Robust cross-links in molluscan adhesive gels: testing for contributions from hydrophobic and electrostatic interactions

Particle binding capacity of snail saliva

Gastropods forage with their radula, a thin chitinous membrane with embedded teeth, which scratch across the substrate to lose food particles. During this interaction, the risk of loosening particles is obvious without having a specialized mechanism holding them on the tooth surface. As mucus secretions are essential in molluscan life cycles and the locomotion and attachment gels are known to have an instant high adhesion, we have hypothesized that the saliva could support particle retention during feeding. As adhesion of snail saliva was not studied before, we present here an experimental setup to test its particle-binding capacity using a large land snail (Lissachatina fulica, Stylommatophora, Heterobranchia). This experiment was also applied to the gels produced by the snail foot for comparison and can be potentially applied to various fluids present at a small volume in the future. We found, that the saliva has high particle retention capacity that is comparable to the foot glue of the snail. To gain some insight into the properties of the saliva, we additionally studied it in the scanning electron microscope, estimated its viscosity in a de-wetting experiment, and investigated its elemental composition using energy dispersive X-ray spectroscopy reveling higher contents of Ca, Zn and other potential cross-linkers similar to those found in the glue.

High Performance Marine and Terrestrial Bioadhesives and the Biomedical Applications They Have Inspired

This study has reviewed the naturally occurring bioadhesives produced in marine and freshwater aqueous environments and in the mucinous exudates of some terrestrial animals which have remarkable properties providing adhesion under difficult environmental conditions. These bioadhesives have inspired the development of medical bioadhesives with impressive properties that provide an effective alternative to suturing surgical wounds improving closure and healing of wounds in technically demanding tissues such as the heart, lung and soft tissues like the brain and intestinal mucosa. The Gecko has developed a dry-adhesive system of exceptional performance and has inspired the development of new generation re-usable tapes applicable to many medical procedures. The silk of spider webs has been equally inspiring to structural engineers and materials scientists and has revealed innovative properties which have led to new generation technologies in photonics, phononics and micro-electronics in the development of wearable biosensors. Man made products designed to emulate the performance of these natural bioadhesive molecules are improving wound closure and healing of problematic lesions such as diabetic foot ulcers which are notoriously painful and have also found application in many other areas in biomedicine. Armed with information on the mechanistic properties of these impressive biomolecules major advances are expected in biomedicine, micro-electronics, photonics, materials science, artificial intelligence and robotics technology.

Metal-binding proteins and cross-linking in the defensive glue of the slug Arion subfuscus

The role of metals in forming the primary cross-links in slug glue was investigated. Several metal-binding proteins were identified in the defensive glue produced by the slug Arion subfuscus. Notably, the C-lectins that are unique to the glue are iron-binding proteins. This is unusual for C-lectins. Dissociating these proteins from iron does not affect the glue's stiffness. Similarly, several proteins that can bind to zinc were identified, but dissociating the proteins from zinc did not weaken the glue. These results suggest that metal coordination is not involved in the primary cross-links of this hydrogel glue. The stable cross-links that provide stiffness are more likely to be created by a catalytic event involving protein oxidation. Cross-linking was unexpectedly difficult to prevent. Collecting the glue into a large volume of ice-cold buffer with reagents aimed at inhibiting oxidative cross-linking caused a slight loss of cross-linking, as demonstrated by the appearance of uncross-linked proteins in native gel electrophoresis. Notable among these was a protein that is normally heavily oxidized (asmp165). Nevertheless, this effect was not large, suggesting that the primary cross-links form before secretion.

