Silver-enhanced colloidal gold probes as markers for scanning electron microscopy

Silver enlargement of small colloidal gold particles has been extensively used for the light microscopical visualization of gold probes. Very recently, a few investigators have employed physical developers in electron microscopy (both pre-embedding and on-grid staining methods). We now demonstrate that physical development of small colloidal gold particles advantageously can be exploited for labelling biological surfaces in scanning electron microscopy. This novel application of silver enhancement of colloidal gold particles is characterized by a high detection efficiency. Thus, specimens are labelled with small gold probes affording high immunocytochemical efficiency but being impossible to detect with the present scanning microscopes. These particles are subsequently scanning electronmicroscopically visualized by silver enhancement.     

Ultrastructural visualization of cytoskeletal mRNAs and their associated proteins using double-label in situ hybridization

We have been able to visualize cytoskeletal messenger RNA molecules at high resolution using nonisotopic in situ hybridization followed by whole-mount electron microscopy. Biotinated cDNA probes for actin, tubulin, or vimentin mRNAs were hybridized to Triton-extracted chicken embryo fibroblasts and myoblasts. The cells were then exposed to antibodies against biotin followed by colloidal gold-conjugated antibodies and then critical-point dried. Identification of mRNA was possible using a probe fragmented to small sizes such that hybridization of several probe fragments along the mRNA was detected as a string of colloidal gold particles qualitatively and quantitatively distinguishable from nonspecific background. Extensive analysis showed that when eight gold particles were seen in this iterated array, the signal to noise ratio was greater than 30:1. Furthermore, these gold particles were colinear, often spiral, or circular suggesting detection of a single nucleic acid molecule. Antibodies against actin, vimentin, or tubulin proteins were used after in situ hybridization, allowing simultaneous detection of the protein and its cognate message on the same sample. This revealed that cytoskeletal mRNAs are likely to be extremely close to actin protein (5 nm or less) and unlikely to be within 20 nm of vimentin or tubulin filaments. Actin mRNA was found to be more predominant in lamellipodia of motile cells, confirming previous results. These results indicate that this high resolution in situ hybridization approach is a powerful tool by which to investigate the association of mRNA with the cytoskeleton.     

Labeling of binding sites for beta 2-microglobulin (beta 2m) on nonfibrillar surface structures of mutans streptococci by immunogold and beta 2m-gold electron microscopy.

As little detail is known about the surface structure of streptococci in the mutans group and the relationship of surface structure to host ligand-binding functions, the twofold purpose of this investigation was to examine in detail, by a range of electron microscopic techniques, the surface structures of streptococci in the different species of the mutans group and to investigate the distribution of beta 2-microglobulin (beta 2m)-binding sites on such structures. Strains representing Streptococcus mutans, S. cricetus, S. rattus, S. sobrinus, and four fresh isolates were studied by shadowcasting and histochemical staining of whole-mounted cells as well as by ultrathin and thick sectioning of embedded specimens. beta 2m-binding site distribution was visualized by indirect immunogold electron microscopy and by direct bacterial binding of beta 2m-conjugated gold probes. Shadowcast preparations revealed binding of gold probes to the cell surface of known beta 2m-binding strains but not to their polar fibrillar appendages. These long fibrils, common to all strains, were trypsin and sonication sensitive and stained with lead citrate but not with uranyl acetate or ruthenium red. More gold particles were bound by the indirect technique. For grid-mounted bacteria, the gold was mostly bound in clusters at the periphery of the cells. When gold probes were reacted in suspension with bacteria before mounting onto grids, a more even distribution of the gold was seen, but the bacteria were aggregated. Heating the bacteria eliminated beta 2m-gold binding but had no effect on the morphology of the fibrils. Thick sections of embedded bacteria prereacted with beta 2m-conjugated gold probes were analyzed by stereo imaging. A wispy, uranyl acetate-stained fuzzy layer, distinct from the fibrils seen by shadowcasting and extending up to one cell diameter from the cell wall, contained the gold probes. These findings introduce a concept that binding sites for some salivary ligands on mutans streptococci may be clustered on very delicate, nonfibrillar structures extending much further from the cell wall than previously appreciated. As for beta 2m, which composes part of the human histocompatibility antigens, part of the bacterial surface would be coated at a distance from its body with a protein not necessarily recognized as foreign by the host.     

