Effect of tunicamycin, an inhibitor of protein glycosylation, on the biological properties of acetylcholine receptor in cultured muscle cells

Construing the metaxin-2 mediated simultaneous localization between mitochondria and nucleolus using molecular viscometry

Fluorescent probes for specific inter-organelle communication are of massive significance as such communication is essential for a diverse range of cellular events. Here, we present the microviscosity-sensitive fluorescence marker, Quinaldine Red (QR), and its dual organelle targeting light-up response in live cells. This biocompatible probe was able to localize in mitochondria and nucleolus simultaneously. While QR was able to sense the viscosity change inside these compartments under the induced effect of an ionophore and ROS-rich microenvironment, the probe's ability to stain mitochondria remained unperturbed even after protonophore-induced depolarization. Consequently, a systematic quantification was performed to understand the alteration of microviscosity. Similar behavior in two distinct organelles implied that QR binds to metaxin-2 protein, common to mitochondrial and nucleolar proteomes. We believe this is the first of its kind investigation that identifies the inter-organelle communications marker and opens up a new dimension in this field.

A genome-wide CRISPR screen identifies DPM1 as a modifier of DPAGT1 deficiency and ER stress

Partial loss-of-function mutations in glycosylation pathways underlie a set of rare diseases called Congenital Disorders of Glycosylation (CDGs). In particular, DPAGT1-CDG is caused by mutations in the gene encoding the first step in N-glycosylation, DPAGT1, and this disorder currently lacks effective therapies. To identify potential therapeutic targets for DPAGT1-CDG, we performed CRISPR knockout screens in Drosophila cells for genes associated with better survival and glycoprotein levels under DPAGT1 inhibition. We identified hundreds of candidate genes that may be of therapeutic benefit. Intriguingly, inhibition of the mannosyltransferase Dpm1, or its downstream glycosylation pathways, could rescue two in vivo models of DPAGT1 inhibition and ER stress, even though impairment of these pathways alone usually causes CDGs. While both in vivo models ostensibly cause cellular stress (through DPAGT1 inhibition or a misfolded protein), we found a novel difference in fructose metabolism that may indicate glycolysis as a modulator of DPAGT1-CDG. Our results provide new therapeutic targets for DPAGT1-CDG, include the unique finding of Dpm1-related pathways rescuing DPAGT1 inhibition, and reveal a novel interaction between fructose metabolism and ER stress.

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Hexosamine biosynthetic pathway mutations cause neuromuscular transmission defect

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.

Aberrant development of neuromuscular junctions in glycosylation-defective Large(myd) mice

Mice deficient in the glycosyltransferase Large are characterized by severe muscle and central nervous system abnormalities. In this study, we show that the formation and maintenance of neuromuscular junctions in Large(myd) mice are greatly compromised. Neuromuscular junctions are not confined to the muscle endplate zone but are widely spread and are frequently accompanied by exuberant nerve sprouting. Nerve terminals are highly fragmented and binding of alpha-bungarotoxin to postsynaptic acetylcholine receptors (AChRs) is greatly reduced. In vitro, Large(myd) myotubes are responsive to agrin but produce aberrant AChR clusters, which are larger in area and less densely packed with AChRs. In addition, AChR expression on the cell surface is diminished suggesting that AChR assembly or transport is defective. These results together with the finding that O-linked glycosylation at neuromuscular junctions of Large(myd) mice is compromised indicate that the action of Large is necessary for proper neuromuscular junction development.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Yeast expression and NMR analysis of the extracellular domain of muscle nicotinic acetylcholine receptor alpha subunit

