Three Streptococcus pneumoniae sialidases: three different products

Synthetic Sialosides Terminated with 8-N-Substituted Sialic Acid as Selective Substrates for Sialidases from Bacteria and Influenza Viruses

Sialosides containing C8-modified sialic acids are challenging synthetic targets but potentially useful probes for diagnostic substrate profiling of sialidases and elucidating the binding specificity of sialic acid-interacting proteins. Here, we demonstrate efficient chemoenzymatic methods for synthesizing para-nitrophenol-tagged α2-3- and α2-6-linked sialyl galactosides containing C8-acetamido, C8-azido, or C8-amino derivatized N-acetylneuraminic acid (Neu5Ac). High-throughput substrate specificity studies showed that the C8-modification of sialic acid significantly changes its recognition by sialidases from humans, various bacteria, and different influenza A and B viruses. Sialosides carrying Neu5Ac with a C8-azido modification were generally well tolerated by all the sialidases we tested, whereas sialosides containing C8-acetamido-modified Neu5Ac were only cleaved by selective bacterial sialidases. In contrast, sialosides with C8-amino-modified Neu5Ac were cleaved by a combination of selective bacterial and influenza A virus sialidases. These results indicate that sialosides terminated with a C8-amino or C8-acetamido-modified sialic acid can be used with other sialosides for diagnostic profiling of disease-causing sialidase-producing pathogens.

Characterisation of anhydro-sialic acid transporters from mucosa-associated bacteria

Sialic acid (Sia) transporters are critical to the capacity of host-associated bacteria to utilise Sia for growth and/or cell surface modification. While N-acetyl-neuraminic acid (Neu5Ac)-specific transporters have been studied extensively, little is known on transporters dedicated to anhydro-Sia forms such as 2,7-anhydro-Neu5Ac (2,7-AN) or 2,3-dehydro-2-deoxy-Neu5Ac (Neu5Ac2en). Here, we used a Sia-transport-null strain of Escherichia coli to investigate the function of members of anhydro-Sia transporter families previously identified by computational studies. First, we showed that the transporter NanG, from the Glycoside-Pentoside-Hexuronide:cation symporter family, is a specific 2,7-AN transporter, and identified by mutagenesis a crucial functional residue within the putative substrate-binding site. We then demonstrated that NanX transporters, of the Major Facilitator Superfamily, also only transport 2,7-AN and not Neu5Ac2en nor Neu5Ac. Finally, we provided evidence that SiaX transporters, of the Sodium-Solute Symporter superfamily, are promiscuous Neu5Ac/Neu5Ac2en transporters able to acquire either substrate equally well. The characterisation of anhydro-Sia transporters expands our current understanding of prokaryotic Sia metabolism within host-associated microbial communities.

Auction market placement and a rest stop during transportation affect the respiratory bacterial microbiota of beef cattle

Background

Bovine respiratory disease (BRD) is a significant health problem in beef cattle production, resulting in considerable economic losses due to mortalities, cost of treatment, and reduced feed efficiency. The onset of BRD is multifactorial, with numerous stressors being implicated, including transportation from farms to feedlots. In relation to animal welfare, regulations or practices may require mandatory rest times during transportation. Despite this, there is limited information on how transportation and rest stops affect the respiratory microbiota.

Results

This study evaluated the effect of cattle source (ranch-direct or auction market-derived) and rest stop duration (0 or 8 h of rest) on the upper respiratory tract microbiota and its relationship to stress response indicators (blood cortisol and haptoglobin) of recently weaned cattle transported for 36 h. The community structure of bacteria was altered by feedlot placement. When cattle were off-loaded for a rest, several key bacterial genera associated with BRD (Mannheimia, Histophilus, Pasteurella) were increased for most sampling times after feedlot placement for the ranch-direct cattle group, compared to animals given no rest stop. Similarly, more sampling time points had elevated levels of BRD-associated genera when auction market cattle were compared to ranch-direct. When evaluated across time and treatments several genera including Mannheimia, Moraxella, Streptococcus and Corynebacterium were positively correlated with blood cortisol concentrations.

Conclusion

This is the first study to assess the effect of rest during transportation and cattle source on the respiratory microbiota in weaned beef calves. The results suggest that rest stops and auction market placement may be risk factors for BRD, based solely on increased abundance of BRD-associated genera in the upper respiratory tract. However, it was not possible to link these microbiota to disease outcome, due to low incidence of BRD in the study populations. Larger scale studies are needed to further define how transportation variables impact cattle health.

Pyrrolo[2,3-e]indazole as a novel chemotype for both influenza A virus and pneumococcal neuraminidase inhibitors

Influenza infections are often exacerbated by secondary bacterial infections, primarily caused by Streptococcus pneumoniae. Both respiratory pathogens have neuraminidases that support infection. Therefore, we hypothesized that dual inhibitors of viral and bacterial neuraminidases might be an advantageous strategy for treating seasonal and pandemic influenza pneumonia complicated by bacterial infections. By screening our in-house chemical library, we discovered a new chemotype that may be of interest for a further campaign to find small molecules against influenza. Our exploration of the pyrrolo[2,3-e]indazole space led to the identification of two hit compounds, 6h and 12. These molecules were well-tolerated by MDCK cells and inhibited the replication of H3N2 and H1N1 influenza A virus strains. Moreover, both compounds suppress viral and pneumococcal neuraminidases indicating their dual activity. Given its antiviral activity, pyrrolo[2,3-e]indazole has been identified as a promising scaffold for the development of novel neuraminidase inhibitors that are active against influenza A virus and S. pneumoniae.

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.