Direct detection of immunogold reactions by real-time video microscopy

Summary Video-enhanced microscopy allows the detection and tracking of individual colloidal gold particles. The analysis of immunogold reactions can also be conducted as a function of time and thus allows the study of dynamic events in living cells. The direct visualization in real time is reported of the reaction of immunogold particles with a surface antigen. This time-resolved immunocytochemistry was achieved by continuous observation of living cells infected with a virus (respiratory syncytial virus) following their incubation with colloidal gold (30 nm) coated with antiviral antibodies. The progress of the immunoreaction was visualized as a sequential deposition of individual gold granules on the viral particles until saturation was reached after 60 min. Binding of colloidal gold was an irreversible event as no elution or dislocation of surface-bound granules took place. Comparative imaging of colloidal gold particles by electron microscopy and by video microscopy demonstrated that the video-imaged immunoreactions represented events involving single gold particles; their signal was sometimes clearly enhanced by secondary depositions taking place in close proximity, i.e. at a distance below the lateral resolution of the light microscope. Our experiments demonstrate that video-enhanced microscopy provides a powerful tool for studying antibody-antigen reactions with a high spatial and temporal resolution.     

Direct detection of immunogold reactions by real-time video microscopy.

Video-enhanced microscopy allows the detection and tracking of individual colloidal gold particles. The analysis of immunogold reactions can also be conducted as a function of time and thus allows the study of dynamic events in living cells. The direct visualization in real time is reported of the reaction of immunogold particles with a surface antigen. This time-resolved immunocytochemistry was achieved by continuous observation of living cells infected with a virus (respiratory syncytial virus) following their incubation with colloidal gold (30 nm) coated with antiviral antibodies. The progress of the immunoreaction was visualized as a sequential deposition of individual gold granules on the viral particles until saturation was reached after 60 min. Binding of colloidal gold was an irreversible event as no elution or dislocation of surface-bound granules took place. Comparative imaging of colloidal gold particles by electron microscopy and by video microscopy demonstrated that the video-imaged immunoreactions represented events involving single gold particles; their signal was sometimes clearly enhanced by secondary depositions taking place in close proximity, i.e. at a distance below the lateral resolution of the light microscope. Our experiments demonstrate that video-enhanced microscopy provides a powerful tool for studying antibody-antigen reactions with a high spatial and temporal resolution.     

Optimizing parameters for correlative immunogold localization by video-enhanced light microscopy, high-voltage transmission electron microscopy, and field emission scanning electron microscopy

Correlative video-enhanced light microscopy, high-voltage transmission electron microscopy, and low-voltage high resolution scanning electron microscopy were used to examine the binding of colloidal gold-labeled fibrinogen to platelet surfaces. Optimal conditions for the detection of large (18 nm) and small (3 nm) gold particles are described.     

Silver-enhanced colloidal gold probes as markers for scanning electron microscopy

Summary Silver enlargement of small colloidal gold particles has been extensively used for the light microscopical visualization of gold probes. Very recently, a few investigators have employed physical developers in electron microscopy (both pre-embedding and on-grid staining methods). We now demonstrate that physical development of small colloidal gold particles advantageously can be exploited for labelling biological surfaces in scanning electron microscopy. This novel application of silver enhancement of colloidal gold particles is characterized by a high detection efficiency. Thus, specimens are labelled with small gold probes affording high immunocytochemical efficiency but being impossible to detect with the present scanning microscopes. These particles are subsequently scanning electronmicro-scopically visualized by silver enhancement.Presented in part at the International Symposium on Biological Regulation of Cell Proliferation, 9th International Chalone Conference, Milano, Italy, March 3–6, 1986     

Epidermal growth factor receptors associated to cytoskeletal elements of epidermoid carcinoma (A431) cells

