Understanding and exploiting the roles of O-GlcNAc in neurodegenerative diseases

O-GlcNAcylation in Endocrinology: The Sweet Link

O-GlcNAcylation is a dynamic posttranslational modification that involves the addition of N-acetylglucosamine (GlcNAc) to the serine and threonine residues of proteins. Over the past 4 decades, this modification has become increasingly recognized as having a critical influence in the field of endocrinology. The carefully controlled hormonal input for regulating sleep, mood, response to stress, growth, development, and metabolism are often associated with O-GlcNAc-dependent signaling. As protein O-GlcNAcylation patterns are heavily dependent on environmental glucose concentrations, hormone-secreting cells sense the changes in local environmental glucose concentrations and adjust hormone secretion accordingly. This ability of cells to sense nutritional cues and fine-tune hormonal production is particularly relevant toward maintaining a functional and responsive endocrine system, therefore emphasizing the importance of O-GlcNAc in the scope and application of endocrinology. This review examines how O-GlcNAcylation participates in hormonal homeostasis in different endocrine tissues and systems, from the pineal gland to the placenta, and underscores the significance of O-GlcNAc in the field of endocrinology.

Downregulation of O-GlcNAcylation enhances etoposide-induced p53-mediated apoptosis in HepG2 human liver cancer cells

Etoposide, an anticancer drug that inhibits topoisomerase II, is commonly used in combination chemotherapy. However, the impact of O-GlcNAcylation regulation on etoposide's anticancer effects has rarely been investigated. This study evaluated the effect of etoposide on cellular O-GlcNAcylation and whether modulating this process enhances etoposide-induced apoptosis. O-GlcNAc expression was measured after 24 h of etoposide treatment, and the effect of O-GlcNAc transferase (OGT) inhibition by OSMI-1 on etoposide's anticancer activity in HepG2 human liver cancer cells was quantitatively analyzed. Additionally, molecular analyses were used to confirm that the observed effects were mediated by p53-induced apoptosis. Etoposide reduced O-GlcNAcylation in a dose-dependent manner without directly interacting with OGT. Cotreatment with 20 μm of OSMI-1 lowered the IC50 value for cell viability by approximately 1.64-fold to 60.68 μm and increased the EC50 value for cytotoxicity by around 4.07-fold to 43.41 μm. Furthermore, this synergistic effect was linked to the activation of the p53/caspase-3/PARP1 pathway. These findings suggest that downregulating O-GlcNAcylation may enhance the efficacy of etoposide-based chemotherapy and help overcome tumor resistance.

Exploring new drug treatment targets for immune related bone diseases using a multi omics joint analysis strategy

