Identification of a pneumococcal glycosidase that modifies O-linked glycans

Neuraminidase-mediated enhancement of Streptococcus pneumoniae colonization is associated with altered mucus characteristics and distribution

Upon entry into the upper respiratory tract (URT), Streptococcus pneumoniae (Spn) upregulates neuraminidases (NA) that cleave sialic acid (SA) from host glycans. Because sialylation is thought to contribute to the physical properties that determine mucus function, we posited that Spn directly alters host mucus through NA activity. By directly imaging the colonized URT, we demonstrated NA-mediated alterations to the characteristics and distribution of mucus along the respiratory epithelium, where colonizing bacteria are found. Mucus exposed to NA showed increased localization within goblet cells and lining the glycocalyx. By contrast, NA-naïve mucus was more likely to be observed sloughing away from the epithelial surface. We also visualized Spn in the URT and observed that NA promoted efficient bacterial localization to the firm mucus layer overlying the glycocalyx, whereas NA-deficient Spn was associated more with loose mucus. By facilitating tighter association with the glycocalyx, NA promoted increased Spn colonization density. The magnitude of the NA-mediated effect on colonization was widened during late colonization by increased evasion of host-mediated clearance mechanisms. Thus, Spn-encoded NAs directly modify the host environment by desialylating mucus, which allows close interaction with mucus at the epithelium, and this is associated with enhanced bacterial colonization.

IMPORTANCE

Although severe illness and death caused by Spn result from secondary invasive diseases including pneumonia, sepsis, and meningitis, stable colonization of the upper respiratory tract (URT) is a prerequisite to invasive disease. Therefore, understanding host-Spn dynamics during asymptomatic colonization of the URT is warranted with respect to the pathogenesis of Spn disease. In this study, we found that Spn NA activity directly alters mucus characteristics that result in increased density and duration of URT colonization. Therefore, targeting Spn NA activity during URT colonization may be a viable strategy to mitigate Spn infection.

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.

Sickly Sweet - How Sugar Utilization Impacts Pneumococcal Disease Progression

Streptococcus pneumoniae is a major human pathogen that can spread to multiple sites in the body. However, the mechanisms dictating disease spread are not well understood. Here we highlight the importance of carbohydrate utilization systems on pneumococcal disease, offering insight into how this pathogen causes a spectrum of disease.

Immunoglobulin A Glycosylation and Its Role in Disease

Human IgA is comprised of two subclasses, IgA1 and IgA2. Monomeric IgA (mIgA), polymeric IgA (pIgA), and secretory IgA (SIgA) are the main molecular forms of IgA. The production of IgA rivals all other immunoglobulin isotypes. The large quantities of IgA reflect the fundamental roles it plays in immune defense, protecting vulnerable mucosal surfaces against invading pathogens. SIgA dominates mucosal surfaces, whereas IgA in circulation is predominately monomeric. All forms of IgA are glycosylated, and the glycans significantly influence its various roles, including antigen binding and the antibody effector functions, mediated by the Fab and Fc portions, respectively. In contrast to its protective role, the aberrant glycosylation of IgA1 has been implicated in the pathogenesis of autoimmune diseases, such as IgA nephropathy (IgAN) and IgA vasculitis with nephritis (IgAVN). Furthermore, detailed characterization of IgA glycosylation, including its diverse range of heterogeneity, is of emerging interest. We provide an overview of the glycosylation observed for each subclass and molecular form of IgA as well as the range of heterogeneity for each site of glycosylation. In many ways, the role of IgA glycosylation is in its early stages of being elucidated. This chapter provides an overview of the current knowledge and research directions.

An in vivo atlas of host-pathogen transcriptomes during Streptococcus pneumoniae colonization and disease

Streptococcus pneumoniae (Spn) colonizes the nasopharynx and can cause pneumonia. From the lungs it spreads to the bloodstream and causes organ damage. We characterized the in vivo Spn and mouse transcriptomes within the nasopharynx, lungs, blood, heart, and kidneys using three Spn strains. We identified Spn genes highly expressed at all anatomical sites and in an organ-specific manner; highly expressed genes were shown to have vital roles with knockout mutants. The in vivo bacterial transcriptome during colonization/disease was distinct from previously reported in vitro transcriptomes. Distinct Spn and host gene-expression profiles were observed during colonization and disease states, revealing specific genes/operons whereby Spn adapts to and influences host sites in vivo. We identified and experimentally verified host-defense pathways induced by Spn during invasive disease, including proinflammatory responses and the interferon response. These results shed light on the pathogenesis of Spn and identify therapeutic targets.

On enzymatic remodeling of IgG glycosylation; unique tools with broad applications

The importance of IgG glycosylation has been known for many years not only by scientists in glycobiology but also by human pathogens that have evolved specific enzymes to modify these glycans with fundamental impact on IgG function. The rise of IgG as a major therapeutic scaffold for many cancer and immunological indications combined with the availability of unique enzymes acting specifically on IgG Fc-glycans have spurred a range of applications to study this important post-translational modification on IgG. This review article introduces why the IgG glycans are of distinguished interest, gives a background on the unique enzymatic tools available to study the IgG glycans and finally presents an overview of applications utilizing these enzymes for various modifications of the IgG glycans. The applications covered include site-specific glycan transglycosylation and conjugation, analytical workflows for monoclonal antibodies and serum diagnostics. Additionally, the review looks ahead and discusses the importance of O-glycosylation for IgG3, Fc-fusion proteins and other new formats of biopharmaceuticals.

Role of BgaA as a Pneumococcal Virulence Factor Elucidated by Molecular Evolutionary Analysis

Streptococcus pneumoniae is a major cause of pneumonia, sepsis, and meningitis. Previously, we identified a novel virulence factor by investigating evolutionary selective pressure exerted on pneumococcal choline-binding cell surface proteins. Herein, we focus on another pneumococcal cell surface protein. Cell wall-anchoring proteins containing the LPXTG motif are conserved in Gram-positive bacteria. Our evolutionary analysis showed that among the examined genes, nanA and bgaA had high proportions of codon that were under significant negative selection. Both nanA and bgaA encode a multi-functional glycosidase that aids nutrient acquisition in a glucose-poor environment, pneumococcal adherence to host cells, and evasion from host immunity. However, several studies have shown that the role of BgaA is limited in a mouse pneumonia model, and it remains unclear if BgaA affects pneumococcal pathogenesis in a mouse sepsis model. To evaluate the distribution and pathogenicity of bgaA, we performed phylogenetic analysis and intravenous infection assay. In both Bayesian and maximum likelihood phylogenetic trees, the genetic distances between pneumococcal bgaA was small, and the cluster of pneumococcal bgaA did not contain other bacterial orthologs except for a Streptococcus gwangjuense gene. Evolutionary analysis and BgaA structure indicated BgaA active site was not allowed to change. The mouse infection assay showed that the deletion of bgaA significantly reduced host mortality. These results indicated that both nanA and bgaA encode evolutionally conserved pneumococcal virulence factors and that molecular evolutionary analysis could be a useful alternative strategy for identification of virulence factors.

