Chemical approaches to understanding O-GlcNAc glycosylation in the brain

Regulation of calcium/calmodulin-dependent kinase IV by O-GlcNAc modification

Similar to phosphorylation, GlcNAcylation (the addition of O-GlcNAc to Ser(Thr) residues on polypeptides) is an abundant, dynamic, and inducible post-translational modification. GlcNAcylated proteins are crucial in regulating virtually all cellular processes, including signaling, cell cycle, and transcription. Here we show that calcium/calmodulin-dependent kinase IV (CaMKIV) is highly GlcNAcylated in vivo. In addition, we show that upon activation of HEK293 cells, hemagglutinin-tagged CaMKIV GlcNAcylation rapidly decreases, in a manner directly opposing its phosphorylation at Thr-200. Correspondingly, there is an increase in CaMKIV interaction with O-GlcNAcase during CaMKIV activation. Furthermore, we identify at least five sites of GlcNAcylation on CaMKIV. Using site-directed mutagenesis, we determine that the GlcNAcylation sites located in the active site of CaMKIV can modulate its phosphorylation at Thr-200 and its activity toward cAMP-response element-binding transcription factor. Our results strongly indicate that the O-GlcNAc modification participates in the regulation of CaMKIV activation and function, possibly coordinating nutritional signals with the immune and nervous systems. This is the first example of an O-GlcNAc/phosphate cycle involving O-GlcNAc transferase/kinase cross-talk.

The hexosamine signaling pathway: O-GlcNAc cycling in feast or famine

The enzymes of O-GlcNAc cycling couple the nutrient-dependent synthesis of UDP-GlcNAc to O-GlcNAc modification of Ser/Thr residues of key nuclear and cytoplasmic targets. This series of reactions culminating in O-GlcNAcylation of targets has been termed the hexosamine signaling pathway (HSP). The evolutionarily ancient enzymes of O-GlcNAc cycling have co-evolved with other signaling effecter molecules; they are recruited to their targets by many of the same mechanisms used to organize canonic kinase-dependent signaling pathways. This co-recruitment of the enzymes of O-GlcNAc cycling drives a binary switch impacting pathways of anabolism and growth (nutrient uptake) and catabolic pathways (nutrient sparing and salvage). The hexosamine signaling pathway (HSP) has thus emerged as a versatile cellular regulator modulating numerous cellular signaling cascades influencing growth, metabolism, cellular stress, circadian rhythm, and host-pathogen interactions. In mammals, the nutrient-sensing HSP has been harnessed to regulate such cell-specific functions as neutrophil migration, and activation of B-cells and T-cells. This review summarizes the diverse approaches being used to examine O-GlcNAc cycling. It will emphasize the impact O-GlcNAcylation has upon signaling pathways that may be become deregulated in diseases of the immune system, diabetes mellitus, cancer, cardiovascular disease, and neurodegenerative diseases.

Direct in-gel fluorescence detection and cellular imaging of O-GlcNAc-modified proteins

We report an advanced chemoenzymatic strategy for the direct fluorescence detection, proteomic analysis, and cellular imaging of O-GlcNAc-modified proteins. O-GlcNAc residues are selectively labeled with fluorescent or biotin tags using an engineered galactosyltransferase enzyme and [3 + 2] azide-alkyne cycloaddition chemistry. We demonstrate that this approach can be used for direct in-gel detection and mass spectrometric identification of O-GlcNAc proteins, identifying 146 novel glycoproteins from the mammalian brain. Furthermore, we show that the method can be exploited to quantify dynamic changes in cellular O-GlcNAc levels and to image O-GlcNAc-glycosylated proteins within cells. As such, this strategy enables studies of O-GlcNAc glycosylation that were previously inaccessible and provides a new tool for uncovering the physiological functions of O-GlcNAc.

Cross-talk between GlcNAcylation and phosphorylation: roles in insulin resistance and glucose toxicity

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a dynamic posttranslational modification that, analogous to phosphorylation, cycles on and off serine and/or threonine hydroxyl groups. Cycling of O-GlcNAc is regulated by the concerted actions of O-GlcNAc transferase and O-GlcNAcase. GlcNAcylation is a nutrient/stress-sensitive modification that regulates proteins involved in a wide array of biological processes, including transcription, signaling, and metabolism. GlcNAcylation is involved in the etiology of glucose toxicity and chronic hyperglycemia-induced insulin resistance, a major hallmark of type 2 diabetes. Several reports demonstrate a strong positive correlation between GlcNAcylation and the development of insulin resistance. However, recent studies suggest that inhibiting GlcNAcylation does not prevent hyperglycemia-induced insulin resistance, suggesting that other mechanisms must also be involved. To date, proteomic analyses have identified more than 600 GlcNAcylated proteins in diverse functional classes. However, O-GlcNAc sites have been mapped on only a small percentage (<15%) of these proteins, most of which were isolated from brain or spinal cord tissue and not from other metabolically relevant tissues. Mapping the sites of GlcNAcylation is not only necessary to elucidate the complex cross-talk between GlcNAcylation and phosphorylation but is also key to the design of site-specific mutational studies and necessary for the generation of site-specific antibodies, both of which will help further decipher O-GlcNAc's functional roles. Recent technical advances in O-GlcNAc site-mapping methods should now finally allow for a much-needed increase in site-specific analyses to address the functional significance of O-GlcNAc in insulin resistance and glucose toxicity as well as other major biological processes.

Recent advances in chemical proteomics: exploring the post-translational proteome

Identification and quantification of multiple proteins from complex mixtures is a central theme in post-genomic biology. Despite recent progress in high-throughput proteomics, proteomic analysis of post-translationally modified (PTM) proteins remains particularly challenging. This mini-review introduces the emerging field of chemical proteomics and reviews recent advances in chemical proteomic technology that are offering striking new insights into the functional biology of post-translational modification.