In the field of treatment and prevention of immune-related bone diseases, significant challenges persist, necessitating the urgent exploration of new and effective treatment methods. However, most existing Mendelian randomization (MR) studies are confined to a single analytical approach, which limits the comprehensive understanding of the pathogenesis and potential therapeutic targets of these diseases. In light of this, we propose the hypothesis that genetic variations in specific plasma proteins have a causal relationship with immune-related bone diseases through the MR mechanism, and that key therapeutic targets can be accurately identified using an integrated multi-omic analysis approach. This study comprehensively applied a variety of analytical methods. Firstly, the protein quantitative trait locus (pQTLs) data from two large plasma protein databases and the Genome-Wide Association Study (GWAS) data of nine immune-related bone diseases were used for Mendelian randomization (MR) analysis. At the same time, we employed the Summary-based Mendelian Randomization (SMR) method, combined with the Bayesian colocalization analysis method of coding genes, as well as the Linkage Disequilibrium Score Regression (LDSC) analysis method based on genetic correlation analysis, as methods to verify the genetic association between genes and complex diseases, thus comprehensively obtaining positive results. In addition, a Phenome-wide Association Study (PheWAS) was conducted on significantly positive genes, and their expression patterns in different tissues were also explored. Subsequently, we integrated Protein-Protein Interaction (PPI) network analysis, Gene Ontology (GO) analysis. Finally, based on the above analytical methods, drug prediction and molecular docking studies were carried out with the aim of accurately identifying key therapeutic targets. Through a comprehensive analysis using four methods, namely the Mendelian randomization (MR) analysis study, Summary-based Mendelian Randomization (SMR) analysis study, Bayesian colocalization analysis study, and Linkage Disequilibrium Score Regression (LDSC) analysis study. We found that through MR, SMR, and combined with Bayesian colocalization analysis, an association was found between rheumatoid arthritis (RA) and HDGF. Using the combination of MR and Bayesian colocalization analysis, as well as LDSC analysis, it was concluded that RA was related to CCL19 and TNFRSF14. Based on the methods of MR and Bayesian colocalization, an association was found between GPT and Crohn's disease-related arthritis, and associations were found between BTN1A1, EVI5, OGA, TNFRSF14 and multiple sclerosis (MS), and associations were found between ICAM5, CCDC50, IL17RD, UBLCP1 and psoriatic arthritis (PsA). Specifically, in the MR analysis of RA, HDGF (P_ivw = 0.0338, OR = 1.0373, 95%CI = 1.0028-1.0730), CCL19 (P_ivw = 0.0004, OR = 0.3885, 95%CI = 0.2299-0.6566), TNFRSF14 (P_ivw = 0.0007, OR = 0.6947, 95%CI = 0.5634-0.8566); in the MR analysis of MS, BTN1A1 (P_ivw = 0.0000, OR = 0.6101, 95%CI = 0.4813-0.7733), EVI5 (P_ivw = 0.0000, OR = 0.3032, 95%CI = 0.1981-0.4642), OGA (P_ivw = 0.0005, OR = 0.4599, 95%CI = 0.2966-0.7131), TNFRSF14 (P_ivw = 0.0002, OR = 0.4026, 95%CI = 0.2505-0.6471); in the MR analysis of PsA, ICAM5 (P_ivw = 0.0281, OR = 1.1742, 95%CI = 1.0174-1.3552), CCDC50 (P_ivw = 0.0092, OR = 0.7359, 95%CI = 0.5843-0.9269), IL17RD (P_ivw = 0.0006, OR = 0.7887, 95%CI = 0.6886-0.9034), UBLCP1 (P_ivw = 0.0021, OR = 0.6901, 95%CI = 0.5448-0.8741); in the MR analysis of Crohn's disease-related arthritis, GPT (P_ivw = 0.0006, OR = 0.0057, 95%CI = 0.0003-0.1111). In the Bayesian colocalization analysis of RA, HDGF (H4 = 0.8426), CCL19 (H4 = 0.9762), TNFRSF14 (H4 = 0.8016); in the Bayesian colocalization analysis of MS, BTN1A1 (H4 = 0.7660), EVI5 (H4 = 0.9800), OGA (H4 = 0.8569), TNFRSF14 (H4 = 0.8904); in the Bayesian colocalization analysis of PsA, ICAM5 (H4 = 0.9476), CCDC50 (H4 = 0.9091), IL17RD (H4 = 0.9301), UBLCP1 (H4 = 0.8862); in the Bayesian colocalization analysis of Crohn's disease-related arthritis, GPT (H4 = 0.8126). In the SMR analysis of RA, HDGF (p_SMR = 0.0338, p_HEIDI = 0.0628). In the LDSC analysis of RA, CCL19 (P = 0.0000), TNFRSF14 (P = 0.0258). By comprehensively analyzing plasma proteomic and transcriptomic data, we successfully identified key therapeutic targets for various clinical subtypes of immune-associated bone diseases. Our findings indicate that the significant positive genes associated with RA include HDGF, CCL19, and TNFRSF14; the positive gene linked to Crohn-related arthropathy is GPT; for MS, the positive genes are BTN1A1, EVI5, OGA, and TNFRSF14; and for PsA, the positive genes are ICAM5, CCDC50, IL17RD, and UBLCP1. Through this comprehensive analytical approach, we have screened potential therapeutic targets for different clinical subtypes of immune-related bone diseases. This research not only enhances our understanding of the pathogenesis of these conditions but also provides a solid theoretical foundation for subsequent drug development and clinical treatment, with the potential to yield significant advancements in the management of patients with immune-related bone diseases.

