Molecular insights into the powerful mucus-based adhesion of limpets (Patella vulgata L.)

Proteomic analysis of the mucus of the photosynthetic sea slug Elysia crispata

Elysia crispata is a tropical sea slug that can retain intracellular functional chloroplasts from its algae prey, a mechanism termed kleptoplasty. This sea slug, like other gastropods, secretes mucus, a viscous secretion with multiple functions, including lubrication, protection, and locomotion. This study presents the first comprehensive analysis of the mucus proteome of the sea slug E. crispata using gel electrophoresis and HPLC-MS/MS. We identified 306 proteins in the mucus secretions of this animal, despite the limited entries for E. crispata in the Uniprot database. The functional annotation of the mucus proteome using Gene Ontology identified proteins involved in different functions such as hydrolase activity (molecular function), carbohydrate-derived metabolic processes (biological processes) and cytoskeletal organization (cell component). Moreover, a high proportion of proteins with enzymatic activity in the mucus of E. crispata suggests potential biotechnological applications including antimicrobial and antitumor activities. Putative antimicrobial properties are reinforced by the high abundance of hydrolases. This study also identified proteins common in mucus samples from various species, supporting a common mechanism of mucus in protecting cells and tissues while facilitating animal movement. SIGNIFICANCE: Marine species are increasingly drawing the interest of researchers for their role in discovering new bioactive compounds. The study "Proteomic Analysis of the Mucus of the Photosynthetic Sea Slug Elysia crispata" is a pioneering effort that uncovers the complex protein content in this fascinating sea slug's mucus. This detailed proteomic study has revealed proteins with potential use in biotechnology, particularly for antimicrobial and antitumor purposes. This research is a first step in exploring the possibilities within the mucus of Elysia crispata, suggesting the potential for new drug discoveries. These findings could be crucial in developing treatments for severe diseases, especially those caused by multidrug-resistant bacteria, and may lead to significant advances in medical research.

Benzodiazepine Delorazepam Induces Locomotory Hyperactivity and Alterations in Pedal Mucus Texture in the Freshwater Gastropod Planorbarius corneus

Benzodiazepines, psychotropic drugs, are ubiquitous in the aquatic environment due to over-consumption and inefficient removal by sewage treatment plants. Bioaccumulation with consequent behavioral and physiological effects has been reported in many aquatic species. However, the responses are species-specific and still poorly understood. To improve the knowledge, we exposed the freshwater snail Planorbarius corneus to 1, 5, or 10 µg/L of delorazepam, the most widely consumed benzodiazepine in Italy. Conventional behavioral tests were used to assess the effects on locomotor and feeding behavior. Histological and biochemical analyses were also performed to detect possible changes in the structure and composition of the foot mucus and glands. The results show a paradoxical response with reduced feeding activity and locomotor hyperactivity. Pedal mucus was altered in texture but not in composition, becoming particularly rich in fibrous collagen-like material, and a significant change in the protein composition was highlighted in the foot. In conclusion, exposure to delorazepam induces disinhibited behavior in Planorbarius corneus, potentially increasing the risk of predation, and an increase in mucus protein production, which, together with reduced feeding activity, would severely compromise energy resources.

Identifying Natural Bioactive Peptides from the Common Octopus (Octopus vulgaris Cuvier, 1797) Skin Mucus By-Products Using Proteogenomic Analysis

The common octopus is a cephalopod species subject to active fisheries, with great potential in the aquaculture and food industry, and which serves as a model species for biomedical and behavioral studies. The analysis of the skin mucus allows us to study their health in a non-invasive way, by using a hardly exploited discard of octopus in the fishing sector. A shotgun proteomics approach combined with liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) using an Orbitrap-Elite instrument was used to create a reference dataset from octopus skin mucus. The final proteome compilation was investigated by integrated in-silico studies, including Gene Ontology (GO), the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, network studies, and prediction and characterization analysis of potential bioactive peptides. This work presents the first proteomic analysis of the common octopus skin mucus proteome. This library was created by merging 5937 identified spectra of 2038 different peptides. A total of 510 non-redundant proteins were identified. Obtained results show proteins closely related to the defense, which highlight the role of skin mucus as the first barrier of defense and the interaction with the environment. Finally, the potential of the bioactive peptides with antimicrobial properties, and their possible application in biomedicine, pharmaceutical, and nutraceutical industry was addressed.

