Cytoplasmically inherited mutations of a human cell line resulting in deficient mitochondrial protein synthesis

Nuclear inheritance of erythromycin resistance in human cells: new class of mitochondrial protein synthesis mutants

The characterization of two new erythromycin-resistant mutants of HeLa cells is described. The strains ERY2305 and ERY2309 both exhibited resistance to erythromycin in growth assays and cell-free mitochondrial protein synthesis assays. The erythromycin resistance phenotype could not be transferred by cybridization. The mutation appeared to be encoded in the nucleus and inherited as a recessive trait. These two mutants, therefore, represent a new class of erythromycin-resistant mutants in human cells that is distinct from the cytoplasmically inherited mutation in strain ERY2301 described previously.

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.

Assignment of the chloramphenicol resistance gene to mitochondrial deoxyribonucleic acid and analysis of its expression in cultured human cells

The mitochondrial deoxyribonucleic acids (mtDNA's) from human HeLa and HT1080 cells differed in their restriction endonuclease cleavage patterns for HaeII, HaeIII, and HhaI. HaeII digestion yielded a 9-kilobase fragment in HT1080, which was replaced by 4.5-, 2.4-, and 2.1-kilobase fragments in HeLa. HaeIII and HhaI yielded distinctive 1.35- and 0.68-kilobase HeLa fragments. These restriction endonuclease polymorphisms were used as mtDNA markers in HeLa-HT1080 cybrid and hybrid crosses involving the cytoplasmic chloramphenicol resistance mutation was used. mtDNA's were purified and digested with the restriction endonucleases, the fragments were separated on agarose gels, and the bands were detected by ethidium bromide staining and Southern transfer analysis. Three cybrids and four hybrids (four expressing HeLa and three expressing HT1080 chloramphenicol resistance) contained 2- to 10-fold excesses of the mtDNA of the chloramphenicol-resistant parent. One cybrid, which was permitted to segregate chloramphenicol resistance and was then rechallenged with chloramphenicol, had approximately equal proportions of the two mtDNA's. Only one hybrid was discordant. These results indicated that chloramphenicol resistance is encoded in mtDNA and that expression of chloramphenicol resistance is related to the ratio of chloramphenicol-resistant and -sensitive genomes within cells.

Efficient selection and characterization of mutants of a human cell line which are defective in mitochondrial DNA-encoded subunits of respiratory NADH dehydrogenase

The mitochondrial NADH dehydrogenase (complex I) in mammalian cells is a multimeric enzyme consisting of approximately 40 subunits, 7 of which are encoded in mitochondrial DNA (mtDNA). Very little is known about the function of these mtDNA-encoded subunits. In this paper, we describe the efficient isolation from a human cell line of mutants affected in any of these subunits. In the course of analysis of eight mutants of the human cell line VA2B selected for their resistance to high concentrations of the complex I inhibitor rotenone, seven were found to be respiration deficient, and among these, six exhibited a specific defect of complex I. Transfer of mitochondria from these six mutants into human mtDNA-less cells revealed, surprisingly, in all cases a cotransfer of the complex I defect but not of the rotenone resistance. This result indicated that the rotenone resistance resulted from a nuclear mutation, while the respiration defect was produced by an mtDNA mutation. A detailed molecular analysis of the six complex I-deficient mutants revealed that two of them exhibited a frameshift mutation in the ND4 gene, in homoplasmic or in heteroplasmic form, resulting in the complete or partial loss, respectively, of the ND4 subunit; two other mutants exhibited a frameshift mutation in the ND5 gene, in near-homoplasmic or heteroplasmic form, resulting in the ND5 subunit being undetectable or strongly decreased, respectively. It was previously reported (G. Hofhaus and G. Attardi, EMBO J. 12:3043-3048, 1993) that the mutant completely lacking the ND4 subunit exhibited a total loss of NADH:Q1 oxidoreductase activity and a lack of assembly of the mtDNA-encoded subunits of complex I. By contrast, in the mutant characterized in this study in which the ND5 subunit was not detectable and which was nearly totally deficient in complex I activity, the capacity to assemble the mtDNA-encoded subunits of the enzyme was preserved, although with a decreased efficiency or a reduced stability of the assembled complex. The two remaining complex I-deficient mutants exhibited a normal rate of synthesis and assembly of the mtDNA-encoded subunits of the enzyme, and the mtDNA mutation(s) responsible for their NADH dehydrogenase defect remains to be identified. The selection scheme used in this work has proven to be very valuable for the isolation of mutants from the VA2B cell line which are affected in different mtDNA-encoded subunits of complex I and may be applicable to other cell lines.

