The mitosome of the anaerobic parasitic protist Entamoeba histolytica: A peculiar and minimalist mitochondrion-related organelle.

The simplest class of mitochondrion-related organelles (MROs) is the mitosome, an organelle present in a few anaerobic protozoan parasites such as Entamoeba histolytica, Giardia intestinalis, and Cryptosporidium parvum. E. histolytica causes amoebiasis in humans, deemed as one of the important, yet neglected tropical infections in the world. Much of the enigma of the E. histolytica mitosome circles around the obvious lack of a majority of known mitochondrial components and functions exhibited in other organisms. The identification of enzymes responsible for sulfate activation (AS, IPP, and APSK) and a number of lineage-specific proteins such as the outer membrane beta-barrel protein (MBOMP30), and transmembrane domain-containing proteins that bind to various organellar proteins (ETMP1, ETMP30, EHI_170120, and EHI_099350) showcased the remarkable divergence of this organelle compared to the other MROs of anaerobic protozoa. Here, we summarize the findings regarding the biology of the mitosomes in E. histolytica, from their discovery up to the present understanding of its roles and interactions. We also include current advances and future perspectives on the biology, biochemistry, and evolution of the mitosomes of E. histolytica.

Entamoeba histolytica EHD1 Is Involved in Mitosome-Endosome Contact.

Interorganellar cross talk is often mediated by membrane contact sites (MCSs), which are zones where participating membranes come within 30 nm of one another. MCSs have been found in organelles, including the endoplasmic reticulum, Golgi bodies, endosomes, and mitochondria. Despite its seeming ubiquity, reports of MCS involving mitochondrion-related organelles (MROs) present in a few anaerobic parasitic protozoa remain lacking. Entamoeba histolytica, the etiological agent of amoebiasis, possesses an MRO called the mitosome. We previously discovered several Entamoeba-specific transmembrane mitosomal proteins (ETMPs) from in silico and cell-biological analyses. One of them, ETMP1 (EHI_175060), was predicted to have one transmembrane domain and two coiled-coil regions and was demonstrated to be mitosome membrane integrated based on carbonate fractionation and immunoelectron microscopy (IEM) data. Immunoprecipitation analysis detected a candidate interacting partner, EH domain-containing protein (EHD1; EHI_105270). We expressed hemagglutinin (HA)-tagged EHD1 in E. histolytica, and subsequent immunofluorescence and IEM data indicated an unprecedented MCS between the mitosome and the endosome. Live imaging of a green fluorescent protein (GFP)-EHD1-expressing strain demonstrated that EHD1 is involved in early endosome formation and is observed in MCS between endosomes of various sizes. In vitro assays using recombinant His-EHD1 demonstrated ATPase activity. MCSs are involved in lipid transfer, ion homeostasis, and organelle dynamics. The serendipitous discovery of the ETMP1-interacting partner EHD1 led to the observation of the mitosome-endosome contact site in E. histolytica. It opened a new view of how the relic mitochondria of Entamoeba may likewise be involved in organelle cross talk, a conserved feature of mitochondria and other organelles in general. IMPORTANCE Membrane contact sites (MCSs) are key regulators of interorganellar communication and have been widely demonstrated between various organelles. However, studies on MCSs involving mitochondrion-related organelles (MROs), present in some anaerobic parasitic protozoans, remain scarce. Entamoeba histolytica, the etiological agent of amoebiasis, possesses an MRO called the mitosome. This organelle is crucial for cellular differentiation and disease transmission, thereby significantly contributing to the amoeba's parasitic lifestyle. Our recent discovery of the interaction between the Entamoeba-specific transmembrane mitosomal protein (ETMP1) and EH domain-containing protein (EHD1) showcases a newly found mitosome-endosome contact site in E. histolytica. This finding reflects the idea that despite their substantially divergent and reduced nature, MROs like mitosomes conserve mechanisms for interorganellar cross talk. We posit lipid and ion transport, mitosome fission, and quality control as potential processes that are mediated by the ETMP1-EHD1-tethered mitosome-endosome contact site in E. histolytica.

Transfer RNA-Derived Small RNAs in the Pathogenesis of Parasitic Protozoa.

