DNA Protection Protein, a Novel Mechanism of Radiation Tolerance: Lessons from Tardigrades

A comparative ultrastructure study of the tardigrade Ramazzottius varieornatus in the hydrated state, after desiccation and during the process of rehydration

Tardigrades can survive hostile environments such as desiccation by adopting a state of anhydrobiosis. Numerous tardigrade species have been described thus far, and recent genome and transcriptome analyses revealed that several distinct strategies were employed to cope with harsh environments depending on the evolutionary lineages. Detailed analyses at the cellular and subcellular levels are essential to complete these data. In this work, we analyzed a tardigrade species that can withstand rapid dehydration, Ramazzottius varieornatus. Surprisingly, we noted an absence of the anhydrobiotic-specific extracellular structure previously described for the Hypsibius exemplaris species. Both Ramazzottius varieornatus and Hypsibius exemplaris belong to the same evolutionary class of Eutardigrada. Nevertheless, our observations reveal discrepancies in the anhydrobiotic structures correlated with the variation in the anhydrobiotic mechanisms.

Tracing the serendipitous genesis of radiation resistance

Free-living organisms frequently encounter unfavorable abiotic environmental factors. Those who adapt and cope with sudden changes in the external environment survive. Desiccation is one of the most common and frequently encountered stresses in nature. On the contrary, ionizing radiations are limited to high local concentrations of naturally occurring radioactive materials and related anthropogenic activities. Yet, resistance to high doses of ionizing radiation is evident across the tree of life. The evolution of desiccation resistance has been linked to the evolution of ionizing radiation resistance, although, evidence to support the idea that the evolution of desiccation tolerance is a necessary precursor to ionizing radiation resistance is lacking. Moreover, the presence of radioresistance in hyperthermophiles suggests multiple paths lead to radiation resistance. In this minireview, we focus on the molecular aspects of damage dynamics and damage response pathways comprising protective and restorative functions with a definitive survival advantage, to explore the serendipitous genesis of ionizing radiation resistance.

Horizontal acquisition of a DNA ligase improves DNA damage tolerance in eukaryotes

Bdelloid rotifers are part of the restricted circle of multicellular animals that can withstand a wide range of genotoxic stresses at any stage of their life cycle. In this study, bdelloid rotifer Adineta vaga is used as a model to decipher the molecular basis of their extreme tolerance. Proteomic analysis shows that a specific DNA ligase, different from those usually involved in DNA repair in eukaryotes, is strongly over-represented upon ionizing radiation. A phylogenetic analysis reveals its orthology to prokaryotic DNA ligase E, and its horizontal acquisition by bdelloid rotifers and plausibly other eukaryotes. The fungus Mortierella verticillata, having a single copy of this DNA Ligase E homolog, also exhibits an increased radiation tolerance with an over-expression of this DNA ligase E following X-ray exposure. We also provide evidence that A. vaga ligase E is a major contributor of DNA breaks ligation activity, which is a common step of all important DNA repair pathways. Consistently, its heterologous expression in human cell lines significantly improves their radio-tolerance. Overall, this study highlights the potential of horizontal gene transfers in eukaryotes, and their contribution to the adaptation to extreme conditions.

Uncharacterized protein C17orf80 - a novel interactor of human mitochondrial nucleoids

Molecular functions of many human proteins remain unstudied, despite the demonstrated association with diseases or pivotal molecular structures, such as mitochondrial DNA (mtDNA). This small genome is crucial for the proper functioning of mitochondria, the energy-converting organelles. In mammals, mtDNA is arranged into macromolecular complexes called nucleoids that serve as functional stations for its maintenance and expression. Here, we aimed to explore an uncharacterized protein C17orf80, which was previously detected close to the nucleoid components by proximity labelling mass spectrometry. To investigate the subcellular localization and function of C17orf80, we took advantage of immunofluorescence microscopy, interaction proteomics and several biochemical assays. We demonstrate that C17orf80 is a mitochondrial membrane-associated protein that interacts with nucleoids even when mtDNA replication is inhibited. In addition, we show that C17orf80 is not essential for mtDNA maintenance and mitochondrial gene expression in cultured human cells. These results provide a basis for uncovering the molecular function of C17orf80 and the nature of its association with nucleoids, possibly leading to new insights about mtDNA and its expression.

