Tridimensional architecture of the Golgi apparatus in the atrial muscle cell of the rat

Three-dimensional reconstruction of the Golgi apparatus in osteoclasts by a combination of NADPase cytochemistry and serial section scanning electron microscopy

The three-dimensional morphology of the Golgi apparatus in osteoclasts was investigated by computer-aided reconstruction. Rat femora were treated for nicotinamide adenine dinucleotide phosphatase (NADPase) cytochemistry, and light microscopy was used to select several osteoclasts in serial semi-thin sections to investigate the Golgi apparatus by backscattered electron-mode scanning electron microscopy. Lace-like structures with strong backscattered electron signals were observed around the nuclei. These structures, observed within the Golgi apparatus, were attributed to the reaction products (i.e., lead precipitates) of NADPase cytochemistry. Features on the images corresponding to the Golgi apparatus, nuclei, and ruffled border were manually traced and three-dimensionally reconstructed using ImageJ/Fiji (an open-source image processing package). In the reconstructed model, the Golgi apparatus formed an almost-continuous structure with a basket-like configuration, which surrounded all the nuclei and also partitioned them. This peculiar three-dimensional morphology of the Golgi apparatus was discovered for the first time in this study. On the basis of the location of the cis- and trans-sides of the Golgi apparatus and the reported results of previous studies, we postulated that the nuclear membrane synthesized specific proteins in the osteoclasts and, accordingly, the Golgi apparatus accumulated around the nuclei as a receptacle.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

Three-dimensional morphology of the Golgi apparatus in osteoclasts: NADPase and arylsulfatase cytochemistry, and scanning electron microscopy using osmium maceration

This study was designed to observe osteoclasts in the rat femora by light and electron microscopic cytochemistry for nicotinamide adenine dinucleotide phosphatase (NADPase) and arylsulfatase, and scanning electron microscopy using osmium maceration to assess the three-dimensional morphology of the Golgi apparatus in osteoclasts. The Golgi apparatus showed strong NADPase activity and surrounded each nucleus with the cis-side facing the nucleus. The Golgi apparatus could be often traced for a length of 20 μm or longer. Observations of serial semi-thin sections confirmed that a single line of reaction products (=lead precipitates) intervened somewhere between any two neighboring nuclei. The nuclear membrane showed strong arylsulfatase activity as well as rough endoplasmic reticulum and lysosomes. Scanning electron microscopy showed that the Golgi apparatus covered the nucleus in a porous sheet-like configuration. Under magnification, the cis-most saccule showed a sieve-like configuration with fine fenestrations. The saccules decreased fenestration numbers toward the trans-side and displayed a more plate-like appearance. The above findings indicate the following. (1) The Golgi saccules of osteoclasts have a three-dimensional structure comparable with that generally seen in other cell types. (2) The Golgi apparatus forms a porous multi-spherical structure around nuclei. Within the structure, in most cases a Golgi stack partitions the room into several compartments in each of which a nucleus fits. (3) The nuclear membrane synthesizes some kinds of proteins more stably and sufficiently than the rough endoplasmic reticulum. Consequently, the Golgi apparatus accumulates around nuclei with the cis-side facing the nucleus.

The Golgi apparatus is a functionally distinct Ca2+ store regulated by the PKA and Epac branches of the β1-adrenergic signaling pathway

Ca(2+) release from the Golgi apparatus regulates key functions of the organelle, including vesicle trafficking. We found that the Golgi apparatus was the source of prolonged Ca(2+) release events that originated near the nuclei of primary cardiomyocytes. Golgi Ca(2+) release was unaffected by depletion of sarcoplasmic reticulum Ca(2+), and disruption of the Golgi apparatus abolished Golgi Ca(2+) release without affecting sarcoplasmic reticulum function, suggesting functional and spatial independence of Golgi and sarcoplasmic reticulum Ca(2+) stores. β1-Adrenoceptor stimulation triggers the production of the second messenger cAMP, which activates the Epac family of Rap guanine nucleotide exchange factors and the kinase PKA (protein kinase A). Phosphodiesterases (PDEs), including those in the PDE3 and PDE4 families, degrade cAMP. Activation of β1-adrenoceptors stimulated Golgi Ca(2+) release, an effect that required activation of Epac, PKA, and the kinase CaMKII. Inhibition of PDE3s or PDE4s potentiated β1-adrenergic-induced Golgi Ca(2+) release, which is consistent with compartmentalization of cAMP signaling near the Golgi apparatus. Interventions that stimulated Golgi Ca(2+) release appeared to increase the trafficking of vascular endothelial growth factor receptor-1 (VEGFR-1) from the Golgi apparatus to the surface membrane of cardiomyocytes. In cardiomyocytes from rats with heart failure, decreases in the abundance of PDE3s and PDE4s were associated with increased Golgi Ca(2+) release events. These data suggest that the Golgi apparatus is a focal point for β1-adrenergic-stimulated Ca(2+) signaling and that the Golgi Ca(2+) store functions independently from the sarcoplasmic reticulum and the global Ca(2+) transients that trigger contraction in cardiomyocytes.