Mitochondrial chloramphenicol-resistant mutants can have deficiencies in energy metabolism

Three pairs of mouse CAP-R PYR-IND OLI-R mitochondrial mutants, and the corresponding CAP-S parental lines, were assayed to determine if cellular expression of these phenotypes was accompanied by changes in cellular energy metabolism: glycolysis, cellular respiration, citric acid cycle activity, and mitochondrial electron transport. Relative to its parental CAP-S line, the SVT2 CAP-R mutant had no significant deficiencies in any of the pathways analyzed. In contrast, the LA9 and SVA31 CAP-R mutants showed significant reductions in cellular respiration. At the biochemical level, respiration deficiency was accompanied by derangements in mitochondrial electron transport. It was also found that the CAP-R mutants had very high levels of glycolysis when the cells were maintained in the presence of chloramphenicol. The possibility is discussed that the sequence changes in the mitochondrial large rRNA gene which determine chloramphenicol resistance can also result, at least in some cases, in reduced levels of mitochondrial biogenesis, leading to respiration deficiency. The PYR-IND and OLI-R phenotypes, which also appear to be encoded by the CAP-R mutations, may result from a compensatory increase in glycolysis-generated ATP or metabolic intermediates.

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.

Induction of mitochondrial mutations in human cells by methotrexate

Inhibition of dihydrofolate reductase by the folate analog, methotrexate (MTX) results in a depletion of tetrahydrofolate dependent one carbon transfer reactions in amino acid and nucleic acid biosynthesis. When human cells (either HeLa or normal skin fibroblasts) are exposed to MTX in a defined medium containing dialyzed fetal calf serum, essential and non-essential amino acids, and purine source, the thymidylate pools alone are depleted. Under these conditions exposure to 10(-6) M MTX induces mitochondrial mutagenesis, measured as an increase in the frequency of chloramphenicol resistant (CAPR) colonies, without altering the rate of nuclear mutation monitored by determining the frequency of 6-thioguanine resistance (TGr). The occurrence of CAPR mutations is time, and MTX concentration dependent and the frequency of CAPR can be decreased quantitatively by adding thymidine to the culture medium. This mitochondrial specific mutagenesis can also be achieved using the thymidylate synthetase inhibitor, 5-fluorodeoxyuridine further implicating thymidylate pools as the mediator of this effect. During the course of exposure to 10(-6) M MTX the thymidine kinase deficient HeLa BU25 cell line exhibits a progressive depletion and degradation of mitochondrial DNA suggesting that the mutagenesis and DNA degradation represent portions of a progressive process. The basis for the selective sensitivity of the mitochondrial genome to thymidylate depletion mutagenesis may be the consequence of its differences from the nuclear genome in mechanisms of DNA replication or repair.

Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes: a distinctive clinical syndrome

We report on two patients who have a mitochondrial myopathy, encephalopathy, lactic acidosis, and recurrent cerebral insults that resemble strokes (MELAS). These two and nine other reported patients share the following features: ragged red fibers evident on muscle biopsy, normal early development, short stature, seizures, and hemiparesis, hemianopia, or cortical blindness. Lactic acidemia is a common finding. We believe that MELAS represents a distinctive syndrome and that it can be differentiated from two other clinical disorders that also are associated with mitochondrial myopathy and cerebral disease: Kearns-Sayre syndrome and the myoclonus epilepsy ragged red fiber syndrome. Existing information suggests that MELAS is transmitted by maternal inheritance. The ragged red fibers suggest an abnormality of the electron transport system, but the precise biochemical disorders in these three clinical syndromes remain to be elucidated.

