Trypanosoma cruzi arginine kinase characterization and cloning. A novel energetic pathway in protozoan parasites

Effectiveness of the repurposed drug isotretinoin in an experimental murine model of Chagas disease

Benznidazole and nifurtimox are the drugs currently used for the treatment of Chagas disease, however its side effects may affect patient adherence. In the search for new alternative therapies, we previously identified isotretinoin (ISO), an FDA-approved drug widely used for the treatment of severe acne through a drug repurposing strategy. ISO shows a strong activity against Trypanosoma cruzi parasites in the nanomolar range, and its mechanism of action is through the inhibition of T. cruzi polyamine and amino acid transporters from the Amino Acid/Auxin Permeases (AAAP) family. In this work, a murine model of chronic Chagas disease (C57BL/6 J mice), intraperitoneally infected with T. cruzi Nicaragua isolate (DTU TcI), were treated with different oral administrations of ISO: daily doses of 5 mg/kg/day for 30 days and weekly doses of 10 mg/kg during 13 weeks. The efficacy of the treatments was evaluated by monitoring blood parasitemia by qPCR, anti-T. cruzi antibodies by ELISA, and cardiac abnormalities by electrocardiography. No parasites were detected in blood after any of the ISO treatments. The electrocardiographic study of the untreated chronic mice showed a significant decrease in heart rate, while in the treated mice this negative chronotropic effect was not observed. Atrioventricular nodal conduction time in untreated mice was significantly longer than in treated animals. Mice treated even with ISO 10 mg/kg dose every 7 days, showed a significant reduction in anti-T. cruzi IgG levels. In conclusion, the intermittent administration of ISO 10 mg/kg would improve myocardial compromise during the chronic stage.

Trypanocidal activity of the anthocyanidin delphinidin, a non-competitive inhibitor of arginine kinase

Arginine kinase from Trypanosoma cruzi (TcAK) catalyzes the interconversion of arginine and phosphoarginine to maintain the ATP/ADP cell balance, and is involved in the parasites' energetic homeostasis and stress responses. Using virtual screening approaches, some plant-derived polyphenolic pigments, such as anthocyanidins, were predicted to inhibit TcAK activity. Here, it was demonstrated that the anthocyanidin delphinidin showed a non-competitive inhibition mechanism of TcAK (Ki arginine = 1.32 µM and Ki ATP = 500 µM). Molecular docking simulations predicted that delphinidin occupies part of the ATP/ADP pocket, more specifically the one that binds the ribose phosphate, and molecular dynamics simulations confirmed the amino acids involved in binding. Delphinidin exerted trypanocidal activity over T. cruzi trypomastigotes with a calculated IC₅₀ of 19.51 µM. Anthocyanidins are low-toxicity natural products which can be exploited for the development of trypanocidal drugs with less secondary effects than those currently used for the treatment of Chagas disease.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Differential expression and activity of arginine kinase between the American trypanosomatids Trypanosoma rangeli and Trypanosoma cruzi

Trypanosoma rangeli is a non-virulent hemoflagellate parasite infecting humans, wild and domestic mammals in Central and Latin America. The share of genotypic, phenotypic, and biological similarities with the virulent, human-infective T. cruzi and T. brucei, allows comparative studies on mechanisms of pathogenesis. In this study, investigation of the T. rangeli Arginine Kinase (TrAK) revealed two highly similar copies of the AK gene in this taxon, and a distinct expression profile and activity between replicative and infective forms. Although TrAK expression seems stable during epimastigotes growth, the enzymatic activity increases during the exponential growth phase and decreases from the stationary phase onwards. No differences were observed in activity or expression levels of TrAK during in vitro differentiation from epimastigotes to infective forms, and no detectable AK expression was observed for blood trypomastigotes. Overexpression of TrAK by T. rangeli showed no effects on the in vitro growth pattern, differentiation to infective forms, or infectivity to mice and triatomines. Although differences in TrAK expression and activity were observed among T. rangeli strains from distinct genetic lineages, our results indicate an up-regulation during parasite replication and putative post-translational myristoylation of this enzyme. We conclude that up-regulation of TrAK activity in epimastigotes appears to improve proliferation fitness, while reduced TrAK expression in blood trypomastigotes may be related to short-term and subpatent parasitemia in mammalian hosts.

Phosphagen kinases from five groups of eukaryotic protists (Choanomonada, Alveolate, Stramenopiles, Haptophyta, and Cryptophyta): Diverse enzyme activities and phylogenetic relationship with metazoan enzymes

Among 28 groups of eukaryotes, apart from Metazoa, phosphagen kinase (PKs) is distributed in only a few protist groups, including the Choanomonada with the closest affinity to metazoans. To clarify the origin of metazoan PKs, we performed a database search and focused on 11 sequences of PK homologs from five groups of protists: the Choanomonada, Alveolata, Haptophyta, Stramenopiles, and Cryptophyta. The recombinant enzymes were prepared to determine their substrate specificity. Emiliania (Haptophyta), Anophryoides, Pseudocohnilembus, Vitrella and Chromera (Alveolata), and Monosiga (Choanomonada) all contained a gene for arginine kinase (AK). In contrast, Aphanomyces, Albugo and Ectocarpus (Stramenopiles), and Guillardia (Cryptophyta) possessed a gene for taurocyamine kinase (TK). The Guillardia TK enzyme exhibited rather strong substrate inhibition toward taurocyamine, which was analyzed using the most likely kinetic model. This was the first report of substrate inhibition in a TK. Together with the research results from other groups, the AK, TK, or creatine kinase (CK) activities have been observed sporadically in at least six groups of protists. However, it is not clear the three enzyme activities were emerged early in the evolution and divergence of protist groups, or some of enzyme activities were introduced to the protists by horizontal gene transfer. In addition, we found that seven protist enzymes examined in this study possess a myristoylation signaling sequence at the N-terminus. The amino-acid sequence around the guanidine-specificity region and the key residue at 89^th position of the protist AK and CK were homologous to those of the metazoan enzymes, but those for protist TKs were different indicating that the latter evolved independently.

A study of Pb2+ induced unfolding and aggregation of arginine kinase from Euphausia superba: kinetics and computational simulation integrating study

Arginine kinase is a crucial phosphagen kinase in invertebrates, which is associated to the environmental stress response, plays a key role in cellular energy metabolism. In this study, we investigated the Pb²⁺-induced inhibition and aggregation of Euphausia superba arginine kinase (ESAK) and found that significantly inactivated ESAK in a dose-dependent manner (IC50 = 0.058 ± 0.002 mM). Spectrofluorimetry results showed that Pb²⁺ induced tertiary structural changes via the internal polarity increased and the non-polarity decreased in ESAK and directly induced ESAK aggregation. The ESAK aggregation process induced by Pb²⁺ occurred with multi-phase kinetics. The addition of osmolytes did not show protective effect on Pb²⁺-induced inactivation of ESAK. The computational molecular dynamics (MD) simulation showed that three Pb²⁺ interrupt the entrance of the active site of ESAK and it could be the reason on the loss of activity of ESAK. Several important residues of ESAK were detected that were importantly contributed the conformation and catalytic function of ESAK. Our study showed that Pb²⁺-induced misfolding of ESAK and the complete loss of activity irreversibly, which cannot be recovered by osmolytes.Communicated by Ramaswamy H. Sarma.

