Reduced protein O-glycosylation in the nervous system of the mutant SOD1 transgenic mouse model of amyotrophic lateral sclerosis

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.

Abnormal protein post-translational modifications induces aggregation and abnormal deposition of protein, mediating neurodegenerative diseases

Protein post-translational modifications (PPTMs) refer to a series of chemical modifications that occur after the synthesis of protein. Proteins undergo different modifications such as phosphorylation, acetylation, ubiquitination, and so on. These modifications can alter the protein's structure, function, and interaction, thereby regulating its biological activity. In neurodegenerative diseases, several proteins undergo abnormal post-translational modifications, which leads to aggregation and abnormal deposition of protein, thus resulting in neuronal death and related diseases. For example, the main pathological features of Alzheimer's disease are the aggregation of beta-amyloid protein and abnormal phosphorylation of tau protein. The abnormal ubiquitination and loss of α-synuclein are related to the onset of Parkinson's disease. Other neurodegenerative diseases such as Huntington's disease, amyotrophic lateral sclerosis, and so on are also connected with abnormal PPTMs. Therefore, studying the abnormal PPTMs in neurodegenerative diseases is critical for understanding the mechanism of these diseases and the development of significant therapeutic strategies. This work reviews the implications of PPTMs in neurodegenerative diseases and discusses the relevant therapeutic strategies.

O-GlcNAcylation and Its Roles in Neurodegenerative Diseases

 As a non-classical post-translational modification, O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) is widely found in human organ systems, particularly in our brains, and is indispensable for healthy cell biology. With the increasing age of the global population, the incidence of neurodegenerative diseases is increasing, too. The common characteristic of these disorders is the aggregation of abnormal proteins in the brain. Current research has found that O-GlcNAcylation dysregulation is involved in misfolding or aggregation of these abnormal proteins to mediate disease progression, but the specific mechanism has not been defined. This paper reviews recent studies on O-GlcNAcylation's roles in several neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, Machado-Joseph's disease, and giant axonal neuropathy, and shows that O-GlcNAcylation, as glucose metabolism sensor, mediating synaptic function, participating in oxidative stress response and signaling pathway conduction, directly or indirectly regulates characteristic pathological protein toxicity and affects disease progression. The existing results suggest that targeting O-GlcNAcylation will provide new ideas for clinical diagnosis, prevention, and treatment of neurodegenerative diseases.

Protein glycosylation and glycoinformatics for novel biomarker discovery in neurodegenerative diseases

Glycosylation is a common post-translational modification of brain proteins including cell surface adhesion molecules, synaptic proteins, receptors and channels, as well as intracellular proteins, with implications in brain development and functions. Using advanced state-of-the-art glycomics and glycoproteomics technologies in conjunction with glycoinformatics resources, characteristic glycosylation profiles in brain tissues are increasingly reported in the literature and growing evidence shows deregulation of glycosylation in central nervous system disorders, including aging associated neurodegenerative diseases. Glycan signatures characteristic of brain tissue are also frequently described in cerebrospinal fluid due to its enrichment in brain-derived molecules. A detailed structural analysis of brain and cerebrospinal fluid glycans collected in publications in healthy and neurodegenerative conditions was undertaken and data was compiled to create a browsable dedicated set in the GlyConnect database of glycoproteins (https://glyconnect.expasy.org/brain). The shared molecular composition of cerebrospinal fluid with brain enhances the likelihood of novel glycobiomarker discovery for neurodegeneration, which may aid in unveiling disease mechanisms, therefore, providing with novel therapeutic targets as well as diagnostic and progression monitoring tools.

Glycosylation and behavioral symptoms in neurological disorders

Glycosylation, the addition of glycans or carbohydrates to proteins, lipids, or other glycans, is a complex post-translational modification that plays a crucial role in cellular function. It is estimated that at least half of all mammalian proteins undergo glycosylation, underscoring its importance in the functioning of cells. This is reflected in the fact that a significant portion of the human genome, around 2%, is devoted to encoding enzymes involved in glycosylation. Changes in glycosylation have been linked to various neurological disorders, including Alzheimer's disease, Parkinson's disease, autism spectrum disorder, and schizophrenia. Despite its widespread occurrence, the role of glycosylation in the central nervous system remains largely unknown, particularly with regard to its impact on behavioral abnormalities in brain diseases. This review focuses on examining the role of three types of glycosylation: N-glycosylation, O-glycosylation, and O-GlcNAcylation, in the manifestation of behavioral and neurological symptoms in neurodevelopmental, neurodegenerative, and neuropsychiatric disorders.