Multivalent binding of the tardigrade Dsup protein to chromatin promotes yeast survival and longevity upon exposure to oxidative damage

Tardigrades are remarkable in their ability to survive extreme environments. The damage suppressor (Dsup) protein is thought responsible for their extreme resistance to reactive oxygen species (ROS) generated by irradiation. Here we show that expression of Ramazzottius varieornatus Dsup in Saccharomyces cerevisiae reduces oxidative DNA damage and extends the lifespan of budding yeast exposed to chronic oxidative genotoxicity. This protection from ROS requires either the Dsup HMGN-like domain or sequences C-terminal to same. Dsup associates with no apparent bias across the yeast genome, using multiple modes of nucleosome binding; the HMGN-like region interacts with both the H2A/H2B acidic patch and H3/H4 histone tails, while the C-terminal region binds DNA. These findings give precedent for engineering an organism by physically shielding its genome to promote survival and longevity in the face of oxidative damage.

The tardigrade Dsup protein enhances radioresistance in Drosophila melanogaster and acts as an unspecific repressor of transcription

The tardigrade-unique damage suppressor protein (Dsup) can protect DNA from ionizing radiation and reactive oxygen species (ROS). In this study, we generated Dsup-expressing lines of Drosophila melanogaster and demonstrated that Dsup increased the survival rate after γ-ray irradiation and hydrogen peroxide treatment in flies too, but reduced the level of their locomotor activity. The transcriptome analyses of Dsup-expressing lines revealed a significant number of DEGs, >99% of which were down-regulated. Moreover, Dsup could bind RNA. These findings suggest that Dsup can act not only as a DNA protector but also as a non-specific transcriptional repressor and RNA-binding protein, that may lead to disturbance of a number of biological processes in D. melanogaster. The obtained data demonstrate features of the Dsup protein action in non-tardigrade organisms and can be used to understand the impact of other unspecific DNA/RNA-binding proteins on ROS and radiation resistance, gene expression, and epigenetic processes.

Chromatin accessibility: methods, mechanisms, and biological insights

Access to DNA is a prerequisite to the execution of essential cellular processes that include transcription, replication, chromosomal segregation, and DNA repair. How the proteins that regulate these processes function in the context of chromatin and its dynamic architectures is an intensive field of study. Over the past decade, genome-wide assays and new imaging approaches have enabled a greater understanding of how access to the genome is regulated by nucleosomes and associated proteins. Additional mechanisms that may control DNA accessibility in vivo include chromatin compaction and phase separation - processes that are beginning to be understood. Here, we review the ongoing development of accessibility measurements, we summarize the different molecular and structural mechanisms that shape the accessibility landscape, and we detail the many important biological functions that are linked to chromatin accessibility.

Application of CRISPR/Cas9 system and the preferred no-indel end-joining repair in tardigrades

Tardigrades are small aquatic animals known for the tolerant ability against various extreme stresses. Recent studies identified several tardigrade-unique proteins as protective factors of biomolecules from extreme stresses. Due to the limitation of the technique available in tardigrades, the function of these protective molecules has largely been studied utilizing the systems of in vitro and the heterologous expression in other organisms. Although RNAi is feasible in tardigrades, their effects are variable and not always sufficient. To analyze the functions of the tardigrade protective proteins, in vivo genetic manipulations have been desired. In this study, we used a tardigrade Hypsibius exemplaris as a model whose genome is available, and developed the delivery method of Cas9 ribonucleoproteins (RNPs) to adult tardigrade cells. Cas9 RNPs containing two kinds of crRNAs were injected to the body cavity of adult tardigrades and subjected to the subsequent electroporation to facilitate the incorporation of RNPs to the cells. Using this delivery method, we detected the deletion of the intervening region between two crRNAs from the genome. Intriguingly, all examined joining sites exhibited no incorporation of insertions/deletions (indels), suggesting that no-indel end-joining is dominant repair system in this tardigrade. We also detected similar removal of the intervening region even in the tardigrades injected with Cas9 RNPs without electroporation and in this case the no-indel end-joining is detected in still dominant but not all examined joining sites. This study provides the development of the delivery method of Cas9 RNPs to tardigrade cells and our data also suggested that simultaneous application of more than two crRNAs/gRNAs are recommended to disrupt the target gene by CRISPR/Cas9 system to avoid scarless repair in the tardigrade.

