O-GlcNAcylation regulates ischemia-induced neuronal apoptosis through AKT signaling

O-GlcNAcylation in ischemic diseases

Protein glycosylation is an extensively studied field, with the most studied forms being oxygen or nitrogen-linked N-acetylglucosamine (O-GlcNAc or N-GlcNAc) glycosylation. Particular residues on proteins are targeted by O-GlcNAcylation, which is among the most intricate post-translational modifications. Significantly contributing to an organism's proteome, it influences numerous factors affecting protein stability, function, and subcellular localization. It also modifies the cellular function of target proteins that have crucial responsibilities in controlling pathways related to the central nervous system, cardiovascular homeostasis, and other organ functions. Under conditions of acute stress, changes in the levels of O-GlcNAcylation of these proteins may have a defensive function. Nevertheless, deviant O-GlcNAcylation nullifies this safeguard and stimulates the advancement of several ailments, the prognosis of which relies on the cellular milieu. Hence, this review provides a concise overview of the function and comprehension of O-GlcNAcylation in ischemia diseases, aiming to facilitate the discovery of new therapeutic targets for efficient treatment, particularly in patients with diabetes.

Hyperglycemia Aggravates the Cerebral Ischemia Injury via Protein O-GlcNAcylation

BACKGROUND

At least one-third of Alzheimer's disease (AD) patients have cerebrovascular abnormalities, micro- and macro-infarctions, and ischemic white matter alterations. Stroke prognosis impacts AD development due to vascular disease. Hyperglycemia can readily produce vascular lesions and atherosclerosis, increasing the risk of cerebral ischemia. Our previous studies have proved that protein O-GlcNAcylation-a dynamic and reversible post-translational modification, protects against ischemic stroke. However, the role of O-GlcNAcylation in hyperglycemia aggravating cerebral ischemia injury remained unclear.

OBJECTIVE

In the present study, we investigated the role and mechanism of protein O-GlcNAcylation in hyperglycemia exacerbating cerebral ischemia injury.

METHODS

High glucose-cultured brain microvascular endothelial (bEnd3) cells were injured by oxygen-glucose deprivation. Cell viability was used as the assay result. Stroke outcomes and hemorrhagic transformation incidence were assessed in mice after middle cerebral artery occlusion under high glucose and streptozotocin-induced hyperglycemic conditions. Western blot estimated that O-GlcNAcylation influenced apoptosis levels in vitro and in vivo.

RESULTS

In in vitro analyses showed that Thiamet-G induces upregulation of protein O-GlcNAcylation, which attenuates oxygen-glucose deprivation/R-induce injury in bEnd3 cells cultured under normal glucose conditions, while aggravated it under high glucose conditions. In in vivo analyses, Thiamet-G exacerbated cerebral ischemic injury and induced hemorrhagic transformation, accompanied by increased apoptosis. While blocking protein O-GlcNAcylation with 6-diazo-5-oxo-L-norleucine alleviated cerebral injury of ischemic stroke in different hyperglycemic mice.

CONCLUSION

Overall, our study indicates a critical role for O-GlcNAcylation in that hyperglycemia aggravates cerebral ischemia injury. O-GlcNAcylation may be a potential therapeutic drug for ischemic stroke associated with AD.

Deep learning from harmonized peptide libraries enables retention time prediction of diverse post translational modifications

In proteomics experiments, peptide retention time (RT) is an orthogonal property to fragmentation when assessing detection confidence. Advances in deep learning enable accurate RT prediction for any peptide from sequence alone, including those yet to be experimentally observed. Here we present Chronologer, an open-source software tool for rapid and accurate peptide RT prediction. Using new approaches to harmonize and false-discovery correct across independently collected datasets, Chronologer is built on a massive database with >2.2 million peptides including 10 common post-translational modification (PTM) types. By linking knowledge learned across diverse peptide chemistries, Chronologer predicts RTs with less than two-thirds the error of other deep learning tools. We show how RT for rare PTMs, such as OGlcNAc, can be learned with high accuracy using as few as 10-100 example peptides in newly harmonized datasets. This iteratively updatable workflow enables Chronologer to comprehensively predict RTs for PTM-marked peptides across entire proteomes.

The role of protein glycosylation in the occurrence and outcome of acute ischemic stroke

Acute ischemic stroke (AIS) is a serious and life-threatening disease worldwide. Despite thrombolysis or endovascular thrombectomy, a sizeable fraction of patients with AIS have adverse clinical outcomes. In addition, existing secondary prevention strategies with antiplatelet and anticoagulant drugs therapy are not able to adequately decrease the risk of ischemic stroke recurrence. Thus, exploring novel mechanisms for doing so represents an urgent need for the prevention and treatment of AIS. Recent studies have discovered that protein glycosylation plays a critical role in the occurrence and outcome of AIS. As a common co- and post-translational modification, protein glycosylation participates in a wide variety of physiological and pathological processes by regulating the activity and function of proteins or enzymes. Protein glycosylation is involved in two causes of cerebral emboli in ischemic stroke: atherosclerosis and atrial fibrillation. Following ischemic stroke, the level of brain protein glycosylation becomes dynamically regulated, which significantly affects stroke outcome through influencing inflammatory response, excitotoxicity, neuronal apoptosis, and blood-brain barrier disruption. Drugs targeting glycosylation in the occurrence and progression of stroke may represent a novel therapeutic idea. In this review, we focus on possible perspectives about how glycosylation affects the occurrence and outcome of AIS. We then propose the potential of glycosylation as a therapeutic drug target and prognostic marker for AIS patients in the future.

Glycophagy mediated glucose-induced changes of hepatic glycogen metabolism via OGT1-AKT1-FOXO1Ser238 pathway

Glycophagy is the autophagy degradation of glycogen. However, the regulatory mechanisms for glycophagy and glucose metabolism remain unexplored. Herein, we demonstrated that high-carbohydrate diet (HCD) and high glucose (HG) incubation induced glycogen accumulation, AKT1 expression and AKT1-dependent phosphorylation of forkhead transcription factor O1 (FOXO1) at Ser238 in the liver tissues and hepatocytes. The glucose-induced FOXO1 phosphorylation at Ser238 prevents FOXO1 entry into the nucleus and the recruitment to the gabarapl1 promoter, reduces the gabarapl1 promoter activity, and inhibits glycophagy and glucose production. The glucose-dependent O-GlcNAcylation of AKT1 by OGT1 enhances the stability of AKT1 protein and promotes its binding with FOXO1. Moreover, the glycosylation of AKT1 is crucial for promoting FOXO1 nuclear translocation and inhibiting glycophagy. Our studies elucidate a novel mechanism for glycophagy inhibition by high carbohydrate and glucose via OGT1-AKT1-FOXO1^Ser238 pathway in the liver tissues and hepatocytes, which provides critical insights into potential intervention strategies for glycogen storage disorders in vertebrates, as well as human beings.

