Cytoplasmic transfer of resistance to antimycin A in Chinese hamster cells

The thankless task of playing genetics with mammalian mitochondrial DNA: a 30-year review

The advances obtained through the genetic tools available in yeast for studying the oxidative phosphorylation (OXPHOS) biogenesis and in particular the role of the mtDNA encoded genes, strongly contrast with the very limited benefits that similar approaches have generated for the study of mammalian mtDNA. Here we review the use of the genetic manipulation in mammalian mtDNA, its difficulty and the main types of mutants accumulated in the past 30 years and the information derived from them. We also point out the need for a substantial improvement in this field in order to obtain new tools for functional genetic studies and for the generation of animal models of mtDNA-linked diseases.

Effects of mycoplasma contamination on phenotypic expression of mitochondrial mutants in human cells

HeLa cells sensitive to the mitochondrial protein synthesis inhibitors erythromycin (ERY) and chloramphenicol (CAP) and HeLa variants resistant to the effects of these drugs were purposefully infected with drug-sensitive and -resistant mycoplasma strains. Mycoplasma hyorhinis and the ERY-resistant strain of Mycoplasma orale, MO-ERYr, did not influence the growth of HeLa and ERY-resistant ERY2301 cells in the presence or absence of ERY. M. hyorhinis also did not affect the growth of HeLa and CAP-resistant Cap-2 cells in the presence or absence of CAP. However, both HeLa and Cap-2 cells infected with the CAP-resistant strain of M. hyorhinis, MH-CAPr, were more sensitive to the cytotoxic effect of CAP. This may be due to the glucose dependence of the cells, which was compromised by the increased utilization of glucose by MH-CAPr in these infected cell cultures. In vitro protein synthesis by isolated mitochondria was significantly altered by mycoplasma infection of the various cell lines. A substantial number of mycoplasmas copurified with the mitochondria, resulting in up to a sevenfold increase in the incorporation of [3H]leucine into the trichloroacetic acid-insoluble material. More importantly, the apparent drug sensitivity or resistance of mitochondrial preparations from mycoplasma-infected cells reflected the drug sensitivity or resistance of the contaminating mycoplasmas. These results illustrate the hazards in interpreting mitochondrial protein synthesis data derived from mycoplasma-infected cell lines, particularly putative mitochondrially encoded mutants resistant to inhibitors of mitochondrial protein synthesis.

Genetics of the mammalian oxidative phosphorylation system: characterization of a new oligomycin-resistant Chinese hamster ovary cell line

The properties of a new type of oligomycin-resistant Chinese hamster ovary (CHO) cell line (Olir 2.2) are described in this paper. Olir 2.2 cells were approximately 50,000-fold more resistant to oligomycin than were wild-type CHO cells when tested in glucose-containing medium, but only 10- to 100-fold more resistant when tested in galactose-containing medium. Olir 2.2 cells grew with a doubling time similar to that of wild-type cells both in the presence or absence of oligomycin. Oligomycin resistance in Olir 2.2 cells was stable in the absence of drug. In vitro assays indicated that there was approximately a 25-fold increase in the resistance of the mitochondrial ATPase to inhibition by oligomycin in Olir 2.2 cells, with little change in the total ATPase activity. The electron transport chain was shown to be functional in Olir 2.2 cells. Olir 2.2 cells were cross-resistant to other inhibitors of the mitochondrial ATPase (such as rutamycin, ossamycin, peliomycin, venturicidin, leucinostatin, and efrapeptin) and to other inhibitors of mitochondrial functions (such as chloramphenicol, rotenone, and antimycin). Oligomycin resistance was expressed codominantly in hybrids between Olir 2.2 cells and wild-type cells. Cross-resistance to ossamycin, peliomycin, chloramphenicol, antimycin, venturicidin, leucinostatin, and efrapeptin was also expressed codominantly in hybrids. Fusions of enucleated Olir 2.2 cells with wild-type cells and characterization of the resulting cybrid clones indicated that resistance to oligomycin and ossamycin results from a mutation in both a nuclear gene and a cytoplasmic gene. Cross-resistance to efrapeptin, leucinostatin, venturicidin, and antimycin results from a mutation in only a nuclear gene.

