Use of colloidal gold particles in double-labeling immunoelectron microscopy of ultrathin frozen tissue sections

An advanced fast method for the evaluation of multiple immunolabelling using gold nanoparticles based on low-energy STEM

We present a powerful method for the simultaneous detection of Au nanoparticles located on both sides of ultrathin sections. The method employs a high-resolution scanning electron microscope (HRSEM) operating in scanning transmission electron microscopy (STEM) mode in combination with the detection of backscattered electrons (BSE). The images are recorded simultaneously during STEM and BSE imaging at the precisely selected accelerating voltage. Under proper imaging conditions, the positions of Au nanoparticles on the top or bottom sides can be clearly differentiated, hence showing this method to be suitable for multiple immunolabelling using Au nanoparticles (NPs) as markers. The difference between the upper and lower Au NPs is so large that it is possible to apply common software tools (such as ImageJ) to enable their automatic differentiation. The effects of the section thickness, detector settings and accelerating voltage on the resulting image are shown. Our experimental results correspond to the results modelled in silico by Monte Carlo (MC) simulations.

Glycoprotein 2 as a gut gate keeper for mucosal equilibrium between inflammation and immunity

Glycoprotein 2 (GP2) is a widely distributed protein in the digestive tract, contributing to mucosal barrier maintenance, immune homeostasis, and antigen-specific immune response, while also being linked to inflammatory bowel disease (IBD) pathogenesis. This review sheds light on the extensive distribution of GP2 within the gastrointestinal tract and its intricate interplay with the immune system. Furthermore, the significance of GP2 autoantibodies in diagnosing and categorizing IBD is underscored, alongside the promising therapeutic avenues for modulating GP2 to regulate immunity and maintain mucosal balance.

GP2-expressing cells: a new guardian with divergent functions in the intestine, eyes, and nose

GP (glycoprotein)-2, originally identified as a predominant membranous component of pancreatic acinar cells, has attracted the interest of researchers in mucosal immunology for its role as a functional molecule specific for antigen-sampling cells in the intestinal Peyer's patches. GP2 is involved in the detection of pathological bacteria and is also histologically useful for the identification of the M cell lineage and their differentiation in lymphoid tissues. Subsequent immunohistochemistry for GP2 has revealed a broad distribution of M cells and related cells in the nasopharyngeal lymphoid tissues, conjunctiva, tear duct, and airway. Especially, GP2 cells in the paranasal sinuses and tear duct have been identified as novel types of epithelial cells. The systematic administration of RANKL can induce extra-M cells in conventional epithelia of body. The production and release of GP2 by conjunctival goblet cells and several mucous glands suggests leading roles for mucous cells in protection, including the entrapment of microorganisms for infections. The ocular surface and conjunctiva are connected to the lacrimal sac, nasolacrimal duct, and further nasal cavity, comprising another canal that passes through the body. The broad distribution of GP2-expressingcells may indicate its function as a new guardian in the intestine, eyes, and nose, all of which are exposed to external milieu.

Bimodal endocytic probe for three-dimensional correlative light and electron microscopy

We present a bimodal endocytic tracer, fluorescent BSA-gold (fBSA-Au), as a fiducial marker for 2D and 3D correlative light and electron microscopy (CLEM) applications. fBSA-Au consists of colloidal gold (Au) particles stabilized with fluorescent BSA. The conjugate is efficiently endocytosed and distributed throughout the 3D endolysosomal network of cells and has an excellent visibility in both fluorescence microscopy (FM) and electron microscopy (EM). We demonstrate that fBSA-Au facilitates rapid registration in several 2D and 3D CLEM applications using Tokuyasu cryosections, resin-embedded material, and cryoelectron microscopy (cryo-EM). Endocytosed fBSA-Au benefits from a homogeneous 3D distribution throughout the endosomal system within the cell, does not obscure any cellular ultrastructure, and enables accurate (50-150 nm) correlation of fluorescence to EM data. The broad applicability and visibility in both modalities makes fBSA-Au an excellent endocytic fiducial marker for 2D and 3D (cryo)CLEM applications.

HPM live μ for a full CLEM workflow

With the development of advanced imaging methods that took place in the last decade, the spatial correlation of microscopic and spectroscopic information-known as multimodal imaging or correlative microscopy (CM)-has become a broadly applied technique to explore biological and biomedical materials at different length scales. Among the many different combinations of techniques, Correlative Light and Electron Microscopy (CLEM) has become the flagship of this revolution. Where light (mainly fluorescence) microscopy can be used directly for the live imaging of cells and tissues, for almost all applications, electron microscopy (EM) requires fixation of the biological materials. Although sample preparation for EM is traditionally done by chemical fixation and embedding in a resin, rapid cryogenic fixation (vitrification) has become a popular way to avoid the formation of artifacts related to the chemical fixation/embedding procedures. During vitrification, the water in the sample transforms into an amorphous ice, keeping the ultrastructure of the biological sample as close as possible to the native state. One immediate benefit of this cryo-arrest is the preservation of protein fluorescence, allowing multi-step multi-modal imaging techniques for CLEM. To minimize the delay separating live imaging from cryo-arrest, we developed a high-pressure freezing (HPF) system directly coupled to a light microscope. We address the optimization of sample preservation and the time needed to capture a biological event, going from live imaging to cryo-arrest using HPF. To further explore the potential of cryo-fixation related to the forthcoming transition from imaging 2D (cell monolayers) to imaging 3D samples (tissue) and the associated importance of homogeneous deep vitrification, the HPF core technology has been revisited to allow easy modification of the environmental parameters during vitrification. Lastly, we will discuss the potential of our HPM within CLEM protocols especially for correlating live imaging using the Zeiss LSM900 with electron microscopy.

Sugar and ice: Immunoelectron microscopy using cryosections according to the Tokuyasu method

Since the pioneering work of Kiyoteru Tokuyasu in the 70ths the use of thawed cryosections prepared according to the "Tokuyasu-method" for immunoelectron microscopy did not lose popularity. We owe this method a whole subcellular world described by discrete gold particles pointing at cargo, receptors and organelle markers on delicate images of the inner life of a cell. Here we explain the procedure of sample preparation, sectioning and immunolabeling in view of recent developments and the reasoning behind protocols including some historical perspective. Cryosections are prepared from chemically fixed and sucrose infiltrated samples and labeled with affinity probes and electron dense markers. These sections are ideal substrates for immunolabeling, since antigens are not exposed to organic solvent dehydration or masked by resin. Instead, the structures remain fully hydrated throughout the labeling procedure. Furthermore, target molecules inside dense intercellular structural elements, cells and organelles are accessible to antibodies from the section surface. For the validation of antibody specificity several approaches are recommended including knock-out tissue and reagent controls. Correlative light and electron microscopy strategies involving correlative probes are possible as well as correlation of live imaging with the underlying ultrastructure. By applying stereology, gold labeling can be quantified and evaluated for specificity.

