neomycin phosphotransferase reporter revealed sequences required for regulatory activity and for secretion in the absence of SycH. Coexpression of SycH and glutathione S-transferase (GST)-YscM1 or GST-YscM2, hybrid GST variants that cannot be transported by the type III apparatus, also relieved repression of Yop synthesis. GST-SycH bound to YscM1 and YscM2 and activated effector yop expression without initiation of the bound regulatory molecules into the type III pathway. Further, regulation of yop expression by YscM1, YscM2, and SycH is shown to act independently of factors that regulate secretion, and gel filtration chromotography revealed populations of YscM1 and YscM2 that are not bound to SycH under conditions where Yop synthesis is repressed. Taken together, these results suggest that YscM1- and YscM2-mediated repression may be relieved through binding to the cytoplasmic chaperone SycH prior to their type III injection into host cells.

Activity-dependent regulation of dendritic synthesis and trafficking of AMPA receptors

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

Visualization of Molecular Activities Inside Living Cells with Fluorescent Labels

The major task of modern cell biology is to identify the function and relation of the many different gene products, discovered by genomics and proteomics approaches, in the context of the living cell. To achieve this goal, an increasing toolbox of custom-designed biosensors based on fluorescent labels is available to study the molecular activities of the cellular machinery. An overview of the current status of the young field of molecular-cellular physiology is presented that includes the application of fluorescent labels in the design of biosensors and the major detection schemes used to extract their sensing information. In particular, the use of the photophysical phenomenon of Förster resonance energy transfer (FRET) as a powerful indicator of cellular biochemical events is discussed. In addition, we will point out the challenges and directions of the field and project the short-term future for the application of fluorescence-based biosensors in biology.