Sialometabolism in Brain Health and Alzheimer's Disease

Serum N-glycoproteomics characterization of differential N-glycosylation in schizophrenia

Glycosylation plays a crucial role in neurotransmission and signaling in schizophrenia; however, comprehensive characterization at the glycoproteome level is still lacking. Here we report our site- and structure-specific quantitative N-glycoproteomics characterization of differential N-glycosylation in the sera of schizophrenia patients at the molecular level of intact N-glycopeptide, where comprehensive qualitative (N-glycosite, monosaccharide composition and sequence structures of N-glycans) and quantitative (fold change) information are obtained. With tandem mass tag labeling, liquid chromatography tandem mass spectrometry analysis and site- and structure-specific DB search using GPSeeker, 7855 intact N-glycopeptides were identified corresponding to 1914 peptide backbones, 1997 N-glycosites and 1671 N-glycoprotein; where 1088 intact N-glycopeptides were differentially expressed in the sera of schizophrenia patients (relative to healthy control) with fold change of no less than 1.5. Function annotation of the corresponding N-glycoproteins was carried out. Neurodegeneration and complement pathway were enriched. These findings provide a comprehensive site- and structure-specific picture of aberrant N-glycosylation in schizophrenia and may foster further function and mechanism studies. SIGNIFICANCE: Schizophrenia, as a complex mental disorder, is affecting an increasing number of individuals globally, yet clinical research has struggled to clearly elucidate its pathogenesis. Current diagnostic and treatment approaches largely depend on patient symptoms and behavior, which lack precision. N-glycoproteomics offers a new dimension of understanding by exploring how schizophrenia alters protein glycosylation patterns in the body. Investigating N-glycoproteins not only contributes to the identification of novel early diagnostic biomarkers but also enhances our knowledge of disease pathogenesis. These molecular insights could pave the way for more accurate diagnostic tools and targeted therapies.

Comprehensive analysis of SLC17A5 variants in large European cohorts reveals no association with Parkinson's disease risk

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss and α-synuclein aggregation. Aging is the primary risk factor, with both rare and common genetic variants playing a role. Previous studies have implicated lysosomal storage disorder (LSD)-related genes, including SLC17A5, in PD susceptibility.

OBJECTIVE

This study aimed to investigate the association of SLC17A5 variants, including rare and common variants and the FSASD-associated p.Arg39Cys missense variant, with PD risk in large European ancestry cohorts.

METHODS

Rare variant burden analyses were performed at minor allele frequency (MAF) thresholds of ≤1 % and ≤0.1 % in 7,184 PD cases and 51,650 controls using whole-genome and whole-exome sequencing data. Association testing of the p.Arg39Cys variant was conducted across five cohorts, encompassing both Finnish and non-Finnish Europeans. Common variant associations were examined using summary statistics from the largest European GWAS of PD.

RESULTS

No significant association was observed between rare SLC17A5 variants and PD at either MAF threshold. The p.Arg39Cys variant, though enriched in Finnish Europeans, showed no significant association with PD across several cohorts. Similarly, common SLC17A5 variants (MAF ≥1%) were not associated with PD risk.

CONCLUSION

Our findings do not support a role for SLC17A5 variants in PD susceptibility. While lysosomal dysfunction is central to PD pathogenesis, its contribution appears pathway-specific, with SLC17A5 unlikely to influence risk. Larger, multiethnic studies and functional analyses are needed to further investigate sialic acid metabolism in PD and related disorders.

Breastfeeding: The Multifaceted Impact on Child Development and Maternal Well-Being

Breastfeeding is recognized as the gold standard in infant nutrition, providing necessary nutrients for optimal growth and development. Beyond its nutritional function, breastfeeding has numerous benefits for both mother and child. This literature review examines the effects of breastfeeding on the development of the nervous and immune systems, its influence on cognitive development, and the impact of stress on lactation. In addition, it explores the emotional effects of breastfeeding on mothers, the challenges associated with exclusive breastfeeding, and the process of weaning along with its implications for both mother and infant. It is indicated that stress significantly affects lactation regulation, with elevated cortisol levels potentially disrupting hormonal balance. Furthermore, the essential roles of oxytocin, sialic acid, and docosahexaenoic acid in infant brain development and cognitive functions are highlighted. Breastfeeding is associated with the regulation of the baby's sleep through the effects of tryptophan, serotonin, and melatonin, which at the same time provide the baby with a sense of security with the mother. It is indicated that women who breastfeed are less likely to suffer from mental health problems and are at a lower risk of hypertension, diabetes, and cardiovascular disease. The weaning process is often a difficult time for mother and child; thus, it should be introduced gradually to minimize stress, anxiety, and potential mood disturbances in the mother.

Elevated neuroinflammation, autoimmunity, and altered IgG glycosylation profile in the cerebral spinal fluid of severe COVID-19 patients

BACKGROUND AND OBJECTIVES

A spectrum of neurologic complications associated with COVID-19 are well documented. While neuroinflammation in the brain of COVID-19 patients likely contributes to these complications, the mechanisms of neuroinflammation and correlates of neurologic complications remain elusive, especially since the etiologic pathogen of COVID-19, SARS-CoV-2, minimally invades the CNS. This study aimed to evaluate markers of neuroinflammation, IgG glycosylation patterns indicative of pro- or anti-inflammatory state, and prevalence of brain auto-reactive antibodies in the CSF of COVID-19 patients and their relationship to brain neuropathology.

METHODS

We evaluated the CSF of 11 deceased unvaccinated COVID-19 donors and 13 matched non-COVID-19 controls. Markers of neuroinflammation, IgG glycosylation patterns, and brain auto-reactive antibodies were assessed, along with their correlation to brain neuropathology. Statistical analyses were performed to compare groups and assess relationships between variables, using non-parametric tests and bootstrap analysis.

RESULTS

COVID-19 CSF showed higher levels of neopterin and ANNA-1, markers of neuroinflammation and autoimmunity, respectively, and lower IFN response compared to non-COVID-19 donors. In brain regions of high microglial activation, IL4 and RANTES were significantly increased. SARS-CoV-2 was undetectable in the CSF and brain of COVID-19 donors, yet anti-SARS-CoV-2 CSF antibodies were detected. Fucosylated IgG were associated with Spike IgG, CSF protein, and soluble CD14, whereas afucosylated bisecting IgG were inversely correlated with Spike IgG. Sialic acid containing IgG were positively correlated with IL1β and TNFα. These associations were not found in non-COVID-19 donors. Inflammatory agalactosylated fucosylated IgG (G0F) were associated with infiltrating CD4 + T cells in the brains of COVID-19 donors. COVID-19 donor CSF displayed higher levels of auto-reactive antibodies to human brain antigens compared to non-COVID-19 donors and donors with positive autoantibodies showed higher levels of neopterin.