Sequence analysis of mitochondrial DNA in a mouse cell line resistant to chloramphenicol and oligomycin

A mouse L-cell line, designated 111-OB3, is described which is resistant to two drugs, chloramphenicol and oligomycin. The cells contain two types of mitochondrial DNA molecules, in roughly equal proportions, which differ in that one is cleaved by endonuclease EcoRI at a novel site within the coding sequence for subunit 6 of the mitochondrial ATPase (ATPase-6). Sequence analysis reveals that the cleavage site was created by a single transversion which predicts a replacement of valine in the wild-type ATPase-6 by glutamic acid. The replacement occurs in a hydrophobic amino acid sequence which is highly conserved in mouse, human, and bovine proteins. The position of the replacement is similar to a substitution observed in one class of yeast mutants resistant to oligomycin. Both of the mitochondrial DNA molecules in 111-OB3 also have a single nucleotide change in the gene encoding the large (16S) rRNA. These observations are consistent with the hypothesis that oligomycin resistance in mammalian cells can be cytoplasmically determined and can result from alterations in ATPase-6. The appearance of the mutation before selection in oligomycin suggests a model for the origin of mitochondrial mutations in mammalian cells.

Sequence analysis of mitochondrial DNA in a mouse cell line resistant to chloramphenicol and oligomycin

A mouse L-cell line, designated 111-OB3, is described which is resistant to two drugs, chloramphenicol and oligomycin. The cells contain two types of mitochondrial DNA molecules, in roughly equal proportions, which differ in that one is cleaved by endonuclease EcoRI at a novel site within the coding sequence for subunit 6 of the mitochondrial ATPase (ATPase-6). Sequence analysis reveals that the cleavage site was created by a single transversion which predicts a replacement of valine in the wild-type ATPase-6 by glutamic acid. The replacement occurs in a hydrophobic amino acid sequence which is highly conserved in mouse, human, and bovine proteins. The position of the replacement is similar to a substitution observed in one class of yeast mutants resistant to oligomycin. Both of the mitochondrial DNA molecules in 111-OB3 also have a single nucleotide change in the gene encoding the large (16S) rRNA. These observations are consistent with the hypothesis that oligomycin resistance in mammalian cells can be cytoplasmically determined and can result from alterations in ATPase-6. The appearance of the mutation before selection in oligomycin suggests a model for the origin of mitochondrial mutations in mammalian cells.

Biochemical genetics of the mammalian oxidative phosphorylation system: analysis of the difference in the sensitivity of various Chinese hamster cell lines to inhibitors of the mitochondrial ATP synthase complex

Seven different Chinese hamster cell lines were found to vary greatly in their sensitivity to inhibitors of the mitochondrial ATPase. In plating-efficiency experiments, Chinese hamster lung V79 and bone marrow M3-1 cells were approximately 10,000-fold more resistant to oligomycin, 100-fold more resistant to efrapeptin, and 10-fold more resistant to ossamycin and leucinostatin than were ovary CHO or peritoneal B14 cells. In vitro experiments indicated that the increased resistance of V79 versus CHO cells to these inhibitors was due to an increased resistance of the mitochondrial ATPase. Heat-inactivation experiments indicated that there was a difference in the structure of the mitochondrial ATPase of V79 and CHO cells. Genetic experiments indicated that the difference in the sensitivity of V79 and CHO cells to inhibitors of the ATPase and the difference in the structure of the mitochondrial ATPase of V79 and CHO cells was due to a difference in both a nuclear and a cytoplasmic gene.

