Evolution of the hydrogenosome

Cell death of Acanthamoeba castellanii following exposure to antimicrobial agents commonly included in contact lens disinfecting solutions

Several antimicrobial agents are commonly included in contact lens disinfectant solutions including chlorhexidine diacetate (CHX), polyhexamethylene biguanide (PHMB) or myristamidopropyl dimethylamine (MAPD); however, their mode of action, i.e. necrosis versus apoptosis is incompletely understood. Here, we determined whether a mechanism of cell death resembling that of apoptosis was present in Acanthamoeba castellanii of the T4 genotype (NEFF) following exposure to the aforementioned antimicrobials using the anticoagulant annexin V that undergoes rapid high affinity binding to phosphatidylserine in the presence of calcium, making it a sensitive probe for phosphatidylserine exposure. The results revealed that under the conditions employed in this study, an apoptotic pathway of cell death in this organism at the tested conditions does not occur. Our findings suggest that necrosis is the likely mode of action; however, future mechanistic studies should be accomplished in additional experimental conditions to further comprehend the molecular mechanisms of cell death in Acanthamoeba.

Holospora-like bacteria "Candidatus Gortzia yakutica" and Preeria caryophila: Ultrastructure, promiscuity, and biogeography of the symbionts

Two already known representatives of Holospora-like bacteria, "Candidatus Gortzia yakutica" from Paramecium putrinum and Preeria caryophila, originally retrieved from the Paramecium aurelia complex, were found in new hosts: Paramecium nephridiatum and Paramecium polycaryum, respectively. In the present study, these bacteria were investigated using morphological and molecular methods. For "Ca. G. yakutica", the first details of the electron microscopic structure in the main and new hosts were provided. Regarding Pr. caryophila, the ultrastructural description of this species was implemented by several features previously unknown, such as the so called "membrane cluster" dividing periplasm from cytoplasm and fine composition of infectious forms before and during its releasing from the infected macronucleus. The new combinations of these Holospora-like bacteria with ciliate hosts were discussed from biogeographical and ecological points of view. Host specificity of symbionts as a general paradigm was critically reviewed as well.

Oxygen levels are key to understanding "Anaerobic" protozoan pathogens with micro-aerophilic lifestyles

Publications abound on the physiology, biochemistry and molecular biology of "anaerobic" protozoal parasites as usually grown under "anaerobic" culture conditions. The media routinely used are poised at low redox potentials using techniques that remove O₂ to "undetectable" levels in sealed containers. However there is growing understanding that these culture conditions do not faithfully resemble the O₂ environments these organisms inhabit. Here we review for protists lacking oxidative energy metabolism, the oxygen cascade from atmospheric to intracellular concentrations and relevant methods of measurements of O₂, some well-studied parasitic or symbiotic protozoan lifestyles, their homeodynamic metabolic and redox balances, organism-drug-oxygen interactions, and the present and future prospects for improved drugs and treatment regimes.

Experimental investigation of D2 conversion to DHO in soil near the Cernavoda nuclear power plant site in Romania

The current Canadian and Romanian model predictions for tritium dose following an atmospheric tritiated hydrogen gas (HT) release is based on a default Canadian Standards Association (CSA) conversion factor of HT to tritiated water (HTO) of 4.3%. The determination of an empirical site specific value for the conversion factor was essential for the CANDU Cernavoda Nuclear Power Plant (NPP) in Romania to verify if the CSA value is appropriate for use at this site. Given the role of soil characteristics on the conversion of HT to HTO, on-site experiments would provide the best evaluation of the conversion factor. The objective of the study was to define the soil HT to HTO conversion parameters specific to the Cernavoda NPP site. In June 2016, a series of experiments were conducted to meet this objective. First, the in situ deposition velocity of D₂ gas, as a surrogate for HT gas, was obtained using an exposure chamber. Diffusion of D₂ into the soil was then evaluated based on the measurements of DHO concentrations in the exposed soil. As soil microbes play a role in the conversion of HT to HTO, this work included a microbiological characterization of the soil, which targeted total soil bacteria (cultivable and gene-based) and hydrogen oxidizing bacteria (cultivable and gene-based). The fraction of hydrogen oxidizing cultivable soil bacteria represented 14-20% of the total cultivable bacteria population estimated as 2.8-29.2 × 10⁵ cfu/g of soil. The empirically derived HT to HTO conversion factor was lower than the default value (4.3%). It fell between 0.9% and 2.0%. The default value is therefore more conservative than the Cernavoda site-specific derived value obtained from the study.

