An improved procedure for immunoelectron microscopy: ultrathin plastic embedding of immunolabeled ultrathin frozen sections

Identification of RSV Fusion Protein Interaction Domains on the Virus Receptor, Nucleolin

Nucleolin is an essential cellular receptor to human respiratory syncytial virus (RSV). Pharmacological targeting of the nucleolin RNA binding domain RBD1,2 can inhibit RSV infections in vitro and in vivo; however, the site(s) on RBD1,2 which interact with RSV are not known. We undertook a series of experiments designed to: document RSV-nucleolin co-localization on the surface of polarized MDCK cells using immunogold electron microscopy, to identify domains on nucleolin that physically interact with RSV using biochemical methods and determine their biological effects on RSV infection in vitro, and to carry out structural analysis toward informing future RSV drug development. Results of immunogold transmission and scanning electron microscopy showed RSV-nucleolin co-localization on the cell surface, as would be expected for a viral receptor. RSV, through its fusion protein (RSV-F), physically interacts with RBD1,2 and these interactions can be competitively inhibited by treatment with Palivizumab or recombinant RBD1,2. Treatment with synthetic peptides derived from two 12-mer domains of RBD1,2 inhibited RSV infection in vitro, with structural analysis suggesting these domains are potentially feasible for targeting in drug development. In conclusion, the identification and characterization of domains of nucleolin that interact with RSV provide the essential groundwork toward informing design of novel nucleolin-targeting compounds in RSV drug development.

Cryosectioning fixed and cryoprotected biological material for immunocytochemistry

Immunocytochemistry for electron microscopy provides important information on the location and relative abundance of proteins inside cells. Gaining access to this information without extracting or disrupting the location of target proteins requires specialized preparation methods. Sectioning frozen blocks of chemically fixed and cryoprotected biological material is one method for obtaining immunocytochemical data. Once the cells or tissues are cut, the cryosections are thawed, mounted onto coated grids, and labeled with specific antibodies and colloidal gold probes. They are then embedded in a thin film of plastic containing a contrasting agent. Subcellular morphology can then be correlated with specific affinity labeling by examination in the transmission electron microscope (TEM). The major advantage of using thawed cryosections for immunolabeling is that the sections remain fully hydrated through the immunolabeling steps, reducing the possibility of dehydration-induced antigen modification. Modern technical advancements both in preparation protocols and equipment design make cryosectioning a routine and rapid approach for immunocytochemistry that may provide increased sensitivity for some antibodies.

Microwave-assisted processing and embedding for transmission electron microscopy

Microwave processors can provide a means of rapid processing and resin embedding for biological specimens that are to be sectioned and examined by transmission electron microscopy. This chapter describes a microwave-assisted protocol for processing, dehydrating and embedding biological material, taking them from living specimens to blocks embedded in sectionable resin in 4 h or less.

Identification of neuromuscular junctions by correlative confocal and transmission electron microscopy

The physiological processes regulating neuromuscular transmission are highly dependent on the structural features of the motor neuron and motor endplate, and detailing the structure of neuromuscular junctions (NMJs) in muscle biopsies is a powerful method for research and diagnostics. The observation of NMJ ultrastructure, however, is complicated by the difficulty in locating NMJs for analysis by electron microscopy. Consequently, a correlative confocal-transmission electron microscopy method was developed. Fixed muscle samples were cryo-protected in sucrose, sectioned on a cryostat, and stained with fluorescent alpha-bungarotoxin for confocal microscopy. Sections containing junctions were mapped and then processed for transmission electron microscopy (TEM). Cryostat sections allowed large expanses of muscle tissue to be rapidly screened and enabled specific junctions to be targeted for TEM. The morphology of the junctions was well preserved with all essential features of the pre- and postsynaptic elements readily identifiable without freeze damage. Unlike NMJ correlative methods using histochemical stains and DAB photo-oxidation, no electron dense precipitate was deposited over the NMJ, enabling an unobstructed view of the pre- and postsynaptic structures.

Preparation of cells and tissues for immuno EM

Transmission electron microscopy (TEM) provides a powerful set of methods to investigate cellular and subcellular structures using thin sections. In this article we summarize some of the different approaches available for researchers interested in using these methods. The essential details involved in specimen preparation for immunolabelling are covered. The best sectioning approach for preserving specimens for structural analysis is Cryo EM of Vitrified Sections (CEMOVIS), a method where still frozen sections are examined in the transmission electron microscope. Because the specimens are kept at low temperature during sectioning and examination, this method is not amenable for immunolabelling, where antibodies are applied to sections at ambient temperature. To combine structural analysis with immunocytochemical analysis of antigens, the approach of freeze-substitution without chemical fixative is the method of choice, at least from a theoretical point of view. In practice, however, the vast majority of electron microscopic (EM) immunocytochemical analyses are carried out using chemically-fixed specimens that have been embedded in specialized resins (such as the Lowicryls) using freeze-substitution or ambient temperature methods. Antibody labelling of thawed cryosections through chemically-fixed specimens (the Tokuyasu method) is also a popular method for preparing cells and tissues for TEM analysis. Here, we provide an overview of all these sectioning methods for EM, focusing mostly on the practical details. Given the space limitation, the fine details necessary to apply these methods have been successfully omitted and will have to be obtained from the technical references we provide.

Cryosectioning fixed and cryoprotected biological material for immunocytochemistry

Immunocytochemistry for transmission electron microscopy provides important information on the location and relative abundance of proteins inside cells. Gaining access to this information without extracting or disrupting the location of target proteins requires specialized preparation methods. Sectioning frozen blocks of chemically fixed and cryoprotected biological material is one method for obtaining immunocytochemical data. Once the cells or tissues are cut, the thawed cryosections can be labeled with specific antibodies and colloidal gold probes. They are then embedded in a thin film of plastic containing a contrasting agent. Subcellular morphology can be correlated with specific affinity labeling by examination in the transmission electron microscope. Modern technical advancements both in preparation protocols and equipment design make cryosectioning a routine and rapid approach for immunocytochemistry that may provide increased sensitivity for some antibodies.

Microwave-assisted processing and embedding for transmission electron microscopy

Microwave processors can provide a means of rapid processing and resin embedding for biological specimens that are to be sectioned and examined by transmission electron microscopy. This chapter describes a microwave-assisted protocol for processing, dehydrating, and embedding biological material, from living specimens to blocks embedded in sectionable resin in 4 h or less.

Improved methods for ultracryotomy of CNS tissue for ultrastructural and immunogold analyses

We examined each step of the protocol for ultracryotomy for central nervous system tissue in order to define and overcome some of the methodological difficulties. The following three steps emerged as critical for the method's success: (1) pretreatment of grids to render them hydrophilic immediately before use; (2) careful collection of ultrathin cryosections during ultracryotomy; (3) removal of the appropriate amount of excess poly(vinyl alcohol)-uranyl acetate (PVA-UA) prior to drying after staining with PVA-UA. By taking account of the three critical steps described above, we succeeded in obtaining ultrathin cryosections, including serial sections, with excellent preservation of ultrastructure, as well as semithin cryosections which are useful for evaluating the quality of the samples and for selecting areas of interest for ultrastructural analysis. Cytoplasmic organelles in neurons and glial cells, and the fine structure of synapses and myelinated fibers were well preserved. The localization of gold particles after immunostaining for astrocytic glutamate transporter (GLAST), metabotropic glutamate receptor 1 (mGluR1) and neurofilament protein was consistent with previous reports and ultrastructure was well-preserved in all cases. These findings should be helpful to researchers wishing to carry out ultrastructural and immunogold analyses of cryosections of nervous tissue.

