The immunofluorescent era of membrane traffic

Quantitative correlative microscopy reveals the ultrastructural distribution of endogenous endosomal proteins

The key endosomal regulators Rab5, EEA1, and APPL1 are frequently applied in fluorescence microscopy to mark early endosomes, whereas Rab7 is used as a marker for late endosomes and lysosomes. However, endogenous levels of these proteins localize poorly in immuno-EM, and systematic studies on their native ultrastructural distributions are lacking. To address this gap, we here present a quantitative, on-section correlative light and electron microscopy (CLEM) approach. Using the sensitivity of fluorescence microscopy, we label hundreds of organelles that are subsequently visualized by EM and classified by ultrastructure. We show that Rab5 predominantly marks small, endocytic vesicles and early endosomes. EEA1 colocalizes with Rab5 on early endosomes, but unexpectedly also labels Rab5-negative late endosomes, which are positive for PI(3)P but lack Rab7. APPL1 is restricted to small Rab5-positive, tubulo-vesicular profiles. Rab7 primarily labels late endosomes and lysosomes. These data increase our understanding of the structural-functional organization of the endosomal system and introduce quantitative CLEM as a sensitive alternative for immuno-EM.

Visualisation of cholesterol and ganglioside GM1 in zebrafish models of Niemann-Pick type C disease and Smith-Lemli-Opitz syndrome using light sheet microscopy

Lysosomal storage diseases are the most common cause of neurodegeneration in children. They are characterised at the cellular level by the accumulation of storage material within lysosomes. There are very limited therapeutic options, and the search for novel therapies has been hampered as few good small animal models are available. Here, we describe the use of light sheet microscopy to assess lipid storage in drug and morpholino induced zebrafish models of two diseases of cholesterol homeostasis with lysosomal dysfunction: First, Niemann–Pick type C disease (NPC), caused by mutations in the lysosomal transmembrane protein NPC1, characterised by intralysosomal accumulation of cholesterol and several other lipids. Second, Smith–Lemli–Opitz syndrome (SLOS), caused by mutations in 7-dehydrocholesterol reductase, which catalyses the last step of cholesterol biosynthesis and is characterised by intralysosomal accumulation of dietary cholesterol. This is the first description of a zebrafish SLOS model. We find that zebrafish accurately model lysosomal storage and disease-specific phenotypes in both diseases. Increased cholesterol and ganglioside GM1 were observed in sections taken from NPC model fish, and decreased cholesterol in SLOS model fish, but these are of limited value as resolution is poor, and accurate anatomical comparisons difficult. Using light sheet microscopy, we were able to observe lipid changes in much greater detail and identified an unexpected accumulation of ganglioside GM1 in SLOS model fish. Our data demonstrate, for the first time in zebrafish, the immense potential that light sheet microscopy has in aiding the resolution of studies involving lysosomal and lipid disorders.

Fis1 deficiencies differentially affect mitochondrial quality in skeletal muscle

Mitochondrial dynamics and mitophagy are important aspects of mitochondrial quality control, and are linked to neurodegenerative diseases and muscular diseases. Fis1, a protein on the mitochondrial outer membrane, is thought to mediate mitochondrial fission. However, Fis1 null worms and mammalian cells only display mild fission defects but show aberrant mitophagy. To assess Fis1 function in vivo, we generated conditional knock-out Fis1 mice to allow for specific Fis1 deletion in adult skeletal muscle. In the absence of Fis1 in Type I muscle, mitochondrial hyperfusion, respiratory chain deficiency, and increased mitophagy were found. Moreover, abnormal mitophagy was aggravated by endurance exhaustive exercise stress (EEE), suggesting that Fis1 is involved in maintaining normal mitophagy in mitochondria-rich Type I muscle during exercise. Additionally, Fis1 loss induced delayed onset muscle ultrastructure change (DOMUC) in Type I muscle and strong inflammation in response to acute exhaustive exercise (EE). Thus, we identify a role for Fis1 in maintaining normal mitochondrial structure and function at rest and under exercise stress.

A Commentary On Immunolabeling

We have witnessed a slow but steady decline in electron microscopy (EM) skills in the biological sciences over the last decade. This decline is illustrated by the continued closures of EM facilities throughout the world. It is, however, difficult to rationalize this decline when the need for these skills is still under a constant demand. Certainly, the introduction of new microscopes has played a part in this. High-resolution confocal light microscopes, two photon imaging and other new technologies are providing impressive amounts of new information on biological systems. Many of these approaches require simple preparation protocols providing information that can be easily interpreted and are thus easily adapted to fast-moving research.

The application of EM techniques is also deceivingly simple. For this reason the field of immunocytochemistry is undergoing an increase in popularity. Laboratories with little or no previous experience in EM are discovering the impressive and convincing results achievable using these high-resolution methods.

The sleeping beauty kissed awake: new methods in electron microscopy to study cellular membranes

Electron microscopy (EM) for biological samples, developed in the 1940–1950s, changed our conception about the architecture of eukaryotic cells. It was followed by a period where EM applied to cell biology had seemingly fallen asleep, even though new methods with important implications for modern EM were developed. Among these was the discovery that samples can be preserved by chemical fixation and most importantly by rapid freezing without the formation of crystalline ice, giving birth to the world of cryo-EM. The past 15–20 years are hallmarked by a tremendous interest in EM, driven by important technological advances. Cryo-EM, in particular, is now capable of revealing structures of proteins at a near-atomic resolution owing to improved sample preparation methods, microscopes and cameras. In this review, we focus on the challenges associated with the imaging of membranes by EM and give examples from the field of host–pathogen interactions, in particular of virus-infected cells. Despite the advantages of imaging membranes under native conditions in cryo-EM, conventional EM will remain an important complementary method, in particular if large volumes need to be imaged.