Characterization of a Chinese hamster ovary cell line resistant to uncouplers

The chemiosmotic theory of oxidative phosphorylation and the action of uncouplers was examined by characterizing a clone, UH5, of Chinese hamster ovary (CHO TK-) cells resistant to 5-chloro-3-tert-butyl-2'-chloro-4'-nitrosalicylanilide (S-13), a potent uncoupler of oxidative phosphorylation. About 9-times and 4-times more S-13 was required to effect growth and respiration respectively of UH5 cells compared to the parental CHO TK- cells. UH5 cells were cross-resistant to the uncouplers SF-6847 (3,5-di-tert-butyl-4-hydroxy-benzylidenemalononitrile), carbonylcyanide p-trifluoromethoxyphenylhydrazone and 2,4-dinitrophenol but not to oligomycin, venturicidin or Tevenel. Size, chromosome number and DNA content indicated that the UH5 cell line was probably pseudotetraploid compared to the parental pseudodiploid CHO TK- cells. Hybrid and cybrid cells formed from crosses of UH5 cells and cytoplasts, respectively, with an uncoupler-sensitive cell line were sensitive to S-13 indicating that resistance is probably nuclear-determined. UH5 cell mitochondria had increased cytochrome oxidase and decreased H+-ATPase activities. A fivefold resistance of oxidative phosphorylation to uncouplers was found at the mitochondrial level with respiration driven by either succinate or ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine. In contrast, no difference in sensitivity was found to valinomycin between mitochondria from UH5 and CHO TK- cells. The oligomycin-sensitive H+-ATPase activity of UH5 and CHO TK- cell mitochondria was equally stimulated by the uncoupler S-13. Uncoupler-resistant mitochondria would not be expected on the basis of the chemiosmotic theory, and the relation of the results to other modes of coupling is considered.

Mitochondrial genetics of mammalian cells: a mouse antimycin-resistant mutant with a probable alteration of cytochrome b

Mouse LA9 antimycin-resistant mutants (ANT-R) were isolated and characterized. Genetic analyses established that this phenotype is encoded within the mtDNA: (1) the ANT-R phenotype showed frequent mitotic segregation and reassortment in hybrid clonal lines; (2) it was transmitted directly in cybrid crosses; and (3) it was cotransmitted in cybrid crosses with the mitochondrial CAP-R marker. Furthermore, the genetic studies suggested that the LA9 CAP-R ANT-R cells were heteroplasmic and contained at least two mtDNA genotypes, cap-r ant-s and cap-s ant-r. Cellular respiration of the ANT-R mutant was markedly more resistant to inhibition by antimycin than that of the parental ANT-S cells. The increased resistance of cellular respiration was entirely accounted for by an increase in the resistance of mitochondrial succinate-cytochrome c oxidoreductase to antimycin inhibition. There was no detectable change in the specific activity of the oxidoreductase in mitochondria of resistant ANT-R cells nor in the sensitivity of the complex to three other specific inhibitors of the complex: TTFA, myxothiazol, and HQNO. Taken together, these studies indicate that the ANT-R phenotype is most likely encoded within the mitochondrial cytochrome b gene and, more specifically, within an antimycin binding domain.

Cytoplasmic modification of nuclear gene expression

A review is presented on 1) the autonomous nature of mammalian cell cytoplasm and 2) the cytoplasmic modification of nuclear gene expression. Topics include a discussion of cytoplasmic suppression of tumorigenicity. It is proposed that alterations in DNA methylation patterns may be a possible mechanism to explain cytoplasmic modification of nuclear gene expression.

Genetics of the mammalian oxidative phosphorylation system: characterization of a new oligomycin-resistant Chinese hamster ovary cell line

The properties of a new type of oligomycin-resistant Chinese hamster ovary (CHO) cell line (Olir 2.2) are described in this paper. Olir 2.2 cells were approximately 50,000-fold more resistant to oligomycin than were wild-type CHO cells when tested in glucose-containing medium, but only 10- to 100-fold more resistant when tested in galactose-containing medium. Olir 2.2 cells grew with a doubling time similar to that of wild-type cells both in the presence or absence of oligomycin. Oligomycin resistance in Olir 2.2 cells was stable in the absence of drug. In vitro assays indicated that there was approximately a 25-fold increase in the resistance of the mitochondrial ATPase to inhibition by oligomycin in Olir 2.2 cells, with little change in the total ATPase activity. The electron transport chain was shown to be functional in Olir 2.2 cells. Olir 2.2 cells were cross-resistant to other inhibitors of the mitochondrial ATPase (such as rutamycin, ossamycin, peliomycin, venturicidin, leucinostatin, and efrapeptin) and to other inhibitors of mitochondrial functions (such as chloramphenicol, rotenone, and antimycin). Oligomycin resistance was expressed codominantly in hybrids between Olir 2.2 cells and wild-type cells. Cross-resistance to ossamycin, peliomycin, chloramphenicol, antimycin, venturicidin, leucinostatin, and efrapeptin was also expressed codominantly in hybrids. Fusions of enucleated Olir 2.2 cells with wild-type cells and characterization of the resulting cybrid clones indicated that resistance to oligomycin and ossamycin results from a mutation in both a nuclear gene and a cytoplasmic gene. Cross-resistance to efrapeptin, leucinostatin, venturicidin, and antimycin results from a mutation in only a nuclear gene.

