Enhancement of Macromolecular Ice Recrystallization Inhibition Activity by Exploiting Depletion Forces

Development of Macromolecular Cryoprotectants for Cryopreservation of Cells

Cryopreservation is a common way for long-term storage of therapeutical proteins, erythrocytes, and mammalian cells. For cryoprotection of these biosamples to keep their structural integrity and biological activities, it is essential to incorporate highly efficient cryoprotectants. Currently, permeable small molecular cryoprotectants such as glycerol and dimethyl sulfoxide dominate in cryostorage applications, but they are harmful to cells and human health. As acting in the extracellular space, membrane-impermeable macromolecular cryoprotectants, which exert remarkable membrane stabilization against cryo-injury and are easily removed post-thaw, are promising candidates with biocompatibility and feasibility. Water-soluble hydroxyl-containing polymers such as poly(vinyl alcohol) and polyol-based polymers are potent ice recrystallization inhibitors, while polyampholytes, polyzwitterions, and bio-inspired (glyco)polypeptides can significantly increase post-thaw recovery with reduced membrane damages. In this review, the synthetic macromolecular cryoprotectants are systematically summarized based on their synthesis routes, practical utilities, and cryoprotective mechanisms. It provides a valuable insight in development of highly efficient macromolecular cryoprotectants with valid ice recrystallization inhibition activity for highly efficient and safe cryopreservation of cells.

De novo designed ice-binding proteins from twist-constrained helices

Attaining molecular-level control over solidification processes is a crucial aspect of materials science. To control ice formation, organisms have evolved bewildering arrays of ice-binding proteins (IBPs), but these have poorly understood structure-activity relationships. We propose that reverse engineering using de novo computational protein design can shed light on structure-activity relationships of IBPs. We hypothesized that the model alpha-helical winter flounder antifreeze protein uses an unusual undertwisting of its alpha-helix to align its putative ice-binding threonine residues in exactly the same direction. We test this hypothesis by designing a series of straight three-helix bundles with an ice-binding helix projecting threonines and two supporting helices constraining the twist of the ice-binding helix. Our findings show that ice-recrystallization inhibition by the designed proteins increases with the degree of designed undertwisting, thus validating our hypothesis, and opening up avenues for the computational design of IBPs.

Synthesis of poly(vinyl alcohol) by blue light bismuth oxide photocatalysed RAFT. Evaluation of the impact of freeze/thaw cycling on ice recrystallisation inhibition

Poly(vinyl alcohol), PVA, is the most potent polymeric ice recrystallisation inhibitor (IRI), mimicking a complex function of ice binding proteins. The IRI activity of PVA scales with its molecular weight and hence broad molecular weight distributions in free radical-derived PVAs lead to activity measurements dominated by small amounts of heavier fractions. Well-defined PVA can be prepared by thermally initiated RAFT/MADIX polymerization using xanthates by the polymerization of the less activated monomer vinyl acetate. The low conversions and molecular weights obtained during this approach, often requires feeding of additional initiator and bulk polymerization. Here we employ bismuth oxide photo-RAFT in solution, using blue light (450 nm), rather than previously reported white light, to obtain a library of PVA's. The use of blue light enabled quantitative conversion and acceptable dispersities. Purple light (380 nm) was also used, but asymmetric molecular weight distributions were obtained in some cases. High concentrations of high molecular weight PVA is known to form cryogels during freeze/thaw which has led to speculation this might limit the use of PVA in environments where the temperature cycles e.g. the construction industry. After 4 freeze/thaw cycles there was only small changes in observable IRI for all synthesised PVAs and two commercial standards. In an extended test, activity was retained after 100 freeze/thaw cycles, mitigating concerns that PVA could not be used in situations where freeze/thaw cycles occur. This work presents a convenient method to obtain well-defined PVAs for cryoscience studies compared to conventional thermal-RAFT and indicates that cryogelation concerns may not prevent their use.

