The expression of sialyltransferases is regulated by the bioavailability and biosynthesis of sialic acids

Ex Vivo Surface Decoration of Phenylboronic Acid onto Natural Killer Cells for Sialic Acid-Mediated Versatile Cancer Cell Targeting

Phenylboronic acid (PBA) has been highly acknowledged as a significant cancer recognition moiety in sialic acid-overexpressing cancer cells. In this investigation, lipid-mediated biomaterial integrated PBA molecules onto the surface of natural killer (NK) cells to make a receptor-mediated immune cell therapeutic module. Therefore, a 1,2-distearoyl-sn-glycero-3-phosphorylethanolamine (DSPE) lipid-conjugated di-PEG-PBA (DSPEPEG-di(PEG-PBA) biomaterial was synthesized. The DSPEPEG-di(PEG-PBA) biomaterial exhibited a high affinity for sialic acid (SA), confirmed by fluorescence spectroscopy at pH 6.5 and 7.4. DSPEPEG-di(PEG-PBA) was successfully anchored onto NK cell surfaces (PBA-NK), and this biomaterial maintains intrinsic properties such as viability, ligand availability (FasL & TRAIL), and cytokine secretion response to LPS. The anticancer efficacy of PBA-NK cells was evaluated against 2D cancer cells (MDA-MB-231, HepG2, and HCT-116) and 3D tumor spheroids of MDA-MB-231 cells. PBA-NK cells exhibited greatly enhanced anticancer effects against SA-overexpressing cancer cells. Thus, PBA-NK cells represent a new anticancer strategy for cancer immunotherapy.

Glycation Interferes with the Expression of Sialyltransferases and Leads to Increased Polysialylation in Glioblastoma Cells

Glioblastoma (GBM) is a highly aggressive brain tumor that often utilizes aerobic glycolysis for energy production (Warburg effect), resulting in increased methylglyoxal (MGO) production. MGO, a reactive dicarbonyl compound, causes protein alterations and cellular dysfunction via glycation. In this study, we investigated the effect of glycation on sialylation, a common post-translational modification implicated in cancer. Our experiments using glioma cell lines, human astrocytes (hA), and primary glioma samples revealed different gene expressions of sialyltransferases among cells, highlighting the complexity of the system. Glycation has a differential effect on sialyltransferase expression, upregulating ST8SIA4 in the LN229 and U251 cell lines and decreasing the expression in normal hA. Subsequently, polysialylation increased in the LN229 and U251 cell lines and decreased in hA. This increase in polysialylation could lead to a more aggressive phenotype due to its involvement in cancer hallmark processes such as immune evasion, resistance to apoptosis, and enhancing invasion. Our findings provide insights into the mechanisms underlying GBM aggressiveness and suggest that targeting glycation and sialylation could be a potential therapeutic strategy.

Glycation Interferes with the Activity of the Bi-Functional UDP-N-Acetylglucosamine 2-Epimerase/N-Acetyl-mannosamine Kinase (GNE)

Mutations in the gene coding for the bi-functional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the key enzyme of the sialic acid biosynthesis, are responsible for autosomal-recessive GNE myopathy (GNEM). GNEM is an adult-onset disease with a yet unknown exact pathophysiology. Since the protein appears to work adequately for a certain period of time even though the mutation is already present, other effects appear to influence the onset and progression of the disease. In this study, we want to investigate whether the late onset of GNEM is based on an age-related effect, e.g., the accumulation of post-translational modifications (PTMs). Furthermore, we also want to investigate what effect on the enzyme activity such an accumulation would have. We will particularly focus on glycation, which is a PTM through non-enzymatic reactions between the carbonyl groups (e.g., of methylglyoxal (MGO) or glyoxal (GO)) with amino groups of proteins or other biomolecules. It is already known that the levels of both MGO and GO increase with age. For our investigations, we express each domain of the GNE separately, treat them with one of the glycation agents, and determine their activity. We demonstrate that the enzymatic activity of the N-acetylmannosamine kinase (GNE-kinase domain) decreases dramatically after glycation with MGO or GO-with a remaining activity of 13% ± 5% (5 mM MGO) and 22% ± 4% (5 mM GO). Whereas the activity of the UDP-N-acetylglucosamine 2-epimerase (GNE-epimerase domain) is only slightly reduced after glycation-with a remaining activity of 60% ± 8% (5 mM MGO) and 63% ± 5% (5 mM GO).

