Antibodies to the Golgi complex and the rough endoplasmic reticulum

Tubulovesicular processes emerge from trans-Golgi cisternae, extend along microtubules, and interlink adjacent trans-golgi elements into a reticulum

Morphological dynamics and membrane transport within the living Golgi apparatus of astrocytes labeled with NBD-ceramide were imaged using both electronically enhanced fluorescence video and laser confocal microscopy. In time-lapse recordings, continuous tubulovesicular processes are observed to emerge from trans-Golgi elements and extend along microtubules at average rates of 0.4 microns/s. In addition, discrete fluorescent particles are observed to emerge from the trans-Golgi and subsequently migrate along microtubules at comparable velocities. Frequently, tubulovesicular processes form stable connections that interlink adjacent trans-Golgi elements into an extensive reticulum. Laser photobleaching-recovery experiments reveal that tubulovesicular processes can provide direct pathways for the diffusion of membrane lipids between joined trans-Golgi elements. These results suggest that microtubule-based transport and membrane fusion can operate to interconnect certain cisternal membranes of adjacent Golgi elements within the cell.

Microtubule-dependent retrograde transport of proteins into the ER in the presence of brefeldin A suggests an ER recycling pathway

Characteristics of brefeldin A (BFA)-induced redistribution of Golgi proteins into the endoplasmic reticulum (ER) and its relationship to an ER retrieval pathway were investigated. Retrograde movement of Golgi proteins into the ER occurred via long, tubulovesicular processes extending out of the Golgi along microtubules. Microtubule-disrupting agents (i.e., nocodazole), energy poisons, and reduced temperatures inhibited this pathway. In BFA-treated cells Golgi proteins appeared to cycle between the ER and an intermediate compartment marked by a 53 kd protein. Addition of nocodazole disrupted this dynamic cycle by preferentially inhibiting retrograde movement, causing Golgi proteins to accumulate in the intermediate compartment. In the absence of BFA, such an ER cycling pathway appeared to be followed normally by the 53 kd protein but not by Golgi proteins, as revealed by temperature shift experiments. We propose that BFA induces the interaction of the Golgi with an intermediate "recycling" compartment that utilizes a microtubule-dependent pathway into the ER.

Immunocytochemistry of the cell nucleus

This electron microscopic review addresses in situ immunocytochemistry of the mammalian cell nucleus with special reference to the use of autoantibodies, which are the major source of antinuclear antibodies. The localization of many key nuclear antigens is documented and immunocytochemical data are related to the major functional processes of transcription and processing of RNA and to replication of DNA.

The CaaX motif of lamin A functions in conjunction with the nuclear localization signal to target assembly to the nuclear envelope

While the nuclear lamin proteins (A, B, and C) assemble specifically at the surface of the nuclear membrane, their sequences do not reveal stretches of hydrophobic amino acids that might explain their association with the nuclear membranes. However, the A and B lamin proteins possess Ras-like C-terminal CaaX sequence motifs, which in Ras proteins are sites of hydrophobic modifications required for membrane association and function. From the analysis of single and double lamin A mutants affecting the CaaX motif, the nuclear localization signal, and higher-order assembly properties, we propose that the CaaX motif functions as a nonspecific, low affinity membrane probe for proteins ultimately segregated to specific cellular membrane systems. Committed association with specific membranes requires additional interactions with membrane-resident factors.

Intracellular degradation of unassembled asialoglycoprotein receptor subunits: a pre-Golgi, nonlysosomal endoproteolytic cleavage

The human asialoglycoprotein receptor is a heterooligomer of the two homologous subunits H1 and H2. As occurs for other oligomeric receptors, not all of the newly made subunits are assembled in the RER into oligomers and some of each chain is degraded. We studied the degradation of the unassembled H2 subunit in fibroblasts that only express H2 (45,000 mol wt) and degrade all of it. After a 30 min lag, H2 is degraded with a half-life of 30 min. We identified a 35-kD intermediate in H2 degradation; it is the COOH-terminal, exoplasmic domain of H2. After a 90-min chase, all remaining intact H2 and the 35-kD fragment were endoglycosidase H sensitive, suggesting that the cleavage generating the 35-kD intermediate occurs without translocation to the medial Golgi compartment. Treatment of cells with leupeptin, chloroquine, or NH4Cl did not affect H2 degradation. Monensin slowed but did not block degradation. Incubation at 18-20 degrees C slowed the degradation dramatically and caused an increase in intracellular H2, suggesting that a membrane trafficking event occurs before H2 is degraded. Immunofluorescence microscopy of cells with or without an 18 degrees C preincubation showed a colocalization of H2 with the ER and not with the Golgi complex. We conclude that H2 is not degraded in lysosomes and never reaches the medial Golgi compartment in an intact form, but rather degradation is initiated in a pre-Golgi compartment, possibly part of the ER. The 35-kD fragment of H2 may define an initial proteolytic cleavage in the ER.

Isolation and partial characterization of heparan sulphate proteoglycan from the human glomerular basement membrane

Heparan sulphate proteoglycan was solubilized from human glomerular basement membranes by guanidine extraction and purified by ion-exchange chromatography and gel filtration. The yield of proteoglycan was approx. 2 mg/g of basement membrane. The glycoconjugate had an apparent molecular mass of 200-400 kDa and consisted of about 75% protein and 25% heparan sulphate. The amino acid composition was characterized by a high content of glycine, proline, alanine and glutamic acid. Hydrolysis with trifluoromethanesulphonic acid yielded core proteins of 160 and 110 kDa (and minor bands of 90 and 60 kDa). Alkaline NaBH4 treatment of the proteoglycan released heparan sulphate chains with an average molecular mass of 18 kDa. HNO2 oxidation of these chains yielded oligosaccharides of about 5 kDa, whereas heparitinase digestion resulted in a more complete degradation. The data suggest a clustering of N-sulphate groups in the peripheral regions of the glycosaminoglycan chains. A polyclonal antiserum raised against the intact proteoglycan showed reactivity against the core protein. It stained all basement membranes in an intense linear fashion in immunohistochemical studies on frozen kidney sections from man and various mammalian species.

