A temperature-sensitive mutant defective in mitosis and cytokinesis

The critical influence of temperature upon trifluorothymidine resistance in mouse lymphoma L5178Y/TK 3.7.2C cells

L5178Y/TK 3.7.2C cells are used for the assessment of chemical mutagenesis caused by presumptive TK gene mutations or multiple loci mutations affecting the TK locus that result in dose-related increases in resistance to the toxic thymidine analog, trifluorothymidine (TFT). This study was based upon our general observation that the incidence of TFTres in these cells could vary with the incubation temperature. As a result of these studies, we found that: (1) a substantial proportion of presumptive TK-/- variants produced by the mutagens 2-aminofluorene (2-AF), N-acetylaminofluorene (AAF), benzo[a]pyrene (B[a]P), 3-methylcholanthrene (3MCA), hycanthone methanesulfonate (Hyc), or methyl methanesulfonate (MMS) are more resistant to TFT at 37 degrees C than at 28 degrees C (or 39 degrees C than at 33 degrees C), (2) the loss of resistance to TFT was most notable in the small-colony variant population, (3) mutagen-derived variants become less resistant as the TFT concentration is increased from 4 micrograms/ml to 50 micrograms/ml, an effect that is more pronounced at 28 degrees C than at 37 degrees C, and (4) stock 3.7.2C cells develop a persistent TFTres due to sharply decreased TK activity when exposed to 40 degrees C for at least 24 h. These data demonstrate two different responses by these cells with respect to temperature stability at the TK locus and suggest that the degree of TFTres is influenced by both temperature and concentration of selective agent in this presumptive gene/chromosomal mutation assay.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Cell division cycle in mammalian cells. VIII. Mapping of G1 into six segments using temperature-sensitive cell cycle mutants

The G1 blocks in three temperature-sensitive (ts) Syrian hamster cell-cycle mutants have been mapped in relation to other G1 landmarks. Two mutants reported here, ts-559 and ts-694, show defective progression only in G1. When shifted from the permissive temperature of 33 degrees C to the non-permissive temperature of 39 degrees C, G1 cells of these two mutants show no further cell cycle progression, while cells in S, G2 and mitosis progress through the cell cycle but become blocked after entering G1. The two mutants complement each other, and also complement the previously reported mutant ts-550C with blocks in both G1 and G2 of the cell cycle. The locations of the G1 blocks in both ts-559 and ts-694 are before the hydroxyurea arrest point. The G1 ts point in ts-694 is prior to the isoleucine deprivation and serum starvation points, while the G1 block in ts-559 is after the serum starvation point but before the isoleucine block. Other G1 block points which have been reported are in mutants of different species and isolated in different laboratories, causing difficulties for relative positioning of the blocks in G1. The mutants for mapping in this study have been isolated from the same cell line. The G1 ts arrest points of ts-559 and ts-694, and that found in ts-550C, together with nutritional deprivations and metabolic inhibitors, provide seven reference points which divide G1 into six segments, each of which is bracketed by two adjacent points: mitosis, ts-694 block, serum starvation arrest point, ts-559 block, isoleucine deprivation arrest point, ts-550C block, hydroxyurea or excess-thymidine arrest segment.

Coexpression of two thermosensitive defects in a Chinese hamster cell line. Manifestation of a G1 block associated with a mitosis defect

Asynchronous cultures of ts12, an anchorage-dependent derivative of the thermosensitive Chinese hamster cell line ts111, show a rapid drop in [3H]thymidine incorporation with accumulation of the cells in the G1 and in the G2 phases of the cycle, when shifted from 34.5 to 39.4 degrees C. Shift-up experiments carried out after either isoleucine deprivation or synchronization at 39.4 degrees C, locate the execution point of a ts function in late G1 (2.5-3 h before S). However, stimulation of proliferation of a high density-arrested population allows a fraction of the cells to enter S. In addition to the G1 ts defect, ts12 expresses a slight cytokinesis defect at 39.4 degrees C (8-15% binucleate cells). The results suggest that altered processes are taking place at a post-metaphasic stage during the first hours after the shift-up. When populations are synchronized by a thymidine block and released at 39.4 degrees C, multinucleate cells in addition to binucleate cells are observed. Part of these multinucleate cells result from abnormal karyokinesis without inhibition of cytokinesis. Evidence is presented suggesting that excess thymidine allows the re-expression of the multinucleation phenotype of ts111.

Mutations in alpha- and beta-tubulin affect spindle formation in Chinese hamster ovary cells

Two Chinese hamster ovary cell lines with mutated beta-tubulins (Grs-2 and Cmd-4) and one that has a mutation in alpha-tubulin (Tax-1) are temperature sensitive for growth at 40.5 degrees C. To determine the functional defect in these mutant cells at the nonpermissive temperature, they were characterized with respect to cell cycle parameters and microtubule organization and function after relatively short periods at 40.5 degrees C. At the nonpermissive temperature all the mutants had normal appearing cytoplasmic microtubules. Premature chromosome condensation analysis failed to show any discrete step in the interphase cell cycle in which these mutants are arrested. These cells, however, show several defects at the nonpermissive temperature that appear related to the function of microtubules during mitosis. Time-lapse studies showed that mitosis was lengthened in the three mutant lines at 40.5 degrees C as compared with the wild-type cells at this temperature, resulting in a higher proportion of cells in mitosis after temperature shift. There was also a large increase in multinucleated cells in mutant populations after incubation at the nonpermissive temperature. Immunofluorescent studies using a monoclonal anti--alpha-tubulin antibody showed that the mutant cells had a high proportion of abnormal spindles at the nonpermissive temperature. The two altered beta-tubulins and the altered alpha-tubulin all were found to cause a similar phenotype at the high temperature that results in mitotic delay, defective cytokinesis, multinucleation, and ultimately, cell death. We conclude that spindle formation is the limiting microtubule function in these mutant cell lines at the nonpermissive temperature and that these cell lines will be of value for the study of the precise role of tubulin in mammalian spindle formation.