Strategies for Assaying Lysosomal Membrane Permeabilization

Late endosomal organelles have an acidic pH and contain hydrolytic enzymes to degrade cargo delivered either from the extracellular environment by endocytosis or from within the cell itself by autophagy. In the event of lysosomal membrane permeabilization (LMP), the contents of late endosomes and lysosomes can be released into the cytosol and then initiate apoptosis. Compounds that can trigger LMP are therefore candidates for the induction of apoptosis, in particular in anticancer therapy. Alternatively, drug-delivery systems, such as nanoparticles, can have side effects that can include LMP, which has toxic consequences for the cells. To determine when, to what extent, and with what consequences LMP occurs is therefore of paramount importance for the evaluation of new potentially LMP-inducing compounds. In this introduction, we provide an overview of some basic assays for assessing LMP, such as staining with lysosomotropic dyes and measurement of cysteine cathepsin activity, and discuss additional strategies for the detection of the release of endogenous lysosomal molecules or preloaded exogenous tracers into the cytosol.

The apicomplexan parasite Babesia divergens internalizes band 3, glycophorin A and spectrin during invasion of human red blood cells

Plasmodium falciparum invades human red blood cells (RBC), while Babesia divergens infects bovine and, occasionally, human RBC. The mammalian RBC is normally unable to endocytose or phagocytose and the events leading to invasion are incompletely understood. Initially, both parasites are surrounded by the RBC plasma membrane-derived parasitophorous vacuolar membrane (PVM) that is formed during invasion. In P. falciparum-infected RBC, the PVM persists at least until parasite replication is completed whereas it has been proposed that the B. divergens PVM is disintegrated soon upon invasion. Here, we have used a B. divergens strain adapted to human RBC to investigate the formation and fate of the PVM. Using ultrastructural analysis and whole-mount or on-section immunofluorescence and immunogold labelling, we demonstrate that the initial vacuolar membrane is formed from protein and lipid components of the RBC plasma membrane. Integral membrane proteins band 3 and glycophorin A and the cytoskeletal protein spectrin are associated with the PVM of the B. divergens, but are absent from the PVM of P. falciparum at the ring or the trophozoite stage. Our results provide evidence that the biophysical properties of the RBC cytoskeleton per se do not preclude the internalization of cytoskeletal proteins by invading parasites.

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.

Streamlined embedding of cell monolayers on gridded glass-bottom imaging dishes for correlative light and electron microscopy

Correlative light and electron microscopy (CLEM) has facilitated study of intracellular trafficking. Routine application of CLEM would be advantageous for many laboratories but previously described techniques are particularly demanding, even for those with access to laser scanning confocal microscopy (LSCM) and transmission electron microscopy (TEM). We describe streamlined methods for TEM of green fluorescent protein (GFP)-labeled organelles after imaging by LSCM using gridded glass bottom imaging dishes. GFP-MAP 1A/1B LC3 (GFP-LC3) transfected cells were treated with rapamycin, fixed and imaged by LSCM. Confocal image stacks were acquired enabling full visualization of each GFP-LC3 labeled organelle. After LSCM, cells were embedded for TEM using a simplified two step method that stabilizes the glass bottom such that the block can be separated from the glass by mild heating. All imaging and TEM processing are performed in the same dish. The LSCM imaged cells were relocated on the block and serial sectioned. Correlation of LSCM, DIC, and TEM images was facilitated by cellular landmarks. All GFP labeled structures were successfully reidentified and imaged by serial section TEM. This method could make CLEM more accessible to nonspecialized laboratories with basic electron microscopy expertise and could be used routinely to confirm organelle localization of fluorescent puncta.

Immunomapping of desmosomal and nondesmosomal adhesion molecules in healthy canine footpad, haired skin and buccal mucosal epithelia: comparison with canine pemphigus foliaceus serum immunoglobulin G staining patterns

Pemphigus foliaceus (PF) is the most common canine autoimmune skin disease. In contrast to human PF (hPF), desmoglein-1 is a minor autoantigen in the canine disease. The major autoantigen(s) of canine PF (cPF) remain(s) unknown, which limits the ability to perform mechanistic studies of lesion formation and the development of novel diagnostic and therapeutic strategies for this disease. The immunofluorescence patterns of selected desmosomal (desmoglein-1, desmoglein-3, desmocollin-1, desmocollin-3, desmoplakin-1/2, plakoglobin and plakophilin-1) and nondesmosomal adhesion proteins (E-cadherin, claudin-1, zona occludens-1 and occludin) in healthy canine footpad, haired skin and buccal mucosal epithelia were determined using hPF and pemphigus vulgaris sera and specific antibodies. The immunostaining patterns were then compared with that of indirect immunofluorescence staining with 66 cPF sera. Most cPF sera (58 of 66; 88%) exhibited positive staining along keratinocyte margins in the stratum spinosum and stratum granulosum of canine footpad. One serum contained autoantibodies binding solely to stratum granulosum keratinocytes. Concurrent intercellular fluorescence in the stratum basale was limited to seven of 66 cPF sera (11%). Only 12 of 66 cPF sera (18%) also exhibited positive IF staining of the buccal mucosa. This study confirms the immunological heterogeneity of cPF immunoglobulin G autoantibodies. Moreover, the major indirect immunofluorescence staining pattern and the inability of most cPF sera to label the buccal mucosa closely matched that of desmocollin-1. These observations warrant further investigation of desmocollin-1 as a potential major cPF autoantigen.

