Identification and cloning of a novel family of coiled-coil domain proteins that interact with O-GlcNAc transferase

Mitochondrial trafficking in neurons

Neurons, perhaps more than any other cell type, depend on mitochondrial trafficking for their survival. Recent studies have elucidated a motor/adaptor complex on the mitochondrial surface that is shared between neurons and other animal cells. In addition to kinesin and dynein, this complex contains the proteins Miro (also called RhoT1/2) and milton (also called TRAK1/2) and is responsible for much, although not necessarily all, mitochondrial movement. Elucidation of the complex has permitted inroads for understanding how this movement is regulated by a variety of intracellular signals, although many mysteries remain. Regulating mitochondrial movement can match energy demand to energy supply throughout the extraordinary architecture of these cells and can control the clearance and replenishing of mitochondria in the periphery. Because the extended axons of neurons contain uniformly polarized microtubules, they have been useful for studying mitochondrial motility in conjunction with biochemical assays in many cell types.

Identification of O-linked N-acetylglucosamine (O-GlcNAc)-modified osteoblast proteins by electron transfer dissociation tandem mass spectrometry reveals proteins critical for bone formation

The nutrient-responsive β-O-linked N-acetylglucosamine (O-GlcNAc) modification of critical effector proteins modulates signaling and transcriptional pathways contributing to cellular development and survival. An elevation in global protein O-GlcNAc modification occurs during the early stages of osteoblast differentiation and correlates with enhanced transcriptional activity of RUNX2, a key regulator of osteogenesis. To identify other substrates of O-GlcNAc transferase in differentiating MC3T3E1 osteoblasts, O-GlcNAc-modified peptides were enriched by wheat germ agglutinin lectin weak affinity chromatography and identified by tandem mass spectrometry using electron transfer dissociation. This peptide fragmentation approach leaves the labile O-linkage intact permitting direct identification of O-GlcNAc-modified peptides. O-GlcNAc modification was observed on enzymes involved in post-translational regulation, including MAST4 and WNK1 kinases, a ubiquitin-associated protein (UBAP2l), and the histone acetyltransferase CREB-binding protein. CREB-binding protein, a transcriptional co-activator that associates with CREB and RUNX2, is O-GlcNAcylated at Ser-147 and Ser-2360, the latter of which is a known site of phosphorylation. Additionally, O-GlcNAcylation of components of the TGFβ-activated kinase 1 (TAK1) signaling complex, TAB1 and TAB2, occurred in close proximity to known sites of Ser/Thr phosphorylation and a putative nuclear localization sequence within TAB2. These findings demonstrate the presence of O-GlcNAc modification on proteins critical to bone formation, remodeling, and fracture healing and will enable evaluation of this modification on protein function and regulation.

TET2 promotes histone O-GlcNAcylation during gene transcription

TET enzymes including TET1, 2 and 3 convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC)¹ and regulate gene transcription^2-5. However, this molecular mechanism by which TET family enzymes regulate gene transcription remains elusive^5-6. Here, using protein affinity purification, we searched for functional partners of TET proteins, and found that TET2 and TET3 associate with OGT, an enzyme that by itself catalyzes O-GlcNAcylation in vivo^7-8. TET2 directly interacts with OGT, which is important for the chromatin association of OGT in vivo. Although this specific interaction does not regulate the enzymatic activity of TET2, it facilitates OGT-dependent histone O-GlcNAcylation. Moreover, OGT associates with TET2 at transcription starting sites (TSS). Down-regulation of TET2 reduces the amount of H2B S112 GlcNAc marks in vivo, which are associated with gene transcription regulation. Taken together, these results reveal a TET2-dependent O-GlcNAcylation of chromatin. The double epigenetic modifications on both DNA and histones by TET2 and OGT coordinate together for the gene transcription regulation.

Cell signaling and mitochondrial dynamics: Implications for neuronal function and neurodegenerative disease

Nascent evidence indicates that mitochondrial fission, fusion, and transport are subject to intricate regulatory mechanisms that intersect with both well-characterized and emerging signaling pathways. While it is well established that mutations in components of the mitochondrial fission/fusion machinery can cause neurological disorders, relatively little is known about upstream regulators of mitochondrial dynamics and their role in neurodegeneration. Here, we review posttranslational regulation of mitochondrial fission/fusion enzymes, with particular emphasis on dynamin-related protein 1 (Drp1), as well as outer mitochondrial signaling complexes involving protein kinases and phosphatases. We also review recent evidence that mitochondrial dynamics has profound consequences for neuronal development and synaptic transmission and discuss implications for clinical translation.

Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease

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.

O-GlcNAc signalling: implications for cancer cell biology

O-GlcNAcylation is the covalent attachment of β-D-N-acetylglucosamine (GlcNAc) sugars to serine or threonine residues of nuclear and cytoplasmic proteins, and it is involved in extensive crosstalk with other post-translational modifications, such as phosphorylation. O-GlcNAcylation is becoming increasing realized as having important roles in cancer-relevant processes, such as cell signalling, transcription, cell division, metabolism and cytoskeletal regulation. However, currently little is known about the specific roles of aberrant O-GlcNAcylation in cancer. In this Opinion article, we summarize the current understanding of O-GlcNAcylation in cancer and its emerging functions in transcriptional regulation at the level of chromatin and transcription factors.