DISCUSSION

These data describe increased neuroinflammation and autoreactive antibody markers in the CSF of COVID-19 donors and suggest that IgG glycosylation and autoimmunity may contribute to COVID-19 pathology, highlighting potential mechanisms underlying the neurologic complications associated with COVID-19.

Surface-enhanced Raman spectroscopy as effective tool for detection of sialic acid as cancer biomarker

Sialic acid, a negatively charged nine-carbon monosaccharide, is mainly located at the terminal end of glycan chains on glycoproteins and glycolipids of cell surface and most secreted proteins. Elevated levels of sialylated glycans have been known as a hallmark in numerous cancers. As a result, sialic acid acts as a useful and accessible cancer biomarker for early cancer detection and monitoring the disease development during cancer treatment which is crucial in elevating the survival rate. The detection of sialic acid has been done by many tools including surface-enhanced Raman spectroscopy (SERS) which gained incredible attention due to its high selectivity and sensitivity. However, currently, comprehensive reviews of sialic acid detection and imaging as a cancer biomarker using SERS are still lacking. Here, we present the significant breakthroughs in SERS-based detection of sialic acid levels on cells, tissues, and body fluids due to the presence of cancer, different cancer metastasis stages, and in response to the external stimuli. This review covers the SERS substrate and novel SERS strategies, using lectin, boronic acid, metabolic glycan labelling and label-free methods, for sialic acid detection as cancer biomarker. The remaining challenges to detect sialic acid and prospect of future development of SERS for other carbohydrate-based cancer biomarker, for instance fucose, are also discussed.

The role of inhibitory immune checkpoint receptors in the pathogenesis of Alzheimer's disease

There is mounting evidence that microglial cells have a key role in the pathogenesis of Alzheimer's disease (AD). In AD pathology, microglial cells not only are unable to remove β-amyloid (Aβ) plaques and invading pathogens but also are involved in synaptic pruning, chronic neuroinflammation, and neuronal degeneration. Microglial cells possess many different inhibitory immune checkpoint receptors, such as PD-1, LILRB2-4, Siglecs, and SIRPα receptors, which can be targeted by diverse cell membrane-bound and soluble ligand proteins to suppress the functions of microglia. Interestingly, in the brains of AD patients there are elevated levels of many of the inhibitory ligands acting via these inhibitory checkpoint receptors. For instance, Aβ oligomers, ApoE4, and fibronectin are able to stimulate the LILRB2-4 receptors. Increased deposition of sialoglycans, e.g., gangliosides, inhibits microglial function via Siglec receptors. AD pathology augments the accumulation of senescent cells, which are known to possess a high level of PD-L1 proteins, and thus, they can evade immune surveillance. A decrease in the expression of SIRPα receptor in microglia and its ligand CD47 in neurons enhances the phagocytic pruning of synapses in AD brains. Moreover, cerebral neurons contain inhibitory checkpoint receptors which can inhibit axonal growth, reduce synaptic plasticity, and impair learning and memory. It seems that inappropriate inhibitory immune checkpoint signaling impairs the functions of microglia and neurons thus promoting AD pathogenesis. KEY MESSAGES: Microglial cells have a major role in the pathogenesis of AD. A decline in immune activity of microglia promotes AD pathology. Microglial cells and neurons contain diverse inhibitory immune checkpoint receptors. The level of ligands for inhibitory checkpoint receptors is increased in AD pathology. Impaired signaling of inhibitory immune checkpoint receptors promotes AD pathology.

Early PSA-NCAM reduction in the dentate gyrus and impaired plasticity in the Alzheimer´s disease 3xTg-mice model

Neurodegenerative diseases such as Alzheimer´s (AD) and physiological ageing are characterized by a decline in neurogenesis and in the polysialylated isoforms of neural cell adhesion molecule (PSA-NCAM) expression within the hippocampus and specifically in the dentate gyrus (DG). In the 3xTG-AD mouse model, which mimics the human disease in both pathological and behavioral features, this decline in PSA-NCAM is associated with the presence of Aβ plaques at 9 months and Tau tangles at 12-15 months. In this work we studied the presence of PSA-NCAM at early ages (1-6 months) in the same model. Our results demonstrated that even as early as the first month of age there is a strong decrease in PSA-NCAM dendritic tree mainly altering the molecular layer (MolL) coverage affecting the synaptic plasticity and furthermore confirmed by the reduction of PSA-NCAM area density (Sv) in the 3xTG-AD. Similar and more marked early changes were seen during aging in both NTG and 3xTg-AD animals. Our results demonstrate for the first time a precipitate decrease of PSA-NCAM cells at such very early phases of the disease. This result suggests an early effect of the disease in the progression of immature and pluripotent cells resulting in an ulterior and early diminution of neurogenesis and therefore an impaired hippocampal cellular and synaptic plasticity.

Dual-emission rare-earth fluorescent nanomaterials for ratiometric and visual detection of N-acetylneuraminic acid and applications in information encryption and anti-counterfeiting

N-acetylneuraminic acid (NANA) can be used as a biomarker for many types of cancers. Currently, there are various methods for detecting NANA but showing some shortcomings that limit the real-time diagnosis of cancer. In contrast, fluorescence analysis has obvious advantages such as low cost, fast response time, and easy operation, and it also enables visual detection for real-time cancer monitoring. Therefore, the establishment of an efficient and rapid detection method is essential for the early prevention and treatment of the disease. Based on the properties of layered rare-earth hydroxide (LRH), we synthesized a dual-emission fluorescent material (NDC/SDS-LEuH), and further fabricated a fluorescent nanoprobe (ANP) for the detection of NANA. The probe has the advantages of high sensitivity (LOD = 32.9 μM) and high selectivity with fast response. During the sensing process, the dual emission of the probe shows opposite changes due to the photoinduced electron transfer (PET) effect and the interaction between NANA and the probe. The color changes of the system can be observed under UV irradiation. Therefore, a visual platform was developed to detect NANA with a LOD of 0.09 mM. In addition, a probe hydrogel was prepared, which can be applied in the anti-counterfeiting to improve the difficulty of counterfeiting and the security of anti-counterfeiting. The probe achieves ratiometric fluorescence detection of NANA, which reduces background interference and improves the accuracy of detection. A visual detection platform was fabricated to realize the real-time detection. In addition, the prepared probe hydrogel showed the potential applications in anti-counterfeiting, which provided new ideas for the design and application of anti-counterfeiting materials.