The immunogenic maturation of goat monocyte-derived dendritic cells and upregulation of toll-like receptors by five antigens of Haemonchus contortus in-vitro

Previously, it was found that several proteins of Haemonchus contortus were involved in the stimulation of the host immune system. However, the information about the selection of superlative antigens with immunogenic efficacies on host DCs is lacking. In the current study, the stimulatory effects of five recombinant proteins (elongation factor-1α, arginine kinase, ES-15, ES-24, and ADP-ribosylation factor 1) of H. contortus on the maturation of goat monocyte-derived dendritic cells (md-DCs) were reported. Recombinant proteins were purified separately in E. coli expression and incubated with isolated goat peripheral blood mononuclear cells (PBMC). Immunofluorescence assay (IFA) results confirmed the binding of these molecules to the md-DC's surface as compared to control groups. In the flow cytometry analysis, recombinant proteins induced md-DC stimulation via the up-regulation of the expression of the costimulatory molecule (CD80) and MHC-II. Quantitative RT-PCR data showed a significant increase in the expression of specific genes of the WNT and toll-like receptor (TLR) signaling pathways. The result of ELISA indicated the higher levels of cytokine (IL-10, IL-12, IFN-γ, and TNF-α) secretion in the md-DC compared to the negative (pET-32a His-Tag) and blank (PBS) control groups. The data gives valuable support in the selection of potential antigens for future studies on the immunomodulation of the host against the infection of H. contortus.

Effect of capsaicin on the protozoan parasite Trypanosoma cruzi

Trypanosoma cruzi is the causative agent of Chagas disease. There are only two approved treatments, both of them unsuitable for the chronic phase, therefore the development of new drugs is a priority. Trypanosoma cruzi arginine kinase (TcAK) is a promising drug target since it is absent in humans and it is involved in cellular stress responses. In a previous study, possible TcAK inhibitors were identified through computer simulations resulting the best compounds capsaicin and cyanidin derivatives. Here, we evaluate the effect of capsaicin on TcAK activity and its trypanocidal effect. Although capsaicin produced a weak enzyme inhibition, it had a strong trypanocidal effect on epimastigotes and trypomastigotes (IC50 = 6.26 µM and 0.26 µM, respectively) being 20-fold more active on trypomastigotes than mammalian cells. Capsaicin was also active on the intracellular cycle reducing by half the burst of trypomastigotes at approximately 2 µM. Considering the difference between the concentrations at which parasite death and TcAK inhibition occur, other possible targets were predicted. Capsaicin is a selective trypanocidal agent active in nanomolar concentrations, with an IC50 57-fold lower than benznidazole, the drug currently used for treating Chagas disease.

Targeting L-Proline Uptake as New Strategy for Anti-chagas Drug Development

L-Proline is an important amino acid for the pathogenic protists belonging to Trypanosoma and Leishmania genera. In Trypanosoma cruzi, the etiological agent of Chagas disease, this amino acid is involved in fundamental biological processes such as ATP production, differentiation of the insect and intracellular stages, the host cell infection and the resistance to a variety of stresses. In this study, we explore the L-Proline uptake as a chemotherapeutic target for T. cruzi. Novel inhibitors have been proposed containing the amino acid with a linker and a variable region able to block the transporter. A series of sixteen 1,2,3-triazolyl-proline derivatives have been prepared for in vitro screening against T. cruzi epimastigotes and proline uptake assays. We successfully obtained inhibitors that interfere with the amino acid internalization, which validated our design targeting the metabolite's transport. The presented structures are one of few examples of amino acid transporter inhibitors. The unprecedent application of this strategy on the development of new chemotherapy against Chagas disease, opens a new horizon on antiparasitic drug development against parasitic diseases and other pathologies.

Broad-complex transcription factor mediates opposing hormonal regulation of two phylogenetically distant arginine kinase genes in Tribolium castaneum

The phosphoarginine-arginine kinase shuttle system plays a critical role in maintaining insect cellular energy homeostasis. Insect molting and metamorphosis are coordinated by fluctuations of the ecdysteroid and juvenile hormone. However, the hormonal regulation of insect arginine kinases remain largely elusive. In this report, we comparatively characterized two arginine kinase genes, TcAK1 and TcAK2, in Tribolium castaneum. Functional analysis using RNAi showed that TcAK1 and TcAK2 play similar roles in adult fertility and stress response. TcAK1 was detected in cytoplasm including mitochondria, whereas TcAK2 was detected in cytoplasm excluding mitochondria. Interestingly, TcAK1 expression was negatively regulated by 20-hydroxyecdysone and positively by juvenile hormone, whereas TcAK2 was regulated by the opposite pattern. RNAi, dual-luciferase reporter assays and electrophoretic mobility shift assay further revealed that the opposite hormonal regulation of TcAK1 and TcAK2 was mediated by transcription factor Broad-Complex. Finally, relatively stable AK activities were observed during larval-pupal metamorphosis, which was generally consistent with the constant ATP levels. These results provide new insights into the mechanisms underlying the ATP homeostasis in insects by revealing opposite hormonal regulation of two phylogenetically distant arginine kinase genes.

Zhang et al. characterize the functions of two distinct arginine kinase genes in flour beetles. Using RNA interference and electophoretic mobility shift assays, they identify Broad-Complex transcription factor as the mediator of opposing hormonal regulation in these genes.

Insight into Structural Aspects of Histidine 284 of Daphnia magna Arginine Kinase

Arginine kinase (AK), a bioenergy-related enzyme, is distributed widely in invertebrates. The role of highly conserved histidines in AKs is still unascertained. In this study, the highly conserved histidine 284 (H284) in AK of Daphnia magna (DmAK) was replaced with alanine to elucidate the role of H284. We examined the alteration of catalytic activity and structural changes of H284A in DmAK. The catalytic activity of H284A was reduced dramatically compared to that in wild type (WT). Thus the crystal structure of H284A displayed several structural changes, including the alteration of D324, a hydrogen-bonding network around H284, and the disruption of π-stacking between the imidazole group of the H284 residue and the adenine ring of ATP. These findings suggest that such alterations might affect a conformational change of the specific loop consisting of G310-V322 at the antiparallel β-sheet region. Thus, we speculated that the H284 residue might play an important role in the conformational change of the specific loop when ATP binds to the substrate-binding site of DmAK.

Apoferritin and Apoferritin-Capped Metal Nanoparticles Inhibit Arginine Kinase of Trypanosoma brucei

The aim of this study was to explore the inhibitory potential of apoferritin or apoferritin-capped metal nanoparticles (silver, gold and platinum) against Trypanosomabrucei arginine kinase. The arginine kinase activity was determined in the presence and absence of apoferritin or apoferritin-capped metal nanoparticles. In addition, kinetic parameters and relative inhibition of enzyme activity were estimated. Apoferritin or apoferritin-capped metal nanoparticles’ interaction with arginine kinase of T. brucei led to a >70% reduction in the enzyme activity. Further analysis to determine kinetic parameters suggests a mixed inhibition by apoferritin or apoferritin-nanoparticles, with a decrease in V_max. Furthermore, the K_m of the enzyme increased for both ATP and L-arginine substrates. Meantime, the inhibition constant (K_i) values for the apoferritin and apoferritin-nanoparticle interaction were in the submicromolar concentration ranging between 0.062 to 0.168 nM and 0.001 to 0.057 nM, respectively, for both substrates (i.e., L-arginine and ATP). Further kinetic analyses are warranted to aid the development of these nanoparticles as selective therapeutics. Also, more studies are required to elucidate the binding properties of these nanoparticles to arginine kinase of T. brucei.