Trafficking kinesin protein (TRAK)-mediated transport of mitochondria in axons of hippocampal neurons

In neurons, the proper distribution of mitochondria is essential because of a requirement for high energy and calcium buffering during synaptic neurotransmission. The efficient, regulated transport of mitochondria along axons to synapses is therefore crucial for maintaining function. The trafficking kinesin protein (TRAK)/Milton family of proteins comprises kinesin adaptors that have been implicated in the neuronal trafficking of mitochondria via their association with the mitochondrial protein Miro and kinesin motors. In this study, we used gene silencing by targeted shRNAi and dominant negative approaches in conjunction with live imaging to investigate the contribution of endogenous TRAKs, TRAK1 and TRAK2, to the transport of mitochondria in axons of hippocampal pyramidal neurons. We report that both strategies resulted in impairing mitochondrial mobility in axonal processes. Differences were apparent in terms of the contribution of TRAK1 and TRAK2 to this transport because knockdown of TRAK1 but not TRAK2 impaired mitochondrial mobility, yet both TRAK1 and TRAK2 were shown to rescue transport impaired by TRAK1 gene knock-out. Thus, we demonstrate for the first time the pivotal contribution of the endogenous TRAK family of kinesin adaptors to the regulation of mitochondrial mobility.

N-acetylglucosamine transferase is an integral component of a kinesin-directed mitochondrial trafficking complex

Trafficking kinesin proteins (TRAKs) 1 and 2 are kinesin-associated proteins proposed to function in excitable tissues as adaptors in anterograde trafficking of cargoes including mitochondria. They are known to associate with N-acetylglucosamine transferase and the mitochondrial rho GTPase, Miro. We used confocal imaging, Förster resonance energy transfer and immunoprecipitations to investigate association between TRAKs1/2, N-acetylglucosamine transferase, the prototypic kinesin-1, KIF5C, and Miro. We demonstrate that in COS-7 cells, N-acetylglucosamine transferase, KIF5C and TRAKs1/2 co-distribute. Förster resonance energy transfer was observed between N-acetylglucosamine transferase and TRAKs1/2. Despite co-distributing with KIF5C and immunoprecipitations demonstrating a TRAK1/2, N-acetylglucosamine transferase and KIF5C ternary complex, no Förster resonance energy transfer was detected between N-acetylglucosamine transferase and KIF5C. KIF5C, N-acetylglucosamine transferase, TRAKs1/2 and Miro formed a quaternary complex. The presence of N-acteylglucosamine transferase partially prevented redistribution of mitochondria induced by trafficking proteins 1/2 and KIF5C. TRAK2 was a substrate for N-acetylglucosamine transferase with TRAK2 (S562) identified as a site of O-N-acetylglucosamine modification. These findings substantiate trafficking kinesin proteins as scaffolds for the formation of a multi-component complex involved in anterograde trafficking of mitochondria. They further suggest that O-glycosylation may regulate complex formation.

[O-GlcNAc glycosylation and regulation of cell signaling]

O-GlcNAcylation corresponds to the addition of N-acetylglucosamine on serine and threonine residues of cytosolic and nuclear proteins. O-GlcNAcylation is a dynamic post-translational modification, analogous to phosphorylation, that regulates the stability, the activity or the sub-cellular localisation of proteins. This reversible modification depends on the availability of glucose and therefore constitutes a powerful means by which cellular activities are regulated according to the nutritional environment of the cell. O-GlcNAcylation has been implicated in important human pathologies including Alzheimer disease and type-2 diabetes. Only two enzymes, OGT and O-GlcNAcase, control the O-GlcNAcylation level on proteins, and thereby regulate signaling pathways. Several lines of evidence indicate that OGT attenuates insulin signal by O-GlcNAcylation of proteins involved in proximal and distal steps in the signaling pathway. This negative feedback may be exacerbated when cells are exposed to elevated glucose concentrations as observed in diabetic patients, and could thereby contribute to insulin resistance and worsening of hyperglycaemia. double dagger.

O-GlcNAc modification, insulin signaling and diabetic complications

O-GlcNAc glycosylation (O-GlcNAcylation) corresponds to the addition of N-acetylglucosamine on serine and threonine residues of cytosolic and nuclear proteins. O-GlcNAcylation is a dynamic post-translational modification, analogous to phosphorylation, that regulates the stability, the activity or the subcellular localisation of target proteins. This reversible modification depends on the availability of glucose and therefore constitutes a powerful mechanism by which cellular activities are regulated according to the nutritional environment of the cell. O-GlcNAcylation has been implicated in important human pathologies including Alzheimer disease and type-2 diabetes. Only two enzymes, OGT and O-GlcNAcase, control the O-GlcNAc level on proteins. Therefore, O-GlcNAcylations cannot organize in signaling cascades as observed for phosphorylations. O-GlcNAcylations should rather be considered as a "rheostat" that controls the intensity of the signals traveling through different pathways according to the nutritional status of the cell. Thus, OGT attenuates insulin signal by O-GlcNAcylation of proteins involved in proximal and distal steps in the PI-3 kinase signaling pathway. This negative feedback may be exacerbated when cells are chronically exposed to elevated glucose concentrations and could thereby contribute to alterations in insulin signaling observed in diabetic patients. O-GlcNAcylation also appears to contribute to the deleterious effects of hyperglycaemia on excessive glucose production by the liver and deterioration of β-cell pancreatic function, resulting in worsening of hyperglycaemia (glucotoxicity). Moreover, O-GlcNAcylations directly participate in several diabetic complications. O-GlcNAcylation of eNOS in endothelial cells have been involved in micro- and macrovascular complications. In addition, O-GlcNAcylations activate the expression of profibrotic and antifibrinolytic factors, contributing to vascular and renal dysfunctions.

