Biosynthesis of N-glycosidically linked glycoproteins during gastrulation of sea urchin embryos

Growth of linear spicules in cultured primary mesenchymal cells of sea urchin embryos is bidirectional

The growth of spicules of the sea urchin embryo was studied in a simple in vitro system in which primary mesenchymal cells attach to the substrata, migrate and fuse via filopodia, and subsequently deposit CaCO3, which in most cases is in the form of linear rods. The use of autoradiographs following 45Ca2+ pulse-chase labeling revealed that in such a system linear spicules that formed had two focal points of growth. Elongation occurred by addition of approximately equivalent amounts of Ca2+ to both ends of each rod. Multiform spicules having variable numbers of elongation sites (tips) also showed a similar pattern of Ca2+ deposition. Thus, the growth of both linear and relatively complex skeletal forms is apparently accomplished by the same basic mechanism.

The 140-kDa fibronectin receptor complex is required for mesodermal cell adhesion during gastrulation in the amphibian Pleurodeles waltlii

We have studied the localization and function of a 140-kDa glycoprotein complex implicated in cell adhesion to fibronectin- and laminin-rich extracellular matrices in Pleurodeles waltlii gastrulae. In particular, we have shown that antibodies directed against highly purified avian fibronectin (FN) receptor complex cross-react with two major polypeptides of apparent molecular weights of 140,000 and 100,000 and a third minor component of 90,000. Using sections of embryos or whole mounts, we have also discovered that the putative FN receptor is widely distributed on the early embryonic cell surface. We have also found that the basal surface of the roof of the blastocoel, a region particularly enriched in an extracellular matrix consisting of fibronectin- and laminin-rich fibrils, is rich in receptor complex. We have prepared monovalent Fab' fragments of this antibody and have found that they cause detachment of cells previously attached to substrata coated with fibronectin, and they also arrest gastrulation when injected into the blastocoel of early gastrulae. Thus, it appears that the fibronectin receptor complex plays a significant functional role in cell attachment of gastrula-stage cells in vitro and in cell migration in vivo during gastrulation.

Migratory and invasive behavior of pigment cells in normal and animalized sea urchin embryos

Pigment cell precursors in the vegetal plate of late mesenchyme blastulae of the sea urchin Strongylocentrotus purpuratus begin to express a cell surface epitope recognized by the monoclonal antibody SP-1/20.3.1. When one-quarter gastrulae are dissociated into ectodermal and mesenchymal fractions, most SP-1/20.3.1 immunoreactive cells separate into the mesenchymal fraction, whereas at the full gastrula and all later stages almost all epitope-bearing cells are in the ectodermal fraction. Exposure of embryos to sulfate-free seawater p-nitrophenyl beta-D-xyloside, and tunicamycin, all of which prevent primary mesenchyme migration, does not inhibit SP-1/20.3.1 immunoreactive cells from distributing similarly to those in controls, although pigment synthesis is completely inhibited in sulfate-free conditions. Time-lapse video sequences reveal that pigment cells, and a small set of rapidly migrating, SP-1/20.3.1 immunoreactive amoeboid cells that appear in the pluteus, remain closely associated with the ectodermal epithelium during most of larval development. Transmission electron microscopy observations of plutei show pigment cells tightly apposed to the ectodermal epithelium at discontinuities in the basal lamina and sandwiched between the basal lamina and the epithelial cells. It is concluded that SP-1/20.3.1 immunoreactive mesenchymal cells invade the ectodermal epithelium and may use migratory substrates other than those used by primary mesenchymal cells.

Measurements of the specific activity of the nucleoside triphosphate pool of sea-urchin embryos following 8-3H-guanosine administration

The nucleoside triphosphate (NTP) pool during the early development of sea-urchin, Paracentrotus lividus, was measured using high-performance liquid chromatography (HPLC). Each NTP pool was measured at four stages between the eight-blastomere stage and the early blastula stage, as was the specific activity of each NTP following an initial administration of 8-3H-guanosine. Unexpectedly high UTP (uridine-5'-triphosphate) concentrations and radioactivity levels in UTP were observed. Studies of the sea-urchin, Strongilocentrotus purpuratus, also revealed elevated levels of 8-3H-guanosine incorporation into UTP. The present procedure was found to be suitable for the unequivocal identification of UTP.

