O-GlcNAcylation is the addition of β-D-N-acetylglucosamine to serine or threonine residues of nuclear and cytoplasmic proteins. O-linked N-acetylglucosamine (O-GlcNAc) was not discovered until the early 1980s and still remains difficult to detect and quantify. Nonetheless, O-GlcNAc is highly abundant and cycles on proteins with a timescale similar to protein phosphorylation. O-GlcNAc occurs in organisms ranging from some bacteria to protozoans and metazoans, including plants and nematodes up the evolutionary tree to man. O-GlcNAcylation is mostly on nuclear proteins, but it occurs in all intracellular compartments, including mitochondria. Recent glycomic analyses have shown that O-GlcNAcylation has surprisingly extensive cross talk with phosphorylation, where it serves as a nutrient/stress sensor to modulate signaling, transcription, and cytoskeletal functions. Abnormal amounts of O-GlcNAcylation underlie the etiology of insulin resistance and glucose toxicity in diabetes, and this type of modification plays a direct role in neurodegenerative disease. Many oncogenic proteins and tumor suppressor proteins are also regulated by O-GlcNAcylation. Current data justify extensive efforts toward a better understanding of this invisible, yet abundant, modification. As tools for the study of O-GlcNAc become more facile and available, exponential growth in this area of research will eventually take place.

Glycans are among the most complex biological molecules found in nature, being composed of monosaccharide units in various combinations and linkages, and featuring diverse and asymmetric types of branching. Until the 1970s, glycan structures were generally represented in scientific publications as chemically accurate drawings, or using standard IUPAC-approved abbreviations (Sharon 1986; McNaught 1997; Chester 1998). However, given the complexity of many glycan structures, it became difficult to represent such molecules in publication figures, especially those involving multistep biosynthetic pathways or multiple structures. Thus, various investigators began to use symbols to represent monosaccharides in glycans. In 1978, Kornfeld and colleagues put forward an elegant and simple system for representation of vertebrate glycans (Kornfeld et al. 1978), and it entered into popular use over the next two decades. This system was eventually adopted and standardized by the editors of the first edition of the textbook Essentials of Glycobiology (Varki et al. 1999). While this adoption increased the popularity of the symbols, the system had some limitations and inconsistencies and did not take advantage of color. Anticipating a revision, the editors developed an updated symbol nomenclature, which was made available to the community in 2005 before the publication of the Second Edition (Varki, Cummings, Esko, Freeze, Stanley, Bertozzi, et al. 2009). This nomenclature was adopted and widely disseminated, especially by the NIGMS-funded Consortium for Functional Glycomics (CFG), which also consulted the scientific community on this issue, thus enhancing acceptance. The official version of the Second Edition nomenclature has since been available in the online version of the textbook. A suggested extended version was then presented at the website Glycopedia. Meanwhile, alternate symbol nomenclature systems have been proposed (Harvey et al. 2009). Issues regarding nomenclature tend to be more controversial than the underlying science (Varki, Cummings, Esko, Freeze, Stanley, Marth, et al. 2009), as there is never one correct answer, and some aspects are matters of taste. For example, the two systems mentioned each have some relative advantages and disadvantages.

The dynamic glycosylation of serine or threonine residues on nuclear and cytosolic proteins by O-linked β- N -acetylglucosamine (O-GlcNAc) is abundant in all multicellular eukaryotes. On several proteins, O-GlcNAc and O-phosphate alternatively occupy the same or adjacent sites, leading to the hypothesis that one function of this saccharide is to transiently block phosphorylation. The diversity of proteins modified by O-GlcNAc implies its importance in many basic cellular and disease processes. Here we systematically examine the current data implicating O-GlcNAc as a regulatory modification important to signal transduction cascades.

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.

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in brains of individuals with Alzheimer's disease (AD) and other tauopathies. Tau pathology is critical to pathogenesis and correlates to the severity of dementia. However, the mechanisms leading to abnormal hyperphosphorylation are unknown. Here, we demonstrate that human brain tau was modified by O-GlcNAcylation, a type of protein O-glycosylation by which the monosaccharide β-N-acetylglucosamine (GlcNAc) attaches to serine/threonine residues via an O-linked glycosidic bond. O-GlcNAcylation regulated phosphorylation of tau in a site-specific manner bothin vitroandin vivo. At most of the phosphorylation sites, O-GlcNAcylation negatively regulated tau phosphorylation. In an animal model of starved mice, low glucose uptake/metabolism that mimicked those observed in AD brain produced a decrease in O-GlcNAcylation and consequent hyperphosphorylation of tau at the majority of the phosphorylation sites. The O-GlcNAcylation level in AD brain extracts was decreased as compared to that in controls. These results reveal a mechanism of regulation of tau phosphorylation and suggest that abnormal hyperphosphorylation of tau could result from decreased tau O-GlcNAcylation, which probably is induced by deficient brain glucose uptake/metabolism in AD and other tauopathies.