Studies on cell division in mammalian cells. VII. A temperature-sensitive cell line abnormal in centriole separation and chromosome movement

A temperature-sensitive Syrian hamster mutant cell line, ts-745, exhibiting novel mitotic events has been isolated. The cells show normal growth and mitosis at 33 degrees C, the permissive temperature. At the nonpermissive temperature of 39 degrees C, mitotic progression becomes aberrant. Metaphase cells and those cells still able to form a metaphase configuration continue through and complete normal cell division. However, cells exposed to 39 degrees C for longer than 15 min can not form a normal metaphase spindle. Instead, the chromosomes are distributed in a spherical shell, with microtubules (MT) radiating to the chromosomes from four closely associated centrioles near the center of the cell. The cells progress from the spherical monopolar state to other monopolar orientations conical in appearance with four centrioles in the apex region. Organized chromosome movement is present, from the spherical shell state to the asymmetrical orientations. Chromosomes remain in the metaphase configuration without chromatid separation. Prometaphase chromosome congression appears normal, as the chromosomes and MT form a stable monopolar spindle, but bipolar spindle formation is apparently blocked in a premetaphase state. When returned from 39 degrees to 33 degrees C, the defective phenotype is readily reversible. At 39 degrees C, the mitotic abnormality lasts 3-5 h, followed by reformation of a single nucleus and cell flattening in an interphase-like state. Subsequent cell cycle events appear to occur, as the cells duplicate chromosomes and initiate a second round of abnormal mitosis. Cell cycle traversion continues for at least 5 d in some cells despite abnormal mitosis resulting in cells accumulating several hundred chromosomes.

Studies on cell division in mammalian cells: VI. A temperature-sensitive mutant blocked in both G1 and G2 phases of the cell cycle

A temperature-sensitive mammalian cell cycle mutant with blocks in G1 and G2 phases of the cell cycle has been isolated in culture. When shifted from the permissive temperature of 33 degrees C to the nonpermissive temperature of 39 degrees C, the fraction of cells initiating DNA synthesis as well as the fraction of cells entering mitosis decreased rapidly. Combined cytophotometric and autoradiographic analysis on the cells at 39 degrees C showed that G1 cells, with the exception of those in late G1, were arrested in that phase. Cells is S phase at the time of temperature shift, together with the late g1 cells which subsequently entered S, continued through S into G2, but were blocked in that phase of the cell cycle and unable to initiate mitosis. Those cells already in mitosis completed cell division at 39 degrees C. The G1 block point of ts-550C was found to be located after the serum starvation and isoleucine deprivation arrest points, approximately 3 h before initiation of DNA synthesis.

Multinucleation and inhibition of cytokinesis in suspended cells: reversal upon reattachment to a substrate

Cell proliferation under nonadhesive conditions was examined in anchorage-dependent mouse fibroblasts (3T3 and 3T6), in epithelial monkey kidney cells (BSC-1) and in four B16 melanoma cell variants that exhibit distinct metastatic properties. Non-adhesive conditions established by suspension culture in methyl cellulose or by plating on poly(2-hydroxyethylmethacrylate)-coated plates cause multinucleation and inhibition of cytokinesis in all cell types. Formation of binucleated cells in suspended 3T3 cells continues for 24 hr until DNA synthesis is completely inhibited. Twenty percent of the cells become binucleated during this time. During a similar period in suspension culture over 60% of a synchronized epithelial cell culture (suspended 8 hr after the initiation of S phase) became binucleated. The melanoma cells that maintain a high level of metabolism while in suspension continued to develop into multinucleated cells. After 3 days in suspension, 50% of the melanoma cells have more than one nucleus. Since cell number during suspension culture remains constant, because of a block in cytokinesis, there is a remarkable increase in cell size in suspended melanoma cells. When replated on tissue culture plates, normal cell division resumes by an early extrusion of the extra nuclei in melanoma cells. The melanoma cell variants, although displaying different metastatic potential in vivo, behave similarly in suspension culture. 3T3 cells require 24 hr of replating before recovering DNA synthesis and initiating extrusion of the extra nucleus. The results suggest a differential sensitivity of cytokinesis and karyokinesis to cell-surface contact with the substrate.

Rapid, quantitative analysis of cell cycle stages of cold-sensitive derivatives of the Chinese hamster cell line CHO-K1

Cell cycle parameters of two cold-sensitive Chinese hamster cell lines have been determined by flow cytometric analysis of cell populations stained with the DNA specific fluorochrome mithramycin. The most striking finding is a depletion of S phase cells, detectable as early as 12 h after a shift to the nonpermissive temperature of 30.0 degrees C and complete by 24 h following the shift. There is a substantial increase in the proportion of cells in the G1 phase of the cell cycle compared to wild type cells under identical conditions but, surprisingly, the proportion of cells having a G2/M DNA content is quite similar in the two populations. The proportion of tetraploid cells present in these populations is not sufficient to account for this observation. Reversibility of the cold-induced block was tested by returning cells held for three days at the nonpermissive temperature to the permissive temperature. Cells having a G1 content of DNA do reenter the S phase, beginning approximately 8 h after a return to the permissive temperature.