Molecular organization of prolactin granules. III. Intracellular transport of sulfated glycosaminoglycans and glycoproteins of the bovine prolactin granule matrix

How Glycosaminoglycans Promote Fibrillation of Salmon Calcitonin

Glycosaminoglycans (GAGs) bind all known amyloid plaques and help store protein hormones in (acidic) granular vesicles, but the molecular mechanisms underlying these important effects are unclear. Here we investigate GAG interactions with the peptide hormone salmon calcitonin (sCT). GAGs induce fast sCT fibrillation at acidic pH and only bind monomeric sCT at acidic pH, inducing sCT helicity. Increasing GAG sulfation expands the pH range for binding. Heparin, the most highly sulfated GAG, binds sCT in the pH interval 3-7. Small angle x-ray scattering indicates that sCT monomers densely decorate and pack single heparin chains, possibly via hydrophobic patches on helical sCT. sCT fibrillates without GAGs, but heparin binding accelerates the process by decreasing the otherwise long fibrillation lag times at low pH and accelerates fibril growth rates at neutral pH. sCT·heparin complexes form β-sheet-rich heparin-covered fibrils. Solid-state NMR reveals that heparin does not alter the sCT fibrillary core around Lys(11) but makes changes to Val(8) on the exterior side of the β-strand, possibly through contacts to Lys(18) Thus GAGs significantly modulate sCT fibrillation in a pH-dependent manner by interacting with both monomeric and aggregated sCT.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

Granulated and non-granulated decidual prolactin-related protein-positive decidual cells in the pregnant mouse endometrium

PROBLEM

Identification of the cell types responsible for the synthesis of decidual prolactin-related protein (dPRP) in the pregnant mouse endometrium.

METHOD OF STUDY

Histochemistry and immunocytochemistry were used to determine peri-implantation dPRP and perlecan distribution in the mouse uterus.

RESULTS

We identified dPRP in pre-decidual and mature decidual cells from days 5 to 12 of pregnancy. On day 8, dPRP immunoreactivity was detected within cytoplasmic granules of a specific population of granulated decidual cells (GDCs). In mesometrial decidual cells, weak immunoreactivity was seen from days 7 to 14. Between days 11 and 14, dPRP was found in cytoplasm and in the extracellular matrix surrounding islands of spongiotrophoblast. Perlecan, a heparan sulfate proteoglycan, was co-localized with dPRP.

CONCLUSION

GDCs are a putative source of dPRP in pregnant mice. Co-localization of perlecan with dPRP suggests that the former acts as a dPRP reservoir and facilitates its paracrine effect in developing placental tissues.

Subcellular distribution of laminin and prolactin in stimulated and blocked prolactin cells in the pituitary of lactating rats

Laminin (LAM), a glycoprotein component of basement membranes, has been previously detected within several subcellular compartments of prolactin (PRL) cells in the pituitary gland. The present work was aimed at comparing the subcellular localization of PRL, a specific secretory product, with that of LAM, in relation to the secretory activity of PRL cells. LAM and PRL were located in parallel, by ultrastructural immunocytochemistry, in PRL cells of lactating female Wistar rats, either stimulated by suckling, or blocked by weaning, or reactivated by suckle following short-term weaning. Variations in physiological conditions were correlated with a redistribution of PRL immunoreactivity within morphologically modified compartments. The Golgi apparatus became hypertrophied, and PRL impressively accumulated within saccules of the Golgi stacks of blocked cells. On the contrary, no apparent changes occurred in LAM distribution, at least at the Golgi level. Only a slight increase of LAM immunoreactivity was observed in rough endoplasmic reticulum after a long weaning period. PRL could be detected in most of the secretory granules and particularly in forming elements, whereas LAM was observable at the peripheral edge of some mature granules. Such a labeling was not markedly influenced by the physiological state. The prominent structures, indicative of crinophagic activity, characteristic of blocked cells, contained masses of dense material, which were always immunopositive with antibodies to PRL, but never to LAM. These observations could suggest that, in PRL cells, intracellular transport and exportation of LAM are controlled by mechanisms independent from those involved in the regulation of PRL secretion.