Defeating the trypanosomatid trio: proteomics of the protozoan parasites causing neglected tropical diseases

Mass spectrometry-based proteomics enables accurate measurement of the modulations of proteins on a large scale upon perturbation and facilitates the understanding of the functional roles of proteins in biological systems. It is a particularly relevant methodology for studying Leishmania spp., Trypanosoma cruzi and Trypanosoma brucei, as the gene expression in these parasites is primarily regulated by posttranscriptional mechanisms. Large-scale proteomics studies have revealed a plethora of information regarding modulated proteins and their molecular interactions during various life processes of the protozoans, including stress adaptation, life cycle changes and interactions with the host. Important molecular processes within the parasite that regulate the activity and subcellular localisation of its proteins, including several co- and post-translational modifications, are also accurately captured by modern proteomics mass spectrometry techniques. Finally, in combination with synthetic chemistry, proteomic techniques facilitate unbiased profiling of targets and off-targets of pharmacologically active compounds in the parasites. This provides important data sets for their mechanism of action studies, thereby aiding drug development programmes.

Bacterial arginine kinases have a highly skewed distribution within the proteobacteria

Phosphagen kinases (PKs) are known to be distributed throughout the animal kingdom, but have recently been discovered in some protozoan and bacterial species. A recent search of the available bacterial genomes revealed 49 unique sequences that appear to code for an arginine kinase (AK). The distribution of sequences was highly skewed with thirty nine out the forty nine sequences being found in six Proteobacteria classes (α, β, δ, γ, ε, and ζ) which represented 46.6% of the 61,335 bacterial genomes available at JGI-IMG/M website. Moreover, twenty one of the unique and metagenome bAK sequences identified were from δ-Proteobacteria despite these representing only 0.88% of the total genomes available. Phylogenetic analyses revealed that the bacterial AK sequences were interpersed between basal species such as cnidarians, sponges and protozoa, displaying an unstable clustering that was dependent upon the parameters chosen for phylogenetic analysis. Three of these putative bacterial AK genes were cloned into the pET45 expression vector, expressed, and biochemically confirmed to be capable of phosphorylating arginine using ATP. Results of the kinetic analyses of the putative bAKs from Ahrensia, D. autotrophicum, and O. profundus show that the catalytic efficiencies with respect to arginine for each enzyme, measured at 10⁴-10⁵ M^-1 s^-1, fall within the range expected for competent arginine kinases.

Aurora kinase protein family in Trypanosoma cruzi: Novel role of an AUK-B homologue in kinetoplast replication

Aurora kinases constitute a family of enzymes that play a key role during metazoan cells division, being involved in events like centrosome maturation and division, chromatin condensation, mitotic spindle assembly, control of kinetochore-microtubule attachments, and cytokinesis initiation. In this work, three Aurora kinase homologues were identified in Trypanosoma cruzi (TcAUK1, -2 and -3), a protozoan parasite of the Kinetoplastida Class. The genomic organization of these enzymes was fully analyzed, demonstrating that TcAUK1 is a single-copy gene, TcAUK2 coding sequence is present in two different forms (short and long) and TcAUK3 is a multi-copy gene. The three TcAUK genes are actively expressed in the different life cycle forms of T. cruzi (amastigotes, trypomastigotes and epimastigotes). TcAUK1 showed a changing localization along the cell cycle of the proliferating epimastigote form: at interphase it is located at the extremes of the kinetoplast while in mitosis it is detected at the cell nucleus, in close association with the mitotic spindle. Overexpression of TcAUK1 in epimastigotes leaded to a delay in the G2/M phases of the cell cycle due a retarded beginning of kinetoplast duplication. By immunofluorescence, we found that when it was overexpressed TcAUK1 lost its localization at the extremes of the kinetoplast during interphase, being observed inside the cell nucleus throughout the entire cell cycle. In summary, TcAUK1 appears to be a functional homologue of human Aurora B kinase, as it is related to mitotic spindle assembling and chromosome segregation. Moreover, TcAUK1 also seems to play a role during the initiation of kinetoplast duplication, a novel role described for this protein.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Characterization and in vitro expression of arginine kinase gene in the invasive western flower thrips, Frankliniella occidentalis

Arginine kinase (AK) plays a critical role in insect energy metabolism and has been proposed to be a potential insecticide target for commercial exploitation. In this study, the full length cDNA encoding a typical group 1 insect AK (FoAK) was isolated from the western flower thrips (WFT), Frankliniella occidentalis (Pergande). Sequence analysis showed that FoAK contains an open reading frame of 1068 nucleotides, which encods a protein of 355 amino acid residues including the signature sequence pattern of ATP-guanidino kinases. Genomic structure analysis showed that the coding region of FoAK contains five exons connected by four introns. RT-qPCR analysis revealed that the mRNA expression of FoAK was developmentally regulated with the lowest level in prepupal stage. Enzymatic activity analysis of the recombinant enzymes expressed in Escherichia coli showed that FoAK is highly stereo specific for L-arginine versus D-arginine and the apparent Michaelis constant for L-arginine (Km^Arg) is comparable to that of AKs from a variety of species. This research should enable further investigation of the function as well as in vitro screening for inhibitors of FoAK.

The Uptake and Metabolism of Amino Acids, and Their Unique Role in the Biology of Pathogenic Trypanosomatids

Trypanosoma brucei, as well as Trypanosoma cruzi and more than 20 species of the genus Leishmania, form a group of flagellated protists that threaten human health. These organisms are transmitted by insects that, together with mammals, are their natural hosts. This implies that during their life cycles each of them faces environments with different physical, chemical, biochemical, and biological characteristics. In this work we review how amino acids are obtained from such environments, how they are metabolized, and how they and some of their intermediate metabolites are used as a survival toolbox to cope with the different conditions in which these parasites should establish the infections in the insects and mammalian hosts.

Arginine kinase from Haemonchus contortus decreased the proliferation and increased the apoptosis of goat PBMCs in vitro

BACKGROUND

Arginine kinase (AK), an important member of phosphagen kinase family has been extensively studied in various vertebrates and invertebrates. Immunologically, AKs are important constituents of different body parts, involved in various biological and cellular functions, and considered as immune-modulator and effector for pro-inflammatory cytokines. However, immunoregulatory changes of host cells triggered by AK protein of Haemonchus contortus, a parasitic nematode of ruminants, are still unknown. The current study was focused on cloning and characterisation of Hc-AK, and its regulatory effects on cytokines level, cell migration, cell proliferation, nitric oxide production and apoptosis of goat peripheral blood mononuclear cells (PBMCs) were observed.

