The bioadhesive ofPhragmatopoma californica tubes: a silk-like cement containingL-DOPA

Bone adhesive materials: From bench to bedside

Biodegradable bone adhesives represent a highly sought-after type of biomaterial which would enable replacement of traditional metallic devices for fixation of bone. However, these biomaterials should fulfil an extremely large number of requirements. As a consequence, bone-adhesive biomaterials which meet all of these requirements are not yet commercially available. Therefore, this comprehensive review provides an extensive overview of the development of bone adhesives from a translational perspective. First, the definition, classification, and chemistry of various types of bone adhesives are highlighted to provide a detailed overview of this emerging class of biomaterials. In this review we particularly focused studies which describe the use of materials that are capable of gluing two pieces of bone together within a time frame of minutes to days. Second, this review critically reflects on i) the experimental conditions of commonly employed adhesion tests to assess bone adhesion and ii) the current state-of-the-art regarding their preclinical and clinical applicability.

Complexity of Spider Dragline Silk

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

Toward bioinspired polymer adhesives: activation assisted via HOBt for grafting of dopamine onto poly(acrylic acid)

The design of bioinspired polymers has long been an area of intense study, however, applications to the design of concrete admixtures for improved materials performance have been relatively unexplored. In this work, we functionalized poly(acrylic acid) (PAA), a simple analogue to polycarboxylate ether admixtures in concrete, with dopamine to form a catechol-bearing polymer (PAA-g-DA). Synthetic routes using hydroxybenzotriazole (HOBt) as an activating agent were examined for their ability in grafting dopamine to the PAA backbone. Previous literature using the traditional coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) to graft dopamine to PAA were found to be inconsistent and the sensitivity of EDC coupling reactions necessitated a search for an alternative. Additionally, HOBt allowed for greater control over per cent functionalization of the backbone, is a simple, robust reaction, and showed potential for scalability. This finding also represents a novel synthetic pathway for amide bond formation between dopamine and PAA. Finally, we performed preliminary adhesion studies of our polymer on rose granite specimens and demonstrated a 56% improvement in the mean adhesion strength over unfunctionalized PAA. These results demonstrate an early study on the potential of PAA-g-DA to be used for improving the bonds within concrete.

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.

Sea anemones (Exaiptasia pallida) use a secreted adhesive and complex pedal disc morphology for surface attachment

Background

The mechanism by which sea anemones attach to surfaces underwater remains elusive, which is surprising given their ubiquitous distribution in the world’s oceans and tractability for experimental biology. Their adhesion is mechanically interesting, bridging the interface between very hard and soft materials. The Cnidaria are thought to have evolved adhesion to surfaces at least 505 Ma ago implying that, among the Metazoa, only Porifera developed this capability earlier. The purpose of this study was primarily to address an existing hypothesis, that spirocysts (a sticky class of cnidocyst) facilitate adhesion to surfaces, as observed during prey capture.

Results

We demonstrated conclusively that spirocysts were not involved in the pedal disc adhesion of Exaiptasia pallida. Second, we applied a variety of imaging methods to develop an understanding of the true adhesion mechanism. Morphological studies using scanning electron microscopy identified a meshwork of adhesive material, unique to the pedal disc. Serial block-face SEM highlighted four classes of cells that could secrete the adhesive from the pedal disc ectoderm. A variety of histochemical techniques identified proteins, glycans and quinones in the cell contents and secreted adhesive, with variation in contents of specific cell-types in different areas of the body.

Conclusions

Spirocysts are not used by Exaiptasia pallida for adhesion to surfaces. Instead, a structurally and compositionally complex secreted glue was observed, firmly attaching the animals underwater. The results of this study provide a basis for further investigations of adhesion in Cnidaria, and establish E. pallida as a new model organism for bioadhesion research.

Silk/Natural Rubber (NR) and 3,4-Dihydroxyphenylalanine (DOPA)-Modified Silk/NR Composites: Synthesis, Secondary Structure, and Mechanical Properties

Silk composites with natural rubber (NR) were prepared by mixing degummed silk and NR latex solutions. A significant enhancement of the mechanical properties was confirmed for silk/NR composites compared to a NR-only product, indicating that silk can be applied as an effective reinforcement for rubber materials. Attenuated total reflection Fourier transform infrared (ATR-FTIR) and wide-angle X-ray diffraction (WAXD) analysis revealed that a β-sheet structure was formed in the NR matrix by increasing the silk content above 20 wt%. Then, 3,4-dihydroxyphenylalanine (DOPA)-modified silk was also blended with NR to give a DOPA-silk/NR composite, which showed superior mechanical properties to those of the unmodified silk-based composite. Not only the chemical structure but also the dominant secondary structure of silk in the composite was changed after DOPA modification. It was concluded that both the efficient adhesion property of DOPA residue and the secondary structure change improved the compatibility of silk and NR, resulting in the enhanced mechanical properties of the formed composite. The knowledge obtained herein should contribute to the development of the fabrication of novel silk-based elastic materials.