Transfer RNA (tRNA)-derived small RNAs (tsRNAs) are newly identified non-coding small RNAs that have recently attracted attention due to their functional significance in both prokaryotes and eukaryotes. tsRNAs originated from the cleavage of precursor or mature tRNAs by specific nucleases. According to the start and end sites, tsRNAs can be broadly divided into tRNA halves (31-40 nucleotides) and tRNA-derived fragments (tRFs, 14-30 nucleotides). tsRNAs have been reported in multiple organisms to be involved in gene expression regulation, protein synthesis, and signal transduction. As a novel regulator, tsRNAs have also been identified in various protozoan parasites. The conserved biogenesis of tsRNAs in early-branching eukaryotes strongly suggests the universality of this machinery, which requires future research on their shared and potentially disparate biological functions. Here, we reviewed the recent studies of tsRNAs in several representative protozoan parasites including their biogenesis and the roles in parasite biology and intercellular communication. Furthermore, we discussed the remaining questions and potential future works for tsRNAs in this group of organisms.

Interorganellar communication and membrane contact sites in protozoan parasites.

A key characteristic of eukaryotic cells is the presence of organelles with discrete boundaries and functions. Such subcellular compartmentalization into organelles necessitates platforms for communication and material exchange between each other which often involves vesicular trafficking and associated processes. Another way is via the close apposition between organellar membranes, called membrane contact sites (MCSs). Apart from lipid transfer, MCSs have been implicated to mediate in various cellular processes including ion transport, apoptosis, and organelle dynamics. In mammalian and yeast cells, contact sites have been reported between the membranes of the following: the endoplasmic reticulum (ER) and the plasma membrane (PM), ER and the Golgi apparatus, ER and endosomes (i.e., vacuoles, lysosomes), ER and lipid droplets (LD), the mitochondria and vacuoles, the nucleus and vacuoles, and the mitochondria and lipid droplets, whereas knowledge of MCSs in non-model organisms such as protozoan parasites is extremely limited. Growing evidence suggests that MCSs play more general and conserved roles in cell physiology. In this mini review, we summarize and discuss representative MCSs in divergent parasitic protozoa, and highlight the universality, diversity, and the contribution of MCSs to parasitism.

Characterization of Plasmodium falciparum Pantothenate Kinase and Identification of Its Inhibitors From Natural Products.

Coenzyme A (CoA) is a well-known cofactor that plays an essential role in many metabolic reactions in all organisms. In Plasmodium falciparum, the most deadly among Plasmodium species that cause malaria, CoA and its biosynthetic pathway have been proven to be indispensable. The first and rate-limiting reaction in the CoA biosynthetic pathway is catalyzed by two putative pantothenate kinases (PfPanK1 and 2) in this parasite. Here we produced, purified, and biochemically characterized recombinant PfPanK1 for the first time. PfPanK1 showed activity using pantetheine besides pantothenate, as the primary substrate, indicating that CoA biosynthesis in the blood stage of P. falciparum can bypass pantothenate. We further developed a robust and reliable screening system to identify inhibitors using recombinant PfPanK1 and identified four PfPanK inhibitors from natural compounds.

Import of Entamoeba histolytica Mitosomal ATP Sulfurylase Relies on Internal Targeting Sequences.

Mitochondrial matrix proteins synthesized in the cytosol often contain amino (N)-terminal targeting sequences (NTSs), or alternately internal targeting sequences (ITSs), which enable them to be properly translocated to the organelle. Such sequences are also required for proteins targeted to mitochondrion-related organelles (MROs) that are present in a few species of anaerobic eukaryotes. Similar to other MROs, the mitosomes of the human intestinal parasite Entamoeba histolytica are highly degenerate, because a majority of the components involved in various processes occurring in the canonical mitochondria are either missing or modified. As of yet, sulfate activation continues to be the only identified role of the relic mitochondria of Entamoeba. Mitosomes influence the parasitic nature of E. histolytica, as the downstream cytosolic products of sulfate activation have been reported to be essential in proliferation and encystation. Here, we investigated the position of the targeting sequence of one of the mitosomal matrix enzymes involved in the sulfate activation pathway, ATP sulfurylase (AS). We confirmed by immunofluorescence assay and subcellular fractionation that hemagluttinin (HA)-tagged EhAS was targeted to mitosomes. However, its ortholog in the δ-proteobacterium Desulfovibrio vulgaris, expressed as DvAS-HA in amoebic trophozoites, indicated cytosolic localization, suggesting a lack of recognizable mitosome targeting sequence in this protein. By expressing chimeric proteins containing swapped sequences between EhAS and DvAS in amoebic cells, we identified the ITSs responsible for mitosome targeting of EhAS. This observation is similar to other parasitic protozoans that harbor MROs, suggesting a convergent feature among various MROs in favoring ITS for the recognition and translocation of targeted proteins.

