Contrasting of Lowicryl K4M thin sections

Visualization of SARS-CoV-2 particles in naso/oropharyngeal swabs by thin section electron microscopy

BACKGROUND

SARS-CoV-2 replicates efficiently in the upper airways of humans and produces high loads of virus RNA and, at least in the initial phase after infection, many infectious virus particles. Studying virus ultrastructure, such as particle integrity or presence of spike proteins, and effects on their host cells in patient samples is important to understand the pathogenicity of SARS-CoV-2.

METHODS

Suspensions from swab samples with a high load of virus RNA (Ct < 20) were sedimented by desktop ultracentrifugation and prepared for thin section electron microscopy using a novel method which is described in detail. Embedding was performed in Epon or in LR White resin using standard or rapid protocols. Thin sections were examined using transmission electron microscopy.

RESULTS

Virus particles could be regularly detected in the extracellular space, embedded in a background of heterogenous material (e.g. vesicles and needle-like crystals), and within ciliated cells. Morphology (i.e. shape, size, spike density) of virus particles in the swab samples was very similar to particle morphology in cell culture. However, in some of the samples the virus particles hardly revealed spikes. Infected ciliated cells occasionally showed replication organelles, such as double-membrane vesicles. The most common cells in all samples were keratinocytes from the mucosa and bacteria.

CONCLUSIONS

The new method allows the ultrastructural visualization and analysis of coronavirus particles and of infected host cells from easy to collect naso/oropharyngeal patient swab samples.

Albumin-based nanoparticles as contrast medium for MRI: vascular imaging, tissue and cell interactions, and pharmacokinetics of second-generation nanoparticles

This multidisciplinary study examined the pharmacokinetics of nanoparticles based on albumin-DTPA-gadolinium chelates, testing the hypothesis that these nanoparticles create a stronger vessel signal than conventional gadolinium-based contrast agents and exploring if they are safe for clinical use. Nanoparticles based on human serum albumin, bearing gadolinium and designed for use in magnetic resonance imaging, were used to generate magnet resonance images (MRI) of the vascular system in rats ("blood pool imaging"). At the low nanoparticle doses used for radionuclide imaging, nanoparticle-associated metals were cleared from the blood into the liver during the first 4 h after nanoparticle application. At the higher doses required for MRI, the liver became saturated and kidney and spleen acted as additional sinks for the metals, and accounted for most processing of the nanoparticles. The multiple components of the nanoparticles were cleared independently of one another. Albumin was detected in liver, spleen, and kidneys for up to 2 days after intravenous injection. Gadolinium was retained in the liver, kidneys, and spleen in significant concentrations for much longer. Gadolinium was present as significant fractions of initial dose for longer than 2 weeks after application, and gadolinium clearance was only complete after 6 weeks. Our analysis could not account quantitatively for the full dose of gadolinium that was applied, but numerous organs were found to contain gadolinium in the collagen of their connective tissues. Multiple lines of evidence indicated intracellular processing opening the DTPA chelates and leading to gadolinium long-term storage, in particular inside lysosomes. Turnover of the stored gadolinium was found to occur in soluble form in the kidneys, the liver, and the colon for up to 3 weeks after application. Gadolinium overload poses a significant hazard due to the high toxicity of free gadolinium ions. We discuss the relevance of our findings to gadolinium-deposition diseases.

Intracellular membranes of bacterial endospores are reservoirs for spore core membrane expansion during spore germination

Bacterial endospores are formed by certain bacteria, such as Bacillus subtilis or the pathogenic Bacillus anthracis and Clostridioides difficile, to allow survival in environmental conditions which are lethal to vegetative bacteria. The spores possess a particular architecture and molecular inventory which endow them with a remarkable resistance against desiccation, heat and radiation. Another remarkable spore feature is their rapid return to vegetative growth during spore germination and outgrowth. The underlying processes of this latter physiological and morphological transformation involve a number of different events, some of which are mechanistically not entirely understood. One of these events is the expansion of the central spore core, which contains the DNA, RNA and most spore enzymes. To date, it has been unclear how the ~1.3- to 1.6-fold expansion of the core membrane surface area that accompanies core expansion takes place, since this occurs in the absence of significant if any ATP synthesis. In the current work, we demonstrate the presence of intracellular membrane structures in spores located just below the core membrane. During spore germination these internal core membranes disappear when the core size increases, suggesting that they are integrated into the core membrane to allow core expansion. These intracellular membranes are most probably present as more or less compressed vesicles or tubules within the dormant spore core. Investigations of spores from different species suggest that these intracellular membrane structures below the core membrane are a general feature of endospore forming bacteria.

Molecular in situ topology of Aczonin/Piccolo and associated proteins at the mammalian neurotransmitter release site

The protein machinery of neurotransmitter exocytosis requires efficient orchestration in space and time, for speed and precision of neurotransmission and also for synaptic ontogeny and plasticity. However, its spatial organization in situ is virtually unknown. Aczonin/Piccolo is a putative organizer protein of mammalian active zones. We determined by immunogold electron microscopy (EM) (i) the spatial arrangement (i.e., topology) of 11 segments of the Aczonin polypeptide in situ, and correlated it to (ii) the positioning of Aczonin-interacting domains of Bassoon, CAST/ELKS, Munc13, and RIM and (iii) the ultrastructurally defined presynaptic macromolecular aggregates known as dense projections and synaptic ribbons. At conventional synapses, Aczonin assumes a compact molecular topology within a layer 35 to 80 nm parallel to the plasma membrane (PM), with a "trunk" sitting on the dense projection top and a C-terminal "arm" extending down toward the PM and sideward to the dense projection periphery. At ribbon synapses, Aczonin occupies the whole ribbon area. Bassoon colocalizes with Aczonin at conventional synapses but not at ribbon synapses. At both conventional and ribbon synapses, CAST, Munc13, and RIM are segregated from Aczonin, closer to the PM, and Aczonin is positioned such that it may control the access of neurotransmitter vesicles to the fusion site.

