Inhibition of glycoprotein processing blocks assembly of spicules during development of the sea urchin embryo

Previous studies have implicated an 130-kD glycoprotein containing complex, N-linked oligosaccharide chain(s) in the process of spicule formation in sea urchin embryos. To ascertain whether the processing of high mannose oligosaccharides to complex oligosaccharides is necessary for spiculogenesis, intact embryos and cultures of spicule-forming primary mesenchyme cells were treated with glycoprotein processing inhibitors. In both the embryonic and cell culture systems 1-deoxymannojirimycin (1-MMN) and, to a lesser extent, 1-deoxynojirimycin (1-DNJ) inhibited spicule formation. These inhibitors did not affect gastrulation in whole embryos or filopodial network formation in cell cultures. Swainsonine (SWSN) and castanospermine (CSTP) had no effect in either system. Further analysis revealed the following: (a) 1-MMN entered the embryos and blocked glycoprotein processing in the 24-h period before spicule formation as assessed by a twofold increase in endoglycosidase H sensitivity among newly synthesized glycoproteins upon addition of 1-MMN; (b) 1-MMN did not affect general protein synthesis until after its effects on spicule formation were observed; (c) Immunoblot analysis with an antibody directed towards the polypeptide chain of the 130-kD protein (mAb A3) demonstrated that 1-MMN did not affect the level of the polypeptide that is known to be synthesized just before spicule formation; (d) 1-MMN and 1-DNJ almost completely abolished (greater than 95%) the appearance of mAb 1223 reactive complex oligosaccharide moiety associated with the 130-kD glycoprotein; CSTP and SWSN had much less of an effect on expression of this epitope. These results indicate that the conversion of high mannose oligosaccharides to complex oligosaccharides is required for spiculogenesis in sea urchin embryos and they suggest that the 130-kD protein is one of these essential complex glycoproteins.

Tumor cell surface beta 1-4-linked galactose binds to lectin(s) on microvascular endothelial cells and contributes to organ colonization

Cell surface carbohydrate structures acting as ligands for tissue specific mammalian lectins have been implicated in cell-cell interactions during embryogenesis, lymphocyte homing, and tumor cell metastasis. In this report, we provide evidence that beta 1-4 linked galactose (Gal) residues in N-linked oligosaccharides on the surface of blood born tumor cells serve as a ligand for binding to microvascular endothelial cells. D36W25, a class 1 glycosylation mutant of the MDAY-D2 lymphoreticular tumor cell line, lacks sialic acid and Gal in cellular glycans due to a defect in the Golgi UDP-Gal transporter. Using UDP-Gal and bovine galactosyltransferase in vitro, beta 1-4 Gal was restored to the surface of the cells and 70% of the galactosylated glycans persisted for 8 h in vitro at 37 degrees C. Compared to mock-treated D36W25 cells, galactosylated D36W25 cells showed an 80% increase in binding to microvascular endothelial cell monolayers in vitro. The enhanced binding of galactosylated D36W25 cells to endothelial cell was inhibited by the addition of lactosamine-conjugated albumin to the assay. Consistent with these observations, swainsonine and castinospermine, two inhibitors of N-linked processing that result in loss of lactosamine antennae inhibited the binding of wild-type MDAY-D2 cells to endothelial cells in vitro. Injection of radiolabeled tumor cells into the circulation of syngeneic mice, showed that galactosylation of D36W25 cells resulted in 2-3 more tumor cells retained in the lungs and livers. In addition, galactosylation of D36W25 cells increased by 30-fold the number of visible liver metastases on inspection 4 wk after tumor cell injection. These results suggest that beta 1-4Gal-binding lectins on microvascular endothelial cells can contribute to retention and secondary tumor formation of blood born tumor cells. With the increasing availability of purified glycosyltransferases, reconstruction of a variety of carbohydrate sequences on the surface of class 1 mutants provides a controlled means of studying carbohydrate-lectin interactions on viable cells.