Multi-omic biomarkers associated with multiple sclerosis: from Mendelian randomization to drug prediction

Currently, the treatment and prevention of multiple sclerosis (MS) continue to encounter significant challenges. Mendelian randomization (MR) analysis has emerged as a crucial research method in the pursuit of new therapeutic strategies. Accordingly, we hypothesize that there exists a causal association between genetic variants of specific plasma proteins and MS through MR mechanisms, and that key therapeutic targets can be precisely identified by integrating multi-omics analytical approaches. In this study, we developed a comprehensive analytical framework aimed at identifying and validating potential therapeutic targets for MS. The framework commenced with a two-sample Mendelian randomization (MR) study utilizing two large plasma protein quantitative trait locus (pQTL) datasets. Building on this foundation, we performed Bayesian co-localization analysis of coding genes, followed by a full phenotype-wide association study (PheWAS) on the co-positive genes identified through both analytical methods. This approach allowed us to explore the functions of key genes and the mechanisms of co-morbidity associated with the disease. Subsequently, we integrated protein-protein interaction (PPI) network analysis, gene ontology (GO) analysis, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis to facilitate drug prediction and molecular docking studies. This study conducted a systematic analysis between two large plasma pQTLs datasets and MS. In the MR analysis, the MR analysis of Icelandic plasma pQTLs and MS identified 88 positive plasma proteins, while the MR analysis of the UK Biobank database pQTLs and MS identified 122 positive plasma proteins. By comparison, uroporphyrinogen III synthase (UROS) and glutathione S-transferase theta 2B (GSTT2B) were found to be the positive proteins shared by the two datasets. After false discovery rate (FDR) correction, signal transducer and activator of transcription 3 (STAT3) was a significantly positive protein in the analysis of Icelandic plasma pQTLs. In the analysis of the UK Biobank database pQTLs, advanced glycosylation end product-specific receptor (AGER), allograft inflammatory factor 1 (AIF1), butyrophilin subfamily 1 member A1 (BTN1A1), cluster of differentiation 58 (CD58), desmoglein 4 (DSG4), ecotropic viral integration site 5 (EVI5), tumor necrosis factor (TNF), and tumor necrosis factor receptor superfamily member 14 (TNFRSF14) were significantly positive proteins. After Bonferroni correction, AGER, CD58, EVI5, and TNF remained significantly positive proteins in the analysis of the UK Biobank database pQTLs. In the Bayesian colocalization analysis, EVI5 (PPH4 = 0.9800), O-GlcNAcase (OGA) (PPH4 = 0.8569), and TNFRSF14 (PPH4 = 0.8904) were the common positive genes in the two analysis methods. In conclusion, EVI5, OGA, and TNFRSF14 may be potential therapeutic targets for MS. Through the comprehensive application of MR analysis and Bayesian colocalization analysis, we have successfully identified that EVI5, OGA, and TNFRSF14 may be key therapeutic targets for MS. These findings may provide a scientific basis for the development of novel immunotherapies, combination treatment regimens, or targeted intervention strategies.

Evidence for Functional Regulation of the KLHL3/WNK Pathway by O-GlcNAcylation

The 42-member Kelch-like (KLHL) protein family are adaptors for ubiquitin E3 ligase complexes, governing the stability of a wide range of substrates. KLHL proteins are critical for maintaining proteostasis in a variety of tissues and are mutated in human diseases, including cancer, neurodegeneration, and familial hyperkalemic hypertension. However, the regulation of KLHL proteins remains incompletely understood. Previously, we reported that two KLHL family members, KEAP1 and gigaxonin, are regulated by O-linked β-N-acetylglucosamine (O-GlcNAc), an intracellular form of glycosylation. Interestingly, some ubiquitination targets of KEAP1 and gigaxonin are themselves also O-GlcNAcylated, suggesting that multi-level control by this posttranslational modification may influence many KLHL pathways. To test this hypothesis, we examined KLHL3, which ubiquitinates with-no-lysine (WNK) kinases to modulate downstream ion channel activity. Our biochemical and glycoproteomic data demonstrate that human KLHL3 and all four WNK kinases (WNK1-4) are O-GlcNAcylated. Moreover, our results suggest that O-GlcNAcylation affects WNK4 function in both osmolarity control and ferroptosis, with potential implications ranging from blood pressure regulation to neuronal health and survival. This work demonstrates the functional regulation of the KLHL3/WNK axis by O-GlcNAcylation and supports a broader model of O-GlcNAc serving as a general regulator of KLHL signaling and proteostasis.

