Composition, morphology and mechanics of hagfish slime

The hagfish genome and the evolution of vertebrates

As the only surviving lineages of jawless fishes, hagfishes and lampreys provide a crucial window into early vertebrate evolution1-3. Here we investigate the complex history, timing and functional role of genome-wide duplications4-7 and programmed DNA elimination8,9 in vertebrates in the light of a chromosome-scale genome sequence for the brown hagfish Eptatretus atami. Combining evidence from syntenic and phylogenetic analyses, we establish a comprehensive picture of vertebrate genome evolution, including an auto-tetraploidization (1RV) that predates the early Cambrian cyclostome-gnathostome split, followed by a mid-late Cambrian allo-tetraploidization (2RJV) in gnathostomes and a prolonged Cambrian-Ordovician hexaploidization (2RCY) in cyclostomes. Subsequently, hagfishes underwent extensive genomic changes, with chromosomal fusions accompanied by the loss of genes that are essential for organ systems (for example, genes involved in the development of eyes and in the proliferation of osteoclasts); these changes account, in part, for the simplification of the hagfish body plan1,2. Finally, we characterize programmed DNA elimination in hagfish, identifying protein-coding genes and repetitive elements that are deleted from somatic cell lineages during early development. The elimination of these germline-specific genes provides a mechanism for resolving genetic conflict between soma and germline by repressing germline and pluripotency functions, paralleling findings in lampreys10,11. Reconstruction of the early genomic history of vertebrates provides a framework for further investigations of the evolution of cyclostomes and jawed vertebrates.

Physiological differences between wild and captive animals: a century-old dilemma

Laboratory-based research dominates the fields of comparative physiology and biomechanics. The power of lab work has long been recognized by experimental biologists. For example, in 1932, Georgy Gause published an influential paper in Journal of Experimental Biology describing a series of clever lab experiments that provided the first empirical test of competitive exclusion theory, laying the foundation for a field that remains active today. At the time, Gause wrestled with the dilemma of conducting experiments in the lab or the field, ultimately deciding that progress could be best achieved by taking advantage of the high level of control offered by lab experiments. However, physiological experiments often yield different, and even contradictory, results when conducted in lab versus field settings. This is especially concerning in the Anthropocene, as standard laboratory techniques are increasingly relied upon to predict how wild animals will respond to environmental disturbances to inform decisions in conservation and management. In this Commentary, we discuss several hypothesized mechanisms that could explain disparities between experimental biology in the lab and in the field. We propose strategies for understanding why these differences occur and how we can use these results to improve our understanding of the physiology of wild animals. Nearly a century beyond Gause's work, we still know remarkably little about what makes captive animals different from wild ones. Discovering these mechanisms should be an important goal for experimental biologists in the future.

The hagfish genome and the evolution of vertebrates

As the only surviving lineages of jawless fishes, hagfishes and lampreys provide a critical window into early vertebrate evolution. Here, we investigate the complex history, timing, and functional role of genome-wide duplications in vertebrates in the light of a chromosome-scale genome of the brown hagfish Eptatretus atami. Using robust chromosome-scale (paralogon-based) phylogenetic methods, we confirm the monophyly of cyclostomes, document an auto-tetraploidization (1RV) that predated the origin of crown group vertebrates ~517 Mya, and establish the timing of subsequent independent duplications in the gnathostome and cyclostome lineages. Some 1RV gene duplications can be linked to key vertebrate innovations, suggesting that this early genomewide event contributed to the emergence of pan-vertebrate features such as neural crest. The hagfish karyotype is derived by numerous fusions relative to the ancestral cyclostome arrangement preserved by lampreys. These genomic changes were accompanied by the loss of genes essential for organ systems (eyes, osteoclast) that are absent in hagfish, accounting in part for the simplification of the hagfish body plan; other gene family expansions account for hagfishes' capacity to produce slime. Finally, we characterise programmed DNA elimination in somatic cells of hagfish, identifying protein-coding and repetitive elements that are deleted during development. As in lampreys, the elimination of these genes provides a mechanism for resolving genetic conflict between soma and germline by repressing germline/pluripotency functions. Reconstruction of the early genomic history of vertebrates provides a framework for further exploration of vertebrate novelties.

Epidermal threads reveal the origin of hagfish slime

When attacked, hagfishes produce a soft, fibrous defensive slime within a fraction of a second by ejecting mucus and threads into seawater. The rapid setup and remarkable expansion of the slime make it a highly effective and unique form of defense. How this biomaterial evolved is unknown, although circumstantial evidence points to the epidermis as the origin of the thread- and mucus-producing cells in the slime glands. Here, we describe large intracellular threads within a putatively homologous cell type from hagfish epidermis. These epidermal threads averaged ~2 mm in length and ~0.5 μm in diameter. The entire hagfish body is covered by a dense layer of epidermal thread cells, with each square millimeter of skin storing a total of ~96 cm threads. Experimentally induced damage to a hagfish's skin caused the release of threads, which together with mucus, formed an adhesive epidermal slime that is more fibrous and less dilute than the defensive slime. Transcriptome analysis further suggests that epidermal threads are ancestral to the slime threads, with duplication and diversification of thread genes occurring in parallel with the evolution of slime glands. Our results support an epidermal origin of hagfish slime, which may have been driven by selection for stronger and more voluminous slime.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

Mechanisms of gill-clogging by hagfish slime

Hagfishes defend themselves from gill-breathing predators by producing large volumes of fibrous slime when attacked. The slime's effectiveness comes from its ability to clog predators' gills, but the mechanisms by which hagfish slime clogs are uncertain, especially given its remarkably dilute concentration of solids. We quantified the clogging performance of hagfish slime over a range of concentrations, measured the contributions of its mucous and thread components, and measured the effect of turbulent mixing on clogging. To assess the porous structure of hagfish slime, we used a custom device to measure its Darcy permeability. We show that hagfish slime clogs at extremely dilute concentrations like those found in native hagfish slime and displays clogging performance that is superior to three thickening agents. We report an extremely low Darcy permeability for hagfish slime, and an effective pore size of 10-300 nm. We also show that the mucous and thread components play distinct yet crucial roles, with mucus being responsible for effective clogging and low permeability and the threads imparting mechanical strength and retaining clogging function over time. Our results provide new insights into the mechanisms by which hagfish slime clogs gills and may inspire the development of ultra-soft materials with novel properties.