Purification of cybrids by fluorescence-activated cell sorting

A general method to isolate and purify substantial numbers of viable cybrids from cultured mammalian cells immediately following cytoplast-cell fusion is described. This method uses cytoplasts whose mitochondria are selectively stained in vivo by the cationic fluorescent rhodamine dye, rhodamine 123. Large numbers of highly purified, rhodamine-stained cytoplasts are fused to appropriate recipient cell lines and then the fusion mixture is sorted based on forward angle scatter and fluorescence parameters. Plating the positively sorted population in culture for as short as 12 h eliminates contaminating cytoplasts which, lacking a nucleus, are unable to adhere or survive. The resultant population, based on an analysis of genetic markers, is 75-100% cybrids, an enrichment of 1000- to 10,000-fold over the initial fusion mixture. Cybrids purified by cell sorting may be useful for detailed molecular studies of mitochondrial DNA gene expression and in the specific induction of new mitochondrial DNA mutants.

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.

Characterization of chloramphenicol- and 8-azaguanine-resistant mutants isolated from a continuous rat-liver epithelial cell line

2 non-tumorigenic, chloramphenicol- and 8-azaguanine-resistant strains have been isolated from the rat-liver cell line K-22, by a 2-step mutagenesis procedure. Their chromosome composition and growth properties have been characterized. Failure of chloramphenicol to inhibit mitochondrial protein synthesis in one of the clones, F1, strongly suggests that resistance to the antibiotic in this strain is due to a mutation in mitochondrial DNA.

Origin, cellular expression, and cybrid transmission of mitochondrial CAP-R, PYR-IND, and OLI-R mutant phenotypes

Chloramphenicol-resistant (CAP-R) mouse and Chinese hamster lines were isolated in a single selection step in drug medium containing pyruvate. Cellular expression of the CAP-R phenotype required pyruvate--or an appropriate substitute--as a nutritional supplement. Subclone lines which were pyruvate independent (PYR-IND) arose in second-step selections at a high frequency. CAP-R PYR-IND Chinese hamster mutants could be directly isolated in single-step selections but at a very low frequency. Subclone lines (OLI-R) which were cross-resistant to oligomycin were isolated in a third selection cycle. The PYR-IND and OLI-R phenotypes were cotransmitted with the CAP-R mtDNA mutation but were expressed at the cellular level only if the number of mutant mitochondrial genomes exceeded a minimum threshold value. Analysis of a mtDNA restriction fragment alteration in one series of mutants supported this model. Threshold limits for cellular expression of mitochondrial mutant phenotypes are likely to be a general phenomenon and will constrain models of the origin and segregation of mtDNA mutations.

Human cell variants resistant to methylglyoxal-bis(guanylhydrazone) display increased sensitivity to chloramphenicol

Four variants of the cultured human cell line VA2 have been isolated which are resistant to the antiproliferative and antimitochondrial effects of methylglyoxal-bis(guanylhydrazone) (MGBG). Each of the four variants is two- to fivefold more sensitive to the mitochondrial protein synthesis inhibitor chloramphenicol (CAP) than wild type when grown in the absence of MGBG, and five- to tenfold more sensitive to CAP when grown in the presence of MGBG. Uptake studies demonstrate that each MGBG-resistant variant cell line is freely permeable to CAP. The in vivo rates of mitochondrial protein synthesis are significantly reduced in each of the variants whether pregrown and labeled in the presence or absence of MGBG. When cytoplasts from a cytoplasmically inherited CAP-resistant mutant are fused to an MGBG-resistant recipient cell line, cybrid clones can be isolated which are functionally resistant to low levels of CAP. With continued growth, the levels of resistance to CAP do not, however, approach the levels of resistance of the CAP-resistant donor cell line. When CAP resistance is subsequently transferred from a CAP/MGBG-resistant cybrid by enucleation and fusion to other human cell lines, then CAP-resistant cybrids can be readily selected in high levels of CAP. It is possible that the substantial decrease in mitochondrial protein synthesis observed in the variants fully accounts for their increased sensitivity to CAP, although the basis for this decreased rate of mitochondrial protein synthesis is not understood.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Assignment of an oligomycin-resistance locus to human chromosome 10