METHODS

The full-length sequence of the Hc-AK gene was amplified by reverse transcription-polymerase chain reaction (RT-PCR) and sub-cloned into the prokaryotic expression vector pET-32a. The biochemical characteristics of recombinant protein Hc-AK, which was purified by affinity chromatography, were performed based on the enzymatic assay. Binding of rHc-AK with PBMCs was confirmed by immunofluorescence assay (IFA). Immunohistochemical analysis was used to detect localisation of Hc-AK within adult worms sections. The immunoregulatory effects of rHc-AK on cytokine secretions, cell proliferation, cell migration, nitric oxide production and apoptosis were determined by co-incubation of rHc-AK with goat PBMCs.

RESULTS

The full-length ORF (1080 bp) of the Hc-AK gene was successfully cloned, and His-tagged AK protein was expressed in the Escherichia coli strain BL21. The recombinant protein of Hc-AK (rHc-AK) was about 58.5 kDa together with the fused vector protein of 18 kDa. The biochemical assay showed that the protein encoded by the Hc-ak exhibited enzymatic activity. Western blot analysis confirmed that the rHc-AK was recognised by the sera from rat (rat-antiHc-AK). The IFA results showed that rHc-AK could bind on the surface of goat PBMCs. Immunohistochemically, Hc-AK was localised at the inner and outer membrane as well as in the gut region of adult worms. The binding of rHc-AK to host cells increased the levels of IL-4, IL-10, IL-17, IFN-γ, nitric oxide (NO) production and cell apoptosis of goat PBMCs, whereas, TGF-β1 levels, cell proliferation and PBMCs migration were significantly decreased in a dose dependent manner.

CONCLUSIONS

Our findings suggested that rHc-AK is an important excretory and secretory (ES) protein involved in host immune responses and exhibit distinct immunomodulatory properties during interaction with goat PBMCs.

Trypanocidal Effect of Isotretinoin through the Inhibition of Polyamine and Amino Acid Transporters in Trypanosoma cruzi

Polyamines are essential compounds to all living organisms and in the specific case of Trypanosoma cruzi, the causative agent of Chagas disease, they are exclusively obtained through transport processes since this parasite is auxotrophic for polyamines. Previous works reported that retinol acetate inhibits Leishmania growth and decreases its intracellular polyamine concentration. The present work describes a combined strategy of drug repositioning by virtual screening followed by in vitro assays to find drugs able to inhibit TcPAT12, the only polyamine transporter described in T. cruzi. After a screening of 3000 FDA-approved drugs, 7 retinoids with medical use were retrieved and used for molecular docking assays with TcPAT12. From the docked molecules, isotretinoin, a well-known drug used for acne treatment, showed the best interaction score with TcPAT12 and was selected for further in vitro studies. Isotretinoin inhibited the polyamine transport, as well as other amino acid transporters from the same protein family (TcAAAP), with calculated IC₅₀ values in the range of 4.6–10.3 μM. It also showed a strong inhibition of trypomastigote burst from infected cells, with calculated IC₅₀ of 130 nM (SI = 920) being significantly less effective on the epimastigote stage (IC₅₀ = 30.6 μM). The effect of isotretinoin on the parasites plasma membrane permeability and on mammalian cell viability was tested, and no change was observed. Autophagosomes and apoptotic bodies were detected as part of the mechanisms of isotretinoin-induced death indicating that the inhibition of transporters by isotretinoin causes nutrient starvation that triggers autophagic and apoptotic processes. In conclusion, isotretinoin is a promising trypanocidal drug since it is a multi-target inhibitor of essential metabolites transporters, in addition to being an FDA-approved drug largely used in humans, which could reduce significantly the requirements for its possible application in the treatment of Chagas disease.

Polyamines are polycationic compounds essential for the regulation of cell growth and differentiation. In contrast with other protozoa, Trypanosoma cruzi, the etiological agent of Chagas disease, is auxotrophic for polyamines; therefore the intracellular availability of these molecules depends exclusively on transport processes. It was previously demonstrated that the lack of polyamines in T. cruzi leads to its death, making the polyamine transporter an excellent therapeutic target for Chagas disease. In this work, the polyamine permease TcPAT12 was selected as a target for drug screening using 3000 FDA-approved compounds and computational simulation techniques. Using two combined virtual screening methods, isotretinoin, a well-known and safe drug used for acne treatment, bound to substrate recognition residues of TcPAT12 and was chosen for further in vitro studies. Isotretinoin inhibited not only the polyamine transport but also all tested amino acid transporters from the same protein family as TcPAT12. Interestingly, isotretinoin showed a high trypanocidal effect on trypomastigotes, with an IC₅₀ in the nanomolar range. Autophagy and apoptosis were proposed as mechanisms of parasites death induced by isotretinoin. These results suggest that isotretinoin is a promising trypanocidal drug, being a multi-target inhibitor of essential metabolites transporters.

Arginine and Lysine Transporters Are Essential for Trypanosoma brucei

For Trypanosoma brucei arginine and lysine are essential amino acids and therefore have to be imported from the host. Heterologous expression in Saccharomyces cerevisiae mutants identified cationic amino acid transporters among members of the T. brucei AAAP (amino acid/auxin permease) family. TbAAT5-3 showed high affinity arginine uptake (K_m 3.6 ± 0.4 μM) and high selectivity for L-arginine. L-arginine transport was reduced by a 10-times excess of L-arginine, homo-arginine, canavanine or arginine-β-naphthylamide, while lysine was inhibitory only at 100-times excess, and histidine or ornithine did not reduce arginine uptake rates significantly. TbAAT16-1 is a high affinity (K_m 4.3 ± 0.5 μM) and highly selective L-lysine transporter and of the compounds tested, only L-lysine and thialysine were competing for L-lysine uptake. TbAAT5-3 and TbAAT16-1 are expressed in both procyclic and bloodstream form T. brucei and cMyc-tagged proteins indicate localization at the plasma membrane. RNAi-mediated down-regulation of TbAAT5 and TbAAT16 in bloodstream form trypanosomes resulted in growth arrest, demonstrating that TbAAT5-mediated arginine and TbAAT16-mediated lysine transport are essential for T. brucei. Growth of induced RNAi lines could partially be rescued by supplementing a surplus of arginine or lysine, respectively, while addition of both amino acids was less efficient. Single and double RNAi lines indicate that additional low affinity uptake systems for arginine and lysine are present in T. brucei.

The Phosphatidylinositol 3-kinase Class III Complex Containing TcVps15 and TcVps34 Participates in Autophagy in Trypanosoma cruzi

Autophagy is a degradative process by which eukaryotic cells digest their own components to provide aminoacids that may function as energy source under nutritional stress conditions. There is experimental evidence for autophagy in parasitic protists belonging to the family Trypanosomatidae. However, few proteins implicated in this process have been characterized so far in these parasites. Moreover, it has been shown that autophagy is involved in Trypanosoma cruzi differentiation and thus might have a role in pathogenicity. Here, we report the cloning and biochemical characterization of TcVps15. In addition, we demonstrate that TcVps15 interact with the PI3K TcVps34 and that both proteins associate with cellular membranes. Under nutritional stress conditions, TcVps15 and TcVps34 modify their subcellular distribution showing a partial co-localization in autophagosomes with TcAtg8.1 and using an active site TcVps15-mutated version (TcVps15-K219D-HA) we demonstrated that this relocalization depends on the TcVps15 catalytic activity. Overexpression of TcVps15-HA and TcVps15-K219D-HA also leads to increased accumulation of monodansylcadaverine (MDC) in autophagic vacuoles under nutritional stress conditions compared to wild-type cells. In addition, the MDC-specific activity shows to be significantly higher in TcVps15-HA overexpressing cells when compared with TcVps15-K219D-HA. Our results reveal for the first time a role of TcVps15 as a key regulator of TcVps34 enzymatic activity and implicate the TcVps15-Vps34 complex in autophagy in T. cruzi, exposing a new key pathway to explore novel chemotherapeutic targets.