Use of polarized PC12 cells to monitor protein localization in the early biosynthetic pathway

A prerequisite for understanding the cellular functions of an unknown protein is the establishment of its subcellular localization. As increasing numbers of novel proteins of the biosynthetic pathway are currently being identified, accessible new methods are required to facilitate their localization. Differentiating rat pheochromocytoma (PC12) cells reorganize their biosynthetic membrane compartments as they develop neurite-like processes. The authors recently showed that polarization of these cells involves the expansion of the intermediate compartment (IC) between the rough endoplasmic reticulum (RER) and the Golgi apparatus. Tubules emerging from the vacuolar parts of the IC move to the developing neurites accumulating in their growth cones, whereas the vacuoles, like RER and Golgi, remain in the cell body. Thus, polarized PC12 cells enhance the resolution for immunofluorescence microscopic mapping of protein localization in the early biosynthetic pathway. The authors also describe here a rapid cell fractionation protocol employing velocity sedimentation in iodixanol gradients that allows one-step separation of the pre-Golgi vacuoles, tubules, and RER.

Image acquisition for colocalization using optical microscopy

Colocalization, in which images of two or more fluorescent markers are overlaid, and coincidence between the probes is measured or displayed, is a common analytical tool in cell biology. Interpreting the images and the meaning of this identified coincidence is difficult in the absence of basic information about the acquisition parameters. In this commentary, we highlight important factors in the acquisition of images used to demonstrate colocalization, and we discuss the minimum information that authors should include in a manuscript so that a reader can interpret both the fluorescent images and any observed colocalization.

Visualization and quantitative analysis of nanoparticles in the respiratory tract by transmission electron microscopy

Nanotechnology in its widest sense seeks to exploit the special biophysical and chemical properties of materials at the nanoscale. While the potential technological, diagnostic or therapeutic applications are promising there is a growing body of evidence that the special technological features of nanoparticulate material are associated with biological effects formerly not attributed to the same materials at a larger particle scale. Therefore, studies that address the potential hazards of nanoparticles on biological systems including human health are required. Due to its large surface area the lung is one of the major sites of interaction with inhaled nanoparticles. One of the great challenges of studying particle-lung interactions is the microscopic visualization of nanoparticles within tissues or single cells both in vivo and in vitro. Once a certain type of nanoparticle can be identified unambiguously using microscopic methods it is desirable to quantify the particle distribution within a cell, an organ or the whole organism. Transmission electron microscopy provides an ideal tool to perform qualitative and quantitative analyses of particle-related structural changes of the respiratory tract, to reveal the localization of nanoparticles within tissues and cells and to investigate the 3D nature of nanoparticle-lung interactions.This article provides information on the applicability, advantages and disadvantages of electron microscopic preparation techniques and several advanced transmission electron microscopic methods including conventional, immuno and energy-filtered electron microscopy as well as electron tomography for the visualization of both model nanoparticles (e.g. polystyrene) and technologically relevant nanoparticles (e.g. titanium dioxide). Furthermore, we highlight possibilities to combine light and electron microscopic techniques in a correlative approach. Finally, we demonstrate a formal quantitative, i.e. stereological approach to analyze the distributions of nanoparticles in tissues and cells.This comprehensive article aims to provide a basis for scientists in nanoparticle research to integrate electron microscopic analyses into their study design and to select the appropriate microscopic strategy.

Cryosectioning fixed and cryoprotected biological material for immunocytochemistry

Immunocytochemistry for transmission 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 thawed cryosections can be 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 be correlated with specific affinity labeling by examination in the transmission electron microscope. 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.

A brief update on lung stereology

Lung stereology has a long and successful tradition. From mice to men, the application of new stereological methods at several levels (alveoli, parenchymal cells, organelles, proteins) has led to new insights into normal lung architecture, parenchymal remodelling in emphysema-like pathology, alveolar type II cell hyperplasia and hypertrophy and intracellular surfactant alterations as well as distribution of surfactant proteins. The Euler number of the network of alveolar openings, estimated using physical disectors at the light microscopic level, is an unbiased and direct estimate of alveolar number. Surfactant-producing alveolar type II cells can be counted and sampled for local size estimation with physical disectors at a high magnification light microscopic level. The number of their surfactant storage organelles, lamellar bodies, can be estimated using physical disectors at the EM level. By immunoelectron microscopy, surfactant protein distribution can be analysed with the relative labelling index. Together with the well-established classical stereological methods, these design-based methods now allow for a complete quantitative phenotype analysis in lung development and disease, including the structural characterization of gene-manipulated mice, at the light and electron microscopic level.

Intra-Golgi transport: A way to a new paradigm?

The morpho-functional principles of intra-Golgi transport are, surprisingly, still not clear, which is in marked contrast to our advanced knowledge of the underlying molecular machineries. Recently, the conceptual and technological hindrances that had delayed progress in this area have been disappearing, and a cluster of powerful morphological techniques has been revealing new glimpses of the organization of traffic in intact cells. Here, we discuss the new concepts around the present models of intra-Golgi transport.

