An ultracytochemical study of the respiratory potency, integrity, and fate of the sea urchin sperm mitochondria during early embryogenesis

Cytochrome oxidase activity via cytochrome c, as demonstrated by the diaminobenzidine procedure, has been employed in this electron microscope cytochemical study to determine the respiratory potency, integrity and fate of the Arbacia sperm mitochondrion at fertilization and during early embryogenesis. The sperm mitochondrion remained intact and was intensely positive for cytochrome oxidase activity both during and after penetration into the egg. The mitochondrion remained highly reactive throughout zygote formation, up to the eight-cell stage. The sperm mitochondrion formed many projections and buds in the cytoplasm of immature oocytes, monospermic and polyspermic eggs, and in blastomeres. At all stages of early embryogenesis, close juxtaposition and structural contact were observed between the highly reactive sperm mitochondrion and the less reactive egg mitochondria. The results suggest that following fertilization the mitochondrion of the sea urchin spermatozoon retains some degree of metabolic autonomy within the ooplasm. The structural integrity of the paternal mitochondrion is maintained along with a functional respiratory enzyme system (cytochrome c-a3). The hypothesis that the fertilizing sperm mitochondrion may have some relevance to sea urchin development is discussed.     

Metabolic maps and functions of the Plasmodium mitochondrion

The mitochondrion of Plasmodium species is a validated drug target. However, very little is known about the functions of this organelle. In this review, we utilize data available from the Plasmodium falciparum genome sequencing project to piece together putative metabolic pathways that occur in the parasite, comparing this with the existing biochemical and cell biological knowledge. The Plasmodium mitochondrion contains both conserved and unusual features, including an active electron transport chain and many of the necessary enzymes for coenzyme Q and iron-sulphur cluster biosynthesis. It also plays an important role in pyrimidine metabolism. The mitochondrion participates in an unusual hybrid haem biosynthesis pathway, with enzymes localizing in both the mitochondrion and plastid organelles. The function of the tricarboxylic acid cycle in the mitochondrion is unclear. We discuss directions for future research into this fascinating, yet enigmatic, organelle.     

Evolutionary transfers of mitochondrial genes to the nucleus in the Populus lineage and coexpression of nuclear and mitochondrial Sdh4 genes

The transfer of mitochondrial genes to the nucleus is an ongoing evolutionary process in flowering plants. Evolutionarily recent gene transfers provide insights into the evolutionary dynamics of the process and the way in which transferred genes become functional in the nucleus. Genes that are present in the mitochondrion of some angiosperms but have been transferred to the nucleus in the Populus lineage were identified by searches of Populus sequence databases. Sequence analyses and expression experiments were used to characterize the transferred genes. Two succinate dehydrogenase genes and six mitochondrial ribosomal protein genes have been transferred to the nucleus in the Populus lineage and have become expressed. Three transferred genes have gained an N-terminal mitochondrial targeting presequence from other pre-existing genes and two of the transferred genes do not contain an N-terminal targeting presequence. Intact copies of the succinate dehydrogenase gene Sdh4 are present in both the mitochondrion and the nucleus. Both copies of Sdh4 are expressed in multiple organs of two Populus species and RNA editing occurs in the mitochondrial copy. These results provide a genome-wide perspective on mitochondrial genes that were transferred to the nucleus and became expressed, functional genes during the evolutionary history of Populus.     

Application of the serial sectioning and tridimensional reconstruction techniques to the morphological study of the Plasmodium falciparum mitochondrion

The use of serial sectioning followed by tridimensional reconstruction is a convenient way to study the spatial morphology of any structure (cell or organelle). This method was applied to the study of organelles of Plasmodium falciparum (FCR3) and enabled clarification of morphological features of the mitochondrion. The mitochondrion is polymorphic; in single sections it may be rounded, elongated or branched in shape. Its matrix may be dense or transparent, and it may or may not possess cristae. The 3-D reconstruction indicated that the mitochondrion is single in P. falciparum. Its form varies according to the age of the trophozoite. It becomes branched, and each lobe of the mitochondrion follows a daughter nucleus during the formation of merozoites.     

