Editorial [Hot topic: Therapeutic Potential of O-GlcNAc Glycosylation Related Signaling Pathways (Guest Editor: Prof. Dr. Brigitte Schmitz)]

It appears to be now fairly well accepted in the field of signal transduction that the monosaccharide N-acetylglucosamine (O-GlcNAc) added posttranslationally through a β-O-glycosidic linkage to serine and threonine residues in cytosolic and nuclear proteins has important regulatory roles. Since its first description in 1984 until today this highly dynamic sugar has been shown to be attached to about thousand proteins. The steady increase in the number of published papers in recent years which deal with O-GlcNAc underlines the importance of O-GlcNAc in most, if not all processes believed in the past to be regulated merely by serine/threonine phosphorylation. But nevertheless, many investigators - who are well aware of phosphorylation as a regulatory modification in processes as diverse as the cell cycle or glycogen metabolism - are still surprised to learn that a sugar residue might act in a similar way, although often in a reciprocal fashion to phosporylation. In a review article on the occasion of the 20th anniversary of the discovery of O-GlcNAc Gerald Hart commented on how long it took for protein phosphorylation to become accepted as an important regulatory modification of proteins from its discovery in 1933. If one considers the mere quantitative output, based on the number of publications concerned with OGlcNAc, one notices that considerably more papers have been published in the last five years than in the twenty years from 1984 - 2004. So there are grounds to hope that general acceptance of O-GlcNAc as a regulatory modification will not take as long as for phosphorylation. Difficulties in detecting O-GlcNAc modification on proteins and in particular in identifying sites of modification have contributed significantly to the initial slow progress in understanding the functions of O-GlcNAc. Recent developments of innovative chemo-enzymatic, as well as mass spectrometric and other techniques have, however, changed this situation greatly. These advances have been extremely important in gaining deeper insights into the function of O-GlcNAc in physiological processes and pathological conditions, such as diabetes type 2, cardiovascular and neurodegenerative diseases, or cancer. I would like to thank the Editors in Chief of CSTT - as will certainly all other O-GlcNAc-“Fans” - for giving me the opportunity of raising the prominence of O-GlcNAc through this Journal, and of giving interested researchers the opportunity of obtaining a concise overview of this exciting and fascinating aspect of signal transduction. It is obvious that O-GlcNAc, as well as the enzymes which catalyse its rapid turnover, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), are excellent targets for signal tranduction therapy. It is to be hoped that this special issue of CSTT will contribute to progress in this important research area, too. In each chapter of this issue authors present specific contributions to the elucidation of the role of O-GlcNAc, in addition to giving an overview of some more general O-GlcNAc-related aspects of processes in which O-GlcNAc is known to be implicated. In Chapter 1 the authors describe the role of O-GlcNAc as a nutrient sensor regulating energy metabolism, not only under diabetic but also under normo- and hypoglycemic conditions. After the discovery of the involvement of the hexosamine pathway in insulin resistance it took another seven years before it was demonstrated that the O-GlcNAc modification of proteins - one of several end-products of this pathway - is causally involved in aberrant insulin signalling. The authors of Chapter 1 and others confirmed much of the early work on glucosamine that its effects were due to increased/decreased OGlcNAcylation of proteins, by over-expressing, for example, either OGT or OGA in vitro or in vivo. The molecular mechanisms of how specific metabolic pathways are regulated by O-GlcNAcylation in addition to those of the better known phosphorylation reactions are described in this chapter as well supporting the view that acute hormone-mediated signalling can be attenuated resulting in cells protecting themselves from excessive nutrient flux or nutrient deprivation. The rapid and dynamic attachment or removal of O-GlcNAc (“O-GlcNAc cycling”) is not the only possible way of how OGlcNAc can regulate cellular functions. O-GlcNAc modification of RNA polymerase II and its transcription factors were discovered early on. In Chapter 2, an overview is given of the wealth of information regarding the regulation of different stages of transcription by O-GlcNAc from chromatin remodelling to e.g. proteasomal degradation of transcription factors supporting that O-GlcNAc has important roles in controlling gene expression in response to glucose levels. A description of recently developed techniques for O-GlcNAc site-mapping is also included in this chapter and, in addition, of screening methods such as two hybrid assays capable of providing much more data on O-GlcNAcylation more rapidly than conventional analyses. These two aspects