A comparison of methods for heat-mediated antigen retrieval for immunoelectron microscopy: demonstration of cytokeratin No. 18 in normal and neoplastic hepatocytes

Post-embedding Mammalian Tissue for Immunoelectron Microscopy: A Standardized Procedure Based on Heat-Induced Antigen Retrieval

We describe a standardized method of fixation, antigen retrieval, and image contrasting for post-embedding immunoelectron microscopy. Tissues are fixed with formaldehyde solutions containing Ca(2+) and Mg(2+) ions at pH 7.4 and then at pH 8.5. After dehydration with dimethylformamide, the specimens are embedded in LR-White resin. For antigen retrieval, ultrathin sections are heated in 0.5 M Tris-HCl, pH 9.0, for 1-2 h at 95 °C. After immunogold labeling, the sections are treated with a mixture of tannic acid and glutaraldehyde, with OsO4 solution, and then double-stained with uranyl acetate and lead citrate. The standardized method yields strong and reproducible immunoreactions for many antigens showing excellent image contrast without destruction of fine structures.

Post-embedding Double-labeling of Antigen-retrieved Ultrathin Sections Using a Silver Enhancement-controlled Sequential Immunogold (SECSI) Technique

In electron microscopy, the post-embedding immunogold technique provides a high degree of resolution and the possibility of quantitation owing to the intrinsic characteristics of the colloidal gold marker. Application of this technique to the subcellular localization of multiple antigens by differential labeling using gold markers of different sizes, or to double labeling using the same primary antibody isotype with serial silver enhancement, has been reported. We have incorporated this double labeling technique into a modified procedure that produces excellent labeling and ultrastructural preservation, even after exposure of ultrathin sections large enough to cover a 300-μm-diameter single-hole grid to hot antigen retrieval solutions and prolonged labeling protocols.

Ultrastructural analysis of testicular tissue and sperm by transmission and scanning electron microscopy

Transmission electron microscopy (TEM) studies have provided the basis for an in-depth understanding of the cell biology and normal functioning of the testis and male gametes and have opened the way to characterize the functional role played by specific organelles in spermatogenesis and sperm function. The development of the scanning electron microscope (SEM) extended these boundaries to the recognition of cell and organ surface features and the architectural array of cells and tissues. The merging of immunocytochemical and histochemical approaches with electron microscopy has completed a series of technical improvements that integrate structural and functional features to provide a broad understanding of cell biology in health and disease. With these advances the detailed study of the intricate structural and molecular organization as well as the chemical composition of cellular organelles is now possible. Immunocytochemistry is used to identify proteins or other components and localize them in specific cells or organelles with high specificity and sensitivity, and histochemistry can be used to understand their function (i.e., enzyme activity). When these techniques are used in conjunction with electron microscopy their resolving power is further increased to subcellular levels. In the present chapter we will describe in detail various ultrastructural techniques that are now available for basic or translational research in reproductive biology and reproductive medicine. These include TEM, ultrastructural immunocytochemistry, ultrastructural histochemistry, and SEM.

Energy filtering transmission electron microscopy immunocytochemistry and antigen retrieval of surface layer proteins from Tannerella forsythensis using microwave or autoclave heating with citraconic anhydride

Tannerella forsythensis (Bacteroides forsythus), an anaerobic Gram-negative species of bacteria that plays a role in the progression of periodontal disease, has a unique bacterial protein profile. It is characterized by two unique protein bands with molecular weights of more than 200 kDa. It also is known to have a typical surface layer (S-layer) consisting of regularly arrayed subunits outside the outer membrane. We examined the relationship between high molecular weight proteins and the S-layer using electron microscopic immunolabeling with chemical fixation and an antigen retrieval procedure consisting of heating in a microwave oven or autoclave with citraconic anhydride. Immunogold particles were localized clearly at the outermost cell surface. We also used energy-filtering transmission electron microscopy (EFTEM) to visualize 3, 3'-diaminobenzidine tetrahydrochloride (DAB) reaction products after microwave antigen retrieval with 1% citraconic anhydride. The three-window method for electron spectroscopic images (ESI) of nitrogen by the EFTEM reflected the presence of moieties demonstrated by the DAB reaction with horseradish peroxidase (HRP)-conjugated secondary antibodies instead of immunogold particles. The mapping patterns of net nitrogen were restricted to the outermost cell surface.