Directed Self-Assembly of Heterologously Expressed Hagfish EsTKα and EsTKγ for Functional Hydrogel

Hagfish slime proteins have long been considered useful due to their potential applications in novel green, environmental, and functional bionic materials. The two main component proteins in the slime thread of hagfish, (opt)EsTKα and (opt)EsTKγ, were used as raw materials. However, the methods available to assemble these two proteins are time- and labor-intensive. The conditions affecting protein self-assembly, such as the pH of the assembly buffer, protein concentration, and the protein addition ratio, were the subject of the present research. Through a series of tests, the self-assembly results of a variety of assembly conditions were explored. Finally, a simplified protein self-assembly method was identified that allows for simple, direct assembly of the two proteins directly. This method does not require protein purification. Under the optimal assembly conditions obtained by exploration, a new gel material was synthesized from the hagfish protein through self-assembly of the (opt)EsTKα and (opt)EsTKγ. This assembly method has the benefits of being a simple, time-saving, and efficient. The self-assembled protein gel products were verified by SDS polyacrylamide gel electrophoresis (SDS-PAGE) and contained (opt)EsTKα and (opt)EsTKγ proteins. Scanning electron microscopy (SEM) was used to investigate the self-assembled protein gel after freeze-drying, and it was observed that the self-assembled protein formed a dense, three-dimensional porous network structure, meaning that it had good water retention. Evaluation of the gel with atomic force microscopy (AFM) indicated that the surface of the protein fiber skeleton show the network-like structure and relatively smooth. Characterization by circular dichroism (CD) and Fourier transform infrared spectroscopy (FT-IR) demonstrated that the two proteins were successfully assembled, and that the assembled protein had a secondary structure dominated by α-helices. The rheological properties of the self-assembled products were tested to confirm that they were indeed hydrogel property.

Purified mucins in drug delivery research

One of the main challenges in the field of drug delivery remains the development of strategies to efficiently transport pharmaceuticals across mucus barriers, which regulate the passage and retention of molecules and particles in all luminal spaces of the body. A thorough understanding of the molecular mechanisms, which govern such selective permeability, is key for achieving efficient translocation of drugs and drug carriers. For this purpose, model systems based on purified mucins can contribute valuable information. In this review, we summarize advances that were made in the field of drug delivery research with such mucin-based model systems: First, we give an overview of mucin purification procedures and discuss the suitability of model systems reconstituted from purified mucins to mimic native mucus. Then, we summarize techniques to study mucin binding. Finally, we highlight approaches that made use of mucins as building blocks for drug delivery platforms or employ mucins as active compounds.

Evolution of a remarkable intracellular polymer and extreme cell allometry in hagfishes

The size of animal cells rarely scales with body size, likely due to biophysical and physiological constraints.^1,2 In hagfishes, gland thread cells (GTCs) each produce a silk-like proteinaceous fiber called a slime thread.^3,4 The slime threads impart strength to a hagfish's defensive slime and thus are potentially subject to selection on their function outside of the body.^5-8 Body size is of fundamental importance in predator-prey interactions, which led us to hypothesize that larger hagfishes produce longer and stronger slime threads than smaller ones.⁹ Here, by sampling a range of sizes of hagfish from 19 species, we systematically examined the scaling of GTC and slime-thread dimensions with body size within both phylogenetic and ontogenetic contexts. We found that the length of GTCs varied between 40 and 250 μm and scaled positively with body size, exhibiting an allometric exponent greater than those in other animal cells. Correspondingly, larger hagfishes produce longer and thicker slime threads and thus are equipped to defend against larger predators. With diameter and length varying 4-fold (0.7-4 μm and 5-22 cm, respectively) over a body-size range of 10-128 cm, the slime threads characterize the largest intracellular polymers known in biology. Our results suggest selection for stronger defensive slime in larger hagfishes has driven the evolution of extreme size and allometry of GTCs.

A bioinspired synthetic soft hydrogel for the treatment of dry eye

Natural soft hydrogels are unique elastic soft materials utilized by living organisms for protecting delicate tissues. Under a theoretical framework derived from the Blob model, we chemically crosslinked high molecular weight hyaluronic acid at a concentration close to its overlap concentration (c*), and created synthetic soft hydrogels that exhibited unique rheological properties similar to a natural soft hydrogel: being dominantly elastic under low shear stress while being viscous when the stress is above a small threshold. We explored a potential application of the hyaluronic acid-based soft hydrogel as a long-acting ocular surface lubricant and evaluated its therapeutic effects for dry eye. The soft hydrogel was found to be biocompatible after topical instillation on experimental animals' and companion dogs' eyes. In a canine clinical study, twice-a-day ocular instillation of the soft hydrogel in combination with cyclosporine for 1 month improved the clinical signs in more than 65% of dog patients previously unresponsive to cyclosporine treatment.