Effects of mycoplasma contamination on phenotypic expression of mitochondrial mutants in human cells

HeLa cells sensitive to the mitochondrial protein synthesis inhibitors erythromycin (ERY) and chloramphenicol (CAP) and HeLa variants resistant to the effects of these drugs were purposefully infected with drug-sensitive and -resistant mycoplasma strains. Mycoplasma hyorhinis and the ERY-resistant strain of Mycoplasma orale, MO-ERYr, did not influence the growth of HeLa and ERY-resistant ERY2301 cells in the presence or absence of ERY. M. hyorhinis also did not affect the growth of HeLa and CAP-resistant Cap-2 cells in the presence or absence of CAP. However, both HeLa and Cap-2 cells infected with the CAP-resistant strain of M. hyorhinis, MH-CAPr, were more sensitive to the cytotoxic effect of CAP. This may be due to the glucose dependence of the cells, which was compromised by the increased utilization of glucose by MH-CAPr in these infected cell cultures. In vitro protein synthesis by isolated mitochondria was significantly altered by mycoplasma infection of the various cell lines. A substantial number of mycoplasmas copurified with the mitochondria, resulting in up to a sevenfold increase in the incorporation of [3H]leucine into the trichloroacetic acid-insoluble material. More importantly, the apparent drug sensitivity or resistance of mitochondrial preparations from mycoplasma-infected cells reflected the drug sensitivity or resistance of the contaminating mycoplasmas. These results illustrate the hazards in interpreting mitochondrial protein synthesis data derived from mycoplasma-infected cell lines, particularly putative mitochondrially encoded mutants resistant to inhibitors of mitochondrial protein synthesis.

Elimination of mitochondrial elements and improved viability in hybrid cells

Experiments were carried out to determine whether the mitochondria-specific dye rhodamine-6G (R6G) can affect transmission of cytoplasmic determinants in mammalian cells. When one parental cell type was treated with R6G prior to fusion with an untreated partner, the subsequent hybridization frequencies in both intra- and interspecific crosses were not adversely affected, even though R6G was extremely toxic to the parental cells. In addition, cells lethally treated with R6G could be rescued by fusion with cytoplasm alone from untreated cells. When chloramphenicol (CAP) resistant cells were used as the R6G-treated parent, the expression of CAP resistance in hybrids and cybrids was greatly reduced. Thus R6G can be used to control the input of cytoplasmic determinants into fused cells. In the interspecific (Chinese hamster x mouse) crosses, it was also seen that the majority of hybrids which had not been R6G pretreated grew poorly or degenerated after a short time. In contrast, nearly all hybrids in crosses where the hamster parent was R6G pretreated grew vigorously. The concomitant elimination of inviability and loss of mitochondrial determinants in R6G-pretreated hybrids suggests that interactions involving mitochondrial gene products or components can influence growth characteristics in interspecific somatic cell hybrids.

Cybrid formation with recipient cell lines containing dominant phenotypes

A clone of Chinese hamster ovary (CHO) cells, BT3, resistant to Tevenel, the sulfamoyl analog of chloramphenicol has been isolated. Resistance was found to be at the mitochondrial level and was shown to be cytoplasmically inherited. This marker was then used to develop a method by which a cell line possessing a dominant nuclear mutation (resistance to 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, DRB) could be used as the recipient in cybrid formation. The unique feature in this procedure was the removal of nucleated cells from the cytoplasts by passage through unipore filters. The dominant character of the DRB- and Tevenel-resistant phenotypes permitted the selection of cybrids immediately after fusion. This initially increased the frequency of cybrid clones 16-fold as compared to a recipient cell line possessing a recessive marker. The possibility of extending the method to recipient cells lacking a selectable drug-resistance marker is discussed.

