The O-GlcNAc Modification on Kinases

Regulation of autophagy: Insights into O-GlcNAc modification mechanisms

Autophagy is a "self-eating" biological process that degrades cytoplasmic contents to ensure cellular homeostasis. Its response to stimuli occurs in two stages: Within a few to several hours of exposure to a stress condition, autophagic flow rapidly increases, which is mediated by post-translational modification (PTM). Subsequently, the transcriptional program is activated and mediates the persistent autophagic response. O-linked β-N-acetylglucosamine (O-GlcNAc) modification is an inducible and dynamically cycling PTM; mounting evidence suggests that O-GlcNAc modification participates in the total autophagic process, including autophagy initiation, autophagosome formation, autophagosome-lysosome fusion, and transcriptional process. In this review, we summarize the current knowledge on the emerging role of O-GlcNAc modification in regulating autophagy-associated proteins and explain the different regulatory effects on autophagy exerted by O-GlcNAc modification.

O-GlcNAcylation regulates tyrosine hydroxylase serine 40 phosphorylation and l-DOPA levels

β-O-linked-N-acetylglucosamine (O-GlcNAcylation) is a post-translational modification (PTM) characterized by the covalent attachment of a single moiety of N-acetylglucosamine (GlcNAc) on serine/threonine residues in proteins. Tyrosine hydroxylase (TH), the rate-limiting step enzyme in the catecholamine synthesis pathway and responsible for the production of the dopamine precursor, l-3,4-dihydroxyphenylalanine (l-DOPA), has its activity regulated by phosphorylation. Here, we show an inverse feedback mechanism between O-GlcNAcylation and phosphorylation of TH at serine 40 (TH pSer40). First, we showed that, during PC12 cells neuritogenesis, TH O-GlcNAcylation decreases concurrently with the increase of pSer40. In addition, an increase in O-GlcNAcylation induces a decrease in TH pSer40 only in undifferentiated PC12 cells, whereas the decrease in O-GlcNAcylation leads to an increase in TH pSer40 levels in both undifferentiated and differentiated PC12 cells. We further show that this feedback culminates on the regulation of l-DOPA intracellular levels. Interestingly, it is noteworthy that decreasing O-GlcNAcylation is much more effective on TH pSer40 regulation than increasing its levels. Finally, ex vivo analysis confirmed the upregulation of TH pSer40 when O-GlcNAcylation levels are reduced in dopaminergic neurons from C57Bl/6 mice. Taken together, these findings demonstrate a dynamic control of l-DOPA production by a molecular cross talk between O-GlcNAcylation and phosphorylation at Ser40 in TH.NEW & NOTEWORTHY This study shows how β-O-linked-N-acetylglucosamine (O-GlcNAcylation) modulates tyrosine hydroxylase (TH) activity, revealing a negative feedback loop with Ser40 phosphorylation both in vitro and ex vivo, which directly influences on l-3,4-dihydroxyphenylalanine (l-DOPA) production. These findings offer insights into neurotransmitter homeostasis regulation, with implications for understanding and potentially treating disorders linked to aberrant catecholamine signaling.

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.

Diverse perspectives on proteomic posttranslational modifications to address EGFR-TKI resistance in non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is the main histological subtype of lung cancer. For locally advanced and advanced NSCLC, epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI)-targeted therapy has been the first choice for NSCLC patients with EGFR mutations. TKIs, as targeted drugs, inhibit kinase activity and autophosphorylation by competitively binding to the ATP binding site of the EGFR tyrosine kinase domain, which blocks the signal transduction mediated by EGFR and thus inhibits the proliferation of tumor cells. However, drug resistance to TKIs is inevitable. EGFR is also a highly glycosylated receptor tyrosine kinase, and a wide range of crosstalk occurs between phosphorylation and glycosylation. Therefore, can the phosphorylation state be altered by glycosylation to improve drug resistance? In this review, we summarize phosphorylation, glycosylation and the crosstalk between these processes as well as the current research status and methods. We also summarize the autophosphorylation and glycosylation sites of the EGFR protein and their crosstalk. By exploring the relationship between EGFR glycosylation and autophosphorylation in targeted TKI therapy, we find that research on EGFR glycosylation is crucial for targeted NSCLC treatment and will become a research direction for identifying potential targets related to regulating TKI drug sensitivity.