Alterations in Lipids and Minerals in Relation To Larval Trematode Infections of Nerita Polita Marine Snails

Marine snails provide an important source of vitamins, proteins, minerals, and essential fatty acids, and their mucus has a therapeutic significance. Parasitic infection of larval trematodes in these snails affects their nutritional value. The present study aimed to screen Nerita polita marine snails for the prevalence of cercarial infections and to evaluate the changes in lipids and some minerals in the infected as well as non-infected ones. Snails were collected randomly from February 2018 to January 2019 from the Red Sea Obhor bay, Jeddah city, Saudi Arabia. The amount of triglycerides, cholesterol, and phospholipids in the Digestive Gland Gonad Complex (DGG), hemolymph, and Snail Conditioned Water (SCW) of non-infected and infected Nerita polita snail was estimated using a spectrophotometer. Minerals content such as Ca, Zn, Pb, Na, Mn, Mg, K, Fe, Cu, and Cd, in the DGG and shell in the infected and non-infected snails were analyzed using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Two types of cercariae were detected, ocellate furcocercus cercaria (Trichobilharzia regent) and xiphidiocercaria (Litorina saxatilis VII). The study showed that there is a fl uctuation in the concentration of lipids and minerals between increase and decrease in DGG, hemolymph, shell, and SCW in infected snails. The nutritive value of the snails is affected with infection through the decrease of some lipids and minerals in infected snails. Further qualitative studies are needed.

Polymer Network Editing of Elastomers for Robust Underwater Adhesion and Tough Bonding to Diverse Surfaces

Tough adhesives with robust adhesion are desperately needed for biomedical and technological applications. However, it is extremely challenging to engineer tough and durable adhesives that are simple to make yet also exhibit strong underwater adhesion as well as tough bonding to diverse surfaces. Here, we report bioinspired elastomers based on water-immiscible polydiolcitrates, where their tough mechanical properties, robust underwater adhesion (80 kPa), and tough bonding performance (with an interfacial toughness >1000 J m^-2 and a shear and tensile strength >0.5 MPa) to diverse solid materials (glass, ceramics, and steel) are actuated by the incorporation of trace amounts of additives. The additives could edit the polymer networks during the elastomer polymerization by dramatically regulating the cross-linking structures of covalent and reversible bonds, the length of polymer chains, and the hydrophobic and hydrophilic motifs, which markedly tuned the mechanical and adhesive properties of the bioelastomers. We also demonstrate versatile applications of the durable elastomers, as tough flexible joints for solid materials, superglue, tissue sealants, hemostatic dressing, and wound repair.

Sea star-inspired recombinant adhesive proteins self-assemble and adsorb on surfaces in aqueous environments to form cytocompatible coatings

Sea stars adhere to various underwater substrata using an efficient protein-based adhesive secretion. The protein Sfp1 is a major component of this secretion. In the natural glue, it is cleaved into four subunits (Sfp1 Alpha, Beta, Delta and Gamma) displaying specific domains which mediate protein-protein or protein-carbohydrate interactions. In this study, we used the bacterium E. coli to produce recombinantly two fragments of Sfp1 comprising most of its functional domains: the C-terminal part of the Beta subunit (rSfp1 Beta C-term) and the Delta subunit (rSfp1 Delta). Using native polyacrylamide gel electrophoresis and size exclusion chromatography, we show that the proteins self-assemble and form oligomers and aggregates in the presence of NaCl. Moreover, they adsorb onto glass and polystyrene upon addition of Na⁺ and/or Ca²⁺ ions, forming homogeneous coatings or irregular meshworks, depending on the cation species and concentration. We show that coatings made of each of the two proteins have no cytotoxic effects on HeLa cells and even increase their proliferation. We propose that the Sfp1 recombinant protein coatings are valuable new materials with potential for cell culture or biomedical applications. STATEMENT OF SIGNIFICANCE: Biological adhesives offer impressive performance in their natural context and, therewith, the potential to inspire the development of advanced biomaterials for an increasing variety of applications in medicine or in material sciences. To date, most marine adhesive proteins that have been produced recombinantly in order to develop bio-inspired adhesives are small proteins from mussels and barnacles. Here, we produced two multi-modular proteins based on the sequence of Sfp1, a major protein from sea star adhesive secretion. These two proteins comprise most of Sfp1 functional domains which mediate protein-protein and protein-carbohydrate interactions. We characterized the two recombinant proteins with an emphasis on functional characteristics such as self-assembly, adsorption and cytocompatibility. We discuss their potential as biomaterials.