O-GlcNAc informatics: advances and trends

As a post-translational modification, protein glycosylation is critical in health and disease. O-Linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), as an intracellular monosaccharide modification on proteins, was discovered 40 years ago. Thanks to technological advances, the physiological and pathological significance of O-GlcNAcylation has been gradually revealed and widely appreciated, especially in recent years. O-GlcNAc informatics has been quickly evolving. Clearly, O-GlcNAc informatics tools have not only facilitated O-GlcNAc functional studies, but also provided us a unique perspective on protein O-GlcNAcylation. In this article, we review O-GlcNAc-focused software tools and servers that have been developed for O-GlcNAc research over the past four decades. Specifically, we will (1) survey bioinformatics tools that have facilitated O-GlcNAc proteomics data analysis, (2) introduce databases/servers for O-GlcNAc proteins/sites that have been experimentally identified by individual research labs, (3) describe software tools that have been developed to predict O-GlcNAc sites, and (4) introduce platforms cataloging proteins that interact with the O-GlcNAc cycling enzymes (i.e., O-GlcNAc transferase and O-GlcNAcase). We hope these resources will provide useful information to both experienced researchers and new incomers to the O-GlcNAc field. We anticipate that this review provides a framework to stimulate the future development of more sophisticated informatic tools for O-GlcNAc research.

Increased cerebrospinal fluid and plasma apoE glycosylation is associated with reduced levels of Alzheimer's disease biomarkers

The apolipoprotein E ( APOE ) ε4 allele is the strongest genetic risk factor for Alzheimer's disease (AD). ApoE is glycosylated with an O-linked Core-1 sialylated glycan at several sites, yet the impact and function of this glycosylation on AD biomarkers remains unclear. We examined apoE glycosylation in a cohort of cerebrospinal fluid (CSF, n=181) and plasma (n= 178) samples from the Alzheimer's Disease Neuroimaging Initiative (ADNI) stratified into 4 groups: cognitively normal (CN), Mild Cognitive Impairment (MCI), progressors and non-progressors based on delayed word recall performance over 4 years. We observed decreasing glycosylation from apoE2 > apoE3 > apoE4 in CSF, and in plasma (apoE3 > apoE4). ApoE glycosylation was reduced in the MCI compared with CN groups, and in progressors compared to non-progressors. In CSF, higher apoE glycosylation associated cross-sectionally with lower total tau (t-tau), p-tau181, and with higher Aβ 1-42 . Similar associations of apoE glycosylation with higher Aβ 1-42 were observed in plasma. In CSF, greater apoE4 glycosylation was associated with lower t-tau and p-tau181. Over a 6-year period, higher baseline levels of CSF apoE glycosylation predicted lower rates of increase in CSF t-tau and p-tau181 and lower rates of decrease in CSF Aβ 1-42 . These results indicate strong associations of apoE glycosylation with biomarkers of AD pathology independent of apoE genotype, warranting a deeper understanding of the functional role of apoE glycosylation on AD tau pathology.

Opportunities for Therapeutic Modulation of O-GlcNAc

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an essential, dynamic monosaccharide post-translational modification (PTM) found on serine and threonine residues of thousands of nucleocytoplasmic proteins. The installation and removal of O-GlcNAc is controlled by a single pair of enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery four decades ago, O-GlcNAc has been found on diverse classes of proteins, playing important functional roles in many cellular processes. Dysregulation of O-GlcNAc homeostasis has been implicated in the pathogenesis of disease, including neurodegeneration, X-linked intellectual disability (XLID), cancer, diabetes, and immunological disorders. These foundational studies of O-GlcNAc in disease biology have motivated efforts to target O-GlcNAc therapeutically, with multiple clinical candidates under evaluation. In this review, we describe the characterization and biochemistry of OGT and OGA, cellular O-GlcNAc regulation, development of OGT and OGA inhibitors, O-GlcNAc in pathophysiology, clinical progress of O-GlcNAc modulators, and emerging opportunities for targeting O-GlcNAc. This comprehensive resource should motivate further study into O-GlcNAc function and inspire strategies for therapeutic modulation of O-GlcNAc.

