Water content influences the vitrified properties of CAHS proteins

Life on the dry side: a roadmap to understanding desiccation tolerance and accelerating translational applications

To thrive in extreme conditions, organisms have evolved a diverse arsenal of adaptations that confer resilience. These species, their traits, and the mechanisms underlying them comprise a valuable resource that can be mined for numerous conceptual insights and applied objectives. One of the most dramatic adaptations to water limitation is desiccation tolerance. Understanding the mechanisms underlying desiccation tolerance has important potential implications for medicine, biotechnology, agriculture, and conservation. However, progress has been hindered by a lack of standardization across sub-disciplines, complicating the integration of data and slowing the translation of basic discoveries into practical applications. Here, we synthesize current knowledge on desiccation tolerance across evolutionary, ecological, physiological, and cellular scales to provide a roadmap for advancing desiccation tolerance research. We also address critical gaps and technical roadblocks, highlighting the need for standardized experimental practices, improved taxonomic sampling, and the development of new tools for studying biology in a dry state. We hope that this perspective can serve as a roadmap to accelerating research breakthroughs and unlocking the potential of desiccation tolerance to address global challenges related to climate change, food security, and health.

An evaluation of thermal tolerance in six tardigrade species in an active and dry state

Tardigrades are known for their ability to survive extreme conditions. Reports indicate that tardigrade thermal tolerance is enhanced in the desiccated state; however, these reports have almost always used a single tardigrade species and drying/heating methods vary between studies. Using six different species of tardigrades we confirm that desiccation enhances thermal tolerance in tardigrades. Furthermore, we show that differences in thermal tolerance exist between tardigrade species both when hydrated and desiccated. While Viridiscus viridianus survives the highest temperatures in the hydrated state of any species tested here, under hydrated conditions, the thermal tolerance of V. viridianus is restricted to an acute transient stress. Furthermore, unlike other stresses, such as desiccation, where mild initial exposure preconditions some species to survive subsequent harsher treatment, for V. viridianus exposure to mild thermal stress in the hydrated state does not confer protection to harsher heating. Our results suggest that while tardigrades have the capacity to tolerate mild thermal stress while hydrated, survival of high temperatures in a desiccated state is a by-product of tardigrades' ability to survive desiccation.

Osmolyte-IDP interactions during desiccation

Desiccation, the extreme loss of water, poses a significant challenge to living organisms. Desiccation-tolerant organisms combat this in part by accumulating desiccation tolerance intrinsically disordered proteins (DT-IDPs) and osmolytes within their cells. While both osmolytes and DT-IDPs help maintain cellular viability on their own, combinations of the two can work synergistically to provide enhanced protection and survival. This review summarises our understanding of the interactions between DT-IDPs and osmolytes during desiccation, and explores possible molecular mechanisms underlying them. Using recent literature on DT-IDPs and on the broader study of IDP-osmolyte interactions, we propose several hypotheses that explain interactions between DT-IDPs and osmolytes. Finally, we highlight several techniques from literature on DT-IDPs that we feel are useful to the study of IDPs in other contexts.

Labile assembly of a tardigrade protein induces biostasis

Tardigrades are microscopic animals that survive desiccation by inducing biostasis. To survive drying tardigrades rely on intrinsically disordered CAHS proteins, which also function to prevent perturbations induced by drying in vitro and in heterologous systems. CAHS proteins have been shown to form gels both in vitro and in vivo, which has been speculated to be linked to their protective capacity. However, the sequence features and mechanisms underlying gel formation and the necessity of gelation for protection have not been demonstrated. Here we report a mechanism of fibrillization and gelation for CAHS D similar to that of intermediate filament assembly. We show that in vitro, gelation restricts molecular motion, immobilizing and protecting labile material from the harmful effects of drying. In vivo, we observe that CAHS D forms fibrillar networks during osmotic stress. Fibrillar networking of CAHS D improves survival of osmotically shocked cells. We observe two emergent properties associated with fibrillization; (i) prevention of cell volume change and (ii) reduction of metabolic activity during osmotic shock. We find that there is no significant correlation between maintenance of cell volume and survival, while there is a significant correlation between reduced metabolism and survival. Importantly, CAHS D's fibrillar network formation is reversible and metabolic rates return to control levels after CAHS fibers are resolved. This work provides insights into how tardigrades induce reversible biostasis through the self-assembly of labile CAHS gels.