Elasticity and energy dissipation in the double network hydrogel adhesive of the slug Arion subfuscus

The slug Arion subfuscus produces a mucus-based defensive secretion that is remarkably tough. This glue appears to be a double network hydrogel, gaining its toughness through the synergistic actions of two networks of polymers, a relatively stiff network and a relatively deformable network. The double network mechanism has great potential to guide the development of synthetic adhesives. Mechanical tests were performed to analyse key predictions of the mechanism. Stress relaxation tests and tensile tests support the presence of stable cross-links. Cyclic stress-strain tests demonstrate that the glue dissipates a great deal of energy through the failure of these cross-links as sacrificial bonds. Energy dissipation by failure of sacrificial bonds rather than viscous processes is supported by the minimal effect of the time course of the experiments on the measured properties. These sacrificial bonds appear able to reform within minutes after failure. Finally, the glue's stiffness decreases at pH values below 5.5, whereas magnesium and calcium rapidly dissociate from the glue at all pH values tested. Thus, these ions may not be the primary cross-linkers generating the glue's stiffness. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Bioinspired Adhesive Architectures: From Skin Patch to Integrated Bioelectronics

The attachment phenomena of various hierarchical architectures found in nature have extensively drawn attention for developing highly biocompatible adhesive on skin or wet inner organs without any chemical glue. Structural adhesive systems have become important to address the issues of human-machine interactions by smart outer/inner organ-attachable devices for diagnosis and therapy. Here, advances in designs of biologically inspired adhesive architectures are reviewed in terms of distinct structural properties, attachment mechanisms to biosurfaces by physical interactions, and noteworthy fabrication methods. Recent demonstrations of bioinspired adhesive architectures as adhesive layers for medical applications from skin patches to multifunctional bioelectronics are presented. To conclude, current challenges and prospects on potential applications are also briefly discussed.

Gastropod parasitic nematodes (Phasmarhabditis sp.) are attracted to hyaluronic acid in snail mucus by cGMP signalling

Phasmarhabditis hermaphrodita is a parasitic nematode of terrestrial gastropods that has been formulated into a biological control agent for farmers and gardeners to kill slugs and snails. In order to locate slugs it is attracted to mucus, faeces and volatile cues; however, there is no information about whether these nematodes are attracted to snail cues. It is also unknown how wild isolates of P. hermaphrodita or different Phasmarhabditis species behave when exposed to gastropod cues. Therefore, we investigated whether P. hermaphrodita (commercial and wild isolated strains), P. neopapillosa and P. californica were attracted to mucus from several common snail species (Cepaea nemoralis, Cepaea hortensis, Arianta arbustorum and Cornu aspersum). We also examined whether snails (C. aspersum) collected from different locations around the UK differed in their attractiveness to wild isolates of P. hermaphrodita. Furthermore, we also investigated what properties of snail mucus the nematodes were attracted to, including hyaluronic acid and metal salts (FeSO4, ZnSO4, CuSO4 and MgSO4). We found that the commercial strain of P. hermaphrodita responded poorly to snail mucus compared to wild isolated strains, and C. aspersum collected from different parts of the UK differed in their attractiveness to the nematodes. We found that Phasmarhabditis nematodes were weakly attracted to all metals tested but were strongly attracted to hyaluronic acid. In a final experiment we also showed that pharmacological manipulation of cyclic guanosine monophosphate (cGMP) increased chemoattraction to snail mucus, suggesting that the protein kinase EGL-4 may be responsible for Phasmarhabditis sp. chemoattraction.

