MuscleJ2: a rebuilding of MuscleJ with new features for high-content analysis of skeletal muscle immunofluorescence slides

Histological analysis of skeletal muscle is of major interest for understanding its behavior in different pathophysiological conditions, such as the response to different environments or myopathies. In this context, many software programs have been developed to perform automated high-content analysis. We created MuscleJ, a macro that runs in ImageJ/Fiji on batches of images. MuscleJ is a multianalysis tool that initially allows the analysis of muscle fibers, capillaries, and satellite cells. Since its creation, it has been used in many studies, and we have further developed the software and added new features, which are presented in this article. We converted the macro into a Java-language plugin with an improved user interface. MuscleJ2 provides quantitative analysis of fibrosis, vascularization, and cell phenotype in whole muscle sections. It also performs analysis of the peri-myonuclei, the individual capillaries, and any staining in the muscle fibers, providing accurate quantification within regional sublocalizations of the fiber. A multicartography option allows users to visualize multiple results simultaneously. The plugin is freely available to the muscle science community.

Deep learning in image-based phenotypic drug discovery

Modern drug discovery approaches often use high-content imaging to systematically study the effect on cells of large libraries of chemical compounds. By automatically screening thousands or millions of images to identify specific drug-induced cellular phenotypes, for example, altered cellular morphology, these approaches can reveal 'hit' compounds offering therapeutic promise. In the past few years, artificial intelligence (AI) methods based on deep learning (DL) [a family of machine learning (ML) techniques] have disrupted virtually all image analysis tasks, from image classification to segmentation. These powerful methods also promise to impact drug discovery by accelerating the identification of effective drugs and their modes of action. In this review, we highlight applications and adaptations of ML, especially DL methods for cell-based phenotypic drug discovery (PDD).

Distinct single-component adjuvants steer human DC-mediated T-cell polarization via Toll-like receptor signaling toward a potent antiviral immune response

The COVID-19 pandemic highlights the importance of efficient and safe vaccine development. Vaccine adjuvants are essential to boost and tailor the immune response to the corresponding pathogen. To allow for an educated selection, we assessed the effect of different adjuvants on human monocyte-derived dendritic cells (DCs) and their ability to polarize innate and adaptive immune responses. In contrast to commonly used adjuvants, such as aluminum hydroxide, Toll-like receptor (TLR) agonists induced robust phenotypic and functional DC maturation. In a DC-lymphocyte coculture system, we investigated the ensuing immune reactions. While monophosphoryl lipid A synthetic, a TLR4 ligand, induced checkpoint inhibitors indicative for immune exhaustion, the TLR7/8 agonist Resiquimod (R848) induced prominent type-1 interferon and interleukin 6 responses and robust CTL, B-cell, and NK-cell proliferation, which is particularly suited for antiviral immune responses. The recently licensed COVID-19 vaccines, BNT162b and mRNA-1273, are both based on single-stranded RNA. Indeed, we could confirm that the cytokine profile induced by lipid-complexed RNA was almost identical to the pattern induced by R848. Although this awaits further investigation, our results suggest that their efficacy involves the highly efficient antiviral response pattern stimulated by the RNAs' TLR7/8 activation.

LocalZProjector and DeProj: a toolbox for local 2D projection and accurate morphometrics of large 3D microscopy images

Background

Quantitative imaging of epithelial tissues requires bioimage analysis tools that are widely applicable and accurate. In the case of imaging 3D tissues, a common preprocessing step consists of projecting the acquired 3D volume on a 2D plane mapping the tissue surface. While segmenting the tissue cells is amenable on 2D projections, it is still very difficult and cumbersome in 3D. However, for many specimen and models used in developmental and cell biology, the complex content of the image volume surrounding the epithelium in a tissue often reduces the visibility of the biological object in the projection, compromising its subsequent analysis. In addition, the projection may distort the geometry of the tissue and can lead to strong artifacts in the morphology measurement.

Results

Here we introduce a user-friendly toolbox built to robustly project epithelia on their 2D surface from 3D volumes and to produce accurate morphology measurement corrected for the projection distortion, even for very curved tissues. Our toolbox is built upon two components. LocalZProjector is a configurable Fiji plugin that generates 2D projections and height-maps from potentially large 3D stacks (larger than 40 GB per time-point) by only incorporating signal of the planes with local highest variance/mean intensity, despite a possibly complex image content. DeProj is a MATLAB tool that generates correct morphology measurements by combining the height-map output (such as the one offered by LocalZProjector) and the results of a cell segmentation on the 2D projection, hence effectively deprojecting the 2D segmentation in 3D. In this paper, we demonstrate their effectiveness over a wide range of different biological samples. We then compare its performance and accuracy against similar existing tools.

Conclusions

We find that LocalZProjector performs well even in situations where the volume to project also contains unwanted signal in other layers. We show that it can process large images without a pre-processing step. We study the impact of geometrical distortions on morphological measurements induced by the projection. We measured very large distortions which are then corrected by DeProj, providing accurate outputs.

Supplementary Information

The online version contains supplementary material available at (10.1186/s12915-021-01037-w).

Unveiling Interindividual Variability of Human Fibroblast Innate Immune Response Using Robust Cell-Based Protocols

The LabEx Milieu Interieur (MI) project is a clinical study centered on the detailed characterization of the baseline and induced immune responses in blood samples from 1,000 healthy donors. Analyses of these samples has lay ground for seminal studies on the genetic and environmental determinants of immunologic variance in a healthy cohort population. In the current study we developed in vitro methods enabling standardized quantification of MI-cohort-derived primary fibroblasts responses. Our results show that in vitro human donor cohort fibroblast responses to stimulation by different MAMPs analogs allows to characterize individual donor immune-phenotype variability. The results provide proof-of-concept foundation to a new experimental framework for such studies. A bio-bank of primary fibroblast lines was generated from 323 out of 1,000 healthy individuals selected from the MI-study cohort. To study inter-donor variability of innate immune response in primary human dermal fibroblasts we chose to measure the TLR3 and TLR4 response pathways, both receptors being expressed and previously studied in fibroblasts. We established high-throughput automation compatible methods for standardized primary fibroblast cell activation, using purified MAMPS analogs, poly I:C and LPS that stimulate TLR3 and TLR4 pathways respectively. These results were in turn compared with a stimulation method using infection by HSV-1 virus. Our "Add-only" protocol minimizes high-throughput automation system variability facilitating whole process automation from cell plating through stimulation to recovery of cell supernatants, and fluorescent labeling. Images were acquired automatically by high-throughput acquisition on an automated high-content imaging microscope. Under these methodological conditions standardized image acquisition provided for quantification of cellular responses allowing biological variability to be measured with low system noise and high biological signal fidelity. Optimal for automated analysis of immuno-phenotype of primary human cell responses our method and experimental framework as reported here is highly compatible to high-throughput screening protocols like those necessary for chemo-genomic screening. In context of primary fibroblasts derived from donors enrolled to the MI-clinical-study our results open the way to assert the utility of studying immune-phenotype characteristics relevant to a human clinical cohort.

Next-Generation Phenotypic Screening in Early Drug Discovery for Infectious Diseases

Cell-based phenotypic screening has proven to be valuable, notably in recapitulating relevant biological conditions, for example, the host cell/pathogen niche. However, the corresponding methodological complexity is not readily compatible with high-throughput pipelines, and fails to inform either molecular target or mechanism of action, which frustrates conventional drug-discovery roadmaps. We review the state-of-the-art and emerging technologies that suggest new strategies for harnessing value from the complexity of phenotypic screening and augmenting powerful utility for translational drug discovery. Advances in cellular, molecular, and bioinformatics technologies are converging at a cutting edge where the complexity of phenotypic screening may no longer be considered a hinderance but rather a catalyst to chemotherapeutic discovery for infectious diseases.