O-GlcNAcylation modulates expression and abundance of N-glycosylation machinery in an inherited glycosylation disorder

Core components of the N-glycosylation pathway are known, but the metabolic and post-translational mechanisms regulating this pathway in normal and disease states remain elusive. Using a multi-omic approach in zebrafish, we discovered a mechanism whereby O-GlcNAcylation directly impacts the expression and abundance of two rate-limiting proteins in the N-linked glycosylation pathway. We show in a model of an inherited glycosylation disorder PMM2-CDG, congenital disorders of glycosylation that phosphomannomutase deficiency is associated with increased levels of UDP-GlcNAc and protein O-GlcNAcylation. O-GlcNAc modification increases the transcript and protein abundance of both NgBR and Dpagt1 in pmm2m/m mutants. Modulating O-GlcNAc levels, NgBR abundance, or Dpagt1 activity exacerbated the cartilage phenotypes in pmm2 mutants, suggesting that O-GlcNAc-mediated increases in the N-glycosylation machinery are protective. These findings highlight nucleotide-sugar donors as metabolic sensors that regulate two spatially separated glycosylation pathways, demonstrating how their coordination is relevant to disease severity in the most common congenital disorder of glycosylation.

O-GlcNAc modification of HSP27 alters its protein interactions and promotes refolding of proteins through the BAG3/HSP70 co-chaperone

Almost all types of cellular stress induce post-translational O-GlcNAc modifications of proteins, and this increase promotes cell survival. We previously demonstrated that O-GlcNAc on certain small heat shock proteins (sHSPs), including HSP27, directly increases their chaperone activity as one potential protective mechanism. Here, we furthered our use of synthetic proteins to prepare biotinylated sHSPs and show that O-GlcNAc modification of HSP27 also changes how it interacts within the sHSP system and the broader HSP network. Specifically, we show that O-GlcNAc modified HSP27 binds more strongly to the co-chaperone protein BAG3, which then promotes refolding of a model substrate by HSP70. We use proteomics to identify other potential HSP27 interactions that are changed by O-GlcNAc, including one that we confirm with another sHSP, αB-crystallin. These findings add additional evidence for O-GlcNAc as a switch for regulating protein-protein interactions and for modifications of chaperones as one mechanism by which O-GlcNAc protects against protein aggregation.

Discovery and clinical translation of ceperognastat, an O-GlcNAcase (OGA) inhibitor, for the treatment of Alzheimer's disease

INTRODUCTION

The aggregation and spread of hyperphosphorylated, pathological tau in the human brain is hypothesized to play a key role in Alzheimer's disease (AD) as well as other neurogenerative tauopathies. O-GlcNAcylation, an important post-translational modification of tau and many other proteins, is significantly decreased in brain tissue of AD patients relative to healthy controls. Increased tau O-GlcNAcylation has been shown to reduce tau pathology in mouse in vivo tauopathy models. O-GlcNAcase (OGA) catalyzes the removal of O-GlcNAc from tau thereby driving interest in OGA inhibition as a potential therapeutic approach to reduce tau pathology and slow the progression of AD.

METHODS

A multidisciplinary approach was used to identify ceperognastat (LY3372689) as a potent OGA inhibitor, including an extensive discovery effort with synthetic chemistry, structure-based drug design, and in vivo OGA enzyme occupancy studies. Preclinical studies assessed the target engagement, inhibition of OGA enzyme activity, OGA enzyme occupancy, and changes in tau O-GlcNAc. Four clinical Phase 1 studies of ceperognastat in healthy participants were performed to assess clinical safety and tolerability, pharmacokinetics (PK), and enzyme occupancy.

RESULTS

Ceperognastat is a potent, central nervous system (CNS)-penetrant, low-dose inhibitor of OGA, which can achieve > 95% OGA enzyme occupancy in animal and human brain. Overall, ceperognastat had an acceptable safety profile in Phase 1 clinical studies with no serious adverse events reported following single and multiple dosing. The PK, enzyme occupancy, and safety profile supported Phase 2 development of ceperognastat.