Identification of Subunits for Novel Universal Vaccines against Three Predominant Serogroups and the Emerging O145 among Avian Pathogenic Escherichia coli by Pan-RV Pipeline

Avian pathogenic Escherichia coli, a causative agent of avian colibacillosis, has been causing serious economic losses in the poultry industry. The increase in multidrug-resistant isolates and the complexity of the serotypes of this pathogen, especially the recently reported emergence of a newly predominant serogroup of O145, make the control of this disease difficult. To address this challenge, a high-throughput screening approach, called Pan-RV (Reverse vaccinology based on pangenome analysis), is proposed to search for universal protective antigens against the three traditional serogroups and the newly emerged O145. Using this approach, a total of 61 proteins regarded as probable antigens against the four important serogroups were screened from the core genome of 127 Avian pathogenic Escherichia coli (APEC) genomes, and six were verified by Western blots using antisera. Overall, our research will provide a foundation for the development of an APEC subunit vaccine against avian colibacillosis. Given the exponential growth of whole-genome sequencing (WGS) data, our Pan-RV pipeline will make screening of bacterial vaccine candidates inexpensive, rapid, and efficient. IMPORTANCE With the emergence of drug resistance and the newly predominant serogroup O145, the control of Avian pathogenic Escherichia coli is facing a serious challenge; an efficient immunological method is urgently needed. Here, for the first time, we propose a high-throughput screening approach to search for universal protective antigens against the three traditional serogroups and the newly emerged O145. Importantly, using this approach, a total of 61 proteins regarded as probable antigens against the four important serogroups were screened, and three were shown to be immunoreactive with all antisera (covering the four serogroups), thereby providing a foundation for the development of APEC subunit vaccines against avian colibacillosis. Further, our Pan-RV pipeline will provide immunological control strategies for pathogens with complex and variable genetic backgrounds such as Escherichia coli and will make screening of bacterial vaccine candidates more inexpensive, rapid, and efficient.

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.

Sialosides Containing 7-N-Acetyl Sialic Acid Are Selective Substrates for Neuraminidases from Influenza A Viruses

Sialidases or neuraminidases are sialic-acid-cleaving enzymes that are expressed by a broad spectrum of organisms, including pathogens. In nature, sialic acids are monosaccharides with diverse structural variations, but the lack of novel probes has made it difficult to determine how sialic acid modifications impact the recognition by sialidases. Here, we used a chemoenzymatic synthon strategy to generate a set of α2-3- and α2-6-linked sialoside probes that contain 7-N-acetyl or 7,9-di-N-acetyl sialic acid as structure mimics for those containing the less stable naturally occurring 7-O-acetyl- or 7,9-di-O-acetyl modifications. These probes were used to compare the substrate specificity of several sialidases from different origins. Our results show that 7-N-acetyl sialic acid was readily cleaved by neuraminidases from H1N1 and H3N2 influenza A viruses, but not by sialidases of human or bacterial origin, thereby indicating that the influenza enzymes possess a distinctive and more promiscuous substrate binding pocket.

Sialidase Inhibitors with Different Mechanisms

Sialidases, or neuraminidases, are enzymes that catalyze the hydrolysis of sialic acid (Sia)-containing molecules, mostly removal of the terminal Sia (desialylation). By desialylation, sialidase can modulate the functionality of the target compound and is thus often involved in biological pathways. Inhibition of sialidases with inhibitors is an important approach for understanding sialidase function and the underlying mechanisms and could serve as a therapeutic approach as well. Transition-state analogues, such as anti-influenza drugs oseltamivir and zanamivir, are major sialidase inhibitors. In addition, difluoro-sialic acids were developed as mechanism-based sialidase inhibitors. Further, fluorinated quinone methide-based suicide substrates were reported. Sialidase product analogue inhibitors were also explored. Finally, natural products have shown competitive inhibiton against viral, bacterial, and human sialidases. This Perspective describes sialidase inhibitors with different mechanisms and their activities and future potential, which include transition-state analogue inhibitors, mechanism-based inhibitors, suicide substrate inhibitors, product analogue inhibitors, and natural product inhibitors.

Screening and purification of NanB sialidase from Pasteurella multocida with activity in hydrolyzing sialic acid Neu5Acα(2–6)Gal and Neu5Acα(2–3)Gal

Study on sialidases as antiviral agents has been widely performed, but many types of sialidase have not been tested for their antiviral activity. Pasteurella multocida NanB sialidase is one such sialidase that has never been isolated for further research. In this study, the activity of NanB sialidase was investigated in silico by docking the NanB sialidase of Pasteurella multocida to the Neu5Acα(2–6)Gal and Neu5Acα(2–3)Gal ligands. Additionally, some local isolates of Pasteurella multocida, which had the NanB gene were screened, and the proteins were isolated for further testing regarding their activity in hydrolyzing Neu5Acα(2–6)Gal and Neu5Acα(2–3)Gal. Silico studies showed that the NanB sialidase possesses an exceptional affinity towards forming a protein–ligand complex with Neu5Acα(2–6)Gal and Neu5Acα(2–3)Gal. NanB sialidase of Pasteurella multocida B018 at 0.129 U/mL and 0.258 U/mL doses can hydrolyze Neu5Acα(2–6)Gal and Neu5Acα(2–3)Gal better than other doses. In addition, those doses can inhibit effectively H9N2 viral binding to red blood cells. This study suggested that the NanB sialidase of Pasteurella multocida B018 has a potent antiviral activity because can hydrolyze sialic acid on red blood cells surface and inhibit the H9N2 viral binding to the cells.

Human Neuraminidases: Structures and Stereoselective Inhibitors

This Perspective describes the classification, structures, substrates, mechanisms of action, and implications of human neuraminidases (hNEUs) in various pathologies. Some inhibitors have been developed for each isoform, leading to more precise interactions with hNEUs. Although crystal structure data are available for NEU2, most of the findings are based on NEU1 inhibition, and limited information is available for other hNEUs. Therefore, the synthesis of new compounds would facilitate the enrichment of the arsenal of inhibitors to better understand the roles of hNEUs and their mechanisms of action. Nevertheless, due to the already known inhibitors of human neuraminidase enzymes, a structure-activity relationship is presented along with different approaches to inhibit these enzymes for the development of potent and selective inhibitors. Among the different emerging strategies, one is the inhibition of the dimerization of NEU1 or NEU3, and the second is the inhibition of certain receptors located close to hNEU.