Chemical Biology Approaches to Understanding Neuronal O-GlcNAcylation

O-linked β-N-acetylglucosamine (O-GlcNAc) is a ubiquitous post-translational modification in mammals, decorating thousands of intracellular proteins. O-GlcNAc cycling is an essential regulator of myriad aspects of cell physiology and is dysregulated in numerous human diseases. Notably, O-GlcNAcylation is abundant in the brain and numerous studies have linked aberrant O-GlcNAc signaling to various neurological conditions. However, the complexity of the nervous system and the dynamic nature of protein O-GlcNAcylation have presented challenges for studying of neuronal O-GlcNAcylation. In this context, chemical approaches have been a particularly valuable complement to conventional cellular, biochemical, and genetic methods to understand O-GlcNAc signaling and to develop future therapeutics. Here we review selected recent examples of how chemical tools have empowered efforts to understand and rationally manipulate O-GlcNAcylation in mammalian neurobiology.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.

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.

Structure-based discovery and development of novel O-GlcNAcase inhibitors for the treatment of Alzheimer's disease

The neurofibrillary tangles (NFTs) formed from hyperphosphorylation of tau protein are closely associated with Alzheimer's disease (AD). O-GlcNAcylation of tau can negatively regulate hyperphosphorylation and the O-GlcNAcase (OGA) catalyzes the removal of O-linked β-N-acetylglucosamine (O-GlcNAc) from tau protein. Therefore, preventing tau hyperphosphorylation by increasing the levels of tau O-GlcNAcylation via OGA inhibitors could be a promising approach. Based on Thiamet-G, a potent OGA inhibitor, and its binding mode to OGA, a novel OGA inhibitor scaffold bearing three parts was designed and hit compound 7j was successfully identified via extensive exploring. Further chemical optimization and diversification of the 7j structure resulted in compound 39 which possesses excellent OGA inhibition, no cytotoxicity, and has good pharmacokinetic properties. In acute AD model mice, 39 was more effective than Thiamet-G in inhibiting OGA activity attributable to its better blood-brain barrier permeability. In addition, 39 restored the cognitive function in mice and reduced amyloid-β (Aβ) concentrations to a greater extent than Thiamet-G. Molecular docking studies demonstrated that 39 was well associated with OGA through H-bonds and hydrophobic interaction. Together, these findings suggest that 39 was promising as a potent OGA inhibitor in the treatment of AD.

Intellectual disability-associated disruption of O-GlcNAc cycling impairs habituation learning in Drosophila

O-GlcNAcylation is a reversible co-/post-translational modification involved in a multitude of cellular processes. The addition and removal of the O-GlcNAc modification is controlled by two conserved enzymes, O-GlcNAc transferase (OGT) and O-GlcNAc hydrolase (OGA). Mutations in OGT have recently been discovered to cause a novel Congenital Disorder of Glycosylation (OGT-CDG) that is characterized by intellectual disability. The mechanisms by which OGT-CDG mutations affect cognition remain unclear. We manipulated O-GlcNAc transferase and O-GlcNAc hydrolase activity in Drosophila and demonstrate an important role of O-GlcNAcylation in habituation learning and synaptic development at the larval neuromuscular junction. Introduction of patient-specific missense mutations into Drosophila O-GlcNAc transferase using CRISPR/Cas9 gene editing leads to deficits in locomotor function and habituation learning. The habituation deficit can be corrected by blocking O-GlcNAc hydrolysis, indicating that OGT-CDG mutations affect cognition-relevant habituation via reduced protein O-GlcNAcylation. This study establishes a critical role for O-GlcNAc cycling and disrupted O-GlcNAc transferase activity in cognitive dysfunction, and suggests that blocking O-GlcNAc hydrolysis is a potential strategy to treat OGT-CDG.

