Cytochemical characteristics of the Golgi apparatus

Microglia in the olfactory bulb of rats during postnatal development and olfactory nerve injury with zinc sulfate: a lectin labeling and ultrastrucutural study

Using isolectin (GSA I-B4) as a marker, this study examined the possible alterations of lectin-labeled membranous glycoproteins in microglial cells in the olfactory bulb of normal development and under experimentally induced degeneration. In light microscopy, several morphological types of microglial cells representing different degrees of cell differentiation were distributed in the bulb laminae. A gradient of microglial differentiation extending from the intermediate to superficial and intermediate to deep occurs in the bulb layers. The differentiation gradient and lectin labeling pattern of microglial cells in the developing bulb resembled those in other areas of the brain tissues. Differentiating microglia showed a gradual diminution of lectin staining when the nascent round cells transformed into the mature ramified cells. Microglia in the external plexiform layer of the olfactory bulb were the first to mature and the cells expressed very weak lectin reactivity. In mature or adult rats, some microglial cells showing intense lectin labeling were observed in the olfactory nerve layer, granule cell layer and subependymal layer. Ultrastructurally, lectin labeling was localized at the trans saccules of the Golgi apparatus. Microglial cells in other bulb laminae, however, exhibited a negative reaction for the isolectin at the Golgi apparatus. Following intranasal irrigation of zinc sulfate, some microglial cells in the olfactory nerve layer and glomerular layer were activated to become phagocytic cells with increased lectin labeling at their ramified processes. GSA I-B4 staining was also localized at their trans saccules of the Golgi apparatus. The lectin labeling pattern of these phagocytic cells resembled that of differentiating microglia in postnatal bulbs, suggesting that bulb microglia in the lesioned sites were activated through cell dedifferentiation into macrophages.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Partitioning of cytoplasmic organelles during mitosis with special reference to the Golgi complex

During mitosis, not only the genetic material stored in the nucleus but also the constituents of the cytoplasm should be equally partitioned between the daughter cells. For this sake, the dividing cell goes through an extensive structural reorganization and transport along the endocytic and exocytic pathways is temporarily arrested. Early in prophase, the radiating array of cytoplasmic microtubules disassembles and the membrane systems of the secretory apparatus start to split up. In metaphase, the nuclear envelope fragments and the condensing chromosomes associate with the forming mitotic spindle. The cisternal and tubular elements of the endoplasmic reticulum and the Golgi complex break down into small vesicles, presumably as the result of an imbalance between vesicle budding and fusion. In anaphase, the two sets of chromosomes are pulled apart and a cleavage furrow forms halfway between the spindle poles. Since most organelles occur in multiple and widely dispersed copies at this stage, they will be evenly distributed between the daughter cells. During telophase and cytokinesis, the preceding fragmentation process is reversed. A nuclear envelope reappears around the chromosomes and cytoplasmic microtubules reassemble. The endoplasmic reticulum is rebuilt as a continuous system of flattened cisternae and tubules. Stacks of Golgi cisternae arise from small vesicles and are rearranged in an interconnected network. In parallel, the biosynthetic functions of the cell are normalized and intracellular membrane traffic is resumed.

Response of amoeboid and ramified microglial cells to lipopolysaccharide injections in postnatal rats--a lectin and ultrastructural study