Mechanistic and Therapeutic Implications of Protein and Lipid Sialylation in Human Diseases

Glycan structures of glycoproteins and glycolipids on the surface glycocalyx and luminal sugar layers of intracellular membrane compartments in human cells constitute a key interface between intracellular biological processes and external environments. Sialic acids, a class of alpha-keto acid sugars with a nine-carbon backbone, are frequently found as the terminal residues of these glycoconjugates, forming the critical components of these sugar layers. Changes in the status and content of cellular sialic acids are closely linked to many human diseases such as cancer, cardiovascular, neurological, inflammatory, infectious, and lysosomal storage diseases. The molecular machineries responsible for the biosynthesis of the sialylated glycans, along with their biological interacting partners, are important therapeutic strategies and targets for drug development. The purpose of this article is to comprehensively review the recent literature and provide new scientific insights into the mechanisms and therapeutic implications of sialylation in glycoproteins and glycolipids across various human diseases. Recent advances in the clinical developments of sialic acid-related therapies are also summarized and discussed.

Emerging roles for ITAM and ITIM receptor signaling in microglial biology and Alzheimer's disease-related amyloidosis

Microglia are critical responders to amyloid beta (Aβ) plaques in Alzheimer's disease (AD). Therefore, the therapeutic targeting of microglia in AD is of high clinical interest. While previous investigation has focused on the innate immune receptors governing microglial functions in response to Aβ plaques, how microglial innate immune responses are regulated is not well understood. Interestingly, many of these microglial innate immune receptors contain unique cytoplasmic motifs, termed immunoreceptor tyrosine-based activating and inhibitory motifs (ITAM/ITIM), that are commonly known to regulate immune activation and inhibition in the periphery. In this review, we summarize the diverse functions employed by microglia in response to Aβ plaques and also discuss the innate immune receptors and intracellular signaling players that guide these functions. Specifically, we focus on the role of ITAM and ITIM signaling cascades in regulating microglia innate immune responses. A better understanding of how microglial innate immune responses are regulated in AD may provide novel therapeutic avenues to tune the microglial innate immune response in AD pathology.

Human apolipoprotein E glycosylation and sialylation: from structure to function

Human apolipoprotein E (ApoE) was first identified as a polymorphic gene in the 1970s; however, the genetic association of ApoE genotypes with late-onset sporadic Alzheimer's disease (sAD) was only discovered 20 years later. Since then, intensive research has been undertaken to understand the molecular effects of ApoE in the development of sAD. Despite three decades' worth of effort and over 10,000 papers published, the greatest mystery in the ApoE field remains: human ApoE isoforms differ by only one or two amino acid residues; what is responsible for their significantly distinct roles in the etiology of sAD, with ApoE4 conferring the greatest genetic risk for sAD whereas ApoE2 providing exceptional neuroprotection against sAD. Emerging research starts to point to a novel and compelling hypothesis that the sialoglycans posttranslationally appended to human ApoE may serve as a critical structural modifier that alters the biology of ApoE, leading to the opposing impacts of ApoE isoforms on sAD and likely in the peripheral systems as well. ApoE has been shown to be posttranslationally glycosylated in a species-, tissue-, and cell-specific manner. Human ApoE, particularly in brain tissue and cerebrospinal fluid (CSF), is highly glycosylated, and the glycan chains are exclusively attached via an O-linkage to serine or threonine residues. Moreover, studies have indicated that human ApoE glycans undergo sialic acid modification or sialylation, a structural alteration found to be more prominent in ApoE derived from the brain and CSF than plasma. However, whether the sialylation modification of human ApoE has a biological role is largely unexplored. Our group recently first reported that the three major isoforms of human ApoE in the brain undergo varying degrees of sialylation, with ApoE2 exhibiting the most abundant sialic acid modification, whereas ApoE4 is the least sialylated. Our findings further indicate that the sialic acid moiety on human ApoE glycans may serve as a critical modulator of the interaction of ApoE with amyloid β (Aβ) and downstream Aβ pathogenesis, a prominent pathologic feature in AD. In this review, we seek to provide a comprehensive summary of this exciting and rapidly evolving area of ApoE research, including the current state of knowledge and opportunities for future exploration.

Altered GM1 catabolism affects NMDAR-mediated Ca2+ signaling at ER-PM junctions and increases synaptic spine formation in a GM1-gangliosidosis model

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate Ca2+ flux across neuronal membranes. The properties of these membrane contact sites are defined by their lipid content, but little attention has been given to glycosphingolipids (GSLs). Here, we show that GM1-ganglioside, an abundant GSL in neuronal membranes, is integral to ER-PM junctions; it interacts with synaptic proteins/receptors and regulates Ca2+ signaling. In a model of the neurodegenerative lysosomal storage disease, GM1-gangliosidosis, pathogenic accumulation of GM1 at ER-PM junctions due to β-galactosidase deficiency drastically alters neuronal Ca2+ homeostasis. Mechanistically, we show that GM1 interacts with the phosphorylated N-methyl D-aspartate receptor (NMDAR) Ca2+ channel, thereby increasing Ca2+ flux, activating extracellular signal-regulated kinase (ERK) signaling, and increasing the number of synaptic spines without increasing synaptic connectivity. Thus, GM1 clustering at ER-PM junctions alters synaptic plasticity and worsens the generalized neuronal cell death characteristic of GM1-gangliosidosis.

Sialylated glycoproteins and sialyltransferases in digestive cancers: Mechanisms, diagnostic biomarkers, and therapeutic targets

Sialic acid (SA), as the ultimate epitope of polysaccharides, can act as a cap at the end of polysaccharide chains to prevent their overextension. Sialylation is the enzymatic process of transferring SA residues onto polysaccharides and is catalyzed by a group of enzymes known as sialyltransferases (SiaTs). It is noteworthy that the sialylation level of glycoproteins is significantly altered when digestive cancer occurs. And this alteration exhibits a close correlation with the progression of these cancers. In this review, from the perspective of altered SiaTs expression levels and changed glycoprotein sialylation patterns, we summarize the pathogenesis of gastric cancer (GC), colorectal cancer (CRC), pancreatic ductal adenocarcinoma (PDAC), and hepatocellular carcinoma (HCC). Furthermore, we propose potential early diagnostic biomarkers and prognostic indicators for different digestive cancers. Finally, we summarize the therapeutic value of sialylation in digestive system cancers.

