Design of an Ice Recrystallization-Inhibiting Polyampholyte-Containing Graft Polymer for Inhibition of Protein Aggregation

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.

Enzyme-Triggered Polyelectrolyte Complex for Responsive Delivery of α-Helical Polypeptides to Optimize Antibacterial Therapy

Responsive nanomaterials hold significant promise in the treatment of bacterial infections by recognizing internal or external stimuli to achieve stimuli-responsive behavior. In this study, we present an enzyme-responsive polyelectrolyte complex micelles (PTPMN) with α-helical cationic polypeptide as a coacervate-core for the treatment of Escherichia coli (E. coli) infection. The complex was constructed through electrostatic interaction between cationic poly(glutamic acid) derivatives and phosphorylation-modified poly(ethylene glycol)-b-poly(tyrosine) (PEG-b-PPTyr) by directly dissolving them in aqueous solution. The cationic polypeptide adopted α-helical structure and demonstrated excellent broad-spectrum antibacterial activity against both Gram-negative and Gram-positive bacteria, with a minimum inhibitory concentration (MIC) as low as 12.5 μg mL-1 against E. coli. By complexing with anionic PEG-b-PPTyr, the obtained complex formed β-sheet structures and exhibited good biocompatibility and low hemolysis. When incubated in a bacterial environment, the complex cleaved its phosphate groups triggered by phosphatases secreted by bacteria, exposing the highly α-helical conformation and restoring its effective bactericidal ability. In vivo experiments confirmed accelerated healing in E. coli-infected wounds.

Advances in Cryopreservatives: Exploring Safer Alternatives

Cryopreservation of cells, tissues, and organs is widely used in the biomedical and research world. There are different cryopreservatives that are used for this process; however, many of them, such as DMSO, are used despite the problems they present, mainly due to the toxicity it presents to certain types of samples. The aim of this Review is to highlight the different types of substances used in the cryopreservation process. It has been shown that some of these substances are well-known, as in the case of the families of alcohols, sugars, sulfoxides, etc. However, in recent years, other compounds have appeared, such as ionic liquids, deep eutectic solvents, or certain polymers, which open the door to new cryopreservation methods and are also less toxic to frozen samples.

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.

Polyampholytes and Their Hydrophobic Derivatives as Excipients for Suppressing Protein Aggregation

Protein aggregation, which occurs under various physiological conditions, can affect cell function and is a major issue in the field of protein therapeutics. In this study, we developed a polyampholyte composed of ε-poly-l-lysine and succinic anhydride and evaluated its protein protection efficacy. This polymer was able to protect different proteins from thermal stress and its performance significantly exceeded that of previously reported zwitterionic polymers. In addition, we synthesized derivatives with varying degrees of hydrophobicity, which exhibited remarkably enhanced efficiency; thus, the polymer concentration required for protein protection was very low. By facilitating the retention of protein enzymatic activity and stabilizing the higher-order structure, these polymers enabled the protein to maintain its native state, even after being subjected to extreme thermal stress. Thus, such polyampholytes are extremely effective in protecting proteins from extreme stress and may find applications in protein biopharmaceuticals and drug delivery systems.

Design of self-assembled glycopolymeric zwitterionic micelles as removable protein stabilizing agents

Developing stabilizers that protect proteins from denaturation under stress, and are easy to remove from solutions, is a challenge in protein therapeutics. In this study, micelles made of trehalose, a zwitterionic polymer (poly-sulfobetaine; poly-SPB), and polycaprolactone (PCL) were synthesized by a one-pot reversible addition-fragmentation chain-transfer (RAFT) polymerization reaction. The micelles protect lactate dehydrogenase (LDH) and human insulin from denaturation due to stresses like thermal incubation and freezing, and help them retain higher-order structures. Importantly, the protected proteins are readily isolated from the micelles by ultracentrifugation, with over 90% recovery, and almost all enzymatic activity is retained. This suggests the great potential of poly-SPB-based micelles for use in applications requiring protection and removal as required. The micelles may also be used to effectively stabilize protein-based vaccines and drugs.