Resistance to Extreme Stresses by a Newly Discovered Japanese Tardigrade Species, Macrobiotus kyoukenus (Eutardigrada, Macrobiotidae)

Simple Summary

Tardigrades are small micrometazoans able to resist several environmental stresses in any stage of their life cycle. The integrated molecular and morphological analysis of tardigrade specimens collected in Tsukuba (Japan) revealed that this population represents a new species, Macrobiotus kyoukenus sp. nov., belonging to the genus Macrobiotus, one of the most speciose and widespread water bear taxon. The stress resistance capabilities of M. kyoukenus sp. nov. have been tested by submitting animals to extreme desiccation, rapid freezing, and high levels of ultraviolet radiations (UVB and UVC). Animals were able to survive desiccation and freezing, and both hydrated and desiccated animals showed a high tolerance to increasing UV radiations. Overall, our findings contribute to the discovery of a larger tardigrade biodiversity in Japan, and the tolerance capabilities of M. kyoukenus sp. nov. show that this new species could become an emerging model for stress resistance studies.

Abstract

Tardigrades are small micrometazoans able to resist several environmental stresses in any stage of their life cycle. An integrated analysis of tardigrade specimens collected in Tsukuba (Japan) revealed a peculiar morphology and a new sensory field in the cloaca. Molecular taxonomy and phylogenetic analysis on different genes (COI, ITS2, 18S and 28S) confirmed that this population is a new species, Macrobiotus kyoukenus sp. nov., belonging to the widespread Macrobiotus hufelandi group. The stress resistance capabilities of M. kyoukenus sp. nov. have been tested by submitting animals to extreme desiccation, rapid freezing, and high levels of ultraviolet radiations (UVB and UVC). Animals were able to survive desiccation (survivorship 95.71 ± 7.07%) and freezing up to −80 °C (82.33 ± 17.11%). Both hydrated and desiccated animals showed a high tolerance to increasing UV radiations: hydrated animals survived to doses up to 152.22 kJ m⁻² (UVB) and up to 15.00 kJ m⁻² (UVC), while desiccated specimens persisted to radiations up to 165.12 kJ m⁻² (UVB) and up to 35.00 kJ m⁻² (UVC). Present data contribute to the discovery of a larger tardigrade biodiversity in Japan, and the tolerance capabilities of M. kyoukenus sp. nov. show that it could become a new emerging model for stress resistance studies.

Deciphering the Biological Enigma-Genomic Evolution Underlying Anhydrobiosis in the Phylum Tardigrada and the Chironomid Polypedilum vanderplanki

Anhydrobiosis, an ametabolic dehydrated state triggered by water loss, is observed in several invertebrate lineages. Anhydrobiotes revive when rehydrated, and seem not to suffer the ultimately lethal cell damage that results from severe loss of water in other organisms. Here, we review the biochemical and genomic evidence that has revealed the protectant molecules, repair systems, and maintenance pathways associated with anhydrobiosis. We then introduce two lineages in which anhydrobiosis has evolved independently: Tardigrada, where anhydrobiosis characterizes many species within the phylum, and the genus Polypedilum, where anhydrobiosis occurs in only two species. Finally, we discuss the complexity of the evolution of anhydrobiosis within invertebrates based on current knowledge, and propose perspectives to enhance the understanding of anhydrobiosis.