Topogenesis and sorting of synaptophysin: synthesis of a synaptic vesicle protein from a gene transfected into nonneuroendocrine cells

Diverse nonneuroendocrine (non-NE) cells were forced to express synaptophysin (SY), the major and typical transmembrane glycoprotein of small (30-80 nm) neurotransmitter vesicles of NE cells, using microinjection of RNA synthesized in vitro from cDNA or transient and stable transfections with cDNA brought under SV40 promoter control. The glycoprotein synthesized in non-NE cells is indistinguishable from SY of NE cells and is integrated with similar, if not identical, orientation in the membranes of a specific, novel type of small cytoplasmic vesicle that structurally resembles synaptic vesicles and in which SY is the only major protein detected. A non-N-glycosylated form of SY generated by site-directed mutagenesis showed the same behavior and specific distribution in small vesicles. The results show that the information contained in this protein alone is sufficient to secure its sorting into a special type of vesicle in a heterotypic context, i.e., in the absence of other NE-specific components.

Construction of the endoplasmic reticulum

To study the construction of the ER, we used the microtubule-disrupting drug nocodazole to induce the complete breakdown of ER structure in living cells followed by recovery in drug-free medium, which regenerates the ER network within 15 min. Using the fluorescent dye 3,3'-dihexyloxacarbocyanine iodide to visualize the ER, we have directly observed the network construction process in living cells. In these experiments, the ER network was constructed through an iterative process of extension, branching, and intersection of new ER tubules driven by the ER motility previously described as tubule branching. We have tested the cytoskeletal requirements of this process. We find that newly formed ER tubules are aligned with single microtubules but not actin fibers or vimentin intermediate filaments. Microtubule polymerization preceded the extension of ER tubules and, in experiments with a variety of different drugs, appeared to be a necessary condition for the ER network formation. Furthermore, perturbations of the pattern of microtubule polymerization with microtubule-specific drugs caused exactly correlated perturbations of the pattern of ER construction. Induction of abnormally short, nonintersecting microtubules with 20 microM taxol prevented the ER network formation; ER tubules only extended along the few microtubules contacting the aggregated ER membranes. This requirement for a continuous network of intersecting microtubules indicates that ER network formation takes place through the branching and movement of ER membranes along microtubules. Cytochalasin B had no apparent effect on the construction of the ER network during recovery, despite apparently complete disruption of actin fibers as stained by phalloidin. Blockage of protein synthesis and disorganization of intermediate filaments with cycloheximide pretreatment also failed to perturb ER construction.

Precursors of chondroitin sulfate proteoglycan are segregated within a subcompartment of the chondrocyte endoplasmic reticulum

Immunocytochemical methods were used at the levels of light and electron microscopy to examine the intracellular compartments of chondrocytes involved in extracellular matrix biosynthesis. The results of our studies provide morphological evidence for the compartmentalization of secretory proteins in the ER. Precursors of the large chondroitin sulfate proteoglycan (CSPG), the major proteoglycan species produced by chondrocytes, were present in the Golgi complex. In addition, CSPG precursors were localized in specialized regions of the ER. Link protein, a separate gene product which functions to stabilize extracellular aggregates of CSPG monomers with hyaluronic acid, was segregated similarly. In contrast, type II procollagen, another major secretory molecule produced by chondrocytes, was found homogeneously distributed throughout the ER. The CSPG precursor-containing ER compartment exhibits a variable tubulo-vesicular morphology but is invariably recognized as an electronlucent, smooth membrane-bounded region continuous with typical ribosome-studded elements of the rough ER. The observation that this ER structure does not stain with antibodies against resident ER proteins also suggests that the compartment is a specialized region distinct from the main part of the ER. These results support recent studies that consider the ER as a compartmentalized organelle and are discussed in light of the possible implications for proteoglycan biosynthesis and processing.

Detection of GTP-binding proteins in purified derivatives of rough endoplasmic reticulum

As a first step in determining the molecular mechanism of membrane fusion stimulated by GTP in rough endoplasmic reticulum (RER), we have looked for GTP-binding proteins. Rough microsomes from rat liver were treated for the release of ribosomes, and the membrane proteins were separated by SDS/polyacrylamide-gel electrophoresis. The polypeptides were then blotted on to nitrocellulose sheets and incubated with [alpha-32P]GTP [Bhullar & Haslam (1987) Biochem. J. 245, 617-620]. A doublet of polypeptides (23 and 24 kDa) was detected in the presence of 2 microM-MgCl2. Binding of [alpha-32P]GTP was blocked by 1-5 mM-EDTA, 10-10,000 nM-GTP or 10 microM-GDP. Either guanosine 5'-[gamma-thio]triphosphate or guanosine 5'-[beta gamma-imido]triphosphate at 100 nM completely inhibited binding, but ATP, CTP or UTP at 10 mciroM did not. Pretreatment of microsomes by mild trypsin treatment (0.5-10 micrograms of trypsin/ml, concentrations known not to affect microsomal permeability) led to inhibition of [alpha-32P]GTP binding, suggesting a cytosolic membrane orientation for the GTP-binding proteins. Two-dimensional gel-electrophoretic analysis revealed the 23 and 24 kDa [alpha-32P]GTP-binding proteins to have similar acid isoelectric points. [alpha-32P]GTP binding occurred to similar proteins of rough microsomes from rat liver, rat prostate and dog pancreas, as well as to a 23 kDa protein of rough microsomes from frog liver, but occurred to distinctly different proteins in a rat liver plasma-membrane-enriched fraction. Thus [alpha-32P]GTP binding has been demonstrated to two low-molecular-mass (approx. 21 kDa) proteins in the rough endoplasmic reticulum of several varied cell types.