Tricine as a Novel Cryoprotectant with Osmotic Regulation, Ice Recrystallization Inhibition and Antioxidant Properties for Cryopreservation of Red Blood Cells

The cryopreservation of red blood cells (RBCs) plays a key role in blood transfusion therapy. Traditional cryoprotectants (CPAs) are mostly organic solvents and may cause side effects to RBCs, such as hemolysis and membrane damage. Therefore, it is necessary to find CPAs with a better performance and lower toxicity. Herein, we report for the first time that N-[Tri(hydroxymethyl)methyl]glycine (tricine) showed a great potential in the cryopreservation of sheep RBCs. The addition of tricine significantly increased the thawed RBCs’ recovery from 19.5 ± 1.8% to 81.2 ± 8.5%. The properties of thawed RBCs were also maintained normally. Through mathematical modeling analysis, tricine showed a great efficiency in cryopreservation. We found that tricine had a good osmotic regulation capacity, which could mitigate the dehydration of RBCs during cryopreservation. In addition, tricine inhibited ice recrystallization, thereby decreasing the mechanical damage from ice. Tricine could also reduce oxidative damage during freezing and thawing by scavenging reactive oxygen species (ROS) and maintaining the activities of endogenous antioxidant enzymes. This work is expected to open up a new path for the study of novel CPAs and promote the development of cryopreservation of RBCs.

Preparation of Poly(vinyl Alcohol) Microparticles for Freeze Protection of Sensitive Fruit Crops

Poly(vinyl alcohol) (PVA) displays ice recrystallization inhibition (IRI) properties as many antifreeze proteins found in cold tolerant organisms. The molecular architecture and composition (molecular weight and distribution of pendant OH and acetate groups) have been studied to improve the antifreezing properties of PVA, suggesting that the molecular architecture of PVA plays an important role in IRI activity. The present work deals with the preparation of PVA microparticles using an alkaline treatment. The effect of PVA molecular weight on the morphology and antifreezeing properties of PVA microparticles was investigated. The antifreezeing property of PVA microparticles on the susceptibility of flower bud tissues to freeze damage was also evaluated. The alkaline treatment of an aqueous PVA solution produced stable polymer chain aggregates with spherical shapes. The average size of the PVA microparticles increased significantly with the increasing molecular weight of the PVA macromolecule precursor. The PVA microparticles inhibited the growth of ice crystals and blocked ice growth at concentrations as low as 0.01 % w/v. The effect of impeding ice crystal growth by preventing the joining of adjacent ice crystals is attributed to the larger size of the PVA particles adsorbed on the ice surface compared to the aggregated PVA macromolecules in saline solution. The thermal hysteresis activity of PVA macromolecules and microparticles was not detected by differential scanning calorimetry analysis. The PVA microparticles reduced the incidence of freeze injuries in flower bud tissues by 55% and their application, considering the low toxicity of PVA, has a high potential for freeze protection in fruit crops.

Bioinspired Ice-Binding Materials for Tissue and Organ Cryopreservation

Cryopreservation of tissues and organs can bring transformative changes to medicine and medical science. In the past decades, limited progress has been achieved, although cryopreservation of tissues and organs has long been intensively pursued. One key reason is that the cryoprotective agents (CPAs) currently used for cell cryopreservation cannot effectively preserve tissues and organs because of their cytotoxicity and tissue destructive effect as well as the low efficiency in controlling ice formation. In stark contrast, nature has its unique ways of controlling ice formation, and many living organisms can effectively prevent freezing damage. Ice-binding proteins (IBPs) are regarded as the essential materials identified in these living organisms for regulating ice nucleation and growth. Note that controversial results have been reported on the utilization of IBPs and their mimics for the cryopreservation of tissues and organs, that is, some groups revealed that IBPs and mimics exhibited unique superiorities in tissues cryopreservation, while other groups showed detrimental effects. In this perspective, we analyze possible reasons for the controversy and predict future research directions in the design and construction of IBP inspired ice-binding materials to be used as new CPAs for tissue cryopreservation after briefly introducing the cryo-injuries and the challenges of conventional CPAs in the cryopreservation of tissues and organs.