Mitochondrial and Nucleolar Localization of Cysteine Desulfurase Nfs and the Scaffold Protein Isu in Trypanosoma brucei

Trypanosoma brucei has a complex life cycle during which its single mitochondrion is subjected to major metabolic and morphological changes. While the procyclic stage (PS) of the insect vector contains a large and reticulated mitochondrion, its counterpart in the bloodstream stage (BS) parasitizing mammals is highly reduced and seems to be devoid of most functions. We show here that key Fe-S cluster assembly proteins are still present and active in this organelle and that produced clusters are incorporated into overexpressed enzymes. Importantly, the cysteine desulfurase Nfs, equipped with the nuclear localization signal, was detected in the nucleolus of both T. brucei life stages. The scaffold protein Isu, an interacting partner of Nfs, was also found to have a dual localization in the mitochondrion and the nucleolus, while frataxin and both ferredoxins are confined to the mitochondrion. Moreover, upon depletion of Isu, cytosolic tRNA thiolation dropped in the PS but not BS parasites.     

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.     

Computational approaches for the prediction of protein function in the mitochondrion

Understanding a complex biological system, such as the mitochondrion, requires the identification of the complete repertoire of proteins targeted to the organelle, the characterization of these, and finally, the elucidation of the functional and physical interactions that occur within the mitochondrion. In the last decade, significant developments have contributed to increase our understanding of the mitochondrion, and among these, computational research has played a significant role. Not only general bioinformatics tools have been applied in the context of the mitochondrion, but also some computational techniques have been specifically developed to address problems that arose from within the mitochondrial research field. In this review the contribution of bioinformatics to mitochondrial biology is addressed through a survey of current computational methods that can be applied to predict which proteins will be localized to the mitochondrion and to unravel their functional interactions. genomic context; proteome     

Frataxin: its role in iron metabolism and the pathogenesis of Friedreich's ataxia

Friedreich's ataxia (FA) is a severe neurodegenerative condition with an incidence of 1:50000 in the European population. In 97% of patients this disease is due to an intronic GAA triplet repeat expansion in the FRDA gene resulting in a marked decrease in its expression. The protein encoded by this gene is known as frataxin which is found within the mitochondrion. Upon deletion of the homologous gene (YFH1) in the yeast, there was an accumulation of iron (Fe) within the mitochondrion. When the YFH1 gene was reintroduced back into the yeast cell Fe was exported out of the mitochondrion and into the cytosol. Evidence that human frataxin is also involved in mitochondrial Fe-overload comes from studies in FA patients that have shown an accumulation of Fe within the heart. While the precise role of human frataxin remains to be determined, the molecule appears to be involved indirectly in regulating the export and/or import of mitochondrial Fe. The finding of mitochondrial Fe-overload suggests that the use of specific Fe chelators which can permeate the mitochondrion may have potential in the treatment of this disease.     

Using Over-Represented Tetrapeptides to Predict Protein Submitochondria Locations

The mitochondrion is a key organelle of eukaryotic cell that provides the energy for cellular activities. Correctly identifying submitochondria locations of proteins can provide plentiful information for understanding their functions. However, using web-experimental methods to recognize submitochondria locations of proteins are time-consuming and costly. Thus, it is highly desired to develop a bioinformatics method to predict the submitochondria locations of mitochondrion proteins. In this work, a novel method based on support vector machine was developed to predict the submitochondria locations of mitochondrion proteins by using over-represented tetrapeptides selected by using binomial distribution. A reliable and rigorous benchmark dataset including 495 mitochondrion proteins with sequence identity ≤25 % was constructed for testing and evaluating the proposed model. Jackknife cross-validated results showed that the 91.1 % of the 495 mitochondrion proteins can be correctly predicted. Subsequently, our model was estimated by three existing benchmark datasets. The overall accuracies are 94.0, 94.7 and 93.4 %, respectively, suggesting that the proposed model is potentially useful in the realm of mitochondrion proteome research. Based on this model, we built a predictor called TetraMito which is freely available at http://lin.uestc.edu.cn/server/TetraMito.     

Apicoplast and mitochondrion in gametocytogenesis of Plasmodium falciparum

Live cell imaging of human malaria parasites Plasmodium falciparum during gametocytogenesis revealed that the apicoplast does not grow, whereas the mitochondrion undergoes remarkable morphological development. A close connection of the two organelles is consistently maintained. The apicoplast and mitochondrion are not components of the male gametes, suggesting maternal inheritance.     

