Concomitant synthesis of membrane protein and exportable protein of the secretory granule in rat parotid gland

Scanning transmission electron microscopic analysis of nitrogen generated by 3, 3'-diaminobenzidine-besed peroxidase reaction with resin ultrathin sections of rhinoceros parotid gland acinar cells

A 3, 3'-diaminobenzidine (DAB)-based method was used to detect the localization of endogenous peroxidase activity in Indian rhinoceros (Rhinoceros unicornis) parotid gland acinar cells. The tissue had previously been resin-embedded in gelatin capsules for routine electron microscopic observations and thus pre-incubation for endogenous peroxidase analysis was not possible. We attempted to demonstrate the relationship between secretory granules (SGs) in resin ultrathin sections of Indian rhinoceros parotid gland acinar cells and endogenous peroxidase activity. A JEM 1400 Plus scanning transmission electron microscope (STEM) was used to conduct energy dispersive X-ray spectroscopy (EDS) analysis of the presence of nitrogen generated by the DAB reaction in bipartite structural SG consisting of a dense body (or core). The mapping patterns of nitrogen were restricted to the dense body. We observed nitrogen localized in the rough endoplasmic reticulum (ER), nuclear envelope (NE) and several components of the Golgi apparatus (G) of rhinoceros parotid gland acinar cells participating in the synthetic pathway of secretory proteins. Moreover, we established a nitrogen-detection method by EDS analysis of rhinoceros parotid gland. The reliability of the method was validated by comparison of the test group (peroxidase detection in ultrathin resin sections) and the control group (ordinary peroxidase detection in semi-thin sections following glutaraldehyde pre-fixation) of rat submandibular gland. The same mapping patterns of nitrogen were detected by DAB reaction in the SG, ER, NE and G in these two groups. Hence, EDS-STEM approaches for endogenous peroxidase post-incubation analysis will prove useful for advanced cytochemical analysis for the identification of any other resin sections.

Electron microscopic immunocytochemical localization of proline-rich proteins in normal mouse parotid salivary glands

Rabbit polyclonal antibodies against isoproterenol-induced mouse proline-rich proteins (PRPs) were used to localize PRPs in the parotid salivary glands of normal adult BALB/c mice. The antibodies recognized both acidic-type and basic-type PRPs. Immunoblotting experiments revealed that the glands contained an acidic-type and a basic-type PRP. Parotid gland tissue was fixed with Karnosky's fixative and embedded in Lowicryl resin at low temperature. PRPs were localized at the electron microscope level using an indirect post-embedding staining technique with protein A-gold. The secretion granules of the acinar cells were strongly labelled. Pre-absorption of the antibody with purified acidic-type and basic-type PRPs indicated that the basic-type PRP is mainly located at the periphery of the granules but that the acidic-type PRP is more evenly distributed within the granules. Pre-absorption of the antibody with alpha-amylase did not affect the staining pattern, suggesting minimal cross-reactivity. PRPs were also detected within the rough endoplasmic reticulum and the Golgi apparatus of acinar cells, within the granules of the proacinar cells and in the lumena of the ducts, but not within the intercalated or striated duct cell granules.

Effects of 5-fluorouracil on protein synthesis and secretion of the rat parotid gland

100 mg/kg of FU were injected intraperitoneally once daily for three days. Animals were anaesthetized with 50 mg/kg of sodium pentobarbital before cannulation of the parotid duct. The total volume, amylase and protein content of the saliva were determined after stimulation with either 5 mg/kg pilocarpine or 5 mg/kg isoproterenol in FU-treated, pair-fed, and control animals. Saliva from FU-treated animals was significantly lower (p less than 0.05) in volume, amylase and protein content than that of both control groups. SDS, anionic and cationic gel electrophoresis of parotid saliva revealed no qualitative changes in the types of proteins secreted. FU reduced the total glandular amylase per unit DNA in both unstimulated and isoproterenol-stimulated parotids (p less than 0.05). Decreased protein synthesis may be the mechanism underlying the depleted secretory protein stores because the contents of isolated secretory granules from experimental glands contained less radiolabelled protein than those of either control group, and whole-gland homogenates had marked reductions in the activities of three lysosomal enzymes and in total RNA content. The secretory granules of experimental animals contained less labelled protein than those of controls, but experimental animals secreted a greater proportion of their total glandular radiolabelled secretory protein into saliva relative to amylase, suggesting that newly synthesized secretory proteins were preferentially secreted.

