Osmolytes as Cryoprotectants under Salt Stress

Elucidating ATP's role as solubilizer of biomolecular aggregate

Proteins occurring in significantly high concentrations in cellular environments (over 100 mg/ml) and functioning in crowded cytoplasm, often face the prodigious challenges of aggregation which are the pathological hallmark of aging and are critically responsible for a wide spectrum of rising human diseases. Here, we combine a joint-venture of complementary wet-lab experiment and molecular simulation to discern the potential ability of adenosine triphosphate (ATP) as solubilizer of protein aggregates. We show that ATP prevents both condensation of aggregation-prone intrinsically disordered protein Aβ40 and promotes dissolution of preformed aggregates. Computer simulation links ATP's solubilizing role to its ability to modulate protein's structural plasticity by unwinding protein conformation. We show that ATP is positioned as a superior biological solubilizer of protein aggregates over traditional chemical hydrotropes, potentially holding promises in therapeutic interventions in protein-aggregation-related diseases. Going beyond its conventional activity as energy currency, the amphiphilic nature of ATP enables its protein-specific interaction that would enhance ATP's efficiency in cellular processes.

How Salt and Temperature Drive Reentrant Condensation of Aβ40

Within the framework of liquid-liquid phase separation (LLPS), biomolecular condensation orchestrates vital cellular processes, and its dysregulation is implicated in severe pathological conditions. Recent studies highlight the role of intrinsically disordered proteins (IDPs) in LLPS, yet the influence of microenvironmental factors has remained a puzzling factor. Here, via computational simulation of the impact of solution conditions on LLPS behavior of neurologically pathogenic IDP Aβ40, we chanced upon a salt-driven reentrant condensation phenomenon, wherein Aβ40 aggregation increases with low salt concentrations (25-50 mM), followed by a decline with further salt increments. An exploration of the thermodynamic and kinetic signatures of reentrant condensation unveils a nuanced interplay between protein electrostatics and ionic strength as potential drivers. Notably, the charged residues of the N-terminus exhibit a nonmonotonic response to salt screening, intricately linked to the recurrence of reentrant behavior in hydrophobic core-induced condensation. Intriguingly, our findings also unveil the reappearance of similar reentrant condensation phenomena under varying temperature conditions. Collectively, our study illuminates the profoundly context-dependent nature of Aβ40s liquid-liquid phase separation behavior, extending beyond its intrinsic molecular framework, where microenvironmental cues wield significant influence over its aberrant functionality.

Adenosine Triphosphate Inhibits Cold-Responsive Aggregation

Adenosine triphosphate (ATP), ubiquitous in all living organisms, is conventionally recognized as a fundamental energy currency essential for a myriad of cellular processes. While its traditional role in energy metabolism requires only micromolar concentrations, the cellular content of ATP has been found to be significantly higher at the millimolar level. Recent studies have attempted to correlate this higher concentration of ATP with its nonenergetic role in maintaining protein homeostasis, leaving the investigation of ATP's nontrivial activities in biology an open question. Here, by coupling computer simulations and experiments, we uncover new insights into ATP's role as a cryoprotectant against cold-salt stress, highlighting the necessity for higher cellular ATP concentrations. We present direct evidence at charged silica interfaces, demonstrating ATP's ability to restore native intersurface interactions disrupted by combined cold-salt stress, thereby inhibiting cold-responsive aggregation in high-salt conditions. ATP desorbs salt cations from negatively charged surfaces through predominant interactions between ATP and the salt cations. Although the mode of ATP's action remains unchanged with temperature, the extent of interaction scales with temperature, requiring less ATP activity at lower temperatures, justifying the reason for reduction in cellular ATP content due to the cold effect, reported in previous experimental studies. The trend observed in inorganic nanostructures is recurrent and robustly transferable to charged protein interfaces. A thorough comparison of ATP's cryoprotective activity with traditionally known biological cryoprotectants (glycine and betaine) reveals ATP's greater efficiency. In retrospect, our findings highlight ATP's additional biological role in cryopreservation, expanding its potential biomedical applications by offering effective protection of cells from cryoinjuries and avoiding the significant challenges associated with the toxicity of organic cryoprotectants.

Osmolyte-Induced Modulation of Hofmeister Series

Natural selection has driven the convergence toward a selected set of osmolytes, endowing them with the necessary efficiency to manage stress arising from salt diversity. This study combines atomistic simulations and experiments to investigate how two osmolytes, glycine and betaine, individually modulate the Hofmeister ion ordering of alkali metal salts (LiCl, KCl, and CsCl) near a charged silica interface. Both osmolytes are found to prevent salt-induced aggregation of the charged entities, yet their mode and degree of relative modulation depend on their intricate interplay with specific salt cations. Betaine's ion-mediated surface interaction maintains Hofmeister ion ordering, whereas glycine alters the relative Hofmeister order of the cation by salt-specific ion desorption from the surface. Experimental validation through surface-enhanced Raman spectroscopy supports these findings, elucidating osmolyte-mediated alterations in interfacial water structures. These observations based on an inorganic interface are reciprocated in amyloid beta 40 dimerization dynamics, highlighting osmolytes' efficacy in mitigating salt-induced aggregation. A molecular analysis suggests that the differential modes of interaction, as found here for glycine and betaine, are prevalent across classes of zwitterionic osmolytes.

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.

A Synergistic View on Osmolyte's Role against Salt and Cold Stress in Biointerfaces

We present our perspective on the role of osmolytes in mitigating abiotic stresses such as hypersalinity and sudden temperature changes. While the stabilizing effect of osmolytes on protein tertiary structures has been extensively studied, their direct impact on abiotic stress factors has eluded mainstream attention. Via highlighting a set of recent success stories of a joint venture of computer simulations and experimental measurements, we summarize the mechanistic insights into osmolytic action, particularly in the context of salt stress and combined cold-salt stress at the interface of biomolecular surfaces and saline environments. We stress the importance of chemical specificity in osmolytic activity, the interplay of differential osmolytic behaviors against heterogeneous salt stress, and the capability of osmolytes to adopt combined actions. Additionally, we discuss the potential of incorporating nanomaterial-based systems to enrich our understanding of osmolyte bioactions and facilitate their practical applications. We anticipate that this discourse will inspire interdisciplinary collaborations and motivate further investigations on osmolytes, ultimately broadening their applications in the fields of health and disease.