Who needs microtubules? Myogenic reorganization of MTOC, Golgi complex and ER exit sites persists despite lack of normal microtubule tracks

A wave of structural reorganization involving centrosomes, microtubules, Golgi complex and ER exit sites takes place early during skeletal muscle differentiation and completely remodels the secretory pathway. The mechanism of these changes and their functional implications are still poorly understood, in large part because all changes occur seemingly simultaneously. In an effort to uncouple the reorganizations, we have used taxol, nocodazole, and the specific GSK3-β inhibitor DW12, to disrupt the dynamic microtubule network of differentiating cultures of the mouse skeletal muscle cell line C2. Despite strong effects on microtubules, cell shape and cell fusion, none of the treatments prevented early differentiation. Redistribution of centrosomal proteins, conditional on differentiation, was in fact increased by taxol and nocodazole and normal in DW12. Redistributions of Golgi complex and ER exit sites were incomplete but remained tightly linked under all circumstances, and conditional on centrosomal reorganization. We were therefore able to uncouple microtubule reorganization from the other events and to determine that centrosomal proteins lead the reorganization hierarchy. In addition, we have gained new insight into structural and functional aspects of the reorganization of microtubule nucleation during myogenesis.

Architecture of the mammalian Golgi

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

Three-dimensional ultrastructure of the Golgi apparatus in different cells: high-resolution scanning electron microscopy of osmium-macerated tissues

The three-dimensional ultrastructure of the Golgi apparatus in different cells of the rat - epithelial principal cells in the epididymal duct, goblet cells in the jejunum, gonadotrophs in the pituitary gland and dorsal root ganglion cells - was studied by scanning electron microscopy (SEM) of osmium-macerated tissues. The Golgi apparatus in the epididymal principal cells took the shape of a candle flame with irregular-shaped cisterns, while those in the goblet cells of the jejunum were cup-shaped or cylindrical with flat cisterns. Gonadotrophs had a large spherical Golgi apparatus; this apparatus was composed of several concentric cisterns with large round windows through which the rough endoplasmic reticulum (rER) and mitochondria extended into the center of the globular Golgi apparatus. Dorsal root ganglion cells had several small Golgi stacks scattered in the cytoplasm. In all Golgi apparatuses of the different cells examined in the present study, the cis-most cistern was generally composed of a flattened sheet with numerous small fenestrations on its wall. On the other hand, the shape of the trans-most cistern varied by cell type; it was generally composed of tubules and/or small sheets which were sometimes connected with each other to form a rather complicated structure. The cis-most cistern and the trans-most cistern were often closely associated with the rER although no direct communication was found between them. These findings indicate that the structure of the Golgi apparatus, especially its overall shape and the ultrastructure of the trans-most cistern, varies by cell type, a point to be considered in relation to the function of the individual cells.

Alternative Splicing of Ryanodine Receptors Modulates Cardiomyocyte Ca 2+ Signaling and Susceptibility to Apoptosis

Ca 2+ release via type 2 ryanodine receptors (RyR2) regulates cardiac function. Molecular cloning of human RyR2 identified 2 alternatively spliced variants, comprising 30- and 24-bp sequence insertions; yet their role in shaping cardiomyocyte Ca 2+ signaling and cell phenotype is unknown. We profiled the developmental regulation and the tissue and species specificity of these variants and showed that their recombinant expression in HL-1 cardiomyocytes profoundly modulated nuclear and cytoplasmic Ca 2+ release. All splice variants localized to the sarcoplasmic reticulum, perinuclear Golgi apparatus, and to finger-like invaginations of the nuclear envelope (nucleoplasmic reticulum). Strikingly, the 24-bp splice insertion that was present at low levels in embryonic and adult hearts was essential for targeting RyR2 to an intranuclear Golgi apparatus and promoted the intracellular segregation of this variant. The amplitude variability of nuclear and cytoplasmic Ca 2+ fluxes were reduced in nonstimulated cardiomyocytes expressing both 30- and 24-bp splice variants and were associated with lower basal levels of apoptosis. Expression of RyR2 containing the 24-bp insertion also suppressed intracellular Ca 2+ fluxes following prolonged caffeine exposure (1 mmol/L, 16 hours) that protected cells from apoptosis. The antiapoptotic effects of this variant were linked to increased levels of Bcl-2 phosphorylation. In contrast, RyR2 containing the 30-bp insertion, which was abundant in human embryonic heart but was decreased during cardiac development, did not protect cardiomyocytes from caffeine-evoked apoptosis. Thus, we provide the first evidence that RyR2 splice variants exquisitely modulate intracellular Ca 2+ signaling and are key determinants of cardiomyocyte apoptotic susceptibility.

Predicting Function from Structure: 3D Structure Studies of the Mammalian Golgi Complex

3D electron tomography studies of the structure of the mammalian Golgi complex have led to four functional predictions (1). The sorting and exit site from the Golgi comprises two or three distinct trans-cisternae (2). The docking of vesicular-tubular clusters at the cis-face and the fragmentation of trans-cisternae are coordinated (3). The mechanisms of transport through, and exit from, the Golgi vary with physiological state, and in different cells and tissues (4). Specialized trans-ER functions in the delivery of ceramide to sphingomyelin synthase in the trans-Golgi membrane, for the regulated sorting via sphingolipid-cholesterol-rich domains. These structure-based predictions can now be tested using a variety of powerful cell and molecular tools.