Dysregulation of hexosamine biosynthetic pathway wiring metabolic signaling circuits in cancer

Metabolite sensing, a fundamental biological process, plays a key role in metabolic signaling circuit rewiring. Hexosamine biosynthetic pathway (HBP) is a glucose metabolic pathway essential for the synthesis of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), which senses key nutrients and integrally maintains cellular homeostasis. UDP-GlcNAc dynamically regulates protein N-glycosylation and O-linked-N-acetylglucosamine modification (O-GlcNAcylation). Dysregulated HBP flux leads to abnormal protein glycosylation, and contributes to cancer development and progression by affecting protein function and cellular signaling. Furthermore, O-GlcNAcylation regulates cellular signaling pathways, and its alteration is linked to various cancer characteristics. Additionally, recent findings have suggested a close association between HBP stimulation and cancer stemness; an elevated HBP flux promotes cancer cell conversion to cancer stem cells and enhances chemotherapy resistance via downstream signal activation. In this review, we highlight the prominent roles of HBP in metabolic signaling and summarize the recent advances in HBP and its downstream signaling, relevant to cancer.

O-GlcNAcylation in Renal (Patho)Physiology

Kidneys maintain internal milieu homeostasis through a well-regulated manipulation of body fluid composition. This task is performed by the correlation between structure and function in the nephron. Kidney diseases are chronic conditions impacting healthcare programs globally, and despite efforts, therapeutic options for its treatment are limited. The development of chronic degenerative diseases is associated with changes in protein O-GlcNAcylation, a post-translation modification involved in the regulation of diverse cell function. O-GlcNAcylation is regulated by the enzymatic balance between O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) which add and remove GlcNAc residues on target proteins, respectively. Furthermore, the hexosamine biosynthetic pathway provides the substrate for protein O-GlcNAcylation. Beyond its physiological role, several reports indicate the participation of protein O-GlcNAcylation in cardiovascular, neurodegenerative, and metabolic diseases. In this review, we discuss the impact of protein O-GlcNAcylation on physiological renal function, disease conditions, and possible future directions in the field.

Nutrient-sensitive protein O-GlcNAcylation shapes daily biological rhythms

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a nutrient-sensitive protein modification that alters the structure and function of a wide range of proteins involved in diverse cellular processes. Similar to phosphorylation, another protein modification that targets serine and threonine residues, O-GlcNAcylation occupancy on cellular proteins exhibits daily rhythmicity and has been shown to play critical roles in regulating daily rhythms in biology by modifying circadian clock proteins and downstream effectors. We recently reported that daily rhythm in global O-GlcNAcylation observed in Drosophila tissues is regulated via the integration of circadian and metabolic signals. Significantly, mistimed feeding, which disrupts coordination of these signals, is sufficient to dampen daily O-GlcNAcylation rhythm and is predicted to negatively impact animal biological rhythms and health span. In this review, we provide an overview of published and potential mechanisms by which metabolic and circadian signals regulate hexosamine biosynthetic pathway metabolites and enzymes, as well as O-GlcNAc processing enzymes to shape daily O-GlcNAcylation rhythms. We also discuss the significance of functional interactions between O-GlcNAcylation and other post-translational modifications in regulating biological rhythms. Finally, we highlight organ/tissue-specific cellular processes and molecular pathways that could be modulated by rhythmic O-GlcNAcylation to regulate time-of-day-specific biology.

Phosphorylation of plant virus proteins: Analysis methods and biological functions

Phosphorylation is one of the most extensively investigated post-translational modifications that orchestrate a variety of cellular signal transduction processes. The phosphorylation of virus-encoded proteins plays an important regulatory role in the infection cycle of such viruses in plants. In recent years, molecular mechanisms underlying the phosphorylation of plant viral proteins have been widely studied. Based on recent publications, our study summarizes the phosphorylation analyses of plant viral proteins and categorizes their effects on biological functions according to the viral life cycle. This review provides a theoretical basis for elucidating the molecular mechanisms of viral infection. Furthermore, it deepens our understanding of the biological functions of phosphorylation in the interactions between plants and viruses.

Systems pharmacology, proteomics and in vivo studies identification of mechanisms of cerebral ischemia injury amelioration by Huanglian Jiedu Decoction

ETHNOPHARMACOLOGICAL RELEVANCE

Huanglian Jiedu Decoction (HLJDD) has the effect of clearing heat and detoxifying, and has been considered as an effective prescription for cerebral ischemia (CI) for thousands of years in traditional Chinese medicine (TCM). It can improve the quality of life of patients with ischemic stroke, but its pharmacological mechanism remains unclear.

AIM OF THE STUDY

The study aimed to explore the pharmacological action and potential mechanism of HLJDD against CI by systems pharmacology, proteomics and in vivo experiments.

MATERIALS AND METHODS

In this study, databases such as TCMIP V2.0 and Genecards were used to predict compounds, targets and CI related targets, and network topology criteria of protein-protein interaction (PPI) network was used to screen core targets. The Database for Annotation, Visualization and Integrated Discovery database (DAVID) was used to discover biological processes and pathways. In addition, molecular docking was performed between the screened core biological active compounds and targets to verify the binding activity. Finally, proteomics and Western blot were performed on cerebral cortex tissues of middle cerebral artery occlusion (MCAO) model rats with HLJDD intervention to further verify the predicted results.

RESULTS

77 compounds and 308 targets of HLJDD were identified, 54 of which were predicted to be associated with cerebral ischemia. PPI network and enrichment results showed that 8 targets, including AKT1, PTGS2 and TLR4, were core targets of HLJDD in CI. And 19 signaling pathways, including Rap1 signaling pathway, cAMP signaling pathway and arachidonic acid metabolism, were identified as key pathways to the therapeutic activity of HLJDD in CI. Combined with proteomics studies, we identified that Rap1 signaling pathway and upstream and downstream targets were the key mechanisms. Molecular biology experiments showed that RAP1A and AKT expression levels were significantly up-regulated in middle cerebral artery occlusion (MCAO) rats treated with HLJDD (P < 0.0001), GRIN1 expression level was significantly down-regulated (P < 0.0001). However, ACTB expression level was slightly down-regulated (P > 0.05), which may be related to the biological function.

CONCLUSION

This study confirms the pharmacological effect of HLJDD on cerebral ischemia. These results indicate that HLJDD mediates various pathways such as inhibition of apoptosis, regulation of oxygen balance, inhibition of excitatory toxicity and maintenance of basic cell functions to improve CI by regulating Rap1 signaling pathway.

Writing and Erasing O-GlcNAc on Casein Kinase 2 Alpha Alters the Phosphoproteome

O-GlcNAc is an essential carbohydrate modification that intersects with phosphorylation signaling pathways via crosstalk on protein substrates or by direct modification of the kinases that write the phosphate modification. Casein kinase 2 alpha (CK2α), the catalytic subunit of the ubiquitously expressed and constitutively active kinase CK2, is modified by O-GlcNAc, but the effect of this modification on the phosphoproteome in cells is unknown. Here, we apply complementary targeted O-GlcNAc editors, nanobody-OGT and -splitOGA, to selectively write and erase O-GlcNAc from a tagged CK2α to measure the effects on the phosphoproteome in cells. These tools effectively and selectively edit the Ser347 glycosite on CK2α. Using quantitative phosphoproteomics, we report 51 phosphoproteins whose enrichment changes as a function of editing O-GlcNAc on CK2α, including HDAC1, HDAC2, ENSA, SMARCAD1, and PABPN1. Specific phosphosites on HDAC1 Ser393 and HDAC2 Ser394, both reported CK2 substrates, are significantly enhanced by O-GlcNAcylation of CK2α. These data will propel future studies on the crosstalk between O-GlcNAc and phosphorylation.