Diverse Mechanisms of Protective Anti-Pneumococcal Antibodies

The gram-positive bacterium Streptococcus pneumoniae is a leading cause of pneumonia, otitis media, septicemia, and meningitis in children and adults. Current prevention and treatment efforts are primarily pneumococcal conjugate vaccines that target the bacterial capsule polysaccharide, as well as antibiotics for pathogen clearance. While these methods have been enormously effective at disease prevention and treatment, there has been an emergence of non-vaccine serotypes, termed serotype replacement, and increasing antibiotic resistance among these serotypes. To combat S. pneumoniae, the immune system must deploy an arsenal of antimicrobial functions. However, S. pneumoniae has evolved a repertoire of evasion techniques and is able to modulate the host immune system. Antibodies are a key component of pneumococcal immunity, targeting both the capsule polysaccharide and protein antigens on the surface of the bacterium. These antibodies have been shown to play a variety of roles including increasing opsonophagocytic activity, enzymatic and toxin neutralization, reducing bacterial adherence, and altering bacterial gene expression. In this review, we describe targets of anti-pneumococcal antibodies and describe antibody functions and effectiveness against S. pneumoniae.

Review of sialic acid's biochemistry, sources, extraction and functions with special reference to edible bird's nest

Sialic acids are a group of nine-carbon α-keto acids. Sialic acid exists in more than 50 forms, with the natural types discovered as N-acetylneuraminic acid (Neu5Ac), deaminoneuraminic acid (2-keto-3-deoxy-nonulononic acid or Kdn), and N-glycolylneuraminic acid (Neu5Gc). Sialic acid level varies depending on the source, where edible bird's nest (EBN), predominantly Neu5Ac, is among the major sources of sialic acid. Due to its high nutritive value and complexity, sialic acid has been studied extensively through acid, aqueous, and enzymatic extraction. Although detection by chromatographic methods or mass spectrometry is common, the isolation and recovery work remained limited. Sialic acid is well-recognised for its bioactivities, including brain and cognition development, immune-enhancing, anti-hypertensive, anticancer, and skin whitening properties. Therefore, sialic acid can be used as a functional ingredient in the various industries. This paper reviews the current trend in the biochemistry, sources, extraction, and functions of sialic acids with special reference to EBN.

Multiple evolutionary origins reflect the importance of sialic acid transporters in the colonization potential of bacterial pathogens and commensals

Located at the tip of cell surface glycoconjugates, sialic acids are at the forefront of host-microbe interactions and, being easily liberated by sialidase enzymes, are used as metabolites by numerous bacteria, particularly by pathogens and commensals living on or near diverse mucosal surfaces. These bacteria rely on specific transporters for the acquisition of host-derived sialic acids. Here, we present the first comprehensive genomic and phylogenetic analysis of bacterial sialic acid transporters, leading to the identification of multiple new families and subfamilies. Our phylogenetic analysis suggests that sialic acid-specific transport has evolved independently at least eight times during the evolution of bacteria, from within four of the major families/superfamilies of bacterial transporters, and we propose a robust classification scheme to bring together a myriad of different nomenclatures that exist to date. The new transporters discovered occur in diverse bacteria, including Spirochaetes, Bacteroidetes, Planctomycetes and Verrucomicrobia, many of which are species that have not been previously recognized to have sialometabolic capacities. Two subfamilies of transporters stand out in being fused to the sialic acid mutarotase enzyme, NanM, and these transporter fusions are enriched in bacteria present in gut microbial communities. Our analysis supports the increasing experimental evidence that competition for host-derived sialic acid is a key phenotype for successful colonization of complex mucosal microbiomes, such that a strong evolutionary selection has occurred for the emergence of sialic acid specificity within existing transporter architectures.

Vaginal sialoglycan foraging by Gardnerella vaginalis: mucus barriers as a meal for unwelcome guests?

Bacterial vaginosis (BV) is a condition of the vaginal microbiome in which there are few lactobacilli and abundant anaerobic bacteria. Members of the genus Gardnerella are often one of the most abundant bacteria in BV. BV is associated with a wide variety of poor health outcomes for women. It has been recognized since the 1980s that women with BV have detectable and sometimes markedly elevated levels of sialidase activity in vaginal fluids and that bacteria associated with this condition produce this activity in culture. Mounting evidence collected using diverse methodologies points to the conclusion that BV is associated with a reduction in intact sialoglycans in cervicovaginal secretions. Here we review evidence for the contributions of vaginal bacteria, especially Gardnerella, in the processes of mucosal sialoglycan degradation, uptake, metabolism and depletion. Our understanding of the impacts of vaginal sialoglycan degradation is still limited. However, the potential implications of sialic acid depletion are discussed in light of our current understanding of the roles played by sialoglycans in vaginal physiology.

Neuraminidase B controls neuraminidase A-dependent mucus production and evasion

Binding of Streptococcus pneumoniae (Spn) to nasal mucus leads to entrapment and clearance via mucociliary activity during colonization. To identify Spn factors allowing for evasion of mucus binding, we used a solid-phase adherence assay with immobilized mucus of human and murine origin. Spn bound large mucus particles through interactions with carbohydrate moieties. Mutants lacking neuraminidase A (nanA) or neuraminidase B (nanB) showed increased mucus binding that correlated with diminished removal of terminal sialic acid residues on bound mucus. The non-additive activity of the two enzymes raised the question why Spn expresses two neuraminidases and suggested they function in the same pathway. Transcriptional analysis demonstrated expression of nanA depends on the enzymatic function of NanB. As transcription of nanA is increased in the presence of sialic acid, our findings suggest that sialic acid liberated from host glycoconjugates by the secreted enzyme NanB induces the expression of the cell-associated enzyme NanA. The absence of detectable mucus desialylation in the nanA mutant, in which NanB is still expressed, suggests that NanA is responsible for the bulk of the modification of host glycoconjugates. Thus, our studies describe a functional role for NanB in sialic acid sensing in the host. The contribution of the neuraminidases in vivo was then assessed in a murine model of colonization. Although mucus-binding mutants showed an early advantage, this was only observed in a competitive infection, suggesting a complex role of neuraminidases. Histologic examination of the upper respiratory tract demonstrated that Spn stimulates mucus production in a neuraminidase-dependent manner. Thus, an increase production of mucus containing secretions appears to be balanced, in vivo, by decreased mucus binding. We postulate that through the combined activity of its neuraminidases, Spn evades mucus binding and mucociliary clearance, which is needed to counter neuraminidase-mediated stimulation of mucus secretions.

