Incorporation of iron into Tritrichomonas foetus cell compartments reveals ferredoxin as a major iron-binding protein in hydrogenosomes

Stem-Loop Structures in Iron-Regulated mRNAs of Giardia duodenalis

Giardia duodenalis is a significant cause of waterborne and foodborne infections, day-care center outbreaks, and traveler's diarrhea worldwide. In protozoa such as Trichomonas vaginalis and Entamoeba histolytica, iron affects the growth, pathogenicity mechanisms, and expression of virulence genes. One of the proposed iron regulatory mechanisms is at the post-transcriptional level through an IRE/IRP-like (iron responsive element/iron regulatory protein) system. Recently, the expression of many putative giardial virulence factors in the free-iron levels has been reported in subsequent RNAseq experiments; however, the iron regulatory mechanism remains unknown. Thus, this work aimed to determine the effects of iron on the growth, gene expression, and presence of IRE-like structures in G. duodenalis. First, the parasite's growth kinetics at different iron concentrations were studied, and the cell viability was determined. It was observed that the parasite can adapt to an iron range from 7.7 to 500 µM; however, in conditions without iron, it is unable to survive in the culture medium. Additionally, the iron modulation of three genes was determined by RT-PCR assays. The results suggested that Actin, glucosamine-6-phosphate deaminase, and cytochrome b5 mRNA were down-regulated by iron. To investigate the presence of IRE-like structures, in silico analyses were performed for different mRNAs from the Giardia genome database. The Zuker mfold v2.4 web server and theoretical analysis were used to predict the secondary structures of the 91 mRNAs analyzed. Interestingly, the iron-induced downregulation of the genes analyzed corresponds to the location of the stem-loop structures found in their UTR regions. In conclusion, iron modulates the growth and expression of specific genes, likely due to the presence of IRE-like structures in G. duodenalis mRNAs.

Iron in parasitic protists – from uptake to storage and where we can interfere

A comprehensive review of iron metabolism in parasitic protists and its potential use as a drug target.

Metabolic reprogramming of hydrogenosomal amino acids in Trichomonas vaginalis under glucose restriction

BACKGROUND

Glucose is the major energy source that is converted to pyruvate for ATP generation in the trichomonad hydrogenosome. Under glucose restriction (GR), the regulation of amino acids metabolism is crucial for trichomonad growth and survival. RNA-sequencing (RNA-seq) analysis has been used to identify differentially expressed genes in Trichomonas vaginalis under GR, leading to significant advances in understanding adaptive responses of amino acid metabolism to GR. However, the levels of amino acid metabolites modulated by GR are unknown in T. vaginalis.

METHODS

Herein, we describe a comprehensive metabolomic analysis of amino acid metabolites in the hydrogenosome using liquid chromatography Fourier transform ion cyclotron resonance mass spectrometry (LC-FT MS). The relative abundance of 17 hydrogenosomal amino acids was analyzed under GR and high-glucose (HG) conditions.

RESULTS

Levels of most amino acids were higher in GR culture. Arginine was not detectable in either HG or GR cultures; however, its metabolic end-product proline was slightly increased under GR, suggesting that the arginine dihydrolase pathway was more activated by GR. Additionally, methionine catabolism was less stimulated under GR because of greater methionine accumulation. Furthermore, branched chain amino acids (BCAA), including leucine, isoleucine and valine, as well as phenylalanine and alanine, markedly accumulated under GR, indicating that glutamate-related metabolic pathways were remarkably enhanced in this setting. Our metabolomic analysis combined with previous RNA-seq data confirm the existence of several amino acid metabolic pathways in the hydrogenosome and highlight their potentially important roles in T. vaginalis under glucose deprivation.

Secondary alcohol dehydrogenase catalyzes the reduction of exogenous acetone to 2-propanol in Trichomonas vaginalis

Secondary alcohols such as 2-propanol are readily produced by various anaerobic bacteria that possess secondary alcohol dehydrogenase (S-ADH), although production of 2-propanol is rare in eukaryotes. Specific bacterial-type S-ADH has been identified in a few unicellular eukaryotes, but its function is not known and the production of secondary alcohols has not been studied. We purified and characterized S-ADH from the human pathogen Trichomonas vaginalis. The kinetic properties and thermostability of T. vaginalis S-ADH were comparable with bacterial orthologues. The substantial activity of S-ADH in the parasite's cytosol was surprising, because only low amounts of ethanol and trace amounts of secondary alcohols were detected as metabolic end products. However, S-ADH provided the parasite with a high capacity to scavenge and reduce external acetone to 2-propanol. To maintain redox balance, the demand for reducing power to metabolize external acetone was compensated for by decreased cytosolic reduction of pyruvate to lactate and by hydrogenosomal metabolism of pyruvate. We speculate that hydrogen might be utilized to maintain cytosolic reducing power. The high activity of Tv-S-ADH together with the ability of T. vaginalis to modulate the metabolic fluxes indicate efficacious metabolic responsiveness that could be advantageous for rapid adaptation of the parasite to changes in the host environment.

