GlcNAcstatin: a picomolar, selective O-GlcNAcase inhibitor that modulates intracellular O-glcNAcylation levels

Structural analyses of enzymes involved in the O-GlcNAc modification

In order to study the O-GlcNAc modification in vivo, it is evident that a range of specific small molecule inhibitors would be a valuable asset. One strategy for the design of such compounds would be to utilise 3-D structural information in tandem with knowledge of catalytic mechanism. The last few years has seen major breakthroughs in our understanding of the 3-D structure of the enzymes involved in the O-GlcNAc modification notably from the study of the tetratricopeptide repeat (TPR) domain of the human O-GlcNAc transferase, of the bacterial homologs of the O-GlcNAc hydrolase and more latterly bacterial homologs of the O-GlcNAc transferase itself. Of particular note are the bacterial O-GlcNAc hydrolase homologs that provide near identical active centres to the human enzyme. These have informed the design and/or subsequent analysis of inhibitors of this enzyme which have found great use in the chemical dissection of the O-GlcNAc in vivo, as described by Macauley and Vocadlo elsewhere in this issue.

GlcNAcstatins are nanomolar inhibitors of human O-GlcNAcase inducing cellular hyper-O-GlcNAcylation

O-GlcNAcylation is an essential, dynamic and inducible post-translational glycosylation of cytosolic proteins in metazoa and can show interplay with protein phosphorylation. Inhibition of OGA (O-GlcNAcase), the enzyme that removes O-GlcNAc from O-GlcNAcylated proteins, is a useful strategy to probe the role of this modification in a range of cellular processes. In the present study, we report the rational design and evaluation of GlcNAcstatins, a family of potent, competitive and selective inhibitors of human OGA. Kinetic experiments with recombinant human OGA reveal that the GlcNAcstatins are the most potent human OGA inhibitors reported to date, inhibiting the enzyme in the sub-nanomolar to nanomolar range. Modification of the GlcNAcstatin N-acetyl group leads to up to 160-fold selectivity against the human lysosomal hexosaminidases which employ a similar substrate-assisted catalytic mechanism. Mutagenesis studies in a bacterial OGA, guided by the structure of a GlcNAcstatin complex, provides insight into the role of conserved residues in the human OGA active site. GlcNAcstatins are cell-permeant and, at low nanomolar concentrations, effectively modulate intracellular O-GlcNAc levels through inhibition of OGA, in a range of human cell lines. Thus these compounds are potent selective tools to study the cell biology of O-GlcNAc.

O-GlcNAc cycling: implications for neurodegenerative disorders

The dynamic post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc), termed O-GlcNAcylation, is an important mechanism for modulating cellular signaling pathways. O-GlcNAcylation impacts transcription, translation, organelle trafficking, proteasomal degradation and apoptosis. O-GlcNAcylation has been implicated in the etiology of several human diseases including type-2 diabetes and neurodegeneration. This review describes the pair of enzymes responsible for the cycling of this post-translational modification: O-GlcNAc transferase (OGT) and beta-N-acetylglucosaminidase (OGA), with a focus on the function of their structural domains. We will also highlight the important processes and substrates regulated by these enzymes, with an emphasis on the role of O-GlcNAc as a nutrient sensor impacting insulin signaling and the cellular stress response. Finally, we will focus attention on the many ways by which O-GlcNAc cycling may affect the cellular machinery in the neuroendocrine and central nervous systems.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

Chemical dissection of the link between streptozotocin, O-GlcNAc, and pancreatic cell death

Streptozotocin is a natural product that selectively kills insulin-secreting β cells, and is widely used to generate mouse models of diabetes or treat pancreatic tumors. Several studies suggest that streptozotocin toxicity stems from its N-nitrosourea moiety releasing nitric oxide and possessing DNA alkylating activity. However, it has also been proposed that streptozotocin induces apoptosis by inhibiting O-GlcNAcase, an enzyme that, together with O-GlcNAc transferase, is important for dynamic intracellular protein O-glycosylation. We have used galacto-streptozotocin to chemically dissect the link between O-GlcNAcase inhibition and apoptosis. Using X-ray crystallography, enzymology, and cell biological studies on an insulinoma cell line, we show that, whereas streptozotocin competitively inhibits O-GlcNAcase and induces apoptosis, its galacto-configured derivative no longer inhibits O-GlcNAcase, yet still induces apoptosis. This supports a general chemical poison mode of action for streptozotocin, suggesting the need for using more specific inhibitors to study protein O-GlcNAcylation.