Lectin-affinity isolation of microvillous membranes from the pigmented epithelium of rat retina

The pigmented epithelium of the vertebrate retina phagocytizes the discarded tips of photoreceptors and it is likely that a specific cellular recognition process is involved in this phenomenon. The apical surface of retinal pigmented epithelium (RPE) contains microvilli which interdigitate with the outer segment regions of photoreceptor cells and it is this apical microvillous surface that is of particular interest with respect to phagocytosis. The present study is a report of a method to isolate a fraction that is enriched in microvilli from the apical surface of this highly polarized epithelial cell. Wheat germ agglutinin (WGA) conjugated sepharose beads are used to remove the microvillous membranes which are then observed with scanning and transmission electron microscopy. The proteins of this RPE-subfraction are separated through use of SDS-polyacrylamide gel electrophoresis. The relative molecular weights (Mr) and lectin binding properties of glycoproteins are examined in Western blots through the use of lectin-peroxidase conjugates as probes for carbohydrate residues. A preliminary comparison of membranes isolated from Long Evans (normal) and Royal College of Surgeons (dystrophic) rat retina RPE shows that the glycoproteins in these two preparations are different with respect to the binding of Concanavalin-A (Con-A) and WGA. In particular a glycoprotein in the normal RPE preparation with a Mr of 175K binds Con-A and WGA, but in the dystrophic RPE preparation binds little or no WGA. A glycoprotein present in the normal RPE preparation with a Mr of 86K binds Con-A and WGA, but both lectins have reduced binding sites in the dystrophic preparation. Limax flavus agglutinin (specific for sialic acid residues) binds to a high molecular weight glycoprotein with a Mr of 195K-196K which is present in both normal and dystrophic RPE membrane preparations and which also binds Con-A and WGA.

Developmental expression of a cell-surface protein involved in calcium uptake and skeleton formation in sea urchin embryos

The developmental expression of a cell-surface protein involved in Ca2+ accumulation and skeleton formation in sea urchin embryos has been studied. In Strongylocentrotus purpuratus, this protein is present in the egg and in all cell types of the early embryo. After gastrulation, its synthesis and expression are restricted to the skeleton-forming primary mesenchyme cells. In Lytechinus pictus, the protein cannot be detected in eggs or in embryos until the mesenchyme blastula stage. Hybrid embryos demonstrate a pattern of expression indistinguishable from that of the species contributing the maternal genome, which suggests that early expression of the protein in S. purpuratus embryos is due to utilization of maternal transcripts from the egg. Later expression of this protein in primary mesenchyme cells is the result of cell-type-specific synthesis, likely encoded by embryonic transcripts. This cell-type-specific expression in primary mesenchyme cells correlates temporally with Ca2+ accumulation during skeleton formation in the embryo.

An autoradiographic analysis of N-linked glycoconjugates in embryonic salivary gland morphogenesis

The synthesis, deposition, and loss of mannose-bearing glycoconjugates during branching morphogenesis of embryonic mouse salivary glands has been evaluated. Day 13 embryonic mouse salivary glands were cultured for 44 hr, pulse labeled 4 hr with [3H]mannose, then fixed after 0, 2, 4, 8, or 24 hr of chase in nonradioactive medium, and processed for autoradiography. Light microscopic autoradiograms of sectioned rudiments reveal extensive label within the epithelium, little label over the mesenchyme, and a concentration of radioactivity at the basal surface of the epithelium. Autoradiograms of "chased" rudiments reveal a) no detectable loss of label from the epithelium or the basal epithelial surface over the first 8 hr, and b) significant label loss by 24 hr of chase at the basal epithelial surface, while moderate amounts of radioactivity remain throughout the rest of the epithelium. The [3H]bound material is insensitive to chondroitinase ABC, a glycosaminoglycan degradative enzyme, but is sensitive to tunicamycin presence in the culture medium. Earlier studies showed that embryonic mouse salivary glands cultured in medium containing tunicamycin (25 ng/ml) continued normal epithelial branching while epithelial growth was inhibited. The present autoradiographic studies of [3H]mannose-labeled rudiments demonstrate that tunicamycin causes a significant decrease in radioactivity, relative to controls. Thus, our results suggest that epithelial branching activity is independent of control levels of mannose-containing/tunicamycin-sensitive, glycoconjugate deposition.