Immune exclusion by naturally acquired secretory IgA against pneumococcal pilus-1

Successful infection by mucosal pathogens requires overcoming the mucus barrier. To better understand this key step, we performed a survey of the interactions between human respiratory mucus and the human pathogen Streptococcus pneumoniae. Pneumococcal adherence to adult human nasal fluid was seen only by isolates expressing pilus-1. Robust binding was independent of pilus-1 adhesive properties but required Fab-dependent recognition of RrgB, the pilus shaft protein, by naturally acquired secretory IgA (sIgA). Pilus-1 binding by specific sIgA led to bacterial agglutination, but adherence required interaction of agglutinated pneumococci and entrapment in mucus particles. To test the effect of these interactions in vivo, pneumococci were preincubated with human sIgA before intranasal challenge in a mouse model of colonization. sIgA treatment resulted in rapid immune exclusion of pilus-expressing pneumococci. Our findings predict that immune exclusion would select for nonpiliated isolates in individuals who acquired RrgB-specific sIgA from prior episodes of colonization with piliated strains. Accordingly, genomic data comparing isolates carried by mothers and their children showed that mothers are less likely to be colonized with pilus-expressing strains. Our study provides a specific example of immune exclusion involving naturally acquired antibody in the human host, a major factor driving pneumococcal adaptation.

Paradoxical Pro-inflammatory Responses by Human Macrophages to an Amoebae Host-Adapted Legionella Effector

Legionella pneumophila has co-evolved with amoebae, their natural hosts. Upon transmission to humans, the bacteria proliferate within alveolar macrophages causing pneumonia. Here, we show L. pneumophila injects the effector LamA, an amylase, into the cytosol of human macrophage (hMDMs) and amoebae to rapidly degrade glycogen to generate cytosolic hyper-glucose. In response, hMDMs shift their metabolism to aerobic glycolysis, which directly triggers an M1-like pro-inflammatory differentiation and nutritional innate immunity through enhanced tryptophan degradation. This leads to a modest restriction of bacterial proliferation in hMDMs. In contrast, LamA-mediated glycogenolysis in amoebae deprives the natural host from the main building blocks for synthesis of the cellulose-rich cyst wall, leading to subversion of amoeba encystation. This is non-permissive for bacterial proliferation. Therefore, LamA of L. pneumophila is an amoebae host-adapted effector that subverts encystation of the amoebae natural host, and the paradoxical hMDMs' pro-inflammatory response is likely an evolutionary accident.

Host-glycan metabolism is regulated by a species-conserved two-component system in Streptococcus pneumoniae

Pathogens of the Streptococcus genus inhabit many different environmental niches during the course of an infection in a human host and the bacteria must adjust their metabolism according to available nutrients. Despite their lack of the citric-acid cycle, some streptococci proliferate in niches devoid of a readily available carbohydrate source. Instead they rely on carbohydrate scavenging for energy acquisition, which are obtained from the host. Here we discover a two-component system (TCS07) of Streptococcus pneumoniae that responds to glycoconjugated structures on proteins present on the host cells. Using next-generation RNA sequencing we find that the uncharacterized TCS07 regulon encodes proteins important for host-glycan processing and transporters of the released glycans, as well as intracellular carbohydrate catabolizing enzymes. We find that a functional TCS07 allele is required for growth on the glycoconjugated model protein fetuin. Consistently, we see a TCS07-dependent activation of the glycan degradation pathway. Thus, we pinpoint the molecular constituents responsible for sensing host derived glycans and link this to the induction of the proteins necessary for glycan degradation. Furthermore, we connect the TCS07 regulon to virulence in a mouse model, thereby establishing that host-derived glycan-metabolism is important for infection in vivo. Finally, a comparative phylogenomic analysis of strains from the Streptococcus genus reveal that TCS07 and most of its regulon is specifically conserved in species that utilize host-glycans for growth.

Worldwide, Streptococcus pneumoniae is the most common cause of community acquired pneumonia with high mortality rates. Interestingly, S. pneumoniae strictly relies on carbohydrate scavenging for energy acquisition, which are obtained from the host. This is a critical step in pathogenesis and a common mechanism among Streptococcal species. In this study, we discover an uncharacterized two-component system that responds to the carbohydrate structures present on the host cells. These are important findings as we describe the molecular mechanism responsible for sensing these host derived glycans, and how this mechanism is linked to virulence, thus highlighting that glycan metabolism is important for infection in vivo, thereby posing a novel target for intervention. Our phylogenetic analysis reveals that the two-component system and the genetic regulon co-occur and are specifically conserved among Streptococcal species capable of degrading host-glycans.

The EngCP endo α-N-acetylgalactosaminidase is a virulence factor involved in Clostridium perfringens gas gangrene infections

Clostridium perfringens is the causative agent of human clostridial myonecrosis; the major toxins involved in this disease are α-toxin and perfringolysin O. The RevSR two-component regulatory system has been shown to be involved in regulating virulence in a mouse myonecrosis model. Previous microarray and RNAseq analysis of a revR mutant implied that factors other than the major toxins may play a role in virulence. The RNAseq data showed that the expression of the gene encoding the EngCP endo α-N-acetylgalactosaminidase (CPE0693) was significantly down-regulated in a revR mutant. Enzymes from this family have been identified in several Gram-positive pathogens and have been postulated to contribute to their virulence. In this study, we constructed an engCP mutant of C. perfringens and showed that it was significantly less virulent than its wild-type parent strain. Virulence was restored by complementation in trans with the wild-type engCP gene. We also demonstrated that purified EngCP was able to hydrolyse α-dystroglycan derived from C2C12 mouse myotubes. However, EngCP had little effect on membrane permeability in mice, suggesting that EngCP may play a role other than the disruption of the structural integrity of myofibres. Glycan array analysis indicated that EngCP could recognise structures containing the monosaccharide N-acetlygalactosamine at 4C, but could recognise structures terminating in galactose, glucose and N-acetylglucosamine under conditions where EngCP was enzymatically active. In conclusion, we have obtained evidence that EngCP is required for virulence in C. perfringens and, although classical exotoxins are important for disease, we have now shown that an O-glycosidase also plays an important role in the disease process.