The ER to Golgi interface is the major concentration site of secretory proteins in the exocrine pancreatic cell

By using quantitative immuno-electron microscopy of two-sided labeled resin sections of rat exocrine pancreatic cells, we have established the relative concentrations of the secretory proteins amylase and chymotrypsinogen in the compartments of the secretory pathway. Their total concentration over the entire pathway was approximately 11 and approximately 460 times, respectively. Both proteins exhibited their largest increase in concentration between the endoplasmic reticulum and cis-Golgi, where they were concentrated 3-4 and 50-70 times, respectively. Over the further pathway, increases in concentration were moderate, albeit two times higher for chymotrypsinogen than for amylase. From trans-Golgi to secretory granules, where the main secretory protein concentration is often thought to occur, relatively small concentration increases were observed. Additional observations on a third secretory protein, procarboxypeptidase A, showed a concentration profile very similar to chymotrypsinogen. The relatively high concentration of amylase in the early compartments of the secretory route is consistent with its exceptionally slow intracellular transport. Our data demonstrate that secretory proteins undergo their main concentration between the endoplasmic reticulum and cis-Golgi, where we have previously found concentration activity associated with vesicular tubular clusters (Martínez-Menárguez JA, Geuze HJ, Slot JW, Klumperman J. Cell 1999; 98: 81-90).

The organization of the Golgi complex and microtubules in skeletal muscle is fiber type-dependent

Skeletal muscle has a nonconventional Golgi complex (GC), the organization of which has been a subject of controversy in the past. We have now examined the distribution of the GC by immunofluorescence and immunogold electron microscopy in whole fibers from different rat muscles, both innervated and experimentally denervated. The total number of GC elements, small polarized stacks of cisternae, is quite similar in all fibers, but their intracellular distribution is fiber type-dependent. Thus, in slow-twitch, type I fibers, approximately 75% of all GC elements are located within 1 micrometer from the plasma membrane, and each nucleus is surrounded by a belt of GC elements. In contrast, in the fast-twitch type IIB fibers, most GC elements are in the fiber core, and most nuclei only have GC elements at their poles. Intermediate, type IIA fibers also have an intermediate distribution of GC elements. Interestingly, the distribution of microtubules, with which GC elements colocalize, is fiber type-dependent as well. At the neuromuscular junction, the distribution of GC elements and microtubules is independent of fiber type, and junctional nuclei are surrounded by GC elements in all fibers. After denervation of the hindlimb muscles, GC elements as well as microtubules converge toward a common pattern, that of the slow-twitch fibers, in all fibers. Our data suggest that innervation regulates the distribution of microtubules, which in turn organize the Golgi complex according to muscle fiber type.

Three-dimensional visualization of the Golgi apparatus: observation of Brunner's gland cells by a confocal laser scanning microscope

The three-dimensional structure of the Golgi apparatus in cells of the Brunner's gland in the mouse was observed by using a confocal laser scanning microscope. Two lectins, FITC-labeled soybean agglutinin and Texas red-labeled Griffonia simplicifolia agglutinin II, were used to visualize the whole Golgi apparatus. Staining with the former lectin, which has been known to label the cis-stacks, showed a lacy dome-like structure situated in the supranuclear region. Staining with the latter lectin, known to label the intermediate-to-trans-stacks and the secretory granules, showed a dome-like structure consisting of network and cobblestone-like patterns in the same region and also granular stainings near the surface of the cobblestone-like patterns and the apical region of a cell. Double-staining demonstrated that the soybean agglutinin-labeled network always surrounded the G. simplicifolia agglutinin II-stained structure. Based on these observations, we propose a new three-dimensional model of the Golgi apparatus: it forms a dome-like structure over a nucleus, a network of cis-stacks forms its outer boundary, and this outer boundary is lined and paved with successive intermediate and trans-stacks. It is thought that secretory granules are released toward the internal space of the Golgi apparatus and transported to the apical cytoplasm through the holes of the network.

Glucose transporter (GLUT-4) is targeted to secretory granules in rat atrial cardiomyocytes

The insulin-responsive glucose transporter GLUT-4 is found in muscle and fat cells in the trans-Golgi reticulum (TGR) and in an intracellular tubulovesicular compartment, from where it undergoes insulin-dependent movement to the cell surface. To examine the relationship between these GLUT-4-containing compartments and the regulated secretory pathway we have localized GLUT-4 in atrial cardiomyocytes. This cell type secretes an antihypertensive hormone, referred to as the atrial natriuretic factor (ANF), in response to elevated blood pressure. We show that GLUT-4 is targeted in the atrial cell to the TGR and a tubulo-vesicular compartment, which is morphologically and functionally indistinguishable from the intracellular GLUT-4 compartment found in other types of myocytes and in fat cells, and in addition to the ANF secretory granules. Forming ANF granules are present throughout all Golgi cisternae but only become GLUT4 positive in the TGR. The inability of cyclohexamide treatment to effect the TGR localization of GLUT-4 indicates that GLUT-4 enters the ANF secretory granules at the TGR via the recycling pathway and not via the biosynthetic pathway. These data suggest that a large proportion of GLUT-4 must recycle via the TGR in insulin-sensitive cells. It will be important to determine if this is the pathway by which the insulin-regulatable tubulo-vesicular compartment is formed.

Trans-Golgi network (TGN) of different cell types: three-dimensional structural characteristics and variability

BACKGROUND

The trans-Golgi network (TGN) is generally considered as a distinct and permanent structural compartment of the Golgi apparatus of various cell types. To verify this postulate we examined and compared the three-dimensional characteristics of the TGNs of 14 different mammalian cell types as presented in our various publications since 1979 when we initially described the trans-tubular network of Sertoli cells.