O-GlcNAcylation Is Essential for Rapid Pomc Expression and Cell Proliferation in Corticotropic Tumor Cells

Pituitary adenomas have a staggering 16.7% lifetime prevalence and can be devastating in many patients because of profound endocrine and neurologic dysfunction. To date, no clear genomic or epigenomic markers correlate with their onset or severity. Herein, we investigate the impact of the O-GlcNAc posttranslational modification in their etiology. Found in more than 7000 human proteins to date, O-GlcNAcylation dynamically regulates proteins in critical signaling pathways, and its deregulation is involved in cancer progression and endocrine diseases such as diabetes. In this study, we demonstrated that O-GlcNAc enzymes were upregulated, particularly in aggressive adrenocorticotropin (ACTH)-secreting tumors, suggesting a role for O-GlcNAcylation in pituitary adenoma etiology. In addition to the demonstration that O-GlcNAcylation was essential for their proliferation, we showed that the endocrine function of pituitary adenoma is also dependent on O-GlcNAcylation. In corticotropic tumors, hypersecretion of the proopiomelanocortin (POMC)-derived hormone ACTH leads to Cushing disease, materialized by severe endocrine disruption and increased mortality. We demonstrated that Pomc messenger RNA is stabilized in an O-GlcNAc-dependent manner in response to corticotrophin-releasing hormone (CRH). By affecting Pomc mRNA splicing and stability, O-GlcNAcylation contributes to this new mechanism of fast hormonal response in corticotropes. Thus, this study stresses the essential role of O-GlcNAcylation in ACTH-secreting adenomas' pathophysiology, including cellular proliferation and hypersecretion.

O-GlcNAcylation in health and neurodegenerative diseases

O-GlcNAcylation is a posttranslational modification that adds O-linked β-N-acetylglucosamine (O-GlcNAc) to serine or threonine residues of many proteins. This protein modification interacts with key cellular pathways involved in transcription, translation, and proteostasis. Although ubiquitous throughout the body, O-GlcNAc is particularly abundant in the brain, and various proteins commonly found at synapses are O-GlcNAcylated. Recent studies have demonstrated that the modulation of O-GlcNAc in the brain alters synaptic and neuronal functions. Furthermore, altered brain O-GlcNAcylation is associated with either the etiology or pathology of numerous neurodegenerative diseases, while the manipulation of O-GlcNAc exerts neuroprotective effects against these diseases. Although the detailed molecular mechanisms underlying the functional roles of O-GlcNAcylation in the brain remain unclear, O-GlcNAcylation is critical for regulating diverse neural functions, and its levels change during normal and pathological aging. In this review, we will highlight the functional importance of O-GlcNAcylation in the brain and neurodegenerative diseases.

O-GlcNAcylation of TDP-43 suppresses proteinopathies and promotes TDP-43's mRNA splicing activity

Pathological TDP-43 aggregation is characteristic of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP); however, how TDP-43 aggregation and function are regulated remain poorly understood. Here, we show that O-GlcNAc transferase OGT-mediated O-GlcNAcylation of TDP-43 suppresses ALS-associated proteinopathies and promotes TDP-43's splicing function. Biochemical and cell-based assays indicate that OGT's catalytic activity suppresses TDP-43 aggregation and hyperphosphorylation, whereas abolishment of TDP-43 O-GlcNAcylation impairs its RNA splicing activity. We further show that TDP-43 mutations in the O-GlcNAcylation sites improve locomotion defects of larvae and adult flies and extend adult life spans, following TDP-43 overexpression in Drosophila motor neurons. We finally demonstrate that O-GlcNAcylation of TDP-43 promotes proper splicing of many mRNAs, including STMN2, which is required for normal axonal outgrowth and regeneration. Our findings suggest that O-GlcNAcylation might be a target for the treatment of TDP-43-linked pathogenesis.

Glycoproteins as diagnostic and prognostic biomarkers for neurodegenerative diseases: A glycoproteomic approach

Neurodegenerative diseases (NDs) are incurable and can develop progressively debilitating disorders, including dementia and ataxias. Alzheimer's disease and Parkinson's disease are the most common NDs that mainly affect the elderly people. There is an urgent need to develop new diagnostic tools so that patients can be accurately stratified at an early stage. As a common post-translational modification, protein glycosylation plays a key role in physiological and pathological processes. The abnormal changes in glycosylation are associated with the altered biological pathways in NDs. The pathogenesis-related proteins, like amyloid-β and microtubule-associated protein tau, have altered glycosylation. Importantly, specific glycosylation changes in cerebrospinal fluid, blood and urine are valuable for revealing neurodegeneration in the early stages. This review describes the emerging biomarkers based on glycoproteomics in NDs, highlighting the potential applications of glycoprotein biomarkers in the early detection of diseases, monitoring of the disease progression, and measurement of the therapeutic responses. The mass spectrometry-based strategies for characterizing glycoprotein biomarkers are also introduced.

