O-acetylation and de-O-acetylation of sialic acids in human colorectal carcinoma

Early in vitro evidence indicates that deacetylated sialic acids modulate multi-drug resistance in colon and lung cancers via breast cancer resistance protein

Cancers utilize sugar residues to engage in multidrug resistance. The underlying mechanism of action involving glycans, specifically the glycan sialic acid (Sia) and its various functional group alterations, has not been explored. ATP-binding cassette (ABC) transporter proteins, key proteins utilized by cancers to engage in multidrug resistant (MDR) pathways, contain Sias in their extracellular domains. The core structure of Sia can contain a variety of functional groups, including O-acetylation on the C6 tail. Modulating the expression of acetylated-Sias on Breast Cancer Resistance Protein (BCRP), a significant ABC transporter implicated in MDR, in lung and colon cancer cells directly impacted the ability of cancer cells to either retain or efflux chemotherapeutics. Via CRISPR-Cas-9 gene editing, acetylation was modulated by the removal of CAS1 Domain-containing protein (CASD1) and Sialate O-Acetyl esterase (SIAE) genes. Using western blot, immunofluorescence, gene expression, and drug sensitivity analysis, we confirmed that deacetylated Sias regulated a MDR pathway in colon and lung cancer in early in vitro models. When deacetylated Sias were expressed on BCRP, colon and lung cancer cells were able to export high levels of BCRP to the cell's surface, resulting in an increased BCRP efflux activity, reduced sensitivity to the anticancer drug Mitoxantrone, and high proliferation relative to control cells. These observations correlated with increased levels of cell survival proteins, BcL-2 and PARP1. Further studies also implicated the lysosomal pathway for the observed variation in BCRP levels among the cell variants. RNASeq data analysis of clinical samples revealed higher CASD1 expression as a favorable marker of survival in lung adenocarcinoma. Collectively, our findings indicate that deacetylated Sia is utilized by colon and lung cancers to engage in MDR via overexpression and efflux action of BCRP.

O-Acetylated sugars in the gas phase: stability, migration, positional isomers and conformation

O-Acetylations are functional modifications which can be found on different hydroxyl groups of glycans and which contribute to the fine tuning of their biological activity. Localizing the acetyl modifications is notoriously challenging in glycoanalysis, in particular because of their mobility: loss or migration of the acetyl group may occur through the analytical workflow. Whereas migration conditions in the condensed phase have been rationalized, little is known about the suitability of Mass Spectrometry to retain and resolve the structure of O-acetylated glycan isomers. Here we used the resolving power of infrared ion spectroscopy in combination with ab initio calculations to assess the structure of O-acetylated monosaccharide ions in the gaseous environment of a mass analyzer. N-Acetyl glucosamines were synthetized with an O-acetyl group in positions 3 or 6, respectively. The protonated ions produced by electrospray ionization were observed by mass spectrometry and their vibrational fingerprints were recorded in the 3 μm range by IRMPD spectroscopy (InfraRed Multiple Photon Dissociation). Experimentally, the isomers show distinctive IR fingerprints. Additionally, ab initio calculations confirm the position of the O-acetylation and resolve their gas phase conformation. These findings demonstrate that the position of O-acetyl groups is retained through the transfer from solution to the gas phase, and can be identified by IRMPD spectroscopy.

Diagnostic usefulness of selected proteases and acute phase factors in patients with colorectal adenocarcinoma

BACKGROUND

Uncontrolled growth and loss of control over basic metabolic functions, leading to invasive proliferation and metastases, are the salient traits of malignant tumors in general and colorectal cancer in particular. Invasion and metastases hinder effective tumor treatment. While surgical techniques and radiotherapy can be used to remove tumor focus, only chemotherapy can eliminate dispersed neoplastic cells. However, the efficacy of the latter method is limited in the advanced stages of the disease. Therefore, recognition of the mechanisms involved in neoplastic cell spreading is indispensable for developing effective therapies.

AIM

To use a number of biomarkers involved in cancer progression and identify a panel that could be used for effective early diagnosis.

METHODS

We recruited 185 patients with colorectal adenocarcinoma (98 men, 87 women with median age 63). Thirty-five healthy controls were sex and age-matched. Dukes’ staging was as follows: A = 22, B = 52, C = 72, D = 39. We analyzed patients' blood serum before surgery. We determined: (1) Cathepsin B (CB) with Barrett's method (fluorogenic substrate); (2) Leukocytic elastase (LE) in a complex with alpha 1 trypsin inhibitor (AAT) using the immunoenzymatic MERCK test; (3) Total sialic acid (TSA) with the colorimetric periodate-resorcinol method; (4) Lipid-bound sialic acid (LASA) with the colorimetric Taut's method; and (5) The antitrypsin activity (ATA) employing the colorimetric test.

RESULTS

In patients, the values of the five biochemical parameters were as follows: CB = 16.1 ± 8.8 mU/L, LE = 875 ± 598 µg/L, TSA = 99 ± 31 mg%, LASA = 0.68 ± 0.33 mg%, and ATA = 3211 ± 1504 U/mL. Except for LASA, they were significantly greater than those of controls: CB = 11.4 ± 6.5 mU/L, LE = 379 ± 187 µg/L, TSA = 71.4 ± 15.1 mg%, LASA = 0.69 ± 0.28 mg%, and ATA = 2016 ± 690 U/mL. For CB and LASA, the differences between the four Dukes’ stages and controls were not statistically significant. The inter-stage differences for CB and LASA were also absent. The receiver operating characteristic (ROC) analysis revealed the potential diagnostic value of CB, TSA, and ATA. The area under ROC, sensitivity, and specificity for these three parameters were: 0.85, 72%, 90%; 0.75, 66%, 77%; and 0.77, 63%, 84%, respectively. The sensitivity and specificity for the three-parameter panel CB-TSA-ATA were equal to 88.2% and 100%, respectively.

CONCLUSION

The increased value of CB, TSA, and ATA parameters are associated with tumor biology, invasion, and metastasis of colorectal cancer. The presented evidence suggests the potential value of the CB-TSA-ATA biochemical marker panel in early diagnostics.

Sialic acid O-acetylation: From biosynthesis to roles in health and disease

Sialic acids are nine-carbon sugars that frequently cap glycans at the cell surface in cells of vertebrates as well as cells of certain types of invertebrates and bacteria. The nine-carbon backbone of sialic acids can undergo extensive enzymatic modification in nature and O-acetylation at the C-4/7/8/9 position in particular is widely observed. In recent years, the detection and analysis of O-acetylated sialic acids have advanced, and sialic acid-specific O-acetyltransferases (SOATs) and O-acetylesterases (SIAEs) that add and remove O-acetyl groups, respectively, have been identified and characterized in mammalian cells, invertebrates, bacteria, and viruses. These advances now allow us to draw a more complete picture of the biosynthetic pathway of the diverse O-acetylated sialic acids to drive the generation of genetically and biochemically engineered model cell lines and organisms with altered expression of O-acetylated sialic acids for dissection of their roles in glycoprotein stability, development, and immune recognition, as well as discovery of novel functions. Furthermore, a growing number of studies associate sialic acid O-acetylation with cancer, autoimmunity, and infection, providing rationale for the development of selective probes and inhibitors of SOATs and SIAEs. Here, we discuss the current insights into the biosynthesis and biological functions of O-acetylated sialic acids and review the evidence linking this modification to disease. Furthermore, we discuss emerging strategies for the design, synthesis, and potential application of unnatural O-acetylated sialic acids and inhibitors of SOATs and SIAEs that may enable therapeutic targeting of this versatile sialic acid modification.