Immunocytochemical studies on translocation of phosphorylated aquaporin-h2 protein in granular cells of the frog urinary bladder before and after stimulation with vasotocin

We have generated a specific antibody against phosphorylated aquaporin-h2 (pAQP-h2) protein to investigate the role of phosphorylation in the translocation of AQP-h2 protein within the granule cells of the urinary bladder of the frog (Hyla japonica). The antibody was generated against a synthetic peptide (ST-160) corresponding to amino acids 255-268, with a phosphorylated Ser-262, a residue that is putatively phosphorylated by protein A kinase. Using this antibody, we found, by Western blot analysis, that phosphorylation of the AQP-h2 protein rapidly increased within 2 min after vasotocin (AVT) stimulation and remained at a higher than normal level for 15 min. Moreover, quantitative immunoelectron microscopy indicated that the location of the AQP-h2 protein dramatically changed after AVT stimulation. Before stimulation, pAQP-h2 protein was localized in only a small number of intracellular vesicles near the nucleus of the granular cells, whereas the labeling density of the intracellular vesicles and the apical membrane rapidly increased after stimulation. This finding was also confirmed by the results of an immunofluorescence study. Thus, phosphorylation of AQP-h2 protein seems to be essential for translocation of the protein from the cytoplasmic pool to the apical plasma membrane of the granular cells in frog urinary bladder.

Exocytotic "constipation" is a mechanism of tubulin/lysosomal interaction in colchicine myopathy

Colchicine, a known microtubule disrupting agent, produces a human myopathy, characterized by accumulation of lysosomes. We have created a reliable animal model of colchicine myopathy that replicates the subacute myopathy seen in humans, reproducing the chronic proximal weakness and vacuolar changes in nonnecrotic myofibers. If a microtubule network plays a role in lysosomal function in muscle, disturbance of it could alter degradation of intrinsic membrane receptors, presumably at some intracellular processing site or at exocytosis. Thus, we examined, as a possible cellular pathogenesis of colchicine myopathy, how the muscle cytoskeleton affects the degradation of membrane proteins, which are processed through the endosomal/lysosomal pathway. We used the acetylcholine receptor as a model membrane component in cultured myotubes allowed to preincubate with colchicine. We tested at which step colchicine interferes with receptor trafficking by accounting for internalization, delivery to lysosomes, hydrolysis, or exocytotic release of debris. We report that colchicine significantly decreases the exocytosis of AChRs but does not affect receptor internalization, lysosomal hydrolysis, or the number of surface membrane receptors. Further, our immunofluorescence observations revealed a morphologic tubulin network in rat skeletal muscle that is more densely distributed in white (mitochondria-poor) muscle fibers than in red (mitochondria-rich) fibers but is present in both. Ultrastructurally, immunogold labeling localized tubulin in the intermyofibrillar region in a long and linear fashion, unassociated with myofibers or mitochondria. Taken together, our findings suggest the following: (1) Microtubules likely play a functional role in the pathway of lysosomal degradation in normal adult skeletal muscle; (2) The observed decrease in overall apparent degradation of membrane receptors by colchicine must be due primarily to inhibition of exocytosis. These data indicate that lysosomal "constipation" underlies colchicine myopathy. (3) An animal model faithful to the human disorder will allow further pathogenetic studies.

Structural integrity of the Golgi stack is essential for normal secretory functions of rat parotid acinar cells: effects of brefeldin A and okadaic acid

We examined the effects of specific inhibitors, brefeldin A (BFA) and okadaic acid (OA), on the ultrastructural organization of the Golgi apparatus and distributions of amylase, Golgi-associated proteins, and cathepsin D in the rat parotid acinar cells. BFA induced a rapid regression of the Golgi stack into rudimentary Golgi clusters composed of tubulovesicules, in parallel with a redistribution of the Golgi-resident proteins and a coat protein (beta-COP) into the region of the rough endoplasmic reticulum (rER) or cytosol. The rapid disruption of the Golgi stack could also be induced by the effect of OA. However, redistribution of the Golgi proteins in rER or cytosol could not be observed and beta-COP was not dispersed but was retained on the rudimentary Golgi apparatus. These findings suggested that the mechanism of OA in inducing degeneration of the Golgi stack was markedly different from that of BFA. In addition, missorting of amylase, a Golgi protein, and cathepsin D into incorrect transport pathways is apparent in the course of the disruption of the Golgi stack by OA. These Golgi-disrupting effects are reversible and the reconstruction of the stacked structure of the Golgi apparatus started immediately after the removal of inhibitors. In the recovery processes, missorting was also observed until the integrated structure of the Golgi apparatus was completely reconstructed. This suggested that the integrated structure of the Golgi apparatus was quite necessary for the occurrence of normal secretory events, including proper sorting of molecules.

GBF1, a guanine nucleotide exchange factor for ADP-ribosylation factors, is localized to the cis-Golgi and involved in membrane association of the COPI coat

Formation of coated carrier vesicles, such as COPI-coated vesicles from the cis-Golgi, is triggered by membrane binding of the GTP-bound form of ADP-ribosylation factors. This process is blocked by brefeldin A, which is an inhibitor of guanine nucleotide exchange factors for ADP-ribosylation factor. GBF1 is one of the guanine nucleotide-exchange factors for ADP-ribosylation factor and is localized in the Golgi region. In the present study, we have determined the detailed subcellular localization of GBF1. Immunofluorescence microscopy of cells treated with nocodazole or incubated at 15 degrees C has suggested that GBF1 behaves similarly to proteins recycling between the cis-Golgi and the endoplasmic reticulum. Immunoelectron microscopy has revealed that GBF1 localizes primarily to vesicular and tubular structures apposed to the cis-face of Golgi stacks and minor fractions to the Golgi stacks. GBF1 overexpressed in cells causes recruitment of class I and class II ADP-ribosylation factors onto Golgi membranes. Furthermore, overexpressed GBF1 antagonizes various effects of brefeldin A, such as inhibition of membrane recruitment of ADP-ribosylation factors and the COPI coat, and redistribution of Golgi-resident and itinerant proteins. These observations indicate that GBF1 is involved in the formation of COPI-coated vesicles from the cis-Golgi or the pre-Golgi intermediate compartment through activating ADP-ribosylation factors.

Application of microwave technology to the processing and immunolabeling of plastic-embedded and cryosections

We have adapted existing microwave irradiation (MWI) protocols and applied them to the processing and immunoelectron microscopy of both plastic-embedded and frozen sections. Rat livers were fixed by rapid MW irradiation in a mild fixation solution. Fixed liver tissue was either cryosectioned or dehydrated and embedded in Spurr's, Unicryl, or LR White resin. Frozen sections and sections of acrylic-embedded tissue were immunolabeled in the MW oven with an anti-catalase antibody, followed by gold labeling. Controls were processed conventionally at room temperature (RT). The use of MWI greatly shortened the fixation, processing, and immunolabeling times without compromising the quality of ultrastructural preservation and the specificity of labeling. The higher immunogold labeling intensity was achieved after a 15-min incubation of primary antibody and gold markers under discontinued MWI at 37C. Quantification of the immunolabeling for catalase indicated a density increase of up to fourfold in the sections immunolabeled in the MW oven over that of samples immunolabeled at RT. These studies define the general conditions of fixation and immunolabeling for both acrylic resin-embedded material and frozen sections.