Effect of Dexamethasone on the Expression of the α2,3 and α2,6 Sialic Acids in Epithelial Cell Lines

N-acetylneuraminic acid linked to galactose by α2,6 and α2,3 linkages (Siaα2,6 and Siaα2,3) is expressed on glycoconjugates of animal tissues, where it performs multiple biological functions. In addition, these types of sialic acid residues are the main targets for the binding and entry of influenza viruses. Here we used fluorochrome-conjugated Sambuccus nigra, Maackia amurensis, and peanut lectins for the simultaneous detection of Siaα2,3 and Siaα2,6 and galactosyl residues by two-color flow cytometry on A549 cells, a human pneumocyte cell line used for in vitro studies of the infection by influenza viruses, as well as on Vero and MDCK cell lines. The dexamethasone (DEX) glucocorticoid (GC), a widely used anti-inflammatory compound, completely abrogated the expression of Siaα2,3 in A549 cells and decreased its expression in Vero and MDCK cells; in contrast, the expression of Siaα2,6 was increased in the three cell lines. These observations indicate that DEX can be used for the study of the mechanism of sialylation of cell membrane molecules. Importantly, DEX may change the tropism of avian and human/pig influenza viruses and other infectious agents to animal and human epithelial cells.

Generation and characterization of a novel gne Knockout Model in Zebrafish

GNE Myopathy is a rare, recessively inherited neuromuscular worldwide disorder, caused by a spectrum of bi-allelic mutations in the human GNE gene. GNE encodes a bi-functional enzyme responsible for the rate-limiting step of sialic acid biosynthesis pathway. However, the process in which GNE mutations lead to the development of a muscle pathology is not clear yet. Cellular and mouse models for GNE Myopathy established to date have not been informative. Further, additional GNE functions in muscle have been hypothesized. In these studies, we aimed to investigate gne functions using zebrafish genetic and transgenic models, and characterized them using macroscopic, microscopic, and molecular approaches. We first established transgenic zebrafish lineages expressing the human GNE cDNA carrying the M743T mutation, driven by the zebrafish gne promoter. These fish developed entirely normally. Then, we generated a gne knocked-out (KO) fish using the CRISPR/Cas9 methodology. These fish died 8–10 days post-fertilization (dpf), but a phenotype appeared less than 24 h before death and included progressive body axis curving, deflation of the swim bladder and decreasing movement and heart rate. However, muscle histology uncovered severe defects, already at 5 dpf, with compromised fiber organization. Sialic acid supplementation did not rescue the larvae from this phenotype nor prolonged their lifespan. To have deeper insights into the potential functions of gne in zebrafish, RNA sequencing was performed at 3 time points (3, 5, and 7 dpf). Genotype clustering was progressive, with only 5 genes differentially expressed in gne KO compared to gne WT siblings at 3 dpf. Enrichment analyses of the primary processes affected by the lack of gne also at 5 and 7 dpf point to the involvement of cell cycle and DNA damage/repair processes in the gne KO zebrafish. Thus, we have established a gne KO zebrafish lineage and obtained new insights into gne functions. This is the only model where GNE can be related to clear muscle defects, thus the only animal model relevant to GNE Myopathy to date. Further elucidation of gne precise mechanism-of-action in these processes could be relevant to GNE Myopathy and allow the identification of novel therapeutic targets.

