Protein-synthesizing structures associated with mitochondria

Mitochondrial homeostatic disruptions are sensitive indicators of stress in neurons with defective mitochondrial DNA transactions

Neurodegeneration and mitochondrial dysfunction are closely linked across many clinical conditions. In genetic diseases that result from defects in mitochondrial DNA (mtDNA) synthesis or maintenance, neurodegeneration is a frequent and major component of the disease pathology. In sporadic neurodegenerative diseases such as Alzheimer's and Parkinson's disease, mtDNA defects have been observed clinically. Mitochondrial stress related to mtDNA dysregulation can produce neuronal dysfunction and death via impaired electron transport chain activity, which results in deficient ATP production and related increases in mitochondrial reactive oxygen species (ROS) production. However, mtDNA dysregulation in post-mitotic neurons may also produce disturbances in mitochondrial homeostasis that are known to impair neuronal function as well. In this study, we used sub-toxic doses of ethidium bromide (EtBr) to induce mtDNA-associated mitochondrial stress in primary cortical neurons and measured several aspects of mitochondrial homeostasis, mitochondrial function and cell death. We found that low-dose EtBr severely depletes mtDNA synthesis and mitochondrial mRNA levels. Furthermore, homeostatic processes are especially disrupted in toxin treated neurons while mitochondrial function is relatively preserved. Mitochondria become fragmented and motility is abolished, while respiration and mitochondrial polarization are partially maintained. Moreover at these doses, cells do not exhibit increased ROS production, clear neurite retraction or loss of viability. These results indicate that mitochondrial homeostasis is a sensitive marker of mtDNA associated stress compared to mitochondria-functional outputs or endpoints related to cellular toxicity. These homeostatic disruptions are expected to contribute to neuronal dysfunction and potentially drive neurodegenerative disease pathology.

Cytochrome c oxidase subunit III: a molecular marker for N-(4-hydroxyphenyl)retinamise-induced oxidative stress in hepatoma cells

N-(4-hydroxyphenyl)retinamide (4HPR), a chemopreventive and chemotherapeutic retinoid, induces apoptosis in various types of cells. Currently, oxidative mitochondrial damage is thought to cause 4HPR-induced apoptosis, although the exact mechanism has not yet been clarified. 4HPR effectively induces apoptosis in hepatoma cells although the susceptibility differs in a cell-specific manner. Hep-3B and PLC/PRF/5 cells were more susceptible to 4HPR than were Hep-G2 and SK-HEP-1 cells, and the resistance to 4HPR seems to be related to growth inhibition (G(1) arrest). We further observed that 4HPR specifically down-regulates cytochrome c oxidase subunit III (CO III) transcript levels through destabilization of its mRNA and thus decreases the activity of cytochrome c oxidase (complex IV). To explore the mechanism whereby the CO III transcript was decreased by 4HPR, we used adenine nucleotide translocator (ANT) ligands, which modulate mitochondrial transmembrane potential (deltapsi(m)) without altering CO III transcription. Intriguingly, bongkrekic acid, a specific ANT inhibitor, enhanced 4HPR-induced deltapsi(m) disruption, which in turn decreased the level of CO III transcripts, which was accompanied by increases in the generation of reactive oxygen species and in apoptosis. In contrast, atractyloside, an activator of ANT, inhibited those 4HPR-induced effects. Taken together, these results indicate that down-regulation of CO III, a molecular marker of oxidative stress, may result from upstream deltapsi(m) disruption and that ligands of ANT may be capable of modulating 4HPR-induced oxidative stress and apoptosis.

Mitochondrial respiratory chain deficiency leads to overexpression of antioxidant enzymes

U937 cell growth in the presence of either chloramphenicol or ethidium bromide rapidly leads to respiratory deficiency. The novel finding of this report is that this response is paralleled by a specific increase in Se-dependent and independent glutathione peroxidase activities as well as of glutathione peroxidase and heme oxygenase mRNAs. Under the same experimental conditions, catalase activity and catalase mRNA do not show appreciable changes. These results can be explained by an increased formation of H2O2 at the early times of development of respiratory deficiency followed by induction of antioxidant enzymes.