Deacetylated sialic acids modulates immune mediated cytotoxicity via the sialic acid-Siglec pathway

Cancers utilize glycans to evade the immune system via the Sialic acid (Sia)-Siglec (Sialic-acid-binding immunoglobulin-like lectins) pathway. Specifically, atypical structural forms of sialic acid bind to inhibitory Siglec receptors on Natural Killer (NK) cells resulting in the suppression of immune cell mediated cytotoxicity. The mechanism of action that governs the Sia-Siglec pathway in cancers is not understood. Specifically, how deviations from the typical form of Sia mechanistically contribute. Here we focused on modulating 9-O and 7,9-O-acetylation of Neu5Ac, via CRISPR-Cas9 gene editing, a functional group that is absent from Sias on many types of cancer cells. The two genes that are responsible for regulating the level of acetylation on Neu5Ac, are Sialic acid acetylesterase (SIAE) and Sialic acid acetyltransferase (CASD1). These genes modulated Siglec binding in colon, lung, and a non-cancerous kidney cell line. In the absence of SIAE, Neu5Ac is acetylated, engagement of cancer associated Siglecs is reduced while binding was increased when the ability to acetylate was removed via CASD1 knock out. In the absence of SIAE NK mediated cytotoxicity increased in both colon and lung cancer cells. In addition to modulating Siglec binding, SIAE expression modulates the level of Sias in a cell, and the α2-6-linkage of Sias - which is specifically upregulated and associated with cancers. Uncovering how functional group alterations on Neu5Ac contribute mechanistically to both Siglec receptor binding, the Sia-Siglec immune evasion pathway, and the production of cancer associated glycosidic linkages -offers a promising avenue for targeted cancer immune therapies in the future.

Role of Sialyl-O-Acetyltransferase CASD1 on GD2 Ganglioside O-Acetylation in Breast Cancer Cells

The O-acetylated form of GD2, almost exclusively expressed in cancerous tissues, is considered to be a promising therapeutic target for neuroectoderm-derived tumors, especially for breast cancer. Our recent data have shown that 9-O-acetylated GD2 (9-OAcGD2) is the major O-acetylated ganglioside species in breast cancer cells. In 2015, Baumann et al. proposed that Cas 1 domain containing 1 (CASD1), which is the only known human sialyl-O-acetyltransferase, plays a role in GD3 O-acetylation. However, the mechanisms of ganglioside O-acetylation remain poorly understood. The aim of this study was to determine the involvement of CASD1 in GD2 O-acetylation in breast cancer. The role of CASD1 in OAcGD2 synthesis was first demonstrated using wild type CHO and CHOΔCasd1 cells as cellular models. Overexpression using plasmid transfection and siRNA strategies was used to modulate CASD1 expression in SUM159PT breast cancer cell line. Our results showed that OAcGD2 expression was reduced in SUM159PT that was transiently depleted for CASD1 expression. Additionally, OAcGD2 expression was increased in SUM159PT cells transiently overexpressing CASD1. The modulation of CASD1 expression using transient transfection strategies provided interesting insights into the role of CASD1 in OAcGD2 and OAcGD3 biosynthesis, and it highlights the importance of further studies on O-acetylation mechanisms.

Sialic Acid-Siglec Axis in Human Immune Regulation, Involvement in Autoimmunity and Cancer and Potential Therapeutic Treatments

Siglecs are sialic acid-binding immunoglobulin-like lectins. Most Siglecs function as transmembrane receptors mainly expressed on blood cells in a cell type-specific manner. They recognize and bind sialic acids in specific linkages on glycoproteins and glycolipids. Since Sia is a self-molecule, Siglecs play a role in innate immune responses by distinguishing molecules as self or non-self. Increasing evidence supports the involvement of Siglecs in immune signaling representing immune checkpoints able to regulate immune responses in inflammatory diseases as well as cancer. Although further studies are necessary to fully understand the involvement of Siglecs in pathological conditions as well as their interactions with other immune regulators, the development of therapeutic approaches that exploit these molecules represents a tremendous opportunity for future treatments of several human diseases, as demonstrated by their application in several clinical trials. In the present review, we discuss the involvement of Siglecs in the regulation of immune responses, with particular focus on autoimmunity and cancer and the chance to target the sialic acid-Siglec axis as novel treatment strategy.

Human neuraminidases have reduced activity towards modified sialic acids on glycoproteins

Multiple levels of diversity in sialic acid presentation can influence the substrate activity of sialosides for glycoside hydrolases. Few reports have investigated the specificity of human neuraminidase (hNEU) activity towards modified sialic acid residues that can occur on glycoproteins. Previously, we evaluated hNEU activity towards 9-O-acetylated sialic acid in glycolipid substrates and found that hNEU generally discriminated against 9-O-acetylated sialic acid over Neu5Ac. Here, we have investigated the substrate specificity of hNEU enzymes for a glycoprotein substrate (bovine submaxillary mucin) containing 9-O-acetylated and Neu5Gc residues. Using this model substrate, we observe a general trend for hNEU tolerance of Neu5Ac > Neu5Gc ≫ Neu5,9Ac₂, consistent with our previous results with glycolipid substrates. These results expand our understanding of hNEU enzyme specificity and suggest that naturally occurring modifications of sialic acids can play a role in regulating hNEU activity.

NK Cell-Based Immune Checkpoint Inhibition

Immunotherapy, with an increasing number of therapeutic dimensions, is becoming an important mode of treatment for cancer patients. The inhibition of immune checkpoints, which are the source of immune escape for various cancers, is one such immunotherapeutic dimension. It has mainly been aimed at T cells in the past, but NK cells are a newly emerging target. Simultaneously, the number of checkpoints identified has been increasing in recent times. In addition to the classical NK cell receptors KIRs, LIRs, and NKG2A, several other immune checkpoints have also been shown to cause dysfunction of NK cells in various cancers and chronic infections. These checkpoints include the revolutionized CTLA-4, PD-1, and recently identified B7-H3, as well as LAG-3, TIGIT & CD96, TIM-3, and the most recently acknowledged checkpoint-members of the Siglecs family (Siglec-7/9), CD200 and CD47. An interesting dimension of immune checkpoints is their candidacy for dual-checkpoint inhibition, resulting in therapeutic synergism. Furthermore, the combination of immune checkpoint inhibition with other NK cell cytotoxicity restoration strategies could also strengthen its efficacy as an antitumor therapy. Here, we have undertaken a comprehensive review of the literature to date regarding NK cell-based immune checkpoints.

