Thermodynamic modeling and cryomicroscopy of cell-size, unilamellar, and paucilamellar liposomes

Emerging Role for Use of Liposomes in the Biopreservation of Red Blood Cells

SUMMARY: Biopreservation is the process of maintaining the integrity and functionality of cells held outside the native environment for extended storage times. The development of red blood cell (RBC) biopreservation techniques that maintain in vitro RBC viability and function represents the foundation of modern blood banking. The biopreservation of RBCs for clinical use can be categorized based on the techniques used to achieve biologic stability, including hypothermic storage and cryopreservation. This review will examine the emerging role of liposomes in the RBC biopreservation, including the incorporation of liposomes into RBC membranes as an effective approach for minimizing RBC hypothermic storage membrane lesion and use of liposomes as a permeabilization strategy for the intracellular accumulation of novel intracellular cryoprotectants. Integration of current biopreservation research with blood banking practices offers enormous potential for future improvements of safety and efficacy of RBC transfusion.

Characterizing the freezing behavior of liposomes as a tool to understand the cryopreservation procedures

Freezing behaviors of egg yolk l-alpha-phosphatidylcholine (EPC) and 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC) large unilamellar vesicles (LUV) were quantitatively characterized in relation to freezing temperatures, cooling rates, holding time, presence of sodium chloride and phospholipid phase transition temperature. Cooling of the EPC LUV showed an abrupt increase in leakage of the encapsulated carboxyfluorescein (CF) between -5 degrees C and -10 degrees C, which corresponded with the temperatures of the extraliposomal ice formation at around -7 degrees C. For the DPPC LUV, CF leakage started at -10 degrees C, close to the temperature of the extraliposomal ice formation; followed by a subsequent rapid increase in leakage between -10 degrees C and -25 degrees C. Scanning electron microscopy showed that both of these LUV were freeze-concentrated and aggregated at sub-freezing temperatures. We suggest that the formation of the extraliposomal ice and the decrease of the unfrozen fraction causes freeze-injury and leakage of the CF. The degree of leakage, however, differs between EPC LUV and DPPC LUV that inherently vary in their phospholipid phase transition temperatures. With increasing holding time, the EPC LUV were observed to have higher leakage when they were held at -15 degrees C compared to at -30 degrees C whilst leakage of the DPPC LUV was higher when holding at -40 degrees C than at -15 degrees C and -50 degrees C. At slow cooling rates, osmotic pressure across the bilayers may cause an additional stress to the EPC LUV. The present work elucidates freeze-injury mechanisms of the phospholipid bilayers through the liposomal model membranes.

Spectrophotometric measurement of intraliposomal trehalose

Trehalose, a non-reducing glucose disaccharide found at high concentrations in many species of anhydrobiotic organisms, shows significant promise in protecting cellular viability and structural integrity during freezing and desiccation. As mammalian cell membranes are impermeable to trehalose, extensive efforts have been taken to introduce trehalose into mammalian cells. In this study, we report on the characterization of trehalose-containing liposomes, with focus on the entrapment of trehalose inside liposomes, as the first step in establishing liposomes as a delivery system in the biopreservation field. Liposomes were synthesized by hydrating a phospholipid/cholesterol lipid bilayer with 200-400 mM trehalose buffer and repeatedly extruding the lipid suspension to form unilamellar vesicles. The trehalose content of the liposomal lysate was determined spectrophotometrically using a commercial kit Megazyme and confirmed with HPLC measurements. The number of liposomes was calculated from the phosphate content of the liposomal preparation and an estimated number of lipid molecules in a 401+/-8 nm liposome. Based on an intraliposomal trehalose content, the calculated liposomal encapsulation efficiency of 200 mM trehalose liposomes was of 92+/-0.7%. This value was in agreement with the 300 and 400 mM trehalose liposomes (91.1+/-8.2% and 102.1+/-9.4%, respectively). The Megazyme method for trehalose measurement is an inexpensive and sensitive technique that does not require specialized instrumentation or extensive technical expertise. Therefore, it can be used to enhance current efforts in the development of alternative strategies for the cryo- and lyoprotection of mammalian cells.