寄生原生动物线粒体与适应性进化

真核生物的线粒体一般具有一定的典型的结构和功能。然而,在单细胞的寄生原生动物中却不断发现从数量、结构到功能均与典型线粒体明显不同的线粒体,表现出线粒体的巨大可塑性和丰富的多样性。该文对寄生原生动物中这些多样的线粒体进行了概述,并对形成这种多样性的根本原因,即这些生物对寄生生活微氧或无氧环境线粒体所发生的种种适应性进化进行了分析探讨。     

Selective identification of the paternal mitochondrion in living sea urchin eggs and embryos by chlorotetracycline

When sea urchin eggs are pretreated with fluorescent chelate probe chlorotetracycline (CTC) and then fertilized with unlabeled sperm, a small, brightly fluorescent particle resembling the mitochondrion of free-swimming sperm both in size and fluorescent staining characteristics appears in the egg cytoplasm. This particle first appears near the base of the insemination cone and, like the paternal mitochondrion identified in previous ultrastructural studies, remains closely associated with the male pronucleus during its microtubule-dependent migration toward the egg center. These similarities strongly suggest that the fluorescent particle observed in the cytoplasm of living, CTC-pretreated sea urchin eggs is, in fact, the mitochondrion of the fertilizing sperm.     

Isolation and respiratory measurements on a single large mitochondrion

Mitochondria ranging in size from 5–8 μm were produced by feeding Cuprizone to mice weaned at 17 days. A small piece of liver was disrupted and a single large mitochondrion isolated with a suction capillary and a micromanipulator and placed in a microchamber attached to a sensitive oxygen electrode. The mitochondrion showed a respiratory rate of 1.4 × 10^?10 μatoms 0/min and a respiratory ratio with and without ADP of 3.7 with succinate. The respiratory ratio is the same as that obtained with normal mitochondria measured in the same apparatus. The respiratory rate of a 7 μm mitochondrion is estimated to be 1.3 × 10^?4 μatoms 0/min/cm² of inner membrane surface area which is very similar to the rate of 0.7 × 10^?4 μatoms 0/min/cm² estimated for a normal mitochondrion.     

线粒体跨膜电位与细胞凋亡

细胞凋亡作为细胞固有的、受机体严密调控的细胞死亡形式,在多细胞生物体清除衰老细胞及无能细胞等方面发挥重要的作用.近年来,细胞凋亡的研究重点已从细胞核转向线粒体.各种死亡信号诱导线粒体膜通透性改变孔（permeability transition pore, PT pore）开放,引起线粒体跨膜电位下降,导致促凋亡物质释放,继而激活caspase,最终使细胞凋亡.Bcl-2和Bcl-X_L通过对线粒体作用而抑制细胞凋亡,而Bax、Bak与Bad通过调节线粒体而诱导细胞凋亡.     

Comparative Analysis of Codon Usage Patterns Among Mitochondrion, Chloroplast and Nuclear Genes in Triticum aestivum L.

In many organisms, the difference in codon usage patterns among genes reflects variation in local base compositional biases and the intensity of natural selection. In this study, a comparative analysis was performed to investigate the characteristics of codon bias and factors in shaping the codon usage patterns among mitochondrion, chloroplast and nuclear genes in common wheat （Triticum aestivum L.）. GC contents in nuclear genes were higher than that in mitochondrion and chloroplast genes. The neutrality and correspondence analyses indicated that the codon usage in nuclear genes would be a result of relative strong mutational bias, while the codon usage patterns of mitochondrion and chloroplast genes were more conserved in GC content and influenced by translation level. The Parity Rule 2 （PR2） plot analysis showed that pyrimidines were used more frequently than purines at the third codon position in the three genomes. In addition, using a new alterative strategy, 11, 12, and 24 triplets were defined as preferred codons in the mitochondrion, chloroplast and nuclear genes, respectively. These findings suggested that the mitochondrion, chloroplast and nuclear genes shared particularly different features of codon usage and evolutionary constraints.     

Ascidian sperm penetration and the translocation of a cell surface glycosidase

Sperm bind to vitelline coat (VC) glycosides of ascidian eggs by means of a sperm surface glycosidase (Hoshi et al.: Zool Sci 2:65, 1985). In the genus Ascidia, N-acetylglucosamine (NAG) is the VC ligand. After initial binding by the tip of the head, sperm pass through the VC and perivitelline space leaving the single mitochondrion outside. This process can also be followed in vitro on a coverslip. Analysis of recorded video images shows that the sperm moves away from the anchored mitochondrion. Our model for sperm penetration suggests that mitochondrial translocation is responsible for driving the sperm into the egg. In the work presented here, we have demonstrated that ascidian sperm have N-acetyl-beta-D-glucosaminidase (NAGase) activity with an acidic pH optimum. This enzyme, which can be removed from the sperm with Triton X-100, binds to concanavalin A, demonstrating that it is glycosylated. Histochemical methods disclose that the enzyme is originally located at the tip of the head but subsequently remains with the surface overlying the mitochondrion during translocation. Fluorescent Con A was used as a second label for localization of the enzyme on the cell surface during translocation. Colocalization of both probes of the enzyme support a crucial facet of our model; the sperm surface VC binding site remains over the mitochondrion during translocation. This would couple mitochondrial translocation with sperm penetration and drive the sperm into the egg.     