Comorbidity of osteoporosis and Alzheimer's disease: Is `AKT `-ing on cellular glucose uptake the missing link?

Osteoporosis and Alzheimer's disease (AD) are both degenerative diseases. Osteoporosis often proceeds cognitive deficits, and multiple studies have revealed common triggers that lead to energy deficits in brain and bone. Risk factors for osteoporosis and AD, such as obesity, type 2 diabetes, aging, chemotherapy, vitamin deficiency, alcohol abuse, and apolipoprotein Eε4 and/or Il-6 gene variants, reduce cellular glucose uptake, and protective factors, such as estrogen, insulin, exercise, mammalian target of rapamycin inhibitors, hydrogen sulfide, and most phytochemicals, increase uptake. Glucose uptake is a fine-tuned process that depends on an abundance of glucose transporters (Gluts) on the cell surface. Gluts are stored in vesicles under the plasma membrane, and protective factors cause these vesicles to fuse with the membrane, resulting in presentation of Gluts on the cell surface. This translocation depends mainly on AKT kinase signaling and can be affected by a range of factors. Reduced AKT kinase signaling results in intracellular glucose deprivation, which causes endoplasmic reticulum stress and iron depletion, leading to activation of HIF-1α, the transcription factor necessary for higher Glut expression. The link between diseases and aging is a topic of growing interest. Here, we show that diseases that affect the same biochemical pathways tend to co-occur, which may explain why osteoporosis and/or diabetes are often associated with AD.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Multifaceted Regulation of Akt by Diverse C-Terminal Post-translational Modifications

Akt is a Ser/Thr protein kinase that regulates cell growth and metabolism and is considered a therapeutic target for cancer. Regulation of Akt by membrane recruitment and post-translational modifications (PTMs) has been extensively studied. The most well-established mechanism for cellular Akt activation involves phosphorylation on its activation loop on Thr308 by PDK1 and on its C-terminal tail on Ser473 by mTORC2. In addition, dual phosphorylation on Ser477 and Thr479 has been shown to activate Akt. Other C-terminal tail PTMs have been identified, but their functional impacts have not been well-characterized. Here, we investigate the regulatory effects of phosphorylation of Tyr474 and O-GlcNAcylation of Ser473 on Akt. We use expressed protein ligation as a tool to produce semisynthetic Akt proteins containing phosphoTyr474 and O-GlcNAcSer473 to dissect the enzymatic functions of these PTMs. We find that O-GlcNAcylation at Ser473 and phosphorylation at Tyr474 can also partially increase Akt's kinase activity toward both peptide and protein substrates. Additionally, we performed kinase assays employing human protein microarrays to investigate global substrate specificity of Akt, comparing phosphorylated versus O-GlcNAcylated Ser473 forms. We observed a high similarity in the protein substrates phosphorylated by phosphoSer473 Akt and O-GlcNAcSer473 Akt. Two Akt substrates identified using microarrays, PPM1H, a protein phosphatase, and NEDD4L, an E3 ubiquitin ligase, were validated in solution-phase assays and cell transfection experiments.

Comparison of Cerebral Cortex Transcriptome Profiles in Ischemic Stroke and Alzheimer’s Disease Models

Ischemic stroke and Alzheimer’s disease (AD) are representative geriatric diseases with a rapidly increasing prevalence worldwide. Recent studies have reported an association between ischemic stroke neuropathology and AD neuropathology. Ischemic stroke shares some similar characteristics with AD, such as glia activation-induced neuroinflammation, amyloid beta accumulation, and neuronal cell loss, as well as some common risk factors with AD progression. Although there are considerable similarities in neuropathology between ischemic stroke and AD, no studies have ever compared specific genetic changes of brain cortex between ischemic stroke and AD. Therefore, in this study, I compared the cerebral cortex transcriptome profile of 5xFAD mice, an AD mouse model, with those of middle cerebral artery occlusion (MCAO) mice, an ischemic stroke mouse model. The data showed that the expression of many genes with important functional implications in MCAO mouse brain cortex were related to synaptic dysfunction and neuronal cell death in 5xFAD mouse model. In addition, changes in various protein-coding RNAs involved in synaptic plasticity, amyloid beta accumulation, neurogenesis, neuronal differentiation, glial activation, inflammation and neurite outgrowth were observed. The findings could serve as an important basis for further studies to elucidate the pathophysiology of AD in patients with ischemic stroke.

Writing and erasing O-GlcNAc from target proteins in cells

O-linked N-acetylglucosamine (O-GlcNAc) is a widespread reversible modification on nucleocytoplasmic proteins that plays an important role in many biochemical processes and is highly relevant to numerous human diseases. The O-GlcNAc modification has diverse functional impacts on individual proteins and glycosites, and methods for editing this modification on substrates are essential to decipher these functions. Herein, we review recent progress in developing methods for O-GlcNAc regulation, with a focus on methods for editing O-GlcNAc with protein- and site-selectivity in cells. The applications, advantages, and limitations of currently available strategies for writing and erasing O-GlcNAc and future directions are also discussed. These emerging approaches to manipulate O-GlcNAc on a target protein in cells will greatly accelerate the development of functional studies and enable therapeutic interventions in the O-GlcNAc field.

Anti-Inflammatory Dipeptide, a Metabolite from Ambioba Secretion, Protects Cerebral Ischemia Injury by Blocking Apoptosis Via p-JNK/Bax Pathway

MQ (l-methionyl-l-glutamic acid), anti-inflammatory dipeptide, is one of the metabolites of monocyte locomotion inhibitory factor, a thermostable pentapeptide secreted by Entamoeba histolytica. Monocyte locomotion inhibitory factor injection has been approved as an investigational drug for the potential neural protection in acute ischemic stroke. This study further investigated the neuroprotective effect of MQ in ischemic brain damage. Ischemia-reperfusion injury of the brain was induced in the rat model by middle cerebral artery occlusion. 2,3,5-triphenyltetrazolium chloride staining assay was used to measure cerebral infarction areas in rats. Laser Doppler measurement instrument was used to detect blood flow changes in the rat model. Nissl staining and NeuN staining were utilized to observe the numbers and structures of neuron cells, and the pathological changes in the brain tissues were examined by hematoxylin–eosin staining. Terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling (TUNEL) staining was used to assess cell apoptosis. The changes in oxidative stress indexes, superoxide dismutase and malondialdehyde (MDA), were measured in serum. Methyl thiazolyl tetrazolium was used to measure the survival rates of PC12 cells. Flow cytometry assessed the apoptosis rates and the levels of reactive oxygen species. Real-time PCR was used to evaluate the mRNA expression levels, and Western blotting was used to analyze the changes in protein levels of p-JNK, Bax, cleaved Caspase3. We revealed that MQ improved neurobehavior, decreased cerebral infarction areas, altered blood flow volume, and the morphology of the cortex and hippocampus. On the other hand, it decreased the apoptosis of cortical neurons and the levels of MDA, and increased the levels of superoxide dismutase. In vitro studies demonstrated that MQ enhanced the cell survival rates and decreased the levels of reactive oxygen species. Compared to the oxygen-glucose deprivation/reperfusion group, the protein and mRNA expressions of p-JNK, Bax, cleaved Caspase3 was decreased significantly. These findings suggested that MQ exerts a neuroprotective effect in cerebral ischemia by blocking apoptosis via the p-JNK/Bax pathway.