Complex carbohydrate composition of large dense-cored vesicles from sympathetic nerve

Highly purified noradrenergic, large, dense-cored vesicles were isolated from bovine sympathetic nerve endings by sucrose-D2O density gradient centrifugation. Their concentration of glycoprotein hexosamine and sialic acid was 6.6 and 3.9 mumol/100 mg lipid-free dry weight, respectively, values which are similar to those previously found in bovine chromaffin granules. However, whereas chromaffin granule glycoproteins are characterized by their high proportion of N-acetylgalactosamine-containing O-glycosidically-linked oligosaccharides (present in the chromogranins), such oligosaccharides accounted for only 17% of those in noradrenergic synaptic vesicle glycoproteins. Fractionation of N-3H-acetylated glycopeptides by sequential lectin affinity chromatography demonstrated that approximately two-thirds of the oligosaccharides were of the tri- and tetraantennary complex type, accompanied by 14% biantennary oligosaccharides and 3% high-mannose oligosaccharides. The vesicles had a relatively low concentration of chondroitin sulfate (less than 5% of that in chromaffin granules) but significant amounts of heparan sulfate (0.4 mumol N-acetylglucosamine/100 mg lipid-free dry weight). No hyaluronic acid was detected. The concentration of ganglioside sialic acid in the noradrenergic vesicles was approximately 1 mumol/100 mg lipid-free dry weight, which is significantly higher than that of a crude membrane mixture from which the vesicles were prepared; the ratio of N-acetyl- to N-glycolylneuraminic acid was 0.8. Several molecular species of gangliosides were detected by thin-layer chromatography, but most of these did not exactly comigrate with bovine brain gangliosides. Cholera toxin binding indicated that approximately half or less of the gangliosides belong to the gangliotetraose series.

Presence of glycoconjugates in prolactin granules of male rats

Prolactin granules in the anterior pituitary glands of male rats contain densely stained materials at the periphery of the matrix. These occur in both small spherical and large polymorphic types of granules. The presence of densely stained materials around secretory granules may be a useful criterion for identification of prolactin cells since the dense structure was observed in 95% of these cells after conventional staining by uranyl acetate and lead citrate. The localization of glycoconjugates in the prolactin granules was examined by applying concanavalin A (Con A) on the ultrathin sections. HRP-Con A or ferritin-conjugated Con A bound specifically to the densely stained materials in the peripheral region of the prolactin granule matrix, indicating that this densely stained matrix contains glycoconjugates; the significance thereof is discussed with reference to the concentration and packaging of secretory product.

Isolation and partial characterization of two populations of secretory granules from rat parotid glands

A method is described for the isolation of two populations of secretory granules from rat parotid glands utilizing differences in their sedimentation characteristics. The granule preparations were analyzed for homogeneity by electron microscopy and chemical analyses. The soluble contents of both types of granules were obtained by hypotonic lysis, and the proteins compared by SDS-PAGE and ion exchange-gel filtration chromatography. Both populations of secretory granules appear to have the same protein composition as that of the parotid saliva. The secretory granules with the smaller apparent buoyant density became labelled with radioactive leucine earlier than the heavier granules when a pulse of this amino acid was supplied to a gland slice system. The lighter granules appear to represent an earlier stage in maturation.

The major tyrosine-sulfated protein of the bovine anterior pituitary is a secretory protein present in gonadotrophs, thyrotrophs, mammotrophs, and corticotrophs

The anterior pituitary is a complex secretory tissue known to contain several sulfated macromolecules. In the present study, we identified the major tyrosine-sulfated protein of the bovine anterior pituitary and investigated its cellular and subcellular localization. This protein consisted of two tyrosine-sulfated polypeptides of molecular weight 86,000 and 84,000 that were highly homologous to each other. In agreement with previous biochemical studies, the tyrosine-sulfated protein of Mr 86,000/84,000 was found to be secretory, as it was observed in the matrix of secretory granules by immunoelectron microscopy. Immunofluorescence studies indicated that the tyrosine-sulfated, secretory protein of Mr 86,000/84,000, referred to as TSP 86/84, was present in all endocrine cells except for some somatotrophic cells. Higher levels of immunoreactivity for TSP 86/84 were observed in gonadotrophic and thyrotrophic than in mammotrophic and corticotrophic cells. This appeared to result from the occurrence of TSP 86/84 in all secretory granules of the former cells and in only some secretory granules of the latter cells. We discuss the possibility that TSP 86/84 may have a role in the packaging of several distinct peptides hormones into secretory granules. One, though not the only, possible function of tyrosine sulfation may concern the sorting of this protein in the Golgi complex.