Molecular analysis of an enigmatic Streptococcus pneumoniae virulence factor: The raffinose-family oligosaccharide utilization system

Streptococcus pneumoniae is an opportunistic respiratory pathogen that can spread to other body sites, including the ears, brain, and blood. The ability of this bacterium to break down, import, and metabolize a wide range of glycans is key to its virulence. Intriguingly, S. pneumoniae can utilize several plant oligosaccharides for growth in vitro, including raffinose-family oligosaccharides (RFOs, which are α-(1→6)-galactosyl extensions of sucrose). An RFO utilization locus has been identified in the pneumococcal genome; however, none of the proteins encoded by this locus have been biochemically characterized. The enigmatic ability of S. pneumoniae to utilize RFOs has recently received attention because mutations in two of the RFO locus genes have been linked to the tissue tropism of clinical pneumococcal isolates. Here, we use functional studies combined with X-ray crystallography to show that although the pneumococcal RFO locus encodes for all the machinery required for uptake and degradation of RFOs, the individual pathway components are biochemically inefficient. We also demonstrate that the initiating enzyme in this pathway, the α-galactosidase Aga (a family 36 glycoside hydrolase), can cleave α-(1→3)-linked galactose units from a linear blood group antigen. We propose that the pneumococcal RFO pathway is an evolutionary relic that is not utilized in this streptococcal species and, as such, is under no selection pressure to maintain binding affinity and/or catalytic efficiency. We speculate that the apparent contribution of RFO utilization to pneumococcal tissue tropism may, in fact, be due to the essential role the ATPase RafK plays in the transport of other carbohydrates.

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.

Identification of Pneumococcal Factors Affecting Pneumococcal Shedding Shows that the dlt Locus Promotes Inflammation and Transmission

Streptococcus pneumoniae (the pneumococcus) is a common cause of respiratory tract and invasive infection. The overall effectiveness of immunization with the organism’s capsular polysaccharide depends on its ability to block colonization of the upper respiratory tract and thereby prevent host-to-host transmission. Because of the limited coverage of current pneumococcal vaccines, we carried out an unbiased in vivo transposon mutagenesis screen to identify pneumococcal factors other than its capsular polysaccharide that affect transmission. One such candidate was expressed by the dlt locus, previously shown to add d-alanine onto the pneumococcal lipoteichoic acid present on the bacterial cell surface. This modification protects against host antimicrobials and augments host inflammatory responses. The latter increases secretions and bacterial shedding from the upper respiratory tract to allow for transmission. Thus, this study provides insight into a mechanism employed by the pneumococcus to successfully transit from one host to another.

Host-to-host transmission is a necessary but poorly understood aspect of microbial pathogenesis. Herein, we screened a genomic library of mutants of the leading respiratory pathogen Streptococcus pneumoniae generated by mariner transposon mutagenesis (Tn-Seq) to identify genes contributing to its exit or shedding from the upper respiratory tract (URT), the limiting step in the organism’s transmission in an infant mouse model. Our analysis focused on genes affecting the bacterial surface that directly impact interactions with the host. Among the multiple factors identified was the dlt locus, which adds d-alanine onto lipoteichoic acids (LTA) and thereby increases Toll-like receptor 2-mediated inflammation and resistance to antimicrobial peptides. The more robust proinflammatory response in the presence of d-alanylation promotes secretions that facilitate pneumococcal shedding and allows for transmission. Expression of the dlt locus is controlled by the CiaRH system, which senses cell wall stress in response to antimicrobial activity, including in response to lysozyme, the most abundant antimicrobial along the URT mucosa. Accordingly, in a lysM^−/− host, there was no longer an effect of the dlt locus on pneumococcal shedding. Thus, our findings demonstrate how a pathogen senses the URT milieu and then modifies its surface characteristics to take advantage of the host response for transit to another host.

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.

First report of two complete Clostridium chauvoei genome sequences and detailed in silico genome analysis

Clostridium (C.) chauvoei is a Gram-positive, spore forming, anaerobic bacterium. It causes black leg in ruminants, a typically fatal histotoxic myonecrosis. High quality circular genome sequences were generated for the C. chauvoei type strain DSM 7528^T (ATCC 10092^T) and a field strain 12S0467 isolated in Germany. The origin of replication (oriC) was comparable to that of Bacillus subtilis in structure with two regions containing DnaA boxes. Similar prophages were identified in the genomes of both C. chauvoei strains which also harbored hemolysin and bacterial spore formation genes. A CRISPR type I-B system with limited variations in the repeat number was identified. Sporulation and germination process related genes were homologous to that of the Clostridia cluster I group but novel variations for regulatory genes were identified indicative for strain specific control of regulatory events. Phylogenomics showed a higher relatedness to C. septicum than to other so far sequenced genomes of species belonging to the genus Clostridium. Comparative genome analysis of three C. chauvoei circular genome sequences revealed the presence of few inversions and translocations in locally collinear blocks (LCBs). The species genome also shows a large number of genes involved in proteolysis, genes for glycosyl hydrolases and metal iron transportation genes which are presumably involved in virulence and survival in the host. Three conserved flagellar genes (fliC) were identified in each of the circular genomes. In conclusion this is the first comparative analysis of circular genomes for the species C. chauvoei, enabling insights into genome composition and virulence factor variation.

The accessory Sec system (SecY2A2) in Streptococcus pneumoniae is involved in export of pneumolysin toxin, adhesion and biofilm formation

In Streptococcus pneumoniae TIGR4, genes encoding a SecY2A2 accessory Sec system are present within a locus encoding a serine-rich repeat surface protein PsrP. Mutant strains deleted in secA2 or psrP were deficient in biofilm formation, while the ΔsecA2 mutant was reduced in binding to airway epithelial cells. Cell wall protein (CWP) fractions from the ΔsecA2 mutant, but not from the ΔpsrP mutant, were reduced in haemolytic (pneumolysin) activity. Contact-dependent pneumolysin (Ply) activity of wild type TIGR4 cells was ten-fold greater than that of ΔsecA2 mutant cells suggesting that Ply was not active at the ΔsecA2 cell surface. Ply protein was found to be present in the CWP fraction from the ΔsecA2 mutant, but showed aberrant electrophoretic migration indicative of protein modification. Proteomic analyses led to the discovery that the ΔsecA2 mutant CWP fraction was deficient in two glycosidases as well as other enzymes involved in carbohydrate metabolism. Taken collectively the results suggest that positioning of Ply into the cell wall compartment in active form, together with glycosyl hydrolases and adhesins, requires a functional accessory Sec system.