Nanoscale Material Heterogeneity of Glowworm Capture Threads Revealed by AFM

Adhesive materials used by many arthropods for biological functions incorporate sticky substances and a supporting material that operate synergistically by exploiting substrate attachment and energy dissipation. While there has been much focus on the composition and properties of the sticky glues of these bio-composites, less attention has been given to the materials that support them. In particular, as these materials are primarily responsible for dissipation during adhesive pull-off, little is known of the structures that give rise to functionality, especially at the nano-scale. In this study we used tapping mode atomic force microscopy (TM-AFM) to analyze unstretched and stretched glowworm (Arachnocampa tasmaniensis) capture threads and revealed nano-scale features corresponding to variation in surface structure and elastic modulus near the surface of the silk. Phase images demonstrated a high resolution of viscoelastic variation and revealed mostly globular and elongated features in the material. Increased vertical orientation of 11–15 nm wide fibrillar features was observed in stretched threads. Fast Fourier transform analysis of phase images confirmed these results. Relative viscoelastic properties were also highly variable at inter- and intra-individual levels. Results of this study demonstrate the practical usefulness of TM-AFM, especially phase angle imaging, in investigating the nano-scale structures that give rise to macro-scale function of soft and highly heterogeneous materials of both natural and synthetic origins.

From reductionism to synthesis: The case of hagfish slime

Reductionist strategies aim to understand the mechanisms of complex systems by studying individual parts and their interactions. In this review, we discuss how reductionist approaches have shed light on the structure, function, and production of a complex biomaterial - hagfish defensive slime. Hagfish slime is an extremely dilute hydrogel-like material composed of seawater, mucus, and silk-like proteins that can deploy rapidly. Despite being composed almost entirely of water, hagfish slime has remarkable physical properties, including high strength and toughness. While hagfish slime has a promising future in biomimetics, including the development of eco-friendly high-performance fibers, recreating hagfish slime in the lab has been a difficult challenge. Over the past two decades, reductionist experiments have provided a wealth of information about the individual components of hagfish slime. However, a reductionist approach provides a limited understanding because hagfish defensive slime, like most biological phenomena, is more than just the sum of its parts. We end by providing some thoughts about how the knowledge generated in the last few decades might be synthesized into a working model that can explain hagfish slime structure and function.

Complex fluids in animal survival strategies

Animals have evolved distinctive survival strategies in response to constant selective pressure. In this review, we highlight how animals exploit flow phenomena by manipulating their habitat (exogenous) or by secreting (endogenous) complex fluids. Ubiquitous endogenous complex fluids such as mucus demonstrate rheological versatility and are therefore involved in many animal behavioral traits ranging from sexual reproduction to protection against predators. Exogenous complex fluids such as sand can be used either for movement or for predation. In all cases, time-dependent rheological properties of complex fluids are decisive for the fate of the biological behavior and vice versa. To exploit these rheological properties, it is essential that the animal is able to sense the rheology of their surrounding complex fluids in a timely fashion. As timing is key in nature, such rheological materials often have clearly defined action windows matching the time frame of their direct biological behavior. As many rheological properties of these biological materials remain poorly studied, we demonstrate with this review that rheology and material science might provide an interesting quantitative approach to study these biological materials in particular in context towards ethology and bio-mimicking material design.

Turning Rubber into a Glass: Mechanical Reinforcement by Microphase Separation

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement. This manuscript presents studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

A New Model of Hagfish Slime Mucous Vesicle Stabilization and Deployment

Hagfishes thwart predators by releasing large volumes of gill-clogging slime, which consists of mucus and silk-like fibers. The mucous fraction originates within gland mucous cells, which release numerous vesicles that swell and rupture when ejected into seawater. Several studies have examined the function of hagfish slime mucous vesicles in vitro, but a comprehensive model of their biophysics is lacking. Here, we tested the hypothesis that vesicles contain polyanionic glycoproteins stabilized by divalent cations and deploy in seawater via exchange of divalent for monovalent cations. We also tested the hypothesis that vesicle swelling and stabilization are governed by "Hofmeister effects". We found no evidence for either hypothesis. Our results show that hagfish mucous granules are only stabilized by multivalent anions, and pH titration experiments underscore these results. Our results lead us to the conclusion that the hagfish slime mucous gel is in fact polycationic in nature.

Concentration effects of three common fish anesthetics on Pacific hagfish (Eptatretus stoutii)

The efficacy of three common fish anesthetics (clove oil, 2-phenoxyethanol, and tricaine methanesulfonate) was evaluated in the Pacific hagfish (Eptatretus stoutii). The overarching aim of our study was to identify the best anesthetic and concentration for the purposes of routine laboratory use of Pacific hagfish (i.e., short and consistent induction and recovery times and minimized stress and safety risk to hagfish). The objectives of our study were fourfold: (1) identify anesthetic stages of Pacific hagfish using clove oil anesthesia; (2) establish standardized anesthesia preparation procedures; (3) determine the optimal anesthetic and concentration for safely achieving stage V anesthesia; and (4) investigate the effects of repeatedly exposing Pacific hagfish to anesthesia. Experimental concentrations, ranging from 50 to 400 mg/L, of each anesthetic were tested on at least three Pacific hagfish individuals. We found the following: (1) Pacific hagfish exhibited similar stages of anesthesia to those described for bony fishes; (2) sufficient mixing of clove oil with seawater had a considerable effect on the consistency and timing of anesthetic induction; (3) concentration and anesthetic significantly impacted induction and recovery timing, whereas body mass had no impact on anesthetic trends; and (4) repeatedly exposing Pacific hagfish to optimal concentrations of clove oil or MS-222 had no effect on induction or recovery timing, whereas exposure number significantly impacted induction timing when using 2-PE. Due to consistent induction and recovery times, low risk of accidental overdose, and high safety margins for both handler and hagfish, we recommend 175 mg/L of clove oil as the ideal anesthetic and concentration for the routine laboratory use of Pacific hagfish.