Characterization of mitochondrial carrier proteins of malaria parasite Plasmodium falciparum based on in vitro translation and reconstitution.

Members of the mitochondrial carrier (MC) family of membrane transporters play important roles in cellular metabolism. We previously established an in vitro reconstitution system for membrane transporters based on wheat germ cell-free translation system. We have now applied this reconstitution system to the comparative analysis of MC proteins from the malaria parasite Plasmodium falciparum and Saccharomyces cerevisiae. We synthesized twelve putative P. falciparum MCs and determined the transport activities of four of these proteins including PF3D7_1037300 protein (ADP/ATP translocator), PF3D7_1004800 protein (ADP/ATP translocator), PF3D7_1202200 protein (phosphate carrier), and PF3D7_1241600 protein (S-adenosylmethionine transporter). In addition, we tested the effect of cardiolipin on the activity of MC proteins. The transport activities of the yeast MCs, ScAac2p, ScGgc1p, ScDic1p, ScPic1p, and ScSam5p, which localize to the mitochondrial inner membrane, were increased by cardiolipin supplementation, whereas that of ScAnt1p, which localizes to the peroxisome membrane, was not significantly affected. Together, this indicates that the functional properties of the reconstituted MCs reflect the lipid content of their native membranes. Except for PF3D7_1241600 protein, these P. falciparum proteins manifested cardiolipin-dependent transport activities. Immunofluorescence analysis showed that PF3D7_1241600 protein is not mainly localized to the mitochondria of P. falciparum cells. We thus revealed the functions of four MC proteins of the malaria parasite and the effects of cardiolipin on their activities.

Genome-Wide Analysis of Known and Potential Tetraspanins in Entamoeba histolytica.

Tetraspanins are membrane proteins involved in intra- and/or intercellular signaling, and membrane protein complex formation. In some organisms, their role is associated with virulence and pathogenesis. Here, we investigate known and potential tetraspanins in the human intestinal protozoan parasite Entamoeba histolytica. We conducted sequence similarity searches against the proteome data of E. histolytica and newly identified nine uncharacterized proteins as potential tetraspanins in E. histolytica. We found three subgroups within known and potential tetraspanins, as well as subgroup-associated features in both their amino acid and nucleotide sequences. We also examined the subcellular localization of a few representative tetraspanins that might be potentially related to pathogenicity. The results in this study could be useful resources for further understanding and downstream analyses of tetraspanins in Entamoeba.

Novel lineage-specific transmembrane ß-barrel proteins in the endoplasmic reticulum of Entamoeba histolytica.

ß-barrel outer membrane proteins (BOMPs) are essential components of outer membranes of Gram-negative bacteria and endosymbiotic organelles, usually involved in the transport of proteins and substrates across the membrane. Based on the analysis of our in silico BOMP predictor data for the Entamoeba histolytica genome, we detected a new transmembrane ß-barrel domain-containing protein, EHI_192610. Sequence analysis revealed that this protein is unique to Entamoeba species, and it exclusively clusters with a homolog, EHI_099780, which is similarly lineage specific. Both proteins possess an N-terminal signal peptide sequence as well as multiple repeats that contain dyad hydrophobic periodicities. Data from immunofluorescence assay of trophozoites expressing the respective candidates showed the absence of colocalization with mitosomal marker, and interestingly demonstrated partial colocalization with endoplasmic reticulum (ER) proteins instead. Integration to organellar membrane was supported by carbonate fractionation assay and immunoelectron microscopy. CD analysis of reconstituted proteoliposomes containing EHI_192610 showed a spectrum demonstrating a predominant ß-sheet structure, suggesting that this protein is ß-strand rich. Furthermore, the presence of repeat regions with predicted transmembrane ß-strand pairs in both EHI_192610 and EHI_099780, is consistent with the hypothesis that BOMPs originated from the amplification of ßß-hairpin modules, suggesting that the two Entamoeba-specific proteins are novel ß-barrels, intriguingly localized partially to the ER membrane.