Functional imaging of a model unicell: Spironucleus vortens as an anaerobic but aerotolerant flagellated protist

Advances in optical microscopy are continually narrowing the chasm in our appreciation of biological organization between the molecular and cellular levels, but many practical problems are still limiting. Observation is always limited by the rapid dynamics of ultrastructural modifications of intracellular components, and often by cell motility: imaging of the unicellular protist parasite of ornamental fish, Spironucleus vortens, has proved challenging. Autofluorescence of nicotinamide nucleotides and flavins in the 400-580 nm region of the visible spectrum, is the most useful indicator of cellular redox state and hence vitality. Fluorophores emitting in the red or near-infrared (i.e., phosphors) are less damaging and more penetrative than many routinely employed fluors. Mountants containing free radical scavengers minimize fluorophore photobleaching. Two-photon excitation provides a small focal spot, increased penetration, minimizes photon scattering and enables extended observations. Use of quantum dots clarifies the competition between endosomal uptake and exosomal extrusion. Rapid motility (161 μm/s) of the organism makes high resolution of ultrastructure difficult even at high scan speeds. Use of voltage-sensitive dyes determining transmembrane potentials of plasma membrane and hydrogenosomes (modified mitochondria) is also hindered by intracellular motion and controlled anesthesia perturbs membrane organization. Specificity of luminophore binding is always questionable; e.g. cationic lipophilic species widely used to measure membrane potentials also enter membrane-bounded neutral lipid droplet-filled organelles. This appears to be the case in S. vortens, where Coherent Anti-Stokes Raman Scattering (CARS) micro-spectroscopy unequivocally images the latter and simultaneous provides spectral identification at 2840 cm^-1. Secondary Harmonic Generation highlights the highly ordered structure of the flagella.

The Physiology of Phagocytosis in the Context of Mitochondrial Origin

How mitochondria came to reside within the cytosol of their host has been debated for 50 years. Though current data indicate that the last eukaryote common ancestor possessed mitochondria and was a complex cell, whether mitochondria or complexity came first in eukaryotic evolution is still discussed. In autogenous models (complexity first), the origin of phagocytosis poses the limiting step at eukaryote origin, with mitochondria coming late as an undigested growth substrate. In symbiosis-based models (mitochondria first), the host was an archaeon, and the origin of mitochondria was the limiting step at eukaryote origin, with mitochondria providing bacterial genes, ATP synthesis on internalized bioenergetic membranes, and mitochondrion-derived vesicles as the seed of the eukaryote endomembrane system. Metagenomic studies are uncovering new host-related archaeal lineages that are reported as complex or phagocytosing, although images of such cells are lacking. Here we review the physiology and components of phagocytosis in eukaryotes, critically inspecting the concept of a phagotrophic host. From ATP supply and demand, a mitochondrion-lacking phagotrophic archaeal fermenter would have to ingest about 34 times its body weight in prokaryotic prey to obtain enough ATP to support one cell division. It would lack chemiosmotic ATP synthesis at the plasma membrane, because phagocytosis and chemiosmosis in the same membrane are incompatible. It would have lived from amino acid fermentations, because prokaryotes are mainly protein. Its ATP yield would have been impaired relative to typical archaeal amino acid fermentations, which involve chemiosmosis. In contrast, phagocytosis would have had great physiological benefit for a mitochondrion-bearing cell.