Imaging of Red-Shifted Light From Bioluminescent Tumors Using Fluorescence by Unbound Excitation From Luminescence

Early detection of tumors is today a major challenge and requires sensitive imaging methodologies coupled with new efficient probes. In vivo optical bioluminescence imaging has been widely used in the field of preclinical oncology to visualize tumors and several cancer cell lines have been genetically modified to provide bioluminescence signals. However, the light emitted by the majority of commonly used luciferases is usually in the blue part of the visible spectrum, where tissue absorption is still very high, making deep tissue imaging non-optimal, and calling for optimized optical imaging methodologies. We have previously shown that red-shifting of bioluminescence signal by Fluorescence Unbound Excitation from Luminescence (FUEL) is a mean to increase bioluminescence signal sensitivity detection in vivo. Here, we applied FUEL to tumor detection in two different subcutaneous tumor models: the auto-luminescent human embryonic kidney (HEK293) cell line and the murine B16-F10 melanoma cell line previously transfected with a plasmid encoding the Luc2 firefly luciferase. Tumor size and bioluminescence were measured over time and tumor vascularization characterized. We then locally injected near infrared emitting Quantum Dots (NIR QDs) in the tumor site and observed a red-shifting of bioluminescence signal by (FUEL) indicating that FUEL could be used to allow deeper tumor detection in mice.

MuscleJ: a high-content analysis method to study skeletal muscle with a new Fiji tool

Background

Skeletal muscle has the capacity to adapt to environmental changes and regenerate upon injury. To study these processes, most experimental methods use quantification of parameters obtained from images of immunostained skeletal muscle. Muscle cross-sectional area, fiber typing, localization of nuclei within the muscle fiber, the number of vessels, and fiber-associated stem cells are used to assess muscle physiology. Manual quantification of these parameters is time consuming and only poorly reproducible. While current state-of-the-art software tools are unable to analyze all these parameters simultaneously, we have developed MuscleJ, a new bioinformatics tool to do so.

Methods

Running on the popular open source Fiji software platform, MuscleJ simultaneously analyzes parameters from immunofluorescent staining, imaged by different acquisition systems in a completely automated manner.

Results

After segmentation of muscle fibers, up to three other channels can be analyzed simultaneously. Dialog boxes make MuscleJ easy-to-use for biologists. In addition, we have implemented color in situ cartographies of results, allowing the user to directly visualize results on reconstituted muscle sections.

Conclusion

We report here that MuscleJ results were comparable to manual observations made by five experts. MuscleJ markedly enhances statistical analysis by allowing reliable comparison of skeletal muscle physiology-pathology results obtained from different laboratories using different acquisition systems. Providing fast robust multi-parameter analyses of skeletal muscle physiology-pathology, MuscleJ is available as a free tool for the skeletal muscle community.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0171-0) contains supplementary material, which is available to authorized users.

Fluorescence imaging host pathogen interactions: fifteen years benefit of hindsight…

We consider in review current state-of-the-art fluorescence microscopy for investigating the host-pathogen interface. Our perspective is honed from years with literally thousands of microbiologists using the variety of imaging technologies available within our dedicated BSL2/BSL3 optical imaging research service facilities at the Institut Pasteur Paris founded from scratch in 2001. During fifteen years learning from the success and failures of introducing different fluorescence imaging technologies, methods, and technical development strategies we provide here a synopsis review of our experience to date and a synthesis of how we see the future in perspective for fluorescence imaging at the host-pathogen interface.

Imaging Intermediate Filaments and Microtubules with 2-dimensional Direct Stochastic Optical Reconstruction Microscopy

The cytoskeleton, composed of actin microfilaments, microtubules, and intermediate filaments (IF), plays a key role in the control of cell shape, polarity, and motility. The organization of the actin and microtubule networks has been extensively studied but that of IFs is not yet fully characterized. IFs have an average diameter of 10 nm and form a network extending throughout the cell cytoplasm. They are physically associated with actin and microtubules through molecular motors and cytoskeletal linkers. This tight association is at the heart of the regulatory mechanisms that ensure the coordinated regulation of the three cytoskeletal networks required for most cell functions. It is therefore crucial to visualize IFs alone and also together with each of the other cytoskeletal networks. However, IF networks are extremely dense in most cell types, especially in glial cells, which makes its resolution very difficult to achieve with standard fluorescence microscopy (lateral resolution of ~250 nm). Direct STochastic Optical Reconstruction Microscopy (dSTORM) is a technique allowing a gain in lateral resolution of one order of magnitude. Here, we show that lateral dSTORM resolution is sufficient to resolve the dense organization of the IF networks and, in particular, of IF bundles surrounding microtubules. Such tight association is likely to participate in the coordinated regulation of these two networks and may, explain how vimentin IFs template and stabilize microtubule organization as well as could influence microtubule dependent vesicular trafficking. More generally, we show how the observation of two cytoskeletal components with dual-color dSTORM technique brings new insight into their mutual interaction.

TrackMate: An open and extensible platform for single-particle tracking

We present TrackMate, an open source Fiji plugin for the automated, semi-automated, and manual tracking of single-particles. It offers a versatile and modular solution that works out of the box for end users, through a simple and intuitive user interface. It is also easily scriptable and adaptable, operating equally well on 1D over time, 2D over time, 3D over time, or other single and multi-channel image variants. TrackMate provides several visualization and analysis tools that aid in assessing the relevance of results. The utility of TrackMate is further enhanced through its ability to be readily customized to meet specific tracking problems. TrackMate is an extensible platform where developers can easily write their own detection, particle linking, visualization or analysis algorithms within the TrackMate environment. This evolving framework provides researchers with the opportunity to quickly develop and optimize new algorithms based on existing TrackMate modules without the need of having to write de novo user interfaces, including visualization, analysis and exporting tools. The current capabilities of TrackMate are presented in the context of three different biological problems. First, we perform Caenorhabditis-elegans lineage analysis to assess how light-induced damage during imaging impairs its early development. Our TrackMate-based lineage analysis indicates the lack of a cell-specific light-sensitive mechanism. Second, we investigate the recruitment of NEMO (NF-κB essential modulator) clusters in fibroblasts after stimulation by the cytokine IL-1 and show that photodamage can generate artifacts in the shape of TrackMate characterized movements that confuse motility analysis. Finally, we validate the use of TrackMate for quantitative lifetime analysis of clathrin-mediated endocytosis in plant cells.

Screening out irrelevant cell-based models of disease

The common and persistent failures to translate promising preclinical drug candidates into clinical success highlight the limited effectiveness of disease models currently used in drug discovery. An apparent reluctance to explore and adopt alternative cell- and tissue-based model systems, coupled with a detachment from clinical practice during assay validation, contributes to ineffective translational research. To help address these issues and stimulate debate, here we propose a set of principles to facilitate the definition and development of disease-relevant assays, and we discuss new opportunities for exploiting the latest advances in cell-based assay technologies in drug discovery, including induced pluripotent stem cells, three-dimensional (3D) co-culture and organ-on-a-chip systems, complemented by advances in single-cell imaging and gene editing technologies. Funding to support precompetitive, multidisciplinary collaborations to develop novel preclinical models and cell-based screening technologies could have a key role in improving their clinical relevance, and ultimately increase clinical success rates.

Phenotypic clustering: a novel method for microglial morphology analysis

Background

Microglial cells are tissue-resident macrophages of the central nervous system. They are extremely dynamic, sensitive to their microenvironment and present a characteristic complex and heterogeneous morphology and distribution within the brain tissue. Many experimental clues highlight a strong link between their morphology and their function in response to aggression. However, due to their complex “dendritic-like” aspect that constitutes the major pool of murine microglial cells and their dense network, precise and powerful morphological studies are not easy to realize and complicate correlation with molecular or clinical parameters.