Construction from Destruction: Hydrogel Formation from Triggered Depolymerization-Based Release of an Enzymatic Catalyst

Biomimetic systems that undergo macroscopic phase transformations by transducing and amplifying external cues are highly desirable for applications such as self-healing. Here, we report self-assembly of polyelectrolyte complexes into a vesicular structure that can accommodate hydrophilic guest molecules, including enzymes. Triggered depolymerization of one of the polyelectrolyte molecules in the complex causes the vesicle to disassemble and release its contents. Such a triggered release of enzymes causes molecular-scale events to be amplified due to the enzyme's catalytic properties. This feature has been utilized to demonstrate construction of hydrogels from the destruction of nanoscopic polymeric vesicles. The design principles developed here are broadly adaptable to other triggerable depolymerization motifs reported in the literature.

Unravelling hagfish slime

Hagfish slime is a unique predator defence material containing a network of long fibrous threads each ∼10 cm in length. Hagfish release the threads in a condensed coiled state known as skeins (∼100 µm), which must unravel within a fraction of a second to thwart a predator attack. Here we consider the hypothesis that viscous hydrodynamics can be responsible for this rapid unravelling, as opposed to chemical reaction kinetics alone. Our main conclusion is that, under reasonable physiological conditions, unravelling due to viscous drag can occur within a few hundred milliseconds, and is accelerated if the skein is pinned at a surface such as the mouth of a predator. We model a single skein unspooling as the fibre peels away due to viscous drag. We capture essential features by considering simplified cases of physiologically relevant flows and one-dimensional scenarios where the fibre is aligned with streamlines in either uniform or uniaxial extensional flow. The peeling resistance is modelled with a power-law dependence on peeling velocity. A dimensionless ratio of viscous drag to peeling resistance appears in the dynamical equations and determines the unraveling time scale. Our modelling approach is general and can be refined with future experimental measurements of peel strength for skein unravelling. It provides key insights into the unravelling process, offers potential answers to lingering questions about slime formation from threads and mucous vesicles, and will aid the growing interest in engineering similar bioinspired material systems.

High concentrations of trimethylamines in slime glands inhibit skein unraveling in Pacific hagfish

Hagfish defend themselves from fish predators by producing large volumes of gill-clogging slime when they are attacked. The slime consists of seawater and two major components that are ejected from the slime glands: mucus and threads. The threads are produced within specialized cells and packaged into intricately coiled bundles called skeins. Skeins are kept from unraveling via a protein adhesive that dissolves when the skeins are ejected from the slime glands. Previous work revealed that hagfish slime glands have high concentrations of methylamines including trimethylamine N-oxide (TMAO), trimethylglycine (betaine) and dimethylglycine (DMG); however, the function of these compounds in the slime glands is unknown. We hypothesized that methylamines have stabilizing effects on the skeins that prevent premature unraveling in the gland. To test this hypothesis, we quantified the effect of methylamines on skein unraveling in Pacific hagfish and found that TMAO and betaine have inhibitory effects on skein unraveling in vitro Furthermore, we found that TMAO is a more effective inhibitor of unraveling than betaine, but the presence of TMAO synergistically boosts the inhibitory action of betaine. Glycine and DMG were far less effective inhibitors of unraveling at natural concentrations. Our results support the hypothesis that high levels of trimethylamines in the slime glands may act to hold the coiled thread skeins together within gland thread cells, and they may do so by stabilizing adhesive proteins. These results advance our knowledge of skein stabilization and deployment and provide yet another example of trimethylamines functioning to stabilize proteins in a marine organism.

Structure and dynamics of hagfish mucin in different saline environments

The defense mechanism of hagfish against predators is based on its ability to form slime within a few milliseconds. Hagfish slime consists of two main components, namely mucin-like glycoproteins and long protein threads, which together entrap vast amounts of water and thus form a highly dilute hydrogel. Here, we investigate the mucin part of this hydrogel, in particular the role of the saline marine environment on the viscoelasticity and structure. By means of dynamic light scattering (DLS), shear and extensional rheology we probe the diffusion dynamics, the flow behavior, and the longest filament breaking time of hagfish mucin solutions. Using DLS we find a concentration-independent diffusion coefficient - characteristic for polyelectrolytes - up to the entanglement regime of 0.2 mg ml-1, which is about ten times higher than the natural concentration of hagfish mucin in hagfish slime. We also observe a slow relaxation process associated with clustering, probably due to electrostatic interactions. Shear rheology further revealed that hagfish mucin possesses pronounced viscoelastic properties at high concentrations (3 mg ml-1), showing that mucin alone achieves mechanical properties similar to those of natural hagfish slime (mucins and protein threads). The main effects of added seawater salts, and predominantly CaCl2 is to reduce the intensity of the slow relaxation process, which suggests that calcium ions lead to an ionotropic gelation of hagfish mucins.

The good, the bad and the slimy: experimental studies of hagfish digestive and nutritional physiology

The hagfishes provide valuable insight into the physiology of feeding, digestion and nutrient absorption by virtue of unusual and unique features of their biology. For example, members of this group undergo long periods of fasting, and are the only vertebrates known to absorb organic nutrients across their epidermal surface. Such properties engender significant attention from researchers interested in feeding and feeding-related processes; however, the practical realities of employing the hagfish as an experimental organism can be challenging. Many of the key tools of the experimental biologist are compromised by a species that does not readily feed in captivity, is difficult to instrument and which produces copious quantities of slime. This Commentary provides critical insight into the key aspects of hagfish feeding and digestive processes, and highlights the pitfalls of this group as experimental organisms. We also suggest key research gaps that, if filled, will lead to better understanding of hagfishes, and we consider how this group may advance our knowledge of feeding, digestion and nutrient absorption processes.