RNA-Seq reveals a central role for lectin, C1q and von Willebrand factor A domains in the defensive glue of a terrestrial slug

The tough, hydrogel glue produced by the slug Arion subfuscus achieves impressive performance through metal-based, protein cross-links. The primary sequence of these proteins was determined through transcriptome sequencing and proteome analysis by tandem mass spectrometry. The main proteins that correlate with adhesive function are a group of 11 small, highly abundant lectin-like proteins. These proteins matched the ligand-binding C-lectin, C1q or H-lectin domains. The variety of different lectin-like proteins and their potential for oligomerization suggests that they function as versatile and potent cross-linkers. In addition, the glue contains five matrilin-like proteins that are rich in von Willebrand factor A (VWA) and EGF domains. Both C-lectins and VWA domains are known to bind to ligands using divalent cations. These findings are consistent with the double network mechanism proposed for slug glue, with divalent ions serving as sacrificial bonds to dissipate energy.

Prediction of anticancer peptides against MCF-7 breast cancer cells from the peptidomes of Achatina fulica mucus fractions

Several reports have shown antimicrobial and anticancer activities of mucous glycoproteins extracted from the giant African snail Achatina fulica. Anticancer properties of the snail mucous peptides remain incompletely revealed. The aim of this study was to predict anticancer peptides from A. fulica mucus. Two of HPLC-separated mucous fractions (F2 and F5) showed in vitro cytotoxicity against the breast cancer cell line (MCF-7) and normal epithelium cell line (Vero). According to the mass spectrometric analysis, 404 and 424 peptides from the F2 and F5 fractions were identified. Our comprehensive bioinformatics workflow predicted 16 putative cationic and amphipathic anticancer peptides with diverse structures from these two peptidome data. These peptides would be promising molecules for new anti-breast cancer drug development.

Double-network gels and the toughness of terrestrial slug glue

The terrestrial slug Arion subfuscus produces a defensive secretion that is sticky and tough, despite being a dilute gel. It is unusual in having high stiffness for a gel, yet retaining the high extensibility typical of mucus. In tensile tests, it sustains an average peak stress of 101 kPa, and fails at an average strain of 9.5. This gives the gel toughness; it requires much greater strain energy to fracture than most gels. This toughness may arise from a double-network type mechanism. In this mechanism, two separate, interpenetrating networks of polymers with different properties combine to give toughness that can be several orders of magnitude greater than either network individually. Native gel electrophoresis suggests that A. subfuscus glue consists of two networks: a network of negatively charged proteins ranging in Mr from 40×10(3) to 220×10(3) that can be dissociated by hydroxylamine and a network of heparan sulfate-like proteoglycans. The two networks are not tightly linked, though proteins of Mr 40×10(3) and 165×10(3) may associate with the carbohydrates. Targeted disruption of either network separately, using enzymatic hydrolysis, disulfide bond breakage or imine bond disruption completely disrupted the glue, resulting in no measurable toughness. Thus, the two networks separately provide little toughness, but together they work synergistically to create a tough material, as predicted in the double-network mechanism.

Characterisation of the secretome of the clam parasite, QPX

Secreted and cell surface-associated molecules play a major role in disease development processes and host-pathogen interactions, and usually determine the virulence of invading organisms. In this study, we investigated proteins secreted by quahog parasite unknown, a thraustochytrid protist that infects the hard clam, Mercenaria mercenaria. In silico analysis of quahog parasite unknown transcripts predicted over 1200 proteins to possess an amino-terminal signal peptide which directs proteins into the classical eukaryotic secretory pathway. Proteomic analysis using LC/MS technology identified 56 proteins present in the extracellular secretion of quahog parasite unknown cells grown in vitro, including six mucin-like molecules, four glycosyl hydrolases and eight peptidases. Transcription levels of 19 quahog parasite unknown extracellular proteins were investigated in clam tissue lesions (in vivo) using quantitative PCR. The overexpression of six of these extracellular proteins in clam tissues compared with in vitro cultures suggests that they are involved in interaction with the clam host.