Deacetylation of sialic acid by esterases potentiates pneumococcal neuraminidase activity for mucin utilization, colonization and virulence

Pneumococcal neuraminidase is a key enzyme for sequential deglycosylation of host glycans, and plays an important role in host survival, colonization, and pathogenesis of infections caused by Streptococcus pneumoniae. One of the factors that can affect the activity of neuraminidase is the amount and position of acetylation present in its substrate sialic acid. We hypothesised that pneumococcal esterases potentiate neuraminidase activity by removing acetylation from sialic acid, and that will have a major effect on pneumococcal survival on mucin, colonization, and virulence. These hypotheses were tested using isogenic mutants and recombinant esterases in microbiological, biochemical and in vivo assays. We found that pneumococcal esterase activity is encoded by at least four genes, SPD_0534 (EstA) was found to be responsible for the main esterase activity, and the pneumococcal esterases are specific for short acyl chains. Assay of esterase activity by using natural substrates showed that both the Axe and EstA esterases could use acetylated xylan and Bovine Sub-maxillary Mucin (BSM), a highly acetylated substrate, but only EstA was active against tributyrin (triglyceride). Incubation of BSM with either Axe or EstA led to the acetate release in a time and concentration dependent manner, and pre-treatment of BSM with either enzyme increased sialic acid release on subsequent exposure to neuraminidase A. qRT-PCR results showed that the expression level of estA and axe increased when exposed to BSM and in respiratory tissues. Mutation of estA alone or in combination with nanA (codes for neuraminidase A), or the replacement of its putative serine active site to alanine, reduced the pneumococcal ability to utilise BSM as a sole carbon source, sialic acid release, colonization, and virulence in a mouse model of pneumococcal pneumonia.

Molecular Characterization of N-glycan Degradation and Transport in Streptococcus pneumoniae and Its Contribution to Virulence

The carbohydrate-rich coating of human tissues and cells provide a first point of contact for colonizing and invading bacteria. Commensurate with N-glycosylation being an abundant form of protein glycosylation that has critical functional roles in the host, some host-adapted bacteria possess the machinery to process N-linked glycans. The human pathogen Streptococcus pneumoniae depolymerizes complex N-glycans with enzymes that sequentially trim a complex N-glycan down to the Man₃GlcNAc₂ core prior to the release of the glycan from the protein by endo-β-N-acetylglucosaminidase (EndoD), which cleaves between the two GlcNAc residues. Here we examine the capacity of S. pneumoniae to process high-mannose N-glycans and transport the products. Through biochemical and structural analyses we demonstrate that S. pneumoniae also possesses an α-(1,2)-mannosidase (SpGH92). This enzyme has the ability to trim the terminal α-(1,2)-linked mannose residues of high-mannose N-glycans to generate Man₅GlcNAc₂. Through this activity SpGH92 is able to produce a substrate for EndoD, which is not active on high-mannose glycans with α-(1,2)-linked mannose residues. Binding studies and X-ray crystallography show that NgtS, the solute binding protein of an ABC transporter (ABC_NG), is able to bind Man₅GlcNAc, a product of EndoD activity, with high affinity. Finally, we evaluated the contribution of EndoD and ABC_NG to growth of S. pneumoniae on a model N-glycosylated glycoprotein, and the contribution of these enzymes and SpGH92 to virulence in a mouse model. We found that both EndoD and ABC_NG contribute to growth of S. pneumoniae, but that only SpGH92 and EndoD contribute to virulence. Therefore, N-glycan processing, but not transport of the released glycan, is required for full virulence in S. pneumoniae. To conclude, we synthesize our findings into a model of N-glycan processing by S. pneumoniae in which both complex and high-mannose N-glycans are targeted, and in which the two arms of this degradation pathway converge at ABC_NG.

Streptococcus pneumoniae (pneumococcus) is a bacterium that causes extensive morbidity and mortality in humans. Vaccines and antibiotics are effective forms of prevention and treatment, respectively, but present challenges as it is a constant race to vaccinate against the enormous and ever evolving pool of different serotypes of the bacterium while resistance to antibiotics continues to trend upwards. It is thus necessary to better understand the molecular aspects of the host-pneumococcus interaction in order to inform the potential generation of alternative treatment strategies. S. pneumoniae relies on its ability to process the carbohydrates presented on the surface of host cells for full-virulence. In this study, we examine the capability of the bacterium to process high-mannose N-linked sugars, a heretofore unknown ability for S. pneumoniae. The results show that the pneumococcal genome encodes enzymes capable of processing these sugars and that, remarkably, the initiating reaction performed by an enzyme that removes terminal α-(1,2)-linked mannose residues is critical to virulence in a mouse model. This study illuminates an extensive pathway in S. pneumoniae that targets N-linked sugars and is key to the host-pathogen interaction, therefore revealing a potential target for therapeutic intervention.

N-acetylglucosamine-Mediated Expression of nagA and nagB in Streptococcus pneumoniae

In this study, we have explored the transcriptomic response of Streptococcus pneumoniae D39 to N-acetylglucosamine (NAG). Transcriptome comparison of S. pneumoniae D39 wild-type grown in chemically defined medium (CDM) in the presence of 0.5% NAG to that grown in the presence of 0.5% glucose revealed elevated expression of many genes/operons, including nagA, nagB, manLMN, and nanP. We have further confirmed the NAG-dependent expression of nagA, nagB, manLMN, and nanP by β-galactosidase assays. nagA, nagB and glmS are putatively regulated by a transcriptional regulator NagR. We predicted the operator site of NagR (dre site) in PnagA, PnagB, and PglmS, which was further confirmed by mutating the predicted dre site in the respective promoters (nagA, nagB, and glmS). Growth comparison of ΔnagA, ΔnagB, and ΔglmS with the D39 wild-type demonstrates that nagA and nagB are essential for S. pneumoniae D39 to grow in the presence of NAG as a sole carbon source. Furthermore, deletion of ccpA shows that CcpA has no effect on the expression of nagA, nagB, and glmS in the presence of NAG in S. pneumoniae.