GP2-expressing cells in the conjunctiva and tear ducts of mice: identification of a novel type of cells in the squamous stratified epithelium

GP2 is a membrane-associated secretory protein originally identified in zymogen granules of pancreatic acinar cells. Recently, this glycoprotein has attracted attention as a marker substance of M cells of Peyer's patches and for its involvement in the selective uptake of pathological bacteria via M cells. When we stained the conjunctiva and tear ducts of mice using a GP2 antibody, all goblet cells in the squamous stratified epithelium of the conjunctiva were intensely immunolabeled, while goblet cells in the intestine and airway were devoid of the immunoreactivity, indicating that the conjunctiva contains a special type of goblet cell. Further immunostaining for GP-2 labeled dispersed cells of peculiar shapes within the stratified squamous epithelium in the lacrimal canaliculi, lacrimal sac, and nasolacrimal duct. The GP2-immunoreactive cells in the tear duct projected arched or branched processes toward the basement membrane. Electron-microscopically, immunogold particles for GP2 outlined the basolateral plasma membrane of both the conjuntival goblet cells and the peculiarly shaped cells in the tear duct. Intracellularly, GP2 products of the goblet cells were localized around secretory granules in the apical cytoplasm and those of the tear duct cells inside the vesicles. The luminal contents close to apical plasma membrane were heavily labeled with immunogold particles, suggesting an exocytosis-based targeting of GP2 to the plasma membrane and its release into the lumen. The possible function of GP2 in tear ducts is discussed in relation to a defense system against invasive microoranisms and antigens.

FluoroNanogold: an important probe for correlative microscopy

Correlative microscopy is a powerful imaging approach that refers to observing the same exact structures within a specimen by two or more imaging modalities. In biological samples, this typically means examining the same sub-cellular feature with different imaging methods. Correlative microscopy is not restricted to the domains of fluorescence microscopy and electron microscopy; however, currently, most correlative microscopy studies combine these two methods, and in this review, we will focus on the use of fluorescence and electron microscopy. Successful correlative fluorescence and electron microscopy requires probes, or reporter systems, from which useful information can be obtained with each of the imaging modalities employed. The bi-functional immunolabeling reagent, FluoroNanogold, is one such probe that provides robust signals in both fluorescence and electron microscopy. It consists of a gold cluster compound that is visualized by electron microscopy and a covalently attached fluorophore that is visualized by fluorescence microscopy. FluoroNanogold has been an extremely useful labeling reagent in correlative microscopy studies. In this report, we present an overview of research using this unique probe.

Correlative Light and Scanning Electron Microscopy for Observing the Three-Dimensional Ultrastructure of Membranous Cell Organelles in Relation to Their Molecular Components

Although the osmium maceration method has been used to observe three-dimensional (3D) structures of membranous cell organelles with scanning electron microscopy (SEM), the use of osmium tetroxide for membrane fixation and the removal of cytosolic soluble proteins largely impairs the antigenicity of molecules in the specimens. In the present study, we developed a novel method to combine cryosectioning with the maceration method for correlative immunocytochemical analysis. We first immunocytochemically stained a semi-thin cryosection cut from a pituitary tissue block with a cryo-ultramicrotome, according to the Tokuyasu method, before preparing an osmium-macerated specimen from the remaining tissue block. Correlative microscopy was performed by observing the same area between the immunostained section and the adjacent face of the tissue block. Using this correlative method, we could accurately identify the gonadotropes of pituitary glands in various experimental conditions with SEM. At 4 weeks after castration, dilated cisternae of rough endoplasmic reticulum (RER) were distributed throughout the cytoplasm. On the other hand, an extremely dilated cisterna of the RER occupied the large region of the cytoplasm at 12 weeks after castration. This novel method has the potential to analyze the relationship between the distribution of functional molecules and the 3D ultrastructure in different composite tissues.

Quantitative immunocytochemistry at the ultrastructural level: a stereology-based approach to molecular nanomorphomics

Biological systems span multiple levels of structural organisation from the macroscopic, via the microscopic, to the nanoscale. Therefore, comprehensive investigation of systems biology requires application of imaging modalities that reveal structure at multiple resolution scales. Nanomorphomics is the part of morphomics devoted to the systematic study of functional morphology at the nanoscale and an important element of its achievement is the combination of immunolabelling and transmission electron microscopy (TEM). The ultimate goal of quantitative immunocytochemistry is to estimate numbers of target molecules (usually peptides, proteins or protein complexes) in biological systems and to map their spatial distributions within them. Immunogold cytochemistry utilises target-specific affinity markers (primary antibodies) and visualisation aids (e.g., colloidal gold particles or silver-enhanced nanogold particles) to detect and localise target molecules at high resolution in intact cells and tissues. In the case of post-embedding labelling of ultrathin sections for TEM, targets are localised as a countable digital readout by using colloidal gold particles. The readout comprises a spatial distribution of gold particles across the section and within the context of biological ultrastructure. The observed distribution across structural compartments (whether volume- or surface-occupying) represents both specific and non-specific labelling; an assessment by eye alone as to whether the distribution is random or non-random is not always possible. This review presents a coherent set of quantitative methods for testing whether target molecules exhibit preferential and specific labelling of compartments and for mapping the same targets in two or more groups of cells as their TEM immunogold-labelling patterns alter after experimental manipulation. The set also includes methods for quantifying colocalisation in multiple-labelling experiments and mapping absolute numbers of colloidal gold particles across compartments at specific positions within cells having a point-like inclusion (e.g., centrosome, nucleolus) and a definable vertical axis. Although developed for quantifying colloidal gold particles, the same methods can in principle be used to quantify other electron-dense punctate nanoparticles, including quantum dots.

The complex ultrastructure of the endolysosomal system

Live-cell imaging reveals the endolysosomal system as a complex and highly dynamic network of interacting compartments. Distinct types of endosomes are discerned by kinetic, molecular, and morphological criteria. Although none of these criteria, or combinations thereof, can capture the full complexity of the endolysosomal system, they are extremely useful for experimental purposes. Some membrane domain specializations and specific morphological characteristics can only be seen by ultrastructural analysis after preparation for electron microscopy (EM). Immuno-EM allows a further discrimination of seemingly identical compartments by their molecular makeup. In this review we provide an overview of the ultrastructural characteristics and membrane organization of endosomal compartments, along with their organizing machineries.