Development and evaluation of multi-functionalized sialic acid conjugated asiatic acid nanoconstruct to mitigate cognitive deficits in Alzheimer's disease

Sialic acid (SA) serves a critical role in neuronal repair and cognitive functions. SA is a nine-carbon carboxylated sugar with a glycoconjugate cap that acts as a ligand and surface decoration with SA facilitates delivery to the target site. The present research aimed to develop SA surface modified AA nanostructured lipid carrier (NLCs) with carbodiimide conjugation method. Sterylamine, poloxamer 188 and tween 80 were used as surfactants and several characterization studies including, differential scanning calorimetry, fourier transform infrared spectroscopy and x-ray photon spectroscopy were analyzed. Further, in vitro, neuroprotective efficiency was evaluated in SH-SY5Y cells and hCMEC/D3 cells and found significant potential effects with the treatments of developed NLCs. Pharmacodynamics studies were also assessed in beta-amyloid-injected rats following quantification of Alzheimer's disease (AD) hallmarks like, Aβ(1-42), tau-protein, glycogen synthase kinase-3β levels, interleukin-6 and tumor necrosis factor-α for neuroinflammatory responses. Characterization studies revealed the conjugation on developed NLCs. The in vitro and in vivo results showed significant effects of SA decorated NLCs in reversing the damage by toxicant which was further characterized by the levels of neurotransmitters like acetylcholinesterase, butyrylcholinesterase. The results revealed significant (p < .05) refurbishment of cholinergic functions after 28 days of treatment of developed NLCs. These preclinical findings support the use of SA as a ligand to deliver the AA at targeted site as well as to mitigate the cognitive deficits in AD.

CSF N-Glycomics Using High-Throughput UPLC-ESI Techniques in Alzheimer's Disease

In this chapter, we will present a high-throughput method applied in our laboratory for the structural elucidation of the cerebrospinal fluid (CSF) N-glycome. This methodology is based on a commercial equipment developed by WATERS™ to speed up N-deglycosylation and N-glycan labeling of glycoproteins of pharmaceutical and biological interest such as monoclonal antibodies. This analytical kit is sold under the trade name of RapiFluor-MS (RFMS). We have slightly modified the methodology, increasing the glycosylation time and using a high-resolution mass analyzer for the analysis of CSF N-glycans, thus obtaining a high-throughput method (up to 96 samples simultaneously), mass accuracy better than 5 ppm, and the ability to separate and identify isomers.

Assessing the Biological Mechanisms Linking Smoking Behavior and Cognitive Function: A Mediation Analysis of Untargeted Metabolomics

(1) Smoking is the most significant preventable health hazard in the modern world. It increases the risk of vascular problems, which are also risk factors for dementia. In addition, toxins in cigarettes increase oxidative stress and inflammation, which have both been linked to the development of Alzheimer's disease and related dementias (ADRD). This study identified potential mechanisms of the smoking-cognitive function relationship using metabolomics data from the longitudinal Wisconsin Registry for Alzheimer's Prevention (WRAP). (2) 1266 WRAP participants were included to assess the association between smoking status and four cognitive composite scores. Next, untargeted metabolomic data were used to assess the relationships between smoking and metabolites. Metabolites significantly associated with smoking were then tested for association with cognitive composite scores. Total effect models and mediation models were used to explore the role of metabolites in smoking-cognitive function pathways. (3) Plasma N-acetylneuraminate was associated with smoking status Preclinical Alzheimer Cognitive Composite 3 (PACC3) and Immediate Learning (IMM). N-acetylneuraminate mediated 12% of the smoking-PACC3 relationship and 13% of the smoking-IMM relationship. (4) These findings provide links between previous studies that can enhance our understanding of potential biological pathways between smoking and cognitive function.

Construction of Porous Perovskite Oxide Microrods with Au Nanoparticle Anchor for Precise Metabolic Diagnosis of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive illness, and early diagnosis and treatment can help delay its progression. However, clinics still lack high-throughput, low-invasive, precise, and objective diagnostic strategies. Herein, the Au nanoparticles anchored porous perovskite oxide microrods (CTO@Au) with designed superior properties is developed to construct a high-throughput detection platform. Specifically, a single metabolic fingerprinting is obtained from only 30 nL of serum within seconds, enabling the rapid acquisition of 239 × 8 high-quality fingerprints in ≈ 2 h. AD is distinguish from health controls and Parkinson's disease with an area under the curve (AUC) of 1.000. Moreover, eight specific metabolites are identified as a biomarker panel, based on which precise diagnosis of AD is achieved, with an AUC of 1.000 in blind test. The possible relevant pathways and potential mechanism involved in these biomarkers are investigated and discussed. This work provides a high-performance platform for metabolic diagnostic analysis.

Amyloid-β: A potential mediator of aging-related vascular pathologies

Aging is one of the most promising risk factors for vascular diseases, however, the precise mechanisms mediating aging-related pathologies are not fully understood. Amyloid beta (Aβ), a peptide produced by the proteolytic processing of amyloid precursor protein (APP), is known as a key mediator of brain damage involved in the pathogenesis of Alzheimer's disease (AD). Recently, it was found that the accumulation of Aβ in the vascular wall is linked to a range of aging-related vascular pathologies, indicating a potential role of Aβ in the pathogenesis of aging-associated vascular diseases. In the present review, we have updated the molecular regulation of Aβ in vascular cells and tissues, summarized the relevance of the Aβ deposition with vascular aging and diseases, and the role of Aβ dysregulation in aging-associated vascular pathologies, including the impaired vascular response, endothelial dysfunction, oxidative stress, and inflammation. This review will provide advanced information in understanding aging-related vascular pathologies and a new avenue to explore therapeutic targets.

Mass spectrometry methods for analysis of extracellular matrix components in neurological diseases

The brain extracellular matrix (ECM) is a highly glycosylated environment and plays important roles in many processes including cell communication, growth factor binding, and scaffolding. The formation of structures such as perineuronal nets (PNNs) is critical in neuroprotection and neural plasticity, and the formation of molecular networks is dependent in part on glycans. The ECM is also implicated in the neuropathophysiology of disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and Schizophrenia (SZ). As such, it is of interest to understand both the proteomic and glycomic makeup of healthy and diseased brain ECM. Further, there is a growing need for site-specific glycoproteomic information. Over the past decade, sample preparation, mass spectrometry, and bioinformatic methods have been developed and refined to provide comprehensive information about the glycoproteome. Core ECM molecules including versican, hyaluronan and proteoglycan link proteins, and tenascin are dysregulated in AD, PD, and SZ. Glycomic changes such as differential sialylation, sulfation, and branching are also associated with neurodegeneration. A more thorough understanding of the ECM and its proteomic, glycomic, and glycoproteomic changes in brain diseases may provide pathways to new therapeutic options.

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Social defeat stress induces genome-wide 5mC and 5hmC alterations in the mouse brain

Stress is adverse experience that require constant adaptation to reduce the emotional and physiological burden, or "allostatic load", of an individual. Despite their everyday occurrence, a subpopulation of individuals is more susceptible to stressors, while others remain resilient with unknown molecular signatures. In this study, we investigated the contribution of the DNA modifications, 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC), underlying the individual differences in stress susceptibility and resilience. Genome-wide 5mC and 5hmC profiles from 3- and 6-month adult male mice that underwent various durations of social defeat were generated. In 3-month animals, 5mC and 5hmC work in parallel and do not distinguish between stress-susceptible and resilient phenotypes, while in 6-month animals, 5mC and 5hmC show distinct enrichment patterns. Acute stress responses may epigenetically "prime" the animals to either increase or decrease their predisposition to depression susceptibility. In support of this, re-exposure studies reveal that the enduring effects of social defeat affect differential biological processes between susceptible and resilient animals. Finally, the stress-induced 5mC and 5hmC fluctuations across the acute-chronic-longitudinal time course demonstrate that the negative outcomes of chronic stress do not discriminate between susceptible and resilient animals. However, resilience is more associated with neuroprotective processes while susceptibility is linked to neurodegenerative processes. Furthermore, 5mC appears to be responsible for acute stress response, whereas 5hmC may function as a persistent and stable modification in response to stress. Our study broadens the scope of previous research offering a comprehensive analysis of the role of DNA modifications in stress-induced depression.