Profiling of O-acetylated Gangliosides Expressed in Neuroectoderm Derived Cells

The expression and biological functions of oncofetal markers GD2 and GD3 were extensively studied in neuroectoderm-derived cancers in order to characterize their potential as therapeutic targets. Using immunological approaches, we previously identified GD3, GD2, and OAcGD2 expression in breast cancer (BC) cell lines. However, antibodies specific for O-acetylated gangliosides are not exempt of limitations, as they only provide information on the expression of a limited set of O-acetylated ganglioside species. Consequently, the aim of the present study was to use structural approaches in order to apprehend ganglioside diversity in melanoma, neuroblastoma, and breast cancer cells, focusing on O-acetylated species that are usually lost under alkaline conditions and require specific analytical procedures. We used purification and extraction methods that preserve the O-acetyl modification for the analysis of native gangliosides by MALDI-TOF. We identified the expression of GM1, GM2, GM3, GD2, GD3, GT2, and GT3 in SK-Mel28 (melanoma), LAN-1 (neuroblastoma), Hs 578T, SUM 159PT, MDA-MB-231, MCF-7 (BC), and BC cell lines over-expressing GD3 synthase. Among O-acetylated gangliosides, we characterized the expression of OAcGM1, OAcGD3, OAcGD2, OAcGT2, and OAcGT3. Furthermore, the experimental procedure allowed us to clearly identify the position of the sialic acid residue that carries the O-acetyl group on b- and c-series gangliosides by MS/MS fragmentation. These results show that ganglioside O-acetylation occurs on both inner and terminal sialic acid residue in a cell type-dependent manner, suggesting different O-acetylation pathways for gangliosides. They also highlight the limitation of immuno-detection for the complete identification of O-acetylated ganglioside profiles in cancer cells.

Sialic acid and biology of life: An introduction

Sialic acids are important molecule with high structural diversity. They are known to occur in higher animals such as Echinoderms, Hemichordata, Cephalochorda, and Vertebrata and also in other animals such as Platyhelminthes, Cephalopoda, and Crustaceae. Plants are known to lack sialic acid. But they are reported to occur in viruses, bacteria, protozoa, and fungi. Deaminated neuraminic acid although occurs in vertebrates and bacteria, is reported to occur in abundance in the lower vertebrates. Sialic acids are mostly located in terminal ends of glycoproteins and glycolipids, capsular and tissue polysialic acids, bacterial lipooligosaccharides/polysaccharides, and in different forms that dictate their role in biology. Sialic acid play important roles in human physiology of cell-cell interaction, communication, cell-cell signaling, carbohydrate-protein interactions, cellular aggregation, development processes, immune reactions, reproduction, and in neurobiology and human diseases in enabling the infection process by bacteria and virus, tumor growth and metastasis, microbiome biology, and pathology. It enables molecular mimicry in pathogens that allows them to escape host immune responses. Recently sialic acid has found role in therapeutics. In this chapter we have highlighted the (i) diversity of sialic acid, (ii) their occurrence in the diverse life forms, (iii) sialylation and disease, and (iv) sialic acid and therapeutics.

Expression of 9-O- and 7,9-O-Acetyl Modified Sialic Acid in Cells and Their Effects on Influenza Viruses

Sialic acids (Sia) are widely displayed on the surfaces of cells and tissues. Sia come in a variety of chemically modified forms, including those with acetyl modifications at the C-7, C-8, and C-9 positions. Here, we analyzed the distribution and amounts of these acetyl modifications in different human and canine cells. Since Sia or their variant forms are receptors for influenza A, B, C, and D viruses, we examined the effects of these modifications on virus infections. We confirmed that 9-O-acetyl and 7,9-O-acetyl modified Sia are widely but variably expressed across cell lines from both humans and canines. Although they were expressed on the cell surfaces of canine MDCK cell lines, they were located primarily within the Golgi compartment of human HEK-293 and A549 cells. The O-acetyl modified Sia were expressed at low levels of 1 to 2% of total Sia in these cell lines. We knocked out and overexpressed the sialate O-acetyltransferase gene (CasD1) and knocked out the sialate O-acetylesterase gene (SIAE) using CRISPR/Cas9 editing. Knocking out CasD1 removed 7,9-O- and 9-O-acetyl Sia expression, confirming previous reports. However, overexpression of CasD1 and knockout of SIAE gave only modest increases in 9-O-acetyl levels in cells and no change in 7,9-O-acetyl levels, indicating that there are complex regulations of these modifications. These modifications were essential for influenza C and D infection but had no obvious effect on influenza A and B infection.IMPORTANCE Sialic acids are key glycans that are involved in many different normal cellular functions, as well as being receptors for many pathogens. However, Sia come in diverse chemically modified forms. Here, we examined and manipulated the expression of 7,9-O- and 9-O-acetyl modified Sia on cells commonly used in influenza virus and other research by engineering the enzymes that produce or remove the acetyl groups.

Metabolomic and glycomic findings in posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is a stressor-related disorder that develops in a subset of individuals exposed to a traumatic experience. Factors associated with vulnerability to PTSD are still not fully understood. PTSD is frequently comorbid with various psychiatric and somatic disorders, moderate response to treatment and remission rates. The term "theranostics" combines diagnosis, prognosis, and therapy and offers targeted therapy based on specific analyses. Theranostics, combined with novel techniques and approaches called "omics", which integrate genomics, transcriptomic, proteomics and metabolomics, might improve knowledge about biological underpinning of PTSD, and offer novel therapeutic strategies. The focus of this review is on metabolomic and glycomic data in PTSD. Metabolomics evaluates changes in the metabolome of an organism by exploring the set of small molecules (metabolites), while glycomics studies the glycome, a complete repertoire of glycan structures with their functional roles in biological systems. Both metabolome and glycome reflect the physiological and pathological conditions in individuals. Only a few studies evaluated metabolic and glycomic changes in patients with PTSD. The metabolomics studies in PTSD patients uncovered different metabolites that might be associated with psychopathological alterations in PTSD. The glycomics study in PTSD patients determined nine N-glycan structures and found accelerated and premature aging in traumatized subjects and subjects with PTSD based on a GlycoAge index. Therefore, further larger studies and replications are needed. Better understanding of the biological basis of PTSD, including metabolomic and glycomic data, and their integration with other "omics" approaches, might identify new molecular targets and might provide improved therapeutic approaches.

Targeting sialic acid-Siglec interactions to reverse immune suppression in cancer

Changes in sialic acids in cancer have been observed for many years. In particular, the increase of sialoglycan density or hypersialylation in tumors has been described. Recent studies have identified mechanisms for immune evasion based on sialoglycan interactions with immunoregulatory Siglec receptors that are exploited by tumor cells and microorganisms alike. Siglecs are mostly inhibitory receptors similar to known immune checkpoints including PD-1 or CTLA-4 that are successfully targeted with blocking antibodies for cancer immunotherapy. Here, we summarize the known changes of sialic acids in cancer and the role Siglec receptors play in cancer immunity. We also focus on potential ways to target these Siglec receptors or sialoglycans in order to improve anti-cancer immunity.

The Interaction of the Gut Microbiota with the Mucus Barrier in Health and Disease in Human

Glycoproteins are major players in the mucus protective barrier in the gastrointestinal and other mucosal surfaces. In particular the mucus glycoproteins, or mucins, are responsible for the protective gel barrier. They are characterized by their high carbohydrate content, present in their variable number, tandem repeat domains. Throughout evolution the mucins have been maintained as integral components of the mucosal barrier, emphasizing their essential biological status. The glycosylation of the mucins is achieved through a series of biosynthetic pathways processes, which generate the wide range of glycans found in these molecules. Thus mucins are decorated with molecules having information in the form of a glycocode. The enteric microbiota interacts with the mucosal mucus barrier in a variety of ways in order to fulfill its many normal processes. How bacteria read the glycocode and link to normal and pathological processes is outlined in the review.