Observation of membrane proteins in situ: AQPcic, the insect aquaporin example

Aquaporins are water-selective channels widely distributed in prokaryotes, plants, and animals. Looking for the presence of a water channel in the filter chamber (FC) of a homopteran insect (Cicadella viridis), we conducted an electron microscopic study. On thin sections, FC displays thin epithelia with developed basal membrane folds (BMFs). Freeze fracture performed on FC shows an amazing network of intramembrane particles. Epithelial cell membranes were purified and observed by negative staining for control purity. Membrane solubilisation followed by PAGE showed that a 25-kDa polypeptide (P25) is the major protein constituent. Using a specific antibody, we located P25 on thin sections on the microvilli and on BMFs of the epithelial cells. Immunogold localisation of P25 on negatively stained membranes and examination of Pt/C shadowed membranes demonstrated that P25 has an asymmetric insertion within the membrane. cDNA cloning and heterologous expression confirmed that P25 is an aquaporin; thus, we called it AQPcic. The native state of crystallisation of this aquaporin in the membrane appeared to be unique and favourable for a structural investigation by negative staining, cryo-electron microscopy, and image processing. We demonstrated that, in the native membrane, AQPcic is a homotetramer forming a regular two-dimensional array.

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.

Functional characterization of Negri bodies (NBs) in rabies virus-infected cells: Evidence that NBs are sites of viral transcription and replication

Rabies virus infection induces the formation of cytoplasmic inclusion bodies that resemble Negri bodies found in the cytoplasm of some infected nerve cells. We have studied the morphogenesis and the role of these Negri body-like structures (NBLs) during viral infection. The results indicate that these spherical structures (one or two per cell in the initial stage of infection), composed of the viral N and P proteins, grow during the virus cycle before appearing as smaller structures at late stages of infection. We have shown that the microtubule network is not necessary for the formation of these inclusion bodies but is involved in their dynamics. In contrast, the actin network does not play any detectable role in these processes. These inclusion bodies contain Hsp70 and ubiquitinylated proteins, but they are not misfolded protein aggregates. NBLs, in fact, appear to be functional structures involved in the viral life cycle. Specifically, using in situ fluorescent hybridization techniques, we show that all viral RNAs (genome, antigenome, and every mRNA) are located inside the inclusion bodies. Significantly, short-term RNA labeling in the presence of BrUTP strongly suggests that the NBLs are the sites where viral transcription and replication take place.

The role of single subunits of the DNA transport machinery of Thermus thermophilus HB27 in DNA binding and transport

Thermus thermophilus HB27 is well known for its extraordinary trait of high frequencies of natural transformation, which is considered a major mechanism of horizontal gene transfer. We show that the DNA translocator of T. thermophilus binds and transports DNA from members of all three domains. These results, together with the data obtained from genome comparisons, suggest that the DNA translocator of T. thermophilus has a major impact in adaptation of Thermus to thermal stress conditions and interdomain DNA transfer in extreme hot environments. DNA transport in T. thermophilus is mediated by a macromolecular transport machinery that consists of at least 16 subunits and spans the cytoplasmic membrane and the entire cell periphery. Here, we have addressed the role of single subunits in DNA binding and transport. PilQ is involved in DNA binding, ComEA, PilF and PilA4 are involved in transport of DNA through the outer membrane and PilM, PilN, PilO, PilA1-3, PilC and ComEC are essential for the transport of DNA through the thick cell wall layers and/or through the inner membrane. These data are discussed in the light of the subcellular localization of the proteins. A topological model for DNA transport across the cell wall is presented.

In situ Abeta pores in AD brain are cylindrical assembly of Abeta protofilaments

According to the amyloid pore hypothesis, pores formed by small oligomers of misfolded amyloidogenic proteins cause membrane leakage with the unregulated rapid influx of ions leading to cell death. Ultrastructurally, pores reconstituted in vitro have mainly been characterised so far, and the presence of in situ pores in the amyloid tissues has not yet been demonstrated. In this study, the presence of in situbeta amyloid (Abeta) pores was shown with high resolution transmission electron microscopy, in the neuronal cell membrane as well as in the membrane of mitochondria-like organelles in the brain with Alzheimer's disease. They are 16 nm wide and 11 nm long flat columnar structures made up of a single cylindrical layer (wall) of laterally associated Abeta protofilaments which surrounds a 10 nm wide opening or lumen. Protofilaments are the basic unit of the fibrils of all amyloid-forming proteins and peptides. Individual extracellular Abeta protofilaments were 2-3 nm wide straight tubular structures with helical wall formed by the tight coiling of 1 nm wide Abeta filaments. These in situ Abeta pores are similar but not identical to in vitro reconstituted Abeta pores.