O-GlcNAcylation: a novel pathway contributing to the effects of endothelin in the vasculature

Glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc) or O-GlcNAcylation on serine and threonine residues of nuclear and cytoplasmic proteins is a posttranslational modification that alters the function of numerous proteins important in vascular function, including kinases, phosphatases, transcription factors, and cytoskeletal proteins. O-GlcNAcylation is an innovative way to think about vascular signaling events both in physiological conditions and in disease states. This posttranslational modification interferes with vascular processes, mainly vascular reactivity, in conditions where endothelin-1 (ET-1) levels are augmented (e.g. salt-sensitive hypertension, ischemia/reperfusion, and stroke). ET-1 plays a crucial role in the vascular function of most organ systems, both in physiological and pathophysiological conditions. Recognition of ET-1 by the ET(A) and ET(B) receptors activates intracellular signaling pathways and cascades that result in rapid and long-term alterations in vascular activity and function. Components of these ET-1-activated signaling pathways (e.g., mitogen-activated protein kinases, protein kinase C, RhoA/Rho kinase) are also targets for O-GlcNAcylation. Recent experimental evidence suggests that ET-1 directly activates O-GlcNAcylation, and this posttranslational modification mediates important vascular effects of the peptide. This review focuses on ET-1-activated signaling pathways that can be modified by O-GlcNAcylation. A brief description of the O-GlcNAcylation biology is presented, and its role on vascular function is addressed. ET-1-induced O-GlcNAcylation and its implications for vascular function are then discussed. Finally, the interplay between O-GlcNAcylation and O-phosphorylation is addressed.

Aberrant O-GlcNAcylation characterizes chronic lymphocytic leukemia

O-linked N-Acetylglucosamine (O-GlcNAc) post-translational modifications originate from the activity of the hexosamine pathway, and are known to affect intracellular signaling processes. As aberrant responses to microenvironmental signals are a feature of chronic lymphocytic leukemia (CLL), O-GlcNAcylated protein levels were measured in primary CLL cells. In contrast to normal circulating and tonsillar B cells, CLL cells expressed high levels of O-GlcNAcylated proteins, including p53, c-myc and Akt. O-GlcNAcylation in CLL cells increased following activation with cytokines and through toll-like receptors (TLRs), or after loading with hexosamine pathway substrates. However, high baseline O-GlcNAc levels were associated with impaired signaling responses to TLR agonists, chemotherapeutic agents, B cell receptor crosslinking and mitogens. Indolent and aggressive clinical behavior of CLL cells were found to correlate with higher and lower O-GlcNAc levels, respectively. These findings suggest that intracellular O-GlcNAcylation is associated with the pathogenesis of CLL, which could potentially have therapeutic implications.

O-GlcNAc signaling in the cardiovascular system

Cardiovascular function is regulated at multiple levels. Some of the most important aspects of such regulation involve alterations in an ever-growing list of posttranslational modifications. One such modification orchestrates input from numerous metabolic cues to modify proteins and alter their localization and/or function. Known as the beta-O-linkage of N-acetylglucosamine (ie, O-GlcNAc) to cellular proteins, this unique monosaccharide is involved in a diverse array of physiological and pathological functions. This review introduces readers to the general concepts related to O-GlcNAc, the regulation of this modification, and its role in primary pathophysiology. Much of the existing literature regarding the role of O-GlcNAcylation in disease addresses the protracted elevations in O-GlcNAcylation observed during diabetes. In this review, we focus on the emerging evidence of its involvement in the cardiovascular system. In particular, we highlight evidence of protein O-GlcNAcylation as an autoprotective alarm or stress response. We discuss recent literature supporting the idea that promoting O-GlcNAcylation improves cell survival during acute stress (eg, hypoxia, ischemia, oxidative stress), whereas limiting O-GlcNAcylation exacerbates cell damage in similar models. In addition to addressing the potential mechanisms of O-GlcNAc-mediated cardioprotection, we discuss technical issues related to studying protein O-GlcNAcylation in biological systems. The reader should gain an understanding of what protein O-GlcNAcylation is and that its roles in the acute and chronic disease settings appear distinct.

The intersections between O-GlcNAcylation and phosphorylation: implications for multiple signaling pathways

A paradigm-changing discovery in biology came about when it was found that nuclear and cytosolic proteins could be dynamically glycosylated with a single O-linked beta-N-acetylglucosamine (O-GlcNAc) moiety. O-GlcNAcylation is akin to phosphorylation: it occurs on serine and/or threonine side chains of proteins, and cycles rapidly upon cellular activation. O-GlcNAc and phosphate show a complex interplay: they can either competitively occupy a single site or proximal sites, or noncompetitively occupy different sites on a substrate. Phosphorylation regulates O-GlcNAc-cycling enzymes and, conversely, O-GlcNAcylation controls phosphate-cycling enzymes. Such crosstalk is evident in all compartments of the cell, a finding that is congruent with the fundamental role of O-GlcNAc in regulating nutrient- and stress-induced signal transduction. O-GlcNAc transferase is recruited to the plasma membrane in response to insulin and is targeted to substrates by forming transient holoenzyme complexes that have different specificities. Cytosolic O-GlcNAcylation is important for the proper transduction of signaling cascades such as the NFkappaB pathway, whereas nuclear O-GlcNAc is crucial for regulating the activity of numerous transcription factors. This Commentary focuses on recent findings supporting an emerging concept that continuous crosstalk between phosphorylation and O-GlcNAcylation is essential for the control of vital cellular processes and for understanding the mechanisms that underlie certain neuropathologies.

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.

O-linked beta-N-acetylglucosamine (O-GlcNAc): Extensive crosstalk with phosphorylation to regulate signaling and transcription in response to nutrients and stress

BACKGROUND

Since its discovery in the early 1980s, O-linked-beta-N-acetylglucosamine (O-GlcNAc), a single sugar modification on the hydroxyl group of serine or threonine residues, has changed our views of protein glycosylation. While other forms of protein glycosylation modify proteins on the cell surface or within luminal compartments of the secretory machinery, O-GlcNAc modifies myriad nucleocytoplasmic proteins. GlcNAcylated proteins are involved in transcription, ubiquitination, cell cycle, and stress responses. GlcNAcylation is similar to protein phosphorylation in terms of stoichiometry, localization and cycling. To date, only two enzymes are known to regulate GlcNAcylation in mammals: O-GlcNAc transferase (OGT), which catalyzes the addition of O-GlcNAc, and beta-N-acetylglucosaminidase (O-GlcNAcase), a neutral hexosaminidase responsible for O-GlcNAc removal. OGT and O-GlcNAcase are regulated by RNA splicing, by nutrients, and by post-translational modifications. Their specificities are controlled by many transiently associated targeting subunits. As methods for detecting O-GlcNAc have improved our understanding of O-GlcNAc's functions has grown rapidly.