Tardigrades and their emergence as model organisms

Experimentally tractable organisms like C. elegans, Drosophila, zebrafish, and mouse are popular models for addressing diverse questions in biology. In 1997, two of the most valuable invertebrate model organisms to date-C. elegans and Drosophila-were found to be much more closely related to each other than expected. C. elegans and Drosophila belong to the nematodes and arthropods, respectively, and these two phyla and six other phyla make up a clade of molting animals referred to as the Ecdysozoa. The other ecdysozoan phyla could be valuable models for comparative biology, taking advantage of the rich and continual sources of research findings as well as tools from both C. elegans and Drosophila. But when the Ecdysozoa was first recognized, few tools were available for laboratory studies in any of these six other ecdysozoan phyla. In 1999 I began an effort to develop tools for studying one such phylum, the tardigrades. Here, I describe how the tardigrade species Hypsibius exemplaris and tardigrades more generally have emerged over the past two decades as valuable new models for answering diverse questions. To date, these questions have included how animal body plans evolve and how biological materials can survive some remarkably extreme conditions.

Interstellar space biology via Project Starlight

Our ability to explore the cosmos by direct contact has been limited to a small number of lunar and interplanetary missions. However, the NASA Starlight program points a path forward to send small, relativistic spacecraft far outside our solar system via standoff directed-energy propulsion. These miniaturized spacecraft are capable of robotic exploration but can also transport seeds and organisms, marking a profound change in our ability to both characterize and expand the reach of known life. Here we explore the biological and technological challenges of interstellar space biology, focusing on radiation-tolerant microorganisms capable of cryptobiosis. Additionally, we discuss planetary protection concerns and other ethical considerations of sending life to the stars.

Contemporary biomedical engineering perspective on volitional evolution for human radiotolerance enhancement beyond low-earth orbit

A primary objective of the National Aeronautics and Space Administration (NASA) is expansion of humankind's presence outside low-Earth orbit, culminating in permanent interplanetary travel and habitation. Having no inherent means of physiological detection or protection against ionizing radiation, humans incur capricious risk when journeying beyond low-Earth orbit for long periods. NASA has made large investments to analyze pathologies from space radiation exposure, emphasizing the importance of characterizing radiation's physiological effects. Because natural evolution would require many generations to confer resistance against space radiation, immediately pragmatic approaches should be considered. Volitional evolution, defined as humans steering their own heredity, may inevitably retrofit the genome to mitigate resultant pathologies from space radiation exposure. Recently, uniquely radioprotective genes have been identified, conferring local or systemic radiotolerance when overexpressed in vitro and in vivo. Aiding in this process, the CRISPR/Cas9 technique is an inexpensive and reproducible instrument capable of making limited additions and deletions to the genome. Although cohorts can be identified and engineered to protect against radiation, alternative and supplemental strategies should be seriously considered. Advanced propulsion and mild synthetic torpor are perhaps the most likely to be integrated. Interfacing artificial intelligence with genetic engineering using predefined boundary conditions may enable the computational modeling of otherwise overly complex biological networks. The ethical context and boundaries of introducing genetically pioneered humans are considered.

Tardigrade Secretory-Abundant Heat-Soluble Protein Has a Flexible β-Barrel Structure in Solution and Keeps This Structure in Dehydration

Secretory-abundant heat-soluble (SAHS) proteins are unique heat-soluble proteins of Tardigrada and are believed to play an essential role in anhydrobiosis, a latent state of life induced by desiccation. To investigate the dynamic properties, molecular dynamics (MD) simulations of a SAHS protein, RvSAHS1, were performed in solution and under dehydrating conditions. For comparison purposes, MD simulations of a human liver-type fatty-acid binding protein (LFABP) were performed in solution. Furthermore, high-speed atomic force microscopy observations were conducted to ascertain the results of the MD simulations. Three properties of RvSAHS1 were found as follows. (1) The entrance region of RvSAHS1 is more flexible and can be more extensive in solutions compared with that of a human LFABP because there is no salt bridge between the βD and βE strands. (2) The intrinsically disordered domain in the N-terminal region significantly fluctuates and can form an amphiphilic α-helix. (3) The size of the entrance region gets smaller along with dehydration, keeping the β-barrel structure. Overall, the obtained results provide atomic-level dynamics of SAHS proteins.

