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

The inositol 1,4,5,-trisphosphate receptor in cerebellar Purkinje cells: quantitative immunogold labeling reveals concentration in an ER subcompartment

The Ca2+ mobilization effect of inositol 1,4,5-trisphosphate, the second messenger generated via receptor-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, is mediated by binding to intracellular receptors, which are expressed in high concentration in cerebellar Purkinje cells. Partially conflicting previous reports localized the receptor to various subcellular structures: elements of ER, both rough and smooth-surfaced, the nuclear envelope, and even the plasma membrane. We have now reinvestigated the problem quantitatively by using cryosections of rat cerebellar tissue immunolabeled with polyclonal monospecific antibodies against the inositol 1,4,5-trisphosphate receptor. By immunofluorescence the receptor was detected only in Purkinje cells, whereas the other cells of the cerebellar cortex remained negative. In immunogold-decorated ultrathin cryosections of the Purkinje cell body, the receptor was concentrated in cisternal stacks (piles of up to 12 parallel cisternae separated by regularly spaced bridges, located both in the deep cytoplasm and beneath the plasma membrane; average density, greater than 5 particles/micron of membrane profile); in cisternal singlets and doublets adjacent to the plasma membrane (average density, approximately 2.5 particles/micron); and in other apparently smooth-surfaced vesicular and tubular profiles. Additional smooth-surfaced elements were unlabeled. Perinuclear and rough-surfaced ER cisternae were labeled much less by themselves (approximately 0.5 particles/micron, two- to threefold the background), but were often in direct membrane continuity with heavily labeled, smooth-surfaced tubules and cisternal stacks. Finally, mitochondria, Golgi cisternae, multivesicular bodies, and the plasma membrane were unlabeled. In dendrites, approximately half of the nonmitochondrial, membrane-bound structures (cisternae, tubules, and vesicles), as well as small cisternal stacks, were labeled. Dendritic spines always contained immunolabeled cisternae and vesicles. The dendritic plasma membrane, of both shaft and spines, was consistently unlabeled. These results identify a large, smooth-surfaced ER subcompartment that appears equipped to play a key role in the control of Ca2+ homeostasis: in particular, in the generation of [Ca2+]i transients triggered by activation of specific receptors, such as the quisqualate-preferring trans(+/-)-1-amino-1,3-cyclopentamedicarboxylic acid glutamatergic receptors, which are largely expressed by Purkinje cells.

Redistribution of synaptophysin and synapsin I during alpha-latrotoxin-induced release of neurotransmitter at the neuromuscular junction

The distribution of two synaptic vesicle-specific phosphoproteins, synaptophysin and synapsin I, during intense quantal secretion was studied by applying an immunogold labeling technique to ultrathin frozen sections. In nerve-muscle preparations treated for 1 h with a low dose of alpha-latrotoxin in the absence of extracellular Ca2+ (a condition under which nerve terminals are depleted of both quanta of neurotransmitter and synaptic vesicles), the immunolabeling for both proteins was distributed along the axolemma. These findings indicate that, in the presence of a block of endocytosis, exocytosis leads to the permanent incorporation of the synaptic vesicle membrane into the axolemma and suggest that, under this condition, at least some of the synapsin I molecules remain associated with the vesicle membrane after fusion. When the same dose of alpha-latrotoxin was applied in the presence of extracellular Ca2+, the immunoreactivity patterns resembled those obtained in resting preparations: immunogold particles were selectively associated with the membrane of synaptic vesicles, whereas the axolemma was virtually unlabeled. Under this condition an active recycling of both quanta of neurotransmitter and vesicles operates. These findings indicate that the retrieval of components of the synaptic vesicle membrane is an efficient process that does not involve extensive intermixing between molecular components of the vesicle and plasma membrane, and show that synaptic vesicles that are rapidly recycling still have the bulk of synapsin I associated with their membrane.

Antibodies directed against the peroxisomal targeting signal of firefly luciferase recognize multiple mammalian peroxisomal proteins

We have previously shown that the peroxisomal targeting signal in firefly luciferase consists of the COOH-terminal three amino acids of the protein, serine-lysine-leucine (Gould, S.J., G.A. Keller, N. Hosken, J. Wilkinson, and S. Subramani, 1989. J. Cell Biol. 108:1657-1664). Antibodies were raised against a synthetic peptide that contained this tripeptide at its COOH terminus. Immunofluorescence and immunocryoelectron microscopy revealed that the anti-peptide antibodies specifically detected peroxisomes in mammalian cells. Further characterization revealed that the antibodies were primarily directed against the COOH-terminal three amino acids of the peptide. In Western blot experiments, the antibodies recognized 15-20 rat liver peroxisomal proteins, but reacted with only a few proteins from other subcellular compartments. These results provide independent immunological evidence that the peroxisomal targeting signal identified in firefly luciferase is present in many peroxisomal proteins.

Characterization of the collagen in the hexagonal lattice of Descemet's membrane: its relation to type VIII collagen

To investigate the nature of the hexagonal lattice structure in Descemet's membrane, monoclonal antibodies were raised against a homogenate of bovine Descemet's membranes. They were screened by immunofluorescence microscopy to obtain antibodies that label Descement's membrane. Some monoclonal antibodies labeled both Descemet's membrane and fine filaments within the stroma. In electron microscopy, with immunogold labeling on a critical point dried specimen, the antibodies labeled the hexagonal lattices and long-spacing structures produced by the bovine corneal endothelial cells in culture; 6A2 antibodies labeled the nodes of the lattice and 9H3 antibodies labeled the sides of the lattice. These antibodies also labeled the hexagonal lattice of Descemet's membrane in situ in ultrathin frozen sectioning. In immunofluorescence, these antibodies stained the sclera, choroid, and optic nerve sheath and its septum. They also labeled the dura mater of the spinal cord, and the perichondrium of the tracheal cartilage. In immunoblotting, the antibodies recognized 64-kD collagenous peptides both in tissue culture and in Descemet's membrane in vivo. They also recognized 50-kD pepsin-resistant fragments from Descemet's membranes that are related to type VIII collagen. However, they did not react either in immunoblotting or in immunoprecipitation with medium of subconfluent cultures from which type VIII collagen had been obtained. The results are discussed with reference to the nature of type VIII collagen, which is currently under dispute. This lattice collagen may be a member of a novel class of long-spacing fibrils.