METHODS

In all these studies we used low and high voltage electron microscopes on thin or thick sections of tissues fixed with glutaraldehyde and postfixed with reduced osmium. The sections were stained with uranyl acetate and lead citrate. Stereopairs, prepared from photographs of tilted specimens, permitted a direct observation of the three-dimensional structure of the various elements of the Golgi apparatus.

RESULTS

The TGNs are multilayered and extensive in cells which do not form large typical secretory granules (Sertoli cells, nonciliated cells of ductuli efferentes, spinal ganglion cells) but have an extensive lysosomal system. The TGN is absent in cells forming very large secretory granules (secretory cells of seminal vesicles and lactating mammary glands). The TGNs are small in cells producing small to medium-size secretory granules and/or appear as residual fragments on the trans aspect of the Golgi stacks (e.g., mucous cells of Brunner's gland, pancreatic acinar cells, etc.). In cells with multiple and extensive TGNs, a continuity of these tubular networks with the two or three transmost saccules of the stack is observed but there are seemingly no connections between the TGNs. Whenever the TGNs are present, they do not form a continuous structure along the Golgi ribbon. However, they do present, in all cases, configurations suggestive of desquamation and renewal.

CONCLUSIONS

The structure of the TGN varies considerably from one cell type to another, being extensive in cells not showing typical secretory granules but having an extensive lysosomal system, while in secretory cells showing small or large secretory granules the TGN is either small or even entirely absent.

A three-dimensional reconstruction study of the rough ER-Golgi interface in serial thin sections of the pancreatic acinar cell of the rat

Distinctive views of the tubulo-vesicular elements interposed between the endoplasmic reticulum (ER) and the Golgi apparatus were obtained in thin sections. The tubules that protrude from the transitional rough ER (tRER) are of dissimilar length. The numbers of tubules and of the nearby omega- and pear-shaped profiles decrease after fasting and are partially restored by refeeding. This formation is designated herein as the budding chamber of the tRER. Close to the budding chamber, clusters of 56 nm diameter vesicles are consistently observed. In some of the cells, convoluted tubules appear enmeshed with the presumptive transport vesicles of 56 nm diameter and with irregular, vesicular formations. Apparently structureless, electron-lucent ellipsoidal areas are found adjacent to these membranous elements. Serial and semi-serial sections show that the budding chamber, the sinuous tubules, the irregular vesicles, the structureless regions and the 56 nm vesicles fill tunnel-like spaces limited by the outermost Golgi cisterna (OGC) on one side and by the tRER on the other. Curved tubules appear to link the lumen of the OGC with that of smooth membranous occupants of these tunnel-like spaces. A presumptive luminal connection between these membranous occupants and the tubules of the budding chamber can also be seen. The predominant configuration of the OGC is that of a perforated, flat saccule. However, OGC regions exhibiting progressively lower densities of fenestrae, including smooth surfaced sectors eventually accumulating an intraluminal content are seen. Two such dilated, saccular portions of the OGC were analyzed through reconstruction of serial sections. Bundles of microtubules run closely apposed to the cis side of the OGC.

Proteins of the Golgi apparatus. Purification to homogeneity, N-terminal sequence, and unusually large Stokes radius of the membrane-bound form of UDP-galactose:N-acetylglucosamine beta 1-4galactosyltransferase from rat liver

The Golgi marker enzyme, UDP-galactose:N-acetylglucosamine beta 1-4galactosyltransferase (beta 1-4GalT) was purified 44300-fold in its intact, membrane-bound form from rat liver membranes. The protein was isolated from detergent extracts as a high-M(r) form, having a Stokes radius approximating a globular protein of M(r) 440,000. It is comprised of a single protein component as observed on SDS/polyacrylamide gels, having an M(r) near 51,000, and does not have intermolecular disulfide cross-links. N-terminal sequencing of the enzyme demonstrated that it contains an N-terminal hydrophobic stretch deduced previously from cDNA encoding for the enzyme. Previous studies have indicated that the protein may be translated at either of two AUG sites near the 5' end of the mRNA [Russo, R. N., Shaper, N. L. & Shaper, J. H. (1990) J. Biol. Chem. 265, 3324-3331], giving rise to two polypeptides, one appended with 13 amino acids. In the work described here, evidence was only found for the sequence of the short form, missing a single methionine at the N-terminus. Mild proteolytic treatment cleaved the enzyme, giving rise to low-M(r) forms which were fully catalytically active and which, upon sequencing, were missing a 66-amino-acid stretch from the N-terminus (as compared to the mouse cDNA). Proteolytic treatment was accompanied by conversion of the form having a large Stokes radius to one approximating a globular protein with M(r) near 50,000. The N-terminal stretch appears to contribute to maintenance of the form having a large Stokes radius. This may be the result of interaction with a detergent micelle, dimerization or oligomerization, or interaction with some other large, non-protein molecule, although a detergent exchange still resulted in a form having a large Stokes radius.

