Survival of unprotected, mammalian plateau-phase cells following freezing in liquid nitrogen

Genetic suppression of cryoprotectant toxicity

We report here a new, unbiased forward genetic method that uses transposon-mediated mutagenesis to enable the identification of mutations that confer cryoprotectant toxicity resistance (CTR). Our method is to select for resistance to the toxic effects of M22, a much-studied whole-organ vitrification solution. We report finding and characterizing six mutants that are resistant to M22. These mutants fall into six independent biochemical pathways not previously linked to cryoprotectant toxicity (CT). The genes associated with the mutations were Gm14005, Myh9, Nrg2, Pura, Fgd2, Pim1, Opa1, Hes1, Hsbp1, and Ywhag. The mechanisms of action of the mutations remain unknown, but two of the mutants involve MYC signaling, which was previously implicated in CT. Several of the mutants may up-regulate cellular stress defense pathways. Several of the M22-resistant mutants were also resistant to dimethyl sulfoxide (Me2SO), and many of the mutants showed significantly improved survival after freezing and thawing in 10% (v/v) Me2SO. This new approach to overcoming CT has many advantages over alternative methods such as transcriptomic profiling. Our method directly identifies specific genetic loci that unequivocally affect CT. More generally, our results provide the first direct evidence that CT can be reduced in mammalian cells by specific molecular interventions. Thus, this approach introduces remarkable new opportunities for pharmacological blockade of CT.

Induction of tolerance to hypothermia and hyperthermia by a common mechanism in mammalian cells

Pretreatment by hypothermic (25 degrees C) cycling (PHC) of attached exponential-phase V79 Chinese hamster cells by Method 4 (24 hr at 25 degrees C + 1.5 hr at 37 degrees C + 24 hr at 25 degrees C + trypsin + 3 hr at 37 degrees C) or by Method 3 (48 hr at 25 degrees C + trypsin + 3 hr at 37 degrees C) make mammalian V79 cells significantly more resistant to 43 degrees C hyperthermia. There is no significant difference in the 43 degrees C curves whether Method 3 or 4 is used for pre-exposure. If pre-exposure at 15 or 10 degrees C, the resistance to hyperthermia is significantly reduced. PHC by Method 4 significantly increases survival of cells exposed to 5 degrees C and, to a lesser extent, to 10 degrees C. The increase in hyper- and hypothermic survival after PHC cannot be accounted for by changes in cell cycle distribution. Heat-shock protein synthesis is not induced by PHC; hence, protection does not result from newly synthesized proteins. When cells are made tolerant to hyperthermia by a pretreatment in 2% DMSO for 24 hr at 37 degrees C (Method 8), the cells are not more resistant to subsequent exposures to hypothermia, either at 5 or 10 degrees C. The results imply that there may be two mechanisms of inducing resistance to hyperthermia, only one of which also confers resistance to hypothermia.

Role of growth phase and ethanol in freeze-thaw stress resistance of Saccharomyces cerevisiae

The freeze-thaw tolerance of Saccharomyces cerevisiae was examined throughout growth in aerobic batch culture. Minimum tolerance to rapid freezing (immersion in liquid nitrogen; cooling rate, approximately 200 degrees C min-1) was associated with respirofermentative (exponential) growth on glucose. However, maximum tolerance occurred not during the stationary phase but during active respiratory growth on ethanol accumulated during respirofermentative growth on glucose. The peak in tolerance occurred several hours after entry into the respiratory growth phase and did not correspond to a transient accumulation of trehalose which occurred at the point of glucose exhaustion. Substitution of ethanol with other carbon sources which permit high levels of respiration (acetate and galactose) also induced high freeze-thaw tolerance, and the peak did not occur in cells shifted directly from fermentative growth to starvation conditions or in two respiratorily incompetent mutants. These results imply a direct link with respiration, rather than exhaustion of glucose. The role of ethanol as a cryoprotectant per se was also investigated, and under conditions of rapid freezing (cooling rate, approximately 200 degrees C min-1), ethanol demonstrated a significant cryoprotective effect. Under the same freezing conditions, glycerol had little effect at high concentrations and acted as a cryosensitizer at low concentrations. Conversely, under slow-freezing conditions (step freezing at -20, -70, and then -196 degrees C; initial cooling rate, approximately 3 degrees C min-1), glycerol acted as a cryoprotectant while ethanol lost this ability. Ethanol may thus have two effects on the cryotolerance of baker's yeast, as a respirable carbon source and as a cryoprotectant under rapid-freezing conditions.