Effect of dimethyl sulfoxide on transformed rat Schwann cells

Cultured rat Schwann cells transformed by Simian Virus 40 (SV40) have previously been shown to retain their ability to synthesize myelin-associated galactosylceramide and sulfatide. Little is known about the mechanism regulating galactosphingolipid synthesis in Schwann cells. We have found that growing the transformed Schwann cells in the presence of dimethyl sulfoxide (DMSO) markedly inhibits the incorporation of [35S]sulfate into sulfatide, in a time- and dose-dependent manner. The concentration of DMSO which resulted in a half-maximal inhibition after 6 days of incubation was 0.5%, and the incubation time required for a half-maximal effect at 1.0% DMSO was approximately 4 days. In contrast, DMSC did not affect the incorporation of [35S]sulfate into glycosaminoglycans. In addition, DMSO treatment has little effect on the synthesis of cellular DNA, proteins and lipids. When transformed Schwann cells were treated with DMSO, a substantial decrease in the incorporation of [3H]galactose into galactosylceramide was observed. The concentration of DMSO which resulted in a half-maximal inhibition of galactosylceramide synthesis was approximately 0.5%, similar to the concentration required for a similar effect on sulfatide synthesis. However, the incubation time required for a half-maximal inhibitory effect on galactosylceramide synthesis at 1.0% DMSO was less than 1 day, which was substantially shorter than the time required for the inhibition of sulfatide synthesis at this concentration. This finding is consistent with the interpretation that treatment with DMSO inhibits the synthesis of galactosylceramide, a precursor of sulfatide, which results in a decrease in the synthesis of sulfatide during a prolonged incubation of DMSO.

Modification of lymphocyte migration by sulfated polysaccharides

The role of sulfated polysaccharides in lymphocyte migration has been analyzed in vivo using lymphocytes labeled with an intracellular DNA-binding fluorochrome Hoechst 33342. The influence of a panel of sulfated polysaccharides on entry (by injecting the sulfated polysaccharide prior to the labeled cells) and displacement from lymphoid organs (by injecting the sulfated polysaccharide after the labeled cells have localized) indicated that different sulfated polysaccharides have selective effects on entry and displacement, and furthermore positioning of subpopulations within organs. Additional experiments suggested that receptors for sulfated polysaccharides on high endothelial venules may interact with complementary structures on lymphocytes. The data supporting this conclusion were: (a) the normal localization behavior of lymphocytes preincubated with sulfated polysaccharides; (b) an inverse relationship between the expression of lymphocyte surface receptors for sulfated polysaccharides and the ability of the lymphocytes to enter lymphoid organs and (c) the selective binding of sulfated polysaccharide-coupled fluoresceinated beads to high endothelial venules. In this case only the beads coupled with the sulfated polysaccharides that inhibited entry bound to the high endothelial venules. These findings are discussed in terms of a fundamental cellular recognition system utilizing sulfated polysaccharides.

N-linked glycoprotein biosynthesis in the developing mouse embryo

We have developed microenzymic assays that have, for the first time, enabled analysis of several enzymes in the pathway for N-linked glycoprotein biosynthesis in pre- and peri-implantation mouse embryos. The in vitro activities of the glycosyl transferases responsible for the formation of N-acetylglucosaminylpyrophosphoryldolichol,N, N'-diacetyl-chitobiosylpyrophosphoryldolichol, mannosylphosphoryldolichol, and glucosylphosphoryldolichol were found to decrease after fertilization before increasing significantly at the blastocyst stage, a stage that was also found to be highly sensitive to the glycosylation inhibitor, tunicamycin. The observed elevation in the activities of these enzymes in blastocysts still occurred when ebbryos were cultured in alpha-amanitin, indicating that de novo mRNA synthesis is unnecessary for the observed increase in their activities. Thus, an elevated capacity for N-glycosylation exists at the blastocyst stage, a time when dramatic increases in cell-cell interactions are known to occur.

Effects of dolichol and dolichyl phosphate on in vitro differentiation of hematopoietic progenitors

The exogenous addition of dolichyl phosphate (Dol-P), an active form of dolichol (Dol) that carries oligosaccharide chains for protein-N-glycosylation, significantly enhanced colony formation of mouse bone marrow hematopoietic progenitors (CFU-e, BFU-e, and CFU-gm) was stimulated by erythropoietin (Epo) and colony-stimulating factor (CSF), but Dol enhanced colony formation of CFU-e only. The effects of Dol or Dol-P on these hematopoietic progenitors were fully dependent on stimulation by Epo or CSF. Other mevalonate-metabolites, such as cholesterol, coenzyme Q10, and isopentenyladenine, had no effect on hematopoietic progenitors. These studies suggest that exogenous Dol-P enhances the frequency of differentiation of hematopoietic progenitors stimulated by Epo or CSF, and there may be a diversity in cellular response of these progenitors to Dol.

