High resolution, fluorescence deconvolution microscopy and tagging with the autofluorescent tracers CFP, GFP, and YFP to study the structural composition of gap junctions in living cells

Spatial and Temporal Localization of Connexins in Cells Using Confocal Microscopy

The 21-member connexin family found in humans is the building block of both single-membrane spanning channels (hemichannels) and double-membrane spanning intercellular channels. These large-pore channels are dynamic and typically have a short life span of only a few hours. Imaging connexins from the time of synthesis in the endoplasmic reticulum through to their degradation can be challenging given their distinct assembly states and transient residences in many subcellular compartments. Here, we describe how connexins can be effectively imaged on a confocal microscope in living cells when tagged with fluorescent proteins and when immunolabeled with high affinity anti-connexin antibodies in fixed cells. Temporal and spatial localization of multiple connexins and disease-linked connexin mutants at the subcellular level extensively informs on the mechanisms governing connexin regulation in health and disease.

Foldscope as a primary diagnostic tool for oral and urinary tract infections and its effectiveness in oral health education

Due to its speed, accuracy and cost-effectiveness, microscopy has become an integral part of clinical examination for disease diagnosis. However, modern microscopes are very costly and require skilled personnel for their operation and maintenance, and specimen processing and analysis is labour-intensive. Further, lack of such expensive diagnostic tools in remote areas is a serious concern. Affordable point-of-care diagnostic tools are the most useful for timely disease diagnosis and management. The Foldscope is an affordable origami-based microscopy device composed of a series of paper clippings, which, upon assembly, can hold a specimen slide for observation, and this specimen can be viewed via a mobile phone camera attached to it. The present study evaluated the use of the Foldscope in the clinical diagnosis of oral and urinary tract infections and evaluated its efficacy as a motivational tool for improving oral health among school children in India. We qualitatively compared the Foldscope to a clinical microscope by examining five different types of clinical samples. Of the different types of clinical samples, the Foldscope was effective in detecting infection in dental plaque samples and urine samples. Thus, we further analysed 31 dental plaque samples of patients aged 3-13 years and 25 urine samples of patients aged 11-62 years. We also evaluated the use of the Foldscope as an educational tool for motivating oral hygiene among 80 school children aged 12 years and found that students in the Foldscope intervention group had better measures of oral hygiene than did students in the nonintervention group. In summary, our study indicated that the Foldscope is useful in detecting urinary tract infections and kidney stones in urine samples and is a useful motivational tool for oral health education among school-aged children. Furthermore, it may also be useful in oral health monitoring in resource poor settings. LAY DESCRIPTION: Poor and remote population often suffers due to lack of capacity to buy products or avail services which are expensive. In such a society people are not able to afford even the basic detection of diseases. Foldscope may come to rescue here! Foldscope is a paper-based, use-and-throw, affordable microscope. This study explores the use of Foldscope in clinics and diseases diagnosis using patient samples. Preliminary detection of disease associated symptoms in dental and urinary infections and digital record keeping via storage in mobile phone memory and hospital OPD records for monitoring patient's health are some of the advantages of Foldscope for clinical use in resource poor settings. It identifies that Foldscope not only can be used by students or teachers for educational purposes but it can also pave a path for promotion of dental hygiene among young children. The study also suggests that further improvement in design or resolution of Foldscope will broaden the scope of its application, in future.

Mobile microscopy as a screening tool for oral cancer in India: A pilot study

Oral cancer is the most common type of cancer among men in India and other countries in South Asia. Late diagnosis contributes significantly to this mortality, highlighting the need for effective and specific point-of-care diagnostic tools. The same regions with high prevalence of oral cancer have seen extensive growth in mobile phone infrastructure, which enables widespread access to telemedicine services. In this work, we describe the evaluation of an automated tablet-based mobile microscope as an adjunct for telemedicine-based oral cancer screening in India. Brush biopsy, a minimally invasive sampling technique was combined with a simplified staining protocol and a tablet-based mobile microscope to facilitate local collection of digital images and remote evaluation of the images by clinicians. The tablet-based mobile microscope (CellScope device) combines an iPad Mini with collection optics, LED illumination and Bluetooth-controlled motors to scan a slide specimen and capture high-resolution images of stained brush biopsy samples. Researchers at the Mazumdar Shaw Medical Foundation (MSMF) in Bangalore, India used the instrument to collect and send randomly selected images of each slide for telepathology review. Evaluation of the concordance between gold standard histology, conventional microscopy cytology, and remote pathologist review of the images was performed as part of a pilot study of mobile microscopy as a screening tool for oral cancer. Results indicated that the instrument successfully collected images of sufficient quality to enable remote diagnoses that show concordance with existing techniques. Further studies will evaluate the effectiveness of oral cancer screening with mobile microscopy by minimally trained technicians in low-resource settings.

