Chloramphenicol-Related Changes in Mitochondrial Ultrastructure

Defective mitochondrial translation differently affects the live cell dynamics of complex I subunits

Complex I (CI) of the oxidative phosphorylation system is assembled from 45 subunits encoded by both the mitochondrial and nuclear DNA. Defective mitochondrial translation is a major cause of mitochondrial disorders and proper understanding of its mechanisms and consequences is fundamental to rational treatment design. Here, we used a live cell approach to assess its consequences on CI assembly. The approach consisted of fluorescence recovery after photobleaching (FRAP) imaging of the effect of mitochondrial translation inhibition by chloramphenicol (CAP) on the dynamics of AcGFP1-tagged CI subunits NDUFV1, NDUFS3, NDUFA2 and NDUFB6 and assembly factor NDUFAF4. CAP increased the mobile fraction of the subunits, but not NDUFAF4, and decreased the amount of CI, demonstrating that CI is relatively immobile and does not associate with NDUFAF4. CAP increased the recovery kinetics of NDUFV1-AcGFP1 to the same value as obtained with AcGFP1 alone, indicative of the removal of unbound NDUFV1 from the mitochondrial matrix. Conversely, CAP decreased the mobility of NDUFS3-AcGFP1 and, to a lesser extent, NDUFB6-AcGFP1, suggestive of their enrichment in less mobile subassemblies. Little, if any, change in mobility of NDUFA2-AcGFP1 could be detected, suggesting that the dynamics of this accessory subunit of the matrix arm remains unaltered. Finally, CAP increased the mobility of NDUFAF4-AcGFP1, indicative of interaction with a more mobile membrane-bound subassembly. Our results show that the protein interactions of CI subunits and assembly factors are differently altered when mitochondrial translation is defective.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Spontaneously Immortalized Mouse Embryo Fibroblasts: Growth Behavior of Wild-Type and Vimentin-Deficient Cells in Relation to Mitochondrial Structure and Activity

Dependent on the presence or absence of vimentin, primary mouse embryo fibroblasts exhibit different growth characteristics in vitro. While most Vim(+/+) fibroblasts stop dividing and die via apoptosis, a substantial fraction of cells immortalize and proliferate almost normally. Vim(-/-) fibroblasts cease to divide earlier, immortalize in vanishingly small numbers and thereafter proliferate extremely slowly. Early after immortalization, Vim(+/+) (imm) fibroblasts appear structurally almost normal, whereas Vim(-/-) (imm) fibroblasts equal postmitotic "crisis" cells, which are characterized by increased cell size, altered cell ultrastructure, nuclear enlargement, genome destabilization, structural degeneration of mitochondria, and diminution of mitochondrial respiratory activity. The differences between immortalized Vim(+/+) (imm) and Vim(-/-) (imm) fibroblasts persist during early cell cloning but disappear during serial subcultivation. At high cell passage, cloned, immortalized vim(-) fibroblasts grow nearly as fast as their cloned vim(+) counterparts, and also resemble them in size, ultrastructure, nuclear volume, and mitochondrial complement; they very likely employ redundancy to cope with the loss of vimentin function when adjusting structure and behavior to that of immortalized vim(+) fibroblasts. Reduction in nuclear size occurs via release of large amounts of filamentous chromatin into extracellular space; because it is complexed with extracellular matrix proteins, it tends to form clusters and to tightly stick to the surface of other cells, thus providing a potential for horizontal gene transfer. On the other hand, cloned vim(+) and vim(-) fibroblasts are equal in showing contact inhibition at young age and becoming anchorage-independent during serial subcultivation, as indicated by the formation of multilayered and -faceted cell sheets and huge spheroids on top of or in soft agar. With this, immortalized vim(-) fibroblasts reduce their adhesiveness to the substratum which, in their precrisis state and early after cloning, is much higher than that of their vim(+) counterparts. In addition, the coupling between the mitochondrial respiratory chain and oxidative phosphorylation is stronger in vim(+) than vim(-) fibroblasts. It appears from these data that after explantation of fibroblasts from the mouse embryo the primary cause of cell and mitochondrial degeneration, including genomic instability, is the mitochondrial production of reactive oxygen species in a vicious circle, and that vimentin provides partial protection from oxidative damage. As a matrix protein with specific in vitro and in vivo affinities for nuclear and mitochondrial, recombinogenic DNA, it may exert this effect preferentially at the genome level via its influence on recombination and repair processes, and in this way also assist the cells in immortalizing. Additional protection of mitochondria by vimentin may occur at the level of mitochondrial fatty acid metabolism.