Sorting and secretion of adrenocorticotropin in a pituitary tumor cell line after perturbation of the level of a secretory granule-specific proteoglycan

A mouse anterior pituitary tumor cell line (AtT-20) that secretes adrenocorticotropin and beta endorphin sorts the proteins it transports to the surface into two exocytotic pathways. AtT-20 cells also synthesize a secretory granule-specific sulfated molecule and secrete it on stimulation (Moore, H.-P., B. Gumbiner, and R. B. Kelly, 1983, J. Cell Biol., 97:810-817). We show here that this molecule is sensitive to proteolysis and that the residual sulfated material co-migrates with a chondroitin sulfate standard on thin-layer electrophoresis. Furthermore, this sulfated molecule is completely sensitive to chondroitinase ABC digestion. Thus the secretory granule-specific sulfated molecule is a proteoglycan with chondroitin sulfate side chains. We examined the role of proteoglycans in the sorting and secretion of adrenocorticotropin in AtT-20 cells by severely decreasing the amount of this vesicle-specific proteoglycan in two ways. First, a xyloside was used to inhibit proteoglycan biosynthesis; second, a variant of the AtT-20 cell line was isolated that synthesized little of the sulfated proteoglycan. In neither case was the sorting or secretion of adrenocorticotropin detectably altered, suggesting that the proteoglycan is not required for these processes.

A subclass of proteins and sulfated macromolecules secreted by AtT-20 (mouse pituitary tumor) cells is sorted with adrenocorticotropin into dense secretory granules

The AtT-20 cell, a mouse pituitary tumor line that secretes adrenocorticotropin and beta-endorphin, sorts the proteins it externalizes into two exocytotic pathways. Cells that are labeled with [35S]methionine or [35S]sulfate can be shown to transport three acidic polypeptides (65,000, 60,000, and 37,000 mol wt) and at least two sulfated macromolecules into storage secretory granules. When the cells are stimulated by the secretagogue 8-bromo-cAMP, these polypeptides are coordinately secreted with mature adrenocorticotropin into the culture medium. In contrast, a completely different set of secreted polypeptides and sulfated macromolecules does not enter a storage form and is transported to the cell surface more rapidly. Their secretion from the cells is constitutive and does not require the presence of secretagogues. These molecules, like a viral membrane glycoprotein described previously (Gumbiner, B., and R. B. Kelly, 1982, Cell, 28:51-59) are not found in isolated secretory granules and therefore must reach the cell surface in a different exocytotic vesicle. The segregation of a subclass of secretory macromolecules into the secretory granules, despite the existence of another potential secretory pathway, suggests that these molecules have specific functions related to regulated hormone secretion or storage. Presumably all of the proteins secreted by the regulated secretory granule pathway share some common property that targets them to the secretory granule.

Metabolic labeling of lutropin with [35S]sulfate

Chemical analyses have previously detected sulfate linked to the oligosaccharides of lutropin isolated from bovine and human pituitaries. To determine whether lutropin could be metabolically labeled with sulfate, isolated bovine and rat pituitaries were incubated with [35S]sulfate. In both species, two major labeled products were immunoprecipitated with antisera specific to lutropin subunits. Incorporation into the subunits occurred posttranslationally since it was not blocked by cycloheximide, which did, however, block the incorporation of radiolabeled methionine. Metabolic labeling with [35S]sulfate provides a valuable approach for examining the biosynthetic processing of lutropin and the physiological role of sulfate in this hormone.