Imaging of intracellular behavior of polymeric nanoparticles in Staphylococcus epidermidis biofilms by slit-scanning confocal Raman microscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy

In drug delivery systems employing polymeric nanoparticles, accurate delivery of drugs to target sites such as bacterial cells, cell tissues, and organelles is essential. In particular, when designing drug delivery systems for the treatment of the biofilm infections, evaluation of the interaction between polymeric nanoparticles and biofilm or bacterial cells using a simple technique is of significant importance. Here we develop two types of novel techniques for the biological imaging of the intracellular behavior of two types of polymeric nanoparticles, biodegradable chitosan-modified poly (dl-lactide-co-glycolide) (PLGA) nanoparticles and chitosan-modified polyvinyl caprolactam - polyvinyl acetate -polyethylene glycol graft copolymer (Soluplus®, Sol) nanoparticles, within a Staphylococcus epidermidis biofilm. As the first technique, Raman imaging of unstained biological materials using slit-scanning confocal Raman microscopy (unstained Raman imaging) was performed, and as the second, field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis of biological materials labeled with quantum dots (SEM-QD imaging) was demonstrated. These analyses revealed differing localization of the respective nanoparticles within the biofilm in accordance with the specific interactions of PLGA nanoparticles and Sol nanoparticles with the biofilm. These novel techniques open the door to biological imaging and analyses with high spatial resolution, which will help to understand the efficacy of drug delivery to target materials.

Molecular mechanisms regulating formation, trafficking and processing of annular gap junctions

Internalization of gap junction plaques results in the formation of annular gap junction vesicles. The factors that regulate the coordinated internalization of the gap junction plaques to form annular gap junction vesicles, and the subsequent events involved in annular gap junction processing have only relatively recently been investigated in detail. However it is becoming clear that while annular gap junction vesicles have been demonstrated to be degraded by autophagosomal and endo-lysosomal pathways, they undergo a number of additional processing events. Here, we characterize the morphology of the annular gap junction vesicle and review the current knowledge of the processes involved in their formation, fission, fusion, and degradation. In addition, we address the possibility for connexin protein recycling back to the plasma membrane to contribute to gap junction formation and intercellular communication. Information on gap junction plaque removal from the plasma membrane and the subsequent processing of annular gap junction vesicles is critical to our understanding of cell-cell communication as it relates to events regulating development, cell homeostasis, unstable proliferation of cancer cells, wound healing, changes in the ischemic heart, and many other physiological and pathological cellular phenomena.

Pannexin-1 channels show distinct morphology and no gap junction characteristics in mammalian cells

Pannexins (Panx) are proteins with a similar membrane topology to connexins, the integral membrane protein of gap junctions. Panx1 channels are generally of major importance in a large number of system and cellular processes and their function has been thoroughly characterized. In contrast, little is known about channel structure and subcellular distribution. We therefore determine the subcellular localization of Panx1 channels in cultured cells and aim at the identification of channel morphology in vitro. Using freeze-fracture replica immunolabeling on EYFP-Panx1-overexpressing HEK 293 cells, large particles were identified in plasma membranes, which were immunogold-labeled using either GFP or Panx1 antibodies. There was no labeling or particles in the nuclear membranes of these cells, pointing to plasma membrane localization of Panx1-EYFP channels. The assembly of particles was irregular, this being in contrast to the regular pattern of gap junctions. The fact that no counterparts were identified on apposing cells, which would have been indicative of intercellular signaling, supported the idea of Panx1 channels within one membrane. Control cells (transfected with EYFP only, non-transfected) were devoid of both particles and immunogold labeling. Altogether, this study provides the first demonstration of Panx1 channel morphology and assembly in intact cells. The identification of Panx1 channels as large particles within the plasma membrane provides the knowledge required to enable recognition of Panx1 channels in tissues in future studies. Thus, these results open up new avenues for the detailed analysis of the subcellular localization of Panx1 and of its nearest neighbors such as purinergic receptors in vivo.

