Biosynthesis of the glycoproteins present in plasma membrane, lysosomes and secretory materials, as visualized by radioautography

Epithelial cell polarity as reflected in enterocytes

Absorptive cells are the main cells present in the intestinal epithelium. The plasma membrane of these tall columnar cells reflects their high degree of polarization, by dividing into apical and basolateral domains with different compositions. The most characteristic structure of these cells consists of closely packed apical microvilli with the same height, looking like a brush, which is why they were named the brush border. The concentrated pattern of some apical markers observed in a restricted brush border domain shows that mature enterocytes are hyperpolarized epithelial cells: the filamentous brush border glycocalyx is anchored at the top of the microvilli and the annexin XIII is concentrated in the lower three fourths. Many studies have been carried out on the biosynthesis and intracellular pathway of domain markers. The results show clearly that the basolateral markers take a direct pathway from the trans-Golgi network to the basolateral membrane. However, the two apical pathways, one direct and one indirect pathway via the basolateral membrane, are used, depending on the apical protein involved. Efficient protein sorting and addressing are essential to the establishment and maintenance of cell polarity, on which the integrity of the epithelial barrier depends.

Radioactive fucose as a tool for studying glycoprotein secretion

The efficiency and reliability of radioactive fucose as a specific label for newly synthesized glycoproteins were investigated. Young adult male rabbits were injected intravitreally with [3H]-fucose, [3H]-galactose. [3H]-mannose, N-acetyl-[3H]-glucosamine or N-acetyl-[3H]-mannosamine, and killed 40 h after injection. In another series of experiments rabbits were injected with either [3H]-fucose or several tritiated amino acids and the specific activity of the vitreous proteins was determined. Vitreous samples were also processed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and histological sections of retina, ciliary body and lens (the eye components around the vitreous body) were processed for radioautography. The specific activity (counts per minute per microgram of protein) of the glycoproteins labeled with [3H]-fucose was always much higher than that of the proteins labeled with any of the other monosaccharides or any of the amino acids. There was a good correlation between the specific activity of the proteins labeled by any of the above precursors and the density of the vitreous protein bands detected by fluorography. This was also true for the silver grain density on the radioautographs of the histological sections of retina, ciliary body and lens. The contribution of radioautography (after [3H]-fucose administration) to the elucidation of the biogenesis of lysosomal and membrane glycoproteins and to the determination of the intracellular process of protein secretion was reviewed. Radioactive fucose is the precursor of choice for studying glycoprotein secretion because it is specific, efficient and practical for this purpose.

The filamentous brush border glycocalyx, a mucin-like marker of enterocyte hyper-polarization

The probably sole constituent of the filamentous brush border glycocalyx, which has been defined on the basis of electron microscopic data as a set of filaments radiating from the tip of rabbit intestinal brush border microvilli, has been purified. It consists of a mucin-type glycoprotein that can be solubilized by either Triton extraction or papain treatment of the brush border membrane vesicles but is insensitive to phosphatidylinositol phospholipase C. The detergent- and papain-solubilized forms both have the same apparent molecular mass of 400 kDa (SDS/PAGE). This suggests that the filamentous brush border glycocalyx may be anchored to the membrane by a small hydrophobic peptidic tail. Ser, Thr, Pro and Ala amount to 65% of the protein core amino acid residues. The glycosidic moiety, which amounts to 73% of the molecular mass, has high O-acetylated sialic acid contents. A monoclonal antibody (3A4) raised against the purified material was produced which specifically recognized the 400-kDa band by immunoprecipitation and immunoblotting, and the filamentous brush border glycocalyx of villus enterocytes when jejunum sections were immunolabelled. The 3A4 determinant was identified with a filamentous brush border glycocalyx-specific carbohydrate structure containing an O-acetylated sialic acid. The fact that the labeled glycocalyx was anchored entirely in a membrane microdomain at the tip of the microvilli shows that mature enterocytes are hyper-polarized epithelial cells.

Traffic through the Golgi apparatus as studied by radioautography

The ability to radiolabel biological molecules, in conjunction with radioautographic or cell fractionation techniques, has brought about a revolution in our knowledge of dynamic cellular processes. This has been particularly true since the 1940's, when isotopes such as 35S and 14C became available, since these isotopes could be incorporated into a great variety of biologically important compounds. The first dynamic evidence for Golgi apparatus involvement in biosynthesis came from light microscope radioautographic studies by Jennings and Florey in the 1950's, in which label was localized to the supranuclear Golgi region of goblet cells soon after injection of 35S-sulfate. When the low energy isotope tritium became available, and when radioautography could be extended to the electron microscope level, a great improvement in spatial resolution was achieved. Studies using 3H-amino acids revealed that proteins were synthesized in the rough endoplasmic reticulum, migrated to the Golgi apparatus, and thence to secretion granules, lysosomes, or the plasma membrane. The work of Neutra and Leblond in the 1960's using 3H-glucose provided dramatic evidence that the Golgi apparatus was involved in glycosylation. Work with 3H-mannose (a core sugar in N-linked side chains), showed that this sugar was incorporated into glycoproteins in the rough endoplasmic reticulum, providing the first radioautographic evidence that glycosylation of proteins did not occur solely in the Golgi apparatus. Studies with the tritiated precursors of fucose, galactose, and sialic acid, on the other hand, showed that these terminal sugars are mainly added in the Golgi apparatus. With its limited spatial resolution, radioautography cannot discriminate between label in adjacent Golgi saccules. Nonetheless, in some cell types, radioautographic evidence (along with cytochemical and cell fractionation data) has indicated that the Golgi is subcompartmentalized in terms of glycosylation, with galactose and sialic acid being added to glycoproteins only within the trans-Golgi compartment. In the last ten years, radioautographic tracing of radioiodinated plasma membrane molecules has indicated a substantial recycling of such molecules to the Golgi apparatus.

Sequence determination of the hemagglutinin-neuraminidase (HN) gene of human parainfluenza type 2 virus and the construction of a phylogenetic tree for HN proteins of all the paramyxoviruses that are infectious to humans

The nucleotide sequence of the hemagglutinin-neuraminidase (HN) gene of human parainfluenza type 2 virus (PIV-2) was determined. The PIV-2 HN gene was 2112 nucleotides excluding poly(A) tail. There was a single large open reading frame in the mRNA which encoded a protein of 571 amino acids with a calculated molecular weight of 63,262. Analysis of the deduced amino acid sequence revealed that there were fourteen potential glycosylation sites and a major hydrophobic region near the N-terminus, which would anchor the protein in the viral membrane. Comparisons of the HN protein sequences of PIV-2 with those of Simian virus 5 (SV5), Sendai virus (SV, parainfluenza virus type 1), human parainfluenza virus type 3 (PIV-3), type 4 (PIV-4), bovine parainfluenza virus type 3 (BPIV-3), mumps virus (MuV), and Newcastle disease virus (NDV) showed definite amino acid sequence relatedness, indicating a common ancestor for these viruses. Furthermore, statistical analysis of the protein sequences suggested a possible evolutionary relatedness among the paramyxoviruses. This is the first time that a phylogenetic tree has been constructed for all the parainfluenza viruses and mumps virus which are infectious to humans. In addition, amino acid sequences involved in hemagglutinating and neuraminidase activities of paramyxovirus were discussed.