The endocytic activity of the flagellar pocket in Trypanosoma brucei is regulated by an adjacent phosphatidylinositol phosphate kinase

Phosphoinositides are spatially restricted membrane signaling molecules. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]--a phosphoinositide that is highly enriched in, and present throughout, the plasma membrane--has been implicated in endocytosis. Trypanosoma brucei has one of the highest known rates of endocytosis, a process it uses to evade the immune system. To determine whether phosphoinositides play a role in endocytosis in this organism, we have identified and characterized one of the enzymes that is responsible for generating PI(4,5)P2. Surprisingly, this phosphoinositide was found to be highly concentrated in the flagellar pocket, the only site of endocytosis and exocytosis in this organism. The enzyme (designated TbPIPKA, annotated as Tb927.10.1620) was present at the neck of the pocket, towards the anterior-end of the parasite. Depletion of TbPIPKA led to depletion of PI(4,5)P2 and enlargement of the pocket, the result of impaired endocytosis. Taken together, these data suggest that TbPIPKA and its product PI(4,5)P2 are important for endocytosis and, consequently, for homeostasis of the flagellar pocket.

Cryosectioning fixed and cryoprotected biological material for immunocytochemistry

Immunocytochemistry for electron microscopy provides important information on the location and relative abundance of proteins inside cells. Gaining access to this information without extracting or disrupting the location of target proteins requires specialized preparation methods. Sectioning frozen blocks of chemically fixed and cryoprotected biological material is one method for obtaining immunocytochemical data. Once the cells or tissues are cut, the cryosections are thawed, mounted onto coated grids, and labeled with specific antibodies and colloidal gold probes. They are then embedded in a thin film of plastic containing a contrasting agent. Subcellular morphology can then be correlated with specific affinity labeling by examination in the transmission electron microscope (TEM). The major advantage of using thawed cryosections for immunolabeling is that the sections remain fully hydrated through the immunolabeling steps, reducing the possibility of dehydration-induced antigen modification. Modern technical advancements both in preparation protocols and equipment design make cryosectioning a routine and rapid approach for immunocytochemistry that may provide increased sensitivity for some antibodies.

Immuno correlative light and electron microscopy on Tokuyasu cryosections

Finding a rare structure by electron microscopy is the equivalent of finding a "needle in a haystack." Correlative light- and immunoelectron microscopy (CLEM) on Tokuyasu cryosections is a sophisticated technique to address this challenge. Hereby, fluorescently labeled structures of interest are identified in an overview image by light microscopy and subsequently traced in electron microscopy. While the direct transfer and imaging of the same sections from optical to electron microscopy enables straightforward correlation, the sample preparation is crucial and technically demanding. We provide a detailed guide outlining the critical steps for sample embedding, cryosectioning, immunolabeling, and imaging. In the example provided, we use CLEM to trace aggregates formed in a zebrafish myopathy model expressing enhanced green fluorescent protein (eGFP) tagged actin. In our case, only a few muscle fibers express eGFP-actin with a subset of fibers containing aggregates. By fluorescence microscopy, we are able to identify the aggregates in the zebrafish tissue, and we subsequently, use immunoelectron microscopy to image the same structures at high resolution. The CLEM method described here using Tokuyasu cryosections can be applied to a large range of samples including small organisms, tissue samples, and cells.

High-resolution microscopy for biological specimens via cathodoluminescence of Eu- and Zn-doped Y2O3 nanophosphors

High-resolution microscopy for biological specimens was performed using cathodoluminescence (CL) of Y(2)O(3):Eu, Zn nanophosphors, which have high CL intensity due to the incorporation of Zn. The intensity of Y(2)O(3):Eu nanophosphors at low acceleration voltage (3 kV) was increased by adding Zn. The CL intensity was high enough for imaging even with a phosphor size as small as about 30 nm. The results show the possibility of using CL microscopy for biological specimens at single-protein-scale resolution. CL imaging of HeLa cells containing laser-ablated Y(2)O(3):Eu, Zn nanophosphors achieved a spatial resolution of a few tens of nanometers. Y(2)O(3):Eu, Zn nanophosphors in HeLa cells were also imaged with 254 nm ultraviolet light excitation. The results suggest that correlative microscopy using CL, secondary electrons and fluorescence imaging could enable multi-scale investigation of molecular localization from the nanoscale to the microscale.

Glutathione-mediated release of Bodipy® from PEG cofunctionalized gold nanoparticles

Gold nanoparticles synthesized via sodium citrate reduction of chloroauric acid (HAuCl₄) were functionalized with either various concentrations of thiol-terminated Bodipy^® FL L-cystine (0.5, 1.0, 1.5, and 2.0 μg/mL) or Bodipy-poly(ethylene glycol) at concentrations of 0.5–18.75, 1.0–12.50, and 1.5–6.25 μg/mL to form a mixed monolayer of BODIPY-PEG. Thiol-terminated Bodipy, a fluorescing molecule, was used as the model drug, while PEG is widely used in drug-delivery applications to shield nanoparticles from unwanted immune responses. Understanding the influence of PEG-capping on payload release is critical because it is the most widely used type of nanoparticle functionalization in drug delivery studies. It has been previously reported that glutathione can trigger release of thiol-bound payloads from gold nanoparticles. Bodipy release from Bodipy capped and from Bodipy-PEG functionalized gold nanoparticles was studied at typical intracellular glutathione levels. It was observed that the addition of PEG capping inhibits the initial burst release observed in gold nanoparticles functionalized only with Bodipy and inhibits nanoparticle aggregation. Efficient and controlled payload release was observed in gold nanoparticles cofunctionalized with only a limited amount of PEG, thus enabling the coattachment of large amounts of drug, targeting groups or other payloads.

Functional inclusion bodies produced in bacteria as naturally occurring nanopills for advanced cell therapies

Inclusion bodies (50-500 nm in diameter) produced in recombinant bacteria can be engineered to contain functional proteins with therapeutic potential. Upon exposure, these protein particles are efficiently internalized by mammalian cells and promote recovery from diverse stresses. Being fully biocompatible, inclusion bodies are a novel platform, as tailored nanopills, for sustained drug release in advanced cell therapies.