N-linked glycoprotein synthesis and transport during G1 are necessary for astrocytic proliferation

The proliferation of astrocytes, purified by a selective detachment technique from mixed glial primary cultures derived from newborn rat cerebrum, was studied. The cells were synchronized by first inducing a quiescent state by removing fetal calf serum (FCS) from the culture medium for 2 days; reversal of the quiescent state by return of serum to the culture medium caused a marked increase in DNA synthesis 12-24 hr later. 2-Deoxyglucose, an inhibitor of dolichol-linked oligosaccharide and thereby N-linked glycoprotein biosynthesis, prevented not only an increase in glycoprotein biosynthesis in G1 phase of the cell cycle but also the burst of DNA synthesis that followed during S phase. Addition of mannose to the culture medium prevented the inhibitions by deoxyglucose of both glycoprotein and DNA syntheses. These data indicated an obligatory relationship in astrocytes between dolichol-linked glycoprotein synthesis and DNA synthesis. To determine whether transport of the newly synthesized glycoproteins to the plasma membrane for incorporation therein or for secretion were necessary for DNA synthesis and astrocytic proliferation, we studied cells treated with monensin, an ionophore for monovalent cations, and an inhibitor of intracellular transport of glycoproteins. The presence of monensin in the first 12 hr after repletion of serum to synchronized astrocytes prevented progression to the S phase and cell proliferation; addition of monensin after the first 12 hr, at the onset of the S phase, had no effect on progression through S phase. Lectin-staining methods combined with fluorescence microscopy demonstrated in monensin-treated cells failure of intracellular glycoproteins to be transported to the plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of (+)-(7S,SS)-1,2,3,5,6,7-hexahydro-7-methyl-8-(p-tolylsulfinyl)-5-indoli zinone

C16H19NO2S, Mr = 289.39, orthorhombic, P2(1)2(1)2(1), a = 11.467 (5), b = 17.207 (8), c = 7.566 (4) A, V = 1493 (2) A3, Z = 4, Dx = 1.29 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 2.07 cm-1, F(000) = 616, T = 296 K, R = 0.049, 928 unique observed reflections. The structure determination combined with the known S configuration at the sulfur site discloses the stereochemistry of the title compound to be 7S and SS.

Pumiliotoxin alkaloids: a new class of sodium channel agents

Pumiliotoxin B (PTX-B) and a variety of congeneric alkaloids and synthetic analogs stimulated sodium flux and phosphoinositide breakdown in guinea pig cerebral cortical synaptoneurosomes. The effects of PTX-B and active congeners and analogs on sodium flux in synaptoneurosomes were potentiated markedly by scorpion venom (Leiurus quinquestriatus). In neuroblastoma cells, PTX-B and active congeners had no effect on sodium flux unless synergized by alpha-scorpion toxin or scorpion venom. Certain inactive congeners, lacking hydroxyl groups in the 6-alkylidene side chain, inhibited sodium flux elicited by PTX-B, scorpion venom, or the sodium channel activator batrachotoxin. Such inhibition appeared different from inhibition by local anesthetics, since pumiliotoxins, unlike local anesthetics, had little or no effect on binding of [3H]batrachotoxinin A benzoate to sodium channels. Thus, it appears likely that some "inactive" congeners bind to the PTX-B binding site, but do not activate sodium channels. In the absence of scorpion venom the stimulation of phosphoinositide breakdown in synaptoneurosomes was consonant with the stimulatory effects of these compounds on sodium flux through voltage-dependent sodium channels.

Synthesis of 1-deoxy-6-epicastanospermine and 1-deoxy-6,8a-diepicastanospermine

Extension of the carbon spine of 2,3;4,5-di-O-isopropylidene-beta-D-fructopyranoside by oxidation at C-1 followed by a Wittig reaction using the phosphorane Ph3P = CHCO2Et gave an oct-4-ulose derivative, which was then transformed into the key intermediate 1-azido-1,2,3-trideoxy-D-arabino-oct-4-ulose. Catalytic hydrogenolysis of this azide, followed by reductive amination between the resulting 1-amino substituent and the 4-keto-group then gave a mixture of pyrrolidines. After sulphonylation at the terminal 8-position, the pyrrolidines were then cyclised further between the nitrogen and C-8 to give 1-deoxy-6-epicastanospermine and 1-deoxy-6,8a-diepicastanospermine.

Post-translational processing of the glycoproteins of lymphocytic choriomeningitis virus

Intracellular events in the synthesis, glycosylation, and transport of the lymphocytic choriomeningitis virus (LCMV) glycoproteins have been examined. We have shown by N-glycanase digestion that LCMV strain Arm-4 bears five oligosaccharides on GP-1 and two on GP-2. By pulse-chase labeling experiments in the presence of drugs which inhibit N-linked oligosaccharide addition and processing we demonstrate that addition of high mannose precursor oligosaccharides is necessary for transport and cleavage of the viral GP-C glycoprotein. Moreover, in the presence of tunicamycin which inhibits en bloc addition of these mannose-rich side chains, virus budding was substantially decreased and infectious virions were reduced by more than 1000-fold in the supernatant medium. Incubation in the presence of castantospermine, which permits addition of oligomannosyl-rich chains but blocks further processing, restored transport and cleavage of GP-C and maturation of virions. Finally, by temperature block experiments we have determined that maturation of GP-C oligosaccharides to an endoglycosidase H resistant form precedes cleavage to GP-1 and GP-2. The latter process is most likely to occur in the Golgi or post-Golgi compartment.