Glycomic Analysis Reveals That Sialyltransferase Inhibition Is Involved in the Antiviral Effects of Arbidol

Due to the high mutation rate of influenza virus and the rapid increase of drug resistance, it is imperative to discover host-targeting antiviral agents with broad-spectrum antiviral activity. Considering the discrepancy between the urgent demand of antiviral drugs during an influenza pandemic and the long-term process of drug discovery and development, it is feasible to explore host-based antiviral agents and strategies from antiviral drugs on the market. In the current study, the antiviral mechanism of arbidol (ARB), a broad-spectrum antiviral drug with potent activity at early stages of viral replication, was investigated from the aspect of hemagglutinin (HA) receptors of host cells. N-glycans that act as the potential binding receptors of HA on 16-human bronchial epithelial (16-HBE) cells were comprehensively profiled for the first time by using an in-depth glycomic approach based on TiO₂-PGC chip-Q-TOF MS. Their relative levels upon the treatment of ARB and virus were carefully examined by employing an ultra-high sensitive qualitative method based on Chip LC-QQQ MS, showing that ARB treatment led to significant and extensive decrease of sialic acid (SA)-linked N-glycans (SA receptors), and thereby impaired the virus utilization on SA receptors for rolling and entry. The SA-decreasing effect of ARB was demonstrated to result from its inhibitory effect on sialyltransferases (ST), ST3GAL4 and ST6GAL1 of 16-HBE cells. Silence of STs, natural ST inhibitors, as well as sialidase treatment of 16-HBE cells, resulted in similar potent antiviral activity, whereas ST-inducing agent led to the diminished antiviral effect of ARB. These observations collectively suggesting the involvement of ST inhibition in the antiviral effect of ARB.

IMPORTANCE This study revealed, for the first time, that ST inhibition and the resulted destruction of SA receptors of host cells may be an underlying mechanism for the antiviral activity of ARB. ST inhibition has been proposed as a novel host-targeting antiviral approach recently and several compounds are currently under exploration. ARB is the first antiviral drug on the market that was found to possess ST inhibiting function. This will provide crucial evidence for the clinical usages of ARB, such as in combination with neuraminidase (NA) inhibitors to exert optimized antiviral effect, etc. More importantly, as an agent that can inhibit the expression of STs, ARB can serve as a novel lead compound for the discovery and development of host-targeting antiviral drugs.

Upregulation of Hallmark Muscle Genes Protects GneM743T/M743T Mutated Knock-In Mice From Kidney and Muscle Phenotype

Background:

Mutations in GNE cause a recessive, adult onset myopathy characterized by slowly progressive distal and proximal muscle weakness. Knock-in mice carrying the most frequent mutation in GNE myopathy patients, Gne^M743T/M743T, usually die few days after birth from severe renal failure, with no muscle phenotype. However, a spontaneous sub-colony remains healthy throughout a normal lifespan without any kidney or muscle pathology.

Objective:

We attempted to decipher the molecular mechanisms behind these phenotypic differences and to determine the mechanisms preventing the kidney and muscles from disease.

Methods:

We analyzed the transcriptome and proteome of kidneys and muscles of sick and healthy Gne^M743T/M743T mice.

Results:

The sick Gne^M743T/M743T kidney was characterized by up-regulation of extra-cellular matrix degradation related processes and by down-regulation of oxidative phosphorylation and respiratory electron chain pathway, that was also observed in the asymptomatic muscles. Surprisingly, the healthy kidneys of the Gne^M743T/M743T mice were characterized by up-regulation of hallmark muscle genes. In addition the asymptomatic muscles of the sick Gne^M743T/M743T mice showed upregulation of transcription and translation processes.

Conclusions:

Overexpression of muscle physiology genes in healthy Gne^M743T/M743T mice seems to define the protecting mechanism in these mice. Furthermore, the strong involvement of muscle related genes in kidney may bridge the apparent phenotypic gap between GNE myopathy and the knock-in Gne^M743T/M743T mouse model and provide new directions in the study of GNE function in health and disease.

The role of sialyltransferases in gynecological malignant tumors

Sialylation is the addition of sialic acids to the terminus of various glycoconjugates, and it is involved in many essential biological processes, such as cell adhesion, signal transduction, immune regulation, etc. The levels of sialylation in a cell are tightly regulated by two groups of enzymes, sialyltransferases (STs, responsible for sialylation) and sialidases (responsible for desialylation). Many studies have reported that the occurrence, development, and survival rates of tumors are significantly associated with STs' abnormal changes. In recent years, the morbidity and mortality rates of gynecological malignant tumors have been continuously rising, which has caused great harm to women's reproduction and health. Abnormal changes of STs in gynecological malignant tumor cell membranes cause the changes of expression of sialic acids, promoting cell migration and, eventually, leading to tumor metastasis. In this review, we outlined the biological characteristics of STs and summarized the expression profiles of 20 STs in different tumors via transcriptome data from Gene Expression Profiling Interactive Analysis (GEPIA) database. Moreover, STs' functions in four common gynecological tumors (ovarian cancer, cervical cancer, endometrial cancer, and gestational trophoblast tumor) were reviewed.