Adeno-associated viral (AAV) vector-mediated therapeutics for diabetic cardiomyopathy - current and future perspectives

Diabetes increases the prevalence of heart failure by 6-8-fold, independent of other comorbidities such as hypertension and coronary artery disease, a phenomenon termed diabetic cardiomyopathy. Several key signalling pathways have been identified that drive the pathological changes associated with diabetes-induced heart failure. This has led to the development of multiple pharmacological agents that are currently available for clinical use. While fairly effective at delaying disease progression, these treatments do not reverse the cardiac damage associated with diabetes. One potential alternative avenue for targeting diabetes-induced heart failure is the use of adeno-associated viral vector (AAV) gene therapy, which has shown great versatility in a multitude of disease settings. AAV gene therapy has the potential to target specific cells or tissues, has a low host immune response and has the possibility to represent a lifelong cure, not possible with current conventional pharmacotherapies. In this review, we will assess the therapeutic potential of AAV gene therapy as a treatment for diabetic cardiomyopathy.

Metabolic reprogramming and epigenetic modifications on the path to cancer

Metabolic rewiring and epigenetic remodeling, which are closely linked and reciprocally regulate each other, are among the well-known cancer hallmarks. Recent evidence suggests that many metabolites serve as substrates or cofactors of chromatin-modifying enzymes as a consequence of the translocation or spatial regionalization of enzymes or metabolites. Various metabolic alterations and epigenetic modifications also reportedly drive immune escape or impede immunosurveillance within certain contexts, playing important roles in tumor progression. In this review, we focus on how metabolic reprogramming of tumor cells and immune cells reshapes epigenetic alterations, in particular the acetylation and methylation of histone proteins and DNA. We also discuss other eminent metabolic modifications such as, succinylation, hydroxybutyrylation, and lactylation, and update the current advances in metabolism- and epigenetic modification-based therapeutic prospects in cancer.

GREB1: An evolutionarily conserved protein with a glycosyltransferase domain links ERα glycosylation and stability to cancer

What covalent modifications control the temporal ubiquitination of ERα and hence the duration of its transcriptional activity remain poorly understood. We show that GREB1, an ERα-inducible enzyme, catalyzes O-GlcNAcylation of ERα at residues T553/S554, which stabilizes ERα protein by inhibiting association with the ubiquitin ligase ZNF598. Loss of GREB1-mediated glycosylation of ERα results in reduced cellular ERα levels and insensitivity to estrogen. Higher GREB1 expression in ERα^+ve breast cancer is associated with greater survival in response to tamoxifen, an ERα agonist. Mice lacking Greb1 exhibit growth and fertility defects reminiscent of phenotypes in ERα-null mice. In summary, this study identifies GREB1, a protein with an evolutionarily conserved domain related to DNA-modifying glycosyltransferases of bacteriophages and kinetoplastids, as the first inducible and the only other (apart from OGT) O-GlcNAc glycosyltransferase in mammalian cytoplasm and ERα as its first substrate.

Butylphthalide Inhibits Autophagy and Promotes Multiterritory Perforator Flap Survival

Multiterritory perforator flap is an important plastic surgery technique, yet its efficacy can be limited by partial necrosis at the choke Ⅱ zone. Butylphthalide (NBP) has been used for many diseases but has not been studied in the multiterritory perforator flap. With the effect of NBP, we observed increasing in capillary density, inhibition of autophagy and oxidative stress, and a reduction in apoptosis of cells, all consistent with increased flap survival. However, the protective effect of NBP on multiterritory perforator flap was lost following administration of the autophagy agonist rapamycin (Rap). Through the above results, we assumed that NBP promotes flap survival by inhibiting autophagy. Thus, this study has found a new pharmacological effect of NBP on the multiterritory perforator by inhibiting autophagy to prevent distal postoperative necrosis and exert effects on angiogenesis, oxidative stress, and apoptosis within the flap.

Potential Roles of O-GlcNAcylation in Primary Cilia- Mediated Energy Metabolism

The primary cilium, an antenna-like structure on most eukaryotic cells, functions in transducing extracellular signals into intracellular responses via the receptors and ion channels distributed along it membrane. Dysfunction of this organelle causes an array of human diseases, known as ciliopathies, that often feature obesity and diabetes; this indicates the primary cilia's active role in energy metabolism, which it controls mainly through hypothalamic neurons, preadipocytes, and pancreatic β-cells. The nutrient sensor, O-GlcNAc, is widely involved in the regulation of energy homeostasis. Not only does O-GlcNAc regulate ciliary length, but it also modifies many components of cilia-mediated metabolic signaling pathways. Therefore, it is likely that O-GlcNAcylation (OGN) plays an important role in regulating energy homeostasis in primary cilia. Abnormal OGN, as seen in cases of obesity and diabetes, may play an important role in primary cilia dysfunction mediated by these pathologies.

O-GlcNAcylation modulates HBV replication through regulating cellular autophagy at multiple levels

O-GlcNAcylation is a form of posttranslational modification, and serves various functions, including modulation of location, stability, and activity for the modified proteins. O-linked-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies the cellular proteins with O-GlcNAc moiety. Early studies reported that the decreased O-GlcNAcylation regulates cellular autophagy, a process relevant for hepatitis B virus replication (HBV) and assembly. Therefore, we addressed the question how O-GlcNAcylation regulates cellular autophagy and HBV replication. Inhibition of OGT activity with a small molecule inhibitor OSMI-1 or silencing OGT significantly enhanced HBV replication and HBsAg production in hepatoma cells and primary human hepatocytes (PHHs). Western blotting analysis showed that inhibition of O-GlcNAcylation-induced endoplasmic reticulum (ER) stress and cellular autophagy, two processes subsequently leading to enhanced HBV replication. Importantly, the numbers of autophagosomes and the levels of autophagic markers LC3-II and SQSTM1/p62 in hepatoma cells were elevated after inhibition of O-GlcNAcylation. Further analysis revealed that inhibition of O-GlcNAcylation blocked autophagosome-lysosome fusion and thereby prevented autophagic degradation of HBV virions and proteins. Moreover, OSMI-1 further promoted HBV replication by inducing autophagosome formation via inhibiting the O-GlcNAcylation of Akt and mTOR. In conclusion, decreased O-GlcNAcylation enhanced HBV replication through increasing autophagosome formation at multiple levels, including triggering ER-stress, Akt/mTOR inhibition, and blockade of autophagosome-lysosome fusion.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Epigenetic activation of O-linked β-N-acetylglucosamine transferase overrides the differentiation blockage in acute leukemia

Background

Overriding the differentiation blockage in acute myeloid leukemia (AML) is the most successful mode-of-action in leukemia therapy – now curing the vast majority of patients with acute promyelocytic leukemia (APL) using all-trans retinoic acid (ATRA)-based regimens. Similar approaches in other leukemia subtypes, such as IDH1/2-mutated AML, are under active investigation. We herein present successful release of the differentiation blockage upon treatment with the natural (−)-Δ⁹-Tetrahydrocannabinol isomer dronabinol in vitro and in vivo.

