Intracellular glycosylation of vitellogenin in the liver of estrogen-stimulated Xenopus laevis

Microscopic Analysis of Nuclear Speckles in a Viviparous Reptile

Nuclear speckles are compartments enriched in splicing factors present in the nucleoplasm of eucaryote cells. Speckles have been studied in mammalian culture and tissue cells, as well as in some non-mammalian vertebrate cells and invertebrate oocytes. In mammals, their morphology is linked to the transcriptional and splicing activities of the cell through a recruitment mechanism. In rats, speckle morphology depends on the hormonal cycle. In the present work, we explore whether a similar situation is also present in non-mammalian cells during the reproductive cycle. We studied the speckled pattern in several tissues of a viviparous reptile, the lizard Sceloporus torquatus, during two different stages of reproduction. We used immunofluorescence staining against splicing factors in hepatocytes and oviduct epithelium cells and fluorescence and confocal microscopy, as well as ultrastructural immunolocalization and EDTA contrast in Transmission Electron Microscopy. The distribution of splicing factors in the nucleoplasm of oviductal cells and hepatocytes coincides with the nuclear-speckled pattern described in mammals. Ultrastructurally, those cell types display Interchromatin Granule Clusters and Perichromatin Fibers. In addition, the morphology of speckles varies in oviduct cells at the two stages of the reproductive cycle analyzed, paralleling the phenomenon observed in the rat. The results show that the morphology of speckles in reptile cells depends upon the reproductive stage as it occurs in mammals.

Vitellogenin in the European cave salamander, Proteus anguinus: Its characterization and dynamics in a captive female as a basis for non-destructive sex identification

Vitellogenin (Vtg) is a precursor protein of egg yolk proteins in oviparous and ovoviviparous vertebrates. Except in a case of exposure to estrogenic endocrine disruptors, Vtg is a female-specific protein and could be used as a molecular marker for sex identification. This would be especially useful in the case of the endangered European cave salamander Proteus anguinus in which sexes are indistinguishable according to external morphology, which hinders the establishment of a successful captive breeding program. Here we describe the identification, partial characterization, and purification of Vtg from P. anguinus. Vtg was identified in the plasma of a vitellogenic proteus female with visible oocytes. The identification of this protein was accomplished by mass spectrometry analysis. Two-dimensional gel electrophoresis revealed proteus Vtg as a mix of 190 kDa isoforms with isoelectric points in the pH range 5.3-6.0. Vtg was purified from proteus blood by gel filtration followed by anion-exchange chromatography. Using specific staining of SDS-PAGE gels, the Vtg was found to be phosphorylated and lipidated. Unlike the case in some other aquatic vertebrates, in P. anguinus, Vtg was not present in detectable amounts in cutaneous mucus. Degradation of oocytes in the captive vitellogenic female was accompanied by simultaneous decrease of Vtg concentration. Over a period of 10 months, the concentration of Vtg dropped from maximal to sub-detectable. Our results show that Vtg is a promising molecular marker for sex identification and ovary maturation in P. anguinus, which could contribute to the development of a viable program for captive reproduction of this unique species.

Metabolic differentiation in the embryonic retina

Unlike healthy adult tissues, cancers produce energy mainly by aerobic glycolysis instead of oxidative phosphorylation. This adaptation, called the Warburg effect, may be a feature of all dividing cells, both normal and cancerous, or it may be specific to cancers. It is not known whether, in a normally growing tissue during development, proliferating and postmitotic cells produce energy in fundamentally different ways. Here we show in the embryonic Xenopus retina in vivo, that dividing progenitor cells depend less on oxidative phosphorylation for ATP production than non-dividing differentiated cells, and instead use glycogen to fuel aerobic glycolysis. The transition from glycolysis to oxidative phosphorylation is connected to the cell differentiation process. Glycolysis is indispensable for progenitor proliferation and biosynthesis, even when it is not used for ATP production. These results suggest that the Warburg effect can be a feature of normal proliferation in vivo, and that the regulation of glycolysis and oxidative phosphorylation is critical for normal development.