Polyampholyte-Based Polymer Hydrogels for the Long-Term Storage, Protection and Delivery of Therapeutic Proteins

Protein storage and delivery are crucial for biomedical applications such as protein therapeutics and recombinant proteins. Lack of proper protocols results in the denaturation of proteins, rendering them inactive and manifesting undesired side effects. In this study, polyampholyte-based (succinylated ε-poly-l-lysine) hydrogels containing polyvinyl alcohol and polyethylene glycol polymer matrices to stabilize proteins are developed. These hydrogels facilitated the loading and release of therapeutic amounts of proteins and withstood thermal and freezing stress (15 freeze-thaw cycles and temperatures of -80 °C and 37 °C), without resulting in protein denaturation and aggregation. To the best of our knowledge, this strategy has not been applied to the design of hydrogels constituting polymers, (in particular, polyampholyte-based polymers) which have inherent efficiency to stabilize proteins and protect them from denaturation. Our findings can open up new avenues in protein biopharmaceutics for the design of materials that can store therapeutic proteins long-term under severe stress and safely deliver them.

Synthesis of Poly(2-(methylsulfinyl)ethyl methacrylate) via Oxidation of Poly(2-(methylthio)ethyl methacrylate): Evaluation of the Sulfoxide Side Chain on Cryopreservation

Conventional cryopreservation solutions rely on the addition of organic solvents such as DMSO or glycerol, but these do not give full recovery for all cell types, and innovative cryoprotectants may address damage pathways which these solvents do not protect against. Macromolecular cryoprotectants are emerging, but there is a need to understand their structure-property relationships and mechanisms of action. Here we synthesized and investigated the cryoprotective behavior of sulfoxide (i.e., "DMSO-like") side-chain polymers, which have been reported to be cryoprotective using poly(ethylene glycol)-based polymers. We also wanted to determine if the polarized sulfoxide bond (S⁺O^- character) introduces cryoprotective effects, as this has been seen for mixed-charge cryoprotective polyampholytes, whose mechanism of action is not yet understood. Poly(2-(methylsulfinyl)ethyl methacrylate) was synthesized by RAFT polymerization of 2-(methylthio)ethyl methacrylate and subsequent oxidation to sulfoxide. A corresponding N-oxide polymer was also prepared and characterized: (poly(2-(dimethylamineoxide)ethyl methacrylate). Ice recrystallization inhibition assays and differential scanning calorimetry analysis show that the sulfoxide side chains do not modulate the frozen components during cryopreservation. In cytotoxicity assays, it was found that long-term (24 h) exposure of the polymers was not tolerated by cells, but shorter (30 min) incubation times, which are relevant for cryopreservation, were tolerated. It was also observed that overoxidation to the sulfone significantly increased the cytotoxicity, and hence, these materials require a precision oxidation step to be deployed. In suspension cell cryopreservation investigations, the polysulfoxides did not increase cell recovery 24 h post-thaw. These results show that unlike hydrophilic backboned polysulfides, which can aid cryopreservation, the installation of the sulfoxide group onto a polymer does not necessarily bring cryoprotective properties, highlighting the challenges of developing and discovering macromolecular cryoprotectants.

Polysaccharide-Derived Ice Recrystallization Inhibitors with a Modular Design: The Case of Dextran-Based Graft Polymers

Ice recrystallization inhibitors inspired from antifreeze proteins (AFPs) are receiving increasing interest for cryobiology and other extreme environment applications. Here, we present a modular strategy to develop polysaccharide-derived biomimetics, and detailed studies were performed in the case of dextran. Poly(vinyl alcohol) (PVA) which has been termed as one of the most potent biomimetics of AFPs was grafted onto dextran via thiol-ene click chemistry (Dex-g-PVA). This demonstrated that Dex-g-PVA is effective in IRI and its activity increases with the degree of polymerization (DP) (sizes of ice crystals were 18.846 ± 1.759 and 9.700 ± 1.920 μm with DPs of 30 and 80, respectively) and fraction of PVA. By means of the dynamic ice shaping (DIS) assay, Dex-g-PVA is found to engage on the ice crystal surfaces, thus the ice affinity accounts for their IRI activity. In addition, Dex- g-PVA displayed enhanced IRI activity compared to that of equivalent PVA alone. We speculate that the hydrophilic nature of dextran would derive PVA in a stretch conformation that favors ice binding. The modular design can not only offer polysaccharides IRI activity but also favor the ice-binding behavior of PVA.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.