Co-Localization of beta-1,3-Glucanases and Callose During Somatic Embryogenesis in Cichorium

During direct somatic embryogenesis in leaves of Cichorium hybrid clone '474', 38 kDa beta-1,3-glucanases are accumulated in the culture medium of the embryogenic hybrid to a higher level when compared with a non-embryogenic cultivar. In the same time, embryogenic cells were surrounded by a cell wall that was characterized by the presence of callose. This callosic deposition disappeared as embryos grew. Callose consisted of beta-1,3-glucan linkages and so represented a possible substrate for beta-1,3-glucanases. Using immunolocalization experiments, we demonstrated that from the three types of callose deposits observed during the culturing of Cichorium leaf explants, only the callose present in the walls surrounding reactivated cells seemed specifically related to somatic embryogenesis. Moreover, callose and the 38-kDa beta-1,3-glucanases were co-localized dispersed throughout the thick and swelled walls of reactivated cells and embryo cell walls. This suggests that callose and beta-1,3-glucanases are implicated in the process of somatic embryogenesis since they were always detected in or quite near embryogenic and embryo cell. This also suggested that beta-1,3-glucanases could be involved in the degradation of this callose.

Rapid diagnostic thin section electron microscopy of bacterial endospores

Emerging infectious diseases such as SARS and the bioterror attacks with anthrax spores that occurred after September 11th, 2001 have highlighted the need to be better prepared for the detection and management of infectious pathogens that threaten public health. Negative staining electron microscopy is one method used to screen environmental and clinical samples for relevant infectious pathogens. Unfortunately, bacterial endospores, like those of Bacillus anthracis, are difficult to identify using this method because of their density that prevents imaging of structural details. Thin section electron microscopy would be an alternative method but this usually requires a few days for preparation and diagnosis. In the present paper we describe the development of a rapid thin section protocol, using mainly Bacillus subtilis spores as a model, which allows an unequivocal diagnosis of endospores within 2 h. The protocol involves chemical fixation assisted by heat or microwaves, rapid dehydration, embedding in the low-viscosity resin LR White and chemically enhanced polymerization. Structural preservation of spores is comparable to preservation after standard Epon embedding. Immunolabeling experiments using B. atrophaeus spores and a specific antibody suggest that the protocol preserves significant antigenicity for on-section immunocytochemistry and therefore offers the possibility for the strain typing of spores using specific antibodies. Further experiments with vegetative bacteria, viruses and cell cultures indicate that the rapid thin section protocol not only preserves spores but also other biological structures. Because of its universality and speed the described protocol complements negative staining electron microscopy as a front line method for the morphology-based diagnosis of pathogens in environmental and clinical samples.

Chemotactic peptide fMetLeuPhe induces translocation of the TRPV2 channel in macrophages

The present study was conducted to characterize the regulation and function of TRPV2 in macrophages. Among six members of the TRPV family channels, only the expression of TRPV2 was detected in macrophages. We then determined localization of TRPV2 using TtT/M87 macrophages transfected with TRPV2-EGFP. In serum-free condition, most of the TRPV2 signal was located in the cytoplasm and colocalized with the endoplasmic reticulum marker. Treatment with serum induced translocation of some of the TRPV2-EGFP to the plasma membrane. Serum-induced translocation was blocked by transfection of short-form TRPV2 (s-TRPV2) lacking a pore-forming region and the sixth transmembrane domain. Addition of a chemotactic peptide formyl Met-Leu-Phe (fMLP) also induced translocation of TRPV2-EGFP to the plasma membrane. The fMLP-induced translocation was blocked by an inhibitor of PI 3-kinase, LY294002, and pertussis toxin. Whole-cell patch clamp analysis showed a Cs+ current in the TtT/M87 cell, which was blocked by an addition of ruthenium red and transfection of either s-TRPV2 or siRNA for TRPV2. fMLP increased the Cs+ current. fMLP induced a rapid and sustained elevation of cytoplasmic Ca2+ ([Ca2+]C), the sustained phase of which was abolished by removal of extracellular calcium. The sustained elevation of [Ca2+]C was also blocked by ruthenium red, and transfection of either s-TRPV2 or siRNA. Finally, fMLP-induced migration of macrophage was blocked by ruthenium red or transfection of s-TRPV2. These results suggest that fMLP induces translocation of TRPV2 from intracellular compartment to the plasma membrane, and this translocation is critical for fMLP-induced calcium entry.

Preferential localization of rat GAPDS on the ribs of fibrous sheath of sperm flagellum and its expression during flagellar formation

The proper assembly of sperm flagellar proteins is fundamental for sperm motility. The sperm- and spermatid-specific isoform of glyceraldehyde 3-phosphate dehydrogenase, GAPDS, is a flagellar protein indispensable for sperm flagellar movement. To obtain information on the assembly of the glycolytic enzyme into the flagellum, the precise localization of rat GAPDS in the flagellum and the stage of incorporation into the flagellum were examined using a monoclonal antibody. Immunolocalization of rat GAPDS was restricted to the fibrous sheath (FS) in the sperm flagellum, and was predominant in the circumferential ribs rather than the longitudinal columns. Immunoreactivity was first detected in the cytoplasm and flagella of the step-16 spermatids during the final step of FS formation. Together with the expression of other FS proteins, the present results indicate the sequential assembly of FS components, suggesting that the expression and transport of GAPDS is regulated in a coordinated manner during sperm flagellar formation.