Enhanced Detection of Charged N-Glycans in the Brain by Infrared Matrix-Assisted Laser Desorption Electrospray Ionization Mass Spectrometric Imaging

N-linked glycosylation represents a structurally diverse, complex, co- and posttranslational protein modification that bridges metabolism and cellular signaling. Consequently, aberrant protein glycosylation is a hallmark of most pathological scenarios. Due to their complex nature and non-template-driven synthesis, the analysis of glycans is faced with several challenges, underlining the need for new and improved analytical technologies. Spatial profiling of N-glycans through direct imaging on tissue sections reveals the regio-specific and/or disease pathology correlating tissue N-glycans that serve as a disease glycoprint for diagnosis. Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) is a soft hybrid ionization technique that has been used for diverse mass spectrometry imaging (MSI) applications. Here, we report the first spatial analysis of the brain N-linked glycans by IR-MALDESI MSI, leading to a significant increase in the detection of the brain N-sialoglycans. A formalin-fixed paraffin-embedded mouse brain tissue was analyzed in negative ionization mode after tissue washing, antigen retrieval, and pneumatic application of PNGase F for enzymatic digestion of N-linked glycans. We report a comparative analysis of section thickness on the N-glycan detection using IR-MALDESI. One hundred thirty-six unique N-linked glycans were confidently identified in the brain tissue (with an additional 132 unique N-glycans, not reported in GlyConnect), where more than 50% contained sialic acid residues, which is approximately 3-fold higher than the previous reports. This work demonstrates the first application of IR-MALDESI in N-linked glycan imaging of the brain tissue, leading to a 2.5-fold increase in the in situ total brain N-glycan detection compared to the current gold standard of positive-mode matrix-assisted laser desorption/ionization mass spectrometry imaging. This is also the first report of the application of the MSI toward the identification of sulfoglycans in the rodent brain. Overall, IR-MALDESI-MSI presents a sensitive glycan detection platform to identify tissue-specific and/or disease-specific glycosignature in the brain while preserving the sialoglycans without any chemical derivatization.

Increased α-2,6 sialic acid on microglia in amyloid pathology is resistant to oseltamivir

Terminal sialic acid residues are present on most glycoproteins and glycolipids, but levels of sialylation are known to change in the brain throughout the lifespan as well as during disease. Sialic acids are important for numerous cellular processes including cell adhesion, neurodevelopment, and immune regulation as well as pathogen invasion into host cells. Neuraminidase enzymes, also known as sialidases, are responsible for removal of terminal sialic acids in a process known as desialylation. Neuraminidase 1 (Neu1) cleaves the α-2,6 bond of terminal sialic acids. Aging individuals with dementia are often treated with the antiviral medication oseltamivir, which is associated with induction of adverse neuropsychiatric side effects; this drug inhibits both viral and mammalian Neu1. The present study tested whether a clinically relevant antiviral dosing regimen of oseltamivir would disrupt behavior in the 5XFAD mouse model of Alzheimer's disease amyloid pathology or wild-type littermates. While oseltamivir treatment did not impact mouse behavior or modify amyloid plaque size or morphology, a novel spatial distribution of α-2,6 sialic acid residues was discovered in 5XFAD mice that was not present in wild-type littermates. Further analyses revealed that α-2,6 sialic acid residues were not localized the amyloid plaques but instead localized to plaque-associated microglia. Notably, treatment with oseltamivir did not alter α-2,6 sialic acid distribution on plaque-associated microglia in 5XFAD mice which may be due to downregulation of Neu1 transcript levels in 5XFAD mice. Overall, this study suggests that plaque-associated microglia are highly sialylated and are resistant to change with oseltamivir, thus interfering with microglia immune recognition of and response to amyloid pathology.

Engineering Adenoviral Vectors with Improved GBM Selectivity

Glioblastoma (GBM) is the most common and aggressive adult brain cancer with an average survival rate of around 15 months in patients receiving standard treatment. Oncolytic adenovirus expressing therapeutic transgenes represent a promising alternative treatment for GBM. Of the many human adenoviral serotypes described to date, adenovirus 5 (HAdV-C5) has been the most utilised clinically and experimentally. However, the use of Ad5 as an anti-cancer agent may be hampered by naturally high seroprevalence rates to HAdV-C5 coupled with the infection of healthy cells via native receptors. To explore whether alternative natural adenoviral tropisms are better suited to GBM therapeutics, we pseudotyped an HAdV-C5-based platform using the fibre knob protein from alternative serotypes. We demonstrate that the adenoviral entry receptor coxsackie, adenovirus receptor (CAR) and CD46 are highly expressed by both GBM and healthy brain tissue, whereas Desmoglein 2 (DSG2) is expressed at a low level in GBM. We demonstrate that adenoviral pseudotypes, engaging CAR, CD46 and DSG2, effectively transduce GBM cells. However, the presence of these receptors on non-transformed cells presents the possibility of off-target effects and therapeutic transgene expression in healthy cells. To enhance the specificity of transgene expression to GBM, we assessed the potential for tumour-specific promoters hTERT and survivin to drive reporter gene expression selectively in GBM cell lines. We demonstrate tight GBM-specific transgene expression using these constructs, indicating that the combination of pseudotyping and tumour-specific promoter approaches may enable the development of efficacious therapies better suited to GBM.

CD33 isoforms in microglia and Alzheimer's disease: Friend and foe

Alzheimer's disease (AD) is the most common form of neurodegenerative disease and is considered the main cause of dementia worldwide. Genome-wide association studies combined with integrated analysis of functional datasets support a critical role for microglia in AD pathogenesis, identifying them as important potential therapeutic targets. The ability of immunomodulatory receptors on microglia to control the response to pathogenic amyloid-β aggregates has gained significant interest. Siglec-3, also known as CD33, is one of these immunomodulatory receptors expressed on microglia that has been identified as an AD susceptibility factor. Here, we review recent advances made in understanding the multifaceted roles that CD33 plays in microglia with emphasis on two human-specific CD33 isoforms that differentially correlate with AD susceptibility. We also describe several different therapeutic approaches for targeting CD33 that have been advanced for the purpose of skewing microglial cell responses.