are also central to Chapter 3. The authors of the other reviews will forgive me for mentioning the last author of this chapter here. Gerald Hart is the pioneer in this exciting research area and he and his coworkers have probably worked in most of the areas in which O-GlcNAcylation plays a role, including cloning of the enzymes OGT and OGA as well as the development of methods for identification and site mapping. His comprehensive review makes clear that O-GlcNAc's roles are essential for most vital cellular functions in healthy situations as well as being involved in many diseases. The power of comparative O-GlcNAc proteome studies, e.g., between normal and diabetic patients, has been demonstrated recently by his group and it is likely that data of such a study could provide the basis for a diagnostic tool for this disease. In this chapter data are shown from a comparative O-GlcNAc proteome study of resting and activated T-cells using SILAC in combination with immunoaffinity enrichment and mass spectrometry. In addition to known O-GlcNAc functions these data reveal new functions as well as having important regulatory implications for T-cell activation. Similar O-GlcNAc proteome approaches are now being carried out more and more, thus catching up with site-mapping and functional analysis of phosphorylation. In Chapter 4 the role of O-GlcNAc in immune regulation is presented with particular emphasis on the role of O-GlcNAcmodified transcription factors during reprogramming of activated T- and B-cells leading to their maturation. Although the link between T- or B-cell receptor activation and enhanced O-GlcNAc modification on transcription factors is not yet clear, nuclear translocation appears to be at least one step requiring O-GlcNAcylation. Altogether, it appears that decreasing O-GlcNAc levels could reduce responses by the adaptive immune system, which would be beneficial in autoimmune diseases but might facilitate virus replication, e.g., in HIV. On the other hand, in the innate immune response increased O-GlcNAc correlates with enhanced neutrophil motility and protection against inflammation. This view parallels evidence presented in Chapter 5 on the cardiovascular system. Increased O-GlcNAcylation mediates at least a short term cytoprotective effect. This is in accord with earlier observations that the O-GlcNAc level on proteins is increased in response to several stress stimuli. Combining these observations made in vitro and in more clinically relevant in vivo models of myocardial infarction with those pathophysiological effects of enhanced O-GlcNAc in diabetes described in Chapter 1, leads to the hypothesis that the initial response to acute stress produces activation of protective pathways including O-GlcNAc, whereas long lasting activation of the same pathway(s) may result in chronic adverse and pathophysiological effects. A highly promising approach to understanding O-GlcNAc's multiple functions in health and disease and its exploitation for therapeutic application is described in Chapter 6. The nematode Caenorhabditis elegans is an excellent model system allowing rapid large scale genomic and proteomic analyses for studying interactions and signalling pathways for O-GlcNAc modified proteins in wild-type and deletion mutants. Besides the study of nutrient sensing and known signalling pathways in which OGlcNAc participates, the discovery of as yet unknown functions of O-GlcNAc and O-GlcNAc's relationships to other signalling networks through systematic genetic interaction analysis provides a tool for investigating novel connections with other genetic networks. The combinatorial use of these techniques, together with the use of C. elegans high throughput screens, has the potential for revealing valuable data on O-GlcNAc essential for increasing knowledge and for therapeutic development. Finally, to understand precisely the mechanisms by which the enzymes OGT and OGA can recognize and catalyze the cycling of O-GlcNAc on probably thousands of proteins without identifiable consensus sequences, is yet another outstandingly important issue. In Chapter 7 mechanistic, structural and kinetic studies are described which address this and related questions. For several reasons much more is known of the structure and catalytic mechanism of OGA. In recent years highly selective inhibitors of OGA have been designed which are also applicable in vivo, as demonstrated in a study proving that a specific inhibitor can pass the blood brain barrier and is capable of reducing hyperphoshorylation of the tau protein, one of the pathological hallmarks of Alzheimer's disease. Structural studies and investigation of the mechanism of enzymatic action for OGT, as well as the search for selective inhibitors for this enzyme are currently subjects of intense research. Results are awaited with high expectations for progress in gaining deeper insights and in developing therapeutic applications. To conclude I should like to thank all those colleagues who have contributed to this Special Issue for providing a cuttingedge, topical and comprehensive review of the importance and biological significance of protein O-GlcNAcylation.