Metabolic competition as a driver of bacterial population structure

Understanding the processes whereby diversity arises and is maintained in pathogen populations is pivotal for designing disease control interventions. A particular problem is the maintenance of strain structure in bacterial pathogen populations despite frequent genetic exchange. Although several theoretical frameworks have been put forward to explain this widespread phenomenon, few have focused on the role of genes encoding metabolic functions, despite an increasing recognition of their importance in pathogenesis and transmission. In this article, we review the literature for evidence of metabolic niches within the host and discuss theoretical frameworks which examine ecological interactions between metabolic genes. We contend that metabolic competition is an important phenomenon which contributes to the maintenance of population structure and diversity of many bacterial pathogens.

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.

Structural Analysis of a Family 101 Glycoside Hydrolase in Complex with Carbohydrates Reveals Insights into Its Mechanism

O-Linked glycosylation is one of the most abundant post-translational modifications of proteins. Within the secretory pathway of higher eukaryotes, the core of these glycans is frequently an N-acetylgalactosamine residue that is α-linked to serine or threonine residues. Glycoside hydrolases in family 101 are presently the only known enzymes to be able to hydrolyze this glycosidic linkage. Here we determine the high-resolution structures of the catalytic domain comprising a fragment of GH101 from Streptococcus pneumoniae TIGR4, SpGH101, in the absence of carbohydrate, and in complex with reaction products, inhibitor, and substrate analogues. Upon substrate binding, a tryptophan lid (residues 724-WNW-726) closes on the substrate. The closing of this lid fully engages the substrate in the active site with Asp-764 positioned directly beneath C1 of the sugar residue bound within the -1 subsite, consistent with its proposed role as the catalytic nucleophile. In all of the bound forms of the enzyme, however, the proposed catalytic acid/base residue was found to be too distant from the glycosidic oxygen (>4.3 Å) to serve directly as a general catalytic acid/base residue and thereby facilitate cleavage of the glycosidic bond. These same complexes, however, revealed a structurally conserved water molecule positioned between the catalytic acid/base and the glycosidic oxygen. On the basis of these structural observations we propose a new variation of the retaining glycoside hydrolase mechanism wherein the intervening water molecule enables a Grotthuss proton shuttle between Glu-796 and the glycosidic oxygen, permitting this residue to serve as the general acid/base catalytic residue.

Bacterial glycosidases in pathogenesis and glycoengineering

Glycosylation is a common post-translational protein modification and many key proteins of the immune system are glycosylated. As the true experts of our immune system, pathogenic bacteria produce enzymes that can modify the carbohydrates (glycans) of the defense mechanisms in order to favor bacterial survival and persistence. At the intersection between bacterial pathogenesis and glycobiology, there is an increased interest in studying the bacterial enzymes that modify the protein glycosylation of their colonized or infected hosts. This is of great importance in order to fully understand bacterial pathogenesis, but it also presents itself as a valuable source for glycoengineering and glycoanalysis tools. This article highlights the role of bacterial glycosidases during infections, introduces the use of such enzymes as glycoengineering tools and discusses the potential of further studies in this emerging field.

Host glycan sugar-specific pathways in Streptococcus pneumoniae: galactose as a key sugar in colonisation and infection [corrected]

The human pathogen Streptococcus pneumoniae is a strictly fermentative organism that relies on glycolytic metabolism to obtain energy. In the human nasopharynx S. pneumoniae encounters glycoconjugates composed of a variety of monosaccharides, which can potentially be used as nutrients once depolymerized by glycosidases. Therefore, it is reasonable to hypothesise that the pneumococcus would rely on these glycan-derived sugars to grow. Here, we identified the sugar-specific catabolic pathways used by S. pneumoniae during growth on mucin. Transcriptome analysis of cells grown on mucin showed specific upregulation of genes likely to be involved in deglycosylation, transport and catabolism of galactose, mannose and N acetylglucosamine. In contrast to growth on mannose and N-acetylglucosamine, S. pneumoniae grown on galactose re-route their metabolic pathway from homolactic fermentation to a truly mixed acid fermentation regime. By measuring intracellular metabolites, enzymatic activities and mutant analysis, we provide an accurate map of the biochemical pathways for galactose, mannose and N-acetylglucosamine catabolism in S. pneumoniae. Intranasal mouse infection models of pneumococcal colonisation and disease showed that only mutants in galactose catabolic genes were attenuated. Our data pinpoint galactose as a key nutrient for growth in the respiratory tract and highlights the importance of central carbon metabolism for pneumococcal pathogenesis.

Unravelling the multiple functions of the architecturally intricate Streptococcus pneumoniae β-galactosidase, BgaA

Bacterial cell-surface proteins play integral roles in host-pathogen interactions. These proteins are often architecturally and functionally sophisticated and yet few studies of such proteins involved in host-pathogen interactions have defined the domains or modules required for specific functions. Streptococcus pneumoniae (pneumococcus), an opportunistic pathogen that is a leading cause of community acquired pneumonia, otitis media and bacteremia, is decorated with many complex surface proteins. These include β-galactosidase BgaA, which is specific for terminal galactose residues β-1-4 linked to glucose or N-acetylglucosamine and known to play a role in pneumococcal growth, resistance to opsonophagocytic killing, and adherence. This study defines the domains and modules of BgaA that are required for these distinct contributions to pneumococcal pathogenesis. Inhibitors of β-galactosidase activity reduced pneumococcal growth and increased opsonophagocytic killing in a BgaA dependent manner, indicating these functions require BgaA enzymatic activity. In contrast, inhibitors increased pneumococcal adherence suggesting that BgaA bound a substrate of the enzyme through a distinct module or domain. Extensive biochemical, structural and cell based studies revealed two newly identified non-enzymatic carbohydrate-binding modules (CBMs) mediate adherence to the host cell surface displayed lactose or N-acetyllactosamine. This finding is important to pneumococcal biology as it is the first adhesin-carbohydrate receptor pair identified, supporting the widely held belief that initial pneumococcal attachment is to a glycoconjugate. Perhaps more importantly, this is the first demonstration that a CBM within a carbohydrate-active enzyme can mediate adherence to host cells and thus this study identifies a new class of carbohydrate-binding adhesins and extends the paradigm of CBM function. As other bacterial species express surface-associated carbohydrate-active enzymes containing CBMs these findings have broad implications for bacterial adherence. Together, these data illustrate that comprehending the architectural sophistication of surface-attached proteins can increase our understanding of the different mechanisms by which these proteins can contribute to bacterial pathogenesis.