The post-embedding method for immunoelectron microscopy of mammalian tissues: a standardized procedure based on heat-induced antigen retrieval

We describe a standardized method of fixation, antigen retrieval, and image contrasting for post-embedding immunoelectron microscopy. Tissues are fixed with formaldehyde solutions containing Ca(2+) and Mg(2+) ions at pH 7.4 and then at pH 8.5. After dehydration with dimethylformamide, the specimens are embedded in LR-White resin. For antigen retrieval, ultrathin sections are heated in 20 mM Tris-HCl buffer (pH 9.0) for 1 h at 95 degrees C. After immunogold labeling, the sections are treated with a mixture of tannic acid and glutaraldehyde, with OsO(4) solution, and then double-stained with uranyl acetate and lead citrate. The standardized method yields strong and reproducible immunoreactions for many antigens showing excellent image contrast without destruction of fine structures.

Establishment of a standardized post-embedding method for immunoelectron microscopy by applying heat-induced antigen retrieval

We have developed a new standardized method for the post-embedding immunoelectron microscopy using the same fixation, antigen retrieval and image contrasting procedures. Tissues were fixed with 4% formaldehyde containing 2.5 mM CaCl(2), 1.25 mM MgCl(2) in a 0.1 M 4-(2-hydroxyethyl)-piperazineethanesulfonic acid (HEPES) buffer (pH 7.4) for 2 h and then with the same fixative composition in 0.1 M HEPES buffer (pH 8.5) overnight at room temperature. Vehicle osmolarity of fixatives was adjusted to 300-330 mOsm by adding glucose. The specimens were dehydrated with dimethylformamide on ice and embedded in LR-White resin. Ultrathin sections were heated in a 20 mM Tris-HCl buffer (pH 9.0) for 1-2 h at 95 degrees C. After immuno-gold labeling, the sections were treated with 2% glutaraldehyde containing 0.05% tannic acid in a 0.1 M phosphate buffer (pH 5.5) for 5 min and with a 1% OsO(4)/0.1 M phosphate buffer (pH 7.4) for 5 min, and then they were double stained with uranyl acetate and lead citrate. The standardized method yielded strong and reproducible immunoreactions for soluble, membrane-bound and filamentous proteins showing an excellent image contrast without destruction of the fine structures.

State of the art in antigen retrieval for immunohistochemistry

The masking effects of antigens by chemical fixation, processing, embedding media interactions, represent a serious problem for immunohistochemical purposes. Fortunately, different approaches in antigen retrieval exist. These techniques are relatively recent and continuously expanding. This review focuses on the present state of the art in antigen retrieval methods for immunohistochemistry in light and electron microscopy. Moreover, a brief discussion on the chemical aspects of fixation, mechanism of retrieval, as well as its efficacy, is given.

