Insights into Design of Biomimetic Glycerol-Grafted Polyol-Based Polymers for Ice Nucleation/Recrystallization Inhibition and Thermal Hysteresis Activity

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.

Poly(l-alanine-co-l-threonine succinate) as a Biomimetic Cryoprotectant

We synthesized a series of [(l-Ala)x-co-(l-Thr succinate)y] (PATs), which are analogous to natural antifreezing glycoprotein with the structure of [l-Ala-l-Ala-l-Thr disaccharide]n, by varying the composition and degree of succinylation while fixing their molecular weight (Mn) and Ala/Thr ratio at approximately 10-12 kDa and 2:1, respectively. We investigated their ice recrystallization inhibition (IRI), ice nucleation inhibition (INI), dynamic ice shaping (DIS), thermal hysteresis (TH), and protein cryopreservation activities. Both IRI and INI activities were greater for PATs with higher l-Ala content (PATs-3 and PATs-4) than those with lower l-Ala content (PATs-1 and PATs-2). DIS activity with faceted crystal growth was clearly observed in PATs-2 and PATs-4 with a high degree of succinylation. TH was small with <0.1 °C for all PATs and slightly greater for PATs with a high l-Ala content. Except for PATs-1, the protein (lactate dehydrogenase, LDH) stabilization activity was excellent for all PATs studied, maintaining LDH activity as high as that of fresh LDH even after 15 freeze-thaw cycles. To conclude, the cryo-active biomimetic PATs were synthesized by controlling the l-Ala content and degree of succinylation. Our results showed that PATs with an l-Ala content of 65-70% and degree of succinylation of 12-19% exhibited the cryo-activities of IRI, INI, and DIS, and particularly promising properties for the cryoprotection of LDH protein.

Engineered Compounds to Control Ice Nucleation and Recrystallization

One of the greatest concerns in the subzero storage of cells, tissues, and organs is the ability to control the nucleation or recrystallization of ice. In nature, evidence of these processes, which aid in sustaining internal temperatures below the physiologic freezing point for extended periods of time, is apparent in freeze-avoidant and freeze-tolerant organisms. After decades of studying these proteins, we now have easily accessible compounds and materials capable of recapitulating the mechanisms seen in nature for biopreser-vation applications. The output from this burgeoning area of research can interact synergistically with other novel developments in the field of cryobiology, making it an opportune time for a review on this topic.

Ice Nucleation Promotion Impact on the Ice Recrystallization Inhibition Activity of Polyols

Heterogeneous ice nucleation occurs vis-à-vis nucleating agents already present in solution yet can occur within a rather broad range of temperatures (0 to ca. -38 °C). Controlling this temperature and the subsequent growth of resulting ice crystals is crucial for the survival of biological organisms (certain insects, fish, and plants that endure subzero temperatures), as well as in the context of medical cryopreservation and food science. In these environments, uncontrolled crystal shape and size can rupture the cell membrane causing irreversible and catastrophic damage. Antifreeze (AF) proteins and synthetic AF analogs address this issue to restrict crystal growth and to shape ice crystals. Yet, if the nucleation temperature is not controlled and occurs in a lower temperature range, nascent ice crystals will have grown to a significantly larger size before the AF agents can be active on their surface to halt or slow the Ostwald ripening process during recrystallization. At a higher nucleation temperature, diffusion of AF macromolecules is enhanced, and dynamic crystal shaping can start earlier, producing smaller crystals overall. While antifreeze proteins, the inspiration for these synthetic analogs, are always applied in a salt buffer aqueous environment (most typically phosphate-buffered saline (PBS) buffer), the heterogeneous nucleation events are stochastic and occur within a wide temperature range. Silver iodide (AgI), however, is a highly effective ice nucleation promoter as its crystal lattice structure is a 98% lattice match to the basal plane of hexagonal ice (Ih) crystals acting as a template for water molecule orientation and decreasing the interfacial free energy. Here, we expose the advantage of purposely seeding such nascent ice crystals with AgI at a defined and higher temperature (-7 °C) in ultrapure water (UPW) such that nucleation can only come from AgI (and also in AgI/PBS), resulting in the most potent synthetic IRI observed to date (at concentrations as low as 0.001 mg·mL^-1).