Glycoproteins Involved in Sea Urchin Temporary Adhesion

Biomedical adhesives, despite having been used increasingly in recent years, still face a major technological challenge: strong adhesion in wet environments. In this context, biological adhesives secreted by marine invertebrates have appealing characteristics to incorporate into new underwater biomimetic adhesives: water resistance, nontoxicity and biodegradability. Little is still known about temporary adhesion. Recently, a transcriptomic differential analysis of sea urchin Paracentrotus lividus tube feet pinpointed 16 adhesive/cohesive protein candidates. In addition, it has been demonstrated that the adhesive secreted by this species is composed of high molecular weight proteins associated with N-Acetylglucosamine in a specific chitobiose arrangement. As a follow-up, we aimed to investigate which of these adhesive/cohesive protein candidates were glycosylated through lectin pulldowns, protein identification by mass spectroscopy and in silico characterization. We demonstrate that at least five of the previously identified protein adhesive/cohesive candidates are glycoproteins. We also report the involvement of a third Nectin variant, the first adhesion-related protein to be identified in P. lividus. By providing a deeper characterization of these adhesive/cohesive glycoproteins, this work advances our understanding of the key features that should be replicated in future sea urchin-inspired bioadhesives.

The Involvement of Cell-Type-Specific Glycans in Hydra Temporary Adhesion Revealed by a Lectin Screen

Hydra is a freshwater solitary polyp, capable of temporary adhesion to underwater surfaces. The reversible attachment is based on an adhesive material that is secreted from its basal disc cells and left behind on the substrate as a footprint. Despite Hydra constituting a standard model system in stem cell biology and tissue regeneration, few studies have addressed its bioadhesion. This project aimed to characterize the glycan composition of the Hydra adhesive, using a set of 23 commercially available lectins to label Hydra cells and footprints. The results indicated the presence of N-acetylglucosamine, N-acetylgalactosamine, fucose, and mannose in the adhesive material. The labeling revealed a meshwork-like substructure in the footprints, implying that the adhesive is mainly formed by fibers. Furthermore, lectins might serve as a marker for Hydra cells and structures, e.g., many labeled as glycan-rich nematocytes. Additionally, some unexpected patterns were uncovered, such as structures associated with radial muscle fibers and endodermal gland cells in the hypostome of developing buds.

Two novel mollusk short-form ApeC-containing proteins act as pattern recognition proteins for peptidoglycan

The Apextrin C-terminal (ApeC) domain is a new protein domain largely specific to aquatic invertebrates. In amphioxus, a short-form ApeC-containing protein (ACP) family is capable of binding peptidoglycan (PGN) and agglutinating bacteria via its ApeC domain. However, the functions of ApeC in other phyla remain unknown. Here we examined 130 ACPs from gastropods and bivalves, the first and second biggest mollusk classes. They were classified into nine groups based on their phylogenetics and architectures, including three groups of short-form ACPs, one group of apextrins and two groups of ACPs of complex architectures. No groups have orthologs in other phyla and only four groups have members in both gastropods and bivalves, suggesting that mollusk ACPs are highly diversified. We selected one bivalve ACP (CgACP1; from the oyster Crossostrea gigas) and one gastropod ACP (BgACP1; from the snail Biomphalaria glabrata) for functional experiments. Both are highly-expressed, secreted short-form ACPs and hence comparable to the amphioxus ACPs previously reported. We found that recombinant CgACP1 and BgACP1 bound with yeasts and several bacteria with different affinities. They also agglutinated these microbes, but showed no inhibiting or killing effects. Further analyses show that both ACPs had high affinities to the Lys-type PGN from S. aureus but weak or no affinities to the DAP-type PGN from Bacillus subtilis. Both recombinant ACPs displayed weak or no affinities to other microbial cell wall components, including lipopolysaccharide (LPS), lipoteichoic acid (LTA), zymosan A, chitin, chitosan and cellulose, as well as to several PGN moieties, including muramyl dipeptide (MDP), N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). Besides, CgACP1 had the highest expression in the gill and could be greatly up-regulated quickly after bacterial challenge. This is reminiscent of the amphioxus ACP1/2 which serve as essential mucus lectins in the gill. Taken together, the current findings from mollusk and amphioxus ACPs suggest several basic common traits for the ApeC domains, including the high affinity to Lys-type PGN, the bacterial binding and agglutinating capacity, and the role as mucus proteins to protect the mucosal surface.

