Flavone-resistant Leishmania donovani Overexpresses LdMRP2 Transporter in the Parasite and Activates Host MRP2 on Macrophages to Circumvent the Flavone-mediated Cell Death

In parasites, ATP-binding cassette (ABC) transporters represent an important family of proteins related to drug resistance and other biological activities. Resistance of leishmanial parasites to therapeutic drugs continues to escalate in developing countries, and in many instances, it is due to overexpressed ABC efflux pumps. Progressively adapted baicalein (BLN)-resistant parasites (pB²⁵R) show overexpression of a novel ABC transporter, which was classified as ABCC2 or Leishmania donovani multidrug resistance protein 2 (LdMRP2). The protein is primarily localized in the flagellar pocket region and in internal vesicles. Overexpressed LdABCC2 confers substantial BLN resistance to the parasites by rapid drug efflux. The BLN-resistant promastigotes when transformed into amastigotes in macrophage cells cannot be cured by treatment of macrophages with BLN. Amastigote resistance is concomitant with the overexpression of macrophage MRP2 transporter. Reporter analysis and site-directed mutagenesis assays demonstrated that antioxidant response element 1 is activated upon infection. The expression of this phase II detoxifying gene is regulated by NFE2-related factor 2 (Nrf2)-mediated antioxidant response element activation. In view of the fact that the signaling pathway of phosphoinositol 3-kinase controls microfilament rearrangement and translocation of actin-associated proteins, the current study correlates with the intricate pathway of phosphoinositol 3-kinase-mediated nuclear translocation of Nrf2, which activates MRP2 expression in macrophages upon infection by the parasites. In contrast, phalloidin, an agent that prevents depolymerization of actin filaments, inhibits Nrf2 translocation and Mrp2 gene activation by pB²⁵R infection. Taken together, these results provide insight into the mechanisms by which resistant clinical isolates of L. donovani induce intracellular events relevant to drug resistance.     

A voltage-gated pore for translocation of tRNA

Very little is known about how nucleic acids are translocated across membranes. The multi-subunit RNA Import Complex (RIC) from mitochondria of the kinetoplastid protozoon Leishmania tropica induces translocation of tRNAs across artificial or natural membranes, but the nature of the translocation pore remains unknown. We show that subunits RIC6 and RIC9 assemble on the membrane in presence of subunit RIC4A to form complex R3. Atomic Force Microscopy of R3 revealed particles with an asymmetric surface groove of ∼20 nm rim diameter and ∼1 nm depth. R3 induced translocation of tRNA into liposomes when the pH of the medium was lowered to ∼6 in the absence of ATP. R3-mediated tRNA translocation could also be induced at neutral pH by a K⁺ diffusion potential with an optimum of 60–70 mV. Point mutations in the Cys₂–His₂ Fe-binding motif of RIC6, which is homologous to the respiratory Complex III Fe–S protein, abrogated import induced by low pH but not by K⁺ diffusion potential. These results indicate that the R3 complex forms a pore that is gated by a proton-generated membrane potential and that the Fe–S binding region of RIC6 has a role in proton translocation. The tRNA import complex of L. tropica thus contains a novel macromolecular channel distinct from the mitochondrial protein import pore that is apparently involved in tRNA import in some species.     

The efficacy and immunogenicity of a live transconjugant hybrid strain of <Emphasis Type="Italic">Shigella dysenteriae</Emphasis> type 1 in two animal models

In our earlier studies, we constructed a hybrid strain of Shigella dysenteriae type 1 by introducing a plasmid vector pPR 1347. After introduction of a lipopolysaccharide (LPS) biosynthesis gene, virulent Shigella dysenteriae type 1 strain became avirulent. In our present study, we have evaluated the immune response and protective efficacy of avirulent live transconjugant Shigella hybrid (LTSH) strain against wild type Shigella dysenteriae type 1, after four doses of oral (rabbit) and intranasal (mouse) immunizations. Serum IgG titers showed exponential increase during immunization and peaking on the 28th day and remained at that level till the 35th day in both the rabbit and the mouse models. When tested, serum IgG titers persisted for 63 days in mice and relatively high for 150 days in case of rabbits. Protection studies showed 100% protection against the challenge with wild type Shigella dysenteriae type 1 strain in rabbits and 80% protection in mice. Our results suggested that the LTSH strain could be a useful vaccine candidate strain in the future.     