Sabellaria alveolata sandcastle worm from the Mediterranean Sea: new insights on tube architecture and biocement

The Atlantic-Mediterranean polychaete Sabellaria alveolata lives in agglutinated tubes adjoined to each other to form discrete reef-like bioconstructions in shallow-water settings characterised by high hydrodynamic energy where sediment particles are constantly resuspended. Tubes are built with sand grains glued by proteinaceous secretions. Analyses of a reef fragment collected near Sampieri (SE Sicily, Sicily Strait) allowed the first detailed description of the tube architecture and biocement of this worms from the Mediterranean. The tube consists of an inner thin organic membrane and three agglutinated layers including: (a) a thin inner layer of flat grains arranged side by side; (b) a thick mid layer with a frame of relatively large sub-rounded grains with cavities partly filled by small grains; and (c) a thin outer layer of large, flat to curved, usually biogenic clasts diverging towards the opening. This particular architecture is distinctive of the family. Morphological and epifluorescence observations revealed that biocement consists of drops at the contact between sub-spherical grains and strips along edges of flat grains. Biocement is a solid foam-like material characterised by high abundance of carbon; the presence of phosphorous and nitrogen confirms its proteinaceous composition. Due to the electrostatic interaction with the proteins, calcium and magnesium are most likely complexed to the cement rather than being trapped in the cells. These elements contribute to the solidification of the glue and stabilisation of the tube structure. However, the organic nature of cement and the high energy of their habitat, make sabellariid reefs dynamic and ephemeral, and the preservation as fossils unlikely, with a confident record only extending back to the Miocene.

Neuroactive compounds induce larval settlement in the scleractinian coral Leptastrea purpurea

Settlement of pelagic coral larvae is commonly induced by chemical cues that originate from biofilms and coralline algae. These natural settlement cues initiate signal pathways leading to attachment and metamorphosis of the coral larva. In order to investigate the settlement process and its natural inducers, it is necessary to gain a better understanding of these signal pathways. At present, the pathways and neurotransmitters involved in this signal transduction are still widely unknown. In this study, we exposed larvae of the brooding coral Leptastrea purpurea to five neuroactive compounds known to be present in cnidarians, and K⁺ Ions. All compounds were applied at different dilutions and settlement behavior of the larvae was documented over 48 h. Dopamine, glutamic acid and epinephrine significantly induced settlement in the coral larvae. The highest observed metamorphosis response was 54% in 10^-5 M dopamine. Serotonin, L-DOPA and K⁺ ions did not have an influence on settlement behavior in our experiments. Exposing larvae to settlement-inducing neurotransmitters and thus bypassing the initial induction could be utilized in coral aquaculture. The active neurotransmitters should be used to further study the settlement process in L. purpurea in greater detail. Their role and relevance should also be assessed for other coral species as they may represent or reveal a universal inducer for coral settlement.

Tick attachment cement - reviewing the mysteries of a biological skin plug system

The majority of ticks in the family Ixodidae secrete a substance anchoring their mouthparts to the host skin. This substance is termed cement. It has adhesive properties and seals the lesion during feeding. The particular chemical composition and the curing process of the cement are unclear. This review summarizes the literature, starting with a historical overview, briefly introducing the different hypotheses on the origin of the adhesive and how the tick salivary glands have been identified as its source. Details on the sequence of cement deposition, the curing process and detachment are provided. Other possible functions of the cement, such as protection from the host immune system and antimicrobial properties, are presented. Histochemical and ultrastructural data of the intracellular granules in the salivary gland cells, as well as the secreted cement, suggest that proteins constitute the main material, with biochemical data revealing glycine to be the dominant amino acid. Applied methods and their restrictions are discussed. Tick cement is compared with adhesives of other animals such as barnacles, mussels and sea urchins. Finally, we address the potential of tick cement for the field of biomaterial research and in particular for medical applications in future.

Mechanical properties of the cement of the stalked barnacle Dosima fascicularis (Cirripedia, Crustacea)

The stalked barnacle Dosima fascicularis secretes foam-like cement, the amount of which usually exceeds that produced by other barnacles. When Dosima settles on small objects, this adhesive is additionally used as a float which gives buoyancy to the animal. The dual use of the cement by D. fascicularis requires mechanical properties different from those of other barnacle species. In the float, two regions with different morphological structure and mechanical properties can be distinguished. The outer compact zone with small gas-filled bubbles (cells) is harder than the interior one and forms a protective rind presumably against mechanical damage. The inner region with large, gas-filled cells is soft. This study demonstrates that D. fascicularis cement is soft and visco-elastic. We show that the values of the elastic modulus, hardness and tensile stress are considerably lower than in the rigid cement of other barnacles.