The alpha subunit of the nicotinic acetylcholine receptor (AChR) from Torpedo electric organ and mammalian muscle contains high affinity binding sites for alpha-bungarotoxin and for autoimmune antibodies in sera of patients with myasthenia gravis. To obtain sufficient materials for structural studies of the receptor-ligand complexes, we have expressed part of the mouse muscle alpha subunit as a soluble, secretory protein using the yeast Pichia pastoris. By testing a series of truncated fragments of the receptor protein, we show that alpha211, the entire amino-terminal extracellular domain of AChR alpha subunit (amino acids 1-211), is the minimal segment that could fold properly in yeast. The alpha211 protein was secreted into the culture medium at a concentration of >3 mg/liter. It migrated as a 31-kDa polypeptide with N-linked glycosylation on SDS-polyacrylamide gel. The protein was purified to homogeneity by isoelectric focusing electrophoresis (pI 5.8), and it appeared as a 4.5 S monomer on sucrose gradient at concentrations up to 1 mm ( approximately 30 mg/ml). The receptor domain bound monoclonal antibody mAb35, a conformation-specific antibody against the main immunogenic region of the AChR. In addition, it formed a high affinity complex with alpha-bungarotoxin (k(D) 0.2 nm) but showed relatively low affinity to the small cholinergic ligand acetylcholine. Circular dichroism spectroscopy of alpha211 revealed a composition of secondary structure corresponding to a folded protein. Furthermore, the receptor fragment was efficiently (15)N-labeled in P. pastoris, and proton cross-peaks were well dispersed in nuclear Overhauser effect and heteronuclear single quantum coherence spectra as measured by NMR spectroscopy. We conclude that the soluble AChR protein is useful for high resolution structural studies.

Downregulation and subcellular redistribution of the gamma-aminobutyric acidA receptor induced by tunicamycin in cultured brain neurons

The significance of N-linked glycosylation and oligosaccharide processing was examined for the expression of gamma-aminobutyric acidA receptor (GABA(A)R) in cultured neurons derived from chick embryo brains. Incubation of cultures with 5 microg/ml of tunicamycin for 24 h blocked the binding of 3H-flunitrazepam and 3H-muscimol, probes for the benzodiazepine and GABA sites on the receptor, by about 20% and 28%, respectively. The loss of ligand binding was due to a reduction in the number of binding sites with no significant changes in receptor affinity. Light microscopic immunocytochemistry also revealed that the treatment reduced approximately 13% of the intensity of GABA(A)R immunoreactivity in the neuronal somata. Furthermore, the fraction of intracellular receptors was decreased to 24% from 34% of control in the presence of the agent, as revealed by trypsinization of cells in situ followed by 3H-flunitrazepam binding. The molecular weight of the receptor subunit protein was lowered around 0.5 kDa after tunicamycin treatment, in accordance with that following N-glycosidase F digestion, indicating the blockade of N-linked glycosylation of GABA(A)R by tunicamycin. Moreover, intense inhibitions of 91% and 44%, respectively, were detected to the general galactosylation and mannosylation in the tunicamycin-treated cells, whereas the protein synthesis was hindered by 13%, through assaying the incorporation of 3H-sugars and 3H-leucine. Nevertheless, treatment with castanospermine or swainsonine (10 microg/ml, 24 h), inhibitors to maturation of oligosaccharides, failed to produce significant changes in the ligand binding. In addition, in situ hybridization analysis showed that these three inhibitors did not perturb the mRNA of GABA(A)Ralpha1-subunit. The data suggest that tunicamycin causes the downregulation and subcellular redistribution of GABA(A)R by producing irregularly glycosylated receptors and modifying their localization. Both galactosylation and mannosylation during the process of N-linked glycosylation may be important for the functional expression and intracellular transport of GABA(A)R.

Detailed structural analysis of asparagine-linked oligosaccharides of the nicotinic acetylcholine receptor from Torpedo californica