The structural basis of the inhibition of human glycosidases by castanospermine analogues

A series of epimers and deoxy derivatives of castanospermine has been synthesized to investigate the contribution of the different chiral centres to the specificity and potency of inhibition of human liver glycosidases. Castanospermine inhibits all forms of alpha- and beta-D-glucosidases, but alteration to any of the five chiral centres in castanospermine markedly decreases the inhibition. 6-Epicastanospermine, which is related to D-pyranomannose in the same way as castanospermine is to D-pyranoglucose, does not inhibit lysosomal (acidic) alpha-mannosidase, but is a good inhibitor of the cytosolic or neutral alpha-mannosidase. Conversely, 1-deoxy-6-epicastanospermine inhibits acidic alpha-mannosidase strongly, but not the neutral alpha-mannosidase. An explanation of this different inhibition based on preferential recognition of different configurations of mannose by the different forms of alpha-mannosidase is postulated. All derivatives of 6-epicastanospermine also have the minimum structural feature for the inhibition of alpha-L-fucosidase, but those with a beta-anomeric substituent do not inhibit the enzyme, or do so very weakly. 1-Deoxy-6,8a-diepicastanospermine, which has four chiral centres identical with alpha-L-fucose, is, however, a potent inhibitor of alpha-L-fucosidase (Ki 1.3 microM).

Castanospermine-glucosides as selective disaccharidase inhibitors

Castanospermine (CS) is a potent but non-selective inhibitor of many glycohydrolases including the intestinal disaccharidases. Several CS-glucosides were synthesized to investigate the effect of an attached glucopyranosyl residue on the potency and selectivity of CS toward inhibition of intestinal disaccharidases. 8 alpha-glucosyl-CS and 7 alpha-glucosyl-CS were nearly as potent against sucrase activity as CS (IC50 values = 30, 40, and 20 nM respectively) but were 1/50 or less as potent as CS against lactase and trehalase activities. 8 beta-glucosyl-CS was 1/20 to 1/140 as potent as CS and 1 alpha-glucosyl-CS was 1/57 to 1/1500 as potent as CS against disaccharidase activities. 1 alpha-glc-CS was less selective than CS, whereas the other CS-glucosides were more selective. 7 alpha-glc-CS and 8 alpha-glc-CS were the most sucrase selective and were particularly ineffective against trehalase and lactase activities. 8 beta-glc-CS was similar to CS except for relatively weaker trehalase inhibition. In summary, selectivity toward certain disaccharidases was achieved by glucosylation of CS hydroxyls. However, a simple structural comparison of the CS-glucoside to a disaccharide substrate did not reliably predict which disaccharidase would be more inhibited by the CS-glucoside.

Histochemical localization and quantification of alpha-glucosidase in the epididymis of men and laboratory animals

The seminal marker of epididymal function alpha-1, 4-glucosidase was localized histochemically in the cytoplasm of the efferent duct epithelium and the brush border of the entire length of the human epididymis. Quantification using the specific inhibitor castanospermine revealed strongest activity in the corpus and cauda regions. Selective inhibition of the brush border enzyme activities by maltotriose identified these as the neutral isoenzymes. Despite detection of alpha-glucosidase in the renal tubules of all the animals studied, the enzyme was not detectable in epididymides of hamsters or mice. In rabbits and monkeys, it was absent from the entire brush border but present weakly in the cytoplasm of the proximal epididymides. An enzyme distribution pattern similar to that in the human epididymis was found in rats, except for the absence of histochemical staining at pH 6.5 from the initial segment and distal cauda epididymidis. Experiments in which endogenous testosterone was depleted in rats demonstrated the dependence of epididymal alpha-glucosidase on androgen, albeit with a low sensitivity. This study suggests the rat to be a suitable model for the investigation of the role of epididymal alpha-glucosidase in fertility.

