The roles of enteric bacterial sialidase, sialate O-acetyl esterase and glycosulfatase in the degradation of human colonic mucin

Ant may well destroy a whole dam: glycans of colonic mucus barrier disintegrated by gut bacteria

The colonic mucus layer plays a critical role in maintaining the integrity of the colonic mucosal barrier, serving as the primary defense against colonic microorganisms. Predominantly composed of mucin 2 (MUC2), a glycosylation-rich protein, the mucus layer forms a gel-like coating that covers the colonic epithelium surface. This layer provides a habitat for intestinal microorganisms, which can utilize mucin glycans present in the mucus layer as a sustainable source of nutrients. Additionally, metabolites produced by the microbiota during the metabolism of mucus glycans have a profound impact on host health. Under normal conditions, the production and consumption of mucus maintain a dynamic balance. However, several studies have demonstrated that certain factors, such as dietary fiber deficiency, can enhance the metabolism of mucus glycans by gut bacteria, thereby disturbing this balance and weakening the mucus barrier function of the mucus layer. To better understand the occurrence and development of colon-related diseases, it is crucial to investigate the complex metabolic patterns of mucus glycosylation by intestinal microorganisms. Our objective was to comprehensively review these patterns in order to clarify the effects of mucus layer glycan metabolism by intestinal microorganisms on the host.

Trait-Based Method of Quantitative Assessment of Ecological Functional Groups in the Human Intestinal Microbiome

We propose the trait-based method for quantifying the activity of functional groups in the human gut microbiome based on metatranscriptomic data. It allows one to assess structural changes in the microbial community comprised of the following functional groups: butyrate-producers, acetogens, sulfate-reducers, and mucin-decomposing bacteria. It is another way to perform a functional analysis of metatranscriptomic data by focusing on the ecological level of the community under study. To develop the method, we used published data obtained in a carefully controlled environment and from a synthetic microbial community, where the problem of ambiguity between functionality and taxonomy is absent. The developed method was validated using RNA-seq data and sequencing data of the 16S rRNA amplicon on a simplified community. Consequently, the successful verification provides prospects for the application of this method for analyzing natural communities of the human intestinal microbiota.

Increase of intestinal bacterial sialidase activity exacerbates acute colitis in mice

The availability of endogenous and dietary carbohydrates in the gastrointestinal tract influences the composition of the gut microbiota. Carbohydrate foraging requires the action of bacterially-encoded glycoside hydrolases, which release mono- and oligosaccharides taken up as carbon sources by multiple microbial taxa. In addition to providing nutrients to the microbiota, the cleavage of host glycans by bacterial glycoside hydrolases may alter the properties of surface glycoproteins involved in cell adhesion and activation processes in the gut lumen. To investigate the impact of bacterial glycoside hydrolase activities on the gut microbial composition and on host glycans during colon inflammation, we increased local glycoside hydrolase activity by supplementing mice with recombinant E. coli expressing specific sialidase, fucosidase and rhamnosidase enzymes during acute colitis induced by dextran sulfate sodium ingestion. Whereas increased fucosidase and rhamnosidase activity did not alter the course of colitis, increased sialidase activity exacerbated disease severity. The effect of increased sialidase activity on inflammation was not caused by changes in the microbial composition given that a similar shift in gut bacteria occurred in all groups of mice supplemented with recombinant E. coli. Increased sialidase activity in the colon of treated mice however significantly altered the distribution of sialic acid on mucosal glycans. Treatment of lamina propria dendritic cells with bacterial sialidase also strongly decreased the density of sialylated ligands to anti-inflammatory siglec lectins, indicating that the remodeling of surface sialylation caused by increased sialidase activity likely accounts for the observed exacerbation of acute colitis in mice.

O-acetylesterase activity of Bifidobacterium bifidum sialidase facilities the liberation of sialic acid and encourages the proliferation of sialic acid scavenging Bifidobacterium breve

Bifidobacterium bifidum possesses two extracellular sialidases (SiaBb1 and SiaBb2) that release free sialic acid from mucin sialoglycans, which can be utilized via cross-feeding by Bifidobacterium breve that, otherwise, is prevented from utilizing this nutrient source. Modification of sialic acids with O-acetyl esters is known to protect mucin glycans from degradation by bacterial sialidases. Compared to SiaBb2, SiaBb1 has an additional O-acetylesterase (Est) domain. We aimed to elucidate the role of the SiaBb1 Est domain from B. bifidum in sialic acid cross-feeding within Bifidobacterium. Pre-treatment of mucin secreted from bovine submaxillary glands (BSM) using His₆ -tagged-Est and -SiaBb2 released a higher amount of sialic acid compared to the pre-treatment by His₆ -SiaBb2. Growth of B. breve increased with an increase in nanE expression when supplemented with both His₆ -Est- and His₆ -SiaBb2-treated BSM. These results indicate that the esterase activity of the SiaBb1 Est domain enhances the efficiency of SiaBb2 to cleave sialic acid from mucin. This free sialic acid can be utilized by coexisting sialic acid scavenging B. breve via cross-feeding. Here, we provide the molecular mechanism underlying the unique sialoglycan degradation property of B. bifidum which is mediated by the complementary activities of SiaBb1 and SiaBb2 in the context of sialic acid cross-feeding.

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.