The present study describes the response of amoeboid and ramified microglial cells in the corpus callosum to intraperitoneal lipopolysaccharide injections in postnatal rats as examined by lectin histochemical staining and electron microscopy. In 1 day old rats receiving endotoxin injections and sacrificed at various time intervals, the lectin labelling of amoeboid/ramified microglia was greatly enhanced. The increased labelling persisted in some ramified microglia in rats killed at 14 and 21 days of age; otherwise in normal or control animals of the corresponding stages, the same cells were very weakly stained. In rats killed at 2 days of age after intraperitoneal lipopolysaccharide injection, the number of microglia appeared to increase, but this was reduced at 7 days of age. The lectin-labelled amoeboid/ramified microglia were frequently seen adherent to the outer walls of the callosal blood vessels where infiltrated lymphocytes were also observed. Ultrastructurally, some lectin-labelled microglial cells underwent degeneration and were engulfed by other lectin-positive cells. After endotoxin injections, microglial cells, notably the amoeboid form, showed extensive ruffling at their cell membrane, massive accumulation of lysosomes and increased staining of lectin at their Golgi apparatus. A similar lectin labelling pattern was also observed in ramified microglia of lipopolysaccharide-injected rats. It is concluded that both amoeboid and ramified microglial cells in postnatal rats responded to endotoxin injections as reflected by their enhanced lectin labelling at the surface membrane, lysosomes and Golgi apparatus. It is suggested that such changes may be involved in synthesis and/or modification of galactosyl glycoconjugates probably for the increased production of membranous glycoproteins or lysosomal enzymes.

Study of surface carbohydrates on isolated Golgi subfractions by fluorescent-lectin binding and flow cytometry

The Golgi complex is a functionally heterogeneous subcellular structure that plays a key role in the synthesis, maturation, and sorting of newly synthesized glycoproteins. Fluorescent lectins have been used extensively to analyze surface glycoproteins by flow cytometry in whole cells and more recently in isolated subcellular organelles, such as mitochondria and chloroplasts. We report here the use of several fluorescein-isothiocyanate-conjugated lectins to detect and quantify specific surface sugars by flow cytometry on isolated elements from purified cis and trans-Golgi fractions from rat liver. Our results show that this approach may be useful to study Golgi composition and function, since it may reveal the intensity of specific binding of different lectins to each Golgi fraction and the percentage of elements binding the lectins specifically. Thus we show here that Golgi elements appear homogeneous in mannose and fucose, whereas galactose and N-acetyl-glucosamine residues are more abundant in the trans-Golgi elements.

Down-regulation of membrane glycoprotein in amoeboid microglia transforming into ramified microglia in postnatal rat brain

The present study describes the ultrastructural localization and labelling pattern of lectin in different microglial cell phenotypes in the postnatal rat brain using the isolectin, GSA I-B4. The nascent round and amoeboid microglial cells (round cells and cells displaying short processes) were labelled at their cytoplasmic membrane and the membrane of the subplasmalemmal vacuoles. In the course of their transformation into ramified forms with age, dense lectin labelling was observed successively at different sites in the differentiating cells. The most striking feature was the staining of the Golgi saccules on the trans face, the trans tubular network and associated vesicles and vacuoles in the 'intermediate' ramified microglia (ramified cells bearing thick and long processes and those with thin and long processes). The vacuoles with accumulated reaction products were closely associated with many microtubules extending into the cytoplasmic processes. At the surface, the lectin-labelled vacuoles and vesicles appeared to fuse with the membrane and their contents communicated with the exterior. In the advanced or most differentiated ramified microglial cells (cells bearing attenuated processes), the lectin staining at all the above mentioned sites became diminished. In conclusion, in the transformation of the round microglia into their ramified derivatives, the glycoconjugates at the cytoplasmic membrane are progressively reduced. It is postulated from this study that the down-regulation of the glycoconjugates of the microglial plasma membrane is due primarily to their internalization during endocytosis. This process would trigger a de novo galactosyl protein synthesis and/or modification at the trans Golgi saccules and trans tubular network probably in an attempt to degrade the internalized membrane glycoproteins or to replenish the consumption of the membrane glycoconjugates.

