Beta-amyloid precursor protein is modified with O-linked N-acetylglucosamine

The O-GlcNAc transferase gene resides on the X chromosome and is essential for embryonic stem cell viability and mouse ontogeny

Nuclear and cytoplasmic protein glycosylation is a widespread and reversible posttranslational modification in eukaryotic cells. Intracellular glycosylation by the addition of N-acetylglucosamine (GlcNAc) to serine and threonine is catalyzed by the O-GlcNAc transferase (OGT). This "O-GlcNAcylation" of intracellular proteins can occur on phosphorylation sites, and has been implicated in controlling gene transcription, neurofilament assembly, and the emergence of diabetes and neurologic disease. To study OGT function in vivo, we have used gene-targeting approaches in male embryonic stem cells. We find that OGT mutagenesis requires a strategy that retains an intact OGT gene as accomplished by using Cre-loxP recombination, because a deletion in the OGT gene results in loss of embryonic stem cell viability. A single copy of the OGT gene is present in the male genome and resides on the X chromosome near the centromere in region D in the mouse spanning markers DxMit41 and DxMit95, and in humans at Xq13, a region associated with neurologic disease. OGT RNA expression in mice is comparably high among most cell types, with lower levels in the pancreas. Segregation of OGT alleles in the mouse germ line with ZP3-Cre recombination in oocytes reveals that intact OGT alleles are required for completion of embryogenesis. These studies illustrate the necessity of conditional gene-targeting approaches in the mutagenesis and study of essential sex-linked genes, and indicate that OGT participation in intracellular glycosylation is essential for embryonic stem cell viability and for mouse ontogeny.

Effect of okadaic acid on O-linked N-acetylglucosamine levels in a neuroblastoma cell line

O-Linked N-Acetylglucosamine (O-GlcNAc) is a major form of post-translational modification found in nuclear and cytoplasmic proteins. Several authors have advanced the hypothesis according to which phosphorylation and O-GlcNAc glycosylation are reciprocally related to one another [1,2]. In order to test this hypothesis we have investigated the effect of a broad spectrum phosphatase inhibitor, okadaic acid (OA), generally used to induce protein hyperphosphorylation, on the GlcNAc content of cellular glycoproteins. We demonstrate that in neuronal cells lines OA decreases the level of O-GlcNAc in both nuclear and cytoplasmic proteins with a greater effect in the nuclear fraction. This phenomenon was demonstrated by the use of three different procedures for the detection of O-GlcNAc in conjunction with a systematic treatment with PNGase F. O-Linked GlcNAc was characterized using respectively lectin staining with WGA, galactosyltransferase labeling and metabolic labeling of cultured cells with [3H]glucosamine. Although the effects on individual proteins varied, a less pronounced effect was observed on HeLa or COS cell total homogenates. When Kelly cells were treated with OA, the major observation was a decrease in O-GlcNAc content of nuclear proteins. The measurement of the UDP-GlcNAc level clearly demonstrates that the decrease on the O-GlcNAc level in the neuroblastoma cell line after treatment with okadaic acid is not a consequence of the modification of the UDP-GlcNAc pool.

O-linked N-acetylglucosamine levels in cerebellar neurons respond reciprocally to pertubations of phosphorylation

The novel intracellular carbohydrate O-linked N-acetylglucosamine (O-GlcNAc) is present on proteins ranging from those of viruses to those of humans and include cytosolic, nuclear and plasma-membrane proteins. In this report we have examined the effect of manipulation of phosphorylation on the levels of O-GlcNAc in cerebellar neurons from early postnatal mice. Our results indicate a reciprocal response of O-GlcNAc levels to phosphorylation. Activation of protein kinase A or C, for example, results in reduced levels of O-GlcNAc specifically in the fraction of cytoskeletal and cytoskeleton-associated proteins, while inhibition of the same kinases results in increased levels of O-GlcNAc. These data are in keeping with a reciprocal action of O-GlcNAc with respect to phosphorylation and suggest that this modification may have a role in signal transduction.

The role of the protein glycosylation state in the control of cellular transport of the amyloid beta precursor protein

The amyloid beta precursor protein can exist as both a membrane-bound and a secreted protein, with the former having the potential to generate the amyloid beta peptide present in the neuritic plaques which are characteristic of Alzheimer's disease. In this study, we have used a clone of the AtT20 mouse pituitary cell line which expresses high levels of the amyloid beta precursor protein to characterize the glycosylation state of the secreted and membrane-bound forms of the protein and to examine the role of post-translational modifications in protein processing. Lectin blot analysis of immunoprecipitated amyloid beta precursor protein demonstrated that the soluble form of the protein contains significant amounts of sialic acid, with the lectin staining being reduced in the particulate cellular fractions. Treatment of the cells with mannosidase inhibitors to interfere with the formation of complex-type N-linked glycans resulted in a decrease in secreted amyloid beta precursor protein and an increase in the level of the cellular form of the protein. The increase in amyloid beta precursor protein levels in the cellular fraction was accompanied by an increase in perinuclear staining. Furthermore, cells overexpressing the alpha2,6(N)-sialyltransferase enzyme also demonstrated an increase in amyloid beta precursor protein secretion. These results suggest that the presence of terminal sialic acid residues on complex-type N-glycans may be required for the optimal transport of the amyloid beta precursor protein from the Golgi to the cell membrane with the subsequent cleavage to generate the secreted form of the protein.