The structural interaction of the epidermal growth factor (EGF) receptor and the cytoskeleton of A431 cells has been studied using a monoclonal anti-EGF receptor antibody. This has been done with immunogold labeling using a variety of electron microscopical preparation procedures and EGF binding studies. By providing an image of the membrane-associated cytoskeleton, the dry cleavage method reveals a preferential localization of EGF receptors superimposed upon cytoskeletal filaments. The colocalization of gold particles with cytoskeletal filaments is not affected when pre-labeled cells are extracted with the non-ionic detergent Triton X-100, as visualized by dry cleavage. Using surface replication, this treatment results in visualization of the cytoskeleton. In these latter preparations, it is also observed that EGF receptor-coupled gold particles remain associated with cytoskeletal elements. Moreover, Triton extraction performed before immunogold labeling of EGF receptors demonstrates that isolated cytoskeletons contained binding sites for anti-EGF receptor antibodies. Using stereo micrographs of replica's obtained from these isolated cytoskeletons, it is shown that gold-labeled EGF receptors are exclusively present on the cortical membrane-associated region of the cytoskeleton and not on more intracellular-located filaments. Scatchard analysis of EGF binding to cells fixed with glutaraldehyde and treated with Triton X-100 before and after EGF binding indicates that a high affinity EGF binding site is associated with the Triton X-100 insoluble cytoskeleton.     

Optimization of immunogold labeling TEM: an ELISA-based method for evaluation of blocking agents for quantitative detection of antigen.

We developed an ELISA-based method for rapid selection of optimal blocking agents to be used in antigen quantification by immunogold labeling electron microscopy. Casein, skim milk, BSA from two sources, acetylated BSA, fish skin gelatin, horse serum, and goat serum were tested for their ability to block nonspecific binding of antibody to recombinant Vitreoscilla hemoglobin (VHb) antigen expressed in Escherichia coli cells by ELISA and the results were confirmed by quantitative immunogold labeling transmission electron microscopy (TEM). Ability to minimize NSB was also evaluated by dot-blot and Western blotting methods. The results demonstrated that ELISA was most accurate in predicting the most efficient blocking agent for TEM. Existing methods could not provide an accurate picture of the ability of various reagents to suppress background labeling. The sensitivity of detection of antigens by immunoelectron microscopy depends on the assay procedure being optimized to obtain the highest possible signal along with as low a background (noise) as possible. Our study indicated that an ELISA-based evaluation of various blocking agents could help in the rapid selection and optimization of a suitable protocol for immunogold localization and quantification of antigens by TEM.     

Colloidal gold probes in immunocytochemistry. An optimization of their application in light microscopy by use of silver intensification procedures

Colloidal gold particles are the markers of choice for ultrastructural localization of antigens. By reducing gold chloride with tannic acid and trisodium citrate, a broad range of narrowly determined particle sizes can be obtained. Such particles can easily be coupled to a number of proteins and the resulting conjugates are conveniently purified on a gel-chromatography column. Their application in light microscopy requires an amplification step with a silver physical developer. Silver-intensified colloidal gold probes can advantageously be used for immunostaining of cryostat, paraffin and plastic sections. Moreover, permeabilized cultured cells and whole-mount preparations can also be stained with gold-silver techniques. Silver intensification does not affect reactivity of a number of tissue antigens, thus permitting double staining combinations with immunoperoxidase or immunofluorescence methods.     

Immunogold electron microscopy of surface antigens of oral bacteria

Colloidal gold particles 3-6 nm in diameter were prepared and stabilized with the IgG fraction of polyspecific rabbit antisera produced against four different oral bacteria. The immunogold markers were used in homologous reactions to label the bacteria in a preembedding procedure for electron microscopy. An indirect immunofluorescence procedure was concurrently used to optimize the labelling conditions before observation with the electron microscope. The immunogold markers labelled fibrillar structures extending outward 50-275 nm from the Gram-positive cell envelopes and a fuzzy 5-10 nm thick capsulelike layer on the outer aspect of Bacteroides gingivalis. The immunogold method appears to be a simple, rapid, and inexpensive procedure suitable for the study of bacterial surface antigens and can be upgraded with the use of monospecific antibodies.     

Suitability of Different Protein A-Gold Markers for Immunogold-Silver Staining in Paraffin Sections

An incubation protocol to immunolabel Lowicryl semithin sections was applied to paraffin probes. To improve the labeling density, colloidal gold complexes of different preparations and sizes were compared. The type of colloidal gold preparation used was found to affect the specificity of the immunostaining. Gold colloid of 5 nm diameter particle size prepared with white phosphorus minimized nonspecific background labeling of β-casein in paraffin embedded sections of the mammary epithelium of pregnant mice. Gold colloids of 5 nm and 9 nm diameter particle size prepared in varying concentrations of tannic acid generated significant nonspecific staining in similar tissue preparations.     