Methods

Using the knock-in mouse model CX3CR1^GFP/+, we developed a 3D automated confocal tissue imaging system coupled with morphological modelling of many thousands of microglial cells revealing precise and quantitative assessment of major cell features: cell density, cell body area, cytoplasm area and number of primary, secondary and tertiary processes. We determined two morphological criteria that are the complexity index (CI) and the covered environment area (CEA) allowing an innovative approach lying in (i) an accurate and objective study of morphological changes in healthy or pathological condition, (ii) an in situ mapping of the microglial distribution in different neuroanatomical regions and (iii) a study of the clustering of numerous cells, allowing us to discriminate different sub-populations.

Results

Our results on more than 20,000 cells by condition confirm at baseline a regional heterogeneity of the microglial distribution and phenotype that persists after induction of neuroinflammation by systemic injection of lipopolysaccharide (LPS). Using clustering analysis, we highlight that, at resting state, microglial cells are distributed in four microglial sub-populations defined by their CI and CEA with a regional pattern and a specific behaviour after challenge.

Conclusions

Our results counteract the classical view of a homogenous regional resting state of the microglial cells within the brain. Microglial cells are distributed in different defined sub-populations that present specific behaviour after pathological challenge, allowing postulating for a cellular and functional specialization. Moreover, this new experimental approach will provide a support not only to neuropathological diagnosis but also to study microglial function in various disease models while reducing the number of animals needed to approach the international ethical statements.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0614-7) contains supplementary material, which is available to authorized users.

Conical diffraction illumination opens the way for low phototoxicity super-resolution imaging

We present a new technology for super-resolution fluorescence imaging, based on conical diffraction. Conical diffraction is a linear, singular phenomenon, taking place when a laser beam is diffracted through a biaxial crystal. We use conical diffraction in a thin biaxial crystal to generate illumination patterns that are more compact than the classical Gaussian beam, and use them to generate a super-resolution imaging modality.

While there already exist several super-resolution modalities, our technology (biaxial super-resolution: BSR) is distinguished by the unique combination of several performance features. Using BSR super-resolution data are achieved using low light illumination significantly less than required for classical confocal imaging, which makes BSR ideal for live-cell, long-term time-lapse super-resolution imaging. Furthermore, no specific sample preparation is required, and any fluorophore can be used. Perhaps most exciting, improved resolution BSR-imaging resolution enhancement can be achieved with any type of objective no matter the magnification, numerical aperture, working distance, or the absence or presence of immersion medium.

In this article, we present the first implementation of BSR modality on a commercial confocal microscope. We acquire and analyze validation data, showing high quality super-resolved images of biological objects, and demonstrate the wide applicability of the technology. We report live-cell super-resolution imaging over a long period, and show that the light dose required for super-resolution imaging is far below the threshold likely to generate phototoxicity.

Shigella subverts the host recycling compartment to rupture its vacuole

Shigella enters epithlial cells via internalization into a vacuole. Subsequent vacuolar membrane rupture allows bacterial escape into the cytosol for replication and cell-to-cell spread. Bacterial effectors such as IpgD, a PI(4,5)P2 phosphatase that generates PI(5)P and alters host actin, facilitate this internalization. Here, we identify host proteins involved in Shigella uptake and vacuolar membrane rupture by high-content siRNA screening and subsequently focus on Rab11, a constituent of the recycling compartment. Rab11-positive vesicles are recruited to the invasion site before vacuolar rupture, and Rab11 knockdown dramatically decreases vacuolar membrane rupture. Additionally, Rab11 recruitment is absent and vacuolar rupture is delayed in the ipgD mutant that does not dephosphorylate PI(4,5)P₂ into PI(5)P. Ultrastructural analyses of Rab11-positive vesicles further reveal that ipgD mutant-containing vacuoles become confined in actin structures that likely contribute to delayed vacular rupture. These findings provide insight into the underlying molecular mechanism of vacuole progression and rupture during Shigella invasion.

A step beyond BRET: Fluorescence by Unbound Excitation from Luminescence (FUEL)

Fluorescence by Unbound Excitation from Luminescence (FUEL) is a radiative excitation-emission process that produces increased signal and contrast enhancement in vitro and in vivo. FUEL shares many of the same underlying principles as Bioluminescence Resonance Energy Transfer (BRET), yet greatly differs in the acceptable working distances between the luminescent source and the fluorescent entity. While BRET is effectively limited to a maximum of 2 times the Förster radius, commonly less than 14 nm, FUEL can occur at distances up to µm or even cm in the absence of an optical absorber. Here we expand upon the foundation and applicability of FUEL by reviewing the relevant principles behind the phenomenon and demonstrate its compatibility with a wide variety of fluorophores and fluorescent nanoparticles. Further, the utility of antibody-targeted FUEL is explored. The examples shown here provide evidence that FUEL can be utilized for applications where BRET is not possible, filling the spatial void that exists between BRET and traditional whole animal imaging.

Real-time label-free detection of dividing cells by means of lensfree video-microscopy

Quantification of cell proliferation and monitoring its kinetics are essential in fields of research such as developmental biology, oncology, etc. Although several proliferation assays exist, monitoring cell proliferation kinetics remains challenging. We present a novel cell proliferation assay based on real-time monitoring of cell culture inside a standard incubator using a lensfree video-microscope, combined with automated detection of single cell divisions over a population of several thousand cells. Since the method is based on direct visualization of dividing cells, it is label-free, continuous, and not sample destructive. Kinetics of cell proliferation can be monitored from a few hours to several days. We compare our method to a standard assay, the EdU proliferation assay, and as proof of principle, we demonstrate concentration-dependent and time-dependent effect of actinomycin D-a cell proliferation inhibitor.

In vitro characterization of Fluorescence by Unbound Excitation from Luminescence: broadening the scope of energy transfer

Energy transfer mechanisms represent the basis for an array of valuable tools to infer interactions in vitro and in vivo, enhance detection or resolve interspecies distances such as with resonance. Based upon our own previously published studies and new results shown here we present a novel framework describing for the first time a model giving a view of the biophysical relationship between Fluorescence by Unbound Excitation from Luminescence (FUEL), a conventional radiative excitation-emission process, and bioluminescence resonance energy transfer. We show here that in homogeneous solutions and in fluorophore-targeted bacteria, FUEL is the dominant mechanism responsible for the production of red-shifted photons. The minor resonance contribution was ascertained by comparing the intensity of the experimental signal to its theoretical resonance counterpart. Distinctive features of the in vitro FUEL signal include a macroscopic depth dependency, a lack of enhancement upon targeting at a constant fluorophore concentration cf and a non-square dependency on cf. Significantly, FUEL is an important, so far overlooked, component of all resonance phenomena which should guide the design of appropriate controls when elucidating interactions. Last, our results highlight the potential for FUEL as a means to enhance in vivo and in vitro detection through complex media while alleviating the need for targeting.

High content analysis of primary macrophages hosting proliferating Leishmania amastigotes: application to anti-leishmanial drug discovery

Background/Objectives

Human leishmaniases are parasitic diseases causing severe morbidity and mortality. No vaccine is available and numerous factors limit the use of current therapies. There is thus an urgent need for innovative initiatives to identify new chemotypes displaying selective activity against intracellular Leishmania amastigotes that develop and proliferate inside macrophages, thereby causing the pathology of leishmaniasis.

Methodology/Principal Findings

We have developed a biologically sound High Content Analysis assay, based on the use of homogeneous populations of primary mouse macrophages hosting Leishmania amazonensis amastigotes. In contrast to classical promastigote-based screens, our assay more closely mimics the environment where intracellular amastigotes are growing within acidic parasitophorous vacuoles of their host cells. This multi-parametric assay provides quantitative data that accurately monitors the parasitic load of amastigotes-hosting macrophage cultures for the discovery of leishmanicidal compounds, but also their potential toxic effect on host macrophages. We validated our approach by using a small set of compounds of leishmanicidal drugs and recently published chemical entities. Based on their intramacrophagic leishmanicidal activity and their toxicity against host cells, compounds were classified as irrelevant or relevant for entering the next step in the drug discovery pipeline.