The Sec15 protein responds to the function of the GTP binding protein, Sec4, to control vesicular traffic in yeast

SEC15 function is required at a late stage of the yeast secretory pathway. Duplication of the gene encoding the ras-like, GTP-binding protein, Sec4, can suppress the partial loss of function resulting from the sec15-l mutation, but cannot suppress disruption of sec15. Analysis of the SEC15 gene predicts a hydrophilic protein product of 105 kD. Anti-Sec15 antibody recognizes a protein of 116-kD apparent molecular mass which is associated with a microsomal fraction of yeast in a strongly pH dependent fashion. Overproduction of Sec15 protein interferes with the secretory pathway, resulting in the formation of a cluster of secretory vesicles, and a patch of Sec15 protein revealed by immunofluorescence. The sec4-8 and sec2-4l mutations, but not mutations in other SEC genes, prevent formation of the Sec15 protein patch. We propose that Sec15 protein responds to the function of the Sec4 protein to control vesicular traffic.

Reconstitution of endoplasmic reticulum in rapidly dividing cells of early Xenopus embryos

The cytology of early blastomeres of Xenopus laevis embryos was examined. Particular attention was given to the organization of the nuclear envelope of karyomeres (chromosome vesicles) and the endoplasmic reticulum (ER) at different stages in early cleavage cycles of frog development. Nuclear envelope formation was observed to occur rapidly around individual chromosomes during early anaphase, and karyomeres fused subsequently to yield the final nucleus during telophase. Endoplasmic reticulum in the perinuclear cytoplasm was observed to be vesicular during metaphase and cisternal in form during telophase. Following microinjection of rat liver rough microsomes into early blastomeres, heterologous ER components were identified by electron microscope immunocytochemistry. The foreign ER was observed as large, reconstituted cisternae at stages in the cell cycle when the nuclear envelope was intact. Therefore, transplanted ER maintained the capacity to reconstitute in the cytoplasm of a rapidly dividing cell. In an attempt to better assess ER structure at the metaphase stage of the cell cycle, we next slowed down the division process by treating Xenopus embryos with anti-microtubule agents. Treatment with critical concentrations of colchicine, nocodazole, or vinblastine led to cleavage arrest but not to inhibition of the nuclear cycle. Following such treatment, homologous ER was observed in a vesicular form at all stages of the nuclear cycle. Heterologous ER, however, identified by immunocytochemistry in microinjected cells treated with nocodazole, displayed both vesicular and cisternal forms. We conclude that microinjected ER membranes exhibit cell-cycle-specific behavior, which is different from that of the host cell ER.

Short cytoplasmic sequences serve as retention signals for transmembrane proteins in the endoplasmic reticulum

The adenoviral transmembrane E3/19K glycoprotein is a resident of the endoplasmic reticulum. Here we show that the last six amino acid residues of the 15-membered cytoplasmic tail are necessary and sufficient for the ER retention. These residues can be transplanted onto the cytoplasmic tail of other membrane-bound proteins such that ER residency is conferred. Deletion analysis demonstrated that no single amino acid residue is responsible for the retention. The identified structural motif must occupy the extreme COOH-terminal position to be functional. An endogenous transmembrane ER protein, UDP-glucuronosyltransferase, also contains a retention signal in its cytoplasmic tail. We suggest that short linear sequences occupying the extreme COOH-terminal position of transmembrane ER proteins serve as retention signals.

Acetylcholine receptor in a C2 muscle cell variant is retained in the endoplasmic reticulum

We have examined the properties and intracellular localization of acetylcholine receptors in the C2 muscle cell line and in a variant (T-) that accumulates AChR intracellularly. On immunoblots, the subunit structures of the AChR from wild-type and T- cells were similar except that the gamma and delta subunits of the variant AChR had altered mobilities. Digestion with endoglycosidases H and F demonstrated that this difference results from a failure of high-mannose N-linked oligosaccharides on AChR subunits to be processed to complex forms in the variant. N-linked glycosylation of other proteins in the variant was normal. When examined by immunocytochemistry, the distribution of internal AChR in wild-type cells was consistent with a location both in the endoplasmic reticulum and in the Golgi. Variant cells, however, showed no evidence of Golgi staining. Subcellular fractionation experiments also demonstrated AChR in the Golgi fractions of wild-type cells, but not in those derived from T- cells. We conclude that in T- myotubes most of the AChR fails to be transported out of the endoplasmic reticulum.

Localization of wheat-germ agglutinin-binding sites in the Golgi complex of cultured rat atrial myocytes

In the Golgi region of cultured rat atrial myocytes, condensed secretory protein was seen in Golgi-associated tubules or cisternae which lay beyond, and often separated from, the remainder of the Golgi stacks. These structures appeared to be involved in packaging of condensed secretory protein into atrial granules. Binding sites of HRP-conjugated wheat-germ agglutinin (WGA) in saponin-treated cultured atrial myocytes were examined by electron microscopy with special reference to atrial granules and the tubular structures associated with the Golgi stacks. HRP reaction products were observed in both trans-cisternae of the Golgi stacks and the associated tubular structures. While the majority of atrial granules were devoid of reaction products, some granules, which were connected to the WGA-positive tubular structures in the vicinity of the Golgi trans-cisternae, showed HRP reaction products at their connected necks. Similar results were obtained when sections of the cells embedded in Lowicryl K4M were labeled with WGA coupled to colloidal gold (G-WGA); the Golgi complex was G-WGA positive, whereas no specific binding of G-WGA to atrial granules was observed. These results suggest that glycoproteins and/or glycolipids with oligosaccharides recognized by WGA in the Golgi transcisternae, may be separated from atrial natriuretic peptides which are packaged into atrial granules.