DISCUSSION

Ceperognastat is an orally available, highly potent, CNS-penetrant OGA inhibitor that achieved high (> 80%) OGA enzyme occupancy and increased brain O-GlcNAc-tau preclinically. Ceperognastat demonstrated > 95% OGA enzyme occupancy in Phase 1 trials. These occupancy data informed the dose selection for the Phase 2 clinical program.

Highlights

Ceperognastat is a highly potent, CNS-penetrant OGA inhibitor.Ceperognastat is both orally available and CNS-penetrant even when given at low doses.Ceperognastat can achieve > 95% OGA enzyme occupancy in the animal and human brain.Ceperognastat had an acceptable safety profile in Phase 1 clinical studies.

The post-translational modification O-GlcNAc is a sensor and regulator of metabolism

Cells must rapidly adapt to changes in nutrient conditions through responsive signalling cascades to maintain homeostasis. One of these adaptive pathways results in the post-translational modification of proteins by O-GlcNAc. O-GlcNAc modifies thousands of nuclear and cytoplasmic proteins in response to nutrient availability through the hexosamine biosynthetic pathway. O-GlcNAc is highly dynamic and can be added and removed from proteins multiple times throughout their life cycle, setting it up to be an ideal regulator of cellular processes in response to metabolic changes. Here, we describe the link between cellular metabolism and O-GlcNAc, and we explore O-GlcNAc's role in regulating cellular processes in response to nutrient levels. Specifically, we discuss the mechanisms of elevated O-GlcNAc levels in contributing to diabetes and cancer, as well as the role of decreased O-GlcNAc levels in neurodegeneration. These studies form a foundational understanding of aberrant O-GlcNAc in human disease and provide an opportunity to further improve disease identification and treatment.

O-GlcNAcylation in tumorigenesis and its implications for cancer therapy

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a dynamic and reversible posttranslational modification that targets serine and threonine residues in a variety of proteins. Uridine diphospho-N-acetylglucosamine, which is synthesized from glucose via the hexosamine biosynthesis pathway, is the major donor of this modification. O-GlcNAc transferase is the sole enzyme that transfers GlcNAc onto protein substrates, while O-GlcNAcase is responsible for removing this modification. O-GlcNAcylation plays an important role in tumorigenesis and progression through the modification of specific protein substrates. In this review, we discuss the tumor-related biological functions of O-GlcNAcylation and summarize the recent progress in the development of pharmaceutical options to manipulate the O-GlcNAcylation of specific proteins as potential anticancer therapies.

Discovery of 4-(Arylethynyl)piperidine Derivatives as Potent Nonsaccharide O-GlcNAcase Inhibitors for the Treatment of Alzheimer's Disease

Inhibiting O-GlcNAcase and thereby up-regulation of the O-GlcNAc levels of tau was a potential approach for discovering AD treatments. Herein, a series of novel highly potent OGA inhibitors embracing a 4-(arylethynyl)piperidine moiety was achieved by capitalizing on the substrate recognition domain. Extensive structure-activity relationships resulted in compound 81 with significant enzymatic inhibition (IC50 = 4.93 ± 2.05 nM) and cellular potency (EC50 = 7.47 ± 3.96 nM in PC12 cells). It markedly increased the protein O-GlcNAcylation levels and reduced the phosphorylation on Ser199, Thr205, and Ser396 of tau in the OA-injured SH-SY5Y cell model, suggesting its potential role for AD treatment. In fact, an in vivo efficacy of ameliorating cognitive impairment was observed following treatment of APP/PS1 mice with compound 81 (100 mg/kg). Additionally, the appropriate plasma PK and beneficial BBB penetration properties were also observed. Compound 81 deserves to be further explored as an anti-AD agent.