Conclusions/Significance

Our assay represents a new screening platform that overcomes several limitations in anti-leishmanial drug discovery. First, the ability to detect toxicity on primary macrophages allows for discovery of compounds able to cross the membranes of macrophage, vacuole and amastigote, thereby accelerating the hit to lead development process for compounds selectively targeting intracellular parasites. Second, our assay allows discovery of anti-leishmanials that interfere with biological functions of the macrophage required for parasite development and growth, such as organelle trafficking/acidification or production of microbicidal effectors. These data thus validate a novel phenotypic screening assay using virulent Leishmania amastigotes growing inside primary macrophage to identify new chemical entities with bona fide drug potential.

Leishmaniases are neglected diseases caused by protozoan parasites that belong to the genus Leishmania. No vaccine exists against any form of leishmaniasis and most of the existing anti-leishmanial drugs have serious side effects. Current strategies for discovering new leishmanicidal molecules are largely using inappropriate types of host cells or employing the irrelevant insect-specific parasite stage. We presented here an innovative and biologically relevant drug screening assay based on the use of the mammalian macrophage host cells and Leishmania pathogenic amastigotes. Our visual fluorescence assay has been established with the objective to screen diverse small-molecules including chemicals and natural compounds that selectively target intra-macrophagic amastigotes without displaying toxicity for the macrophage host cells. The validation of a miniaturized assay, relying on automated handling of biological materials and fluorescent imaging probes, image acquisition, data storage and analysis is presented. Based on robust statistical methods and quality control metrics, the data analysis pipeline allows for the classification of compounds based on their effect on parasite and macrophage survival observed after three days of treatment.

Three-dimensional FRET reconstruction microscopy for analysis of dynamic molecular interactions in live cells

Analysis of cellular pathways requires concentration measurements of dynamically interacting molecules within the three-dimensional (3D) space of single living cells. Förster resonance energy transfer (FRET) microscopy from widefield, from confocal, and potentially from superresolution microscopes can access this information; however, these measurements are distorted by the inherent 3D blurring of optical imaging, spectral overlap of fluorophores, and detection noise. We propose a mathematical model of these processes and demonstrate, through simulation, how these distortions limit the dynamic range and sensitivity of conventional FRET microscopy. Using this model, we devise and validate a new approach (called 3D-FRET stoichiometry reconstruction, 3DFSR) for reconstructing 3D distributions of bound and free fluorescent molecules. Previous attempts to reconstruct 3D-FRET data relied on sequential spectral unmixing and deconvolution, a process that corrupts the detection statistics. We demonstrate that 3DFSR is superior to these approaches since it simultaneously models spectral mixing, optical blurring, and detection noise. To achieve the full potential of this technique, we developed an instrument capable of acquiring 3D-FRET data rapidly and sensitively from single living cells. Compared with conventional FRET microscopy, our 3D-FRET reconstruction technique and new instrumentation provides orders of magnitude gains in both sensitivity and accuracy wherein sustained high-resolution four-dimensional (x,y,z,t) imaging of molecular interactions inside living cells was achieved. These results verify previous observations that Cdc42 signaling is localized to the advancing margins of forming phagosomes in macrophages.

Electron-multiplying charge-coupled detector-based bioluminescence recording of single-cell Ca2+

The construction and application of genetically encoded intracellular calcium concentration ([Ca2+]i) indicators has a checkered history. Excitement raised over the creation of new probes is often followed by disappointment when it is found that the initial demonstrations of [Ca2+]i sensing capability cannot be leveraged into real scientific advances. Recombinant apo-aequorin cloned from Aequorea victoria was the first Ca2+ sensitive protein genetically targeted to subcellular compartments. In the jellyfish, bioluminescence resonance energy transfer (BRET) between Ca2+ bound aequorin and green fluorescent protein (GFP) emits green light. Similarly, Ca2+ sensitive bioluminescent reporters undergoing BRET have been constructed between aequorin and GFP, and more recently with other fluorescent protein variants. These hybrid proteins display red-shifted spectrums and have higher light intensities and stability compared to aequorin alone. We report BRET measurement of single-cell [Ca2+]i based on the use of electron-multiplying charge-coupled-detector (EMCCD) imaging camera technology, mounted on either a bioluminescence or conventional microscope. Our results show for the first time how these new technologies make facile long-term monitoring of [Ca2+]i at the single-cell level, obviating the need for expensive, fragile, and sophisticated equipment based on image-photon-detectors (IPD) that were until now the only technical recourse to dynamic BRET experiments of this type.

High-resolution 3-D imaging of living cells in suspension using confocal axial tomography

Conventional flow cytometry (FC) methods report optical signals integrated from individual cells at throughput rates as high as thousands of cells per second. This is further combined with the powerful utility to subsequently sort and/or recover the cells of interest. However, these methods cannot extract spatial information. This limitation has prompted efforts by some commercial manufacturers to produce state-of-the-art commercial flow cytometry systems allowing fluorescence images to be recorded by an imaging detector. Nonetheless, there remains an immediate and growing need for technologies facilitating spatial analysis of fluorescent signals from cells maintained in flow suspension. Here, we report a novel methodological approach to this problem that combines micro-fluidic flow, and microelectrode dielectric-field control to manipulate, immobilize and image individual cells in suspension. The method also offers unique possibilities for imaging studies on cells in suspension. In particular, we report the system's immediate utility for confocal "axial tomography" using micro-rotation imaging and show that it greatly enhances 3-D optical resolution compared with conventional light reconstruction (deconvolution) image data treatment. That the method we present here is relatively rapid and lends itself to full automation suggests its eventual utility for 3-D imaging cytometry.

A Bayesian reconstruction method for micro-rotation imaging in light microscopy

The authors present a three-dimensional (3D) reconstruction algorithm and reconstruction-based deblurring method for light microscopy using a micro-rotation device. In contrast to conventional 3D optical imaging where the focal plane is shifted along the optical axis, micro-rotation imaging employs dielectric fields to rotate the object inside a fixed optical set-up. To address this entirely new 3D-imaging modality, the authors present a reconstruction algorithm based on Bayesian inversion theory and use the total variation function as a structure prior. The spectral properties of the reconstruction by simulations that illustrate the strengths and the weaknesses of the micro-rotation approach, compared with conventional 3D optical imaging, were studied. The reconstruction from real data sets shows that this method is promising for 3D reconstruction and offers itself as a deblurring method using a reconstruction-based procedure for removing out-of-focus light from the micro-rotation image series.

Over-expression of Leishmania major MAP kinases reveals stage-specific induction of phosphotransferase activity

During the infectious cycle, protozoan parasites undergo various developmental transitions and switch virulence factors in response to extracellular signals in insect vectors and human hosts. Despite the importance of environmental sensing in parasite pathogenicity, little is known about the pathways that transduce extracellular signals into stage-specific gene expression. Here, we used a transgenic approach to gain insight into localisation and activity of three green fluorescence protein (GFP)-tagged Leishmania major mitogen-activated protein kinases, LmaMPK4, 7 and 10. The GFP-LmaMPKs in both L. major and Leishmania donovani transgenic lines showed predominant cytoplasmic localisation and the over-expression had no effect on promastigote morphology, growth and the ability to differentiate into stationary-phase metacyclics for L. major and axenic amastigotes for L. donovani. We isolated the GFP-tagged MPKs from parasite extracts and tested their phosphotransferase activity across various culture conditions. For all three GFP-LmaMPKs, kinase activity was low or absent in promastigote extracts but significantly increased in L. major promastigotes after exposure to pH 5.5 and 34 degrees C, and in axenic L. donovani amastigotes. Enhanced activity correlated with increased GFP-LmaMPK phosphorylation as judged by phospho-specific fluorescent staining of the immuno-precipitated kinases. We could extend these findings to the endogenous LmaMPK10, which accumulated in the phospho-protein fraction of axenic amastigotes but not promastigotes, and thus follows the stage-specific phosphorylation profile of episomally expressed GFP-LmaMPK10. These results provide evidence for the functional conservation of Leishmania MAP kinases in parasite environmental sensing and underscore the potential of transgenic approaches to gain insight into signaling events during the Leishmania life cycle.