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 regulation of RIPK1-dependent apoptosis dictates sensitivity to sunitinib in renal cell carcinoma

Receptor interacting protein kinase 1 (RIPK1) has emerged as a key regulatory molecule that influences the balance between cell death and cell survival. Under external stress, RIPK1 determines whether a cell undergoes RIPK-dependent apoptosis (RDA) or survives by activating NF-κB signaling. However, the role and mechanisms of RIPK1 on sunitinib sensitivity in renal cell carcinoma (RCC) remain elusive. In this study, we demonstrated that the O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) of RIPK1 induces sunitinib resistance in RCC by inhibiting RDA. O-GlcNAc transferase (OGT) specifically interacts with RIPK1 through its tetratricopeptide repeats (TPR) domain and facilitates RIPK1 O-GlcNAcylation. The O-GlcNAcylation of RIPK1 at Ser331, Ser440 and Ser669 regulates RIPK1 ubiquitination and the formation of the RIPK1/FADD/Caspase-8 complex, thereby inhibiting sunitinib-induced RDA in RCC. Site-specific depletion of O-GlcNAcylation on RIPK1 affects the formation of the RIPK1/FADD/Caspase 8 complex, leading to increased sunitinib sensitivity in RCC. Our data highlight the significance of aberrant RIPK1 O-GlcNAcylation in the development of sunitinib resistance and indicate that targeting RIPK1 O-GlcNAcylation could be a promising therapeutic strategy for RCC.

Posttranslational Modifications of -Synuclein, Their Therapeutic Potential, and Crosstalk in Health and Neurodegenerative Diseases

α-Synuclein (α-Syn) aggregation in Lewy bodies and Lewy neurites has emerged as a key pathogenetic feature in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Various factors, including posttranslational modifications (PTMs), can influence the propensity of α-Syn to misfold and aggregate. PTMs are biochemical modifications of a protein that occur during or after translation and are typically mediated by enzymes. PTMs modulate several characteristics of proteins including their structure, activity, localization, and stability. α-Syn undergoes various posttranslational modifications, including phosphorylation, ubiquitination, SUMOylation, acetylation, glycation, O-GlcNAcylation, nitration, oxidation, polyamination, arginylation, and truncation. Different PTMs of a protein can physically interact with one another or work together to influence a particular physiological or pathological feature in a process known as PTMs crosstalk. The development of detection techniques for the cooccurrence of PTMs in recent years has uncovered previously unappreciated mechanisms of their crosstalk. This has led to the emergence of evidence supporting an association between α-Syn PTMs crosstalk and synucleinopathies. In this review, we provide a comprehensive evaluation of α-Syn PTMs, their impact on misfolding and pathogenicity, the pharmacological means of targeting them, and their potential as biomarkers of disease. We also highlight the importance of the crosstalk between these PTMs in α-Syn function and aggregation. Insight into these PTMS and the complexities of their crosstalk can improve our understanding of the pathogenesis of synucleinopathies and identify novel targets of therapeutic potential. SIGNIFICANCE STATEMENT: α-Synuclein is a key pathogenic protein in Parkinson's disease and other synucleinopathies, making it a leading therapeutic target for disease modification. Multiple posttranslational modifications occur at various sites in α-Synuclein and alter its biophysical and pathological properties, some interacting with one another to add to the complexity of the pathogenicity of this protein. This review details these modifications, their implications in disease, and potential therapeutic opportunities.

The OGT-c-Myc-PDK2 axis rewires the TCA cycle and promotes colorectal tumor growth

Deregulated glucose metabolism termed the "Warburg effect" is a fundamental feature of cancers, including the colorectal cancer. This is typically characterized with an increased rate of glycolysis, and a concomitant reduced rate of the tricarboxylic acid (TCA) cycle metabolism as compared to the normal cells. How the TCA cycle is manipulated in cancer cells remains unknown. Here, we show that O-linked N-acetylglucosamine (O-GlcNAc) regulates the TCA cycle in colorectal cancer cells. Depletion of OGT, the sole transferase of O-GlcNAc, significantly increases the TCA cycle metabolism in colorectal cancer cells. Mechanistically, OGT-catalyzed O-GlcNAc modification of c-Myc at serine 415 (S415) increases c-Myc stability, which transcriptionally upregulates the expression of pyruvate dehydrogenase kinase 2 (PDK2). PDK2 phosphorylates pyruvate dehydrogenase (PDH) to inhibit the activity of mitochondrial pyruvate dehydrogenase complex, which reduces mitochondrial pyruvate metabolism, suppresses reactive oxygen species production, and promotes xenograft tumor growth. Furthermore, c-Myc S415 glycosylation levels positively correlate with PDK2 expression levels in clinical colorectal tumor tissues. This study highlights the OGT-c-Myc-PDK2 axis as a key mechanism linking oncoprotein activation with deregulated glucose metabolism in colorectal cancer.

