Reassociation kinetics of polyploid hepatocyte DNA

Isolation and characterization of an avian myogenic cell line

Myogenic cell lines have proven extremely valuable for studying myogenesis in vitro. Although a number of mammalian muscle cell lines have been isolated, attempts to produce cell lines from other classes of animals have met with only limited success. We report here the isolation and characterization of seven avian myogenic cell lines (QM1-4 and QM6-8), derived from the quail fibrosarcoma cell line QT6. A differentiation incompetent QM cell derivative was also isolated (QM5DI). The major features of QM cell differentiation in vitro closely resemble those of their mammalian counterparts. Mononucleated QM cells replicate in medium containing high concentrations of serum components. Upon switching to medium containing low serum components, cells withdraw from the cell cycle and fuse to form elongated multinucleated myotubes. Cultures typically obtain fusion indices of 43-49%. Northern blot and immunoblot analyses demonstrate that each differentiated QM cell line expresses a wide variety of genes encoding muscle specific proteins: desmin, cardiac troponin T, skeletal troponin T, cardiac troponin C, skeletal troponin I, alpha-tropomyosin, muscle creatine kinase, myosin light chain 2, and a ventricular isoform of myosin heavy chain. While all QM lines analyzed to date express at least some myosin light chain 2, only one line, QM7, expresses this gene at high levels. Surprisingly, none of the QM lines reported here express any known form of alpha-actin. The absence of sarcomeric actin expression may explain the absence of myofibrils in QM myotubes. These novel features of muscle gene expression in QM cells may prove useful for studying the role of specific muscle proteins during myogenesis. More importantly, however, the isolation of QM cell lines indicates that it may be feasible to isolate other avian myogenic cell lines with general utility for the study of muscle development.

Growth and aging in the rat: changes in total protein, cellularity, and polyploidy in various organs

The objectives of this study were to determine the influence of growth and aging on ploidy, cell number, and protein content of various organs. Tissue homogenates were prepared at 3, 8, 25, 50, and 100 weeks of age. Samples were analyzed for DNA per nucleus (by flow cytofluorometry), nuclei number, and protein content. Livers of 8- and 100-week-old animals were also perfused with collagenase and the released cells separated into parenchymal and nonparenchymal populations by unit gravity sedimentation. Nuclei of these cells were also analyzed for DNA. In all four zones of the kidney and in thyroid, 4n nuclei diminished in percentage between 3 and 50 weeks and increased at 100 weeks. In the growth phase these probably are cycling cells and after 50 weeks represent an increasing population of nuclei arrested after synthesis of DNA. Constant levels of ploidy were found in brain, heart, rectus abdominis, and adrenal throughout the 3-100 weeks. A dramatic increase in 4n nuclei occurred between 3 and 8 weeks in liver with little change occurring thereafter. Ploidy is a property of only parenchymal cells in liver and this probably is also true in other organs. The 4n nuclei that remain in constant proportion to the total population are established early in life and are not related to aging. They are probably tetraploid and replicate into 4n daughter cells during growth. Cerebrum shows no changes in nuclei number but exhibits a 70% increase in protein between 3 and 100 weeks. Although kidney, liver and adrenal show large increases in number of nuclei (approximately equal to fourfold) with growth, these are not as great as increases in body weight (approximately equal to 11-fold). With regard to organ protein, only liver shows increases approximating those in body weight. Increases in organ nuclei appear to occur in concert for adrenal, kidney, and liver whereas increases in organ protein bear no relationship to each other. Protein content remains at stable levels in organs of 100-week-old animals and little (adrenal, liver) or no (brain, kidney) diminution occurs in nuclei numbers.

Albumin secretion and protein synthesis by cultured diploid and tetraploid rat hepatocytes separated by elutriation

Diploid and tetraploid rat hepatocyte subpopulations were isolated by elutriation and cultured for 24 h. Albumin secretion and protein synthesis rates were two-fold lower in 2n than in 4n hepatocytes. [35S]methionine-labelled proteins analysed by acrylamide gel electrophoresis showed a strikingly similar pattern in the two cell subpopulations. No differences in cellular proteins or in the intensity of labelling were observed. These results show (1) that viable diploid and tetraploid hepatocyte subpopulations can be separated by elutriation under sterile conditions and then cultured; and (2) strongly suggest that the same genes are transcribed and further translated at the same rate in both hepatocyte subpopulations.

Ribosomal RNA sequence conservation and gene number in the larval brine shrimp

The haploid genome size of Artemia is determined to be about 0.9 X 10(12), as evidenced both by Feulgen microspectrophotometry of individual diploid class nuclei, which are but one of five polyploid classes present within the larvae, and by analysis of the reassociation kinetics of the isolated single copy DNA component. Polysomes isolated from 24-h incubation stage larvae contain an average of 10 ribosomes per messenger RNA molecule. Their rRNAs are found to have sedimentation coefficients of 18 S and 26 S, corresponding to molecular weights of 0.70 X 10(6) and 1.40 X 10(6), respectively, as determined by polyacrylamide electrophoresis and also by sucrose density centrifugation. Denaturation in glyoxal followed by agarose gel electrophoresis shows that unlike deuterostome rRNAs, Artemia 26 S rRNA contains a cryptic nick about midway in the molecule, which is not found in the 18 S molecule. Isolated rRNAs were labelled in vitro with 125I and hybridized with filter-immobilized DNA to saturation, which occurred at 0.051% for Xenopus, and at 0.074% for Artemia. From these results, it is calculated that in the haploid Artemia genome there are about 320 copies of the (18 S + 26 S) ribosomal RNA genes. Reciprocal heterologous hybridizations between these two species show that they share about 30% homology between their rDNA coding sequences.

Non-repetitive DNA sequence divergence in phylogenetically diploid and tetraploid teleostean species of the family cyprinidae and the order isospondyli

Non-repetitive DNA of anciently tetraploid teleostean species was analysed for the presence of duplicated sequences. Closely related diploid species were investigated in comparison. From the reassociation kinetics of total nuclear DNA, rate constants and fraction sizes of classes of repetitive and non-repetitive sequences were determined. DNA fractions enriched in the slowest renaturing sequence class were determined. DNA fractions enriched in the slowest renaturing sequence class were prepared and subjected to reassociation. The rate constants of these reactions were compared with the values expected for single-copy DNA from analytical genome size determinations. From reassociated DNA enriched in non-repetitive sequences also the melting temperatures were determined as a measure of internal base sequence heterogeneity. It has been shown that the two ancient tetraploids Cyprinus carpio and Thymallus thymallus are, with regard to the thermal stability of reassociated non-repetitive DNA, and with regard to the correspondence of reaction rates with the values expected for single copy DNA, indistinguishable from diploid controls (Rutilus rutilus, Clupea harengus and Sprattus sprattus). The tetraploid species Salmo irideus, Salvelinus fontinalis and Coregonus lavaretus appear as very recent tetraploids with regard to these criteria. The significance of the results for estimating the time of occurrence of polyploidisation events in these taxa is discussed.