An Entamoeba-Specific Mitosomal Membrane Protein with Potential Association to the Golgi Apparatus.

The aerobic mitochondrion had undergone evolutionary diversification, most notable among lineages of anaerobic protists. Entamoeba is one of the genera of parasitic protozoans that lack canonical mitochondria, and instead possess mitochondrion-related organelles (MROs), specifically mitosomes. Entamoeba mitosomes exhibit functional reduction and divergence, most exemplified by the organelle's inability to produce ATP and synthesize iron-sulfur cluster. Instead, this organelle is capable of sulfate activation, which has been linked to amoebic stage conversion. In order to understand other unique features and components of this MRO, we utilized an in silico prediction tool to screen transmembrane domain containing proteins in the mitosome proteome. Here, we characterize a novel lineage-specific mitosomal membrane protein, named Entamoeba transmembrane mitosomal protein of 30 kDa (ETMP30; EHI_172170), predicted to contain five transmembrane domains. Immunofluorescence analysis demonstrated colocalization of hemagglutinin (HA)-tagged ETMP30 with the mitosomal marker, adenosine-5'-phosphosulfate kinase. Mitosomal membrane localization was indicated by immunoelectron microscopy analysis, which was supported by carbonate fractionation assay. Transcriptional gene silencing successfully repressed RNA expression by 60%, and led to a defect in growth and partial elongation of mitosomes. Immunoprecipitation of ETMP30 from ETMP30-HA-expressing transformant using anti-HA antibody pulled down one interacting protein of 126 kDa. Protein sequencing by mass spectrometry revealed this protein as a cation-transporting P-type ATPase, previously reported to localize to vacuolar compartments/Golgi-like structures, hinting at a possible mitosome-vacuole/Golgi contact site.

Reinventing an Organelle: The Reduced Mitochondrion in Parasitic Protists.

Mitochondria originated from the endosymbiotic event commencing from the engulfment of an ancestral α-proteobacterium by the first eukaryotic ancestor. Establishment of niches has led to various adaptations among eukaryotes. In anaerobic parasitic protists, the mitochondria have undergone modifications by combining features shared from the aerobic mitochondria with lineage-specific components and mechanisms; a diversified class of organelles emerged and are generally called mitochondrion-related organelles (MROs). In this review we summarize and discuss the recent advances in the knowledge of MROs from parasitic protists, particularly the themes such as metabolic functions, contribution to parasitism, dynamics, protein targeting, and novel lineage- specific proteins, with emphasis on the diversity among these organelles.

Hetero-oligomer of dynamin-related proteins participates in the fission of highly divergent mitochondria from Entamoeba histolytica.

Entamoeba histolytica is an anaerobic parasitic protist and possesses mitosomes, one of the most highly divergent mitochondrion-related organelles (MROs). Although unique metabolism and protein/metabolite transport machinery have been demonstrated in Entamoeba mitosomes, the mechanism of mitosomal fusion and fission remains to be elucidated. In this study, we demonstrate that two dynamin-related proteins (DRPs) are cooperatively involved in the fission of Entamoeba mitosomes. Expression of a dominant negative form of EhDrpA and EhDrpB, and alternatively, repression of gene expression of EhDrpA and EhDrpB genes, caused elongation of mitosomes, reflecting inhibition of mitosomal fission. Moreover, EhDrpA and EhDrpB formed an unprecedented hetero-oligomeric complex with an approximate 1:2 to 1:3 ratio, suggesting that the observed elongation of mitosomes is likely caused by the disruption and instability of the complex caused by an imbalance in the two DRPs. Altogether, this is the first report of a hetero-oligomeric DRP complex which participates in the fission of mitochondria and MROs.

Screening and discovery of lineage-specific mitosomal membrane proteins in Entamoeba histolytica.