Prenatal exposure to ethanol alters the synthesis and glycosylation of proteins in fetal hepatocytes

In the present work we have studied the effect of prenatal exposure to alcohol on the synthesis, glycosylation, and transport of proteins in fetal hepatocytes isolated from 21-day-old fetuses derived from control and chronic alcoholic rats. Protein synthesis was evaluated both in a cell-free system and in hepatocytes after (35S)methionine and (3H)leucine incorporation, respectively. Glycosylation was assessed using (3H)mannose and (3H)galactose as precursors. Protein synthesis was significantly decreased in treated hepatocytes. In control hepatocytes, quantitative electron microscope autoradiography showed that both (3H)leucine and (3H)mannose incorporation occur first in the rough endoplasmic reticulum (rER). Later the silver grains appeared over the Golgi apparatus, and, finally there was a transport towards the cell periphery. After pulse, silver grains corresponding to (3H)galactose incorporation appeared over the Golgi apparatus. The label then moved to the hepatocyte periphery. Alcohol treated hepatocytes showed a retention of grains over the Golgi apparatus with a diminution in the label at the cell periphery. These results indicate that prenatal exposure to alcohol induces a decrease in the synthesis of proteins in the hepatocyte as well as an alteration in the process of glycosylation and/or transport of secretory proteins.

Fucosylated glycoconjugates in mouse preimplantation embryos

Preimplantation mouse embryos were metabolically labelled with 3H or 14C-fucose to investigate the synthesis of fucosylated macromolecules. Scintillation counting revealed that there was a progressive increase in both total fucose taken up by the embryo and incorporation of fucose into TCA-precipitable material as embryos developed from the 4-cell to the blastocyst stage. This was reflected in the increasing intensity of bands on autoradiographs of radioactive fucose labelled proteins separated on 10% SDS-PAGs between the 4-cell embryo (at which stage bands were first detectable) and the blastocyst. Minor qualitative changes in fucoproteins were detected at the time of compaction and additional bands appeared at the blastocyst stage. Preliminary analysis of fucolipids in 6- to 8-cell embryos indicated that an approximately equal amount of fucose was incorporated into lipid and protein. Autoradiographs of semi-thin sections of 3H-fucose-labelled embryos showed substantial amounts of radioactive material in the vicinity of the plasma membrane both adjacent to other cells and facing the zona pellucida. These data would support a predominant role for fucoconjugates in cell surface events in the preimplantation embryo from the 8-cell stage.

Radioautographic study of glycoprotein synthesis and fate in the hypothalamo-neurohypophyseal system of vasopressin-deficient Brattleboro rats

L-3H-fucose was injected into the lateral cerebral ventricle of vasopressin-deficient Brattleboro and control Long-Evans rats which were subsequently killed at several time intervals after the injection. The hypothalamus and the neurohypophysis were processed for light- and electronmicroscopic radioautography. Other complementary experiments using immunocytochemical and enzyme-histochemical techniques were also undertaken. L-3H-fucose was incorporated into newly synthesized glycoproteins in the Golgi apparatus of supraoptic and paraventricular neurons, and later on labelled glycoproteins migrated to lysosomes and the plasma membrane surrounding the perikaryon. The Golgi apparatus of the vasopressin-deficient neurons remained heavily labelled as long as 3 days after injection, in sharp contrast with the normal neurons in which there was a remarkable decrease of label in the Golgi region between 4 and 24 h after the isotope administration. Labelled glycoproteins also migrated to the neurohypophysis and were mainly found in the axonal plasma membrane, vesicles and axoplasm. The renewal of glycoproteins in the neurohypophysis of Brattleboro rats was faster than in the normal rats and this was attributed to the lack of formation of products which are normally packaged in secretory granules in the perikaryon and released at the axon terminal in the neurohypophysis. Colchicine caused a disturbance in the topography of the organelles of the perikaryon and the most striking features were the displacement of Golgi stacks to the periphery of the perikaryon and an accumulation of mitochondria in this neuronal region. No secretory granules were observed in the vasopressin-deficient neurons of untreated or colchicine-treated Brattleboro rats. By contrast, secretory granules (most of them labelled with 3H-fucose) were concentrated in the perikaryon of colchicine-treated Long-Evans rats. In these rats, colchicine caused a severe block in the migration of 3H-fucose-labelled glycoproteins to the neurohypophysis, but this did not occur in the Brattleboro rats. The results of the experiments were interpreted in the light of the genetic defect known to occur in Brattleboro rats which causes the inability to produce vasopressin and also remarkable morphological and physiological changes in the affected neurons.

3H-mannose utilization by fibroblasts of the periodontal ligament

The synthesis, intracellular translocation, and secretion of mannose-containing glycoproteins(s) by periodontal ligament fibroblasts have been investigated by means of electron microscopic radioautography. Tritiated mannose was administered to young mice via jugular vein, and radioautographs were prepared at 5, 10, 20, and 35 minutes, 4 and 8 hours after injection. Analysis of electron microscopic radioautographs revealed a maximum labeling (94%) with 3H-mannose of the rough endoplasmic reticulum at 5 minutes. Labeling of the Golgi components started to increase from 10 minutes (14%) and reached a maximum level at 20 minutes (31.2%). At 35 minutes, secretion granules, dense bodies, profiles of intracellular collagen, and the cell surface were labeled. At 8 hours, most labelling (79.2%) was extracellular, and associated either with the collagenous matrix (43.7%) or the cell surface (35.5%). Cytoplasmic vesicles containing dense materials around collagen fibrils were also labeled at 8 hours. It is concluded that mannose is directly incorporated into the rough endoplasmic reticulum (RER), and that mannose-containing glycoprotein(s) are packaged in the Golgi apparatus into secretory granules. Mannose-containing glycoprotein(s) become distributed on the periodontal ligament (PDL) fibroblast cell surface, cytoplasmic dense bodies, and the extracellular matrix.