Suitability of Different Protein A-Gold Markers for Immunogold-Silver Staining in Paraffin Sections

An incubation protocol to immunolabel Lowicryl semithin sections was applied to paraffin probes. To improve the labeling density, colloidal gold complexes of different preparations and sizes were compared. The type of colloidal gold preparation used was found to affect the specificity of the immunostaining. Gold colloid of 5 nm diameter particle size prepared with white phosphorus minimized nonspecific background labeling of β-casein in paraffin embedded sections of the mammary epithelium of pregnant mice. Gold colloids of 5 nm and 9 nm diameter particle size prepared in varying concentrations of tannic acid generated significant nonspecific staining in similar tissue preparations.     

An investigation of optimal gold particle size for immunohistological immunogold and immunogold-silver staining to be viewed by polarized incident light (EPI polarization) microscopy

We investigated the optimal gold particle size for use with polarized incident light (epi polarization) microscopy with immunogold immunohistological preparation in both immunogold indirect (IGS) and silver-enhanced immunogold-silver staining (IGSS) techniques. A range of gold particle sizes from 5 nm-40 nm was used along with tissue of known immunoreactivity with a well-characterized primary monoclonal antibody. Checkerboard titrations were carried out for each technique and for each particle size. The preparations were viewed using a standard polarized incident light microscope and assessed in a semi-quantitative manner. Adequate visualization of gold particles was achieved using the indirect staining method only with a particle size of 40 nm. With silver enhancement (IGSS), particles of all sizes were clearly seen. However, 5-nm particles were considered optimal for this method because of reduced background staining, high titration of antisera possible, and crisp localization of the visual signal.     

Light microscopical detection of leukocyte cell surface antigens with a one-nanometer gold probe.

The potential of ultrasmall gold particles for the light microscopical detection of leukocyte cell surface differentiation antigens was investigated. Suspensions and cytocentrifuge preparations of peripheral blood leukocytes were first incubated with monoclonal antibodies and then with goat antimouse antibodies coupled to colloidal gold particles of 1-nanometer diameter. Cytocentrifuge preparations were made from the cell suspensions. Silver enhancement was performed on all preparations. Then they were counterstained with May-Grünwald Giemsa and examined in light microscopy. The immunostaining appeared as fine dark granules on the surface membrane of the cells. Labeling conditions were determined which gave a dense specific immunostaining and a low background. High dilutions of the ultrasmall gold probe could be used to detect all antigen expressing cells in the samples. The labeling efficiency of the IGSS method with the 1 nanometer probe was comparable to that described earlier for 5 nanometer gold particles. Lymphocyte subsets enumerated with this method in normal peripheral blood were similar to those found with immunofluorescence microscopy. We concluded that one nanometer probes do not offer a major advantage in comparison with 5 nanometer probes for the study of cell surface antigens.     

Light microscopical detection of leukocyte cell surface antigens with a one-nanometer gold probe

Summary The potential of ultrasmall gold particles for the light microscopical detection of leukocyte cell surface differentiation antigens was investigated. Suspensions and cytocentrifuge preparations of peripheral blood leukocytes were first incubated with monoclonal antibodies and then with goat antimouse antibodies coupled to colloidal gold particles of 1-nanometer diameter. Cytocentrifuge preparations were made from the cell suspensions. Silver enhancement was performed on all preparations. Then they were counterstained with May-Grünwald Giemsa and examined in light microscopy. The immunostaining appeared as fine dark granules on the surface membrane of the cells. Labeling conditions were determined which gave a dense specific immunostaining and a low background. High dilutions of the ultrasmall gold probe could be used to detect all antigen expressing cells in the samples. The labeling efficiency of the IGSS method with the 1 nanometer probe was comparable to that described earlier for 5 nanometer gold particles. Lymphocyte subsets enumerated with this method in normal peripheral blood were similar to those found with immunofluorescence microscopy. We concluded that one nanometer probes do not offer a major advantage in comparison with 5 nanometer probes for the study of cell surface antigens.     