Mitochondrial biosynthesis controls the sensitivity of Chinese hamster cells to hydrogen peroxide

The mechanism of H2O2-resistance of Hpr-4, a variant of Chinese hamster V79 cells, was investigated. The effect of H2O2 on the mitochondria of the parental and Hpr-4 cells was compared. First, both biochemical and ultrastructural results showed that mitochondria in the parental cells were damaged by exposure to H2O2, while those in Hpr-4 cells recovered from the damage. Second, the H2O2-resistance of Hpr-4 cells was reversibly reduced or recovered by the addition or removal of inhibitors of mitochondrial biosynthesis, respectively. Third, the parental cells were auxotrophic to pyruvate after exposure to H2O2. Fourth, H2O2-sensitivity of the parental cells was also enhanced by the inhibition of mitochondrial biosynthesis. From these results, it was concluded that the mitochondria of Hpr-4 cells apparently had a greater resistance to H2O2 than those of the parental cells and that functional mitochondria were involved in the recovery of Chinese hamster V79 cells from H2O2-induced damage.

Polyribosomes and messenger RNA from rat liver mitochondria

Rat liver mitochondrial polyribosomes were isolated free from cytoplasmic ribonucleoprotein contaminations in a number of criteria (sedimentation and buoyant density patterns, ribosomal RNA composition). Heterogeneous poly A containing RNA from mitochondrial polysomes was purified by two-stage cellulose chromatography. This RNA was in vitro labelled with 125I up to specific activity approximately 10(6)-10(7) cts.min-1.microng-1 and used for hybridization experiments with separate complementary strands of mitochondrial DNA and nuclear DNA fragments. The proportions of mitochondrial poly A containing RNA that is complementary to heavy and light strands of mtDNA were respectively 31.5% and 8.3%. Besides, a significant RNA fraction was complementary to unique sequences of nuclear DNA (2-3 copies per haploid genome). The hybrids that were formed possessed a high Tm indicative of a perfect base pairing. A dual intracellular origin of mitochondrial messenger RNA is discussed.

Messenger ribonucleic acid metabolism in mammalian mitochondria. Isolation and characterization of polyribosomes from Ehrlich ascites mitochondria

The Mg2+ precipitation procedure of R. D. Palmiter ((1974) Biochemistry 13, 3606) has been used for preparative scale isolation of polysomes from Ehrlich ascites mitochondria. Digitonin-washed metochondria used for isolating the polysomes contain no detectable reduced nicotinamide adenine dinucleotide phosphate-cytochrome c reductase and over 200-fold reduced hexokinase activity. The mitochondrial polysomes exhibit a heterogeneous sedimentation and appear to contain highly aggregated particlses ranging over hexamers. These polysomes are sensitive to RNase, (ethylenedinitrilo)tetraacetic acid and puromycin. Mitochondrial polysomes are active in portein synthesis when supplied with supernatant enzymes from the homologous mitochondrial source or from Escherichia coli. Cytoplasmic enzymes, however, appear to be completely inactive. Protein synthesis by mitochrondrial polysomes is sensitive to chloramphenicol and resistant to cycloheximide and emetine. The procedure yields particles containing intact rRNAs. The extent of cytoplasmic RNA contaminating the total mitochondrial RNA or mitochondrial polysomal RNA has been estimated to be negligible.

Mitochondrial and cytoplasmic ribosomes and their activity in blood and culture form Trypanosoma brucei

Ribosomes of Trypanosoma brucei, a parasitic, flagellated protozoan (order Kinetoplastida), were identified on sucrose density gradients by their radioactively labeled nascent peptides. Ultraviolet absorption revealed only cytoplasmic ribosomes which served as internal sedimentation markers. Synthesis on cytoplasmic ribosomes was completely inhibited by cycloheximide. In the presence of this antibiotic, nascent peptides were associated with ribosomes of lower sedimentation coefficient than the cytoplasmic ribosomes. Chloramphenicol blocked synthesis on these ribosomes which are probably the mitochondrial ribosomes. These ribosomes differed from the cytoplasmic ribosomes in several ways. Their sedimentation coefficient was about 72S rather than 84S. The stability of the 72S ribosomes was less sensitive to pancreatic ribonuclease and low Mg-++ concentrations, dissociating below 0.1 mM Mg++. The 72S ribosomes were more sensitive to elevated KCl concentrations, dissociation above 0.25 M. Protein synthetic activity associated with the 72S class of ribosomes was found in trypanosomes grown in rats. Under these conditions no cytochromes or fully active Krebs cycle is present in these cells and respiration is insensitive to cyanide.