Expression of a Tardigrade Dsup Gene Enhances Genome Protection in Plants

DNA damage is one of the most impactful events in living organisms, leading to DNA sequence changes (mutation) and disruption of biological processes. A study has identified a protein called Damage Suppressor Protein (Dsup) in the tardigrade Ramazzotius varieornatus that has shown to reduce the effects of radiation damage in human cell cultures (Hashimoto in Nature Communications 7:12808, 2016). We have generated tobacco plants that express the codon-optimized tardigrade Dsup gene and examined their responses when treated with mutagenic chemicals, ultraviolet (UV) and ionizing radiations. Our studies showed that compared to the control plants, the Dsup-expressing plants grew better in the medium containing mutagenic ethylmethane sulfonate (EMS). RT-qPCR detected distinct expression patterns of endogenous genes involved in DNA damage response and repair in the Dsup plants in response to EMS, bleomycin, UV-C and X-ray radiations. Comet assays revealed that the nuclei from the Dsup plants appeared more protected from UV and X-ray damages than the control plants. Overall, our studies demonstrated that Dsup gene expression enhanced tolerance of plants to genomutagenic stress. We suggest the feasibility of exploring genetic resources from extremotolerant species such as tardigrades to impart plants with tolerance to stressful environments for future climate changes and human space endeavors.

Mechanisms of Desiccation Tolerance: Themes and Variations in Brine Shrimp, Roundworms, and Tardigrades

Water is critical for the survival of most cells and organisms. Remarkably, a small number of multicellular animals are able to survive nearly complete drying. The phenomenon of anhydrobiosis, or life without water, has been of interest to researchers for over 300 years. In this review we discuss advances in our understanding of protectants and mechanisms of desiccation tolerance that have emerged from research in three anhydrobiotic invertebrates: brine shrimp (Artemia), roundworms (nematodes), and tardigrades (water bears). Discovery of molecular protectants that allow each of these three animals to survive drying diversifies our understanding of desiccation tolerance, and convergent themes suggest mechanisms that may offer a general model for engineering desiccation tolerance in other contexts.

A computational structural study on the DNA-protecting role of the tardigrade-unique Dsup protein

The remarkable ability of tardigrades to withstand a wide range of physical and chemical extremes has attracted a considerable interest in these small invertebrates, with a particular focus on the protective roles of proteins expressed during such conditions. The discovery that a tardigrade-unique protein named Dsup (damage suppressor) protects DNA from damage produced by radiation and radicals, has raised expectations concerning its potential applications in biotechnology and medicine. We present in this paper what might be dubbed a “computational experiment” on the Dsup-DNA system. By means of molecular modelling, calculations of electrostatic potentials and electric fields, and all-atom molecular dynamics simulations, we obtained a dynamic picture of the Dsup-DNA interaction. Our results suggest that the protein is intrinsically disordered, which enables Dsup to adjust its structure to fit DNA shape. Strong electrostatic attractions and high protein flexibility drive the formation of a molecular aggregate in which Dsup shields DNA. While the precise mechanism of DNA protection conferred by Dsup remains to be elucidated, our study provides some molecular clues of their association that could be of interest for further investigation in this line.

Space Radiation Biology for "Living in Space"