Regulation of the human T-cell leukemia virus gene expression depends on the localization of regulatory proteins Tax, Rex and p30II in specific nuclear subdomains

The human T-cell leukemia virus HTLV-1 encodes regulatory proteins, Tax, Rex and p30(II), which are involved in the control of viral gene expression at the transcriptional and post-transcriptional levels. Tax localizes in unique nuclear bodies that contain components of the transcription and splicing complexes. In this work, we studied the relative intracellular localizations of Tax, Rex and p30(II). Run-on transcription assays and immunocytochemistry at light and electron microscopy levels indicated that the Tax nuclear bodies included both de novo transcribed RNA and the RNA polymerase II form that is phosphorylated on its carboxy-terminal domain whereas contacts with chromatin were observed at the periphery of these nuclear bodies. Rex first accumulated in nucleolar foci and then spread across the whole nucleus to display a diffuse and punctuate nucleoplasmic distribution. This distribution of Rex was observed in HTLV-1 transformed lymphocytes and in COS cells expressing the HTLV-1 provirus. Rex colocalized with the cellular export factor CRM-1 in the nucleolar foci as well as in the nucleoplasmic foci that did not overlap with Tax nuclear bodies but were found at the boundaries of the Tax bodies. In addition, we demonstrate that p30(II) interacts with Rex and colocalizes with the Rex/CRM-1 complexes in the nucleoli leading to their clearance from the nucleoplasm. Our results suggest that transcripts originating from Tax-induced activation of gene expression at the boundaries of the Tax bodies are transported out of the nucleus by nucleoplasmic Rex/CRM-1 complexes that are first assembled in nucleolar foci. In addition, p30(II) might exert its negative effect on viral RNA transport by preventing the release of the Rex/CRM-1 complexes from sequestration in nucleolar foci. These data support the idea that the transcriptional and post-transcriptional regulation of HTLV-1 gene expression depends on the concentration of select regulatory complexes at specific area of the nucleus.

Molecular and cellular characterization of a new aquaporin, AQP-x5,specifically expressed in the small granular glands of Xenopusskin

A new toad aquaporin (AQP) cDNA was cloned from a cDNA library constructed from the ventral skin of Xenopus laevis. This AQP (XenopusAQP-x5) consisted of 273 amino acid residues with a high sequence homology to mammalian AQP5. The predicted amino acid sequence contained the two conserved Asn-Pro-Ala motifs found in all major intrinsic protein (MIP) family members and six putative transmembrane domains. The sequence also contained a mercurial-sensitive cysteine and a putative phosphorylation motif site for protein kinase A at Ser-257. The swelling assay using Xenopus oocytes revealed that AQP-x5 facilitated water permeability. Expression of AQP-x5 mRNA was restricted to the skin, brain, lungs and testes. Immunofluorescence and immunoelectron microscopical studies using an anti-peptide antibody (ST-156)against the C-terminal region of the AQP-x5 protein revealed the presence of immunopositive cells in the skin, with the label predominately localized in the apical plasma membrane of the secretory cells of the small granular glands. These glands are unique both in being close to the epidermal layer of the skin and in containing mitochondria-rich cells with vacuolar H+-ATPase dispersed among its secretory cells. Results from immunohistochemical experiments on the mucous or seromucous glands of several other anurans verified this result. We conclude that the presence of AQP-x5 in the apical plasma membrane of the small granular glands suggests its involvement in water secretion from the skins. The physiological roles of the AQP-x5 protein in the small or mucous glands are discussed.

Promotion of cell adherence and spreading: a novel function of RAGE, the highly selective differentiation marker of human alveolar epithelial type I cells

The receptor for advanced glycation endproducts (RAGE) is expressed under pathological conditions in many tissues and has been assigned many functions. We demonstrate, in normal human lung tissue, the preferential and highly abundant expression of RAGE by quantitative polymerase chain reaction. In addition, RAGE expression, as a specific differentiation marker of alveolar epithelial type I cells (AT I cells), and its localization to the basolateral plasma membrane have been confirmed by means of newly raised monoclonal antibodies. The physiological function of RAGE on AT I cells has previously remained elusive. By using HEK293 cells transfected with cDNA encoding for full-length RAGE, we show that RAGE enhances the adherence of epithelial cells to collagen-coated surfaces and has a striking capacity for inducing cell spreading. The preferential binding of RAGE to collagen has been confirmed by assaying the binding of soluble RAGE to various substrates. RAGE might thus assist AT I cells to acquire a spreading morphology, thereby ensuring effective gas exchange and alveolar stability.

Seasonal expression of LHbeta and FSHbeta in the male newt pituitary gonadotrophs

Seasonal changes in LHbeta and FSHbeta mRNA levels were examined in the pituitary gland of the adult male newt, Cynops pyrrhogaster, using in situ hybridization histochemistry and a quantitative real-time RT-PCR method. The annual fluctuation of LHbeta mRNA and FSHbeta mRNA levels in the pituitary gland displayed a close relationship with seasonal changes in testicular function. The values obtained by both methods showed similar fluctuation. The levels of LHbeta mRNA were always exceeded those of FSHbeta. The present immunoelectron microscopic observations support the data on the gene expression levels of the beta-subunits of LH and FSH. Gonadectomy in the summer increased the LHbeta and FSHbeta mRNA levels. Testosterone replacement inhibited the expression of LHbeta mRNA, but not of FSHbeta mRNA, suggesting that the expression of FSHbeta is regulated by some non-steroid factor, probably inhibin. In the case of gonadectomy during any other season, the LHbeta mRNA level increased, but not to the same extent as in summer, and androgen concentrations decreased to the minimum of the year. This finding provides new information about the regulation of annual changes in LHbeta and FSHbeta expression in the pituitary gonadotrophs.

A simple, low-cost device for processing and embedding tissues at sub-zero temperatures

A simple apparatus to maintain tissues samples at sub-zero temperatures during dehydration, infiltration, and polymerization is described. The device uses a conventional siphon-type carbon dioxide gas cylinder to maintain an aluminum block at temperatures as low as -35 degrees C for over 15 hours/cylinder.