Heat-induced antigen retrieval: mechanisms and application to histochemistry

Since the introduction of the fluorescence-labeled antibody method by Coons et al. [Immunological properties of antibody containing a fluorescent group. Proc Soc Exp Biol Med 47, 200-2002], many immunohistochemical methods have been refined to obtain high sensitivity with low background staining at both light and electron microscopic levels. Heat-induced antigen retrieval (HIAR) reported by Shi et al. in the early 1990s has greatly contributed to immunohistochemical analysis for formalin-fixed and paraffin-embedded (FFPE) materials, particularly in the field of pathology. Although antigen retrieval techniques including enzyme digestion, treatment with protein denaturants and heating have been considered tricky and mysterious techniques, the mechanisms of HIAR have been rapidly elucidated. Heating cleaves crosslinks (methylene bridges) and add methylol groups in formaldehyde-fixed proteins and nucleic acids and extends polypeptides to unmask epitopes hidden in the inner portion of antigens or covered by adjacent macromolecules. In buffers having an appropriate pH and ion concentration, epitopes are exposed without entangling the extended polypeptides during cooling process, since polypeptides may strike a balance between hydrophobic attraction force and electrostatic repulsion force. Recent studies have demonstrated that HIAR is applicable for immunohistochemistry with various kinds of specimens, i.e., FFPE materials, frozen sections, plastic-embedded specimens, and physically fixed tissues at both the light- and electron-microscopic levels, and have suggested that the mechanism of HIAR is common to aldehyde-fixed and aldehyde-unfixed materials. Furthermore, heating has been shown to be effective for flow cytometry, nucleic acid histochemistry (fluorescein in situ hybridization (FISH), in situ hybridization (ISH), and terminal deoxynucleotidyl transferase-mediated nick labeling (TUNEL)), and extraction and analysis of macromolecules in both FFPE archive materials and specimens processed by other procedures. In this article, we review mechanism of HIAR and application of heating in both immunohistochemistry and other histochemical reactions.

A comparison of microwave heating and proteolytic pretreatment antigen retrieval techniques in formalin fixed, paraffin embedded tissues

Antigen retrieval (AR) is a technique that re-exposes epitopes in formalin fixed, paraffin embedded sections and makes them detectable by immunohistochemistry. We compared the effects of two AR procedures, enzyme digestion and microwave heating, on immunostaining of vimentin and desmin in formalin fixed, paraffin embedded tissues. Our results showed that AR is necessary for vimentin and desmin immunostaining in tissues fixed in formalin for more than 48 h. With prolonged fixation times, microwave heating showed better results than enzyme digestion for AR. The same results were obtained using 1% zinc sulfate or Citra Plus solution as retrieval solutions for microwave heating. We recommend microwave heating for AR, because it is easier to use and produces better results compared to enzyme treatment.

An antigen retrieval method using an alkaline solution allows immunoelectron microscopic identification of secretory granules in conventional epoxy-embedded tissue sections

Immunoelectron microscopy using chromogranin A-specific antibodies has been proposed as an efficient technique for identification of secretory granules (SGs) in tumor cells with evidence of apparent neuroendocrine differentiation. Using an antigen retrieval (AR) method, we succeeded in immunolabeling SGs with antibodies in ultrathin sections of routinely processed epoxy-embedded blocks of tissue. Samples of an insulinoma were fixed in 2% glutaraldehyde, postfixed in 1% OsO(4), and embedded in epoxy resin. Ultrathin sections were immunostained with chromogranin A-specific antibodies and gold-conjugated second antibodies. There was no significant labeling in the absence of AR. Neither etching with sodium metaperiodate nor microwave irradiation of ultrathin sections in citrate buffer (pH 6.0) or in EDTA buffer (pH 8.0) was effective in improving the efficiency of immunolabeling. However, ultrathin epoxy-embedded sections that were microwaved in alkaline solution (pH 10) were adequately labeled (5.2 +/- 0.34 particles per SG). Moreover, considerably improved efficiency of immunostaining was achieved by microwaving sections in alkaline solution (pH 10) with subsequent immunostaining at 60C (12.2 +/- 0.51 particles per SG). This method can also be applied to epoxy-embedded sections obtained from formalin-fixed, paraffin-embedded blocks of tissue and was even valid for an old epoxy-embedded block of tissue prepared 15 years previously.

Biochemical and anatomical evidence for specialized voltage-dependent calcium channel gamma isoform expression in the epileptic and ataxic mouse, stargazer