Basal body structure in Trichonympha

Trichonympha is a symbiotic flagellate of many species of termites and of the wood-feeding cockroach. Remarkably, this unicellular organism harbors up to over ten thousand flagella on its surface, which serve to propel it through the viscous environment of the host hindgut. In the 1960s, analysis of resin-embedded Trichonympha samples by electron microscopy revealed that the basal bodies that give rise to these flagella are exceptionally long, with a proximal, cartwheel-bearing, region some 50 times longer than that of regular centrioles. In recent years, this salient feature has prompted the analysis of the 3D architecture of Trichonympha basal bodies in the native state using cryo-electron tomography. The resulting ~40 Å resolution map of the basal body proximal region revealed a number of novel features that may be conserved in centrioles of other systems. These include proximal-distal polarity of the pinhead structure that links the cartwheel to centriolar microtubules, as well as of the linker between the A and the C microtubules. Moreover, this work demonstrated that the cartwheel is made of stacked ring-like structures that likely each comprise 18 molecules of SAS-6 proteins.

Environmental selection of protistan plankton communities in hypersaline anoxic deep-sea basins, Eastern Mediterranean Sea

High salt concentrations, absence of light, anoxia, and high hydrostatic pressure make deep hypersaline anoxic basins (DHABs) in the Eastern Mediterranean Sea one of the most polyextreme habitats on Earth. Taking advantage of the unique chemical characteristics of these basins, we tested the effect of environmental selection and geographic distance on the structure of protistan communities. Terminal restriction fragment length polymorphism (T-RFLP) analyses were performed on water samples from the brines and seawater/brine interfaces of five basins: Discovery, Urania, Thetis, Tyro, and Medee. Using statistical analyses, we calculated the partitioning of diversity among the ten individual terminal restriction fragment (T-RF) profiles, based on peak abundance and peak incidence. While a significant distance effect on spatial protistan patterns was not detected, hydrochemical gradients emerged as strong dispersal barriers that likely lead to environmental selection in the DHAB protistan plankton communities. We identified sodium, magnesium, sulfate, and oxygen playing in concerto as dominant environmental drivers for the structuring of protistan plankton communities in the Eastern Mediterranean DHABs.

Biochemistry and evolution of anaerobic energy metabolism in eukaryotes

Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.

The microaerophilic flagellate, Trichomonas vaginalis, contains unusual acyl lipids but no detectable cardiolipin

Previous lipid analysis of trichomonads has led to controversy as to whether these hydrogenosome-containing organisms contain cardiolipin (CL), which is a characteristic component of mitochondria. Here we report a careful lipid analysis of the sexually transmitted protist Trichomonas vaginalis. Major lipids were phosphatidylethanolamine (42%) and phosphatidylcholine (20%) with lesser amounts of phosphatidylglycerol (PG) (12%) and non-polar components. Two unusual lipids, acyl-PG (8%) and ceramide phosphorylethanolamine (2%), were also significant components. The structures of these lipids were confirmed by tandem mass spectrometry following reverse-phase high-performance liquid chromatography. This is the first time ceramide phosphorylethanolamine has been reported in a trichomonad. In contrast, CL (diphosphatidylglycerol) could not be detected either by two-dimensional thin-layer chromatography or by mass spectrometry. These data are discussed in relation to the organism's phylogenetic origin as a parasite showing secondary adaptation to microaerobic conditions.

Tropospheric H(2) budget and the response of its soil uptake under the changing environment

Molecular hydrogen (H(2)) is an indirect greenhouse gas present at the trace level in the atmosphere. So far, the sum of its sources and sinks is close to equilibrium, but its large-scale utilization as an alternative energy carrier would alter its atmospheric burden. The magnitude of the emissions associated with a future H(2)-based economy is difficult to predict and remains a matter of debate. Previous attempts to predict the impact that a future H(2)-based economy would exert on tropospheric chemistry were realized by considering a steady rate of microbial-mediated soil uptake, which is currently responsible of ~80% of the tropospheric H(2) losses. Although soil uptake, also known as dry deposition is the most important sink for tropospheric H(2), microorganisms involved in the activity remain elusive. Given that microbial-mediated H(2) soil uptake is influenced by several environmental factors, global change should exert a significant effect on the activity and then, assuming a steady H(2) soil uptake rate for the future may be mistaken. Here, we present an overview of tropospheric H(2) sources and sinks with an emphasis on microbial-mediated soil uptake process. Future researches are proposed to investigate the influence that global change would exert on H(2) dry deposition and to identify microorganisms involved H(2) soil uptake activity.