SCOPE OF REVIEW

In this review, the functions of GlcNAcylation in regulating cellular processes, its extensive crosstalk with protein phosphorylation, and regulation of OGT and O-GlcNAcase will be explored.

MAJOR CONCLUSIONS

GlcNAcylation rivals phosphorylation in terms of its abundance, protein distribution and its cycling on and off of proteins. GlcNAcylation has extensive crosstalk with phosphorylation to regulate signaling, transcription and the cytoskeleton in response to nutrients and stress.

GENERAL SIGNIFICANCE

Abnormal crosstalk between GlcNAcylation and phosphorylation underlies dysregulation in diabetes, including glucose toxicity, and defective GlcNAcylation is involved in neurodegenerative disease and cancer and most recently in AIDS.

The Role of the O-GlcNAc Modification in Regulating Eukaryotic Gene Expression

O-linked β-N-acetylglucosamine (O-GlcNAc) modification of proteins has been shown to be involved in many different cellular processes, such as cell cycle control, nutrient sensing, signal transduction, stress response and transcriptional regulation. Cells have developed complex regulatory systems in order to regulate gene expression appropriately in response to environmental and intracellular cues. Control of eukaryotic gene transcription often involves post-translational modification of a multitude of proteins including transcription factors, basal transcription machinery, and chromatin remodeling complexes to modulate their functions in a variety of manners. In this review we describe the emerging functional roles for and techniques to detect and modulate the O-GlcNAc modification and illustrate that the O-GlcNAc modification is intricately involved in at least seven different general mechanisms for the control of gene transcription.

Mitochondrial dysfunction in amyotrophic lateral sclerosis

The etiology of motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains to be better understood. Based on the studies from ALS patients and transgenic animal models, it is believed that ALS is likely to be a multifactorial and multisystem disease. Many mechanisms have been postulated to be involved in the pathology of ALS, such as oxidative stress, glutamate excitotoxicity, mitochondrial damage, defective axonal transport, glia cell pathology and aberrant RNA metabolism. Mitochondria, which play crucial roles in excitotoxicity, apoptosis and cell survival, have shown to be an early target in ALS pathogenesis and contribute to the disease progression. Morphological and functional defects in mitochondria were found in both human patients and ALS mice overexpressing mutant SOD1. Mutant SOD1 was found to be preferentially associated with mitochondria and subsequently impair mitochondrial function. Recent studies suggest that axonal transport of mitochondria along microtubules and mitochondrial dynamics may also be disrupted in ALS. These results also illustrate the critical importance of maintaining proper mitochondrial function in axons and neuromuscular junctions, supporting the emerging "dying-back" axonopathy model of ALS. In this review, we will discuss how mitochondrial dysfunction has been linked to the ALS variants of SOD1 and the mechanisms by which mitochondrial damage contributes to the disease etiology.

Control of mitochondrial transport and localization in neurons

Mitochondria play an essential role in ATP generation, calcium buffering and apoptotic signalling. In neurons, the transport of mitochondria to specific locations where they are needed has emerged as an important process for correct nerve cell function. Recent studies have shed light on the mechanisms that control mitochondrial transport and localization in neurons. We describe the machinery that is important for constitutive transport of mitochondria throughout the cell, and highlight recent advances in our understanding of how signalling pathways can converge on this machinery and allow for rapid activity-dependent control of mitochondrial trafficking and localization. Regulation of mitochondrial trafficking might work in concert with mitochondrial tethering systems to give precise control of mitochondrial delivery and localization to regions of high energy and calcium buffering requirements within neurons.

Genome wide high density SNP-based linkage analysis of childhood absence epilepsy identifies a susceptibility locus on chromosome 3p23-p14

Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy (IGE) characterised by typical absence seizures manifested by transitory loss of awareness with 2.5-4 Hz spike-wave complexes on ictal EEG. A genetic component to the aetiology is well recognised but the mechanism of inheritance and the genes involved are yet to be fully established. A genome wide single nucleotide polymorphism (SNP)-based high density linkage scan was carried out using 41 nuclear pedigrees with at least two affected members. Multipoint parametric and non-parametric linkage analyses were performed using MERLIN 1.1.1 and a susceptibility locus was identified on chromosome 3p23-p14 (Z(mean)=3.9, p<0.0001; HLOD=3.3, alpha=0.7). The linked region harbours the functional candidate genes TRAK1 and CACNA2D2. Fine-mapping using a tagSNP approach demonstrated disease association with variants in TRAK1.