Insights into the post-translational modifications in heart failure

Heart failure (HF), as the terminal manifestation of multiple cardiovascular diseases, causes a huge socioeconomic burden worldwide. Despite the advances in drugs and medical-assisted devices, the prognosis of HF remains poor. HF is well-accepted as a myriad of subcellular dys-synchrony related to detrimental structural and functional remodelling of cardiac components, including cardiomyocytes, fibroblasts, endothelial cells and macrophages. Through the covalent chemical process, post-translational modifications (PTMs) can coordinate protein functions, such as re-localizing cellular proteins, marking proteins for degradation, inducing interactions with other proteins and tuning enzyme activities, to participate in the progress of HF. Phosphorylation, acetylation, and ubiquitination predominate in the currently reported PTMs. In addition, advanced HF is commonly accompanied by metabolic remodelling including enhanced glycolysis. Thus, glycosylation induced by disturbed energy supply is also important. In this review, firstly, we addressed the main types of HF. Then, considering that PTMs are associated with subcellular locations, we summarized the leading regulation mechanisms in organelles of distinctive cell types of different types of HF, respectively. Subsequently, we outlined the aforementioned four PTMs of key proteins and signaling sites in HF. Finally, we discussed the perspectives of PTMs for potential therapeutic targets in HF.

Identification of RESP18 Gene Mutations Linked to Hereditary Non-Syndromic Cleft Lip and Palate in a Southern Chinese Family

BACKGROUND Non-syndromic cleft lip with cleft palate (NSCLP) is one of the most common congenital birth defects worldwide; it causes lifelong problems and imposes burdens on patients and their families. This study aimed to describe the genomic analysis and identification of de novo regulated endocrine-specific protein 18 (RESP18) rs2385404 and rs2385405 gene polymorphisms associated with NSCLP in a southern Chinese family and to improve prevention, treatment, and prognosis of NSCLP. MATERIAL AND METHODS We performed a genome-wide association study (GWAS) to investigate the association of NSCLP phenotype with gene mutation. We investigated a 5-persons NSCLP family to screen the genetic variation of Han nationality in southern Chinese. Whole-genome sequencing (WGS) was used to detect all candidate genetic variants, and whole-exome sequencing (WES) was implemented to further verify mutations. The Clinical Variation Data Base (ClinVar) was employed for screening gene mutations. Finally, Sanger sequencing was applied to verify gene variations. RESULTS The combined analysis of WGS, WES, and ClinVar showed that a total of 9 variation positions overlapped among the 3 study cohorts. Sanger sequencing verified Glu amino acid variation in 2 mutation sites (rs2385404, rs2385405) from the RESP18 gene, which caused abnormal RESP18 function and was associated with hereditary NSCLP. CONCLUSIONS The combined genomic results showed that 2 mutations (rs2385404 and rs2385405) of the RESP18 gene were related to NSCLP in the family. The RESP18 gene may play an important role in the etiology and pathogenesis of cleft lip and palate.

Roles of O-GlcNAcylation in Mitochondrial Homeostasis and Cardiovascular Diseases

Protein posttranslational modifications are important factors that mediate the fine regulation of signaling molecules. O-linked β-N-acetylglucosamine-modification (O-GlcNAcylation) is a monosaccharide modification on N-acetylglucosamine linked to the hydroxyl terminus of serine and threonine of proteins. O-GlcNAcylation is responsive to cellular stress as a reversible and posttranslational modification of nuclear, mitochondrial and cytoplasmic proteins. Mitochondrial proteins are the main targets of O-GlcNAcylation and O-GlcNAcylation is a key regulator of mitochondrial homeostasis by directly regulating the mitochondrial proteome or protein activity and function. Disruption of O-GlcNAcylation is closely related to mitochondrial dysfunction. More importantly, the O-GlcNAcylation of cardiac proteins has been proven to be protective or harmful to cardiac function. Mitochondrial homeostasis is crucial for cardiac contractile function and myocardial cell metabolism, and the imbalance of mitochondrial homeostasis plays a crucial role in the pathogenesis of cardiovascular diseases (CVDs). In this review, we will focus on the interactions between protein O-GlcNAcylation and mitochondrial homeostasis and provide insights on the role of mitochondrial protein O-GlcNAcylation in CVDs.