A new high molecular mass protein showing unique localization in desmosomal plaque

A high molecular mass protein of 680 kD was identified and purified from the isolated desmosomes in bovine muzzle epidermal cells. This protein, called "desmoyokin" (from the English, yoke) here, showed no binding ability with keratin filaments in vitro, and its molecule had a characteristic dumbell shape approximately 170 nm in length. We have succeeded in obtaining one monoclonal antibody specific to desmoyokin. By the use of this monoclonal antibody and antidesmoplakin monoclonal antibody, desmoyokin was shown to be colocalized with desmoplakin at the immunofluorescence microscopic level; desmoyokin occurred only in the stratified epithelium, not in the simple epithelium nor in the other tissues. At the electron microscopic level, these two proteins were clearly seen to be sorted out in the plaque of desmosomes with desmoyokin at the periphery and desmoplakin at the center of the disk-shaped desmosomal plaque, suggesting that these two plaque proteins play distinct roles in forming and maintaining the desmosomes in stratified epithelium.

Mitotic Golgi fragments in HeLa cells and their role in the reassembly pathway

Immunoelectron microscopy and stereology were used to identify and quantitate Golgi fragments in metaphase HeLa cells and to study Golgi reassembly during telophase. On ultrathin frozen sections of metaphase cells, labeling for the Golgi marker protein, galactosyltransferase, was found over multivesicular Golgi clusters and free vesicles that were found mainly in the mitotic spindle region. The density of Golgi cluster membrane varied from cell to cell and was inversely related to the density of free vesicles in the spindle. There were thousands of free Golgi vesicles and they comprised a significant proportion of the total Golgi membrane. During telophase, the distribution of galactosyltransferase labeling shifted from free Golgi vesicles towards Golgi clusters and the population of free vesicles was depleted. The number of clusters was no more than in metaphase cells so the observed fourfold increase in membrane surface meant that individual clusters had increased in size. More than half of these had cisterna(e) and were located next to "buds" on the endoplasmic reticulum. Early in G1 the number of clusters dropped as they congregated in the juxtanuclear region and fused. These results show that fragmentation of the Golgi apparatus yields Golgi clusters and free vesicles and reassembly from these fragments is at least a two-step process: (a) growth of a limited number of dispersed clusters by accretion and fusion of vesicles to form cisternal clusters next to membranous "buds" on the endoplasmic reticulum; (b) congregation and fusion to form the interphase Golgi stack in the juxtanuclear region.

Cytochemical studies of lipid metabolism: immunogold probes for lipoprotein lipase and cholesterol

In this article, cytochemical methods are presented for the study of lipid metabolism both in normal cells and in mutant cells with genetic disorders characterized by abnormal lipid metabolism. The benefit of using an immunocytochemical approach to the study of lipase in tissues is discussed, and a review is presented of the results on immunolocalization of lipoprotein lipase in cardiac tissue of normal mice. Immunocytochemical techniques are applied to the study of lysosomal proliferation in hepatocytes from liver of mutant mice with a genetic defect responsible for the lack of hepatic lipase and lipoprotein lipase activity in these animals. Localization of lipids in tissues with structural techniques has been an area of great interest to our laboratory for many years. Attention is called to the development of a technique for the visualization of fatty acids as a function of their ionization state and the production of fatty-acid myelin figures in membranes. Results on the use of filipin to detect unesterified cholesterol in membranes are reviewed. Filipin produces fluorescent filipin-cholesterol complexes but also perturbs cell membranes. Application of this cytochemical probe, in combination with immunocytochemistry of lysosomes, produced useful information on defects in low-density lipoprotein-derived cholesterol translocation in mutant human fibroblasts. Initial results on the application of immunological techniques to the study of cholesterol in lipid model systems indicate a novel approach, which may be applicable to specialized cell systems. Recent advances in cryoultramicrotomy and development of immunoprobes present valuable opportunities for the structural assessment of lipids and lipases in cell organelles and cell membranes.

Lipoprotein lipase in myocytes and capillary endothelium of heart: immunocytochemical study

Lipoprotein lipase was immunolocalized by electron microscopy in hearts of young mice; 78% of lipoprotein lipase was in myocytes, 3-6% in extracellular space, and 18% in capillary endothelium. Lipoprotein lipase in myocytes was located primarily in sarcoplasmic reticulum, Golgi sacs, and transport vesicles and also in secretory vesicles at the cell periphery. Lipoprotein lipase in extracellular space was present near the orifice of secretory vesicles of myocytes and in narrow zones spanning the space between myocytes and capillary endothelium. The lowest concentration of lipase associated with endothelial cells was at the basal plasma membrane, whereas the highest concentration was at the surface of luminal projections. Lipoprotein lipase was associated with chylomicrons at the capillary surface but not with chylomicron remnants. Fasting mice for 48 h increased, in heart, lipoprotein lipase activity by 120% and immunolocalized lipase by 270%. The greatest increase (5-fold) occurred at the surface of intraluminal endothelial projections. The findings indicate that lipoprotein lipase in heart is synthesized by myocytes, transferred across extracellular space along cell surfaces and across endothelial cells via vesicles or intracellular channels, and concentrated at the surface of luminal projections of endothelium where the enzyme hydrolyzes triacylglycerol of chylomicrons and very low-density lipoproteins.