O-GlcNAc Modification of α-Synuclein Can Alter Monomer Dynamics to Control Aggregation Kinetics

The intrinsically disordered protein α-Synuclein is identified as a major toxic aggregate in Parkinson's as well as several other neurodegenerative diseases. Recent work on this protein has focused on the effects of posttranslational modifications on aggregation kinetics. Among them, O-GlcNAcylation of α-Synuclein has been observed to inhibit the aggregation propensity of the protein. Here, we investigate the monomer dynamics of two O-GlcNAcylated α-Synucleins, α-Syn(gT72), and α-Syn(gS87) and correlate them with the aggregation kinetics. We find that, compared to the unmodified protein, glycosylation at T72 makes the protein less compact and more diffusive, while glycosylation at S87 makes the protein more compact and less diffusive. Based on a model of the earliest steps in aggregation, we predict that T72 should aggregate slower than unmodified protein, which is confirmed by ThT fluorescence measurements. In contrast, S87 should aggregate faster, which is not mirrored in ThT kinetics of later fibril formation but does not rule out a higher rate of formation of small oligomers. Together, these results show that posttranslational modifications do not uniformly affect aggregation propensity.

The non-catalytic domains of O-GlcNAc cycling enzymes present new opportunities for function-specific control

O-GlcNAcylation is an essential protein glycosylation governed by two O-GlcNAc cycling enzymes: O-GlcNAc transferase (OGT) installs a single sugar moiety N-acetylglucosamine (GlcNAc) on protein serine and threonine residues, and O-GlcNAcase (OGA) removes them. Aberrant O-GlcNAcylation has been implicated in various diseases. However, the large repertoire of more than 1000 O-GlcNAcylated proteins and the elusive mechanisms of OGT/OGA in substrate recognition present significant challenges in targeting the dysregulated O-GlcNAcylation for therapeutic development. Recently, emerging evidence suggested that the non-catalytic domains play critical roles in regulating the functional specificity of OGT/OGA via modulating their protein interactions and substrate recognition. Here, we discuss recent studies on the structures, mechanisms, and related tools of the OGT/OGA non-catalytic domains, highlighting new opportunities for function-specific control.

The multifaceted role of intracellular glycosylation in cytoprotection and heart disease

The modification of nuclear, cytoplasmic, and mitochondrial proteins by O-linked β-N-actylglucosamine (O-GlcNAc) is an essential posttranslational modification that is common in metozoans. O-GlcNAc is cycled on and off proteins in response to environmental and physiological stimuli impacting protein function, which, in turn, tunes pathways that include transcription, translation, proteostasis, signal transduction, and metabolism. One class of stimulus that induces rapid and dynamic changes to O-GlcNAc is cellular injury, resulting from environmental stress (for instance, heat shock), hypoxia/reoxygenation injury, ischemia reperfusion injury (heart attack, stroke, trauma hemorrhage), and sepsis. Acute elevation of O-GlcNAc before or after injury reduces apoptosis and necrosis, suggesting that injury-induced changes in O-GlcNAcylation regulate cell fate decisions. However, prolonged elevation or reduction in O-GlcNAc leads to a maladaptive response and is associated with pathologies such as hypertrophy and heart failure. In this review, we discuss the impact of O-GlcNAc in both acute and prolonged models of injury with a focus on the heart and biological mechanisms that underpin cell survival.

Post-translational Modification of α-Synuclein Modifies Monomer Dynamics and Aggregation Kinetics

The intrinsically disordered protein α-Synuclein is identified as a major toxic aggregate in Parkinson's as well as several other neurodegenerative diseases. Recent work on this protein has focused on the effects of posttranslational modifications on aggregation kinetics. Among these, O-GlcNAcylation of α-Synuclein has been observed to inhibit the aggregation propensity of the protein. Here we investigate the monomer dynamics of two O-GlcNAcylated α-Synucleins, α-Syn(gT72) and α-Syn(gS87) and correlate them with the aggregation kinetics. We find that, compared to the unmodified protein, glycosylation at T72 makes the protein less compact and more diffusive while glycosylation at S87 makes the protein more compact and less diffusive. Based on a model of the earliest steps in aggregation, we predict that T72 should aggregate slower than unmodified protein, which is confirmed by ThT fluorescence measurements. In contrast, S87 should aggregate faster, which is not mirrored in ThT kinetics of later fibril formation but does not rule out a higher rate of formation of small oligomers. Together, these results show that posttranslational modifications do not uniformly affect aggregation propensity.