The putative Escherichia coli dehydrogenase YjhC metabolises two dehydrated forms of N-acetylneuraminate produced by some sialidases

Homologues of the putative dehydrogenase YjhC are found in operons involved in the metabolism of N-acetylneuraminate (Neu5Ac) or related compounds. We observed that purified recombinant YjhC forms Neu5Ac from two dehydrated forms of this compound, 2,7-anhydro-N-acetylneuraminate (2,7-AN) and 2-deoxy-2,3-didehydro-N-acetylneuraminate (2,3-EN) that are produced during the degradation of sialoconjugates by some sialidases. The conversion of 2,7-AN into Neu5Ac is reversible and reaches its equilibrium when the ratio of 2,7-AN to Neu5Ac is ≈1/6. The conversion of 2,3-EN is irreversible, leading to a mixture of Neu5Ac and 2,7-AN. NMR analysis of the reaction catalysed by YjhC on 2,3-EN indicated that Neu5Ac was produced as the α-anomer. All conversions require NAD⁺ as a cofactor, which is regenerated in the reaction. They appear to involve the formation of keto (presumably 4-keto) intermediates of 2,7-AN, 2,3-EN and Neu5Ac, which were detected by liquid chromatography-mass spectrometry (LC-MS). The proposed reaction mechanism is reminiscent of the one catalysed by family 4 β-glycosidases, which also use NAD⁺ as a cofactor. Both 2,7-AN and 2,3-EN support the growth of Escherichia coli provided the repressor NanR, which negatively controls the expression of the yjhBC operons, has been inactivated. Inactivation of either YjhC or YjhB in NanR-deficient cells prevents the growth on 2,7-AN and 2,3-EN. This confirms the role of YjhC in 2,7-AN and 2,3-EN metabolism and indicates that transport of 2,7-AN and 2,3-EN is carried out by YjhB, which is homologous to the Neu5Ac transporter NanT.

Catalytic and lectin domains in neuraminidase A from Streptococcus pneumoniae are capable of an intermolecular assembly: Implications for biofilm formation

Neuraminidase A from Streptococcus pneumoniae (NanA) is a cell wall-bound modular enzyme containing one lectin and one catalytic domain. Unlike homologous NanB and NanC expressed by the same bacterium, the two domains within one NanA molecule do not form a stable interaction and are spatially separated by a 16-amino acid-long flexible linker. In this work, the ability of NanA to form intermolecular assemblies was characterized using the methods of molecular modeling and bioinformatic analysis based on crystallographic data and by bringing together previously published experimental data. It was concluded that two catalytic domains, as well as one catalytic and one lectin domain, originating from two cell wall-bound NanA molecules, can interact through a previously uncharacterized interdomain interface to form complexes stabilized by a network of intermolecular hydrogen bonds and salt bridges. Supercomputer modeling strongly indicated that artocarpin, an earlier experimentally discovered inhibitor of the pneumococcal biofilm formation, is able to bind to a site located in the catalytic domain of one NanA entity and prevent its interaction with the lectin or catalytic domain of another NanA entity, thus directly precluding the generation of intermolecular assemblies. The revealed structural adaptation is discussed as one plausible mechanism of noncatalytic participation of this potentially key pathogenicity enzyme in pneumococcal biofilm formation.

Psp140: an immunodominant antigen in the supernatant of Streptococcus pneumoniae culture

Background and Objectives:

Streptococcus pneumoniae causes many lethal infections. Due to its reduced sensitivity to commonly used antibiotics, development of new strategies against pneumococcal infections seems to be necessary. We aimed to investigate immunodominant antigens in S. pneumoniae culture supernatant in order to develop novel targets for pneumococcal vaccines.

Materials and Methods:

In this study S. pneumoniae ATCC49619 was sub-cultured into BHI broth from overnight culture at 37°C for 4 h. The supernatant proteins were precipitated using acetone precipitation method. A rabbit was intramuscularly immunized with alum adjuvant and 100 μg pneumococcal supernatant proteins, 6 times at 14 days’ intervals to produce hyperimmune serum. ELISA assay was performed to determine the antibody level response to pneumococcal secretory proteins. Then dot blot applied for rapid evaluation of hyperimmune serum reactivity to pneumococcus supernatant proteins. The western blot was also used to determine the interaction of supernatant proteins with immunogenic rabbit’s hyperimmune-serum.

Results:

According to the western blot analysis, the immunodominant protein had 140KDa molecular weight and designated as pneumococcal secretory protein140 (Psp140).

Conclusion:

The Psp140 protein in the supernatant of S. pneumoniae culture is an immunodominant protein and it is likely related to pneumococcal secretory protein or surface exposed protein which released into culture supernatant during bacterial growth.

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.