Distribution of O-Acetylated Sialic Acids among Target Host Tissues for Influenza Virus

Sialic acids (Sias) are important glycans displayed on the cells and tissues of many different animals and are frequent targets for binding and modification by pathogens, including influenza viruses. Influenza virus hemagglutinins bind Sias during the infection of their normal hosts, while the encoded neuraminidases and/or esterases remove or modify the Sia to allow virion release or to prevent rebinding. Sias naturally occur in a variety of modified forms, and modified Sias can alter influenza virus host tropisms through their altered interactions with the viral glycoproteins. However, the distribution of modified Sia forms and their effects on pathogen-host interactions are still poorly understood. Here we used probes developed from viral Sia-binding proteins to detect O-acetylated (4-O-acetyl, 9-O-acetyl, and 7,9-O-acetyl) Sias displayed on the tissues of some natural or experimental hosts for influenza viruses. These modified Sias showed highly variable displays between the hosts and tissues examined. The 9-O-acetyl (and 7,9-) modified Sia forms were found on cells and tissues of many hosts, including mice, humans, ferrets, guinea pigs, pigs, horses, dogs, as well as in those of ducks and embryonated chicken egg tissues and membranes, although in variable amounts. The 4-O-acetyl Sias were found in the respiratory tissues of fewer animals, being primarily displayed in the horse and guinea pig, but were not detected in humans or pigs. The results suggest that these Sia variants may influence virus tropisms by altering and selecting their cell interactions. IMPORTANCE Sialic acids (Sias) are key glycans that control or modulate many normal cell and tissue functions while also interacting with a variety of pathogens, including many different viruses. Sias are naturally displayed in a variety of different forms, with modifications at several positions that can alter their functional interactions with pathogens. In addition, Sias are often modified or removed by enzymes such as host or pathogen esterases or sialidases (neuraminidases), and Sia modifications can alter those enzymatic activities to impact pathogen infections. Sia chemical diversity in different hosts and tissues likely alters the pathogen-host interactions and influences the outcome of infection. Here we explored the display of 4-O-acetyl, 9-O-acetyl, and 7,9-O-acetyl modified Sia forms in some target tissues for influenza virus infection in mice, humans, birds, guinea pigs, ferrets, swine, horses, and dogs, which encompass many natural and laboratory hosts of those viruses.

Characterization of O-acetylation in sialoglycans by MALDI-MS using a combination of methylamidation and permethylation

O-Acetylation of sialic acid in protein N-glycans is an important modification and can occur at either 4-, 7-, 8- or 9-position in various combinations. This modification is usually labile under alkaline reaction conditions. Consequently, a permethylation-based analytical method, which has been widely used in glycomics studies, is not suitable for profiling O-acetylation of sialic acids due to the harsh reaction conditions. Alternatively, methylamidation can be used for N-glycan analysis without affecting the base-labile modification of sialic acid. In this report, we applied both permethylation and methylamidation approaches to the analysis of O-acetylation in sialic acids. It has been demonstrated that methylamidation not only stabilizes sialic acids during MALDI processing but also allow for characterization of their O-acetylation pattern. In addition, LC-MS/MS experiments were carried out to distinguish between the O-acetylated glycans with potential isomeric structures. The repeatability of methylamidation was examined to evaluate the applicability of the approach to profiling of O-acetylation in sialic acids. In conclusion, the combination of methylamidation and permethylation methodology is a powerful MALDI-TOF MS-based tool for profiling O-acetylation in sialic acids applicable to screening of N-glycans.

Eukaryotic protein glycosylation: a primer for histochemists and cell biologists

Proteins undergo co- and posttranslational modifications, and their glycosylation is the most frequent and structurally variegated type. Histochemically, the detection of glycan presence has first been performed by stains. The availability of carbohydrate-specific tools (lectins, monoclonal antibodies) has revolutionized glycophenotyping, allowing monitoring of distinct structures. The different types of protein glycosylation in Eukaryotes are described. Following this educational survey, examples where known biological function is related to the glycan structures carried by proteins are given. In particular, mucins and their glycosylation patterns are considered as instructive proof-of-principle case. The tissue and cellular location of glycoprotein biosynthesis and metabolism is reviewed, with attention to new findings in goblet cells. Finally, protein glycosylation in disease is documented, with selected examples, where aberrant glycan expression impacts on normal function to let disease pathology become manifest. The histological applications adopted in these studies are emphasized throughout the text.

Harnessing cancer cell metabolism for theranostic applications using metabolic glycoengineering of sialic acid in breast cancer as a pioneering example

Abnormal cell surface display of sialic acids - a family of unusual 9-carbon sugars - is widely recognized as distinguishing feature of many types of cancer. Sialoglycans, however, typically cannot be identified with sufficiently high reproducibility and sensitivity to serve as clinically accepted biomarkers and similarly, almost all efforts to exploit cancer-specific differences in sialylation signatures for therapy remain in early stage development. In this report we provide an overview of important facets of glycosylation that contribute to cancer in general with a focus on breast cancer as an example of malignant disease characterized by aberrant sialylation. We then describe how cancer cells experience nutrient deprivation during oncogenesis and discuss how the resulting metabolic reprogramming, which endows breast cancer cells with the ability to obtain nutrients during scarcity, constitutes an "Achilles' heel" that we believe can be exploited by metabolic glycoengineering (MGE) strategies to develop new diagnostic methods and therapeutic approaches. In particular, we hypothesize that adaptations made by breast cancer cells that allow them to efficiently scavenge sialic acid during times of nutrient deprivation renders them vulnerable to MGE, which refers to the use of exogenously-supplied, non-natural monosaccharide analogues to modulate targeted aspects of glycosylation in living cells and animals. In specific, once non-natural sialosides are incorporated into the cancer "sialome" they can be exploited as epitopes for immunotherapy or as chemical tags for targeted delivery of imaging or therapeutic agents selectively to tumors.

Effects of Sialic Acid Modifications on Virus Binding and Infection

Sialic acids (Sias) are abundantly displayed on the surfaces of vertebrate cells, and particularly on all mucosal surfaces. Sias interact with microbes of many types, and are the targets of specific recognition by many different viruses. They may mediate virus binding and infection of cells, or alternatively can act as decoy receptors that bind virions and block virus infection. These nine-carbon backbone monosaccharides naturally occur in many different modified forms, and are attached to underlying glycans through varied linkages, creating significant diversity in the pathogen receptor forms. Here we review the current knowledge regarding the distribution of modified Sias in different vertebrate hosts, tissues, and cells, their effects on viral pathogens where those have been examined, and outline unresolved questions.

Sialic acids (Sias) are components of cell-surface glycoproteins and glycolipids, as well as secreted glycoproteins and milk oligosaccharides. Sias play important roles in cell signaling, development, and host–pathogen interactions. Cellular enzymes can modify Sias, yet how modifications vary between tissues and hosts has not been fully elucidated.