Methods

Cellular maturation and differentiation were followed in two patients employing whole genome methylation profiling, proteome analyses, NGS deep sequencing and multispectral imaging flow cytometry. For functional studies lentiviral OGT knock-down in vitro and ex vivo cell models were created to evaluate proliferative, apoptotic and differentiating effects of OGT in acute leukemia.

Findings

In here, we provide molecular evidence that dronbinol is capable to override the differentiation blockage of acute leukemia blasts at the state of the leukemia-initiating clone. We further identify the O-linked β-N-acetyl glucosamine (O-GlcNAc) transferase (OGT) to be crucial in this process. OGT is a master regulator enzyme adding O-GlcNAc to serine or threonine residues in a multitude of target proteins. Aberrant O-GlcNAc modification is implicated in pathologies of metabolic, neurodegenerative and autoimme diseases as well as cancers. We provide evidence that dronabinol induces transcription of OGT via epigenetic hypomethylation of the transcription start site (TSS). A lentiviral OGT-knock out approach proves the central role of OGT exerting antileukemic efficacy via a dual-mechanism of action: High concentrations of dronabinol result in induction of apoptosis, whereas lower concentrations drive cellular maturation. Most intriguingly, overriding of the differentiation blockage of acute leukemia blasts is validated in vivo following two patients treated with dronabinol.

Interpretation

In conclusion, we provide evidence for overcoming the differentiation blockage in acute leukemia in subentities beyond promyelocytic and IDH1/2-mutated leukemia and thereby identify O-GlcNAcylation as a novel (drugable) field for future leukemia research.

Funding

Unrestricted grant support by the IZKF Program of the Medical Faculty Tübingen (MMS) and Brigitte Schlieben-Lange Program as well as the Margarete von Wrangell Program of the Ministry of Science, Research and the Arts, Baden-Württemberg, Germany (KKS) and Athene Program of the excellence initiative University of Tübingen (KKS).

The O-GlcNAc Modification on Kinases

O-Linked N-acetyl glucosamine (O-GlcNAc) is a protein modification found on thousands of nuclear, cytosolic, and mitochondrial proteins. Many O-GlcNAc sites occur in proximity to protein sites that are likewise modified by phosphorylation. While several studies have uncovered crosstalk between these two signaling modifications on individual proteins and pathways, an understanding of the role of O-GlcNAc in regulating kinases, the enzymes that install the phosphate modification, is still emerging. Here we review recent methods to profile the O-GlcNAc modification on a global scale that have revealed more than 100 kinases are modified by O-GlcNAc and highlight existing studies about regulation of these kinases by O-GlcNAc. Continuing efforts to profile the O-GlcNAc proteome and understand the role of O-GlcNAc on kinases will reveal new mechanisms of regulation and potential avenues for manipulation of the signaling mechanisms at the intersection of O-GlcNAc and phosphorylation.

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

Centrosomes: Til O-GlcNAc Do Us Apart

The centrosome apparatus is vital for spindle assembly and chromosome segregation during mitotic divisions. Its replication, disjunction and separation have to be fine-tuned in space and time. A multitude of post-translational modifications (PTMs) have been implicated in centrosome modulation, including phosphorylation, ubiquitination and acetylation. Among them is the emerging O-linked N-acetylglucosamine (O-GlcNAc) modification. This quintessential PTM has a sole writer, O-GlcNAc transferase (OGT), and the only eraser, O-GlcNAcase (OGA). O-GlcNAc couples glucose metabolism with signal transduction and forms a yin-yang relationship with phosphorylation. Evidence from proteomic studies as well as single protein investigations has pinpointed a role of O-GlcNAc in centrosome number and separation, centriole number and distribution, as well as the cilia machinery emanating from the centrosomes. Herein we review our current understanding of the sweet modification embedded in centrosome dynamics and speculate that more molecular details will be unveiled in the future.

Sugar as a therapeutic target for the cognitive restoration following traumatic brain injury

PURPOSE OF REVIEW

This review aims to discuss examples of changes in glucose (sugar) metabolism after traumatic brain injury (TBI). It will attempt to provide an understanding of what changes in glucose metabolism mean for the injured brain. It will further identify potential therapeutic target(s) emanating from our growing understanding of glucose pathways and their roles in TBI.

RECENT FINDINGS

Although a significant fraction of glucose is utilized for the energy production in the brain, a small fraction is utilized in other, often ignored pathways. Recent studies have unraveled unexpected biological effects of glucose through these pathways, including redox regulation, genetic and epigenetic regulation, glycation of proteins, nucleotide synthesis and amino acid synthesis.

SUMMARY

A number of regulatory players in minor glucose metabolic pathways, such as folate and chondroitin sulfate proteoglycans, have recently been identified as potential targets to restore cognitive functions. Targeting of these players should be combined with the supplementation of alternative energy substrates to achieve the maximal cognitive restoration after TBI. This multimodal therapeutic strategy deserves testing in various models of TBI.

VIDEO ABSTRACT

Supplemental digital video content 1: Video that demonstrates an effective therapeutic strategy for the cognitive restoration after TBI. http://links.lww.com/CONR/A46.

Mechanical stress alters protein O-GlcNAc in human periodontal ligament cells

Protein O-linked N-acetylglucosamine (O-GlcNAc) is a post-translational modification of intracellular proteins that regulates several physiological and pathophysiological process, including response to various stressors. However, O-GlcNAc's response to mechanical stress has not been investigated yet. As human periodontal ligament (PDL) cells are stimulated by compression force during orthodontic tooth movement that results in structural remodelling, in this study we investigated whether mechanical stress induces any alteration in protein O-GlcNAc in PDL cells. In this study, PDL cells isolated from premolars extracted for orthodontic indications were exposed to 0, 1.5, 3, 7 and 14 g/cm² compression forces for 12 hours. Cell viability was measured by flow cytometry, and protein O-GlcNAc was analysed by Western blot. Cellular structure and intracellular distribution of O-GlcNAc was studied by immunofluorescence microscopy. We found that between 1.5 and 3 g/cm² mechanical compression, O-GlcNAc significantly elevated; however, at higher forces O-GlcNAc level was not increased. We also found that intracellular localization of O-GlcNAc proteins became more centralized under 2 g/cm² compression force. Our results suggest that structural changes stimulated by compression forces have a significant effect on the regulation of O-GlcNAc; thus, it might play a role in the mechanical stress adaptation of PDL cells.