Trypanosoma rhodesiense: mitochondrial proteins of bloodstream and procyclic trypomastigotes

One- and two-dimensional gel electrophoresis of the solubilized mitochondrial proteins of bloodstream and procyclic trypomastigote Trypanosoma brucei rhodesiense and radiolabeling of proteins in the presence of cycloheximide were used to identify proteins synthesized in the trypanosome mitochondrion. The proteins which comprise the mitochondrion were found to be very similar in both bloodstream and procyclic trypomastigotes, but do differ in their level of synthesis. A protein putatively identified as subunit II of cytochrome oxidase (EC 1.9.3.1) was detected in mitochondria from both the procyclic and bloodstream organisms. The presence of this protein in bloodstream trypomastigotes and the overall similarity of protein content in the trypanosome mitochondria is noteworthy in view of the fact that bloodstream trypomastigotes have a repressed mitochondrion with no detectable tricarboxylic acid cycle or cytochrome electron transport chain.     

The Jakobid Flagellates: Structural Features of Jakoba, Reclinomonas and Histiona and Implications for the Early Diversification of Eukaryotes

ABSTRACT. Jakobid flagellates are small, free-living, bacterivorous heterotrophs, with similar morphology, asexual reproduction, and nucleocytoplasmic ultrastructural features at interphase and during cell division. Jakoba is typified by aloricate trophic cells, each containing a branched mitochondrion with prominent nucleoids and flattened cristae. Reclinomonas and Histiona are characterized by loricate trophic cells, each with an unbranched mitochondrion without prominent nucleoids or otherwise differentiated regions and bearing tubular cristae. An undescribed genus is typified by aloricate cells, each with an unbranched mitochondrion bearing discoidal cristae. I propose, on the basis of cytoskeletal architecture and other structural and developmental features, that jakobids are ancestral mitochondrial protists, sister taxa to the amitochondnal retortamonads and ancestral to diverse lineages of mitochondrial eukaryotes. Application of different classification paradigms produces different family-level taxonomies for jakobids.     

Mitochondrial gene transfer in pieces: fission of the ribosomal protein gene rpl2 and partial or complete gene transfer to the nucleus.

Mitochondrial genes are usually conserved in size in angiosperms. A notable exception is the rpl2 gene, which is considerably shorter in the eudicot Arabidopsis than in the monocot rice. Here, we show that a severely truncated mitochondrial rpl2 gene (termed 5' rpl2) was created by the formation of a premature stop codon early in eudicot evolution. This 5' rpl2 gene was subsequently lost many times from the mitochondrial DNAs of 179 core eudicots surveyed by Southern hybridization. The sequence corresponding to the 3' end of rice rpl2 (termed 3' rpl2) has been lost much more pervasively among the mitochondrial DNAs of core eudicots than has 5' rpl2. Furthermore, where still present in these mitochondrial genomes, 3' rpl2 always appears to be a pseudogene, and there is no evidence that 3' rpl2 was ever a functional mitochondrial gene. An intact and expressed 3' rpl2 gene was discovered in the nucleus of five diverse eudicots (tomato, cotton, Arabidopsis, soybean, and Medicago). In the first three of these species, 5' rpl2 is still present in the mitochondrion, unlike the two legumes, where both parts of rpl2 are present in the nucleus as separate genes. The full-length rpl2 gene has been transferred intact to the nucleus in maize. We propose that the 3' end of rpl2 was functionally transferred to the nucleus early in eudicot evolution, and that this event then permitted the nonsense mutation that gave rise to the mitochondrial 5' rpl2 gene. Once 5' rpl2 was established as a stand-alone mitochondrial gene, it was then lost, and was probably transferred to the nucleus many times. This complex history of gene fission and gene transfer has created four distinct types of rpl2 structures or compartmentalizations in angiosperms: (1) intact rpl2 gene in the mitochondrion, (2) intact gene in the nucleus, (3) split gene, 5' in the mitochondrion and 3' in the nucleus, and (4) split gene, both parts in the nucleus.