Mucin-Degrading Microbes Release Monosaccharides That Chemoattract Clostridioides difficile and Facilitate Colonization of the Human Intestinal Mucus Layer

It is widely accepted that the pathogen Clostridioides difficile exploits an intestinal environment with an altered microbiota, but the details of these microbe-microbe interactions are unclear. Adherence and colonization of mucus has been demonstrated for several enteric pathogens and it is possible that mucin-associated microbes may be working in concert with C. difficile. We showed that C. difficile ribotype-027 adheres to MUC2 glycans and using fecal bioreactors, we identified that C. difficile associates with several mucin-degrading microbes. C. difficile was found to chemotax toward intestinal mucus and its glycan components, demonstrating that C. difficile senses the mucus layer. Although C. difficile lacks the glycosyl hydrolases required to degrade mucin glycans, coculturing C. difficile with the mucin-degrading Akkermansia muciniphila, Bacteroides thetaiotaomicron, and Ruminococcus torques allowed C. difficile to grow in media that lacked glucose but contained purified MUC2. Collectively, these studies expand our knowledge on how intestinal microbes support C. difficile.

The sialyl-O-acetylesterase NanS of Tannerella forsythia encompasses two catalytic modules with different regiospecificity for O7 and O9 of sialic acid

The periodontal pathogen Tannerella forsythia utilizes host sialic acids as a nutrient source. To also make O-acetylated sialyl residues susceptible to the action of its sialidase and sialic acid up-take system, Tannerella produces NanS, an O-acetylesterase with two putative catalytic domains. Here, we analyzed NanS by homology modeling, predicted a catalytic serine-histidine-aspartate triad for each catalytic domain and performed individual domain inactivation by single alanine exchanges of the triad nucleophiles S32 and S311. Subsequent functional analyses revealed that both domains possess sialyl-O-acetylesterase activity, but differ in their regioselectivity with respect to position O9 and O7 of sialic acid. The 7-O-acetylesterase activity inherent to the C-terminal domain of NanS is unique among sialyl-O-acetylesterases and fills the current gap in tools targeting 7-O-acetylation. Application of the O7-specific variant NanS-S32A allowed us to evidence the presence of cellular 7,9-di-O-acetylated sialoglycans by monitoring the gain in 9-O-acetylation upon selective removal of acetyl groups from O7. Moreover, we established de-7,9-O-acetylation by wild-type NanS as an easy and efficient method to validate the specific binding of three viral lectins commonly used for the recognition of (7),9-O-acetylated sialoglycans. Their binding critically depends on an acetyl group in O9, yet de-7,9-O-acetylation proved advantageous over de-9-O-acetylation as the additional removal of the 7-O-acetyl group eliminated ligand formation by 7,9-ester migration. Together, our data show that NanS gained dual functionality through recruitment of two esterase modules with complementary activities. This enables Tannerella to scavenge 7,9-di-O-acetylated sialyl residues and provides a novel, O7-specific tool for studying sialic acid O-acetylation.

Fish oil supplementation enhances colon recovery after experimental colitis

INTRODUCTION

Fish oil (FO) has an anti-inflammatory and pro-resolution activity and it has been used to restore physiological disturbances on inflammatory conditions. Here, we investigate whether FO supplementation could, acutely, prevent or restore inflammatory damages on experimental colitis.

METHODS

Wistar rats orally received 2 g.kg^-1.day^-1 of FO for 30 days before induction of experimental colitis. Specimens were collected on the 2nd and 7th days after colitis-induction and intestinal mucus, inflammatory activity and colon integrity were determined.

RESULTS

Experimental colitis did cause colon disruption and FO, acutely, did not prevent the loss of intestinal and fecal mucus, neither the increase of inflammatory activity and intestinal permeability. On the 7th day of colitis, FO soften the perturbations of experimental colitis, increasing histological and fecal mucus and, also decreased inflammatory activity, but this was not accompanied by intestinal permeability.

CONCLUSION

FO did not protect, acutely, intestinal damages from experimental colitis, but at long run promotes higher intestinal recovery.

Repulsive Backbone-Backbone Interactions Modulate Access to Specific and Unspecific Binding Sites on Surface-Bound Mucins

Mucin glycoproteins are the matrix-forming key components of mucus, the innate protective barrier protecting us from pathogenic attack. However, this barrier is constantly challenged by mucin-degrading enzymes, which tend to target anionic glycan chains such as sulfate groups and sialic acid residues. Here, we demonstrate that the efficiency of both unspecific and specific binding of small molecules to mucins is reduced when sulfate groups are enzymatically removed from mucins; this is unexpected because neither of the specific mucin-binding partners tested here targets these sulfate motifs on the mucin glycoprotein. Based on simulation results obtained from a numerical model of the mucin macromolecule, we propose that anionic motifs along the mucin chain establish intramolecular repulsion forces which maintain an elongated mucin conformation. In the absence of these repulsive forces, the mucin seems to adopt a more compacted structure, in which the accessibility of several binding sites is restricted. Our results contribute to a better understanding on how different glycans contribute to the broad spectrum of functions mucin glycoproteins have.

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.