Chemically induced metamorphosis of polychaete larvae in both the laboratory and ocean environment

Planktonic larvae of the marine polychaetePhragmatopoma californica preferentially attach to substrata and metamorphose to the adult form upon contact with cement in tubes built by conspecifics. This gregarious settlement and metamorphosis contributes to the formation of large aggregations or reefs. Larvae also metamorphose upon contact with 2,6-di-tert-butyl-4-methylphenol (DBMP), a possible aromatic analog of cross-linked dihydrox-yphenylalanine (DOPA) residues (present in the polyphenolic protein cement as 2.6% of the amino acid residues). Morphogenesis occurs in the laboratory when larvae are exposed to DBMP either adsorbed to solid surfaces or when dissolved in dimethyl sulfoxide (DMSO) to render it soluble in seawater. Larvae in the ocean were induced to settle and metamorphose on plates coated with DBMP prior to their deployment in the ocean. This is the first report in which a defined organic molecule, identified as an inducer (or precursor to an inducer) of larval settlement and metamorphosis in the laboratory, has been shown to induce these processes in the ocean. Both forskolin and isobutylmethylxanlhine (IBMX) induce metamorphosis ofP. californica larvae, presumably by causing increases in intracellular cyclic AMP (cAMP). A discussion of the pathway controlling chemically mediated metamorphosis and evidence suggesting the possible role of cAMP in the process are presented. Other compounds known to increase intracellular cAMP levels, including arachidonic, linoleic, and palmitoleic acids, found by other workers to induce settlement and metamorphosis ofP. californica, may exert this activity by direct modification of internal cAMP levels in the larvae.

Molecular surface chemistry in marine bioadhesion

This review covers the in situ molecular physicochemical characterisation of bioadhesives at solid/liquid interfaces, with the aim of elucidating the adhesion strategies that lie at the root of marine biofouling. It focuses on three major foulers: mussels, algae and barnacles. The dispersal of these organisms, their colonisation of surfaces, and ultimately their survival rely critically on the ability of the organisms' larvae or spores to locate a favourable settlement site and undergo metamorphosis, thus initiating their sessile existence. Differences in the composition of adhesive secretions and the strategies employed for their temporary or permanent implementation exists between the larval and adult life stages. To date, only a few adhesive secretions from marine fouling organisms have been adequately described in terms of their chemical composition, and a survey revealed the presence of certain recurrent functional groups, specifically catechol, carboxylate, monoester-sulphate and -phosphate. This review will describe the binding modes of such functionalities to wet mineral/metal oxides surfaces. Such functionalities will be ranked based on their ability to bind to hydrophilic surfaces replacing surface-bound water (Langmuir adsorption constant) as well as other adsorbates (competitive adsorption). A plausible explanation for the propensity of the reviewed adhesive functionalities to bind to hydrous metal oxide surfaces will be given on the basis of the Hard and Soft Acids and Bases principle, Hofmeister effects and entropic considerations. From the in situ analysis of marine organism bioadhesives and adsorption studies of functionalities relevant to the bioadhesion process, insights can be gleaned for a knowledge-based innovation of antifouling strategies and the synthesis of strong, durable adhesive materials, which are suitable for implementation in wet environments.

Identification, characterization, and expression levels of putative adhesive proteins from the tube-dwelling polychaete Sabellaria alveolata

The shelter of the tube-dwelling polychaete Sabellaria alveolata is composed of mineral particles assembled with spots of a proteinaceous cement. The adhesive proteins constituting the cement were identified on the basis of their sequence similarity with proteins of a phylogenetically related species, Phragmatopoma californica. Two positively charged proteins, Sa-1 and Sa-2, share common features: they both have a mass of 22 kDa; are rich in glycine, tyrosine and basic residues; and show repeated peptide motifs. The consensus repeat of Sa-1 is KGAYGAKGLGYGNKAGYGAYG (occurring 6-8 times), while Sa-2 displays the consensus heptapeptide VHKAAWG (5 times) and undecapeptide VHKAAGYGGYG (8 times). Two variants of a serine-rich protein, Sa-3A (22 kDa) and Sa-3B (21 kDa), were also identified. Their serine residues account for 75 mol% and are probably phosphorylated, meaning that Sa-3 is very acidic and negatively charged. Moreover, tyrosine residues of all adhesive proteins are presumably modified into DOPA. Although protein sequences are not well-conserved between S. alveolata and P. californica, their main characteristics (including amino acid composition, post-translational modifications, repeated patterns, isoelectric point, and mass) are shared by both species. This suggests that these features are more important for their function than the primary structure of the proteins. The mRNA abundance for each protein was estimated by quantitative real-time PCR, revealing relative expression levels of about 5, 11, 1.5, and 1 for Sa-1, -2, -3A, and -3B, respectively. These levels could be indicative of charge neutralization phenomena or could reflect their function (interface vs. bulk) in the cement.

Unusual adhesive production system in the barnacle Lepas anatifera: an ultrastructural and histochemical investigation

Adhesives that are naturally produced by marine organisms are potential sources of inspiration in the search for medical adhesives. Investigations of barnacle adhesives are at an early stage but it is becoming obvious that barnacles utilize a unique adhesive system compared to other marine organisms. The current study examined the fine structure and chemistry of the glandular system that produces the adhesive of the barnacle Lepas anatifera. All components for the glue originated from large single-cell glands (70-180 μm). Staining (including immunostaining) showed that L-3,4-dihydroxyphenylalanine and phosphoserine were not present in the glue producing tissues, demonstrating that the molecular adhesion of barnacles differs from all other permanently gluing marine animals studied to date. The glandular tissue and adhesive secretion primarily consisted of slightly acidic proteins but also included some carbohydrate. Adhesive proteins were stored in cytoplasmic granules adjacent to an intracellular drainage canal (ICC); observations implicated both merocrine and apocrine mechanisms in the transport of the secretion from the cell cytoplasm to the ICC. Inside the ICC, the secretion was no longer contained within granules but was a flocculent material which became "clumped" as it traveled through the canal network. Hemocytes were not seen within the adhesive "apparatus" (comprising of the glue producing cells and drainage canals), nor was there any structural mechanism by which additions such as hemocytes could be made to the secretion. The unicellular adhesive gland in barnacles is distinct from multicellular adhesive systems observed in marine animals such as mussels and tubeworms. Because the various components are not physically separated in the apparatus, the barnacle adhesive system appears to utilize completely different and unknown mechanisms for maintaining the liquid state of the glue within the body, as well as unidentified mechanisms for the conversion of extruded glue into hard cement.