Autoradiographic study of the production of secretory material by the subcommissural organ of frogs (Rana temporaria) after injection of several radioactive precursors, with special reference to the glycosilation and turnover rate of the secretory material

The suitability of several radioactive precursors for studying the secretory processes in the cells of the subcommissural organ (SCO) of frogs (Rana temporaria) was tested by means of autoradiography. Special attention was paid to: the contributions made by different cellular compartments to the glycosilation of the secretory product, and the intracellular turnover rate of the secretory material. From the results it is concluded that: 3H-glucosamine excellently labels Reissner's fibre (RF) in autoradiographs, much better than any other of the radioactive precursors applied. 3H-glucosamine molecules are attached to the protein moiety of the secretory product within the peri- and subnuclear granular endoplasmic reticulum, whereas 3H-fucose and additional 3H-glucosamine molecules are added to the oligosaccharide moiety in the supranuclear Golgi apparatus, previous to apical release; consequently, the subnuclear secretory material and the material that is released into the brain ventricle are chemically different so far as the oligosaccharide moiety is concerned. The oligosaccharide portion of the apical secretory product belongs (at least partially) to the class of the N-linked complex type oligosaccharides. The intracellular half-life of the subnuclear secretory material is at least 5.5 days. The subnuclear secretory material in the ependymal SCO-cells presumably has to pass through the Golgi apparatus before it can be released; this release probably occurs at the apical cell border.

Synthesis and migration of 3H-fucose-labeled glycoproteins in the retinal pigment epithelium of albino rats, as visualized by radioautography

3H-fucose was injected into the vitreous body of the eye(s) of 250-gm rats, which were then killed by means of an intracardiac perfusion with glutaraldehyde after intervals of 10 min, 1 and 4 hr, and 1 and 7 days. The eyes were removed and further fixed, and pieces of retina were processed for light and electron microscope radioautography. Light microscope radioautography showed that the pigment epithelial cells actively incorporated 3H-fucose label. The intensity of reaction peaked at 4 hr after injection of the label and then slowly declined. Quantitative electron microscope radioautography revealed that, at 10 min after 3H-fucose injection, over 70% of the label was localized to the Golgi apparatus, indicating that fucose residues are added to newly synthesized glycoproteins principally at this site. With time the proportion of label associated with the Golgi apparatus decreased, but that assigned to the infolded basal plasma membrane, the apical microvilli, and various apical lysosomes increased. These results indicate that in retinal pigment epithelial cells newly synthesized glycoproteins continuously migrate from the Golgi apparatus to lysosomes and to various regions of the plasma membrane. In this case, the membrane glycoproteins may play specific roles in receptor functions of the basal plasma membrane or phagocytic activities at the apical surface. Very little label migrated to Bruch's membrane, indicating either a very slow turnover or a paucity of fucose-containing glycoproteins at this site.

Synthesis and migration of 3H-fucose-labeled glycoproteins in the ciliary epithelium of the eye: effects of microtubule-disrupting drugs

3H-fucose was injected intravenously or intravitreously into albino rats. After time intervals of 10, 40, and 50 min, 1, 1.5, and 4 hr, 1, 3, and 7 days, and 1, 2, and 4 weeks after injection, the animals were sacrificed by intracardiac perfusion with glutaraldehyde. Samples of the ciliary body were prepared for light and electron microscope radioautography. Light microscope autoradiographs showed that the cells of both the inner and outer layers of ciliary epithelium actively incorporated 3H-fucose label in a reaction that peaked in intensity at 4 hr after injection, and then progressively declined. Electron microscope radioautographs revealed that, at early time intervals, most of the label was localized to the Golgi apparatus. With time, the plasma membrane of both cell types became increasingly labeled, and accounted for 60-70% of the total silver grains at 4 hr after injection. Adjacent to the basal cell surface of the inner layer cells, the fibers of the zonula became increasingly labeled from 1.5 hr onwards, providing strong evidence that these cells secrete glycoproteins to the zonula. When vinblastine was administered 30 min before 3H-fucose injection, followed by sacrifice 1.5 hr later, a much larger proportion of label remained localized to the Golgi apparatus than in controls, and the plasma membrane and zonula were much less labeled. These results suggest that, as documented in other cell types, microtubules may play a role in the intracellular transport of membrane and secretory glycoproteins in these cells.

The utilization of [3H] sugars by non-malignant and malignant human breast

The utilization of [3H] sugars and leucine by non-malignant and malignant human breast has been assessed using an organ culture technique with subsequent tissue autoradiography. The uptake of sugars by normal and hyperplastic breast was generally constant, with some differences observed in the utilization of galactose by acini of normal and hyperplastic tissues. After 24 h incubation localization was predominantly at the luminal cell periphery. The utilization of sugars by carcinomas was much more variable. Differences were observed between adjacent cells and cell groups of the same tumour. The uptake of individual sugars within a carcinoma was also varied being either similar to, or greater or lesser than normal breast. Variation between carcinomas was also present. No correlation between type and differentiation was noted in this respect, but there was between localization of sugars and differentiation. Better differentiated areas in tumours showed patterns similar to non-malignant breast whilst localization in poorly differentiated cell groups was cytoplasmic. The uptake of leucine was more constant and proved to be a useful indicator of viability. While this approach cannot give information with regard to differences in glycoprotein structure between non-malignant and malignant breast, it has been of value in determining the heterogeneity of tumour cells with regard to the enzymes involved in glycosylation. As such it would be of use in assessing the uniformity of response to agents modifying glycosylation.

Lectin-binding pattern in parotid acinar cells. The fracture-labelling method and post-embedding staining

The localization of complex carbohydrates in the Golgi apparatus, secretory granules and plasmalemma of mouse parotid acinar cells was studied using the fracture-labelling method. The hexose residues of glycoconjugates were identified using ferritin conjugated with Wheat Germ Agglutinin (WGA-), Ricinnus Communis Agglutinin II (RCA-II-), Phaseolus Vulgaris Agglutinin (PHA-) and Limulus Polyphemus Agglutinin (LPA-). We found that the fracture-labelling method allows not only the labelling of membrane faces but also analysis of the compartment's content that is exposed during the fracturing of the tissue. Our results revealed differences in the hexose residues located in the Golgi apparatus, secretory granules and the apical and lateral plasmalemma. Numerous binding sites for WGA-, PHA- and RCA-II-ferritin were demonstrable in the Golgi apparatus. In secretory granules, the WGA- and RCA-II-ferritin binding sites were most numerous, while LPA-ferritin binding sites were very rare. The density of the binding sites for PHA-ferritin showed considerable variation in secretory granules. The apical plasmalemma exhibited a high density of binding sites for all of the lectins used. In the lateral plasmalemma, LPA-ferritin was not bound, and there were fewer binding sites for WGA-, RCA-II- and PHA-ferritin.