Protective effects of amino acids against freeze-thaw damage in mammalian cells

Amino acids were tested for their effectiveness as cryoprotectants. From the results of this study, the mean fractional area loss of amino acid residues upon incorporation in globular proteins, a measure of hydrophobicity, is generally inversely proportional to the freeze-thaw protection by these free amino acids. However, the pattern of protection ("fingerprint") of cells by various amino acids is different from that of the enzymes liver alcohol dehydrogenase and calcium ATPase of the sarcoplasmic reticulum. Furthermore, unlike the case with these enzymes, for cells glutamine is the best cryoprotective agent of the amino acids tested.

Factors influencing survival of mammalian cells exposed to hypothermia. V. Effects of hepes, free radicals, and H2O2 under light and dark conditions

Cytotoxicity resulting from the interaction of fluorescent light from a flow hood with Hepes-buffered cell culture medium at room temperature was demonstrated. Toxicity was prevented by keeping both cells (V79 Chinese hamster) and medium shielded from direct fluorescent light ("dark conditions") or by supplementing the medium with 10 micrograms/ml catalase; this suggests that extracellular hydrogen peroxide is a major cause of the lethal effect under "lighted conditions." No sensitization resulted from the exposure of cells in a sodium bicarbonate (SBC)-buffered medium to fluorescent light, nor in a catalase supplemented SBC-buffered medium. The Hepes/light reaction during routine cell manipulations presensitized cells to hypothermia damage in the dark with the presensitization being more severe for 5 than for 10 degrees C hypothermic exposure. Presensitization was prevented by performing the complete experiment under dark conditions or by supplementing the medium with 10 micrograms/ml catalase. However, catalase did not improve the hypothermic survival when experiments were performed under dark conditions. Hence, 10 micrograms/ml catalase does not protect cells from hypothermic (5 and 10 degrees C) damage per se, but rather from Hepes/light sublethal damage which interacts with hypothermic sublethal damage to result in lethal lesions. Additionally, under dark conditions, superoxide dismutase (SOD), allopurinol, catalase plus SOD, DMSO, or mannitol did not improve survival when present during hypothermic storage, suggesting that extracellular superoxide anion, hydrogen peroxide, or hydroxyl radicals are not the cause of cell killing under conditions of pure hypothermia uncomplicated by prehypothermic ischemia or hypoxia.

Factors influencing survival of mammalian cells exposed to hypothermia. IV. Effects of iron chelation

Survival of V-79 Chinese hamster cells was assessed by colony growth assay after hypothermic exposure in the presence of iron chelators. At 5 degrees C, maximum protection from hypothermic damage was achieved with a 50 microM concentration of the intracellular ferric iron chelator Desferal. A 3-hr prehypothermic incubation with 50 microM Desferal followed by replacement with chelator-free medium at 5 degrees C also provided some protection. This was not observed when the extracellular chelator DETA-PAC (50 microM) was used prior to cold storage. Treating 5 degrees C-stored cells with Desferal just prior to rewarming was ineffective, but treating cells with Desferal during hypothermia exposure after a significant period of unprotected cold exposure ultimately increased the surviving fraction. Submaximal protection during hypothermia was achieved to various degrees with extracellular chelators at 5 degrees C, including 50 microM DETAPAC and 110 microM EDTA. EGTA (110 microM) had little effect. The sensitization of cells at 5 degrees C with 200 microM FeCl3 could be reduced or eliminated with Desferal in accordance with a 1:1 binding ratio. At 10 degrees C, 50 microM Desferal, 50 microM DETAPAC, and 110 microM EDTA were as or less effective in protecting cells than at 5 degrees C. An Arrhenius plot of cell inactivation rates shows a break at 7-8 degrees C, corresponding to maximum survival for control cells and cells in 50 microM Desferal; however, the amount of protection offered by the chelator increases with decreasing temperature below about 19 degrees C, and sensitization increases above that point. It has not previously been shown that iron chelators protect against cellular hypothermia damage which is uncomplicated by previous or simultaneous ischemia. This may be relevant to the low-temperature storage of transplant organs, in which iron of intracellular origin and in the perfusate may be active and damaging.