Directed evolution of a remarkably efficient Kdnase from a bacterial neuraminidase

N-acetylneuraminic acid (5-acetamido-3,5-dideoxy-d-glycero-d-galacto-non-2-ulosonic acid), which is the principal sialic acid family member of the non-2-ulosonic acids and their various derivatives, is often found at the terminal position on the glycan chains that adorn all vertebrate cells. This terminal position combined with subtle variations in structure and linkage to the underlying glycan chains between humans and other mammals points to the importance of this diverse group of nine-carbon sugars as indicators of the unique aspects of human evolution and is relevant to understanding an array of human conditions. Enzymes that catalyze the removal N-acetylneuraminic acid from glycoconjugates are called neuraminidases. However, despite their documented role in numerous diseases, due to the promiscuous activity of many neuraminidases, our knowledge of the functions and metabolism of many sialic acids and the effect of the attachment to cellular glycans is limited. To this end, through a concerted effort of generation of random and site-directed mutagenesis libraries, subsequent screens and positive and negative evolutionary selection protocols, we succeeded in identifying three enzyme variants of the neuraminidase from the soil bacterium Micromonospora viridifaciens with markedly altered specificity for the hydrolysis of natural Kdn (3-deoxy-d-glycero-d-galacto-non-2-ulosonic acid) glycosidic linkages compared to those of N-acetylneuraminic acid. These variants catalyze the hydrolysis of Kdn-containing disaccharides with catalytic efficiencies (second-order rate constants: kcat/Km) of greater than 105 M−1 s−1; the best variant displayed an efficiency of >106 M−1 s−1 at its optimal pH.

Challenges and Pitfalls in Human Milk Oligosaccharide Analysis

Human milk oligosaccharides have been recognized as an important, functional biomolecule in mothers' milk. Moreover, these oligosaccharides have been recognized as the third most abundant component of human milk, ranging from 10-15 g/L in mature milk and up to and over 20 g/L reported in colostrum. Initially, health benefits of human milk oligosaccharides were assigned via observational studies on the differences between breastfed and bottle fed infants. Later, pools of milk oligosaccharides were isolated and used in functional studies and in recent years more specific studies into structure-function relationships have identified some advanced roles for milk oligosaccharides in the healthy development of infants. In other research, the levels, diversity, and complexity of human milk oligosaccharides have been studied, showing a wide variation in results. This review gives a critical overview of challenges in the analysis of human milk oligosaccharides. In view of the myriad functions that can be assigned, often to specific structures or classes of structures, it is very relevant to assess the levels of these structures in the human milk correctly, as well as in other biological sample materials. Ultimately, the review makes a case for a comparative, inter-laboratory study on quantitative human milk oligosaccharide analysis in all relevant biological samples.

Elucidation of a sialic acid metabolism pathway in mucus-foraging Ruminococcus gnavus unravels mechanisms of bacterial adaptation to the gut

Sialic acid (Neu5Ac) is commonly found in terminal location of colonic mucins glycans where it is a much-coveted nutrient for gut bacteria including Ruminococcus gnavus. R. gnavus is part of the healthy gut microbiota in humans but shows a disproportionate representation in diseases. There is therefore a need in understanding the molecular mechanisms underpinning its adaptation to the gut. Previous in vitro work demonstrated that R. gnavus mucin glycan-foraging strategy is strain-dependent and associated with the expression of an intramolecular trans-sialidase releasing 2,7-anhydro-Neu5Ac instead of Neu5Ac from mucins. Here, we have unravelled the metabolism pathway of 2,7-anhydro-Neu5Ac in R. gnavus which is underpinned by the exquisite specificity of the sialic transporter for 2,7-anhydro-Neu5Ac, and by the action of an oxidoreductase converting 2,7-anhydro-Neu5Ac into Neu5Ac which then becomes substrate of a Neu5Ac-specific aldolase. Having generated a R. gnavus nan cluster deletion mutant that lost the ability to grow on sialylated substrates, we showed that in gnotobiotic mice colonised with R. gnavus wild-type and mutant strains, the fitness of the nan mutant was significantly impaired with a reduced ability to colonise the mucus layer. Overall, our study revealed a unique sialic acid pathway in bacteria, with significant implications for the spatial adaptation of mucin-foraging gut symbionts in health and disease.

Identification of Virulence-Associated Properties by Comparative Genome Analysis of Streptococcus pneumoniae, S. pseudopneumoniae, S. mitis, Three S. oralis Subspecies, and S. infantis

Streptococcus pneumoniae is one of the most important human pathogens but is closely related to Streptococcus mitis, with which humans live in harmony. The fact that the two species evolved from a common ancestor provides a unique basis for studies of both infection-associated properties and properties important for harmonious coexistence with the host. By detailed comparisons of genomes of the two species and other related streptococci, we identified 224 genes associated with virulence and 25 genes unique to the mutualistic species. The exclusive presence of the virulence factors in S. pneumoniae enhances their potential as vaccine components, as a direct impact on beneficial members of the commensal microbiota can be excluded. Successful adaptation of S. mitis and other commensal streptococci to a harmonious relationship with the host relied on genetic stability and properties facilitating life in biofilms.

From a common ancestor, Streptococcus pneumoniae and Streptococcus mitis evolved in parallel into one of the most important pathogens and a mutualistic colonizer of humans, respectively. This evolutionary scenario provides a unique basis for studies of both infection-associated properties and properties important for harmonious coexistence with the host. We performed detailed comparisons of 60 genomes of S. pneumoniae, S. mitis, Streptococcus pseudopneumoniae, the three Streptococcus oralis subspecies oralis, tigurinus, and dentisani, and Streptococcus infantis. Nonfunctional remnants of ancestral genes in both S. pneumoniae and in S. mitis support the evolutionary model and the concept that evolutionary changes on both sides were required to reach their present relationship to the host. Confirmed by screening of >7,500 genomes, we identified 224 genes associated with virulence. The striking difference to commensal streptococci was the diversity of regulatory mechanisms, including regulation of capsule production, a significantly larger arsenal of enzymes involved in carbohydrate hydrolysis, and proteins known to interfere with innate immune factors. The exclusive presence of the virulence factors in S. pneumoniae enhances their potential as vaccine components, as a direct impact on beneficial members of the commensal microbiota can be excluded. In addition to loss of these virulence-associated genes, adaptation of S. mitis to a mutualistic relationship with the host apparently required preservation or acquisition of 25 genes lost or absent from S. pneumoniae. Successful adaptation of S. mitis and other commensal streptococci to a harmonious relationship with the host relied on genetic stability and properties facilitating life in biofilms.