Glycan cross-feeding supports mutualism between Fusobacterium and the vaginal microbiota

Women with bacterial vaginosis (BV), an imbalance of the vaginal microbiome, are more likely to be colonized by potential pathogens such as Fusobacterium nucleatum, a bacterium linked with intrauterine infection and preterm birth. However, the conditions and mechanisms supporting pathogen colonization during vaginal dysbiosis remain obscure. We demonstrate that sialidase activity, a diagnostic feature of BV, promoted F. nucleatum foraging and growth on mammalian sialoglycans, a nutrient resource that was otherwise inaccessible because of the lack of endogenous F. nucleatum sialidase. In mice with sialidase-producing vaginal microbiotas, mutant F. nucleatum unable to consume sialic acids was impaired in vaginal colonization. These experiments in mice also led to the discovery that F. nucleatum may also “give back” to the community by reinforcing sialidase activity, a biochemical feature of human dysbiosis. Using human vaginal bacterial communities, we show that F. nucleatum supported robust outgrowth of Gardnerella vaginalis, a major sialidase producer and one of the most abundant organisms in BV. These results illustrate that mutually beneficial relationships between vaginal bacteria support pathogen colonization and may help maintain features of dysbiosis. These findings challenge the simplistic dogma that the mere absence of “healthy” lactobacilli is the sole mechanism that creates a permissive environment for pathogens during vaginal dysbiosis. Given the ubiquity of F. nucleatum in the human mouth, these studies also suggest a possible mechanism underlying links between vaginal dysbiosis and oral sex.

Bacterial mutualism involving the prominent oral bacterium Fusobacterium nucleatum may drive vaginal dysbiosis in women and could help to explain the clinical correlations between vaginal dysbiosis and oral sex.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Metabolic functions of the human gut microbiota: the role of metalloenzymes

Covering: up to the end of 2017 The human body is composed of an equal number of human and microbial cells. While the microbial community inhabiting the human gastrointestinal tract plays an essential role in host health, these organisms have also been connected to various diseases. Yet, the gut microbial functions that modulate host biology are not well established. In this review, we describe metabolic functions of the human gut microbiota that involve metalloenzymes. These activities enable gut microbial colonization, mediate interactions with the host, and impact human health and disease. We highlight cases in which enzyme characterization has advanced our understanding of the gut microbiota and examples that illustrate the diverse ways in which metalloenzymes facilitate both essential and unique functions of this community. Finally, we analyze Human Microbiome Project sequencing datasets to assess the distribution of a prominent family of metalloenzymes in human-associated microbial communities, guiding future enzyme characterization efforts.

Identification and characterization of NanH2 and NanH3, enzymes responsible for sialidase activity in the vaginal bacterium Gardnerella vaginalis

Gardnerella vaginalis is abundant in bacterial vaginosis (BV), a condition associated with adverse reproductive health. Sialidase activity is a diagnostic feature of BV and is produced by a subset of G. vaginalis strains. Although its genetic basis has not been formally identified, sialidase activity is presumed to derive from the sialidase A gene, named here nanH1 In this study, BLAST searches predicted two additional G. vaginalis sialidases, NanH2 and NanH3. When expressed in Escherichia coli, NanH2 and NanH3 both displayed broad abilities to cleave sialic acids from α2-3- and α2-6-linked N- and O-linked sialoglycans, including relevant mucosal substrates. In contrast, recombinant NanH1 had limited activity against synthetic and mucosal substrates under the conditions tested. Recombinant NanH2 was much more effective than NanH3 in cleaving sialic acids bearing a 9-O-acetyl ester. Similarly, G. vaginalis strains encoding NanH2 cleaved and foraged significantly more Neu5,9Ac₂ than strains encoding only NanH3. Among a collection of 34 G. vaginalis isolates, nanH2, nanH3, or both were present in all 15 sialidase-positive strains but absent from all 19 sialidase-negative isolates, including 16 strains that were nanH1-positive. We conclude that NanH2 and NanH3 are the primary sources of sialidase activity in G. vaginalis and that these two enzymes can account for the previously described substrate breadth cleaved by sialidases in human vaginal specimens of women with BV. Finally, PCRs of nanH2 or nanH3 from human vaginal specimens had 81% sensitivity and 78% specificity in distinguishing between Lactobacillus dominance and BV, as determined by Nugent scoring.

De-O-Acetylation of mucin-derived sialic acids by recombinant NanS-p esterases of Escherichia coli O157:H7 strain EDL933

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain EDL933 encodes the single chromosomal 9-O-acetylesterase NanS, and several copies of prophage-encoded 9-O-acetylesterases (NanS-p). These enzymes have recently been shown to cleave 5-N-acetyl-9-O-acetyl neuraminic acid (Neu5,9Ac₂) to yield de-O-acetylated Neu5Ac, the latter of which may serve as a carbon and/or nitrogen source.

In the current study, we investigated the NanS- and NanS-p-mediated digestion of synthetic O-acetylated neuraminic acids and bovine submaxillary glands mucin (BSM)-derived O-acetylneuraminic acids by high-performance thin-layer chromatography (HPTLC) and nano electrospray ionization mass spectrometry (nanoESI MS). Initial HPTLC analyses showed the expected activity of NanS and NanS-p variants for Neu5,9Ac₂. However, all tested enzymes were unable to de-O-acetylate 5-N-acetyl-4-O-acetylneuraminic acid (Neu5,4 Ac₂) in our test system. The nanoESI MS analysis of neuraminic acids after treatment of BSM with NanS-p gave evidence that NanS-p variants of EHEC O157:H7 strain EDL933 cleave off O-acetyl groups from mono-, di-, and tri-O-acetylated Neu5Ac and N-glycolylneuraminic acid (Neu5Gc), regardless of the carbon positions C7, C8 or C9 of the acetate esters. This enzyme activity leads to neuraminidase-accessible Neu5Ac and Neu5Gc on mucin glycans.