Pretreatment of epithelial cells with live Streptococcus pneumoniae has no detectable effect on influenza A virus replication in vitro

Influenza A virus (IAV) and Streptococcus pneumoniae (pneumococcus) are two major upper respiratory tract pathogens responsible for exacerbated disease in coinfected individuals. Despite several studies showing increased susceptibility to secondary bacterial infections following IAV infection, information on the direct effect of S. pneumoniae on IAV in vitro is unknown. This is an important area of investigation as S. pneumoniae is a common commensal of the human upper respiratory tract, present as an important coinfecting pathogen with IAV infection. A recent study showed that S. pneumoniae enhances human metapneumovirus infection in polarized bronchial epithelial cells in vitro. The aim of the current study was to determine whether treatment of epithelial cells with S. pneumoniae affects IAV replication using a standard immunofluorescence assay (IFA). For this study we used four IAV permissive epithelial cell lines including two human-derived cell lines, 12 pneumococcal strains including recent human clinical isolates which represent different genetic backgrounds and serotypes, and six IAV strains of varying genetic nature and pathogenic potential including the pandemic 2009 H1N1 virus. Our results suggested that pretreatment of MDCK cells with 7.5×10⁶ colony-forming units (CFUs) of live S. pneumoniae resulted in gradual cell-death in a time-dependent manner (0.5 to 4 hr). But, pretreatment of cell lines with 7.5×10⁵ and lower CFUs of S. pneumoniae had no detectable effect on either the morphology of cells or on the IAV replication. However, unlike in epithelial cell lines, due to influence of secreted host factors the effect of pneumococci on IAV replication may be different during coinfections in vivo in the human upper respiratory tract, and in vitro with primary human polarized bronchial epithelial cells.

Panel 5: Microbiology and immunology panel

OBJECTIVE

The objective is to perform a comprehensive review of the literature from January 2007 through June 2011 on the virology, bacteriology, and immunology related to otitis media.

DATA SOURCES

PubMed database of the National Library of Medicine.

REVIEW METHODS

Three subpanels with co-chairs comprising experts in the virology, bacteriology, and immunology of otitis media were formed. Each of the panels reviewed the literature in their respective fields and wrote draft reviews. The reviews were shared with all panel members, and a second draft was created. The entire panel met at the 10th International Symposium on Recent Advances in Otitis Media in June 2011 and discussed the review and refined the content further. A final draft was created, circulated, and approved by the panel.

CONCLUSION

Excellent progress has been made in the past 4 years in advancing an understanding of the microbiology and immunology of otitis media. Advances include laboratory-based basic studies, cell-based assays, work in animal models, and clinical studies.

IMPLICATIONS FOR PRACTICE

The advances of the past 4 years formed the basis of a series of short-term and long-term research goals in an effort to guide the field. Accomplishing these goals will provide opportunities for the development of novel interventions, including new ways to better treat and prevent otitis media.

The fundamental link between pneumococcal carriage and disease

Streptococcus pneumoniae (pneumococcus) is a major cause of worldwide mortality and morbidity, and to a large extent is vaccine-preventable. Nasopharyngeal carriage of pneumococcus precedes disease and is the source of pneumococcal spread between people. The use of vaccine effect on carriage as part of the vaccine licensure and post-vaccine introduction evaluation could facilitate and expand the licensure of new, life-saving pneumococcal vaccines and enable a comprehensive estimate of population effects after vaccine introduction. The authors provide a review of the evidence supporting pneumococcal carriage at the individual level as an immediate and necessary precursor to pneumococcal disease. Based on such a causal link between carriage and disease, the authors emphasize the role of information on pneumococcal carriage in vaccine trials and in public health decision-making.

Pneumococcal carbohydrate transport: food for thought

Streptococcus pneumoniae relies exclusively on carbohydrates as a carbon source and devotes 30% of all transport mechanisms to carbohydrate import. Pneumococci utilize at least 32 carbohydrates in vitro. However, some proposed substrates are not human-derived, so it is unclear where they are encountered in the host niche, and other substrates remain unidentified. The majority of transporter loci are conserved, arguing against redundancy and instead for distinct roles during pathogenesis. Despite this, expression and regulation of carbohydrate transporters in vivo remain ill defined. Recent work has also demonstrated that multiple ABC transporters share an ATPase; whether this evolved for genome minimization or for transporter regulation remains unknown. Continued efforts to understand carbohydrate import may reveal novel vaccine and therapeutic targets and increase our understanding of pneumococcal pathogenesis.

Scalable syntheses of both enantiomers of DNJNAc and DGJNAc from glucuronolactone: the effect of N-alkylation on hexosaminidase inhibition

The efficient scalable syntheses of 2-acetamido-1,2-dideoxy-D-galacto-nojirimycin (DGJNAc) and 2-acetamido-1,2-dideoxy-D-gluco-nojirimycin (DNJNAc) from D-glucuronolactone, as well as of their enantiomers from L-glucuronolactone, are reported. The evaluation of both enantiomers of DNJNAc and DGJNAc, along with their N-alkyl derivatives, as glycosidase inhibitors showed that DGJNAc and its N-alkyl derivatives were all inhibitors of α-GalNAcase but that none of the epimeric DNJNAc derivatives inhibited this enzyme. In contrast, both DGJNAc and DNJNAc, as well as their alkyl derivatives, were potent inhibitors of β-GlcNAcases and β-GalNAcases. Neither of the L-enantiomers showed any significant inhibition of any of the enzymes tested. Correlation of the in vitro inhibition with the cellular data, by using a free oligosaccharide analysis of the lysosomal enzyme inhibition, revealed the following structure-property relationship: hydrophobic side-chains preferentially promoted the intracellular access of iminosugars to those inhibitors with more-hydrophilic side-chain characteristics.