Human papillomavirus type 8 interferes with a novel C/EBPβ-mediated mechanism of keratinocyte CCL20 chemokine expression and Langerhans cell migration

Infection with genus beta human papillomaviruses (HPV) is implicated in the development of non-melanoma skin cancer. This was first evidenced for HPV5 and 8 in patients with epidermodysplasia verruciformis (EV), a genetic skin disease. So far, it has been unknown how these viruses overcome cutaneous immune control allowing their persistence in lesional epidermis of these patients. Here we demonstrate that Langerhans cells, essential for skin immunosurveillance, are strongly reduced in HPV8-positive lesional epidermis from EV patients. Interestingly, the same lesions were largely devoid of the important Langerhans cells chemoattractant protein CCL20. Applying bioinformatic tools, chromatin immunoprecipitation assays and functional studies we identified the differentiation-associated transcription factor CCAAT/enhancer binding protein β (C/EBPβ) as a critical regulator of CCL20 gene expression in normal human keratinocytes. The physiological relevance of this finding is supported by our in vivo studies showing that the expression patterns of CCL20 and nuclear C/EBPβ converge spatially in the most differentiated layers of human epidermis. Our analyses further identified C/EBPβ as a novel target of the HPV8 E7 oncoprotein, which co-localizes with C/EBPβ in the nucleus, co-precipitates with it and interferes with its binding to the CCL20 promoter in vivo. As a consequence, the HPV8 E7 but not E6 oncoprotein suppressed C/EBPβ-inducible and constitutive CCL20 gene expression as well as Langerhans cell migration. In conclusion, our study unraveled a novel molecular mechanism central to cutaneous host defense. Interference of the HPV8 E7 oncoprotein with this regulatory pathway allows the virus to disrupt the immune barrier, a major prerequisite for its epithelial persistence and procarcinogenic activity.

Development of a novel set of Gateway-compatible vectors for live imaging in insect cells

Insect genomics is a growing area of research. To exploit fully the genomic data that are being generated, high-throughput systems for the functional characterization of insect proteins and their interactomes are required. In this work, a Gateway-compatible vector set for expression of fluorescent fusion proteins in insect cells was developed. The vector set was designed to express a protein of interest fused to any of four different fluorescent proteins [green fluorescent protein (GFP), cyan fluorescent protein (CFP), yellow fluorescent protein (YFP) and mCherry] by either the C-terminal or the N-terminal ends. Additionally, a collection of organelle-specific fluorescent markers was assembled for colocalization with fluorescent recombinant proteins of interest. Moreover, the vector set was proven to be suitable for simultaneously detecting up to three proteins by multiple labelling. The use of the vector set was exemplified by defining the subcellular distribution of Mal de Río Cuarto virus (MRCV) outer coat protein P10 and by analysing the in vivo self-interaction of the MRCV viroplasm matrix protein P9-1 in Förster resonance energy transfer (FRET) experiments. In conclusion, we have developed a valuable tool for high-throughput studies of protein subcellular localization that will aid in the elucidation of the function of newly described insect and virus proteins.

Green-to-red photoconvertible fluorescent proteins: tracking cell and protein dynamics on standard wide-field mercury arc-based microscopes

Background

Green fluorescent protein (GFP) and other FP fusions have been extensively utilized to track protein dynamics in living cells. Recently, development of photoactivatable, photoswitchable and photoconvertible fluorescent proteins (PAFPs) has made it possible to investigate the fate of discrete subpopulations of tagged proteins. Initial limitations to their use (due to their tetrameric nature) were overcome when monomeric variants, such as Dendra, mEos, and mKikGR were cloned/engineered.

Results

Here, we report that by closing the field diaphragm, selective, precise and irreversible green-to-red photoconversion (330-380 nm illumination) of discrete subcellular protein pools was achieved on a wide-field fluorescence microscope equipped with standard DAPI, Fluorescein, and Rhodamine filter sets and mercury arc illumination within 5-10 seconds. Use of a DAPI-filter cube with long-pass emission filter (LP420) allowed the observation and control of the photoconversion process in real time. Following photoconversion, living cells were imaged for up to 5 hours often without detectable phototoxicity or photobleaching.

Conclusions

We demonstrate the practicability of this technique using Dendra2 and mEos2 as monomeric, photoconvertible PAFP representatives fused to proteins with low (histone H2B), medium (gap junction channel protein connexin 43), and high (α-tubulin; clathrin light chain) dynamic cellular mobility as examples. Comparable efficient, irreversible green-to-red photoconversion of selected portions of cell nuclei, gap junctions, microtubules and clathrin-coated vesicles was achieved. Tracking over time allowed elucidation of the dynamic live-cycle of these subcellular structures. The advantage of this technique is that it can be performed on a standard, relatively inexpensive wide-field fluorescence microscope with mercury arc illumination. Together with previously described laser scanning confocal microscope-based photoconversion methods, this technique promises to further increase the general usability of photoconvertible PAFPs to track the dynamic movement of cells and proteins over time.