Endolyn-78, a membrane glycoprotein present in morphologically diverse components of the endosomal and lysosomal compartments: implications for lysosome biogenesis

A monoclonal antibody (2C5) raised against rat liver lysosomal membranes was used to identify a 78-kD glycoprotein that is present in the membranes of both endosomes and lysosomes and, therefore, is designated endolyn-78. In cultures of rat hepatoma (Fu5C8) and kidney cells (NRK), this glycoprotein could not be labeled with [35S]methionine or with [32P]inorganic phosphate but was easily labeled with [35S]cysteine and [3H]mannose. Pulse-chase experiments and determinations of endoglycosidase H (endo H) sensitivity showed that endolyn-78 is derived from a precursor of Mr 58-62 kD that is processed to the mature form with a t1/2 of 15-30 min. The protein has a 22-kD polypeptide backbone that is detected after a brief pulse in tunicamycin-treated cells. During a chase in the presence of the drug, this is converted into an O-glycosylated product of 46 kD that despite the absence of N-linked oligosaccharides is effectively transferred to lysosomes. This demonstrates that the delivery of endolyn-78 to this organelle is not mediated by the mannose-6-phosphate receptor (MPR). Immunocytochemical experiments showed that endolyn-78 is present in the limiting membranes and the interior membranous structures of morphologically identifiable secondary lysosomes that contain the lysosomal hydrolase beta-glucuronidase, lack the MPR, and could not be labeled with alpha-2-macroglobulin at 18.5 degrees C, a temperature which prevents appearance of endocytosed markers in lysosomes. Endolyn-78 was present at low levels in the plasma membrane and in peripheral tubular endosomes, but was prominent in morphologically diverse components of the endosomal compartment (vacuolar endosomes and various types of multivesicular bodies) which acquired alpha-2-macroglobulin at 18.5 degrees C, and frequently contained substantial levels of the MPR and variable levels of beta-glucuronidase. On the other hand, the MPR was very rarely found in endolyn-containing structures that were not labeled with alpha-2-macroglobulin at the low temperature. Thus, the process of lysosomal maturation appears to involve the progressive delivery of lysosomal enzymes to various types of endosomes that may have already received some of the lysosomal membrane proteins. Although endolyn-78 would be one of the proteins added early to endosomes, other lysosomal membrane proteins may be added only to multivesicular endosomes that represent very advanced stages of maturation.

Subcellular localization of sterol carrier protein-2 in rat hepatocytes: its primary localization to peroxisomes

Sterol carrier protein-2 (SCP-2) is a nonenzymatic protein of 13.5 kD which has been shown in in vitro experiments to be required for several stages in cholesterol utilization and biosynthesis. The subcellular localization of SCP-2 has not been definitively established. Using affinity-purified rabbit polyclonal antibodies against electrophoretically pure SCP-2 from rat liver, we demonstrate by immunoelectron microscopic labeling of ultrathin frozen sections of rat liver that the largest concentration of SCP-2 is inside peroxisomes. In addition the immunolabeling indicates that there are significant concentrations of SCP-2 inside mitochondria, and associated with the endoplasmic reticulum and the cytosol, but not inside the Golgi apparatus, lysosomes, or the nucleus. These results were confirmed by immunoblotting experiments with proteins from purified subcellular fractions of the rat liver cells carried out with the anti-SCP-2 antibodies. The large concentration of SCP-2 inside peroxisomes strongly supports the proposal that peroxisomes are critical sites of cholesterol utilization and biosynthesis. The presence of SCP-2 inside peroxisomes and mitochondria raises questions about the mechanisms involved in the differential targeting of SCP-2 to these organelles.

Use of poly(vinylpyrrolidone) and poly(vinyl alcohol) for cryoultramicrotomy

Specimens infused with or suspended in a mixture of 10-30% poly(vinylpyrrolidone) and 2.07-1.61 M sucrose can often be more easily frozen-sectioned than those infused with sucrose alone. The pH of such a mixture can be efficiently adjusted to neutrality by using Na2CO3. Use of poly(vinylpyrrolidone) causes little or no increase in the background level of immunolabelling. Adsorption staining of ultrathin frozen sections with a mixture of uranyl acetate and poly(vinyl alcohol), i.e. a simple thin-embedding of the sections in such a mixture, produces positive staining effects that are often enough to delineate structures of many organelles. When OsO4-treated frozen sections are stained with uranyl acetate and further adsorption-stained with a mixture of lead citrate and poly(vinyl alcohol), the overall staining effects are similar to those observed in double-stained conventional sections.

Immunoperoxidase methods for the localization of antigens in cultured cells and tissue sections by electron microscopy

We have presented our detailed methods for localizing antigens in cultured cells and tissue sections by IP at the EM level. Immunoperoxidase cytochemistry is particularly well suited for the study of sparse antigens as a result of the enzymatic amplification afforded by the method, and of molecules confined within a membrane-enclosed compartment wherein the DAB reaction produce can accumulate. Although IP is commonly used to localize membrane-compartmentalized molecules, reliable qualitative information can also be obtained on cytoplasmic antigens as well (Anderson et al., 1978; Merisko et al., 1986; Rodman et al., 1984). For these and other reasons, it is likely that IP cytochemistry will continue to be an important tool for the cell biologist especially in the study of membrane traffic. Other inventive combinations of immunocytochemical methods will likely be forthcoming, for example, combining IP localization with postembedding labeling by colloidal-gold conjugates to provide triple EM labeling.

Cryostat sections for coexistence studies and preembedding electron microscopic immunocytochemistry of central and peripheral nervous system tissue

Perfusion-fixed tissue blocks were incubated in high molar sucrose solutions, shock frozen in melting isopentane, and sectioned on a conventional cryostat. Semithin sections (2-4 microns) alternatingly stained for parvalbumin and glutamate decarboxylase enabled us to demonstrate the coexistence of both antigens in the same cell. Thick sections (40 microns) of central and peripheral nervous system tissue were immunostained and processed for correlated light and electron microscopic studies. At the electron microscopic level, the preservation of ultrastructural features such as membranes and synaptic contacts was comparable to that normally seen in vibratome sectioned material. Hence, this technique can successfully be used for preembedding coexistence studies and electron microscopic preembedding immunocytochemistry when vibratome sectioning is problematic.