BMDExpress: a software tool for the benchmark dose analyses of genomic data

BACKGROUND

Dose-dependent processes are common within biological systems and include phenotypic changes following exposures to both endogenous and xenobiotic molecules. The use of microarray technology to explore the molecular signals that underlie these dose-dependent processes has become increasingly common; however, the number of software tools for quantitatively analyzing and interpreting dose-response microarray data has been limited.

RESULTS

We have developed BMDExpress, a Java application that combines traditional benchmark dose methods with gene ontology classification in the analysis of dose-response data from microarray experiments. The software application is designed to perform a stepwise analysis beginning with a one-way analysis of variance to identify the subset of genes that demonstrate significant dose-response behavior. The second step of the analysis involves fitting the gene expression data to a selection of standard statistical models (linear, 2 degrees polynomial, 3 degrees polynomial, and power models) and selecting the model that best describes the data with the least amount of complexity. The model is then used to estimate the benchmark dose at which the expression of the gene significantly deviates from that observed in control animals. Finally, the software application summarizes the statistical modeling results by matching each gene to its corresponding gene ontology categories and calculating summary values that characterize the dose-dependent behavior for each biological process and molecular function. As a result, the summary values represent the dose levels at which genes in the corresponding cellular process show transcriptional changes.

CONCLUSION

The application of microarray technology together with the BMDExpress software tool represents a useful combination in characterizing dose-dependent transcriptional changes in biological systems. The software allows users to efficiently analyze large dose-response microarray studies and identify reference doses at which particular cellular processes are altered. The software is freely available at http://sourceforge.net/projects/bmdexpress/ and is distributed under the MIT Public License.

Immunological and biochemical responses in Mya arenaria (Mollusca Bivalvia) exposed in vivo to estradiol-17beta

Soft-shell clams Mya arenaria were injected with 10, 20 or 40 nmol of estradiol 17beta (E2). We observed a significant inhibiting effect of E2 on phagocytic activity of hemocytes from clams exposed to 10 and 20 nmol. A dose-response increase of the glycogen phosphorylase in the gonad tended to show a remobilisation of glycogen reserves involved in vitellogenesis although the exposure time must have been too short to observe a decrease in glycogen reserves or an increase in RNA concentration. Both results corroborate those of other studies about estrogen involvement in controlling immune capacity and energy metabolism related to vitellogenesis in bivalves. We can assume that immune parameters should now be taken into consideration in assessing endocrine disruption in bivalves. Nevertheless further studies are needed to understand the controlling pathways of E2 with a special regard on its interactions with other effectors involved in bivalve immunity and reproduction as well.

Vitellogenin precursors in the liver of the oviparous lizard, Podarcis sicula

In reptiles, as in the other oviparous vertebrates, vitellogenin (VTG) synthesis is stimulated in the liver by ovarian estrogens. In this article, the presence of VTG precursors was detected in liver subcellular fractions of the oviparous lizard, Podarcis sicula, in the reproductive period. The rough endoplasmic reticulum (RER) and the smooth microsomal fraction (SMF), which includes smooth endoplasmic reticulum and Golgi complex, were separated by means of two different sucrose gradients. The successful separation was controlled at the electron microscope. The contents of the different compartments were extracted by means of n-octyl-beta-D-glucopiranoside detergent and subjected to SDS-PAGE. Western Blotting with homologous anti/VTG antibody revealed two immunoreactive proteins of about 84 and 70 kDa in the RER, and four proteins of about 180, 150, 60, 50 kDa in the SMF; all these proteins appeared phosphorylated and glycosylated. The differences in the molecular weight of these VTG precursors are discussed.