Transient overexpression of human H- and L-ferritin chains in COS cells

The understanding of the in vitro mechanisms of ferritin iron incorporation has greatly increased in recent years with the studies of recombinant and mutant ferritins. However, little is known about how this protein functions in vivo, mainly because of the lack of cellular models in which ferritin expression can be modulated independently from iron. To this aim, primate fibroblastoid COS-7 cells were transiently transfected with cDNAs for human ferritin H- and L-chains under simian virus 40 promoter and analysed within 66 h. Ferritin accumulation reached levels 300-500-fold higher than background, with about 40% of the cells being transfected. Thus ferritin concentration in individual cells was increased up to 1000-fold over controls with no evident signs of toxicity. The exogenous ferritin subunits were correctly assembled into homopolymers, but did not affect either the size or the subunit composition of the endogenous heteropolymeric fraction of ferritin, which remained essentially unchanged in the transfected and non-transfected cells. After 18 h of incubation with [59Fe]ferric-nitrilotriacetate, cellular iron incorporation was similar in the transfected and non-transfected cells and most of the protein-bound radioactivity was associated with ferritin heteropolymers, while H- and L-homopolymers remained iron-free. Cell co-transfection with cDNAs for H- and L-chains produced ferritin heteropolymers that also did not increase cellular iron incorporation. It is concluded that transient transfection of COS cells induces a high level of expression of ferritin subunits that do not co-assemble with the endogenous ferritins and have no evident activity in iron incorporation/metabolism.

Characterization of UT2 cells. The induction of peroxisomal 3-hydroxy-3-methylglutaryl-coenzyme a reductase

In the liver 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is present not only in the endoplasmic reticulum but also in the peroxisomes. However, to date no information is available regarding the function of the peroxisomal HMG-CoA reductase in cholesterol/isoprenoid metabolism, and the structure of the peroxisomal HMG-CoA reductase has yet to be determined. We have identified a mammalian cell line that expresses only one HMG-CoA reductase protein and that is localized exclusively to peroxisomes. This cell line was obtained by growing UT2 cells (which lack the endoplasmic reticulum HMG-CoA reductase) in the absence of mevalonate. The cells exhibited a marked increase in a 90-kDa HMG-CoA reductase that was localized exclusively to peroxisomes. The wild type Chinese hamster ovary cells contain two HMG-CoA reductase proteins, the well characterized 97-kDa protein, localized in the endoplasmic reticulum, and a 90-kDa protein localized in peroxisomes. The UT2 cells grown in the absence of mevalonate containing the up-regulated peroxisomal HMG-CoA reductase are designated UT2*. A detailed characterization and analysis of this cell line is presented in this study.

Amphiphysin II (SH3P9; BIN1), a member of the amphiphysin/Rvs family, is concentrated in the cortical cytomatrix of axon initial segments and nodes of ranvier in brain and around T tubules in skeletal muscle

Amphiphysin (amphiphysin I), a dominant autoantigen in paraneoplastic Stiff-man syndrome, is a neuronal protein highly concentrated in nerve terminals, where it has a putative role in endocytosis. The yeast homologue of amphiphysin, Rvs167, has pleiotropic functions, including a role in endocytosis and in actin dynamics, suggesting that amphiphysin may also be implicated in the function of the presynaptic actin cytoskeleton. We report here the characterization of a second mammalian amphiphysin gene, amphiphysin II (SH3P9; BIN1), which encodes products primarily expressed in skeletal muscle and brain, as differentially spliced isoforms. In skeletal muscle, amphiphysin II is concentrated around T tubules, while in brain it is concentrated in the cytomatrix beneath the plasmamembrane of axon initial segments and nodes of Ranvier. In both these locations, amphiphysin II is colocalized with splice variants of ankyrin3 (ankyrinG), a component of the actin cytomatrix. In the same regions, the presence of clathrin has been reported. These findings support the hypothesis that, even in mammalian cells, amphiphysin/Rvs family members have a role both in endocytosis and in actin function and suggest that distinct amphiphysin isoforms contribute to define distinct domains of the cortical cytoplasm. Since amphiphysin II (BIN1) was reported to interact with Myc, it may also be implicated in a signaling pathway linking the cortical cytoplasm to nuclear function.

Immunocytochemical and immunoelectron microscopical analysis of BCL2 expression in thyroid oxyphilic tumors

In both human lymphoma and carcinoma, BCL2 expression contributes to the neoplastic development by preventing normal turnover due to programmed cell death. In thyroid carcinoma it was shown that BCL2 expression is related to cell differentiation and appears to be associated with a less aggressive behavior. Using immunogold electron microscopy labeling techniques on ultrathin frozen sections, the authors undertook an analysis of bcl-2 protein localization in 7 cases of thyroid tumor with (4 cases) or without (3 cases) oncocytic differentiation, which displayed histopathological features, corresponding to different degree of aggressiveness. All thyroid tumors and a lymphoblastoid cell line overexpressing BCL2 showed a preferential labeling of mitochondrial circumference, while the 4 cases with oncocytic differentiation displayed a bcl-2 protein location, not only on the circumference but also on the cristae of a consistent number of mitochondria. These results may indicate a bcl-2 protein interaction with intramitochondrial constituents related to the neoplastic transformation in oncocytic tumor.

Immunohistochemical localization of cell adhesion molecules and cell-cell contact proteins during regeneration of the rat optic nerve induced by sciatic nerve autotransplantation

BACKGROUND

The central nervous system neurons of adult mammals are known to regenerate into peripheral nerve autograft. The localization of cell adhesion molecules and cell-cell contact proteins were studied during axonal regeneration induced by sciatic nerve autotransplantation.

METHODS

A sciatic nerve autograft was anastomosed to the proximal stump of the transected rat optic nerve. Immunofluorescence microscopy, thin sectioning, and immunoelectron microscopy with the preembedding method and ultrathin cryosections were used to localize cell adhesion molecules (L1; neural cell adhesion molecule, NCAM; myelin-associated glycoprotein, MAG) and cell-cell contact proteins (connexins 32, 43, ZO-1) at 3 days to 4 weeks postoperation.

RESULTS

Most regenerating axons contacted astrocytes in the optic nerve and Schwann cells in the graft. Immunoreactivity of NCAM was widely distributed along the surface of axons, astrocytes, Schwann cells, and perineurial cells. The L1 immunoreactivity was confined to the interface of axon-astrocyte and of axon-Schwann cell. MAG immunoreactivity was seen at the interface of axon and myelin within the graft. Connexins 32, 43, and ZO-1 immunoreactivities were observed at contact sites between axons and Schwann cells within the graft.

CONCLUSIONS

Cell adhesion molecules (L1, NCAM, MAG) are localized at the cell surface of regenerating axons, astrocytes, and Schwann cells during optic nerve regeneration elicited by peripheral nerve graft. Cell-cell contact proteins (connexins 32, 43, ZO-1) are present at the interface between axons and Schwann cells in the graft. Our results suggest that these molecules are involved in cell adhesion events during optic nerve regeneration.

Influence of specimen preparation on the identification of phospholipids by the phospholipase A2-gold method in mineralizing cartilage and bone

The role of phospholipids in biological mineralization has been hypothesized but not fully elucidated. In order to identify phospholipids at the ultrastructural level in the mineralizing extracellular matrix, rat epiphyseal cartilage and metaphyseal bone have been labeled with the phospholipase A(2) (PLA(2))-gold method. The specificity and the efficiency of phospholipid detection have been evaluated by postembedding labeling of sections from epoxy- or hydrophilic resin-embedded samples, and by preembedding labeling of cryosectioned samples. The efficiency of the labeling was higher in cryosections than in hydrophilic resin-embedded specimens, while lower efficiency was found in epoxy resin-embedded samples. A 2- to 6-fold increase of the labeling density in calcified with respect to uncalcified areas of cartilage and bone has been found, depending on the specimen preparation used. The labeling intensity was significantly higher, at the periphery of the calcifying nodules in the epiphyseal cartilage matrix and in the calcifying osteoid, while the fully calcified bone matrix presented a weak labeling. Matrix vesicles, which are considered a possible source of extracellular phospholipids, appeared labeled in cryosections and in epoxy resin-embedded samples after a preincubation with PLA(2), which also increased the labeling of the intracellular membranes. The localization of phospholipids in the areas of initial mineralization suggests some hypotheses on the possible involvement of these molecules in the mineral phase deposition process.

