Monica C. Wusteman

Vitrification media: toxicity, permeability, and dielectric properties ☆

The aim of this study was to select a cryoprotectant for use in attempts to preserve tissues and organs by vitrification. The first step was to select a cell line with which to compare the toxicity of a range of commonly used cryoprotectants. An immortal vascular endothelial cell (ECV304) was exposed to vitrifying concentrations of four cryoprotectants: dimethyl sulfoxide (Me(2)SO; 45% w/w); 2,3 butanediol (BD; 32%); 1,2-propanediol (PD; 45%); and ethanediol (ED; 45%). Three times of exposure (1, 3, and 9 min) and two temperatures (22 and 2-4 degrees C) were studied. After removal of the cryoprotectant, the ability of the cells to adhere and divide in culture over a 2-day period was measured and expressed as a Cell Survival Index (CSI). There was no measurable loss of cells after exposure to the four cryoprotectants but 3-min exposure to BD, PD, or Me(2)SO at room temperature completely destroyed the ability of the cells to adhere and divide in culture. In contrast, exposure to all four cryoprotectants at 2-4 degrees C for up to 9 min permitted the retention of significant cell function, the CSIs, as a proportion of control, being 76.3+/-7.0% for BD, 63.6+/-7.1% for PD, 37.0+/-4.1 for Me(2)SO, and 33.2+/-3.0 for ED. The permeability properties of the cells for these four cryoprotectants was also measured at each temperature. Permeability to water was high, L(p) approximately equal 10(-7) cm/s/atm at 2-4 degrees C with all the cryoprotectants, but there were substantial differences in solute permeability: BD and PD were the most permeable at 2-4 degrees C (P(s)=4.1 and 3.0 x 10(-6) cm/s, respectively). Equilibration of intracellular cryoprotectant concentration was rapid, due in part to high water permeability; the cells were approximately 80% of their physiological volume after 10 min. Treatment at 2-4 degrees C with BD was the least damaging, but PD was not significantly worse. Exposure to vitrifying concentrations of ED and Me(2)SO, even at 2-4 degrees C, was severely damaging. Segments of rabbit carotid artery were treated with vitrifying concentrations of each of the two most favorable cryoprotectants, BD and PD, for 9 min. It was shown that each cryoprotectant reduced smooth muscle maximum contractility to a similar extent and abolished the acetylcholine response. However, vital staining revealed that exposure to BD also caused substantial damage to the endothelial lining, whereas the endothelium was completely intact after PD exposure, raising the possibility that the effect of PD on NO release may be reversible. In later stages of this project it is planned to use dielectric heating to rewarm the tissues and thereby avoid devitrification. The effects of each cryoprotectant on this mode of heating was therefore studied. Gelatin spheres containing vitrifiable concentrations of each cryoprotectant were rewarmed from -60 degrees C in a radiofrequency applicator. Because the uniformity of heating is related to the dielectric properties of the material, these properties were also measured. PD was the most suitable. These physical measurements, combined with the measurements of toxicity and permeability, indicate that PD is the most favorable cryoprotectant of those tested for use in subsequent stages of this study.

Electromagnetic re-warming of cryopreserved tissues: effect of choice of cryoprotectant and sample shape on uniformity of heating.

A method that has been proposed for the cryopreservation of tissues and organs is to add a cryoprotective agent (CPA) in sufficient concentration to allow vitrification, and to use rapid electromagnetic heating to prevent the formation of ice crystals during the re-warming. We have compared the physical and biological properties of four CPAs, measuring the speed and uniformity of heating in a 36 mm sphere placed in a 434 MHz applicator, and the toxicity to ECV304 endothelial cells. Ethanediol and dimethyl sulfoxide were found to be suitable for rapid, uniform heating but toxic to the endothelial cells at vitrifying concentrations. Butane-2,3-diol was less toxic, but the heating patterns were unacceptably non-uniform. Propane-1,2-diol was not significantly more toxic than butane-2,3-diol, and did allow uniform heating. It is therefore the best choice of CPA for the vitrification of tissues. We have shown that the uniformity of heating correlates with the dielectric properties of the perfusate. Furthermore, we have shown that uniform heating is feasible in non-spherical samples provided they are approximately ellipsoidal.

Differences in the Requirements for Cryopreservation of Porcine Aortic Smooth Muscle and Endothelial Cells

One of the basic requirements for the production of tissue-engineered constructs is an effective means of storing both the constructs and the cells that will be used to make them. This paper reports on the cryopreservation of porcine aortic smooth muscle and endothelial cells intended for the production of model vascular constructs. We first determined the cell volume, nonosmotic volume, and the permeability parameters for water and the cryoprotectant dimethyl sulfoxide (Me(2)SO) in these cells at 2-4 degrees and 22 degrees C. The following results were obtained: Table unavailable in HTML format. Using a cell culture assay, both cell types were shown to tolerate threefold changes in cell volume, in either direction, without significant injury. Although these data suggested that single-step methods for the introduction and removal of 10% w/w Me(2)SO should be effective, an additional mannitol dilution step was adopted in order to reduce the time required for removal of the Me(2)SO. Following cooling at 0.3, 1, or 10 degrees C/min and storage at less than -160 degrees C, the survival of porcine aortic smooth muscle cell suspensions, measured by a cell culture assay, was inversely related to cooling rate; at 0.3 degrees C/min, recovery was >80%. The survival rate for aortic endothelial cells was directly related to cooling rate over the range tested and was >80% at 10 degrees C/min.