Immunocytochemistry of acinic cell carcinomas and mixed tumors of salivary glands

Antisera of several secretory products of the salivary gland were used to investigate the histogenesis of acinic cell tumors and mixed salivary gland tumors for comparison. Amylase, lactoferrin, secretory piece, and proline-rich protein (PRP) immunoreactivity was detected in the majority of acinic cell tumors; staining was focal, except for PRP, which was diffuse. Lysozyme immunoreactivity was rare. There was discordance for immunoreactivity with several antisera in identifiable tumor lobules of half of the neoplasms. An antikeratin serum outlined microcystic and follicular areas but rarely solid foci. These findings support the contention that acinic cell tumors derive from a tubular type stem cell. Lactoferrin and secretory piece immunoreactivity was not common in mixed tumors and was confined to scattered ductal cells and luminal contents. Rare small foci of amylase and PRP immunoreactivity were found in two mixed tumors only.

Isolation and compositional analysis of secretion granules and their membrane subfraction from the rat parotid gland

A secretory granule fraction has been isolated from rat parotid by discontinuous gradient centrifugation using hyperosmotic sucrose-Ficoll solutions of low ionic strength. The secretion granule fraction comprises 25% of the total tissue alpha-amylase activity and is judged to be of high purity, both morphologically and by its low level of contamination by enzyme activities associated with other organelles. Secretion granules were lysed by capitalizing on their lability in KCl-containing media, and the low density granule membranes were separated from residual organelle and soluble contaminants by flotation in a sucrose gradient. Residual, poorly extractable secretory contaminants of the granule membrane subfraction were selectively removed by a saponin- (10 micrograms/ml) Na2SO4 (0.3 M) wash, apparently with negligible disruption of granule membrane structure. Based on detailed consideration of the extent of contamination by residual mitochondria and incompletely removed secretory polypeptides, it is possible to estimate that approximately 95% of the protein associated with the purified secretion granule membrane is bona fide granule membrane protein. Further analyses indicate that gamma-glutamyltransferase constitutes a marker enzymatic activity shared by granule membranes and the apical domain of the plasma membrane. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoretograms of radio-iodinated granule membrane polypeptides are characterized by 20-25 radioactive bands of which 5-6 are suggested to be glycoproteins by virtue of their binding of concanavalin A. The limited polypeptide composition of the secretion granule membrane (in comparison to membranes of other cellular compartments) and the high phospholipid-protein ratio (4.4 mg/mg) may reflect the functional specialization of this storage container for secretory proteins.

Quantitation of human salivary acidic proline-rich proteins in oral diseases

Acidic proline-rich proteins (APRP) were quantitated immunochemically in salivary secretions from groups of: caries-resistant (CR) and caries-susceptible (CS) subjects; heavy- and light-calculus-formers; and patients with Sjögren's Syndrome, drug-induced xerostomia, and recurrent parotitis. In all groups except the parotitis patients, there were comparable levels of APRP, about 40-50 mg%, with similar values in parotid and submandibular saliva. In chronic recurrent parotitis, the values were somewhat higher (about 60 mg%). There were no differences in the proportion of APRP-A to C in a subset of CR and CS. Taken as a whole, the data support the view that the secretion of APRP is stable and that caries status and propensity to calculus formation are not associated with abnormal levels of these phosphoproteins.