In the footsteps of sea stars: deciphering the catalogue of proteins involved in underwater temporary adhesion

Sea stars adhere strongly but temporarily to underwater substrata via the secretion of a blend of proteins, forming an adhesive footprint that they leave on the surface after detachment. Their tube feet enclose a duo-gland adhesive system comprising two types of adhesive cells, contributing different layers of the footprint and de-adhesive cells. In this study, we characterized the catalogue of sea star footprint proteins (Sfps) in the species Asterias rubens to gain insights in their potential function. We identified 16 Sfps and mapped their expression to type 1 and/or type 2 adhesive cells or to de-adhesive cells by double fluorescent in situ hybridization. Based on their cellular expression pattern and their conserved functional domains, we propose that the identified Sfps serve different functions during attachment, with two Sfps coupling to the surface, six providing cohesive strength and the rest forming a binding matrix. Immunolabelling of footprints with antibodies directed against one protein of each category confirmed these roles. A de-adhesive gland cell-specific astacin-like proteinase presumably weakens the bond between the adhesive material and the tube foot surface during detachment. Overall, we provide a model for temporary adhesion in sea stars, including a comprehensive list of the proteins involved.

Genome-wide identification m6A modified circRNAs revealed their key roles in skin ulceration syndrome disease development in Apostichopus japonicus

Circular RNAs (circRNAs) are novel endogenous non-coding RNAs (ncRNAs) and can be acted as competing endogenous RNAs (ceRNAs) to regulate microRNA (miRNA) and downstream gene expression. Recently, m⁶A modification has been found in circRNA, and m⁶A circRNAs also play important roles in various biological processes and a variety of diseases. Our previous study had been demonstrated that circRNAs were differentially expressed in skin ulceration syndrome (SUS) diseased sea cucumber Apostichopus japonicus. However, whether the function of circRNAs are dependent on m⁶A levels are largely unknown. Here, we firstly investigated the genome-wide map of m⁶A circRNAs in sea cucumbers with different stages of Vibrio splendidus challenge, that's Control group, SUS-diseased group, and SUS-resistant group. MeRIP-seq revealed that m⁶A abundances were enriched in circRNAs in all three groups, especially for SUS-resistant group. Among them, more than 62% of modified circRNAs harbor only a single m⁶A peak and about 55% of m⁶A sites in circRNAs were derived from sense overlapping in each group. After V. splendidus infection, we found that most of m⁶A peaks in circRNAs were upregulated and less were downregulated in both SUS-diseased and SUS-resistant groups when compared with Control. Furthermore, GO analysis indicated that the host genes of circRNAs with dysregulated m⁶A peaks in SUS-diseased and SUS-resistant groups were both mainly enriched in the adhesion pathway. More importantly, we discovered that more than 50% m⁶A circRNAs showed a positive correlation between the circRNAs expression and m⁶A methylation levels both in SUS-diseased and SUS-resistant groups. Therefore, a core circRNA-miRNA-mRNA (ceRNA) network whether influenced by m⁶A modification was constructed based on conjoint analysis. Our results indicated that several selected m⁶A circRNAs bind with miRNAs were mainly targeting to ubiquitylation system and adhesion pathway. What's more, three candidate m⁶A circRNAs and three target genes were validated by MeRIP-qPCR and qPCR, whose m⁶A levels in circRNA and mRNA expressions were consistent with disease occurrence or disease resistance. All of our current findings suggested that m⁶A circRNAs could play important roles during pathogen infection and might be served as a new molecular biomarker in SUS disease diagnose of A. japonicus.

Sticking Together an Updated Model for Temporary Adhesion

Non-parasitic flatworms are known to temporarily attach to the substrate by secreting a multicomponent bioadhesive to counteract water movements. However, to date, only species of two higher-level flatworm taxa (Macrostomorpha and Proseriata) have been investigated for their adhesive proteins. Remarkably, the surface-binding protein is not conserved between flatworm taxa. In this study, we sequenced and assembled a draft genome, as well as a transcriptome, and generated a tail-specific positional RNA sequencing dataset of the polyclad Theama mediterranea. This led to the identification of 15 candidate genes potentially involved in temporary adhesion. Using in situ hybridisation and RNA interference, we determined their expression and function. Of these 15 genes, 4 are homologues of adhesion-related genes found in other flatworms. With this work, we provide two novel key components on the flatworm temporary adhesion system. First, we identified a Kringle-domain-containing protein (Tmed-krg1), which was expressed exclusively in the anchor cell. This in silico predicted membrane-bound Tmed-krg1 could potentially bind to the cohesive protein, and a knockdown led to a non-adhesive phenotype. Secondly, a secreted tyrosinase (Tmed-tyr1) was identified, which might crosslink the adhesive proteins. Overall, our findings will contribute to the future development of reversible synthetic glues with desirable properties for medical and industrial applications.

Decoding the byssus fabrication by spatiotemporal secretome analysis of scallop foot

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The first secretome about scallop byssal adhesion is profiled based on a new computational strategy.