Helicity of a tardigrade disordered protein contributes to its protective function during desiccation

To survive extreme drying (anhydrobiosis), many organisms, spanning every kingdom of life, accumulate intrinsically disordered proteins (IDPs). For decades, the ability of anhydrobiosis-related IDPs to form transient amphipathic helices has been suggested to be important for promoting desiccation tolerance. However, evidence empirically supporting the necessity and/or sufficiency of helicity in mediating anhydrobiosis is lacking. Here, we demonstrate that the linker region of CAHS D, a desiccation-related IDP from the tardigrade Hypsibius exemplaris, that contains significant helical structure, is the protective portion of this protein. Perturbing the sequence composition and grammar of the linker region of CAHS D, through the insertion of helix-breaking prolines, modulating the identity of charged residues, or replacement of hydrophobic amino acids with serine or glycine residues results in variants with different degrees of helical structure. Importantly, correlation of protective capacity and helical content in variants generated through different helix perturbing modalities does not show as strong a trend, suggesting that while helicity is important, it is not the only property that makes a protein protective during desiccation. These results provide direct evidence for the decades-old theory that helicity of desiccation-related IDPs is linked to their anhydrobiotic capacity.

Water content, transition temperature and fragility influence protection and anhydrobiotic capacity

Water is essential for metabolism and all life processes. Despite this, many organisms distributed across the kingdoms of life survive near-complete desiccation or anhydrobiosis. Increased intracellular viscosity, leading to the formation of a vitrified state is necessary, but not sufficient, for survival while dry. What properties of a vitrified system make it desiccation-tolerant or -sensitive are unknown. We have analyzed 18 different in vitro vitrified systems, composed of one of three protective disaccharides (trehalose, sucrose, or maltose) and glycerol, quantifying their enzyme-protective capacity and their material properties in a dry state. Protection conferred by mixtures containing maltose correlates strongly with increased water content, increased glass-transition temperature, and reduced glass former fragility, while the protection of glasses formed with sucrose correlates with increased glass transition temperature and the protection conferred by trehalose glasses correlates with reduced glass former fragility. Thus, in vitro different vitrified sugars confer protection through distinct material properties. Next, we examined the material properties of a dry desiccation tolerant and intolerant life stage from three different organisms. The dried desiccation tolerant life stage of all organisms had an increased glass transition temperature and reduced glass former fragility relative to its dried desiccation intolerant life stage. These results suggest in nature organismal desiccation tolerance relies on a combination of various material properties. This study advances our understanding of how protective and non-protective glasses differ in terms of material properties that promote anhydrobiosis. This knowledge presents avenues to develop novel stabilization technologies for pharmaceuticals that currently rely on the cold-chain.

Statement of significance

For the past three decades the anhydrobiosis field has lived with a paradox, while vitrification is necessary for survival in the dry state, it is not sufficient. Understanding what property(s) distinguishes a desiccation tolerant from an intolerant vitrified system and how anhydrobiotic organisms survive drying is one of the enduring mysteries of organismal physiology. Here we show in vitro the enzyme-protective capacity of different vitrifying sugars can be correlated with distinct material properties. However, in vivo, diverse desiccation tolerant organisms appear to combine these material properties to promote their survival in a dry state.