Alteration of the biochemical properties of female gonads and vitellins in the clam Mya arenaria at contaminated sites in the Saguenay Fjord

Vitellins (Vn) are the major egg yolk proteins that constitute an important energy reserve for mollusc embryos. The purpose of the present study was to examine whether the relative levels of sugars, lipids, phosphates, and labile IIb metals and calcium normally associated with Vn would differ in clam populations living at contaminated sites. Softshell clams (Mya arenaria) were collected at three sites in the area of the Saguenay Fjord: a marina, a municipal sewer outfall zone, and a reference site. The condition factor (weight:length ratio), metallothionein-like proteins, cytochrome P450 1A1 activity and DNA damage were all determined in the clam's digestive gland. Levels of total sugars, lipids, alkali-labile phosphates, proteins, and labile zinc and calcium were determined in female gonad homogenates and in purified Vn. The results show that clam gonads at the contaminated site by a marina displayed a lower index of gonad activity than the reference site. In addition, the condition factor was significantly lower at the marina site as compared to the reference site, with a concomitant increase in DNA damage and metallothionein (MT) induction. In fact, the condition factor was significantly correlated with DNA damage (R = -0.413, P = 0.045) and MT levels (R = -0.622, P = 0.03). Homogenates of female gonads were found to contain higher levels of labile IIb metals and calcium, with lower lipid content at the marina site compared to the reference site, and much higher levels of alkali-labile phosphates (ALP) and calcium at the municipal outfall site. Vn from the marina site were significantly higher in labile IIb metals but lipid content appeared to be somewhat lower than at the reference site. Vn from the municipal site were found to be highly phosphorylated, with higher levels of lipids and calcium. These results suggest that the chemical composition of the gonads and Vn are altered in contaminated sites.

Evaluation of estrogenic effects of municipal effluents to the freshwater mussel Elliptio complanata

Municipal effluents are an important source of estrogens to the aquatic environment. The purpose of this study was to examine the estrogenicity of municipal effluents to the indigenous freshwater mussel, Elliptio complanata. First, estradiol-binding sites in gonad homogenates were characterized to determine the binding affinity and specificity of estrogens. Mussels were exposed to increasing concentrations of a municipal effluent for 96 h at 15 degrees C. In another experiment, mussels were placed in cages and submerged for 62 days at 1.5 km upstream and 5 km downstream of a municipal effluent plume in the St. Lawrence River. Mussels were harvested for assessment of vitellogenin-like proteins in the hemolymph and determination of total lipid, carbohydrate and protein in the gonad. The presence of specific estrogen-binding sites was found in both male and female gonads. Binding of estradiol to cytosol proteins reached saturation, yielding a dissociation constant of 0.4 nM. Vitellogenin (Vg) levels increased significantly in both the hemolymph and the gonad after exposure to the effluent. Moreover, females appeared to be more sensitive than males to producing Vg. Mussels exposed in situ to contaminated surface waters had higher levels of Vg at the downstream site, again, females had higher levels of Vg than did males. On the other hand, lipid and sugar levels in male gonads were significantly increased at the downstream site. Moreover, mussels at the downstream site had decreased shell growth length and increased total and soft tissue weights. We conclude that municipal effluents contain bio-available xenoestrogens at levels sufficient to elicit effects in freshwater mussels.

Synthesis of vitellogenin polypeptides and deposit of yolk proteins in Anolis pulchellus

The recognition of liver and serum polypeptides in Anolis pulchellus by a polyclonal antibody against S1-lipovitellin confirmed their identity as vitellogenins (Vtg) and demonstrated their structural relationship to yolk lipoproteins. In vivo labeling demonstrated active synthesis of Vtg by vitellogenic females since intracellular incorporation of [3H]-leucine was detected at short periods of label in all five Anolis Vtg forms. Time course analysis of 3H-Vtg levels indicated a 1 hr lag phase between synthesis and secretion. On the other hand, 32P-Vtg appears to be rapidly secreted from the liver into the blood since label was detected simultaneously in both compartments. After 2 hr intracellular 32P-Vtg levels reached a plateau. Decreasing 32P-Vtg levels in the blood were observed several hours after injection. In growing oocytes 32P was detected in yolk phosphoproteins ranging from 37,000 to 75,000 in molecular weight. Based on these results together with previous published data we conclude that in tropical anole the yolk phosphoproteins appear to be derived from the larger highly phosphorylated Vtg forms according to the typical vertebrate Vtg precursor-product relationship. However, the main component of yolk lipovitellin is synthesized in the liver as an independent lipoprotein (Vtg-116) which is taken up by growing oocytes without major proteolytic modifications. This novel mode of lipovitellin biosynthesis and deposit in reptiles has not been reported previously.