Arginine kinase in Toxocara canis: Exon-intron organization, functional analysis of site-directed mutants and evaluation of putative enzyme inhibitors

OBJECTIVES

To determine exon/intron organization of the Toxocara canis (T. canis) AK (TCAK) and to test green and black tea and several other chemicals against the activity of recombinant TCAK in the guanidino-specific region by site-directed mutants.

METHODS

Amplification of genomic DNA fragments containing introns was carried out by PCRs. The open-reading frame (1200 bp) of TCAK (wild type) was cloned into the BamH1/SalI site of pMAL-c2X. The maltose-binding protein-TCAK fusion protein was expressed in Escherichia coli TB1 cells. The purity of the expressed enzyme was verified by SDS-PAGE. Mutations were introduced into the guanidino-specific region and other areas of pMAL/TCAK by PCR. Enzyme activity was measured with an NADH-linked assay at 25 °C for the forward reaction (phosphagen synthesis).

RESULTS

Arginine kinase in T. canis has a seven-exon/six-intron gene structure. The lengths of the introns ranged from 542 bp to 2 500 bp. All introns begin with gt and end with ag. Furthermore, we measured the enzyme activity of site-directed mutants of the recombinant TCAK. The K_m value of the mutant (Alanine to Serine) decreased indicating a higher affinity for substrate arginine than the wild-type. The K_m value of the mutant (Serine to Glycine) increased to 0.19 mM. The K_m value (0.19 mM) of the double mutant (Alanine-Serine to Serine-Glycine) was slightly greater than in the wild-type (0.12 mM). In addition, several other chemicals were tested; including plant extract Azadiracta indica (A. indica), an aminoglycoside antibiotic (aminosidine), a citrus flavonoid glycoside (rutin) and a commercially available catechin mixture against TCAK. Green and black tea (1:10 dilution) produced 15% and 25% inhibition of TCAK, respectively. The extract of A. indica produced 5% inhibition of TCAK. Moreover, green and black tea produced a non-competitive type of inhibition and A. indica produced a mixed-type of inhibition on TCAK.

CONCLUSIONS

Arginine kinase in T. canis has a seven-exon/six-intron gene structure. However, further studies are needed to identify a specific compound within the extract causing the inhibitory effect and also to determine the molecular mechanisms behind inhibition of arginine kinase in T. canis.

Identification of the nicotinamide mononucleotide adenylyltransferase of Trypanosoma cruzi

The intracellular parasite Trypanosoma cruzi is the aetiological agent of Chagas disease, a public health concern with an increasing incidence rate. This increase is due, among other reasons, to the parasite's drug resistance mechanisms, which require nicotinamide adenine dinucleotide (NAD+). Furthermore, this molecule is involved in metabolic and intracellular signalling processes necessary for the survival of T. cruzi throughout its life cycle. NAD+biosynthesis is performed by de novo and salvage pathways, which converge on the step that is catalysed by the enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT) (enzyme commission number: 2.7.7.1). The identification of the NMNAT of T. cruzi is important for the development of future therapeutic strategies to treat Chagas disease. In this study, a hypothetical open reading frame (ORF) for NMNAT was identified in the genome of T. cruzi.The corresponding putative protein was analysed by simulating structural models. The ORF was amplified from genomic DNA by polymerase chain reaction and was further used for the construction of a corresponding recombinant expression vector. The expressed recombinant protein was partially purified and its activity was evaluated using enzymatic assays. These results comprise the first identification of an NMNAT in T. cruzi using bioinformatics and experimental tools and hence represent the first step to understanding NAD+ metabolism in these parasites.

Characterization of Arginine Kinase in the Barnacle Amphibalanus Amphitrite and Its Role in the Larval Settlement

Energy metabolism is a key process in larval settlement of barnacles, but the underlying molecular mechanisms remain ambiguous. Arginine kinase (AK) mainly participates in energy metabolism in invertebrates. So far, its roles in barnacles have not been studied. In the present study, we raised an antibody against AK from Amphibalanus amphitrite Darwin to characterize the roles of AK in the larval settlement process. Among the developmental stages, AK was highly expressed during the cypris stage. Along with the aging process in cyprids, the level of AK decreased. The immunostaining results showed that AK was localized to muscular tissues in cyprids, including antennules, antennular muscles, and thoracic limbs. The larval settlement rate decreased and larval movement was inhibited in response to treatments with high concentrations of AK inhibitors (rutin and quercetin). These results demonstrated that AK was involved in the larval settlement of A. amphitrite through mediating energy supply in muscle tissues. Moreover, further analysis indicated that both the p38 MAPK and NO/cGMP pathways positively mediated the expression of AK in cyprids.

Cooperativity and evolution of Tetrahymena two-domain arginine kinase

Tetrahymena pyriformis contains two arginine kinases, a 40-kDa enzyme (AK1) with a myristoylation signal sequence at the N-terminus and a two-domain 80-kDa enzyme (AK2). The former is localized mainly in cilia and the latter is in the cytoplasm. AK1 was successfully synthesized using an insect cell-free protein synthesis system and subjected to peptide mass fingerprinting (PMF) analysis. The masses corresponding to unmodified N-terminal tryptic peptide or N-terminal myristoylated peptide were not observed, suggesting that N-terminal peptides were not ionized in this analysis. We performed PMF analyses for two other phosphagen kinases (PKs) with myristoylation signals, an AK from Nematostella vectensis and a PK from Ectocarpus siliculosus. In both cases, the myristoylated, N-terminal peptides were clearly identified. The differences between the experimental and theoretical masses were within 0.0165-0.0583 Da, supporting the accuracy of the identification. Domains 1 and 2 of Tetrahymena two-domain AK2 were expressed separately in Escherichia coli and the extent of cooperativity was estimated on the basis of their kinetic constants. The results suggested that each of the domains functions independently, namely no cooperativity is displayed between the two domains. This is in sharp contrast to the two-domain AK from Anthopleura.

The Flagellar Arginine Kinase in Trypanosoma brucei Is Important for Infection in Tsetse Flies

African trypanosomes are flagellated parasites that cause sleeping sickness. Parasites are transmitted from one mammalian host to another by the bite of a tsetse fly. Trypanosoma brucei possesses three different genes for arginine kinase (AK) including one (AK3) that encodes a protein localised to the flagellum. AK3 is characterised by the presence of a unique amino-terminal insertion that specifies flagellar targeting. We show here a phylogenetic analysis revealing that flagellar AK arose in two independent duplication events in T. brucei and T. congolense, the two species of African trypanosomes that infect the tsetse midgut. In T. brucei, AK3 is detected in all stages of parasite development in the fly (in the midgut and in the salivary glands) as well as in bloodstream cells, but with predominance at insect stages. Genetic knockout leads to a slight reduction in motility and impairs parasite infectivity towards tsetse flies in single and competition experiments, both phenotypes being reverted upon expression of an epitope-tagged version of AK3. We speculate that this flagellar arginine kinase is important for T. brucei infection of tsetse, especially in the context of mixed infections and that its flagellar targeting relies on a system equivalent to that discovered for calflagins, a family of trypanosome flagellum calcium binding proteins.