Fetal membrane patch and biomimetic adhesive coacervates as a sealant for fetoscopic defects

Iatrogenic preterm premature rupture of membranes after fetoscopic procedures affects 10-47% of patients, secondary to the non-healing nature of membranes and the separation of layers during the entry. In this study we developed an in vitro model to mimic the uterine wall-fetal membrane interface using a water column with one end sealed with human fetal membranes and poultry breast, and a defect was created with an 11 French trocar. Further, a fetal membrane patch in conjunction with multiphase adhesive coacervates modeled after the sandcastle worm bioadhesive was tested for sealing of an iatrogenic defect. The sealant withstood an additional traction of 12 g for 30-60 min and turbulence of the water column without leakage of fluid or slippage. The adhesive is non-toxic when in direct contact with human fetal membranes in an organ culture setting. A fetal membrane patch with multiphase adhesive complex coacervates may help to seal the defect and prevent iatrogenic preterm premature rupture of the membranes.

Localization of the bioadhesive precursors of the sandcastle worm, Phragmatopoma californica (Fewkes)

The marine sandcastle worm bonds mineral particles together into underwater composite dwellings with a proteinaceous glue. The products of at least four distinct secretory cell types are co-secreted from the building organ to form the glue. Prominent hetereogeneous granules contain dense sub-granules of Mg and the (polyphospho)proteins Pc3A and B, as well as at least two polybasic proteins, Pc1 and Pc4, as revealed by immunolabeling with specific antibodies against synthetic peptides. Equally prominent homogeneous granules comprise at least two polybasic proteins, Pc2 and Pc5, localized by immunolabeling with anti-synthetic peptide antibodies. The components of the sub-micrometer granule types are unknown, though positive staining with a redox-sensitive dye suggests the contents include o-dihydroxy-phenylalanine (dopa). Quantitative PCR and in situ hybridization demonstrated that a tyrosinase-like enzyme with a signal peptide was highly expressed in both the heterogeneous and homogeneous granules. The contents of the granules are poorly mixed in the secreted mixture that forms the glue. Subsequent covalent cross-linking of the glue may be catalyzed by the co-secreted tyrosinase. The first three parapodia of the sandcastle worm also contain at least two distinct secretory tissues. The Pc4 protein was immunolocalized to the anterior secretory cells and the tryosinase-like gene was expressed in the posterior secretory cells, which suggests these proteins may have multiple roles.

Complex coacervates as a foundation for synthetic underwater adhesives

Complex coacervation was proposed to play a role in the formation of the underwater bioadhesive of the Sandcastle worm (Phragmatopoma californica) based on the polyacidic and polybasic nature of the glue proteins and the balance of opposite charges at physiological pH. Morphological studies of the secretory system suggested that the natural process does not involve complex coacervation as commonly defined. The distinction may not be important because electrostatic interactions likely play an important role in the formation of the sandcastle glue. Complex coacervation has also been invoked in the formation of adhesive underwater silk fibers of caddisfly larvae and the adhesive plaques of mussels. A process similar to complex coacervation, that is, condensation and dehydration of biopolyelectrolytes through electrostatic associations, seems plausible for the caddisfly silk. This much is clear, the sandcastle glue complex coacervation model provided a valuable blueprint for the synthesis of a biomimetic, water-borne, underwater adhesive with demonstrated potential for repair of wet tissue.

Natural Underwater Adhesives

The general topic of this review is protein-based underwater adhesives produced by aquatic organisms. The focus is on mechanisms of interfacial adhesion to native surfaces and controlled underwater solidification of natural water-borne adhesives. Four genera that exemplify the broad range of function, general mechanistic features, and unique adaptations are discussed in detail: blue mussels, acorn barnacles, sandcastle worms, and freshwater caddisfly larva. Aquatic surfaces in nature are charged and in equilibrium with their environment, populated by an electrical double layer of ions as well as adsorbed natural polyelectrolytes and microbial biofilms. Surface adsorption of underwater bioadhesives likely occurs by exchange of surface bound ligands by amino acid sidechains, driven primarily by relative affinities and effective concentrations of polymeric functional groups. Most aquatic organisms exploit modified amino acid sidechains, in particular phosphorylated serines and hydroxylated tyrosines (dopa), with high-surface affinity that form coordinative surface complexes. After delivery to the surfaces as a fluid, permanent natural adhesives solidify to bear sustained loads. Mussel plaques are assembled in a manner superficially reminiscent of in vitro layer-by-layer strategies, with sequentially delivered layers associated through Fe(dopa)(3) coordination bonds. The adhesives of sandcastle worms, caddisfly larva, and barnacles may be delivered in a form somewhat similar to in vitro complex coacervation. Marine adhesives are secreted, or excreted, into seawater that has a significantly higher pH and ionic strength than the internal environment. Empirical evidence suggests these environment triggers could provide minimalistic, fail-safe timing mechanisms to prevent premature solidification (insolubilization) of the glue within the secretory system, yet allow rapid solidification after secretion. Underwater bioadhesives are further strengthened by secondary covalent curing.