Role of glycosylation in the transport of recombinant glycoproteins through the secretory pathway of lepidopteran insect cells

Cell lines established from the Lepidopteran insect Spodoptera frugiperda (e.g., Sf9) are used routinely as hosts for the expression of foreign proteins by baculovirus vectors. Previously, we showed that human tissue plasminogen activator (t-PA) was expressed, N-glycosylated, and secreted by Sf9 cells infected with a recombinant baculovirus (Jarvis DL, Summers MD: Mol Cell Biol 9:214-223, 1989). We also showed that t-PA secretion was blocked by tunicamycin (TM), an inhibitor of N-glycosylation, but not by castanospermine (CS) or N-methyldeoxynojirimycin, inhibitors of the initial steps in N-linked oligosaccharide processing. This suggested that the addition, but not the processing, of N-linked oligosaccharides is required for the secretion of recombinant t-PA from baculovirus-infected Sf9 cells. In this study, we present a more generalized evaluation of the role of N-glycosylation in the transport of recombinant glycoproteins through the Sf9 cell secretory pathway. Several different secretory or membrane-bound glycoproteins were expressed in control, TM-treated, or CS-treated Sf9 cells, and their appearance in the medium or on the cell surface was measured. The results showed that TM blocked the transport of some, but not all, of these proteins, whereas CS did not block the transport of any. This suggests that N-glycosylation is sometimes required for the transport of recombinant glycoproteins through the Sf9 secretory pathway, while processing of the oligosaccharides is not. At least two other proteins, p80 and p31, consistently coimmunoprecipitated with the nonglycosylated precursors of recombinant glycoproteins expressed in TM-treated Sf9 cells. Neither was antigenically related to any of the recombinant proteins. Relatively larger amounts of p80 and p31 were coprecipitated when transport was completely blocked by TM compared to when transport was only reduced or was unaffected. These results suggest that p80 and p31 block the transport of some nonglycosylated glycoprotein precursors in TM-treated Sf9 cells by binding to them and producing transport-incompetent heterooligomeric complexes. If this speculation is correct, then p80 and p31 are functionally analogous to the mammalian immunoglobulin heavy chain binding/glucose-regulated 78 kilodalton protein (BiP/GRP78).

Effects of deoxymannojirimycin and castanospermine on the polarized secretion of thyroglobulin

In order to better explore a possible role of oligosaccharide structures in the polarized secretion of thyroglobulin (Tg), we labeled cultured porcine thyroid cells with L-[3,4,5-3H]leucine or D-[2-3H]mannose in the presence or absence of the following inhibitors of N-linked oligosaccharide processing: deoxymannojirimycin (dMM), an inhibitor of mannosidase I expected to give rise to high mannose units exclusively, thus abolishing the formation of lactosaminyl branches, and castanospermine (Cs), an inhibitor of glucosidases which is expected to produce glucosylated high mannose oligosaccharides. [3H]Leucine pulse-chase experiments were performed to study the rate of Tg secretion in the presence and absence of dMM (1 mM) or Cs (1.6 mM). After a 15-h chase period, dMM and Cs did not modify the relative proportions of released Tg (65% in the follicular content, 16% in the medium) and cellular Tg (19%). In contrast, Cs led to a lower rate of secretion in both secretory pathways (t1/2 increased from 42 min to 105 min in the apical pathway and from 108 min to 138 min in the basal pathway). After a long-term labeling (16 h) with [3H]leucine or [3H]mannose, drugs did not notably affect the relative proportions of labeled Tg in the different compartments (follicular content, medium, and cell). The structures of N-glycans borne by Tg, with and without drugs, were checked. After Tg immunoprecipitation, pronase-glycopeptides were fractionated on concanavalin A-Sepharose 4B. The glycopeptides tightly bound to the lectin were treated by endo-beta-N-acetylglucosaminidase H and oligosaccharides separated by HPLC. With dMM, complex-type glycans of Tg were totally replaced by high mannose-type glycans, Man8-9GlcNAc; Cs induced the accumulation of glucosylated high mannose-type structures, Glc3Man7-9GlcNAc and Glc2Man8-9GlcNAc, but the action of this inhibitor was not total. In conclusion the correct secretion of Tg does not require the presence of the sialyllactosaminyl structure; the presence of glucose residues on high mannose-type structures lowers the rate of exocytosis.