Melamine resins and their application in electron microscopy

Melamine resins are derived from the heterocyclic compound triaminotriazine, C3H6N6. Similar to proteins in structure and reactivity, water-soluble melamine resins can be used as water-embedding media for electron microscopy (Bachhuber and Frösch, 1983). The idea behind this approach was to study some of the artefacts of traditional embedding techniques and to work out conditions to eliminate as far as possible denaturing of proteins and extraction of lipids. Sectioned cells and tissues processed in the melamine resin Nanoplast show remarkable preservation of ultrastructure. Because they can be sectioned extremely thinly, melamine resins are particularly suitable for dark-field and electron spectroscopic imaging of unstained molecular suspensions providing in this way an unusually clear reproduction of ultrastructural detail such as the helical structure of isolated unstained double-strand DNA molecules (Frösch et al., 1987b). In 1988, the melamine resin Nanostrat was introduced as an EM-compatible prolific substrate foil for cell culture (Westphal et al., 1988). Cells or bacteria cultivated on this material can be processed for various kinds of follow-up techniques like TEM, SEM, vertical sectioning and immunocytochemistry.

Immunocytochemistry of calciosomes in liver and pancreas

Calciosomes are small cytoplasmic vacuoles identified in various nonmuscle cell types by their content of protein(s) similar to calsequestrin (CS), the Ca2+ storage protein of the muscle sarcoplasmic reticulum (SR). These entities have been interpreted as the "primitive" counterpart of the SR, and suggested to be the organelle target of inositol-1,4,5-triphosphate action (Volpe, P., K. H. Krause, S. Hashimoto, F. Zorzato, T. Pozzan, J. Meldolesi, and D. P. Lew. Proc. Natl. Acad. Sci. USA. 85:1091-1095). Immunoperoxidase and immunogold experiments carried out in both thick and ultrathin cryosections of rat hepatocytes and pancreatic acinar cells by using antimuscle CS antibodies revealed a specific labeling widely distributed in the entire cytoplasm, while nuclei were negative. Individual calciosomes appeared as small (105 nm) membrane-bound vacuoles intermingled with, and often apposed to ER cisternae and mitochondria. Other calciosomes were scattered in the Golgi area, in between zymogen granules and beneath the plasma membrane. The cumulative volume of the CS-positive organelles was measured to account for the 0.8 and 0.45% of the cytoplasm in liver and pancreas cells, respectively. The real total volume of the calciosome compartment is expected to be approximately twice as large. In hepatocytes, structures similar to CS-positive calciosomes were decorated by antibodies against the Ca2+ ATPase of muscle SR, while ER cisternae were not. By dual labeling, colocalization was revealed in 53.6% of the organelles, with 37.6% positive for the ATPase only. CS appeared preferentially confined to the content, and the Ca2+ ATPase to the contour of the organelle. The results suggested a partial segregation of the two antigens, reminiscent of their well-known segregation in muscle SR. Additional dual-label experiments demonstrated that hepatic calciosomes express neither two ER markers (cytochrome-P450 and NADH-cytochrome b5 reductase) nor the endolysosome marker, luminal acidity (revealed by 3-[2,4-dinitroanilino]-3'-amino-N-methyl dipropylamine). Calciosomes appear as unique cytological entities, ideally equipped to play a role in the rapid-scale control of the cytosolic-free Ca2+ in nonmuscle cells.

Differential effect of alpha-latrotoxin on exocytosis from small synaptic vesicles and from large dense-core vesicles containing calcitonin gene-related peptide at the frog neuromuscular junction

The regulatory peptide called calcitonin gene-related peptide (CGRP) was detected by immunofluorescence in frog motor neurons and motor nerve terminals. In motor nerve terminals, CGRP-like immunoreactivity was found to be segregated within large dense-core vesicles. To determine whether exocytosis from acetylcholine-containing small synaptic vesicles and from CGRP-containing large dense-core vesicles can be independently stimulated, nerve-muscle preparations were exposed to alpha-latrotoxin. This toxin induced complete depletion of acetylcholine-containing small synaptic vesicles but did not induce a parallel depletion of CGRP-like immunoreactivity and of large dense-core vesicles. These effects were independent of the presence of extracellular Ca2+ and occurred both at room temperature and at low temperature (1-3 degrees C). These findings suggest that exocytosis from the two vesicle populations is mediated by distinct biochemical mechanisms, which might be differentially regulated by physiological stimuli.

"Calciosome," a cytoplasmic organelle: the inositol 1,4,5-trisphosphate-sensitive Ca2+ store of nonmuscle cells?

Calsequestrin (CS) is the protein responsible for the high-capacity, moderate affinity binding of Ca2+ within the terminal cisternae of the sarcoplasmic reticulum, believed up to now to be specific for striated muscle. The cells of two nonmuscle lines (HL-60 and PC12) and of two rat tissues (liver and pancreas) are shown here to express a protein that resembles CS in many respects (apparent mass and pH-dependent migration in NaDodSO4/PAGE; blue staining with StainsAll dye; Ca2+ binding ability) and is specifically recognized by affinity-purified antibodies against skeletal muscle CS. In these cells, the CS-like protein is shown by immunofluorescence and immunogold procedures to be localized within peculiar, heretofore unrecognized structures distributed throughout the cytoplasm. These structures appear to be discrete organelles, which we propose to be named "calciosomes." By cell fractionation (Percoll gradient and free-flow electrophoresis), the CS-like protein of HL-60 cells is shown to copurify with the markers of the inositol 1,4,5-trisphosphate (Ins-P3)-sensitive Ca2+ store, whereas the markers of other organelles (endoplasmic reticulum, Golgi complex, mitochondria, endosomes) and of the plasma membrane do not. Calciosome might thus be the intracellular target of Ins-P3--i.e., the source of the Ca2+ redistributed to the cytosol following receptor-triggered generation of the messenger.