Glycointeractions in bacterial pathogenesis

Many important interactions between bacterial pathogens and their hosts are highly specific binding events that involve host or pathogen carbohydrate structures (glycans). Glycan interactions can mediate adhesion, invasion and immune evasion and can act as receptors for toxins. Several bacterial pathogens can also enzymatically alter host glycans to reveal binding targets, degrade the host cell glycans or alter the function of host glycoproteins. In recent years, high-throughput screening technologies, such as lectin, glycan and mucin microarrays, have transformed the field by identifying new bacterial-host glycointeractions, which are crucial for colonization, persistence and disease. In this Review, we discuss interactions involving both host and bacterial glycans that have a role in bacterial pathogenesis. We also highlight recent technological advances that have illuminated the glycoscience of microbial pathogenesis.

Two complementary α-fucosidases from Streptococcus pneumoniae promote complete degradation of host-derived carbohydrate antigens

An important aspect of the interaction between the opportunistic bacterial pathogen Streptococcus pneumoniae and its human host is its ability to harvest host glycans. The pneumococcus can degrade a variety of complex glycans, including N- and O-linked glycans, glycosaminoglycans, and carbohydrate antigens, an ability that is tightly linked to the virulence of S. pneumoniae Although S. pneumoniae is known to use a sophisticated enzyme machinery to attack the human glycome, how it copes with fucosylated glycans, which are primarily histo-blood group antigens, is largely unknown. Here, we identified two pneumococcal enzymes, SpGH29^C and SpGH95^C, that target α-(1→3/4) and α-(1→2) fucosidic linkages, respectively. X-ray crystallography studies combined with functional assays revealed that SpGH29^C is specific for the Lewis^A and Lewis^X antigen motifs and that SpGH95^C is specific for the H(O)-antigen motif. Together, these enzymes could defucosylate Lewis^Y and Lewis^B antigens in a complementary fashion. In vitro reconstruction of glycan degradation cascades disclosed that the individual or combined activities of these enzymes expose the underlying glycan structure, promoting the complete deconstruction of a glycan that would otherwise be resistant to pneumococcal enzymes. These experiments expand our understanding of the extensive capacity of S. pneumoniae to process host glycans and the likely roles of α-fucosidases in this. Overall, given the importance of enzymes that initiate glycan breakdown in pneumococcal virulence, such as the neuraminidase NanA and the mannosidase SpGH92, we anticipate that the α-fucosidases identified here will be important factors in developing more refined models of the S. pneumoniae-host interaction.

Role of Pneumococcal NanA Neuraminidase Activity in Peripheral Blood

The most frequent form of hemolytic-uremic syndrome (HUS) is associated with infections caused by Shiga-like toxin-producing Enterohaemorrhagic Escherichia coli (STEC). In rarer cases HUS can be triggered by Streptococcus pneumoniae. While production of Shiga-like toxins explains STEC-HUS, the mechanisms of pneumococcal HUS are less well-known. S. pneumoniae produces neuraminidases with activity against cell surface sialic acids that are critical for factor H-mediated complement regulation on cells and platelets. The aim of this study was to find out whether S. pneumoniae neuraminidase NanA could trigger complement activation and hemolysis in whole blood. We studied clinical S. pneumoniae isolates and two laboratory strains, a wild-type strain expressing NanA, and a NanA deletion mutant for their ability to remove sialic acids from various human cells and platelets. Red blood cell lysis and activation of complement was measured ex vivo by incubating whole blood with bacterial culture supernatants. We show here that NanA expressing S. pneumoniae strains and isolates are able to remove sialic acids from cells, and platelets. Removal of sialic acids by NanA increased complement activity in whole blood, while absence of NanA blocked complement triggering and hemolytic activity indicating that removal of sialic acids by NanA could potentially trigger pHUS.

9-Azido-9-deoxy-2,3-difluorosialic Acid as a Subnanomolar Inhibitor against Bacterial Sialidases

A library of 2(a),3(a/e)-difluorosialic acids and their C-5 and/or C-9 derivatives were chemoenzymatically synthesized. Pasteurella multocida sialic acid aldolase (PmAldolase), but not its Escherichia coli homologue (EcAldolase), was found to catalyze the formation of C5-azido analogue of 3-fluoro(a)-sialic acid. In comparison, both PmAldolase and EcAldolase could catalyze the synthesis of 3-fluoro(a/e)-sialic acids and their C-9 analogues although PmAldolase was generally more efficient. The chemoenzymatically synthesized 3-fluoro(a/e)-sialic acid analogues were purified and chemically derivatized to form the desired difluorosialic acids and derivatives. Inhibition studies against several bacterial sialidases and a recombinant human cytosolic sialidase hNEU2 indicated that sialidase inhibition was affected by the C-3 fluorine stereochemistry and derivatization at C-5 and/or C-9 of the inhibitor. Opposite to that observed for influenza A virus sialidases and hNEU2, compounds with axial fluorine at C-3 were better inhibitors (up to 100-fold) against bacterial sialidases compared to their 3F-equatorial counterparts. While C-5-modified compounds were less-efficient antibacterial sialidase inhibitors, 9-N₃-modified 2,3-difluoro-Neu5Ac showed increased inhibitory activity against bacterial sialidases. 9-Azido-9-deoxy-2-(e)-3-(a)-difluoro- N-acetylneuraminic acid [2(e)3(a)DFNeu5Ac9N₃] was identified as an effective inhibitor with a long effective duration selectively against pathogenic bacterial sialidases from Clostridium perfringens (CpNanI) and Vibrio cholerae.