Multiphase adhesive coacervates inspired by the Sandcastle worm

Water-borne, underwater adhesives were created by complex coacervation of synthetic copolyelectrolytes that mimic the proteins of the natural underwater adhesive of the sandcastle worm. To increase bond strengths, we created a second polymer network within cross-linked coacervate network by entrapping polyethylene glycol diacrylate (PEG-dA) monomers in the coacervate phase. Simultaneous polymerization of PEG-dA and cross-linking of the coacervate network resulted in maximum shear bond strengths of ∼1.2 MPa. Approximately 40% of the entrapped PEG-dA polymerized based on attenuated total reflectance-Fourier transform infrared spectroscopy. The monomer-filled coacervate had complex flow behavior, thickening at low shear rates and then thinning suddenly with a 16-fold drop in viscosity at shear rates near 6 s(-1). The microscale structure of the complex coacervates resembled a three-dimensional porous network of interconnected tubules. The sharp shear thinning behavior is conceptualized as a structural reorganization between the interspersed phases of the complex coacervate. The bond strength and complex fluid behavior of the monomer-filled coacervates have important implications for medical applications of the adhesives.

Biocompatibility of adhesive complex coacervates modeled after the sandcastle glue of Phragmatopoma californica for craniofacial reconstruction

Craniofacial reconstruction would benefit from a degradable adhesive capable of holding bone fragments in three-dimensional alignment and gradually being replaced by new bone without loss of alignment or volume changes. Modeled after a natural adhesive secreted by the sandcastle worm, we studied the biocompatibility of adhesive complex coacervates in vitro and in vivo with two different rat calvarial models. We found that the adhesive was non-cytotoxic and supported the attachment, spreading, and migration of a commonly used osteoblastic cell line over the course of several days. In animal studies we found that the adhesive was capable of maintaining three-dimensional bone alignment in freely moving rats over a 12 week indwelling period. Histological evidence indicated that the adhesive was gradually resorbed and replaced by new bone that became lamellar across the defect without loss of alignment, changes in volume, or changes in the adjacent uninjured bone. The presence of inflammatory cells was consistent with what has been reported with other craniofacial fixation methods including metal plates, screws, tacks, calcium phosphate cements and cyanoacrylate adhesives. Collectively, the results suggest that the new bioadhesive formulation is degradable, osteoconductive and appears suitable for use in the reconstruction of craniofacial fractures.

First insights into the biochemistry of tube foot adhesive from the sea urchin Paracentrotus lividus (Echinoidea, Echinodermata)

Sea urchins are common inhabitants of wave-swept shores. To withstand the action of waves, they rely on highly specialized independent adhesive organs, the adoral tube feet. The latter are extremely well-designed for temporary adhesion being composed by two functional subunits: (1) an apical disc that produces an adhesive secretion to fasten the sea urchin to the substratum, as well as a deadhesive secretion to allow the animal to move and (2) a stem that bears the tensions placed on the animal by hydrodynamism. Despite their technological potential for the development of new biomimetic underwater adhesives, very little is known about the biochemical composition of sea urchin adhesives. A characterization of sea urchin adhesives is presented using footprints. The latter contain inorganic residues (45.5%), proteins (6.4%), neutral sugars (1.2%), and lipids (2.5%). Moreover, the amino acid composition of the soluble protein fraction revealed a bias toward six amino acids: glycine, alanine, valine, serine, threonine, and asparagine/aspartic acid, which comprise 56.8% of the total residues. In addition, it also presents higher levels of proline (6.8%) and half-cystine (2.6%) than average eukaryotic proteins. Footprint insolubility was partially overcome using strong denaturing and reducing buffers, enabling the visualization of 13 proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The conjugation of mass spectrometry with homology-database search allowed the identification of six proteins: alpha and beta tubulin, actin, and histones H2B, H3, H2A, and H4, whose location and function in the adhesive are discussed but require further investigation. For the remaining unidentified proteins, five de novo-generated peptide sequences were found that were not present in the available protein databases, suggesting that they might be novel or modified proteins.