Measuring and evaluating the role of ATP-sensitive K+ channels in cardiac muscle

Since ion channels move electrical charge during their activity, they have traditionally been studied using electrophysiological approaches. This was sometimes combined with mathematical models, for example with the description of the ionic mechanisms underlying the initiation and propagation of action potentials in the squid giant axon by Hodgkin and Huxley. The methods for studying ion channels also have strong roots in protein chemistry (limited proteolysis, the use of antibodies, etc.). The advent of the molecular cloning and the identification of genes coding for specific ion channel subunits in the late 1980s introduced a multitude of new techniques with which to study ion channels and the field has been rapidly expanding ever since (e.g. antibody development against specific peptide sequences, mutagenesis, the use of gene targeting in animal models, determination of their protein structures) and new methods are still in development. This review focuses on techniques commonly employed to examine ion channel function in an electrophysiological laboratory. The focus is on the K(ATP) channel, but many of the techniques described are also used to study other ion channels.

Mapping the distributions and quantifying the labelling intensities of cell compartments by immunoelectron microscopy: progress towards a coherent set of methods

An important tool in cell biology is the combination of immunogold labelling and transmission electron microscopy (TEM) by which target molecules (e.g. antigens) are bound specifically to affinity markers (primary antibodies) and then detected and localised with visualisation probes (e.g. colloidal gold particles bound to protein A). Gold particles are electron-dense, punctate and available in different sizes whilst TEM provides high-resolution images of particles and cell compartments. By virtue of these properties, the combination can be used also to quantify one or more defined targets in cell compartments. During the past decade, new ways of quantifying gold labelling within cells have been devised. Their efficiency and validity rely on sound principles of specimen sampling, event counting and inferential statistics. These include random selection of items at each sampling stage (e.g. specimen blocks, thin sections, microscopical fields), stereological analysis of cell ultrastructure, unbiased particle counting and statistical evaluation of a suitable null hypothesis (no difference in the intensity or pattern of labelling between compartments or groups of cells). The following approaches are possible: (i) A target molecule can be tested for preferential labelling by mapping the localisation of gold particles across a set of compartments. (ii) Data from wild-type and knockdown/knockout control cells can be used to correct raw gold particle counts, estimate specific labelling densities and then test for preferential labeling. (iii) The same antigen can be mapped in two or more groups of cells to test whether there are experimental shifts in compartment labelling patterns. (iv) A variant of this approach uses more than one size of gold particle to test whether or not different antigens colocalise in one or more compartments. (v) In studies involving antigen translocation, absolute numbers of gold particles can be mapped over compartments at specific positions within polarised, oriented or dividing cells. Here, the current state of the art is reviewed and approaches are illustrated with virtual datasets.

Quantifying immunogold localization on electron microscopic thin sections: a compendium of new approaches for plant cell biologists

A review is presented of recently developed methods for quantifying electron microscopical thin sections on which colloidal gold-labelled markers are used to identify and localize interesting molecules. These efficient methods rely on sound principles of random sampling, event counting, and statistical evaluation. Distributions of immunogold particles across cellular compartments can be compared within and between experimental groups. They can also be used to test for co-localization in multilabelling studies involving two or more sizes of gold particle. To test for preferential labelling of compartments, observed and expected gold particle distributions are compared by χ(2) analysis. Efficient estimators of gold labelling intensity [labelling density (LD) and/or relative labelling index (RLI)] are used to analyse volume-occupying compartments (e.g. Golgi vesicles) and/or surface-occupying compartments (e.g. cell membranes). Compartment size is estimated by counting chance events after randomly superimposing test lattices of points and/or line probes. RLI=1 when there is random labelling and RLI >1 when there is preferential labelling. Between-group comparisons do not require information about compartment size but, instead, raw gold particle counts in different groups are compared by combining χ(2) and contingency table analyses. These tests may also be used to assess co-distribution of different sized gold particles in compartments. Testing for co-labelling involves identifying sets of compartmental profiles that are unlabelled and labelled for one or both of two gold marker sizes. Numbers of profiles in each labelling set are compared by contingency table analysis and χ(2) analysis or Fisher's exact probability test. The various methods are illustrated with worked examples based on empirical and synthetic data and will be of practical benefit to those applying single or multiple immunogold labelling in their research.

Multiple-labelling immunoEM using different sizes of colloidal gold: alternative approaches to test for differential distribution and colocalization in subcellular structures

Various methods for quantifying cellular immunogold labelling on transmission electron microscope thin sections are currently available. All rely on sound random sampling principles and are applicable to single immunolabelling across compartments within a given cell type or between different experimental groups of cells. Although methods are also available to test for colocalization in double/triple immunogold labelling studies, so far, these have relied on making multiple measurements of gold particle densities in defined areas or of inter-particle nearest neighbour distances. Here, we present alternative two-step approaches to codistribution and colocalization assessment that merely require raw counts of gold particles in distinct cellular compartments. For assessing codistribution over aggregate compartments, initial statistical evaluation involves combining contingency table and chi-squared analyses to provide predicted gold particle distributions. The observed and predicted distributions allow testing of the appropriate null hypothesis, namely, that there is no difference in the distribution patterns of proteins labelled by different sizes of gold particle. In short, the null hypothesis is that of colocalization. The approach for assessing colabelling recognises that, on thin sections, a compartment is made up of a set of sectional images (profiles) of cognate structures. The approach involves identifying two groups of compartmental profiles that are unlabelled and labelled for one gold marker size. The proportions in each group that are also labelled for the second gold marker size are then compared. Statistical analysis now uses a 2 × 2 contingency table combined with the Fisher exact probability test. Having identified double labelling, the profiles can be analysed further in order to identify characteristic features that might account for the double labelling. In each case, the approach is illustrated using synthetic and/or experimental datasets and can be refined to correct observed labelling patterns to specific labelling patterns. These simple and efficient approaches should be of more immediate utility to those interested in codistribution and colocalization in multiple immunogold labelling investigations.

Antigen storage compartments in mature dendritic cells facilitate prolonged cytotoxic T lymphocyte cross-priming capacity

Dendritic cells (DCs) are crucial for priming of naive CD8(+) T lymphocytes to exogenous antigens, so-called "cross-priming." We report that exogenous protein antigen can be conserved for several days in mature DCs, coinciding with strong cytotoxic T lymphocyte cross-priming potency in vivo. After MHC class I peptide elution, protein antigen-derived peptide presentation is efficiently restored, indicating the presence of an intracellular antigen depot. We characterized this depot as a lysosome-like organelle, distinct from MHC class II compartments and recently described early endosomal compartments that allow acute antigen presentation in MHC class I. The storage compartments we report here facilitate continuous supply of MHC class I ligands. This mechanism ensures sustained cross-presentation by DCs, despite the short-lived expression of MHC class I-peptide complexes at the cell surface.

