Keto-acid oxidoreductases in the anaerobic protozoa

Transcriptional and functional analysis of trifluoromethionine resistance in Entamoeba histolytica

OBJECTIVES

Drug resistance in parasitic protozoa is an obstacle to successful chemotherapy. Understanding how pathogens respond to drugs is crucial in preventing resistance. Previously, we have shown that in Entamoeba histolytica, methionine γ-lyase (EhMGL) downregulation results in trifluoromethionine resistance. The transcriptional response, however, of this parasite to the drug is not known. In this study, we used microarray analysis to determine whether additional genes are involved.

METHODS

The expression profiles of 9230 genes in wild-type and trifluoromethionine-resistant strains were compared. Episomal overexpression of EhBspA1 was performed to verify its role in trifluoromethionine resistance. The transcriptomes of a trifluoromethionine-resistant strain cultured with or without trifluoromethionine, an EhMGL gene-silenced strain, a strain with reduced susceptibility to metronidazole and a wild-type strain under cysteine-deprived conditions were compared to determine the specificity of the changes observed in the trifluoromethionine-resistant strain.

RESULTS

The expression of 35 genes differed at least 3-fold between trifluoromethionine-resistant and wild-type strains. Some of the genes play roles in metabolism, the stress response and gene regulation. EhMGL and EhBspA1 were found to be highly downregulated and upregulated, respectively. Overexpression of EhBspA1 conferred partial resistance to trifluoromethionine. Comparative transcriptome analysis showed that genes modulated in trifluoromethionine-resistant strains were specific.

CONCLUSIONS

E. histolytica has few known resistance mechanisms against drugs. In this study, we showed that aside from EhMGL downregulation, induction of EhBspA1 plays a role in trifluoromethionine resistance. We also showed a unique set of induced genes that could represent the signature profile of trifluoromethionine resistance in E. histolytica.

Transcriptome profiling of female alates and egg-laying queens of the Formosan subterranean termite

Termites are known to have an extraordinary reproductive plasticity and capacity, but the underlying genetic patterns of termite reproductive biology are relatively understudied. The goal of this study was to identify genes for which expression levels differ between dealated precopulatory females (virgins) and egg-laying queens of the Formosan subterranean termite, Coptotermes formosanus Shiraki. We constructed a normalized polyphenic expressed sequence tag (EST) library that represents genomic material from most of the castes and life stages of the Formosan subterranean termite. Microarrays were designed using probes from this EST library and public genomic resources. Virgin females and queens were competitively hybridized to these microarrays and differentially expressed candidate genes were identified. Differential expression of eight genes was subsequently confirmed via reverse transcriptase quantitative PCR (RT-QPCR). When compared to virgins, queens had higher expression of genes coding for proteins related to immunity (gram negative binding protein), nutrition (e.g., termite-derived endo-beta-1,4-glucanase), protein storage, regulation of caste differentiation and reproduction (hexamerin, juvenile hormone binding protein). Queens also had higher transcript levels for genes involved in metabolism of xenobiotics, fat, and juvenile hormone (glutathione-S-transferase-like proteins, and cytochrome P450), among others. In particular, hexamerin, juvenile hormone binding protein, and a cytochrome P450 from the 4C subfamily are likely to be involved in initiating the inactive period during the reproductive cycle of the queen. Vice versa, virgins had higher expression than queens of genes related to respiration, probably due to recent flight activity, and several genes of unknown function.

Lipoic acid metabolism in microbial pathogens

Lipoic acid [(R)-5-(1,2-dithiolan-3-yl)pentanoic acid] is an enzyme cofactor required for intermediate metabolism in free-living cells. Lipoic acid was discovered nearly 60 years ago and was shown to be covalently attached to proteins in several multicomponent dehydrogenases. Cells can acquire lipoate (the deprotonated charge form of lipoic acid that dominates at physiological pH) through either scavenging or de novo synthesis. Microbial pathogens implement these basic lipoylation strategies with a surprising variety of adaptations which can affect pathogenesis and virulence. Similarly, lipoylated proteins are responsible for effects beyond their classical roles in catalysis. These include roles in oxidative defense, bacterial sporulation, and gene expression. This review surveys the role of lipoate metabolism in bacterial, fungal, and protozoan pathogens and how these organisms have employed this metabolism to adapt to niche environments.

Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites

The "amitochondriate" protozoan parasites of humans Entamoeba histolytica, Giardia intestinalis, and Trichomonas vaginalis share many biochemical features, e.g., energy and amino acid metabolism, a spectrum of drugs for their treatment, and the occurrence of drug resistance. These parasites possess metabolic pathways that are divergent from those of their mammalian hosts and are often considered to be good targets for drug development. Sulfur-containing-amino-acid metabolism represents one such divergent metabolic pathway, namely, the cysteine biosynthetic pathway and methionine gamma-lyase-mediated catabolism of sulfur-containing amino acids, which are present in T. vaginalis and E. histolytica but absent in G. intestinalis. These pathways are potentially exploitable for development of drugs against amoebiasis and trichomoniasis. For instance, L-trifluoromethionine, which is catalyzed by methionine gamma-lyase and produces a toxic product, is effective against T. vaginalis and E. histolytica parasites in vitro and in vivo and may represent a good lead compound. In this review, we summarize the biology of these microaerophilic parasites, their clinical manifestation and epidemiology of disease, chemotherapeutics, the modes of action of representative drugs, and problems related to these drugs, including drug resistance. We further discuss our approach to exploit unique sulfur-containing-amino-acid metabolism, focusing on development of drugs against E. histolytica.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Efficacy of antigiardial drugs

The flagellated protozoa Giardia duodenalis is the most commonly detected parasite in the intestinal tract of humans. Infections with the parasite result in diarrhoeal disease in humans and animals, with infants at risk from failure-to-thrive syndrome. The incidence of giardiasis worldwide may be as high as 1000 million cases. Current recommended treatments include the nitroheterocyclic drugs tinidazole, metronidazole and furazolidone, the substituted acridine, quinacrine, and the benzimidazole, albendazole. Paromomycin is also used in some situations, and nitazoxanide is proving to be useful. However, treatment failures have been reported with all of the common antigiardial agents, and drug resistance to all available drugs has been demonstrated in the laboratory. In addition, clinical resistance has been reported, including cases where patients failed both metronidazole and albendazole treatments. The identification of new antigiardial drugs is an important consideration for the future, but maintaining the usefulness of the existing drugs is the most cost-effective measure to ensure the continued availability of antigiardial drugs.

Identification and characterization of two isoenzymes of methionine gamma-lyase from Entamoeba histolytica: a key enzyme of sulfur-amino acid degradation in an anaerobic parasitic protist that lacks forward and reverse trans-sulfuration pathways

To better understand the metabolism of sulfur-containing amino acids, which likely plays a key role in a variety of cell functions, in Entamoeba histolytica, we searched the genome data base for genes encoding putative orthologs of enzymes known to be involved in the metabolism. The search revealed that E. histolytica possesses only incomplete cysteine-methionine conversion pathways in both directions. Instead, this parasite possesses genes encoding two isoenzymes of methionine gamma-lyase (EC 4.4.1.11, EhMGL1/2), which has been implicated in the degradation of sulfur-containing amino acids. The two amebic MGL isoenzymes, showing 69% identity to each other, encode 389- and 392-amino acid polypeptides with predicted molecular masses of 42.3 and 42.7 kDa and pIs of 6.01 and 6.63, respectively. Amino acid comparison and phylogenetic analysis suggested that these amebic MGLs are likely to have been horizontally transferred from the Archaea, whereas an MGL from another anaerobic protist Trichomonas vaginalis has MGL isotypes that share a common ancestor with bacteria. Enzymological and immunoblot analyses of the partially purified native amebic MGL confirmed that both of the MGL isotypes are expressed in a comparable amount predominantly in the cytosol and form a homotetramer. Recombinant EhMGL1 and 2 proteins catalyzed degradation of L-methionine, DL-homocysteine, L-cysteine, and O-acetyl-L-serine to form alpha-keto acid, ammonia, and hydrogen sulfide or methanethiol, whereas activity toward cystathionine was negligible. These two isoenzymes showed notable differences in substrate specificity and pH optimum. In addition, we showed that EhMGL is an ideal target for the development of new chemotherapeutic agents against amebiasis by demonstrating an amebicidal effect of the methionine analog trifluoromethionine on trophozoites in culture (IC50 18 mum) and that this effect of trifluoromethionine was completely abolished by the addition of the MGL-specific inhibitor DL-propargylglycine.

Drug resistance in the sexually transmitted protozoan Trichomonas vaginalis

Trichomoniasis is the most common, sexually transmitted infection. It is caused by the flagellated protozoan parasite Trichomonas vaginalis. Symptoms include vaginitis and infections have been associated with preterm delivery, low birth weight and increased infant mortality, as well as predisposing to HIV/AIDS and cervical cancer. Trichomoniasis has the highest prevalence and incidence of any sexually transmitted infection. The 5-nitroimidazole drugs, of which metronidazole is the most prescribed, are the only approved, effective drugs to treat trichomoniasis. Resistance against metronidazole is frequently reported and cross-resistance among the family of 5-nitroimidazole drugs is common, leaving no alternative for treatment, with some cases remaining unresolved. The mechanism of metronidazole resistance in T. vaginalis from treatment failures is not well understood, unlike resistance which is developed in the laboratory under increasing metronidazole pressure. In the latter situation, hydrogenosomal function which is involved in activation of the prodrug, metronidazole, is down-regulated. Reversion to sensitivity is incomplete after removal of drug pressure in the highly resistant parasites while clinically resistant strains, so far analysed, maintain their resistance levels in the absence of drug pressure. Although anaerobic resistance has been regarded as a laboratory induced phenomenon, it clearly has been demonstrated in clinical isolates. Pursuit of both approaches will allow dissection of the underlying mechanisms. Many alternative drugs and treatments have been tested in vivo in cases of refractory trichomoniasis, as well as in vitro with some successes including the broad spectrum anti-parasitic drug nitazoxanide. Drug resistance incidence in T. vaginalis appears to be on the increase and improved surveillance of treatment failures is urged.