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.

Poly(l-Ala-co-l-Lys) Exhibits Excellent Ice Recrystallization Inhibition Activity

The control of ice recrystallization is very important in cryo-technological fields such as the food industry, biopharmaceuticals, and cell storage. Ice recrystallization inhibition (IRI) compounds are therefore designed to limit the growth of ice crystals, decrease the crystal size, and control the crystal shape. To improve the IRI activity of cryo-systems, various synthetic polymers such as biomimetic polypeptides from polar fish, facially amphiphilic polymers, polyampholytes, poly(vinyl alcohol) derivatives, and block copolymers with hydrophilic-hydrophobic balance have been developed. Except for graphene oxide, poly(vinyl alcohol) has thus far exhibited the best performance among these polymers. Herein, poly(l-alanine-co-l-lysine) (PAK) was shown to exhibit a similar IRI activity to that of poly(vinyl alcohol). Moreover, in contrast to the needle-shaped ice crystals generated by the aqueous PVA solution, the PAK solution was shown to generate cubic-to-spherical shaped ice crystals. Furthermore, neither poly(l-alanine-co-l-aspartic acid) (PAD) nor poly(ethylene glycol) (PEG) with a similar molecular weight provided any significant IRI activity. Examination by FTIR and circular dichroism spectroscopies indicated that the PAK forms α-helices, whereas the PAD forms random coils in water. Further, a dynamic ice shaping study suggested that PAK strongly interacts with ice crystals, whereas PAD and PEG only weakly interact. These results suggest that PAK is an important compound with superior IRI activity and that this activity is dependent upon the functional groups and secondary structure of the polypeptides.

Polymer Colloids: Synthesis Fundamentals to Applications

This special issue of Biomacromolecules highlights research from The International Polymer Colloid Group (IPCG), which was founded in 1972 as a forum for the exchange of ideas and emerging research activities for scientists and engineers from both academia and industry who study or use polymer colloids. The increasing relevance of polymeric structures with colloidal dimensions to biomacromolecules research provided the impetus for organizing this special issue. The IPCG is composed of over 120 researchers from over 20 countries who are elected to membership. Activities comprise annual symposia including a biennial International Polymer Colloid Group Research Conference and a semiannual newsletter that incorporates a summary of recent (including unpublished) research results from our members.

High Sub-Zero Organ Preservation: A Paradigm of Nature-Inspired Strategies

The field of organ preservation is filled with advancements that have yet to see widespread clinical translation, with some of the more notable strategies deriving their inspiration from nature. While static cold storage (SCS) at 2 °C to 4 °C is the current state-of-the-art, it contributes to the current shortage of transplantable organs due to the limited preservation times it affords combined with the limited ability of marginal grafts (i.e. those at risk for post-transplant dysfunction or primary non-function) to tolerate SCS. The era of storage solution optimization to minimize SCS-induced hypothermic injury has plateaued in its improvements, resulting in a shift towards the use of machine perfusion systems to oxygenate organs at normothermic, sub-normothermic, or hypothermic temperatures, as well as the use of sub-zero storage temperatures to leverage the protection brought forth by a reduction in metabolic demand. Many of the rigors that organs are subjected to at low sub-zero temperatures (-80 °C to -196 °C) commonly used for mammalian cell preservation have yet to be surmounted. Therefore, this article focuses on an intermediate temperature range (0 °C to -20 °C), where much success has been seen in the past two decades. The mechanisms leveraged by organisms capable of withstanding prolonged periods at these temperatures through either avoiding or tolerating the formation of ice has provided a foundation for some of the more promising efforts. This article therefore aims to contextualize the translation of these strategies into the realm of mammalian organ preservation.