Vitellogenin induction by estradiol in channel catfish, Ictalurus punctatus

In oviparous vertebrates estrogens induce hepatic synthesis of vitellogenin (VG), a blood protein sequestered in vitellogenic oocytes and from which lipovitellin (LV) and phosvitin are derived. Our objective was to identify VG in the channel catfish, Ictalurus punctatus. An intraperitoneal injection of estradiol-17 beta into adult male fish induced a dose-dependent accumulation of a 150 kDa protein (EP) in the plasma. EP was detectable in Coomassie blue-stained polyacrylamide gels within 24 hr after injection of 2 mg hormone/100 g body weight. During the next 4 days, EP increased from 5 to about 25% of the total plasma protein. Electrophoretic mobility, peptide mapping, and immunological crossreactivity showed EP to be indistinguishable from a plasma protein in adult females with vitellogenic ovaries. Two major yolk polypeptides, YP1 (120 kDa) and YP2 (29.6 kDa), were precipitated by (NH4)2SO4 from a yolk protein extract. YP1 but not YP2 reacted with an anti-EP polyclonal antiserum in Western blots. Peptide mapping after proteolysis with trypsin showed YPs 1 and 2 to be unique and revealed structural homologies between YP1 and EP. Liver but not pancreatic explants from an estradiol-treated male synthesized and secreted a [35S]methionine-labeled, 150 kDa protein beginning about 2 hr after initial exposure to the label. We tentatively conclude that EP and YP1 represent VG and LV, respectively. YP2 remains unidentified.

The rate of bulk flow from the endoplasmic reticulum to the cell surface

Tripeptides containing the acceptor sequence for Asn-linked glycosylation (Asn-X-Ser/Thr) were added to CHO and HepG2 cells. The tripeptides were glycosylated in the ER and then secreted into the medium, via the Golgi complex in which the oligosaccharide chains were processed. The half-time for secretion, approximately 10 min, was faster than that of known proteins transported through the same pathway. Since much evidence suggests that oligosaccharide chains are not signals for transport, it appears that no signal is necessary for rapid and efficient transport from the ER to the Golgi, or from the Golgi to the cell surface. Rather, it appears that proteins retained as permanent residents en route through the ER-Golgi transport pathway must contain specific retention signals.

Processing of pro-vitellogenin in insect fat body: a role for high-mannose oligosaccharide

Several discrete events were resolved in the processing of vitellogenin in Blattella germanica. Using tunicamycin to inhibit the synthesis of high-mannose oligosaccharide, a high molecular weight pro-vitellogenin peptide (apo-proVG, Mr 215,000) was identified in fat body. Dosages of tunicamycin which inhibited glycosylation of vitellogenin by 98% inhibited its synthesis by as much as 59%, yet led to an intracellular accumulation of apo-proVG. Reversibility and dose dependency of these effects on vitellogenin synthesis, glycosylation, proteolytic processing, and secretion were demonstrated. In control insects, glycosylation of apo-proVG yielded a Mr 240,000 pro-vitellogenin peptide (proVG). FITC-Concanavalin A bound to purified proVG but not to apo-proVG, thus confirming an absence of high-mannose oligosaccharide in the apo-protein. Following its glycosylation, proVG was processed rapidly in fat body to Mr 160,000 (VG160) and Mr 102,000 (VG102) peptides which subsequently were secreted into hemolymph. After uptake into developing oocytes, the VG160 peptide was processed further prior to chorionation, yielding subunits of Mr 95,000 and 50,000. Uniqueness of the peptides of mature vitellin (Mr 102,000, 95,000, and 50,000) was indicated by comparison of the CNBr fragments of each purified subunit. Staining of CNBr fragments with FITC-Concanavalin A also indicated that high-mannose oligosaccharides are attached at one or more sites within each vitellin subunit. Resolution of the substructure of this insect vitellin and identification of events involved in the processing and secretion of its fat body apo-protein provide a basis for further study of the assembly and transport of vitellogenin, its packaging in eggs, and utilization during embryogenesis.