Dynamic conformational changes of a tardigrade group-3 late embryogenesis abundant protein modulate membrane biophysical properties

A number of intrinsically disordered proteins (IDPs) encoded in stress-tolerant organisms, such as tardigrade, can confer fitness advantage and abiotic stress tolerance when heterologously expressed. Tardigrade-specific disordered proteins including the cytosolic-abundant heat-soluble proteins are proposed to confer stress tolerance through vitrification or gelation, whereas evolutionarily conserved IDPs in tardigrades may contribute to stress tolerance through other biophysical mechanisms. In this study, we characterized the mechanism of action of an evolutionarily conserved, tardigrade IDP, HeLEA1, which belongs to the group-3 late embryogenesis abundant (LEA) protein family. HeLEA1 homologs are found across different kingdoms of life. HeLEA1 is intrinsically disordered in solution but shows a propensity for helical structure across its entire sequence. HeLEA1 interacts with negatively charged membranes via dynamic disorder-to-helical transition, mainly driven by electrostatic interactions. Membrane interaction of HeLEA1 is shown to ameliorate excess surface tension and lipid packing defects. HeLEA1 localizes to the mitochondrial matrix when expressed in yeast and interacts with model membranes mimicking inner mitochondrial membrane. Yeast expressing HeLEA1 shows enhanced tolerance to hyperosmotic stress under nonfermentative growth and increased mitochondrial membrane potential. Evolutionary analysis suggests that although HeLEA1 homologs have diverged their sequences to localize to different subcellular organelles, all homologs maintain a weak hydrophobic moment that is characteristic of weak and reversible membrane interaction. We suggest that such dynamic and weak protein-membrane interaction buffering alterations in lipid packing could be a conserved strategy for regulating membrane properties and represent a general biophysical solution for stress tolerance across the domains of life.

Water content, transition temperature and fragility influence protection and anhydrobiotic capacity

Water is essential for metabolism and all life processes. Despite this, many organisms distributed across the kingdoms of life survive near-complete desiccation or anhydrobiosis (Greek for "life without water"). Increased intracellular viscosity, leading to the formation of a vitrified state is necessary, but not sufficient, for survival while dry. What properties of a vitrified system make it desiccation-tolerant or -sensitive are unknown. We have analyzed 18 different in vitro vitrified systems, composed of one of three protective disaccharides (trehalose, sucrose, or maltose) and varying amounts of glycerol, quantifying their enzyme-protective capacity and their material properties in a dry state. We find that protection conferred by mixtures containing maltose correlates strongly with increased water content, increased glass-transition temperature, and reduced glass former fragility, while the protection of glasses formed with sucrose correlates with increased glass transition temperature and the protection conferred by trehalose glasses correlates with reduced glass former fragility. Thus, in vitro different vitrified sugars confer protection through distinct material properties. Extending on this, we have examined the material properties of a dry desiccation tolerant and intolerant life stage from three different organisms. In all cases, the dried desiccation tolerant life stage of an organism had an increased glass transition temperature relative to its dried desiccation intolerant life stage, and this trend is also seen in all three organisms when considering reduced glass former fragility. These results suggest that while drying of different protective sugars in vitro results in vitrified systems with distinct material properties that correlate with their enzyme-protective capacity, in nature organismal desiccation tolerance relies on a combination of these properties. This study advances our understanding of how protective and non-protective glasses differ in terms of material properties that promote anhydrobiosis. This knowledge presents avenues to develop novel stabilization technologies for pharmaceuticals that currently rely on the cold-chain.