An oligomycin-resistant variant of human fibrosarcoma HT1080 was isolated and characterized as nuclear and codominant. The mutant was stable, was not cross-resistant to respiratory inhibitors, and it contained a mitochondrial ATPase which was less sensitive to oligomycin. Hybrids formed between the human mutant and a mouse cell line expressed the resistance phenotype. By a detailed karyotypic analysis of these hybrids using trypsin-Giemsa banding it was found that resistance to oligomycin correlated with the retention of two human chromosomes 10. The hybrid lines contained only mouse mitochondrial DNA as shown by analyses of mitochondrially synthesized proteins and mitochondrial DNA. The study assigns an ATPase oligomycin-resistance locus to human chromosome 10 and suggests that mouse and human subunits can combine in a functional enzyme complex.

Assignment of two mitochondrially synthesized polypeptides to human mitochondrial DNA and their use in the study of intracellular mitochondrial interaction

Two mitochondrially synthesized marker polypeptides, MV-1 and MV-2, were found in human HeLa and HT1080 cells. These were assigned to the mitochondrial DNA in HeLa-HT1080 cybrids and hybrids by demonstrating their linkage to cytoplasmic genetic markers. These markers include mitochondrial DNA restriction site polymorphisms and resistance to chloramphenicol, an inhibitor of mitochondrial protein synthesis. In the absence of chloramphenicol, the expression of MV-1 and MV-2 in cybrids and hybrids was found to be directly proportional to the ratio of the parental mitochondrial DNAs. In the presence of chloramphenicol, the marker polypeptide linked to the chloramphenicol-sensitive mitochondrial DNA continued to be expressed. This demonstrated that resistant and sensitive mitochondrial DNAs can cooperate within a cell for gene expression and that the CAP-resistant allele was dominant or codominant to sensitive. Such cooperation suggests that mitochondrial DNAs can be exchanged between mitochondria.

Assignment of the chloramphenicol resistance gene to mitochondrial deoxyribonucleic acid and analysis of its expression in cultured human cells

The mitochondrial deoxyribonucleic acids (mtDNA's) from human HeLa and HT1080 cells differed in their restriction endonuclease cleavage patterns for HaeII, HaeIII, and HhaI. HaeII digestion yielded a 9-kilobase fragment in HT1080, which was replaced by 4.5-, 2.4-, and 2.1-kilobase fragments in HeLa. HaeIII and HhaI yielded distinctive 1.35- and 0.68-kilobase HeLa fragments. These restriction endonuclease polymorphisms were used as mtDNA markers in HeLa-HT1080 cybrid and hybrid crosses involving the cytoplasmic chloramphenicol resistance mutation was used. mtDNA's were purified and digested with the restriction endonucleases, the fragments were separated on agarose gels, and the bands were detected by ethidium bromide staining and Southern transfer analysis. Three cybrids and four hybrids (four expressing HeLa and three expressing HT1080 chloramphenicol resistance) contained 2- to 10-fold excesses of the mtDNA of the chloramphenicol-resistant parent. One cybrid, which was permitted to segregate chloramphenicol resistance and was then rechallenged with chloramphenicol, had approximately equal proportions of the two mtDNA's. Only one hybrid was discordant. These results indicated that chloramphenicol resistance is encoded in mtDNA and that expression of chloramphenicol resistance is related to the ratio of chloramphenicol-resistant and -sensitive genomes within cells.