NPGPx-Mediated Adaptation to Oxidative Stress Protects Motor Neurons from Degeneration in Aging by Directly Modulating O-GlcNAcase

Amyotrophic lateral sclerosis (ALS), the most common motor neuron disease, usually occurs in middle-aged people. However, the molecular basis of age-related cumulative stress in ALS pathogenesis remains elusive. Here, we found that mice deficient in NPGPx (GPx7), an oxidative stress sensor, develop ALS-like phenotypes, including paralysis, muscle denervation, and motor neurons loss. Unlike normal spinal motor neurons that exhibit elevated O-GlcNAcylation against age-dependent oxidative stress, NPGPx-deficient spinal motor neurons fail to boost O-GlcNAcylation and exacerbate ROS accumulation, leading to cell death. Mechanistically, stress-activated NPGPx inhibits O-GlcNAcase (OGA) through disulfide bonding to fine-tune global O-GlcNAcylation. Pharmacological inhibition of OGA rescues spinal motor neuron loss in aged NPGPx-deficient mice. Furthermore, expression of NPGPx in ALS patients is significantly lower than in unaffected adults. These results suggest that NPGPx modulates O-GlcNAcylation by inhibiting OGA to cope with age-dependent oxidative stress and protect motor neurons from degeneration, providing a potential therapeutic axis for ALS.

Emerging molecular mediators and targets for age-related skeletal muscle atrophy

The age-associated decline in muscle mass has become synonymous with physical frailty among the elderly due to its major contribution in reduced muscle function. Alterations in protein and redox homeostasis along with chronic inflammation, denervation, and hormonal dysregulation are all hallmarks of muscle wasting and lead to clinical sarcopenia in older adults. Reduction in skeletal muscle mass has been observed and reported in the scientific literature for nearly 2 centuries; however, identification and careful examination of molecular mediators of age-related muscle atrophy have only been possible for roughly 3 decades. Here we review molecular targets of recent interest in age-related muscle atrophy and briefly discuss emerging small molecule therapeutic treatments for muscle wasting in sarcopenic susceptible populations.

Increased Tau Phosphorylation in Motor Neurons From Clinically Pure Sporadic Amyotrophic Lateral Sclerosis Patients

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motor neurons. There is a pathological and genetic link between ALS and frontotemporal lobar degeneration (FTLD). Although FTLD is characterized by abnormal phosphorylated tau deposition, it is unknown whether tau is phosphorylated in ALS motor neurons. Therefore, this study assessed tau epitopes that are commonly phosphorylated in FTLD, including serine 396 (pS396), 214 (pS214), and 404 (pS404) in motor neurons from clinically pure sporadic ALS cases compared with controls. In ALS lower motor neurons, tau pS396 was observed in the nucleus or the nucleus and cytoplasm. In ALS upper motor neurons, tau pS396 was observed in the nucleus, cytoplasm, or both the nucleus and cytoplasm. Tau pS214 and pS404 was observed only in the cytoplasm of upper and lower motor neurons in ALS. The number of motor neurons (per mm2) positive for tau pS396 and pS214, but not pS404, was significantly increased in ALS. Furthermore, there was a significant loss of phosphorylated tau-negative motor neurons in ALS compared with controls. Together, our data identified a complex relationship between motor neurons positive for tau phosphorylated at specific residues and disease duration, suggesting that tau phosphorylation plays a role in ALS.

The impact of proteostasis dysfunction secondary to environmental and genetic causes on neurodegenerative diseases progression and potential therapeutic intervention

Aggregation of particular proteins in the form of inclusion bodies or plaques followed by neuronal death is a hallmark of neurodegenerative proteopathies such as primary Parkinsonism, Alzheimer's disease, Lou Gehrig's disease, and Huntington's chorea. Complex polygenic and environmental factors implicated in these proteopathies. Accumulation of proteins in these disorders indicates a substantial disruption in protein homeostasis (proteostasis). Proteostasis or cellular proteome homeostasis is attained by the synchronization of a group of cellular mechanisms called the proteostasis network (PN), which is responsible for the stability of the proteome and achieves the equilibrium between synthesis, folding, and degradation of proteins. In this review, we will discuss the different types of PN and the impact of PN component dysfunction on the four major neurodegenerative diseases mentioned earlier. Graphical abstract.