The N terminus of connexin37 contains an alpha-helix that is required for channel function

The cytoplasmic N-terminal domain of connexins has been implicated in multiple aspects of gap junction function, including connexin trafficking/assembly and channel gating. A synthetic peptide corresponding to the first 23 amino acids of human connexin37 was prepared, and circular dichroism and nuclear magnetic resonance studies showed that this N-terminal peptide was predominantly alpha-helical between glycine 5 and glutamate 16. The importance of this structure for localization of the protein at appositional membranes and channel function was tested by expression of site-directed mutants of connexin37 in which amino acids leucine 10 and glutamine 15 were replaced with prolines or alanines. Wild type connexin37 and both substitution mutants localized to appositional membranes between transfected HeLa cells. The proline mutant did not allow intercellular transfer of microinjected neurobiotin; the alanine mutant allowed transfer, but less extensively than wild type connexin37. When expressed alone in Xenopus oocytes, wild type connexin37 produced hemichannel currents, but neither of the double substitution mutants produced detectable currents. The proline mutant (but not the alanine mutant) inhibited co-expressed wild type connexin37. Taken together, our data suggest that the alpha-helical structure of the connexin37 N terminus may be dispensable for protein localization, but it is required for channel and hemichannel function.

Following Tracks of Hemichannels

It has been suggested that plasma membrane-bound hemichannels perform physiological and pathophysiological functions per se. Such functions require the presence of hemichannels on the cell surface and their accessibility to the extracellular environment for at least some limited period of time. We have previously shown that hemichannels can be labeled by means of antibodies directed to an external loop domain of connexin (Cx) 43. We now provide evidence that trafficking of hemichannel vesicles can be visualized upon binding of a labeled homophilic peptide corresponding to a region of the first extracellular loop (EL1) of Cx43. In vivo imaging was performed after labeling hemichannels from the extracellular site with a mimetic peptide tagged with a fluorochrome (Alexa-546). Using a Cx43-CFP transfected HeLa cell line for incubation with the mimetic peptide, a significant number of double-labeled vesicles were found inside the cells. This double labeling indicates that a portion of Cx43 within the cell had accessed the cell surface as hemichannels where it bound to the peptide and was subsequently endocytosed. Pulse labeling with the peptide showed a decrease in the number of dual-labeled vesicles over time, indicating degradation and/or concurrent recycling of hemichannel vesicles.

Calmodulin mediates the Ca2+-dependent regulation of Cx44 gap junctions

We have shown previously that the Ca2+-dependent inhibition of lens epithelial cell-to-cell communication is mediated in part by the direct association of calmodulin (CaM) with connexin43 (Cx43), the major connexin in these cells. We now show that elevation of [Ca2+](i) in HeLa cells transfected with the lens fiber cell gap junction protein sheep Cx44 also results in the inhibition of cell-to-cell dye transfer. A peptide comprising the putative CaM binding domain (aa 129-150) of the intracellular loop region of this connexin exhibited a high affinity, stoichiometric interaction with Ca2+-CaM. NMR studies indicate that the binding of Cx44 peptide to CaM reflects a classical embracing mode of interaction. The interaction is an exothermic event that is both enthalpically and entropically driven in which electrostatic interactions play an important role. The binding of the Cx44 peptide to CaM increases the CaM intradomain cooperativity and enhances the Ca2+-binding affinities of the C-domain of CaM more than twofold by slowing the rate of Ca2+ release from the complex. Our data suggest a common mechanism by which the Ca2+-dependent inhibition of the alpha-class of gap junction proteins is mediated by the direct association of an intracellular loop region of these proteins with Ca2+-CaM.

Photophysical properties of Cerulean and Venus fluorescent proteins

Cerulean and Venus are recently developed fluorescent proteins, often used as a donor-acceptor pair by researchers in Forster resonance energy transfer-based colocalization studies. We characterized the fluorescent properties of these two proteins in a broad spectral range (form ultraviolet to visible region). Excitation spectra, lifetimes, and polarization spectra show significant energy transfer from aromatic amino acids to the fluorescent protein chromophore. High steady-state anisotropy values and the lack of a fast component in anisotropy decays show that the fluorescent protein chromophore is rigidly fixed within the protein structure. Furthermore, we show that the chromophores are not accessible to external quenchers, such as acrylamide or potassium iodide (KI), allowing the removal of "unwanted" background in the environment with external quencher, while leaving the Cerulean/Venus fluorescence unchanged.

