Structural and functional studies of Streptococcus pneumoniae neuraminidase B: An intramolecular trans-sialidase

Carbohydrate binding modules: Compact yet potent accessories in the specific substrate binding and performance evolution of carbohydrate-active enzymes

Carbohydrate binding modules (CBMs) are independent non-catalytic domains widely found in carbohydrate-active enzymes (CAZymes), and they play an essential role in the substrate binding process of CAZymes by guiding the appended catalytic modules to the target substrates. Owing to their precise recognition and selective affinity for different substrates, CBMs have received increasing research attention over the past few decades. To date, CBMs from different origins have formed a large number of families that show a variety of substrate types, structural features, and ligand recognition mechanisms. Moreover, through the modification of specific sites of CBMs and the fusion of heterologous CBMs with catalytic domains, improved enzymatic properties and catalytic patterns of numerous CAZymes have been achieved. Based on cutting-edge technologies in computational biology, gene editing, and protein engineering, CBMs as auxiliary components have become portable and efficient tools for the evolution and application of CAZymes. With the aim to provide a theoretical reference for the functional research, rational design, and targeted utilization of novel CBMs in the future, we systematically reviewed the function-related characteristics and potentials of CAZyme-derived CBMs in this review, including substrate recognition and binding mechanisms, non-catalytic contributions to enzyme performances, module modifications, and innovative applications in various fields.

Functional and structural analyses reveal that a dual domain sialidase protects bacteria from complement killing through desialylation of complement factors

The complement system is the first line of innate immune defense against microbial infections. To survive in humans and cause infections, bacterial pathogens have developed sophisticated mechanisms to subvert the complement-mediated bactericidal activity. There are reports that sialidases, also known as neuraminidases, are implicated in bacterial complement resistance; however, its underlying molecular mechanism remains elusive. Several complement proteins (e.g., C1q, C4, and C5) and regulators (e.g., factor H and C4bp) are modified by various sialoglycans (glycans with terminal sialic acids), which are essential for their functions. This report provides both functional and structural evidence that bacterial sialidases can disarm the complement system via desialylating key complement proteins and regulators. The oral bacterium Porphyromonas gingivalis, a "keystone" pathogen of periodontitis, produces a dual domain sialidase (PG0352). Biochemical analyses reveal that PG0352 can desialylate human serum and complement factors and thus protect bacteria from serum killing. Structural analyses show that PG0352 contains a N-terminal carbohydrate-binding module (CBM) and a C-terminal sialidase domain that exhibits a canonical six-bladed β-propeller sialidase fold with each blade composed of 3-4 antiparallel β-strands. Follow-up functional studies show that PG0352 forms monomers and is active in a broad range of pH. While PG0352 can remove both N-acetylneuraminic acid (Neu5Ac) and N-glycolyl-neuraminic acid (Neu5Gc), it has a higher affinity to Neu5Ac, the most abundant sialic acid in humans. Structural and functional analyses further demonstrate that the CBM binds to carbohydrates and serum glycoproteins. The results shown in this report provide new insights into understanding the role of sialidases in bacterial virulence and open a new avenue to investigate the molecular mechanisms of bacterial complement resistance.

Absolute Affinities from Quantitative Shotgun Glycomics Using Concentration-Independent (COIN) Native Mass Spectrometry

Native mass spectrometry (nMS) screening of natural glycan libraries against glycan-binding proteins (GBPs) is a powerful tool for ligand discovery. However, as the glycan concentrations are unknown, affinities cannot be measured directly from natural libraries. Here, we introduce Concentration-Independent (COIN)-nMS, which enables quantitative screening of natural glycan libraries by exploiting slow mixing of solutions inside a nanoflow electrospray ionization emitter. The affinities (K_d) of detected GBP-glycan interactions are determined, simultaneously, from nMS analysis of their time-dependent relative abundance changes. We establish the reliability of COIN-nMS using interactions between purified glycans and GBPs with known K_d values. We also demonstrate the implementation of COIN-nMS using the catch-and-release (CaR)-nMS assay for glycosylated GBPs. The COIN-CaR-nMS results obtained for plant, fungal, viral, and human lectins with natural libraries containing hundreds of N-glycans and glycopeptides highlight the assay's versatility for discovering new ligands, precisely measuring their affinities, and uncovering "fine" specificities. Notably, the COIN-CaR-nMS results clarify the sialoglycan binding properties of the SARS-CoV-2 receptor binding domain and establish the recognition of monosialylated hybrid and biantennary N-glycans. Moreover, pharmacological depletion of host complex N-glycans reduces both pseudotyped virions and SARS-CoV-2 cell entry, suggesting that complex N-glycans may serve as attachment factors.

Role of Streptococcus pneumoniae extracellular glycosidases in immune evasion

Streptococcus pneumoniae (pneumococcus) typically colonizes the human upper airway asymptomatically but upon reaching other sites of the host body can cause an array of diseases such as pneumonia, bacteremia, otitis media, and meningitis. Be it colonization or progression to disease state, pneumococcus faces multiple challenges posed by host immunity ranging from complement mediated killing to inflammation driven recruitment of bactericidal cells for the containment of the pathogen. Pneumococcus has evolved several mechanisms to evade the host inflicted immune attack. The major pneumococcal virulence factor, the polysaccharide capsule helps protect the bacteria from complement mediated opsonophagocytic killing. Another important group of pneumococcal proteins which help bacteria to establish and thrive in the host environment is surface associated glycosidases. These enzymes can hydrolyze host glycans on glycoproteins, glycolipids, and glycosaminoglycans and consequently help bacteria acquire carbohydrates for growth. Many of these glycosidases directly or indirectly facilitate bacterial adherence and are known to modulate the function of host defense/immune proteins likely by removing glycans and thereby affecting their stability and/or function. Furthermore, these enzymes are known to contribute the formation of biofilms, the bacterial communities inherently resilient to antimicrobials and host immune attack. In this review, we summarize the role of these enzymes in host immune evasion.

Influenza virus and pneumococcal neuraminidases enhance catalysis by similar yet distinct sialic acid-binding strategies

Influenza A viruses and the bacterium Streptococcus pneumoniae (pneumococci) both express neuraminidases that catalyze release of sialic acid residues from oligosaccharides and glycoproteins. Although these respiratory pathogen neuraminidases function in a similar environment, it remains unclear if these enzymes use similar mechanisms for sialic acid cleavage. Here, we compared the enzymatic properties of neuraminidases from two influenza A subtypes (N1 and N2) and the pneumococcal strain TIGR4 (NanA, NanB, and NanC). Insect cell-produced N1 and N2 tetramers exhibited calcium-dependent activities and stabilities that varied with pH. In contrast, E. coli-produced NanA, NanB, and NanC were isolated as calcium insensitive monomers with stabilities that were more resistant to pH changes. Using a synthetic substrate (MUNANA), all neuraminidases showed similar pH optimums (pH 6-7) that were primarily defined by changes in catalytic rate rather than substrate binding affinity. Upon using a multivalent substrate (fetuin sialoglycans), much higher specific activities were observed for pneumococcal neuraminidases that contain an additional lectin domain. In virions, N1 and especially N2 also showed enhanced specific activity toward fetuin that was lost upon the addition of detergent, indicating the sialic acid-binding capacity of neighboring hemagglutinin molecules likely contributes to catalysis of natural multivalent substrates. These results demonstrate that influenza and pneumococcal neuraminidases have evolved similar yet distinct strategies to optimize their catalytic activity.

