Separation of nuclei representing different phases of the growth cycle from unsynchronized mammalian cell cultures

Cell cycle-related variations in UV damage and repair capacity in Chinese hamster (CHO-K1) cells

UV damage to CHO cell DNA, measured by formation of thymine-containing dimers, increases from mitosis to early S phase. Computer simulation of UV absorption by the DNA of an idealized CHO cell at different stages in the cell cycle resembles the cycle dependence of UV damage. Incision at UV damage sites, measured by the accumulation of breaks in preexisting DNA during 30 minutes' post-irradiation incubation with the DNA synthesis inhibitors 1-beta-D arabinofuranosylcytosine and hydroxyurea, increases from mitosis to interphase. Analysis of the dose dependence of DNA break accumulation indicates that both the affinity of the endonuclease for dimer sites and the maximum enzyme activity at saturating levels of dimers are significantly lower in mitosis than in interphase. The killing of CHO cells by UV is enhanced if repair is temporarily inhibited by are C. The DNA gyrase inhibitor novobiocin prevents UV-induced incision.

Separation of intact rat hepatocytes and rat liver nuclei into ploidy classes by velocity sedimentation at unit gravity

A system is described which permits the separation of isolated hepatocytes and isolated rat liver nuclei belonging to different ploidy classes by velocity sedimentation at unit gravity. The problem of obtaining single cells suspensions is discussed and preparations were obtained that contained 96% single hepatocytes. By improving the sedimentation method, it took 2.5 h to separate rat liver nuclei on sucrose gradients into diploid and tetraploid ploidy classes. Recoveries were generally over 95%. The diploid band was 99% pure. DNA and protein content of the ploidy classes were measured. After partial hepatectomy and [3H]thymidine injection it was found that the label moved largely into the tetraploid compartment. Isolated hepatocytes were fractionated in 1 h on Ficoll gradients. Erythrocytes were separated from small nucleated cells and the population of hepatocytes was clearly separated from these two cell populations. Diploid hepatocytes were 80% and tetraploid hepatocytes were 99% pure. Viability was about 80% after fractionation. The gene dosage of NADPH cytochrome c reductase, succinate dehydrogenase and lactate dehydrogenase was estimated in diploid and tetraploid hepatocytes. Gene dosage was equal in diploid and tetraploid hepatocytes for succinate dehydrogenase and NADPH cytochrome c reductase. It is suggested, after correcting for non-viable tetraploid hepatocytes, that the gene dosage of lactate dehydrogenase was significantly lower in diploid than in tetraploid hepatocytes.

The effects of 5alpha-dihydrotestosterone on the kinetics of cell proliferation in rat prostate

The regenerating rat prostate was used as an experimental model to determine the effects of 5alpha-dihydrotestosterone on certain parameters of cell proliferation, including the duration of the phases of the cell cycle and the size of the cellular growth fraction. Rats castrated 7 days previously were treated with daily subcutaneous injections of 5alpha-dihydrotestosterone for 14 days; 48h after the beginning of therapy, cells in the process of DNA synthesis were labelled with a single injection of radioactive thymidine and the progress of these cells through the division cycle was observed. Cell-cycle analysis was performed by fractionating prostatic nuclei according to their position in the cell cycle by using the technique of velocity sedimentation under unit gravity. The results indicate that during regeneration the cell population undergoes 1.8 doublings with a doubling time of 40h, and that the process involves almost four rounds of cell division with a cell-generation time of 20h. The growth fraction at any time is about 0.5, and about half the daughter cells produced do not re-enter the proliferative cycle. All cells present at the start of regeneration eventually undergo at least one division during the course of regeneration, although any given cell can divide from one to four times.

Molecules at the external nuclear surface. Sialic acid of nuclear membranes and electrophoretic mobility of isolated nuclei and nucleoli