Moreover, we could demonstrate by HPTLC analyses that recombinant Bacteroides thetaiotaomicron sialidase (BTSA-His) was able to cleave Neu5Ac and Neu5,9Ac₂ from BSM and that the combination of BTSA-His with both NanS-His and NanS-p-His derivatives enhanced the release of de-O-acetylated core Neu5Ac and Neu5Gc from mammalian mucin O-glycans.

Growth experiments with EHEC wildtype strain EDL933, its nanS and nanS/nanS-p1a-p7 mutant and exogenous BTSA-His in BSM demonstrated that the presence of BTSA-His enhanced growth of EDL933 and the nanS deletion mutant but not the nanS/nanS-p1a-p7 mutant.

Thus, we hypothesize that the expression of sialic acid O-acetylesterases with a broad specificity could be an advantage in competition with the gut microbiota for nutrients and facilitate EHEC colonization in the human large intestine.

Capillary Electrophoresis Analysis of Bovine Milk Oligosaccharides Permits an Assessment of the Influence of Diet and the Discovery of Nine Abundant Sulfated Analogues

Bovine milk oligosaccharides (BMOs), like their analogues in human milk, have important prebiotic functions. Environmental factors have previously been linked to variation in BMO structures, and thus to test the hypothesis that the bovine diet may lead to these changes in relative BMO abundances, a rapid capillary electrophoresis (CE)-based work flow was developed to profile the BMOs extracted from the milk of cows fed distinctly different diets. Over the first week of lactation, few significant differences were observed between the different diet groups, with the dominant changes being clearly linked to lactation period. CE analyses indicated the presence of ten unusually anionic BMOs, which were predicted to be phosphorylated and sulfated species. Nine unique sulfated BMOs were detected by high-resolution accurate mass spectrometry, none of which have been previously described in bovine milk. The biosynthesis of these was in direct competition with 3'-sialyllactose, the most abundant BMO in bovine milk.

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.

Bifunctional properties and characterization of a novel sialidase with esterase activity from Bifidobacterium bifidum

Sialidases catalyze the removal of terminal sialic acid from various complex carbohydrates. In the gastrointestinal tract, sialic acid is commonly found in the sugar chain of mucin, and many enteric commensals use mucin as a nutrient source. We previously identified two different sialidase genes in Bifidobacterium bifidum, and one was cloned and expressed as an extracellular protein designated as exo-α-sialidase SiaBb2. The other exo-α-sialidase gene (siabb1) from the same bifidobacterium encodes an extracellular protein (SiaBb1) consisting of 1795 amino acids with a molecular mass of 189 kDa. SiaBb1 possesses a catalytic domain that classifies this enzyme as a glycoside hydrolase family 33 member. SiaBb1 preferentially hydrolyzes α2,3-linked sialic acid over α2,6-linked sialic acid from sialoglycan, which is the same as SiaBb2. However, SiaBb1 has an SGNH hydrolase domain with sialate-O-acetylesterase activity and an N-terminal signal sequence and C-terminal transmembrane region. SiaBb1 is the first bifunctional sialidase identified with esterase activity. Abbreviations: GalNAc: N-acetyl-D-galactosamine; Fuc: L-fucose; Gal: D-galactose.

Mucus-Pathogen Interactions in the Gastrointestinal Tract of Farmed Animals

Gastrointestinal infections cause significant challenges and economic losses in animal husbandry. As pathogens becoming resistant to antibiotics are a growing concern worldwide, alternative strategies to treat infections in farmed animals are necessary in order to decrease the risk to human health and increase animal health and productivity. Mucosal surfaces are the most common route used by pathogens to enter the body. The mucosal surface that lines the gastrointestinal tract is covered by a continuously secreted mucus layer that protects the epithelial surface. The mucus layer is the first barrier the pathogen must overcome for successful colonization, and is mainly composed of densely glycosylated proteins called mucins. The vast array of carbohydrate structures present on the mucins provide an important setting for host-pathogen interactions. This review summarizes the current knowledge on gastrointestinal mucins and their role during infections in farmed animals. We examine the interactions between mucins and animal pathogens, with a focus on how pathogenic bacteria can modify the mucin environment in the gut, and how this in turn affects pathogen adhesion and growth. Finally, we discuss analytical challenges and complexities of the mucus-based defense, as well as its potential to control infections in farmed animals.

The role of mucus as an invisible cloak to transepithelial drug delivery by nanoparticles

Mucosal administration of drugs and drug delivery systems has gained increasing interest. However, nanoparticles intended to protect and deliver drugs to epithelial surfaces require transport through the surface-lining mucus. Translation from bench to bedside is particularly challenging for mucosal administration since a variety of parameters will influence the specific barrier properties of the mucus including the luminal fluids, the microbiota, the mucus composition and clearance rate, and the condition of the underlying epithelia. Besides, after administration, nanoparticles interact with the mucosal components, forming a biomolecular corona that modulates their behavior and fate after mucosal administration. These interactions are greatly influenced by the nanoparticle properties, and therefore different designs and surface-engineering strategies have been proposed. Overall, it is essential to evaluate these biomolecule-nanoparticle interactions by complementary techniques using complex and relevant mucus barrier matrices.