A monoclonal antibody inhibits calcium accumulation and skeleton formation in cultured embryonic cells of the sea urchin

The assembly of the spicules (primitive skeleton) of the sea urchin embryo is being studied in primary mesenchyme cells cultured in vitro. A monoclonal antibody (1223) has been prepared that inhibits the deposition of CaCO3 into the spicules. This antibody reacts with a 130,000 Mr cell-surface protein that is concentrated on the surface of approximately 5% of the cells of dissociated gastrula stage embryos. When primary mesenchyme cells in the embryo or cells cultured in vitro are examined, the 1223 antigen is detected on the surface of the cells and on the extracellular material associated with the spicule. We conclude that the 1223 antibody recognizes a cell-surface protein that plays an essential role in spicule formation.

Cell surface glycosaminoglycans (GAGs) of cultured chick fibroblasts. Modifications in relation to the stage of embryo development

We have investigated the changes in glycosaminoglycan (GAG) composition between cultured fibroblasts derived from 8- and 16-day chick embryos. GAG composition has been studied after [3H]glucosamine and [35S]sulfate labeling. Both the 8- and 16-day embryo fibroblasts were found to contain hyaluronic acid (HA), dermatan sulfate (DS), heparan sulfate (HS) and chondroitin sulfates (CS), the latter being the major component in 8- and 16-day cells. These four GAGs were quantified after their separation using cellulose acetate electrophoresis. The amounts of HA and CS were respectively shown to increase 2-fold and 4-fold between the 8th and 16th day of development, whereas the amounts of HS and DS resp. diminished 2.5-fold and 1.2-fold. These results show that the relative proportions of the different GAGs alter during embryo development. The fibroblasts from 8-day-old embryos detached more rapidly from the culture dishes than the cells from 16-day-old embryos when treated with trypsin. However, this difference was not directly related to the different GAG content.

Developmental regulation of glycosyltransferases involved in synthesis of N-linked glycoproteins in sea urchin embryos

Previous in vivo studies using drugs that inhibit the N-glycosylation of proteins have demonstrated that newly synthesized N-linked glycoproteins are required for gastrulation in embryos of two species of sea urchins, Strongylocentrotus purpuratus and Arbacia punctulata. To understand the biochemical events regulating glycoprotein synthesis during gastrulation in S. purpuratus embryos, we examined the in vitro activities of enzymes catalyzing several of the early steps in N-linked glycoprotein synthesis. The activities of glycosyl transferases responsible for production of N,N-diacetylchitobiosylpyrophosphoryldolichol and glucosylphosphoryldolichol, two intermediates in the formation of oligosaccharylpyrophosphoryldolichol (the carbohydrate donor for N-glycosylation), were low but detectable in membranes from eggs. After fertilization these activities remained constant or increased slowly up to the blastula stage and thereafter increased rapidly at gastrulation. In agreement with these in vitro findings, in vivo labeling experiments revealed that the rate of incorporation of [3H]Man into oligosaccharylpyrophosphoryldolichol and into protein increased three- to fourfold prior to gastrulation and then slightly more at the prism stage. In contrast, in vitro activity of mannosylphosphoryldolichol synthase, another enzyme in the pathway of N-linked glycosylation, was maximal in membranes from egg and embryos in the early stages of development and declined prior to gastrulation. Furthermore, the level of this activity was at least 100-fold greater than that for enzymes involved in the formation of the chitobiosyl and glucosyl lipids. With the exception of mannosylphosphoryldolichol synthase activity, these data indicate that there is a general activation of the glycosylation apparatus before gastrulation in sea urchin embryos. Possible explanations for the decrease in mannosylphosphoryldolichol synthase activity are discussed.

Identification of a major N-glycosylated protein of rabbit mammary gland and its appearance during development in vivo

A major N-glycosylated protein was purified from hormonally stimulated pregnant and lactating rabbit mammary tissue. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, immunodiffusion and amino acid analysis showed the protein to be transferrin. The rate of synthesis of the protein during tissue development was studied and found to parallel the whey proteins casein and alpha-lactalbumin. The function of the protein during lactation is discussed.