Segregation of secretory material in all elements of the Golgi apparatus in principal epithelial cells of the rat seminal vesicle

At the apex of the epithelial principal cells of the seminal vesicle, there appears to be two types of mature secretory granules, i.e., large and small. Both types of secretory granules showed an eccentric electron-dense spherical body with one pole attached to the delimiting membrane. The remainder of the large granule surrounding the eccentric body showed a granulofilamentous texture, whereas that of the small granule was electron lucent. The formation of these two types of granules was traced back to the various elements of the Golgi stacks. In the case of the large granules, the earliest stage of segregation of the precursor of the eccentric dense body was observed in distensions of the cis-element. Within distensions of all subjacent saccules, the dense bodies continued to be present but progressively increased in size while remaining attached to the saccular membrane. Following separation from the trans-face of the stack, the large prosecretory granules continued to increase in size by fusing with each other. The very large prosecretory granules, as they migrated toward the cell apex to become mature secretory granules, reduced in size prior to exocytosis. The small granules formed exclusively on the trans-aspect of the Golgi stacks and did not appear to fuse with each other. Observations on the formation of the large prosecretory granules within the Golgi apparatus and of the eccentric body in particular, which may be taken as a marker of the saccular membrane, were suggestive of a cis-trans migration and renewal of Golgi saccules.

Golgi apparatus of epithelial principal cells of the epididymal initial segment of the rat: structure, relationship with endoplasmic reticulum, and role in the formation of secretory vesicles

The initial segment of the epididymis of rats, fixed with glutaraldehyde, was postfixed with reduced osmium, a technique that clearly delineates the membranes of cisternae of the endoplasmic reticulum (ER) and the various elements of the Golgi apparatus, or with tannic acid to enhance the coats of vesicles and ribosomes on ER cisternae. The material was also treated to demonstrate various phosphatase activities (NADPase, TPPase, CMPase, G-6-Pase) or impregnated with osmium tetroxide. In osmium-impregnated material, the Golgi apparatus of the epithelial principal cells of the initial segment appeared in the light microscope as a branching, anastomosing ribbon forming a large network in the supranuclear region. In the electron microscope, ER were of two types: the heavily granulated, flattened, rough ER seen in the infranuclear and juxtanuclear regions and the distended, tubular, sparsely granulated ER, showing only few ribosomes, seen interlaced with the Golgi ribbon in the supranuclear region and at the apical pole of the cell. Of particular interest in this cell was the fact that the sparsely granulated ER approximated the Golgi stack on both its cis- and trans-faces. On the cis-face of the Golgi stack, the sparsely granulated ER cisternae showed the usual finger- or bud-like protrusions directed toward the cis element of the Golgi stack and around which numerous small 80 nm vesicles or membranous tubules were clustered. The Golgi stack consisted of the following elements in a cis-trans axis: the cis osmiophilic element, a first saccule slightly dilated, saccules two to four (S2-S4), which were NADPase-positive, and saccules five to seven and the eight Golgi element, which were TPPase-positive. On the trans-aspect of the Golgi stacks, several (up to four) CMPase-positive trans-Golgi networks were observed often in close apposition to the sparsely granulated ER cisternae. One of the trans-Golgi networks showed a "peeling-off" configuration, i.e. part of it was closely apposed to the overlying Golgi element of the stack, whereas the remaining part was separated from the stack by a space occupied by a cisterna of sparsely granulated ER. The other trans-Golgi networks were completely separated from the stack and were often seen sandwiched between sparsely granulated ER cisternae. Thus, ER cisternae showed extensive areas of close apposition but no continuity with the trans-Golgi networks. Although the saccules of the Golgi stacks showed NADPase and/or TPPase activity, the trans-Golgi networks displayed CMPase activity, thus facilitating their identification from the closely associated unreactive sparsely granulated ER cisternae.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural distribution of NADPase within the Golgi apparatus and lysosomes of mammalian cells

Cytochemical studies with over 40 different mammalian cell types have indicated that NADPase activity is associated with the Golgi apparatus and/or lysosomes of all cells. In the majority of cases, NADPase is restricted to saccular elements comprising the medial region of the Golgi stack and an occasional lysosome. There is often weak NADPase activity in other Golgi compartments such as the trans Golgi saccules and/or elements of the trans Golgi network. In some cells, however, strong NADPase activity is found within these latter compartments, either exclusively in trans Golgi saccules or elements of the trans Golgi network, or in combination with medial Golgi saccules and each other including (1) medial Golgi saccules + trans Golgi saccules, (2) medial Golgi saccules + trans Golgi saccules + trans Golgi network, or (3) trans Golgi saccules + trans Golgi network. In some rare cases, no NADPase activity is detectable in either Golgi saccules or elements of the trans Golgi network, but it is observed in an occasional lysosome or throughout the lysosomal system of these cells. It is unclear at present if these variations in the distribution of NADPase across the Golgi apparatus, and between the Golgi apparatus and lysosomal system, are due to differences in targeting mechanisms or to the existence of "bottlenecks" in the natural flow of NADPase along the biosynthetic pathway toward lysosomes. While no clear pattern in the association of strong NADPase activity with lysosomes was apparent relative to the ultrastructural distribution of NADPase activity in Golgi saccules or elements of the trans Golgi network, the results of this investigation suggested that cells having NADPase localized predominantly toward the trans aspect of the Golgi apparatus (in trans Golgi saccules or elements of the trans Golgi network or both) have few NADPase-positive lysosomes. The only exception is hepatocytes which were classified as predominantly trans but had noticeable NADPase activity within medial Golgi saccules and elements of the trans Golgi network as well, and highly reactive lysosomes. Other cells showing highly reactive lysosomes including (1) Kupffer cells of liver and those forming the proximal convoluted tubules of the kidney, both of which also had strong NADPase activity within medial and trans Golgi saccules and elements of the trans Golgi network, (2) Leydig cells of the testis and interstitial cells of the ovary, which also showed strong NADPase activity within medial Golgi saccules, and (3) macrophages from lung, spleen and testis, and Sertoli cells from the testis all of which showed no Golgi associated NADPase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Granule formation and polarity of the Golgi apparatus in neutrophil granulocytes of the rat

