Biochemical and pathophysiological characterization of Helicobacter pylori asparaginase

Characterization of HcaA, a novel autotransporter protein in Helicobacter cinaedi, and its role in host cell adhesion

IMPORTANCE

Helicobacter species are classified as gastric or enterohepatic according to their habitat. Among enterohepatic Helicobacter species, which inhabit the intestine, colon, and liver, Helicobacter cinaedi has been most frequently isolated from humans. H. cinaedi often causes bacteremia and cellulitis in immunocompromised hosts. Here, we focused on the H. cinaedi autotransporter protein A (HcaA), a novel virulence factor in H. cinaedi. We discovered that HcaA contributes to cell adhesion via its Arg-Gly-Asp motif. Furthermore, in animal experiments, bacterial colonization was reduced in mice infected with HcaA-knockout strains, supporting the hypothesis that HcaA contributes to H. cinaedi adhesion to host cells. Our study provides a novel mechanism for the establishment of H. cinaedi infections and provides new insights into the role of autotransporter proteins in the establishment of Helicobacter infection.

A phylogenetic approach to explore the Aspergillus fumigatus conidial surface-associated proteome and its role in pathogenesis

Aspergillus fumigatus, an important pulmonary fungal pathogen causing several diseases collectively called aspergillosis, relies on asexual spores (conidia) for initiating host infection. Here, we used a phylogenomic approach to compare proteins in the conidial surface of A. fumigatus, two closely related non-pathogenic species, Aspergillus fischeri and Aspergillus oerlinghausenensis, and the cryptic pathogen Aspergillus lentulus. After identifying 62 proteins uniquely expressed on the A. fumigatus conidial surface, we assessed null mutants for 42 genes encoding conidial proteins. Deletion of 33 of these genes altered susceptibility to macrophage killing, penetration and damage to epithelial cells, and cytokine production. Notably, a gene that encodes glycosylasparaginase, which modulates levels of the host pro-inflammatory cytokine IL-1β, is important for infection in an immunocompetent murine model of fungal disease. These results suggest that A. fumigatus conidial surface proteins and effectors are important for evasion and modulation of the immune response at the onset of fungal infection.

A phylogenetic approach to explore the Aspergillus fumigatus conidial surface-associated proteome and its role in pathogenesis

Aspergillus fumigatus , an important pulmonary fungal pathogen causing several diseases collectively called aspergillosis, relies on asexual spores or conidia for initiating host infection. Here, we used a phylogenomic approach to compare proteins in the conidial surface of A. fumigatus , two closely related non-pathogenic species, Aspergillus fischeri and Aspergillus oerlinghausenensis , and the cryptic pathogen Aspergillus lentulus . After identifying 62 proteins uniquely expressed on the A. fumigatus conidial surface, we deleted 42 genes encoding conidial proteins. We found deletion of 33 of these genes altered susceptibility to macrophage killing, penetration and damage to epithelial cells, and cytokine production. Notably, a gene that encodes glycosylasparaginase, which modulates levels of the host pro-inflammatory cytokine IL-1β, is important for infection in an immunocompetent murine model of fungal disease. These results suggest that A. fumigatus conidial surface proteins and effectors are important for evasion and modulation of the immune response at the onset of fungal infection.

Gastric Helicobacter species associated with dogs, cats and pigs: significance for public and animal health