Space travel has advanced significantly over the last six decades with astronauts spending up to 6 months at the International Space Station. Nonetheless, the living environment while in outer space is extremely challenging to astronauts. In particular, exposure to space radiation represents a serious potential long-term threat to the health of astronauts because the amount of radiation exposure accumulates during their time in space. Therefore, health risks associated with exposure to space radiation are an important topic in space travel, and characterizing space radiation in detail is essential for improving the safety of space missions. In the first part of this review, we provide an overview of the space radiation environment and briefly present current and future endeavors that monitor different space radiation environments. We then present research evaluating adverse biological effects caused by exposure to various space radiation environments and how these can be reduced. We especially consider the deleterious effects on cellular DNA and how cells activate DNA repair mechanisms. The latest technologies being developed, e.g., a fluorescent ubiquitination-based cell cycle indicator, to measure real-time cell cycle progression and DNA damage caused by exposure to ultraviolet radiation are presented. Progress in examining the combined effects of microgravity and radiation to animals and plants are summarized, and our current understanding of the relationship between psychological stress and radiation is presented. Finally, we provide details about protective agents and the study of organisms that are highly resistant to radiation and how their biological mechanisms may aid developing novel technologies that alleviate biological damage caused by radiation. Future research that furthers our understanding of the effects of space radiation on human health will facilitate risk-mitigating strategies to enable long-term space and planetary exploration.

Differential mechanisms of tolerance to extreme environmental conditions in tardigrades

Tardigrades, also known as water bears, are small aquatic animals that inhabit marine, fresh water or limno-terrestrial environments. While all tardigrades require surrounding water to grow and reproduce, species living in limno-terrestrial environments (e.g. Ramazzottius varieornatus) are able to undergo almost complete dehydration by entering an arrested state known as anhydrobiosis, which allows them to tolerate ionic radiation, extreme temperatures and intense pressure. Previous studies based on comparison of the genomes of R. varieornatus and Hypsibius dujardini - a less tolerant tardigrade - have pointed to potential mechanisms that may partially contribute to their remarkable ability to resist extreme physical conditions. In this work, we have further annotated the genomes of both tardigrades using a guided approach in search for novel mechanisms underlying the extremotolerance of R. varieornatus. We have found specific amplifications of several genes, including MRE11 and XPC, and numerous missense variants exclusive of R. varieornatus in CHEK1, POLK, UNG and TERT, all of them involved in important pathways for DNA repair and telomere maintenance. Taken collectively, these results point to genomic features that may contribute to the enhanced ability to resist extreme environmental conditions shown by R. varieornatus.

The tardigrade damage suppressor protein binds to nucleosomes and protects DNA from hydroxyl radicals

Tardigrades, also known as water bears, are animals that can survive extreme conditions. The tardigrade Ramazzottius varieornatus contains a unique nuclear protein termed Dsup, for damage suppressor, which can increase the resistance of human cells to DNA damage under conditions, such as ionizing radiation or hydrogen peroxide treatment, that generate hydroxyl radicals. Here we find that R. varieornatus Dsup is a nucleosome-binding protein that protects chromatin from hydroxyl radicals. Moreover, a Dsup ortholog from the tardigrade Hypsibius exemplaris similarly binds to nucleosomes and protects DNA from hydroxyl radicals. Strikingly, a conserved region in Dsup proteins exhibits sequence similarity to the nucleosome-binding domain of vertebrate HMGN proteins and is functionally important for nucleosome binding and hydroxyl radical protection. These findings suggest that Dsup promotes the survival of tardigrades under diverse conditions by a direct mechanism that involves binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals.

Tardigrades, also known as water bears and moss piglets, are small animals found in many different environments on land and sea. These animals have the remarkable ability to survive extremes including very low temperatures, high levels of radiation and exposure to chemicals that are harmful to other forms of life. Tardigrades have even been found to survive the harsh conditions of outer space.

X-rays are a type of radiation naturally produced by lightning strikes and are also found in cosmic rays from outer space. High doses of X-rays can cause genetic mutations that may lead to serious illness or death. This is because when X-rays come into contact with water they split the water molecules to make particles known as hydroxyl radicals, which in turn damage the DNA inside cells.

The genomes of animals and plants are made of DNA, which is packaged into a structure called chromatin. Previous studies identified a protein named Dsup in a tardigrade called Ramazzottius varieornatus that can protect human cells from damage by X-rays. However, it was not known whether Dsup binds directly to chromatin or plays a more indirect role in protecting DNA.