Heat-induced changes in the rat parotid gland

Gland size, amylase activity, total proteins and DNA concentrations were measured in parotid gland during chronic heat exposure (34 degrees C). Subcellular fractionation was performed. Exposure to heat resulted in a decrease in gland size and in DNA concentration; amylase activity/100 mg tissue and amylase specific activity to protein concentration ratio increased. Subcellular fractions, except plasma membrane, decreased significantly. Thus, chronic heat exposure slows down the production of gland constituent proteins, whereas acinar function, e.g. the production and storage of exportable proteins, is not disturbed. The reduced gland size is apparently due to hypoplasia.

Secretory protein synthesis in parotid glands of young and old rats

To determine whether the synthesis of secretory proteins changes with age, the incorporation of [14C]-leucine into exportable proteins of the parotid gland was compared in 2- and 24-month-old rats. The proteins were separated by SDS-gel electrophoresis of the post-microsomal supernatant and identified by comparing the banding patterns in gels prepared from unstimulated glands with those from the glands stimulated to secrete. The amount of radioactivity incorporated into the bands corresponding to exportable proteins was significantly less in the older group, indicating that the synthesis of secretory proteins declines with age.

Plasma membrane of the rat parotid gland: preparation and partial characterization of a fraction containing the secretory surface

A plasma membrane fraction from the rat parotid gland has been prepared by a procedure which selectively enriches for large membrane sheets. This fraction appears to have preserved several ultrastructural features of the acinar cell surface observed in situ. Regions of membrane resembling the acinar luminal border appear as compartments containing microvillar invaginations, bounded by elements of the junctional complex, and from which basolateral membranes extend beyond the junctional complex either to contact other apical compartments or to terminate as free ends. Several additional morphological features of the apical compartments suggest that they are primarily derived from the surface of acinar cells, rather than from the minority of other salivary gland cell types. Enzymatic activities characteristically associated with other cellular organelles are found at only low levels in the plasma membrane fraction. The fraction is highly enriched in two enzyme activities--K+ -dependent p-nitrophenyl phosphatase (K+ -NPPase, shown to be Na+/K+ adenosine triphosphatase; 20-fold) and gamma-glutamyl transpeptidase (GGTPase; 26-fold)--both known to mark plasma membranes in other tissues. These activities exhibit different patterns of recovery during fractionation, suggesting their distinct distributions among parotid cellular membranes. Secretion granule membranes also exhibit GGTPase, but no detectable K+ -NPPase. Since Na+/K+ adenosine triphosphatase and GGTPase, respectively, mark the basolateral and apical cellular surfaces in other epithelia, we hypothesize that these two enzymes mark distinct domains in the parotid plasmalemma, and that GGTPase, as the putative apical marker, may signify a compositional overlap between the two types of membranes which fuse during exocytosis.

Salivary proline-rich proteins

Proline-rich proteins are major components of parotid and submandibular saliva in humans as well as other animals. They can be divided into acidic, basic and glycosylated proteins. The primary structure of the acidic proline-rich proteins is unique and shows that the proteins do not belong to any known family of proteins. The proline-rich proteins are apparently synthesized the acinar cells of the salivary glands and their phenotypic expression is under complex genetic control. The acidic proline-rich proteins will bind calcium with a strength which indicates that they may be important in maintaining the concentration of ionic calcium in saliva. Moreover they can inhibit formation of hydroxyapatite, whereby growth of hydroxyapatite crystals on the tooth surface in vivo may be avoided. Both of these activities as well as the binding site for hydroxyapatite are located in the N-terminal proline-poor part of the protein. Little is known about the functions of the glycosylated and basic proline-rich proteins.

Changes in the synthesis of exportable and nonexportable proteins in parotid glands during aging

The age-related differences in the synthesis of exportable and nonexportable proteins of the parotid salivary gland were compared in 2- and 24-months-old rats. Parotid slices from these rats were incubated in the presence of [14C]leucine and the amount of radioactivity incorporated into the water-soluble proteins of the postmicrosomal supernatant was compared. The exportable and nonexportable proteins were identified by electrophoretic separation of these proteins by comparing the banding patterns of the gel preparations from unstimulated glands to those from the glands stimulated to secrete. The radioactivity determination in various protein bands from these rats indicated that the synthesis of exportable secretory proteins declined with age, while that of nonexportable proteins did not appear to change.