Structural diversity of marine anti-freezing proteins, properties and potential applications: a review

Cold-adapted organisms, such as fishes, insects, plants and bacteria produce a group of proteins known as antifreeze proteins (AFPs). The specific functions of AFPs, including thermal hysteresis (TH), ice recrystallization inhibition (IRI), dynamic ice shaping (DIS) and interaction with membranes, attracted significant interest for their incorporation into commercial products. AFPs represent their effects by lowering the water freezing point as well as preventing the growth of ice crystals and recrystallization during frozen storage. The potential of AFPs to modify ice growth results in ice crystal stabilizing over a defined temperature range and inhibiting ice recrystallization, which could minimize drip loss during thawing, improve the quality and increase the shelf-life of frozen products. Most cryopreservation studies using marine-derived AFPs have shown that the addition of AFPs can increase post-thaw viability. Nevertheless, the reduced availability of bulk proteins and the need of biotechnological techniques for industrial production, limit the possible usage in foods. Despite all these drawbacks, relatively small concentrations are enough to show activity, which suggests AFPs as potential food additives in the future. The present work aims to review the results of numerous investigations on marine-derived AFPs and discuss their structure, function, physicochemical properties, purification and potential applications.

Macromolecular cryoprotectants for the preservation of mammalian cell culture: lessons from crowding, overview and perspectives

Cryopreservation is a process used for the storage of mammalian cells at a very low temperature, in a state of 'suspended animation.' Highly effective and safe macromolecular cryoprotectants (CPAs) have gained significant attention as they obviate the toxicity of conventional CPAs like dimethyl sulfoxide (DMSO) and reduce the risks involved in the storage of cultures at liquid nitrogen temperatures. These agents provide cryoprotection through multiple mechanisms, involving extracellular and intracellular macromolecular crowding, thereby impacting the biophysical and biochemical dynamics of the freezing medium and the cryopreserved cells. These CPAs vary in their structures and physicochemical properties, which influence their cryoprotective activities. Moreover, the introduction of polymeric crowders in the cryopreservation media enables serum-free storage at low-DMSO concentrations and high-temperature vitrification of frozen cultures (-80 °C). This review highlights the need for macromolecular CPAs and describes their mechanisms of cryopreservation, by elucidating the role of crowding effects. It also classifies the macromolecules based on their chemistry and their structure-activity relationships. Furthermore, this article provides perspectives on the factors that may influence the outcomes of the cell freezing process or may help in designing and evaluating prospective macromolecules. This manuscript also includes case studies about cellular investigations that have been conducted to demonstrate the cryoprotective potential of macromolecular CPAs. Ultimately, this review provides essential directives that will further improve the cell cryopreservation process and may encourage the use of macromolecular CPAs to fortify basic, applied, and translational research.

Exploring the application and mechanism of sodium hyaluronate in cryopreservation of red blood cells

The cryopreservation of red blood cells (RBCs) is essential for transfusion therapy and maintaining the inventory of RBCs units. The existing cryoprotectants (CPAs) have many defects, and the search for novel CPAs is becoming a research hotspot. Sodium hyaluronate (SH) is polymerized from sodium glucuronate and N-acetylglucosamine, which has good water binding capacity and biocompatibility. Herein, we reported for the first time that under the action of medium molecular weight sodium hyaluronate (MSH), the thawed RBCs recovery increased from 33.1 ​± ​5.8% to 63.2 ​± ​3.5%. In addition, RBCs functions and properties were maintained normally, and the residual MSH could be removed by direct washing. When MSH was used with a very low concentration (5% v/v) of glycerol (Gly), the thawed RBCs recovery could be increased to 92.3 ​± ​4.6%. In general, 40% v/v Gly was required to achieve similar efficiency. A mathematical model was used to compare the performance of MSH, PVA and trehalose in cryopreservation, and MSH showed the best efficiency. It was found that MSH could periodically regulate the content of intracellular water through the “reservoir effect” to reduce the damages during freezing and thawing. Moreover, MSH could inhibit ice recrystallization when combined with RBCs. The high viscosity and strong water binding capacity of MSH was also conducive to reducing the content of ice. This works points out a new direction for cryopreservation of RBCs and may promote transfusion therapy in clinic.