Hepatic Na(+)-K(+)-ATPase enzyme activity correlates with polarized beta-subunit expression

We have examined underlying causes for observations made in hepatocytes in which catalytic subunits of Na(+)-K(+)-ATPase are found both in bile canalicular (apical) and sinusoidal (basolateral) membrane domains, whereas functional activity is associated preferentially with sinusoidal membrane sites. In a series of parallel studies, we determined by both light and electron microscopy that Na(+)-K(+)-ATPase alpha-subunits were localized to both membrane domains of hepatocytes. With the use of purified liver plasma membrane subfractions, ouabain inhibition curves demonstrated similar inhibition constants (inhibition constant 10(-5) M), and immunoblots using alpha 1-, alpha 2-, and alpha 3-polyclonal and monoclonal antibodies demonstrated antigenic sites predominantly for alpha 1 in both membrane fractions. Also, Northern blot hybridization analysis revealed only the alpha 1-isoform in hepatocytes. In contrast to the bipolar distribution of the alpha 1-subunit, the beta-subunit was identified only at the sinusoidal surface using fluorescence labeling with a monoclonal antibody. The beta 1-isoform was demonstrated by Northern blot analysis and was present predominantly at the sinusoidal domain by immunoblotting with polyclonal antibodies. In addition to the bipolar distribution of alpha 1, immunoblotting of liver plasma membrane subfractions demonstrated a symmetrical distribution of fodrin, ankyrin, actin, and E-cadherin at both domains. These results suggest that functionally competent alpha/beta-complexes form at the sinusoidal domain, whereas only alpha 1-subunits are present at the apical pole.

Differential localization and functional role of calsequestrin in growing and differentiated myoblasts

Calsequestrin (CSQ) is the low affinity, high capacity Ca(2+)-binding protein concentrated within specialized areas of the muscle fiber sarcoplasmic reticulum (a part of the ER) where it is believed to buffer large amounts of Ca2+. Upon activation of intracellular channels this Ca2+ pool is released, giving rise to the [Ca2+]i increases that sustain contraction. In order to investigate the ER retention and the functional role of the protein, L6 rat myoblasts were infected with a viral vector with or without the cDNA of chicken CSQ, and stable clones were investigated before and after differentiation to myotubes. In the undifferentiated L6 cells, expression of considerable amounts of heterologous CSQ occurred with no major changes of other ER components. Ca2+ release from the ER, induced by the peptide hormone vasopressin, remained however unchanged, and the same occurred when other treatments were given in sequence to deplete the ER and other intracellular stores: with the Ca2+ pump blocker, thapsigargin; and with the Ca2+ ionophore, ionomycin, followed by the Na+/H+ ionophore, monensin. The lack of effect of CSQ expression on the vasopressin-induced [Ca2+]i responses was explained by immunocytochemistry showing the heterologous protein to be localized not in the ER but in large vacuoles of acidic content, positive also for the lysosomal enzyme, cathepsin D, corresponding to a lysosomal subpopulation. After differentiation, all L6 cells expressed small amounts of homologous CSQ. In the infected cells the heterologous protein progressively decreased, yet the [Ca2+]i responses to vasopressin were now larger with respect to both control and undifferentiated cells. This change correlated with the drop of the vacuoles and with the accumulation of CSQ within the ER lumen, where a clustered distribution was observed as recently shown in developing muscle fibers. These results provide direct evidence for the contribution of CSQ, when appropriately retained, to the Ca2+ capacity of the rapidly exchanging, ER-located Ca2+ stores; and for the existence of specific mechanism(s) (that in L6 cells develop in the course of differentiation) for the ER retention of the protein. In the growing L6 myoblasts the Ca(2+)-binding protein appears in contrast to travel along the exocytic pathway, down to post-Golgi, lysosome-related vacuoles which, based on the lack of [Ca2+]i response to ionomycin-monensin, appear to be incompetent for Ca2+ accumulation.

A novel class of autoantigens of anti-neutrophil cytoplasmic antibodies in necrotizing and crescentic glomerulonephritis: the lysosomal membrane glycoprotein h-lamp-2 in neutrophil granulocytes and a related membrane protein in glomerular endothelial cells

Necrotizing and crescentic glomerulonephritis (NCGN) is frequently associated with circulating antineutrophil cytoplasmic autoantibodies (ANCA). It is established that ANCA are specific for soluble enzymes of granules of polymorphonuclear neutrophil granulocytes (PMN), such as myeloperoxidase (MPO) or protease 3 (PR3). The purpose of this study was to identify membrane proteins of PMNs, and/or glomerular cells, as additional autoantigenic ANCA targets. When membrane protein fractions were prepared from PMNs and isolated human glomeruli, and immunoblotted with ANCA sera of NCGN patients, two bands with apparent molecular masses of 170 and 80-110 kD (gp170/80-110) were labeled in PMNs, and a 130-kD glycoprotein (gp130) in glomeruli. Gp130 was purified, and monoclonal and rabbit antibodies (Abs) were produced which showed the same double specificity as the patient's ANCA. Using these probes, evidence was provided that gp170/80-110 is identical with human lysosomal-associated membrane protein 2 (h-lamp-2), because both proteins were immunologically cross-reactive and screening of a cDNA expression library from human promyelocytic leukemia cells with anti-gp130 Ab yielded a clone derived from h-lamp-2. Gp170/80-110 was localized primarily in granule membranes of resting PMNs, and was translocated to the cell surfaces by activation with FMLP. By contrast, gp130 was localized in the surface membranes of endothelial cells of human glomerular and renal interstitial capillaries, rather than in lysosomes, as found for h-lamp-2. Potential clinical relevance of autoantibodies to gp170/80-110 and gp130 was assessed in a preliminary trial, in which ANCA sera of patients (n = 16) with NCGN were probed with purified or recombinant antigens. Specific reactivity was detected in approximately 90% of cases with active phases of NCGN, and frequently also in combination with autoantibodies specific for PR3 or MPO. Collectively, these data provide evidence that h-lamp-2 in PMNs and a different, structurally related 130-kD membrane protein on the cell surface of renal microvascular endothelial cells are autoantigenic targets for ANCA in patients with active NCGN.