Many viruses use Sias as receptors, with different modifications aiding or inhibiting virus infection. How modified Sias influence viral protein evolution and determine host/tissue tropism are poorly understood, and are important areas of research.

New advances in molecular glycobiology using pathogen proteins to detect varied forms allows for improved study of modified Sias that have otherwise proven difficult to isolate. This opens new avenues of inquiry for virology, as well as host interactions with bacterial and eukaryotic pathogens.

Sialic acids: biomarkers in endocrinal cancers

Sialylations are post translational modification of proteins and lipids that play important role in recognition, signaling, immunological response and cell-cell interaction. Improper sialylations due to altered sialyl transferases, sialidases, gene structure and expression, sialic acid metabolism however lead to diseases and thus sialic acids form an important biomarker in disease. In the endocrinal biology such improper sialylations including altered expression of sialylated moieties have been shown to be associated with disorders. Cancer still remains to be the major cause of global death and the cancer of the endocrine organs suffer from the dearth of appropriate markers for disease prediction at the early stage and monitoring. This review is aimed at evaluating the role of sialic acids as markers in endocrinal disorders with special reference to cancer of the endocrine organs. The current study is summarized under the following headings of altered sialylations in endocrinal cancer of the (i) ovary (ii) pancreas (iii) thyroid (iv) adrenal and (v) pituitary gland. Studies in expression of sialic acid in testis cancer are limited. The future scope of this review remains in the targeting of endocrinal cancer by targeting altered sialylation which is a common expression associated with endocrinal cancer.

Metabolic glycoengineering sensitizes drug-resistant pancreatic cancer cells to tyrosine kinase inhibitors erlotinib and gefitinib

Metastatic human pancreatic cancer cells (the SW1990 line) that are resistant to the EGFR-targeting tyrosine kinase inhibitor drugs (TKI) erlotinib and gefitinib were treated with 1,3,4-O-Bu3ManNAc, a 'metabolic glycoengineering' drug candidate that increased sialylation by ∼2-fold. Consistent with genetic methods previously used to increase EGFR sialylation, this small molecule reduced EGF binding, EGFR transphosphorylation, and downstream STAT activation. Significantly, co-treatment with both the sugar pharmacophore and the existing TKI drugs resulted in strong synergy, in essence re-sensitizing the SW1990 cells to these drugs. Finally, 1,3,4-O-Bu3ManNAz, which is the azido-modified counterpart to 1,3,4-O-Bu3ManNAc, provided a similar benefit thereby establishing a broad-based foundation to extend a 'metabolic glycoengineering' approach to clinical applications.

Advanced Technologies in Sialic Acid and Sialoglycoconjugate Analysis

Although the structural diversity of sialic acid (Sia) is rapidly expanding, understanding of its biological significance has lagged behind. Advanced technologies to detect and probe diverse structures of Sia are absolutely necessary not only to understand further biological significance but also to pursue medicinal and industrial applications. Here we describe analytical methods for detection of Sia that have recently been developed or improved, with a special focus on 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac), N-glycolylneuraminic acid (Neu5Gc), deaminoneuraminic acid (Kdn), O-sulfated Sia (SiaS), and di-, oligo-, and polysialic acid (diSia/oligoSia/polySia) in glycoproteins and glycolipids. Much more attention has been paid to these Sia and sialoglycoconjugates during the last decade, in terms of regulation of the immune system, neural development and function, tumorigenesis, and aging.

Glycosylation characteristics of colorectal cancer

Glycans on proteins and lipids are known to alter with malignant transformation. The study of these may contribute to the discovery of biomarkers and treatment targets as well as understanding of cancer biology. We here describe the change of glycosylation specifically defining colorectal cancer with view on N-glycans, O-glycans, and glycosphingolipid glycans in colorectal cancer cells and tissues as well as patient sera. Glycan alterations observed in colon cancer include increased β1,6-branching and correlating higher abundance of (poly-)N-acetyllactosamine extensions of N-glycans as well as an increase in (truncated) high-mannose type glycans, while bisected structures decrease. Colorectal cancer-associated O-glycan changes are predominated by reduced expression of core 3 and 4 glycans, whereas higher levels of core 1 glycans, (sialyl) T-antigen, (sialyl) Tn-antigen, and a generally higher density of O-glycans are observed. Specific changes for glycosphingolipid glycans are lower abundances of disialylated structures as well as globo-type glycosphingolipid glycans with exception of Gb3. In general, alterations affecting all discussed glycan types are increased sialylation, fucosylation as well as (sialyl) Lewis-type antigens and type-2 chain glycans. As a consequence, interactions with glycan-binding proteins can be affected and the biological function and cellular consequences of the altered glycosylation with regard to tumorigenesis, metastasis, modulation of immunity, and resistance to antitumor therapy will be discussed. Finally, analytical approaches aiding in the field of glycomics will be reviewed with focus on binding assays and mass spectrometry.

Increasing the α 2, 6 sialylation of glycoproteins may contribute to metastatic spread and therapeutic resistance in colorectal cancer

Abnormal glycosylation due to dysregulated glycosyltransferases and glycosidases is a key phenomenon of many malignancies, including colorectal cancer (CRC). In particular, increased ST6 Gal I (β-galactoside α 2, 6 sialyltransferase) and subsequently elevated levels of cell-surface α 2, 6-linked sialic acids have been associated with metastasis and therapeutic failure in CRC. As many CRC patients experience metastasis to the liver or lung and fail to respond to curative therapies, intensive research efforts have sought to identify the molecular changes underlying CRC metastasis. ST6 Gal I has been shown to facilitate CRC metastasis, and we believe that additional investigations into the involvement of ST6 Gal I in CRC could facilitate the development of new diagnostic and therapeutic targets. This review summarizes how ST6 Gal I has been implicated in the altered expression of sialylated glycoproteins, which have been linked to CRC metastasis, radioresistance, and chemoresistance.

Investigations on aberrant glycosylation of glycosphingolipids in colorectal cancer tissues using liquid chromatography and matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF-MS)

Cancer is a leading cause of death and alterations of glycosylation are characteristic features of malignant cells. Colorectal cancer is one of the most common cancers and its exact causes and biology are not yet well understood. Here, we compared glycosylation profiles of colorectal tumor tissues and corresponding control tissues of 13 colorectal cancer patients to contribute to the understanding of this cancer. Using MALDI-TOF(/TOF)-MS and 2-dimensional LC-MS/MS we characterized enzymatically released and 2-aminobenzoic acid labeled glycans from glycosphingolipids. Multivariate data analysis revealed significant differences between tumor and corresponding control tissues. Main discriminators were obtained, which represent the overall alteration in glycosylation of glycosphingolipids during colorectal cancer progression, and these were found to be characterized by (1) increased fucosylation, (2) decreased acetylation, (3) decreased sulfation, (4) reduced expression of globo-type glycans, as well as (5) disialyl gangliosides. The findings of our current research confirm former reports, and in addition expand the knowledge of glycosphingolipid glycosylation in colorectal cancer by revealing new glycans with discriminative power and characteristic, cancer-associated glycosylation alterations. The obtained discriminating glycans can contribute to progress the discovery of biomarkers to improve diagnostics and patient treatment.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

Functions and Biosynthesis of O-Acetylated Sialic Acids

Sialic acids have a pivotal functional impact in many biological interactions such as virus attachment, cellular adhesion, regulation of proliferation, and apoptosis. A common modification of sialic acids is O-acetylation. O-Acetylated sialic acids occur in bacteria and parasites and are also receptor determinants for a number of viruses. Moreover, they have important functions in embryogenesis, development, and immunological processes. O-Acetylated sialic acids represent cancer markers, as shown for acute lymphoblastic leukemia, and they are known to play significant roles in the regulation of ganglioside-mediated apoptosis. Expression of O-acetylated sialoglycans is regulated by sialic acid-specific O-acetyltransferases and O-acetylesterases. Recent developments in the identification of the enigmatic sialic acid-specific O-acetyltransferase are discussed.