Identification and functional characterization of a novel arginine/ornithine transporter, a member of a cationic amino acid transporter subfamily in the Trypanosoma cruzi genome

Background

Trypanosoma cruzi, the etiological agent of Chagas disease, is auxotrophic for arginine. It obtains this amino acid from the host through transporters expressed on the plasma membrane and on the membranes of intracellular compartments. A few cationic amino acid transporters have been characterized at the molecular level, such as the novel intracellular arginine/ornithine transporter, TcCAT1.1, a member of the TcCAT subfamily that is composed of four almost identical open reading frames in the T. cruzi genome.

Methods

The functional characterization of the TcCAT1.1 isoform was performed in two heterologous expression systems. TcCAT subfamily expression was evaluated by real-time PCR in polysomal RNA fractions, and the cellular localization of TcCAT1.1 fused to EGFP was performed by confocal and immunoelectron microscopy.

Results

In the S. cerevisiae expression system, TcCAT1.1 showed high affinity for arginine (K_m = 0.085 ± 0.04 mM) and low affinity for ornithine (K_m = 1.7 ± 0.2 mM). Xenopus laevis oocytes expressing TcCAT1.1 showed a 7-fold increase in arginine uptake when they were pre-loaded with arginine, indicating that transport is enhanced by substrates on the trans side of the membrane (trans-stimulation). Oocytes that were pre-loaded with [³H]-arginine displayed a 16-fold higher efflux of [³H]-arginine compared with that of the control. Analysis of polysomal RNA fractions demonstrated that the expression of members of the arginine transporter TcCAT subfamily is upregulated under nutritional stress and that this upregulation precedes metacyclogenesis. To investigate the cellular localization of the transporter, EGFP was fused to TcCAT1.1, and fluorescence microscopy and immunocytochemistry revealed the intracellular labeling of vesicles in the anterior region, in a network of tubules and vesicles.

Conclusions

TcCAT1.1 is a novel arginine/ornithine transporter, an exchanger expressed in intracellular compartments that is physiologically involved in arginine homeostasis throughout the T. cruzi life cycle. The properties and estimated kinetic parameters of TcCAT1.1 can be extended to other members of the TcCAT subfamily.

2-DE Mapping of the Blue Mussel Gill Proteome: The Usual Suspects Revisited

The Blue Mussel (Mytilus edulis, L. 1758) is an ecologically important and commercially relevant bivalve. Because of its ability to bioconcentrate xenobiotics, it is also a widespread sentinel species for environmental pollution, which has been used in ecotoxicological studies for biomarker assessment. Consequently, numerous proteomics studies have been carried out in various research contexts using mussels of the genus Mytilus, which intended to improve our understanding of complex physiological processes related to reproduction, adaptation to physical stressors or shell formation and for biomarker discovery. Differential-display 2-DE proteomics relies on an extensive knowledge of the proteome with as many proteoforms identified as possible. To this end, extensive characterization of proteins was performed in order to increase our knowledge of the Mytilus gill proteome. On average, 700 spots were detected on 2-DE gels by colloidal blue staining, of which 122 different, non-redundant proteins comprising 203 proteoforms could be identified by tandem mass spectrometry. These proteins could be attributed to four major categories: (i) “metabolism”, including antioxidant defence and degradation of xenobiotics; (ii) “genetic information processing”, comprising transcription and translation as well as folding, sorting, repair and degradation; (iii) “cellular processes”, such as cell motility, transport and catabolism; (iv) “environmental information processing”, including signal transduction and signalling molecules and interaction. The role of cytoskeleton proteins, energetic metabolism, chaperones/stress proteins, protein trafficking and the proteasome are discussed in the light of the exigencies of the intertidal environment, leading to an enhanced stress response, as well as the structural and physiological particularities of the bivalve gill tissue.

Silencing the HaAK gene by transgenic plant-mediated RNAi impairs larval growth of Helicoverpa armigera

Insect pests have caused noticeable economic losses in agriculture, and the heavy use of insecticide to control pests not only brings the threats of insecticide resistance but also causes the great pollution to foods and the environment. Transgenic plants producing double-stranded RNA (dsRNA) directed against insect genes have been is currently developed for protection against insect pests. In this study, we used this technology to silence the arginine kinase (AK) gene of Helicoverpa armigera (HaAK), encoding a phosphotransferase that plays a critical role in cellular energy metabolism in invertebrate. Transgenic Arabidopsis plants producing HaAK dsRNA were generated by Agrobacterium-mediated transformation. The maximal mortality rate of 55% was reached when H. armigera first-instar larvae were fed with transgenic plant leaves for 3 days, which was dramatically higher than the 18% mortality recorded in the control group. Moreover, the ingestion of transgenic plants significantly retarded larval growth, and the transcript levels of HaAK were also knocked down by up to 52%. The feeding bioassays further indicated that the inhibition efficiency was correlated with the integrity and concentration of the produced HaAK dsRNA in transgenic plants. These results strongly show that the resistance to H. armigera was improved in transgenic Arabidopsis plants, suggesting that the RNAi targeting of AK has the potential for the control of insect pests.

Interaction between Gallotannin and a Recombinant Form of Arginine Kinase of Trypanosoma brucei: Thermodynamic and Spectrofluorimetric Evaluation

Current chemotherapies against trypanosomiasis are beset with diverse challenges, a situation which underscores the numerous research efforts aimed at finding newer and effective treatments. Arginine kinase of trypanosome has been validated as target for drug development against trypanosomiasis. The present study investigated the interaction between a recombinant form of the arginine kinase (rTbAK) of trypanosome and gallotannin. The interaction between gallotannin and recombinant arginine kinase of Trypanosoma brucei caused significant decrease of enzyme activity. Kinetic analysis revealed the interaction to be of noncompetitive inhibition. Further thermodynamic analysis showed that the interaction between gallotannin and the recombinant arginine kinase was nonspontaneous and involved hydrophobic forces. The K sv values and the FRET analysis suggest that static quenching of fluorescence intensity by gallotannin was static. Data revealed inhibitory interactions between gallotannin and rTbAK of trypanosome. Although the mechanism of inhibition is not clear yet, molecular docking studies are ongoing to clearly define the inhibitory interactions between the gallotannin and rTbAK. The knowledge of such binding properties would enrich development of selective inhibitors for the arginine kinase of Trypanosoma brucei.