Targeted Protein O-GlcNAcylation Using Bifunctional Small Molecules

Protein O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) plays a crucial role in regulating essential cellular processes. The disruption of the homeostasis of O-GlcNAcylation has been linked to various human diseases, including cancer, diabetes, and neurodegeneration. However, there are limited chemical tools for protein- and site-specific O-GlcNAc modification, rendering the precise study of the O-GlcNAcylation challenging. To address this, we have developed heterobifunctional small molecules, named O-GlcNAcylation TArgeting Chimeras (OGTACs), which enable protein-specific O-GlcNAcylation in living cells. OGTACs promote O-GlcNAcylation of proteins such as BRD4, CK2α, and EZH2 in cellulo by recruiting FKBP12F36V-fused O-GlcNAc transferase (OGT), with temporal, magnitude, and reversible control. Overall, the OGTACs represent a promising approach for inducing protein-specific O-GlcNAcylation, thus enabling functional dissection and offering new directions for O-GlcNAc-targeting therapeutic development.

O-GlcNAcylation determines the translational regulation and phase separation of YTHDF proteins

N6-methyladenosine (m6A) is the most abundant internal mRNA nucleotide modification in mammals, regulating critical aspects of cell physiology and differentiation. The YTHDF proteins are the primary readers of m6A modifications and exert physiological functions of m6A in the cytosol. Elucidating the regulatory mechanisms of YTHDF proteins is critical to understanding m6A biology. Here we report a mechanism that protein post-translational modifications control the biological functions of the YTHDF proteins. We find that YTHDF1 and YTHDF3, but not YTHDF2, carry high levels of nutrient-sensing O-GlcNAc modifications. O-GlcNAcylation attenuates the translation-promoting function of YTHDF1 and YTHDF3 by blocking their interactions with proteins associated with mRNA translation. We further demonstrate that O-GlcNAc modifications on YTHDF1 and YTHDF3 regulate the assembly, stability and disassembly of stress granules to enable better recovery from stress. Therefore, our results discover an important regulatory pathway of YTHDF functions, adding an additional layer of complexity to the post-transcriptional regulation function of mRNA m6A.

O-GlcNAcylation regulates neurofilament-light assembly and function and is perturbed by Charcot-Marie-Tooth disease mutations

The neurofilament (NF) cytoskeleton is critical for neuronal morphology and function. In particular, the neurofilament-light (NF-L) subunit is required for NF assembly in vivo and is mutated in subtypes of Charcot-Marie-Tooth (CMT) disease. NFs are highly dynamic, and the regulation of NF assembly state is incompletely understood. Here, we demonstrate that human NF-L is modified in a nutrient-sensitive manner by O-linked-β-N-acetylglucosamine (O-GlcNAc), a ubiquitous form of intracellular glycosylation. We identify five NF-L O-GlcNAc sites and show that they regulate NF assembly state. NF-L engages in O-GlcNAc-mediated protein-protein interactions with itself and with the NF component α-internexin, implying that O-GlcNAc may be a general regulator of NF architecture. We further show that NF-L O-GlcNAcylation is required for normal organelle trafficking in primary neurons. Finally, several CMT-causative NF-L mutants exhibit perturbed O-GlcNAc levels and resist the effects of O-GlcNAcylation on NF assembly state, suggesting a potential link between dysregulated O-GlcNAcylation and pathological NF aggregation. Our results demonstrate that site-specific glycosylation regulates NF-L assembly and function, and aberrant NF O-GlcNAcylation may contribute to CMT and other neurodegenerative disorders.

Genetically Encoded Boronolectin as a Specific Red Fluorescent UDP-GlcNAc Biosensor

There is great interest in developing boronolectins that are synthetic lectin mimics containing a boronic acid functional group for reversible recognition of diol-containing molecules, such as glycans and ribonucleotides. However, it remains a significant challenge to gain specificity. Here, we present a genetically encoded boronolectin which is a hybrid protein consisting of a noncanonical amino acid (ncAA) p-boronophenylalanine (pBoF), natural-lectin-derived peptide sequences, and a circularly permuted red fluorescent protein (cpRFP). The genetic encodability permitted a straightforward protein engineering process to derive a red fluorescent biosensor that can specifically bind uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), an important nucleotide sugar involved in metabolic sensing and cell signaling. We further characterized the resultant boronic acid- and peptide-assisted UDP-GlcNAc sensor (bapaUGAc) both in vitro and in live mammalian cells. Because UDP-GlcNAc in the endoplasmic reticulum (ER) and Golgi apparatus plays essential roles in glycosylating biomolecules in the secretory pathway, we genetically expressed bapaUGAc in the ER and Golgi and validated the sensor for its responses to metabolic disruption and pharmacological inhibition. In addition, we combined bapaUGAc with UGAcS, a recently reported green fluorescent UDP-GlcNAc sensor based on an alternative sensing mechanism, to monitor UDP-GlcNAc level changes in the ER and cytosol simultaneously. We expect our work to facilitate the future development of specific boronolectins for carbohydrates. In addition, this newly developed genetically encoded bapaUGAc sensor will be a valuable tool for studying UDP-GlcNAc and glycobiology.