The structures of the major oligosaccharide moieties of the nicotinic acetylcholine receptor (AcChoR) protein from Torpedo californica have been reported [Nomoto, H., Takahashi, N., Nagaki, Y., Endo, S., Arata, Y. and Hayashi, K. (1986) Eur. J. Biochem. 157, 233-242] to be high-mannose types. Here we report detailed analyses of the structures of the remaining oligosaccharides in this receptor. The sialylated oligosaccharides released by glycopeptidase (almond) digestion were separated according to the number of sialic acid residues using high-performance anion-exchange chromatography with pulsed amperometric detection. After removal of sialic acid from each fraction, the resulting neutral oligosaccharides were separately pyridylaminated and were analyzed by a combination of sequential exoglycosidase digestion and HPLC, then identified on a two-dimensional sugar map. The structures of two desialylated pyridylamino-oligosaccharides were further analyzed by high-resolution proton NMR. Each oligosaccharide was composed of species containing varying numbers of sialic acids. The desialylated complex-type oligosaccharides of AcChoR consisted of ten, eight and one different biantennary, triantennary and tetraantennary oligosaccharide, respectively. The biantennary oligosaccharides were divided into two groups; oligosaccharides with fucose at the proximal N-acetylglucosamine (six varieties) and oligosaccharides without fucose (four varieties). Each group consisted of species differing in the number of terminal galactose residues. The major component of the biantennary oligosaccharides had two galactose residues at the non-reducing termini. The terminal alpha-galactose residue(s) linked to C3 of beta-galactose were found in the fucose-containing biantennary oligosaccharides (two varieties). The triantennary oligosaccharides were also divided into two groups; oligosaccharides with (four varieties) and without (four varieties) besecting N-acetylglucosamine. These groups were composed of species differing in the number of terminal galactose residues. The major component of the triantennary oligosaccharides was fully galactosylated with three galactose residues. An unusual group, Gal beta 1-3GlcNAc, was present in low levels in the triantennary oligosaccharides. In contrast, the tetraantennary oligosaccharide was composed of only one species, which is fully galactosylated with four galactose residues.

Site-directed mutagenesis of the conserved N-glycosylation site on the nicotinic acetylcholine receptor subunits

The role of the conserved N-glycosylation site on each subunit of the Torpedo acetylcholine receptor (AChR) in the biogenesis and function of the receptor was examined by expressing site-directed mutant subunits in Xenopus oocytes. Different mutant subunits caused different effects. The most striking effect was seen with the mutant gamma subunit which, when co-expressed with alpha, beta, and delta subunits, caused degradation of all the subunits. N-Glycosylation of the other subunits appears to contribute to stability of the subunits and/or efficient insertion of the receptor into the plasma membrane, but is not required for assembly. The AChRs containing the mutant alpha subunit formed functional ion channels in the plasma membrane and their activity could be blocked by alpha-bungarotoxin (alpha-BuTX). Thus, attachment of a carbohydrate moiety at the conserved N-glycosylation site is not an absolute requirement for the formation of ACh and alpha-BuTX binding sites.

Altered patterns of N-linked glycosylation of the Torpedo acetylcholine receptor expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) from Torpedo electroplax is an oligomeric transmembrane glycoprotein made up of four highly homologous subunits in a stoichiometry of alpha 2 beta gamma delta. The role of N-linked glycosylation of the AChR has been studied in several cell lines and these studies have suggested that the addition of carbohydrate may be important for receptor expression. While Xenopus oocytes have proven to be an invaluable tool for studying the AChR, little is known about N-linked glycosylation of the oocyte-expressed receptor. The present report demonstrates that the oocyte-expressed AChR is glycosylated and contains the same number of oligosaccharide residues per subunit as the native receptor. However, unlike the native Torpedo receptor which contains both high mannose and complex oligosaccharides, the oocyte-expressed AChR contains only high mannose oligosaccharide modifications. However, as has been well documented, the Torpedo AChR expressed in oocytes is fully functional, demonstrating that the precise nature of the oligosaccharide modification is not critical for receptor function. The role of the oligosaccharide component of the AChR in receptor function was examined using tunicamycin (TM) to inhibit N-linked protein glycosylation. TM treatment resulted in a 70-80% inhibition of AChR expression in oocytes. Functional, unglycosylated receptors were not expressed; receptors expressed in TM-treated oocytes were functional wild-type, glycosylated AChR, formed only during the initial 12 hr of TM exposure. These data suggest that while glycosylation of the oocyte-expressed Torpedo AChR is required for assembly of subunits into a functional receptor, as has been demonstrated in other cells, oocyte modification of normal Torpedo glycosylation patterns does not affect receptor function or assembly.