Differentially regulated expression of 9-O-acetyl GD3 (CD60b) and 7-O-acetyl-GD3 (CD60c) during differentiation and maturation of human T and B lymphocytes

GD3 (CD60a) and its 9-O-acetylated variant (CD60b) are intracellular regulators of apoptosis in T lymphocytes. Surface expressed 9-O-acetyl- and 7-O-acetyl-GD3 (CD60b and CD60c) may have a functional impact on activated T and B cells. In order to investigate the balance between surface and intracellular expression and synthesis and degradation of these glycosphingolipids in human lymphocytes of various differentiation stages, we analyzed (i) expression of GD3 molecules on native T and B cells and thymocytes by flow cytometry and (ii) activity and regulation of possible key enzymes for CD60a,b,c synthesis and degradation at the transcriptional level. Both, surface and cytoplasmic expression of CD60a and CD60c was highest in tonsillar T cells. In thymocytes, CD60c outweighs the other CD60 variants and was mainly found in the cytoplasm. All lymphocyte preparations contained sialate O-acetyltransferase activity producing 7-O-acetyl-GD3. Sialidase activity was highest in peripheral blood lymphocytes followed by thymocytes and tonsillar T and B cells. Transcription of GD3 synthase (ST8SiaI), the key enzyme for GD3 synthesis, was highest in tonsillar T cells, whereas transcriptional levels of sialidase NEU3 and O-acetylesterase H-Lse were lowest in activated T cells. This balance between enzymes of sialic acid metabolism may explain the strong overall staining intensity for all GD3 forms in T cells. Both CASD1, presumably encoding a sialic acid-specific O-acetyltransferase, and H-Lse showed highest transcription in peripheral B lymphocytes corresponding to the low expression of CD60b and c in these cells. Our data point to regulatory functions of these anabolic and catabolic key enzymes for the expression of GD3 and its O-acetylated variants in lymphocytes at a given differentiation stage.

The human Cas1 protein: a sialic acid-specific O-acetyltransferase?

Sialic acids are important sugars at the reducing end of glycoproteins and glycolipids. They are among many other functions involved in cell-cell interactions, host-pathogen recognition and the regulation of serum half-life of glycoproteins. An important modification of sialic acids is O-acetylation, which can alter or mask the biological properties of the parent sialic acid molecule. The nature of mammalian sialate-O-acetyltransferases (EC 2.3.1.45) involved in their biosynthesis is still unknown. We have identified the human CasD1 (capsule structure1 domain containing 1) gene as a candidate to encode the elusive enzyme. The human CasD1 gene encodes a protein with a serine-glycine-asparagine-histidine hydrolase domain and a hydrophobic transmembrane domain. Expression of the Cas1 protein tagged with enhanced green fluorescent protein in mammalian and insect cells directed the protein to the medial and trans-cisternae of the Golgi. Overexpression of the Cas1 protein in combination with α-N-acetyl-neuraminide α-2,8-sialyltransferase 1 (GD3 synthase) resulted in an up to 40% increased biosynthesis of 7-O-acetylated ganglioside GD3. By quantitative real-time polymerase chain reaction, we found up to 5-fold increase in CasD1 mRNA in tumor cells overexpressing O-Ac-GD3. CasD1-specific small interfering RNA reduced O-acetylation in tumor cells. These results suggest that the human Cas1 protein is directly involved in O-acetylation of α2-8-linked sialic acids.

Synthesis of Substrates for the Characterisation of Sialate-O-Acetyltransferases

The synthesis of a series of α- and β-configured sialosides using a Koenigs–Knorr methodology is described. The target compounds can serve as substrates for the investigation of biologically relevant sialate- O-acetyltransferase activity.

Is permethylation strategy always applicable to protein N-glycosylation study?: A case study on the O-acetylation of sialic acid in fish serum glycans

O-Acetylation is one of the major modifications of sialic acids that significantly alters biological properties of the parent molecule. These O-acetylated forms are components of the cellular membrane and can affect physiological and pathological responses. Understanding the role of N-glycans in physiology is of increasing relevance to cellular biologists in various disciplines who study glycoproteomics yet lack information regarding the function of the attached glycans. However, permethylation, the most common mass spectrometric analytical means, leads to the loss of O-linked acetyl groups in sialic acids. In this chapter, we demonstrated that O-acetylation of sialic acid in Atlantic salmon serum N-glycan can be well investigated by capillary electrophoresis-mass spectrometry.

Assays of sialate-O-acetyltransferases and sialate-O-acetylesterases

The O-acetylation of sialic acids is one of the most frequent modifications of these monosaccharides and modulates many cell biological and pathological events. Sialic acid-specific O-acetyltransferases and O-acetylesterases are responsible for the metabolism of esterified sialic acids. Assays were developed for the analysis of the activities and specificities of these enzymes. The methods had to be varied in dependence on the substrate assayed, the kind of biological source, and the state of enzyme purity. With the new techniques the primary site of O-acetyl incorporation at C-7, catalyzed by the animal sialate-O-acetyltransferases studied, was ascertained. Correspondingly, this enzyme, for example from bovine submandibular gland, can be denominated as AcCoA:sialate-7-O-acetyltransferase (EC 2.3.1.45). Methods for assaying the activity of esterases de-O-acetylating sialic acids and their metabolic cooperation with the O-acetyltransferases are presented.

Inhibition of Aspergillus fumigatus conidia binding to extracellular matrix proteins by sialic acids: a pH effect?

Infection by Aspergillus fumigatus, which causes the life-threatening disease invasive aspergillosis, begins with the inhalation of conidia that adhere to and germinate in the lung. Previous studies have shown that A. fumigatus conidia express high levels of the negatively charged 9-carbon sugar sialic acid, and that sialic acid appears to mediate the binding of A. fumigatus conidia to basal lamina proteins. However, despite the ability of sialic acid to inhibit adherence of A. fumigatus conidia, the exact mechanism by which this binding occurs remains unresolved. Utilizing various free sialic acids and other carbohydrates, sialic acid derivatives, sialoglycoconjugates, glycoproteins, α-keto acid related compounds and amino acids we have found that the binding of A. fumigatus conidia to type IV collagen and fibrinogen was inhibited by (i) glycoproteins (in a sialic acid-independent manner), and (ii) free sialic acids, glucuronic acid and α-keto acid related compounds. However, inhibition by the latter was found to be the result of a shift in pH from neutral (pH 7.4) to acidic (less than pH 4.6) induced by the relatively high concentrations of free sialic acids, glucuronic acid and α-keto acid related compounds used in the binding assays. This suggests that previous reports describing inhibition of A. fumigatus conidia binding by free sialic acid may actually be due to a pH shift similar to that shown here. As previously reported, we found that A. fumigatus conidia express only N-acetylneuraminic acid, the most common sialic acid found in nature. However, A. fumigatus appears to do so by an alternative mechanism to that seen in other organisms. We report here that A. fumigatus (i) does not incorporate sialic acid obtained from the environment, (ii) does not synthesize and incorporate sialic acid from exogenous N-acetylmannosamine, and (iii) lacks homologues of known sialic acid biosynthesizing enzymes.