Entamoeba histolytica, an anaerobic intestinal parasite causing dysentery and extra-intestinal abscesses in humans, possesses highly reduced and divergent mitochondrion-related organelles (MROs) called mitosomes. This organelle lacks many features associated with canonical aerobic mitochondria and even other MROs such as hydrogenosomes. The Entamoeba mitosome has been found to have a compartmentalized sulfate activation pathway, which was recently implicated to have a role in amebic stage conversion. It also features a unique shuttle system via Tom60, which delivers proteins from the cytosol to the mitosome. In addition, only Entamoeba mitosomes possess a novel subclass of ß-barrel outer membrane protein called MBOMP30. With the discoveries of such unique features of mitosomes of Entamoeba, there still remain a number of significant unanswered issues pertaining to this organelle. Particularly, the present understanding of the inner mitosomal membrane of Entamoeba is extremely limited. So far, only a few homologs for transporters of various substrates have been confirmed, while the components of the protein translocation complexes appear to be absent or are yet to be discovered. Employing a similar strategy as in our previous work, we collaborated to screen and discover mitosomal membrane proteins. Using a specialized prediction pipeline, we searched for proteins possessing α-helical transmembrane domains, which are unique to E. histolytica mitosomes. From the prediction algorithm, 25 proteins emerged as candidates, two of which were initially observed to be localized to the mitosomes. Further screening and analysis of the predicted proteins may provide clues to answer key questions on mitosomal evolution, biogenesis, dynamics, and biochemical processes.

A novel Mitosomal ß-barrel Outer Membrane Protein in Entamoeba.

Entamoeba possesses a highly divergent mitochondrion-related organelle known as the mitosome. Here, we report the discovery of a novel protein in Entamoeba, which we name Mitosomal ß-barrel Outer Membrane Protein of 30 kDa (MBOMP30). Initially identified through in silico analysis, we experimentally confirmed that MBOMP30 is indeed a ß-barrel protein. Circular dichroism analysis showed MBOMP30 has a predominant ß-sheet structure. Localization to Entamoeba histolytica mitosomes was observed through Percoll-gradient fractionation and immunofluorescence assay. Mitosomal membrane integration was demonstrated by carbonate fractionation, proteinase K digestion, and immunoelectron microscopy. Interestingly, the deletion of the putative ß-signal, a sequence believed to guide ß-barrel outer membrane protein (BOMP) assembly, did not affect membrane integration, but abolished the formation of a ~240 kDa complex. MBOMP30 represents only the seventh subclass of eukaryotic BOMPs discovered to date and lacks detectable homologs outside Entamoeba, suggesting that it may be unique to Entamoeba mitosomes.

Proteomic analysis of Entamoeba histolytica in vivo assembled pre-mRNA splicing complexes.

The genome of the human intestinal parasite Entamoeba histolytica contains nearly 3000 introns and bioinformatic predictions indicate that major and minor spliceosomes occur in Entamoeba. However, except for the U2-, U4-, U5- and U6 snRNAs, no other splicing factor has been cloned and characterized. Here, we HA-tagged cloned the snRNP component U1A and assessed its expression and nuclear localization. Because the snRNP-free U1A form interacts with polyadenylate-binding protein, HA-U1A immunoprecipitates could identify early and late splicing complexes. Avoiding Entamoeba's endonucleases and ensuring the precipitation of RNA-binding proteins, parasite cultures were UV cross-linked prior to nuclear fraction immunoprecipitations with HA antibodies, and precipitates were subjected to tandem mass spectrometry (MS/MS) analyses. To discriminate their nuclear roles (chromatin-, co-transcriptional-, splicing-related), MS/MS analyses were carried out with proteins eluted with MS2-GST-sepharose from nuclear extracts of an MS2 aptamer-tagged Rabx13 intron amoeba transformant. Thus, we probed thirty-six Entamoeba proteins corresponding to 32 cognate splicing-specific factors, including 13 DExH/D helicases required for all stages of splicing, and 12 different splicing-related helicases were identified also. Furthermore 50 additional proteins, possibly involved in co-transcriptional processes were identified, revealing the complexity of co-transcriptional splicing in Entamoeba. Some of these later factors were not previously found in splicing complex analyses. BIOLOGICAL SIGNIFICANCE: Numerous facts about the splicing of the nearly 3000 introns of the Entamoeba genome have not been unraveled, particularly the splicing factors and their activities. Considering that many of such introns are located in metabolic genes, the knowledge of the splicing cues has the potential to be used to attack or control the parasite. We have found numerous new splicing-related factors which could have therapeutic benefit. We also detected all the DExH/A RNA helicases involved in splicing and splicing proofreading control. Still, Entamoeba is very inefficient in splicing fidelity, thus we may have found a possible model system to study these processes.