Regulation of AβPP Glycosylation Modification and Roles of Glycosylation on AβPP Cleavage in Alzheimer's Disease

The presence of senile plaques in the gray matter of the brain is one of the major pathologic features of Alzheimer's disease (AD), and amyloid-β (Aβ) is the main component of extracellular deposits of the senile plaques. Aβ derives from amyloid-β precursor protein (AβPP) cleaved by β-secretase (BACE1) and γ-secretase, and the abnormal cleavage of AβPP is an important event leading to overproduction and aggregation of Aβ species. After translation, AβPP undergoes post-translational modifications (PTMs) including glycosylation and phosphorylation in the endoplasmic reticulum (ER) and Golgi apparatus, and these modifications play an important role in regulating the cleavage of this protein. In this Review, we summarize research progress on the modification of glycosylation, especially O-GlcNAcylation and mucin-type O-linked glycosylation (also known as O-GalNAcylation), on the regulation of AβPP cleavage and on the influence of AβPP's glycosylation in the pathogenesis of AD.

O-GlcNAc Modification Protects against Protein Misfolding and Aggregation in Neurodegenerative Disease

Post-translational modifications (PTMs) of proteins are becoming the focus of intense research due to their implications in a broad spectrum of neurodegenerative diseases. Various PTMs have been identified to alter the toxic profiles of proteins which play critical roles in disease etiology. In Alzheimer's disease (AD), dysregulated phosphorylation is reported to promote pathogenic processing of the microtubule-associated tau protein. Among the PTMs, the enzymatic addition of N-acetyl-d-glucosamine (GlcNAc) residues to Ser/Thr residues is reported to deliver protective effects against the pathogenic processing of both amyloid precursor protein (APP) and tau. Modification of tau with as few as one single O-GlcNAc residue inhibits its toxic self-assembly. This modification also has the same effect on the assembly of the Parkinson's disease (PD) associated α-synuclein (ASyn) protein. In fact, O-GlcNAcylation ( O-linked GlcNAc modification) affects the processing of numerous proteins implicated in AD, PD, amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD) in a similar manner. As such, manipulation of a protein's O-GlcNAcylation status has been proposed to offer therapeutic routes toward addressing multiple neurodegenerative pathologies. Here we review the various effects that O-GlcNAc modification, and its modulated expression, have on pathogenically significant proteins involved in neurodegenerative disease.

Nutrient regulation of signaling and transcription

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

The Role of O-GlcNAcylation for Protection against Ischemia-Reperfusion Injury

Ischemia reperfusion injury (IR injury) associated with ischemic heart disease contributes significantly to morbidity and mortality. O-linked β-N-acetylglucosamine (O-GlcNAc) is a dynamic posttranslational modification that plays an important role in numerous biological processes, both in normal cell functions and disease. O-GlcNAc increases in response to stress. This increase mediates stress tolerance and cell survival, and is protective. Increasing O-GlcNAc is protective against IR injury. Experimental cellular and animal models, and also human studies, have demonstrated that protection against IR injury by ischemic preconditioning, and the more clinically applicable remote ischemic preconditioning, is associated with increases in O-GlcNAc levels. In this review we discuss how the principal mechanisms underlying tissue protection against IR injury and the associated immediate elevation of O-GlcNAc may involve attenuation of calcium overload, attenuation of mitochondrial permeability transition pore opening, reduction of endoplasmic reticulum stress, modification of inflammatory and heat shock responses, and interference with established cardioprotective pathways. O-GlcNAcylation seems to be an inherent adaptive cytoprotective response to IR injury that is activated by mechanical conditioning strategies.

Ferulic acid ameliorates cerebral infarction by activating Akt/mTOR/4E‑BP1/Bcl‑2 anti‑apoptotic signaling in the penumbral cortex following permanent cerebral ischemia in rats

The aim of the present study was to determine the effects of ferulic acid (FerA) administered immediately following the onset of permanent middle cerebral artery occlusion (MCAo) and then 7 days of ischemia, and also to explore the involvement of protein kinase B (Akt)‑induced signaling in the penumbral cortex. Immediately following the onset of MCAo, FerA was intravenously administered to rats at a dose of 60 mg/kg (FerA‑60 mg), 80 mg/kg (FerA‑80 mg), or 100 mg/kg (FerA‑100 mg). FerA‑80 mg and FerA‑100 mg effectively ameliorated cerebral infarction and neurological deficits 7 days following permanent cerebral ischemia. FerA‑80 mg and FerA‑100 mg significantly upregulated the expression of phospho‑Akt (p‑Akt), phospho‑mammalian target of rapamycin (p‑mTOR), and eukaryotic initiation factor 4E (eIF4E)‑binding protein 1 (4E‑BP1), and the phospho‑4E‑BP1 (p‑4E‑BP1)/4E‑BP1 and mitochondrial Bcl‑2/Bax ratios, and markedly downregulated the levels of cytochrome c‑, cleaved caspase‑3‑, and terminal deoxynucleotidyl transferase‑mediated dUTP‑biotin nick‑end labeling‑immunoreactive cells in the penumbral cortex at 7 days post‑ischemia. LY294002, a selective inhibitor of phosphoinositide 3‑kinase/Akt signaling, was administered 30 min prior to ischemia, which abrogated the upregulating effects of FerA‑100 mg on the expression of p‑Akt, p‑mTOR, 4E‑BP1, p‑4E‑BP1 and eIF4E, the mitochondrial Bcl‑2/Bax ratio and the ameliorating effect of FerA‑100 mg on cerebral infarction. FerA administered at doses of 80 and 100 mg/kg exerted beneficial effects against cerebral ischemia by activating Akt‑induced signaling. The effects of FerA at doses of 80 and 100 mg/kg on mitochondrial B‑cell lymphoma-2 (Bcl‑2)‑associated X protein‑related apoptosis were attributed to the activation of Akt/mTOR/4E‑BP1/Bcl‑2 anti‑apoptotic signaling, and eventually contributed to suppression of the cytochrome c/caspase‑3 activation pathway in the penumbral cortex 7 days following permanent cerebral ischemia.

'O-GlcNAc Code' Mediated Biological Functions of Downstream Proteins

As one of the post-translational modifications, O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) often occurs on serine (Ser) and threonine (Thr) residues of specific substrate cellular proteins via the addition of O-GlcNAc group by O-GlcNAc transferase (OGT). Maintenance of normal intracellular levels of O-GlcNAcylation is controlled by OGT and glycoside hydrolase O-GlcNAcase (OGA). Unbalanced O-GlcNAcylation levels have been involved in many diseases, including diabetes, cancer, and neurodegenerative disease. Recent research data reveal that O-GlcNAcylation at histones or non-histone proteins may provide recognition platforms for subsequent protein recruitment and further initiate intracellular biological processes. Here, we review the current understanding of the 'O-GlcNAc code' mediated intracellular biological functions of downstream proteins.