Castanospermine inhibits the function of the low-density lipoprotein receptor

Castanospermine, a plant alkaloid that inhibits the glycoprotein processing enzyme glucosidase I, has been used to inhibit N-linked oligosaccharide modification, resulting in the production of glycoproteins having Glc3Man7-9(GlcNAc)2 oligosaccharides. This alkaloid caused a significant inhibition of LDL endocytosis in cultured primate smooth muscle cells and human skin fibroblasts. At an optimum concentration of 250 micrograms/mL, castanospermine caused a 40% decrease in cell surface receptor-mediated LDL binding at 4 degrees C, with no apparent change in affinity. Further, the inhibitor had no direct effect on LDL metabolism. This inhibition of LDL receptor expression and function occurred only when the drug was present during de novo receptor synthesis, i.e., during up-regulation. Although the number of cell surface LDL receptors was significantly reduced in the presence of castanospermine, the total number of receptors in the cell was only slightly reduced, indicating that castanospermine induced a redistribution rather than a reduction in the number of receptors. Similarly, subcellular fractionation studies confirmed that castanospermine treatment of fibroblasts results in an altered distribution of receptor activity compared with controls. These findings are consistent with the conclusion that the decrease in specific LDL binding to cells grown in the presence of castanospermine is due to intracellular redistribution of the LDL receptor so that more receptor remains in internal compartments as a result of a diminished rate of transport.

Effect of tunicamycin, an inhibitor of protein glycosylation, on the high-affinity transport of acidic amino acid neurotransmitters in C6 glioma cells

The effect of tunicamycin, an inhibitor of protein glycosylation, on the high-affinity transport of D-aspartate was investigated in C6 astrocytoma cells. A concentration of tunicamycin (1 microgram/ml) that after 24 h exposure inhibited the rate of transport by 70% and incorporation of [3H]mannose by 82-95% had only a small effect on [14C]leucine incorporation into protein and cell growth (20% reduction). Tunicamycin decreased the Vmax for transport without affecting the Km, which suggests that inhibition of glycosylation reduces the number of competent transporters on the surface of the plasma membrane. The decrease in the velocity of uptake was attenuated when C6 cells were treated with tunicamycin in the presence of protease inhibitors, indicating that the underglycosylated carriers are subject to enhanced proteolytic degradation. Incubation in drug-free medium following treatment with 1 microgram/ml of tunicamycin for 24 h resulted in recovery of D-aspartate transport within 48 h. This recovery was prevented by the presence of cycloheximide, which indicates that synthesis of new transporters is necessary for the restoration of normal rates of D-aspartate uptake. These results support our earlier postulate that the high-affinity carriers for amino acid transmitters are transmembrane glycoproteins.

Model for intracellular folding of the human immunodeficiency virus type 1 gp120

The intracellular folding of the human immunodeficiency virus type 1 gp120 has been assessed by analyzing the ability of the glycoprotein to bind to the viral receptor CD4. Pulse-chase experiments revealed that the glycoprotein was initially produced in a conformation that was unable to bind to CD4 and that the protein attained the appropriate tertiary structure for binding with a half-life of approximately 30 min. The protein appears to fold within the rough endoplasmic reticulum, since blocking of transport to the Golgi apparatus by the oxidative phosphorylation inhibitor carbonyl cyanide m-chlorophenylhydrazone did not appear to perturb the folding kinetics of the molecule. The relatively lengthy folding time was not due to modification of the large number of N-linked glycosylation sites on gp120, since inhibition of the first steps in oligosaccharide modification by the inhibitors deoxynojirimycin or deoxymannojirimycin did not impair the CD4-binding activity of the glycoprotein. However, production of the glycoprotein in the presence of tunicamycin and removal of the N-linked sugars by endoglycosidase H treatment both resulted in deglycosylated proteins that were unable to bind to CD4, suggesting in agreement with previous results, that glycosylation contributes to the ability of gp120 to bind to CD4. Interestingly, incomplete endoglycosidase H treatment revealed that a partially glycosylated glycoprotein could bind to the receptor, implying that a subset of glycosylation sites, perhaps some of those conserved in different isolates of human immunodeficiency virus type 1, might be important for binding of the viral glycoprotein to the CD4 receptor.