Comparison of direct fecal smear microscopy, culture, and polymerase chain reaction for the detection of Blastocystis sp. in human stool samples.

OBJECTIVE: To compare the sensitivity and specificity of direct fecal smear microscopy, culture, and polymerase chain reaction in the detection of Blastocystis sp. in human stool. METHODS: Human stool samples were collected from a community in San Isidro, Rodriguez, Rizal, Philippines. These samples were subjected to direct fecal smear microscopy, culture and polymerase chain reaction to detect the presence of Blastocystis sp. RESULTS: Of the 110 stool samples collected, 28 (25%) were detected positive for the presence of Blastocystis sp. by two or more tests. Culture method detected the highest number of Blastocystis-positive stool samples (n=36), followed by PCR of DNA extracted from culture (n=26), PCR of DNA extracted from stool (n=10), and direct fecal smear (n=9). Compared to culture, the sensitivity of the other detection methods were 66.7% for PCR from culture and 19.4% for both PCR from stool and direct fecal smear. Specificity of the methods was high, with PCR from culture and direct fecal smear having 97.3%, while PCR from stool at 95.9%. CONCLUSIONS: In this study, in vitro culture is the best method for detecting Blastocystis sp. in human stool samples.

Profiles of Entamoeba histolytica-specific immunoglobulins in human sera.

OBJECTIVE: To determine the profiles of anti-Entamoeba histolytica (E. histolytica) IgA, IgG, and IgM in sera of diarrheic and non-diarrheic individuals and partially characterize target antigens. METHODS: Serum samples from thirty diarrheic and thirty non-diarrheic individuals were subjected to IgA, IgG, and IgM profiling through enzyme-linked immunosorbent assay (ELISA), flow cytometry, and immunoblot. RESULTS: ELISA titer results showed that both diarrheic and non-diarrheic individuals possess high levels of E. histolytica-specific IgG compared to IgA and IgM. Flow cytometry data showed that diarrheic serum samples had higher mean reaction percentages against E. histolytica cells compared to non-diarrheic samples. Immunoreactive E. histolytica proteins with molecular weights ranging between 7 kDa and 292 kDa were recognized by diarrheic serum IgG, and 170 kDa and 250 kDa by non-diarrheic serum IgG. CONCLUSIONS: Our findings suggest that serum anti-E. histolytica IgG, compared with serum anti-E. histolytica IgA and IgM responses, was generally high in both diarrheic and non-diarrheic sera, indicating a past exposure to the organism both in symptomatic patients as well as in asymptomatic carriers, respectively. In addition, serum IgG from diarrheic and non-diarrheic patients were able to detect immunogenic E. histolytica proteins.

Kinetic analysis of antibody responses to Blastocystis hominis in sera and intestinal secretions of orally infected mice.

The kinetics of antibody production against Blastocystis hominis, an emerging infectious protozoan parasite which causes intestinal disorder in humans and animals, was studied. Sera and intestinal secretions were collected from B. hominis-immunized Balb/C mice for 8 weeks. Flow cytometry was used to monitor the levels of immunoglobulins A (IgA), G (IgG), and M (IgM) from both types of biological samples. The kinetic profile derived from flow cytometry analysis revealed that IgM led the early immune action against B. hominis infection in immune sera while IgA was the predominant antibody isotype in intestinal secretions. Western blotting revealed an array of antigens recognized by both serum and intestinal secretion antibodies. Immunoreactive B. hominis soluble proteins with molecular weights ranging from 28.2 to 77.6 kDa were detected by serum antibodies and 15.1 to 117.5 kDa by secretory antibodies. These antigens may be cytoplasmic or membrane-bound as determined through indirect fluorescent antibody test. Moreover, two immunogens (39.8 and 77.6 kDa) were commonly recognized by serum antibodies, one (70.8 kDa) by secretory antibodies and two (55.0 and 56.2 kDa) by both serum and secretory antibodies, suggesting a possible target in the further understanding of B. hominis pathogenicity, discovery of virulence factors, and development of immunology-based diagnostic protocols and alternative modes of treatment.