Co-infection of the respiratory epithelium, scene of complex functional interactions between viral, bacterial, and human neuraminidases

The activity of sialic acids, known to play critical roles in biology and many pathological processes, is finely regulated by a class of enzymes called sialidases, also known as neuraminidases. These are present in mammals and many other biological systems, such as viruses and bacteria. This review focuses on the very particular situation of co-infections of the respiratory epithelium, the scene of complex functional interactions between viral, bacterial, and human neuraminidases. This intrinsically multidisciplinary topic combining structural biology, biochemistry, physiology, and the study of host-pathogen interactions, opens up exciting research perspectives that could lead to a better understanding of the mechanisms underlying virus-bacteria co-infections and their contribution to the aggravation of respiratory pathology, notably in the context of pre-existing pathological contexts. Strategies that mimic or inhibit the activity of the neuraminidases could constitute interesting treatment options for viral and bacterial infections.

Crystal structure of the Propionibacterium acnes surface sialidase, a drug target for P. acnes-associated diseases

Propionibacterium acnes, though generally considered part of the normal flora of human skin, is an opportunistic pathogen associated with acne vulgaris as well as other diseases, including endocarditis, endophthalmitis and prosthetic joint infections. Its virulence potential is also supported by knowledge gained from its sequenced genome. Indeed, a vaccine targeting a putative cell wall-anchored P. acnes sialidase has been shown to suppress cytotoxicity and pro-inflammatory cytokine release induced by the organism, and is proposed as an alternative treatment for P. acnes-associated diseases. Here, we report the crystal structures of the surface sialidase and its complex with the transition-state mimic Neu5Ac2en. Our structural and kinetic analyses, together with insight from a glycan array screen, which probes subtle specificities of the sialidase for α-2,3-sialosides, provide a basis for the structure-based design of novel small-molecule therapeutics against P. acnes infections.

Sialic acid removal by trans-sialidase modulates MMP-2 activity during Trypanosoma cruzi infection

Matrix metalloproteinases (MMPs) not only play a relevant role in homeostatic processes but are also involved in several pathological mechanisms associated with infectious diseases. As their clinical relevance in Chagas disease has recently been highlighted, we studied the modulation of circulating MMPs by Trypanosoma cruzi infection. We found that virulent parasites from Discrete Typing Units (DTU) VI induced higher proMMP-2 and MMP-2 activity in blood, whereas both low (DTU I) and high virulence parasites induced a significant decrease in proMMP-9 plasma activity. Moreover, trans-sialidase, a relevant T. cruzi virulence factor, is involved in MMP-2 activity modulation both in vivo and in vitro. It removes α2,3-linked sialyl residues from cell surface glycoconjugates, which then triggers the PKC/MEK/ERK signaling pathway. Additionally, bacterial sialidases specific for this sialyl residue linkage displayed similar MMP modulation profiles and triggered the same signaling pathways. This novel pathogenic mechanism, dependent on sialic acid removal by the neuraminidase activity of trans-sialidase, can be exploited by different pathogens expressing sialidases with similar specificity. Thus, here we present a new pathogen strategy through the regulation of the MMP network.

Diversity of sialidases found in the human body - A review

Sialidases are enzymes essential for numerous organisms including humans. Hydrolytic sialidases (EC 3.2.1.18), trans-sialidases and anhydrosialidases (intramolecular trans-sialidases, EC 4.2.2.15) are glycoside hydrolase enzymes that cleave the glycosidic linkage and release sialic acid residues from sialyl substrates. The paper summarizes diverse sialidases present in the human body and their potential impact on development of antiviral compounds - inhibitors of viral neuraminidases. It includes a brief overview of catalytic mechanisms of action of sialidases and describes the origin of sialidases in the human body. This is followed by description of the structure and function of sialidase families with a special focus on the GH33 and GH34 families. Various effects of sialidases on human body are also briefly described. Modulation of sialidase activity may be considered a useful tool for effective treatment of various diseases. In some cases, it is desired to completely suppress the activity of sialidases by suitable inhibitors. Specific sialidase inhibitors are useful for the treatment of influenza, epilepsy, Alzheimer's disease, diabetes, different types of cancer, or heart defects. Challenges and future directions are shortly depicted in the final part of the paper.

Assessing the function of pneumococcal neuraminidases NanA, NanB and NanC in in vitro and in vivo lung infection models using monoclonal antibodies

Streptococcus pneumoniae isolates express up to three neuraminidases (sialidases), NanA, NanB and NanC, all of which cleave the terminal sialic acid of glycan-structures that decorate host cell surfaces. Most research has focused on the role of NanA with limited investigations evaluating the roles of all three neuraminidases in host-pathogen interactions. We generated two highly potent monoclonal antibodies (mAbs), one that blocks the enzymatic activity of NanA and one cross-neutralizing NanB and NanC. Total neuraminidase activity of clinical S. pneumoniae isolates could be inhibited by this mAb combination in enzymatic assays. To detect desialylation of cell surfaces by pneumococcal neuraminidases, primary human tracheal/bronchial mucocilial epithelial tissues were infected with S. pneumoniae and stained with peanut lectin. Simultaneous targeting of the neuraminidases was required to prevent desialylation, suggesting that inhibition of NanA alone is not sufficient to preserve terminal lung glycans. Importantly, we also found that all three neuraminidases increased the interaction of S. pneumoniae with human airway epithelial cells. Lectin-staining of lung tissues of mice pre-treated with mAbs before intranasal challenge with S. pneumoniae confirmed that both anti-NanA and anti-NanBC mAbs were required to effectively block desialylation of the respiratory epithelium in vivo. Despite this, no effect on survival, reduction in pulmonary bacterial load, or significant changes in cytokine responses were observed. This suggests that neuraminidases have no pivotal role in this murine pneumonia model that is induced by high bacterial challenge inocula and does not progress from colonization as it happens in the human host.

Comparative studies of catalytic pathways for Streptococcus pneumoniae sialidases NanA, NanB and NanC

Streptococcus pneumoniae (S. pneumoniae) is a leading human pathogen, which takes large responsibility for severe otitis media, acute meningitis and septicaemia. It encodes up to three distinct sialidases: NanA, NanB and NanC, which are promising drug targets. Recent experimental studies have shown that these three sialidases might work together up to the ultimate step, where NanA and NanB produce N-acetylneuraminic acid (Neu5Ac) and 2,7-anhydro-Neu5Ac following the functions of sialidase and intramolecular trans-sialidase, whilst NanC carries on a ping-pong mechanism that produces or removes 2-deoxy-2,3-didehydro-Neu5AC. It is intriguing that these sialidases have similar active sites but operate via three distinct reaction pathways. To clarify this issue, herein we present the first systematic computational investigation on the catalytic pathways for S. pneumoniae NanA, NanB and NanC based on combined quantum mechanics/molecular mechanics simulations, and propose the most preferred routes for the three S. pneumoniae sialidases. Our findings support the mechanisms of NanA and NanC that were proposed by previous experimental studies, whereas the role of water in NanB was found to differ slightly from our current understandings. The mechanistic insights obtained from this work are expected to assist in the design of potent inhibitors targeting these key enzymes for therapeutic applications.