Proteomic identification of altered protein O-GlcNAcylation in a triple transgenic mouse model of Alzheimer's disease

PET scan analysis demonstrated the early reduction of cerebral glucose metabolism in Alzheimer disease (AD) patients that can make neurons vulnerable to damage via the alteration of the hexosamine biosynthetic pathway (HBP). Defective HBP leads to flawed protein O-GlcNAcylation coupled, by a mutual inverse relationship, with increased protein phosphorylation on Ser/Thr residues. Altered O-GlcNAcylation of Tau and APP have been reported in AD and is closely related with pathology onset and progression. In addition, type 2 diabetes patients show an altered O-GlcNAcylation/phosphorylation that might represent a link between metabolic defects and AD progression. Our study aimed to decipher the specific protein targets of altered O-GlcNAcylation in brain of 12-month-old 3×Tg-AD mice compared with age-matched non-Tg mice. Hence, we analysed the global O-GlcNAc levels, the levels and activity of OGT and OGA, the enzymes controlling its cycling and protein specific O-GlcNAc levels using a bi-dimensional electrophoresis (2DE) approach. Our data demonstrate the alteration of OGT and OGA activation coupled with the decrease of total O-GlcNAcylation levels. Data from proteomics analysis led to the identification of several proteins with reduced O-GlcNAcylation levels, which belong to key pathways involved in the progression of AD such as neuronal structure, protein degradation and glucose metabolism. In parallel, we analysed the O-GlcNAcylation/phosphorylation ratio of IRS1 and AKT, whose alterations may contribute to insulin resistance and reduced glucose uptake. Our findings may contribute to better understand the role of altered protein O-GlcNAcylation profile in AD, by possibly identifying novel mechanisms of disease progression related to glucose hypometabolism.

High glucose reduces megalin-mediated albumin endocytosis in renal proximal tubule cells through protein kinase B O-GlcNAcylation

The role of albumin reabsorption in proximal tubule (PT) cells has emerged as an important factor in the genesis of albuminuria observed in the early stages of diabetes. Evidence has shown that a decrease in megalin expression could be the key mechanism in this process. In the present work, we investigated the molecular mechanism underlying the modulation of albumin endocytosis in LLC-PK1 cells, a model of PT cells. High glucose concentrations (HG) inhibited megalin expression and albumin endocytosis after 48 h of incubation. This inhibitory effect involves the entrance of glucose into PT cells through SGLT located at the luminal membrane. Once inside PT cells, glucose is diverted to the hexosamine biosynthetic pathway (HBP) increasing O-GlcNAcylation of several intracellular proteins, including PKB. This process promotes the inhibition of PKB activity measured by its phosphorylation at Thr-308 and Ser-473 and phosphorylation of specific substrates, glycogen synthase kinase 3β (GSK3β) and tuberous sclerosis complex 2. The decrease in PKB activity led to a decrease in megalin expression and, consequently, reducing albumin endocytosis in LLC-PK1 cells. HG did not change mammalian target of rapamycin (mTOR) C2 activity, responsible for phosphorylated PKB at Ser-473. In addition, HG activated the mTORC1/S6K pathway, but this effect was not correlated to the decrease in megalin expression or albumin endocytosis. Taken together, our data help to clarify the current understanding underlying the genesis of tubular albuminuria induced by hyperglycemia in the early stage of diabetes pathogenesis.

Protein O-GlcNAcylation in Cardiac Pathologies: Past, Present, Future

O-GlcNAcylation is a ubiquitous and reversible post-translational protein modification that has recently gained renewed interest due to the rapid development of analytical tools and new molecules designed to specifically increase the level of protein O-GlcNAcylation. The level of O-GlcNAc modification appears to have either deleterious or beneficial effects, depending on the context (exposure time, pathophysiological context). While high O-GlcNAcylation levels are mostly reported in chronic diseases, the increase in O-GlcNAc level in acute stresses such as during ischemia reperfusion or hemorrhagic shock is reported to be beneficial in vitro, ex vivo, or in vivo. In this context, an increase in O-GlcNAc levels could be a potential new cardioprotective therapy, but the ambivalent effects of protein O-GlcNAcylation augmentation remains as a key problem to be solved prior to their transfer to the clinic. The emergence of new analytical tools has opened new avenues to decipher the mechanisms underlying the beneficial effects associated with an O-GlcNAc level increase. A better understanding of the exact roles of O-GlcNAc on protein function, targeting or stability will help to develop more targeted approaches. The aim of this review is to discuss the mechanisms and potential beneficial impact of O-GlcNAc modulation, and its potential as a new clinical target in cardiology.

Cross-Dysregulation of O-GlcNAcylation and PI3K/AKT/mTOR Axis in Human Chronic Diseases

The hexosamine biosynthetic pathway (HBP) and the phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway are considered as nutrient sensors that regulate several essential biological processes. The hexosamine biosynthetic pathway produces uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), the substrate for O-GlcNAc transferase (OGT), the enzyme that O-GlcNAcylates proteins on serine (Ser) and threonine (Thr) residues. O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and phosphorylation are highly dynamic post-translational modifications occurring at the same or adjacent sites that regulate folding, stability, subcellular localization, partner interaction, or activity of target proteins. Here we review recent evidence of a cross-regulation of PI3K/AKT/mTOR signaling pathway and protein O-GlcNAcylation. Furthermore, we discuss their co-dysregulation in pathological conditions, e.g., cancer, type-2 diabetes (T2D), and cardiovascular, and neurodegenerative diseases.

Antrodia cinnamomea Oligosaccharides Suppress Lipopolysaccharide-Induced Inflammation through Promoting O-GlcNAcylation and Repressing p38/Akt Phosphorylation

Antrodia cinnamomea (AC), an edible fungus growing in Taiwan, has various health benefits. This study was designed to examine the potential inhibitory effects of AC oligosaccharides on lipopolysaccharide (LPS)-induced inflammatory responses in vitro and in vivo. By trifluoroacetic acid degradation, two oligosaccharide products were prepared from AC polysaccharides at 90 °C (ACHO) or 25 °C (ACCO), which showed different oligosaccharide identities. Compared to ACCO, ACHO displayed better inhibitory effects on LPS-induced mRNA expression of pro-inflammatory cytokines including IL-6, IL-8, IL-1β, TNF-α and MCP-1 in macrophage cells. Further, ACHO significantly suppressed the inflammation in lung tissues of LPS-injected C57BL/6 mice. The potential anti-inflammatory molecular mechanism may be associated with the promotion of protein O-GlcNAcylation, which further skewed toward the marked suppression of p38 and Akt phosphorylation. Our results suggest that the suppressive effect of AC oligosaccharides on inflammation may be an effective approach for the prevention of inflammation-related diseases.

Higher O-GlcNAc Levels Are Associated with Defects in Progenitor Proliferation and Premature Neuronal Differentiation during in-Vitro Human Embryonic Cortical Neurogenesis

The nutrient responsive O-GlcNAcylation is a dynamic post-translational protein modification found on several nucleocytoplasmic proteins. Previous studies have suggested that hyperglycemia induces the levels of total O-GlcNAcylation inside the cells. Hyperglycemia mediated increase in protein O-GlcNAcylation has been shown to be responsible for various pathologies including insulin resistance and Alzheimer's disease. Since maternal hyperglycemia during pregnancy is associated with adverse neurodevelopmental outcomes in the offspring, it is intriguing to identify the effect of increased protein O-GlcNAcylation on embryonic neurogenesis. Herein using human embryonic stem cells (hESCs) as model, we show that increased levels of total O-GlcNAc is associated with decreased neural progenitor proliferation and premature differentiation of cortical neurons, reduced AKT phosphorylation, increased apoptosis and defects in the expression of various regulators of embryonic corticogenesis. As defects in proliferation and differentiation during neurodevelopment are common features of various neurodevelopmental disorders, increased O-GlcNAcylation could be one mechanism responsible for defective neurodevelopmental outcomes in metabolically compromised pregnancies such as diabetes.