N-acetylneuraminic acid links immune exhaustion and accelerated memory deficit in diet-induced obese Alzheimer's disease mouse model

Systemic immunity supports lifelong brain function. Obesity posits a chronic burden on systemic immunity. Independently, obesity was shown as a risk factor for Alzheimer's disease (AD). Here we show that high-fat obesogenic diet accelerated recognition-memory impairment in an AD mouse model (5xFAD). In obese 5xFAD mice, hippocampal cells displayed only minor diet-related transcriptional changes, whereas the splenic immune landscape exhibited aging-like CD4+ T-cell deregulation. Following plasma metabolite profiling, we identified free N-acetylneuraminic acid (NANA), the predominant sialic acid, as the metabolite linking recognition-memory impairment to increased splenic immune-suppressive cells in mice. Single-nucleus RNA-sequencing revealed mouse visceral adipose macrophages as a potential source of NANA. In vitro, NANA reduced CD4+ T-cell proliferation, tested in both mouse and human. In vivo, NANA administration to standard diet-fed mice recapitulated high-fat diet effects on CD4+ T cells and accelerated recognition-memory impairment in 5xFAD mice. We suggest that obesity accelerates disease manifestation in a mouse model of AD via systemic immune exhaustion.

Opposing Effects of ApoE2 and ApoE4 on Glycolytic Metabolism in Neuronal Aging Supports a Warburg Neuroprotective Cascade against Alzheimer's Disease

Apolipoprotein E4 (ApoE4) is the most recognized genetic risk factor for late-onset Alzheimer's disease (LOAD), whereas ApoE2 reduces the risk for LOAD. The underlying mechanisms are unclear but may include effects on brain energy metabolism. Here, we used neuro-2a (N2a) cells that stably express human ApoE isoforms (N2a-hApoE), differentiated N2a-hApoE neuronal cells, and humanized ApoE knock-in mouse models to investigate relationships among ApoE isoforms, glycolytic metabolism, and neuronal health and aging. ApoE2-expressing cells retained robust hexokinase (HK) expression and glycolytic activity, whereas these endpoints progressively declined with aging in ApoE4-expressing cells. These divergent ApoE2 and ApoE4 effects on glycolysis directly correlated with markers of cellular wellness. Moreover, ApoE4-expressing cells upregulated phosphofructokinase and pyruvate kinase with the apparent intent of compensating for the HK-dependent glycolysis reduction. The introduction of ApoE2 increased HK levels and glycolysis flux in ApoE4 cells. PI3K/Akt signaling was distinctively regulated by ApoE isoforms but was only partially responsible for the ApoE-mediated effects on HK. Collectively, our findings indicate that human ApoE isoforms differentially modulate neuronal glycolysis through HK regulation, with ApoE2 upregulating and ApoE4 downregulating, which markedly impacts neuronal health during aging. These findings lend compelling support to the emerging inverse-Warburg theory of AD and highlight a therapeutic opportunity for bolstering brain glycolytic resilience to prevent and treat AD.

Altered expression of glycobiology-related genes in Parkinson's disease brain

The precise mechanisms initiating and perpetuating the cellular degeneration in Parkinson's disease (PD) remain unclear. There is decreased expression of the main brain gangliosides, and GM1 ganglioside in particular, in the PD brain along with decreased expression of the genes coding for the glycosyltranferase and the sialyltransferase responsible for the synthesis of these brain gangliosides. However, potentially important pathogenic mechanisms contributing to the neurodegeneration in PD may also include altered levels of expression of genes involved in glycosylation, sialylation and sphingolipid synthesis and metabolism. Although various studies have described pathological lipid and glycolipid changes in PD brain, there have been limited studies of expression of glycobiology-related genes in PD brain. The current study was performed as an initial attempt to gain new information regarding potential changes in glycoprotein and glycolipid-related genes in PD by investigating the gene expression status for select glycosyltransferases, sialyltransferases, sialidases, sphingosine kinases, and lysosomal enzymes in the substantia nigra and putamen from patients with PD and neurologically normal controls. Results showed altered expression of glycosyltransferase genes (B3GALT2 and B4GALT1) potentially involved in microglial activation and neuroinflammation, sphingosine-1-phosphate (S1P) modulators (SPHK1, SPHK2, and SGPL1) involved in sphingolipid synthesis and metabolism, polysialyltransferase genes (ST8SIA2 and ST8SIA4) that encode enzymes responsible for polysialic acid (polySia) biosynthesis, and the sialidase NEU4, expression of which has been linked to the clearance of storage materials from lysosomes. The data presented here underscore the complexity of the glycolipid/sphingolipid dysregulation in the PD brain and continued and expanded study of these processes may not only provide a greater understanding of the complex roles of aberrant glycosylation sialylation, and sphingolipid synthesis/metabolism in the pathophysiology of PD but may identify potential druggable targets for PD therapeutics.

Aberrant Ganglioside Functions to Underpin Dysregulated Myelination, Insulin Signalling, and Cytokine Expression: Is There a Link and a Room for Therapy?

Gangliosides are molecules widely present in the plasma membranes of mammalian cells, participating in a variety of processes, including protein organization, transmembrane signalling and cell adhesion. Gangliosides are abundant in the grey matter of the brain, where they are critically involved in postnatal neural development and function. The common precursor of the majority of brain gangliosides, GM3, is formed by the sialylation of lactosylceramide, and four derivatives of its a- and b-series, GM1, GD1a, GD1b and GT1b, constitute 95% of all the brain gangliosides. Impairments in ganglioside metabolism due to genetic abnormalities of GM-synthases are associated with severe neurological disorders. Apart from that, the latest genome-wide association and translational studies suggest a role of genes involved in brain ganglioside synthesis in less pervasive psychiatric disorders. Remarkably, the most recent animal studies showed that abnormal ganglioside functions result in dysregulated neuroinflammation, aberrant myelination and altered insulin receptor signalling. At the same time, these molecular features are well established as accompanying developmental psychiatric disorders such as attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD). This led us to hypothesize a role of deficient ganglioside function in developmental neuropsychiatric disorders and warrants further gene association clinical studies addressing this question. Here, we critically review the literature to discuss this hypothesis and focus on the recent studies on ST3GAL5-deficient mice. In addition, we elaborate on the therapeutic potential of various anti-inflammatory remedies for treatment of developmental neuropsychiatric conditions related to aberrant ganglioside functions.