UDP-glucose 6-dehydrogenase knockout impairs migration and decreases in vivo metastatic ability of breast cancer cells

Dysregulated metabolism is a hallmark of cancer that supports tumor growth and metastasis. One understudied aspect of cancer metabolism is altered nucleotide sugar biosynthesis, which drives aberrant cell surface glycosylation known to support various aspects of cancer cell behavior including migration and signaling. We examined clinical association of nucleotide sugar pathway gene expression and found that UGDH, encoding UDP-glucose 6-dehydrogenase which catalyzes production of UDP-glucuronate, is associated with worse breast cancer patient survival. Knocking out the mouse homolog Ugdh in highly-metastatic 6DT1 breast cancer cells impaired migration ability without affecting in vitro proliferation. Further, Ugdh-KO resulted in significantly decreased metastatic capacity in vivo when the cells were orthotopically injected in syngeneic mice. Our experiments show that UDP-glucuronate biosynthesis is critical for metastasis in a mouse model of breast cancer.

Trans-sialidase Associated with Atherosclerosis: Defining the Identity of a Key Enzyme Involved in the Pathology

Atherosclerosis is associated with the increased trans-sialidase activity, which can be detected in the blood plasma of atherosclerosis patients. The likely involvement in the disease pathogenesis made this activity an interesting research subject and the enzyme that may perform such activity was isolated and characterized in terms of substrate specificity and enzymatic properties. It was found that the enzyme has distinct optimum pH values, and its activity was enhanced by the presence of Ca2+ ions. Most importantly, the enzyme was able to cause atherogenic modification of lowdensity lipoprotein (LDL) particles in vitro. However, the identity of the discovered enzyme remained to be defined. Currently, sialyltransferases, mainly ST6Gal I, are regarded as major contributors to sialic acid metabolism in human blood. In this mini-review, we discuss the possibility that atherosclerosis- associated trans-sialidase does, in fact, belong to the sialyltransferases family.

The manifold roles of sialic acid for the biological functions of endothelial glycoproteins

Vascular endothelia are covered with a dense glycocalix that is heavily sialylated. Sialylation of vascular glycoconjugates is involved in the regulation of cell-cell interactions, be it among endothelial cells at cell junctions or between endothelial and blood-borne cells. It also plays important roles in modulating the binding of soluble ligands and the signaling by vascular receptors. Here, we provide an overview over the sialylation-function relationships of glycoproteins expressed in the blood and lymphatic vasculature. We first describe cellular interactions in which sialic acid contributes in a stereospecific manner to glycan epitopes recognized by glycan-binding proteins. Our major focus is however on the rarely discussed examples of vascular glycoproteins whose biological functions are modulated by sialylation through other mechanisms.

Robustness in glycosylation systems: effect of modified monosaccharides, acceptor decoys and azido sugars on cellular nucleotide-sugar levels and pattern of N-linked glycosylation