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.

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.

Alteration of O-GlcNAcylation affects assembly and axonal transport of neurofilament via phosphorylation

Neurofilaments (NFs), the most abundant cytoskeletal components in the mature neuron, are hyperphosphorylated and accumulated in the neuronal cell body of AD brain, and the abnormalities of NFs appear to contribute to neurodegeneration. Although previous studies have showed that O-GlcNAcylation and phosphorylation of NFs regulate each other reciprocally, the NFs O-GlcNAcylation and its effects on assembly and axonal transport are poorly explored. Here, we focus on the role of dysregulation of O-GlcNAcylation on structure and function of neurofilaments by corresponding phosphorylation. In the study, we found that decreased O-GlcNAcylation by intracerebroventricular administration of Alloxan, 6-diazo-5-oxonorleucine (Don) and okadaic acid (OA) in the rats resulted in increased phosphorylation with assembly of lower and shorter NFs. In contrast, in the sample of NAG-thiazoline (NAG-Ae) causing increased O-GlcNAcylation, NFs showed elongated filaments fibers and higher proportion of assembly. Furthermore, alloxan treatment induced abnormal accumulation of NFs bodies and delayed time of Fluorescence Recovery After Photobleaching (FRAP) in SK-N-SH cells, but the NAG-Ae treatment speeded up the axonal transport. Our experiments suggest that increased O-GlcNAcylation plays a key role in protecting the structure and function of NFs including filament assembly and axonal transport via decreased phosphorylation. These results expanded the function of O-GlcNAcylation in AD pathogenesis.

Recent advances in mass spectrometry-based glycoproteomics

Protein glycosylation plays fundamental roles in many biological processes as one of the most common, and the most complex, posttranslational modification. Alterations in glycosylation profile are now known to be associated with many diseases. As a result, the discovery and detailed characterization of glycoprotein disease biomarkers is a primary interest of biomedical research. Advances in mass spectrometry (MS)-based glycoproteomics and glycomics are increasingly enabling qualitative and quantitative approaches for site-specific structural analysis of protein glycosylation. While the complexity presented by glycan heterogeneity and the wide dynamic range of clinically relevant samples like plasma, serum, cerebrospinal fluid, and tissue make comprehensive analyses of the glycoproteome a challenging task, the ongoing efforts into the development of glycoprotein enrichment, enzymatic digestion, and separation strategies combined with novel quantitative MS methodologies have greatly improved analytical sensitivity, specificity, and throughput. This review summarizes current MS-based glycoproteomics approaches and highlights recent advances in its application to cancer biomarker and neurodegenerative disease research.

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: A regulator of tau pathology and neurodegeneration

O-GlcNAcylation is the posttranslational modification of intracellular proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). The discovery of O-GlcNAc modification of tau and its impact on tau phosphorylation has attracted recent research interest in O-GlcNAc studies in the Alzheimer's disease (AD) field. Modification of proteins by O-GlcNAc occurs extensively in the brain. The expressions and activities of the enzymes catalyzing O-GlcNAc cycling are several-fold higher in the brain than in the peripheral tissues. The O-GlcNAcylation levels of brain proteins including tau are decreased in AD brain, probably due to decreased brain glucose metabolism. The reduction of brain O-GlcNAcylation appears to mediate the molecular mechanism by which decreased brain glucose metabolism contributes to neurodegeneration. Studies on mouse models of tauopathies suggest a neuroprotective role of pharmacological elevation of brain O-GlcNAc, which could potentially be a promising approach for treating AD and other neurodegenerative diseases.

Adducin at the Neuromuscular Junction in Amyotrophic Lateral Sclerosis: Hanging on for Dear Life

The neurological dysfunction in amyotrophic lateral sclerosis (ALS)/motor neurone disease (MND) is associated with defective nerve-muscle contacts early in the disease suggesting that perturbations of cell adhesion molecules (CAMs) linking the pre- and post-synaptic components of the neuromuscular junction (NMJ) are involved. To search for candidate proteins implicated in this degenerative process, researchers have studied the Drosophila larval NMJ and find that the cytoskeleton-associated protein, adducin, is ideally placed to regulate synaptic contacts. By controlling the levels of synaptic proteins, adducin can de-stabilize synaptic contacts. Interestingly, elevated levels of phosphorylated adducin have been reported in ALS patients and in a mouse model of the disease. Adducin is regulated by phosphorylation through protein kinase C (PKC), some isoforms of which exhibit Ca²⁺-dependence, raising the possibility that changes in intracellular Ca²⁺ might alter PKC activation and secondarily influence adducin phosphorylation. Furthermore, adducin has interactions with the alpha subunit of the Na⁺/K⁺-ATPase. Thus, the phosphorylation of adducin may secondarily influence synaptic stability at the NMJ and so influence pre- and post-synaptic integrity at the NMJ in ALS.