Design, synthesis, and evaluation of curcumin analogues as potential inhibitors of bacterial sialidase

Sialidases are key virulence factors that remove sialic acid from the host cell surface glycan, unmasking receptors that facilitate bacterial adherence and colonisation. In this study, we developed potential agents for treating bacterial infections caused by Streptococcus pneumoniae Nan A that inhibit bacterial sialidase using Turmeric and curcumin analogues. Design, synthesis, and structure analysis relationship (SAR) studies have been also described. Evaluation of the synthesised derivatives demonstrated that compound 5e was the most potent inhibitor of S. pneumoniae sialidase (IC50 = 0.2 ± 0.1 µM). This compound exhibited a 3.0-fold improvement in inhibitory activity over that of curcumin and displayed competitive inhibition. These results warrant further studies confirming the antipneumococcal activity 5e and indicated that curcumin derivatives could be potentially used to treat sepsis by bacterial infections.

Exploration of the Sialic Acid World

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

Applications of a highly α2,6-selective pseudosialidase

Within human biology, combinations of regioisomeric motifs of α2,6- or α2,3-sialic acids linked to galactose are frequently observed attached to glycoconjugates. These include glycoproteins and glycolipids, with each linkage carrying distinct biological information and function. Microbial linkage-specific sialidases have become important tools for studying the role of these sialosides in complex biological settings, as well as being used as biocatalysts for glycoengineering. However, currently, there is no α2,6-specific sialidase available. This gap has been addressed herein by exploiting the ability of a Photobacterium sp. α2,6-sialyltransferase to catalyze trans-sialidation reversibly and in a highly linkage-specific manner, acting as a pseudosialidase in the presence of cytidine monophosphate. Selective, near quantitative removal of α2,6-linked sialic acids was achieved from a wide range of sialosides including small molecules conjugates, simple glycan, glycopeptide and finally complex glycoprotein including both linkages.

Catalytic mechanism of type C sialidase from Streptococcus pneumoniae: from covalent intermediate to final product

Streptococcus pneumoniae is a Gram-positive human pathogenic bacterium, which is the main cause of pneumonia and meningitis in children and the elderly. Three sialidases (or neuraminidases) encoded from Streptococcus pneumoniae could catalyze the cleavage of sialic acid linkages. This mechanism is directly connected with infection, apoptosis, and signaling, and usually considered to be one of the critical virulence factors. Type C neuraminidase (NanC) is unique because its primary product of Neu5Ac2en is considered to be an inhibitor to the other two sialidases. Experimentally, there are two different pathways for the formation mechanism of Neu5Ac2en catalyzed by NanC. In this work, a combined quantum mechanical and molecular mechanical approach was employed in all calculations. Starting from the covalent sialylated intermediate, we first examined the reaction to Neu5Ac2en and found the reaction prefers a direct proton abstraction mechanism rather than the water mediated proton abstraction mechanism. Free energy profiles can confirm that Neu5Ac2en is the major product of NanC. Functional roles of some important residues were also investigated, e.g., D315 acts as the proton acceptor during the formation of Neu5Ac2en, while the general base for the hydrolytic reaction to Neu5Ac. This study can facilitate the understanding of the catalytic mechanism of NanC and has the potential to aid in future inhibitor design studies.

Streptococcus pneumoniae Sialidase SpNanB-Catalyzed One-Pot Multienzyme (OPME) Synthesis of 2,7-Anhydro-Sialic Acids as Selective Sialidase Inhibitors

Streptococcus pneumoniae sialidase SpNanB is an intramolecular trans-sialidase (IT-sialidase) and a virulence factor that is essential for streptococcal infection of the upper and lower respiratory tract. SpNanB catalyzes the formation of 2,7-anhydro- N-acetylneuraminic acid (2,7-anhydro-Neu5Ac), a potential prebiotic that can be used as the sole carbon source of a common human gut commensal anaerobic bacterium. We report here the development of an efficient one-pot multienzyme (OPME) system for synthesizing 2,7-anhydro-Neu5Ac and its derivatives. Based on a crystal structure analysis, an N-cyclohexyl derivative of 2,7-anhydro-neuraminic acid was designed, synthesized, and shown to be a selective inhibitor against SpNanB and another Streptococcus pneumoniae sialidase SpNanC. This study demonstrates a new strategy of synthesizing 2,7-anhydro-sialic acids in a gram scale and the potential application of their derivatives as selective sialidase inhibitors.

Immunization with pneumococcal neuraminidases NanA, NanB and NanC to generate neutralizing antibodies and to increase survival in mice

Purpose. Pneumococcal virulence protein-based vaccines can provide serotype-independent protection against pneumococcal infections. Many studies, including clinical observational studies on Thomsen-Friedenrich antigen exposure and haemolytic uremic syndrome, defined the role of neuraminidases NanA, NanB and NanC in host-pneumococcus interaction. Since neuraminidases are major virulence proteins, they are potential targets for both vaccines and small molecule inhibitors. Here we explored the utility of three neuraminidases as protein vaccine antigens to generate neutralizing antibodies and to increase survival following pneumococcal infections.Methodology. Rabbits and mice were immunized subcutaneously with enzymatically active recombinant NanA, NanB and NanC as individual or a combination of the three neuraminidases. Antisera titres were determined by ELISA. Neuraminidase activity inhibition by antiserum was tested by peanut lectin and flow cytometry. Clinical isolates with serotype 3, 6B, 14, 15B, 19A and 23F were used to infect immunized mice by tail vein injection.Results/Key findings. Presence of high levels of IgG antibodies in antisera against NanA, NanB and NanC indicates that all of the three neuraminidases are immunogenic vaccine antigens. To generate potent NanA neutralizing antibodies, both lectin and catalytic domains are essential, whereas for NanB and NanC a single lectin domain is sufficient. Immunization with triple neuraminidases increased the survival of mice when intravenously challenged with clinical isolates of serotype 3 (40 %), 6B (60 %), 15B (60 %), 19A (40 %) and 23F (30 %).Conclusion. We recommend the inclusion of three pneumococcal neuraminidases in future protein vaccine formulations to prevent invasive pneumococcal infection caused by various serotypes.

Glycan-metabolizing enzymes in microbe-host interactions: the Streptococcus pneumoniae paradigm

Streptococcus pneumoniae is a frequent colonizer of the upper airways; however, it is also an accomplished pathogen capable of causing life-threatening diseases. To colonize and cause invasive disease, this bacterium relies on a complex array of factors to mediate the host-bacterium interaction. The respiratory tract is rich in functionally important glycoconjugates that display a vast range of glycans, and, thus, a key component of the pneumococcus-host interaction involves an arsenal of bacterial carbohydrate-active enzymes to depolymerize these glycans and carbohydrate transporters to import the products. Through the destruction of host glycans, the glycan-specific metabolic machinery deployed by S. pneumoniae plays a variety of roles in the host-pathogen interaction. Here, we review the processing and metabolism of the major host-derived glycans, including N- and O-linked glycans, Lewis and blood group antigens, proteoglycans, and glycogen, as well as some dietary glycans. We discuss the role of these metabolic pathways in the S. pneumoniae-host interaction, speculate on the potential of key enzymes within these pathways as therapeutic targets, and relate S. pneumoniae as a model system to glycan processing in other microbial pathogens.