The relevance of the aldehyde bisulfite toluidine blue reaction and its variants in the submicroscopic carbohydrate research

Carbohydrates are chemical compounds that contain only oxygen, hydrogen and carbon. They are classified by their number of sugar units: monosaccharides (such as glucose and fructose), and disaccharides (such as sucrose and lactose) are simple carbohydrates; oligosaccharides and polysaccharides (such as starch, glycogen and cellulose) are complex carbohydrates. Carbohydrates play a crucial role in diverse biological systems [Hricovín M. Structural aspects of carbohydrates and the relation with their biological properties. Curr Med Chem 2004;11:2565-83]. According to Roseman [Sugars of the cell membrane. In: Weissmann G, Clairborn E, editors. Cell membranes. Biochemistry, Cell Biology, Pathology. New York: H. P. Publ. Co; 1975. p. 55-64], two classes of glycoproteins are described. Free glycoproteins are localised in the surface coat of the membranes and form a thick mobile layer, without any association to the membrane itself. Functionally, however, they are located in a close association with the membrane (e.g. in the duodenal mucosa). The other group consists of the membrane glycoproteins, which are integral to the membranes and are located in the outer layer. The oligosaccharide chains are bound to the N-terminal part of proteins, and are situated in the hydrophilic zone. Glycoproteins have diverse functions. They are important in specific receptor functions, in immunological cell destruction and play a significant role in reactions with lectins, antibodies, as well as in cell association and mutual recognition of the cells. This paper focuses on aspects of a summary of polarisation optical investigations and biological functions of the following three groups of carbohydrates: oligosaccharides, glycoproteins and glycosaminoglycans.

Hydrophobic bile acids, genomic instability, Darwinian selection, and colon carcinogenesis

Sporadic colon cancer is caused predominantly by dietary factors. We have selected bile acids as a focus of this review since high levels of hydrophobic bile acids accompany a Western-style diet, and play a key role in colon carcinogenesis. We describe how bile acid-induced stresses cause cell death in susceptible cells, contribute to genomic instability in surviving cells, impose Darwinian selection on survivors and enhance initiation and progression to colon cancer. The most likely major mechanisms by which hydrophobic bile acids induce stresses on cells (DNA damage, endoplasmic reticulum stress, mitochondrial damage) are described. Persistent exposure of colon epithelial cells to hydrophobic bile acids can result in the activation of pro-survival stress-response pathways, and the modulation of numerous genes/proteins associated with chromosome maintenance and mitosis. The multiple mechanisms by which hydrophobic bile acids contribute to genomic instability are discussed, and include oxidative DNA damage, p53 and other mutations, micronuclei formation and aneuploidy. Since bile acids and oxidative stress decrease DNA repair proteins, an increase in DNA damage and increased genomic instability through this mechanism is also described. This review provides a mechanistic explanation for the important link between a Western-style diet and associated increased levels of colon cancer.

High level of sialate-O-acetyltransferase activity in lymphoblasts of childhood acute lymphoblastic leukaemia (ALL): enzyme characterization and correlation with disease status

Previous studies had established an over-expression of 9-O-acetylated sialoglycoproteins (Neu5,9Ac(2)-GPs) on lymphoblasts of childhood acute lymphoblastic leukaemia (ALL). Here, we report the discovery and characterization of sialate-O-acetyltransferase enzyme in ALL-cell lines and lymphoblasts from bone marrow of children diagnosed with B- and T-ALL. We observed a positive correlation between the enhanced sialate-O-acetyltransferase activity and the enhanced expression of Neu5,9Ac(2)-GPs in these lymphoblasts. Sialate-O-acetyltransferase activity in cell lysates or microsomal fractions of lymphoblasts of patients was always higher than that in healthy donors reaching up to 22-fold in microsomes. Additionally, the V (max) of this enzymatic reaction with AcCoA was over threefold higher in microsomal fractions of lymphoblasts. The enzyme bound to the microsomal fractions showed high activity with CMP-N-acetylneuraminic acid, ganglioside GD3 and endogenous sialic acid as substrates. N-acetyl-7-O-acetylneuraminic acid was the main reaction product, as detected by radio-thin-layer chromatography and fluorimetrically coupled radio-high-performance liquid chromatography. CMP and coenzyme A inhibited the microsomal enzyme. Sialate-O-acetyltransferase activity increased at the diagnosis of leukaemia, decreased with clinical remission and sharply increased again in relapsed patients as determined by radiometric-assay. A newly-developed non-radioactive ELISA can quickly detect sialate-O-acetyltransferase, and thus, may become a suitable tool for ALL-monitoring in larger scale. This is the first report on sialate-O-acetyltransferase in ALL being one of the few descriptions of an enzyme of this type in human.

Sialic acids in human health and disease

The surfaces of all vertebrate cells are decorated with a dense and complex array of sugar chains, which are mostly attached to proteins and lipids. Most soluble secreted proteins are also similarly decorated with such glycans. Sialic acids are a diverse family of sugar units with a nine-carbon backbone that are typically found attached to the outermost ends of these chains. Given their location and ubiquitous distribution, sialic acids can mediate or modulate a wide variety of physiological and pathological processes. This review considers some examples of their established and newly emerging roles in aspects of human physiology and disease.

GLYCAM06: a generalizable biomolecular force field. Carbohydrates

A new derivation of the GLYCAM06 force field, which removes its previous specificity for carbohydrates, and its dependency on the AMBER force field and parameters, is presented. All pertinent force field terms have been explicitly specified and so no default or generic parameters are employed. The new GLYCAM is no longer limited to any particular class of biomolecules, but is extendible to all molecular classes in the spirit of a small-molecule force field. The torsion terms in the present work were all derived from quantum mechanical data from a collection of minimal molecular fragments and related small molecules. For carbohydrates, there is now a single parameter set applicable to both alpha- and beta-anomers and to all monosaccharide ring sizes and conformations. We demonstrate that deriving dihedral parameters by fitting to QM data for internal rotational energy curves for representative small molecules generally leads to correct rotamer populations in molecular dynamics simulations, and that this approach removes the need for phase corrections in the dihedral terms. However, we note that there are cases where this approach is inadequate. Reported here are the basic components of the new force field as well as an illustration of its extension to carbohydrates. In addition to reproducing the gas-phase properties of an array of small test molecules, condensed-phase simulations employing GLYCAM06 are shown to reproduce rotamer populations for key small molecules and representative biopolymer building blocks in explicit water, as well as crystalline lattice properties, such as unit cell dimensions, and vibrational frequencies.