Chemical modification of drug molecules as strategy to reduce interactions with mucus

Many drug molecules possess inadequate physical-chemical characteristics that prevent to surpass the viscous mucus layer present in the surface of mucosal tissues. Due to mucus protective role and its fast turnover, these drug molecules end up being removed from the body before being absorbed and, thus, before exerting any physiologic affect. Envisaging a better pharmacokinetics profile, chemical modifications, to render drug a more mucopenetrating character, have been introduced to drug molecules backbone towards more effective therapies. Mucus penetration increases when drug molecules are provided with net-neutral charge, when they are conjugated with mucolytic agents and through modifications that makes them resistant to enzymes present in mucus, with the overall increase of their hydrophilicity and the decrease of their molecular weight. All of these characteristics act as a whole and influence each other so they must be well thought when drug molecules are being designed for mucosal delivery.

3-Sulfogalactosyl-dependent adhesion of Escherichia coli HS multivalent adhesion molecule is attenuated by sulfatase activity

Bacterial adhesion to host receptors is an early and essential step in bacterial colonization, and the nature of adhesin–receptor interactions determines bacterial localization and thus the outcome of these interactions. Here, we determined the host receptors for the multivalent adhesion molecule (MAM) from the gut commensal Escherichia coli HS (MAM^HS), which contains an array of seven mammalian cell entry domains. The MAM^HS adhesin interacted with a range of host receptors, through recognition of a shared 3-O-sulfogalactosyl moiety. This functional group is also found in mucin, a component of the intestinal mucus layer and thus one of the prime adherence targets for commensal E. coli. Mucin gels impeded the motility of E. coli by acting as a physical barrier, and the barrier effect was enhanced by specific interactions between mucin and MAM^HS in a sulfation-dependent manner. Desulfation of mucin by pure sulfatase or the sulfatase-producing commensal Bacteroides thetaiotaomicron decreased binding of E. coli to mucin and increased the attachment of bacteria to the epithelial surface via interactions with surface-localized sulfated lipid and protein receptors. Together, our results demonstrate that the E. coli adhesin MAM^HS facilitates retention of a gut commensal by attachment to mucin. They further suggest that the amount of sulfatase secreted by mucin-foraging bacteria such as B. thetaiotaomicron, inhabiting the same niche, may affect the capacity of the mucus barrier to retain commensal E. coli.

The sialate O-acetylesterase EstA from gut Bacteroidetes species enables sialidase-mediated cross-species foraging of 9-O-acetylated sialoglycans

The gut harbors many symbiotic, commensal, and pathogenic microbes that break down and metabolize host carbohydrates. Sialic acids are prominent outermost carbohydrates on host glycoproteins called mucins and protect underlying glycan chains from enzymatic degradation. Sialidases produced by some members of the colonic microbiota can promote the expansion of several potential pathogens (e.g. Clostridium difficile, Salmonella, and Escherichia coli) that do not produce sialidases. O-Acetyl ester modifications of sialic acids help resist the action of many sialidases and are present at high levels in the mammalian colon. However, some gut bacteria, in turn, produce sialylate-O-acetylesterases to remove them. Here, we investigated O-acetyl ester removal and sialic acid degradation by Bacteroidetes sialate-O-acetylesterases and sialidases, respectively, and subsequent utilization of host sialic acids by both commensal and pathogenic E. coli strains. In vitro foraging studies demonstrated that sialidase-dependent E. coli growth on mucin is enabled by Bacteroides EstA, a sialate O-acetylesterase acting on glycosidically linked sialylate-O-acetylesterase substrates, particularly at neutral pH. Biochemical studies suggested that spontaneous migration of O-acetyl esters on the sialic acid side chain, which can occur at colonic pH, may serve as a switch controlling EstA-assisted sialic acid liberation. Specifically, EstA did not act on O-acetyl esters in their initial 7-position. However, following migration to the 9-position, glycans with O-acetyl esters became susceptible to the sequential actions of bacterial esterases and sialidases. We conclude that EstA specifically unlocks the nutritive potential of 9-O-acetylated mucus sialic acids for foraging by bacteria that otherwise are prevented from accessing this carbon source.

A Chemical Biology Solution to Problems with Studying Biologically Important but Unstable 9-O-Acetyl Sialic Acids

9-O-Acetylation is a common natural modification on sialic acids (Sias) that terminate many vertebrate glycan chains. This ester group has striking effects on many biological phenomena, including microbe-host interactions, complement action, regulation of immune responses, sialidase action, cellular apoptosis, and tumor immunology. Despite such findings, 9-O-acetyl sialoglycoconjugates have remained largely understudied, primarily because of marked lability of the 9-O-acetyl group to even small pH variations and/or the action of mammalian or microbial esterases. Our current studies involving 9-O-acetylated sialoglycans on glycan microarrays revealed that even the most careful precautions cannot ensure complete stability of the 9-O-acetyl group. We now demonstrate a simple chemical biology solution to many of these problems by substituting the oxygen atom in the ester with a nitrogen atom, resulting in sialic acids with a chemically and biologically stable 9-N-acetyl group. We present an efficient one-pot multienzyme method to synthesize a sialoglycan containing 9-acetamido-9-deoxy-N-acetylneuraminic acid (Neu5Ac9NAc) and compare it to the one with naturally occurring 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac2). Conformational resemblance of the two molecules was confirmed by computational molecular dynamics simulations. Microarray studies showed that the Neu5Ac9NAc-sialoglycan is a ligand for viruses naturally recognizing Neu5,9Ac2, with a similar affinity but with much improved stability in handling and study. Feeding of Neu5Ac9NAc or Neu5,9Ac2 to mammalian cells resulted in comparable incorporation and surface expression as well as binding to 9-O-acetyl-Sia-specific viruses. However, cells fed with Neu5Ac9NAc remained resistant to viral esterases and showed a slower turnover. This simple approach opens numerous research opportunities that have heretofore proved intractable.