A mouse acrosomal cortical matrix protein, MC41, has ZP2-binding activity and forms a complex with a 75-kDa serine protease

Sperm with a large acrosome such as that of guinea pigs and hamsters have a subdomain structure in the anterior acrosome, but the mouse acrosome looks homogeneous and its matrix has not been precisely analyzed. The intra-acrosomal protein MC41 is localized in the cortical region of the mouse anterior acrosome, suggesting a subdomain structure in the mouse acrosome. Thus, the present study was undertaken to analyze the mouse acrosomal matrix using an anti-MC41 antibody. When mouse sperm were treated with 2% Triton X-100, Triton-insoluble matrix components remained in the acrosomal cortical region. Immunogold for MC41 labeled the Triton X-100 and high-salt-insoluble matrix components, demonstrating that MC41 is a subdomain-specific acrosomal matrix protein. We further examined interactions of MC41 with acrosomal proteases and zona proteins. A serine protease of 75 kDa was associated with MC41 under low-salt conditions, presumably forming a complex. Far Western blotting technique indicated that MC41 bound to both ZP2 and ZP2(f) in the presence of high-salt-soluble sperm proteins. In acrosome-reacting sperm, MC41 was present on the hybrid vesicles formed by the fusion of the plasma and outer acrosomal membranes. Presumably, MC41 has a significant role in secondary sperm-zona binding during the acrosomal reaction.

Reappraisal of potassium permanganate oxidation applied to Lowicryl K4M embedded tissues processed by high pressure freezing/freeze substitution, with special reference to differential staining of the zymogen granules of rat gastric chief cells

The high pressure freezing/freeze substitution technique is known to yield a deep vitreous freezing of tissues. Combination of this technique with Lowicryl K4M embedding allows us histochemical studies of dynamic cellular processes with improved structural preservation. The disadvantage of Lowicryl K4M embedding is its poor electron density in electron microscopy. To address this problem, we examined the effects of KMnO4 oxidation applied to Lowicryl K4M embedded rat gastric glands processed by high pressure freezing. The KMnO4 oxidation-uranyl acetate-lead citrate sequence succeeded not only in contrast enhancement of cellular components, but also in differential staining of the zymogen granules of rat gastric chief cells. This technique could be applied to semi-thin sections of Lowicryl K4M embedded rat gastric glands. The KMnO4 oxidation-toluidine blue staining provided sufficient contrast with regard to the zymogen granules. Various experiments used in this study verified that the KMnO4 oxidation plays an essential role in the differential staining of the zymogen granules. Combined use of the KMnO4 oxidation with phospholipase A2-immunostaining demonstrated that gold labeling was localized to the zymogen granules without the loss of immunolabeling. Energy dispersive X-ray microanalysis revealed some manganese depositions on the zymogen granules. It is highly anticipated that the KMnO4 oxidation will become a useful tool for histochemical investigations combined with cryofixation/freeze substitution and low temperature embedding techniques.

A mutation in the insulin 2 gene induces diabetes with severe pancreatic beta-cell dysfunction in the Mody mouse

The mouse autosomal dominant mutation Mody develops hyperglycemia with notable pancreatic beta-cell dysfunction. This study demonstrates that one of the alleles of the gene for insulin 2 in Mody mice encodes a protein product that substitutes tyrosine for cysteine at the seventh amino acid of the A chain in its mature form. This mutation disrupts a disulfide bond between the A and B chains and can induce a drastic conformational change of this molecule. Although there was no gross defect in the transcription from the wild-type insulin 2 allele or two alleles of insulin 1, levels of proinsulin and insulin were profoundly diminished in the beta cells of Mody mice, suggesting that the number of wild-type (pro)insulin molecules was also decreased. Electron microscopy revealed a dramatic reduction of secretory granules and a remarkably enlarged lumen of the endoplasmic reticulum. Little proinsulin was processed to insulin, but high molecular weight forms of proinsulin existed with concomitant overexpression of BiP, a molecular chaperone in the endoplasmic reticulum. Furthermore, mutant proinsulin expressed in Chinese hamster ovary cells was inefficiently secreted, and its intracellular fraction formed complexes with BiP and was eventually degraded. These findings indicate that mutant proinsulin was trapped and accumulated in the endoplasmic reticulum, which could induce beta-cell dysfunction and account for the dominant phenotype of this mutation.

Contrast-enhancement for the image of human immunodeficiency virus from ultrathin section by immuno electron microscopy

A simple contrast-enhancement method is described for electron microscopic imaging of the human immunodeficiency virus (HIV) from a sample embedded in Lowicryl K4M resin, by immuneelectron microscopy. Ultrathin sections were treated with a mixture of ruthenium red dye (RR) and osmium tetroxide (OSO4). This treatment provided good contrast enhancement of the entire ultrastructural image of virus particles without the loss of immunolabelling. RR/OsO4 solution is simple to prepare and provides a better contrast than that which is achieved during conventional post-embedding immunoelectron microscopy. Treatment of ultrathin sections from low temperature-embedded samples with RR/OsO4 solution is recommended.