Isolation and properties of two sialate-O-acetylesterases from horse liver with 4- and 9-O-acetyl specificities

Sialate-O-acetylesterase was purified almost 900-fold from particle-free supernatants of horse liver by gel filtration, ion-exchange chromatography and isoelectric focussing. The native enzyme on gel filtration exhibits a molecular weight of 54,000 Da. It was separated by isoelectric focussing into two forms with pI values of 4.8 and 5.7, respectively. The esterase with a lower pI hydrolyses only 9-O-acetyl groups from sialic acids (K(M) 1.1 mM), while that with the higher pI esterifies both 4- and 9-O-acetylated monosaccharides at similar rates (K(M) 0.3 M and 1.3 mM, respectively). Both forms are inactive with 7-O-acetylated N-acetylneuraminic acid. Enzyme assays were carried out at the pH optimum (pH 8.4-8.6) using free O-acetylated sialic acids followed by direct analysis of the reaction products by isocratic anion-exchange HPLC. Glycosidically bound sialic acids can also be de-O-acetylated. Horse liver esterase seems to be an essential enzyme for the catabolism of 4-O-acetylated sialoglycoconjugates, since sialidase from this tissue cannot act on 4-O-acetylated sialic acids.

Properties and partial purification of sialate-O-acetyltransferase from bovine submandibular glands

The O-acetylation of sialic acids in various positions is a frequent modification of these residues in glycoproteins and glycolipids of higher animals and some bacteria. Sialic acid O-acetylation is involved in the regulation of many cell biological and pathophysiological events. Since the properties and the structural and molecular genetic aspects of the eukaryotic sialate O-acetyltransferases are not yet known, we attempted to isolate the enzyme from bovine submandibular glands. O-Acetyltransferase was solubilised from its microsomal location with a zwitterionic detergent and enriched by approximately 50-fold in three steps, including affinity chromatography on coenzyme A. It exhibits a molecular mass of 150-160 kDa. Evidence was obtained for the putative existence of a low-molecular-mass, dialysable enzyme activator. The enzyme showed best activity with CMP-N-acetylneuraminic acid (CMP-Neu5Ac), followed by N-acetylneuraminic acid (Neu5Ac). These compounds, as well as AcCoA, have high affinity for both the microsome-bound and the partially purified O-acetyltransferase. CoA is a strong inhibitor. N-Acetyl-9-O-acetylneuraminic acid was found to be the main reaction product. No evidence was obtained for the involvement of an isomerase that might be responsible for the migration of O-acetyl groups within the sialic acid side chain.

Characterization of a carbohydrate epitope defined by the monoclonal antibody H185: sialic acid O-acetylation on epithelial cell-surface mucins

Sialic acids comprise a large family of derivatives of neuraminic acid containing methyl, acetyl, sulfate, and phosphate among other groups, which confer specific physicochemical properties (e.g., hydrophobicity and resistance to hydrolases) to the molecules carrying them. Several years ago, a monoclonal antibody, designated H185, was developed, which binds to cell membranes of human corneal, conjunctival, laryngeal, and vaginal epithelia and whose distribution is altered on the ocular surface of patients with keratinizing disease. Recent findings using immunoprecipitation and immunodepletion techniques have demonstrated that, in human corneal epithelial cells, the H185 antigen is carried by the membrane-associated mucin MUC16. In this study, we show that the H185 epitope on human corneal cells and in tear fluid is an O-acetylated sialic acid epitope that can be selectively hydrolyzed in an enzyme-concentration-dependent manner by sialidase from Arthrobacter ureafaciens and to a lesser extent by sialidases from Newcastle disease virus, Clostridium perfringens, and Streptococcus pneumoniae. Binding of the H185 antibody was impaired by treatment of tear fluid with a recombinant 9-O-acetylesterase from influenza C virus. Two O-acetyl derivatives, Neu5,7Ac(2) and Neu5,9Ac(2), were identified in human tear fluid by fluorometric high-performance liquid chromatography (HPLC) and electrospray mass spectrometry (MS). Immunoprecipitation of the H185 epitope from human corneal epithelial cells revealed that Neu5,9Ac(2) was the major derivative on the mucin isolate. These results indicate that exposed wet-surfaced epithelia are decorated with O-acetyl sialic acid derivatives on membrane-associated mucins and suggest that O-acetylation on cell surfaces may protect against pathogen infection by preventing degradation of membrane-associated mucins.

Increased interferon gamma production by peripheral blood mononuclear cells in response to stimulation of overexpressed disease-specific 9-O-acetylated sialoglycoconjugates in children suffering from acute lymphoblastic leukaemia

Disease-specific over-expression of 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) on peripheral blood mononuclear cells (PBMC) of children with acute lymphoblastic leukaemia (ALL, PBMC(ALL)) has been demonstrated using a lectin, Achatinin-H, with specificity towards 9-O-AcSAalpha2-6GalNAc. This study investigated the contributory role of 9-O-AcSGs induced on PBMC(ALL). Stimulation of PBMC(ALL) with Achatinin-H through 9-O-AcSGs led to a lymphoproliferative response with a significantly increased interferon-gamma (IFN-gamma) production when compared with unstimulated cells as demonstrated by enzyme-linked immunosorbent assay and mRNA expression. Under identical conditions, PBMC(ALL) ablated of O-acetylations did not respond to such stimulation. In summary, it may be concluded that stimulation of over-expressed 9-O-AcSGs regulate signalling for proliferation, leading to the release of IFN-gamma. Controlled expression of these molecules may be exploited as potential targets for therapy, promising beneficial effects to children with ALL.

Regulation of sialic acid O-acetylation in human colon mucosa

The expression of O-acetylated sialic acids in human colonic mucins is developmentally regulated, and a reduction of O-acetylation has been found to be associated with the early stages of colorectal cancer. Despite this, however, little is known about the enzymatic process of sialic acid O-acetylation in human colonic mucosa. Recently, we have reported on a human colon sialate-7(9)-O-acetyltransferase capable of incorporating acetyl groups into sialic acids at the nucleotide-sugar level [Shen et al., Biol. Chem. 383 (2002), 307-317]. In this report, we show that the CMP-N-acetyl-neuraminic acid (CMP-Neu5Ac) and acetyl-CoA (AcCoA) transporters are critical components for the O-acetylation of CMP-Neu5Ac in Golgi lumen, with specific inhibition of either transporter leading to a reduction in the formation of CMP-5-N-acetyl-9-O-acetyl-neuraminic acid (CMP-Neu5,9Ac2). Moreover, the finding that 5-N-acetyl-9-O-acetyl-neuraminic acid (Neu5,9Ac2 could be transferred from neo-synthesised CMP-Neu5,9Ac2 to endogenous glycoproteins in the same Golgi vesicles, together with the observation that asialofetuin and asialo-human colon mucin are much better acceptors for Neu5,9Ac2 than asialo-bovine submandibular gland mucin, suggests that a sialyltransferase exists that preferentially utilises CMP-Neu5,9Ac2 as the donor substrate, transferring Neu5,9Ac2 to terminal Galbeta1,3(4)R- residues.