Modeling and optimization of cryopreservation

Meta-analysis of the Boyle van 't Hoff relation: Turgor and leak models explain non-ideal volume equilibrium

There has been much recent attention paid to the interaction of cell volume, its regulation, and the molecular biology of the cell. Cells are generally assumed to behave as linear osmometers, with their water volume linearly proportionate to the inverse of osmotic pressure as described by the Boyle van 't Hoff (BvH) relation. This study evaluates the generality of this and other long-standing assumptions about cell responses to anisotonic conditions. We present alternative models that account for osmoregulation including mechanical resistance to volumetric expansion (the turgor model) and ion-osmolyte leakage (the leak model). To evaluate the generality of the BvH relation and determine the suitability of alternative models, we performed a comprehensive survey of the literature and a careful analysis of the resulting data, and then we used these data to compare among models. We identified 137 articles published from 1964 to 2019 spanning 14 animal species and 26 cell types and determined the BvH relation is not an appropriate general model but is adequate when restricted to the hypertonic region. In contrast, models that account for either mechanical resistance or ion-osmolyte leakage fit well to almost all collected data. The leak model has fitted parameters that are in the same range as the current literature estimate, while the turgor model typically requires an elastic modulus value of one or multiple orders of magnitude larger than literature values. However, confirmation of the underlying mechanism of osmotic regulation is required at the cell-specific level and cannot be assumed a priori.

Rational synthesis of total damage during cryoprotectant equilibration: modelling and experimental validation of osmomechanical, temperature, and cytotoxic damage in sea urchin (Paracentrotus lividus) oocytes

Sea urchins (e.g., Paracentrotus lividus) are important for both aquaculture and as model species. Despite their importance, biobanking of urchin oocytes by cryopreservation is currently not possible. Optimized cryoprotectant loading may enable novel vitrification methods and thus successful cryopreservation of oocytes. One method for determining an optimized loading protocol uses membrane characteristics and models of damage, namely osmomechanical damage, temperature damage (e.g., chill injury) and cytotoxicity. Here we present and experimentally evaluate existing and novel models of these damage modalities as a function of time and temperature. In osmomechanical damage experiments, oocytes were exposed for 2 to 30 minutes in hypertonic NaCl or sucrose supplemented seawater or in hypotonic diluted seawater. In temperature damage experiments, oocytes were exposed to 1.7 °C, 10 °C, or 20 °C for 2 to 90 minutes. Cytotoxicity was investigated by exposing oocytes to solutions of Me2SO for 2 to 30 minutes. We identified a time-dependent osmotic damage model, a temperature-dependent damage model, and a temperature and time-dependent cytotoxicity model. We combined these models to estimate total damage during a cryoprotectant loading protocol and determined the optimal loading protocol for any given goal intracellular cryoprotectant concentration. Given our fitted models, we find sea urchin oocytes can only be loaded to 13% Me2SO v/v with about 50% survival. This synthesis of multiple damage modalities is the first of its kind and enables a novel approach to modelling cryoprotectant equilibration survival for cells in general.

Dimethyl sulfoxide: a central player since the dawn of cryobiology, is efficacy balanced by toxicity?

Dimethyl sulfoxide (DMSO) is the cryoprotectant of choice for most animal cell systems since the early history of cryopreservation. It has been used for decades in many thousands of cell transplants. These treatments would not have taken place without suitable sources of DMSO that enabled stable and safe storage of bone marrow and blood cells until needed for transfusion. Nevertheless, its effects on cell biology and apparent toxicity in patients have been an ongoing topic of debate, driving the search for less cytotoxic cryoprotectants. This review seeks to place the toxicity of DMSO in context of its effectiveness. It will also consider means of reducing its toxic effects, the alternatives to its use and their readiness for active use in clinical settings.

Factors Affecting the Membrane Permeability Barrier Function of Cells during Preservation Technologies

Cellular membranes are exposed to extreme conditions during the processing steps involved in cryopreservation (and freeze-drying) of cells. The first processing step involves adding protective agents. Exposing cells to protective agents causes fluxes of both water and solutes (i.e., permeating cryoprotective agents) across the cellular membrane, resulting in cell volume changes and possibly osmotic stress. In addition, protective molecules may interact with lipids, which may lead to membrane structural changes and permeabilization. After loading with protective agents, subsequent freezing exposes cells to severe osmotic and mechanical stresses, caused by extra and/or intracellular ice formation and a drastically increased solute concentration in the unfrozen fraction. Furthermore, cellular membranes undergo thermotropic and lyotropic phase transitions during cooling and freezing, which drastically alter the membrane permeability and its barrier function. In this article, it is shown that membrane permeability to water and solutes is dependent on the temperature, medium osmolality, types of solutes present, cell hydration level, and absence or presence of ice. Freezing most drastically alters the membrane permeability barrier function, which is reflected as a change in the activation energy for water transport. In addition, membranes become temporarily leaky during freezing-induced fluid-to-gel membrane phase transitions, resulting in the uptake of impermeable solutes.

Concentration dependence of the cell membrane permeability to cryoprotectant and water and implications for design of methods for post-thaw washing of human erythrocytes

For more than fifty years the human red blood cell (RBC) has been a widely studied model for transmembrane mass transport. Existing literature spans myriad experimental designs with varying results and physiologic interpretations. In this review, we examine the kinetics and mechanisms of membrane transport in the context of RBC cryopreservation. We include a discussion of the pathways for water and glycerol permeation through the cell membrane and the implications for mathematical modeling of the membrane transport process. In particular, we examine the concentration dependence of water and glycerol transport and provide equations for estimating permeability parameters as a function of concentration based on a synthesis of literature data. This concentration-dependent transport model may allow for design of improved methods for post-thaw removal of glycerol from cryopreserved blood. More broadly, the consideration of the concentration dependence of membrane permeability parameters may be important for other cell types as well, especially for design of methods for equilibration with the highly concentrated solutions used for vitrification.