Small molecule monosaccharide analogs (e.g. 4F-GlcNAc, 4F-GalNAc) and acceptor decoys (e.g. ONAP, SNAP) are commonly used as metabolic glycoengineering tools to perturb molecular and cellular recognition processes. Azido-derivatized sugars (e.g. ManNAz, GlcNAz, GalNAz) are also used as bioorthogonal probes to assay the glycosylation status of cells and tissue. With the goal of obtaining a systems-level understanding of how these compounds work, we cultured cells with these molecules and systematically evaluated their impact on: (i) cellular nucleotide-sugar levels, and (ii) N-linked glycosylation. To this end, we developed a streamlined, simple workflow to quantify nucleotide-sugar levels using amide-based hydrophilic interaction liquid chromatography (HILIC) separation followed by negative-mode electrospray ionization mass spectrometry (ESI-MS/MS) using an Orbitrap detector. N-Glycans released from cells were also procainamide functionalized and quantified using positive-mode ESI-MS/MS. Results show that all tested compounds changed the baseline nucleotide-sugar levels, with the effect being most pronounced for the fluoro-HexNAc compounds. These molecules depressed UDP-HexNAc levels in cells by up to 80%, while concomitantly elevating UDP-4F-GalNAc and UDP-4F-GlcNAc. While the measured changes in nucleotide-sugar concentration were substantial in many cases, their impact on N-linked glycosylation was relatively small. This may be due to the high nucleotide-sugar concentrations in the Golgi, which far exceed the K_M values of the glycosylating enzymes. Thus, the glycosylation system output exhibits 'robustness' even in the face of significant changes in cellular nucleotide-sugar concentrations.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

GNE myopathy, previously known as hereditary inclusion body myopathy (HIBM), or Nonaka myopathy, is a rare autosomal recessive muscle disease characterized by progressive skeletal muscle atrophy. It has an estimated prevalence of 1 to 9:1,000,000. GNE myopathy is caused by mutations in the GNE gene which encodes the rate-limiting enzyme of sialic acid biosynthesis. The pathophysiology of the disease is not entirely understood, but hyposialylation of muscle glycans is thought to play an essential role. The typical presentation is bilateral foot drop caused by weakness of the anterior tibialis muscles with onset in early adulthood. The disease slowly progresses over the next decades to involve skeletal muscles throughout the body, with relative sparing of the quadriceps until late stages of the disease. The diagnosis of GNE myopathy should be considered in young adults presenting with bilateral foot drop. Histopathologic findings on muscle biopsies include fiber size variation, atrophic fibers, lack of inflammation, and the characteristic "rimmed" vacuoles on modified Gomori trichome staining. The diagnosis is confirmed by the presence of pathogenic (mostly missense) mutations in both alleles of the GNE gene. Although there is no approved therapy for this disease, preclinical and clinical studies of several potential therapies are underway, including substrate replacement and gene therapy-based strategies. However, developing therapies for GNE myopathy is complicated by several factors, including the rare incidence of disease, limited preclinical models, lack of reliable biomarkers, and slow disease progression.

Integration of genetic and metabolic features related to sialic acid metabolism distinguishes human breast cell subtypes

In this report we use 'high-flux' tributanoyl-modified N-acetylmannosamine (ManNAc) analogs with natural N-acetyl as well as non-natural azido- and alkyne N-acyl groups (specifically, 1,3,4-O-Bu3ManNAc, 1,3,4-O-Bu3ManNAz, and 1,3,4-O-Bu3ManNAl respectively) to probe intracellular sialic acid metabolism in the near-normal MCF10A human breast cell line in comparison with earlier stage T-47D and more advanced stage MDA-MB-231 breast cancer lines. An integrated view of sialic acid metabolism was gained by measuring intracellular sialic acid production in tandem with transcriptional profiling of genes linked to sialic acid metabolism. The transcriptional profiling showed several differences between the three lines in the absence of ManNAc analog supplementation that helps explain the different sialoglycan profiles naturally associated with cancer. Only minor changes in mRNA transcript levels occurred upon exposure to the compounds confirming that metabolic flux alone can be a key determinant of sialoglycoconjugate display in breast cancer cells; this result complements the well-established role of genetic control (e.g., the transcription of STs) of sialylation abnormalities ubiquitously associated with cancer. A notable result was that the different cell lines produced significantly different levels of sialic acid upon exogenous ManNAc supplementation, indicating that feedback inhibition of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE)-generally regarded as the 'gatekeeper' enzyme for titering flux into sialic acid biosynthesis-is not the only regulatory mechanism that limits production of this sugar. A notable aspect of our metabolic glycoengineering approach is its ability to discriminate cell subtype based on intracellular metabolism by illuminating otherwise hidden cell type-specific features. We believe that this strategy combined with multi-dimensional analysis of sialic acid metabolism will ultimately provide novel insights into breast cancer subtypes and provide a foundation for new methods of diagnosis.