The effect of castanospermine on the synthesis of synaptic glycoproteins by rat brain slices

Slices were prepared from rat forebrains and the incorporation of [3H]mannose and [35S]methionine into proteins and glycoproteins determined. The incorporation of methionine continued to increase for up to 8 hours whereas mannose incorporation was maximal between 2 and 4 hours and declined thereafter. Glycopeptides prepared by pronase digestion of [3H]mannose-labeled glycoproteins were digested with endoglucosaminidase H (endo H) and analysed by gel filtration. The major endo H-sensitive oligosaccharide eluted in a position similar to standard Man8GlcNAc. In the presence of castanospermine, which inhibits glucosidase I, the first enzymatic step in the processing of N-linked oligosaccharides, a new endo H-sensitive glycan similar in size to standard Glc3Man9GlcNAc2 accumulated. Synaptic membranes (SMs) were isolated from slices which had been incubated with either [3H]mannose or [35S]methionine in the presence and absence of castanospermine. In the presence of inhibitor the relative incorporation of [3H]mannose into high-mannose glycans of synaptic glycoproteins was increased. The incorporation of newly synthesized, [35S] methionine-labeled, Con A-binding glycoproteins into SMs was not affected by the addition of inhibitor. Many of the glycoproteins synthesized in the presence of castanospermine exhibited a decreased electrophoretic mobility indicative of the presence of altered oligosaccharide chains. The results indicate that changes in oligosaccharide composition produced by castanospermine had little effect on the subsequent transport and incorporation of glycoproteins into synaptic membranes.

A block at Man5GlcNAc2-pyrophosphoryldolichol in intact but not disrupted castanospermine and swainsonine-resistant Chinese hamster ovary cells

A mutation in glycoprotein processing inhibitor-resistant (PIR) Chinese hamster ovary (CHO) cells was previously shown to result in a block at the Man5GlcNAc2 stage of the dolichol-oligosaccharide biosynthetic pathway (Lehrman, M.A., and Zeng, Y. (1989) J. Biol. Chem. 264, 1584-1593). These cells had normal mannose-P-dolichol synthase activity and were able to transfer the Man5GlcNAc2 oligosaccharides to protein. We have now characterized the mutation in greater detail. In PIR cells, biosynthesis of GDP-mannose and mannose-P-dolichol was normal, and pulse-chase analysis indicated that the rate of Man5GlcNAc2-P-P-dolichol formation in vivo was similar to that in parental CHO cells but without subsequent formation of larger intermediates. Cell fusion studies demonstrated that the PIR genotype was recessive and that PIR cells could complement the mutation in B4-2-1 cells, which fail to synthesize mannose-P-dolichol. In contrast to the results obtained with intact cells, incubation of membrane preparations of PIR cells with GDP-[3H]mannose resulted in the synthesis of intermediates containing up to 9 mannose residues, indicating that the cells contained active mannosyltransferases VI to IX. With a simplified assay for the formation of intermediates containing 6 to 9 mannoses, it was shown that physical disruption of PIR cells was able to eliminate the block at the pentamannosyl stage. Furthermore, although the temperature requirements of the reactions for the control CHO and PIR membranes were similar, Man5GlcNAc2-elongating activity in CHO membranes was inhibited by alkaline pH treatment, whereas this treatment irreversibly stimulated the activity in PIR membranes. Taken together, these results suggest that the PIR cells have a recessive defect, and that the missing gene product is required by mannosyltransferase VI in vivo for proper utilization of either mannose-P-dolichol or Man5GlcNAc2-P-P-dolichol. Since the defect was manifested in vivo but not in vitro, this requirement appears necessary for intact cells but not for disrupted cells or isolated membranes.

Biosynthesis and processing of bovine cartilage link proteins

We have examined posttranslational modifications which are responsible for converting an apparently single precursor (Hering, T. M., and Sandell, L. J. (1988) J. Biol. Chem. 263, 1030-1036) to the two major forms of link protein in bovine articular cartilage. Resistance to endoglycosidases H and F suggests that Asn-linked oligosaccharides of link protein secreted by bovine chondrocytes in culture are of the complex or hybrid type. There is no evidence for O-linked oligosaccharides. There is no apparent precursor-product relationship between link protein (LP)1 and LP2, since after a short pulse with [3H]leucine two forms are present, consistent with the existence of two glycosylation sites. An immunoprecipitate of LP1 from pulse-labeled chondrocytes was observed to show a decrease in electrophoretic mobility and increased microheterogeneity during transit through the Golgi, whereas LP2 did not change. During processing both LP1 and LP2 become endoglycosidase H resistant. LP1, but not LP2, can be biosynthetically labeled with [35S]sulfate. Incorporation of [35S]sulfate is inhibited by tunicamycin, indicating that the sulfate is associated with Asn-linked carbohydrate. Sulfation may be important for normal processing, secretion, or degradation of link protein and with sialylation may confer considerable charge heterogeneity upon LP1. We conclude that there are considerable biochemical differences between glycoproteins LP1 and LP2 which may provide a basis for functional differences.