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Scallop byssal secretome covered almost all of the known structural elements and functional domains of aquatic adhesives.

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The EGF-like domain containing proteins, the Tyr-rich proteins and 4C-repeats containing proteins are the main components of scallop byssus.

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A novel “nearby secretion” model of scallop byssus secretion and adhesion is proposed.

Secretome is involved in almost all physiological, developmental, and pathological processes, but to date there is still a lack of highly-efficient research strategy to comprehensively study the secretome of invertebrates. Adhesive secretion is a ubiquitous and essential physiological process in aquatic invertebrates with complicated protein components and unresolved adhesion mechanisms, making it a good subject for secretome profiling studies. Here we proposed a computational pipeline for systematic profiling of byssal secretome based on spatiotemporal transcriptomes of scallop. A total of 186 byssus-related proteins (BRPs) were identified, which represented the first characterized secretome of scallop byssal adhesion. Scallop byssal secretome covered almost all of the known structural elements and functional domains of aquatic adhesives, which suggested this secretome-profiling strategy had both high efficiency and accuracy. We revealed the main components of scallop byssus (including EGF-like domain containing proteins, the Tyr-rich proteins and 4C-repeats containing proteins) and the related modification enzymes primarily contributing to the rapid byssus assembly and adhesion. Spatiotemporal expression and co-expression network analyses of BRPs suggested a simultaneous secretion pattern of scallop byssal proteins across the entire region of foot and revealed their diverse functions on byssus secretion. In contrast to the previously proposed “root-initiated secretion and extension-based assembly” model, our findings supported a novel “foot-wide simultaneous secretion and in situ assembly” model of scallop byssus secretion and adhesion. Systematic analysis of scallop byssal secretome provides important clues for understanding the aquatic adhesive secretion process, as well as a common framework for studying the secretome of non-model invertebrates.

The genome sequence of the blue-rayed limpet, Patella pellucida Linnaeus, 1758

We present a genome assembly from an individual Patella pellucida (the blue-rayed limpet; Mollusca; Gastropoda; Patellidae). The genome sequence is 712 megabases in span. The majority of the assembly (99.85%) is scaffolded into 9 chromosomal pseudomolecules. The mitochondrial genome was assembled and is 14.9 kilobases in length.

Comparative proteomics for an in-depth understanding of bioadhesion mechanisms and evolution across metazoans

Bioadhesion is a critical process for many marine and freshwater invertebrate animals. Bioadhesives mainly made of proteins have remarkable adhesive ability underwater. Unraveling the molecular composition of bioadhesives is fundamental to understanding their physiological roles as well as their potential for biotechnology applications and antibiofouling strategies. With the development of high-throughput methods such as proteomics, bioadhesive protein data in diverse taxa are rapidly accumulating, but the common mechanism across species is elusive due to the vast variety of bioadhesives. In this review, bioadhesive proteins from various taxa are reviewed, with the aim of facilitating researchers to appreciate the diversity of bioadhesive proteins (mostly 20-40) across species. By comparing proteomes across species, it was found that glycine-rich, epidermal growth factor, peroxidase, and DOPA together with typical extracellular domains are the most commonly used domains. Additionally, permanent and temporary adhesion show obvious differences in terms of domains or proteins. A basic recipe for bioadhesives composed of six components is proposed: structural elements, extracellular domains, modification enzymes, proteinase inhibitors, cytoskeletal proteins, and others. The extracellular domains are mostly related to interactions with other macromolecules (proteins, carbohydrates, and lipids), suggesting that domain shuffling and macromolecule interaction might be fundamental for bioadhesive evolution.

Exploration of sea anemone-inspired high-performance biomaterials with enhanced antioxidant activity

Antioxidant biomaterials have attracted much attention in various biomedical fields because of their effective inhibition and elimination of reactive oxygen species (ROS) in pathological tissues. However, the difficulty in ensuring biocompatibility, biodegradability and bioavailability of antioxidant materials has limited their further development. Novel bioavailable antioxidant materials that are derived from natural resources are urgently needed. Here, an integrated multi-omics method was applied to fabricate antioxidant biomaterials. A key cysteine-rich thrombospondin-1 type I repeat-like (TSRL) protein was efficiently discovered from among 1262 adhesive components and then used to create a recombinant protein with a yield of 500 mg L^-1. The biocompatible TSRL protein was able to self-assemble into either a water-resistant coating through Ca²⁺-mediated coordination or redox-responsive hydrogels with tunable physical properties. The TSRL-based hydrogels showed stronger 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging rates than glutathione (GSH) and ascorbic acid (Aa) and protected cells against external oxidative stress significantly more effectively. When topically applied to mice skin, TSRL alleviated epidermal hyperplasia and suppressed the degradation of collagen and elastic fibers caused by ultraviolet radiation B (UVB) irradiation, confirming that it enhanced antioxidant activity in vivo. This is the first study to successfully characterize natural antioxidant biomaterials created from marine invertebrate adhesives, and the findings indicate the excellent prospects of these biomaterials for great applications in tissue regeneration and cosmeceuticals.