The SPI-2 type III secretion system restricts motility of Salmonella-containing vacuoles

Intracellular replication of Salmonella enterica occurs in membrane-bound compartments, called Salmonella-containing vacuoles (SCVs). Following invasion of epithelial cells, most SCVs migrate to a perinuclear region and replicate in close association with the Golgi network. The association of SCVs with the Golgi is dependent on the Salmonella-pathogenicity island-2 (SPI-2) type III secretion system (T3SS) effectors SseG, SseF and SifA. However, little is known about the dynamics of SCV movement. Here, we show that in epithelial cells, 2 h were required for migration of the majority of SCVs to within 5 μm from the microtubule organizing centre (MTOC), which is located in the same subcellular region as the Golgi network. This initial SCV migration was saltatory, bidirectional and microtubule-dependent. An intact Golgi, SseG and SPI-2 T3SS were dispensable for SCV migration to the MTOC, but were essential for maintenance of SCVs in that region. Live-cell imaging between 4 and 8 h post invasion revealed that the majority of wild-type SCVs displaced less than 2 μm in 20 min from their initial starting positions. In contrast, between 6 and 8 h post invasion the majority of vacuoles containing sseG, sseF or ssaV mutant bacteria displaced more than 2 μm in 20 min from their initial starting positions, with some undergoing large and dramatic movements. Further analysis of the movement of SCVs revealed that large displacements were a result of increased SCV speed rather than a change in their directionality, and that SseG influences SCV motility by restricting vacuole speed within the MTOC/Golgi region. SseG might function by tethering SCVs to Golgi-associated molecules, or by controlling microtubule motors, for example by inhibiting kinesin recruitment or promoting dynein recruitment.

Quantitative real-time analysis of HIV-1 gene expression dynamics in single living primary cells

Studies on the regulation of viral transcription upon infection of the target cells have provided important information on the viral and host factors that influence pathogenesis. However, these studies have been limited so far to steady-state analysis of gene expression. Here we report an image based photon-counting method that allows real-time quantitative imaging of viral gene expression in infected single cells. Employing an HIV-1 vector bearing the firefly luciferase reporter gene, we exploited a single cell photon imaging methodology (a customized and highly sensitive imaging microscope) to measure viral gene expression following integration into a host genome in situ. Our approach reveals real-time dynamics of viral gene expression in living HIV natural target cells (primary human CD4 T cells and macrophages), and promises itself as a powerful tool for quantitative studies on a wide variety of virus-host cell interactions.

Imaging today's infectious animalcules

The study of pathogens and their interactions with host cells has advanced hand-in-hand with developments in optical microscopy. Whereas microbiology benefits enormously from modern imaging technologies, for example, digital imaging and confocal microscopy, it also presents unique challenges. To overcome these, microbiologists are adept at customising imaging methods, and recently there have been studies using state-of-the-art quantitative imaging methods to probe host-pathogen interactions at the single-cell level. Of particular interest are the studies using combined light and electron microscopy methods, bi-arsenical tetra-cysteine tag labelling and automated image-acquisition and analysis for high-throughput/high-content experimentation. These applications demonstrate how imaging methodologies, adapted for microbiology, continue to open avenues for studies that previously have proven inaccessible.

Actin polymerisation at the cytoplasmic face of eukaryotic nuclei

Background

There exists abundant molecular and ultra-structural evidence to suggest that cytoplasmic actin can physically interact with the nuclear envelope (NE) membrane system. However, this interaction has yet to be characterised in living interphase cells.

Results

Using a fluorescent conjugate of the actin binding drug cytochalasin D (CD-BODIPY) we provide evidence that polymerising actin accumulates in vicinity to the NE. In addition, both transiently expressed fluorescent actin and cytoplasmic micro-injection of fluorescent actin resulted in accumulation of actin at the NE-membrane. Consistent with the idea that the cytoplasmic phase of NE-membranes can support this novel pool of perinuclear actin polymerisation we show that isolated, intact, differentiated primary hepatocyte nuclei support actin polymerisation in vitro. Further this phenomenon was inhibited by treatments hindering steric access to outer-nuclear-membrane proteins (e.g. wheat germ agglutinin, anti-nesprin and anti-nucleoporin antibodies).

Conclusion

We conclude that actin polymerisation occurs around interphase nuclei of living cells at the cytoplasmic phase of NE-membranes.

Visualization of local Ca2+ dynamics with genetically encoded bioluminescent reporters

Measurements of local Ca2+ signalling at different developmental stages and/or in specific cell types is important for understanding aspects of brain functioning. The use of light excitation in fluorescence imaging can cause phototoxicity, photobleaching and auto-fluorescence. In contrast, bioluminescence does not require the input of radiative energy and can therefore be measured over long periods, with very high temporal resolution. Aequorin is a genetically encoded Ca(2+)-sensitive bioluminescent protein, however, its low quantum yield prevents dynamic measurements of Ca2+ responses in single cells. To overcome this limitation, we recently reported the bi-functional Ca2+ reporter gene, GFP-aequorin (GA), which was developed specifically to improve the light output and stability of aequorin chimeras [V. Baubet, et al., (2000) PNAS, 97, 7260-7265]. In the current study, we have genetically targeted GA to different microdomains important in synaptic transmission, including to the mitochondrial matrix, endoplasmic reticulum, synaptic vesicles and to the postsynaptic density. We demonstrate that these reporters enable 'real-time' measurements of subcellular Ca2+ changes in single mammalian neurons using bioluminescence. The high signal-to-noise ratio of these reporters is also important in that it affords the visualization of Ca2+ dynamics in cell-cell communication in neuronal cultures and tissue slices. Further, we demonstrate the utility of this approach in ex-vivo preparations of mammalian retina, a paradigm in which external light input should be controlled. This represents a novel molecular imaging approach for non-invasive monitoring of local Ca2+ dynamics and cellular communication in tissue or whole animal studies.

Use of micro-rotation imaging to study JNK-mediated cell survival inTheileria parva-infected B-lymphocytes

Lymphocytes infected with the protozoan parasiteTheileria parvaare transformed to permanently proliferating cells, an event underlying the pathology of the disease. However, the molecular signalling mediating this process is complex and poorly understood. Here, we show that down-regulation of JNK signalling by transient over expression of a dominant-negative mutant of JNK (JNK-APF) significantly increases Annexin-V-phycoerythrin (V-PE) labelling on infected B cell populations observed using flow cytometry. To establish whether this increase was specifically due to apoptosis, we used a novel single-cell imaging method: micro-rotation (MR)-imaging, designed to allow high-resolution 3-dimensional imaging of single cells in suspension. With this method we visualized subcellular patterns of V-PE uptake and chromatin organization in lymphocytes co-transfected with JNK-APF and GFP-tagged histone-H2B. This single-cell approach allowed us to clearly reveal characteristic apoptotic phenotypes, whose patterns reflected progressive states of programmed cell death due to JNK down-regulation. Our results strongly suggest a role for JNK in the survival ofTheileria-infected B cells, and demonstrate the powerful utility of a new and unique 3-dimensional imaging method for living cells in suspension.