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MSH improved the RBCs recovery in cryopreservation.

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MSH can be removed directly after thawing.

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The properties and functions of RBCs were protected by MSH.

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High RBCs recovery is found using MSH with 5% v/v glycerol.

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The mathematical model is studied for the cryopreservation.

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The mechanism is proposed for cryopreservation using MSH.

Synthesis, self-aggregation and cryopreservation effects of perylene bisimide-glycopeptide conjugates

Three perylene bisimide-glycopeptide conjugates (PBI-AFF-Man, PBI-AFF-Glu and PBI-AFF-Gal) were synthesized, which showed moderate activity in the control of ice crystal growth. Furthermore, the cellular cryopreservation effects of PBI-AFF-Man, PBI-AFF-Glu and PBI-AFF-Gal showed enhancements in cell viabilities, especially for PBI-AFF-Glu with values of 22.77 ± 3.33% (HeLa cells), 19.43 ± 1.90% (A549 cells) and 16.63 ± 1.76% (GES-1 cells) at a dose of 1.0 mg mL^-1. This work will help guide the development of self-assembled cryoprotectants.

Bioinspired Crowding Inhibits Explosive Ice Growth in Antifreeze Protein Solutions

Antifreeze (glyco)proteins (AF(G)Ps) are naturally evolved ice inhibitors incomparable to any man-made materials, thus, they are gaining intensive interest for cryopreservation and beyond. AF(G)Ps depress the freezing temperature (T_f) noncolligatively below the melting temperature (T_m), generating a thermal hysteresis (TH) gap, within which the ice growth is arrested. However, the ice crystals have been reported to undergo a retaliatory and explosive growth beyond the TH gap, which is lethal to living organisms. Although intensive research has been carried to inhibit such an explosive ice growth, no satisfactory strategy has been discovered until now. Here, we report that crowded solutions mimicking an extracellular matrix (ECM), in which AF(G)Ps are located, can completely inhibit the explosive ice growth. The crowded solutions are the condensates of liquid-liquid phase separation consisting of polyethylene glycol (PEG) and sodium citrate (SC), which possess a nanoscale network and strong hydrogen bond (HB) forming ability, completely different to crowded solutions made of single components, that is, PEG or SC. Due to these unique features, the dynamics of the water is significantly slowed down, and the energy needed for breaking the HB between water molecules is distinctly increased; consequently, ice growth is inhibited as the rate of water molecules joining the ice is substantially reduced. The present work not only opens a new avenue for cryopreservation, but also suggests that the ECM of cold-hardy organisms, which also exhibit great water confining properties, may have a positive effect in protecting the living organisms from freezing damage.

The atomistic details of the ice recrystallisation inhibition activity of PVA

Understanding the ice recrystallisation inhibition (IRI) activity of antifreeze biomimetics is crucial to the development of the next generation of cryoprotectants. In this work, we bring together molecular dynamics simulations and quantitative experimental measurements to unravel the microscopic origins of the IRI activity of poly(vinyl)alcohol (PVA)-the most potent of biomimetic IRI agents. Contrary to the emerging consensus, we find that PVA does not require a "lattice matching" to ice in order to display IRI activity: instead, it is the effective volume of PVA and its contact area with the ice surface which dictates its IRI strength. We also find that entropic contributions may play a role in the ice-PVA interaction and we demonstrate that small block co-polymers (up to now thought to be IRI-inactive) might display significant IRI potential. This work clarifies the atomistic details of the IRI activity of PVA and provides novel guidelines for the rational design of cryoprotectants.