The formation of granules in neutrophil (heterophil) progenitor cells was examined with the electron microscope in sections of rat bone marrow fixed in 2% glutaraldehyde and postfixed with reduced osmium (Karnovsky: Proceedings of the 11th Meeting, American Society of Cell Biologists, Abstr. 284, p. 146, 1971). The cells were also osmicated in 2% osmium tetroxide for 36 hours at 37 degrees C to outline the osmiophilic element usually observed on the cis-face of the stacks of saccules of the Golgi apparatus of various cell types. In myeloblasts, which do not produce granules, the cis-osmiophilic element (CE) was found on the concave face of the C-shaped Golgi stacks. In promyelocytes the CE was present on the convex aspect of the C-shaped stacks, while the primary (azurophilic) granules formed in relation to elements on the concave aspects of the stacks. In myelocytes, the situation was reversed: the CE was found on the concave face of the Golgi stacks, while the secondary (specific) granules were seen forming in relation to elements on the convex aspect of the stacks. Finally, in metamyelocytes and mature neutrophils in which no granule formation took place, the appearance on Golgi stacks varied: they were either flat or C-shaped. The CE was indiscriminately found on one face or the other of the flat Golgi stacks of metamyetocytes and on the convex or concave faces of the C-shaped Golgi stacks of mature neutrophils. Using the cis-osmiophilic-element as a marker of the cis-face of the stacked Golgi elements, it thus appeared that despite marked changes in the configuration and orientation of the stacks of the cis-trans polarity of the stacked elements was maintained throughout granulopoiesis. In addition the primary and secondary granules that appeared sequentially in promyelocytes and myelocytes were both seen to form in relation to trans-elements of the Golgi apparatus.

Ultrastructural identification of sulphated glycoconjugates in the Golgi apparatus in human colonic absorptive cells

The subcellular localization of sulphated glycoconjugates was determined at the ultrastructural level by using the high iron diamine (HID) technique for sulphate groups in the absorptive cells of human colonic mucosa. Stained material was observed on the apical plasma membrane, in intracytoplasmic vesicles and in the Golgi complex. In this organelle, the last two or three cisternae of the trans side and the trans-Golgi network (TGN) were labelled, as well as a variable number of coated and noncoated vesicles facing the trans side and surrounding trans-Golgi network. These findings point to the trans side of the Golgi apparatus and trans-Golgi network as the subcompartments functionally involved in the sulphation of glycoconjugates.

Formation of secretion granules in the Golgi apparatus of plasma cells in the rat

The three-dimensional structure of the components of the Golgi apparatus was analyzed in plasma cells of rat duodenum. The spheroidal juxtanuclear Golgi apparatus was formed by a continuous ribbonlike structure composed of the following stacked elements. On the cis-face of the Golgi stack, there was a tubular membranous network referred to as the cis-element and/or a slightly dilated saccule perforated with small pores. The two or three subjacent saccules, which showed few pores, were slightly dilated and contained a fluffy granulofilamentous material. They were also perforated in register by cavities or wells containing 80-nm vesicles. The next one or two underlying elements were fenestrated saccules showing flattened portions as well as distended portions containing a homogeneous material denser than that seen in the overlying saccules. The last two or three elements of the stack showed a partially separated or "peeling off" configuration. These last elements consisted of prosecretory granules attached to flattened, empty-looking saccules showing buds at their surface; detached, more-or-less fenestrated, flattened saccules; and shrivelled residual trans-tubular networks. In the trans-region of the stack, in addition to numerous small vesicles, short membranous tubules, detached prosecretory granules, and denser fully formed secretion granules were also seen. These images were interpreted to indicate that secretory material present in the trans-saccules flows toward the dilated portions which become prosecretory granules. The trans-most elements seemingly peel off the stack to yield prosecretory granules and fragmenting trans-tubular networks.

Formation of secretion granules in the Golgi apparatus of pancreatic acinar cells of the rat

The three-dimensional structure of the Golgi apparatus and its components has been analyzed in sections of pancreatic acinar cells by using stereopairs of electron microscope photographs. Pancreatic tissue fixed in glutaraldehyde was postfixed in reduced osmium, and the sections were stained with lead citrate. Tissues were also treated to demonstrate phosphatase activity (i.e., nicotinamide adenine dinucleotide phosphatase, NADPase; thiamine pyrophosphatase, TPPase; cytidine monophosphatase, CMPase). The following stacked components were observed along the branching, anastomotic, continuous, ribbonlike Golgi apparatus. 1) On the cis-face of the Golgi stack there was a tubular membranous network known to be osmiophilic and referred to as the cis-osmiophilic tubular network or cis-element. 2) A first, poorly fenestrated saccule, unreactive for the phosphatases tested, was slightly distended in places and contained a fluffy granulofilamentous material. 3) The subjacent three or four saccules, reactive for NADPase and/or TPPase, showed dilated portions containing a granulofilamentous secretory material similar to that filling the rest of the saccule. They also showed nondilated portions perforated with large fenestrations, some of which were in register and formed wells containing 80-nm vesicles. The dilated portions of these saccules were present at random along the length of the saccules and were not located exclusively at their edges. 4) The remaining one or two elements of the stack, CMPase positive, showed dilated spheroidal portions or prosecretory granules containing a homogeneous secretory material and flattened fenestrated regions free of secretory material and having the appearance of networks of narrow membranous tubules. 5) Lastly on the trans-aspect of the stack there were detached prosecretory granules reactive for CMPase and surrounded by a corona of small vesicles, and smooth-surfaced spherical CMPase-negative granules having a denser content that were identified as fully formed secretion granules; there were also occasional free trans-tubular networks strongly reactive for CMPase that appeared to undergo fragmentation and numerous small vesicles free from acid-phosphatase activity. These various images were interpreted as indicating that prosecretory granules formed in relation to two or three fenestrated saccules on the trans-side of the stack. Such granules, following their detachment from the trans-face of the stack, their separation from trans-tubular networks, and condensation of their content, yielded mature secretion granules.