Characterization of the oligosaccharides of prolyl hydroxylase, a microsomal glycoprotein

Prolyl hydroxylase is a tetrameric glycoprotein that catalyzes a vital posttranslational modification in the biosynthesis of collagen. The enzyme purified from whole chick embryos (WCE) possesses two nonidentical subunits, alpha and beta, and has been shown by several techniques to reside in the endoplasmic reticulum of chick embryo fibroblasts. The studies described here demonstrate that the larger of the two subunits (alpha) exists in two forms in chick embryo fibroblasts (CEF); these two forms differ in carbohydrate content. The larger alpha subunit, alpha', contains two N-linked high mannose oligosaccharides, each containing eight mannose units; the smaller subunit, alpha, contains a single seven-mannose N-linked oligosaccharide. Both oligosaccharides could be cleaved by endo-beta-N-acetylglucosaminidase H and completely digested with alpha-mannosidase to yield mannosyl-N-acetylglucosamine.

Biosynthesis of prolyl hydroxylase: evidence for two separate dolichol-media pathways of glycosylation

Prolyl hydroxylase is a glycoprotein containing two nonidentical subunits, alpha and beta. The alpha subunit of prolyl hydroxylase isolated from 13-day-old chick embryos contains a single high mannose oligosaccharide having seven mannosyl residues. Two forms of alpha subunit have been shown to exist in enzyme purified from tendon cells of 17-day-old chick embryos, one of which (alpha) appears to be identical in molecular weight and carbohydrate content with the single alpha of enzyme from 13-day-old chick embryos, as well as another form (alpha') that contains two oligosaccharides, each containing eight mannosyl units [see Kedersha, N. L., Tkacz, J. S., & Berg, R. A. (1985) Biochemistry (preceding paper in this issue)]. Biosynthetic labeling studies were performed with chick tendon cells using [2-3H]mannose, [6-3H]glucosamine, [14C(U)]mannose, and [14C(U)]glucose. Analysis of the labeled products using polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that only the oligosaccharides on alpha' incorporated measurable mannose or glucosamine isotopes; however, both alpha subunits incorporated 14C amino acid mix and [14C(U)]glucose [metabolically converted to [14C(U)]mannose] under similar conditions. Pulse-chase labeling studies using 14C amino acid mix demonstrated that both glycosylated polypeptide chains alpha and alpha' were synthesized simultaneously and that no precursor product relationship between alpha and alpha' was apparent. In the presence of tunicamycin, neither alpha nor alpha' was detected; a single polypeptide of greater mobility appeared instead. Incubation of the cells with inhibitory concentrations of glucosamine partially depressed the glycosylation of alpha' but allowed the glycosylation of alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis and secretion of alpha 1 acute-phase globulin in primary cultures of rat hepatocytes

Experimental inflammation in rats led to a sevenfold increase in serum levels of alpha 1 acute-phase globulin. This increase is correlated with elevated levels of translatable mRNA for alpha 1 acute-phase globulin in the liver. Biosynthesis and secretion of alpha 1 acute-phase globulin were studied in rat hepatocyte primary cultures. An intracellular form of alpha 1 acute-phase globulin with an apparent relative molecular mass of 63 500 and a secreted form of 68 000 were found. The intracellular form of alpha 1 acute-phase globulin could be deglycosylated by endoglucosaminidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. The secreted form of alpha 1 acute-phase globulin was not sensitive to endoglucosaminidase H, but was susceptible to the action of sialidase reflecting carbohydrate side-chains of the complex type. Pulse-chase experiments revealed a precursor-product relationship for the high-mannose and the complex type alpha 1 acute-phase globulin. In the hepatocyte medium newly synthesized alpha 1 acute-phase globulin was detected 30 min after the pulse. Unglycosylated alpha 1 acute-phase globulin was found in the cells as well as in the medium when the transfer of oligosaccharide chains onto the polypeptide chains was blocked by tunicamycin. Tunicamycin led to a marked delay in alpha 1 acute-phase globulin secretion.