Characterization of adenohypophysial polypeptides by two-dimensional gel electrophoresis. I. L-[3H]leucine-labeled polypeptides

Homogenates of cow and rat anterior pituitary slices, labeled in vitro with L-[3H]leucine, were analyzed by high-resolution two-dimensional polyacrylamide gel electrophoresis. This technique was also applied to the materials released into the chase medium from bovine anterior pituitary slices. The pattern of both total polypeptides (revealed by Coomassie-Blue staining) and L-[3H]leucine-labeled polypeptides (revealed by fluorography) was found to be more complex than previously demonstrated by different techniques. In particular, the GH band separated by one-dimensional Na-dodecylsulfate--polyacrylamide gel electrophoresis was resolved into 3--5 components; 2 of these, which were highly labeled by L-[3H]leucine, were both identified as GH by immunoprecipitation with specific anti-GH bodies. In addition, we found evidence in favor of the existence of some, previously unsuspected, 'putative' secretory proteins. In fact, besides GH and PRL, several minor components (2 with apparent Mr approximately 70 00--62 000, pI approximately 4.8; others with Mr approximately 50 000, pI between approximately 5.8 and approximately 6.8; and 1 with Mr approximately 26 000, pI approximately 5.7) were found to be synthesized at high rates and to accumulate in the medium, with different kinetics, during chase incubation.

Characterization of adenohypophysial polypeptides by two-dimensional gel electrophoresis. II. Sulfated and glycosylated polypeptides

Adenohypophysial sulfated and glycosylated polypeptides were studied by high-resolution two-dimensional polyacrylamide-gel electrophoresis followed by fluorography. The preparations analyzed were the following: (a) homogenates from cow and rat anterior pituitary slices labeled in vitro either with [35S]sulfate or D-[6-3H]glucosamine; (b) materials released from bovine adenohypophysis slices pulse labeled with [35S]sulfate; and (c) purified fractions of bovine prolactin granules stripped by detergent treatment of their limiting membrane. A heterogeneous family of sulfated components, almost all glycosylated, differing in their peptide moieties as well as in their isoelectric points, was revealed in the glandular tissue. The major of these components (apparent Mr approximately 70 000; pI approximately 4.8), which was also highly labeled by L-[3H]-leucine (Zanini, A., and Rosa, P. (1981) Mol. Cell. Endocrinol. 24), might be a secretory protein because it accumulates in the medium during chase incubation of bovine pituitary slices in vitro. This sulfated component, which was more concentrated in the bovine than in the rat gland, was present in purified bovine prolactin granules stripped of their limiting membrane. However, the available evidence suggests that this might not be the only subcellular location of the sulfated polypeptide in the pituitary tissue.

Molecular organization of prolactin granules. II. Characterization of glycosaminoglycans and glycoproteins of the bovine prolactin matrix

Prolactin (PRL) granules can be isolated from the anterior pituitary gland of adult cows in nearly 50% yield by use of a procedure previously developed for the fractionation of the rat pituitary. Treatment of the isolated bovine granules with 0.2% Lubrol PX results in the solubilization of most membranes present in the fractin but has only a limited effect on the matrices, which remain aggregated and can be recovered and purified by gradient centrifugation. These membraneless PRL granules, studied in detail by morphological and biochemical techniques, were found to contain only small amounts of contaminants (primarily growth hormone granules and small membrane fragments). SDS polyacrylamide gel electrophoresis revealed that, in comparison with other fractions isolated from the bovine pituitary, the membraneless granules have a simpler polypeptide composition including PRL (approximately 85%), growth hormone (approximately 8%), as well as approximately 13 minor bands with apparent mol wt ranging from 80,000 go 45,000. Many of these minor bands are accounted for by glycoproteins, as revealed by their binding of 125I-concanavalin A, and two of these are also stained blue by the stains-all procedure, a reaction specific for acidic glycoconjugates. Chemical analyses of the membraneless granule fractin revealed the presence of a heterogeneous mixture of complex carbohydrates. Among glycosaminoglycans, the major component is heparan sulfate, while hyaluronic acid and chondroitin sulfate ar present in smaller amounts. Moreover, some of the glycoproteins are sulfated and account for over 50% of the nondialyzable 35S radioactivity found in the fraction isolated from labeled slices. Although the concentration of glycosaminoglycans and glycoproteins is relatively low in membraneless granules, the possibility that their presence in the fraction is largely due to cross-contamination and/or artifactual adsorption could be excluded on two grounds. These are: (a) electron microscope radiautography of preparations obtained from [35S]sulfate- and D-[6-3H]glucosamine-labeled slices showed a significant labeling of PRL granules in both intact cells and membraneless granule pellets, and (b) a mixing experiment showed that membraneless granules contain very little macromolecular sulfate radiactivity adsorbed from the soluble glycoconjugates present in the pituitary homogenate.