Fixation-induced redistribution of hyperphosphorylated RNA polymerase II in the nucleus of human cells

RNA polymerase II (pol II) transcribes the most varied group of genes and is present in hypo- and hyperphosphorylated forms, with residues Ser(2) and Ser(5) of the C-terminal domain (CTD) of the largest subunit as main targets of phosphorylation. The elongating (active) form is phosphorylated on Ser(2) and can be specifically recognized with the H5 antibody. It has been found in different nuclear distributions: in discrete sites throughout the nucleoplasm, consistent with a role in transcription, and/or concentrated in "splicing speckles", a nuclear compartment mostly devoid of transcriptional activity. Here, we assess the effects of cell fixation and permeabilization on the distribution of polymerase II and correlate its distribution with the preservation of cellular ultrastructure. We show that phospho-Ser(2) polymerase II can redistribute to, or be differentially retained in, "speckles" in conditions that do not preserve cellular ultrastructure. The fixation protocols that disrupt polymerase II distribution also cause partial or total loss of TATA-binding protein, Sm antigen and PML staining in PML bodies, and have no noticeable effect in the labeling of SC35 in "splicing speckles" or coilin in Cajal bodies. When nuclear ultrastructure is preserved, phospho-Ser(2) polymerase II is found in discrete sites throughout the nucleoplasm, without visible enrichment within splicing speckles. A minor proportion of the total amount of the phospho-Ser(2) form is present in these domains.

Ultrathin cryosections: an important tool for immunofluorescence and correlative microscopy

Here we show that ultrathin cryosections of placental tissue can be used as a substrate in immunofluorescence experiments. A high degree of spatial resolution can be achieved in these preparations because there is essentially no out-of-focus fluorescence. Therefore, immunofluorescence microscopy using ultrathin cryosections provides a very useful method for determining the precise subcellular localization of antigens in tissues. In addition, ultrathin cryosections of placenta also serve as a substrate for correlative immunofluorescence and immunoelectron microscopy using FluoroNanogold as the detection system. In correlative microscopy, the exact same structures in the same ultrathin section were observed by both fluorescence and electron microscopy. Using a particle counting procedure and electron microscopy, we compared the labeling obtained with colloidal gold and FluoroNanogold and found a higher number of particles with silver-enhanced FluoroNanogold than with colloidal gold.

Correlative microscopy of ultrathin cryosections is a powerful tool for placental research

In this report, we describe procedures for correlative fluorescence and electron microscopy in immunocytochemical studies on the human placenta. Ultrathin cryosections of placenta were used for detection of the distribution of antigens by immunofluorescence and subsequently by immunoelectron microscopy of the same ultrathin cryosection. This methodology has certain advantages over conventional immunohistochemistry and immunoelectron microscopy. The advantages are, most notably, that the same exact structures are examined by both imaging modalities. In addition, since the tissue is physically sectioned (50-100 nm thickness), greater resolution for fluorescence can be obtained in the z-dimension than can be obtained by optical sectioning in confocal microscopy. This last point is of particular importance for discriminating between structures closely stacked in the z-dimension. In this report, we have determined the distribution of caveolin-1 in ultrathin cryosections of terminal villi of the human term placenta. We demonstrate that the use of ultrathin cryosections is a powerful approach for immunofluorescence and correlative microscopy for the in situ localization of antigens.

Gut thoughts on the Golgi complex

The new millennium coincides within 1 year of Camillo Golgi's centennial celebrations. It is quite remarkable that the structure and formation of this organelle is as controversial today as was its mere existence from Golgi's time to the 1950s, when EM approaches were introduced. Since the late 1950s, two opposing models of Golgi structure and function have split the Golgi scientific community, namely vesicular transport versus organelle maturation. Although a few years ago Golgi maturation seemed to be 'out for the count', it has recently seen an almost messianic revival. In this review, I argue that this large-scale desertion from the vesicle transport model to the maturation camp is premature. I propose an alternative, dynamic steady-state model, in which transient tubular connections function in parallel to vesicular transport and that the biosynthetic pathway is made up of three major distinct compartments: the ER, the Golgi and the TGN.

Role for dynamin in late endosome dynamics and trafficking of the cation-independent mannose 6-phosphate receptor

It is well established that dynamin is involved in clathrin-dependent endocytosis, but relatively little is known about possible intracellular functions of this GTPase. Using confocal imaging, we found that endogenous dynamin was associated with the plasma membrane, the trans-Golgi network, and a perinuclear cluster of cation-independent mannose 6-phosphate receptor (CI-MPR)-containing structures. By electron microscopy (EM), it was shown that these structures were late endosomes and that the endogenous dynamin was preferentially localized to tubulo-vesicular appendices on these late endosomes. Upon induction of the dominant-negative dynK44A mutant, confocal microscopy demonstrated a redistribution of the CI-MPR in mutant-expressing cells. Quantitative EM analysis of the ratio of CI-MPR to lysosome-associated membrane protein-1 in endosome profiles revealed a higher colocalization of the two markers in dynK44A-expressing cells than in control cells. Western blot analysis showed that dynK44A-expressing cells had an increased cellular procathepsin D content. Finally, EM revealed that in dynK44A-expressing cells, endosomal tubules containing CI-MPR were formed. These results are in contrast to recent reports that dynamin-2 is exclusively associated with endocytic structures at the plasma membrane. They suggest instead that endogenous dynamin also plays an important role in the molecular machinery behind the recycling of the CI-MPR from endosomes to the trans-Golgi network, and we propose that dynamin is required for the final scission of vesicles budding from endosome tubules.