The effect of intraocular injection of tetrodotoxin on fast axonal transport of [3H]proline- and [3H]fucose-labeled materials in the developing rat optic nerve

The fast axonal transport of [3H]proline-labeled proteins and [3H]fucose-labeled glycoproteins delivered to the dorsal lateral geniculate nucleus in the developing rat optic nerve was investigated during tetrodotoxin-induced monocular impulse blockade. Repeated intraocular injections of various dosages of tetrodotoxin or citrate buffer vehicle were made every two days in rats aged 5-21 days postnatal, and the accumulation of rapidly transported radioactivity in the lateral geniculate nucleus measured between three and twelve hours post-injection at each age. The effectiveness of prolonged tetrodotoxin treatment was monitored by loss of the pupillary light reflex and the level of cytochrome oxidase activity in the contralateral superior colliculus and dorsal lateral geniculate nucleus. Numbers of optic axons proximal to the chiasm and the frequency of retinal ganglion cells per unit distance from the optic disc were examined for signs of tetrodotoxin-induced degeneration of the retinofugal pathway. Tetrodotoxin-treatment reduced the amount of fucosyl glycoproteins, but not proline-labeled proteins, axonally transported to the lateral geniculate nucleus during the first three weeks of postnatal development. Other studies indicated that tetrodotoxin significantly reduced the incorporation of [3H]fucose into retinal proteins indicating that the reduction in transport was probably due to a decrease in precursor incorporation into retinal ganglion cells. Electron microscopy of ganglion cells at 21 days revealed dilated and vacuolated Golgi cisternae associated with tetrodotoxin treatment, suggesting that tetrodotoxin may alter fucose metabolism by secondarily disrupting Golgi organization. Other protein synthetic machinery in these cells, including ribosomes and rough endoplasmic reticulum, appeared normal throughout tetrodotoxin treatment. These data indicate that Na+-dependent optic impulse activity may be indirectly related to the axonal transport of glycoproteins during early postnatal development by mediating the incorporation of precursor into glycoproteins at the Golgi apparatus and their subsequent entrance into the fast transport system.

The carbohydrates of secretory granules and the glycocalyx in developing mucoid cells

Complex carbohydrates in secretory granules and at the apical cell surface of mouse gastric mucoid cells were studied during embryogenesis and in the early postnatal period by various cytochemical methods; the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) and tannic acid-uranyl acetate (TA-UA) procedures made neutral mucosubstances (NMS) visible, whereas the hexose residues of glycoconjugates were identified using WGA-, RCA II- and ConA-ferritin. The glycocalyx was stained with ruthenium red (RR). During differentiation of the embryonic mucoid cells the number of secretory granules increased in parallel to the increase in their carbohydrate component. NMS-stainable parts in secretory granules also had binding sites for the conjugates RCA II- and WGA-ferritin, but the binding of ConA could not be identified. The increasing quantity of NMS in secretory granules was correlated with the increased amount of PA-TCH-SP and TA-UA positive substances in the apical glycocalyx only in 14- and 18-day-old embryos. The observed uniform affinity for RR and lectin conjugates in all analysed developmental stages remains to be explained.

Glycoprotein synthesis in the Golgi apparatus of spermatids during spermiogenesis of the rat

During steps 1-7 of spermiogenesis the Golgi apparatus contributes to the formation of the acrosomic system which develops at the surface of the nucleus. Later, in step 8, the Golgi apparatus detaches from the acrosome and remains suspended in the elongated cytoplasm until it degenerates during step 16. Using 3H-fucose as a tracer and the radioautographic technique, we observed that the Golgi apparatus incorporates the tracer and delivers the labeled glycoproteins to the developing acrosomic system during steps 1-7 of spermiogenesis, to multivesicular bodies during steps 1-9, and to the remaining cytoplasm and plasma membrane during steps 1-15. Throughout these steps of spermiogenesis the Golgi apparatus does not show major changes in structure; it is composed of a cortex made up of connected stacks of saccules and a medulla showing a loose aggregate of vesicular profiles. Glycoprotein synthesis in this Golgi apparatus, before and after it contributes lysosomal glycoproteins to the growing acrosomic system, was quantitatively assessed in electron microscope EM radioautographs of tissue sections from animals sacrificed at 1, 4, 8, and 24 h of 3H-fucose injection. The incorporation of the labeled sugar was found to remain quantitatively similar during steps 1-15 of spermiogenesis, and therefore, no shift in glycoprotein synthesis took place following separation of the Golgi apparatus from the acrosomic system. Throughout these steps, fucose molecules are first incorporated in the cortex of the organelle and subsequently transported to the medulla, where they temporarily accumulate before being delivered, depending on the step of spermiogenesis, to the acrosomic system, to the multivesicular bodies, and also, presumably, to the plasma membrane.

Immunopurification and characterization of a neuronal heparan sulfate proteoglycan

We have identified a unique heparan sulfate (HeS) proteoglycan synthesized by the neuronal-like cell line PC12. The proteoglycan, purified with monoclonal antibodies from medium conditioned by PC12 cells, has an apparent molecular weight of 350,000, and it contains a Mr 80,000 core protein and HeS side chains of Mr 15,000 each. The purified molecule has the same apparent size and density as it has in conditioned medium. HeS proteoglycans that are indistinguishable antigenically but very difficult to solubilize are found on the external surface and in the interior of PC12 cells and neurons. Mild proteolysis converts the surface proteoglycan into a molecule closely resembling that found in the medium. The same surface antigens are also present on a subpopulation of T-cells and on a non-neuronal accessory cell found in dorsal root ganglion cultures. The PC12 cell line and the non-neuronal dorsal root ganglion cells secrete a factor into medium that, after adsorption to polylysine-coated surfaces, induces rapid neurite out-growth by primary sympathetic neurons. The monoclonal antibodies used to purify the neuronal HeS proteoglycan from PC12 cells are capable of depleting this conditioned medium of its neurite-promoting activity. These studies suggest that a HeS proteoglycan synthesized and secreted by neurons and certain accessory cells plays a role in regulating neurite outgrowth.

Radioautographic analysis of [3H]-fucose utilization by mouse odontoblasts with emphasis on intracytoplasmic and plasma membrane glycoproteins

[3H]-fucose utilization by odontoblasts was studied by light and electron microscopic radioautography. At 10 min after injection, fucose label was concentrated in the Golgi area. By 20-30 min, there was a progressive decline in Golgi labelling with label present at the plasma membrane, terminal web, odontoblast process and predentine matrix. At 4 h, the predentine and the predentine-dentine junction were heavily labelled. At the ultrastructural level, Golgi labelling at 10 min was mostly localized to cisternal elements and at 20 and 30 min secretory granules and dense bodies were also labelled. Most of the silver grains observed in the terminal web were associated with microfilaments near the plasma membrane. In the predentine, the matrix itself accounted for 23.0 per cent of the label at 4 h and the plasma membrane of the odontoblast process accounted for 19 per cent. The results indicate that odontoblasts, in addition to secreting glycoproteins into the dentinal matrix, also continuously manufacture glycoproteins for incorporation into the cell surface, the lysosomal system and the terminal web.