Cell dynamics during cocoon secretion in the aquatic leech, Theromyzon tessulatum (Annelida: Clitellata: Glossiphoniidae)

One distinguishing feature of clitellate annelids is the presence of specialized segments comprising the clitellum, whose primary function is to secrete a cocoon. Using histological analyses, we have documented cell types (I-V) and cellular processes associated with cocoon secretion in the aquatic leech, Theromyzon tessulatum. Our data indicate that the bulk of the cocoon's biomass arises from precursor cells of a single type that hypertrophy and proliferate approximately 1 week prior to egg laying, and then differentiate into either of two cell types (i.e., Type II or Type III) depending on their position within the clitellum. Type II cells are concentrated along the lateral edges and venter of the clitellum and secrete alcian blue-staining granules that form opercula (i.e., glue-like material that seals both cocoon ends), while Type III cells populate the dorsal midline and secrete azocarmine-staining granules that build the cocoon wall. Both cell types occupy spaces between deep muscle layers and extend long-neck tubules to the surface epithelium as they fill with granules a few days prior to egg laying. Other cell types appear to make minor contributions to the cocoon (e.g., Type I, Type IV) or have supporting or signaling roles (e.g., Type V). Our observations suggest that post-translational modification (i.e., glycosylation) of the same core protein(s) distinguishes the granules of Type II/III cells, and that the default state of the Type II/III precursor may be evolutionarily linked to secretory cells in basal polychaetes.

Novel antifoulants: inhibition of larval attachment by proteases

We investigated the effect of commercially available enzymes (alpha-amylase, alpha-galactosidase, papain, trypsin, and lipase) as well as proteases from deep-sea bacteria on the larval attachment of the bryozoan Bugula neritina L. The 50% effective concentrations (EC(50)) of the commercial proteases were 10 times lower than those of other enzymes. Crude proteases from six deep-sea Pseudoalteromonas species significantly decreased larval attachment at concentrations of 0.03 to 1 mIU ml(-1). The EC(50) of the pure protease from the bacterium Pseudoalteromonas issachenkonii UST041101-043 was close to 1 ng ml(-1) (0.1 mIU ml(-1)). The protease and trypsin individually incorporated in a water-soluble paint matrix inhibited biofouling in a field experiment. There are certain correlations between production of proteases by bacterial films and inhibition of larval attachment. None of the bacteria with biofilms that induced attachment of B. neritina produced proteolytic enzymes, whereas most of the bacteria that formed inhibitive biofilms produced proteases. Our investigation demonstrated the potential use of proteolytic enzymes for antifouling defense.

The role of calcium and magnesium in the concrete tubes of the sandcastle worm

Sandcastle worms Phragmatopoma californica build mound-like reefs by sticking together large numbers of sand grains with cement secreted from the building organ. The cement consists of protein plus substantial amounts of calcium and magnesium, which are not invested in any mineral form. This study examined the effect of calcium and magnesium depletion on the structural and mechanical properties of the cement. Divalent ion removal by chelating with EDTA led to a partial collapse of cement architecture and cement dislodgement from silica surfaces. Mechanical properties examined were sand grain pull-out force, tube resistance to compression and cement adhesive force. EDTA treatment reduced sand grain pull-out forces by 60% and tube compressive strength by 50% relative to controls. EDTA lowered both the maximal adhesive force and energy dissipation of cement by up to an order of magnitude. The adhesiveness of calcium- and magnesium-depleted cement could not be restored by re-exposure to the ions. The results suggest that divalent ions play a complex and multifunctional role in maintaining the structure and stickiness of Phragmatopoma cement.

When quinones meet amino acids: chemical, physical and biological consequences

Quinones and amino acids are usually compartmentally separated in living systems, however there are several junctions in which they meet, react and influence. It occurs mainly in wounded, cut or crushed plant material during harvest, ensiling or disintegrating cells. Diffusing polyphenols are oxidized by polyphenol oxidases (PPOs) to quinonic compounds, which associate reversibly or irreversibly with amino acids and proteins. The reaction takes place with the free nucleophilic functional groups such as sulfhydryl, amine, amide, indole and imidazole substituents. It results in imine formation, in 1,4-Michael addition via nitrogen or sulphur and in Strecker degradation forming aldehydes. The formation and activity of quinone-amino acids conjugates influences the colour, taste, and aroma of foods. Physical and physiological phenomena such as browning of foods, discoloration of plants during processing, alteration of solubility and digestibility, formation of humic substances, germicidal activity, cytotoxicity and more occur when quinones from disintegrating cells meet amino acids. The mechanisms of toxicity and the pathways by which PCBs may be activated and act as a cancer initiator include oxidation to the corresponding quinones and reaction with amino acids or peptides. Sclerotization of insect cuticle is a biochemical process involving also the reaction between quinones and amino acid derivatives.

Cement Proteins of the Tube-building Polychaete Phragmatopoma californica

The mineralized tube of the sandcastle worm Phragmatopoma californica is made from exogenous mineral particles (sand, shell, etc.) glued together with a cement secreted from the "building organ" on the thorax of the worm. The glue is a cross-linked mixture of three highly polar proteins. The complete sequences of Pc-1 (18 kDa) and Pc-2 (21 kDa) were deduced from cDNAs derived from previously reported peptide sequences (Waite, J. H., Jensen, R., and Morse, D. E. (1992) Biochemistry 31, 5733-5738). Both proteins are basic (pI approximately 10) and exhibit Gly-rich peptide repeats. The consensus repeats in Pc-1 and -2 are VGGYGYGGKK (15 times), and HPAVXHKALGGYG (eight times), respectively, in which X denotes an intervening nonrepeated sequence and Y is modified to 3,4-dihydroxyphenyl-l-alanine (Dopa). The third protein, Pc-3, was deduced from the cement to be about 80 mol % phosphoserine/serine, and the cDNA was obtained by exploiting the presence of poly-serine repeats. Pc-3 consists of a family of at least seven variants with 60-90 mol % serine most of which is phosphorylated in the cement. Pc-1, -2, and -3 contain cysteine some of which reacts to form 5-S-cysteinyl-Dopa cross-links during the setting process.