The yeast homolog of H < beta > 58, a mouse gene essential for embryogenesis, performs a role in the delivery of proteins to the vacuole

The highly conserved and ubiquitously expressed mouse gene H < beta > 58, identified through insertional mutagenesis, has been shown to be essential for early postimplantation development in mouse, but the mechanism by which it acts is unknown (Radice et al. 1991; Lee et al. 1992). We report here the identification of a yeast gene related to the H < beta > 58 gene and provide biochemical and genetic evidence for its function within the cell. The gene, PEP8, plays a role in the delivery of proteins to the vacuole. Disruption of the gene did not affect cell viability. However, the disruptants were shown to have a defect in the processing of the soluble vacuolar proteases but not the membrane vacuolar hydrolases. The processing defect appeared to be a consequence of the inability of the soluble vacuolar hydrolase to reach the vacuole. Although a small amount of the vacuolar precursors was mis-sorted to the extracellular medium, mis-sorting did not appear to be the primary defect in these cells. Pep8p was identified by epitope tagging of the protein. Biochemical fractionation indicated that the protein was peripherally bound to membranes. Immuno-gold electron microscopy indicated that the Pep8p localized to vacuolar membranes. Complementation experiments with the mouse H < beta > 58 cDNA revealed that a Pep8p-H < beta > 58 fusion protein in which the carboxy-terminal 85 amino acids of Pep8p were replaced by the carboxy-terminal 115 amino acids of H < beta > 58 was functional.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the integrin alpha 6 beta 4 in peripheral nerves: localization in Schwann and perineural cells and different variants of the beta 4 subunit

Integrin alpha 6 beta 4 is expressed in human peripheral nerves, but not in the central nervous system. This integrin heterodimer has previously been found in perineural fibroblast-like cells and in Schwann cells (SCs), which both assemble a basement membrane but do not form hemidesmosomes. We show here that in SCs, which had formed a myelin sheath, alpha 6 beta 4 was enriched in the proximity of the nucleus, at Ranvier paranodal areas and at Schmitt-Lanterman clefts; alpha 6 beta 4 was also found at the grooved interface between small axons and non-myelinating SCs. Immunoprecipitation of human peripheral nerves, in combination with Western blotting showed that beta 4 is associated with the alpha 6A subunit. Northern blot analysis of human peripheral nerves showed a single beta 4 transcript of 6 kb. Using the reverse transcriptase polymerase chain reaction, we detected two mRNA species, one for the most common (−70, -53) form of beta 4 and the other encoding the (+53) variant of beta 4. Cultured SCs were devoid of alpha 6 beta 4 but expressed alpha 6 beta 1, indicating that SCs lose beta 4 expression when contact with neurons is lost. Thus, resting SCs in contact with axons express alpha 6A in combination with beta 4, irrespective of myelin formation. We suggest that alpha 6 beta 4 expressed in SCs plays a role in peripheral neurogenesis.

The endoplasmic-sarcoplasmic reticulum of smooth muscle: immunocytochemistry of vas deferens fibers reveals specialized subcompartments differently equipped for the control of Ca2+ homeostasis

Cryosection immunofluorescence and immunogold labeling with antibodies against specific markers were used in rat vas deferens smooth muscle fibers to reveal the molecular arrangement of the endomembrane system (referred to variously in the text as ER or sarcoplasmic reticulum [SR]; S-ER or ER/SR) known to participate in the control of Ca2+ homeostasis. The lumenal ER chaperon, immunoglobulin binding protein (BiP), as well as protein disulfide isomerase, and calreticulin, a Ca2+ binding protein expressed by most eukaryotic cells, appeared to be evenly distributed throughout the entire system (i.e., within [a] the nuclear envelope and the few rough-surfaced cisternae clustered near the nucleus; [b] single elements scattered around in the contractile cytoplasm; and [c] numerous, heterogeneous, mainly smooth-surfaced elements concentrated in the peripheral cytoplasm, part of which is in close apposition to the plasmalemma). All other structures, including nuclei, mitochondria, Golgi complex, and surface caveolae were unlabeled. An even distribution throughout the endomembrane system appeared also for the proteins recognized by anti-ER membrane antibodies. In contrast, calsequestrin (the protein that in striated muscles is believed to be the main actor of the rapidly exchanging Ca2+ storage within the lumen of the sarcoplasmic reticulum) was found preferentially clustered at discrete lumenal sites, most often within peripheral smooth-surfaced elements of moderate electron density. Within these elements dual labeling revealed intermixing of calsequestrin with the other lumenal ER proteins. Moreover, the calsequestrin-rich elements were enriched also in the receptor for inositol 1,4,5-trisphosphate, the second messenger that induces Ca2+ release from intracellular stores. These results document the previously hypothesized molecular heterogeneity of the smooth muscle endomembrane system, particularly in relation to the rapid storage and release of Ca2+.

The C-terminal tripeptide of glycosomal phosphoglycerate kinase is both necessary and sufficient for import into the glycosomes of Trypanosoma brucei

Glycosomal phosphoglycerate kinase (gPGK) of Trypanosoma brucei differs from the cytoplasmic isozyme (cPGK) in its higher isoelectric point characterized by clusters of positive charges along the polypeptide chain, and a 20 amino acid C-terminal extension ending in serine-serine-leucine (SSL). While a C-terminal SSL tripeptide is apparently not capable of directing luciferase to the peroxisomes in mammalian cells [J. Cell Biol. 108 (1989), 1657-1664], we show here that it is sufficient for the import of luciferase as well as an unrelated protein, beta-glucuronidase, into the glycosomes of T. brucei, as determined by immunoelectron microscopy. The analysis of luciferase-gPGK fusion proteins indicates that the only targeting signal for import of gPGK into the glycosome resides in this C-terminal SSL sequence.

Overlapping distribution of two glycosyltransferases in the Golgi apparatus of HeLa cells

Thin, frozen sections of a HeLa cell line were double labeled with specific antibodies to localize the trans-Golgi enzyme, beta 1,4 galactosyltransferase (GalT) and the medial enzyme, N-acetylglucosaminyltransferase I (NAGT I). The latter was detected by generating a HeLa cell line stably expressing a myc-tagged version of the endogenous protein. GalT was found in the trans-cisterna and trans-Golgi network but, contrary to expectation, NAGT I was found both in the medial- and trans-cisternae, overlapping the distribution of GalT. About one third of the NAGT I and half of the GalT were found in the shared, trans-cisterna. These data show that the differences between cisternae are determined not by different sets of enzymes but by different mixtures.

Topography of opsin within disk and plasma membranes revealed by a rapid-freeze deep-etch technique

Rod outer segments in fresh rat retinas were examined by a rapid-freeze, deep-etch technique to explore how membrane proteins are organized at the macromolecular level. Cross-fractures revealed that intradiscal membranes are adherent to each other except at the rim. When an isolated fresh retina was incubated in a hypotonic solution for a few minutes, the interdiscal space was expanded and the cytoplasmic surface of the disk membrane was found to be covered with protrusions except at the rim. A few particles were scattered among the protrusions and were attached to the cytoplasmic surface. Since the distribution density of the cytoplasmic surface protrusions was similar to that of the P-face particles, which are known to reflect opsins, the protrusions were considered to be portions of opsins extending into the cytoplasm. The intradiscal surfaces in chemically-fixed retinas were rather smooth and were labelled with anti-opsin antibodies and wheat germ agglutinin. The true surfaces of the plasma membrane were found to be similar in fine structure to those of the disk. A model of the macromolecular organization of rod outer segments is proposed on the basis of these observations. The model shows apposed opsins within a disk membrane adhering to one another except at the rim. These opsins, as well as those in the plasma membrane, are minimally exposed to the extracellular surface, but protrude deeply into the cytoplasm.

Immunohistochemistry of cholinergic receptors

Acetylcholine and its receptors are involved in a variety of important signal transduction processes. As shown here paradigmatically for the human neuromuscular junction and the cerebral cortex, acetylcholine receptors can be visualized immunohistochemically at the cellular and subcellular level under physiological and pathological conditions. At normal motor endplates nicotinic cholinoceptors are localized at the surface of the postsynaptic junctional folds. In myasthenic syndromes investigation of muscle biopsies enables the diagnosis of receptor deficiencies at the ultrastructural level. In normal cerebral cortex pyramidal neurons are equipped with both nicotinic and muscarinic acetylcholine receptors localized to postsynaptic densities. In neuropsychiatric diseases cholinoceptor expression can be monitored at the cellular level by quantitative assessment of immunolabeled cortical neurons.