Evidence for lateral transfer of genes encoding ferredoxins, nitroreductases, NADH oxidase, and alcohol dehydrogenase 3 from anaerobic prokaryotes to Giardia lamblia and Entamoeba histolytica

Giardia lamblia and Entamoeba histolytica are amitochondriate, microaerophilic protists which use fermentation enzymes like those of bacteria to survive anaerobic conditions within the intestinal lumen. Genes encoding fermentation enzymes and related electron transport peptides (e.g., ferredoxins) in giardia organisms and amebae are hypothesized to be derived from either an ancient anaerobic eukaryote (amitochondriate fossil hypothesis), a mitochondrial endosymbiont (hydrogen hypothesis), or anaerobic bacteria (lateral transfer hypothesis). The goals here were to complete the molecular characterization of giardial and amebic fermentation enzymes and to determine the origins of the genes encoding them, when possible. A putative giardia [2Fe-2S]ferredoxin which had a hypothetical organelle-targeting sequence at its N terminus showed similarity to mitochondrial ferredoxins and the hydrogenosomal ferredoxin of Trichomonas vaginalis (another luminal protist). However, phylogenetic trees were star shaped, with weak bootstrap support, so we were unable to confirm or rule out the endosymbiotic origin of the giardia [2Fe-2S]ferredoxin gene. Putative giardial and amebic 6-kDa ferredoxins, ferredoxin-nitroreductase fusion proteins, and oxygen-insensitive nitroreductases each tentatively supported the lateral transfer hypothesis. Although there were not enough sequences to perform meaningful phylogenetic analyses, the unique common occurrence of these peptides and enzymes in giardia organisms, amebae, and the few anaerobic prokaryotes suggests the possibility of lateral transfer. In contrast, there was more robust phylogenetic evidence for the lateral transfer of G. lamblia genes encoding an NADH oxidase from a gram-positive coccus and a microbial group 3 alcohol dehydrogenase from thermoanaerobic prokaryotes. In further support of lateral transfer, the G. lamblia NADH oxidase and adh3 genes appeared to have an evolutionary history distinct from those of E. histolytica.

Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads

Land, K. M., Clemens, D. L., and Johnson, P. J. 2001. Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads. Experimental Parasitology 97, 102-110. In trichomonads, metronidazole is activated to its cytotoxic form in a specialized energy-producing organelle called the hydrogenosome. Electron transport components in the organelle, pyruvate:ferredoxin oxidoreductase and ferredoxin, donate a single electron to the drug, converting it to a cytotoxic free radical. Previous biochemical analyses of enzyme activities of highly resistant strains of both Trichomonas vaginalis and Tritrichomonas foetus reveal undetectable activity for pyruvate:ferredoxin oxidoreductase and another hydrogenosomal enzyme, hydrogenase. We have chosen to analyze a highly drug-resistant strain of T. foetus and its parental drug-sensitive strain from which it was derived to study the molecular basis for these enzyme defects. Quantitation of pyruvate:ferredoxin oxidoreductase and ferredoxin levels in sensitive and resistant cells shows a marked reduction of these proteins in the resistant strain. RNA analysis reveals an approximately 60% reduction in pyruvate:ferredoxin oxidoreductase mRNA and 90-98% reduction in mRNA levels encoding hydrogenosomal proteins hydrogenase, ferredoxin, and malic enzyme. We have measured the levels of transcription of these genes and observed 60% reduction of pyruvate:ferredoxin oxidoreductase gene transcription and 85% reduction in malic enzyme gene transcription in the resistant strain. The reduction or absence of these organellar proteins is likely to reduce or eliminate the ability of the cell to activate the drug, giving rise to the highly resistant phenotype. Ultrastructural analysis of thin sections revealed that resistant cells are 20% smaller in size and hydrogenosomes in resistant cells are approximately one-third the size of those in the drug-sensitive parental strain. These data suggest that altered gene expression of multiple hydrogenosomal proteins results in the modification of the organelle and leads to drug resistance.