This article focuses on the pathogenic significance of Helicobacter species naturally colonizing the stomach of dogs, cats and pigs. These gastric "non-Helicobacter (H.) pylori Helicobacter species" (NHPH) are less well-known than the human adapted H. pylori. Helicobacter suis has been associated with gastritis and decreased daily weight gain in pigs. Several studies also attribute a role to this pathogen in the development of hyperkeratosis and ulceration of the non-glandular stratified squamous epithelium of the pars oesophagea of the porcine stomach. The stomach of dogs and cats can be colonized by several Helicobacter species but their pathogenic significance for these animals is probably low. Helicobacter suis as well as several canine and feline gastric Helicobacter species may also infect humans, resulting in gastritis, peptic and duodenal ulcers, and low-grade mucosa-associated lymphoid tissue lymphoma. These agents may be transmitted to humans most likely through direct or indirect contact with dogs, cats and pigs. Additional possible transmission routes include consumption of water and, for H. suis, also consumption of contaminated pork. It has been described that standard H. pylori eradication therapy is usually also effective to eradicate the NHPH in human patients, although acquired antimicrobial resistance may occasionally occur and porcine H. suis strains are intrinsically less susceptible to aminopenicillins than non-human primate H. suis strains and other gastric Helicobacter species. Virulence factors of H. suis and the canine and feline gastric Helicobacter species include urease activity, motility, chemotaxis, adhesins and gamma-glutamyl transpeptidase. These NHPH, however, lack orthologs of cytotoxin-associated gene pathogenicity island and vacuolating cytotoxin A, which are major virulence factors in H. pylori. It can be concluded that besides H. pylori, gastric Helicobacter species associated with dogs, cats and pigs are also clinically relevant in humans. Although recent research has provided better insights regarding pathogenic mechanisms and treatment strategies, a lot remains to be investigated, including true prevalence rates, exact modes of transmission and molecular pathways underlying disease development and progression.

Molecular Analysis of L-Asparaginases for Clarification of the Mechanism of Action and Optimization of Pharmacological Functions

L-asparaginases (EC 3.5.1.1) are a family of enzymes that catalyze the hydrolysis of L-asparagine to L-aspartic acid and ammonia. These proteins with different biochemical, physicochemical and pharmacological properties are found in many organisms, including bacteria, fungi, algae, plants and mammals. To date, asparaginases from E. coli and Dickeya dadantii (formerly known as Erwinia chrysanthemi) are widely used in hematology for the treatment of lymphoblastic leukemias. However, their medical use is limited by side effects associated with the ability of these enzymes to hydrolyze L-glutamine, as well as the development of immune reactions. To solve these issues, gene-editing methods to introduce amino-acid substitutions of the enzyme are implemented. In this review, we focused on molecular analysis of the mechanism of enzyme action and to optimize the antitumor activity.

Pathogens MenTORing Macrophages and Dendritic Cells: Manipulation of mTOR and Cellular Metabolism to Promote Immune Escape

Myeloid cells, including macrophages and dendritic cells, represent an important first line of defense against infections. Upon recognition of pathogens, these cells undergo a metabolic reprogramming that supports their activation and ability to respond to the invading pathogens. An important metabolic regulator of these cells is mammalian target of rapamycin (mTOR). During infection, pathogens use host metabolic pathways to scavenge host nutrients, as well as target metabolic pathways for subversion of the host immune response that together facilitate pathogen survival. Given the pivotal role of mTOR in controlling metabolism and DC and macrophage function, pathogens have evolved strategies to target this pathway to manipulate these cells. This review seeks to discuss the most recent insights into how pathogens target DC and macrophage metabolism to subvert potential deleterious immune responses against them, by focusing on the metabolic pathways that are known to regulate and to be regulated by mTOR signaling including amino acid, lipid and carbohydrate metabolism, and autophagy.

L-Asparaginase of Leishmania donovani: Metabolic target and its role in Amphotericin B resistance