The tube cement of Phragmatopoma californica: a solid foam

Phragmatopoma californica is a marine polychaete that builds protective tubes by joining bits of shell and sand grains with a secreted proteinaceous cement. The cement forms a solid foam (closed cells) via covalent crosslinking, as revealed by electron and laser scanning confocal microscopy. The cement contains extractable calcium and magnesium,and non-extractable phosphorus. Amino acid analysis demonstrated that the phosphorus is in the form of phosphoserine and that >90% of serine in the cement (i.e. 28 mol% of residues) is phosphorylated. In addition to previously identified basic proteins, the cement contains a highly acidic polyphosphoserine protein as a major component. We propose a model for the structure and bonding mechanism of the cement that has the following major features: (1) within the secretory pathway of cement gland cells, the electrostatic association of the oppositely charged proteins and divalent cations (Ca2+ and Mg2+) condense the cement proteins into dehydrated secretory granules; (2) the condensation of the cement leads to the separation of the solution into two aqueous phases (complex coacervation) that creates the closed cell foam structure of the cement; (3)rehydration of the condensed cement granules after deposition onto tube particles contributes to the displacement of water from the mineral substrate to facilitate underwater adhesion; and (4) after secretion, covalent cross-linking through oxidative coupling of DOPA gradually solidifies the continuous phase of the cement to set the porous structure.

Combinatorial screening for enzyme-mediated coupling. Tyrosinase-catalyzed coupling to create protein--chitosan conjugates

In nature, tyrosinase-generated o-quinones are commonly involved in processes that lead to functional biomaterials. These biomaterials are chemically complex and have been difficult to analyze. Furthermore, the cascade of reactions involving o-quinones is poorly understood, and it has been difficult to mimic ex vivo for materials processing. We report the use of a combinatorial approach to learn how tyrosinase and low molecular weight phenolic precursors can be used to generate biologically active protein-polysaccharide conjugates. Specifically, we screened various phenolic coupling precursors and various reaction conditions for the coupling of proteins onto the polysaccharide chitosan. Several natural phenols were identified as appropriate precursors for the coupling of polyhistidine tagged organophosphorus hydrolase (His-OPH) onto chitosan films. OPH activity was retained upon coupling and subsequent studies indicated that the histidine tag was not necessary for coupling. Using conditions identified for His-OPH coupling, we observed that various biologically active proteins (cytochrome c, OPH, and His-CAT) could be coupled onto chitosan films. The glycosylated protein horseradish peroxidase was not effectively coupled onto chitosan under the conditions studied. In all cases studied, we observed that coupling required a phenolic precursor, suggesting that tyrosinase is unable to couple by reaction with surface tyrosyl residues of the target protein. In conclusion, this study illustrates a combinatorial approach for the "discovery" of conditions to couple biologically active proteins onto chitosan through natural, quinone-based processes.

Chitosan based water-resistant adhesive. Analogy to mussel glue

Using analogies from nature, we investigated the possibility that tyrosinase-catalyzed reactions of 3,4-dihydroxyphenethylamine (dopamine) could confer water-resistant adhesive properties to semidilute solutions of the polysaccharide chitosan. Rheological measurements showed that the tyrosinase-catalyzed, and subsequent uncatalyzed, reactions lead to substantial increases in the viscosity of the chitosan solutions. Samples from these high-viscosity modified-chitosans were spread onto dry glass slides, the slides were lapped and clipped together either in air or after being submerged in water, and the bound slides were held under water for several hours. Adhesive shear strengths of over 400 kPa were observed for these modified chitosan samples, while control chitosan solutions conferred no adhesive strength (i.e., the glass slides separated in the absence of measurable forces). High viscosities and water-resistant adhesive strengths were also observed when semidilute chitosan solutions were treated with the known cross-linking agent, glutaraldehyde. Further studies indicate a relationship between the increased viscosities and water-resistant adhesion. These results demonstrate that the renewable biopolymer chitosan can be converted into a water-resistant adhesive.

Synthetic Polypeptide Mimics of Marine Adhesives

Water soluble copolypeptides containing l-dihydroxyphenylalanine (DOPA) and l-lysine were prepared by ring-opening polymerization of alpha-amino acid N-carboxyanhydride (NCA) monomers. We have prepared a range of different copolymers to probe the effects of functional group composition on adhesive and cross-linking behavior. Aqueous solutions of these copolymers, when mixed with a suitable oxidizing agent (e.g., O2, mushroom tyrosinase, Fe3+, H2O2, or IO4-), formed cross-linked networks that were found to form moisture-resistant adhesive bonds to a variety of substrates (e.g., aluminum, steel, glass, and plastics). It was found that successful adhesive formation was dependent on oxidation conditions, with chemical oxidants giving the best results. Optimized systems were found to form adhesive bonds that rival in strength those formed by natural marine adhesive proteins. Our synthetic systems are readily prepared in large quantities and require no enzymes or other biological components.

