Colloidal gold labeling of intracellular ligands in dorsal root sensory neurons, visualized by scanning electron microscopy

High resolution helium ion scanning microscopy of the rat kidney

Helium ion scanning microscopy is a novel imaging technology with the potential to provide sub-nanometer resolution images of uncoated biological tissues. So far, however, it has been used mainly in materials science applications. Here, we took advantage of helium ion microscopy to explore the epithelium of the rat kidney with unsurpassed image quality and detail. In addition, we evaluated different tissue preparation methods for their ability to preserve tissue architecture. We found that high contrast, high resolution imaging of the renal tubule surface is possible with a relatively simple processing procedure that consists of transcardial perfusion with aldehyde fixatives, vibratome tissue sectioning, tissue dehydration with graded methanol solutions and careful critical point drying. Coupled with the helium ion system, fine details such as membrane texture and membranous nanoprojections on the glomerular podocytes were visualized, and pores within the filtration slit diaphragm could be seen in much greater detail than in previous scanning EM studies. In the collecting duct, the extensive and striking apical microplicae of the intercalated cells were imaged without the shrunken or distorted appearance that is typical with conventional sample processing and scanning electron microscopy. Membrane depressions visible on principal cells suggest possible endo- or exocytotic events, and central cilia on these cells were imaged with remarkable preservation and clarity. We also demonstrate the use of colloidal gold probes for highlighting specific cell-surface proteins and find that 15 nm gold labels are practical and easily distinguishable, indicating that external labels of various sizes can be used to detect multiple targets in the same tissue. We conclude that this technology represents a technical breakthrough in imaging the topographical ultrastructure of animal tissues. Its use in future studies should allow the study of fine cellular details and provide significant advances in our understanding of cell surface structures and membrane organization.

Distribution of immunoreactivity for cytoskeletal (microtubule, microtubule-associated, and neurofilament) proteins in adult human dorsal root ganglia

BACKGROUND

The cytoskeleton of mature neurons consists of three main types of filamentous structures: microtubules (or neurotubules) neurofilaments and microfilaments, and of the so-called associated proteins. Neurotubules are formed by alpha- and beta-tubulin; neurofilaments are comprised of three protein subunits (68, 160, and 200 kDa of molecular weight), referred to here as neurofilament proteins (NFPs). The microtubule-associated proteins (MAPs) and tau-proteins form cross bridges between microtubules and other cytoskeletal constituents, as well as cellular organelles. This study analyzes the distribution of several cytoskeletal proteins in adult human dorsal root ganglia (DRG).

METHODS

Sections of formaldehyde-fixed, paraffin-embedded adult human DRG were processed for PAP immunohistochemistry. Mouse monoclonal antibodies against specific epitopes of alpha- and beta-tubulin, MAP-1, MAP-2, MAP-5, tau-protein, and NFPs (68, 160, and 200 kDa) were used. Furthermore, a quantitative image analysis (optic microdensitometry) was performed to establish the relationship between neuronal size and intensity of immunostaining.

RESULTS

Most of DRG neuron cell bodies displayed immunoreactivity for all assessed antibodies, with the exception of MAP2, which was absent. Nevertheless, the neuronal perikarya showed an heterogeneous pattern of immunoreactivity, which was not related to neuronal profile size. Positive immunolabelling was also observed in satellite cells and Schwann cells for microtubule and MAP1 proteins, and for tau-protein in Schwann cells.

CONCLUSIONS

Adult human primary sensory neurons in DRG express immunoreactivity for neurotubule and neurofilament proteins, as well as for some microtubule-associated proteins. However, since large heterogeneity was observed in the expression of those proteins, we conclude that the expression of cytoskeletal proteins is not a criterion to establish DRG neuronal subtypes.

Advantages of digitonin extraction to reveal the intracellular structure of rat glomerular podocytes for high-resolution scanning electron microscopy

Kidneys of anesthetized rats were perfused with digitonin to extract cytosolic proteins of glomerular podocytes so that the remaining intracellular structures could be examined by three-dimensional stereo high-resolution scanning electron microscopy (HRSEM). Cytoskeleton, consisting of microtubules and intermediate filaments, was preserved with each applied concentration of digitonin. High concentrations of digitonin (1.0 mg/ml) produced a corrugated appearance in plasma membranes likely due to the formation of digitonin-cholesterol complexes. At 1.0 mg/ml digitonin, the Golgi complex became vesicularized, and mitochondria were well extracted and their ultrastructure preserved. Lower concentrations of digitonin (0.1 and 0.2 mg/ml) were less disruptive to both the plasma membrane and the Golgi complex. Mitochondria, rough endoplasmic reticulum, coated vesicles, nuclear membrane, and chromatin were well preserved. Extraction with digitonin, at the optimal concentration and perfusion time, simultaneously maintains both the cytoskeleton and membranous organelles inside the cell and provides a method to elucidate the interactions between these two components. Furthermore, digitonin extraction should preserve antigenic sites, thereby allowing the localization of intracellular proteins by backscattered electron imaging of immunogold labels in the scanning electron microscope.