Biological and pathobiological aspects of the glycocalyx of the small intestinal epithelium. A review

The literature on the glycocalyx of small intestinal epithelium is reviewed. The structure, general and barrier functions, synthesis, and degradation of the glycocalyx, and pathobiological aspects of the glycocalyx in relation to its barrier function are mentioned. Topics for future research are indicated.

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: III. Inhibition of intracellular migration of membrane glycoproteins in rat intestinal columnar cells and hepatocytes as visualized by light and electron-microscope radioautography after 3H-fucose injection

In the first paper of this series (Bennett et al., 1984), light-microscope radioautographic studies showed that colchicine or vinblastine inhibited intracellular migration of glycoproteins out of the Golgi region in a variety of cell types. In the present work, the effects of these drugs on migration of membrane glycoproteins have been examined at the ultrastructural level in duodenal villous columnar cells and hepatocytes. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for radioautography. In duodenal villous columnar cells, 3H-fucose labeling of the apical plasma membrane was reduced by 51% after colchicine and by 67% after vinblastine treatment; but there was little change in labeling of the lateral plasma membrane. Labeling of the Golgi apparatus increased. This suggests that labeled glycoproteins destined for the apical plasma membrane were inhibited from leaving the Golgi region, while migration to the lateral plasma membrane was not impaired. In hepatocytes, labeling of the sinusoidal plasma membrane was reduced by 83% after colchicine and by 85% after vinblastine treatment. Labeling of the lateral plasma membrane also decreased, although not so dramatically. Labeling of the Golgi apparatus and neighboring secretory vesicles increased. This indicates that the drugs inhibited migration of membrane glycoproteins from the Golgi region to the various portions of the plasma membrane. Accumulation of secretory vesicles at the sinusoidal front suggests that exocytosis may also have been partially inhibited. In both cell types, microtubules almost completely disappeared after drug treatment. Microtubules may, therefore, be necessary for intracellular transport of membrane glycoproteins, although the possibility of a direct action of these drugs on Golgi or plasma membranes must also be considered.

Influence of colchicine and vinblastine on the intracellular migration of secretory and membrane glycoproteins: I. Inhibition of glycoprotein migration in various rat cell types as shown by light microscope radioautography after injection of 3H-fucose

Previous studies have shown that colchicine and vinblastine inhibit secretion in many cell types by interrupting the normal intracellular migration of secretory products. In the present work, radioautography has been used to study the effects of these drugs on migration of membrane and secretory glycoproteins in a variety of cell types. Young (40 gm) rats were given a single intravenous injection of colchicine (4.0 mg) or vinblastine (2.0 mg). At 10 min after colchicine and 30 min after vinblastine administration, the rats were injected with 3H-fucose. Control rats received 3H-fucose only. All rats were sacrificed 90 min after 3H-fucose injection and their tissues processed for light microscope radioautography. Examination of secretory cell types such as ameloblasts and thyroid follicular cells in control animals revealed reactions of approximately equal intensity over the Golgi region and over extracellular secretion products, while in drug-treated rats most of the reaction was confined to the Golgi region. In a variety of other cell types, including endocrine cells (e.g., hepatocytes) and cells generally considered as nonsecretory (e.g., intestinal columnar cells), reaction in control animals occurred both over the Golgi region and over various portions of the cell surface. In drug-treated animals, a strong Golgi reaction was present, but reaction over the cell surface was weak or absent. These results indicate that in many cell types, colchicine and vinblastine inhibit migration out of the Golgi region not only of secretory glycoproteins, but also of membrane glycoproteins destined for the plasma membrane.

Comparative radioautographic study of the effects of L-azetidine-2-carboxylic acid on matrix secretion and Golgi of the mouse incisor

The effect of a proline analog, L-azetidine-2-carboxylic acid (LACA), on protein matrix secretion by odontoblasts and ameloblasts was compared by light and electron microscopic radioautography after injection of 3H-glycine in young mice. LACA inhibited the secretion of dentin matrix with consequent accumulation of 3H-glycine labeled procollagen in the cisternae of the rough endoplasmic reticulum. In contrast, LACA had no apparent effect on ameloblasts as enamel matrix continued to be packaged in the Golgi apparatus and secreted from Tomes' process within 30 min after injection of the radioprecursor. Electron microscopy revealed that LACA did not cause any change in ameloblast ultrastructure but produced a marked alteration of the odontoblast Golgi complex. All odontoblast Golgi saccules and collagen secretion granules disappeared within 2 h after LACA administration. Odontoblast Golgi cisternae, however, appeared not to be affected. These observations confirm previous studies conducted in this laboratory showing that Golgi saccules in collagen-secreting cells are the initial staging areas for the formation of secretory granules. These results also indicate that a close correlation exists between form and function in the Golgi apparatus of collagen-secreting cells.

Radioautographic localization of 3H-fucose incorporation into bone cells in cultured calvaria

Radioautographic procedures were used to examine the incorporation of 3H-fucose into macromolecular material in bone organ cultures. Radioautography demonstrated that silver grains were first apparent in the paranuclear area of osteoclasts, and continued to increase in number over osteoclasts throughout the culture period. Silver grains associated with osteoclasts were later found on the side adjacent to bone and the adjacent bone surface. Osteoblasts, pre-osteoblasts and fibroblasts of the fibrous periosteum incorporated some 3H-fucose as evidenced by labelling in the later time periods. At later incubation periods, labelled material was found in the underlying bone below osteoblasts, demonstrating the active elaboration of bone matrix glycoproteins. Osteocytes were labelled only after the longest period of incubation.