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.

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.

An Infant Mouse Model of Influenza Virus Transmission Demonstrates the Role of Virus-Specific Shedding, Humoral Immunity, and Sialidase Expression by Colonizing Streptococcus pneumoniae

The pandemic potential of influenza A viruses (IAV) depends on the infectivity of the host, transmissibility of the virus, and susceptibility of the recipient. While virus traits supporting IAV transmission have been studied in detail using ferret and guinea pig models, there is limited understanding of host traits determining transmissibility and susceptibility because current animal models of transmission are not sufficiently tractable. Although mice remain the primary model to study IAV immunity and pathogenesis, the efficiency of IAV transmission in adult mice has been inconsistent. Here we describe an infant mouse model that supports efficient transmission of IAV. We demonstrate that transmission in this model requires young age, close contact, shedding of virus particles from the upper respiratory tract (URT) of infected pups, the use of a transmissible virus strain, and a susceptible recipient. We characterize shedding as a marker of infectiousness that predicts the efficiency of transmission among different influenza virus strains. We also demonstrate that transmissibility and susceptibility to IAV can be inhibited by humoral immunity via maternal-infant transfer of IAV-specific immunoglobulins and modifications to the URT milieu, via sialidase activity of colonizing Streptococcus pneumoniae Due to its simplicity and efficiency, this model can be used to dissect the host's contribution to IAV transmission and explore new methods to limit contagion.IMPORTANCE This study provides insight into the role of the virus strain, age, immunity, and URT flora on IAV shedding and transmission efficiency. Using the infant mouse model, we found that (i) differences in viral shedding of various IAV strains are dependent on specific hemagglutinin (HA) and/or neuraminidase (NA) proteins, (ii) host age plays a key role in the efficiency of IAV transmission, (iii) levels of IAV-specific immunoglobulins are necessary to limit infectiousness, transmission, and susceptibility to IAV, and (iv) expression of sialidases by colonizing S. pneumoniae antagonizes transmission by limiting the acquisition of IAV in recipient hosts. Our findings highlight the need for strategies that limit IAV shedding and the importance of understanding the function of the URT bacterial composition in IAV transmission. This work reinforces the significance of a tractable animal model to study both viral and host traits affecting IAV contagion and its potential for optimizing vaccines and therapeutics that target disease spread.

Strategies for chemoenzymatic synthesis of carbohydrates

Carbohydrates are structurally complex but functionally important biomolecules. Therefore, they have been challenging but attractive synthetic targets. While substantial progress has been made on advancing chemical glycosylation methods, incorporating enzymes into carbohydrate synthetic schemes has become increasingly practical as more carbohydrate biosynthetic and metabolic enzymes as well as their mutants with synthetic application are identified and expressed for preparative and large-scale synthesis. Chemoenzymatic strategies that integrate the flexibility of chemical derivatization with enzyme-catalyzed reactions have been extremely powerful. Briefly summarized here are our experiences on developing one-pot multienzyme (OPME) systems and representative chemoenzymatic strategies from others using glycosyltransferase-catalyzed reactions for synthesizing diverse structures of oligosaccharides, polysaccharides, and glycoconjugates. These strategies allow the synthesis of complex carbohydrates including those containing naturally occurring carbohydrate postglycosylational modifications (PGMs) and non-natural functional groups. By combining these srategies with facile purification schemes, synthetic access to the diverse space of carbohydrate structures can be automated and will not be limited to specialists.

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.

Triazole-linked transition state analogs as selective inhibitors against V. cholerae sialidase

Sialidases or neuraminidases are enzymes that catalyze the cleavage of terminal sialic acids from oligosaccharides and glycoconjugates. They play important roles in bacterial and viral infection and have been attractive targets for drug development. Structure-based drug design has led to potent inhibitors against neuraminidases of influenza A viruses that have been used successfully as approved therapeutics. However, selective and effective inhibitors against bacterial and human sialidases are still being actively pursued. Guided by crystal structural analysis, several derivatives of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en or DANA) were designed and synthesized as triazole-linked transition state analogs. Inhibition studies revealed that glycopeptide analog E-(TriazoleNeu5Ac2en)-AKE and compound (TriazoleNeu5Ac2en)-A were selective inhibitors against Vibrio cholerae sialidase, while glycopeptide analog (TriazoleNeu5Ac2en)-AdE selectively inhibited Vibrio cholerae and A. ureafaciens sialidases.

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.

Desialylation of Platelets by Pneumococcal Neuraminidase A Induces ADP-Dependent Platelet Hyperreactivity

Platelets are increasingly recognized to play a role in the complications of Streptococcus pneumoniae infections. S. pneumoniae expresses neuraminidases, which may alter glycans on the platelet surface. In the present study, we investigated the capability of pneumococcal neuraminidase A (NanA) to remove sialic acid (desialylation) from the platelet surface, the consequences for the platelet activation status and reactivity, and the ability of neuraminidase inhibitors to prevent these effects. Our results show that soluble NanA induces platelet desialylation. Whereas desialylation itself did not induce platelet activation (P-selectin expression and platelet fibrinogen binding), platelets became hyperreactive to ex vivo stimulation by ADP and cross-linked collagen-related peptide (CRP-XL). Platelet aggregation with leukocytes also increased. These processes were dependent on the ADP pathway, as inhibitors of the pathway (apyrase and ticagrelor) abrogated platelet hyperreactivity. Inhibition of NanA-induced platelet desialylation by neuraminidase inhibitors (e.g., oseltamivir acid) also prevented the platelet effects of NanA. Collectively, our findings show that soluble NanA can desialylate platelets, leading to platelet hyperreactivity, which can be prevented by neuraminidase inhibitors.

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.