Molecular imaging: reporter gene imaging

Non-invasive in-vivo molecular genetic imaging developed over the past decade and predominantly utilises radiotracer (PET, gamma camera, autoradiography), magnetic resonance and optical imaging technology. Molecular genetic imaging has its roots in both molecular biology and cell biology. The convergence of these disciplines and imaging modalities has provided the opportunity to address new research questions, including oncogenesis, tumour maintenance and progression, as well as responses to molecular-targeted therapy. Three different imaging strategies are described: (1) "bio-marker" or "surrogate" imaging; (2) "direct" imaging of specific molecules and pathway activity; (3) "indirect" reporter gene imaging. Examples of each imaging strategy are presented and discussed. Several applications of PET- and optical-based reporter imaging are demonstrated, including signal transduction pathway monitoring, oncogenesis in genetic mouse models, endogenous molecular genetic/biological processes and the response to therapy in animal models of human disease. Molecular imaging studies will compliment established ex-vivo molecular-biological assays that require tissue sampling by providing a spatial and a temporal dimension to our understanding of disease development and progression, as well as response to treatment. Although molecular imaging studies are currently being performed primarily in experimental animals, we optimistically expect they will be translated to human subjects with cancer and other diseases in the near future.

Chapter 5: Imaging in depth: controversies and opportunities

One enduring challenge of biological imaging is achieving depth of penetration-into cells, tissues, and animals. How deeply can we probe and with what resolution and efficacy? These are critical issues as microscopists seek to push ever deeper, while resolving structural details and observing specific molecular events. In this guide to depth-appropriate modalities, standard optical platforms such as confocal and two-photon microscopes are considered along with complementary imaging modalities that range in depth of penetration. After an introduction to basic techniques, the trade-offs and limitations that distinguish competing technologies are considered, with emphasis on the visualization of subcellular structures and dynamic events. Not surprisingly, there are differences of opinion regarding imaging technologies, as highlighted in a section on point-scanning and Nipkow-disk style confocal microscopes. Confocal microscopy is then contrasted with deconvolution and multi-photon imaging modalities. It is also important to consider the detectors used by current instruments (such as PMTs and CCD cameras). Ultimately specimen properties, in conjunction with instrumentation, determine the depth at which subcellular operations and larger-scale biological processes can be visualized. Relative advantages are mentioned in the context of experiment planning and instrument-purchase decisions. Given the rate at which new optical techniques are being invented, this report should be viewed as a snapshot of current capabilities, with the goal of providing a framework for thinking about new developments.

Intracellular calcium regulation of connexin43

The mechanism by which intracellular Ca(2+) concentration ([Ca(2+)](i)) regulates the permeability of gap junctions composed of connexin43 (Cx43) was investigated in HeLa cells stably transfected with this connexin. Extracellular addition of Ca(2+) in the presence of the Ca(2+) ionophore ionomycin produced a sustained elevation in [Ca(2+)](i) that resulted in an inhibition of the cell-to-cell transfer of the fluorescent dye Alexa fluor 594 (IC(50) of 360 nM Ca(2+)). The Ca(2+) dependency of this inhibition of Cx43 gap junctional permeability is very similar to that described in sheep lens epithelial cell cultures that express the three sheep lens connexins (Cx43, Cx44, and Cx49). The intracellular Ca(2+)-mediated decrease in cell-to-cell dye transfer was prevented by an inhibitor of calmodulin action but not by inhibitors of Ca(2+)/calmodulin-dependent protein kinase II or protein kinase C. In experiments that used HeLa cells transfected with a Cx43 COOH-terminus truncation mutant (Cx43(Delta257)), cell-to-cell coupling was similarly decreased by an elevation of [Ca(2+)](i) (IC(50) of 310 nM Ca(2+)) and similarly prevented by the addition of an inhibitor of calmodulin. These data indicate that physiological concentrations of [Ca(2+)](i) regulate the permeability of Cx43 in a calmodulin-dependent manner that does not require the major portion of the COOH terminus of Cx43.

Multi-Modality Molecular Imaging of Tumors

Noninvasive in vivo molecular-genetic imaging uses nuclear, magnetic resonance, and optical imaging techniques. Described and discussed are "direct" imaging of specific molecules and pathway activity, "indirect" reporter gene imaging, and "bio-marker" or "surrogate" imaging. Applications of PET- and optical-based reporter imaging are demonstrated, including imaging of oncogenesis in genetic mouse models, endogenous molecular-genetic-biological properties, and response to therapy in animal models of human disease. Molecular imaging studies complement established ex vivo molecular-biological assays that require tissue sampling by providing a spatial as well as temporal dimension to our understanding of oncogenesis, and the progression and treatment of cancer. Molecular imaging studies being performed in experimental animals will be translated to animals in the near future.