Emergence of Amphotericin B (AmB) resistant Leishmania donovani has posed major therapeutic challenge against the parasite. Consequently, combination therapy aimed at multiple molecular targets, based on proteome wise network analysis has been recommended. In this regard we had earlier identified and proposed L-asparaginase of Leishmania donovani (LdAI) as a crucial metabolic target. Here we report that both LdAI overexpressing axenic amastigote and promastigote forms of L. donovani survives better when challenged with AmB as compared to wild type strain. Conversely, qRT-PCR analysis showed an upregulation of LdAI in both forms upon AmB treatment. Our data demonstrates the importance of LdAI in imparting immediate protective response to the parasite upon AmB treatment. In the absence of structural and functional information, we modeled LdAI and validated its solution structure through small angle X-ray scattering (SAXS) analysis. We identified its specific inhibitors through ligand and structure-based approach and characterized their effects on enzymatic properties (K_m, V_max, K_cat) of LdAI. We show that in presence of two of the inhibitors L1 and L2, the survival of L. donovani is compromised whereas overexpression of LdAI in these cells restores viability. Taken together, our results conclusively prove that LdAI is a crucial metabolic enzyme conferring early counter measure against AmB treatment by Leishmania.

Contribution of Asparagine Catabolism to Salmonella Virulence

Salmonellae are pathogenic bacteria that cause significant morbidity and mortality in humans worldwide. Salmonellae establish infection and avoid clearance by the immune system by mechanisms that are not well understood. We previously showed that l-asparaginase II produced by Salmonella enterica serovar Typhimurium (S Typhimurium) inhibits T cell responses and mediates virulence. In addition, we previously showed that asparagine deprivation such as that mediated by l-asparaginase II of S Typhimurium causes suppression of activation-induced T cell metabolic reprogramming. Here, we report that STM3997, which encodes a homolog of disulfide bond protein A (dsbA) of Escherichia coli, is required for l-asparaginase II stability and function. Furthermore, we report that l-asparaginase II localizes primarily to the periplasm and acts together with l-asparaginase I to provide S Typhimurium the ability to catabolize asparagine and assimilate nitrogen. Importantly, we determined that, in a murine model of infection, S Typhimurium lacking both l-asparaginase I and II genes competes poorly with wild-type S Typhimurium for colonization of target tissues. Collectively, these results indicate that asparagine catabolism contributes to S Typhimurium virulence, providing new insights into the competition for nutrients at the host-pathogen interface.

Leishmania infantum Asparagine Synthetase A Is Dispensable for Parasites Survival and Infectivity

A growing interest in asparagine (Asn) metabolism has currently been observed in cancer and infection fields. Asparagine synthetase (AS) is responsible for the conversion of aspartate into Asn in an ATP-dependent manner, using ammonia or glutamine as a nitrogen source. There are two structurally distinct AS: the strictly ammonia dependent, type A, and the type B, which preferably uses glutamine. Absent in humans and present in trypanosomatids, AS-A was worthy of exploring as a potential drug target candidate. Appealingly, it was reported that AS-A was essential in Leishmania donovani, making it a promising drug target. In the work herein we demonstrate that Leishmania infantum AS-A, similarly to Trypanosoma spp. and L. donovani, is able to use both ammonia and glutamine as nitrogen donors. Moreover, we have successfully generated LiASA null mutants by targeted gene replacement in L. infantum, and these parasites do not display any significant growth or infectivity defect. Indeed, a severe impairment of in vitro growth was only observed when null mutants were cultured in asparagine limiting conditions. Altogether our results demonstrate that despite being important under asparagine limitation, LiAS-A is not essential for parasite survival, growth or infectivity in normal in vitro and in vivo conditions. Therefore we exclude AS-A as a suitable drug target against L. infantum parasites.

Asparagine deprivation mediated by Salmonella asparaginase causes suppression of activation-induced T cell metabolic reprogramming

Salmonellae are pathogenic bacteria that induce immunosuppression by mechanisms that remain largely unknown. Previously, we showed that a putative type II l-asparaginase produced by Salmonella Typhimurium inhibits T cell responses and mediates virulence in a murine model of infection. Here, we report that this putative L-asparaginase exhibits L-asparagine hydrolase activity required for Salmonella Typhimurium to inhibit T cells. We show that L-asparagine is a nutrient important for T cell activation and that L-asparagine deprivation, such as that mediated by the Salmonella Typhimurium L-asparaginase, causes suppression of activation-induced mammalian target of rapamycin signaling, autophagy, Myc expression, and L-lactate secretion. We also show that L-asparagine deprivation mediated by the Salmonella Typhimurium L-asparaginase causes suppression of cellular processes and pathways involved in protein synthesis, metabolism, and immune response. Our results advance knowledge of a mechanism used by Salmonella Typhimurium to inhibit T cell responses and mediate virulence, and provide new insights into the prerequisites of T cell activation. We propose a model in which l-asparagine deprivation inhibits T cell exit from quiescence by causing suppression of activation-induced metabolic reprogramming.