Intracellular transport of a variant surface glycoprotein in Trypanosoma brucei

Trypanosome variant surface glycoproteins (VSGs) have a novel glycan-phosphatidylinositol membrane anchor, which is cleavable by a phosphatidylinositol-specific phospholipase C. A similar structure serves to anchor some membrane proteins in mammalian cells. Using kinetic and ultrastructural approaches, we have addressed the question of whether this structure directs the protein to the cell surface by a different pathway from the classical one described in other cell types for plasma membrane and secreted glycoproteins. By immunogold labeling on thin cryosections we were able to show that, intracellularly, VSG is associated with the rough endoplasmic reticulum, all Golgi cisternae, and tubulovesicular elements and flattened cisternae, which form a network in the area adjacent to the trans side of the Golgi apparatus. Our data suggest that, although the glycan-phosphatidylinositol anchor is added in the endoplasmic reticulum, VSG is nevertheless subsequently transported along the classical intracellular route for glycoproteins, and is delivered to the flagellar pocket, where it is integrated into the surface coat. Treatment of trypanosomes with 1 microM monensin had no effect on VSG transport, although dilation of the trans-Golgi stacks and lysosomes occurred immediately. Incubation of trypanosomes at 20 degrees C, a treatment that arrests intracellular transport from the trans-Golgi region to the cell surface in mammalian cells, caused the accumulation of VSG molecules in structures of the trans-Golgi network, and retarded the incorporation of newly synthesized VSG into the surface coat.

Sorting of three secretory proteins to distinct secretory granules in acidophilic cells of cow anterior pituitary

The distribution of three proteins discharged by regulated exocytosis--growth hormone (GH), prolactin (PRL), and secretogranin II (SgII)--was investigated by double immunolabeling of ultrathin frozen sections in the acidophilic cells of the bovine pituitary. In mammotrophs, heavy PRL labeling was observed over secretory granule matrices (including the immature matrices at the trans Golgi surface) and also over Golgi cisternae. In contrast, in somatotrophs heavy GH labeling was restricted to the granule matrices; vesicles and tubules at the trans Golgi region showed some and the Golgi cisternae only sparse labeling. All somatotrophs and mammotrophs were heavily positive for GH and PRL, respectively, and were found to contain small amounts of the other hormone as well, which, however, was almost completely absent from granules, and was more concentrated in the Golgi complex, admixed with the predominant hormone. Mixed somatomammotrophs (approximately 26% of the acidophilic cells) were heavily positive for both GH and PRL. Although admixed within Golgi cisternae, the two hormones were stored separately within distinct granule types. A third type of granule was found to contain SgII. Spillage of small amounts of each of the three secretory proteins into granules containing predominantly another protein was common, but true intermixing (i.e., coexistence within single granules of comparable amounts of two proteins) was very rare. It is concluded that in the regulated pathway of acidophilic pituitary, cell mechanisms exist that cause sorting of the three secretory proteins investigated. Such mechanisms operate beyond the Golgi cisternae, possibly at the sites where condensation of secretion products into granule matrices takes place.

Firefly luciferase is targeted to peroxisomes in mammalian cells

Although several enzymes known to reside in peroxisomes have been studied extensively, no cis-acting amino acid sequences involved in the transport of these proteins to peroxisomes have been described. As a first step towards the determination of a putative peroxisomal targeting sequence, we have expressed the cDNA encoding the firefly luciferase [Photinus-luciferin:oxygen 4-oxidoreductase (decarboxylating, ATP-hydrolyzing), EC 1.13.12.7] in monkey kidney cells and found that the product of the gene is transported to peroxisomes. Luciferase is derived from the firefly (Photinus pyralis) and is synthesized and stored in the cells of the firefly's lantern organ, where it is also found in peroxisomes. The fact that this protein is similarly targeted in cells from such different organisms suggests that the process of protein transport to peroxisomes has been highly conserved through evolution.

Anti-liver-kidney microsome antibody is a marker for the rat hepatocyte endoplasmic reticulum

Human sera, containing anti-liver-kidney microsome antibody as demonstrated by indirect immunofluorescence, were obtained from a subgroup of young patients with autoimmune chronic hepatitis. The anti-liver-kidney microsome antibody-positive sera were used to study the localization of the liver-kidney microsome antigen in hepatocytes. Immunoblot analysis of microsomal subfractions, lysosomal membranes, plasma membranes, mitochondria and purified ribosomes obtained from rat liver demonstrated that this antibody recognizes a protein of 50 kD present only in endoplasmic reticulum membranes. Immunogold labeling of ultrathin frozen sections and immunoperoxidase staining of 11 to 15 micron cryostat sections were used to detect the liver-kidney microsome antigen in rat liver tissue. The anti-liver-kidney microsome antibody binds to antigenic domains on the cytoplasmic face of smooth and rough endoplasmic reticulum membranes of hepatocytes. No labeling was observed of the Golgi apparatus, peroxisomes, mitochondria, lysosomes, nuclei or plasma membranes. Not only was the antigen recognized by the anti-liver-kidney microsome antibody specific for endoplasmic reticulum membranes, but it was also specific for the endoplasmic reticulum of hepatocytes only, since no labeling was observed in any organelle of Kupffer or endothelial cells. Therefore, the anti-liver-kidney microsome antibody can be considered as a marker for endoplasmic reticulum in rat hepatocytes.