Phosphagen kinase in Schistosoma japonicum: II. Determination of amino acid residues essential for substrate catalysis using site-directed mutagenesis

Phosphagen kinases (PKs) play major roles in the regulation of energy metabolism in animals. Creatine kinase (CK) is the sole PK in vertebrates, whereas several PKs are present in invertebrates. We previously identified a contiguous dimer taurocyamine kinase (TK) from the trematode Schistosoma japonicum (Sj), a causative agent of schistosomiasis. SjTK contiguous dimer is comprised of domain 1 (D1) and domain 2 (D2). In this study, we used SjTK contiguous dimer (SjTKD1D2) or truncated single-domain constructs (SjTKD1 or SjTKD2) and employed site-directed mutagenesis to investigate the enzymatic properties of TK mutants. Mutation in SjTKD1 or SjTKD2 (D1E222G or D2E225G) caused complete loss of activity for the substrate taurocyamine. Likewise, a double mutant (D1E222GD2E225G) in the contiguous dimer (D1D2) exhibited complete loss of activity for the substrate taurocyamine. However, catalytic activity in the contiguous dimer remained in both of D1 inactive mutant (D1D2D1E222G) and D2 inactive mutant (D1D2D2E225G), suggesting that efficient catalysis of SjTKD1D2 is dependent on the activity of D1 and D2. The catalytic efficiency of the mixture of both single domains (WTD1+WTD2) showed same enzymatic properties (Km(Tauro)=0.68;Vmax/Km(Tauro)=137.04) to WTD1D2 (Km(Tauro)=0.47; Vmax/Km(Tauro)=144.30). This result suggests that the contiguous dimeric structure is not essential for the catalytic efficiencies of both domains of SjTK. Vmax/Km(Tauro) of the mixture of wild-type and inactivated domains (78.02 in WTD1+D2E225G and 128.24 in D1E222G+WTD2) were higher than the corresponding mutants (47.25 in D1D2D1E222G and 46.77 in D1D2D2E225G). To identify amino acid residues that are critical for taurocyamine binding, we performed alanine scanning mutagenesis at positions 57-63 on the guanidino specificity (GS) region of the SjTKD1, which is considered to be involved in guanidino-substrate recognition. R63A and R63Y mutants lost activity for taurocyamine, suggesting that these residues are associated with taurocyamine binding. In addition, we investigated the role of Tyr84 in D1 and found an association with substrate alignment. The Y84 residue was replaced with R, H, K, I, A, and G. Although the activities of each mutant were decreased (Vmax=2.36-67.50μmolPi/min/mgprotein), Y84 mutants possess binding affinity for taurocyamine (Km(Tauro)=3.19-10.04mM). The D1Y84R, D1Y84H, D1Y84K, and D1Y84A mutants exhibited low activity for taurocyamine, whereas the D1Y84I and D1Y84G mutants exhibited slightly decreased activity compared with the other Y84 mutants. The D1Y84K mutant lost substrate synergy between taurocyamine and ATP, suggesting that this mutation moves the position of the GS loop, similar to that of lombricine kinase (LK), and interferes with taurocyamine binding. This is the first comprehensive investigation of essential amino acid residues for substrate catalysis in trematode TK.

Two arginine kinases of Tetrahymena pyriformis: characterization and localization

Two cDNAs, one coding a typical 40-kDa arginine kinase (AK1) and the other coding a two-domain 80-kDa enzyme (AK2), were isolated from ciliate Tetrahymena pyriformis, and their recombinant enzymes were successfully expressed in Escherichia coli. Both enzymes had an activity comparable to those of typical invertebrate AKs. Interestingly, the amino acid sequence of T. pyriformis AK1, but not AK2, had a distinct myristoylation signal sequence at the N-terminus, suggesting that 40-kDa AK1 targets the membrane. Moreover, Western blot analysis showed that the AK1 is mainly localized in the ciliary fraction. Based on these results, we discuss the phosphoarginine shuttle, which enables a continuous energy flow to dynein for ciliary movement in T. pyriformis, and the role of AK1 in this model.

Proteome and allergenome of Asian wasp, Vespa affinis, venom and IgE reactivity of the venom components

Vespa affinis (Asian wasp, Thai banded tiger wasp, or local name: Tor Hua Seua) causes the most frequent incidence of medically important Hymenoptera sting in South and Southeast Asia. However, data on the venom components attributable to the sting derived-clinical manifestations (local reactions, IgE mediated-anaphylaxis, or systemic envenomation) are lacking. This study provides the first set information on V. affinis venom proteome, allergenome, and IgE reactivity of individual venom components. From 2DE-gel based-proteomics, the venom revealed 93 protein spots, of which proteins in 51 spots could be identified and classified into three groups: typical venom components and structural and housekeeping proteins. Venom proteins in 32 spots reacted with serum IgE of wasp allergic patients. Major allergenic proteins that reacted to IgE of >50% of the wasp allergic patients included PLA1 (100%), arginine kinase (73%), heat shock 70 kDa protein (73.3%), venom allergen-5 (66.7%), enolase (66.7%), PLA1 magnifin (60%), glyceraldehyde-3-phosphate dehydrogenase (60%), hyaluronidase (53.3%), and fructose-bisphosphate aldolase (53.3%). The venom minor allergens were GB17876 transcript (40%), GB17291 transcript (20%), malic enzyme (13.3%), aconitate hydratase (6.7%), and phosphoglucomutase (6.7%). The information has diagnostic and clinical implications for future improvement of case diagnostic sensitivity and specificity, component-resolve diagnosis, and design of specific Hymenoptera venom immunotherapy.

Molecular cloning and characterization of taurocyamine kinase from Clonorchis sinensis: a candidate chemotherapeutic target

Background

Adult Clonorchis sinensis lives in the bile duct and causes endemic clonorchiasis in East Asian countries. Phosphagen kinases (PK) constitute a highly conserved family of enzymes, which play a role in ATP buffering in cells, and are potential targets for chemotherapeutic agents, since variants of PK are found only in invertebrate animals, including helminthic parasites. This work is conducted to characterize a PK from C. sinensis and to address further investigation for future drug development.

Methology/Principal findings

A cDNA clone encoding a putative polypeptide of 717 amino acids was retrieved from a C. sinensis transcriptome. This polypeptide was homologous to taurocyamine kinase (TK) of the invertebrate animals and consisted of two contiguous domains. C. sinensis TK (CsTK) gene was reported and found consist of 13 exons intercalated with 12 introns. This suggested an evolutionary pathway originating from an arginine kinase gene group, and distinguished annelid TK from the general CK phylogenetic group. CsTK was found not to have a homologous counterpart in sequences analysis of its mammalian hosts from public databases. Individual domains of CsTK, as well as the whole two-domain enzyme, showed enzymatic activity and specificity toward taurocyamine substrate. Of the CsTK residues, R58, I60 and Y84 of domain 1, and H60, I63 and Y87 of domain 2 were found to participate in binding taurocyamine. CsTK expression was distributed in locomotive and reproductive organs of adult C. sinensis. Developmentally, CsTK was stably expressed in both the adult and metacercariae stages. Recombinant CsTK protein was found to have low sensitivity and specificity toward C. sinensis and platyhelminth-infected human sera on ELISA.

Conclusion

CsTK is a promising anti-C. sinensis drug target since the enzyme is found only in the C. sinensis and has a substrate specificity for taurocyamine, which is different from its mammalian counterpart, creatine.