Condensation-sorting events in the rough endoplasmic reticulum of exocrine pancreatic cells

In guinea pig exocrine pancreatic cells intracisternal granules (ICGs) occur at a low frequency within the lumen of the RER. By starving and refeeding guinea pigs or injecting them in CoCl2 solution, the number of these granules is greatly increased. We show here that ICGs contain the complete set of secreted pancreatic digestive enzymes and proenzymes. Two other soluble proteins in the lumen of the RER, GRP 78/BiP and protein disulphide isomerase (PDI), are specifically excluded from ICGs. The formation of ICGs, which occurs without acidification of the RER cisternae, is therefore a sorting event involving the cocondensation of a complete set of secretory enzymes and proenzymes, which for brevity we refer to collectively as the zymogens. With the exception of approximately 50% of the RNase, the zymogens in ICGs are covalently cross-linked by intermolecular disulphide bonds. The synthesis of all three resident ER cisternal proteins--PDI, GRP 78/BiP, and GRP 94--with the carboxy-terminal sequence KDEL, is induced in response to the accumulation of massive amounts of misfolded secretory protein in the ICGs in the lumen of the RER. After injection of rats with large doses of parachlorophenylalanine-methylester, crystals form in the lumen of the RER. We show that these crystals appear to be a lattice of amylase with the other zymogens incorporated between the layers. Both GRP 78/BiP and PDI are excluded from these crystals. The formation of these amylase crystals within the RER and the inclusion of other zymogens is, therefore, also a sorting event. These data establish that in exocrine pancreatic cells zymogens can cocondense in the RER into either amorphous aggregates or crystals that exclude other soluble RER proteins. This demonstrates that cocondensation is a mechanism capable of sorting zymogens within the secretory pathway.

A calsequestrin-like protein in the endoplasmic reticulum of the sea urchin: localization and dynamics in the egg and first cell cycle embryo

Using an antiserum produced against a purified calsequestrin-like (CSL) protein from a microsomal fraction of sea urchin eggs, we performed light and electron microscopic immunocytochemical localizations on sea urchin eggs and embryos in the first cell cycle. The sea urchin CSL protein has been found to bind Ca++ similarly to calsequestrin, the well-characterized Ca++ storage protein in the sarcoplasmic reticulum of muscle cells. In semi-thin frozen sections of unfertilized eggs, immunofluorescent staining revealed a tubuloreticular network throughout the cytoplasm. Staining of isolated egg cortices with the CSL protein antiserum showed the presence of a submembranous polygonal, tubular network similar to ER network patterns seen in other cells and in egg cortices treated with the membrane staining dye DiIC16[3]. In frozen sections of embryos during interphase of the first cell cycle, a cytoplasmic network similar to that of the unfertilized egg was present. During mitosis, we observed a dramatic concentration of the antibody staining within the asters of the mitotic apparatus where ER is known to aggregate. Electron microscopic localization on unfertilized eggs using peroxidase-labeled secondary antibody demonstrated the presence of the CSL protein within the luminal compartment of ER-like tubules. Finally, in frozen sections of centrifugally stratified eggs, the immunofluorescent staining concentrated in the clear zone: a layer highly enriched in ER and thought to be the site of calcium release upon fertilization. This localization of a CSL protein within the ER of the egg provides evidence for the ability of this organelle to serve a Ca++ storage role in the regulation of intracellular Ca++ in nonmuscle cells in general, and in the regulation of fertilization and cell division in sea urchin eggs in particular.

Pre-Golgi degradation of newly synthesized T-cell antigen receptor chains: intrinsic sensitivity and the role of subunit assembly

The T cell antigen receptor (TCR) is a multisubunit complex composed of at least seven transmembrane chains. The predominant species in most T cells has the composition alpha beta gamma delta epsilon zeta 2. The roles of subunit assembly in transport out of the ER and in the recently described process of pre-Golgi degradation of newly synthesized TCR chains were analyzed in a T-cell line deficient in the synthesis of delta chains (delta 2) and in COS-1 fibroblasts transfected with genes encoding individual TCR chains. Studies with the delta-deficient T-cell line showed that, in the absence of delta, the other TCR chains were synthesized at normal rates, but, instead of being transported to the cell surface, they were retained in the ER. Analysis of the fate of TCR chains retained in the ER showed that they were degraded at vastly different rates by a nonlysosomal pathway. Whereas the alpha chains were degraded rapidly, gamma, zeta, and epsilon were relatively long-lived. To analyze whether this selective degradation was because of different intrinsic susceptibility of the individual chains to degradation or to the formation of resistant oligomers, TCR chains were expressed alone or in combinations in COS-1 fibroblasts. These studies showed that (a) individual TCR chains were degraded at different rates when expressed alone in COS-1 cells, and (b) sensitive chains could be stabilized by coexpression with a resistant chain. Taken together, these observations indicate that both intrinsic sensitivity and subunit assembly play a role in determining the rates at which newly synthesized TCR chains are degraded in the ER.

An antibody against secretogranin I (chromogranin B) is packaged into secretory granules

We have investigated the sorting and packaging of secretory proteins into secretory granules by an immunological approach. An mAb against secretogranin I (chromogranin B), a secretory protein costored with various peptide hormones and neuropeptides in secretory granules of many endocrine cells and neurons, was expressed by microinjection of its mRNA into the secretogranin I-producing cell line PC12. An mAb against the G protein of vesicular stomatitis virus--i.e., against an antigen not present in PC12 cells--was expressed as a control. The intracellular localization and the secretion of the antibodies was studied by double-labeling immunofluorescence using the conventional and the confocal microscope, as well as by pulse-chase experiments. The secretogranin I antibody, like the control antibody, was transported along the secretory pathway to the Golgi complex. However, in contrast to the control antibody, which was secreted via the constitutive pathway, the secretogranin I antibody formed an immunocomplex with secretogranin I, was packaged into secretory granules, and was released by regulated exocytosis. Our results show that a constitutive secretory protein, unaltered by genetic engineering, can be diverted to the regulated pathway of secretion by its protein-protein interaction with a regulated secretory protein. The data also provide the basis for immunologically studying the role of luminally exposed protein domains in the biogenesis and function of regulated secretory vesicles.