Induction of endoplasmic reticulum stress and unfolded protein response constitutes a pathogenic strategy of group A streptococcus

The connection between bacterial pathogens and unfolded protein response (UPR) is poorly explored. In this review we highlight the evidence showing that group A streptococcus (GAS) induces endoplasmic reticulum (ER) stress and UPR through which it captures the amino acid asparagine (ASN) from the host. GAS acts extracellularly and during adherence to host cells it delivers the hemolysin toxins; streptolysin O (SLO) and streptolysin S (SLS). By poorly understood pathways, these toxins trigger UPR leading to the induction of the transcriptional regulator ATF4 and consequently to the upregulation of asparagine synthetase (ASNS) transcription leading to production and release of ASN. GAS senses ASN and alters gene expression profile accordingly, and increases the rate of multiplication. We suggest that induction of UPR by GAS and by other bacterial pathogens represent means through which bacterial pathogens gain nutrients from the host, obviating the need to become internalized or inflict irreversible cell damage.

Ammonium metabolism enzymes aid Helicobacter pylori acid resistance

The gastric pathogen Helicobacter pylori possesses a highly active urease to support acid tolerance. Urea hydrolysis occurs inside the cytoplasm, resulting in the production of NH3 that is immediately protonated to form NH4 (+). This ammonium must be metabolized or effluxed because its presence within the cell is counterproductive to the goal of raising pH while maintaining a viable proton motive force (PMF). Two compatible hypotheses for mitigating intracellular ammonium toxicity include (i) the exit of protonated ammonium outward via the UreI permease, which was shown to facilitate diffusion of both urea and ammonium, and/or (ii) the assimilation of this ammonium, which is supported by evidence that H. pylori assimilates urea nitrogen into its amino acid pools. We investigated the second hypothesis by constructing strains with altered expression of the ammonium-assimilating enzymes glutamine synthetase (GS) and glutamate dehydrogenase (GDH) and the ammonium-evolving periplasmic enzymes glutaminase (Ggt) and asparaginase (AsnB). H. pylori strains expressing elevated levels of either GS or GDH are more acid tolerant than the wild type, exhibit enhanced ammonium production, and are able to alkalize the medium faster than the wild type. Strains lacking the genes for either Ggt or AsnB are acid sensitive, have 8-fold-lower urea-dependent ammonium production, and are more acid sensitive than the parent. Additionally, we found that purified H. pylori GS produces glutamine in the presence of Mg(2+) at a rate similar to that of unadenylated Escherichia coli GS. These data reveal that all four enzymes contribute to whole-cell acid resistance in H. pylori and are likely important for assimilation and/or efflux of urea-derived ammonium.

Role of γ-glutamyltranspeptidase in the pathogenesis of Helicobacter pylori infection