Emerging molecular biomarker targets for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a rapidly progressive neurodegenerative disease that affects upper (UMN) and lower motor (LMN) neurons. It is associated with a short survival and there is no effective treatment, in spite of a large number of clinical trials. Strong efforts have been made to identify novel disease biomarkers to support diagnosis, provide information on prognosis, to measure disease progression in trials and increase our knowledge on disease pathogenesis. Electromyography by testing the function of the LMN can be used as a biomarker of its dysfunction. A number of electrophysiological and neuroimaging methods have been explored to identify a reliable marker of UMN degeneration. Recently, strong evidence from independent groups, large cohorts of patients and multicenter studies indicate that neurofilaments are very promising diagnostic biomarkers, in particular cerebrospinal fluid and blood levels of phosphoneurofilament heavy chain and neurofilament light chain. Furthermore, their increased levels are associated with poor prognosis. Additional studies have been performed aiming to identify other biomarkers, which alone or in combination with neurofilaments could increase the sensitivity and the specificity of the assays. Emerging molecular marker targets are being discovered, but more studies with standardized methods are required in larger cohorts of ALS patients.

Intracellular protein O-GlcNAc modification integrates nutrient status with transcriptional and metabolic regulation

The inducible, nutrient-sensitive posttranslational modification of protein Ser/Thr residues with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs on histones, transcriptional regulators, metabolic enzymes, oncogenes, tumor suppressors, and many critical intermediates of growth factor signaling. Cycling of O-GlcNAc modification on and off of protein substrates is catalyzed by the actions of O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. To date, there are less than 150 publications addressing the role of O-GlcNAc modification in cancer and over half were published in the last 2 years. These studies have clearly established that increased expression of OGT and hyper-O-GlcNAcylation is common to human cancers of breast, prostate, colon, lung, and pancreas. Furthermore, attenuating OGT activity reduces tumor growth in vitro and metastasis in vivo. This chapter discusses the structure and function of the O-GlcNAc cycling enzymes, mechanisms by which protein O-GlcNAc modification sense changes in nutrient status, the influence of O-GlcNAc cycling enzymes on glucose metabolism, and provides an overview of recent observations regarding the role of O-GlcNAcylation in cancer.

O-GlcNAc and neurodegeneration: biochemical mechanisms and potential roles in Alzheimer's disease and beyond

Alzheimer disease (AD) is a growing problem for aging populations worldwide. Despite significant efforts, no therapeutics are available that stop or slow progression of AD, which has driven interest in the basic causes of AD and the search for new therapeutic strategies. Longitudinal studies have clarified that defects in glucose metabolism occur in patients exhibiting Mild Cognitive Impairment (MCI) and glucose hypometabolism is an early pathological change within AD brain. Further, type 2 diabetes mellitus (T2DM) is a strong risk factor for the development of AD. These findings have stimulated interest in the possibility that disrupted glucose regulated signaling within the brain could contribute to the progression of AD. One such process of interest is the addition of O-linked N-acetylglucosamine (O-GlcNAc) residues onto nuclear and cytoplasmic proteins within mammals. O-GlcNAc is notably abundant within brain and is present on hundreds of proteins including several, such as tau and the amyloid precursor protein, which are involved in the pathophysiology AD. The cellular levels of O-GlcNAc are coupled to nutrient availability through the action of just two enzymes. O-GlcNAc transferase (OGT) is the glycosyltransferase that acts to install O-GlcNAc onto proteins and O-GlcNAcase (OGA) is the glycoside hydrolase that acts to remove O-GlcNAc from proteins. Uridine 5'-diphosphate-N-acetylglucosamine (UDP-GlcNAc) is the donor sugar substrate for OGT and its levels vary with cellular glucose availability because it is generated from glucose through the hexosamine biosynthetic pathway (HBSP). Within the brains of AD patients O-GlcNAc levels have been found to be decreased and aggregates of tau appear to lack O-GlcNAc entirely. Accordingly, glucose hypometabolism within the brain may result in disruption of the normal functions of O-GlcNAc within the brain and thereby contribute to downstream neurodegeneration. While this hypothesis remains largely speculative, recent studies using different mouse models of AD have demonstrated the protective benefit of pharmacologically increased brain O-GlcNAc levels. In this review we summarize the state of knowledge in the area of O-GlcNAc as it pertains to AD while also addressing some of the basic biochemical roles of O-GlcNAc and how these might contribute to protecting against AD and other neurodegenerative diseases.