Biogenesis of epithelial cell plasma membranes

Polarized monolayers of cultured epithelial cells, such as the kidney-derived MDCK cell line, when infected with enveloped viruses, provide a convenient model system for study of the intracellular routes followed by newly synthesized glycoproteins to reach specific domains of the plasma membrane. The polarized nature of the monolayers is reflected in the asymmetric assembly of enveloped viruses, some of which, such as influenza and simian virus 5 (SV5), bud from the apical surfaces of the cells, while others, such as vesicular stomatitis virus (VSV), emerge from the basolateral surfaces. MDCK cells can sustain double infection with viruses of different budding polarity, and within such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at different sites. Immunoelectron microscopic observations of doubly infected cells show that glycoproteins of influenza and VSV traverse the same Golgi apparatus. This indicates that critical sorting steps must take place during or after passage of the glycoproteins through the organelle. Following passage through the Golgi, the HA glycoprotein accumulates almost exclusively at the apical surface, where the influenza virions assemble. Significant amounts of the G protein, however, are detected on both plasma membranes in singly and doubly infected cells, although VSV virion assembly is limited to basolateral domains. These observations indicate that the site of VSV budding is not exclusively determined by the presence of G polypeptides on a given cell-surface domain. It is possible that other cellular or viral components are responsible for the selection of the appropriate budding domain or that the G protein found on the apical surface must be transferred to the basolateral domain before it becomes competent for assembly.

Cryosectioning and immunolabeling

In this protocol, we describe cryoimmunolabeling methods for the subcellular localization of proteins and certain lipids. The methods start with chemical fixation of cells and tissue in formaldehyde (FA) and/or glutaraldehyde (GA), sometimes supplemented with acrolein. Cell and tissue blocks are then immersed in 2.3 M sucrose before freezing in liquid nitrogen. Thin cryosections, cut in an ultracryotome, can be single- or multiple immunolabeled with differently sized gold particles, contrasted and viewed in an electron microscope. Semi-thin cryosections can be used for immunofluorescence microscopy. We describe the detailed procedures that have been developed and tested in practice in our laboratory during the past decades.

Acinar cell membrane disruption is an early event in experimental acute pancreatitis in rats

OBJECTIVE

To test the hypothesis that disruption of acinar cell membranes is the earliest event that takes place after the onset of acute pancreatitis.

METHODS

Cerulein and taurocholate pancreatitis were induced in rats. Furthermore, stimulation with different doses of bombesin, pilocarpine, and cerulein was performed. Five to 180 minutes after initiation of treatment, animals were killed. Disruption of cell membranes was detected by the penetration of the experimental animal's own albumin or immunoglobulin G (IgG) into acinar cells by immunocytological localization. Tissue was further analyzed by electron microscopy and electron microscopic immunostaining.

RESULTS

Animals with pancreatitis displayed significantly greater antialbumin and anti-IgG immunostaining in the cytoplasm of acinar cells and in vacuoles in comparison with controls, confirming membrane disruption. This was not detectable after stimulation with bombesin, pilocarpine, and nonsupramaximal doses of cerulein. The first changes were seen after 5 minutes of induction of pancreatitis. Results were verified by electron microscopy and electron microscopic immunohistochemistry.

CONCLUSIONS

The penetration of albumin and IgG into acinar cells indicates that wounding of their plasma membrane occurs at the onset of acute pancreatitis. Disruption of the membranes could be expected to allow the influx of calcium ions, causing massive intracellular alterations, and exit of molecules, such as enzymes from acinar cells.

Innate immune recognition triggers secretion of lysosomal enzymes by macrophages

Gamma interferon-induced lysosomal thiolreductase (GILT) is expressed constitutively in antigen-presenting cells, where it reduces disulfide bonds to facilitate antigen presentation. GILT is synthesized as an enzymatically active precursor protein and is processed in early endosomes to yield the mature enzyme. The exposure of the promonocytic cell line THP-1 to Escherichia coli causes a differentiation-dependent induction of GILT expression in which the majority of precursor GILT is secreted as active enzyme. We confirm this result in cultured primary monocytes and macrophages, and demonstrate, as an in vivo correlate of the phenomenon, upregulation of precursor GILT levels in the serum of mice injected with lipopolysaccharide. We show that macrophage differentiation is accompanied by a transcriptional downregulation of mannose-6-phosphorylation, which likely prevents the recognition and proper sorting of soluble lysosomal enzymes by the mannose-6-phosphate receptors. We provide evidence for a mechanism of generalized soluble lysosomal enzyme secretion through the constitutive secretory pathway.

Aberrant infection and persistence of varicella-zoster virus in human dorsal root ganglia in vivo in the absence of glycoprotein I

Varicella-zoster virus (VZV) causes varicella, establishes latency in sensory ganglia, and reactivates as herpes zoster. Human dorsal root ganglia (DRGs) xenografts in immunodeficient mice provide a model for evaluating VZV neuropathogenesis. Our investigation of the role of glycoprotein I (gI), which is dispensable in vitro, examines the functions of a VZV gene product during infection of human neural cells in vivo. Whereas intact recombinant Oka (rOka) initiated a short replicative phase followed by persistence in DRGs, the gI deletion mutant, rOkaDeltagI, showed prolonged replication with no transition to persistence up to 70 days after infection. Only a few varicella-zoster nucleocapsids and cytoplasmic virions were observed in neurons, and the major VZV glycoprotein, gE, was retained in the rough endoplasmic reticulum in the absence of gI. VZV neurotropism was not disrupted when DRG xenografts were infected with rOka mutants lacking gI promoter elements that bind cellular transactivators, specificity factor 1 (Sp1) and upstream stimulatory factor (USF). Because gI is essential and Sp1 and USF contribute to VZV pathogenesis in skin and T cells in vivo, these DRG experiments indicate that the genetic requirements for VZV infection are less stringent in neural cells in vivo. The observations demonstrate that gI is important for VZV neurotropism and suggest that a strategy to reduce neurovirulence by deleting gI could prolong active infection in human DRGs.