γ-Glutamyltranspeptidase and asparaginase have been shown to play important roles in Helicobacter pylori colonization and cell death induced by H. pylori infection. In this study, the association of γ-glutamyltranspeptidase and asparaginase was elucidated by comparing activities of both deamidases in H. pylori strains from patients with chronic gastritis, gastric and duodenal ulcers, and gastric cancer. γ-Glutamyltranspeptidase activities in H. pylori strains from patients with gastric cancer were significantly higher than in those from patients with chronic gastritis or gastric ulcers. There was a wide range of asparaginase activities in H. pylori strains from patients with gastric cancer and these were not significantly than those from patients with other diseases. To identify the contributions of γ-glutamyltranspeptidase and asparaginase to gastric cell inflammation, human gastric epithelial cells (AGS line) were infected with H. pylori wild-type and knockout strains and inflammatory responses evaluated by induction of interleukin-8 (IL-8). IL-8 response was significantly decreased by knockout of the γ-glutamyltranspeptidase-encoding gene but not by knockout of the asparaginase-encoding gene. Additionally, IL-8 induction by infection with the H. pylori wild-type strain was significantly decreased by adding glutamine during infection. These findings indicate that IL-8 induction caused by γ-glutamyltranspeptidase activity in H. pylori is mainly attributable to depletion of glutamine. These data suggest that γ-glutamyltranspeptidase plays a significant role in the chronic inflammation caused by H. pylori infection.

Clinical Application of the DiversiLab Microbial Typing System Using Repetitive Sequence-Based PCR for Characterization of Helicobacter pylori in Japan

We evaluated the DiversiLab (DL) system with universal primers, a semiautomated repetitive extragenic palindromic sequence-based polymerase chain reaction (PCR) (rep-PCR) system, for the characterization of Helicobacter pylori in Japan. All 135 isolates from Japanese patients with gastric cancer (GC, n = 55) or non-GC (n = 80) were used and subjected to the drug susceptibility examinations (amoxicillin, AMPC; metronidazole, MNZ; and clarithromycin, CAM) by E-test. There were 28 MNZ-resistant (20.7%), 35 CAM-resistant (25.9%), and 16 MNZ/CAM-resistant (11.9%) isolates. DL rep-PCR fingerprinting analysis at the level of 95% similarity revealed five major groups (A-E) and the other including 45 isolates. The occupation rates of GC-derived isolates in groups B (54.2%) and E (58.8%) were higher than in the other groups: A (26.7%), C (28.6%), D (30.0%), and the other (40.0%). Relative higher occupation rates of drug resistants, such as MNZ-, CAM- and double MNZ/CAM-resistant isolates, were observed in groups B (45.8%), C (42.6%), and D (40%). Five of eight GC-derived isolates with MNZ/CAM resistance were significantly assigned to group B (P = 0.0312, χ(2) -test). These results suggest that the isolates classified in group B have a potential to contribute to the development of severe gastric disorders. The DL system, rapid and high sensitive technology, would be widely available in clinical laboratory for pathological and epidemiological analyses even in H. pylori.

The periplasmic enzyme, AnsB, of Shigella flexneri modulates bacterial adherence to host epithelial cells

S. flexneri strains, most frequently linked with endemic outbreaks of shigellosis, invade the colonic and rectal epithelium of their host and cause severe tissue damage. Here we have attempted to elucidate the contribution of the periplasmic enzyme, L-asparaginase (AnsB) to the pathogenesis of S. flexneri. Using a reverse genetic approach we found that ansB mutants showed reduced adherence to epithelial cells in vitro and attenuation in two in vivo models of shigellosis, the Caenorhabditis elegans and the murine pulmonary model. To investigate how AnsB affects bacterial adherence, we compared the proteomes of the ansB mutant with its wild type parental strain using two dimensional differential in-gel electrophoresis and identified the outer membrane protein, OmpA as up-regulated in ansB mutant cells. Bacterial OmpA, is a prominent outer membrane protein whose activity has been found to be required for bacterial pathogenesis. Overexpression of OmpA in wild type S. flexneri serotype 3b resulted in decreasing the adherence of this virulent strain, suggesting that the up-regulation of OmpA in ansB mutants contributes to the reduced adherence of this mutant strain. The data presented here is the first report that links the metabolic enzyme AnsB to S. flexneri pathogenesis.