New insights into metabolic signaling and cell survival: the role of beta-O-linkage of N-acetylglucosamine

The involvement of glucose in fundamental metabolic pathways represents a core element of biology. Late in the 20th century, a unique glucose-derived signal was discovered, which appeared to be involved in a variety of cellular processes, including mitosis, transcription, insulin signaling, stress responses, and potentially, Alzheimer's disease, and diabetes. By definition, this glucose-fed signaling system was a post-translational modification to proteins. However, unlike classical cotranslational N-glycosylation occurring in the endoplasmic reticulum and Golgi apparatus, this process occurs elsewhere throughout the cell in a highly dynamic fashion, similar to the quintessential post-translational modification, phosphorylation. This more recently described post-translational modification, the beta-O-linkage of N-acetylglucosamine (i.e., O-GlcNAc) to nucleocytoplasmic proteins, represents an under-investigated area of biology. This signaling system operates in all of the tissues examined and seems to have persisted throughout all multicellular eukaryotes. Thus, it comes with little surprise that O-GlcNAc signaling is an integral system and viable target for biomedical investigation. This system may be a boundless source for insight into a variety of diseases and yield numerous opportunities for drug design. This Perspective will address recent insights into O-GlcNAc signaling in the cardiovascular system as a paradigm for its involvement in other biological systems.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

Chemical approaches to understanding O-GlcNAc glycosylation in the brain

O-GlcNAc glycosylation is a unique, dynamic form of glycosylation found on intracellular proteins of all multicellular organisms. Studies suggest that O-GlcNAc represents a key regulatory modification in the brain, contributing to transcriptional regulation, neuronal communication and neurodegenerative disease. Recently, several new chemical tools have been developed to detect and study the modification, including chemoenzymatic tagging methods, quantitative proteomics strategies and small-molecule inhibitors of O-GlcNAc enzymes. Here we highlight some of the emerging roles for O-GlcNAc in the nervous system and describe how chemical tools have significantly advanced our understanding of the scope, functional significance and cellular dynamics of this modification.

O-GlcNAc modification in diabetes and Alzheimer's disease

Similar to phosphorylation, O-GlcNAcylation (or simply GlcNAcylation) is an abundant, dynamic, and inducible post-translational modification. In some cases, GlcNAcylation and phosphorylation occur at the same or adjacent sites, modulating each other. GlcNAcylated proteins are crucial in regulating virtually all cellular processes, including signaling, cell cycle, and transcription, among others. GlcNAcylation affects protein-protein interactions, activity, stability, and expression. Several GlcNAcylated proteins are involved in diabetes and Alzheimer's disease. Hyperglycemia increases GlcNAcylation of proteins within the insulin signaling pathway and contributes to insulin resistance. In addition, hyperinsulinemia and hyperlipidemia are also associated with increased GlcNAcylation, which affect and regulate several insulin signaling proteins, as well as proteins involved on the pathology of diabetes. With respect to Alzheimer's disease, several proteins involved in the etiology of the disease, including tau, neurofilaments, beta-amyloid precursor protein, and synaptosomal proteins are GlcNAcylated in normal brain. The impairment of brain glucose uptake/metabolism is a known metabolic defect in Alzheimer's neurons. Data support the hypothesis that hypoglycemia within the brain may reduce the normal GlcNAcylation of tau, exposing kinase acceptor sites, thus leading to hyperphosphorylation, which induces tangle formation and neuronal death. Alzheimer's disease and type II diabetes represent two metabolic disorders where dysfunctional protein GlcNAcylation/phosphorylation may be important for disease pathology.

Tautomeric modification of GlcNAc-thiazoline

The potent O-GlcNAcase (OGA) inhibitor GlcNAc-thiazoline has been modified by buffer- or acylation-induced imine-to-enamine conversion and then electrophile or radical addition (Xn = D3, F, N3, OH, SMe, COCF3, CF3). Several functionalized GlcNAc-thiazolines show highly selective inhibition of OGA vs human hexosaminidase and thus have promise as tools for targeted investigations of OGA, an enzyme linked to diabetes and neurodegeneration. A new radical addition/fragmentation reaction of the N-(trifluoroacetyl)enamine has been discovered.

A 1-acetamido derivative of 6-epi-valienamine: an inhibitor of a diverse group of β-N-acetylglucosaminidases

The synthesis of an analogue of 6-epi-valienamine bearing an acetamido group and its characterisation as an inhibitor of beta-N-acetylglucosaminidases are described. The compound is a good inhibitor of both human O-GlcNAcase and human beta-hexosaminidase, as well as two bacterial beta-N-acetylglucosaminidases. A 3-D structure of the complex of Bacteroides thetaiotaomicron BtGH84 with the inhibitor shows the unsaturated ring is surprisingly distorted away from its favoured solution phase conformation and reveals potential for improved inhibitor potency.

Glycosidase inhibitors as conformational transition state analogues

A method for estimating the conformational similarity between hexopyranose rings is presented and used to probe the behaviour of various glycosyl hydrolase inhibitors as conformational transition state analogues.