Protective effect of L-glutamine against freeze-thaw damage in mammalian cells

L-Glutamine at 18 mM protects mammalian cells against freeze-thaw (FT) damage by a factor of about 6, depending on FT conditions, in balanced salt solutions. While not nearly as effective a cryoprotectant as dimethyl sulfoxide (DMSO) or propylene glycol (PG), the mechanism of protection by glutamine appears to be independent from that of DMSO or PG; thus, 18 mM glutamine is effective at reducing FT damage in combination with these agents. These combinations allow lower concentrations of the more toxic agents DMSO and PG to be used in FT medium. There is no pre-FT or post-FT effect of glutamine when cells are exposed to a FT cycle in balanced salt solutions. Hence, protection is due to its presence during the FT-cycle. The presence of 2 mM L-glutamine in Eagle's basal medium is sufficient to account for cryoprotection by this medium.

Effects of butylated hydroxytoluene on membrane lipid fluidity and freeze-thaw survival in mammalian cells

Butylated hydroxytoluene (BHT) increases the fluidity of membrane lipids in the hydrocarbon but not the polar regions, as measured by electron spin resonance spin label probes. BHT also sensitizes nucleated mammalian cells to freeze-thaw damage as measured by colony formation survival assays. Furthermore, the membranes of BHT-exposed cells are more susceptible to physical stress, as reflected by the BHT-induced sensitization to hypotonic stress. Since others have shown that BHT induces hexagonal phase lipids in lipid bilayers, this phenomenon may also influence the above survival results.

Effects of pre- and post-thaw cell-to-cell contact and trypsin on survival of freeze-thaw damaged mammalian cells

When multicellular spheroids, which simulate small bits of tissue, are exposed to a freeze-thaw (FT) cycle, the survival of the individual cells in the spheroid is higher if the cells of the spheroid are trypsinized and plated as single cells immediately after thawing than if the spheroid is allowed to remain intact for 4 hr and then trypsinized for plating. The results imply either that cell-to-cell contact inhibits repair of potentially lethal damage (PLD) or that accumulation of additional lethal or sublethal damage during the post-thaw period for cells in contact is taking place. Pre- and post-FT trypsinization of single cells indicate that trypsin does not enhance repair of PLD caused by a FT cycle.

Factors influencing survival of mammalian cells exposed to hypothermia. II. Effects of various hypertonic media

Survival of Chinese hamster lung (V79) cells, exposed as a function of time to hypothermia in tissue culture, in isosmotic and various hypertonic media was measured using a colony assay. The mechanism of hypothermic cell killing is different above and below 7 degrees C in this cell line. Addition of NaCl or mannitol to increase the tonicity to 400 mOsm greatly decreased the survival at 10 degrees C while addition of KCl had no significant effect. When these experiments were repeated at 5 degrees C, addition of either NaCl, KCl, or mannitol was detrimental to long-term cell survival. Furthermore, addition of mannitol to the medium did not improve survival when cells were stored at 7 degrees C. Addition of KCl at 5 or 10 degrees C or NaCl at 5 degrees C only affected the cells' ability to accumulate sublethal damage, while addition of mannitol at 5 or 10 degrees C affected both of the above and the cold sensitivity of the cells. Addition of NaCl at 10 degrees C only affected the latter. These experiments suggest that prevention of cell swelling by these conditions, while possibly necessary during clinical hypothermic organ storage, is detrimental to single cell survival at these temperatures.

Factors influencing survival and growth of mammalian cells exposed to hypothermia. I. Effects of temperature and membrane lipid perturbers

The Arrhenius plot of the rate of V79 Chinese hamster cell inactivation due to hypothermia has a "break" around 7-10 degrees C with optimum storage temperature for unprotected cells being about 10 degrees C. Addition of the membrane lipid perturber, butylated hydroxytoluene, improves survival of cells when compared to controls at temperatures below this break but not above. Arrhenius plots of growth rates of the cells show breaks at 30 and 40 degrees C. Measurements of membrane fluidity by electron spin resonance or membrane polarization anisotropy by fluorescence spectrophotometry techniques as a function of temperature in these cells also reveal "breaks" centered around 8 and 30 degrees C. Hence, the changes in the rate of cell inactivation and growth as a function of temperature may be related to membrane lipid phase changes.