Mitochondrial and cytoplasmic ribosomes. Distinguishing characteristics and a requirement for the homologous ribosomal salt-extractable fraction for protein synthesis

1. Mitochondrial and cytoplasmic ribosomes of Euglena gracilis differ in their total RNA and protein content. 2. Mitochondrial ribosomes dissociate to subunits at higher Mg(2+) concentrations than do cytoplasmic ribosomes. 3. A separable 5S RNA is obtained from cytoplasmic and chloroplast ribosomes, but not from mitochondrial ribosomes. 4. For protein-synthesizing activity with a natural mRNA, mitochondrial ribosomes use tRNA from any cell compartment and are partly active with supernatant enzymes from cytoplasm. Cytoplasmic ribosomes are partly active with enzymes and tRNA from mitochondria or chloroplasts. 5. Both mitochondrial and cytoplasmic ribosomes show high specificity for the homologous salt-extractable ribosomal fraction for protein-synthesizing activity.

Role of the mitochondrial genome during early development in mice. Effects of ethidium bromide and chloramphenicol

The role of the mitochondrial genome in early development and differentiation was studied in mouse embryos cultured in vitro from the two to four cell stage to the blastocyst (about 100 cells). During this period the mitochondria undergo morphological differentiation: progressive enlargement followed by an increase in matrix density, in number of cristae, and in number of mitochondrial ribosomes. Mitochondrial ribosomal and transfer RNA synthesis occurs from the 8 to 16 cell stage on and contributes to the establishment of a mitochondrial protein-synthesizing system. Inhibition of mitochondrial RNA- and protein-synthesis by 0.1 microg/ml of ethidium bromide or 31.2 microg/ml of chloramphenicol permits essentially normal embryo development and cellular differentiation. Mitochondrial morphogenesis is also nearly normal except for the appearance of dilated and vesicular cristae in blastocyst mitochondria. Such blastocysts are capable of normal postimplantation development when transplanted into the uteri of foster mothers. Higher concentrations of these inhibitors have general toxic effects and arrest embryo development. It is concluded that mitochondrial differentiation in the early mouse embryo occurs through the progressive transformation of the preexisting mitochondria and is largely controlled by the nucleocytoplasmic system. Mitochondrial protein synthesis is required for the normal structural organization of the cristae in blastocyst mitochondria. Embryo development and cellular differentiation up to the blastocyst stage are not dependent on mitochondrial genetic activity.

Atypical pattern of utilization of amino acids for mitochondrial protein synthesis in HeLa cells

The capacity of HeLa cell mitochondria, either isolated or in intact cells, to incorporate different labeled amino acids into proteins was investigated. Eight amino acids (alanine, arginine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, and lysine), which include most of the charged polar ones, showed a very low amount, if any at all, of chloramphenicol-sensitive incorporation, relative to that expected for an "average" HeLa-cell protein. By contrast, the most hydrophobic amino acids (leucine, isoleucine, valine, phenylalanine, and methionine) were the most actively incorporated by HeLa mitochondria. The available evidence suggests that pool effects cannot account for this general pattern of utilization of amino acids; furthermore, this pattern is in good agreement with the known hydrophobic properties of proteins synthesized in mitochondria.

Mitochondrial protein synthesis: RNA with the properties of Eukaryotic messenger RNA

A heterogeneous RNA fraction with properties resembling those of messenger RNA was identified in mammalian mitochondria. Synthesis of contaminating RNA of nuclear origin was suppressed by treatment with camptothecin. Labeling of the messenger-like RNA is completely inhibited by ethidium bromide, a specific inhibitor of mitochondrial functions.Although mitochondrial protein synthesis resembles that of prokaryotes in several regards, the messenger-like RNA is covalently linked to poly(adenylic acid) [poly(A)]. Poly(A) has thus far been found only in eukaryotic cells. The poly(A) segment has a gel electrophoretic mobility of about 4 S, corresponding to a length of 50-80 nucleotides, and thus resembles in size the poly(A) found in some mammalian viral RNAs. The messenger RNA can be released from the mitochondrial protein-synthesizing structure by treatment with puromycin.