Effect of glycosylation inhibitors on N-glycosylpeptides and on invasion of malignant mouse MO4 cells in vitro

Cell surface glycans are believed to play a role in tumour invasion and metastasis. Yet, we have previously shown that the inhibitors of N-linked glycan processing swainsonine (SW) and 1-deoxynojirimycin (dNM) did not prevent invasion of chick heart fragments by MO4 murine fibrosarcoma cells in organ culture. We now present biochemical evidence that these and other inhibitors of processing were indeed effective in remodeling glycans, including those expressed at the cell surface. After metabolic labeling with tritiated mannose or fucose, glycosylpeptides were obtained by Pronase treatment of material released from intact cells by trypsin. Glycosylpeptides were separated by Biogel P-10 chromatography. With all drugs tested, there was a shift towards lower molecular weight of the glycan chains. There were, however, major quantitative differences between the different drugs and also, for monensin (MON; 0.1 microgram ml-1), between fucose-labeled and mannose-labeled chains. The shift in apparent molecular weight affected mainly fucose-labeled peptides after treatment of MO4 cells with SW (0.4 microgram ml-1). The shift induced by dNM (10 mM) + SW (0.4 microgram ml-1) in both fucosylated and mannosylated chains was much larger than that induced by SW given alone. 1-Deoxymannojirimycin (dMM; 1 mM) had major effects on both mannose and fucose-labeled structures and so did N-methyl-1-deoxynojirimycin (MdNM; 2 mM) and castanospermine (CS; 100 micrograms ml-1). With the latter drugs, incorporation of fucose in complex-type glycosylpeptides was dramatically reduced. The effect of SW on fucose-labeled glycosylpeptides of embryonic chick heart was similar to that observed on MO4 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific N-linked oligosaccharides are required for oligodendroglial differentiation but probably not for astrocytic differentiation

Primary cultures of cerebral glia derived from neonatal rat brain were utilized to determine whether specific glycoproteins are involved in oligodendroglial and astrocytic differentiation. Specific emphasis was placed on the oligosaccharide portion of glycoproteins, and inhibitors of glycoprotein processing were studied. Castanospermine, an inhibitor of glucosidase I, and thereby formation of both complex glycoproteins and high mannose glycoproteins, and deoxymannojirimycin (DMM), an inhibitor of mannosidase I and thereby formation of complex glycoproteins, were utilized. Castanospermine exposure prevented the developmental inductions of the two oligodendroglial markers, 2',3'-cyclic nucleotide 3'-phosphohydrolase and glycerol-3-phosphate dehydrogenase. The effect of castanospermine on oligodendroglial differentiation was reversible. In contrast, castanospermine had no effect on the developmental inductions of the two astrocytic markers, glutamine synthetase and lactate dehydrogenase. DMM exposure had no effect on either oligodendroglial or astrocytic differentiation. Although both inhibitors caused a marked decrease in the formation of complex glycoproteins and an increase in high mannose structures, the oligosaccharide composition of these high mannose structures differed markedly. Castanospermine caused an increase in 'abnormal', apparently glucosylated high mannose structures and a decrease in all other 'normal' high mannose oligosaccharides, whereas DMM caused an increase in most high mannose structures, especially those migrating in the region of the Man7GlcNAc standard. The data indicate that oligodendroglial differentiation requires specific N-linked oligosaccharides, probably principally of the high mannose type, and that astrocytic differentiation can proceed normally despite marked alterations in both complex and high mannose glycoproteins.

Quantitative relationship between intestinal sucrase inhibition and reduction of the glycemic response to sucrose in rats

We have investigated the quantitative relationship between sucrase inhibition and reduction in the 0-3 h glycemic response to an oral dose of sucrose in rats. Castanospermine is a quasi-irreversible sucrase inhibitor that did not dissociate from sucrase during tissue preparation or assay for sucrase activity. An oral dose of castanospermine (0.1-3.0 mg/kg body wt) dose-dependently reduced sucrase activity of intestinal segments by 15-90%; 0.4 mg/kg body wt reduced total sucrase activity about 50%. The lower doses inhibited sucrase much more extensively in the proximal than in the distal segments. Castanospermine also dose-dependently reduced the 0-3 h glycemic response to sucrose; 1.5 mg/kg body wt reduced the glycemic response about 50%. Each submaximal castanospermine dose inhibited total sucrase activity more than it reduced the glycemic response. We conclude that intestinal sucrase activity in the rat is in modest excess relative to the rate-determining step of glucose absorption following sucrose administration. Fourteen days of castanospermine treatment (0.2 mg.kg body wt-1.d-1) resulted in sucrase inhibition that was similar to a single castanospermine treatment, suggesting that castanospermine treatment resulted in neither cumulative sucrase inhibition nor induction of sucrase activity.