The effects of cell passages on the cell morphology and the outcome of herpes simplex virus type 1 infection

Because cell cultures are essential in biological research which involves the analysis of virus morphogenesis, this study focused on examining the significance of cell passages. Human embryonic lung fibroblasts (MRC-5) at passage (P) 27 were seeded twice a week to P 32, P 40, and P 48, when just at confluence and then infected with herpes simplex virus type 1 (HSV-1). The structure of the non-virus-infected (MOCK) and HSV-1 infected cells, the amount of cellular infectious virus particles and the capability to express HSV-1 glycoproteins C (gC-1) and D (gD-1) were investigated by phase-contrast and immunofluorescence light microscopy, immunogold cryosection EM, plaque assays, immunoblots, and total protein assays. Modified cell structure including fragmentation of tubulin fibers were visible in MOCK from P 38 onwards. The quantity of vimentin remained unchanged while actin accumulated and beta-tubulin decreased in HSV-1 infected late P cells compared to early P cultures. Cells of high P counts contained significantly fewer infectious virus particles, very likely of lower virulence, and their expression of gC-1 and gD-1 were concordantly reduced. These observations indicate that the number of cell P must be considered in order to reproduce results of cell biology and viral morphogenesis. The MRC-5 cells ought not to be passaged more than ten times beyond P 27 in the laboratory.

Correlative light-electron microscopy reveals the tubular-saccular ultrastructure of carriers operating between Golgi apparatus and plasma membrane

Transport intermediates (TIs) have a central role in intracellular traffic, and much effort has been directed towards defining their molecular organization. Unfortunately, major uncertainties remain regarding their true structure in living cells. To address this question, we have developed an approach based on the combination of the green fluorescent protein technology and correlative light-electron microscopy, by which it is possible to monitor an individual carrier in vivo and then take a picture of its ultrastructure at any moment of its life-cycle. We have applied this technique to define the structure of TIs operating from the Golgi apparatus to the plasma membrane, whose in vivo dynamics have been characterized recently by light microscopy. We find that these carriers are large (ranging from 0.3-1.7 microm in maximum diameter, nearly half the size of a Golgi cisterna), comprise almost exclusively tubular-saccular structures, and fuse directly with the plasma membrane, sometimes minutes after docking to the fusion site.

Stimulated phosphorylation of intracellular connexin43

A monoclonal antibody, Zymed 13-8300, was previously reported to only detect nonphosphorylated connexin43 (Nagy et al., Exp. Cell Res. 236, 127-136, 1997). We show that 13-8300 can detect several phosphorylated species of connexin43 in Western blots after stimulation of two fibroblast cell systems with fresh growth medium, 12-O-tetradecanoyl phorbol-13-acetate, pervanadate, or permolybdate. In one of the cell systems, at least three forms of phosphorylated connexin43 could migrate at the same position during electrophoresis. The comigration of differentially phosphorylated species may complicate the molecular and functional analysis of phosphorylation sites in Cx43. Immunofluorescence experiments indicated that the newly generated phosphorylated Cx43 forms mainly had a perinuclear location. Also, in cells treated with brefeldin A for 8 h, in which the majority of connexin43 was intracellular, phosphorylation was induced by the agents. Phosphorylation of intracellular connexin43 can therefore be induced by several stimuli.

Localization of mouse hepatitis virus nonstructural proteins and RNA synthesis indicates a role for late endosomes in viral replication

The aim of the present study was to define the site of replication of the coronavirus mouse hepatitis virus (MHV). Antibodies directed against several proteins derived from the gene 1 polyprotein, including the 3C-like protease (3CLpro), the putative polymerase (POL), helicase, and a recently described protein (p22) derived from the C terminus of the open reading frame 1a protein (CT1a), were used to probe MHV-infected cells by indirect immunofluorescence (IF) and electron microscopy (EM). At early times of infection, all of these proteins showed a distinct punctate labeling by IF. Antibodies to the nucleocapsid protein also displayed a punctate labeling that largely colocalized with the replicase proteins. When infected cells were metabolically labeled with 5-bromouridine 5'-triphosphate (BrUTP), the site of viral RNA synthesis was shown by IF to colocalize with CT1a and the 3CLpro. As shown by EM, CT1a localized to LAMP-1 positive late endosomes/lysosomes while POL accumulated predominantly in multilayered structures with the appearance of endocytic carrier vesicles. These latter structures were also labeled to some extent with both anti-CT1a and LAMP-1 antibodies and could be filled with fluid phase endocytic tracers. When EM was used to determine sites of BrUTP incorporation into viral RNA at early times of infection, the viral RNA localized to late endosomal membranes as well. These results demonstrate that MHV replication occurs on late endosomal membranes and that several nonstructural proteins derived from the gene 1 polyprotein may participate in the formation and function of the viral replication complexes.

Role of the membrane anchoring and cytoplasmic domains in intracellular transport and localization of viral glycoproteins

Nuclear factor-κB (NF-κB) binds to nucleotide sequences between -80 and -70 bp upstream of the transcriptional start site in the interleukin-8 (IL-8) promoter and is crucial for transcription of the IL-8 gene. We showed that exogenous nitric oxide in the form of a nitric oxide donor significantly reduced IL-8 mRNA in cytokine-activated ECV304. Similarly, nitric oxide significantly reduced migration of polymorphonuclear neutrophils through cytokine-activated ECV304 monolayers, an IL-8-dependent process. Using a luciferase reporter construct containing the NF-κB site of the IL-8 gene, we showed that exposing cytokine-activated ECV304 to exogenous nitric oxide resulted in significant reduction of NF-κB binding. Follow-up studies using a luciferase reporter construct possessing a mutated NF-κB site confirmed that the luciferase activity observed in the NF-κB reporter resulted from NF-κB binding. These studies demonstrate that nitric oxide, supplied exogenously into reactions containing activated endothelium, down-regulates pro-inflammatory activity, such as the secretion of chemokines, and functional activity, such as transendothelial migration of neutrophils. Key words: interleukin-8, nuclear factor κ B, transendothelial migration, nitric oxide.