O-GlcNAcylation: a novel post-translational mechanism to alter vascular cellular signaling in health and disease: focus on hypertension

O-Linked attachment of beta-N-acetyl-glucosamine (O-GlcNAc) on serine and threonine residues of nuclear and cytoplasmic proteins is a highly dynamic posttranslational modification that plays a key role in signal transduction pathways. Preliminary data show that O-GlcNAcylation may represent a key regulatory mechanism in the vasculature, modulating contractile and relaxant responses. Proteins with an important role in vascular function, such as endothelial nitric oxide synthase, sarcoplasmic reticulum Ca(2+)-ATPase, protein kinase C, mitogen-activated protein kinases, and proteins involved in cytoskeleton regulation and microtubule assembly are targets for O-GlcNAcylation, indicating that this posttranslational modification may play an important role in vascular reactivity. Here, we will focus on a few specific pathways that contribute to vascular function and cardiovascular disease-associated vascular dysfunction, and the implications of their modification by O-GlcNAc. New chemical tools have been developed to detect and study O-GlcNAcylation, including inhibitors of O-GlcNAc enzymes, chemoenzymatic tagging methods, and quantitative proteomics strategies; these will also be briefly addressed. An exciting challenge in the future will be to better understand the cellular dynamics of this posttranslational modification, as well as the signaling pathways and mechanisms by which O-GlcNAc is regulated on specific proteins in the vasculature in health and disease.

GABA(A) receptors and their associated proteins: implications in the etiology and treatment of schizophrenia and related disorders

Gamma-aminobutyric acid type A (GABA(A)) receptors play an important role in mediating fast synaptic inhibition in the brain. They are ubiquitously expressed in the CNS and also represent a major site of action for clinically relevant drugs. Recent technological advances have greatly clarified the molecular and cellular roles played by distinct GABA(A) receptor subunit classes and isoforms in normal brain function. At the same time, postmortem and genetic studies have linked neuropsychiatric disorders including schizophrenia and bipolar disorder with GABAergic neurotransmission and various specific GABA(A) receptor subunits, while evidence implicating GABA(A)R-associated proteins is beginning to emerge. In this review we discuss the mounting genetic, molecular, and cellular evidence pointing toward a role for GABA(A) receptor heterogeneity in both schizophrenia etiology and therapeutic development. Finally, we speculate on the relationship between schizophrenia-related disorders and selected GABA(A) receptor associated proteins, key regulators of GABA(A) receptor trafficking, targeting, clustering, and anchoring that often carry out these functions in a subtype-specific manner.

Ca2+-dependent regulation of mitochondrial dynamics by the Miro-Milton complex

Calcium oscillations control mitochondrial motility along the microtubules and in turn, support on-demand distribution of mitochondria. However, the mechanism mediating the Ca(2+) effect remained a mystery. Recently, several papers reported on the Ca(2+)-dependent regulation of mitochondrial dynamics by a Miro-Milton complex linking mitochondria to kinesin motors. Both mitochondrial motility and fusion-fission dynamics seem to be sensitive to a Ca(2+)-dependent switch by this complex. Evidence is emerging that calcium signaling through Miro-Milton is central to coordination of the local oxidative metabolism with the energy demands and protection against Ca(2+)-induced cell injury.

Increasing O-GlcNAc levels: An overview of small-molecule inhibitors of O-GlcNAcase

The O-GlcNAc modification is found on many nucleocytoplasmic proteins. The dynamic nature of O-GlcNAc, which in some ways is reminiscent of phosphorylation, has enabled investigators to modulate the stoichiometry of O-GlcNAc on proteins in order to study its function. Although several genetic and pharmacological methods for manipulating O-GlcNAc levels have been described, one of the most direct approaches of increasing global O-GlcNAc levels is by using small-molecule inhibitors of O-GlcNAcase (OGA). As the interest in increasing O-GlcNAc levels has grown, so too has the number of OGA inhibitors. This review provides an overview of the available methods of increasing O-GlcNAc levels, with a special emphasis on inhibition of OGA by small molecules. Known inhibitors of OGA are discussed with particular attention on those most suitable for cell-based biological studies. Several examples in which OGA inhibitors have been used to study the functional role of the O-GlcNAc modification in biological systems are discussed, highlighting the pros and cons of different inhibitors.

O-GlcNAc modifications regulate cell survival and epiboly during zebrafish development

Background

The post-translational addition of the monosaccharide O-linked β-N-acetylglucosamine (O-GlcNAc) regulates the activity of a wide variety of nuclear and cytoplasmic proteins. The enzymes O-GlcNAc Transferase (Ogt) and O-GlcNAcase (Oga) catalyze, respectively, the attachment and removal of O-GlcNAc to target proteins. In adult mice, Ogt and Oga attenuate the response to insulin by modifying several components of the signal transduction pathway. Complete loss of ogt function, however, is lethal to mouse embryonic stem cells, suggesting that the enzyme has additional, unstudied roles in development. We have utilized zebrafish as a model to determine role of O-GlcNAc modifications in development. Zebrafish has two ogt genes, encoding six different enzymatic isoforms that are expressed maternally and zygotically.

Results

We manipulated O-GlcNAc levels in zebrafish embryos by overexpressing zebrafish ogt, human oga or by injecting morpholinos against ogt transcripts. Each of these treatments results in embryos with shortened body axes and reduced brains at 24 hpf. The embryos had 23% fewer cells than controls, and displayed increased rates of cell death as early as the mid-gastrula stages. An extensive marker analysis indicates that derivatives of three germ layers are reduced to variable extents, and the embryos are severely disorganized after gastrulation. Overexpression of Ogt and Oga delayed epiboly and caused a severe disorganization of the microtubule and actin based cytoskeleton in the extra-embryonic yolk syncytial layer (YSL). The cytoskeletal defects resemble those previously reported for embryos lacking function of the Pou5f1/Oct4 transcription factor spiel ohne grenzen. Consistent with this, Pou5f1/Oct4 is modified by O-GlcNAc in human embryonic stem cells.

Conclusion

We conclude that O-GlcNAc modifications control the activity of proteins that regulate apoptosis and epiboly movements, but do not seem to regulate germ layer specification. O-GlcNAc modifies the transcription factor Spiel ohne grenzen/Pou5f1 and may regulate its activity.

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.