Genetically Encoded Boronolectin as a Specific Red Fluorescent UDP-GlcNAc Biosensor

There is great interest in developing boronolectins, which are synthetic lectin mimics containing a boronic acid functional group for reversible recognition of diol-containing molecules, such as glycans and ribonucleotides. However, it remains a significant challenge to gain specificity. Here, we present a genetically encoded boronolectin, which is a hybrid protein consisting of a noncanonical amino acid (ncAA) p-boronophenylalanine (pBoF), natural-lectin-derived peptide sequences, and a circularly permuted red fluorescent protein (cpRFP). The genetic encodability permitted a straightforward protein engineering process to derive a red fluorescent biosensor that can specifically bind uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), an important nucleotide sugar involved in metabolic sensing and cell signaling. We further characterized the resultant boronic acid-and peptide-assisted UDP-GlcNAc sensor (bapaUGAc) both in vitro and in live mammalian cells. Because UDP-GlcNAc in the endoplasmic reticulum (ER) and Golgi apparatus plays essential roles in glycosylating biomolecules in the secretory pathway, we genetically expressed bapaUGAc in the ER and Golgi and validated the sensor for its responses to metabolic disruption and pharmacological inhibition. In addition, we combined bapaUGAc with UGAcS, a recently reported green fluorescent UDP-GlcNAc sensor based on an alternative sensing mechanism, to monitor UDP-GlcNAc level changes in the ER and cytosol simultaneously. We expect our work to facilitate the future development of specific boronolectins for carbohydrates. In addition, this newly developed genetically encoded bapaUGAc sensor will be a valuable tool for studying UDP-GlcNAc and glycobiology.

Design of OSMI-4 Analogs Using Scaffold Hopping: Investigating the Importance of the Uridine Mimic in the Binding of OGT Inhibitors

β-N-Acetylglucosamine transferase (OGT) inhibition is considered an important topic in medicinal chemistry. The involvement of O-GlcNAcylation in several important biological pathways is pointing to OGT as a potential therapeutic target. The field of OGT inhibitors drastically changed after the discovery of the 7-quinolone-4-carboxamide scaffold and its optimization to the first nanomolar OGT inhibitor: OSMI-4. While OSMI-4 is still the most potent inhibitor reported to date, its physicochemical properties are limiting its use as a potential drug candidate as well as a biological tool. In this study, we have introduced a simple modification (elongation) of the peptide part of OSMI-4 that limits the unwanted cyclisation during OSMI-4 synthesis while retaining OGT inhibitory potency. Secondly, we have kept this modified peptide unchanged while incorporating new sulfonamide UDP mimics to try to improve binding of newly designed OGT inhibitors in the UDP-binding site. With the use of computational methods, a small library of OSMI-4 derivatives was designed, prepared and evaluated that provided information about the OGT binding pocket and its specificity toward quinolone-4-carboxamides.

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

O-GlcNAcylation is an atypical, dynamic and reversible O-glycosylation that is critical and abundant in metazoan. O-GlcNAcylation coordinates and receives various signaling inputs such as nutrients and stresses, thus spatiotemporally regulating the activity, stability, localization and interaction of target proteins to participate in cellular physiological functions. Our review discusses in depth the involvement of O-GlcNAcylation in the precise regulation of skeletal muscle metabolism, such as glucose homeostasis, insulin sensitivity, tricarboxylic acid cycle and mitochondrial biogenesis. The complex interaction and precise modulation of O-GlcNAcylation in these nutritional pathways of skeletal muscle also provide emerging mechanical information on how nutrients affect health, exercise and disease. Meanwhile, we explored the potential role of O-GlcNAcylation in skeletal muscle pathology and focused on its benefits in maintaining proteostasis under atrophy. In general, these understandings of O-GlcNAcylation are conducive to providing new insights into skeletal muscle (patho) physiology.