Enzymatic sulfation of N-glycosidically linked oligosaccharides by endothelial cell membranes

Homogenates of human vascular endothelial cells catalyze the transfer of 35SO4 from 3'-phosphoadenosine 5'-phospho[35S]sulfate into macromolecular endogenous acceptors with properties of both glycoproteins and glycosaminoglycans. Analysis of the 35S-glycoprotein products by both DEAE-cellulose chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that these 35S-glycoproteins correspond to the major sulfated glycoproteins observed when endothelial cell cultures were labeled with 35SO4 [A. Heifetz, C. Watson, A. R. Johnson, and M. K. Roberts (1982) J. Biol. Chem. 257, 13581-13586]. The pH optimum of the glycoprotein sulfotransferase is 7.0-7.5, which is distinctly different than that of endothelial glycosaminoglycan sulfotransferase(s), whose pH optimum is 6.0. The 35S-glycoprotein products were characterized as sialated, triantenary-branched, N-linked 35SO4-oligosaccharides containing an endo-beta-N-acetylglucosaminidase D-sensitive site. The site of sulfation was characterized as the GlcNAc residue on the reducing end of the N-linked oligosaccharide. Thus, these sulfotransferases in a cell-free homogenate appear to preferentially add sulfate moieties to a specific class of glycoprotein acceptors at a specific site on sialated oligosaccharides.

Fine specificity of a monoclonal anti-testicular cell antibody for glycolipids with terminal N-acetyl-D-glucosamine structure

The specificity of a mouse anti-testicular cell monoclonal antibody, J1, was investigated. Previous studies suggested that N-acetyl-D-glucosamine (GlcNAc) was a constituent of the determinant recognized by J1. When the antibody was tested against a variety of purified glycolipids containing this saccharide in terminal, penultimate or internal positions, J1 reacted only with species expressing terminal GlcNAc. The influence of oligosaccharide chain length, branch substitution, and haptenic valence on J1 binding was examined using glycolipids prepared by a weak acid hydrolysis and exoglycosidase digestion of bovine I-active ganglioside. Degree of binding was inversely proportional to chain length and was proportional to hapten valence. Failure of J1 to bind partially deglycosylated transferrin implied binding preference for GlcNAc beta 1----3Gal over GlcNAc beta 1----2Man. Immunofluorescence analysis of J1 binding to human neutrophils failed to detect lactotriosylceramide on their surface, although this glycolipid has previously been isolated from these cells, suggesting that this structure exists in a cryptic or intracellular form. Binding results were consistent with J1 having low affinity for GlcNAc or GlcNAc beta 1----3Gal on a variety of lacto-series glycolipids.

Collagen metabolism and spicule formation in sea urchin micromeres

The role of collagen or collagen-like protein(s) in the in vitro formation of the sea urchin embryonic skeleton was investigated using isolated micromeres of Strongylocentrotus purpuratus. Micromeres were cultured in sea water containing 4% horse serum on tissue culture plastic or an extracellular matrix of type I collagen. The effect of proline analogs and an inhibitor of collagen hydroxylation on in vitro spicule formation in both culture systems was monitored. When micromeres are cultured in the presence of proline analogs L-azetidine-2-carboxylic acid and L-3, 4-dehydroproline which disrupt collagen metabolism, spicule formation is significantly less inhibited on a collagen substratum than on plastic. Culturing micromeres on plastic in the presence of alpha, alpha'-dipyridyl, an inhibitor of collagen hydroxylation, resulted in almost complete inhibition of spicule formation. The inhibition by alpha, alpha'-dipyridyl can be overcome by culturing micromeres on collagen substratum. These results do not support the idea of collagen being the calcified organic matrix of the spicule. Rather, they suggest that micromeres synthesize a collagen-like extracellular matrix which is necessary for spicule formation. Inhibition of this activity by proline analogs or a collagen processing inhibitor can be overcome by providing the cells with a previously deposited extracellular matrix.