Immunoelectron microscopical localization of phospholamban in adult canine ventricular muscle

The subcellular distribution of phospholamban in adult canine ventricular myocardial cells was determined by the indirect immunogold-labeling technique. The results presented suggest that phospholamban, like the Ca2+-ATPase, is uniformly distributed in the network sarcoplasmic reticulum but absent from the junctional portion of the junctional sarcoplasmic reticulum. Unlike the Ca2+-ATPase, but like cardiac calsequestrin, phospholamban also appears to be present in the corbular sarcoplasmic reticulum. Comparison of the relative distribution of phospholamban immunolabeling in the sarcoplasmic reticulum with that of the sarcolemma showed that the density of phospholamban in the network sarcoplasmic reticulum was approximately 35-fold higher than that of the cytoplasmic side of the sarcolemma, which in turn was found to be three- to fourfold higher than the density of the background labeling. However, a majority of the specific phospholamban labeling within 30 nm of the cytoplasmic side of the sarcolemma was clustered and present over the sarcoplasmic reticulum in the subsarcolemmal region of the myocardial cells, suggesting that phospholamban is confined to the junctional regions between the sarcolemma and the sarcoplasmic reticulum, but absent from the nonjunctional portion of the sarcolemma. Although the resolution of the immunogold-labeling technique used (60 nm) does not permit one to determine whether the specific labeling within 30 nm of the cytoplasmic side of the sarcolemma is associated with the sarcolemma and/or the junctional sarcoplasmic reticulum, it is likely that the low amount of labeling in this region represents phospholamban associated with sarcoplasmic reticulum. These results suggest that phospholamban is absent from the sarcolemma and confined to the sarcoplasmic reticulum in cardiac muscle.

Application of frozen thin sectioning immunogold staining to the study of the developing neuroepithelial basal lamina

In order to examine the deposition of basal lamina components in the developing neuroepithelium, a technique for frozen thin sectioning and immunogold staining of early embryonic tissue was developed. Different fixatives and buffer systems were evaluated to determine which best retained immunoreactivity and satisfactory ultrastructure of day 9 and 10 mouse embryos. Fixation in sodium phosphate and sodium bicarbonate buffers did not retain antigenicity, and incubations in TBS (trishydroxymethyl-aminomethane buffered saline) in an effort to 'restore' immunoreactivity were similarly unsuccessful. Fixation in sodium cacodylate buffer, however, did retain the antigenicity of basal lamina components; the pattern of type IV collagen and laminin distribution was clearly determined. These results represent the first report of on-grid immunocytochemistry of early embryonic material.

Extensive laminin and basement membrane accumulation occurs at the onset of bleomycin-induced rodent pulmonary fibrosis

The distribution of laminin was studied during pulmonary fibrosis induced in rodents by bleomycin sulfate. Large accumulations of laminin associated with basement membranes were seen in thickened lung interstitial spaces by immunofluorescence microscopy, starting at 7 days (32-75% increases) and persisting through 28 days (66-79% increase). By electron microscopy, these laminin concentrations were skeinlike masses of reduplicated basement membranes localized at the surface of alveolar capillary endothelial cells. Numerous macrophages were also associated with this basement membrane material. These findings suggest that bleomycin-induced damage to lung cells causes massive local accumulations of basement membranes, which might be involved in the expansion of the interstitial stroma by stimulating attachment and activation of certain inflammatory cells.

Epithelial polyanion (podocalyxin) is found on the sides but not the soles of the foot processes of the glomerular epithelium

The distribution of podocalyxin was determined by several different immunoelectron microscopic methods, which included so-called diffusion methods carried out on cryostat sections and surface methods done on ultrathin sections. When localization was carried out by three different indirect immunogold methods (using ultrathin frozen sections, ultrathin sections cut from tissues embedded in Lowicryl K4M or LR White, or 20- mu cryostat sections for incubations), gold particles were found exclusively on the urinary surfaces of glomerular epithelial cells and, at a lower concentration, on the luminal surface of glomerular endothelial cells. No gold was bound to the basal surfaces (soles) of the epithelial foot processes, the filtration slit diaphragms, the glomerular basement membrane (GBM), or those regions of the endothelial plasmalemma that face the GBM. When podocalyxin was localized by an indirect immunoperoxidase method on cryostat sections, heavy deposits of reaction product were seen on the same cell surfaces as with the immunogold methods. Each of the methods used had certain advantages as well as limitations, but the collective results obtained were convergent and complimentary. It is concluded that podocalyxin is restricted in its distribution to the urinary surfaces of epithelial cells and the luminal surface of endothelial cells; it is missing or present in very low concentrations on the soles (basal surface) of the epithelial foot processes, the slit diaphragms and the basement membrane surface of the endothelium; and the podocalyxin-containing regions of the epithelial and endothelial plasmalemmae constitute microdomains of distinctive membrane protein composition on the corresponding cell surfaces.

3-Hydroxy-3-methylglutaryl coenzyme A reductase localization in rat liver peroxisomes and microsomes of control and cholestyramine-treated animals: quantitative biochemical and immunoelectron microscopical analyses

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a key regulatory enzyme involved in cholesterol biosynthesis, has recently been reported to be present in rat liver peroxisomes (Keller, G.A., M.C. Barton, D.J. Shapiro, and S.J. Singer, 1985, Proc. Natl. Acad. Sci. USA, 82:770-774). Immunoelectron labeling of ultrathin frozen sections of normal liver, using two monoclonal antibodies to purified rat liver microsomal HMG-CoA reductase, indicated that the enzyme is present in the matrix of peroxisomes. This study is a quantitative biochemical and immunoelectron microscopical analysis of HMG-CoA reductase in rat liver peroxisomes and microsomes of normal and cholestyramine-treated animals. Cholestyramine treatment produced a six- to sevenfold increase in the specific activity of peroxisomal HMG-CoA reductase, whereas the microsomal HMG-CoA reductase specific activity increased by about twofold. Using a computer program that calculates optimal linear combinations of marker enzymes, it was determined that between 20 and 30% of the total reductase activity was located in the peroxisomes of cholestyramine-treated animals. Less than 5% of the reductase activity was present in peroxisomes under control conditions. Quantitation of the immunoelectron microscopical data was in excellent agreement with the biochemical results. After cholestyramine treatment there was an eightfold increase in the density of gold particles per peroxisome, and we estimate about a threefold increase in the labeling of the ER.