Evidence for a carbohydrate-binding module (CBM) of Tannerella forsythia NanH sialidase, key to interactions at the host–pathogen interface

Bacterial sialidases cleave terminal sialic acid from a variety of host glycoproteins, and contribute to survival and growth of many human-dwelling bacterial species, including various pathogens. Tannerella forsythia, an oral, Gram-negative, fastidious anaerobe, is a key organism in periodontal disease and possesses a dedicated sialic acid utilisation and scavenging (nan) operon, including NanH sialidase. Here, we describe biochemical characterisation of recombinant NanH, including its action on host-relevant sialoglycans such as sialyl Lewis A and sialyl Lewis X (SLeA/X), and on human cell-attached sialic acids directly, uncovering that it is a highly active broad specificity sialidase. Furthermore, the N-terminal domain of NanH was hypothesised and proved to be capable of binding to a range of sialoglycans and non-sialylated derivatives with Kd in the micromolar range, as determined by steady-state tryptophan fluorescence spectroscopy, but it has no catalytic activity in isolation from the active site. We consider this domain to represent the founding member of a novel subfamily of carbohydrate-binding module (CBM), involved in glycosidase-ligand binding. In addition, we created a catalytically inactive version of the NanH enzyme (FRIP → YMAP) that retained its ability to bind sialic acid-containing ligands and revealed for the first time that binding activity of a CBM is enhanced by association with the catalytic domain. Finally, we investigated the importance of Lewis-type sialoglycans on T. forsythia–host interactions, showing that nanomolar amounts of SLeA/X were capable of reducing invasion of oral epithelial cells by T. forsythia, suggesting that these are key ligands for bacterial–cellular interactions during periodontal disease.

Sialidases from gut bacteria: a mini-review

Sialidases are a large group of enzymes, the majority of which catalyses the cleavage of terminal sialic acids from complex carbohydrates on glycoproteins or glycolipids. In the gastrointestinal (GI) tract, sialic acid residues are mostly found in terminal location of mucins via α2-3/6 glycosidic linkages. Many enteric commensal and pathogenic bacteria can utilize sialic acids as a nutrient source, but not all express the sialidases that are required to release free sialic acid. Sialidases encoded by gut bacteria vary in terms of their substrate specificity and their enzymatic reaction. Most are hydrolytic sialidases, which release free sialic acid from sialylated substrates. However, there are also examples with transglycosylation activities. Recently, a third class of sialidases, intramolecular trans-sialidase (IT-sialidase), has been discovered in gut microbiota, releasing (2,7-anhydro-Neu5Ac) 2,7-anydro-N-acetylneuraminic acid instead of sialic acid. Reaction specificity varies, with hydrolytic sialidases demonstrating broad activity against α2,3-, α2,6- and α2,8-linked substrates, whereas IT-sialidases tend to be specific for α2,3-linked substrates. In this mini-review, we summarize the current knowledge on the structural and biochemical properties of sialidases involved in the interaction between gut bacteria and epithelial surfaces.

Defining the role of pneumococcal neuraminidases and O-glycosidase in pneumococcal haemolytic uraemic syndrome

The host and bacterial factors that lead to development of pneumococcal haemolytic uraemic syndrome (pHUS) remain poorly defined; however, it is widely believed that pneumococcal exposure of the Thomsen-Friedenreich antigen (T-antigen) on host surfaces is a key step in pathogenesis. Two enzymatic activities encoded by pneumococci determine the level of T-antigen exposed. Neuraminidases cleave terminal sialic acid to expose the T-antigen which is subsequently cleaved by O-glycosidase Eng. While a handful of studies have examined the role of neuraminidases in T-antigen exposure, no studies have addressed the potential role of O-glycosidase. This study used 29 pHUS isolates from the USA and 31 serotype-matched controls. All isolates contained eng, and no significant correlation between enzymatic activity and disease state (pHUS and blood non-pHUS isolates) was observed. A prior study from Taiwan suggested that neuraminidase NanC contributes to the development of pHUS. However, we observed no difference in nanC distribution. Similar to previously published data, we found no significant correlation between neuraminidase activity and disease state. Accurate quantification of these enzymatic activities from bacteria grown in whole blood is currently impossible, but we confirmed that there were no significant correlations between disease state and neuraminidase and O-glycosidase transcript levels after incubation in blood. Genomic sequencing of six pHUS isolates did not identify any genetic elements possibly contributing to haemolytic uraemic syndrome. These findings support the hypothesis that while exposure of T-antigen may be an important step in disease pathogenesis, host factors likely play a substantial role in determining which individuals develop haemolytic uraemic syndrome after pneumococcal invasive disease.

Pneumococcal Neuraminidase Substrates Identified through Comparative Proteomics Enabled by Chemoselective Labeling

Neuraminidases (sialidases) are enzymes that hydrolytically remove sialic acid from sialylated proteins and lipids. Neuraminidases are encoded by a range of human pathogens, including bacteria, viruses, fungi, and protozoa. Many pathogen neuraminidases are virulence factors, indicating that desialylation of host glycoconjugates can be a critical step in infection. Specifically, desialylation of host cell surface glycoproteins can enable these molecules to function as pathogen receptors or can alter signaling through the plasma membrane. Despite these critical effects, no unbiased approaches exist to identify glycoprotein substrates of neuraminidases. Here, we combine previously reported glycoproteomics methods with quantitative proteomics analysis to identify glycoproteins whose sialylation changes in response to neuraminidase treatment. The two glycoproteomics methods-periodate oxidation and aniline-catalyzed oxime ligation (PAL) and galactose oxidase and aniline-catalyzed oxime ligation (GAL)-rely on chemoselective labeling of sialylated and nonsialylated glycoproteins, respectively. We demonstrated the utility of the combined approaches by identifying substrates of two pneumococcal neuraminidases in a human cell line that models the blood-brain barrier. The methods deliver complementary lists of neuraminidase substrates, with GAL identifying a larger number of substrates than PAL (77 versus 17). Putative neuraminidase substrates were confirmed by other methods, establishing the validity of the approach. Among the identified substrates were host glycoproteins known to function in bacteria adherence and infection. Functional assays suggest that multiple desialylated cell surface glycoproteins may act together as pneumococcus receptors. Overall, this method will provide a powerful approach to identify glycoproteins that are desialylated by both purified neuraminidases and intact pathogens.

Dual Acting Neuraminidase Inhibitors Open New Opportunities to Disrupt the Lethal Synergism between Streptococcus pneumoniae and Influenza Virus

Secondary infections with Streptococcus pneumoniae cause severe pneumonia and enhance lethality during influenza epidemics and pandemics. Structural and functional similarities with viral neuraminidase (NA) suggest that the highly prevalent pneumococcal NAs, NanA and NanB, might contribute to this lethal synergism by supporting viral replication and that dual acting NA inhibitors (NAIs) will disrupt it. To verify this hypothesis, NanA and NanB were expressed in E. coli. After confirming their activity in enzyme assays, in vitro models with influenza virus A/Jena/8178/09 (Jena/8178) and the recombinant NanA or NanB (rNanA and rNanB) were established in A549 and MDCK cells to mimic the role of these pneumococcal NAs during co-infection. Studies on the influence of both NAs on viral receptor expression, spread, and yield revealed a distinct effect of NanA and NanB on viral replication in these in vitro models. Both enzymes were able to support Jena/8178 replication at certain concentrations. This synergism was disrupted by the NAIs oseltamivir, DANA, katsumadain A, and artocarpin exerting an inhibitory effect on viral NA and NanA. Interestingly, katsumadain A and artocarpin inhibited rNanA and rNanB similarly. Zanamivir did not show activity. These results demonstrate a key role of pneumococcal NAs in the lethal synergism with influenza viruses and reveal opportunities for its effective disruption.