5'-Nucleotidase from bovine caudate nucleus synaptic plasma membranes: specificity for substrates and cations; study of the carbohydrate moiety by glycosidases

We studied 5'-nucleotidase in preparations of synaptic plasma membranes from bovine caudate nucleus. The best substrates for this membrane-bound enzyme were purine nucleotides, particularly 5'AMP. Effects of metal cations and chelating agents suggest that 5'-nucleotidase is a metalloprotein. Optimal conditions for solubilization of the 5'-nucleotidase were found by using a low concentration of the zwitterionic detergent sulfobetaine 14. In contrast, another membrane-bound enzyme, acetylcholinesterase, was not solubilized under these conditions, but only in the presence of Triton X-100. The effects of lectins (concanavalin A, Lens culinaris agglutinin, wheat germ agglutinin, and Limulus polyphemus agglutinin) showed that both enzymes are glycoproteins. Sequential hydrolysis with specific glycosidases produced modifications of the effect of lectins on these enzymes. The results suggest the presence of a complex-type glycosylation, with a fucose residue on the internal N-acetyl-D-glucosamine of the pentasaccharide core.

Epidermal fucosylation of cell surface glycoprotein

When Ulex europeus agglutinin I (UEA) conjugated with fluorescein isothiocyanate is applied to tissue sections from the cutaneous epidermis of the newborn rat, the lectin binds to the surfaces of cells in the layer immediately above the basal layer but not to the cells in the basal layer itself. The latter cells bind the isolectin I-B4, from Griffonia simplicifolia (GS I-B4). The addition of a fucosyl residue to the oligosaccharide of the glycoprotein found on the surface of the basal cell can account for the change in lectin-binding specificity which occurs as the basal cell moves toward the cutaneous surface and becomes a spinous cell. The epidermis of the newborn rat has the necessary transferase to convert a glycoprotein with binding-specificity for GS I-B4 to binding specificity for UEA by adding a fucosyl residue from GDP-L-fucose.

Glutamine fructose-6-phosphate aminotransferase and glycoprotein synthesis in the developing chick eye

Glutamine fructose-6-phosphate aminotransferase is an essential enzyme in the synthesis of glycoproteins and other complex carbohydrates. The specific activity of this enzyme was measured in posterior ocular tissues of the chick embryo from day 6 of development until hatching. The enzyme is present at high concentrations in the early embryo, then decreases 15-fold by day 9 in both retina and retinal pigment epithelium (PE). Thus, aminotransferase activity is highest during the period when retinal cells are migrating and beginning to differentiate; these maturational events may require cell-surface glycoproteins. On day 17-18 of chick embryo development, the neural retina suddenly adheres firmly and irreversibly to the underlying PE. Aminotransferase activity and glucosamine incorporation into retinal macromolecules were examined near this critical time, as were the effects of glycoprotein-synthesis inhibitors upon retinal adhesion. No evidence was found for involvement of complex carbohydrates (on cell surfaces or in the interphotoreceptor matrix, IPM) in formation of retinal adhesion in the chick. Histological studies show an absence of glycosaminoglycans in the IPM of embryonic chicks (at all stages) and of adult chickens, although these compounds are present in mammalian matrix.

Inaccessibility of certain Ricinus lectin binding sites due to the increase in hyaluronic acid during chick embryo development

Chick embryo fibroblasts constitute a useful model for investigating cell surface differentiation using Ricinus lectin as a marker. Fibroblasts from 8-day chick embryos had two classes of Ricinus lectin binding sites, whereas those from 16-day embryos displayed only one class. Hyaluronidase treatment of fibroblasts from 8-day embryos had no effect on their capacity to bind Ricinus lectin; however after this treatment, 16-day cells resembled 8-day cells since the former also exhibited two classes of lectin-binding sites. Treatment with hyaluronidase released 2-5 times more hyaluronic acid from the older cells than from the younger cells. The same hyaluronidase treatment did not change the number of 8-day cells detached by trypsin from the substrate, but increased the number of detached 16-day cells. These observations suggest (i) that the greater adhesiveness to the substrate of the 16-day cells might be due to the presence on the cell surface of a larger amount of glycosaminoglycans at 16 days than at 8 days, and (ii) that the increased accumulation of hyaluronic acid on the cell surface might be involved in an alteration in the cell membrane during differentiation.