Mycobacterium tuberculosis exploits asparagine to assimilate nitrogen and resist acid stress during infection

Mycobacterium tuberculosis is an intracellular pathogen. Within macrophages, M. tuberculosis thrives in a specialized membrane-bound vacuole, the phagosome, whose pH is slightly acidic, and where access to nutrients is limited. Understanding how the bacillus extracts and incorporates nutrients from its host may help develop novel strategies to combat tuberculosis. Here we show that M. tuberculosis employs the asparagine transporter AnsP2 and the secreted asparaginase AnsA to assimilate nitrogen and resist acid stress through asparagine hydrolysis and ammonia release. While the role of AnsP2 is partially spared by yet to be identified transporter(s), that of AnsA is crucial in both phagosome acidification arrest and intracellular replication, as an M. tuberculosis mutant lacking this asparaginase is ultimately attenuated in macrophages and in mice. Our study provides yet another example of the intimate link between physiology and virulence in the tubercle bacillus, and identifies a novel pathway to be targeted for therapeutic purposes.

Distinct physiological roles for the two L-asparaginase isozymes of Escherichia coli

Escherichia coli expresses two L-asparaginase (EC 3.5.1.1) isozymes: L-asparaginse I, which is a low affinity, cytoplasmic enzyme that is expressed constitutively, and L-asparaginase II, a high affinity periplasmic enzyme that is under complex co-transcriptional regulation by both Fnr and Crp. The distinct localisation and regulation of these enzymes suggest different roles. To define these roles, a set of isogenic mutants was constructed that lacked either or both enzymes. Evidence is provided that L-asparaginase II, in contrast to L-asparaginase I, can be used in the provision of an anaerobic electron acceptor when using a non-fermentable carbon source in the presence of excess nitrogen.

L-asparaginase II produced by Salmonella typhimurium inhibits T cell responses and mediates virulence

Salmonella enterica serovar Typhimurium avoids clearance by the host immune system by suppressing T cell responses; however, the mechanisms that mediate this immunosuppression remain unknown. We show that S. Typhimurium inhibit T cell responses by producing L-Asparaginase II, which catalyzes the hydrolysis of L-asparagine to aspartic acid and ammonia. L-Asparaginase II is necessary and sufficient to suppress T cell blastogenesis, cytokine production, and proliferation and to downmodulate expression of the T cell receptor. Furthermore, S. Typhimurium-induced inhibition of T cells in vitro is prevented upon addition of L-asparagine. S. Typhimurium lacking the L-Asparaginase II gene (STM3106) are unable to inhibit T cell responses and exhibit attenuated virulence in vivo. L-Asparaginases are used to treat acute lymphoblastic leukemia through mechanisms that likely involve amino acid starvation of leukemic cells, and these findings indicate that pathogens similarly use L-asparagine deprivation to limit T cell responses.

Genome-wide survey of mutual homologous recombination in a highly sexual bacterial species

The nature of a species remains a fundamental and controversial question. The era of genome/metagenome sequencing has intensified the debate in prokaryotes because of extensive horizontal gene transfer. In this study, we conducted a genome-wide survey of outcrossing homologous recombination in the highly sexual bacterial species Helicobacter pylori. We conducted multiple genome alignment and analyzed the entire data set of one-to-one orthologous genes for its global strains. We detected mosaic structures due to repeated recombination events and discordant phylogenies throughout the genomes of this species. Most of these genes including the "core" set of genes and horizontally transferred genes showed at least one recombination event. Taking into account the relationship between the nucleotide diversity and the minimum number of recombination events per nucleotide, we evaluated the recombination rate in every gene. The rate appears constant across the genome, but genes with a particularly high or low recombination rate were detected. Interestingly, genes with high recombination included those for DNA transformation and for basic cellular functions, such as biosynthesis and metabolism. Several highly divergent genes with a high recombination rate included those for host interaction, such as outer membrane proteins and lipopolysaccharide synthesis. These results provide a global picture of genome-wide distribution of outcrossing homologous recombination in a bacterial species for the first time, to our knowledge, and illustrate how a species can be shaped by mutual homologous recombination.