Enhanced Cross-Linking of Diazirine-Modified Sialylated Glycoproteins Enabled through Profiling of Sialidase Specificities

Sialic-acid-mediated interactions play critical roles on the cell surface, providing an impetus for the development of methods to study this important monosaccharide. In particular, photo-cross-linking sialic acids incorporated onto cell surfaces have allowed covalent capture of transient interactions between sialic acids and sialic-acid-recognizing proteins via cross-linking. However, natural sialic acids also present on the cell surface compete with photo-cross-linking sialic acids in binding events, limiting cross-linking yields. In order to improve the utility of one such photo-cross-linking sialic acid, SiaDAz, we examined a number of sialidases, enzymes that remove sialic acids from glycoconjugates, to find one that would cleave natural sialic acids but remain inactive toward SiaDAz. Using this sialidase, we improved SiaDAz-mediated cross-linking of an antisialyl Lewis X antibody and of endoglin. This protocol can be applied generally to sialic-acid-mediated interactions and will facilitate identification of sialic acid binding partners.

Recombinant sialidase NanA (rNanA) cleaves α2-3 linked sialic acid of host cell surface N-linked glycoprotein to promote Edwardsiella tarda infection

Edwardsiella tarda is one of the major pathogenic bacteria affecting both marine and freshwater fish species. Sialidase NanA expressed endogenously in E. tarda is glycosidase removing sialic acids from glycoconjugates. Recently, the relationship of NanA sialidase activity to E. tarda infection has been reported, however, the mechanism with which sialidase NanA aids the pathogenicity of E. tarda remained unclear. Here, we comprehensively determined the biochemical properties of NanA towards various substrates in vitro to provide novel insights on the potential NanA target molecule at the host cell. GAKS cell pretreated with recombinant NanA showed increased susceptibility to E. tarda infection. Moreover, sialidase inhibitor treated E. tarda showed a significantly reduced ability to infect GAKS cells. These results indicate that NanA-induced desialylation of cell surface glycoconjugates is essential for the initial step of E. tarda infection. Among the natural substrates, NanA exhibited the highest activity towards 3-sialyllactose, α2-3 linked sialic acid carrying sialoglycoconjugates. Supporting this finding, intact GAKS cell membrane exposed to recombinant NanA showed changes of glycoconjugates only in α2-3 sialo-linked glycoproteins, but not in glycolipids and α2-6 sialo-linked glycoproteins. Lectin staining of cell surface glycoprotein provided further evidence that α2-3 sialo-linkage of the N-linked glycoproteins was the most plausible target of NanA sialidase. To confirm the significance of α2-3 sialo-linkage desialylation for E. tarda infection, HeLa cells which possessed lower amount of α2-3 sialo-linkage glycoprotein were used for infection experiment along with GAKS cells. As a result, infection of HeLa cells by E. tarda was significantly reduced when compared to GAKS cells. Furthermore, E. tarda infection was significantly inhibited by mannose pretreatment suggesting that the bacterium potentially recognizes and binds to mannose or mannose containing chains following desialylation. Together, these results suggest that E. tarda may employ endogenous NanA to desialylate α2-3 glycoproteins on host cells, thus revealing one of the potential binding molecules during infection.

Streptococcus pneumoniae NanC: STRUCTURAL INSIGHTS INTO THE SPECIFICITY AND MECHANISM OF A SIALIDASE THAT PRODUCES A SIALIDASE INHIBITOR

Streptococcus pneumoniae is an important human pathogen that causes a range of disease states. Sialidases are important bacterial virulence factors. There are three pneumococcal sialidases: NanA, NanB, and NanC. NanC is an unusual sialidase in that its primary reaction product is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en, also known as DANA), a nonspecific hydrolytic sialidase inhibitor. The production of Neu5Ac2en from α2–3-linked sialosides by the catalytic domain is confirmed within a crystal structure. A covalent complex with 3-fluoro-β-N-acetylneuraminic acid is also presented, suggesting a common mechanism with other sialidases up to the final step of product formation. A conformation change in an active site hydrophobic loop on ligand binding constricts the entrance to the active site. In addition, the distance between the catalytic acid/base (Asp-315) and the ligand anomeric carbon is unusually short. These features facilitate a novel sialidase reaction in which the final step of product formation is direct abstraction of the C3 proton by the active site aspartic acid, forming Neu5Ac2en. NanC also possesses a carbohydrate-binding module, which is shown to bind α2–3- and α2–6-linked sialosides, as well as N-acetylneuraminic acid, which is captured in the crystal structure following hydration of Neu5Ac2en by NanC. Overall, the pneumococcal sialidases show remarkable mechanistic diversity while maintaining a common structural scaffold.

Biochemical characterisation of the neuraminidase pool of the human gut symbiont Akkermansia muciniphila

Since the isolation and identification of Akkermansia muciniphila one decade ago, much attention has been drawn to this gut bacterium due to its role in obesity and type 2 diabetes. This report describes the discovery and biochemical characterisation of all four putative neuraminidases annotated in the A. muciniphila genome. Recombinantly expressed candidate genes, which were designated Am0705, Am0707, Am1757 and Am2085, were shown to cover complementary pH ranges between 4.0 and 9.5. Temperature optima of the enzymes lay between 37 and 42 °C. All four enzymes were strongly inhibited by Cu(2+) and Zn(2+), and loss of activity after the addition of EDTA suggests that all neuraminidases, with the exception of Am0707, require divalent metal ions for their catalytic function. Chemoenzymatically synthesised α2,3- and α2,6-linked indoyl-sialosides were utilised to determine the regioselectivity and substrate promiscuity of the neuraminidases towards C5-modifications of sialic acids with N-acetyl-, N-glycolyl-, N-propionyl-, or hydroxyl-groups. The combination of simple purification procedures and good activities of some of the characterised neuraminidases makes them potentially interesting as tools in bioanalytical or industrial applications.

Discovery of intramolecular trans-sialidases in human gut microbiota suggests novel mechanisms of mucosal adaptation

The gastrointestinal mucus layer is colonized by a dense community of microbes catabolizing dietary and host carbohydrates during their expansion in the gut. Alterations in mucosal carbohydrate availability impact on the composition of microbial species. Ruminococcus gnavus is a commensal anaerobe present in the gastrointestinal tract of >90% of humans and overrepresented in inflammatory bowel diseases (IBD). Using a combination of genomics, enzymology and crystallography, we show that the mucin-degrader R. gnavus ATCC 29149 strain produces an intramolecular trans-sialidase (IT-sialidase) that cleaves off terminal α2-3-linked sialic acid from glycoproteins, releasing 2,7-anhydro-Neu5Ac instead of sialic acid. Evidence of IT-sialidases in human metagenomes indicates that this enzyme occurs in healthy subjects but is more prevalent in IBD metagenomes. Our results uncover a previously unrecognized enzymatic activity in the gut microbiota, which may contribute to the adaptation of intestinal bacteria to the mucosal environment in health and disease.