ZO-1 is required for protein kinase C gamma-driven disassembly of connexin 43

We have previously reported that protein kinase C gamma (PKC-gamma) is activated by phorbol-12-myristate-13-acetate (TPA) and that this causes PKC-gamma translocation to membranes and phosphorylation of the gap junction protein, connexin 43 (Cx43). This phosphorylation, on S368 of Cx43, causes disassembly of Cx43 out of cell junctional plaques resulting in the inhibition of dye transfer. The purpose of this study is to identify the specific role of zonula occludens protein-1 (ZO-1), a tight junction protein with recently established effects on gap junctions, in this PKC-gamma-driven Cx43 disassembly. For this purpose, ZO-1 levels in lens epithelial cells in culture were decreased by up to 70% using specific siRNA. The down-regulation of ZO-1 caused a stable interaction of PKC-gamma with Cx43 even without normal enzyme activation by TPA. However, after TPA activation of the PKC-gamma, the Cx43 did not disassemble out of plaques even though the PKC-gamma enzyme was activated and the Cx43 was phosphorylated on S368. Confocal microscopy demonstrated that the siRNA treatment caused a loss of ZO-1 from borders of large junctional Cx43 cell-to-cell plaques and resulted in the accumulation of Cx43 aggregates inside of cells. Loss of the specific "plaquetosome" arrangement of large Cx43 plaques surrounded by ZO-1 was accompanied by a complete loss of functional dye transfer. These results suggest that ZO-1 is required for Cx43 control, both for dye transfer, and, for the PKC-gamma-driven disassembly response.

Non-invasive molecular imaging and reporter genes

Molecular-genetic imaging in living organisms has become a new field with the exceptional growth over the past 5 years. Modern imaging is based on three technologies: nuclear, magnetic resonance and optical imaging. Most current molecular-genetic imaging strategies are “indirect,” coupling a “reporter gene” with a complimentary “reporter probe.” The reporter transgene usually encodes for an enzyme, receptor or transporter that selectively interacts with a radiolabeled probe and results in accumulation of radioactivity in the transduced cell. In addition, reporter systems based on the expression of fluorescence or bioluminescence proteins are becoming more widely applied in small animal imaging. This review begins with a description of Positron Emission Tomography (PET)-based imaging genes and their complimentary radiolabeled probes that we think will be the first to enter clinical trials. Then we describe other imaging genes, mostly for optical imaging, which have been developed by investigators working with a variety of disease models in mice. Such optical reporters are unlikely to enter the clinic, at least not in the near-term. Reporter gene constructs can be driven by constitutive promoter elements and used to monitor gene therapy vectors and the efficacy of gene targeting and transduction, as well as to monitor adoptive cell-based therapies. Inducible promoters can be used as “sensors” to monitor endogenous cell processes, including specific intracellular molecular-genetic events and the activity of signaling pathways, by regulating the magnitude of reporter gene expression.

Quantitative multiphoton spectral imaging and its use for measuring resonance energy transfer

Protein labeling with green fluorescent protein derivatives has become an invaluable tool in cell biology. Protein quantification, however, is difficult when cells express constructs with overlapping fluorescent emissions. Under these conditions, signal separation using emission filters is inherently inefficient. Spectral imaging solves this problem by recording emission spectra directly. Unfortunately, linear unmixing, the algorithm used for quantifying individual fluorophores from emission spectra, fails when resonance energy transfer (RET) is present. We therefore sought to develop an unmixing algorithm that incorporates RET. An equation for spectral emission incorporating RET was derived and an assay based on this formalism, spectral RET (sRET), was developed. Standards with defined RET efficiencies and with known Cerulean/Venus ratios were constructed and used to test sRET. We demonstrate that sRET analysis is a comprehensive, photon-efficient method for imaging RET efficiencies and accurately determines donor and acceptor concentrations in living cells.

Evidence that bone marrow cells do not contribute to the alveolar epithelium

An ongoing controversy is the role of marrow cells in populating the alveolar epithelium. In this study, we employed flow cytometry and histologic techniques to evaluate this process. Donor bone marrow was harvested from transgenic mice expressing the LacZ or eGFP gene ubiquitously, or under the control of the human surfactant protein (SP)-C promoter, and transplanted into lethally irradiated, neonatal mice. In recipients transplanted with marrow that express eGFP or lacZ ubiquitously, light microscopy revealed cells whose morphology and location were compatible with a type II cell phenotype. Consistent with this, fluorescent microscopy suggested colocalization of eGFP and pro-SP-C proteins in single cells. In mice transplanted with SP-C-eGFP marrow, engraftment was not detectable by histology or flow cytometry. We therefore used deconvolution microscopy to reanalyze histologic sections that were thought to show marrow-derived type II cells. We found that all putative marrow-derived pneumocytes resulted from the overlapping fluorescent signals of an endogenous pro-SP-C+ type II cell and a donor-derived eGFP+ cell. Taken together, our observations underscore the technical difficulties associated with evaluating engraftment in lung, and argue against a contributory role for marrow cells in populating the alveolar epithelium.