Ultrastructural localization of beta-actin and amphoterin mRNA in cultured cells: application of tyramide signal amplification and comparison of detection methods

We describe a nonradioactive preembedding in situ hybridization protocol using digoxigenin-labeled RNA probes and tyramide signal amplification to increase the sensitivity of detection. The protocol is sensitive enough for electron microscopic localization of endogenous messenger RNAs encoding beta-actin and amphoterin. Three visualization methods were compared: diaminobenzidine enhanced by nickel, Nanogold enhanced by silver and gold toning, and fluorescently labeled tyramides. Diaminobenzidine and Nanogold can be used in both light and electron microscopy. The nickel-enhanced diaminobenzidine was the most sensitive visualization method. It is easy to accomplish but a drawback is poor spatial resolution, which restricts its use at high magnifications. Nanogold visualization has considerably better spatial resolution and is therefore recommended for electron microscopy. Fluorescent tyramides, especially TRITC-tyramide, offer a good detection method for fluorescence and confocal microscopy. The methods were used to localize amphoterin and beta-actin mRNAs in motile cells. Both mRNAs were found in the soma and cell processes. In double labeling experiments, beta-actin mRNA localized to filamentous structures that also contained ribosomal proteins. Especially in the cortical cytoplasm, beta-actin mRNA was associated with actin filaments. Direct localization to microtubules was only rarely seen. (J Histochem Cytochem 47:99-112, 1999)

Differential extraction of proteins from paraformaldehyde-fixed cells: lessons from synaptophysin and other membrane proteins

While investigating the localization of synaptophysin in PC12 cells using immunofluorescence microscopy, we noticed a striking difference in its apparent subcellular distribution depending on whether digitonin or Triton X-100 was used as permeabilization agent of paraformaldehyde (PFA)-fixed cells. We found that this difference was due to epitope inaccessibility in the digitonin-treated cells combined with an almost quantitative extraction of the antigen on Triton X-100 permeabilization. Both phenomena were differential with respect to the various synaptophysin-containing compartments. The extraction of antigen from PFA-fixed cells was also seen with other membrane proteins but not with cytosolic proteins and proteins in the lumen of the secretory pathway. Significantly, some of the membrane proteins were extracted from the PFA-fixed cells in higher-molecular-weight forms which we believe represent their in vivo oligomeric states. The implications of our observations are discussed with respect to the method of immunofluorescence microscopy and also to the possible use of paraformaldehyde as an in vivo crosslinker for the study of membrane protein quaternary structure.

Expression of the adrenoleukodystrophy protein in the human and mouse central nervous system

The gene mutated in X-linked adrenoleukodystrophy (ALD), a progressive demyelinating disease, codes for a protein (ALDP) involved in very-long-chain fatty acid (VLCFA) transport. The expression of ALDP and of two peroxisomal enzymes involved in beta-oxidation of VLCFA, acyl-CoA oxidase, and catalase was studied in human and mouse brain. The pattern of expression was similar in both species. While acyl-CoA oxidase and catalase are found in all types of CNS cells, including neurons and oligodendrocytes, ALDP expression is restricted mostly to the white matter and endothelial cells. ALDP is highly expressed in astrocytes and microglial cells in vivo and in regenerating oligodendrocytes in vitro. In contrast, in vivo, ALDP is detected in much fewer oligodendrocytes and quantitative Western blot analysis confirmed the lower abundance of ALDP in these cells than in astrocytes. Only oligodendrocytes localized in corpus callosum, internal capsules, and anterior commissure express ALDP at levels comparable to those seen in astrocytes. In ALD, demyelination is first detected in these white matter regions, suggesting that the ALD gene mutation selectively affects those oligodendrocytes strongly expressing ALDP. Because of their failure to express ALDP, microglia and astrocytes may also contribute to demyelination in ALD patients.

Traffic of dynamin within individual Drosophila synaptic boutons relative to compartment-specific markers

Presynaptic terminals contain several specialized compartments, which have been described by electron microscopy. We show in an identified Drosophila neuromuscular synapse that several of these compartments-synaptic vesicle clusters, presynaptic plasma membrane, presynaptic cytosol, and axonal cytoskeleton-labeled by specific reagents may be resolved from one another by laser scanning confocal microscopy. Using a panel of compartment-specific markers and Drosophila shibire(ts1) mutants to trap an intermediate stage in synaptic vesicle recycling, we have examined the localization and redistribution of dynamin within single synaptic varicosities at the larval neuromuscular junction. Our results suggest that dynamin is not a freely diffusible molecule in resting nerve terminals; rather, it appears localized to synaptic sites by association with yet uncharacterized presynaptic components. In shi(ts1) nerve terminals depleted of synaptic vesicles, dynamin is quantitatively redistributed to the plasma membrane. It is not, however, distributed uniformly over presynaptic plasmalemma; instead, fluorescence images show "hot spots" of dynamin on the plasma membrane of vesicle-depleted nerve terminals. We suggest that these dynamin-rich domains may mark the active zones for synaptic vesicle endocytosis first described at the frog neuromuscular junction.