A blood-resistant surgical glue for minimally invasive repair of vessels and heart defects

Currently, there are no clinically approved surgical glues that are nontoxic, bind strongly to tissue, and work well within wet and highly dynamic environments within the body. This is especially relevant to minimally invasive surgery that is increasingly performed to reduce postoperative complications, recovery times, and patient discomfort. We describe the engineering of a bioinspired elastic and biocompatible hydrophobic light-activated adhesive (HLAA) that achieves a strong level of adhesion to wet tissue and is not compromised by preexposure to blood. The HLAA provided an on-demand hemostatic seal, within seconds of light application, when applied to high-pressure large blood vessels and cardiac wall defects in pigs. HLAA-coated patches attached to the interventricular septum in a beating porcine heart and resisted supraphysiologic pressures by remaining attached for 24 hours, which is relevant to intracardiac interventions in humans. The HLAA could be used for many cardiovascular and surgical applications, with immediate application in repair of vascular defects and surgical hemostasis.

The relative contribution of calcium, zinc and oxidation-based cross-links to the stiffness of Arion subfuscus glue

Metal ions are present in many different biological materials, and are capable of forming strong cross-links in aqueous environments. The relative contribution of different metal-based cross-links was measured in the defensive glue produced by the terrestrial slug Arion subfuscus. This glue contains calcium, magnesium, zinc, manganese, iron and copper. These metals are essential to the integrity of the glue and to gel stiffening. Removal of all metals caused at least a 15-fold decrease in the storage modulus of the glue. Selectively disrupting cross-links involving hard Lewis acids such as calcium reduced the stiffness of the glue, while disrupting cross-links involving borderline Lewis acids such as zinc did not. Calcium is the most common cation bound to the glue (40 mmol l(-1)), and its charge is balanced primarily by sulphate at 82-84 mmol l(-1). Thus these ions probably play a primary role in bringing polymers together directly. Imine bonds formed as a result of protein oxidation also contribute substantially to the stiffness of the glue. Disrupting these bonds with hydroxylamine caused a 33% decrease in storage modulus of the glue, while stabilizing them by reduction with sodium borohydride increased the storage modulus by 40%. Thus a combination of metal-based bonds operates in this glue. Most likely, cross-links directly involving calcium play a primary role in bringing together and stabilizing the polymer network, followed by imine bond formation and possible iron coordination.

Cross-linking by protein oxidation in the rapidly setting gel-based glues of slugs

The terrestrial slug Arion subfuscus secretes a glue that is a dilute gel with remarkable adhesive and cohesive strength. The function of this glue depends on metals, raising the possibility that metal-catalyzed oxidation plays a role. The extent and time course of protein oxidation was measured by immunoblotting to detect the resulting carbonyl groups. Several proteins, particularly one with a relative molecular mass (M(r)) of 165 x 10³, were heavily oxidized. Of the proteins known to distinguish the glue from non-adhesive mucus, only specific size variants were oxidized. The oxidation appears to occur within the first few seconds of secretion. Although carbonyls were detected by 2,4-dinitrophenylhydrazine (DNPH) in denatured proteins, they were not easily detected in the native state. The presence of reversible cross-links derived from carbonyls was tested for by treatment with sodium borohydride, which would reduce uncross-linked carbonyls to alcohols, but stabilize imine bonds formed by carbonyls and thus lead to less soluble complexes. Consistent with imine bond formation, sodium borohydride led to a 20-35% decrease in the amount of soluble protein with a M(r) of 40-165 (x 10³) without changing the carbonyl content per protein. In contrast, the nucleophile hydroxylamine, which would competitively disrupt imine bonds, increased protein solubility in the glue. Finally, the primary amine groups on a protein with a M(r) of 15 x 10³ were not accessible to acid anhydrides. The results suggest that cross-links between aldehydes and primary amines contribute to the cohesive strength of the glue.