Pink1 forms a multiprotein complex with Miro and Milton, linking Pink1 function to mitochondrial trafficking

Recessive mutations in Pink1 lead to a selective degeneration of dopaminergic neurons in the substantia nigra that is characteristic of Parkinson disease. Pink1 is a kinase that is targeted in part to mitochondria, and loss of Pink1 function can alter mitochondrial morphology and dynamics, thus supporting a link between mitochondrial dysfunction and Parkinson disease etiology. Here, we report the unbiased identification and confirmation of a mitochondrial multiprotein complex that contains Pink1, the atypical GTPase Miro, and the adaptor protein Milton. Our screen also identified an interaction between Pink1 and Mitofilin. Based on previously established functions for Miro and Milton in the trafficking of mitochondria along microtubules, we postulate here a role for Pink1 in mitochondrial trafficking. Using subcellular fractionation, we show that the overexpression of Miro and Milton, both of which are known to reside at the outer mitochondrial membrane, increases the mitochondrial Pink1 pool, suggesting a function of Pink1 at the outer membrane. Further, we document that Pink1 expressed without a mitochondrial targeting sequence can still be targeted to a mitochondria-enriched subcellular fraction via Miro and Milton. The latter finding is important for the interpretation of a previously reported protective effect of Pink1 expressed without a mitochondrial targeting sequence. Finally, we find that Miro and Milton expression suppresses altered mitochondrial morphology induced by loss of Pink1 function in cell culture. Our findings suggest that Pink1 functions in the trafficking of mitochondria in cells.

Functional analysis of HvSPY, a negative regulator of GA response, in barley aleurone cells and Arabidopsis

SPINDLY (SPY) is an important regulator of plant development, and consists of an N-half tetratricopeptide repeat (TPR) domain containing 10 TPR motifs and a C-half catalytic domain, similar to O-GlcNAc transferase (OGT) of animals. The best characterised role of SPY is a negative regulator of GA signalling, and all known spy alleles have been isolated based on increased GA response. Of the eight alleles that directly affect the TPR domain, all alter TPRs 6, 8 and/or 9. To test the hypothesis that a subset of TPRs, including 6, 8 and 9, are both essential and sufficient for the regulation of GA response, we overexpressed the full-length barley (Hordeum vulgare L.) SPY protein (HvSPY) and several deletion mutants in barley aleurone cells and in Arabidopsis wild type (WT) and spy-4 plants. Transient assays in barley aleurone cells, that also express endogenous HvSPY, demonstrated that introduced HvSPY and HvTPR inhibited GA(3)-induced alpha-amylase expression. With the exception of HvSPYDelta1-5, the other deletion proteins were partially active in the barley assay, including HvSPYDelta6-9 which lacks TPRs 6, 8 and 9. In Arabidopsis, analysis of seed germination under a range of conditions revealed that 35S:HvSPY increased seed dormancy. Hvspy-2, which lacks parts of the eighth and ninth TPRs, was able to partially complement all aspects of the spy-4 phenotype. In the presence of AtSPY, 35S:HvTPR caused some phenotypes consistent with a decrease in GA signalling, including increased seed sensitivity to paclobutrazol and delayed flowering. These plants also possessed distorted leaf morphology and altered epidermal cell shape. Thus, despite genetic analysis demonstrating that TPRs 6, 8 and 9 are required for regulation of GA signalling, our results suggest that these TPRs are neither absolutely essential nor sufficient for SPY activity.

Mitochondrial transport dynamics in axons and dendrites

Mitochondrial dynamics and transport have emerged as key factors in the regulation of neuronal differentiation and survival. Mitochondria are dynamically transported in and out of axons and dendrites to maintain neuronal and synaptic function. Transport proceeds through a controlled series of plus- and minus-end directed movements along microtubule tracks (MTs) that are often interrupted by short stops. This bidirectional motility of mitochondria is facilitated by plus end-directed kinesin and minus end-directed dynein motors, and may be coordinated and controlled by a number of mechanisms that integrate intracellular signals to ensure efficient transport and targeting of mitochondria. In this chapter, we discuss our understanding of mechanisms that facilitate mitochondrial transport and delivery to specific target sites in dendrites and axons.

A PGC-1alpha-O-GlcNAc transferase complex regulates FoxO transcription factor activity in response to glucose

Metabolic and stress response gene regulation is crucial for the survival of an organism to a changing environment. Three key molecules that sense nutrients and broadly affect gene expression are the FoxO transcription factors, the transcriptional co-activator PGC-1alpha, and the dynamic post-translational modification, O-linked beta-N-acetylglucosamine (O-GlcNAc). Here we identify novel post-translational modifications of PGC-1alpha, including O-GlcNAc, and describe a novel mechanism for how PGC-1alpha co-activates transcription by FoxOs. In liver, in cultured cells, and in vitro with recombinant proteins, PGC-1alpha binds to O-GlcNAc transferase and targets the enzyme to FoxOs, resulting in their increased GlcNAcylation and increased transcriptional activity. Furthermore, glucose-enhanced activation of FoxO1 occurs via this PGC-1alpha-O-GlcNAc transferase-mediated GlcNAcylation. Therefore, one mechanism by which PGC-1alpha can serve as a co-activator of transcription is by targeting the O-GlcNAc transferase to increase GlcNAcylation of specific transcription factors important to nutrient/stress sensing and energy metabolism.