Inherited forms of ataxia and absence seizures in mice have been linked to defects in voltage-dependent calcium channel subunits. However, a correlation between the sites of neuronal dysfunction and the impact of the primary lesion upon calcium channel subunit expression or function has not been clearly established. For example, the mutation in stargazer mice has pleiotropic consequences including synaptic alterations in cerebellar granule cells, hippocampal CA3/mossy fibers, and cortical neurons in layer V that, presumably, lead to ataxia and seizures. Genetic analysis of stargazer mice determined that the defective gene encodes a protein expressed in brain (gamma2) with limited homology to the skeletal muscle L-type calcium channel gamma1 subunit. Although additional gamma isoforms have been subsequently identified primarily in neural tissue, little was known about the proteins they encode. Therefore, this study explored the distribution and biochemical properties of gamma2 and other gamma isoforms in wild-type and stargazer brain. We cloned human gamma2, gamma3, and gamma4 isoforms, produced specific anti-peptide antibodies to gamma isoforms and characterized both heterologously expressed and endogenous gamma. We identified regional specificity in the expression of gamma isoforms by western analysis and immunohistochemistry. We report for the first time that the mutation in the stargazer gene resulted in the loss of gamma2 protein. Furthermore, no compensatory changes in the expression of gamma3 or gamma4 protein were evident in stargazer brain. In contrast to other voltage-dependent calcium channel subunits, gamma immunostaining was striking in that it was primarily detected in regions highly enriched in excitatory glutamatergic synapses and faintly detected in cell bodies, suggesting a role for gamma in synaptic functions. Sites of known synaptic dysfunction in stargazer (the hippocampal CA3 region, dentate gyrus, and cerebellar molecular layer) were revealed as relying primarily upon gamma2, as total gamma isoform expression was dramatically decreased in these regions. Electron microscopy localized anti-gamma antibody immunostaining to dendritic structures of hippocampal mossy fiber synapses, with enrichment at postsynaptic densities. To assess the association of native gamma with voltage-dependent calcium channel or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits, gamma isoforms (gamma2, gamma3 and gamma4) were detergent solubilized from mouse forebrain. Antibodies against a highly conserved C-terminal epitope present in gamma2, gamma3 and gamma4 immunoprecipitated voltage-dependent calcium channel subunits (alpha1B), providing the first in vivo evidence that gamma and voltage-dependent calcium channels form stable complexes. Furthermore, both anti-gamma2 antibodies and anti-alpha1B antibodies independently immunoprecipitated the AMPA receptor subunit, GluR1, from mouse forebrain homogenates. In summary, loss of gamma2 immunoreactivity in stargazer is precisely localized so as to contribute to previously characterized synaptic defects. The data in this paper provide compelling evidence that gamma isoforms form complexes in vivo with voltage-dependent calcium channels as well as AMPA receptors, are selectively and differentially expressed in neuronal processes, and localize primarily to dendritic structures in the hippocampal mossy fiber region.

Amplification methods for the immunolocalization of rare molecules in cells and tissues

The needs to precisely assign macromolecules to specific locations and domains within tissues and cells and to reveal antigens which are present in low or even in trace amounts, led to the elaboration of a wide spectrum of immunocytochemical amplification procedures. These arise from the successive improvements of tissue preparation techniques, of antigen retrieval procedures and of immunological or non-immunological detection systems. Improvement of detection systems may be the most active in the development of amplification techniques. Since the early work of Coons, in which by the introduction of the indirect technique has started amplifying the signal, different systems have succeeded in increasing the sensitivity of antigens detection. Indeed, amplification techniques such as the multiple antibody layers, the multiple bridges, the enzyme complexes, the avidin-biotin, the silver intensification, and the numerous variations and combinations among these have increased the sensitivity for the detection of scarce tissue antigens. However, as shown by the recent progress carried out with new approaches such as the catalyzed reporter deposition (CARD) and the enhanced polymer one-step staining (EPOS), more efficient methods are still needed. In electron microscopy, few techniques have reached the resolution afforded by the post-embedding immunogold approach. In spite of this and in order to further increase its sensitivity, new probes and novel approaches are allowing combination of the gold marker with the amplification capacity of enzymes afforded by the CARD technique. Immunogold amplification strategies, such as the multiple incubations with the primary antibody and the use of an anti-protein A antibody have also led to enhanced signals displaying the advantages in terms of resolution and possibilities of quantification inherent to the colloidal gold marker.