The tardigrade protein CAHS D interacts with, but does not retain, water in hydrated and desiccated systems

Tardigrades are a group of microscopic animals renowned for their ability to survive near complete desiccation. A family of proteins, unique to tardigrades, called Cytoplasmic Abundant Heat Soluble (CAHS) proteins are necessary to mediate robust desiccation tolerance in these animals. However, the mechanism(s) by which CAHS proteins help to protect tardigrades during water-loss have not been fully elucidated. Here we use thermogravimetric analysis to empirically test the proposed hypothesis that tardigrade CAHS proteins, due to their propensity to form hydrogels, help to retain water during desiccation. We find that regardless of its gelled state, both in vitro and in vivo, a model CAHS protein (CAHS D) retains no more water than common proteins and control cells in the dry state. However, we find that while CAHS D proteins do not increase the total amount of water retained in a dry system, they interact with the small amount of water that does remain. Our study indicates that desiccation tolerance mediated by CAHS D cannot be simply ascribed to water retention and instead implicates its ability to interact more tightly with residual water as a possible mechanism underlying its protective capacity. These results advance our fundamental understanding of tardigrade desiccation tolerance which could provide potential avenues for new technologies to aid in the storage of dry shelf-stable pharmaceuticals and the generation of stress tolerant crops to ensure food security in the face of global climate change.

Natural and engineered mediators of desiccation tolerance stabilize Human Blood Clotting Factor VIII in a dry state

Biologics, pharmaceuticals containing or derived from living organisms, such as vaccines, antibodies, stem cells, blood, and blood products are a cornerstone of modern medicine. However, nearly all biologics have a major deficiency: they are inherently unstable, requiring storage under constant cold conditions. The so-called 'cold-chain', while effective, represents a serious economic and logistical hurdle for deploying biologics in remote, underdeveloped, or austere settings where access to cold-chain infrastructure ranging from refrigerators and freezers to stable electricity is limited. To address this issue, we explore the possibility of using anhydrobiosis, the ability of organisms such as tardigrades to enter a reversible state of suspended animation brought on by extreme drying, as a jumping off point in the development of dry storage technology that would allow biologics to be kept in a desiccated state under not only ambient but elevated temperatures. Here we examine the ability of different protein and sugar-based mediators of anhydrobiosis derived from tardigrades and other anhydrobiotic organisms to stabilize Human Blood Clotting Factor VIII under repeated dehydration/rehydration cycles, thermal stress, and long-term dry storage conditions. We find that while both protein and sugar-based protectants can stabilize the biologic pharmaceutical Human Blood Clotting Factor VIII under all these conditions, protein-based mediators offer more accessible avenues for engineering and thus tuning of protective function. Using classic protein engineering approaches, we fine tune the biophysical properties of a protein-based mediator of anhydrobiosis derived from a tardigrade, CAHS D. Modulating the ability of CAHS D to form hydrogels make the protein better or worse at providing protection to Human Blood Clotting Factor VIII under different conditions. This study demonstrates the effectiveness of tardigrade CAHS proteins and other mediators of desiccation tolerance at preserving the function of a biologic without the need for the cold-chain. In addition, our study demonstrates that engineering approaches can tune natural products to serve specific protective functions, such as coping with desiccation cycling versus thermal stress. Ultimately, these findings provide a proof of principle that our reliance on the cold-chain to stabilize life-saving pharmaceuticals can be broken using natural and engineered mediators of desiccation tolerance.

Expression of tardigrade disordered proteins impacts the tolerance to biofuels in a model cyanobacterium Synechocystis sp. PCC 6803

Tardigrades, known colloquially as water bears or moss piglets, are diminutive animals capable of surviving many extreme environments, even been exposed to space in low Earth orbit. Recently termed tardigrade disordered proteins (TDPs) include three families as cytoplasmic-(CAHS), secreted-(SAHS), and mitochondrial-abundant heat soluble (MAHS) proteins. How these tiny animals survive these stresses has remained relatively mysterious. Cyanobacteria cast attention as a "microbial factory" to produce biofuels and high-value-added chemicals due to their ability to photosynthesis and CO₂ sequestration. We explored a lot about biofuel stress and related mechanisms in Synechocystis sp. PCC 6803. The previous studies show that CAHS protein heterogenous expression in bacteria, yeast, and human cells increases desiccation tolerance in these hosts. In this study, the expression of three CAHS proteins in cyanobacterium was found to affect the tolerance to biofuels, while the tolerance to Cd²⁺ and Zn²⁺ were slightly affected in several mutants. A quantitative transcriptomics approach was applied to decipher response mechanisms at the transcriptional level further.