Dissecting tumor cell invasion: epithelial cells acquire invasive properties after the loss of uvomorulin-mediated cell-cell adhesion

The generation of invasiveness in transformed cells represents an essential step of tumor progression. We show here, first, that nontransformed Madin-Darby canine kidney (MDCK) epithelial cells acquire invasive properties when intercellular adhesion is specifically inhibited by the addition of antibodies against the cell adhesion molecule uvomorulin; the separated cells then invade collagen gels and embryonal heart tissue. Second, MDCK cells transformed with Harvey and Moloney sarcoma viruses are constitutively invasive, and they were found not to express uvomorulin at their cell surface. These data suggest that the loss of adhesive function of uvomorulin (which is identical to E-cadherin and homologous to L-CAM) is a critical step in the promotion of epithelial cells to a more malignant, i.e., invasive, phenotype. Similar modulation of intercellular adhesion might also occur during invasion of carcinoma cells in vivo.

The t complex polypeptide 1 (TCP-1) is associated with the cytoplasmic aspect of Golgi membranes

The t complex polypeptide 1 (TCP-1) is a protein of unknown function expressed in large amounts during spermatogenesis. Rat monoclonal antibodies recognizing TCP-1 have been prepared and used to immunoprecipitate and Western blot a 57 kd protein from germ cell and tissue culture cell extracts. In tissue culture cells, indirect immunofluorescent localization of antigen indicated a perinuclear distribution similar to that of the Golgi apparatus. Analysis of the TCP-1 distribution in tissue culture cells showed that the polypeptide was associated with the cytoplasmic aspect of membranes of the trans-Golgi network (TGN). The distribution in spermatids suggested that TCP-1 was localized to structures often associated with the developing acrosome. The TCP-1 antigenic epitopes are highly conserved, allowing the protein to be identified in cells across a wide variety of vertebrate species and tissues. These experiments suggest that TCP-1 may be essential for transport of proteins through the exocytic pathway in all cells and required in large amounts for acrosome formation in developing spermatids.

Involvement of the Golgi apparatus in sorting of materials to opposite ends of frog rod retinal photoreceptors

We have studied the rod cells of retinas of Rana pipiens by phosphatase cytochemistry and immunocytochemistry. We find that the Golgi apparatus of these cells, although different in its intracellular distribution from that of other neurons, has a cis-trans organization like that of other neurons as regards morphological features and the distribution of phosphatase activities. Antibodies against opsin bind to several sacs of the rod Golgi apparatus, especially those at the trans side of the Golgi stack. This suggests that Golgi involvement in the packaging of opsin for eventual delivery to the photoreceptive outer segments of the cell involves passage through trans Golgi systems. Proteins destined for the opposite end of the cell--the presynaptic terminal--also seem to pass through trans Golgi systems, as is indicated both by immunocytochemical localization of the synaptic vesicle protein p38 (synaptophysin) and by the presence of thiamine pyrophosphatase activity in some of the synaptic vesicles. Our findings suggest that sorting of membrane proteins destined for opposite ends of the photoreceptor takes place in systems at or near the trans Golgi face.

Rapid redistribution of Golgi proteins into the ER in cells treated with brefeldin A: evidence for membrane cycling from Golgi to ER

In cells treated with brefeldin A (BFA), movement of newly synthesized membrane proteins from the endoplasmic reticulum (ER) to the Golgi apparatus was blocked. Surprisingly, the glycoproteins retained in the ER were rapidly processed by cis/medial Golgi enzymes but not by trans Golgi enzymes. An explanation for these observations was provided from morphological studies at both the light and electron microscopic levels using markers for the cis/medial and trans Golgi. They revealed a rapid and dramatic redistribution to the ER of components of the cis/medial but not the trans Golgi in response to treatment with BFA. Upon removal of BFA, the morphology of the Golgi apparatus was rapidly reestablished and proteins normally transported out of the ER were efficiently and rapidly sorted to their final destinations. These results suggest that BFA disrupts a dynamic membrane-recycling pathway between the ER and cis/medial Golgi, effectively blocking membrane transport out of but not back to the ER.

Rapid fragmentation and reorganization of Golgi membranes during frustrated phagocytosis of immobile immune complexes by macrophages

The authors have observed the rapid reorganization of the cellular membranes of macrophages during Fc receptor-mediated frustrated phagocytosis of immune complex-coated surfaces. As the macrophages spread, large, clear basal vacuoles and anastamosing tubules were formed, occasionally contiguous with the adherent surface. Coated vesicles also were observed. This process was accompanied by a rapid reorganization of the Golgi complex region of the macrophages, which was observed using trimetaphosphatase histochemistry and an antibody to a Golgi membrane antigen as markers. On contact of the macrophages with the immune complexes, the Golgi complexes, which were tightly clustered around the centrioles, dispersed into vesicles and reorganized near the basal surface. The Golgi cisternae swelled, fragmented, and decreased in number. Golgi membrane antigen was found in the large basal vacuoles and also associated with the adherent basal surface of the macrophages. This indicates that the Golgi complexes were reorganized, in part, by a direct recruitment of their membranes to the increasing basal surface area of the spreading macrophages. The changes in the structure of the Golgi complexes were reversible; by 2 hours, the complexes had recovered their normal organization, with an accompanying decrease in the number of large basal vacuoles. These data suggest that the dynamic interrelationship among the Golgi membranes, intracellular vacuoles, and the plasma membrane can be perturbed by membrane spreading on a nonphagocytosable surface.

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.