An evaluation of antigen retrieval procedures for immunoelectron microscopic classification of amyloid deposits

The advantages of using immunoelectron microscopy in amyloid research and surgical pathology for the classification of amyloid deposits are well documented. The aim of this study was to improve single-labeling postembedding immunostaining by testing different antigen retrieval (AR) techniques. Etching and AR procedures were applied to sections from aldehyde-fixed and Epon-embedded autopsy specimens of patients who had suffered from generalized AA amyloidosis, systemic senile ATTR amyloidosis, or generalized kappa-light chain amyloidosis. The procedures used were no AR, H(2)O(2), saturated aqueous sodium metaperiodate (mPJ), heating in deionized water (dH(2)O), heating in sodium citrate buffer (SCB), heating in EDTA (each 91C, 30 min), and combinations of etching and heating. Little effect was evident after treatment with H(2)O(2), mPJ, and heating in dH(2)O, but the signal density markedly increased after heating in 1 mM EDTA. Heating in SCB affected immunolabeling with anti-transthyretin and anti-kappa-light chain, whereas no effect was achieved for immunolabeling with anti-AA amyloid. We concluded that AR may significantly improve immunostaining of specimens that have undergone conventional fixation and embedding procedures for electron microscopy. The effect of AR on the detection of amyloid fibril proteins was probably mediated in part through chelation or binding of metal ions by the AR medium. (J Histochem Cytochem 47:1385-1394, 1999)

Antigen retrieval of loricrin epitopes at desmosomal areas of cornified cell envelopes: an immunoelectron microscopic analysis

Cell envelopes (CEs) are insoluble, chemically and mechanically tough structures formed during terminal differentiation of keratinocytes, providing skin with a protective barrier against the environment. They are 15 to 20 nm thick structures beneath the plasma membrane and continuous with desmosomal attachment plaques. Sequential deposition of several proteins including involucrin and loricrin leads to a gradual increase in envelope thickness and rigidity. Cross-linking of desmosomal components to other CE-proteins has been demonstrated and desmosomes in the cornified cells have been regarded as a part of CEs. Our previous immunoelectron microscopy studies showed that desmosomal areas of granular cells were loricrin-positive, but those in cornified cells were negative. We asked whether this is due to epitope masking and applied trypsin digestion of the electron microscopy sections to retrieve the possibly masked epitopes. Since this treatment made desmosomal structures obscure, one side of the sections was stained with anti-desmoglein antibody as an indicator of desmosomes. Trypsin was applied on the other side followed by immunolabeling with anti-loricrin antibody. Trypsin digestion indeed unmasked the loricrin epitopes in the desmoglein-positive desmosomal areas of CEs. It seems therefore that loricrin is first accumulated at the desmosomes before the CE-assembly and cross-linking of loricrin occurs at the desmosomal areas of CEs as well as at the non-desmosomal areas.

Fixative-dependent increase in immunogold labeling following antigen retrieval on acrylic and epoxy sections

We examined the increase in immunogold labeling of variably fixed, resin embedded tissue sections following antigen retrieval by heating in citrate solution. Fibrin clots and porcine renal tissue were fixed in glutaraldehyde, paraformaldehyde or ethanol, and specimens were embedded in LR-White or epoxy resin. Immunogold labeling was performed on ultra-thin sections with anti-fibrinogen for the fibrin clots and anti-IgG for the porcine renal tissue. Immunogold labeling increased greatly after heating epoxy sections regardless of the fixative used. The ratio labeling(retrieved)/labeling(nonretrieved) (Lr/Ln) was 2.8 or higher, and the largest increases were obtained for anti-IgG. Heating induced a large increase of immunolabeling for LR-White sections only when the specimens had been fixed in paraformaldehyde (Lr/Ln = 2.2 for anti-IgG and 1.4 for antifibrinogen). LR-White sections showed decreased, insignificant or weakly increased immunolabeling of ethanol or glutaraldehyde fixed tissues following antigen retrieval. Disruption of aldehyde cross-links is not the only mechanism for antigen retrieval when epoxy sections are heated in citrate solution since large increases in immunolabeling were obtained on ethanol fixed tissue. The large heat-induced increases in immunolabeling on epoxy sections are probably caused by the disruption of chemical bonds between the epoxy resin and side groups of proteins.