Time-series transcriptomic screening of factors contributing to the cross-tolerance to UV radiation and anhydrobiosis in tardigrades

BACKGROUND

Tardigrades are microscopic animals that are capable of tolerating extreme environments by entering a desiccated state of suspended animation known as anhydrobiosis. While antioxidative stress proteins, antiapoptotic pathways and tardigrade-specific intrinsically disordered proteins have been implicated in the anhydrobiotic machinery, conservation of these mechanisms is not universal within the phylum Tardigrada, suggesting the existence of overlooked components.

RESULTS

Here, we show that a novel Mn-dependent peroxidase is an important factor in tardigrade anhydrobiosis. Through time-series transcriptome analysis of Ramazzottius varieornatus specimens exposed to ultraviolet light and comparison with anhydrobiosis entry, we first identified several novel gene families without similarity to existing sequences that are induced rapidly after stress exposure. Among these, a single gene family with multiple orthologs that is highly conserved within the phylum Tardigrada and enhances oxidative stress tolerance when expressed in human cells was identified. Crystallographic study of this protein suggested Zn or Mn binding at the active site, and we further confirmed that this protein has Mn-dependent peroxidase activity in vitro.

CONCLUSIONS

Our results demonstrated novel mechanisms for coping with oxidative stress that may be a fundamental mechanism of anhydrobiosis in tardigrades. Furthermore, localization of these sets of proteins mainly in the Golgi apparatus suggests an indispensable role of the Golgi stress response in desiccation tolerance.

Natural and Designed Proteins Inspired by Extremotolerant Organisms Can Form Condensates and Attenuate Apoptosis in Human Cells

Many organisms can survive extreme conditions and successfully recover to normal life. This extremotolerant behavior has been attributed in part to repetitive, amphipathic, and intrinsically disordered proteins that are upregulated in the protected state. Here, we assemble a library of approximately 300 naturally occurring and designed extremotolerance-associated proteins to assess their ability to protect human cells from chemically induced apoptosis. We show that several proteins from tardigrades, nematodes, and the Chinese giant salamander are apoptosis-protective. Notably, we identify a region of the human ApoE protein with similarity to extremotolerance-associated proteins that also protects against apoptosis. This region mirrors the phase separation behavior seen with such proteins, like the tardigrade protein CAHS2. Moreover, we identify a synthetic protein, DHR81, that shares this combination of elevated phase separation propensity and apoptosis protection. Finally, we demonstrate that driving protective proteins into the condensate state increases apoptosis protection, and highlights the ability of DHR81 condensates to sequester caspase-7. Taken together, this work draws a link between extremotolerance-associated proteins, condensate formation, and designing human cellular protection.

Intrinsically Disordered Tardigrade Proteins Self-Assemble into Fibrous Gels in Response to Environmental Stress

Tardigrades are remarkable for their ability to survive harsh stress conditions as diverse as extreme temperature and desiccation. The molecular mechanisms that confer this unusual resistance to physical stress remain unknown. Recently, tardigrade-unique intrinsically disordered proteins have been shown to play an essential role in tardigrade anhydrobiosis. Here, we characterize the conformational and physical behaviour of CAHS-8 from Hypsibius exemplaris. NMR spectroscopy reveals that the protein comprises an extended central helical domain flanked by disordered termini. Upon concentration, the protein is shown to successively form oligomers, long fibres, and finally gels constituted of fibres in a strongly temperature-dependent manner. The helical domain forms the core of the fibrillar structure, with the disordered termini remaining highly dynamic within the gel. Soluble proteins can be encapsulated within cavities in the gel, maintaining their functional form. The ability to reversibly form fibrous gels may be associated with the enhanced protective properties of these proteins.