Purification and properties of bovine liver plasma membrane 5' nucleotidase

5'-Nucleotidase from bovine liver plasma membranes has been extracted by the zwitterionic detergent sulfobetaine 14, and purified to apparent homogeneity. Two affinity chromatographies on concanavalin-A-Ultrogel and 5' AMP-Sepharose 4B followed by AcA-54-Ultrogel filtration resulted in a purification of 16000 times relative to the homogenate. Sodium dodecyl sulphate gel electrophoresis indicates that the apparent molecular weight of the subunit is 70000. Cross-linking of the native enzyme with dimethylpimelimidate followed by gel electrophoresis shows a band with an apparent molecular weight of 140000 indicating that the enzyme is a dimer. 5'-Nucleotidase is a glycoprotein and its activity is inhibited to different degrees by various lectins, indicating a direct interaction with the enzyme. The purified enzyme shows a sevenfold greater affinity for AMP than the membrane-bound enzyme. The optimum activity of the purified enzyme occurs at pH 7.5 while the membrane-bound enzyme showed a wide range of pH optimum (7.5-8.3). An Arrhenius plot of the membrane-bound enzyme shows a break at 28 degrees C, which disappears in the purified enzyme. The enzyme was inhibited by EDTA, and this inhibition was reversed by divalent cations. This, as well as other evidence, indicates that the enzyme contains a highly bound metal cation, perhaps Mn2+ or Mg2+.

Developmental changes in the oligosaccharide composition of synaptic junctional glycoproteins

Synaptic junctions (SJs) were isolated from the forebrains of rats ranging in postnatal age from 10 days to greater than 1 year. SJ glycoproteins that react with Concanavalin A (Con A) were isolated by chromatography on Con A-agarose and separated by gel electrophoresis. The concentrations of the major SJ Con A binding (Con A+) glycoproteins (apparent Mr 180,000, 130,000, and 110,000) increased between 10 and 28 days, with GP180 and GP110 showing greater relative increases than GP130. Con A binding oligosaccharides associated with 10-day SJs were sensitive to digestion with endoglycosidase C11 and alpha-mannosidase, indicating that they were of the high-mannose type, as previously shown for 28-day SJs. Con A+ oligosaccharides from rats of increasing postnatal age were analyzed by chromatography on Biogel P-4. Two major oligosaccharides, containing five and eight mannose residues, were present in SJs of all ages examined. During development the ratio of man5 to man8 oligosaccharides increased, so that man5 constituted the predominant species in 28-day and adult SJs. Peptide mapping experiments showed that GP180, GP130, and GP110 were each associated with a unique polypeptide composition. Little or no change in peptide composition of the major SJ glycoproteins occurred during development.

Role of glycoproteins in neuronal differentiation. Inhibition of neurite outgrowth and the major cell surface glycoprotein of murine neuroblastoma cells by a purified tunicamycin homologue

Mouse neuroblastoma cells in culture can be induced to differentiate morphologically by serum deprivation or by dibutyryl cyclic AMP (db-cAMP), e.g. they appear flattened, adhere more firmly to the culture substratum and extend long neuritic processes, and thus represent a widely used model system for neuronal cells. This differentiation is accompanied by modulation of cell surface components, such as the induction of a high molecular weight (HMW) glycoprotein (200 kD). We have studied the role of glycoproteins in the process of neuronal differentiation, using a purified homologue of the antibiotic tunicamycin (Al-tunicamycin) and neuroblastoma N115 cells grown in culture. Al-tunicamycin markedly inhibited (up to 60-75%) the incorporation of radioactively labelled sugars into cellular proteins of differentiating neuroblastoma cells. Concomitantly, the cells altered their morphology, they became rounded and less adhesive and retracted their neurites. Changes in the appearance, glycosylation and electrophoretic mobility of several cellular and secreted glycoproteins were observed, when cells were incubated in the presence of Al-tunicamycin. The most striking effect of Al-tunicamycin on the composition of cellular glycoproteins was the marked reduction in appearance of the 200 kD glycoprotein. The findings suggest that glycoproteins and in particular the neuron-specific 200 kD glycoprotein, are related to morphological differentiation processes, mainly to cellular adhesion and neurite outgrowth.