Advantages of backscatter electron imaging scanning electron microscopy for intracellular localization of cardiac analytes by gold conjugated antibody

Myoglobin and myosin light chain 1 (MLC1) are intracellular human cardiac marker proteins which are released as a consequence of ischemia. Human cardiomyocytes were isolated from fresh biopsies and also maintained for several passages in cell culture. The cardiomyocytes were fixed in 100% methanol at -20 degrees C, and labeled. The immunolocalization of intracellular antigen by fluorescence conjugated imaging was compared with scanning electron microscopy (SEM) backscatter electron (BSE) imaging of gold conjugated antibody. Ultra-violet light microscopy showed the intracellular distribution of both proteins to be mainly in the nuclear envelope, the cytoplasm immediately surrounding the nucleus and along portions of the cell membrane. To confirm this observed distribution of myoglobin and MLC1, labeling was repeated with antimyoglobin and anti-MLC1 monoclonal antibodies conjugated to colloidal gold particles. The advantage of colloidal gold labeling is that the intracellular antigen-antibody complexes may be more precisely located because of the significant improvement in resolution provided by BSE imaging in the SEM. BSE imaging confirmed the presence and subsarcolemma localization of myoglobin in cardiomyocytes directly isolated from fresh biopsies. The distribution of colloidal gold-conjugated antibodies did not coincide with the intracellular distribution of the two proteins in the cardiomyocytes grown in cell culture as indicated by immunofluorescence. A relatively random, intracellular gold particle distribution was confirmed by x-ray microanalysis. BSE imaging resulted in consistent auto-backscatter labeling patterns very similar to the labeling patterns obtained with immunofluorescent labeling. X-ray microanalysis confirmed that these auto-backscatter labeling patterns were formed by concentrations of intracellular phosphate. Sodium dodecyl sulfate-poly-acrylamide gel electrophoresis (SDS-PAGE) and subsequent Western blotting indicated that myoglobin and MLC1 were no longer present in detectable quantities in these cells after several passages. Polymerase chain reaction (PCR) amplification of mRNA for human myoglobin and cardiac MLC1 confirmed the absence of their transcripts. Electrophoretic analysis of proteins in cardiomyocytes grown in cell culture confirmed an increasing presence of alkaline phosphatase. Staining of this enzyme with 5-bromo-4-chloro-3-indolyl phosphate and nitroblue tetrazolium showed that alkaline phosphatase was distributed in the same intracellular pattern as the fluorescence conjugated anti-body and the phosphatase auto-backscatter. These results indicate that high-resolution backscatter SEM imaging may be used as necessary control to confirm fluorescence light microscope intracellular labeling of antigens.

Distribution of Sertoli cell microtubules, microtubule-dependent motors, and the Golgi apparatus before and after tight junction formation in developing rat testis

Sertoli cells are polarized epithelial cells of the seminiferous epithelium which provide structural and physiological support for differentiating germ cells. They establish different basal and adluminal environments for the selective nurturing of pre- and post-meiotic germ cells within the seminiferous epithelium, segregated by the Sertoli-Sertoli cell tight junctional complex, the blood-testis barrier. Tight junction formation between epithelial cells in vitro is a critical polarizing event associated with changes in polarized targeting of membrane-specific proteins and reorganization of microtubules, centrioles, and the Golgi apparatus. To investigate whether tight junction formation is associated with organelle reorganization in Sertoli cells in vivo, we have characterized distribution patterns of Sertoli cell microtubules, the mechanoenzymes kinesin and cytoplasmic dynein, and the Golgi apparatus during tight junction formation in developing rat testis. Immunocytochemistry on samples taken at 5, 10, 15, 20, and 25 days of age was used to examine the distribution of these proteins during the extensive cellular reorganization that culminates in the formation of the blood-testis barrier at 19 days of age. Our data show that the distribution patterns reflect the extensive intercellular repositioning of tubule cells in developing seminiferous tubules, but that changes in intracellular organization are not temporally associated with formation of the blood-testis barrier.

Molecular regulation of GLUT-4 targeting in 3T3-L1 adipocytes

Insulin stimulates glucose transport in muscle and adipose tissue by triggering the movement of the glucose transporter GLUT-4 from an intracellular compartment to the cell surface. Fundamental to this process is the intracellular sequestration of GLUT-4 in nonstimulated cells. Two distinct targeting motifs in the amino and carboxy termini of GLUT-4 have been previously identified by expressing chimeras comprised of portions of GLUT-4 and GLUT-1, a transporter isoform that is constitutively targeted to the cell surface, in heterologous cells. These motifs-FQQI in the NH2 terminus and LL in the COOH terminus-resemble endocytic signals that have been described in other proteins. In the present study we have investigated the roles of these motifs in GLUT-4 targeting in insulin-sensitive cells. Epitope-tagged GLUT-4 constructs engineered to differentiate between endogenous and transfected GLUT-4 were stably expressed in 3T3-L1 adipocytes. Targeting was assessed in cells incubated in the presence or absence of insulin by subcellular fractionation. The targeting of epitope-tagged GLUT-4 was indistinguishable from endogenous GLUT-4. Mutation of the FQQI motif (F5 to A5) caused GLUT-4 to constitutively accumulate at the cell surface regardless of expression level. Mutation of the dileucine motif (L489L490 to A489A490) caused an increase in cell surface distribution only at higher levels of expression, but the overall cells surface distribution of this mutant was less than that of the amino-terminal mutants. Both NH2- and COOH-terminal mutants retained insulin-dependent movement from an intracellular to a cell surface locale, suggesting that neither of these motifs is involved in the insulin-dependent redistribution of GLUT-4. We conclude that the phenylalanine-based NH2-terminal and the dileucine-based COOH-terminal motifs play important and distinct roles in GLUT-4 targeting in 3T3-L1 adipocytes.