Chavez, Cruz-Becerra, Fei, Kassavetis et al. used biochemical approaches to study Dsup. Their experiments revealed that Dsup from R. varieornatus binds to chromatin to protect the DNA from damage by hydroxyl radicals, and that the Dsup protein in another tardigrade species also works in a similar way. Further analysis showed that a region of Dsup that is needed to bind to chromatin is very similar to a region that had been previously found only in chromatin-binding proteins from humans and other vertebrates (animals with backbones). This connection between Dsup and vertebrate chromatin-binding proteins remains a mystery.

The new findings about tardigrade Dsup may help researchers develop animal cells that live longer under normal or extreme environmental conditions. In this manner, Dsup could be used to expand the range of applications of cells in biotechnology. It could also increase the effectiveness of current methods, such as the production of some pharmaceuticals, that depend upon the use of cultured cells.

Radiation Tolerance in Tardigrades: Current Knowledge and Potential Applications in Medicine

Tardigrades represent a phylum of very small aquatic animals in which many species have evolved adaptations to survive under extreme environmental conditions, such as desiccation and freezing. Studies on several species have documented that tardigrades also belong to the most radiation-tolerant animals on Earth. This paper gives an overview of our current knowledge on radiation tolerance of tardigrades, with respect to dose-responses, developmental stages, and different radiation sources. The molecular mechanisms behind radiation tolerance in tardigrades are still largely unknown, but omics studies suggest that both mechanisms related to the avoidance of DNA damage and mechanisms of DNA repair are involved. The potential of tardigrades to provide knowledge of importance for medical sciences has long been recognized, but it is not until recently that more apparent evidence of such potential has appeared. Recent studies show that stress-related tardigrade genes may be transfected to human cells and provide increased tolerance to osmotic stress and ionizing radiation. With the recent sequencing of the tardigrade genome, more studies applying tardigrade omics to relevant aspects of human medicine are expected. In particular, the cancer research field has potential to learn from studies on tardigrades about molecular mechanisms evolved to maintain genome integrity.

Covalent Modification of Amino Acids and Peptides Induced by Ionizing Radiation from an Electron Beam Linear Accelerator Used in Radiotherapy

To identify modifications to amino acids that are directly induced by ionizing radiation, free amino acids and 3-residue peptides were irradiated using a linear accelerator (Linac) radiotherapy device. Mass spectrometry was performed to detail the relative sensitivity to radiation as well as identify covalent, radiation-dependent adducts. The order of reactivity of the 20 common amino acids was generally in agreement with published literature except for His (most reactive of the 20) and Cys (less reactive). Novel and previously identified modifications on the free amino acids were detected. Amino acids were far less reactive when flanked by glycine residues in a tripeptide. Order of reactivity, with GVG most and GEG least, was substantially altered, as were patterns of modification. Radiation reactivity of amino acids is clearly and strongly affected by conversion of the α-amino and α-carboxyl groups to peptide bonds, and the presence of neighboring amino acid residues.

Cell Biology of the Tardigrades: Current Knowledge and Perspectives

The invertebrate phylum Tardigrada has received much attention for containing species adapted to the most challenging environmental conditions where an ability to survive complete desiccation or freezing in a cryptobiotic state is necessary for persistence. Although research on tardigrades has a long history, the last decade has seen a dramatic increase in molecular biological ("omics") studies, most of them with the aim to reveal the biochemical mechanisms behind desiccation tolerance of tardigrades. Several other aspects of tardigrade cell biology have been studied, and we review some of them, including karyology, embryology, the role of storage cells, and the question of whether tardigrades are eutelic animals. We also review some of the theories about how anhydrobiotic organisms are able to maintain cell integrity under dry conditions, and our current knowledge on the role of vitrification and DNA protection and repair. Many aspects of tardigrade stress tolerance have relevance for human medicine, and the first transfers of tardigrade stress genes to human cells have now appeared. We expect this field to develop rapidly in the coming years, as more genomic information becomes available. However, many basic cell biological aspects remain to be investigated, such as immunology, cell cycle kinetics, cell metabolism, and culturing of tardigrade cells. Such development will be necessary to allow tardigrades to move from a nonmodel organism position to a true model organism with interesting associations with the current models C. elegans and D. melanogaster.