A metabolic view of the diversity-stability relationship

A theoretical analysis of the "portfolio effect" expressed in metabolic terms indicates that the coefficient of variation of total biomass in the ecosystem is influenced by three factors: metabolic diversity, total population size and organism biomass (body mass). The contribution of these factors to ecosystem stability depends on the power scaling of population size to its temporal variance: the Tilman's z. In natural populations, 1 < z < 2 both from a theoretical and an empirical background, and so a higher metabolic diversity, a larger population size and a bigger body mass are expected to increase ecosystem stability. The maximization of any of these factors will enhance ecosystem stability both at ecological (successional) and evolutionary timescales, which could explain a number of trends observed in ecosystems and in the history of life.

The malaria parasite type II NADH:quinone oxidoreductase: an alternative enzyme for an alternative lifestyle

The operation of a type II NADH:quinone oxidoreductase (PfNDH2), also known as alternative Complex I, in the mitochondrion of the human malaria parasite, Plasmodium falciparum, has recently been described. Unlike the Complex I of typical mitochondria, type II NADH:quinone oxidoreductases do not have transmembrane domains and are not involved directly in proton (H(+)) pumping. Here, we present a predictive model of PfNDH2, describing putative NADH-, flavin- and quinone-binding sites, as well as a possible membrane 'anchoring' region. In addition, we hypothesize that the alternative Complex I is an evolutionary adaptation to a microaerophilic lifestyle enabling (proton) uncoupled oxidation of NADH. This adaptive feature has several advantages, including: (i) a reduction of proton 'back-pressure' in the absence of extensive ATP synthesis; (ii) a reduction of mitochondrial superoxide generation; and (iii) a mechanism for the deregulated oxidation of cytosolic NADH.

Eukaryotic Fe-hydrogenases – old eukaryotic heritage or adaptive acquisitions?

All eukaryotes seem to possess proteins that most probably evolved from an ancestral Fe-hydrogenase. These proteins, known as NARF or Nar, do not produce hydrogen. Notably, a small group of rather unrelated unicellular anaerobes and a few algae possess Fe-hydrogenases, which produce hydrogen. In most, but not all organisms, hydrogen production occurs in membrane-bounded organelles, i.e. hydrogenosomes or plastids. Whereas plastids are monophyletic, hydrogenosomes evolved repeatedly and independently from mitochondria or mitochondria-like organelles. A systematic analysis of the various hydrogenosomes and their hydrogenases will contribute to an understanding of the evolution of the eukaryotic cell, and provide clues to the evolutionary origin(s) of the Fe-hydrogenase.

Trichomonas hydrogenosomes contain the NADH dehydrogenase module of mitochondrial complex I

Hydrogenosomes are double-membraned ATP-producing and hydrogen-producing organelles of diverse anaerobic eukaryotes. In some versions of endosymbiotic theory they are suggested to be homologues of mitochondria, but alternative views suggest they arose from an anaerobic bacterium that was distinct from the mitochondrial endosymbiont. Here we show that the 51-kDa and 24-kDa subunits of the NADH dehydrogenase module in complex I, the first step in the mitochondrial respiratory chain, are active in hydrogenosomes of Trichomonas vaginalis. Like mitochondrial NADH dehydrogenase, the purified Trichomonas enzyme can reduce a variety of electron carriers including ubiquinone, but unlike the mitochondrial enzyme it can also reduce ferredoxin, the electron carrier used for hydrogen production. The presence of NADH dehydrogenase solves the long-standing conundrum of how hydrogenosomes regenerate NAD+ after malate oxidation. Phylogenetic analyses show that the Trichomonas 51-kDa homologue shares common ancestry with the mitochondrial enzyme. Recruitment of complex I subunits into a H2-producing pathway provides evidence that mitochondria and hydrogenosomes are aerobic and anaerobic homologues of the same endosymbiotically derived organelle.