Cell-cell contact affects membrane integrity after intracellular freezing

The response of cells to freezing depends critically on the presence of an intact cell membrane. During rapid cooling, the cell plasma membrane may no longer be an effective barrier to ice propagation and can be breached by extracellular ice resulting in the nucleation of the supercooled cytoplasm. In tissues, the formation of intracellular ice is compounded by the presence of cell-cell and cell-surface interactions. Three different hamster fibroblast model systems were used to simulate structures found in organized tissues. Samples were supercooled to an experimental temperature on a cryostage and ice nucleated at the constant temperature. A dual fluorescent staining technique was used for the quantitative assessment of the integrity of the cell plasma membrane. A novel technique using the fluorescent stain SYTO was used for the detection of intracellular ice formation (IIF) in cell monolayers. The cumulative incidence of cells with a loss of membrane integrity and the cumulative incidence of IIF were determined as a function of temperature. Cells in suspension and individual attached cells showed no significant difference in the number of cells that formed intracellular ice and those that lost membrane integrity. For cells in a monolayer, with cell-cell contact, intracellular ice formation did not result in the immediate disruption of the plasma membrane in the majority of cells. This introduces the potential for minimizing damage due to IIF and for developing strategies for the cryoprotection of tissues during rapid cooling.

The osmotic rupture hypothesis of intracellular freezing injury

A hypothesis of the nature of intracellular ice formation is proposed in which the osmotically driven water efflux that occurs in cells during freezing (caused by the increased osmotic pressure of the extracellular solution in the presence of ice) is viewed as the agent responsible for producing a rupture of the plasma membrane, thus allowing extracellular ice to propagate into the cytoplasm. This hypothesis is developed into a mathematical framework and the forces that are present during freezing are compared to the forces which are required to rupture membranes in circumstances unrelated to low temperatures. The theory is then applied to systems which have been previously studied to test implications of the theory on the nature of intracellular ice formation. The pressure that develops during freezing due to water flux is found to be sufficient to cause a rupture of the plasma membrane and the theory gives an accurate description of the phenomenology of intracellular ice formation.

Leakage of a trapped fluorescent marker from liposomes: effects of eutectic crystallization of NaCl and internal freezing

Leakage of trapped carboxyfluorescein from DL-alpha-dipalmitoylphosphatidylcholine multilamellar liposomes (diameter 1-2 microns) in NaCl solutions was measured after rapid freezing to temperatures between -15 and -55 degrees C. Leakage was low after freezing between -15 and -35 degrees C, but increased steeply between -35 and -45 degrees C. From DSC measurements it was found that the increase in leakage was associated with two crystallization processes: Eutectic crystallization of NaCl and freezing of undercooled solvent trapped in the interior of the liposomes ("internal freezing"). Damage caused by the former process could effectively be prevented by small amounts of trehalose (1% less than or equal to w less than or equal to 1.5%). Trehalose in these concentration also decreased damage due to internal freezing, but to a minor degree. In addition to these damaging transitions, a time-dependent process was found to cause leakage from the liposomes at -25 degrees C. The association between leakage and thermal activity suggests that DSC supplements cryomicroscopy and leakage measurements in the characterization of cryostability of liposomes.

Cryomicroscopic analysis of intracellular ice formation during freezing of mouse oocytes without cryoadditives

Kinetics of intracellular ice formation (IIF) under various freezing conditions was investigated for mouse oocytes at metaphase II obtained from B6D2F1 mice. A new cryostage with improved optical performance and "isothermal" temperature field was used for nucleation experiments. The maximum thermal gradient across the window was less than 0.1 degrees C/10 mm at sample temperatures near 0 degrees C. The dependence of IIF on the initial concentration of the suspending medium was found to be pronounced. The mean IIF temperatures were found to be -9.56, -12.49, -17.63, -22.20 degrees C for freezing at 120 degrees C/min in 200, 285, 510, and 735 mosm phosphate-buffered saline, respectively. For concentrations higher than 735 mosm, the kinetics of IIF showed a break point at approximately -31 degrees C. Below -31 degrees C, all the remaining unfrozen oocytes underwent IIF almost immediately over a temperature range of less than 3 degrees C. This dramatic shift in the kinetics of IIF suggests that there were two distinct mechanisms responsible for IIF during freezing. The effect of the cooling rate on the kinetics of IIF was also investigated in isotonic PBS. At 1 degrees C/min none of the oocytes contained ice, whereas, at 5 degrees C/min all the oocytes contained ice. The mean IIF temperatures for cooling rates between 1 and 120 degrees C/min were almost constant with an average of -12.82 +/- 0.6 degrees C (SEM). In addition, constant temperature experiments were conducted in isotonic PBS. The percentages of oocytes with IIF were 0, 50, 60, and 95% for -3.8, -6.4, -7.72, and -8.85 degrees C. In undercooling experiments, IIF was not observed until approximately -20 degrees C (at which temperature the whole suspension was frozen spontaneously), suggesting the involvement of the external ice in the initiation of IIF between approximately -5 and -31 degrees C during freezing of oocytes.