Passive immunity with multi‐serotype heat‐killed Shigellae in neonatal mice

The short‐ and long‐term passive protective efficacy of a mixture of heat‐killed cells of six serogroups/serotypes of Shigella strains (Shigella dysenteriae 1, S. flexneri 2a, S. flexneri 3a, S. flexneri 6, S. boydii 4, and S. sonnei) were studied in neonatal mice. Neonatal mice from immunized dams exhibited significant short‐ and long‐term passive protection against individual challenge by each of the six Shigella strains. High IgG and IgA titers against the lipopolysaccharide from each of the six Shigella strains were observed in sera from immunized dams.     

Artificial Citrate Operon Confers Mineral Phosphate Solubilization Ability to Diverse Fluorescent Pseudomonads

Citric acid is a strong acid with good cation chelating ability and can be very efficient in solubilizing mineral phosphates. Only a few phosphate solubilizing bacteria and fungi are known to secrete citric acids. In this work, we incorporated artificial citrate operon containing NADH insensitive citrate synthase (gltA1) and citrate transporter (citC) genes into the genome of six-plant growth promoting P. fluorescens strains viz., PfO-1, Pf5, CHAO1, P109, ATCC13525 and Fp315 using MiniTn7 transposon gene delivery system. Comprehensive biochemical characterization of the genomic integrants and their comparison with plasmid transformants of the same operon in M9 minimal medium reveals the highest amount of ∼7.6±0.41 mM citric and 29.95±2.8 mM gluconic acid secretion along with ∼43.2±3.24 mM intracellular citrate without affecting the growth of these P. fluorescens strains. All genomic integrants showed enhanced citric and gluconic acid secretion on Tris-Cl rock phosphate (TRP) buffered medium, which was sufficient to release 200–1000 µM Pi in TRP medium. This study demonstrates that MPS ability could be achieved in natural fluorescent pseudomonads by incorporation of artificial citrate operon not only as plasmid but also by genomic integration.     

The Vibrio cholerae Extracellular Chitinase ChiA2 Is Important for Survival and Pathogenesis in the Host Intestine

In aquatic environments, Vibrio cholerae colonizes mainly on the chitinous surface of copepods and utilizes chitin as the sole carbon and nitrogen source. Of the two extracellular chitinases essential for chitin utilization, the expression of chiA2 is maximally up-regulated in host intestine. Recent studies indicate that several bacterial chitinases may be involved in host pathogenesis. However, the role of V. cholerae chitinases in host infection is not yet known. In this study, we provide evidence to show that ChiA2 is important for V. cholerae survival in intestine as well as in pathogenesis. We demonstrate that ChiA2 de-glycosylates mucin and releases reducing sugars like GlcNAc and its oligomers. Deglycosylation of mucin corroborated with reduced uptake of alcian blue stain by ChiA2 treated mucin. Next, we show that V. cholerae could utilize mucin as a nutrient source. In comparison to the wild type strain, ΔchiA2 mutant was 60-fold less efficient in growth in mucin supplemented minimal media and was also ∼6-fold less competent to survive when grown in the presence of mucin-secreting human intestinal HT29 epithelial cells. Similar results were also obtained when the strains were infected in mice intestine. Infection with the ΔchiA2 mutant caused ∼50-fold less fluid accumulation in infant mice as well as in rabbit ileal loop compared to the wild type strain. To see if the difference in survival of the ΔchiA2 mutant and wild type V. cholerae was due to reduced adhesion of the mutant, we monitored binding of the strains on HT29 cells. The initial binding of the wild type and mutant strain was similar. Collectively these data suggest that ChiA2 secreted by V. cholerae in the intestine hydrolyzed intestinal mucin to release GlcNAc, and the released sugar is successfully utilized by V. cholerae for growth and survival in the host intestine.     