Lanthanide Complexes for Tumor Diagnosis and Therapy by Targeting Sialic Acid

Sialic acid (SA) is overexpressed on cell membranes of tumor cells, and increased serum SA concentration has been observed in tumor-bearing patients. Herein, a series of lanthanide-containing bimetallic complexes (TDA-M-Lns) for targeting SA were prepared via coordination among luminescent lanthanide ions (Ln³⁺ = Tb³⁺, Eu³⁺, Dy³⁺, or Sm³⁺), metal ion quenchers (M²⁺ = Cu²⁺ or Co²⁺), and the organic ligand 2,2'-thiodiacetic acid (TDA). SA can competitively coordinate with Ln³⁺, resulting in the "signal-on" of the Ln³⁺. Therefore, the TDA-M-Lns can be simply used for cost-saving detection of SA in the blood samples. Among the TDA-M-Lns, TDA-Co-Eu showed the highest sensitivity to detect SA in the blood of tumor-bearing mice. Furthermore, the TDA-Co-Eu was successfully used to target SA and deposit Eu³⁺ on the surfaces of tumor cells for the inhibition of tumor cell growth and migration. The therapeutic effect of TDA-Co-Eu on a Balb/c mouse liver tumor model was evaluated. It was proved that TDA-Co-Eu can be applied for SA detection as well as for inhibiting tumor growth.

Alzheimer's Disease Risk Variant rs3865444 in the CD33 Gene: A Possible Role in Susceptibility to Multiple Sclerosis

Polymorphisms in genes encoding receptors that modulate the activity of microglia and macrophages are attractive candidates for participation in genetic susceptibility to multiple sclerosis (MS). The aims of the study were to (1) investigate the association between Alzheimer’s disease-linked variant rs3865444:C>A in the CD33 gene and MS risk, (2) assess the effect of the strongest MS risk allele HLA-DRB1*15:01 on this association, and (3) analyze the correlation of rs3865444 with selected clinical phenotypes, i.e., age of onset and disease severity. CD33 rs3865444 was genotyped in a cohort of 579 patients and 1145 controls and its association with MS risk and clinical phenotypes was analyzed by logistic and linear regression analysis, respectively. Statistical evaluation revealed that rs3865444 reduces the risk of MS in the HLA-DRB1*15:01-positive subpopulation but not in the cohort negative for HLA-DRB1*15:01. A significant antagonistic epistasis between rs3865444 A and HLA-DRB1*15:01 alleles in the context of MS risk was detected by the interaction synergy factor analysis. Comparison of allele and genotype distribution between relapsing-remitting MS, secondary progressive MS, and control groups revealed that rs3865444 C to A substitution may also be associated with a decreased risk of transition of MS to its secondary progressive form, irrespective of the HLA-DRB1*15:01 carrier status. On the other hand, no correlation could be found between rs3865444 and the age of disease onset or MS severity score. Future studies are required to shed more light on the role of CD33 in MS pathogenesis.

SARS-CoV-2 Attacks in the Brain: Focus on the Sialome

The epidemiological observations suggest that respiratory and gastrointestinal symptoms caused by severe acute respiratory coronavirus 2 (SARS-CoV-2) are accompanied by short- and long-term neurological manifestations. There is increasing evidence that the neuroinvasive potential of SARS-CoV-2 is closely related to its capacity to interact with cell membrane sialome. Given the wide expression of sialylated compounds of cell membranes in the brain, the interplay between cell membrane sialoglycans and the virus is crucial for its attachment and cell entry, transport, neuronal damage and brain immunity. Here, we focus on the significance of the brain sialome in the progress of coronavirus disease 2019 (COVID-19) and SARS-CoV-2-induced neuropathology.

Human apolipoprotein E isoforms are differentially sialylated and the sialic acid moiety in ApoE2 attenuates ApoE2-Aβ interaction and Aβ fibrillation

The APOE genotype is the most prominent genetic risk factor for the development of late-onset Alzheimer''s disease (LOAD); however, the underlying mechanisms remain unclear. In the present study, we found that the sialylation profiles of ApoE protein in the human brain are significantly different among the three isoforms, with ApoE2 exhibiting the most abundant sialic acid modification whereas ApoE4 had the least. We further observed that the sialic acid moiety in ApoE2 significantly affected the interaction between ApoE2 and Aβ peptides. The removal of sialic acid in ApoE2 increased the ApoE2 binding affinity for the Aβ17-24 region of Aβ and promoted Aβ fibrillation. These findings provide a plausible explanation for the well-documented differential roles of ApoE isoforms in Aβ pathogenesis. Specifically, compared to the other two isotypes, the higher expression of sialic acid in ApoE2 may contribute to the less potent interaction between ApoE2 and Aβ and ultimately the slower rate of brain Aβ deposition, a mechanism thought to underlie ApoE2-mediated decreased risk for AD. Future studies are warranted to determine whether the differential sialylation in ApoE isoforms may also contribute to some of their other distinct properties, such as their divergent preferences in associations with lipids and lipoproteins, as well as their potential impact on neuroinflammation through modulation of microglial Siglec activity. Overall, our findings lead to the insight that the sialic acid structure is an important posttranslational modification (PTM) that alters ApoE protein functions with relevance for AD.

Polysialic Acid in the Immune System

Polysialic acid (polySia) is a highly regulated polymer of sialic acid (Sia) with such potent biophysical characteristics that when expressed drastically influences the interaction properties of cells. Although much of what is known of polySia in mammals has been elucidated from the study of its role in the central nervous system (CNS), polySia is also expressed in other tissues, including the immune system where it presents dynamic changes during differentiation, maturation, and activation of different types of immune cells of the innate and adaptive response, being involved in key regulatory mechanisms. At least six polySia protein carriers (CCR7, ESL-1, NCAM, NRP2, ST8Sia 2, and ST8Sia 4) are expressed in different types of immune cells, but there is still much to be explored in regard not only to the regulatory mechanisms that determine their expression and the structure of polySia chains but also to the identification of the cis- and trans- ligands of polySia that establish signaling networks. This review summarizes the current knowledge on polySia in the immune system, addressing its biosynthesis, its tools for identification and structural characterization, and its functional roles and therapeutic implications.

Correlation between Sialylation Status and Cell Susceptibility to Amyloid Toxicity

The interaction between the cell membrane and misfolded protein species plays a crucial role in the development of neurodegeneration. This study was designed to clarify the relationship between plasma membrane composition in terms of the differently linked sialic acid (Sia) content and cell susceptibility to toxic and misfolded Aβ-42 peptides. The sialylation status in different cell lines was investigated by lectin histochemistry and confocal immunofluorescence and then correlated with the different propensities to bind amyloid fibrils and with the relative cell susceptibility to amyloid damage. This study reveals that expressions of Sias α2,3 and α2,6 linked to galactose/N-acetyl-galactosamine, and PolySia are positively correlated with Aβ-42-induced cell toxicity. PolySia shows an early strong interaction with amyloid fibrils, favoring their binding to GM1 ganglioside containing α2,3 galactose-linked Sia and a loss of cell viability. Our findings demonstrate that cell lines with a prevailing plastic neuron-like phenotype and high monoSia and PolySia contents are highly susceptible to amyloid Aβ-42 toxicity. This toxicity may involve a change in neuron metabolism and promote a compensative/protective increase in PolySia, which, in turn, could favor amyloid binding to GM1, thus exacerbating cell dysmetabolism and further amyloid aggregation.