Neuraminidase A from Streptococcus pneumoniae has a modular organization of catalytic and lectin domains separated by a flexible linker

Neuraminidase A (NanA) of the pathogen Streptococcus pneumoniae cleaves receptors of the human respiratory epithelial surface during bacterial colonization. The full-size structure of NanA that contains one lectin and one catalytic domain within a single polypeptide chain remains unresolved. Both domains are crucial for the microorganism's virulence and considered as promising antimicrobial targets. Methods of bioinformatics and molecular dynamics have been implemented to model NanA's structure and study interaction between the lectin and catalytic domains in three neuraminidases NanA, NanB, and NanC from Streptococcus pneumoniae. A significant difference in spatial organization of these homologous enzymes has been revealed. The lectin and catalytic domains of NanB and NanC form rigid globules stabilized by multiple interdomain interactions, whereas in NanA, the two domains are separated by a 16 amino acids long flexible linker - a characteristic of proteins that require conformational flexibility for their functioning. The biological role of this structural adaptation of NanA as a key virulence enzyme is discussed.

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.

Chemoenzymatic synthesis of para-nitrophenol (pNP)-tagged α2-8-sialosides and high-throughput substrate specificity studies of α2-8-sialidases

para-Nitrophenol (pNP)-tagged α2-8-linked sialosides containing different sialic acid forms were chemoenzymatically synthesized using an efficient one-pot three-enzyme α2-8-sialylation system. The resulting compounds allowed high-throughput substrate specificity studies of the α2-8-sialidase activity of a recombinant human cytosolic sialidase hNEU2 and various bacterial sialidases. The sialoside substrate profiles obtained can be used to guide the selection of suitable sialidases for sialylglycan analysis and for cell and tissue surface glycan modification. They can also be used to guide sialidase inhibitor design.

Sialidase-catalyzed one-pot multienzyme (OPME) synthesis of sialidase transition-state analogue inhibitors

Sialidase transition state analog inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (Neu5Ac2en, DANA) has played a leading role in developing clinically used anti-influenza virus drugs. Taking advantage of the Neu5Ac2en-forming catalytic property of Streptococcus pneumoniae sialidase SpNanC, an effective one-pot multienzyme (OPME) strategy has been developed to directly access Neu5Ac2en and its C-5, C-9, and C-7-analogs from N-acetylmannosamine (ManNAc) and analogs. The obtained Neu5Ac2en analogs can be further derivatized at various positions to generate a larger inhibitor library. Inhibition studies demonstrated improved selectivity of several C-5- or C-9-modified Neu5Ac2en derivatives against several bacterial sialidases. The study provides an efficient enzymatic method to access sialidase inhibitors with improved selectivity.

Influenza-associated thrombotic microangiopathies

Thrombotic microangiopathy (TMA) refers to phenotypically similar disorders, including hemolytic uremic syndromes (HUS) and thrombotic thrombocytopenic purpura (TTP). This review explores the role of the influenza virus as trigger of HUS or TTP. We conducted a literature survey in PubMed and Google Scholar using HUS, TTP, TMA, and influenza as keywords, and extracted and analyzed reported epidemiological and clinical data. We identified 25 cases of influenza-associated TMA. Five additional cases were linked to influenza vaccination and analyzed separately. Influenza A was found in 83%, 10 out of 25 during the 2009 A(H1N1) pandemic. Two patients had bona fide TTP with ADAMTS13 activity <10%. Median age was 15 years (range 0.5-68 years), two thirds were male. Oligoanuria was documented in 81% and neurological involvement in 40% of patients. Serum C3 was reduced in 5 out of 14 patients (36%); Coombs test was negative in 7 out of 7 and elevated fibrin/fibrinogen degradation products were documented in 6 out of 8 patients. Pathogenic complement gene mutations were found in 7 out of 8 patients tested (C3, MCP, or MCP combined with CFB or clusterin). Twenty out of 24 patients recovered completely, but 3 died (12%). Ten of the surviving patients underwent plasma exchange (PLEX) therapy, 5 plasma infusions. Influenza-mediated HUS or TTP is rare. A sizable proportion of tested patients demonstrated mutations associated with alternative pathway of complement dysregulation that was uncovered by this infection. Further research is warranted targeting the roles of viral neuraminidase, enhanced virus-induced complement activation and/or ADAMTS13 antibodies, and rational treatment approaches.

Chemoenzymatic synthesis of Neu5Ac9NAc-containing α2-3- and α2-6-linked sialosides and their use for sialidase substrate specificity studies

O-Acetylation of sialic acid (Sia) modulates its recognition by sialic acid-binding proteins and plays an important role in biological and pathological processes. 9-O-Acetylation is the most common modification of sialic acid in human. However, study of O-acetylated sialoglycans is hampered due to the instability of O-acetyl group towards pH changes and sensitivity to esterases. Our previous studies demonstrated a chemical biology method to this problem by replacing the oxygen atom in the C9 ester group of sialic acid by a nitrogen to form an amide. Here, we synthesized a library of sixteen new 9-acetamido-9-deoxy-N-acetylneuraminic acid (Neu5Ac9NAc)-containing α2-3- and α2-6-linked sialosides with various underlying glycans using efficient one-pot three-enzyme (OP3E) sialylation systems. Neu5Ac9NAc-containing compounds with a para-nitrophenol aglycon have been used together with their 9-O-acetyl analogs in microtiter plate-based high-throughput substrate specificity studies of nine different sialidases including those from humans and bacteria. In general, similar to 9-O-acetylation, 9-N-acetyl modification of sialic acid in the substrates lowers sialic acid-cleavage activity of most sialidases. In most cases, Neu5Ac9NAc is a good analog of 9-O-acetyl sialic acid. However, exceptions do exist. For example, 9-N- and 9-O-acetyl modifications have different effects on the sialosides cleave efficiencies of a commercially available C. perfringens sialidase as well as recombinant Streptococcus pneumoniae sialidase SpNanC and Bifidobacterium infantis sialidase BiNanH2. The mechanism for the difference awaits further investigation.

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

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