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.

Trichomonas vaginalis Hmp35, a putative pore-forming hydrogenosomal membrane protein, can form a complex in yeast mitochondria

An abundant integral membrane protein, Hmp35, has been isolated from hydrogenosomes of Trichomonas vaginalis. This protein has no known homologue and exists as a stable 300-kDa complex, termed HMP35, in membranes of the hydrogenosome. By using blue native gel electrophoresis, we found the HMP35 complex to be stable in 2 m NaCl and up to 5 m urea. The endogenous Hmp35 protein was largely protease-resistant. The protein has a predominantly beta-sheet structure and predicted transmembrane domains that may form a pore. Interestingly, the protein has a high number of cysteine residues, some of which are arranged in motifs that resemble the RING finger, suggesting that they could be coordinating zinc or another divalent cation. Our data show that Hmp35 forms one intramolecular but no intermolecular disulfide bonds. We have isolated the HMP35 complex by expressing a His-tagged Hmp35 protein in vivo followed by purification with nickel-agarose beads. The purified 300-kDa complex consists of mostly Hmp35 with lesser amounts of 12-, 25-27-, and 32-kDa proteins. The stoichiometry of proteins in the complex indicates that Hmp35 exists as an oligomer. Hmp35 can be targeted heterologously into yeast mitochondria, despite the lack of homology with any yeast protein, demonstrating the compatibility of mitochondrial and hydrogenosomal protein translocation machineries.

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Mitochondria and hydrogenosomes are two forms of the same fundamental organelle

Published data suggest that hydrogenosomes, organelles found in diverse anaerobic eukaryotes that make energy and hydrogen, were once mitochondria. As hydrogenosomes generally lack a genome, the conversion is probably one way. The sources of the key hydrogenosomal enzymes, pyruvate : ferredoxin oxidoreductase (PFO) and hydrogenase, are not resolved by current phylogenetic analyses, but it is likely that both were present at an early stage of eukaryotic evolution. Once thought to be restricted to a few unusual anaerobic eukaryotes, the proteins are intimately integrated into the fabric of diverse eukaryotic cells, where they are targeted to different cell compartments, and not just hydrogenosomes. There is no evidence supporting the view that PFO and hydrogenase originated from the mitochondrial endosymbiont, as posited by the hydrogen hypothesis for eukaryogenesis. Other organelles derived from mitochondria have now been described in anaerobic and parasitic microbial eukaryotes, including species that were once thought to have diverged before the mitochondrial symbiosis. It thus seems possible that all eukaryotes may eventually be shown to contain an organelle of mitochondrial ancestry, to which different types of biochemistry can be targeted. It remains to be seen if, despite their obvious differences, this family of organelles shares a common function of importance for the eukaryotic cell, other than energy production, that might provide the underlying selection pressure for organelle retention.

An [Fe] hydrogenase from the anaerobic hydrogenosome-containing fungus Neocallimastix frontalis L2

Hydrogenases, oxygen-sensitive enzymes that can make hydrogen gas, are key to the function of hydrogen-producing organelles (hydrogenosomes), which occur in anaerobic eukaryotes scattered throughout the eukaryotic tree. All of the eukaryotic enzymes characterized so far are iron-only [Fe] hydrogenases. In contrast, it has previously been suggested that hydrogenosomes of the best-studied anaerobic fungus Neocallimastix frontalis L2 contain an unrelated iron-nickel-selenium [NiFeSe] hydrogenase. We have isolated a gene from strain L2 that encodes a putative protein containing all of the characteristic features of an iron-only [Fe] hydrogenase, including the cysteine residues required for the co-ordination of the unique 'hydrogen cluster'. As is the case for experimentally verified hydrogenosomal matrix enzymes from N. frontalis, the [Fe] hydrogenase encodes a plausible amino terminal extension that resembles mitochondrial targeting signals. Phylogenetic analyses of an expanded [Fe] hydrogenase dataset reveal a complicated picture that is difficult to interpret in the light of current ideas of species relationships. Nevertheless, our analyses cannot reject the hypothesis that the novel [Fe] hydrogenase gene of Neocallimastix is specifically related to other eukaryote [Fe] hydrogenases, and thus ultimately might be traced to the same ancestral source.