Autoimmunity induced by lactate dehydrogenase-elevating virus: monoclonal autoantibodies against Golgi antigens and other subcellular elements

Immunocompetent mice infected with LDVAGIA developed autoantibodies against Golgi antigen as early as 6-7 days postinfection preceding anti-viral antibodies for nearly a week. The anti-Golgi antibody titres peaked between two and three weeks postinfection independent of the applied virus dose. Already one week postinfection anti-Golgi autoantibodies were prominent in IgG subclasses IgG2a and b. Spleen cells from these mice were fused with myeloma cells and the culture fluids were screened by indirect immunofluorescence for antibodies reactive with the Golgi antigen of normal cultured cells. A panel of cloned stable antibody-producing hybridomas has been obtained. Some antibodies showed broad cross-species reactivity, recognizing similar antigenic determinants in the Golgi region of mouse, rat and other mammalian cells and also in avian and piscine cells, whereas others recognized determinants in this cell compartment only in mammalian cells and one recognized Golgi antigen only in particular murine tumor cells. From 19 Golgi antibody producing hybridomas 3 secreted IgM-antibodies, 10 synthesized autoantibodies of the subclass IgG2a and 6 of IgG2b. Applied in immunoelectron microscopy mABs decorated the Golgi organelle. A considerable amount of LDVAGIA-induced hybridomas produced antibodies against conserved antigens associated with the cytoskeleton, mitochondria, and the nucleus, LDV-induced monoclonal anti-Golgi antibodies decorated the Golgi area in LDV- and mock-infected macrophages. However, cytoplasmic fluorescence characteristic for LDV-infected cells was not observed indicating that the anti-Golgi autoantibodies do not cross-react with the virus.

Dynamics of the endoplasmic reticulum in living non-muscle and muscle cells

The dynamic changes of the endoplasmic reticulum (ER) in interphase and mitotic cells was detected by the vital fluorescent dye 3,3'-dihexyloxacarbocyanine iodide. Two types of arrays characterize the continuous ER system in the non-muscle PtK2 cell: 1) a lacy network of irregular polygons and 2) long strands of ER that are found aligned along stress fibers. In cross-striated myotubes there was a periodic localization of fluorescence over each I-band corresponding to the positions of the terminal cisternae of the sarcoplasmic reticulum (SR). In contrast to the arrangement in muscle cells, the alignment of the long strands of ER alon stress fibers showed no strict periodicity that could be correlated with the sarcomeric units of the stress fibers. The ER and SR arrays seen in living cells were also detected in fixed cells stained with antibodies directed against proteins of the endoplasmic reticulum and sarcoplasmic reticulum, respectively. Observations of vitally stained PtK2 cells at 1 to 2 minute intervals using low light level video cameras and image processing techniques enabled us to see the polygonal ER units form and undergo changes in their shapes. During cell division, the ER, rhodamine 123-stained mitochondria, and phagocytosed fluorescent beads were excluded from the mitotic spindle while soluble proteins were not. No obvious concentration or alignment of membranes could be found associated with the contractile proteins in the cleavage furrow. After completion of cell division there was a redeployment of the ER network in each daughter cell.

An anti-organelle antibody in pathology. The chromatolytic reaction studied with a monoclonal antibody against the Golgi apparatus

To evaluate the use of an anti-organelle antibody in a pathologic reaction the chromatolytic reaction was chosen for study. Qualitative analysis was made of rat hypoglossal nuclei stained with a cresyl violet method for Nissl substance and a monoclonal antibody against rat Golgi apparatus (10A8) 0 and 3 days, and 1, 2, 3, 4, 5, and 6 weeks after section of the right hypoglossal nerve. Marked dispersion of Nissl substance was noted at 2 weeks and of Golgi apparatus at 4 weeks. Reaggregation of staining had occurred for both organelles by 6 weeks. A quantitative light microscopic analysis was carried out for both stains on randomly selected hypoglossal sections from 0, 2, 4, and 6 weeks. The analysis confirmed the qualitative observations and showed them to be highly significant. In addition, it revealed an increase in nuclear area from 0 to 2 weeks, an increase in cytoplasmic area at 4 weeks, decrease in the total area of Nissl substance from 0 to 2 and 4 to 6 weeks, decrease in the percent cytoplasmic areas occupied by Nissl from 0 to 2 weeks and decreases in both the total and percent cytoplasmic area occupied by Golgi apparatus maximal at 4 weeks. Both qualitative and quantitative analyses confirmed the use of this monoclonal antibody to study morphologic changes of the Golgi apparatus secondary to an experimental pathologic lesion. In addition, a previously unrecognized temporal dissociation between the changes of Nissl substance and Golgi apparatus was described.

Immunoelectron microscopic localization of synaptophysin in a Golgi subcompartment of developing hypothalamic neurons

Synaptophysin, previously identified as an integral membrane glycoprotein (mol. wt 38,000) characteristic of presynaptic vesicles of mature neurons, provides a molecular marker to study the origin, formation and traffic of synaptic vesicles. Using the monoclonal antibody SY38 against this polypeptide we have localized synaptophysin by immunofluorescence and electron microscope immunoperoxidase methods in cultured mouse hypothalamic neurons taken from 16-day-old fetuses which achieve synaptogenesis after 10-12 days in vitro. We have compared the localization of synaptophysin in perikarya and nerve endings as a function of age (2-19 days in vitro) and of treatment of mature neurons with nocodazole. Using immunofluorescence microscopy, synaptophysin was already detected in neuronal soma at 2 days in vitro, where the initiation of neurite development is observed. At the electron microscope level, virtually all mature synaptic boutons and varicosities showed an extensive synaptophysin labeling of synaptic vesicles at 12-13 days in culture whereas neurites showed only very few labeled vesicles. In neuronal soma taken before synapse formation (6 days in vitro), synaptophysin was selectively localized in membranes of the innermost cisternae of the Golgi zone and in vesicles of variable size and shape in the core of the Golgi zone. In contrast, after synapse formation, synaptophysin labeling was barely detected in the Golgi zone of neurons but a very strong labeling of synaptic vesicles in synaptic boutons was observed. Treatment of mature neurons (12 days in vitro) with nocodazole (10(-5) M) resulted in a conspicuous synaptophysin staining of the innermost trans-Golgi cisternae and numerous vesicles in the cytoplasm. Furthermore, an accumulation of labeled synaptic vesicles on the presynaptic membrane of nerve terminals was found. The data suggest that synaptophysin is released from the Golgi apparatus in a vesicular form, after glycosylation, and is then transported to nerve endings by a mechanism which requires integrity of microtubules.