Application of cryoultramicrotomy to immunocytochemistry

This paper reviews the most recent status of immuno-cryoultramicrotomy. The technical aspects of each step of the method are also analysed in detail with the intention of providing a useful source of information for investigators using this method.

Transferrin receptors recycle to cis and middle as well as trans Golgi cisternae in Ig-secreting myeloma cells

The recycling itinerary of plasma membrane transferrin receptors (TFR) was charted in IgG-secreting mouse myeloma cells (RPC 5.4) by tagging surface receptors with either bound anti-transferrin receptor antibodies (anti-TFR) or Fab fragments thereof and determining the intracellular destinations of the tagged receptors by immunocytochemistry. By immunofluorescence, TFR tagged with either probe were seen to be rapidly internalized and translocated from the cell surface to the juxtanuclear (Golgi) region. When localized by immunoperoxidase procedures at the electron microscopic level, the anti-TFR-labeled receptors were detected in all cisternae (cis, middle, and trans) of the Golgi stacks as well as in endosomes and trans Golgi reticular elements. There was no difference in the routing of TFR tagged with monovalent Fab and those tagged with divalent IgG. Tagged receptors were detected in Golgi stacks of approximately 50% of the cells analyzed. The position of the labeled cisternae within a given stack was found to be quite variable with cis and middle cisternae more often labeled at 5 min and trans cisternae at 30 min of antibody uptake. The finding that recycling plasmalemmal TFR can visit all or most Golgi subcompartments raises the likely possibility that any Golgi-associated posttranslational modification can occur during recycling as well as during the initial biosynthesis of plasmalemma receptors and other membrane proteins.

Molecular specialization of astrocyte processes at nodes of Ranvier in rat optic nerve

The HNK-1 and L2 monoclonal antibodies are thought to recognize identical or closely associated carbohydrate epitopes on a family of neural plasma membrane glycoproteins, including myelin-associated glycoprotein, the neural cell adhesion molecule, and the L1 and J1 glycoproteins, all of which have been postulated to play a part in mediating cell-cell interactions in the nervous system. We have used these two antibodies in immunofluorescence and immunogold-electron microscopic studies of semithin and ultrathin frozen sections of adult rat optic nerve, respectively, and we show that they bind mainly to astrocyte processes around nodes of Ranvier. Most other elements of the nerve, including astrocyte cell bodies and large astrocytic processes, are not labeled by the antibodies. To our knowledge, this is the first demonstration that perinodal astrocyte processes are biochemically specialized. We provide evidence that one of the HNK-1+/L2+ molecules concentrated around perinodal astrocyte processes is the J1 glycoprotein; our findings, taken together with previously reported observations, suggest that the other known HNK-1+/L2+ molecules are not concentrated on these processes. Since anti-J1 antibodies previously have been shown to inhibit neuron to astrocyte adhesion in vitro, we hypothesize that J1 may play an important part in the axon-glial interactions that presumably are involved in the assembly and/or maintenance of nodes of Ranvier.

Biosynthesis and intracellular sorting of growth hormone-viral envelope glycoprotein hybrids

Various aspects of the biogenetic mechanisms that are involved in the insertion of nascent plasma membrane proteins into the endoplasmic reticulum (ER) membrane and their subsequent distribution through the cell have been investigated. For these studies chimeric genes that encode hybrid proteins containing carboxy-terminal portions of the influenza virus hemagglutinin (154 amino acids) or the vesicular stomatitis virus envelope glycoprotein (G) (60 amino acids) linked to the carboxy terminus of a nearly complete secretory polypeptide, growth hormone (GH), were used. In in vitro transcription-translation experiments, it was found that the insertion signal in the GH portion of the chimeras led to incorporation of the membrane protein segments into the ER membrane. Effectively, GH became part of the luminal segment of membrane proteins of which only very small segments, corresponding to the cytoplasmic portions of the G or HA proteins, remained exposed on the surface of the microsomes. When the chimeric genes were expressed in transfected cells, the products, as expected, failed to be secreted and remained cell-associated. These results support the assignment of a halt transfer role to segments of the membrane polypeptides that include their transmembrane portions. The hybrid polypeptide containing the carboxy-terminal portion of HA linked to GH accumulated in a juxtanuclear region of the cytoplasm within modified ER cisternae, closely apposed to the Golgi apparatus. The location and appearance of these cisternae suggested that they represent overdeveloped transitional ER elements and thus may correspond to a natural way station between the ER and the Golgi apparatus, in which further transfer of the artificial molecules is halted. The GH-G hybrid could only be detected in transfected cells treated with chloroquine, a drug that led to its accumulation in the membranes of endosome or lysosome-like cytoplasmic vesicles. Although the possibility that the chimeric protein entered such vesicles directly from the Golgi apparatus cannot be ruled out, it appears more likely that it was first transferred to the cell surface and was then internalized by endocytosis.

Correct targeting of the bean storage protein phaseolin in the seeds of transformed tobacco

The storage protein phaseolin accumulates during seed development in protein bodies in cotyledons of the common bean Phaseolus vulgaris. Hall et al. (In L Van Vloten-Doting, TC Hall, eds, Molecular Form and Function of the Plant Genome, 1985 Plenum Press, In press) recently reported the expression of a gene coding for phaseolin and the accumulation of phaseolin protein in developing seeds of tobacco plants regenerated from transformed callus cells. The protein did not accumulate in other organs of the plants. Mature seeds from normal and transformed tobacco plants were obtained and the subcellular distribution of phaseolin in the seeds was examined using both light and electron microscopic immunocytochemical methods. Phaseolin was found in six of seven transformed tobacco embryos examined, but was present in only one endosperm of five. When present, phaseolin was located exclusively in the protein bodies of the embryonic and endospermic cells. Furthermore, phaseolin was restricted solely to the amorphous matrix of the protein bodies and was excluded from the globoid and proteinaceous crystalloid components of these organelles. The subcellular location of phaseolin in seeds from transformed tobacco plants is similar to that seen in mature seeds of the common bean indicating that in the transformed cells the protein is targeted to the right subcellular compartment.