Immunogold Labeling of Cryosections from High-Pressure Frozen Cells

Immunogold labeling of cryosections according to Tokuyasu (Tokuyasu KT. A technique for ultracyotomy of cell suspensions and tissues. J Cell Biol 1973;57:551-565), is an important and widely used method for immunoelectron microscopy. These sections are cut from material that is chemically fixed at room temperature (room temperature fixation, RTF). Lately in many morphological studies fast freezing followed by cryosubstitution fixation (CSF) is used instead of RTF. We have explored some new methods for applying immunogold labeling on cryosections from high-pressure frozen cells (HepG2 cells, primary chondrocytes) and tissues (cartilage and exocrine pancreas). As immunolabeling has to be carried out on thawed and stable sections, we explored two ways to achieve this: (1) The section fixation method, as briefly reported before (Liou W et al. Histochem Cell Biol 1996;106:41-58 and Möbius W et al. J Histochem Cytochem 2002;50:43-55.) in which cryosections from freshly frozen cells were stabilized in mixtures of sucrose and methyl cellulose and varying concentrations of glutaraldehyde, formaldehyde and uranyl acetate (UA). Only occasionally does this method reveal section areas with excellent cell preservation and negatively stained membranes like Tokuyasu sections of RTF material. (Liou et al.) (2) The rehydration method, a novel approach, in which CSF with glutaraldehyde and/or osmium tetroxide (OsO4) was followed by rehydration and cryosectioning as in the Tokuyasu method. Especially, the addition of UA and low concentrations of water to the CSF medium favored superb membrane contrast. Immunogold labeling was as efficient as with the Tokuyasu method.

Characterization of the TGN exit signal of the human mannose 6-phosphate uncovering enzyme

The human mannose 6-phosphate uncovering enzyme participates in the uncovering of the mannose 6-phosphate recognition tag on lysosomal enzymes, a process that facilitates recognition of those enzymes by mannose 6-phosphate receptors to ensure delivery to lysosomes. Uncovering enzyme has been identified on the trans-Golgi network at steady state. It has been shown to traffic to the plasma membrane from where it is rapidly internalized via endosomal structures, the process being mediated by a tyrosine-based internalization motif, Y488HPL, in its cytoplasmic tail. Using immunogold electron microscopy a GFP-uncovering enzyme fusion construct was found to be colocalized with the cation-dependent mannose 6-phosophate receptor in regions of the trans-Golgi network, suggesting that uncovering enzyme might follow a similar pathway of exit from the trans-Golgi network as that of the cation-dependent mannose 6-phosohate receptor. In this study, we identified the signal sequence in the cytoplasmic tail of uncovering enzyme responsible for its exit from the trans-Golgi network. Using GFP fusion constructs of the transmembrane and cytoplasmic domains of uncovering enzyme, we could show, by automated analysis of confocal immunofluorescence images, that residues Q492EMN in the cytoplasmic tail of uncovering enzyme are involved in its exit from the trans-Golgi network. Detailed characterization of the exit signal revealed that residue Q492 is the most important to the exit function while M494 and N495 also contribute. The cytoplasmic tail of the uncovering enzyme does not possess any of the known canonical signal sequences for interaction with Golgi-associated gamma ear-containing adaptor proteins. The identification of a trans-Golgi network exit signal in its cytoplasmic tail elucidates the trafficking pathway of uncovering enzyme, a crucial player in the process of lysosomal biogenesis.

Peroxisomal localization of sulfite oxidase separates it from chloroplast-based sulfur assimilation

Recently, we isolated the sulfite oxidase (SO) gene from Arabidopsis thaliana and characterized the purified SO protein. The purpose of the present study was to determine the subcellular localization of this novel plant enzyme. Immunogold electron-microscopic analysis showed the gold labels nearly exclusively in the peroxisomes. To verify this finding, green fluorescent protein was fused to full-length plant SO including the putative peroxisomal targeting signal 1 (PTS1) 'SNL' and expressed in tobacco leaves. Our results showed a punctate fluorescence pattern resembling that of peroxisomes. Co-labelling with MitoTracker-Red excluded that the observed fluorescence was due to mitochondrial sorting. By investigation of deleted or mutated PTS1, no functional peroxisomal targeting signal 2 (PTS2) could be detected in plant SO. This conclusion is supported by expression studies in Pichia pastoris mutants with defined defects either in PTS1- or PTS2-mediated peroxisomal import.

Mammalian GGAs act together to sort mannose 6-phosphate receptors

The GGAs (Golgi-localized, γ ear–containing, ADP ribosylation factor–binding proteins) are multidomain proteins implicated in protein trafficking between the Golgi and endosomes. We examined whether the three mammalian GGAs act independently or together to mediate their functions. Using cryo-immunogold electron microscopy, the three GGAs were shown to colocalize within coated buds and vesicles at the trans-Golgi network (TGN) of HeLa cells. In vitro binding experiments revealed multidomain interactions between the GGAs, and chemical cross-linking experiments demonstrated that GGAs 1 and 2 form a complex on Golgi membranes. RNA interference of each GGA resulted in decreased levels of the other GGAs and their redistribution from the TGN to cytosol. This was associated with impaired incorporation of the cation-independent mannose 6-phosphate receptor into clathrin-coated vesicles at the TGN, partial redistribution of the receptor to endosomes, and missorting of cathepsin D. The morphology of the TGN was also altered. These findings indicate that the three mammalian GGAs cooperate to sort cargo and are required for maintenance of TGN structure.

Intracellularly expressed single-domain antibody against p15 matrix protein prevents the production of porcine retroviruses

The presence of porcine endogenous retroviruses presents a potential risk of transmission of infectious diseases (xenozoonosis) if tissues and organs from genetically modified pigs are to be used in xenotransplantation. Here, we report that intracellular expression of a llama single-domain antibody against p15, the matrix domain protein of the porcine endogenous retrovirus Gag polyprotein, blocks retrovirus production, providing the possibility of eliminating the risk of infection in xenotransplantation.

Goliath, a ring-H2 mitochondrial protein, regulated by luteinizing hormone/human chorionic gonadotropin in rat leydig cells

We have cloned the rat homologue of the ring-H2 protein Goliath involved in Drosophila development. The rat Goliath mRNA (1.85 kb) was translated as a major ubiquitous protein species of 28-kDa and three larger isoforms (50, 46, and 36 kDa) expressed mainly in liver, lung, stomach, heart, and thymus and barely detectable in other tissues (kidney, skeletal muscle, brain, testis, intestine, and spleen). By immunohistochemistry on rat testis sections, we localized the protein in interstitial tissue and seminiferous tubules. In tubules, Goliath was expressed mainly in postmeiotic germ cells and to a much lesser extent in Sertoli cells. In the interstitium, Goliath was exclusively present in Leydig cells. Using a series of immunolabeling, cellular fractionation, and electron microscopy experiments, we established that Goliath is present in mitochondria of the R2C Leydig cell line. Using short-term hypophysectomized animals, we showed that Goliath is regulated by LH/hCG in Leydig cells but not in germ cells. This regulation in Leydig cells concerned only the 50-kDa isoform. This report is the first description of a differential regulation of the Goliath protein between germ cells and Leydig cells.