The molecules occurring as terminal residues on the external surfaces of nuclei prepared from rat liver by either sucrose-CaCl(2) or citric acid methods and nucleoli derived from the sucrose-CaCl(2) nuclei were studied chemically and electrokinetically. In 0.0145 M NaCl, 4.5% sorbitol, and 0.6 mM NaHCO(3) with pH 7.2 +/- 0.1 at 25 degrees C, the sucrose-CaCl(2) nuclei had an electrophoretic mobility of -1.92 microm/s/V/cm, the citric acid nuclei, -1.63 microm/s/V/cm, and the nucleoli, -2.53 microm/s/V/cm. The citric acid nuclei and the nucleoli contained no measurable sialic acid. The sucrose-CaCl(2) nuclei contained 0.7 nmol of sialic acid/mg nuclear protein; this was essentially located in the nuclear envelope. Treatment of these nuclei with 50 microg neuraminidase/mg protein resulted in release of 0.63 nmol of sialic acid/mg nuclear protein; treatment with 1 % trypsin caused release of 0.39 nmol of the sialic acid/mg nuclear protein. The pH-mobility curves for the particles indicated the sucrose-CaCl(2) nuclei surface had an acid-dissociable group of pK. approximately 2.7 while the pK for the nucleoli was considerably lower. Nucleoli treated with 50 microg neuraminidase/mg particle protein had a mobility of -2.53 microm/s/V/cm while sucrose-CaCl(2) nuclei similarly treated had a mobility of -1.41 microm/s/V/cm. Hyaluronidase at 50 microg/mg protein had no effect on nucleoli mobility but decreased the sucrose-CaCl(2) nuclei mobility to -1.79 microm/s/V/cm. Trypsin at 1 % elevated the electrophoretic mobility of the sucrose-CaCl(2) nuclei slightly but decreased the mobility of the nucleoli to -2.09 microm/s/V/cm. DNase at 50 microg/mg protein had no effect on the mobility of the isolated sucrose-CaCl(2) nuclei but decreased the electrophoretic mobility of the nucleoli to -1.21 microm/s/V/cm. RNase at 50 microg/mg protein also had no effect on the electrophoretic mobility of the sucrose-CaCl(2) nuclei but decreased the nucleoli mobility to -2.10 microm/s/V/cm. Concanavalin A at 50 microg/mg protein did not alter the nucleoli electrophoretic mobility but decreased the sucrose-CaCl(2) nuclei electrophoretic mobility to -1.64 microm/s/V/cm. The results are interpreted to mean that the sucrose-CaCl(2) nuclear external surface contains terminal sialic acid residues in trypsin-sensitive glycoproteins, contains small amounts of hyaluronic acid, is completely devoid of nucleic acids, and binds concanavalin A. The nucleolus surface is interpreted to contain a complex made up of protein, RNA, and primarily DNA, to be devoid of sialic acid and hyaluronic acid, and not to bind concanavalin A.

Studies on histone fraction F2A1 in macro- and micronuclei of Tetrahymena pyriformis

Histone fraction F2A1 has been isolated and purified from macronuclei of the ciliate Tetrahymena pyriformis. It migrates as a single species on sodium dodecyl sulphate-acrylamide gel electrophoresis, with a molecular weight indistinguishable from that of calf thymus F2A1. The solubility properties of Tetrahymena F2A1 are also similar to those of calf thymus F2A1. Electrophoretic analyses on urea-acrylamide gels indicate that Tetrahymena F2A1 consists of four or five subspecies, the two fastest having electrophoretic mobilities identical with those of the two major electrophoretically separable forms of calf thymus F2A1. High resolution (long gel) electrophoresis coupled with incorporation of radioactive acetate both in vivo and in vitro suggest that, as in the case of calf thymus F2A1, differentical acetylation of a parent molecule can explain the observed electrophoretic heterogeneity of Tetrahymena F2A1. Electrophoretic analysis of histones isolated from the micronucleus, which is genetically less active than the macronucleus, indicates that it contains largely the relatively unacetylated (parent) form of histone F2A1.

Transfer of genetic information by purified metaphase chromosomes

Transfer of genetic information from isolated mammalian chromosomes to recipient cells has been demonstrated. Metaphase chromosomes isolated from Chinese hamster fibroblasts were incubated with mouse A(9) cells containing a mutation at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus. Cells were plated in a selective medium, resulting in death of all unaltered parental A(9) cells. However, colonies of cells containing hypoxanthine phosphoribosyl transferase (EC 2.4.2.8) appeared with a variable frequency of about 10(-6) to 10(-7). The enzyme from these cells was indistinguishable from that from Chinese hamster cells, as shown by DEAE-cellulose chromatography and gel electrophoresis, and differed clearly from the mouse enzyme. The colonies, thus, did not result from reversion of A(9) parental cells to wild type, but appeared to represent progeny of individual cells that had ingested chromosomes, replicated, and expressed the hprt gene. These colonies differed from each other in stability of expression of the transferred gene.

( 6 N)methyl adenine in the nuclear DNA of a eucaryote, Tetrahymena pyriformis

DNA isolated from macronuclei of the ciliate, Tetrahymena pyriformis, has been found to contain [(6)N]methyl adenine (MeAde); this represents the first clear demonstration of significant amounts of MeAde in the DNA of a eucaryote. The amounts of macronuclear MeAde differed slightly between different strains of Tetrahymena, with approximately 0.65-0.80% of the adenine bases being methylated. The MeAde content of macronuclear DNA did not seem to vary in different physiological states. The level of MeAde in DNA isolated from micronuclei, on the other hand, was quite low (at least tenfold lower than in macronuclear DNA).