Identification of TRAK1 (Trafficking protein, kinesin-binding 1) as MGb2-Ag: a novel cancer biomarker

The present study aimed to describe the characterization of an antibody MGb2 that reacts with an epitope on gastric cancer cells, and identification of MGb2 antigen (MGb2-Ag). Immunostaining revealed its distribution in human tissues and demonstrated that the positive rate of MGb2-Ag was 81.48% in gastric cancer, 100% in gastric signet-ring cell carcinoma and mucinous adenocarcinoma, 13.16% in precancerous conditions, and 0% in chronic superficial gastritis. Using Western blotting, immunoprecipitation and MALDI-TOF MS (matrix assisted laser desorption/ionization time-of-flight mass spectrometry), MGb2-Ag was identified as TRAK1 (Trafficking protein, kinesin-binding 1), a new molecular gained limited recognition. Both MGb2 and commercial anti-TRAK1 Ab recognized prokaryotic expressed TRAK1. Immunostaining characteristics of TRAK1 were identical with MGb2-Ag in continuous sections of paraffin-embedded tissues of gastric tissues. This is the first report that TRAK1/MGb2-Ag is a promising diagnostic marker for gastric cancer and may help to detect signet-ring cell carcinoma and mucinous adenocarcinoma.

Hypertonia-associated protein Trak1 is a novel regulator of endosome-to-lysosome trafficking

Hypertonia, which is characterized by stiff gait, abnormal posture, jerky movements, and tremor, is associated with a number of neurological disorders, including cerebral palsy, dystonia, Parkinson's disease, stroke, and spinal cord injury. Recently, a spontaneous mutation in the gene encoding trafficking protein, kinesin-binding 1 (Trak1), was identified as the genetic defect that causes hypertonia in mice. The subcellular localization and biological function of Trak1 remain unclear. Here we report that Trak1 interacts with hepatocyte-growth-factor-regulated tyrosine kinase substrate (Hrs), an essential component of the endosomal sorting and trafficking machinery. Double-label immunofluorescence confocal studies show that the endogenous Trak1 protein partially colocalizes with Hrs on early endosomes. Like Hrs, both overexpression and small-interfering-RNA-mediated knockdown of Trak1 inhibit degradation of internalized epidermal growth factor receptors through a block in endosome-to-lysosome trafficking. Our findings support a role for Trak1 in the regulation of Hrs-mediated endosomal sorting and have important implications for understanding hypertonia associated with neurological disorders.

O-linked beta-N-acetylglucosaminyltransferase substrate specificity is regulated by myosin phosphatase targeting and other interacting proteins

O-GlcNAc-transferase (OGT) substrate specificity is regulated by transiently interacting proteins. To further examine the regulation of OGT, we have identified 27 putative OGT-interacting proteins through a yeast two-hybrid screen. Two of these proteins, Trak1 (OIP106) and O-GlcNAcase, have been shown previously to interact with and regulate OGT. We demonstrate here that MYPT1 and CARM1 also interact with and target OGT. MYPT1 and CARM1 are substrates of OGT in vitro and in vivo. MYPT1 and CARM1 also function to alter OGT substrate specificity in vitro. Furthermore depletion of MYPT1 in Neuro-2a neuroblastoma cells alters GlcNAcylation of several proteins under basal conditions, suggesting that MYPT1 regulates OGT substrate specificity in vivo.

A mitotic GlcNAcylation/phosphorylation signaling complex alters the posttranslational state of the cytoskeletal protein vimentin

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.

The O-linked N-acetylglucosamine modification in cellular signalling and the immune system. 'Protein modifications: beyond the usual suspects' review series

The intracellular modification of proteins by the addition of a single O-linked N-acetylglucosamine (O-GlcNAc) molecule is a ubiquitous post-translational modification in eukaryotic cells. It is catalysed by O-linked N-acetylglucosaminyltransferase, which attaches O-GlcNAc to serine/threonine residues, and it is counter-regulated by beta-N-acetylglucosaminidase, which is the antagonistic glycosidase that removes the O-GlcNAc group. O-GlcNAc modification competes with phosphorylation by protein kinases at similar sites, thereby affecting important signalling nodes. Accumulating evidence supports a central role for O-GlcNAc modifications and the corresponding enzymes in the regulation of immune cells, particularly in the activation processes of T and B lymphocytes. Here, we discuss recent advances in the field of O-GlcNAc modifications, focusing on the cells of the immune system.

A C. elegans homolog of huntingtin-associated protein 1 is expressed in chemosensory neurons and in a number of other somatic cell types

Huntingtin-associated protein 1 (HAP1) is a binding partner for huntingtin, the protein responsible for Huntington's disease. In mammals, HAP1 is mostly found in brain where it is expressed in neurons. Although several functions have been proposed for HAP1, its role has not yet been clearly established. In this paper, we report on the identification of a HAP1 Caenorhabditis elegans homolog called T27A3.1. T27A3.1 shows conservation with rat and human HAP1, as well as with Milton, a Drosophila HAP1 homolog. To determine the cellular expression of T27A3.1 (multiple isoforms; a-e), we generated several transgenic worm lines expressing a fluorescent reporter protein [green fluorescent protein (GFP) and DsRed2] under the control of the promoter for T27A3.1. We have found that T27A3.1 is expressed in many cell types including a subset of chemosensory neurons in the head and tail. These include the amphid chemosensory neurons ASKL and R, ASIL and R, ADFL and ASEL, the phasmid neurons PHBL and R, and the CAN neurons that are required for worm survival.

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.

Regulation of the O-linked beta-N-acetylglucosamine transferase by insulin signaling

O-Linked beta-N-acetylglucosamine (O-GlcNAc) transferase (OGT) catalyzes the addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues in response to stimuli or stress analogous to phosphorylation by Ser/Thr-kinases. Like protein phosphatases, OGT appears to be targeted to myriad specific substrates by transiently interacting with specific targeting subunits. Here, we show that OGT is activated by insulin signaling. Insulin treatment of 3T3-L1 adipocytes stimulates both tyrosine phosphorylation and catalytic activity of OGT. A subset of OGT co-immunoprecipitates with the insulin receptor. Insulin stimulates purified insulin receptor to phosphorylate OGT in vitro. OGT is a competitive substrate with reduced and carboxyamidomethylated lysozyme (RCAM-lysozyme), a well characterized insulin receptor substrate. Insulin stimulation of 3T3-L1 adipocytes results in a partial translocation of OGT from the nucleus to the cytoplasm. The insulin activation of OGT results in increased O-GlcNAc modification of OGT and other proteins including, signal transducer and activator of transcription 3 (STAT3). We conclude that insulin stimulates the tyrosine phosphorylation and activity of OGT.