In vivo import of firefly luciferase into the glycosomes of Trypanosoma brucei and mutational analysis of the C-terminal targeting signal

The compartmentalization of glycolytic enzymes into specialized organelles, the glycosomes, allows the bloodstream form of Trypanosoma brucei to rely solely on glycolysis for its energy production. The biogenesis of glycosomes in these parasites has been studied intensively as a potential target for chemotherapy. We have adapted the recently developed methods for stable transformation of T. brucei to the in vivo analysis of glycosomal protein import. Firefly luciferase, a peroxisomal protein in the lantern of the insect, was expressed in stable transformants of the procyclic form of T. brucei, where it was found to accumulate inside the glycosomes. Mutational analysis of the peroxisomal targeting signal serine-lysine-leucine (SKL) located at the C-terminus of luciferase showed that replacement of the serine residue (Serine548) with a small neutral amino acid (A, C, G, H, N, P, T) still resulted in an import efficiency of 50-100% of the wild-type luciferase. Lysine549 could be substituted with an amino acid capable of hydrogen bonding (H, M, N, Q, R, S), whereas the C-terminal leucine550 could be replaced with a subset of hydrophobic amino acids (I, M, Y). Thus, a peroxisome-like C-terminal SKL-dependent targeting mechanism may function in T. brucei to import luciferase into the glycosomes. However, a few significant differences exist between the glycosomal targeting signals identified here and the tripeptide sequences that direct proteins to mammalian or yeast peroxisomes.

The endoplasmic reticulum-sarcoplasmic reticulum connection: distribution of endoplasmic reticulum markers in the sarcoplasmic reticulum of skeletal muscle fibers

The skeletal muscle sarcoplasmic reticulum (SR) was investigated for the presence of well-known endoplasmic reticulum (ER) markers: the lumenal protein BiP and a group of membrane proteins recognized by an antibody raised against ER membrane vesicles. Western blots of SR fractions revealed the presence of BiP in fast- and slow-twitch muscles of the rabbit as well as in rat and chicken muscles. Analyses of purified SR subfractions, together with cryosection immunofluorescence and immunogold labeling, revealed BiP evenly distributed within the longitudinal SR and the terminal cisternae. Within the terminal cisternae BiP appeared not to be mixed with calsequestrin but to be distributed around the aggregates of the latter Ca2+ binding protein. Of the various membrane markers only calnexin (91 kDa) was found to be distributed within both SR subfractions, whereas the other markers (apparent molecular masses of 64 kDa and 58 kDa and a doublet around 28 kDa) were concentrated in the terminal cisternae. These results suggest that the SR is a specialized ER subcompartment in which general markers, such as the ones we have investigated, coexist with the major SR proteins specifically responsible for Ca2+ uptake, storage, and release. The differential distribution of the ER markers reveals new aspects of the SR molecular structure that might be of importance for the functioning of the endomembrane system.

O-glycosylation of intact and truncated ribophorins in brefeldin A-treated cells: newly synthesized intact ribophorins are only transiently accessible to the relocated glycosyltransferases

Ribophorins I and II are type I transmembrane glycoproteins of the ER that are segregated to the rough domains of this organelle. Both ribophorins appear to be part of the translocation apparatus for nascent polypeptides that is associated with membrane-bound ribosomes and participate in the formation of a proteinaceous network within the ER membrane that also includes other components of the translocation apparatus. The ribophorins are both highly stable proteins that lack O-linked sugars but each contains one high mannose N-linked oligosaccharide that remains endo H sensitive throughout their lifetimes. We have previously shown (Tsao, Y. S., N. E. Ivessa, M. Adesnik, D. D. Sabatini, and G. Kreibich. 1992. J. Cell Biol. 116:57-67) that a COOH-terminally truncated variant of ribophorin I that contains only the first 332 amino acids of the luminal domain (RI332), when synthesized in permanent transformants of HeLa cells, undergoes a rapid degradation with biphasic kinetics in the ER itself and in a second, as yet unidentified nonlysosomal pre-Golgi compartment. We now show that in cells treated with brefeldin A (BFA) RI332 molecules undergo rapid O-glycosylation in a multistep process that involves the sequential addition of N-acetylgalactosamine, galactose, and terminal sialic acid residues. Addition of O-linked sugars affected all newly synthesized RI332 molecules and was completed soon after synthesis with a half time of about 10 min. In the same cells, intact ribophorins I and II also underwent O-linked glycosylation in the presence of BFA, but these molecules were modified only during a short time period immediately after their synthesis was completed, and the modification affected only a fraction of the newly synthesized polypeptides. More important, these molecules synthesized before the addition of BFA were not modified by O-glycosylation. The same is true for ribophorin I when overexpressed in HeLa cells although it is significantly less stable than the native polypeptide in control cells. We, therefore, conclude that soon after their synthesis, ribophorins lose their susceptibility to the relocated Golgi enzymes that effect the O-glycosylation, most likely as a consequence of a conformational change in the ribophorins that occurs during their maturation, although it cannot be excluded that rapid integration of these molecules into a supramolecular complex in the ER membrane leads to their inaccessibility to these enzymes.

Articular-cartilage matrix gamma-carboxyglutamic acid-containing protein. Characterization and immunolocalization

Matrix gamma-carboxyglutamic acid (Gla)-containing protein (MGP) was found to be present in articular cartilage by Western-blot analysis of guanidinium chloride extracts of human and bovine cartilage and was further localized by immunohistochemical studies on human and monkey specimens. In newborn articular cartilage MGP was present diffusely throughout the matrix, whereas in growth-plate cartilage it was seen mainly in late hypertrophic and calcifying-zone chondrocytes. In adult articular cartilage MGP was present primarily in chondrocytes and the pericellular matrix. Immunoelectron microscopy studies revealed an association between MGP and vesicular structures with an appearance consistent with matrix vesicles. MGP may be an important regulator of cartilage calcification because of its localization in cartilage and the known affinity of Gla-containing proteins for Ca2+ and hydroxyapatite.

Use of ultrastructural immunohistochemistry in human pituitary pathology

Recent advances in ultrastructural immunohistochemistry have provided insight into not only the subcellular localization of single antigens but also the colocalization of two distinct antigens in the same cellular constituent. In the field of pituitary pathology, precise identification of cell types, mechanism of processing, and dynamic intracellular transportation of hormones, as well as production of multiple hormones in the same cells of nontumorous and neoplastic adenohypophyses, have been documented by use of these techniques. The present review deals with the use of major methods for ultrastructural immunohistochemistry including pre-, post-, and non-embedding methods, particularly focusing on their application to human pituitary pathology. Problems of tissue processing and a protocol for double labeling technique using the protein A-gold complex are also described.

Immunohistochemical localization of Na(+)-dependent glucose transporter in rat jejunum

Glucose is actively absorbed via a Na(+)-dependent active glucose transporter (Na-GT) in the small intestine. We raised a polyclonal antibody against the peptide corresponding to amino acids 564-575 of rabbit intestinal Na-GT, and localized it immunohistochemically in the rat jejunum. By means of immunofluorescence staining, Na-GT was located at the brush border of the absorptive epithelial cells of the intestinal villi. Electron-microscopic examination showed that Na-GT was localized at the plasma membrane of the apical microvilli of these cells. Little Na-GT was found at the basolateral plasma membrane. Along the crypt-villus axis, all of the absorptive epithelial cells in the villus were positive for Na-GT. In addition to the brush border staining, the supranuclear positive staining, which was shown to be the Golgi apparatus by use of electron microscopy, was seen in cells located between the base to the middle of the villus. Cells in crypts exhibited little or no staining for Na-GT. Goblet cells scattered in the intestinal epithelium were negative for Na-GT staining. These observations show that Na-GT is specific to the apical plasma membrane of the absorptive epithelial cells, and that the onset of Na-GT synthesis may occur near the crypt-villus junction.