Ultrastructural localization of calsequestrin in adult rat atrial and ventricular muscle cells

The distribution of calsequestrin in rat atrial and ventricular myocardial cells was determined by indirect immunocolloidal gold labeling of ultrathin frozen sections. The results presented show that calsequestrin is confined to the sarcoplasmic reticulum where it is localized in the lumen of the peripheral and the interior junctional sarcoplasmic reticulum as well as in the lumen of the corbular sarcoplasmic reticulum, but absent from the lumen of the network sarcoplasmic reticulum. Comparison of these results with our previous studies on the distribution of the Ca2+ + Mg2+-dependent ATPase of the cardiac sarcoplasmic reticulum show directly that the Ca2+ + Mg2+-dependent ATPase and calsequestrin are confined to distinct regions within the continuous sarcoplasmic reticulum membrane. Assuming that calsequestrin provides the major site of Ca2+ sequestration in the lumen of the sarcoplasmic reticulum, the results presented support the idea that both junctional (interior and peripheral) and specialized nonjunctional (corbular) regions of the sarcoplasmic reticulum are involved in Ca2+ storage and possibly release. Furthermore, the structural differences between the junctional and the corbular sarcoplasmic reticulum support the possibility that Ca2+ storage and/or release from the lumen of the junctional and the corbular sarcoplasmic reticulum are regulated by different physiological signals.

Immunocytochemical localization of phaseolin and phytohemagglutinin in the endoplasmic reticulum and Golgi complex of developing bean cotyledons

Development of legume seeds is accompanied by the synthesis of storage proteins and lectins, and the deposition of these proteins in protein-storage vacuoles (protein bodies). We examined the subcellular distribution, in developing seeds of the common bean, Phaseolus vulgaris L., of the major storage protein (phaseolin) and the major lectin (phytohemagglutinin, PHA). The proteins were localized using an indirect immunocytochemical method in which ultrathin frozen sections were immunolabeled with rabbit antibodies specific for either PHA or phaseolin. Bound antibodies were then localized using goat-anti-rabbit immunoglobulin G adsorbed onto 4- to 5-nm colloidal gold particles. The sections were post-fixed with OsO4, dehydrated, and embedded in plastic on the grids. Both PHA and phaseolin exhibited a similar distribution in the storage-parenchyma cells, being found primarily in the developing protein bodies. Endoplasmic reticulum and Golgi complexes (cisternal stacks and associated vesicles) also were specifically labeled for both proteins, whereas the cytosol and other organelles, such as mitochondria, were not. We interpret these observations as supporting the hypothesis that the transport of storage proteins and lectins from their site of synthesis, the rough endoplasmic reticulum, to their site of deposition, the protein bodies, is mediated by the Golgi complex.

Basement membrane heparan sulfate proteoglycans are concentrated in the laminae rarae and in podocytes of the rat renal glomerulus

A polyclonal antibody was raised against a proteoglycan fraction purified from extracts of isolated rat renal glomeruli. The antibody was characterized by column chromatography and by NaDodSO4/PAGE analysis of immunoprecipitates and was shown to recognize specifically the core protein of a population of heparan sulfate proteoglycans (Mr 130,000) found in the glomerular basement membrane and in other renal basement membranes. The antibody was used to localize the core proteins of this population of heparan sulfate proteoglycans (HSPG) in the glomerulus by immunoelectron microscopic techniques. These HSPG were found to be concentrated in the lamina rarae interna and externa of the glomerular basement membrane (GBM) and precursor forms were detected in the endoplasmic reticulum and Golgi cisternae of glomerular visceral epithelial cells (podocytes).

3-Hydroxy-3-methylglutaryl-coenzyme A reductase is present in peroxisomes in normal rat liver cells

The location inside rat liver parenchymal cells of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase; EC 1.1.1.34), the key regulatory enzyme in cholesterol biosynthesis, has been examined by immunoelectron microscopy and by subcellular fractionation. Although HMG-CoA reductase is generally thought to be exclusively a microsomal enzyme, we find that a substantial portion of cellular HMG-CoA reductase is localized in peroxisomes. Immunoelectron microscopic labeling of ultrathin frozen sections of normal rat liver, using two monoclonal antibodies to purified HMG-CoA reductase, showed that the enzyme is present in the peroxisomes at a higher concentration than at any other site inside the hepatocytes. Subcellular fractionation studies using Percoll and metrizamide gradients demonstrated a close correspondence of peaks of HMG-CoA reductase activity and of catalase activity, again revealing the presence of the reductase enzyme in peroxisomes. HMG-CoA reductase is therefore localized in peroxisomes in addition to being in the microsomal fraction.

Localization of phytohemagglutinin in the embryonic axis of Phaseolus vulgaris with ultra-thin cryosections embedded in plastic after indirect immunolabeling

We have examined the properties and subcellular localization of phytohemagglutinin (PHA), the major lectin of the common bean (Phaseolus vulgaris.), in the axis cells of nearly mature and imbibed mature seeds. On a protein basis the axis contained about 15% as much PHA as the cotyledons. Localization of PHA was done with an indirect immunolabeling method (rabbit antibodies against PHA, followed by colloidal gold particles coated with goat antibodies against rabbit immunoglobulins) on ultra-thin cryosections which were embedded in plastic on the grids after the immunolabeling procedure. The embedding greatly improved the visualization of the subcellular structures. The small (4 nm) collodial gold particles, localized with the electron microscope, were found exclusively over small vacuoles or protein bodies in all the cell types examined (cortical parenchyma cells, vascular-bundle cells, epidermal cells). The matrix of these vacuoles-protein bodies appears considerably less dense than that of the protein bodies in the cotyledons, but the results confirm that in all parts of the embryo PHA is localized in similar structures.