The "primitive" microaerophile Giardia intestinalis (syn. lamblia, duodenalis) has specialized membranes with electron transport and membrane-potential-generating functions

Here it is shown that the flagellated protozoon Giardia intestinalis, commonly regarded as an early branching eukaryote because of its lack of mitochondria, has membraneous structures that partition the cationic, membrane-potential-sensitive fluorophore rhodamine 123. This organism also reduces a tetrazolium fluorogen at discrete plasma-membrane-associated sites. That these functions occur in distinctive specialized membrane systems supports the growing evidence that G. intestinalis may not be primitive, but is derived from an aerobic, mitochondria-containing flagellate.

Giardia intestinalis, a eukaryote without hydrogenosomes, produces hydrogen

The microaerophilic flagellated protist Giardia intestinalis, the commonest protozoal agent of intestinal infections worldwide, is of uncertain phylogeny, but is usually regarded as the earliest branching of the eukaryotic clades. Under strictly anaerobic conditions, a mass spectrometric investigation of gas production indicated a low level of generation of dihydrogen (2 nmol x min(-1) per 10(7) organisms), about 10-fold lower than that in Trichomonas vaginalis under similar conditions. Hydrogen evolution was O2 sensitive, and inhibited by 100 microM metronidazole. Fluorescent labelling of G. intestinalis cells using monoclonal antibodies to typical hydrogenosomal enzymes from T. vaginalis (malate enzyme, and succinyl-CoA synthetase alpha and beta subunits), and to the large-granule fraction (hydrogenosome-enriched, also from T. vaginalis) gave no discrete localization of epitopes. Cell-free extracts prepared under anaerobic conditions showed the presence of a CO-sensitive hydrogenase activity. This first report of hydrogen production in a eukaryote with no recognizable hydrogenosomes raises further questions about the early branching status of G. intestinalis; the physiological characterization of its hydrogenase, and its recently elucidated gene sequence, will aid further phylogenetic investigations.

Giardia: highly evolved parasite or early branching eukaryote?

The phylogeny of the commonest protozoal agent of intestinal disease, Giardia, is unclear. Although recent intensive research suggests this important human parasite is an early branching eukaryote that evolved before the endosymbiotic origin of mitochondria, there is also evidence to suggest that, as a highly evolved parasite, it has lost many of its ancestral characteristics. In this case, these organisms might have arisen much more recently from aerobic free-living flagellates.

An overview of endosymbiotic models for the origins of eukaryotes, their ATP-producing organelles (mitochondria and hydrogenosomes), and their heterotrophic lifestyle

The evolutionary processes underlying the differentness of prokaryotic and eukaryotic cells and the origin of the latter's organelles are still poorly understood. For about 100 years, the principle of endosymbiosis has figured into thoughts as to how these processes might have occurred. A number of models that have been discussed in the literature and that are designed to explain this difference are summarized. The evolutionary histories of the enzymes of anaerobic energy metabolism (oxygen-independent ATP synthesis) in the three basic types of heterotrophic eukaryotes those that lack organelles of ATP synthesis, those that possess mitochondria and those that possess hydrogenosomes--play an important role in this issue. Traditional endosymbiotic models generally do not address the origin of the heterotrophic lifestyle and anaerobic energy metabolism in eukaryotes. Rather they take it as a given, a direct inheritance from the host that acquired mitochondria. Traditional models are contrasted to an alternative endosymbiotic model (the hydrogen hypothesis), which addresses the origin of heterotrophy and the origin of compartmentalized energy metabolism in eukaryotes.