Functional cooperation between the neural adhesion molecules L1 and N-CAM is carbohydrate dependent

The neural cell adhesion molecules L1 and N-CAM have been suggested to interact functionally by formation of a complex between the two molecules (Kadmon, G., A. Kowitz, P. Altevogt, and M. Schachner. 1990. J. Cell Biol. 110:193-208). To determine the molecular mechanisms underlying this functional cooperation, we have studied the contribution of carbohydrates to the association of the two molecules at the cell surface. Aggregation or adhesion between L1- and N-CAM-positive neuroblastoma N2A cells was reduced when the synthesis of complex and/or hybrid glycans was modified by castanospermine. Fab fragments of polyclonal antibodies to L1 inhibited aggregation and adhesion of castanospermine-treated cells almost completely, whereas untreated cells were inhibited by approximately 50%. Fab fragments of polyclonal antibodies to N-CAM did not interfere with the interaction between castanospermine-treated cells, whereas they inhibited aggregation or adhesion of untreated cells by approximately 50%. These findings indicate that cell interactions depending both on L1 and N-CAM ("assisted homophilic" binding) can be reduced to an L1-dominated interaction ("homophilic binding"). Treatment of cells with the carbohydrate synthesis inhibitor swainsonine did not modify cell aggregation in the absence or presence of antibodies compared with untreated cells, indicating that castanospermine-sensitive, but swainsonine-insensitive glycans are involved. To investigate whether the appropriate carbohydrate composition is required for an association of L1 and N-CAM in the surface membrane (cis-interaction) or between L1 on one side and L1 and N-CAM on the other side of interacting partner cells (trans-interaction), an L1-positive lymphoid tumor cell line was coaggregated with and adhered to neuroblastoma cells in the various combinations of castanospermine-treated and untreated cells. The results show that it is the cis-interaction between L1 and N-CAM that depends on the appropriate carbohydrate structures.

Effect of glycosylation inhibitors on the structure and function of the murine transferrin receptor

The murine transferrin receptor is a disulphide-linked dimer with three N-glycosylation sites. We have investigated the structural and functional properties of the transferrin receptor from murine plasmacytoma cells (NS-1 cells) treated with the glycosylation inhibitor, tunicamycin and the glycosylation-processing inhibitors, swainsonine and castanospermine. 1. Tunicamycin (1 microgram/ml) inhibited mannose incorporation in NS-1 cells by greater than 90%, but also inhibited methionine incorporation by up to 50%. Both swainsonine (1 microgram/ml) and castanospermine (50 micrograms/ml) resulted in mannose incorporation greater than 100% of untreated cells and neither drug affected methionine incorporation. 2. Incubation of NS-1 cells with tunicamycin resulted in a shift in the apparent molecular mass of the transferrin receptor from 96 kDa and 94 kDa to approximately 82 kDa. 3. Peptide N-glycosidase F digestion of the receptor from untreated cells resulted in the fully deglycosylated 82 kDa component as well as an 87 kDa component which represents partially deglycosylated receptor resistant to peptide N-glycosidase F digestion. 4. The receptor from swainsonine-treated cells was equally sensitive to peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase H (endo H; resulting in both 87-kDa and 82-kDa components), whereas the receptor from castanospermine-treated cells was only partially sensitive to endo H. 5. Analysis of mannose- and fucose-labelled cellular glycopeptides by concanavalin-A--Sepharose chromatography showed that swainsonine (1 microgram/ml) treatment resulted in approximately 90% inhibition of the synthesis of complex N-glycans and an accumulation of fucosylated hybrid structures. In contrast, castanospermine (100 micrograms/ml) treatment resulted in only partial inhibition (60%) of the synthesis of complex N-glycans. 6. Analysis of the receptor from tunicamycin, swainsonine and castanospermine treated cells under nonreducing conditions showed a single component corresponding to the dimer, indicating that dimerisation of newly synthesised murine receptor is independent of carbohydrate. 7. The non-glycosylated receptor from tunicamycin-treated cells appears to bind transferrin as demonstrated by interaction with transferrin-Sepharose. 8. Surface expression of the receptor was not significantly altered in the presence of either swainsonine or castanospermine as judged by flow cytometry.