Unraveling inter-species differences in hagfish slime skein deployment

Hagfishes defend themselves from fish predators by producing defensive slime consisting of mucous and thread components that interact synergistically with seawater to pose a suffocation risk to their attackers. Deployment of the slime occurs in a fraction of a second and involves hydration of mucous vesicles as well as unraveling of the coiled threads to their full length of ∼150 mm. Previous work showed that unraveling of coiled threads (or ‘skeins’) in Atlantic hagfish requires vigorous mixing with seawater as well as the presence of mucus, whereas skeins from Pacific hagfish tend to unravel spontaneously in seawater. Here, we explored the mechanisms that underlie these different unraveling modes, and focused on the molecules that make up the skein glue, a material that must be disrupted for unraveling to proceed. We found that Atlantic hagfish skeins are also held together with a protein glue, but compared with Pacific hagfish glue, it is less soluble in seawater. Using SDS-PAGE, we identified several soluble proteins and glycoproteins that are liberated from skeins under conditions that drive unraveling in vitro. Peptides generated by mass spectrometry of five of these proteins and glycoproteins mapped strongly to 14 sequences assembled from Pacific hagfish slime gland transcriptomes, with all but one of these sequences possessing homologs in the Atlantic hagfish. Two of these sequences encode unusual acidic proteins that we propose are the structural glycoproteins that make up the skein glue. These sequences have no known homologs in other species and are likely to be unique to hagfishes. Although the ecological significance of the two modes of skein unraveling described here are unknown, they may reflect differences in predation pressure, with selection for faster skein unraveling in the Eptatretus lineage leading to the evolution of a glue that is more soluble.

Structure and Nanomechanics of Dry and Hydrated Intermediate Filament Films and Fibers Produced from Hagfish Slime Fibers

Intermediate filaments (IFs) are known for their extensibility, flexibility, toughness, and their ability to hydrate. Using keratin-like IFs obtained from slime fibers from the invertebrate Atlantic hagfish ( Myxine glutinosa), films were produced by drop-casting and coagulation on the surface of a MgCl₂ buffer. Drop-casting produced self-supporting, smooth, and dense films rich in β-sheets (61%), whereas coagulation formed thin, porous films with a nanorough surface and a lower β-sheet content (51%). The films hydrated and swelled immediately when immersed in water and did not dissolve. X-ray diffraction showed that the β-crystallites remained stable upon hydration, that swelling presumably happens in the amorphous C-terminal tail-domains of the IFs, and that high salt conditions caused a denser network mesh size, suggesting polyelectrolyte behavior. Hydration resulted in a roughly 1000-fold decrease in apparent Young's modulus from 10⁹ to 10⁶ Pa as revealed by atomic force microscopy nanoindentation. Nanoindentation-based power-law rheology and stress-relaxation measurements indicated viscoelasticity and a soft-solid hydrogel character for hydrated films, where roughly 80% of energy is elastically stored and 20% is dissipated. By pulling coagulation films from the buffer interface, macroscopic fibers with highly aligned IF β-crystals similar to natural hagfish fibers were produced. We propose that viscoelasticity and strong hydrogen bonding interactions with the buffer interface are crucial for the production of such long biomimetic fibers with aligned β-sheets. This study demonstrates that hagfish fiber IFs can be reconstituted into functional biomimetic materials that are stiff when dry and retain the ability to hydrate to become soft and viscoelastic when in water.

Concentration-independent mechanics and structure of hagfish slime

The defense mechanism of hagfish slime is remarkable considering that hagfish cannot control the concentration of the resulting gel directly; they simply exude a concentrated material into a comparably "infinite" sea of water to form a dilute, sticky, cohesive elastic gel. This raises questions about the robustness of gel formation and rheological properties across a range of concentrations, which we study here for the first time. Across a nearly 100-fold change in concentration, we discover that the gel has similar viscoelastic time-dependent properties with constant power-law exponent (α=0.18±0.01), constant relative damping tanδ=G^''/G^'≈0.2-0.3, and varying overall stiffness that scales linearly with the concentration (∼c^0.99±0.05). The power-law viscoelasticity (fit by a fractional Kelvin-Voigt model) is persistent at all concentrations with nearly constant fractal dimension. This is unlike other materials and suggests that the underlying material structure of slime remains self-similar irrespective of concentration. This interpretation is consistent with our microscopy studies of the fiber network. We derive a structure-rheology model to test the hypothesis that the origins of ultra-soft elasticity are based on bending of the fibers. The model predictions show an excellent agreement with the experiments. Our findings illustrate the unusual and robust properties of slime which may be vital in its physiological use and provide inspiration for the design of new engineered materials.

STATEMENT OF SIGNIFICANCE

Hagfish produce a unique gel-like material to defend themselves against predator attacks. The successful use of the defense gel is remarkable considering that hagfish cannot control the concentration of the resulting gel directly; they simply exude a small quantity of biomaterial which then expands by a factor of 10,000 (by volume) into an "infinite" sea of water. This raises questions about the robustness of gel formation and properties across a range of concentrations. This study provides the first ever understanding of the mechanics of hagfish slime over a very wide range of concentration. We discover that some viscoelastic properties of slime are remarkably constant regardless of its concentration. Such a characteristic is uncommon in most known materials.

Effect of ionic strength and seawater cations on hagfish slime formation

The defensive slime of hagfish consists of a polyanionic mucin hydrogel that synergistically interacts with a fiber network forming a coherent and elastic hydrogel in high ionic strength seawater. In seawater, the slime deploys in less than a second entrapping large quantities of water by a well-timed thread skein unravelling and mucous gel swelling. This rapid and vast hydrogel formation is intriguing, as high ionic strength conditions generally counteract the swelling speed and ratio of polyelectrolyte hydrogels. In this work we investigate the effect of ionic strength and seawater cations on slime formation dynamics and functionality. In the absence of ionic strength skeins swell radially and unravel uncontrolled, probably causing tangling and creating a confined thread network that entraps limited water. At high ionic strength skeins unravel, but create a collapsed and dense fiber network. High ionic strength conditions therefore seem crucial for controlled skein unraveling, however not sufficient for water retention. Only the presence of naturally occurring Ca2+ or Mg2+-ions allowed for an expanded network and full water retention probably due to Ca2+-mediated vesicle rupture and cross-linking of the mucin. Our study demonstrates that hagfish slime deployment is a well-timed, ionic-strength, and divalent-cation dependent dynamic hydrogel formation process.