Streptococcus pneumoniae can utilize multiple sources of hyaluronic acid for growth

The mechanisms by which Streptococcus pneumoniae obtains carbohydrates for growth during airway colonization remain to be elucidated. The low concentration of free carbohydrates in the normal human airway suggests that pneumococci must utilize complex glycan structures for growth. The glycosaminoglycan hyaluronic acid is present on the apical surface of airway epithelial cells. As pneumococci express a hyaluronate lyase (Hyl) that cleaves hyaluronic acid into disaccharides, we hypothesized that during colonization pneumococci utilize the released carbohydrates for growth. Hyaluronic acid supported significant pneumococcal growth in an hyl-dependent manner. A phosphoenolpyruvate-dependent phosphotransferase system (PTS) and an unsaturated glucuronyl hydrolase (Ugl) encoded downstream of hyl are also essential for growth on hyaluronic acid. This genomic arrangement is present in several other organisms, suggesting conservation of the utilization mechanism between species. In vivo experiments support the hypothesis that S. pneumoniae utilizes hyaluronic acid as a carbon source during colonization. We also demonstrate that pneumococci can utilize the hyaluronic acid capsule of other bacterial species for growth, suggesting an alternative carbohydrate source for pneumococcal growth. Together, these data support a novel function for pneumococcal degradation of hyaluronic acid in vivo and provide mechanistic details of growth on this glycosaminoglycan.

Bacterial hydrolysis of host glycoproteins - powerful protein modification and efficient nutrient acquisition

Glycoproteins are ubiquitous in nature and fundamental to most biological processes, including the human immune system. The glycoprotein carbohydrate moieties, or glycans, are very diverse in their structure and composition, and have major effects on the chemical, physical and biological properties of these glycoproteins. The hydrolysis of glycoprotein glycans by bacterial glycosidases can have dramatic effects on glycoprotein function and, thereby, be beneficial for the bacteria in different ways. This review gives an introduction to the expanding field of extracellular glycosidases from bacterial pathogens with activity on host glycoproteins, describes some known and proposed consequences for the host and the bacteria and discusses some evolutionary and regulatory aspects of bacterial glycosidases.

Identification of an ATPase, MsmK, which energizes multiple carbohydrate ABC transporters in Streptococcus pneumoniae

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and results in over 1 million deaths each year worldwide. Asymptomatic colonization of the airway precedes disease, and acquisition of carbohydrates from the host environment is necessary for bacterial survival. We previously demonstrated that S. pneumoniae cleaves sialic acid from human glycoconjugates to be used as a carbohydrate source. The satABC genes are required for growth and import of sialic acid. The satABC genes are predicted to encode components of an ABC transporter but not the ATPases essential to energize transport. As this subunit is essential, an ATPase must be encoded elsewhere in the genome. We identified msmK as a candidate based on similarity to other known carbohydrate ATPases. Recombinant MsmK hydrolyzed ATP, revealing that MsmK is an ATPase. An msmK mutant was reduced in growth on and transport of sialic acid, demonstrating that MsmK is the ATPase energizing the sialic acid transporter. In addition to satABC, S. pneumoniae contains five other loci that are predicted to encode CUT1 family carbohydrate ABC transporter components; each of these lacks a predicted ATPase. Data indicate that msmK is also required for growth on raffinose and maltotetraose, which are the substrates of two other characterized carbohydrate ABC transporters. Furthermore, an msmK mutant was reduced in airway colonization. Together, these data imply that in vivo, MsmK energizes multiple carbohydrate transporters in S. pneumoniae. This is the first demonstration of a shared ATPase in a pathogenic bacterium.

Sialic acid transport contributes to pneumococcal colonization

Streptococcus pneumoniae is a major cause of pneumonia and meningitis. Airway colonization is a necessary precursor to disease, but little is known about how the bacteria establish and maintain colonization. Carbohydrates are required as a carbon source for pneumococcal growth and, therefore, for colonization. Free carbohydrates are not readily available in the naso-oropharynx; however, N- and O-linked glycans are common in the airway. Sialic acid is the most common terminal modification on N- and O-linked glycans and is likely encountered frequently by S. pneumoniae in the airway. Here we demonstrate that sialic acid supports pneumococcal growth when provided as a sole carbon source. Growth on sialic acid requires import into the bacterium. Three genetic regions have been proposed to encode pneumococcal sialic acid transporters: one sodium solute symporter and two ATP binding cassette (ABC) transporters. Data demonstrate that one of these, satABC, is required for transport of sialic acid. A satABC mutant displayed significantly reduced growth on both sialic acid and the human glycoprotein alpha-1. The importance of satABC for growth on human glycoprotein suggests that sialic acid transport may be important in vivo. Indeed, the satABC mutant was significantly reduced in colonization of the murine upper respiratory tract. This work demonstrates that S. pneumoniae is able to use sialic acid as a sole carbon source and that utilization of sialic acid is likely important during pneumococcal colonization.

GH101 family of glycoside hydrolases: subfamily structure and evolutionary connections with other families

The GH101 family is composed of endo-alpha-N-acetylgalactosaminidases and their homologues. Pairwise sequence comparison and phylogenetic analysis allowed us to distinguish five to six subfamilies in this family. Diverse domain structures were found among the family members. Usually they have five irreplaceable and some optional domains. Iterative screening of the protein database revealed an evolutionary relationship of the GH101 catalytic domain with glycoside hydrolase domains from GH13, GH31, and GH70 families. Among other homologous proteins we have found representatives of COG1649, as well as members of four new families of predicted glycoside hydrolases (GHL1-GHL4).

The membrane receptors guanylyl cyclase-A and -B undergo distinctive changes in post-translational modification during brain development

Temporal carbohydrate expression patterns at cell surfaces are thought to be of crucial regulatory significance during developmental processes. Hitherto, however, data on individual membrane proteins undergoing development-associated changes in glycosylation are sparsely. Here, we show that the two natriuretic peptide receptors, guanylyl cyclase-A (GC-A) and GC-B are subject to pronounced size alterations in the rat brain between postnatal day 1 and adult. Comparable size changes were not detectable for GC-A and GC-B in peripheral tissues and for three other membrane proteins (insulin receptor, insulin-like growth factor-II/mannose-6-phoshate receptor, neutral endopeptidase) in brain, indicating remarkable specificity. As revealed by treatments with carbohydrate-digesting enzymes, both GC-A and GC-B are hyperglycosylated at N-linked glycosylation sites in the developing brain. At postnatal day 1, the vast majority of GC-B (but not GC-A) molecules contain additionally an O-linked carbohydrate modification of about 1 kDa in mass and a further modification of similar size which is resistant to enzymatic removal. The glycoforms exhibited functional activity in membrane GC assays, indicating proper folding and signaling capability. These data link recently reported roles of natriuretic peptides during brain development for the first time with specific glycosylation states of their cyclic GMP-generating receptors.