Drug targets and mechanisms of resistance in the anaerobic protozoa

The anaerobic protozoa Giardia duodenalis, Trichomonas vaginalis, and Entamoeba histolytica infect up to a billion people each year. G. duodenalis and E. histolytica are primarily pathogens of the intestinal tract, although E. histolytica can form abscesses and invade other organs, where it can be fatal if left untreated. T. vaginalis infection is a sexually transmitted infection causing vaginitis and acute inflammatory disease of the genital mucosa. T. vaginalis has also been reported in the urinary tract, fallopian tubes, and pelvis and can cause pneumonia, bronchitis, and oral lesions. Respiratory infections can be acquired perinatally. T. vaginalis infections have been associated with preterm delivery, low birth weight, and increased mortality as well as predisposing to human immunodeficiency virus infection, AIDS, and cervical cancer. All three organisms lack mitochondria and are susceptible to the nitroimidazole metronidazole because of similar low-redox-potential anaerobic metabolic pathways. Resistance to metronidazole and other drugs has been observed clinically and in the laboratory. Laboratory studies have identified the enzyme that activates metronidazole, pyruvate:ferredoxin oxidoreductase, to its nitroso form and distinct mechanisms of decreasing drug susceptibility that are induced in each organism. Although the nitroimidazoles have been the drug family of choice for treating the anaerobic protozoa, G. duodenalis is less susceptible to other antiparasitic drugs, such as furazolidone, albendazole, and quinacrine. Resistance has been demonstrated for each agent, and the mechanism of resistance has been investigated. Metronidazole resistance in T. vaginalis is well documented, and the principal mechanisms have been defined. Bypass metabolism, such as alternative oxidoreductases, have been discovered in both organisms. Aerobic versus anaerobic resistance in T. vaginalis is discussed. Mechanisms of metronidazole resistance in E. histolytica have recently been investigated using laboratory-induced resistant isolates. Instead of downregulation of the pyruvate:ferredoxin oxidoreductase and ferredoxin pathway as seen in G. duodenalis and T. vaginalis, E. histolytica induces oxidative stress mechanisms, including superoxide dismutase and peroxiredoxin. The review examines the value of investigating both clinical and laboratory-induced syngeneic drug-resistant isolates and dissection of the complementary data obtained. Comparison of resistance mechanisms in anaerobic bacteria and the parasitic protozoa is discussed as well as the value of studies of the epidemiology of resistance.

Inhibition of pyruvate-ferredoxin oxidoreductase gene expression in Giardia lamblia by a virus-mediated hammerhead ribozyme

Giardia lamblia is a primitive eukaryotic microorganism that derives its metabolic energy primarily from anaerobic glycolysis. In trophozoites, pyruvate-ferredoxin oxidoreductase (PFOR) converts pyruvate to acetyl-CoA with the transfer of a pair of electrons to ferredoxin, which can then reduce metronidazole and activate it into a potent antigiardiasis agent. It is unclear, however, whether this anaerobic disposal of electrons is essential for the energy metabolism in Giardia. In the present study, cDNAs encoding hammerhead ribozyme flanked with various lengths of antisense PFOR RNA were cloned into a viral vector pC631pac derived from the genome of giardiavirus (GLV). RNA transcripts of the plasmids showed high cleavage activities on PFOR mRNA in vitro. They were introduced into GLV-infected G. lamblia trophozoites by electroporation and stablized in the transfected cells via serial passages under puromycin selection. PFOR mRNA and enzyme activity in the transfected cells were decreased by 46-60% with the ribozyme PRzS flanked with 20 nt PFOR antisense RNA on each arm and by 69-80% with the ribozyme PRzL flanked with 600 and 1500 nt PFOR antisense RNA. PRzS without the inserted ribozyme or ribozyme flanked with alcohol dehydrogenase E antisense RNA showed no effect on PFOR mRNA and activity. The ribozyme-transfected cells demonstrated significantly enhanced resistance to metronidazole and grew equally well under anaerobic and aerobic conditions. In contrast, the wild-type cells grew slightly better anaerobically than the transfectants but did not grow at all in aerobic conditions. Thus, the reduced PFOR expression enables Giardia to grow under molecular oxygen and the presence of PFOR enhances the anaerobic growth of Giardia with an increased susceptibility towards metronidazole. In addition, this study demonstrated for the first time the feasibility of using a viral RNA vector to express a ribozyme targeted at a specific mRNA in G. lamblia to reduce the expression of a specific gene.

Structure and electron transfer mechanism of pyruvate:ferredoxin oxidoreductase

The first crystal structure of pyruvate:ferredoxin oxidoreductase to be determined has provided significant new information on its structural organization and redox chemistry. Spectroscopic analyses of a radical reaction intermediate have shed more light on its thiamin-based mechanism of catalysis. Different approaches have been used to study the interaction between the enzyme and ferredoxin, its redox partner.