Induction of N-glycosylation activity in cultured embryonic rat brain cells

Developmental changes in protein N-glycosylation activity have been studied using cultures of dissociated fetal rat brain cells as an in vitro model system. These cultures undergo an initial phase of neurite outgrowth and cell proliferation (4-6 days in culture), followed by a period of cellular differentiation. N-Glycosylation activity has been measured by assaying the incorporation of [2-3H]mannose into dolichol-linked oligosaccharides and glycoprotein over a period of 1-25 days in culture. This study revealed a marked induction of N-glycosylation activity beginning at approximately 1 week of culture. [2-3H]Mannose incorporation into the oligosaccharide-lipid intermediate fraction and glycoprotein reached maximal values between 12 and 16 days of culture and declined thereafter. The major dolichol-linked oligosaccharide labeled by the brain cell cultures was shown to be Glc3Man9GlcNAc2 by HPLC analysis. Parallel incorporation studies with [3H]leucine showed that the increase in protein N-glycosylation was relatively higher than a concurrent increase in cellular protein synthesis observed during the induction period. Maximal labeling of glycoprotein corresponded to the period of glial differentiation, as indicated by a sharp rise in the marker enzymes, 2',3'-cyclic nucleotide 3'-phosphohydrolase (an oligodendroglial marker) and glutamine synthetase (an astroglial marker). The results describe a developmental activation of the N-glycosylation pathway and suggest a possible relationship between N-linked glycoprotein assembly and the growth and differentiation of glial cells.

Carbohydrate structures of acetylcholine receptor from Torpedo californica and distribution of oligosaccharides among the subunits

The structure of carbohydrates in acetylcholine receptor (AChR) from Torpedo californica is reported. Oligosaccharides released quantitatively from the whole molecule by N-oligosaccharide glycopeptidase digestion were fractionated by thin-layer chromatography and further purified by high-performance liquid chromatography. We show that more than 70% of the total oligosaccharide chains in Torpedo AChR are of the high-mannose type with the structures (Man)8(GlcNAc)2 and (Man)9(GlcNAc)2. The structure of these oligosaccharides were determined by proton nuclear magnetic resonance spectroscopy. These two types of oligosaccharides were shown to be distributed different proportions in all subunits of Torpedo AChR. We also show that several kinds of complex-type oligosaccharides comprising the rest of the carbohydrate in the protein exist mainly in the gamma and delta subunits. The structure of the carbohydrate moiety that is distributed on the four subunits of AChR was also examined by susceptibility to endo-beta-N-acetylglucosaminidase and sialidase and by binding affinity to lectins, e.g. concanavalin A, leucoagglutinating phytohemagglutinin, and wheat germ agglutinin.

Effect of tunicamycin on hexose transport in mouse embryo fibroblast Swiss 3T3 cells

The role of glycosylation of the carrier in the transporting activity was investigated in Swiss 3T3 cells. Inhibition of protein glycosylation by tunicamycin resulted in the decrease of hexose uptake in a dose- and time-dependent manner without a cytotoxic effect. From kinetic analysis, a decrease in the number or availability of hexose carriers in the plasma membrane was suggested. This was in good correlation with the decrease in the amount of photoaffinity cytochalasin B binding in the plasma membrane by the treatment with tunicamycin. The rate of phorbol 12,13-dibutyrate-induced translocation of the hexose carrier from microsomal to plasma membrane was reduced in tunicamycin-treated cells, which may be correlated with the decrease in the number of the completely glycosylated carrier translocatable from the microsomal membrane. In both tunicamycin-treated and untreated cells, the stimulation of hexose transport by phorbol 12,13-dibutyrate was abolished by the removal of phorbol 12,13-dibutyrate, and upon its readdition the stimulation recovered to the same degree as before the removal. Thus, the recycling of the functionally mature hexose carrier appeared not to be affected by the treatment with tunicamycin. These results suggested that complete glycosylation of the carrier may be necessary for the translocation of the carrier from microsomal to plasma membrane to accomplish its function on the cell surface.

ATP-induced cation influx in myotubes is additive to cholinergic agonist action

Using biochemical methods, ATP was shown to induce an inward flux of 86Rb into cultured chick myotubes. A biphasic dose-response curve was observed, the first part of which was saturable and had an EC50 value of approximately 10 microM. beta, gamma-imido ATP inhibited the ATP-induced uptake. ADP, AMP and adenosine were less potent than ATP in producing influx. ATP-induced influx of 86Rb was found to be additive to carbachol-induced influx and, in contrast to the latter, not blocked by the nicotinic acetylcholine receptor antagonist alpha-bungarotoxin. The results suggest the presence of an ATP recognition-site on myotubes, triggering ion-permeation.