Sumoylation of critical proteins in amyotrophic lateral sclerosis: emerging pathways of pathogenesis

Emerging lines of evidence suggest a relationship between amyotrophic lateral sclerosis (ALS) and protein sumoylation. Multiple studies have demonstrated that several of the proteins involved in the pathogenesis of ALS, including superoxide dismutase 1, fused in liposarcoma, and TAR DNA-binding protein 43 (TDP-43), are substrates for sumoylation. Additionally, recent studies in cellular and animal models of ALS revealed that sumoylation of these proteins impact their localization, longevity, and how they functionally perform in disease, providing novel areas for mechanistic investigations and therapeutics. In this article, we summarize the current literature examining the impact of sumoylation of critical proteins involved in ALS and discuss the potential impact for the pathogenesis of the disease. In addition, we report and discuss the implications of new evidence demonstrating that sumoylation of a fragment derived from the proteolytic cleavage of the astroglial glutamate transporter, EAAT2, plays a direct role in downregulating the expression levels of full-length EAAT2 by binding to a regulatory region of its promoter.

Regulation of protein degradation by O-GlcNAcylation: crosstalk with ubiquitination

The post-translational modification of intracellular proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates essential cellular processes such as signal transduction, transcription, translation, and protein degradation. Misfolded, damaged, and unwanted proteins are tagged with a chain of ubiquitin moieties for degradation by the proteasome, which is critical for cellular homeostasis. In this review, we summarize the current knowledge of the interplay between O-GlcNAcylation and ubiquitination in the control of protein degradation. Understanding the mechanisms of action of O-GlcNAcylation in the ubiquitin-proteosome system shall facilitate the development of therapeutics for human diseases such as cancer, metabolic syndrome, and neurodegenerative diseases.

X-linked dystonia parkinsonism syndrome (XDP, lubag): disease-specific sequence change DSC3 in TAF1/DYT3 affects genes in vesicular transport and dopamine metabolism

X-chromosomal dystonia parkinsonism syndrome (XDP, 'lubag') is associated with sequence changes within the TAF1/DYT3 multiple transcript system. Although most sequence changes are intronic, one, disease-specific single-nucleotide change 3 (DSC3), is located within an exon (d4). Transcribed exon d4 occurs as part of multiple splice variants. These variants include exons d3 and d4 spliced to exons of TAF1, and an independent transcript composed of exons d2-d4. Location of DSC3 in exon d4 and utilization of this exon in multiple splice variants suggest an important role of DSC3 in the XDP pathogenesis. To test this hypothesis, we transfected neuroblastoma cells with four expression constructs, including exons d2-d4 [d2-d4/wild-type (wt) and d2-d4/DSC3] and d3-d4 (d3-d4/wt and d3-d4/DSC3). Expression profiling revealed a dramatic effect of DSC3 on overall gene expression. Three hundred and sixty-two genes differed between cells containing d2-d4/wt and d2-d4/DSC3. Annotation clustering revealed enrichment of genes related to vesicular transport, dopamine metabolism, synapse function, Ca(2+) metabolism and oxidative stress. Two hundred and eleven genes were differentially expressed in d3-d4/wt versus d3-d4/DSC3. Annotation clustering highlighted genes in signal transduction and cell-cell interaction. The data show an important role of physiologically occurring transcript d2-d4 in normal brain function. Interference with this role by DSC3 is a likely pathological mechanism in XDP. Disturbance of dopamine function and of Ca(2+) metabolism can explain abnormal movement; loss of protection against reactive oxygen species may account for the neurodegenerative changes in XDP. Although d3-d4 also affect genes potentially related to neurodegenerative processes, their physiologic role as splice variants of TAF1 awaits further exploration.