(Un)expected Similarity of the Temporary Adhesive Systems of Marine, Brackish, and Freshwater Flatworms

Many free-living flatworms have evolved a temporary adhesion system, which allows them to quickly attach to and release from diverse substrates. In the marine Macrostomum lignano, the morphology of the adhesive system and the adhesion-related proteins have been characterised. However, little is known about how temporary adhesion is performed in other aquatic environments. Here, we performed a 3D reconstruction of the M. lignano adhesive organ and compared it to the morphology of five selected Macrostomum, representing two marine, one brackish, and two freshwater species. We compared the protein domains of the two adhesive proteins, as well as an anchor cell-specific intermediate filament. We analysed the gene expression of these proteins by in situ hybridisation and performed functional knockdowns with RNA interference. Remarkably, there are almost no differences in terms of morphology, protein regions, and gene expression based on marine, brackish, and freshwater habitats. This implies that glue components produced by macrostomids are conserved among species, and this set of two-component glue functions from low to high salinity. These findings could contribute to the development of novel reversible biomimetic glues that work in all wet environments and could have applications in drug delivery systems, tissue adhesives, or wound dressings.

Two Amphioxus ApeC-Containing Proteins Bind to Microbes and Inhibit the TRAF6 Pathway

The apextrin C-terminal (ApeC) domain is a class of newly discovered protein domains with an origin dating back to prokaryotes. ApeC-containing proteins (ACPs) have been found in various marine and aquatic invertebrates, but their functions and the underlying mechanisms are largely unknown. Early studies suggested that amphioxus ACP1 and ACP2 bind to bacterial cell walls and have a role in immunity. Here we identified another two amphioxus ACPs (ACP3 and ACP5), which belong to the same phylogenetic clade with ACP1/2, but show distinct expression patterns and sequence divergence (40-50% sequence identities). Both ACP3 and ACP5 were mainly expressed in the intestine and hepatic cecum, and could be up-regulated after bacterial challenge. Both prokaryotic-expressed recombinant ACP3 and ACP5 could bind with several species of bacteria and yeasts, showing agglutinating activity but no microbicidal activity. ELISA assays suggested that their ApeC domains could interact with peptidoglycan (PGN), but not with lipoteichoic acid (LTA), lipopolysaccharides (LPS) and zymosan A. Furthermore, they can only bind to Lys-type PGN from Staphylococcus aureus, but not to DAP-type PGN from Bacillus subtilis and not to moieties of PGN such as MDPs, NAMs and NAGs. This recognition spectrum is different from that of ACP1/2. We also found that when expressed in mammalian cells, ACP3 could interact with TRAF6 via a conserved non-ApeC region, which inhibited the ubiquitination of TRAF6 and hence suppressed downstream NF-κB activation. This work helped define a novel subfamily of ACPs, which have conserved structures, and have related yet diversified molecular functions. Its members have dual roles, with ApeC as a lectin and a conserved unknown region as a signal transduction regulator. These findings expand our understanding of the ACP functions and may guide future research on the role of ACPs in different animal clades.

Omics-based molecular analyses of adhesion by aquatic invertebrates

Many aquatic invertebrates are associated with surfaces, using adhesives to attach to the substratum for locomotion, prey capture, reproduction, building or defence. Their intriguing and sophisticated biological glues have been the focus of study for decades. In all but a couple of specific taxa, however, the precise mechanisms by which the bioadhesives stick to surfaces underwater and (in many cases) harden have proved to be elusive. Since the bulk components are known to be based on proteins in most organisms, the opportunities provided by advancing 'omics technologies have revolutionised bioadhesion research. Time-consuming isolation and analysis of single molecules has been either replaced or augmented by the generation of massive data sets that describe the organism's translated genes and proteins. While these new approaches have provided resources and opportunities that have enabled physiological insights and taxonomic comparisons that were not previously possible, they do not provide the complete picture and continued multi-disciplinarity is essential. This review covers the various ways in which 'omics have contributed to our understanding of adhesion by aquatic invertebrates, with new data to illustrate key points. The associated challenges are highlighted and priorities are suggested for future research.