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.

Spatiotemporal Proximity Labeling Tools to Track GlcNAc Sugar-Modified Functional Protein Hubs during Cellular Signaling

A fundamental mechanism that all eukaryotic cells use to adapt to their environment is dynamic protein modification with monosaccharide sugars. In humans, O-linked N-acetylglucosamine (O-GlcNAc) is rapidly added to and removed from diverse protein sites as a response to fluctuating nutrient levels, stressors, and signaling cues. Two aspects remain challenging for tracking functional O-GlcNAc events with chemical strategies: spatial control over subcellular locations and time control during labeling. The objective of this study was to create intracellular proximity labeling tools to identify functional changes in O-GlcNAc patterns with spatiotemporal control. We developed a labeling strategy based on the TurboID proximity labeling system for rapid protein biotin conjugation directed to O-GlcNAc protein modifications inside cells, a set of tools called "GlycoID." Localized variants to the nucleus and cytosol, nuc-GlycoID and cyt-GlycoID, labeled O-GlcNAc proteins and their interactomes in subcellular space. Labeling during insulin and serum stimulation revealed functional changes in O-GlcNAc proteins as soon as 30 min following signal initiation. We demonstrated using proteomic analysis that the GlycoID strategy captured O-GlcNAcylated "activity hubs" consisting of O-GlcNAc proteins and their associated protein-protein interactions. The ability to follow changes in O-GlcNAc hubs during physiological events such as insulin signaling allows these tools to determine the mechanisms of glycobiological cell regulation. Our functional O-GlcNAc data sets in human cells will be a valuable resource for O-GlcNAc-driven mechanisms.

Global O-GlcNAcylation changes impact desmin phosphorylation and its partition toward cytoskeleton in C2C12 skeletal muscle cells differentiated into myotubes

Desmin is the guardian of striated muscle integrity, permitting the maintenance of muscle shape and the efficiency of contractile activity. It is also a key mediator of cell homeostasis and survival. To ensure the fine regulation of skeletal muscle processes, desmin is regulated by post-translational modifications (PTMs). It is more precisely phosphorylated by several kinases connecting desmin to intracellular processes. Desmin is also modified by O-GlcNAcylation, an atypical glycosylation. However, the functional consequence of O-GlcNAcylation on desmin is still unknown, nor its impact on desmin phosphorylation. In a model of C2C12 myotubes, we modulated the global O-GlcNAcylation level, and we determined whether the expression, the PTMs and the partition of desmin toward insoluble material or cytoskeleton were impacted or not. We have demonstrated in the herein paper that O-GlcNAcylation variations led to changes in desmin behaviour. In particular, our data clearly showed that O-GlcNAcylation increase led to a decrease of phosphorylation level on desmin that seems to involve CamKII correlated to a decrease of its partition toward cytoskeleton. Our data showed that phosphorylation/O-GlcNAcylation interplay is highly complex on desmin, supporting that a PTMs signature could occur on desmin to finely regulate its partition (i.e. distribution) with a spatio-temporal regulation.

O-GlcNAcylation: The Underestimated Emerging Regulators of Skeletal Muscle Physiology

O-GlcNAcylation is a highly dynamic, reversible and atypical glycosylation that regulates the activity, biological function, stability, sublocation and interaction of target proteins. O-GlcNAcylation receives and coordinates different signal inputs as an intracellular integrator similar to the nutrient sensor and stress receptor, which target multiple substrates with spatio-temporal analysis specifically to maintain cellular homeostasis and normal physiological functions. Our review gives a brief description of O-GlcNAcylation and its only two processing enzymes and HBP flux, which will help to better understand its physiological characteristics of sensing nutrition and environmental cues. This nutritional and stress-sensitive properties of O-GlcNAcylation allow it to participate in the precise regulation of skeletal muscle metabolism. This review discusses the mechanism of O-GlcNAcylation to alleviate metabolic disorders and the controversy about the insulin resistance of skeletal muscle. The level of global O-GlcNAcylation is precisely controlled and maintained in the "optimal zone", and its abnormal changes is a potential factor in the pathogenesis of cancer, neurodegeneration, diabetes and diabetic complications. Although the essential role of O-GlcNAcylation in skeletal muscle physiology has been widely studied and recognized, it still is underestimated and overlooked. This review highlights the latest progress and potential mechanisms of O-GlcNAcylation in the regulation of skeletal muscle contraction and structural properties.