Sodium channels near end-plates and nuclei of snake skeletal muscle

1. The spatial distribution of the two predominant types of voltage-gated channels in the sarcolemma, Na+ channels and delayed K+ channels, was studied in skeletal muscle fibres of garter snakes (Thamnophis sirtalis) using loose-seal patch recordings. 2. The average Na+-current density was five times larger in the perijunctional sarcolemma (within 25 microns of the visible edge of the end-plate) than at distant locations. K+ currents were not larger near end-plates. The apparent membrane capacitance, 3-6 microF/cm2, was the same in perijunctional and extrajunctional regions, indicating that differences in Na+-current density reflect differences in the number of Na+ channels per unit membrane area. 3. Perijunctional Na+ channels had the same voltage dependence, gating kinetics and sensitivity to tetrodotoxin as extrajunctional Na+ channels, suggesting that these cells express a single type of Na+ channel. 4. Myonuclei were found to cluster near end-plates and to avoid regions where a nerve branch or blood vessel crossed a fibre's surface. 5. Na+-current density in the sarcolemma above a nucleus was no larger than away from nuclei, indicating that functional Na+ channels are probably not inserted near nuclei. 6. Maps spanning several millimetres of fibre length showed up to sixfold differences in current density between widely separated patches. Differences between patches separated by 50 or 100 microns were much smaller.

Tridimensional architecture of the Golgi apparatus and its components in mucous cells of Brunner's glands of the mouse

The three-dimensional structure of the Golgi apparatus and its components has been analyzed in thin and thick sections of mucous cells of mouse Brunner's glands by using low- and high-voltage electron microscopes and a stereoscopic approach. In thick sections of glands impregnated with osmium or treated to detect nicotinamide adenine dinucleotide phosphatase (NADPase) or thiamine pyrophosphatase (TPPase) activity, the Golgi apparatus appeared, at low magnification, as a continuous network located in the supranuclear region. At higher magnifications and in thin sections of tissue postfixed with reduced osmium and stained with lead citrate or treated to demonstrate phosphatase activity, the following components were observed: on the cis-face of the Golgi stacks, an osmiophilic tubular network referred to as the cis-element; a cis-saccular-compartment composed of a distended porous saccule slightly reactive for NADPase and three or four underlying NADPase-positive, flattened, poorly fenestrated saccules; a trans-saccular-compartment consisting of four to six TPPase-positive saccules or sacculo-tubular elements, prosecretory granules, and "peeling off" trans-tubular networks. The saccules of the cis-compartment were often perforated by large pores in register. The cavities thus formed in the stacks were called wells and were pan-shaped with a mouth directed toward the cis-face of the stacks and a bottom closed by TPPase-positive saccules. The wells always contained 80-nm vesicles. The saccules of the trans-compartment were involved in the formation of secretory granules according to the following proposed sequence of transformation. The secretion product appeared initially as a granular material evenly distributed throughout a slightly distended, poorly fenestrated saccule. These saccules appeared to transform into fenestrated elements with irregular pores and with parts of them taking on the appearance of a tubular network; they were thus referred to as sacculotubular elements. The secretory material initially distributed throughout these elements accumulated in nodular dilatations randomly distributed along the tubular portions of the elements. The dilatations, considered as prosecretory granules, increased in size as they drained the secretory material from the rest of the sacculotubular elements. Such prosecretory granules, large and irregular in shape, "peeled off" from the stacks of saccules with residual saccular or tubular structures still attached to them, some of the latter forming trans-tubular networks. The prosecretory granules detached from such membranous residues, condensed, and finally transformed into spherical secretion granules.

Ultrastructural localization of concanavalin A-binding sites in the Golgi apparatus of various types of neurons in rat dorsal root ganglia: functional implications

The localization of concanavalin A (con A) binding sites has been determined at the electron-microscopic level in the six types of neurons (A1, A2, A3, B1, B2, C) of rat dorsal root ganglia. In all ganglion cells, con A stained the plasma membrane, the nuclear envelope, the cisternae of the rough endoplasmic reticulum, and the matrix of some multivesicular bodies. In contrast, the con A reactivity of the Golgi apparatus varied according to cell type. In type B1 and B2 cells and possibly in type A3 cells, the lectin was exclusively located in three or four saccules on the cis side of the Golgi stacks, whereas the TPPase-positive saccules and the trans sacculotubular elements were unstained with con A. In type A1, A2, and C neurons, all Golgi saccules as well as the trans sacculotubular elements were stained with the lectin. These results suggest that different types of glycoproteins were produced in these two groups of neurons. In the type A1, A2, and C cells, the persistence of the lectin reactivity in the TTPase-positive saccules or sacculotubular elements on the trans side of the Golgi stacks suggests the presence of glycoproteins with oligosaccharide side chains rich in alpha-D-mannosyl residues in terminal positions. In contrast, the disappearance of the con A reactivity in equivalent elements of the Golgi stacks in type B1, B2, and A3 cells suggests the addition at this level of other sugar residues characteristic of complex oligosaccharide side chains. The majority of the vesicular elements associated with the Golgi apparatus, as well as lysosomes, were unstained with con A.