Mucosal sialoglycans contribute to host–microbe interactions at mucosal surfaces and impact bacterial colonization of the digestive system. Here the authors identify and characterize an intramolecular trans-sialidase produced by the gut bacterium R. gnavus ATCC 29149 that may contribute to adaptation to the mucosal environment.

Production, purification and crystallization of a trans-sialidase from Trypanosoma vivax

Sialidases and trans-sialidases play important roles in the life cycles of various microorganisms. These enzymes can serve nutritional purposes, act as virulence factors or mediate cellular interactions (cell evasion and invasion). In the case of the protozoan parasite Trypanosoma vivax, trans-sialidase activity has been suggested to be involved in infection-associated anaemia, which is the major pathology in the disease nagana. The physiological role of trypanosomal trans-sialidases in host-parasite interaction as well as their structures remain obscure. Here, the production, purification and crystallization of a recombinant version of T. vivax trans-sialidase 1 (rTvTS1) are described. The obtained rTvTS1 crystals diffracted to a resolution of 2.5 Å and belonged to the orthorhombic space group P212121, with unit-cell parameters a = 57.3, b = 78.4, c = 209.0 Å.

Sialic acid-mediated gene expression in Streptococcus pneumoniae and role of NanR as a transcriptional activator of the nan gene cluster

In this study, we investigated the transcriptomic response of Streptococcus pneumoniae D39 to sialic acid (N-acetylneuraminic acid [Neu5Ac]). Transcriptome comparison of wild-type D39 grown in M17 medium with and without sialic acid revealed the elevated expression of various genes and operons, including the nan gene cluster (nan operon I and nanA gene). Our microarray analysis and promoter-lacZ fusion studies showed that the transcriptional regulator NanR acts as a transcriptional activator of nan operon I and the nanA gene in the presence of sialic acid. The putative regulatory site of NanR in the promoter region of nan operon I is predicted and confirmed by promoter truncation experiments. Furthermore, the role of CcpA in the regulation of the nan gene cluster is demonstrated through microarray analysis and promoter-lacZ fusion studies, suggesting that in the presence of sialic acid and glucose, CcpA represses the expression of nan operon I while the expression of the nanA gene is CcpA independent.

Utilisation of mucin glycans by the human gut symbiont Ruminococcus gnavus is strain-dependent

Commensal bacteria often have an especially rich source of glycan-degrading enzymes which allow them to utilize undigested carbohydrates from the food or the host. The species Ruminococcus gnavus is present in the digestive tract of ≥90% of humans and has been implicated in gut-related diseases such as inflammatory bowel diseases (IBD). Here we analysed the ability of two R. gnavus human strains, E1 and ATCC 29149, to utilize host glycans. We showed that although both strains could assimilate mucin monosaccharides, only R. gnavus ATCC 29149 was able to grow on mucin as a sole carbon source. Comparative genomic analysis of the two R. gnavus strains highlighted potential clusters and glycoside hydrolases (GHs) responsible for the breakdown and utilization of mucin-derived glycans. Transcriptomic and functional activity assays confirmed the importance of specific GH33 sialidase, and GH29 and GH95 fucosidases in the mucin utilisation pathway. Notably, we uncovered a novel pathway by which R. gnavus ATCC 29149 utilises sialic acid from sialylated substrates. Our results also demonstrated the ability of R. gnavus ATCC 29149 to produce propanol and propionate as the end products of metabolism when grown on mucin and fucosylated glycans. These new findings provide molecular insights into the strain-specificity of R. gnavus adaptation to the gut environment advancing our understanding of the role of gut commensals in health and disease.

A surface-exposed neuraminidase affects complement resistance and virulence of the oral spirochaete Treponema denticola

Neuraminidases (sialidases) catalyse the removal of terminal sialic acid from glycoconjugates. Bacterial pathogens often utilize neuraminidases to scavenge host sialic acid, which can be utilized either as a nutrient or as a decorating molecule to disguise themselves from host immune attacks. Herein, a putative neuraminidase (TDE0471) was identified in Treponema denticola, an oral spirochaete associated with human periodontitis. TDE0471 is a cell surface-exposed exo-neuraminidase that removes sialic acid from human serum proteins; it is required for T.denticola to grow in a medium that mimics gingival crevice fluid, suggesting that the spirochaete may use sialic acid as a nutrient in vivo. TDE0471 protects T.denticola from serum killing by preventing the deposition of membrane attack complexes on the bacterial cell surface. Animal studies revealed that a TDE0471-deficient mutant is less virulent than its parental wild-type strain in BALB/C mice. However, it causes a level of tissue damage similar to the wild type in complement-deficient B6.129S4-C3(tm1) (Crr) /J mice albeit the damage caused by both bacterial strains is more severe in these transgenic mice. Based on these results, we propose that T.denticola has evolved a strategy to scavenge host sialic acid using its neuraminidase, which allows the spirochaete to acquire nutrients and evade complement killing.

Structural basis of the novel S. pneumoniae virulence factor, GHIP, a glycosyl hydrolase 25 participating in host-cell invasion

Pathogenic bacteria produce a wide variety of virulence factors that are considered to be potential antibiotic targets. In this study, we report the crystal structure of a novel S. pneumoniae virulence factor, GHIP, which is a streptococcus-specific glycosyl hydrolase. This novel structure exhibits an α/β-barrel fold that slightly differs from other characterized hydrolases. The GHIP active site, located at the negatively charged groove in the barrel, is very similar to the active site in known peptidoglycan hydrolases. Functionally, GHIP exhibited weak enzymatic activity to hydrolyze the PNP-(GlcNAc)5 peptidoglycan by the general acid/base catalytic mechanism. Animal experiments demonstrated a marked attenuation of S. pneumoniae-mediated virulence in mice infected by ΔGHIP-deficient strains, suggesting that GHIP functions as a novel S. pneumoniae virulence factor. Furthermore, GHIP participates in allowing S. pneumoniae to colonize the nasopharynx and invade host epithelial cells. Taken together, these findings suggest that GHIP can potentially serve as an antibiotic target to effectively treat streptococcus-mediated infection.

Structural and biochemical characterization of the broad substrate specificity of Bacteroides thetaiotaomicron commensal sialidase

Sialidases release the terminal sialic acid residue from a wide range of sialic acid-containing polysaccharides. Bacteroides thetaiotaomicron, a symbiotic commensal microbe, resides in and dominates the human intestinal tract. We characterized the recombinant sialidase from B. thetaiotaomicron (BTSA) and demonstrated that it has broad substrate specificity with a relative activity of 97, 100 and 64 for 2,3-, 2,6- and 2,8-linked sialic substrates, respectively. The hydrolysis activity of BTSA was inhibited by a transition state analogue, 2-deoxy-2,3-dehydro-N-acetyl neuraminic acid, by competitive inhibition with a Ki value of 35μM. The structure of BSTA was determined at a resolution of 2.3Å. This structure exhibited a unique carbohydrate-binding domain (CBM) at its N-terminus (a.a. 23-190) that is adjacent to the catalytic domain (a.a. 191-535). The catalytic domain has a conserved arginine triad with a wide-open entrance for the substrate that exposes the catalytic residue to the surface. Unlike other pathogenic sialidases, the polysaccharide-binding site in the CBM is near the active site and possibly holds and positions the polysaccharide substrate directly at the active site. The structural feature of a wide substrate-binding groove and closer proximity of the polysaccharide-binding site to the active site could be a unique signature of the commensal sialidase BTSA and provide a molecular basis for its pharmaceutical application.