Intracellular ice formation: cryomicroscopical observation and calorimetric measurement

The formation of ice crystals within biological cells is generally deleterious and results in a severe loss of cellular viability and function. With the aim of circumventing this lethal event, the mechanisms of nucleation and their dependence on governing parameters such as temperature, cooling rate and solute and/or additive concentration, and the correlation with the osmotically induced water transport across the cell membrane were investigated. Quantitative low-temperature light microscopy was used for this purpose as it offers the major advantage of studying the dynamics of the involved processes. To substantiate further the visual observations of the morphological changes associated with intracellular ice formation, supplementary studies by differential scanning calorimetry (DSC) were performed under comparable conditions to measure the quantity of water actually transformed into the crystalline state due to the evolution of latent heat. Human lymphocytes were used as a biological model cell. In particular it could be shown that the twitching type of intracellular ice formation which is evident but difficult to observe under the cryomicroscope can be attributed to a liquid-solid phase change within the cells as determined by DSC. Good agreement was obtained between the results measured by both techniques with respect to the following dependencies of governing parameters: the fraction of cells exhibiting intracellular ice determined as a function of the cooling rate shows a sharp demarcation zone with an increase from 0 to 100% at about the same threshold cooling rate. On the other hand, the temperatures at which intracellular ice forms were found to be only weakly dependent on the cooling rate. With respect to the effect of cryo-additive concentration at a fixed value of the cooling rate, the crystallization temperatures were seen to decrease with concentration. The DSC results may hence be regarded as a validation of the microscopic observations.

Water transport and estimated transmembrane potential during freezing of mouse oocytes

The kinetics of water transport and the changes in transmembrane potential during freezing of mouse oocytes in isotonic phosphate buffered saline (PBS) were simulated using thermodynamic models. The permeability to water at 0 degree C, Lpg, and the activation energy, ELp, of metaphase II mouse oocytes from B6D2F1 mice were determined to be 0.044 +/- 0.008 micron/min-atm and 13.3 +/- 2.5 kcal/mol during freezing at 2 degrees C/min. The inactive cell volume was determined to be 0.214 with a correlation coefficient of 0.995, indicating that the oocytes closely follow the ideal Boyle-van't Hoff relation. The mean value of the oocyte diameter was 79.41 +/- 4.62 microns. These results were used to predict the behavior of mouse oocytes under various freezing conditions. The effect of the cooling rate on the cell volume and cytoplasm undercooling was investigated. The changes in transmembrane potential were also investigated during freezing of mouse oocytes. The computer simulations showed that at the beginning of the freezing process (-1 degrees C), the fast growth of ice in the extracellular solution causes a sharp increase of the membrane potential. It is predicted that the change in membrane potential is substantial for almost all cooling rates. Estimations show that values as high as -90 mV may be reached during freezing. The hyperpolarization of the membrane may cause orientation of the dipoles within the membrane. For membrane proteins with 300 debye dipole moment, the theoretical prediction suggests that the percentage of dipoles aligned with the membrane potential increases from 16% at 0 degrees C prior to freezing to 58% at -8 degrees C after seeding of the external ice followed with a cooling at 120 degrees C/min.

Osmotic response of oocytes using a microscope diffusion chamber: a preliminary study comparing murine and human ova

The hydraulic conductivity (Lp) of the plasma membrane determines how cells respond to the stresses of dehydration encountered during cryopreservation. We have used a microscope diffusion chamber which allows for direct real-time observation of the dynamic osmotic response of individual cells in microvolume suspension to compare the Lp of murine and human unfertilized ova. In this system, the response of an individual cell to the induced osmotic imbalance is documented via a series of photomicrographs or videotape; from these data the Lp can be computed. Donated human preovulatory oocytes were compared with macroscopically normal human ova, 43 hr after insemination, which had failed to fertilize (Ff) and with murine ova collected 13 hr post human chorionic gonadotropin injection. The permeability coefficients were 0.65 +/- 0.43, 0.84 +/- 0.39, and 0.36 +/- 0.07 micron3/micron2/atm/min. The results suggest that it may be possible to use Ff ova for experiments to design suitable cryopreservation procedures.

Phenomena at the advancing ice-liquid interface: solutes, particles and biological cells

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of second-phase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well. A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium. The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0.05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities. The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells. The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization.(ABSTRACT TRUNCATED AT 400 WORDS)