Pyruvate blocks expression of sensitivity to antimycin A and chloramphenicol

Selectivity in Chinese hamster cells with antimycin A and chloramphenicol depends on a metabolic balance which can be modulated by varying the level of exogenous pyruvate. The effects of both inhibitors are most clearly seen in pyruvate-free nutrients. Addition of 1 mM pyruvate in plating assays shifts dose-response curves for antimycin A or chloramphenicol to higher concentration levels and reduces the differential in response between sensitive and resistant cells. In mass populations, growth inhibition by antimycin A is reduced by adding pyruvate, and growth curves for sensitive and resistant cells tend to converge. These observations show that responses to antimitochondrial drugs can be conditioned by extrinsic factors and indicate the need for further definition of selective systems.

Cytoplasmic inheritance of oligomycin resistance in Chinese hamster ovary cells

Oligomycin-resistant clones were isolated from Chinese hamster ovary cells by treatment of cells with ethidium bromide, followed by mutagenesis with ethylmethane sulfonate and selection in oligomycin. One clone (Olir 8.1) was chosen for further study. Olir 8.1 cells grow with doubling time similar to that of wild-type cells, whether grown in the presence or absence of drug (doubling time of 13-14 h). In plating efficiency experiments, Olir 8.1 cells are approximately 100-fold more resistant to oligomycin than are wild-type cells. There is approximately a 32-fold increase in the resistance to inhibition by oligomycin of the mitochondrial ATPase from Olir 8.1 cells. The electron transport chain is functional in Olir 8.1 cells. Oligomycin resistance is stable in the absence of selective pressure. There is little or no cross-resistance of Olir 8.1 cells to venturicidin and dicyclohexylcarbodiimide, other inhibitors of the mitochondrial ATPase, or to chloramphenicol, an inhibitor of mitochondrial protein synthesis. Oligomycin resistance is dominant in hybrids between Olir 8.1 cells and wild-type cells. Fusions of enucleated Olir 8.1 cells with sensitive cells and characterization of the resulting "cybrid" clones indicates that oligomycin resistance in Olir 8.1 cells is cytoplasmically inherited.

Cytoplasmic genetics of mammalian cells: conditional sensitivity to mitochondrial inhibitors and isolation of new mutant phenotypes

We report here that glucose, as a carbon source, and pyruvate are required for the phenotypic expression of cytoplasmically transmitted chloramphenicol-resistance (CAP-R) mutations, recovery of CAP-R mutants, and continuous growth in the presence of oligomycin or antimycin. We assume that glucose supplies additional energy when mitochondrial respiration is diminished and that pyruvate provides intermediates when the Krebs cycle is inhibited. Thus, the requirement for pyruvate is fully satisfied by an exogenous source of purines, and partially by alpha-ketoglutarate or a pyrimidine source. Based upon these findings, we have obtained two types of mutations affecting mitochondrial function--oligomycin resistance and pyruvate-independent expression of chloramphenicol resistance. Both are cytoplasmically transmitted and provide new markers for a genetic analysis of mitochondrial biogenesis.

Oligomycin-resistant mitochondrial ATPase from mouse fibroblasts

Fourteen oligomycin-resistant LM(TK-) clones were isolated following the mutagenesis of minicells. In the absence of oligomycin, the mutants grew with population doubling times similar to that of the wild type (1 day). In 3 or 5 microgram oligomycin/ml the doubling times of the mutants were 1.2-2.5 days. Both stable and unstable classes were represented among the oligomycin-resistant mutants. Mitochondrial ATPase activities of the mutants were 1.3-1130 times more resistant to oligomycin than the wild type. The mitochondrial ATPase of OLI 14 was found to be bound firmly to the mitochondrial membrane, showed no alteration in the pH optimum compared to wild-type, and exhibited increased resistance to DCCD and venturicidin. These results are consistent with the conclusion that oligomycin resistance in these mutants results from altered mitochondrial ATPase.

Cytoplasmic inheritance of erythromycin resistance in human cells

An erythromycin-resistant mutant, ERY2301, was isolated from ethidium bromide-treated HeLa cells in the presence of erythromycin at 300 micrograms/ml. ERY2301 cells were enucleated and the anucleate cytoplasts were fused with D98/AH-2, a hypoxanthine phosphoribosyltransferase-deficient variant of HeLa cells. The resultant cybrids were isolated in a double selective medium containing erythromycin and 6-thioguanine. Cybrid formation occurred at a frequency of 10(-3) to 10(-4). In vitro protein synthesis by intact and Triton X-100 treated mitochondria isolated from ERY2301 was resistant to the macrolide antibiotics erythromycin and carbomycin, but was sensitive to chloramphenicol. These results suggest that the site of erythromycin resistance in ERY2301 may be at the level of mitochondrial protein synthesis and indicate that this trait is cytoplasmically inherited and, therefore, presumably encoded in the mitochondrial genome.