Imaging movement of malaria parasites during transmission by Anopheles mosquitoes

Malaria is contracted when Plasmodium sporozoites are inoculated into the vertebrate host during the blood meal of a mosquito. In infected mosquitoes, sporozoites are present in large numbers in the secretory cavities of the salivary glands at the most distal site of the salivary system. However, how sporozoites move through the salivary system of the mosquito, both in resting and feeding mosquitoes, is unknown. Here, we observed fluorescent Plasmodium berghei sporozoites within live Anopheles stephensi mosquitoes and their salivary glands and ducts. We show that sporozoites move in the mosquito by gliding, a type of motility associated with their capacity to invade host cells. Unlike in vitro, sporozoite gliding inside salivary cavities and ducts is modulated in speed and motion pattern. Imaging of sporozoite discharge through the proboscis of salivating mosquitoes indicates that sporozoites need to locomote from cavities into ducts to be ejected and that their progression inside ducts favours their early ejection. These observations suggest that sporozoite gliding allows not only for cell invasion but also for parasite locomotion in host tissues, and that it may control parasite transmission.

Focusing light on infection in four dimensions

The fusion of cell biology with microbiology has bred a new discipline, cellular microbiology, in which the primary aim is to understand host-pathogen interactions at a tissue, cellular and molecular level. In this context, we require techniques allowing us to probe infection in situ and extrapolate quantitative information on its spatiotemporal dynamics. To these ends, fluorescent light-based imaging techniques offer a powerful tool, and the state-of-the-art is defined by paradigms using so-called multidimensional (multi-D) imaging microscopy. Multi-D imaging aims to visualize and quantify biological events through time and space and, more specifically, refers to combinations of: three (3D, volume), four (4D, time) and five (5D, multiwavelength)-dimensional recordings. Successful multi-D imaging depends upon understanding the available technologies and their limitations. This is especially true in the field of microbiology where visualization of infectious/pathogenic activities inside living host systems presents particular technical challenges. Thus, as multi-D imaging rapidly becomes a common bench tool to the cellular microbiologist, this review provides the new user with some of the necessary technical insight required to get the best from these methods.

Cloning and mRNA expression of the Ca2+-binding DREAM protein in the pituitary

It is well recognized that the level of intracellular calcium governs several cellular processes such as gene expression and secretion in the pituitary. Recently, a novel gene has been identified in neuroendocrine cells that encodes DREAM, a calcium-binding protein that acts as a transcriptional repressor by binding specific downstream regulatory elements (DRE) on DNA. To explore the possibility that DREAM may be expressed in the rat pituitary and may function in endocrine activity, we analyzed its mRNA expression by RT-PCR. Using oligonucleotide primers derived from the mouse DREAM cDNA, we amplified, cloned, and characterized a 852-bp RT-PCR product from rat pituitary tissue. Two splice variants of the rat DREAM gene differing by four nucleotides (tetramer ACAG) were identified. The ACAG(+) variant (ORF1) consisted of 768bp encoding a protein of 256 residues with an estimated molecular weight of 29.5kDa. Amino acid sequence analysis of ORF1 indicated 92.6% and 98.1% identity to the DREAM gene product from human and mouse, respectively. The second variant, ACAG(-) (ORF2), was 567-bp long and was predicted to encode a peptide of 189 residues with a molecular mass of about 20.8kDa. To determine which endocrine pituitary cells were expressing DREAM, we evaluated several different clonal populations containing cells that expressed specific pituitary hormones. We found that both DREAM splice variants were expressed in each pituitary cell types examined, which included the mammotropes (MMQ cells), somatotropes (GC cells), mammosomatotropes (GH(3) cells), gonadotropes (LbetaT2 cells), thyrotropes (TalphaT1 cells), and corticotropes (AtT-20 cells). Interestingly, the levels of the two variants differed between the cell types tested with the ACAG(+) variant comprising about two-thirds of the DREAM expression for the mammotropes, somatotropes, mammosomatotropes, and corticotropes as compared to less than one-half for the thyrotropes and the gonadotropes. Our initial attempts to identify pituitary-specific genes regulated by DREAM revealed that prolactin gene expression was not influenced by DREAM suggesting that an action of DREAM may involve other pituitary hormones or be mediated by other cell processes. When taken together, our findings of DREAM expression in the pituitary in a manner specific to pituitary endocrine cell type raises the possibility that this protein may play a role in determining specific pituitary cell function.

PRL gene expression in individual living mammotropes displays distinct functional pulses that oscillate in a noncircadian temporal pattern

PRL gene expression in the anterior pituitary has been the focus of intensive investigation for many years, but very little information is available on the actual dynamics by which this process occurs in individual mammotrope cells. Here, we used single cell bioluminescent imaging microscopy and a recently refined reporter gene strategy to measure PRL promoter-driven gene expression (PRL-GE) in individual living primary mammotropes. Using this approach we report a new phenomenon involving repetitive on/off gene expression bursts that occurred in a distinctly noncircadian oscillatory pattern. Furthermore, we demonstrate a functional basis for these gene expression oscillations, inasmuch as PRL-GE pulses were sensitive to calcium-dependent modulation, which we show arose exclusively as changes in the shape of individual pulse episodes. Our results provide the first clear evidence that PRL-GE, in its homologous cell environment, displays oscillatory bursts of activity. Moreover, they strongly support the idea that these discrete on/off bursts of activity serve as an important determinant of the timing and level of PRL-GE under both basal and stimulated conditions.

Pulsatile exocytosis is functionally associated with GnRH gene expression in immortalized GnRH-expressing cells

Pulsatile release of GnRH is essential for proper reproductive function, but little information is available on the molecular processes underlying this intermittent activity. Recently, GnRH gene expression (GnRH-GE) episodes and exocytotic pulses have been identified separately in individual GnRH-expressing cells, raising the exciting possibility that both activities are linked functionally and are fundamental to the pulsatile process. To explore this, we monitored GnRH-GE (using a GnRH promoter-driven luciferase reporter) and exocytosis (by FM1-43 fluorescence) in the same, living GT1-7 cells. Our results revealed a strong temporal association between exocytotic pulses and GnRH-GE episodes. To determine whether a functional link existed, we blocked one process and evaluated the other. Transcriptional inhibition with actinomycin D had only a modest influence on exocytosis, suggesting that exocytotic pulse activity was not dictated acutely by episodes of gene expression. In contrast, blockage of exocytosis with anti-SNAP-25 (which obstructs secretory granule fusion) abolished GnRH-GE pulse activity, indicating that part of the exocytotic process is responsible for triggering episodes of GnRH-GE. When taken together, our findings suggest that a careful balance is maintained between release and biosynthesis in GT1-7 cells. Such a property may be important in the hypothalamus to ensure that GnRH neurons are in a constant state of readiness to respond to changes in reproductive function.

Synchronized exocytotic bursts from gonadotropin-releasing hormone-expressing cells: dual control by intrinsic cellular pulsatility and gap junctional communication

Periodic secretion of GnRH from the hypothalamus is the driving force for the release of gonadotropic hormones from the pituitary, but the roles of individual neurons in the context of this pulse generator are not known. In this study we used FM1-43 to monitor the membrane turnover associated with exocytosis in single GT1-7 neurons and found an intrinsic secretory pulsatility (frequency, 1.4 +/- 0.1/h; pulse duration, 17.3 +/- 0.6 min) that, during time in culture, became progressively synchronized among neighboring cells. Voltage-gated calcium channels and gap junctional communication each played a major role in synchronized pulsatility. An L-type calcium channel inhibitor, nimodipine, abolished synchronized pulsatility. In addition, functional gap junction communication among adjacent cells was detected, but only under conditions where pulsatile synchronization was also observed, and the gap junction inhibitor octanol abolished both without affecting pulse frequency or duration. Our results, therefore, provide strong evidence that the GnRH pulse generator in GT1-7 cells arises from a single cell oscillator mechanism that is synchronized through network signaling involving voltage-gated calcium channels and gap junctions.