Emptying and refilling of slime glands in Atlantic (Myxine glutinosa) and Pacific (Eptatretus stoutii) hagfishes

Hagfishes are known for their unique defensive slime, which they use to ward off gill breathing predators. While much is known about the slime cells (gland thread cells and gland mucous cells), little is known about how long slime gland refilling takes, or how slime composition changes with refilling or repeated stimulation of the same gland. Slime glands can be individually electro-stimulated to release slime, and this technique was used to measure slime gland refilling times for Atlantic and Pacific hagfish. The amount of exudate produced, the composition of exudate, and the morphometrics of slime cells were analyzed during refilling, and as a function of stimulation number when full glands were stimulated in rapid succession. Complete refilling of slime glands for both species took three to four weeks, with Pacific hagfish achieving faster absolute rates exudate recovery than Atlantics. We found significant changes in composition of exudate and morphometrics of slime cells from Pacific hagfish during refilling. Over successive stimulations of full Pacific glands, multiple boluses of exudate were released, with exudate composition, but not thread cell morphometrics, changing significantly. Finally, histological examination of slime glands revealed slime cells retained in glands after exhaustion. Discrepancies in volume of cells released that can be explained by contraction of striated muscle alone suggests other mechanisms may be involved in the exudate ejection. Our results provide a first look at the process and timing of slime gland refilling in hagfishes, and raise new questions about how refilling is achieved at the cellular level.

Cellular mechanisms of slime gland refilling in Pacific hagfish (Eptatretus stoutii)

Hagfishes use their defensive slime to ward off gill-breathing predators. Slime gland refilling is a surprisingly slow process, and previous research has shown that the composition of the slime exudate changes significantly during refilling, which likely has consequences for the functionality of the slime. This study set out to expand our understanding of slime gland refilling by examining the cellular processes involved in refilling of the glands, as well as determining where in the gland the main slime cells, the gland thread cells and gland mucous cells, arise. Slime glands were electro-stimulated to exhaust their slime stores, left to refill for set periods of time, and harvested for histological and immunohistochemical examination. Whole slime glands, gland thread cell morphometrics and slime cell proportions were examined over the refilling cycle. Slime glands decreased significantly in size after exhaustion, but steadily increased in size over refilling. Gland thread cells were the limiting factor in slime gland refilling, taking longer to replenish and mature than gland mucous cells. Newly produced gland thread cells underwent most of their growth near the edge of the gland, and larger cells were found farthest from the edge of the gland. Immunohistochemical analysis also revealed proliferating cells only within the epithelial lining of the slime gland, suggesting that new slime cells originate from undifferentiated cells lining the gland. Our results provide an in-depth look at the cellular dynamics of slime gland refilling in Pacific hagfish, and provide a model for how slime glands refill at the cellular level.

No support for Heincke's law in hagfish (Myxinidae): lack of an association between body size and the depth of species occurrence

This study tests for interspecific evidence of Heincke's law among hagfishes and advances the field of research on body size and depth of occurrence in fishes by including a phylogenetic correction and by examining depth in four ways: maximum depth, minimum depth, mean depth of recorded specimens and the average of maximum and minimum depths of occurrence. Results yield no evidence for Heincke's law in hagfishes, no phylogenetic signal for the depth at which species occur, but moderate to weak phylogenetic signal for body size, suggesting that phylogeny may play a role in determining body size in this group.

Hagfish slime exudate stabilization and its effect on slime formation and functionality

Hagfish produce vast amounts of slime when under attack. The slime is the most dilute hydrogel known to date, and is a highly interesting material for biomaterial research. It forms from a glandular secrete, called exudate, which deploys upon contact with seawater. To study slime formation ex vivo and to characterize its material properties, stabilization of the sensitive slime exudate is crucial. In this study, we compared the two main stabilization methods, dispersion in high osmolarity citrate/PIPES (CP) buffer and immersion in oil, and tested the influence of time, temperature and pH on the stability of the exudate and functionality of the slime. Using water retention measurements to assess slime functionality, we found that CP buffer and oil preserved the exudate within the first 5 hours without loss of functionality. For longer storage times, slime functionality decreased for both stabilization methods, for which the breakdown mechanisms differed. Stabilization in oil likely favored temperature-sensitive osmotic-driven swelling and rupture of the mucin vesicles, causing the exudate to gel and clump. Extended storage in CP buffer resulted in an inhibited unraveling of skeins. We suggest that a water soluble protein glue, which mediates skein unraveling in functional skeins, denatures and gradually becomes insoluble during storage in CP buffer. The breakdown was accentuated when the pH of the CP buffer was raised from pH 6.7 to pH 8.5, probably caused by increased denaturation of the protein glue or by inferior vesicle stabilization. However, when fresh exudate was mixed into seawater or phosphate buffer at pH 6-9, slime functionality was not affected, showing pH insensitivity of the slime formation around a neutral pH. These insights on hagfish exudate stabilization mechanisms will support hagfish slime research at a fundamental level, and contribute to resolve the complex mechanisms of skein unraveling and slime formation.