Loss of the intermembrane space protein Mgm1/OPA1 induces swelling and localized constrictions along the lengths of mitochondria

Mgm1 is a member of the dynamin family of GTP-binding proteins. Mgm1 was first identified in yeast, where it affects mitochondrial morphology. The human homologue of Mgm1 is called OPA1. Mutations in the OPA1 gene are the prevailing cause of dominant optic atrophy, a hereditary disease in which progressive degeneration of the optic nerve can lead to blindness. Here we investigate the properties of the Mgm1/OPA1 protein in mammalian cells. We find that Mgm1/OPA1 is localized to the mitochondrial intermembrane space, where it is tightly bound to the outer surface of the inner membrane. Overexpression of wild type or mutant forms of the Mgm1/OPA1 protein cause mitochondria to fragment and, in some cases, cluster near the nucleus, whereas the loss of protein caused by small interfering RNA (siRNA) leads to dispersal of mitochondrial fragments throughout the cytosol. The cristae of these fragmented mitochondria are disorganized. At early time points after transfection with Mgm1/OPA1 siRNA, the mitochondria are not yet fragmented. Instead, the mitochondria swell and stretch, after which they form localized constrictions similar to the mitochondrial abnormalities observed during the early stages of apoptosis. These abnormalities might be the earliest effects of losing Mgm1/OPA1 protein.

Interaction of drugs with extranuclear genetic elements and its consequences

Bacterial ancestry of mitochondria and plastids is now generally accepted. Both organelles contain their own DNA and transcription-translation apparatus of a prokaryotic type. Due to this fact these systems carry bacteria-like properties. Thus organellar DNA and ribosomes are essentially different from nuclear DNA and cytoplasmic ribosomes in physical as well as in functional respects. Due to the bacterial character of both types of organelles they are susceptible to various antibacterial chemicals. Inhibitors of bacterial protein synthesis inhibit mitochondrial (plastidial) biogenesis. Therefore the cellular content of mitochondria (plastids)-made proteins decreases during cytoplasmic turnover or cell division in the presence of these drugs. Such drug activity consequently leads to a reduced capacity for oxidative phosphorylation or photosynthesis. Organellar genomes are less stable and more sensitive to mutagenesis as compared to nuclear genome. It means also that genotoxic agents induce various disorders of mitochondrial (plastidial) functions. Impairments in the respiratory chain are associated with structural as well as functional abnormalities of mitochondria. These are clinically expressed mostly in tissues with a high demand for ATP: brain, heart, skeletal muscle, and retina. On the other hand, some antibacterial inhibitors of mitochondrial biogenesis (e.g., tetracyclines) inhibit selectively tumor cell proliferation. Therefore they may be considered for use in anticancer therapy. The article summarizes the response of mitochondria and plastids in various organisms to drugs and environmental xenobiotics. Various model organisms suitable for detection of xenobiotic effect on mitochondria (plastids) are presented as well as the possible consequences of such interaction.

