Anaerobic protists and hidden mitochondria

Protists: Eukaryotic single-celled organisms and the functioning of their organelles

Organelles are membrane bound structures that compartmentalize biochemical and molecular functions. With improved molecular, biochemical and microscopy tools the diversity and function of protistan organelles has increased in recent years, providing a complex panoply of structure/function relationships. This is particularly noticeable with the description of hydrogenosomes, and the diverse array of structures that followed, having hybrid hydrogenosome/mitochondria attributes. These diverse organelles have lost the major, at one time, definitive components of the mitochondrion (tricarboxylic cycle enzymes and cytochromes), however they all contain the machinery for the assembly of Fe-S clusters, which is the single unifying feature they share. The plasticity of organelles, like the mitochondrion, is therefore evident from its ability to lose its identity as an aerobic energy generating powerhouse while retaining key ancestral functions common to both aerobes and anaerobes. It is interesting to note that the apicoplast, a non-photosynthetic plastid that is present in all apicomplexan protozoa, apart from Cryptosporidium and possibly the gregarines, is also the site of Fe-S cluster assembly proteins. It turns out that in Cryptosporidium proteins involved in Fe-S cluster biosynthesis are localized in the mitochondrial remnant organelle termed the mitosome. Hence, different organisms have solved the same problem of packaging a life-requiring set of reactions in different ways, using different ancestral organelles, discarding what is not needed and keeping what is essential. Don't judge an organelle by its cover, more by the things it does, and always be prepared for surprises.

Mitochondria-derived organelles in the diplomonad fish parasite Spironucleus vortens

In some eukaryotes, mitochondria have become modified during evolution to yield derived organelles (MDOs) of a similar size (hydrogenosomes), or extremely reduced to produce tiny cellular vesicles (mitosomes). The current study provides evidence for the presence of MDOs in the highly infectious fish pathogen Spironucleus vortens, an organism that produces H₂ and is shown here to have no detectable cytochromes. Transmission electron microscopy (TEM) reveals that S. vortens trophozoites contain electron-dense, membranous structures sometimes with an electron-dense core (200 nm-1 μm), resembling the hydrogenosomes previously described in other protists from habitats deficient in O₂. Confocal microscopy establishes that these organelles exhibit autofluorescence emission spectra similar to flavoprotein constituents previously described for mitochondria and also present in hydrogenosomes. These organelles possess a membrane potential and are labelled by a fluorescently labeled antibody against Fe-hydrogenase from Blastocystis hominis. Heterologous antibodies raised to mitochondrial proteins frataxin and Isu1, also exhibit a discrete punctate pattern of localization in S. vortens; however these labelled structures are distinctly smaller (90-150 nm) than hydrogenosomes as observed previously in other organisms. TEM confirms the presence of double-membrane bounded organelles of this smaller size. In addition, strong background immunostaining occurs in the cytosol for frataxin and Isu1, and labelling by anti-ferredoxin antibody is generally distributed and not specifically localized except for at the anterior polar region. This suggests that some of the functions traditionally attributed to such MDOs may also occur elsewhere. The specialized parasitic life-style of S. vortens may necessitate more complex intracellular compartmentation of redox reactions than previously recognized. Control of infection requires biochemical characterization of redox-related organelles.

The organellar genome and metabolic potential of the hydrogen-producing mitochondrion of Nyctotherus ovalis

It is generally accepted that hydrogenosomes (hydrogen-producing organelles) evolved from a mitochondrial ancestor. However, until recently, only indirect evidence for this hypothesis was available. Here, we present the almost complete genome of the hydrogen-producing mitochondrion of the anaerobic ciliate Nyctotherus ovalis and show that, except for the notable absence of genes encoding electron transport chain components of Complexes III, IV, and V, it has a gene content similar to the mitochondrial genomes of aerobic ciliates. Analysis of the genome of the hydrogen-producing mitochondrion, in combination with that of more than 9,000 genomic DNA and cDNA sequences, allows a preliminary reconstruction of the organellar metabolism. The sequence data indicate that N. ovalis possesses hydrogen-producing mitochondria that have a truncated, two step (Complex I and II) electron transport chain that uses fumarate as electron acceptor. In addition, components of an extensive protein network for the metabolism of amino acids, defense against oxidative stress, mitochondrial protein synthesis, mitochondrial protein import and processing, and transport of metabolites across the mitochondrial membrane were identified. Genes for MPV17 and ACN9, two hypothetical proteins linked to mitochondrial disease in humans, were also found. The inferred metabolism is remarkably similar to the organellar metabolism of the phylogenetically distant anaerobic Stramenopile Blastocystis. Notably, the Blastocystis organelle and that of the related flagellate Proteromonas lacertae also lack genes encoding components of Complexes III, IV, and V. Thus, our data show that the hydrogenosomes of N. ovalis are highly specialized hydrogen-producing mitochondria.

Metabolic reconstruction and analysis for parasite genomes

With the completion of sequencing projects for several parasite genomes, efforts are ongoing to make sense of this mass of information in terms of the gene products encoded and their interactions in the growth, development and survival of parasites. The emerging science of systems biology aims to explain the complex relationship between genotype and phenotype by using network models. One area in which this approach has been particularly successful is in the modeling of metabolism. With an accurate picture of the set of metabolic reactions encoded in a genome, it is now possible to identify enzymes or transporters that might be viable targets for new drugs. Because these predictions greatly depend on the quality and completeness of the genome annotation, there are substantial efforts in the scientific community to increase the numbers of metabolic enzymes identified. In this review, we discuss the opportunities for using metabolic reconstruction and analysis tools in parasitology research, and their applications to protozoan parasites.

The Expression of a Plant-type Ferredoxin Redox System provides Molecular Evidence for a Plastid in the Early Dinoflagellate Perkinsus marinus

Perkinsus marinus is a parasitic protozoan with a phylogenetic positioning between Apicomplexa and dinoflagellates. It is thus of interest for reconstructing the early evolution of eukaryotes, especially with regard to the acquisition of secondary plastids in these organisms. It is also an important pathogen of oysters, and the definition of parasite-specific metabolic pathways would be beneficial for the identification of efficient treatments for infected mollusks. Although these different scientific interests have resulted in the start of a genome project for this organism, it is still unknown whether P. marinus contains a plastid or plastid-like organelle like the related dinoflagellates and Apicomplexa. Here, we show that in vitro-cultivated parasites contain transcripts of the plant-type ferredoxin and its associated reductase. Both proteins are nuclear-encoded and possess N-terminal targeting sequences similar to those characterized in dinoflagellates. Since this redox pair is exclusively found in cyanobacteria and plastid-harboring organisms its presence also in P. marinus is highly indicative of a plastid. We also provide additional evidence for such an organelle by demonstrating pharmacological sensitivity to inhibitors of plastid-localized enzymes involved in fatty acid biosynthesis (e.g. acetyl-CoA carboxylase) and by detection of genes for three enzymes of plastid-localized isoprenoid biosynthesis (1-deoxy-D-xylulose 5-phosphate reductoisomerase, (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate reductase, and (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate synthase).

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.