Effect of colchicine on rat small intestinal absorptive cells. II. Distribution of label after incorporation of [3H]fucose into plasma membrane glycoproteins

By means of radioautography the influence was tested of various periods (5, 15, 30, 40 min, 2 hr) of pretreatment with colchicine, administered intraperitoneally to rats at a dosage of 0.5 mg/100 g of body weight, on the intracellular pathway of [3H]fucose in absorptive cells of the small intestine. Administration of colchicine for 30 min and longer time intervals causes delay in the insertion of [3H]fucose into the oligosaccharide chains of glycoconjugates in the Golgi apparatus, and results in redistribution of the label apparent over the different portions of the plasma membrane. In controls, at 2 and 4 hr after administration of [3H]fucose the apical plasma membrane is strongly labeled; 53.7 +/- 3.2% of the silver grains are recorded over apical regions of the plasma membrane that contrast to basolateral portions comprising 25.4 +/- 3.2% of the label. Colchicine causes equalization of the reaction of apical and basolateral regions of the plasma membrane: the number of silver grains attributable to the apical plasma membrane is reduced; following treatment with colchicine, apical portions of the plasma membrane comprise 31.6 +/- 1.8% of the silver grains, 38.6 +/- 3.8% are attributable to basolateral membrane regions. The colchicine-induced equalization of the density of label of apical and basolateral regions of the plasma membrane, in addition to the occurrence of basolateral microvillus borders (demonstrated in the companion paper), suggests microtubules to be important in the maintenance of the polar organization of small intestinal absorptive cells.

Effect of colchicine on rat small intestinal absorptive cells. I Formation of basolateral microvillus borders

Treatment of rats with colchicine (0.5 mg/100 g of body weight) for more than 3 hr causes formation of microvillus borders along lateral and basal surfaces of absorptive cells in the small intestine. Morphologically, these strongly resemble the apical brush border inclusive of the terminal-web region. Formation of basolateral microvilli is restricted to mature absorptive cells. At 6 hr after administration of colchicine, 3.47% (+/- 1.94%) of the basolateral cell surfaces exhibit "implantation" of microvillus borders. The results show that colchicine induces formation of surface differentiations at lateral and basal surface regions that are restricted to the apical cell surface in controls. Redistribution of constituents of the plasma membrane from apical to basolateral membrane portions, as well as rearrangement in the organization of microfilaments can be considered to underlie formation of basolateral microvillus borders. From the antimicrotubular effect of colchicine it may be deduced that microtubules exert a regulative function in the formation of surface differentiations on absorptive cells of the small intestine and in the maintenance of the polarity of the cells.

Association of glycoconjugates with the cytoskeletal framework

The association of glycoconjugates with the cytoskeletal framework was examined in detergent-extracted cells. Sparse cultures of fibroblasts that assemble only minimal amounts of extracellular matrix were extracted under mild conditions with Triton X-100 which remove most of the lipids and soluble cellular proteins. The detergent-resistant framework retains lectin binding sites in the nucleus, in the perinuclear area occupied by the rough endoplasmic reticulum-Golgi system of the intact cell, and in a network throughout the cytoskeletal framework. Fluorescent-antibody staining with antibody against collagen type I and fibronectin reveals extensive perinuclear staining of the remnant rough endoplasmic reticulum-Golgi system. In contrast, only sporadic staining of the pericellular area is obtained with these antibodies, in sparse cultures of whole cells. Lectin binding sites were detected in the nucleus and are attributed to chromatin-associated glycoconjugates. They can be removed by DNase under conditions that preserve the cytoplasmic lectin binding sites and the nuclear matrix. The results suggest a high degree of integration of the membrane residues of the cytoplasmic elements and the nuclear matrix with the skeletal framework and indicate a possible role for the glycoconjugates in this structural integration.

Association of glycoconjugates with the cytoskeletal framework

The association of glycoconjugates with the cytoskeletal framework was examined in detergent-extracted cells. Sparse cultures of fibroblasts that assemble only minimal amounts of extracellular matrix were extracted under mild conditions with Triton X-100 which remove most of the lipids and soluble cellular proteins. The detergent-resistant framework retains lectin binding sites in the nucleus, in the perinuclear area occupied by the rough endoplasmic reticulum-Golgi system of the intact cell, and in a network throughout the cytoskeletal framework. Fluorescent-antibody staining with antibody against collagen type I and fibronectin reveals extensive perinuclear staining of the remnant rough endoplasmic reticulum-Golgi system. In contrast, only sporadic staining of the pericellular area is obtained with these antibodies, in sparse cultures of whole cells. Lectin binding sites were detected in the nucleus and are attributed to chromatin-associated glycoconjugates. They can be removed by DNase under conditions that preserve the cytoplasmic lectin binding sites and the nuclear matrix. The results suggest a high degree of integration of the membrane residues of the cytoplasmic elements and the nuclear matrix with the skeletal framework and indicate a possible role for the glycoconjugates in this structural integration.

Synthesis and secretion of glycoprotein by the epididymal epithelium

The secretory activity of the middle segment of the mouse caput epididymidis was studied using 3H-fucose as a precursor to glycoprotein. Young adult male mice were injected with a concentrated solution of 3H-fucose interstitially, ie beneath the connective tissue capsule of the epididymis. Two animals were killed and prepared for light microscopic radioautography at each of six intervals between 10 minutes and 24 hours after injection. Silver grains were concentrated over the supranuclear Golgi region at 10 minutes and over the apical ends of the cells 30 minutes and 1 hour after injection. Quantitative analysis showed that luminal radioactivity increased greatly beginning with the 2-hour samples. The results indicate that the epididymal epithelium synthesizes and secretes glycoproteins, and that 1 to 2 hours are required for terminal glycosylation, intracellular transport, and release of the secretory product.

Accumulation of [3H]fucose-labelled glycoproteins in the Golgi apparatus of dorsal root ganglion neurons during inhibition of fast axonal transport caused by exposure of the ganglion to Co2+-containing or Ca2+-free medium

Previous in vitro studies have established that Co2+-containing or Ca2+-free media interfere with the initiation of the fast axonal transport of proteins. The present study has used light- and electron-microscope radioautography to compare the distribution of [3H]fucose-labelled glycoproteins in neuronal cell bodies of control dorsal root ganglia and ganglia incubated for 16-17 h in Ca2+-free medium or in medium containing 0.18 mM Co2+. The radioautographic reaction in control cell bodies was diffusely scattered throughout the cytoplasm; grain counts revealed that 22% of the reaction was associated with elements of the Golgi apparatus and 78% was over other organelles and the remainder of the cytoplasm. In most experimental cell bodies, 78% of the silver grains were clustered over elements of the Golgi complex whereas other organelles and the remainder of the cytoplasm were comparatively much less labelled; structural alterations of the Golgi apparatus were also produced by the modified media. In parallel studies where the radioactivity in nerve trunks and ganglia was measured by liquid scintillation counting, it was found that the Ca2+-free medium and the Co2+-containing medium both reduced by approximately 80% the quantity of [3H]fucose-labelled glycoproteins which were carried by the fast axonal transport system; they did so without interfering with the incorporation of [3H]fucose into glycoproteins. The results indicate that in the presence of Co2+ or in the absence of Ca2+ the proteins which are destined for fast axonal transport accumulate at the Golgi apparatus of neuronal cell bodies. These results thus suggest that Ca2+ is required for proteins to leave the Golgi region in transit to the fast axonal transport system.