Cell type-dependent variations in the subcellular distribution of alpha-mannosidase I and II

alpha-mannosidases I and II (Man I and II) are resident enzymes of the Golgi complex involved in oligosaccharide processing during N-linked glycoprotein biosynthesis that are widely considered to be markers of the cis- and medial-Golgi compartments, respectively. We have investigated the distribution of these enzymes in several cell types by immunofluorescence and immunoelectron microscopy. Man II was most commonly found in medial- and/or trans- cisternae but showed cell type-dependent variations in intra-Golgi distribution. It was variously localized to either medial (NRK and CHO cells), both medial and trans (pancreatic acinar cells, enterocytes), or trans- (goblet cells) cisternae, or distributed across the entire Golgi stack (hepatocytes and some enterocytes). The distribution of Man I largely coincided with that of Man II in that it was detected primarily in medial- and trans-cisternae. It also showed cell type dependent variations in its intra-Golgi distribution. Man I and Man II were also detected within secretory granules and at the cell surface of some cell types (enterocytes, pancreatic acinar cells, goblet cells). In the case of Man II, cell surface staining was shown not to be due to antibody cross-reactivity with oligosaccharide epitopes. These results indicate that the distribution of Man I and Man II within the Golgi stack of a given cell type overlaps considerably, and their distribution from one cell type to another is more variable and less compartmentalized than previously assumed.

Ultrastructural distribution of lectin-binding sites on gastric superficial mucus-secreting epithelial cells. The role of Golgi apparatus in the initial glycosylation

Normal human gastric epithelial cells were examined by electron microscopy using each of five biotinylated lectins [Ulex europaeus agglutinin I (UEA-I), peanut agglutinin (PNA), wheat germ agglutinin (WGA), soybean agglutinin (SBA) and Dolichos biflorus agglutinin (DBA)] as a probe. We employed 35 gastric surgical specimens removed from complicated peptic disease. The lectin-binding sites were revealed with streptavidin-colloidal gold complex. All specimens were embedded in Spurr and LR White resins. In superficial foveolar epithelial cells, the lectins used were generally positive in all cell types (mainly UEA-1 and PNA) on the Golgi region and mucus cytoplasmic vacuoles, with many variations among cells in the same case. On the other hand, extracellular mucus was negative for WGA. Labelling with PNA revealed a biphasic pattern (peripheral positivity) on mucous droplets in surface and foveolar cells. The cis side of the Golgi apparatus was labelled with SBA and PNA and rough endoplasmic reticulum with SBA (only five cases). Lectin-binding variability could be related to heterogeneous composition of gastric mucus. Our results with SBA suggest initiation of O-glycosylation at the Golgi apparatus; however a role of the rough endoplasmic reticulum cannot be excluded (N-glycosylation). We propose the following sequence of sugar addition to the carbohydrate side-chains of gastric glycoproteins: (1) GaNAc (Golgi apparatus cis-side), (2) GlcNAc (Golgi apparatus intermediate face), (3) GalNac or Gal, alpha-L-fucose (Golgi apparatus trans-side).

Ultrastructural localization of sialylated glycoconjugates in cells of the salamander olfactory mucosa using lectin cytochemistry

An indirect gold-labeling method utilizing the lectin from Limax flavus was employed to characterize the subcellular distribution of sialic acid in glycoconjugates of the salamander olfactory mucosa. The highest density of lectin binding sites was in secretory vesicles of sustentacular cells. Significantly lower densities of lectin binding sites were found in secretory granules of acinar cells of both Bowman's and respiratory glands. Lectin binding in acinar cells of Bowman's glands was confined primarily to electron-lucent regions and membranes of secretory granules. In the olfactory mucus, the density of lectin binding sites was greater in the region of mucus closest to the nasal cavity than in that closest to the epithelial surface. At the epithelial surface, the density of lectin binding sites associated with olfactory cilia was 2.4-fold greater than that associated with microvilli of sustentacular cells or non-ciliary plasma membranes of olfactory receptor neurons, and 7.9-fold greater than non-microvillar sustentacular cell plasma membranes. Lectin binding sites were primarily associated with the glycocalyx of olfactory receptor cilia. The cilia on cells in the respiratory epithelium contained few lectin binding sites. Thus, sialylated glycoconjugates secreted by sustentacular cells are preferentially localized in the glycocalyx of the cilia of olfactory receptor neurons.