An established avian fibroblast cell line without mitochondrial DNA

Putative chicken "muscle-specific 7 S RNA" is related to the mitochondrial ATPase 6 gene

Sequence analysis of the mitochondrial ATPase 6 gene from chicken revealed that its 3' region is virtually identical with a chicken muscle-specific 7 S RNA which was reported to induce the expression of tissue-specific functions in blastoderm explants. Using chicken and quail cell lines depleted of mitochondrial DNA, we demonstrate that the 7 S RNA is encoded by the mitochondrial genome and not by nuclear (repetitive) DNA as suggested previously. Moreover, no 7 S RNA-homologous transcript of the expected length (about 400 bases) is detected, either in these cell lines or in heart and liver tissues. The only RNA species hybridizing with a 7 S RNA-specific probe is an abundant, 900 base long transcript of mitochondrial origin that we identify as the ATPase 8-ATPase 6 fused messenger. We suggest that the characterized muscle-specific 7 S RNA cDNA is derived from an unrelated contaminant in the blastoderm-inducing fraction.

Development and characterization of continuous avian cell lines depleted of mitochondrial DNA

Populations of quail and chicken cells were treated with ethidium bromide, an inhibitor of mitochondrial DNA replication. After long-term exposure to the drug, the cell populations were transferred to ethidium bromide (EtdBr)-free medium, and cloned. Clones HCF7 (quail) and DUS-3 (chicken) were propagated for more than a year, and then characterized. Analysis of total cellular DNA extracted from these cells revealed no characteristic mitochondrial DNA molecule by Southern blot hybridization of HindIII- or AvaI-digested total cellular DNA probed with cloned mitochondrial DNA fragments. Reconstruction experiments, where a small number of parental cells was mixed with HCF7 cells and DUS-3 cells before extraction of total cellular DNA, further strengthen the notion that the drug-treated cells are devoid of mitochondrial DNA molecules. The cell populations were found to proliferate at a moderately reduced growth rate as compared to their respective parents, to be auxotrophic for uridine, and to be stably resistant to the growth inhibitory effect of EtdBr and chloramphenicol. At the ultrastructural level, mitochondria were considerably enlarged and there was a severe reduction in the number of cristae within the organelles and loss of cristae orientation. Morphometric analysis revealed a fourfold increase of the mitochondrial profile area along with a twofold decrease of the numerical mitochondrial profiles. Analysis of biochemical parameters indicated that the cells grew with mitochondria devoid of a functional respiratory chain. The activity of the mitochondrial enzyme dihydroorotate dehydrogenase was decreased by 95% and presumably accounted for uridine auxotrophy.

Development and characterization of mutant chicken cell lines for somatic cell genetics studies

A series of stable mutants bearing nuclear genetic markers were developed from the established chicken cell line DU24. The mutants were obtained after mutagenesis of DU24 cells with ethyl methanesulfonate (EMS) or arose spontaneously when plated in the appropriate selective medium. Clones resistant to 5-bromodeoxyuridine (BrdU) were obtained following a two-step selection procedure and analyzed. The BrdUr cells were found to be deficient in thymidine kinase activity and were HAT sensitive. Molecular characterization of these mutants revealed no deletions or other rearrangements, but methylation of some cytosine residues was decreased in the mutants. A similar restriction profile was seen in a series of mutants made resistant to BrdU after cultivation of DU24 cells in increasing concentrations of the drug over a period of six months. Selection of EMS-treated BrdUr cells in 10 microM ouabain gave rise to a clone resistant to both drugs and which was still HAT sensitive. Clones resistant to 6-thioguanine were also isolated, but showed wild-type hypoxanthine phosphoribosyltransferase activity and were HAT resistant. A number of the cell lines isolated were found to be suitable for fusion experiments with both chicken cells and cells from other vertebrate species.