The human T-cell leukemia virus type 1 transactivator protein Tax colocalizes in unique nuclear structures with NF-kappaB proteins

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) is a potent activator of viral transcription. Tax also activates the expression of specific cellular genes involved in the control of T-lymphocyte growth via effects on cellular transcription factors, including members of the NF-kappaB/cRel family. Immunocytochemistry and electron microscopy were used to characterize the intracellular localization of Tax and identify cellular factors which are the potential targets for its transcriptional activity. These studies indicated that Tax localizes in discrete nuclear foci in T lymphocytes transformed by HTLV-1 and in cells transduced with Tax expression vectors. The Tax-containing foci are complex nuclear structures comprising a central core in which Tax colocalizes with splicing factor Sm. In addition to splicing factors Sm and SC-35, the Tax-containing nuclear structures also contain transcriptional components, including the largest subunit of RNA polymerase II and cyclin-dependent kinase CDK8. The inclusion of the two subunits of NF-kappaB, p50 and RelA, and the presence of the mRNA from a gene specifically activated by Tax through NF-kappaB binding sites suggest that these unique nuclear structures participate in Tax-mediated activation of gene expression via the NF-kappaB pathway.

Proteolytic processing of pro-opiomelanocortin occurs in acidifying secretory granules of AtT-20 cells

Using antibodies specific for pro-opiomelanocortin (POMC), amidated joining peptide (JP), and the prohormone convertase PC1, we showed immunocytochemically that PC1 in a corticotrophic tumor cell line, AtT-20, was co-localized either with POMC or with amidated JP in secretory granules, and also confirmed that POMC was cleaved mainly in secretory granules. Analysis using DAMP (3- [2,4-dinitroanilino]-3'-amino-N-methyldipropylamine) as the pH probe suggested a correlation between POMC processing and acidic pH in the secretory granules. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, completely inhibited POMC processing and caused constitutive secretion of the unprocessed precursor. By contrast, chloroquine, a weak base that is known to neutralize acidic organelles, was unable to inhibit POMC processing. Electron microscopic analysis revealed that, in AtT-20 cells treated with bafilomycin A1, the trans-Golgi cisternae were dilated and few secretory granules were present in the cytoplasm. These observations suggest that acidic pH provides a favorable environment for proteolytic processing of POMC by PC1 but is not required, and that integrity of the trans-Golgi network and sorting of POMC into secretory granules are important for POMC processing.

Protein glycosylation in the endoplasmic reticulum and the Golgi apparatus and cell type-specificity of cell surface glycoconjugate expression: analysis by the protein A-gold and lectin-gold techniques

High resolution immunolabeling applying the protein A-gold technique and carbohydrate cytochemistry using lectin-gold labeling on Lowicryl K4M and thawed-frozen thin sections are most useful approaches for the detection of protein antigens and lectin binding sites in intracellular organelles and the plasma membrane. They provided the basis for modern electron microscopic studies on protein glycosylation reactions and the identification of their subcellular localization as reviewed here. These studies have demonstrated organelle subcompartments and the cell type-specific compartmentation of endoplasmic reticulum and Golgi apparatus-associated glycosylation reactions. The other subject reviewed in this paper is cell surface glycoconjugates, as they are expressed in relation to specific cell types present in various organs and during cellular differentiation processes.

The silver anniversary of gold: 25 years of the colloidal gold marker system for immunocytochemistry and histochemistry

Since 1971, when W.P. Faulk and G.M. Taylor published "An immunocolloid method for the electron microscope", colloidal gold has become a very widely used marker in microscopy. It has been used to detect a huge range of cellular and extracellular constituents by in situ hybridization, immunogold, lectin-gold, and enzyme-gold labeling. Besides its use in light microscopic immunogold and lectin-gold silver staining, colloidal gold remains the label of choice for transmission electron microscopy studying thin sections, freeze-etch, and surface replicas, as well as for scanning electron microscopy. The year 1996 is the 25th anniversary of the introduction of colloidal gold as a marker in immunoelectron microscopy and this overview outlines some of the major milestones in the development of the colloidal gold marker system.

Intra-acrosomal 155,000 dalton protein increases the antigenicity during mouse sperm maturation in the epididymis: a study using a monoclonal antibody MC101

We found an intra-acrosomal antigen of about 155,000 daltons (155 kDa) in a survey using the monoclonal antibody MC101 raised against mouse cauda epididymal spermatozoa. Morphological studies by means of indirect immunofluorescence and immunogold electron microscopy localized the antigen to the cortex region of the anterior acrosome. Avidin biotin complex immunocytochemistry initially demonstrated a faint signal at the anterior acrosome in the testis spermatozoa that increased in intensity as the sperm moved toward the distal epididymis. This incremental immunoreactivity was also confirmed by immunoblotting following one-dimensional SDS-PAGE. The 155 kDa protein band was immunostained, and it was much more intense in the cauda epididymal than in the caput and corpus epididymal spermatozoa. Only a trace or no immunostain was evident in the caput or testis spermatozoa. The antigen localization did not change during passage through the epididymis, being confined at the cortex region of the anterior acrosome. The epididymal epithelial cells were not immunostained. These findings suggested that the 155 kDa protein is biochemically modified, further implying that the biochemical alteration of intra-acrosomal material is involved in sperm maturation in the epididymis.