New adhesin functions of surface-exposed pneumococcal proteins

Background

Streptococcus pneumoniae is a widely distributed commensal Gram-positive bacteria of the upper respiratory tract. Pneumococcal colonization can progress to invasive disease, and thus become lethal, reason why antibiotics and vaccines are designed to limit the dramatic effects of the bacteria in such cases. As a consequence, pneumococcus has developed efficient antibiotic resistance, and the use of vaccines covering a limited number of serotypes such as Pneumovax^® and Prevnar^® results in the expansion of non-covered serotypes. Pneumococcal surface proteins represent challenging candidates for the development of new therapeutic targets against the bacteria. Despite the number of described virulence factors, we believe that the majority of them remain to be characterized. This is the reason why pneumococcus invasion processes are still largely unknown.

Results

Availability of genome sequences facilitated the identification of pneumococcal surface proteins bearing characteristic motifs such as choline-binding proteins (Cbp) and peptidoglycan binding (LPXTG) proteins. We designed a medium throughput approach to systematically test for interactions between these pneumococcal surface proteins and host proteins (extracellular matrix proteins, circulating proteins or immunity related proteins). We cloned, expressed and purified 28 pneumococcal surface proteins. Interactions were tested in a solid phase assay, which led to the identification of 23 protein-protein interactions among which 20 are new.

Conclusions

We conclude that whether peptidoglycan binding proteins do not appear to be major adhesins, most of the choline-binding proteins interact with host proteins (elastin and C reactive proteins are the major Cbp partners). These newly identified interactions open the way to a better understanding of host-pneumococcal interactions.

Structure and kinetic investigation of Streptococcus pyogenes family GH38 alpha-mannosidase

Background

The enzymatic hydrolysis of α−mannosides is catalyzed by glycoside hydrolases (GH), termed α−mannosidases. These enzymes are found in different GH sequence–based families. Considerable research has probed the role of higher eukaryotic “GH38” α−mannosides that play a key role in the modification and diversification of hybrid N-glycans; processes with strong cellular links to cancer and autoimmune disease. The most extensively studied of these enzymes is the Drosophila GH38 α−mannosidase II, which has been shown to be a retaining α−mannosidase that targets both α−1,3 and α−1,6 mannosyl linkages, an activity that enables the enzyme to process GlcNAc(Man)₅(GlcNAc)₂ hybrid N-glycans to GlcNAc(Man)₃(GlcNAc)₂. Far less well understood is the observation that many bacterial species, predominantly but not exclusively pathogens and symbionts, also possess putative GH38 α−mannosidases whose activity and specificity is unknown.

Methodology/Principal Findings

Here we show that the Streptococcus pyogenes (M1 GAS SF370) GH38 enzyme (Spy1604; hereafter SpGH38) is an α−mannosidase with specificity for α−1,3 mannosidic linkages. The 3D X-ray structure of SpGH38, obtained in native form at 1.9 Å resolution and in complex with the inhibitor swainsonine (K_i 18 µM) at 2.6 Å, reveals a canonical GH38 five-domain structure in which the catalytic “–1” subsite shows high similarity with the Drosophila enzyme, including the catalytic Zn²⁺ ion. In contrast, the “leaving group” subsites of SpGH38 display considerable differences to the higher eukaryotic GH38s; features that contribute to their apparent specificity.

Conclusions/Significance

Although the in vivo function of this streptococcal GH38 α−mannosidase remains unknown, it is shown to be an α−mannosidase active on N-glycans. SpGH38 lies on an operon that also contains the GH84 hexosaminidase (Spy1600) and an additional putative glycosidase. The activity of SpGH38, together with its genomic context, strongly hints at a function in the degradation of host N- or possibly O-glycans. The absence of any classical signal peptide further suggests that SpGH38 may be intracellular, perhaps functioning in the subsequent degradation of extracellular host glycans following their initial digestion by secreted glycosidases.

Mechanistic investigation of the endo-alpha-N-acetylgalactosaminidase from Streptococcus pneumoniae R6

The large (1767-amino acid) endo-alpha-N-acetylgalactosaminidase from Streptococcus pneumoniae (SpGH101) specifically removes an O-linked disaccharide Gal-beta-1,3-GalNAc-alpha from glycoproteins. While the enzyme from natural sources has been used as a reagent for many years, very few mechanistic studies have been performed. Using the recently determined three-dimensional structure of the recombinant protein as a background, we report here a mechanistic investigation of the SpGH101 retaining alpha-glycoside hydrolase using a combination of synthetic and natural substrates. On the basis of a model of the substrate complex of SpGH101, we propose D764 and E796 as the nucleophile and general acid-base residues, respectively. These roles were confirmed by kinetic and mechanistic analysis of mutants at those positions using synthetic substrates and anion rescue experiments. pK(a) values of 5.3 and 7.2 were assigned to D764 and E796 on the basis of the pK(a) values derived from the bell-shaped dependence of k(cat)/K(m) upon pH. The enzyme contains several putative carbohydrate binding modules whose glycan binding specificities were probed using the printed glycan array of the Consortium for Functional Glycomics using the inactive D764A and D764F mutants that had been labeled with Alexafluor 488. These studies revealed binding to galacto-N-biose, consistent with a role for these domains in localizing the enzyme near its substrates.

Characterization of novel beta-galactosidase activity that contributes to glycoprotein degradation and virulence in Streptococcus pneumoniae

The pneumococcus obtains its energy from the metabolism of host glycosides. Therefore, efficient degradation of host glycoproteins is integral to pneumococcal virulence. In search of novel pneumococcal glycosidases, we characterized the Streptococcus pneumoniae strain D39 protein encoded by SPD_0065 and found that this gene encodes a beta-galactosidase. The SPD_0065 recombinant protein released galactose from desialylated fetuin, which was used here as a model of glycoproteins found in vivo. A pneumococcal mutant with a mutation in SPD_0065 showed diminished beta-galactosidase activity, exhibited an extended lag period in mucin-containing defined medium, and cleaved significantly less galactose than the parental strain during growth on mucin. As pneumococcal beta-galactosidase activity had been previously attributed solely to SPD_0562 (bgaA), we evaluated the contribution of SPD_0065 and SPD_0562 to total beta-galactosidase activity. Mutation of either gene significantly reduced enzymatic activity, but beta-galactosidase activity in the double mutant, although significantly less than that in either of the single mutants, was not completely abolished. The expression of SPD_0065 in S. pneumoniae grown in mucin-containing medium or tissues harvested from infected animals was significantly upregulated compared to that in pneumococci from glucose-containing medium. The SPD_0065 mutant strain was found to be attenuated in virulence in a manner specific to the host tissue.