Effect of anisotonic NaCl treatment on cellular ultrastructure of V79 Chinese hamster cells. Part 1. Mitotic cells

The ultrastructural modifications produced by anisotonic NaCl treatment of Chinese hamster mitotic cells were observed at three NaCl concentrations which have been frequently used in radiosensitization studies: 0.05, 0.5 and 1.5 M. After exposure to 0.05 M NaCl, many well-spread chromosomes are visible. The chromatin fibres are well dispersed and membraneous material is associated with the chromosomes. After hypertonic treatment with 0.5 M NaCl, the chromosomes have a uniform, structureless appearance with some coalescing into larger anaphase-like masses. At 1.5 M NaCl, large scale cellular dehydration is apparent, and filamentous structures such as microfilaments are tightly constricted. The degree of chromosome staining is also reduced below the level of the cytoplasm. After both hypo- and hypertonic NaCl treatment the chromosomes appear swollen relative to untreated cells, but hypertonic treatment causes chromosome clumping and dissociates chromatin. Conformational changes in the chromatin may restrict the capacity for DNA repair and be related to cellular radiosensitivity.

Effect of salt solutions on the radiosensitivity of mammalian cells as a function of the state of adhesion and the water structure

The radiation isodose survival curve of attached Chinese hamster (V79) cells, subjected to a wide concentration range of salt or sucrose solutions, is characterized by two maxima separated by a minimum. Cells are radioprotected at the maxima (high and low hypertonic salt concentrations) while they are radiosensitized at the minimum (intermediate hypertonic salt concentrations). Both cations and anions can alter the cellular radiosensitivity above and beyond the (osmotic) effect observed for cells treated with sucrose solutions. However, the basic curve shape, except in the case of sulphate salts, remains the same. When these experiments are repeated with single cells in suspension, the isodose survival curve is quite different in that high salt concentrations (greater than 0.9 M) do not protect cells in suspension unlike the case with attached cells. The curve shape is also altered in that the second maximum is absent with many salt solutions. If multicellular spheroids are used for these experiments, the data resemble those for single cell suspensions rather than for attached cells. The radiation survival data for cells in suspension in salt solutions correlate with water proton spin-lattice relaxation time (T1) and, in hypo- and iso-tonic solutions, with cell volume.

Effect of salt solutions on the radiosensitivity of mammalian cells. IV. Treatment with NaCl solutions containing ouabain, NEM, PNAP, cysteamine or DMSO

The effects of a wide concentration range of NaCl solutions containing either ouabain, ethanol, para-nitroacetophenone (PNAP), N-ethylmaleimide (NEM), cysteamine or dimethyl sulphoxide (DMSO) on cellular radiosensitivity have been examined. Ouabain and NEM treatment increased the radiosensitivity of V79 Chinese hamster cells, but the action of these chemicals did not depend on the concentration of NaCl. PNAP increased cellular radiosensitivity with increasing NaCl concentration reaching a maximum effect at 0.6 to 0.7 M NaCl. The radioprotective properties of cysteamine, DMSO and ethanol were all strongly dependent on the NaCl concentration in a complex but qualitatively similar manner. DMSO (2.0 M) increased radiation survival of cells after a 1380 rad dose by a factor of about 10(4) when present in 0.075 M NaCl and by a factor of 8.7 when present in 1.2 M NaCl.

The radiation response of cultured mammalian V79-S171 cells exposed to a wide concentration range of sulphate salt solutions

The radiation response of Chinese hamster cells (V79) exposed to a wide concentration range of Li2SO4, Na2SO4 or K2SO4 has been examined and compared with the radiation response of cells treated in an identical manner with LiCl, NaCl, or KCl solutions. At hypotonic salt concentrations, cells were radiosensitized by both the chloride and sulphate salts. At high salt concentrations, approximately greater than 0.9 M, a radioprotective effect was observed with both chloride and sulphate salts. At intermediate salt concentrations from about 0.2 to 0.9 M, the cells that were treated with the sulphate salt solutions were radioprotected; cells treated with chloride salt solutions were radiosensitized. The difference in radiation response was attributed to the difference in anions for the two types of salts used.