Nuclear and cytoplasmic glycosylation

O-GlcNAcylation is a form of cytoplasmic and nuclear glycosylation that is found on many diverse proteins of the cell including RNA polymerase II and its associated transcription factors, cytoskeletal proteins, nucleoporins, viral proteins, heat shock proteins, tumor suppressors, and oncogenes. It involves the attachment of a single, unmodified N-acetylglucosaminyl residue O-glycosidically linked to the hydroxyl groups of serine and threonine moieties of proteins. It is a highly abundant and dynamic form of posttranslational modification that appears to modulate function in a manner similar to phosphorylation. All O-GlcNAc-containing proteins are phosphoproteins that are involved in the formation of multimeric complexes, suggesting that O-GlcNAc may play a role in mediating protein-protein interactions. O-GlcNAc sites resemble phosphorylation sites and in many cases the two modifications are mutually exclusive; therefore, O-GlcNAcylation may act as an antagonist of phosphorylation and help to mediate many essential functions of the cell.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

A subpopulation of estrogen receptors are modified by O-linked N-acetylglucosamine

Estrogen receptors (ER) are ligand-inducible transcription factors regulated by Ser(Thr)-O-phosphorylation. Many transcription factors and eukaryotic RNA polymerase II itself are also dynamically modified by Ser(Thr)-O-linked N-acetylglucosamine moieties (O-GlcNAc). Here we report that subpopulations of murine, bovine, and human estrogen receptors are modified by O-GlcNAc. O-GlcNAc moieties were detected on insect cell-expressed, mouse ER (mER) by probing with bovine milk galactosyltransferase, followed by structural analysis. Wheat germ agglutinin-Sepharose affinity chromatography also readily detected terminal GlcNAc residues on subpopulations of ER purified from calf uterus, from human breast cancer cells (MCF-7), or from mER produced by in vitro translation. These data suggest that greater than 10% of these populations of estrogen receptors bear O-GlcNAc. Site mapping of insect cell expressed mER localized one major site of O-GlcNAc addition to Thr-575, within a PEST region of the carboxyl-terminal F domain. Based upon their relative resistance to both hexosaminidase and to in vitro galactosylation, O-GlcNAc moieties appear to be largely buried on native mER. This dynamic saccharide modification, like phosphorylation, may play a role in modulating the dimerization, stability, or transactivation functions of estrogen receptors.

Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Modification of Ser and Thr residues by attachment of O-linked N-acetylglucos-amine [Ser(Thr)-O-GlcNAcylation] to eukaryotic nuclear and cytosolic proteins is as dynamic and possibly as abundant as Ser(Thr) phosphorylation. Known O-GlcNAcylated proteins include cytoskeletal proteins and their regulatory proteins; viral proteins; nuclear-pore, heat-shock, tumor-suppressor, and nuclearoncogene proteins; RNA polymerase II catalytic subunit; and a multitude of transcription factors. Although functionally diverse, all of these proteins are also phosphoproteins. Most O-GlcNAcylated proteins form highly regulated multimeric associations that are dependent upon their posttranslational modifications. Evidence is mounting that O-GlcNAcylation is an important regulatory modification that may have a reciprocal relationship with O-phosphorylation and may modulate many biological processes in eukaryotes.

Identification of O-linked N-acetylglucosamine modification of ankyrinG isoforms targeted to nodes of Ranvier

AnkyrinGs of 270 and 480 kDa are localized at nodes of Ranvier and are candidates to couple the voltage-dependent sodium channel and neurofascin to the spectrin/actin network. This study presents evidence that these ankyrins contain O-linked GlcNAc residues and identifies as the site of glycosylation a serine-rich domain that distinguishes them from other ankyrin isoforms. The 480-kDa ankyrinG, extracted from brain membranes associated with wheat germ agglutinin-affinity columns, was [3H]galactose-labeled with UDP-[3H] galactose and galactosyltransferase, and cross-reacted with an antibody against O-GlcNAc monosaccharides. AnkyrinG-associated sugars are O-linked monosaccharides based on resistance to peptide-N-glycosidase F and analysis of saccharides released by beta-elimination. The serine-rich domain is the site of glycosylation based on wheat germ agglutinin binding activity of polypeptides produced by in vitro translation in reticulocyte lysates. Immunofluorescence revealed co-localization of ankyrinG and O-GlcNAc immunoreactivity at nodes of Ranvier. These observations suggest that ankyrin at the node of Ranvier is O-GlcNAc-glycosylated and are the first demonstration of a post-translational modification that is concentrated at the node of Ranvier and not in adjacent areas of myelinated axons.

The microtubule-associated protein tau is extensively modified with O-linked N-acetylglucosamine

Tau is a family of phosphoproteins that are important in modulating microtubule stability in neurons. In Alzheimer's disease tau is abnormally hyperphosphorylated, no longer binds microtubules, and self-assembles to form paired helical filaments that likely contribute to neuron death. Here we demonstrate that normal bovine tau is multiply modified by Ser(Thr)-O-linked N-acetylglucosamine, a dynamic and abundant post-translational modification that is often reciprocal to Ser(Thr)-phosphorylation. O-GlcNAcylation of tau was demonstrated by blotting with succinylated wheat germ agglutinin and by probing with bovine milk beta(1,4)galactosyltransferase. Structural analyses confirm the linkage and the saccharide structure. Tau splicing variants are multiply O-GlcNAcylated at similar sites, with an average stoichiometry of greater than 4 mol of O-linked N-acetylglucosamine/mol of tau. However, the number of sites occupied appears to be greater than 12, suggesting substoichiometric occupancy at any given site. A similar relationship between average stoichiometry and site-occupancy has also been described for the phosphorylation of tau. Site-specific or stoichiometric changes in O-GlcNAcylation may not only modulate tau function but may also play a role in the formation of paired helical filaments.