Tridimensional structure of the Golgi apparatus in type A ganglion cells of the rat

The three-dimensional structure of the whole Golgi apparatus and of its components in type A ganglion cells was examined in thin and thick sections by low- and high-voltage electron microscopy. At low magnification, in 10-micron-thick sections of osmicated cells, the Golgi apparatus formed a broad, continuous perinuclear network. At higher magnification and in thinner sections of cells impregnated with uranyl acetate-lead-copper citrate or postfixed in K-ferrocyanide-reduced osmium, the Golgi apparatus appeared as a heterogeneous structure in which saccular regions characterized by stacks of saccules alternated with intersaccular regions made up of branching membranous tubules which bridged the saccules of adjacent stacks. The saccular regions consisted of the following superimposed elements: a cis-osmiophilic element made up of anastomosing tubules; two or three saccules negative for the phosphatases tested (i.e., nicotinamide adenine dinucleotide phosphatase = NADPase, thiamine pyrophosphatase = TPPase, and cytidine monophosphatase = CMPase); two saccules showing TPPase activity; and one to three trans-sacculotubular elements showing a "peeling-off" configuration, one of which showed CMPase activity. The saccules (phosphatase-negative) on the cis-side of the Golgi stacks showed, in addition to small circular pores, larger perforations in register. The cavities thus formed in the stacks of saccules, called "wells," always associated with small 80-nm vesicles, had a pan shape with the mouth directed toward the cis-face and the bottom closed by a TPPase-positive saccule. In face views of the saccules, the smallest of these perforations showed either a crescent shape, due to the presence of a bud on one side of the perforation, or a circular shape with a single small 80-nm vesicle in the center which was occasionally attached to the saccule by a filiform stalk. Such smaller cavities were considered as the precursors of the larger perforations and eventually of the wells. The small 80-nm vesicles seen in the small cavities or in the wells appeared to form in situ and possibly migrate toward the cisternae of endoplasmic reticulum seen proximal to the cis-face of the stack of saccules. Small 80-nm vesicles were also numerous in the intersaccular regions, along the lateral- and trans-aspects of the Golgi stacks, while larger, 150-to 300-nm vesicles, coated and uncoated, were seen only on the trans-face of the Golgi stacks in proximity to the trans-sacculotubular elements which appear to "peel off" from the Golgi stacks.

Terminal glycosylation in rat hepatic Golgi fractions: heterogeneous locations for sialic acid and galactose acceptors and their transferases

Endogenous acceptors for N-acetylglucosamine (GlcNAc), galactose (Gal) or sialic acid (NeuAc) transfer were labeled to high activities when purified hepatic Golgi fractions were incubated with the corresponding radiolabeled nucleotide sugar in the absence of detergent. The in vitro conditions which were optimal for the endogenous glycosylation of GlcNAc and Gal acceptors (Mn2+, ATP) also promoted fusion within a subset of Golgi membranes. Electron microscope radioautography revealed that the majority of NeuAc acceptors were associated with unfused Golgi membranes, whereas the majority of Gal acceptors were localized to fused membranes. GlcNAc acceptors were approximately equally distributed between fused and unfused membranes. Under conditions in which Golgi membrane fusion was absent (-Mn2+), only NeuAc transfer was active. The majority of endogenous NeuAc acceptors were consequently assigned to the more trans regions of the hepatic Golgi apparatus as concluded from a combination of radioautography (NeuAc transfer) and acid NADPase cytochemistry (reactive medial and trans Golgi saccules). The distribution of NeuAc and Gal transferases was assessed after Percoll gradient centrifugation of disrupted Golgi fractions. The median density of NeuAc transferase was lower than that of Gal transferase. The studies are indicative of distinct Golgi components harboring the majority of acceptors and enzymes for terminal glycosylation.

Presence of an osmiophilic feltwork distinct from the Golgi apparatus in spinal ganglion cells of the rat

Rat dorsal root ganglia were osmicated for 48 h at 42 degrees C in a 2% unbuffered aqueous solution of osmium tetroxide. In addition to the osmiophilic cis-element of the Golgi apparatus, the osmium stained a cytoplasmic structure which, although present in the perikaryon of all ganglionic neurons, was well developed in type B1 cells. In photographic stereopairs of 1-2 microns thick sections examined with the electron microscope at 100 kV, these osmiophilic elements appeared as long, wavy thread-like elements with fusiform enlargements. Their average diameter was smaller than that of the thinner cisternae of the smooth endoplasmic reticulum. Occasionally branched, these osmiophilic structures were oriented in all directions of space. When examined in 1.5-7 microns thick sections with a high voltage electron microscope, they formed an osmiophilic feltwork which extended from the juxtanuclear to the subplasmalemmal regions of the perikaryon. It was not connected with the osmiophilic cis-element of the Golgi apparatus. In sections of glutaraldehyde-fixed tissues impregnated with the double impregnation technique or postfixed with reduced osmium, the osmiophilic tubules were clearly distinguished from microtubules, cisternae of endoplasmic reticulum or endocytic structures.