Changes in [3H]galactose uptake in human colonic mucosa with carcinoma: an ultrastructural study

As part of our investigation on glycoprotein synthesis in pre-malignant colonic epithelium, changes in the uptake of [3H]galactose were studied at the ultrastructural level. Normal control mucosa from rectal biopsies of patients with no known gastro-intestinal disease and mucosa adjacent to carcinoma ('transitional' mucosa) from specimens resected for colo-rectal cancel were compared. These tissues were incubated in TC 199 medium containing [3H]galactose for various intervals of time and for pulse labelling. Silver grain distribution was statistically analysed. The results showed a reduction in the incorporation of the galactose by 'transitional' mucosa. The uptake by this mucosa was less uniform than normal and showed considerable peaking in the endoplasmic reticulum of the goblet cells and in the Golgi of the absorptive cells, suggesting a blockage or alteration in glycoprotein synthesis. The differences were most marked in the middle crypt (the region of differentiation) and in the upper crypt (the region of maturation).

Uptake of [3H]threonine in human colonic mucosa associated with carcinoma: an autoradiographic analysis at the ultrastructural level

The uptake of D,L-[G-3H]threonine was studied to show cellular protein synthesis, at the ultrastructural level, as part of our investigation into alterations in glycoprotein synthesis which occur in colonic mucosa adjacent to carcinoma ('transitional' mucosa). Threonine uptake, though variable, was higher in 'transitional' than in normal mucosa. Most of the threonine was incorporated into the endoplasmic reticulum of the immature cells at the bottom of the crypt. With longer isotope incubation, activity was found in other organelles in cells which were still undifferentiated or immature or both. From our data, the increased uptake of [3H]threonine in 'transitional' mucosa, seems to be the result of prolonged protein synthesis associated with an extension of the cellular proliferation zone in the crypt, rather than being the effect of increased cell turnover. Thus, variations in [3H]threonine uptake are not related to the changes in the rate of galactose incorporation in mucosa adjacent to carcinoma.

Synthesis and turnover of membrane glycoconjugates in monolayer culture of pig and human epidermal cells

The regression and turnover of the surface glycoconjugates of trypsin-prepared pig and human cultured epidermal cells have been determined using the glycoprotein precursors N-acetyl-D-(I-3H) glucosamine (3H-NAG) and N-(3H)-acetyl-D-mannosamine (3H-NAM). Sialic acid assays have been performed on similar unlabelled cells. The major points which emerged from this study were: (1) Trypsin-damaged cell surfaces are rapidly repaired, probably by normal membrane turnover. There was a 12% regeneration of sialic acid within 2 h and total resynthesis occurred within 24 h. (2) The presence of an internal membrane system, part of which also demonstrates turnover, probably contributed to the speed of surface membrane repair. Some of the glycoprotein/sialic acid of this internal membrane system (30%) remains bound for a considerable length of time. (3) The membrane turnover maintains the cell in equilibrium so that total loss equals the synthesis of glycoprotein. (4) The equilibration of 3H-NAG or 3H-NAM uptake between 24 and 48 h is limited by the relative concentrations of glucose and labelled sugar in the medium at this time. (5) 3H-NAm was a more specific marker of glycoprotein than 2H-NAG. (6) The results for human epidermal cells closely matched those for pig epidermal cells, indicating that pig cells can be used as a model for human cells.

The site of incorporation of sialic acid residues into glycoproteins and the subsequent fates of these molecules in various rat and mouse cell types as shown by radioautography after injection of [3H]N-acetylmannosamine. I. Observations in hepatocytes

To study the site of incorporation of sialic acid residues into glycoproteins in hepatocytes, we gave 40-g rats and 15-g Swiss albino mice a single intravenous injection of [3H]N-acetylmannosamine (8 mCi) and then sacrificed them after 2 and 10 min. To trace the subsequent migration of the labeled glycoproteins, we injected 40-g rats with 4 mCi of [3H]N-acetylmannosamine and sacrificed them after 20 and 30 min, 1, 4, and 24 h, and 3 and 9 d. Concurrent biochemical experiments were carried out to test the specificity of injected [3H]N-acetylmannosamine as a precursor for sialic acid residues of glycoproteins. In radioautographs from rats and mice sacrificed 10 min after injection, grain counts showed that over 69% of the silver grains occurred over the Golgi region. The majority of these grains were localized over the trans face of the Golgi stack, as well as over associated secretory vesicles and possibly GERL. In rats, the proportion of grains over the Golgi region decreased with time to 37% at 1 h, 11% at 4 h, and 6% at 24 h. Meanwhile, the proportion of grains over the plasma membrane increased from 4% at 10 min to 29% at 1 h and over 55% at 4 and 24 h; two-thirds of these grains lay over the sinusoidal membrane, and the remainder were equally divided over the lateral and bile canalicular membranes. Many silver grains also appeared over lysosomes at the 4- and 24-h time intervals, accounting for 15-17% of the total. At 3 and 9 d after injection, light microscope radioautographs revealed a grain distribution similar to that seen at 24 h, with a progressive decrease in the intensity of labeling such that by 9 d only a very light reaction remained. Because our biochemical findings indicated that [3H]N-acetylmannosamine is a fairly specific precursor for the sialic acid residues of glycoproteins (and perhaps glycolipids), the interpretation of these results is that sialic acid is incorporated into these molecules in the Golgi apparatus and that the latter then migrate to secretion products, to the plasma membrane, and to lysosomes in a process of continuous renewal. It is possible that some of the label seen in lysosomes at later time intervals may have been derived from the plasma membrane or from material arising outside the cells.

The site of incorporation of sialic acid residues into glycoproteins and the subsequent fates of these molecules in various rat and mouse cell types as shown by radioautography after injection of [3H]N-acetylmannosamine. II. Observations in tissues other than liver

Biochemical evidence from the preceding paper indicated that [3H]N-acetylmannosamine may be used as a fairly specific precursor for the sialic acid residues of glycoproteins (and perhaps glycolipids) in radioautographs of rat liver and duodenum. In order to study the site of incorporation of this label in cell types of various tissues, we gave 40-g rats and 15-g Swiss albino mice a single intravenous injection of 8 mCi of [3H]N-acetylmannosamine and sacrificed them after 2 and 10 min. To trace the subsequent migration of the labeled glycoproteins, we injected 40-g rats with 4 mCi of [3H]N-acetylmannosamine and sacrificed them after 20 and 30 min, 1, 4, and 24 h, and 3 and 9 d. Light microscope radioautographic analysis revealed that in a great variety of cell types the label was initially localized to the Golgi region. Electron microscope radioautographic analysis of duodenal villous columnar and goblet cells, pancreatic acinar cells and Paneth cells, from rats and mice sacrificed 10 min after injection, showed that the silver grains were localized over Golgi saccules (and adjacent secretion granules). In kidney proximal and distal tubule cells reaction was initially localized to the Golgi apparatus in some areas of the kidney cortex whereas in other areas it was more diffuse. In all cells, the proportion of silver grains over the Golgi apparatus decreased with time after injection while an increasing number of grains appeared over secretion products in secretory cells or over the plasma membrane in other cell types. Lysosomes also became increasingly labeled at later time intervals. The above results suggest that in most cell types sialic acid residues are incorporated into glycoproteins (and perhaps glycolipids), primarily in the Golgi apparatus. With time, these newly synthesized molecules migrate to secretion products, to the plasma membrane, or to the lysosomes.