Membrane dynamics in the parotid acinar cell during regranulation: a stereological study following isoprenaline-induced secretion

Recently weaned male rabbits were injected either with 150 micrograms/kg isoprenaline in saline containing 0.01 M ascorbic acid or simply with the drug vehicle. Groups of drug-injected animals were killed at various time after injection. Parotid gland tissue samples from all animals were fixed, embedded and thin sectioned, and micrographs were prepared at standard magnification. Estimations of membrane areas of each membrane type in parotid acinar cells were made. It was found that in animals killed 2 hours after induced secretion apical area was larger than in controls. In animals killed at successively later times the apical area was progressively less. No elevation of any internal smooth membrane areas was ascertained at any sampling time, though the areas of rough endoplasmic reticulum in 2-12 hour samples were larger. It is suggested that excess apical membrane, though probably removed by interiorization, is afterwards disassembled in side the cell to create fresh macromolecular building units (protein molecules), perhaps after passing through the Golgi apparatus. This cryptic pool of building units can provide about 900 micrometers2 of secretion granule membrane per cell, the supply apparently being exhausted in the first eight hours after degranulation, whilst granule numbers are being increased. Thereafter, apparently, limited granule fusion occurs, so that ultimately the cellular complement of secretion granule membrane comes to enclose a greater volume of secretory product, though the average granule number per cell is small.

Purification of synaptic vesicles from elasmobranch electric organ and the use of biophysical criteria to demonstrate purity

We have purified cholinergic synaptic vesicles from the electric organs of two related marine elasmobranchs, Torpedo californica and Narcine brasiliensis, to a specific activity higher than had previously been obtained. We have demonstrated the homogeneity of the vesicles by biophysical criteria. The purification scheme consisted of differential centrifugation, flotation equilibrium in sucrose density gradients, and permeation chromatography on glass bead columns of average pore size 3000 A. Our criteria for purity were that bound acetylcholine, bound nucleotide triphosphate, protein, and lipid--phosphorus behave identically when vesicles were analyzed by procedures which depend on vesicle size, density, and charge. Contaminants were not detected when vesicles were fractionated by preparative and analytical sedimentation, by preparative equilibrium sedimentation using glycerol density gradients, or by electrophoresis in Ficoll density gradients. Pure synaptic vesicles, which have been purified 290-fold from the initial homogenate, contain per mg of protein: 8 mumol of acetylcholine, 3 mumol of ATP, and 7 mumol of lipid phosphorus. These procedures may be of general value in the purification of membrane vesicles.

Compartmentation of enzymes in the rabbit parotid salivary gland. A study by enzyme histochemical, tissue fractionation and morphometric techniques

Rabbit parotid gland was chosen as a suitable model tissue for studying the role of membranes in enzyme secretion by acinar cells. The study was initiated using subcellular fractionation techniques. During these experiments the effects of various tissue disruption conditions such as the medium and the duration and vigour of homogenization were explored and the results assessed by enzyme and chemical assays and both quantitatively and qualitatively by electron microscopy. A series of basic fractions was isolated and marker enzyme or chemical assays selected for each of the relevant membrane types (rough endoplasmic reticulum, Golgi apparatus, zymogen granule, plasmalemma). A parallel study was effected using enzyme histochemical methods applied to frozen sections. Interesting comparisons could then be made between histochemical and biochemical methods of enzyme demonstration. These comparisons are discussed. The basic fractions provide the material from which specimens of purified membranes of the four species can be obtained. The isolation and characterization of such purified membranes is the subject of another report.