The use of sialidase therapy for respiratory viral infections

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DAS181 is a novel inhaled therapy for the treatment of influenza.

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Treatment targets sialic acid on the cell surface.

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The sialidase removes both α2-3 and α2-6 linked sialic acids.

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The use of an amphiregulin tag to the sialidase anchors it to the cell surface.

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Treatment for 3 days appears effective in treating influenza and parainfluenza.

DAS181 is an inhaled bacterial sialidase which functions by removing sialic acid (Sia) from the surface of epithelial cells, preventing attachment and subsequent infection by respiratory viruses that utilize Sia as a receptor. DAS181 is typical of bacterial sialidases in cleaving Sia α2-3 and Sia α2-6 linkages, and it also has a demonstrated effect against acetylated and hydroxylated forms of Sia. The potency of the compound has been enhanced by coupling the active sialidase with an amphiregulin tag, allowing a longer duration of action and minimizing spread to the systemic circulation. DAS181 is now in Phase II development for the treatment of influenza, and it has also demonstrated activity in individual cases of parainfluenza in immunosuppressed patients. Continued evaluation of the roles and activities of bacterial sialidases is required to expand the range of successful antiviral therapies targeting Sia or its derivatives.

Trypanosoma cruzi trans-sialidase initiates a program independent of the transcription factors RORγt and Ahr that leads to IL-17 production by activated B cells

We identified B cells as a major source for rapid, innate-like interleukin 17 (IL-17) production in vivo in response to Trypanosoma cruzi infection. IL-17⁺ B cells exhibited a plasmablast phenotype, outnumbered T_H17 cells and were required for optimal response to this pathogen. Using both murine and human primary B cells, we demonstrate that exposure to parasite-derived trans-sialidase in vitro was sufficient to trigger modification of the cell surface mucin, CD45, leading to Btk-dependent signaling and IL-17A or IL-17F production via an ROR-γt and AHR-independent transcriptional program. Our combined data suggest that generation of IL-17⁺ B cells may be an unappreciated feature of innate immune responses required for pathogen control or IL-17-mediated autoimmunity.

Cell-permeable probe for identification and imaging of sialidases

Alkyne-hinged 3-fluorosialyl fluoride (DFSA) containing an alkyne group was shown to be a mechanism-based target-specific irreversible inhibitor of sialidases. The ester-protected analog DFSA (PDFSA) is a membrane-permeable precursor of DFSA designed to be used in living cells, and it was shown to form covalent adducts with virus, bacteria, and human sialidases. The fluorosialyl-enzyme adduct can be ligated with an azide-annexed biotin via click reaction and detected by the streptavidin-specific reporting signals. Liquid chromatography-mass spectrometry/mass spectrometry analysis on the tryptic peptide fragments indicates that the 3-fluorosialyl moiety modifies tyrosine residues of the sialidases. DFSA was used to demonstrate influenza infection and the diagnosis of the viral susceptibility to the anti-influenza drug oseltamivir acid, whereas PDFSA was used for in situ imaging of the changes of sialidase activity in live cells.

Optimization of a direct spectrophotometric method to investigate the kinetics and inhibition of sialidases

BACKGROUNDS

Streptococcus pneumoniae expresses three distinct sialidases, NanA, NanB, and NanC, that are believed to be key virulence factors and thus, potential important drug targets. We previously reported that the three enzymes release different products from sialosides, but could share a common catalytic mechanism before the final step of product formation. However, the kinetic investigations of the three sialidases have not been systematically done thus far, due to the lack of an easy and steady measurement of sialidase reaction rate.

RESULTS

In this work, we present further kinetic characterization of pneumococcal sialidases by using a direct spectrophotometric method with the chromogenic substrate p-nitrophenyl-N-acetylneuraminic acid (p-NP-Neu5Ac). Using our assay, the measured kinetic parameters of the three purified pneumococcal sialidase, NanA, NanB and NanC, were obtained and were in perfect agreement with the previously published data. The major advantage of this alternative method resides in the direct measurement of the released product, allowing to readily determine of initial reaction rates and record complete hydrolysis time courses.

CONCLUSION

We developed an accurate, fast and sensitive spectrophotometric method to investigate the kinetics of sialidase-catalyzed reactions. This fast, sensitive, inexpensive and accurate method could benefit the study of the kinetics and inhibition of sialidases in general.

Insights into the recognition of the human glycome by microbial carbohydrate-binding modules

Mammalian glycans are often very complex and consequently both commensal bacteria and bacterial pathogens have developed specialized and often elaborate carbohydrate-active enzyme (CAZyme) systems to interact with these sugars. These enzymes are frequently multimodular, with modular functions most often conferring catalysis (glycoside hydrolase catalytic modules) or carbohydrate-binding (carbohydrate-binding modules or CBMs). Structure-function studies of five CBM families are revealing specificities for complex mammalian carbohydrates. Three of these CBM families (32, 47, and 51) show significant structural identity between their β-sandwich folds, suggesting a shared evolutionary precursor, but have divergent binding specificities. The family 40 and 41 CBMs recognize sialic acid and glycogen, respectively, through different modes of sugar binding, though they also adopt all β-structure folds. A structural view of new models generated for complete CAZymes suggests three distinct modes of CBM deployment: (i) formation of the catalytic site, (ii) coordinated catalysis and binding, and (iii) general substrate adherence.

Abrogation of neuraminidase reduces biofilm formation, capsule biosynthesis, and virulence of Porphyromonas gingivalis

The oral bacterium Porphyromonas gingivalis is a key etiological agent of human periodontitis, a prevalent chronic disease that affects up to 80% of the adult population worldwide. P. gingivalis exhibits neuraminidase activity. However, the enzyme responsible for this activity, its biochemical features, and its role in the physiology and virulence of P. gingivalis remain elusive. In this report, we found that P. gingivalis encodes a neuraminidase, PG0352 (SiaPg). Transcriptional analysis showed that PG0352 is monocistronic and is regulated by a sigma70-like promoter. Biochemical analyses demonstrated that SiaPg is an exo-α-neuraminidase that cleaves glycosidic-linked sialic acids. Cryoelectron microscopy and tomography analyses revealed that the PG0352 deletion mutant (ΔPG352) failed to produce an intact capsule layer. Compared to the wild type, in vitro studies showed that ΔPG352 formed less biofilm and was less resistant to killing by the host complement. In vivo studies showed that while the wild type caused a spreading type of infection that affected multiple organs and all infected mice were killed, ΔPG352 only caused localized infection and all animals survived. Taken together, these results demonstrate that SiaPg is an important virulence factor that contributes to the biofilm formation, capsule biosynthesis, and pathogenicity of P. gingivalis, and it can potentially serve as a new target for developing therapeutic agents against P. gingivalis infection.