Biogenesis of hepatocyte plasma-membrane domains. Incorporation of (3H)fucose into plasma-membrane and golgi-apparatus glycoproteins

1. Rats were injected intracaudally with [3H]fucose and its rate of incorporation into the fucoproteins of serum, Golgi and plasma-membrane subfractions was followed for up tp 2h. 2. Incorporation into the Golgi dictyosome and secretory-vesicular fractions reached a maximum at 15 min or less, but most of the radioactivity was associated with classes of secretory glycoproteins. Incorporation into sinusoidal plasma-membrane fractions reached a maximum at 30 min, coinciding with the maximum release of fucoproteins into the serum. Contiguous and canalicular plasma-membrane fractions were labelled slightly later and at a lower rate and specific radioactivity. 3. Fluorography of fucoproteins separated by polyacrylamide-gel electrophoresis helped to distinguish between the major secretory and membrane-bound glycoproteins. The results show that a major biogenetic sequence is probably from Golgi dictyosomes to Golgi secretory elements to a sinusoidal plasma membrane. 4. The kinetics of incorporation make it unlikely that there is rapid and direct insertion of glycoproteins into the bile-canalicular plasma membrane. A route involving direct transfer of glycoproteins via a membrane-mediated intracellular path from the blood sinusoidal to the bile-canalicular plasma membranes is proposed.

Distribution of glycoproteins containing fucose in normal and psoriatic keratinocytes

Electron microscope autoradiography has been used to compare the fucose-containing glycoproteins of normal and psoriatic epidermis. Normal keratinocytes and uninvolved psoriatic cells have most of their fucose on the plasma membrane, whereas involved psoriatic cells retain more within the cytoplasm. The fucose distribution in other epidermal cell types is also described.

A comparison of [3H]galactose and [3H]fucose uptake with the morphological and histochemical changes observed in mucous secretion in chemically induced rat colonic carcinoma

The alterations in carbohydrate metabolism which occur in the distal colon of rats during carcinogenesis induced by dimethylhydrazine were investigated using [3H]galactose and [3H]fucose as glycoprotein precursors. A statistically significant decrease in [3H]galactose uptake was observed in dysplastic epithelia. These findings are consistent with the alterations in mucin composition with predominance of sialomucins shown in these areas by histochemical methods. Furthermore, changes in the gradient of [3H]galactose incorporation along the crypt epithelium were also found in the histological and histochemically non-involved colonic mucosa of dimethylhydrazine-treated rats, as compared with controls. No significant variations were seen in [3H]fucose incorporation. These results correlate well with our previous histochemical observations and are further evidence of the profound alterations in glycoprotein synthesis affecting the whole colonic mucosa during carcinogenesis.

Synthesis of secretory and plasma membrane glycoproteins by striated duct cells of rat salivary glands as visualized by radioautography after 3H-fucose injection

The ability of the striated ducts of rat salivary glands to incorporate 3H-fucose into glycoprotein was studied by light and electron microscope radioautography. At 3.5 to 20 minutes after intravenous injection, the majority of the radioautographic grains in the ducts of the parotid gland were localized to the Golgi apparatus. By 40 minutes, the percentage of grains over the Golgi apparatus had decreased; a corresponding increase in grains occurred over small (0.1-0.4 micrometer) apical granules and the highly infolded basal and lateral plasma membranes. By two hours, less than 10% of the label was associated with the Golgi apparatus, while 26% and 28% were attributed to the apical granules and plasma membrane, respectively. By 8 to 12 hours after injection, the number of grains over the apical cytoplasm had decreased, suggesint luminal discharge of the apical granules. In contrast, the basal and lateral plasma membranes remained labeled up to 30 hours after injection as judged by the distribution of grains in light microscope radioautographs. Mitochondria appeared capable of independent incorporation of fucose, accounting for about 20% of the grains from ten minutes to two hours after injection. Comparable results were obtained in the striated ducts of the submandibular and sublingual glands. These results indicate that the striated duct cells readily incorporate 3H-fucose into newly-synthesized glycoproteins. A portion of these are secretory glycoproteins which are packaged and stored in the apical granules, and a portion are membrane glycoproteins which are incorporated into the extensive plasma membrane of these cells.

Different aspects of membrane differentiation at the inner side (GERL) of the Golgi apparatus in rabbit luteal cells

After luteinization, during the growth phase, rabbit luteal cells showed a well-developed Golgi apparatus, which was clearly reduced at the end of pseudo-pregnancy. During this whole period, acid phosphatase was demonstrated in the saccules (g) of the Golgi stack and in the innermost Golgi element (G2), which may be part of GERL. Between both acid phosphatase-positive compartments, a negative or slightly positive element (G1) was present paralleling the saccules of the Golgi stack. This element was composed of cisternal (G1 c) and perforated portions (G1 p) and directly bordered the thiamine pyrophosphatase-positive saccules of the Golgi stack (g1 -g2). Arylsulphatase activity was present in two saccules in the middle of the stack (g3 -g4) and in the innermost Golgi element (G2). In the acid phosphatase and arylsulphatase reactions the limiting membrane of the lysosomes was more reactive than the matrix. After phosphotungstic acid staining at a low pH, the inner elements of the Golgi apparatus (G1 and G2) and the border of the lysosomes were heavily contrasted. The lysosomal matrix and the other Golgi stack saccules were either almost unstained or displayed a clearly lower contrast. It is concluded that the cytochemical difference between Golgi (g) and GERL (G) membranes is most probably the result of a specific process of membrane differentiation, which takes place at G1. There is also evidence that the lysosomal matrix hydrolases may be formed in the saccules of the Golgi stack. The strongly phosphotungstic acid-positive inner elements are, although more extended, comparable in large part with the GERL elements as described in neurons (Novikoff et al., 1971).