Environmental bioadhesion: themes and applications

Many marine organisms attach to underwater surfaces using protein adhesives. These are basic proteins with high levels of the amino acid 3,4-dihydroxyphenylalanine and an extended flexible conformation. The hydroxylation of tyrosine residues plays a key role in the chemisorption of these polymers to surfaces and in the setting of the adhesive. These unique proteins are attracting biotechnological attention for application in industry and medicine. Recent development on the immobilization of antigens and antibodies, enzymes, cells and tissues, illustrate the great potential use of these adhesives for diagnostics and medicine. The use of these adhesive proteins as anticorrosive coats for metal also suggests important applications for industry.

Characterization of two high molecular weight catechol-containing glycoproteins from pharate pupal cuticle of the tobacco hornworm, Manduca sexta

Two high molecular weight cuticular proteins (MSCP120 and MSCP246) were extracted in acidic guanidine hydrochloride solution from tanning abdominal cuticle of Manduca sexta pharate pupae and purified by size exclusion high performance liquid chromatography. The apparent molecular weights were ca. 120 and 246 kDa as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Both proteins contained high levels of glutamate/glutamine, glycine, serine, alanine and aspartate/asparagine. MSCP120 was enriched in histidine relative to MSCP246, but the reverse was true for valine and proline. Small quantities of beta-alanine and 3,4-dihydroxyphenylalanine (DOPA), as well as other catechols and carbohydrates, also were detected in the hydrolysates. The proteins became radiolabeled when [1-14C]-beta-alanine was injected into pharate pupae, presumably by the formation of adducts with N-beta-alanyldopamine metabolites during sclerotization. Mild acid hydrolysis released N-beta-alanylnorepinephrine and 3,4-dihydroxyphenylketoethanol from both proteins. Strong acid hydrolysis yielded predominantly 3,4-dihydroxyphenylketoethylamine (arterenone), but also DOPA and dopamine. The N-terminal amino acid sequences of the two cuticular proteins were dissimilar, and that of MSCP246 was more hydrophobic than MSCP120. Both of these proteins were glycosylated with glucose, N-acetylglucosamine and traces of N-acetylgalactosamine, and MSCP246 also contained galactose. These structural glycoproteins, which occur in cuticle undergoing sclerotization, apparently react post-translationally with quinonoid tanning agents to yield catecholamine-protein adducts. Small amounts of peptidyl DOPA probably are formed by hydroxylation of tyrosyl residues. Results from this study are consistent with the hypothesis that these catechol-containing glycoproteins participate in cross-linking reactions in M. sexta pupal cuticle during sclerotization.

Calcium control of metamorphosis in polychaete larvae

The importance of Ca2+ in the control of metamorphosis of a marine invertebrate larva was investigated. An excess of [Ca2+] in the external medium induced metamorphosis of Phragmatopoma californica (polychaete) larvae in a concentration-dependent manner. This effect is specific for calcium, and not simply the result of osmotic changes, as an excess of Mg2+ did not induce metamorphosis. Consistent with this finding, the calcium ionophore, A23187, also induced metamorphosis in a concentration-dependent manner. Paradoxically, however, the aromatic compounds diltiazem, verapamil, D600, and nifedipine, known to block Ca2+ channels in other systems, also induced metamorphosis. When exposed to diltiazem for only 20 h and subsequently washed free of this compound, 95% of the larvae metamorphosed and developed normally. Previous studies have demonstrated that the induction of metamorphosis in Phragmatopoma californica is controlled by chemosensory recognition of an exogenous morphogen and mediated by an excitatory pathway that involves adenyl cyclase and cyclic AMP. Because cellular excitation and cyclic AMP-dependent signal transduction generally involve the participation of calcium ion, the most parsimonious explanation for the results reported here include (1) direct control of the morphogenetic pathway by calcium ion, and (2) complexities of the calcium regulation of this process, or a functional similarity between the structurally related aromatic effectors tested and the natural inducer of metamorphosis.

Eggshell formation in Bdelloura candida, an ectoparasitic turbellarian of the horseshoe crab Limulus polyphemus

The marine triclad Bdelloura candida (Turbellaria) episodically deposits stalked eggshells onto the gill lamellae of the horseshoe crab Limulus polyphemus. Ultrastructural and biochemical analyses indicate that the eggshells consist of primary inner and secondary outer layers. Protein of the primary layer is rich in glycine, aspartate/asparagine, serine, and 3,4-dihydroxyphenylalanine (Dopa). In this regard it resembles the eggshell compositions of other members of the phylum Platyhelminthes. The primary layer appears to be derived from precursors produced in the vitelline cells of the flatworm. Each egg-laying episode consumes all of the precursor-containing vitelline cells. A Dopa-containing protein, vitelline protein 1 (Vp1), has been isolated from B. candida and resembles the primary eggshell layer in its composition. Vp1 has an apparent molecular weight of 34.4 and an acidic pI. Dopa-containing proteins are presumed to be directly involved in the cross-linking reactions that accompany quinone-tanning. The secondary outer layer of the eggshell may serve to adhere the eggshell to the gill. Its amino acid composition is unlike that of the primary eggshell and nothing is known about its precursors.