Peripheral nervous system myelin and Schwann cell glycoproteins: identification by lectin binding and partial purification of a peripheral nervous system myelin-specific 170,000 molecular weight glycoprotein

Radioiodinated lectins were used to detect glycoproteins of peripheral nervous system (PNS) myelin (rat, human, bovine) and cultured rat Schwann cells. Proteins were resolved by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis and transferred to nitrocellulose filters. The filters were overlaid with radioiodinated lectins of known saccharide affinities. These included concanavalin A, Helix pomatia, Limulus polyphemus, Maclura pomifera, peanut, soybean, Ulex europaeus, and wheat germ agglutinins. Inclusion of the appropriate monosaccharide in the overlay solution (0.2 M) inhibited lectin binding to the nitrocellulose-fixed proteins. Fluorography permitted identification of 26 myelin glycoproteins and many more in Schwann cells. All lectins labeled a band present in myelin, but not Schwann cells, corresponding to the major PNS myelin protein, P0. Our attention focused on a high-molecular-weight myelin glycoprotein [apparent molecular weight (Mr) 170,000], which appeared abundant by Coomassie Blue staining and which was heavily labeled by all lectins except concanavalin A. A protein with approximately this Mr and lectin-binding pattern was present in human and bovine PNS myelin as well, but not detected in rat Schwann cells, CNS myelin, liver and fibroblast homogenates, or cultured bovine oligodendroglia. Hence this 170,000 Mr glycoprotein is apparently unique to PNS myelin.

Changes in protein glycosylation during chick embryo development

To investigate the molecular changes in cell-surface glycoproteins during chick embryo development, fibroblasts from 8- and 16-day embryos were extensively digested by pronase after (i) metabolic labeling with radioactive precursors and (ii) external labeling. Two main classes of glycopeptide pronase digestion product were distinguished by Sephadex G-50 column chromatography. The large material excluded was mostly composed of glycosaminoglycans. The small retarded glycopeptides underwent age-related modifications. Those in the 8-day cells were mainly N-linked, whereas 16-day cells contained both O- and N-linked glycopeptides. The evolution of high-mannose chains in younger cells to complex-type chains in the older cells is suggested by (i) the decrease in the mannose-to-galactose and mannose-to-N-acetylglucosamine ratio with embryo development, and (ii) the fact that endo-beta-N-acetylglucosaminidase H treatment released more oligomannosyls from younger than from older embryo cell glycopeptides. Small glycopeptides were also more highly sialylated in 16-day cells than in 8-day cells. The present results provide the first biochemical evidence that both quantitative and qualitative modifications occur in cell-surface glycoconjugates during the late stages of chick embryo development.

Sulfated glycan present in the EDTA extract of Hemicentrotus embryos (mid-gastrula)

Light microscopical observations of the Alcian blue-stained gastrulae of Hemicentrotus pulcherrimus together with the scanning electron microscopical observations of the embryos revealed the presence of highly acidic glycans in the invaginating archenteron (inside surface), the surrounding of secondary mesenchyme cells (pseudopodial protrusions and filamentous structures) and the hyaline layer. In the embryos grown in sulfate-free sea water and thus with arrested gastrulation it was found that the dye stainability in the above regions was markedly reduced. The glycosaminoglycan fraction prepared from the whole embryos (mid-gastrulae) was found to contain various kinds of acidic glycans as analysed by chromatography on DEAE-cellulose. Among these glycan components, the "F" component was mainly recovered in the EDTA extract of the embryos, and was shown to be specifically deleted in the embryos grown in sulfate-free sea water, suggesting that the "F" component may be related to the Alcian blue-stainable material in Hemicentrotus embryos. The component "F" was found to consist of sulfated fucan and acid mucopolysaccharide (unidentified) chains, which are probably linked to a common peptide core, forming macromolecules with larger than 10(6) molecular weights.

Presence of fibronectin during early embryogenesis in amphibian Pleurodeles waltlii

The presence of fibronectin (FN) has been investigated during the early development of Pleurodeles waltlii by using indirect immunofluorescence and gel electrophoresis methods. Between the early blastula and late gastrula stages, fluorescent staining was detected on cell surface, especially in cells from the blastocoele roof and the marginal zone. Through gastrulation, extracellular material which was accumulated in the area of the blastopore occurred heavily stained for FN. Sodium metaperiodate-sodium boro[3H]hydride has been used to label the surface carbohydrate residues. Labelled glycoproteins were characterized by sodium dodecyl sulphate and two-dimensional polyacrylamide gel electrophoresis coupled with fluorography. Autoradiograms revealed a 220K cell surface glycoprotein which was co-migrating with monomers of FN. The results suggested that FN may be involved in the early morphogenesis of the amphibian embryo.