Subcellular distribution of terminal alpha-D- and beta-D-galactosyl residues in Ehrlich tumour cells studied by lectin-gold techniques

We have studied by high resolution in situ light and electron microscopic lectin-gold techniques the subcellular distribution of alpha-D-Gal residues using the Griffonia simplicifolia I-B4 isolectin and compared it with that of beta-D-Gal residues as detected with the Datura stramonium lectin in Ehrlich tumour cells grown as ascites or monolayer. The microvillar but not the smooth plasma membrane regions were labelled with the Griffonia simplicifolia I-B4 isolectin whereas both plasma membrane regions were equally well labelled with the Datura stramonium lectin. Elements of the endocytotic/lysosomal system such as coated membrane invaginations and vesicles, early and late endosomes and secondary lysosomes were positive for both alpha-D-Gal and beta-D-Gal residues. A particular feature of Ehrlich tumour cells is an elaborate tubular membrane system located in the pericentriolar region which is labelled throughout by both lectins and represents part of the endosomal system. In the Golgi apparatus labelling with both lectins was observed to commence in trans cisternae which is indirect evidence for a joint distribution of the sequentially acting beta 1,4 and alpha 1,3-galactosyl-transferases.

Ultrastructural immunogold localization of tumor necrosis factor-alpha to the cytoplasmic granules of rat peritoneal mast cells with rapid microwave fixation

Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional, proinflammatory cytokine, which can be produced by mast cells and several other cell types. We used a newly developed microwave energy-assisted aldehyde fixation method to prepare purified rat peritoneal mast cells for the postembedding immunogold ultrastructural localization of TNF-alpha. These fixation methods were superior to chemical fixation alone in preserving both the ultrastructural morphology and immunoreactive TNF-alpha in rat mast cells. The percent of TNF-alpha-positive mast cells in samples prepared with microwave-assisted fixation in low (84%) and standard (81%) glutaraldehyde concentrations exceeded that for low (56%) and standard (15%) glutaraldehyde concentrations without the assistance of microwave energy. TNF-alpha was identified in the large storage granules of rat mast cells. The percent of positive granules in microwave-assisted standard (44%) and low (40%) glutaraldehyde samples was considerably higher than the percent of positive granules in standard (5%) and low (10%) glutaraldehyde-fixed samples without microwave assistance. This location of TNF-alpha in rat peritoneal mast cells suggests that this cytokine can use the regulated secretory route(s) for release from appropriately stimulated rat mast cells into the microenvironment.

Respiratory syncytial virus nucleocapsid protein (N) expressed in insect cells forms nucleocapsid-like structures

The gene coding for the N protein of RSV strain Long has been cloned and sequenced. It was introduced behind the polyhedrin promoter of the shuttle vector pVL941 and baculoviruses containing the N gene were constructed by homologous recombination. Infection of Spodoptera frugiperda 9 cells resulted in the production of large amounts of a protein similar in size and antigenicity to the authentic N protein. The baculovirus expressed N protein was concentrated in the cytoplasm of the insect cells and could be extracted at low salt concentration. Nucleocapsid structures similar to those purified from RSV-infected cells could be observed by electron microscopy after negative staining of cellular extracts.

A variant of a slam freezing device for electron microscopy

A home-made slam freezing device is presented that allows reproducible results in freezing various unfixed tissues. The heart of the device is an aluminum socket, which harbors a plunger that is set in motion by a spring. At the end of the plunger there is an electromagnet which holds the sample on a sheet metal planchette. During stop freezing the electrical contacts are interrupted and the plunger can be withdrawn leaving the specimen on the cooled copper block. This guarantees freezing of not only solid tissues, but also cell suspensions, such as blood or bone marrow.

Adaptation of a non-radioactive in situ hybridization method to electron microscopy: detection of tenascin mRNAs in mouse cerebellum with digoxigenin-labelled probes and gold-labelled antibodies

In this study we describe a method for the detection of mRNAs at the ultrastructural level using a non-radioactive in situ hybridization method based on digoxigenin-labelled cRNA probes and gold-labelled digoxigenin-specific antibodies. We applied this protocol to an analysis of the expression of the extracellular matrix protein tenascin in the developing cerebellar cortex of the mouse. To gain an impression of the sensitivity attainable with digoxigenin-labelled probes, we first established at the light microscopic level that the hybridization signal obtained with the non-radioactive probe is as sensitive as that obtained with a 35S-labelled probe. The non-radioactive hybridization protocol was then combined with electron microscopic post-embedding and immunogold detection techniques. Tenascin-specific, digoxigenin-labelled cRNA probes were hybridized to ultrathin sections of Lowicryl K4M-embedded tissue and the probe/target mRNA hybrids were detected using gold-labelled antibodies to digoxigenin. In agreement with the observations from in situ hybridization at the light microscopic level, specific labelling was observed in Golgi epithelial cells in the region of the Purkinje cell layer and cells in the internal granular layer, which could be identified as astrocytes by ultrastructural criteria. Labelling was detectable in association with free ribosomes and ribosomes of the rough endoplasmic reticulum. In addition, focal hybridization signals were occasionally found in the nucleus. No signal was observed in Golgi epithelial cells or astrocytes using sense or in any other cerebellar cell type using either sense or anti-sense probes. The described in situ hybridization technique uses ultrastructural criteria to associate the presence of a given mRNA species with a particular cell type. Additionally, it provides information about the target mRNA's subcellular distribution, thus offering the possibility to study intracellular transport of particular mRNAs.