Plasma Membrane Calcium ATPase-Neuroplastin Complexes Are Selectively Stabilized in GM1-Containing Lipid Rafts

The recent identification of plasma membrane (Ca²⁺)-ATPase (PMCA)-Neuroplastin (Np) complexes has renewed attention on cell regulation of cytosolic calcium extrusion, which is of particular relevance in neurons. Here, we tested the hypothesis that PMCA-Neuroplastin complexes exist in specific ganglioside-containing rafts, which could affect calcium homeostasis. We analyzed the abundance of all four PMCA paralogs (PMCA1-4) and Neuroplastin isoforms (Np65 and Np55) in lipid rafts and bulk membrane fractions from GM2/GD2 synthase-deficient mouse brains. In these fractions, we found altered distribution of Np65/Np55 and selected PMCA isoforms, namely PMCA1 and 2. Cell surface staining and confocal microscopy identified GM1 as the main complex ganglioside co-localizing with Neuroplastin in cultured hippocampal neurons. Furthermore, blocking GM1 with a specific antibody resulted in delayed calcium restoration of electrically evoked calcium transients in the soma of hippocampal neurons. The content and composition of all ganglioside species were unchanged in Neuroplastin-deficient mouse brains. Therefore, we conclude that altered composition or disorganization of ganglioside-containing rafts results in changed regulation of calcium signals in neurons. We propose that GM1 could be a key sphingolipid for ensuring proper location of the PMCA-Neuroplastin complexes into rafts in order to participate in the regulation of neuronal calcium homeostasis.

Overview of sialylation status in human nervous and skeletal muscle tissues during aging

Sialic acids (Sias) are a large and heterogeneous family of electronegatively charged nine-carbon monosaccharides containing a carboxylic acid and are mostly found as terminal residues in glycans of glycoproteins and glycolipids such as gangliosides. They are linked to galactose or N-acetylgalactosamine via α2,3 or α2,6 linkage, or to other Sias via α2,8 or more rarely α2,9 linkage, resulting in mono, oligo and polymeric forms. Given their characteristics, Sias play a crucial role in a multitude of human tissue biological processes in physiological and pathological conditions, ranging from development and growth to adult life until aging. Here, we review the sialylation status in human adult life focusing on the nervous and skeletal muscle tissues, which both display significant structural and functional changes during aging, strongly impacting on the whole human body and, therefore, on the quality of life. In particular, this review highlights the fundamental roles played by different types of glycoconjugates Sias in several cellular biological processes in the nervous and skeletal muscle tissues during adult life, also discussing how changes in Sia content during aging may contribute to the physiological decline of physical and nervous functions and to the development of age-related degenerative pathologies. Based on our current knowledge, further in-depth investigations could help to develop novel prophylactic strategies and therapeutic approaches that, by maintaining and/or restoring the correct sialylation status in the nervous and skeletal muscle tissues, could contribute to aging slowing and the prevention of age-related pathologies.

An Oligomannuronic Acid-Sialic Acid Conjugate Capable of Inhibiting Aβ42 Aggregation and Alleviating the Inflammatory Response of BV-2 Microglia

Oligomannuronic acid (MOS) from seaweed has antioxidant and anti-inflammatory activities. In this study, MOS was activated at the terminal to obtain three different graft complexes modified with sialic acid moiety (MOS-Sia). The results show that MOS-Sia addition can reduce the β-structure formation of Aβ42, and the binding effect of MOS-Sia3 is more obvious. MOS-Sia conjugates also have a better complexing effect with Ca²⁺ while reducing the formation of Aβ42 oligomers in solutions. MOS-Sia3 (25–50 μg/mL) can effectively inhibit the activation state of BV-2 cells stimulated by Aβ42, whereas a higher dose of MOS-Sia3 (>50 μg/mL) can inhibit the proliferation of BV-2 cells to a certain extent. A lower dose of MOS-Sia3 can also inhibit the expression of IL-1β, IL-6, TNF-α, and other proinflammatory factors in BV-2 cells induced by Aβ42 activation. In the future, the MOS-Sia3 conjugate can be used to treat Alzheimer’s disease.

Hypoxia/ischemia impairs CD33 (Siglec-3)/TREM2 signaling: Potential role in Alzheimer's pathogenesis

Recent genetic and molecular studies have indicated that the innate immune system, especially microglia, have a crucial role in the accumulation of β-amyloid plaques in Alzheimer's disease (AD). In particular, the CD33 receptor, also called Siglec-3, inhibits the TREM2 receptor-induced phagocytic activity of microglia. CD33 receptors recognize the α2,3 and α2,6-linked sialic groups in tissue glycocalyx, especially sialylated gangliosides in human brain. The CD33 receptor triggers cell-type specific responses, e.g., in microglia, CD33 inhibits phagocytosis, whereas in natural killer cells, it inhibits the cytotoxic activity of the NKG2D receptor. Nonetheless, the regulation of the activity of CD33 receptor needs to be clarified. For example, it seems that hypoxia/ischemia, a potential cause of AD pathology, increases the expression of CD33 and its downstream target SHP-1, a tyrosine phosphatase which suppresses the phagocytosis driven by TREM2. Moreover, hypoxia/ischemia increases the deposition of sialylated gangliosides, e.g., GM1, GM2, GM3, and GD1, which are ligands for inhibitory CD33/Siglec-3 receptors. In addition, β-amyloid peptides bind to the sialylated gangliosides in raft-like clusters and subsequently these gangliosides act as seeds for the formation of β-amyloid plaques in AD pathology. It is known that senile plaques contain sialylated GM1, GM2, and GM3 gangliosides, i.e., the same species induced by hypoxia/ischemia treatment. Sialylated gangliosides in plaques might stimulate the CD33/Siglec-3 receptors of microglia and thus impede TREM2-driven phagocytosis. We propose that hypoxia/ischemia, e.g., via the accumulation of sialylated gangliosides, prevents the phagocytosis of β-amyloid deposits by inhibiting CD33/TREM2 signaling.

The significance of sialylation on the pathogenesis of Alzheimer's disease

Sialylation, one of the most common and complex modes of glycosylation, corresponds with the development of the infant brain and nervous system. The most prevalent neurodegenerative disease is Alzheimer's disease (AD), which is mainly characterized by cognitive decline and behavioral disorders. However, the relationship between sialylation and AD occurrence is poorly understood. In this article, we reviewed the role of sialylation on the occurrence and development of AD, then discussed the value of sialylation modification for AD diagnosis and treatment.