Mitoribosomes from Candida utilis. Morphological, physical, and chemical characterization of the monomer form and of its subunits

Highly purified mitochondrial ribosomes (mitoribosomes) have been obtained from the yeast Candida utilis. Sedimentation analysis in sucrose gradients made in 5 mM MgCl(2), 1 mM Tris, pH 7.4 and 50 mM KCl clearly distinguishes mitoribosomes (72S) from cytoplasmic ribosomes (cytoribosomes) (78S). Mitoribosomes are completely dissociated into 50S and 36S subunits at 10(-4)M MgCl(2) whereas complete dissociation of cytoribosomes into 61S and 37S subunits occurs only at 10(-6)M MgCl(2) Electron microscopy of negatively stained mitoribosomes (72S peak) shows bipartite profiles, about 265 x 210 x 200 A Characteristic views are interpreted as frontal, dorsal, and lateral projections of the particles, the latter is observed in two enantiomorphic forms Mitoribosome 50S subunits display rounded profiles bearing a conspicuous knoblike projection, reminiscent of the large bacterial subunit. The 36S subunits show a variety of angular profiles. Mitoribosomal subunits are subject to artifactual dimerization at high Mg(2+) concentration Under these conditions, a supplementary 80S peak arises. Electron microscopic observation of the 80S peak reveals closely paired particles of the 50S type Buoyant density determinations after glutaraldehyde fixation show a single peak at rho = 1.48 for mitoribosomes and 1.53 for cytoribosomes In the presence of ethylenediaminetetraacetate (EDTA), two species of RNA, 21S and 16S, are obtained from mitoribosomes, while 25S and 17S RNA are obtained from cytoribosomes It is established that the small and large RNA species are derived from the 36S and 50S subunits, respectively, by extraction of the RNA from each subunit The G + C content of the RNA is lower for mitoribosomes (33%) than for cytoribosomes (50%). Incubation of C utilis mitochondria with leucine-(14)C results in the labeling of 72S mitoribosomes. The leucine-(14)C incorporation is inhibited by chloramphenicol and resistant to cycloheximide Puromycin strips the incorporated radioactivity from the 72S mitoribosomes, which is consistent with the view that leucine-(14)C is incorporated into nascent polypeptide chains at the level of mitoribosomes

The effects of erythromycin and chloramphenicol on the ultrastructure of mitochondria in sensitive and resistant strains of Paramecium

The effects on cell structure of 12 hr to 6 days of exposure to erythromycin or chloramphenicol, two antibiotics known to inhibit specifically the mitochondrial protein synthesizing system, have been studied in the ciliate Paramecium aurelia. A wild type strain (sensitive to both antibiotics) and three mutant strains carrying cytoplasmically inherited mutations conferring resistance to one or the other antibiotic have been used In sensitive cells both antibiotics lead to a progressive and profound alteration of mitochondrial structure evidenced by an elongation of the organelle, a considerable decrease in the number of cristae, and the appearance of some abnormal lamellar cristae and of rigid plates of periodic structure. The modifications of cell structure, then, are mainly restricted to mitochondrial cristae. The three resistant mutants studied, on the contrary, retain normal or nearly normal mitochondrial structure in the presence of the antibiotic to which they are resistant. This fact is in good agreement with the postulated location in the mitochondrial DNA of the resistance mutations studied. The results are discussed in the light of present knowledge concerning the function of the mitochondrial protein-synthesizing system.

The effect of ethidium bromide on mitochondrial DNA synthesis and mitochondrial DNA structure in HeLa cells

The synthesis of mitochondrial DNA (mDNA) in HeLa cells is selectively inhibited by relatively low concentrations of ethidium bromide. After exposure of cells to strongly inhibitory concentrations of the drug, the apparent superhelix density of mDNA is rapidly increased, as judged by its buoyant density in CsCl in the presence of ethidium bromide. Mitochondrial DNA synthesized in the presence of partially inhibitory concentrations of ethidium bromide is also altered in its buoyant density in the presence of the dye, but is more heterogeneous in this respect. However, the change in buoyant density of newly synthesized mDNA may be explained by changes in structure other than a change in superhelix density, as indicated by its increased resistance to digestion by pancreatic DNase.

Immunofluorescent localisation of human alpha feto-protein in fetal and neonatal livers and cultured cells from hepatocellular carcinoma

The indirect immunofluorescence technique demonstrated that human alpha fetoprotein (AFP) was present in a focal pattern in the cytoplasm of malignant liver cells of patients whose sera contained AFP. A few fibroblastlike cells in tissue culture of liver biopsies from patients with hepato-cellular carcinoma and AFP in their sera also had the protein. The intracellular localisation of human AFP was confirmed by centrifugation of washed, homogenised and ultra-sonically disrupted neonatal liver cells. Examination of livers of fetuses and neonates showed that AFP was present predominantly in the periportal parenchymal cells.