Safety and Immunogenicity of a Live Oral Recombinant Cholera Vaccine VA1.4: A Randomized,Placebo Controlled Trial in Healthy Adults in a Cholera Endemic Area in Kolkata,India

Background

A live oral cholera vaccine VA 1.4 developed from a non-toxigenic Vibrio cholerae O1 El Tor strain using ctxB gene insertion was further developed into a clinical product following cGMP and was evaluated in a double-blind randomized placebo controlled parallel group two arm trial with allocation ratio of 1∶1 for safety and immunogenicity in men and women aged 18–60 years from Kolkata, India.

Method

A lyophilized dose of 1.9×10⁹ CFU (n = 44) or a placebo (n = 43) reconstituted with a diluent was administered within 5 minutes of drinking 100 ml of a buffer solution made of sodium bicarbonate and ascorbic acid and a second dose on day 14.

Result

The vaccine did not elicit any diarrhea related adverse events. Other adverse events were rare, mild and similar in two groups. One subject in the vaccine group excreted the vaccine strain on the second day after first dose. The proportion of participants who seroconverted (i.e. had 4-folds or higher rise in reciprocal titre) in the vaccine group were 65.9% (95% CI: 50.1%–79.5%) at both 7 days (i.e. after 1^st dose) and 21 days (i.e. after 2^nd dose). None of the placebo recipients seroconverted. Anti-cholera toxin antibody was detected in very few recipients of the vaccine.

Conclusion

This study demonstrates that VA 1.4 at a single dose of 1.9×10⁹ is safe and immunogenic in adults from a cholera endemic region. No additional benefit after two doses was seen.

Trial Registration

Clinical Trials Registry-India, National Institute of Medical Statistics (Indian Council of Medical Research) CTRI/2012/04/002582     

Conjugated eicosapentaenoic acid inhibits human topoisomerase IB with a mechanism different from camptothecin

Conjugated eicosapentaenoic acid (cEPA) has been found to have antitumor effects which has been ascribed to their ability to inhibit DNA topoisomerases and DNA polymerases. We here show that cEPA inhibits the catalytic activity of human topoisomerase I, but unlike camptothecin it does not stabilize the cleavable complex, indicating a different mechanism of action. cEPA inhibits topoisomerase by impeding the catalytic cleavage of the DNA substrate as demonstrated using specific oligonucleotide substrates, and prevents the stabilization of the cleavable complex by camptothecin. Preincubation of the inhibitor with the enzyme is required to obtain complete inhibition. Molecular docking simulations indicate that the preferred cEPA binding site is proximal to the active site with the carboxylic group strongly interacting with the positively charged K443 and K587. Taken together the results indicate that cEPA inhibitor does not prevent DNA binding but inhibits DNA cleavage, binding in a region close to the topoisomerase active site.     

An insight into the active site of a type I DNA topoisomerase from the kinetoplastid protozoan Leishmania donovani

DNA topoisomerases are ubiquitous enzymes that govern the topological interconversions of DNA thereby playing a key role in many aspects of nucleic acid metabolism. Recently determined crystal structures of topoisomerase fragments, representing nearly all the known subclasses, have been solved. The type IB enzymes are structurally distinct from other known topoisomerases but are similar to a class of enzymes referred to as tyrosine recombinases. A putative topoisomerase I open reading frame from the kinetoplastid Leishmania donovani was reported which shared a substantial degree of homology with type IB topoisomerases but having a variable C-terminus. Here we present a molecular model of the above parasite gene product, using the human topoisomerase I crystal structure in complex with a 22 bp oligonucleotide as a template. Our studies indicate that the overall structure of the parasite protein is similar to the human enzyme; however, major differences occur in the C-terminal loop, which harbors a serine in place of the usual catalytic tyrosine. Most other structural themes common to type IB topoisomerases, including secondary structural folds, hinged clamps that open and close to bind DNA, nucleophilic attack on the scissile DNA strand and formation of a ternary complex with the topoisomerase I inhibitor camptothecin could be visualized in our homology model. The validity of serine acting as the nucleophile in the case of the parasite protein model was corroborated with our biochemical mapping of the active site with topoisomerase I enzyme purified from L.donovani promastigotes.