Iron-saturated lactoferrin and pathogenic protozoa: could this protein be an iron source for their parasitic style of life?

Iron is an essential nutrient for the survival of pathogens inside a host. As a general strategy against microbes, mammals have evolved complex iron-withholding systems for efficiently decreasing the iron accessible to invaders. Pathogens that inhabit the respiratory, intestinal and genitourinary tracts encounter an iron-deficient environment on the mucosal surface, where ferric iron is chelated by lactoferrin, an extracellular glycoprotein of the innate immune system. However, parasitic protozoa have developed several mechanisms to obtain iron from host holo-lactoferrin. Tritrichomonas fetus, Trichomonas vaginalis, Toxoplasma gondii and Entamoeba histolytica express lactoferrin-binding proteins and use holo-lactoferrin as an iron source for growth in vitro; in some species, these binding proteins are immunogenic and, therefore, may serve as potential vaccine targets. Another mechanism to acquire lactoferrin iron has been reported in Leishmania spp. promastigotes, which use a surface reductase to recognize and reduce ferric iron to the accessible ferrous form. Cysteine proteases that cleave lactoferrin have been reported in E. histolytica. This review summarizes the available information on how parasites uptake and use the iron from lactoferrin to survive in hostile host environments.

The core components of organelle biogenesis and membrane transport in the hydrogenosomes of Trichomonas vaginalis

Trichomonas vaginalis is a parasitic protist of the Excavata group. It contains an anaerobic form of mitochondria called hydrogenosomes, which produce hydrogen and ATP; the majority of mitochondrial pathways and the organellar genome were lost during the mitochondrion-to-hydrogenosome transition. Consequently, all hydrogenosomal proteins are encoded in the nucleus and imported into the organelles. However, little is known about the membrane machineries required for biogenesis of the organelle and metabolite exchange. Using a combination of mass spectrometry, immunofluorescence microscopy, in vitro import assays and reverse genetics, we characterized the membrane proteins of the hydrogenosome. We identified components of the outer membrane (TOM) and inner membrane (TIM) protein translocases include multiple paralogs of the core Tom40-type porins and Tim17/22/23 channel proteins, respectively, and uniquely modified small Tim chaperones. The inner membrane proteins TvTim17/22/23-1 and Pam18 were shown to possess conserved information for targeting to mitochondrial inner membranes, but too divergent in sequence to support the growth of yeast strains lacking Tim17, Tim22, Tim23 or Pam18. Full complementation was seen only when the J-domain of hydrogenosomal Pam18 was fused with N-terminal region and transmembrane segment of the yeast homolog. Candidates for metabolite exchange across the outer membrane were identified including multiple isoforms of the β-barrel proteins, Hmp35 and Hmp36; inner membrane MCF-type metabolite carriers were limited to five homologs of the ATP/ADP carrier, Hmp31. Lastly, hydrogenosomes possess a pathway for the assembly of C-tail-anchored proteins into their outer membrane with several new tail-anchored proteins being identified. These results show that hydrogenosomes and mitochondria share common core membrane components required for protein import and metabolite exchange; however, they also reveal remarkable differences that reflect the functional adaptation of hydrogenosomes to anaerobic conditions and the peculiar evolutionary history of the Excavata group.

Crusade for iron: iron uptake in unicellular eukaryotes and its significance for virulence

The effective acquisition of iron is a pre-requisite for survival of all organisms, especially parasites that have a high iron requirement. In mammals, iron homeostasis is meticulously regulated; extracellular free iron is essentially unavailable and host iron availability has a crucial role in the host-pathogen relationship. Therefore, pathogens use specialized and effective mechanisms to acquire iron. In this review, we summarize the iron-uptake systems in eukaryotic unicellular organisms with particular focus on the pathogenic species: Candida albicans, Tritrichomonas foetus, Trypanosoma brucei and Leishmania spp. We describe the diversity of their iron-uptake mechanisms and highlight the importance of the process for virulence.

Human hololactoferrin: endocytosis and use as an iron source by the parasite Entamoeba histolytica

Entamoeba histolytica is an enteric protozoan that exclusively infects human beings. This parasite requires iron for its metabolic functions. Lactoferrin is a mammalian glycoprotein that chelates extracellular iron on mucosal surfaces, including the surface of the large intestine, where E. histolytica initiates infection. This work examined the interaction in vitro of E. histolytica trophozoites with human hololactoferrin (iron-saturated lactoferrin). A minimum concentration of 50 microM Fe from hololactoferrin supported growth of the amoeba. Amoebic binding sites for hololactoferrin were different from those for human apolactoferrin, holotransferrin and haemoglobin. One amoebic hololactoferrrin-binding polypeptide of 90 kDa was found, which was not observed after treatment of trophozoites with trypsin. Hololactoferrin-binding-protein levels increased in amoebas starved of iron, or grown in hololactoferrin. Internalization of hololactoferrin was inhibited by filipin. Endocytosed hololactoferrin colocalized with an anti-chick embryo caveolin mAb in amoebic vesicles, and lactoferrin was further detected in acidic vesicles; amoebic caveolin of 22 kDa was detected by Western blotting using this antibody. Cysteine proteases from amoebic extracts were able to cleave hololactoferrin. Together, these data indicate that E. histolytica trophozoites bind to hololactoferrin through specific membrane lactoferrin-binding proteins. This ferric protein might be internalized via caveolae-like microdomains, then used as an iron source, and degraded.