Pyruvate : NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum: a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists

Most eukaryotes perform the oxidative decarboxylation of pyruvate in mitochondria using pyruvate dehydrogenase (PDH). Eukaryotes that lack mitochondria also lack PDH, using instead the O(2)-sensitive enzyme pyruvate : ferredoxin oxidoreductase (PFO), which is localized either in the cytosol or in hydrogenosomes. The facultatively anaerobic mitochondria of the photosynthetic protist Euglena gracilis constitute a hitherto unique exception in that these mitochondria oxidize pyruvate with the O(2)-sensitive enzyme pyruvate : NADP oxidoreductase (PNO). Cloning and analysis of Euglena PNO revealed that the cDNA encodes a mitochondrial transit peptide followed by an N-terminal PFO domain that is fused to a C-terminal NADPH-cytochrome P450 reductase (CPR) domain. Two independent 5.8-kb full-size cDNAs for Euglena mitochondrial PNO were isolated; the gene was expressed in cultures supplied with 2% CO(2) in air and with 2% CO(2) in N(2). The apicomplexan Cryptosporidium parvum was also shown to encode and express the same PFO-CPR fusion, except that, unlike E. gracilis, no mitochondrial transit peptide for C. parvum PNO was found. Recombination-derived remnants of PNO are conserved in the genomes of Saccharomyces cerevisiae and Schizosaccharomyces pombe as proteins involved in sulfite reduction. Notably, Trypanosoma brucei was found to encode homologs of both PFO and all four PDH subunits. Gene organization and phylogeny revealed that eukaryotic nuclear genes for mitochondrial, hydrogenosomal, and cytosolic PFO trace to a single eubacterial acquisition. These findings suggest a common ancestry of PFO in amitochondriate protists with Euglena mitochondrial PNO and Cryptosporidium PNO. They are also consistent with the view that eukaryotic PFO domains are biochemical relics inherited from a facultatively anaerobic, eubacterial ancestor of mitochondria and hydrogenosomes.

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.

Iron-induced changes in pyruvate metabolism of Tritrichomonas foetus and involvement of iron in expression of hydrogenosomal proteins

The main function of the hydrogenosome, a typical organelle of trichomonads, is to convert malate or pyruvate to H(2), CO(2) and acetate by a pathway associated with ATP synthesis. This pathway relies on activity of iron-sulfur proteins such as pyruvate:ferredoxin oxidoreductase (PFOR), hydrogenase and ferredoxin. To examine the effect of iron availability on proper hydrogenosomal function, the metabolic activity of the hydrogenosome and expression of hydrogenosomal enzymes were compared in Tritrichomonas foetus maintained under iron-rich (150 microM iron nitrilotriacetate) or iron-restricted (180 microM 2,2-dipyridyl) conditions in vitro. The activities of PFOR and hydrogenase, and also production of acetate and H(2), were markedly decreased or absent in iron-restricted trichomonads. Moreover, a decrease in activity of the hydrogenosomal malic enzyme, which is a non-Fe-S protein, was also observed. Impaired function of hydrogenosomes under iron-restricted conditions was compensated for by activation of the cytosolic pathway, mediating conversion of pyruvate to ethanol via acetaldehyde. This metabolic switch was fully reversible. Production of hydrogen by iron-restricted trichomonads was restored to the level of organisms grown under iron-rich conditions within 3 h after addition of 150 microM iron nitrilotriacetate. Protein analysis of purified hydrogenosomes from iron-restricted cells showed decreased levels of proteins corresponding to PFOR, malic enzyme and ferredoxin. Accordingly, these cells displayed decreased steady-state level and synthesis of mRNAs encoding PFOR and hydrogenosomal malic enzyme. These data demonstrate that iron is essential for function of the hydrogenosome, show its involvement in the expression of hydrogenosomal proteins and indicate the presence of iron-dependent control of gene transcription in Tt. foetus.