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 transferase is activated by CaMKIV-dependent phosphorylation under potassium chloride-induced depolarization in NG-108-15 cells

Post-translational modification of cellular proteins by beta-o-linked N-acetylglucosamine (o-GlcNAc) moieties plays a significant role in signal transduction by modulating protein stability, protein-protein interactions, transactivation processes, and the enzyme activities of target proteins. Though various classes of proteins are known to be regulated by o-GlcNAc modification (o-GlcNAcylation), the mechanism that regulates o-linked GlcNAc transferase (OGT) activity remains unknown. Here, we report that potassium chloride-induced depolarization provokes the activation of OGT and subsequent o-GlcNAcylation of proteins in neuroblastoma NG-108-15 cells. Moreover, such an induction of protein o-GlcNAcylation was abolished by treating cells with either a voltage-gated calcium channel inhibitor or a calcium/calmodulin-dependent protein kinase (CaMK) inhibitor. In addition, CaMKIV was found to specifically phosphorylate and activate OGT in vivo and in vitro, which implies that CaMKIV is required for depolarization-induced activation of OGT. Furthermore, we found that OGT is involved in depolarization-induced and CaMKIV-dependent activation of activator protein-1 (AP-1) and subsequent tissue inhibitor of metalloproteinase-1 (Timp-1) gene expression. Taken together, our findings suggest that CaMKIV activated OGT, and OGT has an essential role on the process of CaMKIV-dependent AP-1 activation under depolarization in neuronal cells.

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.

Cycling of O-linked beta-N-acetylglucosamine on nucleocytoplasmic proteins

All animals and plants dynamically attach and remove O-linked beta-N-acetylglucosamine (O-GlcNAc) at serine and threonine residues on myriad nuclear and cytoplasmic proteins. O-GlcNAc cycling, which is tightly regulated by the concerted actions of two highly conserved enzymes, serves as a nutrient and stress sensor. On some proteins, O-GlcNAc competes directly with phosphate for serine/threonine residues. Glycosylation with O-GlcNAc modulates signalling, and influences protein expression, degradation and trafficking. Emerging data indicate that O-GlcNAc glycosylation has a role in the aetiology of diabetes and neurodegeneration.

Elevation of the post-translational modification of proteins by O-linked N-acetylglucosamine leads to deterioration of the glucose-stimulated insulin secretion in the pancreas of diabetic Goto-Kakizaki rats

Many nuclear and cytoplasmic proteins are O-glycosylated on serine or threonine residues with the monosaccharide beta-N-acetylglucosamine, which is then termed O-linked N-acetylglucosamine (O-GlcNAc). It has been shown that abnormal O-GlcNAc modification (O-GlcNAcylation) of proteins is one of the causes of insulin resistance and diabetic complications. In this study, in order to examine the relationship between O-GlcNAcylation of proteins and glucose-stimulated insulin secretion in noninsulin-dependent type (type 2) diabetes, we investigated the level of O-GlcNAcylation of proteins, especially that of PDX-1, and the expression of O-GlcNAc transferase in Goto-Kakizaki (GK) rats, which are an animal model of type-2 diabetes. By immunoblot and immunohistochemical analyses, the expression of O-GlcNAc transferase protein and O-GlcNAc-modified proteins in whole pancreas and islets of Langerhans of 15-week-old diabetic GK rats and nondiabetic Wistar rats was examined. The expression of O-GlcNAc transferase at the protein level and O-GlcNAc transferase activity were increased significantly in the diabetic pancreas and islets. The diabetic pancreas and islets also showed an increase in total cellular O-GlcNAc-modified proteins. O-GlcNAcylation of PDX-1 was also increased. In the diabetic GK rats, significant increases in the immunoreactivities of both O-GlcNAc and O-GlcNAc transferase were observed. PUGNAc, an inhibitor of O-GlcNAcase, induced an elevation of O-GlcNAc level and a decrease of glucose-stimulated insulin secretion in isolated islets. These results indicate that elevation of the O-GlcNAcylation of proteins leads to deterioration of insulin secretion in the pancreas of diabetic GK rats, further providing evidence for the role of O-GlcNAc in the insulin secretion.

GRIF1 binds Hrs and is a new regulator of endosomal trafficking

Endosomal sorting of internalized cell surface receptors to the lysosomal pathway plays a crucial role in the control of cell signaling and function. Here we report the identification of GABA(A) receptor interacting factor-1 (GRIF1), a recently discovered protein of unknown function, as a new regulator of endosome-to-lysosome trafficking. Yeast two-hybrid screen and co-immunoprecipitation analysis reveal that GRIF1 interacts with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), an essential component of the endosomal sorting machinery. We have mapped the binding domains of GRIF1 and Hrs that mediate their association and shown the colocalization of GRIF1 with Hrs on early endosomes. Like Hrs, both overexpression and siRNA-mediated depletion of GRIF1 inhibit the degradation of internalized epidermal growth factor receptors and block the trafficking of the receptors from early endosomes to the lysosomal pathway. Our results indicate, for the first time, a functional role for GRIF1 in the regulation of endosomal trafficking. Interestingly, overexpression of full-length GRIF1, but not the Hrs- or kinesin-interacting GRIF1 deletion mutants, causes a perinuclear clustering of early endosomes. Our findings suggest that GRIF1 may also participate in microtubule-based transport of early endosomes by acting as an adaptor linking Hrs-containing endosomes to kinesin.