Structural organization of gap junction channels

Gap junctions were initially described morphologically, and identified as semi-crystalline arrays of channels linking two cells. This suggested that they may represent an amenable target for electron and X-ray crystallographic studies in much the same way that bacteriorhodopsin has. Over 30 years later, however, an atomic resolution structural solution of these unique intercellular pores is still lacking due to many challenges faced in obtaining high expression levels and purification of these structures. A variety of microscopic techniques, as well as NMR structure determination of fragments of the protein, have now provided clearer and correlated views of how these structures are assembled and function as intercellular conduits. As a complement to these structural approaches, a variety of mutagenic studies linking structure and function have now allowed molecular details to be superimposed on these lower resolution structures, so that a clearer image of pore architecture and its modes of regulation are beginning to emerge.

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.

Green fluorescent proteins in receptor research: An emerging tool for drug discovery

In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.

In vivo molecular-genetic imaging: multi-modality nuclear and optical combinations

Multi-modality, noninvasive in vivo imaging is increasingly being used in molecular-genetic studies and will soon become the standard approach for reporter gene imaging studies in small animals. The coupling of nuclear and optical reporter genes, as described here, represents only the beginning of a far wider application of this technology in the future. Optical imaging and optical reporter systems are cost-effective and time-efficient; they require less resources and space than PET or MRI, and are particularly well suited for imaging small animals, such as mice. Optical reporter systems are also very useful for the quantification and selection of transduced cells using FACS, and for performing in vitro assays to validate the function and sensitivity of constitutive and specific-inducible reporter systems. However, optical imaging techniques are limited by depth of light penetration and do not yet provide optimal quantitative or tomographic information. These issues are not limiting for PET- or MRI-based reporter systems, and PET- and MRI-based animal studies are more easily generalized to human applications. Many of the shortcomings of each modality alone can be overcome by the use of dual- or triple-modality reporter constructs that incorporate the opportunity for PET, fluorescence and bioluminescence imaging.

A novel triple-modality reporter gene for whole-body fluorescent, bioluminescent, and nuclear noninvasive imaging

Two genetic reporter systems were developed for multimodality reporter gene imaging of different molecular-genetic processes using fluorescence, bioluminescence (BLI), and nuclear imaging techniques. The eGFP cDNA was fused at the N-terminus with HSV1-tk cDNA bearing a nuclear export signal from MAPKK (NES-HSV1-tk) or with truncation at the N-terminus of the first 45 amino acids (Delta45HSV1-tk) and with firefly luciferase at the C-terminus. A single fusion protein with three functional subunits is formed following transcription and translation from a single open reading frame. The NES-TGL (NES-TGL) or Delta45HSV1-tk/GFP/luciferase (Delta45-TGL) triple-fusion gene cDNAs were cloned into a MoMLV-based retrovirus, which was used for transduction of U87 human glioma cells. The integrity, fluorescence, bioluminescence, and enzymatic activity of the TGL reporter proteins were assessed in vitro. The predicted molecular weight of the fusion proteins (~130 kDa) was confirmed by western blot. The U87-NES-TGL and U87-Delta45-TGL cells had cytoplasmic green fluorescence. The in vitro BLI was 7- and 13-fold higher in U87-NES-TGL and U87-Delta45-TGL cells compared to nontransduced control cells. The Ki of (14)C-FIAU was 0.49+/-0.02, 0.51+/-0.03, and 0.003+/-0.001 ml/min/g in U87-NES-TGL, U87-Delta45-TGL, and wild-type U87 cells, respectively. Multimodality in vivo imaging studies were performed in nu/ nu mice bearing multiple s.c. xenografts established from U87-NES-TGL, U87-Delta45-TGL, and wild-type U87 cells. BLI was performed after administration of d-luciferin (150 mg/kg i.v.). Gamma camera or PET imaging was conducted at 2 h after i.v. administration of [(131)I]FIAU (7.4 MBq/animal) or [(124)I]FIAU (7.4 MBq/animal), respectively. Whole-body fluorescence imaging was performed in parallel with the BLI and radiotracer imaging studies. In vivo BLI and gamma camera imaging showed specific localization of luminescence and radioactivity to the TGL transduced xenografts with background levels of activity in the wild-type xenografts. Tissue sampling yielded values of 0.47%+/-0.08%, 0.86%+/-0.06%, and 0.03%+/-0.01%dose/g [(131)I]FIAU in U87-NES-TGL, U87-Delta45-TGL, and U87 xenografts, respectively. The TGL triple-fusion reporter gene preserves the functional activity of its subunits and is very effective for multimodality imaging. It provides for the seamless transition from fluorescence microscopy and FACS to whole-body bioluminescence imaging, to nuclear (PET, SPET, gamma camera) imaging, and back to in situ fluorescence image analysis.