Origins of hydrogenosomes and mitochondria

Complete genome sequences for many oxygen-respiring mitochondria, as well as for some bacteria, leave no doubt that mitochondria are descendants of alpha-proteobacteria, a finding for which the endosymbiont hypothesis can easily account. Yet a wealth of data indicate that mitochondria and hydrogenosomes - the ATP-producing organelles of many anaerobic protists - share a common ancestry, a finding that traditional formulations of the endosymbiont hypothesis less readily accommodates. Available evidence suggests that a more in-depth understanding of the origins of eukaryotes and their organelles will hinge upon data from the genomes of protists that synthesize ATP without the need for oxygen.

Multiple acquisition of methanogenic archaeal symbionts by anaerobic ciliates

Anaerobic heterotrichous ciliates (Armophoridae and Clevelandellidae) possess hydrogenosomes that generate molecular hydrogen and ATP. This intracellular source of hydrogen provides the basis for a stable endosymbiotic association with methanogenic archaea. We analyzed the SSU rRNA genes of 18 heterotrichous anaerobic ciliates and their methanogenic endosymbionts in order to unravel the evolution of this mutualistic association. Here, we show that the anaerobic heterotrichous ciliates constitute at least three evolutionary lines. One group consists predominantly of gut-dwelling ciliates, and two to three, potentially four, additional clades comprise ciliates that thrive in freshwater sediments. Their methanogenic endosymbionts belong to only two different taxa that are closely related to free-living methanogenic archaea from the particular ecological niches. The close phylogenetic relationships between the endosymbionts and free-living methanogenic archaea argue for multiple acquisitions from environmental sources, notwithstanding the strictly vertical transmission of the endosymbionts. Since phylogenetic analysis of the small-subunit (SSU) rRNA genes of the hydrogenosomes of these ciliates indicates a descent from the mitochondria of aerobic ciliates, it is likely that anaerobic heterotrichous ciliates hosted endosymbiotic methanogens prior to their radiation. Therefore, our data strongly suggest multiple acquisitions and replacements of endosymbiotic methanogenic archaea during their host's adaptation to the various ecological niches.

Trophozoites of Giardia lamblia may have a Golgi-like structure

Trophozoites of the primitive protozoan Giardia lamblia have been considered as cells which do not present the Golgi complex. Using C(6)-NBD ceramide, which has been shown to label the Golgi complex of mammalian cells, labelling of the perinuclear region of G. lamblia was observed by confocal laser scanning microscopy. Transmission electron microscopy of thin sections and of replicas of freeze-fractured cells revealed the presence of concentric perinuclear membranes resembling the Golgi complex.

Hydrogenosomes: eukaryotic adaptations to anaerobic environments

Like mitochondria, hydrogenosomes compartmentalize crucial steps of eukaryotic energy metabolism; however, this compartmentalization differs substantially between mitochondriate aerobes and hydrogenosome-containing anaerobes. Because hydrogenosomes have arisen independently in different lineages of eukaryotic microorganisms, comparative analysis of the various types of hydrogenosomes can provide insights into the functional and evolutionary aspects of compartmentalized energy metabolism in unicellular eukaryotes.

Secondary absence of mitochondria in Giardia lamblia and Trichomonas vaginalis revealed by valyl-tRNA synthetase phylogeny

Nuclear-coded valyl-tRNA synthetase (ValRS) of eukaryotes is regarded of mitochondrial origin. Complete ValRS sequences obtained by us from two amitochondriate protists, the diplomonad, Giardia lamblia and the parabasalid, Trichomonas vaginalis were of the eukaryotic type, strongly suggesting an identical history of ValRS in all eukaryotes studied so far. The findings indicate that diplomonads are secondarily amitochondriate and give further evidence for such conclusion reached recently concerning parabasalids. Together with similar findings on other amitochondriate groups (microsporidia and entamoebids), this work provides critical support for the emerging notion that no representatives of the premitochondrial stage of eukaryotic phylogenesis exist among the species living today.