Functional significance of O-GlcNAc modification in regulating neuronal properties

Post-translational modifications (PTMs) covalently modify proteins and diversify protein functions. Along with protein phosphorylation, another common PTM is the addition of O-linked β-N-acetylglucosamine (O-GlcNAc) to serine and/or threonine residues. O-GlcNAc modification is similar to phosphorylation in that it occurs to serine and threonine residues and cycles on and off with a similar time scale. However, a striking difference is that the addition and removal of the O-GlcNAc moiety on all substrates are mediated by the two enzymes regardless of proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. O-GlcNAcylation can interact or potentially compete with phosphorylation on serine and threonine residues, and thus serves as an important molecular mechanism to modulate protein functions and activation. However, it has been challenging to address the role of O-GlcNAc modification in regulating protein functions at the molecular level due to the lack of convenient tools to determine the sites and degrees of O-GlcNAcylation. Studies in this field have only begun to expand significantly thanks to the recent advances in detection and manipulation methods such as quantitative proteomics and highly selective small-molecule inhibitors for OGT and OGA. Interestingly, multiple brain regions, especially hippocampus, express high levels of both OGT and OGA, and a number of neuron-specific proteins have been reported to undergo O-GlcNAcylation. This review aims to discuss the recent updates concerning the impacts of O-GlcNAc modification on neuronal functions at multiple levels ranging from intrinsic neuronal properties to synaptic plasticity and animal behaviors.

Dyrk2 involved in regulating LPS-induced neuronal apoptosis

The activation of relevant signaling pathways plays a very important role in LPS-induced neuronal damage. Dual-specificity tyrosine-phosphorylation-regulated kinase 2(Dyrk2), as a phosphokinase that can directly or indirectly phosphorylate signal molecules, was recently reported to down-regulate Type I Interferon(TIF) by promoting ser527 phosphorylation of TBK1. To further investigate the role of Dyrk2 in neuroinflammation, we for the first time focused on its function in LPS-induced neuronal damage. We found LPS stimulation increased the expression of Dyrk2 in the nucleus and cytoplasm of neurons. In addition, overexpression of Dyrk2 not only reduced the level of TNF-α induction, but also obviously inhibited LPS-induced neuronal apoptosis. We further found that Dyrk2 promoted the induction of phospho-Akt, phospho-p65 and phospho-p38MAPK (p38 mitogen-activated protein kinase), but immunoprecipitation showed Dyrk2 interacted with and Akt, p38MAPK and IκBα (IkappaB-alpha), except NF-κB subunit p65. These findings suggest Dyrk2 can inhibit LPS-induced neuronal apoptosis and plays key roles in LPS-indcued signaling pathways by its phosphokinase function. These data provide a novel viewpoint that Dyrks family may have an important role in neuroinflammation, and provide a potential molecular target for improving neuronal apoptosis.

Protective effect of TSG against oxygen-glucose deprivation in cardiomyoblast cell line H9c2: involvement of Bcl-2 family, Caspase 3/9, and Akt signaling pathway

OBJECTIVE

This study was designed to investigate the effect of TSG (2, 3, 5, 4'-tetrahydroxystibene-2-O-β-D-glucoside) on ischemic cardiomyopathy (ICM) related cell apoptosis and the mechanism related to it in vitro.

METHODS

Rat cardiomyoblast cell line H9c2 was cultured in oxygen-glucose withdrawal medium for 8 hours to establish an in vitro cell model of oxygen-glucose deprivation (OGD). Cells were pretreated with TSG to test the protective effect of it against OGD. Cell viability, apoptosis, mitochondrial transmembrane potential (ΔΨ_m), and apoptosis related proteins were detected using appropriated methods. Differences between treatments were analyzed.

RESULTS

OGD treatment inhibited cell viability, expression of Akt and Bax, induced loss of ΔΨ_m, cell apoptosis, and triggered expression of Bcl-2 and Caspase-3/9. TSG pretreatment, on the contrary, suppressed OGD-induced cell apoptosis, ΔΨ_m loss, Bcl-2 and Caspase-3/9 expression, and promoted OGD-inhibited cell viability, Bax and Akt expression.

CONCLUSION

We concluded that TSG's protective effect against OGD-induced in vitro ischemic cell model was associated to Akt/Caspase-3 pathway. TSG might be explored as a therapeutic target for ICM.

The emerging link between O-GlcNAcylation and neurological disorders

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is involved in the regulation of many cellular cascades and neurological diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and stroke. In the brain, the expression of O-GlcNAcylation is notably heightened, as is that of O-linked N-acetylglucosaminyltransferase (OGT) and β-N-acetylglucosaminidase (OGA), the presence of which is prominent in many regions of neurological importance. Most importantly, O-GlcNAcylation is believed to contribute to the normal functioning of neurons; conversely, its dysregulation participates in the pathogenesis of neurological disorders. In neurodegenerative diseases, O-GlcNAcylation of the brain's key proteins, such as tau and amyloid-β, interacts with their phosphorylation, thereby triggering the formation of neurofibrillary tangles and amyloid plaques. An increase of O-GlcNAcylation by pharmacological intervention prevents neuronal loss. Additionally, O-GlcNAcylation is stress sensitive, and its elevation is cytoprotective. Increased O-GlcNAcylation ameliorated brain damage in victims of both trauma-hemorrhage and stroke. In this review, we summarize the current understanding of O-GlcNAcylation's physiological and pathological roles in the nervous system and provide a foundation for development of a therapeutic strategy for neurological disorders.

O-GlcNAcylation Reduces Ischemia-Reperfusion-Induced Brain Injury

O-GlcNAcylation is a common posttranslational modification of nucleocytoplasmic proteins with β-N-acetylglucosamine (GlcNAc) and regulates numerous biological processes. By using mouse models of cerebral ischemia induced by permanent and transient middle cerebral artery occlusion (MCAO), we observed an initial elevation (~1.7-fold, 1–4 hours after ischemia) and then decline of O-GlcNAcylation during cerebral ischemia. We found that moderate increase (<3-fold) of brain O-GlcNAcylation by pharmacological means ameliorated cerebral ischemia-reperfusion injury and the consequent motor and neurological deficits. Interference of the transient elevation of O-GlcNAcylation pharmacologically or genetically aggravates the ischemia-induced brain damage, motor deficits and mortality. The alteration of O-GlcNAcylation was also seen in the ischemic areas of postmortem human brains. This study reveals an important regulation of cerebral ischemia-reperfusion injury by O-GlcNAcylation and also provides a possible therapeutic strategy, i.e., by increasing O-GlcNAcylation, to reduce the cerebral damage and improve the clinical outcome of ischemic stroke.