Retention of amylase in the secretory granules of parotid gland after extensive release of Ca++ by ionophore A-23187

The effect of the ionophore A-23187 was tested on isolated secretory granules of rat parotid gland. The ionophore caused extensive release of calcium from the granules without effecting release of amylase or other secretory proteins. It is therefore concluded that the role of calcium in the granules is probably not that of a stabilizing agent.

alpha-Adrenergic, beta-adrenergic and cholinergic mechanisms for amylase secretion by rat parotid gland in vitro

1. Rat parotid gland slices, incubated in a balanced, buffered salt solution, were found to be physiologically stable for up to 2 hr with respect to O2 consumption, water content, extracellular space and cation content. 2. The slices could be stimulated to secrete amylase by activation of alpha-adrenergic, beta-adrenergic or muscarinic cholinergic receptors. 3. The secretion elicited through all three receptors appeared to involve exocytosis as revealed by electron microscopy. 4. The beta-agonist, isoprenaline, increased tissue content of cyclic adenosine 3',5'-monophosphate (cyclic AMP); alpha-adrenergic and cholinergic agents had no effect on the level of this cyclic nucleotide. 5. Secretion via cholinergic or alpha-adrenergic mechanisms required extra-cellular calcium; the beta-adrenergic mechanism did not. 6. It was concluded that stimulation of rat parotid cells activates distinctly separate pathways leading ultimately to exocytosis, one pathway involving cyclic AMP, and the other, external Ca2+ ion.

A review of macromolecular transport and secretion at the cellular level

This is a review of current information concerning mechanisms involved in transport and secretion of macromolecules in exocrine glands. Emphasis has been placed on information available for pancreatic acinar cells. The review was prompted by the availability of considerable amounts of new information developed during the past several years. Exportable proteins in the pancreatic acinar cells are synthesized on ribosomes attached to the endoplasmic reticulum. Following synthesis, nascent proteins are transported from ribosomes attached to the endoplasmic reticulum into intracisternal spaces bound by the endoplasmic reticulum. The proteins are then carried to the Golgi complex by transitional elements. Zymogen granules are formed in the Golgi complex and migrate to the cell apex. Appropriate stimulation leads to fusion of the zymogen granule membrane and apical plasmalemma followed by a break in the membrane and consequent release of the granule content into the ductules. The extact molecular events involved in the process of secretion are not known. The roles of cAMP and cGMP in pancreatic secretion are supported by indirect evidence only. The role of calcium in secretion is apparent, but further investigation is needed to delineate the exact mechanism of its action. Membrane depolarization and associated ionic fluexes seem to play a significant role.

Involvement of vesicle coat material in casein secretion and surface regeneration

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied with emphasis on vesicle coat structures. Typical 40-60 nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles", were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heabily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.

Exocytosis couples to endocytosis of ferritin in parotid acinar cells from isoprenalin stimulated rats

Distribution of acid phosphatase as a marker enzyme for lysosomes was investigated in the isoprenalin stimulated rat parotid gland. The enzyme was localized in lipofuscin-like bodies as well as in non-discharged granules. The appearance of these bodies was correlated in time to the appearance of smooth vesicles and reduction of the acinar lumen. Ferritin, used as a tracer and introduced into the stimulated gland via cannulated parotid ducts, was found in smooth vesicles, vacuoles and lipofuscin-like bodies throughout the cytoplasm of the acinar cells. Very often ferritin-containing vesicles were found in the vicinity of the Golgi complex. In most cases the vesicles containing ferritin also showed acid phosphatase reaction product. A possible correlation between the lysosomal system and the process of recycling and degradation of membranes in the stimulated gland is discussed.

Growth of the nuclear envelope in the vegetative phase of the green alga Acetabularia. Evidence for assembly from membrane components synthesized in the cytoplasm

The primary nucleus of the green alga Acetabularia grows about 25,000-fold in volume while it is separated from the endoplasmic reticulum and the whole cytoplasm by a special paranuclear cisterna of a vacuolar labyrinthum system which shows only very few (two to six per square micrometer) and small (ca. 40-120 nm in diamter) fenestrations. The nuclear envelope does not bear polyribosomes, nor do they occur in the entire zone intermediate between the nuclear envelope and the paranuclear cisterna. It is suggested that this special form of nuclear envelope growth takes place by assembly from cytoplasmically synthesized proteins that are translocated across the paranuclear cisterna in a nonmembrane-structured form.