Quantitative analysis of immunogold labelling for ferritin in liver from control and iron-overloaded rats

The distribution of ferritin antigenicity in control and iron-loaded rat hepatocytes was investigated with an immunogold-ferritin antibody technique. Antibody to horse spleen ferritin showed immunoreactivity as determined by dot blotting with immunogold/silver staining with purified rat liver ferritin but not with rat haemosiderin. The initial site of ferritin degradation was studied by analysing the density of gold labelling in the cytosol and lysosomes in combination with pre-embedding acid phosphatase cytochemistry. Immunoreactive ferritin was present in the cytosol, cytosolic clusters and lysosomes of normal hepatocytes. After iron-loading, the labelling density increased over tenfold in parenchymal cell cytosol with a smaller increase in Kupffer cells. Ferritin clusters contained substantially more immunoreactive ferritin than equivalent areas of lysosomes or cytosol. Analysis of the labelling density in hepatocyte lysosomes showed that, despite a striking increase in iron content, one-quarter of the lysosomes showed less immunolabelled ferritin than the cytosol. The existence of a wide range of ferritin labelling densities in the lysosomes with a large proportion unlabelled suggests that the ferritin protein shell is not degraded at a significant rate either in the cytosol or in clusters but only after incorporation into lysosomes.

Dynamic behavior of endoplasmic reticulum in living cells

Endoplasmic reticulum (ER) was studied by fluorescence microscopy of living CV-1 cells treated with the fluorescent carbocyanine dye DiOC6(3). Using video recording and image processing techniques, several distinct forms of highly localized movements of ER were documented, categorized, and analyzed in terms of mechanism and structural implications. These include tubule branching, ring closure, and sliding. These localized movements have been observed to generate the basic elements of ER: linear tubules, polygonal reticulum, and triple junctions. We propose that as such they act as the mechanism for constructing the polygonal lattice of interconnected membrane tubules that constitutes ER. The nature of these movements suggests possible involvement of the cytoskeleton, and, in view of the close correlations in the distributions of ER and microtubules, and the accompanying paper (Dabora and Sheetz), it is possible that microtubules may play a role in generating ER motility and in constructing and maintaining the ER network in living cells.

Temperature-sensitive Chinese hamster cell mutant with a defect in glycoprotein synthesis: accumulation of the EGF receptor in the endoplasmic reticulum and the role of the glucose-regulated protein GRP78

A temperature-sensitive mutant of Chinese hamster fibroblasts with a defect in glycoprotein synthesis is investigated after transfection and amplification of the gene for the human EGF receptor. We demonstrate that at the nonpermissive temperature a partially glycosylated species of the receptor accumulates in the endoplasmic reticulum. The oligosaccharides present are the high mannose types, since they can be removed completely by treatment with endoglycosidase H. Pulse-chase experiments show that the abnormal species of the receptor cannot be chased to a form that is either resistant to endoglycosidase H, or altered in its mobility on SDS polyacrylamide gels. The abnormal species of the receptor appears within the first hour of a shift to the nonpermissive temperature, and no further changes are observed upon prolonged incubation of cells at 40 degrees C. However, after 3-4 hours immunoprecipitations of the receptor yield another protein, which has properties very similar, if not identical, to the glucose-regulated protein GRP78. The induction of this protein at 40 degrees C can be suppressed completely with an inhibitor of RNA synthesis, without any effect on the glycosylation defect, or on the accumulation of the EGF receptor in the endoplasmic reticulum.

A GTP-binding protein required for secretion rapidly associates with secretory vesicles and the plasma membrane in yeast

SEC4, one of the 10 genes involved in the final stage of the yeast secretory pathway, encodes a ras-like, GTP-binding protein. In wild-type cells, Sec4 protein is located on the cytoplasmic face of both the plasma membrane and the secretory vesicles in transit to the cell surface. In all post-Golgi blocked sec mutants, Sec4p is predominantly associated with the secretory vesicles that accumulate as a result of the secretory block. Sec4p is synthesized as a soluble protein that rapidly (t1/2 less than or equal to 1 min) and tightly associates with secretory vesicles and the plasma membrane by virtue of a conformational change of a covalent modification. These data suggest that Sec4p may function as a "G" protein on the vesicle surface to transduce an intracellular signal needed to regulate transport between the Golgi apparatus and the plasma membrane.

Immunocytochemical localization of mutant low density lipoprotein receptors that fail to reach the Golgi complex

In the low density lipoprotein (LDL) receptor system, blocks in intracellular movement of a cell surface receptor result from naturally occurring mutations. These mutations occur in patients with familial hypercholesterolemia. One class of mutant LDL receptor genes (class 2 mutations) produces a receptor that is synthesized and glycosylated in the endoplasmic reticulum (ER) but does not reach the cell surface. These receptors contain serine/threonine-linked (O-linked) carbohydrate chains with core N-acetylgalactosamine residues and asparagine-linked (N-linked) carbohydrate chains of the high mannose type that are only partially trimmed. To determine the site of blockage in transport, we used electron microscope immunohistochemistry to compare the intracellular location of LDL receptors in normal human fibroblasts with their location in class 2 mutant fibroblasts. In normal cells, LDL receptors were located in coated pits, coated vesicles, endosomes, multivesicular bodies, and portions of the Golgi complex. In contrast, the mutant receptors in class 2 cells were almost entirely confined to rough ER and irregular extensions of the rough ER. Metabolic labeling studies with [3H]glucosamine confirmed that these mutant receptors contain core O-linked sugars, suggesting that the enzymes that attach these residues are located in the rough ER or the transitional zone of the ER. These studies establish that naturally occurring mutations in cell surface receptors can cause the receptors to remain trapped in the ER, thereby preventing their normal function and producing a genetic disease.