Spontaneous calcium oscillatory patterns in mammotropes display non-random dynamics

We previously showed that primary rat mammotropes exhibited four distinct patterns of 'spontaneous' free intracellular calcium ([Ca2+]i) oscillatory behavior: a quiescent state A and three oscillatory states B,C&D, which differed in frequency/amplitude characteristics. When [Ca2+]i was monitored in 10 min windows separated by several hours, these phenotypes were frequently found to interconvert, raising the question about whether these transitions were random or ordered events. We reasoned that if such activity were random, then neither episode duration nor transitional probabilities should differ among phenotypes. We tested this logic in the current study by making long-term, continuous measurements of [Ca2+]i in mammotropes microinjected with Fura-2-dextran and identified by their ability to express a prolactin promoter-driven reporter plasmid. We found that transitions occurred in ~25% of cells (n = 36 from 9 independent experiments) once every 1-5 h and demarcated phenotype episodes of different duration (A, 1.04 +/- 0.2 h; B, 1.64 +/- 0.3 h; C, 2.45 +/- 0.62 h; D, 0.90 +/- 0.2 h, mean +/- SEM). Moreover, some transitions occurred more frequently than others and linked specific phenotypes into a common pattern: C to B to A. Our results demonstrate that the seemingly spontaneous nature of [Ca2+]i phenotype transitions are, in fact, ordered and support the view that they comprise a structured 'code' like that proposed to underlie calcium-dependent regulation of exocytosis and gene expression.

Functional integrity of green fluorescent protein conjugated glycine receptor channels

The alpha subunit (alphaZ1) of the zebrafish glycine receptor (GlyR) has been N-terminus fused with green fluorescent protein (GFP). We found that both pharmacological and electrophysiological properties of this chimeric alphaZ1-GFP are indistinguishable from those of the wild-type receptor when expressed in Xenopus oocytes and cell lines. The apparent affinities of this receptor for agonists (glycine, taurine and GABA), and the antagonist (strychnine) are unchanged, and single channel kinetics are not altered. In the same expression systems, alphaZ1-GFP was visualized using fluorescence microscopy. Fluorescence was distributed anisotropically across cellular membranes. In addition to the Golgi apparatus and endoplasmic reticulum, its presence was also detected on the plasmalemma, localized at discrete hot-spots which were identified as sites of high membrane turnover. Overall, the preservation in alphaZ1-GFPs of the wild type receptor functional properties makes it a promising new tool for further in situ investigations of GlyR expression, distribution and function.

N-methyl-D-aspartate evokes rapid net depolymerization of filamentous actin in cultured rat cerebellar granule cells

Filamentous actin (F-actin) was measured in cultured rat cerebellum granule neurons with the use of fluorescently labeled phallotoxin as a site-specific probe for F-actin, and fluorescence microscopy. The averaged apparent intensity of soma-associated F-actin-derived fluorescence (F(app)) was measured from fixed cells after incubation in either 1) normal Krebs solution containing 2 mM extracellular calcium ([Ca2+]ex) or 2) normal Krebs solution plus N-methyl-D-aspartate (NMDA) for 2 min immediately before fixation. NMDA (10, 50, and 100 microM) decreased F(app) to 63 +/- 5% (mean +/- SE), 53 +/- 4%, and 47 +/- 2%, respectively, of that measured from control cells. This effect was mimicked by treatment of cells with ionomycin. The ability of NMDA to reduce the F(app) in the presence of [Ca2+]ex was abolished when cells were maintained in [Ca2+]ex-free medium. Cells first treated with NMDA for 2 min and then left in normal medium for 30 min before fixation gave F(app) fluorescence similar to control values (91 +/- 12%). However, if the F-actin polymerization inhibitor cytochalasin D was added to cells immediately after NMDA was removed, the F(app) did not recover with time (36 +/- 3%). Cells treated for 30 min with cytochalasin D alone showed a small reduction in staining (approximately 20%). It is concluded that the actin polymerization state of rat cerebellar granule neurons is sensitive to changes in intracellular calcium, and that NMDA receptor activation evokes an initial rapid depolymerization of F-actin.

The effect of extracellular polyvalent cations on bovine anterior pituitary cells. Evidence for a Ca(2+)-sensing receptor coupled to release of intracellular calcium stores

We have investigated the effects of extracellular cations ([ION]ex) on cytosolic free calcium levels ([Ca2+]i) in bovine anterior pituitary (bAP) cells, using single-cell microfluorimetry. Increasing the [Ca2+]ex from 1 mM to 20 mM caused [Ca2+]i to increase in 64 +/- 14% of bAP cells. The [Ca2+]ex-induced [Ca2+]i increase was observed when cells were maintained in the presence of the voltage-gated-calcium-channel antagonist nitrendipine, but not when cells were treated with thapsigargin. Addition of [La3+]ex (5-15 microM) decreased [Ca2+]i, whereas 30 microM-1 mM caused a [Ca2+]i rise in 60.9 +/- 8.8% of bAP cells. [La3+]ex-induced [Ca2+]i changes were abolished by treating bAP-cells with either thapsigargin or ionomycin, but not nitrendipine. [La3+]ex at 15 microM did not increase [Ca2+]i in any cells tested, but when cells were treated with thimerosal, [La3+]ex (15 microM) caused a [Ca2+]i increase in 62.5 +/- 12.2% of bAP cells. In the presence of 1 mM [Ca2+]ex, successive additions of La3+ caused successive [Ca2+]i rises, but in nominally [Ca2+]ex-free medium only the first addition of [La3+]ex caused a [Ca2+]i rise. Addition of thyroliberin (TRH) in the presence of 1 mM [Ca2+]ex, caused [Ca2+]i to increase in 70% of bAP cells; subsequent addition of [La3+]ex (1 mM) only caused [Ca2+]i increases in 75% of those cells which had already responded to TRH. However, all cells which responded to 1 mM [La3+]ex also responded subsequently to TRH. After treatment with TRH in medium that was nominally [Ca2+]ex free, addition of La3+ (0.5-1 mM) did not increase [Ca2+]i in any cells tested. The number of cells which showed [La3+]ex-induced [Ca2+]i increases decreased in culture: only 21.75 +/- 2.2% cells responded after 7-11 days. When cells were cultured for 7-11 days in the presence of tunicamycin, [La3+]ex failed to increase [Ca2+]i in any cells tested. [Mn2+]ex rapidly quenched the Fura-2 signal measured from all bAP cells, but at 10 mM it also triggered a [Ca2+]i rise in about 60% of bAP cells. The Mn(2+)-induced [Ca2+]i rise was specifically abolished in cells cultured in the presence of tunicamycin although quenching was still observed. From these data we suggest that bAP cells may express a polyvalent cation receptor coupled to the release of calcium from intracellular stores.

Simultaneous measurement of [Ca2+]i and secretion-coupled membrane turnover, by single cell fluorescence microscopy