A time-related study of Brefeldin A effects in HSV-1 infected cultured human fibroblasts

Glycoprotein D (gD-1) is an essential virion envelope component of herpes simplex virus type 1 (HSV-1) normally transported to the plasma membrane of the infected cells. In the present study, the intracellular transport of gD-1 was inhibited in cultured HSV-1 infected human fibroblasts by Brefeldin A (BFA) 1 microgram/ml medium added for 12 h after virus adsorption. Immunofluorescence light- and confocal microscopy revealed abolished transport of gD-1 to the plasma membrane, juxtanuclear accumulation of gD-1, and a disorderly arrangement of the tubulin fibres. Withdrawal of BFA influence for more than 60 min resulted in incomplete transport but increasing accumulation of gD-1 in the plasma membrane and in Golgi-like areas close to the nuclei. The tubulin pattern was almost normalized 6 h after removal of BFA. The egress of infectious HSV-1 particles released 9 h post-BFA treatment was not fully reestablished. The results indicate that BFA effects were not completely reversible and caused a sort of cytotoxic influence involving the structure of tubulin.

The endocytic pathway for H-ferritin established in live MOLT-4 cells by laser scanning confocal microscopy

We have established the intracellular destination of the putative immunoregulatory protein, human recombinant H (heavy)-ferritin, in the transformed T-cell line MOLT-4, by laser scanning confocal microscopy of live cells. A series of confocal images was collected over a 60 min time course using indirect immunofluorescence of H-ferritin and transferrin, their respective monoclonal antibodies, and fluorescein isothiocyanate (FITC)-labelled IgG. A marked drop in FITC fluorescence after 40 min of H-ferritin internalization, indicative of an acidic environment, and co-localization with tetramethylrhodamine isothiocyanate-labelled-dextran strongly suggests that H-ferritin is transferred to the lysosome. In contrast, transferrin was observed to return to the cell surface. Electron microscopy confirmed that H-ferritin was transferred to the lysosome. The receptor-mediated endocytosis and lysosomal delivery of H-ferritin may thus potentiate its putative immunoregulatory activity.

Distinct intracellular localization of Lck and Fyn protein tyrosine kinases in human T lymphocytes

Two src family kinases, lck and fyn, participate in the activation of T lymphocytes. Both of these protein tyrosine kinases are thought to function via their interaction with cell surface receptors. Thus, lck is associated with CD4, CD8, and Thy-1, whereas fyn is associated with the T cell antigen receptor and Thy-1. In this study, the intracellular localization of these two protein tyrosine kinases in T cells was analyzed by immunofluorescence and confocal microscopy. Lck was present at the plasma membrane, consistent with its proposed role in transmembrane signalling, and was also associated with pericentrosomal vesicles which co-localized with the cation-independent mannose 6-phosphate receptor. Surprisingly, fyn was not detected at the plasma membrane in either Jurkat T cells or T lymphoblasts but was closely associated with the centrosome and to microtubule bundles radiating from the centrosome. In mitotic cells, fyn co-localized with the mitotic spindle and poles. The essentially non-overlapping intracellular distributions of lck and fyn suggest that these kinases may be accessible to distinct regulatory proteins and substrates and, therefore, may regulate different aspects of T cell activation. Anti-phosphotyrosine antibody staining at the plasma membrane increases dramatically after CD3 cross-linking of Jurkat T cells. The localization of lck to the plasma membrane suggests that it may participate in mediating this increase in tyrosine phosphorylation, rather than fyn. Furthermore, the distribution of fyn in mitotic cells raises the possibility that it functions at the M phase of the cell cycle.

Membrane anchoring domain of herpes simplex virus glycoprotein gB is sufficient for nuclear envelope localization

We have used the glycoprotein gB of herpes simplex virus type 1 (gB-1), which buds from the inner nuclear membrane, as a model protein to study localization of membrane proteins in the nuclear envelope. To determine whether specific domains of gB-1 glycoprotein are involved in localization in the nuclear envelope, we have used deletion mutants of gB-1 protein as well as chimeric proteins constructed by replacing the domains of the cell surface glycoprotein G of vesicular stomatitis virus with the corresponding domains of gB. Mutant and chimeric proteins expressed in COS cells were localized by immunoelectron microscopy. A chimeric protein (gB-G) containing the ectodomain of gB and the transmembrane and cytoplasmic domains of G did not localize in the nuclear envelope. When the ectodomain of G was fused to the transmembrane and cytoplasmic domains of gB, however, the resulting chimeric protein (G-gB) was localized in the nuclear envelope. Substitution of the transmembrane domain of G with the 69 hydrophobic amino acids containing the membrane anchoring domain of gB allowed the hybrid protein (G-tmgB) to be localized in the nuclear envelope, suggesting that residues 721 to 795 of gB can promote retention of proteins in the nuclear envelope. Deletion mutations in the hydrophobic region further showed that a transmembrane segment of 21 hydrophobic amino acids, residues 774 to 795 of gB, was sufficient for localization in the nuclear envelope. Since wild-type gB and the mutant and chimeric proteins that were localized in the nuclear envelope were also retained in the endoplasmic reticulum, the membrane spanning segment of gB could also influence retention in the endoplasmic reticulum.