The biosynthesis of acute-phase proteins in primary cultures of rat hepatocytes

The biosynthesis and secretion of alpha 2-macroglobulin, transferrin, alpha 1-acid glycoprotein and alpha 1-proteinase inhibitor were studied in rat hepatocyte primary cultures. After labeling with [35S]methionine, two forms, which can be separated electrophoretically differing by molecular weight, were found for each of the four glycoproteins. The following molecular weights were estimated for the intracellular precursors and the secreted forms: alpha 2-macroglobulin, 176 000 and 182 000; transferrin, 84 000 and 86 000; alpha 1-acid glycoprotein, 39 000 and 43 000-60 000; alpha 1-proteinase inhibitor, 49 000 and 54 000. Carbohydrate moieties could be removed from intracellular forms by treatment with endoglucosaminidase H indicating that their oligosaccharide chains were of the high-mannose type. The extracellular forms were sensitive to sialidase. They incorporated [3H]galactose and [3H]fucose showing that their oligosaccharide chains were of the complex type. Pulse-chase experiments revealed a precursor-product relationship for the high-mannose and the complex type glycoproteins. In the hepatocyte medium newly synthesized albumin was detected after 30 min and newly synthesized glycoproteins after 60 min. Unglycosylated alpha 2-macroglobulin (162 000), transferrin (79 000), alpha 1-acid glycoprotein (23 000), and alpha 1-proteinase inhibitor (41 000) were found in the cells as well as in the medium, when the transfer of oligosaccharide chains onto the polypeptide chains was blocked by tunicamycin. Tunicamycin led to a marked reduction of the secretion of alpha 2-macroglobulin, alpha 1-acid glycoprotein and alpha 1-proteinase inhibitor, whereas the secretion of transferrin was less affected.

Biosynthesis and secretion of alpha 1-antitrypsin in primary cultures of rat hepatocytes. Characterization of differently glycosylated intracellular and extracellular forms

The biosynthesis and secretion of alpha 1-antitrypsin was studied in rat hepatocyte primary cultures. After labeling with [35S]methionine an alpha 1-antitrypsin with an apparent molecular weight of 49000 estimated by sodium dodecyl sulfate/polyacrylamide slab gel electrophoresis was immunoprecipitated from the cell homogenate. This intracellular form of alpha 1-antitrypsin could be deglycosylated by endoglycosidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. Pulse-chase experiments showed that about 30 min after its synthesis the transformation of the 49000-Mr alpha 1-antitrypsin to a protein with an apparent molecular weight of 54000 began. Only this 54000-Mr protein was secreted by the hepatocytes. The 54000-Mr alpha 1-antitrypsin was not sensitive to endoglycosidase H, but sensitive to neuraminidase, and it incorporated [3H]galactose and [3H]fucose indicating that its oligosaccharide chains were of the complex type. In the presence of tunicamycin, which blocks the formation of N-asparagine-linked oligosaccharide chains, an unglycosylated alpha 1-antitrypsin with an apparent molecular weight of 41000 was found in the cells as well as in the medium. However, tunicamycin decreased the secretion of alpha 1-antitrypsin by 60-70%, whereas the secretion of albumin remained unaffected. In the presence of colchicine the secretion of both alpha 1-antitrypsin and albumin was impaired. The results demonstrate the importance of glycosylation for the secretion of alpha 1-antitrypsin.