Effect of anti-ER antibodies within the ER lumen of living cells

We describe the production and partial characterization of 12 monoclonal antibodies raised against a preparation of endoplasmic reticulum membranes obtained from Xenopus laevis liver. Four of the antibodies cross-react with liver melanocytes; two of the antibodies recognize extracellular antigens, whilst the remaining six recognize antigens present in hepatocytes. The concentrations of these latter antigens increase markedly in livers stimulated by estrogen. Western blotting analysis revealed that the six anti-hepatocyte monoclonal antibodies recognize at least five different antigens whose molecular weights are 14K, 18K, 19K, 43K, and 125K. The possible functional involvement of the various antigens in the secretory pathway was investigated using Xenopus oocytes as a surrogate secretory system. The mRNAs coding for the monoclonal antibodies were injected into oocytes and the resulting immunoglobulin chains were translated and assembled into active anti-ER antibodies inside the lumen of the ER. The effect on secretion was then observed. Our data indicate that the binding of antibodies to most antigens of the endoplasmic reticulum membrane may result in a blockage of secretion.

Muscle cell components dictate hepatocyte gene expression and the distribution of the Golgi apparatus in heterokaryons

Major changes in cytoarchitecture and gene expression were induced in short-term heterokaryons. When human hepatocytes were fused with mouse muscle cells, the hepatocyte Golgi apparatus changed from its usual polar location to a uniformly circumnuclear location typical of striated muscle. Human liver albumin ceased to be expressed, and expression of the human muscle cell-surface antigen 5.1H11 was induced without DNA replication or cell division. Coexpression of liver and muscle proteins was rarely observed. These novel findings provide insight into the regulation of gene expression and the targeting and localization of organelles with a central role in cell polarity, intracellular transport, and secretion.

Immunoelectron microscopic demonstration of human melanosome associated antigens (HMSA) on melanoma cells: comparison with tyrosinase distribution

Previously, we have developed a mouse monoclonal antibody (MoAb) HMSA-1 (human melanosome-associated antigen-1) against the melanosome fraction of human malignant melanoma, and demonstrated the selective distribution of the HMSA-1 in neoplastic melanocytes on routine paraffin sections. This study examined, by using enzyme linked immunosorbent assay (ELISA) and immunoelectron microscopy, the subcellular distribution of the HMSA-1 in malignant melanocytes. Fractionation of cell organelles and ELISA assay indicated that the HMSA-1 is rich in fractions of large granule, melanosome and endoplasmic reticulum (ER) in melanoma cells. Immunoelectron microscopic study showed that the HMSA-1 is localized in the melanosomes of various developmental stages and vacuolar structures, which appeared to be the stage I melanosomes and which contained the matrix protein. Dopa cytochemistry revealed that the distribution of MoAb HMSA-1 reaction product is localized in the area different from that of tyrosinase, indicating that the synthetic processes of melanosomal matrix protein and tyrosinase are different. Furthermore, the reaction product with MoAb HMSA-1 was seen in the rough ER, indicating that the melanosomal matrix protein is synthesized by membrane-bound ribosomes and processed through the channel of ER.

Human proteins IEF 58 and 57a are associated with the Golgi apparatus

A mouse monoclonal antibody (mAB 22-II-D8B) raised against lysed transformed human amnion cells (AMA) has been characterized. The mAB decorated the Golgi apparatus in growing and quiescent cultured monolayer cells (fibroblasts and epithelial cells) of various species as determined by double immunofluorescence labeling and colocalization with galactosyltransferase antibodies. It reacted with the acidic human proteins IEF 58 (Mr = 29,000) and 57a, respectively (Mr = 30,000) (HeLa protein catalogue number; [(1982) Clin. Chem. 28, 766]), Golgi staining was also observed in BS-C-1 cells microinjected with mAB 22-II-D8B suggesting that the epitopes recognized by the antibody are most likely located on the cytoplasmic face of the membranes. The precise localization of the antigens to the various cisternae of the Golgi apparatus could not be demonstrated by immunogold cytochemistry on ultrathin cryosections due to either weak reactivity of the antibody or low concentration of the antigens. Immunofluorescence staining with mAB 22-II-D8B of lymphoid human Molt-4 cells and some human tissues failed to reveal any significant staining even though these expressed high levels of both IEF 58 and 57a. These results are taken to imply that the epitopes recognized by mAB 22-II-D8B may be masked in some cell types.

Sec53, a protein required for an early step in secretory protein processing and transport in yeast, interacts with the cytoplasmic surface of the endoplasmic reticulum

The sec53 mutant is a conditional lethal yeast secretory mutant. At 37 degrees C, precursors to exported proteins become firmly attached to the endoplasmic reticulum membrane and are not released into the lumen in a soluble form. The accumulated precursors are insoluble in the detergent Triton X-100; however, urea, a known protein denaturant, solubilizes them. Using antibody directed against the Sec53 protein, we found that a substantial portion of the Sec53 protein is associated with the cytoplasmic surface of the endoplasmic reticulum membrane. Membrane-bound Sec53 protein is largely insoluble in Triton X-100, but the protein is effectively released from the membrane by urea. We propose that the Sec53 protein may be a member of a complex of proteins required for an early step in protein processing and transport.