Interaction of microbes with mucus and mucins: recent developments

Due to the recent rapid expansion in our understanding of the composition of the gut microflora and the consequences of altering that composition the question of how bacteria colonise mucus layers and interact with components of mucus, such as mucin, is now receiving widespread attention. Using a combination of mucus secreting cells, and a novel mucin microarray platform containing purified native mucins from different sources we recently demonstrated that two gastrointestinal pathogens, Helicobacter pylori and Campylobacter jejuni, colonise mucus by different mechanisms. This result emphasizes the potential for even closely related bacteria to interact with mucus in divergent ways to establish successful infection. Expanding the use of the mucin arrays described in the study to other microorganisms, both pathogenic and commensal, should lead to the discovery of biologically important motifs in bacterial-host interactions and complement the use of novel in vitro cell models, such as mucus secreting cell lines.

Virulent Shigella flexneri affects secretion, expression, and glycosylation of gel-forming mucins in mucus-producing cells. Infect Immun. 2013;81:3632-3643. [PMID: 23876800 DOI: 10.1128/iai.00551-13]

Mucin glycoproteins are secreted in large amounts by the intestinal epithelium and constitute an efficient component of innate immune defenses to promote homeostasis and protect against enteric pathogens. In this study, our objective was to investigate how the bacterial enteropathogen Shigella flexneri, which causes bacillary dysentery, copes with the mucin defense barrier. We report that upon in vitro infection of mucin-producing polarized human intestinal epithelial cells, virulent S. flexneri manipulates the secretion of gel-forming mucins. This phenomenon, which is triggered only by virulent strains, results in accumulation of mucins at the cell apical surface, leading to the appearance of a gel-like structure that favors access of bacteria to the cell surface and the subsequent invasion process. We identify MUC5AC, a gel-forming mucin, as a component of this structure. Formation of this gel does not depend on modifications of electrolyte concentrations, induction of trefoil factor expression, endoplasmic reticulum stress, or response to unfolded proteins. In addition, transcriptional and biochemical analyses of infected cells reveal modulations of mucin gene expression and modifications of mucin glycosylation patterns, both of which are induced by virulent bacteria in a type III secretion system-dependent manner. Thus, S. flexneri has developed a dedicated strategy to alter the mucus barrier by targeting key elements of the gel-forming capacity of mucins: gene transcription, protein glycosylation, and secretion.

Unified theory of bacterial sialometabolism: how and why bacteria metabolize host sialic acids

Sialic acids are structurally diverse nine-carbon ketosugars found mostly in humans and other animals as the terminal units on carbohydrate chains linked to proteins or lipids. The sialic acids function in cell-cell and cell-molecule interactions necessary for organismic development and homeostasis. They not only pose a barrier to microorganisms inhabiting or invading an animal mucosal surface, but also present a source of potential carbon, nitrogen, and cell wall metabolites necessary for bacterial colonization, persistence, growth, and, occasionally, disease. The explosion of microbial genomic sequencing projects reveals remarkable diversity in bacterial sialic acid metabolic potential. How bacteria exploit host sialic acids includes a surprisingly complex array of metabolic and regulatory capabilities that is just now entering a mature research stage. This paper attempts to describe the variety of bacterial sialometabolic systems by focusing on recent advances at the molecular and host-microbe-interaction levels. The hope is that this focus will provide a framework for further research that holds promise for better understanding of the metabolic interplay between bacterial growth and the host environment. An ability to modify or block this interplay has already yielded important new insights into potentially new therapeutic approaches for modifying or blocking bacterial colonization or infection.

Small intestinal goblet cell proliferation induced by ingestion of soluble and insoluble dietary fiber is characterized by an increase in sialylated mucins in rats

The study aimed to examine the effects of insoluble and soluble fibers on mucin sialylation and sulfation in the small intestine. First, diets containing soluble [konjac mannan (KM), psyllium, or guar gum; 50 g/kg) or insoluble (polystyrene foam, wheat bran, or cornhusk; 80 g/kg) fiber were fed to rats for 13 d. The fiber-fed groups had more goblet cells in the ileum than the fiber-free control group. High-iron diamine/alcian blue staining showed more sialylated mucin-producing cells in the fiber-fed groups than in the control, whereas sulfated mucin-producing cells were fewer (insoluble fibers) or unchanged (soluble fibers). Second, feeding KM (50 g/kg) and beet fiber (BF) (80 g/kg) diets for 7 d yielded a higher ileum Siat4C expression than the control, but Gal3ST2 and Gal3ST4 expression was comparable. Luminal mucin content correlated with sialic acid (r = 0.96; P < 0.001) or sulfate (r = 0.62; P < 0.01), but the slope of the sialic acid-derived equation was greater than that of the sulfate-derived equation, indicating a preferred increase in sialylated mucins. Third, rats were fed the control diet for 10 d while receiving antibiotic treatment. Analysis of the luminal mucin showed that sialylated mucins were more vulnerable to bacterial degradation than sulfated mucins. Finally, a study of bromo-deoxyuridine incorporation in rats fed a BF diet indicated that goblet cell proliferation accompanied by increased sialylated mucin appeared to be related to accelerated ileal epithelial cell migration. We conclude that intestinal goblet cell responses to insoluble and soluble fibers are characterized by increases in sialylated mucin production.