Gallus gallus NEU3 sialidase as model to study protein evolution mechanism based on rapid evolving loops

BACKGROUND

Large surface loops contained within compact protein structures and not involved in catalytic process have been proposed as preferred regions for protein family evolution. These loops are subjected to lower sequence constraints and can evolve rapidly in novel structural variants. A good model to study this hypothesis is represented by sialidase enzymes. Indeed, the structure of sialidases is a β-propeller composed by anti-parallel β-sheets connected by loops that suit well with the rapid evolving loop hypothesis. These features prompted us to extend our studies on this protein family in birds, to get insights on the evolution of this class of glycohydrolases.

RESULTS

Gallus gallus (Gg) genome contains one NEU3 gene encoding a protein with a unique 188 amino acid sequence mainly constituted by a peptide motif repeated six times in tandem with no homology with any other known protein sequence. The repeat region is located at the same position as the roughly 80 amino acid loop characteristic of mammalian NEU4. Based on molecular modeling, all these sequences represent a connecting loop between the first two highly conserved β-strands of the fifth blade of the sialidase β-propeller. Moreover this loop is highly variable in sequence and size in NEU3 sialidases from other vertebrates. Finally, we found that the general enzymatic properties and subcellular localization of Gg NEU3 are not influenced by the deletion of the repeat sequence.

CONCLUSION

In this study we demonstrated that sialidase protein structure contains a surface loop, highly variable both in sequence and size, connecting two conserved β-sheets and emerging on the opposite site of the catalytic crevice. These data confirm that sialidase family can serve as suitable model for the study of the evolutionary process based on rapid evolving loops, which may had occurred in sialidases. Giving the peculiar organization of the loop region identified in Gg NEU3, this protein can be considered of particular interest in such evolutionary studies and to get deeper insights in sialidase evolution.

Features and applications of bacterial sialidases

Sialidases, or neuraminidases (EC 3.2.1.18), belong to a class of glycosyl hydrolases that release terminal N-acylneuraminate residues from the glycans of glycoproteins, glycolipids, and polysaccharides. In bacteria, sialidases can be used to scavenge sialic acids as a nutrient from various sialylated substrates or to recognize sialic acids exposed on the surface of the host cell. Despite the fact that bacterial sialidases share many structural features, their biochemical properties, especially their linkage and substrate specificities, vary widely. Bacterial sialidases can catalyze the hydrolysis of terminal sialic acids linked by the α(2,3)-, α(2,6)-, or α(2,8)-linkage to a diverse range of substrates. In addition, some of these enzymes can catalyze the transfer of sialic acids from sialoglycans to asialoglycoconjugates via a transglycosylation reaction mechanism. Thus, some bacterial sialidases have been applied to synthesize complex sialyloligosaccharides through chemoenzymatic approaches and to analyze the glycan structure. In this review article, the biochemical features of bacterial sialidases and their potential applications in regioselective hydrolysis reactions as well as sialylation by transglycosylation for the synthesis of sialylated complex glycans are discussed.

Exposure of Thomsen-Friedenreich antigen in Streptococcus pneumoniae infection is dependent on pneumococcal neuraminidase A

Pneumococcal hemolytic uremic syndrome is recognized in a small portion of otherwise healthy children who have or have recently had Streptococcus pneumoniae infections, including severe pneumonia, meningitis, and bacteremia. As in other types of hemolytic uremic syndrome (HUS), pneumococcal HUS is characterized by microangiopathic hemolytic anemia, and thrombocytopenia, usually with extensive kidney damage. Although not demonstrated in vivo, the pathogenesis of pneumococcal HUS has been attributed to the action pneumococcal neuraminidase exposing the usually cryptic Thomsen-Friedenreich antigen (T-antigen) on red blood cells (RBC), and kidney glomeruli. We evaluated the effect of pneumococcal infection on desialylation of RBC and glomeruli during pneumococcal infections in mice. Following intravenous infection with capsular type 19F pneumococci, CFU levels exceeding 1000 CFU/mL blood by the third day were significantly more likely to result in exposed T-antigen on RBC than lower levels of bacteremia. In a pneumonia model, significantly more T-antigen was exposed on RBC in mice treated with penicillin than in those receiving mock treatment. Utilizing mutant pneumococci, we demonstrated that neuraminidase A but not neuraminidase B was necessary for exposure of T-antigen on RBC in vivo. Thus, pneumococcal neuraminidase A is necessary for the exposure of T-antigen in vivo and treatment with penicillin increases this effect. Interestingly, NanA(-) pneumococci were found in the blood in higher numbers and caused more deaths than wild type, NanB(-), or the NanA(-)/NanB(-) pneumococci.

Three Streptococcus pneumoniae sialidases: three different products

Streptococcus penumoniae is a major human pathogen responsible for respiratory tract infections, septicemia, and meningitis and continues to produce numerous cases of disease with relatively high mortalities. S. pneumoniae encodes up to three sialidases, NanA, NanB, and NanC, that have been implicated in pathogenesis and are potential drug targets. NanA has been shown to be a promiscuous sialidase, hydrolyzing the removal of Neu5Ac from a variety of glycoconjugates with retention of configuration at the anomeric center, as we confirm by NMR. NanB is an intramolecular trans-sialidase producing 2,7-anhydro-Neu5Ac selectively from α2,3-sialosides. Here, we show that the first product of NanC is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en) that can be slowly hydrated by the enzyme to Neu5Ac. We propose that the three pneumococcal sialidases share a common catalytic mechanism up to the final product formation step, and speculate on the roles of the enzymes in the lifecycle of the bacterium.

Assessment of Streptococcus pneumoniae capsule in conjunctivitis and keratitis in vivo neuraminidase activity increases in nonencapsulated pneumococci following conjunctival infection

PURPOSE

The pneumococcal capsule is required for pathogenesis in systemic infections, yet reports show most conjunctivitis outbreaks are caused by nonencapsulated pneumococci, while keratitis infections are caused by encapsulated strains. This study aims to determine the effect of capsule in pneumococcal keratitis and conjunctivitis in rabbit models of infection.

METHODS

A capsule-deficient isogenic mutant was created using homologous transformation. Parent and mutant strains were injected within the upper bulbar conjunctiva (conjunctivitis) or into the corneal stroma (keratitis) of New Zealand white rabbits. Clinical examinations were performed 24 and 48 hr post-infection at which time corneas or conjunctivae were removed, homogenized, and plated to determine the recovered bacterial load. Whole eyes were removed for histological examination. The neuraminidase activity was determined following in vitro and in vivo growth.

RESULTS

There were no significant differences in clinical scores between the eyes infected with the parent or mutant for either infection, nor was there a difference in the amount of bacteria recovered from the cornea. In the conjunctivae, however, the mutant strain was cleared by the host faster than the parent strain. Histological examination showed slightly more infiltrating polymorphonuclear leukocytes (PMN) and macrophages in the conjunctivae infected with the parent strain. The neuraminidase activity of both strains was not significantly different when the strains were grown in vitro. However, the neuraminidase activity of the parent was significantly less than that of the mutant at 3 and 12 hr post conjunctival infection.

CONCLUSIONS

Although more outbreaks of pneumococcal conjunctivitis are tied to nonencapsulated S. pneumoniae strains, this study showed that an encapsulated strain was capable of establishing conjunctivitis in a rabbit injection model and survive attack by the host immune system longer than its nonencapsulated isogenic mutant. Nonetheless, the nonencapsulated pneumococci had an increased neuraminidase activity level in vivo when compared to the parent strain.