Involvement of the endoplasmic reticulum in peroxisome formation

The traditional view holds that peroxisomes are autonomous organelles multiplying by growth and division. More recently, new observations have challenged this concept. Herein, we present evidence supporting the involvement of the endoplasmic reticulum (ER) in peroxisome formation by electron microscopy, immunocytochemistry and three-dimensional image reconstruction of peroxisomes and associated compartments in mouse dendritic cells. We found the peroxisomal membrane protein Pex13p and the ATP-binding cassette transporter protein PMP70 present in specialized subdomains of the ER that were continuous with a peroxisomal reticulum from which mature peroxisomes arose. The matrix proteins catalase and thiolase were only detectable in the reticula and peroxisomes. Our results suggest the existence of a maturation pathway from the ER to peroxisomes and implicate the ER as a major source from which the peroxisomal membrane is derived.

Involvement of the endoplasmic reticulum in peroxisome formation

The traditional view holds that peroxisomes are autonomous organelles multiplying by growth and division. More recently, new observations have challenged this concept. Herein, we present evidence supporting the involvement of the endoplasmic reticulum (ER) in peroxisome formation by electron microscopy, immunocytochemistry and three-dimensional image reconstruction of peroxisomes and associated compartments in mouse dendritic cells. We found the peroxisomal membrane protein Pex13p and the ATP-binding cassette transporter protein PMP70 present in specialized subdomains of the ER that were continuous with a peroxisomal reticulum from which mature peroxisomes arose. The matrix proteins catalase and thiolase were only detectable in the reticula and peroxisomes. Our results suggest the existence of a maturation pathway from the ER to peroxisomes and implicate the ER as a major source from which the peroxisomal membrane is derived.

MHC class II compartments in human dendritic cells undergo profound structural changes upon activation

Immature dendritic cells efficiently capture exogenous antigens in peripheral tissues. In an inflammatory environment, dendritic cells are activated and become highly competent antigen-presenting cells. Upon activation, they lose their ability for efficient endocytosis and gain capability to migrate to secondary lymphoid organs. In addition, peptide loading of MHC class II molecules is enhanced and MHC class II/peptide complexes are redistributed from an intracellular location to the plasma membrane. Using immuno-electron microscopy, we show that activation of human monocyte-derived dendritic cells induced striking modifications of the lysosomal multilaminar MHC class II compartments (MIICs), whereby electron-dense tubules and vesicles emerged from these compartments. Importantly, we observed that MHC class II expression in these tubules/vesicles transiently increased, while multilaminar MIICs showed a strongly reduced labeling of MHC class II molecules. This suggests that formation of the tubules/vesicles from multilaminar MIICs could be linked to transport of MHC class II from these compartments to the cell surface. Further characterization of endocytic organelles with lysosomal marker proteins, such as the novel dendritic cell-specific lysosomal protein DC-LAMP, HLA-DM and CD68, revealed differential sorting of these markers to the tubules and vesicles.

Cooperation of GGAs and AP-1 in packaging MPRs at the trans-Golgi network

The Golgi-localized, gamma-ear-containing, adenosine diphosphate ribosylation factor-binding proteins (GGAs) are multidomain proteins that bind mannose 6-phosphate receptors (MPRs) in the Golgi and have an essential role in lysosomal enzyme sorting. Here the GGAs and the coat protein adaptor protein-1 (AP-1) were shown to colocalize in clathrin-coated buds of the trans-Golgi networks of mouse L cells and human HeLa cells. Binding studies revealed a direct interaction between the hinge domains of the GGAs and the gamma-ear domain of AP-1. Further, AP-1 contained bound casein kinase-2 that phosphorylated GGA1 and GGA3, thereby causing autoinhibition. This could induce the directed transfer of the MPRs from GGAs to AP-1. MPRs that are defective in binding to GGAs are poorly incorporated into AP-1-containing clathrin-coated vesicles. Thus, the GGAs and AP-1 interact to package MPRs into AP-1-containing coated vesicles.

Rab4 function in membrane recycling from early endosomes depends on a membrane to cytoplasm cycle

The monomeric GTPase rab4 is associated with early endosomes and regulates recycling vesicle formation. Because the function of rab proteins in the biosynthetic pathway does not appear to depend on cycling between membranes and cytosol, we were interested to investigate whether or not this holds true for rab function in the endocytic pathway. We created a chimeric rab4 protein (NHrab4cbvn) in which the carboxyl-terminal prenylation motif was replaced by the transmembrane domain of cellubrevin. The chimeric protein was permanently attached to membranes, properly targeted to early endosomes, and bound guanine nucleotide to the same extent as wild type rab4. However, in transport assays we found that basolaterally endocytosed transferrin was less efficiently transported to the apical cell surface in Madin-Darby canine kidney cells transfected with NHrab4cbvn than in cells expressing wild type rab4. Hence, rab4 function requires ongoing cycles of association and dissociation from early endosomes. This cycle is altered during mitosis when rab4 accumulates in the cytoplasm through phosphorylation by a mitotic kinase. We show here, using a rab4 construct that is permanently hooked onto membranes, that the membrane-bound pool of rab4 is targeted by a mitotic kinase.

A novel flat-embedding method to prepare ultrathin cryosections from cultured cells in their in situ orientation

Immunogold labeling of ultrathin cryosections provides a sensitive and quantitative method to localize proteins at the ultrastructural level. An obligatory step in the routine preparation of cryosections from cultured cells is the detachment of cells from their substrate and subsequent pelleting. This procedure precludes visualization of cells in their in situ orientation and hampers the study of polarized cells. Here we describe a method to sample cultured cells from a petri dish or coverslip by embedding them in a 12% gelatin slab. Subsequently, sections can be prepared in parallel or perpendicular to the plane of growth. Our method extends the cryosectioning technique to applications in studying polarized cells and correlative light-electron microscopy.

Expression of the hepatitis C virus structural proteins in mammalian cells induces morphology similar to that in natural infection

Like many positive-strand RNA viruses, replication of the hepatitis C virus (HCV) is associated with cytoplasmic membrane rearrangements. However, it is unclear which HCV proteins induce these ultrastructural features. This work examined the morphological changes induced by expression of the HCV structural proteins, core, E1 and E2, expressed from a Semliki Forest Virus (SFV) recombinant RNA replicon. Electron microscopy of cells expressing these proteins showed cytoplasmic vacuoles containing membranous and electron-dense material that were distinct from the type I cytoplasmic vacuoles induced during SFV replicon replication. Immunogold labelling showed that the core and E2 proteins localized to the external and internal membranes of these vacuoles, but at times were also associated with some of the internal amorphous material. Dual immunogold labelling with antibodies raised against the core protein and against an endoplasmic reticulum (ER)-resident protein (protein disulphide isomerase) showed that the HCV-induced vacuoles were associated with ER-labelled membranes. This report has identified an association between the HCV core and E2 proteins with induced cytoplasmic vacuoles which are morphologically similar to those observed in HCV-infected liver tissue, suggesting that the HCV structural proteins may be responsible for the induction of these vacuoles during HCV replication in vivo.