Silver-enhanced colloidal-gold labelling of rabbit kidney collecting-duct cell surfaces imaged by scanning electron microscopy

The luminal cell surfaces of rabbit kidney cortical collecting-duct cells were labelled with peanut lectin (PNA) and investigated by scanning electron microscopy. Labelling was performed either on 20-microns-thick cryostat sections from prefixed and cryoprotected rabbit kidney tissue or on cultured collecting-duct epithelium using biotinylated PNA and a 6-nm colloidal-gold-coupled antibody against biotin. Colloidal-gold labels were detected at low magnification (2000-4000x) using silver enhancement. Coating with chromium allowed simultaneous imaging of both cell-surface morphology and labelling topography in the backscattered electron imaging mode. Our results show that PNA binding is specific for a subtype of intercalated cells equipped with microvilli on the luminal surface. The presented method promises to be useful for the identification of specific cell types in heterogeneous tissues.

Distribution of snRNPs, splicing factor SC-35 and actin in interphase nuclei: immunocytochemical evidence for differential distribution during changes in functional states

Small nuclear ribonucleoproteins (snRNPs) play an integral role in the processing of pre-mRNA in eukaryotic nuclei. snRNPs often occur in a speckled intranuclear distribution, together with the non-snRNP splicing factor SC-35. snRNPs have also been shown to be associated with actin in the nuclear matrix, suggesting that both actin and snRNPs may be involved in the processing and transport of transcripts. The work reported here was undertaken to compare the spatial relationship of snRNPs, SC-35, and intranuclear actin in neuronal and non-neuronal cell types. In undifferentiated PC12 cells and in non-neuronal cells growing in association with dorsal root ganglion neurons, confocal immunocytochemistry revealed a typical, speckled distribution of snRNP aggregates, which colocalized with the SC-35 splicing factor. In contrast, a unique snRNP distribution was observed in dorsal root ganglion neurons in vitro and in PC12 cells differentiated by nerve growth factor. In nuclei of these cells, snRNPs were predominantly located at the periphery where they formed a spherical shell apposed to the nuclear envelope. Ultrastructural immunogold labelling of snRNPs in dorsal root ganglion neurons in vitro confirmed this distribution. In contrast, SC-35 remained distributed in a speckled pattern throughout nuclei of dorsal root ganglion neurons and PC12 cells, even in cases where snRNPs were almost exclusively positioned at the nuclear periphery. In non-neuronal cells in dorsal root ganglion cultures and in undifferentiated PC12 cells, snRNP aggregates were frequently associated with actin aggregates, as determined by Nearest Neighbor Analyses. In PC12 cells, this spatial relationship was altered during nerve growth factor-induced differentiation, prior to the time at which these cells showed morphological evidence of differentiation. Specifically, Nearest Neighbor Analyses between snRNP and actin aggregates in PC12 cells exposed to nerve growth factor for 4 hours revealed that snRNP and actin aggregates exhibited a closer association than in undifferentiated cells. These results suggest that sites of pre-mRNA processing and transcription may differ between cell types, and that the functions of snRNPs and actin within interphase nuclei may be related. The results also indicate that the distribution of snRNPs is dynamic and that it may depend upon the functional state of the cell as well as upon its state of differentiation.

Germin isoforms are discrete temporal markers of wheat development. Pseudogermin is a uniquely thermostable water-soluble oligomeric protein in ungerminated embryos and like germin in germinated embryos, it is incorporated into cell walls

Nascent synthesis and accumulation of germin and its mRNA mark the onset of renewed growth when wheat embryos are germinated in water. Germin is a water-soluble, pepsin-resistant protein that is not found in immature embryos, or in mature embryos before their germination. An antiserum was raised by injecting rabbits with germin that was freed of other proteins by pepsinization and gel filtration. The antiserum has been used to detect, in extracts of mature embryos from dry, ungerminated wheat grains, a protein that is antigenically related to germin. The antigenically related protein has been named pseudogermin. Pseudogermin accumulates, maximally, between 20-25-days postanthesis, then declines appreciably in amount by 30-days postanthesis, in soluble extracts of immature embryos from several wheat varieties. The antiserum was also used to identify germin and pseudogermin among the proteins extracted from cell walls and to bind immunogold to cell walls preparatory to visualizing freeze-cleaved embryos by scanning electron microscopy. Wall-associated germin accounts for about 40% of the total germin in germinating wheat embryos. Appearance of germin in the apoplast is the most conspicuous germination-related change in the distribution of cell-wall proteins. It seems that germin may act at the level of the apoplast and that pseudogermin may subsume the role of germin at low water potentials during embryogenesis. The N-terminal eicosapeptide sequences in germin and pseudogermin are very similar but SDS/PAGE analysis detects discrete differences between the mobilities of their constituent monomers as well as gross differences between the stabilities of the parent oligomers. Like germin, pseudogermin is a water-soluble, pepsin-resistant protein, but pseudogermin has unprecedented disulphide-independent thermostability properties that have never been previously reported for a water-soluble oligomeric protein. Polysaccharides that co-purify with otherwise pure specimens of germin (and pseudogermin) have been isolated for analysis and shown to be highly substituted glucuronogalactoarabinoxylans. The possible biological significance of selective and tenacious association between germin and glucuronogalactoarabinoxylans is discussed in relation to cell expansion during embryogenic and germinative development of wheat, as are some peculiarities of amino-acid sequence that suggest a possible relation between germin and a proton-specific ion pump: gastric ATPase.