Cold-induced injury to porcine corneal endothelial cells and its mediation by chelatable iron : Implications for corneal preservation

Purpose: During hypothermic storage of the cornea, corneal endothelial damage restricts storage times. We previously reported a new, iron-dependent mechanism of cold-induced injury to cultured liver cells. In this study, we sought to evaluate whether corneal endothelial cells incur a similar kind of injury. Methods: Cultured porcine corneal endothelial cells were exposed to 4°C in either cell culture medium, Krebs-Henseleit buffer, Optisol-GS solution, or McCarey-Kaufman medium for 5 hours to 14 days and then rewarmed under cell culture conditions (3 hours). The cultures were assessed for lethal cell injury (LDH release); cellular, nuclear, and mitochondrial morphologic alterations; lipid peroxidation; and mitochondrial membrane potential. Results: Corneal endothelial cells sustained substantial injury following cold storage and rewarming in cell culture medium (47% ± 8% and 64% ± 20% cell death after 2 and 5 days cold storage, respectively). The injury displayed some apoptotic features, and cells lost mitochondrial membrane potential before cell death occurred. The iron chelators deferoxamine, 1,10-phenanthroline, and 2,2′-dipyridyl and the antioxidant butylated hydroxytoluene completely inhibited this cell injury. Marked iron-dependent cell injury and lipid peroxidation also occurred during and after cold incubation in Krebs-Henseleit buffer and, most importantly, iron-dependent cell injury was also observed after cold incubation in Optisol solution and in McCarey-Kaufman medium. Conclusions: Cultured porcine corneal endothelial cells incur a strong iron-dependent injury elicited by hypothermia. This cold-induced injury might provide an explanation for the known corneal endothelial susceptibility to hypothermic preservation injury, which thus might be amenable to therapeutic interventions (ie, by iron chelators).

Vitrification of ECV304 cell suspensions using solutions containing propane-1,2-diol and trehalose.

In this paper, we report on the suitability of solutions containing propane-1,2-diol (propylene glycol, PD), sugars, and salts for the vitrification of the human cell line, ECV304. Cooling (at 10 degrees C/min) and rewarming (at 80 degrees C/min) were at rates that are practicable for the tissues to be studied later. Under these conditions, 45% PD in phosphate-buffered saline (PBS) sometimes froze during cooling and always devitrified during rewarming but both events were avoided if the PBS salts were replaced by an osmotically equivalent concentration of sucrose or trehalose. The effect of such solutions on cells was evaluated using a cell culture assay in which the number of cells recovered after 3 days of culture was divided by the number cells plated, giving a cell multiplication factor or CMF. In the absence of PD the cells tolerated a low-salt concentration in solutions that were made isotonic with sugars, but they recovered poorly when 45% PD was also present. Trehalose gave significantly better recovery than sucrose. When 39% PD and 15% trehalose were included in a low-salt vehicle solution (LSV) that contained approximately 5% of the total salt concentration of PBS (this solution was designated LSV/39/15), the cells exhibited approximately 40% of untreated control CMF following exposure for 9min. LSV/39/15 vitrifies with a glass transition temperature of -102 degrees C, does not devitrify when warmed at 80 degrees C/min, and has suitable dielectric properties for uniform and rapid dielectric heating. An improved method for adding and removing LSV/39/15 gave a CMF of approximately 55% of untreated controls. Using this method, 1.0ml suspensions of ECV304 cells was cooled to, and stored briefly at, -120 degrees C and then rewarmed by immersion in a 37 degrees C water bath ( approximately 75 degrees C/min). The CMF of the cooled samples was similar to that of the exposure-only controls, approximately 50% of the untreated control CMF in both cases.

The stability during low-temperature storage of an antifreeze protein isolated from the roots of cold-acclimated carrots

Natural antifreeze proteins (AFPs) not only inhibit freezing at high subzero temperatures; they have the additional properties of inhibiting the recrystallization of ice during warming and of preventing devitrification. The natural AFP that occurs in the roots of cold-acclimated carrots can be extracted reasonably simply and is non-toxic: it was selected for study as a possible ingredient of the vitrification mixtures that are being developed for use in tissue cryopreservation. For this application, it would be essential for the AFP to remain active during prolonged storage at very low temperatures. For logistic reasons, it would also be essential to have an effective method of storage of the purified AFP itself. In this study, carrot AFP was isolated and purified, and its ability to inhibit recrystallization was monitored over 40 weeks of storage at -80 or -196 degrees C. The data revealed a progressive decrease in activity during storage, reaching half the original activity in 10-20 weeks and only 2-3% of the original activity at 40 week. These data suggest that carrot AFP will not be effective in tissue cryopreservation.