A 220-kD undercoat-constitutive protein: its specific localization at cadherin-based cell-cell adhesion sites

Recently we developed an isolation procedure for the cell-to-cell adherens junctions (AJ; cadherin-based junctions) from rat liver (Tsukita, Sh. and Sa. Tsukita. 1989. J. Cell Biol. 108:31-41). In this study, using the isolated AJ, we have obtained two mAbs specific to the 220-kD undercoat-constitutive protein. Immunofluorescence and immunoelectron microscopy with these mAbs showed that this 220-kD protein was highly concentrated at the undercoat of cell-to-cell AJ in various types of tissues and that this protein was located in the immediate vicinity of the plasma membrane in the undercoat of AJ. In the cells lacking typical cell-to-cell AJ, such as fibroblasts, the 220-kD protein was immunofluorescently shown to be coconcentrated with cadherin molecules at cell-cell adhesion sites. These localization analyses appeared to indicate the possible direct or indirect association of the 220-kD protein with cadherin molecules. Furthermore, it was revealed that the 220-kD protein and alpha-spectrin were coimmunoprecipitated with the above mAbs in both the isolated AJ and the brain. The affinity-purified 220-kD protein molecule looked like a spherical particle, and its binding site on the spectrin molecule was shown to be in the position approximately 10-20 nm from the midpoint of spectrin tetramer by low-angle rotary-shadowing electron microscopy. Taking all these results together with biochemical and immunological comparisons, we are persuaded to speculate that the 220-kD protein is a novel member of the ankyrin family. However, the possibility cannot be excluded that the 220-kD protein is an isoform of beta-spectrin. The possible roles of this 220-kD protein in the association of cadherin molecules with the spectrin-based membrane skeletons at the cadherin-based cell-cell adhesion sites are discussed.

Evolutionary conservation of a microbody targeting signal that targets proteins to peroxisomes, glyoxysomes, and glycosomes

Peroxisomes, glyoxysomes, glycosomes, and hydrogenosomes have each been classified as microbodies, i.e., subcellular organelles with an electron-dense matrix that is bound by a single membrane. We investigated whether these organelles might share a common evolutionary origin by asking if targeting signals used for translocation of proteins into these microbodies are related. A peroxisomal targeting signal (PTS) consisting of the COOH-terminal tripeptide serine-lysine-leucine-COOH has been identified in a number of peroxisomal proteins (Gould, S.J., G.-A. Keller, N. Hosken, J. Wilkinson, and S. Subramani. 1989. J. Cell Biol. 108:1657-1664). Antibodies raised to a peptide ending in this sequence (SKL-COOH) recognize a number of peroxisomal proteins. Immunocryoelectron microscopy experiments using this anti-SKL antibody revealed the presence of proteins containing the PTS within glyoxysomes of cells from Pichia pastoris, germinating castor bean seeds, and Neurospora crassa, as well as within the glycosomes of Trypanosoma brucei. Western blot analysis of purified organelle fractions revealed the presence of many proteins containing this PTS in both glyoxysomes and glycosomes. These results indicate that at least one of the signals, and therefore the mechanism, for protein translocation into peroxisomes, glyoxysomes, and glycosomes has been conserved, lending support to a common evolutionary origin for these microbodies. Hydrogenosomes, the fourth type of microbody, did not contain proteins that cross-reacted with the anti-PTS antibody, suggesting that this organelle is unrelated to microbodies.

Cu,Zn superoxide dismutase is a peroxisomal enzyme in human fibroblasts and hepatoma cells

The intracellular localization of Cu,Zn superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) has been examined by immunofluorescence using four monoclonal anti-Cu,Zn superoxide dismutase antibodies raised against a recombinant human Cu,Zn superoxide dismutase derivative produced and purified from Escherichia coli. Colocalization with catalase, a peroxisomal matrix enzyme, was used to demonstrate the peroxisomal localization of Cu,Zn superoxide dismutase in human fibroblasts and hepatoma cells. In the fibroblasts of Zellweger syndrome patients, the enzyme is not transported to the peroxisomal ghosts but, like catalase, remains in the cytoplasm. In addition, immunocryoelectron microscopy of yeast cells expressing human Cu,Zn superoxide dismutase showed that the enzyme is translocated to the peroxisomes.

Intracellular Ca2+ stores in chicken Purkinje neurons: differential distribution of the low affinity-high capacity Ca2+ binding protein, calsequestrin, of Ca2+ ATPase and of the ER lumenal protein, Bip

To identify intracellular Ca2+ stores, we have mapped (by cryosection immunofluorescence and immunogold labeling) the distribution in the chicken cerebellar cortex of an essential component, the main low affinity-high capacity Ca2+ binding protein which in this tissue has been recently shown undistinguishable from muscle calsequestrin (Volpe, P., B. H. Alderson-Lang, L. Madeddu, E. Damiani, J. H. Collins, and A. Margreth. 1990. Neuron. 5:713-721). Appreciable levels of the protein were found exclusively within Purkinje neurons, distributed to the cell body, the axon, and the elaborate dendritic tree, with little labeling, however, of dendritic spines. At the EM level the protein displayed a dual localization: within the ER (rough- and smooth-surfaced cisternae, including the cisternal stacks recently shown [in the rat] to be highly enriched in receptors for inositol 1,4,5-triphosphate) and, over 10-fold more concentrated, within a population of moderately dense, membrane-bound small vacuoles and tubules, identified as calciosomes. These latter structures were widely distributed both in the cell body (approximately 1% of the cross-sectional area, particularly concentrated near the Golgi complex) and in the dendrites, up to the entrance of the spines. The distribution of calsequestrin was compared to those of another putative component of the Ca2+ stores, the membrane pump Ca2+ ATPase, and of the ER resident lumenal protein, Bip. Ca2+ ATPase was expressed by both calciosomes and regular ER cisternae, but excluded from cisternal stacks; Bip was abundant within the ER lumena (cisternae and stacks) and very low within calciosomes (average calsequestrin/Bip immunolabeling ratios were approximately 0.5 and 36.5 in the two types of structure, respectively). These results suggest that ER cisternal stacks do not represent independent Ca2+ stores, but operate coordinately with the adjacent, lumenally continuous ER cisternae. The ER and calciosomes could serve as rapidly exchanging Ca2+ stores, characterized however by different properties, in particular, by the greater Ca2+ accumulation potential of calciosomes. Hypotheses of calciosome biogenesis (directly from the ER or via the Golgi complex) are discussed.