Hagfish slime and mucin flow properties and their implications for defense

When hagfish (Myxinidae) are attacked by predators, they form a dilute, elastic, and cohesive defensive slime made of mucins and protein threads. In this study we propose a link between flow behavior and defense mechanism of hagfish slime. Oscillatory rheological measurements reveal that hagfish slime forms viscoelastic networks at low concentrations. Mucins alone did not contribute viscoelasticity, however in shear flow, viscosity was observed. The unidirectional flow, experienced by hagfish slime during suction feeding by predators, was mimicked with extensional rheology. Elongational stresses were found to increase mucin viscosity. The resulting higher resistance to flow could support clogging of the attacker's gills. Shear flow in contrast decreases the slime viscosity by mucin aggregation and leads to a collapse of the slime network. Hagfish may benefit from this collapse when trapped in their own slime and facing suffocation by tying a sliding knot with their body to shear off the slime. This removal could be facilitated by the apparent shear thinning behavior of the slime. Therefore hagfish slime, thickening in elongation and thinning in shear, presents a sophisticated natural high water content gel with flow properties that may be beneficial for both, defense and escape.

The Hagfish Gland Thread Cell: A Fiber-Producing Cell Involved in Predator Defense

Fibers are ubiquitous in biology, and include tensile materials produced by specialized glands (such as silks), extracellular fibrils that reinforce exoskeletons and connective tissues (such as chitin and collagen), as well as intracellular filaments that make up the metazoan cytoskeleton (such as F-actin, microtubules, and intermediate filaments). Hagfish gland thread cells are unique in that they produce a high aspect ratio fiber from cytoskeletal building blocks within the confines of their cytoplasm. These threads are elaborately coiled into structures that readily unravel when they are ejected into seawater from the slime glands. In this review we summarize what is currently known about the structure and function of gland thread cells and we speculate about the mechanism that these cells use to produce a mechanically robust fiber that is almost one hundred thousand times longer than it is wide. We propose that a key feature of this mechanism involves the unidirectional rotation of the cell’s nucleus, which would serve to twist disorganized filaments into a coherent thread and impart a torsional stress on the thread that would both facilitate coiling and drive energetic unravelling in seawater.

Gelation of Soy Milk with Hagfish Exudate Creates a Flocculated and Fibrous Emulsion- and Particle Gel

Hagfish slime is an ultra dilute, elastic and cohesive hydrogel that deploys within milliseconds in cold seawater from a glandularly secreted exudate. The slime is made of long keratin-like fibers and mucin-like glycoproteins that span a network which entraps water and acts as a defense mechanism against predators. Unlike other hydrogels, the slime only confines water physically and is very susceptible to mechanical stress, which makes it unsuitable for many processing operations and potential applications. Despite its huge potential, little work has been done to improve and functionalize the properties of this hydrogel. To address this shortcoming, hagfish exudate was mixed with a soy protein isolate suspension (4% w/v) and with a soy emulsion (commercial soy milk) to form a more stable structure and combine the functionalities of a suspension and emulsion with those of the hydrogel. Hagfish exudate interacted strongly with the soy systems, showing a markedly increased viscoelasticity and water retention. Hagfish mucin was found to induce a depletion and bridging mechanism, which caused the emulsion and suspension to flocculate, making "soy slime", a cohesive and cold-set emulsion- and particle gel. The flocculation network increases viscoelasticity and substantially contributes to liquid retention by entrapping liquid in the additional confinements between aggregated particles and protein fibers. Because the mucin-induced flocculation resembles the salt- or acid-induced flocculation in tofu curd production, the soy slime was cooked for comparison. The cooked soy slime was similar to conventional cooked tofu, but possessed a long-range cohesiveness from the fibers. The fibrous, cold-set, and curd-like structure of the soy slime represents a novel way for a cold coagulation and fiber incorporation into a suspension or emulsion. This mechanism could be used to efficiently gel functionalized emulsions or produce novel tofu-like structured food products.

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.

Self-Assembly of Recombinant Hagfish Thread Keratins Amenable to a Strain-Induced α-Helix to β-Sheet Transition

Hagfish slime threads are assembled from protein-based bundles of intermediate filaments (IFs) that undergo a strain-induced α-helical coiled-coil to β-sheet transition. Draw processing of native fibers enables the creation of mechanically tuned materials, and under optimized conditions this process results in mechanical properties similar to spider dragline silk. In this study, we develop the foundation for the engineering of biomimetic recombinant hagfish thread keratin (TK)-based materials. The two protein constituents from the hagfish Eptatretus stoutii thread, named EsTKα and EsTKγ, were expressed in Escherichia coli and purified. Individual (rec)EsTKs and mixtures thereof were subjected to stepwise dialysis to evaluate their protein solubility, folding, and self-assembly propensities. Conditions were identified that resulted in the self-assembly of coiled-coil rich IF-like filaments, as determined by circular dichroism (CD) and transmission electron microscopy (TEM). Rheology experiments indicated that the concentrated filaments assembled into gel-like networks exhibiting a rheological response reminiscent to that of IFs. Notably, the self-assembled filaments underwent an α-helical coiled-coil to β-sheet transition when subjected to oscillatory shear, thus mimicking the critical characteristic responsible for mechanical strengthening of native hagfish threads. We propose that our data establish the foundation to create robust and tunable recombinant TK-based materials whose mechanical properties are controlled by a strain-induced α-helical coiled-coil to β-sheet transition.

Physiology, Biomechanics, and Biomimetics of Hagfish Slime

Hagfishes thwart attacks by fish predators by producing liters of defensive slime. The slime is produced when slime gland exudate is released into the predator's mouth, where it deploys in a fraction of a second and clogs the gills. Slime exudate is composed mainly of secretory products from two cell types, gland mucous cells and gland thread cells, which produce the mucous and fibrous components of the slime, respectively. Here, we review what is known about the composition of the slime, morphology of the slime gland, and physiology of the cells that produce the slime. We also discuss several of the mechanisms involved in the deployment of both mucous and thread cells during the transition from thick glandular exudate to ultradilute material. We review biomechanical aspects of the slime, along with recent efforts to produce biomimetic slime thread analogs, and end with a discussion of how hagfish slime may have evolved.