Mast cell-dependent B and T lymphocyte activation is mediated by the secretion of immunologically active exosomes

Mitogenic activity of bone marrow-derived mouse mast cells and mast cell lines P815 and MC/9 on B and T lymphocytes is present in their culture supernatants. To identify this activity, mast cells were incubated in serum-free medium and the supernatant was subjected to differential centrifugation, which resulted in two fractions, the hypodense and dense fraction (pellet). When analyzed for their mitogenic activity on spleen cells, all activity was found to be associated with the dense fraction. Electron microscopy studies revealed the presence in this fraction of small vesicles called exosomes with a heterogeneous size from 60 to 100 nm of diameter. When cocultured with spleen cells, purified exosomes induced blast formation, proliferation, as well as IL-2 and IFN-gamma production, but no detectable IL-4. Similar data were obtained by injecting exosomes into naive mice. In contrast to mast cell lines, a pretreatment with IL-4 is required for bone marrow-derived mast cells to secrete active exosomes. Structurally, exosomes were found to harbor immunologically relevant molecules such as MHC class II, CD86, LFA-1, and ICAM-1. These findings indicate that mast cells can represent a critical component of the immunoregulatory network through secreted exosomes that display mitogenic activity on B and T lymphocytes both in vitro and in vivo.

The dileucine motif within the tail of MPR46 is required for sorting of the receptor in endosomes

The cytoplasmic tail of MPR46 carries a leucine-based motif that is required for the sorting of lysosomal enzymes by the receptor. In addition, it is one of three independent, but functionally redundant, internalization signals present in the cytoplasmic tail of MPR46. We have analyzed a mutant of MPR46, in which the dileucine pair was replaced by alanines (MPR46 LL/AA) with respect to its intracellular distribution and trafficking. Ultrastructural analysis of cells expressing the MPR46 LL/AA mutant revealed that the substitution of the dileucine pair causes a shift of the receptor distribution from the TGN, where it is packaged into AP1-containing vesicles, to vesicular structures distributed throughout the cytoplasm. The vesicles could be identified as early endosomes with internalized BSA-gold and rab5 as markers. By analyzing the receptor trafficking biochemically, we found that return of the LL/AA mutant receptor from the plasma membrane/endosome pool back to the TGN was impaired, while recycling from endosomes to the plasma membrane was enhanced. In conclusion, our data indicate that the dileucine motif in the MPR46 tail is required for a sorting event in endosomes.

Follicular dendritic cells carry MHC class II-expressing microvesicles at their surface

Follicular dendritic cells (FDCs) present in lymphoid follicles play a critical role in germinal center reactions. They trap native Ags in the form of immune complexes providing a source for continuous stimulation of specific B lymphocytes. FDCs have been reported to express MHC class II molecules, suggesting an additional role in the presentation of not only native, but also processed Ag in the form of peptide-loaded MHC class II. Adoptive bone marrow transfer experiments have shown that MHC class II molecules are only passively acquired. Up to now the origin of these MHC class II molecules was not clear. Here we show by cryoimmunogold electron microscopy that MHC class II molecules are not present at the plasma membrane of FDCs. In contrast, microvesicles attached to the FDC surface contain MHC class II and other surface proteins not expressed by FDCs themselves. The size and marker profiles of these microvesicles resemble exosomes. Exosomes, which are secreted internal vesicles from multivesicular endosomes, have been shown earlier to stimulate proliferation of specific T lymphocytes in vitro, but their target in vivo remained a matter of speculation. We demonstrate here that isolated exosomes in vitro bind specifically to FDCs and not to other cell types, suggesting that FDCs might be a physiological target for exosomes.

Clathrin-coated lattices and buds on MHC class II compartments do not selectively recruit mature MHC-II

Newly synthesized major histocompatibility complex class II molecules (MHC-II) are transported to MHC-II-containing endosomal and lysosomal compartments (MIICs) for the degradation of associated invariant chain and peptide loading. Subsequently MHC-II is transported to the plasma membrane, in part through direct fusion of MIICs with the plasma membrane. In search of potential alternative pathway(s) we studied the 3-dimensional structure of MIICs and the subcellular distribution of MHC-II by immuno electronmicroscopy on whole-mount preparations and cryosections of Mel JuSo cells. Intracellular MHC-II and invariant chain mainly localized to lamp-1 positive compartments suggesting that the majority of MHC-II exits the endocytic tract at lysosomes. Clathrin-coated lattices and buds were found to be associated with these organelles, but MHC-II was not found to be enriched in the clathrin-coated domains. Moreover, leupeptin, a drug that interferes with Ii-processing and delays delivery of newly synthesized MHC-II to the plasma membrane, was not found to decrease the relative amount of MHC-II in clathrin-coated areas. Together these data indicate clathrin-mediated exit site(s) from lysosomes but suggest that they do not selectively recruit mature MHC-II, consistent with the notion that transport to the plasma membrane occurs independently of the cytoplasmic domains of the MHC-II (α) and (beta) chains.

Synaptic vesicles form by budding from tubular extensions of sorting endosomes in PC12 cells

The putative role of sorting early endosomes (EEs) in synaptic-like microvesicle (SLMV) formation in the neuroendocrine PC12 cell line was investigated by quantitative immunoelectron microscopy. By BSA-gold internalization kinetics, four distinct endosomal subcompartments were distinguished: primary endocytic vesicles, EEs, late endosomes, and lysosomes. As in other cells, EEs consisted of vacuolar and tubulovesicular subdomains. The SLMV marker proteins synaptophysin and vesicle-associated membrane protein 2 (VAMP-2) localized to both the EE vacuoles and associated tubulovesicles. Quantitative analysis showed that the transferrin receptor and SLMV proteins colocalized to a significantly higher degree in primary endocytic vesicles then in EE-associated tubulovesicles. By incubating PC12 cells expressing T antigen-tagged VAMP (VAMP-TAg) with antibodies against the luminal TAg, the recycling pathway of SLMV proteins was directly visualized. At 15 degrees C, internalized VAMP-TAg accumulated in the vacuolar domain of EEs. Upon rewarming to 37 degrees C, the labeling shifted to the tubular part of EEs and to newly formed SLMVs. Our data delineate a pathway in which SLMV proteins together with transferrin receptor are delivered to EEs, where they are sorted into SLMVs and recycling vesicles, respectively.