Defense and adaptation: the complex inter-relationship between Campylobacter jejuni and mucus

Mucus colonization is an essential early step toward establishing successful infection and disease by mucosal pathogens. There is an emerging literature implicating specific mucin sub-types and mucin modifications in protecting the host from Campylobacter jejuni infection. However, mucosal pathogens have evolved sophisticated mechanisms to breach the mucus layer and C. jejuni in particular appears to harbor specific adaptations to better colonize intestinal mucus. For example, components of mucus are chemotactic for C. jejuni and the rheological properties of mucus promote motility of the organism. Furthermore, recent studies demonstrate that mucins modulate the pathogenicity of C. jejuni in a species-specific manner and likely help determine whether these bacteria become pathogenic (as in humans), or adopt a commensal mode of existence (as in chickens and other animals). This review focuses on recent advances in understanding the complex interplay between C. jejuni and components of the mucus layer.

Analysing mucin degradation

Turnover of mucins in supramucosal gels is essential for the removal of surface contaminants, and the maintenance of normal mucosal barrier function. In addition to the well-known processes promoting the physical turnover of mucus gels, extracellular mucin degradation also requires the coordinated action of a range of enzyme activities including glycosidases and proteases. These are collectively termed "mucinase". Derangements of mucinase activity lead to downstream barrier defects and mucosal disease. This chapter is focussed on methods that can be used to assess the degradation of whole mucins and isolated mucin glycans. A range of approaches is described using labelled or unlabelled substrates utilised in assays based on 96-well plates, size exclusion chromatography, and NP-HPLC. These are suitable for defining the extent and progress of mucin degradation in different mucosal systems, and identifying abnormalities and critical control points.

Mucin dynamics and enteric pathogens

The extracellular secreted mucus and the cell surface glycocalyx prevent infection by the vast numbers of microorganisms that live in the healthy gut. Mucin glycoproteins are the major component of these barriers. In this Review, we describe the components of the secreted and cell surface mucosal barriers and the evidence that they form an effective barricade against potential pathogens. However, successful enteric pathogens have evolved strategies to circumvent these barriers. We discuss the interactions between enteric pathogens and mucins, and the mechanisms that these pathogens use to disrupt and avoid mucosal barriers. In addition, we describe dynamic alterations in the mucin barrier that are driven by host innate and adaptive immune responses to infection.

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.

Mining metagenomic data for novel domains: BACON, a new carbohydrate-binding module

Third-generation sequencing has given new impetus to protein sequence database growth, revealing new domains. Description and analysis of these is required to further improve the coverage and utility of domain databases. A novel domain, here named BACON, was discovered from analysis of metagenomic data obtained from gut bacteria. Domain architectures unambiguously link its function to carbohydrate metabolism but a further strong connection to protease domains suggests that many BACON domains bind glycoproteins. Conserved residues in the BACON domain are also characteristic of carbohydrate binding while its biased phyletic distribution and other data suggest mucin as a potential specific target.

YjhS (NanS) is required for Escherichia coli to grow on 9-O-acetylated N-acetylneuraminic acid

The nanATEK-yhcH, yjhATS, and yjhBC operons in Escherichia coli are coregulated by environmental N-acetylneuraminic acid, the most prevalent sialic acid in nature. Here we show that YjhS (NanS) is a probable 9-O-acetyl N-acetylneuraminic acid esterase required for E. coli to grow on this alternative sialic acid, which is commonly found in mammalian host mucosal sites.

Management of the human mucosal defensive barrier: evidence for glycan legislation

The human gastrointestinal barrier comprises several layers which enable protection against the external environment. The mucosal epithelium, lamina propria, glycocalyx and secreted mucus each make a contribution to barrier protection. Glycocalyx and secreted mucins constitute a glycosylated environment which interacts with the enteric microflora. Turnover of the mucus layer and the creation of binding ligands for bacteria are significant factors in gut homeostasis. The gut microbiota is composed of many bacterial species, but improved technology has allowed detection of populations present at different stages of development and in disease. Interaction of the microflora with the gut occurs from birth onwards and enables maturation of gut angiogenesis and glycosylation as demonstrated in mouse models. Glycan legislation regulates the ongoing interaction between the microflora and the host mucosa. This accounts for host glycosylation mechanisms providing a dynamic response to fluctuations in the gut microflora. Evidence for glycan legislation is based on a surgical model where intact mucosa can be compared with and without contact to the faecal microflora. In addition, mucosal cell glycosylation is assessed using inhibitors of O-glycan synthesis. These inhibitors lead to growth arrest in cultured colorectal cancer cell lines through the induction of apoptosis and downregulation of proliferation.