Inhibition of yeast exoglucanases by glucosidase inhibitors

Castanospermine, 1-deoxynojirimycin, and N-methyl-1-deoxynojirimycin, three well-characterized inhibitors of the glucosidases involved in the processing of N-linked oligosaccharides, did not affect the biosynthesis or the secretion of exoglucanases (EC 3.2.1.58) from Saccharomyces cerevisiae and Candida albicans but inhibited the activity itself. Regardless of the substrate used, laminarin or p-nitrophenyl beta-D-glucoside (pNPG), all three inhibitors proved to act in a competitive manner. Castanospermine was the most potent inhibitor, with Ki values ranging from 0.16 to 0.5 microM for three different purified yeast exoglucanases. The inhibition caused by 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin was poorer, but still significant. By contrast, the glucosidase inhibitors did not show any action on a partially purified endoglucanase (EC 3.2.1.39) Candida albicans. A purified exoglucanase from Basidiomycete QM 806, which was specific for laminarin, was unaffected by castanospermine but it was still inhibited in an uncompetitive manner by 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin. The presence of castanospermine in the culture medium of growing yeasts did not have any effect on yeast growth in spite of the fact that, under the conditions used, the external exoglucanase was fully inhibited. None of the yeast exoglucanases hydrolyzed the glucan synthesized in vitro by membrane preparations derived from either yeast. These results support the concept that yeast exoglucanases are glucosidases that also attack laminarin, rather than glucanases capable of attacking pNPG.

Antibody-dependent cytotoxicity mediated by natural killer cells is enhanced by castanospermine-induced alterations of IgG glycosylation

Inhibitors of glycosylation and carbohydrate processing were used to probe the functional consequences of specific, differential alterations in glycosylation of monoclonal IgG secreted by hybridoma clones. Neither the absence of glycosylation nor the presence of atypical oligosaccharides significantly influenced binding of the monoclonal antibody to the cell surface antigen recognized. However, lymphocyte-mediated antibody-dependent cytotoxicity was enhanced significantly, as compared to native (unmodified) IgG-sensitized target cells, when target cells were sensitized with IgG bearing the atypical oligosaccharides induced metabolically by castanospermine, N-methyldeoxynojirimycin, deoxymannojirimycin or monesin, but not by swainsonine. The enhanced cytotoxicity was mediated by natural killer cells but not by monocytes or interferon-activated polymorphonuclear leukocytes. By contrast, antibody-dependent cytotoxicity mediated by activated polymorphonuclear leukocytes against target cells sensitized with the IgG glycosylation phenotypes induced by swainsonine and tunicamycin, but not by castanospermine, was decreased in comparison to cytotoxicity against target cells sensitized with native IgG. The enhanced lymphocyte-mediated cytotoxicity was Fc receptor-dependent. A panel of monoclonal antibodies directed against different human tumor target cells was used to demonstrate that the castanospermine-induced IgG phenotype generally enhanced antibody-dependent tumoricidal activity mediated by natural killer cells. However, differences in lymphocyte response to an alteration in IgG glycosylation were observed.

Catabolic pathway of oligosaccharide-diphospho-dolichol. Subcellular sites of the degradation of the oligomannoside moiety

The degradation of oligosaccharide-diphospho-dolichol leads to the release of oligosaccharide material ranging from (Glc)3(Man)9(GlcNAc)2-P to (Man)3 species and further smaller species. The subcellular location of the glucosidases and mannosidases involved in this catabolic process has been investigated on the basis of their differential sensitivity towards specific inhibitors (castanospermine, deoxymannojirimycin and swainsonine). The results indicate that the first steps of degradation down to the (Man)6 species occurs in the rough endoplasmic reticulum. This result is supported by the fact that the (Man)6 species is the end product when lipid-intermediate-derived glucosylated oligosaccharides are incubated with purified rough endoplasmic reticulum membranes. Swainsonine and lysosomotropic agents (chloroquine and ammonium chloride) do not affect the degradation process, thus indicating that neither Golgi apparatus nor lysosomes are involved in this catabolism. The observation of the same degradation pattern of the released oligosaccharide material in mannosidosis fibroblasts, lacking lysosomal mannosidases, confirms these results. Finally, the subcellular distribution of the released oligosaccharide material indicates that the oligomannosides larger than (Man)6 species are sequestered in the particulate fraction whereas, in contrast, oligomannosides smaller than (Man)6 species are found predominantly in the cytosol. Taken altogether, the experiments demonstrate that the first steps of the degradation of oligosaccharide-diphospho-dolichol occurs in the rough endoplasmic reticulum producing oligomannosides of the (Man)6 species which are then translocated to the cytoplasm to be further degraded.