Detection of gold probes with video-enhanced contrast microscopy: nanovid microscopy

This contribution describes a new microscopic technique aimed at visualizing colloidal gold particles of 20-40 nm diameter as dynamic markers at the light microscopic level. The technique, called nanovid microscopy, is based on the use of contrast enhancement by video techniques and digital image processing. Two applications on living cells are discussed. The first is the receptor-mediated endocytosis of the transferrin receptor, which for the first time can be followed in real time. A second application documents the motion of cell-surface glycoproteins on PTK2-cells. In addition, nanovid microscopy allows the number of gold particles in immunogold staining to be quantified easily. With this tool, the translocation of specific molecules in living cells becomes one of the most enjoyable activities to observe.     

New sensitive light microscopical detection of colloidal gold on ultrathin sections by reflection contrast microscopy. Combination of reflection contrast and electron microscopy in post-embedding immunogold histochemistry.

One simple post-embedding method for combined light- and electron microscopy is presented. Different types of antigens in normal rat and mouse kidneys as well as in tissues from cases of experimental induced nephritis were stained after Lowicryl K4M embedding by an immunogold (silver) method. The (silver-enhanced) gold particles were visualized by light microscopy, e.g. bright-field (BFM)- and reflection contrast (RCM) microscopy, as well as by electron microscopy. The potentials of RCM visualization in this field were investigated, resulting in the successful detection of colloidal gold (15 nm) particles, or silver enhanced gold particles, on ultrathin sections. Furthermore, an increased detection sensitivity of RCM compared with BFM together with an increase in the sensitivity of the immunostaining by RCM visualization was found. The different ways to use RCM, alone or in combination with bright-field- or phase contrast microscopy for visualization of plastic sections varying in thickness, type of plastic and staining, are discussed.     

Immunogold staining method for the light microscopic detection of leukocyte cell surface antigens with monoclonal antibodies: its application to the enumeration of lymphocyte subpopulations

Colloidal gold was used as a marker for the light microscopic detection of lymphocyte cell surface antigens with monoclonal antibodies. Suspensions of peripheral blood leukocytes were first incubated with monoclonal mouse antibodies and then with colloidal gold-labeled goat anti-mouse antibodies. The cells were fixed and cytocentrifuge preparations or smears were made. Granulocytes and monocytes were then labeled by the cytochemical staining of their endogenous peroxidase activity. Lymphocytes reacting with the monoclonal antibody had numerous dark granules around the surface membrane. With electron microscopy, these granules appeared as patches of gold particles. This immunogold staining method proved to be a reliable tool for the enumeration of T-lymphocyte subpopulations in peripheral blood. The results were almost identical to those obtained with immunofluorescence microscopy. The procedure can also be applied on small volumes of capillary blood. This constitutes a good microtechnique for the determination of lymphocyte subsets in children.     

Ultrastructural study of protein kinase C-betaII localization during the cell cycle of human glioma cells

Transmission electron microscopy and immunogold labeling were used to determine how PKC-betaII is localized at stages in the cell cycle of the human glioma cell line U-373MG. Results show that immunogold particles in both dimethylsulfoxide (DMSO) and calphostin C (0.5 microM)-treated cells were mainly located in the cytoplasm. The concentration of gold particles in the nucleus was relatively small and constant throughout the cell cycle of both DMSO and calphostin C treated cells. Micrographs revealed changes in PKC-betaII during the cell cycle. The concentration of gold particles in the DMSO-treated cells was constant until 8 h. Subsequently, cytoplasmic PKC-betaII oscillated with an increased at 10 h, a rapid decrease at 12 h, and a rise at 14 h. The concentration of the gold particles then gradually decreased. In contrast, immunogold labeling in calphostin C-treated cells increased gradually up to 10 h. Subsequently, the pattern of PKC-betaII labeling in calphostin C-treated cells recapitulated those of control cells as seen by the rapid decline of PKC-betaII labeling at 12 h and its re-accumulation at 14 h. Additionally, there was a rapid increase at 20 h. Western blots of PKC-betaII showed constant PKC-betaII immunoreactivity throughout the cell cycle. In comparison to Western blots, in-situ immunogold labeling revealed changes in PKC-II immunoreactivity at 10 h and 14 h. This technique may represent intracellular immunoreactivity of PKC-betaII. The results from the immunogold labeling technique suggest that binding of calphostin C to the regulatory domain of PKC-betaII provokes a conformation change in PKC-betaII, preventing its activation and degradation.