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.

O-GlcNAcylation and Its Role in Cancer-Associated Inflammation

Cancer cells, as well as surrounding stromal and inflammatory cells, form an inflammatory tumor microenvironment (TME) to promote all stages of carcinogenesis. As an emerging post-translational modification (PTM) of serine and threonine residues of proteins, O-linked-N-Acetylglucosaminylation (O-GlcNAcylation) regulates diverse cancer-relevant processes, such as signal transduction, transcription, cell division, metabolism and cytoskeletal regulation. Recent studies suggest that O-GlcNAcylation regulates the development, maturation and functions of immune cells. However, the role of protein O-GlcNAcylation in cancer-associated inflammation has been less explored. This review summarizes the current understanding of the influence of protein O-GlcNAcylation on cancer-associated inflammation and the mechanisms whereby O-GlcNAc-mediated inflammation regulates tumor progression. This will provide a theoretical basis for further development of anti-cancer therapies.

Ogt Demonstrated Conspicuous Clinical Significance in Cancers, from Pan-Cancer to Small-Cell Lung Cancer

The clinical progression of small-cell lung cancer (SCLC) remains pessimistic. The aim of the present study was to promote the understanding of the clinical significance and mechanism of O-linked N-acetylglucosamine (GlcNAc) transferase (OGT) in SCLC. Wilcoxon tests, standardized mean difference (SMD), and Kruskal–Wallis tests were utilized to compare OGT level differences among the experimental and control groups. The univariate Cox regression analysis, Kaplan–Meier curves, and receiver operating characteristic curves were applied to determine OGT's clinical relevance in cancers. The Spearman correlation analysis and enrichment analysis were utilized to explore the underlying mechanisms of OGT in cancers. For the first time in the field, we provide an overview of OGT in 32 cancers using a large number of samples (n = 21,196), determining distinct OGT expression in 25 cancers and its prognosis effects in 12 cancers. Furthermore, using 950 samples from multiple sources, upregulated OGT was found in both mRNA and protein levels in SCLC (SMD = 0.93, 95% CI [0.24, 1.63]). Higher OGT levels represented a more unfavorable disease-free interval for SCLC patients (p < 0.001). The research also identified OGT expression as a potential marker for SCLC prediction (sensitivity = 0.79, specificity = 0.86, and AUC = 0.88). The high expression of OGT in SCLC may result from the positive regulation of two transcription factors—DEK and XRN2. We primarily investigated the underlying mechanisms of OGT in SCLC. Herein, based on the analyses from pan-cancer to SCLC, OGT demonstrated conspicuous clinical significance. OGT may be an underlying biomarker for the treatment and identification of some cancers, including SCLC.

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.

Simultaneously Identifying and Distinguishing Glycoproteins with O-GlcNAc and O-GalNAc (the Tn Antigen) in Human Cancer Cells

Glycoproteins with diverse glycans are essential to human cells, and subtle differences in glycan structures may result in entirely different functions. One typical example is proteins modified with O-linked β-N-acetylglucosamine (O-GlcNAc) and O-linked α-N-acetylgalactosamine (O-GalNAc) (the Tn antigen), in which the two glycans have very similar structures and identical chemical compositions, making them extraordinarily challenging to be distinguished. Here, we developed an effective method benefiting from selective enrichment and the enzymatic specificity to simultaneously identify and distinguish glycoproteins with O-GlcNAc and O-GalNAc. Metabolic labeling was combined with bioorthogonal chemistry for enriching glycoproteins modified with O-GlcNAc and O-GalNAc. Then, the enzymatic reaction with galactose oxidase was utilized to specifically oxidize O-GalNAc, but not O-GlcNAc, generating the different tags between glycopeptides with O-GlcNAc and O-GalNAc that can be easily distinguishable by mass spectrometry (MS). Among O-GlcNAcylated proteins commonly identified in three types of human cells, those related to transcription and RNA binding are highly enriched. Cell-specific features are also revealed. Among glycoproteins exclusively in Jurkat cells, those involved in human T-lymphotropic virus type 1 (HTLV-1) infection are overrepresented, which is consistent with the cell line source and suggests that protein O-GlcNAcylation participated in the response to the virus infection. Furthermore, glycoproteins with the Tn antigen have different subcellular distributions in different cells, which may be attributed to the distinct mechanisms for the formation of protein O-GalNAcylation.