Cytoplasmically retargeted HSV1-tk/GFP reporter gene mutants for optimization of noninvasive molecular-genetic imaging

To optimize the sensitivity of imaging HSV1-tk/GFP reporter gene expression, a series of HSV1-tk/GFP mutants was developed with altered nuclear localization and better cellular enzymatic activity, compared to that of the native HSV1-tk/GFP fusion protein (HSV1-tk/GFP). Several modifications of HSV1-tk/GFP reporter gene were performed, including targeted inactivating mutations in the nuclear localization signal (NLS), the addition of a nuclear export signal (NES), a combination of both mutation types, and a truncation of the first 135 bp of the native hsv1-tk coding sequence containing a "cryptic" testicular promoter and the NLS. A recombinant HSV1-tk/GFP protein and a highly sensitive sandwich enzyme-linked immunosorbent assay for HSV1-tk/GFP were developed to quantitate the amount of reporter gene product in different assays to allow normalization of the data. These different mutations resulted in various degrees of nuclear clearance, predominant cytoplasmic distribution, and increased total cellular enzymatic activity of the HSV1-tk/GFP mutants, compared to native HSV1-tk/GFP when expressed at the same levels. This appears to be the result of improved metabolic bioavailability of cytoplasmically retargeted mutant HSV1-tk/GFP enzymes for reaction with the radiolabeled probe (e.g., FIAU). The analysis of enzymatic properties of different HSV1-tk/GFP mutants using FIAU as a substrate revealed no significant differences from that of the native HSV1-tk/GFP. Improved total cellular enzymatic activity of cytoplasmically retargeted HSV1-tk/GFP mutants observed in vitro was confirmed by noninvasive imaging of transduced subcutaneous tumor xenografts bearing these reporters using [(131)I]FIAU and a gamma-camera.

Distribution and dynamics of gap junction channels revealed in living cells

To study the structural composition and dynamics of gap junctions in living cells, we tagged their subunit proteins, termed connexins, with the autofluorescent tracer green fluorescent protein (GFP) and its cyan (CFP) and yellow (YFP) color variants. Tagged connexins assembled normally and channels were functional. High-resolution fluorescence images of gap junction plaques assembled from CFP and YFP tagged connexins revealed that the mode of channel distribution is strictly dependent on the connexin isoforms. Co-distribution as well as segregation into well-separated domains was observed. Based on accompanying studies we propose that channel distribution is regulated by intrinsic, connexin isoform specific signals. High-resolution time-lapse images revealed that gap junctions, contrary to previous expectations, are dynamic assemblies of channels. Channels within clusters and clusters themselves are mobile and constantly undergo structural rearrangements. Movements are complex and allow channels to move, comparable to other plasma membrane proteins not anchored to cytoskeletal elements. Comprehensive analysis, however, demonstrated that gap junction channel movements are not driven by diffusion described to propel plasma membrane protein movement. Instead, recent studies suggest that movements of gap junction channels are indirect and predominantly propelled by plasma membrane lipid flow that results from metabolic endo- and exocytosis.

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.

Multicolor and electron microscopic imaging of connexin trafficking

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

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

Since its introduction into heterologous organisms as a marker of gene expression, the green fluorescent protein (GFP) has led a dramatic revolution in cell, developmental and neurobiology. By allowing breathtaking visualization of fluorescent fusion proteins as they move within and between cells, GFP has fundamentally transformed the spatial analysis of protein function. Now, new GFP technologies allow far more than simple observations of fusion protein localization. The growing family of fluorescent protein variants is enabling more sophisticated studies of protein function and illuminating wide-ranging processes from gene expression to second-messenger cascades and intercellular signalling. Together with advances in microscopy, new GFP-based experimental approaches are forging a second GFP revolution.

Expression of fluorescently tagged connexins: a novel approach to rescue function of oligomeric DsRed-tagged proteins

A novel, brilliantly red fluorescent protein, DsRed has become available recently opening up a wide variety of experimental opportunities for double labeling and fluorescence resonance electron transfer experiments in combination with green fluorescent protein (GFP). Unlike in the case of GFP, proteins tagged with DsRed were often found to aggregate within the cell. Here we report a simple method that allows rescuing the function of an oligomeric protein tagged with DsRed. We demonstrate the feasibility of this approach on the subunit proteins of an oligomeric membrane channel, gap junction connexins. Additionally, DsRed fluorescence was easily detected 12-16 h post transfection, much earlier than previously reported, and could readily be differentiated from co-expressed GFP. Thus, this approach can eliminate the major drawbacks of this highly attractive autofluorescent protein.