Impaired development of rat cerebellum induced by neonatal injection of the glycoprotein synthesis inhibitor, tunicamycin

The effects of the protein glycosylation inhibitor tunicamycin on the postnatal development of the rat cerebellum were examined in vivo. Tunicamycin (0.2 micrograms) was injected intracranially into 1-day-old rats. Inhibition of glycosylation of the macromolecules in the cerebellum by tunicamycin treatment was suggested by a reduced incorporation of [3H]glucosamine into the trichloroacetic acid (TCA)-insoluble fraction. The tunicamycin treatment did not affect gain in body weight significantly. However the cerebellar weight was significantly reduced by 30-40% compared with that of the controls. Development of GABAergic and cholinergic innervations in the hypoplastic cerebellum was examined by measuring the activities of glutamate decarboxylase (GAD) and choline acetyltransferase (ChAT). The specific activity and the total activity of GAD were significantly reduced in the tunicamycin-treated cerebellum. In contrast the specific activity of ChAT was significantly increased, whereas the total activity of ChAT per cerebellum was identical with that of the controls. These results suggest that the intracranial injection of tunicamycin affects the postnatal development of rat cerebellum, such as GABAergic and cholinergic innervations.

Effects of tunicamycin on NGF binding and neurite outgrowth in PC12 cells

The effects of inhibition of glycosylation on nerve growth factor (NGF) binding and neurite outgrowth response of PC12 cells have been examined. Exposure to tunicamycin (1-10 micrograms/ml) for 24-36 hr eliminates the rapidly dissociating component of NGF binding and decreases the proportion of PC12 cells capable of elaborating neurites in a dose-dependent manner. These decreased cellular responses are probably due to an underglycosylation of the NGF receptor, since the effects of tunicamycin are correlated with a decrease in 3H-fucose incorporation rather than a general decline in cellular metabolism as measured by viability and protein synthesis. These results suggest that carbohydrate side chains are important for the function and/or orientation of the NGF receptor in PC12 cells and that the rapidly dissociating component of NGF binding may be associated with a minimum concentration of functional receptors per cell required for the full biologic response.

Tunicamycin prevents stimulation of protein synthesis by branched chain amino acids in isolated rat muscles

Quarter rat diaphragms were incubated for 150 min. with or without stimulators of protein synthesis and/or inhibitors of protein glycosylation. Tyrosine incorporation into protein was measured during the last 120 min. Branched chain amino acids stimulated protein synthesis by 40-60%. Tunicamycin or 2-deoxy-D-glucose did not affect baseline protein synthesis, but inhibited or abolished stimulation of protein synthesis by branched chain amino acids. Tunicamycin did not prevent stimulation of protein synthesis by insulin or affect amino acid transport under these conditions. The data suggest that a) glycoprotein(s) may be involved in the post-transcriptional stimulation of protein synthesis by leucine, b) leucine and insulin may stimulate peptide chain initiation in muscle through different mechanisms.

Concanavalin A inhibits nicotinic acetylcholine receptor function in cultured chick ciliary ganglion neurons

The effects of various lectins and toxins on neuronal nicotinic acetylcholine receptor function have been studied in primary cultures of chick ciliary ganglion neurons. Neuronal response to acetylcholine receptor activation was measured by a cation flux method at 4 degrees C in a high potassium-low sodium medium designed to stabilize membrane potential near zero, with acetylcholine as the agonist and cesium-137 as the tracer ion. Exposure to 1 mM acetylcholine for 30 s produced a 5-10-fold stimulation of cesium-137 influx. Acetylcholine-stimulated influx was inhibited more than 95% by 10 microM D-tubocurarine, but was insensitive to both 1 microM tetrodotoxin and 1 microM alpha-bungarotoxin. Concanavalin A (50 micrograms/ml) inhibited agonist-induced ion flux by 80% at 4 degrees C. Succinyl-concanavalin A was ineffective at concentrations up to 250 micrograms/ml, and could not protect against the concanavalin A inhibition. However, inhibition by concanavalin A was eliminated by prior incubation of the lectin with 0.2 M alpha-methyl-D-mannoside and subsequent co-incubation with the sugar. Wheat germ agglutinin, lentil lectin, cholera toxin and tetanus toxin were without effect at either 4 degrees C or 37 degrees C. These results suggest a specific interaction between concanavalin A and neuronal nicotinic acetylcholine receptors.