Electron tomographic and ultrastructural analysis of the Cryptosporidium parvum relict mitochondrion, its associated membranes, and organelles

Sporozoites of the apicomplexan Cryptosporidium parvum possess a small, membranous organelle sandwiched between the nucleus and crystalloid body. Based upon immunolabelling data, this organelle was identified as a relict mitochondrion. Transmission electron microscopy and tomographic reconstruction reveal the complex arrangement of membranes in the vicinity of this organelle, as well as its internal organization. The mitochondrion is enveloped by multiple segments of rough endoplasmic reticulum that extend from the outer nuclear envelope. In tomographic reconstructions of the mitochondrion, there is either a single, highly-folded inner membrane or multiple internal subcompartments (which might merge outside the reconstructed volume). The infoldings of the inner membrane lack the tubular "crista junctions" found in typical metazoan, fungal, and protist mitochondria. The absence of this highly conserved structural feature is congruent with the loss, through reductive evolution, of the normal oxidative phosphorylation machinery in C. parvum. It is proposed that the retention of a relict mitochondrion in C. parvum is a strategy for compartmentalizing away from the cytosol toxic ferrous iron and sulfide, which are needed for iron sulfur cluster biosynthesis, an essential function of mitochondria in all eukaryotes.

Mitochondrion‐Derived Organelles in Protists and Fungi

The mitochondrion is generally considered to be a defining feature of eukaryotic cells, yet most anaerobic eukaryotes lack this organelle. Many of these were previously thought to derive from eukaryotes that diverged prior to acquisition of the organelle through endosymbiosis. It is now known that all extant eukaryotes are descended from an ancestor that had a mitochondrion and that in anaerobic eukaryotes the organelle has been modified into either hydrogenosomes, which continue to generate energy for the host cell, or mitosomes, which do not. These organelles have each arisen independently several times. Recent evidence suggests a shared derived characteristic that may be responsible for the retention of the organelles in the absence of the better-known mitochondrial functions--iron-sulfur cluster assembly. This review explores the events leading to this new understanding of mitochondrion-derived organelles in amitochondriate eukaryotes, the current state of our knowledge, and future areas for investigation.

Mitochondrial-type assembly of FeS centers in the hydrogenosomes of the amitochondriate eukaryote Trichomonas vaginalis

Mitochondria are the site of assembly of FeS centers of mitochondrial and cytosolic FeS proteins. Various microaerophilic or anaerobic unicellular eukaryotes lack typical mitochondria ("amitochondriate" protists). In some of these organisms, a metabolically different organelle, the hydrogenosome, is found, which is thought to derive from the same proteobacterial ancestor as mitochondria. Here, we show that hydrogenosomes of Trichomonas vaginalis, a human genitourinary parasite, contain a key enzyme of FeS center biosynthesis, cysteine desulfurase (TviscS-2), which is phylogenetically related to its mitochondrial homologs. Hydrogenosomes catalyze the enzymatic assembly and insertion of FeS centers into apoproteins, as shown by the reconstruction of the apoform of [2Fe-2S]ferredoxin and the incorporation of 35S from labeled cysteine. Our results indicate that the biosynthesis of FeS proteins is performed by a homologous system in mitochondriate and amitochondriate eukaryotes and that this process is inherited from the proteobacterial ancestor of mitochondria.

Mitochondrial-type iron-sulfur cluster biosynthesis genes (IscS and IscU) in the apicomplexan Cryptosporidium parvum

Several reports have indicated that the iron-sulfur cluster [Fe-S] assembly machinery in most eukaryotes is confined to the mitochondria and chloroplasts. The best-characterized and most highly conserved [Fe-S] assembly proteins are a pyridoxal-5'-phosphate-dependent cysteine desulfurase (IscS), and IscU, a protein functioning as a scaffold for the assembly of [Fe-S] prior to their incorporation into apoproteins. In this work, genes encoding IscS and IscU homologues have been isolated and characterized from the apicomplexan parasite Cryptosporidium parvum, an opportunistic pathogen in AIDS patients, for which no effective treatment is available. Primary sequence analysis (CpIscS and CpIscU) and phylogenetic studies (CpIscS) indicate that both genes are most closely related to mitochondrial homologues from other organisms. Moreover, the N-terminal signal sequences of CpIscS and CpIscU predicted in silico specifically target green fluorescent protein to the mitochondrial network of the yeast Saccharomyces cerevisiae. Overall, these findings suggest that the previously identified mitochondrial relict of C. parvum may have been retained by the parasite as an intracellular site for [Fe-S] assembly.