Reactions of the nitroso analogue of chloramphenicol with reduced glutathione

Nitroso-chloramphenicol (NOCAP) was synthetized by reduction of chloramphenicol (CAP) with zinc dust in a modification of the procedure published by Corbett and Chipko. The radioactive derivative was similarly prepared from [dichloroacetamido-1-14C]CAP. NOCAP rapidly reacted with GSH with the formation of hydroxylamino-chloramphenicol (NHOHCAP), D-(-)-threo-1-(p-hydroxylaminophenyl)-2-dichloroacetamido-1,3-propanediol and glutathione disulfide (GSSG). In addition, a hydrophilic sulfinamide was formed (GSONHCAP), D-(-)-threo-1-(p-glutathionesulfinamidophenyl)-2-dichloroacetamido-1, 3-propanediol. Free amino-chloramphenicol (NH2CAP), D-(-)-threo-1-(p-aminophenyl)-2-dichloracetamido-1,3-propanediol, was not detected. The proportion of NHOHCAP formed increased with increasing GSH concn, at the expense of GSONHCAP. Analysis by stopped-flow spectroscopy revealed formation of a labile adduct in the reaction of NOCAP with GSH (k = 5500 M-1 sec-1 at 37 degrees, pH 7.4). This reaction was reversible because nearly all NOCAP could be extracted with ether from the labile intermediate. The equilibrium adduct/NOCAP was dependent on GSH concn (K = 4500 M-1 at 37 degrees, pH 7.4). The labile intermediate either isomerized to the sulfinamide, GSONHCAP (favoured by decreasing pH at constant GSH), or it was thiolytically cleaved by another GSH molecule to NHOHCAP and GSSG (favoured by increasing GSH at constant pH). At acid pH, GSONHCAP readily hydrolyzed to NH2CAP and glutathionesulfinic acid. Thus, NOCAP reacts with thiols similar to nitrosobenzene. A scheme is presented for the proposed reaction mechanism. It is concluded that most of the NOCAP, if formed in the intestine or liver, will be rapidly disposed by reactions with GSH. Hence, toxic concns at the sensitive target, i.e. the bone marrow, may usually be prevented.

Freeze-fracture analysis of the effects of age and 2,4-dinitrophenol on the morphology of flight muscle mitochondria of Musca domestica L

Morphometric comparison of freeze-fractured mitochondria in flight muscles of adult (37-day-old) and old (68-day-old) houseflies revealed a 28% decrease of cristae in the old flies. The major membrane change with age was an increase in the 90-120-A particles in the inner membrane external face concomitant with a loss of particle clusters associated with the openings of the cristae on to the inner membrane. In vitro treatment of flight muscle with 2,4-dinitrophenol, and uncoupler of mitochondrial respiration, did not produce this change but resulted in the formation of smooth particle-free vesicular swellings in the mitochondria. Such swelling were infrequent in the old muscle. The cause for the aging change is not clear, but a reduction in the ability of the intramembranous particles to aggregate, either through modification of altered synthesis, is indicated.

Effect of chloramphenicol on colony formation from erythrocytic precursors

In the present experiments, we compare the behavior of erythropoietic precursors and granulocytic precurosors exposed to chloramphenicol. Subcutaneous injection of 20 mg of chloramphenicol sodium succinate on five successive days induced a reduction in the number of erythrocytic colony-forming units (CFU-E) and erythropoietic burst-forming units (BFU-E) in murine marrow and spleen, while no change in number of granulocytic precursors (CFU-C) in murine hemopoietic organs was observed. In vitro studies demonstrated concentration-dependent inhibition of chloramphenicol or chloramphenicol sodium succinate in the colony formatin of all these precursors from murine and human bone marrow. However, erythropoietic burst formation of murine and human BFU-E was completely suppressed by the addition of chloramphenicol or chloramphenicol sodium succinate to the culture medium at therapeutic concentrations (20--40 microgram/ml). From the present results, it can be concluded that the reduced number of BFU-E in mice after injection of chloramphenicol sodium succinate resulted from the selective toxicity of chloramphenicol to BFU-E at therapeutic concentrations.

Chloramphenicol induced inhibition of platelet protein synthesis: in vitro and in vivo studies

Chloramphenicol (CAP), an antibiotic which causes various blood dyscrasias, was shown to inhibit in vitro protein synthesis of human blood platelets. The effect is dose- and time-dependent, it is reversible after incubation for 2 h, and is comparable with the suppression achieved by cycloheximide (CXM). Electron microscopic examination revealed swelling and destruction of mitochondria. Administration of CAP to dogs produced a progressive inhibition of platelet protein synthesis as shown by a reduction in the rate of [3H]leucine incorporation into protein. The maximal inhibitory effect of the antibiotic was reached 0.5-4.0 h after its administration, when minimal leucine incorporation values of 9-40% of the control were measured. The suppressive effect of CAP was followed by an overshoot in the incorporation of [3H]leucine, up to a maximum of 602% of the value at zero time of the experiment.