Inhibition of O-GlcNAcylation protects from Shiga toxin-mediated cell injury and lethality in host

Shiga toxins (Stxs) produced by enterohemorrhagic Escherichia coli (EHEC) are the major virulence factors responsible for hemorrhagic colitis, which can lead to life-threatening systemic complications including acute renal failure (hemolytic uremic syndrome) and neuropathy. Here, we report that O-GlcNAcylation, a type of post-translational modification, was acutely increased upon induction of endoplasmic reticulum (ER) stress in host cells by Stxs. Suppression of the abnormal Stx-mediated increase in O-GlcNAcylation effectively inhibited apoptotic and inflammatory responses in Stx-susceptible cells. The protective effect of O-GlcNAc inhibition for Stx-mediated pathogenic responses was also verified using three-dimensional (3D)-cultured spheroids or organoids mimicking the human kidney. Treatment with an O-GlcNAcylation inhibitor remarkably improved the major disease symptoms and survival rate for mice intraperitoneally injected with a lethal dose of Stx. In conclusion, this study elucidates O-GlcNAcylation-dependent pathogenic mechanisms of Stxs and demonstrates that inhibition of aberrant O-GlcNAcylation is a potential approach to treat Stx-mediated diseases.

The Metabolic Chemical Reporter Ac46AzGal Could Incorporate Intracellular Protein Modification in the Form of UDP-6AzGlc Mediated by OGT and Enzymes in the Leloir Pathway

Galactose is a naturally occurring monosaccharide used to build complex glycans that has not been targeted for labeling as a metabolic reporter. Here, we characterize the cellular modification of proteins by using Ac₄6AzGal in a dose- and time-dependent manner. It is noted that a vast majority of this labeling of Ac₄6AzGal occurs intracellularly in a range of mammalian cells. We also provided evidence that this labeling is dependent on not only the enzymes of OGT responsible for O-GlcNAcylation but also the enzymes of GALT and GALE in the Leloir pathway. Notably, we discover that Ac₄6AzGal is not the direct substrate of OGT, and the labeling results may attribute to UDP-6AzGlc after epimerization of UDP-6AzGal via GALE. Together, these discoveries support the conclusion that Ac₄6AzGal as an analogue of galactose could metabolically label intracellular O-glycosylation modification, raising the possibility of characterization with impaired functions of the galactose metabolism in the Leloir pathway under certain conditions, such as galactosemias.

O‑GlcNAcylation contributes to intermittent hypoxia‑associated vascular dysfunction via modulation of MAPKs but not CaMKII pathways

Intermittent hypoxia (IH) leads to vascular dysfunction, and O-linked-β-N-acetylglucosamine (O-GlcNAc)ylation may regulate vascular reactivity through the modulation of intracellular signaling. The present study hypothesized that O-GlcNAc modifications contributed to the vascular effects of acute IH (AIH) and chronic IH (CIH) through the MAPK and Ca²⁺/calmodulin-dependent kinase II (CaMKII) pathways. Rat aortic and mesenteric segments were incubated with DMSO, O-GlcNAcase (OGA) or O-GlcNAc transferase (OGT) inhibitor under either normoxic or AIH conditions for 3 h, and arterial function was then assessed. Meanwhile, arteries isolated from control and CIH rats were exposed to 3 h of incubation under normoxic conditions using DMSO, OGA or OGT as an inhibitor, before assessing arterial reactivity. CIH was found to increase the expression of vascular O-GlcNAc protein and OGT, phosphorylate p38 MAPK and ERK1/2, and decrease OGA levels, but it had no effects on phosphorylated CaMKII levels. OGA inhibition increased global O-GlcNAcylation and the phosphorylation of p38 MAPK, ERK1/2 and CaMKII, whereas OGT blockade had the opposite effects. OGA inhibition preserved acetylcholine-induced relaxation in AIH arteries, whereas OGT blockade attenuated the relaxation responses of arteries under normoxic conditions or undergoing AIH treatments. However, the impairment of acetylcholine dilation in CIH mesenteric arteries was improved. CIH artery contraction was increased following angiotensin II (Ang II) exposure. Blockade of p38 MAPK and ERK1/2, but not CaMKII, attenuated Ang II-induced contractile responses in CIH arteries isolated from the non-OGT inhibitor-treated groups. OGT inhibition significantly blocked contractile responses to Ang II and abolished the inhibitory effects of MAPK inhibitors. These findings indicated that O-GlcNAcylation regulates IH-induced vascular dysfunction, at least partly by modulating MAPK, but not CaMKII, signaling pathways.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

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.