Alpha IIb beta 3 integrin dissociation induced by EDTA results in morphological changes of the platelet surface-connected canalicular system with differential location of the two separate subunits

Treatment of human platelets by EDTA (5 mM at 37 degrees C and pH 7.4 for 30 min) induces ultrastructural morphological changes of the surface-connected canalicular system (SCCS). The first consists in dilations of some portions of the channels, whereas the second is represented by collapse of parts of the canaliculi. The collapsed elements of the EDTA treated SCCS are made up of two parallel limiting membranes and a central striated zone. Some of the EDTA treated platelets form microaggregates, the cohesion of which is apparently due to the appearance of pentalaminar interplatelet structures. EDTA treatment is known to induce an irreversible loss of platelet aggregability which is due to irreversible dissociation of the membrane GPIIb-IIIa complexes. In the present study, we looked for involvement of GPIIb-IIIa in the formation of these pentalaminar structures, and were able to demonstrate that the morphological changes described are in fact directly dependent on the EDTA induced dissociation of GPIIb-IIIa complexes. Indeed, we observed that these changes (a) cannot be induced in type I Glanzmann's thrombasthenia, where GPIIb-IIIa complexes are absent, (b) do not appear when human platelets are preincubated with monoclonal anti-GPIIb-IIIa complex-dependent (CD41a) antibodies, which protect the complex from EDTA induced dissociation, (c) appear only at alkaline pH and at 37 degrees C, which corresponds to the range of pH and temperature where EDTA can dissociate GPIIb-IIIa complexes, (d) are accompanied by the disappearance in fluorescence flow cytometry of the heterodimer complex-dependent epitopes, when using anti-CD41a antibodies and (e) do not appear in rat platelets, where GPIIb-IIIa does not dissociate after EDTA treatment. Furthermore, using gold-labeled mAbs concomitantly with the addition of EDTA, we observed that almost only GPIIb was present in the collapsed regions of the canaliculi. Using double labeling studies with polyclonal anti-GPIIb antibodies coupled to 10 nm gold particles and polyclonal anti-GPIIIa antibodies coupled to 20 nm gold particles, we observed that while both 10 and 20 nm particles were present in the dilated portions of the canaliculi almost only the small particles, coupled to the anti-GPIIb antibodies, labeled the collapsed portions of the SCCS. On Lowicryl thin sections, polyclonal antibodies against GPIIb labeled the central striated zone while both GPIIb and GPIIIa were found in the dilated portions of the SCCS. All these observations lead us to suggest that homopolymers of GPIIb could be responsible for "zipping" of the SCCS.

Characterization of the antigen recognized by a monoclonal antibody MN9: unique transport pathway to the equatorial segment of sperm head during spermiogenesis

MN9, a monoclonal antibody raised against mouse spermatozoa, specifically recognizes the equatorial segment of sperm head in several mammalian species, including humans. Colloidal gold-immuno-electron microscopy of mouse spermatozoa has shown that the antigen is localized in the space between the outer and inner acrosome membranes and on the acrosome membranes at the equatorial segment. Immunoblotting after electrophoresis of spermatozoa from the cauda epididymidis has identified two immunoreactive bands: 38 kDa and 48 kDa in mouse, and 48 kDa in rat. During spermiogenesis in rat, this antigen is transported to the equatorial segment via a unique pathway, first appearing in some cisternae of the endoplasmic reticulum and in the Golgi apparatus of spermatids at around step 3. The antigen can further be found on the vesicles at the trans-side of the Golgi apparatus, in the matrix of the head cap, and on the head cap membrane in step-4 to step-7 spermatids. The antigen appears to be concentrated at the equatorial segment during late spermiogenesis. Neither the (pro-)acrosomic granule nor the surrounding membrane are required in this pathway. This pathway can be termed the 'Golgi-head cap tract'.

Specific binding of human fibrinogen fragment D to Aspergillus fumigatus conidia

The interaction of purified human fibrinogen with Aspergillus fumigatus conidia was investigated by immunofluorescence and electron microscopy and binding assays with radiolabeled proteins. We described the localization of the binding sites on the A. fumigatus conidia and on the fibrinogen molecule and determined the binding characteristics. Immunofluorescence revealed that the fixation of purified fibrinogen was selectively associated with conidia and suggested a role for the D domains of the fibrinogen molecule. Binding assays performed with 125I-radiolabeled proteins confirmed that binding sites were located specifically in the D domains. No reaction could be detected with fragment E. The binding of 125I-fragment D to conidia was time dependent, saturable, and specific. Scatchard analysis of the data revealed an average of 1,200 binding sites per conidium, and an apparent dissociation constant (Kd) of 2.2 x 10(-9) M was estimated. Pretreatment of the cells with proteolytic enzymes or heat abolished binding, demonstrating the protein nature of the binding sites. Ultrastructural localization of the fungal receptors was determined by transmission electron microscopy. Labeling appeared to be associated with the outer electron-dense layer of the conidial wall and progressively decreased during the germination process. Labeling of thin sections with fragment D and an antifibrinogen immune serum revealed that binding sites also lay in the inner part of the wall and in vacuoles. These results indicate the presence at the conidial surface of specific receptors for fibrinogen which could act as mediators of conidial adherence to host tissues.

High osmotic pressure for Pneumocystis carinii London Resin White embedding enables fine immunocytochemistry studies: I. Golgi complex and cell-wall synthesis

A method for embedding Pneumocystis carinii in hydrophilic resin (London Resin White) has been developed for immunocytochemistry studies. Using high osmotic pressure (about 850 mosmol) from fixation to embedding, this method improved the preservation of the fine structure as well as the antigenicity of rabbit- and SCID mouse-derived P. carinii. Cytochemistry studies were performed using colloidal gold-conjugated lectins (concanavalin A, glycine max, Ulex europaeus) that reacted with the cytoplasmic components (endoplasmic reticulum, Golgi vesicles). Colloidal gold-conjugated streptavidin was also tested and was found to be reactive with the parasite cell wall and cytoplasmic components, which precludes its indiscriminate use in P. carinii immunocytochemistry studies.