Thyrotropin releasing hormone (TRH), which stimulates prolactin secretion, increases the fluorescence of cultured bovine anterior pituitary (bAP) cells in the presence of the non-permeant membrane indicator dye FM 1-43 [Stafford SJV. Shorte SL. Schofield JG. (1993) Use of a fluorescent dye to measure secretion from intact bovine anterior pituitary cells. Biosci. Rep., 13, 9-17]. FM 1-43 is non-fluorescent in aqueous solution but becomes fluorescent when incorporated into the plasma membrane. The membrane area accessible to FM 1-43 dye, and therefore cell fluorescence, increases during exocytosis as secretory granules fuse with the plasma membrane, and endocytosis as vesicles formed at the plasma-membrane fuse with intracellular organelle membranes. We have here measured changes in FM 1-43 uptake and the intracellular calcium concentration ([Ca2+]i) concurrently in the same cells on exposure to TRH, phorbol myristate acetate (PMA) or NH4Cl. TRH (0.1-10 microM) caused a transient increase in [Ca2+]i in 70-90% of bAP cells and in 60-90% of the responding cells also caused a sustained increased FM 1-43 fluorescence. TRH increased [Ca2+]i but did not affect FM 1-43 fluorescence in GH3 rat pituitary cells, probably because they contain too few secretory granules to give a detectable increase. The dopamine D2-receptor agonist quinpirole (10 microM) had little effect on the TRH-induced [Ca2+]i rise in bAP cells, but abolished the increase in FM 1-43 fluorescence. The phorbol ester PMA (0.3-3 microM) caused a small, transient increase in [Ca2+]i followed by a fall to levels lower than original resting levels in 40-60% of bAP cells and increased FM 1-43 uptake in cells showing these changes. Extracellular NH4Cl, which mobilises calcium from an ionomycin-insensitive calcium store, caused a transient [Ca2+]i increase in over 90% of the bAP-cells and increased FM 1-43 uptake in a subpopulation (> 50%) of these. The Na+/H+ ionophore monensin prevented the increase in FM 1-43 fluorescence but not the [Ca2+]i rise induced by TRH, prevented the increases in both FM 1-43 fluorescence and [Ca2+]i caused by NH4Cl, and reduced the number of cells showing a rise in FM 1-43 fluorescence in response to PMA from 64% to 34%. The Ca(2+)-ATPase inhibitor thapsigargin reduced the number of bAP cells displaying TRH-induced increases in [Ca2+]i and membrane-turnover from 74% to 18%, but did not affect the changes in [Ca2+]i or FM 1-43 fluorescence caused by PMA or NH4Cl. We discuss the relationships between the secretogogue-induced increases in FM 1-43 fluorescence and changes in intracellular [Ca2+]i under these conditions.

Thyroliberin-induced changes in the fluorescence of a membrane probe in individual bovine anterior pituitary cells

1. We have investigated the use of TMA-DPH (1-[4-(trimethylammonio) phenyl]-6-phenylhexa-1,3,5-triene) as an indicator of exocytosis in individual bovine anterior pituitary cells using microfluorimetric imaging. 2. TMA-DPH was photolabile in artificial and cell membranes. In cells incubated in TMA-DPH the distribution of fluorescence depended both on the incubation time and the illumination schedule. If the dye was added while the cells were subjected to repeated cycles of 0.36 s light intermittent with 1-15 s dark, the fluorescence of the peripheral annulus and the central region of individual cells rose in parallel and reached a steady state within 200 s; the annulus was always brighter than the central region. However, using long intervening dark periods (200 s), the central region continued to incorporate dye after the annulus had reached a plateau. 3. When the cells were loaded with TMA-DPH using intermittent light with short dark periods, the dye washed out of the central region and the annulus in parallel when external dye was removed. However, if the cells had been loaded using long dark periods, the dye was washed out of the central region more slowly than from the annulus. 4. When cells were incubated in TMA-DPH in the dark for 1 min and then exposed to constant illumination in the presence of external dye, the fluorescence of the central region and the annulus both decayed in parallel to a new steady state. If the cells were incubated in TMA-DPH in the dark for 240 min the fluorescence from each region fell to a steady state but the falls were larger and were not in parallel. 5. We suggest that TMA-DPH fluorescence was derived from plasma membrane-associated and internalized dye and that the amount of fluorescence from the latter varied because TMA-DPH was photobleached. Thus, when illumination was interrupted by short dark intervals, annular fluorescence was high compared to central fluorescence because bleached dye in the plasma membrane was rapidly replaced by unbleached dye from the medium. However, long dark intervals permitted the dye to be internalized before it was bleached and fluorescence was therefore also present in central regions. 6. The total cell fluorescence, observed using 15 s dark intervals, was increased 5-40% (in single cells) in a dose-dependent fashion by addition of TRH (tripeptide thyrotrophin-releasing hormone; 1-200 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

Use of a fluorescent dye to measure secretion from intact bovine anterior pituitary cells

The fluorescent dye FM1-43 has been used to indicate membrane changes in individual bovine anterior pituitary cells exposed to secretory stimuli. After ten minutes incubation with FM1-43 (2 microM), cells showed three patterns of dye fluorescence: annular, partly filled and uniformly filled. FM1-43 fluorescence was increased in 61% of the cells by TRH (40 nM), a physiological stimulus for prolactin secretion, and in 89% of the cells by 60 mM external K+. The fluorescence also increased when cells incubated in the presence of quinpirole, a dopamine D2-receptor agonist which inhibits prolactin secretion, were exposed to raclopride, a D-2 antagonist. The increases in FM1-43 fluorescence caused by these treatments suggests that the dye acts as an indicator of secretion, possibly through incorporation into secretory vesicle membranes exposed on the cell surface during exocytosis. If the dye was washed away after loading, the fluorescence of partly and uniformly filled cells was retained and a rise in fluorescence could still be seen on stimulation by TRH. This suggests that some dye had been taken up by endocytosis and trapped in an intracellular compartment, which expanded through membrane recapture after TRH stimulation. FM1-43 could therefore be a useful probe for membrane cycling associated with secretory responses.

Thyrotropin-releasing hormone-induced cytosolic calcium transients: characterisation of store refilling in bovine anterior pituitary cells

The intracellular calcium ion concentration ([Ca2+]i) in individual bovine anterior pituitary cells was measured using fura-2 and ratiometric imaging. Addition of thyrotropin-releasing hormone (TRH) in the presence of external calcium ion ([Ca2+]e; 1 mM) caused a rapid transient increase in [Ca2+]i falling to a plateau which remained above pre-stimulation levels in the continued presence of TRH. Decreasing [Ca2+]e to 0.1 microM decreased [Ca2+]i. At 0.1 microM [Ca2+]e, the first TRH addition caused the rapid transient rise in [Ca2+]i but no plateau phase and a second addition of TRH did not cause a second transient rise. However, the second application of TRH in 0.1 microM [Ca2+]e caused a rise in [Ca2+]i if it was preceded by transient exposure of the cells to 2 mM [Ca2+]e. The presence of nitrendipine, 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), or TRH during the re-exposure to external calcium blocked this recovery of subsequent responses to TRH in the presence of only 0.1 microM [Ca2+]e. We conclude that refilling of the calcium stores depleted by TRH occurred only after the removal of agonist, used a tBHQ-sensitive uptake mechanism, and was mainly sustained by voltage-gated calcium entry into the cells.

Ammonium ions mobilize calcium from an internal pool which is insensitive to TRH and ionomycin in bovine anterior pituitary cells

The effects of NH4Cl on cytoplasmic free calcium concentration ([Ca2+]i) and pH (pHi) in single bovine anterior pituitary cells were determined using fluorescence imaging microscopy. Addition of NH4Cl (10-40 mM) in the presence of 1 mM extracellular calcium ([Ca2+]e) increased [Ca2+]i to a peak which then fell to a sustained plateau, returning to resting levels upon removal of NH4Cl. In medium containing 0.1 microM [Ca2+]e, or in 1 mM [Ca2+]e medium containing 0.1 microM nitrendipine, the plateau was absent leaving only a transient [Ca2+]i spike. NH4Cl also increased pHi and this, like the [Ca2+]i plateau, remained elevated during the continued presence of NH4Cl. In medium containing only 0.1 microM [Ca2+]e, to preclude refilling of internal stores by entry of external calcium, repeated exposures to NH4Cl induced repeated [Ca2+]i transients. In contrast, only the initial exposure to thyrotropin releasing hormone (TRH; 20-500 nM) caused a [Ca2+]i rise but, after an additional exposure to NH4CI, TRH responses re-emerged in some cells. Pre-treatment with the calcium ionophore ionomycin abolished the rise caused by TRH, but neither TRH nor ionomycin pretreatment affected the response to NH4Cl. Neither acetate removal nor methylamine increased [Ca2+]i in medium containing 0.1 microM [Ca2+]e, although in both cases pHi increased. We conclude that in bovine anterior pituitary cells NH4Cl raises [Ca2+]i by two independent pathways, increasing net calcium entry and mobilizing Ca2+ from a TRH-insensitive calcium store.