Intestinal barrier dysfunction in inflammatory bowel diseases

The etiology of human inflammatory bowel diseases (IBDs) is believed to involve inappropriate host responses to the complex commensal microbial flora in the gut, although an altered commensal flora is not completely excluded. A multifunctional cellular and secreted barrier separates the microbial flora from host tissues. Altered function of this barrier remains a major largely unexplored pathway to IBD. Although there is evidence of barrier dysfunction in IBD, it remains unclear whether this is a primary contributor to disease or a consequence of mucosal inflammation. Recent evidence from animal models demonstrating that genetic defects restricted to the epithelium can initiate intestinal inflammation in the presence of normal underlying immunity has refocused attention on epithelial dysfunction in IBD. We review the components of the secreted and cellular barrier, their regulation, including interactions with underlying innate and adaptive immunity, evidence from animal models of the barrier's role in preventing intestinal inflammation, and evidence of barrier dysfunction in both Crohn's disease and ulcerative colitis.

Dexamethasone-pDMAEMA polymeric conjugates reduce inflammatory biomarkers in human intestinal epithelial monolayers

The mucoadhesive polymer, poly(dimethylamino)ethyl methacrylate, (pDMAEMA), was synthesised by living radical polymerisation and subsequently conjugated by quaternisation reaction to a functionalised anti-inflammatory corticosteroid dexamethasone, to separately yield two conjugates with either 9:1 or 18:1 molar ratios of dexamethasone:polymer respectively. The hypothesis was to test whether the active agent maintained in vitro bioactivity when exposed to the apical side of human intestinal epithelial monolayers, Caco-2 and mucus-covered HT29-MTX-E12 (E12). HPLC analysis indicated high conjugate purity. Similar to pDMAEMA, fluorescently-labelled dexamethasone-pDMAEMA conjugates were bioadhesive to Caco-2 and mucoadhesive to E12. Apical addition of conjugates suppressed mRNA expression of the inflammatory markers, NURR1 and ICAM-1 in E12 following stimulation by PGE(2) and TNF-alpha, respectively. Conjugates also suppressed TNF-alpha stimulated cytokine secretion to the basolateral side of Caco-2 monolayers. Measurement of dexamethasone permeability from conjugates across monolayers suggested that conjugation reduced permeability compared to free dexamethasone. LDH assay indicated that conjugates were not cytotoxic to monolayers. Anti-inflammatory agents can therefore be successfully conjugated to polymers and they retain adhesion and bioactivity and have potential to be formulated for topical administration.

MUC1 and the MUCs: A Family of Human Mucins with Impact in Cancer Biology

Mucins represent a family of glycoproteins characterized by repeat domains and a dense O-glycosylation. During the last two decades, the gene and peptide structures of various mucins as well as their glycosylation states were partly elucidated. Characteristic tumor-associated alterations of the expression patterns and glycosylation profiles were observed in biochemical, immunochemical, and histological studies and are discussed in the light of efforts to use the most prominent member in this family, MUC1, as a tumor target in anti-tumor strategies. Within this context the present review, focusing on MUC1, describes recent work on the regulation of mucin biosynthesis by cytokines and hormones, the role of mucins in cell adhesion, and their interaction with the immune system. Important aspects of clinical diagnostics based on mucin antigens are discussed, including the application of tumor serum assays and the significance of numerous studies revealing correlations between the expression of peptide cores or mucin-associated carbohydrates and clinicopathological parameters like tumor progression and prognosis.

Thickness and continuity of the adherent colonic mucus barrier in active and quiescent ulcerative colitis and Crohn's disease

BACKGROUND

The colon is covered by a mucus barrier that protects the underlying mucosa and alterations in this mucus barrier have been implicated in the aetiology of inflammatory bowel disease (IBD). This study investigated the thickness and continuity of the mucus barrier in ulcerative colitis (UC) and Crohn's disease (CD) in comparison to normal controls.

METHODS

Rectal biopsies were taken from 59 patients and cryostat sections stained with periodic acid-Schiff's/Alcian blue to visualise the mucus layer. Mucus thickness and continuity and goblet cell density were measured using light microscopy.

RESULTS

An essentially continuous adherent mucus layer was observed in normal human rectum and there was no change in the mucus barrier in quiescent UC. In active UC there was a trend for the mucus layer to become progressively thinner and significantly more discontinuous as disease severity increased. In severe active UC the mucus layer thickness and goblet cell density were significantly reduced compared with normal controls while the percentage discontinuity significantly increased.

CONCLUSION

It is not until severe UC that there is a global change in mucosal protection as a consequence of large regions lacking mucus, a decrease in secretory potential caused by a loss of goblet cells and a thinner, less effective mucus layer even when it is present.

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.

Mucins in the mucosal barrier to infection

The mucosal tissues of the gastrointestinal, respiratory, reproductive, and urinary tracts, and the surface of the eye present an enormous surface area to the exterior environment. All of these tissues are covered with resident microbial flora, which vary considerably in composition and complexity. Mucosal tissues represent the site of infection or route of access for the majority of viruses, bacteria, yeast, protozoa, and multicellular parasites that cause human disease. Mucin glycoproteins are secreted in large quantities by mucosal epithelia, and cell surface mucins are a prominent feature of the apical glycocalyx of all mucosal epithelia. In this review, we highlight the central role played by mucins in accommodating the resident commensal flora and limiting infectious disease, interplay between underlying innate and adaptive immunity and mucins, and the strategies used by successful mucosal pathogens to subvert or avoid the mucin barrier, with a particular focus on bacteria.