Identification, kinetic properties and intracellular localization of the (Ca(2+)-Mg2+)-ATPase from the intracellular stores of chicken cerebellum

The microsomal fraction of chicken cerebellum expresses a large amount of Ca(2+)-ATPase (105 kDa), which is phosphorylated by ATP in the presence of Ca2+. The Ca(2+)-ATPase activity is highly sensitive to temperature and to the presence of detergents. This ATPase has kinetic properties similar to those of chicken skeletal-muscle sarcoplasmic reticulum, as (i) it is activated by low (microM) and inhibited by high (mM) Ca2+ concentrations, (ii) it shows biphasic activation with ATP and (iii) it is inhibited by vanadate. However, the vanadate-sensitivity is at least 10 times greater than that observed in chicken skeletal or cardiac sarcoplasmic-reticulum Ca(2+)-ATPases. Thus, despite cross-reacting with antibodies against the cardiac and skeletal isoforms, the cerebellar microsomal Ca(2+)-ATPase appears to be distinct from both muscle enzymes. The Ca(2+)-ATPase is concentrated in, but not exclusive to, Purkinje neurons. In Purkinje neurons the Ca(2+)-ATPase appears to be expressed throughout the cell body, the dendritic tree (and the spines) and the axons. At the electron-microscope level the Ca(2+)-ATPase is found in smooth and rough endoplasmic-reticulum cisternae as well as in other, yet unidentified, smooth-surfaced structures.

Immunohistochemical studies on the localisation and ontogeny of heart fatty acid binding protein in the rat

The localisation and ontogenic expression of heart fatty acid binding protein (H-FABP) were immunochemically examined in the entire body of the rat. In muscles, H-FABP immunoreactivity was observed not only in cardiac muscle cells but also in red skeletal muscles, where gold particles indicating the immunoreaction sites are distributed evenly in the sarcoplasm except for the interior of membranous organelles. In the urogenital system, the immunoreactivity was preferentially demonstrated in the distal tubular epithelial cells in the kidney, and in the superficial epithelial cells in the urinary bladder. The H-FABP immunoreactivity was further found in cells characterised by numerous lipid droplets, such as interstitial cells of the theca interna in the ovary and those of Leydig in the testis, and cell Element III in the placenta. In the digestive system, parietal cells in the stomach and pancreatic beta-cells were immunoreactive to H-FABP. In addition, capillary endothelial cells were immunostained in the cardiac and red skeletal muscles, the exocrine pancreas, digestive tract and thymus. From the rather wide distribution of H-FABP, it is suggested that H-FABP plays some fundamental roles in the active metabolism of fatty acids in the body. The present findings also indicate that H-FABP is a useful marker for the morphological study of interstitial cells in the ovary and testis and parietal cells in the stomach.

Immunogold labelling in combination with cryoultramicrotomy, freeze-etching, and label-fracture

During the past years, the methods of ultrastructural visualization of intracellular and cell-surface proteins have been improved considerably, mainly as the result of the development of low-temperature preservation in combination with immunocytochemical labelling procedures using poly- or monoclonal antibodies. In this contribution we will discuss the combination of immunogold labelling with cryoultramicrotomy and two replica methods, i.e. freeze-etching and label-fracture. The main advantage of cryoultramicrotomy is that it enables post-sectioning labelling, thus providing complete accessibility of all cellular antigens, located both intracellularly and on the cell surface. Important parameters that influence the labelling (i.e. label-efficiency), including penetration of the label and antibodies in the section, effects of fixatives on antigenicity, and steric hindrance, will be discussed in detail. The replica methods have the advantage of enabling an analysis of the lateral distribution of antigens located at the cell surface. The label efficiency is of particular importance in these studies and in this context several parameters will be discussed, including accessibility and effect of fixatives.

Ultrastructure of the human erythrocyte cytoskeleton and its attachment to the membrane

We attached paraformaldehyde-fixed human erythrocyte ghosts to coated coverslips and sheared them to expose the cytoskeleton. Quick-freeze, deep-etch, rotary-replication, or tannic acid/osmium fixation and plastic embedding revealed the cytoskeleton as a dense network of intersecting straight filaments. Previous negative stain studies on spread skeletons found 5-6 spectrin tetramers intersecting at each actin oligomer, with an estimated 250 such intersections/microns 2 of membrane. In contrast, we found 3-4 filaments at each intersection and approximately 400 intersections/microns 2 of membrane. Immunogold labeling verified that the filaments were spectrin, but their lengths (29-37 nm) were approximately one-third that of extended spectrin dimers. The length and diameter of the filaments were sufficient to accommodate spectrin dimers, but not spectrin tetramers. Our results suggest that, in situ, spectrin dimers may associate as hexamers and octamers, rather than tetramers. We present several explanations that can reconcile our observations on intact cytoskeletons with previous reports on spread material. Extracting sheared ghosts with solutions of low ionic strength removed the cytoskeleton to reveal projections from the cytoplasmic surface of the membrane. These projections contained band 3, as shown by immunogold labeling, and they aggregated to a similar extent as intramembrane particles (IMP) when the cytoskeleton was removed, suggesting a direct relationship between these structures. Quantification indicated a stoichiometry of 2 IMP for each cytoplasmic projection. Cytoplasmic projections presumably contain other proteins besides band 3 since further treatment with high ionic strength solutions extracts peripheral proteins and reduces the diameter of projections by approximately 3 nm.

A major stylar matrix polypeptide (sp41) is a member of the pathogenesis-related proteins superclass

A novel stylar-specific glycosylated protein, sp41, was characterized. Sp41 constitutes greater than 12% of the transmitting tract tissue soluble proteins and is mainly localized in the extracellular matrix. Two cDNA clones corresponding to sp41 mRNA were isolated and sequenced. The decoded sequences are, respectively, 80% and 49% homologous to acidic and basic pathogen-induced (1-3)-beta-glucanases of the leaf. Thus a subfamily of (1-3)-beta-glucanase pathogenesis-related (PR) proteins constitutes one of the major stylar matrix proteins. The accumulation of sp41 transcripts in normally developing and elicitor-treated styles and leaves was followed using an RNase protection assay. During development sp41 transcript accumulation starts well after carpel differentiation. It is first detected in styles at 8 days before anthesis. The maximal level of accumulation is reached during anthesis. Elicitor-treated styles do not accumulate the leaf-type (1-3)-beta-glucanase transcript, although they retain the capacity to synthesize leaf-type pathogenesis-related proteins such as the pathogen-induced acidic chitinase. The developmental regulation of sp41 expression points to a role for them in the normal processes of flowering and reproductive physiology.

Calreticulin is a candidate for a calsequestrin-like function in Ca2(+)-storage compartments (calciosomes) of liver and brain

In a search for the non-muscle equivalent of calsequestrin (the low-affinity high-capacity Ca2(+)-binding protein responsible for Ca2+ storage within the terminal cisternae of the sarcoplasmic reticulum), acidic proteins were extracted from rat liver and brain microsomal preparations and purified by column chromatography. No calsequestrin was observed in these extracts, but the N-terminal amino acid sequence of the major Ca2(+)-binding protein of the liver microsomal fraction was determined and found to correspond to that of calreticulin. This protein was found to bind approx. 50 mol of Ca2+/mol of protein, with low affinity (average Kd approx. 1.0 mM). A monoclonal antibody, C6, raised against skeletal-muscle calsequestrin cross-reacted with calreticulin in SDS/PAGE immunoblots, but polyclonal antibodies reacted with native calreticulin only weakly, or not at all, after SDS denaturation. Immuno-gold decoration of liver ultrathin cryosections with affinity-purified antibodies against liver calreticulin revealed luminal labelling of vacuolar profiles indistinguishable from calciosomes, the subcellular structures previously identified by the use of anti-calsequestrin antibodies. We conclude that calreticulin is the Ca2(+)-binding protein segregated within the calciosome lumen, previously described as being calsequestrin-like. Because of its properties and intraluminal location, calreticulin might play a critical role in Ca2+ storage and release in non-muscle cells, similar to that played by calsequestrin in the muscle sarcoplasmic reticulum.