Integrity of mitochondria in a mammalian cell mutant defective in mitochondrial protein synthesis

A defect in mitochondrial protein synthesis has previously been identified in the respiration-deficient Chinese hamster lung fibroblast mutant V79-G7. The present work extends the characterization of this mutant. A more sensitive analysis has shown that mutant mitochondria synthesize all mitochondrially encoded peptides, but in significantly reduced amounts. This difference is also seen when isolated mitochondria are tested for in vitro protein synthesis. To distinguish between a defect in the translational machinery and a defect in the transcription of mitochondrial DNA, we investigated the synthesis of the 16S and 12S mitochondrial rRNA species and found them to be made in normal amounts in G7 mitochondria. These rRNA species appear to be assembled into subunits whose sedimentation behavior is virtually indistinguishable from that of the wild-type subunits. We also examined the consequences of the defect in mitochondrial protein synthesis on mutant cells and their mitochondria-utilizing techniques of electron microscopy, two-dimensional gel electrophoresis and immunochemical analysis. G7 mitochondria have a characteristic ultrastructure distinguished by predominantly tubular cristae, but the overall biochemical composition of mitochondrial membrane and matrix fractions appears essentially unaltered except for the absence of a few characteristic peptides. Specifically, we identify the absence of two mitochondrially encoded subunits of cytochrome c oxidase on two-dimensional gels and demonstrate a drastic reduction of both cytoplasmically and mitochondrially synthesized subunits of enzyme in immunoprecipitates of G7 mitochondria.

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.

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.

On the effect of inhibitors of mitochondrial macromolecular-synthesizing systems and respiration on the growth of cultured chick embryo cells

We have found that chick embryo fibroblasts (DEF) cultivated in the presence of tryptose phosphate broth (TPB) are inherently resistant to the growth inhibitory effect of ethidium bromide (EB). As demonstrated by cytochrome oxidase activity and oxygen consumption measurements, analyses of reduced-minus-oxidized cytochrome spectra and electron microscopic observations, TPB did not seem to prevent the inhibitory effect of EB on mitochondrial DNA transcription. EB-treated chick cell populations cultivated in the presence of TPB behave essentially the same as populations treated with chloramphenicol (CAM) and grow with mitochondria devoid of a functional respiratory chain. In contrast to CAM-treated CEF populations, however, the respiratory activity of EB-treated cell populations did not reappear when the cells were shifted back to EB-free medium. Attempts to demonstrate that TPB confers resistance to the growth inhibitory effect of carbomycin and mikamycin, inhibitors of the mitochondrial protein-synthesizing system, have failed, the drugs being cytotoxic at doses where protein synthesis on mitoribosomes is not suppressed. On the other hand, the present results demonstrated that chick cell populations proliferate in the presence of the respiratory inhibitors rotenone, antimycin A, amytal and oligomycin whether or not TPB is present in the growth medium.

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.

Studies on Chinese hamster ovary mutants showing multiple cross-resistance to oxidative phosphorylation inhibitors

Several stable Chinese hamster ovary (CHO) mutants were selected after ethylmethane sulfonate mutagenesis for resistance to oligomycin, ruatmycin, venturicidin, or antimycin. These mutants shared a number of common properties. They exhibited cross-resistance to those drugs which act on oxidative phosphorylation, irrespective of the structure and site of action of the drug. All the mutants showed a reduced ability to grow in suspension and to reach high saturation densities. They were also unable to use galactose as a carbon source. The short lag period required for selection (10-15 days), the similarity of the mutation rates for resistance to each of the four drugs, the high variance/mean ratios in fluctuation tests, and the recessive behavior of the resistance marker in hybrids suggest that the mutations responsible for resistance to oxidative phosphorylation inhibitors in CHO cells are coded by nuclear DNA. Segregation experiments indicated no linkage between the oligomycin-resistant marker (OLG) AND Thg (thioguanine resistance). Oxidative phosphorylation, as measured by the rate of respiration coupled to phosphorylation in whole cells remained as sensitive to the drugs in the mutants as in the parental cell line. Glucose transport and the overall Krebs' cycle activities also appeared similar in the mutants and the wild type. All the mutants had an increased rate of lactic acid production (up to twofold), associated with increased specific activities for several glycolytic enzymes when assayed in cell-free extracts.