The food-borne clonorchiasis imposes public health problems on inhabitants in endemic areas. Praziquantel has been employed as an efficacious anthelminthic in large-scale campaigns as well as for individual treatment of Clonorchis sinensis human infections. Although praziquantel continues to have good efficacy, new drug development for this parasite has been recognized as a crucial issue to be investigated intensively. Clonorchis sinensis adults generate energy through glycolysis, actively utilizing exogenous glucose, and produce a large amount of eggs each day. Taurocyamine kinase (CsTK) is distributed abundantly in the locomotive and reproductive organs, and is an important enzyme in energy generation and homeostasis in adult C. sinensis. Enzymes of the glycolytic pathway are also expressed abundantly in these organs and in tegument, implying these organs play central roles which are essential for survival and reproduction of C. sinensis. The TK enzymes, including CsTK, are found only among invertebrate organisms and have substrate specificity for taurocyamine, which are significantly different from phosphagen kinases of vertebrate animals. With these molecular biological, enzymatic, and evolutionary characteristics, we propose here that CsTK could be a target for development of chemotherapeutic agents against C. sinensis and be a biomolecular model for other human-infecting trematodes.

T273 plays an important role in the activity and structural stability of arginine kinase

Arginine kinase (AK) is a key enzyme for cellular energy metabolism, catalyzing the reversible phosphoryl transfer from phosphoarginine to ADP in invertebrates. The amino acid residue C271 is involved in keeping AK's activity and constraining the orientation of the substrate arginine. However, the roles of the C271 interaction amino acid residues in AK's substrate synergism, activity and structural stability are still unclear. The crystal structure of AK implied that the amino acid residue T273 interacted with the residue C271 and might play vital roles in keeping AK's activity, substrate synergism and structural stability. The mutations T273G and T273A led to significantly loss of activity, obviously decreased of substrate synergism and structural stability. Furthermore, spectroscopic experiments indicated that mutations T273G and T273A impaired the structure of AK and led them to a partially unfolded state. The inability to fold to the functional state made the mutations prone to aggregate under environmental stresses. Moreover, the mutations T273S and T273D almost had no effects on AK's activity and structural stability. This study herein indicated that the residue T273 played key roles in AK's activity, substrate synergism and structural stability.

Characterization of a putative oomycete taurocyamine kinase: Implications for the evolution of the phosphagen kinase family

Phosphagen kinases (PKs) are known to be distributed throughout the animal kingdom, but have recently been discovered in some protozoan and bacterial species. Within animal species, these enzymes play a critical role in energy homeostasis by catalyzing the reversible transfer of a high-energy phosphoryl group from Mg⋅ATP to an acceptor molecule containing a guanidinium group. In this work, a putative PK gene was identified in the oomycete Phytophthora sojae that was predicted, based on sequence homology, to encode a multimeric hypotaurocyamine kinase. The recombinant P. sojae enzyme was purified and shown to catalyze taurocyamine phosphorylation efficiently (kcat/KM (taurocyamine) = 2 × 10(5) M(-1) s(-1)) and glycocyamine phosphorylation only weakly (kcat/KM (glycocyamine) = 2 × 10(2) M(-1) s(-1)), but lacked any observable kinase activity with the more ubiquitous guanidinium substrates, creatine or arginine. Additionally, the enzyme was observed to be dimeric but lacked cooperativity between the subunits in forming a transition state analog complex. These results suggest that protozoan PKs may exhibit more diversity in substrate specificity than was previously thought.

Host cell poly(ADP-ribose) glycohydrolase is crucial for Trypanosoma cruzi infection cycle

Trypanosoma cruzi, etiological agent of Chagas’ disease, has a complex life cycle which involves the invasion of mammalian host cells, differentiation and intracellular replication. Here we report the first insights into the biological role of a poly(ADP-ribose) glycohydrolase in a trypanosomatid (TcPARG). In silico analysis of the TcPARG gene pointed out the conservation of key residues involved in the catalytic process and, by Western blot, we demonstrated that it is expressed in a life stage-dependant manner. Indirect immunofluorescence assays and electron microscopy using an anti-TcPARG antibody showed that this enzyme is localized in the nucleus independently of the presence of DNA damage or cell cycle stage. The addition of poly(ADP-ribose) glycohydrolase inhibitors ADP-HPD (adenosine diphosphate (hydroxymethyl) pyrrolidinediol) or DEA (6,9-diamino-2-ethoxyacridine lactate monohydrate) to the culture media, both at a 1 µM concentration, reduced in vitro epimastigote growth by 35% and 37% respectively, when compared to control cultures. We also showed that ADP-HPD 1 µM can lead to an alteration in the progression of the cell cycle in hydroxyurea synchronized cultures of T. cruzi epimastigotes. Outstandingly, here we demonstrate that the lack of poly(ADP-ribose) glycohydrolase activity in Vero and A549 host cells, achieved by chemical inhibition or iRNA, produces the reduction of the percentage of infected cells as well as the number of amastigotes per cell and trypomastigotes released, leading to a nearly complete abrogation of the infection process. We conclude that both, T. cruzi and the host, poly(ADP-ribose) glycohydrolase activities are important players in the life cycle of Trypanosoma cruzi, emerging as a promising therapeutic target for the treatment of Chagas’ disease.

The phosphoarginine energy-buffering system of trypanosoma brucei involves multiple arginine kinase isoforms with different subcellular locations

Phosphagen energy-buffering systems play an essential role in regulating the cellular energy homeostasis in periods of high-energy demand or energy supply fluctuations. Here we describe the phosphoarginine/arginine kinase system of the kinetoplastid parasite Trypanosoma brucei, consisting of three highly similar arginine kinase isoforms (TbAK1-3). Immunofluorescence microscopy using myc-tagged protein versions revealed that each isoform is located in a specific subcellular compartment: TbAK1 is exclusively found in the flagellum, TbAK2 in the glycosome, and TbAK3 in the cytosol of T. brucei. The flagellar location of TbAK1 is dependent on a 22 amino acid long N-terminal sequence, which is sufficient for targeting a GFP-fusion protein to the trypanosome flagellum. The glycosomal location of TbAK2 is in agreement with the presence of a conserved peroxisomal targeting signal, the C-terminal tripeptide ‘SNL’. TbAK3 lacks any apparent targeting sequences and is accordingly located in the cytosol of the parasite. Northern blot analysis indicated that each TbAK isoform is differentially expressed in bloodstream and procyclic forms of T. brucei, while the total cellular arginine kinase activity was 3-fold higher in bloodstream form trypanosomes. These results suggest a substantial change in the temporal and spatial energy requirements during parasite differentiation. Increased arginine kinase activity improved growth of procyclic form T. brucei during oxidative challenges with hydrogen peroxide. Elimination of the total cellular arginine kinase activity by RNA interference significantly decreased growth (>90%) of procyclic form T. brucei under standard culture conditions and was lethal for this life cycle stage in the presence of hydrogen peroxide. The putative physiological roles of the different TbAK isoforms in T. brucei are further discussed.

Gene structure of the two-domain taurocyamine kinase from Paragonimus westermani: evidence for a distinct lineage of trematode phosphagen kinases

Taurocyamine kinase (TK) is an enzyme that catalyzes the reversible transfer of a phosphate between ATP and taurocyamine. Annelid TKs were suggested to have evolved from a CK ancestor. However, TKs from the lung fluke Paragonimus westermani comprised another lineage. Construction of phylogenetic tree and comparison of exon/intron organization showed that P. westermani TK and other trematode TKs evolved from a molluscan arginine kinase (AK) gene. Exon shuffling probably caused the changes in amino acid sequence thereby changing the affinity from AK to TK. The present study provides new insights on the evolution of phosphagen kinases found in trematodes.