Host-defense peptides of the skin with therapeutic potential: From hagfish to human

It is now well established that peptides that were first identified on the basis of their ability to inhibit growth of bacteria and fungi are multifunctional and so are more informatively described as host-defense peptides. In some cases, their role in protecting the organism against pathogenic microorganisms, although of importance, may be secondary. A previous article in the journal (Peptides 2014; 57:67-77) assessed the potential of peptides present in the skin secretions of frogs for development into anticancer, antiviral, immunomodulatory and antidiabetic drugs. This review aims to extend the scope of this earlier article by focusing upon therapeutic applications of host-defense peptides present in skin secretions and/or skin extracts of species belonging to other vertebrate classes (Agnatha, Elasmobranchii, Teleostei, Reptilia, and Mammalia as represented by the human) that supplement their potential role as anti-infectives for use against multidrug-resistant microorganisms.

Slipping vs sticking: water-dependent adhesive and frictional properties of Linum usitatissimum L. seed mucilaginous envelope and its biological significance

Flax seeds produce mucilage after wetting. The mucilage due to its ability to absorb and maintain water is responsible for specific surface properties which are essential for seed dispersal in different ways. In the present paper, we asked how the hydration level affects the adhesive and frictional properties of the mucilage and which role does the mucilage play in seed dispersal? We have experimentally quantified: (1) desiccation dynamics of seeds with a mucilage envelope, (2) desiccation-time dependence of their friction coefficient, and (3) desiccation-time dependence of their pull-off forces on a smooth glass substrate. Freshly-hydrated seeds had an extremely low friction coefficient, which rapidly increased with an increasing desiccation time. Pull-off force just after hydration was rather low, then increased with an increasing water loss. Adhesion and friction experiments show that there is a clear maximum in the force values at certain hydration states of the mucilage. Different hydration levels of the mucilage can be employed in various dispersal mechanisms. Fully hydrated mucilage with its low viscosity gives optimal sliding conditions for endozoochory, whereas water loss provides conditions for the epizoochory. We suggest that the hydration level of the mucilage envelope can determine the potential mode of flax seed dispersal.

Spontaneous unraveling of hagfish slime thread skeins is mediated by a seawater-soluble protein adhesive

Hagfishes are known for their ability to rapidly produce vast quantities of slime when provoked. The slime is formed via the interaction between seawater and two components released by the slime glands: mucin vesicles from gland mucous cells, which swell and rupture in seawater to form a network of mucus strands, and intermediate filament-rich threads, which are produced within gland thread cells as tightly coiled bundles called skeins. A previous study showed that the unraveling of skeins from Atlantic hagfish (Myxine glutinosa) requires both the presence of mucins and hydrodynamic mixing. In contrast, skeins from Pacific hagfish (Eptatretus stoutii) unravel in the absence of both mucins and mixing. We tested the hypothesis that spontaneous unraveling of E. stoutii skeins is triggered by the dissolution of a seawater-soluble protein adhesive and the release of stored strain energy within the coiled thread. Here we show that, as predicted by this hypothesis, unraveling can be initiated by a protease under conditions in which unraveling does not normally occur. We also demonstrate, using high resolution scanning electron microscopy, that the treatment of skeins with solutions that cause unraveling also leads to the disappearance of surface and inter-thread features that remain when skeins are washed with stabilizing solutions. Our study provides a mechanism for the deployment of thread skeins in Pacific hagfish slime, and raises the possibility of producing novel biomimetic protein adhesives that are salt, temperature and kosmotrope sensitive.

Defensive slime formation in Pacific hagfish requires Ca2+- and aquaporin-mediated swelling of released mucin vesicles

Hagfishes defend themselves from fish predators via the rapid deployment of a fibrous slime that adheres to and clogs gills. The slime transforms from a thick glandular exudate to a fully hydrated product in a fraction of a second through a process that involves the swelling and rupture of numerous mucin vesicles. Here we demonstrate that the vesicle membrane plays an important role in regulating the swelling of mucin granules, and provide evidence that the membrane contains proteins that facilitate the movement of ions and water molecules. By exposing isolated mucin vesicles to varying combinations of inorganic ions, organic compounds and membrane channel inhibitors, we found that the majority of hagfish mucin vesicles require Ca(2+) to rupture. We also show that Ca(2+)-dependent rupture can be pharmacologically inhibited, which suggests a role for Ca(2+)-activated membrane transporters. We demonstrate that the aquaporin inhibitor mercuric chloride reduces the rate of vesicle swelling by an order of magnitude, which suggests that aquaporins facilitate the influx of water during vesicle deployment. Molecular evidence of two aquaporin homologues expressed in the slime glands further supports this idea. We propose a model of hagfish slime mucin vesicle rupture that involves Ca(2+)-activated transporters and aquaporins, and suggest that the presence of these proteins is an adaptation for increasing the speed of vesicle rupture and, consequently, the speed of the sliming response of hagfishes.

Coiling and maturation of a high-performance fibre in hagfish slime gland thread cells

The defensive slime of hagfishes contains thousands of intermediate filament protein threads that are manufactured within specialized gland thread cells. The material properties of these threads rival those of spider dragline silks, which makes them an ideal model for biomimetic efforts to produce sustainable protein materials, yet how the thread is produced and organized within the cell is not well understood. Here we show how changes in nuclear morphology, size and position can explain the three-dimensional pattern of thread coiling in gland thread cells, and how the ultrastructure of the thread changes as very young thread cells develop into large cells with fully mature coiled threads. Our model provides an explanation for the complex process of thread assembly and organization that has fascinated and perplexed biologists for over a century, and provides valuable insights for the quest to manufacture high-performance biomimetic protein materials.